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A TEXT-BOOK OF ZOOLOGY

A

TEXT-BOOK OF ZOOLOGY

BY

T. JEFFERY PARKER, D.Sc., F.R.S.

PROFESSOR OF BIOLOGY IN TUB UNIVERSITY OF OTAIiO. DUNKII1N, N.Z.

AND

WILLIAM A. HASWELL, M.A., D.Sc., F.R.S,

PROFESSOR OF BIOLOGY IN THF. UNIVERSITY OF SYDNEY, N.S.W.

IN TWO VOLUMES
VOL. I

WITH ILLUSTRATIONS

ILontion

MACMILLAN AND CO., LIMITED

NEW YORK: THE MACMILLAN COMPANY
1897

All rights reserved

,

_

RICHARD CLAY AND SONS, LIMITED,
LONDON AND BUNGAY.

ADDENDA ET CORRIGENDA

VOL. I

p. 188. Feiija and Aegir have been proved to be nothing more than damaged
examples of a normal Sea-anemone (Hcdcampoides).

p. 214. Line 15 from top, for " Hormosira" read " Hormiphora."
p. 230. The statement that Land-snails are sometimes the second host of the
Liver Fluke is based on error. In addition to Lymniva, however, other fresh-
water genera, certainly Hulinus (Physa) are sometimes the second hosts of this
Tremato.de.

p. 246. Fig. 192: n, nervous system; r, rostella ; r.b, sacs of rostella ; r.ts,
sheaths of rostella. (From Leuckart and Nitsche's Diagrams, after Pintner. )

p. 247. Fig. 195 : d</, vitelline duct ; dat, vitelline glands ; e, excretory
pore ; k, head end ; od, oviduct ; ov, ovary ; p, penis ; r..v, receptaculum
seminis ; t, testes ; vd, vas deferens ; va, vesicula seminalis ; w.y.o, female
genital opening.

p. 247. Fig. 196 : to, excretory opening ; mo, male reproductive opening ;
n, longitudinal nerve; n', junction of longitudinal nerves; n.r, nerve-ring;
o (in front), ovaries ; o (behind), tube leading from opening of vas deferens to
male aperture ; o', central cavity into which ova pass before being received
into the uterus ; p (in front), proboscis ; p (behind), papilla on which vas
deferens opens; r.s, receptaculum seminis; r.s.o, opening of duct leading to
receptaculum seminis ; .s, sucker ; t, lobes of testes ; ut, uterus ; vts, vesicula
seminalis ; yk, vitelline glands.

p. 267. Fig. 213, for " Rhabdoeoelida " read " Rhabdocoelida."j
p. 337. Line 19 from top, for " is " read " in."
p. 356 et seq. Antheneajlavescens should be Anthenea acuta.
p. 367. Last line of page, after "anus" insert "and is."
p. 474. Line 10 from top, for "gonods" read "gonads."
p. 485. Fig. 386, line 4 under cut, for " Broun's " read " Bronn's."
p. 520. Line 19 from top, for "podomore" read "podomere."
p. 533. Fig. 419, first line under cut, after "A, the entire animal" iustrf
" s. carina ; c. scutum ; t. tergum."
p. 546. Fig. 434, at end of line under cut, for " Set, larva" read "4a, larva."

VOL. II

p. 118. Line 3 from bottom, for "(a. I. c.)" read "(a. lat. c.)."
p. 131. Fig. 762, in description under cut, for ".s^. c." read "st. p."
p. 423. Fig. 1019 : The stroke from cor should run vertically downwards,
p. 429. Fig. 1023 : The third stroke from cun is an error ; it points to the
process of the second metatarsal.

PREFACE

IN spite of its bulk, the present work is strictly adapted to the
needs of the beginner. The mode of treatment of the subject is
such that no previous knowledge of Zoology is assumed, and
students of the first and second years should have no more
difficulty in following the accounts of the various groups than is
incidental to the first study of a complex and unfamiliar subject.

There can be little doubt that the study of Zoology is most
profitably as well as most pleasantly begun in the field and by the
sea-shore, in the Zoological Garden and the Aquarium. In a
very real sense it is true that the best zoologist is he who knows
the most animals, and there can certainly be no better foundation
for a strict and scientific study of the subject than a familiarity
with the general appearance and habits of the common members
of the principal animal classes. But Zoology as a branch of
academical study can hardly be pursued on the broad lines of
general natural history, and must be 'content to lose a little in
breadth of view at least in its earlier stages while insisting upon
accurate observation, comparison, and induction, within the limited
field of Laboratory and Museum work.

A not uncommon method of expounding the science of Zoology
is to begin the study of a given group by a definition the very
terms of which it is impossible that the student should under-
stand ; then to take a general survey of the group, illustrated by
casual references to animals and to structures of which it is highly
unlikely he has ever heard ; and, finally, to descend to a survey of
the more important forms included in the group. It will probably
be generally agreed that, from the teacher's point of view, this
method begins at the wrong end, and is hardly more rational than

vi PREFACE

it would be to deliver a course on the general characteristics of
English Literature, suitably illustrated by " elegant extracts," to a
class of students who had never read a single English poet or

essayist.

There can be no question as to the vast improvement effected in
zoological teaching by the practice of preceding the study of a
given group as a whole by the accurate examination of a suitable
member of it. With the clear mental image of a particular animal,
in the totality of its organisation, the comparison of the parts and
organs of other animals of like build becomes a profitable study,
and the danger of the comparative method that the student may
learn a great deal of the systems of organs in a group without
getting a clear conception of a single animal belonging to it is
much diminished.

The method of " types " has, however, its own dangers. Students
are, in their way, great generalisers, and, unless carefully looked
after, are quite sure to take the type for the class, and to consider
all Arthropods but crayfishes and cockroaches, and all Molluscs but
mussels and snails, as non-typical. For this reason a course of
Zoology which confines itself entirely or largely to " types," or, as
we prefer to call them, 1 examples, is certain to be a singularly
narrow and barren affair, and to leave the student with the
vaguest and most erroneous ideas of the animal kingdom as a
whole. This is especially the case when the number of examples
is small, each of the Phyla being represented by only one or two
forms.

In our opinion every gr.oup which cannot readily and intelli-
gibly be described in terms of some other group should be
represented, in an elementary course of Zoology, by an example.
We have, therefore, in the majority of cases, described, in some
detail, an example of every important class, and, in cases where
the diversity of organisation is very great as in Crustacea and
Fishes two or more examples are taken. The student is thus
furnished with a brief account of at least one member usually
readily accessible of all the principal groups of animals.

By the time the example has been studied, a definition of the
class and of its orders will convey some idea to the mind, and will

1 Following a suggestion for which we are indebted to Dr. Alexander Hill,
Master of Downing College, Cambridge.

PREFACE vii

serve to show which of the characters already met with are of
distinctive importance, and which special to the example itself.
In order to bring out this point more clearly, to furnish a connec-
tion between the account of the example and that of the class as a
whole, and to give some idea of the meaning of specific, generic,
and family characters, we have introduced, after the classification,
a paragraph giving the systematic position of the example, some-
times in more, sometimes in less detail.

Following the table of classification with its brief definitions
comes the general account of the group. This is usually treated
according to the comparative method, the leading modifications of
the various parts and organs being described seriatim. In a few
cases this plan has been abandoned and the class described order
by order, but this is done only when the deviations from the type
are so considerable as to lead us to think the comparative method
unsuitable for beginners. On the other hand, when all the classes
of the phylum present a very uniform type of structure, the
phylum is studied comparatively as a whole. The description of
each group usually ends with some account of its ethology and
distribution, and with a discussion of its affinities and of the
mutual relationships of its various sub-divisions.

We have done our best to make the space devoted to each group
proportional to its complexity and range of variation, and to
subdue the natural tendency to devote most attention to the more
recently investigated classes, or to those in which we ourselves
happen to be especially interested. A few lesser groups have been
put into small type, partly to economise space, partly because they
seem to us to be of minor importance to the beginner.

Following out the plan of deferring the discussion of general
questions until the facts with which they are connected have been
brought forward, we have placed the sections on Distribution, on
the Philosophy of Zoology, and on the History of Zoology at the
end of the book. We have, however, placed a general account
of the structure and physiology of animals immediately after the
Introduction, and one on the Craniate Vertebrata before the
description of the classes of that division, but it will be obvious
that these deviations from the strictly inductive method were
inevitable in order to avoid much needless repetition.

After a good deal of consideration we have decided to omit all

viii PREFACE

references to the literature of the subject in the body of the work.
Anything like consistent historical treatment would be out of place
in an elementary book ; and the introduction of casual references
to particular discoveries, while they might interest the more
advanced reader by giving a kind of personal colouring to the
subject, could hardly fail, from their necessarily limited character, to
be misleading to the beginner, and to increase rather than diminish
his difficulties. We have, therefore, postponed all reference to the
history of the science to the concluding Section, in which the main
lines of progress are set forth, and have given, as an Appendix, a
guide to the modern literature of Zoology. The latter is intended
merely to indicate the next step to be taken by the student who
wishes to acquire something more than a mere text-book
knowledge. 1

The various Sections have been written by the authors in fairly
equal proportions, but the work of each has been carefully read
and criticised by the other, and no disputed point has been allowed
to stand without thorough discussion. We are, therefore, jointly
and severally responsible for the whole work.

A very large proportion of the figures have been specially drawn
and engraved for the book. Those in which no source is named
are from our own drawings, with the exception of Figs. 571, 572,
1017, 1018, 1019, 1022, 1059, 1063, and 1071, for which we are
indebted to Mrs. W. A. Haswell. Figs. 1002 bis, 1005 bis, are from
photographs kindly taken for us by Mr. A. Hamilton. Many blocks
have been borrowed from well-known works, to the authors and
publishers of which we beg to return our sincere acknowledg-
ments. All the new figures have been drawn by Mr. M. P. Parker.

We have received generous assistance from Professors Arthur
Dendy, G. B. Howes, Baldwin Spencer, and J. T. Wilson, and from

1 In this connection we cannot resist the pleasure of quoting two passages,
exactly expressing our own views, from the preface to Dr. Waller's Human
Phyxiolixji/, which came under our notice after the above paragraph was in type :
"I have given a Bibliography after some hesitation, feeling that references to
original papers are of no use to junior students, and must be too imperfect to be
satisfactory to more advanced students. . . . Attention has been paid to
recent work, but I have felt that the gradually-formed deposit of accepted know-
ledge must be of greater intrinsic value than the latest ' discovery ' or the newest
theory. An early mental diet in which these items are predominant is an
unwholesome diet ; their function in elementary instruction is that of
condiments, valuable only in conjunction with a foundation of solid food."

PREFACE ix

Mr. J. P. Hill and Dr. Arthur Willcy. Professor W. N. Parker
has very kindly read the whole of the proof-sheets and favoured us
with many valuable suggestions, besides acting as referee in
numerous minor difficulties which would otherwise have cost a
delay of many weeks.

It is a mere truism to say that a text-book can never really
reflect the existing state of the science of which it treats,
but must necessarily be to some extent out of date at the time
of publication. In the present instance, the revises of the earlier
pages, giving the last opportunity for any but minor alterations,
were corrected in the latter part of 1895, and the sheets passed
for press in the middle of 1896. We are, therefore, fully alive to
the fact that much of our work already needs a thorough revision,
and can console ourselves only by reflecting that " to travel hope-
fully is a better thing than to arrive, and the true success is to
labour."

We may mention, in conclusion, that, whatever may be the merits
or demerits of the book, it enjoys the distinction of being unique
in one respect. The two authors have been separated from one
another, during the greater part of their collaboration, by a
distance of 1200 miles, and the manuscript, proofs, and drawings
have had to traverse half the circumference of the globe in their
journeys between the authors on the one hand, and the publishers,
printers, artist, and engravers on the other. It will, therefore, be
readily believed that all persons concerned have had every oppor-
tunity, during the progress of the work, of exercising the supreme
virtue of patience.

CONTENTS

PREFACE

LIST OF ILLUSTRATIONS IN VOL. I xvii

TABLE OF THE CLASSIFICATION OF THE ANIMAL KINGDOM . xxxj

INTRODUCTION 1

SECTION I

THE GENERAL STRUCTURE AND PHYSIOLOGY OF ANIMALS . . 10

1. Amoeba .... .... 10

2. The Animal Cell .... 14

3. The Ovum : Maturation, Impregnation, and Segmentation : the

Germinal Layers 18

4. Tissues . . 21

5. Organs ... . .29

6. The Reproduction of Animals 38

7. Symmetry 39

8. The Primary subdivisions or Phyla of the Animal Kingdom . . 41

SECTION II

PHYLUM PROTOZOA .43

Class I. Rhizopoda 44

1. Example of the Class Amoeba protens . .44

2. Classification and General Organisation 45
Systematic position of the Example . . 46

Class II. Mycetozoa 61

1. Example of the Class Didymium difforme . 61

2. General Remarks 63

Class III. Mastigophora 63

1. Example of the Class Euglena viridis . 63

2. Classification and General Organisation . 65
Systematic Position of the Example . 65

Class IV. Sporozoa 75

1. Example of the Class Monocystis cujilis . 75

2. Classification and General Organisation . . 76

xii CONTENTS

PACB

PHYLUM PROTOZOA continued.

Class Y. Infusoria . ... .79

1. Example of the Class Paramoscium caudtttimi . . 79

2. Classification and General Organisation . . 82
Systematic Position of the Example . . 82
Further Remarks on the Protozoa . 92

SECTION III

PHYLUM AND CLASS PORIFERA .96

1. Example of the Class Si/co)i gelatinosum . . .96

2. Distinctive Characters and Classification ... .102
Systematic Position of the Example ... . 103

3. General Organisation 105

SECTION IV

PHYLUM CCELENTERATA . . . 118

Class I. Hydrozoa . . . 118

1. Example of the Class Obelin . ... . . 118

2. General Structure and Classification . . . 13O
Systematic Position of the Example ... . 132

3. Additional Remarks ... . . . 155

Class II. Scyphozoa . . . 156

1. Example of the Class Aurelia aiirita . . . . 156

2. General Structure and Classification .... . 163
Systematic Position of the Example .... . 164

3. Additional Remarks . . . . 171
Class III. Actinozoa . . . . . 172

1. Example of the Class Tealia crassicornis . . . 172

2. Distinctive Characters and Classification ... . 180
Systematic Position of the Example . . . . 183

3. General Organisation . ... 183
Class IV. Ctenophora . . .198

1. Example of the Class Hormiphora plnmosa . . . 198

2. Distinctive Characters and Classification ........ 207

Systematic Position of the Example . . . . 208

3. General Organisation . 209

Appendix to Ctenophora Ct'enoplana and Codoplana . . . 212

The Relationships of the Ccelenterata 213

Appendix to the Ccelenterata . 216

The Mesozoa . .... 216

Salinella . . . 219

Trichoplax . . 220

SECTION V

PHYLUM PLATYHELMINTHES . . .... . 221

1. Examples of the Phylum . . 222

i. Plnnaria or Dendroccelnm .... . 222

ii. Distomum hepaticum . . 226

iii. Ta'nia sulium 231

CONTENTS xiii

PACK

PHYLUM PLATYHELMIXTHES continued.

2. Distinctive Characters and Classification . . 237
Systematic Position of the Examples . . 238

3. General Organisation . 239

4. Distribution, Mode of Occurrence, and Mutual Relationships . 263
Appendix to Platyhelminthes Class Nemertinea .... 268

SECTION VI

PHYLUM NEMATHELMINTHES . . . . 275

Class I. Nematoda ... . 275

1. Example of the Class Asairis lumbricoides . . 275

2. Distinctive Characters and Classification . . 281
Systematic Position of the Example . . 282

3. General Organisation ... . 283
Class II. Acanthocephala . . 289
Class III. Chatognatha . 293

Appendix to Nemathelminthes 296

Family Chatosomidce . . 296

,, Echinoderidce . 296

,, Desmoscolecidte . . . ... . 296

Affinities and Mutual Relationships of the Nemathelminthes . 297

SECTION VII

PHYLUM TROCHELMINTHES . . . . 298

Class I. Rotifera . . . 299

1. Example of the Class Bmchionus rubens . 299

2. Distinctive Characters and Classification . . . . 303
Systematic Position of the Example . . 304

3. General Organisation . . ... 304
Class II. Dinophilea . . 310
Class III. Gastrotricha . 311

SECTION VIII

PHYLUM MOLLUSCOIDA. . . . 313

Class I. Polyzoa . . . ^*, 313

1. Example of the Class Bugula avicularia . 314

2. Distinctive Characters and Classification . . 319
Systematic Position of the Example . . 320

3. General Organisation ... . 321
Class II. Phoronida . . . 328
Class III. Brachiopoda ,331

\ . Example of the Glass Magdlania lenticnlaris . 331

2. Distinctive Characters and Classification . 337

Systematic Position of the Example . . . 338

M. General Organisation .... . . . 338

Mutual Relationships of the Classes of the Molluscoida . . 343

xiv CONTENTS

SECTION IX

PAGE

PHYLUM ECHINI I>ERMATA . . 346

1. Examples of the Phylum ... . . 346

i. Axf>'i-!<i* rii/if us and Anthenea flavescens .... 346

ii. Strongylocentrotiis or Echinus . . 363

iii. Crucumaria or Colochirus . . . . 369

iv. Anti'il'ni I'nxacea ... 373

2. Distinctive Characters and Classification . . . 377

Systematic Position of the Examples . . . 380

3. General Organisation . . 381

SECTION X

PHYLUM ANXULATA . . 403

Class [. Chaetopoda . . . 403

1. Examples of the Class . . . 404
i. JWyW.s dumerHii . . . 404

ii. Lumliricns .... . . . 417

2. Distinctive Characters and Classification . . 426
Systematic Position of the Examples . . . 428

3. General Organisation . 429

Appendix to Chaetopoda Class Myzostomida . . . 448

Class II. Gephyrea .... . 450

1. Example of the Class Sipuncidus nudus . . 451

2. Distinctive Characters and Classification . . 454
Systematic Position of the Example . . 455

3. General Organisation ... . 455
Class III. Archi-annelida . 462
Class IV. Hirudinea . ... 465

1 . Examples of the Class Hirndo medicinal, is and H. quinquestriata 465

2. Distinctive Characters and Classification .... . 474
Systematic Position of the Examples . 476

.">. General Organisation .... . 476

General Remarks on the Annulata . 481

SECTION XI

PHYLUM AHTHROPODA . . . . . . . 484

Class I. Crustacea . 484

1. Examples of the Class . 484
i. Ap\is or Lepidurus . . 484

ii. Astacus fluviatttis . . . 498

2. Bistinctive Characters and Classification . . 519
Systematic Position of the Examples . 524

.".. General Organisation .... . 525

Appendix to Crustacea Class Trilobita . 558

Class II. Onychophora .. . 559

Class III. Myriapoda . . 566

1. Distinctive Characters and Classification . , 566

2. General Organisation ... . 567

CONTENTS xv

PAGE

PHYLUM ARTHROPODA continued.

Class IV. Insecta .... .571

1. Example of the Class PeriplaM$fatamericana . 571

2. Distinctive Characters and Classification . . 588
Systematic Position of the Example . . 587

3. General Organisation . . . 588
Class V. Arachnida .... . 604

1. Example of the Class Enscurpln or Buthus . . . 604

2. Distinctive Characters and Classification . . .611

3. General Organisation ... 613

Appendix to the Arachnida the Pycnogonida, Linguatulida,

and Tardigrada . . 624

Relationships of the air-breathing Arthropoda . 627

SECTION XII

PHYLUM MOLLUSCA . . . 631

Class I. Pelecypoda . . . . 631

1. Examples of the Class Anodonta and. Unio . . 631

2. Distinctive Characters and Classification . . 645
Systematic Position of the Examples . 647

3. General Organisation . . 647
Class II. Amphineura . 663

1. Distinctive Characters and Classification . . 663

2. General Organisation . 663
Class III. Gastropoda . . 671

1. Example of the Class Triton nudiferus . . 671

2. Distinctive Characters and Classification . . 682
Systematic Position of the Example . 685

3. General Organisation . . . 685
Appendix to the Gastropoda

A. Class Scaphopoda . 705

B. Rhodope . . 707
Class IV. Cephalopoda . 708

1. Examples of the Class . 708
i. Sepia cultmta . . 708

ii. Nautilus pompiliiis .... . . 725

2. Distinctive Characters and Classification . . 736
Systematic Position of the Examples . 737

3. General Organisation ... . . 737
General Remarks on the Mollusca . 751

LIST OF ILLUSTRATIONS

Klli. FAOE

1. Amoeba proteus 10

2. Amoeba polypodia, fission. 13

3. Diagrams illustrating Karyokinesis 17

4. Ovum of Sea-urchin 18

5. Maturation and fertilisation of ovum 19

6. Segmentation of ovum 20

7. Gastrula . . ... 21

8. Various forms of epithelium 22

9. Diagram to illustrate structure of glands 23

o o

10. Gelatinous connective tissue 24

11. Reticular connective tissue 24

12. Fatty tissue . . 25

13. Hyaline cartilage .25

14. Fibro-cartilage 25

15. Bone . . .26

16. Unstriped Muscle . . ... . .27

17. Striped Muscle . . 27

18. Nerve cells . .... 28

19. Nerve fibres . . 28

20. Various forms of spermatozoa . . 28

21. Viscera of Frog 31

22. Bones of human arm with biceps muscle 35

23. Nervous system of Frog .... 36

24. Hydra ... . .39

25. Diagram of axes of body .... 40

26. Radial symmetry 40

27. Amoeba, various species ... 45

28. Protamceba primitiva 47

29. Quadrula, Hyalospheiiia, Arcella, and Difflugia . .47

30. Chlamydomyxa labyrinthuloides .... .48

31. Labyrinthula vitellina . . . . . . . . ... 49

32. Microgromia socialis .50

33. Platoum stercoreum .51

34. Various forms of Foraminifera . . . . .52

35. Shells of Foraminifera ... . .53

36. Hastigerina murrayi .... .54

xviii LIST OF ILLUSTRATIONS

FIO. PACK

37. Actinophrys sol ... .55

38. Actinosphserium eichhornii .56

39. Various forms of Heliozoa ... . .57

40. Lithocircus annularis ... .58

41. Thalassoplancta brevispicula . . 59

42. Aulactinium actinastrum . . 60

43. Actinomma asteracanthioii . . .60

44. Collozoum inerme . 61

45. Didymium difforme . . .62

46. Euglena viridis 64

47. Various forms of Flagellata .... . .66

48. Hamatococcus pluvialis ... 68

49. Pandorina morum ... .69

50. Volvox globator ... . 70

51. Heteromita rostrata .... -71

52. Various forms of Choanoflagellata . . 72

53. Various forms of Dinoflagellata ... . 74

54. Noctiluca miliaris .... .74

55. Monocystis agilis ... . .75

56. Gregarina 77

57. Eimeria and Coccidium .... .78

58. Myxidium and Myxobolus ... .78

59. Sarcocystis miescheri ... . 7'-'

60. Paramoecium caudatum . . ... 80

61. ,, ,, conjugation . . 81

62. Various forms of Ciliata ... 85

63. ,, ,, 86

64. Vorticella ... .87

65. Zoothamnium arbuscula . .88

66. Opalina ranarum . . . .89

67. Various forms of Tentaculifera ... .91

68. Diagram showing the mutual relationships of the Protozoa . . 94

69. Sycon gelatinosum ... .97

70. ,, ,, magnified . 97

71. ,, ,, transverse section . 98

72. ,, ,, vertical section . 99

73. ,, ,, pore membrane . . 100

74. ,, ,, apopyle . . . 100

75. External form of various Sponges . . 106
7<>. Ascetta primordialis . . . . 107

77. Diagrams of canal system of various Sponges ... . 108

78. Vertical Section of Spongilla . 109

79. Culls of ectoderm of Sponge . . 110

80. Skeleton of various Sponges . . Ill

81. Various forms of Sponge Spicules . . 112

82. Development of Sycon raphanus . . 114

83. Obelia . . . li'O

84. ,, vertical section of polype ... . 122

85. Nematocysts of Hydra .... . 123

LIST OF ILLUSTRATIONS xix

FKi

86. Tentacle of Eucopella

87. Obelia, medusa

88. Diagram of medusa . .......

89. Derivation of medusa from polype . . . 127

90. Projections of polype and medusa ....

91. Development of zoophyte . ..... 130

92. Bougainvillea raniosa . . . 134

93. Various forms of Leptolinne . . 135

94. Hydra . . . 13

95. Protohydra leuckartii .... ... 137

96. Various forms of leptoliiie Medusse ..... . 139

97. Diagram illustrating formation of sporosac by degeneration of

medusa ............. 140

98. Early development of Eucope . 141

99. Two Trachymedusse . . 142

100. Two Narcomeduste ........ . 143

101. yEginura, tentaculocyst . ... . . 143

102. Larva of yEginopsis ... . 144

103. Millepora alcicornis, skeleton ... . 145

104. Millepora, diagram of structure ... . . 14(>

105. Stylaster sanguineus, skeleton ... . 147

106. Halistemma tergestinum ....... . . 148

107. Diagram of a Siphonophore .... . 150

108. Development of Halistemma . . . 151

109. Physalia . .152

110. Diphyes campanulata ... . 153

111. Porpita pacifica ........ . 154

112. Graptolites . . 155

113. Aurelia aurita, dorsal and ventral views . . . 157

114. ,, ,, side view and vertical section ..... 159

115. ,, ,, portion of umbrella with tentaculocyst . . . 160

116. ,, ,, development .... . . 162

117. Tessera princeps . ..... . . 165

118. Lucernaria ... .... 166

119. Pericolpa quadrigata . . . . 167

120. Charybttfea marsupialis . ... 168

121. Nausithoe . . . 169

122. Pilemapulmo . . 170

123. Pelagia noctiluca, development ...... .171

124. Tealia crassicornis, dissection and transverse section . 173

125. Diagrammatic sections of Sea-anemone . . . . 175

126. Tealia crassicornis, section of tentacle .... . 1 77

127. Nematocysts of Sagartia ........ . 177

128. Section of mesenteric filament of Sagartia ... . 178

129. Transverse sections of embryos of Actinia .... . 180

130. Zoanthus sociatus ....... . . 184

131. Hartea elegans ....'. .... . -184

132. Corallium rubrum ........... 185

133. Astrsea pallida . . . 185

I -1

xx LIST OF ILLUSTRATIONS

Fid. PACE

134. Pennatula sulcata . . 186

135. Tubipora musica. . . . . . 186

136. Edwardsia claparedii 187

137. Cirripathes anguina . . 188

138. Fenja mirabilis . .189

139. Minyas . 189

140. Alcyonium palmatum ... .... 190

141. Gorgonia verrucosa . 191

142. Structure of simple coral .... 193

143. Dendrophyllia and Madrepora . ...... 194

144. Adamsia palliata . . ...... 196

145. Hormiphora plumosa 198

146. ,, ,, dissection and transverse section . . .199

147. ,, ,, diagrammatic sections 201

148. , , , , section of branch of tentacle .... 202

149. ,, ,, sense-organ . . . ... 203

150. Ovum of Lampetia . . . 204

151. Segmentation of oosperm in Ctenophora . . . 204

152. Development of Ctenophora . . . . .205

153. Development of Callianira ... . 206

154. ,, ,, (later stages) . . 206

155. Three Cydippida . 209

156. Deiopea kalokiienota . . . . . 210

157. Cestus veneris . ... 210

158. Beroe forskalii . . 211

159. Ctenoplana kowalevskii ... 212

160. Sections of embryos of Actinia and Beroe 214

161. Diagram illustrating the mutual relationships of the Coalenterata . 215

162. Dicyema paradoxum with infusoriform embryos .... 217

163. ,, ,, ,, vermiform ,, ... 217

164. ,, ,, embryo . .... 218

165. Rhopalura giardii, male . . ... 218

166. ,, ,, female . . 218

167. ,, ,, development . . ... '-. 219

168. Salinella, longitudinal section ... . 219

169. ,, transverse ,, . ... 219

170. Trichoplax adhjerens . 220

171. Planaria, digestive and excretory systems . ... 223

172. ,, nervous system ... 223

173. ,, reproductive system . ... . . 225

174. Transverse section of a Planarian . .... 226

175. Distomum hepaticum ... . 226

176. ,, ,, section of integument . ... 227

177. ,, ,, internal organisation 228

178. ,, ,, terminal part of reproductive apparatus . 229

179. ,, ,, development . . . 230

180. Tamia solium ... . .232

181. ,, head . . 233
1---. ,, ,, transverse section 233

LIST OF ILLUSTRATIONS xxi

FIG.

183. Trenia solium, proglottis .... . 234

184. ,, ,, ripe proglottis . . . . 236

185. ,, ,, development ... . ... 236

186. Various Planarians . . ... 240

187. Gunda segmentata . . 241

188. Digenetic Trematodes ... . 242

189. Gyrodactylus and Polystomum . . 243

190. Tenmocephala . . .244

191. Actiiiodactylella . . 245

192. Tetrarhynchus . . 246

193. Trenia echinococcus . . 246

194. Ligula ... .247

195. Caryophylheus- . . 247

196. Amphiptyches . . ... 247

197. Archigetes . . . .247

198. Section of body-wall of a Triclad . . 248

199. Parenchyma of Flat-worm .... . 249

200. Diagram of Rhabdoc<ele ... . .251

201. ,, ,, Polyclad . . .251

202. ,, ,, Triclad . . 252

203. Flame-cell . . 253

204. Reproductive organs of Mesostomum ehrenbergii . . . 256

205. Development of a Polyclad . . . 257

206. Miiller's larva . . 258

207. A Cysticercoid . . 260

208. with head evaginated . . 261

209. Cyst of Taenia echinococcus ... . 262

210. Scolices ,, .262

211. Scolex . 262

212. Process of budding in Microstomum . ... 263

213. Diagram of the relationships of the Platyhelminthes and NemcT-

tinea . . . .267

214. Diagram of Nemertine .... ... 268

215. Tetrastemma . . 269

216. Anterior portion of Nemertine . . . 270

217. Proboscis of Hoplonemertean, retracted . . . 270

218. ,, ,, everted . . 270

219. Transverse section of Nemertine ... . 271

220. Vascular and excretory systems of Nemertine . . 271

221. Pilidium . . 272

222. Ascaris lumbricoides . . 276

223. ,, ,, transverse section . 277

224. ,, ,, muscie fibres . 278

225. ,, ,, dissection of female ... . 279

226. Nervous system of Nematoda . . . . 280

227. Ascaris lumbricoides, dissection of male organs . . 281

228. Body-wall of platymyarian Nematode . . 283

229. Dochmius duodenalis .... 284

230. Transverse section of Gordius 284

xxii LIST OF ILLUSTRATIONS

VI i ; PAttE

231. Oxyuris 285

232. Gordius, anatomy . . . . 286

233. Development of Ascaris nigrovenosa . . . 287

234. Trichina spiralis . 288

235. Two species of Echinorhynchus . . . 290

236. Echinorhynchus gigas, dissection of male . . 291

237. ,, ,, ,, female . . 291
1238. ,, ,, female organs . . 292

239. Sagitta hexaptera . 293

240. ,, bipunctata, transverse sections . . . 294

241. ,, head . . 294

242. ,, hexaptera, eye . . 295

243. Development of Sagitta . . . 295

244. A trochosphere . . . 298

245. Brachionus rubens, female . . . 300
241). ,, ., pharynx . . .301

247. ,, ,, male and female, with attached eggs . . 302

248. Diagram of a Rotifer ... .303

249. Typical forms of Rotifera . 305
250 ,, . 307

251. ,, ,, mastax . . 308

252. Dinophilus gyrociliatus . ... 310

253. ChiL'tonotus maximus . . . . 311

254. ,, ,, anatomy . 311

255. Bugula avicularia . . . 315

256. Development of Bugula . . 317

257. ,, - 318

258. Plumatella . . 322
258 b is. Cristatella . . 323

259. Lophopus . 324

260. Pedicellina . 327

261. Phoronis australis .... ... . 328

262. .. ., free end . . :52!>

263. ,, ,, internal organisation ... . 329

264. ,, ,, development .... . 330

265. Magellania flavescens, shell . . 332

266. ., , lenticularis, anatomy .... . 334

267. ,, navescens, lophophore . 335

26S. ,, muscular system . .... . 335

269. Terebratula, nervous system, &c. ... . 336

270. Typical Brachiopods . . . . 339

271. ,, ,, anatomy . 340

_'72. Development of Cistella . . 341

27:;. 342

-74. Lophophore of embryo Brachiopod . . 343

275. Diagrams of phylactolaematous Polyzoons . . . 344

276. Starfish, ventral aspect .... . . 347

277. ,, vertical section of arm ... . 349

278. ,, ambulacral system ... 350

LIST OF ILLUSTRATIONS xxiii

FIfi. PAGE

279. Starfish, portion of vertical section of arm 351

280. ,, diagrammatic sections . . . 352

281. Asterias rubens, digestive system . 353

282. Astropecten, section of stone-canal .... 354

283. Anthenea flavescens, dissection from dorsal aspect . . . 355

284. ,, ,, lateral dissection . . 356

285. ,, ,, dorsal surface . . . 357

286. ,, ,, ventral surface . . . 357

287. Asterina gibbosa, development . . ... 359

288. ,, \ 360

289. ,, ,, bipinnaria . . 360

290. ., 361

291. ,, exigua, young after metamorphosis . . . 361

292. Apical system of young Starfish . . . 362

293. Strongyloceiitrotus . . . 364

294. Corona of Sea-urchin . . 365

295. Apical disc of Sea-urchin . . 365

296. Echinus, lantern of Aristotle . . 366

297. Sea-urchin, anatomy, lateral view . 367

298. ,, ,, oral view . . 368

299. Cucumaria planci ... . 369

300. Anatomy of a Holothurian . . 371

301. Antedon ... 373

302. ,, disc . . 374

303. , , transverse section of pinnule . . 375

304. ,, sagittal section . . . 376

305. Anthenea, ventral view . . 385

306. Ophioglypha lacertosa . . 386

307. Astrophyton arborescens . 387

308. Strongyloceiitrotus .... . 388

309. Diagram of spine of Sea-urchin . . 389

310. Pedicellaria of Arbacia punctulata . 389

311. Hemipneustes radiatus . . 390

312. Clypeaster sub-depressus . 390

313. Antedon . . 391

314. Metacrinus interruptus . . . 392

315. Development of Echinoderms 397

316. ,, ,, Antedon ... . . 398

317. Diagram to illustrate the relationships of the classes of Echino-

dermata .... ... . . 402

318. Nereis dumerilii ... . 404

319. ,, ,, parapoclium . 405

320. ,, ,, setts . 405

321. ,, ,, anatomy . . 407

322. ,, ,, transverse section . . 408

323. ,, nervous system .... . 410
.".24. ,, eye . 411

325. ,, dumerilii, riephridium . . 412

326. ,, development .... . 414

xxiv LIST OF ILLUSTRATIONS

PAOE

327. Nereis development . . . 415

328. Lumbricus agricola . 418
:!2<l. setre . .419

330. ,, transverse section . 41 ( .>

331. ,, herculeus, sagittal section . 420

332. ,, nervous system . . . 422

333. ,. nephriclium . . 423

334. ,, agricola, reproductive organs . . 424

335. ,, development ... . 426

336. Polynoe setosissima . 429

337. Vermilia ccespitosa . . 430

338. Setfe of various Polychreta ... . . 431

339. Section of setigerous sac of an Oligochrete . . 431

340. Polynoe extenuata, anterior end . 432

341. Tubifex . . 433

342. Terebella . 434

343. Aphrodita, enteric canal . . 436

344. Saccocirrus, transverse section . . , 438

345. Diagram illustrating development of gonad of Polyclueta . . 441

346. Spirorbis kevis .... . . 444

347. Eupomatus, development of trochosphere . . 445

348. Autolytus cornutus, budding ... . 446

349. Serpuhe with their tubes . 447

350. Myzostomum . . 449

351. ,, anatomy ... . 450'

352. Sipunculus nudus, anterior extremity . . . . . 451

353. ,, ,, tentacular fold . . 452

354. ,, ,, anatomy . . 453^

355. ,, ,, nervous system . 453^

356. Bonellia viridis, female . . . 456

357. Echiurus . . 456

358. Priapulus . . . 457

359. Bonellia, anatomy . . 458

360. Echiurus, ciliated funnel . . 458
3(51. ,, anatomy . . 459

362. ,, nervous system . . 459

363. Bonellia, male . . 460

364. Echiurus, trochosphere . . 460

365. Polygordius neapolitanus . . . 462

366. Protodrilus . .463

367. Polygordius neapolitanus, transverse section . 463

368. ,, ,, trochosphere . . 464

369. ,, ,, ,, later stage . . . 464

370. Hirudo medicinalis . . 466

371. ,, ,, transverse section . . . 468-

jaw . . . 468

">7-"'. ,, quinquestriata, dissection from dorsal aspect . . . 469

374. ,, ,, ,, ,, left side . 470-

375. ,, medicinalis, nephridiuni . . . 471

LIST OF ILLUSTRATIONS xxv

FIG. I'M, I

376. Hirudo, diagram of blood-channels ... . . 472

377. ,, section of eye . . . 473

378. ,, cocoon ... . 474

379. Three Rhynchobdellida . . 475

380. Proboscis of Clepsine . . 477

381. Transverse sections of three Leeches . . . 478

382. Pontobdella, nephridial system . . . 479

383. Clepsine, development .... . . 47-'

384. Diagram of origin of metamerism 482

385. Diagram illustrating the relationships of the Annulataand Trochel-

minthes 483

386. Apus cancriformis, dorsal aspect . ... 485

387. Lepidurus kirkii, side view . . 486

388. Apus glacialis, ventral aspect 487

389. ,, appendages 488

390. Lepidurus kirkii, sagittal section . 491 )

391. Apus, transverse section 492

392. ,, shell-gland ... 493

393. ,, cancriformis, nervous system .... . 494

394. ,, structure of paired eye ... 495

395. ,, development 496

396. Astacus fluviatilis, male . . . 499

397. ,, ,, appendages .... . 501

398. ,, ,, articulations and muscles of leg . . 503

399. Section of skin and exoskeleton of Lobster . . 504

400. Articulations and muscles of abdomen of Crayfish . . 505

401. Astacus fluviatilis, dissection from right side 506

402. ,, ,, gills ... . 508

403. ,, ,, kidney 510

404. ,, ,, transverse section of thorax . . . 511

405. ,, ,, diagram of circulation . ... 512

406. ,, ,, nervous system . . . 513

407. ,, ,, reproductive organs . . . 515

408. ,, ,, formation of the blastoderm . . . 515

409. ,, ,, early embryo . 516

410. ,, ,, nauplius 517

411. ,, ,, section of embryo . . . 518

412. ,, ,, advanced embryo .... 519

413. Three Euphyllopoda . . ... 526

414. ,, Cladocera . ... 527

415. Cypris ... . 528
41(J. Cyclops and Calocalanus . 529

417. Various forms of parasitic Eucopepoda . . . 531

418. Argulus foliaceus . . 532

419. Lepas anatifera . . ... . 533

420. Balamis . . 534

421. Sacculina carcini . . . 535

422. Nebalia geoft'royi . . . 536

423. Mysis oculata . ... 537

xxvi LIST OF ILLUSTRATIONS

rift.

424. Shrimp and Prawn . 538

425. Scyllarus arctus 539

426. Pagurus bernhardus 539

427. Cancer pagurus . 540

428. Typical Brachyura . 541

429. Squilla . . 542

430. Diastylis stygia . . 543

431. Gammarus neglectus 544

432. Asellus aquaticus . 545

433. Various Amphipoda . 546

434. ,, Isopoda . 546

435. Orchestia cavimana, anatomy 548

436. Euphausia pellucida . 549

437. Nervous system of Crab . . 550

438. Cypris-stage of Lepas . 552

439. Larvre of Crabs ..... . 554

440. Diagram illustrating the mutual relationships of the orders of

Crustacea . .... 557

441. Dalmanites and Phacops . . 558
441-6 is. Triarthrus beckii . . . 559

442. Peripatus capeiisis .... . 560

443. ,, ,, ventral view of head . 560

444. ,, anatomy . 561

445. ,, edwardsii, nephridium . . - 563

446. ., novaezealandiae, development . 564

447. ,, capensis ,, . 565

448. Scolopendrella immaculata . . 567

449. Scolopendra . . . 568

450. Lithobius forficatus . 568

451. Pauropus huxleyi . . . 569

452. Strongylostoma, development - 570

453. Periplaneta americana . 572

454. ., mouth-parts . 573

455. ,, americana, lateral view of head . . 573

456. ,, muscular system . . .5/5

457. ,, anatomy . 576

458. ,, salivary glands . . . 577

459. Trachea of caterpillar 577

460. Periplaneta, tracheal system . . 578

461. ,, nervous system . . 578

462. ,, male reproductive organs . . . 579

463. ,, female reproductive organs . . 579

464. Segmentation of ovum of Insect .... . 580

465. Ventral plate of embryo Cockroach . 581

466. Germinal layers and amnion of Insect . . 582

467. Lepisma . . . 582

468. Podura . . 584

469. Locusta . . 584

470. Ephemera . . 584

LIST OF ILLUSTRATIONS xxvii

FKi. 1'ACIE

471. Aphis rospti 585

472. Cicada ... ..... . 585

473. Pulex and larva . 586

474. Gastrophilus equi .... 58(3

475. Pieris .... . .586

476. Crioceris ... 587

477. Section of integument of Insect . 588

478. Mouth-parts of Honey-bee 589

479. ,, ,, Diptera . . . 590

480. ,, ,, Lepidoptera . 591

481. Digestive organs of Beetle 592

482. Nervous, tracheal, and digestive systems of the Honey-bee . . 593

483. Tracheal gills of Ephemerid .... . 594

484. Heart of Cockchafer . . .... . 594

485. Nervous system of Diptera 595

486. Ocellus of Dytiscus larva . . 596

487. Chordotonal organ of Isopteryx .... . . 597

488. Sexual apparatus of Honey-bee . 598

489. Segmentation of ovum of Insect 599

490. Germinal layers and amnion of Insect . . . . 600

491. Development of Hydrophilus 601

492. . . .601

493. Apis mellitica, queen, worker, and drone 603

494. Formica rufa . 603

495. Euscorpio . . ... 605

496. Ventral surface of cephalothorax and pr;e-abdomen of Scorpion . 605

497. Endosternite of Scorpion ... ... 60(5

498. Scorpion, anatomy, lateral view . . 608

499. ,, ,, dorsal ,, . . . 609

500. ,, development . . . 610

501. Embryo of Scorpion . . 610

502. Chelifer bravaisii . 613

503. Phrynus . . 613

504. Galeodes dastuguei . . .... 614

505. Epeira diadema ... . 615

506. ,, ,, chelicene and pedipalpi of female .... 615

507. ,, ,, -. male 615

508. Sarcoptes scabirei . . . 616

509. Trombidium fuliginosum ... . . 616

510. Limulus .... 617

511. ,, ventral view . 618

512. Eurypterus fischeri .... 619

513. Anatomy of dipneumonous Spider . 620

514. Limulus, sagittal section .... . .621

515. Lung-book of Spider ... . . 621

516. Tracheal system of Spider . . 621

517. Gill-books of Limulus . <>i"J

518. Lateral eye of Euscorpius 622

519. Central . 623

xxviii LIST OF ILLUSTRATIONS

Flli. PACE .

520. Nymphoii hispiduni . ... . 625

521. Pentastomum taenioides . ... 625

522. Macrobiotus hufelandi . ... 626

523. Diagram to illustrate affinities of Arthropoda 629

524. Anodonta cygnea ... .... . 632

525. ,, ,, interior of valve and animal removed from shell. 633

526. ,. section of shell and mantle . ... 634

527. ,. cygnea, animal after removal of mantle-lobe . . 636

528. ,, ,, dissection from left side . . 637

529. ,. ., structure of gills . . 638

530. ,. ,, transverse sections . . 639

531. ,, diagram of circulation . . 641

532. .. otocyst . 642

533. ,. early embryo . . 643

534. , . later embryos . . 643

535. ,, advanced embryo . . 644
536 ,, metamorphosis . 645

537. Anatomy of Pecten . . . 648

538. Valves of Mya, Modiola, and Vulsella . . 649

539. Cardium edule . . 649

540. Venus gnidia . . 650

541. Scrobicularia piperata . 650

542. Solecurtus strigillatus . 651

543. Diagram of concrescence of mantle-lobes . . 651

544. Requieiiia and Hippurites . . 652

545. Teredo navalis . . (152

546. Aspergillum . . . 653

547. Mytilus edulis . . 653

548. Nucula nucleus . .... 654

549. Grills of Pelecypoda . . 655

550. Gill-filaments of Mytilus . 656

551. Dissection of Poromya . 656

552. Donax, enteric canal ... . 657

553. Diagram of Nucula . . 657

554. Nervous system and auditory organs of Nucula . . 658

555. Eye of Pecten ... . 659

556. Development of Ostrea . ... . 66O

557. Veliger of Ostrea . . 6C,i)

558. Embryos of Cyclas . . 661

559. Diagram illustrating the mutual relationships of the Pelecypoda . 662

560. Chsetoderma nitidulum . 664

f>61. Neomenia carinata . . . 664

562. Chiton spinosus, dorsal view . . 665

563. ,, ventral view . . ... 665

564. ,, valves of shell . . 665
56."). Chsetoderma nitidulum, longitudinal section . . 666
566. Chiton, longitudinal section . . . 667
.~>67. Nervous system of Amphineura . . 667

565. Neomenia carinata, reproductive organs . . 668

LIST OF ILLUSTRATIONS xxix

FIG. l'\i:K

569. Chiton, nephridial and genital systems . . . . (id'.)

570. ,, development ... 670

571. Triton iiodiferus, shell . .672

572. Triton iiodiferus, shell, median section . . 673

573. ,, ,, operculum . 674

574. ,, ,, lateral view of body . . 674

575. ,, ,, diagram of introvert . . . 675

576. ,, ,, dissection from dorsal side . . 677

577. ,, ,, buccalmass .... . . 678

578. ,, ,, vertical section of buccal cavity .... 678

579. , , , , nervous system from dorsal side . . . 680

580. ,, ,, ,, ,, and related parts, lateral view . 681

581. ,, ,, section of eye . ... . 682

582. Solarium perspectivum . . . . 685

583. Terebra oculata . . . 686

584. Cyprasa nioneta 687

585. Doris tuberculata . . 687

586. Carinaria mediterranea ... ... . 687

587. Limax . . .687

588. Sigaretus Itevigatus . . . 688
589 Aplysia .... . 688

590. Shell-bearing Pteropoda . . 689

591. Atlanta peronii .... . . 689

592. Pterotrachea scutata ... . 690

593. Helix nemoralis . . 690

594. Pleurophyllidia lineata . . 691

595. Patella vulgata ... .691

596. Pulmonary cavity and related parts in Limax . . . 692

597. Nervous system of Patella . . . . 693

598. Diagrams of displacement of mantle-cavity, &c. . . . 694

599. Nervous system of Aplysia . 695

600. ,, ,, ,, Limmeus . . . . 695

601. Eyes of Gastropoda . . 696

602. Osphradium of Murex . . 696

603. Reproductive organs of Helix 697

604. Hermaphrodite gland of Gastropoda . . 698

605. Forms of egg-cases in Gastropoda . . . 699

606. Segmentation and formation of germinal layers in Gastropoda . 700

607. Early development of Patella . 701

608. Trochospheres of Patella . 702

609. Later trochosphere of Patella . ... 703

610. Veliger of Vermetus . .704

611. Diagram illustrating the relationships of the Gastropoda . 705

612. Dentalium, section of shell . . 706

613. ,, anatomy . . . 706

614. ,, larva* . . 706
614/>/,s. Rhodope . . 707

615. Sepia cultrata . . 709

616. sucker 710

xxx LIST OF ILLUSTRATIONS

FIO. PAOB

617. Sepia, cultrata, shell . 711

618. ,, chromatophore . . . 711

619. ,. cultrata, cranial cartilage . . . 712

620. ,, ,, nuchal cartilage . . 712

621. ,, ,, mantle-cavity . . . .713

622. ,, officinalis, jaws . 714

623. ,, section of buccal mass . . . 715

624. ,, officinalis, enteric canal . . . . 715

625. ,, cultrata, dissection of male from posterior aspect . . 716

626. ,, ,, lateral dissection of male . . ... 717
627- ,, officinalis, longitudinal section of ink-sac .... 718

628. ,, cultrata, vascular system . ... 719

629. ,, ,, cephalic ganglia . . ... 719

630. ,, ,, pedal and pleuro-visceral ganglia .... 719

631. ,, section of eye . . 720

632. ,, cultrata, otolith . . 721

633. ,, officinalis, renal organs .... .... 722

634. ,, ,, diagrammatic sagittal section of female . . . 723

635. ,, male reproductive organs ... . . 724

636. ,, sperms and spermatophore . . 725

637. Nautilus pompilius, section of shell . . 726
.638. ,, ,, female, in shell . . 727

639. ,, ,, spadix . . 728

640. ,, ,, cephalic cartilage . . . 729

641. ,, ,, mantle-cavity of male 730

642. ,, ,, dissection of male from left side . . 731
642 //.-;. ,, ,, arteries . 733

643. ,, ,, renal sacs, ctenidia, &c 733

644. ,, ,, male reproductive organs 735

645. ,, ,, female ., ,, .735

646. Octopus vulgaris . ... 737

647. Loligo vulgaris . ... 738

648. Argonauta argo ... . . . . 739

649. Octopus lentus, male . 73V)

650. Amphitretus pelagicus . ... 740

651. Shell of Spirula . 740

652. Spirula peronii . . 741

653. Ammonite . . 741

654. Shell of Belemnite . . 742

655. ,, Argonauta argo . . 742

656. Segmentation of Loligo . . 745

657. Blastoderm of Sepia . . . . 745

658. ,, ,, sections . . 746

659. Development of Loligo . . 747

660. ,, .748
61)1. ,, .748

t;r,_>. ,, ,, ... . 749

663. Diagram to illustrate the relationships of the Cephalopoda . . 750

\

CLASSIFICATION OF THE ANIMAL KINGDOM
ADOPTED IN THIS BOOK.

KINGDOM ANIMALIA.

PHYLUM I. PROTOZOA.

Class I. RHIZOPODA.

Order 1. LOBOSA.

,, 2. LABYRINTHITLIDEA.
,, 3. FORAMINIFERA.
,, 4. HELIOZOA.
,, 5. RADIOLARIA.

Class II. MYCETOZOA.
Class III. MASTIGOPHORA.
Order 1. FLAGELLATA.
,, -2. CHOANOFLAGELLATA.

Order 3. DINOFLAOELLATA.
,, 4. CYSTOFLAGELLATA.

Class IV. SPOROZOA.

Order 1. GREGARINIDA.
^ 2. COCCIDIIDEA.
;) 3. MYXOSPORIDEA.

,, 4. SAROOCYSTIDEA.

Class V. INFUSORIA.
Order 1. CILIATA.

,, 2. TENTACULIFERA.

PHYLUM II. PORIFERA.

Class PORIFERA.

L Calcarea.

Order 1. HOMOCQSLA.
,, 2. HETEROCCELA.

Sub-class II. Non-Calcarea.
Tribe I. MYXOSPONUI.*:.

Order 1. HEXACTINELLIDA.
,, 2. DESMOSPONOTA;.

PHYLUM III. CGELENTERATA.

Class I. HYDROZOA.

Order 1. LEPTOLIN^E.
Sub-order a. Anthomedusce.
,, /. Leptomedtifice.
Order 2. TRACHYLINJK.

Sub-order a.

,, l>. Narcomeduxa'
Order 3. HYDROCORALLINA.
,, 4. SIPHONOPHORA.
5. GRAPTOLITHIDA.

xxxii CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM III. CCELENTERATA- -.untinn ,,].

Class II. SCYPHOZOA.

Order 1. STAUROMEIH-S.V;.
,, 2. PEROMEDUS,.*:.
,, 3. CUBOMEDUS.E.

,, 4. DlSCOMEDUSJC.

( 'lass 111. ACTINOZOA.
Sub-class I. Zoantharia.
Order 1. ACTIXIARIA.
,, 2. MADREPORARIA.
,, 3. ANTIPATHARIA.
Sub-class II. Alcyonaria.
Order 1. ALCYOXACEA.

Order 2. GORGOXACEA.
,, 3. PENXATULACEA.

Class IV. crENOPHORA.
Order/. CYDIPPIDA.
,, 2. LOB AT A.
,, 3. CESTIDA.
,', 4. BEROIDA.

Appendix to Ctenophora Ctenoptana
and C'doplana.

Appendix to CVelenterata Mesozoa,
SaUneU<(, and Trii'}ij>/nx,

Class I. TURBELLARIA.
Order 1. POLYCLADIDA.
,, 2. TRICLADIDA.
,, 3. RHABDOCCELIDA.

Class II. TREMATODA.

Order 1. MOXOGENETICA.
2. I)K;ENETICA.

PHYLUM IV. PLATYHELMINTHES.

Class III. CESTODA.
Order 1. MOXO/.OA.
2. POLYZOA.

Appendix to Platyhelminthes Class
NEMERTINEA.

Class I. NEMATODA.

Order 1. NEMATOIDEA.
,, 2. GORDIOIDEA.

Class II. ACANTHOCEPHALA.

PHYLUM V. NEMATHELMINTHES.

Class III. CILETOGNATHA.

Appendix toNemathelminth.es Chtvto-

-, Jrhhw<li riifn', and Dt-smo-

PHYLUM VI. TROCHELMINTHES.

Class I. ROTIFERA.

< Inlcr 1. KlII/.oTA.

,, 2. BDELLOIDA.
,, 3. PLOIMA.

Sub-order u. J//n,-!cii/n.
b. Loricata.

Order 4. SOIRTOPODA.
,, 5. TROCHOSPH^ERIDA.

Class II. DINOPHILEA.
III. GASTROTRICHA.

PHYLUM VII. MOLLUSCOIDA.

Class I. POLYZOA.

Sub-class I. Ectoprocta.
( )rder 1. ( !YMM>I,.I.;\I \T.\.
Sul>-order a. Cyclostomatd.
,, b. Cheitotitomata.
c. Ctenostoynata.

Order 2. PHYLACTOL.EMATA.
Sub-class II. Endoprocta.

Class II. PHORONIDA.

,, III. BRACKIOPODA.

Order 1. IXARTICULATA.
2. ARTICULATA.

CLASSIFICATION OF THE ANIMAL KINGDOM

XXXlll

PHYLUM VIII.

Class I. ASTEROIDEA.

Order 1. PHAXEROZOXIA.
,, 2. CRYPTOZOXIA.

Class II. OPHIUROIDEA.

Order 1. OPHIUKIDA.

,, 2. EURYALIDA.

Class III. ECHINOIDEA.

Order 1. PALJEO-ECHINOIDKA.
,, 2. REGULARIA.
,, 3. CLYFEASTRIDEA.
4. SPATAXGOIDEA.

ECHINODERMATA.

Class IV. HOLOTHUROIDEA

Order 1. ELASIPODA.
,, 2. PEDATA.
,, 3. APOD A.

Class V. CRINOIDEA.

Order 1. PALYEOCRINOIDEA.
,, 2. NEO-CRINOIDEA.

Class VI. CYSTOIDEA.
VII. BLASTOIDEA.

PHYLUM IX. ANNULATA.

Class I. CHJETOPODA.

Sub-class I. Polychaeta.
Order 1. ARCHI-CH^TOPODA.
,, 2. ERRANTIA.
,, 3. SEDENTAKIA.
Sub-class II. Oligochaeta.
Order 1. NAIDOMORPHA.

,, 2. LUMBRICOMORPHA.

Appendix to the Chtvtopoda Class
MYZOSTOMIDA.

Class II. GEPHYREA.

Order 1. IXERMIA.

,, 2. ARM ATA.

Class III. ARCHI-ANNELIDA.

,, IV. HIRUDINEA.

Order 1. RHYNCHOBDELLIDA.

2. GXATHOBDELLIDA.

PHYLUM X.
Class I. CRUSTACEA.
Sub-class I. Entomostraca.

Order 1. PHYLLOPODA.

Sub-order a. Euphyllopoda.

,, b. C/adocera.
Order 2. OSTRACODA.
,, 3. COPEPODA.
Sub-order a. Emopepoiln .
,, b. Branchiura.
Order 4. CIRRIPF.DIA.

Sub-order a. Eucirripedia.

Sub-class II. Malacostraca.
Order 1. PHYLLOCARIDA.
,, 2. SCHIZOPODA.
,, 3. DECAPODA.
Sub- order a. Macrura.

,, b. Brachyura.
Order 4. STOMATOPODA.
,. '5. CUMACEA.

ARTHROPODA.

Order 6. ARTHROSTRACA.
Sub-order a. Ampliipoda.

,, b. I*opoda.

Appendix to Crustacea Class TRILO-
BITA.

Class II. ONYCHOPHORA
,, III. MYRIAPODA.
Order 1. SYMPHYLA.
,, 2. CHILOPODA.
,, 3. DIPLOPODA.
,, 4. PAUROPODA.

Class IV. INSECTA.
Order 1. APTERA.

,, 2. ORTHOPTERA.

,, 3. NEUROPTERA.

,, 4. HEMIPTEKA.

,, 5. DIPTERA.

,, 6. LEPIDOPTERA.

,, 7. COLEOPTERA.

,, 8. HYMEXOPTERA.

c

XXXIV

CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM X. ARTHROPODA continued.

Class V. ARACHNIDA.
Order 1. SCORPIOXIDA.

2. PSEUDOSCORPIOXIDA.
,, 3. PEDIPALPIDA.
,, 4. SOLPUGIDA.
,, 5. PHALAXGIDA.
6. ARANEIDA.

Order 7. ACARIDA.
,, 8. XIPHOSURA.
,, 9. EURYPTERIDA.

Appendix to the Arachnida The
PYCXOGOXIDA, LIXGUATULIDA, and TAR-

DIGRADA.

PHYLUM XI. MOLLUSCA.

Class I. PELECYPODA.

Order 1. PROTOBRAXCHIA.

,, 2. FlLIBRAXCHIA.
,, 3. PSEUDO-LAMELLIBRANCHIA.
4. EULAMELLIBRAXCHIA.

Sub-order a. Integripalliafa.
,, b. Sinupalliata.

Class II. AMPHINEURA.
Order 1. PLACOPHOKA.
,, 2. APLACOPHORA.

Class III. GASTROPODA.

Sub-class I. Streptoneura.
Order 1. ASPIDOBRAXCHIA.
Sub-order a. JJo<'Of//oxa.
,, b. Rhipidoglossa.

PHYLUM XII.

SUB-PHYLUM I. ADELOCHORDA.
Class. ADELOCHORDA.

SUB-PHYLUM II. UROCHORDA.

Class. UROCHORDA

Order 1. LARVACEA.
,, 2. THALIACEA.
Sub-order a. Cyrlomyarin.
,, b. Hf.mimyn ri.
,, <. Pyrosomata,
Order 2. ASCIDIACEA.

Sub-order a. Axriili.tr xii,tj,/ir<:.

Order 2. PECTIXIBRAXCHIA.
Sub-order .

S[-I;-I-HYLUM III. VERTEBRATA.

Division A. Acrania.
Class. ACRANIA.

Sub-class II. Euthyneura.
Order 1. OPISTHOBRAXCHIA.
Sub-order a. Tectibranchiata.
,, b. Nudibranchiata.
Order 2. PULMOXATA.

^Appendix to the Gastropoda Class
SCAPHOPODA and RHODOPE.

Class IV. CEPHALOPODA.

Sub-class I. Dibranchiata.
Order 1. DECAPODA.
,, 2. OCTOPODA.

Sub-class II. Tetrabranchiata.

CHORDATA.

Division B. Craniata.

Class I. CYCLOSTOMATA.

Order 1. PETROMYXONTES.
,, MYXIXOIDEI.

Class II. PISCES.

Sub-class I. Elasmobranchii.
Order 1. CLADOSELACHEA.
,, 2. PLEURACAXTHEA.
,, 3. ACAXTHODEA.
,, 4. SELACHII.
Sub-order a. Protoxelarhii.

,, b. Eiixthivhii.
Section a. Squalida.
,, /S. Rajida.

Sub-class II. Holocephali.

,, III. Teleostomi.
Order 1. CROSSOPTERYGII.
2. CHOXDROSTET.

CLASSIFICATION OF THE ANIMAL KINGDOM

XXXV

PHYLUM XII. CHORDATA continued.

< k'der 3. HOLOSTEI.
,, 4. TELOSTEI.
Sub-order <<. Physostomi.
/>. A ii'i'-iinthini.
c. Acaiithopteri.
,, '/. Pharyngognaihi.

i. Pfectogognathi.
,, f. Lophobranchii.

Sub-class IV. Dipnoi.

Order 1. MOXOPXEUMOXA.
,, 2. DIPXEUMOXA.

Appendix to Pisces The Ostracodermi.

Class III. AMPHIBIA.
Order 1. URODELA.
,, 2. AM:RA.
,, 3. GYMXOPHIOXA.
,, 4. STEGOCEPHALA.

Class IV. REPTILIA.
Order 1.

Hub-order a. L>c 4 t_rf//i.
,, li. Ophidia.

Order 2. RHYXCHOCEPHALIA.
,, 3. CHELOMA.
,, 4. THEROMORPHA.
,, 5. CROCODILIA.
,, 6. SAUROPTERYGIA.
,, 7. ICHTHYOSAURIA.

,, 8. DlJfOSAVRIA.

,, 9. PTEROSAURIA.

Class V. AVES.

Sub-class I. Archaeornithes.

,, II. Neornithes.
Section A. Ratitae.

Order 1. MECISTANES.

,, 2. RHE.E.

,, 3. STRUTHIOXES.

,, 4. ^EPYORXITHES.

,, 5. GASTORXITHES.

Section B. Carinatse.

Order 1. STEREORXITHES.
,, 2. ODOXTOLCJE.
,, 3. ICIITHYORXITHES.
,, 4. PYGOPODES.
5. IMPENNES.

Order 6. TUKBIXARES.

,, 7. STEGAXOPODES.

,, 8. HERODIOXES.

,, 9. AXSERES.

., 10. ACCIPITRES.

., 11. CRYPTURI.

,, 12. GALLIXJ:.

,, 13. GRALLJK.

.. 14. GAVI.K.

., 15. LIMICOLJE.

,, 16. PTEROCLETES.

,, 17. COLUMB^;.

,, 18. PSITTACI.

,, 19. STRKJES.

,, 2'. PlC'ARI.T-:.

,, 21. PASSEKES.

Class VI. MAMMALIA.
Sub-class I. Prototheria.

II. Theria.

Section A. Metatheria (MARSUPIALIA).
Order 1. POLYPROTODOXTIA.

,, 2. DlPROTODOXTIA.

Section B. Eutheria.
Order 1. EDENTATA.

,, 2. SlREXIA.

,, 3. CETACEA.
,, 4. UXUTJLATA.

Division A. Ungulala Vera.

Sub-order a. Artioiladyla.
,, !>.

Division B. Subungulata.

Sub-order a. Hynu'oiilea.
,, b. Prolioxi'idea.
,, <. Amlifi/pocla.
,, d. Gondyfanrihra.
, , e. Toxodontia.
(4roup Tillodontia.
Order 5. RODEXTIA.
,, 6. CARXIVORA.
Sub-order a. Candvora vera.
,, ft. Pinnipedia.
, , <. Creodonta.
Order 7. IxsECTrvoRA.
,, 8. CHIROPTERA.
,, 9. PRIMATES.
Sub-order . Lemuroidea.
,, h. Anthropoidea.

ZOOLOGY

INTRODUCTION

Zoology, the branch of Natural History which deals with
animals, is one of the two subdivisions of the great science Biology,
which takes cognisance of all organisms, or things having life, as
distinguished from such lifeless natural objects as rocks and
minerals. The second of the two subdivisions of Biology is
Botany, which deals with plants.

The subject-matter of Zoology, then, is furnished by the animals
which inhabit the land-surface, the air, and the salt and fresh
waters of the globe : the aim of the science is to find out all that
can be known of these animals, their structure, their habits, their
mutual relationships, their origin.

The first step in the study of Zoology is the recognition of the
obvious fact that the innumerable individual animals known to
us may be grouped into what are called species, the members of
which resemble one another so closely that to know one is to know
all. The following example may serve to give the reader a fairly
accurate notion of what Zoologists understand by species, and of
the method of naming species which has been in use since the time
of the great Swedish naturalist Linnreus.

. The Domestic Cat, the European Wild Cat, the Ocelot, the Leopard,
the Tiger, and the Lion are animals which agree with one another in
the general features of their organisation in the number and form
of their bones, and teeth, in the possession of retractile claws, and
in the position and characters of their internal organs. No one
can fail to see that these animals, in spite of differences of size,
colour, markings, &c., are all, in the broad sense of the word,
" Cats." This is expressed in the language of systematic Zoology
by saying that they are so many species of a single genus.

Ac-cording to the system of binomial nomenclature introduced by
Linnoeus, each kind of animal receives two names one the generic

IE B

2 ZOOLOGY

name, common to all species of the genus ; the other the spcci/ir
mi me, peculiar to the species in question. Both generic and specific
names are Latin in form, and are commonly Latin or Greek in
origin, although frequently modern names of persons or places, with
Latinised terminations, are employed. In giving the name of an
animal, the generic name is always placed first, and is written
with a capital letter, the specific name following it, and being
written, as a rule, with a small letter. For instance, to take the
examples already referred to, the Domestic Cat is called Fclis
domestica, the European Wild Cat F. catus, the Leopard F. pardus,
the Tiger F. tiyris, the Lion F, ko. Thus the systematic name of an
animal is something more than a mere appellation, since it indicates
the affinity of the species with other members of the same genus :
to name an animal is, in fact, to classify it.

It is a matter of common observation that no two individuals of
a species are ever exactly alike : two tabby Cats, for instance,
hoAvever they may resemble one another in the general characters
of their colour and markings, invariably present differences in
detail by which they can be readily distinguished. Individual
variations of this kind are of universal occurrence. Moreover, it
often happens that the members of a species are divisible into
groups distinguishable by fairly constant characters : among
Domestic Cats, for instance, we find white, black, tabby, gray, and
tortoiseshell Cats, besides the large long-haired Persian breed, and
the tailless Manx Cat. All these are distinguished as varieties of
the single species Felis domestica.

It is often difficult to decide whether 'two kinds of animals should
be considered as distinct species or as varieties of a single species, and
no universal rule can be given for determining this point. Among
the higher animals mutual fertility is a fair practical test, the
varieties of a species usually breeding freely with one another and
producing fertile offspring, while distinct species either do not
breed together or produce infertile hybrids or mules. Compare,
for instance, the fertile mongrels produced by the union of the
various breeds of Domestic Dog with the infertile mule produced by
the union of the Horse and Ass. But this rule is not without
exception, and in the case of wild animals is, more often than not,
impossible of application : failing it, the only criterion of a " good
species " is usually the presence of constant differences from allied
species. Suppose, for instance, that a naturalist receives for
description a number of skins of wild Cats, and finds, after an
accurate examination, that in some specimens the tail is two-thirds
the length of the body and the skin of a uniform reddish tint with
a few markings on the head, while in the rest the tail is nearly half
as long as the body, and the skin tawny with black stripes. If
there are no intermediate gradations between these two sets of
individuals, they will be placed without hesitation in distinct

INTRODUCTION 3

species : if, on the other hand, there is a complete series of grada-
tions between them, they will be considered to form a single
variable species.

As, therefore, animals have to be distinguished from one another
largely by structural characters, it is evident that the foundations
of a scientific Zoology must be laid in Morphology, the branch of
science w r hich deals with form and . structure. Morphology may be
said to begin with an accurate examination of the external
characters ; the divisions of the body, the number and position of
the limbs, the characters of the skin, the position and relations of
the mouth, eyes, ears, and other important structures. Next the
internal structure has to be studied, the precise form, position,
&c., of the various organs, such as brain, heart, and stomach being
made out : this branch of morphology is distinguished as Anatomy.
And, lastly, the various parts must be examined by the aid of the
microscope, and their minute structure, or Histology, accurately
determined. It is only when we have a fairly comprehensive
knowledge of these three aspects of a given animal its external
characters, its rough anatomy, and its histology that we can with
some degree of safety assign it to its proper position among its
fellows.

An accurate knowledge of the structure of an animal in its
adult condition is not, however, all-sufficient. Nothing has been
made more abundantly clear by the researches of the last half-
century than that the results of anatomy and histology must be
checked, and if necessary corrected, by Embryology i.e. by the
study of the changes undergone by animals in their develop-
ment from the egg to the adult condition. A striking instance is
afforded by the common Barnacles which grow in great numbers on
ships' bottoms, piers, &c. The older zoologists, such as LinnaBus,
grouped these creatures, along with Snails, Mussels, and the like,
in the group Mollusca, and even the great anatomical skill of
Cuvier failed to show their true position, which was made out only
when Vaughan Thompson, about fifty years ago, proved, from
a study of the newly hatched young, that their proper place
is among the Crustacea, in company with Crabs, Shrimps, and
Water-fleas.

Given a sound knowledge of the anatomy, histology, and em-
bryology of animals, their Classification may be attempted that
is, we may proceed to arrange them in groups and sub-groups,
each capable of accurate definition.

The general method of classification employed by zoologists is
that introduced by Linnasus, and may be illustrated by reference
to the group of Cats which we have already used in the explanation
of the terms genus, species, and variety.

We have seen that the various kinds of true Cat Domestic Cat,
Lion, Tiger, &c. together constitute the genus Fclis. Now there

B 2

4 ZOOLOGY

is one member of the cat-tribe, the Cheetah, -or Hunting Leopard,
which differs from all its allies in having imperfectly retractile
claws and certain peculiarities in its teeth. It is therefore placed
in a distinct genus, Cyncdurus, to mark the fact that the differences
separating it from any species of Felis are of a more fundamental
character than those separating the species of Felis from one
another.

The nearest allies of the Cats are the Hyaenas, but the presence
of additional teeth and of non-retractile claws to mention only
two points makes the interval between Hyaenas and the two
genera of Cats far greater than that between Felis and Cynselurus.
The varying degree of difference is expressed in classification by
placing the Hyaenas in a separate family, the Hyccnida:, while
Felis and Cynaelurus are placed together in the family Fclidcc.
Similarly, the Civets and Mongooses form the family Viverridcc ;
the Dogs, Wolves, Jackals, Foxes, &c., the family Canidcc ; Bears,
the family Ursidcc ; and so on.

All the foregoing animals have sharp teeth adapted to a flesh
diet, and their toes are armed with claws. They therefore differ
fundamentally from such animals as Sheep, Deer, Pigs, and Horses,
which have flat teeth adapted for grinding vegetable food, and
hoofed feet. The differences here are obviously far greater than
those between any two of the families mentioned above, and are
emphasised by placing the flesh-eaters in the order Carnivora,
the hoofed animals in the order Ungulata. In the same way
gnawing animals, such as Rats, Mice, and Beavers, form the order
Eodentia ; pouched animals, such as Kangaroos and Opossums, the
order Marsupialia ; and so on.

Garni vora, Ungulata, Rodentia, Marsupialia, &c., although
differing from one another in many important respects, agree in
the possession of a hairy skin and in the fact that they all suckle
their young. They thus differ from Birds, which have a covering
of feathers and hatch their young from eggs. The differences here
are considerably more important than those between the orders
of quadrupeds referred to, and are expressed by placing the latter
in the class Mammalia, while Birds constitute the class Avcs. In
the same way the scaly, cold-blooded Lizards, Snakes, Tortoises, &c.,
form the class Rcptilia; the slimy-skinned, scaleless Frogs, Toads, and
Salamanders the class Amphibia ; and the finned, water-breathing
Fishes the class Pisces.

Mammals, Birds, Reptiles, Amphibians, and Fishes all agree with
one another in the possession of red blood and an internal skeleton
an important part of which is the backbone or vertebral column
and in never having more than two pairs of limbs. They thus
differ in some of the most fundamental features of their organisation
from such animals as Crabs, Insects, Scorpions, and Centipedes,
which have colourless blood, a jointed external skeleton, and

INTRODUCTION 5

numerous limbs. These differences far greater than those be-
tween classes are expressed by placing the backboned animals
in the phylum or sub-kingdom Chordata, the many-legged
armoured forms in the phylum Arthropoda. Similarly, soft-bodied
animals with shells, such as Oysters and Snails, form the phylum
MoUusca, Polypes and Jelly-fishes the phylum Coelenterata. And
finally the various phyla recognised by zoologists together con-
stitute the kingdom Animalia.

Thus the animal kingdom is divided into phyla, the phyla into
classes, the classes into orders, the orders into families, the families
into genera, and the genera into species, while the species themselves
are assemblages of individual animals agreeing with one another
in certain constant characters. It will be seen that the individual
is the only term in the series which has a real existence : all the
others are mere groups formed, more or less arbitrarily, by man.

To return to the animal originally selected as an example, it will
be seen that the zoological position of the Domestic Cat is expressed
as follows :

Kingdom ANIMALIA.
Phylum CHORDATA.
Class MAMMALIA.

Order CARNIVORA.
Family Fc/icfa.
Genus Felis.

Species F. domestica.

The object of systematic zoologists has always been to find a
natural as opposed to an artificial classification of animals.
Good instances of artificial classification are the grouping of Bats
with Birds on the ground that they both possess wings, and of
Whales with Fishes on the ground that they both possess fins and
live in the water. An equally good example of a natural classi-
fication is the grouping of both Bats and Whales under the head of
Mammalia because of their agreement, in all essential points of
anatomy, histology, and embryology, with the hairy quadrupeds
which form the bulk of that class.

With the older zoologists the difficulty was to find some general
principle to guide them in their arrangement of animals some
true criterion of classification. It was believed by all but a few
advanced thinkers that the individuals of each species of animal
were descended from a common ancestor, but that the original
progenitor of each species was totally unconnected with that of
every other, having, as Butfon puts it, " participated in the grace
of a distinct act of creation." To take an instance all Wolves
were allowed to be descended from a pair of ancestral Wolves, and
all Jackals from a pair of ancestral Jackals, but the original pair in
each case was supposed to have come into being by a supernatural

6 ZOOLOGY

process of which no explanation could or ought to be offered.
Nevertheless it was obvious that a Jackal was far more like a
Wolf than either of them was like a Tiger, and that in a natural
system of classification this fact should be expressed by placing the
Wolf and Jackal in one family, the Tiger in another.

All through the animal kingdom the same thing occurs : no
matter what group we take, we find the species composing it
resemble one another in varying degrees, or, as it is sometimes ex-
pressed, have varying degrees of relationship to one another. On
the view that each species was separately created the word relation-
ship was used in a purely metaphorical sense, as there could of
course be no real relationship between two groups of animals
having a totally independent origin. But it was assumed that
creation had taken place according to a certain scheme in the
Divine Mind, and that the various species had their places in tin's
scheme like the bits of glass in a mosaic. The problem of classifica-
tion was thus to discover the place of each species in the pattern of
the unknown design.

The point of view underwent a complete change when, after the
publication of Darwin's Origin of fyecies in 1859, the Doctrine
of Descent or of Organic Evolution came to be genera 11 v
accepted by biologists. A species is now looked upon, not as an
independent creation, but as having been derived by a natural
process of descent from some pre-existing species, just as the
various breeds of Domestic Fo\vl are descended from the little
Jungle-fowl of India. On this view the resemblances between
species referred to above are actually matters of relationship, and
species are truly allied to one another in varying degrees since
they are descended from a common ancestor. Thus a natural
classification becomes a genealogical tree, and the problem of
classification is the tracing of its branch*'-.

This, however, is a matter of extreme difficulty. Representing
by a tree the whole of the animals which have ever lived on the
earth, those existing at the present day would be figured by the
topmost twigs, the trunk and main branches representing extinct
forms. Thus the task of arranging animals according to their
relationships would be an almost hopeless one but from two
circumstances : one, that remains of many extinct forms have been
preserved : the other, that the series of changes undergone by an
animal in its development from the egg often forms an epitome of
the changes by which, in the course of ages, it has been evolved
from an ancestral type. Evidence furnished by the last-named
circumstance is, of course, furnished by embryology : the study of
extinct animals constitutes a special branch of morphology to
which the name Palaeontology is applied.

The solid crust of the earth is composed of various kinds of
rocks divisible into two groups : (1) Igneous rocks, such as granite

INTRODUCTION 7

and basalt, the structure of which is due to the action of the
internal heat of the globe, and which originate below the surface
and are not arranged in layers or strata . (2) Aqueous or sedimentary
rocks, which arise by the disintegration, at the surface of the earth,
of pre-existing rocks, the fragments or debris being carried off by
streams and rivers and deposited at the bottom of lakes or seas.
Being formed in this way by the deposition of successive layers or
strata, the sedimentary rocks have a stratified structure, the lowest
being in every case older than the more superficial layers. The
researches of geologists have shown that there is a general order <>f
succession of stratified rocks : that they may be divided into three
great groups, each representing an era of time of immense but
unknown duration, and that each group may be subdivided into
more or fewer systems of rocks, each representing a lesser period of
time. The following table shows the thirteen rock-systems usually
recognised, arranged under the three great groups in chronological
order, the oldest being at the bottom of the list.

j 13. Quaternary and Recent.

m/-i m , 12. Pliocene.

. Uamozoic or lertiary. .< ,, -.,.

I 11. Miocene.

( 10. Eocene.
j 9= Cretaceous.
II. Mesozoic or Secondary . \ 8. Jurassic.

7. Triassic.
| 6. Permian.

5. Carboniferous.

I. Palaeozoic or Primary . . j J ?-f OniaiL

3. oilurian.

2. Cambrian.
1. Laurentian.

Imbedded in these rocks are found the remains of various extinct
animals in the form of what are called fossils. In the more recent
rocks the resemblance of these to the hard parts of existing
animals is perfectly clear : we find shells hardly differing from
those we pick up on the beach, bones easily recognisable as those
of Mammals, Birds, or Fishes, and so on. But in the older rocks the
fossils are in many cases so different in character from the animals
existing at the present day as to be referable to no existing order.
We find Birds with teeth, great aquatic Reptiles as large as Whales,
Fishes, Molluscs, Crustacea, &c., all of an entirely different type from
any now existing. We thus find that the former were in many
cases utterly unlike the present animal inhabitants of the globe,
and we arrive at the notion of a succession of life in time, and are
even able, in exceptionally favourable circumstances, to trace back
existing forms to their extinct ancestors.

By combining the results of comparative morphology, embryology,

8 ZOOLOGY

and paleontology we get a department of Zoology called Phylo-
geny, the object of which is to trace the pedigrees of the various
groups. There are, however, very few cases in which this can be
done with any approach to exactness : most " phylogenies " are
purely hypothetical, and merely represent the views at which a
particular zoologist has arrived after a more or less exhaustive
study of the group under discussion.

Animals may also be studied from the point of view
of Distribution. One aspect of this study is inseparable from
Palaeontology, since it is obviously necessary to mention in con-
nection with a fossil the particular system or systems of rocks in
which it occurs : thus we distinguish geological distribution or
distribution in time.

The distribution of recent forms may be studied under two
aspects, their horizontal or geographical distribution, and their
vertical or bathymetrical distribution. To mention the latter
first, we find that some species exist only on plains, others hence
called alpine forms on the higher mountains ; that some marine
shells, fishes, &c., always keep near the shore (littoral species), others
live at great depths (abyssal species), while others (pelagic
species) swim on the surface of the ocean. Among aquatic
animals, moreover, whether marine or fresh-water, three principal
modes of life are to be distinguished. These are animals, such
as Jelly-fishes, which float on or near in the water, and are
carried about passively by currents: such forms are included
under the term Plankton. Most Fishes, Whales, and Cuttle-fishes,
on the other hand, are strong swimmers, and are able to traverse
the water at will in any direction ; they together constitute the
Xi-l'ton. Finally, such animals as Crabs, Oysters, Sponges, Zoo-
phytes, &c., remain permanently fixed to or creep over the surface
of the bottom, and are grouped together, as the Benthos.

Under the head of geographical distribution we have such facts
as the absence of all Land-mammals, except Bats, in New Zealand
and the Polynesian Islands, the presence of pouched Mammals,
such as Kangaroos and Opossums, only in some parts of America
and in Australia and the adjacent islands, the entire absence of
Finches in Australasia, and so on. We find, in fact, that the
fauna i.e. the total animal inhabitants of a country is to a
large extent independent of climate, and that the fauna? of
adjacent countries often differ widely. In fact, it is convenient
in studying the geographical distribution of animals largely to
ignore the ordinary division into continents, and to divide the
land-surface of the globe into what are called zoo-geographical
regions. The characteristics of these regions will be discussed in
a future section ; at present it is only necessary, for convenience of
reference, to give their names and boundaries.

INTRODUCTION 9

1. The Holarctic Region includes the whole of Europe, Asia as
far south as the Himalayas, Africa north of the Sahara, together
with the corresponding portion of Arabia, and North America as
far south as Mexico. For convenience of reference it is often
customary to divide this region into two : its Eurasian portion is
then called the Palc&arctic, its American portion the Ncarctic
region.

2. The Ethiopian Region includes Africa south of the Sahara,
Southern Arabia, and Madagascar with the adjacent islands.

3. The Oriental Region includes India, Ceylon, South China,
the Malayan Peninsula, and what are known as the Indo-Malayan
islands, i.e. those islands of the Malayan Archipelago which lie to
the west of a line called Wallace's line passing to the east of
the Philippines, between Borneo and Celebes and between Bali
and Lombok.

4. The Australian Region includes Australia, Tasmania, and the
Austro-Malayan islands, i.e. the islands of the Malayan Archipelago
lying to the east of Wallace's line.

5. The New Zealand Region includes New Zealand and the
adjacent islands, such as the Chatham, Auckland, and Campbell
groups.

6. The numerous groups of islands lying between Australia
and Southern Asia to the west, and America to the east, are
conveniently grouped together as the Polynesian Region.

7. The Neotropical Region includes the whole of South and
Central America and part of Mexico.

There are still two departments of zoological science to be
mentioned. As it is impossible to have a right understanding of
a machine without knowing something of the purpose it is in-
tended to serve, so the morphological study of an animal is im-
perfect without some knowledge of its Physiology, i.e. of the
functions performed by its various parts, and the way in which
they work together for the welfare of the whole. It is hardly
possible to give more than occasional references to physiological
matters in a text-book of Zoology, but in order to pave the way
for such references a brief account of the general principles of
Physiology will be given in the next section.

Not only may we study the action of a given animal's organs,
but also the actions of the animal as a whole, its habits, its
relations to other animals, whether as friends, as enemies, or as
prey, to the vegetable kingdom, and to its physical surroundings,
such as temperature, humidity, &c. In a word, the whole question
of the relation of the organism to its environment gives us a final
and most important branch of Natural History which has been
called Ethology or Bionomics.

SECTION I.

THE GENERAL STRUCTURE AND PHYSIOLOGY

OF ANIMALS

1. AMCEBA.

IF we examine under the microscope a drop of water containing
some of the slimy deposit which collects at the bottom of pools of
rain-water and in similar situations, we occasionally find it to
abound in microscopic life ; and among the minute moving creatures
in such a drop we frequently find examples of a remarkable or-
ganism the Amelia or Proteus Animalcule (Fig. 1). This is

a little particle of irregular
shape, which we should be
likely, on a cursory examina-
tion, to put down as motion-
less; it appears somewhat like
an irregular particle of some
colourless glass-like substance
with a more granular central
portion. If, however, we make
an exact drawing of the out-
line of the Amoeba, and, after
an interval, compare the draw-
ing with the original, we find
that the drawing appears no
longer to represent what we
see ; a change has taken place
in the shape of the Amoeba ;
and careful observation shows that this change is constantly going
on : the Amoeba is constantly varying in shape. This change is
effected by the pushing out of projections or processes, called
pseudopods (psd.), which undergo various alterations of size
and shape, and may become withdrawn, other similar processes
being developed in their place. At the same time careful

FIG. 1. Amoeba proteus, a living specimen,
c. fin: contractile vacuole ; nv. nucleus;
}>.*'. pseudopods. (From Parker's Bioloyii,
after Gruber.)

SECT, i STRUCTURE AND PHYSIOLOGY OF AXIMALS 11

watching shows that the Amoeba is also, with extreme
changing its position. This it effects by a kind of streaming-
motion. A projection forms itself on one side, and the entire
substance of the Amoeba gradually streams into it ; a fresh
projection appears towards the same side, the streaming move-
ment is repeated, and, by a constant succession of such move-
ments, an extremely gradual locomotion, which it often takes very
close watching to detect, is brought about. In these movements,
it is to be noticed, the Amoeba is influenced to some extent by
contact with other minute objects ; when the processes come in
contact with small grains of sand or other similar particles their
movements are modified in such a way that the Amoeba, in its
slow progress onwards, passes on one side of them, so that it
might be said to feel its way among the solid particles in the drop
of sediment.

Judging from the nature of these movements, we are obliged to
infer that the substance of which this remarkable object is com-
posed must be soft and semi-fluid, yet not miscible with the water,
and, therefore, preserving a sharp contour, The.se and other
characteristics to be mentioned subsequently enable us to conclude
that we have to do with the substance of complex chemical com-
position termed protoplasm, which constitutes the vital material of
all living organisms whether animals or plants. In Amoeba the
protoplasm is clearly distinguishable into two parts, an outer
homogeneous, glassy-looking layer completely enclosing a more
granular internal mass.

Examination of the Amoeba with a fairly high power of the
microscope reveals the presence in its interior of two objects which
with a low power we should be likely to overlook. One of these
is a small rounded body of a homogeneous appearance, which
preserves its form during all the changes which the Amoeba as a
whole undergoes. This is termed the nucleus (Fig. 1, iiu.}: it is
enclosed in an extremely delicate membrane, and consists of a
protoplasmic material differing from that which forms the main
bulk of the Amoeba in containing a substance which refracts the
light more strongly and which has a stronger affinity for certain
colouring matters. The other minute object to be distinguished
in the interior appears as a clear rounded space (c. me.) in the
protoplasm. When this is watched it will be observed to increase
gradually in size till it reaches a maximum of, let us say, a fifth of
the total diameter of the Amoeba, when, by a contraction of its walls,
it suddenly disappears, to reappear presently and gradually grow
again to its maximum size. This pulsating clear space is the
contractile vacuolc^

By watching the Amoeba carefully for some time we may be
enabled to observe that the movements of the protoplasm of the
body not only effect locomotion, but are connected also with the

12 ZOOLOGY SECT.

reception of certain foreign particles of organic nature i.e. either
entire minute animals or plants, or minute fragments of larger
forms into the interior of the protoplasm. A process of the
protoplasm is pressed against such a particle, which becomes sunk
in the soft substance, and passes gradually into the interior. Here'
it becomes surrounded by a little globule of watery fluid, and by
degrees partially or wholly disappears ; the part, if any, which
remains subsequently passes outwards from the protoplasm into
the surrounding water. The matter which disappears evidently
mixes with the protoplasm and adds to its bulk. All, in fact, of
the matter of the foreign body that is capable of it becomes
digested and assimilated by the protoplasm. The globule of watery
fluid enclosing the food-particle (for such is the true nature of the
foreign body) probably contains some ingredient of the nature of a
ferment, capable of acting on certain substances and rendering
them more soluble or capable of being more readily taken up by
the protoplasm. This w r e infer mainly from what we know of the
digestion and absorption of food in the higher animals ; but the
fact, which has been established by experiment, that the Amoeba
is able readily to digest certain classes of organic substances, while
others, when taken into the interior of the protoplasm, remain
unaltered, seems to indicate that some special property, similar to
those possessed by the digestive ferments of the higher animals,
is present in the watery fluid surrounding the food-particle.

The movements of the Amoeba, slow and gradual though they
are, must involve a certain expenditure of energy or working power :
this can only be derived from the energy of chemical affi/tif//
which the protoplasm possesses in virtue of its complex chemical
composition. The protoplasm loses some of this energy by its
conversion into energy of movement. This loss implies the break-
ing up of the complex chemical ingredients of which protoplasm
is made up into simpler ones ; the protoplasm falls a grade in the
scale of chemical compounds, and by its fall generates the force by
means of which the Amoeba moves. The energy of chemical
affinity which the protoplasm possesses is thus analogous to the
potential energy which the weight of a clock has when it is wound
up. As the weight, by virtue of its position, is able as it falls to
deal out working power so as to cause the movement of the
machinery of the clock, so the protoplasm is able, by the degra-
dation or decomposition of its complex compounds, to deal out
working power enabling the Amoeba to move. In the case of the
clock- weight there comes a time when all the potential energy is
expended ; the weight reaches its lowest limit, and unless it is
wound up again the clock stops. The like holds good of the
Amoeba : the protoplasm is continually being used up broken up
into compounds of a lower order and, in course of time, the whole
potential energy would become exhausted, were it not that a new

STRUCTURE AND PHYSIOLOGY OF ANIMALS

13

supply is being constantly received. This new supply of energy i-
derived from the substance of the food-particles ; and this at the
same time maintains the bulk of the Amoeba, which, if food par-
ticles are absent from the water, gradually diminishes.

Accompanying the degradation, or destructive metabolism as it
is termed, of the protoplasm, and intimately connected with it, is
the passage inwards of oxygen from the air dissolved in the water,
and the passage outwards of carbonic acid gas. Oxygen is a
necessary agent in the process of destructive metabolism, and

FIG. 2. Amoeba polypodia in successive phases of division. The light spot is the contractile
vacuole ; the dark the nucleus. (From Lang's Text-Book, after F. B. Schulze.)

carbonic acid is a constant waste-product of such action. This
interchange of oxygen and carbonic acid is the essence of the pro-
cess of respiration observable in all living things. In addition to
the carbonic acid given off in this process other waste-products are
formed and have to be got rid of. In all probability the contract II
vacuole already referred to has to do with this process the process
of excretion since uric, acid, which in higher animals is the typical
form assumed by such waste-products, is said to have been detected
in the interior of the contractile vacuole in the case of certain near
relatives of Amoeba.

When food is abundant the Amoeba increases in bulk more

14 ZOOLOGY

SECT.

food being ingested than is required for simply maintaining the
size unaltered and soon a remarkable change takes place. The
processes become withdrawn, and a fissure appears dividing the
Amoeba into two parts (Fig. 2). This fissure grows inwards, and the
two parts become more and more completely separated from one
another till eventually the separation becomes complete, and we
have two distinct Amoeba? resulting from the division of the one.
While the protoplasm has been undergoing this division into two
halves the nucleus also divides, and each of the two new Amoebse
possesses a nucleus similar to the original one, and developed from
it by division. It is mainly by this simple process of division
into two, or Unary fission as it is called, that reproduction or
multiplication takes place in the Amoeba.

In spite of the great simplicity of its structure, the Amoeba
thus carries on a number of different functions. The practically
structureless particle of protoplasm is able to act on matter
absorbed as food in such a way as to alter the chemical com-
position of the latter and to assimilate it : it is able to carry on
movements of locomotion as well as movements those involved
in the taking in of food particles which may be looked upon as
movements of prehension : it exhibits a certain degree of sensitive-
ness or irritability, as shown by the modifications of its movements
which result from contact with foreign bodies ; it is able to respire ;
it carries on processes of excretion ; and, finally, it is capable of
reproducing its kind. It is these functions that characterise living
beings as distinguished from non-living matter. The power of
locomotion, the capacity for assimilating organic substances, and
the absence of two special compounds chlorophyll and crlhdosc-
are specially characteristic of the animal as distinguished from the
plant.

2. THE ANIMAL CELL.

In all but the lowest animals the various functions just enume-
rated are carried on by means of a more or less complex machinery
of organs muscles, alimentary or enteric canal, glands, heart and
blood-vessels, gills or lungs, nervous system, organs of excretion, and
organs of reproduction. But in all animals, however complex,
the same substance, protoplasm, which in Amoeba constitutes
the bulk of the body, is the essential and active part. Wherever
in the body active functions are being discharged and active
changes are going on, there we find protoplasm present : where
there is no protoplasm there is no vital activity. In the
earliest stages of their existence all animals are formed entirely
of pi'otoplasm. Every animal consists at first of a single minute
particle of protoplasm, not widely different from an Amoeba. Soon
this particle divides into a number of parts which, instead of

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 15

separating completely from one another, like the parts of a
divided Amoeba, remain associated together, forming a clump
of minute particles of protoplasm. Such minute protoplasmic
particles are termed cells ; every animal consists, at first, of a
single cell, and afterwards, in all higher animals, this single cell
becomes converted by division and subdivision into a little cluster
or clump of cells.

It is time that we should inquire more particularly as to the
meaning of these two terms cell and protoplasm evidently so
important in the study of both plants and animals. Protoplasm,
we have already seen, is a semi-fluid, gelatinous, clear or finely
granular substance of complex chemical composition. It is known
not to be a definite compound, but to be a somewhat varying
mixture of chemical compounds, the most essential of which are
bodies of the class of proteids highly complex substances, into the
composition of which the elements carbon, hydrogen, oxygen,
nitrogen, and sulphur all enter. Living protoplasm always con-
tains a large amount of water. It is soluble in weak acids or
weak alkalies ; and is capable of being coagulated rendered firmer
and more opaque by the action of heat and of strong alcohol. Its
reaction is slightly alkaline. As regards its minute structure, it
is generally acknowledged that there are two kinds of substance in.
the protoplasm, in some cases more, in others less, distinctly marked
off from one another. One of these substances (mitome) is less
fluid than the other, and appears to be arranged in the form of a
network of threads, composed of numerous minute rounded gran-
ules, enclosing the second, more fluid substance {paramitomc} in
its meshes.

To a particle of protoplasm, usually containing a nucleus in its
interior, constituting the entire body of such a simple organism as
Amoeba, and forming one of the constituent elements of which a
higher plant or animal is made up, the term cell is applied. The
word was first employed in reference to the microscopic struc-
ture of plants, in connection with which it is much more appro-
priate than in connection with the microscopic structure of animals ;
for a plant-cell has, nearly always, a definite, firm, enclosing
envelope or cdl-wcdl (Fig. 3, I, c.w) a structure which is only
exceptionally present in the case of animals. In the interior
of the cell-protoplasm, or cytoplasm, is a body termed the nude it*
similar to the nucleus of Amoeba ; usually of rounded shape, with
a thin enclosing nuclear membrane (A, nu.m), which is perforated
by numerous minute apertures. In the nucleus is a single coiled
thread, or a network of threads, or one or more rounded clumps,
of a substance chromatin (chr.) which differs from ordinary
protoplasm in having a stronger affinity for most staining
agents. A rounded body termed the nudeolus (nu), which usually
occurs in the interior of the nucleus, is formed either of a

16 ZOOLOGY SECT.

solid mass of chromatin, or of a substance achromatin differing
somewhat from chromatin in its properties and less strongly
affected by staining agents. Allied to the achromatin of which
such nucleolar clumps may be composed is a constituent of the
nucleus to which the name of linin is applied. This assumes the
form of a network of delicate threads linin filaments which
usually have associated with them, embedded in their substance
or adhering to their surfaces, rows of chromatin granules, the
interstices being filled with a granular material the nuclear sap.
When the nucleus divides during the process of division of the
cell, its contents, more particularly the chromatin, in many cases
go through a remarkable series of changes, to which the term
Iccwyokinesis or mitosis is applied.

At the time when this mitotic division is about to be initiated
either one or two minute bodies (Fig. 3, A, c) are to be distinguished
situated close together in the cytoplasm in the immediate neigh-
bourhood of the nucleus. These are the centrosomes minute
masses of a protoplasmic substance which seems to resemble the
matter of the nucleolus. The centrosomes, a,t first close together,
gradually separate from one another, a spindle-shaped bundle of
very fine fibres of achromatic l material the nuclear spindle ex-
tending between them(Fig. 3,C). At the same time each centrosome
becomes the centre of a system of fine achromatin fibres (ap-
parently made up, like the fibres of the spindle, of rows of granules)
which are arranged round it in a radiating manner forming a
structure termed the attraction-sphere or astrosphere (Fig. 3, A, s).
Meantime important changes have been in progress in the nucleus.
The chromatin first becomes arranged in a close tangle, and then
becomes divided up into a number of parts the chromatin segments
or chromosomes which frequently have the form of loop-like threads
(Fig. 3, C, chr), but often assume other forms. The nuclear mem-
brane disappears. Each of the chromatin segments splits length-
wise into two parts the daughter-segments of the chromatin or
daughter-chromosomes (Fig. 3, D, E), and with these the filaments of
the spindle become connected.

At this point the segments of the chromatin form a single group
the equatorial plate extending across the axis of the spindle.
The latter has shifted its position, so that its fibres now run
across the original site of the nucleus, each of them having
become interrupted and divided into two halves, each of which
extends inwards from the corresponding centrosome, and has
become connected with one of the daughter-chromosomes. The
spindle-fibres now contract, and, apparently as a result of this con-
traction, half the daughter-chromosomes become drawn towards

1 The term achromatin is usually applied to all the matter of the nucleus
that has not the special characteristics of chromatin ; but it applies to cytoplasm /<
structures i.e. structures belonging to the body of the cell as well.

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 17

one of the centrosomes and half towards the other (Fig. 3, F, G, H),
so that the}- are now separated into two distinct groups. When the
groups have approached the extremity of the spindle, the
segments of each unite, and eventually the entire chromatin of
each of the two groups assumes the arrangement which the
chromatin of the original nucleus exhibited before division began.

FIG 3. Diagrams illustrating karyokinesis. A, the resting cell ; B, C, D, successive phases in
the formation and arrangement of the chromatin loops and of the nuclear spindle ; E, F, G,
separation of the two sets of daughter-chromosomes and their passage towards the poles of
the spindle ; H, 1, division of the cell-body and formation of the two new nuclei ; c. centro-
sonie ; (/.,-. c-hromatin ; '/'' cell-plate ; -iti'i nucleoli ; ///<. //(. nuclear membrane ; s. astrosphere ;
*i>. spindle (From Parker's liiotoriii, after Flemming, Rabl, &c.)

A new nuclear membrane becomes formed around each chromatin
group, and the whole assumes the character of a complete nucleus
the daughter-nucleus. A furrow which appears on the surface of
the cell-protoplasm (Fig. 3, H, I), surrounding it in the form of a
ring in a plane at right angles to the long axis of the spindle,
deepens gradually so as to give rise to a cleft, eventually com-
pletely separating the substance of the cell into two halves. Each
VOL. I C

18

ZOOLOGY

SECT.

of these halves encloses one of the daughter-nuclei and has
assumed the character of a complete daughter-cell. In some
instances the division of the nucleus is direct or amitotic, the
nucleus simply becoming separated into two equal parts, without
disappearance of the nuclear membrane, and without any compli-
cated re-arrangement of the chromatin.

3. THE OVUM : MATURATION, IMPREGNATION, AND SEGMENTATION :

THE GERMINAL LAYERS.

Amoaba is simply an independent animal cell ; or, to express the
same meaning in allot her way, is a unicellular animal, and as such
it is a member of the phylum of the Protozoa or unicellular
animals. All the rest of the animal kingdom, forming the
division Metazoa, are multicdltilar in the fully developed condition:

but each of these multicellular
animals or Metazoa originates from
a single cell the ovum. The
ovum is a typical cell (Fig. 4),
usually spherical in shape, with
one or more enclosing membranes,
with cell protoplasm enclosing a
nucleus (germinal vesicle) in which
are contained one or more rounded
masses of chromatin (germinal spot
or spots). The ovum may contain
in addition to the protoplasm a
quantity of non-protoplasmic nu-
trient material or yollc.

Before the process of impregna-
tion or fertilisation which gives the
impulse to development, the ovum
undergoes a change which is termed
maturation (Fig. 5, A). This con-
sists in essence, of the throwing out of portions of the nucleus
Lhe latter approaches the surface and divides, mitotically, into two
parts one coming to project on the surface and finally becoming
completely separated off from the ovum as a rounded partu-le-
the first polar lody (pol). A second division of the nucleus
results m the throwing off of a second polar lody ; and, after this
has been formed, the portion which remains in the ovum resumes
its central position and forms what is termed the fcmal- mo-
nucleus ( 9 pro it.).

In the proces.s of impregnation a very minute body, the mah

cell sperm-cell, or sperm, penetrates into the interior of "the female

I or wwm,and the nucleus whir-h it contains the male pro-

Fir;. 4. Ovum of a Sea-Urchin, showing
the radially striated cell-membrane,
the protoplasm, containing y. ilk-
granules, the large nucleus (germinal
vesicle), with its network of chro-
matin and a large nucleolus (ger-
minal spot). (From Balfour's Eia-
bryology, after Hertwig.)

i STRUCTURE AND PHYSIOLOGY OF ANIMALS lf

nucleus (g pron.} coalesces with the female pro-nucleus to form a
single nucleus called the segmentation nucleus (scg. nucl.}. The
principal part in the process of fertilisation is thus played by the

B

mem,

icenl

D

mem

FIG. 5. Diagnim illustrating the maturation and fertilization of the ovum. A, formation of fii>t
polar globule ; B, beginning of fertilization, spermatozoa approaching the micropyle ; C. for-
mation of the male prouueleus ; I), approximation 'of the male and female proimclei ; E. for-
mation of segmentation-nucleus ; 9 cro<. female centrosome ; $ ee.i. male centrosome ; mem,
egg-membrane; nurroji. micropyle ; pol. polar bodies ; $ fit-on, female pronucleus ; 3 pron.
male pronucleus ; .<>;/. ,ti>ct. segmentation nucleus.

two nuclei; but the centrosomes, one (^ cent.} derived from the
sperm, and the other ( o cent.} from the ovum, also seem to take a
share. Sometimes each of these divides into t\v<>, and the two
centrosomes resulting from the division of that derived from the

C 2

X( >OLOGY

SECT.

sperm coalesce with the two formed by division of that belonging .
to the ovum ; but, more commonly, the centrosome of the ovum
disappears before the two nuclei come into contact. The result of
these changes is the formation of the impregnated ovum, OT oosperm
as it is called. The oosperm, it is to be noted, before development
begins, consists of the primary ovum minus the portions of the
substance of its nucleus removed in the polar bodies, and also,
usually, minus its centrosome, and plus the sperm with its nucleus
and centrosome.

On impregnation follows shortly the process of division already
briefly referred to, which is known as segmentation (Fig. 6).
This either affects the entire substance (holobkistic or complete
segmentation) or only a part (meroblostic or incomplete seg-
mentation) of the oosperm. In the former case the ovum usually
contains little or no food-yolk, consisting exclusively, or nearly
so, of protoplasmic matter. The first stage in the process of

':. 6. Various stages in the segmentation of the ovum. (From Gegenbaur's Compant,,,

Anatomy.)

segmentation is the mitotic division of the segmentation-nucleus,
accompanied by the division into two parts of the substance
of the protoplasm the result being the formation of two cells,
each with its nucleus (Fig. 6). Each of these two cells then divides
-four cells being thus formed ; the four divide to form eight ;
the eight divide to form sixteen, and so on, until, b}' the proc. sss
of division and subdivision, the oosperm becomes segmented into
a large number of comparatively small cells which are termed the
Uastomeres. This mass of cells is spherical in shape, and the
rounded blastomeres of which it is composed, project on its sur-
face so as to give it somewhat the appearance of the fruit of
the mulberry, whence it is termed the mnUirrry lm/// or morn la
stage. The blastomeres next become arranged regularly in a
single layer the embryo assuming the form of a hollow sphere,
the blastosphere or blastula,with a wall composed of a single layer
of cells enclosing a cavity the segmentation <"///// or lil<i*toc,,'lc.

One side of the hollow blastula next becomes pushed inwards or
invaginated, as one might push in one side of a hollow india-rubber
ball the result of this process of invagination, or <!<!*i rulitn>n as

STRUCTURE AND PHYSIOLOGY OF ANIMALS

it is termed, being the formation of a cup the yastrula (Fig. 7)
with a double wall. The cavity of the cup-shaped gastrula is the
archenteron or primitive digestive cavity : the opening is termed the
blastcporc, the outer layer of the wall of the
cup is the ectoderm (or epiblasf), the inner
the cndoderm (or l<ui>nl>lst}. The ectoderm
and endoderm axe the primary germinal A'//(vx
of the embryo: from one or both of them are

t/ '

developed the cells of a third layer the
mesoderm (mcsoblasf] which is subsequently
formed between them.

This mode of formation of the primary
germinal layers in holoblastic oosperm.s by a
process of gastrulation prevails in a number
of different sections of the animal kingdom.
In many animals, however, it becomes modi-
fied or disguised in various ways ; and in many
meroblastic oosperm.s it is doubtful if there
occurs anything of the nature of true gastru-
lation.

The cells of the three germinal layers give
rise to the various organs of the body of the fully-formed animal-
each layer having a special part to play in the history of the de-
velopment. As the various parts of the embryo become gradually
moulded from the cells of the germinal layers, it becomes evident
on comparison that their internal structure the form and arrange-
ment of their constituent cells is undergoing gradual modifica-
tions, the nature of which is different in the case of different parts.
A differentiation of the cells is going on in the developing organs,
resulting in the formation of a variety of different kinds of
tissues.

F]<;. 7. Gastmla in longi-
tudinal M_vTii>n. n,
muth : '', enterou ;
<, eiidodovm ; </, ecto-
derm. (From Gegeii-
b; ur's t''ji,tjni,-ntii\Ait-

UtOlllll.)

4. TISSUES.

The cells of the tissues of the animal body differ greatly in
form in different cases. Some are rounded, others cubical, others
polygonal; some are shaped like a pyramid, others like a cone,
others like a column or cylinder ; others are flattened and tabular
or scale-like. Cells situated on free surfaces are in many cases
beset at their free ends with delicate, hair-like structures or cilia
which vibrate to and fro incessantly during the life of the cell
(Fig. 8, <7.) ; sometimes there is on each cell a single, relatively
long, whip-like cilium, which is then termed a ji /////'/// (//,//).
Cells provided with cilia are termed ciliated, such as bear flagvlla
fldtjdlate cells.

Some tissues are composed entirely of cells. Others, though
originating from cells or by the agency of cells, consist in greater

22

ZOOLOGY

SECT.

or less measure of non-protoplasmic matter formed between the
cells. Tissues composed entirely of cells take the form, for the
most part, of membranes covering various surfaces external and

internal. Such mem-
branes are known under
the general name of
epithelia (Fig. 8) ; they
may consist of a single

V j

layer of cells (a-h) or
may be many-layered
( / ) ; the former are
termed non-stratified, the
latter stratified, epithelia,
The cells of an epithe-
lium may be flattened
(c, d), their edges being
cemented together, so as
to form a continuous
membrane ; or they may

i/ ,

be cubical or cylindrical
or prismatic (((,b); in
the case of a stratified
epithelium the cells
may be of different
forms in different strata
(*). The epidermis, which
covers the outer sur-
face of the body of an
animal, is an example
of an epithelium ; some-
times it is stratified,
sometimes unstratified :
its cells sometimes pos-
sess cilia, sometimes are
devoid of them. Lining
the internal cavities of
the body are layers of
cells, or epithelia, some-
times in a single layer,
sometimes in several
layers, sometimes cili-
ated, sometimes non-
ciliated.

Glands (Fig. !') :nv formed for the most part by the modifica-
tion of certain cells of epithelia. 'In many cases a single cell of the
epithelium forms a gland, which is then termed a unicellular <//f/i//
(Fig. 9, A}. The secretion (or .substance which it is the function of

Fie.. 8. Various forms of epithelium, u, cilititerl epi-
thelinm ; /., ci.lunnuiv; /, suvfaeu view of the same ;
r, tesselated ; t 1 , the same from the surface ; /, flagel-
late epithelium with enllars ; 11, flagellate epithelium
without ei >llars; A, epithelium f intestine with
pseudopodia ; <', .stratified epithelium; /,-, ilerie e]>i-
thelium of a marine planarian with pigment cells,
rod-cells, ami sul.-epitheli-il glands. '

COK ''' Aim*" i

STRUCTURE AND PHYSIOLOGY OF ANIMALS

23

the gland to form or collect) gathers in such a case in the interior
of the cell, and reaches the surface of the epithelium through a
narrow prolongation of the cell which serves as the duct of the
gland (11). In other cases the gland is multicellular formed of a
number of cells of the epithelium lining a depression or infolding,
simple or complex in form, of the latter (D-G). In the central
cavity of such a gland the secretion collects to reach the general
surface or cavity lined by the
epithelium through the passage
or dud.

A series of tissues in which the
cells are, in most instances, sub-
ordinate, as regards bulk, to sub-
stances formed between them, is
the group known as the con-
nective tissues, including gela-
tinous connective tissue, retiform
connective tissue, fibrous connective
tissue, cartilage, and bone. In the
majority of forms of connective
tissue the cells lie embedded in
an intermediate substance called
the matrix or ground-substance
of the connective tissue.

In the case of gelatinous con-
nective tisst/i' (Fig. 10) the ground
substance (//) is of a gelatinous
character, sometimes supported
by systems of fibres (i 2 ), and the
cells are usually stellate or star-
shaped with radiating processes.
Retiform or reticulate connective
tissue ( Fig. 11) consists of stellate
or branching cells with process^
which are prolonged into fibivs

the fibres from neighbouring cells joining so as to form a net-
work. In this form of connective tissue there is no true ground-
substance the interspaces between the cells being filled with
other tissue elements.

Fibrous connective tissue, which is a very common form, has a
ground-substance containing gelatin, and consisting of numerous
fibres, usually arranged in bundles. Thicker yellow elastic fibres
may be present among the others, and may be so numerous as to
give the entire tissue an elastic character. Associated with fibrous
tissue, and produced by modification of its cells is adipose tissue or
fat (Fig. 12). Fat consists of masses of large cells in which the
protoplasm has more or less completely become replaced by fat,

Fn;. V. Diagram to illustrate the structure
of glands. A, unicellular glands in an
epithelium ; B, unicellular glands lying
below epithelium and communicating
with the surface by narrow processes
(ducts) ; C, group of gland-cells ; D,
group of gland-cells lining a depression ;
E and F, simple multicellular gland :
G, -.branched multicellular gland. (From
Lang.)

24

ZOOLOGY

SECT.

the cells being bound together into groups and masses or lobules
by means of fibrous connective tissue.

b, 9

^ - /

../ , / ^

Q

KK. 10. 'iuhitiimus connective tissue of ;i Jullj--nsh ; ., epithelium ; </, xvl:itii..ius matrix :
6 2 branching cells ; ,/, elastic fibres. (From Lang's Comparativt Anatomy.)

In the case of cartilage the matrix is of a firm but elastic
character, sometimes quite homogeneous in appearance (hyaline

FIG. 11. Reticular connective tissue. (Frum Lang.)

cartilage, Fig. 13), sometimes permeated by systems of fibres (jibro-
cartilage,Yig. 14), which may be of an elastic nature (t/cllotv clastic
cartilage). The cells are usually rounded, and as a rule several

STRUCTURE AND PHYSIOLOGY OF ANIMALS

25

occur together in spaces scattered through the matrix : sometimes
condensation of the matrix round each of the spaces in Avhich the
cells are contained forms a cell-capsule. The outer surface is
covered over by a fibrous membrane the perichondrium. Carti-

FIG. 12. Fatty tissue ; F, fat cells ; B, connective-tissue fibrils. (From Lang. ::ftcr Hanvicr.)

lage is frequently hardened' by the deposition in the matrix of salts
of lime and is then known as calcified cartila</<\

In lone or osseous connective tissue (Fig. 15) the matrix is exceed-
ingly dense and hard owing to its being strongly impregnated with
carbonate and phosphate of lime. It consists of numerous thin
plates or lamella?, which are arranged partly parallel with the sur-
face, partly concentrically around certain canals (c) the Naccrsian

Fie. 1:5. Hyaline rartilage.

Fie. 1-1. Fil>ro-f;:v:;'

canals in which blood vessels lie. The cells, or bone-corpuscles, lie
in minute spaces the lacuna' between the lamellae, and a system
of exceedingly fine channels the canalicida: extend from lacuna
to lacuna, containing fine protoplasmic processes by means of which
neighbouring cells are placed in communication with one another.
The outer surface of the bone is covered by a vascular fibrous

ZOOLOGY

SECT.

membrane the periosteum which takes an active part in its
growth and nutrition.

The connective tissues are all more or less passive in the
functions which they perform, serving mainly for support and for
binding together the various organs. Muscular tissue, on the

other hand, has an active part to
play this being the tissue by
means of which, in general, all
the movements of the body of
an animal are brought about.
Muscular tissue varies greatly in
minute structure in different
groups of animals, and even in
different parts of the same ani-
mal. It consists of microscopic
fibres aggregated together into
large bundles or layers. These
fibres are composed of a sub-
stance the muscle substancc-
which when living has the special
property of contractility, contract-
ing or becoming shorter and
thicker on the application of a
stimulus. There are two princi-
pal varieties of muscular tissue
to be distinguished, termed re-
spectively non-striated and strwlfl
muscle. Each fibre of non-striated
muscle (Fig. 16) is usually a
single, greatly elongated cell,
sometimes branched, with a single
nucleus; it may contain a core
of unaltered protoplasm, or all
except the nucleus may be altered
into muscle substance; cross-
striation is absent. A fibre of
striated muscular tissue (Fig. 17)
is formed by the union together
of several cells which are repre-
sented by their nuclei (,). Some-
times there is a core of proto-
plasm : but more usually the entire fibre is composed of muscle
substaner. with perhaps a remnant of protoplasm in the neigh-
bourhood of each nucleus. The substance of the fibre is crossed
by numerous transverse bands and stria?, the precise significance
of which is a matter of controversy. The fibre is usually en-
1 in a delicate sheath the sarcolemma. Striated muscular

-*>*

"-'.td-tt^iT^ jVj r

**^Si335wT Wsfe*."

^r^m=n'w^f.i *<*/
kM 8sESaSisS3*3*

section of compact
bone, a, lamella? concentric with the
outer surface ; l>, lamella? concentric
with the surface of the marrow cavity ;
c, sn-tiun of Haversian canals; c', sec-
tion of a Haversian canal just dividing
into two : d, interstitial lamella;.
Jiuxl' / ns "> Pin

STRUCTURE AND PHYSIOLOGY OF ANIMALS

27

tissue is specially characteristic of parts in which rapid movement
is necessary.

The principal elements of nervous tissue arc ncrcc cells and
nerve fibre*.

(Fig. 18) vary greatly in form ; they are relatively large

Fii;. lii. Non striated muscle cell; /. sub-it.-nn-e of fibre ; )>. nucleus ; />. unaltered protoplasm in
the neighbourhood of the nucleus. (From Huxley's ..S,VOK.S in Physiology.)

cells with large nuclei, and one or several processes produced into
nerve fibres.

The in'i'Ci' nl'cs (Fig. 19), which are to be looked upon as greatly
produced processes of nerve cells, are arranged for the most part
in strands which are termed no-res. The fibres themselves vary

i/

greatly in structure in different classes of animals. In the higher

animals the most characteristic form of nerve fibre is that which is

termed the meduUated nerve-fibre. In this there is a central cylinder

the axis-cylinder* QT ne urn-vis (A, an} which is the essential part

B

-7?

-6
-cl

6

Km. 17. Striated muscle. A, part of a muscular fibre of a Frog; B, portion of stri;ited muscle
teased out to show separation into fibrillas. (From Huxley's X-.s.<o,i.s i,i PliiitMouii.)

of the fibre, and is made up of numerous extremely fine primitive
fibrillcK ; this is surrounded by a layer of a white glistening
material the wldt? substance of ftchwann or medullary slienti!
(med), enclosed in turn in a very delicate membrane the
'nc in'U<: 'ii i ma (ncur).

The blood, the lymph, and other similar fluids in the body of an
nnini:il may be looked upon as liquid tissues, having certain cells

28

ZOOLOGY

SECT.

the corpuscles disseminated through a liquid plasma, which
takes the place of the ground substance of the connective tissues.

FK-. is. Nerve cells. A. niultipolar;
, bipolar.

]"].. I.'. Xcrve fibre.-. A. iiicilnlluted ;
B, npn-medullated ; ax, n. .,
ii .'. medullary sheath :
neurilemma.

In a large proportion of cases such corpuscles are similar to
Amcebae in their form and movements ("/nt.i/i/</ corpuscles, leuco-
cytes). In the blood of Vertebrates leucocytes occur along with
coloured corpuscles of definite shape containing the red-colouring
matter (hccmoglobin) of the blood. The leucocytes arc able like
Amcebse to ingest solid particles, and under certain conditions a

number of them may unite to-
gether to form a single mass of
protoplasm, with many nuclei,
termed a plasm mHi'm.

The characteristic cells of the
reproductive tissues are the ova
and the spermatozoa or sperms. The
ova (Fig. 4\ when fully formed, are
relatively large, usually spherical
cells, sometimes composed entirely
of protoplasm, sometimes with an
addition of mitri-ent food-yolk. Each
ovum, as already mentioned, en-
closes a large nucleus (//. / -m-iiinl
c< *i<-]<) and in the interior of that
one or more nucleoli or germinal
spots. The sperms (Fig. 20) are
extremely minute bodies, nearly

always motile, usually slender and whip-like, tapering towards
one extremity, and commonly with a rounded lu:ad at the other.

KH.. 20. Various forms of si>crmato/,,;i.
(i. i >f a Mammal ; l>. of a Turbullariaii
worm ; c, and -'.and e, of Xematode
worms; /, of a Crustacean; <i. of a
Salamander; A, the commonest form
with oval head and long- flagellurn.
(From Lang's Comparativi Anatomy.)

i STRUCTURE AND PHYSIOLOGY OF ANIMALS ^

The sperms are developed by a succession of cell-divisions from
certain cells the prim it ire male cells similar in character t<>
immature ova.

5. ORGANS.

The chief systems of organs of an animal are the integumen-
tary, the skeletal, the muscular, the alimentary or digestive, the
vascular, the respiratory, the nervous, the excretory, and the repro-
ductive.

The skin or integument consists in the majority of animals
of a cellular membrane the epidermis to which reference has
already been made, with, superficial to it, in many animals a non-
cellular layer the cuticle, and below it usually a fibrous layer which
is known as the dcrmis. The epidermis may consist of a single
layer or may be stratified ; it is frequently ciliated, and some of
its cells frequently assume the form of unicellular glands. Modi-
fication of its superficial layers of cells gives rise frequently to the
formation of hard structures contributing to the development of
an exoskeleton (vide infra}.

The cuticle, when present, varies greatly in thickness and con-
sistency. Sometimes it is very thin and delicate ; in many
animals it becomes greatly thickened and hardened so as to form
a strong protecting crust, sometimes of a material termed cJiitin,
somewhat akin to horn in consistency, sometimes solidified by the
deposition of calcareous salts. The cuticle is to be looked upon as
a secretion from the cells of the epidermis; but the term is
frequently applied in the case of the higher animals, in which a
cuticle in the strict sense of the term is absent, either to a super-
ficial part of the epidermis, in which the cells have become altered
and horny, or to the whole of that layer.

The layer or layers of the integument situated beneath the
epidermis consist of fibrous connective tissue and muscular fibres,
constituting the derm or dcrmis.

The term skeleton or skeletal system is applied to a system
of hard parts, external or internal, which serves for the protection
and support of softer organs and often for the attachment of muscles.
This system of hard parts may be external, enclosing the soft
parts, or it may lie deep within the latter, covered by integument
and muscles : in the former case it is termed an exoskeleton or
external skeleton ; in the latter an endoskeleton or internal skeleton.
In many groups of animals both systems are developed. An
exoskeleton is formed by the thickening and hardening of a part
or the whole of one of the layers of the integument enumerated
above; or more than one of these layers may take part in its
formation. In many invertebrate animals, such as Insects,
Crustaceans, and Molluscs, it is a greatly thickened and hardened

oO ZOOLOGY SECT.

cuticle which forms the exoskeleton. The horny scales of Reptiles,
the feathers of Birds, and the fur of Mammals are examples of
an exoskeleton derived from the epidermis, while the bony
shell of Turtles and the bony scales of Fishes are examples of a
dermal exoskeleton.

When an endoskeleton is present, it usually consists either of
cartilage or bone or of both ; but sometimes it is composed of
numerous minute bodies (s^iculcs) of carbonate of lime or of a
siliceous material.

A skeleton, whether internal or external, is usually composed
of a number of pieces which are movably articulated together,
and which thus constitute a system of jointed levers on which the
muscles act.

The alimentary or digestive system consists of a cavity or
system of cavities into which the food is received, in which it is
digested, and through the wall of which the nutrient matters are
absorbed ; together with certain glands.

In the lowest groups in which a distinct alimentary or enteric cavil ;/
is present it is not distinct from the general cavity of the bodv.
but in all higher forms there is an enteric enal which is sus-
pended within the cavity of the body, and the lumen of which is
completely shut off from the latter. It may have simply the form of
a sac or bag with a single opening which serves both as month and
anas; in other cases the sac becomes branched and may take the
form of a system of branching canals. In most animals, however,
the alimentary canal has the form of a longer or shorter tube
beginning at the mouth and ending at the anal opening (Fig. 21),
In most cases there are organs in the neighbourhood of the mouth
serving for the seizure of food : these may be simply tentacles or
soft finger-like appendages, or they may have the form of Ja/rx, by
means of which the food is not only seized but torn to pieces or
pounded up to small fragments in the process of mastication. The
alimentary canal itself is usually divided into a number of regions
which differ both in structure and in function.

In general there may be said to be three regions in the ali-
mentary canal the ingestive, the digestive and absorbent, and the
egestive or efferent. The ingestive region is the part following
behind the mouth, by which the food reaches the digestive and
absorbent region. But, besides serving as a passage, it may also
act as a region in which the food undergoes certain processes,,
chiefly mechanical, which prepare it for digestion. This ingestive
region may comprise a mouth cavity or buccal cavity, a pJiart/ri.!',
an a'Xi>/ia</us or yulhi , with sometimes a muscular ///::>////, which
may be provided with a system of teeth for the further breaking
up of the food, and sometimes a crop or food-pouch.

The digestive and absorbent region is the part in which the
chemical processes of digestion go on, and from which takes place

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 31

the absorption of the digested food-substances. Into this part are
poured the secretions of the various digestive glands, which act on
the different ingredients of the food so as to render them more
soluble. Through the lining membrane of this part the digested
nutrient matter passes, to enter the blood system. This region
may present a number of different parts ; nearly always there are
at least two a wide sac, the stomach, and a narrow tube, the
intestine.

The cgestive or efferent part of the alimentary canal is the
posterior part of the intestine in which digestion and absorption
do not go on, or only go on to a limited extent, and which serves

Fm. 21. General view of the viscera of a male Frog, from the right side, a, stomach ; 6, urinary
bladder; c, small intestine; rl, cloaca! aperture; < 7, large intestine; <, liver ; /, bile cl'irt ;
</, gall bladder ; h, spleen ; i, lung ; /,, larynx ; I, fat body ; //<, testis ; ,*, ureter ; o, kidney ;
f, pancreas; ., cerebral hemisphere; sp, spinal cord; t, tongue; c, auricle; c/-, umstyle :
-, ventricle ; I-.K, vesicula seminalis ; te, optic lobe ; x, cerebellum ; it, Eustachian recess ;
:, nasal sac. (From Marshall.)

merely for the passage to the anal opening of the fccces or
unabsorbed effete matters of the food.

The whole of the interior of the alimentary canal is lined
by a layer of cells the alimentary or enteric epithelium. The
form and arrangement of the cells of this epithelium vary greatly
in different groups of animals. Usually, they are vertically
elongated, prismatic, or columnar, or pyramidal in shape ; fre-
quently they are ciliated. In some lower forms, the cells lining
the alimentary cavity have the power, like Amoeba, of thrusting
forth processes of their protoplasm (Fig. 8, It}, and of taking minute
particles of food into their interior to become digested and absorbed
(intracellular digestion}. Sometimes they are all more or less
active in secreting a fluid destined to act on the food and render
it more soluble ; sometimes this function is con Kned to certain ot
the cells, which have a special form : very often the secreting cells

: ZOOLOGY SECT.

line special little pouch-like, simple or branched glands, opening
by a passage or duct into the main cavity of the alimentary
canal. Besides these glands formed from specially modified cells
of the enteric epithelium there are nearly always present certain
large special glands, separate from the alimentary canal itself but
opening into it by means of ducts. Of these the most generally-
occurring are the glands termed salivary glands, liver, and pancreas.
The sahcai'ii glands have the function of secreting a fluid called
the saliva, which, in many cases at least, has a special action on
starchy matters, converting them into sugar. The ducts of these
glands open always, not into the digestive, but into some part
of the ingestive part of the alimentary system.

The most important function of the liver properly so called
is one distinct from the process of digestion; its secretion the
bile has, however, at least a mechanical effect on this process,
and assists the secretion of the pancreas in its effects upon fat.
In lower fi >rms the organ to which the term liver is commonly applied
appears in many cases to combine the functions of a true liver
with that of a pancreas, and is thus more appropriately termed
hepato-jiancreas or liver-pancreas.

The pancreas secretes a fluid the pancreatic juice which has a
very important effect in digestion. It renders substances of the
nature of albumins soluble by converting them into modifications
termed peptones; it converts starch into the soluble substance sugar ;
it acts on fatty matters in such a way as to convert them into
emulsions which are capable of being taken up and absorbed, and
it effects the splitting up of part of the fat into fatty acids and
glycerine.

When the food has been acted on by the various digestive
secretions, the soluble part of it is fitted to be taken up and
absorbed through the wall of the alimentary canal into the blood
(in animals in which a blood-system exists), or into the fluid
which takes its place. In the higher animals a part of the
soluble matter of the food passes directly into the blood contained
in the blood-vessels ; while another part is taken up by a set of
special vessels, the lacteals, which are a part of the lymphatic
system, and reaches the blood indirectly.

In some of the lower groups of animals there isno system of blood-
vessels, and the nutrient matter of the food, absorbed through
the alimentary canal merely passes from cell to cell throughout
the body, or is received into a space or series of spaces containing
fluid intervening between the alimentary canal and the wall of
tin 1 body. But in the majority of animals there is a system of
branching tubes containing a special fluid the blood and it is
into this that the nutrient matter absorbed from the food sooner
or later finds its way. The blood has for one of its principal
functions the conveyance of the nutrient matters from the

STRUCTURE AND PHYSIOLOGY OF ANIMALS 33

alimentary canal throughout the body, so that the various organs
may select from it the material which they require for the carrying
on of their functions. To carry out this office the blood is contained
in a complicated system of branching tubes or blood-vessels.

The essence of the process of respiration, as we have already
seen, is an interchange of oxygen and carbonic acid, which goes on
between the tissues of an organism and the surrounding medium,
whether air or water. During the vital changes which go on in
the bodies of all animals, as in Amoeba, oxygen is constantly
being used up and carbonic acid being formed. The necessary
supply of oxygen has to be got from the air, or, in the case of
aquatic animals, from the air dissolved in the surrounding water.
At the same time the carbonic acid has to be got rid of. In the
lowest animals as for instance Amoeba, and many of higher
organisation the oxygen passes inwards and the carbonic acid
outwards through the general surface of the body. But in the
great majority of animals there is a special set of organs the
organs of respiration having this particular function. In some
animals these organs of respiration are processes, simple or
branched, lined by a very delicate membrane, and richly supplied
with blood-vessels. Such processes are called gills or branchiae;
they are specially adapted for the absorption of oxygen dissolved
in water.

In other animals the oxygen is obtained directly from the air ;
and in such air-breathing forms the organ of respiration is very
often a sac, either simple or compound, termed a lung. The
interior of this sac is lined with an epithelium of extreme delicacy,
immediately outside of which is a network of microscopic blood-
vessels or capillaries with thin walls, and the oxygen readily passes
from the air in the cavity of the lung through the membrane and
the thin wall of the blood-vessel into the blood. In other air-
breathing forms the organs of respiration are trachecc, which are
ramifying tubes, by means of which the air is conveyed to all parts
of the body. In such forms, of which the Insects are examples, the
air is conveyed, by means of these tubes, from openings on the
surface of the body to all parts, and respiration goes on in all the
organs.

In order that the air or water in contact with the surface of the
lungs or gills may be renewed, there are usually special mechanical
arrangements. In many gill-bearing animals the gills are attached
to the legs, and are thus moved about when the animal moves its
limbs. In others certain of the limbs are constantly moving in
such a way as to cause a current of water to flow over the gills.
In air-breathing forms there is usually a pumping apparatus, by
means of which the air is alternately drawn into and expelled
from the lungs.

In a great number of animals there is in the blood a substance

D

34 ZOOLOGY

SECT.

called hcemoglobin, which has a strong affinity for oxygen ; and the
oxygen from the air, when it enters the blood, enters into a state
of loose chemical combination with it. In this state, or simply
dissolved in the fluid plasma of the blood, the oxygen is conveyed
throughout the body.

Thus the blood, besides receiving the solid and liquid food from
the alimentary canal and carrying it throughout the body for
distribution, receives also the oxygen or gaseous food, and supplies
it to the parts requiring it. In all parts of the body in which
vital action is taking place chemical changes are constantly going
on. These chemical changes in the tissues, having for their result
the production of heat, of motion, secretion, and nerve-action, are
for the most part of the .nature of oxidations, and involve a constant
consumption of oxygen, while a product which becomes formed as
a result of this action is carbonic acid gas.

To carry out all the functions which it has to perform as a
distributor of nourishment and oxygen, and a remover of carbonic
acid, the blood has to be moved about through the vessels to
circulate throughout the various organs. In the lowest forms in
which a definite blood-system is to be recognised, this movement
is effected in great measure by the general movements of the
body of the animal. In others certain of the vessels contract and
drive the blood through the system ; such contractions are of a
peristaltic character, the contractions being of the nature of con-
strictions running in a definite direction along the course of the
vessel with an effect similar to that produced by drawing the hand
along a compressible india-rubber tube.

In all higher forms the movement of the blood is effected by
means of a special organ the heart. The heart is a muscular
organ which by its contractions forces the blood through the
system of vessels. In its simplest form it usually consists of two
chambers, both with muscular walls, the one, called the auricle,
receiving the blood and driving it into the other, which is called
the ventricle. The latter, in turn, when it contracts, drives the blood
through the vessels to the various parts of the body the return
of the blood backwards to the auricle from the ventricle being
prevented by the presence of certain valves, which act like folding
doors opening from the auricle towards the ventricle, but closing
when pressure is exerted in the opposite direction. In the higher
animals the heart becomes a more complex organ than this, with a
larger number of chambers and a more elaborate system of valves.

Carbonic acid, as already mentioned, is a waste-product con-
stantly being produced in the tissues, and being carried off by the
blood to pass out by the gills or lungs. Besides the carbonic
acid, there are constantly being formed waste-substances of another
class viz., substances containing nitrogen, of which uric acid and
urea are the principal ultimate forms. These are separated from

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 35

the blood and thrown out of the body by a distinct set of organs
called renal organs, or organs of urinary excretion. The

form of these organs varies greatly in the different groups :
in many cases they are more or less intimately connected with
the genital system.

In place of the simple contractions and extensions of the proto-
plasm which constitute the only movements of Amoeba, the higher
animals are capable of complex and definite movements. These
are brought about by the agency of a set of organs termed the
muscles. A muscle is a band or sheet of muscular fibres
endowed in the living state with the property of contmct'diti/, by
virtue of which, when stimulated in certain ways, it contracts in
fche direction of its length, becoming shortened, and, at the same
time, thickened (Fig. 22). The extremities of the muscle are

FIG. 22. Bonas of the human arm aud fore-arm with the biceps muscle, showing the shortening
and thickening of the muscle during contraction and the consequent change in the relative
position of the bones viz. , flexion of the fore-arm on the upper arm. (From Huxley's Physiology. )

frequently composed, not of contractile muscular fibres, but of a
form of strong fibrous connective tissue the tendon of the muscle.
The ends of the muscle are usually firmly attached to two different
parts of the jointed framework or skeleton, external or internal,
and, when the muscle contracts and becomes shortened, these two
parts are drawn nearer to one another.

In all but the most lowly-organised animals there is a system
of organs the nervous system by means of which a communi-
cation is effected between the various parts of the body, enabling
them to work in harmony, and by means of which also a communi-
cation is established between the organism and the external world,
The two essential elements of the nervous system the nerve-cells
and nerve-fibres have a regular arrangement which varies in the
different animal types, both as regards structural details and 'the
relations borne to the other systems of organs ; but there are
always to be recognised two chief parts or sets of parts the
central and the peripheral.

D 2

36

ZOOLOGY

SECT.

ol. n,

<vc

au,

The central parts of the nervous system consist (Fig. 23) of
certain aggregations of nerve matter, known as nerve-ganglia,
containing a large number of nerve-cells ; when a relatively large

mass of this matter is
collected together it is
termed the brain. To or
from these central parts
pass all the systems of
nerve-fibres, constituting
the peripheral part of the
system ; the former have
the office both of re-
ceiving impressions con-
veyed by the nerve-fibres
from the surface, from
the organs of special
sense, and from the in-
ternal organs, and of
sending off messages
through similar channels
to the various parts of
the body, to muscles, to
glands, to alimentary
canal, and to vascular
system. When a move-
ment is to be effected
a message passes from
the nerve-centre along a
nerve-fibre to a muscle
and causes it to contract ;
when an organ requires
the amount of blood sup-
plied to it to be in-
creased or diminished a
message is combed
along a nerve-fibre and
causes the dilatation or
contraction of the blood-
vessels of the part ; and
a similar initiatory or
(From controlling influence is
exerted over the activities
of all the organs.

In certain groups of animals all the impressions from the
external world are received through the integument of the general
surface, and this is the case in all animals with the general
impressions of touch and of heat and cold. The sensitiveness of

FJ.G. 23, Nervous system of the Frog.
Huwes's Atlas.)

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 37

the integument to such general impressions may be increased by
the presence in it of a variety of tactile papillae or corpuscles
having nerve-fibres terminating in them. In most animals, how-
ever, there are certain organs, the organs of special sense,
adapted to receiving impressions of special kinds eyes for the
reception of the impressions produced by light, cars for the recep-
tion of those produced by the waves of sound, olfactory organs or
organs of smell, and gustatory organs or organs of taste. The most
rudimentary form of eye is little more than a dot of pigment
which absorbs some of the rays of bright light these producing
a nerve-disturbance in certain neighbouring nerve-cells. To this
may be added clear, highly-refracting bodies which intensify the
effect. In the higher types of eye there are the same essential
parts -the clear, highly-refracting substance, the pigment and the
nerve-cells ; but each has undergone a development resulting in
the construction of an organ adapted to the reception of light-
impressions of a very definite character. The highly-refracting
body assumes the form of a lens for the focussing of the light-rays ;
the nerve-cells are arranged in a regular layer, the retina, from
which nerve-fibres pass to the central part of the nervous system ;
the pigment is so arranged as to absorb the light rays and prevent
their passage beyond the retina, and in certain cases lines also a
diaphragm, the iris, with a central aperture through which the
rays of light are admitted to the central parts of the eye. In
some animals (Insects, Crustacea) the eye consists of a very large
number of independent elements, each with its refracting apparatus,
its nervous element, and its absorbing pigment.

The car in its simplest form is a membranous sac or otocyst with
internally projecting stiff cilia, and containing a liquid in which
there lie a number of particles of carbonate of lime. The sound-
waves evidently set in vibration the liquid and its contained cal-
careous particles, and by means of these vibrations acting on the
cilia, an impression of a definite character is produced in the cells
of a neighbouring nerve-ganglion. In higher forms the apparatus
for receiving the vibrations becomes extremely complex, and there
is elaborated a nervous mechanism by which sounds of different
pitch and intensity produce impressions of a distinct character.
The organ of hearing frequently assumes the additional function
of an organ ministering to the sense of rotation and thus has an
important part to play in the maintenance of the equilibrium of
the body.

The essential elements of the reproductive organs the ova
and spermatozoa have already been briefly alluded to (p. 28).
The ova are developed in an organ termed the ovary, and the
sperms in an organ termed the spcrmary or tcstis. Sometimes
ovaries and testes are developed in the same individual, when the
arrangement is termed moncecious or hermaphrodite ; sometimes

38 ZOOLOGY

SECT.

the ovaries occur in one set of individuals the females and the
testes in another set the males, when the term unisexual or
dioecious is employed. Very frequently the male differs from the
female in other respects besides the nature of the reproductive
elements in size, colour, and the like ; when such differences are
strongly marked the animal is said to be sexually dimorphic. The
ova and sperms are usually conveyed to the exterior by canals
or ducts the ovarian ducts or oviducts, and the tcsticular ducts,
spermiducts, or vasa dcfcrentia. In some instances the ova are
impregnated after being discharged from the oviducts, and the
development of the young takes place externally ; in other cases
the impregnation takes place in the oviduct, and the young
become fully developed in the interior of a special enlargement
of the oviduct termed the uterus. In the former case the animal
is said to be oviparous, in the latter viviparous ; but there are
numerous intermediate gradations between these two extremes.

6. THE REPRODUCTION OF ANIMALS.

In a limited number of groups of animals reproduction takes
place by means of cells corresponding to ova developed in organs
similar to ovaries, but without impregnation by means of sperms.
This phenomenon is known as parthenogenesis.

Besides the sexual process of reproduction by means of ova and
spermatozoa, there are in many classes of animals various asexual
modes of multiplication. One of these the process of simple
fission has been alreadynoticed in connection with the reproduction
of Amoeba. The formation of spores is an asexual mode of multi-
plication which occurs only in the Protozoa, and will be described
in the account of that group. Multiplication by budding takes
place in a number of different classes of animals. In this form of
reproduction a process or bud (Fig. 24, Id) is given off from some
part of the parent animal ; this bud sooner or later assumes the
.form of the complete animal, and may become detached from
the parent either before or after its development has been
completed, or may remain in permanent vital connection with the
parent form.

When the buds, after becoming fully developed, remain in vital
continuity with the parent, a sort of compound animal, consisting
of a greater or smaller number of connected units, is the result.
Such a compound organism is termed a colony, and the component
units are termed zooids. In some cases such a colony is produced
by a process which is more correctly termed incomplete fission
than budding.

Alternation of generations ; heterogamy ,- paedogenesis.

In the life-history of a considerable number of animals, a stage in
which reproduction takes place by a process of budding or fission

STRUCTURE AND PHYSIOLOGY OF ANIMALS

39

alternates with a stage in which there occurs a true sexual mode
of reproduction. Such a phenomenon is termed alternation of
generations or -metagenesis. The term hcterogamy is applied to
cases in which two different sexual generations usually a true
sexual and a parthenogenetic alternate with one another.
Pcedogcncsis, or the development of young by a sexual process from

PIG. 24. Fresh-water polype (Hydra), two specimens, the one expanded, the other contracted
showing multiplication by budding, lid.* M.- M.3 buds in various stages of growth. (From
Parker's Biology. ,)

individuals that have not attained the adult condition, is a
phenomenon which is to be observed in some groups of animals.

7. SYMMETRY.

The general disposition or symmetry of the parts in an animal
presents two main modifications the radial and the bilateral.
The gastrula (p. 21) is the simplest and most generalised form among
multicellular animals or Metazoa ; but no adult animal retains
this simple shape. In the gastrula we may imagine a central
primary axis (Fig. 25,AJJ) passing through the middle of the blas-
topore and of the archenteric cavity, and a series of secondary axes
(ab, cd,) running at right angles to this to the outer surface. In a
symmetrical gastrula the secondary axes would be all equal. Many

40

ZOOLOGY

SECT,

animals are in the ad alt condition similar in their symmetry to the
gastrula, except that there are special developments along a series
of regularly arranged radiating secondary axes; these radial
developments may be in the form of tentacles or radially arranged
processes (Fig. 26), or may assume the character of a radial arrange-
ment of internal parts. Such an animal is said to be radially
symmetrical. The body of a radially symmetrical animal is capable

c

FIG. 25. Diagram of the axes of the body.
AB, primary axis ; ab, cd, secondary
axes. The lower figure is a transverse
section of the upper one showing its two
secondary axes. (From Gegeubaur.)

FIG. 26. Radial symmetry. Letters as
in Fig. 25. The processes at A are
the tentacles ; the lower figure repre-
sents the upper or oral surface. (From
Gegenbaur.)

of being divided into a series of equal radial parts or antimcres,
each of which is symmetrically disposed with regard to one of the
secondary or radial axes.

In animals which are not permanently fixed, locomotion usually
takes place in the direction of the primary axis of the body, and
one side, habitually directed downwards, becomes modified differ-
ently from the other which is habitually directed upwards ; a lower
or ventral surface becomes distinguishable from an upper or dorsal.
The radial symmetry now becomes disturbed ; the secondary axes
have become unequal ; the dor so-ventral or vertical secondary axes

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 41

are, to a greater or less extent, different from the transverse or
horizontal secondary axes, and the body of an animal having such
a disposition of the parts is divisible into two equal lateral halves
or hemisomcs by a median vertical plane passing through the
primary axis. This is the bilateral symmetry observable in all but
a few types of animals.

Sometimes the bilaterally symmetrical animal is unsegmented ;
sometimes it is divided into a series of segments or metamcrcs.
A distinct head may be present or absent. The head end or
anterior end is that which, save in exceptional cases, is directed
forwards in locomotion. It is towards this end that the organs of
special sense are situated, as well as the opening of the mouth and
the organs for the prehension and mastication of food. A head is
developed when the anterior part bearing these structures is
separated off externally from the rest. In segmented animals the
head consists of a number of segments amalgamated together, and
it contains the brain or the principal central ganglia of the nervous
system.

8. THE PRIMARY SUBDIVISIONS OR PHYLA OF THE ANIMAL

KINGDOM.

The various systems of organ) digestive, circulatory, nervous,
excretory, etc. present under one form or another in all the higher
groups of animals, are variously arranged and occupy various
relative positions in different cases, producing a number of widely
different plans of animal structure. According as their structure
conforms to one or another of these great plans, animals are referred
to one or another of the corresponding great divisions or phyla of
the animal kingdom. That animals do present widely differing
plans of structure is a matter of common knowledge. We have
only to compare the true Fish, such as Cod, Haddock, etc., in a fish-
monger's shop with the Lobsters and the Oysters, to recognise the
general nature of such a distinction. The first-named are charac-
terised by the possession of a backbone and skull, with a brain and
spinal cord, and of two pairs of limbs (the paired fins) ; they belong
to the great vertebrate or backboned group the division Vcrtc-
brata of the phylum Chordata. The Lobsters, on the other hand, in
which these special vertebrate structures are absent, possess a
jointed body enclosed in a hard jointed case, and a number of pairs of
limbs also enclosed in hard jointed cases, and adapted to different
purposes in different parts of the body some being feelers, others
jaws, others legs : their general type of structure is that which
characterises the phylum Artliropoda. The Oysters, again, with
their hard calcareous shell secreted by a pair of special folds
of the skin constituting what is termed the in antic, and with a
special arrangement of the nervous system and other organs which

42 ZOOLOGY SECT, i

need not be described here, are referable to the phylum Mollusca.
Other familiar animals are readily to be recognised as belonging to
one or other of these great phyla. A Prawn, a Crab, a Blue-bottle
Fly, a Spider, are all on the same general plan as the Lobster : they
are jointed animals with jointed limbs, and they have the internal
organs occupying similar positions with relation to one another.
They are all members of the phylum Arthropoda. Again, a Mussel,
a Snail, a Squid are all to be set side by side with the Oyster as
conforming to the same general type of structure ; they are all
members of the phylum Mollusca. While a Dog, a Lizard, a Fowl
are obviously nearer the Fish : they all have skull and back-
bone, brain and spinal cord, and two pairs of limbs ; they are all
members -of the great group Chordata.

Altogether twelve phyla are to be recognised, viz. :

I. Protozoa VII. Mottuscoida

II. Porifara VIII. Echinodcrmata

III. Ccelenterata IX. Annulata

IV. Platyhelminthes X. Arthropoda
V. Ncmathdmintlies XL Mollusca

VI. Trochelminthes XII. Chordata

SECTION II
PHYLUM PROTOZOA

IN the preceding section we learnt the essential structure of an
animal cell, and it was pointed out that in the lowest organisms
the entire individual consists of a single cell. All such unicellular
animals are placed in the lowest primary subdivision of the animal
kingdom the phylum Protozoa.

We have also learnt that cells vary considerably in character.
They may be amceboid or capable of protruding temporary processes
of protoplasm called pseudopods ; flagellate, or produced into one
or more always a small number of threads having an intermit-
tent lashing movement ; ciliated, or produced into numerous
rhythmically moving threads of protoplasm ; or encysted, the proto-
plasm being enclosed in a cell-wall. Moreover, under certain
circumstances, amoeboid cells may fuse with one another to form
a plasmodium.

These well-marked phases in the life of the cell allow us to
divide the Protozoa into subdivisions called Classes. The same
organism may be amoeboid, flagellate, encysted, and plasmodial
at various stages of its existence, but nevertheless we find
certain forms in which the dominant phase in the life-history is
amoeboid, others which are characteristically flagellate or ciliated,
others again in which the tendency to form plasmodia is a
distinctive feature. In this way five well-marked groups of
unicellular organisms may be distinguished.

Glass 1. RHIZOPODA. Protozoa in which the amoeboid form is
predominant, the animal always forming pseudopods. Flagella
are often present in the young, and occasionally in the adult.
Encystation frequently occurs.

Class 2. MYCETOZOA. Terrestrial Protozoa in which the plas-
modial phase is specially characteristic, as also is the formation
of large and often complex cysts.

Class 3. MASTIGOPHORA. Protozoa in which the flagellate form

44 ZOOLOGY SECT.

is predominant, although the amoeboid and encysted conditions
frequently occur.

Class 4. SPOROZOA. Parasitic Protozoa without organs of loco-
motion in the adult. Encystation is almost universal, and the
young may be flagellate or amoeboid.

Class 5. INFUSORIA. Protozoa which are always ciliated, either
throughout life or in the young condition.

CLASS I. RHIZOPODA.

1. EXAMPLE OF THE CLASS Amoeba proteus.

Amoeba has been fully described in the preceding chapter ; it
will therefore be unnecessary to do more than recapitulate the
most essential features in its organisation.

It is an irregular mass of protoplasm (Fig. 27, E) about ^ mm.
in diameter, produced into irregular processes or pscudopods (psd)
of variable size and form, and capable of being protruded and
retracted often with considerable rapidity. The protoplasm is
divisible into a granular internal substance or cndosarc and a clear
outer layer or ectosarc ; the difference between the two is hardly a
structural one, but depends simply on the accumulation of granules
in the central portion. The granules are, for the most part, various
products of metabolism proteinaceous or fatty.

Imbedded in the endosarc is a large nucleus (mi), of spherical
form, and consisting of a clear achromatic substance, enclosed in a
membrane, and containing minute granules of chromatin. The
contractile vacuolc (c. vac.'), a very characteristic organ of the
Protozoa, lies in the ectosarc, and exhibits rhythmical movements,
contracting and expanding at more or less regular intervals.

Amoeba feeds by ingesting minute organisms (Fig. 27, C,/. vac.)
or fragments of organisms i.e., by enveloping them in its substance,
retaining them until the proteids they contain are dissolved and
assimilated, and then crawling away and leaving the undigested
remnants behind.

Amoebae are sometimes found to undergo cncystation ; the
pseudopods are withdrawn and the protoplasm surrounds itself
with a cell-wall or cyst (D, cy), from which, after a period of rest,
it emerges and resumes active life. The cyst is formed of a
chitinoid material i.e., a nitrogenous substance allied in composi-
tion to horn and to the chitin of which the armour of Insects,
Crayfishes, etc., is composed.

Reproduction takes place by simple or Unary fission ; direct or
amitotic division of the nucleus is followed by division into two of
the cell-body (i). Rarely two Amoebas have been observed to

II

PHYLUM PROTOZOA

45

conjugate, or undergo complete fusion, but nothing is known of the
result of this process or of its precise significance in this particular

case.

Fio. 27. Amoeba. A, A. quartet; B, the same killed and stained; C, A. proteus; D, encysted
specimen ; B, A. proteus ; F, nucleus of same, stained ; G, A. i-wiicosa; H, nucleus of same,
stained ; I, A. proteus, undergoing binary fission ; a, point of union of enclosing pseudopods ;
c. vac. contractile vacuole ; cii. cyst ; f. vac. food vacuole ; nu. nucleus ; psd. pseudopod. (From
Parker's Biology, after Leidy, Gruber, and Howes.)

2. CLASSIFICATION AND GENERAL ORGANISATION.

The Rhizopoda differ among themselves in the character of
their pseudopods, which may be short and blunt or long and

46 ZOOLOGY SECT.

delicate ; in the number of nuclei ; and in the presence or absence
of a hard shell within or around the protoplasm. The following
five orders may be distinguished :

ORDER 1. LOBOSA.
Rhizopoda with short, blunt pseudopods.

ORDER 2. LABYRINTHULIDEA.

Rhizopoda having a network of fine pseudopods, in which
corpuscles travel to and fro.

ORDER 3. FORAMINIFERA.

Shelled Rhizopoda with fine, branched, and anastomosing
pseudopods.

ORDER 4. HELIOZOA.

Rhizopoda with fine, stiff, radiating pseudopods.

ORDER 5. RADIOLARIA.

Rhizopoda having a shell in the form of a perforated central
capsule, and usually, in addition, a siliceous skeleton : the pseudo-
pods are long and delicate.

Systematic Position of the Example.

Amoeba proteus is one of many species of the genus Am-ceba,
belonging to the family Amosbidce, of the order Lobosa. The blunt
pseudopods not uniting to form networks place it among the
Lobosa : the absence of a shell among the Amcebidse. The genus
Amoeba is distinguished by the presence of one or more nuclei,
and of a contractile vacuole. In A. proteus the pseudopods are
of considerable length and sometimes branched, and there is a
single nucleus, having its chromatin in the form of scattered
granules.

ORDER 1. LOBOSA.

General Structure. The members of this group all agree
with Amoeba in essential respects, their most characteristic feature
being the short, blunt pseudopods. The chief variations in struc-
ture upon which the genera and species are founded have to do
with the number and character of the nuclei, the form of the
pseudopods, and the presence or absence of a shell.

In Amoeba itself there may be one (Fig. 27, E) or several (B)
nuclei, the chromatin of the nucleus may be arranged in various
ways (F, H), and the pseudopods may be prolongations of con-

II

PHYLUM PROTOZOA

47

siderable relative size (c), or mere wave-like elevations of the
surface (G). Sometimes specimens are found in which neither
nucleus nor vacuole is present ; these are placed in the genus

PIG. 28. Protamoeba primitiva. Showing changes of form and three stages in binary fission.

(After Haeckel, from Parker's Biology.)

Protamoeba (Fig. 28). Very probably, however, future investigation
will show this and other non-nucleate forms to possess a potential
nucleus in the form of minute scattered granules of chromatin.

The largest of the naked or shell-less Lobosa is Pelomyxa, which
may be as much as 8 mm. in diameter ; it is multi-nucleate and is
further distinguished by numerous non-contractile vacuoles in the
endosarc.

D

FIG. 29. A, Quadrula symmetrica; B. Hyalosphenialata; C, Arcella vulgar is }
D, Difflugia pyriformis. (From Lang's Comparative Anatomy.)

Skeleton. We may understand the relation of the shelled to
the shell-less Lobosa by supposing an Amoeba to draw in the
pseudopods from the greater part of its body, and to secrete, from
that part only, a cell- wall ; such a cell-wall or capsule would differ

48

ZOOLOGY

SECT.

from a cyst in having an aperture at one end to allow of the
protrusion of pseudopods from a small naked area. This is exactly
what we find in Arcella and its allies (Fig. 29, A-c), in which the
shell is chitinoid. A different kind of shell is found in Difflugm
(D), which secretes a gelatinous coating to which minute sand-
grains and other foreign particles become attached.

\

',

,

, '

t

\

-, ',

, !

^ )

1

\

1 .

n < i
t.

..'

\

;

I ' \

/i

"/"-A v;

> ' ( M y

i

\
\

FIG. 30. Chlamydomyxa labyrinthuloides. A, active phase; c.ia. ceU-waU ; /. frag-
ment of Alga ingested as food ; sp. spindles in course of pseudopods ; B, resting stage
numerous individuals in the cells of a fragment of Sj>ha{inion; a, specimen completely enclosed
in cell ; 6 aud r, specimens which have emerged through the ruptured cell-wall ; C, specimen
multiplying by budding ; D, by binary fission ; E, by internal fission. (A after Archer, B B
after Geddes.)

ORDER 2. LABYRINTHULIDEA.

In this group there are only two genera Laln/rinthula and Chlamydomyxa.

Chlamydomyxa (Fig. 30) has hitherto been found only in Ireland, where it

occurs in association with the common Bog-moss (Sphagnum). Like Amceba, it

II

PHYLUM PROTOZOA

49

may exist either in the active or in the resting condition. In the resting stage
(B, a, b, c) it consists of a mass of protoplasm surrounded by a laminated wall
of cellulose and coloured green by chlorophyll the ordinary pigment of green
plants. There are also specks of bright red, due to a pigment called hcvmato-
chrome, allied to chlorophyll, and small globular bodies of a bluish tint. In
the young condition (a) the resting cells are globxilar and microscopic, lying
enclosed within the cells of the Sphagnum, but as they grow in this confined
space they become elongated and irregular, and finally Irarst through the Avail
of the moss-cell, forming masses (b, c) quite visible to the naked eye. These
may bud (C) or undergo binary fission (D) ; or the protoplasm, retreating from
the cell-wall, may divide into numerous small masses, each of which surrounds
itself with a new cell-wall (E).

During the whole of the resting stage there is nothing to distinguish Chlamy
domyxa from a plant, and it would certainly be placed among the lower Algce
if the active phase of its existence were unknown.

In the active stage (A) the protoplasm protrudes from the ruptured cell-wall
in the form of pseudopods produced into a complex network of extremely
delicate filaments, which may unite to form larger masses of protoplasm at a
considerable distance from the original cell. At the same time the bluish
spheres (.y). ) found in the resting stage take on a spindle shape and travel slowly
along the filaments.

The filaments are used to capture living organisms (/.) which are digested by
the protoplasm surrounding them, the products of nutrition being conveyed
along the network to all parts of the organism. Thus in the active condition the
nutrition of Chlamydomyxa is holozoic, i.e. strictly like that of an animal, the
food consisting of living protoplasm. In the resting stage, on the other hand,
nutrition is purely holophytic, i.e. like that of an ordinary green plant, the food
consisting of the carbon dioxide and various mineral salts dissolved in the water.

Labyrinthtila (Fig. 31) differs in many respects from Chlamydomyxa. In the
resting stage (B) it consists of a heap of small nucleated cells (c. ) connected by a

B

FIG. 31. Labyrinthula vitellina. A, specimen crawling on a fragment of Alga (.) ; c. cells
travelling in the filaments ; B, part of specimen in resting condition with heap of cells (c.) ;
C, a single cell from an actively moving specimen with connecting threads; an. nucleus.
(From Biitschli's Protozoa, after Cienkowsky.)

homogeneous substance. In the active condition (A) it is produced into long
delicate filaments, not formed of protoplasm, along which the cells (c. ) travel, in
the same manner as the spindles of Chlamydomyxa. Labyrinthula has, therefore

E

50

ZOOLOGY

SECT.

the character, not of a single cell, but of a cell-colony, formed of numerous cells
connected by a non-protoplasmic substance. Chlamydomyxa, on the other hand,
hus the character of a single cell, and no nuclei have been found in the spindles.
Thus further investigation is necessary before the" association of these two
organisms in one group is fully justified, and it has recently been proposed to
include Labyrinthula among the Mycetozoa.

ORDER 3. FORAMIXIFERA.

General Structure. The members of this order differ from
the Lobosa and agree with the active phase of Chlamydomyxa in
the fact that their pseudopods are long and delicate, and unite to
form networks ; moreover,, with few exceptions, they agree with
Arcella and its allies in possessing a shell. In the majority of
cases this shell is formed of calcimn carbonate.

One of the simplest members of the group is Micro gromia (Fig.
32). It consists of a protoplasmic body (B), with a single nucleus

FIG. 32. Microgromia socialis. A, entire colony; B, single zooid ; C, zooid which
has undergone binary fission, with one of the daughter-cells creeping out of the shell :
D, flagellula ; c. rac. contractile vaeuole ; a a. nucleus; sh. shell. (From Biitschli's Prc/to-un,
after Hertwig and Lesser.)

(nu.) and contractile vaeuole (c. vac.), enclosed in a chitinoid cell-
wall or shell (sh.) with an aperture at one end through which the
protoplasm protrudes and is produced into delicate radiating
pseudopods. The animal multiplies by binary fission, and the
individuals or zooids thus produced remain united in larger or
smaller clusters or cell-colonies (A). Sometimes the cell-body of a
zooid divides, and one of the daughter-cells creeps out of the cell-
wall (C), and, after moving about for a time like an Amoeba, draws
in its pseudopods, assumes an oval form, and sends out two
fiagella by means of which it is propelled through the water (D).
We shall find other instances in which the young of a Rhizopod is

ii PHYLUM PROTOZOA 51

a flagellula, i.e. a cell provided with one or more flagella, which,
if its history were not known, would be included among the
Mastigophora.

Platoum (Fig. 33, A) is a form resembling Microgromia, but
illustrating a very interesting type of colony. The protoplasm
flows out of the mouth of the shell in the form of a long plate (B)

c. vac

^^TX^\

-c. va,c

FIG. 33. Platoum stercoreum. A, single zooid ; B, formation of colony ; e. vac. contractile
vacuole ; ./'. food particles ; nu. nucleus ; s/j. shell. (From Butsckli's Protozoa, after
Cieukowsky.)

which sends off rounded side branches, and each of these, acquiring
a cell-wall, becomes a zooid of the simple cell-colony.

Grromia (Fig. 34, 1} leads us to the more typical Foraminifera.
The protoplasm of this form protrudes from the mouth (a) of the
chitinoid shell (.s7f.) and flows around it so that the shell becomes
an internal structure. The pseudopods are very long and delicate,
and unite to form a complicated network, exhibiting a streaming
movement of granules, and serving, as usual, to capture prey.

Skeleton. Squammulina (Fig. 34,5") differs from Gromia mainly
in having the shell formed of calcium carbonate, so as to have the
character of a hollow, stony sphere, with an aperture at one end.
It appears that all the calcareous Foraminifera begin life in this
simple form ; but in the majority of cases the adult structure
attains a considerable degree of complexity. The protoplasm of
the original globular chamber overflows, as it were, through the
aperture ; but, instead of forming an elongated plate from which
side buds are given off, as in Platoum, the extended mass rounds
itself off, and secretes a calcareous shell in organic connection with
the original shell, and communicating with it by the original
aperture. In this way a two-chambered shell is produced, and a
repetition of the process gives us the many-chambered shell found
in most genera. New chambers may be added in a straight line
.(Fig. 35, o), or alternately on opposite sides of the original
chamber (-5), or with each new chamber enclosing its predecessor
(4}, or in a flat spiral, each new chamber being larger than its
predecessor (7, 5), or in a spire in which the newer chambers

E 2

52

ZOOLOGY

overlap the older (9, 10\ or in an irregular spiral of globular
chambers (), or in an extremely compact spiral in which the new
chambers completely enclose their predecessors (11\ In all cases

- -^mvv; iifM mm^ - :

^//y?///liMm ' \^w^'-

' /^/Mffll/lllin^ " ; 5wN 2 - R :
|j|Jilll\ ' N\

' ' ! ; ! I \ \ 'A\

' \\K\\\ \\
\ \ \

i-.- i \M\

3.Squammulina

4.M i I i o I a

FIG. 34. Various forms of Foramiuifera. In It, Miliola, a, shows the living animal ;
b, the same killed and stained ; a. aperture of shell ; /. food particles ; n. nucleus ; sh. shell.
(From Biitschli's Protozoa and Claus's Zoology.)

adjacent chambers communicate with one another either by a
single large hole or by numerous small ones : the protoplasm
is thus perfectly continuous throughout the organism. With the

II

PHYLUM PROTOZOA

53

increase in the number of chambers there is a multiplication of
the nucleus (Fig. 34, 4, b, nu).

The shell presents two leading types of structure, apart from

tSoccammina

2.Lagena

3-Nodosaria

4.Frondicularia G.GIobigerina

7. Discorbina

9.Planorbulina

ll.Nummulites

FIG. 35. Shells of Foraminifera. In 3, 4, and 5, a shows the surface view, and b a section ;
Sa is a diagram of a coiled cell without supplemental skeleton ; Sb of a similar form
with supplemental skeleton (s. sJ-.) ; and 10 of a form with overlapping whorls ; in lid half
the shell is shown in horizontal section ; b is a vertical section ; a. aperture of shell ; 1 15,
successive chambers, 1 being always the oldest or initial chamber. (After Carpenter, Brady,
and Butschli.)

the form and arrangement of the chambers : either it is of a
porcelain-like texture and provided with a single terminal aperture,
(Fig. 34, 4), or the texture is glassy and the whole shell perforated

ZOOLOGY

SECT.

with very minute apertures, through which, as well as through the
terminal aperture, pseudopods are protruded (Fig. 34, #).

In many cases additional complexity is attained by the develop-
ment of what is called the supplemental skeleton (Fig. 35, Sb, s. sk.).
This consists of a deposit of calcium carbonate outside the original
shell : it is traversed by a complex system of canals, containing
protoplasm, and is sometimes produced into large spines.
Foraminifera in which this secondary skeleton occurs are some-
times of considerable size 2-3 cm. in diameter and of extra-
ordinary complexity.

Many Foraminifera resemble Difflugia in having a skeleton
formed of sand-grains, sponge-spicules, and other foreign bodies
cemented together by a secretion from the protoplasm (Fig. 35, 1).
Some of these are formed on the imperforate type, having the

sfi

FIG. 36. Hastigerina murrayi. plsrn. vacuolated protoplasm surrounding shell : psd.
pseudopods ; sh. shell ; sp. spines. (After Brady.)

protoplasm protruded from a single terminal aperture ; others on
the perforate type, small pseudopods being protruded between the
particles forming the shell.

In many cases the pseudopods are the only portions of proto-
plasm outside the shell, whereas in Gromia, as we saw, the shell
is invested with a layer of protoplasm, and is thus in strictness
an internal structure. In one of the calcareous forms with
perforated spiral shell, called Hastigerina (Fig. 36), a very remark-
able modification of this condition of things obtains. The shell
(sh.) is surrounded with a mass of protoplasm (plsm.) many times
its own diameter, and so full of vacuoles as to present a bubbly or
frothy appearance. The shell itself, moreover, in this and allied

II

PHYLUM PROTOZOA

forms is provided with numerous delicate, hollow, calcareous spines
(sp.), which are only to be seen in perfect, freshly-caught specimens.

Many Foraminifera exhibit the phenomenon of dimorphism :
the individuals of a single species occur under two distinct forms
differing from one another in the size of the central chamber,
the shape and mode of growth of the succeeding chambers, and
the character of the nuclei.

The reproduction of the Foraminifera is very imperfectly
known ; but in some forms the protoplasm has been observed to
divide into flaydlulce or swarm-cells, minute masses of protoplasm,
each provided with a flagellum : usually these are of uniform size,
but in some cases large and small spores are produced. In some
species young forms, provided with a shell, are formed in the
terminal chamber of the adult.

Distribution. Gromia, Microgromia, and a few other forms are
found in fresh- water : one species has been found in damp earth,
but the great majority of the Foraminifera are marine, some being
pelagic, i.e. occurring at or near the surface of the ocean, others
abyssal, i.e. living at great depths. In the Atlantic, large areas
of the sea-bottom are covered with a gray mud called Globigerina-
ooze from the vast number of Globigerinae contained in it.

From the palasontological point of view, the Foraminifera are a
very important group. Remains of their shells occur in various
formations from the Silurian period to the present day, certain
rocks such as the White Chalk (Cretaceous period) and the Num-
vnulitic limestone (Eocene) being largely made up of them.

ORDER 4. HELIOZOA.

General Structure. The Heliozoa are at once distinguished
from the preceding groups by the
character of their pseudopods, which
have the form of stiff filaments
radiating outwards from the more
or less globular cell-body, present-
ing very little movement beyond
the characteristic streaming of
granules, and not uniting to form
networks.

One of the simplest forms is
the common " Sun-animalcule," Ac-
tinoplirys sol (Fig. 37). The body
is nearly spherical, and contains
a large nucleus and numerous
vacuoles, some of which, near the
surface, are contractile. Each of the stiff, radiating pseudopods
has a delicate axis, which is traceable through the protoplasm

FIG. 37. Actinophrys sol. a. axial
filaments of pseudopods ; n. nucleus ;
p. pseudopod. (From Lang's Com-
paratire Anatomy, after Grenacher.)

56

ZOOLOGY

SECT.

as far as the nucleus. Living organisms are devoured in much
the same way as in Amceba: each is ingested along with a
droplet of water, and is thus seen, during digestion, to lie in a
definite cavity of the protoplasm, called a food-vacuolc.

Actinosphcerium (Fig. 38, A), another fresh-water form, is more
complex. The protoplasm is distinctly divided into a central mass,
the medulla or endosarc (B, mcd.), in which the vacuoles are small,
and an outer layer, the cortex or ectosarc (cort.), in which they are

N

PIG. 38. Actinosphserium eichhornii. A, the entire organism ; B, a small portion
highly magnified ; (/</. chromatopliore ; cort. cortex ; c. rac. contractile vacuole ; med. medulla ;
nu. nuclei. (From Biitschli's Protozoa, after Hertwig and Lesser.)

very large. There are numerous nuclei (nu.) and chromatophores
(chr.), the latter coloured green by chlorophyll.

Many genera form colonies. Numerous zooids may be united
by bridges of protoplasm into an open network, or the connecting
bridges may be shorter and the zooids more numerous, giving the
colony a more compact appearance.

Transitional stages occur between the naked genera already re-
ferred to and forms with a distinct skeleton. Sometimes the body
simply surrounds itself with a temporary gelatinous investment
(Fig. 39, 2, g.), in other cases it is surrounded by a capsule of loosely
woven fibres through which the pseudopods pass, thus reminding
us of the state of things characteristic of perforate Foraminifera.

II

PHYLUM PROTOZOA

57

One genus has a shell formed of agglutinated sand-grains ; in
another (Fig. 39, A) the skeleton consists of loosely matted needles

\1.Raf>hidr'o{jhrys

c.vcu?

2. Nude aria

S.CIaHirulina

Fia. 39. Various forms of ITeliozoa. 3a, the entire animal; 3b, the flagellula ; c. rac.
contractile vacuole ; (7. gelatinous investment; ww. nucleus jisiJ. pseudopods ; sA-. siliceous
skeleton ; sp. spicules. (From Butschli's Protozoa, after Schulze and Greeff.)

of silica. Lastly, in the graceful Clatlirulina (3) the body is
enclosed in a perforated sphere of silica, quite like the skeleton of
many of the Radiolaria (vide infra}.

58

ZOOLOGY

SECT,

Reproduction ordinarily takes place by binary fission, but
spore-formation also occurs. Actinosphserium, for instance, encloses
itself in a gelatinous cyst and undergoes multiple fission, forming
numerous spores each enclosed in a siliceous cell-wall. These
resting spores remain quiescent throughout the winter, and in spring
the protoplasm emerges from each and assumes the form of the
ordinary active Actinosphasrium. In Clathrulina spore-formation
takes place in the active condition, and the spores (Fig. 39, 3 b) are
tlagellulse, each being an ovoid body provided with two flagella.
Conjugation has been observed in some instances, but the precise
nature and significance of the process is still imperfectly known.

ORDER 5. RADIOLARIA.

The Radiolaria are a large and well-defined group of Rhizopods,
noticeable, in most instances, by the presence of a siliceous skeleton
of great beauty and complexity. They are all marine.

General Structure. The most important characteristic of
the group is the presence of a perforated membranous sac, called
the central capsule (Fig. 40, cent, caps.), which lies embedded in the
protoplasm, dividing it into intra-capsular (int. caps, pr.) and extra-
capsular (e,:d. caps.pr.) regions. In the intra-capsular protoplasm is
a large and complex nucleus (nu.), or sometimes many nuclei : from
the extra-capsular protoplasm the pseudopods(^sc/.) are given off in
the form of delicate radiating threads, which in some cases remain
free, in others, e.g. Lithocircus, anastomose freely, i.e. unite to form

networks. There is no con-
tractile vacuole, but in many
forms the extra-capsular pro-
toplasm contains numerous
large non-contractile vacuoles,
which give it the frothy or
bubbly appearance noticed
previously in Hastigerina.
The vacuolated portion of

SKei.' yg^v*. ' K - fT the protoplasm has a gela-

z ' l/\ \f tinous consistency, and is dis-

tinguished as the cali/mna.

The central capsule may
be looked upon as a chitinoid
internal skeleton, reminding
us of the shell of Gromia and
of the perforated calcareous

shell of Hastigerina with its investment of vacuolated proto-
plasm. It is found in its simplest form in Thalassoplancta
(Fig. 41), in which it is spherical and uniformly perforated with
minute holes. In other forms, such as Litliocircus (Fig 40), it is

nu

Inl: caps.pr
cent caps

FIG. 40. Lithocircus nnmilaris c,, it. caps.
central capsule ; ext. caps, j>r. extra-capsular
pn itoplism ; iiif. cnjix. jn: intra-capsular pro-
toplasm ; nu. nucleus ; /..<>/. pseudopods ; skel.
skeleton ; -. cells of Zooxanthella. (After
Butschli, from Parker's Bioloyy.)

II

PHYLUM PROTOZOA

59

more or less conical in form, and the apertures are restricted to the

Hat base of the cone. Lastly, in the most complex forms (Fig. 42),

the membrane of the capsule is double,

and there are three apertures a principal

one having a central position and provided

with a lid or opcrcaliun (op.), and two

subsidiary ones on the opposite side. In

relation with the principal or lidded

aperture there is found in the extra-

capsular protoplasm a heap of pigment

called the phceodium (ph.}.

In some genera the central capsule is
the only skeletal structure present, but in
most cases there is in addition a skeleton
mainly external formed, as a rule, of
silica, but in one subdivision of the class
of a chitinoid substance called acanthin, so
transparent that it can only be distin-
guished from silica by chemical tests. The
siliceous skeleton may consist of loosely
woven spines (Fig. 41), but usually (and
the acanthin skeleton always) has the
form of a firm frame-work of globular,
conical, stellate, or discoid shape, fre-
quently produced into simple or branched
spines. A very beautiful form of skeleton
is exhibited by Actinomma (Fig. 43), in
which there are three concentric per-
forated spheres (A, sk. 1, sk. 2, sk. 3) con-
nected by radiating spicules. The outer
of these spheres occurs in the extra-
capsular protoplasm (B, ex. caps, jjr.),
the middle one in the intra-capsular protoplasm, and the inner
one in the nucleus (mi.).

Colonial forms are comparatively rare in this order, but occur
in some genera by the central capsule undergoing repeated divi-
sions while the extra-capsular mass remains undivided. In this
way is produced in Collozoum for instance (Fig. 44, A, B, C) a
firm gelatinous mass, the calymna or vacuolated extra-capsular
protoplasm (D, vac.) common to the entire colony, having embedded
in it numerous central capsules (c. caps.) each indicating a zooid of
the colony. Collozoum may attain a length of 3 or 4 cm.

Reproduction by binary fission has been observed in some cases,
and is probably universal. The nucleus divides first, then the
central capsule, and finally the extra-capsular protoplasm.

Spore-formation has been observed in Collozoum and some other
genera : the intra-capsular protoplasm divides into small masses,

FIG. 41. Thalassoplancta
brevispicula, part of a
section, km. central cap-
sule ; ip. jntra-capsular
protoplasm , n. nucleus,
containing nl. numerous
nucleoli ; ot. oil drops ; ca.
calymna ; />. protoplasm
surrounding calymna ; s.
spicules. (From Lang's
Comparative Anatomy, after
Haeckel).

60

ZOOLOGY

SKCT

I' ; >

* ?

*%

FIG. 42. Aulactinium actinastrum. c. calymna ; km. central capsule ; n. nucleus ; op-
opsrculum ; p/t. phteodium. (From Lang's Comparative Anatomy, after Haeckel.)

cent, caps

FIG. 43. Actinomma asteracanthion. A, the shell with portions of the two outer
spheres broken away ; B, section showing the relations of the skeleton to the animal :
c, nt. caps, central capsule ; ex. caps. pr. i-xtm-cap-mlar rirotnplasm : nu. nucleus ; sk. 1, outer,
sk. -2, middle, sk. 3, inner sphere of skeleton. (From Blitschli's Prntu:oa, after Haeckel and
Hertwig.)

II

PHYLUM PROTOZOA

61

each of which becomes a flagellula (Fig. 44, E, F) provided with a
single flagellum. In some instances all the spores produced aiv
alike (E), and each encloses a small crystal (f.): in other cases (F)
in the same species the spores are dimorphic, some being small
(microspores) others large (megaspores). Their development has not
been traced.

Symbiosis. One most characteristic and remarkable feature
of the group has yet to be mentioned. In most species there occur
in the extra-capsular protoplasm minute yellow cells (Fig. 40, :. )
which multiply by fission independently of the Radiolarian. It is

vac

*&%' .>'~*f*&rf ; j^

^llspr

Qfiuifi

c.caps

nu

FIG. 44. Collozoum inerme, A C, three forms of the entire colony, nat. size; D, a small
colony showing the numerous central capsules (c. caps.) and extra-capsular protoplasm with
vacuoles(mr.) ; E, spores containing crystals (c.) ; F, niega- and microspore. (From Butschli's
Protozoa, after Hertwig and Brandt.)

now known that these are unicellular plants belonging to the class
of Algse and to the species Zooxantliella nutricola. This intimate
association of two organisms is called symbiosis : it is a mutually
beneficial partnership, the Radiolarian supplying the Alga with
carbon dioxide and nitrogenous waste matters, while the Alga gives
off oxygen and produces sugar and other food-stuffs, some of which
must make their way by diffusion into the protoplasm of the
Radiolarian.

CLASS II. MYCETOZOA.

1. EXAMPLE OF THE CLASS Didymium di forme.

Didymium occurs as a whitish or yellow sheet of protoplasm (Fig. 45, <:
often several centimetres across, which crawls, like a gigantic Amoeba, over
the surface of decaying leaves. It shows the characteristic streaming move-
ments of protoplasm and feeds by ingesting various organic bodies, notably the
Bacilli which always occur in great numbers in decaying substances. Numerous
nuclei are present.

ZOOLOGY

SECT.

After leading an active existence for a longer or shorter time, the protoplasm
aggregates into a solid lump, surrounds itself with a cyst, and undergoes multiple
fission, dividing into an immense number of minute spores. The cyst (Fig. 45,
A, spy. 1, sprj. 2) is therefore not a mere resting capsule, like that of Amoeba,
but a sporangium or spore-case. Its wall consists of two layers, an inner of a
dark purple colour and membranous texture, formed of cellulose, and an outer of
a pure white hue, formed of calcium carbonate. Thus the whole sporangium,

FIG. 45. Didymium difforme. A, two sporangia (spg. 1 and 2) cm a fragment of leaf(/.);

B, section of sporangium, with ruptured outer layer (a.) ; and threads of capillitium (c/>.);

C, a flagellula with contractile vacuole (c. vac.) and nucleus (/!.); D, the same after loss
of flagellum ; b, an ingested Bacillus ; E, an amcebula ; F, conjugation of amoebute to form
a small plasmodium ; G, a larger plasmodiuru accompanied by numerous amosbulse ; sp.
ingested spores. (After Lister.)

which may attain a diameter of 3 or 4 mm. , resembles a minute egg. From the
inner surface of the wall of the sporangium spring a number of branched
filaments of cellulose, which extend into the cavity among the spores and together
constitute the capillitium (B, cp. ).

The spores consist of nucleated masses of protoplasm surrounded by a thick
cellulose wall of a dark reddish-brown colour. After a period of rest the proto-
plasm emerges in the form of an amceboid mass which soon becomes a flagellula
(C); provided with a single flagellum, a nucleus (.), and a contractile vacuole

ii PHYLUM PROTOZOA 03

(c. vac.). The flagellulse move freely and ingest Bacilli (D, 1>.) ; then, after a
time, they become irregular in outline, draw in the flagellum, and become
amoeboid (E). The amcebulte thus formed congregate in considerable numbers
and fuse with one another (F), the final result being the production of the
great amoeboid mass (G) with which we started. There is no fusion of the
nuclei of the amcebulse. Thus Didymium in its active condition is aplasmodium,
i.e. a body formed by the concrescence of amcebulse.

2. GENERAL REMARKS ON THE MYCETOZOA.

Speaking generally, the Mycetozoa differ from all other Protozoa in their
terrestrial habit. They are neither aquatic, like most members of the phylum,
nor parasitic, like many other forms, but live habitually a sub-aerial life on
decaying organic matter. Thej 7 are also remarkable for their close resemblance
in the structure of the sporangia and spores to certain Funyi, a group of parasitic
or saprophytic plants in which they are often included, most works on Botany
having a section on the Myxomycetex or " Slime-fungi," as these organisms are
then called. They are placed among animals on account of the structure and
physiology of the flagellate, amoeboid, and plasmodial phases which exhibit
automatic movements and ingest solid food. On the other hand, the Mycetozoa
are sometimes included among the Rhizopoda, a course which their very peculiar
reproductive processes appears to render inadvisable.

An interesting organism, called Protomyxa, probably belongs to this group. In
its plasmodial phase it consists of orange-coloured masses of protoplasm, about
1 mm. in diameter, which crawl over sea-shells by means of their long, branched
pseudopods, and ingest living prey. No nuclei are known. The protoplasm
becomes encysted and breaks up into naked spores, which escape from the cyst
as flagellulte, but soon become amoeboid and fuse to form the plasmodium.

CLASS III. MASTIGOPHORA.

1. EXAMPLE OF THE CLASS finglena mridis.

Euglena (Fig. 46) is a flagellate organism commonly found in
the water of ponds and puddles, to which it imparts a green colour.
The body (E, H) is spindle-shaped, and has at the blunt anterior
end a depression, the gullet (F, ccs.}, from the inner surface of which
springs a single long flagellum (_/?.). According to recent observa-
tions the flagellum is not a simple thread, but is beset with delicate
cilium-like processes. The organism is propelled through the
water by the lashing movements of the flagellum, which is always
directed forwards ; it can also perform slow worm-like movements
of contraction and expansion (A D), but anything like the free
pseudopodial movements which characterise the Rhizopoda is
precluded by the presence of a very thin skin or cuticle which
invests the body. There is a nucleus (nu.) near the centre of the
body, and at the anterior end a contractile vacuole (H, c. vac.},
leading into a large non-contractile space or reservoir (r.) which
discharges into the gullet.

The greater part of the body is coloured green by the charac-
teristic vegetable pigment, chlorophyll, and contains grains of

64

ZOOLOGY

SECT.

paramylum (H, p.), a carbo-hydrate allied to starch. In contact
with the reservoir is a bright red speck, the stigma (p<J-\ formed of
a pigment allied to chlorophyll and called liccmatocliromc. It seems
probable that the stigma is a light-perceiving organ or rudimentary
eye.

Euglena is nourished like a typical green plant : it decomposes
the carbon dioxide dissolved in the water, assimilating the carbon
and evolving the oxygen. Nitrogen and other elements it absorbs
in the form of mineral salts in solution in the water. But it has
also been shown that the movements of the flagellum create a
whirlpool by which minute fragments are propelled down the

FIG. 4(1. Euerleiia viridis. A D, four views illustrating euglenoid movements; Band H,
enlarged views ; F, anterior cud further enlarged ; G, resting form after binary fission ; c. vac*
contractile vacuole in H, reservoir in E and F ; cti. cyst ; fl. flagellum ; in. mouth ; ni'. nucleus ;
ft, gullet ; p. paramylum bodies; pg. pigment spot; r. (ill H), reservoir: (From Parker's
Bioloyy, after Kent and Klebs.)

gullet and into the soft internal protoplasm. There seems to be
no doubt that in this way minute organisms are taken in as food.
Euglena thus combines the characteristically animal (holozoic) with
the characteristically vegetable (holophytic) mode of nutrition.

Sometimes the active movements cease, the animal comes to-
rest and surrounds itself with a cyst or cell-wall of cellulose (G),.
from which, after a quiescent period, it emerges to resume active
life. It is during the resting condition that reproduction takes
place by the division of the body in a median plane parallel to
the long axis (G). Under certain circumstances multiple fission
takes place, and flagellulre are produced, which, sometimes after
passing through an amoeboid stage, develop into the adult
form.

ii PHYLUM PROTOZOA

2. CLASSIFICATION AND GENERAL ORGANISATION.

The Mastigophora form a very extensive group, the genera and
species of which show a wonderful diversity in structure and habit.
The only character common to them all is the presence of one or
more flagella. Some approach plants so closely as to be claimed
by many botanists ; others are hardly to be distinguished from
Rhizopods ; while the members of one order present an interesting
likeness to certain otherwise unique cells found in Sponges.

The class is divisible into four orders as follows :

ORDER 1. FLAGELLATA.

Mastigophora having one or more flagella at the anterior end
of the body.

ORDER 2. CHOANOFLAGELLATA.

Mastigophora having a single flagellum surrounded at its base
by a contractile protoplasmic collar.

ORDER 3. DINOFLAGELLATA.

Mastigophora having two flagella, one anterior, the other en-
circling the body like a girdle.

ORDER 4. CYSTOFLAGELLATA.

Mastigophora having two flagella, one of which is modified into
a long tentacle, while the other is small and contained within
the gullet.

Position of the Example.

Euglena viridis is one of several species of the genus
and -belongs to the Family Euglcnidcc, sub-order Euglcnoidca, and
order Fla-gdlata.

The presence of an anterior flagellum and the absence of a
collar, transverse flagellum, or tentacle, indicate its position among
the Flagellata. It is placed among the Euglenoidea in virtue of
possessing a single flagellum and a small gullet into which the
contractile vacuole opens. The genus Euglena is distinguished
by its centrally placed nucleus, green chromatophore, red stigma,
and euglenoicl movements. E. viridis is separated from other
species of the genus by its spindle-shaped -body with blunt ante-
rior and pointed posterior end, and by the flagellum being somewhat
longer than the body.

ORDER 1. FLAGELLATA.

The cell-body is usually ovoid or flask-shaped (Fig. 47, 6, 7, 9,
&c.), but may be almost globular (!}, or greatly elongated (5).

F

66

ZOOLOGY

SECT.

Anterior and posterior ends are always distinguishable, the flagella
being directed forwards in swimming, and, as a rule, dorsal and

3.Asfosiopis
2.0ikomonas (?)

II.Dinobryon

13. Anhho|)hysa 14.Rhi|>idodendron

FIG. 47. Various forms of Flacrellata. In 3, flagellate (a) rmd amreboid (b) phases are
shown ; in 5, flagellate () and helizoan (I) phases ; in 8 are shown two stages in the in-
gestion of a food particle (/.) ; clir. chroruatophores ; c. vac. contractile vacuole ; f. food par-
ticle g. gullet; iiu. nucleus ; /. lorica ; p. protoplasm ; per. peristome ; v. i. vacuole of ingestiou.
(Mostly from Butschli's Protozoa, after various authors.)

ventral surfaces can be distinguished by the presence of a mouth
or by an additional flagellum on the ventral side. They are,

ii PHYLUM PROTOZOA 67

therefore, usually bilaterally symmetrical or divisible into equal and
similar right and left halves by a vertical antero-posterior plane.

Some of the lower forms have no distinct cuticle, and are able,
under certain circumstances, to assume an amoeboid form (2).
The curious genus Mastiyamceba (4) has a permanently amoeboid
form, but possesses, in addition to pseudopods, a single long
flagellum. It obviously connects the Mastigophora with the
Rhizopoda, and indeed there seems no reason why it should be
placed in the present group rather than with the Lobosa. Simi-
larly, Dimorpha (-5) connects the Flagellata with the Heliozoa : in
its flagellate phase (a) it is ovoid and provided with two flagella,
but it may send out long stiff radiating pseudopods, while retaining
the flagella, or may draw in the latter and assume a purely
helizoan phase of existence provided with pseudopods only (&).

The number of flagella is subject to great variation. There
may be one (Fig. 47, l-3\ two (9, 10), three (6\ or four (7).
Sometimes the flagella show a differentiation in function ; in
Hctcromita, e.g. (Fig'.' 51) the anterior flagellum (ft. 1} only is
used in progression, the second or ventral flagellum (jfi. 2) is trailed
behind when the animal is swimming freely or is used to anchor
it to various solid bodies.

There are also important variations in structure correlated with
varied modes of nutrition. Many of the lower forms, such as
Heteromita, live in decomposing animal infusions: they have
neither mouth nor gullet and take no solid food, but live by
absorbing the nutrient matters in the solution ; their nutrition is,
in fact, saprophytic, like that of many fungi. A few live as para-
sites in various cavities of the body of the higher animals. One
Euglena-like form lives as an mtra-cellular parasite within the
cells of one of the lower worms.

Hccmatococcus (Fig. 48), Pandorina (Fig. 49), Volvox (Fig. 50), and
their allies present us with a totally different state of things.
The mouthless body is surrounded by a cellulose cell- wall (c.w.\
and contains chromatophores (chr.) coloured either green by chloro-
phyll or red by hsematochrome. Nutrition is purely holophytic,
i.e. takes place by the absorption of a watery solution of mineral
salts and by the decomposition of carbon dioxide. It is, therefore,
not surprising that these chlorophyll-containing Flagellata are
often included among the Algse or lower green plants.

Other genera live in a purely animal fashion by the ingestion of
solid proteinaceous food, usually in the form of minute living
organisms : in these cases there is always some contrivance for
capturing and swallowing the prey. In Oikomonas (Fig. 47, S) we
have one of the simplest arrangements : near the base of the
flagellum is a slight projection containing a vacuole (v.i.)', the
movements of the flagellum drive small particles (/.) against this
region where the protoplasm is very thin and readily allows the

F 2

68

ZOOLOGY

SECT.

particles to penetrate into the vacuole, where they are digested.
In Euglena, as we have seen, there is a short, narrow gullet, and
in some genera (9, g) this tube becomes a large and well-marked
structure.

Skeleton. While a large proportion of genera are naked or
covered only by a thin cuticle, a few fabricate for themselves a
delicate chitinoid shell or lorica (10, /.), usually vase-shaped and
widely open at one end so as to allow of the protrusion of the

FIG. 48. Hsetnatococcus pluvialis. A, motile stage; B, resting stage ; C, D, two modes
of fission ; E, Hmnatococcus lacustris, motile stage ; F, diagram of movements of flagellum ;
ehr. chromatopliores ; <-. vac. contractile vacuole ; c.ic. cell- wall ; nu. nucleus; /!<<'. nucleolus ;
pyr. pyrenoids. (From Parker's

contained animalcule. In the chlorophyll-containing forms there
is a closed cell-wall of cellulose (Fig. 48, c.v. ).

In many genera Colonies of various forms are produced by
repeated budding. Some of these are singularly like a zoophyte
(see Sect. IV.) in general form (Fig. 47, 11}, being branched colonies
composed of a number of connected monads, each enclosed in
a little glassy lorica ; or green (chlorophyll-containing) zooids are
enclosed in a common gelatinous sphere, through which their
fiagella protrude (12) ; or tufts of zooids, reminding us of the
flower-heads of Acacia, are borne on a branched stem (13). In
Vohox (Fig. 50) the zooids of the colony are arranged in the form
of a hollow sphere, and in Pamlorina (Fig. 49) in that of a solid
sphere enclosed in a delicate shell of cellulose. Lastly, in Wiipido-

II

PHYLUM PROTOZOA

69

dcndron (Fig. 47, 14} a beautiful branched fan-shaped colony is
produced, the branches consisting of closely adpressed gelatin-
ous tubes each the dwelling of a single zooid.

Binary fission is the ordinary mode of asexual multiplication,
and may take place either in the active or in the resting condition.
Hsematococcus (Fig. 48) and Euglena (Fig. 46), for instance,
divide while in the encysted condition ; Heteromita (Fig. 51)

\

FIG. 40. Pandorina morum. A, entire colony; B, asexual reproduction, each zooid
dividing into a daughter-coluny : 0, liberation of gametes ; D F, three stages in conjugation
of gametes ; G, zygote ; H- -K, development of zygote into a new colony. (From Parker's
Bioloyy, after Goebel.)

and other saprophytic forms while actively swimming : in the
latter case the divison includes the almost infinitely fine flagellum.
In correspondence with their compound nature, the colonial
genera exhibit certain peculiarities in asexual multiplication. In
Dindbryon (Fig. 47, 11} a zooid divides within its cup, in which
one of the two products of division remains ; the other crawls out
of the lorica, fixes itself upon its edge, and then secretes a new
lorica for itself. In Pandorina (Fig. 49) each of the sixteen zooids
of the colony divides into sixteen (B), thus forming that number of
daughter-colonies within the original cell- wall, by the rupture of

70

ZOOLOGY

SECT.

which they are finally liberated. In Voho.r (Fig. 50), certain
zooids, called partlienogonidia (A, a), have specially assigned to them
the function of asexual reproduction : they divide by a process
resembling the segmentation of the higher animals (D 1 D 5 ), and
form daughter-colonies which become detached and swim freely
in the interior of the mother-colony.

A very interesting series of stages in sexual reproduction is
found in this group. In Heteromita two individuals come together

H

FIG. 50. Volvox globator. A, entire colony, enclosing several daughter-colonies ;
B, the same during sexual maturity; C, four zooids in optical section; pi D?, develc.p-
meiit of parthenogonidium ; E, ripe spermary ; F. sperm; G, ovary containing uvuni and
sperms; H, oosperm ; a, parthenogouidia : rf. nagellum : oc. ovum ; ni-n. ovaries ; pjr. pigment
spot ; gjiy. spermaries. (From Parker's Biology, after Colin and Kirchuer.)

(Fig. 51, E 1 ) and undergo complete fusion (E 2 E 4 ) : the result of
this cuvjinjiit.ion of the two f/amctrs or conjugating cells is a thin-
walled sac, the zygote (E 5 ), the protoplasm of which divides by
multiple fission into very minute spores. These, when first
liberated by the rupture of the zygote (E (i ), are mere granules,
but soon the ventral or trailing flagellum is developed, and after-
wards the anterior flagellum (F 1 F 4 ). In Pandorina (Fig. 49)
the cells of the colony escape from the common gelatinous envelope
(C) and conjugate in pairs (D, E), forming a zygote (F, G), which,
after a period of rest (H), divides and forms a new colony (K).

II

PHYLUM PROTOZOA

71

In some cases the conjugating cells are of two sizes, union always
taking place between a large cell or megagamete and a small cell

nu

E

FIG. 51. Heteromita rostrata. A, the positions assumed in the springing movements
of the anchored form ; B, longitudinal fission of anchored 'form ; C, transverse fission of
the same ; D, fission of free-swimming form ; E, conjugation of free-swimming with anchored
form ; E 3 , zygote ; E s , emission of spores from zygote ; F, development of spores ; fl.l, ante-
rior ; fl.2, ventral flagellum. (From Parker's Biology, after Dallinger.)

or microgamete. In Volvox (Fig. 50) this dimorphism reaches its
extreme, producing a condition of things closely resembling what

72

ZOOLOGY

SECT.

we find in the higher animals. Certain of the zooids enlarge and
form megagametes (B, ovy.), others divide repeatedly and give rise
to groups of microgametes (B, spy. E, F), each in the form of an
elongated yellow body with a red pigment-spot and two flagella.
These are liberated, swim freely, and conjugate with the stationary
megagamete (G), producing a zygote (H), which, after a period of
rest, divides and reproduces the colony. It is obvious that the
megagamete corresponds with the ovum of the higher animals,
the microgamete with the sperm, and the zygote with the oosperm
or impregnated egg.

It should be noticed that in the more complex cases of repro-
duction just described we meet with a phenomenon not seen in
cases of binary fission, viz., development, the young organism being
far simpler in structure than the adult, and reaching its final form
by a gradual increase in complexity.

LMonosiga.

2.Salpingoeca.

S.Polyoeca. 4.Prof-erospongi8.

FIG. 52. Various forms of Choanoflaeellata. Sb illustrates longitudinal fission ; 2c, the pro-
duction of flagellulre ; c. collar ; c. roc. contractile vacuole ; .rf. flagellum ; I. lorica ; tin. nucleus.
(After Saville Kent.)

ORDER 2. CHOANOFLAGELLATA.

General Structure. The members of this group are distin-
guished by the presence of a vase-like prolongation of the proto-
plasm, called the collar (Fig. 52, 1, c.\ surrounding the base of the
single flagellum (/?.). The collar is contractile, and, although its
precise functions are not yet certainly known, there is evidence to

11 PHYLUM PROTOZOA 7 .;

show that its movements cause a flow of water, with minute
particles in suspension, up the outside of the collar an* I down the
inside, the solid particles being then ingested in tin- soft proto-
plasm between the base of the flagellum and that of the collar.
The animalcule may draw in both collar and flagellum and assume
an amoeboid form.

The nucleus (nu.) is spherical, and there are one or two run-
tractile vacuoles (c. vac.}, but no trace of mouth <>r gullet. S<M in-
forms are naked (1), others (2) enclosed in a chitinoid shell or
lorica of cup-like form. A stalk (s.) is usually present in the
loricate and sometimes also in the naked forms.

The genera mentioned in the preceding paragraph are all simple,
but in other cases colonies are produced by repeated fission. 1 1 1
Polyaxa (3) the colony has a tree-like form, which may reach
a high degree of complexity by repeated branching. A totally
different mode of aggregation is found in Protcrosponyia (4), in
which the zooids are enclosed in a common gelatinous matrix of
irregular form.

Reproduction. The "collared monads," as these organiMns
are often called, multiply by longitudinal fission (.?&). In some
cases multiple fission of encysted individuals has been observed
(2c), small simple flagelluke being produced which gradual ly
develop into the perfect form.

The order is especially interesting from the fact that, with the
exception of Sponges and the larva of a Sea-Urchin, it is the only
group in the animal kingdom in which the collar occurs.

ORDER 3. DINOFLAGELLATA.

The leading features of this group are the arrangement of the two flagella
which they always possess, and the usual presence of a remarkable and ot'tt :,
very beautiful and complex shell.

The body (Fig. 53, 1) is usually bilaterally asymmetrical, i.e. it may be
divided into right and left halves which are not precisely similar. On the
ventral surface is a longitudinal groove, (I. gr.), extending along the anterior half
only, and meeting a transverse groove (t. gr.), which is continued round the body
like a girdle. From the longitudinal groove springs a large flagellum (fl. 1),
which is directed forwards and serves as the chief organ of propulsion ; a second
flagellum (fl. 2) lies in the transverse groove, where its wave-like movemfiii -.
formerly caused it to be mistaken for a ring of small cilia.

The body is covered with a shell (2) formed of cellulose, and often of very
complex form, being produced into long and ornamental process, and marked
with stripes, dots, &c. Besides a nucleus and a contractile vacuole, the pr.it o-
plasm contains chromatophores (1, chr.) coloured with chlorophyll or an allied
pigment of a yellow colour, called diatomin. Nutrition is holophytic or holozoic.

The foregoing description applies to all the commoner genera. /'//
(3) is remarkable for the absence of the transverse groove, while Polykrikc
has 110 fewer than eight transverse grooves and no shell. The lattrr genus
also has stinging-capsules or nematocysts (a, b) in the protoplasm, iv-rmlthm;
those of Zoophytes (see Sect. IV.), and has numerous nuclei of two
distinguished as mega/nuclei (int.), and micronuelei (nu.).

74

ZOOLOGY

SECT.

Reproduction is, as usual, by binary fission, the process taking place some-
times in a free-swimming individual, sometimes in one which lias lost its flagella
and come to rest.

vctf

2-Ceratium 3.Prorocenfrum

4.Polykrikos

Fir;. 53. Various forms of Dinoflaerellata. 2 shows the shell only ; kn, is an undischarged,
and b a discharged stinging-capsule; chr. chromatophores ; ./?. 1, longitudinal fiagellum ;
/'. .'.transverse fiagellum ; I. <ir. longitudinal groove; ntr. iiematocyst ; iiu. meganucleus;
/a'/, micronucleus ; p<j. pigment spot ; t. <ji-. transverse groove. (From Biitschli's Protozoa.)

The Dinoflagellata are mostly marine. Some are phosphorescent. Certain
kinds occasionally occur in such abundance in bays and estuaries as to cause a
deep brownish or red discoloration of the sea-water.

ORDER 4. CYSTOFLAGELLATA.

This group includes only two genera, Xortitiica and Leptofllxnix. A descrip-
tion of Noctiluca miliaris, the organism to which the diffused phosphorescence

of the sea is largely due, will serve
to give a fair notion of the leading
characteristics of the order.

Noctiluca (Fig. 54) is a nearly
globular organism, about ^ mm. in
diameter. It is covered with a
delicate cuticle, and the medullary
protoplasm is greatly vacuolated.
On one side is a groove from
which springs a very large and
stout flagellum or t< >itfh: (1>f/. ), no-
tiecable for its transverse striation.
Xcarthe base of this flagellum is
the mouth (m. ), leading into a short
gullet in which is a second flagel-
lum (f. ), very small in proportion
to the first. On the side opposite
to the mouth is a strongly marked
superficial ridge. The light-giving
region is the cortical protoplasm.

Reproduction takes place by binary fission, the nucleus dividing indirectly.
Spore-formation also occurs, sometimes preceded by conjugation, sometimes not.

FIG. .M. Noctiluca miliaris. . the adult
animal ; l>, c. fiagellulie ; lt. tentacle ; /. flagel-
lum ; HI. mouth ; n. nucleus. (From Lang.)

II

PHYLUM PROTOZOA

75

The spores (6, c), formed by the breaking up of the protoplasm of the parent escape
in a form very unlike the adult, the tentacle or large flagellum hem" repre
sented by a short thick process, while the main swimming organ of the ila-ullula
becomes the small oral flagellum of the adult.

CLASS IV. SPOROZOA.

1. EXAMPLE OF THE CLA.ssMonoeystis agilis.

One of the most readily procured Sporozoa is the microscopic
worm-like Monocystis agilis (Fig. 55, A, B), which is commonly
found leading a parasitic life in the vcsiculae seminales of the
common Earthworm. It is flattened, greatly elongated, point -d
at both ends, and performs slow movements of expansion and

D

Fio. 55. Monocystis agilis. A, B, two individuals in different stages of contraction :
C, cyst containing spores; DF, development of young (M) in a group of sperm-. < 11s
of the Earthworm ; G, newly liberated Monocystis surrounded by sperms of tin. 1 Karthwonn ;
,!/, young Monocystis ; nu. nucleus ; sp. sperms or sperm-cells of Earthwoi in. (After Biitschli
and Huxley.)

contraction, reminding us of those of Euglena. The protoplasmic
body is covered with a firm cuticle, and is distinctly divided into
a denser superficial portion, the cortex, and a central semi-fluid
mass, the medulla. There is a large clear nucleus (nu.) \\itli a
distinct nucleolus and nuclear membrane, but the other organs of
the protozoan cell-body are absent: there is no trace of contractile
vacuole, of flagella or pseudopods, of mouth or gullet. Nutrition
is effected entirely by absorption.

Reproduction takes place by a peculiar and characteristic proces
of spore-formation. Either a single individual, or two individuals
closely applied together but not actually fused, become encysted.
Multiple fission then takes place (C), the protoplasm becoming
divided into an immense number of spindle-shaped spores, each
surrounded with a strong chitinoid coat, and thus differing

76 ZOOLOGY SECT.

markedly from the naked spores of the Rhizopoda and Mastigo-
phora. The protoplasm of each spore then undergoes fission,
becoming divided into a number of somewhat sickle-shaped
bodies, which are arranged within the spore-coat somewhat like
a bundle of sausages. By the rupture of the spore-coat these
falciform young are liberated and at once begin active move-
ments, the thin end of the body moving to and fro like a clumsy
flagellum. The falciform young appear, in fact, to be greatly
modified flagellulse. They make their way to the clumps of
developing sperms, bore their way in, and are thus found sur-
rounded by sperm-cells in various stages of development (D F).
After thus living an intra-cellular life for a time, they escape into
the cavity of the vesicula (G) and grow into the adult form.

2. CLASSIFICATION AND GENERAL ORGANISATION.

The Sporozoa are exclusively parasitic, being the only group of
Protozoa of which this can be said. They have no organs of
locomotion and always multiply by spore-formation. The class is
divisible into the following four orders :

ORDER 1. GREGARINIDA.
Sporozoa in which the adult is free and motile.

ORDER 2. COCCIDIIDEA.
Sporozoa in which the adult is a minute intra-cellular parasite.

ORDER 3. MYXOSPORIDEA.

Sporozoa in which the adult is amoeboid.

ORDER 4. SARCOCYSTIDEA.
Elongated Sporozoa, usually found in muscle.

Systematic Position of the Example.

Monocystis agilis is a species of the genus Monocystis, belonging
to the Family Monocystidce, of the order Gregarinida. It is placed
in the Gregarinida on account of being free and motile in the
adult state. The absence of partitions dividing the protoplasm
into segments indicates its position among the Monocystidse.
Monocystis is distinguished by its elongated form, by the absence
of any special apparatus in the C}*st for the liberation and dispersal
of the spores, and by its spindle-shaped spores with thickened
ends, each producing 4 8 falciform young. The differences
between the species of Monocystis depend largely upon size.

II

PHYLUM PROTOZOA

77

ORDER 1. GREGARIXIDA.

All the more typical members of the class In-long to thi-
group. With the exception of Monocystis, already described, the
only genus to which it will be necessary to draw attention is
Gregarina (Fig. 56), the various species of which are parasitic in
the intestines of Crayfishes, Cockroaches, Centipedes, and other
articulated animals. It differs from Monocystis in having the
medullary protoplasm of the adult divided into two sections, an
anterior, the protomcritc (pr.), and a posterior, the deutomerite
(dcu.\ in which the nucleus is situated. Anteriorly to tli- proto-
merite there is sometimes found, especially in young indi\ iduals,

dou.

spd

FIG. 56. Gregrarina. A, two specimens of C-t. liintturiini partly eiiiK-<M<;>l in
epithelial cells of Cockroach ; B 1 , B 2 , two specimens of ft. dujardini ; in 1.5- tin epii
is cast off ; C, cyst of G. llattarum, from which must of the spores have li.vn discharged :
D, four stages iii the development of G. r/;</<i,>/,<i : cy. cyst ; deu. deutomerite ; p. epimerite ;
(/. gelatinous investment of cyst; nu. nucleus ; jn: protomerite ; px,i. 1, sluu-t pM-u.l'>]M><l ;
ps<L 2, long pseudopod ; j). mass of spores ; spd. sporoducts. (From Biitschli's l',-ot<>:.i><i. )

a third division, the epimeritc {e^>.}, which is sometimes provided
with hooks (B 1 ), serving to attach the parasite to the epithelium
of the intestine of its host. As maturity is reached the epimerite
is thrown off (B : ), and the parasite then lies freely in the cavity of
the intestine.

The cysts of Gregarina (C) are often very complex and are
provided with delicate ducts (spd.) in the thickness of the wall,
through which the spores escape. In Grc<jrintt gigantea of the
Lobster, the young is liberated from the spore in the form of a non-
nucleated amcebula (D 1 ), with one l..ng and one short p>nido|.od
(D 2 ); this divides by the long pseudopod (psd.2) becoming sepa-
rated off, and each product of fission, developing a nucleus, passi -
into the adult form (D 3 , D 4 .)

78

ZOOLOGY

SECT.

ORDER 2. COCCIDIIDEA

The members of this, order are extremely minute and simple forms which
occur as parasites, not in the intestine, but in the actual cells of various animals.
E nutria, (Fig, 57, 1), for instance, is found in the intestinal epithelium of the

IE;

imeria

2.Coccidiutn

FIG. 57. Coccidiidea. A, adult Eimerla (E) in enteric epithelial cell (ep.) of mouse ;
B, encysted form ; C, encysted form, the protoplasm contracting to form a spore ; D, formation
of falciform young (/.) in interior of spore (up.) ', E, spore with falciform young ; F, adult
encysted form of Coccidiitm from liver of rabbit ; G, division into spores ; H, cyst containing
ripe spores (sp.), each with a single falciform young ; I, single spore with falciform young (/).
(From Butschli's Protozoa, after Leuckart and Eimer.)

mouse and the sparrow, Coccidium (2) in the rabbit's liver, and Klossia in the
epithelium of the kidney of molluscs. They are not locomotive, but remain
quiescent in the cell (A), finally encysting (C), and producing one or more spores
(D), in each of which one or more falciform young (E) are developed. The
remarkable parasite, Dr&panidiwm ranarum, found in the blood corpuscles of the
frog, is probably the falciform stage of some unknown member of this order.

ORDER 3. MYXOSPORIDEA.

This group includes a small number of genera, which differ from other
Sporozoa in being amoeboid (Fig. 58, A). Many nuclei are present, but whether

FIG. 58. A, Myxidium lieberkiihnii, amoeboid phase ; B, Myxobolns imilleri,

spore with discharged nematocysts (ntc.) ; C, spores (psorospenus)

ntc. nematocysts. (From Butschli's Protozoa.)

s) of a Myxosporidian ;

this condition is due to the multiplication of a single nucleus or to the organism
being a plasmodium is not known. A good example of the order is Myxidium,
found in the urinary bladder of the pike.

ii PHYLUM PROTOZOA ;

The spores are often very complex ; in some cases (B) they posse-
like the trichocysts of Infusoria and the nematocysts of /oopli\ t.^ ,;,-;,,
in others they have the form of curious twisted bodies called psorospt //,.
in the gills, kidneys, &c., of fishes ; they have been seen to liberate a

ORDER 4. SARCOCYSTIDEA.

The best known form of this order is Sarcoryxfis (Fig. 59), which occurs in ti..-
llesh of mammals, each parasite having the form of a long spindle embedded in a

.-JiV

"-

FIG. 59. Sarcocystis miescheri. adult form (*) in striped muscle of pit: (From
Biitschlis Protozoa, after Kuiiiey.)

striped muscular fibre. They are often known as Raimy's or J// .,/,, ,-\
corjjitsdes. The protoplasm divides into spores from which falciform young are
liberated.

CLASS V.-INFUSORIA.
1. EXAMPLE OF THE CLASS Pammcecium tiaudatum.

Structure. Pa.ramcecium, the " slipper-animalcule," is tolerablv
common in stagnant ponds, organic infusions, &c. The bodv(FiL;
60) is somewhat cylindrical, about ^ mm. in length, rounded at
the anterior and bluntly pointed at the posterior end. On the
ventral face is a large oblique depression, the biiccal groove (buc.
gr.), leading into a short gullet (yul.), which, as in Euglena, ends in
the soft internal protoplasm.

The body is covered with small cilia arranged in longitudinal
rows and continued down the gullet. The protoplasm is vn-v
clearly differentiated into a comparatively dense cortc,'- (co/1.) and
a semi-fluid -nif<]nll (m<d.), and is covered externally by a thin
cuticle (cu.) continued down the gullet. The cilia are prolongations
of the cortex, and perforate the cuticle.

In the cortex are found two nuclei, the relations of which arc
very characteristic. One, distinguished as the meganudeui-^nu.),
is a large ovoid body, staining evenly with aniline dyes, which,
when it divides, does so directly by a simple process of constrict ion.
The other, called the micronuclcus (pa. mi.), is a vt TV small body
closely applied to the meganucleus: when it divides it goes
through the complex series of stages characteristic of mitosis
(p. 16).

The contractile vacuoles (c. vac.) are two in number, and arc very
readily made out. Each is connected with a series of radia tin--
spindle-shaped cavities in the protoplasm which serve as feeders
to it. After the contraction of the vacuole these cavities are seen
gradually to fill, apparently receiving water from the siiiToundin-

80

ZOOLOGY

SECT.

protoplasm : they then contract, discharging the water into the
vacuole, the latter rapidly enlarging while they disappear from

B

TClC ^,'fa

luc. C/T:

c.vac. -

im.

FIG. CO Paramoecium caudatum. A, the living animal from the ventral aspect ; B, the

ame in optical section: the arrow shows the course taken by food-particles ; (', a specimen

which has discharged its trichocysts ; D, diagram of binary fission; Ice. gr. buccal groove;

cart, cortex ; CK. cuticle ; c. vac. contractile vacuole ; /. rue. food vacuole ; iiul. gullet ; rued.

medulla; ,i .'. inegauucleus ; pa. -/( it.micronucleus; trek, trichocysts. (From Parker's Biology.)

view ; finally the vacuole contracts and discharges its contents
externally.

The cortex contains minute radially arranged sacs called
(//'<:]/.). When the animal is irritated, more or fewer of

II

PHYLUM PROTOZOA

81

these suddenly discharge a long delicate thread, which, in the
condition of rest, is very probably coiled up within the sac. In ,i
specimen killed with iodine or osmic acid the threads can In -
quently be seen projecting in all directions from the surface (C).

Food, in the form of smalt living organisms, is taken in by
means of the current caused by the cilia of the buccal groove. The
food-particles, enclosed in a globule of water or " food-vacuole "
(/. vac.), circulate through the protoplasm, when the soluble parts
are gradually digested and assimilated. Starchy and fatty matters,
as well as proteids, are available as food, the digestive powers of
Paramcecium being thus considerably in advance of those of Amoeba.
Effete matters are egested at a definite anal spot posterior to t lu-
men th, where the cortex and cuticle are less resistent than else-
where. The whole feeding process can readily be observed in this
and other Infusoria by placing in the water some insoluble colour-
ing matter, such as carmine or indigo, in a fine state of division.

Reproduction. Multiplication takes place by transverse
fission (D), the division of the body being preceded by that of both
nuclei. As already mentioned, the meganucleus divides directly,
the micro-nucleus indirectly.

It has been proved, however, that multiplication by binary
fission cannot go on indefinitely ; but that after it has been repeated

Mg.ntl

mi.nu

FIG 61 -Paramcecium caudatum, stages in conjugation. <jul. gullet ; m<7. nu. me-
mi. nv.. micronuck-us ; Mg. nu. reconstructed mcganucleu, ; Mi. nu. recon
nucleus. (From Parker's Jiiolofiii, after llertwig.

a certain number of times it is interrupted by co?// In

this very remarkable and characteristic process two Paramcecia

G

.82 ZOOLOGY SECT.

become applied by their ventral faces (Fig. 01, A), but do not fuse.
The ineganucleus (mg. tin.) of each breaks up into small masses,
which disappear, being apparently absorbed into the protoplasm.
At the same time the micronucleus (mi. nu.) of each divides,
each product of division immediately dividing again, so that each
gamete or conjugating body is provided with four rnicronuclei (B).
Two of these (mi. nu.', mi. nu.") disappear; of the remaining two
one is distinguished as the stationary pro nucleus, the other as the
active pronucleus. The active pronucleus of each Infusor now
passes into the body of the other and fuses with its stationary
pronucleus (D), each individual thus coming to possess a single
nuclear body derived in equal proportions from the two conjugat-
ing cells (E). The animalcules then separate from one another,
and the nucleus of each divides and gives rise to the permanent
mega- (G, Mg. nu.} and micronuclei (Mi. nu.).

2. CLASSIFICATION AND GENERAL ORGANISATION.

In the majority of the Infusoria the body is ciliated throughout
life, but in certain forms cilia are present only in the immature
condition, the adult being provided with peculiar organs of
prehension or tentacles. We thus get two orders, viz. :

ORDER 1. CILIATA.
Infusoria provided with cilia throughout life.

ORDER 2. TENTACULIFERA.

Infusoria possessing cilia in the young condition, tentacles in
the adult.

,V//.sA ni i' 1 1<- Position of the

Paramoecium aurelia is one of several species of the genus
Famine fin in, belonging the Family Pai'ni<it:i<l<i', of the sub-order
TrirJtnxfi'inata, and order Ciliata. The presence of cilia in the
adult condition places it among the Ciliata : the presence of a
permanently open mouth into which food particles are swept by
the movement of the cilia, among the Trichostomata. The Para-
mcecidre are free-swimming, asymmetrical, uniformly ciliated, with
a ventrally placed mouth. P. caudatum is about \ i mm. in length,
its length about four times its breadth, rounded in front, and
bluntly pointed behind, and has a single micronucleus.

ORDER 1. CILIATA.

This order presents a wider range of variations some of them
of a truly extraordinary character than any other group of
Protozoa.

ii PHYLUM PROTOZOA

The form of the body is very varied : it may be ovoid (Fig.
62, 1), kidney-shaped (2), trumpet-shaped (3), vase or cup-shaped
{4, 9); produced into a long, flexible, neck-like pm,-.-> (5\ or into
large paired lappets (6); flattened from above downwards, or
elongated and divided into segments reminding us of those of an
articulated worm (8).

Most species are free-swimming, but some are attached to weeds
stones, &c., by a stalk. This may be a purely cuticular structure
(9\ or may contain a prolongation of the cortex in the form of a
delicate contractile axial fibre (Figs. 64 and 65, ax. /.), which serves
to retract the Infusor, its contraction causing the stalk to coil up
into a close spiral.

The arrangement of the cilia is also subject to great varia-
tion, and presents four chief types. In the holotricJtotis ////*, of
which Paramoecium is an example, the cilia are all small, equal -
sized or nearly so, and arranged in longitudinal rows (Fig. 60, Fig.
62, 1}. The second or hderotriclious type is seen in its simplest
form in Nyctotherus (Fig. 62, 2), in which the left side of the
peristome is bordered by a row of specially large adored cilia, the
rest of the body being covered with small cilia. In >S'A ntor (3}
the peristome is situated on the broad distal end of the trumpet-
shaped body, and the adoral band of cilia takes a spiral course.
This leads us to the peritrichous type of ciliation : in Vorticelln.
(Fig. 64) the vase-shaped body is, for the most part, quite bare of
cilia, but around the thickened edge of the peristome passes one
limb of a spiral band of large cilia, the other limb being continued
round a raised lid-like structure, or disc, into which the distal
region is produced. This arrangement of cilia reaches its greatest
complexity in Epistylis plicatilis (Fig. 62, 0), in which the cilia ry
spiral makes no fewer than four turns.

But it is in the hypotrichous type that the most extraordinary
modifications are found. The flattened body bears on its dorsal
surface mere vestiges of cilia in the form of very minute pn ess
of the cuticle, while on the ventral surface the cilia take the form
of large hooks, fans, bristles, and plates with fringed ends ( Fi-\ (i'2.
?). The hooks and plates do not vibrate rhythmically like ordi-
nary cilia, but are moved as a whole at the will of the animal,
thus acting as legs. The heterotrichous Ciliata, in fact, in addition
to swimming freely in the water, creep over the surface OF' weeds,
&c., very much after the manner of Woodlice. One of the mosl
extraordinary forms in this group is Diophrys, the size and arrange-
ment of its polymorphic cilia giving it a very grot, sque appear-
ance. In another genus (10} the distal end of the flasl shaped
body bears a circlet of large fringed cilia, giving the animal
appearance of a Rotifer (vide infra, Section VII. ).

In addition to cilia, many genera possess delicate
protoplasm or undtdatimj membranes in connt.rtmn \\\ the

G 2

84 ZOOLOGY SECT, ir

peristome. They contract so as to produce a wave-like movement
which aids in the ingestion of food. In some cases (Fig. 62, 11}
the undulating membrane (u. ml.) is a very large and obvious
structure.

Certain peculiar forms have yet to be mentioned. Multicilirt (Fig.
62, 13) has an irregular body of varying form, and bears a small
number of very long fiagellum-like cilia. Another genus in which
the cilia approach to flagella is Lnphomonas (18), the ovoid body of
which bears a tuft of close-set cilia at its anterior end. Actino-
bolus (14) is remarkable for the possession, in addition to cilia, of
long retractile tentacles used for attachment. In Didinium ( 15)
the barrel-shaped body is encircled by two hoops of cilia.

As we have seen, the meganucleus in Paramcecium is ovoid : in
other genera it may be elongated and band-like (3, mg. mi.), horse-
shoe-shaped (9), very long and constricted at intervals so as to
look like a string of beads (16), or much convoluted and branched
(17). In some genera the meganucleus undergoes repeated
divison, forming at last a very great number of small bodies only
discoverable by staining : this process of fragmentation of the nucleus
may proceed so far that the protoplasm of a stained specimen has
the appearance of being strewn with granules of chromatin. The
discovery of this phenomenon has tended to throw doubt on the
reported total absence of a nucleus in some Rhizopods.

In nearly all species one or more micronuclei are present, the
number sometimes reaching nearly thirty. In Opalina (Fig. 66)
numerous nuclear bodies (nu.) are present which divide by mitosis,
and therefore resemble micronuclei : if they are to be considered
as such, this genus must be held to differ from the other Ciliata
in the total absence of a meganucleus.

In Vorticclla- and other peritrichous genera there is a single
contractile vacuole (Fig. 64, c. vac), which, like that of Euglena,
opens through the intermediation of a reservoir into the gullet.
In the remaining Ciliata there may be one, two, or many some-
times a hundred contractile vacuoles They may be scattered
all over the cortex (Fig. 62, IS), or arranged in one or two rows
(8). The star-like arrangement of radiating canals, described in
Paramcecium, occurs in several genera : or there may be two long
canals, or the number of these channels in the protoplasm may
reach thirty (19, c). In some instances the protoplasm is hollowed
out by numerous non-contractile vacuoles (18, vac.) so as to
have a reticulate appearance, reminding us of the extra-capsular
I irotoplasm of Radiolaria.

Trichocysts, like those of Paramcecium, are found in many
holotrichous forms, but are rarely present in the other subdivisions
of the order. In the peritrichous _}>wfy7/s iiiii/n'.'/nria, however,
thciv art.- found numerous minute capsules (Fig. 62, 9, ntc.)
in pairs, each containing a coiled thread. They are

Tnlh

nti

C.Folliculina

!2.Mu!Hci!ia IS.Lophomonss

, 14. A c f i n o b o ! u s
^^ j*^*

^a^-c .-. , fir w IS.Trachelius mophryoelena

D .d.n.u m e " ld %!85S i r M ,.,.

PIG. (12. Various forms <.f Ciliata. 9a shu\vs ]uvt c.f : id, and

c a couple of nematocysts ; <i. anus; e. (in is) cuticle: <. (in I'M excretory canals;
contractile vacuole; /. /-KC. foml vacuole; .'/. Kullct : . micrn-

inu'lous ; milt, mouth; ,/c. nucleus; ,<(<-. ncmatocyst ; />. (in 15) seized \>y

l>i 'ni'in in ; t. tentacle; n. ,,<>'. uinlulatiny niciiiiiraiic ; vac. Qon-contractile \acn..].
vestibule. (From Biitschli's /Y<I':-<, after various autliurs.)

86

ZOOLOGY

SECT.

obviously structures of the same character as trichocysts, and
their resemblance to the nematocysts so characteristic of Coelenterata
(vide infra, Section IV.) is singularly close.

Digestive Apparatus. Many parasitic forms (Fig. 62, 8, 17 ;
Fig. 66 ) have no mouth or gullet, and are nourished by absorp-
tion of the digested food in the intestine of their host. The
simplest condition of the ingestive apparatus is found in Prorodon
(Fig. 62, 1.} and its allies, in which the mouth (mth.) is at one pole
of the ovoid body and is closed except during the ingestion of
food, and the gullet (g.) is a short, straight tube. Such forms,
on account of the symmetrical disposition of their organs and the
want of differentiation of their cilia they are all holotrichous
may be considered as the lowest or least specialised of the Ciliata.

LDicfyocysFa

S.Thuricola 4.0p'nrydium 5. SMchoMcha
2. Pyxicola

FIG. i'i3. Various forms of Ciliata. In 1 the shell alone is shown ; m. contractile fibre ; Of.
operculum. (From Butschli's Protozoa, after various authors.)

From them there is a fairly complete gradation to genera, like
Paramcecium, having the permanently open mouth on the left side
of the ventral surface, at the end of a well-marked buccal grove
or peristome. V< i rti <<!! n (Fig. 64) and its allies are peculiar in
having the edge of the peristome (per.) thickened so as to form a
projecting rim, and in the development of an elevated disc (cL) from
the area thus enclosed : the mouth (mth.) lies between the peri-
stome and the disc, and between it and the gullet proper (gull. ) is
interposed a section of the ingestive tube called the vestibule, into
which the contractile vacuole opens, and which contains the anal
spot. The distal end of Vorticella, with the peristome and disc, is
considered to be dorsal, the narrow end, to which the stalk is
attached, ventral : the mouth is, as usual, to the left. In Nycto-
thcrus (Fig. 62, ,'?) and some other genera there is, instead of the

II

PHYLUM PROTOZOA

87

temporary anal spot describe. I in Paramcecium, a distinct anal
aperture (.).

Most of the Ciliata are naked, having no shell or other form of
skeleton ; but in a few forms the body is provided with a shell or
lorica, formed of a chitinoid material, and reminding us ,,f the

per.

H*

FIG. 04. Vorticella. A, B, living specimens in different positions ; 0, optical section; D 1 , I)-,
diagrams illustrating coiling of stalk; E 1 , E-, two stages in binary fission; E :i , five /nol>l ;
FI, F-, division into mega- and microzooids ; Gi, G-, conjugation; in, multiple fission i
encysted form; H-, H 3 , development of spores; ax. f. axial fibre; t-oi-t. cortex; re. cuticle;
c. rac. contractile vacuole ; d. disc; (lull, gullet; t. microzonid ; mtk. mouth; nu. mega-
nucleus ; pi:/', peristumo. (From Parker's Binli><in.)

similar structure found in so many of the Mastigophora. Some
(Fig. 62, 4} have bell-like shells, variously ornamented, ;tn<l in
others (Fig. 63, _?) the similarly shaped shell is peril traifd ;md
resembles the skeleton of some of the Radiolnria. A chitinoid
plate or operculum (Fig. 63, 2, op.} may be fixed to the edge of the
peristome, and, when the animal is retracted in its case accurately
closes the mouth of the latter, or a similar opereu urn (->') is

88

ZOOLOGY

SECT.

attached to the interior of the tube, and is closed by a contractile
thread of protoplasm (w.), which acts as a retractor muscle.

Compound forms or colonies are common among the Peritricha,
rare in the other subdivisions. Many peritrichous forms occur as
branched, tree-like colonies, often of great complexity (Fig. 62, 9 ;
Fig. 65). The stem of these may be a purely cuticular structure
and non-contractile (Fig. 62, 9, 6), or may contain an axial
fibre or muscle, like that of Vorticella (Fig. 64, fa'./.). In Opliridium
(Fig. 63, 4) the colony is an irregular mass, sometimes 3-4 cm. in
diameter, consisting of a gelatinous substance in which a delicate,
branching stem is embedded, each branch terminating in a zooid.
Some genera (Fig. 63, 5) secrete a hollow, brown, gelatinous tube,
branched dichotomously ; the end of each branch is the habitation
of one of the zooids.

Reproduction. Transverse fission is the universal method of
reproduction, the entire process taking from half an hour to two

7/.Z.

FIG. 65. Zoothamnium arbuscula. A, entire colony ; B, the same, natural size ; C, the
same, retracted ; D, nutritive zouid ; E, reproductive zooid; FI, F-. development of reproduc-
tive zooid; .r./. axial fibre; e. rac. contractile vacuole ; <ic. nucleus; n.g. nutritive zooid;
,-.:. reproductive zooid. (From Parker's Biology, after Saville Kent.)

hours in different species. In Vorticella (Fig. 64, E) and other
Peritricha the plane of division is parallel to the long axis of the
bell-shaped body, but as the distal surface probably corresponds
with the dorsal surface of such forms as Paramcecium, fission
is really transverse in this case also. In such simple Peritricha
as Vorticella division proceeds until tw r o zooids are produced on
a single stalk ; one of the two then acquires a second circlet
of cilia near its proximal end, becomes detached (E 3 ), and, after
leading a free-swimming life for a time, settles down and develops
a stalk : in this way the dispersal of the non-locomotive species is
ensured. In many species of Zootham/ti/o/i (Fig. 65) the zooids

II

PHYLUM PROTOZOA

89

are dimorphic: the ordinary bell-shaped forms ,/.;. divide in
the usual way, but as they remain attached, the process results only
in the increased complexity of the colony, not in the developm
of a new one. The larger zooids (r. 2.) are globular and monthlies :
they become detached, swim off, and, after a short five exist. -nee,
settle down, develop a stalk (F), divide, and so form a new colony.
In Vorticella multiplication by budding also occurs: a small
process is given off from one side (Fig. 64, F), develops a basal
circlet of cilia, and swims off as a microzooid, the parent individual

FIG. 66. Opalina ranarum. A, living specimen; B, stained specimen showing nuclei; C,
stages in nuclear division; DF, stages in fission; G, final product of fission ; H, encysted
form; I, young form liberated from cyst; K, the same after multiplication of the nucleus
has begun ; nu. nucleus. (From Parker's Bioloyy, after Saville Kcut aud Zeller.)

or mcgazooid being left attached to the stalk. Obviously this
process is simply a modification of binary fission, the products of
division being of very different dimensions, instead of equal-sized as
in the more usual case.

Spore-formation take place in Colpoda. The Infusor becomes
encysted, and divides into two, four, and finally eight masses, each
of which, becoming surrounded by a special investment, becomes
a spore. A somewhat similar process has been described in
Vorticella (Fig. 64, H).

A peculiar kind of spore-formation, specially adapted fco the
requirements of an internal parasite, takes plac. in '

90 ZOOLOGY SECT.

(Fig. 66), a parasite in the intestine of the Frog. Binary fission
takes place (D, E, F), and is repeated again and again so rapidly
that the daughter-cells are unable to grow to the adult size before
the next division. The final results of the process are small bodies
(G), each with only two or three nuclei instead of the large number
characteristic of the adult. These become encysted (H), and in
this passive condition are passed out of the Frog's intestine with
its faeces, frequently being deposited on water-weeds. All this
takes place during the Frog's breeding season : the tadpoles or Frog-
larva? feed upon the water-plants, and in doing so frequently take
in the spores or encysted Opalinae along with their food. When
this occurs the cyst is dissolved by the digestive juices of the host,
and the protoplasm of the spore is set free as a rounded body
with a single nucleus (I), which rapidly grows into an adult
Opalina (K).

Conjugation, in the form of a temporary union, accompanied by
interchange of micronuclei, has been described in Paramcecium, and
takes place in many other Ciliata. In Stylonychia histrio there
is, instead, a complete union of the two gametes. In Vorticella union
is also permanent, and takes place, not between two ordinary forms,
but between one of the ordinary stalked individuals, or mega-
gametes, and a free-swimming, small form, or microgamete,
produced, as described above, by budding (G 1 , G : ). The essence of
conjugation is the reception of nuclear material derived from
another individual : its effect appears to be increased activity in
multiplication by fission.

ORDER 2. TENTACULIFERA.

Judged from the adult structure alone, the members of this
order would certainly be placed in a separate class of the Protozoa :
it is only in virtue of the facts of development that they are
united in a single class with the Ciliata.

The body may be globular (Fig. 67, Ja), ovoid (ll>), or cup-
shaped (2), but presents nothing like the variety of form met
with among the Ciliata. The distinguishing feature of the group
is furnished by the tentacles which are always present in greater
or less number, and which, in some cases at least, are the most
highly differentiated organs found in the whole group of Protozoa.
The characters of the tentacles vary strikingly in the different
genera.

In the common forms Acincta (2), and Podopliryu (1), the ten-
tacles spring either from the whole surface or in groups from the
angles of the somewhat triangular body. Each tentacle is an elon-
gated cylindrical structure (_?c), capable of protrusion and retrac-
I ion, and having its distal end expanded into a sucker. It is, more-
over, practically tubular, the axial region consisting of a semi-fluid

II

PHYLUM PROTOZOA

91

protoplasm, while the outer portion is tolerably firm and resistant.
When partially retracted, a spiral ridge is sometimes observable

C-i/ac

l.Podobhrya

2. A c i n e ^ a

6. S phoer o (shrya

7,0bhryodendron

S.Epholofa

9. DendroGoma

Fio. 67. Various forms of Tentaculifera. 7 anil /., two sjiL-cifs of Podophrya; c, a
tentacle much enlarged; 2a, Acineta jolyi ; 2l>, A. tuberosa ; in '/' tin' animal lias captured
several small Cilia ta ; Sn, a specimen multiplying b}- budding ; \/\ a fivr ciliated i'u>l ; :'/. tin-
entire colony ; 96, a portion of the stem ; 9c, a liberated hud: ", urg.tniMM i-a]>tmvd as
6. e. brood-cavity ; ini. bud: r rue. contractile vacuole ; )/;;/. /!<(. meganucleus ; mi. nu. micro-
nucleus ; t. tentacle. (After Biitschli and Saville Kent.)

around the tentacle : this may indicate the presence of a band of
specially contractile protoplasm, resembling the axial fibre in the

<92 ZOOLOGY SECT.

stalk of VorticcUa. Infusors and other organisms are caught by
the tentacles (4, 0), the cuticle of the prey is pierced or dissolved
where the sucker touches it, and the semi-fluid protoplasm can
then be seen flowing down the tentacle into the body of the
captor. A single tentacle only may be present (3\ or the tentacle
maybe branched (4), the extremity of each branch being suc-
torial. In some forms there are no terminal suckers (J), and the
tentacles are waved about to catch the prey instead of standing
out stiffiv as in Acineta. In other cases there are one or more

/

long striated tentacles with tufted ends (7).

The nucleus may be ovoid (la), horseshoe-shaped, or branched
(8,9) : in some cases a micronucleus (1 a, mi. nu.) has been found.
There are one or more contractile vacuoles (c. vac.).

Some genera are naked (./) : others form a stalked shell or
lorica (;.? a) like that met with in many of the Mastigophora,

The only colonial form is the wonderful Dcndrosoma (9), in
which the entire colony attains a length of about 2 mm., and bears
an extraordinary resemblance to a zoophyte (ride Infra, Sect. IV.).
It consists of a creeping stem from which vertical branches spring,
and the various ramifications of these are terminated in Podo-
phrya-like zooids with suctorial tentacles. The nucleus is very
remarkable, extending as a branched axis throughout the
colony (b, nu.).

Reproduction by Unary fission takes place in many species.
In Ephdota ycmmipam (8) a peculiar process of budding occurs:
the distal end of the organism grows out into a number of pro-
jections or buds, into which branches of the nucleus extend. These
become detached, acquire cilia on one surface, and swim off (b).
After a short active existence tentacles appear and the cilia are
lost. In this case budding is external, but in Acincta fulia-n*/' ('2 b)
the buds become sunk in a depression, which is finally converted
into a closed brood-cavity (b,c.) : in this the buds take on the form
of ciliated embryos, which finally escape from the parent. In
Dendrosoma the common stem of the colony produces both internal
and external buds (b, Id.).

Further Iu: UK tries on the Protozoa.

The majority of the Protozoa are aquatic, the phylum being
equally well represented in fresh and salt water. They occur
practically at all heights and depths, from 8,000 to 10,000 feet
above sea-level, to a depth of 2,000 to 3,000 fathoms. Some
forms, such as species of Amoeba and Gromia, live in damp sand
and moss, and may therefore be almost considered as terrestrial
organisms. In accordance with their small size and the readiness
with which they are transported from place to place a large pro-
portion of genera and even of species are universally distributed,

n PHYLUM PROTOZOA .).;

being found in all parts of the world where the microscopic ta
has been investigated.

Numerous parasitic forms are known. Besides the entire class
of Sporozoa, species of Rhizopoda and of Infusoria occur both as
internal and external parasites. Species of Amoeba are common
in the intestines of the higher animals, and one species has lieeii
found in connection with a cancerous disease in Sheep. Parasitic
Vorticella? are said to give rise to the skin-disease eczema in .Man.
A ciliate Infusor, Ichthyophthirius, is found in the skin of fresh-
water Fishes, where it gives rise to inflammation and death.

Many instances have been met with in our survey of the
Phylum, of compound or colonial forms, the existence of which
seems at first sight to upset our definition of the Protozoa as
unicellular animals. But in all such cases the zooids or unicellular
individuals of the colony exhibit a quasi-independence, each, as a
rule, feeding, multiplying, and performing all other essential
animal functions independently of the rest, so that the onl\
division of labour is in such forms as Zoothamnium and Volvox,
in which certain zooids are incapable of feeding, and are set apart
for reproduction. In all animals above Protozoa, on the other
hand, the body is formed of an aggregate of cells, some of which
perform one function, some another, and none of which exhibit
the independent life of the zooid of a protozoan colony. It cannot,
however, be said that there is any absolute distinction between a
colony of unicellular zooids and a single multicellular individual :
Proterospongia and Volvox approach very near to the border-land
from the protozoan side, and a similar approach in the other
direction is made by certain animals known as Mesozoa, which will
be discussed hereafter (Sect. IV.). Moreover, the Mycetozoa, the
plasmodia of which are formed by the fusion of Amoebulte, the
nuclei of the latter remaining distinct and multiplying, are rather
non-cellular than tmi-cellular. This point will also be refenvc I t <
at the conclusion of the section on Sponges (Sect. III.).

In each division of the Protozoa we have found comparatively
low or generalised forms side by side with comparatively high or
specialised genera. For instance, among the Rhizopoda, there
can be no hesitation in placing the Lobosa, and especially Pi <
mceba, at the bottom of the list, and the Radiolaria at the top.
Similarly, among the Mastigophora, such simple Flagellata as
Oikomonas (Fig. 47, .? and 8) and Heteromita are obviously the
lowest forms, Noctiluca and the Dinoflagellata the highest.
whether the Rhizopoda, as a whole, are higher or lower than the
Flagellata, is a question by no means easy to answer. A na^'llmn
certainly seems to be a more specialised cell-organ than a
pseudopod, and some of the Mastigophora rise above tin- highest
of the Rhizopoda in the possession of a firm cortex and cuticle,

94

ZOOLOGY

SECT

and the consequent assumption of a more definite form of body
than can possibly be produced by the flowing protoplasm of a
Foraminifer or a Radiolarian. On the other hand, the nucleus
of the Radiolaria is a far more complex structure than that of
the Mastigophora, and in Foraminifera, Radiolaria, and Heliozoa
the organism frequently begins life as a flagellula, a fact which,
on the hypothesis that the development of the individual recapitu-
lates that of the race, appears to indicate that these orders of
Rhizopoda are a more recently developed stock than at any rate
the lower Flagellata. These circumstances, and the fact that
Mastigamoeba might equally well be classed as a lobose Rhizopod
with a flagellum or as a Flagellate with pseudopods, seem to
indicate that the actual starting-point of the Protozoa was a form

Radiolaria
Foraminifera

Lobosa

Mycet

Heliozoa C hoano-
Flagellata

Flagellata

Dinoflagellata

Cystofiagellata

Tentaculifera

Ciliata

-Sporozoa

FIG. 68. Diagram showing the mutual relationships of the chief groups of Protozoa.

capable of assuming either the amoeboid or the flagellate phase.
From such a starting-point the Lobosa, Foraminifera, Heliozoa,
Radiolaria, and Flagellata diverge in different directions, the first
four keeping mainly to the amoeboid form, but assuming the
flagellate form, in the young condition, in the case of Foraminifera,
Heliozoa, and Radiolaria.

The Choanoflagellata, Dinoflagellata, and Cystoflagellata are
obviously special developments of the Flagellate type along
diverging lines.

As to the Ciliata, Lophomonas and Multicilia (Fig. 62, 12 and 13}
appear to indicate the derivation of the order from the Flagellate
type, since their cilia are long and flagellum-like, but the evidence
is not strong and no other is at hand. The derivation of the Tenta-
culifera from a ciliate type appears to be clear. The Tentaculifera
and the hypotrichous Ciliata are undoubtedly the highest develop-

ii PHYLUM PROTOZOA 95

ment of the Protozoan series, since they show a degree of
differentiation attained nowhere else by a single cell.

The Mycetozoa appear to have been derived from the ommiK'n
amceboid-flagellate stock, since they are all predominantly amce-
boid in the adult condition, flagellate when young. The Spomzoa
probably had a similar origin, but the characters of this class have
evidently been profoundly modified in accordance with their
parasitic mode of life.

The diagram on the previous page is an attempt to express
these relationships in a graphic form.

SECTION III
PHYLUM AND CLASS PORIFERA.

THE microscopic animals described in the preceding section
are, as already repeatedly pointed out, characterised by their
unicellular character, and in this respect stand in contrast to the
remainder of the animal kingdom. The animal kingdom is thus
capable of division into two great subdivisions, the Protozoa
or unicellular animals, and the Metazoa or multicellular the latter
comprising all the groups that remain to be dealt with. In the
earliest stage of their existence all the multicellular animals or
Metazoa are, as already pointed out (p. 18), in a unicellular
condition, originating in a single cell, the fertilised ovum or
oosperm. By the process of segmentation or yolk-division the
unicellular oosperm becomes converted in all the Metazoa
into a mass of cells from which the body of the adult animal is
eventually built up. Of the Metazoa, the group which approxi-
mates most closely to the Protozoa is that now to be dealt with
the Porifera or Sponges.

1. EXAMPLE OF THE CLASS Syecn yckttitwstcm.

General External Appearance and Gross Structure

Sycon gelatinosum, 1 one of the Calcareous Sponges, has the form of a
tuft, one to three inches long, of branching cylinders (Fig. 69), all con-
nected together at the base, where it is attached to the surface of a
rock or other solid body submerged in the sea. It is flexible, though
of tolerably firm consistency ; in colour it presents various shades
of gray or light brown. To the naked eye the surface appears
smooth, but when examined under the lens it is found to exhibit
a pattern of considerable regularity, formed by the presence of

1 This species is an inhabitant of southern seas. In all essential respects the
account of it given above will apply to 8. ci/iafinn, a common European species
which differs chiefly in the absence of the pore-membranes.

SECT. Ill

PHYLUM AND CLASS PORIFERA

97

innumerable elevations of a polygonal
whole surface and are separated off
from one another by a system of de-
pressed lines. In these depressions
between the elevations are to be de-
tected, under the microscope, groups
of minute pores the inhalant po r r<*.
At the free end of each of the cylin-
drical branches is a small but distinct
opening, surrounded by what appears
like a delicate fringe. When thq.
branches are bisected longitudinally
(Fig. 70), it is found that the terminal
openings (0) lead into narrow passages,
wide enough to admit a stout pin,
running through the axes of the
cylinders ; and the passages in the

shape, which cover the

Fio. 70. Sycon gelatiaosum. A portion slightly
magnified; one cylinder (that to the right) bisc, i, J
longitudinally to show the central paragastric cavity
opening on the exterior by the osculum, and the
position of the incurrent and radial canals ; the
former indicated by the black bands, the latter,
dotted ip. marks the position of three of the groups
of inhalant pores at the outer ends of the incurrent
canals ; o. osculum.

VOL. I

Fir.. GO. Sycon gelatinosum.

Entire sponge, consisting of a
group of branching cylinders
(natural size).

interior of the various
branches join where the
branches join the pas-
sages thus forming a
communicating system.
On the wall of the
passages are numerous
line apertures which re-
quire a strong lens for
their detection. The
larger apertures at the
ends of the branches
are the oscula of the
sponge, the passages the
paragastric cavities. If
a living Sycon is placed
in sea-water with which
has been mixed sm in-
carmine powder, it will
be noticed that the
minute particles of the
carmine seem to be at-
tracted towards the sur-
face of the sponge, ami
will often be seen to
pass into its substance
through the minute in-
halant pores already
mentioned as occurring
in groups between the
elevations on the outer
II

98

ZOOLOGY

SECT.

surface. This would appear to be due to the passage of a
current of water into the interior of the sponge through these
minute openings dotted over the surface ; and the movement of
the floating particles shows that a current is at the same time
flowing out of each of the oscula. A constant circulation of
water would thus appear to be carried on currents moved by
some invisible agency flowing through the walls of the sponge
to the central paragastric cavities, and passing out again by
the oscula.

If a portion of the Sycon is firmly squeezed, there will be
pressed out from it first sea-water, then, when greater pressure is

R

\ I

FIG. 71. Sycon gelatinosum. Section through the wall of a cylinder taken at right angles
to the long axes of the canals, highly magnified ; co, colleucytes ; 1C, incurrent canals ;
oc. young ova ; R, radial canals ; sp. triradiate spicules.

exerted, a quantity of gelatinous-looking matter, which, on being
examined microscopically, proves to be partly composed of . a
protoplasmic material consisting of innumerable, usually more or
less broken, cells with their nuclei, and partly of a non-protoplasmic,
jelly-like substance. When this is all removed there remains
behind a toughish felt-like material, which maintains more or less
completely the original shape of the sponge. This is the skeleton
or supporting framework. A drop of acid causes it to dissolve
with effervescence, showing that it consists of carbonate of
lime. When some of it is teased out and examined under the
microscope, it proves to consist of innumerable, slender, mostly
three-rayed microscopic bodies (Figs. 71 and 72, sp) of a clear
glassy appearance. These are the calcareous spicules which form

Ill

PHYLUM AND CLASS PORIFERA

the skeleton of the
Sycon.

The arrangement of the
spicules, their relation to
the protoplasmic parts,
and the structure of the
latter, have to be studied
in thin sections of hard-
ened specimens (Figs. 71
and 72). An examination
of such sections leads to
the following results.

Microscopic struc-
ture. Covering the outer
surface of the sponge is
a single layer of cells the
ectoderm (Fig. 72, cc)
through which project
regularly-arranged groups
of needle-like and spear-
like spicules (.$//), form-
ing the pattern of poly-
gonal elevations on the
outer surface. The cells
of the ectoderm are in the
form of thin scales, which
are closely cemented to-
gether by their edges to
form a syncytium, or mem-
brane consisting of cells
so intimately united that
their boundaries are not
readily distinguishable.
The paragastric cavities
are lined by a layer of
cells (en) which are like
those of the ectoderm,
but are somewhat thicker
and more granular: this is
the endoderm of the para-
gastric cavity. Running
radially through the thick
wall of the cylinders are a
large number of regularly-
arranged straight passages.
Of these there are two sets,
those of the one set the
incurrcnt canals (Figs. 71

j

PG

FIG. 72. Sycon celatinosum. Tin

through the wall of a cylinder (parallel with tin-
course of the canals), showing one inrunvnt i-:inal
(1C), and one radial (It) throughout (heir 1, n
gp. triradiate spicules ; sp'. oxeote spicules of do-mal

cortex ('/'.); Sp". tetraradiatr spicules of iriMral
cortex (",.); cc. ectoderm; en. endodenu ; pro. pore
membrane; pp. prosopyics ; op. apopyle; oN. dia-
phragm; ase. excurrent passage; P.O. paraga

cavity: em. early embryo ; em. laic cml.ryo. The
arrows indicate the course of the water 'tin
the sponge.

100 ZOOLOGY SECT.

and 727(7) narrower, and lined by ectoderm similar to the ectoderm
of the surface ; those of the other set the radial or flagellate canals
(R) rather wider, octagonal in cross-section, and lined by endoderm
continuous with the lining of the paragastric cavity. The incurrent
canals end blindly at their inner extremities not reaching the
paragastric cavity; externally each becomes somewhat dilated,
and the dilatations of neighbouring canals often communicate.
These dilated parts are closed externally by a thin membrane
the pore membrane (Fig. 72, pin, and Fig. 73), perforated by three
or four small openings (Fig. 73, p) the inhalant pores already
referred to. The flagellate canals are blind at their outer ends,
which lie at a little distance below the surface opposite the
polygonal projections referred to above as forming a pattern on
the outer surface ; internally, each communicates with the para-

i-

FIG. 73. Sycon gelatinosum. Sur- FIG. 74. Sycon gelatinosum.

face view of a pore membrane highly An apopyle surrounded by its di;v

magnified ; p. inhalant pore ; R, posi- phragm ; m. contractile cells,
tioii of the outer end of a radial canal.

gastric cavity by a short wide passage the excurrent canal
(Fig. 72 cxc). Incurrent and flagellate canals run side by side
separated by a thin layer of sponge substance, except at certain
points, where there exist small apertures of communication the
prosopylcs (pp), uniting the cavities of adjacent incurrent and
flagellate canals.

The ectoderm lining the incurrent canals is of the same character
as the syncytium of the outer surface. The endoderm of the
flagellate canals, on the other hand, is totally different from that
which lines the paragastric cavity. It consists of cells of columnar
shape ranged closely together so as to form a continuous layer.
Each of these flagellate endoderm cells, or collared cells, as they are
termed, is not unlike one of the Choanoflagellate Protozoa (p. 72);
it has its nucleus, one or more contractile (?) vacuoles, and, at the
inner end, a single, long, whip-like flagellum, surrounded at its base
by a delicate, transparent, collar-like upgrowth, similar to that which
has already been described as occurring in the Choanoflagellata. If

111 PHYLUM AND CLASS PORIFERA iQl

a portion of a living specimen of the sponge is teased out in sea-
water, and the broken fragments examined under a tolerably high
power of the microscope, groups of these collared cells will be
detected here and there, and in many places the movement of the
flagella will be readily observed. The flagellum is flexible, but
with a certain degree of stiffness, especially towards the base ' and
its movements resemble those which a very supple fishing-rod is
made to undergo in the act of casting a long line the movement
being much swifter and stronger in the one direction than in the
other. The direction of the stronger movement is seen, when
some of the cells are observed in their natural relations, to be
from without inwards. It is to these movements that the forma-
tion of the currents of water passing along the canals in due. The
collars of the cells in specimens teased in this way become for the
most part drawn back into the protoplasm.

The short passage or excurrent canal, which leads inwards from
the flagellate canal to the paragastric cavity, differs from t In-
former in being lined by flattened cells similar to those of the
paragastric cavity ; it is partly separated from the flagellate canal
by a thin diaphragm (Fig. 72, di, and Fig. 74), perforated by a
large circular central aperture the apopyk(ap} which is capable
of being contracted or dilated : its opposite aperture of com-
munication with the paragastric cavity, which is very wide, is
termed the 'gastric ostium of the excurrent canal.

The effect of the movement of the flagella of the cells in the
flagellate canals is to produce currents of water running from
without inwards along the canals to the paragastric cavity. This
causes water to be drawn inwards through the prosopyles from
the incurrent canals, and, indirectly, from the exterior through the
perforated membranes at the outer ends of the latter.

Between the ectoderm of the outer surface and of the incurrent
canals and the endoderm of the inner surface and of the flagellate
canals are a number of spaces filled by an intermediate layer the
mcsodcrm or mcsoqlcca in which the spicules of the skeleton ar--

<J J.

embedded. Each spicule is developed from a single cell of the
middle layer, the remains of the cell the sdcroUast being some-
times distinguishable on the surface of the fully developed spicule
as a thin investment. The spicules (Figs. 71 and 72, sp) are
regularly arranged, and connected together in such a way as to
protect and support the soft parts of the sponge. Most are. as
already noticed, of triradiate form. Large numbers, however, are
of simple spear-like or club-like shape ($_//); these, which are
termed the oxeotc spicules, project on the outer surface beyond the
ectoderm, and are arranged in dense- masses, one oppo-ite tin-
outer end of each of the ciliated canals, this arrangement pro-
ducing the pattern already referred to as distinguishable on tin-
outer surface. The thick outer layer in which the bases of these

102 ZOOLOGY SECT.

oxeote spicules lie embedded, is termed the dermal cortex (dc). A
thick stratum at the inner ends of the canals and immediately
surrounding the paragastric cavity is termed the (/astral cortex (gc).
It is supported by triradiate and also by tetraradiate spicules, one
ray of each of which (sp") frequently projects freely into the para-
gastric cavity, covered over by a thin layer of flattened endoderm
cells.

The mesoderm itself, as distinguished from the spicules which
lie embedded in it, consists of a clear gelatinous substance con-
taining numerous nucleated cells of several different kinds. Most
of these are small cells of stellate shape, with radiating processes
the. connective-tissue cells or collencytes (Fig. 71, co); others are fusi-
form ; a good many, the amoeboid wandering cells, are Amoeba-like,
and capable of moving about from one part of the sponge to another.

Around the inhalant pores and the apopyles are elongated cells
(Figs. 73 and 74), sometimes prolonged into narrow fibres. These
are contractile effecting the closure of the apertures in question,
and are therefore to be looked upon as of the nature of muscular
fibres. In the case of the inhalant pores they are ectodermal ; in
that of the apopyles they are endodennal. A band of similar
fibres surrounds the osculum the oscular sphincter.

The sexual reproductive cells the ova (Figs. 71 and 72, ov) and
sperms are developed immediately below the flagellate endoderm
cells of the flagellate canals, and in the same situation are to be
found developing embryos (em, em'), resembling in their various
stages those of Sycon raplianus, as described below.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

Sponges are plant-like, fixed, aquatic Metazoa, all, with the
exception of one family, inhabitants of the sea. The primary form
is that of a vase or cylinder, the sides of which are perforated by a
number of pores, and in the interior of which is a single cavity ;
but in the majority of Sponges a process of branching and folding
leads to the formation of a structure of a much more complex
character. The surface of the Sponge is covered by a single layer
of flattened cells the ectoderm and the internal cavities, or a
part of them, are lined by a second single layer the endoderm
part or the whole of which consists of a single layer of columnar
cells each provided internally with a long flagellum. Between these
two layers is a quantity of tissue usually of a gelatinous consistency
the rncxixli / ///, or mesoglcea containing a number of cells of
various kinds, some of which secrete the elements of the skeleton.
The skeleton or supporting framework, developed in the mesoderm,
consists in some cases of fine flexible fibres of a material termed
spongin ; in others of spongin fibres supplemented by microscopic
siliceous spicules ; in others of siliceous spicules alone ; in others

in PHYLUM AND CLASS POR1FERA

of spicules of carbonate of lime. Reproduction takes place both
asexually by the formation of gemmuks, and sexually l>y moans of
ova and sperms. The ovum develops into a ciliated free-swimming
larva, which afterwards becomes fixed and develops into the
plant-like adult Sponge.

The Sponges are sufficiently far removed in structure from their
nearest allies the Protozoa on the one hand and the Ccelentrnit.-i
on the other to justify us in looking upon them as constituting one
of the great divisions or phyla of the animal kingdom. At 1 1 1 '
same time there is so much uniformity of structure within tin-
group that a division into classes is not demanded; the phylum
Porifera contains a single class.

The class Porifera is classified as follows :

Sub-Class I. Calcarea.

Sponges with a skeleton of calcareous spicules, and with com-
paratively large collared cells.

ORDER 1. HOMOCCELA.

Calcareous Sponges in which the endoderm consists throughout
of flagellate collared cells.

ORDER 2. HETEROCCELA.

Calcareous Sponges in which the endoderm consists partly of
flattened cells, the collared cells being restricted to flagellate canals
or chambers.

Sub-Class II. Non-Calcarea.

Sponges in which the skeleton is either absent, or composed of
.spongin fibres, or of siliceous spicules.

TRIBE LMYXOSPONQIjE.
Xon-Calcarea devoid of skeleton.

TRIBE ILSILICISPONGI.-E.
Non-Calcarea provided with a skeleton.

ORDER 1. HEXACTIXELLIDA.
Silicispongioe with six-rayed siliceous spicules.

ORDER 2. DESMOSPOXGLE.
Silicispongise devoid of six-rayed spicules.

Systematic Position of the

Si/con gelatinosiun is one of many species of the genus Si/con.

Sycon is one of several genera of the j'">ni/.'/ Sycettidce ; and the

104 ZOOLOGY SECT.

family Sycettidce is one of several families of the order Heteroccda
of the class Calcarea. Among the families of the Heteroccela,
that of the Sycettidce is distinguished by the following features,
which characterise all its members:

" The flagellate chambers are elongated, arranged radially around
a central paragastric cavity, their distal ends projecting more or less
on the dermal surface, and not covered over by a continuous cortex.
The skeleton is radially symmetrical."

Of the genera into which the Sycettidce are divided, Sycon is
characterised as follows :

" The flagellate chambers are not intercommunicating ; their
distal ends are provided each with a tuft of oxeote spicules."

The members of one of the other genera of the family Sycetta
win'le possessing the general characteristics of the family, differ from
those of the genus Sycon in wanting the tufts of oxeote spicules ;
those of a third Sycantha have the flagellate chambers united
in groups ; the chambers of each group intercommunicating by
openings in their walls, and each group having a single common
opening into the gastric cavity. The members of this genus re-
semble Sycon, and differ from Sycetta in the presence of tufts
of oxeote spicules at the distal ends of the flagellate chambers.

These distinctions between classes, orders, families, and genera
are of an entirely arbitrary character. No such divisions exist in
nature ; and they are merely established as a convenient way of
grouping the sponges and facilitating their classification. But a
classification of this kind, if carried out on sound principles, should
nevertheless have something corresponding to it in nature, inas-
much as the grouping of the various divisions and subdivisions
aims at expressing the relationships of their members to one
another. The members, for example, of the family Sycettidce are
all regarded, on account of the features which they possess in
common, as being more nearly related to one another than to the
members of the other families, and as- having been derived from a
common ancestor which also possessed those features the diver-
gences of structure which we observe in the different genera and
species being the result of a gradual process of change.

Within the limits of the genus Sycon, S. gelatinosum is distin-
guished from the rest as a group of individual Sponges all possess-
ing certain specific characters which it will be unnecessary to
detail here. But the individual Sponges referable to this species
frequently differ somewhat widely from one another: there aiv
numerous individual variations. If we compare a number of
specimens all possessing the species-characters of Sycon gelatino-
sum, we find that they differ in the number of branches, in the
shape of the cylinders some being relatively narrow, some re-
latively wide in the degree of development of the oscular crown
of spicules, in the ratio of the thickness of the wall to the width

in PHYLUM AND CLASS PORIFERA 105

of the contained paragastric cavities, and in many other more
minute points ; in fact, we find as a result of the comparison that no
two specimens are exactly alike. These differences are so great
that some very distinct races or varieties of S. gelatinosuni have
been recognised, and some have received special names. HIT.
again, as in the case of the families and orders, the distinctions
are of an arbitrary character some writers on Sponges setting
down as several species what others regard merely as varieties of
one species. It is impossible, in fact, to draw a hard and fast line
of distinction between species and varieties. In the higher groups
of animals the attempt is made to establish a physiological dis-
tinction ; all the members of a species are regarded as being fertile
inter se, and capable of producing fertile offspring as a result of
their union ; but such a mode of distinguishing species is impos-
sible of application among lower forms such as the sponges. In
these lower groups, accordingly, a species can only be defined as
an assemblage of individuals which so closely resemble one an-
other that they might be supposed to be the offspring of a parent
form similar to themselves in all the most essential features.
And, according to the view taken of the relative importance of
different points of colour, shape, and internal structure, the con-
ceptions of the species and their varieties and mutual relationships
formed by different observers must often differ widely from one
another.

3. GENERAL ORGANISATION.

General Form and Mode of Growth. The simplest Sponges
are vase-shaped or cylindrical in form, either branched or un-
branched, and, if branched, with or without anastomosis or
coalescence between neighbouring branches. But the general
form of the less simple Sponges diverges widely from that of such
a branching cylinder as is presented by Sycon gelatinosum
(Fig. 69).

From the point to which the embryonic sponge becomes
attached it may spread out horizontally, following the irregulari-
ties of the surface on which it grows, and forming a more or less
closely adherent encrustation like that of an encrusting lichm
(Fig. 75, A). The surface of such an encrustation may br smooth ;
more commonly it is raised up into elevations rounded bosses
cones, ridges or lamellae ; and the edges may be entire or lnl>r.|.
In other cases the sponge grows at first more actively in tin-
vertical than in the horizontal direction, and the result m;t\ ln> M
long, narrow structure, cylindrical or compressed, and more or Irs
branched (Fig. 75, B}. Sometimes vertical and horizontal growth
is almost equal, so that eventually there is formal ,-i thick
mass of a rounded or polyhedral shape (Fig. 75, C), with an even,
or lobed, or ridged surface. Very often, after active vertical growth

100

ZOOLOGY

SECT.

has resulted in the formation of a comparatively narrow basal
part or stalk, the Sponge expands distally, growing out into lobes
or branches of a variety of different forms, and frequently anasto-
mosing. Sometimes, after the formation of the stalk with root-
like processes for attachment, the Sponge grows upwards in such
a way as to form a cup or tube with a terminal opening. Such a

- --',"" . '"""Vaa

KBR

s~ -. . -

S . '

'- ' ',

"

?!%& ^*.*Z^.&

A.Qscaria

C.E us^ongia

B.Psammoclema

&^

D. Poherion

FIG. 75. External form of various Sponges. A, Osearia. an encrusting form, with the
upper surface raised up into a number of rounded prominences; JB, Psammoclema a
nullifying subcyliiidrical Sponge ; C, Eusponeria (toilet sponge), a massive form with
a broad base ; D, Poterion (Neptune's Cup), an example of a complex Sponge assuming
the form of a vase. (After Vosmaer.)

cup-shaped Sponge, exemplified in the gigantic Neptune's Cup
(Poterion, Fig. 75, D), is not to be confounded with the simple
vase or cup referred to above as the simplest type of Sponge,
being a much more complex structure with many oscula. Some-
times the Sponge grows from the narrow base of attachment into
a thin flat plate or lamella ; this may become divided up into a
number of parts or lobes, which may exhibit a divergent arrange-

Ill

PHYLUM AND CLASS PORIFERA

107

merit like the ribs of an open fan. Often the lamella becoiin ^
folded, and sometimes there is a coalescence between the folds
resulting in the development of a honey-comb-like form of
sponge.

Sponges resemble plants, and differ from the higher groups of
animals, in the readiness with which, in many cases, their form
becomes modified during growth by
external conditions (environment).
Different individuals of the same
kind of Sponge, while still exhibiting
the same essential structure and the
same general mode of growth, may
present a variety of minor differences
of form, in accordance with differ-
ences in the form of the supporting
surface or in the action of waves and
currents.

Leading Modifications of
Structure. Sycon gelatinosum be-
longs to a type of Sponges interme-
diate between the very simplest forms
on the one hand, and the more com-
plex on the other. The simplest and
most primitive of known Sponges is
one named Ascctta primordialis (Fig.
76). It is vase-shaped, contracted at
the base to form a sort of stalk by
the expanded extremity of which it
is attached ; at the opposite or free
end is the circular osculum. So far
there is a considerable resemblance
to Sycon gelatinosum ; but the struc-
ture of its wall in Ascetta is ex-
tremely simple. Regularly arranged
over the surface are a number of
small rounded apertures, the inhalant
or incurrent pores ; but, since the
wall of the Sponge is very thin, these
apertures lead directly into the cen-
tral or paragastric cavity (Fig. 77, A},
the long passages or canals through which the communication is
effected in Sycon being absent. The wall consists of the same
three layers as in Sycon, but the middle one, though it contains
a small number of spicules, is very thin. The ectoderm is a syn-
cytium ; the endoderm, which lines the paragastric cavity, consists
throughout of flagellate collared cells similar to those of the fla-
gellate canals of Sycon.

Fio. 76. Ascetta primordialis.

A portion of the wall of the vase-
like sponge removed to slmw
the paragastric cavity. (After
Haeokel.)

108

ZOOLOGY

SECT.

A somewhat more complex type of structure than that of Asce'tta

is exhibited lay those
Sponges in which the
wall

becomes thick-
ened and perforated
by radially-arranged
canals, which open
directly on the outer
surface by means of
inhalant pores, and
lead directly into the
paragastric cavity by
means of apopyles
the whole inner sur-
face as wt-11 as the
radial canals being
lined with flagellate
endoderm cells. In
forms which may be
regarded as repre-
senting the next stage
of development (Fig.
77, B: see also the
figures of Sycon gela-
tinosum), there are
formed by infolding
of the surface, in the
intervals between the
radial canals, canal-
like spaces, the incur -
rent canals, lined by
ectoderm and com-
municating with the
exterior on the one
hand, either by a
Avide opening or by
pores perforating a
pore-membrane, and
on the other by means
of small openings, the
prosopyks, with the
radial canals. In
some Sponges of this
grade, as in those of
the last described, the
whole endoderm may
consist of flagellate

FIG. 77. Diagram of the canal system of various sponges, the
ectoderm denoted by a continuous narrow line; the flat-
tened endoderm by an interrupted line ; the flagellate
endoderm by short parallel strokes. A, cross-section
through a part of the wall of an Ascon ; ]i, cross-section
through a part of the wall of a Sycon ; C, cross-section
through a part of the wall of L ucilla c<>,, ,-, : xa : J), vertical
section through 0*rr< I in , n, spaces of the incurrent canal
sy-tx-ni ; >/, spaces of the excurreiit canal system ; os. oscu-
lum. (After Korschelt and Heider.)

Ill

PHYLUM AND CLASS PORIFERA

109

collared cells, but in many, as in Sycon, these cells are found
only in the radial canals, and not in the paragastric cavity,
which is lined with flattened cells like those of the ectoderm.
Sponges similar to Sycon gelatinosum, but with Iraiuihiinj flagel-
late canals (Fig. 77, C), afford us the next grade of advancing
complexity. In these the incurrent as well as the flagellate
canals may form a branching system. In all the higher groups
of Sponges (Fig. 77, D, and Fig. 78) the flagellate endock-rni
cells are confined to certain special enlargements of the canals
the so-called "ciliated chambers" C and the rest of the canals HIV
lined by flattened cells.

Special names have been applied to the main types of canal-
system briefly sketched above. Forms in which the paragastric
cavity is lined by flagellate cells are said to belong to the Ascon

In

FIG. 78. Vertical section of a fresh-water sponge (Spongilla), showing the arrangement of the
canal-system. C. ciliated chambers ; DP. dermal pores ; Ex. excurrent canals ; GO. openings
of the' excurrent canals ; PG. paragastric cavity ; SD. subdermal cavities ; 0. osculum.
(Modified from Leuckart and Nitsche's diagrams.)

type, whether the paragastric cavity communicates directly or by
flagellate canals with the exterior. Forms in which there is a
paragastric cavity lined by flattened cells, and a system of radially
arranged flagellate chambers, are said to possess the Sycon type of
structure. Such Sponges as have small rounded flagellate cham-
bers (" ciliated chambers "), communicating in most cases by
narrow branching incurrent canals with the exterior (directly or
indirectly) on the one hand, and by similar excurrent canals with
the paragastric cavity on the other the flagellate cells being
confined to the flagellate chambers are said to possess the Ehagon
type of canal-system.

The development of branches from the originally simple Sponge,
.and the coalescence of neighbouring branches with one another,
greatly obscure the essential nature of the Sponge as a colony of
zooids similar to the branches of Sycon gelatinosum, and thiseffecl
is increased by the development of a variety of infoldings of the

110

ZOOLOGY

SECT.

ectoderm which appear in the higher forms. The oscula dis-
tributed over the surface of the mass may indicate the component
zooids, but these are not always recognisable, being carried inwards
by the infoldings or closed up altogether.

A thicker or thinner specialised outer layer the dermal cortex
situated immediately below the superficial ectoderm, is present
in many Sponges. This is a layer of mesoderm with special
skeletal elements, usually containing spaces and canals lined by
ectoderm suMcrmal cavities (Fig. 78, 8D) which communicate
directly with the exterior, and, internally, usually with more deeply
situated spaces (stibcortical cavities), from which the incurrent canals
lead to the ciliated chambers. This dermal cortex is present,
though not highly developed, in Sycon gelatinosum (Fig. 72, dc),
and the enlarged outer ends of the incurrent canals lying in the
dermal cortex, and closed externally by the pore-bearing mem-
brane, may be regarded as representing dermal cavities. In most
higher sponges a special inner layer is developed; this is the
gastral cortex, represented in a rudimentary form in Sycon gelatino-
sum (Fig. 72, gc.) as the internal layer with special spicules,
in which the excurrent canals are situated.

Histology. In the protoplasmic elements or cells of the various
groups of Sponges there is little variation, except in minor points.

The cells of the ectoderm (Fig.
79) are flattened and form a
syncytimn ; very rarely they as-
sume other forms ; in some cases
each flattened ectodermal cell is
provided with a flagellum. The
endoderm consists of flattened
cells similar to those of the ecto-
derm, or of flagellate collared
cells. In the gelatinous sub-
stance of the mesoderm are em-
bedded connective-tissue cells,
amoeboid wandering cells, and,
in certain positions (around
orifices), muscle cells. Uni-
cellular glands (see p. 22) are present in some sponges, both cal-
careous and siliceous ; also cells containing the pigment to which
the bright colour of many sponges is due, though in most cases
the pigment is not confined to special cells, but occurs scattered
through the connective-tissue cells and flagellate cells. Fresh-
water Sponges are green, owing to the presence of chlorophyll, the
colouring matter to which the prevailing green colour of plants
is due. Sensory cells or nerve cells have been described ; but the
nature of the elements which have been so regarded cannot be
said to have been placed beyond question.

FIG. 79. Cells of the ectoderm, very highly
magnified. (After Von Lendenfeld.)

Ill

PHYLUM AND CLASS PORIFERA

111

The elements of the skeleton differ in character in the different
classes. In the Calcarea they consist of calcareous spicules, usually
triradiate in form. Each of these spicules is developed from a single
cell the sckroblast. In the Non-Calcarea the skeleton either con-
sists of spongin fibres alone (Fig. 80, A), or of siliceous spicules alone,

A.EusJDongia

B. Pachychalina

FIG. SO. Microscopic structure of the skeleton in various sponges. A, Eusponiria network
of spongin fibres; li, Pachychaliiia, spongin strengthened In- sili.vnus .-.pirnlcs: C,
Spongelia, spongin strengthened by various foreign siliceous bodies, fragments of spiculus
of other sponges, &c. (After Vosmaer.)

or of a combination of spongin fibres with siliceous spiciilrs ( 15) : in
some Myxospongise skeletal parts are altogether absent. Spminin
is a substance allied to silk in chemical composition : the fibn s aiv
exceedingly fine threads, consisting of a soft ^ranul;ir core and an
outer tube of concentric layers of spongin. These ilnvads

112

ZOOLOGY

SCT.

and anastomose, or are woven and felted together in such a way as
to form a firm, elastic supporting structure. They are secreted by
the activity of certain cells of the mesoderm, which are called the
spongin-blasts. In certain exceptional cases the spongin assumes
the form of spicules. The siliceous spicules (Fig. 81) are much
more varied in shape than the spicules of the Calcarea, and in a
single kind of Sponge there may be a number of widely differing
forms of spicules, each form having its special place in the skeleton
of the various parts of the Sponge-body. In most Non-Calcarea
siliceous spicules and spongin fibres combine to form the support-
ing framework, the relative development of these two elements
varying greatly in different cases. But in certain groups of the
Non-Calcarea, including the common Washing-sponges (Fig. 80, A),
spicules are completely absent, and the entire skeleton consists of
spongin. In some Non-Calcarea which are devoid of spicules, the

FIG. 81. Various forms of sponge spicules. (From Lang's Text-Book.)

place of these is taken by foreign bodies shells of Radiolaria,
grains of sand, or spicules from other sponges (Fig. 80, C). In
others, again, such as the Venus's Flower-Basket (Eiiphctclla), the
Glass-Rope Sponge (Hyaloncma), and others, the skeleton consists
throughout of siliceous spicules bound together by a siliceous
cement.

Reproduction in the Sponges is effected either sexually or
asexually. The process by which, in all but the simplest forms
of Sponges, a colony of zooids is formed from the originally
simple cylinder or vase, may be looked upon as an asexual
mode of reproduction by budding. Asexual multiplication
also assumes the form in some cases of a process of production
of internal buds in the shape of groups of cells called gemmules,
which eventually become detached and develop into new indi-
viduals. In the Fresh-water Sponges (Spongilliche) multiplication
takes place very actively by means of such gemmules, each of which
is a spherical group of cells enclosed in an envelope composed of
peculiarly shaped siliceous spicules, termed ampliidiscs (Fig. 81,

i" PHYLUM AND CLASS PORIFERA

right side). These gemnmles are formed in the substance of the
Sponge towards the end of the year; they are set free by the
decay of the part of the parent sponge in which they are de-
veloped, and fall to the bottom. In spring the contained mass
of protoplasmic matter reaches the exterior through an aperture-
the micropyh in the wall of the gemmule, and develops into the
adult form.

All Sponges multiply by a sexual process by means of
male cells, or sperms, and female cells, or ova. These arc
developed from certain of the amoeboid wandering cells of the
mesoderm, which take up a special position, usually immediately
below the collared cells of the endoderm. Ova and sperms are
developed in the same Sponge, but rarely at the same time. The
amoeboid cell destined to form sperms divides into a number of
small cells, giving rise to a rounded mass of sperms. The latter,
when mature, have oval or pear-shaped heads and a long tapering
appendage or tail. Each amoeboid cell destined to form an ovum
enlarges, and eventually assumes a spherical form. After a sperm
has penetrated into its interior and effected impregnation, the ovum
usually becomes enclosed in a brood-capsule formed for it by certain
neighbouring cells, and in this situation, still enclosed in the parent
Sponge, it undergoes the earlier stages of its development.

In Sycon the course of the development is as follows. Imme-
diately after impregnation the ovum divides into two cells ; each
of these again divides into two, the plane of the second division
being at right angles to that of the first. A vertical radial fissure
then appears, dividing each of these four cells into two ; so that the
embryo (Fig. 82, b, c) now consists of eight cells, which are of a
pyramidal shape, and arranged in one layer in a radiating manner,
in such a way as to form a flat cone with a central aperture. The
apices of the eight pyramidal cells are next separated off as a
ring of eight small cells from the rest of the cells, which
remain as eight larger cells. The eight small cells are the endoderm
cells, the eight larger are the ectoderm cells. The cells are arranged
so as to form the wall of a sphere the Uastula (Fig. 82, d)
with a central cavity, the ectoderm cells being on one side of the
sphere and the endoderm cells on the other. The endoderm cells soon
increase greatly in number by further division, and remain clear ;
the ectoderm cells divide more slowly, and become granular. The
clear cells become elongated, and flagella are developed at their
outer ends (Fig. 82, e). The granular cells become pushed
inwards so as to be partially enclosed by the clear cells, the space
(segmentation cavity or Nastoccele) in the interior of the blastula he-
coming greatly reduced. In this stage of development termed the
amphiblastula (Fig. 82, e) the embryo Sycon escapes from the
enclosing capsule into the flagellate canal and reaches the exterior.
It is now an oval body consisting of a mass of cells, of whi"h
VOL I. I

114

ZOOLOGY

SECT.

those on the one side are numerous, clear, narrow, arranged
parallel with one another, and provided with cilia at their free
ends ; while those on the other are fewer and larger, of rounded
shape, coarsely granular and devoid of cilia : between these two sets

FIG. 82. Development of Sycon raphanus. a, ovum ; 6, c, ovum segmented 6, as seen from
above, c, lateral view; d, blastula ; c, amphiblastula ; /, commencement of invagination ;.
g, gastrula attached by its oral face ; /(, i, young sponge A, lateral view ; i, as seen from above.
(From Sollas, after Schulze.)

of cells is a cavity in which are a few cells the beginning of the
middle layer. The clear cells next become pushed in or in-
vaginated within the granular cells (Fig. 82, /) so that the embryo
becomes converted into a double-walled cup the gastrula the
outer layer of the wall of which is formed by the granular ectoderm
cells, and the inner by the clear endoderm. The flagella of the

in PHYLUM AND CLASS PORIFERA 115

clear cells disappear at this stage, and the ectoderm cells become
amoeboid and lose their granular character. The opening of the
cup or gastrula the Uastoporc at first a wide opening, soon
becomes narrowed, and eventually closes up completely. A clear
layer containing cells the mesogloaa has now become developed
between the other two, and in this the first spicules become
developed. The embryo has meanwhile become fixed by the side
on which the blastopore was situated (Fig. 82, #), and sooii assumes
a cylindrical form (Fig. 82, h, i). An aperture which is developed
at the free end becomes the osculum, and small perforations in
the sides of the cylinder form the inhalant apertures. As the wall
of the cylinder increases in thickness by the growth of the
mesoglcea the radial canals are formed, the endoderm extending
into them and its cells becoming flagellate.

The amphiblastula type of larva is characteristic of the Calcarea,
and is probably universal in that sub-class, except in such primi-
tive forms as Ascetta. In the latter there is an oval blastula with
a wall composed of a single layer of flagellate cells. From the
posterior pole of the blastula, where the cells are more granular,
cells pass into the segmentation cavity, which they eventually
completely fill. The central mass of cells thus formed gives rise
to the collar-cells of the flagellate chambers, the outer layer to the
flattened ectoderm.

In the Silicispongia?, on the other hand, the typical larva is a
solid body with a superficial layer of ciliated, and an internal mass
of granular cells. From the former, apparently, the collared cells of
the flagellate chambers are formed : from the latter the external
ectoderm and the flattened cells lining the canals. The granular
cells break through the ciliated cells at one end and grow over
the latter as an investing layer. This is a remarkable reversal of
what, as will be seen subsequently, is to be observed in the
Coelenterata and in fact in the rest of the Metazoa, but is readily
reconcilable with what takes place in Sycon and the more complex
Calcarea.

Distribution and Mode of Occurrence of Sponges, and
their Position in the Animal Series. Fossil remains of Sponges
have been found in various formations from those of the Cambrian
period onwards, the greatest abundance being found in the
Chalk. No extinct class or order has been detected, the fossil
forms being all members of existing groups. Some of the orders
of existing Sponges such as the Myxaspongias are incapable of
being preserved as fossils, and the fossil forms belong, as we should
expect, to the more highly silicified Non-Calcarea and to the more
complex groups of the Calcarea.

Fresh-water Sponges (Spongillidce) occur in rivers, canals, and
lakes in all the great divisions of the earth's surface. Marine
Sponges occur in all seas, and at all depths, from the shun

116 ZOOLOGY SECT.

between tide-marks to the deepest abysses of the ocean. The
Calcarea and the true horny sponges (Geratosa) are most abundant
in shallow water, and have not been found below 450 fathoms.
The Sponges found at the greatest depths are members of the
groups Hexactinellida and Choristida of the Non-Calcarea.

Sponges do not appear to be edible by Fishes or even the higher
Crustaceans or Molluscs. Countless lower animal forms, however,
burrow in their substance, if not for food, at least for shelter, and
the interior of a Sponge is frequently found to be teeming with small
Crustaceans, Annelids, Molluscs, and other invertebrates. None of
the Sponges are true parasites. The little Boring Sponge, Cliona,
burrows in the shells of Oysters and other bivalves, but for pro-
tection and not for food. But a Sponge frequently lives in that
close association with another animal or plant to which the term
messmat&ism, or commensalism, is applied, associations which benefit
one or both. Thus some species of Sponge are never found growing
except on the backs or legs of certain Crabs. In these cases the
Sponge protects the Crab and conceals it from its enemies, while the
Sponge benefits by being carried from place to place and thus ob-
taining freer oxygenation. Certain Cirripede crustaceans (members
of the order to which the Barnacles and Acorn-shells belong) are in-
variably found embedded in certain species of Sponge. Frequently
a Sponge and a Zoophyte grow in intimate association, so that they
seem almost to form one structure. Thus the Glass-rope Sponge
(Hyalonema) is always found associated with a Zoophyte (Palythoa),
and there are many other instances. Sponges often also grow in
very close association with certain low forms of plants (Algae).

The position of the Porifera in the animal series is unquestion-
ably among the Metazoa. The view that they are compound
Protozoa is now no longer maintained, since the significance of
the facts of their development has been fully recognised. A
Sponge is to be regarded as a colony of Protozoa only in the sense
in which the same may be said of one of the higher animals. It
consists of a complex of cells, some of which have a considerable
degree of independence, and some of which have a close re-
semblance to certain Protozoa ; but the same is true of one of
the higher animals, the difference being one of degree and not
of kind. Like the rest of the Metazoa, the Sponge develops from
the oosperm by a process of yolk-division.

But the Porifera are perhaps somewhat nearer the Protozoa
than are any of the other types of Metazoa ; and among the
Protozoa they appear to approach nearest to certain colonial
Flagellata. The genus Proterospongia (Fig. 52), already referred
to (p. 73), appears to be the member of the latter group which
of all known forms most closely resembles a sponge. Proterospongia
consists of a colony of collared Flagellates (Choano-Flagellata)
em bedded in a mass of gelatinous substance, in which there are

in PHYLUM AND CLASS PORIFERA 117

also amoeboid zooicls similar to the amoeboid wand* ring- cells of
Sponges.

But, while the Porifera are clearly Metazoa, and not Protozoa,
there is some room for difference of opinion as regards i licit-
relationships to the Coelenterata, with which great type they are
sometimes amalgamated. The reasons for and against such an
arrangement will be discussed in considering the general relation-
ships of the Coelenterata.

SECTION IV
PHYLUM CCELENTERATA

IN the previous section we saw that the simplest type of Sponge
has the general character of a cylinder, closed at one end and open
at the other, and having walls perforated by minute pores, and
composed of three layers ectoderm, mesoglrea, and endoderm, the
last consisting of collared flagellate cells.

In such an organism as this, imagine the pores to disappear,
the internal cavity thus coming to communicate with the exterior
by a single terminal aperture ; the mesoglrea to be replaced by a
very thin structureless layer containing no cells; the endoderm
cells to lose their collars ; and a circlet of arm-like processes, or
tentacles, formed of the same layers as the body-wall, to be
developed around the terminal aperture. The result would be a
polype, and would serve as a type of the general structure of the
group of animals with which we are now concerned.

The most famnar examples of Coelenterata are the horny,
seaweed-like " Zoophytes," or, as they are sometimes called,
" Corallines," to be picked up on every sea-beach, Jelly-fishes,
Sea-anemones, and Corals. The phylum is divided into four classes
as follows :

Class 1. HYDROZOA, including the Fresh-water Polypes, Zoo-
phytes, many Jelly-fishes mostly of small size a few Stony
Corals, and the peculiar Paleozoic fossils known as Graptolitcs.

Class 2. SCYPHOZOA, including most of the large Jelly-fishes.

Class 3. ACTINOZOA, including the Sea-anemones, and the vast
majority of Stony Corals.

Class 4. CTENOPHORA, including certain peculiar Jelly-fishes
known as " Comb-jellies."

CLASS I. HYDROZOA.

1. EXAMPLE OF THE CLASS OMia.

General Structure. Obelia is a common zoophyte occurring
in the form of a delicate, whitish or light brown, almost fur-like

SECT, iv PHYLUM CCELENTERATA 119

growth on the wooden piles of piers and wharfs. It consists of
branched filaments about the thickness of fine sewing-cotton : of
these, some are closely adherent to the timber, and serve for
attachment, while others are given off at right angles, and present
at intervals short lateral branches, each terminating in a bud-like
enlargement.

The structure is better seen under a low power of the
microscope. The organism (Fig. 83) is a colony, consisting of a
common stem or axis, on which are borne numerous zooids. The
axis consists of a horizontal portion, resembling a root or creeping
stem, and of vertical axes, which give off short lateral branches in
an alternate manner, bearing the zooids at their ends. At the
proximal ends of the vertical axes the branching often becomes
more complex : the offshoots of the main stem, instead of ending
at once in a zooid, send off branches of the third order on which
the zooids are borne. In many cases, also, branches are found to
end in simple club-like dilatations (Bd. 1, 2) : these are immature
zooids.

The large majority of the zooids have the form of little conical
structures (P. 1 P. 4), each enclosed in a glassy, cup-like invest-
ment or hydrotlicca (h.tli), and produced distally into about two
dozen arms or tentacles (t) : these zooids are i\\epoly%)cs or hydrantlis.
Less numerous, and found chiefly towards the proximal region of
the colony, are long cylindrical bodies or Uastostyles (bis), each
enclosed in a transparent case, the gonothcca (g.tli), and bearing
numerous small lateral offshoots, varying greatly in form according
to their stage of development, and known as medusa-buds (m.bd). By
studying the development of these structures, and by a comparison
with other forms, it is known that both blastostyles and medusa-
buds are zooids, so that the colony is trimorph-ic, having zooids of
three kinds.

To make out the structure in greater detail, living specimens
should be observed under a high power. A polype is then seen
to consist of a somewhat cylindrical, hollow tody, of a yellowish
colour, joined to the common stem by its proximal end, and pro-
duced at its distal end into a conical elevation, the manubrium or
liypostomc (mnb), around the base of which are arranged the twenty-
four tentacles in a circle. Both body and manubrium are hollow,
containing a spacious cavity, the cntcron (cnt), which communicates
with the outer world by the mouth (mth), an aperture placed at
the summit of the manubrium. The mouth is capable of great
dilatation and contraction, and accordingly the manubrium appears
now conical, now trumpet-shaped. Under favourable circum-
stances small organisms may be seen to be caught by the polypes
and carried towards the mouth to be swallowed.

The hydrotheca (h.tti) has the form of a vase or wine-glass, and
is perfectly transparent and colourless. A short distance from its

Fie. 83. Obelia sp. A, portion of a colony with certain parts shown in longitudinal section;
B, medusa ; C, the same with reversed umbrella ; D, the same, oral aspect : Ed. 1, 2, buds ;
bis. blastostyle ; cce. crenosarc ; ect. ectoderm ; end. endoderm ; ent. enteric cavity ; g.th.
gonotheca; h.th. hydrotheca ; I, lithocyst ; m.bd. medusa-bud; mnb, manubrium ; rnsgl
mesogloea ; ruth, mouth ; p. perisarc ; P. 1, 2, 3, polypes ; rod. c. radial canal ; t. tentacle ,.
vl. velum.

SECT, iv PHYLUM CCELENTERATA

narrow or proximal end, it is produced inwards into a sort of
circular shelf (sA), perforated in the centre: upon this the base of
the polype rests, and through the aperture it is continuous with the
common stem. When irritated by a touch or by the addition of
alcohol or other poison the polype undergoes a very marked con-
traction : it suddenly withdraws itself more or less completely into
the theca, and the tentacles become greatly shortened and curved
over the manubrium {P. 2}.

The various branches of the common stem show a very obvious
distinction into two layers : a transparent, tough, outer membrane,
of a yellowish colour and horny consistency, the perisarc (p), and
an inner, delicate, granular layer, the ccenosarc (cos), continuous
by a sort of neck or constriction with the body of each hydranth.
The coenosarc is hollow, its tubular cavity being continuous with
the cavities of the polypes, and containing a fluid in which a
flickering movement may be observed, 'due, as we shall see, to the
action of cilia. At the base of each zooid or branch the perisarc
presents several annular constrictions, giving it a ringed appear-
ance : for the most part it is separated by an interval from the
coenosarc, but processes of the latter extend outwards to it at
irregular intervals, and in the undeveloped zooids (Bd. 2) the two
layers are in close apposition.

In the blastostyle both mouth and tentacles are absent, the
zooid ending distally in a flattened disc: the hydrotheca of a
pjlype is represented by the gonotheca (g.th), which is a cylindrical
capsule enclosing the whole structure, but ultimately becoming
ruptured at its distal end to allow of the escape of the medusa-
buds. These latter are, in the young condition, mere hollow off-
shoots of the blastostyle : when fully developed they have the
appearance of saucers attached by the middle of the convex
surface to the blastostyle, produced at the edge into sixteen very
short tentacles, and having a blunt process, the manubrium,
projecting from the centre of the concave surface. They are ulti-
mately set free through the aperture in the gonotheca as little
medusae or jelly-fish (B D), which will be described hereafter.

The microscopical structure of a polype (Fig. 84) reminds us,
in its general features, of that of such a simple sponge as Ascetta,
but with many characteristic differences. The body is com )!-,< I
of two layers of cells, the ectoderm (ect) and the endoderm (end) :
between them is a very delicate transparent membrane, the
mesoglcea or supporting lamella (msgl), which, unlike the inter-
mediate layer of sponges, contains no cells and is practically
structureless. The same three layers occur in the manubrium,
the ectoderm and endoderm being continuous with <mr aiu>t : at
the margin of the mouth. The tentacles are formed of an outer
layer of ectoderm, then a layer of mesogloea, and mm 11
core of large endoderm cells arranged in a single scries The

122 ZOOLOGY SECT.

coenosarc, blastostyles, and medusa-buds all consist of the same
layers, which are thus continuous through the entire colony.

The perisarc or transparent outer layer of the stem shows no
cell-structure, but only a delicate lamination. It is, in fact, not a
cellular membrane or epithelium, like the ectoderm and endoderm,
but a cuticle, formed, layer by layer, as a secretion from the ectoderm
cells (see p. 29). It is composed of a substance of chitinoid or horn-
like consistency, and, like the lorica of many Protozoa, serves as a
protective external skeleton. When first formed it is of course in
contact with the ectoderm, but when the full thickness is attained

FIG. 84. Obelia sp. Vertical section of a polype, highly magnified ; ect. ectoderm; end. endo-
derm ; cut. enteric cavity ; h.th. hydrotheca; msp/.-mesogloea ; inth. mouth ; ntc. nematocysts ;
s/j. shelf -like prolongation of hydrotheca ; t. tentacles.

the latter retreats from it, the connection being maintained only
at irregular intervals. In the same way the hydro- and gonothecse
are cuticular products of the polypes and blastostyles respectively :
in the young condition both occur in the form of a closely fitting
investment of the knob-like rudiment of the zooid (Fig. 83, B, 1,2}.
The ectoderm has the general character of a columnar epithelium
(see p. 22), but exhibits considerable differentiation of its component
cells. It is mainly composed of large conical cells with their bases
outwards, and having between their narrow inner ends clumps of
small rounded interstitial cells, and occasional large branched nerve-

IV

PHYLUM CCELENTERATA

123

cells (Fig, 86, nv.c). The tentacles and the manubrium contain,
in addition, a layer of unstriped muscle-fibres between the ectoderm
and the mesogloea : they are arranged longitudinally, and serve
for the rapid shortening of the tentacles (Fig. 86, m.f). This
muscular layer is a derivative of the ectoderm, and may be looked
upon as a rudimentary mesoderm.

nb

FIG. So. Xematocysts of Hydra. A, undischarged ; B, discharged ; C, nerve-supply ; cnb.
cnidoblast ; cue. cnidocil ; nu. nucleus; ntc. nematocyst ; nv.c. nerve-cell. (From Parker's
Biolorry, after Schneider.)

Embedded in the ectoderm are numerous clear ovoid bodies, the
stinging-capsules or nematocysts (Figs. 83 86, ntc), organs closely
resembling those of Epistylis umbellaria (p. 84), and like them
serving as weapons of offence. Each consists (Fig. 85, A) of a tough
ovoid capsule, full of fluid, and invaginated at one end in the form
of a hollow process continued into a long, coiled, hollow thread.
The whole apparatus is developed in an interstitial cell called a
cnidoblast (cnb), which, as it approaches maturity, migrates towards

124

ZOOLOGY

SECT.

nv.c

the surface and becomes embedded in one of the large ectoderm
cells. At one point of its surface the cnidoblast is produced into
a delicate protoplasmic process, the cnidocil or triggcr-liai-r (cnc) :
when this is touched for instance by some small organism
brought into' contact with the waving tentacle the cnidoblast
undergoes a sudden contraction, and the pressure upon the stinging
capsule causes an instantaneous eversion of the thread (B), at the
base of which are minute barbs. The threads are poisonous and

exert a numbing effect on the
animals upon which Obelia
preys.

The endoderm also has the
general character of a columnar
epithelium. In the body of the
polype the cells are very large
and have the power of sending
out pseudopods at their free
ends (Fig. 84), which apparently
seize and ingest minute portions
of the partly-digested food. As
in many Protozoa, the pseudo-
pods may be drawn in and long
tiagella protruded, the contrac-
tion of which causes a constant
movement of the food particles
in the enteron. Amongst these
large cells are narrow cells with
very granular protoplasm : they
are gland-cells, and secrete a
digestive juice. In the inanu-
brium a layer of endodermal

/

muscle-fibres has been described
taking a transverse direction,
and so serving to antagonise
the longitudinal muscles and
contract the cavity. In the
tentacles (Figs. 84 and 86) the
endoderm (end) consists of a

single row of short cylindrical cells, nearly cubical in longitudinal
section: their protoplasm is greatly vacuolated and their cell-walls
so thick that they may be considered as forming a sort of internal
skeleton to the tentacles.

The structure of the Medusae formed as we have seen by the

development of medusa-buds liberated from a ruptured gonangium

yet remains to be considered. The convex outer surface of the

bell or umbrella (Fig. 83, B D) by which the zooid was originally

attached to the blastostyle is distinguished as the ex-umbrella, the

FIG. 86. Tentacle of Eucopella. The
lower part of the figure shows the ex-
ternal surface, in the middle part the
ectoderm is removed, and the muscular
and nervous layer exposed, in the upper
part these latter are removed so as to
show the core of endoderm cells ; ect.
ectoderm ; end. endoderm ; m.f. muscle-
fibres ; ntc. nematocyst ; nu. nucleus ;
nv.c. nerve-cell. (After von Lendeufeld.)

IV

PHYLUM OlELENTERATA

125

concave inner surface as the sub-umbrella. From the centre of
the sub-umbrella proceeds the manubrium (mnb\ at the free end
of which is the four-sided mouth (mth). Very commonly, as the
medusa swims the umbrella becomes turned inside out, the sub-
umbrella then forming the convex surface and the manubrium
springing from its apex (Fig. 83, C, and Fig. 87).

The mouth (Figs. 83, 87, and 88, mth) leads into an enteric
cavity which occupies the whale interior of the manubrium, and
from its dilated base sends off four delicate tubes, the ra<Ii<tt
canals (real, c), which pass at equal distances from each other
through the substance of the umbrella to its margin, where they all
open into a circular canal (circ. c), running parallel with and close
to the margin. By means of this system of canals the food, taken

FIG. ST. Obelia sp. A, mature medusa swimming with everted umbrella ; B, one quarter
of the same, oral aspect ; circ.c. circular canal ; gon. gonad ; I. lithocyst ; mnb. manubrium ;
mth. mouth ; rad. c. radial canal ; t. tentacle. (After Haeckel.)

in at the mouth and digested in the manubrium, is distributed to
the entire medusa.

The edge of the umbrella is produced into a very narrow fold or
shelf, the velum (Fig. 88, vl), and gives off the tentacles (t), which
are sixteen in number in the newly-born medusa (Fig. 83), very
numerous in the adult (Fig. 87). At the bases of eight of the
tentacles two in each quadrant are minute globular sacs (I),
each containing a calcareous particle or lithite. These are the
marginal scn&e-orgcms or lithocysts : they were formerly considered
to be organs of hearing, and are hence frequently called otocysts :
in all probability their function is to guide the medusa by
enabling it to judge of the direction in which it is swimming.
The marginal organs, in this case, may therefore be looked upon
as organs of the sense of direction.

The manubrium (Fig. 88, mnb) of the medusa consists of

120

ZOOLOGY

SECT.

precisely the same layers as that of the hydranth ectoderm,,
mesogloea, and endoderm. The ectoderm is continued on to the
sub-umbrella, and then round the margin of the bell on to the
ex-umbrella, so that both surfaces of the bell are covered with
ectoderm. The endoderm is continued from the base of the
enteric cavity into the radial canals, and so to the circular canal,
so that the whole canal-system is lined by endoderm. In the
portions of the bell between the radial canals there is found,
between the outer and inner layers of ectoderm, a thin sheet of
endoderm, the cndodcrm-lamclla (end. lam), which stretches
between adjacent radial canals and between the circular canal
and the enteric cavity. In the bell, as in the manubrium, a

errfi ' /nrn

i

FIG. 88. Dissection of a medusa with rather more than one-quarter of the umbrella and manu-
brium cut away (diagrammatic). The ectoderm is dotted, the endoderm striated, and the
inesoglcea black ; circ. c. circular canal ; mil. lam. endoderm lamella ; yon. gonad ; /. lithocyst ;
in nil. manubrium; mth. mouth; rod. c. radial canal; -cl. velum.

layer of mesogloea everywhere intervenes between ectoderm and
endoderm.

The velum (vl) consists of a double layer of ectoderm and a
middle one of mesogloea : there is no extension of endoderm into
it. The tentacles, like those of the hydranth, are formed of a
core of endoderm covered by ectoderm, the cells of the latter
being abundantly supplied with stinging-capsules.

Comparison of Polype and Medusa. Striking as is the
difference between a polype and a medusa, they are strictly
homologous structures, and the more complex medusa is readily
derivable from the simpler polype form. It is obvious, in the
first instance, that the apex of the umbrella corresponds with the
base of a hydranth (Fig. 89, A and D), being the part by which
the zooid is attached in each case to the parent stem : the mouth
and the manubrium are also obviously homologous structures in.

IV

PHYLUM CCELENTERATA

127

the two cases. Suppose the tentacular region of a polype to be
pulled out, as it were, into a disc-like form (B), and afterwards to
be bent into the form of a saucer (C) with the concavity distal.

ecf

B

eel

i IQ. 89. Diagrams illustrating the derivation of the medusa from the polype. A, lungitudinal,
and A', transverse section (along the line 06) of polype-form ; B, polype-form with extended ten-
tacular region ; C, vertical, and C', transverse section (along the line ab) of form with tentacular
region extended into the form-of a bell ; D, vertical, and D', transverse section (along the line cd>)
of medusa. The ectoderm is dotted, the endoderm striated, and the mesoglcea black ; dr. <.
circular canal; ect. ectoderm; end endoderm; end. lain, endoderm lamella; >,ni. cav. rut
cavity; hyp. hypostome or manubrium ; ninb. manubrium ; instil, mesoglcea ; mlfi. mouth;
nv. nv', nerve-rings ; t. tentacle ; i: velum. (From Parker's Biology.)

i.e. towards the manubrium. The result of this will be a medusa-
like body (C, C') with a double wall to the entire bell, the narrow
space between the two layers containing a prolongation of the

128

ZOOLOGY

SECT.

enteron (ent. cav'} and being lined with endoderm. From such a
form the actual condition of things found in the medusa would be
produced by the continuous cavity in the bell being for the most
part obliterated by the growing together of its walls so as to form

sub-radius --___
fier-radius

FIG 00. Projections of polype (A) and medusa (B), showing the various orders of radii;
ffon. gonad ; mnb. manubrium.

the endoderm-lamella (D', end. lam), and remaining only along
four meridional areas the radial canals (rad. c), and a circular
area close to the edge of the bell the circular canal (dr. c).

While both polype and medusa are radially symmetrical, the increase in
complexity of the medusa is accompanied by a differentiation of the structures
lying along certain radii. If a polype is projected on a plane surface (Fig. 90, A),

iv PHYLUM CCELENTERATA 129

taken at right angles to its long axis, a large number of radii about twenty-
four can be drawn from the centre outwards, all passing through similar parts,
i.e. along the axis of a tentacle and through similar portions of the body and
manubrium. But in the medusa (B) the case is different. The presence of the
four radial canals allows vis to distinguish four principal radii or per-radii. Half
way between any two per-radii a radius of the second order, or inter-radius, may
be taken ; half way between any per-radius and the inter-radius on either side a
radius of the third order, or ad-radius, and half way between any ad-radius and
the adjacent per- or inter-radius, a radius of the fourth order, or sub-radius. Thus
there are four per-radii, four inter-radii, eight ad-radii, and sixteen sub-radii.
In Obelia the radial canals, the angles of the mouth, and four of the tentacles are
per-radial, four more tentacles are inter-radial, and the remaining eight tentacles,
bearing the lithocysts, are ad-radial. The sub-radii are of no importance in this
particular form.

Reproduction. In the description of the fixed Obelia-colony
no mention was made of cells set apart for reproduction, like
the ova and sperms of a sponge. As a matter of fact, such sexual
cells are found only in their fully developed condition, at least in
the medusae. Hanging at equal distances from the sub-umbrella,
in immediate relation with the radial canal, and therefore per-
radial in position, are four ovoid bodies (Figs. 87 and 88, gon).
each consisting of an outer layer of ectoderm, continuous with
that of the sub-umbrella, an inner layer of endoderm, continuous
with that of the radial canal and enclosing a prolongation of the
latter, and of an intermediate mass of cells which have become
differentiated into ova or sperms. As each medusa bears organs
of one sex only (testes or ovaries, as the case may be), the individual
medusas are dicecious. It will be noticed that the gonad has the
same general structure as an immature zooid an outpushing
of the body-wall consisting of ectoderm and endoderm, and
containing a prolongation of the enteric cavity.

Development. When the gonads are ripe the sperms of the
male medusae are shed into the water and carried by currents to
the females, impregnating the ova, which thus become oosperms
or unicellular embryos. The oosperm undergoes complete seg-
mentation (Fig. 91, A F), and is converted into an ovoidal body
called a plamda (G, H), consisting of an outer layer of ciliated
ectoderm cells and an inner mass of endoderm cells in which a
space appears, the rudiment of the enteron. The planula swims
freely for a time (H), then settles down on a piece of timber, sea-
weed, &c., fixes itself by one end (K), and becomes converted into
a hydrula or simple polype (L, M), having a disc of attachment at
its proximal end, and at its distal end a manubrium and circlet of
tentacles. Soon the hydrula sends out lateral buds, and, by a
frequent repetition of this process, becomes converted into the
complex Obelia-colony with which we started.

This remarkable life-history furnishes the first example we have
yet met with of alternation of generations, or metagenesis (see p. 39).

K

130

ZOOLOGY

SECT.

The Obelia-colony is sexless, having no gonads, and developing
only by the asexual process of budding ; but certain of its buds
the medusae develop gonads, and from their impregnated eggs

FIG. 91. Stages in the development of two Zoophytes (A H, Laomedea I M, Euclen-
driuni i allied to Obelia ; A P, stages in segmentation ; G, the planula enclosed in the
maternal tissues ; H, the free-swimming planula ; I M, fixation of the planula and develop-
ment of the hydrula. (From Parker's Biology, after Allmau.)

new Obelia-colonies arise. We thus have an alternation of an
asexual generation, or agamobium the Obelia-colony. with a
sexual generation, or gamobium the medusa.

2. GENERAL STRUCTURE AND CLASSIFICATION.

The Hydrozoa may be defined as multicellular animals in which
the cells are arranged in two layers, ectoderm and endoderm,
separated by a gelatinous, non-cellular mesogloea, and enclosing
a continuous digestive cavity which communicates directly with
the exterior by a single aperture the mouth and is lined through-
out by endoderm. The ectoderm consists of epithelial cells, inter-
stitial cells, muscle-fibres, and nerve cells. Certain of the inter-
stitial cells give rise to characteristic organs of offence the
stinging-capsules. The endoderm consists of flagellate or amoeboid
cells, gland-cells, and sometimes muscle-fibres. There are two
main forms of zooids, polypes or nutritive zooids, which are
usually sexless, and medusae or reproductive zooids. In corre-
spondence with its locomotive habits, the medusa attains a higher

iv PHYLUM CCELENTERATA 131

degree of organisation than the polype, having more perfect
muscular and nervous systems, distinct sense organs, and a diges-
tive cavity differentiated into central and peripheral portions, the
latter taking the form of radial and circular canals. The repro-
ductive products are discharged externally, and are very commonly,
though not always, of ectodermal origin.

Many Hydrozoa agree with Obelia in exhibiting alternation of
generations, the asexual generation being represented by a fixed,
more or less branched hydroid colony, the sexual generation by a
free-swimming medusa. In other forms there are no free medusae,
but the hydroid colony produces fixed reproductive zooicls. In
others, again, there is no hydroid stage, the organism existing only
in the medusa-form. Then, while in most instances the only
skeleton or supporting structure is the horny perisarc, there are
some forms in which the coenosarc secretes a skeleton of calcium
carbonate, forming a massive stony structure or cored. Lastly,
there are colonial forms which, instead of remaining fixed, swim
or float freely on the surface of the ocean, and such pelagic species
are always found to exhibit a remarkable degree of polymorphism,
the zooids being of very various forms and performing diverse
functions.

Thus we have zoophyte colonies known to produce free medusae,
zoophyte colonies known not to produce free medusae, and medusas
known to have no zoophyte stage. Moreover, there are many
medusae of which the life-history is unknown, so that it is un-
certain whether or not a zoophyte stage is present. It is also
found that in some cases closely allied zoophytes produce very
diverse medusa?, while similar medusa?, in other cases, may spring
from very different zoophytes. For these reasons a sort of double
classification of the Hydrozoa has come about, some zoologists
approaching the group from the point of view of the zoophyte,
others from that of the medusa. On the whole the following
scheme seems best adapted for bringing before the beginner the
leading modifications of the class.

ORDER 1. LEPTOLIN.E.

Hydrozoa in which there is a fixed zoophyte stage, and in which
the sense organs are exclusively ectodermal.

Sub-Order a. Antliomedusce.

Leptolina? in which the polypes are not protected by hydrothecse or the
reproductive zooids by goiiothecce : the medusas bear the gonads in the manu-
brium and have no lithocysts.

Suit-Order l>. Leptom eelusec.

Leptolinse in which hydro- and gonotheca? are present : the medusae bear the
gonads in connection with the radial canals and usually have lithocysts.

K 2

132 ZOOLOGY SECT.

ORDER 2. TRACHYLIN^E.

Hydrozoa in which no fixed zoophyte stage is known to occur,
all members of the group being locomotive medusae, some of which
have been proved to develop directly from the egg. The sense
organs are formed partly of endoderm.

Sub- Order a . Trachymedusce.

Trachylinse in which the tentacles spring from the margin of the umbrella,
and the gonads are developed in connection with the radial canals.

Sub-Order b. Narcomedusce.

Trachylinse in which the tentacles spring from the ex-umbrella, some dis-
tance from the margin, and the gonads are developed in connection with the
manubrium.

ORDER 3. HYDROCORALLINA.

Hydrozoa in which a massive skeleton of calcium carbonate is
secreted from the ccenosarc, the dried colony being a coral.

ORDER 4. SIPHONOPHORA.

Pelagic Hydrozoa in which the colony usually exhibits extreme
polymorphism of its zooids.

ORDER 5. GRAPTOLITHIDA.

An extinct group of Hydrozoa, found only in rocks of palaeozoic
age, in the form of the fossilised perisarc of the branched colonies.

Systematic Position of the Example.

Obelia, in virtue of the possession of gono- and hydrothecse, and
of gonads formed in connection with the radial canals, belongs to
the sub-order Leptomedusae. It is placed in the family Campanu-
lariidcv, distinguished by having cup-shaped thecas borne at the
ends of distinct branchlets: the genus Obelia is distinguished
from other genera of the same family by the fact that the
reproductive zooids are free-swimming medusae.

ORDER 1. LEPTOLIX^E.

The more typical members of this group agree in all essential
respects with Obelia, consisting of branched colonies bearing two
principal forms of zooids, which serve for nutritive and reproductive
purposes respectively.

General Structure. The form and size of the colonies are
subject to great variation : they may be little insignificant tufts
growing on shells, sea- weeds, &c., or may take the form of com-
plex trees three feet in height, and containing many thousand

iv PHYLUM CCELENTEEATA 133

zooids, The hydranths may be colourless and quite invisible to
the naked eye, or, as in some Tubularise (Fig. 93, 5), may be bril-
liantly coloured, flower-like structures, nearly an inch in diameter.
The medusae may be only just visible to the naked eye, or, as in
jfflquorea, may attain a diameter of 38 mm., or about 15 inches :
they are often seen with great difficulty owing to the bubble-like
transparency of the umbrella, but frequently the manubrium is
brightly coloured, or brilliant dots of colour the ocelli or eye-spots
may occur around the margin of the umbrella. They are also
frequently phosphorescent, the phosphorescence of the ocean being
often due to whole fleets of medusae liberated in thousands from
the hydroid colonies beneath the surface.

The two sub-orders of Leptolinae are distinguished by the
arrangement of the perisarc. In the Anthomedusee, of which
Bougainvillea (Fig. 92) is a good example, the cuticle stops short at
the bases of the hydranths, and the reproductive zooids are not
enclosed in gonothecae. It is for this reason that, in classifications
founded on the zoophyte stage, the Anthomedusae are called Gymno-
blastca or naked-budded zoophytes (see also Fig. 93, 1, 4, 5}. In
the Leptomedusse the cuticle is usually of a firmer consistency than
in the first sub-order and furnishes hydrothecae for the hydranths
and gonothecae for the reproductive zooids : they are hence often
classified as Calyptoblastea or covered-budded hydroids. To this
group belong the commonest species of hydroids found on the sea-
shore, and often mistaken for sea-weeds, the " Sea-firs " or Sertu-
larians.

The medusae also exhibit characteristic differences in the two
sub-orders. In the Anthomedusoe the umbrella is usually strongly
arched, and may even be conical or mitre-shaped (Fig. 93, 7 ; Fig.
96, 1 and 2) : its walls are thick owing to a great development of
the gelatinous mesogloea of the ex-umbrella, that of the sub-umbrella
remaining thin : and the velum is considerably wider than in Obelia,
But the most important characteristics are the facts that the
gonads (gori) are developed in the manubrium and that lithocysts are
absent. Sense organs are, however, present in the form of specks
of red or black pigment at the bases of the tentacles. These ocelli
(pc) consist of groups of ectoderm cells containing pigment, and it
has been proved experimentally that they are sensitive to light :
they are, in fact, the simplest form of eyes. In the Leptomedusaej|
the umbrella is usually less convex, thinner, and of softer consist-
ency than in the Anthomedusse, the gonads are developed as buds
formed in connection with the radial canals and projecting from
the sub-umbrella, the velum is feebly developed, and sense organs
take the form sometimes of ocelli, but usually of lithocysts.

In the majority of Leptolince the coenosarc, as in Obelia,
consists of a more or less branched structure attached to stones,
timber, seaweeds, shells, &c., by a definite root-like portion. The

134

ZOOLOGY

SECT. IV

curious genus Hydractinia (Fig. 93, 1} is remarkable for possessing
a massive ccenosarc, consisting of a complex arrangement of
branches which have undergone fusion so as to form a firm
brownish crust on the surfaces of dead gastropod shells inhabited
by Hermit-crabs. The constant association of Hydractinia with

FIG. Pi. Bougainyillea ramosa. A, entire colony, natural size ; B, portion of the same
magnified; C, immature medusa; dr. c. circular canal; <<>. cuticle or perisarc ; tut. car.
enteric cavity; hi/il. polype or hydrant h ; /<///>. hypostome or manubrium ; -uttd. medusa ; ///'/.
uianubrimii ; /<!'. c. radial canal; t. tentacle; v, velum. '(From Parker's Biology, after
Allman.)

Hermit-crabs is a case of commensalism : the hydroid feeds upon
minute fragments of the Hermit-crab's food, and is thus its com-
mensal or messmate, and the Hermit-crab is protected from its
enemies by the presence of the inedible, stinging hydroid.
Hydractinia belongs to the Anthomedusse : the Leptomedusan

p

'.:

,\V .-

a

1

l>

ffcf|fe|:

%:);;; :f ;:;

b f"f ,'@:

>/!. ii

',' / -'. .^- F

'i'' : *
MUs

a

,v'lK;,

1. HydracMnia

mfh

^,-'i^

/??

/.,-{<

m b vW

<-tK ^P

i < "^^

1 ^

2.Myriorhela

-ninb 3. Corymorpha

'^

. Clavafella

FIG. 93. Various forms of Leptolinse. In 1, a shows the entire colony, I a portion highly
magnified ; in 7, is a species producing medusa-buds from the manubrium, b from the bases
of the tentacles ; d2, dactylozooids ; -m. and M. meduste ; mnb. manubrium ; mtlt. mouth ;
oc. eye-spots ; rad. c. radial canals ; . sporosacs ; s;;. spines ; t, ft, t-, tentacles.

136

ZOOLOGY

SECT.

Clatlirozoon, an Australian genus, resembles it in having branched
and intertwined coenosarcal tubes, the perisarc of which under-

msyt

' ' ' ' ' ' SCALE FOB A

"155" "> ">

ntc

cnc

FIG. 94. Hydra. A, vertical section of entire animal; B, portion of transverse section, highly
magnified ; C, two large ectoderm cells ; D, endoderm cell of H. viridis ; E, large nematocyst ;
F, small nematocyst ; G, sperm ; a, ingested diatom ; bd. 1, bd. ^buds ; chr. chrouiatophores ;
cnbl. cnidoblast ; cue. cnidocil ; ect. ectoderm ; end. endoderm ; ent. cav. enteric cavity ; ent.
cav'. its prolongation into the tentacles ; fl. fiagellum ; hyp. hypostome or manubriiun ; int. c.
interstitial cells ; m. pr. muscle processes ; ruth, mouth ; insyl. mesoglcea ; ntc. large, and ntc'.
small nematocysts ; nu. nucleus; of. ovum; ory. ovary ; psd. pseudopods ; spy. spermary,
vac. vacuole.

goes fusion, but the complex mass thus produced, instead of
forming an incrustation on a shell, is a large, abundantly branched,

iv PHYLUM CCELENTERATA 137

tree-like structure, resembling some of the fan-corals or Gorgonacea
(vide infra).

A great simplification of the colony is produced in Mi/riothela
(Fig. 93, 2) in which the short coenosarc bears a single large
terminal hydranth, and gives off numerous slender branches which
bear the reproductive zooids (s). Even greater simplicity is found
in Corymorpha (3), in which the entire organism consists of a
single stalked polype, from the tentacular region of which the
medusae (in) arise.

But the simplest members of the whole class, with the exception
of one or two imperfectly known forms which will be referred to
below, are the Fresh-water Polypes of the genus Hydra. The entire
organism (Figs. 24 and 94) consists of a simple cylindrical body
with a conical hypostome and a circlet of six or eight tentacles.
It is ordinarily attached, by yirtue of a sticky secretion from the
proximal end, to weeds, &c., but is capable of detaching itself
and moving from place to place after the manner of a looping
caterpillar. The tentacles are hollow, and communicate freely
with the enteron. There are no distinct muscle-fibres, but the
large ectoderm cells are produced into muscle processes (C, in. pr)
which serve the same functions. There is no perisarc. Buds
(bd. 1, ltd. 2) are produced which develop into Hydree, but these are
always detached sooner or later, so that a permanent colony is
never formed. There are no special reproductive zooids, but
simple ovaries (ovy) and testes (spy) are developed, the former at
the proximal, the latter at the distal end of the .body. Even
simpler than Hydra are Protohydra (Fig. 95) and Microhydra, in
which the tentacles are absent.

!

FIG. 95. Protohydra leuckartii. (From Chun, after Greeff.) The mouth is to the left, the

disc of attachment to the right.

The polypes are usually cylindrical, as in Obelia, but in some
genera they are widened out into a vase-like form (Fig. 93, 5), in
others elongated into a spindle-shape (4). The tentacles may be
disposed in a single circlet, as in Obelia and Hydra, or there may
be an additional circlet round the hypostome (3, 5) or at the base of
the polype, or they may be scattered irregularly over the whole
surface (4). In Myriothela (2) they are short and so numerous
as to have the appearance of close-set papilla?. In some forms
they are knobbed at the ends, the knobs being loaded with stinging-
capsules (4).

In some species a dimorphism of the hydranths obtains, some
of them being modified to form protective zooids. In Hydractinia

138 ZOOLOGY SECT, iv

(1) these are simply mouthless hydranths with very short ten-
tacles abundantly supplied with nematocysts, capable of very active
movements, and called dactylozooids (dz). In Plumularia there
are small structures called " guard-polypes," resembling tentacles
in structure, and each enclosed in a theca. In Hydractinia the
coenosarc is also produced into spines (sp), which may be much
modified zooids.

But the most remarkable modifications occur in the reproduc-
tive zooids. In a large proportion of genera, both of Anthome-
dusa? and LeptoinedussB, these take the form of locomotive medusa?,
agreeing in general structure with the descriptions already given,
but exhibiting endless variety in detail. As to size they vary
from about 1 mm. in diameter up to 400 mm. (16 inches). The
number of tentacles may be very great (Fig. 96, 2) or these
organs may be reduced to two (Fig. 96, 1), or even to one
(Fig. 93, 8} ; in the last-named cases it will be noticed that the
medusa is no longer radially, but bilaterally symmetrical, i.e.
it can be divided into two equal and similar halves by a single
plane only, viz., the plane passing through the one or two
tentacles. With the increase in the number of the tentacles a
corresponding increase in that of the radial canals often takes
place (Fig. 96, 3).

Some medusa? creep over submarine surfaces, walking on the
tips of their peculiarly modified tentacles (Fig. 93, 6} but the
majority propel themselves through the water in a series of jerks
by alternately contracting and expanding the umbrella, and so,
by rhythmically driving out the contained water, moving with
the apex foremost. In correspondence with these energetic move-
ments there is a great development of both muscular and
nervous systems. The velum and the sub-umbrella possess
abundance of muscle-fibres, presenting a transverse striation,
and round the margin of the umbrella is a double ring of nerve-
cells and fibres, one ring being above, the other below the at-
tachment of the velum (Fig. 89, D, nv, nv'}. The medusa? thus
furnish the first instance we have met with of a central nervous
systci.i, i.e. a concentration of nervous tissue over a limited area
serving to control the movements of the whole organism. It has
been proved experimentally that the medusa is paralysed by
removal of the nerve-ring. Over the whole sub-umbrella is a
loose network of nerve-cells and fibres connected with the nerve-
ring, and forming a peripheral nervous system.

In some medusas the circular canal communicates with the
exterior by minute pores placed at the summits of papilla?, the
endoderm cells of which contain brown granules. There seems to
be little doubt that these are organs of excretion, the cells with-
drawing nitrogenous waste matters from the tissues and passing
them out through the pores. If we except the contractile

140

ZOOLOGY

SECT.

vacuoles of Protozoa, this is the first appearance of specialised
excretory organs in the ascending series of animals.

Besides producing gonads, some medusae multiply asexually by
budding, the buds being developed either from the manubrium
(Fig. 93, 7), or from the margin of the umbrella (7&). The buds
always have the medusa form.

In many Leptolina? the reproductive zooids undergo a degrada-
tion of structure, various stages of the process being found in
different species. Almost every gradation is found, from perfect
medusa?, to ovoid pouch-like bodies called sporosacs (Fig. 93, 11),
5, s), each consisting of little more than a gonad, but showing an in-
dication of its true nature in a prolongation of the digestive cavity
of the colony, representing the stomach of the manubrium (Fig. 97).
We thus have a reproductive zooid reduced to what is practically
a reproductive organ. It is obvious that a continuation of the

as-

FIG. 97. Diagram illustrating the formation of a sporosac by the degradation of a medusa. A,
medusa enclosed in ectodermal envelope (es) ; B, intermediate condition with vestiges of
umbrella () and radial canals (ra) ; C, Sporosac, ec, ectoderm ; m, endoderm ; //;, manubrium ;
ov, ovary ; t, tentacle ; i; velum. (From Lang's Comparative Anatomy.)

same process might result in the production of a simple gonad
like that of Hydra : there is, however, no evidence to show that
the Fresh-water Polype ever produced medusa?, and the probabili-
ties are that its ovaries and testes are simply gonads, and not
degenerate zooids. The case is interesting as showing how a
simple structure may be imitated by the degradation of a com-
plex one. It is quite possible, on the other hand, that the
reproductive organs of the Leptomedusas (Fig. 88) are sporosacs,
i.e. reproductive zooids, not mere gonads.

In Obelia we found the medusae to be budded off from pecu-
liarly modified mouthless zooids the blastostyles. This arrange-
ment, however, is by no means universal : the reproductive zooids
whether medusas or sporosacs may spring directly from the
coenosarc, as in Bougainvillea (Fig. 92), or from the ordinary
hydranths (Fig. 93, 4 and o). The primitive sex-cells, from which
ova or sperms are ultimately developed, are sometimes' formed

IV

PHYLUM CCELENTERATA

141

from the endoderm or (more usually) ectoderm cells of the gonad ;
but in many cases originate in the coenosarc, and slowly migrate
to their final destination in the gonad, where they metamorphose,
in the usual way, into the definitive reproductive products.

The development of the Leptolinae frequently, but not always,
begins within the maternal tissues, i.e. while the oosperm or im-
pregnated egg-cell is still contained in the gonad of the medusa or
in the sporosac. The oosperm divides into two cells, then into
four, eight, sixteen, &c. Fluid accumulates in the interior of the
embryo, resulting in the formation of a blastula or hollow globe
formed of a single layer of cells (Fig. 97, A). The blastula
elongates, and the cells at one pole undergo division, the daughter-
cells passing into the cavity, which they gradually fill (B). At

FIG. 98. Early development of Eucope. A, blastula<-stage ; B, plauula with solid eudoderm ;
C, planula with enteric cavity ; al. enteric cavity ; ep. ectoderm ; Inj. endoderm. (From
Balfour's Embryology, after Kowalevsky.)

this stage the embryo is called a plamda : it consists of an outer
layer of cylindrical cells the ectoderm which acquire cilia, and an
inner mass of polyhedral cells the endoderm. In some cases the
planula arises by a different process : a solid morula is formed, the
superficial cells of which become radially elongated and form
ectoderm, the central mass of cells becoming endoderm. By
means of its cilia the pianula swims freely, and before long a
cavity appears in the middle of the solid mass of endoderm, the
cells of which then arrange themselves in a single layer around
the cavity or enteron (C, al}. The planula then comes to rest, fixes
itself at one end to some suitable support, and becomes con-
verted into a simple polype or liydrula by the attached end
broadening into a disc and the opposite extremity forming a
manubrium and tentacles. The hyclrula soon begins to send off
lateral buds, and so produces the branched colony.

142

ZOOLOGY

SECT.

In Tubularia the oosperm develops, while still enclosed in
the sporosac, into a short hydrula, which, after leading a free
existence fora short time, fixes itself by its proximal end, buds, and
produces the colony. In Hydra development begins in the ovary,
and is complicated by the fact that the ectoderm of the morula
gives rise to a sort of protective shell : in this condition the
embryo is set free, and, after a period of rest, develops into the
adult form.

ORDER 2. TRACHYLIX^E

General Structure. The members of this order are all
medusae : no zoophyte stage is certainly known in any of them, and
several species have been proved to develop directly from the egg.

FIG. 99. Two Trachy medusae, dr. c. circular canal; rion. gonad ; mnl>. inniml>rium ; mth.
mouth ; rad. c. radial canal ; re. c. recurrent canal ; t. tentacle ; tc. tentaculocyst ; ty. torigue ;
ill. velum. (After Haeckel.)

They thus differ from the members of the preceding order in the
fact that there is no alternation of generations in their life-
history.

Most species are of small or moderate size, the largest not
exceeding 100 mm. (4 inches) in diameter. The gelatinous tissue
or mesoglcea of the ex-umbrella is usually well developed, giving
the medusa a more solid appearance than the delicate jelly-fish of
the preceding order : this is well shown in Fig. 99, in which the
apical region of the umbrella has a comparatively immense thick-
ness. The tentacles are also stiff and strong, and are always solid

IV

PHYLUM CCELENTERATA

143

in the young condition, although they may be replaced in the
adult by hollow tentacles.

But the most characteristic anatomical feature of the group is
the structure of the sense-organs, which are club-shaped bodies

rad.c

mth
l.Cunarcha

2.Polycol|ja

PIG. 100. Two Narcomedusae, 2 in vertical section, (/on. gonad ; mnb. maiiubriuui : mth.
mouth; jir. peronium ; t. tentacle; tc. teutaculocyst ; t.r. tentacle-root; el. velum. (After
Haeckel.)

(Figs. 99 and 100, tc) consisting of an outer layer of ectoderm
enclosing a central axis of endoderm cells (Fig. 101) : they have,
therefore, the structure of tentacles. They contain one or more
lithites, which are always
derived from the endoderm.
To distinguish them from the
lithocysts of Leptomedusse,
and to mark the fact that
they are modified tentacles,
they are called tentaculocysts.
They may either project
freely from the margin of the
umbrella, or may become en-
closed in a pouch-like growth
of ectoderm and more or less
sunk in the tissue of the
umbrella.

The two sub-orders of
Trachylinse are characterised
by the mode of origin of
the tentacles. In Trachy-
medusas, as in the preceding

order, they arise near the edge of the umbrella (Fig. 99), but in
the Narcomedusse they spring about half-way between the edge
and the vertex (Fig. 100), and are continued, at their proximal

-end

. 101. .Siginura myosura, a tentaculo-
cyst highly magnified, ect. ectoderm; end.
endoderm ; I. lithites ; ntc. nematocysts ;
nv.c. group of nerve-cells. (After Haeckel).

144 ZOOLOGY

SECT.

ends, into the jelly of the ex-umbrella in the form of " tentacle-
roots ; ' (t.r).

As to the position of the reproductive organs, there is the
same difference between the two sub-orders of Trachylinae as
between the two sub-orders of Leptolinse. In the Trachymedusae
the gonads (Fig. 99, gon) are developed in the course of the radial
canals : in the Narcomedusae (Fig. 100) they lie in the manubrium,
sometimes extending into pouch-like offshoots of its cavity.

There is always a well-developed velum, which, as in Fig. 100, 1.
may hang down vertically instead of taking the usual horizontal
position. In the Narcomedusse the manubrium is short ; in the
Trachy medusae it is always well developed, and is sometimes (Fig.
99, B) prolonged into a long, highly contractile peduncle, having
its inner surface produced into a tongue-like process (tg) which
protrudes through the mouth.

The simplest case of the development of Trachylinae is seen in
is, one of the Narcomedusse. The oosperm gives rise to

FIG. 102. Larva of .ffiginopsis. m. mouth ; t. tentacle. (From Balfour, after iletsclinikoff.)

i

a ciliated planula, which forms first two (Fig. 102), then four
tentacles, and a mouth, hypostome, and stomach. The larva of
zEginopsis is thus a hydrula, closely resembling the corresponding
stage of Tubularia. After a time the tentacular region grows out,
carrying the tentacles with it, and becomes the umbrella of the
medusa. Thus the actual formation of the medusa from the
hydrula of /Eginopsis corresponds precisely with the theoretical
derivation given above (p. 127). It will be seen that in the present
case there is no metagenesis or alternation of generations, but that
development is accompanied by a metamorphosis that is, the egg
gives rise to a larval form differing in a striking manner from the
adult, into which it becomes converted by a gradual series of
changes.

Metagenesis is, however, not quite unknown among the Trachy-
linse. In a parasitic Narcomedusa (Cunina parasitica) the planula

IV

PHYLUM CCELENTERATA

145

fixes itself to the manubrium of one of the Trachymedusse which
serves as its host, and develops into a hydrula. But the latter, in-
stead of itself becoming metamorphosed into a medusa, retains the
polype form and produces other hydrulse by budding, these last
becoming converted into medusas in. the usual way.

ORDER 3. HYDROCORALLINA.

The best-known genus of Hydroid Corals is Millepora, one species
of which is* the beautiful Elk-horn Coral , M. alcicornis. The dried
colony (Fig. 103 A) consists of an irregular lobed or branched mass

FIG. 103. Millepora alcicornis. A, part of skeleton, natural size; B, portion of surface,
magnified ; C, vertical section, magnified ; d.p. dactylopores ; g.p. gastropores ; tb. tabulae.
(After Nicholson and Lydekker.)

of carbonate of lime, the whole surface beset with the numerous
minute pores to which the genus owes its name. The pores are
of .two sizes : the larger are about 1 or 2 mm. apart, and are called
gcistroporcs (B, y.p) ; the smaller are arranged more or less
irregularly round the gastropores, and are called dactylopores (d.p).
The whole surface of the coral between the pores has a pitted
appearance. Sections (C) show that the entire stony mass is
traversed by a complex system of branched canals, which com-
municate with the exterior through the pores. The wide vertical
canals in immediate connection with the gastropores are traversed
by horizontal partitions, the tabulcc- (tb).

In the living animal each pore is the place of origin of a zooid :
from the gastropores protrude polypes (Fig. 104, P) with hypostome

146

ZOOLOGY

SECT.

and four knobbed tentacles; from the dactylopores long, filamentous,
mouthless dactylozooids or feelers (D.Z), with irregularly disposed
tentacles : the function of these latter is probably protective and
tactile, like that of the guard-polypes of Phimularia and the
dactylozooids of Hydractinia. The bases of the zooids are con-
nected with a system of delicate tubes, which ramify through the
canals of the coral, and represent a much-branched coenosarc,
recalling that of Hydractinia (p. 134).

ccf

PIG. 104. Millepora. Diagrammatic view of a portion of the living animal, partly from the
surface, partly in vertical section. In the sectional part the ectoderm is dotted, the endoderm
striated, and the skeleton black, cct. ectoderm ; end. endoderm ; il.p. dactylopore ; D.Z.
dactylozooid ; g.p. gastropore ; rut It. mouth ; P. polype ; t. tentacle. (Altered from Moseley.)

The coenosarcal tubes have the usual structure, consisting of
ectoderm and endoderm, with an intervening mesogloea. From
the relative position of the parts it will be obvious that the cal-
careous skeleton is in contact throughout with the ectoderm of the
colony : it is, in fact, like the horny perisarc of the Leptolinae, a
cuticular product of the ectoderm.

The only other genus to which we shall refer is Stylastcr (Fig.
105), which forms a remarkably elegant tree-like colony, abund-
antly branched in one plane, and of a deep pink colour. On the

IV

PHYLUM CCELENTERATA

147

branches are little cup-like projections, with radiating processes
passing from the wall of the cup towards the centre, and thus
closely resembling the true cup-corals belonging to the Actinozoa
(vide infra). But in the case of Stylaster each " cup " is
the locus, not of one, but of several zooids : a polyp projecting
from its centre, and a dactylozooid from each of the compartments
of its peripheral portion.

The gonads of Millepora are formed in small capsules, occurring
in the course of the ccenosarcal canals ; in Stylaster there are

B

FIG. 105. Stylaster sanguineus. A, portion of skeleton, natural size ; B, small portion,
magnified ; a. ampullte ; d.j>. dactylopores ; ;/.^;. gastropores. (After Nicholson and Lydekker.)

sporosacs or degraded reproductive zooids lodged in special cham-
bers () of the coral.

The Hydrocorallina occur only in the tropical portions of the
Pacific and Indian Oceans, where they are found on the " coral
reefs " partly or entirely surrounding many of the islands in those
seas. Fossil forms are found as far back as the Triassic epoch.

ORDER 4. SIPHONOPHORA.

The diversity of form exhibited by the members of this order is
so great that anything like a general account of it would only be
confusing to the beginner, and the most satisfactory method of
presentation will be by the study of a few typical genera.

Halistemmd (Fig. 10G A) occurs in the Mediterranean and other

seas, and consists of a long, slender, floating stem, to which a

number of structures, differing greatly in form, are attached. At one

the uppermost end of the stem is an ovoid, bubble-like bodv con-

L 2 '

B

FIG. 100.

. 100. Halistemraa tergestinum. A, the entire colony ; B, a single group of zooids ;
ex. ccenosarc; dz. dactylozooid ; hjih. hydrophyllium or bract; net. nectocalyx or swimming-
bell : ntc. battery of uematocysts ; p. polype: pn. pueuinatophore or float: . '. sporocysts ;
t. tentacle. (After Glaus.)

SECT, iv PHYLUM CCELENTERATA 149

taining air the float or pneumatophore (pn}. Next come a number
of closely set, transparent structures (net), having the general char-
acters of unsymmetrical medusne without manubria, each being a
deep, bell-like body, with a velum and radiating canals. During life
these &wimming-bells or ncdocali/ccs contract rhythmically i.e. at
regular intervals drawing water into their cavities, and immedi-
ately pumping it out, thus serving to propel the entire organism
through the water. Below the last nectocalyx the character of the
structures borne by the stem changes completely : they are of
several kinds, and are arranged in groups which follow one
another at regular intervals, and thus divide the stem into seg-
ments, like the nodes and internodes of a plant.

Springing from certain of the " nodes " are unmistakable polypes
(p), differing however from those we have hitherto met with in
having no circlet of tentacles round the mouth, but a single long
branched tentacle () arising from its proximal end, and bearing
numerous groups or " batteries " of stinging-capsules (ntc). In
the remaining nodes the place of the polypes is taken by dactylo-
zooids or feelers (dz) mouthless polypes, each with an unbranched
tentacle springing from its base. Near the bases of the polypes
and dactylozooids spring groups of sporosacs (B, s, s'), some male,
others female ; and finally delicate, leaf-like, transparent bodies
the bracts or hydropliyllia (lipli) spring from the "internodes" and
partly cover the sporosacs.

It is obvious that, on the analogy of such a hydroid polype as
Obelia, HaHstemma is to be looked upon as a polymorphic floating
colony, the stem representing a coenosarc, and the various struc-
tures attached to it zooids the polypes nutritive zooids, the
feelers tactile zooids, the sporosacs reproductive zooids, the bracts
protective zooids, and the swimming-bells locomotory zooids. The
float may be looked upon as the dilated end of the stem, which
has become invaginated or turned-in so as to form a bladder
filled with air, its outer and inner surfaces being furnished by
ectoderm, and the middle portion of its wall by two layers of
endoderm, between which the enteric cavity originally extended
(Fig. 107, pn). The upper or float-bearing end is proximal i.e.
answers to the attached end of an Obelia-stem : it is the opposite
or distal end which grows and forms new zooids by budding.

In some Siphonophora the bracts contain indications of radial
canals, so that these structures, as well as the swimming-bells
and sporosacs, are formed on the medusa-type, while the hydranths
and feelers are constructed on the polype-type.

It will be noticed that the radial symmetry, so characteristic
of most of the Hydrozoa previously studied, gives way, in the
case of HaHstemma, to a bilateral symmetry. The swimming-bells
are placed obliquely, and the mouth of the bell is not at right
angles to the long axis, so that only one plane can be taken

150

ZOOLOGY

SECT.

dividing these structures into two equal halves : the same applies
to the polype and feelers with their single basal tentacle. When
first formed the various zooids are all on one side of the stem, but

fin

net

FIG. 107. Diagram of a Siphonopbore : the thick line represents endoderm ; the space ex-
ternal to it, ectoderm : the internal space, the enteric cavity, cce. ccenosarc ; </;. dactylozooid ;
hph. hydrophyllium ; ,>ct. net', neetoealyces ; ntc. battery of ncmatocyst.s ; ;' polype; pn.
pneumatophore ; t tjntaeb. (After Clans.)

the latter becomes spirally twisted during growth, and so causes
them to arise irregularly.

The egg of Halistemma gives rise to a ciliated planula re-
sembling that of the other Hydrozoa. At one pole the ectoderm
becomes invaginated to form the float (Fig. 108, cp), the opposite
extremity is gradually converted into the first polype (po), and

IV

PHYLUM CCELENTERATA

151

a bud appears on one side which becomes the first tentacle (t).
By gradual elongation, and the formation of new zooids as lateral
buds, the adult form is produced ; the various zooids are all
formed between the first polype and the float, so that the two
become further and further apart, being always situated at the
distal and proximal ends of the colony respectively.

In an allied form (Agalma) the first structure to appear in the embryo is not
the float, but the first bract, which grows considerably and envelops the growing
embryo in much the same way as the umbrella of a medusa envelops the manu-
brium. On this and other grounds some zoologists look upon the Siphonophore-
colony as a medusa the manubrium of which has extended immensely and
produced lateral buds after the manner of some Anthomedusoe (Fig. 93, 7 a).

FIG. 10S. Two stages iu the development of Halistemma : the endoderm is shaded, the
ectoderm left white, ep. pneumatocyst or air-chamber of pneumatophore ; hy. endoderm
surrounding pneumatophorc ; po. polype ; pp. pneumatophore ; t. tentacle. (From Balfour,
after Metschuikoff.)

On this theory the entire ccenosarc is an extended manubrium, and the first or
primary bract is the umbrella. But frequently as in Halistemma a primary
bract is not formed, and when present there appears to be no reason against
regarding it as a lateral bud of the axis, of quite the same nature as the remaining
zooids.

In the well-known " Portuguese man-of-war " (Physalia) there
is a great increase in proportional size of the float and a corre-
sponding reduction of the rest of the coenosarc. The float (Fig.
109, pn) has the form of an elongated bladder, from 3 to 12 cm.
long, pointed at both ends, and produced along its upper edge
into a crest or sail (cr) : as a rule it is of a brilliant peacock-blue
colour, but orange-coloured specimens are sometimes met with.
At one end is a minute aperture communicating with the exterior.
There are no swimming-bells, but from the under size of the float
hang polypes (p), feelers, groups of medusoids looking like bunches

152

ZOOLOGY

SECT.

of grapes of a deep blue colour, and long retractile tentacles,
sometimes several feet in length and containing batteries of
stinging-capsules powerful enough to sting the hand as severely
as a nettle. The male reproductive zooid remains attached, as in

.--

V

y

f /

:

FIG. 109. Physalia : the living animal floating on the surface of the sea. <v. crest : j>. polype ;
pn. pneumatophore. (After Huxley.)

Halistemma, but the female apparently becomes detached as a
free medusa.

In Diphyes the float is absent. Two swimming-bells (Fig. 110, m }
of proportionally immense size are situated at the proximal end of
the coenosarc, and are followed by widely-separated groups of

IV

PHYLUM CCELENTERATA

153

zooids (), each group containing a polype (n) with its tentacles ('),
a medusoid (g), and a large enveloping bract (t). The stem often
breaks at the internodes, and the detached groups of zooids then
swim about like independent organisms.

Porpita is formed on a different type, and has a close general
resemblance to a medusa. It consists (Fig. Ill) of a -discoid

Parker's Bioloijn, after Gegeubaur.)

body, enclosing a chambered chitinoid shell (sli) containing air, and
obviously corresponding with the float of Physalia. The edge of
the disc is beset with long tentacles (t), and from its lower surface
depend numerous closely set feelers or dactylozooids (hy'} and blasto-
styles bearing medusae, while in the centre is a single polype (7w/),

154

ZOOLOGY

SECT.

which is the only nutritive zooid, taking in food for the entire
colony. The closely allied genus Velclla is of rhomboidal form,
and bears on its upper surface an oblique sail.

The reproductive zooids are liberated as free medusae. The
eggs give rise to young which have a close resemblance to flat
medusa? with maiiubrium, marginal tentacles, and an air-chamber
or float developed in the ex-umbrella. Thus it is quite possible
that the Siphonophora of the Porpita-type may be medusae the
sub-umbrella of which has given rise to buds forming the feelers

FIG. 111. Porpita pacifica. A, from beneath; B, vertical section; 7i.;/. polype ; Inf. dactylo-
zooids ; sit. chambered shell; t. tentacles. (From Parker's Biology, after Duperry and
Koelliker.)

and blastostyles. But, as their early development is not known, it
is still quite legitimate to describe them in the same terms as the
other Siphonophora /.!-'. to consider them as hydroid colonies in
which the coanosarc is represented by the discoid or rhomboid
bodv with its contained air-chamber.

ORDER 5. GRAPTOLITHIDA.

The " Graptolites " are fossil Hydrozoa found in the Upper Cambrian and
Silurian rocks. They are known only by their fossilised chitinoid skeleton, all
tract- of tlu- soft parts having, as in the majority of fossils, disappeared.

IV

PHYLUM CCELENTERATA

155

With one doubtful exception they are compound, consisting of an elongated
tube, the perisarc of the common stem, having attached to it, either in a single
or a double row, numerous small projections, the hydrothecw (Fig. 112, h.th).
The ccenosarcal skeleton is strengthened by a
slender axis, the vinjnla (v), the proximal end of
which, is connected with a small dagger-shaped body,
the .iirula (?), supposed to be the skeleton of the
primary zooid by the budding of which the colony,
was produced. In connection with some species
oval or cup-like capsules have been found : these
may probably be of the nature of gonothecte.

ADDITIONAL REMARKS ox THE HYDROZOA.

The vast majority of Hydrozoa are
marine, the only exceptions being Hydra,
found all over the world ; Microhydra, at
present known only in North America ;
Cordyloplwra, one of the Anthomedusse,
found in Europe, America, Australia, and
New Zealand ; Polypodium, also an Antho-
medusa, found in the Volga, where in
one stage of its existence it is parasitic on
the eggs of a Sturgeon; Limnocodium, a
doubtful Trachymedusa, hitherto found
only in a tank in the Botanical Gardens,
Regent's Park, where it was probably in-
troduced from the West Indies ; and Limnocnida, found in
Lake Tanganyika, Africa.

The oldest known Hydrozoa are the Graptolites, found first in
the Cambrian rocks ; Hydractinia occurs in the Cretaceous epoch,
and Hydrocorallinas from the Cretaceous onwards.

Parasitism, although rare, is not unknown in the class. Poly-
podium, one of the AnthomedusaB, is parasitic during part of its
existence, in the ovary of the Sturgeon ; and Cunina, one of the
Narcomedusa?, is parasitic on a Trachymedusa.

In the section on the Protozoa we saw that while the majority
of species are independent cells, each performing alone all the
essential functions of an animal, others, such as Pandorina,
Volvox, and Proterospongia, consist of numerous unicellular
zooids associated to form a colony in which a certain division of
labour obtains, the function of reproduction, for instance, being
assigned to certain definite cells and not performed by all alike.
Thus the colonial Protozoa furnish an example of individual ion,
numerous cells combining to form a colony in which the several
] cuts are dependent one upon another, and which may therefore
be said to constitute, from the physiological point of view, an
individual of a higher order than the cell.

FIG. 112. Graptolites.

A, Monograptus colonus ;

B, DimorpJiograptus, both
magnified ; hy. ill. hydro-
theca ; *. siculi ; v. vir-
gula. (After Nicholson
and Lydekker.)

15G ZOOLOGY SECT, iv

This is still more notably the case in the lower Metazoa, such
as Ascetta and Hydra, in which we have numerous cells combined
to form a permanent two-layered sac with a terminal aperture,
some of the' cells having digestive, others tactile, others repro-
ductive functions. Thus while an Amceba or a Paramoecium is
an individual of the first order, Hydra and Ascetta are individuals
of the second order, each the equivalent of an indefinite number of
individuals of the first order.

In the Hydrozoa we see this process carried a step further.
Budding takes place and colonies are produced, the various zooids
of which each the equivalent of a Hydra instead of remaining
all alike, become differentiated both morphologically and physio-
logically, so as to differ immensely from one another both in form
and function. In Obelia, for instance, reproduction is made over
exclusively to the medusw, while in Halistemma we have zooids
specially set apart, not only for reproductive, but for tactile and
protective purposes. Thus in Halistemma and the other Siphono-
phora there is a very complete subordination of the individual
zooids to 'the purposes of the colony as a whole, the colony thus
assuming, from the physiological point of view, the characteristics
of a single individual, and its zooids the character of organs. In
this way we get an individual of the third order, consisting of an
aggregate of polymorphic zooids, just as the zooid or individual
of the second order is an aggregate of polymorphic cells or
individuals of the first order.

CLASS II. SCYPHOZOA.

1. EXAMPLE OF THE CLASS THE COMMON JELLY-FISH

(Aurelia aurita).

Aurelia is the commonest of the larger jelly-fishes and is often
found cast up on the sea-shore, when it is readily recognisable by
its gelatinous, saucer-shaped umbrella, three or four inches in
diameter, and having near the centre four red or purple horseshoe-
shaped bodies the gonads lying embedded in the jelly.

External Characteristics. The general arrangement of the
parts of the body is very similar to what we are already familiar
with in the hydrozoan jelly-fishes (Figs. 113 and 114, A). Most
conspicuous is the concavo-convex umbrella, the convex surface of
which, or ex-umbrella, is uppermost in the ordinary swimming-
position. The outline is approximately circular, but is broken by
eight notches, in each of which lies a pair of delicate processes,
the marginal lappets (my. lp} : between the pairs of lappets the
edge of the umbrella is fringed by numerous close-set marginal
tentacles (t).

<z on

B

FIG. 113. Aurelia aurita. A, dorsal view, part of the ex-umbrella cut away to show part of
the stomach and one of the four gastric pouches ; B, ventral view two of the oral anus are
removed ; .?. c. adradial canal ; g. /. gastric filaments ; cinn. gonads ; 17. p. gastric pouch ;
i.v. c. inter-radial canal ; my. J-p. marginal lappet ; mth. mouth ; or. a. oral ann ; p.r. c. per-
radial canal ; s.g. p. sub-genital pit ; st. stomach ; t. tentacles.

158 ZOOLOGY SECT.

In the centre of the lower or sub-umbrellar surface is a four-
sided aperture, the mouth (mtli), borne at the end of an extremely
short and inconspicuous manubrium: surrounding it are four long
delicate processes, the oral arms (or. a), lying one at each angle
of the mouth and uniting around it. Each arm consists of a
folded membrane, tapering bo a point at its distal end, beset
along its edges with delicate processes, and abundantly provided
with stinging-capsules. The angles of the mouth and the arms
lie in the four per-radii, i.e. at the end of the two principal axes
of the radially symmetrical body : of the marginal notches with
their lappets, four are per-radial and four inter-radial.

At a short distance from each of the straight sides of the
mouth, and therefore inter-radial in position, is a nearly circular
aperture leading into a shallow pouch, the' sub-genital pit (s.g. p),
which lies immediately beneath one of the conspicuously coloured
gonads (gon).

Digestive Cavity and Canal-System. The mouth leads by
a short tube or gullet (gul), contained in the manubrium, into a
spacious stomach (st\ which occupies the whole middle region of
the umbrella, and is produced into four wide inter-radial gastric
pouches (g. p), which extend about half-way from the centre to
the circumference and are separated from one another by thick
pillar-like portions of the umbrella-jelly. In the outer or peri-
pheral wall of each gastric pouch are three small apertures,
leading into as many radial, canals, which pass to the edge of
the umbrella and there unite in a very narrow circular <//<//
(circ. c). The canal, which opens by the middle of the three
holes, is of course inter-radial (i.r. c) : it divides immediately
into three, and each division branches again : the canals from the
other two holes are adradial (a.r. c), and pass to the central canal
without branching. There is also an aperture in the re-entering
angle between each two gastric pouches: this leads into a per-
radial canal (p.r. c), which, like the inter-radial, branches
extensively on its way to the edge of the umbrella,

The general arrangement of the cell-layers in Aurelia is the
same as in a hydroid medusa (Fig. 114, B). The main mass of
the umbrella is formed of gelatinous mesoglcea, Avhich, however,
is not structureless, but is traversed by branching fibres and
contains amoeboid cells derived from the endoderm. Both ex-
and sub-umbrellse are covered with ectoderm, and the stomach and
canal-system are lined with endoderm, which is ciliated through-
out. Some observations seem to show that the short tube
described above as a gullet is lined, not by endoderm, but by
an in-turned portion of the ectoderm, as we shall see to be the
case in Actinozoa and Ctenophora ; but this matter cannot be
considered as definitely settled.

It was mentioned above that in the free medusa the gonads

IV

PHYLUM CCELENTERATA

appear through the transparent umbrella as coloured horseshoe-
shaped patches. Their precise position is seen by cutting away a
portion of the ex-umbrella so as to expose one of the gastric
pouches from above (Fig. 113, A). It is then seen that the
gonad (ffon) is a frill-like structure lying on the floor of the
pouch and bent in the form of a horse-shoe with its concavity
looking inwards, i.e. towards the mouth. Being developed from

'0/t

FIG. 114. Aiirelia aurita. A, side view, one-foil thof the umbrella cut away ; B. diagrammatic
vertical section, ectoderm dotted, endoderm striuted, mesoglcea black ; circ. c. circular canal ;
g.f. gastric filaments ; r/on. gonad ; rj. p. gastric pouch ; <7/. gullet ; h. hood ; ;'./. c. inter-radial
canal ; mrj. lp. marginal lappet ; mth. mouth ; or. a. oral arm ; s.y. p. sub-genital pit ; st.
stomach.

the floor of the enteric cavity, the gonad is obviously an
endodermal structure : when mature, its products ova or sperms
are discharged into the stomach and pass out by the mouth.
Here, then, is an important difference from the Hydrozoa, in
which the generative products are usually ectodermal, and are
always discharged directly on the exterior. The sexes are lodged
in distinct individuals.

loo

ZOOLOGY

SECT.

Lying parallel with the inner or concave border of each gonad
is a row of delicate filaments (g. /), formed of endoderm with a
core of mesogloaa and abundantly supplied with stinging-capsules.
These are the gastric filaments ; their function is to kill or
paralyse the prey taken alive into the stomach. No such endo-
dermal tentacles are known in the Hydrozoa.

Muscular and Nervous Systems. The contractions of the
bell by which the animal is propelled through the water are
effected by means of a muscular zone round the edge of the sub-
umbrella. The nervous system is formed on a different plan
from that of the hydroid medusae. Instead of a double nerve-ring
round the margin of the umbrella, there are eight groups of nerve-
cells in connection with the marginal notches. The nerve-cells
lie between the bases of the epithelial cells, and external to the
muscular layer : they are obviously ectodermal structures.

The sense organs are lodged in the marginal notches in close
relation with the nerve-patches : like the latter, therefore, four of

B

oe

FIG. 115. Aurelia aurita. A, small portion of edge of umbrella, showing the relations of the
tentaculocyst ; B, vertical section of the same region (diagrammatic) ; h, hood ; I, lithite ;
///:/. I)*, marginal lappet ; oc, ocellus ; olf. 1, olf. 2, olfactory pits. (Altered from Lankester.)

them are per-radial and four inter-radial. Each consists of a
peculiar form of sense-club or tentaculocyst, containing a prolonga-
tion of the circular canal, and thus representing a hollow instead
of a solid tentacle. At the extremity are calcareous concretions
or lithites (/) derived from the endoderm, and on the outer side
is an ectodermal pigment-spot or ocellus (oc). The tentaculocysts
are largely hidden by the marginal lappets (nig. Ip) and by a
hood-like process (k) connecting them ; and in connection with
each are two depressions, one on the ex-umbrella (olf. 1), the other
immediately internal to the sense-club (olf. 2) : these depressions
are lined with sensory epithelium and are called olfactory pits.

The development and life-history of Aurelia present several
striking and characteristic features. The impregnated egg-cell

iv PHYLUM CCELENTERATA 101

or oosperm divides regularly and forms a morula, which, by accumu-
lation of fluid in its interior, becomes a blastula a closed sac with
walls formed of a single layer of cells. One end of this sac becomes
invaginated to form the gastrula. The blastopore or gastrula-
mouth closes, the embryo being converted into a closed two-
layered sac or planula (Fig. 116, A), indistinguishable from that of
a Hydrozoon, although formed by a totally different process.

The planula swims about by means of the cilia with which its
ectodermal cells are provided, and, after a brief free existence,
settles down, loses its cilia, and becomes attached by one pole.
At the opposite pole a mouth is formed, the process taking place
by a sinking-ill or invagination of the surface so as to produce a
depression lined 'Ivith ectoderm (B, st.), the .bottom of which
becomes perforated so as to communicate with the enteric cavity
(C, st.) : the depression is the stomodceum, a structure of which
there is no trace in the Hydrozoa. On two opposite sides of the
mouth hollow processes grow out, forming the first two tentacles :
soon two others appear at right angles to these, the organism
thus being provided with four per-radial tentacles. Subsequently
four inter-radial and eight adradial tentacles appear. At the
same time the attached or proximal end is narrowed into a stalk-
like organ of attachment (E), and the endoderm of the enteric
cavity is produced into four longitudinal ridges, inter-radial in
position, and distinguished as the gastric ridges or tcvniolcs (D, tn.).
The mouth (E, intli.) assumes a square outline, and its edges become
raised so as to form a short manubrium (mnb.\ and, finally, the
ectoderm of the distal surface i.e. the region lying between the
mouth and the circlet of tentacles becomes invaginated in each
inter-radius so as to produce four narrow funnel-like depressions
the septal funnels or infundibula (E andF, s. /.) sunk in the four
gastric ridges.

The outcome of all these changes is the metamorphosis of the
planula into a polype (E), not unlike a Hydra or the hydrula-stage
of the Leptolina3, but distinguished by a pronounced differentia-
tion of structure, indicated by the sixteen tentacles developed in
regular order, the stomodaeum, and .the four gastric ridges with
their septal funnels. The Scyphozoon-polype is called a Scyphula
or Scyphistoma.

The Scyphula may grow to a height of half an inch, and some-
times multiplies by budding. After a time it undergoes a process
of transverse fission (G), becoming divided by a series of constric-
tions which deepen until the polype assumes the appearance of a
pile of saucers, each with its edge produced into eight bifid lobes,
four per- and four inter-radial. Soon the process of constriction
is completed, the saucer-like bodies separate from one another,
and each, turning upside down, begins to swim about as a small
jelly-fish called an Ephyrula (H, I). The umbrella of the ephyrula
VOL i M

162

ZOOLOGY

SECT.

is divided into eight long bifid arms (a.) with deep (adradial)
notches : it has of course carried away with it a segment of the
stomach with the gastric ridges of the Scyphula : during the process

Fio. 116. Aurelia aurita, development. A, planula ; B, C, formation of stomodseum ; D,
transverse section of young Scyphula ; E, Scyphula ; F, longitudinal section of same : the
section passes through a per-radius on the left of the dotted line, through an inter-radius on
the right ; G, division of Scyphula into ephyrulre ; H, ephyrula from the side ; L, the same
from beneath. In A D and F the ectoderm is unshaded, the endoderm striated, and the
mesoglcea dotted, a. lobes of umbrella; nmb. manubrium ; mth. mouth; s.f. septal funnel;
st. stomodseum ; t. tentacle ; in. tsenioles. (From Korschelt and Heider's Embryology.)

of constriction this becomes closed in on the proximal or ex-
umbrellar side, while on the sub-umbrellar side it remains open,
and its edges grow out to form a manubrium. On each gastric

IV

PHYLUM CCELENTERATA 163

ridge appears a single gastric filament, soon to be followed by
others, and in the notches at the extremities of the eight arms
tentaculocysts make their appearance. In the meantime the
spacious enteric cavity is continued into the eight arms in the
form of wide radiating canals.

-As the ephyrula grows the adradial regions at first deeply
notched grow more rapidly than the rest, the result being that
the notches become gradually filled up, and the umbrella, from an
eight-rayed star, becomes a nearly circular disc. Four oral arms are
developed, and numerous marginal tentacles, and the ephyrula
gradually assumes the form of the adult Aurelia. It seems
probable that the sub-genital pits of the medusa are formed
from sections of the septal funnels of the Scyphula.

Thus the life-history of Aurelia differs in several marked
respects from that of any of the Hydrozoa. There is an alternation
of generations, as in Obelia, the gamobium being represented by
the adult Aurelia, the agamobium by the Scyphula. But instead
of the medusa being developed either as a bud on a branched
colony, as in Leptolinas, or by direct metamorphosis of a polype,
as in "Trachylina?, it is formed by the metamorphosis of an ephyrula
developed as one of several transverse segments of a polype.

It has been shown that, under exceptional circumstances, the
egg of Aurelia develops directly i.e. without the interposition of
a Scyphula-stage into the adult medusa. As we shall see, this is
the normal mode of development of many allied forms.

2. GENERAL STRUCTURE AND CLASSIFICATION.

The Scyphozoa may be defined as medusoid Coelenterata, having
the same general structure and arrangement of the layers as the
medusoid Hydrozoa, but differing from them in the possession of
endodermal gastric tentacles; in having endodermal gonads dis-
charging their products into the digestive cavity ; and, in nearly
all cases, by the absence of a velum, and in the presence of sense-
organs in the form of hollow sense-clubs or tentaculocysts. How
far a stomodEeum or ectodermal gullet is characteristic of the
group is uncertain. As in the Hydrozoa, the medusa develops
directly from the egg in some species, while in others there is an
alternation of generations, a polype-form (agamobium) giving rise
to the medusa-form (gamobium) by a process of transverse fission.
In the majority of cases, however, nothing is known of the life-
history, the process of development having been worked out only
in a few cases.

As far as is known the segmenting embryo gives rise to a gastrula
by imagination : by the closure of the blastopore a planula is
produced, at one end of which a second invagination takes place,
forming the stomodseum.

si '2

164 ZOOLOGY SECT.

The Scyphozoa are divisible into four orders, as follows :

ORDER 1. STAUROMEDUS^E.

Scyphozoa having a conical or vase-shaped umbrella, sometimes
attached to external objects by an ex-umbrellar peduncle : no
tentaculocysts.

ORDER 2. PEROMEDUS^E.

Scyphozoa having a conical umbrella divided by a transverse
constriction : four inter-radial tentaculocysts.

ORDER 3. CUBOMEDUS.E.

Scyphozoa with a four-sided cup-shaped umbrella: our per-
radial tentaculocysts.

ORDER 4. DISCOMEDUS.E,

Scyphozoa with a flattened saucer- or disc-shaped umbrella :
not fewer than eight tentaculocysts, four per- and four inter-radial.

Sub-Order a. Cannostomce.

Discomedusce with a simple square mouth devoid of oral arms.

Sub-Order b. Scmostomce.

Discomedusa? in which the square mouth is produced into four long oral
arms.

Sub- Order c. Rliizostorn a\

Discomedusae having the mouth obliterated by the growth across it of the
oral arms : the stomach is continued into canals which open by funnel-shaped
apertures on the edges of the arms.

Systematic Position of the Example.

Aurelia aurita is one of several species of the genus Aurelia,
and is placed in the family Ulmarida', the sub-order Semostomcc,
and the order Discomedusce.

Its saucer-shaped umbrella and eight tentaculocysts place it at
once among the DiscomedusaB : the presence of a distinct mouth
surrounded by four oral arms excludes it from the first and third
sub-orders of Discomedusas and place it in the second sub-order or
Semostoma?. The latter group contains six families, characterised
mainly by differences in the canal-system : the Ulmarida? are
distinguished by narrow branched radial canals opening into a
circular canal. Of the seven genera in this family, Aurelia stands
alone in having its tentacles attached on the dorsal or ex-umbrellar
side of the margin, and in the oral arms showing no trace of bi-
furcation. Eight species of Aurelia are recognised, A. aurita
being distinguished by having the oral arms slightly shorter

IV

PHYLUM CCELENTERATA

105

than the radius of the umbrella, and by having a trichotomous
inter-radial canal and two unbranched adradial canals springing
from each gastric pouch.

ORDER 1. STAUROMEDUS.E.

The simplest member of this order, and indeed of the whole class of Scyphozoa,
is Tessera (Fig. 117), a small medusa about 4 mm. in diameter. It is interesting

i.r.

FIG. 117. Tessera princeps. A, external view; B, vertical section ; g.f. gastric filament;
gon. gouad ; -i.r. t. inter-radial tentacle; mnb. manubrium ; mth. mouth; p.r. t. per-radial
tentacle ; st. stomach ; tn. tsemole. (After Haeckel.)

as having the same general characters as the Scyphula-stage of Aurelia, except
that the bell-shaped body is free-swimming. The edge of the umbrella is
surrounded by eight tentacles, four per- (j).r. t) and four inter-radial (i.r. t.), and
movement is effected by a well-developed system of circular and radial muscles.
The simplicity of the genus is well shown in the total absence of sense-organs.
The manubrium (mnb.) leads into a spacious stomach (tit.), from which four wide
per-radial pouches are continued into the umbrella, and are connected with one
another by a spacious cavity passing round its margin and called the circular
sinus. There are only four gastric filaments (g.f.), one springing from each of the
four inter-radial gastric ridges or tsenioles (tn. ). The gonads (gon. ) are horseshoe-
shaped, with their concavities directed towards the margin of the umbrella.

166

ZOOLOGY

SECT.

Lucernaria (Fig. 118), a genus not uncommon on the British coasts, is in one
respect even more like a Scj'phula, since it is attached by a peduncle developed
from the centre of the ex -umbrella. The margin of the umbrella is prolonged
into eight short hollow adradial arms, bearing at their ends groups of short
adhesive tentacles (t. ). As in the Scyphula, each gastric ridge contains an

FIG. 118. Lucernaria. A, oral aspect ; B, from the side ; g. foot-gland ; 17. /. gastric filaments
j707i. gonad; ,nth. mouth ; t. tentacles ; tn. taenioles. (After Glaus.)

infundibulum, lined with ectoderm and opening on the sub-umbrella. The
gastric filaments (g. f. ) are very numerous a distinct advance on Tessera and
the gonads (gon.) are band-like. There are no sense-organs in Lucernaria, but
in an allied genus degenerate tentaculocysts are present.

ORDER 2. PEROMEDUS^E.

This group includes a small number of rai'e and beautiful Medusae of curiously
complex structure, of which Pericolpa. may be taken as an example. The umbrella
(Fig. 119) is always conical, and is divided by a horizontal furrow into an apical
region or cone (en. ) and a marginal region or crown ; the crown is again divided
by a second, rather irregular horizontal furrow into a series of pedal lobes (pd. I.),
adjacent to the cone, and a series of marginal lappets (mcj. lp.), forming the free
edge of the bell.

Four of the pedal lobes, inter-radial in position, bear tentaculocysts (tc.), four
others, per-radially situated, give origin to long, hollow tentacles (t.). In the
more complex genera there are eight additional adradial tentacles.

The mouth (mth.) is very large, and leads by a wide manubrium (mnb.)into a
spacious stomach (st. ), which is continued quite to the apex of the cone. In the
wall of the stomach are four wide per-radial slits, leading into an immense
circular sinus (circ. .$.). As in Lucernaria, there are four wide inter-radial in-
fundibula. The gastric filaments (<j. f. } are very numerous, and the elongated
U-shaped gonads (yon.) are eight in number and adradial.

ORDER 3. CUBOMEDUS.K.

The Jelly-fishes forming this order are, as the name implies, of a more or less
cubical form, resembling a deep bell with somewhat flattened top and square
transverse section. They resemble the hydrozoan Medusae more than any of the
other Scyphozoa. The best known species, Charybdcea marsupial is (Fig. 120), is
about 5 cm. in diameter and of very firm consistency.

IV

PHYLUM CCELENTERATA

167

As in the lower Peromedusa?, the margin of the umbrella bears four ten-
tacles (t. ) and four tentaculocysts (f<: ), but the position of these organs is reversed,
the tentaculocysts being per- radial, the tentacles inter-radial. The tentaculo-
cysts are set in deep marginal notches, and the tentacles spring from conspicuous

Fie. ll'.i. Pericolpa quadrigata. A, external view; B, vertical section; c!rc. s. circular
sinus ; en. cone; ft. f. gastric filaments; <ii>. y<>n:uls; UKJ. Iji. marginal lappets; m,il>. manu-
brium ; ml//, mouth; /'. t. pedal lobes; st. stomach; t. tentacles; tc. tentaculocysts; tn.
taenioles. (After Haeckel.)

gelatinous lobes (/. ), which probably answer to the pedal lobes of the preceding
order.

The margin of the umbrella is produced, in most cases but not in all, into a
horizontal shelf (rl. ), resembling the velum of the hydroid Medusa?, biit differing
from it in containing a series of branched vessels (end. lam'.} continuous with the
canal-system, and of course lined with endoderm. In the Hydrozoa, it will be
remembered, the velum is formed simply of a double layer of ectoderm with a
supporting layer of mesogkea. Owing to this fundamental difference, the velum-
like organ of the Cubomedusae is distinguished as the rdarium.

1G8

ZOOLOGY

SECT.

The mouth is situated at the end of a short manubrium (mnl>.) leading into a
wide stomach, from which go off four very broad per-radial pouches (rad. p-),
occupying the whole of the four fiat sides of the umbrella, and separated from one
another by narrow inter-radial septa or partitions placed at the four corners.
These pouches are equivalent to wide radial canals, and the partitions between

gon.

B

end lam

arc.

cul.fi

.yon

rrtnL

FIG. 120. Cnarybdsea marsupialis. A. side view of the entire animal ; B. vertical section
passing on the left side through an inter-radius, 011 the right through a per-radius ;
C, transverse section ; </',<. <. circular canal ; < </</. ln.m. endoderrn lamella ; /"/. Inm'. its pro-
longation into the velarium; (i. /. gastric filaments; ;i<>,>. gonacl ; <//>'. septum separating
gonads ; !. lappet; >in>li. rnaiiuliriuni ; /</.;>. radial pouch; t. tentacle; ti: tontaculocyst ;
?/. velarium. (After Glaus, somewhat altered.)

them to a poorly developed endoderm lamella (m<1. 7a/.). At the margin of
the umbrella the pouches communicate with one another by apertures in the
septa, so that a kind of circular canal is produced (r/'/r. <.). JJear the junction
of the gastric pouches with the stomach are the usual four groups of gastric
filaments (;/. f. ).

The gonads (r/on.) are four pairs of narrow plate-like organs, attached one
along each side of each inter-radial septum. The nervous system takes the form

IV

PHYLUM CCELENTERATA

1G9

of a sinuous nerve-ring round the margin of the bell, bearing a distinct group of
nerve-cells at the base of each tentaculocyst and tentacle. The Cubomedusse are
the only Scyphozoa which, like the Hydrozoa, have a complete nerve-ring. The
tentaculocysts are very complex, each bearing a lithocyst and several eye-spots.

ORDER 4. DISCOMEDUS^E.

The preceding orders are all small ones, i.e. include a small number of genera
and species. The vast majority of Scyphozoa belong to the present order the
" Disc-jellies " or " Sea-blubbers " as ordinarily understood.

The umbrella is always comparatively flat, having the form of an inverted
saucer. The edge is produced primitively into eight pairs of marginal lappets,
but in some of the more highly differentiated forms the number both of lappets
and of tentaculocysts becomes greatly increased. Many of the species belonging

FIG. 121. Nausithoe. The entire animal from the oral aspect, cu: adradii ; g. gonads ; r/.f.
gastric filaments ; <Y. inter-radii ; //<. circular muscle of sub-umbrella ; pr. per-radii ; rl. tenta-
culocysts ; sr. sub-radii ; t. tentacles. The black cross in the centre represents the mouth.
(From Lang's Comparative Anatomy.)

to the lowest sub-order the Cannostomse are small, not exceeding a few
millimetres in diameter, but most of the Semostomae and Rhizostomte are large,
and one of the former group Ci/anea arctica may attain a diameter of 2 metres
and upwards, while its marginal tentacles reach the astonishing length of
40 metres, or about 130 feet. But in spite of their size and apparent solidity,
the amount of solid matter in these great Jelly-fishes is extraordinarily small ;
some of them have been proved to contain more than 99 per cent, of sea-water.

The marginal tentacles are short and solid in the Cannostomre (Fig. 121),
hollow and often of great length in the Semostomse (Fig. 113), and altogether
absent in the Rhizostomse (Fig. 122). The oral arms are absent in the Canno-
stoma 1 (Fig. 121), where there is a single square or four-rayed mouth : in the
Semostomtf there are four oral arms (Fig. 113, or. a.), each resembling a leaf

170

ZOOLOGY

SECT.

folded along its midrib, and having more or less frilled edges : in the Rhizostoma-
each of the original four arms (Fig. 122, or. a.) becomes divided longitudinally in
the course of development, the adult members of the group being characterised
by the presence of eight arms, often of great length, and variously lobed and
folded so as to present a more or less root-like appearance.

The arrangement of the enteric cavity and its offshoots presents an interest-
ing series of modifications. In the Cannostomse (Fig. 121) the resemblance to the
Ephyrula-stage of Aurelia is very close, the stomach giving oft eight pouches
which bifurcate and enter the marginal lappets. In the Semostoma? (Fig. 113)
the stomach lobes give off well-defined radial canals, which are frequently more
or less branched, often unite into complex networks, and sometimes open into a
circular canal round the margin of the umbrella.

In the Rhizostomaj (Fig. 122, B) a similar network of canals is found in the
umbrella, but an extraordinary change has befallen the oral or ingestive portion

st

ff'f

-FiG 122. Pilemapulmo. A, side view of the entire animal ; B, vertical section, diagrammatic :
C, one of the suctorial mouths, magnified ; c. arm canal ; g.f. gastric filaments ; gon. gonads ;
or. a. oral arms ; rod. c. radial canal ; s. mth. suctorial mouths ; st. stomach ; tl, tS.tS, tentacles
on oral arms. (After Cuvier, Claus, and Huxley.)

of the enteric system. Looking at the oral or lower surface of one of these Jelly-
fishes, such as Pilema, no mouth is to be seen, but a careful examination of the
oral arms shows the presence of large numbers hundreds, or even thousands in
some cases of small funnel-like apertures (B, C, s.mth.) with frilled margins.
Rhizostomes have been found with prey of considerable size, such as fishes, em-
braced by the arms and partly drawn into these apertures, which are therefore
called the suctorial mouths. They lead into canals in the thickness of the arms
(B, c.), the lesser canals unite into larger, and then finally open into the stomach
(at. ). We thus get a, polyatomatow or many-mouthed condition which is practi-
cally unique in the animal kingdom, the only parallel to it being furnished
by the Sponges, in which the inhalant pores are roughly comparable with the
suctorial mouths of a Rhizostome.

IV

PHYLUM CCELENTERATA

171

It has been found that this characteristic arrangement is brought about by
certain changes taking place during growth. The young Rhizostome has a single
mouth in the usual position, and more or less leaf-like arms, folded along the
midrib so as to enclose a deep groove, from which secondary grooves pass, like
the veins of a leaf, towards the edge of the arm. As development proceeds, these
"rooves become converted into canals by the union of their edges, thus forming
a system of branching tubes opening proximally into the angles of the mouth and
distally by small apertures the suctorial mouths on the edges of the arms.
At the same time the proximal ends of the arms grow towards one another and
finally unite across the mouth, closing it completely, and forming a strong
horizontal brachial disc, which in the adult occupies the centre of the sub-
umbrellar surface.

In Ephyra, the lowest of the Cannostoma?, only four gastric filaments are
present, as in Tessera (p. 165) or a newly liberated Ephyrula (p. 163), but as a
rule these characteristic structures are very numerous. The lower forms, also, have
no sub-genital pouches, or indeed anything corresponding to the septal funnels of
the preceding orders. In the higher Rhizostomse a remarkable modification is
produced in connection with these cavities : the four pouches approach the centre
and fuse with one another, forming a single spacious chamber, the sub-yenital

B

FIG. 123. Pelagia ncctiluca : Three developmental stages, m. mouth ; ,-. marginal lappet :
s. teutaculocyst. (From Korschelt and Heider, after Krohii.)

2)ortico, which lies immediately below the floor of the stomach and above the
brachial disc.

In many of the Discomeduspe development takes place in the same general
way as in Aurelia, i.e. the impregnated egg gives rise to a Scyphula or asexual
polype stage, which, by transverse division, produces sexual medusa?. But in
other cases there is no alternation of generations, and development is direct. For
instance, in Pelayia (Fig- 123) one of the Semostomse a blastula is formed
which becomes invaginated at one end, forming a gastrula. The blastopore or
gastrula-mouth remains open, and a considerable space is left between the
invaginated endoderm and the ectoderm. Next the mouth region becomes
elevated, forming a manubrium, and around this a circular depression appears
the rudiment of the sub-umbrellar cavity surrounded by a raised ridge, the
umbrella margin, which soon becomes divided into lobes, the marginal lappets.
Up to this time the embryo is ciliated externally, but soon the cilia disappear,
and the little creatures assume somewhat the form of an Ephyrula, which
gradually develops into the adult Pelagi-a.

ADDITIONAL REMARKS ON THE SCYPHOZOA.

The Scyphozoa are all marine, and the majority are pelagic, i.e.
swim freely on the surface of the ocean. A few inhabit the deep
sea, and have been dredged from as great a depth as 2,000 fathoms.

172 ZOOLOGY SECT.

Nearly all are free-swimming in the adult state : some, however,
live on coral-reefs or mud-banks, and are found resting, in an
inverted position, on the ex-umbrella : and a few, such as Lucern-
aria, are able to attach themselves at will by a definite ex-
umbrellar peduncle.

Considering the extremely perishable nature of these organisms,
and the fact that many of them contain not more than 1 per cent,
of solid matter, it is not to be expected that many of them should
have left traces of their existence in the fossil state. Nevertheless,
in the finely grained limestone of Solenhofen, in Bavaria, belong-
ing to the Upper Jurassic period, remarkably perfect impressions
of Jelly-fishes have been found, some of them readily recognisable as
Discomednsae.

Many of the Scyphozoa are semi-transparent and glassy, but
often with brilliantly coloured gonads, tentacles, or radial canals.
In many cases the umbrella, oral arms, &c., are highly coloured,
and some species, e.g. Pelagia noctiluca, are phosphorescent. They
are all carnivorous, and although mostly living upon small
organisms, are able, in the case of the larger species, to capture
and digest Crustaceans and Fishes of considerable size.

CLASS III. ACTINOZOA

1. EXAMPLE OF THE CLASS A SEA-ANEMONE (Tcalia crassicornis).

Sea-anemones are amongst the most abundant and best known
>f shore-animals. They are found attached to rocks, sea-weeds,
shells, &c., either in rock-pools or on rocks left high and dry by the
ebbing tide. Usually their flower-like form and brilliant colour
make them very conspicuous objects, but many kinds cover them-
selves more or less completely with sand and stones, and contract
so much when left uncovered by water, that they appear like soft
shapeless lumps stuck over with stones, and thus easily escape obser-
vation. Any of the numerous species will serve as an example of
the group : the form specially selected is the " Dahlia Wartlet "
( Tcalia crassicornis), one of the commonest British species.

External characters. Tealia (Fig. 124, A) has the form of a
cylinder, the diameter of which slightly exceeds its height. It is
often as much as 3 inches (8 cm.) across, is of a green or red colour,
and habitually covers itself with bits of shell, small stones, &c. It
is attached to a rock or other support by a broad sole-like base,
sharply separated from an upright cylindrical wall or column, the
surface of which is beset with rows of adhesive warts or tubercles :
at its upper or distal end the column passes into a horizontal plate,
the disc or pcristomc. In the middle of the disc, and slightly
elevated above its surface, is an elongated slit-like aperture, the

IV

PHYLUM CCELENTERATA

173

ost. /

tries, f

<z

-V

V./n

FIG. 124. Tealia crassicornis. A, dissected specimen ; B, transverse section, the half
above the line ab through the gullet, the lower half below the gullet ; d. //its. directive
mesenteries ; </o>i. gonads ; gul. gullet ; 1. m. longitudinal muscle ; I p. lappet ; Hits. 1, primary ;
JH.CS. 3, secondary ; -nies. 3, tertiary mesenteries ; mcs. /. mesenteric filaments ; mtli. mouth ;
ost. 1, ost. 2, ostia ; p. m. parietal muscle ; .<tji/i. siphonoglyphe ; s. i:i. sphincter muscle ; t. m.
transverse muscle.

174 ZOOLOGY SECT.

mouth (mth.), from wiiich streaks of colour radiate outwards.
Springing from the disc and encircling the mouth are numerous
short conical tentacles (.), which appear at first sight to be arranged
irregularly, but are actually disposed in five circlets, of which the
innermost contains five, the next five, the third ten, the fourth
twenty, and the fifth or outermost forty, making a total of eighty.

Obviously the Sea-anemone is a polype, formed on the same
general lines as a Hydra or a Scyphula, but differing from them in
having numerous tentacles arranged in multiples of five, and in
the absence of a hypostome, the mouth being nearly flush with the
surface of the disc. Its great size and bulk, and the comparative
firmness of its substance, are also striking points of difference
between Tealia and the polypes belonging to the classes Hydrozoa
and Scyphozoa.

Enteric System, Still more fundamental differences are found
when we come to consider the internal structure. The mouth does
not lead at once into a spacious undivided enteric cavity, but into
a short tube (gul.), having the form of a flattened cylinder, which
hangs downwards into the interior of the body, and terminates in
a free edge, produced at each end of the long diameter into a
descending lobe or lappet (/>.). This tube is the gullet or stomodcvum,
a structure we have already met with in the Scyphozoa, but which
here attains a far greater size and importance. Its inner surface is
marked with two longitudinal grooves (A and B, sc/ph.\ placed one
at each end of the long diameter, and therefore corresponding with
the lappets : they are known as the gullet-grooves or siphonogly plies.

The gullet does not simply hang freely in the enteric cavity, but
is connected with the body-wall by a number of radiating
partitions, the complete or primary mesenteries (mes. 1) : between
these are incomplete secondary mesenteries (mes. 2), which extend
only part of the way from the body-wall to the gullet, and
tertiary mesenteries (mes. 5), which are hardly more than
ridges on the inner surface of the body- wall. Thus the entire
internal cavity of a Sea-anemone is divisible into three regions:
(1) the gullet or stomodcewth. communicating with the exterior
by the mouth, and opening below into (2) a single main digestive
cavity, the stomach or mcsentcron, which gives off (3) a number of
radially arranged cavities, the inter-mesenteric chambers or metentera.
It is obvious that we may compare the gullet and stomach with
the similarly named structures in the Scyphula-stage of Aurelia,
and the mesenteries with the gastric ridges ; indeed, there seems to
be little doubt that these structures are severally homologous. A
further correspondence is furnished by the presence of an aperture
or ostium (ost. 1} in each mesentery, placing the adjacent inter-
mesenteric chambers in direct communication with one another:
in Tealia a second ostium (ost. 2) is present near the outer edge
of the mesentery. Moreover, the free edge of the mesentery

IV

PHYLUM CCELENTERATA

175

below the gullet is produced into a curious twisted cord, the
in < 'tenteric filament (mes./.), answering to the gastric filaments of
the Scyphozoa.

The* general arrangement of the cell-layers is the same as in
the two preceding classes. The body-wall (Fig. 125) base, column,
and disc consists of a layer of ectoderm outside, one of endoderm
within, and between them an intermediate layer or mesogloea,
which is extremely thick and tough. The gullet (fful.\ which, like
that of the Scyphula, is an in-turned portion of the body- wall, is
lined with ectoderm, and its outer surface i.e. that facing the
inter-mesenteric chambers is endodermal. The mesenteries (mes.)
consist of a supporting plate of mesogloea, covered on both sides by

B

c/t

'n. mes. c

FIG. 125. Diagrammatic vertical (A) and transverse (B) sections of a Seaj-anemone. The
ectoderm is dotted, the endoderm striated, the mesoglosa. black, ac. acontium ; en. cinclis ;
iinl. gullet; int. mes. c. inter-mesenteric chamber; mes. mesentery; mes. /. mesenteric
filament ; mth. mouth ; ost. ostium ; p. pore ; t. tentacle.

endoderm. The tentacles (t) are hollow out-pushings of the disc,
and contain the same layers.

Muscular System. Sea-anemones perform various charac-
teristic movements : the column may be extended or retracted, the
tentacles extended to a considerable length, or drawn back and
completely hidden by the upper end of the column being folded
over them like the mouth of a bag; the gullet, and even the
mesenteries, may be partially everted through the mouth ; and
lastly, the whole animal is able, very slowly, to change its position
"by creeping movements of its base.

These movements are performed by means of a very well-
developed set of muscles. A mesentery examined from the surface

176 ZOOLOGY SECT.

is seen to be traversed by definite fibrous bands, the two most
obvious of which are the longitudinal or retractor muscle (Fig.
124, I. m.), running as a narrow band from base to disc, and the
parietal muscle (p.m.), passing obliquely across the lower and outer
angle of the mesentery. Both these muscles are very thick, and
cause a projection or bulging on one side of the mesentery,
specially obvious in a transverse section (B. l.m.) : a third set of
fibres, forming the transverse muscle (t.m.), crosses the longitudinal
set at right angles, but is not specially prominent. The longi-
tudinal muscles shorten the mesentery, and draw the disc
downwards or towards the base, thus retracting the tentacles ; the
parietal muscles approximate the column to the base, and the
transverse fibres produce a narrowing of the mesentery, and thus,
opposing the action of the longitudinal muscles, act as extensors of
the whole body. The withdrawal of disc and tentacles, during
complete retraction, has been compared to the closure of a bag by
tightening the string, and is performed in much the same way, the
string being represented by a very strong band of fibres, the
circular or sphincter muscle (s.m), which encircles the body at the
junction of the column and disc.

The foregoing muscles can all be seen by the naked eye, or
under a low magnifying power. They are supplemented by fibres,
only to be made out by microscopic examination, occurring both in
the body-wall and in the tentacles. The latter organs, for instance,
are able to perform independent movements of extension and re-
traction by means of delicate transverse and longitudinal fibres.

It was mentioned above that the thickness of the longitudinal
and parietal muscles produces a bulging 011 one surface of the
mesenteries. A transverse section shows that the arrangement of
the mesenteries and of their muscles is very definite and charac-
teristic (Fig. 124, B). At each end of the gullet, opposite the
siphonoglyphe, are two mesenteries (d. mcs), having their longi-
tudinal muscles turned away from one another : they are distin-
guished as the directive mesenteries, and, in the case of Tealia,
there are two couples of directive mesenteries, one at each end of
the long axis of the gullet. Of the remaining complete or
primary mesenteries there are four couples on each side (mcs. 1),
differing from the directive couples in having the longitudinal
muscles turned towards one another. The secondary and tertiary

7

mesenteries (mes. 2, mes. &) are also arranged in couples, and in all
of them the longitudinal muscles of each couple face one another.

Symmetry. It will be noticed that Tealia, unlike the typical
hydrozoan and scyphozoan polypes, presents a distinct bilateral sinn -
mctry, underlying, as it were, its superficial radial symmetry. It
is divisible into equal and similar halves by two planes only, viz. a
vertical plane taken through the long diameter of the gullet, and a
transverse plane taken through its short diameter.

IV

PHYLUM CtELENTERATA

177

The general microscopic structure of a Sea-anemone is well-
shown by a section through a tentacle (Fig. 126). Both ectoderm
(cr/.) and endoderm (end.) consist mainly of very long columnar,
ciliated, epithelial cells, and the mesogloea (nisgl.) is not only ex-
tremely thick, but has the general characters of connective tissue,
being traversed by a network of delicate fibres with interspersed
cells. The middle layer has, in fact, ceased to be a mere gelatinous
supporting lamella or mesogloea, and has assumed, to a far greater

ntc

Fin. 12G. Tealia crassicornis. Trans-
verse section of tentacle, cet. ectoderm ;
end. endoderm; l.m. longitudinal muscles ;
msgl. mesoglcea ; ni:e. nerve cells; nv.f.
nerve fibres; utr. iiematocysts ; t. m.
transverse muscles. (After Hertwig.)

FIG. 127. Three nematocysts of
Sagartia. (After Hertwig.)

extent than in any of the lower groups, the characters of an inter-
mediate cell-layer or mesoderm.

Stinging-capsules occur in the ectoderm, and are also very
abundant in the mesenteric filaments. They (Fig. 127) resemble
in general characters the nematocysts of Hydrozoa, but are of
a more elongated form, and the thread is usually provided at
the base with very numerous slender barbs (B). Very fre-
quently the coiled thread is readily seen in the undischarged
capsule (A). Gland-cells (Fig. 128, gl) are very abundant
in the ectodermal lining of the gullet and in the mesenteric
filaments : the latter are trilobed in section, and the gland-
cells are confined to the middle portion, the lateral divisions

VOL I. N

178

ZOOLOGY

SECT.

being invested with ordinary ciliated cells (c.). In virtue of
possessing both stinging-capsules and gland-cells, the mesenteric
filaments perform a double function. The animal is very voracious,
and is able to capture and swallow small Fishes, Molluscs, Sea-
urchins, &c. The prey is partly paralysed, before ingestion, by the
nematocysts of the tentacles, but the process is completed, after
swallowing, by those of the mesenteric filaments. Then as the
captured animal lies in the stomach, the edges of the filaments
come into close contact with one another and practically surround

nlc

Fir;. !>. Transverse section of mesenteric filament of Sagartia. r. ciliated cells ; #?. glaiul-

cells ; ,>tc. uematocysts. (After Hertwig.)

it, pouring out, at the same time, a digestive juice secreted by their
gland-cells.

The muscles described above consist partly of spindle-shaped
nucleated fibres, and partly of muscle-processes, like those of
Hydra : the latter occur chiefly in the transverse muscular layer of
the tentacles and are endodermal, the longitudinal layer is formed
of distinct fibres of ectodermal origin : the great muscles of the
mesenteries are of course endodermal. Although always derived
either from the ectoderm or endoderm, many of the muscle-fibres
of Tealia undergo a remarkable change of position by becoming-
sunk in the mesogloea, and thus appearing to belong to that
layer {Fig. 126, /. m.). This fact is significant from the circum-

iv PHYLUM CCELENTERA.TA 179

stance that, as we shall see, the muscles of all animals above
Coelenterata are mesodermal structures.

The nervous system is very simple. It consists of a layer of
delicate fibres lying between the epithelial and muscular layers of
the ectoderm. Among the fibres are found nerve-cells (Fig. 126,
nv.c.), often of large size, and occurring chiefly in the disc and
tentacles. Thus, as in the polype-forms previously described, the
nervous system is in a generalised condition, and shows no con-
centration into a definite central nervous system such as occurs
in Medusfe.

Reproductive organs. Sea-anemones are dioecious, the sexes
being lodged in distinct individuals. The gonads ovaries or testes
are developed in the substance of the mesenteries (Fig. 124, gon.),
a short distance from the edge, and, when mature, often form very
noticeable structures. The reproductive products are obviously, as
in the Scyphozoa, endodermal. The sperms, when ripe, are dis-
charged into the stomach and escape by the mouth : they are
then carried, partly by their own movements, partly by ciliary
action, down the gullet of a female, where they find their way
to the ovaries and impregnate the eggs.

The development of Sea-anemones resembles, in its main features,
that of Scyphozoa. The oosperm undergoes more or less regular
division, the details differing considerably in individual cases, and
becomes converted into a planula, an elongated ovoidal body with
an outer layer of ciliated ectoderm, and an inner layer of large
endoderm cells, surrounding a closed enteric cavity, usually filled
with a mass of yolk, which serves as a store of nutriment.

In this condition the embryo escapes from the parent, through
the mouth, swims about for a time, and then settles down, becom-
ing attached by its broader or anterior end. At the opposite or
narrow end a pit appears, the rudiment of the stomodseum ; this
deepens and, its lower or blind end becoming perforated, effects a
communication with the enteron.

The mesenteries are developed in regular order, but in a way which would
certainly not be suspected from their arrangement in the adult. First of all, a
single pair of mesenteries (Fig. 129, A, 1) grow from the body- wall to the gullet,
being situated one on each side of the vertical plane, at right angles to the long
diameter of the stomodseum, and near one end of that tube. The enteron thus
becomes divided into two chambers, a larger or dorsal and a smaller or ventral,
and the embryo acquires a distinct bilateral symmetry. Next a pair of mesen-
teries (2) appear in the dorsal chamber, dividing it into a median and two lateral
compartments ; then a third pair (3) in the ventral chamber, producing a similar
division ; then a fourth pair (4) in the middle compartment of the dorsal
chamber ; then a fifth pair (B, 6) in the lateral compartments of the dorsal
chamber ; and a sixth (6) in the lateral compartments of the ventral chamber.
Soon the longitudinal nruscles are developed, and the fate of these primitive
pairs of mesenteries can be seen. The third and fourth pairs become the two
directive couples of the adult ; another couple of primary mesenteries is consti-
tuted, on each side of the vertical plane, by one of the mesenteries of the lirst

N 2

180

ZOOLOGY

SECT.

and one of the sixth pair ; a third couple is similarly formed by a mesentery of
the second and one of the fifth pair. Thus it is only in the case of the directive
mesenteries that an adult couple coincides with an embryonic pair : in other

sld,

A

B

FIG. 129. Transverse sections of early (A) and later (B) stages of an embryo ^ca-n

(Actinia.) The mesenteries are numbered in the order of their development ; aid. stomo-
daurn. (After Korschelt and Heider.)

instances the two mesenteries of a couple are of different orders, belonging:
to distinct embryonic pairs.

The tentacles are developed in a somewhat similar order. The first to make
its appearance is connected with the larger or dorsal enteric chamber mentioned
above : for some time it remains much longer than any of its successors, and
thus accentuates in a marked degree the bilateral symmetry of the embryo.

It will be noticed that the development of the Sea-anemone is
accompanied by a well-marked metamorphosis, but that there is no
alternation of generations. In this respect its life-history offers a
marked contrast with that of Obelia or of Aurelia.

2. DISTINCTIVE CHAKACTERS AND CLASSIFICATION.

The Actinozoa are Coelenterata which exist only in the polype-
form, no medusa-stage being known in any member of the class.
The actinozoan differs from the hydrozoan polype mainly in
possessing a stomoda?um the relative size and physiological
importance of which are far greater than in the Scyphozoa, the
first group in which this structure is met with : it differs from
both hydrozoan and scyphozoan polypes in the possession of
mesenteries or vertical radiating partitions, which extend inwards
from the body-wall, and in some cases join the stomod;vum. The
free margins of the mesenteries bear coiled mesenteric filaments,
which appear to answer to the gastric filaments of Scyphozoa, but
may be partly ectodermal in origin. The mesenteries are developed

iv PHYLUM CCELENTERATA 181

in pairs symmetrically on each side of a vertical plane : their final
radial position is secondary.

The body-wall consists of ectoderm and endoderm separated by
a stout mesogloea containing fibres and cells. The stomodaeum
consists of the same layers reversed i.e. its lining membrane is
ectodermal. The mesenteries are formed of a double layer of
endoderm with a supporting plate of mesoglcea. Nematocysts,
frequently of a more complex form than those of Hydrozoa and
Scyphozoa, are present in the tentacles, body- wall, stomodseum,
and mesenteric filaments. The muscular system is well developed,
and contains both ectodermal and endodermal fibres and enclo-
dermal muscle-processes. The nervous system consists of irregu-
larly disposed cells and fibres ; there is no concentration of these
elements to form a central nervous system.

The gonads are developed in the mesenteries, the sex-cells are
endodermal, and the ripe sexual products are discharged into the
enteron. The impregnated egg develops into a planula, which, after
a short free existence, settles down and undergoes metamorphosis
into the adult form. Except in one doubtful instance there is no
alternation of generations.

In some Actinozoa the animal remains simple throughout life,
but in most members of the class an extensive process of budding
takes place, the result being the formation of colonies of very various
form and often of great size. Some kinds, again, resemble Tealia
in having no hard parts or skeletal structures of any kind ; but the
majority possess a skeleton, formed either of carbonate of lime or
of a horn-like or chitinoid material, and developed, in most cases
though not in all, from the ectoderm.

The Actinozoa are classified as follows :

Sub-Class I. Zoantharia.

Actinozoa in which the tentacles and mesenteries are usually
very numerous and are arranged, as a rule, in multiples of five or
six. The tentacles are usually simple, unbranched, hollow cones.
There are commonly two siphonoglyphes and two pairs of directive
mesenteries : the remaining mesenteries are usually arranged
in couples with the longitudinal muscles of each couple facing one
another.

ORDER 1. ACTINIARIA.

Zoantharia which usually remain simple, but in a few instances
form small colonies. The tentacles and mesenteries are numerous,
and there is no skeleton. This order includes the Sea- anemones.

ORDER 2. MADREPORARIA.

Zoantharia which resemble the Actiniaria in the general
structure of the soft parts, but which usually form colonies, and

182 ZOOLOGY SECT.

always possess an ectodermal calcareous skeleton. This order
includes the vast majority of Stony Corals (Figs. 133 and 143).

ORDER 3. ANTIPATHARIA.

Compound, tree-like Zoantharia in which the tentacles and
mesenteries are comparatively few (6 24) in number. A skeleton
is present in the form of a branched chitinoid axis, developed from
the ectoderm, which extends throughout the colony. This order
includes the " Black Corals" (Fig. 137).

Sub-Class II. Alcyonaria.

Actinozoa in which the tentacles and mesenteries are always
eight in number. The tentacles are pinnate, i.e. produced into
symmetrical branchlets. There is never more than one siphono-
glyphe, which is ventral in position, i.e. faces the proximal end of
the colony. The mesenteries are not arranged in couples, and
their longitudinal muscles are all directed ventrally, i.e. towards the
same side as the siphonoglyphe.

ORDER 4. ALCYONACEA.

Alcyonaria in which the skeleton usually consists of calcareous
spicnlcs or small irregular bodies, found in the mesoglcea, but
probably originating from wandering ectoderm cells. The common
' Dead men's fingers " (Alcyonium, Fig. 140) has a skeleton of this
type. In some cases the spicules become aggregated so as to pro-
duce a coherent skeleton, which may form a branched axis to the
whole colony, as in the precious Red Coral (Corallium, Fig. 132),
or a series of connected tubes for the individual polypes, as in
the Organ-pipe Coral (Tulipora, Fig. 135). In the "Blue Coral"
(Heliopora) the skeleton is a massive ectodermal structure resem-
bling that of the Madreporaria. Most genera are compound ; a
few, such as Hartca (Fig. 131), are simple.

ORDER 5. GORGONACEA.

Compound tree-like Alcyonaria, with a calcareous or horny
skeleton of ectodermal origin forming a branched axis throughout
the colony. Spicules are present in the mesoglcea, There is no
siphonoglyphr. The beautiful ' Sea-fans " belong to this group
(Fig. 141 ).

ORDER 6. PENNATULACEA.

Alcyonaria in which the colony is usually elongated, and has
one end embedded in the mud at the sea-bottom, while the
opposite or distal end bears the polypes, usually on lateral

iv PHYLUM CCELENTERATA 183

branches. The stem is supported by a calcareous or horny
skeleton. The polypes are dimorphic. The " Sea-pens" (P< // natula ;
are the commonest members of this group (Fig. 134).

Sysfrnt<'<fit' 7'uxifion of the Example.

Tealia crassicornis is one of several species of the genus
Tealia: it belongs to the family TcaUdce, which, with several
other families, make up the tribe Hexactinicv, of the order
Actiniaria, of the sub-class Zqantharia.

The presence of numerous tentacles, arranged in multiples of
five, places it at once among the Zoantharia. The fact that it is
simple and devoid of a skeleton causes it to be assigned to the
Actiniaria. This order is divided into tribes characterised by
differences in the arrangement of the mesenteries, especially by
the presence of one or two couples of directive mesenteries, and
by the direction in which the longitudinal muscles are turned.
In the Hexactinise the mesenteries are all arranged in couples
with the longitudinal muscles of each turned towards one another,
except in the case of the two directive couples. The mesenteries
are in multiples of five, and the stomodasimi has two siphonoglypb.es
and two lappets.

The family TealidaB is characterised by the possession of
numerous mesenteries, tentacles of moderate length, which are
completely covered by the closed-in disc during retraction, and
by the presence of a large endodermal sphincter muscle. The
genus Tealia is distinguished from other members of the same
family by being broader than high, by having numerous retractile,
equal-sized tentacles, and by the presence of longitudinal series
of warts on the column. The species crassicornis is distinguished
from other species of the genus by the warts being of approxi-
mately equal size.

3 GENERAL ORGANISATION.

The chief variations in the external form of the Actinozoa are
due to the diverse modes of budding : as we shall see, the structure
of the individual polypes or zooids is remarkably uniform at
least as regards all the essentials of their organisation.

Nearly all the Actiniaria or Sea-anemones are simple, and, in the
few instances where colonies are formed, these are usually small,
and contain a very limited number of zooids. In Zoriiitkii*
(Fig. 130), for instance, the original polype sends out a horizontal
branch or stolon (st.), from which new polypes arise. Besides the
Sea-anemones the only simple forms are certain Madreporarian
corals, such as MaMlvm (Fig. 142, A, B), and three genera of
Alcyonacea, of which Hartca (Fig. 131) may be taken as an
example.

184

ZOOLOGY

SECT.

The simplest mode of budding is that just described in Zoan-
thus, in which new zooids are developed from a narrow band-like

B

sgph/

FIG. 130. Zo an thus sociatus. A, entire colon}' ; st. stolon. B, transverse section; s : i/>li.
siphonoglyphes ; il. <!. dorsal, and v. d. ventral directive mesenteries. (After McMumch and
Korschelt and Heider.)

or tubular stolon (Fig. 130, st.). A more usual method resembles
that we are already familiar with in Hydrozoa, new buds being

formed as lateral outgrowths,
and a tree-like colony arising
with numerous zooids spring-
ing from a common stem or
coenosarc. Corallium and Gor-
gonia (Figs. 132 and 141) are
good examples of this type of
growth. In other cases the
buds grow more or less paral-
lel with one another, producing
massive colonies either of close-
set zooids or of zooids separ-
ated by a solid ccenosarc. As
examples of this type we may
take Pcdytlioa, the most com-
plex of the Actiniaria, and
many of the common Madre-
poraria, such as Astrcva (Fig.
133). In the Sea-pens (Penrta-
talacca) the proximal end of
the elongated colony (Fig.
134) is sunk in the mud, and
the distal end bears zooids
springing either directly from

sf,

ri<.:. 131. Hartea eleerans. rivl. pullet :
mes. mesentery ; .;>. spicules ; t. tentacles.
(After Perceval Wright.)

IV

PHYLUM CCELENTERATA

is;,

the coenosarc or, as in Pennatula itself, from flattened lateral
branches.

A very peculiar mode of budding occurs in the Organ-pipe
Coral (Tubipora). The base of the original
polype (Fig. 135) grows out into a flattened
expansion from which new polypes arise,
diverging slightly from one another as they
grow, and separated by tolerably wide inter-
vals. The distal ends of the polypes then
grow out into horizontal expansions or plat-
forms (/>/.), formed at first of ectoderm and
mesoglcea only, but finally receiving prolonga-
tions of the endoderm. The platforms extend,
come in contact with one another, and fuse.
In this way platforms of considerable ex-
tent are formed (A, pi.}, uniting the polypes
with one another. From the upper surfaces of the platforms,
between the older polypes, new buds arise, and in this way
the colony tends to assume the form of an inverted pyramid,
the number of zooids, and consequently the diameter of the
colony, increasing pari passu with the vertical growth of the

FIG. 132. Corallium
rubrum, portion
of a branch. (After
Claus.)

FIG. 133. Astreea pallida , the living colony. (After Dana.)

latter. The skeleton of this remarkable coral will be referred to
hereafter.

Although the general structure of the individual polypes

of the Actinozoa is, as mentioned above, very uniform, the varia-
tions in detail are numerous and interesting, especially among
the Actiniaria. One of the most important points to consider
is the arrangement of the mesenteries. In Edwardsia (Fig. 136),
a genus which burrows in sand, instead of attaching itself to

186

ZOOLOGY

SECT,

'

1 ^^ irf.'^X' "'iVv

S*~~ ~- ^^*^ \ , fa S -

^

'

- '.- -

B

1 I ''

I

^^ -

FIG- 134. Pennatula sulcata. A, entire colony; B, portion of the same magnified.
1. lateral bir.ncli ; p. polype ; . siphonozooid. (After Koelliker.)

Fir; ]35. Tubipora musica. A. skeleton of entire colony ; B, transverse sections ..f polype ;

single polype with tuK- ; n.l t ..uniiciK-eiiieiit .,f platL.n.i : I). fnMwth .if new jmlyiPL'S from
platform ; /. ,. longitudinal muscles; ,>i. p-i. polypes; /-/. platform : /,,/,. siphono flyphe .*//.
spicules ; st.J. stomodseum. (After Cuvicr, Quoy and Gaimard, and Hickson )

IV

PHYLUM CCELENTERATA

187

B

-t

rocks, &c., there are only eight mesenteries (B), the usual two
couples of directives, and two others on each side of the vertical
plane, having their longitudinal muscles directed ventrally, and
therefore not arranged in couples. The adult Edwardsia thus
corresponds with a temporary stage in the development . >f one of
the more typical sea-anemones, viz., the stage with eight mesen-
teries shown in Fig. 129, A; it is probably to be looked upon as
the most primitive or generalised member of the order. In
Zoanthus (Fig. 130, B) the dorsal
directives (d.d.) do not reach the
gullet, and each lateral couple con-
sists of one perfect and one small
and imperfect mesentery. In Ceri-
anthus, another burrowing form,
there is a couple of very small
ventral directives, and the remain-
ing mesenteries are very numerous,
not arranged in couples, and all
directed ventrally at their outer
ends, so as to have a very obviously
bilateral arrangement : in this genus
as growth proceeds, new mesen-
teries are added on the dorsal side,
and not, as is usual, between already
formed couples. On the other hand,
the newly discovered Cri/ractis ex-
hibits a perfectly radial arrange-
ment : the mesenteries are all
arranged in couples with the longitudinal muscles facing one
another. Lastly, in all the more typical Sea-anemones (forming
the tribe Hcxactinicv) there are either six, eight, or ten pairs of
perfect mesenteries, which, as well as the secondary and tertiary
cycles, are all arranged in couples, the longitudinal muscles of
all but the one or two directive couples facing one another.

In the Madreporaria the mesenteries are arranged, so far as is
known, in the way just described for the Hexactinia?. In the
Antipatharia there are six primary, and sometimes either four or
six secondary, mesenteries. In the whole of the Alcyonaria the
mesenteries are eight in number: they are not arranged in
couples, and their longitudinal muscles are all turned the same
way, viz., towards the ventral aspect (Fig. 135, B). In this
whole sub-class, therefore, the resemblance to Edwardsia is very
close, the main difference being that the longitudinal muscles
of the ventral directives are turned inwards in the Alcyonaria,
outwards in Edwardsia.

The tentacles in Zoantharia are usually very numeroii>. and in
nearly all cases have the form of simple glove-finger-like out-

Pio. 136. Edwardsia claparedii.

A, the entire animal ; f. tube. B.
transverse section. (After Andres,
and Korschelt, and Holder. )

188

ZOOLOGY

SECT.

FIG. 137. Cirripathes anguina,

portion of colony. (After Broun.)

pushing* of the disc. In Edwardsia, however, they may be
reduced to sixteen, and in some genera of Sea-anemones they are
branched. In the Antipatharia (Fig. 137) they vary in number

from six to twenty-four. In the

t/

Alcyonaria on the other hand, the
tentacles, like the mesenteries, are
eight in number and are always
pinnate, i.e. slightly flattened and
with a row of small branchlets
along each edge (Fig. 131). Many
Actiniaria have the tentacles per-
forated at the tip (Fig. 125, A, >.),
and in some species these organs
undergo degeneration, being re-
duced to apertures on the disc,
which represent the terminal
pores of the vanished tentacles,
and are called stomidia.

Many Sea-anemones possess
curious organs of offence called
acontia (Fig. 125, A, and Fig. 144,

ac.). These are long delicate threads springing from the edges
of the mesenteries : they are loaded with nematocysts, and can
be protruded through minute apertures in the column, called
' port-holes " or cindidcs (en.).

Enteric System, The gullet in the Actiniaria presents some
remarkable modifications. It is usually a compressed tube with
two siphonoglyphes, but in Zoanthus and some other genera the
ventral gullet-groove alone is present (Fig. 130, B), and in Gyractis
both grooves are absent, and the tube itself is cylindrical with a
circular mouth. The ordinary compressed form of gullet often
assumes, in the position of rest, a x-shaped transverse section,
owing to its walls coming together in the middle and leaving the
two ends wide open. In a deep-sea form, Halcampoides, there is a
longitudinal partition dividing the stomodaeum into dorsal and
ventral tubes, the latter of which is said to serve for the egestion
of waste matters, and so act as an intestine. In some forms the
bluntly-pointed proximal or aboral end of the body is perforated by
a small aperture which seems to serve as an anus. In two recently
described genera, Fc-nja (Fig. 138) and ^Kyir, a very remarkable
modification is described : the gullet is continued to the aboral
end, when it opens on the exterior by an anus (a.), thus forming
a complete digestive tube. By this arrangement the inter-
mesenteric chambers are shut off from all communication with the
digestive tube, and together constitute a cavity surrounding the
latter and reminding us of the body-cavity met with in most of
the higher animals. In Fenja each inter-mesenteric chamber

IV

PHYLUM CCELENTERATA

189

communicates separately with the exterior by an aperture near
the anus, through which the genital products escape.

Fixed and Free Forms. A large proportion of Actinozoa are
permanently fixed, such, for instance, as most of the Stony Corals,

FIG. 13S. Fenja mirabilis. A, the<entire animal ; B, with the body-wall divided longitudinally

a. anus. (After Daniel.)

the Sea-fans, Black Corals, &c. Most Sea-anemones are temporarily
attached by the base, but are able slowly to change their position :
some forms, such as Edwardsia (Fig. 136) and Ceriantlms, usually
live partly buried in sand enclosed in a tube formed of discharged
stinging-capsules, the oral end with its crown of tentacles alone
being exposed : others, such as Peachia and Fenja, live an actually
free life, habitually lying on the sea-bottom with the longitudinal
axis horizontal like that of a worm : a few, such as Minyas (Fig. 139),
have the aboral end dilated into a sac containing air and serving
as a float ; by its means these animals
can swim at the surface of the sea, and
are thus, alone among the Actinozoa,
pelagic.

Dimorphism. With the exception of
one genus of Stony Corals, the Zoantharia
are all homomorphic, i.e. there is no dif-
ferentiation of the zooids of a colony. But
in the Alcyonaria dimorphism is common :
the ordinary zooids or polypes are ac-
companied by smaller individuals, called

sipli07iozooids (Fig. 134, s.), having no tentacles, longitudinal
muscles, or gonads.

None of the Actiniaria have a true skeleton : in some, how-
ever, there is a thick cuticle, and several kinds enclose themselves
in a more or less complete tube (Fig. 136), which may be largely

FIG. 130. Minyas. /. float.
(After Andre.s.)

190

ZOOLOGY

SECT.

formed of discharged nematocysts. The simplest form of skeleton
is found in the solitary Alcyonarian genus Hartea (Fig. 131 ), already
referred to, in which minute irregular deposits of calcium carbonate,
called spicules (sp.), are deposited in the mesoderm. A similar
spicuh.ir skeleton occurs in the " Dead-men's fingers " (Alcyonium.
Fig. 140). where spicules of varying form are found distributed
throughout the mesoderm of the ccenosarc. In TuUpora (Fig. 135).
the " Organ-pipe Coral," the mesodermal spicules become closely
fitted together, and form a continuous tube for each polype, the

PH.. 140. Alcyonium palmatum, A, entire colony ; B, spicules (After Cuvier.)

tubes being united by horizontal calcareous platforms (pi.) formed
by deposits of spicules in the expansions of the same name already
referred to. The skeleton of Tubipora is, therefore, an internal
skeleton, and in the living state is covered by ectoderm. In the
Red Coral of commerce (CoraUiii.m, Fig. 132) the originally separate
spicules are embedded in a cement-like deposit of carbonate of
lime, the result being the production of an extremely hard and
dense branched rod, which extends as an axis through the ccenosarc.
Another type of skeleton is found in the Antipatharia (Fig. 137)
and in the Gorgonacea (Fig. 141). It also consists of an axial rod.
extending all through the colon}- and branching with it, but is

IV

PHYLUM CCELENTERATA

191

formed of a black horn-like material. Moreover it is not meso-
dermal, but ectodermal in origin : in close contact with it is an
epithelium, from the cells of which it is produced as a cuticular
secretion, and this epithelium is formed as an invagination of the
base of the colony. In addition to its axis, Gorgonia contains
numerous spicules in the mesoderm of the ccenosarc. In some

FIG. 141. Gorgonia verrucosa. A, entire colony; B, portion of the same magnified; c.
ccenosarc ; p. polype. (After Koch and Cuvier.)

of the Gorgonacea the axial skeleton is partly horny, parti}'
calcareous.

In the Sea-pen (Pennatula, Fig. 134) and its allies the stem of
the colony is supported by a horny axis which is unbranched, not
extending into the lateral branches. In this case the axis is
contained in a closed cavity lined by an epithelium, the origin of

192 ZOOLOGY SECT.

which is still uncertain. Spicules occur in the mesoderm, some of
them microscopic, others readily visible to the naked i-yi .

In the Madreporaria we have a skeleton of an entirely different
type, consisting, in fact, of a more or less cup-like calcareous
structure, secreted from the ectoderm of the base and column of
the polypa When formed by a solitary polype, such a " cup-
coral " is known as a corallite : in the majority of species a large
number sometimes many thousands of corallites combine to
form a core///////, the skeleton of an entire coral-colony.

The structure of a corallite is conveniently illustrated by that
of the solitary genus Flabellum (Fig. 142, A, B). It has the form
of a short conical cup, much compressed so as to be oval in section-
Its wall or tkeca (tli.) is formed of dense stony calcium carbonate,
white and smooth inside, rough and of a brownish colour outside,
except towards the margin, where it is white. Its proximal or
aboral end is produced into a short stalk or peduncle, by which the
Coral is attached in the young state, becoming free when adult :
in many other simple Corals there is no stalk, but attachment to
the support is effected by means of a flattened proximal surface
or basal plate (C, b. pi.}. From the inner surface of the theca a
number of radiating partitions, the septa (sep.}, proceed inwards or
towards the axis of the cup, and, like the mesenteries of a polype,
are of several orders, those extending furthest towards the
centre being called primary septa, the others secondary, tertiary,
and so on. Towards the bottom of the cup the primary septa
meet in the middle to form an irregular central mass, the columella
(col.). In some Corals the columella is an independent pillar-like
structure arising from the basal plate (D, col.).

In many Corals there is a distinct calcareous layer investing the
proximal portion of the theca, and called the cpithcca (C, c.th.). Some
species have the inner portions of the septa detached so as to form
a circlet of narrow upright columns, the pali. In others there are
horizontal partitions or dissepiments passing from septum to septum,
and in others, again, complete partitions or tabulae, like those of
Millepora (p. 145), extending across the whole corallite. In the
Mushroom-coral (Funffia), the corallite is discoid, the theca is con-
fined to the lower surface, and small calcareous rods, the synapticulcK,
connect the septa with one another.

In the living condition the polype fills the whole interior of the
corallite and projects beyond its edge to a greater or less degree
according to its state of expansion (C). The proximal part of the
body- wall is thus in contact with the theca, which has the relation
of a cuticle, and is, in fact, a product of the ectoderm. The free
portion of the body-wall is frequently, in the extended state, folded
down over the edge of the theca so as to cover its distal portion.
The septa alternate with the mesenteries, each lying in the space
between the two mesenteries of one couple, and each being in-

IV

PHYLUM CCELENTERATA

193

vested by an in-turned portion of the body-wall (E, F). Thus the
septa, which appear at first sight to be internal structures, are
really external : they lie altogether outside the enteric cavity, and
are in contact throughout with ectoderm.

The ectodermal nature of the entire corallite is further proved by
its development. The first part to appear is a ring-shaped

FIG. 142. A, B, two views of Flabellum curvatum. C, semi-diagrammatic view of a simple
coral ; I), portion of a corallite ; E, F, diagram of a simple coral in longitudinal and transverse
section ; ectoderm dotted, endoderm striated, skeleton black, b. pi. basal plate ; col. colum-
ella ; e. tk. epitheca ; <n<l. gullet ; mis. mcs. 1, //.. ~, mesenteries ; mes. f. mesenterio filaments ;
sep. septa ; t. tentacle ; th. tlieca. (A and B after Moseley ; C and D after Gilbert Bourne.)

deposit of carbonate of lime between the base of the polype and the
body to which it adheres : sections show this ring to be formed by
the ectoderm cells of the base. The ring is soon converted into a
disc, the la sal plate, from the upper surfaces of which a number of
ridges arise, arrayed in a star-like fashion : these are the rudiments
of the septa. Here, again, sections show that each septum corre-
VOL. 1 O

194

ZOOLOGY

SECT.

spends with a radial in-pushing of the base, and is formed as a
secretion of the invaginated ectoderm. As the septa grow they
unite with one another at their outer ends, and thus form the theca.
In some cases, however, the theca appears to be an independent
structure.

The almost infinite variety in form of the compound corals is
due, in the main, to the various methods of budding, a subject
which has already been referred to in treating of the actinozoan
colony as a whole. According to the mode of budding, massive
Corals are produced in which the corallites are in close contact
with one another, as in Astrffia (Fig. 133): or tree-like forms, such

9$ -&$* ^K&

FIG. 143. Dendrophy Ilia nigrescans, B, Madrepora aspera. <v>. contllite* :
eg. coenosaiv : />. pulypes. (After Dana.)

as Dendrophyllia (Fig. 143, A), in which a common calcareous stem,
the ccenencliyma is formed by calcification of the coenosarc (cs.), and
gives origin to the individual corallites. It is by this last-named
method, the coanosarc attaining great dimensions and the indivi-
dual corallites being small and very numerous, that the most
complex of all Corals, the Madrepores (AI</trj>ora, Fig. 143, B)
are produced.

The microscopic structure of corals presents two main varieties.
In what are called the aporosc or poreless corals, such as Flabellum,
Astnea, &c., the various parts of the corallite are solid and stony,
while in the perforate forms, such as Madrepora, all parts, both of

iv PHYLUM CCELENTERATA 195

the corallites and of the connecting coenenchyma, have the charac-
ters of a mesh-work, consisting of delicate strands of carbonate of
lime united with one another in such a way as to leave interstices,
which in the living state are traversed by a network of interlacing
tubes, representing the coenosarc, and placing the polypes of the
colony in communication.

The Blue Coral (Heliopora) one of the Alcyonacea, has a massive
corallum having the same general appearance as a Madreporarian.
The lobed surface bears apertures of two sizes, the larger being for
the exit of the ordinary polypes, the smaller for the siphonozooids.
Tabuke are present, and septum-like ridges, which, however, have
no definite relations to the mesenteries, and are inconstant in
number.

Colour. The Actinozoa are remarkable for the variety and
brilliancy of their colour during life. Every one must 1 lave noticed
the vivid and varied tints of sea-anemones ; but most dwellers in
temperate regions get into the habit of thinking of Corals as white,
and have no conception of their marvellously varied and gorgeous
colouring during life. The Madrepores, for instance, may be pink,
yellow, green, brown, or purple : Tubipora has green polypes, con-
trasting strongly with its crimson skeleton ; and the effect of the
bright red axis of Corallium is greatly heightened by its pure white
polypes. In Heliopora the whole coral is bright blue ; the tropical
Alcyonida? are remarkable for their elaborate patterns and gor-
geous colouration; and Pennatula, in addition to its vivid colours,
is phosphorescent.

In most cases the significance of these colours is quite unknown.
In some species, however, "yellow-cells" or symbiotic Algas have
been found in the endoderm, where they probably serve the same
purpose as the similar structures which we have already studied
in Radiolaria (p. 61).

Many Actinozoa, like many sponges (p. 116), furnish examples of
commensalism, a term used for a mutually beneficial association
of two organisms of a less intimate nature than occurs in symbiosis.
An interesting example is furnished by the Sea-anemone Adamsia
palliata (Fig. 144). This species is always found on a univalve shell
such as that of a Whelk inhabited by a Hermit-crab. The
Sea-anemone is carried from place to place by the Hermit-crab, and
in this way secures a more varied and abundant food-supply than
would fall to its lot if it remained in one place. On the other
hand, the Hermit-crab is protected from the attack of predaceous
Fishes by retreating into its shell and leaving exposed the Se;i-
anemone, which, owing to its toughness, and to the pain caused
by its poisonous stinging-capsules, is usually avoided as an article
of food.

Other Sea-anemones^such as the gigantic Discosoma of the
great Barrier Reef are found associated with Small Fishes or

196

ZOOLOGY

SECT.

Crustacea, which have their abode in the enteric cavity. In this

t/

case the Fish secures shelter in a place where it is very unlikely to
be disturbed, and the two animals are strictly commensals or " mess-
mates" since they share a common table. A somewhat similar
instance is furnished by the Blue Coral (Heliopora), already referred
to more than once. The corallum contains, not only the apertures
for the polypes and siphonozooids, but also tubular cavities of

p ' -^

FIG. 144. Adamsia palliata, four individuals attached to a Gastropod shell inhabited by a
Hermit-crab, ac. ci. acontia ; sk. shell of Gasteropod, (After Andres.)

an intermediate size, in each of which is found a small chaetopod
Worm, belonging to the genus Zeucodore. As the polypes are
frequently found retracted at a time when the Worms are protruded
from their holes in search of food, it is not surprising that the
latter should have been credited with the fabrication of the coral.
Trapezia, a genus of Crabs, always lives in interstices of a par-
ticular species of Madrepore.

The distribution of the Actiniaria is world-wide, and in

IV

PHYLUM CCELENTERATA 197

many cases the same genera are found in widely separated
parts of the world. They are, however, larger and of more varied
form and colour in tropical regions, for instance on coral-reefs. The
largest reef-anemone, Discosoma, found also in the Mediterranean,
attains a diameter of 2 feet. Most members of the order are
littoral, living either between tide-marks or at slight depths, but a
few are pelagic, and several species have been dredged from depths
of from 10 to 2,900 fathoms.

The Madreporaria, taken as a whole, have also a wide distribu-
tion ; but the number of forms in temperate regions is small, and
the majority including the whole of what are called reef-building
Corals are confined to the tropical parts of the Atlantic, Indian,
and Pacific Oceans, flourishing only where the lowest winter tem-
perature does not sink below 68 F. (20 C.). Thus their northern-
most limits are the Bermudas in the Atlantic, and Southern Japan
in the Pacific ; their southernmost limits, Rio and St. Helena in
the Atlantic, Queensland and Easter Island in the Pacific : in other
words, they extend to about 30 on each side of the equator.
Moreover, they have a curiously limited bathy metrical distribu-
tion, flourishing only from high-water mark down to a depth of
about 20 fathoms, but not lower.

Many of the Pacific Islands are formed entirely of coral rock,
others are fringed with reefs of the same, and the whole east coast
of Northern Queensland is bounded, for a distance of 1,250 miles,
by the Great Barrier Reef, a line of coral rock more or less parallel
to and at a distance of from 10 to 90 miles from the land.
Such reef's consist of gigantic masses of coral rock fringed by living
coral, the latter growing upon a basis of dead coral, the interstices
of which have been filled up with ddbris of various kinds, so as to
convert the whole into a dense limestone.

The Antipatharia, and many of the Alcyonaria, such as the Gor-
gonacea and Pennatulacea, have also a world-wide distribution,
and, even in temperate regions, both Black Corals and Sea-fans may
attain a great size : the members of both these groups, as well as
the Sea-pens, are found at moderate depths. The Red Coral is found
only in the Mediterranean, at a depth of 10 to 30 fathoms. Tubi-
pora and Heliopora have the same distribution as the reef-building
Corals.

From the palseontological point of view, corals are of great im-
portance : they are known in the fossil condition from the Silurian
epoch upwards, and in many formations occur in vast quantities,
forming what are called coral limestones. The majority of fossil
forms are referable to existing families, but in the Palaeozoic era
the dominant group was the Eugosa, the affinities of which are still
very obscure. The corallites are usually bilaterally symmetrical,
the septa are arranged in multiples of four, and the cup presents
on one side a pit, the fossula, where the septa are greatly reduced.

198

ZOOLOGY

SECT. IV

CLASS IV. CTENOPHORA.

1. TYPE OF THE CLASS Hormiplwra plumosa.

External Characters. Hormiphora is a pear-shaped organism,
about 5-20 mm. in diameter, and of glassy transparency (Figs. 145
and 146). The species H. plumosa, is found in the Mediterranean ;
allied forms belonging either to the same genus (often called
Oydippe) or to the closely allied genus Pleurobrachia are common
pelagic forms all over the world.

From opposite sides of the broad end depend two long tentacles
(t.\ provided with numerous little tag-like processes, and springing
each from a deep cavity or sheath, into which it can be completely

Fio. 14.0. Hormiphora plumosa. A, from the side, B, from the aboral pole. utth. mouth ;
.<. pi. swimming plates ; t. and 1. tentacles. (After Chun.)

retracted (Fig. 146, t.sh.). At the narrow end where the stalk
of a pear would be inserted is a slit-like aperture, the mouth-
(mth.): this end is therefore oral. At the opposite or aboral pole is
a slight depression, in which lies a prominent sense-organ (s.o.\ to-
be described hereafter.

But the most striking and characteristic feature in the external
structure of Hormiphora is the presence of eight equidistant meri-
dional bands (s.pl.), starting from near the aboral pole, and extending
about two-thirds of the distance towards the oral pole. Each band
is constituted by a row of transversely arranged comb-like struc-
tures, consisting of narrow plates frayed at their outer ends. During
life the frayed ends are in constant movement, lashing to and fro.

5.0

ovy

Fio. 146. Hormiphora plumosa. A, dissected specimen having rather more than one
quarter of the body cut away. B, transverse section ; Diagrammatic. //. >. adradial
canal; inf. infundibulum ; inf. c. infundibular canal; int. c. inter-radial canal; mrd. c.
meridional canal; mtlt. mouth; ov/. ovary; per. c. per-radial canal; . o. sense-organ ; .. /</.
swimming-plate ; spy. spermary ; *td. stomodpeum ; xt<i. c. stomodseal canal ; std. r. stomodseal
ridges ; t. tentacle ; t. b. base of tentacle ; (. c. tentacular canal ; t. sh. tentacular sheath.

200 ZOOLOGY SECT, iv

and so propelling the animal through the water. The combs are,
in fact, rows of immense cilia, fused at their proximal ends : their
presence and mode of occurrence arranged in meridional comb-
ribs or swimming-plates are strictly characteristic of the class,
and indeed give it its name.

It will be seen at once that apart from all considerations of
internal structure Hormiphora presents a similar combination of
radial with bilateral symmetry as some Hydrozoa, such as Ctenaria
(Fig. 96, 1], and as the majority of Actinozoa. The swimming-plates
are radially arranged, and mark the eight adradii, but the slit-like
mouth and. the two tentacles indicate a very marked and character-
istic bilateral symmetry. A plane passing through the longitudinal
axis of the body, parallel with the long axis of the mouth, is called, as
in Actinozoa (see p. 176), the vertical plane : it includes two per-radii,
which are respectively dorsal and ventral. A plane at right angles
to this, passing through both tentacles, and including right and
left per-radii, is called the transverse plane.

Enteric System. The mouth -leads into a flattened tube (Fig.
146, std.),often called the stomach, but more correctly the gullet or
stomodaum. It reaches about two-thirds of the way towards the
aboral pole, and its walls are produced internally into ridges (std. r.\
which increase the area for the absorption of digested food.
Living prey is seized by the tentacles, ingested by the aid of the
mobile edges of the mouth and digested in the stomodaeum, which
is thus physiologically, though not morphologically, a stomach.
The products of digestion make their way into the various parts
of the canal-system, presently to be described, and indigestible
matters are passed out at the mouth.

Towards its upper or aboral end the stomodasum gradually
narrows and opens into a cavity called the infundibulum (inf.),
which probably answers to the stomach of an Actinozoon or a
medusa, and is flattened in a direction at right angles to the
stomodreum i.e. in the transverse plane. From the infundibulum
three tubes are given off: one, the infundibular canal (inf. c.), passes
directly upwards, and, immediately beneath the aboral pole, divides
into four short branches, two of which open on the exterior by
minute apertures, the excretory >om (Fig. 147, A, ex. p.\ The two
other canals given off from the infundibulum are the per-radial
canals (per. c.}: they pass directly outwards, in the transverse plane,
and each divides into two inter-radial canals (int. c.), which in their
turn divide each into two adradial canals (a dr. c.). These succes-
sive bifurcations of the canal-system all take place in a horizontal
plane (Fig. 147, B), and each of the ultimate branches or adradial
canals opens into a meridional canal (mdr. c.), which extends up-
wards and downwards beneath the corresponding swimming-plate.
Furthermore, each per-radial canal gives off a stomodceal canal
(std, c.), which passes downwards, parallel to and in close contact

int.o

adxc

s.Jbl

std.c

sfd-

spl B

mrd.c

inf

1-' 1( , . 147. Hormiphorapluxnosa, diagrammatic longitudinal (A) and transverse (B)sections.
The ectoderm is dotted, the endoderm striated, the mesoglcea black, and the muscular axis of
the tentacles gray. Lettering as in Fig. 140, except ex. j>., excretory pore.

202

ZOOLOGY

SECT.

with the stomockeum, and a tentacular caned (t. c.) which ex-
tends outwards and downwards into the base of the correspond-
ing tentacle. Each tentacle presents a thickened base (f. >.),
closely attached to the wall of the sheath, and giving off a long-
flexible filament, beset with processes of two kinds one simple
and colourless, the other leaf-like, beset with branchlets, and of a
yellow colour.

Cell-layers. The body is covered externally by a delicate
ectodermal epithelium (Fig. 147), the cells from which the combs
arise being particularly large. The epithelium of the stomodaeum
is found by development to be ectodermal, that of the infundibulum
and its canals endodermal : both are ciliated. The interval between
the external ectoderm and the canal-system is filled by a soft jelly-

ad c

-s c

Fitt. 148. Hormiphora plumosa. A, transverse section of one of the branches of a tentacle :
B, two adhesive cells (ml. r) and a sensory cell (g. c) highly magnified. CM. cuticle; nu.
nucleus. (After Hertwigand Chun.

like mesoglcea. The tentacle-sheath is an invagination of the ecto-
derm, and the tentacle itself is covered by a layer of ectoderm,
within which is a core or axis formed by a strong bundle of longi-
tudinal muscular fibres, which, as we shall see, are of mesodermal
origin, and which serve to retract the tentacle into its sheath.

Delicate muscle-fibres lie beneath the external epithelium and
beneath the epithelium of the canal-system, and also traverse
the mesoglcea in various directions. The feeble development
of the muscular system is, of course, correlated with the fact
that the swimming-plates are the main organs of progression,
the Ctenophora differing from all other Coelenterata in retaining
cilia as locomotory organs throughout life.

A further striking difference between our present type and the
Ccelenterata previously studied is the absence, in Hormiphora, of
stinging-capsules. The place of these structures is taken by the

IV

PHYLUM CCELENTERATA 203

peculiar adhesive-cells with which the branches of the tentacles
are covered. An adhesive-cell (Fig. 148, ad. c.) has a convex surface
produced into small papillae, which readily adheres to any object
with which it comes in contact and is with difficulty separated.
In the interior of the cell is a spirally coiled filament, the
delicate inner end of which can be traced to the muscular axis of
the tentacular branch. These spiral threads act as springs, and
tend to prevent the adhesive -cells being torn away by the struggles
of the captured prey.

Both the central nervous system and the principal sense-
organ are represented by a peculiar apparatus situated, as already
mentioned, at the aboral pole. In this region is a shallow depres-
sion (Fig. 149, c.|>.) lined by ciliated epithelium and produced in
the transverse plane into two narrow ciliated areas, the polar
plates (p.pl.\ From the depression arise four equidistant groups
of very large S-shaped cilia (sp~), united to form as many sprinys (sp.),
which support a mass of calcareous particles (I.), like the lithites

C.fff

Fio. 14!). Hormiphora plumosa, Sense-organ : b. bell ; c. p. ciliated plate ; <:. <//. ciliated
groove ; i.e. f. excretory pore ; 1. lithites ; p. pi. polar plate ; sp. spring. (Modified from Chun.)

of Hydrozoa and Scyphozoa. From each spring a cilint<'<l groove
(c.gr.) proceeds outwards, bifurcates, and passes to the two swimming-
plates of the corresponding quadrant. The otolithic mass, with
its springs, is enclosed in a transparent case or bell (b), formed of
coalesced cilia. It appears that the whole apparatus acts as a
kind of steering-gear, or apparatus for the maintenance of equili-
brium. Any inclination of the long axis must cause the otolithic
mass to bear more heavily upon one or other of the springs: the
stimulus appears to be transmitted by the corresponding ciliated
groove to a swimming-plate, and results in a vigorous movement
of the combs. Thus the sensory pit acts as a central nervous
system, and the ciliated grooves as nerves.

Reproductive Organs. The animal is hermaphrodite, the
organs of both sexes being found in the same individual. The
yonads&YQ. developed in the meridional canals (Fig. 146, B), each of
which has an ovary (ovy.) extending along the whole length of one
side, a spermary (spy.) along the whole length of the opposite side.
The organs are so arranged that in adjacent canals those of the

204

ZOOLOGY

SECT.

v .m

rui,

same sex face one another. It will be seen that the reproductive
products have, as in Scyphozoa and Actinozoa, the position of
endoderm-cells : whether they are developed, in the first instance,

from that layer is uncertain.
When ripe the ova and
sperms are discharged into
the canals, make their way
to the infundibulum, thence
to the stomodgeum,and finally
escape by the mouth. Im-
pregnation takes place in the
water.

Development. The pro-
cess of development has been
traced in several genera
closely allied to Hormiphora,
so that there is every reason
to believe that, in all essen-
tial particulars, the following description will apply to that genus.
The egg (Fig. 150) consists of an outer layer of protoplasm (jj/sw.)
containing the nucleus (nu.), and of an internal mass of a frothy
or vacuolated nature (yk.}: the vacuoles contain a homogeneous
substance which serves as a store of nutriment to the growing-
embryo, and apparently corresponds with the yolk which we shall
find to occur in a large proportion of animal eggs. Enclosing
the egg is a thin vitclline membrane (v.m.), separated by a consider-
able space, filled with a clear jelly, from the protoplasm.

After impregnation the oosperm segments, but the details of
the process are very different from those we are familiar with in

FIG. 150. Ovum of Lampetia. nu. nucleus
plsm. protoplasm ; r. in. vitelliue membrane
yk. yolk. (After Chun.)

FIG. 151. Segmentation of the oosperm in Ctenophora. nig. megameres ; ml. micromeres ; plsw..
protoplasm ; yk. yolk. (Modified from Korschelt and Heider.)

the other Coelenterata. The protoplasmic layer accumulates on
the side which will become dorsal, and the oosperm divides along

iv PHYLUM CCELENTERATA 205

a vertical plane, forming two cells each with a sort of protoplasmic
cap (Fig. 151, A, plsm.}. A second division takes place at right
angles to the first, producing a four-celled stage (B), and each of
the four cells divides again into daughter-cells of unequal size, the
result being an eight-celled embryo, each cell with a protoplasmic
cap at its dorsal end (C, D). Next a horizontal division takes
place, dividing off the protoplasmic caps as distinct cells, and so
producing a sixteen-celled stage (E, F) in which we can dis-
tinguish eight large, ventral, yolk-containing cells or megameres
(mg.), and eight small, dorsal, protoplasmic cells or micromeres (mi.).
The micromeres increase rapidly in number by division, and are
further added to by new, small cells being budded off from the
megameres (Fig, 151, G, H, and Fig. 152, A). The result of this
increase is that the micromes gradually overspread the megameres
(Fig. 152, C), the final result being the production of an embryo
consisting of a central mass of large yolk-containing cells (ma.),
partly surrounded by an epithelium-like layer, incomplete below,

FIG. 152. Three stages in the development of Ctenophora. ;un. megameres ; mi.

(From Lang's Coiiiparatisc Anatomy.)

micromeres.

of small cells (mi.). This stage corresponds with the gastrula of
preceding types, the micromeres forming the ectoderm, the mega-
meres the endoderm, and the ventral edge of the ectodermal
'investment representing the blastopore. There is, however, no
archenteron or gastrula-cavity, and the stage has been produced,
not by a process of invagination or tucking-in, but by one of cpiboly
or overgrowth.

The endoderm-cells increase in number, and become much
elongated, and arranged obliquely, their long axes radiating,
upwards and outwards, from the long axis of the entire embryo
(Fig. 153, A). Their lower (ventral) ends then become divided off,
forming a number of small cells, which constitute the rudiment
of a true middle cell-layer or mesoderm (A, me.). A kind of in-
vagination of the megameres with their mesoderm cells then takes
place, resulting in the formation of a cavity the infundibulum
(B, d.) bounded below by the megameres, now placed horizontally,
and above by the mesoderm. The mesoderm gradually retreats to
the dorsal surface (C), finally spreading out between the dorsal
ectoderm and the infundibulum. At the same time the ectoderm
cells bounding the aperture of the infundibulum grow into it so

206

ZOOLOGY

SECT.

as to line its ventral portion : in this way the stomodseum (st.) is
produced. The remainder of the cavity widens out and becomes

me

me

FIG. 153. Three stages iu the development of Callianira. -'. iufundilmlum ; co. ectodenn ;
i ,'. endoderm ; me. mesoderm ; st. stomodseum. (From Lang's Comparativi Anatomy.)

the definitive infundibulum (d.), and before long sends off four
adradial pouches, the rudiments of the canal-system. At the same
time a gelatinous layer (Fig. 154, g.), the mesoglcea, makes its

appearance between the ectoderm and
endoderm.

The later processes of development
may be described very briefly. The
canal-system gradually assumes its
adult complexity and the swimming-
plates appear. A thickening of the
ectoderm on each side of the body
gives rise to the epithelium of the
tentacle and of its pouch. The muscle-
fibres forming the axis of the tentacle
(B, me.) are derived from the mesoderm,
which also gives rise to the contractile
fibres of the mesoglcea (me 1 .). The
otoliths are formed in the ectoderm-
cells of the apical pole, but gradually
make their way on to the free surface
of the cells and become supported on
four groups of fused cilia. Four outer
groups of cilia unite with one another
to form the bell (.$/,'.).

The most noteworthy points in this
somewhat complex process of develop-
ment are the following :

1. The distinction between a purely
protoplasmic part of the egg and a yolk-
^etop^nt^lsSa^ira! containing portion. In the Hydrozoa

d.infundibulum ; c. endoderm ; anc { Actinozoa the Volk-material is
a. mesoglcea; /' . mesoderm; ... , ^ , -.. .-. -,

si-, st-nse-organ ; st. st,,m,.d;<-uui ; small in amount ana evenly distributed.
te "^;X;o La " g ' ! the egg being described as alecithal or

v PHYLUM CCELENTERATA 207

yolkless. In the present instance the yolk is at first accumulated
in the centre of the egg, which is thus centrolecitkal or mid-yolked,
but soon the protoplasm accumulates at one end and the yolk at
the opposite end of the developing embryo, producing a telolecithal
or end-yolked condition.

2. The fact that segmentation is wtcqi/n/, there being a distinc-
tion into large cells or megameres, containing yolk, and purely
protoplasmic small cells or micromeres.

3. The formation of a peculiar type of gastrula by epiboly or
overgrowth, the ectoderm cells (micromeres) growing over and
partly enclosing the endoderm cells (megameres).

4. The presence, for the first time in the ascending animal
series, of a true middle embryonic layer or mesoderm. In the
other Coelenterata, as well as in the Sponges, two embryonic layers
only are formed, and the intermediate layer of the adult is formed
by the comparatively late separation of muscle-cells and connec-
tive-tissue fibres either from ectoderm or endoderm. In the
present case a definite layer of mesoderm cells becomes separated
from the endoderm during the gastrula stage.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Ctenophora are pelagic Coalenterata in which the formation
of colonies is entirely unknown. No indication of a polype-stage,
so characteristic of the remaining Coelenterata, can be detected
either in the adult or in the embryonic condition. Ciliary move-
ment, instead of being a merely embryonic form of locomotion as
in the preceding classes, is retained throughout life, the cilia being
fused to form comb-like structures, which are arranged in eight
meridional rows or swimming-plates. Tentacles, when present,
are usually two in number, situated in opposite (right and left)
per-radii, and retractile into pouches. The enteron communicates
with the exterior by a large stomodgeum which functions as the
chief digestive cavity. From the enteron is given off a system of
canals, the ultimate branches of which are adradial and have a
meridional position, lying beneath the swimming-plates ; a single
axial canal is continued to the aboral pole, where it commonly
opens by two excretory pores. There are no gastric filaments.
The central nervous system is represented by a ciliated area on
the aboral pole, and is connected with a single sensory organ,
having the character of a peculiarly modified lithocyst. The
organs of both sexes are lodged in the same individual, the ovaries
and testes being formed on opposite sides of the meridional canals.
The oosperm undergoes unequal segmentation, the gastrula is
formed by epiboly or overgrowth, and a definite mesoderm is
established during the gastrula stage. There is no alternation of

208 ZOOLOGY SECT.

generations ; but in some cases development is accompanied by a
well-marked metamorphosis.

The Ctenophora are divisible into four orders as follows :

ORDER 1. CYDIPPIDA.

Ctenophora having two tentacles, retractile into sheaths, and
unbranched meridional and stomodaeal vessels. The body is
either circular in section or is slightly compressed in the trans-
verse plane (Figs. 145 and 155).

ORDER 2. LOBATA.

Ctenophora having numerous non-retractile lateral tentacles
contained in a groove : the bases of the two principal tentacles are
also present, but have no sheaths. The stomoda?al and meri-
dional vessels unite with one another. The body is compressed in
the vertical plane, and is produced into two large oral lobes or
lappets and into four pointed processes or auricles (Fig. 156).

ORDER 3. CESTIDA.

Ctenophora having a band-like form owing to the extreme
compression of the body in the vertical plane. The bases of the
two principal tentacles are present, enclosed in sheaths, and there
are also numerous lateral tentacles contained in a groove. Union
or anastomosis of the meridional and stomoda?al vessels takes
place (Fig. 157).

ORDER 4. BEROIDA.

Ctenophora having no tentacles. The mouth is very wide, and
the gullet occupies the greater part of the interior of the body.
The meridional vessels are produced into a complex system of
anastomosing branches (Fig. 158).

Systematic Position of the Example.

Hormiphora plumosa is a species of the genus Hormiphora, be-
longing to the family Pleurobraehiadce and to the order Cydippida.

The presence of two tentacles, retractile into sheaths, and of
unbranched meridional canals places it in the order Cydippida.
In this order there are three families, amongst which the Pleuro-
brachiadcv are distinguished by the absence of any compression of
the body, the transverse section being circular. The genus
Hormiphora is distinguished by having a rounded body somewhat
produced at the oral pole, and by the aperture of the tentacle-
sheath being on a higher level than the funnel. In the species
plumosa the stomodieal ridges are of a brown colour, and the leaf-
like branchlets of the tentacles yellow.

IV

PHYLt'M CCELENTERATA

20!)

3. GENERAL ORGANISATION.

Compared with the two former classes of Ca'lenterates, the Hydrozoa and
Actinozoa, the organisation of the Ctenophora is remarkably uniform. This is
due to the fact that all the species are pelagic, none are colonial, and none form
skeletons. Nevertheless a very great diversity of form is produced in virtue of
differences in proportion and modifications of the tentacular and canal systems.

The Cydippida agree in all essential respects with Hormiphora, the most
important deviation from the type-form being the compression of the body in the
transverse plane in some genera, e.ij. Euch/ora (Fig. 15.">, 2), the result being an
oval instead of a circular transverse section, with the tentacles at the end of the

mtfi
l.Callianira

2.Euchlora

3. L arrive Ha

KH-. 1 ">">. TlnvL 1 Cydippida. >>. /'. al>iMl l>rui-c.ss ; mil,, niuutli. (Af.i,-;- C'.i'in )

long axis. The aboral pole may be produced into wing-like appendages, as in
Cattianira (1), and in Lampetia (3) the mouth is so dilatable as to form, when
expanded, a sole-like plate by which the animal retains itself on the surface of the
water or creeps over submarine objects. In Enchlora nthra minute nematocysts
have been found, and there is reason to believe that it was \)y the modification
of these characteristic coelentei'ate organs of offence that the adhesive cells of
Cteuophora were evolved.

The Lobata, for instance Deiopea, are distinguished, as their name implies,
by the presence of a pair of large lappets (Fig. 15(3, //>.), into which the oral
sin-face is produced at either end of the vertical plane. Four of the swimming

VOL. I. P

210

ZOOLOGY

SECT.

plates are shorter than the others, and at their bases arise elongated processes
called aitfir/i * {mo:), which bear swimming-plates. The meridional canals (mdr.c)
unite with one another, and, with the cesophageal canals, are continued into the
lappets, where they become curiously coiled. The principal tentacles are
usually absent in the adult, but are represented by their basal portions, which

rnrd.c

Fi<;. 15(.i. Deiopea kaloknenota. A. adult ; B. young, m'.,: auricle ; I p. lappet ; I. t. lateral
tentacles; mrd. c. meridional canal ; mth. mouth. (After Chun.)

are small, situated at the oral end, and devoid of sheaths. From each tentacle-
base grooves are continued along the oral surface to the auricles, and from the
grooves depend numerous small lateral tentacles (/./.). In the young condition
the Lobata resemble such compressed Cydippida as Euchlora, having a pair of
long principal tentacles, no lappets, and unbranched vessels (B).

The Ce*tida are represented by the remarkable " Venus's Girdle" (Cent-its
venerin), a band-shaped Ctenophore (Fig. 157) which sometimes attains a length
of li- metre, or nearly 5 feet. The body is greatly elongated horizontally in the

FIG. 157. Cestus veneris. A, adult, B. young. /. t. lateral tentacles ;
.. t ii:- swimming-plates ; t. tentacle. (After Chun.)

mouth :

vertical, and compressed in the transverse, plane, so as to have the form of a
ribbon, which progresses by undulations of the whole body as well as by the
action of its swimming-plates. Four of the swimming-plates (*.pl. 1 ) are very
small ; the other four (n.pl. 2 ) are continued all along the aboral edge of the body.
The bases of the two principal tentacles (t.) are large and are enclosed in sheaths,
and, as in Lobata, numerous small lateral tentacles (l.t.) spring from grooves

IV

PHYLUM CCELENTERATA

which, in the present case, are continued the whole length of the oral edge.
The young of Cestus (B) resembles a compresssd Cydippid which undergoes
gradual elongation in the median plane.

Beroe, the principal genus of the Beroida, lias the form of a cylinder (Fig. 1.18),
one end of which is rounded and bears the sense-organ, the other truncated
and occupied entirely by the immense mouth (mth.). The greater part of the
body is taken up by the huge gullet ; the mfundibulum (inf.), per- radial and
infundibular canals, &c. , being all crowded into
a small space at the aboral pole. The meri-
dional canals send off branches which unite
with one another, forming a complex net \vork-
of tubes, and at their oral ends the four meri-
dional canals of each (right and left) side and
the corresponding stomoda?al canal unite into
a horizontal tube, which runs parallel with the
margin of the mouth. There is no trace of
tentacles either in the adult or in the embryonic
condition.

The Ctenophora are usually per-
fectly transparent, and quite colour-
less, save for delicate tints of red,
brown, or yellow, in the tentacles and
stomodteal ridges. Cestus has, how-
ever, a delicate violet hue, and when
irritated shows a beautiful blue or
bluish-green fluorescence. Beroe is
coloured rose-pink.

Ctenophora are found in all seas
from the arctic regions to the tropics.
As is to be expected from their
perishable nature, there is no trace of
the group in the fossil state.

A very remarkable fact has been
made out with regard to Bolina
hydatina, one of the Lobata, a Cteno-
phore which attains a diameter of

25-40 mm. While still in the larval or cydippid condition,
and not more than 0'5-2 mm. in diameter, it becomes sexually
mature, the gonads producing ripe ova and sperms, and the
eggs are impregnated and develop in the usual manner. Soon
the gonads degenerate, the larva metamorphoses into the adult
form, and a second period of sexual maturity supervenes. This
precocious ripening of sex-cells occurs, as we shall see, in other
animal groups and is called pcvdogcnesis.

FIG. 158. Beroe forskalii

i, >f. iufimdibuluin ; mtli. mouth :
.. ill. swimming-plates. (After
Chun.)

P 2

212

ZOOLOGY

APPENDIX TO CTEXOPHORA

CTEXOPLAXA AND OELOPLANA.

Before leaving the Ctenopkora mention must be made of two remarkable
organisms which have been supposed to connect the present class with the
Turbellaria Polj-cladida, or Planarians, a group of worms to be described in
the following section.

(.'t< noplana (Fig. 159) is a small marine animal, nearly circular in outline,
flattened dorso-ventrally, and about 6 mm. in diameter. It has hitherto been
found only twice once in the Indian Ocean and once in New Britain. Instead
of swimming freely, like a Ctenophoran, it creeps on its ventral surface, like
a worm. In the centre of the dorsal surface is a vesicle (.s\o. ) containing
an otolith surrounded by eight radiating ridges (r.r.), alternating with which
are as many clefts ('/.), each containing a protrusible row of stiff processes,

2

\gBix ; ' . f-

,

' ' :''"''"

Fu.. l.v.>. Ctenoplana kowalevskii, A. hvm above, II. fi-ma the side. ct. clefts; r. r.
radiating ridges ; *. <i. seiisu-ury.iii. lAt'tei- K'ii-"tiieft'.)

resembling the swimming-plates .of Ctenophora. The mouth is in the centre
of the ventral surface, and leads into a stomach, from which are given off
numerous anastomosing canals, as well as a vertical canal which passes upwards
and ends beneath the sense-organ. In diverticula of this system are formed the
testes, which have independent ducts opening on the exterior. There are two
solid tentacles contained in sacs, and a nerve-centre lies beneath the sense-organ
(*.o. ). Beneath the ectoderm is a basement -membrane, which acts as an organ
of support, and the muscular system is complex. Near each tentacle is an
aperture leading into a branched canal which is probably excretory, like the
nephridial tubes of flat-worms.

< 'ulu/Jiiua is found in the Pved Sea. It is also flattened dorso-ventrally, but
is oval instead of circular in outline, its dimensions being about 6 by 4 mm. It
resembles Ctenoplana in its ventral mouth, dorsal sense-organs, paired retractile
tentacles, and complex system of anastomosing canals from the stomach. There
are, however, no swimming-plates, and the ectoderm is ciliated.

Nothing is known of the development of either genus.

iv PHYLUM C(ELENTERAT\

THE RELATIONSHIPS OF THE CtELEXTERATA

There can be little doubt that the lowest ccelentevate form
known to us is the simple hydrozoan polype, represented by
Hydra and by the hydrula stage of many Hydrozoa. Somewhat
more complex, in virtue of its stomodseum and gastric ridges and
filaments, is the scyphozoaii polype, represented by the Scyphula
of Aurelia, Still more complex is the actinozoan polype, or
Actinula, as it may be called, with its large stomodseum, mesen-
teries and mesenteric filaments, and elaborate muscular system.
Speaking generally, one may say that these three polype-forms
represent as many grades of organisation along a single line of
descent.

The medusa-form in the Hydrozoa is, as we have seen, readily
derived from the hydrula by the widening out of the tentacular
region into an umbrella. 'We may thus conceive of the Trachy-
lime, or hydroid medusae with no fixed zoophyte stage, as being
derived from a pelagic hydrula.

The Leptolinse may be considered to have arisen in consequence
of the adoption of asexual multiplication, by budding, during the
larval or hydrula stage. Instead of the hydrula giving rise
directly to a medusa, we may suppose it to have formed a temporary
colony, by budding, after the manner of Hydra, the individual
zooids being ultimately set free as medusa?. The next stage
would be the establishment of a division of labour, in virtue
of which a certain proportion only of the zooids became medusa?,
the rest retaining the polype-form, remaining permanently
attached, and serving for the nourishment of the asexual colony.

The Hydrocorallina appear to be a special development of the
leptoline stock, the nearest affinities of the order being with such
forms as Hydractinia.

The Siphonophora may be conceived as having originated from
a hydrula specially modified for pelagic life by the conversion of
the basal disc into a float something after the fashion of Minyas
(Fig. 139). In such a form extensive budding, accompanied by
division of labour, would give rise to the complex siphonophoran
colony.

The lowest Scyphozoa are the Stauromedusse, some of which,
however, show evidence of degeneration, so that it is quite possible
to conceive them as having been derived from more highly
organised forms, instead of springing directly from simple polypes
of the Scyphula type. The Cannostomse, Semostom;e, and
Rhizostomse clearly represent three grades of increasing com-
plexity along the same general line of descent. So little is known

214

ZOOLOGY

SKI T.

of the development of the remaining Scyphozoa that it is advisable
to leave their position an open question.

The close similarity of Edwardsia and the Alcyonaria in the
number and arrangement of the mesenteries seems to indicate the
derivation of both Zoantharia and Alcyonaria from a common
ancestor in the form of a simple actinozoan polype or actinula.
Edwardsia clearly leads us to the Hexactinia? or typical Sea-
anemones, and the Madreporaria are undoubtedly to be looked
upon as skeleton- forming Hexactinia?.

The relationships of the Ctenophora to the other Coelenterata are
very doubtful. Cteriaria,one of the Anthomedusa3(Fig. 96,^), presents
some remarkable resemblances to a Cydippid, such as Hormiphora.
It has two tentacles, situated in opposite per-radii, and each having
at its base a deep pouch in the umbrella resembling the sheath of
Hormosira. There are eight radial canals formed by the bifur-
cation of four inter-radial offshoots of the stomach, and corre-
sj M mding with them are eight bands of nematocysts diverging
from the apex of the ex-umbrella. If these striking resemblancer
indicate true homologies we must compare the whole sub-umbrellar
cavity of Ctenaria with the stomodseum of Hormiphora, the
margin of the bell of Ctenaria with the mouth of Hormiphora,
and the mouth of Ctenaria with the aperture between the
stomodaeum and the infundibulum of Hormiphora. But, as we
have seen, the gullet of Ctenophora is a true stomodoeum de-
veloped as an in-pushing of the oral ectoderm, and has there-
fore a totally different origin from the sub-umbrella of a
medusa. Moreover the tentacles of Ctenaria have no muscular
base contained in the sheath, but spring from the margin of
the umbrella as in other Hydrozoa : its gonads are developed in
the manubrium, not in the radial canals, and there is no trace of
an aboral sense-organ.

On the other hand, the resemblance between transverse sections

A

end

B

end

J-'i .. !<:(.>. Transverse section of uiiilnyo.- of Actinia (A) ;md Beroe (11). n't. ectoderm ;
end. endocierm ; i,if. infmidiliulum. (After Chun.)

of an embryo Ctenophore (Fig. 100, B) and of an embryo Actinian
(A) is very striking, and the presence of a well-developed stomo-
dneum, and of gonads developed in connection with the endoderm

IV

PHYLUM CCELENTERATA

215

and discharging- their products through the mouth, may be taken
as further evidences of affinity between the Ctenophora and the
Actinozoa.

The special characteristics of the Ctenophora are, however, so
numerous and so striking, and their development so utterly unlike
that of any of the other Ccelenterata, that in our present state of
knowledge it is impossible to determine their affinity with the
other classes with any degree of certainty.

As to the orders of Ctenophora, it seems tolerably clear that
both Lobata and Cestida are derived from cydippid forms, since
they both pass through, in the course of development, a stage
closely resembling the lower Cydippida. The Beroida are more
highly organised in certain respects, e.g. in the details of their
histology, than the other Ctenophora, and it seems quite possible
that they may be derived from tentaculate forms.

These relationships are expressed in the following diagram :

Madrefjoraria

Hexachnia /

\ / Alcy Onaria

Cesh'da

B e ro'i da

EcLuiarcLsra. \
Rhizost"omce \ '

ACTINULA

Cannosf-omCB

Shau.ro medusae Y

SCYPHULA

Hydrocorallina.

JTrachylincc

H YD RULA

Frc;. 101. Diagram illustrating the mutual relationships of the Coelentemtu.

By many authors the Sponges have been looked upon as so
closely related to the Ccelenterata to be capable of being regarded
as members of the same great phylum. The points of resemblance
are readily to be recognised: the simple structure, with the large cen-
.tral cavity into which a wide opening the mouth or the osculum,
as the case may be leads ; the absence of a well-developed meso-
derm, the fixed mode of life, and, associated with it, the tendency
to form compound structures or colonies by a process of budding.
In addition, the occurrence in both groups of the planula and

216 ZOOLOGY SECT.

gastrula larval stages constitutes an important connecting link.
But a closer examination of the -subject shows that some of these
apparent points of resemblance are superficial only, and establishes
a number of differences between Sponges and Ccelenterates too
important to allow us to suppose that a close relationship exists.
One of these differences stands out beyond the others as the most
radical. The osculum of a sponge is found, when we trace the
development of the larva, to correspond in no sense with the
mouth of the ccelenterate. The latter corresponds with the blas-
topore or gastrula mouth. In the Porifera the gastrula mouth is
(Fig. 82, p. 114) found in all cases in which the details have
been made out with certainty to become applied to the substratum
when the larva fixes itself, and the osculum is developed at the
opposite extremity of the body. This alone, apart from important
differences in the adult structure, such as the presence in the wall
of the sponge of the system of inhalant apertures, the presence of
the peculiar collared endoderm cells, and the absence of stinging
capsules would suffice to remove the Sponges from the Ccelenterata,
and place them in a phylum apart. On the other hand, that the
Sponges and Ccelenterates were originally derived by a common
root from the Protozoa i.e. possessed a common metazoan ancestor
is rendered very probable when we consider the similarity that
"xists between the members of the two groups in the earlier stages
of their development.

APPENDIX TO THE CCELENTERATA

THE MESOZOA.

Under the designation MESOZOA have been comprised certain lowly organised
animal forms, formerly supposed to afford us something of the nature of a
connecting link between the Protozoa and the Metazoa, but now more generally
looked upon as degenerate members of the latter sub-division. It has been
proposed to term them the Planuloidea, from the resemblance which they bear
to the Planula larva of the Ccelenterates.

They are all multicellular, with an ectoderm composed of a single layer of
cells ciliated in whole or in part, and an endoderm either composed of a single
elongated cell or of sevei-al cells ; a mesogkea is not represented. The Mesozoa
comprise three families, the Dicyemidd', the If<t< rofi/t miilic, and the Orthonectida .
all the members of which are internal parasites.

The Difj/tniidn' are parasites in the kidneys of various Cuttle-fishes and
Octopi (Cephalopoda). The animal (Fig. 162), the length of which is between
0'7o and (j or 7 millimetres, consists of a head-part and an elongated body.
The form of the head varies a good deal, according to age : in young specimens
it is isotropic (i.e. symmetrical around the long axis) ; in the adult condition
ventral and dorsal sides are distinguishable. It consists of a swollen disc of
four cells and a ring of four or five pole cells. The cells of the head all bear
cilia, which are shorter and thicker than those of the body cells.

The body consists of a single large axial endoderm cell and of a single layer
of ectoderm cells, which completely invest the axial cell. The ectoderm cells
which follow immediately on the head are distinguishable from the rest by their
granular contents, and by their being dilated internally in such a way that the
apex of the axial cell is constricted. Originally all the ectoderm cells are

IV

PHYLUM CCELENTERATA

cubical ; afterwards they become drawn out so as to be more spindle-shaped, and
at the same time become bent in such a way as to present a concave internal and
a convex external surface ; the latter is always covered with long cilia. The
two most posterior are usually semi-cylindrical and surround the posterior end
of the endoderm cell.

The endoderm cell is either almost completely cylindrical or spindle-shaped.
It is covered in its entire extent by ectoderm cells. There is a differentiated
cortical layer, beneath which the finely giannlar gelatinous contents are at first
homogeneous, but afterwards become vacuolated. In the middle of the cell is a

Km. ir,-_>._ Dicyema paradoxum,
with infusorifonn embryos. (From
'

FKJ. lt>3. Dicyema paradoxum,
witli vermiform embryos. (From
Uronn's TIiii.i-i-t.ii-li.)

large oval or ellipsoidal nucleus. In addition to the nucleus of the endoderni
cell itself other nuclei also occur in it ; these are the nuclei of the i/trw-a {/<.

Two forms of adults are to be distinguished, termed respectively the
the iiematogene and the rhombor/eite : the former (Fig. 163) give rise to r> rm/form,
latter (Fig. 16'2) to infusoriform embryos. The rhombogenes are shorter and thicker
than the nematogenes ; the number of ectoderm cells is smaller, and the germ-
cells small and produced endogenously in the axial cell, instead of being formed
in special cells enclosed in the axial. The rhombogene and iiematogene forms
may be phases in the life-history of the same individual ; but some always
possess the iiematogene form.

The first-formed germ-cells are derived by cell-division, accompanied by
mitosis, from the nucleus of the axial cell. Subsequently they increase in
numbers by division. A germ destined to give rise to one of the vermiform

218

ZOOLOGY

SECT.

embryos goes through a process similar to segmentation. Of the cells thus
formed one gives rise to the axial cell ; the others, increasing in numbers and
becoming smaller, gradually grow over the axial cell until they at length
completely enclose it. This process is a simple form of epiboly, and the part
which is last to be covered by ectoderm cells (the posterior
end) corresponds to the blastopore. The embryo increases
greatly in length, and escapes from the interior of the parent
by perforating the body-wall.

The germ destined to give rise to an infusoriform embryo
likewise undergoes segmentation and epibolic gastrulation.
The fully formed infusoriform embryo (Fig. 164) is pear-
shaped, the broader (head) end being that which is directed
forward in swimming, and is completely bilaterally symmetri-
cal. The ectoderm cells enclose an axial cell containing several
smaller multi-nucleated cells which can be ejected apparently
voluntarily. The fate of these embryos is not known ; it has
been suggested that they may be males, and that the cells
thrown out may be male cells.

The Heteroct/emiiln; which are also parasites of the Cephalopoda, resemble the
Dicyemidse in most respects, but want the head.

The family Orthonectidce comprises only one genus Rhopalura with two

FIG. 104. Embryo
of Dicyema
paradoxum.

(From Bromi's
Thierreicli.)

FIG. K.,. Rhopalura Giardii,

(From Bruim's Thicmich.)

FIG. 100. Rhopalura Giardii,

(From Broun's Tliierrciclt.)

species, one of which is parasitic in the genital pouches of an Ophiuroid, while
the other is a parasite of a Nemeitean worm. In form they are spindle-shaped,

IV

PHYLUM CCELENTERATA

usually rounded at the ends, without any trace of bilateral symmetry. The
body is divided into several rings or segments. Beneath the single layer of cells
constituting the ectoderm is a layer of muscular fibres. In the centre is a mass
of endoderm cells. There are distinct male (Fig. 165) and female (Fig. 166)
individuals, the former fusiform in shape, the latter oval, cylindrical, or com-
pressed. In one of the species there are two different forms of females, the one
producing only male young, the latter only female.

Motile sperms are developed in the interior of a mass of sperm-cells the
teatis. When mature these pass out between the muscular fibres ; the ectoderm
cells disintegrate and the sperms es-
cape, the animals perishing. In the
cylindrical female the conical anterior
segment of the body becomes separated
oft' from the rest as a sort of operculum,
permitting of the discharge of the ova.
In the compressed forms the body be-
comes broken up into fragments, in
which the ova lie embedded. After
impregnation the eggs are developed
in the interior of protoplasmic masses,
one such mass in some cases containing
only male or only female embryos, in
other cases both. The male ovum (Fiu.
167) divides into two cells, a smaller
and a larger ; the latter remains for
a time undivided, but subsequently

segments to form the cells of the endoderm. The former at once divides to form
a number of small cells, which grow over the large cell to form a continuous
ectoderm layer by a process of epiboly. There is some discrepancy between the
statements of different authors as to the development of the females ; but there
appears to be unequal segmentation followed by epiboly, the peripheral layer
of the endoderm afterwards giving rise to the middle fibrous stratum (mesoderm).

SALINELLA.

Perhaps having more claim than the Orthonectidse and Dicyemidse to a position
intermediate between the Protozoa and the Metazoa is a remarkable infusorian-

^s^^sjs^-tg,;

FIG. Uis. Salinella, Imiyitudinfil section. (After Frenzd.)

Fn.. 10','. Salinella, transverse section. (After Frenzel.)

220

ZOOLOGY

SECT. 1\

like animal, tiallm.f/u, found in brackish water from the Argentine Republic.
Salinella (Figs. KiN and 160) is a minute animal in the form of a somewhat de-
pressed cylinder, open at both ends and with a wall composed of a single layer
of cells. The anterior end is somewhat pointed ; around the anterior opening or
mouth, which is ventrally directed, is a circlet of fifteen to twenty long whip-
like cilia. The posterior aperture (anus), which is usually closed, is surrounded
by a few stiff setae. The ventral surface is flattened, and is covered with tine
vibratile cilia, while on the dorsal surface and the sides are regularly arranged
rows of straight setae (non-motile cilia). The internal cavity (enteron) is found
to contain sand, plant fragments, and Bacteria ; its surface is beset with lung
cilia. Multiplication takes place by transverse fission ; and a process of
conjugation followed by encystation has also been observed.

TRICHOPLAX.

Another aberrant multicellular animal having affinities with the Protozoa, and
not assignable to any of the recognised groups of the Metazoa, is Trichoplax (Fig.
170). Trichoplax is a compressed plate-like body of irregular and extremely vari-
able shape, but circular in the resting condition, without trace of radial or bi-
lateral symmetry. The whole surface is covered with cilia, by means of which the
animal performs slow gliding movements, always in close contact with the sub-
turn. Frequently it puts forth a process, or more than ' 'ik-h may

Fii.. iTo. Trichoplax adbaerens, \rn-t f a vurn<-il section.
(From Lang, after F. E. ^chnlzc. )

extend to a considerable distance and assume a variety of -hapes. There art-
three distinct cell-layers an upper epidermis, consisting of flattened cells closelv
united by their edges so as to form a syncytium like that of the ectoderm of
Sponges ; a lower epidermis, composed of pyramidal cells, the apices of which are
produced and unite with the elements of the middle layer ; and a middle layer
of spindle-shaped or slightly branched cells, which anastomose to form a network
containing in its meshes hyaline, fluid ground-substance. In addition to their
nuclei, these cells of the middle layer enclose several kinds of highly refracting
granules and spherules. There are no stinging-cells, and no trace of internal
organs of any kind. Little hyaline papilla- which appear round the edges may
be for temporary fixation. Multiplication by simple fission has alone been
observed.

The position of Trichoplax and of the nearly allied T,\jnoj>lax is quite
uncertain. It cannot be a larval form, as it lias been kept under observation
for nearly a year without exhibiting any change. It is possible that both it and
S<i/'in<Jla, neither of which have been observed in quite natural circumstances, 1
maybe abnormal modifications brought about by the influence of exceptional

CO i :

1 Trichoplax has only been observed in marine aquaria ; Salinella only in
water in which salts from certain saline springs in South America had been
dissolved.

SECTION V
PHYLUM PLATYHELMINTHES

A NUMBER of classes of Metazoa, some a little, others very de-
cidedly, higher in organisation than the Coelenterata, are commonly
regarded as constituting one great sub-kingdom or phylum the
Vcrmes or Worms. The groups ordinarily referred to the Vermes
differ, however, very widely from one another: points of agree-
i in 'lit, except such as are merely negative, are, in fact, frequently
hardly recognisable: and rather than group together under one
common designation such a heterogeneous assemblage of forms, it
sci -ins better to avoid the term Vcrmes altogether and to endeavour
to divide the "worms" into phyla the members of which shall
have points of positive resemblance to one another. The four
phyla Platyhelminthes, Nemathelminthes, Troclielminthcs, Mollns-
coida, and Annulata, with their appendices, all consist of forms
which are or have been comprised in the Vermes. They differ
from the Coelenterata in the presence of three well-developed
body-layers of which the middle one, or mesoderm, is of relatively
predominant importance: and, for the most part, in the much
higher stage of complexity attained by the various systems of
organs. The first four phyla present no metameric segmentation:
in^the Annulata, metamerism is more or less strongly pronounced.

The Platyhelminthes or Flat-Worms are a group of soft-bodied,
bilateral, usually flattened animals, which are devoid of true
metameric segmentation (p. 41). With a sufficient degree of uni-
formity of structure to render the phylum a fairly compact and
well-defined one, there is yet a considerable range in complexity,
from the simplest forms, certain of which have been supposed to be
nearly connected with the Ctenophora among the Coelenterata, to
the highest, which have all the various systems of organs very
mueh more highly developed. The body is built up from
three embryonic lavers ectoderm, mesoderm, and i-ii<!n,l,Tm as in

222 ZOOLOGY

SECT.

all higher groups of animals. An excretory vascular system of
a peculiar kind the water-vascular system is present in all
members of the phylum. A body-cavity or ccelome (p. 279) is not
present, the spaces between the various organs and the wall of
the body being filled up with a peculiar form of connective-tissue
termed the parenchyma. The egg is composite, the egg-shell
enclosing not only the oosperm or impregnated ovum, but a
quantity of nutrient material or food -yolk, derived in most in-
stances from a special set of glands, the yolk or vitdline glands.

The main features which distinguish the Platyhelminthes from
the Coelenterata are, the pronounced bilateral symmetry with
the many secondary features which it involves,* the presence
of a middle embryonic layer or mesoderin, and the n on -occurrence
of fixed colonies formed by budding.

1. EXAMPLES OF THE PHYLUM.
i. A Fresh-water Triclad (Planaria or Dcndroccelum). 1

General Features. Species of fresh-water Planarians of the
genera Planaria and Dcndroccelum are common in the mud at
the bottom of ponds of fresh-water in all quarters of the globe..
They are small, thin, flattened worms a few millimetres in length,,
broader at one end, the anterior, than at the other, the posterior,.
which is more or less pointed. The animal (Figs. 171-173) is
very readily recognised to be bilaterally symmetrical, with an uj>^er
or dorsal and a lower or central surface, riylit and left borders.
and anterior and posterior ends. The colour varies in different,
species and in different individuals; bat is usually gray, red,
brown, or black. Movements of locomotion in the direction of the
long axis of the body are recognisable in the living animal. Some-
times this is a steady gliding movement, which is brought about
by the action of a coating of vibratile cilia on the surface ; some-
times the worm moves along somewhat after the fashion of a
Leech, the ventral surface of the anterior end of the body being
of a sticky adhesive character, and performing the part of the
anterior sucker of the Leech.

Close to the anterior extremity on the dorsal surface are two
rounded black spots, the eyes (Fig. 172). On the ventral surface,,
a considerable distance behind the middle of the body, is the
opening of the mouth (Fig. 171, mo.), and further back still, near
the posterior pointed end, is a smaller median opening, the genital
aperture (Fig. 173).

Digestive System. The mouth (Fig. 171, MO.) leads through
a short mouth-cavity into a cylindrical thick-walled chamber, the

1 The account is siifficieutly general to apply to species of either of these

PHYLUM PLATYHELMINTHES

J23

pharyiw (ph.), which is highly mobile, and is capable of being
thrust out as a proboscis through the mouth, beyond which it

FKI. 171. Planar ia. Digestive and excre-
tory systems, ex. openings of excretory
system ; int. intestine ; mo. mouth ;
n. ph. opening of pharynx. (After Jijima
and Hatschek.)

FIG. 172. Planaria. Nervous system-
/'/. brain ; tyt eye ; /. n<:. longitu-
dinal nerve ; pit. pharynx. (After
Jijima and Hatschek.)

may then be extended to a relatively considerable -distance,
When retracted it lies within an enclosing muscular sheath. The

i'i'4 ZOOLOGY SECT.

cavity of the pharynx opens in front into the intestine (int.), which
almost immediately divides into three narrow main branches, one
running forward in the middle line, the other two running back-
wards. Each of these three main branches gives off numerous
smaller branches, which in turn become branched, so that the
whole intestine forms a ramifying system, extending throughout
the greater part of the body ; all the branches terminate blindly,
an anal aperture being absent.

A system of vessels the water-vessels or vessels of the excre-
tory $i/*fciti(ex.) sends ramifications through all parts of the body.
There are two main, considerably coiled, longitudinal trunks, right
and left, which open externally on the dorsal surface by means of
several minute pores ; in front they are connected together by a
transverse vessel. Each main trunk gives origin to a number of
branches, which in turn give off a system of extremely fine capillary
vessels, many of which terminate in flame-cells (Fig. 203, p. 253).
A flame-cell is a nucleated cell having in its protoplasm a small
space, into which one of the capillaries leads; in this space lies a
bundle of vibratile cilia, or a single thick cilium, which performs
regular undulating movements, giving it somewhat the appearance
of a flickering candle-flame. Cilia also occur in the course of
some of the capillaries. This system of vessels is usually regarded
as excretory; but it may also have a respiratory function.

A well-developed nervous system (Fig. 172) is present. At
the anterior end is a central knot of nerve-matter, the brain (br),
from which proceed backwards two longitudinal nerve-cords (I. ne.}.
The brain consists partly of transverse fibres connecting to-
gether the two longitudinal nerve-cords, partly of groups of nerve-
cells situated at the ends, or in the course of, the nerve-fibres.
The nerve-cords give off both internally and externally numerous
transverse branches, which divide into finer twigs ; the internal
branches of the two cords frequently anastomose, thus forming
commissures or connecting nerve-strands between the two. A
number of nerves extend forwards to the anterior margin, which
is highly sensitive.

Reproductive System. The reproductive organs (Fig. 173)
are hermaphrodite or moncecious in their arrangement, both male
and female organs occurring in the same individual. The genital
aperture leads into a small chamber, the genital cloaca, which
is common to both the male and the female reproductive
systems.

The male part of the apparatus consists of tcstes, rasa deferentia,
and cirrus ov penis. The testes (tes.) are numerous rounded glands,
situated near the right and left borders. Two ducts, the right
and left vasa dcferentia (v. d.\ run backwards from the neighbourhood
of the te^tes and unite in the middle line posteriorly. The median
duct formed by the union of the two vasa deferentia traverses a

PHYLUM PLATYHELMINTHES

protrusible muscular organ, the r
the genital cloaca, At the base

i 'if

Flo. 173. Planaria. Reproductive system.
in. muscular sac ; or. ovary ; ;i/<. pharynx ;
j>. penis ; pi-, prostate ; tes. testes ; ul.
uterus ; c. it. vas deferens ; fit. \-itelline
glands. (After Jijima and Hatschek.)

the embryo leaves the shell it
shape of the parent.
VOL. I

i mis or penis (p), to open into
of the penis, where the vasa
differentia meet, the median
canal is slightly enlarged to
form a rounded dilatation, the
Tcsicuia seminahs. Into the
median canal open the narrow
ducts of a number of unicellu-
lar glands, the prostate glands
(pr\

The female part of the re-
productive apparatus consists
of ovaries, oviducts, vitelline
!/lands, uterus, and muscular sac.
The ovaries (or.) are two in
number small rounded bodies
situated near the anterior end,
each connected with an elong-
ated duct, the oviduct. The
two oviducts (od.) unite pos-
teriorly to form a short median
common oviduct opening into
the genital cloaca. With this
cavity are connected also the
uterus (ut.}, a median rounded
chamber, and a thick-walled
muscular body, the muscular
sac (m.). Numerous branching-
tubes the vitelline glands (vit.}
open into the oviducts.

Reproduction is entirely
sexual. The oosperm is en-
closed within a protecting case
or shell, which contains also
a quantity of food-yolk derived
from the vitelline glands.
When the larva has reached
a certain stage it develops a
temporary larval mouth and
gullet and swallows the food-

tolk, by the aid of which it
rows rapidly. The larval
mouth disappears, and a new
one the permanent mouth is
developed in its place. When
has assumed the characteristic

Q

226

ZOOLOGY

SECT.

{jar

eel

test

circ.mus

ext lanysrut-s

int long, ma

^v f l

dors rent. ??tu

sckr

Fn;. 174. Transverse section of a Planarian. <.-<,/. //<.. circular muscular fibres; <-<. in
testinal cceca ; dors. cent. unn. dorso-ventral mascular fibres; cet. ei.-tndt.-rin: ext. limit, mus.
external layer of longitudinal muscular fibres ; int. central lumen of the intestine ; int. long.
/,/".--. internal layer of longitudinal muscular fibres; ne. nerve-cords; o. .7. oviducts; j>u,-.
parenchyma; tuxt. testes ; ,-. </,/. vas deferens ; vit. vitelline glands. (From Hatschek's
Lehrbuch.)

ii. The Liver-Fluke (Distomum hepaticum).

General Features. The Liver- Fluke of the Sheep, which is to
be found in the interior of the larger bile-ducts of the infested
animal, is a soft-bodied worm of flattened leaf-like shape (Fig.
175), with a triangular process, the Innl-lul,,:. projecting from the
broader end. The symmetry of the parts is distinctly bilateral as
in the Planarian. Externally the body is quite equilateral, the
right and left portions exactly balancing one another, but, as will

appear subsequently, this complete symmetry
does not extend to all the internal organs.

The surface is devoid of vibratile cilia, but
is covered with innumerable minute spin H lex
or pcvpilloe, which are prolongations of the
homogeneous external layer or mt.irlc in-
vesting the whole animal. At the extreme
anterior end of the triangular head-lobe is
the small opening of the ,/n,///// (,,/n.} sur-
rounded by a muscular oral .S/T/V/-. Some
distance back on the ventral surface, just
behind the head-lobe, is a second much larger
posterior sucker (sckr.). Between the two
suckers, but rather nearer the posterior one,
is a median aperture, the </' :n it<d opening (rejir.),
through which a curved muscular process, the
cirrus or penis may be protruded. In the middle of the posterior
end of the body is a minute opening, the excretory pore (cxer.).

Body-wall. The body-wall (Fig. 176) is found on section to
comprise three layers: (1) A homogeneous rntidc (ct'.t.) of which
the spinules (sp.) are special developments; (2) a layer of circularly

tx.cr

I'll.. 17'.. Distomum
hepaticum, natural
size. excr. excretory
jiorc : am. mouth ; n pr.
reproductive aperture ;
trk,: posterior sucker.

PHYLUM PLATYHELMIXTHES

227

disposed muscular fibres (circ. mits. ): (3) a layer of longitudinal
muscular fibres (lomj. /////*.). A cellular epidermis is \\anting.
Beneath the muscles are numerous unicellular glands (///.), the
ducts of which, in the form of processes of the cells, open on the

FIG. 17(>. Distomum hepaticum. Section "f the integument, circ. mus. layer nt circular
muscular fibres ; re/, cuticle ; <il. unicellular glands ; lout/, nuts, layer of longitudinal
muscular fibres ; xji. spiiiulcs. (After Br.um.)

outer surface. Internally, the interspaces between the organs are
filled by a peculiar form of connective-tissue, the parenchyma.

Digestive System. The mouth (Fig. 177) leads to a small
rounded bulb-like body, the pliar/in > (/'//.), with thick muscular
walls and a small cavity. From this a short passage, the oesophagus
leads to the intestine. The latter (int.) is frequently a very
conspicuous structure, owing to its being filled with the dark
biliary matter on which the Fluke feeds. It divides almost imme-
diately into two main limbs, right and left, and from each of
these are given off, both internally and externally, a number of
blind branches or cwea, those on the inner side being short and
simple, while those on the outer side are longer and branched.
The two limbs of the intestine with their branches thus form, as
in the Planarian, a complicated system, the ramifications of which
extend throughout the whole of the body. There is no aperture
of communication between the intestine and the exterior, the only
external opening of the alimentary system being through the
mouth.

A branching system of vessels the water- vessels or vessels of
the excretory system ramify throughout the body. A longi-
tudinal main trunk opens behind by means of the excretory pore
already mentioned as occurring at the posterior end. In front it
gives off four large trunks, each of which branches repeatedly, the
branches giving off smaller vessels, and these again still smaller
twigs, until we reach a system of extremely fine microscopic vessels
or capillaries. Each of these ends internally in a slight enlarge-
ment situated in the interior of a large cell, an excretory cell or
flame-cell, similar to the flame-cells of the Planarian.

Q 2

228

ZOOLOGY

!SE(.T.

The Liver-Fluke has a well differentiated nervous system,
which shares in the prevailing bilateral arrangement of the parts.
The central part of this system consists of a ring of nerve matter
which surrounds the oesophagus, and presents two lateral thicken-
ings, or ganglia, containing nerve-cells, and a single ganglion in the
middle line below. From this are given off a number of nerves,

vii

pb.,

'p

n

FH... 177. Distomum hepaticum. Internal organisation. General view of the anterior

(th

nil. shell
lobes of

gland; to. testes ; v.t. uterus; nfl-. left vas deferens ; . right vas deferens :
vitelline glands ; vs. vesicula seminalte. - (After Sommer.)

of which the chief are a pair of lateral cords running back to the
posterior end and giving off numerous branches. There are no
organs of special sense.

The reproductive organs (Fig. 177) are constructed on the
hermaphrodite plan, i.e. both male and female organs occur in
the same individual. The male part of the apparatus consists of

PHYLUM PLATYHELMINTHES

229

v.s

Fit

testcs, casa deferentia, and cirrus. The tcsfc* (fi>.) are two greatly
ramified tubes, which occupy the middle part of the body, one
situated behind the other. From each testis there runs forwards
a duct, the i~as dc/crens, the two vasa deferentia (v.d.) opening
anteriorly into an elongated sac, the vesicula seminalis (v. s.), from
which a narrow tube the cjacnlatory duct (Fig. 178 cj.d.) leads
to the male aperture at the ex-
tremity of the penis. The female
part of the reproductive apparatus
consists of a single ovary, an ovi-
duct, a uterus, vitelline glands, vitel-
line ducts, and shell-glands. The
ovary (Fig. 177, ov.) is a branched
tube situated on the right-hand
side in front of the testes ; the
branches open into a common
narrow tube, the oviduct (od.). The
vitellinc glands (vit.') consist of very
numerous, minute, rounded folli-
cles, which occupy a considerable
zone in the lateral regions of the
body. On each side are two large
ducts, anterior and posterior, unit-
ing to form a single main lateral
duct, right or left ; and these run
nearly transversely inwards to open
into a small sac, the yolk reser-
voir. From this a single median

vitclline duct runs backwards for a short dis