L ru a S J 0* s a m a A STUDENT'S TEXT. BOOK OF ZOOLOGY. By ADAM SEDGWICK, M.A., F.R.S., Professor of Zoology and Comparative Anatomy in the University of Cam bridge, Professor of Zoology (elect) at the Imperial College of Science and Technology. In Three Volumes. Profusely Illustrated. Vol. I. Protozoa to Chsetognatha xii., 620. 18/ (1898). Vol. II. Amphioxus and Vertebrata, xvi., 705. 21/- (1905). Vol. III. Tunicata, Enteropneusta,Echinodermata, and Arthropoda. The Introduction to Arthropoda,the Crustacea and Xiphosura by J. J. LISTER, M.A., F.R.S. The Insecta and Arachnida by A. E. SHIPLEY, M.A., Hon. D.Sc., Princeton, F.R.S. xii., 906. 24/- (1909). SOME OPINIONS OF THE PRESS ON VOLS. I AND II. ' Both as a successful teacher of many years' standing and as the translator of Glaus' " Text-Book of Zoology," a work which has for long held its own against younger rivals, the author is well A STUDENT'S TEXT-BOOK OF ZOOLOGY SOME OPINIONS OF THE PRESS continued. equipped for his task. He has produced, as was to be expected, a well-planned, well-illustrated, and trustworthy manual, a book which is so good, and so likely to exert a considerable influence by later editions, that our best plan will be rather to indicate the capabilities for improvement than to eulogize the successes at- tained." Athenceum (1898). " Zoological students owe a deep gratitude to Professor Sedgwick for this excellent and very comprehensive work." Spectator (1898). " As an exposition of the modern ideas of the morphological rela- tions of the coelom and the animals possessing it, Mr. Sedgwick's work will take a high rank." Literature (1898). " The book is a most valuable contribution to zoological literature and will be of special use to advanced students and teachers as well as to those working at special groups, who wish to keep in touch with general zoology." Lancet (1898). ' It makes an admirable and handy book of reference to others interested in natural history, who will here find the general nature and habits of a large number of animals described in a readable style. No efforts have been spared to make the path of the young zoologist as easy and pleasant as possible. The book is beyond question one of the best volumes on zoology at present available. "- Knowledge (1898). " Mr. Sedgwick is an original thinker with a clear, terse and incisive style." Guardian (1898). '' It is without doubt the completest work of its kind accessible to English readers." Pall Mall Gazette (1898). A STUDENT'S TEXT-BOOK OF ZOOLOGY SOME OPINIONS OF THE PRESS continued. " Mr. Sedgwick's book is a very good one, ably put together and likely to be extremely useful. It is, in fact, not only the last but the best zoological text-book in the language." Prof. Ray Lankester in Nature (1898). " Zoologists are to be congratulated on the appearance of the second volume of what promises to be a monumental work." Athenceum (1905). " There is all the information that the student, and even the advanced student, requires about the anatomy and the classification of vertebrated animals ; moreover all the salient facts are illus- trated by numerous figures in the text. We know of no English text-book that is so full." LanceJ (1905). " The student will find everything that a text-book of zoology should contain." British Medical Journal (1905). " This cautious and excellent text-book."- Zoologist (1905). "It is a fine piece of work which will fully uphold the reputa- tion that the first volume has gained."- Cambridge Review (1905). " The first volume has a well-deserved reputation for accuracy, clearness, terseness and independence, and in the crowd of text-books it has filled a definite place to the satisfaction of teachers as well as of students. " The scholarliness, clearness and carefulness of statement are obvious, but those who work with it will discover other virtues. A suggestive scepticism, a mature judgment and a more indefinable quality which we can only hint at in the phrase morphological insight." Nature (1905). A STUDENT'S TEXT-BOOK OF ZOOLOGY SOME OPINIONS OF THE PRESS continued. " It is marked by all the qualities of accuracy, lucidity and orderly method which were conspicuous in his first volume, and encourages us to hope that when the work is complete it will compare favourably with any of the books on a similar scale which have hitherto been produced more commonly in other countries. Mr. Sedgwick is not merely a biologist ; he is also a cosmical philosopher, who has ever kept in mind the relation of his particular subject to the vast scheme of things, and so is able to perceive the proper proportion of its details." Spectator (1905). " There is never any doubt what he means, and the lucidity of his exposition can only come from a thorough knowledge of the facts he has to lay before his readers." Guardian (1905). A STUDENT'S TEXT-BOOK OF ZOOLOGY CORRIGENDA "x Page27. Line 10-11 of paragraph 2, for " Rtiodooma " read " Rhodosoma." 167. Legend of Fig. 117, for " Parkinson " read " Parkinsonl." 177. Legend of Fig. 128, for " hedatic " read " hepatic." 183. Line 10 from bottom, for " Gymnasteria" read " Gymnasterias." 189. Fam. 1 delete the following genera Gnathaster Slad., Mimaster Slad., Leptogonaster Slad. 189. Line 8 of Fam. 1, for " tenuispiins " read " tenuis pints." 266. Line 10 from bottom, delete " Pseudostichopus Theel." ,. 269. Line 4 from bottom, for " Myriotroeha " read " Myriotrochus." 487. Line 10 from bottom, for " Armadillium " read " Armadillidium." 540. Last line, for " Coenobita " read " Cenobita," or alter the other mentions of the genus to Coenobita. ,, 573. Line 6 from bottom, for " Opisthoptaus " read " Opisthopatm." 716. From 25 line 12, 13. Taleporia and Solenobia are now classified with the Tineidae, which see. J2_- A STUDENT'S TEXT-BOOK OF ZOOLOGY BY ADAM SEDGWICK, M.A. F.R.S. FELLOW AND FORMERLY TUTOR OF TRINITY COLLEGE, CAMBRIDGE, AND PROFESSOR OF ZOOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY. PROFESSOR OF ZOOLOGY (ELECT) AT THE IMPERIAL COLLEGE OF SCIENCE AND TECHNOLOGY VOL. Ill THE INTRODUCTION TO ARTHROPODA, THE CRUSTACEA, AND XIPHOSURA, By J, J. LISTER, M.A. F.R.S. FELLOW OF ST. JOHN'S COLLEGE THE INSECTA AND ARACHNIDA By A. E. SHIPLEY, M.A. Hon. D.Sc. Princeton, F.R.S. FELLOW OF CHRIST'S COLLEGE AND READER OF ZOOLOGY IN THE UNIVERSITY LONDON SWAN SONNENSCHEIN AND CO. LTD NEW YORK: THE MACMILLAN CO. 1909 PREFACE IN parting with the present volume I have the satisfaction of feeling that I have accomplished the first part of the task which I set myself fourteen years ago. When I began I looked forward to executing the whole work, both Special and General Parts, myself in seven or eight years. That expectation has not been realized. Indeed had it not been for the assistance given me by my colleagues, Mr. J. J. Lister and Mr. A. E. Shipley, the present volume would have been far from completion. Thanks to their generous co-operation it is now finished, and I am in a position to turn my attention to the General Part. It is satisfactory to have finished it, but it is impossible to avoid a feeling of regret that owing to circumstances which were unforeseen when the work was commenced it has occupied so manv years and that the General Part, to write which the Special Part was undertaken, is not yet begun. The distribution of the work of this volume has been as follows : The chapters on Tunicata, Enteropneusta, Echinodermata. Onychophora and Myriapoda are by my- self ; the chapters on the Arthropoda in General and on the Crustacea, and the section on the Xiphosura are by Mr. Lister ; and those on the Insecta and the Arachnida by Mr. Shipley. Our thanks are especially due to Professor Herdman, Mr. Punnett, Dr. Bather, Professor MacBride, Mr. Sinclair, Dr. Caiman, Mr. L. A. Borradaile, Mr. David Sharp, Mr. Hugh Scott, Professor Imms, Mr. C. Warburton and Mr. C. C. Dobell for reading the proofs and for the advice and assist- ance which they have given in different parts of the volume. Our thanks are further due to those authors and pub- lishers who have allowed us to use illustrations which have VI PREFACE appeared in other works. The sources of these are acknow- ledged, but we must especially mention Claus's Lehrbuch, Shipley & MacBride's Zoology, Bronn's Thierreich, Kor- schelt & Heider's Textbook of Embryology, Delage & Herou- ard's Zoologie Concrete, Perrier's Traite de Zoologie, the Cambridge Natural History, Lang's Textbook of Comparative, Anatomy, Zittel's Grundziige der Palaeontologie , Lankester's Treatise on Zoology. I have also to state that the drawings from which figures 105 and 132 were engraved were made by Mr. J. C. Simpson under the direction of Professor MacBride. Now that the work is completed something must be said as to its architectural plan. This has been criticized (see, for example, the review in Nature, November 23, 1905) OIL the ground that the Arthropods are separated from the Annelids, and that the Tunicates and Enteropneusta are placed at the end of the Chordata. In the first volume the clue given by the coelom is mainly followed and this leads from the Annelids to the enterocoelic phyla the Phor- onidea, the Brachiopoda and the Chaetognatha. Having reached this point it was not possible to treat the Arthropoda until after the Enterocoela were finished. This accounts for the position of the Arthropoda at the end of the work. It may be objected that the Arthropoda should have come after the Annelida in volume I. But this would have involved the inconvenience of separating the Gephyrea Achaeta from the Gephyrea Armata, and the Brachiopoda and Polyzoa from the Chaetopoda. Moreover, the Arthro- poda differ so fundamentally from the Annelida in their coelomic arrangements and are such an enormous and self- contained group that it did not appear that any practical disadvantage would follow upon their separation from the latter which in some features of their organization they so closely follow. In fact we are here confronted with a difficulty which the systematic Zoologist meets at every turn and to which attention has often been called in the course of the work. When there is more than one clue, as there nearly always is, which shall we follow ? We are obliged to adopt a linear arrangement, whereas Nature PREFACE Vll is content with nothing less than arrangement in three dimensions. Following then the clue of the coelom and having reached so near the Chordata, it seemed natural and proper to take them after the Chaetognatha. I might of course have placed the Echinoderms here and treated the Enteropneusta after them. That was in fact the original intention, but after careful consideration there seemed no reason based on zoological affinity why this should be done and it would have been almost an outrage to have placed the Echinoderms next to Sagitta. It was accordingly decided to take the Chordata at this point, beginning with Amphioxus and re- serving the Enteropneusta to the end to lead on to the Echinoderms. Though I offer this explanation I quite recognize the validity of the criticism. All zoological arrangements are compromises, and none of them can be, now or ever, entirely natural. A. SEDGWTCK. CAMBRIDGE, November 25, 1908. TABLE OF CONTENTS CHAPTER I PAGE PHYLUM TUNICATA .... 1 Order 1. Ascidiacea. . . 25 Tribe 1. Ascidiae simplices (Monascidia) . . 26 2. Ascidiae composi- tae (Synascidia) 30 3. Ascidiae Salpae- fonnes (Asc. Luciae) ... 39 Order 2. Thaliacea ... 44 Sub-order 1. Hemimyaria (Safpida) . . 45 ,, 2. <>ydotnyaria (Doliolida}. . 54 Order 3. Appendiculariae . 60 CHAPTER II PHYLUM ENTEROPNEUSTA . 66 Order 1. Balanoglossida . 68 2. Cephalodiscida . 104 CHAPTER III PHYLUM ECHINODERMATA .115 Class I. ASTEROIDEA . . 166 Order 1. Encrinasteriae . . 188 2. Euasteriae. . .189 Sub-order 1. Phanerozonia 189 ,, 2. Cryptozonia . 191 Class II. OPHIUROIDEA . 194 Order 1. Lysophiurae . . 205 2. Zygophiurae (Ophiurae) . . .205 3. Streptophiurae . 206 , 4. Cladophiurae (Euryalae) . . . 207 Class III. ECHINOIDEA . Order 1. Palaechinoidea 2. Euechinoidea . Sub-order 1. Cidaroidu ,, 2. Diadematoida Section 1. Streptosoiuata 2. Stereosomata . Sub-order 3. Holectypoida . ,, 4. Clypeastroida 5. Spatangoida . Section 1. Asternata 2. Sternata Class IV. HOLOTHUROI- DEA . . Order 1. Actinopoda Sub-order 1. Aspidochiro- tae . 2. Dendrochirn- tae . Order 2. Paraetinopoda . Class V. CRIXOIDEA . Order 1. Larviformia 2. Fistulata . . 3. Camerata . . ,, 4. Flexibilia . . 5. Articulata . Class VI. CYSTIDEA. . Order 1. Amphorida 2. Rhombifera . ,, 3. Diploporida 4. Edrioasterida . Class VII. BLASTOIDEA CHAPTER IV PHYLUM ARTHROPODA PAGE 208 238 239 231? 239 . 240 . 240 242 243 244 24 245 247 265 266 268 269 270 293 293 295 297 298 303 309 309 309 310 310 . 314 60276 CONTENTS CHAPTER V PAGE Class I. CRUSTACEA . . 342 Sub-class 1. EXTOMOSTRACA 359 Order 1. Trilobita . . .361 2. Branchiopoda . . 3(57 Sub-order 1. PhyUopoda . 375 Tribe 1. Anostraca . . 376 2. Xotostraca . . 376 3. Conchostraca . . 377 Sub-order 2. Cladocera. . 377 Order 3. Ostracoda . . . 382 Sub- order 1. Myodocopa . 389 2. Podocopa. . 391 Order 4. Copepoda . . . 392 Sub-order 1. Eucopepoda . 404 2. Branchiura . 410 Order 5. Cirripedia . . . 413 Sub-order 1. Cirripedia Genuina . . 423 Tribe 1. Pedunculata . . 423 2. Operculata . . 425 Sub-order 2. Acrothoracica 426 3. Apoda . . 428 4. Rhizocephala 428 5. Ascotkoracica 434 Sub-class 2. MAI.ACOSTRACA 435 Order 1. Leptostraca . . 455 2. Syncarida . . . 459 3. Schizopoda . . 463 Tribe 1. Hemitropha . . 470 2. Holotropha . . 471 Order 4. Cumacea . . . 472 5. Tanaidacea . . 477 6. Isopoda . . . 479 Section 1. Isopoda genuina 487 Tribe 1. Oniscoidea . . 487 2. Asellota ... 487 3. Phreatoieidea . . 488 4. Valvifera ... 488 5. Flabellifera . . 488 6. Epiearidea . . 489 Section 2. Isopoda anomala 491 Order 7. Amphipoda. . . 492 Sub-order 1. Amphipoda genuina Division 1. Gam.mari.na Tribe 1. Corophiina ,, 2. Gammarina Genuina 499 500 500 500 PAGE Division 2. Hyperina . . 502 Tribe 1. Hyperina anomala 502 2. Hyperina normalia 503 Sub-order 2. Laemodipoda 503 Order 8. Stomalopoda . . 504 9. Decapoda . . .511 Section 1. Macrura Natantia 527 Tribe 1. Penaeidea . . 528 2. Stenopidea . . 531 3. Caridea ... 531 Section 2. Macrura Reptantia . . 534 3. Anomura . . 536 Tribe 1. Thalassinidea . . 537 2. Paguridea . . 539 3. Galatheidea . . 541 4. Hippidea . . . 542 Section 4. Brachyura . . 542 Tribe 1. Brachyura Anomala . . 543 2. Oxystomata . . 544 3. Oxyrhyncha . . 544 4. Catometopa . . 546 5. Cyclometopa . . 548 CHAPTER VI Class II. ONYCHOPHORA . 550 CHAPTER VII Class III. MYRIAPODA . . 578 Order 1. Pauropoda . . 580 2. Diplopoda . . 582 Sub-order 1. Psdapliog- nntha . . . 591 2. Chilo. IH't:}. p. 11!). t De Selys-Long^huinps. Arch. liiol., 17, I'.tol. p. 499. RENAL AND REPRODUCTIVE ORGANS. 17 beneath the endostyle, giving off near its origin a branch to the test ( Fig. 12). The other after giving off a branch to the test, which accompanies the branch from the endostylar vessel, is distribute:! to the digestive viscera, gonads and body wall. The endo- stylar vessel communicates with the sinuses in the pharyngeal wall and these again with a dorsal vessel running along the dorsal lamina. This posteriorly is distributed to the viscera and body wall, whence the blood is returned to the posterior end of the heart. It is a peculiarity very generally observable in Tunicata that the heart contracts a certain number of times in one direction and then a similar number of times in the opposite direction. Thus for a certain number of beats it acts as a respiratory heart driving the blood to the respiratory organs and thence to the system. It then reverses its action and becomes a systemic heart driving the blood first to the system and thence to the respiratory organs.* The blood is colourless and contains nucleated corpuscles. These are generally colourless and amoeboid, but some of them generally contain pigment, either yellow, red, brown or blue. In the Botryllidae the terminal branches of the test-vessels are dilated into ampulla-like sacs which have rhythmically contractile walls and assist in the circulation of the colony. The renal organs f are but little understood and appear to have no relation with the coelom. The only structures to which a renal function has been ascribed are some vesicular bodies containing concretions of uric acid and other substances, and placed in the walls of the intestine, in the mantle and sometimes in other places. These structures which have not been found in all Tunicates are without a duct ; so that the excretory matters cannot escape. In the Molgulidae there is a large saccular body of this nature on the right side of the body. It has been suggested that the neural gland plays a part in renal excretion. Reproductive Organs. The Tunicata are with very few exceptions (e.g. Oiko pleura dioica) hermaphrodite, and as a general rule the female organ ripens first. So far as can be ascertained the gonads have no relations with the coelom, either developmentally or otherwise. The glands are continuous with * Schultze, JcnaZeitsch., 35, 1901, p. 2'Z1 f Dahlgriin. Arch. f. mik: Anat. 58. 1901, p 60S z in c 18 PHYLUM TUNICATA (UROCHORDA). their ducts, the terminal parts of which are usually single and open into the atrial cavity. The ovaries and testes are generally placed near one another and contained either amongst or near the digestive viscera or in the mantle wall. In the Potyclinidae they are in the post-abdomen. In dona (Fig. 13) the testis is a branched gland ramifying on the wall of the intestine between the pylorus and the rectum. Its tubules gradually collect into one main duct, which accompanies the rectum and opens into the atrium in front of the anus by several openings, the walls of which contain a red pigment, consisting of red renal vesicles of the kind referred to above. The ovary is a rounded mass placed in the intestinal loop, and the oviduct accompanies the vas deferens to open close by it, far forward into the atrium. The gonads arise from a common mesodermal rudiment. In the synas- cidians they do not as a rule appear in the zooid which develops from the egg, and they may be absent from the first-formed generation of budded zooids (BotryUus. etc.). Reproduction by budding occurs in a large number of Tunicata. Sometimes, as in the synascidians. the budded individuals remain em- bedded in a common tunic with the parent ; sometimes, as in Tinliacea, they become eventually separate and lead for a time an independent existence. In the latter case the life history is complicated by the phenomenon of alternation of generations of the variety known as metagenesis ; for the indi- viduals which bud proceed from the egg and do not develop sexual organs. Budding does not occur in the Appcwliculnriae and monascidians. In all cases excepting BotryUus the budding is effected by the division of a ventrally placed process of the body called the stolon. The stolon contains a diverticulum of the pharynx known as the spicardium and given off between the endostyle and the oesophageal opening (p 15). The stolon also con- tains an extension of the mesodermal and vascular tissues of the parent. For a more detailed account of the phenomenon and of the origin of the ortmii- ach: / intestinal limp: ij n'ctiini. DEVELOPMENT. 19 to the accounts given below under each family or order. Here we will only remark that there is considerable variability- in the mode of formation of the organs. The pharynx, atrium, diges- tive organs and pericardium are usually derived from the epi- cardial process of the pharynx, while the ganglion in the synascidians usually develops from the endoderm, but it may arise from the mesoderm (Pyrosoma) or from the ectoderm (Thaliacea). In the Botnjllidac the endoderm of the parent does not participate in the budding process. Development. In most monascidians (except Ci/nl/il't, etc.) the eggs are fertilised in the sea or in the atrium and undergo their whole development outside the body of the parent. This is also the case in Doliolum and in the Appendiculariae. In the synascidians on the other hand the early development usually takes place in the atrial cavity or in incubatory pouches of it. In the salps the egg undergoes its early development in the ovary ; in the later stages it emerges into the atrial cavity but remains connected with the parent by the placenta The eggs are frequently laid in their follicle, which is somewhat com- plicated. It is formed of two layers (Fig. 14), the outer of which consists of vacuolated cells ; these are prolonged into papillae, and help to float the egg in the sea. The inner layer consists of follicle-cells which have migrated inwards, and are called the test-cells because formerly they were supposed to give rise to the test of the adult. The two layers are separ- ated by a structureless chorion. The development generally leads to the formation of a free-swimming tailed larva, the tadpole larva, by means of which the species is distributed over a wider area. The tad- pole larva is nearly always formed in the monascidians (it is absent in some species of Molgula) and in the synas- cidians (absent in Pyrosoma). Tt is akn fnnnrl in Dnlinhim FIG. 14. Mature egg from the oviduct of Ciona in JJOllOlUm, i,i>,-xtinalis (after Kupffer). < i<>llir!r cells but not in thp aln< im,,m-like cells); d chorion; . test-cells; _ baipb. / ovum ; z gelatinous substance. The eggs usually have but little yolk. In the synascidians they are however richer in yolk. and in Pyrosoma the cleavage is actually meroblastic 20 PHYLUM TUNICATA (UROCHORDA). Development of the tadpok larva. The segmentation is complete and leads to the formation of a blastosphere, from which the gastrula arises by invagination. The gastrula FIG. 15. Development of r/nil/nxiii niniininllatii (alter Kowalevsky from Glaus), a Com- mencemeut of invayination ; / h cleavage-cavity. b Gastrula with blastopore ; ch rudi- ment of notochonl ; hi/ endodenn. (. Later stage ; Ek ectoderm ; ^V rudiment of neural- canal ; Eil' eiulodenn of future tail region, d Stage with body and tail ; Ed' endoderm of tail ; M muscular cells in tail, <> Just-hatched larva ; A eye ; Bl blood corpuscles ; _D k com- mencing intestine ; !;,! endo^uie. ; F opening of cerebral vesicle into mouth ; Ob cerebral vesicle with otolith projecting from its floor ; Hp papilla for attachment ; Kl one of the atrial invaginations ; O mouth ; I'll pharynx ; R g anterior swelling of post-cerebral region of nerve tube ; lim posterior part of nerve tube. / Two days larva, only the anterior part of the tail is represented ; 1 ks, 2 ks branchial stigmata Bb blood shins between them ; D in- testine. elongates in the future antero-posterior axis and the blastopore comes to lie on the posterior end of the dorsal surface (Fig. 15 b). A flat median groove of ectoderm appears along the DEVELOPMENT. 21 dorsal side of the already bilaterally symmetrical embryo extending from the blastopore forwards. This groove, into the hind end of which the blastopore opens, is the first rudiment of the central nervous system. It is known as the medullary groove. Its edges project and form the medullary folds which grow round and close the narrow blastopore, and gradually unite with one another from this point forward in such a manner as to convert the groove into a canal, the walls of which separate from the external ectoderm and give rise to the central nervous system. This canal is the medullary canal ; behind it is shut off from the exterior, ,>ut communicates with the cavity of the gastrula by way of the blastopore which is now called the neurenteric canal ; while in front it remains open for some time, but eventually clos3S. Before these changes are completed, the medio-dorsal endoderm cells of that part of the gastric wall which immediately underlies the posterior part of the neural canal (Fig. 15, c and rf, Ch) become different from the remaining endoderm cells and constitute the first rudiment of the notochord. Meanwhile the latero-dorsal endoderm, on each side of the notochord. has separated off the mesoderm as a solid * plate of cells (Fig. 16, ms). Tnes3, in the trunk, later become converted into a mesenchyme, occupying the space which now makes its appearance be- tween the ectoderm and endoderm, and give rise tD the blood-corpuscles, musculature, genital, and excretory organs of the body, while the caudal part becomes the musculature of the tail. The distinction between the caudal and trunk region of the em- bryo is now very apparent. The notochord is confined to the tail. The caudal part of the enteron be- FlG 16 ._ Tran ^ e rse section through cnlirl /TTirr 1 ^ rl J7rJ ' \ flir irrli an mbryo of Clavetina (after V. SOlld (JUg. Ida, MO, ), tllOUgll Beneden and Julin. from Korschelt rprnainincr rnntinnnnn with flip and Heider). eft rudiment of noto- remaining COnilllUOUS Wlttl me chord ; ec ectoderm :? endoderm ; inprlnllirv tnhp rnnnrl flip Viirirl pnrl ms rudiment of mesoderm; mia lllar y medullary folds ; n medullary plate. of the notochord. Eventually it disappears, apparently giving rise to blood corpuscles. * According to van Benedeii and Julin there is at first an enteric pro- longation in the front part of this. 22 PHYLUM TUNIC AT A (UROCHORDA). The anterior part of the enteron dilates and constitutes the rudiment of the pharynx (Fig. 15, Ph], from the hind end of which the intestine is developed as an outgrowth (D). In the further course of development the tail becomes greatly elongated and curved ventrally on the trunk (Fig. 15 e), and some ectodermai papillae are formed at the anterior end of the body for the future attachment of the larva (Hp). The anterior end of the medullary tube becomes dilated into the cerebral vesicle, in the wall of which two sensory structures -the eye and auditory organ are developed (Fig. 15 /, Gb). The part of the medullary tube immediately behind this acquires thickened walls and is called the trunk ganglion (Ry). This is followed by the narrow caudal extension of the tube which ultimately disappears. The mouth is formed as a perforation on the dorsal surface of the front end and the cerebral vesicle acquires an opening into the anterior part of the alimentary canal (Fig. 15 /, o). The atrium arises as two dorso-lateral invaginations of ectoderm (kl) into the left of which the anus opens. The atrial invaginations spread laterally round the pharynx but remain separate ventrally ; dorsally they coalesce, so that the single atrial aperture arises. The gill-slits or branchial stigmata arise as a pair of perforations of the wall separating the atrial cavity from the pharynx. They subsequently become more numerous, partly by formation of new I ic :f orations and partly by division of those already existing. The endostyle arises as a groove on the anterior (Willey) or antero-ventral wall of the pharynx, but subsequently, as a con- sequence of the rotation which the body undergoes at the meta- morphosis (Fig. 18). becomes entirely ventral. The development of the t /limr/Httm. which as we have seen above is probably to be regarded as the coelom of the animal, seems to take place in different ways in different forms. In CioiKi it arises from the hind end of the pharynx as two diverticula which remain separate throughout life and invest the digestive viscera like a perivisceral cavity. In ('In r, linn (Fig. 17, ep) it appeals to arise as a single diverticulum of the pharynx between the end of the endostyle and the oesophagus, the front end of which becomes double. In some cases however it is apparently delainiiiated from the phaiynx and is at first solid. The peri- cardium in a great number of eases, if not universally, is nipped of) from 1 he epieardium either from its posterior unpaired portion DEVELOPMENT OF GANGLION AND NEURAL GLAND. 23 or from one of its anterior limbs. The heart arises as an in- vagination of the dorsal wall of the pericardium (Fig. 17, pc). The tunic is laid down as a cuticular excretion of the ectoderm, and the larva is hatched. After a few hours of free life, during which it swims by the movement of its tail, it attaches itself by the three anterior ectodermal papillae and the tail shrinks up to a stump and atrophies (Fig. 18). The cerebral vesicle largely breaks down. The dorsal part of it which communicates with the pharynx by the new neuropore persists and gives rise to the neural gland. The dorsal wall of this thickens and forms the -_j .___ ur . -,^* " e-y pp ft, FIG. 17. Left side view of a Clarelina embryo (after Seeliger from Korschelt and Haider* uu eye ; ch notochord ; e atrial aperture ; ed rectum ; ep epicardial outgrowth of pharynx es endostyle ; / folding of the body surface in anticipation of the rotation that take place after fixation ; fg duct of neural gland ; h adhering papillae ; i mouth ; ks gill slits ; m stomach ; mz muscle cells of the tail ; oe oesophagus ; ot auditory organ ; p atrial cavity ; pc pericardium ; s larval tail ; sb cerebral vesicle. ganglion of the adult. If this account is correct it follows that the neural gland is a part of the original cerebral vesicle. It has been compared to the hypophysis (pituitary body) of the Vertebrata ; but it differs from this in the fact that it is, in its origin, actually a part of the embryonic brain, which the pit- uitary body never is. The trunk ganglion becomes solid and persists as the visceral nerve. At the same time the fixed larva undergoes a peculiar change which is illustrated in the annexed diagram (Fig. 18). The mouth which was close to the point of attachment becomes displaced to the opposite end, and the original hind end becomes placed close to the point of attach- ment. In a few monascidians (Culeolus, etc.) this change does 24 PHYLUM TUMCATA (UROOHORDA). FIG. 18. Diagram illustrating the metamorphosis ot the larva of Clavelitia during and after fixation (f ri 1 1 Eorschell ami Eeider, after Seeliger). A Free-swi inji larva. B Larva just attached. C Older metamorphosed stage, ch notochord ; e atrial aperture ; ed in- testine ; c/( r|ijcardium ; cs cndnsU lr ; / i-ctudcrmal fold ; ft duct of neural gland ; g ganglion ; /< heart ; hp adhesive papillae ; i mouth ; A-.S gill-slits ; // atrial cavity ; r trunk-ganglion ; v part j tii. 11 wall of stolon ; sb cerebral vesicle ; . cast cell 11 1< e ti>>m' of tail ; st stolo prolifer ; r larv.-il tail degenerating. ASCIDIACEA. 25 not take place, the animal remaining throughout life attached by a stalk which arises close to the mouth (Fig. 19, 2). The following classification has been adopted : Order 1. Ascidiacea. Tribe 1. Ascidiae simplices, Monaseidia. ,, 2. Ascidiae compositae, Synascidia. ,, 3. Ascidiae salpaeformes, Ascidiae Luciae. Order 2. Thaliacea. Sub-order 1. Hemimyaria, Salpida. 2. Cyclomyaria, Doliolida. Order 3. Appendiculariae (Perennichordata, Larvacea, Cope- lata). Order 1. ASCIDIACEA (TETHYODEA). Fixed or free-sivimming , solitary or colonial Tunicnta, which in the adult are never provided with a tail and have no trace of a notochord. The free- swimming forms are colonies and the solitary forms are fixed. The test is permanent and well developed ; as a rule it increases with the age of the animal. The musculature of the mantle is in the form of an irregular network, there being no regular circu- lar bands. The pharynx is large and well developed. Its walls are perforated by numerous apertures opening into a single atrial (peribranchial) cavity, into which the anus opens and which communicates with the exterior by an atrial aperture. The colonial forms reproduce by gemmation, and in most the sexually produced embryo develops into a tailed larva. The order is divided into three groups, the Ascidiae Simplices, the Ascidiae Compositae and the Ascidiae Salpaeformes. These three groups can only be regarded as tribes, for they are closely interrelated, and are distinguished, the two first oy the presence or absence of the power of budding and the last by being free-swimming. If other and more general anatomical characters w r ere taken, quite a different grouping of the families would be obtained, and there can be no question that some of the families of synascidians are more closely related to cer- tain families of the monascidians than to each other. But if in the grouping of the families account were taken of these facts it is difficult to say that a more natural system of classification would be obtained ; cross-relation- ships between the groups constituted would still exist, and would by many be considered to be of sufficient importance to justify a different 26 PHYLUM TUNIC AT A (UROCHORDA) grouping, so that it seems best to adopt a plan which has at least the merit of being simple and easy of application. Tribe 1. ASCIDIAE SIMPLICES (MONASCIDIA). Solitary usually fixed forms, incapable of budding in the adult state ; with large 'pharynx, and numerous branchial stigmata. These are the typical sea-squirts. They are solitary forms usually of considerable size and attached by their tunics to rocks and sea-weeds. A few however, e.g. Molgula, are not attached save to grains of sand by processes of the test, and in a few stalked forms the stalk arises from near the mouth (Boltenia. Culeolus, Fig. 19). The tunic is usually somewhat thick and opaque and may have a cartilaginous consistency. When touched they frequently discharge two streams of water which proceed from the two openings, the one. the mouth or inhalent opening which is terminal and at the free end, the other the cloacal or atrial aperture, which is placed on the dorsal surface a little distance from the free end. The protective covering of mud or sand is generally associated with fibrous processes of the test (Molgulidae, Polycarpa molguloides. Ascidia conchilega), but in some cases the sand adheres directly to the test. In Culeolus, Fungal us and Bath your us. abyssal forms belonging to different sub-families of the Cynthiidae, the pharynx presents the remarkable modification of being without the fine longitu- dinal bars (Fig. 20). The dorsal lamina varies from the con- dition of a membrane to that of a series of languets ; an inter- mediate condition of a toothed membrane being found. The viscera are placed in the body- wall at the level of the hinder part of the pharynx (except in Ciona), usually on the left side. They project into the atrial cavity, sometimes being attached to its wall by a mesentery. The atrial cavity is always traversed by vascular strands passing from the pharynx wall to the mantle \vall. The vascular system is well developed, the sinuses sometimes having the aspect of vessels. There is usually a tailed larva. Fain. I. Ascidiidae. Usually sessile, rarely pedunculate d : month usually with S lohes, atrial aperture \\ it li <> : pharyngcal wall without folds, \\itli internal Inngit u< linal hars hearing papillae; stigmata straight 01 enr\ ec I ; tentacles unl>raiiclu > d : gonads pi act d in t he intcst inal loop. ( 'iona approximate^ t<> the ( 'la \ elinidae l>\ the position c t' its \ is< era ;iiid the presence of an epieanlillin (pp. !.">. '.{'). MONASCIDIA. 27 Pharynx with internal longitudinal bars and straight stn/ma/a, viscera on the left side of pharynx, dorsal lamina as a membrane. Ascidia L. (Figs. 2, 12), test soft, A. mentula O.F.M. : Phallusia Sav. , P. mammillata Cuv., pharynx recurved on itself posteriorly on the left side, Seas of E u r . ; Pachylaena Herd. , test hard. P h a r ij n x a s above, dorsal lamina as languets. dona F 1 e m . (Figs. 1, 3, 1, GA, 11, 13, 14), viscera posterior to phar- ynx, tunic gelatin- ous, renal cells grouped near tl it- generative orifice, C. intestinalis L. most seas. Rhodo- oma E h r e n b erg (Chev reulius L.- Duth.) (Fig. ID), test folded at the anterior end jso as to form a kind of bivalve operculum covering the mouth and atrial opening, viscera on right side, Med. Abyssas- cidia Herd., at- tached by ventral surface, apertures far apart, deep-sea, 2,000 to 2,(KlOfms., viscera on either right or left side of FIG. 19. 1. Holgula (Anurella) so.'enota. from the right side (nat. nharvnv size). ^1 atrial pore, B mouth, at the end of papilliform prominences Ti, Ti ; ai, r alimentary canal: /s, f* liver ; r reproductive gland?. Pharynx with 2. Boltenia wiformis x j. 3. Rliutlnxnitin (Chevreulius) callense x 3 ; internallongiludinal a moutll: n 8 an 8 lion : o atrial aperture ; y operculum (from Perrier). bars, curved stigmata and viscera on right side of pharynx, bars. inata straight ; tentacles simple or branched ; intestine on the left side, only slightly or not at all attached to the mantle ; goiiads on the inner surface of the mantle, either on. both sides or on one only. In the genera. Culeolus, Ci/xtint/i. 1-1. J''l<;. % J1.- Filanii nt> nt tin' tuiiii- i grains nf >aml arr attarln'il (from M<:/:/u/ii riixi-nritn. shnvvin 1 -' tin 1 - an IOHL; stalk, fioltona Sav. (Fig. 19), pharynx wall \\ith the line longitudinal bars, stalk arises on ventral surface near mouth. X. Atlantic, .\nstial.i-ia. Arctic. ('t ra Herd., stigmata not in spirals, lobes of the apertures plain, abyssal, test without processes, stalked. Ker- iHielen. Ctenicella L.-Duthiers, apertures laciniated. Aled. Eiigyra Alder and Hancock, pharynx not folded, but with saccular divert i- cula in longitudinal rows, gonads impaired, most seas. Paramolr/ula Traustedt, pharynx without folds, stigmata spirally coiled and in in- fundibula. gonads paired. Bostrichobrunrlnix Traustedt: (l(mni*ll Coelocormus) colonial forms, tin individuals of n-li i<-li n /it-oil nci by gemmation and are embeddnl in common test (except Claveli>iir in a special incubatory pouch which they do not leave until they have developed into the tailed larva. Vain. 1. Botryllidae. <'olon\ usuallv thin and encrusting, sometime- in the form of thick flesh \ masses; zooids arranged in systems (Fig. 23) circular or elliptical or in branching lines, the x.ooids of a >\stem openinu int 1 1 one CO! on cloaca ; C moil cloaca I openings (list ii let . usually lobed : /.odi the sui-face nf the colons- : intestine on the left side of the posterior part of 'I' In folds arc really longitudinal rows of saccular project ions ( inhnnli - i> nli i) of t lie phar\ ns_eul \\ all into t lie peril ira nchial ca\ il \ . i' I'i/.oii, Histoire dc la Uastonenese chc/. les Hotryllidcs. Ann. Sd. \n/. (7). 14, 1SII3. /S4. SYNASCIDIA. 31 the pharynx ; test usually soft, with numerous vessels ending in terminal knobs and joined to the body of each zooid at two points ; pharynx large with 3 internal longitudinal bars on each side and numerous stigmata ; dorsal lamina as a membrane ; tentacles simple, not more than 1 6 ; gonads on both sides in the mantle (except Symplegma) ; gemmation lateral, from the bodies of the zooids* ; the neural gland is dorsal to the ganglion in Botryllus ; the stomach has an hepatic caecum. The budding of Botryllus differs from that of other synascidians in that the endoderm does not participate ; the bud being formed as an outgrowth of the atrial cavity and consisting only of outer ectoderm of the body, ectodermal lining of the atrial cavity and interposed mesoderm (Fig. 24). The process begins in the larva before hatching, as a pair of ventral out- growths of the atrial cavity. After fixation of Hie larva the left of these atrophies and the right alone de- velops. The zooid produced from it gives rise to two buds by a process which is described below and is re- peated in all the subsequent budding. The zooids of the colony thus in- crease in geometrical ratio, but in all cases when the buds are developed, the form which has produced them dies ; thus the fixed larva dies when it has produced its bud, and the latter dies when it has developed and produced its two buds. The zooids produced arrange themselves so that their atrial cavities are tiirned to- wards one another and open into the common cloaca which is a depression of the surface of the colony as in Pyrosoma. A system of zooids thus arises. The number of zooids in a system is limited ; when the limit is reached, of the two buds which each zooid produces one atrophies, and the other, instead of taking up its Fl - 23. Botryllus violaceus (liter M. Ed- wards from Clans). mouth ; A opening position in the system, moves away O f common cloaca of a system. from it and becomes the centre of a new system in the same colony. The budded zooids do not separate from the parent as in most synascidian colonies. It is true that the pharynx of the bud loses its connection with the atrium of the parent, but the outer c todermal connection persists and becomes an elongated and slender tube by which the vascular systems of the parent and bud remain in continuity. On the atrophy of the parent zooid the two tubes which connected her to her offspring become directly continuous owing to the fact that though the internal organs of the parent break down, its ectoderm persists. As stated above the buds are formed as diverticula of the atrial cavity on its ventral side (Fig. 24). They form hollow vesicles, the cavity of each of which divides into two ; one of these becomes the pharynx and th<- * In Sarcobotrylloides Herdman describes stolonial budding from the vessels of the test. PHYLUM TTJNICATA (UROCHORDA). aie other, saddle-shaped, becomes the atrium of the bud. The organs formed in the usual way, the pericardium and intestine as outgrowths oi the pharynx and the from The Eel -1" FIG. 24. Diagram <>i a ilorsal view of a young oozoid or of a bud of Botryllus violaeeus (after Pizon, from Perrier). B fu- ture mouth ; bl rudiment of new blasto-zooid of one side ; c.vib vibratile tube ; ect ectoderm ; gh gonad ; i intestine ; perb peribranchial sac (atrial cavity) ; pv epicardical di- verticulum ; vb pharynx ; Vcl cloacal part of future atrial cavity. nervous system the endoderm. original connection re- mains throughout life as the vascular ecto- dermal tube referred to above. Botryllus Gartner and Pallas (Fig. 23), colony thin, encrust- ing, systems circular, gonads paired, placed laterally, littoral, Eur., Med., N. Amer. Poly- cyclus Lamarck, as last, but colony thick and fleshy, Eur. and Med. Botrylloides H. Milne - Edw., colony thin encrusting, sys- tems elongated or branched irregularly, gonads paired, lateral. Sarcobotrylloides v. Symplegma Herdman, Drasche, like *last, but colony thick and fleshy. colony stalked, gonads unpaired, in intestinal loop, Bermuda. Fain. 2. Distomidae. Colony rounded or massive, rarely encrusting, either sessile or with long peduncle ; systems irregular, inconspicuous or absent, both mouth and atrium usually opening on the surface of the colony. Zooids divided into thorax and abdomen, and sometimes provided with long vas- cular ectodermal appendages ; test gelatinous or cartila- ginous, sometimes with non-stellate calcareous spicules ; pharynx without internal longitudinal bars ; dorsal lamina as langxiets ; gonads and heart in or alongside intestinal loop, spermatic vesicles numerous, vas de- ferens straight. In the Distomidae the budding is epicardial. The epieardiuni arises as a pair of diverticula of the pharynx, one v tissioii and eventually develop into new zooids.* In < 'olclla some of the buds, those placed in the deeper p irts of the stalk, have a store of reserve food material in their ectoderm. FIG. 25. Distaplia (after Herdman ironi Delage and Herouard). , op. c'lt. J uliii, 1 nt (Jong. Zoology, Leyden, JStKi. SYNASCIDIA. 33 Distaplia Delia Valle (Fig. 25), with a common cloaca and atrial languets, incubatory pouch as diverticulum of cloaca, colony sessile or only shortly peduncnlated ; Med., AtL, Ind. Ocean. Julinia Caiman, Antarctic. Dis- toma Gartner, with atrial siphon, without, pinnies in th t.p=t-, and incubatory pouch, colony sessile or only shortly pedunculated ; Med., Eur., Torres Straits. Heterotrema Fiedler, Cystodites v Drasche, Colella Herd., with an incubatory pouch and a well- marked peduncle, S. Ocean. Oxycornia v. Drasche. Chondro- stachys Macdonald, the zooids project beyond the test, Aust. Archi- distoma Garstang, the zooids arise from an incrusting basal layer, either singly or united in a common test, with- out common cloaca ; covered with sand ; may be compared to a Glavelina in which the tests of the zooids have fused in groups ; Plymouth. Finn. 3. Polyclini- dae. Colony usually massive, sometimes en- crusting, sometimes lobed or even stalked; systems of various shapes, sometimes ir- regular or absent ; common cloaca! aper- tures usually indistinct or absent ; zooids placed perpendicular- ly to surface, usually divided into thorax, abdomen, and post- abdomen (Fig. 26) ; mouth 6 or 8-lobed, atrial aperture often with atrial languet ; test sometimes stiffened by embedded sand grains; pharynx small with small stigmata, without internal longitudinal bars, with horizontal membranes ; tentacles small, few ; dorsal lamina as languets; gonads in post-abdomen, testis as spermatic sacs attached to FIG. 26. Diagrammatic section through a portion of a Poly- clinid colony (after Delage and Herouard). 1 openum of common cloaca ; 2 atrial openings ; 3 ganglion ; 4 mouths ; 5 tentacles; 6 peripharyngeal band ; 7 endostyle ; 8 languets; y anus; 10 male; 11 female generative opening; 12 epic;inli.i! tubes; 13 stomach; 14 pyloric gland ; 15 ovary ; Ifi tr.-ti- : 17 epicardium ; IS dorsal; 19 ventral limb of pericardium ; 20 posterior bifurcation of epicardium ; 21 heart. Z III D 34 PHYLUM TUNICATA (UROCHOBDA). large vas deferens ; gemmation from post-abdomen which lias the heart at its extremity. The post-abdomen contains the epicardium, which divides it into a right and left portion, the heart and pericardium, and in its upper part the gonads (Fig. 26). The pericardium (19) is U-shaped and contains the heart (also U-shaped) in its posterior part. The epicardium (17) bifurcates in front into two tubes which abut against the pharynx but do not open into it in the adult ; posteriorly it bifurcates so as to grasp the bend of the pericardium (20). Budding is effected by the separation of the post-abdomen from the abdomen and its fission into pieces (Fig. 27). The portion of the heart and pericardium in eacli piece atrophies and the epicardium develops in the usual way into pharynx, atrium and intes- tine of the bud. In Amaroucium, and possibly others, budding takes place in the summer ; towards winter this ceases and sexual organs are formed. The budding zooid regenerates a new post-abdomen. This method of bud- ding is clearly of the stolonic kind, such as we find in Clavelina and Distomidae, differing only by the presence of the heart in the stolon. With intestinal loop twisted and generally smooth-walled stomach. Polyclinum Sav. (Fig. 26). Systems simple, stomach smooth-walled, most seas. (Jlossophorum Lahille, hori- zontal membranes of pharynx denticulated, each colony of a single system ; Med. Aptogaster Herd., Polyclinoides v. Drasche, Aplidiopsis Lahille, with non-twisted intes- tine and smooth-walled stomach ; Med., Atl. Pharyngo- dictyon Herd., pharynx with simple meshwork of rectangular meshes, fine longitudinal bars supposed to be ab- sent (see p. 11); Antarctic, 1,600 fms. ; Tylobranchion Herd. Intestinal loop not twisted, wall of stomach grooved or areolated. Ajiiliilinni Sav., without languets, most seas. Psammapilidium Herd. Amaroucium M.-Edw. (Amaroecium) (Fig. 27), atrial aperture with a laii- guet, post-abdomen not separated off by constriction, most seas. Sigillina Sa\ ., Fragarium Giard and Fragaroides Maur.. with 8 buccal lobes, Med. Sid- nyum Sav.. <> buccal lobes, areolated stomach ; N\ arid S. Atl. Morchellium Giard. Synoiciim Phillips, each colony forms a separate club-.shapod mass. Parascidia M.-Edw. Cir- cinalium Giard. Morchellioides Herd. Polyclinopsis Gottschaldt. Fain. 4. Didemnidae. Colony usually thin and encrusting, rarely thick, never stalked : systems irregular, inconspicuous or absent, common cloacal apertures usually conspicuous : zooids placed perpendicularly or "Uiquely to surface, divided into tlmr.ix ami abdomen; mouth 6-lobed, atrial opening plain or with atrial languet ; test usually with stellate cal- careous spicules ; ectodermal processes well developed, with muscles which FIG. 27. Amaroucium -hi '\\ing seg- mented post-abdomen, JT, >j lunU. k anterior swollen end of epicanlium in the bud (after Kowalevsky >. SYNASCIDIA. 35 act as retractors ; pharynx small, with 3 or 4 (rarely (i) rows of stigmata ; gonads alongside intestinal loop, ova large, single testis round which vas deferens is coiled spirally ; gemmation from pyloric region, thorax and abdomen formed from separate buds ; larval gemmation feeble. The budding* (Fig. 28) in the Didemnidae is very remarkable. The buds appear to arise in two separate portions from distinct parts of the body ; the one of these, known as the thoracic bud (b), arises as a diverti- culum of the outer wall of the lower end of the atrial cavity, on the right side opposite the stomach ; the other, called the abdominal bud (c), is a diverticulum of the oesophagus which also projects towards the right side. The thoracic bud is formed of outer ectoderm, ectodermal lining of the atrial cavity and interposed mesoderm (like the bud of the Botryllidae) ; it gives rise to the thoracic portion of a new zooid, i.e. to the pharynx, atrial cavity, rectum and a portion of the oesophagus (Fig. 28, -B). It separates from its place of origin and acquires new relations : these are as follows : the mouth opens on the surface of the colony, the atrial aper- 28. Three successive stages in the budding of Didemnuin (from Delage and Herouard, 4 . after D. Valle). a parent ; b pharyngeal bud ; c abdominal bud ; cl atrial aperture ; O oeso- phagus ; r rectum ; s mouth of bud. ture into the common cloaca of the colony, the portion of oesophagus joins the oesophagus and the rectum j oins the rectum of the parent. The abdominal bud meanwhile has formed itself into a loop connected at both ends with the oesophagus of the parent ; one end however separates itself from the oesophagus, joins the rectum close to the point of union of the latter with the rectum of the thoracic bud ; and the whole loop forms itself into a new oesophagus, stomach and intestine. A new heart and pericardium are formed in the abdominal bud. Later when the new pharynx and new intestine have developed so as to be equal in size to those of the parent, they become detached from its oesophagus and rectum and join up in such a way that the oesophagus and rectum of the abdominal bud become continuous with the oesophagus and rectum of the thoracic bud. Such is this extraordinary phenomenon. The details are not yet fully worked out, and we await future observations for a more complete understanding of it. It sometimes happens that the two parts of the bud are not equally * Delia Valle, Mem. R. Accad. Lincei (3), 10, 1881, and Arch. Ital. Bio- logie, 3. 1883. Caulery, Comp. Rend., 1895-97. Salensky, Naples Mit., U, 1895. 36 PHYLUM TUXICATA (UROCHORDA). 13 developed, or one part alone may develop, so that zooids may be found with two sets of intestines. In such cases the old intestines disappear and the phenomenon may be regarded as one of regeneration. Didemnum Sav., colony thick and fleshy, pharynx with three rows of stigmata, most seas. Diilrioides\'. Drasche. Sarcodidemnoides Oka. Leptoclnnnn M.-Edw., colony thin and encrusting, 4 rows of stig- mata. Tetradidenmum Delia Valle. Poli/- fiijiirrutoii Xott. Kuiwlitim Sav.. rows of stigmata, Med. and Red Sea. Hy purr/on Sollas, with faecal pellets included in the test, Malay Peninsula. Fain. 5. Diplosomidae. Colony usually thin, transparent and encrusting, usually without spicules in the test, pharynx with 4 rows of stigmata ; body divided into thorax and abdomen ; vas deferens not spirally coiled ; gemmation* as in the Didemnidae : the larva produces a well-developed bud be- fore fixation. Diplosoma Macdonald. Med., Atl., Pac., Austr. Diplosomoides Herd., with spicules, Med. Brvrixtt Ilium Jourdain. As- tellium Giard. Pseudodidemnum Giard. Fain. f>. Coelocormidae. Colony massive, deeply concave on upper surface, not at- tached ; zooids large, scattered all over the surface, mouth .Vlobed : test soft with cal- careous spicules near surface of colony ; pharynx large, dorsal lamina as languets : intestine extending behind pharynx but not forming distinct abdomen ; testis as pyriform vesicles which join a spirally coiled vas de- ferens. Coeloconnus Herdman, 8. Atlantic, GOO fms. : . $$ UIJSJ IS' - G FIG. 29. A blastozooite of Clavelina, side view, diagrammatic (after Delage and Herouard), 7 atrial aperture ; 2 subneural gland ; 3 inoutli; 4, 5, epicardial tube ; G heart ; 7 stolon ; S sto- lonic septum ; 9 stomach ; in ovary ; // test is ; 12 oesophagus ; 13 anus. the recent account of Pizon (('<>n> i>t<- Ri'iidus, 137, 1903, p. 750) which is somewhat condensed, the budding appears to be as follows : The larva buds before fixation. Of the two individuals so formed, O 1 and B l , the oozoid O 1 or zooid into which the larva itself develops buds a new thorax (pharynx, oesophagus and rectum) and so becomes bithorurir. After twenty tour hours the old thorax degenerates, while its abdomen V persists and retains its connection with the budded thorax. There is thus formed a new individual O-. which in its turn produces two buds; from one of 1 hese proceeds a ne\\ t borax O 3 from the other a new abdomen V-. After about twenty-four hours these separate in such a way that the new thorax O :! takes I he old abdomen V, \\ hile the thorax of O 2 keeps the new abdomen V-. ()-' having thus acquired a new abdomen buds a new thorax ()' and then after twenty-four hours loses its old thorax. A ne\\ y.ooid \\ ith t he old a I id omen V- and t he new thorax O 1 is thus formed. ( )' L">es through the same scries of changes as those described for O 2 ; i.e. it first doubles itself and nixes rise to t \\ o new x.ooids, of which that with the old thorax O 3 becomes hithoracic ;md then loses its old thor-ix. The yooid B 1 produced by the free |ar\ a behaves like O 2 . SYNASCIDIA. 37 Fam. 7. Polystyelidae. Colonies massive or encrusting, rarely stalked or formed of small masses connected by stolons, without common cloacal cavities ; both apertures 4-lobed, opening directly to the exterior ; pharynx large, with strong internal longitudinal bars ; tentacles small, numerous ; dorsal lamina as membrane ; intestine alongside pharynx, rarely extend- ing behind it ; gonads as polycarps in mantle projecting into atrium ; gem- mation doubtful, probably from vascular stolons ; in Goodsiria it is stated to be pallial (Ritter). It is not certain that all the genera here included have the power of budding, and it is possible that some of them are close aggregations of simple Ascidians. By their structure they approach Poly- carpn (Styelinae) of that group. Goodsiria Cunningham, Chorizocormu? Herd., Oculinaria Gray, Thylacium Cams, Poly sty ela Giard, Synstyela Giard. Fam. 8. Clavelinidae. The zooids are not embedded in a common test but are attached to creeping stolons or to a stolonial mass from which new ascidiozooids are formed by gemmation ; test xisually gelatinous, thin and transparent ; pharynx often without internal longitudinal bars, which are without papillae ; tentacles simple, dorsal lamina as languets ; intestine usually behind pharynx as abdomen ; gonads in intestinal loop. This family comes closest to the genus Ciona of the simple Ascidians, with which groxip it is often united. The stolon of* the oozoiteof Clavelina .(form produced from the egg) is divided into two parts by a septum, the stolonic septum. This is a collapsed continuation of the epicardium and therefore contains endoderm. At the free end of the stolon the septum ceases so that the cavi- ties (blood spaces) on either side of it are in communication. At the other end the septum is continuous with the hind end of the single epicardial tube (in the blastozooite with the hind end of the pericardium, Fig. 29), the pos- terior part of which is applied to the dorsal side of the pericardium, while the front end forks to open by two openings into the hind end of the pharynx just ventral to the oesophagus. In the blastozooites or forms which ha,ve been produced by budding (Fig. 29), the arrangement is the same except that the stolonic septum is connected with the pericardium and not with the epicardium ; but this is not a matter of any great importance, inasmuch as the pericardium develops from the hind end of the epicardium, with which it remains in close contact. The point is that the stolonic septum is an endodermal structure continuous with or developed from the pharyngeal wall. The form produced from the egg remains asexual, the zooids (blastozooites) which are budded from the stolon of this become sexual (Fig. 29). The budding takes place in this way. The stolon produces on its upper side a small diverticulum (Fig. 31), into which the septum sends a hollow endodermal prolongation. This FIG. 30. Diagrammatic transverse section through the posterior part of a bud of Clavelina (from Kor- schelt and Heider). ep epicardium ; h heart ; i intestine ; m stomach ; n viscera] nerve cord ; pc pericardial vesicle. ' Kowalevsky, Sur le bourgeonnement du Perophorn listcri, Rev. Sci. Nat. Monlpellier, 1874, and Ueb. d. Knospung d. Ascidii-!). Arch./. JI//A-. Anat., 10, 1874. 38 PHYLUM TUNIC ATA (UKOCHORDA). endoderrnal vesicle becomes constricted into two parts which remain connected by a narrow neck : the upper of these gives rise to the pharynx and atrial cavity, and the lower to the epicardium from which the pericardium subsequently proceeds. The digestive tube arises from the pharynx in the ordinary way, while the central nervous system appears to develop from the pharyngealendodenn,* as it does also in the buds of Distaplia t and Botryllus i and probably in those of all Ascidiae Compositae. It would thus appear that in the Ascidiae Compositae the atrial cavity and the central nervous system,which in the embryo develop from the ectoderm, in the bud de- velop from the endoderm. This contrast is highly remarkable, and due weight will have to be given to it, when we are considering in the General Part, the theory of the embryonic layers. Clavelina Sav., stolons delicate and branched, body divided into thorax and abdomen, without pe- duncle, pharynx without inner longitudinal bars ; 2-3 cm. in length ; Eur. and Med. Podocla- i-illn Herd., with peduncle, Arctic. Australia. Stereoclavetta Herd., stolons and hind end of body in a common mass of test, Atlantic, Australia. Pycnw:lurdla Garstang, (.I.M.B.A. (-2), 2, 1891), similar to preceding, Plymouth. Perophora Lister, without abdomen, short peduncle, pharynx with 4 rows of stigmata, transverse bars of pharynx with papillae which branch and may form imperfect internal longitudi- nal liars (Fig. 8), zooids 3-5 mm. ; All.. ,Med.. X. Amer., Austr. Pero- l>lif>r<>/iN/ft Lahille, with 15 or 16 rows of stigmata, Med. Ecteina- ni-iilin Herd., without abdomen. test without blood-vessels, pharynx with internal longitudinal bars, most seas. Slttftcn'a v. Ben. Dia- zoiiii Sav.. many zooids united by tests posteriorly to form a large colony, abdomen present and embedded in con i test. phar\n\ with internal longitudinal bars, zooids 3-6 cm. FIG. 31. Portion of a proliferating .-toloii > of i'l'm/ilinrii latti-r l\n\\:ili-\-k\ troin K..r- l"-!l .-MM! llchli'l |. 61 toili-nil ; i')l cll- doderiu ; kn lm KM. Ritter, Jowrw. 1896. t Kowalcvsky. Arch. Mil:. Ann/.. Id, 1x71. i Hjort. Znnl. Anz., 10, 18!)2, p. 32s. PYROSOMA. 39 Eur. and Med. Rhopalaea Philippi, zooids few, not united by tests, stolons as foliaceous expansions ; 5-12 cm. ; Med. Rhopalopsis Herd. Tribe 3. ASCIDIAE SALPAEFORMES (ASCIDIAE LUCIAE). Free-swimming pelagic colonial Ascidians. The colonies have the form of hollow cylinders, closed at one end, open at the other, and slightly tapering towards the closed end. The closed end is rounded ; the open end is flat and its edges project inwards to form a diaphragm (Fig. 32). The cavity of the cylinder is the common cloaca of the colony, and its opening, which can vary in size, is the common cloacal opening. The test is trans- parent and gelatinous ; it bears on its outer surface a number of projections (Fig. 31 bis), while the inner surface is perfectly smooth. The zooids are elon- gated antero-posteriorly and placed in the thickness of the wall of the cylinder in a single layer at right angles to the surface. Their mouths open on the outer surface, each at the base of one of the test processes ; and their atrial apertures are at the opposite end and open into the common cloaca. The colonies vary in length from a few inches to four feet (Pyrosoma spinosum). They float horizontally in the sea, and have a slight power of movement with the closed end forward. The movement appears to be caused by feeble longi- tudinal Contractions of the Wall of the FW. 31 tns. Pyrosoma elegans, x i (from Perrier). a cloacal hind end of the cylinder. They are opening ; a buccai appendages J of the zooids. phosphorescent. When they are at rest the light is very feeble, but on stimulation by contact or other means, the light, passing through a red and green stage, be- comes white. Moseley * states that he traced his name on the * Notes by a Naturalist on the " Challenger," London, 1879, p. 574. 40 PHYLUM TUNICATA (UROCHORDA). surface of a large specimen with his finger, and describes how "in a few seconds his name came out in letters of fire." The light is emitted by two groups of cells in the region of the mouth. There is a single genus Pyrosoma Peron. The zooids usually lie with their ventral surfaces towards the closed end of the colony, but in P. elegans they have the opposite position. There is a buccal and atrial sphincter muscle and a few delicate bands of muscular fibre in the body wall. The test contains cellular elements and some fibrous bands, and the zooids near the common cloacal opening give off two tubular prolongations of the dorsal body-wall containing muscular fibres, which appear to have the function of acting on the dia- phragm. The structure of the zooids is well shown in Fig. 33. The viscera are behind the pharynx, and the anus opens into the posterior part of the atrial cavity. The atrial cavity sends forwards two diverticula, one on each side of the pharynx, but t hese are not continuous with one another on the dorsal side of the pharynx, as in other Ascidians. The lateral walls of the pharynx consist of from 20 to 50 transverse bars, crossed by from 15 to 30 longitudinal bars. The stigmata are quadrangular, and the longitudinal bars are supposed to be inter nal, fine longitudinal bars being absent, as in Culeolus, etc., among simple Ascidians and in Pharyngodictyon among the Synascicliaiis. however if this interpretation is correct (see p. 12). There is the N FIG. 32. Diagram of a section through a Py- rosoma colony (after Delage and Herouard). 1 buccal appendage ; 2 common cloaca ; 3 zooids ; 4 diaphragm ; 5 cloacal opening. It is doubtful of of Wh A usual row tentacles which the ven- tral median is the longest (Fig. M). The dorsal 1 am i 11 a has the form of I _' I II II U' II e t s which slio\\ no l-i'lat ion to the transverse liars. Tin- intest ine is curved, there is a pylorie gland ra 1 1 1 i f y i n g on the intest ine. The heart is placed a little behind the end of the endostyle (C). The gonada are placed in the body wall on the ventral side behind the intes- The ovary contains one ovum which ripens before the test is. The Br C T End FIG 33 \ zooi'l df I'l/nixiiimi (from Pt-rrior). A e\h;ileut aperture : 4/anus Br pharynx . Cheart ; Endendostyle : N nerve ganglion ; ,0 mouth Or civiiry ; >' *t.i].m ; 7' test is ; \\'l, p.-ripharyngeal ring. ASCIDIAE SALPAEFORMES. 41 ft budding stolon (st) is a projection of the ventral body wall and contains a diverticulum of the pharynx which comes off just behind the endostyle, a genital cord, and a prolongation of the pericardium which latter structure is said not to give rise to any organ in the bud. The ganglion carries on its ventral side a pigmented organ which is interpreted as an eye, and the phosphorescent organs (Fig. 33) are paired (right and left) masses of cells containing a fatty substance at the level of the peripharyngeal band in the blood space there found. The development* of Pyrosoma is very remarkable. The ovum is large and full of yolk. The cleavage is partial and gives rise to a disc of cells lying upon the yolk. There is no tailed stage and the development is very different from that of other Ascidians. It takes place within the egg- follicle and leads to the formation of an imperfect individual called by Huxley the cyathozooid. This while still in the maternal tissues produces a sto- lon (Fig. 34), which immediately undergoes imperfect transverse fission into four parts. Each of these develops into a typical Ascidian zooid, called by Hux- ley the ascidiozooid (Fig. 35). The four ascidiozooids arrange p la . 34. Diagram of a Pyrosoma embryo showing the devel- themselves equatori- P m g cyathozooid lying on the yolk with some of its organs developed, and the stolon st with commencing fission. ally round the parent ,/ enteron ; do yolk ; e atrial aperture ; g ganglion ; h heart ; cvathozooid (Fig 35) fi ciliated pit, of cyathozooid ; r edge of germinalfdisc grow- ing over the yolk of cyathozooid ; stfstolon. acquire a common cloaca and form the first individuals of a new colony. The cyathozooid now disappears, the ascidiozooids lose their primary connection, and the little colony passes into the atrium of the parent and thence to the ex- terior. Each of the zooids composing it possesses a ventral stolon which immediately begins to bud. As all the successively produced zooids have ventral stolons, the colony constantly increases in size and number of individuals until the limit of growth is reached. Budding by which the adult colony is formed, f Each of the f our primary ascidiozooids possesses a stolon at the hind end of its endostyle. This stolon is the pedicle which in an earlier stage connected the zooid to that next it in the chain, i.e. it is a part of the original stolon which does not develop into an ascidiozooid. It contains (Fig. 36) a diverticulum of the pharynx (d), two lateral bands of mesoderm, an unpaired cord of mesoderm called the genital cord (g) and the elaeoblast. Of these structures the mesodermal bands break up and are said not to give rise to organs, while the cord of genital mesoderm which is at first * Kowalevsky, Arch. f. Mik. Anat., 11, 1875, p. 597. t Seeliger, Jen. Zeitsch., 23, 1889, p. 595. 42 PHYLUM TUNIC ATA (UROCHORDA). en. FIG. 35. Two stages in the development of Pyrosoma (after Kowalevsky from Knrsi licit and Heider). In A the yolk of the ovum is partly surrounded by the cyathozooid c which has developed on it (see Fig. 34) ; in B the yolk is entirely enclosed by the cyathozooid. c cyatho- zooid, d its atrial pore, d its alimentary canal, do its yolk ; ec ectoderm ; el elaeoblast (a raesoblastic mass in the buds) ; en endostyle of ascidiozooid ; ft ciliated pit and g ganglion of cyathozooid ; i mouth, ks gill-slits of ascidiozooid ; I vascular space of cyathozooid; TO test ; ganglion of ascidiozooid ; p peribranchial (atrial) cavity of ascidiozooid : xn lateral nerve ; v endodermal canal connecting aseidiozooids ; z mesoblast cells. *?*- y FIG. 36. Three stages in the development of the proliferating stolon of Pyrosmim (frmu sclielt mid Ileider, after Seeliger). In C the division of the stolon into two buds, I and II. is indicated, d pharyngeal process (epicardium) ; ec ectoderm; es endostyle of parent; i genital (mesoderm) band ; kx first gill-slits ; m rudiment of digestive canal ; . rudiment eural tube. ASCIDIAE SALPAEFORMES. 43 continuous with the rudiment of the genital organs of the parent, gives rise to the peribranchial (atrial) cavity, to the central nervous system (n), to the gonads, and to the mesodermal structures of the budding zooid. The pharyngeal diverticulum becomes the pharynx and develops the intestine as an outgrowth (Fig. 37). It is clearly homo- logous with the epicardium found in many synascidians. The pericardium is developed as a vesicle formed of mesodermal tissue on the right side. It comes to lie on the dorsal side of the pharyngeal process which projects into the stolon of the fully formed zooid (Fig. 37 pc), and thus differs in FIG 37 A chain of three zooids in the budding of Pyrosoma (from Korschelt and Heider after Seeliger), I youngest, II middle and III oldest bud (nearly developed). D point at which the pharyngeal process of the parent enters ; d pharyngeal process (epicardium) ; dc rudiment of digestive canal ; dm elongated cell mass (? phosphorescent and haema poietic organs) ; e rudiment of atrial aperture ; eb elaeoblast ; EC ectoderm of parent ; oe e< derm of new stolon ; Es endostyle of parent ; es endostyle of bud ; fg ciliated pit ; g gangli h testis ; hd intestine ; hz heart ; i rudiment of mouth ; U atrium ; ks gill-clefts ; If internal longitudinal gill bars : Im phosphorescent organ ; m stomach ; ms genital strand ; nient of nervous system ; o ovary (egg follicle with egg) ; oe oesophagus ; p dotted dicating the boundary of the atrial cavity (peribranchial sac) ; pc pericardium ; rz langue tentacle rudiment ; v tube connecting the enteron of the first and second I position from the pericardium of other forms, which is placed ventral to the epicardium (see p. 15). The nervous system develops from the distal end of the genital cord (Fig. 36) which passes round the front end of the pharyngeal process on to the dorsal surface of the bud and becomes hol- lowed out to form a vesicle. The ganglion arises from the thickened dorsal wall of this vesicle, the cavity of which acquires a tubular communication with the pharynx and forms the subneural gland and duct (Fig. 37, fg} The elaeoblast also develops from the mesoderm, as paired masses which 44 PHYLUM TUN 1C ATA (UROCHORDA). join round the pharyngeal process (Fig. 37, eb). The test of the primary tetrazooid colony is said to be formed entirely by the cyathozooid, but in the subsequent budding each new zooid plays its part in adding to it. The stolon undergoes fission by transverse constrictions into buds. First one bud is marked off (Fig. 36 A), thenanother between the first and the parent (Fig. 36 C), and so on until five have been formed, the youngest bud being always next the parent. When five zooids have thus been marked off, the distal one has acquired full development and becomes de- tached and a new constriction is formed at the base of the stolon. The new zooids when detached pass round and take up their position near the common cloacal opening of the colony, where the youngest zooids lie. As they increase in age they become further and further removed from this opening by the interposition of the continually forming new zooids. The points in the above account which must be received with caution are those relating to the origin of the nervous system and peribranchial tubes. In the stolon of the cyathozooid these organs arise as ectoderm invaginations : in the stolons of the later zooids which are directly derived from that of the cyathozooid, they are said to arise from mesoderm. It is difficult to believe that this difference really exists. Order 2. THALIACEA.* Free-swimming solit'iry pelagic forms, which in the, adult are never provided with a tail or a notochord. Metagenesis always occurs, and the sexual forms ty pica/I ;/ r< main for some time con- nected to a process of the body of the asexual form by which they are budded. The mouth and atrial apertures are at opposite ends of the body. Tin- test is permanent and transparent, and closely adherent to the body. The musculature of the mantle is in the form of * Traustedt, Spolia atlantica, Bidray. til Kundskah om Salperne, Danskc \'iil. >'. /,s7.-. Sl.Tiit.. INS."). p. :53'.. Kowalevsky and Barrois, .Matcriaux pour s -i-vir a I'liistoire dc /' . 1 nrltiitin. .lonni. de V Anal. Phys., 19, lss:i. rijanin. l>i< Arten der Hutimiii Doliolion ///< <;<>!/< v. Xm/tfl, Leipzig, 1884. I'.arrois, Sui- le eycli<|\ic n<'- net i< pic ct la bourgeonnement de 1'Anchinie, Journ. de I'Anat. /'/;.:!. Seeli^ -r. Die Knospung der Salpcn, .l'ii. Zeitsch., 19, 188(i. Korotnet'f, Die Knospung der Anrliii/i'* Mit., 10, I s'.il . p. IST. Id.. Kml>r\ nlotiie der Solpa >'< mocratica, Z./.ir.Z.. .">!). p. _>!), L895. Id.. Tunicatenstudien, etc., JVapZes JWii., 11 and 12, 1895-6. Id., Zur Einb. v. Salpa runcinala Eusiformis, /..i.n-.K., (i'2. ISiKi. Brooks, The Genus Salpa. M> m. liinl. Lnli. .latin* ff'>/>kni.-< I r niveraity,2, I8!C?. Saleiisky, A scries of pa | HTS on the de\ eli .pi MCI it and I >u< l< IIML' "!' SH//IU, I'iih', Z.f.ir.T,., 17, l*7i, :'.(i. ls7s. Morph. Jahrb., :t. ls77, -'(), is'.i:?. \!'>_ ic von Salpa t'lisifonnis, \>>h. Senck. Oes. Fr. THALIACEA. 45 more or less complete circular bands, by the contraction of which locomotion is effected. The pharynx has either two large, or many small apertures leading into a single atrial cavity, which opens to the exterior by the atrial aperture. The anus opens into the atrial cavity. Alternation of sexual and asexual genera- tions (metagenesis) occurs in the life-history and may be com- plicated by polymorphism (Doliolidae). They are only occa- sional visitors to British shores. A few species of Salpa and of Doliolum have been taken in the seas around the Hebrides. They are divided into two groups, the Cydomyaria and the Hemimyaria, which seem to be sufficiently distinct to merit the rank of sub-orders. Sub-order 1. Hemimyaria (Salpida). Thaliacea in which the pharynx is reduced to its median dorsal and ventral (endostylar) walls, the lateral icalls being absent. The asexual form produces chains of sexual individuals, which give rise again to the asexual form. The muscular rings are usually incom- plete ventrally and a tailed larva is not developed. The salps are transparent pelagic organisms, coloured and opaque at one spot the nucleus, where the digestive organs and heart are placed. The test is soft and gelatinous and the mantle or body- wall closely adheres to its inner surface. The body is some- what elongated, with the mouth in front and the atrial aperture behind and slightly dorsal. The muscular tissue (Fig. 38) of the body-wall is arranged in hoop-like bands usually six to nine (there may be as many as twenty and as few as four) which are continuous dorsally, but usually not ventrally, except behind where they constitute the atrial sphincter. At the mouth they are modified in a different way, being prolonged forward into the lips. All these muscles are transversely striated. Dorsally some of them frequently join or approach one another. It is by the contractions of these bands with the mouth closed that water is expelled from the atrial cavity, causing the movement of the animal in a forward direction. The mouth is not lobed, but is bounded by mobile upper and lower lips. It leads into the pre- branchial part of the pharynx, which contains but a single ten- tacle the so-called languet on its dorsal side (Fig. 40, 11} 46 PHYLUM TUNIC ATA (UROCHORDA). Emb J FIG. 38. a sexual, & asexual form of Saipa (Thalia) democratica-miicronata (from Clans). mouth ; A atrial aperture ; N ganglion ; Br gill ; End endostyle ; Wg ciliated pit ; Ma mantle ; .V nucleus ; C heart ; Emb embryo ; st.p stolon. w I- ni. 39. a Himl c-nd nt Sul/m democratica, ventral view. St.p stolon; iVw nucleus, b Ter- minal part (it stolon lyuiiiin chain) nia^niliril. o mouth ; A atrial aperture ; A T ganglion,'; ll'i/ ciliated pit ; II / peripharyngeal band ; End endostyle ; Af anus ; Br gill ; ^Vw nucleus ilr nvar\ ; (' heart (Irnin Clans). SALPIDA. 47 Behind this is the peripharyngeal band (16), which is grooved in the usual manner. The nervous system and the organs of sense, in correspondence with the power of free locomotion, present a higher grade of development than in the Ascidiacea. The ganglion (13) with its numerous nerves lies dorsal to the anterior attachment of the : ' gill " and is of considerable size. It has on its dorsal side a horseshoe-shaped brownish-red pigment band, in which are con- tained besides the pigment numerous rod-shaped structures, the whole directly resting on and being part of the ganglion. There can be but little doubt that this structure is an eye. The dorsal tubercle (ciliated pit) is a short diverticulum of the pharynx in front of the peripharyngeal band ; it is without a FIG. 40. Sal'pa in longitudinal vertical section, seen from the left side, diagrammatic. 1 mouth ; 2 atrial aperture ; 3 anus ; 4 pharynx (branchial sac) ; 5 " gill " (dorsal lamina) ; 6 duct of subueural gland ; 7 endostyle ; 8 heart ; 9 oesophagus ; 11 languet ; 12 muscle - bands ; 13 nerve ganglion ; 14 embryo in ovisac ; 15 atrial cavity ; 16 peripharyngeal ring ; 17 stomach ; 18 testis ; 19 test ; 20 subneural gland ; 21 dorsal tubercle. glandular part and does not extend as far back as the ganglion (21). On the ventral side of the ganglion there are two glands (subneural glands) which open by separate orifices into the pharynx (or atrial cavity ?) just in front of the peripharyngeal band (6) and behind the representative of the dorsal tubercle. There are no otocysts in salps. The pharynx is without any side walls or stigmata and freely communicates with the atrial cavity on each side. The endo- style (7) is present in the usual form, and the dorso-median wall of the pharynx is represented by the so-called gill (5) which takes an oblique course across the body from near the ganglion to the opening of the oesophagus where it terminates. The oesophagus (.9) leads into a gut which possesses a stomach and pyloric gland. 48 PHYLUM TUNICATA (UKOCHORDA). is twisted on itself and constitutes the nucleus. The anus (3) opens into the atrial cavity in the posterior region. In Cyclo- salpa there is no distinct nucleus ; for the intestine is not twisted but passes forward, along the endostyle in the sexual form, and along the gill in the asexual form, to open anteriorly into the atrial cavity. The heart is placed in the nucleus in front of the stomach. In the asexual or solitary form (see below) the pharynx gives off a diverticulum at the hind end of the endostyle which enters the stolon. The stolon is a process of the ventral body- wall anterior to the nucleus. It lies in an excavation of the test and contains in addition to this pharyngeal diverticulum a number of other structures which will be described below. Its function is to segment into a number of zooids, which develop sexual organs and differ, when fully formed, in certain features from the asexual animals which arise from the egg. The sexual forms which originate from the stolon remain adherent by pro- cesses of the test, and form chains, from which they break off in sections. They are therefore generally found joined to others, but in some cases they become eventually entirely separate from one another. We thus get in the salps the regular alter- nation of a sexual and an asexual generation. The asexual form is solitary, the sexual usually joined to others in chains. As these two forms differ slightly in anatomical structure and have often been described before their genetic connection was known, they have generally received different specific names. To indicate this connection when it has been discovered and to render the matter perfectly clear, both these specific names are used in the name of the animal ; thus Thalia democratica mucronata is a species of which the asexual form was originally described as Salpa democratica and the sexual or chain-form as 8. mucronata, their genetic connection being subsequently dis- covered. The alternation of generation in salps was discovered at the beginning of last century by the poet Chamisso,* and sub- sequently rediscovered by Steenstrup. The sexual form, often called chain-form, proles gregaria, or blastozoite,! dillers from the asexual form, often called solitary / ' I if uiii/intliliiix i/ii\ different authors, >.jr. the blastozoite is applied to the sexual form 1 ause it is produced liy liiiddinu niid t> (lie iisexiiid form heeause it '"ids. It is host therefore to discard these terms. SA.LPIDA. e , proles solitaria or oozoite, in a number of characters, among the most important of which must be mentioned the presence of generative organs and the absence of a stolon. The salps, as a rule, develop only one ovum (lasis produces several ova and embryos). The ovary is in the young form placed in the nucleus by the right side of the intestine and is connected with the epithelium of the atrial cavity by a long stalk, consisting for the most part of a single cord of cells (Fig. 41). The . J X \^ .,/ opening of the oviduct will be formed later at the point where the stalk joins the atrial epithe- lium on the dor- sal side of the atrial cavity, somewhat to the right behind the penultimate muscular ring. Before fertilisation the cord con- necting the ovary to the atrial wall shortens and draws the ovary from the nucleus to the dorsal side. At the same time it acquires a lumen which opens into the atrial cavity. The ovary now has the form of a simple epithelial sac opening into the atrium and containing the ovum. Soon after fertilisation the oviduct loses its opening and the whole epithelial wall both of oviduct and sac constitutes the follicle of the ovum (Fig. 42 C, b). The testes which develop and ripen after the ovary are a pair of branched tubular glands lying in the nucleus on each side of the digestive organs. They open separately into the atrium. Amongst other points of difference between the sexual and asexual form attention may be called to the adhesive papillae on its ventro-lateral surfaces by which the sexual form is attached to its fellows in the chain. There are eight pairs of these. In isolated individuals they disappear. The muscles are less numerous and less developed, and finally the visual apparatus is FIG. 41. Side view of Salpa democratica-mucronata, sexual form (combined after Claus and Salensky from Korschelt and Heider). d atrial cavity ; e atrial aperture ; end endo- style ; / peripharyngeal baud ; i mouth ; k gill ; n ganglion ; nu nucleus ; od oviduct ; ov ovary ; ph pharynx-cavity ; x aperture of oviduct. z in E 50 PHYLUM TUNICATA (UKOCHORDA). much more elaborate, consisting of eight pigmented structures on the dorsal side of the ganglion. The development takes place within the follicle described above and is remarkable for the fact that the follicle cells play an important part in transferring nourishment from the maternal organism to the embryo. They do this in two ways : (1) the follicle cells, on the side of the embryo which future development shows to be ventral, proliferate and form a thick mass which soon assumes a cavernous structure. This is the pla- centa (Fig. 43). Maternal blood passes through its spaces and it is con- tinually detaching small masses which, charged with nutritive matter, pass into the vascular system of the embryo and afford nourishment for the growing tissues. ' (2) The follicle-cells in other parts of the follicle pro- liferate and migrate in amongst the blastomeres, with which they become inextricably intermingled, so that it is difficult if not impossible to dis- tinguish between them. These cells are called calymnocytes (gonoblasts). Their fate is much disputed, but there can be but little doubt that they, like the plac.-ntal tissues, afford nourishment to the growing embryo. FIG. 42. Stages in the cleavage of Salpa democratica-miicronata (after Salensky from Kor- schelt and Heider). In A and B the oviduct is open, in C it is closed, and the oviducal and follicular epithelia have combined to form the follicle, a atrial epithelium ; b shortened oviduct ; c follicle-epithelium ; / blastomeres ; z calymnocytes. In C, c points to the rudi- nic-iit ni the placenta. Many views have been held as to the nature of the calymnocytes. Sal- ensky is of opinion that they actually form the embryo, and that the em- bryo of Salpa is a follicular bud and not a true embryo. Brooks thinks that for a time they actually form the embryonic organs but are eventually replaced and probably consumed by the blastomeres. Meanwhile the embryo has been growing and projecting more and more into the atrial cavity. The follicle cells and the atrial epithelium over it become thin and eventually rupture so that the embryo lies free in the atrial cavity attached only to the placenta (Fig. 43). The formation of the layers is impossible to follow, but the organs gradu- ally make their appearance in the confused mass of embryonic and follicular cells. With regard to them it is only possible to note a few points here. The pericardium is. according to Salensky, developed from the mesoderm ; but Korotneff asserts that it arises as a diverticulum of the pharynx, which is perhaps the more usual way in Tunicata. The ganglion is by most observers described as arising from a mesodermal mass which becomes secondarily penetrated by an endodermal diverticulum of the pharynx. It is at first hollow and its cavity communicates with the pharynx. SALPIDA. 51 Later it becomes solid and detached from the pharyngeal diver ticuluin, which persists as the ciliated pit. The two subneural glands arise as two invaginations of endoderm further back where the ganglion is in contact with the pharyngeal epithelium. The elaeoblast (Fig. 43, eb) is a mass of apparently large mesodermal cells in the nucleus in later embryonic life. The large cells of which it consists are filled with nutritive matter, and it is probably to be regarded as reserve of food material. A similar tissue is found in the stolon of Pyrosoma. By Salensky it is regarded as a vestige of the tail and notochord of a larval stage. The stolon makes its appearance in the embryo as a diverticulum of the ventral wall of the pharynx between the end of the endostyle and c FIG. 43. Late stage of the embryo of Sal-pa democratica-mucronata (after Salensky from Kor- schelt and Heider). e atrial aperture ; eb elaeoblast ; ed intestine ; es endostyle ; ft ciliated pit ; i mouth ; k gill ; m stomach caecum ; n ganglion ; oe oesophagus ; p pericardium ; pi placenta ; st stolon ; t basal plate of placenta. the opening of the oesophagus (Fig. 43, st). This soon raises the ectoderm and a small process of the ventral body-wall is formed projecting into the tunic. As this process lengthens a cavity appears between it and the tunic, and acquires an opening to the exterior so that the young stolon lies in a tube formed by the tunic. In some species this tube is directed straight forwards along the ventral side of the body (Cyclosalpa pinnaia. Salpa affinis, etc.). In others it is at first directed forwards, then turns to the left and runs backwards on the left side of the nucleus, opening behind this organ (lasis tilesii, etc.). In yet others (Fig. 39) the tube winds spirally round the nucleus (Thalia democratica-mucronata, etc.). With later growth the stolon extends beyond this tube and projects freely uncovered by tvinic. Later on the zooids produced on the stolon acquire tunics as a result of their own activity. The projecting part of the chain of zooids is usually 52 PHYLUM TUNIC ATA (UROCHORDA). in the form of an elongated band, but in Cyclosalpa it has the form of a ring of from seven to twelve zooids attached together. The first rudiment of the stolon contains, in addition to the endodermal tube, a mass of meso- derm derived from the mesoderm of the parent in the neighbourhood of the elaeoblast and possibly from the elaeoblast itself. Stolons a little older contain on the ventral side of the pharyngeal diverticulum a mass of cells called the genital cord, a pair of peribranchial tubes, one on each side of the pharyngeal diverticulum, a pair of mesoderm bands placed immediately outside the peribranchial tubes, a nerve tube on the dorsal side, and a pericardial tube ventral to the pharyngeal diverticulum on the right side. In addition there are two blood-sinuses, one on the dorsal and the other on the ventral side of the pharyngeal diverticulum ; these communicate with each other at the free end of the stolon and proximally with the blood sinuses in the body of the parent. The neural and peribranchial tubes and mesodermal and genital bands are confined to the stolon and are not continuous with the tissues of the parent. Their origin is doubtful : according to Seeliger they are all differ- entiations of the mesoderm of the stolonic rudiment ; accord- ing to Brooks the peribranchial and neural tubes are derived from the ectoderm of the base of the stolon, while Korotneff maintains that the peribranchial tubes are derived from the en- doderm tube. The fate of these structures is as follows : the en- dodermal tube becomes the ^Transverse section through a young p h arvnx au d give rises to the mi nt Halpa (after Brooks, troin Korschelt J and Heider). c perihranchial tubes ; ec ecto- digestive canal, the neural tube derm; e endoderm tube ; genital cord ; , yields the ganglion and ciliated mesoderm hand; n neural tube ; <> upper, and // lower i.l 1-sjniis. pit, the peribranchial tubes the atrial cavity, while the genital cord gives rise to the genital organs., and the mesoblastic bands to the muscles, pericardium, heart, elaeoblast and mesoderm generally of the future zooids. In t he st met nre of the stolon ami it s relation to the parent, and in the for- mation of t he organs, the budding of salps is not unlike that of Pyrosoma. Tli" ph.ii vn-eal diverticulum clearly corresponds to the epicardial tube. The two processes differ mainly in the much larger number of buds pro- dueed liy salps before separation from the stolon. The formation of the buds takes place in the following way. The stolon becomes marked into segments by transverse constrictions due to unfold- in _' of the ectoderm. These .ire numerous (50 to 100), very close together, am I all appear at the same ti (Kiii. ">) They nre t he buds which gradu- ally enlarge and develop into the sexual zooids. All the organs of the Flo. SALPIDA. 53 stolon are divided by the segmentation except the outer ectoderm, the blood-sinuses, and the endodermal tube. These are continuous from end to end of the stolon, being found in the narrow stalks which connect the buds as well as in the buds themselves (Fig. 45). The stolon has therefore now the form of a chain of developing zooids connected together, the dorsal surface of one to the ventral surface of the next, by short narrow stalks. Meanwhile, the proximal part of the stolon, between the zooids and the parent, has been growing, and undergoing a similar process of segmenta- tion into a group of buds. This process is repeated, so that eventually a chain is formed consisting of groups of from 50 to 100 zooids. The indi- viduals of each group are of the same age, but the groups next the parent consist of younger individuals than the distal and earlier formed groups. As development proceeds a change in the attachment of the zooids is effected. At first they are arranged in a single row, the intermediate con- stricted part of the stolon passing from the ventral surface of one to the dorsal surface of the next (Fig. 45). Soon, as a result of growth, by which the relations of the parts are changed, and of rotation, they become arranged in a double row, the zooids of one row alternating with those of FIG. 45. Diagram representing a stolon of Salpa as it would appear if no secondary shifting of the individuals were to take place (after Brooks, from Korschelt and Heider). P solitary form (parent) ; /, //, ///, first, second and third group of individuals (the number shown in each group is much smaller than actually occurs) ; b, b", b'" pharynx ; c", c'" atrium ; d digestive canal ; ee ectoderm ; el elaeoblast ; en endoderm of the connecting stalks ; es endostyle ; g gill ; h heart ; ganglion ; o", o'" ovary. the other. Later the intermediate constricted part of the stolon, which joins them, atrophies and the zooids remain adherent by processes of the body- wall and test, which have become developed for the purpose. When this happens the zooids of the group affected become detached from the proxi- mal part of the stolon and form a free-swimming chain of sexual individuals all of the same age and connected by processes of the body wall and test. The sexual zooids when full grown are of about the same size as the asexual. The ovaries are formed early and the ova are fertilised before the testes are developed. Fertilisation therefore must be effected by spermatozoa produced by another chain. Cyclosalpa Blainville, digestive tube running antero-posteriorly and not coiled up to form a nucleus ; the chain-forms are attached together in a circle. G. pinnata Forsk., with linear bands, C. affinis Cham., without linear bands. Salpa Forskal, digestive tube coiled up in the nucleus, one embryo, chain as elongated band, S. africana-maxima Forsk. Thalia 54 PHYLUM TUNIC AT A (UROCHORDA). Blumenbach, very like Salpa, Th. detnocratica-mucronata Forsk. Pegea Sav., very like Salpa but with supposed traces of stigmata (hemitremata) on its dorsal wall. lasis Sav., like Salpa, but with several embryos at different stages of development in the same sexual animal. The abyssal form Octacnerntts Moseley, discovered by the Challenger Expedition and described by Moseley* is placed here ; why, it is difficult to for it does not appear to possess any important salp-like feature. Octacnemus bythiu* .Moseley, body octoradiate, medusa-like, flattened antero-posteriorly, probably attached, test gelatinous, thin, transparent ; pi larynx with no stigmata or openings into the atrial cavity ; digestive 'ir-ans coiled up to form with the reproductive organs a nucleus which is pl.ir-ed posteriorly ; S. Pacific. Our knowledge of its structure is limited, .MM! nothing is known about its budding and reproduction. A second imperfectly known colonial speciesf O. patagoniensis Metcalfe from 1,000 fms. off the coast of Patagonia has been assigned to this genus. Sub-order 2. Cyclomyaria (Doliolida). Barrel-shaped Thaliacea with thin test, and pharynx with two rows of stigmata on its posterior watt. The asexual form has a ventral stolon which buds and produces three kinds of zooids, one of which develops sexual organs. The muscular rings are complete ventralli/ and a tailed larva is ahvays developed. Doliolum is a transparent pelagic organism with a thin test in \vhich there are no protoplasmic elements and no cellulose. The pharynx (Fig. 46) occupies the anterior part of the body and the atrial cavity the posterior, the digestive canal and the heart (5) together with, in the sexual form, the gonads being aggregated together in an inconspicuous nucleus behind the pharynx on the ventral side of the atrial cavity into which they project. The mouth (1) and atrial (17} openings are at opposite ends of the body and terminal, and the edges of both are lobed. There are no tentacles The dorsal tubercle (22) is surrounded by the spirally coiled, dorsal ends of the peripharyngeal bands, and the pharynx, which has an endostyle (3) but no dorsal lamina, possesses only two rows of stigmata placed in its posterior wall (20). There is a ganglion and closely adjacent subneural gland (21), which opens in front of the peripharyngeal band by the dorsal tubercle (22). The muscular bands of the body-wall are in the form of complete hoops, of which there are nine in the Trim*. Linn. Soc. {-). I. p. -_'s7, IX7t'>. See also Herdman Challenger I, 1 ' ports, Timi.-iitM. Pt. Ill, 1888. t Metealfe, Jo////> Hopkins Univ. ('ire., 12. IS't.X Fie. 8. DOL1OLIDA. 55 20- asexual and eight in the sexual forms, the anterior and posterior of them acting as sphincters. A tailed larva is formed, and develops into an asexual form which produces a budding stolon. The buds are polymorphic, there being three kinds, one of which alone becomes sexual. The asexual form (Fig. 46) presents the following features : It has nine muscular rings, and the ganglion is placed behind the fourth. The mouth is surrounded by ten lobes, the atrium by twelve. There is an otocyst (4) in the body-wall on the left side which contains an oto- lith and in some species opens to the exterior. The body carries two median appendages, a dorsal one (cadophore), arising far back near the atrial opening (19). and a ventral one, the stolon (8), arising behind the fifth muscular band. The stolon will be described below. The dorsal process contains a blood space divided into two by a septum, and its dorsal ectoderm is columnar. In the sexual form (Fig. 47) the mouth is surrounded by twelve lobes, the atrial aperture by ten ; there are eight muscular rings and the ganglion is placed behind the third. There is no otocyst, both the stolon and dorsal appen- dage are absent, and there is a greater number of stigmata. There is an ovary and a testis, both unpaired ; they open a little to the left of the middle line behind the anus. The ovary never contains more than one ripe egg at the same time, but as soon as this is expelled a second is formed, and after that a third. After the third egg is laid the ovary atrophies 15 J9 16 FIG. 46. Diagram of a longitudinal-vertical sec- tion through the asexual (budding) form of Doliolitm (after Delage and Herouard). ] mouth ; 2 peripharyngeal band ; 3 endostyle ; 4 otocyst ; 5 heart ; 6 left pharyngeal diverti- culum ; 7 mesoderm ; S stolon ; 9 diverticulum from left side of cloaca into stolon ; 10 aper- ture of oesophagus ; 11 pyloric glands ; 12 sto- mach ; 13 intestine ; 14 anus ; 15 nerve ; 76 tac- tile process ; 17 exhalent aperture ; 19 dorsal appendage ; 20 hinder wall of pharynx with stigmata ; 21 subneural gland with the closely adjacent ganglion ; 22 opening of duct of sub- neural gland (dorsal tubercle). 56 PHYLUM TUXICATA (UROCHORDA). and the testis ripens. The egg surrounded by its layer of follicle cells passes into the cloaca and thence into the sea. After ' at I SO si ov FIG. 47. Doliolum denticulatiim, sexual form, from the left side (after Herdmau). at at rial aperture ; at.l atrial lobes ; br mouth ; br.l buccal lobes ; brs pharynx ; dt dorsal tubercle ; end endostyle ; h heart : i intestine ; >;' m* muscular rings ; n nerve ; ng ganglion ; ov ovary ; pbr atrial cavity; pp peripharyng"al liand ; g stigmata ; sgl subneural gland ; so sense organs ; st stomach ; tes testis. fertilisation it secretes a vitelline membrane and falls to the bottom. Here it develops into a tailed larva by a process, of which all the stages have not been followed. It begins to swim by means of its tail while still within the vitelline membrane, Flo. 48. Old larva of Duliolinn i-/ifi-ni>r,;/i, ;itt.-r I'ljanin, from Korschelt and Heider). ch notochord ; d dorsal appendage ; ' endnstyle ; h heart and pericardium ; m vitelline mem- brane ; n ganglion ; r rudiment "I -tnlmi in^cttc-shaped organ). DOLIOLIDA. 57 which eventually ruptures and allows the larva to go free. The anterior end of the body assumes the barrel-shaped form of the adult, with its dorsal process, but still preserves its larval tail and notochord (Fig. 48). These eventually atrophy, persisting for a time as a stump, which resembles in structure and position the organ called elaeoblast in the salps and Pyrosoma. The principal features in the development are as follows. Cleavage is total and is followed by an invaginate gastrula. In the next stage observed the embryo appears to contain three cell-groups which constitute the rudi- ments of the nervous system, the mesoderm and the notochord. The alimentary canal and endoderm are formed later as a se- condary invagination of ectoderm. The atrium is formed by another invagination of ectoderm. The nervous r u d i ment elongates. Its median part remains bulky and gives rise to the ganglion of the adult and the subneural gland (ventral lobe) ; its anterior end narrows, acquires a lumen which opens into the pharynx, and forms the duct of the subneural gland and the dorsal tubercle ; the posterior portion also becomes narrow and persists as the posterior unpaired nerve (nervus branchialis). The pericardium is developed as an excavation of a portion of the mesoderm, and the stolon is formed as a small ventral process. The stolon in its earliest stage in the embryo consists of a small mass of mesoderm applied against the ectoderm in the neighbourhood of the heart. This is very soon reinforced by two pairs of out- Arvf f'"' ''^^ growths, one from the atrium and the other from P "$?. ".l|a the pharynx. These, carrying with them the mesoderm, cause a projection of the ventral body wall, which extends into, but does not pierce, the tunic. Later the stolon pierces the tunic and pro- jects freely, and the five strands contained in it be- come increased, by development of the cloacal and now FIG. 49. Dorsal region of a larva of Doliolum Mullen (after Uljanin, from Korschelt and Heider). cl atrium ; fl, dorsal tubercle and duct of subneural gland ; m muscle-hoops ; ganglion ; nb branchial nerve. R I FIG. 50.- CCL -Probudof Dolio- ; showing its trans- pharyngeal tubes, to seven. The free end " wn 1 DeSe 1 and segments into small bodies (Fig. 50), which become Herouard). < trans- detached and are known as probuds. The probuds f contain a portion of the seven cell masses sur- rounded by ectoderm. They wander by means of the pseudopodial activity of certain of their ectoderm cells (Fig. 50, ca) along the right side of the animal to the dorsal side of the base of the dorsal process (Fig. 51). Here they divide into from fourteen to twenty buds, which attach themselves on each side of the middle dorsal line. This 58 PHYLUM TUNICATA (UROCHORDA). the stolon (Fig. 52). attachment is an epithelial one ; they perforate the tunic and their ecto- derm becomes adherent to the columnar ectoderm of the dorsal side of By the growth of the stolon in girth they become displaced outwards and give rise to the lateral buds (Fig. 52, G) of the dorsal stolon. As the stolon is continually growing in length the buds which first affix themselves are carried away from the body and new buds are at- tached between them and the base of the stolon. Thus it happens that the buds increase in age as we travel from the body along the stolon. The probuds which arrive first at the dorsal process give rise to lateral btids (G). Those which arrive later give rise to median buds, which are attached nearer the middle line and are not displaced so far outwards (Fig. 52, P). The fate of the lateral and median buds is as follows. The lateral buds develop into small doliolum - like zooids with the rudiments of gonads, which however soon atro- phy, and with a well-de- veloped alimentary canal, but without a closed atrial cavity (Fig. 52, G). They actively take in nourish- ment, and serve for the nutri- tion of the growing stolon. They are therefore called qastrozooids. They are in osmotic relation, through their epithelial attachment to the dorsal stolon, with the blood of the parent. This relation, is of some im- portance, because the a- sexual parent (oozoite) has lost its pharynx and diges- tive canal by atrophy and. retaining only its ner\ mis system, heart, and muscles, has become converted into u. ML. .">!.- -Dorsal view of the posterior part "t an asexual form (nurse) of Doliolum showing the migra- tion of the probuds (after Barrels, from Korschelt and Heider). I lateral liuds ; m median buds ; ;/ ..f.M.S., 4, lSf>(>. Fol, Etudes sur les Appendiculaires tin detroit de .Messiue. Mnn. de la So<-ihioxus. The blood-channels are without epithelial walls. APPENDICULARIAE. 63 and the blood appears to be entirely devoid of cellular elements. The mouth opens into a spacious pharynx (Fig. 55), and leads behind by a short oesophagus into a dilated stomach, from which passes forward an intestine to open on the ventral surface by the anus. The hind end of the intestine may be marked off as a rectum. There is no pyloric gland or liver except in Stegosoma and Megalocercus in which a gland, identified as liver, opens by a narrow duct into the stomach. The pharynx has, on the anterior part of its ventral wall, a short endostyle (o), consisting of a groove bounded by large glandular cells without cilia except at its front end. From the front end of the endostyle there arises on each side a ciliated band which passes backwards and dorsal - wards to join its fellow at the oesophageal opening. There is no dorsal lamina and no row of tentacles. From the hind end of the endostyle a ventral band of cilia, corresponding to the ventral groove of other Tunicata, passes backwards to the oeso- phageal opening. In the ventral wall of the pharynx behind the endostyle are the internal openings of the two gill-apertures ; these lead into two short ciliated tubes which open on the ventral side of the body by the spiracles (n). There is a small ciliated diverticulum of the dorsal wall of the pharynx, placed on the right side of the ganglion and corresponding to the dorsal tubercle of other Tunicata. The nervous system consists of a ganglion (Fig. 55) placed on the dorsal side of the anterior wall of the pharynx, and of a dorsal nerve cord (d) which passes back from the ganglion on the right side of the stomach to the tail. In the tail, to the hind end of which it extends, the dorsal cord lies on the left side of the notochord and presents a variable number of swelhngs due to the presence of nerve cells and called caudal ganglia. The number of these caudal ganglia varies from eight to forty. The first of them is usually larger than the others. Nerves are given off by the cerebral ganglion, and by the caudal ganglia. It has been stated that the cerebral ganglion and dorsal cord contain a minute canal, but this is doubtful The position of the caudal nerve cord on the left side of the notochord has suggested the view that the tail has undergone rotation through 90 so that its true dorsal surface has come to lie to the left. An otolithic vesicle (a) lies on the left side of the ganglion, so closely in contact with it that at one place the inner wall of the 64 PHYLUM TUNICATA (tJROCHORDA). vesicle appears to be formed by the ganglion. There are no visual organs. The notochord (h) is confined to the tail. It consists of an axial hyaline substance of cartilaginous consistency, surrounded by a protoplasmic nucleated membrane. The nuclei project on the inner side into the hyaline axial substance. The musculature of the tail is well developed. It consists of two broad bands of muscular substance, one on each side of the noto- chord. Each band consists of an outer protoplasmic layer con- taining the nuclei and an inner layer of striated contractile substance. The nuclei are ten in number in each band, arranged in a row at regular intervals, thus indicating that the muscular bands are composed of ten cells. The limits of these cells can- not be made out in the adult ; though it is said that after certain treatment indications of them may be seen as transverse lines through the muscle substance between the nuclei. (Fig. 55, k). It has been suggested that these lines represent the limit of segments and that the tail is segmented. This however is very doubtful. There is no correspondence between the caudal ganglia and the supposed muscle segments. The pericardium is a simple epithelial sac placed on the ventral side of the stomach. Its dorsal wall is contractile and slightly invaginated into its cavity. A concave contractile lamella (Fig. 55) bounding a blood sinus, the dorsal wall of which is formed by the stomach, is thus established. This concave contractile membrane constitutes the heart. When it contracts it propels the blood from the super jacent blood sinus into the blood channels generally and so acts as the central organ of the circulation. It is said to reverse the direction of its action as in other Tunicates, but this is doubtful. The wall of the pericardium is a simple protoplasmic membrane con- taininn nuclei. Dorsally it contains in its outer layer striated contractile fibres. It differs from the heart of other Tunicates merely in the small <-\tent to which the dorsal wall of the pericardium is invaginated. The gonads are contained at the posterior end of the body (Fig. 55, /, g). Excepting Oikopleura dioica all members of the group are hermaphrodite, the male organs maturing first. There is a single or double ovary contained between two testes. There is no oviduct. The eggs when ripe dehisce into the vascular cavity from which they escape by dehiscence of the body-wall, APPENDICULARIAE. 65 causing the death of the animal. The testes acquire an opening to the exterior at the time of sexual maturity. Budding does not take place. What little is known of the development * appears to indicate that it does not differ essentially from that of other Ascidians. The first Appendicu- larian was discovered by Chamisso, who gave it the name Appendicularia. For a long time the systematic position of these forms remained obscure. By many zoologists they were regarded as larval Tunicates, but it was Huxley who dis- covered their spermatozoa and to whom belongs the merit of having first recognized them as members of the tunicate phylum. Appendicularia Cham., capsule ovoid, stomach unilobed, rectum enor- mous, testis single. Oikopleura Mertens, capsule large, stomach bilobed, mouth with ventral lip (Fig. 54) ; tail very long. Vexillaria J. Muller, special muscles traverse the body and are inserted on to the viscera. Stego- soma Chun, capsule unknown, stomach with liver, surface-waters and deep sea to 1,000 fms. Megalocercus Chun, the largest of all known genera, length of body 8 mm., total length 30 mm., capsule unknown, with a body- wall musculature similar to that of the Salps ; Med., 600-900 fms. Folia Lohmann, capsule unknown, tail long, genital mass single ; Atl. Althoffia Lohmann, warm parts of Atl. Fritillaria Q. and G., body elongated, con- stricted in the middle at the attachment of the tail, a fold of the dorsal integument forming a hood over the head (Fig. 55), testis usually single. Kowalevskia Lahille, capsule large with single orifice, and interior marked with projecting ribs ; endostyle, peripharyngeal band, pericardium and heart absent ; pharynx with 4 longitudinal rows (a dorsal and ventral on each side} of solid, ciliated comb-like processes which act as strainers ; Med. and E. Atl. Goldschmidt, Biol. Centralbl., 23, 1903, p. 72. Z III CHAPTER II. PHYLUM ENTEROPNEUSTA. Unsegmented Chordata with a tripartite division of the body rind coelom, a dorsal preoral lobe, and a notochord-like structure which is confined to the anterior (proboscis) region of the body. Pharyngeal branchial apertures are present in all except Rhab- popleura. The Enteropneusta are Chordates * which present the fol- lowing important features. The body is divided into three regions, the proboscis which is a dorsal preoral lobe, the collar, and the trunk. In the anterior part of the trunk paired lateral apertures the gill-slits are present putting the alimentaiy canal in communication with the exterior (except in Rhabdo- pleura). The coelom, in the cases in which its development is known, arises as five diverticula of the embryonic enteron, viz., one unpaired pouch, which extends into the proboscis and is called the proboscis cavity ; one pair of pouches which occupy the collar-region and are known as the collar- cavities ; and finally a posterior pair which occupies the whole of the trunk region and constitutes the trunk-cavities. The proboscis cavity opens to the exterior by a single or double pore ; the collar Cavities each by a pore ; while the trunk-cavities are devoid of an external opening. The anterior part of the alimentary canal sends into the base of the proboscis an unpaired diverticulum which has a charac- teristic structure and has been identified as a notochord. The central nervous system lies in the ectoderm, and there is a special concentration of it in the dorso-median line of the collar. This concentration comes, in the majority of forms, to lie in the wall of a canal which is open at both ends. There is always a * See vol. ~2. oh. i. 66 AFFINITIES. NOMENCLATURE. 67 system of tubes which are identified as vascular, and in all there is a curious organ in the base of the proboscis which consists partly of the before-mentioned gut diverticulum and partly of vascular and glandular tissue. This organ is called the central- or proboscis-complex. A remarkable feature of the phylum and one which it has in common with the Cephalochorcla is the absence or small amount of stellate or other connective tissue cells between the coelomic epithelium and the epithelium of the ectoderm and endod'erm (p. 91). The group Enteropneusta was established by Gegenbaur in 1870 and was placed among the Vermes. They have been re- garded by some authors as especially related to the Nemertines and by others as possessing leanings towards the Annelids. Aletschnikof}' * in 1869 pointed out their resemblances to Echinoderms, while Gegenbaur (1874) and Huxley f (1877) were the first to call attention to the Tunicate affinities, Huxley going so far as to include the two in a new group, the Pharyngo- pneusta. The connexion with the Vertebrata was suggested by Sedgwick 1: in 1884, and their inclusion in the group Chordata was accomplished by Bateson in 1885. The Echhioderm affinities first pointed out by Metschnikoff undoubtedly exist and are dealt with below (p. 99) arid in the section dealing with affinities in the general account of Echinoderms. The suggested affinities with Phoronis are discussed at the end of this chapter. The phylum is divided into two orders, the Balanoglossida and the Cephalodiscida. The establishment of these orders has been necessitated by the discovery of the genera,' RJiabdopleura and Cephalodiscus, which present all the important Enteropneust features (except the gill apertures in Bhdbdopleura) . A great variety of names has been proposed for the phylum here called Enteropneusta. We have selected Gegenbaur' s term on the grounds of priority and usage. The discovery of Cephalodiscus and Bhabdopleura, and consequent necessity for the establishment of two orders within the phylum, have induced some authors to restrict the term Enteropneusta to the forms which we have here included under Balanoglossida, and to employ another name, Hemichordata, for the phylum. We object to this terminology for two reasons : in the first place Cephalodiscus is as much ' Ueber Tornaria, Nachr. K. Ges. Wiss. u. Univ. Gottingen, 1809, p. 287- "292, and Z. f. w . Z., 20, 1870, pp. 131-144. t Journal Linn. Soc., Zool., 12, 1877, pp. 199-226. J Q.J.M.S., 24, p. 70. Q.J.M.S., 25 sup., p. 111. 68 PHYLUM ENTEROPNEUSTA. enten.pneiist as Itiiluiiniiluxxii.i or Pttjchodera and cannot logically be divorced from the group to which that name is applied. In the second place, Hemichordata is a bad term, because it is not certain that the struc- ture identified as notochord is really such, and because, even if it is a noto- , it cannot fairly be said to be " hemi." It might perhaps be termed ri." but no one. s<> far as we know, has suggested this or applied tin- nanif Proboscichordata to the phylum. Order 1. BALAXOGLOSSIDA.* Enteropneusta irilh vermiform, cloifjahd body and tim/i// /' ilu . I mi rican Acad. of Arts and Sciences, 9, 1873. W. Bateson, A Series of Studies in the Anatomy and Development of Balanoglossus in the Q.J.M.S., 24, 25, and 26, 1884-6. J. W. Spengel, Die Enteropneusten, Fauna und Flora des Golfes von Neapel, 18th Monograph, 1893. Id., Die r-'-iiennung der Enteropneusten Gattungen, Zool. Jahrb., Syst., 15, 1901, p. 209. Id., Neue Beitrage, etc.. ii.. iii., iv., Zool. Jahbr., Anat., 20, p. 1, 1 1. .'!!."), p. 413. J. P. Hill, The Enteropnexista of Funafuti, Memoir* Aux- ti-nlnni Miiximn, 3, 1898, pp. '2(1.") and 335. A. Willey, Enteropneusta tnuu the S. Pacific, Willci/'* Zoological Result.*, |>t. 3, 1899, p. 32. R, C. I'mmcft, The Entcropneusta, < luril iii<'/(it/. S. F. Harnicr. Hemichordata, in Cambridge Natural History vol. 7. l!(4. p. 3. REGIONS OF THE BODY. SKIN. 09 , slightly over the anterior end of the trunk, so that there is a circular groove at the junction of the collar and trunk. In some species this hind end of the collar may project so far back as to cover three or four gill- slits (Dolichoglossus koivalevskii), and in this case the projection has been called by Bateson the atrial fold, and the space which it encloses the atrial cavity (cf. Ampliioxus). The trunk itself is also indistinctly divided into re- gions : these are the bran- chiogenital immediately succeeding the collar (Fig. 56, k, g) ; next, the hepatic (vb), in which the intestinal walls contain a green or brown pigment ; and lastly the abdominal region which forms the hind end of the body. The branchiogenital re- gion possesses in its anterior part on each side of the dorsal middle line a double row of pores ; these are the branchial pores which place the gill-apertures of the alimentary canal in com- munication with the ex- terior. The branchial pores are placed in a slight groove, the branchial groove. Posteriorly the branchiogenital region is without branchial apertures. The genital organs are also found in. the lateral walls of the body in this region. The skin consists of a layer of ciliated epidermis or ectoderm beneath which is a structureless basement membrane. The epidermis is in most parts a thick layer and appears to consist of very long and narrow cells, extending indeed through the whole thickness (Fig. 57) with interspersed unicellular glands. Moreover the nuclei occur at different levels, thus suggesting at FIG. 56. Dolichoglossus kowalevskii (after A. Agas- siz, from Korschelt and Heider). e proboscis ; kr collar ; k anterior part of branchial region ; g genital region ; db dorsal, vb ventral middle line. 70 BALANOGLOSSID A . first sight the view that the epidermis is many-layered. These long ceUs, into which the epidermis breaks up when teased or otherwise ill-treated, are connected together by numerous lateral processes (Fig. 57), and are prolonged internally into fine fibres which enter the nervous felt-work found in the deeper part of the epidermis over the greater part of the body. The thickness of the epidermis and the number and disposition of the gland cells varies in different places. The muscles and connective tissue are entirely derived from the walls of the coelomic sacs and will be considered in connexion with those struc- tures. The nervous system. In the lower stratum of the ectoderm just external to the basement mem- brane there is the feltwork of fine fibres almost en- tirely without nuclei, which has just been men- FlO. 57. Preparations of the epidermis of the collar of BuJano- tidied. This tlS- ylosstis clavigerus (after Bateson). A, a similar preparation more highly magnified, showing the long spindle-shaped cells g\jg jg Supposed to passing internally into fine fibres ; -B, portion of epidermis teased and pressed out. showing elongated cells united at their fog nei'VOUS in outer ends l>y numerous connection-. function and strongly recalls the corresponding layer found in many Xcmertini. It is thickened along the dorsal middle line of the collar and trunk and along the ventral middle line of the trunk. These constitute the dorsal and ventral nerve cords (the collar portion of the dorsal cord is invaginated, see below). They are connected at the junction of the collar and trunk by lateral commissures which like them are special concentrations of the nerve plexus and lie in the ectoderm at the base of the collar or atrial groove. The dorsal nerve cord extends forwards on to the base of the proboscis to become continuous with the general NERVOUS SYSTEM. 71 epidermal nerve-plexus of that organ. This is specially de- veloped round the stalk of the proboscis to form the somewhat FIG. 58. Transverse section through the collar nerve cord of Glossobalanus sarniensis. showing the central canal c with its cuticular lining ; ga giant ganglion cell, dw dorsal rout. ill-defined structure which is frequently spoken of as the proboscis nerve-ring. The part of the dorsal cord contained in the collar has different relations from the rest of the nerve trunks. It nc FIG. 59. Transverse section of the collar nerve cord of Glossobalanus minutus (after Spengel). showing two of the isolated cavities with their cuticular lining, and the radiating lines sur- rounding them ; nc canals in the fibrous substances which contained processes of the giant cells. lies in the ventral wall of a tube or rod (Figs. 58, 59, 60) of ectoderm which is derived in the embryo from the delamination 72 BALANOGLOSSIDA. or imagination (according to the species) of a dorsal median tract of collar ectoderm. This collar nerve-cord, which is called the medullary cord, is always continuous with the outer ecto- derm at the front and hind end of the collar (and in some species by the dorsal roots, see below). At these points (Fig. 60) there is almost always a canal the medullary canal which in some species is continued through the whole length of the cord. The central canal, whether it is confined to the anterior and pos- terior ends or extends throughout the whole length of the cord, opens to the exterior by the anterior and posterior nerve pores, the former being placed at the base of the proboscis stalk within the anterior projecting rim of the collar, and the latter at the Imttom of the groove (atrial) which is found at the junction of the collar and trunk (Fig. 60). The fibrous matter of the trunk dorsal nerve-cord is continued along the ventral side of the medullary cord. In the genus Glossobalanus, however, the fibrous matter is continued right round the medullary cord, being found on its dorsal surface also (Figs. 58, 59). In those cases in which the central canal is not present as a continuous tube, it appears to be represented by a number of isolated cavities which are frequently surrounded by elongated radiating cells.* In Schizocardium peruvianum and Dolichoglossus merechkovskyi the medullary cord is quite solid, there being neither neuropores nor isolated cavities. There is frequently present at the anterior and posterior ends of the medullary cord an ectodermal pit, with the ectodermal lining of which the nerve cord is continuous. The anterior and posterior neuropores when present open into these pits, or if the cord is entirely solid, as in the two >l>ecies just named, the medullary cord is continuous with the ectoderm lining them. It is difficult to say whether the pits should be regarded as terminal parts of the central canal or as independent structures. Some importance has been attached to them by Spengel, but as in many cases it is exceedingly difficult to distinguish them from the adjacent parts of the medullary canal, it is perhaps better to regard them as the terminal and, if the central canal is elsewhere obliterated, as persistent portions of this 'anal. The dorsal nerve cord lias the same relation to these pits as it has to the cord, i.e. it lies in their \entral \\alls. They are sometimes dilated and the anterior pit may project backwards so as to overlap the dorsal side of the adjacent p : irt of the medullary cord (Spengelia alba, Schizo- i-nriii mil ). thus recalliii!_ r in some decree a cerebral vesicle of the vertebrate brain. The condition of the central canal varies much even in different species of the same genus. It is present as a complete canal, traversing the whole length of the medullary cord and opening at each end by a neuropore, only in the genus Ptyclwdera(Pt. flam, erythrea, Imhiinx //*/*). in Glossobalanufi In some, cases these isolated ca\ itics are said to coexist with a central canal without however communicating with this latter (e.g. S'c/i. brasil- " n-' in some individuals). NERVOUS SYSTEM. 73 tiarniensis, hedleyi, ruficollis, and in Balanoglossus apertus. In other species it is as a rule only present at the anterior and posterior ends, being repre- sented in the middle by a number of cavities (sometimes arranged in a double row [Fig. 59] ). In two cases, already mentioned, all traces of the central canal are absent (unless the anterior and posterior pockets are regarded as parts of it), the cord being solid. But even within the limits of the same species it frequently happens that the central canal or its repre- sentative exhibits considerable variation. In the Ptvchoderida. 1 the dorsal side of the medullary cord is / *. connected with the dorsal ectoderm by a variable number of cords which constitute the dorsal nerve roots (Figs. 60, 8). These roots are frequently hollow (Fig. 58), particularly those which arise from the anterior end of the cord, and their axial canal communicates with the central canal of the cord or with one of the isolated cavities which represent that structure. The axial canals of the roots do not however open to the exterior, but in some species they may be continued for a short distance in the epidermis as intraepidermal canals (Balanoglossus canwsus). The dorsal nerve roots vary much in number and in the course which they take in reaching the ectoderm. In Glossobalanus rnfiro//fs there may be from 12 to 18 of them, in Ptychodera flava 1. to 6. As to course, they may be straight or directed obliquely backwards or even curved. In the other families dorsal roots are rarely present. There seems to be a trace of one such structure (not, however, reaching the skin) in SpengeMa porosa. In Dolichoglossus kowalevskii the posterior part of the medullary cord possesses a dorsal keel-like projection, which is solid and does not reach the epidermis. The roots whether solid or hollow consist of cellular prolongations of the cord and are coated with fibrous matter which is a prolongation of that of the cord and is continuous with the nervous stratum of the epidermis. The ectoderm over the dorsal and ventral nerve cords of the trunk is thickener) and sometimes projects as a ridge : in some cases it is partially invaginated and so placed at the bottom of a groove. Moreover it differs from the rest of the epidermis in containing fewer gland cells. As an indi- cation of the fact that the nerve cord of the collar is derived from the com- pletion of such an invagination by the union of the sides of the groove over the gland-free nerve-cord ectoderm, may be mentioned the fact that the ectoderm of the floor of the central canal of the collar cord is more or less free from gland cells, while they abound in the roof. Further, as already mentioned, fibrous nerve matter is usually found only in ^he ventral wall of the ventral canal. The ectodermal lining of the central canal and iso- lated medullary cavities possesses a cuticle-like border. It is impossible to arrive at any satisfactory conclusions as to the structure of the nerve tissue. The most that can be said is that the external ends of some of the superjacent ectoderm cells extend into and branch in it. According to Bateson, processes from the fibrous matter of the nerve cords may be traced through the basement membrane into the adjacent muscles. There do not appear to be anv organs of special sense. 74 BALANOGLOSSIDA. The alimentary canal extends as a straight tube from mouth to anus. The mouth (p. 68) leads into a wide tube which traverses the collar region and is called the buccal cavity. The buccal cavity passes at the junction of collar and trunk into the pharynx in the side walls of which the branchial slits are placed ; this leads in the hinder part of the branchiogenital region into the oesophagus which passes behind into the intestine. The anterior part of the intestine is distinguished as the hepatic intestine from the posterior part which opens by the anus. The part of the intestine next the anus is distinguished as the rectum. The wall of the hepatic part of the intestine contains a greenish or brown pigment, and in the Ptychoderidse and in Schizocardium fit*. 60. Diagrammatic Longitudinal-vertical section of Ptychodera (from MacBride). C col- lar ; Pr proboscis ; Tr trunk. 1 proboscis coelom ; 2 glomerulus ; 3 heart ; 4 pericardium ; i proboscis pore ; 6 collar coelom ; 7 collar nerve-cord ; 8 dorsal nerve roots of 1 ; D dorsal iiluncl-vfssrl ; 10 dorsal mesentery ; 11 external opening of the branchial sac (not clearly shown), 12 U-shaped internal opening of the same ; 13 ventral blood-vessel; 14 ventral part (hypobrauchial groove) of the branchial region (pharynx) of the alimentary canal: 15 dorsal part of the branchial region of the same ; 16 buccal cavity ; 17 mouth ; 75 noto- chord ; 19 euticular sheath of the same ; 20 ventral pocket of proboscis coelom ; 21 ventral n nt 1 proboscis coelom. possesses on each side a row of hepatic diverticula which cause the hepatic sacculations of the body wall visible in these genera . The alimentary canal is lined by a more or less columnar epithe- lium and is ciliated throughout. It is attached to the body wall in the dorsal and ventral middle lines by longitudinal mesen- teries ; which however in the adult are deficient in certain parts of the body (see below, pp. 87, 89). The buccal cavity gives off from the anterior part of its roof, in the dorsal middle line, a forwardly directed caecal diverticulum (Figs, 60, 64), which, following Bateson, we shall call the noto- chord. The notochord is a tubular structure which extends through the neck of the proboscis and projects into its base. It is divisible into two regions : the narrower neck which lies NOTOCHORD. PHARYNX. 75 in the stalk and opens into the buccal cavity and the wider head which projects into the proboscis-cavity and in which the lumen may be much reduced and not easy to see. At the junction of the two there is a ventrally directed csecaldiverticulum (Figs. 60, 64). The walls of this structure are formed of endoderm which is continuous with the epithelium lining the buccal cavity and consists of cells which have undergone a peculiar modifica- tion, very similar to that of the notochord cells in Amphioxus and in the embryos of the Vertebrata. That is to say, they are much vacuolated and the protoplasm and nucleus occupy but a small portion of them. There is a basement membrane round the whole structure. This, which may be compared to the sheath of the vertebrate notochord. is much developed as a skeletal tissue ventrally and posteriorly (Fig. 60, 19, Fig. 64, n). We shall describe it later on. There is considerable variability in the structure of the notochord. In transverse section the neck is often semilunar, but its cavity may be broken up into several separate spaces. This is particularly liable to happen in the neighbourhood of the ventral appendage which in some species is very massively developed. In some species portions of the proboscis skeleton may penetrate the substance of the notochord (Harrimania kupfferi, Balanoglossus biminiensis). In Ralanoglossus carnosus some of the noto- chordal cells lining parts of its lumen, which are quite cut off from the rest, are densely ciliated. In the same species the hind end of the notochord splits up into three minute tubes which unite again before joining the mouth. In Stereobalanus canadensis the neck is absent, and the head is therefore quite disconnected from the buccal wall. Speaking generally, it may be said that the notochord behind its ventral diverticulum or caecum, quickly loses its chorda-like character. It is interesting to notice that it is in this posterior portion the skeletal development of the sheath is found. May we draw the inference that anteriorly it has a supporting function, while posteriorly this function is taken over by its sheath ? In Schizo- cardium, Glandiceps, and Spengelia the head is continued forwards as a narrow process which traverses the axis of the proboscis and is either solid or contains a small indistinct lumen. This anterior extension is called the vermiform process of the notochord. The ventral caecum of the noto- L'hord frequently has a considerable lateral extension, whence arise its lateral pouches (e.g. Glossobalanus ruficollis). The lumen of these struc- tures is often cut off from the main lumen. The pharynx. The anterior part of the branchiogenital region is occupied by the pharynx, which, as in other Chordata, con- stitutes the branchial * portion of the alimentary canal. The gill- : Punnett suggests, as it seems to us with some truth, that the 'unction of the branchial wall with its perforations is more that of a sieve than of a branchial organ, allowing the water which enters by the alimentary 76 BALANOOLOSSIDA. apertures are, except in Ptychodera and Stereobalanus canadensis, tubes, which lead outward to open externally by the branchial pores placed in the branchial groove on each side of the middle dorsal line. The internal openings (gill-slits) are U-shaped, the 1011 1 .0. 111. aliifsoliiiltniiis iiinnitiix. transverse section through the lirauchuil region, .-oinewliat diagrammatic (alter Spem-el in mi Lang). / dorsal nerve eord ; 2 dorsal blood-vessel: 3 bran- chial (submedian) groove ; /ectoderm; .j^ dorsal P ar t "I <-avil\ oi pharynx; Hi h\ pobraiu-liial groove of pharynx, in this region very largely developed. dors il margin having a tongue-like projection, forming a tonguc- 1 r (Figs. 60, 62) as in Amphioxus. This loads into a pouch canal \vith the food to escape. On this view, the skin, which contains a rich plexus lit' liliiud \-(.-sscls. vvmild art as tlu> main nroan of respiration. PHARYNX. 77 1 having the same vertical extent as the gill-slit and placed close to the pharyngeal wall (Fig. 61). This branchial pouch opens to the exterior by a small aperture the branchial pore. The whole branchial passage is lined by endoderm and is developed as a pharyngeal diverticulum. In the forms just mentioned, the U-shaped gill-slits open directly to the exterior and there are no branchial pouches or pores distinct from them. This is due to the fact that the original pharyngeal outgrowth remains short and its external opening becomes as wide as the internal. The gill- apertures are added to throughout life and arise as small round perforations at the hind end of the pharynx. The tongue is de- veloped as a down-growth of the dorsal wall as in Amphioxus, but hangs freely, not as in that animal becoming con- nected with the ventral margin (Figs. 60, 62). It receives a prolongation of the trunk coelom (p. 92). Synapticula (Fig. 62) are present in some species, traversing the two limbs of the U-shaped aperture. They are connected to the tongue bar, and vary considerably in number in the different species ; in Bal. carnosus there may be as many as 30 on each side of a tongue bar ; 10 to 1 1 seems however to be the usual num- ber. They are absent in the Harri- maniidae and in Glandiceps. In most forms the gill - slits are . not quite straight, being slightly bowed with the concavity directed forward. The successive branchial pouches are placed close together, and their opposed walls constitute the branchial septa. The first of them always receives the opening of the collar pore (see below), and in Bal. carnosus the first branchial pore opens into the hind end of the medullary canal. The gill-apertures are very numerous. The number varies of course with the growth of the animal, and appears to be highly variable in the different species. It varies from 10 or 11 pair;;, the number found in Dolickoglossus sulcatus, to 700. the number recorded for Bal. aurantiacus. FIG. 62. Diagram of two gill- slits of Ptychodem viewed from the inner sick- (from MacBride). 1 gill-slit; 2 tongue-liar ; 3 synapticulum ; 4 septal or primary liar : -J skeletal rod of primary bar forking ventrally; H skeletal rod of tongue-bar, double. 78 BALAJSTOGLOSSIDA. The walls of the pharynx are supported by a definite skeleton recalling the branchial skeleton of Amphioxus (see p. 91). The gill-slits of the two sides are separated from one another dorsally by a narrow streak of endoderm called the epibranchial streak (Fig. 63, 1). Their ventral extension is more variable. In Schizocardium they almost meet ventrally, being separated only by a narrow band of endoderm which is called the hypobranchial streak (Fig. 63, C). In Dolichoglossus and Glandiceps they extend half way down the pharyngeal wall, being separated by a wide tract of continuous endoderm, which may also be called the hypobranchial streak (Fig. 63, A and B). Lastly in the Ptychoderidie and in Spengelia. they only extend half way down the pharyngeal wall, and the hypobranchial portion is wide as it is in Dolichoglossus ; but at the junction of hypobranchial streak (from MacBride). the two parts the lining of the pharynx is much thickened, form- ing the so-called parabranchial ridges (Fig. 61, 13} and its lumen is constricted in such a way that the whole pharynx has in section a figure-of-8 shape. The result of this arrangement is that the pharynx is almost divided into two tubes a dorsal tube in connexion with the gill-slits and a ventral tube com- parable to the hypobranchial streak of other forms. In ffldiidiceps two conditions are found ; in some species the arrange- ment resembles that found in Dolichoglossus, in others the condition described for Schizocardium is repeated. It seems obvious to compare the epibranchial streak with the hyper- l>liupyngeal groove of Amphioxus and the hypobranchial which is so much developed in Ptychodera and Spengelia with the hypopharyngeal irroove or endostyle* of that form. The pharynx sometimes retains its * The term oesophagus is sometimes applied to the ventral portion of the pharynx, but this is obviously a misnomer and must be rejected. INTESTINE. 79 distinctness for a short distance behind the gill-slits as a groove on the dorsal wall of the oesophagus ; this has been called the postbnnn-hinl canal. The pharynx is followed by the oesophagus (or afferent in- testine as it has been called), which extends throughout the hinder part of the branchiogenital region and leads into the hepatic intestine. This structure occupies the anterior part of the abdominal region of the body and is distinguished by the fact that its walls contain green or brown pigment globules. In the Ptychoderidfe and in Schizocardium, it gives off the hepatic diverticula already referred to. These are arranged in two dorso-lateral rows, one on each side, and open into the intestine by transverse slits. The boundary between the oesophagus and hepatic intestine is not very sharp, especially when the hepatic diverticula are not present. In Glandiceps hacksi there is a kind of siphonal tube or accessory in- testine in the hepatic region on the dorsal side. It leaves the hepatic in- testine at about the middle of its length and joins it again at its hind end. Spengelia alba is distinguished by possessing hepatic diverticula which do not cause external sacculations of the body wall. In some species the oesophagus and anterior part of the hepatic intestine - sends off dorso-laterally short canals which open to the exterior by pores which are placed on each side of the dorsal surface in a line with the branch- ial pores. These pores are in two sets. The anterior set comprises the so-called unpaired intestinal pores. These are placed in the part of the branchiogenital region immediately following the pharynx, i.e. on the anterior part of the oesophagus, and as their name implies are usually unpaired. They are found in Schizocardium brasiliense, \ 3 on one side and 16 on the other ; in Glandiceps hacksi 9 altogether, one on the right side and the rest on the left. The posterior set is known as the paired intestinal pores. These are placed at the hind end of the. branchiogenital region, or front end of the hepatic, and communicate either with the hind end of the oesophagus or front end of the hepatic intestine. They are found in Schizocardium brasiliense (one pair), Glandiceps hacksi (three pairs), Dolichoglossus kowalevskii (four to six pairs) ; also in D. mere- schkowskii and in Spengelia alba. The hepatic intestine passes without any strong line of de- marcation into the intestine proper or efferent intestine which occupies the hinder part of the abdominal region. This section becomes somewhat narrower behind where it forms the rectum which opens outward by the wide terminal anus. In most Ptychoderidse a median rod of endoderm is marked <> ft' from the ventral wall of the hinder part of the intestine. It occurs as a thickening of the gut wall, or as a tube with an interrupted lumen opening at both ends into the gut, and has been called the pygoehord. It lies in what may be called the caudal part of the body and extends back to the anus. 80 BALANOGLOSSIDA. The coelom, though largely filled up by muscular and con- nective tissues in the adult, presents the arrangement which, as we have seen in the first chapter of the second volume (p. 7), is on the whole characteristic of the Chordata (see Fig. 72). It is present in three distinct parts which remain separate from one another : these are the unpaired preoral or proboscideal sac, the I cured collar sacs, and the paired trunk sacs. In the embryo these sacs all contain distinct cavities and have epithelial walls (Figs. 72, 73). In the adult the walls have increased in thickness and have become differentiated into the muscular and connective tissues of the body ; while the cavitiss are much reduced, being encroached upon and partly obliterated by these tissues. The coelom of the proboscis. The wall of the coelomic sac of the proboscis consists of a thin layer of circular muscles lying next the basement membrane of the ectoderm and inside this of a thick layer of longitudinal muscles, which are often arranged in bands and which surround the central cavity (Fig. 65). Within the longitudinal muscles and extending amongst them is a layer of loose stellate connective tissue which partly fills up the proboscis anteriorly. Into the posterior region of the proboscis cavity there projects a complicated structure known as the central complex or the basal organ of the prDboscis (Figs. 60, 64). The central complex. \\ liich consists of the notochord and certain vascular organs, is almost entirely covered towards the proboscis cavity by a layer of coelomic epithelium, and is attached to the body wall in the dorsal and ventral middle lines by the dorsal and ventral septa of the proboscis (Fig. 65, i, 6). The posterior part of the pro- boscis cavity is therefore double and lined, at any rate posteriorly in the region of the dorsal and ventral canals (see below), with coelomic epithelium ; while the anterior part (solid in Harrimania kupfferi) is single (except in Willeyia in which it is double) and without, so far as can be seen, a distinct layer of coelomic epithe- lium. Each of the posterior halves of the proboscis cavity is further subdivided, in consequence of lateral expansions of the notochord which meet the body- wall, into a dorsal and a ventral portion (Fig. 66).* In this way are formed the two nl and the two ventral canals of the proboscis coelom. In Fig. 06 the t\vo ventral canals are joined, see below. PROBOSCIS COELOM. 81 The two dorsal canals are lined by coelomic epithelium and extend back into the proboscis stalk (eh), where the left of them, and sometimes the right also, opens to the exterior on the dorsal side close to the junction of the stalk with the body, by the proboscis pore or pores. The ventral canals also extend back to the hind end of the proboscis stalk as far as the mouth, where they end blindly (racemose organ) or, at most, join one another owing to a deficiency in the ventral septum posteriorly (Fig. 66, eh v). The anterior end of the central complex projects into the proboscis coelom to a variable extent in the different species. anp. pup. vrv TIG. 64. Diagrammatic reconstruction of the anterior end of Spengelia porosa, x 10 (after Ptmm-tt). The dotted line in front of the notochord indicates the forward extension of the pericardia! auricles. The heart is omitted, anp anterior neuropore ; en posterior cornu of nuchal skeleton ; ds dorsal septum ; mdv dorso-ventral muscles (muscle-plate) ; ml longitudinal muscles ; n nuchal or proboscis skeleton ; p proboscis coelom ; -per peri- cardium : ///,') posterior neuropore : r dorsal root of medullary nerve cord ; vrv ventral recurrent vessel ; vs ventral septum. Ill some species the notochord is continued forwards in front of the other organs of the central complex for a considerable dis- tance as the vermiform process (p. 75). However far it may extend this anterior extension of the notochord in front of the pericardium (see below) is connected to the dorsal and ventral proboscis wall by a median septum consisting of dorso-ventral muscle fibres which traverse the proboscis cavity and are in- serted on to the notochord. These muscle fibres constitute the dorso-ventral muscle plate of Spengel (Fig. 64, mdv).* An- * The transverse sections figured pass behind this muscular septum, excepting possibly dorsally in Fig. 65 (see footnote, p. 83). /. Ill O 82 BALANOGLOSSIDA. 11 teriorly this septum ends with a free edge, the anterior part of the proboscis cavity being single, but posteriorly it overlaps, though it does not join, the dorsal and ventral septa above referred to. In this overlapping part the muscle fibres of the dorso-ventral muscle plate diverge on reaching the central complex and em- brace that structure. The ventral septum consists of a layer of basement membrane which is continuous with the basement membrane of the body wall and with that surrounding the notochord (Fig. 64, vs ; Fig. 65. 6 ; Fig. 67. .r). It ends with a free edge in front, rarely extending be- yond the front end of the central complex, some- times not so far. Posteriorly it either extends to the very hind end of the ventral canals, c o m - pletely separat- ing them, or it stops short of this, ending with a free edge, so that the ventral canals join pos- teriorly (Fig. 66). The basement membrane of the ventral sep- tum at the point where it joins the basement membrane of the ventral proboscis wall is pierced by the fibres of the circular muscular layer. The dorsal septum, which separates the dorsal canals, is formed by the walls of the pericardium. 1 lie basement membrane of which is continuous with the basement membrane of the body wall in the dorsal middle line (Fig. 65). It is usually continued forwards as an effective septum by the fibres of the dorso-ventral muscle plate (Fig. 64), which may overlap the anterior portion Kic. ita. Diagram of a transverse section through the proboscis "i ;i Itulunoglossid (from L;mg after Spengel). 1 dorsal septum ; 3 ectoderm; '', i lood vessels of the integument ; 4 circular, .; longitudinal muscular layer; 6 vcnti.-il -I'ptuin : 7 noto- chord ; 8 coelom ; 9 heart ; 10 alomerulus ; 11 pericardium. PROBOSCIS COELOM. 83 of the pericardium,* in the region anterior to the dilated part of the notochord, where the dorsal canals acquire their coelomic epithelial lining. f The extension of the ventral septum and of the median muscle septum of the proboscis exhibits considerable diversity in the group. It is now necessary to describe more in detail the posterior more definite portions of the proboscis coelom, where an epithelial lining is pre- sent. First of all the ventral canals ; they nearly always join posteriorly t o f o r m the ventral ccectun which extends back to the root of the proboscis (Fig. 66, ehv), and the hind end of which in --eh Sk 3 ehv FIG. 66. Transverse section through the neck of the proboscis of Balanoglossus gigas behind the ventral septum (after Spengel). eh dorsal canal of coelom, leading on the left side to the pore ; h tissue in the pericardium ; div notochord (buccal diverticuluni) ; sk pro- boscis skeleton ; ehv hind end of ventral canal (ventral caecum). The efferent vessels of the proboscis are shown on each side dorso- laterallv of the notochord. some species forms a small projection (the racemose or- gan) from the roof of the buc- cal cavity. The dorsal canals are more complicated. They pass backwards in the neck of the proboscis as narrow canals lined with a ciliated epithe- lium (Fig. 66 eh), and they either end blindly or open, at the hind end of the neck, into terminal bladder-like dilatations. These two end-vesicles open to the exterior by the proboscis pores. As a rule there is only one proboscis pore and end- vesicle, viz. on the left side, but when there are two, those of the * It appears to have done this in the section from which Fig. 65 was taken. f It will be remembered that anteriorly the proboscis coelom appears to be without epithelium in the adult (see p. 80). 84 BALANOGLOSSIDA. left side are nearly always larger than those of the right. The lining of the hind end of the dorsal canals, whether they end blindly or not. appears to give off nests of cells into the chondroid skeletal tissue (see below), which is found in the neck of the proboscis. There is the greatest variation in the proboscis pores and in the dorsal IM mils ;ui) The right vesicle is larger than the left, and neither of them are in communication with their dorsal canals. Finally, in Bal. carnosus, in which only the left pore and vesicle are present, the end vesicle is continued behind the an- terior neuropore as two csecal pockets placed ventral to the neural tube, and in some specimens the pore opens into the front end of the medullary t nl ie (for the behaviour of the first branchial pore in this species, see p. 77). \Vhen there are two pores the right is said to arise later in development than the left. The lining of the end vesicles is said to be due to an ingrowth of ectoderm : if this is true the actual coelomic pore would be th" aperture connecting; the vesiele \\ith the dorsal eoelomic canal. The central complex of the proboscis (Figs, 60, 64, 65, 67). Before proceeding to our account of the coelom in the collar, it will be convenient to describe this remarkable structure. It occupies almost the whole of the neck of the proboscis and in the dorsal middle line is in contact with the basement membrane of the ectoderm (Fig. <>7). On the ventral side of it is the ventral extension of the proboscis coelom. In some species (Ptychodera flava and cn/!//rt, etc.) the ventral canal of the coelom extends back as far as the union of the notochord with the buccal epithelium, but in most it stops short of this and the posterior pirt of the central complex is in contact in the ventral middle CENTRAL COMPLEX. 85 lm line with the basement membrane of the ventral ectoderm of the proboscis stalk. The central complex consists of the following parts : (1) ventrally the notochord and, in the stalk, the skeletal develop- ments of the basement membrane of the notochord (Fig. 64, n ; Fig. 66, sk ; Fig. 60, 19), (2) the heart (Fig. 67, b, see also Figs. 60, 65), which is a blood vessel lying on the dorsal side of the notochord, between it and (3) the pericardium (Fig. 64, per, Fig. 67, A. see also Figs. 60, 65), a closed sac of con- siderable size and of doubtful meaning, and (4) the glomerulus or proboscis gland which is the well - developed and folded coelomic epithelium lying on the front wall and anterior part of the sides of the pericar- dium (Fig. 65, 10, Fig. 60, 2). The notochord has already been described. [t extends along the whole length of the cen- tral complex and varies very considerably throughout the group. The basement membrane which lies immediately outside its epithelium forms its sheath and is especially thickened on the ventral side of the posterior part of it (Fig. 64, n, Fig. 66, sk). This specially thickened part of the notochordal sheath is continuous with a specially thickened part of the basement membrane of the dorso-lateral buccal wall (Fig. 64 en), and the whole constitutes the proboscis or nuchal skeleton (Fig. 68). The proboscis skeleton consists of a median body and of two limbs diverging backwards. The body is placed ventral to the posterior part of the notochord in the neck of the epi 7' sv FIG. 67. Transverse section through the hind end of the proboscis, near the.neck, of Glossobalanus minutus (after Spengel). The section goes behind the glomerulus. The epidermis is dotted and the nervous layer of the epidermis is diagonally shaded, the basement mem- brane of the epidermis "is unshaded, adv blood vessels of skin ; 6 heart ; div notochord ; eh proboscis coelom divided here into a right and left half by the anterior end of ventral septum sv ; epv coelomic epithelium ; h pericardium ; lm longitudinal muscles of proboscis wall ; rev efferent vessels of proboscis ; rm circular muscles ; sv ventral septum. 86 BALANOGLOSSIDA. i proboscis. It extends from the hinder side of the notochordal caucum in front to the junction of the notochord with the mouth behind (Fig. 64, n). At this point it divides into the two limbs which diverge backwards into the collar region lying immediately within the buccal epithelium. The limbs may be long, extending al- most through the whole length of the collar, or they may be short reaching but a little way. The proboscis skeleton is strengthened laterally in the hinder part of the neck of the proboscis by the chon- droid tissue. This tissue is the enlarged basement membrane be- longing to the anterior wall of the collar coelom and the posterior part of the dorsal canals of the FIG. 68. Proboscis skeleton of oios- proboscis coelorn. It differs from sobalanus sarniensis (after Spengel). the rest of the skeletal tissue of the body in containing nests of cells which have migrated into it from the adjacent coelomic epithelial walls (p. 84). The chondroid tissue, which has a certain resemblance to hyaline cartilage, is specially developed in Schizocardium and ''/(indiceps. The heart is a blood vessel lying in the basement membrane between the notochord and the pericardium (Figs. 65, 67). Its connexions will be described later on when we are dealing with the vascular system. The pericardium or proboscis sac is a closed vesicle lying over the anterior end of the notochord and extending only for a short distance backwards into the stalk (Figs. 64, 65, 67). This posterior part of it is traversed by transversely directed fibres ; its anterior part is filled up with a peculiar loose tissue of unknown function while its main body contains a clear fluid. It possesses an epithelial lining which in some cases may proliferate so as almost entirely to fill up its cavity. It has no connexion with the vascular system, and the latest observers have asserted that it is quite closed off from the coelom, but Bateson maintained that its cavity com- municates with the coelom through the loose tissue filling up its anterior part. This loose tissue sometimes occupies a large GLOMERULUS. COLLAR COELOM. 87 portion of its cavity (e.g. Glossobalanus ruficollis, etc.). Its ventral wall is concave, slightly enveloping the dorsal sides of the notochord, and contains a number of transverse muscular fibres which being in contact with the heart tube very likely cause the pulsations by means of which the blood fluid is moved. In Schizocardium (hence the name) and in Glandiceps the anterior lateral portions of the pericardium are produced into two tubes which extend along the vermiform process of the notochord and constitute the auricles of the pericardium. In Bal. carnosus the pericardium is bifid anteriorly, being produced into two pouches, accompanied by glomerulus tissue, beyond the anterior limit of the notochord. As already stated the dorsal wall of the posterior part of the pericardium is in contact with the basement membrane of the ectoderm, thus giving rise to the dorsal septum ; but its anterior wall and the anterior part of its dorsal wall, and its side walls are covered by coelomic epithelium which is much folded and composed of large cells containing pigment grains. This is the glomerulus. Within the folds, i.e. between their epithelium and the pericardial wall are some blood vessels which are in direct communication with the heart. If the glomerulus is an excre- tory organ, as is supposed by some, its secretion must pass into the proboscis coelom, reaching the exterior by the proboscis pore. The collar coelom is in many species almost entirely filled up by the muscular and connective-tissue development of its walls, and, except in the parts of it known as the collar canals, is devoid of a lining epithelium. The two lateral sacs of which it originally consists meet dorsally above the collar nerve-cord and ventrally below the gut (Fig. 69). The longitudinal mesenteries formed by the opposed walls of these sacs persist only in part : the ventral mesentery is present in the posterior region for a short distance, but is deficient over the greater part of the collar ; the dorsal mesentery which extends between the nerve cord and the skin persists over the greater part of its length, but is deficient anteriorly. In Harrimania kupfferi both mesenteries are absent in the adult, and it is quite possible that there may be consider- able variation in them in other species. In some genera (Ptychodera, Schizocardium, Spengelia and Dolichoglossus), the coelomic sacs of the collar are in parts separated from the gut wall by the peripharyngeal cavities (Fig. 69) 88 BALANOGLOSSIDA. \vhich are forward extensions of the trunk coelom and will be described later. The collar canals are two short tubes lined by a ciliated epithe- lium, and placing the collar-coelom in communication with the .-xterior. They occur dorso-laterally at the hind end ot the collar, and they open externally by the collar pores into the first branchial pouch. Internally they open by a funnel-shaped aperture into the collar coelom, which, in the neighbourhood of the funnel, is free ..-- 1 ---8 from the tissue ele- ments which pervade it elsewhere. The col- lar canals are covered towards the coelom by a layer of flat epithelium, and their dorsal walls are often infolded so as to pro- ject into the cavity of the canal.* The visceral wall of the collar coelom pre- sents along a certain line on each side a fold containing blood vessels and called the vascular fold. In the Ptychoder ida? the vascular fold begins at the hind end of the collar region and extends along the ventral surface of the alimentary canal in the middle line to a point not far removed from the anterior end of the collar. Here it divides into two folds which pass transversely one on each side to the dorsal surface. In the other genera the fold is double throughout its course. It begins at the hind end of the collar ventrally and ascends obliquely dorsalwards and forwards on each side to the point where the neck of the proboscis joins the body. The vascular folds contain blood vessels which unite the median FIG. (19. Diagram of a transverse section through collar region of a Balanoglossid to show the relations of the body-cavities in the collar (from MacBride). The coelo- mic Spaces are represented as clear of tissue and the ventral mesentery as persistent. 1 medullary ner\e cord ; 2 dorsal blood vessel ; 3 perihaemal cavity ; 4 notochord at its point of attachment to the huccal wall ; 5 posterior diverging crura of the proboscis skeleton; 6 collar coduin ; r Imccal cavity; g peripharyngeal cavity; 9 ventral vessel. This fold ivi-.ill-. .1 similiir -inn-lure projecting into the sand canal of TRUNK COELOM. 89 ventral vessel of the trunk with the efferent vessels of the heart. They are obviously to be compared with the glomerulus of the proboscis region. The trunk coelom extends throughout the whole of the body behind the collar. It is a more continuous cavity than that of the collar and is lined throughout by an epithelium and does not open * to the exterior. The dorsal and ventral mesenteries are persistent over the greater part of their course. The coelom of the trunk, though completely separated from that of the collar, sends forwards diverticula into the collar region. There are two pairs of these extensions. Dorsally on each side of the dorsal blood vessel of the collar, between the collar nerve-cord and the gut, there is a tubular extension of the trunk coelom which extends throughout the whole collar region as far as the insertion of the neck of the proboscis ; these are the parihaemal cavities (Fig. 69, j). They are largely occupied by longitudinal muscular tissue which is almost entirely developed from their dorsal walls. The other pair of collar extensions of the trunk coelom constitute the peripharyngeal cavities (Fig. 69, 8). These are not present in all genera (see above, p. 87). They lie between the ventral wall of the collar coelom and the gut. In the Ptychoderidse they entirely surround the gut except in the dorsal and middle ventral lines as far forwards as the transversely directed vascular folds described on p. 88. In the other genera in which they are present, they also extend as far forwards as the vascular folds, but these being obliquely directed, they occupy a triangular area on each side of the pharynx. In the Ptychoderidce the anterior part of the trunk coelom is divided on each side into a dorsal and ventral portion by a septum called the lateral septum or accessory mesentery (Fig. 70). The lateral septum passes in the hinder part of its extent from the body-wall, where it is inserted along the submedian line, to the gut wall ; while anteriorly it shifts its gut- wall attachment to the body wall near the dorsal mesentery (Fig. 70, B). Moreover, the dorsal section of the coelom gradually dwindles anteriorly and ceases altogether in the branchial region. Posteriorly the lateral septum ends freely and the dorsal cavity opens into the posterior part of the coelom. * Willey has described structures in Spengelia, in relation with the first gill-pouch and the perihaemal cavities, which he regards as vestiges of a pair of coelomic pores for the trunk coelom. 90 BALANOGLOSSIDA. FIG. 70. Diagram- <>f transverse s.-rti<>ns through the trunk of Ptychoderu in show the insertions dy wall of the collar are developments of the parie- tal wall uf the collar coelom. They consist of an external layer of longi- tudinal fibres and in the anterior region of an inner circular layer as well. In the trunk the body wall muscles diminish in strength posteriorly. They consist of a layer of longitudinal fibres only, except in the Ptychoderidce in which there is an outer layer of circular fibres as well. The layer of longitudinal fibres is interrupted in the dorsal and ventral middle lines and in the submedian lines (where the branchial pores and gonads open). The visceral muscles of the collar are rather complicated, part of them being derived from the extensions of the trunk coelomic sacs. They are partly longitudinal and partly transverse. Briefly, it may be said that the visceral wall of the collar coelom where it is in contact with the outer wall of the peripharyngeal extensions consists of longitudinal muscular fibres, whereas, where it is in contact with the gut wall, it gives rise to trans- versely directed fibres. For instance, in Ptychodera all the buccal wall behind the vascular fold possesses longitudinal fibres, whereas in front of the vascular fold, transverse fibres only are found. In the other genera the longitudinal fibres are found over the triangular area referred to above (p. 89), while only the transverse fibres are found on the part in front of the vascular folds. The inner wall of the peripharyngeal cavities con- sisN dt' circular fibres which, together with the partial layer of circular fibres belonging to the visceral wall of the collar coelom, complete the trans- verse layer of fibres round the gut in the collar region. In addition to the above muscles, radial fibres are found passing across the collar coelom from the visceral to the parietal wall. In the trunk 1'egioii the coelomic space i> traversed by radial muscular band* and the visceral muscles. The dorsal walls of the periliaemal cavities give rise to longitudinal muscles which almost entirely iill them. Muscles art? also in some genera i le\ eloped in the ventral walls oi these cavities. There are also fibres. \\hidi traverse the periliaemal cavitio. CONNECTIVE AND SKELETAL TISSUES. 91 The muscular fibres seem to be unstriated, but Bateson de- tected signs of a faint transverse striation in Dolichoglossus koivalevskii. In addition to the muscles above described, muscular fibres are present in the walls of the branchial pouches and in the anterior wall of the collar coelom. The connective and skeletal tissues and basement membrane. The Balanoglossida present the remarkable feature of having no connective tissues in the ordinary sense of the word, excepting the delicate reticulate tissue found in the coelomic cavities and extending amongst the muscles. This connective tissue is, as already mentioned, a product of the walls of the embryonic coelomic sacs and appears to be of the nature of mesenchyme. On the other hand all the epithelia of the body possess a kind of internal cuticle, that is to say they secrete on their inner- surfaces a continuous structureless membrane devoid of nuclei and fibrous structures. This structureless layer is called the basement membrane. It is formed not only on the internal side of the ectoderm cells but also on the inner side of the endo- derm and of the walls of the coelomic sacs, even in cases in which these walls have become entirely converted into connective tissue and muscles. It follows from this that not only must there be a layer of basement membrane interposed between all the organs of the body, but also that this interposed membrane must, theoretically at any rate, be in all cases a double membrane, one of its laminae being derived from one and the other from the other of the two organs which are in contact. As a matter of fact this doubleness of the basement membrane is not as a rule discernible, the two laminae of which it is theoretically composed having completely fused. But a separation persists in places and the cavities thus formed constitute the blood vessels (see below). The skeletal tissues are entirely derived from this membrane. The proboscis skeleton has already been described. Here it is only necessary to call attention to the fact that one part of this skeleton the chondroid tissue differs from the rest of the basement membrane of the body in containing strings and nests of nuclei which have migrated into it from the adjacent coelomic walls. The branchial or pharyngeal skeleton consists of a special 92 BALANOGLOSSIDA. thickening of the basement membrane of the walls of the pharynx on each side the gill-slits. It is made up on each side of a series of structures placed vertically in the walls of the pharynx and presenting a resemblance to a three-pronged fork (Fig. 62). The three prongs of each piece are joined together dorsally and end freely ventrally. The central prong is single (though presenting the appearance when closely examined of being composed of two fused rods) and forks ventrally. It is contained in the septum separating two gill-slits. The anterior and posterior prongs lie in the adjacent tongue bars, which also contain a corresponding prong of an adjacent skeletal piece. The anterior and posterior prongs do not fork ventrally. It would thus appear that each tongue bar contains two skeletal rods belonging to adjacent skeletal pieces. The whole skeleton lies close to the wall of the pharynx. Coelom is present in the tongue bars and its epithelium contributes with that of the adjacent endoderm to the formation of this skeletal basement membrane (Fig. 61). In the primary or septal bars there is no coelom in the adult, so that the skeletal rods in them are exclusively formed by the pharyngeal epithelium. The vascular system consists, as already explained, of channels hollowed out in the basement membranes of the body. It might be said to represent a persistent portion of the spare which theoretically occurs between the two laminse of which all or nearly all the basement membranes consist. But as the basement membranes do not show a composition of two lamella 1 it will be perhaps safer at this point to take the first statement as representing all that we actually k>itr on this subject. The blood is usually described as colourless, but it appears to be coloured red in some forms at least, if we may judge from the fact that a red line can be seen through the body wall along the lines of the chief blood vessels ; and it contains a few floating amoeboid cells of the ordinary type. An epithelial lining is present in the blood vessels of a few forms (Ptychodcra, etc.), but frequently nothing of the kind can be detected. Some of the larger vessels are provided with muscles (usually circular), which arc furnished by the walls of adjacent portions of the coelom. The principal blood vessels are as follows : (1) A longitudinal dorsal vessel running through the body and collar, and passing into the proboscis to become VASCULAR SYSTEM. GONADS. continuous with the heart. The proboscis portion of this vessel is called the afferent vessel of the proboscis. It runs in the dorsal mesentery, or in what remains of it, in the trunk, while in the collar it is placed between the two perihaemal cavities (Fig. 69). In the proboscis it runs between the notochord and the hind end of the pericardium where it is enlarged to form the heart (Figs. 60, 65). (2) A ventral longitudinal vessel extending through the whole trunk and into the hinder part of the collar (Fig. 69). It runs in the ventral mesentery or in what remains of it. In the collar the ventral vessel divides and becomes con- tinuous with the two plexuses of vessels which are contained in the vascular folds already described. These pass in front into the efferent vessels of the proboscis (Fig. 67 rev) which run, one on each side, through the chondroid tissue in the neck of the proboscis to the hind end of the glomerulus, with the blood spaces of which they are continuous. There is in all the basement membranes a capillary network by means of which these main vessels are connected. This network is principally developed in the body wall, in the gut wall, particularly in the hepatic region, and in the gonads. That part of this network which lies in the pharynx wall and is presumably of importance in respiration is of course directly connected with the dorsal vessel. The dorsal vessel is supposed to be contractile and its contractions prob- ably travel forwards, so that the blood passes from it to the heart. From the heart it passes, driven by the contraction of the transverse muscle in the ventral wall of the pericardium, to the spaces within the folds of the glomerulus. Thence it passes into the efferent vessels of the proboscis which are continued from the hind end of the glomerulus through the chondroid tissue into the vessels of the vascular folds and so into the ventral trunk vessel. There are other main vessels in addition to the two mentioned : e.g., in Ptychodera there is on each side in the lateral septa a vessel which arises in the skin in front and enters the intestinal plexus behind ; it supplies the i:i>ii,ids. There are also definite main vessels both in the tongue and prim- ary bars of the pharynx, supplying the capillary network. The reproductive organs. The sexes are separate and the reproductive organs are similar in form and arrangement in the two sexes. They are simple or branched sacs which project into the coelom and are placed in the lateral part of the trunk in the 94 BALANOGLOSSIDA. branchiogenital region (p. 69). In the simplest cases they form on each side one series, the series of primary gonads, which open to the exterior on the dorsal surface along a line which may be called the gonaducal or sub-median line. The submedian line lies along the insertion of the lateral septum, where the longi- tudinal muscular layer is broken, and in the branchial region frequently coincides with the branchial groove, but in Ptycho- deridae it is placed at a greater or less interval from the branchial groove and the branchial pores perforate the dorsal longitudinal muscles. In the branchial region the primary genital sacs open externally to the branchial pores (Fig. 61), with which they correspond roughly in number. The wall of the genital sacs consists internally of a layer of germinal epithelium which is continuous with the ectoderm through the external opening. Outside this is a layer of base- ment membrane containing an abundant capillary plexus or even a continuous sinus. Then come some muscular fibres and a layer of coslomic epithelium towards the body cavity. It would appear that new gonadial sacs are continually being formed at the hind end of the genital region. In Ptychodera asymmetrica gonads are present only on the left side * (Punnett). The arrangement just described, in which the so-called primary series of gonads alone is present, is the simplest found and is possibly charac- teristic of all in the young state. It usually becomes more complicated with growth in the following way. The genital sacs become lobed and the lobes acquire independent openings to the exterior (Schizocardium, brasil- iense, Glandiceps talaboti). The secondary pores, as these are called, are not placed in the line of the primary pores, but are internal or external to them, except in the branchial region, where there are no secondary pores internal to the primary pores. The secondary pores usually open on the submedian line, but some of them may perforate the dorsal longitudinal muscles. A further complication is reached by the complete separation off of some of the lobes, to form a number of accessory glands in addition to the original gland which is called the primary gland. These accessory glands are not pl;irrd in the series of primary glands but in longitudinal rows there may bo several either external or internal to the primary row. They acquire their own external openings, which, inasmuch as they belong to the accessory grate the longitudinal muscles. * Agreeing in this rcsjr-ct with some species of Amphioxus (see Gold- -'hmidt on Amphioxid.es, Wiss. Ergebnisse der " Valdivia " Expedition, Bd. \->, Lf. 1, 1905, and Zo,,l. !/,;, iger, 30, 1906, p. 443). GONADS. OVA. TORN ARIA. 95 In Stereobalanus canadensis there are several rmvs of gonads, both inside and outside the branchial pores. Their openings are all in the submedian line which is much widened out. In Ptychodera there are usually several rows of accessory gonads which are for the most part external to the primary gonads. The extension of the genital organs varies in the different species. In some species (e.g. Ptychodera flava) they are found throughout the branchio- genital region, beginning just behind the collar ; in other species (e.g. Glandiceps abyssicola) they are confined to that part of the branchio-genital region which lies behind the branchial pores ; while in Stereobalanus cana- densis they are coextensive with the branchial region. In the majority of species, however, they begin in the branchial region at a variable distance behind the collar. In almost all cases the branchio-genital region over- laps the hepatic region, so that genital sacs are found at the anterior part of the latter. In Bal. carnosns, however, the transition is abrupt and no genital sacs are found in the hepatic region. In most of the Ptychoderidse the side walls of the body in the branchiogenital region are prolonged into winglike folds the genital pleurae. The genital sacs are for the most part con- tained in these wing-like outgrowths, but not entirely, for in some species they are found in the body as well, mediae! of the insertion of the lateral septum. The genital pleurse arc extremely mobile structures and may be bent up so as to cover the back of the animal. In Bal. carnosus their free opposed edges may become united by mucus, so that a cavity, widely open behind and receiving the branchial pores, is formed on the dorsal side of the branchiogenital region. The gametes are produced from the walls of the gonads and when ripe are passed out to the exterior through the genital pores. The spermatozoa have spherical or oval heads and active flagelliform tails. The ova are provided with a close- fitting egg-membrane, and are of two sizes according to the amount of yolk and manner of development. In most genera the egg is small, varying from '06 mm. in diame- ter in Ptychodera flava to 15 mm. in Bal. carnosus. The develop- ment of such eggs is probably indirect, passing through a free larval stage, called the Tornaria (Fig. 74). In some cases, however, the eggs are much larger, varying in their longest diameter from - 4 mm. in Dolichoglossus kowalevskii to 1*5 in Harrimania kupfferi. The early stages of the small eggs which probably develop into tornaria are not known, but thanks to the researches of Bateson all the stages of the development of Dolichoglossus 96 BALANOGLOSSIDA. kowalevskii are known. The eggs of this species are laid singly and deposited in the muddy sand which the animal inhabits. Segmentation is regular and complete and leads to the forma- tion of a spherical blastosphere which becomes a gastrula by i imagination (Fig. 71, -4). The blastopore narrows and a circle of cilia is formed round it ; it eventually closes up at that pole of the egg which will become, as shown by the persistent ciliary circle, the future hind end ; i.e. it closes in the posi- tion of the future anus. The embryo becomes covered with fine cilia and a tuft of longer cilia is formed at the anterior end. The embryo elongates and 1 B - e Ar ~ two transversely directed grooves appear, marking out the three regions ; proboscis, collar, and trunk (Fig. 71, B). The embryo hatches at about this stage and lives as a free larva which at first swims in the mud by means of its cilia, and later burrows in the mud by means of its proboscis (Fig. 71, C). A groove appears in the collar re- gion in the dorsal middle line marking the position of the medullary nerve- cord which is now being developed. This groove is a temporary structure and does not appear to participate in the formation of the collar nerve-cord which arises by delamination from the ectoderm. The first gill-slits now appear just behind the collar as a pair of perforations, and the mouth is formed as a minute perforation in the groove between the collar and proboscis. The anus is formed rather later at the hind end. Meanwhile the five coelomic pouches have made their appear- ance ; one unpaired pouch in front and two pairs behind (Fig. 72). These become closed from the gut and persist as the coelomic sacs. All tin- mesodermal tissues are derived from the walls of these sacs. n;. 71. Free-swimming larva; of Dolichoglossus kii'i-ntevskii in different stages of development (after Bateson, from Korschelt and Holder), e proboscis; & branchial pore; kr collar. DEVELOPMENT. 97 The collar nerve cord is delaminated from the ectoderm beneath t-lio transient groove already mentioned. It remains however connected with the ectoderm throughout life at its front and hind ends. It appears to increase in length at the front and hind ends of the collar by invagination of the median ectoderm, by which pro- cess the continuous central canal pre- sent at these points is formed. In the subsequent changes the gill- slits increase in number from before backwards and the adult form is gradu- ally assumed (Fig. 71, C). A larval organ, constituting a suctorial tail for attachment, is developed and lasts for a short time ; it is placed at the hind end ventral to the anus. The noto- chord is formed as a dorso-median groove of the anterior part of the gut which becomes partially constricted off from before backwards to form a tube ; it later extends into the base of the proboscis. The pericardium is devel- oped from a solid mass of cells derived FIG. 72. Diagram of a longitudinal sec- from the wall of the posterior part of the proboscis coelom. The first trace of the generative organs is seen when there are 10 gill-slits. Their exact ori- gin was not made out in this species, but from observations of other species they are probably mesodermal, though by some observers they are believed to be derived from the ectoderm, with which in any case they soon become connected. So far they have not been traced into connexion with the coelom. The proboscis pore is first indicated by a thickening of ectoderm on the base of the ":,7 proboscis. This thickening acquires a cavity which later opens to the exterior and to the proboscis coelom. The collar pores arise by the per- foration of thickened patches of ectoderm in close connex- FIG. 73. Transverse section through the middle ion with the opening of the part of the collar of a larva of Dolichogtosstts kowa- levskii which is at about the stage of Fig. 71 B tion through a larva of Dolichoglossus kowalevskii (after Bateson. from K. and H.). ei anterior (proboscis), en middle (collar), em posterior (trunk) coelomic diverticula, d enteron. n. fir ,, _;il di't At tho nnst prior farst g ul - sllt - At tne P . (after Bateson, from K. and H.). Above is seen the end of the proboscis coelom coelom r n ; f the point where the coelom was connected with the gut in an earlier stage (Fig. 72), a proliferation of the coelomic wall occurs in the dorsal middle line ; this causes a projection into the cavity which Z III. H 98 BALANOGLOSSIDA. Fm. 74. Tornaria larva (after Metschnikoff), a from the side ; 6 from the dorsal surface ; A anus ; C pericardium ; P, P' coelomic sacs ; O mouth ; S apical plate ; W rudiment of the proboscis coeloni (so-called water-vascular sac). eventually reaches the ventral wall. The hinder part of the proboscis coelom thus becomes divided into two parts by a vertical septum. of the cells of this septum give rise to the pericardium, which wh< formed is solid, and later on the notochord grows into it. The tornaria larva. In most species which have eggs of the smaller size, the development is indirect and a transparent pela- gic larva known as the tornaria is developed. This larva was first described by J. Miiller, who took it for an Echinoderm larva. Its real nature was determined by Metschnikoff. The early stages of this development are not known, but the later stages by which the larva passes into the adult have been more or less worked out. In the youngest stage known, the tornaria has a somewhat ovoid form with a ventral mouth, a terminal anus, and an alimentary canal divided into three re- gions, viz., oesophagus, stomach and intestine (Figs. 74, 75). There are two ciliated bands, one of which is preoral and encircles the preoral lobe, and the other postoral but longitudinal in direc- tion. These two bands touch one another dorsally and anteriorly, the ectoderm at the point of junction being thickened to form the apical plate (S). The apical plate possesses nerve fibres and ganglion cells and soon acquires a tuft of immobile cilia. It also contains a pair of pigmented eye spots. Internally there is a spacious blastocoel, in the front part of which is a vesicle called the water-sac (Fig. 74). This is the FIG. 75. Early stage of a tornaria larva in longitudinal section (from Balfour after Goette). m mouth ; an anus ; tv rudiment of pro- boscis coelom (so-called water-vascular vesicle). TORNARIA. ECHINODERM AFFINITIES. 99 rudiment of the proboscis coelom. It lies on the hinder part of the oesophagus and is connected to the apical plate by a muscu- lar band, and opens on the dorsal surface by a short ciliated tube, the rudiment of the proboscis canal and pore of the adult. The pericardium (heart- vesicle), the origin of which is uncertain, lies at first near the skin to the right of the proboscis pore. It soon leaves the surface and becomes surrounded by the proboscis coelom. By some authors it is regarded as the right proboscis cavity. Later a transverse circumanal ciliated ring is formed, and sometimes a second less dis- tinct ring behind this. Moreover the course of the two first described ciliated bands fre- quently becomes complicated and folded, and in Tornaria grena- cheri they acquire ciliated lobe-like projections. In the later development the form of the adult is gradually at- tained ; the cili- ated rings disap- pear and the two posterior pairs of coelomic sacs are developed in a manner which has not been thoroughly ascertained. The collar nerve cord, gill-slits etc., are formed as they are in the direct de- velopment. The tornaria has by its ciliated bands and general form a strong external resemblance to the bipinnaria larva of an echino- derm, a resemblance which is increased by the presence of the so-called water- vesicle with its dorsal pore ; and when first dis- covered it was described as belonging to that group. A close examination of its structure, though revealing points of difference FIG. 76. Two later stages in the development of Tornaria larva, side view. A larva with one pair of gill apertures (after Metsch- nikoff ) ; B larva, with four pairs of gill apertures (after A. Agas- siz) ; Bo gill apertures ; C heart ; O mouth ; P coelomic sac ; W rudiment of proboscis-coelom. 100 BALANOGLOSSIDA. such as the presence of eye-spots and an apical plate, does not nullify this resemblance and renders it extremely probable that the Enteropneusta have affinities with the Echinodermata as well as with the Vertebrata a view which is expressed in this work by the juxtaposition of the chapters dealing with them. The Balanoglossida live in sand or mud* and are found in most seas. Their burrows, the walls of which appear to be cemented into some consistency by the mucus which the animal secretes, appear frequently to be U-shaped, opening on the surface at two points. At one of the openings the casts formed by their faeces are discharged, while the anterior end is in relation with the other opening. In Dolichoglossus kowalevskii, Bateson describes them as being highly coloured (collar a bright orange) and as living at about a depth of eight inches with the anterior end of the body (to the branchial region) and the hind end both vertical, the middle portion being coiled in a corkscrew spiral. They usually possess a disagreeable smell, described as re- sembling that of iodoform or sometimes that of chloride of lime with a faecal admixture. It frequently happens that two species live in association : thus Willey records that Glossobalanus ruficollis inhabits the same burrow as Bed. carnosus in New Britain, while the latter is taken with Spengelia porosa at Lifu. They actively burrow in the ;sand by means of their proboscis and collar, in which the chief muscular development occurs, drawing the hinder part of the body passively after them. The proboscis and collar can be readily rendered turgid or the reverse, and they clear out the sand and mud by swallowing them. Thus their locomotion and alimentation are effected by the same means. It is probable that the inflation of the proboscis and collar which appears to be a necessary condition of their use in locomotion is effected by the taking in of water through the proboscis and collar pores. The body very readily breaks into pieces particularly the hinder part, and some species appear to be able to practise autotomy. In presence of this fragility of body, we should expect the power of regeneration to be considerable, and this appears to be the case. Animals are frequently taken showing * See, however, note on p. 103. REGENERATION. PARASITES. 101 features which are only explicable on the view that injuries are being repaired. Thus individuals are found with a very small proboscis and collar, or with an imperfectly formed collar indicating that these parts are being regenerated. It is asserted that in the regeneration of the collar of Ptychodera flava the collar nerve cord is formed as an open groove which is gradually con- stricted off, and that in the regeneration of the proboscis of the same species the proboscis pores may be equal in size. The first fact has its- counterpart in some tornaria larvae in which the medullary cord arises by the constriction off of an open groove, but the latter has no counterpart so far as is known in the early developmental history, no tornaria being known with two water-pores. Various parasites (Gregarines, flagellate Protozoa, Trema- todes, Nematodes, etc.) are found in the Balanoglossida, the most remarkable perhaps being a parasitic Copepod Ive balano- glossi sometimes found in the genital pleurae. The Balanoglossida are remarkable for the complexity of their structure in contrast with the simplicity of their mode of life and the absence of organs of special sense. There are no known organ- isms whether animal or vegetable, at present living, which pre- sent such complexity of organization combined with such simplic- ity of habits. They possess all the three features of the Chordata (see vol. 2, ch. i.) : namely a notochord developed from the dorsal wall of the enteron ; a tubular central nervous system ; and paired branchial apertures leading outwards from the anterior part of the alimentary canal. In addition they possess complex vascular and muscular systems, and a coelom which, largely filled up by muscular and connective tissue in the adult, consists of three main divisions, viz. an unpaired chamber in the pro- boscis, two paired chambers in the collar, and a similar number of chambers in the trunk. All the mesodermal structures including the gonads develop from the walls of the coelomic sacs, but no relation has as yet been traced between the cavities of the gonads and those of the coelom. There is no relation between the repetition of any of the organs and that of the coelomic sacs. Indeed all the repeated structures of the adult, viz. the gill-slits and the gonads occur in the region of the posterior or trunk sacs. Although there is no regularity in this repetition it is interesting that it should occur here, for it is these posterior sacs which in AmpJiioxus 102 BALANOGLOSSIDA and possibly in the Vertebrata become segmented into the mesoblastic somites. Finally, we must not forget to call attention to the great variability presented by different members of the group. This variability not only occurs among different species, but is also shown by different individuals of the same species. Moreover, and this is the remarkable point, it affects features of structure which to judge by the standard of higher Chordata are of great importance, and are usually perfectly constant through large groups. In illustration of this we may call attention to the condition of the central canal and dorsal roots in the nerve cord of the collar (many species of Ptychoderidse), to the canals and pores which connect the proboscis coelom with the exterior (species of Ptychodera), and to the length of the branchial region as compared with the rest of the body (macro- and brachy- branchiate varieties of Ptychodera) : these features are variable in some cases within the limits of the same species. As examples of characters which vary in allied species and which we should otherwise judge to be important, we may refer to the presence or absence of liver diverticula, to the condition of the notochord, and to the presence or absence of external protective covering of the gill-slits, and finally to the presence or absence of genital pleurae. It is in consequence of this remarkable variability that in our treatment of the group we have entered into much greater detail than has been our custom in this work. Farn. 1. Ptychoderidae. Proboscis usually shorter than the collar: cornua of proboscis skeleton do not extend backwards beyond the middle of the collar. Dorsal unpaired roots unite the medullary cord of the collar with the epidermis. Efferent vessels of proboscis united in one transverse plane by a circular vessel with the ventral blood vessel of the collar. Peri- pharyngeal spaces contain circular muscles and completely surround the buccal cavity continuously up to the level of the mouth opening. Peri- haemal cavities without transverse muscles. Circular muscles outside the longitudinal are usually present in the body wall of the trunk. Hypo- pharyngeal streak as a well-marked groove on the ventral side of the pharynx (Fig. (il). Genital pleurae well developed or small. Lateral mesenteries present in the trunk coelom. External liver-saccules present (except in (llnssobalanus ruficollis, Willey). Ptychodera Esch., the gill- slits open directly to exterior and the genital pleurae have a ventral origin, generally with continuous axial canal in the medullary cord of the collar, Pt. fiava Esch., erythraea Speng., bahamensis Speng. Balanoglossus D. Chi., gill-slits open into pouches which discharge to exterior by dorsal i_nll pm-es, genital pleura* with dorsal origin, B. apertus Speng., clavigerus D. Chi., gifja* F. v. Mull., aurantiacus Girard, australiensis Hill, carnosus BALANOGLOSSIDA. 103 Willey, bimimensis Willey, jamaicensis Willey. Glossobalanus Speng., gill apertures as in Balanoglossus, genital pleurae reduced to ridges, Gl. minutus Kow., sarniensis Koehler, hedleyi Hill, ruftcollis Willey. Fam. 2. Glandicipitidae. Proboscis longer than collar. Notochord fre- quenthy produced anteriorly into a vermiform process. Cornua of proboscis skeleton extend to posterior region of collar. Nerve -roots absent or vestigial. Efferent vessels of proboscis pass obliquely downwards to posterior end of collar. Peripharyngeal spaces separate, vestigial or absent. Perihaemal cavities contain transverse muscles. Circular muscles of body-wall lie inside the longitudinal muscles. Genital pleurae and lateral septa of trunk coelom absent. External liver saccules present or absent. Schizocardium Speng., right and left peripharyngeal cavities and synap- ticula present, ventral septum of proboscis extends to end of vermiform process, external liver saccules present, medial gonads absent, pericardial auricles highly developed, hypopharyngeal streak of pharynx reduced to narrow band (Fig. 63) ; Sch. brasiliense and peruvianum Speng. Spen- gelia Willey, peripharyngeal cavities and synapticula as in Schizocardium, ventral septum of proboscis does not extend to the vermiform process, external liver saccules absent, medial gonads present or absent, pericardial auricles reduced (Fig. 64), dermal pits in the genital region, hypopharyngeal groove deep and well marked as in Ptychodera ; Sp. porosa and alba Willey. Glandiceps Speng., peripharyngeal cavities and synapticula absent, ventral septum of proboscis and external liver saccules and medial gonads as in Sp., pericardial auricles reduced, hypopharyngeal streak of pharynx re- duced to broad tract (Fig. 63) ; G. talaboti and hacksi Marion, G. abyssicola- Speng. Willey ia Punnett, branchial part of pharynx small compared with ventral portion, without dermal pits, synapticula, medial gonads. Fam. 3. Harrimaniidae. Boreal forms with large eggs and direct development, vermiform process of notochord and dorsal roots of medul- lary cord absent ; cornua of proboscis skeleton, efferent vessels of proboscis and perihaemal cavities as in Glandicipitidae ; perihaemal spaces present or absent ; no circular muscles in body-wall of trunk ; synapticula and external liver saccules absent. Harrimania Hitter, proboscis short, proboscis pores paired, peripharyngeal spaces absent, medial gonads present ; H. kupfferi v. Will.-Suhm ; H. maculosa Hitter. Dolicho- glossus j Speng., proboscis long, proboscis pore unpaired, peripharyngeal spaces present, medial gonads absent ; D. kowalevskii A. Ag., D. mereschkoivskii N. Wag., D. sidcatus * Speiigel, with a dorsally grooved proboscis and 10 or 11 pairs of gill-slits; D. ruber Tattersall, from west coast of Ireland. Stereobalanus Speng., proboscis short, two proboscis pores, two pairs of genital pleurae. Fam. 4. Protobalanidae.t The coelom preserves its primitive arrange- ment, is free from mesenchyme and its mesenteries persist. Lateral septa are absent from the trunk, and perihaemal and peripharyngeal cavities from the collar. Gonads in a single row. The other characters as in the Harrimaniidae. Protobalanus Caullery and Mesnil, P. koehleri, 4-6 cm. in length, St. Martin's Bay, Cap de la Hague, north coast of France. * Dolichoglosstis otagoensis Benham (Q.J.M.S., 42, 1899, p. 497), a form recently described from New Zealand, creeps on seaweed by means of a very contractile proboscis, which is grooved dorsally as in D. sulcatus ; it possesses only 12 pairs of gill-slits. t Caullery et Mesnil, Zool. Jahrb. Anat., 20, 1904, p. 227. 104 PHYLUM ENTEROPNETJSTA. Order 2. CEPHALODISCIDA. Enteropneusta in which the collar is prolonged into paired tenta- culiferous arms and the trunk is much shortened in its antero- posterior axis, with at most one pair of branchial apertures. The animals live in colonies and inhabit tubes which are formed by the proboscis. The Cephalodiscida comprise two genera, Cephalodiscus Mc- Intosh and Ehabdopleura Allman. They are colonial animals which possess the power of budding and inhabit tubes secreted by the ectoderm of the flattened proboscis. The two genera while agreeing in most points of structure differ in the presence or absence of branchial apertures. In Cephalodiscus there is one pair of lateral openings leading outwards from the pharynx ; in Rhahdopleura branchial apertures are absent. In the following account the two genera are dealt with separately. Cephalodiscus* resembles the Balanoglossids in the main plan of its organisation, but differs from them in the fact that the trunk region, though exceedingly shortened antero-posteriorly, is much elongated in the dorso-ventral direction, and in the restriction of certain trunk organs, which are repeated in the Balanoglossida, to a single pair, e.g. gill-slits and gonads. More- over, it has the power of reproducing itself by budding ; and several individuals live in association in a single tube system. Cephalodiscus was discovered by the Challenger in 1876, at a depth of 245 fathoms, in the Strait of Magellan. It has since been found in other localities (Japan, and from comparatively shallow water in the Malay Archipelago and in the Antarctic). It was at first thought to be a compound Ascidian. Then it was referred to the Polyzoa, and it was not until 1887 that its structure was satisfactorily elucidated by Harmer and its real nature as an ally of the Enteropneusta demonstrated. A considerable number of individuals, probably all produced by budding from a single original individual, live together in a system of ramifying, sometimes anastomosing tubes which ; \V. ('. .Mdiitosh, Cephalodiscus, ('Imll, ifjer Reports, vol. 20, 1887. S. F. Harmer, Appendix to the preceding. Id., On the Notochord of Cophalodiscus, Zoolog. Anzeiger. 1897. Id., The Pterobraiichia of the Siboga-Expodition, Sihfnja-E.>-/>crlitie, vol. 2(1 bis, 1905. A. T. Masterman, On the Structure of Cephalodiscus, Q.J.M.S., 40, p. 340, and Transactions of Royal Soc. of Edinburgh, 39, ISDN. Id., Q.J.M.S., 46, 1903, p. 715. CEPHALODISCTJS. 105 consist of a flexible, brownish semitransparent material. The tubes are composed of superposed lamellae and are probably secreted by the proboscis of the animal (see below). The organ- isms are not attached to each other or to the wall of the tube in anyway, but appear to have the power of moving freely about inside it. Scattered about here and there on the tubes are large rounded apertures, near which the individuals are often found, and through which they can protrude their tentacular tufts. The tubes are covered with tapering spinous processes of their walls. The cavity of the tube may be continuous or it may be divided up into cham- bers, one for each in- dividual or zooid. With regard to the size of a colony it may be mentioned that in Ceph- alodiscus dodecalophus the network of tubes covered an area of 9 inches by 6 inches. The stems have a dia- meter of from 4 to 10 mm., and the whole colony appears to have been attached to mar- ine objects such as stones, sponges, etc., by vertical stems which descend from the underside of the net- work to the substratum. The natural position of the colonies would thus appear to be horizontal. The full-grown individuals of C. dodecalophus measure about 2 mm. in their longest diameter. Cephalodiscus may be described as an animal in which the or- anal axis of the body is very short and the ventral surface behind the mouth is produced into a large hump in which the alimentary canal is continued (Figs. 77, 78). This ventral hump terminates at its ventral and anterior end in a pedicle on which buds are continually being formed. The mouth is ventral and anterior and is overhung by a large FIG. 77. Cephalodiscus dodecalophus, anterior view (after Mclntosh). 1 tentacles ; 2 proboscis (buccal shield) ; 3 pigment baud on proboscis ; 4 buds ; 5 pedicle ; 6 trunk. 106 PHYLUM ENTEROPNEUSTA. preoral lobe (Figs. 77, 2, 78, 18) called the Duccal disc or proboscis. Behind this there is a region of the body which, except in the young bud, is not very distinctly marked off either from the proboscis in front or from the trunk behind. This is the collar region. 16 17 8 F IG yg. Median longitudinal vertical section through Cephalodiscus dodecalophus (after Harmer, from Lang). The figure is diagrammatic, especially in the fact that certain struc- tures not in the middle line are shown. 2 nervous system ; 3 collar coelom ; 5 postoral lamella (operculum) ; A' trunk coelom ; 9 pharynx ; 10 oesophagus ; 11 stomach ; 12 in- testine 13 buccal cavity ; 14 pedicle ; 15 ovary ; IP, anus ; 17 oviduct ; IS proboscis (buccal shield) ; 19 proboscis coelom ; 20 one of the proboscis pores ; 21 notochordal diver- ticulum of the pharynx. The collar forms no projection round the base of the proboscis in the mid-dorsal region, but its ventral and lateral portions pro- ject forward, forming a kind of lip (Figs. 78, 79). This lip is called the operculum* : it forms the ventral and lateral edge of the * The operculum can be folded backwards, a peculiarity which com- bined with the inronspicuousness of the ventral part of the collar has led some authors to describe it as a projection of the posterior edge of the collar. CEPHALODISCUS. 107 mouth- opening which lies between it and the base of the proboscis stalk. The arms are prolongations of the dorso-lateral parts of the collar commencing at the point where this lip-like projection passes into the dorsal part of the collar. They are tentaculi- ferous and vary in number from four to six pairs (one pair in the males of C. sibogae). In C. dodecalophus they are slightly swollen at their ex- tremities. Each arm contains a pro- longation of the collar body- cavity (Fig. 79) and is grooved on its ventral surface, the grooves uniting in pairs and converging to the corners of the mouth, near which they terminate. The ectoderm of the swellings at the end of the arms of C. dodecalo- phus contains a number of ovoid globules of a clear substance which have been variously interpreted as organs akin to the rhabdites of Turbellaria, and as the refractive structures of a rudimentary visual organ. Similar refringent vesicles are found along the whole course of the arms of the males of C. sibogae. FIG. 79. Transverse section through C'ephalo- discus dodecalophus (after Harmer). 1 central nervous system of the collar region ; 2 oper- culum turned backwards ; 3 pharynx ; 4 oesophagus ; 5 stomach ; 6 intestine ; 7 trunk-coelom ; S gill-slit ; 9 collar-pore ; 10 collar-coelom ; 11 arm. Behind the collar region is the trunk, the ventral portion of which is prolonged into the huge hump before referred to. The single pair of gonads (Fig. 78, J5) are placed in the anterior part of the trunk, just behind the collar, and open dorso-laterally one on each side. The anus is at the hind end of the dorsal surface (Fig. 78, 16). The proboscis presents a broad disc-like surface towards the front (Fig. 77) and is marked by a crescentic band of pigment in its ventral portion (j). Its anterior wall is covered with a glandular ectoderm, which appears to play some part in the formation of the tube, as in Rhabdo pleura. The mouth is on the ventral side at the junction of the proboscis and collar (Fig. 78) 108 PHYLUM ENTEROPNEUSTA. and is usually entirely hidden by the ventral portion of the over- hanging proboscis. The alimentary canal (Fig. 78) consists of mouth-cavity, pharynx, oesophagus, stomach, and intestine which opens by the posterior and dorsal anus. The bulk of it is contained in the great ventral hump so characteristic of this animal. The anterior end of the pharynx gives off a forward diverti- culum which extends through the collar region into the proboscis : this is the notoehord (Fig. 78, 21). There are two gill- slits, one on each side leading outward from the pharynx and opening ventro-laterally on the anterior part of the trunk. The body- cavity is arranged as in other Enter opneusta, i.e. it consists of an unpaired chamber in the proboscis which opens to the exterior by a pair of pores at the junction of the proboscis and collar region of the body (Fig. 78, 20) ; of a pair of chambers in the collar region, which also open outwards by a pair of collar pores (Fig. 79, 9), and of another pair in the trunk which are in rela- tion with the greater part of the alimentary canal and with the ovaries, and which do not open to the exterior. The two halves of the body-cavity of the collar overlap dorsally the hinder part of the proboscis region (Fig. 78, j). The dorsal and ventral mesenteries appear to persist, completely or incompletely, in the collar and trunk regions. The sexes are separate and all the individuals of one colony are of the same sex (except in C. nig- rescens, p. 109). The ovaries are paired sacs placed dorsally in the anterior part of the trunk region. They open to the exterior by two short oviducts, the walls of which are richly pigmented (Fig. 78, ij}. The male colonies appear, in some species (e.g. sibogae), to be dimorphic, containing male individuals which are characterized by possessing two arms without tentacles, a long pedicle and an abortive alimentary canal, and neuter individuals which are like the females but without ovaries. The coelom of the proboscis and collar are to some extent obliterated by muscle and connective tissue, as is the prolongation of the trunk body- cavity in the pedicle. The organs appear to be separated by a basement membrane as in the Balanoglossida and there is no connective tissue of the usual kind except in. the coelomic sacs in the dorsal region of the collar. The central nervous system is contained in the ectoderm. It extends forwards on to the hinder part of the proboscis and the proboscis pores perforate its CEPHALODISCUS. 109 anterior portion (Fig. 78). It is moreover continued as a nervous tract along the dorsal side of each arm. The pedicle is a process from the ventral surface. It is said to be largely filled with longitudinal muscular fibres, but it appears to contain a prolongation of the posterior body-cavity. Buds are formed upon its terminal portion. From one to three buds are found upon almost all full-grown individuals. The endo- dermal tissues of the parent do not extend into the pedicle and play no part in the formation of the bud, the alimentary canal of the latter being entirely formed by an invagination of ecto- derm. Cephalodiscus has, in its proboscis and in close contact with the noto- chord, structures corresponding to the pericardial sac, glomerulus, and central blood-sinus (heart) of the Balanoglossida. The pericardial sac is at the anterior end of the notochord and contains the heart in its interior. Blood vessels have been made out in other parts of the body as spaces the walls of which are probably formed by the walls of the mesodermic cavities, or they may be spaces between the same structures and the ectoderm or.endoderm. Free eggs and embryos are found in the tubes of some species. The ova are of a fair size and contain a considerable quantity of yolk. The cleavage is complete and the embryos leave the colony at an early stage as ciliated planulas. The following species are known. C. dodecalophus M'Intosh, with 12 arms, with end bulbs and vesicles ; cavity of tube continuous ; Straits of Magellan, 448 metres. C. levinseni Harmer, with 12 arms, without end- bulbs or vesicles ; cavities of the tubes divided up into chambers, one for each zooid ; sea between Japan and Corea, 183 metres. C. gracilis Harmer, very small ; with 10 arms, apparently without end-bulbs and vesicles in adult ; E. coast of Borneo, reef. C. sibogae Harmer, male-colony only known, with dimorphic individuals (see p. 108) ; S.E. of Celebes, 75-94 metres. C. nigrescens Lankester,* colony large and massive, nearly transparent, with tubes projecting from its surface, each tube being cut off from the rest and containing one full-grown zooid ; the zooids are deeply pigmented and large (4' 5 mm. X 1 mm.) and have 6 to 8 pairs of arms which are without terminal swellings ; each colony contains male, female, and hermaphrodite individuals, the latter having one ovary and one testis ; Antarctic Ocean, in 100 fms. The genus Ehabdo pleura^ Allman must also, in view of Fowler's * Proc. Roy. Soc., 1905, 76 B, p. 400. t Allman, Q.J.M.S., 9, 1869, p. 57. Sars, Q.J.M.S., 14, 1874, p. 23. Lankester, Q.J.M.S., 24, 1884, p. 622. Fowler, Proc. Roy. Soc., 52, 1893, p. 132, also in Leuckart's Festschrift, Leipzig, 1892, p. 293, and Q.J.M.S., 48, 1904, p. 23. Schepotieff, A. Zur Organisation von Rhabdopleura, Bergens Museums Aarbog., 1904, No. 2, and Zool. Anzeiger, 28, 1905, p. 795. L10 PHYLUM ENTEROPNEUSTA. work, be placed in alliance with the Enteropneusta. It differs, however, from the other members of the phylum by the absence of pharyngeal apertures. It exists in the form of colonies, the zooids of which are con- nected with one another by living substance and are contained in trans- parent tubes of a chitin- like material. The colo- nies have the form of a branching axis which lies upon the substratum and gives off at irregular intervals the terminal branches. These, for the most part, after adhering for a short distance to the substratum, rise up and project freely into the water. At their free ends are found the openings of the tubes by which the zooids come into relation with the exte-rnal world. The zooids consist of a body and a stalk (Fig. 81) The body is small ("12 mm. in diameter) and possesses at its anterior end a preoral lobe the buccal disc or proboscis, a pair of tentaculiferous arms arising laterally at the level of the mouth from the collar region (see below), a mouth on the ventral side of the buccal disc (Fig. 82, 8) and an anus (j) placed at about the same level as the mouth on the dorsal side. The stalk is the narrow ventral end of the body which passes down to join the common axis of the colony in the creeping stolon. Its connexion with the body is somewhat an- Fiu. 80. Portion of a living colony of Rhabdopleura normani (Allman), Lofoiru Islands, x 16. a the terminal branches with the zooids in different states of protrusion : c the proboscis (cephalic disc) ; d the two tentacular arms ; / the stomach ; g the intestine; h the stalks of the zooids ; i the axial rod in the creeping stolon. EHABDOPLEURA. Ill teriorly placed. The tube is secreted by the buccal discs of the zooid. It consists of a series of rings, each of which is secreted separately by the buccal disc and added to its predecessor. It is therefore quite independent of and separate from the bodies and FIG. 81. Rhabdopleura nor- mani from the right side (after Lankester, from Lang). 7 buccal disc (pro- boscis) ; 2 arms with ten- tacles ; 3 region of the collar pore ; 4 anus ; 5 trunk ; 6 stalk or pedicle. stalks of the zooids, and the body of the animal can move freely within its tube. It can be retracted into the tube by the con- traction (into a spiral) of a muscle in the stalk, and it can crawl up the inside of its tube by help of its buccal disc and can protrude its arms from the terminal opening. 112 PHYLUM ENTEROPNEUSTA. The common axis consists of the stalks of the zooids in earlier stages of the growth of the colony. It lies within the tube and the older parts of it possess a cuticle on its ectoderm. This cuticle becomes hardened and of a dark colour. It fuses second- arily with one side of the wall of the tube and forms a con- spicuous object as a dark brown, thin rod easily visible to the naked eye in the creeping stolon of the colonies. Some of the terminal branches of the colony do not rise up, but the tubes with their contained axis continue along the substratum and end in open mouths. These are actively growing proliferous branches. The axis in them terminates in an imperfect zooid, the proboscis of which adds to the tube, while its stalk has lateral wart-like buds. Transverse septa are formed across the tube between the buds, and the buds increasing in size burst through the wall of the tube and grow outwards from the creeping axis as imperfect zooids. Most of the new zooids so formed rise up and develop into the perfect form ; a few no doubt adhere to the substratum and form new proliferous branches. It follows from what has been said that the creeping (stolonic) part of the tube is divided up into chambers by septa which do not however interrupt the con- tinuity of the living substance of the stalk, and from each of which one zooid-bearing branch arises. There are no branchial apertures, but there is on each side a groove, between the inser- tion of the proboscis and that of the arms, which leads through the mouth into the oesophagus, on the side-wall of which it can be traced. This groove is called the branchial groove. The anatomy of the animal may be understood at a glance by inspection of Fig. 82. The alimentary canal is bent on itself. The buccal cavity is provided with an anteriorly directed diverti- culum which is continuous with a rod-like structure, apparently half-cellular and half-gelatinoid (Fig. 82, 9). This, which is clearly comparable to the notochordal diverticulum of other Enteropneusta, projects into the base of the preoral disc (pro- boscis). The body cavity, which appears to have a cellular lining, is divided into five chambers arranged as in other Entero- pneusta ; viz. an unpaired chamber (10) in the preoral disc (pro- boscis), a pair of chambers separated from one another by dorsal and ventral median septa and placed in what may be called the collar region of the body (Fig. 82, 2), and finally a pair of chambers in the trunk in relation with the greater part of the alimentary RHABDOPLEURA. 113 canal (./). The collar-cavities are continued into the arms, which arise from the collar region, and the trunk cavities appear to be continued into the stalk. At any rate the stalk contains in addi- tion to its muscular tissue two cavities separated by a septum.* The proboscis-cavity possesses a pair of pores, and the collar- cavities communicate with the exterior, each by a collar canal. On the dorsal side of the collar region behind the collar coelom (Fig. 82), there is an ectodermal thickening which seems to corre- spond with the nerve plate of Cephalodiscus and the medullary nerve cord of the Balanoglossida. Mesoblastic skeletal tissue, ap- parently similar in character and relations to that of the Balano- glossida, occurs in the arms and tentacles, and in the axis of the stalk. Most of the specimens hitherto found have been devoid of generative organs, but in a few there was a testis in the form of an elongated sac lying parallel to the intestine on the right side, and forming a projection on the surface of the body ; it opened close behind the anus. A vascular system consisting of a dorsal and ventral vessel has been described, and there appears to be a pericardial vesicle and heart at the anterior end of the notochord as in other Enteropneusta. Rhabdo pleura^ is a marine animal and is found in compara- c Fowler thinks that the stalk contains a cord of endoderm which is continuous with that of the alimentary canal. If this is so, the endoderm may participate in the budding. t It has been suggested that the section of the Graptolites known as Monograptidae are nearly related to Rhabdopleura (see Alhnan, On the Morphology and Affinities of Graptolites, Ann. and Mag. Nat. Hist., 1872. and Schepotieff, Neues Jahrbuch /. Mineralogie, 2, 1905, pp. 79-98). Z III I FIG. 82. Rhabdopleura normani in longi- tudinal vertical section (after Fowler from Lang). 1 arm of one side, indi- cated by dotted lines ; 2 anterior paired (collar) coelom ; 3 anus ; 4 posterior paired (trunk) coelom ; 5, 6 alimentary canal ; 7 buccal cavity ; 8 mouth ; 9 anterior diverticulum of buccal cavity(notochord); 10 anterior impaired (proboscis) coelom ; 11 buccal shield. 114 PHYLUM ENTEROPNEUSTA. tively deep water (40 to 120 fms.). It is widely spread, having been recorded from the Norwegian Fiords, off the Lofoten and Shetland Islands, off Tristan d'Acunha, South Australia the Malay Archipelago, the Azores, Ireland and Brittany. The question of the relationship of Phoronis to the Enteropneusta was dealt with on p. 547 of vol. i. of this work. Since that passage was written several memoirs * dealing with the development and structure of the Actinotrocha larva have been published. The upshot of these is to justify the criticism there set forth. To take the points seriatim, the exist- ence of a " neuropore " and " subneural gland " has not been confirmed : on the contrary it has been denied by Ikeda and Goodrich. The relations of the stomach caecum of Actinotrocha have been shown to be not those of a notochord. No evidence is forthcoming to contradict Caldwell's original statement that the cavity in the preoral lobe of Actinotrocha is haemocoelic (so-called primary body cavity of the trochosphere), and not a coelomic sac. It seems fairly clear that nothing comparable to proboscis pores or to collar pores are present. The only new point that has come out in this connexion is that the preseptal body-cavity of the adult is present in the old larva as a horseshoe -shaped cavity which underlies the ring of tentacles and send extensions into them, but the development of this cavity has not been ascertained. Finally, it has been shown that Cald- well was right in his account of the larval excretory organ as a ciliated canal not opening into the body-cavity, but terminating internally in some peculiar cells which recall the so-called flame cells of Platyhelminthes and Annelid larvae, etc. (solenocytes of Goodrich, see vol. 2., pp. 27, 28). * Masterman, Q.J.M.S., 43, 1900, p. 375. Ikeda, Journ. Coll. Sci. Imp. Univ. Japan, 13, 1900-1, p. 507. Goodrich, Q.J.M.S., 47, 1903, ]>. 103. Schultz, Z.f. w. Z., 75, 1903, pp. 391, 473. M. de Selys Long- champs, Mem. Classe Sci. Acad. Belgique, i. 1904. CHAPTER III. PHYLUM ECHINODERMATA* With a radial, usually pentamerous arrangement. The body- uraU contains calcareous plates and generally bears spines. The coelom is divided into two well-marked portions the perivisceral cavity and the watervascular system, and the gonads are noi con- nected with it in the adult. The Echinodermata, like the Coelenterata, present a radial arrangement of their principal organs and were for that reason united by Cuvier with the polyps and medusae in the group Radiata. With the progress of anatomical knowledge, it soon became apparent that the organization of the Echinoderms differs totally from that of the Coelenterates and belongs to a much more complex grade of development. This fact, which was first recognized by R. Leuckart, led to the separation of the two groups and to the establishment of two independent phyla, the Coelenterata and Echinodermata. A trace of the old view * Fr. Tiedemann, Anatomic der Rohrenholothurie, des pommeranzfar- benen Seesternes u. des Steins eeig els, Heidelberg, 1820. De Blainville, Manuel tf Actinologie, Paris, 1834. L. Agassiz, Monogr. d' Echinoderm.es vivants et fossiles, Neuchatel, 1 838-42. L. Agassiz et E. Desor, " Catalogue raisonne des families, des genres et des especes de la classe des Echinodermes" Ann. Sci. nat. (3), 6, 7, a,nd 8, 1846-7. E. Forbes, A History of British Starfishes, London, 1841. R. Leuckart, Ueb. d. Morphologic u. Verwandt- jchaftverhaltnisse d. wirbellosenthiere, Braunschweig, 1848. J. Miiller, "Ueb. d. Bau d. Echinodermen," Abh. d. Berlin Akad., 1853. Id., " Sieben Abh. iib. die Larven u.d. Entw. d. Echinodermen," Abh. d. Berlin Akad.. 1846, 1848, 1849, 1850, 1851, 1852. A. Agassiz, " On the Embryology 01 Echinoderms." Memoirs of the American Academy, 1864. E. Metschnikoff, *' Studien ub. die Entwickelungsgesch. der Echinodermen u. Nemertinen," St. Petersburg, 1869. H. Ludwig, " Morphologische Studien an Echino- dermen," Z. /. w. Z., 1876-82. Id., " Echinodermen," in " Bronns Klassen u. Ordnungen," in progress. O. Hamann, " Beitrage z. Histologie d. Echinodermen," Jena. Zeitschrift., 1884-89. L. Cuenot, "Etudes morphologiques sur les Echinodermes," Arch, de Biologic, 11. 1891. ^ H. E. Durham, " On Wandering Cells in Echinoderms, etc.," Q.J.M.S., 33, 1892. P. H. Carpenter, several memoirs in the Q.J.M.S. and other Journals, 1875-1890. E. Ray Lankester, A Treatise on Zoology, Pt. 3, The Echino- derma, London, 1900. E. W. MacBride, Echinodermata. in vol. I of the Cambridge Natural History, 1906. 115 116 PHYLUM ECHINODERMATA. has until quite recently survived in the juxtaposition of the two phyla which used frequently to be found in works on Zoology and Comparative Anatomy. But of late years it has been recognized more and more clearly that this juxtaposition is not warranted by the facts and that the affinities of the Echino- dermata, in so far as any can be traced, are rather with the higher phyla of the Metazoa, than with the lower. Expression was given to this view in 1877 by Huxley in his Anatomy of Inrcrtcbrata and in 1880 by F. M. Balfour, who in his Comparative Eni'ift/ology placed the Echinodermata at the end of the volume dealing with the invertebrate groups, in the neighbourhood of the Enteropneusta and Chordata. This example was followed in 1890 by A. Lang in his textbook of Comparative Anatomy, and now we, in the light of the most recent work on the subject, have thought it right to take the same course. The Echinodermata are radiately symmetrical animals (see p. 117) in which the number of radii is nearly always five or some multiple of five. This symmetry is, however, characteristic of the adult only, for in the youngest state all members of the group are bilaterally symmetrical and in nearly all there is a free-swimming bilateral larva. The view usually taken and adopted by us when referring to the matter in the chapter on Mollusca in the first volume of this work (p. 317) is that the bilateral symmetry is the primitive symmetry possessed by some adult ancestor and that the radial arrangement is to be regarded as a distortion from the original condition. Now, however, after a more com- plete study of the group, we see reason to suspend our judgment on this matter, and though we should hesitate to adopt the view that the Echinodermata have been derived from asym- metrical or from radiately symmetrical forms, and that the larva has been especially produced and modified for a free- s\\ i mining life, we are of opinion that there is at least as much to be said for it as for the older and more usually adopted view that the ancestral form was a form in which bilateral symmetry had been completely evolved.* The radial symmetry is expressed not only in the external appearance, but also in the arrangement of most of the internal organs. It is, however, never completely carried out, and in ' For a discussion of this question, see the section on " Affinities,"' p. 160. RADIAL STRUCTURE. 117 some forms (certain Echinoids) there is a tendency to bilateral symmetry. Speaking generally the body may be described as spherical or discoidal in form, with the mouth in the centre of the lower * surface and the anus at or near the centre of the upper surface. An oral and aboral surface or pole may thus be distinguished. The anus varies more in position than does the mouth. The mouth is nearly always in the centre of the oral surface or at the oral pole ; in a few Holothurians, some Echinoids and in Actinometra among Crinoids it is slightly shifted from this position. The anus on the other hand is central only in Holothurians, and even in some of these it is slightly displaced ; in Asteroids and regular Echinoids it is very near the centre of the aboral surface, but always slightly excentric ; in irregular Echinoids it is at some distance from the central point and sometimes on the oral surface ; and in Crinoids it is always on the oral surface. In a few Asteroids (Astropectinidae, etc.) and all Ophiuroids the anus is absent in the adult. The radial structure is indicated externally by the rows of tube- feet (p. 129) which extend outwards from the mouth towards the aboral pole. The surface of the body is thus marked into radii along which the tube-feet are arranged, and into interradii the portions between the tube-feet rows. In Asteroids, Ophiu- roids, and Crinoids the radial portions of the disc are prolonged into processes, which constitute the arms ; in Holothurians and Echinoids the radii are not so prolonged and there are no arms. The rows of tube-feet never extend quite to the aboral pole ; it is therefore possible to distinguish that portion of the surface of the body from which tube-feet project, as ambulacra! or actinal, from the antambulacral or abambulacral or abactinal surface which is without tube-feet. In the brachiate Echiiioderms, that is in Asteroidea, Ophiuroidea and Crinoidea, these two regions or surfaces are about equal in extent, the whole of the oral surface being ambulacral, and the whole of the aboral surface antambulacral. In such cases it is customary to call the lower or ambulacral surface ventral, and the upper or antam- bulacral surface dorsal. But it is better not to use the terms dorsal or ventral in adult Echinoderm morphology without prefixing the word adult, because dorsal and ventral are used * In Crinoids and their allies the oral surface is turned upwards in the natural position of the animal. 118 PHYLUM ECHINODERMATA. in an entirely different sense in describing the larvae and in adult Holothuriaiis, and cannot be used satisfactorily in Echin- oids at all. In Holotlmrians and Echinoids the tube-feet extend almost to the aboral pole, and the antambulacral surface of the body is restricted to the very small region round the anus. Moreover in Holotlmrians, in which the body is cylindrical and lies with its whole length applied to the substratum, it is usual to call one surface of the cylinder, viz. that on which the single genital opening and madreporite are placed, the dorsal, and the other side the ventral. In many Holothurians the animal always- lies with the three radii (trivium) of the so-called ventral surface directed towards the substratum and uses the tube-feet of these radii for adhesion, the tube-feet of the two dorsal radii (bivium} being without suckers and probably used for respiration and sensation only. In pentameral Echinoderms the words bivium and trivium are frequently used to designate the two groups into which the five radii may be divided. These words are used however in a somewhat vague sense and it must not be supposed that the arms of the bivium and trivium of one class are necessarily the same as the bivial and trivial arms of another class. The water-pore or madreporite is generally single and always interradial in position. It is generally abactinal, but in Crinoids and Ophiuroids it is on the ambulacral surface. In Holothurians the madreporitic interradius occupies the middle of the so-called dorsal surface. In Ophiuroids as in Asteroids the water-pore is generally single. but in Crinoids it is always multiple, there being one (Rhizo- crunt*} or more than one in each interradius. The number of water-pores (madreporites *) is however subject to variation in all classes of Echinoderms except Echinoids, in which normally there is never more than one. In Echinoids, Holothuroids. Neocrinoids and Blastoids the number of radii is constantly live except in abnormal individuals. t In Asteroids. Ophiuroids, Crinoids and Cystids this number may romplete cxpla.nat inn df the terms water-pore and madreporite sen i>fl(>\v. ]>. 1:>7. It may be mentioned here that the numerous perfora- tions in the madreporitic plate of Echinoids and Asteroids represent one water-pore only. '<- See Bateson, Mutcn'rifn far thr Study of Variation, London, 1894. p. 43.J et seq. In Echinoids 4- and 6-rayed abnormalities are not uncommon, and in Holothurians Ludwijj; found half a dozen 6-rayed individuals in ].">(> specimens of Cin-mnaria plane/. NUMBER AND ENUMERATION OF RADII. 119 be departed from.* In most cases the number of radii is deter- mined early in development, but in a few forms (e.g. Labidiaster) the number increases with the growth of the animal, at any rate in the early stages of the adult (Perrier). In Cystids the number of radii varies more than in any class, the number two or three sometimes occurring. Enumeration of Radii. After some hesitation we have decided to adopt a definite enumeration of the radii for all classes of Echinoderms. Our enumeration, which is shown in Fig. 83, is based on the assumption that the position of the stone-canal and primary water-pore is the same in all classes. If that assumption is incorrect, which it may very well be, j" the homolo- gies which might be deduced from it fall to the ground. In any case we desire to warn the reader against attaching too much importance to the determination of homologies based solely on this assumption. We have adopted the enumeration because it conduces to clearness and enables us to impart a greater precision to our descriptions and not because we think that it is of any importance from the point of view of determining homologies. The interradius in which the hydrocoel closes in the larva presents remarkable variations, in relation to the position of the madreporite, as the diagram indicates, and it is probable that later researches will show an even greater variability in this respect, the number of species hitherto examined from this point of view being small. Our enumeration is identical with that of Loven excepting in its applica- tion to Crinoids (see p. 272). The first-formed water-pore of Criiioids and the anus are in the same interradius. As our enumeration is based on the position of the water-pore, we assign the anus of Crinoids to inter- radius II. Ill, and not to interradius I.V, as is done by Loven. One of the most important characteristics of the Echino- dermata is the presence of calcareous plates in the dermis. These may have various forms, from the isolated plates and * In Qphiuroids and Crinoids, though the number of radii is usually five, the arms in some cases branch. f It is quite likely, as suggested by MacBride, that the interradius of closure is the fixed point, the position of the madreporite being variable. He points out to me that the stone-canal opens on the inner side of the hydrocoel ring, and that the position of the opening might easily shift. The madreporite being on the dorsal surface might also easily change its interradius, as the anus undoubtedly does. 120 PHYLUM ECHINODERMATA. spicules of Holothurians, which are remarkable for their definite and symmetrical form, to the compact and complete dermal skeleton of Echinoids. The plates arise as deposits in the form Trivium r in CLOSURE IN OPHIURIDS AND HOLOTHURIANS ? CLOSURE IN ANTEDON IV CLOSURE IN ECHINIDS AND HOLOTHURIANS ? POSTERIOR INTERRADIUS CLOSURE IN ASTERINA ANUS OF EXOCYCLICA Bivium FlO. 83. Diagram uf an Kchinoderm viewed from the aboral pole, to show the enumeration of the radii adopted in this work. The diagram also shous the interradius in which the liydrocoel closes in certain observed eases (vide Bury. (J..I.M.S.. '2'.1. p. 431). I-V the radii ; M the inadrejiorite. In MacBride's enumeration of the liydrocoel lolies adopted hen 1 in the succeeding account of the development of Asterhui, lulie no. 1 is radius II. lolie no. ~ is radius III, ami lolic no. f> is radius I. In Cue-not 's enumeration the equivalence is A = V. B = I, C = II, D = III. and E = IV. Radius I is the right anterior, and radius IV is the left anterior radius of Crinoids. The plane of symmetry of Holothurians and Crinoids passes through radius V and interradius II. Ill, that of Asteroids through IV and I. II. and that of exocyclic Echinoids through 111 and V.I (Loven's plane of symmetry). SKELETAL SYSTEM. 121 of networks or spongeworks of calcareous matter in the con- nective tissue. In many of the classes (Asteroids, Ophiuroids and Echinoids) these plates carry spines and processes which project on the surface, and in the young state at least are covered by the epidermis. The majority of the spines are movably articulated with the plates, and in Asteroids and Echinoids some of them are specially modified as snapping organs the pedicellariae. The function of the spines is probably mainly protective, but in the case of the long spines of Echinoids it is locomotory. The pedicellariae are protective, seizing foreign organisms (p. 224). They are said not to bite animals of the same species (autodermophily, Uexkiill). The epidermis is generally ciliated, but in Ophiuroids, and on the aboral surface of Crinoids it is difficult to distinguish it as a layer distinct from the cutis. The description of the skeletal system will be best dealt with in connexion with the different classes of the phylum. It will be well however to call attention here to certain plates which are supposed to be homologous throughout the group : these are the plates of the oral and apical systems. The oral plates are five in number and are placed interradially round the mouth on the oral surface. They can be distinguished in many Crinoids and in Ophiuroids, but are not clearly distinguishable in any other class. The plates of the apical system are placed at the aboral pole, and when present are usually discernible at a comparatively early stage of development. In typical cases (Ophiuroids, Fig. 84, many Crinoids, Fig. 85) they consist of a central plate surrounded by ten plates, five of which are radial in position and five interradial : the former are called infrabasals, the latter basals. Beyond this circle there are five radially placed plates called the radials. Of these plates the infrabasals, or under- basals as they are sometimes called, are frequently absent. In Echinoids the central plate is not pierced by the anus which is placed on one side of it ; moreover it is frequently very difficult to distinguish it, owing to the presence of a number of small plates, called the periproct plates, at the apical pole. In Crinoids the position of the central plate is not certainly known, but it is supposed to be represented by the so-called dorso-central which is found at the peripheral end of the stalk. In Asteroids a complete 122 PHYLUM ECHINODERMATA. apical system can sometimes be made out, but as a rule the plates are indistinguishable from the other plates of the antambulacral surface. In Holothurians the plates of the oral and apical systems are absent both in the larva and in the adult. In Echinoids, in which a central plate can often be made out, the five interradial basals (genital plates) are always present, but infrabasals and radials are not found. The oculars of Echinoids. which are often called radials, are rather comparable to the terminals, which must now be referred to. Whereas the plates of the apical system, viz., the central, the infrabasals, the \n/-Mifi3Ite( ce ism^Fl Flo. 84. Apical system of a young Ophiurid Ainphiura squamata (after P. H. Carpenter), ba basal : ce cen- tral : ib int'rabasal ; r radial ; t ter- minal. FIG. 85. Apical system of a Crinoid (Cyatho- crini'x). IKI basal ; c2 second costal (primi- brach) : ib infrabasal ; ian anal interradial ; r radial (from Lang). basals and the radials are developed round the right coelomic vesicle of the larva, the terminals are five radially placed plates on the oral wall of the left coelomic sac. They become the oculars of Echinoids and those plates of Asteroids and Ophiu- roids which are placed at the ends of the arms on the aboral side of the projecting end of the radial water- vascular canal. The question as to whether these plates are in all eases homologous is a very difficult one to answer. It was originally suggested by Loven and has been maintained by many of the later students of the group- notably by P. H. Carpenter that there is a general homology between these plates in the different classes ; but recently some doubt has \>i-> -ii thrown upon, this view of them (see p. 292). It is possible NERVOUS SYSTEM. 123 that the very remarkable similarity in their arrangement may be clue to the similarity in the general structural conditions, viz. the presence of a central point round which the plates are regularly arranged. This view derives some support from a consideration of the fact that when the matter is minutely examined there is a considerable amount of variation in the arrangement of the apical plates. For in- stance the apical pole may be occupied by a central plate (many Echin- oids, larva of Antedon, many Asteroids and Ophiuroids), or by five interradially placed plates (basals in many irregular Echinoids), or by five radially placed plates (infrabasals of some Crinoids). Again the number of circles of these plates varies considerably. In many Crinoids there are three, viz. infrabasals, basals, radials. The same arrangement occurs in many Asteroids and Ophiuroids, but in these classes there is the greatest variation in the arrangement of the plates near the apical pole, and in many of them the plates in this region are small and numerous and the typical apical plates cannot be recognized. In Echinoids, on the other hand, there is never more than one circle, the interradially placed basals. The alimentary canal. The variations in the position of the mouth and anus have already been indicated (p. 117). The alimentary canal passes between the two, and is chiefly remarkable for the very general absence of separate glandular appendages. The anus is absent in the adult of Ophiuroids and of a few Asteroids. For details the reader is referred to the account of the different classes. The central nervous system consists of three parts which are variously developed in the different classes. (1) The ventral, (2) the deep oral, (3) the apical. The ventral system consists of a concentration of a diffuse subepithelial plexus, found in most parts of the body in both ectoderm and endoderm. It is mainly a sensory system and supplies the skin, the feet and the gut. The ectodermal part of this plexus, which is continuous with the endodermal and may be called the ectoneural, is especially concentrated in an annular tract round the mouth (circumoral nerve ring) and in prolongations of this along the whole length of the radii (radial or ambulacral nerves). These concentrations constitute what we have called the ambulacral central nervous system. The general disposition of the ectoneural plexus is well shown in Fig. 86 : it extends into the tube-feet, in some of which, especially in the pointed variety, it is well developed, as well as into the spines, papulae and pedicellariae. The endodermal part of this plexus, which is called the endoneural and presents in Asteroids special concentrations round the edge of the mouth 124 PHYLUM ECHINODERMATA. opening (perioesophageal nerve ring), is found throughout the endoderm and is apparently continuous at the mouth and possibly at the anus with the ectoneural plexus. In Echinoids, Holothurians and Ophiuroids, the circumoral ring and radial nerve trunks are separate from the superficial ectoderm and placed in the wall of a canal, the epineural canal (Figs. 140, 169, 180), which is actually formed in development by the invagination of the larval ectoderm along the centre of each radius* (p. 150). So that in these forms the ectodermal part of the central nervous system attains its internal position by invagination as it does in Enteropneusta and Verte- Flo. 86. Scheme of the nervous system of the arm of a starfish (after Cuenotl. a wall; b body-cavity of arm ; c ampulla of tube foot ; rftube foot ; e radial canal of water- vascular system ; 1 radial portion of ectoneural central nervous system ; 2 ectoneural plexus of tube-foot ; 3 ectoneural plexus of skin ; J Lange's nerve cords (deep oral) ; 5 mesoneural plexus just beneath the longitudinal muscle. brata, and the epineural canal may be compared to the central canal of the nervous system of those animals. Special nerve trunks pass from the central parts of this system to the skin, tube-feet, etc. At the end of the radii these radial trunks pass to the surface and lie in the ectoderm covering the terminal tentacle, if such is present. In the same three classes the apical nervous system (see below) is not present, or at any rate not developed in the same marked manner that it is in Crinoids and Asteroids. The deep oral nervous system consists of a double cord in each * This has been shown for Echinoids by Mac-Bride, for Ophiuroids by Grave, and for Holothurians by Clark. SENSE ORGANS. 125 radius just within the radial nerve thickening of the ectoneural system, being only separated from the latter by a thin layer of connective tissue (Fig. 86, 4). These are called Lange's cords after their discoverer. They lie in the outer wall of the peri- haemal canal and are mesodermal in origin. They are said to be exclusively motor in function. Round the mouth there ap- pears to be a more or less complete ring belonging to this system in Asteroids and Ophiuroids, but this is absent or much less developed in Echinoids and Holothurians. The deep oral system is not present in Crinoids, or, if it is present, it lies deep on each side of the water-vascular canal (Fig. 197). The apical nervous system is also mesodermal and motor. It has the form of a cord in the dorsal middle line and is developed from the dorsal peritoneum with which it sometimes remains continuous (Asteroids, Fig. 86). It is best developed in Crinoids (p. 283), where it is separate from the peritoneum ; it is not found in Holothurians. The sense organs, so far as they are understood, are mainly of a tactile nature. There are the tube-feet, which are all highly sensitive, and in some cases (Crinoids, Ophiuroids, certain tube- feet of Echinoids and Holothurians) exclusively concerned with sensation (and respiration). There is the unpaired tentacle, formed by the projecting end of the radial water-vascular trunk, at the end of the radii ; in the early larvae this is the only tentacular or tube-foot structure present. Lastly there are the circumoral tentacles of Holothurians, which contain prolonga- tions of the water-vascular system and are used for grasping, and the buccal tube-feet of regular Echinoids. In some Crinoids there are special tube-feet near the mouth which may be regarded as circumoral tentacles. The circumoral tufts of Echinoids are respiratory in function and not connected with the water- vascular system. Of other tactile structures we must mention the spines which are richly provided with nervous tissue. Special organs of sense are not numerous. The pigment spots at the end of the arms of Asteroids are probably visual in function (p. 175), as are also the shining spots on the skin of Diadema (anEchinoid, p. 230). The ocular plates of Echini have no visual structures on them and are merely perforated by the terminal tentacle of the water- vascular canal. 126 PHYLUM ECHINODERMATA. Otocysts are found in Holothurians (Synaptidae and Elasi- poda), in connexion with the radial nerves (p. 256). Finally the sphaeridia of Echinoids (p. 227) are probably organs of some special sense. They are modified spines, and are richly provided with nervous tissue. It has been suggested that they are organs of orientation, enabling their possessor to perceive its position in space. The muscular system is very variously developed in the different classes. In Holothurians. only, is there a well-marked derrno- muscular body-wall. In all other Echinoderms the skin is not contractile or but slightly contractile, and the muscles are restricted to special bands acting on particular skeletal plates or other part of the body, and to the muscular elements in the walls of the tube-feet and water-vascular system generally and of the viscera. In the spiniferous forms there are special muscles attached to the base of the spines and pedicel] ariae. The muscular tissue consists of smooth contractile fibres with- out transverse striation, except in the case of the muscles of some of the pedicellariae and of a few spines. The Coelom. Our knowledge of the coelom of Echinodermata has been of slow growth. It began witli the discovery of A. Agassiz * that the enteron of the larva of Asterias berylinns gave rise not only to the alimentary canal of the adult, but also to the body-cavity and water-vascular system. This discovery was established and extended by the work of Metsclmikoff on Asteroids. Echinoids, and Ophiuroids, of Kowalevsky and Selenka on Holothurians. of Bury on Crinoids. and of Ludwig on Asteroids. Finally the recent work of MacBride has not only elucidated the relations of the genital organs and of the so-called perihaemal spaces, but has thrown light upon the nature of that organ which was formerly called the '"heart" but is now more commonly referred to as the axial organ or ovoid gland. The coelom arises from the enteron of the embryo as a single pouch. This pouch soon separates from the enteron and divides into a number of sacs, one or one pair of which constitutes the hydroeoel, the others the splanchnocoel. The left hydrocoelf gives rise to the water- vascular system, while the splanchnocoel * " Embryology of the Starfish," Contributions to the Natural History of the United States, 5, 1864. f The right hydroeoel, when present, is always very small, and takes no part in forming the water-vascular system. COELOM. 127 becomes transformed into the perivisceral cavity and its associated spaces (axial sinus, aboral sinus, perihaemal canals) of the adult. All the coelomic spaces are lined by an epithelium, which in the case of the water-vascular system and perivisceral cavity is ciliated, and they all contain an albuminous fluid in which amoeboid cells float. Nothing of the nature of nephridia is known in the group, but one of the divisions of the splanchnocoelic part of the coelom, viz. that known in the larva as the anterior coelom, has an open- ing to the exterior. This opening is called the primary water pore or the madreporitic pore. The primary water-pore, which is frequently in special relation with a dermal plate called the madreporite, and is always interradial in position, may be a small simple opening or it may be subdivided (Asteroids, Echinoids, many Ophiuroids) into a large number of minute secondary pores, which are more or less closely aggregated together on the madre- porite. Though the generative organs are separate from the coelom in the adult, the primitive germ cells which give rise to them have a coelomic origin. We may now proceed to consider the different parts of the coelom in greater detail, and first of all we will treat of the spaces derived from its splanchnocoelic division. These are three in number : the perivisceral cavity, the axial sinus and the perihaemal spaces. 1. The perivisceral cavity or body cavity proper is always well developed and in relation with the alimentary canal and principal viscera. It is developed from the right and left posterior coeloms of the larva and it never communicates with the exterior except in Crinoids in which the water-pores open into it (see p. 287). It is often traversed by complete or incomplete mesenteries (Holothurians) or by strands of connective tissue, which pass from the body wall to the wall of the alimentary canal. In the s / brachiate forms the arms always contain prolongations of the perivisceral cavity. As stated above, it usually has a ciliated lining and contains an albuminous corpusculated fluid. The Amoebocytes of the coelomic fluids and possibly of other organs play an important part, as was first shown by Durham (op. cit.), in remov- ing foreign bodies from the organism. They act as phagocytes and pass to the exterior by diapedesis through the walls of the papulae on the outside of which they disintegrate. PHYLUM ECHIXODERMATA. Several kinds of amoebocytes,* differing in size, character of granula- tions and shape, are found in the coelomic fluids and tissues of the body. In some cases spherical cells provided with a long cilium have been ob- served (e.g. coelomic fluid of Dorocidaris). In Echinus the amoeboid cells have been observed to unite and give rise to plasmodia and networks, f In the larva they play an active part in the absorption of the calcareous skeleton, and in both larva and adult isolated amoeboid cells take part in tissue formation. 2. The axial sinus is distinct from the perivisceral cavity in all classes except in the adults of Crinoids and some Holothur- ians (see pp. 129, 152). It is developed from the anterior coelom of the larva and communicates with the exterior by the water- pore or pores, and the stone-canal opens into it. It will be convenient to reserve what we have to say about the two latter structures until we have dealt with the water-vascular system. In Asteroids the axial sinus (i.e. the space into which the stone- canal and axial organ project) is continued ventrally round the mouth and forms the so-called inner circumoral perihaemal space (Fig. 131). This space is not connected with and must not be confused with the perihaemal space about to be described. 3. The perihaemal spaces (sinus system) usually consist of an annular circumoral space which is called the outer in contra- distinction to that just mentioned, and of five tubes placed along the radii between the radial nerve cord and the radial water-vascular trunk (Fig. 131). This system of spaces appears to be unrepresented in Crinoids. It is lined by an epithelium and has been definitely traced in Asteroids to outgrowths of the posterior and anterior body-cavities of the larva (p. 145). The perihaemal system must not be confused with the epineural canal of Ophiuroids, Echinoids, and Holothurians (p. 124), which lies superficial to the radial nerve cord and has an epithelial lining which must be regarded as ectodermal. The aboral circular sinus (Fig. 131) might be included amongst the perihaemal spaces ; it is a development of the left posterior coelom and is in relation with the generative rachis (see p. 146). The water-vascular system is derived from the left hydrocoel (p. 144). It is very similarly developed in all classes of Echino- clerms. It is the organ which pre-eminently displays the radial structure of the body, and is the first to show it in the larva. It consists of a circular vessel round the mouth with as many * Cuenot, Arch. Zool. K.rp. (2), 1), 1891, p. (i!3. f Theel, Festskrift Lilljeborg Upsala, 1896, 3, p. 47 WATER- VASCULAR SYSTEM. 129 tubular prolongations as there are radii. These are the radial water- vascular vessels ; they give off lateral branches to the tube-feet all along their course (Fig. 133). The tube-feet are hollow, cylindrical or conical processes of the body wall, and the space within them is continuous with the water- vascular system by the just mentioned lateral branches of the radial vessels. At their inner ends they are connected with small vesicles the ampullae (Fig. 129, 26). Ampullae are absent in Crinoids and Ophiuroids ; they have muscular walls and their function is to drive the fluid into the tube-foot and so to cause its extension. The retraction of the foot is brought about by the contraction of the muscles in its wall, the fluid in the tube- foot passing into the ampulla.* In the forms with ampullae- the tube-feet are locomotive and adhesive organs. Their free ends terminate in sucker-like discs which adhere to foreign bodies. When ampullae are absent (Crinoids and Ophiuroids} the tube-feet are purely sensory and respiratory in function and cannot be used for adhesion and locomotion. Calcareous; bodies are often present in the connective tissue layer of the tube-feet, particularly at the sucker-like termination. The circumoral vessel frequently possesses accessory structures opening into it in its interradial portions ; these are the bladder - like polian vesicles (p. 184) and the gland-like Tiedemann's bodies (p. 185). The water-vascular system is indirectly connected with the exterior by a canal called the stone- or sand- canal (Figs. 133, 136). The stone-canal owes its name to the fact that it frequently contains in its walls a large amount of calcareous matter, which readily breaks through the lining and falls into the canal, where it is found as gritty matter. The connexion with the exterior is effected in the following way (Fig. 132) : the stone-canal passes off from the circumoral vessel in one of its interradii to open, in the larva, into the anterior coelom which is a portion of the splanchnocoel. The anterior coelom, which, as already explained (p. 127), opens to the exterior by the water-pore, persists into the adult in Asteroids, Ophiuroids, and Echinoids as a distinct space, the axial sinus, f In Crinoids it is a distinct space in the * For the valve assisting in this process, see p. 182. f In the adult many of the pores of the madreporite come to open directlr into the dorsal end of the stone-canal, but the opening of the stone-canal into the axial sinus is always maintained. Z III K 130 PHYLUM ECHINODERMATA. young larva, but later becomes merged in the general perivisceral cavity, so that in the adult both the stone-canals and the water pores open into the perivisceral cavity. In Holothurians the anterior coelom is very feebly developed, but as Bury has shown it can be traced through the whole development (p. 152) and has the appearance of a small appendage of the stone-canal (Fig. 108). In a few Holothurians it retains its communication (by the water-pore) with the exterior, but in the majority the water-pore closes in the adult, and the walls of the anterior body-cavity largely break down and give rise to the so-called internal madreporite. It thus comes about that in most Holo- thurians the stone-canal appears to open directly into the general body cavity. It sometimes happens (some Asteroids and Ophiuroids, Crinoids) that there is more than one primary water-pore ; in such cases there is a cor- responding increase in the number of madreporites and stone-canals, and they are generally placed in different interradii. In Crinoids there are never less than five primary water-pores, and generally they are much more numerous. In the former case there is one water-pore and one stone-canal in each interradixis ; in the latter case there are many of both structures in all the interradii, and an exact correspondence between the stone-canals and water-pores, though it may exist, cannot be shown. The water- vascular system is lined by a flat ciliated epithelium and contains an albuminous fluid with leucocytes very similar to that found in the perivisceral cavity and sinus system. Its function is mainly locomotory, but to this must be added, especially in forms in which the tube-feet are without ampullae, a tactile and respiratory function. By some authors it has been regarded as in part excretory, but there seems to be little evidence in support of this. There might be something to be said for this view, if the current through the madreporite set outward, but the reverse appears to be the case (Ludwig). If there is any current through the water-pore, it appears to be an inward one as a result of which sea-water is drawn into the system. But it may be that on occasion the direction of this current is reversed. Particles of carmine injected into the water-vascular system are taken up by the cells of Tiedemann's bodies (Kowalevsky). The axial organ, which goes by various names, e.g. dorsal organ, ovoid gland, heart, etc., is found in Echinoids, Asteroids (Fig. 130), Ophiuroids and Crinoids. It is developed as a fold EXCRETION. VASCULAR SYSTEM. 131 of the wall of the axial sinus (except in Crinoids, p. 159), and consists of connective tissue and of cells which have grown into it from the genital rudiment of the larva, It is covered towards the axial sinus by the coelomic epithelium. Its walls are folded so that it appears to be penetrated by tubular pro- longations of the epithelium of the axial sinus and of the peri- visceral cavity. Some observers have attributed to the axial organ a lymphatic gland function, and have supposed that it buds out amoeboid cells into the axial sinus, but this is ex- tremely doubtful. In Echinoids it takes up carmine injected into the body cavity (Kowalevsky). As stated above the primitive germ cells grow into it, so that it is connected in the adult with the generative rachis. The axial organ is absent in Holothurians. Excretion. Very little is known about excretion in Echino- derms. There do not appear to be any special organs devoted to it. It is possible that there may be some organ in connexion with the water-vascular system or with the axial sinus which is concerned with the elimination of the nitrogenous waste, for these organs open to the exterior by the water-pore but no such organs have been certainly identified. The so-called vascular system, which is found in all classes and appears to be specially well developed in the Holothurians and Echinoids, is formed of a peculiarly modified connective tissue in which the fibres are sparse, and which contains intercom- municating spaces without an epithelial lining. The fluid in these spaces does not appear to undergo any definite movement. The real nature of this tissue is doubtful. By some observers it has been regarded as a lymphatic gland, a view which is sug- gested by the appearance, sometimes found, of amoeboid cells being budded off from it. Typically there is a circumoral tract of it with radial prolongations which lie between the radial water- vessel and the radial nerve cord ; an annular aboral tract of it, in which the generative rachis is embedded and which sends extensions to the genital organs ; and in Holothurians and Echinoids a considerable development of it in the mesentery and on the gut wall. An account of its occurrence will be found in the description of the different classes. The generative glands * of Echinoderms are peculiar in the * G. W. Field, on the Morphology and Physiology of the Echinoderm spermatozoa, Journ. Morph., Boston, 2, 1895, p. 235. 132 PHYLUM ECHINODEEMATA. fact that they open directly to the exterior and apparently have no relation to the coelom. The researches of MacBride have however shown us that the cells composing them are, in Asteroids, Echinoids and Ophiuroids, coelomic in origin, being derived from lining cells of the body-cavity, and this will probably be found to hold throughout the phylum. Another peculi- arity of them consists in the fact that in all classes, with the possible exceptions of Holothurians (see below), they are connected with a cellular cord, disposed in different ways in the different classes and called the generative rachis. Indeed they may be regarded as swollen, fertile portions of this rachis, the rest of which is sterile and does not produce generative cells. As stated above the axial organ contains a prolongation of the generative rachis. There are no accessory glands of any kind. The generative products when ripe are in nearly all cases discharged directly into the sea, where the ova are fertilized and undergo their development. In rare cases the eggs are retained in special brood pouches or on the skin of the mother. Secondary sexual characters are as a rule not developed. The generative organs usually consist of tufts of branched tubes which open directly on the surface. In Crinoids they are contained in the pinnules as hollow structures without any opening to the exterior, and the manner of escape of the genera- tive cells is not certainly known, but it probably takes place by dehiscence. In Asteroids, Ophiuroids, and Echinoids they are pentamerously arranged in the interradii and parti- cipate in the radial symmetry of the body ; they open inter- radially and sometimes dorsally. In Holothurians, which are sometimes said to be without the generative rachis,* they are present as a single tuft of tubes which open in the adult-dorsal middle line. The Echinoderms are exclusively marine animals. f They are found on the floor of the ocean from the littoral to the abyss. Very few pelagic forms are known. They are almost all slowly * Holothurians possess a cord containing germ cells and passing from the point of union of the genital tubes along the generative duct towards the body wall (Theel, Bih. Svenska Akad. Handl., 27, Afd. 4, No. 6, 1902). This is probably the generative rachis. | Synapta similis inhabits brackish water of the mangrove swamps of Bohol (Ludwig). CLASSIFICATION. 133 creeping creatures, moving by means of their tube-feet or by their arms (Comatulids, Ophiuroids). Most Crinoids how- ever are attached by a stalk which is a prolongation of the aboral surface of the body, but they may become detached and acquire a new attachment (Pentacrinus). From a psychical or nervous point of view, the Echinoderms are extremely low in the scale of life, but judging them from the complexity of their organization apart from the nervous system, they approximate to the so-called higher animals. Geologically they are of great interest, the structure of the body wall lending itself very readily to their preservation as fossils. They make their appearance in the Cambrian and in the Silurian with a range of structure not unlike that which characterizes living forms. The power of regenerating lost parts is considerable in almost all Echinoderms, and many of them, especially those with long arms, possess the power of autotomy. It is least developed in Echinoids, but in Asteroids and Ophiuroids it is in some cases so extensive that not only may arms and portions of the disc be regenerated, but the whole body may be reformed from a single arm. In Crinoids and Holothurians the viscera even are capable of regeneration. Asexual reproduction is found in many Asteroids, Ophiuroids and Holothurians. It takes the form of fission into two equal halves, in the two former classes the plane of fission passing through the disc, in the latter transversely to the long axis of the body. In this work the Echinodermata are divided into seven classes : Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea, Crinoidea, Cystidea, Blastoidea. The mutual relationships of these are discussed under the accounts of each class. The group Pelmatozoa is considered and the reasons for not adopting it are stated on p. 303 et seq. Development.* In the great majority of Echinoderms the * For a more complete account of the developmant of the Echinoder- mata, the readsr is referred to the following works, where the older litera- ture will be found : F. M. Balfour, A Treatise on Comparative Embryology, vol. 1, London, 1885; E. Korschelt and K. Heider, Textbook of the Em- bryology of Invertebrates, English Translation, London, 1895. A. Lang, Text-Book of Comparative Anatomy, English Translation, vol. 2, London, 1896. Of recent workers the following may be cited : E. W. MacBride, The development of Asterina gibbosa, Q.J.M.S., 38, 1896, p. 134 PHYLUM ECHINODERMATA. egg is small and the entire development takes place in the sea. and in most of them there is a free-swimming, externally bisym- metrical larval stage. In about fifty species however the young remain in connexion with the mother during their development and care of the brood occurs, and in some cases (e.g. Asterina gibbosa) a free-swimming larva is not formed. In such cases the egg is usually larger than when a free-swimming larva is formed, owing to the presence of a greater quantity of food-yolk.* When care of the brood occurs, f the young may either creep about freely on the surface of the mother, or they may be attached to special parts of the body wall, e.g. the neighbourhood of the mouth, among the spines of the back, in special depres- FlG. 87. Two stages in the development of Holuthitriu tiilndnxa viewed in optical section (from Balfour, after Seleuka). A, Blastosphere at the close of segmentation. B, com- mencing gastrula stage, ae archenteron ; bl wall of blastosphere ; ep ectoderm ; ft vitelline membrane ; hy endoderm ; mr micropyle ; ms protoplasmic immigration into the seg- mentation cavity from the invaginating endoderm ; sc segmentation cavity. sions of the skin. In some Holothurians the development takes place in the body cavity, and in the dioecious Chiridota contorta in the genital tubes. In the starfish Stichaster nutrix the young 339. Id., The development of Echinus esculentus, Phil. Trans., 1903, p. 285. H. Bury, The Metamorphosis of Echiiioderms, Q.J.M.S.* 38, 1895. H. Theel, The development of Echinocyamus pusillus, Nova Acta R. Soc. Sci. Upsala, 1892. Id. Prelim. Account of the development of Echinus miliaris, Proc. Roy. Swedish Acad., 28, 1902. G. W. Field, The larva of Asterias vulgaris, Q.J.M.S., 34, 1893, p. 105. S. Goto, The metamorphosis of Asterias pallida, etc., Journal Coll. Sci. Imp. Univ., Tokyo, 10, 1898, p. 239. Id., Metamorphosis of Asterina gibbosa, Ibid.. 12, 1898, p. 227. Th. Mortensen, Echinodermlarven, Ergeb. d. Plankton- Exped. d. Humboldt-Stiftung, 2, 1898. * In Benthodytes sanguinolenta the eggs measure 2 mm. in diameter. f Ludwig, Zoolog. Jahrb., Suppl. Bd., 7, 1904, p. 683. DEVELOPMENT. 135 undergo their early development in the stomach of the mother. Care of the brood is found most frequently in forms inhabiting the colder seas. With a few exceptions the early development, so far as it is at present known, may be summarized as follows. The egg is fertilized in the sea, undergoes a total cleavage, and becomes transformed into a hollow one-layered blastosphere. At one pole of the blastosphere an in vagination makes its appearance and a typical gastrula is formed (Fig. 87). The invaginated cells constitute the endoderm, the cavity bounded by them the archenteron, and the opening of the archenteron the blastopore. The archenteron rarely if ever fills the segmentation cavity, but the lat- ter soon becomes traversed by a nucleated proto- plasmic network (Fig. 88), which is continuous both with the endoderm and with the ecto- derm (outer cells of the gastrula), and constitutes the first trace of the meso- derm. This meso- dermal network, which is reinforced from the later appearing enterocoelic vesicles is commonly called mesenchyme and gives rise to the connective tissues, to some of the muscles, and to the calcareous structures of the body. Different views may be held as to the origin and- structure of this early appearing mesodermal network. The usual view is that it consists of amoeboid cells which arise by the proliferation of the epithelial cells of the embryo in the blastosphere and later stages, and wander into the hlastocoel. Another view for which there is much to be said is that the blastocoel at its very first appearance is traversed by protoplasmic pro- cesses of its walls,* and that the apparent proliferation of cells of the \ FIG. 88. Gastrula stage of Tox&pneustes bremgpinosus (from Korschelt and Heider, after Selenka). The mesodermal net- work (so-called mesenchyme) is shown traversing the blasto- coel and continuous with both ectoderm and endoderm. * See C. Shearer on the connexions between the blastomeres of embryos in the Proc. Roy. Soc., 1906. 136 PHYLUM ECHINODERMATA. endodermal pole of the blastosphere or of the invaginating endoderm, etc., is really only a proliferation of nuclei, and that the migration of cells into the blastocoel is nothing more than the shifting of these nuclei along the strands of the protoplasmic reticulum which has traversed the blastocoel from its first appearance. On this view the embryo must be regarded not as an aggregation of separate cells, but as a continuous structure with different densities of protoplasm in different parts. Thus in the blasto- sphere stage the bulk of the protoplasm is concentrated in a peripheral layer to which the nuclei are confined, the central portion being occupied by a sparse non-nucleated reticulum. As to the meaning of this peculiar peripheral aggregation of the protoplasm and nuclei, which is characteris- tic of the blastosphere of so many pelagically developing animals, we are quite in ignorance. It has been suggested that it represents some ances- tral stage of structure common to all the organisms in which it occurs. On the other hand it may be argued, perhaps with greater force, that the condition is due to some physiological necessity, possibly of a nutri- tive nature, which is felt by all organisms developing in the open sea. The undoubted fact of the presence of free leucocytes in the fluids of the body must not be held to be at variance with the view of the nature of the mesenchyme mentioned above. The view of the essential 'continuity of the mesenchyme is based upon Comparative Embryology. In all cases in which its structure has been thoroughly made out, it has the form of a network continuous both with ectoderm and endoderm. The free leucocytes which are found in the body fluids are products of this network, formed no doubt for a definite purpose. What that purpose is has been largely explained by the remarkable and penetrating observations of Metschiiikoff,* though the causes which govern the budding off of the leucocytes have not been ascertained. The eggs when laid are enclosed in the vitelline membrane (Fig. 87), but they soon acquire a coat of cilia and become free. The gastrula has a somewhat oval form with the blastopore at one end and one surface slightly flattened. The end carrying the blastopore is posterior and the flattened surface is ventral. The archenteron now bends towards the ventral surface anteriorly, where, its wall having fused with the ectoderm, it acquires an opening to the exterior ; this is the larval mouth, the posterior opening or blastopore persisting as the larval anus (except in Crinoids). The fate of the larval mouth and anus varies in the different classes. In Holothuroidea they usually persist into the adult. In Ophiuroids the larval mouth persists but the anus closes, the adult being without one. In Asteroids and Echinoids the larval mouth and anus close and the oesophagus atrophies, the adult mouth, oesophagus, and anus being new formations. It is possible, however, that in some Asteroids they may persist. In Crinoids there is 110 larval mouth or anus, and the blastopore closes at an early stage. * See the numerous papers by this author dealing with the importance of the amoeboid cells in the organism and especially his great work on Immunity, English Translation, Cambridge, 1905. DEVELOPMENT. 137 pr.c Later the anus shifts on to the ventral surface, and the cilia become confined to definite tracts or bands. According to the arrangement of these ciliated bands, two main types of larvae can be distinguished. In one (A, Fig. 89) there is a single longitudinal band of cilia which passes across the ven- A 8 tral surface immediately in front of the mouth and in front of the anus, and bounds a depression on the ventral surface into which the mouth opens. This is the Auricularia type of larva which is found in Holothurians ; a modifica- tion of it is found in Echinoids and Ophiuroids, where it is known as the Pluteus larva (Fig. 94). In the other (B, Fig. 89) there are two bands of cilia, the one preoral and encircling the preoral lobe, the other longitudinal in appearance, but really postoral, forming a complete circuit of the body between the mouth and the anus ; this is the Bipinnaria type of larva and is characteristic of Asteroids. The larvae of both these types are externally bilaterally symmetrical and have been so since the ventral sur- face became distinct. They remain symmetrical externally for some time longer, but at about this stage, or in some cases even sf- FlO. 89. A, the larva of a Holothurian (Auricularia type) ; B of an Asteroid (Bipin- naria type), both seen from the left side (from Balfour). a anus ; m mouth ; st stomach ; l.c longitudinal ciliated band ; pr.c preoral ciliated band. tn. ten FIG. 90. A series of diagrams representing the evolution of an auricularia larva from the simplest Echinoderm larval form, ventral view (from Balfour). The black line represents the ciliated band. The shaded part is the oral side of the ring (oral depression, see Fig..89), the clear part the aboral side of the larva, m mouth ; are anus. before this stage is reached, their internal structure begins to show traces of that asymmetry which is so characteristic of the later larvae and eventually leads to the establishment of the radial symmetry of the adult. Before, however, considering 138 PHYLUM ECHINODERMATA. these internal changes it will be well to consider briefly the various forms which the bilateral larva may assume. The auricularia larva. In the Holothurians the ciliated band becomes elongated and sinuous, as is shown in the series of diagrams in Fig. 90. A completely developed auricularia larva of a Holothurian is shown in Fig. 91. The sides of the body are prolonged into processes which correspond to the arms of the bipinnaria and pluteus, and there is a well-marked preoral lobe. Calcareous structures in the form of spheres, wheels and star-shaped bodies are formed, but there are no calcareous rods. In some species (Auricularia stelligera and sphaerigera) peculiar elastic spheres, the nature of which is not known, are present. -ffl FIG. 91. Auricularia stelligera, ventral view (after J. Mullen. 1 frontal area ; 2 preoral pro- cess ; 3 anterior, 4 posterior portion of the ciliated band : 5 postoral process ; 6 anal area ; 7 postero-lateral pro- cess ; 8 postero-dorsal pro- cess ; 9 oral depression ; 10 dorso-median process ; 11 antero-dorsal process. FlO. 92. Pupal stage of the larva of Synapta digitata (from Kor- schelt and Heider, after Se- inon). ed hind-gut ; ent right body-cavity ; m oral funnel ; w water-vascular ring, with out-growths into tentacles t and radial vessels p. The larva now enters the so-called pupa-stage, in which it has the form of a barrel with five ciliated hoop-like bands (Fig. 92). The pupa is formed from the auricularia in the following way : The ciliated ring breaks up (Fig. 93) into pieces which rearrange themselves into the five rings of the pupa and the oral ring ; the mouth and surrounding parts, including the oral ring, retreat into the interior, giving rise to the oral vestibule (atrial cavity), the opening of which narrows and passes to the left side. Event- ually the atrial opening becomes terminal by the atrophy of he small preoral lobe, and the epithelium of the oral ring DEVELOPMENT. 139 bscomes the epithelial covering of the tentacles which at first project into the atrial cavity. When the adult condition is FIG. 93. Synapta larva, showing the break up of the ciliary band of the auricularia. a anus ; ed prootodaeum ; ent euterocoel ; kr calcareous wheels ; m oral funnel or atrium ; mg stomach ; n nerve bands ; 10 water-vascular ring with prolongations (from Korschelt and Heider). attained the ciliated bands disappear, the oral vestibule opens out and the tentacles project. The pluteus larva. In the Ophiuroidea and Echinoidea the larva resembles the auricularia in possessing a single ciliated band, but differs from it by the long arm-like processes of the FIG. 94. Diagrammatic figures showing the evolution of an ophiuroid pluteus from a simple Echinoderm larva (from Balfour, after J. Miiller). The calcareous skeleton is not represented, m mouth ; an anus ; d antero-lateral arms ; d' the long postero-lateral arms ; ' postoral arms ; g' postero-dorsal arms. margin of the body on to which the ciliated band is continued, by the small size of the preoral lobe (frontal area) and by the 140 PHYLUM ECHTNODERMATA. 8 FIG. 95. Echinopluteus of a Spatangid (after J. Miiller). 1 frontal area ; 2 preoral arm ; 3postoral arm ; 4 anterior, 5 posterior transverse portion of ciliated band ; 6 unpaired posterior ami ; 7 anal area ; 8 postero-lateral arm ; 9 oral area ; 10 postero-dorsal arm ; 11 antero-dorsal arm ; 12 antero-lateral arm. great development of the postanal part of the body (Fig. 94). The arms are supported by calcareous rods. The plutei of Ophiu- roids and Echinoids are distinguished as ophio- plutei and echinoplutei respectively. In ophio- plutei the postero-lateral arms are always the largest and directed for- wards (Fig. 94), and pre- oral and antero-dorsal arms are absent. In echinoplutei (Fig. 95) the postero-lateral arms when present are directed back- wards or outwards, preoral arms are present, and in Spatangid larvae (Fig. 95) there is an unpaired posterior arm on to which the ciliated band is not continued. Moreover in some genera (Echinus, Strongylocentrotus, Sphaer echinus) there are in old larvae four ciliated projec- tions, called ciliated epau- lettes (Fig. 96), at the base of the postoral and postero- dorsal arms, and sometimes an additional pair at the hind end of the body. Ciliated lobes (auricular appendages, auricles) are cutaneous expansions be- tween some of the arms in certain echinoplutei, and pedicellariae may appear before the larval charac- ters are lost. In ophio- plutei the calcareous skele- FlG . 96 ._p lu t eus larva of Echinus KM with ton i=? in two Vmlvps parli four ciliated epaulettes TF (after Metschnikoff) -On IS m tWO naiVCS, eaCIl from the ventr al side. mouth ; A anus. u DEVELOPMENT. 141 half proceeding from one calcification centre ; in echinoplutei the skeleton arises from five or six centres. The bipinnaria larva, found only in the Asteroidea, has two ciliated rings, one preoral and the other longitudinal and postoral (p. 137 and Fig. 89 B). In possessing a well-marked preoral lobe it resembles the auricularia, and its two ciliated rings must be regarded as having been derived by the division of the single band of the auricularia, a view which is supported by the fact that in some species (e.g. Asterias rubens, A, glacialis, Astropecten) they are at first connected dorsally. Further, Driesch* has shown that in some artificially reared bipinnariae there is only one band. As in the pluteus a series of arms is formed along the lines of the two ciliated bands, and sometimes three arms, not connected with the bands and covered with warts, are formed in front of the preoral arms at the anterior end of the frontal area ; these are the brachiolar arms and the larva bearing them the Brachiolaria larva, f The arms are with- out calcareous rods. 12 The median brachiolar arm replaces the ven- tral median arm and bears at its base a sucker J by wliich the larva attaches itself during the metamorphosis. The ciliated band appears, as a rule, not to pass on to these arms (except in Bipinnaria papillata). FlG. 97. Bipinnaria elegans. frontal area ; 2 preoral arm ; 3 anterior, 4 posterior trans- verse portion of the ciliated band ; 5 postoral, 6 postero- lateral, 7 postero-dorsal arm ; 8 anal area ; 9 oral depression ; 10 antero-dorsal, 11 ventro- median, 12 dorso-median arm (after Mortensen). In the bipinnaria (and brachiolaria) the frontal area is well developed and surrounded by the preoral ring of cilia. There is a median ventral anterior arm and a median dorsal anterior (Fig. 97), neither of which is present in the auricularia or in the * Arch. f. Entwickelungsmechanik, 20, 1906, p. 13. f It is possible that the brachiolar arms, which serve for temporary attachment, are present in a later stage of all bipinnariae, for in the only two cases in which the life-history is fully known (Asterias glacialis, A. vulgaris) they are formed. J It is probable that in all bipinnariae this disc, which serves for fixation during the metamorphosis, is present at a later stage in all cases, for not only is it present in the two cases mentioned in the last note, but it occurs in all cases thus far examined where a shortened development due to food yolk has brought these stages within range of easy observation (Ludwig, op. cit. p. 134). (Cf. Anasterias, Asterias antarctica, Asterina gibbosa, Cribrella sanguinolenta.) U2 PHYLUM ECHINODEEMATA. pluteus. Antero-lateral arms are never present. The arms have muscles and are contractile, a fact which renders it diffi- cult to preserve these larvae without distortion. These are the principal types of larvae of the Echinodermata, but we must not omit to mention the vermiform Ophiurid larvae which are pelagic and the vermiform larvae of those Asteroids in which care of the brood occurs. The larva of Asterina gibbosa may be classed with the latter. Echinoderm larvae are found principally near the coast and in small waters : they are not characteristic of the plankton of the high seas. Only a few of the larvae are taken in more than one locality. In the nomenclature of Echinoderm larvae when the adult is not known, the name of the kind of larva is used as that of the genus and a specific name is added. In this sense there are four genera of larvae, viz. Auricu- laria (confining this term to Holothurian larvae), Ophiopluteiis, Echino- pluteus, and Bipinnaria (including brachiolaria). When the adult is known specific names are not required. j-j G . 98. Longitudinal-horizontal sections through three successive stages of the larva of Asterina gibbosa (from Korschelt and Heider, after Ludwig). A, showing the first origin of the enterocoel Vp from the enteron ; B, a later stage, the enterocoel is still in communi- cation with the enteron D ; C enterocoel cut off from the enteron and consisting of a median unpaired portion in front the anterior body cavity, and two paired portions behind, the right and left posterior body cavities. Bl blastopore; D intestine; Vp entero- coel (vasoperitoneal vesicle) ; r and I right and left side of the larva. The hydrocoel has not yet made its appearance. Development of internal organs and the metamorphosis. The coelom arises soon after the establishment of the gastrula as a single evagination of the enteron* (Figs. 98, 99). This soon separates from the enteron, and becomes divided into a number * The term vaso-peritoneal is sometimes applied to the enteric pouch DEVELOPMENT. 143 of sacs, one* of which is called the hydrocoel because it gives rise to the water-vascular system and its lining ; the others constituting the splanehnocoel, because they give rise to the body-cavity and its associated spaces of the adult. The mode of division is very similar throughout the group except in Crinoids, and is described below. In Asteroids we may take as type Asterina gibbosa in which the development has been so fully worked out by Ludwig and later by FIG. 99. Longitudinal section of three stages in -. .- T- i mi i i the formation of the enterocoel of Echinus mili- MaC-Bride. 1 he eggS Which am (from Korschelt and Heider). are 0'5 mm. in diameter are attached to stones to which they adhere by means of the vitelline membrane. On about the fourth day the embryo ruptures the vitelline membrane and escapes. It is then found to possess a large preoral lobe (Fig. 100) the edge of which is thickened, constituting the larval organ. The larval organ surrounds a central depression and is covered with specially long cilia, by means of which the larva can swim. Later, in the centre of the concavity of the larval organ, a small eleva- tion is formed. The ectoderm of this contains gland-cells which secrete an adhesive substance by means of which the larva fixes itself during the m e t a m o rphosis. The temporary fixation which is possible during larval life appears to be due to a kind of cupping action brought about by the application of the preoral lobe to the sub- stratum and the retract- ion of its central portion. FIG. 100. 'Larvae of Asterina gibbosa (after Ludwig). A, a younger stage, oblique ventral view. B, older stage from the side. Lu larval organ ; m larval mouth. and to the sacs which arise from it, the word enterocoel being retained for the peritoneal or body-cavity portions of the latter. We, however, prefer to call the totality of sacs enterocoelic, distinguishing the water- vascular portion as hydrocoel and the perivisceral portion as splanehnocoel. * Or one pair (see footnote on p. 126). 144 PHYLUM ECHINODERMATA. The early development and formation of the larval mouth and anus (p. 135) have already been dealt with. The enterocoel or coelom has the form of an anterior unpaired diverticulum of the enteron extending into the preoral lobe and sending back two lateral prolongations, one on each side of the larval gut (Fig. 98) . This soon separates from the gut, and the posterior prolonga- tions of it eventually meet one another ventrally, their opposed walls forming a ventral mesentery. The primary water- pore is now formed as a pocket of the unpaired portion of the coelom. It meets the skin of the dorsal surface just to the left of the middle line, and a perforation is formed at the point of contact. At the same time there is formed, first on the left side of the enteron and then on the right, a septum, by which the hinder parts of the two coelomic prolongations referred to above become separated off from the anterior part and form closed sacs called the right and left posterior coeloms respectively. The unpaired portion of the coelom, together with the anterior portions of the two prolongations, constitute the anterior coelom of the larva into which opens the primary water-pore. The hydrocoel is developed as an outgrowth from the hinder end of the anterior coelom on the left side, and while it is yet but faintly marked indications of its five prirnarj 7 lobes appear. These, which are numbered* 1, 2, 3, 4, and 5, Xo. 1 being the most dorsal, are arranged in a curve open anteriorly, the hydrocoel sac assuming a horseshoe shape (Figs. 101, 102). The two limbs of the horseshoe eventually come together, enclosing the stalk of the preoral lobe and the oesophagus of the adult, when that is formed on the left side of the larva. In this way the circumoral water-vascular vessel is developed from the horseshoe-shaped hydrocoel. The radial canals are developed from the lobes of the hydrocoel. Shortly after the formation of the left hydrocoel or hydrocoel proper, a rudimentary right hydrocoel is developed as an outgrowth from the hinder wall of the anterior coelom to the right of the middle line (Fig. 101, rJiy). It loses its connexion with the anterior coelom and persists into the adult as a small thin-walled sac beneath the madreporite. * The relations of these numbers to the enumeration adopted for the adult is explained on p. 120. The numbering here adopted has relation to the fact that the hydrocoel before it closes to form a ring has two ends, one of which may be called anterior. DEVELOPMENT. 145 The stone-canal arises as a groove along the anterior face of the posterior wall of the anterior coelom. The central portion of this groove be- comes closed to form a canal which opens at its posterior end into the hydrocoel between lobes 1 and 2 and at the other into the anterior coelom, which as we have seen opens to the exterior by the primary water-pore. Save for this communication the anterior coelom and the hydrocoel become completely separate. The anterior coelom persists into the adult as the axial sinus. When the primary water-pore becomes con- verted by subdivision into the numerous pores of the madreporite of the adult, most of these are found to lead directly into the stone-canal, but some open into the axial sinus. The stone-canal retains its opening into the axial sinus (anterior coelom). The left posterior coelom undergoes a very complicated development. It gives off dorsal and ventral outgrowths which grow on to the right side of the larval body, and a diverticulum which extends round the oesophagus and is called the oral coelom. These all become indistinguish- able in the adult and merely persist as part of the perivisceral cavity. In addition to these the left posterior coelom gives off four interradial prolongations, each of which bifurcates to proceed to the adjacent arm rudiments. These are the first traces of the outer perihaemal ring and of the radial perihaemal canals (Fig. 131). There is a fifth interradial coelomic prolongation, viz. that which lies between lobe 1 and 2 of the hydrocoel and is distributed to arms 1 and 2 ; this lies in the interradixis of the axial sinus and stone-canal and is an evagination of the anterior coelom. Lastly the aboral sinus is an outgrowth of the left posterior coelom, from which it becomes completely cut off in the adult (p. 146). The right posterior coelom remains much smaller than the left. It persists in the adult, as the part of the body-cavity (sometimes called epigastric) which lies between the gut and the aboral body wall of the starfish. It is prolonged into each arm as the space between the two mesenteries of each pyloric caecum, Z III L FIG. 101. Longitudinal horizontal section of a larva of Asterina gibbosa showing the origin of the hydrocoel and the relation of the coelomic sacs (after MacBride). a anterior body-cavity in the preoral lobe ; Ipc left pos- terior coelom ; Ihy 1. 2. 3 lobes of the hydrocoel, no. 1 being the most dorsal, no. 3 the posterior, no. 5 (not shown) is at the other end of the incipient ring and is the most ventral ; or.coral coelom ; rhy right hydrocoel ; rpc right pos- terior coelom. 146 PHYLUM ECHINODERMATA. so that these mesenteries are really remains of the septum between the right and left posterior coeloms. In the central region of the disc this septum seems to have disappeared in the adult. The anterior coslom, with the atrophy of the preoral lobe, becomes much reduced in size. As stated above, it persists as the axial sinus. The inner perihaemal ring is an outgrowth of the axial sinus, with the ventral end of which it remains continuous (Fig. 105, int.p.r.). The axial organ makes its appearance as a ridge projecting into the axial sinus and containing jelly, fibres and leucocytes. Later there is formed a thickening of the epithelium of the left posterior coelom near the aboral end of the incipient axial organ. The cells of this thickening are the primitive germ-cells. They become invaginated into the septum separating the axial sinus from the left posterior coelom, and thence grow out in two directions, viz. (1) into the ridge of the axial organ forming its core, and (2) as a cord extending in a direction parallel to the surface of the disc and constituting the generative rachis. The generative rachis becomes enclosed by a flap of peritoneum in a space cut off from the left posterior coelom and known as the aboral sinus. It forms an aboral ring on the dorsal side of the stomach and sends off, as it passes each interradius, two branches enclosed in corresponding branches of the aboral sinus. These branches end in swellings which become hollowed out, acquire a communication with the exterior and form the generative glands. The portion of the aboral sinus round the gonads be- comes cut off by a septum from the rest. The metamorphosis of Asterina gibbosa begins on the eighth day of its development. The larva fixes itself by its preoral lobe to the substratum by means of a thin mucilage which appears to be secreted by the adhesive disc, and it remains attached during the whole of the metamorphosis (Figs. 102 to 104). The following changes occur : (1) The constriction of the body into disc and stalk, the latter being formed from the preoral lobe. (2) The sharp flexure of the disc on the stalk, the disc being bent obliquely and to the left, so that the left side of the body is turned towards the substratum. (3) The prepon- derating growth of the organs of the left side, the left posterior coelom and the left hydrocoel having both sent out dorsal and ventral horns, which meet so as to form complete circles, while the right hydrocoel and the right posterior coelom remain small. (4) The gradual atrophy of the stalk. (5) The out- METAMORPHOSIS OF ASTERINA. 147 growth of the adult oeso- a phagus and the formation of the new mouth on the left side. Even before the meta- morphosis the lobes of the hydrocoel are visible on the left-hand side of the larva in a curve open anteriorly (Fig. 102), the dorsal lobe being no. 1, the posterior no. 3 and the ventral no. 5. On the right side are visible the first rudiments of the arms, lettered A-E, the dorsal being A and the ven- tral E ; these arm rudiments are developed over the left posterior coelom. The open curve of the hydrocoel eventually closes into a circle, the two ends coming together round the base of the preoral lobe (stalk), so that the preoral lobe arises from the oral surface of the developing disc (Fig. 105). In this closure the rudiment of arm E shifts so that it comes to lie directly on lobe no. 1 of the hydrocoel. When this fitting together of the two surfaces of the future disc is complete, the oral and aboral sur- faces of the starfish are fashioned, and it is seen that the oral surface is derived from the left and slightly ventral side of the larva, the aboral from the right and slightly dorsal side. Meanwhile the first cal- FlO. 103. Larva of Asterina gibbosa of T T i n nine days from the right side. The CareOUS plates are laid down Oil letters denote the rudiments of the j_\ i i arms, the arabic numerals the lobes of tne aboral Surface as SllOWn in the hydrocoel. The first formed tube- -pi- 1A/1 75 .C snip. Lees. ..../' snt.C FIG. 105. Longitudinal vertical section through a metamorphosing larva of Asterina gibbosa (after an original drawing by E. W. MacBride). The section passes through the interradius of the preoral lobe and through the opposite radius, and shows that the preoral lobe of the larva is attached to the oral surface of the adult, ad. in. adult mouth ; amp. ampulla of a tube-foot; ant.c. anterior coelom in the preoral lobe; ii/t./i.r. internal perihaemal ring- canal ; l.oes. vestige of larval oesophagus; l.p.c. left posterior coelom; p.or.c. perioral coelom; pyl. pyloric sac; r. />./. right posterior coelom; slum, stomach; t.f. tube-foot; w.v.ring water-vascular ring canal. completely double (Fig. 106), whereas in Asterina the anterior coelom is single from the first, the posterior alone being double. Later, after the division of the coelom into anterior and posterior in bipinnaria, the two COMPARISON WITH ENTEROPNEUSTA. 149 anterior coeloms unite in the front part of the preoral lobe to form a single cavity.* In bipinnariae with brachiolar arms, which are it will be remembered processes of the preoral lobe, temporary fixation f during development takes place by these arms. During the metamorphosis of bipinnariae fixation occurs by means of a median oval sucker-like disc on the preoral lobe (p. 141). Attachment by the preoral lobe has also been noticed by Perrier in Asterias spirabilis, in which the larvae adhere to the buccal membrane of the mother. From the above account f it is clear that in the larvae of Aste- roids which become attached dur- ing development, the fixation is effected by the preoral lobe, and that the stalk so formed is sur- rounded by the hydrocoel and springs from what will become the oral surface of the starfish. The knowledge of this fact, so impor- tant for a proper comprehension of the morphology of the class, we owe to MacBride. The further statement may be made that the coelom, which arises by a single diverticulum from the enteron, becomes segmented into three pairs of chambers, viz;, the anterior coelom, the hydrocoel, and the posterior coelom. Of these the anterior coelom is at first single or soon becomes so, and, as in the Enteropneusta, acquires an opening or two openings to the exterior (water pores). Of the second pair the chamber of the left side becomes much larger than that of the right, retains its connexion with the anterior chamber by the stone-canal, and becomes the hydrocoel, while the right remains as a small apparently functionless sac ; this pair of chambers may be compared to the collar-cavities of the * There are according to Field (op. cit. ) two water-pores in many of the bipinnariae of Asterias vulgaris, but it is doubtful if this can be regarded as a normal occurrence (see note, p. 166). t Bury, op. cit., 1895 ; Delage, Arch. Zool. Exp. (4), (2), 1905, p. 27. f The preceding account of the development of Asterina is taken largely (often directly quoted) from the important work of MacBride (loc. cit.). 4-f \- FIG. 106. Optical section of an Asteroid larva (Bipinnaria) seen from the dorsal surface (after Bury). There are two anterior coelomic sacs, the water pore is shown opening into the left of them. The hydrocoel is indicated but not yet completely separated from the left anterior coelom. 1 left anterior coelom ; 2 water pore ; 3 hydrocoel : 4 second terminal plate (arm B) ; 5 left posterior coelom ; 6 mesentery separating the two posterior coeloms ; 7 right posterior coelom ; 8 right anterior coelom ; 9 euteron. PHYLUM ECHINODERMATA. Enteropneusta, which in the Cephalodiscida furnish the tentacles. The chambers of the third pair both persist and furnish the perivisceral cavities of the adult, but the left be- comes, in accordance with the predominance of the organs of the left side of the larva, much larger than the right, and alone furnishes the germ cells from its lining. The knowledge of these facts we owe mainly to Bury and MacBride. Bury was the first to grasp the importance of the anterior coelom and its trans- ference into the axial sinus, while MacBride went a step further in showing that the coelom was segmented into three chambers on each side, though Metschnikoff * preceded him in having demonstrated the presence of the right hydrocoel in Amphiura squamata. In Echinoids (Fig. 99) the single enterocoel divides into two, one on each side. Each of these again di vides into two, the hindermost of which lie at the sides of the stomach and constitute the posterior coeloms. The left anterior division develops a water-pore, which is placed on the left side of the dorsal surface, and then becomes constricted into two parts, of which the anterior retains the water-pore and persists into the adult as the madreporitic ampulla and axial sinus, corresponding to the left anterior coelom of Asterids, while the posterior becomes the left hydrocoel. The separation between these two vesicles appears not to become complete, the connecting tube persisting as the stone-canal. The left hydrocoel has at first the form of a disc which is soon transformed into a ring (ap- parently by bacoming notched on one side) through which the adult oesophagus later grow s. The right anterior division likewise constricts into two parts, but does not develop a water-pore. The anterior of these is the right anterior coelom ; its fate is unknown. The posterior portion is the right hydrocoel, which remains small, never develops lobes, and persists into the adult as the " dorsal sac " which lies beneath the madre- porite. The segmentation of the coelom therefore proceeds in a very similar manner to that of Asterids, and as in them the segmentation of the left side precedes that of the right. The two posterior coeloms give rise to the general perivisceral cavity. The lantern coelom, which is homologous with the outer perihaemal ring of Asterids, develops as five evaginations of the left posterior coelom. The teeth and jaws are developed from the walls of these pockets and the radial perihaemal canals are outgrowths of them. The genital rachis, genital organs and aboral sinus are developments of the left posterior coelom, exactly as in Asterids. An invagination of ectoderm which becomes closed is formed on the left-hand side of the larva. Its cavity is known as the amniotic cavity, its outer wall becomes thin and is called the amnion, while its inner wall or floor applies itself to the hydrocoel and forms the ambulacra! surface of the adult. The epineural canals are developed as imaginations of the floor of this cavity. There is no fixation of the larva during the metamorphosis. Lastly it must be men- * Studien iib. d. Eat. d. Echinodermen u. Nemertinen, Mem. de VAcad. de. St. Petersbourg, 14, 1869. DEVELOPMENT OF OPHIUBIDS AND HOLOTHURIANS. 151 tioned that there is a well-developed nervous system in the larva of Echinus. It has the form of an apical plate of neuro-epithelium, placed on the preoral portion of the body dorsal to the ciliated band and between the preoral arms. It is not recognizable till the larva is three weeks old. In the metamorphosis the animal falls to the bottom, the amnion rup- tures and shrivels up, the larval arms are absorbed by phagocytic amoe- bocytes, the larval mouth and anus close and the somewhat spherical form of the adult is assumed. At first the metamorphosed animal is with- out a mouth and anus ; these soon appear, the mouth first. The anus is formed in the centre of the antambulacral surface which is at first equal in area to the ambulacral. In Ophiurids a single coelomic sac is budded off from the anterior end of the archenteron. This soon divides into a right and left sac. Each of these divides into anterior and posterior coeloms. The hydrocoels FIG. 107. Longitudinal vertical sections of the two stages of the larva of Synapta digitate showing the formation of the enterocoel and of the primary water-pore P. Bl blastopore (after Selenka). then arise from the hind end of the anterior coelom in the usual way (MacBride) and the left anterior coelom develops a water-pore. The left hydrocoel alone acquires lobes and develops into the water-vascular system. As in Asterina the rudimentary right hydrocoel in abnormal specimens occasionally acquires a form similar to the left. In Holothurians the single pouch acquires an opening to the exterior by a pore placed on the dorsal surface and just to the left of the middle line. This is the primary water-pore. In Synapta it may even be formed before the enterocoel has separated from the enteron (Fig. 107). Soon after the formation of the pore, the enterocoel divides into two portions, an anterior and a posterior ; the anterior remains in connexion with the water-pore and constitutes the combined anterior coelom and hydrocoel, while the 152 PHYLUM ECHESTODERMATA. posterior is the splanchnocoel. The splanchnocoel then divides into two sacs which apply themselves to the gut, one on the right and the other on the left, and give rise to the perivisceral cavity and its lining. The tube connecting the combined anterior coelom and hydrocoel with the water- pore elongates and a small swelling appears on its 5 anterior wall (Fig. 108). This is supposed to be the reduced anterior coelom. The vesicle itself becomes lobed and forms the hydrocoel. It eventually surrounds the oesophagus to form the circumoral water-vascular vessel and gives off five outgrowths which become the radial canals. The canal connecting the hydrocoel with the small anterior coelom must be regarded as the stone-canal. In the forms with a so-called inter- Fio. 108. Diagram of the na [ madreporite, it must be supposed that the hvdrocoel and anterior , . , , /. ., n coelom of an old Holo- canal (water-pore) distal 01 the small anterior thurian auricularia (after coelom breaks down and that the anterior coelom Bury). 1 water-pore ; 2 .^.-^1^1 i pore canal ; 3 sand-canal ; acquires a tree communication with the general 4 polian vesicle ; 5 an- perivisceral cavity by the rupture of its walls, as it terior coelom. j n -j / i KO\ does in Urmoids (p. 158). Development of Crinoids. The development of Crinoids differs considerably from that of the other classes and requires separate treatment. The principal points of difference concern (1) the form of the larva ; (2) the relation of the larval preoral lobe to the adult surfaces ; (3) the fact that, though the hydro- coel at first undergoes a displacement to the left side, the oral surface of the adult is the posterior surface of the larva and the left posterior coelom does not exceed the right in size ; (4) the posterior coeloms arise from the enteron independently of the common rudiment of the anterior coelom and hydrocoel ; and (5) the absence of any trace of a right hydrocoel. In Antedon, the only Crinoid the development of which is known, the egg, as in most other Echinoderms, is fertilized and undergoes its whole development in the sea-water, but it remains for some time within the vitelline membrane attached to the pinnules of the parent. The total cleavage leads to the forma- tion of a hollow blast osphere from which a gastrula arises by invagination. The blastopore closes completely at or near the hind end of the embryo, and the uniform ciliation gives place to five ciliated bands which encircle the body transversely and to a ciliated tuft at the anterior end (Fig. 109). The ciliated tuft springs from a thickened patch of ectoderm which con- stitutes a neural apical plate. In the deeper layers of this neural plate nerve-cells and fibres are formed and constitute the larval DEVELOPMENT OF ANTEDON. 153 C.T nervous system. The embryo usually leaves the egg-membrane on the seventh day, and becomes the free-swimming larva, which shows bilateral symmetry, the ventral surface being slightly flattened. The anterior ciliated ring is incomplete ventrally, and between the second and third, which are separated by a wider interval than the others, there is a ciliated depression (Lm), called the vestibular depression (so-called larval mouth) and supposed to correspond with the now closed blastopore. On the ventral surface between the first and second rings, there is a small pit, the adhesive pit (6V), by the secretion of which the larva, after a free-swimming life of from 12 to 48 hours, attaches itself. After attachment the larva at first lies with its entire ventral surface turned towards the surface to which it is attached ; soon however it erects itself and projects at right angles to the substratum. The attached, i.e. anterior, end of the larva now be- comes narrow and elongated into the stalk, while its free, i.e. posterior, end becomes broader and constitutes the rudiment of the calyx (Fig. 115). The vestibular depression has during these changes become cut off from the ectoderm and forms a closed ecto- dermal vesicle (Fig. 114, 4), which constitutes the larval vestibule. At first this vesicle is placed on the ventral surface, but soon it comes to occupy the free end (Fig. 115). This change in posi- tion is shared by certain internal organs, and is doubtless due to the relative growth of parts by which the ventral surface of the larva comes to occupy the free end. The floor of the vestibule applies itself to the adjacent internal organs and eventually becomes perforated by the mouth opening. Mean- while processes from the water-vascular ring, the tentacular canals, push before them the floor and project as tentacles into the cavity of the vestibule. Eventually the roof of the vesti- Flo. 109. Larva of Antedon rosacea with ciliated bands, anterior tuft of cilia and rudiments of the skeletal plates (from Korschelt and Heider). Gr. adhesive pit by which the larva attaches itself ; Lm. the vestibular depression (larval mouth). 154 PHYLUM ECHINODERMATA. bule disappears and the tentacles and mouth become freely exposed. We thus reach the so-called Cystid stage of develop- ment (Fig. 110). The larvae of Cri- noids, then, become attached by the ventral side of the anterior end, and, as was shown by Bury, the stalk of the adult is a de- velopment of the preoral lobe. They therefore resemble the larvae of Aste- roids in the fact that attachment takes place by the preoral lobe, but differ entirely from them in the relation which the preoral lobes bear to the arrangement of the organs in the adult ; for whereas in Aste- roids the preoral lobe is encircled by the water- vascular ring, and its with- ered vestige springs from the oral sur- face of the adult disc (Fig. 105), in Crinoids it is quite FIG. 110. Cystid larva of Antedon (after Thomson). free of the circum- oral vessel and arises from the apical or aboral surface of the adult. Our knowledge of the development of the coelom is mainly due to Bury, who in his memoir on the development of DEVELOPMENT OF ANTEDON. 155 Antedon* first made us familiar with the conception of the ante- rior coelom and so paved the way for the modern views, largely due to him and MacBride, on the segmentation of the coelom into three chambers on each side of the body (p. 149). B _-*r"v VT mes-. A> <^.: \ ^S?BB3j3 $T $#. * M<^&iM -mot _sr t.1 Fia. 111. 4 longitudinal vertical section of gastrula of Antedon at the end of the second day showing the formation of mesenchyme. bl blastopore (after Bury). B longitudinal section of a later stage showing the division of the archenteron. mes mesenchyme ; ent first-formed enterocoel which gives rise to the right and left posterior coeloms (after Barrois). After the closure of the blastopore the archenteron which is placed at the hind end of the embryo divides by a constriction into an anterior and a posterior portion (Fig. Ill B). The posterior portion is the first enterocoel vesicle ; it lies close to the hind end of the embryo and soon divides into a right and left part, which constitute the right and left posterior coeloms respec- tively. The anterior portion or vesicle into which the archenteron has divided de- velops two outgrowths, a dorsal and a ventral (Fig. 112, 5, 7), which give origin to the intestine ; and itself becomes constricted into two portions, a ventral one which is the rudiment of the hydrocoel, and an anterior one which is the anterior coelom (Fig. 112). These become separated, in * Phil. Trans., 179, 1888. ia. 112. Posterior end of an embryo of Ante- don of sixty hours, from the right side. 1 the outline of the right posterior coelom ; 2 rudi- ment of anterior coelom ; 3 rudiment of hydrocoel ; 4 mesenchyme ; 5 ventral, 7 dorsal part of enteron, which clasps 6 the still persistent connecting portion between the incipient right and left posterior coeloms (from Lang, after Seeliger). 156 PHYLUM ECHINODERMATA. continuity with one another, from the rudiment of the intestine. Soon after this has happened the anterior coelom separates from the hydrocoel, which, placed between the enteron and the ectoderm on the left-hand side of the body (Fig. 113), at once acquires the characteristic horseshoe form. The anterior coelom then acquires its external opening, the water-pore, which is placed on the left side of the body just in front of the fourth ring of cilia. Later the hydrocoel develops an anteriorly directed outgrowth which acquires an opening into the anterior coelom and forms the stone-canal. The posterior coeloms now shift ; the left-hand one moves posteriorly and comes to lie like a cup over the hind end of the enteron (Fig. 113, <5), while the right sac extends ante- riorly and following the enteric wall reaches on to the left side (Fig. 114). The right posterior coelom gives off from its anterior end five forwardly directed diverticula (Fig. 113, 2). FIG. 113. Longitudinal-vertical section of a free- Tlif>f> hppnmp nut nfif from swimming larva of Antedon.. 1 stem-joints; 2 anterior prolongation of right posterior coelom if a f r, lofor eta o-p anrl to form the chambered organ ; 3 right posterior ] coelom ; 4 enteron ; 5 left posterior coelom 0iyp rise to the Chamb8r6d 6 hydrocoel ; 7 anterior coelom. organ. The calcareous plates make their appearance in the embryo on the sixth day. They are shown in Fig. 114, which however is taken from a larva after attachment. There are five orals (Fig. 114, or) arranged in a horseshoe curve near the posterior end. The horseshoe is set obliquely to the long axis, its dorsal end being posterior to its anterior end, and the open end of it is directed ventrally. Parallel to this row, but anterior, are the five basals (ba) set in a similarly disposed horse- shoe curve. Both the orals and basals are, as shown by the later development, interradially placed. In front of the basals ---.-6 DEVELOPMENT OF ANTEDON. 157 but deeper and in the axis of the body is a row of stem-joints. The anterior of these is the dorsocentral (Fig. 114, 7), which becomes the terminal joint of the stem. The stem-joints rapidly increase in number, the new pieces being added at the posterior (proximal or calycine) end of the row. A little later, on the seventh day, the underbasals (ib), three in number (rarely four or five), are formed. They lie in front of the basals and a good deal deeper and eventu- ally fuse with the top (posterior) stem joint to form the centro- dorsal plate. At about this stage the larva hatches and undergoes its brief free-swimming life. It then attaches itself (p. 153), the ciliary rings, preoral tuft and apical plate atrophy, and the anterior end begins to become narrower and longer, and to mark itself out as the stem from the pos- terior end, which becomes the calyx. In fact the larva be- comes club-shaped, the swollen free end forming the rudiment of the calyx (Fig. 114). The vestibular depression (p. 153) on the ventral surface becomes deeper and converted into an ectodermal invagination which occupies the greater part of the ventral surface, remaining open for some time anteriorly. It eventually closes, and soon after attachment shifts on to the posterior end of the larva (Fig. 115, 5). The hydrocoel follows this shift of the vestibule and lies at the posterior end immediately beneath the floor of the vestibule (Fig. 114, 3). It is still an open horseshoe, the opening being towards the water-pore ; but its ends have approximated and its five lobes, each of which soon becomes trilobate, pushing up the ectoderm of the floor of the vestibule, project into the vestibular cavity as the primary tentacles. Five additional pairs of tentacles are formed later at the base of these (Fig. 115, 7). FIG. 114. Young attached larva o Antedon from the left side (from Lang, after Seeliger). The vestibule is closed. ba 1-3 basals ; or 13 orals ; ib under- basals ; 1 dorsocentral plate ; 2 anterior coelom ; 3 lobes of hydrocoel ; 4 vestibule ; 5 enteron ; 6 left, 7 right posterior coelom ; 8 stem-joints ; v. ventral ; d dorsal surface of the larva. 158 PHYLUM ECHINODERMATA. The mouth is formed as a funnel-shaped depression of the vestibule, which passes through the hydrocoel ring and opens into the enteron (Fig. 115, 8). On the shifting of the vestibule to the hind end, the oral plates which, like the basals, have become arranged in a circle, also shift backwards. They come to lie in the thin roof of the vestibule, and when the latter ruptures and splits into five lobes, each lobe contains one oral plate. The stage we have now reached is sometimes called the cystid stage (Fig. 116, b). It is characterized by having a mouth overhung by five oral plates, an absence of arms, and an anus which has been formed as a lateral perforation through the body wall outside the circle of the orals. There are twenty-five ten- tacles, which at first arose in five groups, but now all spring sepa- rately from the water - vascular ring. As to internal changes, we may mention that the mesenteries be- tween the right and left posterior coeloms, which, by shifting, have some time before become aboral and oral, break down so that the perivisceral cavity is a continuous cavity. Further, the anterior coelom loses its walls and becomes merged in the general body-cavity. The result of this is that the primary water-pore and the sand- canal, which at the previous stage both opened into the anterior coelom, now open directly into the body-cavity. The primary water-pore and the anus lies in the same interradius. Later each of the other interradii Flo. 115. Attached larva of Antedcm after the separation of the stalk and calyx (from Lang, after Seeliger). The calcareous plates are not shown. d dorsal, v ventral side. 1 right pos- terior (aboral) coelom ; 2 stomach ; 3 left posterior (oral) coelom ; 4 sac- culi ; 5 vestibule ; (i primary ten- tacles, 15 in number derived from the 5 lobes of the hydrocoel each of which becomes 3-lobed ; 7 the secondary interradial tentacles (in 5 pairs) ; 8 oesophagus ; 9 hind gut ; 10 axial organ ; 11 fibrous strands in the stalk. DEVELOPMENT OF ANTEDON. 159 acquires a water-pore and sand-canal which lead from their first appearance into the general body cavity. The genital stolon (axial organ) develops as a thickening in FlO. 116. Larvae of Antedon (after Thomson), a free-swimrning larva with rings of cilia; b attached cystid stage ; c older stage described as Pentacrinus europaeus with arms and cirri. the epithelial wall of the aboral coelom. The genital rachis is probably developed as an outgrowth of this. The larva becomes converted into the pentacrinoid larva 160 PHYLUM ECIIINODERMATA. (Fig. 116, c) by the formation of the arms, which grow out from the sides of the body aborally to the water-pore and anus and between the oral and basal plates. The result of their out- growth is that the oral surface of the body is much enlarged by the formation of the tegnien calycis beyond the circle of the oral plates which become reduced in size and eventually disappear. On the aboral side of the calyx, calcareous plates, the radials, are formed to support the growing arms. Finally the calyx be- comes detached from the stalk, and the free adult state is reached. The Crinoids, in so far as relates to the development of the coelom, differ from all other Echinoderms in the fact that the rudiment of the posterior coeloms is budded off from the gut independently of the anterior coelom and hyclrocoel. They resemble certain of the Holothurians in the fact that the anterior coelom becomes merged in the general body cavity, but differ from these in the retention of the water-pore. In becoming attached by the preoral lobe they resemble the larvae of certain Asterids, but they differ, as already explained (p. 154), from these in the relation which the attaching surface bears to the adult structure. Affinities. The fundamental fact in the moi-phology of the Echinoderms is the enterocoelic origin of the coelom. In this, as has been already pointed out in the second volume of this work (chap, i., p. 7) they are associated with the Brachiopoda Chaetognatha, Chordata and probably the Phoronidea. Whether we are to regard this fact as indicating affinity it is difficult to say. In the absence of evidence tending to unite any of these groups more closely or as closely with any other group of the animal kingdom, we may perhaps consider this common feature as sufficient justification for treating them in immediate succession to one another, but we must not attribute too much importance to it, for it is absent from both vertebrate and tunicate development, nor is it found in Annelids, Arthropods and Molluscs, the coelom of which is clearly the homologue of the coelom of enterocoelic forms. The question now presents itself, do the Echinodermata possess any features which enable us to associate them more closely with any particular phylum of the Enterocoela than with the others ? It has been pointed out by some zoologists, amongst whom I may specially mention MaoBride, that in the primitive disposition of their coelomic AFFINITIES. 101 sacs they present a certain resemblance to the Chordata. In all the members of that great group with the exception of the Tunicata, the coelom in its first state in the embryo presents traces more or less marked of three divisions : these are (1) the anterior or proboscis coelom, which in Vertebrata and Entero- pneustais single, in Amphioxus double, (2) the collar or middle coelom which is always double, and (3) the trunk coelom which is double and which in Vertebrata and Amphioxus becomes metamerically segmented. In Echinodermata we seem to be able to make out indications at least of a similar tripartite division. We have (1) the anterior coelom which is sometimes single (Asterina), sometimes double (Echinus], (2) the hydrocoel which is probably fundamentally double though in some cases only one hydrocoel sac is formed (Holothurians, Crinoids), and (3) the posterior coelom which is always paired. Of these the hydrocoel presents the peculiarity of growing out into ten- tacles a feature which is also presented by the middle division of the coelom in the enteropneust genera, Cephalodiscus and Rhabdopleura. But with these similarities we have to note certain differences. In the first place in the Chordata, in which the enterocoelic origin of the coelom is clearly presented, these three divisions of it always come off from the enteron separately, whereas in Echinoderms the enteron at most gives off only one pair of coelomic sacs. Further, whereas in the Chordata the middle coelom (collar) is never associated more closely with the anterior than with the posterior, in the Echinoderms it is always closely associated with the anterior coelom, being developed from it and remaining connected with it by the stone-canal throughout life. With regard to these differences we have only to say this : that they are differences such as we might expect from the greater remoteness of the Echinoderms from the Chor- data than of any of the Chordata from each other, but that they are not sufficiently great to put out of court the homologies suggested by the comparison. To turn to other points of resemblance : we have the resem- blance (1) in the central .nervous system, (2) in the skeletal system, (3) in the shifting of the mouth and in the asymmetry of the body, and (4) in the larval form. To take these in order : (1) In the Chordata, as is well known, the central nervous system z in M 162 PHYLUM ECHINODERMATA. never becomes separated by mesodermal tissues from the tract of ectoderm which gave it origin in the embryo. This is a feature of all Echinoderms in so far as the ventral nervous system, which is the predominant central nervous system, is concerned. When this nervous system is removed from the sur- face, the removal is effected by invagination (p. 124). (2) The presence of calcined skeletal tissue in the meso- derm of the body wall is a character found in Echinodermata and Vertebrata alone among Coelomata. This has already been pointed out by MacBride, and though not perhaps a very important indication of affinity is one which from its rarity deserves mention here. (3) In all Echinoderms the mouth shifts from the ventral surface of the larva on to the left side of the body. This can be demonstrated in all classes except Crinoids, and in Crinoids it may fairly be inferred. In Chordates a similar though not identical phenomenon is presented by > Amphioxus. In this animal the mouth actually makes its appearance on the left side in an animal otherwise bilaterally symmetrical, but the phenomenon differs from that of Echinoderms in the fact that the left-sided position of the mouth is not preceded by a con- dition in which it is in the middle ventral line. The feature then which Echinoderms have in common with Amphioxus is the sinistral position of the mouth. Here again we have a character which strikes us from its very rarity, for it is found in no other Coelomate nor so far as we know in any other mem- ber of the animal kingdom. It also strikes us by its strangeness and inexplicableness. In Amphioxus no serious attempt has been made to explain it. In attempting to explain peculiarities of this kind we are accustomed to take into consideration two factors which must be kept distinct. Firstly we have peculiarities in habit, secondly associated peculiarities in other organs. Now in Amphioxus the asymmetry of the mouth is accompanied by no peculiarity in habit, for the animal while it has this monstrous mouth behaves more after the fashion of a bilaterally symmetrical animal than it does in later life, when the mouth has acquired a more median position and it has taken to burrowing in sand. Nor do the peculiarities in some of the other organs lend us any assistance, for no one, so far as we know, has ever attempted to bring the RADIAL STRUCTURE : ASYMMETRY. 163 extraordinary features in the development of the gill clefts, of the endostyle, of the head-cavities, the asymmetric positon of the anus and olfactory pit, into relation with the asymmetry of the mouth. The thing cannot be done. There is no sort of con- nexion between these various asymmetries. They seem to> occur without rhyme or reason. The mouth which should be- a median structure is from the first on the left side ; the gill- clefts, which are on the left in the adult, appear in the median line and at once pass on to the right side ; the endostyle which is a median structure in the adult appears as an entirely dextral organ. In Echinoderms on the other hand the asymmetry of the mouth is accompanied by changes in habit and by change of other organs which seem to be connected with the change in the mouth. The animal here becomes sessile or semi-sessile and acquires an entirely different symmetry in which other organs of the body participate in an intelligible manner. But though we can understand to a certain extent that the shift of the mouth might indent the left hydrocoel and bring about an inequality in the posterior coeloms, no adequate attempt has ever been made to show how the sessile habit and the radial structure is connected with the shifting of the mouth on to the left side. We have here three factors : the sinistral mouth, the radial structure and the sessile habit. Can these factors be brought into the relation of cause and effect ? (1) Can the sessile habit be regarded as the cause of the other two, even if we accept the view that all Echinoderm classes have passed through a fixed stage in their phylogeny. We can only point out in reply that no such results have followed fixa- tion in any other group of the Coelomata : they have not followed in Cirripedes, Brachiopods or in Tunicates. (2) Can the left-handed mouth be regarded as the cause ? In Amphioxus, the only other animal in which the mouth is sinistral, it is accompanied neither by the sessile habit nor by the radial symmetry. (3) Lastly, can the acquisition of radial symmetry, to whatever cause due, have brought in its train the shifting of the mouth and the sessile habit ? In the only other animals which can lay claim to a radial symmetry, the Coelenterata, no such result has followed. 161 PHYLUM ECHINODEEMATA. It is therefore no more possible to explain the sinistral position of the mouth in Echinoderms than it is to account for the same phenomenon in Amphioxus. But that conclusion does not in any degree diminish the importance of the character as an indi- cation of affinity. On the contrary it increases it. For, if it cannot be shown to be connected with habit of life or with other peculiarities of structure in the animals presenting it, the pre- sumption that it is a property which was possessed by the ancestral matrix from which Echinoderms and Amphioxus have emerged is increased. We are thus brought back to the ques- tion which we touched upon on p. 116, are the Echinoderms descended from asymmetrical or from bilaterally symmetrical forms ? This discussion of the asymmetry of Amphioxus and Echinoderms has elicited facts which are not without a bearing upon this question. We have seen that in Amphioxus there is hardly a single organ of the body which displays complete bilateral symmetry at all stages of existence, and in the adult traces of this asymmetry slight traces, it is true, but all the more striking on that account are present (position of olfactory pit and anus just to the left of the middle line, preoral hood and other small distortions). We have also seen that the asymmetry of one organ is entirely independent of the asymmetry of the others. Very similar statements may be made about Echino- derms : in these also development begins with a transitory bilateral symmetry which is almost at once followed by asym- metry, at least of the internal organs, and the asymmetry then initiated is never completely got rid of, for the radial symmetry of the adult is in all classes imperfect (least so in Holothurians), and some of the adult distortions, such as the position of the anus, recall the similarly slight distortions found in the adult Amphioxus. Now the upshot of these considerations is to make us pause in accepting as final the conclusion that the ancestral Echinoderm was a bilaterally symmetrical animal. (4) The striking resemblance of the bipinnaria larva to the tornaria of Enteropneusta has already been referred to (p. 99). It is impossible to estimate its value, but it clearly cannot be passed over in a discussion of this kind, and taken in conjunc- tion with the other facts mentioned must be admitted to have considerable weight. We have now passed in review all the points of resemblance AFFINITIES : PHYLOGENY. 165 between the Eehinodermata and the Chordata and we have seen that they all, except possibly the last, are concerned with fundamental, not superficial, traits. Some of them link the Echinoderms to all the Chordates or to all except the Tunicata, e.g. the relations of the central nervous system and the general relations of the coelomic sacs ; another they hold in common only with Vertebrata, viz. the presence of calcified plates in the mesoderm of the body wall ; a third the sinistral position of the mouth is found again only in Amphioxus ; and lastly there is the striking resemblance to Enteropneusta by the tornaria larva. The accumulated weight of these facts is over- whelming and leaves us no choice but to consider not only that the Chordata are the nearest allies of the Eehinodermata, but that the Eehinodermata are of all Coelomata the nearest to the Chordata. 8 '-7 '4 '5 Fia. llGbii. Diagrammatic representation of the supposed Dipleurula ancestor of Echino- derms, seen from the left side, with the ventral surface towards the substratum (after Bather). 7 right and left water-pore; 2 preoral lobe ; 3 nerve plate of preoral lobe; 4 anterior coelom of the left side ; 5 month ; 6 left posterior coelom ; 7 anus ; 8 right posterior coelom ; 9 right hydrocoel ; 10 left hydrocoel. We now come to a consideration of the so-called Dipleurula, a hypothe- tical form which has been imagined by some zoologists as the bilateral ancestor of all Echinoderms. The structure of this hypothetical animal will be understood at a glance from an inspection of Fig. \\Qbis, which represents it as seen from the left side with its ventral surface turned towards the substratum. It is a bilaterally symmetrical animal with a preoral lobe (2) carrying a nervous plate (.3), a ventral mouth (5), a terminal or ventral anus (7), three coelomic vesicles on each side (4, 10, 6), and two water-pores (1). How has this form been arrived at ? It is not arrived at by selecting features common to all the free-swimming bilateral larvae (sometimes called Dipleurula larvae) of Echinoderms ; but by picking and choosing from among the characters of different larvae those which, according to the preconceived ideas of its authors, the common ancestor might be supposed to have possessed, and adding one or two characters which none of them possess. For instance, the preoral lobe is. very, small in pluteus larvae, though well developed in bipinnaria and- in the larva of Crinoids ; a preoral nervous system has been detected 166 PHYLUM ECHINODEEMATA. only in Crinoid and Echino-pluteus larvae, no Echinoderm larva has a right hydrocoel equal in size to the left, and in Crinoids and Holo- thurians there is no trace of a right hydrocoel at any stage of existence. Lastly, in no normal* Echinoderm does the right hydrocoel ever possess a water-pore. We do not wish to be unduly critical, but we think it not unreasonable to point out that, in the absence of any test which enables us to decide which characters are ancestral and which secondarily acquired, an ancestor constructed by this somewhat one-sided application of the recapitulation theory can have very little value in advancing zoological knowledge. We do not say that this kind of speculation has no value, for it is a source of delight and stimulus to many minds ; but we think that it is most important that its value should not be overrated and that it should not be allowed to divert attention from more important and more practicable problems. To continue the imaginary history of the dipleurula ancestor. The next change is due to its fixation, which is supposed by Bather to have taken place by the right side of its preoral lobe, though the fixation actual occurs in the middle line. This led to the passage of the mouth to the left side and to the establishment of the radiate structure of most organs except the genital and to the shifting and asymmetry of the coelomic sacs. This brings us to the so-called Pentactaea.t Now came the diver- gence into types. The Holothurian type in which the generative organs never acquire a radial arrangement, was the first to separate. In this type the attachment was entirely lost from the whole life history. Next, after the acquisition of radial structure by the gonads, the Asteroids and Echinoids separated off ; of these the Echinoids entirely lost their attach- ment, while the Asteroids appear to have retained it in some if not in all casss (larval attachment, p. 149). Lastly, or perhaps as a continuation of the main stem but little modified, came the Crinoids, in which the attach- ment is retained. This type further presents the following remarkable feature which may or may not have been primitive ; the mouth shifts from the left side to the hind end, where it lies alongside the anus. Class ASTEROIDEA J Star-shaped or pentagonal forms with the body flattened in the or-anal axis. The arms are not sharply marked off from the disc and have an ambulacral groove from which the tube feet project. The madreporite is on the abactinal surface. * Brooks and Field have asserted that in bipinnaria a second madre- poric pore normally occurs, but this statement has not been confirmed. f See Lang's Comparative Anatomy, Pt. 2, p. 548. j E. W. MacBride, " The development of Asterina gibbosa," Q.J.M.S., 38, 1896, p. 339. H. Ludwig, " Asteroidea " in Bronris Thierreich, Leipzig, 18948. Id., Die Seesterne des Mittelmeeres, Neapel, 1897. S. Goto, Metamorphosis of Asterias pallida, etc., Journ. Coll. Sc. Japan, 10, 1898, p. 239. Id., Metamorphosis of Asterina gibbosa, ibid., 12, 1898, p. 227. W. P. Sladeri, " Report on the Asteroidea," Challenger Reports, vol. 30, 1889. E. Perrier, " Echinodermes," in Exped. Sci. du Travailleur et du Talisman, Paris, 1894, and in Mission Scient. du Cap Horn, vol. vi., Paris, 1891. See also works of Lxidwig, Cuenot, Hamann, Delage et Herouard, loc. cit. ASTBROIDEA. 167 The form of body varies from that of a pentagonal disc in which the rays are only marked by the angles (Pentagon- asteridae, Fig. 117, Pterasteridae, species of Culcita), to that of a star (Fig. 139), in which the disc is small and the arms as sharply marked off from it as in the Ophiurids (Brisingidae). The number of rays varies in living forms from five to forty-five (Labi- diaster). Five is the most usual number and is especially constant in the discoidal forms and in those with well-developed marginal plates (Palmipes rosaceus is exceptional in having eleven and Culcita tetragona in having four). Four is sometimes found as an individual variation. The number of rays shows a distinct tendency to increase in families, in which the arms are long, the disc- small, and the marginal plates feebly developed (e.g. Brisingidae, Heliasteridae, Asterii- dae, Echinasteridae). When there are more than six rays individual variations in their number are fairly common. In Labidiaster in which the arms are very numerous, the number of them increases with the growth of the animal ; but in most if not all other cases the full num- ber is laid down in the embryo. The body is usually compressed dorso-ventrally. On the actinal surface, reaching the whole length of the radii and terminating in the centre of the disc in the oral area or depression, are the ambulacral grooves, and from them project the two, more rarely four, rows of J Fia. 117. Pent'tgonaster Parkinson. tube-feet. The mouth is placed Forbes (after Perrier). Seen from be- low and trom the side. on the actinal surface in the centre of the disc in the oral depression. There are no circumoral tentacles or tentacular prolongations of any kind round the mouth, but at the distal end of each ambulacral groove there is a red pigment spot which is called the eye or ocellus and over which projects an unpaired tentacle-like structure ; this is the ocular tentacle and contains the end of the radial water- vascular trunk. The dermal skeleton is well developed, and carries, especially on the abactinal surface, numerous spines and usually pedicellariae. The anus, which is absent in the Astropectinidae and probably in the Porcellanasteridae, is on 168 PHYLUM ECHINODEKMATA. the abactinal surface of the disc in interradius I. II, very nearly, but not quite, at the central point (Fig. 83). The flattening of the body is carried furthest in Palmipes membranaceus, which has the form of a pentagonal sheet of card- board. While the oral surface is generally flat, the aboral sur- face is often arched, sometimes considerably so (Pteraster, Hymenaster, Marginaster, Pentaceros, Culcita, many Solasteridae FIQ. 118.Echinastersentus from the oral surface (after A. Agassiz). mouth. A f ambulacra! feet. and Asteriidae). The rays are usually approximately equal in size ; inequality generally implies recent mutilation and re- generation. Starfishes vary in size from those with an arm-radius (centre of disc to extremity of ray), of 1 cm. or less (Marginaster pentagonus, '3 cm.) to those in which the same dimension measures 45 centimetres (Luidia savignyi to 37 cm., Freyella remex to 45 cm.). ASTEROIDEA. 169 The body- wall consists (1) of a single layer of ciliated columnar ectoderm, with a cuticle on the outer side and a basement mem- brane on the inner ; (2) of a dermis formed of a gelatinous matrix containing fibrillar connective tissue and calcareous plates and muscular elements ; (3) of a layer of ciliated peritoneal epithelium which lines the body cavity. In the ectoderm are sense-cells and gland-cells, and in some parts of the body nerve- fibres and nerve-cells are present in the deeper parts of the same layer. The integument is more or less hard and stiff owing to the presence of the calcareous plates. These structures may be regularly arranged and in contact with each other, or they may be irregularly disposed rods forming a kind of loose network, through the variously shaped meshes of which such delicate processes of the body-spaces as the tube-feet and dermal branch- iae project, or they may be isolated from one another. The calcareous plates may be deeply imbedded in the dermis and not visible from the exterior, or they may lie just beneath the epidermis, so that their shape is more or less completely dis- cernible in surface view. As a general rule some or all of the dermal plates bear granules, or processes and spines of various shapes, or pedicellariae. When these structures project from the surface, as they generally do, the skin has a rough or even spiny appearance ; but sometimes, especially when the plates lie deep, they project but little in the fresh state, and the skin is nearly smooth ; though even here, in dried specimens, the skin is rough and the spines are discernible from the surface (e.g. Tylaster willei, Porania, Culcita). In addition to these skeletal structures of the general in- tegument, small calcareous pieces are found in the walls of the tube-feet. The skeleton of the plates or ossicles falls under two heads, (a) the auibulacral, (b) the ambital. (a) The ambulacra! skeleton consists of the ambulacral and adambulacral ossicles, together constituting four rows of plates in each ray (Fig. 110). They form the roof and sides of the ambulacral groove (Fig. 121). The ambulacral ossicles (Fig. 119, A) are two rows of rod-shaped structures which meet and are articulated together in the middle line above (Fig. 121), and diverge from one another on each side, abutting at their outer ends upon the adambulacral ossicles (A'). These, which correspond in number with the ambulacral ossicles, though they usually alternate with them in position (Fig. 119), form the edges of the ambulacral grooves and carry 170 PHYLUM ECHINODERMATA. the adambulacral spines (Fig. 121, 8). The ambulacral ossicles are always without any form of spinous appendage, and the tubes connecting the ampullae and tube-feet pass be- tween them in such a way that never more than one tube passes between two successive ossicles. When these apertures are ar- ranged in a straight line on each side, as usually happens, there are two rows of tube-feet in each ambulacrum (Fig. 119) (biseriate arrangement). When on the other hand the apertures are alternately near to and remote from the middle line of the ray (Fig. 120), there appear to be four rows of tube feet (quadrise- riate arrangement, Asteriidae, Heliasteridae). In certain fossil starfishes (Palaeasteroidea) the ambulacral ossicles of the two sides are not opposite one an- other, but alternate. In living forms (Euasteroidea) they are opposite. The ossicles of both series increase in number with the growth of the starfish in such a way that the oldest, i.e. first formed, ossicles are next the mouth, while the youngest are at the end of the arm. The young- MV IG. 119. Diagram showing the skeletal pieces of the arm of a starfish with adambulacral peristom and biserial tube-feet, when viewed from the actinal surface (from Ludwig). .41, A2, etc., first, second, etc., ambulacral ossicle ; A'l, A' 2, A' 3, first, second, third adambula- cral ossicle ; inner intermediate piece (oral plate); MV inframarginal plates; T terminal plate; VI, V2, VZ ventrolateral plates. est ambulacral ossicle lies on the ventral side of the terminal plate. The superambulacral pieces, which may be mentioned here, are internally placed and connect the ambulacral ossicles with the marginal plates (Fig. 121, 5) ; they are found in the Astropectinidae, many species of Linckiidae, and in some species of the Porcellanas- teridae and Archasteridae. The first two ambulacra] ossi- cles of each side are more or less fused with one another (they are represented separate in the diagram) and form with the first of the adambulacral series the dental apparatus of the peris- tom. When the adambulacral pieces of this system are more prominent than the ambulacral, and form the buccal angles, the peristom is called adambulacral (Fig. 119) ; when, on the other hand, the .ambulacral pieces are the more prominent and the buccal angles are less FIG. 120. Diagram showing skeletal pieces of the proximal part of the arm of a starfish with ambulacral peristom and quadriserial tube- feet (from Ludwig). Al, A2, etc., first, second, etc., ambulacral ossicle ; A 1, A'2, A'3, first, second, third adambulacral ossicle ; o inner intermediate piece (oral plate). ASTEROIDEA. 171 FIG. 121. Diagram of a transverse section through the arm of Astropecten, the hepatic diverticula being omitted. 7 ambulacral, 2 adambulacral ossicle ; 3 lower, 4 upper marginal plate ; 5 superambulaeral ossicle ; 6 integument of abactinal surface ; 7 paxil- li ; S adambulacral spines ; 9 spines of the lower side of the ini'ramarginal plate ; 10 lower, 11 upper marginal spine ; a radial water-vascular vessel ; 6 so-called radial blood-vessel ; c radial nerve ; d ampulla of tube- foot ; e tube-foot ; / perivisceral cavity of arm. developed, we get the ambulacral peristom (Fig. 120). Whatever may be the condition in the adult, all young starfishes begin by possessing an ambulacral peristom. The adambulacral pieces of the peristom carry opines. The terminal plates may be mentioned here. They belong to the actinal surface, and one of them is found at the end of each arm on the abactinal side of the last ambulacral ossicles (Fig. 119 T). They are especially conspicuous and important when the skeletal plates are first making their appearance ; and it can then be seen that they are laid down round the left coelomic ;sac, i.e. on the actinal surface of the future star- fish (p. 147). (b) The ambital skeletal. The rest of the skeletal plates are classified as ambital. They consist of the interambulacral plates and the antambulacral. The interambulacral plates are of three kinds : ( 1 ) the inner intermediate pieces (o, Fig. 119), of which there is one in each interradius ; they lie on the abactinal side of the two adambulacral pieces which form the buccal angles ; (2) the ventrolateral plates, which lie between the adambulacral and the inferior marginals (V, Fig. 119); the ventro-lateral plates are often only found on the interradial portions of the actinal surface of the disc ; (3) the inferior marginals, which constitute a row of plates placed at the edge of the arm, just external to the ventro-laterals (Fig. 119, MV). The antambulacral plates (Fig. 122) constitute the bulk of the ambital skeleton. They consist of (1) the superior marginals '(M), which may either be in contact with the inferior marginals or separated by intercalated plates; (2) the eleven primary plates of the apical system, viz. five radial (R), five interradial (JR), and one central (C) ; (3) the secondary radial plates of the arms and disc (Rb and Rd) (carinalia of Perrier, medioradials of Sladen) ; these are often not distinguishable in adults from the other antambulacral plates, e.g. many Archasteridae, Porcellanasteridae, Asteriidae, Solasteridae, Echinasteridae ; (4) the dorsolateral plates (dl), corresponding to the ventro-laterals of the interambulacral skeleton ; (5) the supplementary plates of the arms and disc (s) which may effect the connexion of the dorsolaterals into a meshwork (hence reticularia of Perrier) ; (6) the madreporic plate (Md) placed in the left anterior in- terradius (the mouth being downward, and the anal interradius forward) ; the madreporic plate may be either the interradial plate of the apical system (JR, MD), or it may be outside this plate (Md), or it may be fused with it (Md 1 ). The primary plates of the apical system are quite distinct in the young starfish, but in later growth are only rarely distinguishable by size or form .(some Asterinidae and Pentacerotidae, many Pentagonasteridae) from 172 PHYLUM ECHINODERMATA. V ra the other plates of the antambulacral surface. Sometimes they remain throughout life as the sole plates of the antambulacral system (Cnemidaster wyvillei], and sometimes only a few other plates are developed between them (Neomorphaster talismani, Korethraster setosiis, different species of Marginaster}. The dorsolaterals may be connected into an irregular network, and fit closely into one another, and when the marginals are not dis- tinctly developed they pass gradually into the ventrolaterals at the sides of the arms. The external skeletal structures are spines, spinelets, scales, gran- ules and pedicellariae. Like the plates they are dermal structures and are covered by a layer of ectoderm. They are processes of or movably - attached to the subjacent skele- tal plates. The spines are elon- gated pointed rods, which project, usually singly, from the sub- jacent plates, to which they are attached by muscles and often by a ligament. They are found mainly on the larger plates of the abactinal surface and on the marginal plates. The spinelets are simply small spines. They often occur in tuft-like FIG. 122. Diagram of a starfish viewed from the abactinal side to show the antambulacral skeleton (after Ludwig). The arms are numbered i-v (see Fig. 83). A anus ; C central plate ; dl dorso-lateral plates of arms ; ill, I ditto of disc ; JR interradial plate of apical system ; JRA inter- radius in which the anus is placed ; JRM iuterradius con- taining the madreporite ; JR, Md madreporite and inter- radial as one and the same plate ; M marginal ; Mil madreporite separate from the interradial : Md' madre- porite as fused with the interradial ; R radial ; Rbl. rtr. secondary radials of arms ; Rdl, etc. secondary radials of disc ; s supplementary plates ; T terminal plate. aggregations. Some- times the plates carry- ing such tufts project in a column-like manner : such projecting columns with their tufts of spines are called paxilli * (Fig. 121). The spinelets are found principally upon the adambulacral plates and upon the actinal and abactinal intermediate plates ; they are attached to the subjacent * For a somewhat similar arrangement of spines in the Solasteridae and Pterasteridae, see p. 191. ASTEROIDEA. 173 plates by muscles and are covered with a glandular and nervous epider- mis. The scales are flattened spinelets. The granules are short rounded spinelets ; they occur set closely together on the skeletal plates. The pedicellariae are pincer-like calcareous structures consisting of two, rarely three, blades articulated to a plate in the clermis, and capable of executing snapping movements by means of a special set of muscles at- tached to their base. They are contained in the dermis and are covered FIG. 123. Pedicellariae of Asteroids (from Lang). A-F grouped spines resembling pedice lariae. G sessile pedicellaria of Gymnasteria carinifera (after Cuenot). H stalked straight pedicellaria, diagrammatic (after Cuenot). J basal piece of a stalked crossed pedicellaria of Asterias rubens (after Perrier). K stalked crossed pedicellaria of Asterits glacialis (after Cuenot). 1 calcareous blade of forceps ; 2 basal piec3 ; 3 occlusor muscle ; 3 axial muscle of the blades attached to the basilar piece and acting as an occlusor muscle ; 4 opening muscle ; 5 axial band ; in K this band divides, each branch being inserted into the base of one of the blades in such a way as to strengthen the grip when the pedicellaria is pulled ; 6 ectoderm ; 7 body wall ; S stem. by ectoderm. They may attain a size of 4 mm., but are usually much smaller. They are modified spines, and sometimes small groups of spines ar,e so associated that they can be moved towards one another and act like pedicellariae (Fig. 123, A-F). Pedicellariae are entirely absent in the genera Solaster, Echinaster, Cribrella, Mithrodia, Ophidiaster, Scytaster, Astropecten. Pedicellariae are of two kinds, sessile and stalked. Sessile pedicellariae (Fig. 123, (?) arise direct from the integument, to one of the plates of which 174 PHYLUM ECHENODEKMATA. they are attached. Stalked pedieellariae are at the end of a short soft stalk, the blades articulating with a calcareous piece at the end of the stalk (Fig. 123, H and K, 2). Stalked pedieellariae are either straight (forficiform) or crossed (forcipiform) (Fig. 123, H and K). In the forcipi- form pedieellariae the blades cross at their lower ends. Three-bladed pedieellariae constructed after the fashion of the straight variety are occasionally found. The blades of pedicellariae may be longer than they are broad, in which case they ar& said to be forcipate ; or they may be broader than they are long, in which case they are valvulate (Fig. 124). Valvulate pedicellariae are always sessile.. ^^^ Alveolate pedicellariae are sessile pedicellariae in which the 'blades are inserted into a depression or Fm. 124. Valvulate ,. , pedicellariae of alveolus in the calcareous plate. otalked pedicel- Culcita grex (after l ar i ae are confined to the Brisingidae, Pedicellas- IVrner). a one of .. . ' ... the five pedicel- teridae, Heliasteridae, Astemdae, Zoroastericlae, lariae shown in Stichasteridae, which are united together by Perrier the figure. as Forcipulata. Spines are movable : those at the edges of the ambulacral grooves, can be bent over the groove. Pedicellariae are no doubt related to such movable spines. However this may be, it is highly probable that they are defensive weapons, seizing and damaging other organisms which come in contact with them, and that they also serve for keeping the skin clear of foreign growths (see the account of pedicellariae under Echinoidea). The starfishes have some power of bending their arms and the tips are generally turned upwards so as to expose the eye to the light. Muscles of the body wall. Muscular fibres are found in the inner parts of the dermis near the peritoneum on the dorsal and lateral parts of the arm and disc. They are arranged in an inner longitudinal and outer circular layer. Some of the fibres of the circular layer pass outwards to be inserted into the cal- careous plates. The longitudinal layer is specially thickened on the abactinal side of the arms and disc to form a kind of longitudinal muscular band. These bands unite at a central point in the disc. Skeletal muscles. Special muscles passing between various, parts of the skeletal system are present. The most important are : (1) Muscles of the spines and pedicellariae, (2) muscles which pass between the two ambulacral ossicles of a pair, (3) muscles which connect the successive ambulacral ossicles. All three parts of the central nervous system are present, viz. the superficial, the apical and the deep oral (Fig. 125 and p. 123). The ectoneural tracts consist of a circumoral ring and of radial ASTEROIDEA. 175 nerve trunks, the whole lying in the deeper parts of the ecto- derm. It is in connexion with a diffuse ectoneural plexus found throughout the ectoderm, and at the mouth with an endoneural plexus. The apical or mesoneural nervous system and the deep oral system have already been sufficiently described (p. 123). a FIG. 125. Scheme of the nervous system of the arm of a starfish (after Cuenot). a wall, b body-cavity of arm ; c ampulla of tube-foot ; d tube-foot ; e radial canal of water- vascular system ; 1 radial portion of ectoneural central nervous system ; 2 ectoneural plexus of tube-foot ; 3 ectoneural plexus of skin; 4 Lange's nerve cords (deep oral) ; 5 mesoneural plexus just beneath the longitudinal muscle. The structure of the ectoneural plexus and its relation to the ectoderm are best seen by examining the annular (circumoral) nerve ring or the radial prolongations of it. The ectoderm here is much thickened and consists of elongated columnar cells with their nuclei near their outer ends ; the inner ends of the majority of the cells, which may be called supporting cells, taper and form a supporting tissue for a plexus of fine nerve fibres and scattered ganglion cells, which are here especially conspicuous and cause the thickening of the ectoderm. Some of the ectoderm cells are sense cells, and their inner ends do not form supporting fibres but branch out and join the nervous plexus. There are special aggregations of these round the ends of the tube-feet. The ectoderm along these central concentrations of the ectoneural sys- tem is much thickened, and its cells contain yellow pigment grains which give the whole tract in both its annular and radial por- FlG i26.Astroi>ecten aurantia- tions the appearance of a yellow streak. At the ends of the arms, this thickened tract of nervous ectoderm is continued over the terminal tentacle-like process (ocular tentacle), which forms the pro jecting end of the radial water-vascular trunk. The eye is on the oral side of the base of this tentacle ; it is coloured bright red and formed by a special thickening of the radial nerve tract Oc ctis, end of ray with the eye Oc surrounded by spicules (from Claus, after Haeckel). J76 PHYLUM ECHLNODERMATA. and subjacent connective tissue. In a typical case* each eye consists of a number (50 to more than 100) of funnel-shaped ectodermal pits, the cells of which contain the red pigment and end in a clear highly-refractile rod which projects into the cavity of the pit. The pit is closed towards the exterior by the cuticle, which may have on its inner side a lens-like thickening. The pigment cells are continued internally as fine fibres which join the nervous tissue of the radial nerve. The pits increase in number with the growth of the animal and they appear to contain a transparent gelatinous tissue. In some cases there are no pits and the pigment cells are distributed uniformly over the surface of the ocular cushion (Astro- pecten pentacanthus, miilleri}. The alimentary canal begins with a mouth which is placed in the centre of the actinal surface of the disc in the buccal membrane, and leads into a short oesophagus or directly into the spacious stomach (Figs. 127, 132). The oesophagus passes quite gradually into the stomach from the abactinal part of Nd FIG. 127. Longitudinal section through the disc and an arm of Solaster endeca (from Clans, modified after G. 0. Sars). mouth leading directly into the wide stomach ; A anus ; L hepatic diverticulum of the stomach ; G gonad ; Aid madreporite ; Js rectal caecum ; Aj tube-foot. which two caecal diverticula (hepatic) are given off into each arm. From the stomach a short rectum, which receives the rectal caeca, leads to the anus, which opens on the abactinal surface in interradius /. // (Figs. 83, 122). The buccal membrane is the part of the body-wall round the mouth in the oral depression and is devoid of calcareous structures ; the circular muscular fibres in it act as a constrictor and the longitudinal as dilators of the mouth opening. The oesophagus passes without any line of de- marcation into the stomach ; it is beset with ten glandular diverticula in Echinaster and Cribrella. The stomach is a spacious, thin-walled sac, divided by a horizontal fold (absent in the Astro jii'i-tiindae without an anus) into an oral (cardiac) and aboral portion (pyluric- s K ). Its walls are often considerably folded, so that it appears lobed. From its aboral portion the stomach gives off the hepatic or pyloric caeca (Fig. 128), one pair into each arm. These are tubular structures beset with numerous * Pfeffer, Zool. Jahrb. Anat., 14, 1901, p. 523. ASTEROIDEA. 177 secondary and tertiary glandular diverticula, and suspended to the abactinal body-wall of the arm by two mesenteries (Fig. 129, 24). The two caeca of a pair usually arise separately from the pyloric sac, but in some cases (Asterias, etc.) they are united near the stomach and arise from it by a common tube. The proximal end of each of these caeca gives off on its actinal side a pouch, called Tiedemanris pouch (specially developed in the Echinasteridae and Asterinidae). The abactinal side of the stomach is closely applied to the body wall, but between the two is a variable number of inter- radially placed glandular diverticula : these are the rectal caeca (Fig. 127, Js). They vary considerably in form, size and number (from two to five, in Luidia they are absent), and they open into the rectum except in the Astro pectinidae, in which the rectum and anus are absent and the caeca open directly into the stomach. The rectum is a short tube which leads from the stomach to the anus. The anus (p. 167) is absent in the Astropectinidae and Porcel- lanasteridae. The alimen- tary canal is lined throughout by a glandular ciliated epithelium, and, in some forms at least, a few calcareous bodies are found (Culcita, Ophidi- aster chinensis) both in its walls and in the mesenteries which attach it to the body-wall. The mesenterial attachments of the alimentary canal are as follows : ( 1 ) a pair of muscular strands run from the oral wall of the stomach in each radius to be inserted along the ambulacral ossicles of the ray (Fig. 132, ret) ; they serve to retract the stomach when, it has been evaginated for the prehension of food ; (2) a number of fine cords passing from the aboral wall of the stomach to the body-wall (Fig. 132, mes) ; these are remains of the septum between the right and left posterior coeloms of the larva ; (3) the mesenteries of the hepatic caeca (p. 177 and Fig. 129, 24) ; these fuse with one another distally, but proximally end freely so that the space between them is in open communication with the general body-cavity. They are probably remains of the mesentery which separated the right and left posterior body-cavities of the larva. On this view the spaces between the two mesenteries of a pair are parts of the right posterior body-cavity, and, if the division between the two body-cavities were retained in the adult, the two mesenteries of one caecum should join respectively the mesenteries of the caeca on each side of it in the disc, so as to cut off just above the stomach a circular patch of body-cavity with two prolongations into each arm. (4) Two bands which pass from the aboral end of the wall of the axial sinus to the stomach. These were formerly supposed to be processes of the aboral sinus. Z III N FlO. 128. Asterina gibbosa with the ahactinal body- wall removed (from Claus). Ld hedatic caecum G gonad. 178 PHYLUM ECHINODERMATA. The coelom. The relations and development of the coelom have been more fully elucidated in Asteroids than in any other class of Echinodermata. It represents the following parts : * (1) the general or perivisceral body-cavity, (2) the axial sinus and inner circumoral perihaemal ring, (3) the so-called outer circumoral perihaemal ring with its radial prolongations between IS 27 FIG. 129. Diagrammatic view of a transverse section through the arm of an Asteroid (fro 11 Lang). 1 trunks of the deep oral nervous system (Lange's nerves) ; 2 radial water-vascular trunk ; 3 tissue of so-called radial blood-vessel ; 4 radial nerve of the superficial oral system ; 5 radial perihaemal canal ; G and 7 branches of the same to the walls of the tube- feet ; 8 stalked pedicellaria ; 9 spine ; 10 genital aperture ; 11 papula with contained body cavity 13 ; 12 sessile pedicellaria ; 14 hepatic caecum; 15 peribranchial cavity ; 16 supra- marginal; 17 inframarginal ; IS adamlmlacral plate; 19 marginal canal communicating at 20 with the perivisceral coelom ; 21 peritoneal epithelium ; 22 genital sinus ; 23 gonad ; 24 mesenteries of hepatic diverticula ; 25 tube connecting tube-foot with 26 ampulla ; 27 cavity of tube-foot ; 28 upper and lower transverse muscles connecting the ambulacral plates; 29 branches of the deep oral nerve trunks ; 30 ambulacral plates; 31 perivisceral cavity ; 32 longitudinal muscle ; 33 apical nervous system. the ambulacral nerves and the radial water-vascular trunks, (4) the aboral sinus surrounding the generative rachis and the sinuses in the walls of the gonads, (5) the water- vascular system, and (6) the gonads. The general body-cavity which is derived from the right and * For the general relations and development of these structures the reader is referred to p. 126 et seq. and p. 142 et seq. ASTEROIDEA. 179 - c 4 left posterior body-cavities of the larva is a perivisceral space in relation with the stomach, rectum and pyloric caeca : it is a continuous space, is found in the disc and in the arms on the abactinal side of the ambulacral ossicles and is traversed by certain mesenteries which have already (p. 177) been described It is lined by a ciliated epithelium, and contains an albuminous fluid with floating amoeboid cells. The papulae or dermal branchiae (Fig. 129, 13) are thin projections of the body- wall found principally on the abactinal surface of the disc and arms and con- taining prolongations of the perivisceral cavity. Round the bases of the papulae there is generally present in the body- wall a space lined by an epithelium and supposed to be developed as a diverticulum of the perivisceral cavity. These St.... a FIG. 130. Diagrammatic transverse section through the axial sinus of a starfish (from Lud wig), a, b the lamellae of the inter- brachial septum ; C axial organ ; St stone-canal ; H axial sinus. The upper side of the section is the body-wall side. Flo. 131. Diagram showing the arrangement of the pcrihaemal spaces, etc., of a starfish, seen from the aboral side (after MacBride). a' axial sinus and inner perihaemal ring ; ab aboral circular sinus; yen.r generative rachis ; ov.g axial organ; ph. I. II . . . ph.IV.V the five parts of the so-called outer perihaemal ring; rhy vestige of right hydrocoel; lY the arms numbered. 180 PHYLUM ECHINODERMATA. spaces are called the peribranchial cavities (Fig. 129, 15, Fig. 132, pbr). The axial sinus is a tube passing from the abactinal to the actinal surface in the interradius of the madreporite (Fig. 132, ax.s). It is contained in an irrterbrachial septum (Fig. 130) and is lined by a ciliated epithelium. The stone-canal and axial organ project into it, being attached to its inner wall, i.e. to its wall next the oro-anal axis, by membranes (st.c). Orally the me* Mils st.c my tf&i.r y f "' h fl ft mv < -;'" ' ,jf> br ip.c. FIG. 132. Longitudinal vertical section through the disc and one arm of a young starfish (after an original drawing by Prof. E. W. MacBride, F.R.S.). The section passes through the interradius of the stone-canal and madreporite. ab aboral sinus ; ax.s axial sinus ; az.t terminal unpaired tentacle ; br papula (dermal gill) ; ext.p.r external perihaemal ring- canal ; gen.r genital rachis ; gen.st. genital stolon (prolongation of rachis into axial organ) ; int.p.r internal perihaemal ring-canal; l.p.c left posterior coelom of larva, hypogastric coelom of adult ; mes remains of the mesentery separating the left posterior (hypogastric) and right posterior (epigastric) coeloms ; mp madreporic pores ; mv madreporic vesicle (right hydrocoel) ; nr nerve ring ; oc eye ; pbr peribrauchial space ; per.rad.c radial peri- haemal canal ; p.or.c perioral coelom, an outgrowth of the hypogastric coelom, from its walls are formed the retractors of the stomach ; pyl pyloric sac ; pyl' the opening of a pyloric caecum ; rail. n radial nerve ; reel rectum ; ret retractor of the stomach ; r.p.c right posterior coelom (epigastric of the adult) ; st.c stone-canal ; stow, stomach ; tf tube-foot ; w.v.rad.c water-vascular radial canal ; w.v. ring water-vascular ring canal. axial sinus is continuous with the so-called inner perihaemal space which surrounds the mouth (Fig. 131, a l , Fig. 132, int.pr). At its aboral end it communicates with the stone-canal on the one hand and with the exterior through some of the pores of the madreporite on the other (Fig. 132, mp.). The axial sinus is derived from the anterior body-cavity of the larva. The outer circumoral perihaemal ring, which is separated by a slightly oblique septum from the inner perihaemal ring just described, is not really a ring at all. It is (Fig. 131, ph. I. II. etc.) ASTEROIDEA. 181 made up of as many interradial pieces as there are arms, each piece being prolonged into two adjacent arms. It thus comes about that each arm has a prolongation of two pieces of the sa-called outer perihaemal ring, the septum between these two radial prolongations being the septum (Fig. 129, 3) which divides the radial perihaemal canal into two parts and in which the so-called radial blood-vessel runs. This outer circumoral peri- haemal ring is derived for the most part from the left posterior body-cavity of the larva (see p. 145). It is lined by a flat epithelium, whether ciliated or not is unknown. There is said to be a canal in the body-wall at the edge of the ambulacra! groove which communicates at intervals with the radial perihaemal space and with the perivisceral cavity (Fig. 1'29, 19). The aboral sinus (Figs. 131and 132a&) is a circular or penta- gonal sinus placed on the aboral side of the stomach between it and the skin, and giving off in each interradius two prolonga- tions * to the generative organs (gen.r). In this sinus and its prolongations lies a peculiar cord of tissue, consisting partly of generative rachis and partly of vascular tissue. The space, formerly supposed to be a blood-vessel, in the wall of the gonads, is de- veloped as a part of this system, but in the adult there is a septum which shuts it off from the rest. The whole of the aboral sinus with its prolongation to the gonads and the sinus in the walls of the latter is developed as a part of the left pos- terior body cavity of the larva. The water-vascular system is lined throughout by a ciliated epithelium, has thin muscular walls, and contains a colourless albuminous fluid in which float amoeboid cells. It consists of a circumoral vessel, which is placed on the inner side of the buccal membrane close to the calcareous pieces of the peristom, sends prolongations the radial vessels into the radii, and communicates with the exterior through the stone-canal and axial sinus. The stone-canal passes aboralwards in interradius //. /// (the so-called left anterior). At its aboral end it opens into the axial sinus on the one hand and to the exterior through the pores of the madreporite on the other. The madreporite is * The two cords which pass from the point where the axial sinus abuts upon the aboral sinus to the stomach are not processes of the generative rachis and aboral sinus as was formerly supposed, but of the wall of the axial sinus (see p. 177). 182 PHYLUM ECHINODERMATA. a calcareous sieve-like plate perforated by many pores the secondary water-pores. Though in the adult most of the pores of the madreporite lead directly into the stone-canal, some of them open into the axial sinus. In the larva the primary water-pore opens into the axial sinus. With later growth this one pore becomes divided by folding into many, the epithelium of some of which becomes directly continuous with the epithelium of the stone -canal, but some of the pores retain their direct connexion with the axial sinus. The circumoral vessel bears two kinds of appendages the polian vesicles (p. 184) and Tiedemann's bodies (p. 185). The radial canals, which lie between the heads of the ambu- lacral ossicles in the angle formed by their apposition (Fig. 129), give off on each side as many lateral branches as there are tube- fcet. Each of these branches goes to a tube-foot and is con- nected by a canal which passes between two adjacent ossicles, with the internally placed am- pulla. In some forms there are two ampullae to each tube-foot (many Astropectinidae). The radial canals end blindly at the end of the arm in the ocular tentacle. The tube-feet are always pointed in the young, but in the adult they often terminate in suctorial disc-like expansions. Sometimes the distal feet of an arm are pointed and the proximal suctorial. The pointed feet are tactile, while the others are adhesive as well. The tube-feet have a well-developed ectoneural nerve layer, and longitudinal muscles only, except in the sucking disc where there are radial fibres, the contraction of which brings about the adhesion of the disc. At the junction of the lateral branches with the radial trunk there is a valve Flo. 133. Diagrammatic representation of the water-vascular system of a starfish (from Clans). Re circumoral vessel ; Ap polian vesicle ; Stc stone- canal ; M madreporite ; P tube-feet ; Ap' ampullae of the same. ASTEROIDEA. 183 which prevents the fluid passing back into the radial canal, so that when the ampulla contracts, it drives its contents into and so extends the tube-foot. The retraction of the foot is caused by the contraction of the longitudinal muscles of its walls, the fluid being driven back into the ampulla. If the tube-foot has a sucker, it is able to be attached to external objects and then by its contraction to draw the body of the starfish towards the object. Of course movement of the body in this way can only be effected when a number of feet are acting together in a co- ordinated manner. If a starfish be removed violently from its substratum, the attached sucker will be broken off and left on the stone and water will be ejected from the lacerated ends of the contracting tube-feet. FIG. 134. Diagrams of transverse sections through the stone-canals of various Asteroids (after Lang). 1 membrane by which the stone-canal is attached to the wall of the axial sinus ; 2 epithelium of the axial sinus ; 3 epithelium of stone-canal ; 4 connective tissue of wall of stone -canal. The stone-canal projects into the axial sinus (p. 180). On one side of it, viz. on the side opposite that by which it is attached to the wall of the axial sinus (Fig. 130), there is a longitudinally disposed fold of its lining membrane. In the simplest cases this fold projects into the canal as a ridge (Echinaster purpureus, Brisinga coronata (Fig. 134, .4). In other forms (Asterina gibbosa Penn, Cribrella, oculata Linck, etc.) the free edge of the fold splits into two lamellae (Fig. 134, B), which in yet other species (of Asterias, Pentaceros, Gymnasteria, etc. ) become coiled (C). A further complication is introduced by the fusion of the ridge with the opposite wall of the canal and the formation, from each surface of the septum so constituted, of a spirally coiled lamella (species of Astro- pecten, etc., D). Finally there are forms (Astropecten aurantiacus, species of Luidia and Culcita)in which these septa are present in great number and divide the whole lumen into many irregular chambers. All these ridges and lamellae vanish at the lower end of the canal, where it joins the circular vessel. The walls of the stone-canal and the lamellae, etc., which project into it, contain a strong deposit of calcareous matter. 184 PHYLUM ECHINODERMATA. The small sac beneath the madreporite (Fig. 132, mv), sometimes called the ampulla, is the small right hydrocoel (p. 144). The discovery of the communication between the upper end of the stone-canal and the axial sinus, we owe to the work of Perrier, Durham, and MacBride. The number of madreporites and stone-canals varies considerably. In the majority of cases there is only one of each, but there may be more, and this increase in their number is usually, though not always, associated either with an increase in the number of arms, whether such appears as an individual variation or as a constant specific character, or with the power of asexual reproduction by fission across the disc which some starfishes possess. Though the increase in the number of madreporites is generally associated with a greater number of arms than five, it is by no means always found in such forms. For instance, in the genera Heliaster and Labidiaster which normally have a large number of arms there is only a single madreporite. The increase in the number of madreporites is found most frequently in the families Asteriidae, Stichasteridae, Echinasteridae, and Linckiidae, and the following table* in which the abbreviation M is used for madre- porite shows some of the most conspicuous instances of it : Asterias calamaria Gray, with 6-12, usually 7 arms, has in 7-armed forms usually 1, rarely 2 M ; but in a 12-armed specimen 2 M and in an 11-armed one, 4 M have been observed. Asterias tenuispina Lam., has in 5- to 9-armed forms 1 to 3, rarely 4 M. Asterias capensis Perr., has 6 (rarely 5) arms and 3 M. Asterias rubens L., exceptionally 2 M in 5-armed forms. Stichaster polyplax M.Tr., has 7 arms and 1 to 5, usually 3 M. Stichaster albulus Stimps., has 5 to 7 arms and 1 to 2 M. Acanthaster echinites Ellis and Solander, has in 13- to 20-armed speci- mens 5 to 16 M. Acanthaster ellisii Gray, has with 11 to 19 arms 5 to 15 M. Echinaster eridanella M.Tr. and E. purpureus Gray, have in 5-armed individuals 1 M, in 6- or 7-armed 2 M. Ophidiaster germani Perr., has in 5-armed forms 2 M. Linckia multifora Lam., has often 2 M in 5-armed specimens. Linckia pacifica Gray, var. diplax M. Tr., and L. guildingii Gray, have as a rule 2 M in 4- to 7-armed specimens. In the above instances the madreporites are in different, either conti- guous or remote, interradii, but cases are known in which there are two madreporites and canals in the same interradius as an individual varia- tion (Linckia multifora Lam., Heliaster multiradiata Gray, and in a 6-armed example of Asterias glacialis O.F.M.), and Giard has described a specimen of Asterias rubens with one madreporite and two stone-canals passing off from it. Finally it must be mentioned that sometimes the madreporite is divided into several pieces, all however connected with the same stone-canal. Tiedemann's bodies are small yellowish glandular bodies attached to the inner wall of the circumoral vessel into which they open (Figs. 135, 136). They consist of branching tubes of * Taken from Ludwig's excellent account in Bronn's Thierreich to which the reader is referred for the facts and literature relating to the variation in the number of madreporites. ASTEROIDEA. 185 .T FIG. 135. Vertical section through an interradial region of the peri- stom of Asterias rubens L. (after Ludwig), showing the connexion of one of Tiedemann's bodies with the water-vascular ring. B tissue of the so-called vascular ring ; E outer perihaemal ring ; J inner perihaemal ring ; Mi interradial muscle of peristom ; M h huccal membrane ; N circ'umoral nerve- ring ; T Tiedemann's body ; 1!' water- vascular ring ; Zoral portion of deep oral nervous system (Lange's nerve). cubical ciliated yellow epithelium. There are usually two in each inter- radius, except that of the madre- porite, which has only one, and they are supposed to be of the nature of lymphatic organs and to bud off cells into the water-vascular sys- tem. Like so many other features of Asteroid anatomy the polian vesicles vary considerably both in number and arrangement throughout the class, even in closely allied forms. They are large vesicular structures with muscular walls and long stalks which open interradially into the circumoral vessel (Fig. 136). Some- times (Asterias rubens and glacialis, etc.) they are absent altogether ; sometimes there is one in each interradius, except that of the madreporite, in which there are none at all or two (species of Astropecten) ; finally there are cases of two, three or four or even more in each i n t e r r a dius ; in the latter case it is common to find several vesicles opening by one stalk. It is sup- posed by some that the polian vesicles are con- tractile structures acting as central organs to vary the pressure in the 10 FIG. 136. Circumoral water-vascular vessel with appendages of Asterina gibbosa (after Cuenot, from Lang), seen from the aboral side. 1 mouth in the centre of the buccal membrane ; 2 stone- canal ; 3 axial sinus ; 4 transverse muscles of the ambulacral ossicles ; 5 ambulacral ossicles ; 6 poliau vesicles ; 7 Tiede- A , r l ir .1 c n T TQ( C , 7a f m fe j bl [11 > mann's bodies ; 8 circumoral vessel ; 9 ring of supposed vascular fi, c fV, f fl,,, tissue; 10 ampullae of tube-feet. Otlieib tnat tliey 186 PHYLUM ECHINODERMATA. are of the nature of lymphatic glands, and that their lining membrane which has the form of a connective tissue network containing dividing cells in its meshes buds off amoeboid cells which enter the fluid of the system. The axial organ (ovoid gland, heart) is a fold of the wall of the axial sinus, into which it projects. Its walls are secondarily folded so that it appears in section to contain prolongations of the axial sinus. It consists mainly of connective tissue and of cells derived from a prolongation of the generative rudiment (pp. 131, 146). The so-called vascular system (lacunar system) of Asteroids consists of tracts of connective tissue in which the fibres are sparser, the ground substance stains more deeply and the leucocytes are more numerous than in the ordinary connective tissue. There is a cord of it in the vertical septum dividing the radial perihaemal canal (Fig. 129, 3) : this is the so-called radial blood-vessel. This is continuous with a circumoral cord of the same tissue (Fig. 135, B). There is a certain amount of it in the wall of the stomach, which presumably is in connexion with the circumoral tract, and it is possible that the latter may send prolongations on to the axial organ. There is also a tract of it in close association with the generative rachis in the wall of the aboral sinus. The vascular tissue is sometimes described as consisting of bundles of anastomosing canals without any epithelial lining and containing a coagul- able fluid. A definite circulation of this fluid has never been observed. The sexes are separate. Asterina gibbosa however has been said * to be a protandrous hermaphrodite, the same gland pro- ducing in young specimens spermatazoa and in old ones ova. The generative organs consist of bunches of tubes which are attached on each side of the interbrachial septa to the abactinal body-wall (Fig. 127). There are therefore twice as many gonads as there are arms. Each gonad consists of one or of several tufts of tubes. In the latter case, the gonads extend into the arms, to the dorso-lateral wall of which they are attached. Each gonad opens to the exterior by one simple opening (rarely subdivided into several) in the abactinal wall of the disc, or, when there are several gonadial tufts, by as many pores as * Cuenot, L. Arch. Zool. Exp. et gen. 5 bis, 1888. ASTEROIDEA. 187 there are tufts, along the sides of the abactinal surface of the proximal parts of the arms. The gonad tubes are lined by a simple epithelium which gives rise to the generative cells, and the external openings are always on the dorsal surface except in Asterina gibbosa and Asterina pancerii Gasco, in which they open on the actinal surface, no doubt in correspondence with the fact that these animals attach their ova to foreign objects. The composition of the gonads of a single tuft or of several tufts is often a generic character, but sometimes both conditions are found in the same genus (e.g. Echinaster). The gonads are really the peripheral parts of the generative rachis. This structure consists of a cellular cord placed in the wall of the aboral sinus on the dorsal side of the stomach. It gives off a prolongation into the axial organ, and in each inter- radius two cords proceed from it to the gonads. These cords are accompanied by a prolongation of the aboral sinus, which reaches as far as the gonads and surrounds them (p. 181). Except in those forms in which there are arrangements for the care of the brood, external sexual differences are only occa- sionally present, and are then usually confined to differences in colour. A brood pouch is developed on the dorsal surface of the Pterasteridae (see p. 192). In some of the Astropectinidae the eggs pass into the interstices between the stalks of the paxilli and there undergo their development. In Stichaster nutrix Studer describes the young as being at first in outgrowths of the stomach where they undergo their early development, and then as passing on to the edges of the mouth. In Asterias perrieri Smith and other species the young are described as being attached to the oral surface of the disc in the neighbourhood of the mouth, and there undergoing their development ; the arms being slightly bent over them for protection. In species of Di/i/nnfrrins similar phenomena appear to occur. They are all marine and crawl upon the bottom of the sea. They capture their food by means of their tube-feet and many of them have the power of partially everting their stomach, the inner surface of which is applied to their prey. Many of them have the power of autotomously severing their arms from the disc and of regenerating arms so lost. The power of regenerating lost parts is great in all members of the 188 PHYLUM ECHINODERMATA. group, and in some cases it appears that a single arm can re- produce the whole animal. This appears to be the explanation of the so-called comet-forms, which consist of a large arm carry- ing a small disc with four small arms. Fission of the body through the centre of the disc into two parts sometimes occurs. The power of regeneration possessed by a wounded surface sometimes leads to the production of curious forms, e.g. in Linckia multifora the wounded surface of an arm has been described as forming a new disc with four arms. The development (p. 133) is rarely direct, and the young usually pass through the free-swimming larval stage called bipinnaria. Including brachiolaria larvae about twenty bipin- naria larvae are known. Most of these have not been related to their adults. The larva of Asterina gibbosa may be regarded as a much modified bipinnaria ; it has the power of swimming feebly with the cilia of its larval organ. Bipinnaria asterigera is the larva of Luidia sarsii ; it is the largest bipinnaria known (1-licm.). B. metschnikoffi and mulleri probably belong to species of Astropecten. B. russoi and buryi have been assigned to Asterias glacialis. The Asteroidea are found fossil from the Cambrian onwards, but the known fossil forms are not nearly so numerous as in the case of Oinoids and Echinoids. This is doubtless due to the fact that their tissues do not lend themselves so readily to pre- servation as do those of the above-named classes. The early forms do not differ essentially from those now living. The class is divided into two orders the Encrinasteriae, in which the ambulacral plates alternate on the two sides of the arm and the madreporite is on the lower surface ; and the Euasteriae, in which the ambulacral plates are opposite one another and the madreporite is on the dorsal surface. The Encrinasteriae, are exclusively Palaeozoic, while the Euasteriae include all the living forms and make their first appearance in the Silurian. Order I. ENCRINASTERIAE. With characters as above. Aspidosoma Goldf. lower Devonian ; Palaeaster Hall (Archasterias J. Mull), Silurian, Devonian and Carboniferous; Urasterella M'Coy (Stenas- ter Billings), lower Silurian ; Palasterina McCoy ; Palaeodiscus Salter ; Palaeocoma Salter, upper Silurian ; Salteraster, etc. ASTEROIDEA. ISO Order II. ETJASTEKIAE. With characters as above. Sub-Order 1. PHANEROZONIA. With large marginal plates. The supramarginal and inframarginal plates are in contact. Papulae restricted to the abambulacral surface within the area bounded by the supramarginal plates. Ambulacral plates usually broad. Tube-feet in two rows in each arm. Oral adambulacrals prominent. Pedicellariae when present sessile. Fam. 1. Archasteridae. Marginal plates thick, with spines or spini- form papillae. Adambulacral plates large and not compressed. Ventro- laterals and marginals with spines or paxilli. Superambulacral plates absent. Pararchaster Slad. ; Pontaster Slad. ; Cheiraster Studer ; Pecti- naster Perr. ; Lonchotaster, Dytaster, Plutonaster Slad. ; Archaster Miiller and Troschel ; Grnathaster Slad. ; Asterodon Perr. ; Odontaster Verrill ; Mimaster Slad. ; Goniopecten Perr. ; Leptogonaster Slad. ; Pseudar -chaster, Aphroditaster Slad. British species : Pontaster tenuispiins, Scilly, Faeroe Channel, etc. 90-60 fms. Plutonaster bifrons, Faeroe Channel, etc., 200-1,300 fms. ; PI. bifrons, N. of Ireland, 1,360 fms. Fam. 2. Poreellanasteridae. Marginal plates well developed, but thin and porcellanous in appearance, and apparently naked or covered only with a thin epidermal layer. Abactinal area covered with membrane and carrying in its centre an epiproctal prominence (Fig. 137). Anus said to be absent. Actinal surface of disc is covered interradially with squamiform plates. Cribriform organs (1 to 14 in each inter- radius) present. Adambulacral plates large with simple mar- ginal armature uniserially dis- posed. Excepting Ctenodiscus all genera are exclusively from the deep sea. Cribriform organs are situated on the marginal plates in the inter-brachial region of the disc, and extend when numerous on to the base of the arms. They consist of a number of parallel vertically arranged calcareous lamellae equal in length to the height of the two series of marginal plates. Porcellanaster W. Thorns. (Fig. 137) ; Styracaster, Hyphalaster, and Thoracaster Slad. ; Pseudaster Perr. ; Ctenodiscus Miill and Trosch. British species : Ctenodiscus cristatus, Faeroe Channel, 312 fms. Fam. 3. Astro pectinidae. With large marginals bearing spines or spiniform papillae. Actinal interradial areas small. Abactinal skeleton with paxilli (Fig. 121). Tube-feet conical. Superambulacral plates present. Anus absent. Pedicellariae rarely present. In Leptoptychaster kerguelenensis the eggs pass into the spaces between the groups of paxilli and there develop (W. Thomson, J. Lin. Soc. London, 1876, 13). Craspidaster Slad. ; Leptoptychaster Smith ; Moiraster Slad. ; Blaki- aster Perr. ; Astropecten Linck ; Psilaster Slad. ; Phoxaster Slad. ; Bathy- biaster Dan. and Kor. ; Ilyaster Dan. and Kor. ; Luidia Forbes ; Platas- terias Gray. British species : Leptoptychaster arcticus, Faeroe Channel, 1,312 fms. Astropecten irregularis (aurantiacus) Atl. and Med., 10-1,000 FIG. 137. Porcellanaster gracilis Sladen, side view, showing three cribriform organs and the epiproctal prominence. 190 PHYLUM ECHINODERMATA. fms. Luidia ciliaris, E. N. Atl., to 87 fms., L. sarsii, E. N. Atl., to 374 fms. Fam. 4. Pentagonasteridae. Interbrachial region of disc well de- veloped, so the body is pentagonal with more or less concave sides. Mar- ginals well developed. All the plates, both dorsal and ventral, form a close mosaic, and are granular or naked. Anus present, but often hidden by paxilli. Pentagonaster Linck. (Fig. 117) ; Stephanaster Ayres ; Astrogonium M. and T. ; Calliaster Gray ; Chitonaster Slad. ; Calliderma Gray ; Iconaster Slad. ; Gnathaster Slad. ; Nymphaster Slad. ; Paragonaster Slad. ; Mediaster Slad. ; Nectria Gray ; Stellaster Gray ; Ogmaster v. Martens ; Leptogonaster Slad. ; Goniodiscus M. and T. ; Mimaster Slad. ; FIG. 138. Porania glaber Sladcn, abaetinal view (after Sladen). Anihenoides Perr. ; Ho plaster Perr. ; British species : Pentagonaster greeni, Faeroe Channel, 440 fms. Mimaster tizardi, Faeroe Channel, 550 fms. Fam. 5. Antheneidae. With well developed marginals, which may bear granules or tubercles. Actinal interradial areas large and covered with pavement-like plates, which bear large valvate pedicellariae. Anus distinct. Anthenea Gray ; Goniaster L. Ag. ; Hippasteria Gray. Fam. 6. Pentacerotidae. Dorsal marginals smaller than the ventral, often more or less hidden. Actinal interradial areas with large pavement- like plates which bear unequal sized granules. With small valvate pedi- cellariae. Abactinal skeleton reticulate. Anus distinct. Pentaceros Linck, Nidorcllia Gray ; Amphiaster Verrill ; Pentaceropsis ASTEROIDEA. 191 Slad. ; Culcita L. Ag. ; Asterodiscua Gray ; Choriaster Liitken ; Paulia Gray. Fam. 7. Gymnasteriidae. Marginal plates large. The whole body is covered with a thick membrane. Arms usually short. Actinal inter- radial areas with large regular plates. Abactinal skeleton tesselate. Anus distinct. Asteropsis M. and T. ; Dermasterias Perr. ; Gymnasterias Gray ; Tylaster D. and K. ; Porania Gray (Fig. 138) ; Marginaster Perr. ; Rhegaster Slad. ; Poraniomorpha D. and K. ; Lasiaster Slad. British species : Porania pulvillus. E. N. AtL, to 106 fms. Fam. 8. Asterinidae. With small, sometimes inconspicuous marginal plates. Abactinal skeleton composed of imbricating plates notched on one side and bearing spines on the free margin. Actinal interradial areas with imbricating plates bearing spines. No pedicellariae. Cycethra J. Bell ; Ganeria Gray ; Patiria Gray ; Nepanthia Gray ; Asterina Nardo ; Disasterina Perr. ; Palmipes Linck ; Stegnaster Slad ; Tremaster Verr. British species : Asterina gibbosa, E. N. AtL, to 35 fms. Palmipes placenta, shores of Britain, etc., to 30 fms. Sub-Order 2. CRYPTOZONIA. Marginal plates inconspicuous. The supra- and infra-marginal plates are often separated by intermediate plates. Papulae not confined to the area bounded by the supra-marginals, but found also between the mar- ginals and on the ambulacral surface. Ambulacral plates crowded and narrow. Tube-feet often in four rows. Ambulacrals or adambulacrals of the oral skeleton prominent. Pedicellariae stalked or sessile. Fam. 1. Linckiidae. Marginal plates comparatively well developed, and in contact. Disc small, arms long. Abactinal skeleton tesselate. Pedicellariae (rarely present) excavate or foraminate. Chaetaster M. and T. ; Fromia Gray ; Ferdina Gray ; Ophidiaster Ag. ; Pharia Gray ; Leiaster Peters ; Linckia Gray ; Phataria Gray ; Nardoa Gray ; Narcissia Gray ; Metrodira Gray. Fam. 2. Zoroasteridae. Marginal plates in contact. Disc small ; arms long, cylindrical, and tapering. Integumentary skeleton spiny. Abactinal skeleton tesselate, arranged in regular longitudinal and trans- verse series. Primary apical plates persistent and distinct in the adult. Tube-feet conical, terminated by a small sucker ; they are arranged in four series at the base of the arm, in two series distally. Pedicellariae (forcipulate) stalked. Zoroaster W. Thorn., for the most part from great depths ; Cnemidaster Slad. ; Pholidaster Slad. ; Mammaster Perr. ; Caly- caster Perr. Fam. 3. Stichasteridae. Marginal plates in contact. Disc small ; arms long, cylindrical and tapering. Integumentary skeleton for the most part granular. Abactinal skeleton tesselate arranged in longitu- dinal rows. Primary apical plates less distinct. Tube-feet usually cylindrical and with large terminal sucker, arranged in four rows all along the arms. Pedicellariae forcipiform and forficiform. Coelasterias Stimpson ; Stichaster M. and T. ; Tarsaster Slad. ; Neornor- pluister Slad. , Tonia Gray ; Nanaster Perr. ; Granaster Perr. British species : Stichaster roseus, Brit, coast, to 200 fms. Fam. 4. Solasteridae. Abactinal skeleton reticulated, with plates carrying on a projecting tubercle a bundle of divergent spines. Actinal intermediate plates more or less developed. Anus distinct. No pedi- 192 PHYLUM ECHINODERMATA. cellariae. Crossaster M. and T. ; Solaster Forbes ; Rhipidaster Slad. ; Ctenaster Perr. ; Lophaster Verrill ; Korethraster W. Thorns. ; Peribolaster Slad. British species: Solaster papposus, E. N. Atl. to 640 fms., S.endeca, ditto to 150 fms. Fam. 5. Pterasteridae. With a reticulated dorsal skeleton bearing paxilliform groups of spines. These spines are united together by a membrane (supradorsal membrane) which forms a continuous canopy over the dorsal surface. The chamber so enclosed is said to be a brood-chamber, and opens to the exterior centrally by a valvular aperture, and by a number of small contractile pores in the supradorsal membrane, and at the side of the arms by apertures regularly recurring over each adambulacral plate and called the segmental apertures. The canopy is present in all specimens hitherto examined ; it is uncertain whether these specimens are females or hermaphrodites or whether the canopy is present in both sexes. The supradorsal membrane may in some forms be wholly or partially aborted. Actinolateral spines when present united by mem- brane so as to form a web on the actinal surface. Pedicellariae absent. Pteraster M. and T. ; Retaster Perr. ; Marsipaster Slad., from the deep sea; Calyptraster Slad.; Hymenaster W. Thorns., almost entirely an abyssal form ; Benihaster Slad., from the deep sea ; Myxaster Perr. ; Cryptaster Perr. The above are disco-pentagonal in form and have a supradorsal membrane with segmental apertures. Pythonaster Slad., stellate forms without supradorsal membrane, actino-lateral spines and segmental apertures, from the deep sea. Fam. 6. Echinasteridae. Dorsal skeleton formed of plates disposed in longitudinal and transverse series, or in an irregular network bearing spines. Spines moderate, pointed, naked or covered by a thin membrane containing calcareous granulations. Arms long. Pedicellariae present only in Acanthaster and Valvaster. Acanthaster Gervais, with numerous arms (more than 10) and several (5-16) madreporic plates ; Mithrodia Gray ; Cribrella Ag. (Henricia Gray) ; Perknaster Slad. ; Echinaster M. and T. (Fig. 118) ; Plectaster Slad. ; Valvaster Perr., with wide-meshed arrangement of the calcareous plates and large groups of papulae. British species : Cribrella sanguinolenta, E. N. Atl., to 1.350 fms. Fam. 7. Heliasteridae. Arms very numerous (more than 25) and short, disc large. Abactinal skeleton reticulate. Tube-feet in four rows. Double interbrachial septa. Heliaster Gray. Fam. 8. Pedicellasteridae. Disc small, not sharply marked off from the arms. Abactinal skeleton of the arms reticulated. Tube-feet in two rows. With numerous, large forcipiform pedicellariae. Genital organs open on the disc. Pedicellaster Sars ; Coronaster Perr., with numerous arms ; Lytaster Perr. ; Gastraster Perr. Fam. 9. Asteriidae. With reticulate abactinal skeleton, bearing isolated or grouped spines. Tube-feet in four rows. Pedicellariae for- ficiform or forcipiform. Members of this family which have more than five arms are called heteractinides : in such species the number may be constantly six, or the number may be subject to individual variation. Pycnopodia Stimpson ; Coscinasterias Verrill ; Polyasterias Perr. ; Stolasterias Slad. ; Leptasterias Verrill ; Asterias L. ; Diplasterias, Perr. ;. Smilasterias Slad. ; Sporasterias Perr ; Anasterias Perr. ; Hydrasterias Slad. ; Cosmasterias Slad. ; Podasterias Perr. ; Uniophora Gray ; Cal- vasterias Perr. British species : Asterias glacialis, E. N. Atl., to 66 fms., ASTEROIDEA. 193 A. rubens, ditto to 110 fms., A. muelleri, ditto, 53 to 433 fins., A. murrayi, W. coast Scotland and Ireland. Fam. 10. Brisingidae. Arms long, marked off from the disc (Fig. 139). Marginals absent or vestigial. Abactinal skeleton absent or present only on the ovarian regions. Tube-feet biserial. Genital organs opening on the sides of the arms. Somewhat ophiurid-like in appearance. For the most part in deep water. FIG. 139. Odinia elegans E. Per., abactinal view (after E. Perrier). Hymenodiscus Perr. ; Gymnobrisinga Studer ; Brisinga Asbjornsen ; Odinia Perr. ; Freyelki Perr. ; Colpaster Slad. ; Brisingaster de Loriol ; Labidiaster Ltitken. British species : Brisinga endecacnemos and coronata, E. N. Atl., 100 to 1,300 fms. The starfishes of the abyssal regions of the ocean belong to the families Brisingidae. Pedicellastridae, Zoroaster idae, Stichasteridae, Pterasteridae, Pentagonasteridae, Ar chaster idae, Porcellanasteridae. But these are not exclusively abyssal, having littoral representatives in various parts of the world. Z HI O 194 PHYLUM ECHINODERMATA. Class OPHIUROIDEA * Brachiate Echinoderms with the body flattened in the or-anal axis. The arms are sharply marked off from the disc and are without an ambulacral groove. The madreporite is on the oral surface. The Ophiurids are brachiate Echinoderms, and the arms which are very rarely more than five in number (e.g. Ophioglypha hexactis with 6 arms, Ophiocantha vivipara 6 or 7 arms, 0. anomala and nodosa 6 arms etc.) are sharply marked off from the disc. The ambulacral grooves are absent or so slightly marked as not to be noticeable, and in most forms the ambula- cral nerve tracts are separated from the ectoderm by calcareous plates. The generative organs usually open into special pockets developed on the oral side of the disc at the base of the arms and called the genital bursae. The alimentary canal is without an anus and is not prolonged into the arms. The water-pore (or pores) is on the oral surface of the disc, and interradial in posi- tion. There is an axial sinus, axial organ, and generative rachis. The tissue of the so-called vascular system is feebly developed, and the tube-feet are without ampullae. As stated above the arms are rarely more than five in number, but in the Cladophiurae they may be much branched (Fig. 146). The integument is not ciliated, and except in the Cladophiurae and perhaps some Streptophiuridae the ectoderm is not present as a layer distinct from the dermis. The dermis is without muscles, but in all Ophiuroids, except the Cladophiurae, is richly provided with calcareous plates. As a general rule these plates form a complete dermal armour which may or may not be * A. Ljungman, Ophiuridea viventia hue usque cognita, Stockholm, 1867. H. Ludwig, Trichaster elegans, Z.f.w.Z., 31, 1878. Id., Das Mimdskelet der Asterien u. Ophiuren, Ibid., 32, 1879. Id., Zur Ent. d. Ophiurenskeletes, Ibid., 36, 1882. Id. Ophiopteron elegans etc., Ibid., 47, 1888. Th. Lyman, Report on the Ophiuridea, Challenger Reports, 5, 1882. Id., Ophiuridae and Astrophytidae Illustrated Catalogue of the Museum of Comp. Anat. of Harvard College, I. Cambridge, Mass., 1865. L. Cuenot, Etudes anatomiques sur les Ophiures, Arch. Zool. Exper. et gen (2), 6, 1888, p. 33. E. W. MacBride, Development of the genital organs, etc., in Amphiura squamata, Q.J.M.S., 34, 1893. F. Jeffrey Bell, A Contribution to the Classification of Ophiuroids, etc., Proc. Zool. Soc., 1892, p. 175. J. W. Gregory, Classification of the palaeozoic Echinoderms (Ophiuroidea), Proc. Zool. Soc., 1896, p. 1028. Liitken et Mortensen, The Ophiuridae, Mem. Mus. Harvard College, 23, 1899. O. Hamann, Ophiuroidea, in Bronn's Thierreich, 1900-1901. OPHIUROIDEA. 195 covered by a soft integument, but in the Cladophiurae and some Streptophiuridae the skeletal plates of both the disc and the arms are much reduced, and the integument is soft and thick. The skeleton of the arms consists of a double row of internally- placed ossicles which are generally fused with each other in pairs and are comparable to the ambulacral ossicles of Asteroids . 140. Transverse section through the arm of an Ophiurid, diagrammatic (from Lang). 1 tube-foot (ambulacral tentacle) ; 2 its cavity ; 3 epineural circular canal ; 4 circular ganglion, both at the base of the tentacle ; 5 under arm plate ; 6 radial epineural canal ; 7 radial nerve trunk of the superficial oral system (ambulacral nerve) ; S radial blood- vessel ; 9 radial trunk of the deep oral nervous system (Lange's nerve) ; 10 radial peri- haemal canal ; 11 peripheral branch of the radial nerve ; 12 spine ; 13 lower intervertebral muscle ; li lateral plate ; 15 ambulacral (vertebral) ossicle ; 16 upper intervertebral muscle ; 17 brachial coelom ; 18 specially ciliated strip of peritoneum ; 19 upper arm plate ; 20 radial water-vascular trunk ; 21 lateral portions of the brachial coelom which are repeated in each brachial segment ; 22 branch of water-vascular trunk to tube-foot ; 23 ganglion at base of spine ; 24 motor nerve from deep oral nervous system. (Fig. 140, 15). The water- vascular trunk, perihaemal canal and nerve cords lie on the lower side of these ossicles and a prolongation of the perivisceral space on the dorsal (77). The outer or ambital skeleton of the arm is segmented. Each segment consists of four plates ; a median under arm-plate, a median upper arm-plate, and two lateral arm-plates (Fig. 140). These plates join one another and the corresponding plates of 196 PHYLUM ECHINODERMATA. the adjacent segments, except in those forms in which the skeletal plates are deficient (Cladophiurae, some Strep tophiuridae) and the arms have a soft integument which contains only small skeletal pieces. The lateral arm-plates, which are generally compared to the adambulacral plates of Asteroids, carry spines, the others do not. The tube-feet emerge through openings between the under and lateral plates, one pair in each arm- segment. At the edges of these apertures are small scales. The ambulacra! or vertebral ossicles arise as separate pieces, which generally fuse together in pairs. In certain deep sea forms (Ophiohelus) each of these has the form of a curved rod joined to its fellow at each end. The successive ossicles movably articulate with one another and are attached by muscles (Fig. 140, 13 and 16). The articular surfaces vary in form and may develop processes and pits analogous to the zygosphenes and zygantra of an ophidian vertebra ; but sometimes the articulating surfaces are simple and the arms have a greater power of movement (Astrophyton, etc.). The radial water-vascular trunk lies in a groove on the lower side of the ambulacral ossicles, and its branches in passing to the tube-feet have a curved course through these structures (Fig. 140). Spines are present on the lateral plates of the arms, and occasionally on the upper surface of the disc and on the lower surface between the arms. In Ophiopteron elegans some of the arm-spines of each arm-joint are united together by a thin transparent membrane, thus forming a series of lateral fins. Pedicellariae of the ordinary type are not present, but in some forms (Cladophiurae) movable hooks, generally articulated to a pedicle, are found on certain parts of the arm. The hooks which occur in pairs are not opposed but move parallel to one another. Such modified pedicellariae are found in Astrophyton, Ophio- thrix fragilis, Trichaster elegans. The absence of true pedi- cellariae in Ophiurids would appear to militate against the view- that pedicellariae play an indispensable part in keeping the skin of Echinoderms clear of foreign growths and debris. The lower skeletal pieces of the disc constitute the oral skeleton and are very complicated. They comprise the proximal ambu- lacral and adambulacral (lateral) brachial plates, and the inter- radial buccal shields, on one or all of which are the water pores (see below), and a number of accessory pieces which belong to the ambital skeleton. The integument on the lower side of the disc between the insertions of the arms is either soft and contains small isolated skeletal pieces or granules, or is provided with a layer of imbricating plates. The bursal OPHIUROIDEA. 197 apertures which in some species are double (Ophioderma) are placed on the lower side of the disc, one on either side of the insertion of each arm. The genital plate (Fig. 141, gp) is a skeletal piece placed on the radial side of each of these slits. In a view from the inner side (Fig. 141) the angles are seen to consist of four plates; the two oral-angle plates (am , + ad Y ) and the peristomial plates (am 1 ). The oral -angle plates meet at the torus angularis (ta) and diverge outwards and towards the radii. Each of them has on the side towards the buccal fissure two depressions for the oral tube-feet (Fig. 141, pteb) and carries on its lower edge a number of small spines ; of these some project into the buccal fissures and are called oral papillae (Fig. 142, 10), ptel FIG. 141. Oral skeleton of Ophiopyren longispinus Lym., from within (from Lang after Lyman). am ambulacral ossicle ; am i peristomial plates, supposed to be the ambulacral ossicle of the first brachial segment ; am 2 + adi oral-angle plates ; D teeth ; fb bursal aper- tures ; gp genital plate ; ibr interbrachial region ; pteb depressions for oral tube-feet ; sge bursal plate ; to torus angularis. while others arise nearer the angle and project towards the centre of the mouth and are called dental papillae. The dental papillae are above the teeth of the torus angularis. The oral-angle plates are supposed to con- sist of the adambulacral (lateral) plate of the first brachial segment (next the angle) and of the ambulacral ossicle of the second brachial segment (outer part of oral-angle plate). The peristomial plates (am^) are on the inner side of the oral-angle plates and are supposed to represent the ambulacral ossicles of the first brachial segment. At the outer end of each buccal fissure is a plate (Fig. 142, 8) which is supposed to be the under plate of the second brachial segment) ; dorsally to this there may be sometimes made out another piece which is supposed 198 PHYLUM ECHINODERMATA. to represent the under plate of the first brachial segment. The mouth is a star-shaped aperture with the five interradial angles projecting into it (Fig. 142). These angles are formed externally of four plates : the oral plate or buccal shield (7), of two lateral buccal plates (5) lying on the sides of the buccal shield and meeting on the oral side of it, and of the torus angularis, which carries teeth. The lateral buccal shields are supposed to be the modified adambulacrals (laterals) of the second brachial seg- ment. The torus angularis consist of a vertical row of pieces which may fuse together (Fig. 141, ta). The slits between the angles are called the buccal fissures. Fm. 142. Lower surface of disc and base of arms of Ophiactis poa Lym. seen from the out- side (from Lang after Lyman). 1 under plates ; 2 spines of the lateral plates 4; 3 ten- tacle scales ; 5 lateral buccal shields ; 6 apertures of bursae ; 7 buccal shields ; 8 first under plate of arm (supposed to belong to tne 2nd brarhial segment) ; 9 torus angularis ; 10 oral papillae. The skeleton of the disc. The upper skeletal plates of the disc consist principally of the plates of the primary apical system, but there are generally other plates as well, and these may be so numerous as to completely obscure the primary plates. Moreover the completeness of the system of primary apical plates varies even in the same genus. In a typical case the ar- rangement of plates on the upper side of the disc is as follows (Fig. 143) : a central plate surrounded by five radials (r) and five basals (ba) ; the radials are separated from the central by the OPHiUEOIDEA 199 infrabasals, beyond this system are the ten radial shields (rs) and the five second interradials (ir), which do not belong to the primary apical system. The completeness of the apical system varies much in the group. In some forms, the embryonic condition is retained, but even here it may be reduced to the central and five radials, or central and five basals. Not infrequently it ( happens that the integument of the disc is soft and the plates small, scat- tered and incon- spicuous. The radial shields are perhaps the most constant and con- spicuous of the upper skeletal plates of the disc (including the apical system). They sometimes reach from the base of the arm to near the centre of the disc. FIG. 143. Plates of the upper surface of the disc of Ophiomusium validum (from Lang, after P. H. Carpenter), ce central plate ; r radial, ba basal plates ; the dotted plates betweeu the basals and the central are the infrabasals ; ir interradial plates ; rs radial shields ; br arms. At the end of each arm there is a median termi- nal plate which differs from the terminal plate of Asteroids in the fact that it not only lies over the terminal unpaired ten- tacle of the water-vascular system, but completely surrounds it. The alimentary canal, the mouth of which has already been described, leads by a short oesophagus into a large stomach (Fig. 144). The stomach is without any special glandular appendages and there is no anus. The nervous system is arranged very much as in Asteroids, except that the ectoneural plexus is absent in those forms in which the epithelium of the ectoderm is reduced, and that the ventral system (circumoral ring and ambulacral nerves) is re- moved from the surface and lies in the wall of an epineural canal (Fig. 140, 7). This canal, which is lined by ectoderm, being developed as an ectodermal groove, is covered ventrally by calcareous plates. It is found in connexion with the circumoral 200 PHYLUM ECH1NODERMATA. ring as well as with the radial cords. The latter are swollen into ganglia at the points where the nerves come off. The deeper oral system consists of two trunks in each arm placed close together and just above the radial trunks of the superficial system (Fig. 140, 9). They innervate the inter- vertebral muscles (Fig. 140) and are continued into a circum- oral band in the disc. The apical nervous system appears not to be represented, unless the genital nerve ring which runs in the wall of the aboral sinus belongs to it. The tube-feet are all sensory structures, and are supplied by a branch from the ambulacral nerve (Fig. 140, 4), which dilates into a ganglion at aB P# JCW PMS'PH JSi~ T Z Z T ' ! B 2 B* B 4 B FIG. 144. Diagrammatic vertical section through the disc and one radius of an Ophiurid (0 phinglypha), from Perrier, after .Ludwig. A^ peristomial plate (1st ambulacral) ; A' 2 - A 6 2nd to fith ambulacral plates ; aB generative rachis ; B l -B$ the 1st to the 6th ventral plates (under arm plates) ; Bi mesenterial filaments attaching the stomach to the body- wall ; Br radial blood-vessel ; D stomach ; ePH circumoral perihaemal sinus ; iPH perioesophageal part of perivisceral cavity ; K W body wall ; L body-cavity of disc ; L 1 of arm ; Li lip ; M, A/i 1 , .Mi 11 muscles of the oral skeleton ; ME oral angle plate ; MF and MF 1 first and second oral feet ; MS buccal shield ; j.V nerve ring ; A T r radial nerve ; mouth ; oB circular blood-vessel ; P polian vesicle ; PH aboral sinus ; rPH radial perihaemal canal ; S septum which separates the perioesophageal sinus from the rest of the perivisceral cavity ; T torus angularis ; IF water-vascular ring ; Wr radial water vessel ; Z teeth. their base. There is no eye, but the ambulacral nerve trunk becomes epithelial in position on the terminal tentacle. The general arrangement and relations of the coelom are the same as in Asteroids, the principal difference consisting in the ventral position of the madreporite. The general body-cavity or perivisceral cavity is in relation with the stomach in the disc and is prolonged into the arms on the dorsal side of the ambulacral ossicles (Fig. 140). The portion in the disc is traversed by connective tissue strands (Fig. 144), and is divided into two parts by a septum connecting the oesophagus with the oral skeleton (S). A small perioesophageal OPHIUROIDEA. 201 sinus (iPH) is thus cut off from the main portion. The brachial portion is dilated segmentally over each pair of ambulacral ossicles. The walls between these dilatations or chambers are imperfect and are traversed by calcareous plates which connect the ambulacral ossicles with the plates of the ambital skeleton. There is a streak of epithelium carrying specially strong cilia in the upper wall of the brachial continuation of the peri visceral coelom (Fig. 140, 18). The water-vascular system is almost exactly as in Asteroids. It consists of a circumoral vessel sending off a prolongation along each arm. This gives off lateral branches to the tube-feet which however are without ampullae and are purely sensory in function ; it ends in the terminal tentacle. The first two pairs of tube- feet are in relation with the mouth as oral tentacles (see p. 197), and are supplied by canals which arise from the circumoral vessel. There is a polian vesicle in each interradius except that of the stone-canal. In Ophiactis virens, which has several stone- canals, there are not only two or three polian vesicles in each interradius, but also a number of tubular prolongations (canals of Simroth) of the circumoral vessel which encircle the intestine and penetrate between the generative organs. These tubes are supposed to be respiratory in function, a view which is sug- gested by the fact that the genital bursae are absent in this species. The stone-canal (Fig. 145, 2), which however is without any calcareous matter in its walls, passes ventralwards to open into the ampulla (3), which corresponds to the whole of the axial sinus of Asteroids and opens to the exterior by the pore-canal (4) on the ventral surface of the disc, on one of the buccal shields (oral plates). The opening of the water-pore is placed asymmet- rically on the oral plate on an edge of it adjacent to a bursal slit. As a rule there is only one water-pore, but in some species of many genera of Ophiurae (Amphiura, Ophiolepis, Ophiopocus Ophionereis, Ophiocnida), and in all Astrophytidae there are several pores on the buccal shield concerned. In Trichaster elegans, there are five stone-canals and five water-pores, one in each interradius. In Ophiactis virens, which re- produces itself by division, the stone-canals are repeated in several interradii. In these cases of repetition of the stone- canals, the young forms are said to have only one. 202 PHYLUM ECHINODERMATA. Some or all of the corpuscles in the water-vascular fluid of Ophiactis virens are coloured red with haemoglobin. The axial organ has the same structure as in Asteroids. It is found on the side of the stone-canal turned away from the mouth (Fig. 145). It is in relation witli a section of the body cavity (Fig. 145, 7), which is quite unconnected * with the ampulla (axial sinus) and is probably the diverticulum of the left posterior body-cavity of the larva which is formed in the invagination of the primitive germ-cells (MacBride). The axial organ (Fig. 145, 8) is continuous with the generative rachis, which is contained in the aboral sinus (Fig. 144, PH) 16 (a portion of the left larval coelom as in Asterids) and takes a somewhat peculiar course round the disc. Radially it lies in the aboral part of the disc FIG. 145. Diagram of a vertical section through the j lp f wppn fV,p stomach cae- madreporitic interradius of Ampkium squamata, Between 1 showing the relations of the axial organ, stone- ,,, Qrir l f] 1P nrmpr in- canal and neighbouring sinuses (after MacBride). ' 1 circumoral water-vascular vessel; 2 stone- f pmlTrlf vnf wViprpn in pirli canal; 3 ampulla (axial sinus) ; 4 pore-canal ; tegument, Wiiei 5 closed sac which appears to represent the right infpwa/line if r1ir> rlnwn hydrocoel; 6 aboral sinus in its ventral position; mtCliaC 7 sinus derived from the left posterior coelom and U p 4- wppn f! 1p etnmaoli rappa sometimes called the axial sinus ; 8 axial organ ; DetWCen W16 9 genital rachis; 10 genital bursa ; 11 wall of f^.,, r ,1 Q f| 1p Inwpr <5iirfflf>p stomach ; 12 perioesophageal sinus ; 13 inter- OWarClS U16 radial muscle; 14 nerve ring; 15 teeth; 16 /f^: CT ~\ AA\ Tf liac Vippti mouth ; 17 oral surface of disc. suggested that this course is due to the fact that, in the interradii, structures which were originally on the upper side of the disc have moved on to the lower surface ; as an instance of this may be cited the water- pore which in the young form is dorsal, but in all adults has passed on to the ventral surface. The perihaemal system consists of a circumoral canal which is prolonged into a radial tube in each arm on the upper side of the nerve cords (Figs. 144, ePH ; 140, 10). There does * Cuenot, who thought that the two are in communication, appears to have been in error. His mistake is reproduced, together with an errone- ous figure, on p. 820 of the section on Echinoderms in Bronn's Thierreich (1900). OPHIUROIDEA. 203 not seem to be any representative of the inner oral perihaemal ring of Asteroids. The vascular tissue seems to be arranged very much as in Asteroids. In some forms the radial vessels are said not to be present. The generative organs. Most Ophiurids are dioecious, but Amphiura squama-ia is hermaphrodite. The generative glands are simple sacs which open, in all except Ophiopus and Ophiactis virens, into the genital bursae. The genital bursae are five pairs of sac-like invaginations of the interradial portions of the lower wall of the disc, and project into the body-cavity between the bulgings of the stomach. They open by slit-like apertures (double in Ophiura) placed one on either side of the insertions of the arms into the disc (Fig. 142). They have thin walls, lined by a ciliated epithelium and often containing calcareous matter. The gonads are small sacs placed on the walls of the bursae and opening into them. Each generative sac is connected with a branch from the generative rachis, the course of which has been described. The aboral sinus is continued with the rachis to each gonadial sac and invests it as in Asteroids. The generative cells pass into the bursae and outwards by their slit-like apertures. In many Ophiurids (Amphiura squamata and magellanica, Ophiacantha vivipara and marsupialis ; Ophiomyxa vivipara, etc.) the bursae act as brood pouches and the eggs develop in them ; but the principal function of the bursae seems to be respiratory, water being continually drawn in and ejected by the ciliary currents, and in some cases by the muscular elevations and depressions of the dorsal surface. In Ophiactis virens the bursae are absent * and the gonads open directly on the lower surface of the disc. They are replaced by the canals of Simroth, which have been already described (p. 201). The power of regenerating lost parts, e.g. arms, even a portion of the disc, is considerable, but apparently the disc cannot be regenerated from a single arm as in Asteroids. The brittle stars readily lose their arms, so that the power of regeneration is very important. Reproduction by fission through the centre of the disc has been observed in a few genera (e.g. Ophiactis). * Cuenot, Arch. Biologic, 11, 1891, p. 303. 204 PHYLUM ECHINODERMATA. Among preserved Ophiurids many specimens are found in which the dorsal surface of the disc is absent.* The significance of this fact is not understood. The larval form is the Ophiopluteusf (p. 140). In those forms in which care of the brood occurs there is no free larva. The ophiopluteus may present modifications in which the arms are reduced. Such are Ophiopluteus metschnikoffi and daparedei. The larvae known as Ophiopluteus annulatus,% 0. krohnii, O. oblongus are vermiform, without ciliated band and without or with only one skeletal rod. These larvae are pelagic. In- cluding the vermiform larvae about seventeen ophioplutei are known, most of which are as yet unrelated to any adult. The Ophiuroidea live upon the bottom of the sea and feed upon the minute organisms and organic matter contained in the surface mud, which they take up by means of the buccal tube-feet. They move fairly actively by means of the lateral flexion of their arms. In some forms the arms possess a power of vertical movement as well, especially towards the end. When the arms are very long they can be moved in a serpentine manner. The group first makes its appearance in the Upper Cambrian (Ordovician). Its affinities are with the Asteroids, with which it is sometimes united under the superclass Stelleroidea. It is indeed difficult to separate them, especially when the palaeo- zoic genera (Eophiura, Bohemura, etc.), recently described by Jaekel, || and such a form as Astrophiura are considered. The principal points of difference relate to the closing over of the ambulacral groove (a feature which is but slightly marked in * Jeffrey Bell, in Gardiner's Maldive and Laccadive Expedition, 1903, p. 223. Prof. MacBride states that he has seen an Amphiura squamata throw off the whole dorsal surface of the disc with the stomach. t Mortensen, op. cit. j This is J. Miiller's vermiform asteroid larva, with segmented body (Ueb. d. Larven u. d. Metamorphose d. Echinodermen, Abh. 3, p. 26, Abh. d. Pr. Akad. d. Wiss. zu Berlin, Abh. i.-vii., 1846, 1848-53), and appar- ently the larva described by Grave ( Mem. Johns Hopkins, 1900). The latter is at first uniformly covered with cilia, which later become restricted to four bands. These give it a resemblance to an Antedon larva, a resemblance which is heightened by the fact that the last trace of the larval organ (preoral lobe) is found at the edge of the aboral surface of the disc. The author does not state whether this organ is encircled by the water- vascular ring as it is in Asteroids. Preyer, Naples Mittheilungen, 7, 1886, p. 27. || Asteriden und Ophiuriden aus dem Silur Bohmens, Zeitschr. der deut- schen geol. Ges., 55, 1903, p. 106. See also Bather's description of a Devonian genus, Sympterura (Geol. Mag., 1905, p. 161.) OPHIUROIDEA. 205 the Lysophiurae and Ophioteresis), the sharp differentiation of the arms from the disc, the absence from the arms of any prolongation of the alimentary system, and the ventral position of the madreporite. Lastly the free larva has the pluteus form. The closure of the ambulacral groove and the presence of an epineural canal is a feature of some importance and one which the class has in common with Echinoids and Holo- thurians. The Ophiuroidea are divided into four orders : (1) the Lyso- phiurae, which are palaeozoic forms in which the ambulacral ossicles alternate ; (2) the Zygophiurae (brittle-stars) in which the system of dermal plates is well developed and in which the arms cannot be rolled up ; (3) the Streptophiurae, which are also brittle-stars, but which approach in some of their characters the Cladophiurae ; (4) the Cladophiurae (gorgon-heads) which have a thick integument with granular deposits without regular dermal plates. Order 1. LYSOPHIURAE. The ambulacral ossicles are alternate and are not united into pairs, but those of each segment are separate. There are no ventral arm plates. All extinct, Silurian and Devonian. Eophiura, Bohemura, Sympterura, Protester, Bundenbachia, Sturtzura, Eugaster, Ptilonaster, Taeniura, Palaeo- phiura. Order 2. ZYGOPHIURAE (OPHIURAE). Brittle-Stars. The surfaces by which the ambulacral ossicles of the arms articulate with one another are provided with processes and pits which fit into one another and limit the movement of the ossicles upon one another. Upper, under and lateral arm plates are present, and the arms are incapable of coiling round straight rods. The lateral arm-plates bear spines. 1. With arm-spines short, parallel to the long axis of the arms. Fam. 1. Ophiodermatidae. With numerous oral papillae, without dental papillae, with arm incisions on the disc. Ophioderma M. and T., Ophioncus Ives, Ophiogona Stud., Pectinura Forbes, Opliiopezella Ljg., Ophiopinax Bell, Ophiopeza Ptrs., Ophiopyren Lym. (Fig. 141), Ophioconis Ltk. Fam. 2. Ophiolepidae. With 3-6 oral papillae of which the innermost is rarely infradental, without dental papillae, with arm incisions on the disc. Ophiotrochus Lym. ; Ophiopaepale Ljg. ; Ophioceramis Lym. ; Ophiothyreus Ljg. ; Ophiolepis M. and T. ; Ophioplocus Lym. ; Ophio- zona Lym. ; Ophioplinthus Lym. ; Ophiolipus Lym. ; Ophiernus Lym. ; Ophiophyllum Lym. ; Ophiochaeta Ltk. ; Ophiopleura Dan. ; Ophiopyrgus Lym. ; Ophiomastus Lym. ; Ophiomusium Lym. (Fig. 143), no tentacle pores beyond the basal arm-joints ; Ophiotypa Khlr. ; Ophiura Lm. ; 206 PHYLUM ECHINODERMATA. Ophiocten Ltk. ; Gymnophiura Mrtsn. British species : Ophiura ciliaris, E. X. Atl., 7-100 fms. ; 0. albida, ditto, to 250 fms., O. affinis, ditto, 10-192 fms. 2. Spines at right angles to the arm axis. Fam. 3. Amphiuridae. With 1 to 5 oral papillae, the innermost often infradental. Arms inserted on the ventral side of the disc. Dental papillae absent. Ophiambyx Lym. ; Ophiopholis M. and T. ; Ophiostigma Ltk. ; Ophiochiton, Hemipholis, Amphilepis, Ophiocnida, Ophiophragmus, Ophiopla.r, Ophiochytra, Ophiomyces, all Lym. ; Ophiopits~Ljg. ; Ophiactis (Fig. 142) and Opliionereis Ltk.; Amphiura and Ophiopsila Forbes; Paramphiura Khlr. ; British species: Amphiura chiajii, to 120 fms ^ A. filijormis, to 120 fms. ; A. elegans, to 120 fms. Ophiactis abyssicola, 64 to 767 fms. ; 0. balli, to 203 fms. Ophiopholis aculeata to 300 fms. Ophiacantha bidentata, 20-2,335 fms. Fam. 4. Ophiohelidae. Disc with scales and sharp or blunt spines,, with teeth and oral papillae, without dental papillae. Ophiomitra, Ophiothamnus, Ophiocamax, Ophiotholia, all Lym. Fam. 5. Ophiacanthidae. Disc covered by soft skin, which more or less hides the subjacent scales ; dental papillae absent or few. Ophia- cantha il. and T. ; O. vivipara Ljn., 6 or 7 arms ; 0. anomala G. O. Sars, 6 arms. Ophiolebes, Ophiotoma, Ophiogeron, Ophiosciasma, Lym. : Ophio- blenna, Ophionema, Ophionephthys Ltk. ; O phiocentrus Ljg. ; Ophioscolex M. and T. ; Ophiotrema Khlr. Brit. sp. : Ophioscolex glacialis, 100-300 fms., 0. purpurei, 64 to 76 fms. Fam. 6. Ophioeomidae. With oral and dental papillae. Ophio- cymbium Lym. ; Ophiocoma L.Ag. ; Ophiarachna and Ophiomastix M. and T. ; Ophiopteris E. Sm. ; Ophiarthrum Ptrs. British species r Ophiocoma nirjra. to 87 fms. Fam. 7. Ophiotrichidae. With 8-10 dental papillae, without oral papillae. Ophiopteron and Opltiotrichoides Ludw. ; Ophiothrix and Ophiocnvmis M. and T. ; Ophiocampsis Dune. ; Ophiomaza and Ophio- psammium Lym. ; Ophiothela Verr. ; Ophiogymna Ljg. ; Luetkenia, Gymnolophus, Ophioaethiops, Ophiosphaera Brock. Brit. sp. : Ophiothrix frag His, to 52 fms. Order 3. STEEPTOPHIUEAE. Astrophytoii-like Ophiurae. The ambulacral ossicles articvilate with one another by means of a more or less simple ball-and-socket joint, and the arms can be moved in a vertical direction and be coiled towards the mouth. Upper, under, and side plates are more or less regularly developed : the side plates bearing spines. Fam. 1. Ophiomyxidae. With 3-7 oral papillae, without teeth. Arms covered with soft skin. Neoplax Bell ; Ophioteresis Bell, no under arm-plates ; Ophiobyrsa, Ophiochondrus, Sigsbeia and Ophiobrachion Lym. ; Ophiomyxa M. and T. ; Hemieuryale Martens ; Astrophis A. M. Edw. ; Ophiohclus. Several extinct families come here, viz., Ophiurinidae, Lapworthidae, Eoluididae, Onychasteridae, Eucladiidae. The Eucladiidae from th& middle Silurian of England have short arms, and a ventral madreporite. Each arm has 2 or 3 arm -like branches ; these are tube-feet with a flexible^ armour of minute spiny scales. Eudadia H. Woodward, Euthemon. Sollas. OPHIUROIDEA. 207 FIG. 146. Trickaster elegans (after Ludwig). Order 4. CLADOPHIUEAE (EURYALAE). Gorgon-heads. The ambulacral ossicles articulate with one another by means of hour glass-shaped surfaces and are covered by granular deposits in the thick integument. The arms may be simple or branched repeatedly. They can be moved in the vertical plane and 'coiled towards the mouth. There are no spines on the sides of the arms. Most of those with ion- branched arms have a mouth-shield at the inner angle of each lower interbrachial space, one of which serves as the madreporite. Those with branched arms have often no mouth-shields, and the madreporites, sometimes single, sometimes five in number, are found in various regions of the lower interbrachial spaces. Pedicellaria-like processes are sometimes present. Fam. 1. Astro phytidae. With simple arms. Astrotoma Lym.* ; Astronyx M. andT.* ; Astro- chele Vll.* ; Astrogomphus Lym.f ; Astroporpa Orst. and Ltk.f; Ophiocreas Lym.| ; Astrocherna Oerst. and Ltk.J; Astro- ceras Lym.f. Brit. sp. : Astronyx loveni to 350 fms. Fam. 2. Trichasteridae. The arms branch a few times near their free ends. Trichaster L. Ag. (Fig. 146) ; Astroclon Lym.; Astrocnida Lym. Fam. 3. Euryalidae. The arms branch much and from near their base. Euryale Lmk. ; Gorgonocephalus Leach (Fig. 147) ; Astropliyton Linck. Brit, sp. : Gorgonocephalus lincM and eucnemis. The genus Astrophiura Sladen though undoubtedly an Ophi- uran presents some Asteroid features. The family Astro phiu- ridae has been created for its reception. The disc is penta- gonal and the greater part of the arms are included in it. The free portion of the arms is short, reduced and without tube-feet. There are no teeth and the buc- cal armature is simple and im- perfect. Under arm-plates are present and the cavities for the retracted feet are spacious. The madreporite appears to be ven- FIG. 147. Young Gorgonocephalus agassizi, ventral view (after Lyman). tral. Madagascar and neighbouring islands. the Disc large. Disc moderate (about one-tenth of length of arms). { Disc small. 208 PHYLUM ECHINODERMATA. Class ECHINOIDEA * Spherical, oval, or discoidal Echinoderms with a shell composed of calcareous plates usually closely fitting and carrying movable spines. The mouth is on the under surface, and the anus either within the apical system or between the apical system and the mouth. The five ambulacra are indicated by rows of pores and usually extend almost to the aboral pole. There are typically five inter- radial gonads. The body of an Echinoid is typically spherical, but is often heart-shaped, oval, or flattened. There are no arm-like pro- longations, but there is a five-rayed symmetry in the build of the body as is shown by the disposition of the water-vascular, nervous and other systems of organs. The dermal skeletal plates, which are pentagonal or hexagonal in shape, are con- nected together so as to form, with rare exceptions (Echino- thuridae, which see, and certain Palaeechinoidea, plates of posterior ambulacrum of some Spatangids, peristomial plates of Cidaroida), a firm immovable skeleton, the test or shell. The mouth is on the lower surface of the body, generally in the centre, but sometimes shifted towards what is called the anterior end ; the apical system is on the upper surface and the anus is either within it (Endocyclica) or outside it (Exocyclica}. * A. Agassiz, Revision of the Echini, Cambridge U.S.A., 1872-4. Id. Report on the Echinoidea, Challenger Reports, 1881. Id. Panamic deep- sea Echini, Mem. Mus. Harvard College, 31, 1904. Cotteau, Echinides, Paleontologie Francaise, 7, 9, 10, Paris 1862-79. P. Martin Duncan, A revision of the genera and groups of the Echinoidea. Journ. Linnean Society, 23, 1891. S. Loven, Etudes sur les Echinoides, Kongl. Svenska Vetenskaps-Akademiens Handlingar, 11, 1872; a translation in Ann. and Mag. Nat, History (4), 10, 1872. C. F. and P. B. Sarasin, Ub. d. Anat. d. Ecliinothuriden etc. Ergebnisse Nat. Forschungen Ceylon, 1, 1888. Id., Die Auge etc. der Diadeniatiden, ibid. 1, 1887. J. Muller, L. Cuenot, O. Hamann, Delage Op. cit. Prouho, Recherches sur lo Dorocidaris papillata, etc., Arch. Zool. Exp. et gen. (2), 5, 1888, p. 213. O. Hamann, Echinoidea in Bronn's Thierreich, 1901-1905. Th. Mortensen, Echinoidea (Pt. l),in the Danish Ingolf -Expedition, I, vol. 4, 1903. F. Leipoldt, Das angebliche Excretions-organ der Seeigel, Z.f.w.Z., 55, 1893, p. 585. S. Loven, on Pourtalesia, K. Svenska Vet. Akad. Handl. 19, 1884. Id., Echinologica, Bih. Svenska Vet. Akad. ForhdL, 13, 1887. Id., Echinologica, Bih. Svenska Vet. Akad. Handl., 18, 1892. H. Theel, On the development of Echinocyamus pusillus, Nova Acta R. Soc. Sci. Upsala, 1892. Id., Prel. ace. of the devel. of Echinus miliaris, Bih. Svenska Akad. Handl. ,28, 1902. E. W. MacBride, Dev. of Echinus esculentus etc., Phil. Trans., 1903, p. 285. ECHINOIDEA. 209 When the anus is outside the apical system, it always lies in what is called the posterior interradius (V. I). The outline of an Echinoid shell when viewed from the apical pole is called the ambitus. The plates of the skeleton are covered by the ciliated epidermis and lie entirely superficial to the nervous and water- vascular systems ; they are perforated by apertures for the passage of the tube-feet, and bear prominences and tubercles to which the variously shaped spines are movably articulated. The apical system is very limited in extent and takes up the whole of the abambulacral surface of the body (Fig. 149). The XOJ-* ^-^ft^^y^''-- -'- ^ftit^^fS ^t/i^^-^S~ : FIG. 148. Test of a young regular sea-urchin Strong -jlocentrotus droebachiensis (from Claus). a from the aboral side ; b from the oral side. PR rows of pores in the anterior radius. The peristomial membrane contains the mouth with 5 teeth in the centre and 5 pairs of plates perforated by pores for the oral tube-feet. madreporite is on the upper surface and is one of the apical plates, generally the basal of what is called the right anterior interradius (II, III). For purposes of description, there may be said to be two principal kinds of Echinoids, the regular forms, in which the body is more or less spherical and the anus is within the apical system (Endocyclica, Regularia), and the irregular forms, in which the body is oval, or heart-shaped, and more or less flattened in the principal axis, and in which the anus is outside the apical system in the posterior interambulacrum (Ectocyclica, Irregularia). The test consists of the plates of (1) the apical system, and (2) the corona or rest of the shell ; z m P 210 PHYLUM ECHINODERMATA. to these may be added the peristomial plates, i.e. the plates in the peristomial membrane in the middle of which the mouth is placed (Fig. 148 b). In the regular forms or Endocyclica (Diadematoida, Cidaroida, most Palaeechinoidea), the apical system (Fig. 149) consists of (1) the periproct area containing a number of small plates, the periproct plates, amongst which, towards the right posterior radius (No. I) or in- terradius (I. II) lies the amis (an) ; (2) the five interradially placed basals (genitals), which are usually per- forated each by a genital opening (go), and one of which (m) is perforated by the water - pores of the m a d r e p orite. With this system, though not belonging to it, must be mentioned the five radially placed ocular plates (radials) which are perforated for the small terminal tentacles of the water- vascular system : these plates are really the terminals and belong to the ambulacral surface. Some or all of the radials may be wedged in between the basals and assist in forming the boundary of the periproct. Occasionally, as a specific or individual character, more than one genital opening is present on each genital plate. Thus two pores have been found in Cidaris perornata, Arbacia punctulata, etc., five on the madreporite of Echinus acutus. In short, among both fossil and recent species there is some variability in this character.* In the young of the Saleniidae the centre of the apical system is occupied by a central, on one side of which, in the right posterior interradius, the anus is placed. This condition is repeated in most young sea-urchins (Fig. 150), but in adults a number of small plates the periproct or Flo. 149. Apical plates of young Strongylocentrotus droe- bachiensis (from Lang, after Loven), an anus among the periproct plates ; go genital openings on the basal or genital plates ; m madreporite (right anterior basal) ; r radial plates (ocular). * In the cretaceous and tertiary genus Goniopygus (Arbaciidae) the genital openings are outside the apical system. ECHINOIDEA. 211 anal plates are added and the central becomes indistinguishable. The anus is always excentric, being displaced towards the right posterior interradius (as in Asterids). The test or corona consists of ten double meridional rows of plates passing round from the apical to the oral pole (Fig. 148). Five of these double rows are radial in position and constitute the ambulacra, while five are in- terradial and are called the inter - ambulacral plates. This statement is true of all Euechi- noidea* In the Palaeechinoidea, how- ever, the number of rows of plates in the interambulacra is either one (Bothrioci- daris) or more than two (from 3 to 11). The plates are pentagonal in F ig. iso. Apical system of a young e i i -, ,1 ,i Echinus (from Delage, after Loven). lorm aild are SO placed that the An anus; c central] gtx one of the v i ,1 genital plates. I. to V. indicates the median SUture between the tWO enumeration of the radii adopted in rOWS of an ambulacrum is zigzag ^rdTinteiradiS No^AlVand the /-TV i PiQ. 157. Scutella subrotundakt Lamk. Miocene, Bordeaux (after Zittel), a from below ; b from above ; c section. Natural size. The apical system of the Exocyclica is much modified. There is a frequent tendency for the genital opening of the posterior interradius to disappear, and in many Spatangoids the posterior basal plate is also absent. The anal area is occupied either by an extension of the right anterior basal with its madreporitic pores (etkmophract condition, Holectypus), or the central is said to persist and to fuse with the right anterior basal and posterior basal, the plate so formed being perforated by madreporitic pores (ethmolysian condition, many Spatangoids, Fig. 160), or 218 PHYLUM ECHINODERMATA. IV FIG. 159. Clypeaster rosaceus L. apical system and adjacent parts (from Delage after Loven). The radials and basals are fused to form the single plate pa covered with pores, at the apical pole, and the genital openings gtx have shifted into the interambulacra, I-V ambulacra numbered. the radial and basal plates all fuse to form a single plate at the apical pole on which the madreporitic pores are distri- buted with very various arrange- ment (Clypeast- roids, Fig. 159). Further, the mad- reporitic pores, though typically connected with the right anterior basal, often ex- tend beyond it on to the posterior basal (Fig. 160) or even on to all the basals, and in rare cases beyond the apical system on the adjacent interambulacral plates. Lastly in some Clypeastroids the genital openings are found beyond the apical system. In Clypeaster itself (Fig. 159) they lie at some little distance from the basals in the interambulacra, and sometimes they are found be- tween the basals and the first inter- ambulacral plates. It would appear that the association of the genital and water pores with the basal plates is not necessarily a fundamental fea- ture of Echinoid anatomy. In the case of the genital pores this association is un- doubtedly not an essential feature, for not only are these pores in some forms dis- sociated from the basals, as we have just seen, but in the young of all forms which have been examined the basals are not so perforated. The water-pore likewise is some- times dissociated from the basals in the adult, and in development appears before them. S. Loven* puts the matter in this way. "The madreporite is not an integral part of the calycinal system, but an extra- neous accessory, and there exists no such thing as a madreporic plate." In most Holectypoida the genital aperture of the posterior basal is * Monograph on Pour tale sia, loc. cit., p. 76. llf Fio. 160. Apical system and ad- jacent parts of young Spatan- gus purpureus showing ethmo- lysian condition (from Delage a'fter Loven). r radials ; b basals ; mdp plate with madre- poritic pores formed by fusion of right anterior basal, central plate, and posterior basal ; gtx genital pore ; I-V radii numbered. ECHINOIDEA. 219 absent, and in Pygaster the posterior basal itself is absent. In the Clypea- stroids there are usually five genital openings, but sometimes that of the posterior interradius is absent. In Spatangoids there are never more than four genital openings, that of the pos- terior interradius being absent. In many of the geologically older forms the corresponding basal has also disap- peared, but it is present in most recent and living Spatangoids (p. 217), though without a genital opening. In some Spatangoids the genital opening of the right anterior basal is also absent, and s o in e t i m e s nv IF V g FIG. 162. Apical system of CoUyrites elliptica Lamk. (from Lang, after Loven). I-V the ambulacra num- bered ; a, b the corre- sponding rows of ambula- cra lettered (see Loven's law). FIG. 161. Apical system and adjacent parts of Holaster suborbicularis Defr. (after Loven, from Lang), m madre- poritic basal ; r radials ; 6 basals ; v radius v ; 2 and 5 2nd and 5th interradius. that of the left anterior as well, so that only two genital open- ings are left. In some Spatangoids the apical system pre- sents still more curious variations. The typical arrangement which is found in the Endocyclica and in the Exocyclica described above, and in which the plates are all grouped round a centre, is called compact. But in certain Spatangoids it may be elongated, i.e. the plates are arranged in two rows extending in an antero-posterior direction (Fig. 161). As a result of this the rays of the trivium are removed by a consider- able interval from those of the bivium. This modification is carried still further in the Colly- ritidae, in which the radials of the bivium are separated, by the junction of the plates of the right and left posterior interradii, from the rest of the apical system (viz. 3 radials and 4 basals, the posterior basal being absent). Such an apical system is said to be disjunct (Fig. 162). Loven's law. In the preceding description of the Echinoid shell, a certain enumeration of the radii and orientation of the shell has been adopted. One of the radii has been called ante- rior, and one of the interradii posterior, while the three anterior have been spoken of as the trivium and the two posterior as the bivium. When the madreporite is distinct, this orientation can be easily determined ; the anterior radius being that to the left of the madreporitic basal (Figs. 148, 161). The orientation is also easy when the anus is outside the apical system, because it 220 PHYLUM ECHINODERMATA. always lies in the posterior interradius. But in other cases, in which these marks are not so distinct or are not available, the orientation can be determined, as well as confirmed in the preceding cases, by certain IV V FIG 163. Brissopsislyrifera'S'oTt). The plates of the shell spread out and viewed from the oral surface (after Loven). I to V the radii numbered ; 1 to 5 the interradii numbered. In the interradii 1, 4, 5, the three plates next the peristome are numbered to show corre- sponding plates (note asymmetry in 1 and 4), a and b mark the rows in each radius and interradius. In interradius 5, 1 marks the plastron and is placed on the labrum, -2 (st) the sternum, and 3 (ep) the episternum. The apical system is shown in 2, the inter- radius of the madreporite. an anus. peculiarities of a very remarkable character in the peristomial ambulacral plates. These peculiarities were discovered by Loven, and the statement of them constitutes Loven's law. In all Echinoids, except some Pour- talesiidae, the ambulacral plates which border the peristome of the bivium ECHINOIDEA. 221 are, with regard to their size or pores, symmetrical with each other, while the corresponding plates of the trivium are not symmetrical. To parti- cularize further let us take the case of Spatangoids (Fig. 163) : if the shell be viewed from the oral pole and the trivium be directed forward and the bivium backward and the radii be numbered according to the plan used above for Echinoids, viz. in such a way that the right bivial radius (to the left of the observer in this view which is ventral) be numbered I and the left bivial radius V, the other radii being numbered II, III, and IV, and if further the two rows of plates (Fig. 163) in each radius be called a and b, in such a way that row a in radius I is the row next the posterior interradius, and so on all the way round, so that in radius V the row next the posterior interradius is row b, then it is found that the ambulacral marginal plates I a, II a, III b, IV a, V b possess two pores and are larger than the others, viz. I b, II b, III a, IV b, V a, which also only possess one pore. Further it will be observed that the ambulacral marginal plates of the bivial radii, viz. I and V, are symmetrical with each other, while the corresponding plates in the lateral radii of the trivium, II and IV, are asymmetrical. This law, namely that the marginal ambulacral plates of radii I and V, are symmetrical with each other, while those of radii II and IV are asym- metrical holds for most Echinoidea. In the regular forms, e.g. Strongylocentrotus droebachiensis (Fig. 164), the same law is followed, the difference in the marginal ambulacral plates consisting in their size and in the number of primary plates of which they are composed. In Clypeastroids Loven's law is followed only with regard to the size of the plates, and not with regard to the pores, and in some genera its application is very difficult to make out. In the Pourtalesiidae it does not hold at all for the majority of the species, the constitution of the peristome in this family being different from that of other Echinoids. If the law has the importance which Loven attributes to it, it is most remarkable that in the same family it should hold for some species and not for others. Pedicellariae * are always present. They have stalks contain- ing a calcareous rod, and three, rarely four, calcified blades (Fig. 166). The bases of the blades are broad and usually concave on the inner side, the concavity being traversed by a vertically directed crest, the a-pophysis (Fig. 167, a). The blades are articulated together basally and not with a special calcareous piece. There is a special musculature for moving the blades, and the stalks are movable on the shell. Glands are frequently present on the stalk or on the outer sides of the blades. The pedicellariae are covered with a ciliated epithe- lium. On the inner sides of the blades patches of this epithe- lium may be modified as special organs of sense (Fig. 165), the * Mortensen, op. cit., p. 4. Von Uexkiill cited on p. 224. Prouho, Arch. Zool. Exp. et gen (2), 5, 1887, p. 213. C. Stewart, Journ. R. Mic. Soc. 3, 1880, p. 909. 222 PHYLUM ECHINODERMATA. cells of which are provided with sensory hairs. When these are not present scattered sense-cells are present in the epithelium on the inner sides of the blades. Nerves pass along the stalk u Fid. 164. Strongylocentrotus droebachiensis. The skeleton of a young individual of 4 mm. spread out and viewed from the oral surface. I to V the radii numbered ; 1 to 5 the inter- radii numbered ; a, b mark the rows in each radius and interradius. The apical system is shown in 2, the interradius of the madreporite (right anterior). ECHINOIDEA. 223 for the inner vation of the muscles and sensory epithelium. Pedicellariae are found on all parts of the surface amongst the spines. They are smaller and less numerous in the Irregularia (Spatangoids and Clypeastroids) than in the Regularia. They present considerable variation in structure and are of great im- portance in classification (Morten- sen). There are four kinds of pedicel- lariae : the globiferous (genimi- form), the tridentate (tridactyle), the ophicephalous, and the triphyl- lous (trifoliate). (l)The globiferous pedicellariae (Figs. 165, 167, 1) have long stalks and are pro- vided with a gland, which shows indica- tions of being double, on the outer side of each blade (on the inside in the Cida- ridae) and in some cases (Sphaer echinus granularis) with three glands half-way up the stalk. The blade ends in a tooth which is grooved on its outer side. The gland of the blade opens on this groove near the tip of the tooth and secretes a viscid fluid which is supposed to be poisonous. There is a patch of sen- sory epithelium on the inner side of each blade near its base and sometimes a second nearer the apex. The axial cal- careous rod extends the whole length of the stalk and the blades are attached to the end of it by a ligament. The so-called globifers are globiferous pedicellariae, the distal parts of which beyond the stalk-glands are absent or reduced. They have the form of short stalks ending in a trilobed swelling and have been found in Sphaerechinus granu- laris, Centrostephanus longispinus, etc. Traces of blades are sometimes dis- cernible on them. (2) The tridentate pedicellariae (Figs. 166 C, 167, 4) are the largest and most movable of these organs. Their mobility is due to the fact that the distal end of the stalk is occupied by a rod of elastic tissue embedded in a sheath of smooth muscular fibres, the axial calcareous rod not reaching the whole way. The blades are long, broad at the base and narrow distally, and they are provided with teeth along their edges. They are usually without glands, and the adductor muscles are cross-striped. FIG. 165. Diagram showing the struc- ture of a globiferous pedicellaria (from Lang). 1, 3 sense organs ; 2 aperture of the gland of the blade ; i adductor muscle ; 5 skeleton of the blade ; 6 epithelium of the blade ; 7 gland of the blade, 8 its epithe- lium ; 9 muscle-layer of the gland ; 10, 11 divaricator muscles ; 12 nerve ; 13 calcareous rod of the stalk ; 15, 16 gland of the stalk, 14 its aper- ture. 224 PHYLUM ECHINODERMATA. There are no special organs of sense, but sensory cells are scattered among the ciliated ectoderm cells on the inner sides of the blades. (3) The ophicephalous pedicellariae, which are distributed all over the shell and on the buccal membrane, are smaller than the preceding. The stalk contains a calcareous rod proximally and an elastic band distally in the axis, and the blades, which are short and broad and toothed along the edges (not shown in the figure), carry at the base a calcareous semicircular rod which crosses those of the two other blades and ensures a good arti- culation (Fig. 167, 2). The blades are without glands and patches of sensory epithelium, but there are sometimes mucous glands on the stalk in the Diadematidae. These glands may be much developed and the blades reduced, in which case they approximate to the globifer condition. FIG. 166. Pedicellariae of Echinoids. A, 4-bladed pedicellaria of Schizaster canaHferus ; B globiferous pedicellaria of Sphaerechinus granularis with glands on the stalk ; C longi- tudinal section of a decalcified tridentate pedicellaria of Centrostephanus longispinus. 1 adductor muscle ; 2 nerve ; 3 elastic column ; 4 calcareous rod ; 5 longitudinal muscular fibre (from Lang). (4) The triphyllous pedicellariae are the smallest kind and have short broad leaf-like blades without teeth or with very fine teeth (Fig. 167, 4). The stalks are very flexible, the calcareous stalk only reaching half way and being continued by elastic tissue. They are without glands and special sense organs. The function * of pedicellariae is to seize upon foreign bodies or organisms which approach or touch the shell and spines. The globiferous pedicellariae by means of their poison glands are probably able to deal with the more powerful organisms. * J. von Uexkiill, Zeitsch. f. Biol. (2), 19, 1899, p. 334. ECHINOIDEA. 225 In some cases the tube-feet have been observed to transfer to the mouth organisms held by the pedicellariae. The triphyllous pedicellariae are supposed to be particularly concerned in keep- ing the shell and spines clear of smaller particles such as sand grains which may fall upon them. It is possible that all the pedicellariae may assist in keeping the animals clear of foreign organisms, whether animal or vegetable, which would naturally attach themselves to the spines and shell (see, however, p. 196), thus accounting for the clean condition in which the spiny shell sf bl. FIG. 167. Blades of pedicellariae seen from the inner side ; 1 of a globiferous (Parechinus miliaris), 2 of an ophicephalous (Strongylocentrotus droebaehiensis), 3 of a triphyllous (Pareehinus miliaris), 4 of a tridentate pedicellaria (Strong, droebaehiensis), a apophysis; 6 basal part of bl blade ; et end- tooth ; st lateral tooth ; I articular surface (after Mortensen). is usually found. It is said that pedicellariae which have once got hold cannot leave go, so that they must be torn off with the bitten object. The spines are of various sizes and shapes. They are movably articulated to tubercles on the shell plates. The larger tubercles are called primary tubercles and carry the larger or primary spines ; they consist of a mamelon which may be perforate or im perforate according to the presence or absence of a ligament, and of a boss which is the eminence supporting the mamelon ; the scrobicule is the smooth area of the test round the boss, z m Q 226 PHYLUM ECHINODERMATA. The larger spines are attached to their tubercles by an elastic ligament which is inserted into a pit on the tubercle and into another pit in the socket of the spine. The smaller spines are covered by a ciliated epithelium, as are the larger spines in their growing state. The epithelial covering is absent from the full grown larger spines except at their base. The spines are attached by ligamentous and muscular sheaths which pass from the part of the spine round the socket to the smooth part of the shell plate round the tubercle. Around the base of the large spines there is a ring of nervous tissue just beneath the ectoderm. The large spines * are used in locomotion. The small spines are protective and are arranged round the large spines, and round the pores of the ocular plates, the anal and genital apertures, etc. They can be bent over the protected object, and are without the nerve ring. In the Clypeastroids and Spatangoids the spines are small and seta-like. In Asthenosoma special poison spines have been described by the Sara- sins. They have swollen heads containing the poison gland. It is quite possible that the spines are often poisonous. In Centrostephanus longispinus there are about fifteen short spines on the interambulacra near the anus, which are in a continual state of rotation, describing a circle with their tips. The term epistroma is applied to calcareous deposits which are found on the plates of the test in some forms. Clavulae are minute spines with swollen ends and covered with a ciliated epithelium. They are found in the Spatangoida only and are arranged in definite tracts called fascicles or Semites. The arrangement of these tracts is of importance in classifica- tion. The tube-feet within a fasciolar area always differ in structure from those outside it. The principal kinds of fascicles are as follows (Fig. 168). (1) The peri- petalous (p), which encloses the petaloid portions of the ambulacra ; (2) the subanal (sa), which encloses a space on the oral side of the anus ; (3) the marginal (m) along the border of the shell parallel to the ambitus ; {4) the internal (i) which crosses the petals near the apical region ; (5) the laterals (I), which run one on each side from a point on the peripetalous towards the periproct. They are not all present in the same species. * For the physiology of the spines see J. v. Uexkiill. Zeit. Biol. (2), 21, 1899, p. 73, and ibid. 22, p. 447. ECHINOIDEA. 227 The structure of the spines affords important systematic characters for the diagnosis of families and to a certain extent of genera.* Sphaeridia are small, densely calcareous, glassy, spherical bodies, composed of a stratified vitreous substance and placed upon short stalks of which the calcareous tissue is more reticu- lar ; the stalks are articulated to prominences on the test. They are found on the ambulacral plates only, and particularly on the ambulacral plates bordering the peristome. They are covered by a ciliated epithelium and round their base is a muscular sheath and a sub -epithelial circular nerve tract as in the case of the large spines. They may project freely, or be placed in FIG. 168. Diagram of a Spatangoid showing the fascicles (after Gregory), i internal, I lateral, m marginal, p peripetalous, sa subanal fascicle. depressions of the test, which are sometimes completely closed (Clypeastroida, Cassidulidae). They are probably sensory structures, and, from their position near the mouth, they have been supposed to be olfactory or gustatory. On the other hand it has been suggested that they are for orientation. They are to be regarded as modified spines. They are present in all Echinoids except Cidaris. The jaws and five teeth appear to be present in all Echinoids except Spatangoids. In the typical sea-urchins they form the structure known as the lantern of Aristotle. The jaws consist of a complicated framework of calcareous plates and rods by means * E.Hesse, NeuesJahrb. f. Mineral. Geol. u. Palaeont. Beilageband, 13, 1889-1901, p. 185. 228 PHYLUM ECHINODERMATA. of which, with the aid of muscles and ligaments, the teeth are brought into action. The perignathic girdle, which is absent in Spatangoida consists of pro- cesses inwards of the ambulacral and interambulacral peristomial marginal and sometimes adjacent plates. It is said to be continuous when the ambulacral processes turn towards one another and unite so as to form an arch the auricle through which the radial water-vascular canals and nerves, etc., pass. The auricles are connected by the ridges which are processes of the interambulacral plates. The girdle is said to be inter- rupted when the ambulacral processes are absent or very small, while the interambulacral processes are tall and diverge from one another, in such a way as to tend to approximate over the ambulacrum ; when they touch and form an arch over the latter, they constitute a false auricle. These processes are for the attachment of the muscles of the jaws. They have been compared to the ambulacral ossicles of Asteroidea. The body is covered with a ciliated ectoderm, which extends over the smaller spines, the clavulae, sphaeridia and pedicellariae, and on to the bases of the larger spines. Beneath it is the dermis which contains pigment cells and a nerve-plexus, and the plates of the skeleton. Inside the dermis is the ciliated peritoneal epithelium. Muscles are not present in the body -wall, except in those forms with flexible shell plates in which there are five pairs of longitud- inal muscles, running meridionally within the test. Nervous system.* The arrangement of the ventral nervous system is very similar to that found in Ophiurids. It is removed from the surface, both the circumoral ring and the radial nerves being contained in the epithelial wall of an epineural canal (Fig. 169). It is connected by nerves, which pass through the pores for the tube-feet, with the general sub-epithelial plexus of the ectoderm (7, 8). The epineural canal is developed in the larva by the closure of an ectodermal groove. The circumoral ring, which is connected with a sub-epithelial or epithelial plexus over part of the intestine, lies between Aris- totle's lantern and the peristomial membrane, close to the mouth. The radial trunks lie between the epineural and perihaemal canals, and give off nerves to the tube-feet, etc., and as stated above to the integumentary plexus. They end by passing on to * For the physiology of the nervous system see J. v. Uexkiill, Zeit. Biol. (2), 21, 1899, p. 73, and 22, p. 447 ; also the article Echinodermata in the Cambridge Natural History, p. 519. ECHINOIDEA. 229 the papilliform termination of the radial water-vascular trunk which perforates each of the ocular plates of the periproct. The deeper oral system is restricted to five radially-placed patches, close to the circumoral nerve ring, and innervates the muscles of the jaw apparatus. It is absent in Spatangoids. .10 18 17 16 14 FIG. 169. Transverse section through a radial part of the body-wall of an Echinoid (diagram- matic after Delage and Herouard). 1 tubercule for the articulation of 2 a spine ; 3 musculus externus, 4 musculus interims of the base of a spine ; 5 nerve ring at the base of a spine ; 6 ambulacral plate ; 7 ectoneural plexus ; S pedal nerve ; 9 nerve ring of the sucker, 10 of a tube-foot ; 11 skeletal piece in the pedal sucker ; 12 muscle of foot ; 13 cavity of tube- foot ; 14 ampulla traversed by muscular bands ; 15 canal passing from radial water- vascular canal, 16 to ampulla ; 17 radial blood-strands ; IS radial perihaeraal canal ; 19 radial nerve ; 20 epineural canal. The apical nervous system appears to be unrepresented, unless an annular nerve trunk in connexion with the aboral circular sinus belongs to it. Sense organs. The tube-feet, and pedicellariae and smaller spines are all highly sensitive and provided in the case of the two latter structures with special aggregations of nervous tissue. The terminal tentacle projects slightly on the ocular plate and 230 PHYLUM ECHINODERMATA. its tip is coloured with pigment ; there is no evidence that it has a visual function. In the Diadematidae the skin is provided with numerous shining spots which have the structure of com- pound eyes (vide Sarasin loc. cit.). The sphaeridia have been supposed to have an orientating function. The alimentary canal winds through the body from the mouth to the anus (Fig. 170). It is suspended to the body wall by a perforated mesentery, and sometimes the coils are con- nected by a mesentery. Pharynx, stomach, intestine and rectum may be distinguished but are little marked off from one another. The junction of the oesophagus and intestine is often marked by a swelling and in Spatangus by a caecum. There is an assessory intestine or siphon (absent in Cidaroida) which is FIG. 170. Sea-urchin divided equatorially (after Tiedemann, from Claus). D alimentary canal, fixed to the shell by the mesentery ; G gonads ; J interambulacral plates. given off from it near its commencement (oesophageal region), accompanies it along its inner (axial) wall, and joins it again lower down. It is supposed to allow of the passage of re- spiratory currents of water. In a few genera (Schizaster, Brissus, etc.) there is a second siphon. The walls of the alimentary canal contain muscular elements and, in the first part of the intestine at least, blood lacunae. In the Spatangoida the alimentary canal is always found distended with sand. The coleom presents essentially the same parts as in Asteroids. It consists of water- vascular, perihaemal, and perivisceral por- tions, and of the axial sinus. The perivisceral portion occupies the greater part of the interior of the body and is in relation with the coils of the alimentary canal. ECHINOIDEA. 231 The perihaemal system consists of (1) five radial canals, extend- ing the whole length of the radii and lying between the radial nerve and the radial water- vessel (Fig. 169, 18), and (2) a large sinus round the mouth. This is the perioesophageal sinus or lantern- coelom, so called because, in the forms with teeth, the jaws (lantern of Aristotle) lie within it. It is homologous with the outer perihaemal ring of other classes, though it is completely separate in the adult from the radial perihaemal vessels which reach right up to it. The external gills, of which there are five pairs, one pair in each interradius, are processes of the outer part of the peri- stomial membrane and contain prolongations of this sinus. They pass through notches on the interambulacral marginal plates and are present in most Endocyclica, but are absent in the Cidaroida. The internal gills or Stewart's organs are found in Cidaroida and Echinothuridae. They are hollow processes of the lantern membrane into the body cavity and their cavity is a prolonga- tion of the perioesophageal sinus. They are usually five in number and radial in position. Organs of a similar nature, eight or nine in number, have been described by Cuenot in some Clypeastroids. The axial sinus ends blindly, ventrally, at some distance from the oral region, while dorsally it communicates with the stone- canal. It is contained in the axial organ, which is wrapped round it. There is no inner circumoral perihaemal ring. There is also an aboral circular sinus in the walls of which lies the generative rachis (p. 234). This sinus sends prolonga- tions to the generative organs. The water- vascular system is arranged very much as in Asteroids. There is a circumoral vessel (Fig. 171, Eg) placed at the upper end of the pyramids of the jaws and giving off the five radial canals which, passing beneath the intermediate plates, travel oralwards within the lantern-membrane to the test and then, after giving off a branch to the oral tube-feet (absent in Cidaroida and Echinothuridae) which perforate the plates in the peristomial membrane, turn outwards through the auricles to run along the radii within the shell plates to terminate in the short un- paired tentacle which perforates the ocular plate. In most Endocyclica the circumoral vessel possesses in each interradius 232 PHYLUM ECHINODERMATA. a small outgrowth, which has a spongy structure and has been identified as a polian vesicle ; it enters into close relations with branches from the circumoral vascular ring. The radial vessels give off transverse vessels, each of which opens into an ampulla. The ampullae are placed within the shell plates and each of them communicates by two canals with a tube-foot * (Fig. 169). In those cases in which the pores are single, it would appear that the ampulla is only connected by one canal with its tube-foot The circular vessel is connected by a stone-canal, the walls of M I FIG. 171. Diagram illustrating the relations of the different systems of organs in an Echinus (after Huxley). A amis ; Am tube-foot ; Aur auricle ; M madreporite ; mouth ; Pe pedicellariae ; Po polian vesicle; prprotractor, re retractor muscles of the lantern; R radial vessel of the water-vascular system; Eg circumoral water- vascular vessel ; Sc stone-canal ; St spine ; Z teeth. which may or may not contain calcareous matter, with an ampulla placed just below the madreporic plate and opening outwards through the pores of this structure. The ampulla communicates with and is really the upper part of the axial sinus, which in Echinoids is surrounded by the axial organ. In Echinocyamus pusillus the madreporite is peculiar in the fact that it is pierced by only one water-pore. In Spatangoids the stone-canal is short and its opening into the axial sinus is separated by a wide interval from the madreporite. In the Endocyclica the tube-feet terminate in sucking discs and are * See footnote, p. 214. ECHLNOIDEA. 233 supported by calcareous pieces, which form a ring round the margin of the sucker (Fig. 169). In many forms the abactinal tube-feet are pointed and are without the calcareous ring, and in all cases the suckers of the abactinal feet are weaker than the others. The suctorial tube-feet together with the spines form the organs of locomotion. In the Cidaridae and Echinothuridae the feet which come through the ambulacral plates of the peristomial membrane are similar to the other feet. In forms with only ten perforated ambulacral plates on the peri- stome, the tube-feet which they transmit, the oral tube-feet or buccal tentacles, are small and terminate in an oval disc. They move actively when in the neighbourhood of food, without, however, touching it. They are supposed to be olfactory or gustatory in function. In the Clypeastroids and Spatangoids the feet of the petals differ from those of the rest of the ambulacral system. They are broadened at the base, and their sides are indented or sacculated ; their walls are without calcareous bodies. They are said to be respiratory in function and are called ambulacral gills (see p. 214). The tube-feet issuing from the fine pores of Clypeastroids are locomo- tive. They are cylindrical in form, are provided with calcareous bodies, and end in a sucker (which may be supported by a calcareous ring). In Spatangoids the feet are very various in shape, according to the part of the body in which they are placed. There are (1) the respiratory feet of the petals without terminal suckers or calcareous bodies ; (2) ordinary locomotive feet with suckers and calcareous supports ; (3) simple tactile feet without suckers ; (4) brush-like tactile feet found round the mouth and the anus and in the Cassidulidae on the phyllodes ; they terminate in an expanded disc which carries a number of club-shaped filaments, each of which is supported by a calcareous rod ; (5) the rosette- feet of the anterior ambulacrum ; these end in discs the edges of which are drawn out into short processes supported by calcareous rods ; they are often of great length and are prehensile in function, seizing food which is to be conveyed to the mouth. The feet of the petals are in connexion with double pores in the shell, the other feet only having one pore. According to J. Miiller the locomotive tube-feet of the Spatangoids are less numerous, while those of Clypeastroids are far more numerous, than in regular Echinoids. The vascular system attains a development somewhat similar to that which it has in Holothurians. It consists of a plexus in the intestinal wall connected with two longitudinal intestinal trunks, which lie in the mesenteries. These open into a cir- cumoral vessel * from which pass five radial vessels. The latter lie between the water- vascular canals and the radial perihaemal space, and give off vessels to the tube-feet. The circumoral blood-vessel is close to the circumoral water-vascular trunk. As in other cases the significance of this system is obscure : it consists largely of lacunar tissue. * The word vessel is not perhaps correctly applied to the various tracts and branches of this lacunar tissue. 234 PHYLUM ECHINODERMATA. The axial organ is well developed and lies close to the stone- canal in the axial sinus. The dorsal end of it projects into a sinus just below the madreporic plate. This is called the madreporic vesicle, and is the right hydrocoel of the larva. The axial organ consists largely of connective tissue, and, its wall being folded, it appears to be penetrated by epithelial diverticula of the body-cavity on the one hand and the axial sinus on the other. It contains a number of cells which in the larva were derived from a downgrowth into it of the genital rudiment.* Its relation to the axial sinus is described on p. 231. They are dioecious. The gonads are typically five in number ; but in many Spatangoids the number is reduced to 4, 3, or even 2, and in some Clypeastroids to 4. They are branched racemose glands and are interradial in position. With a few exceptions in Clypeastroids, their ducts open through the basal plates. The genital organs arise as they do in Asteroids as an out- growth of the genital rudiment (p. 146), which, becoming enclosed by a fold of the wall of the left coelomic sac, encircles the apical pole of the animal and constitutes the genital rachis. The genital organs themselves are developed as outgrowths of the rachis. The part of the left coelom enclosed by the fold above referred to becomes cut off from the rest and persists as the aboral sinus (p. 146). The eggs are small and very numerous ; they are generally discharged into the sea, where fertilization occurs. In a few forms (species of Cidaris) the eggs become attached to the apical part of the test amongst the spines and undergo their develop- ment there. In some Spatangoids, some or all of the dorsal parts of the ambulacra are sunk in and serve as brood pouches (marsupia). The free larva has the pluteus form ; for a description of it and of the main features of the development the reader is re- ferred to pp. 140, 150. Echinoids have a considerable power of repairing injuries, but not of forming new individuals from broken-off pieces. Asexual reproduction is unknown in the group. The Endocydica for the most part creep upon a rocky bottom, * The genital rudiment is developed as a solid outgrowth of the epithelium of the left posterior coelomic sac close to the septum separating it from the left anterior coelom. ECHINOIDEA. 235 and have the power of climbing steep slopes, by means of their powerful suckers. They also use the large spines for locomo- tion. In the genus Cidaris, in which the spines are very large, the feet are not important in locomotion and have feeble suckers. Hence this genus prefers deep water in which it is not rolled about. They frequently live in cavities, which they hollow out of quite hard rock by mechanical action of their teeth or other- wise. The Exocydica on the other hand for the most part live in sand, and the locomotive tube-feet are relatively feebly developed. In Echinocardium cordatum, which lives buried about eight inches deep in sand, the tube-feet of the anterior ambulacrum near the apex are very long, several times the length of the animal. They project up through the sand through a hole above the apex and end in frilled suckers. They catch food, shrink back through the hole, and hand it to the buccal tentacles.* There can be little doubt that the affinities of the Echinoidea are with the Asteroids. The general plan of structure and the relations of the chief systems of organs are the same in the two classes. The anatomical differences are small and relate to comparatively unimportant features, such as the structure of the alimentary canal and the calcareous covering of the body. The most important difference is the closure of the ambulacral groove and its conversion into the epineural canal, so that the ectoneural central nervous system is placed in the wall of a closed canal as it is in Chordates, but this peculiarity is shared by Ophiurids. The differences in external form, though con- siderable at the first glance, are much diminished on a close inspection. If the oro-anal axis of a pentagonal Asteroid such as Asterina be elongated and the antambulacral surface reduced we get the body-form of a typical sea-urchin. The madreporite in being aboral and interraclial occupies a similar position in the two classes, and in both Asteroids and regular Echinoids the anus though close to the aboral pole is always slightly excentric. This position of the anus is highly significant not only as in- dicating the fundamental asymmetry of the body but also be- cause it is identical or nearly identical in the two groups. In Asteroids it is placed in the interradius adjacent to that of the madreporite (Fig. 122) ; in the regular Echinoids it is either * An observation of the late Dr. Robertson of Cumbrae communicated by Dr. MacBride. 236 PHYLUM ECHINODERMATA. in this interradius or in the next radius (radius No. I of our enumeration, Fig. 83). In the irregular Echinoids the anus lies in the next interradius but one (interradius No. V. I of our enumeration, Fig. 83) the so-called posterior interradius of the Exocyclica, and the question arises which position is most primitive, the subapical position in interradius I. II, or in radius I of the Endocyclica, or the position in the Exocyclica in which it lies remote from the apex in interradius V. I. It is impossible to answer this question. On the one hand the similarity between the Asteroids and regular Echinoids on this point suggests that the subapical position is primitive, but this similarity may be apparent only. On the other hand there is plausibility in Loven's contention that its position in the posterior interradius, recalling as it does the condition found in Palaeozoic Crinoids, is really primitive. Both these arguments, however, rest upon an unproved assumption, viz. that the homologies between the different radii and interradii of the classes of Echinodermata have been determined. We know nothing on this subject, but it must be admitted that, taking into account the obvious affinities between the Asteroids and Echinoids, there is more probability in favour of the homologies implied by the first view than in those implied by the second ; for the Crinoids are the most out- lying group of living Echinoderms. The development of Echinoidea, though differing in many points from that of Asteroids, bears out on the whole the view as to their affinities suggested by their adult structure.* On account of their marine habits and the structure of their body-wall the Echinoidea are well suited for preservation as fossils, and an immense number of extinct species are known. Writing in 1881 A. Agassiz estimated that 2,000 fossil and 225 recent species were known and the number is now probably much greater. The study of these forms and the elucidation of their affinities present problems of the greatest importance to the student of organic evolution. Of the extant families the Cida- ridae alone are found in Palaeozoic formations. The Diadema- toida, Holectypoida, Cassidulidae and Collyritidae begin in the Jurassic. The Spatangidae are not found till the Cretaceous, and the Clypeastridae do not make their appearance before the upper * See MacBride, Phil. Trans., 195, 1903, p. 316. JSCHINOIDEA. 237 layers of the Cretaceous. The palaeozoic Palaeechinoidea are very imperfectly known, but they comprise exocyclic as well as endocyclic forms, though the majority are Endocyclic. They make their appearance in the Upper Cambrian. From the late appearance of Spatangidae and Clypeastridae it has been commonly assumed that the Endocycilca preceded the Ectocyclica in evolution. This view is borne out by the fact that Holectypus and the Cassidulidae, which are intermediate between the regular and irregular type, preceded the Spatangidae in the geological succession. But bearing in mind the fact that Echinocystites is an exocyclic form from the Upper Silurian, we should be prudent in suspending our judgment on this point, until the Palaeozoic Echinoid fauna has been more fully inves- tigated an attitude which is still further justified when we remember that Collyrites, which is more modified in some re- spects in the Spatangid manner than any of the Spatangidae, preceded the latter in its first appearance, and is contemporan- eous with (? before) Holectypus. A. Agassiz (Challenger Echinoids, p. 19) in discussing the origin of living Echinoids calls attention to the hopeless nature of the attempt to re- present the geological succession of forms either diagrammatically or descriptively, and points out that this hopelessness is due to the great number of different combinations of the various characters which have existed in extinct forms. The structural features of living Echinoids are the same essentially as those of extinct forms, but they are combined differently. Features which have apparently disappeared reappear quite suddenly and apparently in no connexion with the types which have immediately preceded them. " We cannot hope," he says, " to trace the development of any type through a series of forms each slightly different from its pre- decessor ; we must only expect to be able to follow the changes of a single feature and study it in its combination with other features, combinations which from their very nature can never form an unbroken series, as their terms are not synchronous. " If we examine in the same manner [i.e. by tracing it through the Echinoids of all time] any one of the structural features which have once made their appearance, we find that, without exception, they are either persistent to the present day or can be traced in a somewhat modified form in some one of the types now living, though the peculiar combination of any definite number of these may have disappeared." Finally Agassiz goes on to say (p. 23) that " adopting this method of tracing the development of a single structural feature at a time such as the growth of the poriferous zone from the simple paired zone to the com- plicated ambulacral zone of a Spatangoid, we shall find that the most primitive ambulacral zone known still exists side by side with the exist- ence at the present day of the resultants, if we may so say, of all the com- binations which have taken place." 238 PHYLUM ECHINODERMATA. The classification of the Echinoidea is at present under revision at the hands of Mortensen and others (op. cit.). Pending the completion of this work, we have after some hesitation decided to adopt, in its main features, the classification propounded by Duncan (op. cit.) in 1891. It has two considerable advantages : it holds the field, having been adopted by Zittel, Delage and Lang in their valuable textbooks ; and it is extremely simple, introducing the smallest possible number of new terms. The system is as follows : Order 1. PALAEECHINOIDEA. ., 2. EUECHINOIDEA. Sub-order 1. Cidaroida. ,, 2. Diadematoida. Section 1. Streptosornata. ,, 2. Stereosomata. Sub-order 3. Holectypoida. 4. Clypeastroida. ,, 5. Spatangoida. Section 1. Asternata. ,, 2. Sternata. Order 1. PALAEECHINOIDEA.* With only one or with more than two vertical roivs of plates in each of the five interradii, and with two or many vertical roivs of simple or compound plates in each of the five ambulacra. This sub-class comprises exclusively extinct and for the most part palaeozoic t forms. To the characters mentioned in the definition the following may be added. The peristome is in the middle of the oral surface. Jaws are present. The plates may or may not overlap. The anal area is either within the apical system, or outside it in the posterior interambulacrum. Echino- cystites alone is known to be exocyclic. The sub-class dates from the Upper Cambrian (Bothriocidaris). The most important genera are as follows : Bothriocidaris Eichw., interambulacral plates in one row. Upper Cam- brian. Echinocystites W. Thorns., exocyclic, the anus and madreporite * K. A. Zittel, Handbuch der Palaeontologie, Leipzig, 1880 ; also A. Agassiz, op. cit., P. Martin Duncan, op. cit. f Tiarechinus is from the Trias, and Tetracidaris, which is here placed with the Euechinoidea among the Cidaridae, but is by some regarded as belonging to the Palaeechinoidca is from the Cretaceous. ECHINOIDBA. 239 are on the posterior interradius, interambulacra multiserial, Silurian. Palaeodiscus Salter, with flattened body, Silurian. Lepidocentrus J. Miill., Devonian. KonincTcocidaris Dollo and Buis., Carboniferous. Perischodornus * McCoy, Carb. Archaeocidaris McCoy, Garb. Lepido- cidaris Meek and Worthen, Carb. Lepidechinus Hall, Dev., Carb. Palae- echinus McCoy, Sil., Carb. RhoecMnus W. Keeping, Carb. Melonites Norwood and Owen, Carb. Oligoporus Meek and Worthen, Carb. Lep-i- desthes Meek and Worthen, Carb. Tiarechinus Neumayr, with only four plates in each inter ambulacrum, one at the peristomium and three extending side by side from the peristoniial to the apical system ; apical system unusually large. Order 2. EUECHINOIDEA f With two vertical rows of plates in each of the five interradii, and a similar number of vertical rows of simple or compound plates in each of the five radii. The peristome is on the oral side and rarely placed anteriorly towards the edge of the shell. Jaws and teeth are present or absent. The anus is either within the apical system, or in the posterior interradius. The sub-class comprises some extinct and all recent forms. The Cidaroida and Diadematoida are Endocyclica or regular sea-urchins, the Holectypoida, Clypeas- troida and Spa.tangoida are Ectocyclica or irregular forms. Sub-Order 1. CIDAROIDA. Test spheroidal. Ambulacra narrow, usually composed of primary plates, rarely compound. Moiith central, anus within the apical system. With internal branchiae only (Endobranchiata). With jaws and more or less vertically placed teeth and a discontinuous perignathic girdle. With large spines and tubercles. The interradial as well as the ambulacral plates are continued on to the oral area to the mouth, and are imbricated on^the peristome. Sphaeridia, ophicephalous and triphyllous pedicellariae are absent. Carboniferous to the present day ; principal distribution in the Jurassic and Cretaceous. Fam. 1. Cidaridae with the characters of the order. Dorocidaris A. Ag., N. part of Atlantic Ocean, 50-1,500 fms., D. papillata Leske, W. coast Ireland. Cidaris Leske, cosmopolitan in the warm seas, littoral to 300 fms. Phyllacanthus Brdt., Red Sea to Australia, littoral. Porocidaris Desor, P. purpurata W. Th., N. Atl., 300-1,500 fms. Extinct genera : Orthocidaris Cotteau, Temnocidaris Cotteau, Diploci- daris Desor, Tetracidaris Cotteau, Cretaceous, see note, p. 238. Sub-Order 2. DIADEMATOIDA. Mouth central, anus within the apical system. J Internal branchiae well developed, reduced, or absent. With external branchiae (Ectobran- * Sollas, Quart. Journ. Geol. Soc., 55, 1899, p. 70. f P. Martin Duncan, op. cit. j In one extinct genus, Heterodiadema, the posterior basal is absent, and the periproct is pushed back a slight distance into the posterior inter- radius. 240 PHYLUM ECHINODERMATA. chiata) and incisions in the peristome margin. With jaws and teeth and continuous perignathic girdle. Ambulacral plates alone continued on to the oral area, where they may appear as separate buccal plates. Sphaeridia, ophicephalous and triphyllous pedicellariae present. Jurassic to the present day. Section 1 . STREPTOSOMATA. Test more or less flexible, with external and internal branchiae. Peri- stomial ambulacra! plates in several rings. Fam. Eehinothuridae. With the characters of the sub-order. The thin and flexible tests are large and tumid, or depressed. Some of the spines of the interambulacra have poison sacs near their ends and are poisonous. The plates of the apical system are usually separate. Coronal plates feebly calcareous and with membranous edge, with open reticulate structure. Internal longitudinal muscles for moving the plates. A flexible echinoid was described in 1863 by S. P. W'oodward from the chalk. Asthenosoma, the first recent form was described by Grube * in 1868, and rediscovered by Wy. Thomson t in the dredgings of H.M.S. Porcupine. The large internal branchiae were discovered by the Sara- sins t in 1888. Pelanechinus Keeping, apical plates absent, Oolitic. Echinothuria S. P. Woodward, apical plates absent, Upper Cretaceous ; Phormosoma Wy. Thorns., recent, 120-2,750 fathoms, N. Atlantic to Azores, in most seas; Asthenosoma Gr., recent, 100 to 450 fathoms, N. Altantic and in most seas. Echinosoma, Calveria, Araeosoma, etc. Section 2. STEREOSOMATA. Test rigid, with external branchiae, and reduced or absent internal branchiae. With 5 pairs of isolated peristomial ambulacra! plates (buccal plates). Fam. 1. Saleniidae. Ectobranchiate, with persistent central or centrals. Ambulacra narrow ; the plates are primaries, rarely compound actinally. Interradial plates few, tubercles large. Sphaeridia present. Jaws with the foramina of the pyramids unarched by epiphyses, teeth with a keel. Jurassic to recent. Mostly extinct forms. Peltastes Ag., AcrosaleniaAg., extinct ; Salenia Gray, f and r, Caribbean Sea, etc., 60-1,700 fathoms. Fam. 2. Hemicidaridae. Exclusively fossil forms, Permian to Cre- taceous. Hemicidaris L. Ag., Acrocidaris L. Ag., Goniopygus L. Ag., genital openings outside the apical system in the interradii ; Circopeltis Pomel, Cidaropsis Cotteau, Glypticus L. Ag. Leptocidaris Quenstedt, allied here. Fam. 3. Aspidodiadematidae. With spheroidal test and large, narrow, ringed apical system formed by broad basals and broad intervening radials. Interradial plates few. Ambulacra with low primary plates ; pores in straight series, one pair in each plate. Peristome incised ; branchiae bifid with ten large buccal plates. Tentacles heteropodous. Aspido- diadema A. Ag., 100 to l,700fathoms, Caribbean Sea, N. part of S. Atlantic, Philippine Sea. Dermatodiadema A. Ag. * Jahresb. d. Schles. Ges. f. Vaterl. Cult. 1868, p. 42. f Phil. Trans. 144, 1874, p. 737. j P. and F. Sarasin, Erg. Nat. For. auf Ceylon, 1, 1888, p. 129. ECHINOIDEA. CIDAROIDA. 241 Fam. 4. Diadematidae. Regular, ectobranchiate, with or without vestiges of internal branchiae ; shell highly ornamented ; ambulacra usually narrower than interambulacra, with vertical rows of primary tubercles, and usually consisting of compound plates, all the components of which are primaries ; the pore-pairs are usually in a simple row, and sometimes in double rows only near the mouth and apex. Interambxi - lacra also with vertical rows of primary tubercles. Teeth grooved, jaws without a closed pyramidal foramen, feet heteropodous. Chief distribution in Jurassic, Chalk and Tertiary. Diadema Schynvoet, with blue patches (ocellar) on the shell, f and r, most seas ; Centrostephanus Ptrs., r ; Placodiadema Duncan, f ; Hetero- diadema Cotteau, f ; Codiopsis L. Ag., f ; Pleurodiadema De Loriol, f ; Magnosia Michelin, f ; Goltaldia Desor, f ; Diplopodia McCoy, f ; Pedi- nopsis Cotteau, f ; Acanthechinus Duncan and Sladen, f ; Phymechinus Desor, f ; Asteropsis Cotteau, f ; Diplotagma Schliiter, f ; Micropyga A. Ag., r, 100 to 600 fathoms, Philippines, Fiji ; Plistopliyma Peron and Gauthier, f ; Pedina L. Ag., f ; Echinopedina Cotteau, f ; Stomechinus Desor, f ; Micropedina Cotteau, f ; Heterocidaris Cotteau, f ; Echinothrix Peters, r, East coast Africa, Pacific Islands, Red Sea, etc. ; Astropyga Gray, r, Panama, California, Zanzibar, etc., with overlapping plates ; Poly- cyphus L. Ag., f ; Codechinus Desor, f ; Orthopsis Cotteau, f ; Eodiadema Duncan, f ; Peronia Duncan, f ; Echinopsis L. Ag., f ; Gymnodiadema De Loriol, f. Progonechinus Duncan and Sladen, f, allied here. Fam. 5. Cyphosomatidae. Contains only one recent genus, Coptosoma Desor, f and r ; and the following extinct genera. Cyphosoma L. Ag., Gauthieria Lambert, Thylechinus Pomel, Micropsis Cotteau. Fam. 6. Arbaciidae. Test depressed abactinally, flat actinally ; epistroma with granules, projecting ridges, sessile glassy knobs. Apical system large ; periproct oval and oblique, composed of four triangular plates ; pore in ocular plates double. Ambulacra straight, narrow, ex- panding near the peristome ; pore-pairs simple or in large arcs or crowded actinally ; plates compound near the ambitus ; in the compound plates the middle component is a large primary, while the aboral and adoral components are demiplates, or the primary is adoral and the demiplates are aboral to it. Sphaeridia solitary or numerous. Peristome large, in- curved at the sides of the ambulacra. Teeth keeled ; auricles not closed above. Tertiary and recent. Arbacia Gray, f and r ; Echinocidaris Duncan and Sladen, r, most seas ; Coelopleurus L. Ag., f and r ; Podoci- daris A. Ag., r, 150 to 1,075 fms., Caribbean, Philippines ; Dialithocidaris A. Ag., deep sea. Fam. 7. Temnopleuridae. Regular ectobranchiate with the teeth keeled, and auricles closed. Ambulacra with triple compound plates. The suture of the ambulacral and interradial plates and of the apical system grooved and may be pitted, or there may be a raised ornamentation, costulate or reticulate or in ridges, the sutures being furrowed or not. Cretaceous to recent. Glyphocyphus J. Haime, f ; Dictyopleurus Duncan and Sladen, f ; Arachniopleurus Dun. and Slad., f ; Ortholophus Duncan, f. ; Paradoxechinus Laube, f ; Eckinocyphus Cotteau, f ; Zeuglopleurus Gregory, f ; Lepidopleurus Dun. and Slad., f ; Coptophyma Peron and Gauthier, f ; Trigonocidaris A. Ag., r, Florida, Caribbean, etc. ; Temno- pleurus L. Ag., f and r, Indian Ocean, Persian Gulf, Pacific ; Pleurechinus L. Ag., f and r ; Temnechinus Forbes, f and r ; Salmacis L. Ag., f and r, Red Sea, Ind. Ocean, etc. ; Salmacopsis Doderlein, r ; Mespilia Desor, Z III R 242 PHYLUM ECHINODERMATA. r ; Microcyphus L. Ag., r ; Aniblypneustes L. Ag., r ; Goniopneustes Dim- can, r ; Holopneustes L. Ag., r ; Hypsiechinus Mrtsn, r ; Grammechinus Dun. and Slad., f. Fam. 8. Stomopneustidae. Large forms with closed auricles and powerful spines. Globiferous pedicellariae without end-tooth. Stomo- pneustes L. Ag., littoral, Indian Ocean, Australia. Fam. 9. Echinidae. Regular shell with ambulacra and interambulacra of equal width ; tube feet similar. Ambulacral plates compound with three pairs of pores which are arranged in arches of triplets. Peristomial notches small. Coronal plates without pits or grooves, and their opposed surfaces are plain. Globiferous ped. with an end-tooth and one or several lateral teeth on each side. Teeth keeled. Cretaceous to recent. Pare- chinus Mrtsn., pores trigeminate, primary tubercle on all the ambulacral plates, globiferous pedicellariae without neck and no cross beams connect the edges across the inside of the blade, numerous short greenish spines ; P. miliaris Miill., North Sea, etc. Loxechinus Des. Echinus L., pores trigeminate, primary tubercle on every or only on every other ambula- cral plate, spines upon the whole long and strong, the actual primary spines not curved at the point, globiferous pedicellariae generally with the edges connected across the inside of the blade, no ocular plate reaches to the periproct. E. esculentus L., primary spines short, mainly littoral to about 100 fms. E. acutus Lmk., primary spines much longer than secondary, to 1,350 fms. Sterechinus Koehler, Paracentrotus Mrtsn., P. lividus. Fossil genera, Stirechinus, Glyptechinus, Sporotaxis, etc. Fam. 10. Toxopneustidae. Globiferous pedicellariae with end-tooth but without lateral teeth, the edges of the blade quite coalesced on the inside so that the blade is tubular, usually 1-2 oculars reach the periproct. Psammechinus L. Ag. Gymnechinus Mrtsn. Toxopneustes L. Ag. (Boletia Des.), littoral forms, Ind. -Pacific Oc. Tripneustes Ag. Sphaerechinus Des., Channel Islands, Med., etc. Pseudoboletia Trosch. Pseudocen- trotus Mrtsn. Strongylocentrotus Brdt., S. droebachiensis Miill. Antho- cidaris Ltk. Parasalenia A. Ag. Fam. 11. Echinometridae. Globiferous pedicellariae with end-tooth and one impaired, strong lateral tooth, the edges of the blade almost always connected by cross-beams across the inside ; no neck ; all littoral. Pseudechinus Mrtsn. Heliocidaris Desml. (Evechinus Ver.), New Zealand. Echinostrephus Ag., Indo-Pac. Toxocidaris Ag., Australia. Echinometra Rond., cosmop. in warm zone. Heterocentrotus Brdt., Indo-Pac. Colo- bocentrotus Brdt., Indo-Pac. Sub-Order 3. HOLECTYPOIDA. Mouth central, anus outside the apical system in the posterior inter- radius, either dorsal and close to the apical system (Pygaster, Pygastrides) or ventral (Holectypus, etc.). The posterior genital opening usually absent. The madreporite may extend back and occupy the place of the anal area. With external branchiae, apetaloid ambulacra, and a pair of pores or only one pore on each ambulacral plate. The plates of the corona are not pro- longed on to the peristome. With feeble jaws and vertical teeth, or without these structures. Sphaeridia present. The perignathic girdle is variable : it may be weak, or it may form a strong collar, the interradial portions of it being wide and bent upwards and outwards from the peri- stome internally. All the genera included in this group are Jurassic and ECHINOIDEA. 243 Cretaceous, except Pygastrides * Loven from the Caribbean Sea, 200-300 fathoms. The fossil genera are Holectypus Desor, Pileus Desor, Pygaster L. Ag., without the posterior basal ; Discoidea Klein ; Conoclypeus L. Ag. Galeropygus Cotteau is allied here. Sub-Order 4. CLYPEASTROIDA (Cake-urchins). Mouth central or nearly so ; anus outside the apical system in the posterior interradius and is either on the lower side or at the margin. With external branchiae and well-developed jaws and jaw skeleton. Tube-feet heteropodous. The edge of the corona is usually close to the mouth and there are no perforated ambulacra! plates in the peristomial membrane. Peristomial margin consists of 10 ambulacral f and usually 5 interambulacral plates. Teeth visually more or less horizontal, rarely vertical. Sphaeridia are present, few, and covered. The test is usually flat, rarely arched dorsally ; its margin may be notched. Its cavity is traversed by calcareous pillars and septa which pass from its upper to its lower walls. The apical system is much reduced. The madreporite is central (Fig. 159) and the basal plates are coalesced with each other and sometimes with the oculars. The madreporite may spread out on to all the apical plates, and the generative openings may descend into the interambulacral areas (p. 218). There are at least four genital openings, visually five ; when only four the posterior is absent. There are five petals (Fig. 152) with pairs of pores, which pass out between the plates and are generally yoked. There are numerous small simple tube-feet, each one in relation with one pore ; for the arrangement of these see p. 215. The interambulacral plates may be interrupted (p. 217) and often have small single pores, at least on the lower surface of the shell. The interambulacral marginal peristomial plates are single ; the perignathic girdle is discon- tinuous. In the young state the shell is regular and there are five equal interradii. Cretaceous to the present day. Fam. 1. Fibulariidae. Small forms with rudimentary, widely open, few-pored petals, pores of petals not yoked ; jaws rather high, teeth superior and slanting. Perignathic processes broad, low, one on each interradius. Interambulacra small, terminating in a single apical and a single peristomial plate. Periproct visually on the lower surface. Slightly developed vertical partitions within the test, limiting the ambulacra at their side on the oral surface and radiating towards the peristome. A sphaeridium in each ambulacrum, covered. Echinocyamus van Phels., f and r, madre- porite with only one pore, pore-pairs not yoked. E. pusillus Gray, N.E. Atlantic and Brit. Seas, Mediterranean, etc. ; Sismondia Desor, Runa A. Ag. ; Fibularia Lamk. r and f ; Moulinsia L. Ag. ; Rotuloidea R. Etheridge. Fam. 2. Clypeastridae. Petals well developed ; visually unequal ; actinal furrows straight. Small pores scattered, generally on interam- bulacra as well as on ambulacra and not specially confined to furrows. Interambulacra actinally discontinuous, one peristomial and two apical plates in each. Sphaeridia, two in each ambulacrum, covered. Perignathic * Loven, Bih. Svenske Akad. Hand., 13, 1888. f In some Clypeastroids two small tube-feet, each with a separate pore, perforate the edge of each pair of marginal ambulacral plates (J. Miiller, op. tit., p. 39). These seem to be the only representatives of the marked oral tube-feet of Spatangoids. 244 PHYLUM ECHINODERMATA. processes tall, narrow, two on each ambulacrum, fitting in below the jaws. Genital pores sometimes outside the apical system (see p. 218). ", Clypeaster Lamk. (Fig. 172), Red Sea, Indian Ocean and warmer seas generally ; Diplothicanfhus Duncan, Plesianthus Duncan, all recent and fossil ; Anomalan- thus J. Bell, r. Fam. 3. Laga- nidae. Test flat ; petals unequal, nar- row, lanceolate ; ambulacra beyond them very wide ; pore-pairs for bran chial tube-feet few, and between them are minute pores for prehensile ten- tacles. Interradii small, continuous ; each with a single apical and peristo- mial plate. Peri- proct between the peristome and pos- terior margin. Peri- gnathic processes single on the inter- radial peristomial plates, situated so as to be beyond not below the jaws. Klein, r FIG. 172. Clypeaster rosaceus (Rgne animal). Laganum and f. Fam. 4. Scutel- lidae. Test very flat, often lobed or perforated. Ambulacral furrows on lower side bifurcating and branching (pore-fasciae); small pores on actinal surface only found in furrows. Radiating partitions internally. Scutella Lamk. (Fig. 157), fossil ; Echinarachnius Leske, r ; Echinodiscus Brey- nius, f and r ; Encope L. Ag., f and r ; Mellita Klein, f and r ; Lenita Desor, f; Mortonia Desor, f; Eotula Klein, r; Aracknoides .Breynius, f and r. Sub-Order 5. SPATANGOIDA (Heart-urchins). Test often more or less heart-shaped. Mouth central or subcentral, or at the front end of the lower surface of the shell. Anus outside the apical system in the posterior interradius (Fig. 153). External gills, jaws, teeth and perignathous ring absent. The plates of the shell are not con- tinued on to the peristome. Sphaeridia present. Large spines are never found. Fascicles and clavulae are frequently present. There is usually an ambulacra! rosette on the upper surface ; sometimes with only four petals, the anterior ambulacrum not being petaloid. When the mouth is shifted in the direction of the anterior radius, it is transversely elongated and possesses on its hinder border a lip formed by ECHINOIDEA. SPATANGOIDA. 245 the enlarged peristomial plate (labrum) of the posterior interradius. They are usually sand-burrowers and the tube-feet show considerable diversity. In the young state the form approaches that of the regular urchins in the position of the mouth and the form of the ambulacra. Jurassic to the present time. The Spatangidae are first found in the Cretaceous. Section 1. ASTERN ATA. Shell ova], mouth central or sub-central, without sternum and fascicles ; ambulacra all alike, simple or petaloid. Apical system compact or elongate. Interradii some or all with a single peristomial plate. With- out plastrons, with or without floscelles. Through the Echinoneidae they are related to the regular forms and through the Cassidulidae to the Clypeastrids. Fam. 1. Echinoneidae. Ambulacra simple, all alike, without petals. Mouth central, without floscelle. With four perforated basal plates. Mostly begin in Cretace- ous, but a few in the ;]) Jurassic. Echinoconus Breyn., f, Cretaceous ; Lanieria Duncan, f ; Echi- noneus van Phel., f and r, Caribbean Sea, Australia, Zanzibar, etc. ; Amblypy- gus L. Ag., f ; Caratomus L. Ag., f ; Pygaulus L. Ag., f ; Pyrina Desm., f, Jurassic to Eocene ; Nucleopygus L. Ag., f ; Anorthopygiis Cotteau, f ; Haimea Mich, f ; Oligo- pygus De Lor., f ; Echi- nobrissus Breyn. (Nucleo- lites Lm.), f ; Anochanus Grube, r ; Botriopygus d'Orb. f ; Ilariona Dames, f. Fam. 2. Cassidulidae. Ambulacra petaloid or not, closed or open below. Peristome with floscelle (Fig. 173). With four genital pores, the basals are sometimes fused ; the madreporite much extended. Jurassic to present day. Cassidulus Lamk., f ; Echinanthus Breyn., f ; Studeria Dune. (Catopygus L. Ag.), f and r; Clypeus Klein, f ; Py gurus d'Orb., f ; Echinolampas Gray, f and r ; Conolampas A. [Ag. r ; Neolampas A. Ag., r ; Rhynchopygus d'Orb., r. Section 2. STERNATA. Peristome excentric anteriorly. Sternum well developed. Anterior ambulacrum different from the rest (except in Ananchytidae). Fascioles present or absent. Without floscelle. Fam. 1. Collyritidae. Without floscelle ; apical system elongate (Fig. 162), disjunct ; the two posterior radials being separated by a con- I FIG. 173. Oral region of Cassidulus pacificus showing phyllodes (after Loven from Delage). 6 mouth ; I-V radii numbered. 246 PHYLUM ECHINODERMATA. lit FIG. 174. Apical region of Spatan- gus purpureus (after Loveu from Delage). gtx basal ; mdp niad- reporite ; I-V radii numbered. siderable interval, which is occupied by supplementary plates, from the rest of the apical system. Ambulacra similar, Jurassic, Cretaceous. All fossil. Dysaster L. Ag. ; Collyrites Desm. Fam. 2. Plesiospatangidae. Fossil. Fam. 3. Ananchytidae. Apical system elongate or compact. Apetal- ous. With or without an anterior groove. Echinocorys Breyn., f ; Holaster L. Ag. (Fig. 161), f ; Offaster Desor, f ; Cardiaster Forbes, f ; Urechinus A. Ag., 422-1,800 fathoms, Pacific ; Cystechinus A. Ag., f and r ; 1,000-2,000 fathoms ; Calymne W. Thorns., r, 2,650 fathoms, N. of Bermuda. Fam. 4. Spatangidae. Usually heart- shaped and with an anterior groove. Apical system with four or less perforated plates ; compact or with the madreporite variable in its posterior extension ; radials five and external. The test is longer than broad and bilateral symmetry is marked. The anterior ambulacrum is always different from the other four, which may be petaloid abactinally. Fascicles are present in most genera, but they may be absent ; and the genera may be grouped according to this feature and their arrangement if present. They begin in the Lower Creta- ceous, and reach their highest development at the present day. Recent Genera. Platybrissus Grube ; Palaeopneustes A. Ag., Caribbean Sea ; Hemiaster Desor, f and r ; Faorina Gray, China ; Linthia Merian, f and r ; Schizaster L. Ag., f and r ; Agassizia Val., f ^ s and r ; Moira A. ^ Ag., f and r ; Moi~ r ops is A. Ag. ; Bris- sus Klein, f and r ; Meoma Gray, f and r ; Metalia Gray, f and r ; RMnobris- sus A. Ag. ; Bris- sopsis L. Ag., f and r ; Spatangus Klein, f and r, S. purpureus Leske, 5-530 fms., Channel Islands, North Sea, etc. ; Maretia Gray, f and r, 25 to 800 fathoms ; Eupa- tagus L. Ag., f andr, Australia ; Macropneustes L. Ag., f and r, Caribbean ; Nacospatangus A. Ag., Linopneustes A. Ag. ; Cionobrissus A. Ag. ; Echino- cardium Gray, f and r, littoral to 2,675 fathoms, worldwide ; Breyn in Desor, f and r ; Lovenia L. Ag. and Desor, f and r, 10-28 fathoms. The following are apetalous : Genicopatayus A. Ag., Antarctic; Palaeo- FlG. 175. Pourtalesia jef/reysi (from Delage, after Loven). / anal Semite ; prc periproct ; 3, J, o third, fourth and fifth in- terradii. ECHINOIDEA. SPATANGOIDA. 247 brissus A. Ag., Caribbean ; Aceste W. Thorn., 600 to 2,600 fathoms ; Aerope W. Thorn., 800 to 1,750 fathoms; Palaeotropus Loven, 82 to 375 fms. ; Homolampas A. Ag., 32 to 2,475 fms. ; Argopatagus A. Ag., 800 fms. Fam. 3. Leskiidae. Test thin, ovoid. Api- cal system with three basals fused into one ; two large genital pores upon conical promin- ences. Peristome in front, pentagonal, with five angular buccal plates. A peripetalous fasciole. Palaeostoma Loven, China. Fam. 4. Pourtalesiidae. Elongated and Holothurian-like in appearance. Test thin, transparent. Mouth terminal in front. Anus behind, above the posterior projecting rostrum if that is present, or terminal on the actinal surface. Apical system variable, compact or disjunct. Peristomial margin also variable : in some species Loven's law is carried out, in others it is not. Ambulacra not petaloid. With four or three genital openings. Plates for the most part with single pores ; tube- feet homoiopodous. Is the living representa- tive of the Cretaceous genus Infulaster Hag. Atlantic and Pacific, 345 to 3,000 fms. Pourtalesia A. Ag., Spatagocystis A. Ag., Echinocrepis A. Ag. FIG. 176. Apical region of Pour- talesia jeffreysi (after Loven, from Delage). gtx genital opening ; mdp madreporite. Class HOLOTHUROIDEA.* Sea-cucumbers. Elongated, worm-like Echinoderms with a dermo-muscular body wall containing small isolated calcareous bodies. With con- tractile tentacles surrounding the mouth and containing prolonga- tions of the water -vascular system. The apical system of plates is not developed at any stage of life, and the water -vascular pore * J. Miiller, Ueb. Synapta digitata u. ub. die Erzeugung von Schnecken in Holothurien, Berlin, 1852. G. F. Jaeger, De Holothuriis. Diss. Inaug. Zurich, 1833. De Quatrefages, Mem. sur la Synapte de Duvernoy, Ann. Sci. Nat. (2), 17, 1842. A. Baur, Beit. z. Naturgesch. d. Synapta digitata, Dresden, 1864, and Jena, 1865. Semper, Holothurien, Reisen in Archipel der Phillipinen, II. 1, 1868. H. Ludwig, " The Holothuroidea," Mem. of the Museum o/Comp. Zoology Harvard College, vol. 17, 1894. Id. "Holo- thuroidea " in Bronn's Klassen und Ordnungen des Thierreichs, Bd. 2, Abt. 3 , 1889-1892. Theel, Hj., " Report on the Holothuroidea," Challenger Reports, Pt. 1, in vol. 4, 1882, and Pt. 2, in vol. 14, 1886. Id., " Report on the Holothuroidea," Bull. Mus. Com. Zool. Harvard College, 13, 1886. R. Perrier, Holothuries, Exped. Sc. Travailleur et Talisman, 2, 1902, p. 273. A. Kowalevsky, Beit. z. Ent. d. Holothurien, Petersbourg, 1867. R. Semon, Entwick. d. Synapta, Jena. Zeitschrift, 22, 1888, p. 175. H. Ludwig, Zur Entwick. der Holothurien (Cucumaria planci), Sitzber. k. Preuss. Acad. d. Wiss. Berlin, 1891. E. Herouard, Recherches sur les Holothuries des cotes de France, Arch. Zool. Exp. (2), 7, 1889, p. 555, and Holothuries provenant des campagnes de la " Princesse Alice," Res. camp. sc. Monaco, Fasc. 21, 1902. Romanes and Ewart, Observations on the locomotor system of Echiiiodermata, Phil. Trans., 1882, p. 829. 248 PHYLUM ECHINODERMATA. T is usually absent in the adult. The axial organ and generative rachis are not present. In the Holothurians or sea-cucumbers the body is elongated in the direction of the long axis, at or near the two ends of which are the mouth and anus respectively, and the radial water- vessels are disposed in five equidistant meridional rows, extending from the oral to the anal pole. The mouth is surrounded by a row of tentacles and the animal lies with its long axis parallel to the substratum. Typically the mouth and anus are terminal, and there are five usually double rows of tube- feet which mark the radii and pass from the oral to the anal end of the body (Fig. 177). In such cases the symmetry is pentameral and there is apparently no- thing to mark one side of the body from the other. But as a matter of fact there are two structures which are not radially disposed. These are the stone-canal and the generative opening. The generative opening, which is single, is placed in the centre of an interradius, usually not far behind the tenta- cular circlet, and the stone- canal is in the middle line of the same interradius. This intei radius is called dorsal. It enables us to distin- guish a ventral radius, a right and left ventral radius, and a right and left dorsal radius. Our enumeration of the radii of Holothurians (Fig. 182) is the same as that adopted for other classes. It is based on the assumption that the madreporitic interradius is the same in all cases. We need not repeat our warning (p. 119) as to the inadvisability of basing important speculations as to homologies on this assumption. It sometimes Af Fio. 17". Cucumaria with extended dentriticalJy branched tentacles T. Af tube-feet (from Claus). HOLOTHUROIDEA. 249 happens that the side of the body carrying the three ventral radii and two ventral interradii (trivium) is flattened and modi- fied into a sole-like creeping surface (Fig. 185), while the dorsal side with its two dorsal radii (bivium) and three interradii is arched. In such cases the tube-feet of the trivium have dis- coidal ends and are suctorial in function, while those of the bivium are pointed and probably have a respiratory or tactile function : such non-locomotory pointed tube-feet are called ambulacral papillae. It will be noted that the radii of the bivium are not the same as in Asteroids and Echinoids (Fig. 182, cf. with Fig. 83). The body may be circular, or pentagonal in section ; when pentagonal the radii occupy the angles. More rarely the body is flask-shaped (Rhopa- lodina) or spherical. The ventral sole may occupy the whole length of the animal (Colochirus, Stichopus, Millleria etc.), or be confined to the middle portion (Psolus, Psolidium). Sometimes the dorsal interradius is much shortened ( Ypsilothuria) and concave, the ventral surface being correspondingly elongated and convex. In such cases the oral and aboral poles are approximated. In Rhopalodina this modification is carried to an extreme in that the dorsal interradius is practically obliterated. In this case the body is flask-shaped, the mouth and anus being closely ap- proximated, with the genital opening between them, at the end of the neck of the flask, and the body appears to have ten radii. In other cases the two ends of the long axis are bent ventralwards so that the mouth and anus appear to be on the ventral surface at each end of the flattened sole-like surface. Finally in certain deep-sea forms processes may be developed from the dorsal surface of the body. In Psychropotes the hinder region of the body projects back over the anus (Fig. 185), and in Peniagone dorsal lobes are developed over the anterior part of the body. In the pelagic form Pelagothuria (Fig. 186) a kind of umbrella is formed round the oral region. There are three kinds of ambulacral appendages : the tube- feet and ambulacral papillae already mentioned, and the ten- tacles. The tentacles contain a prolongation of the water-vascular system, usually of the radial canals, and are modified tube-feet. They vary in number from 10 to 30. The number is usually a multiple of 5, except in the Synaptidae, in which there are frequently 12, and is usually constant in the same species and even genus ; but there are genera and even species in which the number is variable. In the Dendrochirotae with ten tentacles, the two ventral (adult) tentacles are usually smaller than the others (Fig. 177). They may be pinnate (Molpadiidae, Synap- tidae), shield-shaped (Aspidochirotae), or dentritically branched 250 PHYLUM ECHINODERMATA. (Dendrochirotae). The tentacles and the part of the body carrying them are retractile. The ambulacra! feet terminate in a suctorial disc which is provided with a perforated calcareous plate, while the ambula- tory papillae have pointed ends and the calcareous plates are reduced or absent. In the Elasipodidae calcareous plates are only exceptionally present even on the feet. No sharp line can be drawn between these two kinds of appendages ; and it is often impossible to say whether we have to do with the one or the other. The ambulacral feet are essentially locomotory, while the papillae are respiratory and sensory. The distribution of these structures varies considerably even in the same genus. They may be arranged in radial rows, which may be single, double or multiple ; or they may be scattered on radii and FIG. 178. Calcareous bodies from the integument of Holothurians. a Calcareous wheels of ChirikOta ; h anchor with supporting plate of Synapta ; c stool-like body; d plate of Holothtiria impatiens : e hooks of Chiridota. interradii alike. If there is a ventral sole, the three ventral radii are provided with feet and the dorsal radii with papillae, scat- tered or in rows. Sometimes the feet are absent from the median row of the sole (some Elasipodidae, Fig. 184), and in Psolus the dorsal surface is altogether without ambulacral appendages. In the Molpadiidae, Synaptidae and Pelago- thuria both feet and papillae are absent, the tentacles being the only representative of ambulacral appendages. There is a dermo- muscular body wall, which contains isolated calcareous spicules of various form (Fig. 178). Calcareous plates such as are found in other Echinoderms are feebly if at all developed and no representatives of the oral and apical systems of plates are found at any stage of life. The skin has a leathery consistency and may be covered with warts and ridges. HOLOTHUROIDEA. 251 In Myriotrochus and many species of Synapta and Chiridota it is thin and transparent. The body wall consists of a single layer of non-ciliated epithelial cells which carry a cuticle, a thick cutis which consists of connective tissue and contains the calcareous todies, a layer of circular muscles which is often interrupted in the radii, five radial bands of longitudinal muscles (Fig 181), each of which may be double, and finally the layer of peritoneal epithelium lining the body cavity (Fig. 180). The cutis consists of a ground substance containing branched cells and fibres. The calcareous bodies of the cutis are minute in size and definite in shape* : they have the form of anchors, wheels, rods, perforated plates, stools, etc., and their shape and arrangement is of im- portance for the determination of species. They are found in the cutis of the tube-feet, ambulacral appendages, and tentacles, as well as in the body wall. In a few forms, e.g. Psolus, Theelia, the calcareous bodies of the dorsal side are large and plate-like and appear like protective scales. In the Dendrochirotae in which the anterior part of the body is invaginable, there are at the base of the invaginable part five calcareous plates the oral valves, which cover over the aperture when the proboscis is withdrawn. Similar plates are sometimes found round the anus. Both oral and anal plates may be radial or interradial in position. Retractor muscles capable of retracting the anterior part of the body are found in the Dendrochirotae and some other forms (Molpadia, species of Chiridota and Synapta). They are muscular bands detached from the longitudinal muscles at about the middle of the body and inserted into the radial pieces of the calcareous ring (Fig. 179). The calcareous ring consists of a circle of ten calcareous pieces five radial and five interradial which surround the oeso- phagus (Fig. 181, 28). It is placed in the outer wall of the perioesophageal sinus (Fig. 179) on the oral side of the water- vascular ring but aboral of the nerve ring. The interradial pieces of the ring may be wanting or there may be more than five interradial pieces. The water- vascular system consists of (1) a circular vessel * In some Holothurians they appear to change their shape with advancing age, e.g. Stichopus Japonicus, Ann. Zool. Japonenses, 1, 1897, p. 35. 252 PHYLUM ECHINODERMATA. round the oesophagus placed aboral of the calcareous ring in the outer wall of the perioesophageal sinus (Fig. 179) ; (2) five FlO. 179. Diagram of a longitudinal section through the oral region of a Holothurian. The section passes through a radius on the right side, and through an interradius on the left (from Lang). 1 cutis, 2 ectoderm, 3 oral tentacle cut off, 4 canal of the oral tentacle. 5 blood-vessel of the oral tentacle, 6 tentacle nerve, 7 circumoral nerve, 8 oral portion of perioesophageal sinus, 9 mouth, 10 oesophagus, 11 perioesophageal sinus, 12 interraclial piece of calcareous ring, 13 circumoral water-vascular vessel, 14 so-called blood-vascular ring, Id ventral intestinal vessel of the so-called vascular system, 76' epithelium of alimen- tary canal. 77 polian vesicle. IS ampulla of oral tentacle, 19 peritoneal epithelium, 20 circular muscles of body wall, 21 body-cavity, 22 and 2(J radial vessel of vascular system. 23 radial trunk of superficial nervous system. 21 radial cpineiiral canal. 25 radial peri- haemal canal, 27, 29 radial canal of water-vascular system, 28 longitudinal muscles, 30 radial piece of calcareous ring, 31 retractor muscle, 32 dorsal intestinal vessel of vascular system. radial vessels which travel oralwards to the anterior end of the body and then aboralwards in the radii of the body wall, just HOLOTHUROIDEA. 253 outside the longitudinal muscular bands to the apical pole, where they terminate blindly in the integument near the anus. There is no projecting terminal tentacle as in Asteroids and Echinoids. The radial canals are five in number ; they are absent only in Synaptidae. The circular vessel has two appen- dages the polian vesicle and the stone-canal. The polian vesicle (Fig. 181) is generally single and may be of large size; it is usually attached to the circular canal in the left adult- ventral interradius. Exceptionally, additional polian vesicles are present, generally in the adult-ventral region of the body. The stone-canal is usually single (always in Molpadiidae, Pela- gothuridae and Elasipodidae), but in the Synaptidae, Aspido- chirotae and Dendrochirotae it is occasionally multiple. When it is single it lies in the dorsal mesentery ; when multiple, the primary canal alone is in the mesentery, and the accessory stone- canals which are very variable in number (2 to 160) project, mostly from the dorsal half of the ring-canal and on either side of the mesentery, freely into the body-cavity into which they open (see below). Occasionally the primary stone-canal is in- dependent of the dorsal mesentery, and projects into the body cavity on the right-hand side (many Aspidochirotae). In a few species (Thyone chilensis, Synapta beselii) the stone-canal is branched, with a body-cavity opening at the end of each branch. The wall of the stone-canal is without muscles and usually contains calcareous deposits ; the internal lining consists of a ciliated epithelium which on one side of the tube is composed of much more columnar cells than on the other. The termina- tion of the stone-canal presents the most remarkable variations. In some it is attached to the body- wall and opens to the exterior in the dorsal middle line just in front of the generative opening either by a single pore (Pelagothuria and some Elasipodidae) or by more than one pore (2-50 or more, many Elasipodidae). In other cases (certain Elasipodidae and Molpadiidae) the stone- canal ends blindly in the body wall in the dorsal middle line, and opens, not to the exterior, but into the body-cavity by a number of pores which perforate its walls just before its blind end. In all other Holothurians it has lost its connexion to the body wall altogether, and opens into the body-cavity by a large number of pores placed upon its slightly swollen termination. It is remarkable that species of the same genus may differ in the 254 PHYLUM ECHINODERMATA. mode of termination of the stone-canal, as the following state- ment shows. The stone-canal opens to the exterior by a single pore in Pelagothuria and in species of the following genera of Elasipodidae, Scotoplanes, Kolga, Parelpidia, Elpidia, Peniagone, Benthodytes ; it opens to the exterior by more than one pore in the following genera of Elasipodidae, Benthodytes, Psychropotes, Laetmogone, Iliodaemon ; it ends blindly in the body wall, opening into the body-cavity by several pores close to its blind end in species of the following genera of Elasipodidae Irpa, Elpidia, Oneirophanta, Orphnurgus, Benthodytes, and in the molpadian genera Trochostoma and Ankyroderma. In other Molpadiidae, in Synaptidae and Dendro- chirotae it opens into the body-cavity as in the last named, but is without the blind part and the connexion to the skin ; lastly in the Aspidochiro- tae the numerous pores lead from the body -cavity into a sac with which the stone-canal communicates by one or more openings. It is possible that this last arrangement gives the key to the explanation of these strange variations in the termination of the stone-canal. As has been fully described the stone-canal in other Echinoderms does not open directly to the exterior, but into a portion of the body -cavity, the axial sinus, which opens to the exterior by the water-pore or pores (madreporite) and is derived from the anterior body-cavity of the larva. In adult Holothurians there is apparently no trace of axial sinus or other derivate of the anterior body-cavity. But in the larvae, as Bury has shown, a repre- sentative of this cavity which has the appearance of being merely a small appendage of the stone-canal (p. 152, Fig. 108) is present. It is possible that the sac, into which the stone-canal of the Aspidochirotae opens and the small dilatation into which the body-cavity pores lead in some other forms, is the representative of the anterior body-cavity, which in other Holothurians is so much reduced that it is not even discernible as a dilatation on the stone-canal in the adult. On this view the pores of the so-called internal madreporite are secondary perforations in the septum which separates the much-reduced anterior body-cavity from the general body-cavity, the real water-pore being aborted ; whereas in Holothurians with an external madreporite, the water-pore of the larva has persisted, but the anterior body-cavity into which it opens has become indistinguish- able from the stone-canal. The appendages of the radial canals consist of prolongations into the tube-feet and tentacles, and of some prolongations which ramify and end blindly in the body wall. The tentacular canals arise from the radial canals soon after their origin from the ring canal (Fig. 179). They are provided (except in Elasipodidae) with ampullae which project into the body-cavity. In the Synaptidae alone do they arise direct from the ring canal. The tube-feet prolongations arise alternately on each side of the radial canals. Ampullae are always present and either project into the body-cavity or are embedded in the body-wall between HOLOTHUROIDEA. 255 the cutis and circular muscles (Elasipodidae). There is appar- ently no representative of the axial organ. v-.u. .VTU ^f-'^^-^r fn \ \nOEoi I l ' ~J ^V, 10 radial longitudinal muscle ; 11 left respiratory tree ; 12 dorsal blood-vessel of intestine ; 13 circular muscles of body-wall ; li cuvierian organs ; 15 cloaca ; 16 anus ; 17 radial muscles of cloaca : IS cut edge of body-wall ; 19 right respiratory tree ; 20 posterior edge of the dorsal mesentery ; 21 median ventral longitudinal muscles : 22 third part of intestine : 23 first part of intestine ; 24 gonad ; 21 so-called internal madreporites of two stone-canals ; 26 dnrsal mesentery ; 27 genital duct ; 2S inter- racliaL 29 radial piece of the calcareous ring ; 30 genital opening. S 258 PHYLUM ECHINODEEMATA. DORSAL Left Right RM RH anterior part of intestine is attached by a dorso median (adult) mesentery (Fig. 181, 26) to the body wall in the dorsal inter- radius (Fig. 182, J/J. At the first bend the mesentery passes across the left dorsal radius to the left dorsal interradius, where it is attached all along the second or forward reach of the intestine (M 2 }. At the second bend the mesentery passes across the intervening radii and interradii into the right ventral interradius. where it is attached all along the third backwardly directed reach of the intestine (M 3 ). In Synapta the alimentary canal is straight, but as is shown by the attachment of the mesentery it has the same spiral course round the body wall as that just described. The anus, which is typically terminal, may, like the mouth, be apparently shifted on to the dorsal or ventral surface. The wall of the gut consists of (1) an internal epithelium which lias a cuticle and may be ciliated, (2) a layer of connective tissue con- taining blood spaces, (3) a muscular layer consisting of longitudinal and circular fibres, (4) an outer connec- tive tissue layer, and (5) the peri- toneal epithelium. The respiratory trees (Fig. 181) are two hollow much- branched structures placed right and left in the body cavity and opening together or by separate openings into the cloaca. The ultimate branches of the organ end in ampulla-like dilatations which may also te found along the course of the branches themselves. The walls consist of (1) an inner, probably ciliated, epithelium often in more than one layer ; projections into the cavity, caused by cells con- taining yellow pigment granules, are present ; (2) a connective tissue layer, (3) a muscular layer, (4) an outer connective tissue layer, and (5) a layer of peritoneal epithelium. The respiratory trees do not communi- cate with the body-cavity, and they are absent in the Synaptidae and Elasipodidae, but in the latter group there is a forwardly directed caecum VENTRAL FIG. 182. Diagrammatic transverse section through a Holothurian seen from the aboralpole, to show the course of the gut, the attachment of the mesentery and the enumeration of the radii adopted in the text. The vertical dotted line gives the position of the median plane, and the x the position of the transversely cut verti- cal axis. 1, 2, 3 represent" the three stretches of the alimentary canal, and M. 3/2, 3/3 the mesenteries attaching them. RI-RV mark the radii numbered, and IRl-IRs the interradii. //fo is the dorsal interradius of the bivium, which contains the water-pore and generative opening. HOLOTHUROIDEA. 259" which opens into the cloaca and may represent them. The number of respiratory trees is never more than two, the apparent exceptions to this rule being caused by some of the branches acquiring a great distinctness. The function of these organs is probably respiratory, the cloaca apparently having the power of sucking up water and of driving it into them, and then of expelling it. Rhythmical inspiratory and expiratory movements of the cloaca and to a certain, extent of the body appear to effect this. They may also be partly excretory, for the expelled water besides carrying faeces also contain various kinds of cell debris including cells with brown granulations, which probably originate on the walls of the " trees." The cuvierian organs (Fig. 181, 14) are tubular organs which open into the terminal parts of the respiratory trees. The number varies in different species, but as many as 100 have been counted in one individual. They are usually unbranched, but they may be branched or even racemose. They are found mainly in the Aspidochirotae, especially in the genera Holothuria and Mulleria, but they appear to be entirely absent in Labi- dodemas, Pseudostichopus, Paelopatides, and Stichopus (St. paradoxus excepted). They are unknown in Synaptidae and Elasipodidae and are only exceptionally found in other families (e.g. Molpadia chilensis, Cucumaria frondosa and nigricans). They are probably to be regarded as modified branches of the respiratory trees. The unbranched tubes are lined with an epithelium, outside which is a layer of connective tissue. This is followed by an internal circular and an external longitudinal muscular layer, the internal circular layer consisting of a closely wound spiral fibre. Then comes an outer connective tissue layer and finally the peritoneal epithelium, which in the case of the glandular cuvierian organs- appears to be peculiarly modified and to secrete a sticky substance. In some Holothurians (the so-called cotton-spinners) the cuvierian organs can be ejected from the cloaca when the animal is irritated and used as organs of defence. This phenomenon has been studied in Holothuria nigra (forskali),* and in H. poli,-f and in other forms. J When the skin is irritated a small number of these organs make their way, blind end forwards, through a rent which is formed in the dorsal wall of the cloaca. On emerging from the anus they rapidly undergo elongation to twenty or thirty times their original length, darting about in all directions and be- coming attenuated in the process. They stick by their viscid surface to- everything they touch (except the surface of the animal itself). If the Holothurian now moves away they become detached from its body by rupture. The cause of the active elongation and movement of the tubes is not clearly known. It is not apparently due to injection of fluid from the respiratory trees because it can take place if the tubes are detached from the animal. It would appear to be caused by some process taking place in the wall of the tube itself. When first ejected each tube is thicker at the free end than at the base. The elongation appears to take place at the expense of this " head " which diminishes in length during the process. The elongation is said to begin while the organs are still in the body-cavity and can be brought about by direct irritation of the tubes themselves as well as of the skin. The tubes after elongation cannot be shortened and must therefore be cast off, new tubes being presumably * Minchin, Ann. and Mag. Nat. Hist. (6), 10, 1892, p. 273. t Barthels, Verh. Nat. Ver. Bonn, 53, 1896, p. "('.. J Semper, op. tit. Peach, Ann. and Mag. Nat. Hist., 15, 1845, p. 171. 260 PHYLUM ECHINODERMATA. formed from the respiratory trees. The branched cuvierian organs have not got the viscid wall and their function is not understood. The coelom presents the usual main divisions, viz. (1) the am- bulacral system (2) the perivisceral cavity, and (3) the perihae- mal spaces. The ambulacra! system has already been described. The perivisceral cavity is spacious and is traversed by the dorsal (larval) mesentery (see p. 257), and by muscular and connective strands. It is lined by a ciliated epithelium and contains a corpusculated fluid. It has so far as is known no communica- tion with the exterior, but in many Holothurians it communicates with the water-vascular system through the so-called inner madreporite. There is a special section of the body-cavity round the oesophagus, called the perioesophageal sinus (Fig. 179, 8, 11). This is separated from the rest of the body-cavity by a membrane, which however is in most cases perforated by apertures putting it in communication with the general body-cavity, and it is tran versed by strands of tissue passing from the wall of the oesophagus to this outer wall of the sinus, except just round the mouth opening where it is free from trabeculae and constitutes the peribuccal sinus. In the Elasipodidae the outer membrane is complete and the perioesophageal sinus is completely shut off from the rest of the body-cavity. In the Synapticlae there is a number of funnel-shaped ciliated organs attached by a stalk to the mesentery and body wall. The cavity of these organs, which is freely open to the body cavity, is lined by a ciliated epithelium and ends blindly in the stalk. The stalk may be branched and carry many ciliated funnels. The perihaemal spaces, which are almost certainly parts of the coelom, are lined by a flat epithelium and contain a fluid similar to that of the body-cavity. They consist of five radial canals (Fig. 180, Sin.rd) placed just internal to the radial nerves, between these and the radial blood-vessels. A circumoral peri- haemal canal has been described in some cases (Synapta, Cucumaria, etc.). In the Synaptidae the perihaemal canals do not extend very far from the oral region, and the circumoral perihaemal space is separated by a septum from the radial perihaemal spaces. The perihaemal spaces appear not to communicate with the perivisceral cavity. The vascular system. Immediately aborad (Fig. 179, 14) of the water-vascular ring is the circumoral vessel of the so- called vascular system. This sends off to each radius a vessel which extends to the aboral end of the body just external to the water- vascular canal (Fig. 180, lac.rd). It is also connected HOLOTHUROIDEA. 261 Ov D with two intestinal vessels, one of which lies on the mesenterial side of the intestine close to the insertion of the mesentery, and the other on the op- posite, non-mesente- rial side (Fig. 181). \ it t JH V 111 F\\X\\ \ in ~B " XVX^ "*- In many Holothurians (most Aspidochirotae, some Dendrochirotae and Molpadiidae) the dorsal gut vessel (mesen- terial) becomes detached in part of its course from the intestinal wall (Fig. 183), but remains con- nected with the blood lacunae in the intestinal wall by a plexus of ves- sels passing across the body-cavity. This plexus is the rete mirabile. The dorsal vessel and rete mirabile lie on the same side of the mesentery. It often forms a kind of \wb which loosely in- vests the terminal bran- ches of the left respira- tory tree. These vessels are connected with lacunae in the walls of the neighbouring organs. The intestinal vessels are connected with lacunae in the inner con- nective tissue coat of the alimentary canal ; the circular vessel sup- plies the tentacular canals, the stone canal and the polian vesicle, FIG. 183. Holothuria tubulosa opened longitudinally (from the op^rmhao-iis nH t'laus, after 51. Edwards). mouth in the midst of the sopnagus tentacles T ; D alimentary canal ; Sc stone-canal; P often the gonad ; the polian vesicle ; Rg circular vessel of the water-vascular radi-il vp^pl OTTO off system ; Ov ovary ; Ag radial water-vascular canal ; M longitudinal muscles ; Gf intestinal vessel ; Cl cloaca ; 117 branches to the tube- respiratory trees. feet and papillae ; and lastly the lacunae in the walls of the gonad may be supplied from the dorsal (mesenterial) intestinal vessel, or from the circular vessel. The vascular system is composed of tissue similar to that found in Asteroids and Echinoids, but containing better defined Wl Cl 262 PHYLUM ECHINODERMATA. channels. The vascular trunks though called vessels are nothing more than a system of communicating spaces or bundles of anastomosing tubes. They are without any epithelial lining, and contain a coagulable fluid. Reproductive organs. The Holothurians are for the most part dioecious, but a few of the Synaptidae and Molpadiidae are hermaphrodite (ova and spermatozoa arising in the same tubes). The gonad is single and lies in the adult-dorsal interradius (Fig. 181). It consists of a tuft of branched tubes projecting into the body-cavity, on both sides of the mesentery (on the left side only in Holothuria, M idler ia, Labidodemas and some Elasipodidae). The genital duct lies in the dorsal mesentery and opens to the exterior in the middle, adult-dorsal line in the anterior region of the body. In the Dendrochirotae it is between two of the tentacles or even within the tentacular circlet ; in the Molpadiidae and Synaptidae it is immediately behind the ten- tacular circlet ; it is furthest removed from the tentacles in the Elasipodidae (in Psycliropotes longicauda it lies in the posterior half of the body). The genital duct is always single, but in some Elasipodidae it divides so as to open by several apertures (in some species the number varies in different individuals). The genital opening may be at the end of a small papilla. The genital rachis, if present, is represented by a cord near the genital duct (p. 132). As a rule there are no external sexual differences, but the sexes can sometimes be distinguished by inspection of the generative organs. In a few species the male alone possesses a genital papilla (Thyone aurantiaca, Cucumaria laevigata, etc.). In the connective tissue layer of the wall of the gonad, which lies next the inner epithelium, there is an extensive development of the lacunar spaces of the blood-system. They are all marine, and with the exception of one pelagic form (Pelagothuria) they live on the sea bottom, usually attached to external objects by their tube-feet. Most of them are able to crawl by means of their tube-feet, though many of them move but little. Many of those which have been observed are said to be more active during night than in the day time, but no visual organs are known. They are found in all seas and at all depths, many being littoral and a considerable number abyssal. The Elasipodidae are almost entirely deep-sea forms. The Synap- HOLOTHUROIDEA. 263 tidae and Molpadiidae, which are without tube-feet, are burrowers. The skin in some forms secretes a slimy substance, which in some cases appears to be used for entangling small food organ- isms. In Pseudostichopus mollis and occultatus the body is covered with the shells of Foraminifera. They feed on the smaller marine animals, which in the Dendrochirotae are collected and carried to the mouth by the large tentacles. These tentacles are used for this purpose one at a time. The two small tentacles are applied alternately over the mouth so as to close it or wipe it in the intervals between the introduction of food by the large tentacles. The Aspido- chirotae fill their intestine with sand, which they eject from the anus together with the current of water from the respiratory trees. It is worthy of notice that many Holothurians (especially the Aspidochirotae) have the power when irritated of ejecting through the anal opening the alimentary canal (and its appen- dages) which breaks off behind the vascular ring and in the region of the cloaca. They are able to regenerate the parts so lost, sometimes with considerable rapidity. In some cases this ejection includes the gonad, the calcareous ring and tentacles. The ejected organs seem to be distasteful to other animals, and if taken up are soon rejected. This ejection of the viscera is said not to occur in Cucumaria or in the Elasipodidae. In the Synaptidae the body, when irritated, breaks into several pieces by violent muscular contractions of the body wall. The an- terior of the pieces so produced is able to regenerate the rest of the body. Asexual reproduction by division is said to take place in Cucumaria planci, etc.* Of parasites found in Holothurians may be mentioned Ento- valva mirabilis, a bivalve mollusc living in the pharynx of species of Synapta (vol. i. p. 349), the Gastropods, Eulirna and Stylifer in the gut and on the skin of various Aspidochirotae, Entoconcha in the body cavity (Synapta and Holothuria), and Entocolax from the inner side of the body wall of Myriotrochus rinkii (vol. i. p. 403). Finally the remarkable commensal Ficras/er, a Teleostean fish, lives in the right respiratory tree of Aspidochirotae (vol. 2, p. 227). They vary in length from 5 to 60 cm. (Cucumaria frondosa * Chadwick, Proc. and Trans. Liverpool Biol. Soc., 5, 1891, p. 81. 264 PHYLUM ECHINODERMATA. Gunn.) but the Synaptidae may attain a length of from one to two metres. Certain species of the genera Holothuria and Stickopus form an important article of commerce in the East, being used by the Chinese for the preparation of a highly esteemed soup, etc. They are known as Trepang and Beche de rner. They are not suitable for preservation as fossils, but remains of their spicules have been described from the Carboniferous, the Jurassic, the Cretaceous and the Eocene formations (Synapta, Chiridota, Myriotrochus). The development is sometimes direct, but more usually there appears to be a bilateral larva called Auricularia which arrives at the adult condition by passing through a barrel-shaped pupa stage. In Phyllophorus urna Gr., Synapta vivipara Oerst., and Chiridota rotifera Pourt., the eggs make their way into the body-cavity, where they are fertilized and undergo their development. This phenomenon has been examined in the case of Synapta vivipara by Clark * who states that the eggs escape into the body-cavity through the Avails of the genital tubes and that the spermatozoa make their way into the sea through the genital duct and then enter the cloaca, through the walls of which, either by means of fine pores or by actual pene- tration, they enter the body-cavity and fertilize the ova. The young escape from the body-cavity by rupture of the body wall or of the intestinal wall. In some forms the eggs are received into two vent rally placed pouches (Cucumaria nu'nuta Fabr. (glacialis) and laevigata Verr.), in others into a dorsal pouch (Psolus ephippifer W. Thorns.). In some they are attached to the dorsal integument (Cucumaria crocea Less.), and in Cucumaria planci the eggs are retained for some time amongst the tentacles. As a rule, however, both ova and spermatozoa are spawned direct into the sea, and the young are developed independently of the mother. Direct development within the egg membranes occurs in Cucu- maria kirchsbergii (Psolinus brevis, Kowalevsky, op. cit.), and possibly in those forms which develop in brood-pouches. In Phyllophorus urna (Kowalevsky, op. cit.) a ciliated larva is formed, which swims about in the body-cavity and possesses 5 tentacles and 2 tube-feet when it leaves the parent. In Cucumaria planci * Mem. Boston Soc. Nat. Hist., 5, 1898, p. .">:{. HOLOTHUROIDEA. 265 the embryo passes at once into the stage of the barrel-shaped larva. It is said that the eggs of Cucumaria Mrchsbergii are already fertilized when they leave the body of the mother. With regard to affinities we think that there can be no doubt that the Holothuriam must be placed apart from the three preceding classes of Echinodermata. They differ from these in a number of anatomical features which we cannot but regard as of fundamental importance. For instance, in the embryo, the inconspicuousness of the anterior coelom ; in the adult, the absence of radial repetition of the gonad, and the absence of the axial sinus and axial organ, and the absence at all times of life of the plates of the apical system. The absence of pentamerous structure in the alimentary canal cannot be re- garded as so important, for the same negative feature is charac- teristic of Echinoidea. Neither can the fact that the blastopore gives rise to the permanent anus, or that the larval mouth and anus both persist into the adult, be regarded as of funda- mental importance having regard to the varied behaviour of these structures in other animals. But although it is undoubted that the Holothurians must be placed apart from Asteroids and Echinoids, still it cannot be said that they approximate to the Crinoids, for those very features which separate them from the first, separate them also from the last. The anterior coelom, the radial repetition of the gonads, the axial organ and the apical plates are all present and well-developed in Crinoids. though the anterior coelom disappears in the adult. Indeed we may go further and say that Holothurians stand further from Asteroids and Echinoids than do the Crinoids, for whereas Asteroids and Echinoids agree with Crinoids in the above- named anatomical characters, the only point of importance which they have in common with Holothurians and which is not also found in Crinoids is the form of the free larva. For these reasons we hold that out of the primeval matrix of the Echino- dermata three main groups have emerged and persisted to the present day; these are (1) the Asteroids, Ophinroids and Echinoids, (2) the Holothuroids and (3) the Crinoids. Order 1. ACTINOPODA. All external appendages of the water-vascular system arise from the radial canals, and have the form of tentacles round the mouth and of ambulacral feet and papillae on the body. Tentacles are always present, but feet and papillae may be absent. 266 PHYLUM ECHINODERMATA. Sub-order 1. ASPIDOCHIROTAE. With 18-30 shield shaped tentacles (except Molpadiidae). Ampullae of the tentacular canals, respiratory trees, and cuvierian organs present or absent. Without retractor muscles of the pharynx (present in Molpadia). Fam. 1. Holothuriidae. Body cylindroidal. Solelike ventral surface usually but slightly developed. The ampullae of the tentacular canals are well developed and project into the coelom. The calcareous ring is formed of 5 radial and 5 interradial pieces. Otolithic vesicles are absent. Stone-canals often numerous, opening into the body-cavity. Longitudinal muscles usually divided ; retractor muscles absent. Respiratory trees well developed, the left being usually covered by the plexus of the intes- . FIG. 184. Dorsal and ventral view of Deima atlanticum (after Herouard). tinal vascular system. Cuvierian organs present. Mainly in the indo- pacific region and littoral. Mutteria Jager, anus with calcareous teeth ; the other genera are without these. Holothiiria L., trepang, ambulacra! appendages over whole body, not in rows, cosmopolitan ; Labidodemas Selenka, with feet only which are confined to the radii ; Pseudostichopus Theel ; Stichopus Brandt, trepang, with flat ventral surface on which there are usually three rows of feet. British genus, Holothuria with species : With pedicels only, H. intestinalis A. and R. ,, pedicels and papillae, H. tremula Gunn. ,, pedicels almost entirely ventral, H. nigra auct. ,, two rows of pedicels on either side of body, H. aspera Bell. Fam. '2. Synallactidae. Body rarely cylindrical, generally flattened and with a ventral sole. Tentacular canals without ampullae. The stone- HOLOTHUROIDEA. 267 canal is single and usually joins the body-wall. Neither of the respiratory trees are connected with the vascular network of the intestine. Abyssal forms. Synallactes Luclw. Meseres Ludw. Bathyplotes Oest. Bathyher- pystikes Sluiter. Benthothuria R. Perrier. Pseudostichopus Theel. Mesothuria Ludw. Paelopatides Theel. Zygothuria R. Per. Fam. 3. Elasipodidae. Mouth more or less ventral. Ventral surface usually flattened toasolelike surface with three rows of feet (trivium). With 10, 15 or 20 more or less shield-shaped tenta- cles, without ampullae. Calcareous ring of 5 radial pieces or of 5 radial and 5 inter- radial complete or incomplete pieces. Otocysts generally present on the radial nerves. Stone-canal attached to the skin and often opening to exterior. Longi- tudinal muscles simple, retractors absent. Respiratory trees absent or vestigial. Cuvierian organs absent. For the most part abyssal. Sub-fam. 1. Deimatinae. Body generally elongated. Calcareous ring well developed. Ventral surface flat- tened, the unpaired radius being with- out or with reduced feet. Tube-feet serially arranged. Deima Theel (Fig. 184). Oneirophanta, Orphnurgus, Laet- mogone, Iliodaemon, and Pannychia Theel. Scotodeima, Laetmophasma, and Capheira Ludwig. Benthogone Koehler. Sub-fam. 2. Elpidiinae. Usually with 10 tentacles. Ventral surface flattened, the unpaired radius being always without feet. The dorsal ambulatory appendages much re- duced in number and the latero- ventral feet are frequently confined to the hinder part of their radii. The calcareous ring is without the interradial pieces. Peniagone Theel. Elpidia and Parelpidia Theel. Irpa and Kolga Dan. and Ivor. Scoto- planes, Achlyonice, Scotoannassa, and Enypniastes Theel. Rhipidothuria Herouard. Sub-fam. 3. Psychropotinae. With paired ventral ambulacrum with two The dorsal ambulacra with papillae all along their length or only in front. A large appendage sometimes projects from the hinder part of the dorsal surface. Calcareous ring with 5 separate calcareous pieces. Benthodytes, Euphronides, Psychropotes (Fig. 185), Psychro- trephes Theel. Fam. 4. Pelagothuriidae. Pelagic forms of medusa-like appearance. J FIG. 185. Psyc/iru/ititi's Imnji- caiicla (from Lang, after Theeli. 1 oral tentacles, 2 mouth, 3, i, S, ambulacral appendages of the trivium (adult-ventral) ; 5 anus ; 6 dorsal appendage with its two posterior processes (7). 10 to 20 tentacles. Un- rows of feet, rarely naked. 268 PHYLUM ECHINODERMATA. Tube-feet and ambulacra! papillae absent. Mouth and anus terminal. Body cylindrical ; round the tentacular circlet it widens out into a thin disc, the edge of which is produced into long rays ; 13 to 16 tentacles ; the tentacular canals arise from the well-developed radial canals, and they each give off at their base a canal (? ampulla) into the disc. Calcareous ring absent. The stone-canal opens on the surface of the body. Longi- tudinal muscles simple ; retractors absent. Transverse muscles interrupted in the radii. Re- spiratory trees, cili- ated cups and Cuvi- erian organs absent. Genital organs right and left of the dorsal mesentery. The}' swim on the surface of the sea liy means of their disc. Pelagothuria Ludwig. Fam. 5. Molpa- diidae. Burrowers. Feet and ambulac- ral papillae absent. Mouth terminal. Hinder end of body often reduced to a tail-like appendage. Tentacles small, usually 15 in number, cylindrical or provided with some small branches near the end ; tentacular ampullae present, pro- jecting into body-cavity. Calcareous ring of 5 radial and 5 interradial pieces. Stone-canal always single, often fastened to the body-wall. Otolithic vesicles absent. Longitudinal muscles divided, retractors well developed in Molpadia only. Respiratory trees present. Cuvierian organs only in one species. Principally on mud or clay. Molpadia Cuv., Eupyrgus Liitken, Haplodactyla Grube, Caudina Stimp., Trochostoma D. and K., Ankyroderma D. and K., anchor shaped spicules in this genus. Sub-order 2. DENDROCHIROTAE, with branched tentacles and re- tractor muscles of the pharynx. Tentacular ampullae (not projecting), and respiratory trees always present. Fam. Cucumariidae. With feet, rarely with ambulacra! papillae. Mouth usually dorsal or terminal. Anus often dorsal. Body cylindrical or pentagonal or with a ventral sole. With 10-30 branched tentacles often unequal in size, the two ventral being generally smaller than the rest (Fig. 177) ; tentacular ampullae not distinct. Calcareous ring of 5 radial and 5 interradial pieces. Otolithic vesicles absent. Stone-canal never opening to exterior. Retractor muscles well developed. Respira- tory trees always, Cuvierian organs only occasionally present. Cucumaria Blainville (Fig. 177), 10 tentacles, feet in rows on the radii ; Till/one Semper, feet over whole body ; Orcula Troschel, Phgllophoru* Grube, with an inner row of smaller tentacles ; Pseudocucumis Ludwig : FIG. 186. Pelagothuria natatrix (from Lang, after Ludwig), seen from the aboral pole. HOLOTHUROIDEA. 269 Actinocucurnis Ludwig ; Colochirus Troschel, feet in rows on ventral surface ; Psolidium Ludwig ; Theelia Ludwig ; P solus Oken with 10 tentacles, a ventral creeping sole, dorsal surface arched without ambula- cral appendages, often with calcareous scales ; principally arctic and antarctic. Eliopalodina J. E. Gray, mouth and anus close together at the end of stalk-like process, the dorsal interradius much shortened ; on account of the peculiar course of the radial canals there appear to be ten radii. On the Congo coast. Ypsilothuria E. Per. (Sphaerothuria Ludw.)- British genera and species : Gucumaria : A. Tube-feet confined to ambulacra. I . Tube-feet non retractile. Skin smooth . . C. hyndmani Forbes. Skin stiff C. lactea F and G. II. Tube-feet retractile. a Skin smooth. Attenuated at either end . . C. pentactes Mont. Body sac-like or elongated . C. planci Gmel. b Skin rough C. hispida Barr. B. Tube-feet scattered C. frondosa Gunn. Thyone : Body not curved on itself, T. fusus O. F. M. Body curved on itself, T. raphanus D. and K. Psolus : Tube-feet in 3 complete rows, P. phantapus Struss., tube-feet of median row few or none, P. jabricii D. and K. Phyllophorus : P. pellucidus D. and K., P. drummondi Thomp. Order 2. PARACTINOPODA. The oral tentacles arise from the circumoral vessel. Tube-feet and ambulacral papillae, respiratory trees and cuvierian organs are not present. Fam. Synaptidae. Radial canals absent in the adult. Mouth terminal. Body cylindrical, worm-like. 10-27 (often 12) feathered tentacles ; tentacular ampullae only indicated. Calcareous ring often with more than 5 interradial pieces. Anchor-shaped spicules are present in Synapta and its allies. Wheel-shaped spicules in most other genera. Stone-canal sometimes multiple. Otocysts pn the origin of the radial nerves. Longi- tudinal muscles usually undivided ; retractor muscles sometimes present. Respirator}' trees absent. Ciliated organs on the wall of the body-cavity present, cuvierian organs absent. Generative organs often hermaphrodite. Synapta Eschscholtz, in sand and mud ; Euapta, Chondroclaea, and Labidoplax Oestergren ; Anapta Semper ; Chiridota Eschsch. ; Trocho- dota Ludwig ; Trochoderma Theel ; Myriotrocha Steenstrup ; Acantho- trochus D. and K. British species : Synapta inhaerens O. F. Mi.il]. ; S. buski Maclnt., rare, 6'. digitata Montagu, British and French coasts, etc. 270 PHYLUM ECHINODERMATA. Class CRINOIDEA* Brachiate Echinoderms attached during the whole or part of life by the aboral apex of the body. The arms are usually provided with pinnules and branched, the tube-feet are tentacle-like and with- out ampullae, the water-pore is always multiple, and the anus is interradially placed on the oral surface. All living Crinoids are, so far as is known, attached, either in the young state only (Comatulids, Thaumatocrinus) or through- out life, by a jointed stalk proceeding from the apical point of the body. The body consists of a calyx or disc, and of radial branched prolongations of the disc the arms. The branches of the arms may be all alike, or some of them, known as pin- nules, may differ in structure from the others. In the natural position of the animal the abactinal (dorsal of adult) surface is turned towards the substratum, to which indeed it is, as already stated, generally fixed by a stalk, while the actinal (ventral of adult) surface is directed upwards. The actinal surface of the calyx is called the calyx-cover (tegmen calycis) and bears, usually j" in its centre, the mouth, from which radiate towards the periphery on the calyx-cover the ambulacra! grooves (food grooves). The portions of the * J. S. Miller, A Natural History of the Crinoidea or Lily-shaped Animals, Bristol, 1821. J. V. Thompson, " Sur le Pentacrinus Europaeus, Vetat de jeunesse du genre de Comatula," L'Institut, 1835. Id., " Memoir on the Starfish of the genus Comatula," Edinburgh New Phil. Journ., 20, 1836. J. Miiller, " Ueu d. Bau. v. Pentacrinus caput medusae," Abhand. d. Berlin Akad., 1841. Id. " Ueb. d. Gattung Comatula u. ihre Arten," ibid., 1847. W. B. Carpenter, Researches on the Structure, Physiology and Development of Antedon rosaceus, Phil. Trans., 156. H. Ludwig, Crinoiden, Z.f.w.Z., 1877, 28, p. 257; and 29, p. 47. P. H. Carpenter, "Report on the Crinoidea," Challenger Reports, I, 'The stalked Crinoids," 1884, and II, "The Comatulae," 1888. M. Neumayr, Die Stamme des Thierreicb.es, 1, 1889. C. Wachsmuth and F. Springer, " Revision of the Palaeocrinoidea," Proc. Ac-ad. Nat. Sci. Philadelphia, 1879, 1881, 1885. Id., " Discovery of the ventral Structure of Taxo- crinus and Haplocrinus, etc." Ibid., 1889. Id. "The perisomic plates of Crinoids," Ibid. 1890. Id., North American Camerata, Mem. Mus. Harvard, 20 and 21, 1897. E. Perrier, "Memoire sur 1' Organisation et le developpement de la Comatule de la Mediterranee," Nouv. Archives Mus. Hist. Nat. Paris, 1886-92. H. Bury, " The early stages in the development of Antedon rosacea," Phil. Trans. 179, 1888. O Seeliger, " Studien zur Entwick. der Crinoiden," Zool. Jahrb., Abth. f. Anat. 6, 1892. H. Ludwig, loc. cit. F. A. Bather, " British fossil Crinoids," a series of papers in Ann. and Mag. Nat. Hist. (6), 5, 1890. Id., The Crinoidea of Gotland, Pt. 1. Kongl. Svenska Vetenskaps-Akad. Hand., 25, 1893. Id., Crinoidea, in Lankester's Zoology', Pt. 3, 1900 (Literature given). Reirhensperger, " Anatomie von Pentacrinus," Z.f.w.Z.. 80, 1905, p. 22. ) InActinometra the mouth is excentric, and the anus is nearly central. CRINOIDEA. 271 calyx-cover between the ambulacral grooves are called the inter- ambulacral regions, and in one of these the so-called posterior- is situated a papilla, at the end of which is the anus (Fig. 188). The stalk carries whorls of jointed appendages, the cirri ; the seg- ments to which such whorls are attached being called nodal, and the intervening segments internodal. In the unattached forms, the apex of the calyx where the stalk would be attached, if present, carries more than one whorl of similar jointed cirri. The ambula- cral grooves are con- tinued from the calyx-cover along the whole length of the arms and their branches (excepting in a few cases, e.g. some of the posterior arms of Actinometra and some of the proximal pinnules of Antedon), and tube- feet project from their sides. As in Ophiurids the tube- feet are without ampullae and are not used as locomo- tive organs. The arms and pinnules contain prolongations of all the most im- portant organs of the calyx, excepting the alimentary canal, which as in Ophiurids is confined to the disc. The mouth is surrounded by small tentacles, the cavities of which open directly into the circumoral vessel of the water- vascular system. Orientation and numbering of the rays. As stated above the interradius in which the anus is placed is called posterior. If the animal be drawn from the ventral surface with the anal in- FIG. 187. Pentacrinus caput medusae (from Clans, at'tc: J. Miiller). O mouth ; A anus. The lower figure is the tegmen calycis, the arms heiug cut off. 272 PHYLUM ECHINODERMATA. terradius downward in the figure, then in Crinoid nomenclature the three radii which project upwards constitute the trivium, the middle one being called anterior, while the two lower radii con- stitute the bivium. Further the right posterior radius is that radius of the bivium to the observer's right, the radius of the trivium on the same side being the right antero-lateral radius. On comparing this with the enumeration of the radii adopted in Fie;. 188. Antedon bifii/n, actinal surface (from Claus). mouth, A anus. The pinnules are tilled with the generative products. this work (p. 119), it would appear that the right posterior radius would correspond to radius No. II, and the left posterior radius to radius No. III. Following out the same comparison the right antero-lateral radius of Crinoids is No. I of our enumera- tion, the anterior radius No. V, and the left antero-lateral No. IV. The use of the words right and left in this orientation is the exact opposite of their application in other Echinoderms.* * As Loven considers that the anus of Crinoids is in interradius V. I {as in exocyclic Echinoids), his nomenclature of the radii of Crinoids CRINOIDEA. 273 Excepting for the anal interradius, which also contains the primary water-pore of the larva, pentameral symmetry of the Crinoid body is externally complete in almost all living Crinoids.* In extinct Crinoids, however, it is frequently disturbed by the insertion of the so-called anal plates in the posterior (anal) interradius. (See p. 276.) As in most other Echinoderms the integument contains a large amount of calcareous matter in the form of, for the most part, regularly arranged plates, but movable spines t and pedicellariae are conspicuous by their absence. The number of genera now living is comparatively small, but, judged from the Crinoid standpoint, their variation in structure is considerable ; indeed, in this respect they may be said to be fairly representative of the immense number of forms which lived in the Palaeozoic epoch, when the group attained its greatest development. It is on account of these extinct forms that the study of the skeletal parts has attained in this class that rela- tively exaggerated importance which we found it to have in the case of Echinoids. It is a subject of great difficulty and the results arrived at, with regard to the structure of extinct forms, are constantly undergoing modification. Moreover the struc- ture of the skeleton varies in closely allied forms in char- acters which it is customary to regard as having a high morphological value. For instance in the Camerate families Platycrinidae and Actinocrinidae the ambulacra of the calyx cover may be exposed or subtegminal even within the limits of the same genus (Wachsmuth and Springer), and the constitution of the apical system of plates may vary in the most important particulars in genera of the same family. Crinoids have indirectly had an important influence upon our knowledge of the ocean. Until quite recently but two living genera of fixed Crinoids were known, Pentacrinus and Holopus. The discovery of Rhizocrinus in deep water by G. O. Sars in 1864, by the interest it excited on account of its stalked charac- ter and general resemblance to extinct forms, led to the expedi- tions of H.M.S. Porcupine, in 1868, and H.M.S. Lightning in 1869, would be as follows: right posterior radius = radius V, left posterior = radius I, anterior radius = radius III, and right antero - lateral = radius IV. : Thaurnatocrinus is the most striking exception, see p. 301. f Spines have been described in one extinct genus (Arthrocanihd). Z III T 274 PHYLUM ECHINODERMATA. and the following years, and indirectly to the despatch of the Challenger expedition in 1872. FIG. 189. Developmental stages of Antedon, much enlarged (from Claus, after W. Thomson). a Free-swimming larva, with tufts and rings of cilia Wr, and with rudimentary calcareous plates. 6 Attached cystid stage of the same animal. O orals, R radials, B basals, Cd centro-dorsal plate, c older stage described as Pentacrinus europaeus with arms and cirri. The general form of the body and skeletal plates of Crinoids may be considered under three heads : (1) the calyx and its cover, (2) the arms, and (3) the stem. CRENOIDEA. 275 FIG. 190. Analysis of the calyx of a dicyclic in- aduuate form (from Lang). ' ba basal ; ib infra- basal; C2 second arm plate (primibrach 2); ian anal interradial (special anal). In the larva of Antedon and in what we may call the simplest forms i.e. the Larviformia and Marsupites amongst extinct forms, Holopus and the Hyocrinidae amongst recent the calyx is composed entirely of the plates of the apical sys- tem ; whereas in the majo- rity of Crinoids the lower ends of the arms are incor- porated in it and together with some interradial plates assist in forming its walls. The apical system when fully developed con- sists of the following plates (Fig. 190) : five infrabasals (ib) in contact at the apical pole and radial in position, five basals (ba) outside the infrabasals, in contact with each other, and interradial in position, five * radials (r) beyond the basals, in contact with each other and radial in position. Of these plates the infrabasals are sometimes absent, in which case the apical system is said to be monoeyclie, as opposed to dicyclic when they are present. This character the presence or ab- sence of infra-basals seems to be vari- able in most of the orders of Crinoidea. In the Flexibilia the base is said always to be dicyclic, but in the other orders both conditions are found. It is known that in many supposed monoeyclie forms the infra-basals are really present in a reduced condition or have fused with the top joint of the stem (pseudo-monocyclic forms, e.g. Antedon). Wachsmuth and Springer assert that the mono- or dicyclic character of the calyx may be determined by inspection of the stem. In dicyclic forms the lobes of the chambered organ are radial and the cirri are attached to the stem radially, and, if the stem be pentagonal, the angles are interradial ; with monoeyclie calices the reverse arrangement holds. * Sometimes called primary radials. FIG. 191. Analysis of the calyx of Marsupites testudinarius (from Zit- tel). erf central; ib iufratasal ; 6 basal ; r radial. 276 PHYLUM ECHINODERMATA. In Marsupites there is a central plate in the middle of the five infra- basals (Fig. 191). There is a central plate also in the larva of Antedon, which however lie at the attached end of the stalk : it is called the dorse-central and was once supposed to be comparable with the central plate of other Echinoderms. It is not certain whether the central plate in the calyx of Marsupites is this plate or the top segment of a larval stem. In Antedon the central plate of the adult is a composite structure consisting of the top piece of the stem and the infrabasals : it is called the centro-dorsal (Fig. 189 Cd.). The number of radials in the apical system is normally five, but the basals and infrabasals may be reduced to four, three or two. The reduction of the basals is supposed to be specially characteristic of the ancient Crinoids ; all living Crinoids have five, except Hyocrinus which has three.* In living Comatulidae except Atelocrinus neither the radials nor the basals appear on the surface ; the basals are united into a small plate, the rosette plate, which encloses the chambered organ. It frequently happens that the regular radial symmetry of these simple calyces is disturbed by the enlargement of the posterior basal and by the presence of an extra plate or of extra plates between the radials which border the posterior (anal) mterradius (Fig. 190, ian). The anal inter- radial plates have been much discussed. When there is only one it rests upon the posterior basal and lies between the right and left posterior radials ; it is called the special anal (Fig. 190, ian}. When there is more than one, the lowest of them that which intervenes between the radials and reaches the circle of basals is called the radianal (Fig. 201, ra), the rest being simply plates of the anal tube, of which the lowest is sometimes called the special anal plate. The irregularity of the calyx caused by this peculiarity of the anal interradius is never found in living forms f or in their allies, and was supposed to be generally characteristic of palaeozoic Crinoids. There are however, many of these in which it is not found. * Fused to one in Gepliyrocrinus. the ally of Hyocrinus. f For the asymmetry of the living Thaumatocrinus see p. 301. FIG. 192. - - Poteriocrinus (after Zittel) showing a simple calyx with dicyclic has;'. There are two priini- braehs but neither enter into the composition of the cup. CRINOIDEA. 277 The origin of the radianal is supposed to be as follows. The right posterior radial (p. 272) is sometimes divided horizontally into an upper (oral) and lower (aboral) part. These are called the superradial and in- ferradial respectively. The inferradial is often shifted so as to lie in the anal interradius, between the right posterior radial (now superradial) and the special anal plate ; it is then called radianal. This horizontal division of the radial may appear in other radii, but no shifting of the lower element occurs except in the radius specified. The skeleton of the arms (Fig. 193). The arms are supported by a row of plates called brachials. They are placed on the abactinal side and are often grooved on their actinal or ventral FIG. 193. Caetocrinus proboscidatis Hall. Calcareous Limestone, Iowa. A, the calyx cover is broken away, so as to show the subtegrainal skeleton ; a the covering pieces of the ambulacra! grooves. The opening of the ambulacra! grooves are shown on the interradial ; the plates marked (ft on each side of the anal interradial are interradials. C, cast of a calyx-cover showing the impressions of the ambulacral canals a leading to the mouth o ; an anus. D covering plates of an ambulacral groove (from Zittel, after Meek and Wortheu). . tt surfaces for the reception of the soft parts of the arms ; more- over they contain a canal for the axial cord of the apical nervous system (Fig. 197, 8). The first brachial of an arm is carried by the corresponding radial plate of the apical system. When the arms divide once dichotomously, the brachials before the division are called primibrachs (costals) (Fig. 193, r 2 , r 3 ) and the brachial im- mediately before the division is called the axillary primibrachial or primaxil (r 3 ). When the arms divide a second time the brachials which occur between the first and second divisions are called secundibrachs (distichals), the axillary secundibrach being 278 PHYLUM ECHINODERMATA. the secundaxil ; and so on, tertibrachs, quartibrachs with tertaxil, quartaxil, etc., for the axillary pieces. In Crinoids with a simple larviform calyx, viz. the Larviformia, some Fistulata, Marsupites, Hyocrinus, etc., the whole of the arm is free, no part of it assisting in forming the calyx. In this case all the brachials are said to be free. But in most Crinoids a certain portion of the lower part of the arm is incorporated in the calyx, which is thereby enlarged. In this case those brachials which are incorporated in the calyx are called fixed brachials. As a general rule when the calyx is thus enlarged a number of interradial plates are found in the wall of the calyx between the fixed brachials : these may be called interprimi- brachs, inter secundibrachs, etc., according to their position be- tween primibrachs, secundibrachs, etc. (Fig. 203). In the Larviformia and Fistulata there are no interradial plates in the calyx excepting the anal plate or plates in the posterior interradius (Fig. 201). In Thaumatocrinus and the Rhodocrinidae there are five interradial s regularly disposed around the calyx between the radials, and touching the basals ; in Thaumatocrinus the anal interradius is marked by a short external jointed process. In the Camerata and Flexibilia, calyx interradials are generally present between the brachials (Figs. 193, 203), but not between the radials except in the Rhodocrinidae and except in the anal interradius, in which anal plates can often be made out. In the Articulata, calyx interradials are generally absent ; they are present in some Apiocrinidae between the primibrachs , but are absent in the Encrinidae (Fig. 205), Bourgueticrinidae, Pentacrinidae, and Comatulidae. In the Encrinidae no part of the arm is incorporated in the calyx, and in the last three families mentioned the body-wall which connects together those portions of the arms which are incorporated into the calyx resemble the calyx cover in either being without definite plates, or in having loosely arranged irregular plates. It thus appears that the radials form an uninterrupted circle in Articulata. In most other Crinoids they are interrupted in the anal interradius only, excepting in Thaumatocrinus and the Rhodo- crinidae, in which they are interrupted in every interradius. The brachial plates at the base of the arms are always in a single row, and in most Crinoids this arrangement is continued throughout the arms : this is the uniserial arrangement. But in some of the later palaeozoic forms the distal plates may be wedge-shaped, the broad end of the wedge being alternately on the right and left side of the arm : this is the alternate arrangement. Finally the plates may be in two series, the contact surfaces interlocking so as to give rise to a zigzag line ; this is the biserial arrangement and often follows the alternate arrangement as the end of the arm is approached. The pinnules are modified arm branches. They alternate on the two sides of the arm and are often closely crowded. They contain all the organs found in the arms, are jointed like the CEINOIDEA. 279 arms, and with the exceptions mentioned on p. 271 possess an ambulacral groove and tentacles. This view of them is sug- gested by their mode of origin in the growth of the arm, as branches at the growing end.* The first indication of a pinnule is the formation of a fork at the growing point of the arm ; one of these branches grows faster than the other and forms the continuation of the arm, while the other becomes a pinnule. In Hyocrinus the proximal pinnules are almost as long as the arms and the pinnule-bearing joints have the appearance of axillaries. In the Cyathocrinidae there are no pinnules, but the arms are much branched and what would be pinnules in other forms are merely arm-branches. The second brachial is the first arm joint which bears a pinnule (Thaumatocrinus, Eudiocrinus). Pinnules are always absent from axillary joints, from the hypozyal of every syzygy (see p. 283), and from the lower of every pair of joints that are united by ligamentous articulation. The pinnules contain the generative organs, but the so-called oral pinnules of Antedon and its allies are sterile. In Mefa- crinus the ambulacral grooves of the proximal pinnules may start directly from the margin of the mouth or from the portion of the grooves on the calyx cover ; so that the pinnules appear to be appendages of the calyx. The calyx-cover (tegmen calycis) in the simplest cases con- sists only of the large triangular, interradially placed oral plates (sometimes called deltoids), which are arranged in such a manner as to form a pyramid-like projection over the ventral side of the calyx (Fig. 194). The outer sides of these plates are in contact with the radials of the calyx, and the posterior of them is, in Haplo- crinus at any rate, perforated by the anus fterior" 1 aide 6 (Fig. 194). This arrangement is found in the J e h rforatg is "the Larviformia. In Holopus among living forms, there' 1 " between a very similar condition is found, there being a radiaf ht and te the very few small plates between the large orals Casals _^ a^ _piate and the edge of the calyx (the position of the 1 or radu * In Comatulids, however, the proximal pinnules are formed la-ter than the distal, and are not therefore formed at the growing point as arm- branches. 280 PHYLUM ECHINODERMATA. anus of Holopus is not known). In the living genera Hyocrinus and Thaumatocrinus, the orals are very similar to those of the Larviformia, but between them and the edge of the calyx there are a few rows of interambulacral plates (Fig. 209). In Rhizocrinus also the orals are present, but much smaller than in those first mentioned, the part of the calyx-cover occupied by interambulacral plates being still more extensive. In the Larviformia the oral pyramid is supposed to have been closed, the oral plates being actually connected with one another along the lines of contact, except at the base of the arms, where a small opening exists through which the ambu- lacral grooves pass out on to the arms. In such cases the mouth is said to be subtegminal. This condition is not found in any living form : in Holopus, Hyocrinus, etc., the oral plates are not continuous and the mouth opens between them. But a very similar state of affairs is found for a short period in the development of Antedon. In the larva of this animal there is a stage in which the mouth surrounded by its fifteen tentacles opens into a closed sac, the oral vestibule in the walls of which are contained the large oral plates (p. 157). In the majority of forms, however, both living and extinct, the orals are either comparatively in- conspicuous or absent in the adult. In the Articulata (except Rhizocrinus) orals are absent. In the Fistu- lata they are distinct in some forms and not in others. When they are distinct they are in the centre of the calyx-cover and the posterior is the largest and is placed partly between the others. In. some forms with subtegminal mouth (some Cyathocrinidae) the mouth is covered by 5 proximal ambulacral plates which simulate orals, the true orals (deltoids) being outside these. In the Flexibilia they are present in Taxocrinus and surround the open mouth (Fig. 195). In the Camerata they can generally be distinguished at the apex of the vault into which the calyx- cover is produced, but their identification is sometimes uncertain. In the forms with small orals the greater part of the calyx - cover is occupied by compactly or loosely arranged interambu- lacral plates, which are continuous with the interradial plates of the calyx, if such are present, and by the ambulacral grooves iii their passage outwards to the arms. FIG. 195. Calyx cover of Taxocrinus intermedius (after W. and Sp., from Delage and He'rouard). CRINOIDEA. 281 The ambulacral grooves themselves are often protected by covering plates which arise at their sides and project over them : these could apparently be erected and depressed and are alter- nate on the two sides of the groove (Fig. 193, A, D, 195). The grooves are sometimes unprotected and open. In the Camerata it frequently happens that the interambulacral plates of the calyx-cover project over and cover up the ambulacral grooves (and their covering plates) which are thus converted into canals open at the edge of the disc where the arms are given off (Fig. 193, A}. The mouth, being covered by the firmly united orals, communicates with the exterior, in such cases, only through these canals which branch as often as the arm branches before leaving the calyx. There is, however, as has already been stated, considerable variation amongst the Camerata with regard to this character. Covering plates are found in some living forms, e.g. Hyocrinus, Holopus, Rhizocrinidae, some species of Antedon. It appears, therefore, if the present accounts of Palaeontologists are to be trusted, that the mouth of the Camerata is, like that of the Larvi- formia, subtegminal. In both these groups, therefore, the only com- munication of the mouth with the exterior is by the ambulacral grooves which issue through openings between the bases of the central tegminal plates. In some Fistulata the grooves are open the whole way, and the mouth is not subtegminal ; in others some of the central covering plates are enlarged and fixed, so that the grooves open im- mediately beyond them and the mouth is subtegminal. In some of the Camerata the ambulacral grooves open to the exterior only at or near the edge of the calyx (Fig. 193, A), the first part of their course- being covered up by the encroachment of the interambulacral plates. Further particulars as to the structure of the calyx-cover. In the Fistxi- lata the calyx-cover is usually flat, except in the posterior interradius in which it is prolonged into an enormous sac-like process which no doubt contained a considerable part of the animal's viscera and had the anus at or near its apex. Both process and interambulacra generally are firmly plated. In the Camerata the whole calyx-cover is symmetrically prolonged into a vault-like process (Fig. 202), at the end of which was the mouth covered up by the rather indistinct orals, and on the posterior side of which was the anus. The vault is firmly plated and its covering consists of the interambulacral plates and their extensions over the ambulacral grooves. In the Flexibilia the calyx-cover is only known in Taxocrinus. It was flat and flexible with numerous loosely arranged interambulacral plates (Fig. 203). The ambulacral grooves are exposed and the orals are dis- tinct and surround the freely open mouth. In Holopus and Hyocrinus the structure of the calyx-cover has been sufficiently explained (p. 280). In most other recent forms the calyx-cover is membranous and has only loosely connected plates in the interambula- 282 PHYLUM ECHINODERMATA. era. The ambulacra! grooves however have covering plates or, as in some Comatulids, covering folds without plates. The anus is at the end of a papilla in the posterior interradius, and its walls resemble in structure the interambulacral area from which it arises. The covering plates of the ambulacral grooves are either attached to the brachials or to a special set of lateral or side plates. The stem is composed of a number of ossicles united by close sutures or by articulation. It is traversed by an axial canal (see p. 288), and it may bear at intervals whorls of jointed cirri, which contain a prolongation of the axial canal. The cirri of the lowest pieces are in some forms root-like in appear- ance and ramify in the muddy or sandy bottom on which the animals live (Rhizocrinus) ; in this case the normal cirri may be absent. In other cases the lowest ossicle is attached to the substratum by a kind of cement (Pentacrinus). In growth the addition of new pieces is confined to the upper end of the stem, where they arise by intercalation between existing pieces and (except in Flexibilia and some Articulata) between the stem and the cup. In Flexibilia, etc., the top segment of the stem is often fused with the infrabasals.. In Uintacrinus, Marsupites, Thaumatocrinus and Holopus the stem is absent. In the Comatulidae it is present in the young, but in later life the animal breaks away from it, retaining only the top joint, on which several whorls of cirri are formed. This top piece (Fig. 189, Cd) which remains attached to the calyx in Comatulids may be formed of two or more joints fused, as is suggested by the numerous whorls of cirri on it ; it is called the centrodorsal piece of the calyx and fuses with the infra-basals. It is uncertain whether Thaumatocrinus has a stem in the young state ; probably it has. Whether Marsupites and Uintacrinus, which were also without stems, were fixed or not cannot be certainly determined. Holopus is attached by the broad base of its calyx, but it is without a stem. The connexions between the skeletal pieces of Crinoids are of various kinds. In studying them it must be remembered that the plates are laid down as calcareous films in the connective tissue of the body, and that these, as they increase into plates, remain connected by the uncalcified connective tissue. When this tissue is well marked, the joint is said to be a loose suture ; if it is contractile in function (muscular) we have a muscular articulation ; such joints permit of movement of the connected plates on one another. When the plates are closely applied together and the intervening connective tissue is sparse, we have a close suture. CRINOIDEA. 283 In a close suture, which is also called a synostosis, the plates are immov- ably connected together. A syzygial joint or suture, or a syzygy as it is sometimes called, is a close suture of two adjacent brachials, and is charac- terized by the fact that the proximal plate of the pair, i.e. the one next the calyx, does not bear a pinnule, while the distal one does. The proxi- mal non-pinnuliferous piece of a syzygy is called the hypozygal, the distal one the epizygal. Syzygial suture is also found in the stem : in this case the lower piece of the pair or hypozygal is without cirri, while the upper piece or epizygal bears cirri. In anchylosis the plates are cemented to- gether and the line of separation is difficult to distinguish or absent. As in Ophiurids the ectodermal epithelium of the abactinal side of the arms and calyx is not to be distinguished. The epithelium of the ambulacral grooves is ciliated ; elsewhere it is non-ciliated. The cutis contains the skeleton and its connective tissue is very largely replaced by calcareous plates. There is no dermo-muscular system. The muscles are in bundles connecting the movably-articulated skeletal plates. The central nervous system consists of a ventral ectoneural portion, a deep oral portion and a dorsal apical system. The ventral ectoneural system (Fig. 197, 1) very closely re- sembles that of Asterids and as in them consists of an epithelial plexus, especially concentrated in the epithelium of the open ambulacral grooves of both arms and pinnules and of the ecto- derm immediately surrounding the mouth opening. The apical nervous system consists of a cap-like sheath of nerve fibres and cells surrounding the chambered organ (p. 288) and giving off interradially nerves, which bifurcate in the basals (Antedon, Ehizocrinus) or amongst the radials (Bathycrinus) into two strands, which. diverge and pass to join the correspond- ing strands of neighbouring nerves (Fig. 196). The single cords so formed are radial in position and called the radial nerves of the apical system ; they run to the tips of the arms and of their branches (Fig. 197, 8), and give off cords which similarly traverse the pinnules (Fig. 198). In the Articulata the whole system lies in canals the so-called axial canals in the skeletal pieces of the calyx and arms, viz. the infra-basals, the basals, the radials and the brachials. In some forms (Antedon, etc.) there is in the primaxil (Fig. 196, E 3 ) a chiasma and a commissure connecting the two nerves which result from the bifurcation of the main nerve, and the same nerves are in all cases connected, at the level of the radials, both with each other and with those 284 PHYLUM ECHINODERMATA. of adjacent radii by a commissure which forms a ring round the cup (Fig. 196). In Encrinus and Pentacrinus the apical nerve cords are double. In many Palaeozoic Crinoids the canals for these axial cords are not separ- ated from the ventral grooves on the brachials. In others they are present in the brachials, but exist only as grooves on the radials and basals. The apical nerves give off branches which ramify in the ossicles (Fig. 197), and supply the muscles of the ossicles and the inte- gument ; some of them are connected with the branches of the deep oral system. A prolongation of the ner- vous sheath, which surrounds the chambered organ and forms the centre of this system, accompanies the prolongation of the chambered organ into the stalk and cirri, or if the stalk is absent into the cirri of the centro-dorsal plate. FIG. 196. Antedon bifida. Diagram showing the arrangement of the apical nervous system in the calyx, aaxial cords, the black disc in the centre of the figure represents the central sheath which surrounds the chambered organ ; CD centro- dorsal plate ; B\ first secundi- brach ; RI radial ; #2, -#3 primi- brachs (from Perrier after Lud- wig). As in Asterids the apical nervous sys- tem appears to originate in connexion with the coelomic epithelium. It was dis- covered by Dr. W. B. Carpenter * in Comatula as a result of his experiments on the animal. His observations were for some time discredited, but were eventually confirmed by Marshall ,f who showed that the nervous aggregation round the cham- bered organ governs the movements of the arms and that the nerve cords proceeding from it contain both sensory and motor fibres. The deep oral nervous system is placed below the epithelium ; it consists of a ring round the mouth and two radial nerves in each arm (Fig. 197, 4). The ring also gives off nerves which ramify in the connective tissue strands of the body-cavity. Branches of the brachial nerves of this system anastomose with branches of the apical cords. Sense organs. There are no terminal tentacles nor special organs of sense. The tube-feet must be regarded as specially sensitive and possibly the whole arm as well. The tube-feet are supplied by both the superficial and the deep oral nervous system. * Proc. Roy. ,S'oc. 24, 1876, p. 211, and vol. 37, 1884, p. 67. t Q.J.M.S., 24, 1884, p. 507. CEINOIDEA. 285 The alimentary canal is tubular and passes from the central or subcentral mouth to the excentric anus. In this passage it executes, in Antedon, one complete coil in the calyx. Its lining is ciliated and its central portion is slightly dilated. In some forms caecal outgrowths of the canal are present. In Actinometra the digestive tube executes four complete coils in the calyx before the anus is reached. Moreover the mouth is excentric, being shifted anteriorly, while the anus is subcentral. The coelom presents the usual division into perivisceral cavity and water- vascular system. The perivisceral cavity occupies the calyx and extends into the arms. The calycine portion is for the most part traversed by a connective tissue network in which calcareous structures may be present. In some Comatulidae three parts of the body cavity may be distinguished. (1) an axial portion, in which there are no connective tissue strands ; this occupies the axis of the calyx ; (2) a perivisceral portion around the gut coils, and (3) a subcutaneous portion just beneath the integument and marked off from (2) by a kind of septum. These parts do not appear to have any special importance. The perivisceral cavity is continued into the arms as three distinct sets of cavities, which however communicate at inter- vals. These are (Fig. 197), (1) the dorsal or coeliac canal (7), {2) the canal containing the generative rachis (6), and (3) the ventral or sub tentacular canal (-5). The last is divided by a septum into two, and on the dorsal wall of the dorsal canal de- pressed patches of ciliated epithelium are occasionally met with, especially in the pinnules. All these parts of the body-cavity are continued into the pinnules (Fig. 198), the only difference being that in the pinnules the generative rachis is swollen up into the generative glands. The ventral canal, on reaching the calyx, opens into the axial part of the perivisceral space. The genital section is lost in the meshes of the body-cavity round the oesophagus, and the dorsal canal opens into the subcutaneous part of the body- cavity of the calyx. There is nothing corresponding to the axial sinus of Asterids and Echinids. There is in the arms and pinnules a canal * (2) between the * This canal is not always distinguishable. It may be due to shrinkage. 286 PHYLUM ECHINODERMATA. water-vascular trunk and the ambulacral groove which is sup- posed to represent the perihaemal space of other types. It appears to end blindly at each end, and there is no circumoral representative of it. The water-vascular system consists of a circular vessel round the mouth giving off a radial vessel into each arm. The radial 9 FIG. 197. Diagram of a transverse section through the arm of a Crinoid (from Lang). 7 Radial nerve of the superficial oral system, 2 perihaemal canal (?), 3 radial water-vascular trunk, 4 the radial trunks of the deep oral nervous system, 5 and 11 the ventral or subtentacular canals, 6 the canal round the generative rachis, 7 the dorsal or coeliac canal, 8 radial trunk of the apical nervous system, 9 peripheral cutaneous termination of nerves of 8, 10 connecting nerve between Sand 4, 11 subtentacular canal, 12 nerve to tube-foot, 13 water- vascular canal of tube-foot, 14 sensory process on tube-foot, 15 ambulacral groove. vessels branch with the arms and extend into the pinnules. They terminate short of the end of the arms and there are no terminal tentacles. The water-vascular trunks have muscular walls and are sometimes traversed by muscular fibres. The lining epithelium is not, as in other Echinoderms, ciliated. The circular vessel gives off tubes which pass into the circumoral tentacles, and a number of other tubes which hang down and CRINOIDEA. 287 open into the body-cavity. These are the representatives of the stone-canal of other types. They are usually very numerous and are without calcareous deposits in their walls. They open into the general body-cavity, the portion corresponding to the axial sinus of other types having become continuous in the adult with the perivisceral space. The calycine pores, each of which represents a madreporite of other Echinoderms, also open as we have seen into the body-cavity. The radial canals give off lateral branches, each of which, in Antedon, supplies three tube-feet. In correspondence with this, the tube-feet are placed in groups of three at the sides of the ambulacral grooves and are without ampullae. They are to be regarded as purely sensory and respiratory structures, and are often called tentacles. The water-pores are lined by a ciliated epithelium. In Rhizocrinus lofotensis there are only five, one in each interradius, and they open into the perivisceral cavity close to the opening of the stone-canals which are also five in number. In other Crinoids * the number of water-pores is very numerous (sometimes over 100), and it has not been shown that there is any relation between their openings and those of the numerous stone-canals. They are placed on the interambulacral portions of the calyx- cover, and they perforate the interambulacral plates if such are present. In Actinometra they have been observed on some of the proximal pinnules as well as on the calyx. The axial organ (genital stolon) occupies the axial portion of the perivisceral cavity. Apically it has the form of a thin strand in the axis of the chambered organ ; from this point it ascends in the body-cavity, where it widens. Its oral end is narrowed again to a few strands which are continuous (see below) with the generative rachis in the arms. It consists of convoluted canals lined by columnar epithelium and embedded in connective tissue of the vascular modification. The canals anastomose and end without leading to any organ. The vascular system is present and has the usual form of anas- tomosing spaces in the mesoderm. It is richly developed in the wall of the alimentary canal, over the axial organ, in a ring round the mouth, and round the genital rachis and the genital * In Cya.thocrinus and other Fistulata there is said to have been a multiporous madreporite. 288 PHYLUM ECHINODERMATA. organs. It is not clear whether there is any radial branch of this system into the arms. The intestinal network and that over the axial organ communicates with the circumoral tract, and on the circumoral tract on the rectal side is a special development of this tissue containing cellular elements and called the spongy organ. The chambered organ is a portion of the embryonic coelom (p. 156). In the adult it is completely cut off from the rest of the coelom and consists of five radially disposed chambers separated by interradial septa, the whole being surrounded by the central organ of the apical nervous system (p. 283). It is placed at the apex of the calyx, in the Comatulids in the centro-dorsal plate. The chambered organ is continued, with its nerve investment, into the stalk in the stalked forms, and into the cirri if such are present. The sacculi are globular sacs containing highly refractile spherules. They are found in the connective tissue at the edge of the ambulacral grooves of the arms, pinnules and calyx, and sometimes in other parts. Each spherule is in its origin related to one cell, the remains of which can be traced round it. Their meaning and function is unknown. They are absent in Actino- metra, Tkaumatocrinus and Holopus. The generative organs may be described under two heads : (1) the generative rachis, and (2) the gonads. These two struc- tures are continuous and form part of one structure of which the rachis is the sterile portion. The rachis is contained in the arms (Fig. 197) : it is a cord of cells containing a small lumen and surrounded by vascular tissue. It lies in the genital divi- sion of the arm body-cavities and is continuous in the disc, through a circular cord (Ante.don] or a network of strands (Pentacrinus), with the axial organ. The generative organs are developments of the terminal portions of the generative rachis, i.e. of the portion contained in the pinnules. In exceptional cases the generative rachis of the arms also gives rise to generative cells. In the pinnules the generative rachis swells up, its cavity becomes larger, and its lining cells become ova or spermatozoa (Fig. 198). These escape into the water probably by dehiscence. So far as is known Crinoids are always of separate sexes, and the development, which has been followed in Antedon only CRINOIDEA. L>S!) . T and is described at p. 152, takes place in the water and never in brood pouches. Arms very readily break off so that the animals can escape if they become entangled or seized by enemies ; the rupture takes place at a syzygy. Arms so lost are regenerated, but no cases of asexual reproduction have been met with. Regenera- tion of the viscera takes place after evisceration, and sponta- neous evisceration is said some- times to occur. The number of radii, i.e. of primary arms, is nearly always five, but some extinct forms show indications of having had a different number, e.g. Plicato- crinidae with 4, 6, or 8 (rarely 5 or 7) radials.* Moreover speci- mens of Rhizocrinus are found with 4, 6, or 7 radii, and of the few specimens of Holopus (Fig. 210j that have been found one was tetramerous. Among Coma- tulae also forms with 4 and 6 rays are very rarely met with. Except in Rhizocrinus, it is rare to find the number five departed from. The Crinoids, especially those living near the shore and to a depth of 150 fathoms are gregarious in their habits, and the remains of the Crinoid forests in the Silurian and Carboniferous rocks show that this habit is not confined to living forms. In the Crinoid forests found in Palaeozoic rocks different genera and species, belonging even to different orders, are associated, whereas after Palaeozoic times the Crinoid forests consist of associations of individuals of the same species. Antedon (Fig. 188) is free but can anchor itself by its cirri ; Rhizocrinus (Fig. 207) is attached by a branching root, while the lowest joint FIG. 198. Transverse section through a pinnule of an adult female of Ante- don (after Ludwig). a sacculus ; 6 septum between the subtentacular canals ; Br perihaemal canal ; CD coeliac canal ; CO genital canal ; CV subtentacular canal ; d mem- brane separating the coeliac canal from the genital canal ; e ciliated pit of the coeliac canal ; E epithelium of ainlmlacral groove ; I' trunk of apical nervous system ; G cavity of the ovary ; K calcareous plate ; Nr radial nerve ; q membrane en- veloping the ovary ; T tentacles (tube-feet) ; Wr radial water-vascular canal. * The living Prornachocrinus is peculiar in having 10 radials. Z III. U 290 PHYLUM ECHINODERMATA. of the stem of Pentacrinus (Fig. 208) is attached to the sub- stratum, but this attachment may be lost by the breaking of the stem or other cause, and the animal may move freely, anchoring itself by its stalk cirri like an Antedon. Locomotion in the case of Antedon is effected by the movement of the arms. It is possible that some of the extinct forms, e.g. Marsupites, and Uintacrinus were free. Food is brought to the mouth by ciliary currents along the arm grooves in the form of floating organisms. The principal external parasite is Myzostoma (vol. I, p. 492), which infests the disc, stalk and arms often in great numbers ; it may be free or enclosed in a cyst in the body wall. The class Crinoidea is divided into five orders, Larviformia, Fistulata, Camerata, Flexibilia, and Articulata. The living forms are all contained in the order Articulata. The grouping of Crinoids into Palaeocrinoids comprising the Larvi- formia, Camerata, Flexibilia and Fistulata, and Neocrinoids (Articulata) must be given up. As in the case of Echinoids the living forms interdigi- tate with the extinct. The most important character of extinct forms not found in the living is the pentameral asymmetry of the calyx brought about by the presence of interradial plates in the posterior interradius, but this is by no means shared by all the Palaeocrinoids (Platycrinidae, Calyptocrinidae, etc.) and indications of it are present in the jointed process of the posterior interradius of the living Thaumatocrinus. As stated above, the forms grouped under our order Articulata are, with the exception of this character, a very fair sample of all Crinoids that are known. The following are some of the characters which were considered to mark the ancient forms. In the Palaeocrinoids in which some of the arm plates (above the radials of the calyx) enter into the composition of the calyx, the plates which are so absorbed are united by interradially placed plates (interprimi- brachs etc.) ; in Neocrinoids when the lower arm joints are so taken into the disc, there are usually no interradial plates between them, the arm plates being in contact or the intervening body wall flexible ; though this does not hold in all cases, e.g. Apiocrinus, Guettardicrinus, Uintacrinus. The perforation of the radials by the axial cord is also said to be a char- acteristic of the Neocrinoids. In most Neocrinoids with divided arms, the axillary is the second primibrach (Metacrinus, Plicatocrinus excepted), while in Palaeocrinoids the axillary varies from the radial to the Oth primibrach this is the only important character which separates the Palaeocrinoids Erisocrinus, Phialocrinus and Stemmatocrinus from Encrinus. Affinities. The Crinoidea stand far apart from the other classes of living Echinodermata. Their important distinctive CRINOIDEA. 291 characters are as follows. (1) They are all, with the partial exception of the Comatulids and possibly one or two others, attached throughout life, and the oral pole which also carries the anus is turned upwards. (2) The gonads are removed altogether from the disc and lie in the ultimate branches of the arms. (3) The anterior coelom becomes merged in the general perivisceral cavity, and there is no axial sinus in the adult. (4) The anterior coelom is given off from the enteron separately from the posterior. (5) The absence of any trace of a right hydrocoel. (6) The fact that the oral surface of the adult is derived from the posterior surface of the larva. (7) The form of the larva. (8) The open condition of the ambulacral grooves. Some of these are absolutely distinctive, viz. (4), (6), (7). Some of them are shared either wholly or partly by Asteroids, viz. (1), (2), (8). Holothurians present (3), and so far as is known (5). The bias is therefore on the whole towards Asteroids, as we have already pointed out in discussing the affinities of Holo thurians, but the bias is very slight, for (8) cannot be regarded as an important character seeing that Ophiuroids, so closely related to Asteroids, do not share it, and (2) is partly shared by Echinoids, so far as the pentamerous arrangement of the gonads is concerned. So that (1) only is left. But this resemblance carries with it an important difference. It is true that Asteroids are the only living Echinoderms outside the Crinoids which pre- sent fixation at any time of life and that the fixation is effected by the preoral lobe, but as shown by MacBride there is this im- portant difference : whereas in Asteroids the pedicle of attach- ment is found to arise from the oral surface of the adult and is surrounded by the hydrocoel ring (Fig. 105), in Crinoids it springs from the aboral surface and is outside and far removed from the circumoral water-vessel. This discrepancy undoubtedly receives its explanation by a consideration of the fact that fix- ation takes place in both classes some time before the hydrocoel ring becomes complete. But it is none the less significant, especially when taken in conjunction with another fact. In all Echinoderms the mouth shifts during the development. In all classes except Crinoids it shifts on to the left side, so that the left side or left ventral side of the larva becomes the ventral side of the adult. In Crinoids however it shifts further ; not only does it move on to the left side indenting the left hydrocoel 292 PHYLUM ECHINODERMATA. and the left posterior coelom, but it continues its movement, carrying with it the left coelom and hydrocoel, until it comes to lie beside the anus at the posterior end (see account of develop- ment). Having reached this point, far removed from the preoral lobe, the hydrocoel ring closes. As a result of this move- ment, which must be due to a torsion of the whole of the pos- terior part of the body, the right posterior coelom has also shifted and come to lie on the aboral (originally anterior) side of the gut, and the preoral lobe of the larva becomes enclosed by the row of skeletal elements (apical plates) which are developed outside the right posterior coelom in all Echinoderms except Holo- thurians. These plates as is well known are laid down at first in the larva in an open curve, which later closes, as does the hydro- coel, to form a complete ring. In Asteroids the closure of this curved row of plates is effected far from the point of origin of the preoral lobe, on the right or right dorsal (larval) side of the body. In Crinoids it is effected at the anterior (larval) end of the body and encloses the preoral lobe, just as the hydrocoel does in Asteroids. If these considerations are sound, it follows that the rejection of the hypothesis as to the homology between the apical plates of Crinoids and those of other Echinoderm classes, in so far as that rejection depends upon the difference in the point of origin of the stalk in Asteroids and Crinoids, is not justified. The fact that in Crinoids alone is the rudiment of the posterior coeloms given off at the posterior end of the larva and separately from that of the anterior becomes to a certain extent intelligible in view of the foregoing considerations. This peculiarity must be regarded as a back-thrust from the adult form, in which the posterior end comes to hold all the viscera. Pursuing the same line of thought the fact that the hydrocoel does not share in this impress of adult arrangements and in like manner develop from the posterior end of the enteron but comes off in front with the anterior coelom is significant as showing that the connexion in origin of the hydrocoel and anterior coelom is a fundamental feature of Echinoderm morphology.* The fact that the left posterior coelom does not at an early stage come to exceed the right in size is another feature of the larva of * This holds even in Ophiurids, see p. 151, and MacBride, Q.J.M.S. 51, 1907, p. 557. CRINOIDEA. 293 Crinoids in which they differ markedly from Asteroids, though not so far as is known from other classes.* Order 1. LARVIFORMIA (Inadunata larviformia Wachsm.) The calyx-cover consists entirely or almost entirely of five triangular oral plates which are applied together so as to form a pyramid and cover the ambulacral grooves of the disc as well as the mouth, j- The calyx is monocyclic and consists of basals and radials only except in Cupresso- crinidae and Stephanocrinidae, there being no interradial plates except in the anal interradius. The anal interradius is frequently unlike the other interradii. The plates are immovably connected by smooth sutural surfaces. The arms are weakf and entirely free. The large orals and simply constructed calyx are characteristic of larvae of living Crinoids. Upper Cambrian to Carboniferous. Fam. 1. Haplocrinidae. Calyx small, irregular, with five unbranched arms. Some of the radials are composed of two pieces (Fig. 199). Anus is in the posterior oral. Haplocrinus Steininger (Fig. 199), Devonian. Fam. 2. Allagecrinidae. Calyx small ; basals and radials 5. Oral plates triangular. Allagecrinus Eth. and Carp. Fam. 3. Pisocrinidae. Pisocrimis de Kon. Tria- crinus Miinst. Fam. 4. Sy mbathocriniclae. Symbathocrinus Phillips, Phimocrinus Schultze, Stylocrinus Sandb., Stortinyo- FlG 199 Haplo- crinus Schultze, Lageniocrinus de Kon. crinus mespili- Fam. 5. Cupressocrinidae. Calyx pentamerally DelTge and nlrou- symmetrical, 5 basals, 5 radials, no interradials. The ard, after W. basals surround a pentagonal centro-dorsal plate ( ? top ^nal side/" 1 ^ segment of stalk). 5 broad and thick arms, which are traversed by a nerve canal. Cupressocrinus Goldfuss, Devonian. Fam. 6. Stephanocrinidae. Calyx composed of 3 high basals, 5 deeply forked radials, and 5 small interradials ; with branching arms (each arm- joint gives off side arms which are non-pinnulate). U. Cambrian, Silurian. Stephanocrinus Conrad. Order 2. FISTTTLATA (Inadunata Fistulata W. and S.) Calyx covered with thin plates which easily fall apart from one another and may be prolonged in the anal interradius into a usually high balloon- shaped, or short conical tube. The ambulacral furrows are covered by alternating covering pieces but are not subtegminal. The mouth is eccentric and sometimes subtegminal (see p. 280). Basals and radials of the calyx immovable, connected by simple suture. Arms free, no brachial included in the calyx, uni- or bi-serial, usually branched, the segments connected by simple sutures, with or without pinnules. Interradial anal plates may be present in the anal interradius, one of them ' It is somewhat surprising, having regard to their early inequality in Asteroids, that in Holothurians and Echinoids, in which the ambulacral surface of the adult so much exceeds in area the antambulacral, the right and left posterior coeloms should remain approximately equal in size in the larva. t Exception Cupressocrinidae. 294 PHYLUM ECHINODERMATA. being usually a radianal, and another a special anal (p. 276). U. Cambrian to Permian. Other interradials are not present. Fam. 1. Hyboerinidae. Monocyclic, 5 high basals. The right pos- terior radial compound ; the superradial small or absent, the interradial (radianal) large. Arms simple unbranched, uniserial. Hybocrinus Billings, Hoplocrinus Grewingk. Baerocrinus Volborth. FIG. 200. Cyathocrinus longimanus Ang. (from Zittel). a, calyx with arms, nat. size (after Angelin). b, arm fragment of C. ramosus Ang. from the side, c from above (showing the covering plates), d, calyx cover of C. malvaceus Hall ; e the same after removal of the calyx plates which lie on the interambulacral plates (after Meek and Worthen). Fam. 2. Heterocrinidae. Monocyclic, 5 basals, some of the radials con- sisting of 2 pieces connected by a horizontal suture. Arms uniserial, Heterocrinus Hall, Ihocrinus Hall, Ohiocrimis Waschsm. Anomalocrinidae. Belemnocrinidae. Catillocrinidae. Caleeocrinidae. Gasterocomidae. Dicyclic, Infra basals fused into a disc. branched. Fam. 3. Fam. Fam. Fam. Fam. 4. 5. 6. With one interradial anal plate. Anal opening low down above the anal plate and between two radials. Gasterocoma Goldf., Myrtillocrinus Sanclb., Nanocrinus Mull. Fam. 8. Cyathocrinidae. Dicyclic (Fig. 190). 1 or 2 interradials in the CRINOIDEA. 295 anal interradius and often a radianal. 5 oral plates and high ventral tube. Arms long, much branched, uniserial, without pinnules. U. Cambrian to Carboniferous. Cyathocrinus Miller (Fig. 200), Barycrinus W. and S., Homocrinus Hall, Lecythocrimis Mull., Arachnocrinus M. W., Gissocrinus Ang., Porocrinus and Carabocrinus Billings. Fam. 9. Crotalocrinidae. Dicyclic ; calyx of 5 infrabasals, 5 basals, 5 radials, and a small anal interradial. Lower brachials laterally in con tact. Arms much branched, and branches connected. Xo pinnules. Axial canals in arm-plates. Silurian. Crotalocrinus Austin, Enallocrinus d'Orb. Fam. 10. Poteriocrinidae. Dicyclic. 5 infrabasals sometimes hidden by the column, 5 basals, 5 radials ; 1-2 interradial anals and often a radianal. Ventral sac large. Arms simple or branched, with long pin- nules ; uniserial, alternate, rarely biserial, Devonian and Carboniferous. Poteriocrinus Miller (Figs. 192,201), Woodocrinus de Kon., Scaphiocrinus Hall, Agassizocrinus Troost, Cromyocrinus and Phialocrinus Trautsch., Erisocrinus M.W., Stemmatocrinus Trautsch. In the last two genera the interradial plate of the anal interradius is very small or absent. FIG. 201. Analysis of the cup of Poteriocrinus (from Zittel, after Bather), a special anal, a' interradial anal, ra radianal. r radial, 6 basal, ib infrabasai, br' primibrach. Order 3. CAMEKATA. The calyx is enlarged by the incorpora- tion of the proximal brachials : * it con- sists of a monocyclic or dicyclic base, a circle of radials, and of a certain number of the proximal brachials. The latter are connected by interradial (iiiterbrachial) plates ; the plates of the anal interradius being more numerous than the others. The radials are in contact all round (Melo- crinidae, Calyptocrinidae, Platycrinidae) ; they are separated only in the anal interradius by an anal plate (Thysanocrinidae, Batocrinidae, Actinocrinidae, Crotalocrinidae, Hexacrinidae) ; they are separated all round (Rhodocrinidae). Infrabasals are present or absent. The calyx-cover is a vault of solid plates firmly connected together. The mouth is central and covered with 5 firmly united oral plates the hinder- most of which is often the largest and projects in between the four others. The orals are sometimes quite inconspicuous. The interambulacral plates of the calyx-cover sometimes project over and cover the ambulacral plates (Fig. 193). The mouth being covered up by the orals, its only communication with the exterior is through the ambulacral canals, which open at the base of the arms and branch as often as the arms branch before leaving the calyx. Anus excentric or subcentral, often at the end of a proboscis-like prolongation. The plates of the calyx are connected by simple, smooth sutural surfaces. They are sometimes continued without any break into the interambulacrals of the calyx-cover. Arms with one or two rows of brachials, usually with pinnules. Dorsal canals have not been observed in the brachials. U. Cambrian to Carboniferous. Except in some Platycrinidae. 296 PHYLUM ECHINODERMATA. FIG. 202. Platycrinus trigin- tidactylus (restored after de Koninck from Zittel). Fam. 1. Platycrinidae. Calyx formed of a monocyclic base and a circle of 5 large radials. Interradials restricted almost exclusively to the calyx-cover, which consists of firmly connected, usually thick plates and is usually much arched (Fig. 202). No anal plate. Arm-branches 10, 20 or more, generally free from the distichals upwards. There is one primibrach (Br. 1) attached to the radial ; the following brachials are free and the arm branches close to its base. Pinnules well developed. The cover- ing plates of the ambulacra are often exposed on the surface of the calyx-cover. Silurian to Car- boniferous. Platycrinus Mill. (Fig. 202), Carboni- ferous, Marsipocrinus Bather. Culicocrinus J. Miill., Cordylocrinus Ang. Fam. 2. Hexacrinidae. Monocyclic, 2 or 3 basals, first anal plate resting on basals and similar in form to radials. Other plates as in Platycrinidae. Devonian and Carboniferous. Dichocrinus Miinst. ; Arthracantha Williams (Hystricrinus Hinde), calyx plates beset with mobile spines. Hexacrinus Austin, Talarocrinus W. and Sp. Fam. 3. Actinocrinidae (Fig. 193). Mono- cyclic. Calyx composed of 3 basals, 5 radials, 5x2 primibrachs, and 5 X a variable number of secundibrachs. First anal interradial plate resting on basals, the other first interradials upon the radials. Arms 5 to 30 or more, uni- or bi-serial with long pinnules. Calyx-cover usually much arched and ambulacra of disc hidden by inter- ambulacral plates. With or without anal tube. U. Cambrian to Carboni- ferous. Carpocrinus Miill. Actinocrinus Mill. Cactocrinus W. and Sp. (Fig. 193). Strotocrinus M. and W. Batocrinus Casseday. Dorycrinus Roem. Desmidocrinus Ang. Agaricocrinus Troost. Fam. 4. Barrandeocrinidae. Monocyclic, 3 basals. Arms biserial, laterally fused with each other, and bent back so as to lie with their dorsal sides against the calyx. Silurian. Barrandeocrinus Ang. Fam. 5. Reteocrinidae. Monocyclic or dicyclic. Infrabasals when present 5, basals 4 or 5. Radials separated by a large special anal which supports a vertical row of anals. The spaces on either side of this row, as well as the other four interradii paved with minute pieces. U. Cam- brian. Reteocrinus Bill. Xenocrinus Miller. Fam. 6. Thysanocrinidae. Dicyclic, radials in contact laterally ex- cept at the posterior side, where they are separated by an anal plate, U. Cambrian, Silurian. Dimerocrinus Phill. (Gly piaster, Thysanocrinus, Eucrinus). Cyphiocrinus etc. Rhodocrinidae. Dicyclic. Calyx composed of 5 infrabasals, X 2 primibrachs and 10 X 1-3 secundibrachs ; interradials The first interradials inserted between the radials and touching The anal interradius is hardly distinguished from the others. U. Cambrian to Carbon. Lyriocrinus Hall, Rhipidocrinus Beyrich, Rhodocrinus Miller. Fam. 8. Melocrinidae. Monocyclic, 3-5 basals, radials in contact all round, neither anal nor interradials touching the basals, 2x5 primi- brachs. and 2 to 3 X 10 secundibrachs. Arms 5x2 with secondary branches which bear the pinnules. U. Cambrian to Devonian. Melo- Fam. 7. 5 basals, 5 numerous, the basals. CRINOIDEA. 297 crinus Goldf. (Ctenocrinus Bronn), Xenocrinus Miller, Patelliocrimis Ang., Glyptocrinus Hall, Corymbocrinus Ang. (Polypeltes Ang). Stelidio- crinus Ang. (Harmocrinus Ang.). Fam. 9. Calyptocrinidae. Monocyclic. Calyx regular, composed of 4 basals, 5 radials in contact, 2x5 primi- brachs, 5 secundibrachs ; all interradii alike. Arms 20, biserial. Silurian, Devonian. Callicrinus Ang., Eucalypto- crinus Goldf., Hypanthocrinus Phill. Order 4. FLEXIBILIA. Calyx cover flat, flexible, with loosely arranged inter- ambulacral plates. Ambu- lacra with alternating cover- ing plates, their calycine por- tions apparently not covered by interambulacral plates (Fig. 195). Mouth central, open, surrounded by 5 orals. Anus excentric. Dicyclic, 3 or 5 small infrabasals, often hidden by top joint of stalk. The calyx extends to the lower brachials (primibrachs, secundibrachs and sometimes terti- brachs being included) and the brachial plates of the calyx are united by articulation, not by suture. The brachials incorporated in the calyx are either in contact laterally or separated by interradials extending to the basals or first radials. A single azygos (anal) plate sometimes present in the posterior interradius. Arms strongly branched distallv, uniserial CO FIG. 203. Taxocrinus splendens Mill, and Gurley (from Lang), ir, ?>i, i>9 interradials, di secundi- brachs (distichals), ba basals, ib infrabasals, co stalk, r radials, ci, 02, 03 primibrachs (costals). FIG 204. Marsupites testudinarius Schloth. (after Zittel). a calyx, nat. size ; b radial with the first arm segment ; c upper part of arms. with or without pinnules. All brachialia with dorsal canal (except some- times in the distal brachials), and united by articulation. U. Cambrian to Carboniferous. 298 PHYLTJM ECHINODERMATA. Fam. 1. Ichthyoerinidae. With characters of order. Ichthyocrinus Conrad, Lccanocrinus Hall, Taxocrinus Forbes (Fig. 203), Forbesiocrinus de Kon. The following genera are allied here : Marsupites Mant. (Fig. 204), Upper Cretaceous of Europe and Asia. Calyx dicyclic, large, unstalked, composed of large thin plates, viz. central (? centrodorsal), 5 infrabasals, 5 basals and 5 radials (Fig. 191). Interradials and anals absent. Arms entirely free, branched, uniserial with dorsal canal. Uintacrinus Grinnell. Calyx pseudo-monocyclic, symmetrically penta- merous, unstalked, composed of thin plates. Infrabasals sometimes preserved, bxit usually atrophied. 5 basals enclosing a small pentagonal centrodorsal, 5 radials, and 5x2 primibrachs. The axillary primibrach carries two rows of secundibrachs which gradually pass into the arms. Intersecundibrachs, usually 2, may rise to 8. Arms long, thin, uniserial, with numerous pinnules, connected over the calyx wall by large inter- brachials. Upper Cretaceous of Kansas and Westphalia. Order 5. AKTICULATA J. Mull. (Neocrinoidea P. H. Carp. Canalic-ulata Chapman.) Calyx-cover membranous, or with usually flat, loose plates. Ambula- cral furrows and mouth open. Orals are present in the young state, and sometimes in the adult. Calyx regular (all interradii alike), pseudo- monocyclic (i.e. the infrabasals usually not separate, but atrophied or fused with the top stem- joint) ; 5 basals sometimes not visible externally ; radials laterally in contact except in Thaumatocrinus. 5x2 primibra- chials. Anal plates always absent. Interradials rarely present. Arms branched or unbranched. Stalk pro- bably always present in the young, but absent in some adults. Basals, radials and brachials perforated by dorsal canals. Arms (with one exception) uniserial or alternate, with pinnules. Trias to present time. The group includes all living, tertiary, and mesozoic Crinoids except Marsupites and Uintacrinus. They are mainly characterized by the open mouth and ambulacral grooves and by the dorsal canal in the arm plates. Fam. 1. Encrinidae. Calyx dicyclic, 5 small infra- basals stuck on to the top stem- joint, 5 basals, 5 radials, interradials absent. Calyx cover arched and plated. Arms divide once or twice, close together, biserial or alternate. Trias. Encrinus Miller (Fig. 205). Fam. 2. Apiocrinidae. Calyx regular, composed of very thick plates, 5 large basals, primibrachs or 2 x 5, and sometimes secundibrachs. Pseudomono- cyclic, the infrabasals being fused with the centro- dorsal. Interradials are sometimes present, but above the radials. Calyx cover plated. Arms branched with long pinnules. Stalk long, circular, rarely pentagonal, without cirri ofteu much expanded near the calyx. Jura, Chalk, present day. Apiocrinus Miller, f ; Guettardocrinus d'Orb., f ; Millericrinus d'Orb., f ; Acrochordo- crinus Trautschold, f ; Calamocrinus A. Ag.* recent, Galapagos Island. * A. Agassiz, Calamocrinus diomedae, Mem. Museum Comp. Zoology, 1892. FIG. 205. Encri- nus hliiformis (from Clans). CRINOIDEA. 299 Fid. 206. Bathycrinus aldrichwius, \V. Thom- son x 3 (from W. Thomson). FIG. 207. Rhizocrinus lofotensis M. Sars. x It. 300 PHYLUM ECHINODERMATA. FIG. 208. Pentacrinus wyviUe-thomsoni Jeffreys, nat.-size (from Wyville Thomson). Fam. 3. Bourgueti- crinidae. Calyx small, consisting of 5 basals, 5 radials, and sometimes 2x5 primibrachs. Calyx cover membranous with 5 orals. Ambulacra with covering plates, but without side plates. Stem with branching root. Jurassic to pre- sent time. Bourgueti- crinus d'Orb, Tertiaries ; Rhizocrinus Sars (Cono- crinus d'Orb.), the arms are very variable in number, out of 75 speci- mens examined by Sars, 15 had 4 arms, 43 had 5, 15 had 6, 2 had 7 (W. Thomson, Depths of the Sea, p. 448) ; Eocene, and present time at great depths ; Meso- c.rinus H. Carp., Cretace- ous ; Baihycrinus W. Thorns. (Fig. 206) recent at great depths, 10 arms, orals aborted. Rhizocrinus (Fig. 207) was discovered by G. O. Sars among the Lofoten Islands in 1864. The interest of this discovery led to the expedition of H.M.S. Lightning in 1868, of the Porcupine 1869-70, when Bathy- crinus was discovered, and later of the Chal- lenger in 1874. Fam. 4. Saecocomidae. Calyx small, hemispheri- cal, non-pedunculate. 5 thin radials, elevated into ridges. Arms 5x2, arm plates cylindrical with wing-like expan- sions, Upper Jurassic. Saccocoma Ag. Fam. 5. Pentacrinidae. Calyx small, consisting of 5 basals, 5 radials, CRINOIDEA. 301 and 5 x 2-3 primibrachials (with 5 small infrabasals in Extracrinus). Arms divided 1-10 times, with pinnules. Calyx cover membranous, containing thin loose plates. Without orals in adult. Stalk long, penta- gonal, rarely cylindrical, with whorls of cirri, without root-like processes ; two joints are united by syzygyat each node, of which the upper bears the cirri. Triassic to present time. Pentacrinus Miller (Isocrinus v. Meyer 1837) (Fig. 208), the 2nd primibrachial is axillary. Triassic to present time, P. asterius L. (caput medusae Mill.), Caribb. Sea, 120 fms. ; P. miilleri Oerst, W. Indies 50-531 fms. ; P. wyville-thomsoni Jeffreys, 800-1,100 fms. Extracrinus Austin (Pentacrinus Blum. 1837,) f- Metacrinus P. H. Carp., recent, Pacific. Balanocrius Ag., f. Dadocrinus v. Meyer, Trias. Fam. 6. Gomatulidae. In the young state stalked, later unstalked and freeswimming. Calyx composed of a centrodorsal plate with cirri ; infrabasals visible only in larva, fused with centrodorsal (top stem-segment) in adult ; 5 more or less reduced basals, which may be visible externally or hidden, 5 radials, and 5. X 2 or more primibrachs. Interradials absent. Calyx-cover membranous, rarely with thin plates. Orals absent in adult. Arms simple or branched, with pinnules, brachials alternate. More than 150 living species, mostly in shallow water. Lias to present time. Antedon Fremink. (Comatula Lmk. etc. ) (Fig. 188). Arms fork once or more, the second primibrach axillary, Lias to present time ; Eudiocrinus P. H. Carpenter, 5 undivided arms, f. and r. Pacific and Bay of Biscay, 50-900 fms. Actinometra Miiller, mouth excentric, Jura to present time, most seas, littoral to 800 fms. Atelecrinus P. H. Carp., with basals visible on outside of calyx, r, trop. Atl. and Pac. Promachocrinus P. H. Carp., with 10 radials (basals 5), recent. Pacific and S. Sea, 70 to 1,800 fms. Thiolliericrinus Etallon, Jura and Chalk. The position of Tliaumatocrinus P. H. Carpenter (one specimen only, S. Sea, 1,800 fathoms) is very uncertain. It is unstalked, has a calyx composed of a centrodorsal with cirri, 5 basals, 5 radials and 5 inter- radials which touch the basals, the anal interradial carries a tapermg 4-jointed process ; 5 arms, unbranched with pinnules ; mouth central with 5 large orals separated from the edge of the calyx by 2 or 3 rows of small irregular plates. In the separation of its radials laterally by inter- radials it recalls the Camerate family Rhodocrinidae. In Reteocrinus and Xenocrinus, of the same order, the radials are separated by a number of small interradials. In some recent Crinoids interradials are present but always above the radials ; they are therefore interprimibrachs, etc. In the structure of its calyx and in the entire freedom of its arms, it resembles the Larviformia. The following families may be mentioned here, though where they should be really placed in the system is a difficult question. Fam. Eugeniacrinidae. Calyx composed of 5 (rarely 4) thick, firmly connected radials ; basals are absent and the calyx cover is not known. Stalk short, composed of long, cylindrical broad joints, without cirri. Axial canals are present, and the basals are probably covered by the radials. Lias, Jurassic and lower Cretaceous of Europe. Eugeniacrinus Miller, Eudesicrinus Loriol, Tetracrinus Miinst., Phyllocrinus d'Orb. Fam. Plicatocrinidae. Calyx composed of 4, 6, or 8 (rarely 5 or 7) high, thin radials and of a 4- to 6-sided undivided base. Cavity of cup wide and deep. Calyx cover unknown. The radials carry an axillary primibrach from which 2 unbranched arms arise. Stalk thin with long, cylindrical joints. Plicatocrinus Miinst. Upper Jurassic. 302 PHYLUM ECHINODERMATA. Fam. Hyocrinidae. Calyx high, composed of 3 basals, sometimes fused, and 5 radials nearly equal in length. Each radial bears a small undivided arm, brachials united by syzygy A into groups of 2 or 3, only the distal of which bear pinnules. The proximal pinnules are longer than the distal and reach to the end of the arms. Calyx cover plated, with 5 large orals round the mouth, with covering plates. Water pores perforating the orals as in Khizo- crinus, but more numerous. Hyocrinus W. Thorns. (Fig. 209), 1,600 to 1,900 fms., Atlantic, By its calyx Hyocrinus is related to the Larvi- formia,, and by its pinnules which all terminate at about the same level, the proximal being B uu-- am FlQ. 209. Hyjcrinus bethe'lhnus \V. Thorns, (from Zittel, after AY. Thorns.). A side-view, twice nat. size. B calyx-cover magnified ; am ambulacral grooves of arms, c axial canal of arm joints, an anus, ni mouth, o oral plates. longer than the distal, to Cyathocrinus, where, however, we have to do with branching arms, not pinnules. Gephyrocrinus * Koehler and Bather, 1786 metres, Canaries. Fam. Holopidae. Calyx cup-shaped, containing all the viscera and fixed by its base to the sub- stratum ; no stalk. The calyx shows no distinct sutures dividing it into areas, so that it is impos- sible to say of what plates it is composed, though there are slight indications of a composition of radials at the upper end of cup. 10 massive arms closely rolled in upon the calyx cover (Fig. 210). Calyx cover with 5 large orals and marginal plates. Anus not observed. There is one primibrachial (if we may so call it) articulated to the edge of the cup and 10 arms-branches. Pinnules present. Holopus d'Orb. Deep water, Caribbean Sea. -n,- 1 angi The first specimen found in 1857 was tetrara- d Orb. Side view, show- ing the 10 arms bent diate ; more specimens subsequently came to over the calyx cover ]iand and it was eventually found by the Blake. (after Agassiz trom De- lageand Herouard). Ine Blake specimens were dredged at 100 fms. * Mem. Soc. Zool. France, 15, 1902, p. 68. CBINOIDEA. 303 Cyathidium Steenstr., Cretaceous and Tertiary ; Cotylederma Quenst., Lias. Holopus is quite unlike any other Crinoid and it is impossible to fix the systematic position.* We do not even know the constitution of the calyx or the position of the anus. If the calyx is composed of basals and radials only, the genus ought to be placed with the Larviformia. CYSTIDEA AND BLASTOIDEA. The Cystids and Blastoids are entirely extinct and their fossils have only been found in the Palaeozoic rocks. They differ so much from living Echinoderms that it is by no means easy to interpret their structural features or to assign them to their proper systematic position. It has been customary with zoolo- gists in recent years to associate them with the Crinoids in a subphylum Pelmatozoa, the remaining Echinoderm classes being united in a second subphylum which has been called Eleutherozoa (Bell) or Echinozoa (P. H. Carpenter). We cannot think that it is for the advantage of Zoology to adopt this classification. In the first place the Crinoids are not sufficiently distinct from other living Echinoderms to justify their assignment to a separate group of the dignity of a subphylum. In the second place our knowledge of Crinoid anatomy is detailed and com- plete, and based upon a minute study of living forms, while our knowledge of Cystids and Blastoids is vague and unsatis- factory to an exasperating degree. To take the forms assigned to the Cystidea alone, we cannot even be certain whether we are dealing with a single class or whether the range of structure met with would not more properly be distributed over several classes equal in value to the other Echinoderm classes. To the zoologist the great interest attaching to the study of Cystids consists in obtaining an answer to these questions. Their association with Crinoids seem to us to make it more difficult to obtain an answer. It places us in an entirely false position with regard to them, for it implies that we have a considerable know- ledge of their anatomy and so may cause us, in the light of our complete knowledge of Crinoids, to strain our interpretation of difficult or doubtful Cystidean structures in a manner and to an extent which may lead us far from the truth. For these reasons ' It is placed by Jaekel and Bather near the Eugeniacrinidse on account of its arm structure. 304 PHYLUM ECHINODERMATA. we prefer to subordinate the Cystidea and Blastoidea to no section of the Echinodermata and to treat them as independent classes, which in respect of their upturned mouth, their orally placed anus, and their aboral peduncle of attachment recall Crinoids, but which in other features of their anatomy show considerable diversity, some having obvious leanings towards Echinoids and Asteroids, and some seeming to be representative of a stage of structure in which the symmetry of the modern Echinoderm had not been evolved. It is indeed highly probable that the Cystids as at present constituted contain heterogeneous elements, which should be assigned to more than one independent class. Already highly competent authorities have separated from them the Edrioasteroidea * and there is much to be said for separating other members of the class. Class CYSTIDEA.t Stalked or unstalked forms with calyx which is usually composed of irregu- larly arranged plates. Arms are imperfectly developed or absent, and the radiate symmetry is often very imperfect or absent. The plates of the calyx usually possess pores. The Cystidea are entirely extinct and only known to us by their fossil remains, which so far as our present information goes are confined to the Palaeozoic rocks. Forthis reason their structure is and must remain very imperfectly known, and the class presents great difficulties to the zoolo- gist. These cannot be overcome and we must content ourselves in this work with the description of a few forms which seem to represent the considerable range of structure found in the group. For fuller information we refer the reader to the excellent account contained in the two works cited. The most significant characters of the Cystids are (1) the irregularity in the arrange- ment of the thecal plates which is so often found in them ; (2) the absence or feeble development of arm-like structures ; (3) the fact that the generative organ seems usually to have been single and not radially arranged ; (4) the fact that in some of them there does not appear to have been a radial symmetry. If we are to have phylogenetic speculations these characters must obviously be taken into verv careful consideration. FIG. 211. Cystaster graniilatus from the posterior side show- ing the oral surface in perspec- tive. The two left-hand rays retain the covering plates which are absent from the others (from Bather) x 3. As anus ; cp covering plates ; o peristomial plates ; sp side plates. * See Bather, op. cit. t Zittel's Text-Book of Palaeontology, vol. 1, London, MacMillan & Co., 1900. F. A. Bather, chapter on Pelmatozoa in Lankester's Treatise on Zoology, London, A. and C. Black, 1900. O. Jaekel, Stammesgeschichte der Pelmatozoen, Berlin, 1899. CYSTIDEA. 305 BT.JF The body consists of a calyx which is either prolonged aborally into a stalk (Fig. 215, etc.) or is without a stalk (Fig. 211, etc.) In the latter case the body was probably attached by its aboral surface to the substratum. When a stalk is present it frequently has rather the appearance of a taper- ing aboral continuation of the body (Fig. 212) than of a sharply differen- tiated stem like that of the stalked Crinoids. The stem, which is often very short and with- out cirri or roots, does not, as a rule, appear to have served for attachment. It is frequently coiled. The calcareous plates of the body wall are usually numerous and irregularly arranged ; but sometimes they are larger, few in number and arranged in definite cycles (e.g. Cystoblastus Fig. 213). They are united by sutures. There is as a rule no sharp line of demarcation between the oral and aboral surfaces, or between the plates of the radial and interradial areas. The mouth is at or near the centre of the oral surface and is sometimes covered by oral plates. In the simplest forms ambulacra! grooves are not visible (Figs. 212, 214) and no radial structure can be made out, but it is asserted by Barrande that such forms probably have subtegminal grooves. Usually ambulacra! grooves are pre- sent, and they are placed either on the surface of the calyx (Fig. 217) or on processes of the edge of the mouth (Fig. 216). They vary in number from two to five and may branch. They frequently possess covering plates (Fig. 219) which in life must have been capable of being folded back so as to expose the groove. Arms are often quite absent, and when present are usually small. They either project from the edge of the mouth (Fig. 216) or further out from the calyx (Fig. 220). The arms vary in number from two to thirteen. In some cases the so- called arms resemble pinnules, as in Glypto- sphaera, Protocrinus (Fig. 217) in which the ambulacra! grooves branch and end in small arms : these may perhaps be called brachioles, though it is often impossible to settle whether to apply the term brachiole or pinnule to an arm- like process. Undoubted pinnules arise from the edges of the ambulacra! grooves in some cases and the ambulacra! grooves are continued on to them (Fig. 215). The anus is on the same sur- face as the mouth but excentrically placed in an interradius (Figs. 211, 217, etc.) Between it and the mouth two openings can in some cases be made out : one of these, that nearest the anus, is interpreted as the genital opening ; the other is supposed to be the water-pore (Fig. 214). If the interpretation of the first of these openings is correct, it would appear that the genital organ of Cystids is in the calyx and is not radially Z m. X B -Dr /-St FIG. 212. Dendrocystis Sedgwifki (after Barrande from Bather). As anus Br the arm-like appen- dage ; st stem. 306 PHYLUM ECHINODERMATA. arranged. But the so-called genital opening has only been observed in a few cases, and it may be that in the forms with pinnules the gonads were in them. The identification of the other opening is too precarious to FIG. 213. Cysi.obla.stus Leuchtenbergi (after Volhorth, from Bather). 1 oral; 2 posterior; 3 aboral view; 4 analysis, with plates numbered. As anus;* anal interradius in 1; M water-pore or generative opening; sp plates on the floor of the arm grooves; Sf attachment of stem. allow of our drawing any inference as to the relative position of the water-pore and anus, but when the madreporite can undoubtedly be dis- tinguished, it is placed in the same interradius as the anus. The calcareous plates of Cystids often seem to be composed of three distinct layers : a smooth and thin outer layer, the epistereom ; a thick A B .of o/n- FlO. 214. Aristocystis bohemicus. A, from the left side, B from the oral face (after Bar- rande). 071 anus ; b mouth ; gtx generative opening ; rndp hydropore. CYSTIDEA. 307 middle layer traversed by canals, the mesostereom ; and a thin smooth inner layer, the hypostereom. In many Cystidea the middle layer is traversed at right angles to the surface by canals, the ends of which are closed by the epistereom and hypostereom, if these layers are preserved. If the latter are weathered off the canals appear to open in surface pores. BcL Flo. 215. Asteroblastus Volborthi (after Schmidt and Bather, from Delage and Herouard). dlt oral plates ; p pinnules; pd stalk ; R radial plates. FIG. 216. Echinosphaera aurantium (after Volborth). an anus ; b mouth ; mdp water-pore. The stalk projects below. These pores may occur singly (haplopores) or in pairs (diplopores). When the canals are in pairs, the external openings of a pair are placed in a common pit on the stereom. *| In some forms, classed as Rhombifera, canals are found in the meso- stereom traversing the plates parallel to the surface ; these canals extend across the suture, at right angles to it, to be continuous with similarly arranged canals on the neighbouring plates (Fig. 218). They are so ar- ranged that their terminations form a rhombic figure, the diagonal of which is occupied by the suture between the plates concerned. For this reason and because the ter- minations of the canals often bend outwards and appear on the surface as pores, the figures caused by them are called pore-rhombs (Fig. 218). At their ends, and sometimes also near the suture, the canals may also bend or send a branch inwards to the inner surface of the meso- stereom. Prof. Jaekel compares these terminal vertical canals with the two canals of a diplopore, and supposes the horizontal canal to represent the peri-poral depression greatly extended. In some forms, however, the canals of the rhombs have the form of grooves, the sides of which project on the external surface of the plates, so that the stereom appears as though thrown into folds. Pectini -rhombs are pore-rhombs in which such folds or grooves are very deep and at the FIG. 217. Protocrinus oviformis (after Vol- borth from Delage and Herouard). an anus ; b mouth ; mdp water-pore. 308 PHYLUM ECHINODERMATA. same time restricted in area, and often surrounded by a raised rim. While normal pore-rhombs are usually found on all or most of the plates, pectini-rhombs only occur on a few plates (Fig. 219) in definite posi- tions. The grooves of a pectini-rhomb are frequently filled up or bridged over near the suture, so that the rhomb as seen from outside appears to consist of isolated halves. These halves, however, remain connected within the mesostereom. Fio. 218. Pore-rhombs (a) of Echinosphaera, (b) of Caryocrinus (magnified, from Zittel). The left half of a is abraded, so that the canals appear as open grooves. It appears that the canals, whether of the ordinary pores or of the pore-rhombs are typically in the mesostereom, and do not open on the surface, being covered by the epistereom and hypostereom respectively, though if these layers are absent through weathering or other cause, the canals appear in some cases to be open. It is possible that they are due to tracts of stroma containing blood spaces traversing the stereom, and Mr. Bather has suggested that the canals of the pore-rhombs are develop- A B FIG. 219. Callocystites Jewetti Hall. Upper Silurian, Lockport, New York. A from the side (natural size). B ambnlacral grooves and two pectinated rhombs rh. an anus ; g genital opening ; o mouth. ments from foldings of the mesostereom, such as exist in many Crinoids, being due to the natural tension of the stroma fibres in the integument as it becomes calcified. These foldings are covered towards the surface by the secondary deposition of epistereom and hypostereom. When the epistereom was very thin or absent, as appears sometimes to have been the case, or when it has been removed by weathering, these grooves of the mesostereom appear to be open and their edges project on the surface of the plates as ridges. Whether this view of the real nature of the canals is CYSTIDEA. 309 correct or not, it appears certain that they could not have been tubes leading from the exterior into the body-cavity of the animal, because they are in so many cases closed by the layers of epi- and hypostereom above referred to. Structures very similar to pore-rhombs, having the form of deep folds crossing the sutures between the plates are found in many fossil Crinoids (see especially Porocrinus, Carabocrinus and Hybocrinus}. The ordinary structure of the mesostereom, simulating folds, is best seen in genera with large plates such as Marsupites and Crotalocrinus. In no Cystid except the Edrioasterida are pores present along the ambu- lacral grooves which can have served for the passage of tube -feet. Whether tube-feet were present is, however, another question. The Cystids make their appearance in the Cambrian in which and in the Silurian they are represented by a great diversity of forms. They die out in the Permian. Order 1. AMPHOBIDA. Forms without radial symmetry. Often with irregular arrangement of thecal plates ; usually with a stalk. Body often bilaterally compressed with two food-grooves (Trochocystis). The plates may possess canals, but their ends are not open. A variable number of arm-like processes may be present in some genera. Aristocystis Bar. (Fig. 214), upper Cambrian. Dendrocystis Bar. (Fig. 212), U. Cambrian. Order 2. RHOMBIFERA. Usually with a stalk. With radial symmetry of the ambulacral grooves and in some forms of the thecal plates. The ambulacral grooves usually extend on to processes of the theca (brachioles, arms) arising at the edge of the mouth (exothecal) (Fig. 216) or they may extend outwards for a certain distance on the theca, but in this case also they are exothecal as they do not lie on the thecal plates, but on special plates. With pore-rhombs. When the plates are large and regular they are arranged in cycles and the base is dicyclic, i.e. infrabasals, basals, and radials are present. In Caryocrinus and Hemicosmites there are 6 basals and 6 radials, in most others 5. E 'chinos phaera Wahl. (Fig. 216), arms unknown, U. Cambrian. Arachnocystis Neum., usually three arms, U. Cambrian. Cystoblastus Volb. (Fig. 213), U. Cambrian, may possibly have possessed pinnules arising from the edges of the ambulacral grooves, with 4 infrabasals, 5 basals and 5 radials. Pleurocystis Bill, U. Cambrian, with two arms. Callocystites Hall (Fig. 219), Silurian, with 4 pectinated rhombs, with ambulacral grooves on the calyx, some of which bifurcate. Echinoencrinus H. v. Mayer, 3 small arms, U. Cambrian. Lepadocrinus Conrad, Silurian. Caryo- crinus Say, arms 6-13 in number, Silurian (Fig. 218). Hemicosmites \. Buch, U. Cambrian. Order 3. DIPLOPORIDA. With radial symmetry ; with ambulacral grooves on the theca (epithecal, Fig. 221), provided with lateral pinnules (Fig. 215), and continued on to ter- minal brachioles (Fig. 220) ; with diplopores, with- out rhombs. Eucystis Ang. (Fig. 220), U. Cam- brian. Protocrinus Eich. (Fig. 217), U. Cambrian. Mesocystis Bather (Mesites Hoff.), U. Cambrian, with 5 c ambulacra with covering plates FIG. 220. Eucystis rari- punctata (after Ange- liu, from Delage and Herouard). An anus. 310 PHYLUM ECHINODERMATA. A s and probably with pinnules attached to the covering plates ; the pinnules distinguish them from Echinoids which they otherwise resemble (Fig. 221). The ambulacra! grooves extend almost to the aboral pole, and there are furrows passing between the covering plates which may have served for tube -feet ; they do not appear to have had a stalk but they may have been fixed by the aboral pole. Asteroblastus Eichw. (Fig. 215), U. Cambrian. Blastoidocrinus Billings, U. Cambrian. The two last named genera are placed by Bather with the Blastoids. Order 4. EDB.IOASTERTDA. With radial symmetry and, in some forms, pores between the ambulacra! plates as though for the passage of tube- feet ; the theca is composed of irregular plates, the madreporite is well marked and lies near the mouth in the same interradius as the anus (Fig. 222) ; no pinnules. There does not appear to have been a stalk, though in some cases they may have adhered by the aboral surface. They present some resemblances to Asteroids. U. Cambrian to Carboniferous. Cystaster Hall (Fig. 211), U. Cambrian. Edrioaster Bell., U. Cambrian (Fig. 222). Agelacrinus Vanuxem, U. Cambrian, Silurian, Devonian, Carboniferous. FlO. 221. Mesocystis Pusirefskii (re- stored after Hoffmann and Nitikin, from Bather). As anus ; M water- pores or perforations caused by a parasite ; mouth. amb - amb P amb FIG. 222. Edrioaster Bigsbyi (after Bather). 1 oral view. 2 section through a radius and interradius of the same specimen. 3 Section across an ambulacrum. ad flooring plates of the ambulacra! grooves ; amb covering plates of grooves ; As anus ; ia inter- ambulacral plates ; M madreporite ; p pores between the ambulacra! plates ; ps peri- stome with covering plates ; v.g ventral groove. Class BLASTOIDEA.* Pentamerous forms without arms, with radial ambulacra bearing pinnules, with a well-plated monocyclic calyx, with hydrospires, without pores. The Blastoids are entirely extinct, being confined to the Palaeozoic * Etheridge and Carpenter, Catalogue of the Blastoidea in the Geological Department of the British Museum, 1886. Bather, op. cit. ; Zittel, op. cit. BLASTOIDEA. 311 period. They make their appearance in the Silurian, and reach their richest development in the Carboniferous. As at present constituted the class is a well denned one.* They approach the Cystids through the Diploporida. The calyx is somewhat ovoid and either has a short stalk FIG. 223. Analysis of calyx of Pentremites florealis (from Zittel). Aboral view, the posterior deltoid is downwards in the figure, b basals ; r radials; ir deltoids (interradial). FIG. 224. Apical region of Eleutherocrinus Cassedayi (after Etheridge and Car- penter), a. y axis passing through the anal interradius and the opposite radius (III) ; II-V four of the radii. or is without a stalk. It consists of three rows of plates (Fig. 223) ; three basals, two of which are larger than the third as though composed of two pieces fused ; five radials (r) each of which is forked at its radial end ; and five interradial deltoids (ir), which surround the peristome. The spaces between the forks of the radials and between the deltoids are airta. FIG. 225. Codaster trilobatus (after Bather). 1 oral surface of young form x 6. 2 slightly restored section through a radius. In 1 the central parts of the deltoids are prominent, amb ambulacrum; As anus ; br pinnule (brachiole) ; c.p covering plates of ambulacral groove ; h hydrospire slits ; the hydrospires of the anal interradius are imperfect in this genus ; L lancet plate, containing canal ; mouth ; R radial ; s.p side plate ; A deltoid. occupied by the ambulacra (Fig. 225 amb). The ambulacra are petaloid and are traversed by a median ambulacral groove. The floor of this * Bather has established a subclass of Blastoids to which lie applies the name Protoblastoidea to include Asteroblastus and Blastoidocrinus (p. 310). These genera, however, are without hydrospires and possess diplopores. 312 PHYLUM ECHINODERMATA. groove is formed by a median plate called the lancet plate (Fig. 225, 2, L) and on each side by lateral plates (sp). The lancet-plate contains a canal (Fig. 225) which in the neighbourhood of the mouth opens into the cavity of the calyx. The ambulacral groove possesses cover- ing plates (c.p) which can only be discerned in a few specimens and which are continued over the mouth. Crossing, at right angles, the sutures between the radials and deltoids, are some deep slits (Fig. 225 h) wliich lead into pouches projecting into the calyx-cavity (Fig. 225, 2). These pouches are the hydrospires ; they are disposed parallel to the ambulacral grooves (Fig. 225). Pinnules or brachioles (br) are attached to the side plates and furrows pass from the ambulacral groove across the exposed part of the lancet plate on to them. When preserved, they are usually folded over the grooves. There is no evidence for the existence of tube-feet. The anus is in an interradius at the junction of a deltoid with two radials (As). It is not in the interradius of the small basal. The arrangement of the hydrospires described above is that found in Ccdaster , in other Blast oids it is slightly different. They are usually arranged more compactly and their openings become covered over by ex- tensions outwards of the lancet plate and the side plates of the ambulacral groove (Fig. 22t>). A canal the hydro- spire canal is thus formed. It opens to the exterior by a series of apertures between the side plates on each side of the lancet plate, called the hydros pire- pores, and by a larger opening near the mouth between the deltoid, the proximal side-plate and the lancet plate ; these openings are called the spiracles. They are ten in number, but frequently they become united in pairs, one joining with that of the adjacent radius, so The spiracles adjoining the anus are often FIG. 226. Section across a radius of Pentremites (after Bather), br pin- nule (brachiole) ; c.p covering plate ; o.s.p outer side-plate ; R radial; s.l sub-lancet plate (in many genera a sub-lancet plate is found beneath the lancet plate) ; s.p side plate. The hydrospires here open into a canal beneath the side plates and lancet plate called the hydrospire canal. that there are five only, confluent with it. In some genera (Astrocrinus and Eleutherocrinus) one of the ambulacra, is different from the rest. The meaning of the hydrospires is quite un- known. They have been compared to the genital bursae of Ophiurids and supposed to have been respiratory in function. They have also been compared with the canals of the pore-rhombs of Cystids. Their relation to the sutures between the deltoids and radials which is so clearly seen in Cadaster no doubt suggests this comparison, and if there is anything in the suggestion on p. 308 that the canals of the pore-rhombs are due to foldings of the stereom, there may be something in it. In the general form of their body the Blastoids present a certain resemblance to Echinoids, but they differ from these in the fact that they are usually stalked, in the presence of pinnules, and in the composition of the calyx. The principal genera are : Pentremites Say (Fig. 227), Devonian and Carboniferous. Mesoblastus E. and C., Carboniferous. Troostocrinus Shum., Silurian. Tricoelocrinus M. and \V., Carboniferous (Fig. 227). Nucleocrinus Conrad (Elaeacrinus Roomer), Devonian. Orbi- tremites Austin (Granatocrinus Hall), Carboniferous (Fig. 227). Heterc- BLASTOIDEA. 313 blastus E. and C., Carboniferous Limestone. Cadaster McCoy, Silurian to Carboniferous. Phaenoschisma E. and C., Devonian, Carboniferous. Orophocrinus v. Seebach, Carboniferous. Eleutherocrinus Shum. and Yand., Devonian. Astrocrinus Austin, Carboniferous Limestone. 1 FIG. 227. Thecas of typical Blastoids (after Bather). 1 Pentremites robustus. 2 Tricoelo- crinus woodmani. 3 and 4 Orbitremites (Granatocrinus) orbicularis from the side and from below. All nat. size. CHAPTER IV. PHYLUM ARTHROPODA* Segmented animals generally with a firm external skeleton, jointed appendages, and foot-jaws. The general disposition of the chief organs is the same as in Annelids, but the coelom, though present and discharging important functions, does not develop a perivisceral portion. The perivisceral cavity consists entirely of blood-sinuses. THE relationship of the Arthropoda to the Annelida has led to their classification in one group, the Annulosa of M'Leay. * The literature on the Arthropoda is referred to under the headings of the several groups. There are however two topics dealt with in this section, to the literature of which it may be convenient to the reader to have his attention drawn. On the segmentation of the Arthropod head and other cognate ques- tions see : K. Kishinouye, Note on the coelomic cavity of the spider, Journ. Coll. Sc. Tokyo, vol. vi. (1894), p. 287. E. S. Goodrich, The relation of the Arthropod Head to the Annelid Prostomium (Q.J.M.S., vol. 40, 1898, p. 247). E. R. Lankester, The structvire and classification of the Arthro- poda, Q.J.M.S., vol. 47 (1904), p. 523, reprinted from the Encyclopaedia Britannica, Tenth Ed., New Volume I.. 1902, Article Arthropoda. R. Heymons, Die Entwickehmgsgeschichte der Scolopender, Zoologica, Heft 13 (1901). Essays by G. H. Carpenter on the relationships between the classes of the Arthropoda, Proc. E. Irish Academy, vol. 24 B. (1902-4), p. 320 (in which a bibliography of much recent work bearing on the matter will be found) and Segmentation and Phylogeny of the Arthro- poda, with an account of the maxillae of Polyxenus, Q.J.M.S., vol. 49 (1905-6), p. 469. Sedgwick A., A monograph on the development of Peripatus capensis, Q.J.M.S., vols. 25-28 (1885-8). On the Eyes of Arthropods see : Lankester and Bourne, The minute structure of the lateral and central eyes of Scorpio and Limulus, Q J.M.S., 1883, p. 177. Watase, S., On the morphology of the compound eyes of Arthropods, Studies from the Biolo- gical Laboratories, Johns Hopkins University, iv (1889), p. 287. Kishi- nouye, K., On the development of Araneina, Journ. Coll. Sc. Imp. Univ. Tokyo, vol. 4, pt. 1, 1891. Grenadier, H., Unt. uber das Sehorgan d. Arthropoden, Gottingen, 1879. Exner, Sig., Die Physiologic der facettirten Augen von Krebsen u. Insecten, Leipzig u. Wien, 1891. Chun, Atlantis, Zoologica, Bd. vii. Heft, 19 (1896). Parker G. H., The retina and optic ganglia in Arthropods, Mitth. aus d. zool. Stat. zu Neapel, Bd. xii, 1897, p. 1. Szczawinska, \V., Contrib. a 1'etude d. yeux de quelques crustacees . . Arch. Biol, T. 10, p. 523. 314 COMPARISON WITH ANNELIDA. 315 It is displayed in the metameric segmentation of the body, the lateral extension of the segments into processes which sub- serve locomotion, the presence of a ventral nerve cord surrounding the oesophagus anteriorly and of a dorsal heart. We may therefore begin our review of the characters of the Arthropoda as a whole by drawing a comparison between them and the Annelida. In the Annelida the body is covered by a soft and chitinous covering. The mouth opens on the first segment of the body and a prestomial lobe projects in front of it on the dorsal side. Each segment contains, in the Polychaeta, a spacious coelomic cavity, that of the first extending forwards into the prestomium.* The supraoesophageal ganglion lies in the prestomial lobe, and the first member of the ventral ganglionic chain behind the mouth in the first segment (peristomial region). The pre- stomium bears a pair of tentacular appendages (sometimes with a median tentacle in addition), and the parapodia which project at the sides of the body, although differentiated for locomotory and tactile functions, are never jointed, and none of them are modified as jaws to assist the introduction of food into the mouth. The characteristic annelidan disposition of the nephridia and gonads has been fully treated in an earlier part of the present work and need not be here recapitulated. The cuticle in the Arthropoda is more rigid than that of the Annelida and is generally hardened by a deposit of salts of lime. It is secreted by a layer of cells, the epi- or hypo- dermis, and from time to time during the period of growth of the animal the hard outer layers of the cuticle are separated from the inner layers, and ruptured, the animal emerging from its cast " skin " (ecdysis). The soft inner layers then expand to accommodate the growing body. Except in some cases, and usually in the posterior region, the segments are produced laterally into limbs, and these, like the body from which they spring, are divided into segments by annular tracts of flexible cuticle intervening between the hard and allowing movement between the successive segments and of the whole limb upon the body. Movement is effected by a muscular system, which is contained * See, however, the remarks on this subject in the chapter on Annelida, vol. I, p. 448. 316 PHYLUM ARTHROPODA. within the firm external cuticle, and the bands of it pass from point to point across the articulations. The Arthropoda are thus markedly contrasted with the Chordata, in which the skeletal structures are internal and covered by the muscles.* There is no contractile dermomuscular body wall, except in Peripatus. On the vastly increased mechanical possibilities afforded by such an exoskeleton, with its plates of varying degrees of rigidity and delicately adjusted systems of levers the active many-sided life of the Arthropoda depends. Associated with the increased powers of locomotion which such a skeleton con- fers we find a high development of the sense organs and nervous system, and, in Insects, the elaborate social instincts which have ever excited the wonder of mankind. FIG. 228. Squilla mantis. A', A" first and second antennae ; B', B", B"' the three jiairs .of biramous appendages (6th-8th thoracic) ; Kf, Kj" first and second maxillipeds (after Claus). In no Arthropod is the body-cavity coelomic. It consists of blood-spaces, constituting a haemocoele. The development of members of the several groups however reveals the fact that a coelom exists, in a more or less modified form, in all. In Peripatus the somites are formed from the mesoblastic bands as paired structures, a pair to each segment at the sides of the elongated blastopore (p. 570). They are hollow from their first appearance and the cavity which each contains is the coelomic cavity. From them are formed (a) the nephridia, a pair to each trunk segment, with their end-sacs ; and (6) the tubular generative organs. The latter arise by the junction of the dorsal portions of the paired coelomic cavities of certain of the posterior segments, and their connexion with a posterior * The fibre-cartilaginous endosternum of Limulus and other Arachnids is however comparable, as regards its relation to muscles, with the chordate skeleton. COELOM. NEPHRIDIA. HEART. 317 pair of nephridia, which thus furnish the opening to the exterior. These ducts in the female retain a ciliated lining (Gaffron), the only known instance of the occurrence of a ciliated tract among the Arthropoda. The other groups of Arthropods present various stages of modification of this primitive arrangement. In the Arachnida segmented coelomic sacks (somites) are formed, and in their walls the gonads are developed. In Scorpio five pairs of seg- mental ducts appear in the embryo (in segments 3-6 and in the 8th *). Some of these disappear in the adult, but those of the fifth persist, though with loss of the external aperture, as the coxal glands, and those of the eighth as the ducts of the gonads (Brauer). In other Arachnids a single pair of nephridia (cor- responding to the fifth pair of appendages) also persists as the coxal glands. In Myriapoda and Insects coelomic sacks are also developed and from the walls of some of these the gonads are formed, but, the excretory function having in these groups been in most cases taken over by diverticula of the alimentary canal, no trace of nephridia has been found. In Crustacea two pairs of nephridia persist, though they are rarely found to coexist, as the antennal and shell glands, but the mesoblast presents in this group the extreme of differentiation from the annelidan arrangement, there being scarcely any traces of its segmentation or of general coelomic cavities. In all however the genital ducts are mesoblastic in origin and it is thus open to us to regard them as derived from a pair of nephridia, as analogy with Peripatus and Scorpio would suggest. Connected with the presence of a haemocoelic body cavity in place of the coelomic body-cavity of Annelids is another charac- teristic feature of the Arthropoda, the relation of the heart to the pericardial sinus. The heart in this group is a longitudinal dorsal vessel, per- forated by one or more pairs of lateral ostia. These admit blood from the pericardium, a special compartment of the system of haemocoelic spaces which is separated from the rest by a horizontal septum lying beneath it. * Counting, with Brauer, the cheliceral segment as the first. It is however counted as the second (cf. p. 323) in the table on p. 525. 318 PHYLUM ARTHROPODA. In Arthropods one or more of the anterior appendages is modified to form jaws (foot- jaws), and in association with this we find a concentration and fusion of some of the anterior seg- ments of the body by which they form a group distinct from the segments behind and constitute a head. From the study of development it is clear that, as pointed out by Lankester, there occurs in Arthropods a shifting of the position of the mouth backwards, in relation to the segments of the head ; so that it may come to lie in the adult on the ventral side of the segment which is apparently * the second, third or fourth. The first pair of appendages having the character of jaws lies either at the sides of or immediately behind the mouth, and hence in the several groups the jaws are the appendages of the apparent second, third or fourth segment, the appendages anterior to them taking on other and generally tactile functions or in some cases disappearing altogether. Bearing in mind the fact that in Peripatus the blastopore ex- tends, at one stage in development, as a fissure along the ventral median line between mouth and anus, it may without difficulty be conceived how such an alteration in the position of the mouth may have occurred. The segmentation of the arthropod head. It would be inter- esting, if it were possible, to determine the relation between the anterior segments of the body which in the several divisions of Arthropods form the head and also between them and the anterior segments of the Annelida. The post-cephalic segments are very variable in number in both groups. The body is divided up, here into a larger, there into a smaller number of segments, and the problem of the precise homology of a particular segment in one class with the numerically corresponding segment in another is probably without reality. With regard to the head however the case appears to be different. Within the divisions of the Arthropoda (Insecta, Crustacea, etc.) the number of head-segments shows a remarkable constancy ; the segmentation has become, as it were, numeric- ally stereotyped, and we may inquire how far the segments correspond in the several divisions, and whether a numeri- * The difficulty of determining which segment of an Arthropod corre- sponds with the first in the Annelida is pointed out below. SEGMENTATION OF HEAD 319 FIG. 229. Blastoderm of Scolopendra cingulata; A ventral aspect. B Longitudinal section through a blastoderm at a somewhat later stage, on one side of the middle line. A Preoral region, lateral region of the acron ; 1 pre-antenna ; 2 antenna ; 3 intercalated segment ; 4 mandible ; 5 first, and 6 second maxilla ; l'-6' the coelomic sacs corresponding to these appendages ; 7-29 post-cephalic segments ; 30 mouth ; 31 anus ; 32 telson ; y yolk ; yn yolk nuclei (after Heymons). 320 PHYLUM ARTHROPODA. cal correspondence can be traced between Arthropoda and Annelida. In the development of the head, as of the rest of the body, evidence of the existence of a segment is afforded by the forma- tion in development (1) of a pair of mesoblastic somites, which may retain their coelomic cavities, (2) of a pair of ganglia (forming a neuromere) corresponding in position with the somite, and (3) of a pair of appendages. Among the higher Arthropoda whose development is known to us, the Chilopod Scolopendra appears to present the simplest and most primitive condition of the segments of the body. It has been investigated by Heymons, and Figs. 229.4 and B are taken from his monograph. In front of the antennal segment and marked, like it and all the segments behind it, by a pair of coelomic sacks as well as by a neuromere, is a preantennal segment from which for a short time small tentacular preantennal appen- dages (1) project. Between antennae (2) and mandibles (4) a segment is formed, marked by neuromere and coelomic sacks but without appendages the premandibular or as it has been called intercalated * segment (3). Then come the segments of the mandibles and of the two maxillae of Scolopendra. The twenty- three post-cephalic segments follow, the appendages of the first (7) being transformed into the poison claws. The preantennae are formed at the sides of or a little behind the mouth, though the latter subsequently moves back and lies between the mandibles. In front of the mouth is formed (at a later stage than that figured) the labrum, projecting back- wards from the clypeus ; behind it the bilobed hypopharynx subsequently appears. An unpaired median thickening of epiblast is formed from the clypeal region. It is called by Heymons the archicerebrum. Two paired pitted thickenings of the epiblast on either side of it are the medial and lateral brain rudiments of Heymons. From these five centres, one median and two paired, the syncere- brum of Heymons is formed. * If the name " intercalar-segment " were to be taken literally, as mean- ing that a new segment existed in a given individual between, two segments, which were adjacent in its parents, the designation of the segments by number would be at once recognized as inapplicable. We have however no very satisfactory grounds for asserting or denying the possibility of such an intercalation. SEGMENTATION OF HEAD. 321 The two paired thickenings are so disposed that they continue on either side the line of the paired ganglia of the neuromeres belonging to the postoral part of the body, so that the arrangement raises the question whether we have not in these medial and lateral paired rudiments the representatives of segments anterior to the preantennal. This latter however is the first in relation with which coelomic sacs and appendages are formed, and we may, provisionally at least, reckon it, with Heymons, the first metamere. With the complex syncerebrum the neuromere of the pre- antennal segment (the protocerebrum s. str. of Heymons *) becomes fused to form the dorsal lobes of the adult brain, the procerebrum of Heymons, from which the optic nerves arise, while the neuromere of the antennae forms the anterior paired lobes giving off the antennary nerves the mesocerebrum or deuto- cerebrum. The neuromere of the premandibular ganglion forms the posterior and ventral pair of lobes the metacerebrum or tritocerebrum, from which the commissures pass to the sub- oesophageal ganglion. The suboesophageal ganglion is formed by the union of three neuromeres, namely those of the mandi- bular and two maxillary segments of Scolopendra. In the lower Insects (Apterygota, Orthoptera) the segments of the head are laid down in a very similar manner, but no trace has been found of the coelomic sacks or the appendages of the preantennary segment. The antennal segment is here the first to bear an appendage and to contain distinct meso- blastic sacks. The premandibular segment is also well developed, and in Forficula the nuclei of the mesoblastic somite belonging to it are arranged in two layers, though an actual cavity has not been recognized. The only cases in. which appendages are known to be borne by this segment are those of the primitive Thysanuran genus Campodea, in which Hansen has found a pair of small tubercles representing them in the adult ; and the Collembolan Anurida, in which Wheeler and others have seen them for a short embryonic period. The supraoesophageal ganglion of Insects is likewise developed from three paired masses (Wheeler, Viallanes), the procerebrum, mesocerebrum and metacerebrum. Of these the two latter arise ' Heymons uses the word protocerebrum, sensu stricto, for the neuromere of the preantennary segment. The mass consisting of this neuromere combined with the syncerebrum he calls procerebrum, but also " protocerebrum, sensu lato." It will be called procerebrum in this work. z rn. Y 322 PHYLUM ARTHROPOD A. from the neuromeres of the antennal and premandibular seg- ments respectively, the procerebrum representing the proto- cerebrum together with the complex syncerebrum of Scolopendra (Heymons). With regard to the postoral segments of the head of Insects the evidence is somewhat conflicting. In the development of the Orthoptera investigated by him, Heymons finds three consecutive segments, each with a well developed neuromere and pair of coelomic sacs, which he identifies as belonging to the mandibles and two pairs of maxillae, and which would thus correspond with the appendages so named in Scolopendra. On the other hand several observers (Hansen, Folsom, Carpenter and others) have shown that paired structures are present in the mouth of the lower insects, between mandibles and first maxillae, sometimes uniting with the median ligula or hypopharynx but sometimes distinct from it, and presenting (especially in the Thysanura) all the characters of appendages. Thus in Machilis maritima Carpenter shows that each ends in two lobes, comparable with the galea and lacinia of the succeeding segment, and bears exter- nally a palp (unsegmented in Machilis, Fig. 372, but 3-segmented in Japyx). These appendages are named by Hansen maxillulae (superlinguae by Folsom). It seems impossible to resist the evidence that they represent a head somite, and Folsom does in- deed find, in a later stage of development of the Collembolan Anurida, a neuromere corresponding to them. The only diffi- culty in the acceptance of this view is that Folsom himself finds in an earlier stage of development of Anurida no trace of this segment and figures the mandibular and first maxillar segment in juxtaposition. Moreover it is remarkable that this segment should be apparently unrepresented in the develop- ment of the Orthoptera so carefully investigated by Heymons. Nevertheless the balance seems to incline in the direction of the true segmental nature of the maxillulae. Accepting this view we conclude that the maxillulae, first maxillae and labium of insects correspond respectively with the two pairs of maxillae and the poison claws of Scolopendra. The fact that the labial (2nd maxillary) segment of Insects is less completely fused with the segments in front of it than they are with one another, affords some confirmation of this view. In the Crustacea the evidence for segmentation afforded SEGMENTATION OF HEAD. 323 by the coelomic sacks in other groups fails us, for the meso- blastic somites though segmentally divided contain no coelomic cavities. Among the Malacostraca the supraoesophageal ganglionic mass is laid down in Crangon (according to Kingsley) as three consecutive pairs of ganglia, in addition to the optic ganglia, which lie in front of and external to the first pair. Of these three pairs the anterior is preoral, the others originally postoral. The second and third pairs of ganglia are those of the first and second antennae respectively. The development of the corresponding parts in Astacus appears to present no essential difference. From these facts, and from the similarity of the shapes of the mandibles and the positions of the paired eyes in Crustacea and Insects, we are led to regard the segments bearing the two pairs of antennae of Crustacea as homologous with the antennal and premandibular segments of insects, the appendages of the latter attaining in the Crustacea a full development which is denied them in Insecta. The mandibular and two maxillary segments of the Crustacea correspond with the mandibular, maxillular and first maxillary segments of Insects, the first thoracic appendages finding their homologue in the labium. In the lower Crustacea (e.g. Daphnia, Fig. 250), the ganglion of the second antennary (premandibular) segment retains a postoral position throughout life. Turning now to the Arachnida (including the Merostomata) we find that the chelicerae and the six segments of the cephalo- thorax posterior to them (the segmental significance of the chilaria has been demonstrated by the researches of Kishinouye and Brauer) are represented in development by coelomic sacks, neuromeres and appendages. In front of the cheliceral neuro- mere is the large paired ganglionic mass of the head lobes. No distinct mesoblastic somite anterior to the cheliceran is found in Limulus or Scorpio, but among the true spiders evi- dence of such sacks has been obtained by Kishinouye and others. Indications of paired appendages in front of the chelicerae have been recognized by some observers in the paired origin of the " labrum " in scorpions and spiders, and in prominences formed in relation with the folding in of the brain of spiders, but in neither case is the evidence conclusive. In most Arachnids the cheliceral neuromere fuses with the 324 PHYLUM ARTHROPOD A. paired mass in front of it to form the supraoesophageal ganglion, the pedipalpi being innervated from the suboesophageal mass.* The absence of resemblance between arachnid appendages and those of the other groups of Arthropods renders recogni- tion of the homology of the segments exceedingly dubious. We are reduced to the order of the segments, reckoning as 'first the pre-cheliceral in Arachnida and the preantennal in Trachea ta and Crustacea ; though when it is remembered how slender the evidence is of the existence of these segments, the insecurity of the proceeding is apparent. It remains to consider the segmentation of Peripatus which is perhaps better marked than that of any other Arthropod. As shown by Sedgwick the coelomic sacks are formed in pairs, and the members of the anterior pair, though postoral at their origin move forward and come to lie in apposition in front of the mouth, forming the first segment. The antennae of the adult are the appendages of the first, the mandibles of the second, the oral papillae of the third segment, and in relation with each of these appendages, as with their successors, a pair of nephridia, opening into the corresponding coelomic sacks is formed. At no stage of development is the central nervous system of Peripatus divided up into distinct ganglia, the lateral thickenings of the ectoderm on either side of the ventral median line " are from their origin continuous from somite to somite," and they are continuous with one another in front of the mouth. The brain of the adult consists of two simple swellings produced behind into posterior lobes. They are in apposition with one another, and there are slight swellings of the lateral cords op- posite each of the postoral appendages. The antennary and optic nerves spring from the anterior part of the brain, the nerves to the jaws from the ventral cords as they leave the brain, those to the oral papillae from the ventral cords posterior to the oesophagus. The relation between the segments of the head (in the case of the Arachnida of the cephalothorax) in the main groups of the Arthropoda, which the evidence at our disposal appears to indicate as probable, are set forth in the following table, though * It is stated however that in the Phalangidae and the Acarina (Gama- sidae) the innervation of the chelicerae is suboesophageal. SEGMENTS OF HEAD. 325 -p c CD ~ Arachnida (Scorpio) Protocephalo CD O2 02 S & tl Q^ O 02 5 g 8 _,-, O O -p 05 o 'eg CDmCDmCD'^'CD^CD^H o 45 42 I g IS a 3 o S aj*5 cs 02 O2 CO CO CC 02 I < O2 O2 O2 J -2 02 -r 02 - 8 o "o S CD JD cc g -^ CO a CD 5 S C ' ^< C ^3 ^ "^ o s p. S s. no bC (C 02 "3 ^^ lala Isle I < PH ^ 02 Sj *~W ^ ,J^ -^ -C CM 326 PHYLUM ARTHROPOD A. the numbers in the first column must, in our view, be regarded as provisional. It will be seen that in Peripatus we have evidence of three head segments, in the Crustacea and Myriapoda (though the case of the Diplopoda is doubtful) of six. In the Insecta a seventh segment, that of the labium, appears to have been added, while in Arachnida the anterior part of the body, here called the cephalothorax, is apparently composed of eight segments. The term acron occurring in the first and second columns of the table was introduced bv Janet for the region of the embryo */ *. surrounding the mouth (as the telson surrounds the anus). It is used here, following Heymons, in a more restricted sense for the region in front of the mouth, containing as its median element the clypeal shield, from which the labrum pro- jects backwards over the mouth. From the epiblast of the acron the syncerebrum of Scolopendra is developed. Protocephalon is the name given by Heymons to the region occupied by the procerebrum. The acron is regarded by Heymons as the homologue of the prestomial lobe of Annelids. This conclusion leaves out of consideration the case of Peripatus, which from the distinctness of the coelomic sacs and the persist- ence of nephridia cannot be disregarded in discussing the primi- tive segmentation of Arthropods. It is difficult to believe, notwithstanding the fact that the antennary somites of Peripatus are postoral in origin, that mesoblastic structures have ever existed in front of them, and if this is so these somites represent the most anterior segment of the primitive Arthropods, whether differentiated into prestomial lobe and peristomium or not so differentiated. However, as stated in an earlier part of the present work (Vol. I, p. 448), the relation of the prestomium to the segments of the body is far from clear in the Annelids them- selves, and we may here confine ourselves to pointing out that the evidence remains conflicting when considered from the point of view of Arthropods. We have to conclude then that the homology of the seg- ments in Myriapods, Insects and Crustacea, though not free from difficulty is fairly clear ; that the homology is obscure in the case of Arachnids, and still more so in that of Peripatus. The problem of the relationship between the anterior segments COMPARISON WITH HEAD OF ANNELIDS. 327 in Annelids and any group of Arthropods remains, for the present, undetermined. On another debated point the embryological evidence obtained in recent years also throws light. The anterior antennae of the lower Crustacea, and the antennae of Peripatus, Myriapoda and Insecta differ markedly from the appendages of other segments in their simple, many- jointed and generally uniramous character, and these characters in the adults are emphasized by the con- stantly uniramous condition of the first antennae in the Nauplius larva of Crustacea. The question has arisen Have we not in these appendages the homologues of the paired tentacles of the prestomium of Annelids ? In the case of Peripatus it is possible that we have, but in the other groups the answer of embryology is in the negative. In FIG. 230. Peripatus capensis (after Sedgwick). each they are innervated from the mesocerebrum, and there is evidence in Myriapoda that there has existed at least one seg- ment anterior to that which bears them. Notwithstanding the peculiar character of the first antennae we have to conclude that they, like the other appendages of the head, were origin- ally postoral limbs. The facts that in Peripatus the antennae are the appendages of the first somite, and therefore, presumably, the representa- tives of the transitory preantennae of the Myriapoda, and that the mandibles belong to the second segment, would seem to place this remarkable genus in a category apart from the other groups of the Arthropoda. As the mouth moves backwards the segments originally postoral become preoral, and their ganglia, fusing together, form the brain or supraoesophageal gangliom'c mass of the adult. 328 PHYLUM ARTHROPODA. The brain is thus in all cases a composite structure containing the neuromeres of the preoral segments fused together. The mode of its development in Scolopendra would indicate that there is anteriorly, in addition, a median unpaired element (the archicerebrum of Heymons) together with lateral elements (forming, with the archicerebrum, the syncerebrum of Heymons) with which the neuromeres have united, though how this con- dition is related to that presented by Peripatus remains for the present obscure. The number of neuromeres entering into the brain appears to be two in Peripatus, most Arachnids, and in Daphnia (Fig. 250) and Limnadia (Fig. 241 D) among the Crustacea, three in the Malacostraca, Myriapoda and Insecta. For the further discussion of the segmentation of the arthropod head and other cognate questions the reader is i-eferred to the literature on this subject quoted at the beginning of the chapter. The conclusions here arrived at are to a large extent in accordance with those of Heymons, Goodrich and Lankester, but the designation of the segments by a numerical nomenclature, adopted by Lankester, has been as far as possible avoided, because of the uncertainty of deciding in the several groups, which is the first segment. The Eyes of Arthropods. The eyes of Peripatus are vesicular structures which arise by invagination of a portion of the brain rudiment (while it is still part of the skin) with which they retain their connexion by the optic nerve. In this they appear to resemble the simple eyes of Arachnida. Little has been ascertained throwing light on the origin of the unpaired " nauplius eyes" of the lower Crustacea. Apart from Peripatus the simplest form in which the paired eyes are met with in Arthropods is that presented by the stemma or ocellus of Insects (Fig. 231). The chitinous cuticle is thick- ened to form a lens, and beneath this the cells of the hypo- dermis are disposed in a cup-shaped manner, the adjacent cells being highly pigmented. The cells in the floor of the cup form the retina, and are in connexion with the nerve ; while the transparent ends of the surrounding cells, bending over the retina from the sides, form a clear medium, which has been called the vitreous body, intervening between the retina and the lens. It appears evident from the arrangement of the parts of such a simple eye that the light entering through the lens is focussed on the layer of retinal cells, and hence that some representation EYES. 329 St Rz of the outer world may be conveyed to the brain. From the small number of the cells in the recipient layer, however, it would appear that the perception of surrounding objects thus conveyed must be of a low order. The eyes of Myriapods and the Thysanura consist of a number of such ocelli, col- lected into groups. Although we have to conclude that the Insecta and Crustacea be- long to two distinct divisions of the phylum, the highly developed c o m - pound eyes of each agree closely in the plan on which they are formed. They may be derived from a group of ocelli, the number of which is greatly increased, while the number of cells in each is decreased (Grenadier). Each ocellus, or single eye, which thus with its fellows makes up the com- pound eye, is known as an ommatidium (Fig. 232). They are usually disposed so as to present to the exterior an even convex surface. The minute structure of the eyes of Branchipus and Polnemon may be taken as typical of the compound eyes of Arthropods (Fig. 232). Each ommatidium is limited externally by a corre- sponding portion of the transparent cuticle which may be thick- ened in the centre to form a biconvex lens (d in A), as in the facetted eyes of Insects, Decapods, Isopods, etc., or maintain a uniform thickness, as in Branchipus (c in E). Under it lie the two corneagen or lentigen cells by which it is secreted. Beneath are four (2 in the Isopod Sphaeroma] cells, the vitrellae or crystal cells (kz) grouped about the crystalline cone, the product of their secretion. In Branchipus its shape is fusiform, but it is often conical, with the point directed inwards. In Palaemon (Fig. 232, A k and kj) it is represented by two distinct refractive ia. 231. Section through the ocellus of a Dytiscus larva (after Grenadier, from Claus). CL corneal lens ; Gk the subjacent hypodermis cells forming the " vitreous body " ; P pigment in the peripheral cells of the latter ; Rz retinal cells ; St cuticular rods of the latter. 330 PHYLUM ARTHKOPODA. bodies, an outer and inner. About the vitrellae 2 (or 4) pigmented cells (cf. Fig. 233) are disposed. They are the iris pigment cells (the distal retinular cells of Parker). The vitrellae abut internally against a group of elongated cells FIG. 232. The compound crustacean eye. A two onimatidia of Palaemon squilla. The pigment is removed from the right-hand ommatidium. C Isolated crystalline liody of an ommatidium. consisting of four elements. D Transverse section through a retinula about the middle of its length. B Section through the stalked eye of Bnun-hiinis ; E two omma- tidia of the same on a larger scale. bm basal membrane ; c cornea ; cl corneal lens ; go optic ganglion ; h (in E) len- tigen cells ; hy unmodified hypodermis ; k crystalline cone ; k, outer crystalline body ; kz vitrellae ; m muscle ; nf nerve fibres ; no optic nerve ; p (in E) pigment in retinula cells, (in A) pigmented hypndermis cells ; ;>, (in A) mesoblastic pigment strands beween the omma- tidia ; re retinular cells, deprived <>t piumi-nt in D ; rg retinal ganglion ; rh rhabdom. (From Lang's Textbook, A C and D after Grenadier ; B and E after Claus.) forming the retinula (re) which constitutes the innermost element of the ommatidium. The retinular cells in varying number (4 in My sis, 5 in Branchipus, 7 in Palaemon) are arranged about the optical axis of the ommatidium, and, like the vitrellar and COMPOUND EYE. 331 lentigen cells of the external layers, are in relation with a re- fracting element, the rhabdomere, which forms their axial border. They are also abundantly invested by black pigment. The several rhabdomeres of a retinula make up the rhabdom (Fig. 232, A andD rh). Internally the retinular cells are continuous through apertures in a basement membrane (bm) with the nerve fibres of the optic ganglion. The crystalline cone (C) like the rhabdom bears evidence of being divided longitudinally. In some cases (Sphaeroma) the rhabdom contains a centi^al space, occupied by one or two hyaline cells (Watase). Moreover the rhabdomere has been found in some cases (Schizopoda, Deca- poda) to present very fine striations perpendicular to its margin (Fig. 232 A rh) which are regarded by some authors as the ultimate ends of nerve fibrillae. The distribution of the pigment in both sets of cells is, as we shall see, dependent in some animals on the degree of illumination to which the eye is sub- jected. Beside the cells forming the ommatidia proper other hypoder- mic cells in varying number are present in the interspaces be- tween them (Figs. 232 A p, and 233, 6). These (accessory pig- ment cells) may contain flakes of pigment which is not black but white by reflected and yellow by transmitted light. The function of this pigment appears to be not to absorb, but to reflect the light. It forms the tapetum of crustacean eyes. In the eyes of Insecta, adapted for vision by night (see below), tufts of minute tracheae invest the retinulae, and by their shining surfaces likewise serve as a tapetum (Exner). According to Lord Avebury some 4,000 ommatidia compose the eye of the common house-fly, while that of a Dragon-fly (Aeschna) contains 20,000. The mode of working of the compound eye was explained in part by the theory of mosaic vision first propounded by Johannes Miiller. The only direction from which the light is able to penetrate to the retinula- of an ommatidium, ensheathed in pig- ment in the manner above described, is that in and near the line of its axis. The light which thus enters, concentrated on the pigmeiited retinular cells by the refracting media of the ommatidium and reflected from the surrounding tapetum, will set up changes in them, according to its nature, and give rise to a corresponding stimulus in the nerve fibres with which they are 332 PHYLUM ARTHROPOD A. connected. Thus the retinula of one ommatidium receives a single resultant impression from the light which reaches it. But the adjacent ommatidia being directed to a different, though adjoining, region of the outer world, may transmit a different impression, and the stimuli from all the ommatidia which make up a compound eye will correspond in greater or less degree to the whole of the visible outer world which subtends their several optic axes. The sum of the resulting images which we may thus suppose to be transmitted to the brain may be compared to a mosaic in which the effect is given by a large number of separate pieces, of one size and each of uniform colour. It is evident on the one hand that the smaller the angle of each ommatidium and the larger the number of omma- tidia in an eye, the more perfectly will the resulting stimulus correspond with the details of surrounding objects. On the other hand the loss of light by absorption in the pigment of such an eye is very great and increases for each unit of surface with the number of ommatidia it contains. Our knowledge of the functions of the compound eyes of Arthropods has been extended by the work of Exner, Szczawinska, Chun, Parker and others. It has been shown that in a variety of Arthropods inhabiting shallow water, or the land, the pigment contained in the iris pigment cells and the retinulae occupies very different positions in accordance with the degree of illumina- tion (Fig. 233). In bright light the pigment invests the ommatidia in the manner described above, and though a FIG. 233. Longitudinal sections of two omm-itidia of Astacus flnvi'ttilis showing the ar- rangement, of the pigment as influenced by light (A) and by darkness (B) (after Parker) ; 1 cornea ; 2 nucleus of corneagen cell ; 3 nucleus of vitrella ; 4 nucleus of iris pigment cell ; 5 crystalline cone ; 6 nucleus of cell (if tupetum ; 7 rhalidoni ; 8 nucleus of retinal cell ; 9 basement membrane ; 10 reti- nal nerve fibre. FORMATION OF IMAGE. 333 great deal of light is absorbed by the pigment some reaches the retinulae, and forms the erect mosaic or " apposition image " in the manner indicated. But when the eyes of these animals are fixed by reagents after being exposed to darkness, it is found that the pigment blinds which separate the ommatidia from one another are withdrawn. The pigment in the iris pigment cells is drawn up towards the cornea, that in the retinulae has retreated below the basement membrane towards the nerve fibres.* It has been shown that under these conditions the ommatidia no longer act separately, but that a combined image is thrown on the retinular layer, the crystalline cones being so disposed that the light from a given point falling on a consider- able area of the eye, no longer obstructed in its course by the blinds of pigment, is brought to a focus on that layer. In this manner an erect " superposition image " is formed, the rays refracted by a large number of crystalline cones being super- posed at the focus on the retina, and a stimulus far stronger in proportion to the intensity of the illumination than that of the apposition image, though probably much less distinct in details, is given to the retinulae. The eyes of insects such as fireflies and many moths are permanently in this condition, and are " day blind." On the other hand the eyes of butterflies have the pigment permanently expanded, and are "night blind." A very interesting confirmation of these results has been fur- nished by the beautiful researches of Chun on the pelagic Schizo- pods inhabiting the dark waters of the ocean at a depth of 300- 600 fathoms (Fig. 290). In these the retinular pigment has dis- appeared altogether, while the distribution of that of the iris pigment cells varies in different parts of the eye, according as the ommatidia are directed sideways (lateral eye) towards objects which may be illuminated by the phosphorescent organs carried by the animal itself, or forwards (frontal eye) into the dark region from which the rays of these organs are by their position excluded. In the frontal eye moreover the number of retinulae is far in excess of that of the crystalline cones, a con- dition which is in harmony with the theory of the formation of a superposition image, but unmet by the mosaic theory. * As in the pigment cells of the frog, the chromatophores of Deca- pods, etc., the movement of the pigment takes place within the cell, and the shape of the latter is unaffected by the movement. 334 PHYLUM ARTHROPOD A. The lateral eyes of Limulus offer an interesting intermediate condition between the compound eye and a group of simple eyes. They are raised reniform areas on the sides of the cephalo- thorax, the cuticle over them being transparent. They consist of a number of pits in the hypodermis (Fig. 234) the bottoms of which are occupied by bulb-like retinulae consisting of some 10-15 cells grouped about a central ganglion cell. The inner margin of each retinular cell is highly refracting. The group of these refracting bands in a retinula evidently corresponds t-t FIG. 234. Three ommatidia of the lateral eye of Limulus (after Watase). lu^la retinula is divided longitudinally ; in B and C whole retinulae are represented, c central ganglion cell ; ch cornea ; hyp hypodermis ; I lens ; met mesoderm ; n nerve ; rh segment < f the rhabdom ; rt retinula. with the rhabdom of an ommatidium of the compound eye. The cells are continued below the basement membrane, as is the central ganglion cell, into nerve fibres. The columnar cells of the hypodermis surrounding the retinulae and forming the walls of the pit are bordered with pigment. The cavity of the pit is filled by a rounded process of the under surface of the cuticle, which apparently acts as a lens, but each is continuous above with the common investment of cuticle forming the cornea. The lateral eyes of Scorpions (Fig. 235) form a group on either EYES OF SCORPIONS. 335 side of the cephalothorax consisting of 2-7 distinct eyes, the number varying according to the species. The whole group however passes through a stage in development which strikingly resembles the permanent condition in Limulus (Fig. 234). The hypodermis is thickened and pigmented over a considerable area, and the surface becomes invaginated to form pits corre- sponding in number to the separate eyes of the adult. The cells lining the floors of the pits be- come differenti- ated into inter- neural cells (in) and retinal cells (sz), which are continued below into nerve fibres, and form rhab- doms at their adjacent borders, though they do not become grouped together to form retinu- lae. Pigmented perineural cells line the sides of the cup and a convex cuticular lens (I) is secreted above it. As 11US. .1. I r> o n f FIG - 235. Sections through stages in the development. proc e e d s the the lateral eyes of Scorpio in two . A earlier, B a single element at a later stage ; somewha't diagrammatic (from Korschelt and Heider, after Paiker and Laurie); II-V optic invaginations; h \iy\>- li \mnrl f^rmie derails ; in interneural cells; / lens; mes mesodermal tissue; 1 vl J ^ n optic nerve; pn perineural cells; r retina ; rh , rliabilmn ; sz tervening be- retinal cells - tween the pits assumes the ordinary character. According to Kishinouye the posterior median (Fig. 237, B) and the lateral eyes of Spiders likewise arise as simple ectodermal depressions, in which case they would belong to the same category as the lateral eyes of Limulus and Scorpions. The paired arthropod eyes hitherto considered belong to a 336 PHYLUM ARTHROPODA. type to which the name monostichous * has been applied (Lan- kester). They are all to be regarded as developments of a single layer of the hypodermis. The median eyes of Limulus and Scorpio (Fig. 237), and the an- terior median or " principal " eyes of Spiders (Fig. 237, -4) belong to a different type, the diplostichous . In these eyes two layers of hypodermis are concerned in the formation of the optical appa- ratus (Fig. 236). They are formed in ontogeny by the folding in of the hypodermis from the side of the area which will be occupied by the eye. The outer of the two layers thus involuted becomes a. c. FIG. 236. Section through the median eye of Scorpio in three stages of development. From Korschelt and Heider. A after Parker, B and C diagrammatic, g ? brain (?) ; gl (g in C) vitreous body ; h hypodermis ; / lens ; n optic nerve ; prpostretinal layer ; r retina ; rh rhabdom. the retina (r, Fig. 236), its columnar cells secreting rhabdoms (rh), and the inner, when it persists, the post-retinal membrane. The layer of hypodermis external to the fold becomes the vitreous body and secretes the lens. The nerve, by secondary shifting of its position, enters the under surface of the retina. There are thus three layers of hypodermis concerned in the development of the diplostichous eye, though the dioptric layers are formed from only two of them. The involutions from which these eyes are formed are closely associated with those forming the brain, but the morphological significance of this mode of development is quite obscure. , a row, line. SENSE ORGANS. 337 The optical arrangement of the lateral and median eyes of Scorpions and Spiders and of the median eyes of Limulus is such as to throw a reversed image on the retina. The development of the lateral eyes of the Scorpion supports the view of Lankester and Bourne that they are derived from an eye very like that of Limulus, the several elements of which have become separate. It is easy to imagine, on purely theo- retical grounds, that the compound eyes of Insects and the Crustacea might be derived from an eye such as the lateral eye of Limulus by the opposite process, namely by the specializa- tion of the several depressions into ommatidia, their increase PIG. 237. .1 anterior, B posterior median eyes of a spider (diagrammatic from Korschelt and Heider, after Grenadier and Bertkau). ch chitinous cuticle, passing into cuticular lens (/); the first and seventh ambulatory pendage IS pai'Oral, leps (thoracic legs 2-8) ; A'/ maxilliped ; S/' first abdominal a condition appar- ently permanent in the Trilobites, in which group they formed the first pair of jaw^s. The antennal excretory gland opens when present on the coxopodite, or when there are three seg- ments of the protopodite, on the second of them. The mandible is characterized by the development of the coxopodal endite into a cutting and masticatory blade, the remainder of the appendage constituting the mandibular palp. This usually consists of the basipodite and a few-jointed endo- podite, but in some Copepods (Fig. '260) and Ostracods (Fig. 253) an exopodite is found in addition. The palp is in some cases absent altogether. The two pairs of maxillae (Fig. 240 c and d) are generally short and jaw-like, consisting mainly of the protopodite and a short endopodite, the segments of which are produced into cutting blades (endites) ; a simple lobed exopodite is however frequently present. Hansen concludes that the two inner blades of the first maxilla belong to the 1st and 3rd segments of the protopodite. The maxillary excretory gland (the " shell gland " of many Entomostraca) opens on the 2nd maxilla. Often associated with the formation of a cephalothorax, modi- fication of one or more of the anterior thoracic limbs has 346 CRUSTACEA. occurred in many groups, resulting in their loss, in varying degrees, of a locomotory function, and their adaptation to subserve the transference of food to the mouth. The thoracic limbs thus modified are known as maxillipeds. The appendages of the thorax vary greatly in number and shape. In the Polycopidae (Ostracoda) they are absent alto- gether, locomotion being effected entirely by the limbs of the head ; in other Ostracoda there are two pairs, in the Cladocera 4-6, and in the Phyllopoda 11-19 pairs. In the Malacostraca the number is almost constantly (except in obviously degraded forms) eight. In the ^^-MPffiP 5 ^ R Apodidae there are 40-63 pairs of post- cephalic appendages, presenting a gradual transition in form and size ; but the presence of the open- ing of the genital duct on the segment bear- ing the 11 th permits us to regard the eleven anterior as thoracic. The modifications which befall the epi- podites of the thoracic limbs present an in- teresting series. They are generally branchial in character and may be simple lobed plates (Phyllopoda, Anas - pides(Fig. 238), Am- phipoda) or variously subdivided (Schizopoda, Decapoda) ; but in certain groups of Malacostraca (" Peracarida," cf. p. 4=55) the epipodites of certain of the legs appear to have become modified in the female as oostegites, to form, together with their fellows, a brood pouch beneath the thorax, in which the young are protected. Except in some of the Branchiopoda (as A pus, to which FIG. 240. Larva of Lobster (after G. (). *arsi. a the larva seen from the side ; A' ' , A" first and second antennae; F' the chelate first ambulatory legs (4th thoracic). Like the other thoracic limbs they carry exopodites in the larva. K f" third maxilliped ; b mandible, with palp : c anterior or first maxilla : il posterior or second maxilla (with scaphoguathite) ; e and / first and second maxillipeds. NERVOUS SYSTEM. EYES. 347 allusion lias just been made) abdominal legs are not found in the Entomostraca, but in the Malacostraca they are nearly always present as biramous swimming limbs (pleopods), the posterior pair (uropods) often forming with the telson the powerful caudal fin. In Isopods and Stomatopods the respira- tory function is carried on by the abdominal appendages. The central nervous system. In the Phyllopoda (Fig. 241 D) the ganglia of the ventral chain are wide apart and each is con- nected with its fellow by double transverse commissures. There is a pair to each pair of postoral appendages. Many of the lower Malacostraca present little advance on this arrangement, but various degrees of concentration are presented by the several groups. The composition of the supraoesophageal ganglionic mass differs, as pointed out above (p. 323), in the Malacostraca and in some members at least of the Entomostraca. In the former there is evidence that three neuromeres join with the optic ganglia to form the brain. The middle and the posterior neuromeres give off nerves to the first and second antennae respectively. In the Branchiopod genera Daphnia and Limnadia the neuro- mere of the second antennary segment retains its post-oesopha- geal position (Figs. 241 D and 250) and forms the first ganglion of the ventral chain. Hence we have in recent Crustacea two stages of the process by which the compound brain of Arthropods is formed ; one (represented by Daphnia and Limnadia) in which the second antennary neuromere is post-oesophageal, the other, the malacostracan stage, in which it has become pre-oral, and merged in the mass of the brain, although in many Malacostraca the paired ganglia forming it are apparently connected by a transverse commissure (Fig. 241 y] passing behind the oesophagus. Frontal sense organs. Attention may here be drawn to small paired sense organs, found in many Entomostraca on the front of the head. In the larva of Apus they consist of papillae with dilated bases. The unpaired or median eye often called the nauplius eye is a characteristic feature of the Crustacea, occurring alone or in association with the compound eyes in all the groups of the Entomostraca, and in the larval stages of the Schizopoda, Decapoda and Stomatopoda among the Malacostraca. In some Copepods it consists of 3 groups of cells, a median ventral and FIG. 241. Central Nervous Systems of various Crus- tacea. A Euphcmsia pel- I nc'al i (after G. O. Sars). B Axt'irux ft it ri 'it His (after Vogt and Yung). C Ap- L'ltrcillii (combined from several figures hy Claus). D Limnadia (after Klunzinger), anterior por- tion. F Maid squinado (after Milne Edwards). a' nerve of first antenna ; 2 of second ; ayfj (in D) ganglion of the second antenna ; au nerve to paired eye ; by suboeso- phageal ganglion, con- sisting of several fused neuromeres ; cj commis- sural ganglion ; g brain ; optic ganglion ; m (in F) stomach ; md mandi- bular ganglion ; mx\ and mi2 ganglia of first and second maxillae ; s vis- ceral nervous system ; sc oesophageal commissure ; sg (in F) visceral ganglion, ii'i nerve to median eye ; y post-oesophageal com- missure (? of ganglia of 2nd antennae). I-VIII thoracic ganglia ; 1-6 ab- dominal ganglia. (From Lang.) NATURE OF PAIRED EYES. 349 two lateral, closely aggregated together and each with a pigment mass in relation with it. In the Corycaeidae large dorsal cuti- cular lenses are developed * in relation with the paired lateral elements of the eye ; but our knowledge of the morphology of the median eye is very defective. The structure of the paired eyes has been already described (p. 329). In many Branchiopoda they sink into the body and are covered by a fold of the epidermis (Grobben). The nature of the paired eyes. The morphological nature of the compound eyes of Crustacea has been much discussed, and the evidence still appears to be. conflicting. In the Decapoda and other groups of higher Crustacea they are mounted on jointed stalks and have therefore, to this extent, the character of limbs. As has recently been clearly shown by Herbst,f they may be replaced after injury by an antenna-like appen- dage. On these grounds it has been urged that the crus- tacean paired eye is, in fact, the modified appendage of the first segment of the head. On the other hand the eyes are sessile in Trilobites, an ancient and apparently primitive form of the crustacean stock. The compound eyes of Insects have a similar structure to those of the Crustacea and they are apparently derived from the groups of simple ocelli such as we meet with in the Myriapoda, and the Collembola. If these are to be regarded as limbs they represent the arthropod appendage in an extreme modi- fication indeed. Moreover the preantennary segment, of which the protocerebrum is the neuromere, bears a vestigial appen- dage in the embryo of Scolopendra (Fig. 229, 1). The argument for regarding the paired crustacean eye as a modified limb, which appears at first sight $o be most cogent, is that emphasized by the experiments of Herbst. The phenomenon is apparently allied to that of homoeosis.J In the same way, to take one of many instances, the first antenna of an Asellus may, as an individual variation, assume the character of a mandible. The force of this evidence in its bearing on the appendicular nature of the crustacean eye depends on the extent to which homceosis is confined to truly homologous parts. The appar- ently analogous case, quoted by Bateson (I.e. p. 148), in which the hind leg : Hartog shows that these are present also, though small, in relation with the lateral elements of the eye of Cyclops. f Archiv. f. Entimckelungsmechanik d. Organismen, vols. 2, 9 and 13. 1 Bateson, Materials for the Study of Variation, p. 84. Homoeosis is defined as " the assumption by one member of a meristic series of the form or characters proper to other members of the series." 350 CRUSTACEA. of a Burnet Moth is represented by a hind wing appears to show that it is not always so confined. In view of such a case can it be assumed that a lobe of the head not originally a limb, but lying in the line of the limbs, cannot take on the character of a limb ? The amount of weight which we allow to the results of Herbst's experiments in their bearing on the appendicular nature of the paired eyes depends on the answer to this question. Vesicular sense organs, usually opening to the exterior and containing delicate sensory hairs and hard particles, are found in the basal segment of the first antenna in many Decapods, and closed vesicles of similar structure occur in the uropods of the Mysidae. The open sacks contain sand, which is renewed at each ecdysis. The closed vesicles are said to contain con- cretions of calcium fluoride. It has been generally assumed that these are auditory organs ; * but Kreidl | has shown, by an ingenious experiment, that they have another function. Specimens of the prawns Palaemon xiphius and P. squilla, shortly before an ecdysis, were kept in an aquarium in which sand at the bottom was replaced by iron in a state of fine subdivision. At the ecdysis the lining of the sack of the sense organ was shed, with the rest of the outer cuticle, and the sack was furnished afresh by the Palaemon from the materials available, namely iron particles. On bringing a magnet into its neighbourhood the Palaemon was now found to incline the body, so that the median plane was directed obliquely according as the particles were attracted to one side or the other of the sack, and gave rise to a corresponding stimulus to the hairs in it. It was thus apparent that a function of the sack was to inform the animal, normally by the action of gravity on the contained particles, of its relations in space. DelageJ has come to similar conclusions on the function of the organs in the tail of Mysidae. Beer concludes that all the organs, generally regarded until lately as otocysts in the Crustacea, are in reality of similar function, i.e. that they are not auditory but, to use Verworn's name, statocysts. It would appear however, from the possession * Cf. Hensen, Studien lib. d. Gehororgan der Decapoden, Zeit. f. win. Zool. Bd. 13, 1863. | Physiologic der Ohr-labyrinth, Sitz. Ber. Akad. Wien., Bd. 102, Abth. 3, p. 149. J Comptes Eendus, T. 103 (1886). Statocysten-function, Arch. f. ges. Physiologic, Bd. 73 (1898). ALIMENTARY CANAL. 351 of a stridulating apparatus by some Decapoda (Ocypod Crabs and the Hermit Crabs Coenobita], which live largely in air,* that a corresponding auditory organ must be present some- where in the body ; and the possibility is not excluded that the sacks in question have, as in Vertebrata, a double function, informing their owner of the presence of audible vibrations, and also of their own relation in space. The alimentary canal consists of fore- mid- and hind-guts, of which the first and last are lined by chitin and derived re- spectively from the stomodaeal and proctodaeal involutions (ectodermal) of the embryo, while the mid-gut is endodermal. The remarkably small extent of the mid-gut in the Isopoda and some Decapoda, and the corresponding increase in length of the hind-gut have been already mentioned (p. 340). The fore-gut is usually a short and simple tube in the Ento- mostraca, but in most Malacostraca its posterior region is modi- fied into the masticatory stomach. Salivary glands are usually wanting. From the upper end of the oesophagus the alimentary canal usually runs straight to its termination, though it is coiled in some Cladocera, and Caiman finds that the Cumacean Platycuma Holti has a tract of the gut (? fore-gut) coiled. The mid-gut is marked off by constrictions anteriorly and posteriorly ; in front it is produced into diverticula, which are usually called the liver, but as their walls contain ferment cells the term hepato- pancreas probably more nearly expresses their function. They may be paired (Fig. 251) or unpaired in the Entomostraca, and in the Malacostraca are represented by one to four pairs and they may remain simple, or be subdivided as in Decapods. In Stomatopods it has recently been shown by Orlandi f that there are two of these diverticula opening into the anterior part of the mid-gut, lying parallel with the alimentary canal, and expanding into voluminous lateral branches in the posterior segments of the body including the telson. The supposed segmental and hence exceptional relation of these branches with the gut in this group is thus shown not to exist. In some Amphipods a single or paired caecum is present at the * Of. Alcock, Ann. and Mag. of Nat. Histi/., ser. 6, vol. 10, p. 336. t S. Orlandi, Sulla Struttura dell' intestine clella Squitta manli* Rond., Atti della Soc. Ligustica di Sc. nat. e geog, vol. xii. N. 2 (I'.HH), p. 112. 352 CRUSTACEA. posterior end of the mid-gut, but the excretory function attri- buted to it is doubtful. In any case it is not comparable with the malpighian tubes of Insects, which are diverticula of the hind-gut. The hind-gut in Crustacea is without appendages. The peristaltic contractions of the gut subserve the circu- lation of the blood, and it appears to be the chief agent for this purpose in the small Entomostraca which are without a heart. The chitinous linings of the fore- and hind-guts are shed at each ecdysis. The remarkable paired calcareous concretions, known as crabs' eyes and found in the walls of the stomach of Decapods are also shed at this time and, being ground up by the stomach, apparently furnish the lime salts for the hardening of the new external cuticle. In the strangely distorted and blood-sucking females of some parasitic Isopods (Epicarida) the alimentary canal ends blindly, as it does in some aberrant Cirripedes. In the Rhizocephala and adult Monstrillidae the canal is absent altogether. The blood of Aslacus and of other Decapod Crustacea and of Squilla is a clear fluid containing haemocyanin, a respiratory substance in which copper is present in combination with a proteid. It is colourless when deoxidized, but bright blue when oxidized (oxyhaemocyanin). A red substance may also be present " tetronerythrin ' but it is doubtful if the latter possesses a respiratory function. In the Branchiopods Daphnia and Chirocephalus, however, Lankester found that the respiratory substance of the blood plasma is haemoglobin, and this is the case also in Apus, Cypris, and in the parasitic Copepod Lernan- thropus* Floating in the plasma are colourless amoeboid corpuscles. The Crustacean circulatory system consists of a heart (absent however in the Cirripedes and many Ostracods and Copepods) lying dorsal to the alimentary canal, a system of arterial vessels more or less extensive, and of the lacunar haemocoele, of which the pericardium is part. The Phyllopod Branchipus (Fig. 247) possesses a condition of the circulatory system, which may well be regarded as primi- tive. The heart is a long vessel traversing the thorax and abdomen, and provided with a pair of valvular ostia in each * Cf. Halliburton, Blood of Decapoda, Journ. of Physiology, vol. vi. 1885. CIRCULATION. EXCRETORY ORGANS. 353 segment, through which the blood enters from the surrounding pericardium. A terminal ostium is situated at the posterior end of the heart and the blood is propelled forwards by rhythmic contractions. The " aortic " artery leading forwards from the anterior end opens into the system of lacunae which pervades the body, especially the superficial regions beneath the integu- ment. In the limbs the lacunae are so arranged that the blood flows down one side and up the other, supplying the epipodial branchiae in its course. An incomplete transverse septum dorsal to the alimentary canal separates the pericardial sinus from the lacunae ventral to it. The variants on this arrangement met with in other groups of Crustacea consist mainly in the shortening of the heart and pericardium in various degrees (until we reach the capsular form of heart found in the Euphausiidae and Decapods on the one hand, and the Cladocera on the other) and in the develop- ment of the system of arteries between the heart and haemocoele. This system is most complete in the Decapods where the trunks become subdivided into arterial capillaries. A remarkable vessel found in this group, and in the Schizopods and Stomatopods is the sternal artery which passes ventrally from the heart on one side of the intestine and between the parallel strands of the nerve cord to communicate with the subneural artery from which the limbs are supplied. In Decapods the gills are supplied by a 'system of lacunae independent of the vessels to the limbs, an afferent set leading from the large cephalo-thoracic sinus to the gills, and an efferent conducting the blood to the pericardium, where it mixes with the venous blood returned from other parts. In this group the blood is propelled backwards as well as forwards, a superior abdominal artery extending backwards from the heart above the intestine. As the heart is abbreviated in the several groups the number of ostia diminishes. The excretory organs usually consist of a more or less coiled tube ending internally in a sack and opening at the base of an appendage. They are probably homologous with the segmental tubes of Annelida and other coelomate animals. In the Mala- costraca the' antennal gland (the green gland of Astacus) opening at the base of the second antenna is usually the excretory organ of adult life, though the maxillary gland is found in the Z III A A 354 CRUSTACEA. larva (and, in Cumacea, and Stomatopods in the adult). In the Entomostraca the latter is usually the functional gland in the adult. It opens at the base of the second ma,xilla, and is frequently contained in the fold of the shell, and hence known .as the shell gland. Reproductive Organs. The sexes are distinct in the great majority of the Crustacea. The Cirripedia, however, which lose their locomotive powers early in larval life and become attached, are in nearly all cases hermaphrodite, thus offering a parallel with many, though by no means all, other sessile animals (Tunicata, Phoronis, many Polyzoa). Among Cirripedes also occurs (Scalpellum, Ibla) a remarkable form of dimorphism, the association of supplemental and degenerate dwarf males with the hermaphrodite individuals (p. 424). Several genera of the parasitic Cymothoidae and Epicaridea (Isopoda) are also hermaphrodite, and protandrous, i.e. a male phase precedes the female phase in the life of the individual (p. 484). The remarkable tendency to a hermaphrodite condition in male Decapods, when they recover from the suppression of the re- productive organs induced by cirripede and other parasites is alluded to below (p 445). The reproductive organs are formed on the same type in the two sexes, and usually occupy similar positions, dorsal to the intestine. In both they consist of a single pair of gonads witli the corresponding ducts, though fusion to a greater or less extent between those of opposite sides may occur. The ducts (mesodermal) are continuous with the gonads, and meet distally an invaginated ectodermal tract opening to the exterior. In Cyclops the rudiments of the gonad (" stem-cell ") can be distinguished at an early stage of segmentation. The spermatozoa vary widely in character. They are amoeboid in Polyphemus (Cladocera), oval or sausage-shaped in the Copepods, spherical and beset with stiff radial processes in some Decapods, Euphausiidae and Stomatopods. They are often filiform (though not always, e.g. Danalia) in Isopods and Am- phipods, in the Cirripedes, and the Ostracods, in which latter group they are of gigantic size (up to 10 lines), exceeding the whole body in length three or four times. In many cases they are apparently not mobile, but it is not the case, as sometimes REPRODUCTION. LARVAE. 355 stated, that they are always motionless in Crustacea ; for those of Polyphemus, Cirripedes and Ostracods have been observed actively mobile. The non-mobile condition finds a parallel in the Chilognatha and in some Arachnids. In Decapods fertilisation is effected, according to Koltzoff, by the action of an explosive capsule which is carried by the spermatozoon (cf. p. 525). In many groups of Crustacea the spermatozoa are encased in spermatophores, secreted by the terminal portions of the male ducts. The remarkable manner in which the eggs of the Cladocera are nourished at the expense of neighbouring germ cells is alluded to in the description of that group (p. 379). In the Entomostraca the genital ducts of both sexes usually terminate at a segment lying at the limit between thorax and abdomen whatever the numerical position of this segment may be ; though the Cirripedes are exceptional in that the oviducts open on the basal segment of the first thoracic appendage. In the Malacostraca the positions of the genital openings are fixed, like the number of the segments, the oviducts opening on the 6th, the vasa deferentia on the 8th thoracic segment, either on the base of the appendage or near by on the sternum. It follows, on the view that the genital ducts are derived from segmental organs, that in the Malacostraca the segmental organs of different segments subserve the transmission of the reproductive products in the two sexes. In many Copepods paired or single sperm passages are found in the female leading to the receptaculum seminis and distinct from the direct orifices of the oviducts. To these openings (or opening) the spermatophores are attached by the male. The remarkable birth-aperture described by Schobl in woodlice is referred to in the section dealing with the Isopoda (p. 483). The secondary sexual characters in the shape of sense organs and prehensile modifications of the limbs, together with the many remarkable forms of sexual dimorphism found among the Crustacea are described under the several subdivisions. Larval histories. One of the most interesting features of the Crustacea is the occurrence in all the chief groups, however diverse the forms of the adult may be, of a larva with cer- tain constant characters, the Nauplius (Fig. 242). The name was originally given by 0. F. Miiller towards the end of the 356 CRUSTACEA. eighteenth century to an early stage in the development of Cyclops, under the supposition that it represented a distinct generic type. It is now used for a larva, to whatever group of Crustacea it may belong, having the following characters. The body is oval in shape, wider in front than behind, and shows no trace of external segmentation. A dorsal shield is usually absent, though it occurs in the nauplius of some Cirripedes and of the Cypridae (Ostracoda) (Fig. 256). A pair of setae projects on either side of the hind end. Three pairs of appendages are borne by the nauplius larva ; the anterior (Fig. 242, a ) the 1st antennae of the adult, and placed in front of the mouth. being unbranched, while the two posterior, which become the 2nd antennae and mandibles, are bira- mous and paroral and postoral respectively. The 2nd antennae carry a masticatory e n d i t e directed inwards and acting as a jaw, but the mandibles are, as yet, usually without such a process. A large upper lip projects in front of iH HIP ili cU FIG. 242. Sauplius of Cudop*. dorsal aspect (from ^ Korschelt and Heider, after. Claus). ' first antenna; L a" second antenna; at antennal gland ; (Is mid-gut TnM1 4. flrv pn n r,l with excretory cells ; md mandible ; o median eye. ' a division into oeso- phagus, mid- and hind-gut, though the anus is not always open on hatching. The innervation of the 2nd antennae is from a postoral pair of ganglia, as in some adult Branchiopods. The unpaired median eye is the sole organ of vision, and a heart is not yet formed. The little larva moves rapidly through the water, by the strokes of all three pairs of limbs. When it is recognized that a nauplius stage occurs in the development of the free-swimming Copepods and of their un- gainly parastic relations, in the Barnacles and SaccuHna, in LARVAE. CARE OF YOUNG. 357 Cypris, Apus, Branchipus and Leptodora (winter eggs) ; and among the Malacostraca in Euphausia, Leucifer and Penaeus it is astonishing what diverse forms are knit together by this three- limbed larva. In all these the young enter on a free life as soon as they are hatched, and the adult form is attained in a series of moults by a more or less gradual metamorphosis. But in other cases, as in the Cladocera, many Ostracods, Nebalia and the ' Peracarida " the young are protected, in one way or another, during their development, by the mother, and the metamorphosis is greatly abbreviated. Nevertheless some indication of the 3-limbed nauplius stage, and the throwing off of a nauplius skin occur in all. In the Entomostraca with free swimming larvae the advance to the adult state is by a series of gradual changes, but in the Malacostraca a number of remarkable larval forms are found, belonging to stages which succeed the nauplius, and adapted to a pelagic existence (p. 448 ff). In most, though not in all, cases (and markedly not in the Zoaea larva) the segments and appendages appear in order from before backwards, the former being differentiated from a bud- ding zone at the posterior end of the larva. Various contrivances are found among the Crustacea for the protection of the eggs and of the young. In several groups the batch of eggs is contained in a sack formed by the hardened secretion of the oviduct. This may project freely as in the Copepods, and some Euphausiidae, or lie in the space between the dorsal shield and the body as in Cirripedes. In the Cladocera many Ostracods and Estheria the eggs lie free in this space and the whole of the development may occur in it. When the slowly-developing winter eggs of the Cladocera are produced the walls of the dorsal shield about them become thick and hard, forming the saddle-like " ephippium" which being shed at the next moult with the eggs contained in it, forms a bivalved case for their protection. In the Apodidae two lobes of one appen- dage become opposed to form the egg case on either side. In many Malacostraca a brood pouch is formed as we have seen by the modification of epipodites of some of the thoracic limbs into oostegites, and between them and the ventral surface of the mother the young are protected. In the Decapoda the eggs are attached by a sticky secretion to the long setae on the 358 CRUSTACEA. abdominal legs of the female (many Macrura) or enclosed be- tween the flexed abdomen and the thorax. Fossil forms. The differentiation of the main types of Crus- tacea had occurred before the palaeontological record begins. The Carboniferous rocks contain examples of four out of the five Entomostracan orders, the Branchiopods being represented among other forms by Estheria, a genus which has existed from the Devonian period to the present day, the Cirripedes by Pollicipes and Scalpellum, which go back to the Ordovician, and the Ostracods by a variety of genera which already peopled the Cambrian waters, the genera Cy there and Bairdia extending from the Ordovician to the present day. The fourth order, the Trilobites, is well established in the Cambrian rocks, attains its highest development in the Ordovician and by Devonian times was already on the wane. It is doubtful if they extended beyond the Carboniferous period. The remaining Entomostra- can order, the Copepods, consisting of small and thin-shelled forms, have not been recognized in any geological formation. A number of genera found in the Palaeozoic rocks from the Cambrian to the Carboniferous appear to be related to the most primitive group of the Malacostraca Nebalia and its allies. Among these may be mentioned Ceratiocaris (Cambrian to Carboniferous) and Hymenocaris (Cambrian), Echinocaris (Devonian) and Dithyrocaris (Carboniferous). They have thin bivalved or at least bilobed shells and the abdomen is apparently without appendages though ending in a well-marked caudal fork. They have been referred by many authors to the Phyllo- pods, but the presence in Ceratiocaris of a rostral plate appears to justify the association of this genus at least with Nebalia. The Carboniferous genera Palaeocaris and Acanthotelson and the Permian Gampsonyx and Nectotelson (Brocchi) and perhaps also Gasocaris (Fritsch) find their ally, as was first ably shown by Caiman, in the existing Tasmanian genus Anaspides, and form, with it, the group of the Syncarida (Packard). The caridoid type of Malacostraca shrimp-like forms with dorsal shield, well-developed swimming abdomen ending in a caudal fin, stalked eyes and a scale-like exopodite to the 2nd antenna is represented in the Carboniferous rocks by a number of well-preserved fossils : Palaeopalaemon Whitfield, appearing in the Devonian, Crancjopsis (Palaeocrangon) and Anthrapalae- GEOLOGICAL RECORD. mon Salter. Whether these are to be regarded as true Decapods or as belonging to either of the groups of Schizopods is an open. question, as is, indeed, the case with regard to some existing species. Pygocephalus, from the Coal Measures, appears to be a true Schizopod, belonging to the division of the order classed with the Peracarida.* Clearly marked Macrura appear in the Trias and are well represented in Jurassic rocks. The first undoubted Anomura and Brachyura occur in Cretaceous strata, though Palaeinachus has fair claims to represent the latter suborder in the Jurassic period. Sculda (Jurassic) and Squilla (Cretaceous) give the first clear evidence of the Stomatopods. Cyclosphaeroma from the Great Oolite is undoubtedly an Isopod, but it is far from certain that we possess any earlier representatives of the order. It is interesting to recognize the characteristic tumours caused by parasitic (Bopyroid ?) Isopods in the fossil crabs (Palaeocorystes) of the Cambridge Greensand. The first unquestionable Amphipod is not met with until the Tertiary (Miocene) strata, though a number of obscure Palaeozoic forms have been referred to this order. No fossil Cumacea have been recognized. f On reviewing the palaeontological history of the several orders of Crustacea, the Entomostraca appear to have been differenti- ated at a period before the record begins, and four of their five orders were well established together with certain main types of the Malacostraca (Leptostraca, Syncarida and the Caridoid type) by the Carboniferous period. It may however fairly be claimed that the order of appearance of the subdivisions of the Malacostraca is consistent with the conclusions as to their relationship to which we are led on purely morphological grounds. Sub-class 1. ENTOMOSTRACA. The name Entomostraca, though established by long usage, is without etymological significance as contrasted with that of * Cf. H. Woodward on the genus Pygocephalus Huxley, a primitive Schizopod Crustacean, from the Coal Measures. Geol. Magazine, decade 5, vol. iv, p. 339, Sept. 1907. f The author desires to express his indebtedness to Mr. H. Woods of St. John's College, Cambridge, for his assistance in preparing this notice of the palaeontological record of the Crustacea. 360 CRUSTACEA. the other great division of the Crustacea, the Malacostraca.* This latter was applied by Aristotle to crabs, lobsters and other members of the higher Decapoda, and found its application in contrast, not with the Entomostraca, which were probably unknown to Aristotle, but with a group of " shell-fish " with a still harder shell the oysters, and other bivalves. Aristotle's {wa eVroyUu f included insects, arachnids, myria- pods and apparently the land isopods, as well as annelids. Writers of the 18th and early 19th centuries used the name Crustacea for Aristotle's Malacostraca, and the name Entomos- traca (first employed by 0. F. Miiller, 1785) was applied, though without very direct antithesis, to a group of animals regarded as distinct from the Crustacea, as thus understood, and approx- imating more nearly to insects. Even so late as 1840, Erichson distinguished between Entomostraca and Crustacea and included in the former Limulus, together with A pus, Bran- chipus, Daphnia, Cypris, the Cirripedes, Cyclops, Lernaea, etc. Some time before this date, however, many naturalists had recognised the necessity of establishing a comprehensive group to which the name Crustacea was applied. Aristotle's name Malacostraca was revived to designate, though in a larger sense, the higher division of Crustacea, while that of Entomos- traca was employed for the lower division, with the exclusion of Limulus. It has thus come about that the names Entomostraca and Malacostraca stand for the two divisions of the Crustacea, though the members of the former group are not more con- spicuously segmented than those of the latter, and the Malacos- traca have as a whole firmer shells than the Entomostraca. There are not many positive characters which distinguish the Entomostraca as a group. They are for the most part animals of small size as compared with the Malacostraca. The number of body segments, though fixed in Cirripedes and approximately constant in Copepods, varies widely in the other orders. The abdomen commonly terminates in a caudal fork. The excretory glands of both second antennary and second maxillary segments are developed in the course of the life-history, but it is the latter which (as the " shell-gland " in some groups) becomes the excre- tory organ of the adult a relation the reverse of that usually * /ua\a/cos soft. offrpaKov shell. j- fwoj< a living thing ; ^JTO^OS cut in pieces, or, as we say, " segmented." EJSTTOMOSTRACA. TRILOB1TA. 361 obtaining in the Malacostraca.* The median eye of the larva nearly always persists, though often with compound lateral eyes in addition. The stomach is usually without a masticatory apparatus, though a regular " mill " is present in some Ostracods, notably in Bairdia. The ganglion of the second antennary segment retains its suboesophageal position at any rate in two divisions of the Branchiopods. Among Phyllopods also we find a very simple condition of the heart, it being (in Branchipus) a uniform tube with a pair of ostia corresponding to each of the segments in which it lies. On the whole the Entomostraca are distinguished by a simple and apparently more primitive grade of organization than is found in the Malacostraca, and also by the absence of those characters by the possession of which the Malacostraca are united. They are however a much less homogeneous group than the latter, and it is the fact that most divisions of the Entomos- traca differ more from one another than Nebalia does from some members of the Phyllopoda. Each group which has left any palaeontological record at all was already well differentiated by the Ordovician period. The Trilobites died out early, but certain genera of the Branchiopoda, Ostracoda and Cirripedia have persisted from Ordovician times to the present day. Order 1 . TEILOBITA. f Pa'.aeozoic Crustacea with one 'pair of antennae and (apparently) jour other pairs of cephalic appendages, the gnaihobases of the latter serving as jaws. Of the numerous (up to 30) segments of the trunk the anterior are free and the posterior are united into a pygidium. All the trunk segments except the last (telson) bear biramous appendages. * In the Ostracoda (Cypris) however both antennary and maxillary glands are found by Glaus to be present in the adult. f Burmeister, Die Organisation der Trilobiten, etc., Berlin, 1843. Beyrich, Unters. ilb. Trilobiten, Berlin, 1845, 1846. Barrande, Systeme silurien du centre de la Boheme, Prague, 1852. Salter, S. W., A mono- graph of the British Trilobites, London, 1864-1866. Walcott, C. D., Fossils of the Utica Slate, Trans. Albany Inst., vol. X, 1883 (separate copies 1879). Id., Note on some appendages of Trilobites, Proc. Biol. Soc. Washington, vol. 9. Beecher, C. E., Several papers (in American Journ. of Sc., ser. Ill, vol. 46 and 47, and in American Geologist, vols. 13, 15 and 16), collected in Studies in Evolution, Yale Univ. Publ., London, 1901. 362 CRUSTACEA ENTOMOSTRACA. Trilobites abound in Cambrian and Silurian strata in many parts of the world, but appear not to have survived to the secondary period. Though the preservation of the head and trunk is often as perfect as can be desired no traces of appendages were for a long time found, but within the last few years the Utica shales (Lower Silurian) of the State of New York have yielded abundant examples of a trilobite, Triarthrus Becki, the organic parts of which are replaced by iron pyrites. These have been made the subject of a series of reports by Beecher. Many features of the appendages of these speci- mens are preserved in minute detail, and their discovery has put our knowledge of the group on a new level. Unfortunately, however, notwithstanding the care which has been devoted to their elucidatien, we are still left in tantalizing uncer- tainty on several points. The body of Trilobites is oval, and dorso-ventrally flat- tened. It consists of a head and of a segmented trunk the anterior somites of which were movable on one another, while the posterior are united, as in many Isopoda, to form the pygidium ; and, like woodlice, the animals possessed the power of rolling themselves into a ball, in which position they are often preserved. In both head and trunk a central part is divided by longitu- dinal grooves from pleura! regions, causing the tripartite division of the body in allusion to which the group was named. On the central part of the head, known as the glabella, trans- verse grooves usually indicate a division into five segments, of which the posterior or occipital segment is most distinct (Fig. 244). The anterior and lateral regions of the head end in a sharp FIG. 243. Diagram of Dnlmanites. Ge gena ; 6/glabella ; eye ; PI. pleuron ; Py pygidium ; Rh rhachis ; Sf facial or ocular suture (the leader extends a little beyond the suture). From Claus, after Pictet. TRILOBITA. 363 border, and the postero-lateral angles are often produced into long backward-directed processes. The pleural regions of the head are known as the genae or cheeks, and bear the large reniform, many-facetted eyes which are in some cases raised on prominences of the head shield. In Harpes the eyes are replaced by groups of two or three ocelli, and in some genera they were absent altogether. The gena of either side is traversed by the facial or ocular suture, which runs forwards from the hinder or outer mar- gin, passing on the median side of the eye, and is continuous with its fellow in the middle line in front of the glabella. From the fact that in decayed specimens the regions of the head fall apart at this suture it is con- jectured that some small amount of movement between the head regions may have been admitted during life and the terms " fixed " and " mov- able cheeks " have been applied to the areas of the genae internal and external to the ocular suture. In addition to the eyes on the upper surface of the head, some trilo- bites possess structures on the under surface which are also, apparently, eyes. They are situated on either side of the posterior third of the hypostome. Each consists of a small oval area, called a macula by Lindstrom. In some genera they are smooth ; in others they are partly or entirely facetted, and resemble in structure the dorsal eyes.* The free trunk-segments vary in number from two (Agnostus) to twenty-nine. Those of the pygidium also vary in different genera (from 2-28, though the number is by no means com- plementary to that of the free segments), as does the complete- ness of their fusion into a uniform plate. The terminal segment (telson) is without appendages and the anus is situated on its under surface. The sternal region of * G. Lindstrom, Researches on the Visual Organs of Trilobites, Kongl. Svenska Vetenskaps Akad. Bandet 34, 1901. FIG. 244. Dorsal surface of Becki. After Beecher. Triartlirnx 364 CRUSTACEA ENTOMOSTRACA. -en the trunk segments is sometimes traversed by longitudinal ob- lique ridges, interpreted by Beecher as marking the apodemes for muscular insertion. Beneath the head a large labrum (hy postoma] projects back over the mouth, and a small lower lip is placed behind it. Appendages. A pair of long annulated antennae (Fig. 244), with a large basal joint, are inserted on either side of the labrum. From the region of the mouth a series of appendages, set wide apart, at the ends of broad sternites, extends to the end of the pygidium. In the post-cephalic region they have, in Triar- thrus, the following characters. They are biramous but peculiar among crustacean limbs in being deeply cleft down to the coxal process (Fig. 245). The endopodite is cylindrical in the anterior part of the trunk, but its basal segments become more and more lamellar as the series is followed back. The exo- podite consists of a long basal segment and a multiarticulate ter- minal portion, both beset with an abundant fringe of long setae. From the base of the limb what appears to be a coxal endite ex- tends inwards towards its fellow, as in Apus and its allies ; but as, apart from this, it is not clear that there is any uncleft basal portion of the limb (protopo- dite), the homology of this element of the limb remains for the present obscure. In the head Beecher concluded that there were four pairs of . appendages in addition to the antennae, though, owing to the difficulty in determining their points of insertion, the number assigned to the head is in part a deduction from the number of the segmental grooves on the dorsal surface of the glabella. The two posterior head appendages appear to have had a structure similar to that of the legs behind them, though with a longer and denticulated gnathobase. The two appendages in front of them had also, apparently, a large gnathobase, but the conclusion that their appendages, like those of the segments FIG. 245. Dorsal view of the right second and third thoracic legs of Triar- thrus, as restored by Beecher. In 11. the setae are omitted, en endopodite, with segments 1-6 : e.i exopodite, with segments 1-2. 7 the problematical coxal process or gnathobase. TRILOBITA. 365 behind them, were also biramous appears to rest on insecure foundation. The Utica shales have also yielded specimens of Trinucleus in which the pygidial appendages are displayed, and these resemble the corresponding structures in Triarihrus, though considerably shorter. Before the discovery of this material much light was thrown on the nature of the appendages of Trilobites by Walcott who examined a large number of fossils of Calymene by means of sections. He concluded that two long and sometimes spirally twisted epipodial lobes were appended to the outer side of the base of the biramous limb. In view of the difficulty of arriving at certain results by this method, and of the fact that Triarihrus yields no trace of such structures, their existence in Calymene appears very doubtful. Development. Remains of young trilo- bites are sometimes found associated with those of adults, and the successive stages of the development of the head and trunk have been traced by Barrande and by Beecher. In the earliest which has been recognized, the -protaspis stage of the latter FIG. 246. Larva of manites sonalis Bar. author, they are minute oval bodies - 4-l mm. in the "ana-protas- pis " stage. A - . in length, with indications of the five divi- dium of three segments is present, but there sions of the glabella, and of one succeeding ar e as yet no free thoracic segments. segment. The eyes are anterior and mar- x so. From Beecher, after Barrande. gmal when they first appear, the subse- quent change in position causing the indentation of the ocular suture of the adult. The appendages of the young have not been seen. The fact, pointed out by Barrande, that in Trinucleus, and other genera a pygidium is formed before the number of free thoracic segments is complete, does not, as might at first appear, necessarily show that the region of the formation of fresh segments was in front of the pypdium. We are at liberty to suppose that the formation of fresh segments occurred at the posterior end of the body, as in most other young Crustacea, and that segments which at one stage of growth belonged to the pygidium became in a later stage, after casting a shell, free thoracic segments. Occurrence. The remains of Trilobites are associated in marine strata with those of Crinoids, Brachiopods and Cephalo- pods. The absence of eyes in some cases is perhaps an indica- tion that the species so characterized inhabited deep water. In view of the high interest attaching to Trilobites as ancient 366 CRUSTACEA ENTOMOSTRACA . and primitive members of the Crustacean series, it is most undesirable to press the evidence for more than it is worth ; and in the foregoing account of the appendages an attempt has been made to show where, as appears from the published descriptions, the ground is still insecure. It seems certain that they possessed only one pair of antennae, and, taking into account the indications of segmentation in the glabella of the larva and of the adult in addition to the evidence afforded by the appendages, the existence of four other cephalic limbs, provided with gnathobases, appears probable. The establishment of these results would confirm in the most striking manner the conclusions which have been arrived at from the study of the existing fauna on the nature of the second an- tennae of Crustacea namely that they are postoral appendages, which have in recent forms become preoral. Besides the characters of the second cephalic limbs and the indication of segmentation of the glabella into five segments, we may recognize as primitive features the presence of limbs on all the segments except the telson, the varying and usually large number of the body segments and the small degree of specialization in the series of appendages, though how far this applies to the head appendages must remain for the pre- sent undecided. In the two last features Trilobites resemble the Phyllopods among the Branchiopoda, to which group Burmeister had pointed out their resemblance before the discovery of the appendages. It is also shown in the large hypostoma and the gnathobases of the post-cephalic limbs ; and the absence of a carapace finds a parallel in Branchipus and its allies. On the other hand the deeply cloven character of the limbs removes the Trilobites from the immediate neighbourhood of Phyllopods, as also from all other recent forms. Until the appendages were discovered Trilobites were usually classed with Limulus and its allies, on account of the shape of the head shield, the position of the eyes and the so-called " trilobite stage ' : in the development of Limulus. This association is indeed still retained by many authors, but the indication of five pairs of cephalic appendages, as in the Crustacea, not seven as in the cephalo -thorax of the Merostomata, and the evidence which we now have on the BRANCHIOPODA. 367 characters of the appendages, the antennae and the others, appear to remove them widely from the latter group and to bear indisputable testimony to their Crustacean affinity. Order 2. BRANCHIOPODA.* Crustacea with an elongated and often distinctly segmented body ; usually wiili a flat shield-like carapace or laterally com- pressed bivalve shell, formed by a reduplicature of the skin. The mandible is without a palp in the adult. There are at least four and generally many pairs of swimming feet, which are in nearly all cases leaf-like and lobed. The animals belonging to this order differ very considerably in form and size, in the number of their segments and appendages, as well as in their internal anatomy. They agree however in the lobed and leaf-like character of their feet. Of the two groups into which they are divided, the Phyllopoda are probably the most primitive form of Crustacea which has survived. Their primitive character is seen in the large and varying number of segments of the body, the small degree of differentiation through- out the series of their appendages, the tubular heart with the segmentally arranged ostia, the simple character of the ganglionic chain, and the persistence in all of them of the nauplius larva. The other group, the Cladocera, may be regarded as an off-set from this primitive stock (probably from a form allied to the Conchostraca) in which the number of appendages and the size of the body have been reduced and its segmentation obscured. The body is either cylindrical, elongated and clearly seg- * Besides the works of O. Fr. Miiller, Jurine, M. Edwards and Dana compare Zaddach, De apodis cahcri/ormis anatome et historic/, evolutionis, Bonnae, 1841. E. Grube, Bemerkungen viber die Phyllopoden, Arch, fin- Naturgeschichte, 1853 and 1855. Fr. Leydig, Monographic der Daph- niden, Tiibingen, 1860. C. Glaus, Zur Kenntniss d. Baues u. Entwick. v. Branchipus u. Apus, Gottingen, 1873. Zur Kenntniss d. Organisation etc. der Daphniden, Zeit. /. wiss. Zool. Bd. 27. Zur Kenntniss d. Baues u. d. Organisation, der Polyphemiden, Denkschr. Akad Wien, Math. Nat. Classe. Bd. 37, 1887. Branchipus and Artemia, Arb. Zool. Inst. Wien. vi. A. Weismann, Naturgesch. d. Daphnoiden, Leipzig, 1876-79. Grobbsn, C., Embryonalentwick. v. Moina rectirostris, Arb. Zocl. Inst. Wien. ii. Packard, Monog. N. Amer. Phyllopod Crustacea, Washington, 1883. P. Samassa, Keimblatter-bildung bei Cladocera, Arch. f. mikr. Anal., Bd. 41. G. O. Sars. Fauna Norvegiae, Vol. 1, Phyllocarida and Phyllopoda, Christiania, 1896. M. T. Sudler, Devt. Penilia, Pr. Boston ,s'oc'. N. H., Vol. 29, 1899, p. 109. Samter, Entw. Leptodora, Zcit. i. wiss. Zool. Bd. 68 (1900). Lilljeborg, Cladocera Suecica, NovaActa Soc. Sc. Upsala, 1900. 368 CRUSTACEA EXTOMOSTRACA. mented, without free reduplicature of the skin (Branchipus, Fig. 247), or it may be covered by a broad and flattened shield, an extension of the cephalic integument, which only allows the posterior part of the body to project uncovered (Apus, Fig. 248). In other cases the body is laterally compressed and enclosed by a bivalve shell from which the anterior part of the head projects (Cladocera) (Fig. 251) ; or finally the head together with the laterally compressed body is completely covered by a bivalve shell which is closed by a shell muscle passing between the valves (Conchostraca). Sometimes the head is sharply distinct from the rest of the body, and in one family, the Bran- chipodidae, the anterior part, bearing the eyes and antennae, is divided by a transverse groove from the posterior, bearing the other three pairs of cephalic ap- pendages (Sars). As a rule the posterior segments only are without appendages. The hind end of the body is often curved forwards and may bear two rows of posteriorly directed claws, the last pair of which arise at the point of the tail, and are by far the strongest (Cladocera). In other cases a pair of simple (Branchipus) or articulated (Apus) appendages are present, constituting the caudal fork. Appendages. On the head there are two pairs of antennae, which however, in the adult animal, may be rudimentary or T FIG. 217. Male of Bnntc/ii/nis statjinilis. R-j heart or dorsal vessel with a pair of slit-like ostia in each segment ; D intestine ; M man- dible ; Sd shell gland ; Br distal branchial appendages of the eleven pairs of legs ; Ttestis. The anterior antenna is seen as a slender appendage curving forwards in front of the eye over the base of the strong, prehensile posterior antenna. The long tentacular and short foliaceous appendages of the latter are also seen. BRANCHIOPODA. 369 sff>l -sh.gl peculiarly modified. The anterior antennae are small, and bear the delicate olfactory hairs. The posterior antennae frequently have the form of large biramous swimming appendages, but they may be modified as prehensile organs, e.g. in the males Bran- chipus and its allies (Fig. 247). In other cases (A pus) they are rudimentary (Fig. 248) and may even be entirely absent. A pair of large mandibles is always present beneath the well developed upper lip ; they possess a toothed, biting edge, and in the fully developed condition are destitute of palps.* The man- dibles are followed by one or two pairs of slightly developed maxillae. A kind of underlip is in many cases present, in the form of a bilobed prominence be- hind the mandibles. FIG. 248. Apitsglacinlis, ventral aspect of female, abd.f abdominal limbs ; ant.l first antenna ; ant. 2 second antenna ; Ibr lalirum ; md mandible ; mx first maxilla ; ov brood rouch ; s.f.pl sub-frontal plate : sh.ql shell-gland ; th.fl and tli.f thoracic feet. (From Parker and Haswell. after Bernard.) It is remarkable that while the tho- racic appendages of the Branchiopods conform so uniformly to a common and, as it appears, primitive type of structure, the appendages about the mouth present the greatest divergence from that type which is met with in any group of Crustacea. The absence of the mandibular palp in the adult is all the more striking because of its size and frequently biramous character in other Entomostracan groups, the Copepods and Ostracods. Again, the two pairs of maxillae, which even in the Malacostraca conform more closely than any other appendage to the primitive " Phyllopod " type of limb, are here, in the Branchiopods themselves, perfectly simple lobes, and one pair is often missing. In adult Cladocera and in Limnetis among the Conchostraca there is only one pair, in Branchipnsthe second pair is rudimentary, and in this genus, as also in Apus where two pairs are present, they are retarded in develop- ment, appearing later than the thoracic appendages which follow them in position. It is interesting to note that in Cypris, among the Ostracods, the second maxillae are also retarded in their development. The late appearance of the maxillae is perhaps a confirmation of the view taken * Ekman finds a vestigial palp in Polyartemia forcipata (Bih. K. svevska vet Akad. Handlingar, Bd. 28 [1902]). Z III B B 370 CRUSTACEA ENTOMOSTRACA. here, that the simple condition of these appendages in the Branchiopods is a specialized and not a primitive character. The small size and simple character of the maxillae is perhaps dependent on the peculiar habit of members of this genus, alluded to below, of passing the food forwards along the ventral groove. The postcephalic appendages (legs or feet) are usually very numerous in the Branchiopoda Phyllopoda, and are smaller to- wards the posterior end of the body. In this division of the order they consist, as its name implies, of flattened leaf-like appendages which act as swimming feet, and by the eddies they set up in the water assist in procuring food. They are set transversely on either side of the ventral middle line, and the inner and outer mar- gins are produced into endites and exites. In the mid-ventral line of the body, between the closely ranked legs of either side there is a deep groove, limited at the sides by the endites, and closed above by the sterna of the body segments. Along this groove the material containing the food is, in most if not all Branchiopods, passed forward to the mouth, assisted on its way by the movements of the appendages. It can thus be acted on by the opposed endites of the latter, set on either side. The basal endites, and especially the first or " coxal " endite, have accordingly the character of masticatory lobes, being short, firm and beset with coarse setae. There are usually six endites on a leg (Fig. 249) ; those near the base directed inward, and those situated distally directed more and more in the line of the axis' of the leg, so that the most distal form terminal lobes. The exites are two or three in number. The most distal of them, having in several cases a triangular shape, is known as the ftabellum. The proximal exite or exites (2 in Poly- artemia and Chirocephalus) are generally simple lobes, devoid of setae.* A respiratory function has been especially attributed to them, but, as the justice of this is disputable, the term bracts may be conveniently retained for them. It is not at all obvious how the short multilobed thoracic legs of the Branchiopoda conform to the biramous type of limb so widely found among the Crustacea ; and, as a fact, now one and now another pair of the terminal lobes has been regarded * In cases where two are present they appear to belong to the two proximal divisions of the protopodite. BRANCHIOPODA. 371 by different authors as corresponding with the two branches usually met with. By a comparison of the undoubtedly bira- mous second antennae with the developing thoracic legs of the larval Apus, Lankester * has shown good reason for regarding the fifth and sixth (the two terminal) endites as the homo- logues of the endopodite and exopodite respectively. It follows that the flabellum must be regarded, not as an exopodite (its jointed flagellar termina- tion in Limnetis notwithstanding) but as a distal epipodite. In Daphnia among the Cladocera (Fig. 252) the epipodites are reduced to a single bract and the distal endites are diminished in size. The coxal en- dite on the other hand is large in the first pair of thoracic legs, and in the second and third pairs it forms a great comblike, backwardly directed plate. In the Polyphemidae the anterior or all the legs have a cylindrical shape, and in Leptodora no epipodite is present. It is remarkable that in Apus, which possesses a large number of appendages (40-63 pairs), while the anterior pairs correspond in number with the segments (as indi- cated by the annular constrictions and rings of cuticular spines on the surface of the body), being attached one to each, the posterior far outnumber the segments which bear them, so that one segment carries as many as five or six pairs of limbs. The central nervous system is composed of a supra-oesophageal mass connected by commissures with a ventral chain of ganglia. The latter in the Phyllopoda presents nearly the same simple arrangement which is found in the Annelida, consisting of a pair of ganglia to each pair of appendages (Fig. 241, D). There are, however, in the Phyllopoda, two transverse commissures to each pair of ganglia. Even Apus with its large number of * Q.J.M.S., vol. 21. The evidence here cited, and that brought for- ward by Thiele (Betrachtungen lib. die. Phylogenie der Crust aceenbeine. Zeit. /. wiss. Zoologie. Bd. 82 (1905) p. 445), who regards the flabellum as the exopodite, appear to be conflicting. FIG. 249. Second thoracic leg of Apus cxncriformis. 1 first, 2 second stein joint ; En 1-6 endites ; ep flabellum ; e-p' bract (from Korschelt and Heider, after Lankester). 372 CRUSTACEA EXTOMOSTRACA. appendages forms no exception to this rule. In Simoce phalus and Sida among the Cladocera the ventral chain consists of two longitudinal cords united by transverse commissures, and giving off separate nerves to mandibles, first maxillae, and the swimming appendages. In the adults of these genera the central nervous system is not more than half the length of the body, though in the young state the proportion is much greater.* In Leptodora the ventral ganglia are fused into a common mass. The motor nerves to the first antennae arise from, or can be traced to (A pus) the brain, those of the second antennae arise from the oesopha- geal commissures, or the sub- oesophageal ganglion (Fig. 250). The Branchiopods possess a pair of large eyes which are frequently compound and some- times fused together in the middle line. In the Anostraca they are stalked and movable (Sars). In other forms, though superficial in the larva, the eye is covered in by a fold of skin which, growing from behind, forms an open (Apus and Estheridae) or closed (Clado- cera) chamber over it (Grobben). In addition a small median simple eye (nauplius eye) may persist. The first antennae bear olfactory papillae in the Cladocera. A group of sensory hairs in front of the median eye constitutes the frontal organ ; and groups of ganglion cells in connexion with the skin are found on the forehead (Branchipus) or at the sides of the neck (Cladocera). Many members of the group have the power of attaching themselves to surrounding objects by means of peculiar patches of glandular cells situated on the dorsal surface (cervical gland), near the fold separating the carapace from the head shield. * Cf. Cunnington, Simocephalus, Jen. Zeits., Bd. 37. FIG. 259. Ventral aspect of the brain of Daphnia similis (from Korschelt and Heider, after Claus). c 1 and c 2 divisions of the supraoesophageal ganglion ; c 3 suboesophagsal ganglion ; go optic ganglion ; n nerve to the sensory organ of the neck ; na' nerv tc> 1st antenna; n' and na" nerves to 2nd antenna ; sc oesophageal commissure. BRANCHIOPODA. 373 The heart is elongated and segmented in the Phyllopoda (Fig. 247), saccular in the Cladocera (Fig. 251 ). Coiled excretory organs known as shell glands (Figs. 248 and 251, s d) from the fact that their coils often lie between the lamellae of the shell, are always present, and in the cases where the duct has been traced (Limnadia and Apus), open to the exterior on the posterior maxillae. The antennary glands, though present in the larvae of Phyllopods (Glaus), are not functional in adult Branchiopoda. Respiration is performed by the entire surface of the body the area of which is much increased by the reduplicature of the skin forming the carapace, by the foliaceous swimming feet, as also by their more specialized lobes, the proximal epipodites. Reproduction. The Branchiopoda are of separate sexes.* The males are distinguished from the females by the structure of the first pair of antennae which are larger and more richly provided with olfactory hairs, or by the prehensile character of the second antennae (Anostraca), or (Cladocera, Conchostraca) by -the character of their anterior swimming feet which are armed with prehensile hooks. In general the males are less frequently met with than the females, and, in some cases only at certain seasons of the year. In the Cladocera, and in the genera Apus, Artemia and Limnadia among the Phyllopoda, many generations consisting entirely of females and reproducing parthenogenetically succeed one another. In the Cladocera this is generally the case during the early summer. In the late summer and autumn mixed broods consisting of males as well as females, and broods of males are produced. From the fertilized " resting " or " winter " eggs which are now produced, the parthenogenetic broods of the following year arise. Weismann concludes that in the Daphnidae the succession of broods forming the life cycle is definite for each species, and adapted to its mode of life. Thus in species inhabiting large bodies of water, such as lakes, many parthenogenetic female broods succeed one another during the summer, and it is only on the onset of cold weather in autumn that males are produced. Species which multiply in puddles on the other hand may present only a single parthenogenetic generation. Issakowitsch has recently published f a preliminary account of some * The statement which has recently been made, that Aptis is hermaph- rodite, does not rest on satisfactory evidence. Hermaphrodite forms sometimes, however, make their appearance in the Cladocera, cf. p. 379. f Geschlechtsbestimminde Ursachen bei den Daphniden, Biol. Central- blatt, Bd. 25, No. 16, 1905. 374 CRUSTACEA ENTOMOSTRACA. interesting researches, undertaken with the purpose of finding the answer to the question, " On what does the onset of the sexual period in the life- cycle of the Daphnidae depend ? " and has arrived at results differing considerably from those of Weismann. He finds that when the partheno- genetically produced offspring of one parent (Simocephalus vetulus O.F.M.) are divided into two batches, and, while kept at a warm temperature, are submitted, the one to conditions of abundant nourishment, the other to a starvation diet, those in the first batch continue to reproduce females by parthenogenesis, while those in the second produce males, or winter eggs. The second result is also produced by cold, even when food is abundant, but Issakowitsch is inclined to attribute it to the falling off in nutrition resulting from the lower rate of metabolism. He concludes that "nourish- ment and temperature (the latter by its influence on nourishment) are the determining factors for the occurrence or disappearance of the sexual in- dividuals ; and that there is no cyclical succession of generations in the Daphnidae." The conditions under which males are produced, in the above named Phyllopod genera, are more obscure. The presence of males in a colony of the genus Apus is of rare occurrence. Thus an examination of the successive generations of A. cancriformis inhabiting pools in certain clay pits, carried on for six consecutive years failed to reveal to von Siebold a single male among 8,521 specimens which he examined. Colonies are however occasionally found in which males are present, though always in smaller numbers than the females. In some species of Artemia males are also of rare occurrence, while in others they are plentiful. The males of Limnadia Hermanni Brongn. are unknown. The females usually carry the eggs about with them on ap- pendages specially modified for this purpose (Apus}, or in a brood pouch beneath the shell on the dorsal surface (Daphnidae), or in ventral marsupia (Branchipus). In the Phyllopoda the young leaves the egg as a somewhat modified nauplius larva, with three pairs of appendages (the mandibular palp, which is absent in the adult, being at this stage well developed) ; and the mature condition is reached by a, complicated metamorphosis. In the Cladocera, on the other hand, whose large eggs contain abundant yolk and are protected in the brood pouch of the mother, the young are hatched in the 'orm of the adult, though passing through a nauplius stage within the egg. That remarkable and aberrant form Leptodora offers however an exception to this rule in that while the summer eggs develop in this manner, the winter eggs (which are fertilized) hatch out as nauplius larvae (Sars.) BRANCHIOPODA. PHYLLOPODA. 375 A few Cladocera live in the sea, but the greater number of Branchiopods inhabit bodies of freshwater where there is little or no current. Some of them are found in brine pools. Sub-Order 1. PHYLLOPODA.* Branchiopoda, with dearly segmented body, often enclosed in a ftat shield-shaped or laterally compressed bivalve shell, with from ten to thirty or more pairs of foliaceous swimming feet bearing saccular epipodites. The alimentary canal is provided with two lateral hepatic appendages which are as a rule branched and racemose and only exceptionally short and simple. The heart is a long dorsal vessel with numerous paired lateral slits which may extend along the whole length of the body (Fig. 247). The genital organs which are always paired are placed by the side of the alimentary canal and open at the boundary between the thorax and abdomen, a limit which may or may not be marked by other structural features. In the Branchipodidae they are simple, but in Apus they are racemose glands in both sexes. In the females the genital openings are small slits ; in the males of the Anostraca there are protrusible copulatory organs at the openings. The males may be distinguished from the females in the Conchostraca by the fact that the anterior or two anterior pairs of legs are armed with hooks, and in the Anostraca by the large size of the posterior antennae, which in Branchipus and Branchi- necta are moreover beset with peculiar appendages. In Apus they are distinguished by the absence from the llth pair of appendages, of the brood pouch, to be referred to directly. The eggs are generally protected during development, being carried about the body of the mother either in a projecting uterine dilatation of the united oviducts, at the base of the abdomen (Branchipodidae), which, unlike the egg-sack, of the Copepoda, is a cellular structure opening by muscular lips ; or between the valves of the shell attached to filiform processes * Schaffer, Der Krebsartiger Kief er fuss etc., Regensburg, 1856. A. Kozubowski, Ueber den maiinlichen Apus cancriformis, Arch, filr Natur- gesch., Bd. 23, 1857. C. Claus, Zur Kenntniss des Baues und der Entwickelung von Branchipus und Apus etc., Gottingen, 1873. The same, Untersuchungen liber die Organisation und Entwickelung von Branchipus und Artemia, Arbeiten aus dem zool. Institute, Wien., Bd. 6, 1886. A. S. Packard, A monograph of North American Phyllopod Crus- tacea, Washington, 1883. 376 CRUSTACEA ENTOMOSTRACA. of the 9th and 10th pairs of legs (Estheria), or in box-like brood pouches borne on the llth pair of thoracic appendages, at the base of which the oviducts terminate. These are formed by the apposition of the flabellum and a rounded concave plate pro- jecting from the distal portion of the limb (Apus). In the eggs of Branchipus the segmentation is complete at any rate in the early stages. The iiauplius larva is charac- terized by several peculiarities. Its body is, in Branchipus and Estheria, distinctly divided into a cephalic and post-cephalic region ; the upper lip is extraordinarily large ; the first pair of antennae is usually rudimentary and sometimes even absent, while the second pair is exceptionally large (Balfour). Almost all the Phyllopoda belong to inland waters, and principally inhabit shallow fresh-water pools. When the latter dry up the eggs lying in the dry mud may retain their power of development for years. Tribe 1. ANOSTRACA. Body cylindrical, without carapace ; eyes pedunculated ; second antennae prehensile in the male; 11-19 pairs of thoracic legs bearing two or three exites ; small paired copulatory appendages in the male ; the female carries the eggs in a ventral egg sac in which the oviducts terminate. Widely distributed in fresh and salt water lakes. Fani. 1. Branchipodidae. Abdomen distinctly segmented in both sexes, with paired caudal appendages : prehensile antennae of male distinctly jointed : 1 1 pairs of thoracic legs, usually with 2 exites (3 in Chirocephalus). Artemia ~Le&c\i, in salt water lakes * : ArtemiopsisG.O. Sars ; Branchinecta Verrill ; Branchipus Schaff (Fig. 247) ; Chirocephalus Prev.. occurs in Britain ; Branch i pod ops is G.O.S. : Branchiopsilus G.O.S. Fam. 2. Thamnocephalidae. Abdomen terminates in a broad swim- ming plate. Thamnocephalus Packard, N. America. Fam. 3. Polyartemiidae. Abdomen of female imperfectly segmented ; prehensile antennae of male not jointed ; 19 pairs of thoracic appendages, with three exites. Polyartemia Fischer, with vestigial mandibular palp in P. forcipata. Tundras of N. Europe and Asia and Alaska. Tribe 2. NOTOSTRACA. Shell shield-shaped fused in front with the head shield ; lateral eyes * It has been stated that it is possible, by diminishing the salinity of the water inhabited by a colony of Artemia salina to induce changes in their characters so that they assume those of the genus Branchipus. The evidence in support of this statement has however been shown to be quite insufficient. Cf. Bateson, Materials for the Study of Variation, p. 96 ; Grochowski M. Ueber eine neue im Siisswasser lebende Species von Artemia, Verh. Zool. Bot. Ges., Wien, 1895, p. 95. See also Samter and Heymons, Variationen bei Artemia salina, Abh. Akad. Berlin, 1902, Anh. 2.' BRANCHIOPODA. CLADOCERA. 377 dorsal, not stalked ; caudal fork consisting of two long jointed appendages ; both pairs of antennae simple and rudimentary ; two pairs of maxillae are present. There are 40-63 pairs of legs diminishing from before backwards, and furnished with two exites ; the terminal appendages of the most anterior pair are long and antenna-like : and the llth are modified to focm round brood-pouches in the female (cf. p. 376). Single fam. Apodidae. A pus Schaff. without median caudal lamella. Lepidurus Leach, with a median caudal lamella, and larger carapace, but the 2 genera closely allied. Many spp. known from fresh waters in many parts of the world. Tribe 3. CONCHOSTRACA. A bivalve shell is present in which the whole of the rest of the body may be completely enclosed. Lateral eyes not stalked, approximated. First antennae simple, small or rudimentary ; second large and biramous. 10-28 pairs of post-cephalic limbs are present, of which the first, or the first and second pairs, are provided with prehensile hooks in the male. The eggs are carried between the hinder part of the body and the shell, attached to dorsally directed processes of some of the limbs. Fain. 1. Limnadiidae. Shell compressed, marked with lines of growth ; body elongated, with 16-28 pairs of thoracic legs. Limnadia Brong. ; Eulimnadia Packard ; Estheria Riipp. ; Leptestheria G.O.S. Cyclestheria G.O.S. Fam. 2. Limnetidae. Shell tumid, without lines of growth : head very large ; body short ; not more than 12 pairs of legs. Limnetis Loven. Sub-Order 2. CLADOCERA. Water-fleas.* Small laterally compressed Branchiopoda, whose body, ivith the exception of the head, which projects freely, is usually enclosed in a bivalve shell. The second antennae are large and are used in swimming, and there are four to six pairs of swimming feet. The epipodites may be absent. The Cladocera are small simply organized Branchiopoda, whose resemblance to the larvae of the shelled Phyllopods, par- ticularly to that of Estheria with its six pairs of legs, probably gives the best indication of the origin of the group. Unlike ' Besides the work of F. Leydig already quoted compare H. E. Strauss- Diirkheim, " Memoire sur les Daphnia de la classe des Crustaces," Mem. du Mus. d'hist. nut., Tom. V and VI, 1819 and 1820. Leydig, Natur- geschichte der Daphniden, Tubingen, 1860. P. E. Miiller, Bidrag til Cladocerernes Fortplantings historic, Kjobenhavn, 1868. G. O. Sars, " Om en- dimorph Udvikling samt Generationsvexel hos Leptodora," Vidensk. Selsk.Forh.,Christianii\, 1873, p. 1. A Weismann, Beitrage zur Kenntniss der Daphnoiden, I VII, Z. f. w. Z., Tom. 27, 28, 30 supl. and 33, 1876-1880. C. Claus, Zur Kenntniss der Organisation, etc., der Daphniden, ibid., 27, 1876. C. Claus, Zur Kenntniss des Baues und der Organisation der Poly- phemiden, Wien, 1877. C. Grobben, Die Embryonalentwickelung von Moina rectirostris, Arbeiten aus dem zool. Instititt. Wien, II Band, 1879. W. A. Cunnington, Studien an einer Daphnide, Jena. Zeits., T. 37, p. 447, 1903. 378 CRUSTACEA ENTOMOSTEACA. the anterior antennae, which are usually short, the posterior are modified to form biramous swimming appendages beset with numerous long setae. The four to six pairs of legs are not always foliaceous swimming feet, but in some cases have the form of cylindrical ambulatory or prehensile appendages. The abdomen, which is ventrally flexed, develops on its dorsal side several FIG. 251. Dapfinia. A anus ; Br brood-pouch beneath the dorsal reduplicature of the shell ; C Heart the slit-like opening of one side is visible ; D alimentary canal ; G cerebral ganglion; L paired hepatic diverticulum ; paired eye; Sd shell plaiid. (After Claus.) prominences, which serve to close the brood pouch. It usually consists of three free segments, together with the terminal anal portion, which is beset with rows of hooks. The anal portion begins with two dorsal tactile setae and ends with two hooks or styles, representing the caudal fork of other Entomostraca (Fig. 251). The internal organization is simple in correspondence with the small size of the body. The compound eyes (0) fuse together in BRAXCHIOPODA. CLADOCERA. 379 the middle line to form a large, continually trembling, frontal eye, largely developed in the Polyphemidae. Beneath this the unpaired simple eye, reduced in Daphnia to a streak of pigment in connection with the brain, usually remains. A special sensory apparatus, whose function is not quite clear, appears in the region of the neck, in the form of an aggregation of ganglion cells. The heart has the form of an oval sac, with a pair of trans- verse lateral venous ostia and an anterior arterial opening. Its pulsations are rhythmic, and succeed one another quickly. In spite of the want of arteries and veins, the blood, which contains amoeboid cells, circulates along definite channels in the body. The looped and coiled shell gland (Sd) is always present. The cervical gland, which functions as an organ of attachment, is less widely distributed. The sexual glands lie in the thorax as paired tubes by the side of the alimentary canal. In the ovaries groups of four cells are separated ; in the formation of the summer-eggs one cell of each group, usually the third from the front end, becomes an ovum, while the rest are employed as nutritive cells for the nourishment of the ovum, which in- creases in size and absorbs fat globules. In the formation of the winter-eggs every other group of four cells, or a larger number, likewise breaks up and subserves the nourishment of the cells destined to become the eggs (Weismann). The ovary is directly continuous with the oviduct, which opens dorsally beneath the shell into the brood-pouch. The testes, like the ovaries, lie at the sides of the intestine and are continuous with the vasa- deferentia, which open to the exterior ventrally by a common opening behind the last pair of appendages or at the extreme end of the body, the openings being sometimes situated on small slightly protrusible prominences. The smaller males usually appear in the autumn ; they may, however, also be present at any other time of the year, and, as recent investigations have proved in a tolerably satisfactory manner, always when the conditions of life and nourishment are unfavourable (vide p. 373). Before the appearance of the males, hermaphrodite forms * are sometimes produced with an organization which is partly male and partly female. At the season when males are not present, normally in the * Compare especially \V. Kurz, Ueber androgyne Missbildung bei Cladoceren, Sitzungsber der Akad. der Wissensch. Wien, 1874. 380 CRUSTACEA ENTOMOSTRACA. spring and summer, the females produce the so-called summer eggs, which contain a large quantity of oil globules and are surrounded by a delicate vitelline membrane. They develop rapidly within the brood-pouch between the shell and the dorsal surface of the mother, and after the space of only a few days develop into a fresh generation of young C'ladocera, which escape from the brood-pouch. The embryonic develop- ment takes place accord- ingly under extremely favourable conditions, which depend upon the rich supply of food yolk in the large eggs, and sometimes upon addi- t iona 1 food material secreted within t h e brood-pouch. At the season when the males appear, the females, independently of copulation, begin to pro- duce so-called winter eggs, which are incap- able of developing with- out fertilization. The number of these winter eggs is always relatively small. They are distinguished from the summer eggs by their larger size and the greater quantity of food yolk, and their formation in the ovary is accompanied by much more extensive processes of absorption. Before the eggs pass into the brood-pouch, the walls of the latter, which as above stated are formed by the dorsal and pos- terior portions of the bivalve shell, become modified over a FIG. 252. Appendages of Daphnia (after Claus). a, anterior antenna of male ; b, maxilla ; c, first leg of female, c'.of male; d,a leg of the 2nd pair ; Br bract ; Ex exopodite. BRANCHIOPODA. CLADOCERA. 381 saddle-shaped area, the cuticle acquiring a firm consistency and brown colour. At the next moult after the eggs have entered the brood-pouch, they are contained in the shed cuticle, the bivalved thickened region or ephippium remaining as a protective case for the eggs after the rest of the cuticle has disintegrated. The Cladocera live for the most part in fresh water, and certain species inhabit deep inland lakes. Others live in brackish water and the sea. They swim quickly, and usually with a jumping movement. Some of them attach themselves to fixed objects by means of the dorsal " cervical gland " (p. 372). When the body is thus fixed, the swimming feet are able by their rhythmic movements to set up currents in which small food particles are swept towards the animal. Fam. 1. Sididae. Heart elongated. Gut straight. t> pairs of similar lamellate legs, with well developed epipodites. Latona Straus, Daphnella Baird, Penilia Dana, Limnosida Sars, Sida Straus, with large cervical adhering apparatus. Holopedium Zadd. , 2nd antennae unbranched. Fam. 2 and 3. Daphnidae and Lynceidae. Heart shortly oval, gut with a direct course in Daphnidae, coiled in Lynceidae ; 5 or pairs of limbs, the lamellar character and the branchial appendages becoming more developed from before backwards. (Daplmidae) llyocryptus Sars, Acantholeberis Lilljeb. ; Bosmina Baird, anterior antennae long, a sixth (rudimentary) pair of legs is present ; Drepanothrix Sars, Macrothrix Baird, Lathonura Lilljeb. ; Mo'ina Baird, anterior antennae large and prehensile in the $ of M. paradoxa : Scapholeberis Schodl., Ceriodaphnia Dana, Simocephalus Schodl. ; Daphnia Mull., the head shield not separated from the shell by a groove (Fig. 251). (Lynceidae) Eurycercus Baird, with 6 pairs of legs, Camptocercus Baird, Acroperus Baird, Alonopsis Sars, Alona Baird, Phrixura Mtill., Pleuroxus Baird, Chydorus Leach, Monopsilus Sars. Fam. 4. Polyphemidae. The shell does not enclose the body and legs as in the other families of the Cladocera, but is small and usually only contains the brood chamber. Head bluntly rounded, with very large compound eyes. Legs slender, distinctly jointed, branchial appendages rudimentary. Marine and freshwater. Podon Lilljeb. and Evadne Loven with 4 pairs of short legs crowded together. Abdomen rudi- mentary, covered by the shell. Polyphemus Mull, and Bythotrephes Lilljeb. The second antennae, four pairs of legs, and the abdomen are much elongated. The legs with rudimentary branchial appendages. Leptodora Lilljeb. The head and posterior part of the body elongated and the latter distinctly segmented. The 6 pairs of cylindrical \mbranched legs on the contrary are crowded together, the anterior pair being long. 1st antennae long and comb-like (sensory) in the J , 2nd antenna large, with stout basal joints. The summer eggs develop directly in the brood- pouch into the form of the adult, but the (fertilized) winter eggs hatch out as nauplius larvae. Fr. w. lakes of N. Europe. 382 CRUSTACEA ENTOMOSTRACA. Order 3. OSTRACODA.* Small, usually laterally compressed Entomostraca. with a bivalve shell and usually with seven pairs of appendages, ivhich serve as antennae, jaws, creeping and swimming legs. There is a large mandibular palp, and a short abdomen ending in a simple plate or a caudal fork. 16 1.2 13 15 11 FIG. 253. Stenocij-pris malculmsonii (Brady). ( = Cijpris cylindrica Baird). The right half of the shell has been removed. 1 median eye ; 2 anterior antenna ; 3 posterior antenna ; 4 mandible ; 5 mandibular palp ; 6 first maxilla ; 7 second maxilla ; 8 and 9 thoracic appendages; 10 caudal fork; 11 ovary; 12 advanced ova; 13 bundles of fibres of ad- ductor muscle; 14 upper lip; 15 stomach; the caecal diverticulum of the left side is seen in t.he fold of the shell below 11; 16 the leader points a little below the intestine. (After G. O. Sars.) The body of these small Crustacea is unsegmented and is completely enclosed in a bivalve shell, which gives the animal a resemblance to a mussel. The surface of the shell may be smooth, or variously sculptured, or beset with setae. In the * Besides Straus-Diirkheim, Fischer, Lilljeborg, Baird and others compare \V. Zenker, Ivlonog. d. Ostracoden, Arch. f. Naturgesch., XX, 1854. G. O. Sars, Oversigt af Norges marine Ostracoder, Vid. Selsk. Forh. Christiania, 1865. C. Glaus, Beit. z. Kenntniss d. Ostracoden. Ent- wicklungsgesch. v. Cypris, Marburg, 1868. Id. Die Halocypriden des atlantischen Oceans und Mittelmeeres, Wien, 1891. Id. Beitr. z. Kenntniss d. Siisswasserostracoden, I and II, Arb. Zool. Inst. Wien, 1892 and 1895. G. S. Brady, A monograph of the recent British Ostracoda, Trans. Linn. Soc., London, XXVI., 1868. Id. Ostracoda, Challenger Hep., 1, 1880. A. Kaufmann, Beitr. z. Kenntniss d. Cytheriden, Receuil Zool. Suisse, iii, 1886. O. Nordqvist, Beitr. z. Kenntniss d. inneren mannlicheii Gesch- lechtsorgane d. Cypriden, Act. Soc. Sc. Fenn, xv, Helsingfors, 1885. G. S. Brady and A. M. Norman, Monog. of the Marine and Fresh-water Ostracoda of the N. Atlantic and of N.W. Europe. 1. Podocopa. Trans. R. Dublin Soc. (2) vol. 4 (1889), p. 61. G. W. Mtiller, Die Ostracoden d. Golfes v. Neapel. (Naples Monograph No. 21), Berlin, 1894. Id. Deutsch- lands Siissvvasserostracodeix, Zooloyica. 1900. OSTRACODA. 383 Halocypridae it is richly supplied with glands. The two valves of the shell are joined together by an elastic ligament along the middle third of the back, and the action of this ligament is opposed by a two-headed adductor muscle, which passes from one valve of the shell to the other and causes impressions discernible from without (Fig. 253, 13 ). The tendon common to the two heads of the muscle lies nearly in the middle of the body. The edges of the valves are free at both ends and along the ventral side. In the free-swimming marine Cypridinidae (Fig. 254) and Halocypridae there is a deep indentation in the edges of the valves, to allow the antennae to pass out. When the valves of the shell are open, several pediform appendages can be protruded on the ventral side, which enable the animal to move in the water either by crawling or by swimming. The abdomen can also be protruded ; it either ends in a caudal fork (Cypris and CytJiere), or has the form of a plate armed with spines and hooks on its posterior margin (Cypridina). Appendages. Anterior antennae, uniramous. Posterior antennae, biramous. Mandibles, with a usually biramous palp. First maxillae, usually jaws (in Polycopidae short bira- mous legs). Second maxillae, jaws, maxillipeds, or legs. Sixth pair of appendages, jaws, or legs, or 0. Seventh pair of appendages, legs, or specially modified, or 0. Eighth pair of appendages, represented by the brush- shaped organs in the males of Podocopa, by the penis in Cypridinidae (Miiller). The two pairs of antennae are placed on the anterior region of the body (Figs. 253, 2 and 3, and 254, A', A"), and are used as creeping or swimming legs. In the Cypridinidae and Halocy- pridae the anterior pair is provided with large olfactory hairs. The second pair of antennae are generally the most important organs of locomotion. In the exclusively marine Cypridinidae and Halocy- pridae they have the form of biramous swimming feet, and consist of a broad triangular basal plate, a many-jointed exopodite beset with long swimming setae, and a reduced endopodite, which, however, is stronger in the male and furnished with hooks of a considerable size. The Polycopidae have the two rami approximately equal in size, while in the other families the endopodite is the principal ramus, the exopodite being reduced 384 CRUSTACEA ENTOMOSTRACA. or absent altogether (G. W. Miiller). In these (Fig. 253, 3) the appendages resemble legs, and end with strong hooked bristles, by help of whi?h the animal can attach itself to sur- rounding objects. In the region of the mouth, beneath and to the side of a toler- ably large upper lip, there are two mandibles usually with a broad and strongly toothed biting edge. In one division of the Ostracods the Myodocopa the mandibles have the form of legs rather than jaws. In the Cypridinidae even the masticatory process on the basal joint may disappear, or be represented by a small setose lobe (Fig. 254), while the palp has the form of a strong leg. In the other division, the Podocopa, the palp though still pediform has more moderate proportions, and may be biramous (Fig. 253, -5). In exceptional cases (Paradoxo- stoma), the mandibles are styliform and are enclosed in a suctorial proboscis formed from the upper and under lips. The appendages which follow the mandibles are very vari- ously modified in the different families of Ostracods in relation to the function of mastication, or to locomotion or to both combined. The fourth pair, or first maxillae, are jaws in all except the small family Polycopidae, where they have the form of short legs carrying a distinct outer ramus. In the Podocopa they carry a large comb-like setose plate which by its move- ment promotes the process of respiration, though it does not itself act as a gill (Fig. 253, 6). The second maxillae (fifth pair) are jaw-like in the Cypridinidae and carry an enormous respiratory plate (Fig. 254). In the Halocypridae and Cypridae they have an intermediate character between jaws and locomotory limbs, the basal joint being stout and carrying a respiratory fan (small in Cypris), and the endopodite forming a jointed appendage directed backwards from it and provided with stiff claw-like setae. In the Cytheridae the whole appendage is more slender and leg-like. In the Polycopidae, where this is the last pair of appendages, they are short and leg-like, and carry a respi- ratory fan. The appendages of the sixth pair are again jaw-like in the Cypridinidae, in the Cypridae and Cytheridae they are leg-like, while in the Halocypridae they have an intermediate character. The seventh pair are leg-like in the Cypridae and Cytheridae. In the former family they occupy a peculiar position (Fig. 253, 9), OSTRACODA. 385 being turned upwards and backwards over the hind part of the body and carry a brush of setae at the end. In the Cypridinidae A F" FIG. 254. Cyjiridina mediterraneu. a Female ; b mate. A', A" first and second antennae ; F', F" first and second thoracic legs; Fu caudal fork; G brain; H heart; M stom- ach; Mdf mandiliular palp; MJC' lirst maxilla; MX" second maxilla; O eye; O' un- paired eve; P copulatory organ; SM adductor muscle; Stz frontal organ; T testis. (After Clam.) (Fig. 254, F"} this pair of appendages is long, cylindrical and many -jointed ; they also carry a terminal brush of setae, and as z in c c 386 CRUSTACEA ENTOMOSTRACA. in the Cypridae are directed upwards over the hind part of the body and beneath the shell. In both families they probably have the function of keeping the hinder part of the interior of the shell clean (Putzfuss). In the Halocypridae these ap- pendages are represented by long slender flagella. A pair of short and stalked " brush-shaped organs '' with sensory functions, which are present in the males of the Podocopa, are regarded by Miiller as the representatives of an eighth pair of appendages. In the Cypridae and Cytherellidae they lie behind the seventh, but in other Podocopa they are situated in front of them. In the Cypridinidae they probably enter into the formation of the penis (Fig. 251&, P). The nervous system consists of a bilobed cerebral ganglion and a ventral chain with closely approximated pairs of ganglia. A suboesophageal mass supplies nerves to the mandibles and first maxillae. Sense organs. In addition to the olfactory hairs and " brush- shaped organs " already mentioned eyes are generally present. Except in the Halocypridae the unpaired or nauplius eye is present, consisting of a ventral median and two lateral elements which may be united in the middle line or, as in some Cope- pods, quite distinct. In the Cypridinidae (Fig. 254, 0) there is in addition a pair of compound lateral eyes consisting of a number (4-50 or more) of separate elements. The Halocypridae and Poly- copidae are without eyes. In the Cyprinidae a peculiar median, rod-shaped organ, the frontal process (Stz) pro- jects forward, in the neigh- bourhood of the nauplius FIG. 255. Alimentary canal and generative organs of a female Cypris (from Clans, after W. Zenker). ^yV. D intestine. The niMMiinn of the anus, which AHrYnr*oi'ir oonol Tl is not seen here, is dorsal to the caudal fork. Fn rtiniltJIlUiry Galiai. ovan-; f pF ; cpfT e :r ; ,su mouth, which is frequently (Cypris} armed with toothed lateral bands, leads through a narrow oesophagus into a dilated crop-like portion of the alimentary canal (Fig. 255), which in Bairdia is developed into a regular mastica- tory mill. This is followed by a broad and long stomach pro- OSTRACODA. 387 vided with two long lateral hepatic tubes (L), which may project between the lamellae of the shell. The anus opens near the base of the abdomen either dorsal or ventral to the caudal fork. A saccular heart is present in most Myodocopa on the dorsal surface, where the shell is connected with the body. It is absent in most if not all Podocopa. The function of respiration is performed by the inner lamella of the shell, as well as by the whole surface of the body, over which an uninterrupted current of water is maintained by the swinging movements of the fan-shaped setose plates. There are no true branchiae en the appendages, but in some Cypridinidae there is a double row of leaf-like branchial structures on the back, near the last pair of limbs. Two sets of glandular structures which appear to correspond with the excretory organs of other Entomostraca are described by Claus. An extensive gland, sending a process between the lamellae of the shell, has been traced into the base of the posterior antennae, though the opening was not detected. Smaller glands opening on the basal joint of the second maxillae correspond with the shell glands of other forms, though in the Ostracods they do not extend into the shell. The so-called Spinn- drusen of the Cytheridae which open on the elongated exopodite (vide infra, Cytheridae) of the second antenna belong, apparently, to the category of cutaneous glands. Generative organs. The sexes are always separate and are distinguished by well-marked differences in structure. The males possess appliances on different appendages in Cypridina on the second antennae, in Cypris on the maxilliped for holding the females ; or a pair of legs (the first thoracic) may be modified for this purpose (Halocypridae). In addition a large copulatory organ, often possessing a complicated structure, is always present. It is, Jiowever, not homologous in the several families, as will appear below. In the Cypridinidae the testes are simple rounded bodies and the vasa deferentia run directly to a median opening common to them both just in front of the anus. A pair of appendages situated on either side of the opening and probably representing the brush- shaped organs of some of the Podocopa is, in this family, modified as copulatory organs. In the Cypridae there are four elongated or rounded lobes of the testis on either side, and the vasa deferentia are connected by a canal passing from one to the other, which may be of great length and thrown "388 CRUSTACEA ENTOMOSTRACA. into complex folds. The part of the vas deferens nearer the opening has a chitinous wall, and at its commencement a peculiar ejaculatory apparatus is situated which in the fresh- water forms is a large and complex organ. This was formerly known as the " mucous gland." At its termination the vas deferens traverses the corresponding half of the (paired) penis, the end of it being protrusible. In Halocypridae and Poly- copidae the penis is traversed by the vas deferens but is single, and situated to one side of the middle line. The spermatozoa are very long in some species, in Pontocypris monstrosa they are, according to Miiller, 5-7 mm. in length, or 8-10 times as long as the body of the animal. The female of Cypris possesses two ovarian tubes which project between the la- mellae of the shell (cf. Fig. 253), two receptacula semiiiis, and the same number of genital openings at the base of the abdomen. As in the Copepoda the receptacula seminis are often provided with two ducts ; one by which the spermatozoa are introduced, and the other communicating with the oviduct. In addition to the sexual characters noted above differences in the shape of the shell, and the richer endowment of the male with sensory organs may be noted. Development. The greater number of Ostracoda lay eggs which they either attach to water-plants (Cypris) or, as in Cypridina, carry about with them between the shell valves until the young are hatched. Parthenogenesis has been recognized as occurring among the Cypridae by Weismann and by G. W. Miiller. According to Woltereck * it is, in some species, of the kind found in the Daphnidae, in which a number of partheno- genetic generations succeed one another during the summer, to be followed in the autumn by a sexual generation. In Cypris reptans however the sexual generation has never been seen. Cultures of this species have been under observation for eighteen years in the Freiburg laboratory, yet a male has never been recognized. The free development of Cypris consists of a complicated metamorphosis. The larvae, when hatched, possess, like nauplius larvae, only three pairs of appendages, but they are strongly compressed laterally, and are already enclosed in a thin * R. Woltereck Zur Bildung und Entwickelung d. Ostrakoclen Eies, Ze.it. f. wiss. Zool., Bd. 64 (1898), p. 596. OSTRACODA. 389> bivalve shell (Fig. 256). In the marine Ostracoda the develop- ment is simplified so that the metamorphosis is entirely absent. The Ostracoda feed partly on animal and partly on vegetable matter. Numerous fossil Ostracods are known from almost all formations, but only the remains of their shells are pre- served. They abounded in the Silurian seas and attained there a much larger A f size (90 mm.) than any known existing genera. The deep-sea expeditions of FIG. 256. very young larva of , ! Cypris (after Claus). jS T auplius recent years have however brought to stage,with three pairs of appen- i i j , , -i ,. . dages. A', A" first and second our knowledge the large free-swimming antennae ; D intestine ; M r\ T i ft stomach ; Ml mandible ; Lypridinid Gigantocypns (q.v.), which SM shell muscle. attains a length of 23 mm. Ostracods resemble one or other of the groups of Phyllopods in the following features --the bivalve shells closed by an adductor muscle ; the saccular heart, situated in front of the point of reflection of the shell ; the presence of compound eyes in the Cypridinidae ; the large setose plates attached to some of the postoral appendages ; the large upper lip of the nauplius larva and the retarded development of the second maxillae. On the other hand the highly specialized condition of the mouth parts of the adult Phyllopoda the absence of the mandibular palp and the reduced maxillae finds no parallel in the Ostracods, for in these the mandibular palp is more leg-like in character than in any other group of Crustacea, and the same is true, though in a less degree, of both maxillae. In association with these apparently primitive features in the Ostracods, we may bear in mind the leg-like character of the second anten- nae, and in the Myodocopa, of the mandibles, though these may very possibly be adaptive features. The presence of a bivalve shell in the nauplius larva is another feature in which the two groups stand contrasted. Sub-order 1. MYODOCOPA.* The shell is notched in front to give play to the antennae. The basal joint of the second antenna is usually wide, and the * The name Myodocopa (uwi<5??s muscular, KWTTTJ an oar) is given by Sars in allusion to the expanded muscular base of the second antenna, which 390 CRUSTACEA ENTOMOSTRACA. many jointed exopodite is the longer of the two rami. Tlie mandibular palp is long and pediform. A respiratory plate is absent from the first maxilla, but present on the second. The halves of the caudal fork are plates bearing spines along their margins, gradually increasing in size towards the terminal extremities of the plates. Fam. 1. Cypridinidae. Shell hard, the notch for the antennae situated at or below the middle of the dorso-ventral extent of the shell. First antenna strongly flexed at the articulation between the basal joint and the next. Mandible without a regular masticatory process, this being represented by a small lobed setose plate connected with the basal joint. The large 4-jointed pediform palp bears a small exopodite. The three following appendages are all short and more or less jaw-like and aggregated about the mouth region. The 7th is a many-jointed worm- like structure, the end beset with bristles. Generative orifice single and median. In the male the copulatory organ consists of a pair of structures probably representing a pair of appendages, which arise on either side of the orifice. A heart, two large compound eyes, together with a median eye and rod-like " frontal organ," are present. GypridinaM.. Edw. (Fig. 254), some spp. phosphorescent. Pyrocypris Mull., C rosso pliorns Brady, Gigantocypris Mull., G. Agassizii, obtained by U.S. ss. " Albatross " and since by the " Valdivia," is probably a free-swimming inhabitant of deep water. It is nearly globular in shape, orange in colour, and pro- vided with a pair of mother-of-pearl like plates over the eyes. It attains a length of 23 mm. Philomedes Lilljeb., Pseudophilomedes Miill., Cylin- droleberis Brady, Sarsiella Norm. Fam. 2. Haloeypridae. Shell calcified ; the notch for the antennae situated above the middle of the dorso-ventral extent. Numerous uni- cellular glands, opening on the surface, are present along the margins of the shell. Mandible with well-developed masticatory process. 1st max- illae jaw -like ; 5th pair of appendages (mx. 2) with a backwardly directed endopodite, and a respiratory fan on the protopodite ; the sixth are leg-like ; while the seventh are slender, dorsally directed structures bearing a few bristles. The vasa deferentia unite in a duct traversing the penis which lies on the right of the middle line. A heart and " frontal organ " are present, but eyes are absent. Pelagic. Thaumatocypris Miill., Conchoecia Dana, Halocypris Dana, Euconchoecia Miill., Archiconchoecia Miill. Fam. 3. Polycopidae. The halves of the shell are round or oval, and the antennary notch shallow or absent. Second antennae with a comparatively narrow basal joint, and approximately equal rami. Mandible with a weak masticatory process. The fourth pair of appendages (mx. 1 ) are not masticatory and bear a well-developed exopodite ; the fifth (which is also the last pair) are short and carry a large respiratory fan. The penis is markedly unsymmetrical. Heart, eyes and frontal organ are absent. Minute Ostracods living at the bottom of the sea. Polycope Sars, Poly- copsis Miill. is here a swimming organ and, as in some other Entomostraca, one of the principal organs of locomotion. In the Podocopa these appendages are leg-like. OSTRACODA. 391 Sub-order 2. PODOCOPA.* The shell is not notched in front, and, in accordance with the creeping habit, is flattened along the ventral border. The second antenna articulates with a prominence at the side of the upper lip. The endopodite forms the strong leg-like ter- mination of this appendage, while the exopodite is short, and generally much reduced. The mandible has a short palp which may be biramous (Fig. 253). Fam. 4. Cypridae. Shell generally not sculptured. The eiiclopodite of the second antenna is strongly flexed on the basal joint and terminates in strong claw-like setae ; the exopodite is reduced to a small scale. The fourth pair of appendages (1st maxillae) is jaw-like, having masticatory processes from the basal joint, a short palp and a large respiratory fan (epipodite). The next pair has a masticatory process and a respiratory fan on the basal joint, a short exopodite directed forwards and a longer claw-like endopodite directed backwards. The latter is modified as a prehensile organ in the male. The sixth pair is long and pediform : the seventh, is also pediform but slender and directed dorsally. The halves of the caudal fork slender and rod-like, bearing setae at their ends, or reduced. There is no heart. The testes and ovaries are partly contained between the lamellae of the shells. In. the males a peculiar ejaculatory apparatus formerly known as the " mucous gland," invests the vas deferens near its termination. The greater number of species live in fresh water. Notodromas Lilljeb., with the three parts of the median eye widely separated, Cypris Miill., Stenocypris Sars (Fig. 253), Pontocypris Sars, Pontocypria Miill., Ilyodromus Sars, An/illoecia Sars, Paracypris Sars, Gandona Baird, Macrocypris Brady, Aglaia Brady, Goniocypris Brndy, Metacypris Brady, Nesidea Costa. Fam. 5. Bairdiidae. A small group of marine Podocopa with char- acters intermediate between those of the Cypridae and Cytheridae. The shell is frequently much arched dorsally and the two halves are unsym- metrical. The crop forms a well-developed masticatory mill. Bairdia Mi-Coy, Bythocypris Brady, Anchistrocheles Brady and Norman. Fam. <>. Cytheridae. Shell strongly calcified. Second antenna like that of the Cypridae, but the exopodite is an elongated and slender organ, bent at the tip, and perforated by the duct of the large unicellular "spinning-gland," the secretion of which is, according to Miiller, spread as a web of fibres over surrounding objects. The fourth pair of appendages (nix. 1) has a masticatory function, but the three following are exclusively locomotory. Divisions of the caudal fork small and weak. There is no heart and neither the testes nor ovaries extend between the lamellae of the shell. All the species are marine. Cythere O. F. Miill., Encijthere Brady, Cythereis Sars, Cytherideis Jones, Cytheretta G. W. Miill., Micro- cytherura G. W. Miill., Cyprideis Jones, Krithe Brady, Crosskey and Robertson, Cytheropteron Sars, Loxoconcha Sars, Pseudoloxoconcha G. W. Miill., Cytheroma G. W. Miill., Xestoleberis Sars, Microxestoleberis G. W. Miill., Paracytheridea G. W. Miill., Polycheles Brady, Cytherura Sars, * See footnote on p. 389. 392 CRUSTACEA ENTOMOSTRACA. Eucyiherura G. W. Miill., Sclerochilus Sars ; Cytherois G. W. Miill. , the members of this and the two following genera have suctorial mouth parts and live on the juices of water-plants. Paradoxostoma Fisch., Para- cytherois G. W. Miill. ; Microcythere G. W. Miill., Bythocythere Sars, Pseudocy there Sars, Paracythere G. W. Miill., Jonesia Brady, Normania Brady, Limnicythere Brady. Fam. 7. Cytherellidae. This family, consisting of the single marine genus Cytherella Bosq. apparently represents a primitive form of the Podocopa. The regularly arranged thickened bands in the hinder part of the body give rise to a suggestion of segmentation, but this appears not to correspond with the segmentation indicated by the limbs. The outer ramus of the 2nd antenna is larger than in other Podocopa, and the brush-shaped organs of the male, which probably represent an eighth pair of appendages, are situated behind the last pair of limbs. The divisions of the caudal fork are lamellar and fringed with stout bristles. Fam. 8. Darwtnulidae. This consists of a single fresh-water species Darii'inula stevensoni Brady and Robertson. The anatomy is not com- pletely known, but the species appears to lie allied to the Cypridae. Order 4. COPEPODA.* Entomostraca with elongated, usually ivell segmented body, without shell-forming reduplications of the skin, and ivith biramous swim- ming feet. f The abdomen is without appendages, and the eggs are usually carried by the female in single or paired sacks attached to the anterior abdominal segment. The order consists of a very large number of species of small active free-swimming Crustacea, and of a variety of forms which have to a large extent lost their power of free locomotion and live as external parasites on other animals. While many of the latter present no great departures from the type of structure met with in the free-living groups, in others the Copepod and * 0. Fr. Miiller, Entomostraca sen Insecta testacea, quae in aquis Daniae et Norvegiae reperit, descripsit, Lipsiae, 1785. Jurine, Histoire des Monocles, Geneve, 1820. W. Lilljeborg, De crustaceis ex ordinibus tribus : Clado- cera, Ostracoda et Copepoda, in Scania occurrentibus, Lund, 1853. C. Claus, Die freilebenden Copepoden, Leipzig, 1863. C. Grobben, Die Entwickelungsgeschichte von Cetochilus septentrionalis, Arb. des Zool. Instituts, Wien., Tom 3, 1881. C. Claus, Ueber die Maxillarfiisse der Copepoden etc., ibid. 11, Heft. 1 (1895). M. H. Hartog, The Morphology of Cyclops, etc., Trans. Linn. Soc. Vol. V. pt. I. p. 1 (1888). W. Giesbrecht, Die pelagischen Copepoden etc., Fauna and Flora des Golfes von Neapel, 1892. Idem., Asterocheridae, ibid., Bd. 25, 1899. Giard, Sur le parasitisme des Monstrillidae, Comptes Rendus, T. 123 (1896), p. 836. Malaquin, Le parasitisme evolutif des Monstrillidae, Arch. Zool. exp. (3), T. 9, p. 81. Hansen, Choniostomatidae , Copenhagen, 1897. t The name Copepoda has reference to the oar-like character of the divisions of the swimming feet in the free-living forms. COPEPODA. 393 f indeed the Arthropod char- acter is entirely masked in the females, the bodies pre- senting an ungainly and monstrous appearance with little indication of limbs 01- of segments. The males of these species are however of more normal structure and the affinities are always clearly indicated in the characters of the young. The free-swimming forms superficially resemble the macrurous Decapod Crus- tacea in the general contour of their body (Fig. 257). This consists of a more or less oval anterior portion, including the cephalothorax and a number (3-5) of movable thoracic segments, bearing swimming feet ; and of a narrower posterior por- tion, the abdomen, which is without appendages. It ends in a caudal fork, consisting of two lateral processes, beset with setae. The first abdominal segment carries the generative orifices. The hind part of the body is generally divided off by a constriction which in some forms (Gymnoplea) lies behind the last (5th) thoracic segment, but in others (Cyclopidae, Harpactidae) in front of it. The region be- hind the constriction is called by some authors the urosome. In the latter case the last thoracic segment is included in the nrosome. In the former the urosome is identical with the abdomen. The cephalothorax and the anterior free thoracic segments are produced laterally, in Cyclops, into low pleural folds (Har- tog). In female Notodelphyidae (q.v.) a dorsal brood sack is formed in the thoracic region. In the free-swimming forms the number of segments of the body, as indicated by paired appendages or by the jointing of FIG. 257. Female of Cyclops cumitutus seen from the dorsal surface (after Clans). A', A" the anterior and posterior antennae ; D alimen- tary canal ; OrS egg sacks. 394 CRUSTACEA ENTOMOSTRACA. the cuticular skeleton, or both, is fairly constant, though depar- tures from the usual number are frequently met with, especially in the abdomen, and the number may differ in the sexes of a single species. The cephalothorax terminates in front in a ventrally directed pointed or forked rostrum. It bears two pairs of antennae, mandibles, two pairs of maxillae and a pair of maxillipeds. Pos- teriorly it may be distinct (some Calani d a e ) f r o m the second tho- Fio. 258. An anU-rinr antenna of the male Cyclnps serrulatua (after Claus). Sf olfactory organs, M muscles. racic seg- ment (Fig. 260), but generally two a n - terior tho- racic segments are fused with the head. Behind the second come four other tho- racic segments, bearing swimming feet, but the two last may be fused together. In the females of many Calanidae the sixth thoracic feet are absent, and in the ( 'yclopidae and Harpactidae they are rudimentary, the segment to which they belong being included, as we have seen, in the urosome. From the early investigation of Claus into the anatomy and development of members of the Cyclopidae it was concluded that the two pairs of appendages following the first maxillae are not two pairs but the inner and outer divisions of a single pair. It was pointed out that though in the majority of Copepods they are inserted separately on the body, in Cyclops their bases are actually united (Fig. -Jo!)). This <;. -!59. Mouth parts of Cy- riops. Kj' second maxilla ; Kf" maxilliped (first tho- racic appendage) ; 31 man- dible ; MX first maxilla (after Clans). COPEPODA. 395 view was resisted by Hansen and by Giesbrecht, and was finally aban- doned by Glaus himself.* It has been shown that in the larval history of members of the Cala- nidae (Fig. 263) these two pairs of appendages are not only separate from their origin, but that a segmental furrow of the body lies between them which is, in fact, the division between head and thorax and that there are distinct ganglia correspond- ing to them. It is clear there- fore that the approximation of these appendages found in Cy- clops and its allies is a depar- ture from the more generalized condition found in the Calan- idae, a conclusion which is borne out by a general view of their relationship. We are now, therefore, able to name the anterior pair (the inner in Cyclops) second maxilla, while the posterior (outer in Cyclops) or "hand" of Juririe, must be regarded as the first thoracic appendage, modified to carry food to the mouth, and hence to be named the maxilliped. The abdomen is gener- Mn.P. - Th ally five-jointed in males FIG. 260. Ventral aspect of the anterior part of the body of Calamis finmarcJiicus L. A' first an- tenna ; A* second antenna ; me median eye ; Mii.p mandiliular palp ; jl/:r' first maxilhi ; .!/./" second maxilla ; Mz.p maxilliped : T/i- liases of the first pair of swimming feet, united by " coupler " : n.l. upper lip. (After Sars.) and four-j o i n t e d in females. The first seg- ment carries the openings of the generative ducts, and the anus is situated on the last segment, in the angle between the two divisions of the caudal fork. The abdomen, especially in the parasitic forms, very frequently undergoes a considerable reduction. Appendages. Anterior antennae, uniramous. Posterior antennae, xmi- or biramous. Mandibles, generally with a well-developed and sometimes biramous palp. First maxillae. Second maxillae. Maxillipeds or 1st thoracic appendages, uniramous. Second thoracic appendages (biramous swimming 1<'K S )- Third Fourth Fifth Sixth The anterior antennae arising on either side of the base of the * Arb. aus. d. zool. Inst. Wien, xi, 1. 396 CRUSTACEA ENTOMOSTRACA. rostrum are usually many-jointed (25 in Gymnoplea) and, by their powerful strokes, propel the animal by leaps through the water. They are beset with olfactory structures, and in the male one or both may be modified as a prehensile organ for catching and holding the female (Fig. 258). They are never biramous. The posterior antennae, which are always the shorter, are in some families biramous (Fig. 260). In the parasitic Lernaeidae and their allies, they serve as organs of attach- ment to the host (Fig. 266). The mandibles, in the free-living forms have a toothed blade (basal joint) projecting inwards on either side into the mouth, and a simple or biramous (Fig. 260) palp, which however in Cyclops is reduced to little more than a group of bristles (Fig. 259). An upper lip projects in front of the mouth and a bilobed lower lip may be present behind it. In the parasitic forms the lips are produced and applied together to form a conical suctorial appara- tus within which the palpless, styliform and piercing mandibles are contained (Lernaeopodidae and Caligidae). In Philichtliys and its allies only the upper lip is produced. The first maxillae lie behind and external to the mandibles, and are in the Calanidae lobed structures resembling a Phyllopod appendage (cf . Fig. 260). In Cyclops they are more jaw-like (Fig. 259), and in the parasitic forms are much reduced, and situated on the outer sur- face of the sucking tube. The second maxillae and the maxillipeds are uniramous. In the free forms the maxillipeds are generally the longer and the terminal portion can be folded on the basal joints (the " hand " of Jurine). They are especially modified as prehensile organs in the males of the Corycaeidae in which group the anterior antennae are not adapted in this manner. In the females of the parasitic Lernaeopodidae they are generally long and arm-like, ending in a horny disc, common to the pair, by which the animal is attached to the host (Fig. 264, c). The swimming feet consist of a two- jointed basal portion, and two three-jointed, setigerous, flattened rami (Fig. 261). The coxopodites of these appendages are often intimately con- nected by a median plate, or " coupler,' 1 regarded by Hartog as a prolongation downward of the sternite of the segment (Fig. 260). In the parasitic groups the swimming feet are much reduced, and in some entirely absent. In several free forms while the anterior swimming feet are well developed COPEPODA. the fifth pair is much reduced (Cyclops) or absent. In many Calanidae though absent in the female, it is in the male modified, unsymmetrically, as a pair of copulatory organs. It will be noticed that when five pairs of swimming feet are present, the number of the appendages corresponds with that found in the Cirriped.es. The nervous System consists in the Calanidae of a supra- oesophageal gan- glion, supplying the eyes and anterior antennae, a circumoesophageal ring giving off nerves to the second antennae, and a chain of seven or fewer median ganglia be- hind the oesophagus, of which the two anterior supply the mandibles, maxillae and maxillipeds. In the Corycaeidae Ex ' W\M\\ H \t.bM\\\ En however and in the parasitic forms the ganglia are concentrated into a ring round the oesophagus from which a single j i i j . i-i T FIG. 261. A swimming foot or double cord, in which ganglion cells are of Cyclops. En endopo- , 11 i dite ; Ex exopodite. present, is continued backwards. Sense organs. A median frontal eye (nauplius or Cyclops eye) is present (Figs. 257 and 260), though lost in the later stages of parasitic forms. It is divided into three parts, paired dorsal and median ventral, and the former are provided with cuticular lenses. In the pelagic Pontellidae the median eye is highly developed and provided with deep and cuticular lenses. The lateral divisions of it are also largely developed in the Corycaeidae. In the Branchiura alone among the Copepoda compound lateral eyes are present (Fig. 267) in addition to the median eye. Delicate olfactory hairs and rods are present on the anterior antennae, principally in the male (Fig. 258). Phosphorescent organs. Some pelagic Copepods (some Pontel- lidae and Oncaea) emit bright sparkles of greenish or bluish light, produced by the contact of the secretion of skin-glands with the water. These are present also in the larvae, and the arrangement of the glands and the colour of the light is charac- teristic of the species.* * Giesbrecht, Mittli. lib. Copepoclen 8. Ueb. d. Leuchten cl. pelagischen Copepoden, Naples, Mitth., Bd. 11, p. 648 (1895). 398 CRUSTACEA ENTOMOSTRACA. The alimentary canal is divided into a short narrow oeso- phagus, a wide stomach often with two anterior diverticula, which are sometimes ramified, and an intestine opening in the angle of the caudal fork. Hepatic cells lie in the walls of the stomach, and posterior to these is a tract the cells of which contain urinary secretions. The stomodaeal cuticle only extends over the anterior one-third of the stomach (Hartog). Excretory organs. The antennal glands which open at the base of the 2nd antenna of the nauplius are only present in the larva of these as of most other Entomostraca. Glands which also have an excretory function have been found in some genera at the sides of the anterior part of the cephalothorax, and in the Eucopepoda they have been found to open, as usual, on the 2nd maxillae (Richard). A saccular heart is present in the Calanidae and Pontellidae, but generally a heart is absent, the movement of the blood being effected by the regular oscillations of the intestine. There are no special respiratory organs (unless the lateral pro- cesses of the hind part of the body in the parasitic Pennella are to be so regarded). Generative organs. The Copepoda are of separate sexes. In both the generative organs lie in the cephalothorax and the free thoracic segments and open (except in the Choniostomatidae, q.v.) right and left in the basal segment of the abdomen. The males as a rule are smaller and more active than the females. Sexual differences in the form and structure of different parts of the body, such as the modifications in the males of the anterior antennae, maxillipeds or sixth thoracic appendages have been already noted. In the free-swimming genus Copilia (Corycaeidae) the body is cylindrical in the female, flattened in the male. .But sexual dimorphism of a different nature may also be present to a marked degree. Some species among the pelagic families are beset with setae of such elaborate development and brilliancy of colouring that it appears impossible to regard them as other than ornamental (vide Plates I-IV in Giesbivcht's Pelagischen Copepoden des Golfes von Neapel, Fauna and Flora, vol. xix.). What makes the occurrence of such elaborate ornamentation in this group COPEPODA. 399 more remarkable is, besides the minute size of the animals, the fact that the structure of the eyes is so simple that it appears inconceivable that their owners should be in any degree aware of the ornamentation presented by their fellows. In some species both sexes are similarly ornamented (e.g. Augaptilus filigerus, Claus), in others the male alone is brilliantly coloured (Sapphirina), but in several of the most marked instances it is the female which is conspicuously decorated (Calocalanus, Copilia) while the males are comparatively plain. Some details of the ornamentation are given below under the separate families. In contrast with this kind of dimorphism is that found among the parasitic Copepods. While in some families the sexual differences are slight, in Lernaea (as the result of fertilization) and in the Chondracanthidae and Lernaeopodidae (indepen- dently of that event, Fig. 265) the female grows to a large size, acquiring a monstrous, distorted appearance in which its Copepod characters are completely masked, while the minute males, dwarfs in comparison, retain more of their original characters and frequently live, attached by their hooked appendages to the body of the female. The testes and vasa defer entia may be single or paired. The spermatozoa are elongated bodies twisted about their long axis. They are contained in spermatophores, which are formed in the lower part of the vas defereiis. During copulation, which is only an external approximation of the two sexes, the male fastens one or more spermatophores on the genital segment of the female. The ovary, like the testis, may be single or paired, but the oviduct is always paired. A cement gland is generally present at its termination which also performs the office of a spermatheca where this organ is not separately developed. A separate duct, to the orifice of which the spermatophore is at- tached by the male, leads into the spermatheca (or cement gland). Through this the spermatozoa are injected by the expansion (owing to the inhibition of water) of a fluid contained in the spermatophore. The eggs are fertilized in the lower end of the oviduct, and a mass of them becomes enveloped in the secretion of the cement glands, which hardens in the water. The egg- sacks so produced are single or paired, and may be oval or 1 fili- form (Figs. 257 and 266, d). In some cases however the eggs are laid singly, and in the Nbtodelphyidae they develop in the 400 CRUSTACEA ENTOMOSTRACA. dilated oviducts (uteri) contained in a dorsal expansion of the posterior thoracic region. In Cyclops one fertilization suffices for many broods (Hartog). Firm-shelled eggs are produced by Diaplomus towards the end of the reproductive period, and pass through a latent stage before hatching (Wolf). Development takes place by means of a complicated meta- morphosis, which, in many parasitic forms, is a retrograde one. The larvae have, when hatched, the nauplius form, with an FIG. 262. Metamorphosis of Cyclops, a Xauplius larva of Cyclops serrulatus, after hatching ; b older stage, more highly magnified ; c very young Cyclops form. A anus ; A', A" first and second antennae ; AD rectum ; D intestine ; F',F" first and second swimming feet (second and third thoracic appendages) ; G rudimentary gonads : He urinary concretions ; Mil, Mf mandible ; MX first maxilla ; ifa;/ maxilliped (partially hiding the second maxilla) ; 01 upper lip ; SD antennal gland. (From Claus.) impaired frontal eye and three pairs of appendages. Hooked setae on the second and third pairs of appendages serve to con- duct the food into the mouth, which is covered by a large upper lip (Figs. 242 and 262, a). The posterior region of the body is destitute of appendages, and terminates with two setae at the sides of the anus ; it includes the thorax and abdomen, which are as yet undifferentiated. The antenna ry glands act as excretory organs. The alterations undergone by the larvae in the course of COPEPODA. 401 their further growth are connected with a number of suc- cessive moults, and consist principally in an elongation of the body and the appearance of fresh appendages. In the next larval stage (metanauplius, Fig. 262, b), a fourth pair of appendages, the future first maxillae, makes its ap- pearance behind the three original pairs, which develop into the an- tennae and mandibles. In a later stage four fresh pairs of appendages are formed (Fig. 263). Of these the first and second correspond to the second maxillae and maxillipeds, while the third and fourth represent the first rudiments of the an- terior swimming feet. The functional limbs still resemble those of a nauplius, and it is after another moult that the transformation into the first Cyclops-like form occurs. It then resembles the adult animal in the structure of the an- tennae and mouth parts, although the number of the appendages and of the body rings is still incomplete (Fig. 262, c). The last two pah's of appendages already have the form of short biramous swimming feet, and the rudiments of the third and FJ( J. 263. Metanaupiius of fourth pairs of swimming feet have made their appearance as projections beset with tlree n an^oY Swimming setae.* The body consists in this stage of ^ SS^e^nTi the oval cephalothorax ; of the second, third ^-"^'^mandibiet" and fourth thoranin qpcrmpnt anrl nf an Mz', J/*" first and second Segments , ana OI an maxiUae ; Mx.p maxilli- elongated terminal portion, which gives dagej aiS After r aauf Ppen " rise to the last thoracic segment and to all the abdominal segments by progressive segmentation. It already terminates in the caudal fork. Many forms of parasitic Copepoda, for example Lernanthropus and Chondracanthus (Fig. 265), do not get beyond this stage of body segmentation, and obtain neither the swimming feet of the * Hartog finds that the setae are formed inverted, and that they become everted when a larval skin is shed. Z III D D 402 CRUSTACEA ENTOMOSTRACA. third and fourth pairs, nor a fifth thoracic segment distinct from the stump-like abdomen ; others, for example Aclithcres, by the loss of the two anterior pairs of swimming feet, sink back to a still lower stage (Fig. 264). All the non-parasitic and many of the parasitic Copepoda pass in successive moults through a larger or smaller number of developmental stages, in which the still undeveloped segments and appendages make their appearance, and the appendages already present undergo further segmentation. Many parasitic Copepoda, however, pass over the later series of nauplius forms, Mxf FIG. 264. Achtkeres percarum. a Nauplius form. 6 Larva in the youngest Cyclops stage ; Kf second maxillae and Kf" maxillipeds ; c Female seen from the ventral side ; Ov, ovaries ; KD cement glands ; d The smaller male seen from the side ; M :>/'. A?:r/" second maxillae and maxillipeds. From Claus. and the larva, as soon as hatched, undergoes a moult, and appears at once in the youngest Cyclops form, with antennae adapted for adhering and mouth parts for piercing (Fig. 264). From this stage they undergo a retrogressive metamorphosis, in which they become attached to a host, lose more or less completely the segmentation of the body which grows irregular in shape, cast off their swimming feet, and even lose the eye, which was originally present (Lernaeopoda). The males, however, in such cases often remain small and dwarfed, and adhere firmly (fre- quently more than one) to the body of the female in the region of the genital opening (Fig. 265). COPEPODA. 403 In Lernaea (Fig. 266) such pigmy males were for a long time vainly sought for upon the peculiarly shaped body of the large female (Fig. 266, c, d) which carries egg tubes. At last it was discovered that the small Cyclops-like males (Fig. 266, a) lead an independent life and swim about freely by means of their four pairs of swimming feet, that the females (Fig. 266, b), in the copulatory stage resemble the males, and that it is only after copula- tion that they (the females) become parasitic and under- go the considerable increase in size and modification of form which characterizes the female with egg-tubes. Monstrilla and its allies pass through a very re- markable life-history. The adult forms are pelagic animals, destitute of alimen- tary canal, mouth parts and second antennae. The nauplius attaches itself to the bodies of sedentary Poly- chaetes, and casting off its appendages penetrates into the vascular system of the host, in which all further stages 'in the development supervene, though without casting a skin (Malaquin*). Nourishment is taken in by means of the second antennae and Sp FIG. 265. The two sexual animals of Chon- dracanthus gibbosus magnified about six diameters, a Female seen from the side ; b from the ventral surface with adhering males : c male strongly magnified. A eye ; An' anterior antennae ; An" posterior antennae (for attachment) ; D intestine ; f, F" the two pairs of thoracic feet. M mouth parts ; Oe oesophagus ; Ov egg-tubes ; Sp spermato- phore ; T testis ; Vd vas deferens. From Claus. * Malaquin, Le parasitisme evolutif des Monstrillides, Arch. Zool. exp. 3, T. 9, p. 81. 404 CRUSTACEA ENTOMOSTRACA. sometimes by the mandibles as well, which reappear and grow out into long tentacular appendages, comparable in their function with the root-like appendages of the Cirripede Rhizo- cephala. Having attained the adult stage the animals throw off these appendages, and leave the body of the host to assume their brief pelagic existence, which comes to an end after the maturation and discharge of the sexual products. Sub-order 1. EUCOPEPODA. Copepoda ivith masticatory or suctorial mouth parts, and sivim- ming feet the branches of which are two- or three-jointed. The seventeen families into which the Eucopepoda have been divided are arranged (following Gerstaecker) in six series. The first consists of free-swimming forms ; the fourth, fifth and sixth of parasitic forms. In the second and third series while some members are free-living, others are adapted to a commensal or semiparasitic mode of life. A much larger number of families is recognized by more recent writers, but no complete classification of the sub-order is at present available. The first two families, the Pontellidae and Calanidae, are included by Giesbrecht in his group Gymnoplea, which is characterized by the fol- lowing features. They are free and powerfully swimming pelagic Copepods with long and many-jointed (24-25) anterior antennae, and biramous pos- terior antennae. 5th and 6th thoracic segments closely united and separ- ate from the urosome, and in the male the appendage of the 6th modi- fied, often unsymmetrically, as copulatory organs ; urosome, 5-jointed in male ; male organs unsymmetrical ; heart usually present : the eggs are laid separately or carried by the female in a single egg-sack until they are hatched. First series (Fams. 1-4). Fam. 1. Calanidae. (Amphaskandria of Giesbrecht). The anterior antennae of the male are nearly or quite symmetrical, and more abun- dantly beset with sensory organs than in the female. In the female the 5th pair of swimming feet may be reduced or entirely absent, and the 4th and 5th segments are generally fused. Calanus Leach, C. finmarchicus Gunner (17l>-~>). abundant in X. Atlantic (Fig. 260). Eucalanus Dana, Rhincalanus Dana. MecynoceraThomp., Para- calanus Boeck, Acrocalanus Giesb., Calocalanus Giesb., C. pavo Dana ; in the female (iv. 15)* the anterior antennae bear long bristles, of which three * These numbers refer to the plates and figures in Giesbrecht's Mono- graph on Pelagic Copepods, Fauna and Flora <>f tin- (inlf of Xaples, vol. 19. COPEPODA. 405 at the base are feathery and have a yellow metallic lustre while the ter- minal one is scarlet. The two divisions of the caudal fork also bear four fan-like feathered appendages almost as long as the body and having a metallic orange colour. The male (I. 13)* is much less strikingly ornamented. The anterior antennae bear very numerous sensory vesicles, and are scarlet at the ends, and the posterior antennae, maxillipeds and the caudal fork bear long narrowly feathered scarlet bristles. In the female of C. /i/iiinulosits Claus (III. 5) * the anterior antennae and caudal fork bear feathered bristles somewhat similar to those of C. pavo, but one of those on the left side of the caudal fork is produced to a length nearly six times that of the body (5'8 mm.) and beset with orange coloured and clubbed setae, forming a delicate flexible appendage of extraordinary beauty. Clausocalanus Giesb., Ctenocalanus Giesb., Pseudocalanus Boeck, Drepano- pus Brady, Mobianus Giesb., Spinocalanus Giesb., Aetidius Brady, Gaetanus Giesb., Chiridius Giesb., Undeuchaeta Giesb., EucMrella Giesb., Euchaeta Philippi, Scolecithrix Brady, Xanthocalanus Giesb., Phaenna Claus. Fam. -2. Pontellidae (Heterarthrandria of Giesbrecht). One of the anterior antennae of the male, generally the right, modified as a pre- hensile organ. The 5th pair of swimming feet of the female may be reduced but is never absent. The eyes are often large, sometimes with dorsal and ventral cuticular lenses. The last thoracic and anterior abdominal segments of the male are often unsymmetrical. Centropages Kroyer, Isias Boeck, Temora Baird, Pleuromma Claus, Leuckartia Claus, Isochaeta Giesb., Disseta Giesb., Heterochaeta Claus, Hemicalanus Claus, Augaptilus Giesb., Phyllopus Brady, Candace Dana, Calanopia Dana, Labidocera Lubbock. Pontella Dana, with one pair of eyes provided with cuticular lenses, on the dorsal surface of the head, and additional lenses on the anterior and posterior surfaces of the rostrum. The median ventral element of the eye is also present forming a projection on the ventral surface behind the rostrum. Anomalocera Templeton, with each of the lateral elements of the eye double. Monops Lubbock, Pontellina Dana, Acartia Dana, Corynura Brady. Fam. 3. Harpacticidae. Free-swimming. Body linear, cylindrical, completely segmented. Last thoracic segment included in the urosome. Both anterior antennae of the male modified as prehensile organs. Posterior antennae usually biramous and with bent setae. Mandibular palp and first maxilla biramous and short. Maxillipeds ending in a hook. The anterior pair of swimming feet may be modified as an additional pair of maxillipeds, the 5th pair leaflike and alike in both sexes. Heart absent. Eye median, simple. Openings of oviducts near the ventral middle line, and egg sack generally median. Amymone Claus, Euterpe Claus, Tachidius Lilljeb., Longipedia Claus, Cantliocamptus Westw. Dactylopus Claus, Thalestris Claus, Laophonte Phil., Lillje- borgia Claus, Jurinia Claus, Harpacticus O. F. Mull., H. chelifer, North Sea. Tisbe Lilljeb., Westwoodia Claus, Setella Dana, pelagic. Clytemnestra Dana, Ectinosoma Boeck, Sunaristes Hesse, Amenophia Boeck, Stenhelia Boeck, Ameira Boeck, Nitocra Boeck, Mesochra Boeck, Miracia Dana, Metis Phil., Aenippe Phil., Clausia Clap., Euryte Phil., Idomene Phil. These numbers refer to the plates and figures in Giesbrecht's Mono- graph on Pelagic Copepods, Fauna and Flora of the Gulf of Naples, vol. 19. 406 CRUSTACEA ENTOMOSTRACA. Fam. 4. Peltidiidae. Free -living marine forms, with feeble swimming power. The body is broad and flattened and the margins of the anterior segments are often produced into overlapping lobes. The cuticle is thick. In other respects they resemble the Harpactidae. Scutellidium Glaus, Zaus Goods., Alteutha Baird, Eupelte Claus, Peltidium Phil. Porcellidium Claus, body incompletely segmented. Hersilia Phil. Second series (Fams. 5-8). Fam. 5. Cyclopidae. Marine and freshwater forms with active swimming powers. Body pear-shaped completely segmented. Last thoracic segment included in urosome. Anterior antennae of moderate length, both of them symmetrically modified as prehensile organs in the male (Fig. 258). Posterior antennae uniramous, 4-jointed. Mandibular and maxillary palps reduced. 5th pair of swimming feet cylindrical and alike in both sexes. Heart generally absent. Eye median simple. Egg sacks paired. Cyclops Mull. (Fig. 257), mandibular palp reduced to a few bristles (Fig. 259, M). Abundant in freshwater pools. Cyclopsina Claus. Oithona Baird, 0. plumi/era Baird, pelagic. The appendages of the female are beset with long feathery scarlet setae not present in the male. Misophria Boeck, Thorellia Boeck. Fam. 6. Notodelphyidae. Cyclops-Mke Copepods, with diminished powers of movement, commensal in the interior of Tunicates and other marine animals. Body fully segmented. The anterior antennae of moderate length or with the number of joints reduced to 5. The eggs develop in a wide expansion of the united oviducts (uterus) contained in a large dorsal expansion of the fused 5th and 6th thoracic segments. Cement glands absent. The males are often much smaller than the females. Noto- delphys Allm. differs from the other genera in having the anterior antennae as long as the cephalothorax. Doropygus Thor., Botachus Thor., Gonio- delphys Buchh., Notopterophorus Costa, with lamellate dorsal processes on the thoracic segments. Gunentophorus Costa, Chonephilus Sars, and (?) Gastrodes Hesse, and Ophthalmopaches Hesse. Fam. 7. Ascidicolidae. Resembling the Notodelphyidae in mode of life and in structure except that the body is more elongated, and the anterior antennae are shorter 3-jointed. Ascidicola Thor., Botryllo- philus Hesse, Narcodes Hesse, Ischnogrades Hesse, Ophioseides Hesse, Entetocola Bened., Mychophilus Hesse, Ad.rane.sius Hesse, Aplopodus Hesse, and (?) Podolabis, Cryptopodus, Hypnodes and Lygephilus Hesse. Enterognathus Giesb., E. comatulae an entoparasite on Comatula. Allied to the Ascidicolidae are the forms Thaumaleus Kroyer, Mon- strilla Dana and Haemocera Malaquin, whose remarkable life-history is described above (p. 403). Fam. 8. Buproridae. Consisting of the single species Buprorus loveni Thor., commensal with Ascidia aspera Mull. Allied to the last two families but the body of the female unsegmented. Third series (Fams. 9-11). Fam. 9. Corycaeidae. Active, marine Copepods, mostly free through- out life, though in some species the females become temporarily attached to other animals. Body cylindrical or flattened. Anterior antennae short, usually 6-jointed, alike in both sexes. The posterior antennae and COPEPODA. 407 maxillipeds are symmetrically modified as prehensile organs, the latter especially in the male. Heart absent. The lateral elements of the eye are often large. The openings of the oviducts are dorsal or lateral. Egg sacks paired. Lubbockia Claus ; Oncaea Philippi ; Corycaeus Dana. Copilia Dana, bodies of glass-like transparency ; strongly marked sexual dimorphism ; female cylindrical, male flattened; C. vitraea Haeckel, in the female (II. 1)* the thoracic appendages bear broad plumose setae which, like the stomach, are of a bright orange colour, and the lateral elements of the eyes have large lenses ; the male (L. 8) * is colourless and without lateral eyes. Pachysoma Claus. Sapphirina J. V. Thompson. Lateral elements of the eyes with cuticular lenses present in both sexes, male much broader than female and brilliantly iridescent (I. 7).* Sapphirinella Claus. Fam. 10. Ergasilidae. Copepods in which the females though retaining the power of swimming are parasitic on the gills of fishes and the bodies of mollusca, annelids, etc. The males are probably free at any rate during part of their life. Body cylindrical or flattened, segmented. Anterior antennae 5-7-jointed, postr. antennae modified as grappling hooks, maxillipeds prehensile. The mouth parts are riot modified into a sucking tube, and the eye is simple and median. Egg sacks paired. Doridicola Leyd., parasitic on Doris. Sepicola Claus, on squids. Eolidicola Sars. Lichomolgus Thor. in branchial sacks of tunicates. Terebellicola Sars, on Terebellidae. Eucanthus Glaus. Bomolochus Nordm., on pleuronectid fishes. Ergasilus Nordm. on teleostean fishes. Thersites Pagenst. Fam. 11. Ascomyzontidae (Asterocheridae). Structure and mode of life resembling that of the Ergasilidae, but the lips are modified into a suctorial apparatus containing the styliform mandibles. Anterior antennae 5-'20-jointed, geniculated in the c? , ant", mx" and mxped prehensile. The eye may be absent. Nicothoe Aud., N. astaci with a discoidal proboscis ; lives on the gills of the lobster. Asterocheres Boeck (Ascomyzon Thor.) with a long sucking tube, on tunicates, sponges and echinoderms. Dyspontius Thor., Artotrogus Boeck, and many others. Fourth series (Fams. 12-14). Fam. 12. Caligidae. Both sexes live as parasites on the skins of fishes, but are able to swim rapidly. They are broad flattened Copepods of comparatively large size and incompletely segmented. There is a shield-like cephalothorax and the free thoracic segments are often pro- duced into lateral lobes. The genital segment is swollen, especially in the female, and the rest of the abdomen reduced. Anterior antennae short, with 2 or 3 free segments ; the basal segment fused with the margin of the cephalothorax. Posterior antennae reduced to a grappling hook, not projecting beyond the margin of cephalothorax. Mandibles styliform, enclosed in a sucking tube. First maxillae free, second maxillae and maxillipeds modified as hooks. 4 pairs of usually biramous swimming feet are present, the 5th pair being often rudimentary or absent. Heart pro- vided with paired valves. Eye, when present, simple and median, or with paired elements. Egg sacks paired and elongated. Hermilius Hell., Para - petalus Stp.Ltk., Synestius Stp. Ltk., Caligodes Hell., Caligus Mull., Lepeoph- * These numbers refer to the plates and figures in Giesbrecht's Mono- graph on Pelagic Copepods, Fauna and Flora of the Gulf of Naples, vol. 19. 408 CRUSTACEA ENTOMOSTRACA. theirus Nordm., Anuretes Hell., Gloiopotes Stp. Ltk., Lutkenia Claus, Nesippus Hell., Nogagus Leach, Demoleus Hell., Dysgamus Stp. Ltk., Euryphorus Norclm., Trebius Kr., Elytrophora Gerst., Alebion Kr., Dine- matura Latr., Echthrogaleus Stp. Ltk., Cecrops Leach, PhyUophorus Edw., Gangliopus Gerst., Pandarus Leach, Laemargus Kr., Perissopus Stp. Ltk., Lepidopus Dana., Caligeria Dana, Calistes Dana, Caligina Bened., Specil- ligus, Dana. Fam. 13. Dichelesthiidae. Like the Caligidae, they are generally external parasites at- tached to the gills and soft parts of the skin of fishes. They differ from the Caligidae in the greater reduction in size of the swimming feet, the elongated body and in the characters of the antennae, the anterior being many jointed and the posterior, which are chelate or bear a hook, being lengthened so as to pro- ject beyond the margin of the cephalothorax. M y tilicola Steuer, M. intestinalis, internal parasite in gtit of mussels in Adriatic ; its blood is red. Anthosoma Leach., Tucca Kr., Norion Nordm. , Epachthes Nordm., Lernanthropus Nordm., with red blood ; Stalagmus Xordm., Dichelesthium Herm., D. stiiricnis on the gills of the sturgeon. Lonchidium Gerst., on the gills of sharks. Baculus Lubb., probably a larval form of a Lernaeid allied to Pennella. Philichthys Stp., P. xiphiae. The sexes differ markedly in the adult state. The female in- habits the mucous canals in the head of the sword-fish (Xiphias gladius), but the male leads a free existence. Ler- naeascus Claus, on soles. Clavella Oken, Nemesis Roux, Cycnus Edw., Pseudocycnus Hell., Eudactylina Bened., Lam- proglena Nordm. , D onus a Nordm., Aethon Kr., Ergasilina Bened., and (?) Polyclinophilus Hesse, and Sabellacheres Sars. Fam. 14. Lernaeidae. Before they are fertilized the females (Fig. 2G6, b) resemble the males and conform to the type of the last two families, having the anterior antennae short, the posterior hooked and as in the last family projecting beyond the front margin of the FlO. 266. Lernaea branchialis. a male (about 2-3 mm. in length) ; 6 female at the stage in which fertilization occurs (5-6 mm. in length) ; e female in process of metamorphosis ; d the same, with egg sacks formed (c and d natural size). A' , A" the two pairs of antennae (the hooked pair is the 2nd) ; D gut ; F', f' r the four pairs of swimming feet ; G brain ; Go region of the generative apertures ; M stomach ; .l/.i7 maxilliped ; Oc eye; II suctnrial pro- boscis ; S/> sack of spermatophores ; T testes ; Vd points to the vasa deferentia on their way from the testes to the genital (1st abdominal) segment. (After Claus.) COPEPODA. 409 cephalothorax, the mouth parts suctorial, small hooked maxillipeds and four pairs of small swimming feet. The genital segment of the female is much elongated and the hind part of the body is unsegmented. A median eye is present throughout life. The development of the male ceases at this stage, but after fertilization the body of the female increases enormously in size, growing worm-like, while the anterior part is buried in the tissues of the animal on which it is parasitic (Fig. 266 c, d) The region in front of the genital segment is, in many genera, con- stricted, and the eggs sacs are either oval or much elongated, being in the latter case straight or thrown into a closely packed spiral. Lernaeocera Blainv., Therodamas Kr., Peniculus Nordm. Pennella Oken., P. sagitta, L. The female lives nearly buried in the flesh of the pelagic fish Antennarius, only the posterior part of the body carrying the straight egg tubes and beset on either side with a series of long pro- cesses (respiratory ?) projecting beyond the body of the host ; two long backward pointing processes from the sides of the thoracic region anchor it in position. P. filosa L. attains a length of 67 inches. Lernaeonema Edw., Lerneaenicus Les., Echetus Kr., Lophura K61L, Lernaeolophus Hell., Lernaea L (Fig. 266). The females are parasitic on the gills of fishes (rarely on tunicates). The genital segment is swollen, ventricose and S-shaped. Rootlike processes extend from the head into the ulcerated tissues of the branchial arches in which it is embedded. The males re- main small and lead a free life. Haemobaphes Stp. Ltk., Peroderma Hell., Naobranchia Hesse, and (?) Pseudulus Nordm. Fifth series (Fams. 15-16). Fam. 15. Chondracanthidae. The males and females differ much in form and size. The females are comparatively large animals attached to the gills or other soft parts of the skin of fishes. The abdomen is stump-like and the body produced into stout irregular processes (in which lobes of the ovaries may be lodged) giving it a monstrous misshapen ap- pearance. There is no suctorial proboscis and the mandibles are sickle shaped. Maxillipeds small and hooked. The swimming feet are reduced to two pairs of bifid lobes. Egg sacs elongated or saccular. The dwarf males are found attached to the body of the female. In them the cephalo- thorax is arched dorsally and the abdomen conical and segmented. Lesteira Kr., Medesicaste Kr., Strabax Nordm., Trichthacerus Kr., BUas Kr., Chondracanthus la Roche (Fig. 265), Splanchnotrophus Hanc., Diocus Kr., Ismailia Bergh., Tanyplgurus Stp. Ltk. Fam. 16. Lernaeopodidae. As in the last family there is marked sexual dimorphism. The bodies of the females are large and unsegmented. Maxillipeds in some genera short and thick and in some cases completely fused together, biit generally long and arm-like and only united at their ends in a single horny disc, common to both, by which the animal is attached to the host (Fig. 264). The toothed mandibles are contained in a suctorial proboscis. The swimming feet have completely disappeared. Egg sacks paired and saccular. The males resemble those of the Chon- dracanthidae in shape, but the abdomen is unsegmented, the mouth begins in a suctorial tube, and both second maxillae and maxillipeds are strongly hooked. They are attached to the bodies of the females. Thysanote Kr., Basanistes Nordm., Vanbenedenia Malm., Charopinus Kr., Achtheres Nordm., Lernaeopoda Kr., Tracheliastes Nordm., Brachiella Cuv. : Anchor- ella Cuv., on the gills and mouth parts of fishes (?) Herpyllobius Stp. Ltk. 410 CRUSTACEA ENTOMOSTRACA. Sixth series (Fam. 17). Fam. 17. Choniostomatidae. Minute Copepods which live in the brood or branchial cavities or elsewhere about the bodies of different Maiacostraca, whose blood they suck. The body, especially that of the fully developed female, is more or less globular, and without seg- mentation, though a transverse groove on the ventral surface marks the limit of the cephalothorax, and in one genus the abdomen is distinct. It is often partially clothed with hairs. The anterior antennae are small, the posterior very minute. Mandibles simple and contained in a conical proboscis terminating in a membranous cup. 1st maxillae minute and biramous, on the outer sides of the proboscis. The second maxillae and maxillipeds are generally powerful and prehensile. Two pairs of minute biramous legs and the pair of caudal appendages project from the globular posterior region. The male is often less than ^rd the length of the female and is attached to it or to the body of the host by a stalk secreted by a gland lying in front of the proboscis. The vasa deferentia open, according to Hansen, between the swimming legs of the anterior pair. The eggs are contained in numerous ovisacs which are generally not attached to the body of the female. A " nauplius " stage, though with only two pairs of appendages (cf. Achtheres, Lernaeopodidae) is passed through in the egg and the larva is hatched in the " cyclops stage " with the mouth and anterior appendages like those of the adult, but with large swimming feet, and 1 or 2 free thoracic seg- ments and a 3-jointed abdomen. The larvae attach themselves to a host by a secretion of the frontal border of the cephalothorax, and in some species either the female alone or both sexes pass through a pupal stage, in which the appendages are ill-developed or absent, and the contents of the body appears to undergo a process of histolysis. After the pupal skin is shed the adult structure is complete. Stenothocheres Hans., abdomen distinct, in marsupia of amphipod family Stenothoidae. Homoeoscelis Hans., in branchial cavity of Cumacea. Sphaeronella Salensky, in marsupia of amphipods, isopods and Cumacea, 34 spp. de- scribed. Choniostoma Hans., maxillipeds rudimentary ; in branchial cavity of the decapod Hippolyte. Mysidion Hans., in marsupia of schizopods. Aspidoecia Giard and Bonn., attached to the outer surface of the schizopod Erythrops. Sub-order 2. BRANCHIURA.* Carp-lice. Copepods with large compound eyes, and a suctorial tube about the mouth ; with four pairs of elongated * Jurine, Memoire sur 1'Argule foliace, Annales du Museum d'hist. not., Tom. VII., 1806. Fr. Leydig, Ueber Argulus foliaceus, Zeitschr fur wiss. Zool., Tom. II., 1850. E. Cornalia, Sopra una nuova specie di crostacei sifonostomi, Milano, 1860. C. Glaus, Uber die Entwickelung, Organization und systematische Stelhmg der Arguliden, Zeitschr. jur wiss. Zool., Tom XXV., 1875. F. Leydig, Ueber Argulus foliaceus, Arch. f. Mikrosk. Anat., T. 33, 1889. Nettovich L. v. Neue Beitrage zur Kenntniss d. Argulidae Arb.aus dem Zool. Inst. Wien. Bd. 13 (1900) p. 1. Wilson C. B., North American parasitic Copepods of the family Argulidae (with Bibliography) Proc. U.S. Nat. Museum, vol. 25 (1903) p. 635, and a new COPEPODA BRANCHIURA. 411 J? biramous and many-jointed swimming feet. The abdomen is represented by a broad two-lobed caudal fin. The Branchiura though actively swimming animals are para- sitic on marine and freshwater fish. They are often placed near the Caligidae, but though they certainly resemble this group in the general form of the body they differ from them and from the Eucopepoda in several essential particulars. They have an oval flattened shape enabling them to adhere closely to the bodies of their hosts. The region bearing the first pair of swimming feet is fused with the anterior part of the body forming a cephalo thorax. The abdomen is represented by the short un jointed bilobed tail fin, in the notch at the end of which the short caudal furca is found (Fig. 267). Both pairs of Sj\ antennae are s^mall. The anterior is three- jointed and carries a hook and the posterior, though large and biramous in the larva, is simple in the adult. A large suctorial tube projects from about the mouth, and in if ttrp pnnppnlprl finplv in it are concealed nnely serrated mandibles and the first maxillae which, unlike those of the parasitic Eucopepods, are styliform.* A little anterior to this proboscis there is inserted a long cylindrical tube, terminating in a retractile styliform spine, which serves as a sting (Wilson). It is, however, not present in Dolops. species of Argulus. . . . Ibid. vol. 27 (1904) p. 627. Thiele J. Beit, z. Morphologie d. Argulidae. Mitth. aus dem Zool. Mus. Berlin. Bd. 2. Hft, 4(1904). * Thiele. however (I.e.), considers that only mandibles are contained in the suctorial tube. He regards the appendages here named second maxillae and maxillipeds as the two pairs of maxillae a result with which the posi- tion of the opening of the shell gland would be in harmony. T 267 ._ Youngle Argulm foliaceus (from Claus). A' Anterior antennae; D intestine ; Kf . / liaxilliped . R rostrum; Sf swim ming feet : s <> sucker and second maxilla ; st s T testes. 412 CRUSTACEA ENTOMOSTRACA. Powerful organs of attachment are placed on each side of and behind the mouth ; they consist of two parts (1) of an anterior pair of appendages which apparently correspond to the second maxillae and are in Argulus modified into large sucking discs ; the hook-bearing terminal portion which is present in the newly hatched larva, and in Dolops throughout life, being reduced ; and (2) of a posterior pair, which corresponds to the maxillipeds and is provided with numerous spines on its broad basal portion and a tactile protuberance and two curved terminal claws at its extremity. Xext to these come the four paired swimming feet of the thoracic region, which, with the exception of the last, are, as a rule, covered by the sides of the cephalo- thoracic shield. Each of these consists of a large basal portion, consisting in the adult of several segments, and two much narrower rami, which are beset with long swimming setae and in their form and setigerous investment are not unlike the biramous appendages of the Cirripedia, being like them derived from the copepod feet of the larva (Fig. 267). The internal organization recalls that of the Phyllopoda. The nervous system is distinguished by the great size of the cerebral ganglion, and by the close approximation of the six ganglia of the ventral chain. Two large compound lateral eyes are present in addition to the unpaired tri-lobed median eye. The alimentary canal consists of a short arched ascending oesophagus, a wide stomach with two lateral ramified appendages, and a rectum which runs directly backwards and opens to the exterior in the median indentation of the caudal fin above the two divisions of the caudal fork. The shell gland has a well- developed end-sac and opens, not as in most Entomostraca (but cf. footnote, p. 411) on the sucking disc (rax"), but at the base of the maxilliped. The heart is a tube suddenly expanded behind, extending from the region of the brain to the base of the caudal fin, against which it ends abruptly. The blood circulates in sinuses. It leaves the heart by anterior and posterior apertures and returns to it by a ventral median and two posterior lateral ostia (Leydig). The entire surface of the cephalothorax functions as a respiratory organ. There seems, however, always to be a strong current of blood in the caudal fin, so that this part of the body may be regarded as especially respiratory a circumstance from which the group is named. CIRRTPEDIA. 413 Reproduction. The males are smaller and more agile than the females. The paired testes lie in the two lobes of the abdomen and the vasa efferentia lead to a median vesicula seminalis from which paired ducts pass to the median genital papilla at the base of the abdomen. Fertilization is effected by means of seminal sacs which are situated at the distal ends of the basal segments of the third pair of swimming feet, an arrangement recalling those found in spiders and cephalopods. Copulatory hooks are placed in a corresponding position on the fourth pair. The oviduct leading from the median ovary opens at the base of the abdomen. The females do not carry their eggs about in sacs in the typical copepod manner, but fasten them to surrounding objects. The vitelline membrane of the deposited eggs acquires a vesicular structure. The young when hatched have the general form of the adult, but the appendages, as shown by Claus, resemble those of later copepod larvae. They undergo a metamorphosis. Fain. Argulidae, Carp-lice. Argulus O. Fr. Mull. The anterior pair of maxillipeds modified into large suckers. There is a styliform spine in front of the mouth. A. foliaceus L. (Poudepoissons, Baldiier) parasitic on carp and sticklebacks. A. coregoni Thor., A. giganteus Luc., Dolops Aud. (= Gyropeltis Hell.). The maxillipeds end in a claw; stylliform spine absent. D. Kollari Hell., parasitic on the branchiae of Hydrocyon, Brazil. D. Doradis Corn. Chonopeltis Thiele. Order 5. CIRRIPEDIA.* Fixed, and for the most part hermaphrodite Crustacea with indistinctly segmented body enclosed by a dorsal shield (mantle] which generally contains calcareous shell plates. As a rule there are six pairs of biramous thoracic appendages. A remarkable change in the position of the body within the shell occurs in most cases during the metamorphosis. * Compare J. V. Thompson, Zoological Researches, vol. 1, 1829. H. Burmeister, Beitrage zur Naturgeschichte der Rank en fussier, 1832. Ch. Darwin, A monograph of the Sub-Class Cirri pedia, 2 vols., London, 1851- 185 1. A. Krohn, Die Entwickelung der Cirripedien, Archiv. fur Natur- gesch., 1860. C. Claus, Die Cypris-ahnliche Larve der Cirripedien, etc., Schriften Ges. Naturw. Marburg, Suppl. Heft. V. 1869. R. Kossman, Sttc- toria und Lepadina, Wiirzburg, 1873. Yves Delage, Evolution de la Sacculine, Arch, de Zool. exp., (2), II., 1884. Hoek, Report on the Cirri- pedia, Challenger Reports VIII (1883) and X (1884). Berndt, Zur Biologic u. Anatomie v. Alcippe larnpas Hancock. Z. f. w. Z., 74 (1903), p. 396. Gruvel, A., Monographic des Cirrhipedes, Paris, 1905. G. Smith, Rhizo- cephala, Fauna u. Flora d. Golfes von Neapel, Monograph 29, 1906. 414 CRUSTACEA ENTOMOSTRACA. a H , 07 FlO. 268. a, Late nauplius, b, metanauplius larva of Balanus before the moult. Beneath the skin are the rudiments of the lateral eyes (0) and all the appendages F< : to F ri of the cypris stage ; A anus ; A', A" 1st and 2nd antennae ; A' the antennae with suctorial disc ; D intestine ; Dr gland cells of the anterior horns ; Ff frontal filament ; H frontal horns ; Mdf mandibular foot (third pair of appendages) ; MX rudiment of maxilla ; 0' unpaired eye ; 01 proboscis with mouth. (From Claus.) The Cirripeds were included by the older natural- ists in the Mol- lusca,to the Lamel- libranch group of which the bivalve shell of the Lepadidae with its calcareous plates and the siphon - 1 i k e peduncle give this division a certain external r e s e in - blance. Their relation- ship to the Crus- tacea was fi r s t demonstrated by J. V. Thompson, by the discovery in the year 1829 of the nauplius larva of Balanus. The monograph of the Cirripedia by Charles Darwin (Ray Society 1851-53) first placed our know- ledge of the struc- ture of the group on a satisfactory basis. The structure of the adult Cirripedes, as well as the natural affinities of the CIRRIPEDIA. 415 group are best understood by the study of their develop- ment. The nauplius larva (Fig. 268) is provided with the character- istic three pairs of appendages and median eye, but it also possesses a very delicate dorsal shield prolonged laterally into two peculiar " frontal horns " to which in the later stages, in Lepas, long median and lateral horns are added. The upper lip is large, recalling that of the Phyllopod larva. After undergoing several moults in this guise the larva sud- denly emerges in the Cypris stage (Fig. 269). It is enclosed in a bivalve shell, the margins of which are flattened along the ventral surface, and a pair of compound eyes is present in ad- dition to the median eye. The anterior antennae are four- jointed, and bear a characteristic disc at the end of the second segment on which opens the duct of the cement gland, whose lobules lie in the anterior part of the body. The posterior an- tennae of the nauplius have disappeared, and the man d i b 1 e s and two pairs of maxillae are rudimentary. There are six pairs of biramous and setose thoracic appendages, recalling those of Copepods. The short abdomen ends- in a caudal fork. The position of the animal in relation to its shell is already peculiar. The mouth lies behind the middle of the ventral region and is directed backwards, so that, as shown in Fig. 269, the axis of the animal is bent into a U with unequal limbs. The fold where the inner lining of the carapace joins the dorsal body-wall, which we may call the dorsal fold, is situated on the dorsal side, and some of the soft parts are included in the fold of the carapace. The median eye lies a little behind the compound eyes, and at the bases of the antennae the chitinous covering forms two strong plates (apodemes) projecting deeply into the FIG. 269. Cypris larva of Lepas fasciculate, ab abdomen ; pa paired eye; rf thoracic feet; ua median (nauplius) eye, 1 anterior antennae. From Lang, after Clans. 416 CRUSTACEA ENTOMOSTRACA. body, for the attachment of the antennary muscles. They are indicated at a later stage in the plane of the paired eye in Fig. 270. After a short time passed in the cypris stage, now swimming rapidly and now crawling by means of its leg-like antennae, the larva becomes fixed with the ventral margins of the shell applied to the object to which it is attached, and passes into the pupal stage. The attachment is effected by the secretion of the cement glands by which the discs of the first antennae are glued on to the object selected by the larva for its resting-place. During this stage the compound eyes, the abdomen and other organs undergo a retrogressive metamorphosis, while the adult structures are developing. The curvature of the axis of the larva already marked, undergoes a considerable increase, the region bearing the mouth becoming directed still further backward (Fig. 270). In this marked flexure of the region of the body which lies about the alimentary canal, the anterior part, bearing the antennae and con- taining the cement gland, the compound eyes and the rudiments of the ovaries, does not participate. The result is that a partial separation of the body into two divisions is brought about : the space between the dorsal body wall and the carapace extends downwards (if we regard the ventral edge of the shell as horizontal, as in Fig. 269) and finally downwards and back- wards, as it follows the curvature of the posterior region, until what we have called the dorsal fold (Fig. 271, -4 and B, x) comes to lie near the ventral surface of the larva. When the pupal skin is shed the remains of the compound eyes, the larval swimming appendages and the above men- tioned apodemes for the insertion of the antennary muscles are shed witli it. As the result of the shedding of these chitinous plates a deep notch is left (Fig. 271, B, y) in the ventral side of the anterior end, lined but not filled by the delicate cuticle of the succeeding stage. The subsequent growth of this region is accompanied by a further change in the relative positions of the parts. The notch gradually opens out (Fig. 271, C), with the result that the part of the body posterior to it swings round through nearly a right angle, the ventral margin of the shell being now directed perpendicular to the surface of attachment, instead of parallel to it as heretofore. CIRRIPEDIA. 417 The anterior part of the body becomes elongated in the Lepadidae into the stalk or peduncle and the ovaries and cement gland are contained in it. The antennae become shorter and CCi FIG. 270. Pupa of Lepas pectinata in optical section. (From Lang, after Claus.) ca carina ; cd cement gland ; d intestine ; L liver ; o mouth ; 'pa paired eye ; // [tho- racic appendages ; sc scutum ; sm adductor muscle ; t tergum ; ua nauplius eye ; 1 an- terior antennae. stouter, and soon the region around them becomes flattened into a disc applied to the supporting object, though the com- f -o rf o tta. \ J "i / i,/ a' FIG. 271. Diagrams illustrating the metamorphosis of Lepas. A, Cypris stage ; B, attached pupa ; C, young Lepas still surrounded by the loosened Cypris shell (*). a' first antenna ; ab abdomen; c carina; d intestine ; TO mouth ; o nauplius eye ; pa paired eye ; rf thoracic imbs ; s Cypris shell : sc scutum ; t tcrgum ; a- 'dorsal fold ' ; y ventral fold previously loccupied by the apodemes of the antennary muschs (from Korschelt and Heider). paratively minute antennae can be distinguished even in the full-grown barnacle. z m E E 418 CRUSTACEA ENTOMOSTRACA. In the Operculata or sessile barnacles the anterior part of the body does not undergo this elongation, but increases very largely in width, its " basis " or disc of attachment being the widest part of the body. In them the region of the body morphologically posterior is sunk in the wide anterior part, being surrounded by a fold of the outer wall (Fig. 272 6). Calcareous plates developed in this fold and in the wall of the anterior part below it con- stitute the shell or testa. The calcareous plates of the Operculata are described below, but it may here be pointed out that the i-Tu A FIG. 272. (/, Lejiax after removal of the right shell; b, Balanua tintinnabulum (after Ch. Danvin), one half of the shell has been removed. The following references apply to Fig. 272 a and b, ani Fig. 273. A' anterior antennae at the end of the stalk in a ; Ad adductor muscle ; C carina ; Cd Cement gland and duct ; Cf thoracic appendages ; F caudal fork ; L liver ; M muscle; Mk oral cone ; Orf oviduct ; Oe opening of oviduct; On opary ; P penis ; Sc scutum ; T testis ; Te terguiu ; Tit section of the outer shell ; Vd vas deferens. (After Claus.) fold in question is distinct from that forming the mantle, being situated externally to it, and completely surrounding the body. After the shedding of the bivalve shell of the pupa the fold of skin which supported it is covered by a thin cuticle in which however five local thickenings of chitin have already appeared. These are the provisional " valves " of the shell ; they are always the first plates to appear, however many maybe present in the adult shell, and in the Lepadidae the calcareous shells of the adult are formed under and about them. The adult Lepas, whose relation to the larva may now be CIRRIPEDIA. 419 understood (Figs. 272 and 273) is made up of the hollow muscular peduncle or stalk, containing the ovaries and the remains of the cement gland, and of the capitulum. The latter consists of the mantle, or dorsal shield, with its calcareous valves, together with the contained body, which is pear-shaped and formed, as we have seen, from part of the cephalic region of the larva, to- gether witli the thorax and rudimentary abdomen. The valves of the shell of the Lepadidae consist of a pair of large scuta (Sc) situated anteriorly, posterior and dorsal to them of a pair of terga, and of a single median and dorsal carina (C). The narrow end of the piriform body is divided by grooves into segments bearing the five posterior pairs of thoracic appendages. In front is the rounded and unsegmented prosoma bearing the first pair of thoracic and the oral appendages. It is connected with the mantle for a short distance in front of the mouth, and through this region passes transversely the great adductor scutorum muscle (Fig. 273, M). Various modifications of the outer form of the body are met with. In Concho- derma the mantle remains to a large extent membranous, the valves being reduced in size. In Anelasma, which lives embedded in the skin of sharks, the valves are absent, and rootlike processes extend into the tissues of the host from ot the peduncle. The Pollicipedidae are j transitional between the simple condition of Lepas and that presented by the Bala- nidae. In addition to five mantle plates homologous with those of Lepas, small calcareous plates are developed, in the cuticle of the peduncle, and larger accessory plates at the base of the capitulum. Among the latter a median ventral plate, the rostrum, is situated opposite to the dorsally placed carina. In the Balanidae the carina together with the accessory (side) plates form the outer ring constituting the testa of this family. The carina, detached in this family from the true mantle, and the rostrum lie at either end of the median plane, and side plates are present in vary- ing number (carino-lateral, lateral and rostro-lateral) overlapping one another at their sides. The scuta and terga on the other hand retain their position on the mantle covering the mov- able posterior part of the body, and together form the two halves of the FIG. 273. The organization of Le/ms, after removal of the integument. (After Claus.) References as in Fig. 272. 420 CRUSTACEA ENTOMOSTRACA. operculum, which lies within the aperture of the testa, and when closed over the retracted thorax by the action of the adductor scutorum muscle forms a protection to the contained soft parts of the animal. The remarkable departure from the symmetrical type presented by the Verrucidae is described below. The mouth opens on a prominent oral cone which projects backwards (Fig. 272, M K). In front is the large hood-like labrum, and at the sides are situated the toothed mandibles with their palps (Fig. 274), the toothed first maxillae, and the setose u.l.-- Mr\.p-' B FIG. 274. Mouth parts of Lepus. A seen from the right. B oral appendages of left side isolated and seen from the inner aspect ; Mn inaudible : Mn. /i and /> inandibular palp ; Mx l 1st maxilla; M.r- 2nd maxilla; u.l. labrum. swollen second maxillae which together limit the mouth cavity posteriorly. The six pairs of long, black, many -jointed, biramous. thoracic appendages '(cirri) are curled towards the mouth, and richly beset with setae. With those of the opposite side they form a hand-like structure, which is rhythmically thrust out of the aperture of the mantle and swept through the water with a grasping motion, producing currents and capturing food. The abdomen has almost entirely disappeared, but from the ventral side of it there springs the long probosciform penis. A caudal fork represented by two short lobes is usually present in the Pedunculata and usually absent in the Operculata. In some genera of the former (Ale pas, Ibla and Lithotrya), in the Ascothoracica and some Acrothoracica its lobes are longer and articulated. In Lepas two or more hollow processes situ- ated near the bases of the anterior pair of thoracic- feet are CIRRIPED1A. 421 regarded as branchial, and in Conchoderma a similar process is found at the base of each of the other thoracic feet. A fold of the inner lining of the mantle projecting backward on either side of the attachment of the prosoma to the carapace is the " ovigerous frenum " of the Lepadidae, and it is apparently a homologous structure, though no longer ovigerous, which is converted into a respiratory organ in the Operculata (Darwin) The tubular oesophagus extends forward from the mouth to open into the dilated stomach, the walls of which are prolonged into hepatic diverticula. Bending with the curvature of the body the alimentary canal narrows into the intestine which runs back, in some cases with a clearly defined rectum, to open between the rudimentary abdomen and the base of the penis. In Alcippe it ends blindly. Nervous system. A paired supra-oesophageal ganglion is present, and in the Lepadidae a ventral chain of five ganglia. In the Balanidae these latter are represented by a single large ganglion. A vestige of the unpaired eye of the larva persists. A heart appears to be absent. The excretory organs are represented, as usual in Entomostraca, by the maxillary glands. Hoek described wide paired spaces, with definite walls, leading to apertures in the second maxillae. But it has recently been shown by Bruntz * that there are in addition glandular sacks (simple in Balanus, divided into alveoli in Lepas) lined by an excretory epithelium and opening into the spaces of Hoek, which are in fact their dilated ducts. Similar spaces are described by Berndt in Alcippe, but they are said to be closed. Generative organs. In relation probably with their fixed habit the Cirripedes are almost exceptional among Crustacea in the fact that they are in the majority of cases hermaphrodite. The testes (Fig. 273, T) are branched, glandular tubes lying at the sides of the alimentary canal and extending into the bases of the thoracic appendages. The vasa defer entia, after dilating at their commencement to form vesiculae seminales, run back to unite at the base of the long penis, which is traversed by the common duct. The spermatozoa have a rounded head and * Contrib. a 1'etude de 1'excretion chez les Arthropodes. Arch, de Biol. T. xx (1904) p. 219. 422 CRUSTACEA ENTOMOSTRACA. long tail ending in two filaments. "Giant spermatozoa" have been observed, though rarely, in Balanus perforatus, by Gruvel. The ovaries as already stated lie as a single mass in the peduncle of the Lepadidae, and in the corresponding basal region of the body in the Operculata. The paired oviducts open, not near the base of the abdomen, as usual in the Crustacea, but on pro- minences on the basal joints of the anterior pair of thoracic appendages. The eggs undergo their development in the mantle space, contained in two flattened gelatinous sacs, com- parable to the ovisacs of the Copepods, which lie one on each side and are attached (in the Lepadidae) to the ovigerous frena. The mode of fertilization is described by Gruvel. In Lepas two individuals of a cluster come together, one of them, assuming the part of the male, deposits a viscous mass of spermatozoa on either side of the mantle cavity of the other, in the region of the orifice of the oviduct. The penis of a large individual may attain a length of 4-5 centimetres. A similar process has been observed in Balanus. Gruvel has observed self-fertiliza- tion to occur in an isolated specimen of Pollicipes. The species of the genera Ibla and Scalpellum offer remarkable instances of sexual relationship. Most if not all are dimorphic ; The species consists in some cases of hermaphrodite forms resembling those of allied genera of Lepadidae, but with certain dwarf male forms in addition, the " complemental males." In other cases the two forms constituting the species are male and female the latter resembling the hermaphrodite forms of their allies, though without the male generative organs. In all, the male forms are small and are attached either in a pouch within the scutum or elsewhere about the mantle of the other form. They exhibit various degrees of arrested development and degeneration. Further details are given below. The Acrothoracica are also dioecious, the males being degener- ate and much redueed in size. The Cirripedia are marine animals and attach themselves to various foreign objects. They are found fixed, usually in groups, to logs of wood, rocks, mussel shells, Crustacea, the skin of whales, Hydrozoan colonies, etc. Some as Liihotrya, Alcippe and the Acrothoracica, are able to bore into the shells of Molluscs and Corals. The members of the sub-orders Apoda, Rhizocephala and CIKRIPEDIA. 423 Ascothoracica are parasitic. The relations between the Rhizo- cephala and their Crustacean hosts are among the most astonish- ing examples of parasitism to be found in natural history. The Lepadidae are represented in Ordovician strata by examples of the Pollicipedidae (including the existing genus Pollicipes) and attain their culminating point during the Cretaceous period. The curious unsymmetrical Verrucidae appear in the Cretaceous, but the other groups of Operculata are not known prior to the Tertiary period. Sub-order 1. CIRRIPEDIA GENUINA. Tribe 1. PEDUNCULATA. Body stalked, with six pairs of biramous feet. Scuta, terga and a carina are usually formed on the mantle, and when other plates are present they are not united into an immovable ring. Fam. 1. Lepadidae. The stalk is sharply marked off from the capi- tulum, and calcareous plates are not developed on it. The plates on the capitulum are thin, their number does not usually exceed five, and the terga lie behind the scuta. Hermaphrodite. Lepas L. (Figs. 272 and 273) attached to floating objects ; L. anatifera L., like most of the species of the genus, cosmopolitan, from arctic to tropical seas. Megalasma Hoek, Poecilasma Darw., generally attached to Crustacea. Oxynaspis Darw. , Dichelaspis Darw., the calcareous plates on the mantle are separated from one another by wide intervals, and the scuta and terga are deeply notched ; they live attached to sea-snakes or crabs. Conchoderma Olfers, capi- tulum in the main membranous, the plates are small and may be reduced to two (scuta). Cosmopolitan, attached to floating objects, living or in- organic. Alepas Rang., capitulum without plates, or with horny almost hidden scuta ; attached to various floating objects ; A. parasita Rang. on medusae. Anelasma Darw., A. squalicola Loven, the peduncle is em- bedded in the skin of the sharks Squalus maximus and Spinax living in the North Sea, the skin of the cirripede being prod\iced into branching rootlike processes, knobbed at their ends, which ramify in the tissues of the fish. The capitulum is witjhout plates and has a wide aperture. The six pairs of legs have a shapeless appearance ; they are obscurely arti- culated and without setae. Gymnolepas Auriv., pelagic, on medusae ; cirri articulated and setose. Chaetolepas Studer, on sertularians. Fam. 2. Pollicipedidae. The stalk usually obscurely divided from the capitulum, and covered with calcareous scales or chitinous hairs. Capitulum with numerous massive plates, frequently exceeding five in number ; the terga are rather dorsal than posterior to the scuta. Many species are hermaphrodite, some with complemental males, and some are dioecious. Pollicipes Leach, stalk closely covered with scales or spines ; in addition to the five plates of the Lepadidae, rostral and lateral plates are strongly developed, and many smaller additional plates (18 to 100 or more) clothe the base of the capitulum ; hermaphrodite ; at- tached to fixed or floating objects in the warmer seas of the globe. P. signatus Aur., occurs in Silurian of the I. of Wisby in the Baltic. 424 CRUSTACEA ENTOMOSTRACA. Lithotrya Sow., with an elongated peduncle covered with scales, and eight plates on the capitulum ; the body is sunk in the cavity of the peduncle ; the animal lives in deep burrows which it excavates in cal- careous rocks, corals or shells ; tropical, hermaphrodite. Ibla Leach, attached to littoral objects in the warm seas of the eastern hemisphere. /. Cumingii Darw. Contrary to the general rule among cirripedes the sexes are separate and exhibit marked dimorphism. In the female the scuta and terga only are developed and they are not calcareous but horny. The peduncle is covered with spines, and the body is partly sunk within its cavity. The first pair of cirri is separated by a consider- able interval from the remainder. The males are minute degenerate creatures, and one or more are attached within the mantle cavity of the female. The capitulum is almost undeveloped, but the peduncle is comparatively large and tapers to a point, which is embedded in the tissues of the female and bears the characteristic prehensile antennae. The mouth parts are well developed and a complete alimentary canal is present, but the thoracic appendages are reduced to two pairs, apparently the 5th and 6th, and these are small and irregular. There are well de- veloped testes and vesiculae seminales but no penis. Philippines and Burmah, attached in groups to the peduncles of Pollicipes mitella. The other species, /. quadrivalvis (Cuv.), from the Australian seas, consists of hermaphrodite forms and " complemental males." The hermaphrodite forms resemble the female of I. cumingii except that, like most Cirripedes, they possess the male reproductive organs in addition to the female. The males also resemble those of the other species except that there are a distinct penis and a caudal fork the halves of which are divided into three segments. The caudal appendages of the hermaphrodite form are remark- ably long. Scalpelhim Leach, presents similar remarkable instances of sexual relations. In the hermaphrodite or female form there are 12-15 calcareous plates on the capitulum and the peduncle is nearly always squamiferous. In all the living species that were known when Darwin wrote his monograph, small male forms are attached to the number of two or more about the body of the other form. In some cases these are dis- tinctly pedunculated, the capitulum carries calcareous plates, and an alimentary canal and 6 pairs of cirri are present. In S. ornatum and S. vul- gar e however the males are reduced to flask-shaped bodies, without an alimentary canal, with 4 minute calcareous plates, and only four pairs of cirri which are nonprehensile. In others again the valves have completely disappeared. They always however retain the characteristic cirripede antennae, by which they are attached. The more degenerate males are contained in small pocket-like cavities on the inner surfaces of the scuta of the other form, they are without a functional alimentary canal and it is probable that many of these short-lived forms successively occupy the scutal pouches. In S. ornatum the larger form is, according to Darwin, female, and this may be the case in one other species, but in S. vulgare, rostratum, peronii and villosum the larger form is herma- phrodite, although, possibly in relation to the presence of the com- plemental males, the male system of the hermaphrodite form is in some cases under -developed. The species are found attached to the slender branches of hydrozoan colonies. S. vulgare, British and adjacent coasts. Many species are found in the deep sea (over 2,000 fms.). Among 41 new species ofScalpellum in the " Challenger " collections (about half of them represented by a single specimen), Hoek found the reduced C1BRIPEDIA. 425 male in 19, continuing the results which Darwin arrived at on much more meagre material. Tribe 2. OPERCULATA. The peduncle is rudimentary or absent. The body is surrounded by a ring of plates (testa) the entrance to which can be closed by the scuta and terga which together form an operculum and (except in Verrucidae) are provided with depressor muscles. Fam. 1. Coronulidae. Scuta and terga when present freely movable, but not articulating together. Rostrum overlapping the adjacent plates laterally. Base of the shell membranous. The paired branchiae each consist of two folds. On cetacea and other pelagic vertebrates. Coronula Lam. , attached to the skins of cetacea. Testa not so high as it is broad ; it consists of 6 similar broad pieces of shell the walls of which are thin and deeply folded, the cavities of the folds are turned towards and are filled by the epidermis of the host ; terga and scuta small, not filling the aperture of the testa ; three species. Platylepas Gray, resembling Coronula, but the pieces of the shell are bilobed ; in the warmer seas, attached to turtles, sea-snakes and manatee. Tubicinella Lam., testa much higher than it is broad, formed of six amalgamated pieces ; these basket-like cirripedes are embedded in the skins of whales in the S. Ocean, often associated with Coronula balaenaris. Stephanolepas Fischer, on Chelone imbricata. Xenobalanus Steenstr., shell a shallow six-rayed ring embedded in the superficial layers of the skin of the porpoise on which this cirripede lives ; the body is much elongated (nearly 2 inches), and externally resembles one of the Pedunculata, only the base of it being contained in the shell ; it consists however of the elongated mantle the cavity of which extends down to the cavity of the shell ; the margins of the aperture are reflexed forming a collar, and there are no shell plates ; N. Atlantic. Fam. 2. Balanidae. Scuta and terga freely movable, and articulating with one another. The paired branchiae each consist of a single fold with subordinate lateral folds. Balanus Lister, testa cylindrical or conical, consisting of 6 pieces ; from the upper limit of the tidal zone to 50 fms., in arctic to tropical seas throughout the world ; 41 species. Acasta Leach, lives attached to sponges. Tetradita Sclrum., testa composed of four pieces (carina, rostrum and 2 lateral) permeated by pores ; T. porosa Gmel., the number of segments in the cirri is very variable. Elminius Leach, testa composed of four pieces not porous. Pyrgoma Leach, testa formed of a single piece ; the scutum and tergum of each side are more or less completely joined together. Lives embedded in corals, chiefly in tropical seas v Creusia Leach, like Pyrgoma, but the testa consists of four pieces. Chelonobia Leach, testa of 6 pieces, one of them consisting of the rostrum and two rostro -lateral elements vuiited together. The pieces are thick -walled and not infolded from the base ; scuta narrow united to the terga by a horny articulation ; attached to turtles, Crustacea and smooth gastropod shells, throughout the warmer and tropical seas. Fam. 3. Chthamalidae. Rostrum overlapped laterally by the ad- jacent plates. Chthamalus Ranz., Chamaesipho Darw., Pachylasma Darw. found in deep water. Octomeris Sow., testa formed of 8 pieces. Catophragmus Sow., testa formed of 8 large pieces, with imbricated series of smaller plates set round about them, becoming smaller towards the base. Littoral ; W. Indies and Australia. Fam. 4. Verrucidae. Scuta and terga without depressor muscles. Those of one side only (right or left) move freely, their fellows having 426 CRUSTACEA ENTOMOSTRACA. coalesced with the carina and rostrum to form one unsymmetrical ring of 4 plates, resembling the testa of other Operculata. The four species of the genus Verruca which constitute the living representatives of this family present a remarkable departure from the bilateral symmetry characteristic of other Cirripedes. Superficially the shell appears to be formed on the same plan as in other Operculata, but the operculum which lies in the aperture of the shell is formed of the scutum and tergum of one side only, those of the other side having taken their places in the outer wall of the shell, which is completed by the carina and rostrum. The prehensile antennae of the larva are situated at about the middle of the (membranous) basis of the shell, but the body of the animal, which is symmetrical about its own median plane, lies on its side, with that plane parallel to the surface of attachment. The early larval stages are symmetrical and the terga and scuta of opposite sides are alike at their first formation. The lobes of the caudal fork are unusually long. The shape, mode of growth and articulation of the tergum and scutum show affinities with the Lepadidae. The shells are generally attached to living bodies and are found down to a depth of 90 fathoms, from Iceland to Cape Horn. V. strdmia (O. Mull.) is British. A fossil species is found in the Chalk. Sub-order 2. ACROTHORACICA. Minute Cirripedes of separate sexes. The females are enclosed in a flask-shaped mantle beset with chitinous points and live in hollows which they excavate in the shells of Molluscs. The thoracic appendages of the first pair are palpiform or rudimentary, and two to four pairs of cirriform feet are borne at the posterior end of the body. The intermediate appen- dages are absent. Mouth parts and alimentary canal usually well developed (the latter ends blindly in Alcippe). The males where known are dwarfed, without alimentary canal, and spend their short existence attached to the mantle of the female. The genus Cryptophialus was placed in a distinct order of Cirripedes, the Abdominalia, by Darwin on the indication afforded by the apparent segmentation of the body. As seven segments appear to intervene be- .tween that bearing the maxillipeds (first thoracic) and the region from which the three pairs of biramous appendages, at the posterior end of the body, arise, it seemed evident that the latter could not be homologous with the 4th, 5th and 6th thoracic appendages ofi other Cirripedes. There are however reasons for not regarding the apparent segments as indicative of the true segmentation. The disc by which the adult is at- tached to its burrow must include, at its anterior end, the region from which the first antennae of the pupa sprang (the 1st antennary segment) ; yet the disc is borne on the apparent segment posterior to that bearing the maxillipeds together with the other oral appendages. Hence, in the anterior part, the apparent is no guide to the true segmentation.* The close resemblance between Cryptophialus and Alcippe was fully recognized by Darwin, and the subseqtient discovery of Lithoglyptes and Kochlorine The same argument applies to Proteolepas, the representative of the following sub-order. In it the larval antennae persist throughout life, and are borne behind the mouth on the second ring as indicated by the superficial appearance of segmentation. CIRREPEDIA. 427 link these genera even more closely together. In them, as in Alci/i/i* there is no appearance of the segmentation of the anterior part of the body indicated in Cryptophialus. If we disregard this apparent segmen- tation, the four genera fall into a natural group for which the name Abdominalia becomes misleading and for which Gruvel has proposed the name Acrothoracica, in allusion to the fact that the terminal feet are confined to more or fewer of the apical (terminal) segments of the thorax. The group so formed has affinities, as pointed out by Darwin in the case of Alcippe, with the Lepadidae. There are however indications that they belong to a more generalized type than any of the Thoracica. If we take the oral end of the disc of attachment as representing the region of the 1st antennary segment, there is no such wide separation of the mouth from this region as occurs in that group. Hence the relations of the parts of the adult body are more nearly those which obtain in other groups of Crustacea. Associ- ated with the same condition is the fact that the fold (Fig. 271 x ) between the mantle and the dorsal body wall is not extended into the cephalic region, dividing it into an oral and preoral part, as occurs in the metamorphosis of the Cirripedia Genuina. The pres- ence of an articu- lated caudal fork in Lithoglyptes and Ko chlorine is a generalized feature which they share with Lithotrya, Ibla and Alepas among the Pedunculata. Berndt * has recently brought evidence to show that the appendages which have been regarded as the caudal fork in Alcippe are the 6th pair of thoracic appendages. Fam. 1. Alcippidae. The females (Fig. 275 &) live in hollows in the columella of the shells of Fusi(s and Buccinuni (British) to the wall of which they are attached by a large horny disc, the plane of which is parallel with that of the orifice of the burrow. Their position in relation to the surface of attachment is the same as that of the pupa of Lepas (cf. Figs. 270 and 275). First pair of thoracic appendages large and palpiform, and the three posterior appendages uniramous. They probably a FlG/275. Alcippe lampas (after Ch. Darwin), a. male.'very strongly magnified ; 6, longitudinal section through female. A' the right antenna (the left is seen through the transparent body) ; Cf three posterior pairs of appendages ; D lobe of the mantle ; F maxilliped (first thoracic appendage) ; eye ; Ov ovary ; P penis, projecting from the orifice of the flask-shaped mantle cavity, at the lower (anterior) end of which is situat?d T the testis ; Vs seminal vesicle. The thickened band to the left of Ov is the section of the large disc by which Alcippe is attached to the wall of its burrow. * Zeits. fur wiss. Zool., Bd. 74, p. 396. 428 CRUSTACEA ENTOMOSTRACA. represent the 4th, 5th and 6th thoracic appendages (the 2nd and 3rd being absent). The dwarf males (Fig. 275, a) are without alimentary canal and hence are short-lived, and have a long probosciform penis. Several may be found in the neighbourhood of the upper part of the disc. Alcippe Hanc. A. lampas. Fam. 2. Lithoglyptidae. 3 spp. of the single genus Lithoglyptes Aur. have been described by Aurivillius, living in burrows which they excavate in coral or in the shells of molluscs. There are 5 pairs of thoracic appendages the 2nd only being absent, and 3-4 jointed caudal appendages. The plane of the disc of attachment is nearly at right angles to that of the orifice of the burrow. Alimentary canal complete. E. Indies. Fam. 3. Cryptophialidae. Three pairs of biramous cirriform feet at the posterior end of the body. Cryptophialus Darw., C. minutus Darw. The female attached by a disc, as in Alcippe, in the shell of the gasteropod Concholepas peruviana ; W. coast of S. America. There appear to be 10 post-cephalic segments of the body. The anterior thoracic appendages rudimentary. Alimentary canal complete. The dwarf males resemble those of Alcippe. Fam. 4. Kochlorinidae, contains the single genus Kochlorine Noll., K. hamata Noll.* in the shells of Haliotis and other molluscs, at Cadiz. Female attached to the edge of its burrow by hooks only. The body not definitely segmented ; anterior thoracic appendages large and palpiform, as in Alcippe. Behind the large cirriform feet is a pair of jointed caudal appendages. Males not certainly known. Sub-order 3. APODA. With the characters of the family. Fam. Proteolepadidae. Proteolepas bivincta Darwin, the sole representa- tive of this sub-order is a small maggot-like animal about >,th of an inch long, which lives attached by its antennae in the mantle cavity of the pedunculate cirripede Alepas cornutus. The antennae have the characteristic Cirripede shape, but the mantle and all appendages, except those of the mouth, are absent, and the body is divided, by constrictions, into 1 1 rings, which how- ever, in view of the facts that the mouth, with its 3 pairs of appendages, is borne on the first body ring, and the antennae on the second, cannot be regarded as representing primary segments. It is hermaphrodite and the body is mainly occupied by the largely developed ovaries. The mouth is suctorial and the alimentary canal ends blindly. St. Vincent, W. Indies. Sub-order 4. RHIZOCEPHALA. f The Rhizocephala are parasites on Malacostracan, and mainly on Decapod Crustacea. In the adult state they con- * Noll. F. C., Kochlorine hamata N. ein bohrendes Cirriped. Zeits. f. Wiss. Zool., Bd. 25 (1874-5), p. 114. f W. Lilljeborg, Les genres Liriope et Peltogaster, Nova Acta. reg. soc. scient., Upsala, Ser. 3, vol. hi., 1860. Fr. Miiller, Die Rhizocephaliden, Arch, fiir Naturgesch., 1862 and 1863. R. Kossmann, Beitrage zur Anatomie der schmarotzenden Raiikeiifiissler, Verhandl. der med.-phys. Oesellsch. Wiirzburq, Neue Folge, Tom. IV. Yves Delage, Evolution de la Sacculine, Arch, de Zool. Exp., 2 Ser., Tom. II., 1884. Smith, G., Rhizocephala, Fauna u. Flora d. dol/es von NeapeL, Monog. 29 (190(>). CERRIPEDIA. 429 sist of a swollen body, which projects from the host through an aperture on the ventral surface, and of a system of roots which ramify through the tissues of the host (Fig. 277). As in the case of the Cirripedia Genuina the structure of the adult is best elucidated by the study of development. FIG. 276. Consecutive larval stages of Sacculina carcini (from Lang, after Delage). A, nauplius after first moult ; B, free swimming Cypris-stage ; C, Cypris-stage after the larva has become attached to a seta (b b) of the host ; D, formation of the Kentrogon larva ; E, the Kentrogon larva after the Cypris shell has been thrown off and the pointed process formed ; F, the process has pierced the cuticle of the host. 1, 2, 3 the nauplius limbs ; I- VI the thoracic limbs of the Cypris stage ; ab abdomen ; 66 seta of the host ; / fat globules ; fs frontal sensory organ ; gl glands of the frontal horns ; ov rudiment of the ovary ; pf pointed process ; ua nauplius eye. Much light has been thrown on the structure and life-history of this group by the admirable researches of Yves Delage on Sacculina carcini Thomps. In the nauplius larva (Fig. 276, A) the mouth and alimentary canal are absent, but a mass of " primitive ova " can already be distin- guished. In the Cypris stage the antennae are prehensile and bear two large sense organs, and the 6 pairs of biramous swimming legs are well developed. After swimming freely for two or three days the larva attaches 430 CRUSTACEA ENTOMOSTRACA. itself to a young crab, most frequently (in the neighbourhood of Roscoff, where Delage carried on his researches) to Carcinus moenas. The larva grasps the base of one of the crab's setae with one of its antennae, and there remains attached (Fig. 276, C). The whole of the pos- terior parts of the body of the larva including the swimming appendages and their muscles, together with the eye, ganglion and excretory organs now break down and are shed by rupture of the ectoderm. There remain the ectoderm, which is rapidly made whole, the mass of primitive FIG. 277. Sacculina carcini in situ on the host (from Lang after a diagrammatic draw- ing by Delage). hr branchial, d intestinal and I hepatic regions of the crab. Jcs the body and p the pedicle of the Sacculina (external) ; m&basilar membrane from which the roots of the parasite proceed throughout the body. ova and a small number of other mesoblastic cells (Fig. 276, D, E). The bivalve shell of the Cypris stage is also shed, but the antenna remains connected at its base with the new cuticle secreted by the ectoderm, and still holding on to the seta. This process is com- pleted in about 3 hours. The soft-parts now shrink away from the anterior part of the old cuticle and form a new one within it. The new cuticle is produced in front into a pointed process open at the end, and behind is invaginated around the base of the process (Fig. 276, E). At this stage the young Cirripede is known as the Kentrogon larva. As growth proceeds the base of the process becomes evaginated, with the result that its point is thrust forward along the hollow antenna (Fig. 276 F) and pierces the soft cuticle of the crab at the base of the seta to which the larva is attached. The soft parts of the larva, consisting of the mass of primitive ova and other mesoblast cells, surrounded by a layer of ectoderm, now travel along the hollow process of the cuticle and enter the body of the crab. CIRRIPEDIA. 431 At the stage at which this remarkable Cirripede can next be recognized it has taken up its position in the connective tissue of its host between the intestine and the muscles lying in the ventral wall of the abdomen.* The internal Sacculina, as it is now called, consists of a rounded mass of cells, containing a minute compact body, the primitive ova, and con- tinued at its edges into long root-like processes which ramify throughout R. Fm. 278. Longitudinal sections through two stages of development of Sacculina carcini (from Korschelt and Heider after Delage). A Sacculina interna ; B Sacculina externa ; a atrium (widening of the oviduct) ; am outer 'mantle layer ; b brood-cavity (mantle cavity) ; B basal membrane ; C central tumour ; cl cloacal opening ; D intestinal wall of host ; dr cement glands of the ovarian sac ; / aperture of the perivisceral cavity ; (/ ganglion ; im inner mantle layer ; L body-wall of host ; ov ovary ; p psrivisceral cavity ; pe perivisceral ectodermal layer ; R root processes (some in cross section) ; t rudiment of testes. the soft tissues of the host, even to the tips of the extremities. The crab's heart and branchiae alone are free from the ramifications of its parasite (Fig. 277). As the central mass slowly grows, it begins in time to press against the ventral wall of the abdomen of the crab, which softens and * Mr. G. Smith has recently recognized the parasite when it formed a mass not more than 2 mm. in diameter, and lay considerably in front of its final position. 432 CRUSTACEA ENTOMOSTBACA. gives way before it. Through the aperture so produced the body of the parasite projects into the outer world (Figs. 277 and 278, B). The Sacculina now enters on its final phase of existence in which it is known as the external Sacculina. It forms a flattened oval mass about the size of the terminal joint of the little ringer, whose long axis is trans- verse to that of the abdomen. It is connected with its host by a short pedicle which passes from one end of the shorter axis through the abdom- inal wall, and is continued into the system of roots (cf. Fig. 278, B). At the opposite end of the short axis from the pedicle is the cloaca, which, until a brood of young has been produced, is closed by a chitinous plate projecting at the sides, beyond the lips of the cloaca. This leads into the brood-chamber (Fig. 278, B, 6) surrounding a central i-isceral mass which projects from the region of the pedicle into it. The wall bounding the brood-chamber externally is known as the mantle. It is connected with the visceral mass by a mesentery which is of small breadth but ex- tends from the region of the pedicle nearly to the cloaca along the side which is turned towards the right side of the crab. The Sacculina thus lies in a definite relation to its host. In the visceral mass lie the large paired ovaries, the ducts of which are connected with multilobed digi- tate cement glands (dr) and open on either flattened face (Fig. 278 B). Near its base lie two cylindrical testes. The vasa def erentia open into the brood chamber. A ganglion is situate 011 one side of the plane of the mesentery in the visceral mass and supplies nerves to it and to the mantle, which they reach through the mesentery. According to Delage's view all but the first batch of eggs are fertilized by the spermatozoa of the animal which produces them, and this is effected before the ova leave the oviducts ; the spermatozoa finding their way into the latter from the brood-chamber. When a batch of eggs is ripe the cuticular lining of each of the cement glands is shed all in one piece, and the multilobed digitate bag so produced becomes distended with the eggs in its passage to the brood-chamber. The two batches of eggs each contained in its cuticular sack, lie in the brood-chamber on either face of the flattened visceral mass. They are held in position by minute hooked prominences (retinacula) which project from the inner lining of the brood- chamber, and are supplied with oxyeen by the regular contraction of the mantle. From the eggs emerge the nnuplius larvae above described. It appears that no very long time elapses between the entry of the parasite into the crab and its taking up its position under the gut. Accord- ing to Delage the Sacculina becomes external at the age of 20-22 months, the host being about four months older. The first brood is pro- duced four months later, and other broods succeed, during the summer at intervals of 4 or 5 weeks. A Sacculina becomes external and begins to produce broods in the late summer, and the production of broods is con- tinued during the next summer. At the end of this second summer, being aged rather more than three years, it dies. While the Sacculina is ex- ternal and producing its broods of naiiplii the drain on the resources of the host is greatest, and the Crab does not increase in size or moult though it is not necessarily prevented from producing its own young.* * In the case, of the crab Inachus which is infested by a species of Sacculina, permanent infertility results from the action of the parasite. Cf. p. 445. CIREIPEDIA. 433 The effect of the presence of the parasite on the host is referred to on p. 445. It remains to notice the fact that when the Sacculina has become ex- ternal, but before the plate of chitin has disappeared from the cloacal opening, numbers of Cypris larvae are found to attach themselves by their antennae in the angle between the mantle and the projecting edge of the plate covering the cloacal opening. These have not been seen alive and nothing is known of their internal structure beyond the fact that they do not shed their swimming appendages or develop the pointed process of the cuticle formed by larvae which attach themselves as parasites. It is conjectured that these larvae are males, which in some way, at present unknown, fertilize the first batches of ova. The name Rhizocephala and the term " mantle " as above used imply definite homologies with the parts of other Cirripedes, and the question arises, How far is the use of these terms justified ? Notwithstanding the complexity of the metamorphosis undergone by these remarkable Cirripedes there appears to be nothing in the life-history to render unten - able the view that there exist in the fully formed Sacculina parts cor- responding to the mantle and the head region of other forms. With regard to the mantle, its relation to the visceral mass bears no doubt a certain resemblance to the relation of the mantle fold of other Cirripedes to the contained body ; but the account which Delage gives of the origin of the layers lining the brood-chamber, by delamination from an outer epithelial layer, lends no support to this homology. The ganglion, again, is formed as an ingrowth from this same outer layer, not in the plane of the mesentery but on one side. It must be confessed that the homology of the " mantle " of the Rhizocephala with that of other Cirripedes is uncertain ; and if this is uncertain there is no satisfactory reason for regarding the pedicle and the region from which the roots spring as anterior. The name Rhizocephala therefore, though retained here as that by which the group is usually known, implies a view of the homologies of the adult structure which is at least insecure. The Rhizocephala, parasites of other Crustacea, are themselves liable to be infested by members of the Epicarida, a parasitic group of the Isopoda. Fam. Rhizocephalidae. Degenerate Cirripedes, parasitic on Crustacea, and undergoing a remarkable metamorphosis. Peltogaster Rathke, irregularly cylindrical, uncompressed ; cloacal opening anterior in relation to host ; on Decapoda Anomala. Parthenopea Kossmann, roughly spherical, mantle opening lateral ; on Callianassa and Gebia. Sacculina Thompson, much compressed laterally, cloacal opening posterior, on Decapoda Brachyura. Heterosaccus O. Smith, like Sacculina, but with the mantle opening widely gaping ; on Decapoda Brachyura. Lernaeo- discus Miiller, mantle expanded laterally into lappets, opening posterior and median ; on Decapoda Anomura. Triangulus G. Smith, resembles Lernaeodiscus in many respects, but mantle opening asymmetrically situated ; on Decapoda Anomura. Sylon Kroyer, egg-shaped, mantle opening paired, anterior ; on Decapoda Macrura. Clistosaccus Lilljeborg, irregularly oblong, mantle opening absent ; on Decapoda Anomala. Incertae sedis. Duplorbis G. Smith, on the Isopod Calathura Apeltes Lilljeborg ; Thompsonia Kossmann ; Thylacoplethus Coutiere. z in F F 434 CRUSTACEA ENTOMOSTRACA. Sub-order 5. ASCOTHORACICA. These are parasitic, hermaphrodite or dioecious Crustacea generally living embedded in the tissues of their hosts. They are perhaps allied to Cirripedes but present no very clear affinities with any of the other sub -orders. A nauplius larva is found in Laura, and in Dendrogaster a later larval stage is known which somewhat resembles the Cypris larva of the Cirripedes. The antennae are, however, formed on a quite different plan, and no peduncular attachment is found in any of the four genera. ov. FIG. 279. Laura gerardiae Lacaze-Duthiers. a, body partly removed from the sack; b, a papilla from the outer surface of the sack ; c, complete sack attached to the skeletal stem of Gerardia (G), with the orifice towards the spectator, and in profile, ant. antennae ; /cau- dal fork ; F hepatic diverticulum contained between the layers of the mantle sack ; g.s. supra-oesophageal ganglion ; i intestine; o