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ITY

TEXT-BOOK OF ZOOLOGY

BY

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

PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTA(iO. DINKDIX. X.7.

AND

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

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

IN TWO VOLUME*
VOL. I

WITH ILLUSTRATIONS

\

ILonnon

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. Fenja and Aegir have been proved to be nothing more than damaged
examples of a normal Sea-anemone (Hatcanvpoidea).

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 Lymiwa, however, other fresh-
water genera, certainly Bulinus (Physa] are sometimes the second hosts of this
Trematode.

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

p. 247. Fig. 195 : dg, vitelline duct ; (1st, vitelline glands ; e, excretory
pore ; A', head end ; od, oviduct ; ov, ovary ; p, penis ; r.s, receptaculum
seminis ; t, testes ; vd, vas deferens ; vs, vesicula seminalis ; w.g.o, female
genital opening.

p. 247. Fig. 196 : eo, 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 ; », sucker ; t, lobes of testes ; ut, uterus ; vs, vesicula
seminalis ; yk, vitelline glands.

p. 267. Fig. 213, for " Rhabdoeoelida " read " Rhabdocoelida."!

p. 337. Line 19 from top, for " is " read "in."

p. 356 et seq. Antheneaflavescens 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" insert
" s. carina ; c. scutum ; t. tergum."

p. 546. Fig. 434, at end of line under cut, for " 3a, larva " read " 4a, larva."

VOL. II

pv. 118. Line 3 from bottom, for "(a. L c.)" read "(a. lat. c.)."
p. 131. Fig. 762, in description under cut, for " st. c." read " st. p."
p. 423. Fig. 1019 : The stroke from cor should run vertically downwards,
p. 429. Fig. 1023 : The third stroke from can 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 imporbant 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 group 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 diversify of organisation is very great — as in Crustacea and
Kishes — two nr nmre examples are taken. The student is thus
furnished with a luief account of at least one member — usually
readily arerssible— of all the principal groups of animals.

By the time tin- example has been studied, a definition of the
class and of its orders will e<>mvy some idea to the mind, and will

1 Following a suggestion for which we are indebted to Dr. Alexander Hill,
nt 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 tin1
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 Us, 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
Physiology, 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 be1 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 Willey. 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 treatsr
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.

- -

V 01' THK

UNIVERSITY

CONTENTS

I'AGK

PREFACE v

LIST OF ILLUSTRATIONS IN VOL. I xvii

TABLE OF THE CLASSIFICATION OF THE ANIMAL KINGDOM . . . xxxi
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

7. Symmetry 39

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

SECTION II

PHYLUM PROTOZOA 43

Class I. Rhizopoda . .

1. Example of the Class — Amoeba protetis .

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

Class II. Mycetozoa

1. Example of the Class — Didymmm difforme .

2. General. Remarks . . . •''

Class III. Mastigophora . . .

1. Example of the Class — Euylena viridis

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

Class IV. Sporozoa .

1. Example of the Class — Monocystis agilis .

2. Classification and General Organisation ... 76

xiv CONTENTS

SECTION IX

I'M IK

PHYLUM ECHINODEBMATA . 346

1. Examples of the Phylum . 346

i. Arterias rnbens and Anthenea Jiavescens ..... 346

ii. Strongylocentrotus or Echinus ... .... 363

iii. Gncnmana or Colochirus . . . . . . . . . 369

iv. Antedon rosacea .... 373

2. Distinctive Characters and Classification 377

Systematic Position of the Examples . . . 380

3. General Organisation 381

SECTION X

PHYLUM ANNULATA 403

Class I. Chaetopoda 403

1. Examples of the Class . . . ... 404

i. Nen'!x diimerilii . . 404

ii. Lumbricus 417

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

3. General Organisation 429

Appendix to Chsetopoda— Class Myzostomida . . . . 448

Class II. Gephyrea 450

1. Example of the Class — Sipuucidus 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 — Hirudo medicinalis 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

PiiYi.ru AuTiiKoi-ohA 484

Class I. Crustacea 484

1. Examples of the Class , 484
i. Apus or Lepidurus .... 484

ii. Asiacus JtwviatUis ' 498

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

3. General Organisation . 525

Appendix to Crustacea — Class Trilobita . . 558

(Mass 11. Onychophora . . 559

(Mass II I. Myriapoda . . .... 566

1. Distinctive Characters and Classification 566

2. General Organisation .... 567

CONTENTS xr

PAGE

PHYLUM ARTHROPODA — continued.

Class IV. Insecta . . . . . . . 571

1. Example of the Class — Periplaneta americana .... 571

2. Distinctive Characters and Classification 583

Systematic Position of the Example 587

3. General Organisation 588

Class V. Arachnida 604

1. Example of the Class — Euscorpio 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 nodiferus 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

IV. Cephalopoda .

1. Examples of the Class 708

i. Sepia cultrata 708

ii. Nautilus pompilius 725

2. Distinctive Characters and Classification . . . . . . 73fr

Systematic Position of the Examples

3. General Organisation «3»

General Remarks on the Mollusca 751

LIST OP ILLUSTRATIONS

1. Amoeba proteus

2. Amoeba polypodia, fission ....

3. Diagrams illustrating Karyokinesis

4. Ovum of Sea-urchin

5. Maturation and fertilisation of ovum .

6. Segmentation of ovum

7. Gastrula

8. Various forms of epithelium ....

9. Diagram to illustrate structure of glands

10. Gelatinous connective tissue ....

11. Reticular connective tissue ....

12. Fatty tissue

13. Hyaline cartilage

14. Fibro-cartilage

15. Bone

16. Unstriped Muscle

17. Striped Muscle

18. Nerve cells

10. Nerve fibres

20. Various forms of spermatozoa

21. "Viscera of Frog

22. Bones of human arm with biceps muscle

23. Nervous system of Frog

24. Hydra

25. Diagram of axes of body

26. Radial symmetry

27. Amoeba, various species . . . . .

28. Protamceba primitiva ...

29. Quadrula, Hyalosphenia, Arcella, and Difflugia

30. Chlamydomyxa labyrinthuloides

31. Labyrinthula vitellina

32. Microgromia socialis

33. Platoum stercoreum . .

34. Various forms of Foraminifera . • .

35. Shells of Foraminifera

36. Hastigerina murrayi . . . . .

PAGE

10
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24
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26
27
27
28
28
28
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35
36
$9
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40
45
47
47
48
49
50
51
52
53
54

xviii LIST OF ILLUSTRATIONS

FIO. PAOB

37. Actinophrys sol 55

38. Actinosphaerium eichhornii 56

39. Various forms of Heliozoa 57

40. Lithocircus amiularis 58-

41. Thalassoplancta brevispicula 59

42. Aulactinium actinastrum 60

43. Actinomma asteracanthion .60

44. Collozoum inerme 61

45. Didymium diiForme 62

46. Euglena viridis 64

47. Various forms of Flagellata 66

48. Hsematococcus pluvialis 68

49. Pandorina morum 69

50. Volvox globator . . ^-. .70

51. Heteromita rostrata j, . .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 79

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. Cells of ectoderm of Sponge 110

80. Skeleton of various Sponges ... .... Ill

81. Various forms of Sponge Spicules 112

82. Development of Sycon raphanus .114

K3. Ol.clia . • 120

84. ,, vertical section of polype 122

«r>. Ni-m:itocysts of Hydra .... . . 123

LIST OF ILLUSTRATIONS xix

PACK

86. Tentacle of Eucopella .... 124

87. Obelia, medusa 125

88. Diagram of medusa 12fr

89. Derivation of medusa from polype 127

90. Projections of polype and medusa 128

91. Development of zoophyte 130

92. Bougainvillea ramosa 134

93. Various forms of Leptolinre ... 135

94. Hydra . 13G

95. Protohydra leuckartii 137

96. Various forms of leptoline Medusae 13i>

97. Diagram illustrating formation of sporosac by degeneration of

medusa 140

98. Early development of Eucope 141

99. Two Trachymedusse . 142

100. Two Narcomedusai 143-

101. ^Eginura, tentaculocyst 143-

102. Larva of JEginopsis 144

103. Millepora alcicornis, skeleton 145

104. Millepora, diagram of structure 146

105. Stylaster aanguineus, skeleton . . 147

106. Halistemma tergestinum 148

107. Diagram of a Siphoiiophore . . . 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 quadriga ta 167

120. Charybdrea marsupialis 168

121. Nausithbe 169

122. Pilemapulmo 170

123. Pelagia iioctiluca, development 171

124. Tealia crassicornis, dissection and transverse section . . . 173

125. Diagrammatic sections of Sea-anemone 175

126. Tealia crassicornis, section of tentacle 177

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. Astnea pallida 185

b 2

xx LIST OF ILLUSTRATIONS

Kl<;. PAOE

134. Pennatula sulcata 186

135. Tubipora niusica 186

13(5. Edwardsia claparedii 187

137. Cirripathes anguina 188

138. Fenja mirabilis .189

130. Minyas .189

140. Alcyonium palniatum . , 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 . . . 19'.)

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

ir>6. Deiopea kaloknenota 210

157. Cestus veneris 210

158. Beroe forskalii - . . .211

l.V.». Ctenoplana kowalevskii 212

160. Sections of embryos of Actinia and Beroe 214

161. Diagram illustrating the mutual relationships of the Coelenterata . 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 adluerens . . . . . ... . . 220

171. Planaria, digestive and excretory systems 223

172. ,, nervous system . - 223

173. ,, reproductive system 225

174. Transverse section of a Planarian 226

175. Distoinum hepaticum . . 226

176- ,, section of integument 227

177. „ internal organisation 228

terminal part of reproductive apparatus . 229
development • . • . - 230

180. Ta-nia soliiun .... 232

181. ,, ,, head . . . 233

182. ,, ,, transverse section 233

LIST OF ILLUSTRATIONS xxi

FIG. I'AliK

183. Trenia solium, proglottis . . 234

184. ,, ,, ripe proglottis 236

185. ,, ,, development 236

186. Various Planariaiis 240

187. Gunda segmentata 241

188. Digeiietic Trematodes 242

189. Gyrodactylus and Polystomum - . . 243

190. Temnocephala 244

191. Actinodactylella 245

192. Tetrarhynchus • 246-

193. Tsenia echinococcus .... 246

194. Ligula 247

195. Caryophyllseus' 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 Rhabdocoele 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 evagiiiated 261

209. Cyst of Tseiiia echinococcus 262

210. Scolices ,, ,. 262

211. Scolex ,, ,, 262

212. Process of budding in Microstomum 263

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

tinea 267

214. Diagram of Nemertine 268

215. Tetrastemma - 2(5!)

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. ,, ,, muscle 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

FI<;. l'A«;K

231. Oxyuris .285

232. Gordius, anatomy . 286

233. Development of Ascaris nigrovenosa . 287

234. Trichina spiralis • .

235. Two species of Echinorhynchus .... ... 290

236. Echinorhynchus gigas, dissection of male 291

237. ,, ,, ,, female 291

238. ,, ,, 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

246. ., ,, 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. Chpetonotus maximus .... 31]

254. ,, ,, anatomy 311

255. Bugula avicularia 315

256. Development of Bugula 317

257. „ „ 318

258. Plumatella 322

258/>/'.s. (Yistatella 323

259. Lophopus 324

260. Pedicellina 327

261. Phoroiiis australis . 328

262. ., ,, free end 329

263. ., ,, internal organisation .'529

204. ,, ,, development 330

205. Magellania navescens, shell 332

200. ,, leiiticularis, anatomy 334

267. ,, navescens, lophophore 335

20.S. ,, muscular system 335

20!>. Terebratula, nervous system, &c . 336

270. Typical Brachiopods 339

271. ,, ,, anatomy -. . . . 340

272. Development of Cistella 341

27:;. „ „ 34L>

274. Lophophore of embryo Brachiopod . 343

2~'~>. Diagrams of phylactol;ematous Polyzoons 344

270. Starfish, ventral aspect 347

277. ,, vertical section of arm 349

278. ,, ambulacra! system 350

LIST (3F ILLUSTRATIONS xxiii

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. Strongylocentrotus ........... 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 Holothuriaii ......... 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. Strongylocentrotus ........... 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. ,, ,, parapodium . ...... 405

320. ,, „ setse . .405

321. ,, ,, anatomy .... 407

322. ,, ,, transverse section . . . . . . 408

323. ,, nervous system ..... . . ... 410

324. ,, eye ....... 411

325. ,, dumerilii, nephridium

326. ,, development ..... • 414

xxiv LIST OF ILLUSTRATIONS

FH;. PAOE

327. Nereis development 415

328. Lumbricus agricola . . 418

329. „ setse 419

330. ,, transverse section ... .... 419

331. ,, herculeus, sagittal section . 420

332. ,, nervous system 422

333. ,, nephridium 423

334. ,, agricola, reproductive organs 424

335. ,, development 426

336. Polynoe setosissima 429^

337. Vermilia ccespitosa 430

338. Setre of various Polychceta 431

339. Section of setigerous sac of an Oligochaete 431

340. Polynoe extenuata, anterior end 432

341. Tubifex 433^

342. Terebella .... ...... 434

343. Aphrodite, enteric canal ... 436

344. Saccocirrus, transverse section 438

345. Diagram illustrating development of gonad of Polychaeta • . . 441

346. Spirorbis lams 444

347. Eupomatus, development of trochosphere 445

348. Autolytus cornutus, budding . . . 446

349. Serpulse with their tubes 447

350. Myzostomum 449

351. ,, .anatomy 450

352. Sipunculus iiudus, 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

361. ,, anatomy 459

•»62. ,, nervous system . . . .... . . . 459

363. n.mrllia, male 460

364. Echiurus, fcrochosphere .... .... 460

.'I6.V Polygordius neapolitanus 462

.'Kit). Protodrilus 4^3

.'>(57. Polygon 1ms iu:;ipolitanus, transverse section . . . 4(53

365. ,, ,, trochosphere . . ... 464
•"»69. ,, ,, ,, later stage .... 464

370. Hirudo medicinalis 455

371. ,, ,, transverse section 468

372. ,, ,, jaw 468

,, quinquestriata, dissection from dorsal aspect . . 469

»» » 11 left side .... 470

375. ,, medicinalis, nephridium 47^

LIST OF ILLUSTRATIONS xxv

PAGE

T6. 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 Annulata and 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 490

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 w . . .526

414. „ Cladocera 527

415. Cypris 528

416. Cyclops and Calocalanus '.. 529

417. Various forms of parasitic Eucopepoda 531

418. Argulus foliaceus 532

419. Lepas anatifera .... .....-• 533

420. Balanus 534

421. Sacculina carcini ^,,.,,.^===^--1^=^.^^^ . . . 535

422. Nebalia geoffroyi .... /%j&\v vL^^\ ' 536

423. Mysisoculata /flT N TVER SIT Y N ' ' 537

LIST OF ILLUSTRATIONS

KKi.

1'AOE

424. Shrimp and Prawn

425. Scyllarus arctus . 539

4 2« '• . 1 'agurus 1 )ernhardus 539

427. Cancer pagurus . . 54°

428. Typical Brachyura 541

420. Squilla 542

430. Diastylis stygia . 543

431. Gammarus neglectus . .... . 544

4.'52. 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. Larvse of Crabs 554

440. Diagram illustrating the mutual relationships of the orders of

Crustacea 557

441. Dalmanites and Phacops . . 558
441 />/'.s. Triarthrus beckii . . 559

442. Peripatus capensis ... 560

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

444. ., anatomy . . 561

445. ., edwardsii, nephridium 563

446. ,, iiovsezealandise, development 564

447. ,, capensis ., 565

448. Scolopendrella immaculate . . 567

449. Scolopeiidra 568

450. Lithobius forficatus . . . .568

451. Pauropus huxleyi 569

452. Strongylostoma, development . . .... 570

453. Periplaneta americana ... 572

454. ,, mouth-parts . . 573

4-"),"). . , americana, lateral view of head 573

456. .. muscular system 575

457. ,, anatomy . . . 576

458. , , salivary glands . 577

459. Trachea of caterpillar . . 577

460. Periplaneta, tracheal system ... 578

461. , , nervous system 578

4(51.'. ., male reproductive organs 579

4(>3. ,, female reproductive organs 579

4«;4. Segmentation of ovum of Insect . . . . . . . . 580

•ir>f>. Ventral plate of embryo Cockroach 581

466. Germinal layers and amnion of Insect 582

467. Li-pisma 582

-tr.s. n.,1 lira . 584

•ir.'J. Locusta . 584

470. Ephemera . . 584

LIST OF ILLUSTRATIONS xxvii

FIG. PAGE

471. Aphis rosa3 535

472. Cicada . 585

473. Pulex and larva 586

474. Gastrophilus equi ; _ 586

475. Pieris 586

476. Crioceris m 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 mellifica, queen, worker, and drone 603

1:94. Formica rufa 603

495. Euscorpio 605

496. Ventral surface of cephalothorax and prae-abdomen of Scorpion . 605

497. Endosternite of Scorpion 606

498. Scorpion, anatomy, lateral view 608

499. ,, ,, dorsal ,, 609

500. ,, development 610

501. Embryo of Scorpion 610

502. Chelifer bravaisii 613

503. Phrynus 613

Galeodes dastuguei • . . . . . 614

505. Epeira diadema 615

506. , , , , chelicerae and pedipalpi of -female .... 615

507. ,, ,, ,, ,, male . . . .615
)8. Sarcoptes scabisei 616

Trombidium fuligiiiosum 616

110. Limulus 617

111. ,, ventral view 618

>12. Eurypterus fischeri . . .... . • .

)13. Anatomy of dipneumonous Spider . . . ... . . 620

)14. Limulus, sagittal section - 621

)15. Lung-book of Spider . • 621

>16. Tracheal system of Spider

)17. Gill-books of Limulus

Lateral eye of Euscorpius 622

Central • • 623

xxviii LIST OF ILLUSTRATIONS

TIG. i';u;J-

520. Nymphon hispidum 025

521. Pentastomum t<enioides 62;>

522. Macrobiotus hufelandi 626

523. Diagram to illustrate affinities of Arthropoda ....

524. Anodonta cygnea • 632

525. ,, ,, interior of valve and animal removed from shell. (53:>

526. .. section of shell and mantle 034

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

528. ,. ,, dissection from left side 637

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

530. ,, ,, transverse sections 631)

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. Requienia and Hippurites 652

545. Teredo navalis . . . .652

546. Aspergillum 653

547. Mytilus edulis 653

548. Nucula nucleus 654

549. Gills of Pelecypoda . • 655

550. Gill-filaments of Mytilus 656

551. Dissection of Poromya (556

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 660

557. Veliger of Ostrea 060

558. Embryos of Cyclas 601

55<J. Ding-nun illustrating the mutual relationships of the Pelecypoda . 662

560. Cheetoderma nitidulum 6(54

r>6l. BTeomenia carinata 6(54

562. Chiton spinoeus, dorsal view . . . 665

56:5. ,, ventral view 6(55

564. ,, valves of shell (565

5(5."j. Ohfietoderma nitidiiluni, longitudinal section (5(5(5

5(5(5. Chiton, longitudinal section (5(57

567. Nervous system of Amphineura 667

568. Neomenia carinata, reproductive organs 668

LIST OF ILLUSTRATIONS xxix

II". PACK

569. Chiton, nephridial and genital systems 669

570. ,, development . . . 670

571. Triton nodiferus, shell 672

572. Triton nodiferus, shell, median section 673

573. ,, ,, operculum 674

574. , , , , lateral view of body 674

575. , , , , diagram of introvert 675

576. ,, ,, dissection fron^ 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. Cyprsea moneta 687

585. Doris tuberculata . . . . . . . . . . .687

586. Carinaria mediterranea " . . . 687

587. Limax -. . 687

588. Sigaretus Irevigatus 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

006. 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 . . 70o
(512. Dentalium, section of shell . . 706

613. ,, anatomy 706

614. *„ larvae 706

<>147m. Rhodope

615. Sepia cultrata 709

(5 Hi. sucker . 710

xxx LIST OF ILLUSTRATIONS

Fir,. PAUF;

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
(542 /)/.*. ,, ,, arteries 733

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

644. ,, male reproductive organs 735

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

646. Octopus vulgaris . . . 737

647. Loligo vulgaris 73$

648. Argonauta argo .... 739,

649. Octopus lentus, male 739

650. Amphitretus pelagicus .... 740

651. Shell of Spirula ....... . . . '. 740

652. Spirula peronii . . 74^

653. Ammonite 741

654. Shell of Belenmite .... 74i>

655. ,, Argonauta argo 742

656. Segmentation of Loligo . 745

657. Blastoderm of Sepia .... 745

658. ,, ,, sections .... 746

659. Development of Loligo . . 747

660. „ ' 748,

<'><'>1. „ .* . 748

<''<»± „ it . 749

(',);:;. 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. EHIZOPODA. -

Order 1. LOBOSA.

,, 2. LABYRINTH ULIDE A.

,, 3. FORAMINIFERA.

,, 4. HELIOZOA.
,, 5. RADIOLARIA.
Class II. MYCETOZOA.
Class III. MASTIGOPHOEA. -v
Order 1. FLAGELLATA.

2. CHOANOFLAGELLATA.

Order 3. DINOFLAGELLATA.

,, 4. CYSTOFLAGELLATA.
Class IV. SPOEOZOA. •>
Order 1. GREGARINIDA.

,, 2. COCCIDIIDEA.

,, 3. MYXOSPORIDEA.

,, 4. SARCOCYSTIDEA.

Class V. INFUSORIA. -»
Order 1. CILIATA.

2. TENTACULIFERA.

PHYLUM II. PORIFERA.

Class POEIFEEA.
Sub-class I. Calcarea.

Order 1. HOMOCCELA.

2. HETEROCCELA.

Sub-class II. Non-Calcarea.
Tribe I. MYXOSPONGI^E.
,, II. SILICISPONGI^.
Order 1. HEXACTINELLIDA.
2. DESMOSPONGT^].

PHYLUM III. CCELENTERATA.

Class I. HYDEOZOA.

Order 1. LEPTOLIN^E.
Sub-order a. AnthomeduscK.
,, 6. Leptomedwce.
Order 2. TRACHYLINJE.

Sub-order a. Trachymeduxa'.

,, b. Narcomeduwr.
Order 3. HYDROCORALLINA.
,, 4. SIPHONOPHORA.
5. GRAPTOLITHIDA.

XX XI I

CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM III. CCELENTERATA— confirmed

Olass II. SCYPHOZOA.

Order 1.

,, 2. PEROMEWS.K.

,, 3. ClTBOMEIH'S.i;.

,, 4. Disco MEDUS.E.

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

Order 2. GoRGONACEA.

,, 3. PENNATULACEA.

Class IV. CTENOPHORA.

Order 1. CYDIPPIDA.

„ 2. LOBATA.

,, 3. CESTIDA.

,, 4. BEROIDA.

Appendix to Ctenophora — Ctenoplana
and Ccdoplana.

Appendix to Ca^lenterata — Mesozoa,
Salinella, and Trichoplax.

( 'lass I. TURBELLARIA.
Order 1. POLYCLADIDA.
,, 2. TRICLADIDA.
„ 3. RHABDOCOSLIDA.
Class II. TREMATODA.

Order 1. MOXOCENETICA.

,, 2. DlOENETICA.

PHYLUM IV. PLATYHELMINTHES.
Class III. CESTODA.
Order 1. MONOZOA.

2. PoLYr/OA.

Appendix to Plat yhelmint lies — Class
NEMERTINEA.

PHYLUM V.

Class L NEMATODA.

Order 1. NEMATOIDEA.
,, 2. GORDIOIDEA.

(lass II. ACANTHOCEPHALA.

NEMATHELMINTHES.

Class III. CHJETOGNATHA.

Appendix toNemathelminthes — Ch<xto-
Echinoderidcv, and

(lass I. ROTIFERA.

Order 1. RIII/OTA.

,, 2. 1>I)KLL(I1DA.

,, 3. PLOI.MA.
Suit-order a. llloricata.
I >. Loricata.

PHYLUM VI. TROCHELMINTHES.

Order 4. SCIRTOPOD \.
,, 5. TROCHOSPH^RIDA.

Class II. DINOPHILEA.
, III. GASTROTRICHA.

PHYLUM VII. MOLLUSCOIDA.

Class I. POLYZOA.

Sub-class I. Ectoprocta.

Order 1. ( i YMNOI,.T:MATA.
Sul>-order <i. ( '//<•/<,.-</<>, nafa.
,, I. Cheiloxtomata.

c.

Order 2.
Sub-class II. Endoprocta.
Class II. PHORONIDA.
„ III. BRACKIOPODA.

Order 1. Ix ARTICULATA.
,, 2. ARTICULATA.

CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM VIII.

Class I. ASTEROIDEA.

Order 1. PHAXEROZONIA.

,, 2. CRYPTOZONIA.
ass II. OPHIUROIDEA.
Order 1. OPHIURIDA.
,, 2. EURYALIDA.
Class III. ECHINOIDEA.

Order 1. PAL/EO-ECHIXOIDEA.
,, 2. REGULARIA.

3. CLYPEASTRIDKA.

4. SPATAMJOIDEA.

ECHINODERMATA.

Class IV. HOLOTHUROIDEA

Order 1. ELASIPODA.

,, 2. PEDATA.

,, 3. APODA.

Class V. CRINOIDEA.

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

Class VI. CYSTOIDEA.
, VII. BLASTOIDEA.

PHYLUM IX. ANNULATA.

Class I. CHJETOPODA.

Sub-class I. Polychseta.
Order 1. ARCHI-CHA:TOPODA.
,, 2. ERRANTIA.
,, 3. SEDENTARIA.
Sub-class II. Oligochaeta.
Order 1. NAIDOMORPHA.

,, 2. LUMBRICOMORPHA.

Appendix to the Chtetopoda — Class
MYZOSTOMIDA.

Class II. GEPHYREA.

Order 1. INERMIA.

,, 2. ARM AT A.

Class III. ARCHI-ANNELIDA.

„ IV. HIRUDINEA.

Order 1. RHYKCHOBDELLIDA.
2. GNATHOBDELLIDA.

PHYLUM X. ARTHROPOD A.

Class I. CRUSTACEA.

Sub-class I. Entomostraca.
Order 1. PHYLLOPODA.

Sub-order a. Euphyttopoda.

, , b. Cladocera.
Order 2. OSTRACODA.
,, 3. COPEPODA.
Sub-order a. Eucopepodu. ,.
,, b. Branchiura.
Order 4. CIR.RIPEDIA.

Sub-order a. Encirripedid.
, , b. Rhizocephala.

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

,, b. Brachynra.
Order 4. STOMATOPODA.
5. CUM ACE A.

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

,t 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. HEMIPTERA.
,, 5. DIPTERA.
,, 6. LEPIDOPTERA .

,, 7. COLEOPTERA.

8. HYMENOPTERA.

CLASSIFICATION OF THE ANIMAL KINGDOM
PHYLUM X. ARTHROPOD A— continued.

Class V. ARACHNIDA.

Order 1. SCORPIOXIDA.

± PSEUDOSCORPIOXIDA.

„ 3. PEDIPALPIDA.

,, 4. SOLPUGIDA.

,, 5. PHALANGIDA.

6. ARAXEIDA.

Order 7. Ac ARID A.
,, 8. XIPHOSFRA.
,, 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-LAMELLIBRAXCHIA.

,, 4. EULAMELLIBRAXCHIA.

Sub-order a. Inteyripatliata.
,, 1). Sinupalliata.

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

Class III. GASTROPODA.

Sub-class I. Streptoneura.
Order 1. ASPIDOBRAXCHIA.
Sub-order a. Docoy/o«*a.

PHYLUM XII.

SUB-PHYLUM I. ADELOCHORDA.
Class. ADELOCHORDA.

SUB-PHYLUM II. UROCHORDA.

Class. UROCHORDA

Order 1. LARVACEA.
,, 2. THALIACKA.
Sub-order a. Cydomyari'i.
,, b. Hemimyaria.
,, <: Pyroxonutta.
Order 2. ASCIDIACKA.
Sub-order a. A*r/', /,',,-
,, b.

St r.-iMivi.i M III. VERTEBRATA.

Division A. Acrania.
Class. ACRANIA.

Order 2. PECTIXIBRAXCHIA.
Sub-order a. Platypoda.
,, b. Heteropod<i.
Sub-class II. Euthyneura.
Order 1. OPISTHOBRAXCHIA.
Sub-order a. Tectibranchiata.
,, b. Nudibranchiata.
Order 2. PULMONATA.

Appendix to the Gastropoda— Class
SCAPHOPODA and RHODOPE.

Class IV. CEPHALOPODA.

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

CHOKDATA.

Division B. Craniata.
Class I. CYCLOSTOMATA.

Order 1. PETROMYZONTES.
,, MYXIXOIDEI.

Class II. PISCES.

SiTb-class I. Elasmobranchii.

Order 1. CLADOSELACHKA.

,, 2. PLEURACAXTHEA.

,, 3. ACAXTHODEA.

,, 4. SELACHII.
Sub-order a. Protoselachii.

,, b. Euselachii.
Section a. Squalida.

,, 0. Rajida.
Sub-class II. Holocephali.

,, III. Teleostomi.

Order 1. CROSSOPTERYGII.

,, 2. CHONDEOSTBI.

CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM XII. CHORDATA— continued.

Order 3. HOLOSTEI.
,, 4. TELOSTEI.
Sub-order a. Phy*o>stomi.
h. Anacaiythini.
,, c. Acanthopteri.
,, d. Pharyngognathi.
,, e. Plecfogognathi.
,, • /. Lophobranchii.

Sub-class IV. Dipnoi.

Order 1. MOXOPXEUMOXA.
,, 2. DIPXEUMOXA.

Appendix to Pisces— The Ostracodermi.

Class III.

» Order
"

AMPHIBIA.
1. URODELA.
•2. AXURA.

3. GYMNOPHIONA.

4. STEGOCEPHALA.

Class IV. EEPTILIA.
Order 1. SQUAUATA.
Sub-order a. Lacertilia.
, , b. Ophidia.
,, r. Pythonomorpha.
Order 2. RHYXCHOCEPHALIA.
,, 3. CHELOXIA.
,, 4. THEROMORPHA.
,, 5. CROCODILIA.
,, 6. SAUROPTERYGIA.

. ,, 7. ICHTHYOSAURIA.
,, 8. DlXOSAURIA.

,, 9. PTEROSAURIA.

Class V. AVES.

Sub-class I. Archaeornithes.

,, II. Neornithes.
Section A. Ratitae.

Order 1. MEGISTAXES.
,, 2. RHE.E.
,, 3. STRUTHIOXES.
,, 4. ^EPYORXITHES.
,, 5. GASTORXITHES.

Section B. Carinatae.

Order 1. STEREORXITHES.

,, 2. ODOXTOLOE.

,, 3. ICHTHYORXITHES.

,, 4. PYGOPODES.

5. IMPEXXES.

Order 6. TURBIXARES.
,, 7. STEGAXOPODES.
., 8. HERODIOXES.
,, 9. AXSERES.

,, 10. ACCIPITRES.

,, 11. CRYPTURI.

,, 12. GALLIX.*;.

,, 13.

,, 14.

,, 15. LIMICOL^E.

,, 16. PTEROCLETES.

,, 17. COLUMB^E.

,, 18. PSITTACI.

,, 19. STRIGES.

,, 20. PICARIJE.

,, 21. PASSERES.

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

„ II. Theria.

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

,, 2. DlPROTODOXTIA.

Section B. Eutheria.
Order 1. EDEXTATA.
,, 2. SIREXIA.
,, 3. CETACEA.
„ 4. UXGULATA.

Division A. Ungulata Vera.
Sub-order a. Artiodactyla.
,, b. Peristodactyla.

Division B. Subungulata.
Sub-order a. Hyracoidea.
„ b. Probotcidea.
,, c. Amblypoda.
,, d. Condylarthra.
,, e. Toxodontia.
Group Tillodontia.
Order 5. RODEXTIA.
,, 6. CARXIVORA.
Sub-order a. Camivora vera.
,, b. Pinnipedia.
,, c. Creodonta,
Order 7. IXSECTIVORA.
„ 8. CHIROPTERA.
,, 9. PRIMATES.
Sub-order a. Lemuroidea.
,, b. 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 Linnaeus.

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.

According to the system of binomial nomenclature introduced by
Linnaeus, each kind of animal receives two names — one the generic

E B

ZOOLOGY

'name, common to all species of the genus ; the other the specific
•,it(.m-e, 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 Fells
domestica, the European Wild Cat F. catus, the

Leopard F.

the Tiger F. tiyris, the Lion F. leo. 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,
however 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 Feiis 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 tin- head, while in the rest the tail is nearly half
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 which 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,
«£rc., 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 Linnaeus,
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

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 Linnaeus, 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 Felis. 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 Hyenas 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 Hycenidce, while
Felis and Cynaalurus are placed together in the family Felidcc.
Similarly, the Civets and Mongooses form the family Vivcrridce ;
the Dogs, Wolves, Jackals, Foxes, &c., the family Canidce ; Bears,
the family Ursidce ; 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 Carniwra,
the hoofed animals in the order Ungulata. In the same way
gnawing animals, such as Rats, Mice, and Beavers, form the order
Rodentia ; pouched animals, such as Kangaroos and Opossums, the
order Marsupialia ; and so on.

Carnivora, 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 Aves. In
the same way the scaly, cold-blooded Lizards, Snakes, Tortoises, &c.,
form the dassReptilia; 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

numeroi

INTRODUCTION

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
Mollusca, 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 — CHORD ATA.
Class — MAMMALIA.

Order— CARNIVORA.
Family — Felidcc.
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 Buffon 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 ajsupernatural

ff^ c

( UNI ry }

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 this
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 whe'n, after the
publication of Darwin's Origin of Species in 1859, the Doctrine
of Descent or of Organic Evolution came to be generally
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 Fowl 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 branches.

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 of
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.

f 13. Quaternary and Recent.
III. Camozoio or Tertiary. . JJ jg££ '

( 10. Eocene*
j 9= Cretaceous.

II. Mesozoic or Secondary . \ 8. Jurassic.

I 7. Triassic.
! 6. Permian.
5. Carboniferous.

1 Paleozoic or Primary,, J

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, farms — 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
Xrkton. 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 Palcearctic, its American portion the Nearctic
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. AMOEBA.

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 Amoeba 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 carefui

FlO. 1.— Amoeba proteus, a living specimen.
c. vac. contractile vacuolo; nu. nucleus;
pseudopodB, (From Parker's Biology,
after Gruber.)

SCT. i STRUCTURE AND PHYSIOLOGY OF ANIMALS 11

:

watching shows that the Amoeba "is also, with extreme slowness,
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. These 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, nu.)\ 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. vac.) in the
protoplasm. When this is watched it will be observed to increase
gradually in size till it r^^kes a maximum of, let us say, a fifth of
the total diameter oJ^hJll^ba . when, by a contraction of its walls,
it suddenly disapp<S"s|| « Mppear presently and gradually grow
again to its inaxiiimmlljK This pulsating clear space is the
contractile vacuolc. Jjj&

By watching th^Pmceba carefully for some time we may be
enabled to observe that the mqdtenents of the protoplasm of the
body not only effect locomotionfjui 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 t(b 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 we 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 affinity
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 coinpju^ompounds, to deal out
working power enabling the Amu -ha to move, in the case of the
clock-weight there comes a time when all the potential energy is
expended; the weight reaches its lowBFlimit, 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 ivould become exhausted, were it not that a new

STRUCTURE AND PHYSIOLOGY OF ANIMALS

13

supply is being constantly received. This new supply of energy is
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
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
ecessary 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. E. 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 contractile
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 Amoebae 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 Amoeba?
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 cany 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 i-iciny
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 cellulose —
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
<>f organs — muscles, alimentary or enteric canal, (jlti.inh, heart and
blood-vessels, ////As or /vrv/./ys, fn,cm'ri!,s system, or was of e-.'.rrction, and
f'i'i/ans of reproduction. But in all animals, however complex,
the same substance, protoplasm, which in Aiim-b;' 'ites

the bulk of the body, is the essential and active pan. Wher<
in the body active functions are being discharged and active
changes an- i;"'>ing on, there we find protoplasm present: when-
then- is no protoplasm there is no .vital activity. In the
liesl stages of their existence all animals are formed entirely
of protoplasm. Kvery animal consists at first of a single mil
part icle of protoplasm, not widely different from an Amu-ba. {-
this particle divides into a number of parts which, instear

sp-namf-

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 protcids — 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 (mitcmc) 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 (paramitome) 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 cell-wall (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 n-iich'u*
similar to the nucleus of Amoeba ; usually of rounded shape, with
a thin enclosing nudcar 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 nuckolm (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— achwm a tin— 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 s«p.
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
I'd i'ij oldncsis 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, at 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 pt«fc — 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
that has not the special characteristics of chromatin ; but it applies to cytojt?rixnn'<-
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 they 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 jiivi§ion bggan,

\ chr

\

X G

H

•

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 ; II, 1, division of the cell-body and formation of the two new nuclei ; c. centro-
some ; <••/<;•. chrnmatin ; <•/</. cell-plate ; iii'i nnaleoli ; nc . ///. nuclear membrane ; s. astrosphere ;
sp. spindle (From Parker's Biology, after Flernming, 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-
pletery 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.

Amoeba is simply an independent animal cell; or, to express the
same meaning in another 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 multicellular 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 jn 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 yolk.

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 m essence, of the throwing out of portions of the nucleus.
I he latter approaches the surface and divides, mitotically, into two
parts -one coming to project o%the surface and finally becoming-
completely separated off from the ovum as a rounded particle^
the first polar lody (pol). A second division of the nucleus
results in the throwing off ol I polar lody; and, after this

has been formed, the portion which n>mains in the ovum resumes
its central position and fomw what is termed the female pro-
nucleus (<j> pr (>n.}.

In the process of impregnation a very minute body, the mak
cell werm-cell, or sperm, penetrates into the interior of the female
cell or Gtwm,and the nucleus ^hk-li it contains— the male pro-

FIG. 4.— Ovum of a Sea-Urchin, showing
the radially striated cell-membrane,
the protoplasm, containing yolk^
granules, the large nucleus (germinal
vesicle), with its network of chro-
matin and a large nucleolus (ger-
minal spot). (From Balfour's E,n-
bryology, after Hertwig.)

STRUCTURE AND PHYSIOLOGY OF ANIMALS

19

nucleus (3 pron.) coalesces with the female pro-nucleus to forma
single nucleus called the segmentation nucleus (seg. nucl.). The
principal part in the process of fertilisation is thus played by the

FIG. 5.— Diagram illustrating the maturation and 'fertilization of the ovum. A, formationTof first
polar globule ; B, beginning of fertilization, spermatozoa approaching the micropyle ;*C, for-
mation of the male proimcleus ; D, approximation of the male and female pronuclei ; E, for-

mation of segmentation-nucleus ; $ ceii-t. female ceiitrosome ; 3 cent, male centrosome ; mem.
egg-membrane; microp. micropyle ; pol. polar bodies ; $ pron. female pronucleus; cJ pron.
male pronucleus ; seg. nucl. segmentation nucleus.

two nuclei; but the centrosbmes, one (<j 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 two, and the two
centrosomes resulting from the division of that derived from the

c 2

20 ZOOLOGY

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, or 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, sad 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 (holoblastic or complete
segmentation) or only a part (merollastic 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

FIG. 0. — Various stages in the sagmentation of the ovum. (From Gegenbaur's Comparative

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, by the process
of division and subdivision, the oosperm becomes segmented into
a large number of comparatively small cells which are termed the
hlcstomercs. 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 mullic-n'ti lo</// or morn hi
stag*-. Tin1 blastomeres next become arranged regularly in a
single layer — the embryo assuming the form of a hollow sphere,
the blffxtiixpjHTc or llastula,with a wall composed of a single layer
of cells enclosing a cavity — the scyniml.nlnm cavity or blastocaele.

One side of the hollow blastula next becomes pushed inwards <>••
invaginated, &s one might push in one side of a hollow indi
ball — the result of this process of imagination, or fjastrul

STRUCTURE AND PHYSIOLOGY OF ANIMALS

21

it is termed, being the formation of a cup — the gastrula (Fig. 7) —

with a double wall. The cavity of the cup-shaped gastrula is the

arckenteron or primitive digestive cavity ; the opening is termed the

llastopore, the outer layer of the wall of the

cup is the ectoderm (or epiblast), the inner

the cndoderm (or Jiypoblast). The ectoderm

and endoderm are the primary germinal lay< ,•*

of the embryo ; from one or both of them are

developed the1 cells of a third layer — the

mesoderm (mesdblast) — which is subsequently

formed between them.

This mode of formation of the primary
germinal layers in holoblastic oosperms 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 oosperms 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.

FK ;. 7. — Gastmla in longi-
tudinal section. a,
mouth ; 7', enteron ;
c, endoderm; <7, ecto-
derm. (From Gegen-
baur's ComparoMve An-

atOnilf.)

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 rases
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, «); sometimes there is on each cell a single, relatively
long, whip-like cilium, which is then termed a flandlnrn (g, k).
Cells provided with cilia are termed ciliated, such as bear Hagella
flagellate 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
layer of cells (a-h) or
may be many-layered
(i) ; 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
be cubical or cylindrical
or prismatic (a, &) ; in
the case of a stratified
epithelium the cells
may be of different
forms in different strata
(i). The epidermis.whicli
covers the outer sur-
face of the body of an
animal, is an example
of an epithelium ; some-
times it is stratified,
sometimes unstratih'cd :
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 < Fi-. <)) are formed for the most part by the modifica-
tion of certain cells <>f epithelia, In many cases a single cell of the
epithelium f..rms a -hind, which is then termed a unicellular gland
'- !)- 4 • 'I'1"' secretion (or substance which it is the function of

FIG. 8. — Various forms of epithelium, a, ciliated epi-
thelium ; />, columnar ; ,l, surface view of the same ;
•i'lted : « , the same from the surface ; f, flagel-
late epithelium with collars ; ;i, flagellate epithelium
without collars; /,, epithelium of intestine with
peeudopodiai i, stratified epithelium; /-, deric epi-
thelium of a marine planarian with pigment cells
Us, arid sub-epithelial giancU. (From Lang's

[ui

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 (B). 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 duct.

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 tissue (Fig. 10) the ground
substance (g) is of a gelatinous
character, sometimes supported
by systems of fibres (52), 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 processes
which are prolonged into fibres
—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,

FIG. 9.— 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,-.braiiched multicellular gland. (From
Lang.)

ZOOLOGY

SECT. "

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

fe (S-

ef k 3

Fi<: 10. — 'ielatinous connective tissue of a Jelly-fish; e, epithelium; <j, gelatinous matrix :
60 branching cells ; cf, elastic fibres. (From Lang's Comparative Anatomy.)

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

v

Fn;. 11.- Keticular ummective tissue. (From Lang.)

cartilage,Fig. 13), sometimes permeated by systems of fibres (fibro-
cartilage, Fig. 14), which may be of an elastic nature (yellow clastic
cartilage). The cells are usually rounded, and as ;i 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 which the
cells are contained forms a cell-capsule. The outer surface is
covered over by a fibrous membrane — the perichondriu/n. Carti-

2. — Fatty tissue ; F, fat cells ; B, connective-tissue fibrils. (From Lang, after Ranvier.)

lage is frequently hardened by the deposition in the matrix of salts
of lime — and is then known as calcified cartilage.

In bone 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 Haccrsian

Fie. 13. — Hyaline cartilage.

Fit:. 14.— Fibm-ca; '

canals — in which blood vessels lie. The cells, or lonc^corpuscks, lie
in minute spaces — the lacunoe — between the lamella3, and a system
of exceedingly fine channeTs^-the c<matoite— 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-

;*m&

.*rw

W s

&

part

mal. It consists of microscopic
fibres aggregated together into
large bundles or layers. These
fibres are composed of a sub-
stance— the muscle substance —
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 striated
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 (n). Some-
times there is a core of proto-
plasm : but more usually the entire fibre is composed of muscle
substance, with perhaps a remnant of protoplasm in the neigh-
bourhood of each nucleus. The substance of the fibre is crossed
bynumemu- transverse bands and striae, the precise significance
of which is a matter of controversy. The fibre is usually en-
closed in a delicate sheath — the sareolemma. Striated muscular

ver>e section of compact
bone, c, lamella- concentric with the
outer surface ; /*, lamella; concentric
with the surface of the marrow cavity ;
c, section of llaversian canals; c', sec-
tion of a Haversian canal just dividing
into two ; ./, interstitial lamella-. (From
Huxley's lestomin Physiology.)

.

STRUCTURE AND PHYSIOLOGY OF ANIMALS 27

tissue is specially characteristic charts in which rapid movement
is necessary.

The principal elements of nervous tissue^re nerrc cells and

nerve fibres. ^^B

Nerve cells (Fig. 18) vary greatly in form ; the^^ffe relatively large

K, It). — Xon striated muscle cell ; /. substance of fibre ; n. nucleus ; p. unaltered protoplasm in
the neighbourhood of the nucleus. (From Huxley's Lessons in Physiology.)
is with large nuclei, and one or several processes produced into
nerve fibres.

The nerve fibres (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 nerves. The fibres themselves vary
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 medullatcd nerve-fibre. In this there is a central cylinder
— the H.i'ix-cifUndcr or ncn-mxis (A, an] — which is the essential part

B

FIG. 17;— Striated muscle. A, part of a muscular fibre of a Frog ; B, portion of striated muscle
teased out to show separation into fibrillaj. (From Huxley's Lessons in Physioloyi/.)

of the fibre, and is made up of numerous extremely fine primitive
fibrillce : this is surrounded by a layer of a white glistening
material — the white substance of Schumann or medullary sheath
{med), enclosed in turn in a veiy delicate membrane — the
neurilemma (neur).

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

28

ZOOLOGY

SECT.

itlr •

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

med
i f

I IB

Fi<;. 18. — Nerve cells. A, multip >>ir ;
B, bipolar.

Fie. I1.'.— Nerve fibres. J, medullated
B, non-niedullated ; a.r, iie\iraxis
med, medullary sheath; neur.
neurilemma.

In a large proportion of cases such corpuscles are. similar to
Amoebae in their form and movements (ii'inu'loitJ eurpu^ies, 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 are able like
Amoebae 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 plosmodium.

The characteristic cells of the
reproductive tissues are the ova
and the spermatozoa or $j>ci'i/is. The
ova (Fig. 4\ when fully formed, are
relatively large, usually spherical
cells, sometimes composed entirely
of protoplasm, sometimes with an
addition of nutrient fooil-yolk. Eacl
ovum, as already mentioned, ei
closes a large- nucleus (gernyim
vesicle) and in the interior of tht
one or more nucleoli or germii
_ spots. The sperms (Fig. 20)

extremely minute bodies, nearl

always motile, usually slender and whip-like, tapering towai
extremity, and commonly with a rounded head at the other.

Fi«i. 'Jo. Various forms of spcTmato/na.

K. of a Mammal; l>, of :i Turbellarian

00 ; C, and i/, and ., of Nc-niatodu

QB ; .'', of a ( 'nistarcan ; <i. of ;i

Salamander; //,tlie commonest form

with «>val head and lun^ tlayellum.
(Fr.au Land's Comparative Anatomy.)

i STRUCTURE AND PHYSIOLOGY OF ANIMALS i>y

The sperms are developed by a succession of cell-divisions from
certain cells — the primitive male cells — similar in character to
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 trie 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 dertnis. 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 chitin,
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 dermis.

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

:)<) 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 cndoskclcton is present, it usually consists either of
cartilage or bone or of both ; but sometimes it is composed of
numerous minute bodies (spiculcs) of carbonate of lime or of ai
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- J
digested, and through the wall of which the nutrient matters are /
absorbed ; together \vith certain glands.

In the lowest groups in which a distinct alimentary or enteric cavity
is present it is not distinct from the general cavity of the body,
but in all higher forms there is an enteric canul which is sus-
pended within the cavity of the body, and the lumen of which i
completely shut off from the latter. It may have simply the form o
a sac or bag with a single opening which serves both as mouth an
anus ; in other cases the sac becomes branched and may take th
form of a system of branching canals. In most animals, however
the alimentary canal has the form of a longer or shorter tu
beginning at the mouth and ending at the anal opening (Fig. 21).
In most cases there are organs in the neighbourhood of the mout'
serving for the seizure of food : these may be simply tentac/cx <>
soft finger-like appendages, or they may have the form ot\/V//'-x, b
means of which the food is not only seized but torn to pieces o
pounded up to small fragments in the process of mastication. Th
alimentary canal itself is usually divided into a number of region
which differ both in structure and in function.

In general there may be said to be three regions in the all
mentary canal — the ingestive, the digestive and absorbent, and the
cgestive or efferent. The ingestive region is the part followin
behind the mouth, by which the food reaches the digestive an'
absorbent region. But, besides serving as a passage, it may als
act as a region in which the food undergoes certain processes
chiefly mechanical, which prepare it for digestion. This ingestiv
region may comprise a mouth cavity or buccal cavity, a pharynx,
an (esophagus or guild, with sometimes a muscular ///::v/v/, which
may be provided with a system of teeth for the further breakin
tip 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

FIG. 21. — General view of the viscera of a male Frog, from the right side. «, stomach ; 1>, urinary
bladder ; c, small intestine ; cl, cloacal aperture ; </, large intestine ; c, liver ; /, bile duct ;
ft, gall bladder ; h, spleen ; ?, lung ; / , larynx ; I, fat body ; m, testis ; n, ureter ; o, kidney ;
p, pancreas; s, cerebral hemisphere; sp, spinal cord; t, tongue; c, auricle; v. r, urostyle ;
»•, ventricle ; r.«, vesicula seminalis ; iv, optic lobe ; x, cerebellum ; y, Eustachiaii recess ;
~, nasal sac. (From Marshall.)

merely for the passage to the anal opening of the fccccs 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, k), 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 confined to certain of
the cells, which have a special form ; very often the secreting cells

32 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 salivary 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
fo/6— has, however, at least a mechanical effect on this process,
and assists the secretion of the pancreas in its e'ffects upon fat.
In lower forms 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-pancreas 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
the body. But in the majority of animals there is a system of
branching tubes containing a special fluid — the Mood — 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 branchiw ;
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 tracJiece, 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 haemoglobin, 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 contractility, by
virtue of which, when stimulated in certain ways, it contracts in
the direction of its length, becoming shortened, and, at the same
time, thickened (Fig. 22). The extremities of the muscle are

FIG. 22. — Bones of the human arm and 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.

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 lorain. 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 oft 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 conveyed
along a nerve-fibre and
causes the dilatation or
contraction of the blood-
vessels of the part; and
a similar initiatory or
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

FIG. 23. — Nervous system of the Frog.
Howes's Atlas.)

(From

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, ears 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 ear 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 spermary or testis. Sometimes
ovaries and testes are developed in the same individual, when the
arrangement is termed monoecious 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 testicular ducts,
spermiducts, or vasa defer entia. 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 already noticed 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, bd) 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 licterogamy 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

FIG. 24. — Fresh-water polype (Hydra), two specimens, the one expanded, the other contracted
showing multiplication by budding. bdJ bd.~ bd.% 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, AB) 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 adult 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

d-

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 Gegenbaur.)

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 antimeres,
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 dorso-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 hemisomes 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 metameres.
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 Vertc-
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 mantle, 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 arQ all
members -of the great group Chordata.

Altogether twelve phyla are to be recognised, viz. : —

I. Protozoa VII. Molluscoida

II. Porifera VIII. EcTiwwdermata*

III. Ccelenterata IX. Annulata

IV. Platyhelminthes X. Arthropoda
V. Ncmathdminthes XI. Molhisca.

VI. TrocJielruintlus XII. Cliordata

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^thaf cells vary considerably in character.
They may be amoeboid 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.

Class 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 flasrellate 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 J mm.
in diameter, produced into irregular processes or pseudopods (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 endosarc 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. me.), 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 ency station ; 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 Amoeba have been observed to

PHYLUM PROTOZOA

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

case.

FIG. 27.— Amoeba. A, A. quarto, ; B, the same killed and stained ; C, A. proteus; D, encysted
specimen ; E, A. proteus; F, nucleus of same, stained ; G, A. verrucosa; H, nucleus of same,
stained ; I, A. proteus, undergoing binary fission ; a, point of union of enclosing pseudopods ;
c. vac. contractile vacuole ; cy. cyst ; /. vac, food vacuole ; mi. 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 Amceba,
belonging to the family Amcebidce, of the order Lobosa. The blunt
pseudopods not uniting to form networks place it among the
Lobosa : the absence of a shell among the Amoebida?. 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-

,

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

FIG. 28.— Protamceba primitiva. Showing changes of form and three stages in binary fission.
(After Haeckel, from Parker's Biology.)

Protamaiba (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.

FIG. 29

.—A, Quadrula symmetrical B, Hyalosphenia lata; C, Arcella vulgaris
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 Difflugia
(D), which secretes a gelatinous coating to which minute sand-
grains and other foreign particles become attached.

V lj$M/liL

A ^

FIG. 30.— Chlamydomyxa labyrinthuloides. A, active phase; c.w. ceU-wall; /. 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 Sphagnum; a, specimen completely enclosed
in cell ; It and <-, 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— E
after Geddes.)

ORDER 2. — LABYRINTHULIDEA.

In this group there are only two genera — Labyrinthula and Chlamydomyxa.

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

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

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 hcemato-
chrome, allied to chlorophyll, and small globular bodies of a bluish tint. In
the young condition (a) the resting cells are globular 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 burst through the wall
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 Algae
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 (sp. ) 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 hotophytic, i.e. like that of an ordinary green plant, the food
consisting of the carbon dioxide and various mineral salts dissolved in the water.

Labyrinthula (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 (a.) ; c. cells
J, part of specimen in resting condition with heap of cells (c.) ;

travelling in the filaments ; B, _

C, a single cell from an actively moving specimen with

(From Biitschli's Protozoa, after Cienkowsky.)

lecting threads ; nu. nucleus.

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,
has 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 M'ycetozoa.

ORDER 3. — FORAMINIFERA.

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 calcium carbonate.

One of the simplest members of the group is Microgromia (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. vac. contractile vacuole ; nu. nucleus ; sh. shell. (From Butschli's Protozoa.
after Hertwig and Lesser.)

(mi.) and contractile vacuole (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
its pseudopods, assumes an oval form, and sends out two

in

tiagella by means of which it is propelled through the water (D).
We shall find other instances in which the young of a Rhizopod is

UNIVERSE

OF

PHYLUM PROTOZOA 51

a flagcllula, 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)

va,c

FIG. 33.— Platoum stercoreum.

vacuole ; '/• food particles; n u
Cienkowsky.)

A, single zooid ; B, formation of colony ; c. -cad. contractile
nucleus ; sh. shell. (From Btitschli's Protozoa, after

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

G-romia (Fig. 34, 1) leads us to the more typical Foraminifera.
The protoplasm of this form protrudes from the mouth (a) of the
chitinoid shell (sh.) 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. — Sguammulina (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, 3\ 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, 8), or in a spire in which the newer chambers

E 2

52

ZOOLOGY

SECT.

overlap the older (9, 10\ or in an irregular spiral of globular
chambers (6), or in an extremely compact spiral in which the new
chambers completely enclose their predecessors (11\ In all cases

3.Squammulina

4.M i I i o I a

FIG. 34. — Various forms of Foraminifera. In A, Miliola, a, shows the living animal ;
6, the same killed and stained ; a. aperture of shell ; /. food particles ; nu. nucleus ; sh. shell.
(From BUtschJi'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

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

2.Lagena

4.Frondicularia G.CIobigerina

Q.PIanorbulina

ll.Nummulil'es

FIG. 35. — Shells of Foraminifera. In 3, k, and 5, a shows the surface view, and b a section ;
8a is a diagram of a coiled cell without supplemental skeleton ; 8b of a similar form
with supplemental skeleton (s. s&.) ; and 10 of a form with overlapping whorls ; in lla 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, 8b, 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

jbsd

Fio. 36.— Hastigerina murrayi. plsm. 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

PHYLUM PROTOZOA 55

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 flagellulce 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 Globigerime contained in it.

From the palaeontological 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-
mulitic 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-
tinophn/s sol (Fig. 37). The body

2 J, i -& i i , - FIG- 37.— Actinophrys sol. a. axial

IS nearly Spherical, and Contains filaments of pseudopods ; n. nucleus ;

a large nucleus and numerous SSffS3LJ!3S£SL&>'
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

56

ZOOLOGY

SECT.

as far as the nucleus. Living organisms are devoured in much
the same way as in Amoeba: 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 &focd-vacuole.

Actino&phcerium (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

FIG. 38.— Actinosphserium eichhornii. A, the entire organism; B, a small portion
highly magnified ; chr. chromatophore ; cort. cortex ; c. vac. contractile vacuole ; med. medulla ;
nu. nuclei. (From BUtschli's Protozoa, after Hertwig and Lesser.)

very large. There are numerous nuclei (nu.) and chroma tophores
(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.

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

3.Cla>hru.lina

FIG. 39.— Various forms of Helicrzoa. 8a, the entire animal; Sb, the flagellula ; c. vac.
contractile vacuole ; g. gelatinous investment ; nu. nucleus psd. pseudopods ; sk. siliceous
skeleton ; sp. spicules. (From Butschli's Protozoa, after Schulze and Greeff.)

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

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 ActinosphaBrium. In Clathrulina spore-formation
takes place in the active condition, and the spores (Fig. 39, 3 I) are
tiagellulse, 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 (ext. 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 (psd.) 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
the protoplasm has a gela-
tinous consistency, and is dis-
tinguished as the calymna.

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

'Inl.caps.fr
cent caps

**•<*•**

FIG. 40.— Lithocircus annularis. cent. caps.
central capsule ; <j-t. cnjix. />,-. extra-capsular
protopl isrn ; i,>t. caps. pr. intra-capsular pro-
toplasm ; nu. nucleus ; psd. pseudopods ; skd.
skeleton ; z. cells of Zooxanthella. (After
Biitschli, from Parker's Biology.)

PHYLUM PROTOZOA

59

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

flat 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 opcrculum (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 phwodiwn (pli.\

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, sL 1, sk 2, sk. 3} con-
nected by radiating spicules. The outer
of these spheres occurs in the extra-
capsular protoplasm (B, ex. caps. pr.\

the middle one in the intra-capsular protoplasm, and the inner
one in the nucleus (nu.).

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 T>y 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 ; if), intra-capsular
protoplasm ; n. nucleus,
containing nl. numerous
nucleoli ; ot. oil drops ; ca.
calymna ; rp. protoplasm
surrounding calymna ; s.
spicules. (From Lang's
Comparative Anatomy , after
Haeckel).

60

ZOOLOGY

SECT.

i

71—-

FIG. 42. — Aulactinium actinastrum. c. calymna ; km. central capsule ; n. nucleus ; op-
operculum ; ph. phseodium. (From Lang's Comparative Anatomy, after Haeckel.)

cent, caps

Fie. 43.— Actinomma asteracanthion. A, the shell with portions of the two outer

spheres broken away ; J5, suction showing the relations of the skeleton to the animal ;
Cent, i-n./ix. central capsule ; ex. caps, pr, cxtra-caiisubu- jirntujilasni ; mi. nucleus ; sk. 1, outer,
sk. 3, middle, sk. J, inner Fphere of skeleton. (From ijiitschli's Protozoa, 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 are
alike (E), and each encloses a small crystal (c.) : 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, z.)
multiply by fission independently of the Radiolarian. It is

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(vac.) ; E, spores containing crystals (c.) ; F, mega- 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 ZooxantJiella 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 difforme.

Didymium occurs as a whitish or yellow sheet of protoplasm (Fig. 45, G),
often several centimetres across, which crawls, like a gigantic Amreba, 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.

62

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, spg. 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) on a fragment of leaf(/.);

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

C, aflagellula with contractile vacuole (c. vac.) and nucleus (JIM.); D, the same after loss
of flagellum ; b, an ingested Bacillus ; E, an amcebula ; F, conjugation of amoabulse to form
a small plasmodium ; G, a larger plasmodium accompanied by numerous amosbulte ; 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 amoeboid mass which soon becomes a flagellula
(C), provided with a single flagellum, a nucleus (mi.), and a contractile vacuole

(c. vac. ). The flagellulie move freely and ingest Bacilli (D, b.) ; then, after a
time, they become irregular in outline, draw in the flagellum, and become
amoeboid (E). The amcebula? 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 110 fusion of the
nuclei of the amcebulre. Thus Didymium in its active condition is aplasmodium.
i.e. a body formed by the concrescence of amoabulae.

!

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. - They are also remarkable for their close resemblance
in the structure of the sporangia and spores to certain Fungi, a group of parasitic
or saprophytic plants in which they are often included, most works on Botany
having a section on the Myxomycetes 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 flagellulae, but soon become amoeboid and fuse to form the plasmodium.

CLASS III.— MASTIGOPHORA.

1. EXAMPLE OF THE CLASS — Euglena, mridis.

Euglena (Fig. 46) is a flagellate organism commonly found io
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, ces.), from the inner surface of which
springs" a single long flagellum (fl.). 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 (pg^), formed of
a pigment allied to chlorophyll and called JicematocJirome. 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

C.VTZC

FIG. 46.— Eugleua viridis. A— D, four views illustrating cuglenoid movements; Band H,
enlarged views ; F, anterior end further enlarged ; G, resting form after binary fission ; c. vac.
contractile vacuole in H, reservoir in E and F ; cy. cyst ; jl. flagellum ; m. mouth ; nv. nucleus ;
ces, gullet ; p. paramylum bodies ; py. pigment spot ; r. (in H), reservoir. (From Parker's
Biology, 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 flagellulaB are produced, which, sometimes after
passing through an amoeboid stage, develop into the adult
form.

PHYLUM PROTOZOA 65

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.

Systematic Position of the Example.

Euglena viridis is one of several species of the genus Euglena,
and belongs to the Family Euglenidcv, sub-order Euglenoidea, and
order Flagellata.

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 euglenoid 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 (1\ or greatly elongated (3).

6G

ZOOLOGY

SECT.

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

3.Asrasiof>is

2 Oikomonas (?)

II.DJnobpyon 12. Sy ncry |>ra 13. An^ho|)hysa 14.Rhi|>idodendron

FIG. 47.— Various forms of Plagellata.— In S, flagellate (a) and amoeboid (6) phases are
shown ; in 5, flagellate (a) and helizoaii (I) phases ; in 8 are shown two stages in the in-
gestion of a food particle (/.) ; c/r*\ chromatophores ; c. vac. contractile vacuole ; /. food par-
ticle (j. gullet; n u. nucleus ; 1. lorica ; p. protoplasm ; per. peristome ; v.i. vacuole of ingestion.
(Mostly from Biitschli'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 Mastigamceba (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 (6).

The number of flagella is subject to great variation. There
may be one (Fig. 47, 1-3), two (9, 10), three (6), or four (7).
Sometimes the flagella show a differentiation in function ; in
Heteromita, e.g. (Fig. 51) the anterior flagellum (fl. 1) only is
used in progression, the second or ventral flagellum (fl. 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 intra-cellular parasite within the
cells of one of the lower worms.

Hcvmatococcus (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.u\),
and contains chromatophores (ch/r.) 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 Fiagellata are
often included among the Algae 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, 8) we
have one of the simplest arrangements : near the base of the
flagellum is a slight projection containing a vacuole (y.i.)\ the
movements of the flagellum drive small particles (/.) against this
region where the protoplasm is very thin and readily allows the

F 2

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, L), usually vase-shaped and
widely open at one end so as to allow of the protrusion of the

200 TTl.Tn

FIG. 48.— Hsematococcus plnvialis. A, motile stage ; B, resting stage ; C, D, two modes
of fission ; E, Hwmatococcus lacustris, motile stage ; F, diagram of movements of flageUum ;
clir. chromatophores ; c. vac. contractile vacuole ; c.v:. cell-wall ; mi, nucleus ; nu'. nucleolus ;
pyr. pyrenoids. (From Parker's Biology.)

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

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
flagella protrude (18) ; or tufts of zooids, reminding us of the
flower-heads of Acacia, are borne on a branched stem (13). In
Volwx (Fig. 50) the zooids of the colony are arranged in the form
of a hollow sphere, and in Pandorina (Fig. 49) in that of a solid
sphere enclosed in a delicate shell of cellulose. Lastly, in Rhipido-

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.
HaBmatococcus (Fig. 48) and Euglena (Fig. 46), for instance,
divide while in the encysted condition ; Heteromita (Fig. 51)

FIG. 49.— Fandorina morum. A, entire colony; B, asexual reproduction, each zopid
dividing into a daughter-colony ; C, 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
Biology, 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
Dinobryon (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 Volvox (Fig. 50), certain
zooids, called partlienogonidia (A, «), have specially assigned to them
the function of asexual reproduction : they divide by a process
resembling the segmentation of the higher animals (D^D5), 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

FIG. 50.— Volvox globator. A, entire colony, enclosing several daughter-colonies ;
B, the same during sexual maturity ; C, four zooids in optical section ; D1 — D5, develop-
ment of parthenogonidium ; B, ripe spermary ; F, sperm; G, ovary containing ovum and
sperms; H, oosperm ; a, parthenogonidia ; fl. flagelluru ; or. ovum ; or//, ovaries ; pg. pigment
spot ; spy. spermaries. (From Parker's Biology, after Cohn and Kirchner.)

(Fig. 51, E1) and undergo complete fusion (E2 — E4) : the result of
this conjugation of the two gametes or conjugating cells is a thin-
walled sac, the zygotc (E5), the protoplasm of which divides by
multiple fission into very minute spores. These, when first
liberated by the rupture of the zygote (E°), are mere granules,
but soon the ventral or trailing flagellum is developed, and after-
wards the anterior flagellum (F1 — F4). 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).

:

PHYLUM PROTOZOA

71

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

c.vac

FIG. 51. — Heteromita rostrata. A, the positions assumed in the .springing movements
of the anchored form ; B, longitudinal fission of anchored form ; Cj transverse fission of
the same; D, fission of free-swimming form ; E, conjugation of free-swimming with anchored
form ; E-5, zygote ; E<>, emission of spores from zygote ; F, development of spores; fi.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.

I.Monosiga. 2.Salpingoeca.

S.Polyoeca. 4.Proterospongia.

Fio. 52. — Various forms of Choanoflaprellata. 26 illustrates longitudinal fission ; %c, the pro-
duction of flagellulse ; c. collar ; c. vac. contractile vacuole ; ft. flagellum ; 1. lorica ; mi. 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, /, 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

ir PHYLUM PROTOZOA 73

show that its movements cause a flow of water, with minute
particles in suspension, up the outside of the collar and down the
inside, the solid particles being then ingested in the 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 con-
tractile vacuoles (c. vac.), but no trace of mouth or gullet. Some
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. In
Polyceca (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 Proterospongia (4), in
which the zooids are enclosed in a common gelatinous matrix of
irregular form.

Reproduction. — The "collared monads," as these organisms
are often called, multiply by longitudinal fission (#&). In some
cases multiple fission of encysted individuals has been observed
(2c), small simple flagellulse being produced which gradually
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 often
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 movements
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 proto-
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. Prorocentrum
(3) is remarkable for the absence of the transverse groove, while Polykrikos (4)
has no fewer than eight transverse grooves and no shell. The latter genus
also has stinging -caps ides or nematocysts (a, b) in the protoplasm, resembling
those of Zoophytes (see Sect. IV.), and has numerous nuclei of two sizes,
distinguished as meganudei (nu.), and micronudei (nu'.).

ZOOLOGY

SECT.

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

C le n o d inium

S.Cerahum 3.Prorocenlrum

4.Polykrikos

Fio. 53.— Various forms of Dinoflaerellata. 2 shows the shell only; kn is- an undischarged,
and b a discharged stinging-capsule ; chr. chromatophores ; fl. 1, longitudinal flagellum ;
fl. 2, transverse flagellum ; 1. gr. longitudinal groove ; ntc. nematocyst ; nu. meganucleus ;
nu'. micronucleus ; pg. pigment spot ; t. gr. transverse groove, (From Blitschli'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, yoctiluca and Leptodiscus. 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 tentacle (by. ), 110-
tiecable for its transverse striation.
Near the base of this flagellum is
the mouth (m. ), leading into a short
gullet in which is a second flagel-
lum (/. ), very small in proportion
to the first. On the side opposite

animal ; b, c. flagellulse ; bt/. tentacle ; /. flagel- to the mouth is a strongly marked

luiu ; m. mouth ; n. nucleus. (From Lang.)

IK. M.— Noctiluca miliaris.

adult

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.

.

PHYLUM PROTOZOA

75

The spores (&, c), formed by the breaking up of the protoplasm of the parent, escape
a form very unlike the adult, the tentacle or large flagellum being repre-
jnted by a short thick process, while the main swimming organ of the flagellula
monies the small oral flagellum of the adult.

CLASS IV,— SPOKOZOA,

1. EXAMPLE OF THE CLASS — Monocystis 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 vesiculee seminales of the
common Earthworm. It is flattened, greatly elongated, pointed
at both ends, and performs slow movements of expansion and

C

FIG. 55. — Monocystis agilis. A, B, two individuals in different stages of contraction;
C, cyst containing spores ; D — F, development of young (M) in a group of sperm-cells
of the Earthworm ; G, newly liberated Monocystis surrounded by sperms of the Earthworm ;
M, young Monocystis ; nu. nucleus ; sp. sperms or sperm-cells of Earthworm. (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.) with 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 process
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 flagellulas. 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
divisiblelnto 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 cyst 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
l)t 'tween the species of Monocystis depend largely upon size.

PHYLUM PROTOZOA

77

ORDER 1. — GREGARINIDA.

All the more typical members of the class belong to this
group. With the exception of Monocystis, already described, the
only genus to which it will be necessary to draw attention is
Grcgarina (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 protomerite (pr.\ and a posterior, the deutomerite
(deu.), in which the nucleus is situated. Anteriorly to the proto-
merite there is sometimes found, especially in young individuals,

D4

FIG. 56.— Gregrarina. A, two specimens of G. blattarum partly embedded in enteric
epithelial cells of Cockroach ; Bi, B2, two specimens of G. dujardini ; in B2 the epimerite (<;*.)
is cast off ; C, cyst of G. blattarum, from which most of the spores have been discharged ;
D, four stages in the development of G. gigantea ; cy. cyst ; deu. deutomerite ; cp. epimerite ;
p. gelatinous investment of cyst ; nu. nucleus ; pr. protomerite ; psd. 1, short pseudopod ;
;/*.w. 2, long pseudopod ; sp. mass of spores ; si>d. sporoducts. (From Biitschli's Protozoa.)

a third division, the epimerite (cp.), which is sometimes provided
with hooks (B1), serving to attach the parasite to the epithelium
of the intestine of its host. As maturity is reached the epimerite
is thrown off (B2), 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 Gregarina gigantea of the
Lobster, the young is liberated from the spore in the form of a non-
nucleated amcebula (D1), with one long and one short pseudopod
(D2) ; this divides by the long pseudopod (psd. 2) becoming sepa-
rated off, and each product of fission, developing a nucleus, passes
into the adult form (D3, D4.)

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.
Eimeria (Fig, 57, 1), for instance, is found in the intestinal epithelium of the

1 Eimcria

2.CoccIdiuin

FIG. 57.— Coccidiidea. A, adult Eimeria (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 (sp.) ; E, spore with falciform young ; F, adult
encysted form of Coccidium 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, Drepanidium 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 lieberkuhnii, amreboid phase ; B, MyxoboUis mtilleri,

spore with discharged nematocysts (ntc.) ; C, spores (psorosperms) of a Myxosporidian ;
ntc. nematocysts. (From BUtschli's Protozoa.)

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 Myxi<li>nn,
found in the urinary bladder of the pike.

PHYLUM PROTOZOA 79

RThe spores are often very complex ; in some cases (B) they possess organs
ke the trichocysts of Infusoria and the iiematocysts of Zoophytes (vide infra) ;
in others they have the form of curious twisted bodies called psorosperms, found
in the gills, kidneys, &c. , of fishes ; they have been seen to liberate amcebula?.

ORDER 4. — SARCOCYSTIDEA.

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

FIG. 59.— Sarcocystis miescheri, adult form (s) in striped muscle of pig. (From
Blitschli's Protozoa, after Raiuey.)

striped muscular fibre. They are often known as Ramey's or Miescher's
corpuscles. The protoplasm divides into spores from which falciform young are
liberated.

CLASS V, -INFUSORIA.

1. EXAMPLE OF THE CLASS — Paramcecium caudatum.

Structure. — Paramceciuwi, the " slipper-animalcule," is tolerably
common in stagnant ponds, organic infusions, &c. The body (Fig.
60) is somewhat cylindrical, about J mm. in length, rounded at
the anterior and bluntly pointed at the posterior end. On the
ventral face is a large oblique depression, the luccal groom (hue.
gr.); leading into a short gullet (gul.), "which, as in Eugiena, 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 very
clearly differentiated into a comparatively dense cortex/ (cort.) and
a semi-fluid medulla (ined.), 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 are
very characteristic. One, distinguished as the meganudcus (nu.\
is a large ovoid body, staining evenly with aniline dyes, which,
when it divides, does so directly by a simple process of constriction.
The other, called the micronuclcus (pa. nu.\ is a very 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. me.) are two in number, and are very
readily made out. Each is connected with a series of radiating
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 surrounding

80

ZOOLOGY

SECT.

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

c.vac.

Fi,;. tin— Paraxnoecium caudatum. A, the living animal from the ventral aspect; B, the
same in optical section: the arrow shows the course taken by food-particles ; (X a specimen
which has discharged its trichocysts ; D, diagram of binary fission ; inn-. </,-. buccal groove ;
rih-t. cortex; <-i'. cuticle; c, rue. contractile vacuole;.?'. vdc. fond vacunle ; <n<l. gullet; „<«!.
medulla; nu. meganucleua ; pa. nu. micronucleus ; trek, trichocysts. (From Parker's Biology.)

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

The cortex contains minute radially arranged sack called
trichocysts (trch.). When the animal is irritated, more or fewer of

.

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 a
specimen killed with iodine or osmic acid the threads can fre-
quently be seen projecting in all directions from the surface (C).

Food, in the form of small 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-vaciiole "
(/. 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
Paramoecium being thus considerably in advance of those of Amoeba.
Effete matters are egested at a definite anal spot posterior to the
mouth, where the cortex and cuticle are less resistent tharl 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 micronucleus indirectly.

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

ng.ftu

51.— Paramoecium caudatum, stages in conjugation, gul. gullet ; mg. nu. meganucleus ;
mi, nu. micronucleus ; M<j. nu. reconstructed meganucleus ; Mi. nu. reconstructed micro-
nucleus. (From Parker's Biolony, after Hertwig.)

a certain number of times it is interrupted by conjugation. In
this very remarkable and characteristic process two Paramoecia

32 ZOOLOGY SECT.

become applied by their ventral faces' (Fig. 61, A), but do not fuse.
The meganucleus (ing. nu.) 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 micronuclei (B).
Two of these (mi. nu.', mi. nu.") disappear ; of the remaining two
one is distinguished as the stationary pronucleus, 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.

Systematic Position of the Example.

Paramcecium aurelia is one of several species of the genus
Paramaicium, belonging the Family Para/maecidce, of the sub-order
Trichostomata, 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 ;iiv swept by
the movement of the cilia, among the Trichostomata. The Para-
mcecidse are free-swimming, asymmetrical, uniformly ciliated, with
a ventrally placed mouth. P. caudatum is about \ — -1- 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 83

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 process (J), 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.f.), 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 holotrichous type, 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 Jietcrotrichous 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 Stcntor (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 peritriclwus type of ciliation : in Vorticella
(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, 9), in which the ciliary
spiral makes no fewer than four turns.

But it is in the liypotricJious 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 processes
of the cuticle, while on the ventral surface the cilia take the form
of large hooks, fans, bristles, and plates with fringed ends (Fig. 62,
7). 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 most
extraordinary forms in this group is Dioplirys, the size and arrange-
ment of its polymorphic cilia giving it a very grotesque appear-
ance. In another genus (10) the distal end of the flask-shaped
body bears a circlet of large fringed cilia, giving the animal the
appearance of a Rotifer (vide infra, Section VII.).

In addition to cilia, many genera possess delicate sheets of
protoplasm or undulating membranes in connection with the

84 ZOOLOGY SECT, n

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. nib.) is a very large and obvious
structure.

Certain peculiar forms have yet to be mentioned. Mnlticilia (Fig.
62, 12) has an irregular body of varying form, and bears a small
number of very long flagellum-like cilia. Another genus in which
the cilia approach to flagella is Zophonumas (IS), the ovoid body of
which bears a tuft of close-set cilia at its anterior end. Aciino-
l)olus (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 bo^olongated and band-like (3, mg. nu.), 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 (mi.) 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 Vorticella and other peritrichous genera there is a single
contractile vacuole (Fig. 64, c. inc.), 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
Paramoecium, 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
protoplasm of Radiolaria.

Tricho cysts, like those of Paramoecium, are found in many
holotrichous forms, but are rarely present in the other subdivisions
of the order. In the peritrichous Epistylis nmbcllaria, however,
there are found numerous minute capsules (Fig. 62, 9, ntc.)
arranged in pairs, each containing a coiled thread. They are

.Pro rod on 2.Nycrorherus

S.Anoplophryo. c

9.E pisry I Is

mgr.nu

12.Mulh'cilia 13.Lobhomonas

•^^r-r-we- w IS.Trachelius mophryoglena

iinirf 16.Condy osl-oma

iZOpalinopsis

FIG. 62. — Various forms of Ciliata. 9a shows part of a colony, h a single zooid, and
c a couple of nematocysts ; a. aims ; c. (in IS) cuticle ; c. (in 19) excretory canals ; c. cac.
contractile vacuole ; /. vac. food vacuole ; g. gullet ; my. nu. meganucleus ; mi. nu. micro-
nucleus ; mth. mouth; nu. nucleus; ntc. iiematocyst ; p. (in 15) a Paramoedum seized by
DiiUniium ; t. tentacle ; u. mb. undulating membrane ; vac. non-contractile vacuole ; vst.
vestibule. (From Biitschli's Protozoa, after various authors.)

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.

LDictyocysl-Q

3.ThuHcola ^.Obhrydium
2.Pyxicola

5. Sri cholricha

FIG. 63.— Various forms of Ciliata. In 1 the shell alone is shown ; m. contractile fibre ; op.
operculum. (From Biitschli's Protozoa, after various authors.)

From them there is a fairly complete gradation to genera, like
Paramoeciurn, having the permanently open mouth on the left side
of the ventral surface, at the end of a well-marked buccal grove
or peristome. Vorticdla (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 (d.) from
the area thus enclosed : the mouth (m-th.) 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-
therus (Fig. 62, 2) and some other genera there is, instead of the

II

PHYLUM PROTOZOA

temporary anal spot described 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
loric-a, formed of a chitinoid material, and reminding us of the

FIG. (U.— Vorticella. A, B, living specimens in different positions , C, optical section ; Di, D^,
diagrams illustrating coiling of stalk; E1, E-, two stages in binary fission; E3, free zooid ;
Fl, F-, division into mega- and microzooids ; G1, G-, conjugation; H1, multiple fission of
encysted form ; H-, H3, development of spores ; ax. f. axial fibre ; cort. cortex ; cu. cuticle ;
c. rac. contractile vacuole ; <l. disc; <jvll. gullet'; m. microzooid ; mth. mouth; nu. mega-
nucleus; per. peristnme. (From Parker'

similar structure found in so many of the Mastigophora. Some
(Fig. 62, 4} have bell-like shells, variously ornamented, and in
others (Fig. 63, 1) the similarly shaped shell is perforated and
resembles the skeleton of some of the Radiolaria. 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 opercu um (3) is

•88

ZOOLOGY

SECT.

attached to the interior of the tube, and is closed by a contractile
thread of protoplasm (m.), 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, 7?), or may contain an axial
fibre or muscle, like that of Vorticella (Fig. 64, ax.f.). In Ophridium
(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

PIG. 65. — Zoothamnium arbuscula. A, entire colony ; B, the same, natural size ; C, the
same, retracted ; D, nutritive zooid ; E, reproductive zooid ; F1, F-, development of reproduc-
tive zooid ; ax.f. axial fibre ; c. vac. contractile vacuole ; na. nucleus ; n.z. nutritive zooid ;
r.z. 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 Paramoecium, fission
is really transverse in this case also. In such simple Peritricha
as Vorticella division proceeds until two zooids are produced on
a single stalk; one of the two then acquires a second circlet
of cilia near its proximal end, becomes detached (E3), 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 Zoothamnium (Fig. 65) the zooids

PHYLUM PROTOZOA

89

are dimorphic : the ordinary bell-shaped forms (n.z.) divide in
the usual way, but as they remain attached, the process results only
in the increased complexity of the colony, not in the development
of a new one. The larger zooids (r. z.) are globular and mouthless :
they become detached, swim off, and, after a short free existence,
settle down, develop a stalk (F), divide, and so form a new colony.
In Vorticella multiplication by ItMing 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

J1U

FIG. 66.— Opalina ranarum. A, living specimen ; B, stained specimen showing nuclei ; C,
stages in nuclear division ; D— F, stages in fission ; G, final product of fission ; H, encysted
form ; I, you^g form liberated from cyst ; K, the same after multiplication of the nucleus
has begun ; mi. nucleus. (From Parker's Biology, after Saville Kent and Zeller.)

or megazooid 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, IT).

A peculiar kind of spore-formation, specially adapted to the
requirements of an internal parasite, takes place in Opalina

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-
larvae feed upon the water-plants, and in doing so frequently take
in the spores or encysted Opalinse 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 (G1, G2). 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, Jet}, ovoid (11)}, 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 Podophrya (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 (lc\ capable of protrusion and retrac-
tion, and having its distal end expanded into a sucker. It is, more-
over, practically tubular, the axial region consisting of a semi-fluid

PHYLUM PROTOZOA

91

protoplasm, while the outer portion is tolerably firm and resistant.
When partially retracted, a spiral ridge is sometimes observable

\

C.VGO

7.0t>hryodendron

S.Epholol-a

FIG. fi7. — Various forms of Tentaculifera. la and h, two species of PodopJirva: c, a
tentacle much enlarged ; 2a, Acineta jolyi ; 2!>, A. tuberosa ; in 6 the animal has captured
several small Ciliata ; 8a, a specimen multiplying by budding ; ,V6, a free ciliated l>ud ; 9a, the
entire colony ; .%, a portion of the stem ; 9c, a liberated bud ; a, organism captured as food ;
b.c. brood-cavity ; >><(. bud ; c roc. contractile vacuole ; my. nu. meganucleus ; mi. nu. micro-
nucleus ; t. tentacle. (After Butschli 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 VortMla. Infusors and other organisms are caught by
the tentacles (4, 6), 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
may be 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 stiffly 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
(#, 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 (3 a) like that met with in many of the Mastigophora.

The only colonial form is the wonderful Dendrowma (9), in
which the entire colony attains a length of about 2 mm., and bears
an extraordinary resemblance to a zoophyte (vide infra, Sect. IV.).
It consists of a creeping stem from which vertical bra'nches 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 Ephclota gemmipara (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 Acineta tiiberosa (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
Dcndrosoma the common stem of the colony produces both internal
and external buds (b, Id.).

further Remarks 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 arid even of species are universally distributed,

WJ

ii PHYLUM PROTOZOA 93

being found in all parts of the world where the microscopic fauna
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 been
found in connection with a cancerous disease in Sheep. Parasitic
Vorticellse are said to give rise to the skin-disease eczema in Man.

ciliate Infusor, Iclithyophthirius, 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 only
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 o£ Amcebulse, the
nuclei of the latter remaining distinct and multiplying, are rather
non-cellular than uni-cellular. This point will also be referred to
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 Prota-
mceba, at the bottom of the list, and the Radiolaria at the top.
Similarly, among the Mastigophora, such simple Flagellata as
Oikomonas (Fig. 47, 2 and 8) and Heteromita are obviously the
lowest forms, Noctiluca and the Dinoflagellata the highest. But
whether the Rhizopoda, as a whole, are higher or lower than the
Flagellata, is a question by no means easy to answer. A flagellum
certainly seems to be a more specialised cell-organ than a
pseudopod, and some of the Mastigophora rise above the 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

Fo ram in if era

Lobosa

\

Mycetozoa

Dinoflagellata

Cystoflagellata

Heliozoa Choano-
Flagellata

Flagellata

Tentaculifera

Ciliata

FIG.

-Sporozoa

5. — 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 amosboid 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-

PHYLUM PROTOZOA

95

of

ment of the Protozoan series, since they show a degree
differentiation attained nowhere else by a single cell.

The Mycetozoa appear to have been derived from the common
amoeboid-flagellate stock, since they are all predominantly amoe-
boid in the adult condition, flagellate when young. The Sporozoa
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
jhese 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 gl*)ups that temain 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 wit li-
the Porifera or Sponges.

1. EXAMPLE OF THE CLASS — Syccn gdatiiu.mm.

General External Appearance and Gross Structure —

Sycon gelatinosum?- 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 appmrs
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 S. rl/iatinn, a common European species
which differs chiefly in the absejgptof the pore-membranes.

SECT. Ill

PHYLUM AND CLASS PORI

>%v
' K\

RA

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 pores.
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 the
branches are bisected longitudinally
(Fig. 70), it is found that the terminal
openings (o) 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 gelatiraosum.— A portion slightly
magnified ; one cylinder (that to the right) bisected
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 i p., marks the position of three of the groups
of inhalant pores at the outer ends of the incurrent
canals ; o. osqulum.

VOL. I

FIG. 69.— 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
fine 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 some
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, and
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

H

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

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, collencytes ; 1C, incurrent canals ;
ov. 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

PHYLUM AND CLASS POR1FERA

99

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, ec) —
through which project
regularly-arranged groups
of needle-like and spear-
like spicules (sp), 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

FIG. 72.— Sycon gelatinosum. Transverse section
through the wall of a cylinder (parallel with the
course of the canals), showing one incurrent canal
(1C), and one radial (R) throughout their length ;
sp. triradiate spicules ; sp'. oxeote spicules of dermal
cortex (dc.) ; sp". tetraradiate spicules of gastral
cortex ((/P.) ; ec. ectoderm ; en. endoderm ; pm. pore
membrane ; pp. prosopyles ; ap. apopyle ; di. dia-
phragm ; exc. ^excurrent passage ; P.G. paragastric
cavity ; em. early embryo ; em', late embryo. The
arrows indicate the .course of the water through
the sponge.

H 2

100 ZOOLOGY SECT.

and 72 1C) — 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, pm, 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-

FIG. 73.— Sycon gelatine sum. Sur- FIG. 74.— Sycon gelatinosum.

face view of a pore membrane highly An apopyle surrounded by its dia-

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 exc). 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
prosopyles (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

.

a TDortio

PHYLUM AND CLASS POUIFIjlflA , , , 101

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 i§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 the
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 apopyle (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
mesoderm or mesoglcea — in which the spicules of the skeleton are
embedded. Each spicule is developed from a single cell of the
middle layer, the remains of the cell — the sd^gbla^t^—beiug 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 (sp') ; these, which are
termed the oxeotc spicules, project on the outer surface beyond the
ectoderm, and are arranged in dense masses, one opposite the,
outer end of each of the ciliated canals, this arrangement pro-
ducing the pattern already referred to as distinguishable on the
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 6f stellate shape, with radiating processes — >
the connective-tissue cells or collencytcs (Fig. 71, co); others are fusi-
form ; a good many, the amoeboid toandering 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 endodermal. 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, cm'), resembling in their various
stages those of Sycon raphanus, 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 ftagellum. Between these
two layers is a quantity of tissue usually of a gelatinous consistency
— the mesoderm 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 103

of spicules of carbonate of lime. Reproduction takes place both
asexually by the formation of getiimules, and sexually by means 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 Coelenterata
on the other — to justify us in looking upon them as constituting one
of the great divisions or phyla of the animal kingdom. At the
same time there is so much uniformity of structure within the
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 L—MYXOSPONGI^E.

Non-Calcarea devoid of skeleton.

TRIBE II.—SILICISPONGIdE.
Non-Calcarea provided with a skeleton.

ORDER 1. — HEXACTINELLIDA.
Silicispongioe with six-rayed siliceous spicules.

ORDER 2.— DESMOSPONGLE.
Silicispongise devoid of six -rayed spicules.

Systematic Position of the Example.

Sycon gelatinosum is one of many species of the genus Sycon.
Si/con is one of several genera of the family Syccttidce ; and the

104 ZOOLOGY SECT.

family Syccttidce is one of several families of the order Heterocoela
of the class Calcarea. Among the families of the Heterocoda,
that of the Sycettidm 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 Sycettidai 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—
while 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 — SycantTia — 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 Sycettidw 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 are
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

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. gelatinosum have
been recognised, and some have received special names. Here
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 lichen
(Fig. 75, A). The surface of such an encrustation maybe smooth ;
more commonly it is raised up into elevations — rounded bosses,
cones, ridges or lamellaB ; and the edges may be entire or lobed.
In other cases the sponge grows at first more actively in the
vertical than in the horizontal direction, and the result may be a
long, narrow structure, cylindrical or compressed, and more or less
branched (Fig. 75, B). Sometimes vertical and horizontal growth
is almost equal, so that eventually there is formed a thick, solid
mass of a rounded or polyhedral shape (Fig. 75, C), with an even,
or lobed, or ridged surface. Very often, after active vertical growth

IOC

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

B.Psammoclema

D, Poherion

FIG. 75.— External form of various Sponges. A, Oscaria, an encrusting form, with the
upper surface raised up into a number of rounded prominences; ]1, Psammoclema, a
ramifying subcylindrical Sponge ; C, Euspongia (toilet sponge), a massive form with
a broad base ; 2), 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-

.,

PHYLUM AND CLASS PORIFERA

107

ment like the ribs of an open fan. Often the lamella becomes
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 Ascetta 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.

FIG. 76.— Ascetta primordialis.

A portion of the wall of the vase-
like sponge removed to show
the paragastric cavity. (After
Haecl "

108

ZOOLOGY

SECT.

A somewhat more complex type of structure than that of Ascetta

is exhibited by 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 well 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
wide opening or by
pores perforating a
pore-membrane, and
on the other by means
of small openings, the
prosopyles, 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 ; B, cross-section
through a part of the wall of a Sycon ; C, cross-section
through a pa^pf the wall of Leucilla convexa ; D, vertical
section through Oxr<i,-<.//n ; «., spaces of the incurrent canal
system ; l>, spaces of the excurrent canal system ; os. oscu-
lum. (After Korschelt and Heider.)

PHYLUM AND CLASS PORIFERA

109

>llared 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 branching 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 endoderm
cells are confined to certain special enlargements of the canals —
the so-called " ciliated chambers " C — and the rest of the canals are
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

PG

L GC

}. 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 c'ells, 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 tha 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 Eliagon
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 this effect
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 — subdermal cavities (Fig. 78, SD) — which communicate
directly with the exterior, and, internally, usually with more deeply
situated spaces (sulcortical 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)t
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
syncytium ; very rarely they as-
sume other forms ; in some cases
each flattened ectoderrnal 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 connectivertissue 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.)

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 sclcrdblast. In the Non-Calcarea the skeleton either con-
sists of spongin fibres alone (Fig. 80, A), or of siliceous spicules alone,

B.Pachychaiina

FIG. 80. — Microscopic structure of the skeleton in various sponges. A, Euspongla, network
of spongin fibres ; B, Pachychalina, spongin strengthened by siliceous spicules ; C,
Spongelia, spongin strengthened by various foreign siliceous bodies, fragments of spicules
of other sponges, &c. (After Vosmaer.)

or of a combination of spongin fibres with siliceous spicules (B) : in
some MyxospongiaB skeletal parts are altogether absent. Spongin
is a substance allied to silk in chemical composition : the fibres are
exceedingly fine threads, consisting of a soft granular core and an
outer tube of concentric layers of spongin. These threads branch

112 ZOOLOGY SECT.

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-llasts. 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

m

u ' A

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 (Euplcctella), the
Glass-Rope Sponge (Hyalonema), 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 detache.d and develop into new indi-
viduals. In the Fresh-water Sponges (Spongillidce) 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 amphidiscs (Fig. 81,

i

in PHYLUM AND CLASS PORIFERA 113

right side). These gemmules 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 micropyle — 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 are
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, &, 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
as to form the wall of a sphere — the blastula (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 Uastoccele) in the interior of the blastula be-
coming greatly reduced. In this stage of development — termed the
ampliiblastula (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 which
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 ; b, c, ovum segmented — 6, as seen from
above, c, lateral view ; d, blastula ; e, amphiblastula ; /, commencement of invagination ;
{>, gastrula attached by its oral face ; h, i, young sponge — h, 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 blastopore — at first a wide opening, soon
becomes narrowed, and eventually closes up completely. A clear
layer containing cells — the mesogloea — 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, g), and soon 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
mesogloea 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-class5 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 Silicispongise, 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
Ccelenterata 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 Myxospongise — 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 (Sppngillidce) 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 shore

T 2

116 ZOOLOGY SECT.

between tide-marks to the deepest abysses of the ocean. The
Calcarea and the true horny sponges (Ceratosa) 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 Hexadinellida 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
messmateism, 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
(ffyalonema) is always found associated with a Zoophyte \Palytlioa),
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)
en) bedded in a mass of gelatinous substance, in which there are

PHYLUM AND CLASS PORIFERA

117

also amoeboid zooids similar to the amoeboid wandering- cells of
Sponges.

But, while the Porifera are clearly Metazoa, and not Protozoa,
there is some room for difference of opinion as regards their
relationships to the Ccelenterata, with which great type they are
sometimes amalgamated. The reasons for and against such an
arrangement will be discussed in considering the general relation-
the Coelenterata.

SECTION IV
PHYLUM CGELENTERATA

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, mesogloaa, 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 mesogloea 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 glasses
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 Palaeozoic 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 — Olelia.

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
aJrtachment, while others are given off at right angles, and present
a^lntervals short lateral branches, each terminating' in a bud-like
enlargement.

^e 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, %)\ 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 hydrotlieca (h.th), and produced distally into about two
dozen arms or tentacles (t) : these zooids are the polypes or Jiydrantlis.
Less numerous, and found chiefly towards the proximal region of
he colony, are long cylindrical bodies or blastostylcs (bis), each
^closed in a transparent case, the gonotheca (g.tJi), and bearing
numerous small lateral offshoots, varying greatly in form according
to their stage of development, and known as medusq-luds (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 trimorphic, 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 body, 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
hypostome (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 enteron (cnt), which communicates
with the outer world by the mouth (mth\ an aperture placed at
the summit of tho manubrium. The mouth is capable of great
dilatation and contraction, and accordingly the manubrium appears
LOW conical, now trumpet-shaped. Under favourable circum-
tances small organisms may be seen to be caught by the polypes
and carried towar Is the mouth to be swallowed.

i h< • , \ i ne-glass, and

is perfectly trai and coK> A short distance froi<; its

FIG. 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 ; BiL 1, 2, buds ;
bis. blastostyle ; cce. coenosarc ; cct. ectoderm ; end. endoderm ; ait. enteric cavity ; g.th.
gonotheca; h.th. hydrotheca ; I, lithocyst; m.bd. medusa-Jmd; mnb, manubrium ; mjfjl.
mesogloea; mth. mouth; p. perisarc ; P. 1, 2. 3, polypes; rod. c. radial canal; t. teuTncie;
vl. velum.

E

r>rc

p. iv PHYLUM CCELENTERATA 121

•ow or proximal end, it is produced inwards into a sort of
ircular shelf (sh), 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 ar?d horny consistency, the perisarc (p), and
an inner, delicate, granular layer, the coenosarc (cos), continuous
by a sort of neck or constriction with the body of each hydranth.
The coenosarc is n*ollow, 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. A,t the base of each zooid or branch the perisarc
presents several annular constrictions, giving it a ringed appear-

.ce : 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 zobids (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
polype 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 composed
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 (msgfy which, unlike the inter-
mediate layer of- sponges, contains no cells and is practically
structureless. The sai$e three t layers occur in the manubrium,
the ectoderm and endoderm being continuous with one another at
the margin of the mouth. The tentacles, are formed of an outer
layer of ectoderm, hen a hiyc-i ea, and fii ;i solid

core o in a singlr 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 ; f-nt. enteric cavity ; h. th. hydrotheca ; TOs^.-mesoglcea ; mth. mouth ; ntc. nematocysts ;
sh. 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 gonotheca?
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, /, /).
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 iniler ends clumps of
small rounded interstitial ce//s,-and occasional largo 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 mesoglcea : they are arranged longitudinally, and serve
for the rapid shortening of the tentacles (Fig. 86, tn.f). This
muscular layer is a derivative of the ectoderm, and may be looked
ipon as a rudimentary mesoderm.

cnb

FIG. 85.— Xemafcocysts of Hydra. A, undischarged ; B, discharged ; C, nerve-supply ; cnb.
cnidoblast ; cnc. cnidocil ; nu. nucleus ; ntc. nematocyst ; nv.c. nerve-cell. (From Parker's
Biology, after Schneider.)

Embedded in the ectoderm are numerous clear ovoid bodies, the
stinging -capsules or ncmatocysts (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.

nVmC

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 trigger-hair (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
fiagella 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 manu-
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 medti^ft-buds liberated from a ruptured gonangium
—yet remains to be c< Asidered. The convex outer surface of the
bell or umbrella (Fig. £^B— D) by which the zooid was originally
attached to the blastost^N^ distinguished as the fo>umbrelfa. the

ntc

FIG. 86.— Tentacle of Eucopella. The
lower part of the figure snows the ex-
ternal surface, in the middle part the
ectoderm is removed, and the muscular
and nervous layer exposed, in the upper
p.'irt 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 Lendenfeld.)

IV

PHYLUM CCELENTERATA

125

I I

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 whole interior of the manubrium, and
from its dilated base sends off four delicate tubes, the radial
nals (rad. c), which pass at equal distances from each other
hrough 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

Tntrb

FIG. 87. — Obelia sp. A, mature medusa swimming- with everted umbrella ; B, one quarter
of the same, oral aspect ; circ.c. circular canal ; gon. gonad ; L 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 sense-organs 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 r
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 endodcrm-lamella (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

end. lam

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
mesogloaa black ; circ. c. circular canal ; end. lam. endoderm lamella ; yon. gonad ; I. lithocyst ;
mnb. manubrium ; tuth. mouth ; rod. c. radial canal ; vl. 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 wn-ich
the zooid is attached in each case to the parent stem : the mouth
and the manubrium are also obviously homologous Fjructures ini

IV

PHYLUM CCELENTERATA

the two cases. Suppose the tentacular region of a polype to be
lied out, as it were, into a disc-like form (B), and afterwards to
bent into the form of a saucer (C) with the concavity distal,

FIG. 89.— Diagrams illustrating the derivation of the medusa from the polype. A, longitudinal,.

of medusa. The ectoderm is dotted, the endoderm striated, and the mesogloea black ; dr. c.
circular canal ; ect. ectoderm ; end endoderm ; end. lam. endoderm lamella ; ent. cav. enteric
cavity ; hyp. hypostome or manubrium ; mnb. manubrium ; msgl. mesogloea ; mth. mouth ;.
nv. nv\ nerve-rings ; t. tentacle ;. v. 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-radtiLJ
a*d- radiws

sub-radius --
per-radius

FIG 90.— Projections of polype (A) and medusa (B), showing the various orders of radii;
ffon. gonad ; mn!>. niaiiubrium.

the endoderm-lamella (D', end. lam), and remaining only along
four meridional areas — the radial canals (rod. 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
complexity of the medusa is accompanied by a differentiation of tin si ires
lying along certain radii. If a polype is projected on a plane surface (Fig. 90, A),

IV

PHYLUM CCELJNTERATA 129

taken at right angles to its long axis, a laige 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 us to distinguish four principal radii or per -radii. Half
wa}r 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 pef-
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
medusae are dmcipus. It will be noticed that the gonad has the
same general structure as an immature zo'oid — 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 ov'oidal body
called a planida (G, H), consisting of an outer laj^er 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 liydrula 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
ret met with of alternation of generations, or metagenesis (see p. 39).

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

FTC:. 91.— Stages in the development of two Zoophytes (A— H, Laomedea I— M, Euden-
driuxn) allied to Obelia ; A— F, 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 Allman.)

new Obelia-colonies arise. We thus have an alternation of an
asexual generation, or ago/niobium — 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 mesoglcea, 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 CGELENTERATA 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 zooids. 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 coral. 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 medusas,
zoophyte colonies known not to produce free medusae, and medusae
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 medusae, while similar medusae, 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. — AntJiomedusce.

Leptolinae in which the polypes are not protected by hydrothecas or the
reproductive zooids by gonothecse : the medusae bear the gonads in the manu-
brium and have no lithocysts.

Sub-Order b. — Lepiomedusce.

Leptolinse in which hydro- and gonothecaa are present : the medusas 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. — Tracliymeduscv.

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 1}. — Narcomcdiisce.

Trachylince 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 hydrothecas, and
of gonads formed in connection with the radial canals, belongs to
the sub-order Leptomedusae. It is placed in the family Campanu-
lariidce, distinguished by having cup-shaped thecae 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 medusas.

ORDER 1. — LEPTOLIN^.

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

PHYLUM CKELENTEBATA 133

zooids, The hydranths may be colourless and quite invisible to
the naked eye, or, as in some Tubulariae (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
^Zquorea, 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 Anthomedusae, of which
Bougainmllea (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-
llastea or nak,ed-budded zoophytes (see also Fig. 93, 1, 4, 5). In
the Leptomedusae 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 Anthomedusae 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 (gon) 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
(oc) 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 Leptomedusas
the umbrella is usually less convex, thinner, and of softer consist-
ency than in the Anthomedusae, 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 Leptolinge the ccenosarc, 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 coenosarc, 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 Hjdractinia with

FIG. 92. — Bougainvillea raxnosa. A, entire colony, natural size ; B, portion of the same
magnified ; C, immature medusa ; dr. c., circular canal ; cu. cuticle or perisarc ; ent. car.
enteric cavity; hyd. polype or hydranth ; hyp. hypostome or manubrium ; med. medusa; mnb.
maiiubrium ;" rud. 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

6. Clavafella

FIG. 93.— Various form* of t.eptolinae. In 1, a shows the entire colony, ft a portion highly
magnified ; in 7, a is a species producing niedusa-buds from the manubrium, ft from the bases
of the tentacles ; . 12, dactylozooids ; m. and M^medusse ; mnb, manubrium ; mth. mouth ;
or. eye-spots ; rad. r. * ^dial canals ; s. sporosacs ; s?J- spines ; t, fl, ft, tentacles.

136

ZOOLOGY

SECT.

Clathrozoon, an Australian genus, resembles it in having branched
and intertwined coenosarcal tubes, the perisarc of which under-

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 ; ltd. 1, bd. 2, buds ; chr. chromatophores ;
xaW. cnidoblast ; cnc. cnidocil ; ect. ectoderm ; end. endoderm- ; ent. cav. enteric cavity ; ent.
cav. Its prolongation into the tentacles ; fl. flagellum ; hyp. hypostome or manubrium ; int. c.
interstitial cells ; m. pr. muscle processes ; mth. mouth ; msgl. mesogloea ; ntc. large, and ntc'.
small nematocysts; nu. nucleus; ov. ovum; ovy. 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,

PHYLUM CCELENTERATA

137

ree-like structure, resembling some of the fan-corals or Gorgonacea
(vide infra).

A great simplification of the colony is produced in Myriothela
(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 (m) 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 virtue 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. T^e 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, m. pr)
which serve the same functions. There is no perisarc. Buds
(bd. 1, bd. 2) are produced which develop into Hydra, 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, J), in
others elongated into a spindle-shape (£). 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 papillae. 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-
dusaa and Leptomedusae, 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, 3) ; 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, 8).

Some medusae 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 medusae thus
furnish the first instance we have met with of a central nervous
system, 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 medusae the circular canal communicates with the
exterior by minute pores placed at the summits of papillae, 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, 7a), or from the margin of the umbrella (76). The buds
always have the medusa form.

In many Leptolinae 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
medusae to ovoid pouch-like bodies called sporosacs (Fig. 93, lbt
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

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 (u) and radial canals (ra) ; C, Sporosac, ee, ectoderm ; en, endoderm ; m, manubrium ;
ov, ovary ; t, tentacle ; v, 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 medusae, 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 Leptomedusae (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 medusae or sporosacs — may spring directly from the
ccenosarc, as in Bougairivillea (Fig. 92), or from the ordinary
hydranths (Fig. 93, 4 and 5). The primitive sex-cells, from which
ova or sperms are ultimately developed, are sometimes formed

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 Leptolinas 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, planula with solid endoderm ;
C, planula with enteric cavity ; al. enteric cavity ; ep. ectoderm ; hy. endoderm. (From
Balfour's Embryology, after Kowalevsky.)

this stage the embryo is called a planula : 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 planula 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 hydrula by the attached end
broadening into a disc and the opposite extremity forming a
manubrium and tentacles. The hydrula 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 for a 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. — TRACHYLIN^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; gon .. gonad ; mnb. maimbrium ; mth.
mouth ; rail. c. radial canal ; re. c. recurrent canal ; t. tentacle ; tc. tentaculocyst ; tg. tongue ;
vl. 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

the young condition, although they may be replaced in the
lult by hollow tentacles.

But the most characteristic anatomical feature of the group is
structure of the sense-organs, which are club-shaped bodies

rad.c

nith
t.Cunarcha

2.Polycolf>a

ect

FIG. 100. — Two Narcomedusse, 2 in vertical section, gon. gonad ; mnb. manubrium ; mth.
mouth ; pr. peronium ; t. tentacle ; tc. tentaculocyst ; t.r. tentacle-root ; vl. 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 Leptornedusae,
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
Trachylinae are characterised
byv the mode of origin of
the tentacles. In Trachy-
medusse, 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

ntc

FIG. 101.— -ffiginura 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 Trachylinse as
between the two sub-orders of Leptolinse. In the Trachymedusse
the gonads (Fig. 99, gori) are developed in the course of the radial
canals : in the NarcomedusaB (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 Narcomedusa3 the manubrium is short ; in the
Trachymedusse 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 Trachylinaa is seen in
sEginopsis, one of the Narcomedusae. The oosperm gives rise to

FIG. 102.— Larva of JEginopsis. m. mouth; t. tentacle. (From Balfour, after Metschnikoff.)

a ciliated planula, which forms first two (Fig. 102), then four
tentacles, and a mouth, hypostome, and stomach. The larva of
^Eginopsis is thus a liydrula, 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 hydrula? by budding, these last
becoming converted into medusoe 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. — IMEillepora 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
gastropores (B, g.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 tabulae (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).

'^f&r&JIm m

-VT*§f :!teA^M»

end eef

FIG. 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, ect. ectoderm ; end. endoderm ; d.p. dactylopore ; D.Z.
dactylozooid ; g.p. gastropore ; mth. mouth ; P. polype ; t. tentacle. (Altered from Moseley.)

The ccenosarcal tubes have the usual structure, consisting of
ectoderm and endoderm, with an intervening mesoglcea. 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 Leptolinaa, 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

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
the course of the coenosarcal canals ; in Stylaster there are

in

Fir,. 105. — Stylaster sanguineus. A, portion of skeleton, natural size ; B, small portion,
magnified ; a. ampullae ; d.j). dactylopores ; g.p. gastropores. (After Nicholson and Lydekker.)

sporosacs or degraded reproductive zooids lodged in special cham-
bers (a) 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. — SIPHOXOPHORA.

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.

Halistemma (Fig. 106 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 body con-

L ^

B

FIG. 106.— Halistemma tergestinum. A, the entire colony; B, a single group of zooids ;
CQ». cu'iiMsaiv : </:.. dactylozooid ; hph. hydrophyllium or bract; net. nectocalyx or swimming-
bell ; ntc. battery of nematocysts ; y. polype ; pn. pneumatophore or float ; s. s'. sporocysts ;
t. tentacle. (After Claus.)

SECT. IV

PHYLUM CCELENTERATA 149

taining air — the float or pneumatophore (jm). Next come a number
of closely set, transparent structures (ncf), having the general char-
acters of unsymmetrical medusa without manubria, each being a
deep, bell-like body, with a velum and radiating canals. During life
these swimming-bells or nectocalyces 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 (t) 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 hydrophyllia (hph) — spring from the "internodes" and
partly cover the sporosacs.

It is obvious that, on the analogy of such a hydroid polype as
Obelia, Halistemma is to be looked upon as a polymorphic floating
colony, the stem representing a ccenosarc, 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, ^m). 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 Halistemma, 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

fan

TICt

FIG. 107. — Diagram of a Siphonophore : the thick line represents endoderm ; the space ex-
ternal to it, ectoderm ; the internal space, the enteric cavity, coe. coenosarc ; dz. dactylozooid ;
hjrfi. hydrophyllium ; net. net', nectocalyces ; ntc. battery of ncmatocysts ; p. polype ; pn.
pneumatophore ; t tentacle. (After Glaus.)

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, ep), the opposite
extremity is gradually converted into the first polype (po), and

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 Anthomedusse (Fig. 93, 7 a).

FIG. 108.— Two stages in the development of Halistemma : the endoderm is shaded, the
ectoderm left white, ep. pneumatocyst or air-chamber of pneumatophore ; hy. endoderm
surrounding pneumatonhorc ; po. polype ; pp. pneumatophore ; t. tentacle. (From Balfour,
after Metschmkoff.)

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, ^m) 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 (#>), 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

FIG. 109.— Physalia : the living animal floating on the surface of the sea. cr. crest ; p. polype ;
pn. pneumatophore. (After Huxley.)

Halistemma, but the female apparently becomes 'detached as a
free medusa.

In Dipliyes the float is absent. Two swimming-bells (Fig. 110, m)
of proportionally immense size are situated at the proximal end of
the ccenosarc, and are followed by widely-separated groups of

PHYLUM CCELENTERATA

153

>ids (B), each group containing a polype (»)with its tentacles (i)}
a inedusoid (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

FIG. 110.— Diphyes campanulata A, the entire colony ; B, single group of zooids ; a,
ccenosarc ; c, cavity of swimming-bell ; t, groups of zooids ; g, medusoid ; i, grappling line or
tentacle ; in, swimming-bell ; n, polype ; o, mouth of swimming-bell ; t, bract. (From
__ Parker's Biology, after Gegenbaur.)

body, enclosing a chambered chitinoid shell (sh) 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 (hy1) and blasto-
styles bearing medusa?, while in the centre is a single polype (hy),

154

ZOOLOGY

SECT.

which is the only nutritive zooid, taking in food for the entire
colony. The closely allied genus Veldla is of rhomboidal form,
and bears on its upper surface an oblique sail.

The reproductive zooids are liberated as free medusse. The
eggs give rise to young which have a close resemblance to flat
medusae with manubrium, 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 medusaB the
sub-umbrella of which has given rise to buds forming the feelers

Trry

FIG. 111.— Porpita pacifica. A, from beneath; B, vertical section; hy. polype ; hy'. dactylo-
zooids ; sh. 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 — i.e. to consider them as hydroid colonies in
which the coenosarc is represented by the discoid or rhomboid
body 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
trace of the soft parts having, as in the majority of fossils, disappeared.

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 hydrotheca? (Fig. 112, h.th}.
The coenosarcal skeleton is strengthened by a
slender axis, the virgida (v), the proximal end of
which is connected with a small dagger-shaped body,
the sicula (s), 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 KEMARKS ON 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;
Cordylophora, one of the Anthomedusae,
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 Hydrocorallina3 from the Cretaceous onwards.

Parasitism, although rare, is not unknown in the class. Poly-
podium, one of the Anthomedusae, is parasitic during part of its
existence, in the ovary of the Sturgeon ; and Cunina, one of the
Narcomedusse, 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 individuation,
numerous cells combining to form a colony in which the several
parts 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, Dimorphograpius, both
magnified ; hy. th. hydro-
theca ; s. sicula ; v. vir-
gula. (After Nicholson
and Lydekker.)

150 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 Amoeba 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 medusae, 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 (mg. lp): between the pairs of lappets the
edge of the umbrella is fringed by numerous close-set marginal
tentacles (t).

qon

?nth

n

B

a.7rc

?p

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 arms are
removed ; a.r. c. adradial canal ; g, f. gastric filaments ; gon. gonads ; g. p. gastric pouch ;
i.r. c. inter-radial canal ; mg. lp. marginal lappet ; mth. mouth ; or. a. oral arm ; p.r. c. per-
radial canary 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 (mth), borne at the end of an extremely
short and inconspicuous manultrium : 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 to 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. ^;),
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 canal
(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 mesogloea, which, however,
is not structureless, but is traversed by branching fibres and
contains amoeboid cells derived from the endcderm. 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

PHYLUM CCELENTERATA

159

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 (gori) 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

FIG. 114. — Aurelia aurita. A, side view, one-fotr.th of the umbrella cut away ; B. diagrammatic
vertical section, ectoderm dotted, endoderm striated, mesogloea black ; circ. c. circular canal ;
g.f. gastric filaments ; gon. gonad ; g. p. gastric pouch ; gal. gullet ; h. hood ; i.r. c. inter-radial
canal; mg. Ip. marginal lappet ; mth. mouth; or. a. oral arm; s.g. p. sub-genital pit; st.
stomach.

I

the floor of the enteric cavity, the gonad is obviously an
ondodermal 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.

100 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 mesoglcea 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

Fi<;. ll.'i. — Aurelia aurita. A, small portion of edge of umbrella, showing the relations of the
fentaculocyst ; B, vertical section of the same region (diagrammatic) ; h, hood ; I, lithite ;
//<.'/. //>, marginal lappet; oc, ocellus; olf. 1, off. 2, olfactory pits. (Altered from Lankester.)

them are per-radial and four inter-radial. Each consists of a
peculiar form of sense-club or tcntaculocyst, 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 lithite§ (I) 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 (mg. Ip) and by a
hood-like process (h) 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

PHYLUM CGELENTERATA 161

or oosperm divides regularly and forms a mojcida, which, by accumu-
lation of fluid in its interior, becomes a blastuld — a closed sac with
walls formed of a single layer of cells. One end of this sac becomes
invaginated to form the g^strula. The blastopore or gastrula-
mouth closes, the embryo being converted into a closed two-
layered save OT planufa (Fig. 116, A), indistinguishable from that of
a Hydrozoon, although formed by a totally different process.

The planula swims about by means of the. cijig? 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-in or invagination of the surface) so as to produce a
depression lined with 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 tcenioles (D, tn.).
The mouth (E, mth.) assumes a square outline, and its edges become
raised so as to form a short manubrium (imib.\ 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 Leptolinse, but distinguished by a pronounced differentia-
tion of structure, indicated by the sixteen tentacles developed in
regular order, the stomodseum, and the four gastric ridges with
their septal funnels. The Scyphozoon-polype is called a Scyjahula
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 Epliyrula (H, I). The umbrella of the ephyrula
VOL T 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 ephyrulse ; H, ephyrula from the side ; L, the same
from beneath. In A— D and F the ectoderm 'is unshaded, the endoderm striated, and the
mesogloea dotted. u. lobes_pf umbrella ; mnb. manubrium ; mth. mouth ; .?./'. septal funnel ;
st. stomodajum ; t. tentacle ; tn. taenioles. (From Korschelt and Holder's Enibnjol(»j;i.)

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

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 Leptolinse, 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 Las 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 stomodseum 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-forij^(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 invagination : by the closure of the blastopore a planula is
produced, at one end of which a second invagination takes place,
forming the stomodseum.

M 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: •jt>ur 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.

Discomedusae with a simple square mouth devoid of oral arms.

Sub-Order 1. — Semostomce.

Discomedusse in which the square mouth is produced into four long oral
arms.

Sub-Order c. — Rliizostomw.

Discomedusse 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 Ulmaridce, the sub-order Semostomce,
and the order Discomedusce.

Its saucer-shaped umbrella and eight tentaculocysts place it at
once among the Discomedusse : the presence of a distinct mouth
surrounded by four oral arms excludes it from the first and third
sub-orders of Discomedusae and place it in the second sub-order or
Semostomse. The latter group contains sixiamilies, characterised
mainly by differences in the canal -system : the Ulmaridse 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

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. — STAUROMEDUSJL

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

iff-

i.r:

FIG. 117. — Tessera princeps. A, external view; B, vertical section ; g.f. gastric filament;
gon. gonad ; i.r. t. inter-radial tentacle ; mnb. manubrium ; mth. mouth ; p.r. t. per-radial
tentacle; st. stomach; tn. tseniole. (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- (p.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 (st.), 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 (hi. ). 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 Scyphula, 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. — XiUCeraaria. A, oral aspect ; B, from the side ; g. foot-gland ; (/. /. gastric filaments
gon. gonad ; mth. mouth ; t. tentacles ; tn. tsenioles. (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 rare 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 (mg. Ip. ), forming the free
edge of the bell. x ,

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 (yiite to the apex of the cone. In the
wall of the stomach are four wide "p'er-radial slits, leading into an immense
circular sinus (circ. #.). As in Lucernaria, there are four wide inter-radial in-
fundibula. The gastric filaments (g. f. ) are very numerous, and the elongated
U-shaped gonads (gon.) are eight in number and adradial.

ORDER 3. — CUBOMEDUS/E.

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 Medusse more than any of the
other Scyphozoa. The best known species, Charybdcea marsupialits (Fig. 120), is
about 5 cm. in diameter and of very firm consistency.

PHYLUM CCELENTERATA

167

As in the lower Per.omeduso?, the margin of the umbrella bears four ten-
tacles (/. ) and four tentaculocysts (tc. ), 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

imnb

^

FIG. 110.— Pericolpa quadrigata. A, external view; B, vertical section; clrc. s. circular
sums ; en. cone ; fj. f. gastric filaments ; yon. gonads ; my. Ip. marginal lappets ; r,inb. manu-
brium ; mth. mouth ; pd. 1. pedal lobes ; st. stomach ; t. tentacles ; tc. tentaculocysts ; tn.
tsenioles. (After Haeckel.)

gelatinous lobes (L), 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 (vl. ), resembling the velum of the hydroid Medusae, but 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 mesogloea. Owing to this fundamental difference, the velum-
like organ of the Cubomedusse is distinguished as the velarium.

168

ZOOLOGY

The mouth is situated at the end of a short manubrium (miib. ) leading into a
wide stomach, from which go off four very broad per-radial pouches (rad. p. ),
occupying the whole of the four flat 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

on.

end. lam

circ.

Fro. 120.— C
pass:. „

C, transverse secti
longatioii into the

gonads ; i. lappet; num. iu;uuu>rmm ; nm. /
vl. velarium. (After Glaus, somewhat altered.)

them to a poorly developed endoderm lamella (end. lam.). 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 (circ. c. ). Near the junction
of the gastric pouches with the stomach are the usual four groups of gastric
filaments (y. f. ).

The gonails (//OH.) are four pairs of narrow plate-like organs, attached one
along each side of eacli inter-radial septum. The nervous system takes the form

PHYLUM CCELENTERATA

1G9

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, ar. adradii ; g. goiiads ; g.f.
gastric filaments ; ir. inter-radii ; 7/». 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 Semostomse and Rhizostomae are large,
and one of the former group — Cyanea 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 CannostomaB (Fig. 121),
hollow and often of great length in the Semostomog (Fig. 113), and altogether
absent in the Rhizostomoe (Fig. 122). The oral arms are absent in the Canno-
stoimv (Fig. 121), where there is a single square or four-rayed mouth : in the
Semostomiv 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 Cannostoma? (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 Rhizostomre (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

B

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. gastrie filaments ; gon. gonads ;
or. a. oral arms ; rad. c. radial canal ; s. inth. suctorial mouths ; st. stomach ; tl, t2, t3, tentacles
on oral arms. (After Cuvier, Glaus, 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 mai'gins.
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
(.s£. ). We thus get a polyatomatous 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
grooves 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 Toy 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 Cannostomse, 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 Rhizostomae 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-genital

m

FIG. 123. — Pelagia noctiluca : Three developmental stages, m. mouth ; /•. marginal lappet ;
s. tentaculocyst. (From Korschelt and Heider, after Kro'hn.)

portico, which lies immediately below the floor of the stomach and above J}he
brachial disc. '

In many of the Discomedusae 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 medusae. But in
other cases there is no alternation of generations, and development is direct. For
instance, in Pelagia (Fig. 123) — one of the Semostomae — 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 manubriimi, 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 tin\e 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-
urn brellar 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
Discomednsa?.

Many of the Scyphpzoa 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 (Tealia crassicornis).

Sea-anemones are amongst the most abundant, and best known
of 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, arid 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 "
(Tealia 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

PHYLUM CCELENTERATA

173

^

$. fft

,,-

gjr/ 771 \

sgph ,c

t. 7TL

*VL,L*

Zf> h]zd

dL. ntee

B

mee

d.mes

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 ; >'. -/«w. directive
mesenteries ; gon. gonada ; (fill. guUet ; /. m. longitudinal muscle ; Ip. lappet ; mes. 1, primary ;
mes. 3, secondary ; IUM. • tertiary mesenteries ; mes. f. mesenteric filaments ; mth. mouth ;
ost. 1,'ost. 2, ostia ; p. m. parietal muscle ; sgph. siphonoglyphe ; s. jn- 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 (t.), 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 differauces 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 (lp.). This tube is the gullet or stomodwum,
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, sgph*), placed one
at each end of the long diameter, and therefore corresponding with
the lappets : they are known as the gullet-grooves or siphonoglyphes.
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. 3}, which are hardly more than
ridges on the inner surface of the body-wall. -Thir the entire
internal cavity of a Sea-anemone is divisible into three regions:
(1) the gullet or stomodcBum, communicating with the exterior
by the mouth, and opening below into (2) a single main digestive
cavity, the stomach or mesenteron, which gives off (3) a number of
radially arranged cavities, the inter-mescnteric 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 (pst. 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

PHYLUM CCELENTERATA

175

below the gullet is produced into a curious twisted cord, the
mescntcric 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 mesoglcea,
which is extremely thick and tough. The gullet (<jul.\ 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 mesoglcea, covered on both sides by

B

FIG. 125. — Diagrammatic vertical (A) and transverse (B) sections of a Sea.-anemone. The
ectoderm is dotted, the endoderm striated, the mesoglo3a> black, ac. acontium ; en. cinclis ;
1 1 vl. gullet; int. mes. c. inter-mesenteric chamber; mes. mesentery; mes. f. meseiiteric
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, l.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 tthe tentacles ; the
42ajietal 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 on 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). ^et each end of the gullet, opposite the
siphonoglyphe, are two mesenteries (d. mes.), 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 (mes. 1),
differing from the directive couples in having the longitudinal
muscles turned towards one another. The secondary and tertiary
mesenteries (mes. 2, mes. 3) 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 sym-
metry, 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 CCELENTERATA

177

The general microscopic structure of a Sea-anemone is well
shown by a section through a tentacle (Fig. 126). Both ectoderm
(ect.) and endoderm (end.) consist mainly of very long columnar,
ciliated, epithelial cells, and the mesoglcea (msgl.) 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 mesoglcea, and has assumed, to a far greater

ntc

end

FIG. 126. — Tealia crassicornis. Trans- FIG. 127. — Three nematocysts of

verse section of tentacle, ect. ectoderm ; Sagartia. (After Hertwig.)

end. endoderm; l.m. longitudinal muscles ;
msgl. mesogloaa ; nv. c. nerve cells ; nv.f.
nerve fibres ; ntc. nematocysts ; t. m.
transverse muscles. (After Hertwig.)

extent than in any of the lower groups, the characters of an inter-
mediate cell-layer or mescderm.

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. x

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

ntc

FIG. 128.— Transverse section of mesenteric filament of Sagartia. c. ciliated cells ; gl. gland-
cells ; ntc. nematocysts. (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 mesoglcea, and thus appearing to belong to that
layer (Fig. 126, 1. m.). This fact is significant from the circum-

PHYLUM CGELENTERATA 179

stance that, as we shall see, the muscles of all animals above
Coalenterata 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 Medusae.

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 stomodoeum, 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, 5) in the lateral compartments of the dorsal
chamber ; and a sixth (6) in the lateral compartments of the ventral chamber.
Soon the longitudinal muscles 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 first

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

Std

FIG. 129. — Transverse sections of early (A) and later (B) stages of an erpbryo Sea-anemone
(Actinia.) The mesenteries are numbered in the order of their development ; std. stomo-
dseum. (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 CHARACTERS 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 stomodaeum 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 stomodaeum. 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

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 mesoglcea containing fibres and cells. The stomodseum
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, stomodaeum,
and mesenteric filaments. The muscular system is well developed,
and contains both ectodermal and endodermal fibres and endo-
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
spicules or small irregular bodies, found in the mesogloea, 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 (TuHpora, 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 Hartea (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 mesogloea, There is no
siphonoglyphe. 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" (Pennatula]
are the commonest members of this group (Fig. 134).

Systematic, Position of the Example.

Tealia crassicornis is one of several species of the genus
Tealia : it belongs to the family Tealidce, which, with several
other families, make up the tribe Hexactiniw, of the order
Actiniaria, of the sub-class Zoantharia.

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 stomodaeum has two siphonoglyphes
and two lappets.

The family Tealidse 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 Zoanthus
(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 Flabellum (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

sgph,

FIG. 130.— Zo an thus sociatus. A, entire colony ; st. stolon. B, transverse section ; sgph.
siphonoglyphes ; d. d. dorsal, and r. d. ventral directive mesenteries. (After McMurrich 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
ccenosarc. 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 Palythoa, the most com-
plex of the Actiniaria, and
many of the common Madre-
poraria, such . as Astrcva (Fig.
133). In the Sea-pens (Penna-
talacca) the proximal end of
the elongated colony (Fig.
134) is sunk in the mud, and

FIG. 131.— Hartea elegans. ftul. gullet ;
,it '.<. mesentery ; sp. spicules ; t. tentacles.
(After Perceval Wright.)

the distal end bears zooids
springing either directly from

IN7

PHYLUM CCELENTERATA

185

the ccenosarc 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 (pi.), 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, pL), 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.— Astrsea 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.

FIG. 134.— Pennatula sulcata. A, entire colony ; B, portion of the same magnified.
1. lateral branch ; p. polype ; s. siphonozooid. (After Koelliker.)

1.70.

FIG. 135. — Tu"bipora musica. A, skeleton of entire colony ; B, transverse sections of polype ;
(', single polype with tube and commencement of platform ; 1), growth of new polypes from
platform ; /. m. longitudinal muscles ; pi. ?/-'. polypes ; pi. platform ; sity/i. siphonoglyphe ; sp.
spicules ; std. stomodseum. (After Cuvier, Quoy and Gaimard, and Hickson.)

IV

PHYLUM CCELENTERATA

187

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 of 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 G-ymctis 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 Hexactinice) 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 Hexactiniae. 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 numerous, and in
nearly all cases have the form of simple glove-finger-like out-

FIG. 136.— Edwardsia claparedii.

A, the entire animal ; f. tube. B,
transverse section. (After Andres,
and Korschelt, and Heider.)

188

ZOOLOGY

SECT.

FIG. 137.— Cirripathes anguina,

portion of colony. (After Bronn.)

pushings 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
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, p.),
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,

etc.). 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 cinclidcs (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 inGyractis
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 oo-shaped transverse section,
owing to its walls coming together in the middle and leaving the
two ends wide open. In a deep-sea form, Halcampoidcs, there is a
longitudinal partition dividing the stomodseum 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, Fenja (Fig. 138) and jflgir, 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. 133.— 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 tlieTTongitudinal
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
siphonozooids (Fig. 134, s.), having no
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. 139.— Minyas. /. float.
(After Andres.)

tentacles, longitudinal

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
spicular 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 Tubipora (Fig. 135).
the " Organ-pipe Coral," the mesodermal spicules become closely
fitted together, and form a continuous tube for each polype, the

FIG. 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 (Gvrallium, 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 colony and branching with it, but is

PHYLUM COELENTERATA

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 coenosarc. In some

FIG. 141. Gorgonia verrucosa. A, entire colony; B, portion of the same magnified;
coanosarc ; p. polype. (After Koch and Cuvier.)

of the Gorgonacea the axial skeleton is partly horny, partly
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 eye.

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 polype. 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 corallum, the skeleton of an entire coral-colony.

The structure of a corallite is conveniently illustrated by that
of the solitary genus Fldbellum (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 theca (th.) 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, &. 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 epitheca (C, e.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 tabulce, like those of
Millepora (p. 145), extending across the whole corallite. In the
Mushroom-coral (Fungia), the corallite is discoid, the theca is con-
fined to the lower surface, and small calcareous rods, the synapticulw,
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-

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

SCf>

FIG. 142. — A, B, two views of Flabellum curvatum. C, semi-diagrammatic view of a simple
coral ; D, portion of a corallite ; E, F, diagram of a simple coral in longitudinal and transrerse
section ; ectoderm dotted, endoderm striated, skeleton black, b. pi. basal plate ; col. colum-
ella ; e. th. epitheca ; yul. gullet ; mcs. rues. 1, mcs. 2, mesenteries ; mcs. f. mesenteric filaments ;
sep. septa ; t. tentacle ; th. theca. (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 basal 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. i 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 Astrsea (Fig. 133) ; or tree-like forms, such

FIG. 143.— Dendrophy Ilia nigrescans, B, Madrepora aspera. co. corallites ;
cs. ccenosarc ; p. polypes. (Aftei- Dana.)

as Dendrophyllia (Fig. 143, A), in which a common calcareous stem,
the ccenenchyma is formed by calcification of the coenosarc (cs.), and
gives origin to the individual corallites. It is by this last-named
method, the coenosarc attaining great dimensions and the indivi-
dual corallites being small and very numerous, that the most
complex of all Corals, the Madrepores (Madrepora, Fig. 143, B)
are produced.

The microscopic structure of corals presents two main varieties.
In what are called the aporose. or poreless corals, such as Flabellum,
Astrsea, &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 CGELENTERATA 195

the corallites and of the connecting ccenenchyma, 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 ccenosarc, 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.
Tabulae 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
Alcyonidae are remarkable for their elaborate patterns and gor-
geous colouration; and Pennatula, in addition to its \ivid colours,
is phosphorescent.

In most cases the significance of these colours is quite unknown.
In some species, however, " yellow-cells " or symbiotic Algae 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 les;s 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 Sea-
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
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

FIG. 144.— Adaxxxsia palliata, four individuals attached to a Gastropod shell inhabited by a
Hermit-crab, etc. «ci. acontia ; ah. shell of Gasteropod. (After Andres.)

an intermediate size, in each of which is found a small chsetopod
Worm, belonging to the genus Leucodore. 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 bathymetrical 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 reefs 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 Paleozoic 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. TV

CLASS IV.— CTENOPHORA.

1. TYPE OF THE CLASS — Hormiphora 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
Cydippe) 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

FIG. 145. — Hormiphora plumosa. A, from the side, B, from the aboral pole. mth. mouth;
s. pi. swimming plates ; t. and 6. 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, m 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 fror

5.0

FIG. 146. — Hormiphora plumosa. A, dissected specimen having rather more than one
quarter of the body cut away. B, transverse section ; Diagrammatic, adr. c. adradial
canal ; inf. infundibulum ; inf. c. infundibular canal ; int. c. inter-radial canal ; mrd. c.
meridional canal ; mth. mouth ; or?/, ovary ; per. c. per-radial canal ; s. o. sense-organ ; «. pi.
.swimming-plate ; spy. spermary ; std. stomodamm ; std. c. stomodseal canal ; std. r. stomodaeal
ridges ; t. tentacle ; t. b. base of tentacle ; t. 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 class1
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
stomodceum. It reaches attout 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 stomodseum, 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 stomodseum 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
stomodaeum — 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 pwes (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 (adr. 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

t.c

FIG. 147.— Hormiphora plumosa, diagrammatic longitudinal (A) and transverse (B) sections.
The ectoderm is dotted, the endoderm striated, the mesogloea black, and the muscular axis of
the tentacles gray. Lettering as in Fig. 14(3, except ex. p., excretory pore.

202

ZOOLOGY

SECT.

with the stomockeum, and a tentacular canal (t. c.) which ex-
tends outwards and downwards into the base of the correspond-
ing tentacle. Each tentacle presents a thickened base (t. &.),
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 stornodaeum
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

FIG. 148.— Hormiphora plumosa. A, transverse section of one of the branches of a tentacle ;
B, two adhesive cells (a<L <•) and a sensory cell (s. c) highly magnified, cu. cuticle ; mi.
nucleus. (After Hertwig and 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 Ccelenterata 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

I

PHYLUM CCELENTERATA 203

culiar adhesive-cells with which the branches of the tentacles
are covered. An adhesive-cell (Fig. 148, ad. c.) has a convex surface
roduced into small papillae, which readily adheres to any object
ith which it comes in contact and is with difficulty separated,
n 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
f 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.p.) 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 springs (sp.),
which support a mass of calcareous particles (L), like the lithites

5. 149.— Hormiphora plumosa, Sense-organ: b. beli'- ciliated plate; c. gr. ciliated
groove ; ex. p. excretory pore ; 1. lithites ; p. pi. polar plate ; sp. spring. (Modified from Chun.)

)f Hydrozoa and Scyphozoa. From each spring a ciliated 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
gonads&re 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.

no,

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 stomoda3um,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 (plsm.)
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 vitelline 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.
plsm. protoplasm ; r. TO.
yk. yolk. (After Chun.)

in. nucleus

„.. plsm. protoplasm ; r. TO" vitelline membrane
Ik.

FIG. 151. — Segmentation of the oosperm in Ctenophora. mg. megameres ; mi. micromeres ; plsm.
protoplasm ; yk. yolk. (Modified from Korschelt and'Heider.)

the other Ccelenterata. The protoplasmic layer accumulates on
the side which will become dorsal, and the oosperm divides along

PHYLUM CCELENTERATA

205

UJLJ

::

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
'ing.\ and eight small, dorsal, protoplasmic cells or micromeres (mi.).
The micromeres increase rapidly in number by division, and are
rther 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 rnicromes pfra.dna.11y 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. ma. megameres ; mi. micromeres.
(From Lang's Comparative Anatomy.)

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 blast opore. 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 epiboly
05 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 mesod-erm (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

FIG. 153.— Three stages in the development Of Callianira. d. infundibulum ; ec. ectoderm ;
en. endoderm ; me. mesoderm; st. stomodseum. (From Lang's Comparative 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 (me1.). The
otoliths are formed in the ectoderm-
cells of the apical poyie, 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 (sk.)'.

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-
containing portion. In the Hydrozoa
and Actinozoa the yolk-material is
«/-. sfiisc-ni-gim ; tt. stomodseum small in amount and evenly distributed.

t. tentacle. (From Lang's Com , i •> • -> • -> -> " , .,, 7

•,MI rut ;,-c Anatomy.) the egg being described as alecitnal or

PHYLUM CCELENTERATA 207

kless. In the present instance the yolk is at first accumulated
in the centre of the egg, which is thus centrolecithal 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 unequal, 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 firsit_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 Coelenterata 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 stomodseum 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 stomodaeal 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 stomodasal 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 Pleurobrachiadce and to the order Gydippida.

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-
brachiadce 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 stomodaeal ridges are of a brown colour, and the leaf-
like branchlets of the tentacles yellow.

PHYLUM CCELENTERATA 209

3. GENERAL ORGANISATION.

Compared with the two former classes of Coelenterates, 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.g. Euchlora (Fig. 155, 2), the result being an
oval instead of a circular transverse section, with the tentacles at the end of the

mm

l.Callianira

2.Euchlora

S.LamjaeHa

Fir;. 1 :>.>.— Three Cydippida. ah. i>. aboral process ; ;nth. mouth. (After I'iiui. )

long axis. The aboral pole may be produced into wing-like appendages, as in
CaUifinira (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 Euchlora rubra minute nematocysts
have been found, and there is reason to believe that it was by the modification
of these characteristic coelenterate organs of offence that the adhesive cells of
Ctenophora were evolved.

The Lobata, for instance Deiopea, are distinguished, as their name implies,
by the presence of a pair of large lappets (Fig. 156, lp.)t into which the oral
surface is produced at either end of the vertical plane. Four of the swimming

VOL. I. P

210

ZOOLOGY

plates are shorter than the others, and at their bases arise elongated processes
called auricles (aur.), 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

mnd.c

FIG. 156.— Deiopea kaloknenota. A, adult ; B. young, aur. auricle ; lp. 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 (l.t.). 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 Cevtida are represented by the remarkable " Venus's Girdle" (Cestus
venerw), a band-shaped Ctenophore (Fig. 157) which sometimes attains a length
of 1^ metre, or nearly 5 feet. The body is greatly elongated horizontally in the

\

Jiiitfii^

B

FIG. 157.— Cestus veneris. A, adult, B. young. /. t. lateral tentacles ; ,,dk. mouth; .<.
s. pi:± swimming-plates ; t. tentacle. (After Chun.)

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 (8. pi.1) are very
small ; the other four (».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

211

which, in the present case, are continued the whole length of the oral edge.
The young of Cestus (B) resembles a compress 3d Cydippid which undergoes
gradual elongation in the median plane.

-Beroe, the principal genus of the Beroida, has the form of a cylinder (Fig. 158),
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 infundibulum (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 network | j
of tubes, and at their oral ends the four meri-
dional canals of each (right and left) side and
the corresponding stomodseal 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 | E1£|X\
condition. K : B9L- hf

The Ctenophora are usually per-
fectly transparent, and quite colour-
less, save for delicate tints of red,
brown, or yellow, in the tentacles and
stomodaeal 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 Bolinct
Jiydatina, 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 O5-2 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 pcedogenesis.

tntft

FIG. 158. — Beroe forskalii

inf. infundibulum ; mth. mouth ;
s. pi. swimming-plates. (After
Chun.)

P 2

•JlL> ZOOLOGY SECT

APPENDIX TO CTENOPHORA

CTKNOPLANA AND C<KL<>PLANA.

Before leaving the Ctenophora mention must be made of two remarkal)le
organisms which have been supposed to connect the present class with the
Turbellaria Polycladida, or Planarians, a group of worms to be described in
the following section.

Cttuoplaim (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 (cL ), each containing a protrusible row of stiff processes,

•5 G

A

B

Vic,. 1 :>•.>.— Ctenoplana kOwalevskii. A, from above, B, from the side. cL clefts; r. r.

radiating ridges ; *. /•. sen.se-org;in. (After Korotneff.)

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
(x.o. ). Beneath the ectoderm is a basement-membrane, which acts as an organ
of support, and the muscular system is complex. Xear each tentacle is an
aperture leading into a branched canal which is probably excretory, like the
nephridial tubes of flat-worms.

< '<,-l<>/,l(iiiri, is found in the Red 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-pistes, and the ectoderm is ciliated.

Nothing is known of the development of either genus,

PHYLUM C(ELENTERAT\ 213

THE RELATIONSHIPS OF THE CCELENTERATA

There can be little doubt that the lowest coelenterate 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 stomodaeum and gastric ridges and
filaments, is the scyphozoan polype, represented by the Scyphula
of Aurelia. Still more complex is the actinozoan polype, or
Actinula, as it may be called, with its large stomodreum, 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 Stauromedusa1, 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 Cannostonue, Semostoma^,. and
Rhizostomae clearly represent three grades of increasing com-
plexity along the same general line of descent. So little is known

214 ZOOLOGY SECT.

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 Ctenaria, 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
Hormosipa. There are eight radial canals formed by the bifur-
cation of four inter-radial offshoots of the stomach, and corre-
sponding with them are eight bands of nematocysts diverging
from the apex of the ex-umbrella. If these striking resemblance^
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
stomodaaum and the infundibulum of Hormiphora. But, as wo
have seen, the gullet of Ctenophora is a true stomodseum 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 Hyclrozoa : 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

end

Fi .. J CO. —Transverse section <>f embryos of Actinia (A) and Beroe (13), u-t. ectoderm ;
oi''. endoderm ; inf. iufuiidibulum. (After Chun.)

of an embryo Ctenophore (Fig. 160, B) and of an embryo Actinian
(A) is very striking, and the presence of a well-developed stomo-
, and of gonads developed in connection with the endoderm

PHYLUM CCELENTERATA

215

iVA

:

d discharging their products through the mouth, may be taken
further evidences of affinity between the Ctenophora and the

ctinozoa.

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

her classes with any degree of certainty.

As to the orders of Ctenophora, it seems tolerably clear that
oth 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 /

\ / Alcyonaria

Cesh'da

Lobata

BeroYda

Edwardsia, I
Rhizpst'omae \

Semosl-omcB ACTINULA

Cannosf-omcs

HVDRULA

Fro. 101. — Diagram illustrating the mutual relationships of the Goelenterata.

By many authors the Sponges have been looked upon as so
closely related to the Coelenterata 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,1 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,

fistrula larval stages constitutes an important connecting link,
ut 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 Coelenterates 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 apertures2Jbhe 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 Coelenterates 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
exists between the members of the two groups in the earlier stages
of their development.

APPENDIX TO THE CCELENTERATA

THE MESO/OA.

Tinder 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 Coelenterates.

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 several cells ; a mesogloea is not represented. The Mesozoa
comprise three families, the Dicyemidce, the Heterocyemidce, and the Orthonectidcet
all the members of which are internal parasites.

The Dicyemidce are parasites in the kidneys of various Cuttle-fishes and
Octopi (Cephalopoda). The animal (Fig. 162), the length of which is between
0'75 and 6 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 live polt 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

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 granular gelatinous contents are at first
homogeneous, but afterwards become vacuolated. In the middle of the cell is a

IK;. 1 ''>•->.— Dicyema paradoxum,

with infnsoriform embryos. (From
Brunn's Tliicmich.

FIG. 163.— Dicyema paradoxum,

with vermiform embryos. (From
Broim's 77<a/ve<V/,.

large oval or ellipsoidal nucleus. In addition to the nucleus of the endoderm
cell itself other nuclei also occur in it ; these are the miclei of the i/erm-celfa.

Two forms of adults are to be distinguished, termed respectively the
the nematogene and the rhombof/ene : the former (Fig. 163) give rise to vermiform,
latter (Fig. 162) to infuxoriform 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 nematogene forms
may be phases in the life-history of the same individual ; but some always
possess the nematogene 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

Jend) 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 Heterocyemidce, which are also parasites of the Cephalopoda, resemble the
I)ic3remida3 in most respects, but want the head.

The family Orthonectidce comprises only one genus — Rhopalura — with two

_Fia. 104. — Embryo
of Dicyema
paradoxum.

(From Bronn's
Thicrrcich.)

Fi<;. it.;."'.— Rhopalura Giardii, male.
(From Broim's Thierreich.)

FIG. 1GO.— Rhopalura Giardii, female.
(From Broim's Thlerreich.)

species, one of which is parasitic in the genital pouches of an Ophiuroid, wrhile
the other is a parasite of a Nemeitean worm. In form they are spindle-shaped,

IV

PHYLUM CCELENTERATA

219

illy rounded at the ends, without any trace of bilateral symmetry. The
ly 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 eiidoderm 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
ffxfis. When mature these pass out between the muscular fibres ; the ectoderm
Cecils disintegrate and the sperms es-
cape, the animals perishing. In the
cylindrical female the conical anterior
segment of the body becomes separated
•off 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 (Fig.
167) divides into two cells, a smaller

Fro. 167.— Stages in the development of
Rhopalura Giardii. ( From Bronn's

Tliitrrcich.)

and a larger ; the latter remains for
a time undivided, but subsequently

segments to form the cells of the eiidoderm. 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 eiidoderm afterwards' giving rise to the middle fibrous stratum (mesoderm).

SALINELLA.

Perhaps having more claim than the Orthonectidae and DicyemidaB to a position
intermediate between the Protozoa and the Metazoa is a remarkable infusorian-

FIG. 10$. — Salinella, longitudinal section. (After Frenzel.)

FIG. l(jy.— Salinella, transverse section. (After Frenzel.)

220 ZOOLOGY SECT, iv

like animal, Saline.Ua, found in brackish water from the Argentine Republic.
Salinella (Figs. 168 and 169) 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 seta?. The ventral surface is flattened, and is covered with fine
vibratile cilia, while on the dorsal surface and the sides are regularly arranged
rows of straight setre (non-motile cilia). The internal cavity (<: nitron) is found
to contain sand, plant fragments, and Bacteria ; its surface is beset with long
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-
stratum. Frequently it puts forth a process, or more than one, which may

FKJ. 170.— Trichoplax adhserens, part of a vert;
(Fruiu hang, after F. E. Sehul/e.)

extend to a considerable distance and assume a variety of shapes. There arc-
three distinct cell-layers — an upper epidermis, consisting of flattened cells closely
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 papillae 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 Trtptopla.v is quite
uncertain. It cannot be a larval form, as it has been kept under observation
for nearly a year without exhibiting any change. It is possible that both it and
XalintUa, neither of which have been observed in quite natural circumstances,1
may be abnormal modifications brought about by the influence of exceptional
conditions.

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.

•JTION V
PHYLr.M PLATYHELMINTHES

A NVMBER of c! ''.etazoa, some a little, others very de-

cidedly, higher is nation than the Coelenterata, are commonly

regarded as coi> one great sub-kingdom or phylum — the

Vermcs or Worn.- oroups ordinarily referred to the Vermes.

differ, however, very widely from one another: points of agree-
ment, except sucl merely negative, are, in fact, frequently
hardly recoe , rather than group together under one
common de- i such a heterogeneous assemblage of forms, it
si 'cms be1 d the term Vermcs altogether and to endeavour
to divide th« as1' into phyla the members of which shall
have points of p, »itive resemblance to one another. The four
phyla Pint [i "'.<?, Nematkelminthcs, Trochelminthes, Mollus-
coida, and A . with their appendices, all consist of forms
which are or hav been comprised in the Vermes. They differ
from tb ita in the presence of three well-developed
body-layers— -()f \\Jiich the middle one, or mesoderm, is of relatively
predoiu: , portance: and, for the most part, in the much,
higher si complexity attained by the various systems of
organs. -ur phyla present no metameric segmentation:
in th • A . metamerism is more «»r less strongly pronounced.

. T / intltes or Flat-Worms are a group of soft-bodied,

bilr i\* flattened animals, which are devoid of true

metal ^.aentatioii (p. 41). With a sufficient degree of uni-

formi Tucture to render the phylum a fairly compact and

well one, there is yet a considerable range in complexity,

fron ; -st forms, certain of which have been supposed to be

nea i-red with the Ctenophora among the Coelenterata, to

•»vhich have all the various systems of organs very

highly developed. The body is built up from

-. v-nic layers — sctoihr'in, mesoderm, and civlodcrm — as in

42±-> 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 coelome (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
losing not only the oosperm or impregnated ovum, but a,
quantity <_>f ;^Ht'nt material or foot -yolk, derived in most in-

•es from a special set of glands, th^j yolk or vitelhne glands.
Tip- main features which distinguish vthe Platyhelminthes from
the Ccelenterata are, the pronounced bilateral symmetry with
the many secondary features which it involves, the presence
of- a middle embryonic layer or mesodernNi, and the n on -occurrence:
of fixed colonies formed by budding.

V

1. EXAMPLES OF THE PHYLUM.
i. A Fresh- water Triclad (Planaria or Dendroccelum).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 upper
or dorsal and a lower or ventral surface, right 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 biought 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 ftody 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
o] icning 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 sufficiently general to apply to species of either of these;
genera.

PHYLUM PLATYHELMINTHES

223

pharynx (ph.), which is highly mobile, and is capable of being
thrust out as a proboscis through the mouth, beyond which it

L.r.c

./J/l s

171,— Planar ia. Digestive and excre-
tory systems, ex. openings of excretory
system ; int. intestine ; //<«. month ;
n. ph. opening of pharynx. (After Jijima
;md Hatschek.)

may then be extended to a

len retracted it lies within an enclosing muscular sheath.

FlG. 172.— Planar ia. Nervous system.
In-, brain ; eye eye ; /. ne. longitu-
dinal nerve ; ph. pharynx. (After
Jijima and Hatschek.)

relatively considerable distance.

The

-2'24 /OOLOGY 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 system (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 (l.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 monoecious in their arrangement, both male
yand 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 testes, vasa deferentia,
and cirrus w penis. The testes(^s.) are numerous rounded glands,
situated near the right and left borders. Two ducts, the right
and left ccttt deferentia (v.d.), run backwards from the neighbourhood
of the testes and unite in the middle line posteriorly. The median
duct formed bv the union of the two vasa de.terentia traverses a

PHYLUM PLATYHELMINTHES

225

vit

protrusible muscular organ, the cirrus or penis (p), to open into
the genital cloaca. At the base of the penis, where the vasa

differentia meet, the median
canal is slightly enlarged to
form a rounded dilatation, the
vesicula seminalis. 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
glands, uterus, and muscular sac.
The ovaries (ov.) 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 (lit.), 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

kxl^J^QQy from the vitelline glands.

f^^^ oW When the larva has reached

>? -V a certain stage it develops a

temporary larval mouth and
gullet and swallows the food-
Planaria. Reproductive system. J0^? ^y the aid of which it
m. muscular sac- ; o v. ovary ; pl< . pharynx ; PTOWS ramd 1 V Th P la TVfl 1

p. penis; pr. prostate; tv*. testes ; ut. o " J;*rluV'

uterus; ,-. ,/. vas deferens ; ,-;t. vitelline mouth disappears, and a new
glands. (After Jijima and Hatschek.) ,-, ,

one — the permanent mouth — is
developed in its place. When

the embryo leaves the shell it has assumed the characteristic

shape of the parent.
OL. I

226

ZOOLOGY

SECT.

par

ect

test .

circ.mu<s

od. rdef

Fin. 174. — Transverse section of a Planarian. c//v:. mu*. circular muscular fibres ; cxc. in
testinal coeca ; dors. vent. mus. dorso-ventral mascular fibres; ect. ectoderm; cxt. long, fiiux.
external layer of longitudinal muscular fibres ; int. central lumen of the intestine ; int. lonrt.
mus. internal layer of longitudinal muscular fibres; ne. nerve-cords; o. <L oviducts; JMIT.
parenchyma ; test, testes ; r. dtf. vas deferens ; rit. vitelline glands. (From Hatschek's
Lehrbuclt.)

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 head-lobe, 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 spinules
or papillce, which are prolongations of the
homogeneous .external layer or cuticle in-
vesting the whole animal. At the extreme
anterior end of the triangular head-lobe is
the small opening of the mouth (mo.) sur-
rounded by a muscular oral sucker. 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 genital opening (repr.),
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- (excr.).

Body- wall. — The body-wall (Fig. 176) is found on section to
comprise three layers: — (1) A homogeneous cuticle (cut.) of which
the spinules (sp.) are special developments; (2) a layer of circularly

FIG. 175.— Distomum
hepaticum, natural
size. excr. excretory

•pore ; mo. mouth ; repr.
reproductive aperture ;

.sv/,v. posterior sucker.

PHYLUM PLATYHELMINT.

disposed muscular fibres (circ. mus.) ; (3) a layer of longitudinal
muscular fibres (long. mus.). A cellular epidermis is wanting.
Beneath the muscles are numerous unicellular glands (gl.), the
ducts of which, in the form of processes of the cells, open on the

cut

FIG. 176. — Distomum hepatic urn. Section of the integument, circ. mus. layer of circular
muscular fibres ; cut. cuticle ; <il. unicellular glands ; long. mus. layer of longitudinal
muscular fibres ; sp. spmules. (After Brauii.)

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 pharynx (ph.}, 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 cceca, 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

ZOOLOGY SECT.

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,

-int

vit

%Wr^^ i:df

^£*5tjwl^

od— £S«4- v.d

4-te

FIG. 177.— Distomum hepaticum. Internal organisation. General view of the anterior
] tort ion of the body, .showing the various systems of organs as seen from the ventral aspect.
'./'. ejaculatory duct ; /. female reproductive aperture ; int. anterior portion of the intestine-
(the rest is not shown) ; od. commencement of oviduct ; or. ovary ; /i. penis ; ,>l. pharynx ;
xli. shell gland; to. testes ; vt. uterus ; r^1. left vas cleferens ; •<•(/-. right vas deferene
lobes of vitelline glands ; vs. vesicula seminalis. (After Somuier.)

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

tcstes, vasa deferentia, and cirrus. The tcstes (to.) 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 vas deferens, the two vasa deferentia (v.d.) opening
anteriorly into an elongated sac, the vesicula seminalis (v. s.), from
which a narrow tube — the ejaculatory duct (Fig. 178 ej.cl.) — 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
vitelline 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

vitelline duct runs backwards for a short distance to join the
oviduct. Around the j unction are grouped a mass of unicellular
shell-glands (sh. gl.), each of which is produced into a narrow
process or duct opening into the end of the oviduct or the be-
ginning of the uterus. The uterus (ut.) is a wide convoluted tube,
formed by the union of the oviduct and median vitelline duct ;
in front it opens close to the base of the penis. When the penis
is withdrawn, a small cavity, the genital sinus or cloaca, is formed,
common to the external apertures of both male and female ducts.
A canal, termed the canal of Laurer, leads from the junction of
the oviduct and median vitelline duct to open externally on the
dorsal surface.

Development. — Each ovum on impregnation bceomes sur-
rounded by a mass of vitelline matter or yolk derived from the
yolk-glands. It then becomes enclosed in a chitinous shell, the
substance of which is derived from the shell-glands. The com-
pleted egg remains for a little time in the uterus while the con-
tained ovum is undergoing the process of segmentation ; eventually
it is discharged, and passing down the bile-ducts of the Sheep

FIG. 178.— Distcmum hepaticum.

Terminal part of the reproductive
apparatus. <;/. ejaculatory duct ; /'/
female aperture ; g. unicellular
glands ; od. terminal part of oviduct ;
p. penis ; ps. penis sheath ; s. sucker ;
V. ii. vasa deferentia ; c. s. vesicula
seminalis. (After Sommer.)

230

ZOOLOGY

SECT.

into the intestine, reaches the exterior with the fyeces. A por-
tion of the egg-shell at one end then becomes separated off as
a sort of lid or opcrculum, and gives exit to the contained embryo.
This, the ciliated embryo (Fig. 179, A], is a somewhat conical body
covered all over with vibratile cilia, and with two spots of pig-
ment, the eye-spots (eye], near the broader or anterior end, which is
provided with a triangular head-lobe (pap.). There is no vestige of

FIG. 179.— J— D. Development of Distomum hepaticutn. A, ciliated
larva; 5, sporocyst, containing redise in vaiious stages of development;
C, redia, containing a daughter redia, and cercariai ; D, fully developed
cercaria. />. op. birth opening ; c/^.»enteron of redia ; eye. eye -spots ; »/«.*/.
gastrula stage of redia ; gei-m. early stages in the formation of cercariic :
int. intestine of cercaria; mo/: morula stage in the development cf
cercarise ; a-n. oesophagus ; or. aw. oral sucker ; /><i,>. head-papilla of ciliated
embryo ; j>li. pharynx ; proc. processes of redia ; t-vnt. *;<. ventral sucker.
(After Thomas.)

internal organs, with the exception of a pair of flame-cells. The
ciliated larva swims about in water, or moves over damp herbage
for a time, and perishes unless it happens to reach a Pond-snail
(Lyrnnaxi') or ;i Land-snail ( J-falios), as a parasite of which it is
alone able to enter upon the next phase in its life-history.
When it meets with the Snail, the embryo bores into it by
means of the head-lobe. Established in the interior of the

;

sn

PHYLUM PLATYHELMINTHES 231

.ail, it loses its ectoderm and grows rapidly into the form
of an elongated sac, the sporocyst (Fig. 179, B), with an in-
ternal cavity, with remnants of the eye-spots, and with flame-
cells. The sporocyst may divide into two similar bodies by a
process of transverse fission, but this is exceptional. Eventually
cells are budded off from the layer that lines the internal cavity
of the sporocyst, and these undergo a process of segmentation
similar to the holoblastic segmentation of the impregnated ovum,
resulting in the formation of a morula, which becomes converted
into a stage resembling a gastrula, The gastrula elongates and
gives rise to a body called a redid (C), which begins to move about,
and eventually forces its way out of the interior of the sporocyst.
When fully formed, the redia is a cylindrical body with a pair
of short processes (proc.) near the posterior end, and with a
circular ridge near the anterior end (C). It possesses a mouth
leading to a pharynx and simple sac-like intestine, and there
is a system of excretory vessels. In the interior of the redia
cells are budded off and develop into gastrulse, exactly as in
the case of the sporocyst ; these gastrulse either develop into
a fresh generation of redise, or give rise to bodies termed
•cercarice. The latter (D) are provided with long tails : they
have anterior and posterior suckers, and a mouth and • pharynx,
followed by a bifid intestine. An opening, birth-opening (b. op.),
is formed in the wall of the redia near the circular ridge, and
through this the cercariue escape ; they move actively by means
of their tails, and force their way out of the body of the Snail.
They then, losing the tail, become encysted, attached to blades
of grass or leaves of other herbage. The transference of the
larval Fluke in this stage to its final host, the Sheep, is effected
if the latter swallow the grass on which the cercaria has become
encysted. The young Fluke then escapes from the cyst and forces
its way up the bile-ducts to the liver, in which it rapidly grows,
and, developing reproductive organs, attains the adult condition.

iii. The Common Tape- Worm of Man (Tcenia soliuni].

General Features. — Tcenia solium occurs as a parasite in the
intestine of man. It has the form of a narrow ribbon (Fig. 180),
which may attain a length of several yards, attached at one end to
the wall of the intestine, the remainder hanging freely in the
interior. Towards the attached end the ribbon becomes very much
narrower than it is towards the opposite end ; and at this narrower
extremity is a small, rounded, terminal knob, which is known as the
.head or scolex ; the rest of the animal is termed the body or
•strobila ; the narrow part immediately behind the head is some-
times termed the neck. The attachment of the Tape-worm to the

232

ZOOLOGY

SECT.

wall of the intestine is slight and temporary ; it is effected by
certain organs of adhesion, the hooks and suckers 011 the head.

The head (Fig. 181) maybe roughly described as pear-shaped, but
becomes four-sided at the broader end. In the middle of this

Fie. ]>:(>.— Tsenia solium. Entire specimen, reduced ; cap. head. (After Leuckart.)

broader, anterior end is a rounded prominence, the rostellum, round
the base of which there is a double row of usually about twenty-eight
curved and pointed chitinous hooks. The rostellum is capable of
being protruded and retracted to a slight extent, and the position

PHYLUM PLATYHELMINTHES

233

'

of the hoo! - es accordingly: when the rostellum is fully

rt-tracted th - of the hooks are directed forwards, and may

I'vrn meet LI litre : as the rostellum

is protruded thfe hooks become rotated until

their apico he directed backwards.

Four cup-flu)p*'(l Mi.'kcrs project slightly

from the behind the circlet of

hooks.

The fax? -''it" has a jointed appear-

ance, owii; ide up of a

string of segii -—about

850 altogether. Tin--- . and

shorter in front, gradually h
towards the posterior free extr The

neck or part immediately following the h
is devoid of any trace of segmentation. The
two surfaces of the proglottides -are not to
be distinguished by any differences visible
to the unassisted eye ; but that side to-
wards which the female reproductive organs
are more nearly approximated is regarded as the ventral, the
opposite as the dorsal surface. On one border, sometimes the
right, sometimes the left, of each proglottis, is a little prominence,
the genital papilla, on which is the opening of a chamber, the
ycnital cloaca, into which both the male and female reproductive
ducts open.

An examination of entire living, and of preserved and stained
Tape- Worms under the microscope shows (1) that an alimentary

ad of

solium, maginl
(After Leuckart.) \

FIG. 182.— Transverse section of Tsenia solium. (After Shipley.)

system is not present ; (2) that nervous and excretory systems are
represented ; (3) that there is a complete set of reproductive
organs, constructed on the same general plan as those of the
Liver-Fluke, present in each of the proglottides. •

The nervous system consists of two not very well-defined
<jaiif)lia — united by a broad transverse commissure — in the head ; of

2:u

ZOOLOGY

SECT.

slender nerves passing from these to the sucki -rs. and of two
longitudinal nerves which run backwards through all the proglot-
tides to the posterior end of the body. The ganglia obviously
correspond to the ganglia on the nerve-ring of th< Li\ er-Fluke.

The excretory organs consist of a richly bra) id- d system of
excretory vessels. There are four main trunks ( \ can. excrct.^,

which extend throughout the entire length <>; ,dy, two near

each lateral margin. The two pairs of longii ! vessels are

connected together in the head by a ring-like vessel, and in each
proglottis near its posterior margin by a straight, transverse,
connecting branch. Posteriorly the longitudinal trunk opens into
a pulsatile caudal vesicle, comrnunicajklg with the exterior. These
main trunks of the • stem give origin to a number of

bran in rum give oft' numerous fine canalicules, or

capillaries, terminating in flame-cells similar to those of Distomum
hepatiemi,.

The reproductive organs (Fig. 183) repeated in each fully
'formed proglottis, are in essential respects very similar to those of

can,, excret

ca,n,.ejccret

•va.s.def

por.gerf.

r'n.. 1S3. — A proglottis of Tsenia solium with mature reproductive npp .'iratus. can. •

longitudinal excretory canals with transverse connecting vessels ; </!. vit. vitultHre-ghnrds ;
/. longitudinal nerves; or. or. ovaries; p<><\ gen. genital pi .re ; xctifii. shell-glands;
wter. u t urns ; rn;/. vagina; I-H.X. */</. vas deferens. The numerous smaU round bodies are thu
loljes of the testes. (After Leuckart.)

the Liver-Fluke. In the most anterior proglottides they are not
developed ; it is only at about the 200th proglottis that they first
appear: at first the male parts of the system are alone differen-
tiated : then in the succeeding proglottidesjtill we approach near
the posterior extremity of the body, the female organs are like-
wise developed. In the most posterior segments modifications and
reductions of some of the parts take place, owing to the great
increase in size of the uterus. Tin1 male portion of the apparatus
consists of the testes with their cffnrnt <lm:tx, the vas /A/r/ry/x {vas.
'/';/.), and the cirrus or _/>/•>/ /x, with its we. The testes consists of
numerous rounded lobes situated nearer the dorsal than the ventral

PHYLUM PLATYHELMINTHES 235

I surface, and extending throughout the greater part of the length
: and breadth of the proglottis. With each lobe is connected a fine
efferent duct ; the ducts of neighbouring lobes unite together to
form somewhat larger ducts ; and the larger ducts, receiving
numerous tributaries, eventually open into the inner extremity of
the vas deferens, or main duct of the testis. The vas deferens is a
convoluted tube which extends outwards towards the lateral
margin (right or left as the case may be) of the proglottis.

The terminal part of the vas deferens, which is somewhat nar-
rower than the rest, traverses a narrow protrusible process, the
cirrus or penis, and opens at its extremity by the male genital
aperture. The cirrus is enclosed at the base by a muscular sac, the
cirrus §ac^

The ovary (or.) differs from that of the Liver-Fluke in being a
paired organ, consisting of two approximately equal, right and
left, halves. It is situated towards the posterior border of the pro-
glottis. Like that of the Liver-Fluke, it consists of a number of
branching tubes, in the interior of which the ova are developed.
From opposite sides these tubes converge towards the median
line, where they open into the oviduct. A yolk-gland (gl. vit.), of less
relative extent than in the Liver-Fluke, consists of a number of
minute lobules ; a duct, the yolk-duct, which runs forward from it,
opens into the oviduct. The numerous lobules of a rounded shell-
gland (schld.) surround the yolk-duct where it passes forward to
join the oviduct ; and the uterus (liter.) opens into it at this point.
A short distance from its origin the oviduct -presents a dilatation,
the reccpt(.«:tflttj)i seminis, in which the secretion of the testes is
stored during copulation. The remainder of the oviduct, frequently
known as the vagina (vag.), is a long narrow tube leading to the
female generative opening. The uterus is, in the segments in which
it first makes its appearance, a simple cylindrical diverticulum
of the oviduct ; it retains its simple form as far back as about the
600th proglo^JL where it begins to branch, the ramifications
increasing jfl ^M1 and volume in the posterior segments.

The test^BnHlecretion (probably from the same proglottis)
passes in theri^of copulation along the vagina to the rcceptacu-
lum seminis. Here, or in the proximal part of the oviduct, it
fertilises the ova as they ripen and become discharged from the
ovary. As in the case of the Liver-Fluke, the oosperm proper be-
comes surrounded by a quantity of food-yolk developed in the
yolk-glands, and is then enclosed in a firm chitinous shell formed
for it by the secretion of the shell-gland. It then passes into the
uterus. The first completed eggs are found in the uterus in some
proglottis between the 400th and the 500th. From this point 'back-
wards they rapidly accumulate, until the cavity of the uterus,
which now . becomes branched, is filled and distended with them.
Eventually in the most posterior, so-called " ripe " proglottides

23(>

ZOOLOGY

-SECT.

(Fig. 184), the uterus, with its contained accumulation of eggs,

becomes so large as to fill the greater part of

the interior of the proglottis, the remainder

ci£&; .-.!'> of the reproductive apparatus meanwhile

having become absorbed.

Development. — When the ripe proglottides
are detached they pass to t{ie exterior with the
faeces of the host. For a time they exhibit
movements of contraction. The embryos con-
tained within the eggs have meantime assumed
the form of rounded bodies, each armed with six
chitinoid hooks — the six-hooked or hexaeanth em-
bryo (Fig. 185, A) — enclosed within two mem-
branes. If the proglottides or the eggs which
have escaped from them, should now be taken
into the alimentary canal of the Pig, which forms the ordinary
second host of the parasite, the hooked embryos, becoming freed

P

FIG. 184.—" Ripe " pro-
glottis of Taenia
solium. (After
Leuckart.)

Fii:. 1>C).— Development of Tapeworm. A, hexaeanth eml>ry«>: />'. 1'roscolex of T^mia
.««,; ,,fitu,- C—E, stages in the formation of the scolex of the same : C, The imagination before
the hooks and suckers have become developed; />. after the appearance of the hooks :m<l
suckers; E, partly evaginated ; F, fully evaluated srokx of T. solium with caudal vesicle;
(i. scolux <>f T. » rrata with remains of the vesicle ; //, young tapeworn of T. *< n-ata. (After
Leuckart.)

from their coverings, bore their way with the aid of their hooks
through the wall of the alimentary canal, and reach the voluntary

PHYLUM PLATYHELMINTHES 237

muscles. Here they increase greatly in size, and develop into
rounded cysts with a large cavity filled with watery fluid — the
/n-tixcolex stage (B). On the wall of the proscolex, at one side, is
formed a hollow ingrowth, or invagination (C) ; and on the inner
surface of this are developed the hooks and suckers characteristic
of the head or scolex of the adult (D). When these are fully formed
the hollow ingrowth becomes everted (E), the suckers and hooks thus
coming to be situated on the outer surface (-F). The whole embryo
has now the form of a bladder or vesicle, with Avhich is connected
at one point a process haying all the characters of the head and
neck of the mature Tsenia solium ; this is the bladder-worm stage,
or Cysticerc'iis. If a portion of Pig's muscle containing Cysticerci
which have not been killed by cooking, is taken into the stomach
of Man, the bladder is thrown off, the scolex attaches itself to the
wall of the intestine by its hooks and suckers, and develops the
series of proglottides of the adult Tape-Worm.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Platyhelminthes are bilaterally symmetrical, usually dorso-
ventrally compressed animals, devoid of hard supporting skeleton,
either external or internal, and devoid of metameric segmentation ;
with three embryonic layers — ectoderm, mesoderm, and endoderm
— entering into the formation of the body. A body-cavity is not
present. There is a system of excretory vessels, communicating in«
the majority of cases with the exterior, and furnished with ciliary
flames. There is no blood-vascular system. An enteric cavity
may be absent, may be rudimentary, or may be highly developed :
it is never provided with an anal aperture. The completed egg
contains, in addition to the oosperm, a quantity of yolk-matter,
usually in the form of definite yolk-cells, and usually produced by
a special set of yolk-glands. Development is "sometimes direct,
sometimes accompanied by a metamorphosis.

CLASS I— TURBELLARIA.

Mostly non-parasitic Platyhelminthes with j/ ciliated cellular
epidermis ; with a digestive cavity (except -m the sub-division
Accela).

ORDER 1. — POLYCLADIDA.

Flattened leaf-shaped Turbellaria, without separate yolk-glands :
testes and ovaries numerous; male and female genital apertures
usually separate ; intestine complexly branched.

ORDER 2. — TRICLADIDA.

Turbellaria with elongate depressed body ; with numerous yolk-
- glands, two ovaries, numerous testes : a single genital aperture :

238 ZOOLOGY SECT.

intestine consisting of a median anterior division and two lateral
posterior limbs which are provided with side branches.

ORDER 3. — RHABDOCCELIDA.

Comparatively small Turbellaria, with the body usually elongate
and cylindrical or compressed : with simple, or nearly simple, sac-
like intestine: with or without yolk-glands: with one or two
ovaries and two or many testes.

CLASS II.— TREMATODA.

Ecto- or endo-parasitic Platyhelminthes devoid of cilia,1 or of a
cellular epidermis ; 2 with a well-developed digestive apparatus.

ORDER 1. — MONOGEXETICA.
Mostly ecto-parasitic Trematodes ; with direct development.

ORDER 2. — DIGEXETICA.
Endo-parasitic Trematodes with complicated life-history.

CLASS III.— CESTODA. '

1 .Is "?

Endo-parasitic Platyhelminthes without cilia and without di-
gestive cavity, the animal consisting in most cases of a Bounded
head bearing organs of adhesion in the form of suckers and hooks,
j^nd an elongated compressed body consisting of a string ofsimilar
jproglottides, each containing a complete set of hermaphrodite
weproductiyje organs.

ORDER 1. — MOXOZOA.
The body not divided into proglottides.

ORDER 2. — POLYZOA. •
The body consisting of head or scolex, and string of proglottides.

Systematic Position of the Examples.

Planar ia and Dendroccelum are genera of the family Planaridce,
or fresh- water Planarians, which is one of the two families of the
order Tricladida, differing from the other family, the Geoplanidce
or Land Planarians, mainly in having. the body less elongated and
more dorso-ventrally compressed.

Distomum hepaticum is one of the very many species of the
genus Distomum, which is distinguished from the other three

1 Except in certain species of Temnocephala.

2 Except in the Temnoctphak<e and Actinodactyhlhi.

,

PHYLUM PLATYHELMINTHES 239

genera of the family iJixtoniidri: by being hermaphrodite (two of
the genera — HUlnu'iia and A?/7///,r/vV> — having the sexes distinct),
and by having both anterior and posterior suckers, but no retractile
tentacles (present in the genus Rhopalopliorus). The Distomidce
are a family of the order Digcfncfi<-<t, and differ from the remaining
families by the union of the following characters : — -The anterior
sucker is perforated for the mouth : the posterior sucker is on the
ventral surface and not terminal : the sexual aperture is usually on
the ventral surface and in the anterior third of the body, in front of
the posterior sucker ; the eggs are almost always furnished with
an operculum.

Tccnia solium is one of the many species of the genus Tcvnia,
of the family Twniadie, which is distinguished from the other
families of Cestodes by the possession of four suckers, with or
without a circlet of hooks, and by the development of well-defined
proglottides which become separated off when mature.

3. GENERAL ORGANISATION.

General External Features. — As the name of the phylum
denotes, the body in the Platyhelminthes is,, in the great majority
of cases, much compressed in the dorso-ventral direction : very
thin, so that Avhen very short it may be described as leaf-
like, or, when more elongated, as ribbon-like : or thickish in
the middle and becoming thinner towards the margin. Some,
however, have the body comparatively thick, usually with a certain
amount of dorso-ventral compression ; a few are approximately
cylindrical 'or fusiform. The symmetn~ is always bilateral (p. 41),
the radial arrangement of parts so prevalent in the Coelenterata
and primarily, as we have seen, associated with a fixed or stalked
condition, never being observable. A Flat- Worm has dorsal and
ventral surfaces, right and left sides or borders, and anterior
and posterior ends. The anterior end is that which is directed
forwards in ordinary locomotion : it usually has one or otjier of
the characters of a head-end : but a distinct head is rarely dis-
tinguishable, and the mouth, when present, is usually placed some
distance back on the ventral surface.

In the Turlicllaria, (Fig. 186) the leaf- form is the prevailing one,
a general shape resembling that of Planaria being very common.
In many, however, the body is greatly elongated, and it may
assume the shape of a thin ribbon with puckered edges, as
in some marine forms: or may be thickened and band-like,
as in the Land Planarians ; or. it may approach the shape of
a cylinder, as in some Rhabdocoeles. A head-region is not
usually distinct ; but there is always something - to mark off
the anterior from the posterior end — a difference in shape, the
presence of eyes, and, sometimes, of a pair of short tentacles : J

240

ZOOLOGY

SKCT.

in some a slight constriction separates off an anterior lobe, on
which the eyes are borne, from the rest of the body. In others
the anterior end is retractile, and may be everted as a pro-
boscis. The mouth is never at the extreme anterior end, but
always ventrally placed, sometimes behind the middle. In some
Polycladida there is a large ventral sucker serving for temporary
fixation ; and in some Rhabdocoeles both the anterior and posterior
ends, though not provided with suckers, are adhesive, so that the
animal can loop along like a Hydra or a Caterpillar. There is never

FIG. ISO. — Various Planarians. A, Convoluta ; B, Vortex; C, Moiiotus ; D, Thysanozoon ;
E, Rhynchodemus ; F, Bipalium ; G, Polycelis. All natural size. (After Von Graff.)

any external appearance of segmentation, though in at least one
exceptional instance (Gunda segment at a, Fig. 187) the internal parts
may be so disposed as to approximate to the metameric arrange-
ment (pseudo-metamerism). A number of transverse muscular septa
are present, imperfectly dividing the body internally into a series of
segments ; and various internal organs — intestinal cceca, gonads,
transverse commissures of the nervous system — are arranged in
pairs following this division. A few multiply b}* budding, and
these form long chains, having something in 'common with the
string of proglottides of a Cestode, but differing radically, as will
be shown later, in the mode of development. Colour is very
general in the Turbellarian, though some are transparent and

PHYLUM PLATYHELMTNTHES

241

eye

br

itit

Fie. 187. — Gunda segmentata. General view of the organisation. -f>r. brain ; eye eye

gen. ap. genital aperture ; int. intestine with its coaca ; long. ne. longitudinal nerve cord
or. ovary ; ovd. oviduct ; pc. penis ; ph. pharynx ; te. testes ; v.t. uterus. (After Lang.)

VOL. I

R

242 ZOOLOGY SECT,

colourless. The most vivid coloration characterises some of the
marine Planarians, the Rhabdocoeles being comparatively obscure.
The surface is covered with a coating of fine vibratile cilia, the
vibration of which subserves respiration as well as (in the smaller
forms) locomotion. Among the ordinary cilia are frequently dis-
posed longer whip-like cilia or flagella, likewise motile ; and some-
times non-motile (sensory) cilia may occur here and there.

The Trematodes, nearly related to the Turbellarians in internal
organisation, resemble them also in external form, with certain
modifications connected with a parasitic mode of life. As in the
latter class, the leaf-shape prevails ; an elongated form also occurs,

B

Fio. 188. Digenetic Trematodes. A, Amphistomum ; B, Homalogaeter. <i. p. genital

aperture; m. mouth; *. posterior sucker ; te. testes ; vit. vitelliue glands. (After M. Brauii.)

though more rarely. The body is usually thicker and more solid
than in most Turbellaria. The anterior end is distinguished from
the posterior by its shape, by the arrangement of the suckers, and,
in many of those Trematodes that are external parasites, by the
presence of eyes. Suckers, present in the Turbellaria only in some
of the Polycladida, are universal in their occurrence. They are
always ventrally placed, their chief function being to fix the
parasite to the surface of its host in such a way as to facilitate the
taking in by the mouth of animal juices and epithelial debris.
Their number and arrangement vary considerably. There are
nearly always present an anterior set of suckers (or a single
anterior sucker surrounding the mouth) and a posterior set, or a
single large posterior sucker. The arrangement already described
as characterising the Liver-fluke is that which is typical in the
digenetic forms : a single anterior and a single posterior sucker.

PHYLUM PLATYHELMINTHES

243

In the Monogenetica the suckers are often more numerous ; in the
family G-yrodactylidce (Fig. 189, A) there is no anterior sucker, but

FIG. 189.— Monogenetic Trematodes. A, G>rro<'act >//!'.«. cil. disc bearing hooks and pro-
cesses at the posterior end ; eat. intestine ; gl. unicellular glands whose ducts open on the
surface about the anterior end ; mo. mouth ; oosp. oosperm ; ov. ovary ; j>, penis ; pit. pharynx ;
te. testes. B, Polystojjutm. en. intestine ; y. p. genital pore ; mo. mouth ; ph. pharynx ; or
ovary ; te. testes ; u. uterus ; r., v. d. vas deferens ; fit. vitelline glands ; -fit. </. vitelline ducts ;
x. sperm-duct probably directly connecting the testes with the. oviduct. (From M. Braun.)

at the posterior end one or two discs armed with hooks : in the
Polystomece (Fig. 189, B) there is also a posterior disc on which are

R 2

244

ZOOLOGY

SECT.

six suckers with several hooks ; in the Temnocephalece (Fig. 190)
there is no anterior sucker, but the anterior end develops a row of
adhesive tentacles, while in Actinodactylella (Fig. 191) a series of
marginal tentacles are present in addition to both anterior and
posterior suckers. In general the monogenetic forms, being ex-

l-'io. 100.— Temnocephala. c. cirrus ; /. intestine ; j/i. mouth ; or. ovary ; ph. pharynx ; r. ,-.
receptaculuin vitelline ; t. testes ; t. s. terminal sac of excretory system ; v. vitellarium ; <-. a.
vesicula seminalis. (After Haswell.)

ternal parasites, require more powerful organs of adhesion, and
have the suckers better developed than the digenetic, and often
armed with bristles or hooks. The suckers are used also in
locomotion, and here also there is necessity for greater develop-
ment in the case of the external parasites, which in many cases
move about with considerable activity after the looping fashion of

PHYLUM PLATYHELMINTHES L'45

>eches. Save in two exceptional cases (Temnoc&phala) vibratile
cilia are not known to occur on the surface in the adult condition ;
in some there are groups of non-motile cilia, situated on little
conical elevations — the tactile cones. Pigment is rare in the
endoparasitic Digenetica, save in a few that live in the interior of
transparent animals, though many appear coloured variously by

FIG. 191.— Actinodactylella. b. c. bursa copulatrix ; in: brain ; c. penis ; /.intestine: or.
ovary; ph. pharynx; r. r. receptaculum vitelli ; t., t. testes ; r. vitelline glands; r. s.
vesicula seminalis. (After Haswell.)

the internal organs shining through the translucent body wall, or
are stained by some fluid derived from their host. But pigment
is found in some of the ectoparasitic forms.

The relationship of the Cestoda to the Trematoda is, as will be
subsequently shown, fairly close ; but, though there are connecting
forms between the two classes, the shape of the average Cestode

ZOOLOGY

SECT.

is very different from that of such an average Trematode as the
Liver-fluke. The body of an ordinary Cestode is of great length,
sometimes extending even to a good many feet, and relatively narrow,
being compressed into the form of a ribbon. The anterior end
is attached to the host by means of suckers and hooks placed on
a rounded lobe, the head or scolex, connected with the body by a
narrow part or neck. The head is usually rather radially than

rb

FIG. 19-2.— Tetrarhynchus.

FIG. 193.— Tsenia echinococcus.

(After Cobbold.)

bilaterally symmetrical, witWourjsjiekers and a circlet of hooks.
The hooks, when present, are borne on a longer or shorter retractile
process, the rostellwm, the long axis of which is in line with the
long axis of the body. In Tetrarhynchus (Fig. 192) there are four
very long and narrow rostella covered with booklets.

The Cestoda are devoid^^ofjuouth, and in most of them the
genital apertures are marginally placed, so that, externally, there
is, except in the case of a few in which the genital apertures are
not marginal, nothing to distinguish the dorsal . surface from the
ventral. The body, or strobila, which is narrower in front than it
becomes further back, is made up throughout its length of a series
of segments, or proglottides, which become larger and more dis-
tinctly marked off from one another as we pass backwards. Tcenia

PHYLUM PLATYHELMINTHES

247

iindcoccus (Fig. 193) is exceptional in possessing only three or
four proglottides. In a few (Ligula and iLs allies — Fig. 194),

r-

FIG. 194.— Ligula. (After Leuckhart.)

though the body has the normal elongated ribbon-like form, the
segments are not distinct, and in Caryophyllcem (Fig. 195),

FIG. 195.— Caryophyllseus.

(After Leuckhart.)

FIG. 190.— Amphiptyches.

(After Spencer.)

Fia. 197.—
Archigetes.

(After Leuckhart.)

Amphilina, Amphiptyches (Fig. 196), and Archigetes (Fig. 197),
(Monozoa). segmentation is entirely absent, the whole body in

248

ZOOLOGY

SECT.

c.l.rn

i.l.-m

these genera consisting of a single proglottis. The surface in the
Cestodes is devoid of cilia, and there is no pigment.

Integument and Muscular Layers. — In the Platyhel-
minthes in general there are integumentary layers and underlying
layers of muscle, which are more highly differentiated than in the
Ccelenterates. But considerable differences exist between the
members of the three classes. In the Turbellaria (Fig. 198) there is,
as already noticed in the account given of the Planarian, a distinct
epidermis (Fig. 198, ep) in the form of a layer of cells, most of

which are ciliated. A cleli-

rh cate cuticle is usually, though

not always, distinguishable,
investing the epidermis ex-
ternally. In one family the
cuticle is developed, along
the margin of the body, into
a series of chitinous bristles.
Among the ordinary epi-
dermal cells there are in the
Polycladida numerous cells
containing short rod-like
bodies — the rhabdites (rh.) ;
in the other orders of the
Turbellaria these rhabdite-
forming cells are sunk deeply
within the parenchyma, and,
in the Rhabdoccela, have very
long ducts, formed of pro-
cesses of the cells, by means
of which the rods, together
with a viscid matter, reach
the exterior at certain points

of the surface, chiefly around the anterior extremity. The func-
tion of these rhabdites is not in all cases certain ; they have been
supposed to add to the sensitiveness of the parts in which they
are situated after the fashion of hairs or nails, or to have a
skeletal function. In the Rhabdoccela and Tricladida they un-
doubtedly aid in adhesion, and probably have the function of
assisting in the entanglement and capture of food. In certain of
the Turbellaria stinging capsules occur similar to those of the
Ccelenterata, and transition forms between rhabdites and stinging
capsules occur in some cases. Adhesive cells with processes also
frequently occur in the epidermis. Beneath the epidermis is a
basement membrane (b. m.\ which in the Polycladida is of a thick
resistent character, and contains stellate cells.

In a small number of the Trematoda three distinct layers are
distinguishable in the integument — a homogeneous, or nearly

rh. c

d. v.m.

FIG. 198.— Section of the body-wall of a Triclad.
6. m. basement membrane ; c. m. circular
muscles ; d. v. m. dprso-ventral muscles ;
e. I. m. external longitudinal muscles ; ep.
epidermis ; i. 1. m. internal longitudinal
muscles ; p. parenchyma ; rh. rhabdites ;
rh. c. rhabdite-forming cells. (After Jijima.)

PHYLUM PLATYHELMINTHES

249

)mogeneous, outer cuticle; a cellular, or at least, nucleated,
epidermis, and a basement membrane ; but the cellullar epidermal
layer is absent as such in the adult condition in the majority of
the Trematodes, and there is only a homogeneous, non-nucleated
outer layer, which may be the modified epidermis, or may be the
cuticle, with or without a basement-membrane. Rhdbdite-forming,
and other unicellular glands derived from the epidermis, are
frequently present beneath the integument.

In the Cestodes, as in the majority of the Trematodes, no
definite epidermis is present. The external layer, sometimes
divided into two, is of a homogeneous non-cellular character, and
is usually termed cuticle, though it perhaps partly corresponds to
the basement membrane of other groups. Beneath this is a thin
layer which appears to consist of elastic fibres. Beneath this again
is a layer of fusiform cells, narrow prolongations of which pass to
the surface. It is possible these may be concerned in the absorp-
tion of nutrient matter, but some of them are undoubtedly of the
nature of nerve-cells and have nerve-fibres connected with them.

The muscular layers of the body-wall vary somewhat in their
arrangement in the different groups of Platyhelminthes. Most
commonly there is an external layer of circularly arranged, and
an internal layer of longitudinally arranged fibres ; frequently
layers of fibres running in a diagonal direction are present also.

Characteristic of the flat-worms is a peculiar form of connective
tissue, the parenchyma (Fig. 199), mention of which has^already

ct,

FIG. 199. — Parenchyma, a, I, intercellular spaces ; bm. basement membrane ; <', nuclei ; c?, nuclei ;
ep. epidermis. (After -Braun.)

been made in the descriptions of the examples, presenting many
varieties, filling up the interstices between the organs— leaving

250 ZOOLOGY SECT.

only, in some instances, very small spaces — sometimes regarded as
representing the body-cavity, or caelome, which we shall meet with
in other groups of worms. Sometimes the parenchyma appears to
consist of distinct large cells with greatly vacuolated protoplasm,
with interspaces here and there in which groups of rounded cells
are enclosed. Sometimes the constituent cells run together, and
the parenchyma then appears as a nucleated, finely fibrillated,
vacuolated mass in which the boundaries of the cells are not
recognisable. Pigment occurs in the parenchyma in some Rhab-
doccele Turbellarians and a few Monogenetic Trematodes. In
some Turbellaria — species of Convoluta and Vortex — the paren-
chyma contains numerous cells enclosing chlorophyll corpuscles ;
these are symbiotic unicellular Algas, similar in their mode of
occurrence to the yellow cells which have been referred to as found
in the Radiolaria. Running through the body, for the most part
in a dorso- ventral direction, are numerous slender muscular fibres,
the fibres of the parenchyma muscle ; many of these become in-
serted externally into the basement membrane.

Great differences exist between the various groups of Platy-
helminthes as regards the development of the , alimentary
system, differences which are, broadly, to be correlated with
differences in the mode of nutrition. Some of the Flat-worms
—the Turbellaria and some of the Monogenetic Trematodes —
procure their food, in the shape of small living animals or vege-
table organisms, or floating organic debris, by their own active
efforts. Others — the Digenetic Trematodes and the Cestodes —
having reached a favourable situation in the interior of their host,
remain relatively or completely passive. An alimentary canal is
completely absent in the last-named group, nutrition being effected
by the absorption of digested matter from the interior of the
animal in which the Cestode lives. In all the rest of the Platy-
helminthes there is an alimentary canal, which never opens on the
exterior by an anal aperture. All the Turbellaria and Trematoda
have an alimentary apparatus consisting of two well-defined parts
— a muscular pharynx and an intestine. The pharynx is usually
a rounded muscular bulb, but is sometimes (some Turbellaria) of a
cylindrical shape ; it is usually capable of eversion and retraction.
Actinodactylella (Fig. 191) is exceptional in having an extensile
proboscis with a1 pin-shaped style, which becomes retracted within
the opening of the mouth. Unicellular (" salivary ") glands open
into the pharynx in most cases.

The mouth is always ventral, but varies greatly in its position
on the ventral surface, being sometimes central, sometimes behind,
sometimes in front of, the middle of the length of the body.

In the most lowly organised group of Turbellaria the intestine
is represented merely by a vacuolated nucleated mass of proto-
plasm without lumen. In the others it is sometimes a simple

PHYLUM PLATYHELMINTHES

251

(Rhabdoccele Turbellaria (Fig. 200), a few Trematoda), with or
ithout short lateral diverticula. In the maj ority of the Trematodes
consists of a pair of simple canals ; but in some, as in the Liver-

IG. 200.— General plan of the
structure of a Rhabdoccele
Turbellarian. b. c. bursa
copulatrix ; en. brain ; e. eye ;
g. gennarium ; i. intestine ;
In. longitudinal nerve ; m.
mouth; ph. pharynx; rs.
receptaculuin seminis ; g. uni-
cellular glands; t . testis ; u.
uterus ;. vs. vesicula semin-
alie ; <J ejaculatory duct ;
<J $ common genital aper-
ture. (After Von Graff.)

OV

>1. — General plan of the structure of a Poly clad. en. brain ;

:ye ; i. intestine ; In. longitudinal nerve cord ; m. mouth ;

FIG. 201.— General

ovary; ph. pharynx; phi. sheath of pharynx; t. testes ; u.
uterus ; i-d. vas deferens ; vs. vesicula seminalis ; <$ male
aperture ; $ female aperture. (After Von Graff.)

fluke, there is a pair of canals which give off numerous branches.
In the Polycladida (Fig. 201) there is a central cavity from which
numerous branching canals are given off. In the Tricladida (Fig.
202) one median canal passes forwards from the pharynx,

252

ZOOLOGY

SECT.

In

and a pair of canals backwards from it, all three giving off branches
which again branch. In some Polyclada there are minute pores,
by means of which certain of the canals are placed in communica-
tion with the exterior. A number of unicellular glands, which
probably produce a digestive secretion, open in many Trematodes

and Rhabdocceles, at the junc-
tion of pharynx and stomach-
intestine.

A bilateral nervous system
is developed in all the Platy-
helminthes. Its elements are
nerve-fibres, or rather nerve-tubes,
and nerve-cells. The former are
tubes containing a very delicate,
finely fibrillated, material. The
nerve-cells, which are usually
bipolar, more rarely uni- or multi-
polar, lie in the course of these
tubes, with the contained delicate
material of which the substance
of the cells is in continuity. The
degree of development of a cen-
tral part of the nervous system,
or brain, varies in the different
groups ; it is rudimentary in the
Turbellaria Accela and in the
Cestodes, and best developed in
some Polycladida and some Mono-
genetic Trematodes. It consists
of numerous nerve-tubes which
here converge from the various
parts of the body and pass across
from one side to the other, to-
gether with a central mass of
matter similar to that contained
in the nerve-tubes, and a number
of nerve-cells. It is situated in
the anterior portion of the body,
almost invariably in front of the
mouth. When the peripheral
part of the nervous system is

best developed, as it is in the Polycladida, the Tricladida, and
some Trematodes, there are three pairs of longitudinal ncrvc-
cords running backwards from the brain throughout the body,
connected together by frequent transverse connecting nerves, or
commissures. To these there are sometimes super-added fine net-
works or plexuses of nerves, situated superficially under the dorsal

FIG. 202. — General plan of the structure of
a Triclad. en. brain ; e. eye ; </.
germariuni ; i. median limb of the in-
testine ; ?'2. right limb ; is. left limb ;
In. longitudinal nerve-cord ; m. mouth ;
od'. oviduct ; ph. pharynx ; t. testes ;
tc. tentacles ; vd.vas deferens ; u. uterus ;
<J ejaculatory duct ; 9 vagina ; $ 9
common genital aperture. (After Von
Graff.)

PHYLUM PLATYHELM1NTHES

253

[tegument, or on both dorsal and ventral surfaces. Sometimes
nerves run forwards from the brain as well as backwards. In the
Rhabdocoeles and some of the Trematodes the whole system is
simpler, and the number of longitudinal cords fewer. In the
Cestodes there are two longitudinal trunks which run throughout
the length of the body, and are connected together in the head by
a commissure which represents the brain of other Platyhelminthes.
In addition to the tactile cones of some Trematodes and the
sensory c&7rajef""t&e Turbellaria, already referred to, the sensory
organs^f^fiQ Platyhelminthes are the eyes and the octocysts.
Eyes occur in the Turbellaria and some Monogenetic Trematodes,
but are wanting in the Digenetic

In some of the Polyclada they are extremely numerous, collected
into groups over the brain, and frequently arranged also round the
margin of the body. In the Rhabdocceles and Monogenetic
Trematodes, they are much less numerous — usually two or four.
In some cases each eye simply consists of a pigment spot ; to
this may be added a refractive body. When most highly de-
veloped the eye is still of very simple structure, consisting of a
cup of pigment enclosing refractive bodies, and having nerve-
cells in close relation to it. The otocysts are sacs containing
otoliths of carbonate of lime. The function of these bodies, which
occur only in a small number of the Turbellaria, is unknown ;
there is no sufficient evidence that, as their name is meant to
imply, they are organs of hearing.

The only^ascularsystein^) present in the Platyhelminthes is the
system of waiter-vessels wftich are commonly regarded as perform-
ing an excretory function. The arrangement of these, the mode
of ending internally of the finest branches,
and the way in which the system com-
municates with the exterior vary some-
what in the different groups. A series
of main longitudinal trunks give off
branches which subdivide to form a system
of minute interlacing branches or capil-
laries. In little spaces at the ends of the
capillaries are a number of highly charac-
teristic structures — the ciliary flames. Each
ciliary flame consists of aJbun<llfi-fl£jdhEa-
tile cilia ; typically each is situated in the
interior of a cell — the flame-cell (Fig.
203) — terminating one of the capillary
branches. The finer branches, and in
some cases the larger trunks also, are

intra-cellular, and are to be looked upon as perforations in
linear rows of elongated cells. In the Cestoda, at least the
larger trunks are inter-cellular, being lined by an epithelium of

FIG. 203.— A flame-cell. /.
processes; k. termination
of capillary ; n, nucleus
v. vacuoles ; vf. ciliary
flame. After Lang.)

ex*

fe

254 ZOOLOGY SECT

small cells. This system of water-vessels opens on the exterior
in a variety of different ways : sometimes it opens by a number
of minute pores ; sometimes, as in the Liver-fluke, there is a single
posterior aperture; frequently there are two. In the Tricladida
there are two longitudinal canals which open on the exterior
through special branches by a series of pores. In the Rhabdoccelida
there are either two longitudinal main vessels or a single median
one ; the communication with the exterior in the former case may
be by a pair of ventral apertures, or indirectly through the
pharynx ; or there may be a common short passage in which the
two trunks unite, opening by a posterior median aperture. When
a single main trunk is present it opens at the posterior end of
the body. In the Trematodes there are two principal longitudinal
trunks, which either unite behind to open at the posterior end of
the body,or(Monogenetica) remain separate and open independently
on the dorsal surface, each having, where it opens, a contractile

'cretory sac.

In the Cestodes there are usually four longitudinal trunks, which
open through a contractile excretory sac at the posterior end of
the body. In many cases it has been shown that the main trunks
communicate with the exterior at intervals by means of fine canals.
The excretory sac is thrown off when the last proglottis becomes
separated off and does not in most cases become renewed, though
in at least one species of Tape-worm (Tccnia cucumerina), a new
vesicle is developed again and again at the end of the body as a
fresh segment is thrown off. The main trunks are connected
together by a ring vessel in the head and by a transirr*: Iranch
in each proglottis, and where the latter originate from the main
trunks are valves formed by folds of the wall of the vessel.

The sexes are united in all the Platyhelminthes, with only
one or two exceptions, and the reproductive organs are
sometimes somewhat complicated — presenting a remarkable ad-
vance on those of the Ccelenterata. The male part of the
apparatus consists of testes, with their ducts, the vasa dcfcrcntia, often
with a contractile terminal enlargement or vesicula scminalis, a
cirrus or penis, and often prostate or granule glands. The female
part comprises ovary or ovaries, receptaculum seminis, oviduct, uterus
or ootype, often a bursa copulatmx, shell glands, vitelline or yolk
glands, and cement glands. In most, though not in all, there is an
ovary or ovaries in which the ova are formed, and a set of vitelline
glands or yolk glands producing material which surrounds each of
the mature impregnated ova before it becomes enclosed in its
shell. The shell glands secrete the chitinoid substance of the egg
shells. The cement glands secrete a viscid material for causing
the eggs to adhere together, enclosing them in a cocoon or fasten-
ing them to some foreign body. The oviduct is the passage by
which the ova reach the exterior from the ovary, but an enlarged

v PHYLUM PLATYHELMINTHES 255

part of this passage, into which (usually) the shell glands and
the ducts of the vitelline glands open, is termed uterus or ootype,
while a terminal portion leading from the uterus towards the-
exterior is known as the vagina. The uterus receives the ova
with their investing yolk matter, and retains them frequently
for some considerable time while the shell is being formed and
the earlier stages of development are proceeding : often the uterus ;
is found to contain large numbers of ova, each enclosed in its-
chitinoid shell, in other instances it may enclose only one egg
at a time. The vagina is sometimes a copulatory apparatus for
receiving the penis, and is often provided internally with horny
spines or teeth; sometimes a special sac or bursa copulatrix, lined
with spines, acts as the female copulatory organ. A sac opening
into the distal part of the oviduct, or that most remote from,
the genital opening, is termed receptaculum seminis or receptaculum
vitelli (Fig. 190, r.v.) according as it serves as a reservoir for the1
semen received in copulation or for the vitelline matter or yolk..
Male and female ducts sometimes have separate and independent
openings ; but very commonly there is a common chamber or
genital cloaca into which both lead, opening on the exterior by
a single aperture.

In the Polyclad Turbellaria (Fig. 201) the testes are numerous,,
and there are a corresponding number of fine tubes which
combine to form the two vasa deferentia leading to the male
aperture with its penis. The ovaries consist of numerous small
rounded masses of cells, and there are no separate yolk-glands.
Numerous narrow oviducts lead from the ovaries, and unite to
form larger ducts ; these, in turn, open into elongated uteri, in
which numerous eggs collect. The uteri open into a median
vagina, into which the ducts of the shell glands open, and in which
the eggs receive their chitinoid investment.

In the Tricladida (Fig. 202) there are also numerous testes, but,
the fine tubes connecting them with the two vasa deferentia are
absent. There are only two ovaries, situated far forwards, but
numerous yolk glands. Two oviducts, into which the yolk is dis-
charged from the yolk glands by a series of lateral apertures, lead
from the ovaries to unite in a median chamber or uterus receiving
the ducts of the shell glands.

In the Rhabdocceles (Figs. 200 and 204) there are usually only
two testes and two vasa deferentia leading to the unpaired male
aperture at the extremity of the cirrus. The prostate or granule
glands — a set of unicellular glands, which secrete round bright
granules destined to mix with the sperms — are specially well
developed in the Rhabdocceles, though present in some other
Turbellaria, and in certain Trematodes. Ovaries alone are present
in some, ovaries and yolk glands in others ; there are either two*
ovaries or one only.

25(5

ZOOLOGY

SECT.

Ut

The Trematodes nearly all have two testes, sometimes compact
sometimes branched ; in a few instances there are four. The vasa
deferentia unite into a median duct, which is dilated at the base

of the penis to form a vesicula
seminalis. There is a single
oval or branched ovary, and
two sets of vitelline glands. A
canal termed Laurers canal
leads from the exterior to the
oviduct or vitelline duct.

In the ordinary Cestodes
each segment or proglottis con-
tains a set of reproductive
organs similar to those of a
Trematode. There may be a
single genital aperture leading
into a genital cloaca into which
both male and female ducts
open ; or the male and female
apertures may be distinct
though close together. The
testis is divided into a number
of minute lobes, from which
proceed a number of fine canals,
joining together to form the
vas deferens, at the extremity
of which is the chitinous cirrus.
There are two ovaries, and
either a single vitelline gland,
or two. One or two vitelline
ducts and the two oviducts open into the ootype, from which
the vagina leads to the external aperture. There is in the
Cestodes a uterus distinct from the ootype in the form of a diver-
ticulum of the latter into which the eggs are received; this
becomes greatly enlarged, lobed, and branched in the " ripe "
posterior proglottides, so that it soon becomes by far the most
conspicuous part of the reproductive apparatus — the rest eventually
becoming absorbed. In Bothriocephalus and its allies the uterus
has an independent duct of its own, opening at some distance
from the common genital aperture ; in Caryophyllceus this uterine
duct joins the vagina.

The development of some of the Platyhelminthes (Rhabdoccela,
Monogenetic Trematodes) is direct — i.e., not complicated by the
occurrence of a metamorphosis ; in the Digenetic Trematodes, the
Cestodes, and some of the Planarians a metamorphosis occurs.

The ovum of a Polyclad (Fig. 205) on impregnation divides
first into two equal parts, then into four. From each of these

FIG. 204. Reproductive organs of Ifieso-
stomum Ehrenbergii. dg. duct of
vitelline glands ; do. vitelline glands ; go.
common reproductive aperture ; or. ovary ;
j). penis ; rs. receptaculuni seminis ; " x.
pharynx ; t.,t. testes ; ut. uterus ; vd. vas
deferens. (From Claus, after von Graff
and Schneider.)

PHYLUM PLATYHELMINTHES

257

four cells is then separated off a small cell. The embryo at
this stage consists of eight cells, four large — the megameres, and
four small — the micromeres. The four micromeres increase rapidly
by division, and extend over the embryo, forming a layer, the
ectoderm, completely covering it in all parts except for a median
fissure, the Uastopcre, which runs along what is destined to become
le middle ventral line : this soon closes up. The ectoderm cells

mes

FIG. 205.— Early stages in the development of a Polyclad. A, stage of four cells, of which
those lettered v and h correspond to the anterior and posterior portions of the body ; B to D,
later stages ; B and C, seen from above ; D, from the side ; E, earlier, and F, later stage of
epibolic gastrula, lateral view. ec. ectoderm ; en, endoderm ; mes. mesoderm ; o. en. and u. en.
upper and lower endoderm. (From Korschelt and Heider, after Lang.)

soon develop a coating of cilia. The fourrmegameres then give off
four more small cells or micromeres. which increase in number by
division, and eventually form the middle layer or mesoderm of the
embryo. The megameres give off a number of additional micro-
meres, which form the endoderm layer, giving rise to the epithelium
of the intestine ; finally they become disintegrated, and their sub-
stance goes to nourish the cells of the developing embryo. The
process by which the germinal layers have become formed is, as in
the Ctenophora (p. 205), a process of epibolic gastrulation, and is to

VOL. I S

258

ZOOLOGY

SECT.

be contrasted with the enibolic gastrulation of Sycon (p. 114). In
many cases the embryo develops into a characteristic larval form
known as Mutter's larva (Fig. 206). It assumes an oval shape, with
a series of eight elongated processes, covered with long cilia, and
connected together by a ciliated band. There are eye-spots at
the anterior end, and a mouth in the middle of the ventral

FIG. 20*3. — M tiller's larva. A, longitudinal section; B, lateral view. cc. ectoderm; t'/i..
endoderm ; g. brain ; hd. enteron ; o. mouth ; ph. pharynx ; pt. pharyngeal pouch ; sn. suckeiv
(From Lang).

'

surface. The form of the body alters after a time, becoming

Gradually longer and flatter, and the arms become gradually re-
uced in length, till, eventually, they become completely absorbed.
Little is known of the development of the Rhabdocceles or of
the Monogenetic Trematodes : in both groups development is
direct, and in many the young animal, when it escapes from the
interior of the egg, has already assumed the form of the parent.
Gyrodactylus (Fig. 189, A) is peculiar in being viviparous ; before
the embryo is born a second has become developed in its interior
and a third in the interior of that again. Even more remarkable
is the case of Diplozoon, in which two young animals become fused
together to form a double mature individual.

An account has already been given (p. 230, Fig. 179) of the
development and metamorphosis of the Liver-Fluke (Distomum-
hepaticwm), which may b^4ootoi-aipjQn as typical of thejjigenetic
Trematodes in general.

As in the majority of the Platyhelminthes, the egg-shell of the
Digenetic Trematodes, formed from the secretion of the shell-
glands, encloses not only the ovum derived from the ovary, but
also a number of small yolk-cells contributed by the yolk-glands ;
these soon lose their ceil character. The ovum has been fertilised
by a sperm before the shell is formed, and undergoes division into
a number of cells which gradually displace and absorb the yolk,
the latter taking no direct part in the development of the embryo,,
but serving only for its nourishment in the earliest stages. The
embryo escapes from the egg as a ciliated larva, which develops in
the interior of the second or intermediate host into a sporocyst.
This may multiply by budding or fission. The central cells of the
larva, which are regarded as ova that develop parthenogenetically,.

.

PHYLUM PLATYHELMINTHES 259

give rise to the development of rcdice in the interior of the
sporocyst, and these give rise, either directly or through the inter-
mediation of a second generation of recline, to tailed cercaricc. The
cercaria3 escape from the body of the intermediate host, and, in
many cases, instead of merely becoming attached to herbage, as
in the Liver-Fluke, make their way into the body of a second
intermediate host (usually an invertebrate), and there become
encysted. The encysted larva is eventually taken into the interior
of the final host (generally a vertebrate) by the second intermediate
host being swallowed by the latter : the cyst-wall becomes dis-
solved, and the young Trematode becomes free, and develops into
the sexually mature condition, either in the interior of the alimen-
tary canal itself, or in some related part. There is thus to be
recognised in the Digenetic Trematodes an alternation^of^eneratiGns
comparable to that which has been described as so general in the
Ccelenterata. In the Trematoda, however, it is 1xrbe-observed, it
is~~anT alternation of a sexual, not with art-asexual, but with a
parthenogenetic generation (the "sporocyst), the ova of which develop
into a second parthenogenetic generation (the redire) ; and these
finally produce larva? (the cercarise) capable of developing into the
sexually mature form. The term heterogeny is applied to a life-
history of this kind, in which several distinct generations succeed
one another in a regular series.

The egg of a Cestode is similar in essential respects to that of
a Trematode : there is a tough, chitinoid membrane or egg-shell,
which encloses not only the ovum but a number of yolk-cells, the
latter becoming absorbed as the process of segmentation of the
ovum goes on. The result of segmentation is the formation of a
superficial layer of cells (ectoderm) and a central mass, all enclosed
in a membrane composed of a single layer of cells thrown off when
vthe embryo escapes from the egg. The ectodermal cells become
ciliated, so far as is known, only in Botlirioceplialus ; in the others
they become thrown off or ultimately absorbed without develop-
ing cilia. The central mass of cells alone forms the embryo. The
embryo, while still consisting of a small number of cells, develops
a series of six chitinous hooks. These early changes all take place
in the majority while the egg is still in the uterus of one of the
most posterior of the proglottides of the parent worm. When the
progiottis in question becomes separated off, and has passed out
from the body of the final host, the eggs become discharged.
In order that development may proceed further the embryo must
reach the interior of a second or intermediate host. This is a
passive migration, since the embryo of the Cestode is still confined
within the egg-shell, and the transference has to take place in the
water or food. The digestive fluids of this intermediate host
dissolve the egg-shell and set free the contained six-hooked
Kexacanth embryo, which bores its way by means of its hooks to

s 2

2GO

ZOOLOGY

SECT.

some part- of the body in which it is destined to pass through the
next phase in its life-history, and there becomes encysted. The
phase which follows presents two main varieties. In cases in
which the intermediate host is an invertebrate animal the hooked
embryo develops into a form to which the name of Cysticercoid
is given ; when, on the other hand, the intermediate host is a
vertebrate, the form assumed is nearly always that termed Cysti-
I eercus, or bladder-worm. The cysticercoid (Figs. 207 and 208)
( form is to be regarded as the more primitive and less modified.
Cysticercoids of various tape-worms occur in a great variety of
different invertebrates, e.g. Insects of all kinds, Water-fleas, Cen-
tipedes, Earthworms. The hooked embryo loses its hooks and
develops into the cysticercoid in some part of the invertebrate
intermediate host. The cysticercoid consists of three parts —
\ I a tape-worm head or scolex, with the hooks and suckers of the
U mature worm, a so-called body, and a caudal vesicle. Sometimes
^ there is a tail recalling to some extent the tail of a cercaria.
Sometimes the caudal vesicle is absent : when present it, either

from the first, or as a result

<3^ of later changes, encloses the.

head as well as the body after
the manner of a cyst. While
undergoing these changes the
cysticercoid is usually en-
closed in an adventitious cyst
formed for it by the tissues
of its host, but it often lies
free in the body cavity. The
transference to the final host
is effected by the intermediate
host, or the part of it con-
taining the cysticercoid, being
taken into the alimentary
canal of the final host. Some-
times, if the intermediate host
is a relatively small animal,
such as a water-flea, this may
take place " accidentally " ; in
other cases the invertebrate
intermediate host actually
forms the food of the final

host. Thus a cysticercoid having as an intermediate host an
Earthworm is taken with the latter into the alimentary canal of
a Sea-Gull — its final host. In this way the cysticercoid is set
free in the alimentary canal of the final host, the head becomes
pushed out from the enclosing caudal vesicle and body (probably
owing to the stimulus of the higher temperature) so that the

FIG. 207.— A Cysticercoid (Polyccrcus) wfth
the head and rostellum enclosed by the
caudal vesicle, a. aperture through which
evagination takes place ; Id. body ; c. cavity
of cyst ; caml. caudal vesicle ; ex. aperture
of excretory system ; ros. rostellum ; s.
sucker.

PHYLUM PLATYHELMINTHES

261

res

suckers and hooks come into play and attach the young tape-
worm to the wall of the alimentary canal.

The cystieercus or bladder-worm differs from the cysticercoid
mainly in its much greater sizei
and in the development of a re-f
latively large caudal vesicle oy
caudal bladder. When the hookem
embryo has reached that part of
the vertebrate host in which it
is destined to develop into the
cystieercus it undergoes a re-
markable change ; it becomes
greatly enlarged, and a cavity,
rilled with fluid or with a very
loose form of connective-tissue,
appears in its interior, so that
it assumes the appearance of a
relatively large, bladder. On one
side of this bladder appears a
small invagination with a cavity
opening freely on the exterior.
On the bottom of this is formed
an elevation projecting into its
interior ; this is the rudiment of
the rostellum on which the
hooks are borne ; at its base, on
the inner surface of the side
walls of the invagination, ap-
pear the suckers. When everted this invagination corresponds
closely with the head and body of the cysticercoid ; the bladder
corresponds to the caudal vesicle. The chief difference between a
cystieercus and a cysticercoid is that in the former the caudal 1
vesicle is relatively very large, and that the order of development \
of the parts is somewhat modified.

A very small number both of cysticercoids and cysticerci
multiply by proliferation — by the formation of more than one
tape-worm head from one embryo. In the few instances in which
this occurs among the cysticercoids the hooked embryo gives rise,
not directly to a1 cysticercoid, but to a mass of cells, from which
are given off a number of buds, each developing into a cysticercoid
\vith the three parts already described. One such form occurs in
certain Earth-worms ; another in a Myriapod (Glomeris Hnibatus).

Tcenia ccenumis of the Dog has a bladder- worm . stage, in the
Sheep and Rabbit, which gives rise to several tape-worm heads.
But the best known instance of multiple production of scolices in a
cystieercus is Tcenia ecJiinococcus — well known as cause of the
disease termed hydatids, common in Man and in various domestic

ccutd

Fin. -'OS.— A Cysticercoid, with the
head evaginated. ro*. rostellum ; s., s.
suckers ; mt'.d. caudal vesicle.

ZOOLOGY

SECT.

animals. In this case the hooked embryo develops into a 'large
mother-cyst, from the interior of which daughter-cysts are' budded off
(Fig. 209). Eventually from the walls of these daughter-cysts there

. -JO1.'.— Cyst of Tsenia ecbinococcus with the developing daughter-cysts and scolices.

(After Leuckart.)

are formed numerous tape-worm heads, or scolices (Figs. 210 and 211)
which, when fully formed, assume the appearance of cysticercoids
without the caudal vesicle. These are readily detached, and, should
the organ in which the cyst has been developed be devoured by a
Dog — which is the final host of the parasite — some of these scolices

become attached to the wall
of the intestine and de-
velop into the — as compared
with the size of the cyst, and
as compared with other tape-
worms— very small adult
Tcrnia cchinococcus. The eggs

.' 10.— Scolices of T. echinococcus.
(After Cobbold.)

FK;. 211. — Separate scolux of
T. echinococcus. (After
Cobbold.)

which the latter produces, passing out with the faeces of the Dog,
may be taken into the digestive canal "of Man or- of one of the
domestic animals, and the minute embryos escaping reach some
organ, such as the liver or lung, in which they are capable of
developing into a comparatively enormous cyst.

PHYLUM PLATYHELMINTHES

263

Asexual reproduction also occurs in some Platyhelminthes.
In some Rhabdocosle Turbellaria (Microstomum) a process of bud-
ding (Fig. 212) results in the formation of strings of sexual indivi-
duals which may eventually separate ; the new bud is always
formed from the posterior end of the last individual of the string.

The sporocyst stage in the Trematodes may, as already men-
tioned, multiply by budding or fission. The formation of new
proglottides in the Tape-worm may be
looked upon either simply as growth ac-
companied by segmentation, or as asexual
multiplication, according as we regard
the proglottides as segments of a simple i

animal or zooids of a colony. There is
this essential difference between this
formation of proglottides and the asexual
multiplication by budding in Micro-
stomum, that in the former the proglot-
tides, when they have been formed by
segmentation of the undivided part be-
hind the head, do not in turn give rise by
budding to new proglottides. • Spontane-
ous transverse fission has been observed
in certain Tricladjda, and has often been
observed to be followed by the regenera-
tion of the lost portion.

6. DISTRIBUTION, MODE OF OCCURRENCE,
AND MUTUAL RELATIONSHIPS.

Of all the great^*roups of the animal
kingdom above the Protozoa the Platy-
helminthes are the widest in their distri-
bution. Members of the phylum occur on
land, in fresh-water down to the bottom
•of some of the deepest lakes, on the sea-
shore, in deep sea, and on the surface of
the ocean ; and parasitic Flat-worms live,
in one phase or another, in animals
of nearly every class of the Metazoa.

As regards their mode of life, they present almost every possible
gradation between free-living forms which procure their food, con-
sisting of minute animals and plants, by their own exertions, and
forms that are only capable of living in a special part of the
interior of ascertain other animal, and are quite incapable of pro-
curing food for themselves, living by the passive absorption of the
juices of their host or of its digested food. The Turbellaria are
for the most part free living, and their food consists of small

FIG. 212.— Process of budding
in Microstomum. c., c.'
ciliated groove ; e. eye-spot ;
i. intestine ; m., m.', m.", m."'
mouth. .(After Von Graff.)

264 ZOOLOGY SECT.

Crustacea or the larvae of larger forms, Insect larvae, Water-mites,
Rotifers, small Worms, and the like ; sometimes of Diatoms and
minute Algae of various kinds. Some, however, live a life of true
parasitism. Such are certain Rhabdocoeles which are parasitic in
the alimentary canal of various Holothurians and Gephyreans
(vide infra, Sect. IX.). In these there is, correlated with the inactive
mode of life, a tendency to degradation of structure, a degradation
which is characteristic of parasites in general ; the pharynx is
reduced in size as compared with that of non-parasitic allied
forms, not being required for the capture and swallowing of living
prey ; and the eyes, useless to an animal living in complete dark-
ness, are absent. Some of the Turbellaria, though not parasitic
in the strict sense, live in a state of commensalism with another,
larger animal, that is to say, are more or less constantly associated
with it, living on its surface or in one of its cavities that open
freely on the exterior, and often sharing its food. An example of
this mode of life is the Triclad Bdelloura, which lives on the surface
of the King-Crab (Limulus).

While a free existence is the rule in the Turbellaria, true
parasitism is the rule in the Trematodes, and is universal in the
Cestodes. The majority of the Monogenetic Trematodes are ex-
ternal parasites, living on some part of the outer surface of some
larger animal, and feeding on mucus and other secretions of the
integument. Many are parasites on the gills of Fishes. A few,
however, inhabit the interior of various organs, and are true
internal parasites : one, for example (Polystomuni), lives in the
urinary bladder of the Frog ; another, (Aspidogaster) lives in the
pericardial cavity of a Fresh-water Mussel. At least one family of
the Monogenetica (the TemnocepTialece) are not parasites at all in
the strict sense of the term, living on the surface of the " host "
animal, depositing their eggs there, and being carried about by it,
but subsisting on minute living animals captured in the water.

The Digenetic Trematodes are all internal parasites, and in the
adult condition inhabit, in nearly all cases, the alimentary canal, or
the liver, or the lungs of some vertebrate animal, swallowing the
digested food or various- secretions of their host. But, as mentioned
before in the account given of their development, they are internal
parasites, not only in the adult condition, but throughout the
greater part of their life. After a short period of freedom as
ciliated larvae, they again enter into a state of parasitism as sporo-
cysts or rediae in a second host, and, after a second free interval as
cercariae, may enter the body of a third to become encysted. The
second host is, very generally, a Mollusc, and the cercaria may
become encysted in the same animal.

The Cestodes are, of all the Platyhelminthes, those that are most
modified in accordance with the condition of internal parasitism
in which they remain throughout life. The adult Cestode is

v PHYLUM PLATYHELMINTHES 265

almost always an inhabitant of the alimentary canal of a verte-
brate. The intermediate host is frequently also a vertebrate —
commonly of a kind which is liable to become the prey of the final
host. In the case of Tcenia crassicollis of the intestine of the
domestic Cat, for example, the Cysticercus stage occurs in the
livers of Rats and Mice ; the Cysticercus of Tcenia scrrata of the
Dog is found in Hares and Rabbits ; and so with other tape-worms.
But in many cases the intermediate host is an invertebrate. In
either case the passage from one host to another is a passive
translation, not an active migration as in the Trematodes.

A few human parasites belong to theTrematoda; but none that are
of very common occurrence among Europeans. Distomum hepaticum
has occasionally been found in the human liver. D. ratlwusii
is a common intestinal parasite in China. D. sinense occurs in
the liver of Man in China and Japan. Distomum lanccolatum
and various other species of the genus occasionally occur in the
human subject. BUharzia hcematobium, which differs from most
other Trematodes in being unisexual, is found in the portal system
of veins among the natives of Abyssinia.

The commonest human Cestode parasites among Europeans are
Tcenia solium and T. saginata (otherwise called T. mediocancllata).
The Cysticercus stage of T. solium — Cysticercus cellulosce — occurs,
as already stated, chiefly in the muscles of the Pig; that of
T. saginata in the muscles of the Ox ; and the relative prevalence
in different countries of these two Tape-Worms varies with the
habits of the people with regard to flesh-eating : where more
swine's flesh is eaten in an imperfectly cooked state Tcenia solium
is the more prevalent ; where more beef, T. saginata. Bothrioceplialus
latus, a very large tape-worm without hooks, and with a pair of
longitudinal sucking-grooves on the head, instead of ordinary
suckers, is a common human parasite in eastern countries. Its
Cysticercus, which is elongated and solid, occurs in the Pike and
certain other fresh-water Fishes.

Of all the Cestode parasites of man, however, the most formid-
able is one which occurs in the human body, not in the sexually
mature or strobila condition, but in that of the Cysticercus. This
is Tcenia echinococcus, the presence of which produces what is termed
hydatid disease. The adult Tcenia echinococcus is a very small tape-
worm with only three or four proglottides, occurring in the intes-
tine of the domestic Dog. The eggs passing out with a liberated
proglottis in the faeces, may reach the alimentary canal of Man
uninjured in drinking-water, on the surface of salad vegetables, and
the like; and, the egg-shells becoming dissolved,, the contained
hooked embryos bore their way to the liver or the lungs or some
other organ. Arrived at its final destination, the embryo develops
into a cyst, which may become of enormous size. In the interior
of the primary or mother-cyst are developed a number of secondary

26(5 ZOOLOGY SECT.

or daughter-cysts, and from the walls of these, both internally and
externally, are formed very numerous scolices in the way that has
already been described (p. 262). Hydatid cysts are very common in
some domestic animals — Oxen, Sheep — as well as in Man. Various
other Cestodes occur in the bladder-worm stage occasionally in
Man — e.g. the Cysticercus cellulosoe of Tcenia solium.

The most primitive of the Platyhelminthes are, without doubt,
some of the simplest Turbellaria, and it is among these that we
are to look for the nearest existing relatives to the Ccelenterata.
In none, however, is the relationship very close. Cmloplana
and Ctenoplana (p. 212) are, probably, rather to be looked upon
as Ctenophores specially modified in accordance with a creeping
mode of progression than as intermediate forms between Cteno-
phores and Turbellaria. The relationship with the Coelenterata is
shown, perhaps, most strikingly when we take into account the
development of the Turbellaria, in the earlier stages of which there
is to be recognised a marked tendency towards a jcadial symmetry.
In their development the Turbellaria, that is to say the Planarians,
show some special points of resemblance to the Ctenophora ; the
ectoderm cells are formed and spread over the rest in a similar
way, and the bands of cilia have a disposition and mode of move-
ment that strongly bring to mind the ciliary swimming plates of
the Ctenophora. But though there is much to be said in favour
of the view that the Turbellaria and the Ctenophora were derived
from a common, not very distant stock, the latter are too specially
modified to be looked upon as the direct ancestors of the former.

The connection between the Turbellaria and the Monogenetic
Trematodes is very close — so much so that it is difficult to give
any characters of universal occurrence distinguishing all the
members of the two classes. The Trematodes are, in fact, to be
looked upon as Turbellaria some of whose external characteristics
and, in the case of the Digenetica, whose life-history, have been
specially modified in accordance with a parasitic mode of life. It
is not unlikely that the Trematodes may be a pplyphyletic group —
i.e., that different families may have become developed from
different families of Turbellaria altogether independently, some
of them appearing to be nearer the Rhabdocoeles, others nearer
the Polyclads, others, again, nearer the Triclads, in the majority of
their characters.

The remarkable life-history of the Digenetic Trematodes is, as
already pointed out, to be looked upon as a special form of alter-
nation of generations — the alternation of a sexual with a pcedo-
genctic and parthenogenetic, one (heterogeny). The sporocyst and
redia are to be looked upon as intercalated stages — as cercarise
which exhibit paedogenesis. The cercaria is the characteristic
]ar*;il stage of the Trematodes, and corresponds to the cysticercus

PHYLUM PLATYHELMINTHES

267

or cysticercoid of the Cestode. The most important difference
between these is in the presence in the former of an enteric cavity,
.and its absence in the latter. There seems to be something to be
said in favour of the view that the enteric cavity of the cercaria is
represented by the frontal sucker of some scolices, and by the
tellum of the majority.

Between the adult Cestodes and the Trematodes an intimate
relationship is traceable. Ca.ryophyllceus (Fig. 195) is a Cestode
which, but for the absence of an enteric cavity and the want

NemerMnea

Poly clad I da.
Trlcladida / Monogeneh'ca

Digeneh'ca
Cesfoda

Rhabdoeoelida
Ctenophora

Lower Coelenfer&l'es

FIG. 213. — Diagram of the relationships of the Platyhelminthes (together with the Ncmertinea).

of organs of adhesion at the posterior end, is not far distant
from the Trematodes ; and the same might be said of Amphip-
tyches (Fig. 196), Amphilina, and Archigetes (Fig. 197). The most
important differences between a Cestode and a Trematode, in
addition to the absence of an enteric cavity in the former and
its presence in the latter, is the occurrence in the Cestodes of stro-
bilation. Ligula in a certain sense forms a connecting link in this
respect between the Trematode and the ordinary Cestode, thp
oody being elongated, and the reproductive organs repeated as
in the normal Tape- Worm, but without the corresponding division
of the body into a string of definitely separated proglottides.

268

ZOOLOGY

SECT.

Ir

m,

APPENDIX TO PLATYHELMINTHES.

CLASS NEMERTINEA.

General Features. — The Nemerteans are non-parasitic, unseg-
mented worms, most of which are marine, only a few forms living
on land or in fresh-water. They are commonly looked upon as nearly
related to the Turbellaria, and were formerly included in that
class ; but they are in some respects higher in organisation than

the Turbellaria, and they ex-
hibit certain special features
distinguishing them from the
rest of the lower Worms.

The body (Fig. 214) is nar-
row and elongated, cylindrical
or depressed, unsegmented, and
devoid of appendages. In length
it varies from a few millimetres
to as much as ten metres. The
entire surface is covered with
vibratile cilia. Frequently the
integument is vividly coloured.
The cells of the epidermis secrete
a mucous matter, which may
serve as a sheath or tube for the
animal. The mouth (m.) is at or
near the anterior extremity on
the ventral aspect. Close to it
above there is an opening,
through which can be protruded
a very long muscular organ, the
proboscis (pr.), the possession of
which is one of the most char-
acteristic features of this class of
Worms. The proboscis is hollow ;
when it is extended to its ut-
most, a part still remains which
is not capable of being everted,,
and at the junction between the
eversible and non-eversible parts,,
i.e. at the extremity of the
organ when it is fully protruded,,,
there is in many of the Nemer-
teans a pointed or serrated stylet (Figs. 215. 217, and 218),
which probably permits of the proboscis being used as a weapon :
when a stylet is absent, th« surface of the extremity is sometimes

ctir

Fio. 214.— Diagram of the organs of a,
Nemertine, from below, a. anus ; Itr.
brain ; div. coeca ; long. ne. longitudinal
nerve-cords ; m. mouth ; n. nephridia ;
ov. ovaries ; pr. proboscis. (After
Hubrecht.)

PHYLUM PLATYHELMINTHES

269

abundantly provided with
stinging-capsules ; sometimes
it is beset with glandular
adhesive papilla;. Posteriorly,
this non-eversible part of
the proboscis passes into a
retractor muscle, by means of
which the whole organ is
capable of being retracted
within the interior of an in-
vesting sheath, the proboscis
sheath (Figs. 216 and 219,
p. s.). It is by the contraction
of the muscular walls of this
sheath that the proboscis is
everted ; sometimes the con-
traction takes place with
such force that the proboscis
is broken off entirely from
the body. The abundant
nerve-supply of the proboscis
points to its use being
mainly as a tactile organ.

The alimentary canal is
a simple tube distinguish-
able into oesophagus (Fig. 214)
with longitudinally folded
walls, and intestine with
lateral cceca (div.). It ends in
an anal opening (a) situated
near the posterior extremity
of the body.

The outermost layer of
the integument is an epi-
' dermis of ciliated cells, with
enclosed unicellular glands
containing bodies similar to
the rhaldites of the Turbel-
laria. Beneath this is some-
times a thin homogeneous
basement membrane ; then a
layer of longitudinally ar-
ranged muscular fibres,
among which are unicel-
lular glands with long ducts
that perforate the epidermis,
together with pigment. From
this layer there is a gradual

f>ro I afo

proV-

retr.mus

an

FIG. 215.— Tetrastemma. General view of the
internal organs. an. anus ; ac. st. accessory
stylet ; cer. g. brain ; cil. gr. ciliated groove ;
dors. res. dorsal vessel ; lat. ne. lateral nerve ;
tat.ves. lateral vessel ; neph. nephridium ; op.neph.
nephridial aperture ; probl. eversible part of
proboscis ; proV*. non-eversible part of proboscis ;
prob. ap. aperture for the protrusion of the pro.
boscis ; re.tr. mus. retractor muscle of the pro.
boscis ; st. stylet. (From Hatschek's Lehrbuch.)

270

ZOOLOGY

SECT.-

transition to a layer consisting altogether of longitudinal muscular-
fibres. A layer of circularly arranged muscular fibres always is
present ; but the arrangement of the
layers of muscle varies in different forms.
The muscular layers are embedded in
compact connective-tissue, and a mass of"
the same tissue fills all the space between
the body- wall and the enteric cavity,
there being no body-cavity. Vertical
muscular dissepiments extend across be-
tween the intestinal coeca and produce an
appearance of internal segmentation.

The Nemerteans possess a system of
blood-vessels (Fig. 220) with" well-de-
fined walls formed of an epithelium and
a layer of muscle. There are three prin-
cipal longitudinal trunks — a median dorsal
(mcd. U. v.) and two lateral (lat. U. v.). The

FIG. 216.— Anterior portion of
the body of a Nemertine.
br. brain-lobes; n. lateral blood follows no regular COUrS6 through

nerves ; p. o. external open-
ing through which the pro-
boscis is everted ; p. s. pro-
boscis sheath ; pr. proboscis.
(Esophagus and mouth shown
by dotted lines. (After
Hubrecht.)

the vessels, but is moved about by the
muscular contractions of the body. The
blood is colourless, and contains rounded
or elliptical corpuscles.

The excretory vessels of the Platy-

helminthes are represented by a pair of greatly coiled and
branched tubes (Fig. 220, neph.), opening on the exterior : the-
fine terminal branches of the system are provided with ciliary

FIGS. 217 and 218. — Proboscis of a Hoplonemertean, with stylet reserve-pr.cs and muscular •
bulb. Fig. 217 retracted, Fig. 218 everted. (After Hubrecht.)

flames, and cilia occur also in the course of the vessels them-
selves.

The nervous system is in some respects more highly developed
than in the Turlellaria. The brain (Fig. 216, br.) is composed of"
two large ganglia with lobed surfaces, connected together by two*

PHYLUM PLATYHELMINTHES 271

commissures, dorsal and ventral, between which pass the proboscis
and its sheath. From the brain pass backwards a pair of thick
nerves which run throughout the length of the* body; usually.

c.t

c.t

lang.n

long.ne

l.Tyv

FIG. 219. — Diagrammatic transverse section of a Nemertean (Carinella). a,b,c. layers of
body-wall ; c. t. connective tissue between body-wall and enteron ; 1. bv. lateral blood-vessels ;
long. ne. longitudinal nerve ; p. proboscis ; p. s. proboscis sheath. (After Hubrecht.)

these are lateral in position, sometimes approximated dorsally,
sometimes ventrally. In the Nemerteans devoid of stylet there is
a nerve-plexus between the muscular layers. In the sty let -bearing
forms such a plexus is absent, but metamerically arranged branches
are given off by the nerve-cords ; these divide into smaller nerves-

mph

tetblv

mzd.hlv

FIG. 220.— Anterior portion of a Nemertean (Drepanophorus), showing the blood-vascular
and excretory systems. lot. bl. v. lateral blood-vessels ; rued. bi. v. median blood-vessels ;
neph. nephridial (excretory) tubes. (After Oudemans.)

for the supply of the various organs. Sometimes the lateral cords
unite behind above the anus.

A remarkable apparatus connected with the nervous system is
the system of lateral organs. These consist of a pair of ciliated
tubes (Fig. 215, cil. gr.) opening externally at the sides of the head,

ZOOLOGY SECT.

usually in a groove or slit, and ending internally in the interior of
a pair of posterior brain lobes, which are sometimes distinct from,
sometimes united with, the rest of the brain. The posterior brain
lobes are developed in the larva as outgrowths from the oeso-
phagus ; the ciliated tubes appear quite independently of them
as ingrowths from the epidermis of the head, the two structures

FIG. 221. — A, Pilidium with advanced Xcmerthie worm ; B, Rips embryo of Nemertes from
interior of Pilidium. an. amnion, or investment of the embryo ; i. intestine ; lp. lateral pit ;
/'.nervous system ; «. gullet ; p>: proboscis ; st. stomach. (From Balfour, after Butschli.)

only subsequently coming into relation with one another, and the
outgrowths from the oesophagus usually altering their cellular
structure so as to become converted into nerve-tissue. This
apparatus is probably a special arrangement for oxygenating the
brain substance ; but perhaps it has also a sensory function.

Eyes are present in the majority of Nemerteans, and, in the
most highly organised species occur in considerable numbers.

:

PHYLUM PLATYHELMINTHES 273

metimes they are of extremely simple structure, consisting
merely of spots of pigment ; in other cases they are more highly
developed, having a spherical refractive body with a cellular
" vitreous body," and a " retina " consisting of a layer of rods
enclosed in a sheath of dark pigment, each rod having a separate
nerve-branch connected with it. Otocysts containing otoliths
have been found in only a few of the Nemerteans.

Reproductive System. — Most species are dioecious. The ovaries
(Fig. 214, ov.) and testes are situated in the intervals between the
intestinal cceca. The ovary or testis is a sac the cells lining which
give rise to ova or spermatozoa ; when these are mature each sac
opens by means of a narrow duct leading to the dorsal surface,
where it opens by a pore.

Development.— Some of the Nernerteans go through a meta-
morphosis ; in the others the development is direct. The charac-
teristic larval form is the Pilidium (Fig. 221.) This is a helmet-
shaped body with side lobes representing ear-lappets, and a bunch
of cilia representing a spike. In the metamprphosis two pairs of
ectodermal invaginations, growing inwards around the intestine,
fuse together and form the integument and body- wall of the future
worm, Avhich subsequently frees itself from its investment and
develops into the adult form. In others there is a ciliated
creeping larva called the " larva of Desor" in the interior of
which the larval worm is developed much as in the, case of the
Pilidium.

Though none of the Nemerteans exhibit metameric segmenta-
tion, yet in some of them there is, as in Gunda segmentata
(p. 241) among the Turbellaria, a serial repetition of the in-
ternal parts (pseudo-metamerism). Transverse fission is of
frequent occurrence.

DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Nemertinea are ciliated, unsegmented, worms with elongated
body, without distinct coelome. There is an eversible proboscis
enclosed in a sheath and capable of being protruded to a great
length through an aperture situated in front of and above the
mouth. The intestine usually has distinct lateral diverticula, and
there is a posteriorly situated anus. There is a blood- vascular
system and also a system of excretory vessels.

Sub- Class I — Palseonemertinea.

Nemertinea in which the head is devoid of deep lateral longi-
tudinal grooves, and in which the proboscis is not armed with a

stylet

VOL. I T

274 ZOOLOGY SECT, v

Sub-Class II, — Schizonemertea.

Nemertinea in which there is a deep lateral longitudinal groove
on each side of the head and in which the proboscis is devoid of
stylets.

Sub-Class III. — Hoplonemertea.

Nemertinea in which there are no deep lateral grooves on the
head and in which (except in Malacobdellci) the proboscis is armed
with a stylet.

The Nemerteans are almost exclusively marine, only a small
number of species living in fresh water or in damp localities on
land. None appear to be true parasites; but a few are com-
mensals, living for the most part in the pharynx or atrial cavity of
Ascidians or Sea-Squirts. All are carnivorous, and either capture
living prey in the shape of small-bodied invertebrates of various
kinds, or feed on dead fragments.

SECTION VI
PHYLUM NEMATHELMINTHES

THE members of the preceding phylum are characterised, as a
whole by a marked dorso-ventral flattening In the Worms in-
cluded in the present group the body is elongated and cylindrical,
whence their general name of Round- or Thread-worms. The
phylum includes the following classes : —

Class 1. NEMATODA. — The Round- worms in the strict sense of
the term. The best known forms are internal parasites, but many
genera and species are extremely abundant in fresh- and salt-
water.

Class 2. ACANTHOCEPHALA. — The " Hook-headed Worms," a
group of formidable internal parasites.

Class 3. CELETOGNATHA. — The " Arrow- worms," a small group of
pelagic organisms.

The affinities of the Acanthocephala and Chastognatha with the
Nematoda are somewhat doubtful, and the association of the three
classes is largely a matter of convenience.

CLASS I,— NEMATODA.

IL. EXAMPLE OF THE CLASS — THE COMMON ROUND-WORM OF
MAN. (Ascaris Innibricoides).
Ascaris lumbricoides is a common parasite in the human intes-
ne : a closely allied if not identical form (A. suilla) occurs in the
ig, and another (A. megalocephala) in the Horse. The following
uescription will apply to any of these. The female worm is about
12-28 mm. (5-11 inches) long, and about 6-8 mm. (J inch) in
diameter ; the male is considerably smaller.

External Characters. — When fresh the animal is of a light
yellowish-brown colour: it is marked with four longitudinal
streaks,' two of which, very narrow and pure white in the

T 2

276

ZOOLOGY

SECT.

living Worm, are respectively dorsal and ventral in position, and
are called the dorsal (Fig. 222, d.l.) and ventral (v. 1.) lines : the other
two are lateral in position, thicker than the former, and brown in
colour, and are distinguished as the lateral lines. The mouth is
anterior and terminal in position, and is bounded by three lobes,
or lips, one median and dorsal (d. lp.), the other two ventro-lateral
(v. lp.). A very minute aperture on the ventral side, and about 2
mm. from the anterior end, is the excretory pore (ex. p.). At about
the same distance from the pointed and down-turned posterior end
is a transverse aperture with thickened lips, the anus (an.), which
in the male serves also as a reproductive aperture and gives exit
to a pair of needle-like chitinoid bodies, the penial setw (pn. s.).
In the female the reproductive aperture or gonorpore is separate
from the anus, and is situated on the ventral surface about one-

FIG. 222.— Ascaris lumbricoides. A, anterior end from above ; B, the same from below ;
C. posterior end of female, D.'of male, side view an. anus ; <>. //>. dorsal lip ; </. 7. dorsal line ;
ex. p. excretory pore ; p. papillse ; pn. s. penial seta? ; v. I. ventral line ; v. tp. ventral lip.
(After Leuckart.)

third of the length of the body from the anterior end (Fig. 225,
gnp.). The sexes are also distinguished externally by the form of
the short tail, or post-anal portion of the body, which in the male
is sharply curved downwards (Fig. 222, D), while in the female (C)
its ventral contour is nearly straight.

Body-wall. — The outer surface of the body is furnished by a
delicate, transparent, elastic membrane, of a chitinoid nature, the
cuticle (Fig. 223, cu.). It is divisible into several layers, ancl is
wrinkled transversely, so as to give the animal a segmented ap-
pearance. Beneath the cuticle is a protoplasmic layer (der. epthm.)
containing scattered nuclei and longitudinal fibres, and represent-
ing a syncytial ectoderm — i.e. an ectoderm in which the cell-bodies
are not differentiated, and its cellular nature is recognisable only
by the nuclei. The cuticle is, as usual, a secretion of the
ectoderm.

Beneath the ectoderm is a single layer of muscular fibres (m.\

VI

PHYLUM NEMATHELMINTHES

277

arranged longitudinally, and bounding the body-cavity. The
structure of the muscles is very peculiar : each (Fig. 224, A) has
the form of a spindle, striated longitudinally, and produced on its
inner face (i.e. towards the ccelome) into a large and almost blad-
der-like mass of protoplasm (p) containing a nucleus (me.). Ap-
parently the whole of this structure is derived from a single cell,
part of which has become differentiated into contractile substance
(e), the rest remaining protoplasmic. In transverse section the
contractile portion (B. c) has the form of a plate bent upon itself
so as to be, as it were, wrapped round the protoplasmic portion
(p). The protoplasmic processes project to a greater or less extent

iitt

ovy

v.w

FIG. 223.— Ascaris lumbricoides, transverse section, cu. cuticle ; d. 1. dorsal line ; der. epthm.
deric epithelium or epidermis ; ex. r. excretory vessel ; int. intestine ; hit. I. lateral line ; m.
muscular layer ; ory. ovary ; ut. uterus ; v. v. ventral line. (After Vogt and Yung.)

into the body-cavity, sometimes practically obliterating it, and are
produced into delicate filaments (/.) which take a transverse
direction and are mostly inserted into the dorsal and ventral
lines.

The muscular layer is not continuous, but is divided into four
longitudinal bands or quadrants, two dorso-lateral and two ventro-
lateral, owing to the fact that at the dorsal, ventral, and lateral
lines the ectoderm undergoes a great thickening and projects
inwards, between the muscles, in the form of four longitudinal
ridges (Fig. 223, d.L, v.v., lat. /.). It is this arrangement that gives
rise to the lines seen externally. The ridges forming the lateral
lines ^tre much more prominent than the other two.

278 ZOOLOGY SECT.

Digestive Organs. — The mouth leads into the anterior
division of the enteric canal, the pharynx or stomodceum (Fig. 225
ph.) : its walls are very muscular, its cavity is three-rayed in cross-
section, and it is lined by a cuticle secreted from its epithelial layer
and continuous, at the mouth, with that of the body-wall.
Posteriorly the pharynx opens into the intestine (int.), a thin
walled tube, flattened from aboVe downwards, and formed of a
layer of epithelial cells bounded both internally and externally
by a delicate cuticle : it has no muscular layer (Fig. 223, int.).
Posteriorly the intestine narrows considerably to form the short
rectum, which has a few muscular fibres in its walls and opens
externally by the anus (an.). The food, consisting of the semi-

FIG. 224. - Ascaris lumbricoides. A, a single muscle fibre ; B, several fibres in transverse
section with portion of ectoderm (below) ; c. contractile substance ; /. fibrous processes ; nu.
nucleus ; p. protoplasmic portion. (After Leuckart.)

fluid contents of the intestine of the host, is sucked in by move-
ments of the pharynx, and is then absorbed into the system
through the walls of the intestine. The food being already
digested by the host, there is no need of digestive gland-cells,
such as occur in animals which prepare their own food for
absorption.

It will be noticed that in the above description the pharynx is
also called stomodaeum. This must not be taken' to indicate that
the two terms are synonymous, but that, in the present instance, the
epithelial lining of the pharynx is derived from the ectoderm,
being formed as an in-turned portion of the outer layer of the
body- wall. The epithelium of the intestine, on the other hand, is

PHYLUM NEMATHELMINTHES

in

th

m<

endodermal, this portion of the
canal being derived from the
archenteron of the embryo.

Between the enteric canal and
the body-wall is a distinct space,
the coelome or body -cavity, con-
taining a clear fluid and more or
less encroached upon by the pro-
toplasmic processes of the muscle-
cells. The cavity is bounded ex-
ternally by these processes, in-
ternally by the outer cuticle of the

testine : there is no trace of epi-

elial lining such as occurs in

ost of the higher animals.

The excretory system presents
a certain resemblance to that of
Platodes. It consists of two longi-
tudinal canals (>,>'. v.\ one in each
lateral line. Anteriorly they pass
to the ventral surface, unite with
one another, and open by the
minute excretory pore (ex. p.) al-
ready noticed.

The nervous system consists
•of a ring (nv. r.) surrounding the
pharynx and giving off six nerves
forwards and six backwards (Fig.
226). Of the latter two are of
considerable size and run in the
dorsal and ventral lines respectively
(/////, lin.y. They are connected' with
one another by transverse commis-
sures (c.), and the ventral nerve
swells into a ganglion just in front
of the anus. The pharyngeal nerve-
ring contains nerve-cells, and its
ventral portion (im.) is thickened
and ganglion-like. The only sense-
organs are little elevations, the sen-
sorypapillce(Fig. 222,^.), on the lips.

The reproductive organs are
formed on a peculiar and very
characteristic pattern. The testis
(Fig. 227, ts.) is a long coiled thread,
about the thickness of fine sewing-
cotton, and occupying a consider-

279

2 -

280

ZOOLOGY

SECT.

— din

able portion of the body-cavity. At its posterior end it is con-
tinuous with the vas defcrens, the two passing insensibly into one
another so that the junction is not visible externally. The vas
deferens, in its turn, becomes continuous with a wide canal, the

vesicula seminalis (vs. sem.), which opens
by a short, narrow muscular tube, the
ductus cjacidatorius, into the rectum.
Behind the rectum, and opening into its
dorsal wall, are paired muscular sacs (s.\
containing the penial setce (pn. s.) already
noticed. The anterior end of the testis
consists of a solid mass of sex-cells :
passing backwards there is found a cord
or rachis occupying the axis of the tube
and having the sperm-cells attached to
it : still further back the sperms become
gradually differentiated, and are finally
set free in the vas deferens. The sperms
are peculiar rounded cells (Fig. 20, p.
28, c. d. e.) ; when transferred into the
body of the female they exhibit amoeboid
movements, but as long as they remain
in the male ducts they are non-motile :
they have no trace at any stage of the
characteristic tail of the typical sperm.
In this connection it may be mentioned
that the tissues of Ascaris are remark-
able for the total absence of cilia.

The organs of the female (Fig. 225)
resemble those of the male, but are
double instead of single. There are two
coiled, thread-like ovaries (ovy.), each pass-
ing insensibly into a uterus (ut.). In
the ovary, as in the testis, the eggs are
developed in connection with an axial
cord or rachis. The two uteri unite in
a short muscular vagina (vctg.) which
opens, as already seen, on the ventral
surface of the body (ffnp.) at about one-
third of the entire length from the head.

Development. — The eggs are pro-
duced in immense . numbers — at the

rate, it has been reckoned, of about 15,000 a day. They
are fertilised in the upper part of the uterus, each becoming
enclosed in a chitinoid egg-shell, and are passed out of the body
of the host with its fyeces. Segmentation is complete, but the
details of development are not known in this species, neither is

FK;. 2'2G. — Diagram of Xervous
system of Nematoda. c.
commissxires ; <!(n. dorsal
nerve ; /<.«/«. posterior lateral
nerve; o. n. upper and vm.
under portion of nerve-
ring ; s. (i. lateral swellings ;
vln. ventral nerve. (From
Lang, after Biitschli.)

:

PHYLUM NEMATHELMINTHES

281

the precise manner in which the worm gains access to the human
intestine. It is possible that the eggs containing developing
embryos, or the embryos themselves, after liberation from the egg-
shell, may be taken in by drinking, without previous filtering, water
into which fecal matter has been discharged. On the other
hand it is quite possible that there may be an intermediate host

derepthm,

FIG. 227. — Ascaris lumbricoides, posterior extremity of male, dissected, an. arms ;
cu. cuticle ; der. epthm. epidermis ; m. muscular layer ; p. n. peuial seta ; s. sacs containing
penial seta ; ts. testis ; rs. MIL vesicula seminalis.

such as we have met with in the Flukes and Tape-worms, and shall
also find to occur in several members of the class now under
discussion.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Nematoda are Nematelminthes haying a cylindrical body
of great length in proportion to its diameter, and pointed at both
ends. The body-wall consists of a tough external cuticle, an
ectoderm in the form of a syncytium or protoplasmic layer con-
taining nuclei and rarely exhibiting cell-structure, and a single
layer of longitudinal muscular fibres which are interrupted along
one or more (dorsal, ventral, and lateral) lines. The body-wall
encloses a body-cavity containing a clear fluid and more or less
encroached upon by processes of the muscle-cells or other meso-
dermal tissues. The enteric canal is straight, and consists of
pharynx, intestine, and rectum : the pharynx is a stomodseum.
The mouth is anterior and terminal, the anus ventral and situated
a short distance from the posterior end. Excretory canals, running
in the lateral lines, are usually present. The nervous system con-
sists of a pharyngeal ring containing nerve-cells and giving off
nerves forwards and backwards: of the latter a single ventral
nerve-cord, or two cords, respectively dorsal and ventral, are of
considerable size and extend to the posterior end of the body.
The Nematoda are in nearly all cases dioecious : eggs are pro-

282 ZOOLOGY SECT.

duced in immense numbers, and are impregnated within the body
of the female. The sperms are non-motile, or perform amoeboid
movements only after entering the female organs. Cilia are
wholly absent.

A large proportion of Nematoda are free-living, spending their
whole life in fresh- or salt-water, damp earth, decaying matter,
&c. ; the remainder are parasitic during the whole or a part of
life.

The class is divided into two orders.

ORDER 1. — NEMATOIDEA.

Nematoda in which the coelome is not lined by epithelium, but
is bounded directly by the body-muscles. There are two chief
nerve-cords given off backwards from the pharyngeal ring and
lying in the dorsal and ventral lines. There are two excretory
canals lying in the lateral lines and opening anteriorly and
ventrally. The gonads are continuous with their ducts and con-
sist of long, more or less convoluted cords. This order includes
the whole of the free-living Nematodes as well, as the large
majority of parasitic forms.

ORDER 2. — GORDIOIDEA.

Nematoda in which the coelome is lined by a distinct epithelium.
The pharyngeal nerve-ring sends off a single large ventral nerve-
cord well supplied with nerve-cells. The gonads, or at least the
ovaries^ are arranged metamerically, and the reproductive products
ate discharged into the body-cavity and pass thence into the
gonoducts. This order includes a small number of greatly
elongated thread-like worms which are parasitic in the asexual,
free-living in the sexual stage.

Systematic Position of the Example.

Ascaris lumbricoides is one of many species of the genus Ascaris,
and belongs to the family Ascaridce of the order X<'nifit<>iili'n.

The absence of an epithelial lining to the body-cavity, and the
presence of elongated gonads continuous with their ducts, indicate
its position as one of the Neinatoidea. Among the numerous
families constituting this order the Ascarida? are distinguished by
the possession of three lips furnished with papilla?, and by the
body of the male being curved ventrally. Ascaris is distinguished
from the other genera of the family by the absence of a bulb-like
enlargement at the posterior end of the pharynx, by the posterior
extremity of the body having the form of a short blunt cone, and
by the presence of two penial seta? in the male.

vi PHYLUM NEMATHELMINTHES 283

3. GENERAL ORGANISATION.

External Characters. — The Nematoda vary much in size : the
little Anguilhda, one of the commonest of aquatic animals, does
not exceed 1 mm. in length, while the dreaded parasite known as
the Guinea-worm (Dracunculus) is sometimes as much as 2 metres
(6 feet) long, The length is always great in proportion to the
diameter, and the body is always bluntly pointed at the anterior
end and either pointed or forked posteriorly. One of the most
striking cases of disproportion between length and breadth is
•exhibited by the free, sexual form of Gordius, one of the Gordiacea ;
it is found in earth or water and resembles a tangle of brown
string, the length being frequently as much as 15 or 16 cm., while
the diameter does not exceed 0'5 mm.

Body- wall. — The body is always covered by a cuticle secreted
by the deric epithelium or external ectoderm : the lajbter usually
takes the form of a protoplasmic layer with scattered nuclei, but
in the Gordiacea it consists in part of a true epithelium — a single
layer of distinct cells. Beneath the ectoderm is a muscular layer,
which in many genera has the same structure as in Ascaris, i.e.
consists of a single layer of longitudinal fibres, interrupted at the
dorsal, ventral, and lateral lines,
each fibre being spindle-shaped
and produced into a proto-
plasmic process which projects
into the body-cavity. But in
many, forms (e.g. Sirongylus) the
muscle-cells are flat rhomboidal
plates (Fig. 228), and each quad-
rant contains only two rows, the
total number in a transverse
.section being therefore eight.
In the Gordiacea the muscles
are interrupted along the ventral

,. , /, , p IMC. 228.— The body-wall of a platymyarian

line Only, the dorsal and lateral Xematode, spread out. lat. I. lateral lines.

lines being absent. (Fig. 230.)

Moreover the muscular layer

in this order is lined by a layer of epithelial cells which bounds

the body-cavity.

Enteric Canal. — The mouth is frequently armed with spines
(Fig. 229, C), by means of which the worms draw blood from the
intestinal blood-vessels of their host. Many free-living forms have
a sharp stylet for piercing the tissues of the plants on which they
feed, and a suctorial apparatus for absorbing their juices. The
posterior end of the pharynx is often dilated to form a globular
chamber with muscular walls, the gizzard (Fig. 231, #2.). The only

\

284

ZOOLOGY

SECT,

specially interesting variation in the structure of the intestine
that occurring in Trichina, one of the Nematodes parasitic
in Man, in which this part of the enteric canal consists of a single

FIG. 229.— Dochmius duodenalis.

cv. gl. cervical glands ; ph. pharynx,
bursa. (After Leuckart).

A, male and female in coitu. B, anterior end, showing,
C, mouth with spines ; D, posterior end of male, with

row of perforated cells : the lumen is therefore not inter- but
w£?Y&-cellular, like the gullet of an Infusor. In the sexual stage
of Gordius the enteric canal undergoes more or less complete
degeneration. There are never any digestive glands, but in
Dochmius a pair of pear-shaped bodies of unknown function, the
cervical glands (Fig. 229, B, cv. gl.\ lie one on either side of the
pharynx and probably open externally near the mouth.

In Nematoidea the coelome or body-cavity is always a single'
continuous chamber crossed in various directions by delicate fibres,

FIG. 230. — Transverse section of Gordius. l>m. ventral nerve-cord; c. cuticle et. ccelom
epithelium ; /<?/. epiderrn ; Ut. ccelome ; Im. muscular layer ; md. intestine ; mcs. mesentery; o
ovary ; u. uterus. (From Lang, after Vejdovsky.)

but in Gordius sheets of coelomic epithelium or mesenteries (Fig.
230, mes.) extend longitudinally through it, dividing it into several
compartments. The most important of these are a median ventral

VI

PHYLUM NEMATHELMINTHES

285

int

compartment containing the intestine and the nerve-cord, a pair
of large lateral compartments containing the ovaries, and a pair
of small dorso-median canals which act as oviducts. It is stated
that the median ventral compartment acts as an excretory canal
and opens posteriorly along with the
oviducts : in the Gordiacea there are no
lateral excretory canals like those of
Ascaris and the other typical Nema-
todes

In the Nematoidea the nervous system
has the structure already described in
Ascaris : it is, however, apparently ab-
sent in some free-living forms. But in
Gordius it is much more highly de-
veloped : the pharyngeal ring is of great
thickness and is continued into a single
ventral cord (Fig. 230, bm.), containing
nerve-cells, which compares very well in
size with the corresponding organ in the
higher Worms. Eye-spots have also been
described in the sexual form of Gordius.

The reproductive organs in all the
Nematoidea resemble those of Ascaris,
the only important variation depending
upon the fact that in the smaller forms
the entire genital tube (gonad plus
gonoduct)is short and not coiled (Fig. 231,
ts. and v. df.\ A few forms are herma-
phrodite, but, instead of having a double
set of reproductive organs, as in Plat odes,
organs of the ordinary female nematode
type are present, and the gonads produce
first sperms and afterwards ova. Such
animals are said to be protandrous (male
products ripe first), and self-impregnation
is as effectually prevented as if the organs
of the two sexes were distinct. A totally
different arrangement is met with in the
Gordiacea : the female having numerous
pairs of ovaries (Fig. 232, A, c,vy.) arranged
segmentally and attached to one of the
partitions (mes.) of the body-cavity. The
ripe eggs are discharged into large egg- sacs, formed by the
lateral compartments of the body-cavity, and finally make their
way into the medio -dorsal compartments which act as uteri (C,
ut.) and are continued posteriorly by short vaginae (vag.) into a
median chamber. The latter opens externally, and also receives

FIG. 231.— Oxyuris, from the
right side. gz. gizzard ; int.
intestine ; ph pharynx ;
pn. s. penial sitse ; ts. testis ;
r. >Jf. vas defereiis. (From
Shipley, after Galeb.)

286

ZOOLOGY

SECT.

the duct of a large spermatlicca (spth.) or chamber for storing the
sperms received in copulation. In the male Gordius the testes
are not known: they seem to disappear very early, after discfiarg-

spth

FIG. 232. — Gordius. A, horizontal section of female, showing ovaries (0-77) attached to mesen-
tery (in -x.) ; /'. >r. body-wall ; B,.. posterior extremity of male, sagittal section. I. <:. bursa
copulatrix ; cl. cloaca ; int. intestine ; t. tail ; p. »«. <•</. ventral nerve cord ; <•.<. sem. vesicula
seminalis ; C, posterior extremity of female, sagittal section. (//(/>. gonopore ; .~<j>t/t.. sperma-
theca ; at. viterns ; cay. vagina ; <•. ,ic. c<L ventral nerve cord. (After Vejdowsky.)

ing their contents into large reservoirs or vcsicidce scminales (B,
vs.sem^ : from these vasa deferentia are continued into a cloaca (cl.)
or dilated extremity of the intestine, part of which can be everted
as a bursa copulatrix (b.c.).

The development of Nematoda has been best worked out in
Ascaris nigrovenosa. Segmentation is complete, but somewhat
unequal (Fig. 233, A, B), and the ectoderm cells grow over the
endoderm (C), forming an epibolic gastrula with a long slit-like
blastopore (D-F). The mesoderm is developed from a single pair
of endoderm cells (D., mes.), which enlarge, multiply, and form a
distinct cell-layer between ecto- and endoderm (E-H). TJie epithe-
lium of the pharynx is formed by an invagination of ectoderm,
so that this division of the enteric canal is a true stomodseum
(I, stdm.). . The nervous system is developed from certain cells
(G-I, n.) which bud off from the ectoderm at the anterior end of
the body. The reproductive organs originate from a single meso-
derm cell on each side (H, I, #.).

Many of the Nematoda have a curious and complex life-
history : a few examples will be selected for description.

Ascaris nigrovenosa lives, in the sexual condition, in the lungs
of Frogs and Toads : it is remarkable among members of the class

PHYLUM XEMATHELMINTHES

287

being hermaphrodite. The eggs are laid and the embryos pass
from the lungs into the enteric canal of the host, are expelled with its
faeces, and develop in water into a sexual Nematode, called the
Bhabditis-foTm, in which the sexes are separate: in this the ferti-
lised eggs develop in the body of che female, and, when fully formed,
make their way through the wall of the uterus and proceed to devour
the whole of the maternal tissues, leaving nothing but the cuticle.

Fio. 238. — Development of Ascaris nigrovenosa. Up. blastopore ; cct. ectoderm ; •;•,?(?. endo-
derm ; cut. enteron ; //i;«. mesoderm ; /i. nervous system ; stdm. stomodseum. (From Korschelt
and Heider, after Goette.)

Being set free, they live in mud until they succeed in gaming
access to a frog's mouth, when they pass into the lung, develop
hermaphrodite reproductive organs, and so re-commence the cycle.
It will be seen that we have here a peculiar form of alternation
of generations, distinguished not by the alternation of a sexual
with an asexual form (metagenesis) as in Hydrozoa, but by the
alternation of a hermaphrodite with a dioecious form. This type
of alternation of generations is distinguished as heterogeny

288

ZOOLOGY

SECT.

One of the most terrible parasites of man is Trichina spirtilis
(Fig. 234), a minute worm, the male (C) a little over 1 mm. (TV in.)
in length, the female (B) about 3 mm. (-|- in.). In the adult or

Fig. 234.— Trichina spiralis. A, encysted form, in muscle of host ; B, female ; C. male. Mi.
connective tissue envelope; cy. cyst; >/>. ejaculatory duct ; c. embryos ; /'. fat globules; /;.
testis ; m. r. muscle fibre ; o<i pharynx ; or. ovary ; vo. gonopore ; zli. cell-masses in intestine.
(From Lang's Comparative Anatomy, after Claus.)

sexual condition it lives in the intestine of Man, the Pig, and other
mammals. Internal impregnation takes place, the eggs develop in
the uterus of the female, and the minute young (B, e.), to the

PHYLUM NEMATHELMINTHES 289

imber of at least a thousand, are born alive. Soon after birth
young worms migrate through the walls of the intestine, and,

llowing the course of the connective-tissues, reach the voluntary

mscles of the host, such as those of the limbs, back, tongue, etc.

ach Worm then penetrates the ^af coj^mrn,a of a muscle-fibre and
coils itself up in the muscle-substance (A) : a spindle-shaped cyst
(cy.) is formed round it, and the muscle undergoes more or less
degeneration. This process gives rise to various morbid symptoms
in the host, but, after some months the cysts become calcified and
the danger to the infected individual is over. The flesh of a " trichi-
nised" human subject has been estimated to contain 100,000,000
encysted worms, and that of an infected pig 85,000 to the ounce.
In order that further development of the encysted and sexless
Trichinae should take place, it is necessary for the infected flesh of
the host to be eaten by another animal in which the Worm is
capable of living, e.g. that of Man by a Pig or Rat, or that of a Pig
by Man. When this is done the cysts are dissolved by the
digestive juices, the worms escape, develop reproductive organs,
and copulate, the young migrating into the muscles and producing
the disease as before. The result of eating an ounce of " trichi-
nised " or " measly " pork, improperly cooked, might be the
liberation in the human intestine of perhaps 8-0,000 worms, and,
if half of these were females, each producing 1,000 embryos, some
40,000,000 worms would shortly begin to migrate into the muscles,
and produce the various symptoms of " trichiniasis."

It will be noted that in this case the parasite is able to exist in
various hosts, and that both sexual a,nd asexual stages are passed
through in the same host, dispersal of the species taking place by
the flesh of an infected animal being eaten by another, either of
the same or a different species.

The female Guinea-worm (Dracunculus medinensis) attains a
length of 30-200 cm. (1-6 ft.), and lives in the subcutaneous
connective-tissue of Man. The eggs develop in the uterus, and
the new-born young pass out of the body of the host through
abscesses caused by the presence of the parasite. If, as must
often be the case, they escape into water, they make their way
into the body of a Water-flea (Cyclops), and in this condition
probably reach their human host once more jn his unfiltered
drinking-water. v

CLASS IL-ACANTHCCEPHALA.

This class contains a few genera of parasitic worms, of which EchinorJiynchu*
is the chief. The present section will be devoted to this genus,, a not uncommon
parasite in the intestine of Mammals, Birds, Reptiles, Amphibians, and Fishes.
The largest species, E. gigas, is found in the Pig (Fig. 235, A), and has once
been recorded in the human subject : it may attain, in the female, a length of 50
cm., or more than half a yard. Most species are small,\not exceeding 1 cm. in
length.

VOL. I \ U

290

ZOOLOGY

SECT.

External Characters. — The body is cylindrical, and ends in front in a
protrusible portion, the pr6bo*ri* (A, p. ; B, pr.), which is also cylindrical and is
covered with many rows of recurved chitinoid hooks. The worm lies with the
proboscis sunk in the wall of the intestine of its host, which is sometimes riddled
with holes formed in this way. In some species there is a distinct neck (B. n. )
between the proboscis and, the. trunk, and there may be a globular dilatation at
the anterior end of the neck. At the hinder end of the body is a single
aperture, the gonopore or reproductive aperture (f/np.) : connected with this, in
the male, is a protrusible, bell-like structure, the Inirxa (b. ), which acts as a

-fir

FIG. 235.— A, Echinorhynchus gifiras. female, from the Pig (nat. size); B, E. lesini-

f ormis, male, from the edible Frog (magnified). It. bursa ; c. gl. cement glands ; gnp.
gonopore ; lm. lenmisci ; n. neck ; j>. or pr. proboscis ; /•. in. retractor muscle of proboscis. ;
«. Ig. suspensory ligament ; t. testis ; *-.

copulatory organ, like the somewhat similar organ in certain Nematoda. There
is no trace of mouth, anus, or excretory pore.

The body-wall is covered with a stout cuticle, beneath which is a striated
protoplasmic layer, probably representing the ectoderm. Then comes a layer of
transverse, and then one of longitudinal muscles. The body-wall thus constituted
encloses a spacious body-cavity containing a clear fluid.

In correspondence with the absence of mouth and anus there is no trace of
enteric canal, the Acanthocephala resembling, in this respect, the Cestoda, the only
other class of Metazoa which is entirely anenterous. Food is thus, as in tape-
worms, taken entirely by absorption by the general surface of the body.

The proximal end of the proboscis is contained in a muscular sheath sunk
in the anterior end of the trunk, and is provided with four' retractor muscles.

PHYLUM NEMATHELMINTHES

291

(Fig. 235, /•.;».). The muscles of the sheath are circular and act as protractors.
At the sides of the base of the proboscis two club-shaped organs, the lemniici
(fm.), hang down into the body-cavity. Their
function is quite unknown, but they have been
compared with the cervical glands of Nema-
todes (p. 284).

Tn the body-wall run two longitudinal
vessels (r.) containing a granular fluid, and
connected with a network of fine canals in the
proboscis, bursa, &c. The function of these
vessels is not known with certainty : they may
have to do with, the absorption and circulation
of nourishment.

The central nervous system (Fig. 236, nv.)
is represented by a single large ganglion placed

FIG. 236.— Echinorhynchus gigas -Dissec-
tion of male. b. bursu ; <•. </t. cement glands ;
im. lemiiisci ; nv. nerve ganglion ; pi: pro-
boscis ; s. hi . suspensory ligament ; ts. testis ;
v. df. vas deferens. (After Leuckart.)

Fia. 237.— Echinorhynchus
gigas. Dissection of female
(semi-diagrammatic), b. bell ;
/in. lemniscf ; ~~pr. proboscis ;
x. it, -I/, swimming ovaries ; ut.
uterus ; vg. vagitia.

the base of the proboscis, and sending off nerves in various directions.
In the male there are also two ganglia supplying the reproductive organs.
ans of sense are wholly absent.

u 2

292

ZOOLOGY

A pair of remarkable excretory organs or nephridia have been found to
occur in Echiiiorhyiichus gigas. These consist of a pair of ramified protoplasmic
masses situated in the body-cavity at the posterior end near the genital aperture.
In the interior is a system of branching canals, the terminal branches of which,
each contained in one of the terminal lobes of the tree-like nephridium, are pro-
vided with ciliary flames ; at the end of each lobe are a number of fine perforations
placing the contained canal in communication with the body-cavity. The stalk
of each nephridium contains a single main canal ; these unite to form a wide
median dorsal channel which opens behind in the female into the unpaired
portion of the oviduct and in the male into the ejaculatory duct.

The greater part of the body-cavity is occupied by the reproductive organs.
The sexes are separate, aiidjthe female is larger than the male. In. both sexes
the gonads and their ducts are connected with a great suspensory ligament (*.Iy.),
which extends backwards from the end of the proboscis-sheath.

In the male there are two ovoidal te«fe* (Fig. 236, ts.) connected with the
suspensory ligament. From each a ra-s deferem (v.df.), furnished with several
ve«icula> neminales or sacs for the storage of the spermatic fluid, passes backwards
and unites with its fellow to form an ejaculatory duct,
with which are connected about half a dozen cement
glands (c.gl.}. The ejaculatory duct opens into the
bursa or bell-like copula tory organ (b), and has at its
opening a small papilla acting as a penis.

In the female the ovary is connected with the sus-
pensory ligament (Figs. 237 and 238,*. ft/.)- When ripe
groups of ova — known as the " swimming ovaries "
(s.ovy.} — become detached and swim freely in the
body-cavity, where they are impregnated. The ducts
are very peculiar. On each side of the body is a
muscular uterine bell (b), widely open anteriorly (Fig.
238, x) into the ccelome, and having towards its
posterior end a small aperture, also communicating
with the ccelome (?/). Each bell is connected with an
oviduct, and the two oviducts open into a uterus (ut.),
which itself opens by the genital aperture at the
posterior end of the body. The uterine bells perform
rhythmical swallowing movements, and as the eggs —
containing partty developed embryos — float in the
coelome they are swallowed by the bells. The im-
mature eggs, which are globular, are passed back into
the coelome through the posterior aperture (y) of the
bell ; but the mature eggs, which are spindle-shaped
and covered with a chitinous investment, make their
way from the bell to the uterus, and so to the vagina.
T ) The early stages of development take place in the

\ T~^y coelome. Segmentation is regular, and, according to

— £rl— recent researches, a peculiar form of gastrula is pro-

duced, having neither archeiiteron nor blastoccele — in
other words the ectoderm and endoderm are in close
contact with one another, and no central cavity is
enclosed by the latter. The ectoderm cells secrete
a cuticular membrane investing the embryo, then a
second membrane is formed within the first, and a
third within the second ; the embryo thus comes to be

enclosed in a triple case, which differs from an egg-shell in being formed by the
developing ectoderm. At what will become the anterior end chitinoid hooks
appear.

At about this period the embiyo is born, and reaching the intestine of the
host, is extruded with its fajces. Its further development depends upon its

-uf

FIG. 238. — Female organs of
Ectiinorhynchus.

//. uterine bell ; *. /<>.
suspensory liniment ;
I't. uterus ; >•</. vagina. ;
X, 'i. apertures of bell.
(After Hertwig.)

VI

PHYLUM NEMATHELMINTHES

293

t—OV

being swallowed by an intermediate host, which, in the case of E. gigas of the
Pig is a maggot, the larva of a Beetle, Cetonia aiirata. The Echinorhynchi of
fresh-water Fish have for their intermediate host
certain small fresh-water Crustacea belonging to
the genera Gammarus and Asellus.

Having reached the intestine of the inter-
mediate host, the chitinoid embryonic membranes
are dissolved by its digestive juices, and the
embryo either fixes itself to the wall of the
intestine or makes its way into the ccelome ; in
either case it soon begins to undergo further de-
velopment. The endoderm, hitherto a solid
mass of cells, undergoes a process of splitting,
becoming divided into an outer layer in contact
with the ectoderm and a solid central axis. The

latter gives rise to the reproductive organs and jit: ^ _£#

the suspensory ligament, the outer layer to a •*

ccelomic epithelium, from which the body-
muscles arise ; the cavity formed by the splitting
of the endoderm is the ccelome. Part of the
proboscis and its sheath are also of endodermal
origin. The ectoderm gives rise to the proto-
plasmic layer of the body-wall, to the whole
system of vessels, and to the lenmisci. The
larval cuticle is thrown off and a new one
formed. The larva reaches adult proportions
and attains sexual maturity only if the inter-
mediate is eaten by the permanent host. pt/(£_||/J 1 JjJ— ovd

CLASS III.— CILffiTOGNATKA,

The present group, like that just discussed,
is a very small one, containing only two genera
(Sayitta and Spaddla) of curious arrow-shaped
worms, all but one species of which are pelagic.

External Characters.— The body (Fig. 239)
is elongated and nearly cylindrical, and is divided
into head, trunk, and tail, the head being marked
off by its somewrhat rounded form, while the
junction of trunk and tail is indicated by the
ventrally placed anus (a). The tail bears a hori-
zontal expansion, or caudal Jin (s.Jl.), and there
are also horizontal lateral ./m.s (Ji. ) — a single pair
in Spadella, two pairs in Sagitta.

Body-wall.— There is no cuticle, but the
outer layer of the body-wall is formed by an
epidermis or cleric epithelium (Fig. 240, d.ejithm.),
which, instead of being syncytial as in the two
preceding classes, is formed of several layers of
epithelial cells. Next comes a delicate basement
membrane, and then a layer of muscles (m.), the
fibres of which are striated and disposed longi-
tudinally in four bands — two dorso-lateral and
two ventro-lateral — an arrangement which recalls
that of the corresponding layer in Nematoda.
Beneath the muscle comes a delicate layer of coelomic epithelium (ccel. eptlim).

Enteric Canal. — The mouth (Fig. 239, m.} is a longitudinal slit-like aperture
on the ventral surface of the head ; on either side of it are several sickle-shaped

FK;. i

230. —Sagitta hexaptera,

from the ventral aspect, «.
anus ; fy/. ventral ganglion ;
<i. intestine ; ,rf. lateral fins ;
ho. testis; m. mouth ; or. ovary;
ocii. oviduct ; M'. fesophageal
connective ; */>. vesicula semin-
alis ; s. .rf. tail fin ; sL spermi-
duct. (From Lang's Comparat-
ive Aiiatom>>, after Hertwig.)

294

ZOOLOGY

SECT.

chitmoid hooks (Fig. 241, yh.) which are moved l>y muscles in a horizontal plane
and serve as jaws. The anterior region of the head also bears spines, and is
strengthened by chitinoid plates and parti}- covered by a hood-like fold of the
integument.

The mouth leads by a muscular pharynx or stomodajum into a straight intes-
tine (d), which extends through the trunk and opens by the anus (a) at the junction

d.ejblh/m

FIG. 2-10. — Sagitta bipunctata. Transverse .sections, A, of trunk ; B, of tail. cod. coelome ;
cod. epthm. coelomic epithelium ; (1. cptlaii. deric epithelium ;/. fin ; int. intestine ; rn. muscles ;
ovy. ovary ; ts. testis. (After Hertwig.)

of trunk and tail. The wall of the intestine is made of two layers of cells — an

inner of columnar cells, the enteric epithelium ; and an outer of very delicate

flattened cells, the coelomic epithelium.

Coelome. — At the junction of the head with the trunk, and of the trunk with

the tail, are transverse partitions or septa, dividing the coelome into compart-
ments. The trunk-region of that cavity
is further sub-divided In- two longitu-
dinal partitions, the dorsal and ventral
mesenteries, which connect the dorsal
and ventral surfaces respectively of the
intestine with the body-wall. The form-
ation of the mesenteries is best seen in
a transverse section (Fig. 240, A), which
shows that at the middle dorsal line the
layer of coelomic epithelium lining the
body-wall (parietal layer] becomes de-
flected downwards, forming a two-
layered membrane, the dorsal mesentery :
the two layers of this, on reaching the
intestine, diverge and pass one on either
side of it, forming the visceral layer of
ccelomic epithelium : uniting again below

i.'i<;. 241.— Head of Sagritta bipunctata,

from above, an. optic nerve ; ni>. eye ;
g. brain ; (th. hooks ; rn. olfactory nerve ;
ro. olfactory organ ; sc. oesophageal
connective. (From Lang's Comparative
Anatomy, after Hertwig.)

the intestine, they are continued down-
wards as the vciifrti.l im-stntery, and on
reaching the body- wall diverge once

more to join the parietal layer. The tail-region of the coelome (B) is similarly
divided into right and left chambers by a longitudinal vertical partition.

There is no trace of vascular system or of excretory canals. The
nervous system, on the other hand, is much better developed than in either of
the preceding classes, in accordance with a free life and active movements. On

PHYLUM NEMATHELMINTHES

295

VI

the dorsal side of the pharynx is a comparatively large brain (Fig. 241, y), which
sends off on each side a long nerve-cord, the cesophageal connective (sc. ). The two
connectives sweep round the enteric canal and unite on the ventral surface, not
far from the middle of the trunk, in an elongated ventral ganglion (Fig. 239,
by.), from which numerous nerves are given off. The brain sends nerves to the
eyes (Fig. 241, an.) and to the olfactory organs (ro.), and is also connected with
two pairs of ganglia in the head,
which lie deeply sunk in the meso-
derm : all the rest of the nervous
system retains its primitive con-
nection with the ectoderm.

Sensory Organs. — On the
surface of the body are numerous
little papilla? carrying stiff bristle-
like processes, and probably serv-
ing as organs of touch. There
are two eyes (Fig. 242), situated
one on each side of the dorsal sur-

face of the head : each is globular

and contains three biconvex lenses

(I. ), separated by pigment (p. ) and

surrounded by rod-like sensory

cells (rz.). The dorsal surface of

the head also bears an annular

ridge of peculiarly modified and in part ciliated cells (Fig. 241, ro.) : to this an

-olfactory function has been assigned.

Reproduction. — The Chffitognatha are monoecious. The ovaries (Fig. 239, or.,
Fig. 240, ovy. ) are elongated organs situated one on each side of the trunk-region
of the ccelome, and opening by a narrow oviduct just in front of the posterior
septum. The testes (Fig. 239, ho., Fig. 240, ts.) are similarly situated in the tail-
region of the coelome, and have the form of narrow ridges from which immature
seminal cells are given off and develop into sperms in the coelome. The spermi-
ducts or vasa deferentia are delicate tubes (si. ) opening at one end into the ccelome
by a ciliated funnel-like extremity, and at the other end dilating into a reservoir
or vesicula seminal is (sb. ), which opens externally in the posterior region of the tail.

Development. — Internal impregnation takes place, and the oosperm, seg-
menting completely and regularly, forms a typical gastrula by imagination (Fig.

FIG. 242.— Section of eye of Sagitta hexaptera.
ep. epiderm ; 1. lens ; p. pigment ; rz. visual
cells; st. rods. (From Lang's Comparative
Anatomy, after Hertwig.)

FIG. 243. — Three stages in the development of Sagitta. bl. blastopore ; cs. coelomic sacs ; d.
mesenteron ; g, sexual cells ; p,n. parietal layer of mesoderm ; st. stomodseum ; vrn. visceral
layer of mesoderm. (From Lang's Comparative Anatomy.)

243, A). Two endoderm cells (g.) at the anterior end of the archeiiteron, i.e. the
end opposite to the blastopore, soon increase greatly in size, and are the rudiments
of the gonads. This precocious differentiation of the sex-cells is a point of con-
siderable importance, as will be seen hereafter. Before long these cells migrate
into the archeiiteron and divide, forming a group of four cells (B, y. ), two of which
subsequently become the ovaries and two the testes. At the same time two folds

296 ZOOLOGY

of endoderm grow into the archeiiteron from its anterior end, partly dividing the
cavity into three parts — a middle division or mesenteron (d), the rudiment of the
intestine ; and two lateral divisions, the metentera, or coelomic sacs (ca). There
is some doubt as to the fate of these lateral divisions. According to the account
which is usually accepted they become the right and left compartments of the
coelome of the trunk. According to another account, however, their cavities
completely disappear and the trunk portion of the coelome arises from a fissure
which appears subsequently between the ectoderm and the endoderm ; the
tail-region of the body-cavity is formed from the posterior, undivided portion of
the archenteron. The blastopore (bl.) now closes and an imagination of ectoderm
— the stomodffium (st. ) — takes place at the anterior end, and finally communicates
with the mesenteron.

From this it will be seen that the ectoderm of the gastrula gives rise to the
deric epithelium of the adult and to the epithelium of the pharynx, which is
therefore a stomodffium ; from the same layer the nervous system arises at a later
stage. The epithelium of the intestine arises from the mesial (inwardly-turned)
layers of the two endodermal ridges; their lateral (outwardly-turned) layers
form the visceral layer of coelomic epithelium. The muscular layer of the body
wall and the parietal layer of ccelomic epithelium arise from the rest of the
endoderm, i.e. that portion of it which remains in immediate contact with
the ectoderm. Thus, in Sagitta the mesoderm is entirely derived from the
endoderm of the gastrula.

APPENDIX TO NEMATHELMINTHES.

1. Family Chcetosomidce.

This family includes three genera of small worms, Chcetoxoma, Tristicochceta
and Rhabdogaster, which are sometimes included among the Nematoda.

The body is elongated, its anterior region sometimes dilated to form a head.
Either the whole body, or the dorsal surface only, is beset with fine seta?, and
there may be a double row of movable chitinoid hooks round the head, reminding
us of the "jaws" of Sagitta. The ventral surface bears curious locomotor
rods, either hooked or with knobbed ends : by these the animals crawl. The
mouth is anterior- and terminal, the anus posterior and ventral ; there is a
muscular pharynx. The sexes are separate. The male has a single testis : the
vas deferens opens along with the anus : there are two penial seta?. The female
has paired ovaries and a single vagina opening near the middle of the body
on the ventral side.

2. Family Echinoderidce.

Echinoderes is a minute marine worm of cylindrical form with a flattened
ventral surface. The body is segmented or divided into rings, eleven or twelve
in number, all strongly cuticularised, and most of them bearing spines. The
mouth is placed at the anterior, the anus at the posterior end of the body : the
former opens into a sac, which can be everted so as to form a proboscis or
introvert, and is armed with spines. The enteric canal consists of a muscular
pharynx and a straight intestine. A pair of sacs opening by ciliated ducts on the
tenth segment appear to be excretory organs. The sexes are separate : the gonads
are paired sacs opening at the posterior end of the body.

3. Family Desmofscolecidce.

1)< xinoscolex is also a minute marine worm, having a globular head and a
variable number of segments. The head bears four movable chitinoid rods or
setae, and a pair of similar structures occurs on many of the other segments. The
terminal mouth leads by a muscular pharynx into a straight intestine : the anus
is dorsal in position. The animal is dioecious: the gonads have the form of
simple sacs, the testis opening along with the anus, the ovary on the ventral
surface anterior to the anus. The male has a pair of penial seta'.

PHYLUM NEMATHELMINTHES 297

AFFINITIES AND MUTUAL RELATIONSHIPS OF THE
NEMATHELMINTHES.

The affinities of all the classes of Nemathelminthes are very
obscure, and the propriety of grouping them into a single phylum
is extremely doubtful. They all agree in being elongated, cylin-
drical worms with a coelome ; there is a certain resemblance
between Nematoda and Chaatognatha in the muscular system ;
and the lemnisci of Acanthocephala have been compared with the
cervical glands of Nematoda. Beyond these points there is little
to unite the three classes, and, on the other hand, the proboscis of
Acanthocephala recalls the rostellum of Cestoda.

The three families placed as an Appendix to the phylum present
some undoubted resemblance to the Nematoda : this is especially
the case in the reproductive organs of the ChaetosomidaB, and still
more in those of Desmoscolex. But the segmentation of the body
in both Desmoscolecidas and Echinoderidse and the presence of
setae show a certain resemblance to higher worms or Annulata,
which will be more fully appreciated when that phylum has been
studied.

SECTION VII

PHYLUM TROCHELMINTHES

THE typical larval form of a number of the groups which have
yet to be studied is a form which is known as the Trochosphere or
Trochoplwrc. It is necessary that a clear idea should be formed
at this stage of this important larva, reference to which will very
frequently be made in the sections that follow. The general
shape of a typical trochosphere is oval or pear-like (Fig. 244) with
a broader and a narrower end and distinct bilateral symmetry.
Encircling the body about the middle, or rather nearer the broad
than the narrow end, is a double circlet of strong cilia, the prce-
oral circlet (pr.or.ci.) vv prototroch, situated on a corresponding ring-
like thickening of the ectoderm;
behind the mouth is often a second
circlet of cilia, the post-oral circlet,
and a ciliated groove or ciliated
streak usually runs backwards from
it along the middle of the ventral
surface. The mouth, situated just
behind the prasoral circlet, leads into
an alimentary canal, which at first
runs nearly transversely, and then
bends round so as to run back to-
wards the narrow end, near which
it opens on the exterior in an
anal aperture. About the middle
of the broader (anterior) end of

the trochosphere is a thickening, the apical plate (Br), project-
ing from which are usually a number of sensory cilia; and in
many trochospheres eye-spots and a pair of short tentacles occur
in close relation with the apical plate, which is the nerve-centre
of the larva. A pair of ciliated tubes which may be present are
the excretory organs or nephridia.

Jii.ci

FIG. 244. — A Trochosphere. >/,>,. ci. uiial
cilia ; Br. brain ; cnt. stomach ;
Msd1. mesodermal bands ; pro. (///<.
intestine ; pr. or. ci. prse-oral circlet
of cilia ; stdni. gullet. (From
Parker's Biology, after Fraipont.)

vii PHYLUM TROCHELMINTHES 299

In the higher groups in which this form of larva occurs, the
adult condition is attained by modifications and new developments
of so radical a nature that the transition from larva to adult is of
the nature of a metamorphosis. Sometimes the narrow part of the
larva elongates and becomes divided into a series of sections fore-
shadowing the metameres of the adult animal : in other cases, in
which no metamerism occurs, radical changes of other kinds lead
to the adult form. But in all these higher groups, whatever the
nature of the changes involved, there is a metamorphosis, and the
adult animal is totally unlike the larva. In a small number of
forms now to be dealt with, however, there is no such radical
change, and the adult may be looked upon as a somewhat modi-
fied trochosphere. The groups thus associated together may not
be genetically related : they may have become independently
developed from trochosphere-like ancestors, but the possession of
the general characters which have been referred to above renders
it convenient to group them together and regard them as con-
stituting a small but well-marked phylum. The groups referred
to are the Rotifera or Wheel-animalcules, together with the
Dinophilca and the Gastrotricha.

CLASS I.-ROTIFERA.

The Rotifers or " Wheel-animalcules " are microscopic creatures,
very abundant in pools, gutters, &<?., and formerly classed with the
Infusoria, to which several of them bear a superficial resemblance.
But in spite of their minute size they are multicellular animals,
having an enteric canal, a coelome, nephridial tubes, gonads, a
nervous system, and sense-organs, and have therefore no real
relationship with the Protozoa.

1. EXAMPLE OF THE CLASS — BraMonus rubcns.

External Characters. — Brachionus (Fig. 245) is one of the
commonest members of the class, being frequently found in
abundance in ponds, ditches, &c. It is about J mm. (TXT in.) in
length, and is divisible into two distinct parts — a broad anterior
region, the trunk, and a slender movable tail (t.). The trunk is
enclosed in a glassy cuirass or lorica (lr.), formed by a thickening
of the cuticle and produced into several spines : the tail is
wrinkled superficially and ends in two slender processes, together
forming a kind of forceps. One surface of the trunk is flattened,
and owing to the position of the mouth is considered as ventral,
the opposite or dorsal Surface is convex both from before back-
wards, and from side to side.

The anterior portion of the body projects from the lorica in the
form of a transverse, disc (tr.d.\ with a prominent edgej^inged with

^l^jBa^,

OF THF ^ .

TVERSITY ]

300

ZOOLOGY

cilia : this is the trochal disc and is one of the most characteristic
organs of the class. By the action of the cilia the animal is
propelled through the water, and, as in Vorticella, their successive
flexion gives an appearance of rotation to the disc or " wheel-
organ " whence the name of the class is derived. Within the circlet
of cilia arise three prominences (c.L) covered with cilia of large
size. The trochal disc is not perfectly symmetrical, but has at one

part of its circumference a depression in which the mouth lies :
this marks the ventral surface. The anus («.) is dorsal in position,
and is placed at the junction of the tail with the body-trunk.

The body- wall consists of an epidermal layer covered by a
chitinoid cuticle : it is by a thickening of the latter in the region
of the trunk that the lorica is produced. There is no continuous
muscular layer, but several bands of unstriped muscle (???,) pass
from the lorica to the trochal disc in front and to the tail behind,
and act as retractors of those organs.

vii PHYLUM TROCHELMINTHES 301

Digestive Organs. — The mouth (Fig. 248, mth.) lies, as already
mentioned, in the ventral region of the trochal disc, anterior to the
ciliary circlet, but posterior to the three ciliated lobes ; it leads by
a short luccal cavity into a pharynx (ph.) of peculiar structure,
known as the mastax, and constituting one of the most .character-
istic organs of the class. The mastax is a muscular chamber
(Fig. 246) of rounded form, and contains, as a thickening of its
cuticular lining, an elaborate
apparatus for triturating the
food. In the middle line is a
forked structure, the incus,
consisting of a small base
or fulcrum (/.) and of two
branches or rami (?•.). On
either side of the incus is a
hammer-like structure, the
malleus, consisting of a handle
or manulrium (m.) and of a F[o 24f)._Pharyll!C of Bracblonus rabens.

toothed, head Or UnCUS (U.). f. fulcrum ; m. manubrium ; u. uncus ; r.

By means of the muscular

walls of the chamber the

heads of the mallei are worked backwards and forwards upon the

forked incus, and thus reduce the organisms taken as food to a

fine state of division.

The pharynx leads by a short gullet into a spacious stomach (st.),
having . a wall composed of very large epithelial cells, ciliated
internally: with it are connected paired digestive glands. The
stomach opens into a rounded intestine (int.), also ciliated internally,
which communicates, by means of a short cloaca (cl.), with the ex-
terior. The stomach and intestine are formed from the archenteron
of the embryo and are therefore lined by endoderm : the rest of the
enteric epithelium is ectodermal, the pharynx being derived from
the stomodaeum, the cloaca from the proctodseum. Between the
body-wall and the enteric canal is a spacious coelome containing
a fluid which serves the purpose of blood and contains minute
granules.

The excretory system consists of paired nephridial tubes
(Figs. 245 and 248, nph.) resembling those of the Platyhelminthes.
Their general direction is longitudinal, but they are a good deal
coiled and give off little tag-like processes ending in flame-cells.
The lumen of the tubes is intra-cellular : it is uncertain whether or
not the cavities of the flame-cells communicate with the coelome
by apertures in their walls. Posteriorly the nephridial tubes open
into a bladder or contractile vesicle (c. v.), the contents of which
are discharged, by periodical contractions, into the cloaca.

Nervous System and Sense Organs. — There is a single
ganglion or brain (br.), of proportionally large size, situated at the

302

ZOOLOGY

anterior end of the body, above (dorsal to) the mouth and
pharynx. On the dorsal surface of the brain, where it comes
into contact with the body-wall, is a small red eye -spot (e.).
The only other organs which can be considered as sensory are
three structures known as tactile rods or feelers ; one of these (d.f.)
.is a small cylindrical process tipped with stiff hair-like bodies,
which projects from the dorsal surface just behind the trochal disc :
the other two (/./.) are paired, situated on the dorsal surface of
the lorica and not prominent.

The tail contains a pair of cement glands (c. gl.) by the secretion
of which the animal is able temporarily to attach itself.

Reproduction and Development. — The sexes are lodged
in distinct individuals, which present a striking degree of sexual
dimorphism. The preceding description applies to the female,
which is the form most commonly met with. In addition to the
organs already mentioned it has an ovary (ov), connected with a
large vitellarimn (vt) and opening by an oviduct into the cloaca.

The male (Fig. 247, A) is a very minute creature, not more than
one-fourth the size of the female, and is strangely degenerate in

Fie. -JIT.— Brachionus rubens. A, male; B, female, with attached e^-s ; ,>/>!<. nephridial
tube ; c. gl. cement glands ; or. ovum in body ; or.l ova attached to base of tail ; p. penis ;
t. tail ; ts. testis. (After Hudson and Gosse.)

structure. The enteric canal is absent, the trochal disc simple in
structure, the nervous system and nephridial tubes greatly reduced,
and the greater part of the body occupied by a large testis (ts.)
which opens by a duct at the extremity of a protrusible, dorsal ly
placed penis (p.).

After extrusion the eggs are attached to the base of the tail
of the female (B, ov'.), where they undergo development : they are
of two sizes, the larger giving rise to females, the smaller to

VII

PHYLUM TROCHELM1NTHES

303

males. Probably both kinds develop parthenogenetically, but in
the autumn thick-shelled winter eggs are produced which appear
to require fertilisation. These remain quiescent during the winter,
. and in the spring develop into females.

c.yl

FIG. 248. — Diagram of a Rotifer, n. aim.* ; ///•. brain ; r1. pras-oral ; c2. post-oral circlet of cilia ;
'•. ///. cement gland ; cl. cloaca ; cw. cuticle ; d.ep. deric epithelium ; <l. f. dorsal feeler ; e. eye ;
H. r. flame-cells ; int. intestine ; ui. muscles ; mtJt. mouth ; ,iph. iiephridial tube; ov. ovum ;
oi-il.. oviduct; ory. ovary ; jik. pharynx ; .sf. stomach ; vt. vitellarium.

2. — DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Rotifera are Trochelminthes of microscopic size. The ante-
rior end is modified into a retractile trochal disc, with variously
arranged cilia : the posterior end usually forms a mobile and
often telescopically jointed tail. The mouth is anterior and more
or less ventral in position, the pharynx contains a chitinous
masticatory apparatus, and the anus is placed dorsally at the
junction of the trunk with the tail. There is a spacious coelome.
The excretory organs are a pair of nephridial tubes provided with
flame-cells. The , nervous system consists of a single dorsal
ganglion. The sexes are separate, and the males are, in nearly all
cases, smaller than the females and degenerate in structure.

The class is divided into five orders as follows : —

ORDER 1. — RHIZOTA

Rotifera which are fixed in the adult state by the truncated end
of the non-retractile tail.

Including Floscularia, Stcphanoceros, Melicerta, etc.

ORDER 2. — BDELLOIDA.

Rotifera which both swim freely by means of the cilia of the
disc and creep after the manner of a Leech. The tail is telescopic
and forked distally.

Including Rotifer, Pliilodina, etc.

304 ZOOLOGY SECT, vn

ORDER 3. — PLOIMA.

Rotifera in which locomotion is performed by the ciliated disc
only. The tail is usually forked and more or less retractile.

Sub-order a. — Illoricata

Plo'ima in which the trunk is not covered by a lorica.
Including Hydatina, Polyarthra, Asplanchna, etc.

Sub-order b. — Loricata

Ploima in which a lorica is present.
Including Brachionus, Euchlanis, etc.

ORDER 4. — SCIRTOPODA.

Rotifera provided with setose appendages moved by striped
muscles : skipping movements are performed by the aid of these
as well as swimming movements by the trochal disc. The tail is
either absent or is represented by a pair of ciliated processes.

Including Pcdalion and Hexarthra.

ORDER 5. — TROCHOSPH^RIDA

Globular Rotifera having the trochal disc represented by an equa-
torial circlet of cilia ; tail absent.
Including Trochospkwra only.

Systematic Position of the Example.

Brachionus rubens is one of several species of the genus
Brachionus : it belongs to the family Brachionidce, and to the
sub-order Loricata of the order Ploima.

It is placed in the order Ploima in virtue of its active swimming
habits and the absence of looping or skipping movements. The
presence of a distinct lorica places it in the sub-order Loricata.
The family Brachionidse is distinguished by having a box-like
lorica open at both ends, and a long, flexible, retractile tail with
wrinkled surface and forceps-like termination. In the genus
Brachionus the lorica is not marked with ridges, and the tail is
very long and perfectly retractile. In B. rubens the anterior edge
of the lorica is produced dorsally into six spines and is sinuous
ventrally.

3. GENERAL ORGANISATION.

External Characters. — The majority of the Rotifera are free-
swimming, being propelled rapidly through the water by the action
of the trochal disc. But in the Bdelloida (Fig 249, 5), in addition
to this mode of progression, the animal performs looping move-
ments like those of a leech : the tail in this order is freely jointed,

l.Floscularia 2, Srefahan oceros 3.Melicerra

4.MeNcerrct 5. Phi lod ina 6. A s f) la nchna

T.HydaMna

10. N orholca

FIG. 249.— Typical forms of Rotifera. 9 and 10 show the lorica only, arc.oanus; ci. c2. ciliary
circlets ; int. intestine ; m. muscle ; ph. pharynx. (After Hudson and Gosse.)

VOL. 1 X

306 ZOOLOGY

SECT.

the various segments fitting into one another like the tubes of ;i
telescope, and the body is fixed alternately by it and by the anterior
end, the trochal disc being kept retracted while the animal moves
in this way. Many of the Ploima also have a telescopic tail, but
in some, e.g. Asplanchna (Fig. 249, 6), this organ is absent. In
Pedalion (Fig. 250, 1) curious skipping movements are performed
by the aid of six hollow limbs or appendages, one dorsal, one
ventral, and two on each side. These curious organs are ter-
minated by feathered setae, and closely resemble the limbs of
some of the lower Crustacea : each is moved by two opposing
muscles which extend into its cavity (1, B, m}. Three pairs of
similar appendages are present in the other genus of Scirtopoda,
Hexarthra (Fig. 250, £), the resemblance of which to the Nauplius
larva of Crustacea is very striking (see Fig. 395), and four genera
of unarmoured Ploima, e.g. Polyarthra (Fig. 249, 8) possess simple
or fringed setae moved by muscles attached to their bases.

In the Rhizota the adult is permanently fixed (Fig. 249, 1-4)-
The end of the tail is devoid of the characteristic fork, and is
attached to plants or other supports. Moreover the animal is
surrounded by a tube, into which it can retract itself completely,
protruding the anterior end with the trochal disc when undis-
turbed. In most instances, as for example in Floscidaria (1) and
Stephanoceros (2), the tube is formed of a delicate, transparent,
gelatinous secretion of the epidermis, but in Melicerta (3) it is
built up of rounded pellets, which the animal moulds in a cup-like
depression on the dorsal surface and places in position one by one.
The pellets are usually formed of foreign particles, but in some
species are made of the animal's own faeces.

The ciliation of the trochal disc is subject to considerable
variation. In its simplest form the disc is surrounded by a single
circlet of cilia, within which lies the rnouth. A modification of
this type may be produced by the prolongation of the ciliary
crown into long arm-like processes fringed with cilia, as in
Stephanoceros (#), or, as in Floscularia (1), into blunt elevations
bearing long stiff cilia like those of the Heliozoa. The single
circlet may be folded upon itself, or a second type may be pro-
duced by the addition of a second circlet within and parallel to
the first. The mouth in this case is always placed between the two
circlets on the ventral side (Fig. 248), so that the inner or anterior
circlet is prae-oral and corresponds with the chief ciliary band
of a Trochosphere larva, while the outer or posterior circlet corre-
sponds with the post-oral band found in many worm larvae. In the
curious globular Trochosphcera (Fig. 250, 3} there is a single
equatorial circlet, which is prae-oral, and a few post-oral cilia : here
the correspondence with the typical worm larva is singularly
close. Lastly both the prae- and post-oral circlets may be pro-
duced into more or less complex lobes, as in Melicerta (Fig. 249, 4),

PHYLUM TROCHELMINTHES

307

or may be interrupted as in Brachionus, in which the proe-oral
circlet is represented by three distinct lobes, or as in Pedalion.
in which both circlets are divided into right and left moieties,
one genus the trochal disc is absent.

npk

S.Trocho s £h a e r a

Fio. 250.— Typical forms of Rotifera. In ./, a shows the outer form, b the muscular system.
o. anus ; br. brain : c1. c-. ciliary circlets ; cl. cloaca ; d. gl. digestive gland ; d. I. dorsal limb ;
e. eye-spot; 1. L, LI', lateral limbs; -,,i. muscles; nph, nephridial tube; or. ovary ; ph.
pharynx ; s. sense organ ; v. 1. ventral limb. (After Hudson and Gosse (1 and 2) and
Korschelt and Heider (3).)

Digestive Organs. — The typical form of mastax orpharyngeal
mill is that described in Brachionus (Fig. 246). There is an un-
paired incus consisting of a short stem or fulwrum (/), and of two-
broad branches or rami (r), and a pair of mallei, each consisting

x 2

308

ZOOLOGY

SECT

of a stout handle or mamibrium (m) and a broad, toothed head or
uncus (u). In some forms all the parts of the apparatus become
very slender, the incus assuming the form of forceps (Fig. 251, A).
Or the mallei may be absent and the two rami movable upon
one another so as to convert the incus into a pair of forceps (B)

B

y

FIG. 251. — Typical forms of mastax. A, forcipate type ; B, incudate type ; C, ramate type
/. fulcrum ; m. manubrium ; /•. ramus ; c.. uncus. (After Hudson and Gosse.)

used to seize prey, the mastax being in this case protrusible.
Lastly, the fulcrum and manubrium may be absent, and the unci
and rami very strong and massive (C).

The stomach is always large, and usually has a pair of digestive
glands opening into it : it may pass insensibly into the intestine,
or the latter may be a distinct chamber of more or less globular
form. In the Rhizota the intestine turns forwards so as to allow of
the anus being brought over the edge of the tube in defalcation
(Fig. 249, 4> ft)- In -Asplanchna (&) the stomach ends blindly, the
intestine, cloaca, and anus being absent.

The excretory system is very uniform in structure. It con-
sists of a pair of more or less coiled nephridial tubes, placed
longitudinally and giving off lateral branchlets which end in
flame-cells. Frequently, but not always, the two tubes open
posteriorly into a contractile vesicle or bladder which discharges
into the cloaca.

Nervous System and Sense Organs. — The nervous system
always consists of a single ganglion (Fig. 248, br) towards the
dorsal aspect of the anterior part of the body, and representing the
brain or supra-oesophageal ganglion of the higher Worms : it sends
nerves to the muscles, trochal disc, and tactile organs. One or
more eye-spots (e) are usually present, and are always mere spots
of pigment in close relation with the brain. The only other organs
of sense are the tactile rods (d.f., If), of which there is usually one
on the dorsal surface near the anterior end of the body, and
frequently two others, one on each side of the trunk. They are

vii PHYLUM TROCHELM1NTHES 309

more or less rod-like structures, tipped with delicate sensory hairs
and receiving nerves from the brain.

Reproduction and Development. — In most cases the female
reproductive organs have the same general character as in Brachi-
onus, i.e. the gonad is unpaired (ov\ consists of germarium and
vitellarium, and is provided with an oviduct. But in some of the
Bdelloida, such as Philodina, there are two ovaries, not divisible
into germ-gland and yolk-gland, and the oviduct is absent. The
males are smaller than the females and degenerate in structure,
the enteric canal being atrophied (Fig. 247, A). There is a large
testis (£) with a duct opening at the end of a protrusible penis (_£?).
Apparently hypodermic impregnation sometimes takes place, i.e. the
body- wall of the female may be perforated at any place for the
entrance of the sperms.

Three kinds of eggs are produced : large and small summer eggs,
which always develop parthenogenetically, the larger giving rise to
females, the smaller to males ; and thick-shelled winter eggs, which
probably require impregnation and remain in an inert condition all
through the winter, finally developing in the spring. Most Rotifers
are oviparous, but some (Philodina, &c.) bring forth living young,
which are born by breaking through the body- wall or through the
cloaca, thus causing the death of the parent.

Segmentation is total and irregular, the oosperm dividing into
megameres and micromeres. An epibolic gastrula is formed, the
blastopore closes, and invaginations of ectoderm give rise to the
stomodaeum and proctodaeum. The tail is formed as a prolongation
of the postero-ventral region of the embryo, and contains at first
an extension of the endoderm. No metamorphosis is known to
take place in any member of the class.

Ethology. — A few Rotifers live in the sea, but the majority
are fresh-water forms, occurring in lakes, streams, ponds, and even
in puddles the water of which is rendered foul and opaque by mud
and sewage. Frequently the water in which they live is dried up,
and the thick-shelled winter eggs may then be widely dispersed
by wind. It is even stated that the adult animals may survive
prolonged desiccation and resume active life when again placed in
water. Many forms cling to the bodies of higher animals in order
to obtain a share of their food, thus leading a kind of commensal
existence. Others go a step further and become true external
parasites, like Drilophaga on a fresh-water Oligochsete (vide infra},
or Seison on the little crustacean Nebalia (Fig. 422). Others, again,
are internal parasites, such SisAlbertia in the coelome of Earth-worms,
and the intestines of fresh-water Oligochsetes (Nais) or Notommata
werneckii in the cells of the fresh-water Alga Vaucheria.

Affinities. — The affinities of the Rotifera are very obscure.
Their general resemblance to the free-swimming larva? of Annelids
is extremely close, and, in particular, the curious Trochospha^ra is,

310

ZOOLOGY

SECT.

to all intents and purposes, a sexually mature trochosphere with a
mastax. The excretory organs recall those of the Platyhelminthes,
and also resemble the provisional nephridia or head-kidneys of
Annulate larvae. Lastly, the hollow muscular appendages of Peda-
lion and Hexarthra give those genera a certain resemblance,
which is probably, however, merely adaptive, to the Nauplius or
free-swimming larva of Crustacea.

Class II. — DINOPHILEA.

The various species of the genus Dinophilus are to be looked upon, like the
Kotifera, as modified Trochospheres.

Dinophilus (Fig. 252) is a minute worm-like animal with a head or pro-
stomium, a body composed of five to eight segments separated from one another

by constrictions, and a short ventral tail.
The prostomium bears two eye-spots (a)
and some sensory hairs ; it is either
covered uniformly with cilia or bears two
or three annular ciliated bands, apparently
representing the prototrocli of the Tro-
chosphere. The body is in some of the
species uniformly ciliated ; in others the
cilia are disposed in rings (it'k) correspond-
ing to the segments, except on the ventral
surface, where the ciliation is always uni-
form. The mouth (m), which is situated
on the ventral aspect of the prostomium,
leads into an alimentary canal consisting
of pharynx, oesophagus, stomach, and in-
testine, all of which are ciliated ; the anus
(an) is placed dorsally over the tail. A
protrusible muscular proboscis lies when
retracted in a recess opening close to the
mouth. There is a c< elome which is crossed
by strands of connective tissue. A nervous
system is present, and consists of a large
ganglion in the prostomium, giving off two
anterior and two posterior nerves, or two
lateral cords (sometimes segmented into
a series of ganglia) all situated in the
epidermis.

In one species (D. </iyax) there is an
excretory system which is comparable with
that of the flat-worms, and contains flame-
cells, the internal openings of which are
provided with triangular ciliated appen-
dages ; in others there are five pairs of
tubular nephridia (??). The sexes are
separate. In the male there is a conical
penis ; the last pair of nephridia act as
vesiculffi seminales. In the ovary two
sets of ova are developed, larger ones
destined to give rise to females, and

smaller destined to form males. They pass into the ccelome and reach the
exterior by an aperture on the ventral surface in front of the anus. A process
of unequal segmentation is followed by the formation of an epibolic gastrula.

nd

Fi«; -2i>:L— Dinophilns gyrociliatus,

female. <',>.. aims ; - . eye ; t<i. hind-gut ;
month ; UK/, stomach ; n. nephridia ;
o. ovary; ,>!>. pharynx ; t>/t</. pharyngeal
glands; irk. ciliated rings. (From Lang.
after E. Meyer.)

PHYLUM TROCHELMINTHES

311

The various species of Dinophilus are marine, with the exception of one which
is an inhabitant of brackish water. In certain of its characters— the tendency
to a segmentation of the body, and the disposition of nephridia in pairs corre-
sponding to the imperfectly separated segments — Dinophilus approximates
towards a phylum that has yet to be dealt with — the Annulata — and is some-
les looked upon as a member of the class Archi- Annelida of that phylum.

Irn-

FIG. 253. — Chaetonotus maximus.

Highly magnified. (After Zelinka,)

FIG. 254.— Chaetonotus maximus, or-
ganisation, brn. brain ; <jld. adhesive
gland ; men. mesenteroii ; mo. mouth ;
as. oesophagus; 01: ovum; ocar. ovary;
retr. retractor muscles ; -cent. mus. ventral
muscle. (After Zelinka.)

Class III. — GASTROTRICHA.

The Gastrotricha (Figs. 253 and 254) are a small group of minute fresh-water
animals, which are apparently allied, though certainly not very closely, to the
Rotifera, and are on that account placed in the present phylum. The body is

312 ZOOLOGY SECT. VH

spindle-shaped with flattened ventral surface. The ventral surface bears two
longitudinal bands of cilia ; the dorsal is non-ciliated, but bears a number of
longitudinal rows of slender pointed cuticular processes. The aboral end is
narrow and bifurcated. The mouth, situated at the anterior end, is provided
with a circlet of hooked setae ; it leads into a muscular pharynx ; the intestine
is straight, and terminates in an anal aperture situated near the aboral end.
The nephridia are a pair of unbranched coiled tubes. There is a cerebral
ganglion at the anterior end, giving off a pair of ventral longitudinal nerves ; a
number of sensory cilia occur at the anterior extremity.

SECTION VIII
PHYLUM MOLLUSCOIDA

THE Phylum Molluscoida comprises the three classes of the
Polyzoa (including, provisionally, the Endoprocta), the Brachiopoda,
and the PhoronidcL The members of these three classes are
tolerably widely divergent, so that it is somewhat difficult to
frame a general account of the entire phylum ; but the following
are the most important common features : —

There is, except in the Endoprocta, a true body-cavity, lined in
most cases with a coelomic epithelium, within which the alimen-
tary canal is suspended by means of mesenteries or by means of
funicular strands taking their place. The dorsal region of the
body is abbreviated, being represented only by a short space
between the mouth and anus, which are closely approximated.
There is a lophophore or tentacle-bearing ridge, usually of a horse-
shoe shape, containing1 a special compartment of the coelome, and
overhanging the mouth on its anal side there is in most cases a
sensitive process — the ep'istomc-* — also containing a special com-
partment of the body-cavity. The central part of the nervous
system consists of a single ganglion (supra-ossophageal), or of two
ganglia ( supra-oesophageal and infra-oesophageal), or of a nerve-
ring The nephridia when present are in nearly all cases a single
pair of ciliated tubes, which act also as gonoducts.

CLASS I.— POLYZOA.

The Polyzoa form colonies known, as " Sea-mats," or " Coral-
lines," which in many cases bear a close general resemblance to
Hydroid Zoophytes, and are only on a more minute inspection
found to differ totally from the latter, and to exhibit a very much
higher type of structure.

314 ZOOLOGY SECT.

1. EXAMPLE OF THE CLASS. — BUGULA AVICULARIA.

Bugula avicularia, the common Bird's-Head Coralline (Fig. 255),
occurs in brown or purple bushy tufts, two or three inches long, on
rocks, piles of jetties, and similar situations on the sea-shore in all
parts of the world. On a naked-eye examination it presents a con-
siderable resemblance to a Hydroid Zoophyte, and might readily
be taken for a member of that group. It consists of dichotomously
branching narrow stems, which are rooted by a number of slender
root-filaments. Each stem is found, when examined with a lens, to
be made up of a number of elements, the zocecia of the colony,
which are closely united together and arranged in four longitudinal
rows. The zocecia are approximately cylindrical in shape, but
broader distally than proximally, four or five times as long as
broad, with, near the distal end, a wide crescentic aperture — the
" mouth " of the zocecium — on either side of which is a short blunt
spine. A rounded structure — the ocecium — in many parts of the
colony lies in front of each zocecium. (Fig. 255, owe.') On each
zooecium, except a few at the extremities of the branches, is a
remarkable appendage, the amcularium (avic), having very much
the appearance of a bird's head supported on a very short stalk :
if the Bugula is examined under the microscope in the living
condition, the avicularia will be found to be in almost constant
movement, turning from side to side, and a movable part, repre-
senting the lower jaw of the bird's head, will often be seen to be
moved in such a way that the mouth of the avicularium becomes
opened very widely and then becomes closed up with a quick
" snap." All the parts hitherto mentioned can be shown by using
appropriate tests, to be composed of some material akin to chitin
in composition. The chitinous wall of the zooecia is the hardened
and thickened cuticle of the zooids, having beneath it the soft body
wall.1 The anterior region of the body of the zooid forms an
introvert, i.e. is capable of being involuted like the finger of a
glove within the more posterior part : the cuticle covering this, con-
tinuous behind with the thick ectocyst, is quite thin and flexible.
When the introvert is everted it is seen to bear at its anterior end a
circlet of usually fourteen long, slender filiform tentacles (tent) on a
circular ridge or lopliophorc, surrounding the mouth of the zooid. The
tentacles are densely ciliated except along their outer surfaces : the
cilia vibrate actively in such a way as to drive currents of water,
and with them food-particles, towards the mouth (mo). They are
also capable of being bent in various directions. In the interior of
each is a narrow prolongation of the ecelome. In all probability,
besides bringing minute particles of food to the mouth of the zooid

1 The terms <-Hocii*t and endocyxt are commonly applied respectively to the
hardened cuticle of the zooid and its soft body wall.

VIII

PHYLUM MOLLUSCOIDA

315

by the action of their cilia, the tentacles are prehensile as well as
tactile, and also act as organs of respiration. When retracted they
>come enclosed by the walls of the introvert as by a sheath—

aotc

FIG. 255.— Bugula avicularia. Two zooids, magnified, an. aims ; avic. avicularia ; emb.
embryo enclosed in the ooecium ; juiiic. fuhiculue ; gast, muscular bands passing from the
stomach to the body wall ; int. intestine ; mo. mouth ; oa>. oxjecium ; o-s. oesophagus ; ov. ovary ;
ph. pharynx ; ret. parieto-vaginal muscles ; sp. spermatidia ; atom, stomach.

the tentacle-sheath. A pair of bands of muscular fibres' — iheparietc-
vaginal muscles (ret.) — passing to the introvert from the body- wall,
serve to retract the introvert and tentacles.

316 ZOOLOGY SECT.

The body- wall consists, in addition to the cuticle, of an epidermis
composed of a single layer of large flattened cells, two muscular
layers, the outer circular and the inner longitudinal, and a layer of
an irregular cellular tissue, or parenchyma.

The coelome is extensive ; it is lined externally by the parietal
layer of parenchyma forming the innermost layer of the body- wall,
and internally by a visceral layer of the same tissue ensheathing
the alimentary canal. Across the cavity between the pariebal and
visceral layers of the parenchyma pass numerous strands of spindle-
shaped cells. A large double strand (fume) passes from the
proximal or aboral end of the alimentary canal to the .aboral
wall of the zocecium ; this is the funiculus. The coelomic fluid
contains a number of colourless corpuscles or leucocytes.

Alimentary Canal. — The mouth (mo) leads into a wide chamber
— the pharynx (ph) — just behind the bases of the tentacles ; from
this a somewhat narrower short tube, separated by a constriction
from the pharynx, leads to the stomach (stom) from which it is
also separated by a constriction. The stomach gives off a long
conical prolongation or ccecum passing towards the aboral end of
the zocecium, to which it is attached by the funiculus. The
intestine (int) comes off from the oral aspect of the stomach, not
far from the oesophagus, with which it lies nearly parallel : it ter-
minates in a rounded anal aperture (an) capable of being dis^
tended to a considerable size, situated not far from the mouth,
but outside the lophophore. The entire alimentary canal is lined
by an epithelium, which is ciliated throughout except in a portion
of the stomach : the cells of the epithelium, which are arranged
in a single layer, vary in length in different regions, being longest
in the pharynx, which is comparatively thick-walled. A pair of
slender muscles (gast) passing from the body-wall to the stomach
act as retractors of the alimentary canal when the introvert is
drawn back.

There are no blood-vessels.

A nervous system has not been traced in Bugula avicularia ;
but in many other Polyzoa a small rounded ganglion is distinguish-
able between the mouth and the anus, giving off nerves to the
various parts ; organs of special sense are absent. Definite ex-
cretory organs do not occur in Bugula, the function of excretion
(i.e. the collection of the nitrogenous waste-matters) being appar-
'ently carried on by the leucocytes and the cells of the funicular
tissue.

Reproductive Organs. — Ovary and testis are found to occur
together in the same zooid. They are both formed from specially
modified cells of the parenchyma, either of the funiculus or of
the body-wall. The testis, developed from the cells of the funicular
tissue, gives origin to spherical masses of cells — the spermatidia
(sp) — which develop into sperms with very long motile tails. These

VIII

PHYLUM MOLLUSCOIDA

317

become free from one another and move about in the body-cavity
or in its prolongations into the tentacles. There is no spermiduct,
and it is doubtful if the sperms pass to the exterior. The ovary
(ov) is a small rounded body formed from the parietal layer of the
parenchyma about the middle of the zocecium ; it consists of only
a small number of cells of which only one at a time becomes a
mature ovum, certain smaller cells forming an enclosing follicle.
The mature ovum is perhaps fertilised in the coelome : it passes

end.

end

eel

FIG. 256.— Early stages in the development of Bugula. cent, central mass of cells ; cor. corona ;
cct, ectoderm ; end, eiidoderm ; sej/, segmentation cavity. (After Vigelius.)

into the interior of a rounded outgrowth of the zooecium — the
ocecium (ocec) — lined with parenchyma, and forming a sort of brood-
pouch in which it undergoes its development.

Development. — Segmentation (Fig. 256) is complete and nearly
regular. A blastula is formed having the shape of a bi-convex lens.
In the interior of the blastoccele or cavity of the blastula, four cells
— the primitive endoderm cells — become distinguishable : these
increase in number by division and form a mass of free cells which
almost completely fill the blastocoele ; this mass apparently re-
presents both endoderm and mesoderm. Small cavities which
appear in it subsequently unite together to form the primitive

318

ZOOLOGY

SECT.

coelome. A very broad ring-shaped thickening — the coronet (G,
cor,) — is formed round the equator of the embryo and becomes
provided with cilia. A sac-like, afterwards beaker-shaped, invagi-
nation of the ectoderm on what is destined to become the oral
side of the ciliated ridge, forms a larval structure, termed the sncke/\
(Fig. 257 suck) which afterwards serves to fix the larva. A second
inv agination of the ectoderm in the region of the corona forms
the ectodermal groove. At the aboral pole is developed, also from
the ectoderm, a second larval structure — the calotte or retractile disc
(disc) on which motionless sensory cilia appear. A glandular organ
is developed by ingrowth of the cells lining the ectodermal groove.
An alimentary canal is wanting in the embryo when it escapes
from the ocecium, but develops at a later stage. The sucker

cor

cent

cor

suck

FIG. 257. — A Larva of Buguia plumosa ; B. Sagittal section of larva of Bugrula (diagram-
matic), cent, parenchyma ; cor, corona; disc, retractile disc; pall, ectodermal groove; svck,
sucker. (From Korschelt and Heider.)

becomes everted by a strong contraction of the body, and fixes the
larva to some foreign body. The aboral side of the larva becomes
greatly extended, so that almost the entire integument of the
primary zooid is developed from this part. The retractile disc,
ectodermal groove, and glandular organ are now withdrawn into
the interior. The corona and glandular organ disappear: the
retractile disc takes part in the development of the organs of the
primary zooid. A sac, the wall of which is composed of two layers,
now becomes formed, and gives rise to the organs of the adult
zooid : the inner layer of this sac is formed by invagination from
the retractile disc, while the outer layer appears to be formed from
the central mass of tissue, the remainder of which partly goes
to form the parenchyma of the adult, but partly contributes to
the formation of a rounded granular mass — the brown body — in

PHYLUM MOLLUSCOIDA 319

iich the organs of the larva become merged. The outer layer of
sac is in continuity with the mesoderm layer of the body-wall
gives rise to the mesodermal parts of the zooid. The inner
layer, derived from the ectoderm, forms the deric epithelium, the
nervous system, and the entire enteric epithelium. A diverticulum
of the sac constitutes the first rudiment of the stomach and
intestine ; a second diverticulum forms the rudiment of the
oesophagus ; these become applied to one another and fuse to
form the continuous alimentary canal. The ganglion arises as an
invagination of the ectoderm in the space between the mouth and
the anus. The upper part of the cavity of the primitive sac, after
the rudiment of the alimentary canal has become separated off,
forms a space termed the atrium ; the walls of this become con-
verted into the tentacle sheath, while on its base appear the rudi-
ments of the tentacles and lophophore. During the development
of the organs of the adult zooid the brown body becomes closely
applied to the stomach and gradually absorbed.

The primary zooid thus formed gives rise asexually by a process
of repeated budding to the branching structure which has been
described. In many of the zooecia of a fully-developed colony no
zooid is found to be present, but instead there is a dark brown
body similar to that which occurs in the primary zooecium. These
are zooids that have undergone degeneration — the lophophore,
tentacles, and alimentary canal having become absorbed. Such
degenerated zooids are capable of regeneration, the organs becoming
re-developed and the brown body re-absorbed.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Polyzoa are Molluscoida which, with one exception, form
colonies of zooids connected together by a common organic sub-
stance. There is a lophophore bearing a series of slender, ciliated,
post-oral tentacles. The anterior part of the body forms, in the
majority, a short introvert, within which the lophophore and the
tentacles are capable of being withdrawn. In some the pro-
stomium is represented by a small lobe — the epistome. The
alimentary canal is U-shaped, and the anus is anterior, within, or
just outside of, the tentacular circlet. In most the nervous system
is represented only by a small ganglion between the mouth and
the anus. A cuticle, sometimes gelatinous, sometimes horny,
sometimes calcified, forms a firm exoskeletal layer for the support
of the colony. Nephridia (corresponding to the head-nephridia of
the Trochosphere) occur only in the Endoprocta. ' There is no
vascular system. The sexes are usually united. The majority of
Polyzoa occur in the sea ; a limited number are inhabitants of
fresh water.

320 ZOOLOGY SECT.

Sub-Class I. — Ectoprocta.

Colonial Polyzoa with the anus outside the lophophore, with a
well-developed introvert and a spacious coelome.

ORDER 1. — GYMNOL^EMATA.

Almost exclusively marine Ectoprocta, with a circular lopho-
phore, and without an epistome.

Siib-order a. — Cyclostomata.

Gymnolsemata with tubular calcareous zocecia having circular
apertures devoid of closing apparatus.
Including Crisia, Idmonea, &c.

Sub-wder b. — Cheilostomata.

Gymnolsemata with calcareous or chitinous zocecia usually pro-
vided with opercula.

Including Bugula, Flustra (" Sea-mat ") Membranipora, Cellepora,
Selenaria.

Sub-order c. — Ctenostomata.

Gymnolaemata with chitinous or gelatinous zocecia provided with
a series of tooth-like processes closing the aperture when the
tentacles are retracted.

Including Alcyomdium, Serialaria, Paludicella.

ORDER 2. — PHYLACTOL^MATA.

Fresh-water Ectoprocta with horse-shoe-shaped lophophore and
with an epistome.

Including Cristatella, Plumatella, Fredericella.

Sub-Class II. — Endoprocta.

Colonial or solitary Polyzoa multiplying by the formation of
buds which in Loxosoma soon become separated off, while in
Pedicellina they remain connected together by a creeping stolon.
The anus, as well as the mouth, is internal to the lophophore.
The introvert is slightly or not at all developed. A pair of ciliated
nephridial tubes are present.

Systematic position of the Example.

Bugula avicularia is an example of the sub-order Cheilostomata
of the Gymnolsemata. It is a member of the family Bicellariidae
which is characterised by the erect plant-like colony, with narrow
compressed branches, and attached by root-like fibres : by the

vin PHYLUM MOLLUSCOIDA 321

avicularia, when present, being stalked and birds-head shaped ; and
by the wide oblique apertures of the zooecia all facing in the same
direction. Bugula differs from the other genera of the family in
the arrangement of the zooecia in double or multiple rows, in their
close union, and in the avicularia, when present, being on the side
on which the mouth is situated. The various species differ in the
exact shape of the zooecia and of the avicularia.

3. GENERAL ORGANISATION.
Sub-Class I,— Ectoprocta.

The Ectoprocta and the Endoprocta differ so considerably from
one another that it is advantageous to deal with them separately.
The Ectoprocta are all colonial — the colonies being capable, in
most cases, like the colonies of hydroid zoophytes, of increasing in
size to an apparently indefinite extent by continuous budding.
The thickened cuticle which forms the support of the colony is
sometimes gelatinous, sometimes chitinous, sometimes chitinous
with sand-grains affixed, sometimes calcareous. The form of the
colony varies in different families and genera in accordance with
differences in the shape of the constituent zooecia, and differences
in their mode of budding and consequent arrangement. The
zooecia are sometimes tubular, sometimes ovoid, sometimes poly-
hedral. In some cases the buds are so developed that the colony
assumes the form of a thin flat expansion, which may be encrusting,
and consist of a single layer of zooecia in close contact with one
another or connected together by tubular processes ; or may be
erect, and with the zooecia either in one or two layers : sometimes
the lamellar colony thus formed may be fenestrated or divided into
lobes ; sometimes it is twisted into a spiral. In other cases the
colony, instead of being lamellar, has the form of an erect, shrub-
like structure, consisting of numerous cylindrical, many-sided, or
strap-shaped branches arising from a common root. Sometimes
there is a creeping cylindrical stolon, simple or branched, having
the zooids arranged along it in a single or double row. The colony
is free only in Cristatella (Fig. 258, bis) in which it performs
creeping movements, and in one family of the Cheilostomata — the
Selenariidce — in which it moves along with 'the aid of certain
peculiar appendages — the vibracula — to be described subsequently.

The zooecia open on the exterior by means of circular, or semi-
circular, or crescentic, apertures, which in the Phylactolaemata and
the Cyclostomata among the Gymnolsemata are devoid of any special
closing apparatus, while in the Cheilostomata there is a movable
lid or operculum closed by a pair of occlusor muscles when the
introvert is retracted, and in the Ctenostomata there is a series of
lobes or teeth which close in together over the opening. The

VOL. T Y

322 ZOOLOGY SECT,

cavities of the neighbouring zooecia are in some forms completely
cut off from one another by a continuation of the chitinous or
calcareous exoskeleton; in others there is free communication, in
others againyrfiere is communication through a number of minute
perforations.

The oral (anterior) part of the body of each zooid is, as already
described in the case of Bugula, covered only with a thin and
flexible cuticle, and forms an introvert capable of being retracted
into the interior of the zooecium. At the free end of the introvert
is the mouth surrounded by a lophophore bearing tentacles. The
tentacles are always simple, filiform, and hollow, each containing a
narrow diverticulum of the coalome. They are beset with vibratile

state

FIG. 258. — Plumatella. Portion of a colony magnified, gang, ganglion ; int. intestine ; mo.
mouth ; ce. oesophagus ; repr. reproductive gland ; retr. retractor muscle ; st. stomach ;
stato. statoblasts. (After Allman.)

cilia by means of which currents are created subserving alimenta-
tion and respiration. They are also highly sensitive; and are
capable of being bent about in various directions by the contraction
of muscular fibres in their walls, so that they are capable of being
used for prehension. In the PhylactolaBmata (Fig. 258) the
lophophore is horse-shoe-shaped, in the Gymnolaamata (Fig. 255)
circular : in the former, but not in the latter, there is a ciliated
lobe — the epistome (Fig. 259 cp) — which may have a sensory func-
tion— overhanging the mouth on the anal side. The retraction of
the introvert is effected by a pair of bands of muscular fibres, the
parieto-vaginal muscles, passing to it from the body-wall, and
by a pair of retractor muscles passing from the latter to the ali-
mentary canal.

:

PHYLUM MOLLUSCOIDA

323

Structure of body-wall. — Beneath the cuticle is an epidermis,
consisting of a single layer of flattened polygonal cells, firmly
united together by their edges, so as to form a syncytium comparable
to that of the ectoderm of the Porifera. Beneath this there is
usually, but not always, a layer of muscle, which, when present, is
arranged in two strata — an external composed of circular, and an
internal of longitudinal fibres. There is an extensive coelome
lined in some forms by a definite coelomic epithelium, in part
ciliated, while in others there is no such definite epithelium, but

FIG. 258l>is.— Cristatella mucedo. Entire colony. (After Allman.)

its place is taken by thin parietal and visceral layers of an
irregular cellular tissue — the parenchyma. The body-cavities of
contiguous zooids are in some cases in free communication. Cross-
ing the coelome are strands, in some instances very numerous, of
spindle-shaped cells. In some cases two mesenteric bands suspend
the alimentary canal — an anterior attached near the mouth and
a posterior passing from the coecum to the aboral end of the
zooecium ; in most cases the latter, to which the special name
of funiculus is given, is alone present.

The alimentary canal has in all species the parts that have

Y 2

324

ZOOLOGY

SECT.

been already described in the case of Bugula. In some
Ctenostomata there is in addition a thick-walled chamber—
the ghzard — with chitinous teeth, between the ossophagus and
stomach.

The nervous system consists of a single, sometimes bilobed,
ganglion (Fig. 258, gang, and Fig. 259, yd) placed between the
mouth and the anal aperture, and nerves passing from it to the
various parts. There are never any organs of special sense, unless
the epistome of the Phylactolaemata be of that nature.

Nephridia are not known with certainty to exist in any of
the Ectoprocta. In some there is a pore through which water
enters the body-cavity : or a ciliated intertentacular tube opening
at the base of the tentacles. In Cristatella there is a pair of

ciliated canals with funnel-
like internal apertures and
opening on the exterior by a
common bladder-like excre-
tory duct, and similar ciliated
tubes occur in other Phylac-
tolaemata.

Excretion appears to be
performed by certain cell
of the funicular tissue and o
the parenchyma or ccelomi
epithelium. These becomi
loaded with the products o
excretion, and become fre<
as leucocytes in the ccelome
whence they probably pas
out through the interten
tacular tubes or ciliate(
canals.

In many Ectoprocta th
colony bears a series of re
markable appendages — th
avicularia — which are of the nature of modified zooids. Ir
typical cases the avicularium has the bird's-head-like form tha
has been already described in the case of Bugula ; sometimes i
is completely sessile. A second set of movable appendages fouiic
in some forms are the vibracula ; these are long tapering whip
like appendages which execute to-and-fro movements. The avicu
laria are frequently found to have seized in. their jaws minut
Worms or Crustaceans, and it is probable that their function, a
well as that of the vibracula, is defensive : in the case of th
Selenariidce, which form unattached colonies, the movements of th
vibracula subserve locomotion.

Tin- impregnated ova in many cases undergo the early stages of

FIG. 259. — Anterior portion of the body of
Lophopus from the right side. an. anus ;
c/i. epistome ; pa. ganglion ; o. mouth ; pr. in-
testine ; st. oesophagus ; t. tentacles cut off
near the base. (From Lang's Text-Book.)

viii PHYLUM MOLLUSCOIDA 325

their development in certain dilatations of the colony (Fig. 255,
oosc.), and in many of the Gymnolsemata (Cheilostomata) these
ovicells or ocecia^ as they are termed, take on a very definite
shape.

Reproduction and Development. — As a general rule the
Ectoprocta are hermaphrodite. Both ovary and testis are derived
from the layer lining the coelome (parenchyma or coelomic
epithelium as the case may be), or from the funicular tissue. The
testis may be single or double. In some cases there is a spermi-
duct continuous with each; in most this is absent, and the
spermatidia, as in Bugula, or the mature sperms, become free in
the coelome. The ovary is very generally situated towards the
oral end or about the middle, the testis towards the base. The
mature ova escape into the ccelome, and in some forms become
impregnated there apparently by the spermatozoa of the same
individual. The development of the larva may take place in the
ccelome or a special diverticulum of it ; in the Cheilostomata the
fertilised ova pass into the ovicells ; in some cases, both among
the Phylactolasmata and the Gymnolaemata, they are received into
a sheath formed by the tentacles of an imperfectly-developed
zooid formed in a zocecium in which the original zooid had
undergone degeneration.

In those cases in which the early stages of the development are
passed through in the body-cavity of the parent, the ciliated
embryos may either escape through the zooacial aperture after the
zooid has undergone degeneration, or through a special opening at
the base of the tentacles. In some the fertilised ova pass out
through the intertentacular tube. In Crisia and other Cyclo-
stomata each of the ripe ooecia is found to contain a large number
of embryos, developed from one ovum. The ovum in this genus
segments to form a mass of cells from which finger-like processes
are given off, the end of each of these becoming constricted off to
form an embryo.

Segmentation is total and approximately equal. The form of the
free-swimming larva varies considerably, but in most there is a
circular band with very long cilia, the corona, which may represent
the tentacular crown of the adult ; this divides the surface into
two regions — oral and aboral — the mouth as a rule opening on
the former, and the anus on the latter. The aboral portion of the
body presents a ciliated retractile disc or calotte ; on the oral side is
the sucker by which the larva afterwards becomes fixed. In the
Cyclostomata the larva is barrel-shaped, with the mouth at one
end, and at the other a prominence corresponding to .the retractile
disc. In the Phylactolsemata the larva is in the form of a ciliated
hollow cyst from which the colony is formed by gemmation. A
special form of asexual multiplication by means of bodies termed
statoblasts (Fig. 258, stato) is observable in the Phylactolaemata.

326 ZOOLOGY SECT.

The statoblasts are internal buds formed from the funiculus and
enclosed in a chitinous shell; they are set free eventually by
the death and decay of the parent colony, and in spring each
gives rise to a small zooid which fixes itself and develops into
a colony.

Ethology and Distribution. — None of the Ectoprocta are
parasites in the strict sense of the term, but very many of them
live in intimate association with other organisms, often growing
over and through them so as to form with them one complex
structure. Certain genera are able by some means to excavate
minute burrows in the shells of bivalves.

The majority of Ectoprocta are marine; but all the Phylacto-
laemata, together with Paludicella of the Ctenostomata, are in-
habitants of fresh water. The fresh-water forms inhabit both
running and stagnant waters ; they occur at all elevations and
are represented in all the great regions of the earth's surface.
The marine forms are most abundant at moderate depths :
but representatives of the group have been dredged from as
great a depth as over 3,000 fathoms. In certain localities the
larger kinds grow in great luxuriance, so as to form miniature
forests.

Geologically the Ectoprocta are a very ancient group, being
represented in the Cambrian and later Paleozoic formations by
forms which appear to have belonged mainly, if not exclusively,
the Cyclostomata. In the later formations of the Mesozoic peric
the Cheilostomata are also abundantly represented, and in th<
Tertiary the latter sub-order greatly outnumbers the Cyclostomat
The Tertiary Polyzoa flourished in certain localities in sue]
luxuriance that their remains form calcareous deposits of verj
.great extent.

Sub-Class II. — Endoprocta.

While the sub-class of the Ectoprocta comprises a large. numbei
of genera, that of the Endoprocta includes only Pedicellina (Fig. 260),
Loxosoma, and Urnatella, with one or two other less completely
known forms. They are all marine except Urnatella — an Ameri-
can fresh-water genus. The feature indicated by the nanu
of the sub-class — viz. the position of the anus within th(
circlet of the tentacles, is an important point of difference froi
the rest of the Class ; but there are others of as great or greatei
importance.

In none of the Endoprocta is there a distinct introvert. Th<
body is cup-shaped, with a rim which is capable of being invert*
over a cavity — the vestibule — within which the tentacles can be
withdrawn, and which contains both mouth and anus. An epistome

VIII

PHYLUM MOLLUSCOIDA

327

overhangs the mouth. The coelome is almost or quite obliterated,
the space between the alimentary canal and the wall of the body
being filled, more or less completely, with a gelatinous hyaline
matrix. A pair of nephridia are present. In Loxosoma they lie
one on each side of the oesophagus and open separately on the
exterior; they are ciliated intra-cellular tubes, each of which
probably begins in a flame cell. In Urnatella the two nephridial
tubes unite to open into the cloaca — a diverticulum of the
vestibule. The ganglion (gang), situated between mouth and anus
as in the Ectoprocta, is bilobed in Loxosoma. Tcstes and ovaries
occur in the same individual in some, but appear to mature at
different times : they are provided with special ducts ; in others
he sexes are separate.

Pedicellina and Urnatella are colonial, Loxosoma solitary. In
"icellina (Fig. 260) there is a creeping stolon with which a

tent

lent

_ jdicellina. Showing successive stages : (numbered 1 to 6) in the development of
zooids by budding, an. anus ; gang, ganglion ; mo. mouth ; tent, tentacles (retracted). (After
Hatsphek.)

number of zooids are connected ; a diaphragm separates the body
of each zooid from the stalk. Urnatella has a disc of attachment
with one to six, jointed, branching stems. In Loxosoma, which is
found attached to various Annulata, two parts are distinguishable
—the calyx oT.body and the stalk. In the base of the latter is the
so-called fool-gland, consisting of a small number of granular cells
arranged around a central space opening on the exterior. Buds
are formed, but become detached before reaching maturity. Seg-
mentation of the ovum is complete, and a gastrula is formed by
invagination.

328

ZOOLOGY

SECT.

The Endoprocta are only doubtfully to be included in the same
class with the Ectoprocta. The position of the anus, the absence
of the introvert, and the presence of typical nephridia would not in
themselves be sufficient to justify their removal from the Polyzoa,
and perhaps not even the obliteration of the coelome. But should
certain statements, which have been published with regard to
their development and metamorphosis, be con-
firmed, we should be driven to the conclusion
that the Endoprocta are not directly related
to the Ectoprocta at all. The gist of these
statements is that the line joining mouth and
anus in the Endoprocta is ventral and not
dorsal, the ganglion infra-oesophageal, and the
tentacular circlet prse-oral ; and if this should
be established the structures named cannot be
homologous in the two groups.

CLASS II.— PHORONIDA.

The position of Phoronis — a wortn-iike marine
animal — is a matter of uncertainty ; but it ex-
hibits some unmistakable points of resemblance
to the Polyzoa, more particularly to the Phy-
lactoloernata, and it may very fairly be dealt
with as a third class of the Molluscoida,

Phoronis (Fig. 261) lives in associations con-
sisting of a number of individuals, all of which
are developed from ova, there being no process
of asexual formation of buds. Each worm is
enclosed in a membranaceous or leathery tube,
within which it is capable of being completely
retracted. The body is cylindrical, elongated,
and unsegmented. At one end there is a
crown of numerous slender ciliated tentacles
borne on a horse-shoe-shaped lophophore, the
lateral cornua of which are spirally coiled in
the larger species ; these are supported by a
mesodermal skeleton and are non-retractile.
Both mouth and anus (Fig. 262, mo, an} are
situated at this tentacular extremity of the
body, separated from one another by only a short space. A small
lobe — the epistome (ep) — overhangs the mouth arid lies between
it and the anus. Near the anus open two ciliated nephridial
tubes (neph) of mesodermal origin, which open internally into the
posterior chamber of the body-cavity. The coelome is lined with
a peritoneum from which there proceed three mesenteries (Fig. 263)
— a ventral longitudinal and two transverse, the latter dividing

FIG. 261.— Phoronis
australis, natural
size.

PHYLUM MOLLUSCOIDA

Fir:. -2fr2.— Phoronis aus trails, free end,
magnified, an. anus ; ep. epistome ; mo. mouth ;
nephr. nephridial aperture ; n<ph. nephridium.
(After Benham.)

tent

the cavity of the body into three chambers. The alimentary
canal (Fig. 263) is divisible into oesophageal, gastric, and intestinal
regions There is a closed
system of blood-vessels •
with contractile walls con-
taining red blood-corpus-
cles. The nervous system
is not completely separated
from the epidermis. Its
central part consists of a
horseshoe - shaped post-
oral nerve-ring at the

base of the tentacles, in ^_^ — ,

front of which lie two cili-
ated grooves, apparently
organs of sense. Collections
of nerve-fibres and gan-
glion cells lie in the integument and are connected with
nervous processes of the epidermal cells.

Phoronis is hermaphrodite. Ova and sperms are developed

in the anterior left chamber of
the coelome from cells on the
wall of one of the large blood
vessels. When mature these pass
out through the nephridia to
the spaces enclosed by the ten-
tacles, where the ova are im-
pregnated (or, according to
another account, fertilisation
takes place in the ccelome) and
go through the early stages of
their development fixed to the
tentacles. The segmentation is
complete and slightly irregular :
when four blastomeres are formed
two larger, darker, endoderm,
and two smaller, clearer, ecto-
derm cells are to be distin-
guished. A blastula is formed
with clearer ectoderm cells on
one side; invagination takes
place; and, as the embryo elon-
gates, the blastopore is drawn
out into a slit which eventually
becomes closed up behind, the
anterior portion alone persisting and giving rise to the mouth.
The cells bordering upon the posterior part of the blastopore take

nephr. d.

FIG. 263.— Phoronis australis, internal
organisation, af. bl. afferent blood vessel ;
an. anus ; ef. bl. efferent blood vessel ; ep.
epistome ; rues, mesentery ; mo. mouth ;
nephr. p. nephridiopore ; nephr. d. duct
of nephridium ; nephrost. nephrostome (in-
ternal opening of nephridium ; oes. oeso-
phagus ; rect. rectum ; rect. mes. rectal
mesentery ; sept, septum ; tent, tentacles
(cut short). (After Benham.)

330 ZOOLOGY SECT.

a share in the formation of the mesoderm. Two lateral pouch -
like diverticula of the endoderm sac give rise to the mesoderm ;
in these a cavity appears — the rudiment of the ccelome. A
second pair of sacs of the same character becomes developed
further back. A large pra3-oral lobe is formed, and the anus
makes its appearance at a little distance behind the mouth as an

FK;. 'Jd4. — Phoronis, Development. A, young larva ; B, larva after the formation of the post-
oral circlet of tentacles ; C, larva with commencing pit-like involution ; I), larva with invagina-
tiou partly everted ; E, invagination completely everted, -m. mouth ; an. anus ; ic. involution
to furni the body. (From Balfour's Embryology.)

invagination of the ectoderm ; it becomes surrounded by a circlet
of cilia (Fig. 264, A). The part of the body on which the anus
is situated becomes elevated into a conspicuous process. Behind
the mouth there is a circlet of cilia and from this region grow out
a circlet of processes — the rudiments of the larval tentacles (B).
The larva has now reached the stage to which the term Actino-
trocha is applied. It has a large hoodlike lobe overhanging the

vin PHYLUM MOLLrSCOLDA 331

mouth, and a circlet of ciliated larval . tentacles ; the anus is
situated on a prominent process.

The ectoderm of the process on which the anus is situated
subsequently becomes involuted to form a deep pit (C, iv). The
metamorphosis from this point is completed with great rapidity.
The pit at the side of the anal elevation becomes everted (D),
and the alimentary canal of the larva is drawn into it (E}, the pro-
jection thus formed, which grows out at right angles with the long
ixis of the larva, becoming the body of the future animal ; the
mtacles are drawn into the stomodseum and there become broken
up to form the lophophore of the adult : the hood-like prse-oral
obe is taken into the stomodseum and there digested.

CLASS III.— BRACHIOPODA.

The Brachiopoda are the fabricators of the well-known " Lamp-
shells " found in most parts of the world. They occur in the sea
at various depths, and were formerly classed under Mollusca, their
characteristic bivalved shell being compared with that of oysters,
mussels, &c.

1. EXAMPLE OF THE CLASS — Mayellania ( Waldheim-ia) lenticularis

or M. flavescens.

Magdlania lenticularis is found in great numbers, at moderate
depths, off the coast of New Zealand. An allied species, M. flavescens,
is equally common in the Australian seas, and several other species
are known in various parts of the world.

The body is entirely covered by a shell (Fig. 265) of oval form,
and pink colour, composed of two pieces or valves, one of which, dis-
tinguished as the ventral valve (v. v), projects beyond the other
or dorsal valve (d. v), in the form of a short conical beak (b) perfor-
ated at the end by an aperture, the foramen (b), through which
passes a dark brown stalk or peduncle (Fig. 266,E,pd) of horny
consistency. In the natural state the peduncle is attached to a
rock or other support, and the animal lies with the ventral valve
uppermost and with the valves gaping slightly. The pointed or
peduncular end of the shell is considered to be posterior in posi-
tion, the opposite end or gape anterior.

It will be convenient to consider the shell first. Both valves are
deeply concavo-convex, of a pinkish colour outside, white within.
The ventral valve (Fig. 265), as already stated, is produced poste-
riorly into a beak (b), terminating in a foramen (/) for the peduncle.
The distal margin of the foramen is left incomplete by the shell
proper, but is closed by a small double plate, the deltidium (d}.
Immediately anterior to the beak is the curved liinge-line along
which the valve articulates with its fellow, and just anterior to

332

ZOOLOGY

SECT.

the hinge-line the inner, surface of the shell is produced into a pair
of massive, irregular hinge-teeth (f). On the inner surface of the
valve, towards its posterior end, are certain shallow depressions
marking the attachment of muscles (ad. ra, d. m).

The dorsal valve (D) has no beak, but its posterior edge forms
a hinge-line which is produced in the middle into a strong cardinal

d.m'

FIG. 265.— Magellania flavescens. A, the entire shell from the dorsal aspect ; B. from the
left side ; C, interior of ventral valve ; D, of dorsal valve ; ad. in. adductor impressions ; b.
beak ; c. p. cardinal process ; d. deltidium ; d. m. divaricator impressions ; d. r. dorsal valve ; /.
foramen ; p. m. protractor impressions ; s. tooth-socket ; s. L shelly loop ; up. septum : t. tooth ;
v. v. ventral valve. (After Davidson.)

process (c. p) with a curiously folded surface : when the two valves
are in position this process fits between the hinge-teeth of the
ventral valve, the hinge-teeth in their turn being received into de-
pressions (s) placed on each side of the cardinal process. The inner
surface of the dorsal valve is produced into a median ridge or
septum (sp\ continuous posteriorly with the cardinal process, and
attached on either side of the base of the latter are the two ends
of a delicate calcareous ribbon, the shelly loop (s. I), which projects

•

vin PHYLUM MOLLUSCOIDA 333

freely into the cavity enclosed between the two valves, and has the
form of a simple loop bent upon- itself. The inside of the dorsal
valve also has muscular impressions.

Externally both valves present a series of concentric markings
parallel with the edge or gape : these are lines of growth, the
shell being built up by new layers being deposited within those
previously formed, and projecting beyond them so as to form a
series of outcrops.

Microscopically the shell consists of prismatic rods or spicules
of carbonate of lime, placed obliquely to the surface, and separated
from one another by a thin layer of membrane. It is also tra-
versed, perpendicularly to the surface, by delicate tubules which
begin on the inner surface in microscopic apertures, and extend
to within a short distance of the outer surface.

'The actual body of the animal (Fig. 266, B) lies at the posterior

d of the shell, occupying not more than a third of the space
nclosed between the two valves : it is consequently more or less
wedge-shaped in form, and presents dorsal and ventral surfaces in
contact with the two valves, and an anterior surface looking
towards the gape. The dorsal is of greater extent than the
ventral surface, so that the anterior surface is placed obliquely.

The dorsal and ventral regions are continued each into a flat
reduplication of the body-wall, closely applied to the correspond-
ing valve and containing a prolongation of the ccelome. The two
flaps thus formed are the dorsal (d. m) and ventral (v. m) mantle-
lobes. They are fringed with minute setae (s) lodged in muscular
sacs, like those of Chsetopods (vide infra), and give off from their
outer surfaces hollow processes which extend into the tubules of
the shell mentioned above.

The large wedge-shaped space or mantle-cavity, bounded by the
mantle-lobes above and below, and behind by the anterior surface
, is occupied by a huge and complex lophopliore (Figs.
ancT^247, Iph), which springs from the anterior surface of the
body, and, like that of the fresh-water Polyzoa and of Phoronis,
has the general foim of a horse-shoe. It is, however, peculiarly
modified : the two limbs of the horse-shoe curve towards one
another so as to adapt themselves to the mantle-cavity, and the
middle of the concave edge, which is dorsal in position, is pro-
duced into a spirally coiled offshoot (Iph'} which lies between the
two arms, and is coiled towards the dorsal side. The lophopliore
is hollow, containing a spacious cavity or sinus : its two main arms
also receive prolongations of the ccelome into which the digestive
glands project : it is fringed throughout its whole extent with
long ciliated-tentacles which form the outer boundary of a ciliated
food-groove, bounded on the inner side by a wavy ridge or lip
(Ip, lp'}. By the action of the cilia microscopic particles are swept
along the food-groove to the mouth.

334

ZOOLOGY

JSECT.

Digestive Organs. — The moutli (mtJi) is a narrow crescentic
aperture situated in the middle of the lophophore, towards its

.si

FK;. 2ii(i.— A. Body of Magrellania lenticularis, removed from shell; B, sagittal section
of the entire animal. Both semi-diagrammatic, the lophophore being represented as of smaller
proportional size than in the actual animal (cf. Fig. 207). >L <jl. digestive gland ; d. //i. dorsal
mantle-lobe ; </. v. dorsal valve of shell ; r/o/ii, (ton'*, gonads ; lit. heart ; int. intestine ; Ip, tyi.
lip; /^.lophophore; lpl,i, its coiled process; mth. mouth; nph. in B, nephridium, in A
nephridial aperture ; pd. peduncle ; jtl. xi. pallial sinuses ; «. setae ; v. m. ventral lobe of
mantle ; r. c. ventral valve of shell.

convex or ventral edge, and is bounded dorsally by the lip. It
leads into a V-shaped enteric canal which consists of a gullet,

VIII

PHYLUM MOLLUSOOIDA

335

passing upwards from the mouth, an expanded stomach, (st) and a

straight intestine (int.) which extends from the stomach downwards

and backwards towards the ventral surface and ends blindly,

there being no anus. On

each side of the stomach

and opening into it by a

duct, is a large, branched

digestive gland (d. gl~). The

whole canal is lined with

ciliated epithelium.

The body-wall consists
externally of an epidermis
formed of a single layer of
cells, then of a layer of
connective tissue, of a car-
tilaginous consistency in
many parts, and finally of a
ciliated coelomic epithelium
lining the body-cavity. On
the outer surfaces of the
mantle-lobes,where they are
in contact with the shell, the
epidermis is replaced by a thin membrane showing no cell structure.

The muscular system (Fig. 268) is well developed. Two large
adductor muscles (ad. m} arise on each side from the dorsal valve

ad.m

'.sh.

Fj<;. _'»,:. — Magellania flayescens,the ventral
valve removed ; c. p. cardinal process ; Iph. arm
of lophophore ; Iph.1 its coiled process having
the tentacles removed on the right side ; mth.
mouth. (After Davidson.)

dim,

FIG. 268. — Muscular system of Magellania. ad. m. adductors; b. beak; d. aj. m. dorsa
adjusters; d. m, d. m' divaricators ; d. r. 'dorsal valve; int. intestine,; mth. mouth; pd,
peduncle ; pd. sh. sheath of peduncle ; p. m. protractor ; si. shelly loop ; i: a. j. m. ventral
adjusters ; v. v. ventral valve. (After Hancock.)

and, passing downwards, unite with one another so as to have a
single insertion on the ventral valve : their action is to approxi-
mate the valves and so to close the shell. A large and a small

336

ZOOLOGY

SECT.

pair of di'varicators (d. m, d. m') arise from the ventral valves, and
are inserted into the cardinal process, which they depress : as this
process is situated posteriorly to the hinge-line, its depression
raises the rest of the dorsal valve, and so opens the shell. Two
pairs of muscles arising, one from the ventral, the other from the
dorsal valve, and inserted, into the peduncle, are called adjusters
(aj.m): the peduncle being fixed, they serve to alter or adjust
the position of the animal as a whole by turning it in various
directions.

The coelome is a spacious cavity more or less encroached upon
by the muscles and other organs, and traversed by sheets and

in.. -_'i'''.'.— Anterior body-wall of Terebratula, to show nervous system, &c.: dm. dorsal mesen-
tery : (i. brain ; gt. genital folds ; n. nephridium ; nt. nephrostome ; ces. gullet ; or. ovary ; sc.
CBBOpnageal connective ; usg. infra-cesopkageal ganglion ; vm. ventral mesentery ; dmn, hn, ian,
san. nerves. (From Lang's Comparative Anatomy.)

bands of membrane which connect the enteric canal with the
body- wall, and thus act as mesenteries. The coelome is continued
into each of the mantle-lobes in the form of four canals or pallial
sinuses (Fig. 266, pi. s), the two outer of which are extensively
branched.

Blood System. — Attached to the posterior region of the
stomach is a small, almost globular sac (h\ which has been proved
to be contractile, and is to be considered as a heart. Vessels have
been traced from it to various parts of the body, but the relations
of the whole circulatory system and the course of the circulation
arc very imperfectly known.

vin PHYLUM MOLLUSCOIDA 337

The excretory organs consist of a pair of very large nephridia
(nph) lying one on each side of the intestine. Each is funnel-
shaped, having a wide inner opening or nephrostome, with plaited
walls, opening into the coelome, and a narrow curved outer portion
which opens into the mantle-cavity not far from the mouth. As
in many cases which have already come under our notice the
nephridia act also as gonoducts.

The nervous system is a ring (Fig. 269) round the gullet pre-
senting supra- (g) and infra- (usg) oesophageal swellings or ganglia
of which the infra-oesophageal is the larger. Nerves are given off
to the mantle, lophophore, etc. No special sense-organs are
known.

Reproductive Organs. — The sexes are separate. There are
two pairs of gonads (Fig. 266, gori), one dorsal, one ventral, in
the form of irregular organs sending off branches into the pallial
sinuses.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Brachiopoda are Molluscoida in which the body is enclosed
il a shell formed of two parts or valves which are respectively
dorsal and ventral in position. The body occupies only a small
portion of the space enclosed by the shell, and is usually attached
to foreign objects by a posteriorly placed stalk or peduncle : it
gives off dorsal and ventral reduplications, the mantle-lobes, which
line the valves of the shell and enclose a large mantle-cavity.
From the anterior surface of the body is given off a lophophore
which surrounds the mouth, and is beset with ciliated tentacles.
There is a ridge-like prse-oral lip which is continued on to the
lophophore. The enteric canal is usually V-shaped, and is
divisible into gullet, stomach, and intestine : there is a pair
of digestive glands. The coelome is spacious, and is continued
into the mantle-lobes. A heart is usually present, attached to
the stomach. The excretory organs are one or two pairs of
nephridia which act also as gonoducts. The nervous system is a
ganglionated circum-oesophageal ring : sense-organs are usually
absent in the adult. The sexes are separate or united. Develop-
ment is accompanied by a metamorphosis.

The class is divided into two orders : —

ORDER 1. — INARTICULATA.

Brachiopoda in which the shell is not composed of oblique
prisms : the valves are not united by a hinge, and there is no
shelly loop for the support of the lophophore. An anus is
present.

Including Lingula, Crania, Discina, etc.

VOL. I Z

338 ZOOLOGY SECT.

ORDER 2. — ARTICULATA.

Brachiopoda in which the shell is formed of oblique prisms of
calcium carbonate : the two valves unite by a definite hinge, and
there is usually a shelly loop, for the support of the lophophore,
developed in connection with the dorsal valve. The intestine ends
blindly.

Including Magellania, Terebratula, Bhynchonellct, Cistella
(Argiope), etc.

Systematic position of the Example.

The genus Magellania, of which there are several species,
belongs to the family Terebratulidse, and to the order Articulata.

The dissimilar valves of the shell articulated by teeth and
sockets, and the absence of an anus, place it among the Articulata,
The Terebratulidse are distinguished by an oval or rounded shell,
the structure of which is punctate, the dots corresponding with
blind tubes receiving processes of the mantle ; the beak of the
ventral valve is prominent, and has a foramen partly bounded by a
deltidium of one or two pieces : there is a shelly loop springing
from the hinge-line of the dorsal valve. The genus Magellania is
characterised by having the shelly loop fully half as long as the
shell itself, and by the presence of a median septum on the inner
face of the dorsal valve.

The specific differences between M. lenticularis and M. flavescens
are largely matters of detail, depending upon the precise form of
the shell and loop. More obvious differences are the pink, evenly-
rounded, short-beaked shell of M. lenticularis, that of M. flaves-
cens being horn-coloured, almost pentagonal, and with a pro-
minent beak.

3. GENERAL ORGANISATION.

The shell presents two distinct types ; in the Articulata, the
order to which Magellania belongs, the dorsal and ventral valves
are dissimilar, the dorsal valve having a cardinal process and usually
a shelly loop, the ventral a spout-like beak for the peduncle ; while
in the Inarticulata, of which Lingula is a good example (Fig. 2 70, A),
the two valves are nearly alike, and there is no shelly loop, and no
beak. These differences are accompanied by differences in micro-
scopic structure ; in the Articulata the shell is a dense stony
structure formed of obliquely placed calcareous prisms, while in
the Inarticulata it has no prismatic structure, but usually con-
sists of a chitinoid material more or less strengthened by calcareous
spicules. Among the Articulata the loop may be absent ; when
present, it varies greatly in form and size, being sometimes very
small and simple (Fig. 270, C, D), sometimes bent upon itself, as

PHYLUM MOLLUSCO1DA

339

in Magellania, sometimes attached to the septum or to the interior
of the dorsal valve (E), sometimes, as in the extinct Spirifcra,
represented by a complex double spiral (F), sometimes reduced to
short paired rods springing from the septum (G).

The majority of both orders are attached by a longer or shorter

peduncle which passes between the proximal ends of the valves in

Lingula (Fig. 270, A), through a perforation in the ventral valve in

>iscina (C), and through a foramen in the spout-like posterior end

FIG. 270.— Typical Brachiopoda, A, Lingula; B, Crania; C, Discina j D, Terebratula;
E, Cistella ; F. Spirifera ; G, Kraussina. (After Bronn.)

of the ventral valve in the Articulata. Crania (B) has the ventral
valve fixed directly to foreign objects, the peduncle being absent.

The lophophore is found in its simplest form in Cistella
(Fig. 271, A) in which it is a horse-shoe-shaped disc, with very
short arms, attached to the dorsal mantle-lobe, and surrounded
with flexible tentacles which project between the vajves. From
this the lophophore of Magellania, which may be considered as
typical for the Articulata, is easily derived by an increase in size,
and by the prolongation of the middle region of the concave edge
into a coiled offshoot. In the Inarticulata (C), and in Rhyn-
chonella (B) among the Articulata each arm of the horse-shoe is
coiled into a conical spiral, which in some cases can be protruded
between the valves.

The most noteworthy point about^the muscular system is the
fact that the shell is both opened and closed by muscular action.

z 2

340

ZOOLOGY

SECT.

The dorsal valve may be taken to represent a lever of which the
hinge-line is the fulcrum, the cardinal process the short arm, and
the main portion of the valve the long arm. The muscles all arise
from the ventral valve, the adductors being inserted into the inner
face of the dorsal valve, which they depress, the divaricators into
the cardinal process, their action depressing it and thus elevating
the valve itself. In Lingula there is a very complex muscular
system by means of which the valves can be rubbed upon one another,
or moved laterally as well as opened and shut.

In the Articulata the enteric canal is V-shaped, as in Magel-
lania, the intestine being straight or nearly so, and ending blindly.

FIG. 271.— Dissections of A, Cistella; B, Rhynchonella ; and C, Lingula. a. anus; Iph.
lophophore ; mtk. mouth. (After Schulgin and Hancock.)

In the Inarticulata, on the other hand, the intestine is usually
coiled, and always ends in an anus (Fig. 271, C, a), which generally
opens into the mantle-cavity, but in one genus (Crania) into a
pouch or sinus at the posterior end of the body between the
valves.

A heart is usually present, but the function of blood is per-
formed mainly by the coelomic fluid, which is propelled by the
cilia lining that cavity and circulates both in the coelome itself and
in the pallial sinuses, each sinus presenting — in Lingula at least
— both an outgoing and an ingoing current.

A single pair of nephridia, resembling those of Magellania,

PHYLUM MOLLUSCOIDA

341

urs in all known genera except Rhynchonella, in which there are
two pairs, one dorsal and one ventral. Besides discharging an
excretory function they act as gonoducts.

The nervous system always takes the form of a circum-ceso-
phageal ring with ganglionic enlargements, the largest of which
is ventral or sub-cesophageal in position. Otocysts have been
described in Lingula, rudimentary eyes in Megerlia, and patches
of sensory epithelium in Cistella : with these exceptions sensory
organs are unknown.

There are usually four gonads, two dorsal and two ventral,
sending prolongations into the pallial sinuses. Some genera are
dioacious, others hermaphrodite, the epithelium of the gonads,
producing, in the latter case, both ova
and sperms.

The development of the Brachiopoda

best known in Cistella, in which the
first stages of development are passed
through in a pair of cavities, the brood-
pouches, situated at the base of the
lophophore. Segmentation is regular
and complete, and results in the forma-
tion of a blastula which is converted
into a gastrula by imagination (Fig.
272, A). Paired sacs, the coelomic pouches
(p.v), grow out from the archenteron,
and the blastopore closes. The coelomic
sacs separate from the mesenteron (B, me]
or middle portion of the archenteron, and
extend between it and the ectoderm,
forming the right and left divisions of
the coelome : their outer walls thus be-
come the somatic, their inner walls the
splanchnic layer of mesoderm. The

mesenteron remains closed and surrounded by the coelomic sacs
during the whole of larval life.

The embryo now elongates and becomes divided by an annular
groove into two divisions, an anterior and a posterior : a second
groove soon appears in the anterior division, the embryo then con-
sisting of three regions (B), which, from a superficial point of view,
might be looked upon as metarneres. But as the segmentation
affects only the body-wall and not the internal parts, the process
is not one of metamerism, and the three apparent segments are
called respectively the head-region (Fig. 273, vs), the body-region,
(ms) and the peduncular region (As).

Next the head-region grows out into an umbrella-like disc sur-
rounded with cilia and bearing four eye-spots (A), and on the
body-region a backwardly-directed annular fold (m) appears, bear-

FIG. 272.— Two stages in the
development of Cistella
(Argiope), b. provisional setfe ;
bl. blastopore ; me. mesen-
teron ; pi: coslomic pouches.
(From Balfour's Embryology,
after Kowalensky.)

342

ZOOLOGY

SECT.

ing four groups of provisional setae. Soon this mantle-fold divides
into dorsal and ventral lobes, which, being directed backwards,
cover the peduncular region.

In this condition the larva swims freely like a trochosphere.
After a time it comes to rest arid fixes itself by its peduncular seg-
ment (B). The two lobes of the mantle-fold (m) become reflexed so.

as to point forwards instead of back-
wards, thus leaving the peduncular
region exposed and covering the
head-region : by this process the
outer surface of the larval mantle
becomes internal, and vice versa. A
stomodaeum is formed on the head-
region, and, communicating with the
mesenteron, establishes the enteric
canal. The umbrella-like head-region
decreases in size, and perhaps forms
the lip, which is at first confined to
the region immediately dorsal to the
mouth. The lophophore appears at
first on the inner surface of the
dorsal mantle-lobe, but gradually ex-
tends and surrounds the mouth. In
its early stages it is circular, but
afterwards assumes the horseshoe
form by sending out paired exten-
sions. In genera, like Magellania.
with a complex lophophore, this
organ has at first a simple horse-
shoe form (Fig. 274, Ipli.). A shell is
secreted by the mantle-lobes, and the
peduncular region becomes the pe-
duncle of the adult.

Distribution. — The Brachiopoda
are all marine. They are widely
distributed geographically, and live
at various depths — from between tide-
marks to 2,900 fathoms. At the
present day the class includes only
about twenty genera and 100 species,

but in past times, the case was very different. Brachiopods appear
first in the lower Cambrian rocks, where the existing genera,
Lingula and Discina, are found. No more striking examples can
be adduced of persistent types — organisms which have existed
almost unchanged for the vast period during which the whole of
the fossi lite roiis rocks have been in process of formation. Alto-
l(M) genera are known from the palaeozoic rocks, thirty-four

— rad

FK,. [273.— Two later stages in the
development of Cistella. A,
free-swimming; B, after fixation.
Its. peduncular region ; m. mantle
ms. body-region ; nut. mesenteron
/'•/•. ciliated ring ; r*. head-region
(From Lang's Compnratire Anat
num. after Kowalevsky.)

VIII

PHYLUM MOLLUSCOIDA

343

from the mesozoic, and twenty-one in the cainozoic and recent
periods. Obviously the group is tending, though slowly, towards
extinction.

Recent researches on fossil and recent forms have shown the
Brachiopoda to illustrate, in a remarkable manner, the Recapitu-
lation theory already referred to : the theory, that is, that
ontogeny or individual development
is a more or less modified recapitula-
tion of phylogeny or ancestral develop-
ment. It has been shown that there
is a striking and almost complete
parallelism between the stages in the
development of the shelly loop in such
highly organised forms as Magellania,
and the entire series of articulated
Brachiopods, from those with the sim-
plest to those with the most complex
loop.

Iph

MUTUAL RELATIONSHIPS OF THE
CLASSES OF THE MOLLUSCOIDA.

mth

d.gl

FIG. 274. — Lophophore of embryo
of Terebratulma. it. </!. di-
gestive gland ; int. intestine ;
(p. lip ; Iph. lophophore ; mth.
month. (From Korschelt and
Heider, after Morse.)

In adult structure Phoronis ex-
hibits marked resemblances to the
Ectoprocta, more especially to the
Phylactolsemata — resemblances which
will be rendered clear by a compari-
son of the diagrams A and B of Fig.
275. In both the ventral side of the

body is greatly produced and elongated, and, by the approximation
of the mouth and anus, the dorsal surface is reduced to a very
short space between those two apertures. The form of the lopho-
phore, the presence of an epistome having similar relationships in
the two groups, and the fact that the coslome is similarly developed
in the two groups, point in the same direction. Some points
which are supposed to indicate relationships with the Annulata
and with the Chordata are referred to at a later stage.

The resemblances between the Brachiopoda and the other two
classes of the phylum are somewhat disguised by the development
of the shell, but are very obvious, more particularly when we take
into account certain of the features of the development. One of
the most striking points of resemblance between the three classes
is the presence of the lophophore with its tentacles ; in the earlier
stages of its development in the Brachiopod, as we have seen, this
structure (Fig. 274) has the horseshoe shape which it preserves in
the adult Phoronida and Phylactolsemata, and a lobe — the arm-
fold or lip (/})} — comparable to the epistome, is present, overhanging

344

ZOOLOGY

SECT.

the mouth. The end of the body of the Brachiopod with which
the peduncle is connected must correspond to the aboral extremity
in the Polyzoa, since this represents the part by which the larval
Polyzoan becomes fixed, the everted " sucker " of the latter being
evidently homologous with the foot-segment of the larval Brachi-
opod. The end of the body of the Brachiopod from which the
peduncle proceeds is thus the ventral portion. From the position

FIG. 275. — A, Diagrammatic median section of a phylactoltematous Polyzoan. «,>.. anus;
ep. epistome ; tp. ca,v. epistome cavity ; funic. funiculus ; gang, ganglion ; int. intestine ;
mo. mouth ; neph. nephridium ; us. oesophagus ; st. stomach ; tent, tentacles. B, Diagram-
matic median section of Phoronis. nr. nerve-ring. Other letters as in A. (From Korschelt
and H eider, after Cori.)

of the epistome and lophophore, it follows that the dorsal valve
of the Brachiopod, being on the same side of the mouth as the
epistome, lies on the side of the body corresponding with the anal
side of the Polyzoan, though the intestine is bent round in the
opposite direction and directed towards the ventral valve. The
supra-oesophageal ganglion of the Brachiopod represents the single
ganglion of the Polyzoa, though it is subordinate in importance to

PHYLUM MOLLUSCOIDA

345

the infra-cesophageal ganglion not represented in the latter group.
Other important points of resemblance between the Brachiopoda
and the Phoronida are in the character of the nephridia, and the
presence in both of larval forms which may very well be looked
upon as modified Trochospheres.

The setae of Brachiopods, sunk in muscular sacs, are marks of
annulate affinities, since such organs are found elsewhere only
among Chastopoda and Gephyrea (Sect. X.). The form of the
larva tells in the same direction, the eye-bearing head region
or prostomium and the provisional setae being very striking charac-
ters But the segmentation of the Brachiopod is quite different
from that of the annulate larva, in which new segments are always
added behind those previously formed, and in which metamerism
always affects the mesoderm.

SECTION IX
PHYLUM ECHINODERMATA

THE phylum Echinodermata comprises the Starfishes (AsterMea),
Sea-urchins (Echinoidea\ Brittle-stars (Opliiuroided), Feather-stars
(Crinoidea), and Sea-cucumbers (Holothuroidea). , All exhibit a
radial arrangement of parts, which is recognisable as well in the
globular Sea-urchins and elongated Sea-cucumbers, as in the star-
shaped Starfishes, Brittle-stars and Feather-stars. Another uni-
versal feature is the presence of a calcareous exoskeleton, sometimes
in the form of definitely shaped plates, which may fit together by
their edges so as to form a continuous shell ; sometimes merely in
the form of scattered particles or spicules. In very .many the
surface is beset with tubercles or spines, from which feature the
name of the phylum is derived. *<The_^various systems of organs
attain a comparatively high degree of complexity. The Echino-
derms are rarely capable of rapid locomotion, and are sometimes
permanently fixed by means of a stalk : they never give rise to
colonies by budding. Without a single exception, all the members
of this phylum are inhabitants of the sea.

1. EXAMPLE OF THE ASTEROIDEA.
A Starfish (Asterias rulens or Anthema flavescens).

General External Features of Asterias rubens. — The

body of the Starfish is enclosed in a tough, hard integument,
containing numerous plates, or ossicles as they are termed, of
calcareous material. This exoskeleton is not completely rigid in
the fresh condition, but presents a certain limited degree of flexi-
bility. The body (Fig. 276) is star-shaped, consisting of a central
part, the centred disc, and five symmetrically arranged processes,
the arms or rays, which, broad at the base, taper slightly towards
th^ir outer extremities. There are two surfaces — one, the dorsal,

IX

PHYLUM ECHINODERMATA

347

or abactinal, directed-upwards in the natural position of the living
animal : the other, the ventral, or actinal, directed downwards.
The dorsal surface is convex, the ventral flat ; the* colour of the
former is much darker than that of the latter.

In the centre of the ventral surface (Fig. 276) is a five-fayed
aperture, the actinostome, and running out from this in a radiating
manner are five narrow grooves, the ambulacral grooves, each
running along the middle of the ventral surface of one of the
arms to its extremity. Bordering each of the ambulacral grooves
there are either two or three rows of movable calcareous spines,
the (i-mlndriat'id s)nncx. At the central ends of the grooves the
ambulacral spines of
contiguous sides of ad-
jacent grooves form
five groups, the mo-n.th *
papUloe, one at each
angle of the mouth.
External to the- am-
bulacral spines are
three rows of stout
spines, which are not
movable ; and a third
series runs along the
border separating the
ventral from the dorsal
surface.

Qn the convex <h/--
-y d surface" there are a
number of short stout
spines arranged in ir-
regular rows parallel
with the long axes
of the rays. These • '

are supported on irregularly-shaped ossicles buried in the in-
tegument. In the soft interspaces between the ossicles are a
number of minute pores, the denned pores, scarcely visible with-
out the aid of a lens. Through each of these pores projects*
a very small, soft, filiform process, one of the dermal branchiae
or papula1- (Fig. 280, jRes}}. cat), which is capable of being entirely
retracted.

Very nearly, though not quite, in the centre of the dorsal sur-
face is an aperture, the amis (Fig. 285), wide enough to admit
of the passage of a moderately stout pin. On the. same surface,
midway between the bases of two of the rays, is a flat, nearly
circular plate, th< surface of which is marked by a number of
radiating narrow, straight, or slightly wavy grooves: this is the
madreporite. Tne presence of this structure interferes to some

Fit.. :>7<'>. — Starfish. General view of the ventral surface,
showing the tube-feet. (From Leuckart and XitscheV
Diagrams.)

348 ZOOLOGY SECT;

extent with the radial symmetry of the Starfish, two of the
me£es (p. 40), viz. those between which the madreporite is placed,
being different from the rest. There thus arises a Hint era I sym-
metry, there being one vertical plane, and only one — that passing
through the middle of the madreporite and through the middle of
the opposite arm — along which it is possible to divide the starfish
into two equal — right and left — portions.1 The two rays between
which the madreporite lies are termed the bivium, the three
remaining the trivium.

Attached to the spines of the ventral surface, in the intervals
between them, and in the intervals between the spines of the
dorsal surface, are a number of very small, almo. i microscopic
bodies, which are termed the pedicellarice (Fig. 285 and -Fig. 280,
Ped). Each of these is supported on a longer or shorter flexible
stalk, and consists of three calcareous pieces — a basilar piece at
the extremity of the stalk, and two jaws, which are movably articu-
lated with the basilar piece, and are capable of being moved by
certain sets of muscular fibres, so as to open and close on one
another like the jaws of a bird. In some of the j)edicellaria? the-
jaws, when closed, meet throughout their entire length, while in
the case of others, mostly arranged in circles round the spines on
the dorsal surface, one jaw crosses the other at the end like the
mandibles of a Cross-bill.

In a well-preserved specimen there will be seen in each of the
ambulacral grooves two double rows of soft tubular bodies ending
in sucker-like extremities ; these are the tube-feet (Fig. 276). In
a living specimen they will be seen to act as the locomotive organs
of the animal. They are capable of being greatly extended, and
when the Starfish is moving along, it will be observed to do so by
the tube-feet being extended outwards and forwards (i.e. in the
direction in which the animal is moving), their extremities be-
coming fixed by the suckers, and then the whole tube-foot con-
tracting so as to draw the body forwards ; the hold of the sucker
then becomes relaxed, the tube-foot is stretched forwards again,
and so on. The action of all the tube-feet, extending and con-
tracting in this way, results in the steady progress of the Starfish
»over the surface. With the aid of the tube-feet the Starfish is
also able to right itself if it is turned over on its back.

At the extremity of each of the ambulacral grooves is to be
distinguished a small bright red speck, the eye (Fig. 280, A, oc),
with over it a median process, the tentacle (t), simitar 'to the tube-
feet, but smaller and without the terminal sucker. The tentacles
have been ascertained by experiment to be olfactory organs, the
Starfish being guided to its food much more by this means than
by the sense of sight.

1 The slightly eccentric position of the anal aperture introduces a
ingly slight inequality between the right and left portions.

PHYLUM ECHINODERMATA

349

ransverse Section of an Arm. — If one of the arms be cut
across transversely (Fig. 277 and Fig. 280, B) and the cut surface
examined, the dorsal part of the thick, hard wall of the arm will
present the appearance of an arch (with its convexity upwards),
and the ventral part the form of an inverted V, the ends of the
limbs of which are connected with the ventral ends of the dorsal
arch by a very short, flat, horizontal portion. Enclosed by these
parts is a space, a part of the ccelome or body -cavity, and below,
between the two limbs of the V, is the ambulacral groove. The
dorsal arch is supported by a number of irregular ossicles. It is
perforated by the numerous small dermal pores, through which the
dermal branchiaB pro-
ject. The Y-shaped ven-
'tral part of the body-
wall — i.e. the walls of
the ambulacral groove
— is supported by two
rows of elongate ossicles,
the ambulacral ossicles
(Fig. 280,^lm6.-os),which
meet together at the
apex or summit of the
groove like the rafters
supporting the roof of
a house, but with a
movable articulation al-
lowing of separation or
approximation of the
two rows so as to open
or close the groove. At
the end of the ray the
ambulacral ossicles end
in a median terminal
ossicle. At the edges

of the groove a row of ossicles supports the ambulacral spines and
prominent tubercles. Between the ambulacral ossicles of each row
are a series of oval openings, the ambulacral pores, one betwjeen
each contiguous pair of ossicles, and so arranged" that they form
two rows on each side, one row higher than the other, the pores
of the higher row alternating with those of the lower. In the
ventral groove lie the contracted tube-feet (t. /.) : each tube-
foot is found to correspond to one of the ambulacral pores, so
that the former, like the latter, are arranged in a double alter-
nating row on each side of the groove. VWhen the tube-fool is
drawn upon, it is seen to be contiguous with one of a series of
little bladder-like bodies, which lie on the other side of the ambu-
lacral ossicles, i.e. in the cavity of the arm.; These — the ampullw

FIG. 277.— Starfish. Vertical section through an arm ;
amp. ampullae ; ep. epidermis, r<«/. <inti>. radial vessel
of the ambulacral system, rad. H. r. (erroneously so
lettered) points to the septum dividing the blood-
vessel into two pai'ts. rad. ne. radial nerve of the
epidermal system, sp. spaces in mesoderm of body
wall, t.f., tube-feet. (From Leuckart, after Hamann.)

350

ZOOLOGY

SECT,

(amp. Figs. 277 and 280 ; ap, Fig. 278)— are arranged like the tube-
feet, in a double row on each side, a higher row and a lower,
there being one opposite each ambulacral pore. ( When one of
them is squeezed the corresponding tube-foot is distended and
protruded, the cavities of the tube- foot and ampulla being in
communication by means of a narrow canal running through the
ambulacral pore ; and it is in this way that the foot is protruded
in the living animal : the corresponding ampulla being contracted
by the contraction of the muscular fibres in their walls, the con-
tained fluid is injected into
the tube-foot and causes its
protrusion./

Vascular and Nervous
System. — Running along the
ambulacral groove, immedi-
ately below where the ambu-
lacral ossicles of opposite sides
articulate, is a fine tube, the
radial ambulacral vessel (Fig."
277, rad. amb\ Fig. 278, r),
which appears in the trans-
verse section as a small rounded
aperture. From this short side-
branches (r', Fig. 278) pass out
on either side to open into the
bases of the tube-feet. Below
the radial ambulacral vessel
is a median thickening of the
integument covering the am-
bulacral groove; this marks
the position of (the rad'u'l nerve i
(Fig. 277, rad: ne.) of the
epidermal nervous system, and
is traceable as a narrow thick-
ened band running throughout the length of the groove, and
terminating in the eye at its extremity, while internally it becomes-
continuous with one of the angles of a pentagonal thickening of a
similar character, the nerve-pentagon, which surrounds the mouth?)
In thin sections (Fig. 279) the ventral median thickening, or
radial nerve (rad. nerv.), as well as the nerve-pentagon, are seen to
be thickenings of the epidermis, consisting of numerous vertically-
placed, fibre-like cells, with their nuclei at their outer (lower)
ends, intermixed with longitudinal nerve-fibres and with nerv(
cells. Above this, on each side of the epidermal nerve-thickening
constituting the radial nerve, is a band of cells (d. nerv.} also of
nervous character. These more deeply placed •nerve-bands are the
radial parts of the deep nervous system : like the epidermal th(

Fi<:. 278.— Ambulacral system of a Starfish.
-(. ampullae ; rtp.«Polian vesicles; c. circular
canal ; m. madreporite ; 'm', madreporic
canal ; t. tube-feet ; p, radial vessels ; /,
branches to ampullae. (After Gegenbaur.)

IX

PHYLUM ECH1NODERMATA

deep nervous system has a central part in the form of a pentagon,
which in this case is double, surrounding the mouth. A third set
of nerve elements (the coelomic nervous system) extend along the
roof of the arm superficial to the muscles.

The two radial nerve-bands of the deep nervous system are
thickenings of the lining membrane of a space overlying the
radial nerve and underlying the radial ambulacral system. This
space (rad. bl. v.), extending, like the other parts that have been
mentioned, throughout the length of the arm, forms part of a
system of channels which are usually regarded as constituting a
blood-vascular system,. This radial blood-vessel, as it is termed, is
divided longitudinally by a vertical septum (sept.) into two lateral
halves. Internally it com-
municates with an oral ring-
vessel surrounding the mouth
and likewise divided into two
by a septum. The inner
division of this ring-vessel
communicates with the coe-
lome ; the outer is connected
with the axial sinus referred
to below.

Structure of the Disc.
— When the dorsal wall of
the central disc is dissected
away, the remainder of the
organs come into view (see
Fig. 283). The rows of am-
bulacral ossicles appear on
this view as ridges, the am-
bulacral ridges, one running

along the middle of the ventral surface of each arm to its ex-
tremity, and extending inwards to the corresponding angle of the
mouth. At the sides of each of these ridges appear the rows of
ampullae. Within the pentagonal actinostome is a space, the
peristome, covered with a soft integument, and in the centre of
this is a circular opening, the true mouth, the size of which is
capable of being greatly increased or diminished.

Body- wall and Coelome. — The entire outer surface is covered
with a layer of ciliated epithelium, the epidermis or cleric epi-
thelium (Fig. 280, Der. Epithm.), which is continued over the
various appendages and processes — the tubercles and spines, the
pedicellaria3, the dermal branchiae, and the tube-feet. Beneath
it is a network of nerve-fibrils with occasional nerve-cells. The
mesoderm (Derm) of the wa1*^ of-^JUody beneath this consists of
two layers, between which ^1 waTT Frorti ^aces : the ossicles (os.)
are all, except the amb ^vo hollow appendage inter-radial par-

Fi<i. -27H.— Starfish. Lower part of a vertical
section through the arm, to show the structure
of the radial nerve and the position of the
deep nervous system and of the radial blood-
vessels ; d. nerr. strand of deep nervous sys-
tem ; rail. hi. ,-. radial blood-vessel; /•<«/. nerv.
radial nerve ; sept, septum of radial blood-
, thickening of the septum some-

vessel ; sept'

times taken for the radial blood-v

Cuenot.)

(After

352

ZOOLOGY

SECT.

titions, developed in the outer of these two layers. Each ossicle
consists of a close network of calcareous rods. Between contiguous
ossicles extend bands of muscular fibres.

The interior of the ccelome (ccel.) or body-cavity is lined by a
ciliated epithelium, the ccelomic epithelium (Cod. Epithm.\ which
not only covers the inner surface of the body-wall as the parietal
layer, but forms an investment also for the contained organs —
the various parts of the alimentary canal and its appendages, the
genital glands, the madreporic canal, ampullae, Polian vesicles, etc.

Mdj.

Ovd

FIG. 280. — J)iagrammatic sections of a Starfish. A, vertical section passing on the right through
a radius, on the left through an inter-radius. .The off-side of the ambulacral groove with the
tube-feet (T. F.) and ampullae (Amp.) are shown in perspective. B, transverse section through
an arm. The ectoderm is coarsely dotted, the nervous system finely dotted* the endoderm
radially striated, the mesoderm evenly shaded, the ossicles of the skeleton black, and the
ccelomic epithelium represented by a beaded line. amb. os. ambulacral ossicles ; amp. am-
pullae ; an. anus ; C. Amb. V. circular ambulacral vessel ; C. B. V. septum of ring blood-
vessel ; Cd. cce. cardiac cseca ; Gael, ccelome ; Cad. JSpithm. ccelomic epithelium ; Der. Epitlnn.
deric epithelium ; Derm, mesoderm ; Ent. Epthm. enteric epithelium ; Int. cai. intestinal cseca ;
M<1 i>r. madreporite ; Mes. mesentery ; Mth. mouth ; JVr. R. nerve ring ; oc. eye ; os. ossicles of
body wall ; ord. oviduct ; Ped. pedicellarise ; ph. perihsemal spaces ; pyl. cctc. pyloric caeca ;
A'"-/. n,,>/>. <•. radial ambulacral vessel ; Rod. B. V. points to septum in the radial blood-vessel ;
lia<l. Nr. radial nerve ; Resp. cce. dermal branchiae ; St. stomach ; St. c. stone-canal ; t. ten-
tacle ; T. F. tube-feet. (From Parker's Biology.)

In addition to this visceral layer of the peritoneum, the wall of
the alimentary canal and its caeca consists of a muscular layer and
an internal lining, the enteric epithelium or endoderm (Ent. Epthm.}.
The ccelome is filled with a fluid, the co&lomic fluid, consisting
mainly of sea-water, but containing a number of amoeboid cor-
puscles (amcebocytes) containing a brown pigment. The dermal
branchiae consist of a musmlo^ '^ ^ an external epidermal layer,
and an internal portion^ m^e ^''^al cavities of the hollow
branchi* being in fre th the ccelome.

IX

PHYLUM ECHINODERMATA

Digestive System. — The mouth is found to open through a
short passage, the oesophagus, into a wide sac, the cardiac division
of the stomach (Fig. 280, St, Figs. 282, 284,_ card. st). This is a
five-lobed sac, each of the lobes of which is opposite one of the
five arms. The walls of the sac are greatly folded, and the whole
is capable of being everted through the opening of the mouth,
wrapped over some object desired as food, and then retracted into
the interior, the retraction being effected by means of special
retractor muscles (Fig. 283, retr) which arise from the sides of
the ambulacral ridges. This cardiac division of the stomach com-
municates dorsally with a much smaller chamber, the pyloric
division of the stomach,
and this in turn opens
into a very short coni-
cal intestine, which leads
directly upwards to
open at the anal aper-
ture. The pyloric divi-
sion of the stomach is
pentagonal, each angle
being drawn out to
form a pair of large
appendages, the pyloric
cceca (Figs.' 280, 281,
283, and 284, pyl ccec\
Each pair of pyloric
cseca commences as a
cylindrical canal or
duct, the lumen of
which is continuous
with the cavity of the
pyloric chamber. This
soon bifurcates to form
two hollow stems, ex-
tending to near the extremity of the cavity of the arm, and giving
off laterally two series of short branches, each having connected
with it a number of small bladder-like pouches. The walls of the
pyloric caeca are glandular : they secrete a digestive fluid, and are
therefore to be looked upon as digestive glands. It is found by
experimenting with this digestive fluid that it has an action on
food-matters similar to that exerted by the secretion of the
pancreas in the Vertebrata, converting starch into sugar, proteids
into peptones, and bringing about the emulsification of fats.
While the pouches of the cardia^jliyigionof the stomach are
attached to the ventral^jxaJl^Tthe bod^jblie pyloric creca are
connected with the dorsalwair From the short intestine are
given off inter-radially two hollow appendages, the intestinal caca

VOL. I A A

FIG. 281.— Asterias rub ens. Digestive system; an,
anus; m/v/. xt. cardiac division of. the stomach; int.
(•!«•. intestinal cteca ; ma</r. madreporite ; pyl. c«<c.
jiylnric cteca ; pt/l. st. pyloric division of the stomach.
(From Leuckart.)

354 ZOOLOGY SECT.

(Fig. 281, int. ccc-c), each with several short branches of irregular

shape.

Ambulacral System. — Running downwards from the madre-

porite to near the border of the mouth is an S-shaped cylinder,.

the madreporic or stone-canal
(Figs. 278, m, 284, mad. can),
enclosed in a membranous sac.1
The walls of this canal are sup-
ported by a series of calcareous
rings, projecting from which
inwards is a ridge which bifur-
cates to form two spirally rolled

xxJ5 a^WP'SW! iSiSy^ lamellae occupying a consider-
Xv^; able part of the lumen of the

canal. In some Starfishes, such

lie,. 282.— Transverse section through the . /_. rto«\ ,-,

inadreporic-canal of a Starfish (Axt.-r,- US Astl'Opecten (V iff. z8z) the

reuscher.) int.emal structure is more com-
plicated owing to the branching

of the lamellae. The interior of the madreporic canal communicates
above with the exterior through the grooves of the madreporite.
At the bottom of each of the grooves is a row of pores leading into
a sac, the ampulla, which in turn leads to the madreporic canal.
Below, the latter opens into a wide, five-sided, ring-like canal, the
ring-vessel of the ambulacral system. From this are given off the five
radial ambulacral vessels, passing to the extremities of the arms.
From the pentagonal canal are given off also a series of five pairs of
appendages, the Polian vesicles (Fig. 278, ap ; Fig. 283, pol. ves) —
pear-shaped, thin-walled bladders with long narrow necks — which
are placed inter-radially. On the sides of the neck of each Polian
vesicle (except in the inter-radius containing the madreporic
canal, where there is one on one side only) project inwards a pair
of little rounded glandular bodies, the racemose vesicles, or Tiede-
manns vesicles, the interior of each of which is divided into a
number of chambers.

The various parts of the ambulacral system of vessels have a
muscular wall and an internal lining epithelium, in addition to the
coverings which they may derive, according to their situation,
either from the external epidermis or the internal coelomic epi-
thelium. The muscular layer is most strongly developed on the
tube-feet, where it consists of two strata, and is also well developed
on the ampulla? and Polian vesicles.

Accompanying the madreporic canal there is an organ — the
ovoid gland — the relationships and function of which have given
rise to a considerable amount of difference of opinion. It is a
fusiform body, the interior of which is divided up into a number
of freely-communicating spaces. In the interior of these spaces

1 The so-called "pericardium."

PHYLUM ECHINODERMATA

355

lie numerous large cells, some of which are pigmented, and the
walls of the spaces are lined with an epithelium of flattened cells.
At its dorsal extremity the organ terminates in a narrow pro-
longation with a single central canal, which ends in a ring-like
canal surrounding the anus ; this gives off five radial branches
which pass to the reproductive organs. Surrounding the body of

. case

. 283. — Anthenea flavescens. Dorsal view of a dissection of the internal organs. The
dorsal wall of the body, with the exception of a small portion round the anus and the madre-
porite, has been completely removed. One of the five intestinal cseca has been removed with
the exception of its proximal part. All the ovaries have been removed except one pair, and
four of the pairs of pyloric caeca have been cut away close to their bases. 1 — 5, the five rays
witb their ambulacral ridges ; amp. ampulla? ; an. anus ; int. ca>c. intestinal caeca ; /. p. cut
ends of the inter-radial partitions ; mad. madreporite with the madreporic canal ; or. ovaries ;
pot. ves. Polian vesicles ; put. ca-c. pyloric cseca ; retr. retractor muscles inserted into the
cardiac division of the stomach.

the gland is a blood -sinus — the axial sinus — connected orally with
the oral ring blood-vessel ; and connected with this at its aboral
end is a ring-like vessel enclosing the ring-like canal' in continuity
with the ovoicl gland, and gr off prolongations to the repro-
ductive organs. The cavity te ovoid gland communicates by
a minute aperture with the madreporic canal, and thus, indirectly.
ith the exterior.

w

356

ZOOLOGY

SECT..

Functionally the ovoid gland, with its dependencies, appears to
be an organ for the production of the amoeboid corpuscles* which
abound in the ccelome and the ambulacral system. This function
it shares with the Polian vesicles and Tiedemann's vesicles.
Morphologically it is a genital stolon; the narrow prolongations
which pass to the genital organs being so many genital rachides,
the expanded extremities of which form the ovaries or testes.

Reproductive System. — The Starfish is unisexual, each in-
dividual possessing either ovaries (Figs. 283 and 284, ov) or
testes, which appear very similar until they are examined micro-
scopically. They consist of masses of rounded follicles, like
bunches of minute grapes — a pair in each inter-radial interval.

pyl.ccec

cunf}

FIG. 284. — Anthenea flavescens. Lateral view of a dissection in which one of the rays ;
a portion of a second have been removed, and in which the alimentary canal has been laid
open; amp. ampullae; an. anus; card. st. cardiac pouch of the stomach; ii>t. cose, intestinal
csecum ; ip. inter-radial partition ; mad. madreporite ; m«<l. <•«,!. madreporic canal ; or. ovary;
pyl. ccec, pyloric cseca ; r. cut ends of the ring-vessel of the ambulacral system ; ring r. posi-
tion of the ring-vessel ; retr. retractor muscle of cardiac pouch of stomach ; .*. cavity of the
stomach.

Ova and sperms are alike developed from cells of the same
character as those which become the amcebocytes of the coelomic
and other cavities of the body. The ducts, by means of which
the ova or sperms reach the exterior, open on the dorsal surface
through a number of perforations on a pair of sieve-like plates,
situated inter-radially close to the bases of the arms.

Anthenea flavescens (Figs. 283-286), a common Australian star-
fish, which may be taken as an example instead' of Asterias rubens,
differs from the latter in the following main points.

The animal consists of a relatively large central disc and five
relatively short arms, which taper rapidly towards their extremities.
On the ventral surface the comparatively broad flat surfaces be-
tween the ambulacral grooves are rougLish, owing to the plate-like

OSJ

wl
gn

Kt
i
D

PHYLUM ECHINODERMATA

357

number of minute rounded tubercles,
neighbourhood of the ambulacra]

cell

t

ossicles being beset with a

which, in the immediate

grooves, assume the char-
ter of short blunt
ines. Here and there
mong the tubercles,

usually one in the middle

of each ossicle, are pedi-
II aria', which differ
idely from those of
.sterias. Each pedicel-

laria in Anthenea is a

small, narrow-oblong, cal-
careous body, consisting

of two parallel narrow

valves or jaws : these, in-
stead of being supported

on a flexible stalk, are

articulated with the edges

of a slit-like depression

on the surface of the flat

ossicle, and are thus on

a level with the general

surface. The term val-

mdatc is applied to pedicellariae of this description. In a living

Anthenea many of the pedicellariaB will be found to have their values

widely open; when they
are touched the valves
close together, gradually
opening again after a little
time. The ambulacral
spines bounding the am-
bulacral groove are flat-
tened and blunt, and
arranged in fan-like fasci-
culi. Round the border
separating the dorsal and
ventral surfaces the plates
are arranged in two some-
what irregular rows.

The dorsal surface is
strongly convex, but not
uniformly so, there being
a more or less distinct
depression in the form of

a shallow open groove,
FIG. 286.— Anthenea. view of ventral surface. ,-, • , -j • j 7 '

(After siaden.) the inter-radial depression,

FIG. 285. — Anthenea, view of dorsal surface.
(After Siaden.)

358 ZOOLOGY SECT.

opposite each of the intervals between the arms. The surface is
dotted over with numerous small rounded tubercles, arranged in
somewhat irregular radiating lines. These dorsal tubercles, though
fewer than those on the ventral surface, are for the most part more
prominent, so that they assume the character of short spines. The
ossicles on which they are borne are star-shaped with six rays.
a spine being borne in the centre of each ossicle, and one on
each of the rays. Between the ossicles the surface is covered
with a soft slimy skin, perforated by a large number of minute
dermal pores, each of which is enclosed by a minute irregular ring of
calcareous matter ; each pore serves for the lodgment of one of the
dermal branchiae. Numerous pedicellariae, similar to those on
the ventral surface, but smaller, are borne on the ossicles, usually
taking the place normally occupied by the central spine. The
tube-feet are arranged in a single row on each side of each ambu-
lacral groove ; but the ampullce are in two rows, an upper and a
lower, and each tube-foot has two ampullae, one of the upper
row and one of the lower row, connected with it.

There are vertical calcarious inter-radial partitions not de-
veloped in Asterias. There are five pairs of intestinal cccca, which
are narrow tubes slightly enlarged and lobed at the extremities.

Development of a Starfish (Asterina gibbosa or A.
exigua1)- — In these Starfishes the reproductive apertures are
placed on the ventral surface. When the ova have been dis-
charged aiid impregnated, they adhere by means of a viscid
matter to the surface (rock or stone) on which they are laid, and
go through all the stages of their development in this position,
never passing through a free pelagic stage. The eggs are about
half a millimetre in diameter, and of a spherical shape. Each con-
sists of a perfectly opaque central mass of yellow or orange yolk,
and of a glassy layer enclosing this. After fertilisation the process
of segmentation begins by the division of the ovum into two blasto-
meres almost equal in size, but one, which may be termed cell
I., slightly smaller than the other, cell II. Both I. and II. soon
afterwards divide, I. somewhat earlier than II. The resulting four
cells again divide, the result being the formation of an eight-celled
stage (Fig. 287, A), in which the four cells derived from I. form
an incomplete ring not closed below, and the four derived from
II. form an incomplete ring open above.

The eight cells then divide by meridional fissures into
sixteen, and a further division results in the formation of thirty-
two. The thirty-two cells become arranged in such a way as to
enclose a central cavity which had been present in the four-celled
stage : this stage (B) is the blastula ; the cavity is the segmenta-
tion-cavity or olastoccele. The number of cells in the wall of this

1 The development of these has been described in preference to that of the
examples, as it is better known and more readily studied.

PHYLUM ECHINODERMATA

359

cavity increases by further divisions, and the whole surface becomes
covered with vibratile cilia. A process of imagination then,
follows ; one side of the blastula becomes pushed inwards to form
a doubled-walled cup or gastrula (C) opening on the exterior by
an opening, the blastopore, which, at first very wide, gradually
becomes narrowed. At the same time the shape of the larva alters,
becoming somewhat elongated, the blastopore, lying at first midway
between the two poles, afterwards gradually drawing nearer to
what becomes the lower or ventral pole.

Of the two layers of the gastrula (D and E} the outer is the
toderm, the inner the endoderm ; between them is a space, at first

B

cis-ch

FIG. 287.— Early stages in the development uf a Starfish (Asterina gibbosa). A, eight-celled
stage ; B, stage of about thirty-two cells seen in section ; C, gastrula stage ; D, section of
early gastrula ; E, section of later gastrula. arch, archenteron ; l>!nxto<\ blast<>c<ele ; <>lji.
blastopore; ect. ectoderm; <:,i<i. endoderm. (Modified after Ludwig.)

filled with gelatinous matter, in which cells soon appear, giving
rise subsequently to a third or middle layer, the mcsoderm.

The cavity in the gastrula early becomes distinguishable into
two parts (Fig. 288, B) — that part into which the blastopore leads
(arch), and a wider terminal part (enf) ; the former becomes the
posterior part of the alimentary canal of the larva, the blastopore
becoming the anus ; the latter is termed the enteroccele. The wall
of the enterocoale becomes thinner, and it gives off two lateral
swellings, the right and left enteroccelic pouches (G, ent), which
become closely applied to the sides of the larval alimentary canal :
the left pouch soon becomes larger than the right. The entero-
cosle then becomes completely closed off from the enteric canal.
Towards the anterior end of the larva there is now formed a new
invagination from the surface, the stomodccum, which meets the

360

ZOOLOGY

SECT.

rudimentary alimentary canal, and forms the mouth and oesophagus
of the larva.

The enterocoelic pouches grow round the alimentary canal : in

larv.org

"9ian>.mo

FIG. 28S. — Later stages in the development of the Bipinnaria of Asterina gibbosa A,
newly hatched larva, ventral surface with the beginning of the larval organ at the anterior
end and with the larval mouth. B, dorsal half of an embryo of the same age as A. C, some-
what older larva with larger larval organ, the ectoderm of the left side removed to expose the
alimentary canal and the walls of the body-cavity, arch, archenteron ; II. p. blastopore ; ect.
ectoderm ; ent. enterocoele ; lam: mo. larval nicmth ; Ian: onj. larval organ. (From Ziegler's
models.)

front they unite to form a common cavity, behind they remain
separated by a thin partition or septum. The left enteroccelic
pouch gives rise now to the rudiment of the ambulacral system

g lory, org

\ ""< "

of,?.

Ki,;. -JMI._ Bipinnaria of Asterina gibbosa. A. Diagrammatic lateral view; the alimentar
canal dotted, the ambulacral system striated, the ectoderm shaded. E. Bipinnaria seen from
the left as an opaque object, the body-wall of the left side removed. Hydroccele separated
off from left enteric sac and partly surrounding oesophagus, all. alimentary canal ; amh.
ambulacra] system or hydroccele; iiors.p, dorsal pore ; cut. enteric sacs and coelome ; Ian:
nu>. larval mouth; laro. org. larval organ ; ox. u-sophagus of adult ; r. r. lobes of hydroccele ;
tept. septum between the enteroccelic sacs. (A, after Ludwig ; B, from Ziegler's models.)

in the form of an outgrowth, the hydroccele. The lower border
of this becomes five-lobed (Fig. 289, A\ each of the lobes (r)
developing later into one of the radial ambulacral vessels; the
central part of the hydroccele subsequently forms a ring — the ring-

PHYLUM ECHINODERMATA

361

tasis.org

vessel of the ainbulacral system. A pore, the dorsal p <ore (dors, p),
es its appearance as an invaginatioii of the ectoderm nearly

opposite the larval mouth ; this
opens internally into the left entero-
coelic pouch. A groove is formed on
^e inner surface of the hydrocoele,
and becomes closed to form a canal,
the rudiment of the madreporic
canal ; this subsequently enters into
communication with the dorsal pore,
the external opening of which corre-
sponds to the future madreporite.

A change has meanwhile taken
place in the general form of the body
of the larva. At the upper end has
been formed an anterior short lobe
(anterior head-lobe) and a posterior
longer lobe (posterior head- lobe).
These two lobes together form what is termed the larval organ
(larv. org), which later on becomes absorbed in the transition to
the starfish form. As the hydrocoele develops, its form influences
the external shape of

"":

290.— Bipimutriu of Asterina,
view of the left -hand side, showing
the tive-lobed prominence, aruli,
formed by the developing ambu-
lacral system on what is destined
to bec'>me the ventral surface of
the b,,dy of the Starfish.

fen?

t.f.

the larva ; on the left-
hand side there grows
out a fiye-lobed eleva-
tion (Fig. 290, ami),
oach of the lobes cor-
responding to one of
the five lobes of the
hydrocoele. Each of
the latter then be-
comes divided, first
into three rounded
processes (Fig. 289, B,
amb), and then into
five, and these project
freely on the surface;
the middle one is the
rudiment of the ten-
tacle, the lateral pro-
cesses are the first two
pairs of tube-feet. At
the same time five
elevations of the op-
posite wall, which have become evident, give rise to the beginnings
of the dorsal regions oi the arms.

The transition from the larval Bipihnaria stage, as it is termed,

FIG. 291.— Asterina exigua. Young Starfish shortly
after 'the metamorphosis has been completed, viewed
from the ventral side. circ. ami. circular ambulacral
vessel ; dors. p. dorsal pore and madreporic canal ; rad.
amlt. radial ambulacral vessel ; st. stomach ; tciit. tentacle ;
t. f. tube-feet.

362

/OOLOGY

SECT.

to the condition of the five-rayed starfish (Fig. 291) is effected by the
abortion of the larval organ, the further development of the arms
and tube-feet, and certain changes which take place in the internal
organs. Of these, one of the most important is the formation of a
new mouth and oesophagus (Fig. 289, B, ces), the larval mouth and
oesophagus becoming abolished during the metamorphosis. Round
this new mouth grows the ring- vessel of the ambulacral system.
From the stomach, diverticula grow out radially into the developing
arms to give rise to the caeca; and later the permanent anal
opening is formed on the dorsal surface.

When the first ossicles are definitely formed they present the
following arrangement (Fig. 292). In the middle of the abactinal

. 2l>2. — Diagram showing the relations of the chief plates of the apical system in the young
Starfish, an. anus ; bas. basals ; (?o/'s.=dorso-central ; intu/.r madreporite; rt«(. nulials ; .$<.<:.
mil. secondary radials.

surface is a single dorso-central plate (dors). Around this are five
basals (bas), one of which becomes converted into the
madreporite. External to these, five radials (rad.*) appear some-
what later. At the end of each developing arm is a single
terminal or ocular plate (term), which is carried outwards
as the ambulacral and adambulacral ossicles .of the arm are
developed, supporting the corresponding eye and tentacle. A ring
of secondary radials (sec. rad) is developed between the radials and
the dorso-central. In the adult, by the intercalary development of
numerous additional ossicles, these primary plates of the apical
system, as it is termed, lose their original arrangement, and become,
with the exception of the madreporite and the terminal plates, no

PHYLUM ECH1NODERMATA 363

longer recognisable. Five oral plates, which when they first appear
are on the abactinal surface, pass round to the actinal as develop-
ment proceeds.

2. EXAMPLE OF THE ECHINOIDEA.
A Sea-Urchin. — (Strongylocentrotus or Echinus.)

General External Features. — The Sea-urchin (Figs. 293 and
294) is globular in shape, but somewhat compressed in one direc-
tion, so that two poles are distinctly recognisable. At one of these
the degree of flattening is greater than at the other ; this is the
oral pole, the opposite pole being termed the anal or aborctl. At
the oral pole is a rounded aperture, the mouth, through which may
be seen projecting five hard white points?*the extremities of the
teeth. Surrounding the mouth is a thin soft membrane known as
the peristome or peristomial membrane. At the anal pole is a much
smaller aperture, the anus, the space immediately surrounding
which is termed the periproct.

The entire surface,, with the exception of the peristome and
periproct, is bristling with spines — cylindrical, pointed, solid ap-
pendages, the surface of which is longitudinally fluted. These are
movably articulated with the body so that they may be turned
about in all directions. When one of them is removed (see Fig.
309, p. 389), it is found that the joint is of the character of a ball
and socket, a concavity on- the base of the spine fitting over a
hemispherical elevation of the surface of the Sea-urchin, and the
spine Tbeing retained in place and caused to move by means of a
capsule of muscular fibres enclosing the joint. /Around the bases
of the large spines are a number of very small spinules. Here and
there among the spines are to be observed minute pedicellarice
(see Fig. 310, p. 389), which are comparable to the stalked
pedicellarise of Asterias, but have each three jaws instead
of two, and have a relatively long stalk, which is supported by a
slender calcareous rod. Here and there are to be found also small
rounded bodies termed the sphceridia, which are perhaps, like the
pedicellaria3, to be looked upon as modified spines : they contain
ganglion-cells and are apparently organs of special sense, having
perhaps the function of detecting changes in the composition of
the water.

Projecting from the surface among the spines all the way from
the peristome to the periproct will be observed five double rows of
tube-feet (Fig. 293), which in a living specimen will be found to
be capable of great extension. These are similar to the tube-
feet of the Starfish, and have similar functions ; the sucker-like
extremity of each is supported by a perforated sieve-like plate of
calcareous matter. Each double row of tube-feet occupies a
meridional zone of the surface, termed the ambulacrctl area.,

364 /OOLOGY SECT.

corresponding to the ambulacra] groove of the Starfish : the inter-
mediate zones are termed the inter-ambulacral areas. At the oral
end of each ambulacral area on the peristome is a pair of ap-
pendages similar to tube-feet, but without suckers and termed
tentacles. Ten shrub-like appendages, the dermal branchice, are
situated in the peripheral part of the peristome, a pair opposite
each inter-ambulacral area.

When the spines are removed, the body is found to be enclosed
in a rigid globular shell, or corona (Fig. 294) as it is termed,

FIG. 293,— Strongylocentrotus, entire ainmal with the tube-feet extended. (From Brehm's

Thierleben.)

formed of a system of plate-like ossicles, the edges of which fit
accurately and firmly together, and the surfaces of which are
ornamented with the rounded elevations or tubercles for the articu-
lation of the spines. These plates are arranged in ten zones,
each consisting of two rows, running in a meridional direction
from the edge of the peristome to the neighbourhood of the peri-
proct. Of the zones of plates there are two sets, each consisting
of five, the members of which alternate with one another. In

PHYLUM ECHINOI <]RMATA

365

ie case of one of these sets of .zones — the ambulacral zones or
tmbulacral areas already referred to (amb) — each of the plates is
forated towards its outer end by two minute pores, the ambidacral

' >

Int.asnl)

Am,b

FIG. 294.— Corona of Sea-urchin with 1*e spines removed to show the arrangement of the
plates, lateral view. And>. ambulacral zone with its perforated plates; Ap. apical (aboral)
pole ; Int. amb. inter-anil ulacral zones. (From Bronn's Thierreich.)

pores, for the protrusion of the tube-feet. The other five zones,
the inter -ambulacra! zones or areas (int. amb), have the plates not
perforated. At its anal end each area, ambulacral or inter-
ambulacral, ends in a single apical plate, so that the periproct is
surrounded by a ring of ten plates,
the apical system of plates (Fig.
295). Of these, the five that are
situated at the ends of the am-
bulacral areas are termed the
ocular plates (oc), owing to the fact
that each of them bears a rudi-
mentary eye ; while the five oppo-
site the inter-ambulacral areas, are
termed the genital plates (gen), each
of them being perforated by an
opening which is the aperture of
one of the five genital ducts, — the
ducts of the ovaries or testes as
the case may be. One of these
genital plates (madr) has a swollen

and spongy appearance, which distinguishes it from the others :
this is the madreporite, through which, as in the case of the
structure of the same name in the Starfishes, the madreporic

•mJ>

FIG. 295. — Apical system of plates and
aboral extremities of zones of the shell
of a Sea-urchin : <im/>. anibulacral
zones; gen. genital plates; ii>t. nn//>.
inter-ambulacral zones ; mdil,: madre-
porite ; oc. ocular plates ; ^,-ipr. peri-
proct. (After Leuckart.)

366

ZOOLOGY

SECT.

canal communicates with the exterior. The two ambulacral areas
between which the madreporite lies constitute the bimum, the
remaining three the trivium.

On the inner surface of the shell, close to the edge of the peri-
stome, there project inwards five processes, the auricles (Fig. 297,
aur), one opposite each ambulacral area. Within the ring of auricles
lies a complex structure termed Aristotle's lantern (Fig. 296).
This consists of the five teeth (e) the apices of which are to be
seen projecting through the mouth, together with a system of
ossicles. The teeth are long, curved, and pointed : proximally each
is supported by and partly embedded in a pyramidal ossicle, the
alveolus (a), consisting of two halves united by a longitudinal suture.
Firmly united to the base of the alveolus is a stout bar, the
epiphysfa (b). Adjacent epiphyses are in close contact with one

FIG. -i','6. — Lantern of Aristotle of Echinus. A, Two of the five chief component parts apposed
and viewed laterally. B, Lateral and C internal view of a single part, a, alveolus ; «', suture
with its fellow ; b, epiphysis ; b', suture with alveolus ; c, rotula ; d, radius ; e, tooth. (From
Huxley's Invertebrates, after Miiller.)

another, and running inwards from their points of union are five
radially-directed, stout bars, the rotulce (c), the inner ends of which
unite to bound a circular aperture through which the oesophagus
passes. With the inner end of each rotula is movably articulated
a more slender bar, the radius (d), which runs outwards, parallel
with, and closely applied to, the rotula, to end in a free, bifurcated
extremity. Aristotle's lantern as a whole is in the shape of a five-
sided pyramid, at the apex of which project the five teeth; the
pyramid is hollow, containing a passage which .is the beginning of
the oesophagus. The base has the appearance of a wheel, the tyre
of which is represented by the five epiphyses, the spokes by the
five rotula3, with the five radii in close contact with them, and the
hub by the rounded central aperture. Passing between the various
ossicles of the lantern, and from them to the auricles, are systems
of muscles by means of the contractions of some of which the
lantern as a whole can be protruded or retracted, while the action

PHYLUM ECH1NODERMATA

367

coei

others is to cause the movements of the alveoli by which the
teeth are brought to bear on the food.

Nervous System. — Passing outwards through each auricle,
and running along the inner surface of the corona opposite the
middle of each am-
bulacral area, is a
radial nerve (Fig. 297,
rad. ne). Within the
ring of auricles the
five radial nerves are
connected with a
nerve-ring (nerv.r) sur-
rounding the mouth.
At its^distal end each
radial nerve is con-
nected with the eye
(oc), borne by the
corresponding ocular
plate. These"" parts
correspond to the epi-
dermal nervous sys-
tem of the Starfish ;
the deep and ccelo-l
mic systems ;uv only
feebly developed.

Ambulacral Sys-
tem. — Internal to T
each radial nerve, and pursuing a corresponding course, runs a
radial ambulacral vessel (rad. ami)). From this are given off on
each side a series of short branches to the tube-feet, with each of
which is connected one of a series of compressed sacs, the ampullae
(amp), by two canals, one passing through each of the two pores.
At their oral extremities the five radial ambulacral vessels unite
with a ring-vessel surrounding the oesophagus. [Appended to the
ring-vessel are five Polian vesicles (pol. ves). in the form of small
mammillated bodies. A madreporic caila.1 (mad. can), corresponding
to that of the Starfish, but with soft membranous walls devoid of
ossicles, runs from the madreporite at the side of the periproct
to the ring-canal surrounding the mouth.

Accompanying the madreporic canal is an ovoid gland (plex)
similar in essential character to that of the Starfish and having \
similar relations, except that the connection with the reproductive \
organs has disappeared in the adult.

The enteric canal (Fig. 298, ali) is devoid of the radial caeca
which it presents in the Starfish : it is a wide, soft-walled tube, \y
which winds round the interior of the corona in its passage from
the mouth to the anus held in place by a band of threads the

vrad.n

rada

FIG. 297. — Lateral view of the internal organs of a Sea- 7*5 nl~® Ut\
urchin as seen on the removal of a half of the shell. ^
ab. r. -res. aboral ring blood-vessel ; amp. ampulla} ; an.
anus , tiv.r. auricle; int. intestine; int. ves. intestinal
blood-vessels ; mad. madreporite ; mad. can. madreporic
canal ; mo. mouth ; mus. muscles passing from the
auricles to Aristotle's lantern ; nen: r. nerve ripg ; oc.
ocular plate ; o/v /•. /•. oral ring blood-vessel; plex. ovoid
gland ; pol. ves. Polian vesicle ; rad. amb. radial ambulacrail
vessel ; rat', ne. radial nerve ; siph. siphon ; sp. radial
extension of the coelome surrounding the nerve ; t. f.
tube-feet. (From Leuckart, after Hamann.) (

C?

ZOOLOGY

SECT,

\

mesentery, passing out from it to the inner surface of the shell,
It gives off a short blind diverticulum, the siphon (siph) ; •• this,
together with the intestine itself, probably acts as an organ for the
respiration of the coelomic fluid.

The ccelome contains a fluid in which, as in the Starfish,
there are numerous corpuscles. Of these there are two kinds,
(1) amoeboid corpuscles (amcebocytes) with long pseudopodia, (2) the
vibratile corpuscles, which closely resemble sperms, having a rounded
head and a slender vibratile tail : the latter aid in bringing about
a constant circulation of the ccelomic fluid.

The part of the ccelome in front of Aristotle's lantern is com-
pletely cut off from the rest by the arrangement of the membrane

enclosing the lantern,
and the function of
the branchiae on the
peristome is evidently
the oxygenation of
the coelomic fluid en-
closed in this com-
partment.

A blood- vascular
system is also pre-
sent, and has an
arrangement corre-
sponding to that al-
ready described in

rect

ati

FIG. 208. — Alimentary canal and other organs of Sea- V

urchin as seen when the oral half of the corona has n*n,^ it h tile addition

been removed: «/>. ,-. vet. aboral ring- vessel of the blood- 1 Q£ two lll'O'e

vascular system ; all. alimentary canal ; amp. ampullae; ! . 6

int. ves. intestinal blood-vessels ; I ant. lantern of Aristotle ; I tmal Vessels (r IP'S.
ces. (esophagus; or. r. r. oral ring-vessel of the blood- oci'i i o.no • , °y
vascular system; or. ovary: reet. rectum; .</>/*. siphon. A" • aim Zyo, lilt. VCSj.

(From Leuckart, partly after Cuvier.) The reproductive

organs consist o

- five masses of minute rounded follicles (Fig. 298, cv) situated in
the anal portion of the shell, and each communicating with the
exterior by its duct, which perforates the corresponding genital
plate. The sexes are distinct ; as in the Starfish, there is little
difference to be observed between the ovaries of the female and
the testes of the male until we come to examine their microscopic /
structure. The genital rachides which in the Starfish connect
the gonads with the genital stolon (oyoid gland) are, as alread
noticed, aborted in the adult Sea-Urchin.

The early stages in the development of the Sea-Urchin a
very similar to the corresponding stages in the development
of the Starfish described on page 359. The bilateral larva
of the Sea-Urchin, which is termed a Pluteus, is provided with
a number of elongated arms or processes supported by delicate

I

>

I

HC /

wt/

q

ire
int

Tf

PHYLUM ECHINODERMATA

369

ilcareous rods. A metamorphosis, in which the bilateral larva
becomes converted into the radial adult, takes place as in
Starfish.

3. EXAMPLE OF THE HOLOTHUROIDEA.
A Sea-Cucumber. — Cucumaria or Colochirus.

General External Features.— The body (Fig. 299) is elon-
gated, in shape not unlike a miniature cucumber, somewhat
irregularly five- sided, with an
opening at each end. One
end is somewhat thicker

than the other, and the open-
ing at this thicker (oral or
anterior) end is the mouth,
that at the opposite (aboral
or posterior) end is the anus.
The body is five-sided, and
along each side there extends
a double row of tube-feet. In
Colochirus there is a very
distinct ventral surface, into
which three of the five sides
enter, distinguished by the
absence of the rows of tuber-
cles that occur on the dorsal
portion of the surface, and
by the presence of three dis-
tinct bands of tube-feet.
This ventral part of the body
with its three ambulacra!
areas is the equivalent of the
trivium of the starfish, the
rest representing the bivium.
On the dorsal surface instead
of typical tube-feet there are
papillse devoid of sucking

extremities, and similar appendages take the place of tube-feet at
the ends of the three ventral bands. In Cucumaria the ventral
surface is less distinctly defined, but its position is always to be
determined by reference td the tentacles (vide infra) ; there are no
papiHse. The ventral surface is, it is to be noticed, parallel with
the axis joining mouth and anus, not at right angles with it as in
e Starfish and Sea-urchin, and the body is, when compared with
eirs, greatly drawn out in the direction of the line joining mouth
d anus.

FIG. 299.— Cucumaria planci. Entire animal
seen from the ventral surface. (From Hurtwig s
Lthrbuch, after Ludwig.)

VOL. I

B B

370 ZOOLOGY SECT.

There are no definite calcareous plates; but the integument is
tolerably hard, owing to the presence in its substance of innun id--
able microscopic calcareous spicules, very variable in shape in
different species of Cucumaria, and in Colochirus having the form
of sieve-like or lattice-like plates, some of which are to be found
even in the walls of the tube-feet. The tube-feet are, like those
of the Starfish, used in locomotion, progression being effected by
creeping with the ventral surface applied to the ground. In a
Sea-cucumber living undisturbed under natural conditions there
will be found protruded through the mouth a circlet of ten
tentacles, which are to be looked upon as greatly developed and
specially modified tube-feet. These are tree-like in shape — a
central stem giving off a number of short branches, which may in
turn be branched — and they are highly sensitive and contractile.
Two of these tentacles will be observed to correspond to each of
/the ambulacra! areas. The pair situated opposite the middle
\ ambulacral area of the ventral surface are very much smaller than
) the others, and will be observed to perform the special function of
(pushing the food-particles into the mouth. All the tentacles are
drawn completely back within the mouth when 'the animal is
disturbed.

Structure of Body-wall. — When the wall of the body is
divided, it is found to consist, in addition to the hardened integu-
iiK'uhtry layer, of two layers of muscle in addition to a thin layer
of cells, the peritoneum or ccelomic epithelium, lining the coelome.
The outer layer of muscle is a complete, continuous layer of
muscular fibres which have a circular arrangement, i.e. are
arranged in a ring-like manner around the long axis of the body;
while the inner layer is not continuous, consisting, in fact, merely
of five flattened bands which run longitudinally from the oral to
the anal extremities, each underlying one of the ambulacral areas.
In close contact with each of these bands, on its inner surface,
runs a redid ambulacral vessel (Fig 300, nnl. (i-nili.) together with a
rod I'll nerve.

Ambulacral System. — Just behind the bases of the tentacles,
and surrounding the beginning of the oesophagus is a circ
ambulacral vessel (ring, res.) which gives off the five nnllol vessels;
these first run forwards and give off branches to the tentacles,
and then run backwards, passing along the ambulacral areas anc~
giving off branches to the tube-feet, each of which is provid<
with its ampulla. From the ring-vessel is also given off a lar^
pear-shaped 7W/////. vesicle (pol. ves.), and a short sinuous canal, tl
madreporic canal (mad. can.), which ends in a perforated extremity,
not situated, like the madreporite of the Starfish or the Sea-urchin,
on the outer surface of the body, but in the interior of the coelome.
A nerve-ring surrounds the mouth and gives off the five r«<li<n
nerve*. A vascular ring (//. 11. /•/-,<?.) lies close to the nerve-ring

PHYLUM ECHINODERMATA

id sends off five radial vessels, as well as dorsal and ventral vessels
(int. ves.) accompanying the enteric canal, and a plexus surrounding
' Jie left respiratory tree.

•cross

radamh
ringv&s

ri.bl.ves
pol.ves
stom
int-ves

cL.oJb

. — Internal organs of a Holothurian as seen when the body-wall is divided along the
middle of the dorsal surface : b. v. body-wall ; circ. mv.s. circular layer of muscle ; cl. cloaca ;
cl. op. cloacal opening with five teeth ; Cur. org. Cuvieran organs ; //</(,. <«/>. genital aperture ;
gen. </u. genital duct ; <ien. gl. genital gland ; 'int. intestine; inter, oss. inter-ambulacral ossi-
cles ; int. ves. intestinal vessels ; I<H><J. ,,,cx. longitudinal band of muscle ; mad. can. madreporic
canals; mes. mesentry ; ,»»/. ves. I'oliaii vesicles; ,•<«>. u,,,l,. radial ambulacral vessel; ri. bl.
v-.v. ring blood-vessel ; resp. respiratory trees ; ring-res, ring- vessel of the ambulacral system ;
stom. stomach. (After Leuckart.) ^

The coelome contains a fluid in which float numerous amcebo-
cytes similar to those of the Starfish, and also a number of
flattened nucleated corpuscles containing a red colouring matter

B B 2

372 ZOOLOGY SECT,

— haemoglobin — almost identical with that which gives the red
colour to the blood of the higher animals.

The enteric canal is, as already mentioned, surrounded at
its oral extremity by the circlet of tentacles, and within these, when
they are fully exserted, is a narrow peristome with the mouth in
the centre. When the tentacles are retracted the peristome be-
comes inverted, so that peristome and tentacles become enclosed
within a chamber, the buccal chamber, into which the mouth leads.
Surrounding the oesophagus, which lies immediately behind the
buccal chamber, is a circlet of ten circum-cesophagcal ossicles, five
ambulacral (rad. oss.) in position, and five inter-ambulacral (inter,
oss.). Through each of the former pass the corresponding radial
ambulacral vessel, blood-vessel, and nerve. The alimentary canal
itself is a simple cylindrical tube, only indistinctly marked out
into oesophagus, stomach (stom.), and intestine. It forms several
coils within the ccelome, to the wall of which it is attached by a
thin membranous dorsal mesentery, and terminates behind in a
comparatively wide chamber, the cloaca (cl).

Opening into the cloaca is a pair of remarkable organs of
doubtful function, the so-called respiratory trees (resp.).- Each of
these, beginning behind in a single tubular stem, becomes elabo-
rately branched in front, some of the branches reaching nearly to
the anterior end of the body-cavity. Each of the terminal branches
ends in a ciliated funnel opening into the coelome. Besides having
to do, most probably, with the respiration of the ccelomic fluid
and with the excretion of waste-matters, these organs also have a
hydrostatic function ; it is through them that, when the tentacles
are withdrawn, the overplus of fluid which would impede the
process is got rid of, and through them, in like manner, that the
quantity is again increased when the tentacles are protruded
again.

Reproductive Organs. — The Sea-cucumber, like the Starfisl
and Sea-urchin, has the sexes separate. Ovaries and testes (gen.gl.]
are very like one another, and consist of bunches of tubulj
follicles, which communicate with the exterior by means of a duel
opening on the dorsal surface some little distance behind the 01
end.

The early stages of development are very similar to those of th(
Starfish (p. 358). The bilateral larva, however, assumes a shaj
somewhat different from the Bipinnaria of the Starfish, and
termed the Auricularm (Fig. 315) : it has, a number of shoi
processes developed in the course of the ciliated bands. Th(
larval mouth and oesophagus, instead of being abolished as in th(
Bipinnaria, persist to the adult condition.

PHYL1

:ATA

4. TYPE OF THE CRINOIDEA.
A Feather-Star. — Antedon rosacea.

General External Features. — In the Feather-star (Fig. 301),
.as in the Starfish, there are to be recognised a central disc and a
series of five radiating arms. In the natural position of the animal
the side of the disc which corresponds to the ventral or actinal
surface of the Starfish is directed upwards, and the dorsal or
.abactinal surface downwards. The five arms are bifurcated at
their bases ; they are feather-like, and highly flexible ; they act as
the locomotive organs of the animal, their alternate flexions and

[tensions resulting in a slow movement through the water.

On

. 301.— Antedon. Side view of entire animal. (From Leuckart and Xitsche's Diagrams.)

€bled
)n t
tain

the dorsal side of the disc are whorls of slender curved cylindrical
appendages, the dorsal cirri, by means of which the feather-star is
to anchor itself temporarily to a rock or a sea- weed.

)n the ventral side of the disc the body- wall is soft and flexible ;

taining only scattered irregular spicules of calcareous matter,
and in the centre of this surface is an opening, the mouth (Fig.
302, mo.). From the mouth five very narrow grooves, the ambulacral
grooves, radiate outwards towards the bases of the arms, near which
they bifurcate, so that ten grooves are formed, one passing along
the ventral surface of each of the ten arm-branches to its extremity.
The anal opening (an.) is likewise ventral, and is situated on a
papilliform elevation in the interspace between two of the radi-
ting canals.

illlllg L'clilcl

374

ZOOLOGY

SECT.

The dorsal side of the disc is occupied by a large flat pentagonal
ossicle, the centro-dorsal ossicle,1 (Fig. 304, C, D) bearing on its
outer surface a number of little cup-like depressions, with which
the bases of the cirri are connected. The cirri (cirr.) consist each
of a row of slender ossicles, covered, like all the rest of the animal,
with epidermis, and connected together by means of muscular
fibres. Concealed from view by the centro-dorsal ossicle is a thin
plate termed the " rosette " (ros.), formed by the coalescence of the
basals of the larva. At the sides are five first radial (H.1) ossicles,
also concealed by the centro-dorsal ossicles: with each of these
articulates a second radial (-R.2), which is visible beyond the centro-
dorsal. With each of the second radials articulate two third

radials (-ft.3), each
forming the base of
the corresponding
arm -branch.

The ossicles of
the arms — braehials
(£r.l,t£r.z)— are ar-
ranged in a single
row in each arm.
They are somewhat
elongated in the
direction of the long
axis of the arm,
strongly convex on
their dorsal sur-
faces, longitudinally
grooved ventrally,
connected together
by the investing
epidermis, and by
bundles of muscular

fibres, by the contractions of which the movements of the arms
are brought about. Fringing the sides of each arm are two rows
of side-branches, or pinnules, each supported by its row of con-
nected ossicles, and each grooved along its ventral surface.

The coelome contains numerous strands of connective tissue
which serve to suspend the various organs.

Extending through the arms and pinnules between the sup-
porting ossicles and the ambulacral grooves are. three canals which
are prolongations of the coelome (Fig. 303, ccel. can.). Two of these
—the sub-tentacular canals — form a pair separated from one another
by a median septum underlying the ambulacral groove. The

1 This has nothing to do with the dorso-central ossicle referred to above
(p. 362) as one of the primary apical plates of the Starfish : the dorso-central
is not represented in the Feather-star.

FIG. 302.— Antedon, ventral (upper) surface of the central
disc ; an. anus ; mo. mouth. (From Vogt and Jung.)

PHYLUM ECHINODERMATA

375

rod. amb

otner — the cceliac caned — runs between these and the supporting
ossicles (pss.). The sub-tentacular canals and the coeliac canal
communicate with one another at the extremity of the arms.

The enteric canal begins with a wide funnel-shaped oeso-
phagus leading to a spacious stomach having a small caecum con-
nected with it. Distally it becomes contracted and opens into
a wide intestine, which winds round the coelome, giving off at its
gastric end a number of small csecal diverticula, and becoming
narrower where it passes upwards to open on the exterior, the
terminal part, or rectum, projecting as a tubular papilla on the
surface. In the living animal the rectal tube is observed to
undergo frequent movements of contraction and dilatation by
means of which water is
drawn into and expelled from
the intestine, so that here, as
in the Sea-urchin, there
would appear to be a process
of intestinal respiration.

The ambulacral system
consists of a ring-vessel sur-
rounding the mouth, and a
series of radial vessels (Fig.
303, rod. amb.) which run in
the ambulacral grooves, giving
off branches to the pinnules.
Connected with the radial
vessels and their branches are
a series of minute tubular
appendages, the so-called ten-
tacles (Fig. 304, tent.), which
are homologous with the tube-
feet of the Star-fishes and Sea-
urchins, but are devoid of ter-
minal suckers. These are not

organs of locomotion, they bear numerous sensory papillae, and are
therefore to be looked upon as tactile organs ; but they probably
also have a respiratory function. Connected with the ring-vessel
are a number of ciliated, branched, tubular diverticula, the water-
tubes, which are suspended within the coelome, and may open freely
into it at their extremities. A large number of vessels with
minute ciliated openings — the water -pores (ivat. p.) — lead through
the ventral wall of the disc : these and the ciliated tubes are to be
looked upon as together representing the madreporic canal and its

>enings in the Star-fish and Sea-urchin.

The nervous system consists of two perfectly distinct parts.

nerve-ring (ect. ne.) surrounds the mouth, and from it are given
)ff a series of ambulacral nerves — thickenings of the epidermis of

055

FIG. 303. — Antedon, transverse section of a'
pinnule, amb. ne, ambulacral nerve ; ax. n e.
axial nerve ; ccel. can, sub-tentacular and
coeliac canals ; mus. muscles ; neur. res. blood
vessel ; rad. amb. radial ambulacral vessel.
(After Teuscher.)

ope,
A n

r

376

ZOOLOGY

SECT.

the ambulacra! grooves and their offsets — which extend throughout
the length of the arms and pinnules. In the axis of the support-
ing ossicles of the arm is an axial nerve (ax. co.\ which gives off
branches (Fig. 303, ax. ne.) running through the axes of the ossicles
of the pinnules. The axial nerves are connected internally, not
with the circum-oral nerve-ring, but with a central body situated
below the rosette, in the interior of the centro-dorsal ossicle. This,
the central capsule (Fig. 304, cent, caps.), forms the investment of
a body termed the five-chambered organ (chamb. org.\ divided into
five parts by radial septa, and continuous with the dorsal end of
the genital stolon. Processes from the five angles of the central
capsule combine to form a pentagonal ring from which pass out-

len-l

amb

c7ux.mb.org

cent, caps

FIG. 304. — Antedon, Diagrammatic view of a median vertical section through the disc, passing
through one radius and one inter-radius. aml>. ambulacral vessels ; ax. co. axial nerve cord
passing through the ossicles of the arm ; Br^ Br,* brachial ossicles ; CD. centro-dorsal
ossicle; cent. m/w. central capsule; clm;nl>. o/y/. chambered organ; cirr. cirri ;' ect. ne.
ambulacral (epidermal) nerve-ring and radial nerve ; (/en. st. genital stolon ; int. intestine '.
mo. mouth; /f,i R^ J{,3 radials ; ros. rosette; tent, tentacles; v:at. p. water-pores. (After
Mimes Marshall.)

wards the axial nerves of the arms. The ambulacral nerves and
their central ring represent the epidermal nerves of the Starfish ;
the axial nerves are represented in the latter only by the com-
paratively feebly-developed coelomic system.

A blood-vascular system is present, with a circum-oral rin|
vessel giving off radial vessels to the arms, and a plexus of vessel
surrounding the oesophagus, and enclosing the genital stolon.

Numerous bodies termed the sacculi, the character of whicl
has given rise to much discussion, occur regularly arranged alon^
the ambulacral grooves, and also in other parts. They are small
spherical bodies which become vividly coloured by means of
staining agents. They are sometimes supposed to be parasitic

ix PHYLUM ECHINODERMATA 377

Algae ; but the regularity of their arrangement is opposed to
such a view. It has been suggested with more appearance of
probability that they may be masses of reserve materials, stored
up for the nutrition of the animal

The reproductive organs — ovaries or testes, as the case may
be — are lodged in the dilated bases of the pinnules, which become
•considerably enlarged as the ova or sperms mature, those next to
-the bases of the arms alone remaining sterile. When mature, the
.sexual elements escape by means - of short ducts. Each gonad
is one of the terminal parts of a system of tubes lined by an
•epithelium, and extending from a central part or genital stolon
{gen. si) lodged in the vascular plexus that surrounds the oesopha-
gus and connected dorsally with the chambered organ, outwards
through the arms, the terminal portions, lying in the pinnules,
becoming dilated to form the reproductive organs, and the cells
of their epithelium becoming developed into ova or sperms, while
the rest constitute a non-fertile connecting rachis. This system
is enclosed throughout by a plexus of blood-vessels.

Like the rest of the Echinoderms, the Feather-star undergoes a
metamorphosis (Fig. 316). The larva is at first oval and
covered uniformly with cilia. Afterwards the cilia become
restricted to four transverse bands with a bunch of longer cilia
at one end ; the body becomes bent towards the ventral side,
ossicles begin to be formed, and the posterior extremity becomes
drawn out into a narrow process in which supporting ossicles
soon appear. At the end of this posterior, or more correctly
dorsal, process, a terminal disc is formed, and by means of this
the larva fixes itself, the process forming a supporting stalk.
The larva now assumes the form to which the term l' pentacrinoid"
has been applied, owing to the fact that, in its most essential
features, it resembles the adult form of Pentacrinus, one of the
stalked Crinoids (See Fig. 314), with a central disc, giving off five
bifurcated arms with their pinnules, and supported on a narrow
stalk springing from the middle of the dorsal surface. This fixed
pentacrinoid larva passes into the adult, free-swimming Feather-
star by the development of the dorsal cirri, the greater elongation
of the arms, and the absorption of the stalk, the uppermost ossicle
-of which is represented in the adult by the centro-dorsal ossicle.

5. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Echinodermata are radially symmetrical animals, the radial
•arrangement of whose parts imperfectly conceals a more obscure
.and more primitive bilateral symmetry. The surface is covered
with an exoskeleton of calcareous plates or ossicles, which
usually support a system of movable or immovable calcareous
opines. There is a large body-cavity or coelome, and well-developed

378 ZOOLOGY SECT.

alimentary, nervous, and vascular systems. A characteristic
system of vessels, the ambulacral system, is connected with the
locomotion of the animal, as well as with other functions : the
organs of locomotion are elastic and contractile tubular bodies,
the tube-feet, which are appendages of the ambulacral system.
Nearly all the systems of organs of the animal partake to a greater
or less extent of the general radial form of the body. Repro-
duction is entirely sexual. In the course of its development from
the egg the Echinoderm passes through a peculiar larval stage,
in which the symmetry of parts is bilateral instead of radial as
in the adult animal. All the Echinodermata are marine.
The Echinodermata are classified as follows : —

CLASS I,— ASTEROIDEA.

Free Echinoderms with star-shaped or pentagonal body, in
which a central disc and usually five arms are more or less readily
distinguishable, the arms being hollow, and each containing a
prolongation of the coelome and of the contained organs. There
are distinct dorsal and ventral surfaces, on the former of which
the anus and the madreporite are situated, and on the latter the
mouth and five narrow ambulacral grooves lodging the tube-feet.
The larva has the form .either of a Bipinnaria or of a Brachiolaria
(see p. 397). This class includes the Starfishes.

ORDER 1. — PHANEROZONIA.

Asteroidea with large marginal ossicles. The dermal branchiae
are present only on the dorsal surface. The ambulacral ossicles
not closely crowded. PedicellariaB sessile.

ORDER 2. — CRYPTOZONIA.

Asteroidea with the marginal ossicles inconspicuous. Dermal
branchiae not restricted to the dorsal, but often present on the
oral surface. Ambulacral ossicles crowded together. Pedicellariae
stalked or sessile.

CLASS II.— OPHIUROIDEA.

Star-shaped free Echinoderms, with a central disc and five arms,
which are more sharply marked off from the disc than in the
Asteroidea, and which contain no spacious prolongations of the
coelome. There are distinct dorsal and ventral surfaces. The
anus is absent ; the mouth, as well as the madreporite, ventral.
There are no ambulacral grooves. The larva is a Plutcus. This
class includes the Sand-stars and Brittle-stars.

ORDER 1.— OPHIURIDA.
Ophiuroidea in which the arms are simple.

ORDER 2. — EURYALIDA.

Ophiuroidea in which the arms are branch od.

u ;

PHYLUM ECHINODERMATA

.

CLASS III,— ECHINOIDEA.

Free Echinoderms with globular, heart-shaped, or disc-shaped
body enclosed in a shell or corona of close-fitting, firmly united
calcareous plates. The mouth is nearly always polar ; the anus
usually at the opposite (aboral) pole ; the madreporite is close to
the latter. There are no ambulacral grooves ; but the surface is
divided into alternating ambulacral and inter-ambulacral zones or
areas, which usually run from pole to pole. The larva is a Pluteus.
This class includes the Sea-urchins, with the Heart-urchins and
Cake-urchins.

ORDER 1. — PAL^EO-ECHINOIDEA.

Fossil Echinoidea in which the number of rows of plates in the
corona is variable, and in which the plates overlap one another.

CLASS IV.— HOLOTHUROIDEA.

ORDER 2. — REGULARIA.

Echinoidea with globular corona containing twenty meridional
rows of plates. Mouth and anus polar. A lantern of Aristotle is
present. This order includes the Sea-urchins.

ORDER 3. — CLYPEASTRIDEA.

Echinoidea with more or less flattened corona, with the mouth
central, the anus excentric. A lantern of Aristotle is present.
This order includes the Cake-urchins.

ORDER 4. — SPATANGOIDEA.

Heart-shaped Echinoidea with the mouth and anus excentric.
No lantern of Aristotle. This order includes the Heart-urchins.

Free Echinoderms with elongated, cylindrical or five-sided body,
ving the mouth and anus at opposite extremities. The body-
wall is usually only supported by scattered ossicles or spicules.
There is no external opening to the madreporic canal (except in
some Elasipoda). The surface usually exhibits five ambulacral
areas ; but these may be absent. There is a circlet of large oral
tentacles. The larva is an Auricularia. This class includes the
Sea-cucumbers and Beche-de-mer.

ORDER 1. — ELASIPODA.

Holothuroidea with well-marked bilateral symmetry, with tube-
feet on the ventral surface (which is flattened) and papillae on the
dorsal. Confined to the deep sea.

ORDER 2. — PEDATA.

Holothuroidea with tube-feet either in longitudinal rows or
scattered irregularly over the surface.

380 ZOOLOGY SECT.

ORDER 3. — APODA.
Holothuroidea devoid of tube-feet and of radial ambulacral

vessels.

CLASS V.— CRINOIDEA.

Temporarily or permanently stalked Echinoderms with star-
shaped body, consisting of a central disc and a series of five
bifurcate or more complexly branched arms, bordered with pin-
nules. There are distinct dorsal and ventral surfaces ; the latter
bearing the mouth and anus, and the inner ends of a series of
narrow ambulacral grooves. This class comprises the Feather-
stars and Sea-lilies.

ORDER 1. — PAL^OCRINOIDEA.

Stalked Crinoidea in which the disc is large as compared with
the arms, a number of inter-radial plates being present and often
united with the disc. The ventral surface usually concealed by a
"" vault " of calcareous plates.

Extinct, pabeozoic.

ORDER 2. — NEO-CRTNOIDEA.

Stalked or free Crinoidea in which the disc is small as com-
pared with the arms, and in which inter-radials, when present, do
not combine with the plates of the disc. There is no " vault "
covering the ventral surface.

Comprising all the living forms, together with several extinct
mesozoic families.

CLASS VI.— CYSTOIDEA,

Fossil Echinoderms with globular body, sometimes sessile, sometimes stalked,
enclosed in usually irregular, polygonal plates. Mouth central ; five radiating
ambulacral grooves. Palaeozoic.

CLASS VII,— BLASTOIDEA.

Fossil Echinoderms with ovate stalked body, central mouth, and five ambu-
lacral areas. Palaeozoic.

Systematic Position of the Examples.

Asterias rubens is a species of the genus Asterias, which, with
several others, constitutes the family Asteriidce of the order
Cryptozonia. The family Asteriidce is characterised among the
families of the Cryptozonia by the following distinctive features > —
The ossicles of the dorsal surface are small, unequal, reticulate
plates, bearing isolated or grouped spinelets (paxillaB). The margin
of the actinostome is defined by the ambulacral plates. The
pedicellaria3 are of two forms, forceps-like and scissors-like. The

ix PHYLUM ECHINODERMATA 381

tube-feet are in four rows. Asterias differs from the other genera
of the family in having well-developed reticulate dorsal ossicles
bearing definite spines.

The Sea-urchins of which a short description has been given
are the genera Strongylocentrotus and Echinus, but the description
is sufficiently general to apply to any member of the family
Echinidce, to which these genera, with a number of others, belong.
The family Ecliinidcc is one of about five families of the sub-order
JEctobranchiata, the members of which all differ from the other
sub-order — Entobranchiata — of the JRegularia, or regular Sea-
urchins, in the possession of dermal branchiae, and in having the
auricles in the form of complete arches.

The Sea-cucumber (Cucumaria or Colochirus) is a member of
the Stichopoda — one of the families of the sub-order Dcndrochirotce
of the Pedata, or foot-bearing Holothurians. The Dendrochirotw
differ from the Aspidochirotm — the other sub-order — mainly in
having arborescent instead of shield-shaped tentacles, and the
Stichopoda differ from the rest of the Dendrochirotce in having the
ljube-feet arranged in five regular zones. The genus Cucumaria
is distinguished from the rest by the ten tentacles with the two-
ventral smaller than the others. Colochirus is closely allied to
Cucumaria., the principal distinction being the presence in the
former of papillae taking the place of tube-feet in certain
situations as already noted.

The Feather Star (Antedon rosacea) is a member of the family
Comatulidce, which is distinguished from the four other living
families comprised in the order Neocrinoidea, by the absence of a
stalk in the adult condition.

6. GENERAL ORGANISATION.

General Form and Symmetry. — Like the Coelenterata, the
Echinodermata are radially symmetrical, the body being capable of
division into a series of sub-equal antimeres along a series of
radiating planes at right angles to the principal axis. In the
majority of existing forms (Asteroidea, Ophiuroidea, and Crinoidea)
the radial symmetry is expressed in the external form of the body,
which is produced into a number of radially disposed parts, the arms*
or rays, arranged around a smaller or larger central disc. But in the
Echinoidea the body is sub-spherical, and in the Holothuroidea
sub-cylindrical, the radiate arrangement being in these classes
indicated externally only by the distribution of the tube-feet, and
internally by that of certain of the systems of organs.

Although, however, the general external form and the arrange-
ment of some of the internal organs in the Echinodermata indicates
a radial symmetry, it is invariably found that this radial arrange-
ment serves to hide a more primitive and more fundamental

382 ZOOLOGY SECT.

bilateral symmetry. This is best marked in the larva, which has
pronounced bilateral, instead of radial, symmetry, but is quite
recognisable in the adult. In all Echinoderms there is, passing
through the primary axis, a plane — the median plane — along
which, and along which alone, the body is capable of being divided
into two equal, or, to speak more correctly, approximately equal,
right and left, halves. The existence of such a single median
plane is, as already explained (p. 348), indicative of the bilateral
form of symmetry.

The body is most usually five-rayed (Ophiuroidea, most Aste-
roidea, Crinoidea), cylindrical (most Holothuroidea) or globular
(most Echinoidea), the surface in the two last cases being marked
by five bands or zones of tube-feet, which divide it into five
ambulacral and five inter-ambulacral areas. In the Ophiuroidea
and Asteroidea two of the rays — constituting the bivium — have
between them the madreportte, marking the position of the
madreporic canal of the ambulacral system ; the remaining three
rays form the trivium. The median plane passes through the
madreporite, and thus midway between the two rays of the
bivium, and bisects longitudinally the middle ray of the trivium.
A corresponding disposition of the parts is traceable also, as
will be subsequently shown, in the cylindrical and globular
Echinoderms.

In all the Echinodermata dorsal or abactinal and ventral or
actinal surfaces are more or less distinctly recognisable. In the
Asteroidea, Ophiuroidea, and Echinoidea, the ventral surface is
that in the middle of which the mouth is situated, and which is,
in the natural position of the animal, directed downwards or
towards the surface to which it is clinging. The opposite dorsal
or abactinal surface is, in the majority of the Asteroidea and
Echinoidea, marked by the presence of the anal aperture : in the
Ophiuroidea and some Asteroidea the anus is absent, in some
Echinoidea it is situated on the border between the two surfaces,
or even on the oral surface. In the Crinoidea the ventral surface,
which is habitually directed upwards in the natural position of the
animal, bears both mouth and anus, the former central, the latter
eccentric and inter-radial. In the fixed Crinoids the dorsal surface
lias attached to its centre the distal end of the stalk ; in the free
forms it has connected with it whorls of slender curved appen-
dages, the dorsal cirri, by means of which temporary attachment
is effected. In the Holothurians owing to the elongation of the
body in the direction of the line joining mouth and anus, dorsal
and ventral surfaces corresponding to those of the other classes are
not recognisable ; but in many, as for example in Colochirus, there
is a marked difference between one surface which is habitually
directed upwards, and another which is habitually directed down-
wards : these dorsal and ventral surfaces in the Holothurian, it

PHYLUM ECHINODERMATA 383

to be specially noticed, extend in the direction of the axis
joining mouth and anus, and not at right angles to it as in the

«rfishes and Sea-urchins,
n considering the general external form in the various classes
Uchinoderms, we have to take into account the arrangement of
the tube-feet — the organs of locomotion — as these have important
relations to the other parts, and to the whole plan of organisation
of the animal. These organs, as previously explained, are tubular
appendages with highly elastic and contractile muscular walls,
capable of being stretched out so as to extend a long way from the
surface of the body. In the majority of cases the tube-foot has at
its extremity a sucking-disc, by means of which it can be attached ;
in a few, however, this sucking-disc is absent.

The entire surface is covered with a ciliated epidermis. In the
subjacent dermal layers there are always present calcareous bodies
or ossicles, varying very greatly in form and arrangement in the
different groups. Movable or immovable calcareous spines or
tubercles projecting on the surface are very general. Peculiarly
modified spines, termed pedicellarice, are commonly, though not
universally, present in certain parts in the Echinoidea and
Asteroidea. A pedicellaria consists in essence of two or three
calcareous jaw-like pieces or valves, movably articulated together,
and capable of being separated or approximated by the con-
traction of bundles of muscular fibres. Sometimes there is a long
stalk ; sometimes (as in the case of Anthenea, p. 357) a stalk is
absent. During life the jaws or valves keep opening and closing.
That such specialised structures have some important function to
perform there can be no doubt, but there is some uncertainty as
to what their special purpose is. According to some observers,
the pedicellarise of the Sea-urchin have been seen passing from
one to another the particles of fbecal matter discharged from the
anus, and their function would thus appear to be a cleansing one.
On the other hand, it is stated that when a Sea-urchin is attacked
the spines may be bent aside from the assailed portion of the
surface, so as to allow of the pedicellaria3 being brought to bear as
defensive weapons on the assailant, and from these and other
observations that have been recorded, .both on Asteroids and on
Echiaoids, it is concluded that the main function of these append-
ages is to act as defensive organs. Pedicellarise are absent in the
Ophiuroids, but in the Euryalida there are peculiar hook-like
organs of adhesion, most abundant on the ventral surface and
towards the extremities of the arms. The sphceridia, which have
already been referred to as occuring in the Sea-urchin, are only
doubtfully to be regarded as modified spines ; they are confined
to the Echinoidea. Also confined to that class are the clavulce —
slender spines covered with strong cilia, which occur in bands on
the surface of the Spatangoids. Larger spines resembling the

384 ZOOLOG/ SECT,

clavulse in being covered with strong cilia, occur also in the
Clypeastroids and some Asteroids. The currents produced by the
action of their cilia serve to keep constantly renewed the water
in the neighbourhood of the anus and of the branchiae.

There are two principal systems of plates to be recognised,.
an oral and an apical ; the former corresponding with the oral or
actinal* and the latter with the aboral or abactinal surface. The
former vary considerably in the different classes : the constant
elements are five orals, which may or may not be recognisable in
the adult. The apical system consists (1) of a dorso-central plate -r
(2) of five basals which are inter-radial in position; (3) of five
radials, which are radial in position. In the Asteroidea (Fig. 292)
the radials are late in making their appearance ; before they are-
developed five terminal plates have become distinct, one at the
end of each rudimentary arm ; these become carried outwards by
the extension of the arm, and each supports the corresponding
tentacle. As a rule these plates of the apical system are only
distinct in the young condition. In the Ophiuroidea the arrange-
ment resembles that observable in the Asteroidea. In the Echi-
noidea (Fig. 295) the basals (genitals) are perforated by the ducts
of the reproductive glands ; the radials (oculars) are perforated for
the eye and tentacle : the dorso-central (anal) rarely persists as a.
single plate in the adult, usually becoming broken up into a series
of irregular plates. In the stalked Crinoidea the term dorso-
central has been applied to a plate which is transformed into the
disc of attachment at the base of the stalk, but the correspondence
between this and the similarly named plate in the other classes
is very doubtful; the ossicles of the stalk intervene between,
it and the basals. In the free forms the uppermost segment
of the larval stalk is transformed into a centro-dorsal plate, and th
basals nearly always unite into a rosette plate, which is conceale
from view by the centro-dorsal and the radials. The apical syste
of plates is apparently not represented in the Holothuroidea.

Modifications of Form in the Five Classes.— The general
shape in the Asteroidea is, as already pointed out, that of a star..
There is a central part, or central disc, from which proceed a series
of radially disposed arms or rays. The central disc and the rays
are usually compressed dorso-ventrally, as in Anthenea and
Asterina, but in some Starfishes the rays are approximately
cylindrical ; they nearly always taper distally. In the majority of
Starfishes, as in the examples described, the arms are five in nam
ber, except in malformed individuals ; but in some they are six, i
others seven, eight, or more. The proportions borne by the arm
to the central disc are subject to considerable variation. In som
as in Asterias, the arms are long and the central disc appears
little more than their point of union ; in others, again, the whole
Starfish has the form of a five-sided disc, in which the arms arc-

nt

i

IX

PHYLUM ECHINODERMATA

385

represented only by the five angles ; while between these two
extremes there are numerous intermediate gradations. The
Brisingidce differ from all the rest of the class in having the arms
almost as sharply separated off from the central disc as in the
Ophiuroids.

The dorsal or aboral, and the ventral or oral surfaces are always
distinctly marked off from one another. In the middle of the
latter is the mouth, running out from which are five or more
narrow ambulacral grooves, one of which is continued along the
ventral surface of each arm to its extremity. Near to, but not
quite in, the middle point of the dorsal surface is the anal
aperture, absent in a few
instances ; and on the
same surface, nearer the
margin, between the two
rays of the bivium in the
five-rayed Starfishes, is
the madreporite, a finely
grooved calcareous plate
perforated by a number of
minute apertures. In some
fossil Starfishes it is situ-
ated on the ventral sur-
face. Sometimes instead
of one madreporite there
are several.

The wall of the body
in the Starfishes contains
a number of calcareous
ossicles, movably articu-
lated together and con-
nected by bands of muscle,
so that, though the body
is firm, and in the dried

condition often quite rigid, the arms are capable during life of
slow movements of flexion and extension, enabling the animal to
creep through comparatively small fissures and crannies. A
special system of ossicles — the ambulacral ossicles — are arranged
in a double row along each ambulacral groove, the ossicles of the
two rows articulating movably with one another at the apex of
the groove. At the end of the arm the two rows of ambulacra!
ossicles end in a terminal ossicle which supports the unpaired
tentacle. Spines .are invariably present, but are sometimes con-
fined to the margins of the ambulacral grooves, in which position
they are movably articulated with the underlying ossicles.
Tubercles take the place of spines over most of the surface in
many forms. In Astropecten the ossicles of the dorsal surface
VOL. I c c

FIG. 30,'). — Anthenea. View of ventral surface.
(After Sladen.)

386

ZOOLOGY

SECT.

take the special form to which the term paxillcc is applied. Each
paxilla is a plate which is produced into a short rod, divided at its
extremity into a number of radiating processes.

The tube-feet are arranged in a double row along each of the
ambulacral grooves, each connected through an aperture between
the ambulacral ossicles with an ampulla, or, exceptionally, with
two, situated in the coelome. Each double row of tube-feet
terminates at the extremity of the arm in an unpaired appendage,
the tentacle, which is tactile and olfactory, and not locomotive, in

. 30(i.— OphiOglypha lacertosa. A, outline, of the natural size. £, central disc, dc
view. C, the disc, ventral view showing the mouth and genital fissures. (From Nichols
and Lydekker's Paleontology.')

function. The tube-feet are provided (except in Astropecten) wit!
terminal suckers.

In the Ophiuroidea (Fig. 306) the central disc is much more
sharply marked off from the arms than in the Asteroidea. The
arms, which are five in number, are comparatively slender,
cylindrical, tapering towards the free extremities ; in one group,
the Eitrt/ft/idd (Fig. 307), they are branched. The mouth is in th
middle of the ventral surface of the disc, as in the Asteroidea, bu1
there are no ambulacral grooves and there is no anal aperture.
Five pairs of slits on the oral surface (Fig. 306, C) lead into the
genital sacs, which receive the sperms and ova from the gonads,
and which appear also to act as organs of respiration and perhaps
also of excretion. The surface is covered with thin plate like

PHYLUM ECHINODERMATA

387

ossicles, usually beset along their edges with longer or shorter-
spines; sometimes irregular calcareous granules take the place
of plates. Hook-like organs of adhesion are present only in the
Euryalida. Each of the arms is supported by a row of internally
situated ambulacral ossicles. Tube-feet are present and are pro-
truded at the sides of the arms between the lateral plate-like
ossicles; but they have no sucking-discs and no ampulla?, and
locomotion is effected in the majority of the Ophiuroids by active
flexions and extensions of the arms. In one genus there is a pair

FIG. 307.— Astrophy ton arborescens, dorsal view. (After Ludwig.)

fin-like appendages, supported by slender spines, on each joint
of the arms. The madreporite is situated inter-radially on the
ventral surface, and not on the dorsal as in the Asteroidea, In
the Euryalida there are five madreporites and five madreporic
canals. ^\

In the Echinoidea the body is either globular or heart-shaped, v
or flattened and disc-like ; dorsal and ventral surfaces are always
distinctly recognisable. The exoskeleton is in the form of a rigidly
articulated system of calcareous plates, fitting in closely together

c c 2

388

ZOOLOGY

SECT.

by sutures so as to form a continuous shell or corona. Astkenosoma,
a deep-sea genus, differs from all the rest in having a corona
possessing a certain degree of flexibility, and performing move-
ments which are brought about by the contractions of five
longitudinal bands of muscle running along the ambulacral areas
on the inner surface.

In the globular forms, or regular Sea-urchins, the mouth is situ-
ated at the ventral pole of the globe, the anus at the dorsal, and

FIG. 308.— Strongylocentrotus, entire animal with the tube-feet extended. (From Brehm'i

Thierleben.)

the plates of the corona are in twenty regular meridional
arranged in ten zones, five ambulacral and five inter-ambulaci
as described in the account of Echinus, with peristome, periproct,
ocular and genital plates, and madreporite. Spines (Fig. 309^
pedicellarice (Fig. 310), and sphceridia are present, as already
described (p. 363), the last-named appendages, however, bein
absent in one group. The spines are usually defensive organs
simply, but in some Sea-urchins they act also as the locomotive
organs, the animal moving by their agency along the sea-bottom.

IX

PHYLUM ECHINODERMATA

389

FIG. 309.— Diagram of spine
of Sea-Urchin showing
mode of articulation.
tn. muscle ; b. ligament.
(From Leuckart.)

The tube-feet, which are arranged in a double row in each
ambulacra! zone, are extremely extensible, and terminate in suck-
ing-membranes strengthened by a calcareous rosette. An unpaired
tentacle, corresponding to those of the Asteroidea, is supported on
each of the ocular plates at the ends of
the ambulacral zones. Two tube-feet in
each double row, situated on the peristome,
are likewise of the nature of tentacles,
being devoid of sucking-membranes. Cor-
responding to the dermal branchiae of the
Asteroidea are, in the majority, five pairs
of branched hollow appendages surrounding
the peristome.

Surrounding the mouth are five teeth,
supported by an elaborate system of ossicles
(Aristotle's lantern, see p. 366), and a ring
of processes, the auricles, from the interior
of the corona surrounds this and gives
attachment to some of the muscles by
which the ossicles are moved.

In the heart-shaped forms or Hear.tr
urchins (Fig. 311) the corona is heart-shaped,
the mouth is usually more or less eccentri-
cally placed on the oral surface, and the peristome is usually trans-
versely elongated ; the anus is on or near the border between the
two surfaces. The ambulacral areas do not run continuously, but
stop short at the margin (petaloid ambulacra) ; one of them, the
anterior, is usually unlike the others and frequently devoid of
pores. The genital and ocular plates are in the middle of the
aboral surface, where the ambulacra con-
verge, and are thus widely separated from
the anus ; there are usually only four genital
plates, and the genital apertures may be
reduced to two. Slender spines beset the
entire, surface and are the chief organs of
locomotion. Modified spines, the clavulce,
surround the anus in a ring and are dis-
tributed elsewhere. A few pedicellarise
are present in the neighbourhood of the
mouth, and sphseridia also occur. A series
of tree-like dermal branchias surround the
peristome. The " lantern of Aristotle," with
its teeth, is not represented.

In the Clypeastridea or Cake-urchins the whole corona (Fig. 312)
is usually greatly compressed so as to assume the form of a disc,
sometimes notched at the edges or pierced by fenestraB. The mouth
is in the middle of the flat or concave ventral surface, the anus

FIG. 310.— Pedicellaria of
Arbacia punctulata.

(From Leuckart.)

390

ZOOLOGY

SECT.

eccentrically situated near the margin. The ambulacra are petaloid.
The genital and ocular plates are usually more or less fused

together at their edges,
C and the genital apertures
are often not in the geni-
tal plates, but in the
corresponding ambulacral
zones. The spines are ex-
ceedingly fine and hair-
like. Sphaeridia are pres-
ent, but pedicellarias and
clavulae are absent. An
"Aristotle's lantern" with
teeth is present, as in
the globular forms.

In the Holothuroidea
the body is more or less
elongated in the direc-
tion of the axis joining
mouth with anus, which
are placed at opposite
extremities (anterior or
oral, and posterior or
anal) of the body. The
shape is sometimes com-
pletely cylindrical, some-
times five-sided; in many
there is more or less
dorso-ventral compres-
sion, and the dorsal and
ventral surfaces may
differ greatly from one

another. A flattened sole-like ventral surface bearing the three
rows of tube-feet of the trivium is, as already stated, often dis-
tinguishable : it is most distinctly de-
veloped in Psolus and allied genera. In
some Holothuroids the surface is en-
closed in an armour of close-fitting
plates; but in the vast majority the
body- wall is comparatively soft, being
strengthened merely by a great number
of minute ossicles of a variety of shapes.
In Synapta (Apoda) numerous minute
anchor-like spicules, each connected with
a latticed plate, project from the surface,
and cause the animal to adhere to soft surface showing the petaioia.

, ,. .., T - i ., .-iinliulacTa. (From Hertwig's

bodies with which it comes in contact. /.,/,, •/,*/,•/«.)

FIG. 311.— Hemipneustes radiatus. A, from
above. B, from below. C, apical plates. (From
Bromi's T/ticrreich.)

PHYLUM ECHINODERMATA

391

und the mouth is a whorl of tentacles — pinnate, shield-shaped, or
rescent. The tube-feet are sometimes entirely absent. When
nt they are usually uniform in character throughout, and
may be arranged in five regular longitudinal rows, or scattered
over the entire surface. Sometimes, as has already been stated in
the account of Colochirus, the tube-feet of the dorsal surface and
even some of those of the ventral may assume the form of papillae.
In the Elasipoda the tube-feet of the dorsal surface are remarkably
modified, taking the form of greatly elongated processes.

In the Crinoidea the general shape is that which has been
ribed in the case of the feather-star — star-like, with a central

FIG. 313.— Ante don.

'Side view of entire animal,
diagrams.)

(From Leuckart and Nitsche's wall-

disc and a series of radiating arms, which usually branch dicho-
tomously. In the stalked forms (Fig. 314) a stalk, consisting of
a row of elongated ossicles connected together by bundles of
ligamentous fibres, attaches the animal to the sea-bottom. Along
some of the joints of the stalk are usually arranged a number
of slender, many-jointed appendages — the cirri. At its base the
stalk usually breaks up into a number of root-like processes ; distally
it becomes continuous with the central disc. The ossicles forming
the skeleton of the central disc are the basals and the radials : with
the latter articulate externally the brachials, a single row of which
gives support to each of the arms and its branches, while similar rows
of smaller ossicles support the pinnules — the lateral appendages
which fringe the arms in a double row. In the free forms (Feather-
stars) the stalk is absent in the adult condition, though present

392

ZOOLOGY

SECT.

Fia. 314.— Metacrinus interruptus.

(After P. H. Carpenter.)

in the larva, and the
place of its terminal ossicle
is taken by a plate — the
eentro-dorsal ossicle of the
disc. To the eentro-dorsal
ossicle are attached whorls
of many-jointed, slender,
curved dorsal cirri.

The mouth in all the
Crinoidea, with one ex-
ception (Actinometra), is
situated in the centre of
the ventral (upper) sur-
face, and the anus in all,
with the same exception,
is excentric and inter-
radial. Running outwards
from the mouth are a
series of very narrow am-
lulacral grooves, one of
which runs outwards on
the ventral surface of each
arm, giving off branches
to the arm-branches and
to the pinnules. Bordering
the ambulacral grooves
and their branches are a
pair of rows of short
tubular tentacles, which
correspond morphologi-
cally with the tube-feet
of the other classes, but
are devoid of the terminal
suckers, and are not loco-
motor, but probably sen-
sory and respiratory in
function.

The coelome in the
Echinoderms is a wide
cavity lined by a ciliated
coelomic epithelium and
containing a corpusculated
fluid. Prolongations of it
pass out into the rays,
and, in the Ophiuroidea
and Asteroidea, between
the layers of the body-
wall. In the Crinoidea

PHYLUM ECHINODERMATA 393

contains numerous strands of connective tissue. Special
us providing for the respiration of this fluid are the dermal
nchice or papulae, the Stewart's organs and the respiratory
The first of these, which are confined to the Asteroidea
d Echinoidea, have been described in the accounts of the
Starfish and Sea-urchin. In most Asteroidea they occur only on
the dorsal surface, but in some forms they are present on the
ntral surface as well. In some of the Echinoids the place of
rmal branchiae in providing for the respiration of the compart-
ent of the coelome between the peristome and Aristotle's lantern
is taken by Stewart's organs, arborescent bodies which project
inwards from the peristome. The respiratory trees are referred
to below in connection with the enteric canal.

Some reference has already been made, in describing the general
TIB of the body, to the ambulacral system of vessels. A
ring-like circum-oral vessel (ring-vessel) in nearly all cases sends off
a series of radial branches, one passing along each of the rays or
ambulacral areas and giving off branches to the ampullse of the
tube-feet or to the tentacles. In most of the Holothuroidea
branches pass forwards to the circlet of shield-shaped or branched
oral tentacles, and in some cases there are vesicles or ampullce at
the bases. In the Apoda, in which tube-feet are wanting, radial
vessels are also absent, and the vessels to the tentacles come off
directly from the ring-vessel. In all the classes, except Crinoidea,
one or more bladder-like appendages — the Polian vesicles — are con-
nected with the ring-vessel. The racemose vesicles, or Tiedemanns
vesicles (p. 354), are characteristic of the Asteroidea. In all, .
except the Crinoidea and the majority of the Holothuroidea, there
is a communication between the ring-canal and the surrounding
water through the madreporic canal. In the Asteroidea, and in
Cidaris among the Echinoidea, the wall of this tube is strengthened
by numerous calcareous ossicles. In the Asteroidea, Ophiuroidea,
and Echinoidea, the communication with the exterior is through
the madreporite ; in the few Holothuroids in which such a com-
munication exists (ISlasipoda) there is usually a simple opening,
but sometimes a number of pores crowded together. In the
remainder of the Holothuroidea the distal end of the madre-
poric canal, or canals, lies free in the interior of the body-cavity,
with which it is placed in communication by a number of per-
forations. In the Crinoidea there is no madreporic canal ; but
the ring-vessel is placed in communication with the coelome by
means of a system of ciliated water-tubes, while the coelome com-
municates with the exterior through a number of minute water-
pores, which perforate the ventral body- wall. The fluid contained in
the ambulacral system is similar to that in the coelome and contains
similar corpuscles. In one Ophiuroid, however, the ambulacral
system contains corpuscles coloured red with haemoglobin.

394 ZOOLOGY SECT.

The system of vessels and sinuses to which the designation
blood-vascular system is applied are specialised extensions of
the ccelome, from the main cavity of which they are not completely
separated off. Their walls are for the most part lined by a ciliated
epithelium by means of which the movement of the contained
fluid which does not differ from that in the ccelome, is brought
y about. There is never any contractile part acting as a heart. The
general disposition of the parts of this system in the various classes
has already been referred to in the descriptions of the examples.
The arrangement in the Ophiuroidea resembles that described in
the Starfish. In the Holothuroidea and Crinoidea the axial sinus
and aboral ring-vessel, present in the other three classes, are
absent, and there are large intestinal vessels accompanying the
enteric canal.

The enteric canal varies in the five classes more than any
of the other systems of organs. It is a simple tube in the Holo-
thurians and Echinoids, passing spirally through the body from
the mouth at the oral to the anus at the opposite pole. In most
of the latter group a complex masticatory apparatus with five
teeth — the so-called " lantern of Aristotle " — is situated at its
anterior extremity ; the corresponding region in the Holothurians
is surrounded by a circlet of ossicles, which protect the nervous
and vascular rings, and into which the longitudinal muscles of the
body- wall are inserted.

In the Echinoidea there is a tubular c?3ecum, the siphon, con-
nected with the intestine. In the Holothurians the so-called
" respiratory trees " (absent in the Elasipoda and the Apoda) are
branched appendages — usually two in number, sometimes single
— of the cloaca or posterior wider portion of the intestine, and
the " Cuvierian organs " are simple filiform glandular tubes, also
connected with the cloaca.

The functions of the siphon and of the respiratory trees have
already been referred to in the accounts of Echinus and Cucu-
maria. The Cuvierian organs, which occur only in a limited
number of Holothurians, correspond to undivided basal branches
of the respiratory trees : they are defensive organs, the animal
when attacked throwing out numbers of these long filaments,
which are very viscid and have the effect of entangling and
hampering the assailant.

In the Crinoidea the alimentary canal is simply a coiled tube
with both mouth and anal opening on the same (ventral) surface
of the body. In the Ophiuroids the central mouth leads into a
simple sac, giving off short diverticula, and there is no anal
aperture. In the Asteroidea the alimentary canal is more complex
than m the other classes. The stomach is divided, as already
described in the account of tlie examples, into two portions, the
cardiac and the pyloric, the former giving off five large rounded

,a,li

PHYLUM ECHINODERMATA 395

ial diverticula, the cardiac pouches or cardiac caeca, and the latter
giving off five pairs of very long branched diverticula, the pyloric
or hepatic ca3ca. The intestine is short and conical, and opens, in
all but a few, by an anal aperture. In some Asteroidea (as in
Anthenea, Figs. 283 and 284) the intestine has connected with it
a system of five elongated bifurcated inter- radial intestinal casca ;
in others (as in Asterias, Fig. 281) these are represented only by
two or three lobed diverticula. In one member of the class there
are also ten caeca connected with the oesophagus.

In the nervous system of the Echinodermata three distinct
parts, the relative development of which differs in the different
classes, are to be recognised. These are the epidermal or super-
ficial, the deep, and the ccelomic. The epidermal system is wellf£)
developed in all the classes : its principal parts are a circum-oral/
nerve -ring and radial branches, but a plexus of nerve-fibres with7
occasional nerve-cells extends from it through the epidermis. In
the Ophiuroids the radial nerves and the ring nerve are similar
in their arrangement to what is to be observed in the Asteroids,
but are more deeply placed, being covered over by the investing
calcareous plates. The deep-lying nervous system is absent in the
Crinoidea, very feebly developed in the Echinoidea, but well
developed in the Asteroidea, Ophiuroidea, and Holothuroidea.
Its general arrangement has already been described in the account
of the Starfish. The coelomic system is best developed in the
Crinoidea and is absent altogether in the Holothurians.

The sexes are distinct in all the Echinoderms, with one or two
exceptions ; but there is very rarely any trace of sexual dimorphism.
Asterina gibbosa, the Starfish the development of which has been
described (p. 358), is one of the exceptional hermaphrodite forms ;
the young animals of this species are male, producing sperms, but
at a later stage they become female and produce only ova. In the
family Synaptidae of the Apoda there are also numerous examples
of hermaphroditism, the animal at first producing ova, later only
•sperms. In Amphiura squamata, an Ophiuroid, both ovaries and
testes are present at once. The gonads, ovaries or testes as the
•case may be, are branching bodies inter-radial in position, and
usually in pairs. In the Asteroidea there are five pairs, the ducts
from which open usually on a special plate on the dorsal surface,
but in one or two species open on the ventral surface. In the S "
Echinoidea there are five ovaries or testes, the five ducts of
which open on the genital plates of the apical system. In the
Ophiuroidea there are five pairs of gemtai glands, a pair in the
wall of each of five genital bursce, which open on the exterior by
slits on the ventral surface close to the mouth. In the Holo-
thuroidea there is only a single branched gland, sometimes imper-
fectly divided into two, with a duct opening on the dorsal surface
not far from the mouth. In the Crinoidea the ovaries and testes

396 ZOOLOGY SECT.

occupy a remarkable position, being situated in the dilated bases
of the pinnules ; but as in the other classes they are connected
by means of a genital rachis running through the arm with a
centrally situated genital stolon.

Development and Metamorphosis. — A few of the members
of each class of Echinoderms are viviparous, in the sense that the
development of the young takes place in some sheltering cavity,
or 'brood-pouch, on the surface of the body of the parent. But in
most development takes place externally, and the larva? are free-
swimming. The ovum in all undergoes regular and nearly equal
segmentation, resulting in the formation of a ciliated blastula,
which becomes invaginated so as to form a typical gastrula, like
that of some Coelenterata (p. 161). The invaginated cells form the
lining membrane (the endoderm layer) of an internal cavity — the
primitive alimentary cavity or archenteron; the enclosing cells
form the ectoderm; between the endoderm and ectoderm, and
derived from the former, appear the cells of the mesoderm or middle
layer. From the archenteron is given off a single or double
hollow out-growth, the vaso-peritoncal vesicle or vesicles, from which
are derived the body-cavity with its enclosing peritoneal mem-
brane, and the vessels of the ambulacral system with their various
appendages. In the Crinoidea the vesicle destined to form the
ambulacral system is developed independently of the peritoneal
vesicles destined to form the body-cavity. A canal opening on
the exterior by a dorsally situated opening, the dorsal pore (some-
times double), is formed by invagination from the surface ectoderm,
and comes into relation with a canal formed by out-growth from
the rudimentary ambulacral system to form the foundation of the
madreporic canal of the adult. In the Crinoidea there are formed
five dorsal pores and five canals, but the two sets of structures do
not enter into direct communication (see p. 375).

The part of the vaso-peritoneal vesicle (Jiydroccele) destined to
give rise to the ambulacral system, at first rounded, becomes com-
pressed, and subsequently divided round the border into five lobes.
Each of these lobes grows outwards to become developed subse-
quently into one of the five radial ambulacral vessels of the Echino-
derm ; the central part of the hydroccele gives rise to the ring-
vessel surrounding the oesophagus.

The cilia, which at first (in the gastrula stage) covered the sur-
face of the larva uniformly, become restricted to two definite bands,
one in front of the mouth or prse-oral, the other in front of the anus
or prse-anal. These bands undergo characteristic changes in the
different classes, and the form of the larva at the same time
undergoes modification by the formation, except in the Crinoidea,
of variously arranged processes along the course of the ciliated bands.
The resulting larva, or Echinopcedium, exhibits always marked
bilateral symmetry.

PHYLUM ECHINODERMATA

397

In the Asteroidea the larva is either a Bipinnaria (Fig. 315,
4 to 6) or a BracTiiolaria, The former has a series of bilaterally

Fin. 315.— Diagrams of the development of the larvae of Echinoderms : 1, Primitive form of
fichinoderm larva; 2 and 3, Development of an Ani-i<-nla.,-ia (Holothuroidea) ; 4, 5, and 0,
Development of a Bipinnaria (Asteroidea) ; 7, 8, and £>, Development of a Pluteus (Echinoidea
and Ophiuroidea). (Prom Lenckart and Nitsche's Diagrams.)

arranged processes or arms ; the latter has, in addition, three
processes not developed in the course of the ciliated ridges. The
Bipinnaria is usually free-swimming, but sometimes, as in the

398

ZOOLOGY

SECT.

case of Asterina (p. 361), creeps on the surface of a rock by means
of the larval organ. In at least one form the Bipinnaria, devel-
oped in a brood-pouch, adheres to the parent by means of the
larval organ which takes the form of a short stalk. In both the
Ophiuroidea and the Echinoidea (Fig. 315, 7 to 9) the larva has
the form which is known as the Pluteus. The Pluteus has a series
of slender arms directed forwards and supported by a skeleton
of delicate calcareous rods. A remarkable feature of the Pluteus
in one case at least (Ecliinocyamus pusiUus) is that the ciliated
bands consist of rows of flagellate collared cells, similar to those
of the endoderm of Sponges. The larva of the Holothuroidea, the
Auricularia (2 and 3), has a number of short processes developed
in the course of the ciliated bands; subsequently, in the pupa

FIG. 316.— Development of Antedpn. A, larva with ciliated band, posterior tuft of cilia, and
mouth (to the right) ; B, larva with the developing plates of the peiitacrinoid stage ; C, penta-
crinoid stage.

stage, the ciliated bands become broken up into a series of ciliated
hoops encircling the body. Of the Crinoidea the development
of Antedon alone is known, and has been already described (p. 377).
The larva (Fig. 316) is barrel-like, with four transverse bands
of cilia and a bunch of stronger cilia at the posterior end. The
posterior end of the larva becomes drawn out to form a narrow
process or stalk, by means of which it becomes attached to some
foreign object. The attached condition of Antedon, termed the
pentacrinoid stage, is only temporary; as the. development ap-
proaches completion, the stalk is absorbed, and the Feather-Star
becomes free.

In the transition from the bilateral larva — Pluteus, Bipinnaria,
Brachiolaria, or Auricularia — to the radial adult there is a marked
metamorphosis. As the adult form becomes developed on one side
of the larva, the larval arms or processes become absorbed. In the

PHYLUM ECHINODERMATA 399

[olothuroidea and Ophiuroidea all the organs of the larva are
carried on into the adult ; in the Asteroidea and Echinoidea the
larval mouth and oesophagus are abolished, and a new permanent
mouth and oesophagus formed as a fresh invagination from the
surface. In the very limited number of Echinoderms that are
viviparous there is no such marked metamorphosis ; but even in
these the larva is at first distinctly bilateral in its symmetry.

Ethology, etc. — The Echinodermata are, without exception,
inhabitants of the sea. In the adult condition the majority
creep on the sea-shore or on the sea-bottom, the stalked Crinoids
being exceptional in their permanently attached condition ; but
the larvae of the great majority Q^Q pelagic — i.e. live swimming in
the upper strata of the ocean.

Echinoderms inhabit all depths of the sea, ranging from the
shore between low- and high-water limits to the greatest depths.
Members of all the classes are found at all depths ; but the stalked
Crinoids, and the Elasipoda among the Holothuroidea are virtually
confined to the deepest waters of the ocean, only one genus of the
former and one species of the latter occurring in comparatively
shallow water. Echinoderms are found in the seas of all parts of
the globe. Like the majority of marine invertebrate groups, the
phylum is more abundantly represented, as regards the number of
genera and species, as well as of individuals, in the warmer regions ;
the Crinoidea, the Holothuroidea and the Echinoidea are all much
more abundant in tropical and warm temperate seas than in colder
latitudes.

Echinoderms are of gregarious habits, large numbers of the
same species frequently being found closely associated together in
a comparatively narrow area. The movement of locomotion in
the Starfishes is, as previously described (p. 348), a slow creeping
one through the agency of the tube-feet : the same holds good of
the Echinoidea and those of the Holothuroidea (Pedata) that possess
tube-feet. The footless Holothurians (Apoda, such as Syriapta)
creep along with the help of the tentacles. Most of the Ophiuroids
move by lateral flexions, sometimes sluggish, sometimes remark-
ably rapid, of the arms. The Comatula3, on the other hand, swim
along by the dorso-ventral flexion and extension of the pinnate
arms propelling them through the water. Many Asteroids, Ophiu-
roids, and Echinoids bury themselves in sand or mud ; others creep
into naiTOw fissures in rock or coral. Movements of manducation
are performed by the tentacles in the Holothurians : in the Star-
fishes the mouth papillae are separated from one another and the
cardiac part of the stomach everted in order to enfold the prey,
often of relatively large size. In those Echinoidea that possess a
lantern of Aristotle there are very powerful and efficient move-
ments of mastication. On the whole, as might be expected from
the comparatively highly developed muscular and nervous systems.

400 ZOOLOGY SECT.

the co-ordination of movement is very much more complete in the
Echinodermata than in the groups already dealt with.

A remarkable characteristic of the Echinoderms is the faculty
of self-mutilation which many of them possess, together with
the capacity for replacing parts lost in this way or by acci-
dental injury. This is most marked in many Ophiuroids, some
Asteroids, and some Holothurians, and does not occur at all among
the Echinoids. Many Brittle-stars and some Starfishes, when
removed from the water, or when molested in any way, break off
portions of their arms piece by piece until, it may be, the whole
of them are thrown off to the very bases, leaving the central disc
entirely bereft of arms. A central disc thus partly or completely
deprived of its arms is capable in many cases of developing a new
set ; and a separated arm is capable, in some instances, of develop-
ing a new disc and a completed series of arms. In some Star-
fishes (Ophiuroids and Asteroids) a process of separation of the
arms and their development into complete individuals frequently
occurs altogether independently of injury, and seems to be a
regular mode of reproduction in these exceptional cases. Many
Crinoids, also, readily part with their arms when touched, and are
capable of renewing them again ; and some, at least, are capable
of renewing the visceral sac of the central disc when it has become
accidentally removed.

In the case of many Holothurians it is the internal organs, or
rather portions of them, that are capable of being thrown off and
replaced — the oesophagus, or the cloaca with the Cuvierian organs,
or the entire alimentary canal, being ejected from the body by
strong contractions of the muscular fibres of the body-wall, and in
some instances, at least, afterwards becoming completely renewed.

Two out of the seven classes of the plylum Echinodermata —
the Cystoidea and Blastoidea — are represented only by fossil forms ;
and these are found only in rocks of the older (Palaeozoic) forma-
tions, no representatives having survived to more recent times.
Of the five classes that have living members, one, the Crinoidea,
was very much more abundantly represented in the older geo-
logical periods than it is at the present day, the remains of
stalked Crinoids forming great beds of limestone of Silurian to
Carboniferous age : the free ComatulaB only appeared at a much
later period. The other classes, or at least the Echinoidea,
Asteroidea, and Ophiuroidea, were represented at a very early
period by forms not very widely different from those now living :
but the earliest Echinoids were peculiar in having the number of
rows of plates variable, and in the plates overlapping one another.
The Holothuroidea, owing to their comparatively soft integument,
were less fitted to leave any remains in the form of fossils, and it
is not till we come to the Mesozoic Period that undoubted traces
of their existence are found.

ix PHYLUM ECHINODERMATA 401

Affinities. — The presence of radial symmetry was formerly
regarded as involving a near relationship with the Coelenterata,
which were grouped with the Echinoderms under the comprehen-
sive class designation of Eadiata (see section on the History of
Zoology). But leaving out of account the presence of a bilateral
symmetry underlying and partly concealed by the radial, we are led
by a study of the anatomy of the various systems of organs to the
conclusion that the Echinoderms are in no way closely or directly
related to the Coelenterates. One very great and very important
difference between the two phyla consists in the presence in the /,
Echinodermata of an extensive ccelome or body-cavity between .
the alimentary canal and the body- wall. In addition to this the .7, •
Echinodermata are characterised by the possession of highly
elaborated systems of organs — alimentary, vascular, and nervous — -V — • ^
such as occur in none of the Coelenterates, all of which exhibit^
extreme simplicity in their internal structure. A further point of
difference, not perhaps of so much importance, is the absence in «
the Echinoderms of any tendency to form colonies of zooids by '
asexual multiplication by means of buds : all Echinoderms are
simple, i.e. non-colonial, animals, and each of them is developed,
save in certain very exceptional cases, as a result of a sexual
process from an impregnated ovum. In spite, then, of the radial
symmetry, we are forced to the conclusion that the Echinodermata
are not more nearly related to the Coelenterata than to some of
the groups of Worms They are, in fact, a singularly isolated
group, and we look in vain among the known members, living and
fossil, of other phyla, for any really close allies. The intermediate
forms — whatever they may have been like — between the Echino-
derms and other groups have become extinct, and have left no
remains in the form of fossils, or such remains have not yet been
discovered. So difficult has it been found to connect the Echino-
derms with other animal types that it has even been proposed to
regard an Echinoderm as a radially arranged colony of zooids
connected together centrally, each ray being a zooid equivalent
to an entire simple worm-like animal. But the history of the
development is entirely at variance with such a view.

Whatever may have been the group of animals from which the
Echinodermata were developed, there is every probability that it
was a group with bilateral and not radial symmetry. The radial
symmetry is evidently, as has already been pointed out, of a
secondary character ; it is only assumed at a comparatively late
period of development, and even in the adult condition it does
not completely disguise a more primitive bilateral arrangement of
the parts. Accordingly, within the phylum itself, it is reasonable
to regard those classes as the more ancient which have the radial
symmetry less completely developed. Again, the free condition
which characterises all existing Echinoderms, with the exception

VOL. i D D

402

ZOOLOGY

SECT. IX

of a few Crinoids, is probably less primitive than the attached,
since in other phyla the radial symmetry is co-ordinated with,
and seems to be developed on account of, a fixed, usually stalked
condition. Probably then stalked Echinoderms were the pro-
genitors of the existing free forms. The fossil Cystoidea, some of
which were stalked, and in which the radial symmetry is not
always strongly marked, are to be looked upon as the most primi-
tive of all the classes of Echinodermata, and a probable genealogy
of the whole phylum is indicated in the diagram below.

Cysfoidea

PrimiNve Echinoderms

FIG. 317. — Diagram to illustrate the relationships of the classes of Echinodermata.

According to another view, however, the most primitive of
existing Echinoderms are Synapta and its allies (Holothuroidea
apoda). The other Holothuroids are supposed, according to this
conception of the relationships of the various classes, to have been
derived from a Synapta-like ancestor. From the primitive stock
of the Holothuroids is supposed to have been derived a form
which gave origin to all the stalked classes. From this ancestral
stalked Echinoderm, again, the remainder of the free classes — the
Echinoidea, Asteroid ea and Ophiuroidea — are regarded as having
been descended.

SECTION X
PHYLUM ANNULATA

THE phylum Annulata comprises four classes of Worms — the
Chct'topoda or Earthworms and marine Annelids, the ArcJn -annelid a,
the Gephyrea, and the Hirudinca or Leeches. All of these, except
the Gephyrea, have the elongated body divided externally into a
number of rings, which represent a division of the internal parts
into a series of segments or metameres. There^ is usually an
extensive ccelome, and there is in most a system JJ^p^ood-vessels.
The nervous system consists of a cerebral ganJIB cesophageal
connectives, and a double ventral nerve-cord, wmcrSi all but the
Gephyrea is segmented into a series of ganglia. The organs of
excretion are in the form of metamerically arranged pairs of tubes,
the nephridia or segmented organs, leading from the ccelome to
the exterior, and all these, or certain specially modified pairs of
them, may have the function of permitting of the passage outwards
of the reproductive elements.

CLASS I.— CILffiTOPODA.

The Chaetopoda, comprising the Earthworms, Fresh-water
Worms, and Marine Annelids, are Worms the body of which, un-
like that of a Flat-worm or a Round-worm, is made up of a series
of more or less completely similar segments or metameres^ each
containing a chamber or compartment of the body-cavity and a
section of the alimentary canal and other organs. Af the sides
of each are a pair of muscular processes, the parapodia, which do
uty as limbs, bearing bundles of setae or bristles, and bearing also,

ually, certain tactile appendages, the cirri. There is an extensive

lome, incompletely divided into a series of chambers correspond-
ing to the segments, by a series of muscular partitions, which act
also as mesenteries, being attached internally to the alimentary
canal. The latter extends throughout the length of the body ;

e intestine is usually constricted between the segments. There

D D 2

404

ZOOLOGY

SECT.

is a well-developed blood -vascular system in the majority of the
Chastopoda; and organs of respiration in the form of gills or
branchiae are usually developed. The excretory organs are in the
form of segmentally arranged pairs of tubes, the nephridia. The
nervous system consists of a bilateral principal ganglion or brain,
situated in the prostomium, and a double chain of ganglia extend-
ing throughout the body. The sexes are in some distinct, in others
united. When a definite larval form occurs it is a Trochospherc.

1. EXAMPLES OF THE CLASS.
a. Nereis dumerilii.1

General External Features. — Various species of Nereis occur
abundantly between tide-marks on the sea-shore, under stones, and

among sea-weed in all parts of the
world. The worm varies consider-
ably in colour even in the same
species, the differences being partly
due to differences in the stage of
development of the sexual ele-
ments. In N. dumerilii the pre-
vailing colour is some shade of
violet, with a blush of red in the
more vascular parts due to the
bright red colour of the blood. Ii
shape (Fig. 318) the body, whi<
may be about 7 or 8 centimeti
in length, is long and narrow
approximately cylindrical, som<
what narrower towards the pos
terior end. A very distinct keac
bearing eyes and tentacles, is
cognisable at the anterior en(
the rest is divided by a series
ring-like narrow grooves into
corresponding series of segments
metameres, which are about eight]
in number altogether; and eacl
of these bears laterally a pair
movable muscular processes call(
the parapodia, provided wit!
bundles of bristles or setce. The
head (Fig. 321) consists of
parts, the prostomium (prcest.) and the peristomium (perist.]

1 Though Nereis dumerilii is here named as the example, and the majority
the figures refer specially to that species, the description given would
almost equally well to a considerable number of species of the genus.

FIG. 318.— Nereis dumerilii. Natural
size. A, Nereis phase ; B, Heteronereis
phase. (After Claparede.)

PHYLUM ANNUL AT A

405

^^ neuro

vent.clrr

319.— Nereis dumerilii. A single para-
podium magnified : ac. aciculum ; dors. cirr.
dorsal cirrus ; neuro. neuropodium ; noto. noto-
podium ; vent. cirr. ventral cirrus. (After
Claparede.)

former bears on its dorsal surface four large rounded eyes

in front a pair of short cylindrical tentacles (tent.), and further back

a pair of somewhat longer
stout appendages or palpi
(palp). The peristomium,
which bears some resem-
blance to the segments of the
v _. 7ZC^. body, though wanting the
parapodia, bears laterally
four pairs of long slender
cylindrical tentacles (perist.
tent.) : on its ventral aspect
is a transversely elongated
aperture, the aperture of the
mouth. The segments of the
body differ little in external

characters from one another throughout the length of the worm.

Each bears laterally a pair of parapodia, which in the living animal

are usually in active movement, aiding in

creeping, or acting as a series of oars for

propelling it through the water. When

one of the parapodia (Fig. 319) is examined

more attentively it is found to be biramous,

or to consist of two distinct divisions — a

dorsal, which is termed the notopodium

(noto), and a ventral, which is termed the

neuropodium (neuro). Each of these is

further subdivided into several lobes, and

each bears a bundle of setae. Each of the

bundles of setae is lodged in a sac formed

by invagination of the epidermis, the .s&tigcr-

ous sa-c, and is capable of being protruded

or retracted and turned in various directions

by bundles of muscular fibres in the interior

of the parapodium. In each bundle there

is, in addition to the ordinary setae, a stouter,

straight, dark-coloured seta (ac.), the pointed

apex of which projects only a short distance

on the surface ; this is termed the aciculum.

The ordinary setae (Fig. 320) are exceedingly

fine, but stiffish, chitinous rods, of which

two principal kinds are recognisable ; both

have a terminal Nade articulating with the

main shaft of the seta by a distinct joint;

but in the one variety the shaft of the seta

is finer than in the other, and the terminal blade long, slender.

and nearly straight, whereas in the other variety it is short arid

FIG. 320.— Nereis dumer-
ilii. Meta: highly magni-
fied. (After Claparede.)

400 ZOOLOGY SECT.

slightly hooked. On the dorsal side of the parapodium is a short
cylindrical, tentacle-like appendage, the dorsal cirrus (Fig. 319,
dors. cirr.\ and a similar, somewhat shorter, appendage, the ventral
cirrus (vent, cirr.), is situated on its ventral side. The last seg-
ment of the body, the anal segment, bears posteriorly a small
rounded aperture, the anus ; this segment is devoid of parapodia, *
but bears a pair of appendages, the anal cirri, similar in character
to the cirri of the ordinary segments, but considerably longer.

On the ventral surface, near the bases of the parapodia, there is
in each segment a pair of very fine apertures, the openings of the
nephridia.

. The enteric canal is a straight tube running throughout the
length of the body from the mouth to the anus. Between the
outer surface of this tube and the inner surface of the wall of the
body is a considerable space — the coelome, body cavity, or peri-
visceral cavity — filled with a fluid, the ccelomic fluid. The walls
of the coelome (Fig. 322) are lined with a thin membrane, the
ccelomic epithelium or peritoneum, of which the outer layer — that
lining the body- wall — is the parietal layer (par. pefi.\ that cover-
ing the outer surface of the alimentary canal the splanchnic or
visceral layer (vise. peri?). The space is divided by a series of trans-
verse partitions or septa passing inwards from the body-wall to the
wall of the alimentary canal opposite the grooves between' the
segments, and thus dividing the coelome into a series of chambers,
each of which corresponds to one of the segments. These parti-
tions are not complete, spaces being left around the alimentary
canal and elsewhere, through which neighbouring chambers
communicate.

The mouth leads into a wide cavity, the l)uccal cavity, con-
tinued back into a pharynx (Fig. 321, ph.) These two chambers
extend through the peristomium and the first to the fourth seg-
ments of the body. They are lined with a tolerably thick cuticle,
continuous with a similar layer lining the outer surface of the body,
and in the pharynx are a number of very small dark brown chitinous
denticles, which are very regularly arranged. The posterior part
of the pharynx (dentary region) has very thick walls composed of
bundles of muscular fibres, which are concerned in the movements
of a pair of laterally placed chitinous jaws. Each jaw is elongated
in the direction of the long axis of the body, rounded' at the posterior
end or base, where it is embedded in muscle, pointed at the apex,
which is strongly incurved ; the inner edge is divided into a
number of strong serrations or teeth : the whole jaw might be
compared to a pruning-hook with its cutting edge deeply serrated.

Behind the pharynx the alimentary canal narrows considerably
to form a tube, the oesophagus (ces.), which runs through about five
segments to open into the intestine.

Running backwards and inwards from the wall of the peristomium

PHYLUM ANNULATA

407

the wall of the buccal cavity and pharynx are a number of
bands or sheets of muscle,
the protractor muscles, by
the contraction of which
this anterior part of the
alimentary canal is capable
of being everted as a
proboscis until the jaws are
thrust forth and thus ren-
dered capable of being
brought to bear on some
small living animal, or
fragment of animal matter,
to be seized and swallowed
as food. The proboscis is
withdrawn again by a
retractor sheet of muscle,
which passes inwards and
forwards to be inserted
into the wall of the ali-
mentary canal at the junc-
tion of the pharynx and
phagus.

Into the oesophagus open

pair of large unbranched
glandular pouches, or cceca
(gL), which probably are
of the nature of digestive
glands. The intestine (int.)
is a straight tube of nearly
uniform character through-
out, regularly constricted
between the segments —
the constrictions becoming
much deeper towards the
posterior end of the body.
The part of the intestine
which lies in the last
segment is termed the
rectum.

• The wall of the ali-
mentary canal (Fig. 322)
consists (1) of the visceral
layer of the peritoneum
(vise, peri.) ; (2) of a layer of
longitudinal muscular fibres' (long, mus.) ; (3) of a layer of circular
muscular fibres (circ. mus.}: (4) of the enteric epithelium (ent. ep.),

:i

ve/tf.

ne. cc~

FIG. 321.— Nereis dumerilii. Semi-diagrammatic
view of the anterior portion of the body, with the
dorsal body-wall removed, so as to show the ali-
mentary canal, the septa, the blood-vessels and the
liephridia ; a portion of the intestine removed so as
to show the ventral blood-vessel and nerve-cord
which lie below : dors. vcs. dorsal vessel ; gl. ceso-
phageal glands ; int. beginning of intestine ; ne. co.
nerve cord ; neph. nephridia ; o>s. oesophagus ; palp.
palp ; para.-parapodia ; perist. peristome ; perist. tent.
peristomial tentacles ; ph. pharynx with its jaws ;
praist. prostomium ; rent. res. ventral vessel.

408

ZOOLOGY

SECT.

consisting of close-set, long, narrow cells. To these layers is super-
added in the buccal cavity and the pharynx an internal chitinous
cuticle, continuous with that of the general outer surface.

Developrn en tally the buccal cavity and the pharynx constitute
the stomodceum, the rectum the proctodceum, the rest of the alimen-
tary canal the mesenteron.

The wall of the body consists of a cuticle, an epidermis or
deric epithelium, muscular layers, and the parietal layer of the
peritoneum referred to above. The cuticle (cut.) is a thin chitinous

dors to™/

i*en£. long. THUS

vent.vesg ne.co

FIG. 322.— Nereis dumerilii. Semi-diagrammatic transverse section of the: middle region of
the body : circ. mus. (external), circular layer of muscle of body-wall ; circ. mus. (internal),
circular layer of muscle of wall of enteric canal ; ccel. coelom ; cut. cuticle ; dorm. loiiii. //< as. dorsal
longitudinal muscles of body -wall ; dors. ves. dorsal vessel ; ent. ep. enteric epithelium ; ep. epider-
mis ; long. mus. longitudinal muscle of wall of enteric canal ; ne. co. nerve cord ; neph. nephri-
dium ; neur. set. neuropodial setae and aciculum with their muscles ; not. set. notopodial setae
and aciculum ; obi. mus. oblique muscle ; ov. ovary ; par. peri, parietal layer of ccelomic epithe-
lium ; vent. long. mus. ventral longitudinal muscle ; vent. ves. ventral vessel ; vise. peri, visceral
layer of peritoneal epithelium.

layer which exhibits an iridescent lustre due to the presence of
two intersecting systems of fine lines ; it is perforated by numerous
minute openings, the openings of the epidermal glands. The
epidermis (ep.) is very thin, except on the ventral surface, where
it becomes considerably thickened. It consists of a layer of cells
containing numerous twisted unicellular glands, which are most
abundant on the ventral surface, particularly near the bases of the
parapodia ; on the dorsal surface the epidermis contains plexuses
of fine blood-vessels. The muscular layers are two in number —
an external in which the fibres run circularly (circ. mus.), and an

aii

£

CO!

PHYLUM ANNULATA 409

ternal in which they run longitudinally. The latter is not a
tinuous layer, but consists of four bundles of fibres, two clorso-
eral (dors. long, mus.) and two ventro-lateral (vent. long. mus.).
Nereis has a well-developed system of vessels filled with blood
of a bright red colour. A main dorsal vessel (Figs. 321 and 322,
dors, ves.) runs from one end of the body to the other above the
alimentary canal, and is visible in places through the body-wall in

:e living animal. It, as well as the majority of the vessels, un-
rgoes contractions which are of a peristaltic character — waves of
ntraction passing along the wall of the vessel so as to cause the
movement of the contained blood. These peristaltic contractions
are more powerful in the case of the dorsal vessel than in that of
any of the others, and run with great regularity from behind for-
wards, so as to drive a current of blood in that direction. The
contractions are brought about partly by a series of muscular fibres
which are arranged in rings round the wall of the vessel at short
intervals ; but the wall of the vessel is itself contractile.

Along the middle of the ventral surface below the alimentary
1 runs another large longitudinal vessel, the ventral vessel (vent.
), in which the current of blood takes a direction from before
kwards. Connecting the dorsal and ventral vessels, there are in
h segment two pairs of loop-like transverse vessels which give
branches to the parapodia, the alimentary canal, and neighbour-
parts. Some of these branches communicate with plexuses
of fine vessels in the interior of the lobes of the parapodia and in
the integument of the dorsal surface, and with dilatations or sinuses
situated in the bases of the parapodia. A delicate longitudinal
neural vessel accompanies the nerve-cord.

Nereis is devoid of any bronchia? ; but there can be little doubt
that the lobes of the feet with their rich blood-supply, and the
areas of integument occupied by plexuses of blood-vessels, subserve
the function of respiration.

There is a well-developed nervous system (Fig. 323) which is
bilateral and metameric in its arrangement, like the other systems
of organs. Situated in the prostomium is a large bilobed mass
of nerve-matter containing numerous nerve-cells, the cerebral
ganglion or brain (c). This gives off tentacular nerves to the tentacles
and palpi, and two pairs of short thick optic nerves to the eyes.
Behind, two thick nerve-strands, the ossophageal connectives (d), curve
round the mouth in the peristomium to meet on the ventral
aspect behind the mouth and below the pharynx. The cesopha-
geal connectives, with the cerebral ganglion, thus form a ring
around the anterior part of the enteric canal. Running back-
wards from the point of union of the oesophageal connectives,
along the entire length of the body of the worm, on the ventral
aspect, is a thick cord of nerve-matter, the ventral nerve-cord (h)
In each segment this cord presents a little dilatation from which

410

ZOOLOGY

SECT.

nerves are given off to the various parts of the segment : and
each of these enlargements is really double, consisting of a pair
of closely-united ganglia. The intermediate parts of the cord,
between successive pairs of ganglia, are also double, consisting
of a pair of longitudinal connectives enclosed in a common sheath.
Given off behind from the cerebral ganglion is a system of fine

Fi:;. 323. — Nereis. Anterior portion of nervous system, comprising the brain, the oesophs
connectives, and the anterior part of the ventral nerve-cord. (After Quatrefages.)

nerves with occasional small ganglia, the stomato gastric or visceral
system, distributed to the anterior part of the alimentary canal.

The tentacles and palpi, as well as the cirri, are probably organs
of the sense of touch. The only other sense-organs are the four
eyes, situated on the prostomium. The eye (Fig. 324) consists
of a darkly pigmented cup, the retina (re.), with a small rounded
aperture, the pupil, and enclosing a mass of gelatinous matter,
the lens (L). The wall of the cup is composed of numerous long
and narrow cells lying parallel with one another in a radial

PHYLUM ANNULATA

411

direction. The outer end of each cell narrows into a nerve-fibre
forming part of the optic nerve ; near this end is a nucleus ; the
main body of the cell is densely pigment ed ; the inner part projects
towards the lens as a clear hyaline rod (r.). The cuticle of the
general surface passes over the eye, and a continuation of the
epidermis, with its cells somewhat flattened, constitutes the
cornea (co.).

The organs which are supposed to perform the function of
excretion are a series of metamerically arranged pairs of tubes,
the segmented organs or nephridia (Figs. 321 and 322, neph., Fig. 325
occurring in all the segments of the body. The nephridium
consists of two parts — a body and a narrow anterior prolongation.

FIG. 324. — Nereis. Section through one of the eyes. co. cornea; cu. cuticle ; /. lens ; r. layer
of rods ; re. retina. (After Andrews.)

The body is of an irregular oval shape directed nearly transversely,
but slanting somewhat. The outer end, situated in the base of
the parapodium near its middle, is much the narrower. The inner
end is continuous with a narrow prolongation about equal in
length to the body, which runs forwards and inwards to become
attached to the mesentery.. The external opening or nephri-
diopore (cxt. op.) is a fine circular pore capable of being widened or
contracted, situated on the ventral surface not far from the base
of the ventral cirrus. , This leads into a canal which runs through
the anterior prolongation to its extremity, where it bends sharply
back again and runs to the body, through which it pursues an
extremely tortuous course to the outer end, and then bends back
again and runs in the anterior prolongation to the extremity of
the latter, where it opens into the coelome through a ciliated bell or
funnel (fun.), the nephrostome, projecting through the mesentery

412

ZOOLOGY

SECT.

into the cavity of the segment next in front of that in which the
body of the organ lies. Throughout its course the canal is exca-
vated in a mass of nucleated material of a granular character not
distinguishable into cells.

Nereis is unisexual. The sexual elements, ova or sperms,
are formed from temporary masses of cells, ovaries or testes, which
are developed towards the breeding season by a proliferation of
the cells of the membrane (peritoneum) lining the coelome and the

structures it contains. In Nereis
dumerilii there is in the male
only a single pair of these
proliferating masses of cells
(testes), situated in one of the
segments between the nine-
teenth and the twenty-fifth.
These, during the season of their
active development, give off
groups of cells which become
disseminated throughout the
ccelomic fluid. The original
cells (mother-cells) undergo
division into smaller cells, each
of which develops into a sperm
with a minute rod-shaped head
and a long vibratile flagellum
or tail. In the female the
ovaries (Fig. 322, ov.), formed by
a similar process of prolifera-
tion, take the form of rounded
masses of cells, metamerically
arranged, surrounding the prin-
cipal vessels throughout the
length of the body. The young
ova become detached from the
ovaries, and attain their full de-
velopment while floating about
in the ccelomic fluid. Both

ovaries and testes dwindle after they have given off the sexual
cells, and at the non-breeding season of the year are not to be
detected.

Ova and sperms, when fully ripe, are discharged, reaching the
exterior, in the case of the sperms probably through the nephridia.
in the case of the ova, which are much too large to pass out in
this way, probably through apertures temporarily formed by
rupture of the body-wall ; and impregnation takes place by contact
between the two sets of elements while floating freely in the
sea- water.

FIG. 32u. — Nereis dumerilii. One of the
nephridia. ext. op. external opening or
nephridiopore ; fun. internal funnel or
nephrostome opening into the coelome ;
mes. mesentery or septum.

PHYLUM ANNULATA 413

ereis dumerilii is an extremely variable species. If we
compare a number of specimens, we find numerous individual
differences between them. The most striking of these are
differences of colour and of the number of segments in the body ;
but a careful examination reveals many other points in which
individuals differ. Thus the precise form of the lobes of the^
parapodia varies, together with the number of setae in the two
bundles ; so also do the relative length of the tentacles, the
number of teeth on the jaws, and the number and arrangement
of the denticles in the pharynx. Not only are such individual
differences common, but the species occurs in two distinct forms
or phases, which differ from one another so widely that they have
been referred to distinct genera. One of these is the Nereis phase,.
which is that described in the preceding paragraphs. A Nereis
dumerilii may become sexually mature in this form, or may first
undergo a series of changes by which it becomes converted into
the second or Heteronereis phase (Fig. 318, B). The principal
changes which take place during this metamorphosis are a great
increase in the size of the eyes, and a great modification of the
parapodia in the posterior portion of the body, the lobes becoming
larger and more leaf-like, and the setae of the Nereis becoming
superseded by others which are considerably longer, more nume-
rous, and somewhat oar-shaped. The Heteronereis, instead of
creeping about on the bottom, swims about actively through the
water by wriggling movements of the body combined with active
paddling movements of the parapodia with their long setae. After
a time the Heteronereis, like the Nereis, becomes sexually mature,
developing ova and sperms, the latter of which differ remarkably
in shape from those of the Nereis phase.

Development. — The egg of Nereis when first discharged is
enclosed in a transparent thick gelatinous envelope, within which
are two membranes — an outer very thin and delicate, and an inner
(zona radiata) thicker and very distinctly striated in a radial
direction. The protoplasm of the ovum contains a number of
oil-drops and yolk-spherules. When fertilisation takes place
the yolk-spherules move away from what is destined to become
the upper pole of the egg, leaving a polar area composed of
granular protoplasm. The zona radiata disappears, and the
contents of the ovum undergo for a time amoeboid changes of
form. Then the spherical form is reassumed, two small bodies —
the polar globules — are thrown off at the upper pole, and the process
of segmentation (Fig. 326) begins. Up to a fairly advanced
stage this corresponds very closely with the segmentation of the
Polyclad oosperm as described on page 256. The oosperm divides
first into two parts, then into four. From these four cells — the
megameres — there are separated off in succession three sets of
micromeres, making twelve in all. One of these, belonging to the

414

ZOOLOGY

SECT. X

second set, somewhat larger than the others, and differing from
them in its subsequent history, is termed the first somatoblast
(som. 1); & second somatoblast (som. 2) is soon given off from
the same megamere that gave origin to 'the first.

The germinal layers are now all established. The micromeres
constitute the ectoderm, destined to give rise to the epidermis and
all its derivatives, to the cerebral ganglion and nerve cord, to the1
oesophagus and rectum. The megameres eventually give origin
to the cells of the endoderm, forming the internal epithelium of
the alimentary canal. The second somatoblast gives rise to
the entire mesoderm of the Annelid. The micromeres extend..

micro

.micro

macro

micro

som.l

micro

macro

som.2

som.l

Ft \. :5-2(i. — Nereis. Early stages in the development. A, lateral view of eight-celled stage ;
Ji, the same from above ; (.', stage of the formation of the first somatoblasr ; />. stage at which
both somatoblasts are present; -macro, megameres; micro, micromeres; torn. l,gom. 2, first
and second somatoblasts. (After Westinghausen.)

as they multiply by division, at first as a cap of small cells over
the upper pole of the embryo ; eventually the cap extends itself
so as completely to cover the four megameres and the descendants
of the somatoblasts except at one point, the blastoporc, at the
lower pole, where the investment remains for a time incomplete.
When the blastopore closes, the process of epib'6lic gastrulation is
completed. A thickening of the layer of ectoderm cells, the apical
p/(i.te, in the middle of what is destined to form the head-end of the
embryo, is the rudiment of the cerebral ganglion ; in close relation
to it are formed a pair of pigment spots, the larval eyes. From
the middle of the head-end projects a tuft of cilia (Fig. 327, A,

ap.pl

Jr. bod

sens h

pro

para'

neur.pl

setis.h

}. 327.— Nereis. t __,..._ L

apical tuft of cilia first become distinct; //, somewhat larJr stageV'in'which tl
in valuation is being formed, and the rudiments of the ni< soderm bands are distinct :
troch< in which there are rudiments of the ; ,eti t later

stage, in which the parapodia have begun to become prominent and 1. d setae

project freely; E, larva with three segments: an. ami, s ; «p. ,_•>'/. apical cilia; «p. p/. apical
f>/f, eye ; //-. liod. frontal bodies; int. intestine; /. j/r/'x. longitudinal muse!.

mo. mouth ; neur. ///. neural plate;
prof, prototroch ; .^>?.i. //. sensory hah
gro\ip of cells formed from it ; st. ston. fcer E H. Wils

Later stages in the development. A, sta go at which the pr«.tntruch and tl.c-

ich th

416 ZOOLOGY SECT.

ap. cil\ Encircling the body of the larva behind this is a thick-
ened ridge, the prototroch (prot.), the cells of which develop strong
cilia, Just behind the prototroch the cells of the ectoderm
become pushed inwards, in the middle of what will eventually
become the ventral surface, so as to line a sort of depression or
pouch ; this is the stomodceum (st.) or rudiment of the mouth and
oesophagus. The anus (an.) does not appear until later ; the position
which it will subsequently occupy is indicated at this stage by a
pigment ed area (pig. ar.) marking the point at which the blasto-
pore becomes closed. The first and second somatoblasts divide
to form a mass of small cells which extend on the ventral surface
behind the prototroch and mouth, constituting what is termed
the ventral plate; of this plate the more superficial cells are
descendants of the first somatoblast — one of the twelve original
micromeres ; and those situated more deeply are derived from
the second somatoblast or mesomere. A superficial thickening
along the middle of the ventral plate is the rudiment of the
ventral nerve-cord (neur. pi.) ; the deeper cells divide and extend
to form a pair of mesoderm bands or muscle plates, from which
the muscles of the body-wall are developed ; the muscular layers
of the wall of the alimentary canal are derived from certain of the
same set of cells which migrate inwards from the lower end.

A pair of micromeres separated from the rest at an early stage
are destined to form the larval excretory organs, the head kidneys :
at first situated at the upper end, they sink below the surface and
migrate downwards till they come to lie below the prototroch ;
each then elongates, and a number of vacuoles which have become
formed in the interior coalesce in such a way as to form a long
narrow canal. The embryo has now reached the completed Tro-
chosphere stage.

The endoderm cells become arranged so as to bound a canal-
like space, the beginning of the lumen of the middle part of the
alimentary canal (oesophagus and intestine) (int.), the cells subse-
quently giving rise to the enteric epithelium. This canal becomes
continuous in front with the stomodasum, and behind with a
second smaller ectoderrnal invagination, the proctodceum, which
arises in the position o* the former pigment area. The part of
tlv larva behind the prototroch now elongates, and two pairs of
imaginations, the setigerous sacs (set. sacs), appear at its sides: in
the interior of these, to which a third pair is soon added, are
developed setae which grow out to a great relative length as the
l«>'c«l or provisional seta . Constrictions soon appear marking off
the first three segments.; ml at the same time the mesoderm bands
undergo a corresponding division into three pairs of mesoderm
"he mesoden H segments of each pair grow inwards
tow. another and surround the alimentary canal; in the

interior <»f each appenr-; a cavity which is the beginning of a

PHYLUM ANNULATA 417

segment or chamber of the coelome. As the two mesoderm
segments become closely applied to one another and unite around
the alimentary canal, their two cavities also come into close
relation, and eventually are separated from one another only by
thin vertical septa, which afterwards form the dorsal and ventral
mesenteries. Successive mesoderm segments also come into close
relationship with one another, their cavities eventually only
remaining separated by thin transverse partitions, which form the
intersegmental septa or mesenteries.

The region in front of the prototroch becomes modified to form
the prostomium of the adult. The body increases in length, and
additional segments with their setigerous sacs become distinguish-
able (E) until, on the development of the tentacles,, the outgrowth
of the parapodia (para.} with their cirri and the permanent setse
(which replace those first formed), the formation of the full number
elf segments, and the completion of the internal organs, the adult
condition of the worm is attained.

.

b. THE EARTHWORM (Lumbricns).

General External Features. — The Earthworm (Fig. 328)
has a' long narrow body, which may be described as approximately
cylindrical, but slightly depressed towards one end, the posterior.
Dorsal and ventral surfaces are readily recognisable, the latter
being much paler in colour than the former, and exhibiting a
slight flattening ; an anterior end is distinguishable in the living
animal as that which is directed forwards in the ordinary creeping
movements of the worm. The surface, as in the case of Nereis,
is very distinctly marked out into segments or metameres by a
series of ring-like constrictions; the segments, which are very
numerous, amounting to about 150, are somewhat longer towards
the anterior end than they are further back.

At the extreme anterior end is a rounded lobe, the prostcmium,
immediately behind which is the opening of the mouth. Next to
the prostomium is the most anterior segment, the pcristoriiinm.
which bounds the mouth behind. The eyes and tentacles present in
Nereis are not represented. On the most posterior segment, the
anal segment,^ is a small median opening, the anal aperture. A
limited region of the body in front of the middle, comprising seg-
ments from the thirty-second to the thirty-seventh, has a swollen

appearance ; this is termed the dUdl-u-m. There are no parapodia

"oFY
"worm are to be recognised with the aid of a lens four double rows

like those of Nereis, but running along the lowc- surface of the

of short bristles or setas (Fig. 329), a pair of each row occurring
in each segment, which thus posses h eighi altogether. The
exfremities of all these setae are directed backwards, and they act
as fulcra for the forward movements of the worm on the surface

VOL. I E E

\

418

ZOOLOGY

of the ground or in the interior of its burrow. The setae in the
clitellum, and those in the neighbourhood of the genital apertures,
are much slenderer than the rest. Along the middle line of the
dorsal surface, from about the eleventh segment backwards, is a
row of small apertures, one at the line of division between each
contiguous pair of segments : these, which are termed the dorsal
pores, perforate the body-wall and open internally into the ccelome.
On the ventral surface are two rows of minute apertures — a "pair

B

-'*. — Lumbricus agriCOla. A, entire specimen, lateral view ; «, ventral view of anterior
portion -if the body, magnified. 1, la, 33, first, fifteenth, and thirty-third segment*. Tlu-

r Yogt

on each segment — the < On

,'iiiral sumce of the fifteenth segment (Fig. 328, 15), is

pair of slit-liko. -apertures with somewhat tumid lips, the

reprodi res; and on the segment immediately in front,

the fourteenth, are two smaller rounded apertures, the fcin«l<

Ir the intervals bf-t\\een the ninth and

tenth, and tenth and eleventh segments are two pairs of small
3, the openings of thr

PHYLUM ANNULATA

419

The body-wall (Fig. 330) consists of a
V. an epidermis or derie^ epithelium, a
derm is, riniscultr layers with associated con-
nective-tissue, and, lining the inner surface,
a thin cellular membrane, the peritoneum or
ccelomic epithelium. The cuticle (cut.) is similar
to that of Nereis, and has a similar iridescent
lustre : it is perforated by numerous minute
apertures. The epidermis consists, except on
the clitellum, of a single layer of cells
elongated in the vertical direction : many of
these cells have the character of unicellular
glands ; many others are nerve-cells, and are
connected by fine nerve-fibres with the nerve-
cord. On the clitellum the epidermis is
thickened, and blood-vessels extend between the

FIR. 329.— Lumbricus,

setae, highly magnified.

cells. Below

the epidermis is a layer of connective- tissue, the derm/is. The

dors, v

cut

hap

set

Lmmbricus, transverse section of the middle region of tin -.nvn. layer of

circular muscular fibres; cosl. cr»'!..niu; cut. cuti .epidermis;

epli. afephridioporc ..<. longitudinal muscle;

nephridium ; nep)

. ••. ventral vessel. (After Marshall and Hurst.1)

.uscular fibres which make up the greater part of the thickness
of the body-will are arranged in two principal sets— a layer of

E E 2

420

ZOOLOGY

circularly arranged fibres (circ. mus.) situated externally, imme-
diately below the dermis, and a layer of longitudinally arranged
fibres (long, mus.) situated internally. The circular layer is in-
terrupted at arl' the in-
tervals between the seg-
ments ; the longitudinal
layer is interrupted along
a series of longitudinal
lines, so as to be divided
into seven bundles.

The setae are lodged
in sacs, the setigerous sacs
(see Fig. 339), lined by
a continuation of the
epidermis. In the re-
gion of the body in whJch
the reproductive organs
are lodged some of these
sacs are -enlarged and
glandular, and receive
the special name of the
capsulogenous rilun<h.

The enteric canal
(Fig. 33 3) is, as in
Nereis, a tube which
runs through the entire
I length of the body from
the mouth at the an-
'terior to the anas at the
posterior end. As in the
case of Nereis, it. lies in
a cavity, th< lined

by a thin cellular mem-
brane, the petition
and filled with a fluid,
the caelomic fluid, con-
taining colourless cor-
puscles. It is divided. •
into a series of cham-
bers corresponding to
the segments by a series
of delicate transvers
partitions, the x/'j'ta or
mesenteries, consisting of folds of the peritoneal membrane en-
closing muscular fibres.

The mouth leads into a small lnn'cnl c«ml ij. This is followed by
;i much kirger thick- walled, rounded chamber, tlu JM//'V///M.- (ph.\

PHYLUM AXNULATA\\

423

It.

The Earthworm is devoid of organs of sight or hearing. It
-exhibits sensitiveness to bright light, which may be due to direct
action on the central parts of the nervous system. The sense of
hearing appears to be absent ; but a faculty analogous to taste or
smell, enabling the animal to distinguish between different kinds
of food, is well developed. The
yoblet-shaped liu<lic$, groups of
narrow epidermal cells, most
abundant on the prostomium
and peristomium, have probably
to do with this faculty.

The organs of excretion—
the segmented organs or nephrid'ui.
— (Fig. 333) are similar to those ,
of Nereis, but somewhat more I
complicated. They are slender
tubes which occur in pairs in all
the segments of the body except
the first three and the last. Ex-
ternally each nephridium opens
by one of the small excretory
pores which have already been
mentioned as occurring on the
ventral surface ; internally it ends
in a funnel-shaped ciliated ex-
tremity with an aperture, the
ne-phrost.orm:., opening into the
cavity of the corresponding seg-
ment. The tube is thrown into
several loops attached to the-,
posterior surface of the corre-
sponding mesentery by a fold of
membrane. Two parts' are clearly
recognisable — an inner narrow
and an outer thick part : in the

former the narrow central lumen is a perforation through the
axis of a string of cells, and is thus in/rcrdl/dar : it is lined
in ] tarts with cilia arranged in two rows: in the latter the
,e is lined by cells, and is thus inter-edlulwr, and there is a
thick muscular investment. The nephridia are abundantly
supplied with blood by means of nephridial branches of the
ventral ves

Reproductive Organs. — The Earthworm is • hermaphrodite.
There are two pairs of very small flattened testes (Fig. 334, te, te')t
partly divided^into a number of digitate lobes, situated in the
tenth and eleventh sr/ments. A pah\of comparatively Inrg-
the anterior r , m. .sv ///.), lie partly in the

FIG. 333. — Nephridium of an Oligoi

'/,.-•. mi's.-ntery ; •:!>. terminal dilatation
(absent in the Earthworm), //-•. Tiody
wall; /'.'M, thick; <i<i\, narrow part of
the tube; ^.funnel. (From Lang's
Text-Book.)

424

ZOOLOCJY

SECT.

cavity of the ninth segment, but extend into the tenth, where
they coalesce in the middle to form a large median sac of some-
what irregular form, the anterior sperm reservoir (ant. sp. res.).
The anterior pair of testes project into this, and the cells destined
to form the sperms, developed in the former, pass by dehiscence
into the large median cavity. On either side is a large ciliated
•tcl, or rosette (fun.), leading outwards from the interior of the
reservoir. A second pair of vesicular seminales (mid. ves. sem.),
situated in the eleventh segment, also open into the anterior

cint. fes. sent
'ftl \ int n.co

ctnt. sp.res

rec

post vss. «£?/

rrTcS£S3JT^^^r^V ^' '

/
of.fi

ov.cL

-4. — Immbricus aerricola. reproductive organs. • interior sperm reservoir ;

•interior left vesicula senrinalis ;/<//>. funnel-like opcM>; cfT<jrentia :

o'/. intermuscular partitions ; . miiidlc vesiciil.-i - >. CD. nerve cord ; or.

- ''. nviiliR't HI reservoir: • ,,<. posterior

ihi seniiniilis ; rec. recejitacula seniinis; if, anterior; /L', posterior tcste.s ; v. </</. vusa
deferentia. (After Vogt and .luny.)

sperm reservoir. A third pair (post. -vcs. sem.), situated in the
twelfth segment, unite in front to form the posterior sp- rvoir

(vast. sp. res.), which lies' in the middle of the cavity of the
eleventh si-trmi'iit. The posterior pairs of testes have tthe same
relation to this as the anterior pair have to the anterior reservoir ;
and a posterior pair of ciliated funnels (fun.) leads outwards from
uvity. Each ciliated funnel leads to a narrow, somewhat in-
voluted duct, tli •'• rens, and the two vasa efferentia of each
side unite t<> form a vas <l<j\'i'cnx or spcrn i<lnct (<'. def.). right or
left a* the ease may be. which passes almost straight backwards

A PHYLUM ANNULATA 425

to npi-n by the corresponding male aperture on the fifteenth
segment.

The female reproductive organs consist of a pair'1 of ovaries, a
pair of oviducts with a pair of receptacula ovorun and two pairs of
receptacula seminis. The ovaries (ov.) axe minute Dear-shaped bodies,
which are situated in the thirteenth segment, attached to the
septum between the twelfth and thirteenth. r;'he oviducts (ov. d.)
are a pair of short tubes, each with a comparatively wide funnel-
shaped opening into the cavity of the thirteenth segment, and
extending backwards and outwards in the fourteenth segment, to
open at the female aperture on the ventral surface of the latter.
jThe receptacula ovorum are a pair of reniform sacs which open into
the funnel-shaped ends of the oviducts. The* receptacula seminis
(rec.) are two pairs of rounded sacs which uj en on the exterior
in the intervals between the ninth and tenth, and tenth and
eleventh segments.

Though hermaphrodite the Earthworm is not self-impregnating,
but two individuals provide for mutual fertilisation by an act
of copulation. The copulating individuals become applied together
by their ventral surfaces, the heads pointing in opposite directions,
and become attached in this position by the setae of the genital
region and by a viscid secretion from the clitellum and of certain
glands, called the capsulogenous glands, situated in the neighbour-
hood of the reproduction organs. The sperms from the male
apertures of each pass along temporarily formed grooves to the
receptacula seminis of the other.

When the ova are mature they are discharged from the ovary
into the cavity of the thirteenth segment, whence they pass out
to the exterior through the oviduct, to be enclosed in the cocoon,
after having been detained for a time in the receptaculum
ovorum.

Development. — The oosperms or fertilised ova of the Earthworm
are enclosed, together with a quantity of an albuminous fluid
derived from the capsulogenous glands, in a cocoon, the wall of
which is formed of a viscid secretion from the glands of the
clitellum, hardened and toughened by exposure to the air. The
cocoon is deposited in the earth and the embryos develop into
complete, though minute, worms before they make their escape.
The segmentation is somewhat unequal. A flattened blastula
(Fig. 335^1) is formed, with a large but flattened segmentation
cavity. This becomes invaginated to form a cylindrical gastrula
(B): the blastopoiv becomes narrowed and, subsequently gives
rise to the mouth of the adult. A pair of large . //t^^erm cells
(m) early become marked off from the other cells of the gastrula ;
these become divided to form a pair of mesoderni hands composed
of several rows of small cells, which grow forwards towards the
mouth. By swallowing movements the embryo at this .ikes

426

ZOOLOGY

SECT.

in the albuminous fluid in the interior of the cocoon, and increases
rapidly in size, bursting^. th£ enclosing vitelline membrane. As
the emlayS(jjifcmgates. tin- mesoderm bands become divided into
segments, and the subsequent history of these is essentially similar
to what has been already described in the case of Nereis. The

vet

srtes

a si

rn.

Tries

n.e-7*

FIG. 33.J. — Early stages in the development of Immbricus. A, lateral view uf flattened blastula :
Ji, ventral view of gastrula with slit-like blastopore ; C, lateral view of later
Mast.ocofile : Waxtop. blastopore ; ec<. eetodt n ; //<, primary nn'soderm cell:

nesoderm bands ; /«.,•. cell from which the primiti\c aerve cord (»e. co.) rakes -n-iu-in :
••i;lls taking part in the formation of the nephridia ; .^.

most essential difference between the two forms consists in the
non-occurrence in the Earthworm of any free-swimming Trochn-
sphere stage.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Chaetopoda are Annulata with the body made up-of distinct
, which are usually numerous and similar throughout

Sub-Class I.— POLYCILETA.

x PHYLUM ANNULATA 427

The metameres are provided with chitinous setae developed in sacs
(setigerous sacs) of the epidermis, and usually elevated on muscular
appendages, the parapodia. There is a large ccelome divided
internally into chambers by transverse septa or mesenteries, and
not in communication with the blood-vascular system, which is
nearly always highly developed. The ventral nerve-cord consists
i of a chain of ganglia. The reproductive cells are formed by a
proliferation of certain parts of the peritoneum or membrane lining
the ccelome, and usually reach the exterior through modified
or unmodified nephridia.

Chaetopoda with the sexes distinct and the ovaries and testes of
simple character and metamerically repeated. Highly developed
parapodia are present, in most instances, bearing numerous long
setae. There is usually a definite head with eyes and tentacles,
and often cirri and branchiae on the segments of the body. A
metamorphosis takes place : the larva is a Trochosphere. All the
Polychaeta are marine.

ORDER 1. — ARCHI-CH^ETOPODA.

Aberrant or primitive Polychaeta l in which the nervous system
is not separated from the epidermis and the ventral cord is not
segmented into ganglia. Only one genus (Saccocirrus).

ORDER 2. — ERRANTIA.

Carnivorous free Polychaeta with protrusible pharynx usually
armed with chitinous jaws. There is a well-developed head. The
segments are completely or nearly similar throughout the length
of the body, and the parapodia are equally developed throughout,
and provided with cirri. The branchiae, when present, are not
confined to the anterior end.

ORDER 3. — SEDENTARIA.

Vegetable-feeding Polychaeta which permanently inhabit tubes ;
devoid of protrusible pharynx and of jaws or teeth. The head is
frequently very small, and sometimes devoid of eyes or of tentacles.
The body is distinguishable, by differences in the form of the seg-
n"»nts and of the parapodia, into two or even three regions. Thfex
parapodia are little prominent in the posterior parts, and usually/
without cirri. The branchiae, when present, are usually confined
to the anterior end, and are sometimes represented by modified
cephalic tentacles.

1 The Archi-Chcetojwda are usually classed with the Pclychceta, but their
alliances are perhaps quite as close with the Olifjochfrfa.

428 ZOOLOGY SECT.

i pai

an

Sub-Class II.— OLIGOCHJETA.

Chaetopoda with the sexes united, the ovaries and testes com-
pact and few in number. No definite parapodia are developed

d no cirri, and only a small number of simple setae on each
segment. The head is not distinct. There is no metamorphosis.
Mostly terrestrial or fresh-water forms.

ORDER 1. — NAIDOMORPHA.

Small Oligochaeta with relatively few segments, with asexual
as well as sexual reproduction. Male genital pores on, or in front
of, the seventh segment. The anterior part of the body is often
distinguished from the rest by a difference in the form or
arrangement of the setae. Eye-spots are frequently present.

ORDER 2. — LUMBRICOMORPHA.

Reproduction is only sexual. The anterior part of the body is
never specialised, and the setae are similar throughout (except in
special parts, such as the clitellum). The male genital pores are
behind the seventh segment. There are no eye-spots.

Systematic Position of the Examples.

Nereis dumerilii is one of many species of Nereis, differing from
one another in certain minor details of their structure — such as
the relative length of the palpi and tentacles, the size and form of
the eyes, the shape of the parapodia, the form of the setae, and the
like. * The genus Nereis differs from the other genera of the
family Nereidm, to which it belongs, in having the parapodia
biramous, and the cirri simple, and in the presence of a series o
denticles in the pharynx in addition to the pair of jaws. The
family Nereidae differs from all the other families of the Errantia
in the union of the following characters : — -The body is always
elongated and made up of a considerable number of segments
The prostomium is well developed, and bears a pair of tentacles
a pair of palpi, and four eyes. The peristomium is devoid o
parapodia, and has four pairs of tentacles. The parapodia are
either uniramous or biramous ; both dorsal and ventral cirri are
present ; the setae are compound (articulated). . There is a pair o
anal cirri. In the pharynx there is always a pair of horny jaws
and usually a number of denticles as well.

There are several species of the genus Lmnbricus, differing from
one another in the general form of the body, the number of the
segments, the shape of the prostomial lobe, and other minor
points. All of them agree in the presence of the following features,

PHYLUM ANNULATA

429

which characterise the genus and distinguish it from the many
other genera of the family Lumbricidcc : —

The prostomium is dovetailed completely into the peristomium.
The setaB are always in couples. The clitellum begins between
the twe.nty-sixth and the thirty-second segment, and extends over
six or seven segments. The male apertures are always on the
fifteenth segment. There are three pairs of vesicular seminales,
in the ninth, eleventh, and twelfth segments, connected across the
middle line in the tenth and eleventh by sacs enclosing the
ciliated funnels.

The family Lumbricjdas is distinguished from the other families
of the sub-order Megadrili, which comprises all the Earthworms,
by the combination of the following features : —

The clitellum usually begins behind the twentieth segment and
occupies from six to nine segments; it is incomplete ventrally.
The male apertures are not situated further back than the fifteenth
segment. There are three or four pairs of vesiculae seminales,
in the ninth to the twelfth segments. The testes and ciliated
funnels are in the tenth and eleventh segments. The female
apertures are on the fourteenth.

3. GENERAL ORGANISATION.

The general form of the body in the Chastopoda is cylindrical,
but in many, e.g. some members of the
families Polynoidce (Fig. 336) and Amphi-
nomidce, there is a very considerable degree
of dorso-ventral compression. In most the
body is very long in comparison with its
breadth; but this is not a universal rule,
the length being in some cases not more
than five or six times the breadth. The
surface is marked out by a number of
more or less distinct annular constrictions
or impressed lines into a corresponding
series of segments or metamercs, which are
usually very numerous, often some hundreds
in number, though in some cases there are
not more than from twenty to thirty.
These segments are usually very similar
throughout the length of the body ; but
in the Tubicolous Polychasta (Fig. 337)
there may be two or even more regions
distinguishable from one another by the
form of the segments and of their appen-
dages. In the Oligochaeta there is a thickened zone, the clitellum,
comprising sometimes only one segment, sometimes a number.

Fio. 336.— Polynde seto

sissima. Dorsal view
of entire animal, with
the "pharynx protruded.
(After Quatrefages.) _,

430

ZOOLOGY

SECT.

Each segment, with certain exceptions to be noted presently,
bears either a pair of parapodia or merely certain setae repre-
senting the parapodia. Parapodia are lateral hollow processes
of the body-wall bearing a number of bristles or setae. Fre-
quently the parapodium is divided horizontally into two distinct
lobes or branches — a dorsal which is termed the notopodium,
and a ventral which is termed the neuropodium. Even .when
this is not the case there may be two bundles of setae repre-
senting the two parts. The setae are nearly always chitinous ;

abd

FIG

. 337. — A Serpulid(Vermilia coespitosa). Lateral view of animal removed from its tube
abd. abdomen ; br. branchiae ; op. operculum ; tk. thorax.

in Euphrosyne they are calcined. They are hollow or solid,
entire or divided into a number of joints. In shape (Fig. 338)
they vary greatly in different groups; often several very dis-
tinct forms of setae are present in different parts of each para-
podium of a single worm, or in parapodia of different regions of
the body. Some are exceedingly delicate and hair-like, others
needle-shaped, others compressed and sabre-like, others bayonet-
like. Very often there is a long, straight, narrow part or handle
with which is articulated a terminal blade, or bayonet, or hook.

PHYLUM ANNULATA

431

Sometimes the setae are quite short, projecting little beyond

the parapodia, and are

hook-like or comb-like.

Usually each bundle

contains, in addition to

the ordinary setae, a

stouter straight simple

seta, which scarcely

projects on the sur-

face ; this is termed

the aciculum. Each

seta or each bundle of

setae is lodged in a sac,

the setigerous sac (Fig.

339), formed by an

invagination of the in-

tegument, and lined by

cells continuous with

the epidermis. Each

seta is derived from

one of these cells, and

is to be looked upon as

a specially developed

part of the cuticle of

the general outer sur-

face. The setigerous

sacs are usually pro-

vided with protractor and retractor muscles, by the action of

which the setae may be thrust out or retracted.

In addition to the setae
the parapodium bears very
commonly certain soft ap-
pendages of a sensory char-
acter, the cirri (Fig. 319,
dors. cirr.,vent. cirr.) There
are usually both dorsal
and ventral cirri, the latter
nearly always much smaller
than the former. The cirri
are usually filamentous ;
sometimes jointed ; some-
times they are laterally
compressed and leaf-like.

In Potyndc (FigS. 336 and
'34,fA anrl it<s nllip« portiin

of the parapodia bear

-

FIG. 338.— Setae of various Polychseta. (From Claparede.)

FIG. 330.— Section of the setigerous sac of an Oligo-
chrete. fci, setigerous sac; &2, supplementary
follicle with sett ; e, deric epithelium ; lm, longi-
tudinal muscles of body wall ; Mi, m, muscles of
the setigerous sac ; rm, circular muscular layer
of body wall. (From Hatschek, after Vejdovsky).

-IIP i

instead OI ClOrSal

432

ZOOLOGY

SECT,

flattened scales, the elytra (el.), richly supplied with nerves :
these are sometimes looked upon as modified dorsal cirri ; but in
some members of the group cirri and elytra occur together on
the same segment.

In Sternasrjis a ventral shield formed by a thickening of the
cuticle in the posterior region of the body bears a number of setae
round its edge.

In the Oligochaeta (Fig. 341) the parapodia are absent as pro-
cesses of the body-wall, and are merely represented by a small

rlst len£

dors.ci

FIG. 340.— Polynoe extenuata. Dorsal view of anterior extremity1 dors. cirr. dois-al

t/, elytra ; ptrift. tent, peristcmial tentacles ; prast. prostcmii;m. (After Clapartde.)

number of short setae each lodged in its sac ; and cirri are not
developed.

The first segment or prostcmium, together -with the second
peristcmium, forms in many Polychseta a very distinct head ; the
prostomium in such a case bears eyes and tentacles and contaii
the cerebral ganglion ; on the peristomium is the opening of th<
mouth, and also certain tentacles, the peristcmial tentacles.
ventral pair of prostomial tentacles, somewhat thicker than th(
rest, are sometimes to be distinguished, and are termed the palpi.
Neither prostomium nor peristomium bears parapodia, though an

PHYLUM ANNULATA

433

aciculum is sometimes developed
in the latter ; the prostomium,
in fact, is not quite correctly
termed a segment, being different
from the true segments both in
structure and in mode of develop-
ment. In the Oligochaeta there
is no definite head, tentacles are
entirely absent, and in the terres-
trial forms the prostomium does
not lodge the cerebral ganglion.
In Sternaspis spinosa the pro-
stomium is elongated and bifur-
cated like the proboscis of the
Gephyrea armata (vide infra).

The last segment is termed
the anal segment, owing to its
bearing the anal opening; it
differs from the preceding seg-
ments also, usually, in wanting
the parapodia and in having a
pair of special cirri, the anal
cirri.

Branchiae are borne on the
dorsal surfaces of more or fewer
of the segments in many of
the Polychaeta. Sometimes they
occur on all, or nearly all, the
segments ; sometimes they are
confined to the middle region of
the body ; sometimes they are
present only at the anterior end,
as in the majority of the Tubicola
(Figs. 337 and 342). In the
Terebellidw the branchiaa are
situated on the dorsal surfaces
of some of the anterior segments.
In the Serpulidce they form two
incomplete lateral circlets of
elongated appendages, situated
at the anterior end of the body,
and apparently appendages of
the prostomial segment, sup-
ported sometimes by a cartila-
ginous skeleton; one of them
is enlarged to form a stopper, or
opercuhwn (op.), often armed with
calcareous plates and spines,

VOL. I

F F

434

ZOOLOGY

SECT.

for the closure of the mouth of the tube in which the annelid
lives. In shape the branchiae are sometimes filiform, sometimes
compressed and leaf-like, sometimes branched in a tree-like
manner, sometimes pinnate. In Serpula, (Fig. 349) and its allies
each branchia consists of an elongated stem, on which are borne
two rows of short filaments. The surface of the branchiae is,
usually ciliated. They are richly supplied with blood-vessels, when

I : ; i 3

FIG. 342 — Terebella. (After Quatrefages.)

a blood- vascular system is developed : in Glyccra, in which there
are no blood-vessels, each branchia contains a diverticulum of the
ccelome.

In the Oligochaeta branchiae are rarely present ; but in certain
worms of the family Tiibificidce there are metamerically arranged
simple branchiae on the segments of the posterior region, which is-
the part ordinarily protruded from the tube.

The body-wall consists of a cuticle, an epidermis, muscular

x PHYLUM ANNULATA 435

layers, and a layer of peritoneum. The cuticle, composed of a .
chitinoid material, usually presents two systems of fine lines inter-
secting one another at right angles : it is perforated in many
places by the ducts of the unicellular glands of the epidermis.
The epidermis consists of a single row of cells, with, in some cases,
smaller cells of replacement intercalated between their inner ends.
In shape the cells vary greatly in different families and often in
different parts of the body of the same worm, being sometimes
flattened, sometimes cubical or polyhedral, but more usually more
or less vertically elongated. Cilia occur on the surface in certain
parts in many Chsetopoda. Among the ordinary cells of the
epidermis there are usually numerous unicellular glands often
containing rod-like bodies. In the tubicolous forms these unicel-
lular glands are active in secreting the material for the construc-
tion of the tube. In addition the epidermis frequently contains
sensory cells, which are in many cases contained in certain special
elevations or sensory papilla?.

The muscular part of the body-wall consists of two layers, in the
outer of which the fibres are disposed circularly, while in the
inner their arrangement is longitudinal. The circular layer is
continuous, or, more usually, interrupted opposite the intervals
between the segments. The longitudinal layer is disposed in four
bands in the Polychaeta, two dorso-lateral and two ventro-lateral.
In the Oligochseta it is divided by the setigerous sacs which pass
through it.

The peritoneum or ccelomic epithelium consists of a single layer of

cells. These are usually non-ciliated, but are ciliated in the

- Aphroditea, Glycera, and some others, the movement of the cilia

bringing about an active circulation of the fluid in the coelome,

the ccelomic or pcrivisceral fluid.

The body-cavity or coelome, a wide space intervening between
the wall of the body on the one hand and that of the enteric
canal on the other, is divided in many ChaBtopoda by a series of
transverse septa or mesenteries into a series of chambers corre-
sponding to the segments. The septa are not complete partitions,
there being always apertures of greater or less extent by which
the cavities of neighbouring segments communicate. The septa
consist of double folds of the peritoneum enclosing muscular
bres.

The enteric canal is an elongated, and nearly always straight,
ube, running through the entire length of the body from mouth

anus. A number of different parts are usually distinguishable ;
but their disposition varies to a very great extent in the different

oups. The buccal cavity, into which the mouth leads, is followed
y a muscular pharynx ; these are both formed in the embryo by

vagination of the ectoderm; and therefore correspond to a stomo-
m. The muscular pharynx is absent in some of the tubicolous

F F 2

436

ZOOLOGY

SECT.

Polychseta ; when present it is frequently protrusible to a greater
or less extent (see Fig. 336); around its extremity, when it is
fully^protruded, are to be seen a circlet of papillae in some forms ;
and in many one or more horny teeth, situated in its interior, are
brought into play. A gizzard with thick walls may follow upon
this protrusible pharynx, sometimes preceded by an oesophagus,
which may be dilated behind into a crop. The intestine is nearly
always more or less deeply constricted between the segments, and
in the Aphroditea, or Sea-mice (Fig. 343), there are in each of the
segments (with the exception of one or two of the most anterior
and one or two of the most posterior) a
pair of cceca which are to a greater or less
extent branched at their extremities. In
the Hesionida and Syllida a pair of caaca
which open into the anterior part of the
intestine frequently contain gas, and prob-
ably have a hydrostatic function. In some of
the terrestrial Oligochaeta (Earthworms) a
fold of the intestinal wall, the typhlosole,
projects into its lumen. The intestine is
straight in most, but is somewhat coiled in
the Chlorcemidce, Sternaspis, and others. The
wall of the alimentary canal consists (1) of
the visceral layer of peritoneum ; (2) of
longitudinally arranged muscular fibres :

(3) of circularly arranged muscular fibres ;

(4) of enteric epithelium. The peritoneum
on the surface of the intestine has in many
Chastopoda its cells enlarged and granular
to form the so-called chloragen cells, which
probably have an excretory function. The
epithelium is very generally ciliated ; it con-
tains numerous gland-cells. In addition the
stomodaBum and the proctodasum are lined
internally by a cuticular layer, which is con-
tinuous with the cuticle of the general surface. The anus is
usually terminal in position, sometimes directed towards the
dorsal aspect. There is, in most instances, a longitudinal mesen-
tery running to the alimentary canal from the dorsal body- wall ;
and sometimes a ventral mesentery is also present bearing a
corresponding relation to the ventral surface.

Some Chsetopoda are entirely devoid of blood-vessels. In one
family in which this occurs (the Glyceridce among the errant Poly-
chseta), the perivisceral fluid, which assumes some of the functions
of the blood, contains numerous red corpuscles, the red colour of
which is due to the presence of haemoglobin (see p. 34). In the
majority of the Chastopoda there is a highly developed vascular

FIG. 343.— Enteric canal of
Aphrodita. «, mouth ;
b, pharynx ; c, branching
cceca of intestine; d, anus.
(From Gegenbaur's Com-
parative Anatomy.)

x PHYLUM ANNULATA 437

system. Sometimes the blood is colourless : very commonly it
is bright red in colour, owing to the presence of haemoglobin^
which is not confined to the corpuscles, but is dissolved in the
plasma. In Serpula and its allies the blood is bright green, owing
to the presence of a green colouring matter, which has an affinity
for oxygen similar to that possessed by hsemoglobin.

The chief blood-vessels are usually dorsal and ventral longi-
tudinal trunks. These are connected together by metamerically
arranged transverse branches. In some of the Tubicola the dorsal
vessel is not present, its place being taken by a circum-intestinal
sinus or a circum-intestinal plexus of vessels lying in the wall of the
alimentary canal. The movement of the blood is effected in most
instances by peristaltic contractions of the dorsal vessel or of a
circum-intestinal sinus or plexus, or of a short and wide dorsal
cardiac sac given off by the latter anteriorly, having the effect of
driving the blood from behind forwards. In some instances, as in the
Earthworms and some Tubicola, specially dilated lateral vessels are
contractile, and by their pulsations bring about the circulation of
the blood through the system of vessels. Plexuses of fine capillary
vessels in the integument of various parts frequently aid in
respiration, and are particularly well developed in certain forms in
which definite organs of respiration are absent.

The nervous system consists of a cerebral ganglion or brain
and a double ventral chain of ganglia. The cerebral ganglion is
distinctly bilobed, and may be looked upon as composed of two
intimately united ganglia. It is almost invariably situated in the
prostomium, though placed a little further back in the Earth-
worms ; it gives off branches to the eyes and tentacles. From it
there run backwards and downwards the paired aesophageal con-
nectives, which embrace the anterior part of the alimentary canal
between them, and below join the anterior end of the ventral chain
of ganglia. The latter always exhibits indications of being made
up of two lateral halves, in the double character of the connecting
commissures, and frequently of the ganglia themselves. One of
these double ganglia occurs in each segment, and from it a number
of nerves pass out to the various parts of the segment. In certain
Tubicola (Serpula and others) the two halves of the chain are
separated from one another by a wide space, across which trans-
verse commissures pass between the ganglia. Connected with the
cerebral ganglia, or with the cesophageal connectives, or with both,
there is a system of delicate stomatogastric nerves passing to the
walls of the anterior part of the alimentary canal. In the majority
of the Cha3topoda the cerebral ganglion and the ventral chain are
separated from the epidermis by muscular layers ; in some, how-
ever, the ventral chain is in contact with the epidermis ; and in
certain primitive or aberrant forms, the Archi-Cha3topoda (Fig.
344) and Sternaspis, the cerebral ganglion is in close union with

438

ZOOLOGY

SECT.

the epidermis ; in these also the ventral cord is not segmented
into ganglia. Running longitudinally through the ventral cord in
many forms are certain giant fibres of very large size ; though these
may have rather a skeletal than a nervous function, they are simply
greatly enlarged and modified nerve-fibres. Nerve-cells may be
confined to the ganglia, or may be distributed over the entire sur-
face of the ventral cord. Giant nerve-cells occur in some forms in
certain regions. Small ganglia occur frequently in various peri-

FIG. 344. — Saccocirrus, transverse section, to show the position of the nerve cords, dors. vess.
dorsal vessel ; int. intestine ; ne. co. nerve cord ; set. set?e. (After Fraipout.)

pheral parts, at the bases of cirri or of sensory papillae more
especially.

The organs of special sense are eyes, tentacles and cirri, and
otocysts. Eyes, absent in all the Oligocha^ta with a few exceptions,
and in some of the tube-forming Polychseta, as well as in a few
free forms of that sub-class, are very general in their occurrence.
Their structure is, as a rule, very simple, but in some reaches quite
a high grade of development. Usually they are confined to the pro-
stomium, but Polyophthalmus, in addition to the prostomial eyes, has
pairs of eye-like organs on many of the segments of the body.
Leptochone has a pair on each segment, and in Fabricia there
is a pair on the anal segment, while in many species of Sabella
and all the species of Dasi/chone there are eyes or eye-like organs
on the branchial filaments.

x PHYLUM ANNULATA 439

Most usually the eye is (as in Nereis, p. 410, Fig. 324) a
spherical capsule with a wall composed of a single layer of cells,
which are elongated on the inner side, i.e. the side turned towards
the brain, while on the outer side they are usually flattened. The
outer thin part of the wall of the capsule or cornea, is some-
times united with the epidermis; when the two layers remain
distinct, the outer one is the outer cornea, the inner the inner
cornea. In many cases a thickening of the surface cuticle over the
cornea forms a cuticular lens. The cells of the inner portion of
the wall of the capsule form the elements of the retina ; they are
long narrow cells, sometimes composed of three distinct segments
—(1) a clear rod, directed towards the central cavity ; (2) a middle
segment which is densely pigment ed ; and (3) a segment contain-
ing the nucleus of the cell and directed towards the brain or the
optic ganglion, with which it is connected by a nerve-fibre. Fre-
quently the second and third segments are not to be separately
recognised, the whole of that part of the cell which contains the
nucleus being densely pigmented.

A refractive mass fills the interior of the capsule, and is some-
times distinguishable into a firmer outer part, the lens, and a more
fluid inner part, the vitreous ~body. This refractive mass is often
continuous with the cuticle externally, and internally may be in
continuity with the rods. In some cases the structure of the eye
is very much simpler.

Otocysts are only exceptionally present. They consist of capsules
of ciliated cells, in the fluid contained in which there is one or
several calcareous qtoliths.

Ciliated grooves occur on the prostomium of many forms ; in
Aricia they are present on all the segments ; they have a special
nerve-supply, but their function can only be conjectured. Tactile
cells of the epidermis, with or without a projecting tactile hair or
stiff cilium, are very common, especially on the prostomium in the
Oligochseta and on the tentacles and cirri in the Polychaeta.
•Groups of these are often aggregated together in papillce or
goblet-bodies with special nerve-supply, and often with a ganglion
or a single nerve-cell at the base.

The organs of excretion of the ChaBtopoda are a series of
segmentally arranged tubes, the nephridia, of which a pair, as a
rule, occur in each of the segments of the body, with the exception
usually of a few at the anterior and a few at the posterior end.
In its simplest form the nephridium is a curved tube, ciliated
internally, opening on the exterior by a laterally placed pore at
the one extremity, and at the other ending in a ciliated funnel
or nephrostome, which opens into the cavity of the corresponding
segment. The nephridia thus effect a communication between
the coelome and the Exterior, and serve to carry off waste products
which have passed into the ccelomic fluid ; but in many instances

440 ZOOLOGY SECT.

the cells lining the tube are active in separating out such waste
matters, and are loaded with granules and concretions. The com-
munication which they establish with the exterior also enables
the nephridia to take on the additional function of reproductive
ducts, as will be more particularly noticed in discussing the repro-
ductive system. The outer part of the tube is usually more or
less dilated ; the inner narrower part in some cases consists of a
row of perforated cells, the lumen being intra-cellular. The ex-
ternal apertures are nearly always ventral in position ; in Polynoe
and its allies they are situated on short tubercles or papilla:.
Sometimes the nephridia are confined to a certain region of the
body. In some families of Polychseta there is a single pair of large
nephridia in the anterior region of the body, with or without
smaller pairs in the posterior segments, the former alone appear-
ing to have an excretory function. In Sternaspis only a single
pair of nephridia is present. In the great majority of the Poly-
chseta the nephridia have the function of reproductive ducts as
well as of excretory organs. In the Oligochaeta the ordinary
nephridia do not possess this function : usually they are simple
elongated and coiled tubes, a pair or sometimes more than one in
each segment ; but in some these are replaced or supplemented
in certain of the segments, or in all, by a branching system of
tubes with scattered ciliated funnels. In one genus, though paired
nephridia are present, they do not open directly on the exterior,
but communicate with a branching system of tubes consisting of
four principal longitudinal vessels and segmentally repeated trans-
verse branches. Sometimes the ordinary nephridia are not de-
veloped in the segments lodging the reproductive organs, their place
being there taken by three pairs of tubes (mesonephridia) which
become modified to give rise to the reproductive ducts ; but ordinary
nephridia may be present in these segments as well. In some
Oligochseta the nephridia of the most anterior segments open
into the mouth or pharynx, and have apparently taken on the
function of digestive glands (peptonephridia).

The mode of development of these tubes varies in different
instances. In some cases they are formed, like the excretory tubes
of Platyhelminthes and Troche) minthes, from cells of ectodermal
origin (ecto-nephridia). In others their source is mesodermal, each
tube being formed as a funnel-like outgrowth of the peritoneum
(peritoneal funnel) which subsequently comes to open on the
exterior (mesonephridia). In other cases, again, both ectoderm
and mesoderm take part in their formation, the peritoneal funnel
becoming connected with an ingrowth of the ectoderm (diplo-
nephridia). The ordinary nephridia of the Oligochaeta are ecto-
nephridia ; those of the Polychseta for the most part diplonephridia.

The permanent nephridia of the adult Chsetopod are preceded
in the larva by a series of provisional or embryonic nephridia of a

PHYLUM ANXULATA

441

temporary character. These have been found to occur in the head
(prostomium) of many larval Oligochseta and Polychseta. They
are ciliated intra-cellular tubes, sometimes branched, which do
not open into the cavity of the prostomium. Sometimes ciliary
flames projecting into the lumen occur at the inner ends of the
branches or of the undivided tube. Embryonic nephridia have
also been shown to occur in the body in certain forms.

In the arrangement of the reproductive organs in the
Chaatopoda there is an essential difference between the two sub-
classes, the Oligochseta being hermaphrodite, and the Polychseta.

refine

fie rib

fierib

1

FIG. 345. — Diagram to illustrate the development of a gonad from the peritoneal (coelomic) epithelium
in one of the Polychteta. per it. peritoneal membrane ; repr. gl. gonad (reproductive organ) ;
vent. vess. ventral vessel. (After B. Meyer.)

with only a very few exceptions, unisexual. In the latter the
gonads, ovaries or testes as the case may be, are masses of cells
which are developed as the result of a sort of proliferation of the
coelomic epithelium in certain positions (Fig. 345). Usually these
organs, which are only conspicuous about the breeding season, occur
in the great majority of the segments of the body ; sometimes they
are confined to a certain region. The exact place which they
occupy in the interior of the segment varies in different cases.
Sometimes they surround one of the principal blood-vessels ;
sometimes they are situated laterally, in the bases of the para-
podia. The sperms frequently undergo the final stages in their
development after they have become detached from the testes

442 ZOOLOGY SECT.

while floating in the coelomic fluid, and the same sometimes
holds good of the ova. Both sperms and ova appear to reach the
exterior, in the majority of cases, through the nephridia, which
may become modified and enlarged at the breeding season, though
in some forms it is stated that the reproductive cells escape
through temporary or permanent openings in the body- wall. Im-
pregnation takes place externally in nearly all.

In the Oligochseta the reproductive organs are confined to a
certain limited region of the body. There are either, as in the
Earthworms, two pairs of testes, or a single pair, as in the aquatic
forms. The testes are small, and frequently become reduced to
mere vestiges in the adult animal, having mainly become broken
up into sperm-mother-cells, which in some way reach the vesiculse
seminales to undergo development into mature sperms. The
vesicular seminales are comparatively large sacs, which vary in
number and arrangement in the different genera. One or two
median sperm-sacs, formed by the coalescence of pairs of vesiculaB,
may be present or absent. In the same segments as the testes,
and opening into the sperm-sacs when the latter are developed, are
either two or four ciliated funnels (mesonephridia), according to
the number of the testes, leading into efferent ducts. All the
four ducts, when four are present, may remain distinct, or the
two ducts of each side may open into a common atrium, or they
may unite to form a common elongated vas deferens, opening at the
male genital aperture.

There are never more than two ovaries, which, like the testes,
are of very small size. The ova may become mature in the ovary,
or groups of cells may become detached from the latter and one
cell in each group ripen into an ovum. A receptaculum ovorum
occasionally receives the ova after they leave the ovary. There
are two oviducts, which open by funnel-shaped apertures into the
ccelome.

Development. — The Oligochseta deposit the eggs in cocoons,
either buried in the earth or attached to water-plants. The
cocoon contains, in addition to a number of fertilised ova, a quan-
tity of an albuminous fluid which serves as nourishment to the
developing embryos. Segmentation is always unequal. In the forms
in which food-yolk is scanty there is a process of embolic in-
vagination (Lumbricus rubellus): in the others (Tubifex, &c.) the
process is of the epibolic type. In the former case a blastula and
an invaginate gastrula are formed in the way already described
in the case of the Earthworm. In Lumbricus trapezoides the
gastrula divides into two, each half subsequently giving rise to
an embryo. In the latter the micromeres spread over the mega-
meres very much as in the PolychaBta. A pair of mesoderm cells
early appear, and by their division form the mesoderm bands. No
free larval stage similar to the Trochosphere occurs in any of the

PHYLUM ANNULATA 443

Oligochaeta, but the stage intervening between the completion of
the gastrula and the commencement of the segmentation of the
mesoderm bands corresponds to the Trochosphere in essential
respects, and in some forms there is recognisable a feebly de-
veloped circlet of cilia comparable to the prototroch, and in some
a pair of head nephridia.

Impregnation and the development of the embryo take place
externally in all the Chsetopoda, with a very few exceptions, in
which development takes place in the ccelome or in the interior
of a dilated segmental organ. In the Polychseta, in the great
majority of cases, fertilisation takes place by the sperms coming
in contact with the ova when both have become discharged,
and the development of the embryos goes on while they are
floating freely in the sea. There are a few cases in which the
impregnated ova are received into a sort of brood-pouch and
there pass through at least the earlier stages of their development.
Such a brood-pouch is formed in certain Errantia by the raising
up of the integument on the ventral surface. In some species of
Polynoe and allied genera, the fertilised ova and the resulting
embryos adhere in masses to the dorsal surface under the shelter
of the elytra. In some other Errantia they are stuck by means
of some viscid secretion all over the dorsal surface, or they may
adhere singly to the ventral cirri. In certain Tubicola (Fig. 346)
they develop in a cavity in the operculum ; in others, in the
interior of the tube, between the body of the worm and the inner
surface of the latter, or on its outer surface. In some, again,
though the ova do not remain in any way attached to the parent
worm, they may be deposited in clumps or packets enclosed in
gelatinous matter. Usually they have no other covering but
the egg-membrane.

The segmentation of the ovum in the Polychseta is unequal.
In the great majority the inequality between the megameres and
micromeres is very marked. In some Serpulids, however, the differ-
ence is very slight, and the. two sets of cells are at first scarcely
distinguishable. In such cases the cells arrange themselves in
such a way as to form the wall of a hollow sphere, the Nastula,
with an internal closed cavity, the segmentation cavity. The
megameres, which may or may not have been distinct from
the first, lie on one side of the blastula ; and soon this side
becomes invaginated (Fig. 347, A), the result being the forma-
tion of an emlolic gastrula. In the great majority of forms,
however, an epibolic gastrula is formed after the manner already
described in the case of Nereis; but forms of the process
of gastrulation intermediate between these two extremes have
been observed to occur. The blastopore of the gastrula, however
formed, does not give rise directly either to the mouth or to the
anus. It becomes elongated into a slit which becomes closed up,

444

ZOOLOGY

SECT.

and the anus and proctodsem are formed by a fresh imagination
in the original position of its posterior end, while another in-
vagination of the ectoderm further forwards gives rise to the

FIG. 34(i.— Spirorbis Isevis, a hermaphrodite tubicolous Polychaet. Lateral view of entire
animal, ant. ncph. anterior nephridium ; br. branchise ; as. oesophagus ; op. operculum with
developing embryos in its anterior ; or. ova ; sp. sperms ; st. stomach. (After Claparede.)

mouth and stomodseum. The embryo then passes into t
Trochosphere stage.

The arrangement of the cilia on the surface of the Trochosphe
varies in different Polychseta. Sometimes, though rarely, the prse-

:

PHYLUM ANNULATA

445

oral circlet is absent, and the surface is covered uniformly with
cilia ; such larvae are said to be atrochal. Sometimes there are
two circlets close together, the one immediately in front of, and
the other immediately behind, the mouth. Sometimes, in addition
to the prae-oral circlet, there is a peri-anal circlet round the anal
end (telotrochal larvae). In some cases, instead of a praa-oral circlet,
there is one further back round the middle of the body (meso-
trochal), or there may be several between the mouth and the anal
end (polytrocJial).

The post-oral portion of the larva elongates, and traces of
segmentation become visible ; sometimes a series of constrictions

P08

347. — A, B, C, three stages in the development of the Trochosphere of Eupomatus, from
the side. an. anus ; fli, blastocoele ; TO, polar cells of the mesoderm ; md, mid-gut ; n, larval
head-nephridium ; ot. otolith ; sp, neural plate ; si. stomodaeum ; wk, preoral ciliated ring ; icki
t oral ciliated ring. (From Lang's Comparative Anatomy.)

are developed before there is any trace of parapodia, sometimes rudi-
ments of the latter with their setae are developed first. The number
of segments, at first very small, becomes added to from behind as the
body gradually elongates. The establishment of external segmenta-
tion is accompanied by the division of the mesoderm bands into
a series of segments, the history of which has been sketched in
describing the development of Nereis. The ectoderm of the ventral
plate develops a median thickening which gives rise to the ventral
nerve-cord. Anteriorly this becomes connected by a pair of thick-
enings at the sides of the mouth, the rudiments of the oasophageal
connectives, with the developing cerebral ganglion.

The completion of the metamorphosis is brought about by the
increase in length of the body, and concomitant increase in the

446

ZOOLOGY

SECT.

number of segments, by the full development of the various
systems of internal organs, and by the formation of the tentacles
and other appendages. The parapodia, when first formed, very
usually bear relatively long provisional seta1, which are subsequently
thrown off to make way for those of the adult.

Asexual reproduction by simple fission followed by regenera-
tion of the lost segments, or by proliferation followed by fission,
occurs in certain groups of Chaetopoda both among the Oligochaeta
and the Polychseta. Simple fission occurs in Salmacina, one of
the Serpulids : a constriction becomes formed at a certain point
towards the posterior end ; rudiments of a new set of cephalic
branchiae bud out on one side at this point, said this posterior
part becomes a distinct zooid, which is eventually separated off
and develops the full number of segments characteristic of the
adult. This is not in any way a case of alternation of generations,
as both parent and offspring are similar and sexual (hermaphro-
dite). In Nais and Chcetogaster (Oligochaeta) there is multiplica-
tion by proliferation of the segments at the
posterior end ; then the appearance of a con-
striction separating off five or six of the most
posterior segments, followed by a fresh pro-
liferation in front of the constriction, and
this in turn by the appearance of a second
constriction five or six segments further
forwards — the result being the development
of a chain of zooids which remain for a time
connected together. The sexual cells be-
come fully developed only after the zooids
have become separated from one another,
In some of the Syllidae (Errantia) there is
a distinct alternation of generations. Thfc
asexual worm developed from the ovum gives
rise by a process of posterior proliferation and
constriction (Fig. 348) to sexual zooids, a
number of which may remain for a time con-
nected together in a string before becoming
separated. These sexual zooids become de-
veloped into mature males or females, which
may be remarkably unlike the parent form
in the shape of the parapodia, the character
of the setae, and similar points ; and in some
instances the two sexes not only differ from
the asexual parent form but from one another,

so that the three forms, before their relationship was known, have
been set down as representing three distinct genera.

Syllis ramosa, which occurs in the interior of certain .deep-sea
sponges, is exceptional among the Chaetopoda in giving rise by

FIG. 348.— Budding in one

cornutus), parent stock
with a male zooid nearly
ready to become de-
tached. (After Agassiz. )

PHYLUM ANNULATA

447

lateral branching to a colony, from which sexual zooids afterwards
become separated off.

Mode of Life, etc. — Very few Chsetopoda are true parasites ;
but a considerable number are to be set down as commensals
habitually associating with another animal for the sake of food
and shelter. The Earthworms burrow in soil containing decaying
vegetable matter, passing the mould through their intestine and
subsequently throwing it off in the shape of castings on the
surface. They also feed on de-
caying leaves, and sometimes on
animal substances. Some of the
fresh-water Oligochaeta (Tubiji-
cidce) manufacture tubes of mud
held together by a tenaceous secre-
tion from the epidermal unicel-
lular glands. Some of the Errant
Polychseta form temporary tubes
of a gelatinous character, or more
permanent parchment-like tubes
sometimes strengthened by means
of agglutinated sand-grains. But
the majority of the Errantia, which
live for the most part on other
small animals, are not confined to
tubes, but move about freely. Some
burrow in sand ; others even in
harder substances, such as the
shells of Mollusca, or in limestone,
shale, or sandstone. The Tubicola
secrete tubes the substance of
which is derived from the epider-
mal glands. These tubes are some-
times membranaceous or parchment-like, sometimes membran-
aceous but hardened by the deposition of grains of sand or particles
of broken shells or bits of sea-weed ; sometimes (Fig. 349) thev are
of a hard, shelly, calcareous character, sometimes composed entirely
of foreign particles cemented together ; very frequently they are
permanently fixed to foreign objects. Some Tubicola, such as
species of Polydora and Stylarioides, near relatives of which con-
struct tubes, excavate galleries in rock or coral, or in the shells of
various Mollusca. The Tubicola are vegetable-feeders.

A few Polycha3ta, such as the Alciopidce and Tomopteris, as well
as, in a certain phase, the Nereidw and Syllidce, are pelagic, but the
majority live on the sea bottom. They occur in greatest abundance
near the shore ; but are also found at all depths in the ocean, the
Tubicola being more abundant than the Errantia in the deeper
zones.

FIG. 349.— Serpulse with their tubes.
(After Quatrefages,)

448 ZOOLOGY SECT.

Owing to the soft character of most of their parts, there are
comparatively few actual remains of Cha3topoda in the older
geological formations, though there are many burrows and tracks
which have been ascribed to members of that class. Tubes of
tubicolous Polychseta have, however, been found in formations
dating from the Cambrian period onwards. Some tubes not
distinguishable from those of the existing genus, Spirorlis, are
found as far back as the Silurian ; and others, apparently closely
related to the living Serpula, as far back as the Carboniferous.
In addition there are a number of tubes of extinct forms ascribed
to the tubicolous Polychseta. The horny jaws of various Errantia
have been detected in strata from the Cambrian period onwards ;
and many tracks and burrows occurring in rocks of all ages are
ascribed, some with more, some with less certainty, to this group
of worms. No fossil remains of Oligochseta are known.

APPENDIX TO THE CH^ETOPODA.
CLASS MYZOSTOMIDA.

The Myzostomida are a group of worms which appear to have
their nearest relatives in the Chsetopoda, though with certain
special features of their own. They are all external parasites of
various Crinoids, both of the stalked and the free varieties or
internal parasites of certain Starfishes. They are disc-shaped
animals (Fig. 350) devoid of any trace of external segmen-
tation. There are patches of cilia here and there on both
dorsal and ventral surfaces. At the sides there are five pairs of
parapodia (p), each with a chitinous hook and a supporting rod ; in
the intervals between these there are in Myzostoma four pairs of
small suckers ; and round the margin are a series of ten or
more pairs of cirri provided terminally with motionless sensory
eilia, and with a ventral groove lined by adhesive cells. The
mouth, situated at the anterior extremity, leads into a muscular
pharynx (Fig. 351, pli) capable of being protruded as a proboscis ;
from this a narrow oesophagus leads to the stomach, which gives off a
number of branched lateral diverticula (da). A short cloaca (Klo)
leading from the stomach opens on the exterior at the posterior
end of the body. There is no distinct ccelome, the space between
the alimentary canal and the body-wall being filled by connective
tissue (parenchyma), leaving only the cavities in which the sexual
elements are lodged. Bundles of dorso-ventral muscular fibres
form imperfect transverse septa as in some Platyhelminthes.

There is no blood- vascular system, and specialised organs of
respiration are likewise wanting. No nephridia have been de-
tected, The nervous system comprises a large stellate ganglion

PHYLUM ANNULATE

449

situated ventrally, probably representing a number of fused ganglia,
and giving off a number of nerves ; and of two nerve-rings, one
round the oesophagus, the other round the pharynx, the two rings
being connected together by a series of longitudinal nerves : the
cesophageal ring presents a very obscure dorsal thickening, which
is the only representative of a cerebral ganglion.

Most of the Myzostomida are hermaphrodite. There is some
doubt about the position of the ovary, but in the sexually mature

FIG. 350.— Myzostoma. (After von Graff.)

imai branching spaces in the parenchyma between the caeca are
found to be filled with ova (o). A large uterus (u) opens into the
cloaca by three ducts. There are two elongated and usually
branched testes (h), each of which has two vasa deferentia leading
to a vesicula seminalis (sb) which opens near the lateral margins.
A few species are dioecious. In one species, in addition to the
ordinary hermaphrodite individuals, there are much smaller males
—the " complemental males."

VOL. I G G

450

ZOOLOGY

SECT.

The development of the Myzostomida closely resembles that of
the Polychseta. A trochosphere larva is first formed, and this
becomes metamorphosed into a larva with provisional setoe, bearing
a close resemblance to the larva of Nereis (p. 415).

V

^

\

£B»

— 0

M« f

FIG. 351.— Myzostoma. Diagrammatic view of the internal organs, c, cirri ; da, branches of
the stomach ; ed, hind-gut ; h, testes ; Uo, aperture of cloaca ; TO, stomach ; mo, male genital
aperture; o, ovaries; p, parapodia, with hooks and supporting rod ; ph, pharynx; php,
pharyngeal tentacles ; pht, pharyngeal poxich ; sb, vesicula seminalis ; u, uterus ; wo, foma"
genital aperture. (From Lang's Text-book.)

CLASS IL— GEPHYREA.

| The Gepliyrea are marine Annulata devoid of any trace of segment-
ation in the adult condition, without parapodia, and either without
setae or with only a limited .number ; with either an invaginable
anterior body region or introvert, at the extremity of which is the
mouth surrounded by tentacles, or with a long, highly retractile
proboscis representing the praB-oral lobe of the larva, and having
the mouth situated at the base. The anus is sometimes terminal
and posterior, sometimes anterior and dorsal. There is an exten-
sive coelome filled with a corpusculated fluid, and not divided by

PHYLUM ANNULATA

451

septa. The ventral nerve-cord is not made up of a series of

figlia. There is, as a general rule, only a single pair of nephri-
. The sexes are separate ; the ovaries and testes simple masses
cells ; the nephridia act as reproductive ducts. The larva is
Tochosphere.

1. EXAMPLE OF THE CLASS — Sipunculm nudus.

»

General External Features. — Sipunculus occurs on sand at
moderate depths off the coast in most countries outside of the
tropics. It is an elongated worm of a cylindrical shape, somewhat
narrower towards one — the anterior — end. There is no trace of
division into segments. The anterior portion of the body, to the
extent of about a sixth of the total length, is capable of being
involuted within the part be-
hind. The surface of this an-

rior part, which is termed the

trovcrt (Fig. 352), differs in
appearance from that of the
rest of the body in being
covered more or less closely
with chitinous papillae. The
papillae of the posterior portion
of the introvert are shaped
like the bowl of a spoon, with
the concavity turned towards
the body-wall and the tip
directed backwards; they are
so closely arranged as to overlap
one another like the shingles
of the roof of a house : further
back they become longer and
narrower, mammilliform, and
more scattered. When the in-
trovert is fully evaginated,

there appears at its extremity a horseshoe-shaped fold of the
integument, the tentacular fold (tent.), which is lobed and plaited
(Fig. 353) so that it assumes somewhat the appearance of a circlet
of tentacles. For a little space immediately behind the tenta-
cular fold the surface of the introvert is free from papilla?.

The posterior portion of the body is devoid of papillae, but is
marked out by a number of narrow impressed lines into a number
of elongated four-sided areas.

Body-wall. — The surface is covered by a chitinoid cuticle
having an iridescent lustre similar to that presented by the cuticle
of Nereis and Lumbricus, and due to the same cause — viz., the
presence of two systems of intercrossing lines. The papillae on

G G 2

FIG. 352. — Anterior extremity of Sipun-
culus nudus. a,it. pii(i. anterior papil-
lary region ; post. pap. posterior papillary
region; tt nt. tentacular fold. (After Ward.)

452

ZOOLOGY

SECT.

cer.org

FIG. 353.— Tentacular fold of Sipunculus
xmdus. cer. org. cerebral organ. (After
Ward.)

the introvert are local thickenings of this cuticular layer. Beneath
the cuticle is an epidermis consisting of a single layer of cells,
usually sac-like, but capable of being altered as a result of con-
traction or compression into a spindle-like shape. Below the

epidermis is a layer of connective-
tissue, the dermis, in which, as
well as to some extent in the
epidermis itself, are a number of
denned bodies. Of these there
are three kinds — bicellular glands,
contained in papilla ; multicellu-
lar glands, scattered through the
integument, and not contained
in papillae ; and sense-papillae,
small rounded thickenings of the
epidermis in the anterior region
of the introvert, with their sum-
mits covered with cilia. There
are also numerous pigment cells.
Numerous canals branch through
the dermis. Beneath this are
three layers of muscle — (1) an outer circular layer, continuous
in the introvert, but divided into annular bands in the rest of the
body; (2) an oblique layer, well developed only between the
origins of the two retractor muscles of the introvert ; (3) a longi-
tudinal layer, which is separated by spaces into a series of parallel
bands. Between the bundles of the longitudinal layer of muscle
runs a series of canals which communicate with the body-cavity
by transverse branches.

There is a spacious ccelome, but it is traversed in all directions
by filaments and strands of connective-tissue, with which are mixed
very fine muscular fibres ; these mostly run from the wall of the
body to the alimentary canal. Floating in the ccelomic fluid are
(1) colourless corpuscles; (2) reproductive elements; (3) peculiar
ciliated bodies, the urns, which may be parasitic ciliate Protozoa,
but whose nature is obscure.

The blood-vascular system is very feebly developed. It
consists of dorsal and ventral longitudinal sinuses (i.e. channels
differing from true vessels in wanting an internal epithelium),
communicating in front with a circular sinus at the base of the
tentacular fold.

The alimentary canal (Fig. 354) is a cylindrical tube of uniform
character throughout. It is twice the length of .the body, running
back from the mouth towards the posterior end, and then bending
sharply round to run forwards to the anus, the two limbs being
twisted spirally round one another. Eunning along the entire
length of the alimentary canal, with the exception of the terminal

rt or rectum, is a

PHYLUM ANNULATA

453

narrow groove. Twisted round the posterior
part is a long narrow ccecum, which
opens into the beginning of the
rectum. Two groups of rectal
glands occur close to the anal
opening.

The nervous system (Fig. 355)
differs considerably from that of
the rest of the Annulata. There
is a relatively small bilobed cere-
bral ganglion situated on the dorsal
aspect just behind the tentacular
circlet, to which it gives off on
each side several pairs of nerves.
-recb

oes

FIG. 354.— Dissection of the internal organs
of Sipunculus nudus. dors, ret,:
dorsal retractor muscles of the intro-
vert ; int. intestine ; m. n. co. muscles
accompanying the nerve cord ; n. co.
nerve cord ; neph. nephridium ; <*«.
oesophagus ; rect. rectum ; tent, tenta-
cular fold. (After Vogt and Jung.)

FIG. 355.— Anterior part of the nervous system
of Sipunculus nudus. can. o. ccb, cere-
bral organ ; corns, ce, ossophagal connective ;
n.m i'. ret, nerves to retractor muscles ; n. spl.
splanchnic nerves ; n. ta, 1-lt, nerves to ten-
tacular fold ; /, //, nerves from ventral cord.
(After Ward.)

454 ZOOLOGY SECT.

Anteriorly and dorsally it gives off a number of digitate pro-
cesses lying in the coelome. The cesophageal connectives (corns, ce)
which it gives off behind are greatly elongated ; each gives off
muscular nerves (n. mu. ret), and also a visceral nerve (n. spl) pass-
ing to the alimentary canal. The two commissures unite behind
to form a ventral cord, which extends throughout the rest of the
length of the body. The ventral cord presents no appearance of
ganglia : it sends off laterally a large number of pairs of nerves
(I., II.); on section it appears distinctly double. Two delicate
muscular bands (Fig. 354 m. n. co.)} which take orgin anteriorly
from the body -wall, become attached to the nerve-cord, and follow
it throughout its length, giving off small branch bands to accom-
pany the lateral nerves. A canal with folded and pigmented walls,
which opens in the middle line of the dorsal surface just behind
the tentacular fold (Fig. 353 cer. org.\ extends backwards to the
anterior ventral surface of the cerebral ganglion, where it ends
blindly. It is possible that this, the cerebral organ, may be a
sensory organ of some kind. Eyes are entirely absent. The digi-
tate processes of the cerebral ganglion, which bear a number of
ciliated cups along their edges may be sensory in character.

Sipunculus has only a single pair of nephridia which like those
of the majority of the Polychceta are of the character of diplo-
nephridia. These (Fig. 354, ncpli.) are situated tolerably far
forwards, the external openings being about 2 cm. in front of
the anus. They are long, nearly straight tubes, of a brown or
yellowish colour, and very mobile in the living condition. Near
the external opening, which is situated at the anterior end, is the
internal opening into the coelome. The sexes are separate. There
are no definite gonads except at a certain season of the year,
when cellular elevations developed in the connective tissue cover-
ing the ventral retractor muscles of the introvert represent ovaries
or testes as the case may be. These give origin to cells which
become detached and develop into the fully-formed sexual elements
while floating about in the ccelomic fluid. The segmental organs
act as gonoducts.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Gephyrea are Annulata with the body devoid of any ap-
pearance of segmentation in the adult condition. There is a large
ccelome, which is not divided into chambers by mesenteries or
septa. A blood- vascular system is sometimes present, sometimes
absent. The ventral nerve-cord is not composed of a chain of
ganglia. There is usually only one pair of nephridia. The scx< >
are separate, the gonads simple, and the nephridia act as
gonoducts.

PHYLUM ANNULATA 455

The larva is in most cases a typical trochosphere, and may
develop a metameric segmentation which disappears as develop-
ent proceeds.

ORDER 1. — INERMIA.

"

Gephyrea with an introvert, and usually tentacles or a tentacular
fold. The anus is dorsal. Setae are_absent. Nephridia a single
ir, or absent altogether.

ORDER 2. — ARMATA.

Gephyrea with an elongated prostomial proboscis. The anus
is posterior. Two or rn&Ee_set8e. A single nephridium, or two
or three pairs.

Systematic Position of the Example.

Sipunculusnudus is one of several species of the genus Sipunculus.
Sipunculus differs from other genera of the family Sipunculidce,
of which it is a member, mainly in having a tentacular fold around
the mouth, instead of a series of distinct tentacles. The family
Sipunculidce is one of two families of the order Inermia and differs
from the other, the Priapulidce, in the presence of either tentacles
or a tentacular fold at the oral, and the absence of filiform appen-
dages at the aboral end.

3. GENERAL ORGANISATION.

The Gephyrea are a class of worms whose position among the
Annulata is determined more from a consideration of their develop-
ment than of their structure in the adult condition, though the
latter suggests a tolerably close affinity with the Chaetopoda. The
body of a Gephyrean is unsegmented, usually more or less com-
pletely cylindrical, broadest behind and narrowing towards the an-
terior end. The surface is covered with a chitinous cuticle developed
often into papilla, or tubercles, or hooks. In the Armata, setae, are
present, but they are always very few in number and not implanted
in parapodia ; in Bonellia there is only a single pair, situated about
the middle of the ventral surface ; in most species of Echiurus
(Fig, 357), in addition to this ventral pair, there are a number
arranged in one or two circlets around the posterior end. In the
Inermia the anterior part of the body is capable of being invagi-
nated within the part behind ; at the extreme anterior end of this
invaginable part or introvert, when it is evaginated, is the mouth
surrounded by a circlet of sometimes pinnate, sometimes simple,
tentacles, or by a lobed and plaited tentacular fold. The prostomium
is in such forms quite rudimentary. In the Armata there is no
introvert, but an elongated, highly contractile, simple or bifurcated

456

ZOOLOGY

SECT.

proboscis, which is the greatly produced prostomium ; in Bonellia
(Fig. 356) the proboscis, when fully extended, is five or six times
the length of the body : in Echiurus (Fig. 357) it is much shorter :
at the base of the proboscis on the ventral side is the opening of
the mouth. In Priapulus (Fig 358) there is at the posterior end
an elongated simple or bifurcated caudal appendage covered with
hollow papillae. The anus is situated at the posterior end of the
body in the Armata ; in the Inermia it lies far forwards on the

-prob

G/U SZf

FIG. 356.— Bonellia yiridis. Entire
animal (female) with the proboscis
moderately extended. (After Greef.)

FIG. 357. — Echiurus, entire animal.
Ant. set. anterior setse ; po*t. set. pos-
terior setse ; prob. proboscis. (After
Greef.)

dorsal surface, except in the case of Priapulus, in which it is
terminal.

Body- wall. — Beneath the cuticle is an epidermis, which is com-
posed of a single layer of cells. Among the cells are unicellular,
rarely multicellular, glands, and sensory cells. Various colouring
matters, such as the bright green characteristic of Bonellia, are
contained in the cells of the epidermis. The muscular wall of the
body consists of external circular and internal longitudinal layers,
sometimes with oblique and internal circular layers superadded.
There is an extensive undivided ccelome, covered, as in the case of

PHYLUM ANNUL AT A

457

ie Chaetopoda, with a ccelomic epithelium, which is sometimes

Hated.

The alimentary canal in the Incrmia consists of a muscular
pharynx, intestine, and rectum ; in the Sipunculid.ee (Fig. 354) the
intestine is bent on itself, and spirally twisted as it runs forwards
to the anal opening, which, as already noted, is situated far forwards
on the dorsal surface : at the junction
of intestine and rectum is a single simple
caecum or a pair ; and a number of small
branching caeca are connected with the
rectum close to the anal opening. Re-
tractor muscles pass from the body- wall
to the pharynx. In the Armata, (Figs.
359 and 361) there is a thin-walled
buccal cavity, an elongated and coiled
intestine, opening at the posterior ex-
tremity of the body into a dilated rec-
tum : in most there is an elongated
cwcum or siphon applied to the ventral
aspect of the intestine proper. Into
the rectum there open a pair of remark-
able caeca, the posterior nepliridia (Figs.
359 and 3Q\,post. neph.), supposed to be
excretory in function ; these open into
the coelome by means of a number of
ciliated funnels (Fig. 360).

There are no specialised organs of
respiration in the Gephyrea. A blood-
vascular system is sometimes present,
sometimes absent. When present, as it
is in most Gephyrea, it usually com-
prises a contractile dorsal vessel closely
applied to the intestine, and a peri-
pharyngeal ring or plexus. Cilia are
present in places in the interior of the
vessels.

The nervous system (Figs. 355 and
362) consists of a nerve ring, sometimes
greatly elongated, surrounding the an-
terior part of the alimentary canal, with sometimes a dorsal and
anterior thickening representing a cerebral ganglion; and of a
nerve-cord, devoid of ganglia, running backwards from this along
the middle of the ventral surface, and giving off pairs of branches
at regular intervals; the branches of the same pair sometimes
form complete rings (Fig. 362, ne. ri.) by uniting dorsally. Eyes
of a very simple character, consisting of mere spots of pigment,
are present in some of the Inerrnia.

re,sjb

FIG. 358.— Priapulus. entire
animal, rcsp. posterior papillae.
(After Ehlers.)

458

ZOOLOGY

SECT.

oes

neph.fun

ant.neph

ventyess

Priapulus is devoid of nephridia. In the Armata a pair of

appendages of the rectum are, as already mentioned, to be regarded

as posterior nephri-
dia. In addition
there are present
anterior nephridia.
In Bonellia (Fig.
359, ant.neph.), and
in some Inermia,
there is only one.
In the majority of
cases there is one
pair, while in vari-
ous species of Tha-
lassema there are
from one pair to
four. They are
tubes which open
externally on the
ventral surface, and
internally commu-
nicate with the
coelome by means
of ciliated aper-
tures, the form and
position of which
varies in different
cases. They act as
efferent ducts for
the reproductive

elements (gonoducts) ; but their function as excretory organs

has not been definitely established.

, The sexes are usually distinct, and the reproductive organs

are of very simple character, consisting

merely of ridges or clumps of cells (gonads),

sometimes enclosed in a membrane, de-

veloped at various points on the body-wall,

or on the wall of one of the main blood-

vessels. The cells of these ovaries or

testes may develop in situ into perfect ova

or sperms ; more usually they become deT

tached, and undergo the later stages of

their development while floating in the

ccelomic fluid.

A remarkable instance of extreme

sexual dimorphism occurs in Bonellia.

The ordinary large individuals (Fig. 356),

post. neph.

FIG. 350.— Bonellia, general view of the internal organs.
an. anus ; ant. neph. anterior nephridium ; int. intestine ;
neph. fun. nephrostome ; ms.i oesophagus ; or. ovary ; • ph.
pharynx ; post. neph. posterior nephridium ; prol>. proboscis ;
vent. vess. ventral vessel. (After Greef.)

Flo. 360._One of the ciliated
funnels of the posterior

'

PHYLUM ANNULATA

459

to various points in the structure of which reference has been
already made, are females. The single, greatly enlarged
nephridium contains a spacious cavity, which has been termed
the uterus. In the interior of this is found the very small male
(Fig. 363). This is not unlike a Planarian in appearance, com-
pressed and covered with cilia, with a pair of ventral hook-like
seta3. In the interior of the body bundles of dorso-ventral
muscular fibres placed at regular intervals give an appearance
of rudimentary segmentation. The alimentary canal is rudi-

ant.nep

post.neph

FIG. 361.— Echiurus, internal organisation, an,
anus ; ant. neph, anterior nephridia ; int, in-
testine ; int. vess, intestinal vessel ; ces, oeso-
phagus ; post . ncph, posterior nephridia ; rent.
r.;.s.<, ventral vessel. (After Greef.)

FIG. 302. — Echiurus, general out-
line of the animal with the
nervous system (diagrammatic)
ne. co, nerve cord ; ne. ri, nerve
ring. (After Greef.)

mentary and completely closed, both mouth and anus being
absent. There is a pair of nephridia placed posteriorly. The
sperms, developed from modified ccelomic cells, reach the exterior
through a duct, dilated externally into a vesicula seminalis, and
opening internally into the ccelome by a funnel-shaped aperture.
In Hamingia, also, there are imperfectly developed males which
are lodged in the nephridia of the female.

Development. — The larva of Echiurus (Fig. 364) has a well-
developed prse-oral or prostomial lobe with a prae-oral and post-
oral circlet of cilia, and in other respects closely resembles the
trochosphere embryo of a ChaBtopod. The posterior part of the

4GO

ZOOLOGY

SECT.

body elongates, and the mesoblast-bands, developed as in the
Chaetopoda, become divided into as many as fifteen segments. A
circlet of setae is developed at the anal end, and subsequently
the two ventral setae are formed in the same manner as in
the Chaetopoda. The prae-oral lobe becomes narrowed to form the
cylindrical proboscis of the adult ; and the rudimentary segmenta-
tion gradually disappears as development advances.

In Bonellia there is unequal segmentation, as in most Chaetopoda,
resulting in the formation of four large megameres and eight small

micromeres: the latter multiply

.repr.ap rapidly, and grow over the mega-

.,-v.^s. meres so as eventually to enclose

the latter in a complete layer of
ectoderm, save at one point,
where there is a gap, the llasto-
pore. Here the ectoderm bends

coel

-ves.sem
all

J. C(HI

stem

FIG. 3(53.— Male of Bonellia. all,
alimentary-canal; coel, groups of
coelomic cells destined to give
rise to sperms ; repr. ap, repro-
ductive aperture ; res. sem, vesi-
cuLi seminalis. (After Greef.)

Vic.. 304.— Trochospherc of Echiurus.
an. anus; ap. pi. apial plate; int. in-
testine ; mo. mouth ; ne. co. rudiment
of nerve cord; &s. oesophagus; ot«,
••<},>, i. rasophageal connective; .<fo,,t.
stomach. (After Hatschek.)

inwards to give rise to a continuous mesoderm layer super-
ficial to the megameres. The blastopore soon closes up. The
megameres divide to form the cells of the endoderm, among
which a lumen only appears comparatively late ; mouth and
stotnodceum are developed as an outgrowth, at first solid, from
the endoderm. The anus becomes formed still later by in-
vagination at the hinder end of the body ; and a pair of epidermal
vesicles which appear at its sides form the rudiments of the

x PHYLUM ANNULATA 461

posterior nephridia. A rudimentary prse-oral lobe becomes esta-
blished. The mesoderm remains unsegmented, but splits into
.somatic and splanchnic layers going to form the muscular system,
blood-vessels, and other mesodermal organs. Before the alimentary
canal is formed the larva, which had previously been spherical
with two bands of cilia and a pair of eye-spots, becomes elongated
and dorso-ventrally compressed ; and becomes covered uniformly
with cilia so as to present the general appearance of a Planarian.
It becomes converted into the adult female by a metamorphosis,
including the elongation of the prse-oral lobe to form the proboscis,
and the development of the pair of setae of the adult. The
male never goes through this metamorphosis, but remains in the
Planarian stage : it at first adheres to the proboscis of a female,
then enters the oesophagus, and afterwards, when sexually mature,
enters the cavity of the nephridium.

In the Inermia the early stages of the development closely
resemble those of the embryo of one of the PolychaBta, and a
trochosphere is formed, closly resembling the corresponding stage
in the latter class. But at no stage in the development has any
trace been observed of the temporary segmentation which forms so
marked a feature in the development of Echiurus.

Distribution, Affinities, etc. — The Gephyrea are all marine.
They are only capable of very slow creeping locomotion, and live
for the most part either in natural rock-fissures, or in burrows
which they excavate for themselves either in sand or mud, coral or
rock. Their distribution is general ; and they occur at considerable
depths as well as in shallow water.

The differences between the Inermia and the Armata are so
considerable, that there is some doubt whether they ought to be
united together in one class. The Inermia diverge most widely
from the Cha3topoda in the entire absence of setae and in the
absence of segmentation at any stage.

Affinities between Phoronis (p. 328) and the unarmed Gephyrea
have often been supposed to exist, and it has by some zoologists been
proposed to regard Phoronis as an outlying member of that class.
It seems probable, however, that the very manifest resemblances
which undoubtedly exist do not indicate a near relationship, but
are the result of converging modifications of originally widely
different stocks. The most striking of these points of resemblance
are two — (l)the approximation of the anus towards the oral aperture,
and (2) the presence of the tentacular circlet. But a study of the
development shows that these common features are developed in
totally different ways in the two cases. The forward position of
the anus in the Sipunculida is brought about by a gradual displace-
ment resulting from the growth of the aboral region of the body ;
and the invagination and evagination by which the corresponding
result is attained in Phoronis do not occur. Again, while in Phoronis

462

ZOOLOGY

SECT.

the tentacles of the adult may be looked upon as formed by the
development of processes along the line occupied by the post-oral
circlet of cilia, in the Sipunculida the tentacular lobes have
nothing to do with the post-oral circlet, but are formed by the
growth of a series of lobes from the margin of the mouth itself.
The larva of the Sipunculida again is, as already pointed out, very
nearly related to the larva of the Chsetopoda, and is a typical
trochosphere ; while the Actinotrocha larva of Phoronis diverges
somewhat widely from that type.

CLASS III.— ARCHI-ANNELIDA.

More primitive in some respects than the other Annulata are the Archi-
Annelida, comprising only the two families of the Polyyordiidw and the Hl^trio-

i-'ic. 3(15. — Polygordms neapolitanus. A, the living animal, dorsal aspect, about five times
natural size ; B, anterior end, lateral view ; C, ventral view of the same ; D, portion of the
body showing the metameres ; £, ventral view of the posterior extremity ; A ,i. anus ;
An. seg. anal segment ; c. p. ciliated pit ; gr. grooves between metameres ; Mth. mouth ;
Mtmr. metameree ; p. papillae ; pti-.st. peristornium ; pr.st. prostomium ; s. seta? on tentacles (/).
(From Parker's Biology, after Fraipont.)

drilidce. They are all marine and all of small size. The prostomium (Fig. 365,
pr. st) is small, the peristomium (per. st) large. ' The segments (Mtrnr) are only
faintly marked off externally for the most part, though the internal division of

PHYLUM ANNULATA

403

the coelome by means of septa is complete. Parapodia and setae are absent,
but theprostomium bears a pair of tentacles (t). Several pairs of simple nephridia
are present. The position of the nervous system (Fig. 367) is more primitive than
in any of the rest of the Annulata, with the exception of the Archi-Chsetopoda ;
it is continuous with the epidermis, and not separated from it by mesodermal
elements as in the others. A pair of ciliated grooves (c. p.) are probably to be
looked upon as organs of special sense.

The Histriodrilidce are minute worms which live as parasites on the lobster
(Astacus), feeding on its eggs. The body consists of only about eight narrow
segments. The head has sensory tubercles and two short processes having the

function of limbs : a similar but larger pair of
limbs occurs on the last segment of the body.
The alimentary canal consists of oesophagus, in-
testine, and rectum ; the oesophagus contains
three horny jaws. A pair of closely united cere-
bral ganglia are situated in the prostomium, and
are connected behind by cesophageal connectives

D.V

FIG. 366.— Protodrilus, en-
tire animal, int. intestine ;
mu3. re. muscular append-
age of oesophagus ; <v.s. oeso-
phagus. (After Hatschek.)

FIG. 367. — Polygordius neapolitanus, transverse
section of a male specimen. Cod. Epthm. parietal layer
of ccelornic epithelium ; CoeL Epithm.' visceral or
splanchnic layer of the same ; Cu. cuticle ; J)er.
Epithm. deric epithelium ; D. V. dorsal vessel ; Ent.
Epthm. enteric epithelium ; M. PL muscle plates ;
0. M. oblique muscles ; Spy, immature gonads ;
V. Nr. Cd. ventral nerve cord continuous with deric
epithelium; V. V. ventral vessel. (From Parker's
Biology, after Fraipont.)

with a ventral cord, which is dilated into a ganglion in each segment. There
are foar pairs of simple segmental organs in the female, five in the male.
The sexes are separate ; the gonads, whether ovaries or testes, are formed by the
germination of the visceral layer of the peritoneum. Genital ducts are present
in both sexes.

The family Polygordiidce includes two genera — Polygordius and Protodrilus.
There are a pair of prostomial tentacles, long in Protodrilus, short in Polygor-
dius. The segmentation is only very indistinctly marked externally in Proto-
drilus ; in Polygordius it is indistinct in front, but better marked behind. In
Polygordius lacteus a series of tooth-like processes occur round the anus, and in
front a circlet of adhesive papillae. In Protodrilus there is a ventral ciliated

464

ZOOLOGY

SECT

groove. There is a vascular system with dorsal and ventral longitudinal vessels.
In each segment is a pair of simple nephridia. In Protodrilus there are two

Br

FIG. 368.— Trochosphere of Polygordius neapolitanus ^4, lateral view of entire larva ;
B, diagrammatic vertical section ; C, transverse section through the plane ab in B ; An. anus ;
An. ci. anal cilia ; Bl. blastoccele ; Br, apical plate ; Ent. enteron ; Msd. mesoderm ; Msd.
bd. mesodermal bands ; Nph, head-kidney ; Oc, eye-spot ; Pr. or. ci. cilia of prototroch ; Prc.
dm. proctodseum ; Pt. or. ci. post-oral cilia ; St. dm. stomodseum ; V. Nr. C'd. ventral nerve cord.
(From Parkers Biology, partly after Fraipont.)

Mid (ft

V.WtCd
Pr.ftn.ci~
An.ci

FIG. 369.— Later stage in the development of Polygordius neapolitanus, in which the
posterior part of the trochosphere has become elongated and segmented ; A, entire larva ;
B, vertical section ; c, transverse section along the plane ah in B ; D* — J)3. three stages in the
development of the somatic mesoderm ; CW. crelome ; cce.l. Epthm. coalomic epithelium ; Der.
Epthm.d&fic epithelium; M.,,l. muscle plate; M.«l. (.•«>, ,,.\ somatic mesoderm; M*<i. (vy)/\
splnnchnic mesoderm ; 0. eye ; t. tentacle. Other letters as hi preceding figure. (From
Parker's Bioloyii, partly after Fraipont.)

ventral nerve-cords, connected together by transverse commissures : in Polygor-
dius the cord (Fig. 367, V. Nv. cd) is single ; in neither genus is there any trace

;

PHYLUM ANNULATA 465

ganglia. The sexes are united in Protodrilus, ovaries occurring in all the first
seven segments, and testes in some of those immediately following. In Poly-
gordius the sexes are separate ; the ovaries or testes (Fig. 367, spy] are developed
in the posterior segments. There are no special reproductive ducts.

The larva of Polygordius is a typical trochosphere (Fig. 368), and its meta-
morphosis into the adult worm (Fig. 369) takes places as in the Polychseta in all
itial respects.

CLASS IV.— HIRUDINEA.

EXAMPLE OF THE CLASS — The Medicinal Leech (Hirudo medici-
nalis and H. quinquestriata).

The medicinal Leech is found in ponds, swamps, and slowly
lowing streams in many parts of the world. H. medicinalis is the
common British species : H. quinquestriata is an allied Australian
form.

External Characters. — It is a vermiform animal, some
6-10 cm. (2-3 inches) in length, but is capable of contracting and
elongating itself so as to produce great alterations in form and
proportion. It moves by "looping" movements, and is also
good swimmer. The body (Fig. 370) is depressed or flattened dorso-
ventrally, the dorsal surface convex, the ventral flattened. The
anterior end presents a ventrally directed cup-like hollow, the
anterior sucker (a. &), in the middle of which is a small aperture,
the mouth (mth.). The hinder end bears a disc-like posterior sucker
(p. s.), also directed downwards, and at its junction with the trunk,
on the dorsal surface, is the very small median anus (a.). The
animal is brightly coloured, the dorsal surface in H. medicinalis
being longitudinally banded with alternate stripes of greenish grey
and rusty red, the ventral surface greenish yellow, spotted with
black : in H. quinquestriata the whole under-surface is rust-
coloured.

The whole body is encircled by close-set transverse grooves,
dividing it into annuli. These, like the annuli of some Earthworms,
are more numerous than the true segments or metameres, the
study of the internal organs showing that, except at the two extre-
mities, each segment contains five annuli. There are also external
characters by which the actual segmentation is plainly indicated.
The rust-coloured streaks on the back of If. medicinalis are spotted
with black, and at every fifth annulus the spots are larger than on
the intervening rings : the annuli thus marked are the fifth or last
of their respective segments. Moreover, the same rings bear on
the ventral surface minute paired apertures, the nephridiopores or
excretory apertures (n.p. 1-17): of these there are altogether
seventeen pairs, marking the fifth rings of the sixth to the twenty-
second segments.

VOL. I H H

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FIG. 370.— Hirudomedicinalis. A, dorsal ; B, ventral aspect ; a. anus ; «. «. anterior sucker ;
e. 1. first pair of eyes ; e. 5. fifth pair ; g. p. $ . male gonopore ; g. p. $ . female gonopore ;
•nit/,, mouth; n. p. 1. first pair of nephridiopores ; n. p. 17. seventeenth pair; p. s, posterior
sucker ; s. p. sensory papillae ; I— XXVI. segments. (Partly after Whitman.)

: PHYLUM ANNULATA 467

In front of the first and behind the last pair of nephridiopores
one important external mark of segmentation fails, but a further
indication is furnished by the presence on the first ring of each
undoubted metamere of a number of delicate transparent elevations,
the segmental papillce (s. p.), which have probably a sensory func-
tion. These structures are found along the whole length of the
body, and as they mark the first ring of all those segments the
extent of which can be checked by the nephridiopores, it is legiti-
mate to assume their segmental value in the anterior and posterior
regions, where the controlling excretory apertures are absent. By
the clue thus furnished it is found that there are five segments in
front of that bearing the first pair of nephridiopores, and four
behind that bearing the last pair, making a total of twenty-six
metameres : of these the first six and the last four have less than
the normal number of rings.

The anterior sucker bears on its dorsal surface five pairs of small
black spots, the eyes (e. 1, e. 5), the arrangement of which shows
them to be special modifications of sensory papillae, since they
occupy in the first five segments the precise position occupied in
the sixth and following segments by segmental papillae.

The perfectly definite and comparatively small number of
metameres in the leech offers a striking point of contrast with
what we have met with in the Cha3topoda, and is to be looked
upon as a mark of higher differentiation.

Body-wall. — The body is covered externally by a thin cuticle
(Fig. 371, cu.), which is constantly being cast off in patches and
renewed. Beneath it is an epidermis (d. ep.) consisting of hammer-
shaped cells, separated at their inner ends by spaces in which
blood capillaries run. The blood is thus brought into close relation
with the surrounding water, and the skin becomes a highly effi-
cient respiratory organ. The space between the epidermis and
the enteric canal is filled by a peculiar form of connective-tissue,
consisting of a gelatinous matrix with interspersed cells and fibres.
Many of the cells are large, branched, and abundantly pigmented,
and have their branches directly connected with the smaller blood-
vessels. Surrounding the enteric canal is the peculiar and charac-
teristic botryoidal tissue (b. t.) consisting of branched blood-vessels,
the walls of which are formed of large cells loaded with black
pigment.

Numerous unicellular glands are produced from the epidermis :
the gland-cells themselves lie in the connective-tissue, and are con-
tinued into long ducts which open on the surface. Special glands
in the ninth, tenth, and eleventh segments secrete the substance
from which the cocoon is formed (vide infra, p. 474) : the segments
in question therefore constitute the clitellum.

The muscular system is well developed, and consists of
outer layer of circular (c. in.} and an inner of longitudinal (l.m.)

H H 2

468

ZOOLOGY

SECT.

fibres. There are also dorso-ventral fibres (d. v. m.) passing vertically
between the pouches of the crop (vide infra), and radial fibres ex-

d.s
i-m,

.d.v.m.

b.t

njbh

v.d

nat n.svs'

t *-.&

FIG. 371.— HirudO medicinalis ; Transverse section. 1. t. botryoidal tissue ; c. m. circular
muscles ; cr. crop ; cr'. diverticula of crop ; cu. cuticle ; d. ep. epidermis ; d. s. dorsal sinus ;
d. v. m. dorsal- ventral muscles ; I. m. longitudinal muscles ; I. r. lateral vessel ; n. c. nerve-
cord ; nph. 1 — A, nephridium ; n. s. nephrostomial sinus ; nst. iiephrostome ; ts. testis ; r. d.
vas deferens ; vs, vesicle of nephridium ; r. s. ventral sinus. (After Marshall and Hurst.)

tending from the wall of the enteric canal to the integument :

these take the place of the septa of Chsetopods.

The alimentary organs are greatly modified in accordance

with the blood-sucking habits of the animal. Surrounding the
mouth are three jaws, one median and
dorsal (Fig. 374, d. /.), the other two
ventro-lateral (v. /./.). Each (Fig. 372)
has the form of a compressed muscular
cushion, with a sharp, evenly curved, free
edge covered with chitin, which is pro-
duced into numerous serrations or teeth :
by means of its muscles each jaw can
be moved backwards and forwards through
a certain arc, and the three, acting to-
gether, produce the characteristic triradi-
ate bite in the skin of the animal upon
which the Leech preys.

The mouth leads into a muscular

FIG. 372.— one of the jaws of pharynx (Figs. 373 arid 374, ph.) situated
(Aflte?L?uc?aer?.lcinalis< in the fourth to the seventh segments.

PHYLUM ANNULATA

,diating muscles pass
from its walls to the
integument, and by
their contraction dilate
its cavity and suck
in blood from the
wounds made by the
jaws. Around the
pharynx are numer-
ous unicellular salivary
glands., which open
close to the mouth :
their secretion has the
effect of preventing
the coagulation of the
blood taken as food.
The pharynx com-
unicates by a very
small aperture with
the second and largest
division of the enteric
canal, the huge crop
(cr.), a thin- walled
tube extending from
the eighth to the
eighteenth segment,
and produced into
eleven pairs of lateral
pouches (cr. 1, cr. 11),
the first ten of which
are directed outwards
and correspond each
to a segment, while
the eleventh (cr. 11)
passes directly back-
wards as far as the
twenty - fourth seg-
ment. The crop is
capable of great dila-
tation, and its form
varies greatly accord-
ing to whether it is
empty or gorged with
blood. Posteriorly the
crop communicates by
a minute aperture with
the stomach (st.), a

ph.

crff.

cr.ff-

ZOOLOGY

SECT.

II

.31

tubular chamber with a dilated an-
terior end, and having its wall pro-
duced internally into a spiral fold :
this is the digestive portion of the
canal; the blood is passed into it
from the crop with extreme slow-
ness, and undergoes an immediate
change, its colour turning from red
to green. The digestion of a whole
cropful of blood takes many months.
The stomach is continued into a
narrow intestine (int.} : this passes
into a somewhat dilated rectum (ret.)
which turns slightly upwards and
opens by the anus (an.) in the last
annulus.

The excretory system consists
of seventeen pairs of ectonephridia
(nph. 1-17), situated in segments
6-22. A typical nephridium (Fig.
375) has the general form of a loop
passing upwards from the ventral
body- wall, produced into an offshoot
which extends inwards (mesially) to
the corresponding testis, and con-
nected posteriorly with a small blad-
der or vesicle (vs.). The principal loop
is divisible into two chief parts, the
main lobe (m. I.) and the apical lobe
(a. I), connected with one another
by a short recurrent lobe (r. I.) : the
offshoot to the testis is known as
the testis-lobe (t. I.) ; it is absent in
H. quinquestriata

All these parts are formed of a
close-set mass of gland-cells, tra-
versed by a complex system of
minute intra-cellular passages or
ductules, which finally unite into
comparatively wide inter-celluk
tube or duct : this winds througl
the main and apical lobes, and final b
enters the vesicle, which opens pos
teriorly in the last annulus of th(
segment. The free end of the testis-
lobe is swollen into a lobed im
which lies in a blood sinus (Fij

PHYLUM ANNULATA

471

371, nst) in connection with the testis: comparison with other
Hirudinea shows that this dilated end of the nephridium re-
presents a nephrostome which has lost its open funnel-like end
in correlation with the absence of a distinct coelome. The cells
of the botryoidal tissue appear to carry waste matter to the
nephridia.

There is a complex vascular system, containing, like that of
the Earthworm, red blood, the plasma coloured with haemoglobin
and containing sparsely distributed colourless corpuscles. But a

Diking difference from the preceding annulate types is found in

FIG. 375.— Xephridium of Hirudo medicinalis. a. 1. apical lobe ; m. I. middle lobe :
np. iiephridiopore ; nst. nephrostome ; r. 1. recurrent lobe ; t. I. testis lobe ; vs. vesicle
vs. d. vesicle duct. (After Bourne.)

the fact that the blood-containing spaces are of two kinds — blood-
vessels proper, having muscular walls ; and blood-sinuses, the walls
of which are devoid of muscle.

The two principal blood-vessels are lateral in position (Figs.
373 and 376, /. v.\ running fore and aft at the level of the middle
of the nephridia and uniting with one another at the anterior and
posterior ends of the body. They send off branches both dorsally
and ventrally, some of which anastomose with one another. The
ultimate branches break up into capillaries in the integument,
nephridia, &c.

The two principal sinuses are respectively dorsal (d. s.) and
ventral (v. s.), the former lying just above the enteric canal in the
middle dorsal line, the latter occupying a similar position on the

472

ZOOLOGY

SECT.

ventral side, and enclosing the ventral nerve-cord. The two sinuses
are in connection with one another posteriorly, and are also in com-
munication, by means of their branches, with the capillaries of the
skin. There is thus an indirect connection, by means of capil-
laries, between the blood-vessels and the sinuses, but no direct
communication exists. The sinuses in which the nephrostomes
are lodged open into the ventral sinus. As we shall see more par-
ticularly in the general account of the class, the sinuses represent
a greatly reduced ccelome.

The nervous system is of the usual annulate type. There is
a small brain (Figs. 373 and 374, &r.) situated above the anterior
end of the pharynx immediately behind the median dorsal jaw.

FIG. 376. — Diagram of principal blood-channels of Leech, d. s. dorsal sinus ; 1. v. lateral vessel ;
v. s. ventral sinus containing nerve-cord.

It is connected by a very short pair of oesophageal connectives
with the ventral nerve-cord, which consists of twenty-three well-
marked rounded ganglia (gn. 1-23) united by delicate double
connectives. The first, or sub-cesophageal ganglion is larger
than the others, and is shown by development to be made up of
five united pairs of embryonic ganglia : the last ganglion is also of
unusual size, and results from the fusion of six pairs of ganglia
distinct in the embryo. The whole ventral nerve-cord is contained
in the ventral sinus. Nerves are given off from the ganglia, but
not, as in the Earthworm, from the connectives, in which also,
nerve-cells are wholly absent.

The principal sense-organs are the eyes, of which there are five
pairs (Fig. 377) situated round the margin of the anterior sucker,
on the dorsal side, one pair in each of the first five segments.
They occupy positions taken in the succeeding segments by lateral

PHYLUM ANNULATA

473

UJ.J

i

sh

al

st_
se£

Sr

sense-organs, with which they are obviously homologous. The

structure of the eyes is peculiar : they are cylindrical in form

(Fig. 377), the long axis of the cylinder being at right angles to

the surface of the body. The outer

layer is formed of black pigmented

tissue {pi.), surrounding a layer of

large, clear, refractive cells {p.),

which occupy the greater part of the

organ. A nerve {n.) enters at one

side, and is continued up the axis of

the cylinder by a row of sensory cells.

The margin of the anterior sucker

so bears a large number of gobld-
shaped organs, which are very prob-
ably organs of taste. The minute
structure both of these and of the
segmental sense-organs is very simi-
to that of the eyes. The function

the segmental sense-organs is un-
known.

Reproductive Organs. — The
Leech is monoecious. There are nine
pairs of testes (Figs. 373 and 374, ts.),
in the form of small spherical sacs,
situated in segments 12-20. Each
gives off from its outer surface a
narrow efferent duct, which opens into
a common vas deferens {v. d.). In the
tenth segment the vas deferens in-
creases in width and forms a complex coil, the vesicula seminalis
(v. sem.), from which is continued anteriorly a somewhat dilated
muscular tube, the ductus ejaculatorius {d. ej.). From each ejacula-
tory duct a narrow tube passes to the base of thepe?ws {p.), a curved
eversible muscular organ which opens on the ventral surface
of the second annulus of the tenth segment, in the middle line.
The base of the penis is surrounded by a number of unicellular
glands, which constitute the prostate and secrete a substance by
which the sperms are aggregated into masses called spermatophores.

The ovaries are coiled filamentous bodies, each enclosed in a
small globular ovarian sac {ov. s.), situated in the eleventh segment.
From each ova.rian sac a short oviduct passes inwards and back-
wards, and unites with its fellow into a median duct, the walls of
which are supplied with albumen-secreting gland-cells. The
common oviduct opens into a curved muscular tube, the vagina {va.),
which opens in the middle line on the ventral surface of the
second annulus of the eleventh segment, i.e. one segment behind
the male aperture.

FIG. 377.— Section of eye of Leech
c. cuticle ; d 3, gland-cells ; cp. epi-
dermis ; g, nerve-cells ; n . nerve ;
p, refractive cells ; pi. pigment.
(From Lang's Compamtict Anat-
omy.)

474 ZOOLOGY SECT.

It will be noticed that the ovaries of the Leech form a single
pair, while the testes are multiple and segmental : also that, while
the gonads and efferent ducts of both sexes are paired, the penis
and the vagina are median and unpaired. In the latter respect
the contrast between the Leech and the Annulata previously dis-
cussed is very striking. Further important peculiarities are the
enclosure of the ovary in a sac from which a duct leads directly to
the exterior, and the fact that the testes are hollow sacs discharg-
ing the sperms into a cavity from which they pass directly to the
efferent ducts. In Chsetopods, it will be remembered, the gonods
lie freely in the ccelome, their products — ova or sperms — are dis-
charged from their external surfaces and carried off either by
ordinary nephridia or by nephridia specially modified into gono-
jjucts. It seems tolerably certain that the cavities both of the
Ovarian sacs and of the testes represent shut-off portions of an
almost obsolete ccelome, and that their ducts are meso-nephridia.
Development. — When breeding two Leeches copulate^ and one
impregnates the other by passing spermatophore^ through its

penis into the vagina, Simultaneous
mutual impregnation has also been
described. The clitellar segments
(ninth to eleventh) secrete a cocoon
(Fig. 378), into which spermatophores,
ova, and a quantity of albumen, se-
creted by the albumen-glands, are

FIG. s-s.-The cocoon of mrudo. passed. The animal then withdraws
^entire ; B, in section. (After ^s head from the cocoon, the two ends

of which close up by their own elas-
ticity, producing a closed capsule in

which embryonic development takes place. Segmentation is
unequal, and results in the formation of a globular embryo, which,
after hatching, swims about in the cocoon, actively devouring
its albuminous contents, and finally escaping in a form closely
resembling the adult.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Hirudinea are Annulata in which the body consists of a
limited and definite number of segments, and is marked externally
by secondary rings or annuli, a variable number of which go to
a segment. The anterior end of the body is suctorial, and several
of the hindmost segments are fused to form a powerful sucking
disc, which is directed downwards and backwards. The mouth
lies in the anterior sucker, the anus is usually dorsal and imme-
diately in front of the posterior sucker. The ccelome is always
more or less obliterated by connective-tissue, and is represented
by sinuses of varying dimensions which contain blood. True

PHYLUM ANNULATA

475

l-vessels, with muscular walls, are also present. The nervous
system consists of a brain united by short oesophageal connectives
to a ganglionated ventral nerve-cord. The excretory organs are
segmentally arranged nephridia. The sexes are united, the
testes numerous and usually segmentally arranged, the ovaries
a single pair. The testes have the form of sacs, and discharge
their products internally ; the ovaries either have a similar struc-
ture or are band-like and enclosed in ovarian sacs into which the
ova are set free. The penis and the vagina are unpaired, and open
by median apertures, the male anterior to the female, on the
ventral surface of the body. Development is usually direct, i.e.
unaccompanied by a metamorphosis. Leeches are either free-
living or are permanently or intermittently parasitic : they inhabit
either the land, fresh-water, or the sea.

The class is divided into the following two orders :—

ORDER 1. — RHYNCHOBDELLIDA.

Hirudinea in which the anterior part of the body can be pro-
truded and retracted so as to form a proboscis* or introvert.

fir

S.CIejasine

S.Branc hellion

I.Ponfobdella

FIG. 379. —Three Khynchobdellida. l>r. gills ; pr. everted proboscis. (1, after Bourne, 2 and 3,

after CuvierJ

476 ZOOLOGY

SECT.

This order includes Clepsine, parasitic on Snails, Frogs, &c. :
Piscicola, on fresh-water Fishes ; Pontobdella and Branchdlion, on
marine Fishes (Fig. 379).

•
ORDER 2. — GNATHOBDELLIDA.

Hirudinea in which there is no proboscis : the mouth is usually
provided with three toothed jaws.

This order includes Hirudo, the common Leech, parasitic on Ver-
tebrata ; Aulostoma, the Horse-leech, free-living and carnivorous ;
Trocketa, of subterranean habits ; Hcemadipsa, the Land-Leech.

Systematic Position of the ^Example.

Hirudo belongs to the family Hirudinidce, of the order Gnatho-
bdellida.

The absence of a proboscis places it in the order GnatTiobdellida :
the possession of ten eyes, and the presence of five rings to all the
segments except a few at the anterior and posterior ends, dis-
tinguishes it as a member of the family Hirudinidse : the genus
Hirudo is distinguished by the constant presence of twenty-six
segments and of 102 annuli.

3. GENERAL ORGANISATION.

In the essential features of their organisation the Leeches are a
very uniform group : there are, however, a few interesting modifi-
cations of structure which must be referred to.

Form and Size. — Most kinds do not exceed a few centimetres
in length, but the American species Macroldella valdiwana is said
to attain a length of 76 cm. (2-J- feet). The number of annuli to
a segment varies from three to five, but the general form of the
body is remarkably uniform, the external differences between
various species depending largely on colour and on the develop-
ment of papillae, which in some cases are large and prominent.

The proboscis (Fig. 380), the possession of which is distinctive
of the Rhynchobdellida, is simply the retractile anterior end of the
body, which, by the action of special muscles, can be drawn back
into a temporary sheath. The organ is thus an introvert, like that
of Gephyrea Inermia.

The chief differences in the structure of the enteric canal
depend upon the varying number, or, in some cases, the total
absence, of lateral pouches to the crop ; for instance, the horse-
leech has only a single pair, corresponding to the eleventh pair in
Hirudo, while Nephelis has none at all. In the Rhynchobdellida
there is a distinct slender gullet (Fig. 380, gul) leading from the
pharynx to the crop (cr.), and thrown into a coil when the pro-
boscis is retracted. Among the Gnathobdellida the median jaw

PHYLUM ANNULATA

477

-nvth

is absent in some land-leeches, and in other species all three jaws
are rudimentary or absent.

The varying development of the blood-vessels and sinuses
presents many points of interest tending to explain the condition
of things described above in the medicinal leech. In the latter,
as we have seen, there are lateral vessels with contractile muscular
walls, and dorsal and ventral sinuses with non-don tractile walls.
In Pontobdella, one of the Rhynchobdellida, there are dorsal
(Fig. 381, 2, d.v.) and ventral (v.v.) as well as lateral vessels, and
lateral (l.s.) as well as dorsal and ventral sinuses, and in each case
the vessel is enclosed in the corresponding sinus. The ventral
sinus (v.s.) also con-
tains the nerve-cord
(n.c.) and the ovaries
(ov.)t and offshoots
of it surround the
testes (ts.') and the
nephrostomes (nst.).
This arrangement
clearly suggests the
partial obliteration,
by growths of con-
nective tissue, of an
originally continu-
ous coelome. Another
interesting condi-
tion occurs in Ne-
phelis (3), in which
the middle region
of the body contains
a series of paired,
metamerically ar-
ranged spaces (&),
surrounded by bo-

tryoidal tissue, and containing the nephrostomes. Development
seems to show that these cavities are derived from true ccelomic
spaces in the embryo, formed, as in Cha3topoda, by a splitting of
the mesoderm in each segment.

In most instances the skin, with its abundant supply of capil-
laries, constitutes the only respiratory organ, but in Branchellion
(Fig. 379, 3) a Rhynchobdellid parasitic on the Electric Rays
(Torpedo and Hypnos) and on one of the Australasian Skates (Baja
nasuta), differentiated respiratory organs or gills (br.) are present
in the form of delicate lateral outgrowths of the segments.

In most members of the class the nephridia are formed on
the same general type as those of Hirudo, but differ in the
structure of the nephrostomes,, which may be ordinary ciliated

cr

FIG. 380.— Proboscis of Clepsine. A, retracted ; B, everted ;
cr. crop ; gul. gullet ; mth. mouth ; pr, introvert ; s. gl.
salivary glands. (After Bourne.)

478

ZOOLOGY

SECT.

l.v

funnels, or may be more or less degenerate, as in Hirudo. In the
Rhynchobdellid Pontobdella a very interesting modification of the
nephridial system occurs. Instead of distinct nephridia, there is

found on the ventral
surface of the body a
very complex network
(Fig. 382, nph.), which
sends off on each side
of each segment a
short branch termin-
ating in a nephro-
stome (nst.), and a
similar branch which
opens externally (np.).
This arrangement re-
minds us of the ex-
cretory system of cer-
tain Earthworms and
of Turbellaria.

The nervous sys-
tem always closely
resembles that of
Hirudo, as also do the
sense-organs. The
number of eyes is sub-
ject to considerable
variation : they may
be developed on the
posterior sucker, or
may be absent alto-
gether.

Reproductive
Organs. — The testes
usually have the seg-
mental arrangement
found in Hirudo, their
number varying from
five to twelve pairs.
But in Nephelis they
are very numerous,
and are not arranged
segmentally. In the
Rhynchobclellida the

muscular penis is absent, its place being taken by an eversible
sac or bursa copulatrix. The form of the ovary with its
containing sac in Hirudo is exceptional. As a rule, there is
an elongated hollow ovary, producing ova from its epithelial

- nph

a.Ponfobdella

Fio. 381.— Transverse sections of three Leeches ; diagram-
matic, c. ccelomic spaces ; cr. crop ; d. s. dorsal sinus ;
d. v. dorsal vessel ; 1. s. lateral sinus ; I. -c. lateral vessel ;
n. c. nerve-cord ; neph. nephridium ; nst. nephrostome ;
ov. ovary ; ts. testis ; r. s. ventral sinus ; v. r. ventral
vessel. (After Bourne.)

-OAt/3

f ))

PHYLUM ANNULATA 479

ling, and thus agreeing very closely in structure with the
testis.

In Clepsine, a fresh-water Rhynchobdellid, copulation in the
ordinary sense of the word has
never been observed, but one indi-
vidual has been seen to deposit one
or more spermatophores on any part
of the body of another — often on
the back. The spermatophore, which
is nearly 3J mm. long, apparently
exerts a solvent action on the skin,
since, after a short interval, the
spermatic fluid streams through the
skin into the coelomic spaces, prob-
ably making its way at last to the
ovaries. This extraordinary process
of hypodermic impregnation prob-
ably takes place in other genera,
but has been most closely followed
in Clepsine.

It is in Clepsine that the early stages of development are
best known. Segmentation is unequal, the embryo consisting, in
the eight-celled stage (Fig. 383, A), of four large ventrally placed
megameres (mg.) and four dorsal micromeres (mi.). One of the
legameres, posterior in position, divides into two cells (B) ;.

FIG. 382.— Nephridial system of Pon-
tobdella. gn. U, gn. 17, ganglia
of nerve-cord ; np. nephridiopore ;.
nph. nephridial network ; nst. ne-
phrostome. (After Bourne.)

FIG. 383. — Six stages in the development of Clepsine. g. b. germinal bands ; mg. "megameres ;:
7/ii. micromeres ; mth. mouth. (After Whitman.)

one of these divides again, and its products of division give rise
to paired germinal lands (C, g.b.), like those of the Earthworm.
The micromeres increase in number, forming a cap of cells, the
commencement of the ectoderm, on the dorsal pole of the embryo,

480 ZOOLOGY SECT.

.and at the same time the germinal bands grow forwards (D),
diverging as they go, and take up a position beneath the
margin of the ectoderm-cap : as the latter extends its area
they converge anteriorly, and thus furnish it with a thickened
margin, the bands themselves being in contact in front and
behind, and divergent in the intervening region, so as to
enclose a nearly circular space. The ectoderm-cap, accompanied
by the germinal bands, now grows over the megameres, finally
enclosing them completely by uniting on the ventral surface.
This process is obviously one of epibolic gastrulation.

The ectoderm of the embryo gives rise, as usual, to the epidermis
of the adult as well as to the stomodseum (E, F, mth.) and procto-
dseum. From the germinal bands are formed the ventral nerve-cord
and the nephridia. The endoderm arises from small cells budded off
from the megameres, which gradually grow round what is left of
the latter. The remains of the megameres thus become enclosed
in the enteron of the embryo and undergo gradual absorption,
serving simply as food, and not giving rise to any part of the
tissues. The cocoon contains no albumen, and the yolk of the
megameres supplies the whole of the nutriment required by the
embryo up to the time it leaves the cocoon. The young is
hatched at a comparatively advanced state of development, and,
.after escaping from the cocoon, adheres by its suckers to the body
of the parent.

In the Gnathobdellida the young are hatched at an early stage
of development, and their megameres contain but little yolk :
they are nourished up to the time of leaving the cocoon on the
albumen with which the latter is filled. One member of this
order, Nephelis, is remarkable for undergoing a metamorphosis :
the anterior end of the embryo is ciliated, and it possesses a
provisional pharynx and several pairs of provisional nephridia.
Paired masses of cells, the head-germs, are developed in the head,
and from these and the germinal bands the whole body of the
adult is produced, the greater part of the larval body being cast
off. This process closely resembles the development of the
Pilidium larva of certain Nemerteans (p. 273).

Habits, Distribution, &c. — The majority of the Hirudinea
are inhabitants of fresh-water and live, like the Medicinal Leech.
by sucking the blood of higher animals — Vertebrates or Molluscs.
It is doubtless in correlation with this intermittent parasitism — the
chance of finding a vertebrate host being an infrequent one — that
the crop has attained such vast dimensions, holding, in the case of
the medicinal leech, as much blood as takes it. a year to digest:
The allied species Hirudo sanguisuga has been found in the
nasal passages of man, producing serious results, and being, to all
intents and purposes, an internal parasite. The same is the case
with the horse-leech, Hcemopsis vorax, taken in, when young, by

PHYLUM ANNULATA 481

>rses and cattle while drinking. It attaches itself to the pharynx,
and may even descend the trachea. Others are permanent ecto-
parasites : for instance, Branchellion is found on the outer surface
of the Skate, Electric Ray, and other Fishes, entire families of the
leech, including individuals of all sizes, being sometimes found
crowded together on a small area of skin, which is distinctly
marked by their powerful posterior suckers. Other fish-parasites
are Pontobdella, on Rays, and Piscicola, on fresh-water Fish.
•Aulostcma, to which, as well as to Efemopsis, the name Horse-
leech is applied, is carnivorous, feeding on snails and other
Mollusca ; so also are Clepsine, Nephelis, and the gigantic Macro-
bdella. The last-named genus and some others are of subter-
ranean habits, living in moist earth. The Land-leeches (ITcemadipsa)
live in the forests of many parts of the world, and in spite of
their small size, which does not exceed 30 mm. in length and
5 mm. in diameter, are much dreaded for the persistent attacks
they make on men and cattle.

Many genera are very widely distributed : for instance, the
Land-leeches (Hsemadipsa) occur in India, Ceylon, the East Indies,
Japan, Australia, and South America, a distribution which seems
to indicate that the group is one of great antiquity. Hitherto no
member of the class has been found in New Zealand, with the
exception of the marine Branchellion.

RA special feature of the Annulata, as distinguished from the
yla previously dealt with, is metameric segmentation. In some
of the Platyhelminthes, as we have seen, there obtains a con-
dition to which the term pseudo-metamerism is applied. In such
cases there is a serial repetition of certain of the organs — gonads,
diverticula of the intestine, nerve-commissures, &c. — in such a way
as to produce a jointed appearance, though the body is not
divided into definite segments. An appearance resembling seg-
mentation is produced also in certain Rhabdocceles that multiply
by budding, chains of zooids remaining connected together
for a time. In the strobila of the Cestodes we recognise a con-
dition which might be described as combining pseudo-metamerism
with the formation of a chain of zooids. The condition of true
metamerism, as we observe it in the Annulata, is capable of being
deduced from a condition of pseudo-metamerism as it occurs in
G'unda (p. 241), the pseudo-metameres becoming converted into
true metameres by the development of inter-segmental constric-
tions and the completion of internal partitions. On the other
hand, it is deducible from the condition of a linear colony of
zooids proliferating at the posterior end, the zooids, though

VOL. I I I

GENERAL REMARKS ON THE ANNULATA.

482

ZOOLOGY

SECT.

becoming each complete in itself, not, under ordinary circum-
stances, becoming detached. The establishment of a closer
connection between the various organs of such a colony with the
special differentiation of the anterior end would result in a con-
dition closely resembling the metamerism of the Annulata. It is
conceivable that a condition of pseudo-metamerism was followed
by that of a linear series, not of zooids, but of comparatively
independent parts capable of readily reproducing the animal
when detached by accidental injury, and that a secondary closer
connection established between the organs of all the series of parts
resulted in the metameric condition.

Metamerism is not universal in the phylum. In some (Archi-
Annelida)it maybe said to be incipient or rudimentary ; in others

FIG. 384.— Diagram to illustrate possible relations of the unsegmented to the metamerlcally
segmented worm. A, unsegmented worm with differentiated head end ; B, pseudo-meta-
merism ; C, linear series of zooids in which the first zooid differs in character from the others,
and in which the formation of new zooids takes place at the posterior end ; D, metanierically
segmented worm.

(Gephyrea) vanishing or vestigial. The Archi- Annelida are in this,
as in some other respects, the most primitive of the Annulata, and
through them it seems possible to connect the higher members of
the phylum with such lower forms as Dinophilus (p. 310). The
general occurrence of the trochosphere larva may be taken as
pointing to descent from an unsegmented ancestor having re-
semblances to the trochosphere, and a form like Dinophilus would
afford us an intermediate link between such a hypothetical ances-
tor and Polygordius or Protodrilus.

The position of the unarmed Gephyrea in the Annulata is, as
already noticed, a matter of doubt ; if we dissociate them from
the Armata there is little to connect them positively with the
other members of the phylum. But, on the whole, perhaps the
evidence in favour of regarding them as allied to the Armata,
and through them with the Chaetopoda, is sufficiently strong.

PHYLUM ANNULATA

483

In adult structure, particularly in the absence of parapodia and
setae and of a coelome, the Hirudinea diverge somewhat widely
from the Chaetopoda ; but a study of their earlier developmental
stages shows unmistakably their close connection with the latter
group, more particularly with the Oligochseta.

The following diagram will serve to illustrate this view of the
relationships of the various groups referred to : —

Polychaeha

Myzosfomida
Gefshyr

Oligochaefa

Hirudinea

Arch i -Annelida

Dinofihilea

Gasfrc^richa

Trocho(ohore

FIG. 385 —Diagram illustrating the relationships of the Annulata and the Trochcliuinth

I I 2

SECTION XI
* PHYLUM ARTHROPODA

IN this large and important group of animals we meet with
a characteristic feature of the Chsetopoda, viz. metameric seg-
mentation, as also with more or less perfect bilateral symmetry,
mouth and anus at opposite ends of the elongated body, and a
nervous system formed of a dorsal brain, and a double ventral
chain of ganglia. There is, however, an important advance on the
segmented Worms in the circumstance that each typical segment
bears a pair of appendages, distinguished from the simple foot-
stumps or parapodia of the Polychseta in being divisible into
distinct limb-segments or podomcres, separated from one another by
movable joints and acted upon by special muscles. Arthropods
are also characterised by the almost universal absence of cilia, by
their muscles being nearly always of the striped kind, by their
sperms being usually non-motile, and by the body-cavity being
largely represented by spaces, the blood-sinuses, in free communica-
tion with the circulatory system.

The following are the most important subdivisions of the
phylum.

Class 1. CRUSTACEA, including Crayfishes, Crabs, Shrimps, Wood-
lice, Barnacles, Water-fleas, &c.

Class 2. ONYCHOPHORA, including only a single genus, th(
curious caterpillar-like Peripatus.

Class 3. MYRIAPODA, including the Centipedes and Millipedes.

Class 4. INSECTA, including the true or six-legged Insects, sucl
as Cockroaches, Locusts, Flies, Beetles, Butterflies, and Bees.
s Class 5. ARACHNIDA, including Spiders, Scorpions, Mites, &c.

CLASS I.— CRUSTACEA.

1. EXAMPLES OF THE CLASS.
a. Apus or Lepidurus.

Apus and Lepidurus are two closely allied Crustaceans foum
the fresh-waters of most parts of the world, but curiously local ii

SECT. XI

PHYLUM ARTHROPODA

485

thf-1

distribution and by no means common. They are so much alike
that, save in minor details, the same description will apply to any
species of either genus.

External Characters.— The animal (Fig. 386) is from 20 to
30mm. in length, and has the anterior two-thirds of the dorsal surface
covered by a thin chitinous shell or carapace, beyond the
edge of which the hinder part of the body (dbd.) projects
nearly cylindrical structure distinctly divided into segments.
last or anal segment *

bears a pair of long „ e

processes, the caudal
styles (a. /.) between
which, in Lepidurus, is
a flat scale-like post-
anal plate (Fig. 387).
On the dorsal surface of
the carapace, near its
anterior border, are the
paired eyes (E\ closely
approximated in front,
diverging posteriorly.
Immediately in front
of them is a small
black median eye (e.),
and between their di-
verging posterior ends
is a semi-transparent
oval area, the dorsal
organ (d. o.). Passing
transversely across the
carapace, a short dis-
tance behind the dor-
sal organ, is a shallow
furrow, the cervical fold,
immediately posterior
to which a pair of
coiled tubes (sh.gl.) are

seen, one on each side of the carapace : these are the shell-glands
or excretory organs.

The carapace is attached only as far back as the cervical fold :
behind that level it is free, and. when lifted up or cut away
(Fig. 387), shows the greater part of the body of the animal, divided
into segments like the posterior portion. From the cervical groove
backwards about twenty-eight or thirty segments can be counted :
the region in front of the cervical groove shows no sign of segmen-
tation, and is distinguished as the head. The segments have the
form of chitinous rings, often produced into small spines : each

FIG. 386. — Apus cancriformis, dorsal aspect. aM.
abdomen ; a. /. caudal styles ; d. o. dorsal organ ; E.
paired eye; e. median eye; sh. gl. shell -gland; th.f.l,
endites of first thoracic foot. (From Broun's Thier-reich.)

486

ZOOLOGY

SECT.

XI

PHYLUM ARTHROPODA

487

ring slightly overlaps its successor, and is connected with it by a
narrow area, the articular membrane, the chitinisation of which is
less pronounced than that of the rings themselves. By this
arrangement the segments are freely movable upon one another
in all directions, the articular membranes acting as joints.
The last or anal segment is pierced by the terminal anus
(Fig. 390, an.\

The ventral surface of the head is formed by a flattened sub-
frontal plate (Fig. 388, s.f.pL), con-
tinuous marginally with the cara-
pace. The posterior edge of this
plate is convex backwards, and is
produced in the middle line into a
shield-shaped process, the labrum
or upper lip (/&?'.), which over-
hangs the mouth. From the sub-
frontal plate also arise, on each
side, two delicate processes, the
innermost called the antcnnule
(ant. 1\ the outermost the an-
tenna (ant. 2) : these are the first
two pairs of appendages. The
third pair consists of two strong
toothed bodies of a deep brown
colour, placed one on each side of
the mouth, and called the man-
dibles (md.). The remaining ap-
pendages form two rows of deli-
cate leaf-like processes, attached
to the segmented portion of the
body, and overlapping one another
from before backwards : their
number varies from forty to nearly
seventy (th. /., dbd. /.).

Appendages. — The antennule
(Fig. 389, 1) consists of a bent
rod bearing delicate chitinous
bristles or seta? at its tip, and presenting, at the bend, a joint,
due to the presence of an articular membrane. The appendage
is thus made up of two podomeres or limb-segments, movably
articulated together. Its function is probably tactile.

The antenna (2) is absent in some species both of A pus and
Lepidurus : in A. cancriformis it is a very delicate hook-shaped
unjointed structure, probably functionless. As we shall see from
the study of development, it is a vestigial organ.

The mandible (3) is also an unjointed appendage. It has the
form of a deeply concavo-convex plate, strongly chitinised, and pro-

FIG. 388.— Apus glacialis, ventral
aspect, abd. /. abdominal feet; ant*.
antennule ; ant*, antenna ; //>,: labrum ;
md. mandible; mx. first maxilla; or.
aperture of oviduct ; s. f. pi. sub-frontal
plate ; sh. <j(. shell-gland ; th. f. thoracic
feet ; th. f. 1, first thoracic foot, (After
Bernard.)

438

ZOOLOGY

SECT.

dticed along its inner edge into strong teeth. The mandibles lie
one on each side of the mouth, and are so articulated that, by
means of muscles, their toothed edges can be brought together in
the middle line, so as to rend the food.

The fourth and fifth appendages are very small, and are prob-
ably functionless or nearly so : they follow one another just behind
the mandible, and are called the first and second maxillae. The
first maxilla (4) consists of two curved chitinous plates, the second
of a basal portion produced into two branches (5). Between the
first maxilla and the mandibles are a pair of delicate unjointed

-5.2ndMaxilla

Jl ^

a.J^Thoracic Foot

r*^y?
10. I^Abdomina! Foot

FIG. 389. — Typical appendages of Apus. 1 — k, podomeres of axis ; br. bract ; en. 1, en. 7, endites.
ji. flabellum ; or. ova. (After Lankester.)

processes, the paragnatha (Fig. 387, pgn^) : they form together a
sort of lower lip, and are not usually reckoned as appendages.

The foregoing appendages all spring from the unsegmented
anterior portion of the body or head. As we shall see, however,
the succeeding limbs spring each pair from its own segment,
so that the presence of five pairs of appendages on the head may
be taken provisionally as an indication that this region of the body
is composed of five fused segments.

The sixth appendage (6) springs from the ventro-lateral region
of the first clearly marked segment, and is the first of the long
row of appendages plainly visible in a ventral view. It consists
of an axis formed of four podomeres (l-4)> and bearing a number
of offshoots : six of these, called endites (en. 1 — en. 6), spring from

xi PHYLUM ARTHROPODA 489

its inner or mesial border; two, called exiles (br.,fl.\ from its outer
or lateral border. The proximal endite (en. 1) is small, and bears
strong spines: in conjunction with its fellow of the opposite side
it is used to seize food-particles and pass them on to the mouth : it
is therefore conveniently distinguished as the gnathobase. The
distal endite is rudimentary (en. 6) : the remaining four (en. 2-5}
are long jointed filaments. The proximal exite is nearly trian-
gular, and is called the flabellum (fl.)\ the distal exite is oval, and
is known as the bract (br.)\ both probably serve a respiratory
function.

The seventh appendage (7) has only two podomeres in the axis,
and the endites are comparatively short and flat. The next eight,
i.e. those borne on the third to the sixteenth free segments, closely
resemble one another : each (8) has an unjointed axis and short leaf-
like endites, the whole appendage having a distinctly foliaceous
character. The sixteenth appendage — that of the eleventh free
segment — resembles its predecessors in the male, but in the
female (9) is peculiarly modified. The distal portion of the axis
forms a hemispherical cup, over which the flabellum (fl.) fits
like a lid : in this way a capsule or "brood-pouch is produced,
which serves for the reception of the eggs, and the appendage
is distinguished as the oostegopod or brood-foot. The brood-feet
and the adjacent genital apertures allow of a very convenient
division of the body : all that region from the first free or post-
cephalic segment to that bearing the oostegopods, both inclu-
sive, is called the thorax, and its appendages the thoracic feet : it
consists of eleven metameres. The remaining segments, from the
twelfth to the last inclusive, constitute the abdomen, and their
appendages are called the abdominal feet.

The abdominal resemble the thoracic feet in general characters,
having the same foliaceous form (10\ with unjointed axis, small
leaf-like endites, and large flabellum and bract. They gradually
diminish in size from before backwards, and, from the third abdo-
minal segment onwards, two or more pairs of appendages spring
from each segment, so that while the total number of abdominal
segments, in A. cancriformis, is twenty- two, and the five hinder-
most of these are without appendages, there are altogether fifty-
two pairs of abdominal feet. It seems probable that segments
bearing more than one pair of appendages represent two or more
fused, or, perhaps one should rather say, imperfectly differentiated,
metameres.

Body-wall. — The whole body is, as already mentioned, covered
by a layer of chitin of varying thickness, which constitutes an
t'..roslcektvn or external supporting structure. Immediately under-
lying it is the deric epithelium or epidermis, from which the chitin
is secreted layer by layer. Thus the exoskeleton of Apus is
a continuous cuticular structure, exhibiting segmentation in virtue

490

ZOOLOGY

SECT.

of the fact that, while comparatively thick and strong in places where

no movement is required,
it is thin and flexible in
the intervening spaces, and
thus allows of the move-
ment of the harder parts
upon one another.

The setae, which occur
on many parts of the body,
and in particular fringe
the appendages, are hollow
offshoots of the chitinous
cuticle, containing a proto-
plasmic core continuous
with the epidermis (Fig.
399). They thus differ
fundamentally from the
setae of Chaetopods, which
are solid rods sunk in
muscular sacs.

The muscular system
is well developed (Fig.
390). Underlying the epi-
dermis is a layer of con-
nective tissue, and beneath
this is found, in the pos-
terior or limbless part of
the abdomen, a layer of
longitudinal muscles (Fig.
390) encircling the body,
and attached by connec-
tive-tissue to each seg-
ment. In this way the
muscular layer is itself
segmented, being divided
by the connective-tissue
insertions into muscle-seg-
ments or myomeres. The
action of these muscles is
to approximate adjacent
segments : according as the
fibres on the dorsal, ven-
tral, or lateral regions con-
tract, the abdomen will be
raised, lowered, or turned
sideways. In the limb-
bearing portion of the abdomen and in the thorax there is no

,e

xi PHYLUM ARTHROPODA 491

longer a continuous muscular tube, but paired dorsal (d./ii.)
and A^entral bands, which pass respectively above and below the
origins of the limbs : the dorsal bands arise in front from the
head-region, the ventral from a strong fibrous plate, the cephalic
apodeme (c.ap.), lying just behind the gullet.

Each appendage is moved as a whole by muscles passing into it
from the trunk : its various parts are acted upon by delicate
muscular slips running to the various podomeres of the axis and
to the endites, thus rendering them separately movable. The only
example we have yet met with of appendages moved by definite
muscular bands is that of the curious rotifer Pedalion (p. 307). The
muscles are all striped, a character which applies to the Arthropoda
generally, with the exception of the Onychophora.

Digestive Organs. — The mouth (Fig. 390, mth.) is situated on
the ventral surface of the head, and is bounded in front by the
labrum (lbr.)t on each side by the mandibles, and behind by the
paragnatha. ; The food appears to be pushed forwards towards
the mouth by the toothed bases of the thoracic feet, and is
subdivided by the mandibles, which work laterally. The maxillae
probably functionless, or nearly so.

The mouth leads into a narrow gullet (gul.), which passes
upwards and forwards into the head and enters a wide
stomach^ (st.), from which a straight intestine (int.) is continued
back to the terminal anus (an.). From each side of the
stomach is given off a wide tube (d.gl.) which branches exten-
sively, its ramifications finally ending in delicate caeca. The
larger branches of these digestive glands contain food in process of
digestion : their ultimate caeca secrete a digestive juice : the walls
of the stomach itself are non-glandular. The walls of the enteric
canal consist of an inner layer of epithelium and an outer layer of
connective-tissue and muscle. In the gullet and in the posterior
end of the intestine the epithelium secretes a thin cuticle, which
thus comes to form the actual lining of the cavity. It is shown
by development that the portion of the canal devoid of a chitinous
lining is formed from the archenteron of the embryo : the gullet is
developed from the stomodaeum, the posterior end of the intestine
from the proctodaeum.

The body-cavity is divided into several parts by membranous
partitions (Fig. 391) : there is a large median cavity in which the
enteric canal (i) lies, called the intestinal sinus : on each side of this
are lateral sinuses containing the muscles : and in the dorsal region is
a median cavity, the pericardial sinus. All these spaces are
devoid of an epithelial lining, and contain blood : there is reason
for thinking that they do not correspond with the' coelome of the
higher worms but this subject will be more conveniently discussed
hereafter (p. 547).

The central organ of the circulatory system is the heart (Fig.

492

ZOOLOGY

SECT.

390 lit, and Fig. 391, h), a narrow tube contained in the pericardia!
sinus. It is pierced laterally by several pairs of apertures or ostia,
provided with valves opening inwards, and is continued in front
into a narrow tube, the cephalic artery (c. art.), which extends into
the head and gives off near its origin a pair of arteries to the shell-
glands. When the heart contracts, the blood is driven through
these arteries to the head and carapace : it then travels backwards
in the intestinal sinus, passes to the limbs, and is returned to the
pericardia! sinus, finally re-entering the heart, during its diastole,
through the ostia. The plasma of the blood is coloured red by
hemoglobin, and contains amoeboid corpuscles.

As already mentioned, the function of respiration is discharged
by the flabella and bracts of the feet, which are abundantly sup-

FIG. 301. — Transverse section of Apus. cm. muscles to feet ; dv. dorso-ventral muscles ; e. eggs ;
tl'iii. dorsal muscles ; g. ovary; di: dorso-ventral muscles ; li. heart ; i. intestine ; m. partition
between intestinal and lateral sinus ; vm. ventral muscles. (From Bernard.)

plied with blood, and the movements of which ensure a constant
renewal of the water in their neighbourhood. The. renal organ
or shell-gland (Fig. 392) consists of a coiled urinary tube (uc.)
lying between the two layers of the carapace and lined by gland-
cells. At one end the tube is connected with an e^id-sac (ts.\
also lined with glandular epithelium ; at the other it dilates into
a small bladder (b.) which opens on the second maxilla (m.).

The nervous system (Fig. 393) is constructed on the annulate
type. There is a squarish brain (br.) situated in the dorsal region
of the head, beneath the eyes. From it a pair of cesophageal
connectives pass backwards and downwards to join the ventral nerve-
cord, which consists of a double chain of ganglia (gn. 1-4) united
by longitudinal connectives and transverse commissures so as to
have a ladder-like appearance. The first pair of ganglia lies

XI

PHYLUM ARTHROPODA

493

immediately behind the mouth, and sends off visceral nerves which
join to form a ring round the gullet, swollen in front into a small
visceral ganglion (v. gn.). Passing backwards, the nerve-chain
diminishes in size, and comes to an end at about the level of the
3t pair of abdominal feet (Fig. 390).

The origin of the nerves given off from the central nervous
stem presents many points of interest. From the fourth ganglion
the ventral cord backwards each pair of appendages has its own
pair of ganglia, the metameric correspondence between the limbs
and the nervous system being complete. The mandibles and the
first maxillae also receive nerves, each from their own pair of

FIG. 392. — Shell-gland of Apus, diagrammatic, ac. cephalic artery ; b. bladder ; h. heart
m. second maxilla ; is. end-sac ; uc. urinary tube. (From Bernard.)

ganglia, their serial homology with the more typical appendages
being thus confirmed. But the second maxillae receive their
nerves (mn. 2) from the connectives between the third and fourth
ganglia : the ganglion belonging to their segment may be assumed
to have atrophied. The antenna is supplied by a nerve (ant. 2)
which springs from the oesophageal connective, but which can be
traced backwards to the first ganglion of the ventral chain : this
fact may be taken as an indication that the antennae are serially
homologous with the jaws and feet, that they are in fact meta-
meric or post-oral appendages which have shifted, forwards, one
on each side of the mouth, thus becoming prae-oral. The nerve of
the antennule (ant. 1) also springs from the oesophageal connec-
tive, but is traceable forwards to the brain, where it is connected

494

ZOOLOGY

CLftt.f

ant.z

tnd

•nuz.f

with a special group of nerve-cells. This has been explained by
supposing that the aiitennule is a post-oral appendage the ganglion
of which has moved forwards, along the oesophageal connective,
and fused with the brain — a process which actually takes place

with the ganglia of the antennae
in the higher Crustacea. But it
is also possible to consider the
antennules as prse-oral append-
ages, belonging, like the pro-
stomial tentacles of Chastopods, to
the prostomial region, and there-
fore receiving their nerves from
the brain or prostomial ganglion.
The median and paired eyes are
also supplied by nerves from the
brain.

Organs of Sense.— The seta3
which occur on so many parts of
the body, and especially as fringes
to the limbs, are to be considered
as organs of touch : the only other
organs of special sense are the
eyes. The paired eyes are, as we
have seen, situated on the dorsal
surface of the liead, just over
the brain: they are covered by
transparent cuticle forming the
cornea, beneath which is a narrow
space or water-sac, communicating
with the exterior by a pore, and
therefore filled with water. The
eye itself is made up of a large
number of radially arranged ele-
ments called ommatidia (Fig.
394), each of which consists of an outer and an inner por-
tion. The outer portion is a group of clear glassy cells (ce.)
enclosing a transparent homogeneous vitreous . body (cr.) : the
whole of this portion of the eye serves to refract the rays of
light ; it is the dioptric apparatus, like our own lens and
vitreous humour. The inner portion is a group of sensory
cells, constituting a retinula (re.), and enclosing a refractive rod,
the rliabdome (rh.) : the retinula is the actual percipient parj;
of the ommatidium, its cells being comparable to our own rods
and cones. The retinulaB of adjacent ommatidia are separated
from one another by cells full of black pigment (p.\ so that each
ommatidium is in a state of optical isolation from its fellows,
and the whole eye is what is called a compound eye. The optic

thj.1

Fio. 303. — Nervous system of Apus
cancriformis. ant.' nerve to an-
tennule ; ant." to antenna ; br. brain ;
rjn. 1 — It, first four ganglia of ventral
nerve-cord ; md. mandibular nerve ;
mx. 1, nerve of first maxilla; mx. 3, of
second maxilla ; th.f. 1, of first thoracic
foot; r. fin. visceral ganglion. (After
Lankester and Pelseneer.)

PHYLUM ARTHROPODA

495

nerve springing from the brain dilates into an cptic ganglion, from
which fibres pass to the retinuhe.

The median eye is an ovoid body, and consists of four groups of
large sensory cells enclosing a mass of pigmented tissue : it is in
immediate contact with the brain, and receives a narrow canal
from the water-sac beneath the cuticle of the paired eyes.

Reproductive Organs. — The large majority of individuals
>th of Apus and Lepidurus are females; males are of corn-

FIG. 394. — Diagram of two ommatidia from the paired eyes of Apus. re. vitreous cells ; cr, vit-
reous body ; cl, connective-tissue fibre ; hy, epiderm cells ; p. pigment cells ; re. retinul* ;
rh. rhabdome. (From Bernard.)

paratively rare occurrence. The owry (Fig. 390, ovy.) is a branched
tube occupying a considerable portion of the body-cavity in
sexually mature individuals. The walls of the tube are lined
with epithelium, and give rise to ova, which pass into the lumen
of the tube and thence to a duct (ovd.) opening on the eleventh or
last thoracic segment. As in Leeches (p. 474), there is reason
for thinking that the cavity of the ovarian tube represents a
shut-off portion of the coelome, and the oviduct a nephridium.
One species has been shown to be hermaphrodite : in others
males are occasionally found, but reproduction appears to be, as a
rule, parthenogenetic.

496

ZOOLOGY

SECT.

Development. — The eggs are centrolecithal, i.e., have an
accumulation of yolk in the centre surrounded by a superficial
layer of protoplasm. The process of segmentation and the forma-
tion of the germ-layers, has not been observed.

The embryo is hatched in the form shown in Fig. 395, A. The
body is oval, and is divisible into three regions — a large anterior or
head-region ; an intermediate trunk-region, the hinder part of which

Fio. 395. — Three stages in the development of Apus. ./>. frontal sensory organ ; L, digestive
gland ; s. carapace ; 1 — A, cephalic appendages ; I — XIII, body segments and appendages.
(From Lang's Comparative Anatomy.)

already shows signs of segmentation (/- V) and a posterior bilobed
f iKt I region. The head-region bears a single median eye, and a
pair of small unjointed appendages (1), each with two large setae
at its extremity, which become the antennules of the adult. The
trunk-region bears two pairs of appendages, the first of which (#)
is very large and fringed with setaa, but is chiefly remark-
able for being biramous or two-branched — being formed of a
proximal portion or stem, the protopodite ; a small inner branch, the
endopodite ; and a large outer branch, the exopodite. This second

XI

PHYLUM ARTHROPODA 497

appendage becomes the antenna of the adult, and may be called
the antennary foot : it is the chief organ of locomotion of the
larva. The second trunk-appendage is the mandibular foot (3), so
called because it becomes converted into the mandible of the
adult : it is also biramous. The only internal structure to be
noted is the straight enteric canal with its dilated anterior end
or stomach : the mouth opens between the bases of the antennary
and mandibular feet, and is bounded in front by a large labrum :
the anus lies at the extremity of the anal region. This very
peculiar and characteristic larval form is called a Nauptius.1

The Nauplius swims freely, chiefly by vigorous strokes of
the great antennary feet, and after a time undergoes a series of
moults or ecdyses, the cuticle being cast off and the animal
emerging in the form shown in Fig. 395, B. The trunk-region has
elongated, new segments having been added, as in Chsetopods,
between those previously present and the anal region. The
antennules have become shifted backwards, and rudiments of a
fourth pair of appendages, the first maxillae (4), have appeared.
The carapace has grown out from the dorsal region of the head,
and a peculiar paired sense-organ (fs.) has appeared on the
heai

After two more ecdyses the larva has assumed the form shown
in Fig. 395, C. Several new segments have been added, and the
anterior of these all bear leaf-like thoracic feet. The antennary
feet are still very large, and the bases of the mandibular feet have
become enlarged and toothed so as to form biting jaws. The
carapace (s) has increased greatly, and the caudal styles have
attained a considerable size. Further moults occur, new seg-
ments are added with their appendages, the antennules and
antennae degenerate, — the latter sometimes disappearing alto-
gether— the mandibles become reduced to the enlarged basal
segment, and the larva passes by almost insensible gradations
into the adult form.

It will be seen that the development of Apus proves clearly
that the antennae and mandibles are ordinary trunk-appendages,
homologous with the thoracic and abdominal feet : a comparison
of the antennary and thoracic feet of the larva supports the view
that the endopodite of the former corresponds with the fifth endite
of the latter, and the exopodite with the sixth endite. The
antennules are from the first unbranched or uniramous, and are
originally situated quite at the anterior region of the body : they
do not, therefore, show a complete correspondence with the remain-
ing appendages, and, as was inferred from their nerve supply,
may perhaps be considered as prostomial and not metameric
appendages.

1 More strictly Metanauplius : the typical Nauplius exhibits no segmentation
of the trunk region.

VOL. I K K

498 ZOOLOGY SECT,

1. The Fresh-water Crayfish (Astacus fluviatilis).

Astacus fluviatilis is common in streams and rivers in England
and the continent of Europe ; allied species occur in Asia and
North America ; and fresh- water Crayfishes belonging to other
genera, but agreeing with Astacus in all essential features_, are
found in America, Australia, and New Zealand.

External Characters.— The body of the Crayfish (Fig. 396) is
divided into two regions — an anterior, the ccphalothorax (cth.),
which is unjointed, and is covered by a carapace resembling that of
Apus, but of smaller proportional size ; and a posterior, the abdo-
men (ab\ which is divided into distinct segments, movable upon one
another in a vertical plane. The cephalothorax is again divided
into two regions — an anterior, the head] and a posterior, the thorax
—by a transverse depression, the cervical groove. The divisions of
the body are thus the same as in Apusx but the abdomen alone is
movably segmented, owing to the fact that the carapace, instead
of being a purely cephalic structure continued backwards as a
loose fold over the thorax, is developed from the dorsal and
lateral regions of both head and thorax, and is free only afrthe
sides of the thorax, where it forms a flap or gill- cover (kd) on each'
side, separated from the actual body-wall by a narrow space in
which the gills are contained (Fig. 404). The carapace is made of
chitin, strongly impregnated with carbonate of lime so as to be
hard and but slightly elastic.

The abdomen is made up of seven segments: the first six
(XIV-XIX) of these are metameres in the strict sense of the
word, and have a ring-like form presenting a broad dorsal region or
tergum, a narrow ventral region or sternum, and 'downwardly
directed lateral processes, the pleura — the latter quite unrepre-
sented in Apus. The seventh division of the abdomen is th'e tehon :
it is flattened horizontally, and divided by a transverse gro(5ve~Tnto
anterior and posterior portions, All seven segments are calcified,
and are united to one another by chitinous articular membranes :
the first segment is similarly joined to the thorax. Thus the
exoskeleton of Astacus resembles that of Apus in being a con-
tinuous cuticular structure, but differs from it in being discon-
tinuously calcified, so as to have the character of a hard jointed
armour.

It has been stated that the abdominal segments are movable
upon one another in a vertical plane — i.e., the whole abdomen can be
extended or straightened, and flexed or bent under the cephalo-
thorax : the segments are incapable - of movement from side to
side. This is due to the fact that, while adjacent segments are
connected dorsally and ventrally by flexible articular membranes,
they present at each side a hinge (Fig. 400, h), placed at the

PHYLUM ARTHROPODA

499

junction. of the tergum and pleuron, and formed by a little peg-
l&e process of one segment fitting into a depression or socket in
the other. A line drawn between the right and left hinges con-
stitutes the avis of artic/'/ction, and the only possible movement
is in a plane at right angles to this axis.

Owing to the presence of the carapace, the thoracic region is
imm<fyable, and shows no distinction into segments either on its
dorsal (tergal) or lateral (pleural) aspect. But on the ventral
surface the sterna of the thoracic segments are clearly mar^ke'd
off by transverse grooves, and the hindmost of them is slightly'
movable. Altogether eight thoracic segments can be counted.

The ventral and lateral regions of the thoracic exoskeleton are
produced into the interior of the body in the form of a segmental

Fio. 396.— Astacus fluviatilus, side view of male, a1, antemmle ; «'-', antenna ; alt, abdomen
ct It., cephalothorax ; kd, gill-cover; r. rostrum; 8, third m axillipede ; 9, first leg; 10 — 13,
remaining legs ; 19, uropod ; XIV, first abdominal segment ; XIX, sixth abdominal segment.
(From Lang's Coniparatifc Anatomy.)

I

series of calcified plates, so arranged as to form a row of lateral
chambers in which the muscles of the limbs lie, and a median
tunnel-like passage or sternal canal, containing the thoracic portion
of the nervous system. The entire endopkragmal system, as it is
called, constitutes a kind of internal skeleton: its anterior—end is
formed by a plate, the cephalic apodeme, having the same anatomical
relations as the similarly named structure in Apus.

The head exhibits no segmentation : its sternal region is
formed largely by a shield-shaped plate, the epistoma, nearly vertical
in position. The ventral surface of the head is, in fact, bent so as
to face forwards instead of downwards. The epistoma is bounded
laterally by the free edge of the carapace instead of passing
insensibly into it like the sub-frontal area of ApUs, with
Inch however it agrees in having the labrum attached to the

K K 2

500 ZOOLOCV

middle of its posterior border. The cephalic region of the cara-
pace is produced in front into a large median spine, the rostriif/i
(Fig. 396, r) : immediately below it is a plate from which *sprmg
two movably articulated cylindrical bodies, the eye-stalks, bearing
it their (.'lids.

The appendages are seen at a glance to_ differ from • those of
Apus in their vastly greater ^degree of differentiation : obvitus at
a glance are the long feelers (Fig. 396, a. 1, a. 2) attached to the
head, the five pairs of legs (9-18} springing from the thorax, and
the little fin-like bodies arising from the sterna of the abdomen.
It will be convenient to begin with the last-named region.

The third, fourth, and fifth segments of the abdomen bear
each a pair of small appendages, the swimming feet or

(Fig. 397, 10), the resemblance of which to the biramousfi!,
of the larval Apus is obvious. There is an axis or protopodifa
consisting of a very short proximal (pr. JP) and a long distal
(pr. 2) podomere, and bearing at its free end two jointed pl;i
fringed with setae, the endopoditc, (en) and exopodite (ex). These
appendages act as fins, moving backwards and*forwards with a
regular swing, and probably aiding in the animal's forward
movements.

In the female a similar appendage is borae on the second seg-
ment, while that of the first is more or less rudimentary. In the
male the first and second pleopods (9) are modified into incom-
plete tubes which act as copulatory organs, serving to .transfer
the spermatophores to the -body of the female. The sixth pair of
abdominal limbs (11) are alike in the two sexes : they are very
large, both endo- and exopodite having the form of broad flat
plates : in the natural position of the parts they lie one on each
side of the telson, forming with it a large five-lobed tail fin : they
are therefore conveniently called uropods or tail-feet. The telson
itself bears no appendages.

The thoracic appendages are very different. The four posterior

meiits bear long slender, jointed legs (8), upon which the animal

\valks : in front of these is a pair of very large legs terminating

in huge claws or chelcu, and hence called chelipeds (Fig. 396, 9).

The three anterior segments bear much smaller appendages,

more or less leg-like in form, but having their bases toothed to

ve as jaws : they are distinguished as maxillipeds or foot-jaws

( Pig. 397, 5-7).

The structure of these appendages is best understood by a con-
sideration of the thin/ nK^rillipal (/').. The main portion of tin
limb is formed ot podomeras arranged in a single

strongly calcified, and, with th- ion of the second and third,

which are fused, movably articulated with one another. The secom
pi )(.!«> anting from the proximal end .1 many-joinl

aid from the first springs a thin folded

PHYLUM ARTHROPODA

501

plate (ep) having a plume-like gill (g) attached to it. Obviously,
such an appendage is biramous, but with one of its branches
greatly in excess of the other : the first two segments of the axis
(pr. 1 form the protopodite, its remaining five segments

(en. 1-6) the endopodite, and the feeler, which is directed out-
wards, or away from the median plane, the exopodite (ex). The
folded plate (ep) is called the epipodite : in the natural position

en.3

5. 2".d Maxilla 6. I* Maxilliped

e.Cobulafory Organs lO.Swimming Feot

11. Uropod

of Astacus.

•vs of protopodita ;
tley.j

of the parts it is directed upwards, and lies in the gill -cavity
between the proper wall of the thorax and the gill-cover (Fig. 404).
Its position is thus very similar to that of the flabelluni of Apus,
while the gill attached to it is comparable to the bract.

The five legs (S) differ from the third maxillipecl in their gr.
size, and in having no exopodiic : in the fifth or last the epipo-
also is abs-nu The first three of them have undergone a cur

502 ZOOLOGY SECT.

modification, by which their ends are converted into pincers or
rlt.dtc : the fourth segment (en. 4) of the endopodite (sixth of the
entire limb) is produced distally so as to form a claw-like projec-
tion (en. 4l)> against which the terminal segment (en. 5) bites. The1
first leg is much stouter than any of the others, and its chela is of
immense size and forms an important weapon of offence and
defence. The second maxilliped resembles the third, but is con-,
siderably smaller: the first (6) has its endopodite greatly reduced,
the two segments of its protopodite large and leaf-like, and no
gill is connected with the epipodite.

As in Apus, the head bears a pair of mandibles and two pairs of
maxillae in relation with the mouth, and in front of that aperture
a pair of antennules and one of antennae. The hindmost appendage
of the head is the second maxilla (5), a markedly foliaceous append-
age: its protopodite (pr.-l,pr. 2) is cut up into lobes comparable
with the four proximal endites in the thoracic feet of Apus :
its endopodite (en) corresponds with the fifth endite, while the
sixth endite is represented by the exopodite (ex), modified into
a boomerang-shaped plate, which, as we shall see, is an important
accessory organ of respiration. The first maxilla (4)' is a very
small organ, having neither exo- nor epipodite. The inaudible- (3)
is a large strongly calcified body, toothed along its inner edge,
and bearing on its anterior border a little three-jointed feeler-
like body, the palp, the two distal segments (en. 1, en. 2) of
which represent the endopodite, its proximal segment (jn:
together with the mandible proper (pr. 1), the protopodite.

The antenna (2) is of great size, being nearly as long as the
whole body. It consists of an axis of five podomeres, the fifth or
last of which bears a long flexible, many-jointed structure, or
flc.r/ellum (fl), while from the second segment springs a scale-like
body or squame (ex). It is fairly obvious that the two proximal
segments represent the protopodite. the remaining three, with the
flagellum, the endopodite, and ihe squame the exopodite.

The antennule (1) has an axis of three podomeres (1-3) ending
in two many-jointed flagella (fl. 1. and #), which are sometimes
considered as endo- and exopodite. But in all the other limbs,
as we have seen, .the exopodite springs from the second segment
of the axis, and the probabilities are that there is no exact corre-
spondence between the parts of the antennule and those of the
remaining appendages.

The eye-stalks, already noticed, arise just above the antennules,
and are formed each of a small proximal and a large distal segment.
They are sometimes counted as appendages serially homologous
with the antennae, 'legs,, &c. But, as we- have seen in the case of
Apus, the appendages of Crustacea are always formed in regular
order from before backwards: the eye-stalks, on the other hand,
always appear later, both in individual development and in the

PHYLUM ARTHROPODA

503

en. 4-

tacean series, than the normal anterior appendages. They are

'fore more properly to be looked upon as articulated processes

he prostomium, developed in

sction with the need for an

ased range of vision. The prob-
umoy of the ant ennui es being also
>rostomial structures has already
en referred to : assuming this to
the case, it will be seen that the
y of the Crayfish consists of a
tomium, eighteen rnetameres,
and a telson which is probably com-
posed of an anal segment, plus a
post-anal plate. The prostomium
bears eye-stalks and antennules : the
first four metameres are fused with
the prostomium to form the head,
and bear the antenna?, mandibles,
first maxilla3, and second maxillae :
the next eight metameres (5th-l 2th)
constitute the thorax, and bear the
three pairs of maxillipeds and the
five pairs of legs : the remaining six
metameres (13th-18th), together
with the telson, constitute the abdo-
n, and bear five pairs of pleopods
d one of uropods.

The articulation of the various
podomeres of the appendages is on
the same plan as that of the ab-
dominal segments (p. 498). The
podomeres are, it must be remem-
bered, rigid tubes : they are con-
nected with one another by flexible
articubi r membranes (Fig.398, art. w.),
but at two points the adjacent ends
of the tubes come into contact with
one another and are articulated by
peg and-socket joints (&.), the two
joints being at opposite ends of a
diameter which forms the axis of
articulation. The two podomeres
can, therefore, be moved upon one
another in a plane at right angles

to the axis of articulation and in no other direction, the joints
being pure hinge-joints. As a rule, the range of movement
is from the perpendicular to a tolerably extensive flexion on

wit
anc

FIG. 30S.— Portion of leg of Astacus,
with the exoskeleton partly re-
moved, showing articulations and
muscles, art. -m. articular mem-
brane ; en. 2 — 5, podomeres of eiido-
podite ; ext. extensor muscles ; 1.
flexors ; h. hinge.

504

ZOOLOGY

one side — the articulations are single-jointed, like our own
elbows and knees. The whole limb is, however, capable of
universal movement, owing to the fact that the ax<
articulation vary in direction in successive joints: the first joint
of a limb bending, for instance, up and down, the next back-
wards and forwards, the next obliquely, and so on. In some <
e.g. the pleopods, peg-and-socket joints are absent, the articulation
being formed merely by an annular articular membrane and move-
ment being therefore possible in any plane.

Body-wall. — The exoskeleton is produced into spines of
varying form and size, and many parts of it bear tufts or
fringes of setae, which also exhibit a wide variation in size and
form. It is composed of a thick laminated chitinous membrane
(Fig. 399, cu.), more or less impregnated with lime-salts, and is

shed periodically — once a year during
adult life. Beneath it is the epidermis
(ep.)t composed of a single layer of
cells, from which the chitin is secreted,
and underlaid by a layer of connective-
tissue (c, t.) to which the muscles are
attached.

The muscular system, like the
exoskeleton, shows a great advance
in complexity over that of Apus. In
the abdomen (Fig. 400) the muscles
are of great size, and are divisible
into a smaller dorsal and a larger
ventral set. The dorsal muscles (d. m. }
are paired longitudinal bands, divided
into myomeres, and inserted by con-
nective tissue into the anterior border
of each segment : anteriorly they are

traceable into the thorax, where they arise from the side-walls of
that region. When these muscles contract, they draw the anterior-
edge of each tergum under the posterior edge of its prede<
and thus extend or straighten the abdomen.

The trntrd/ muscles are extraordinarily complex. Omitting de-
tails, there is on each side a wavy longitudinal band of muscle (c. m.\
nearly circular in section, which sends ott" a slip (ex.) to be ii.--
into each segment above the hinge (k.): the contraction of this
muscle must obviously tend to approximate the terga, and so aid
the dorsal muscles in extending the abdomen. Around thisw
muscle is wrapped, in each segment, a band of •muscle (cnv. //>.) in
the form of ;i loop, the outer limb of which turns forwards and is
••1 into a sternum, while the inner limb turns backwards and
is inserted into another and more posterior sternum. The con-
>n of thi- /'/ produces an approximation of

FIG. 899.— Vertical section of skin
and exoskeleton of Lobster.
c.t. connective tissue ; cu. cuticle ;
seta. (After

'(./>. epidermis ;
Gerstaecker.)

PHYLUM ARTHROPODA

505

the sterna, and thus Ilexes the abdomen, the central muscle always
keeping the middle of the loop in place. The ventral muscles
are, like the dorsal, traceable into the thorax, where they arise
from the endophragmal system : their various parts are connected
by a complex system of fibres extending between the central and
enveloping muscles, and connecting both with their fellows of the
opposite side. The flexor muscles are immensely pc .verful, and
produce, when acting together, a sudden and violent bending of

••!>.— Four segments of abdomen of Crayfish in sagittal section, with muscles (diagram-
matic). A, extension; B, flexion; «,-f. m., art. ;/«/., articular membranes; c. m. central
muscl' Mi-sal muscle: ex. extensor slip of central muscle; em: m. enveloping'

muscle ; /?., jU, flexor slips ; h. hinge ; si. sternum ; iff. tergum.

the abdomen upon the cephalothorax, causing the Crayfish to dart
backwards with great rapidity.

It will be seen that the -^ocj^-muscles of the Crayfish cannot be

said to form a layer of the boay-wall, as in ChaBtopods, the abdomen-

of Apus, &c., but constitute an immense fleshy mass, filling up the

i.ter part of the body-cavity, and . leaving a very small space

around the enteric carol

In the limbs (Fig, 398), each podomere is acted upon by two
muscles situated in the next proximal podornere. - These muscles
arc inserted, by chitinous and often calcified tendons, into the

506

ZOOLOGY

SECT.

:oi.— Astacus flu vi at ills,

dissection from the right side,
aiiteiinary .artery ; nh. abdo-
men ; an. anus ;• I. <L bile duct ;
',cheliped; but, ventral nerve
cord ; e*. cardiac division of
stomach ; <-tit. cephalo-thorax ;

. dorsal muscles; ///,, ventral
muscles ; ;/, Vjrain ; ft. heart ; /«/,

small ini' •••-tluiii ; mi.

ophthalmic art' ;>..'riov

abdominal an> ullet;

f>l. \B ; /)'. ''', mvijiud ;

pyloric division of *tomach

is and

minal

male genital aperture. ( •
J.any. after Huxley.)

proximal edge of the segment to be
moved, the smaller on the extensor
(ext.), the larger on the flexor (fl.)
side, in each case half-way between
the two hinges, so that a line join-
ing the two muscular insertions is
at right angles to the axis of articu-
lation.

The digestive organs are con-
structed on the same general^ plan
;is those of Apus, but present many
striking differences (Fig. 401). The
in '>uili lies in the middle ventral line
of the head, and is bounded in front
by the labrum, at the sides by the
mandibles, and behind by a pair of
delicate lobes, the paragnatka. It
leads by a short wide gullet (oe) into
a capacious stomach, which occupies
a great part of the interior of the
head, and is divided into a large an-
terior or cnrcliac division (c. s), and
a small posterior or pyloric division
(j>s): the latter passes into a narrow
and very short small intestine (md),
from which a somewhat wider large
intestine (lid) extends to the anus (an.),
situated on the ventral surface of the
' telson.

The outer layer of the enteric canal
consists of connective tissue contain-
ing striped muscular fibres : within
this is a single layer of columnar
epithelial cells. In the gullet and
stomach, and in the large intestine,
the epithelium secretes a layer of
chitin, which thus constitutes the
innermost lining of those cavities. It-
is proved by development that the
small intestine, which has no chitinous
lining, is the only part of the enteric
canal developed from the inesriit.
the gullet and stomach arise from
the stomod?»'inn, the large intestine
from the proctoda-um. Thus a \
small portion of the enteric epithelium
is cndodermal.

PHYLUM ARTHROPODA 507

the cardiac division of the stomach the chitinous lining is
kened and calcified in certain parts, so as to form a complex
culated framework, the gastric mill, on which are borne a
median and two lateral teeth, strongly calcified and projecting
into the cavity of the stomach. Two pairs of strong muscles
arise from the carapace, and are inserted into the stomach : when
they contract they move the mill in such a way that the three
teeth meet in the middle and complete the comminution of the
food begun by the jaws. The separation of the teeth is effected
partly by the elasticity of the mill, partly by delicate muscles in
the walls of the stomach. The pyloric division of the stomach
forms a strainer: its walls are thickened and produced into
numerous setae, which extend quite across the narrow lumen and
prevent the passage of any but finely divided particles, into the
intestine. Thus the stomach .has no digestive function, but is
merely a masticating and straining apparatus. On each sid<T~oT
the cardiac division is found at certain seasons of the year a
plano-convex mass of calcareous matter, the gastrolith.

The digestion of the food and to some extent the absorption of
the digested products are performed by a pair of large glands (lr.),
lying one on each side of the stomach and anterior end of the
intestine. They are formed of finger-like sacs or cceca, which
discharge into wide ducts opening into the small intestine, and
are lined with glandular epithelium derived from the endoderm
of the embryo. The glands are often called livers, but as the
yellow fluid they secrete digests proteids as well as fat, the name
hepato-pancreas is often applied to them, or they may be called
simply digestive glands. The Crayfish is carnivorous, its food con-
sisting largely of decaying animal matter. Microscopic glands
occur in the wall of the gullet.

The digestive organs and other viscera are surrounded by a
'body-cavity, which is in free communication with the blood-
vessels and itself contains blood. As will be pointed out more
particularly hereafter, this cavity is to be looked upon as an
immense blood-sinus, and not as a true ccelome.

There* are well-developed respiratory organs, in the form of
gilhl contained in a harrow branchial chamber, bounded internally
by the proper wall of the thorax (Fig. 404, ep)y externally by the
gill-cover or pleura! region of the carapace (I'd). Each gill con-
sists of a stem giving off niTiiit ^us branchial filaments, so that
the whole organ is plume-like. 'i'L0 filaments are hollow, and
communicate wit-h [;,\,, parallel canals In the stem — an external,
the filial vein, and ah internal, the cj/'c/'cnt branchial

rein. Th°. gil- is to be considered as an out-pushing of the
body-will, and contains the same layers — a thin layer of
chitin externally, then a single layer of epithelial cells, and
beneath this connective tissue, hollowed out for the blood

ben<

508

ZOOLOGY

channels and containing gland-cells, which will be referred to
presently.

According to their point of origin, the gills are divisible into
three sets — first, podobra ncliice or foot-gills, springing from the
epipodites of the thoracic appendages, from which they are only

FJC. 40-2.— Respiratory organs of Astacus fluviatilis. In A the gill-covi
' the gills undisturbed; in JJ the pi" are removed and the outer orthrol

turned down, oj., antennule ; '(.,., antrim;i : abi.\ tirst ;,//..,.. srcond

inner arthn< nathite ;

11 — 13, pleurobmndiiie ;

. 7—13, podobranchs ; p>. 1, first pi-
r Huxley.)

partially separable ; SOCOIK^, <trtli.mlmincli''f<' <">r joint-gills,

ing from the articular' incinbianes connectin the

appendages with the trunk; and thirdly, pl&u

gills, spi-ino-ing from the lateral walls of the th«)hix, ;\bo\c il

attachment of th<- a])[)cndagcs. ' It is infernxl fr(,ni tin- study •

othr !i<is, that a tv]»ical thoracic segment bcuis four

one pod<>-. two anlii-o-, and one pleurobrancbia. 1 ',ut in Pofamobia

;' \val

PHYLUM ARTHROPODA

509

• mo or more of the, gills in every segment are absent or vestigial,
mid the following table, or " branchial formula/' shows the actual
number and arrangement of these organs, ep standing for epipodite,
and r for the vestige of a gill.

v.

THORACIC
SEGMENTS.

I. It III. IV. V.

VI.

VII.

VIII.

TOTAL.

Ddobranchise...

Q+ep 1 + ep 1 + ep

l+ep

l+ep

l+ep

l+ep

0

6+7ep«

rthrobranchise •

o r* 2 2

2

2

2*

0

11

leurobranchiae

0 0 0 0 0\

';

r

1

l + 2r

TOTAL

Q + cp 2 + €••}>

3 + ep

3 + ep

S + r + ep

3 + r + ep

1

l£+2r + 7fip

By adding up the columns vertically we get the number of gills
in each segment ; by adding them horizontally, the number of each
kind of gill ; and by adding together the results obtained by either
method, the total number of gills, viz., eighteen complete gills
with two vestiges and seven epipodites.

The excretory organs differ both in position and in form
from those of Apus. There are no shell-glands, but at the base
of each antenna is an organ of a greenish colour, the antennary
•or green gland, by which the function of renal excretion is per-
formed. The gland (Fig. 403) is cushion-shaped, and consists of
three parts — (1) a central saccule (s.) of a yellowish colour, occupy-
ing the mid-dorsal region, and consisting of a sac divided into
numerous compartments by partitions, and communicating with
{2) the outer or cortical portion (c. p.), of a green colour, consisting
of a glandular network formed of anastomosing canals, and com-
municating in its turn with (3) a white portion (w. p.), formed of a
single tube partly converted into a sponge-work by ingrowths
of its walls. The whole organ is lined by glandular epithelium,
and the white portion discharges into a thin-walled sac or urinary
bladder (U.) which opens by a duct (d.) on the proximal segment of
the antenna. The glands already referred to as occurring in the
gills are also supposed to have an excretory function.

The circulatory organs are in a high state of development.
The &crtr£ (Figs. 401, .404, A.) is situated in the dorsal region of the
thorax, and is a roughly polygonal muscular organ pierced by
three pairs of apertures or~os&q (p.), guarded by valves which open
inwards. It is enclosed in "jT^pacious 'pcr-iw.rdwl sinus (Fig. 404,
pc.), which contains blood. From the heart spring a number of
narrow tubes, called arteries, which serve to convey the blood to
various parts of the body. At the origin of each artery from the

510

ZOOLOGY

SECT..

heart are valves which allow of the flow of blood in one f^e*
only, viz. from the heart to the artery. From the anterior end
of the heart arise five vessels — the median ophthalmic artery
(Fig. 401, oa.), which passes forwards to the eyes ; paired an-
tennary arteries (aa.\ going to the antennules, antenna, green
glands, &c., and sending off branches to the stomach ; and paired

FIG. 403.— Diagram of kidney of Astacus flu vi at ills. I, unravelled ; II, the parts in their
natural relations. II. bladder ; c. p. cortical portion ; d. duct; s. sacciili ; w. p. white
portion. (After Marchal.)

hepatic arteries, going to the digestive glands. The posterior end
of the heart gives off two unpaired arteries practically united
at their origin, the dorsal abdominal artery (oaa.), which passes
backwards above the intestine, sending branches to it and to the
dorsal muscles ; and the large sternal artery (sa.), which passes
directly downwards, indifferently to right or left of the intestine,
passing between the connectives, uniting the third and fourth

XI

PHYLUM ARTHROPODA

511

thoracic ganglia, and then turns forwards and runs in the sternal
canal, immediately beneath the nerve-cord, and sends off branches
to the legs, jaws, &c. At the point where the sternal artery turns
forwards it gives off the median ventral abdominal artery (itaa.),
which passes backwards beneath the nerve-cord, and supplies th*
ventral muscles, pleopods, &c.

All these arteries branch extensively in the various organs they
supply, becoming divided into smaller and smaller offshoots, which
finally end in microscopic
vessels called capillaries.
These latter end by open
mouths which communicate
with the Uood-sinuscs (Fig.
405, s.), spacious cavities lying
among the muscles and vis-
cera, and all communicating,
mediately or immediately,
with the sternal sinus (st.s.),
a great median canal run-
ning longitudinally along the
thorax and abdomen, and
containing the ventral nerve-
cord and the sternal and
ventral abdominal arteries.
In the thorax the sternal
sinus sends an offshoot to
each gill in the form of a
well-defined vessel, which
passes up the ou^er side of
the gill and is called the
afferent branchial vein (af.br. v.]
see also Fig. 404). , Spaces
in the gill-filaments place the
afferent in. communication
with the efferent branchial
vein (ef.br.v.), which occupies
the inner side of the gill-
stem. The eighteen efferent branchial veins open into six
(>j-anchio-cardiac veins (br.c.v.), which pass dorsally in close contact
with th<- lateral wall of the thorax and open into the pericardial

H'nus (pcd.s.).
The whole of this system of cavities is full of blood, and the
sart is rhythmically contractile. When it contracts, the blood
contained in it is prevented from entering the pericardial sinus by
the closure of the valves of the ostia, and therefore takes the only
other course open to it, viz., into the arteries. When the heart
relaxes, the blood in the arteries is prevented from regurgitating

FIG. 404.— Transverse section of thorax of Cray-
fish, diagrammatic, abm. ventral abdominal
muscles; bf, leg; bui, ventral nerve cord; </,
intestine ; dbm. dorsal muscles of abdomen ;
ep, wall of thorax ; h. heart ; k, gills ; M, gill-
cover ; /. liver; or. ovary ;' j)c. pericardial
sinus ; sa. sn, sternal artery ; vs. ventral sinus.
The arrow shows the direction of the blood -
current. (From Lang's Comparative Anatomy.)

512 ZOOLOGY SECT.

by the valves at their origins, and the pressure of blood in the
pericardial sinus forces open the valves of the ostia and so fills
the heart. Thus in virtue of the successive contractions of the
heart, and of the disposition of the valves, the blood is kept con-
stantly moving in one direction — viz., from the heart by the
arteries to the various organs of the body, where it receives car-
bonic acid and other waste matters ; thence by sinuses into the
great sternal sinus ; from the sternal sinus by afferent branchial
veins to the gills, where ifc exchanges carbonic acid for oxygen ;
from the gills by efferent branchial veins to the branchiocardiac
veins, thence into the pericardial sinus, and so to the heart once
more.

It will be seen that the circulatory system of the Crayfish con-
sists of three sections — (1) the heart or organ of propulsion; (2/a

pcd.s

af . br v

St.S

FIG. 405. — Diagram of the circulation in the Crayfish j heart and arteries scarlet, veins and
sinuses containing non-aerated blood, blue ; those containing aerated blood, pink. a. artery ;
itj. l>,-.c. afferent branchial vein; br.c.c. branchio-cardiac vein; ef.hr. r. efferent branchial
vein; lit. heart; pcd.s. pericardial sinus; x. sinus; xt.x. sternal sinus; f1, ostium with
valves ; v2. arterial valves. The arrows show the direction of the current.

system of out-going channels, the arteries, which carry the blood
from the heart to the body generally ; and (3) a system of return-
ing channels, some of them, the sinuses, mere irregular cavities ;
others, the veins, with definite walls, which return it from the
various organs back to the heart. The respiratory organs, it
should be observed, are interposed in the returning current, so
that blood is taken both to and from the gills by veins.

Comparing the blood-vessels of Astacus with those of a
Cha3topod, it would seem that the ophthalmic artery, heart, and
dorsal abdominal artery together answer to a dorsal vessel, part
of which has become enlarged and muscular, and discharges the
whole function of propelling the blood. The horizontal portion of
the sternal artery, together with the ventral abdominal, represent
a ventral vessel, while the vertical portion of the sternal artery is

PHYLUM ARTHROPODA

513

I commissure, developed sometimes on the right, sometimes on
e left side, its fellow being suppressed.
The blood when first drawn is colourless, but after exposure to
the air takes on a bluish-gray tint. This is owing to the presence
of a colouring matter called hcemocyanin,
which becomes blue when combined with ^

oxygen ; it is a respiratory pigment, and
serves, like haemoglobin, as a carrier of
oxygen from the external medium to the
tissues. The hasmocyanin is contained in
the plasma of the blood: the corpuscles
are all colourless leucocytes.

The nervous system (Fig. 406) con-
sists, like that of Apus, of a brain (g) and
a ventral nerve-cord, united by cesophageal
connectives (sc). But the right and left
halves of the ventral cord have undergone
partial fusion, so that the ganglia, and in
.the abdomen the connectives also, appear
single instead of double. Moreover, the
brain supplies not only the eyes and anten-
nules, but the antennae as well, and it is
found by development that the two pairs
of ganglia belonging to the antennulary
and antennary segments have fused with
the brain proper. Hence we have to dis-
tinguish between a primary brain or arcki-
cerebrum, the eptfiglion of the prostomium,
and a secomary brain or syn-cerebfuni
formed by the union of one or more pairs
of ganglia of the ventral cord with the
archi-cerebrum. A further case of con-
crescence of ganglia is seen in the ventral
nerve-cord, where the ganglia of the last
three cephalic and first three thoracic seg-
ments have united to form a large com-
pound sub-cesopliageal ganglion (bg). All
the remaining segments have their own
ganglia, with the exception of the telson,
which is supplied from the ganglion of the
preceding segment. There is a viscer.al
system of nerves (s) supplying the stomach,
originating in part from the brain and in part from the oeso^
phageal connectives.

Sensory Organs. — The eyes have the same essential structure
as the compound eye of Apus. The chitinous cuticle covering
the distal end of the eye-stalk is transparent, divided by delicate

VOL. I L L

FIG. 406.— Nervous system of
Astacus fluviatilis.

bff. sub-03sophageal gang-
lion ; eg. commissural
ganglion ; ft, brain ; .«, vis-
ceral nerve ; sc, oesopha-
geal connective ; ?/, post-
cesophageal commissure ;
IV — VIII, thoracic gang-
lia ; 1 — 6, abdominal gang-
lia. (From Lang's Com-
•/>«i-«fi,-i- Anatomy, after
Vogt and Yung.)

514 ZOOLOGY SECT.

lines into square areas or facets, and constitutes the cornea. Be-
neath each facet of the cornea is an ommatideum, optically
separated from its neighbours by black pigment, and consisting
of an -•outer segment or vitreous body, and an inner segment or
retimtla formed of sensory cells enclosing a rhabdome.

The antennules contain two sensory organs, to which are assigned
the functions of smell and hearing respectively. The olfactory
organ is constituted by a number of extremely delicate olfactory
setae, borne on the external flagellum, and supplied by branches of
the antennulary nerve. /• The auditory organ is a sac "formed by
invagination of the dorsal surface of the proximal segment, and is
in free communication with the surrounding water by a small
aperture. The chitinous lining of the sac is produced into delicate
feathered auditory setm, supplied by branches of the antennulary
nerve, and in the water which fills the sac are minute sand-
grains, which take the place of otoliths, but, instead of being
formed by the animal itself, are taken in after each ecdysis,
when the lining of the sac is shed. Many of the setae on the
body generally have a definite nerve-supply, and are probably
tactile organs.

Reproduction. — The Crayfish is dioecious, and presents a very
obvious sexual dimorphism. The abdomen of the female is much
broader than that of the male : the first and second pleopods of
the male are modified into tubular or rather spout-like copulatory
organs (Fig. 297, 9) ; and the reproductive aperture is situated in
the male on the proximal podomere of the fifth leg, in the female
on that of the third.

The testis (Fig. 407, B, t, u) lies in the thorax, just beneath the
floor of the pericardial sinus, and consists of paired anterior lobes
(t) and an unpaired posterior lobe (u). From each side goes off a
convoluted vas deferens (vd), which opens on the proximal segment
of the last leg. The sperms are curious non-motile bodies pro-
duced into a number of stiff processes (Fig. 20, f) : they are
aggregated into vermicelli-like spermatophores by a secretion of the
vas deferens.

The ovary (A, ov. u) is also a three-lobed body, and is similarly
situated to the testis : from each side proceeds a thin- walled
oviduct (od), which passes downwards, without convolutions, to
open on the proximal segment of the third or antepenultimate
leg. The eggs are of considerable size and are centrolecithal.

As in Apus, both ovary and testis are hollow organs, discharging
their products internally. The ova, when laid, are fastened to the
setoe on the pleopods of the female by the sticky secretion of
glands occurring both on those appendages and on the segments .
themselves : they are fertilised immediately after laying, the male
depositing spermatophores on the ventral surface of the female's
body just before oviposition.

XI

PHYLUM ARTHROPOD A

515

Development. — The process of segmentation of the oosperm pre-
sents certain striking peculiarities. The nucleus (Fig. 408, A, nu)
divides repeatedly, but no corresponding division of the protoplasm
takes place, with the result that the morula-stage, instead of being

FIG. 407.— Reproductive organs of Astacus fluviatilis. A, female ; B, male ; od. oviduct ;
re, ts external opening ; or. ovary ; t. testis ; u. unpaired posterior portion of gonad ; f<J. vas
deferens. (From Lang's Comparative Anatomy, after Huxley.)

a heap of cells, is multinucleate but non-cellular. Soon the nuclei
thus formed retreat from the centre of the embryo, and arrange
themselves in a single layer close to the surface (B) : around each
of these protoplasm accumulates, the central part of the embryo
sisting entirely of yolk-material. We thus get a superficial

FIG. 40S

—Three stages in the formation of the blastoderm of Astacus fluviatilis. nv,
nuclei ; y. p. -yolk-pyramids. (From Korschelt and Heider, after Morin and Reichenbach.)

segmentation, characterised by a central mass of yolk and a super-
ficial layer of cells collectively known as the blastoderm (C). Sub-
sequently the yolk itself undergoes a process of segmentation,
becoming divided into radiating yjlk-pyramids(y.p.), each with its
base in contact with one of the cells of the blastoderm and its

L L 2

516

ZOOLOGY

apex pointing to the centre of the egg : before long, however,
these pyramids fuse into an undivided mass of yolk.

The first indications of the future Crayfish take the form of
thickenings on what will become the ventral surface. There are
at first five of these thickenings — two anterior, the head-lobes
(Fig. 409, K), on which the eyes subsequently appear ; two some-
what further back, the thoracico-abdominal rudiments (TA)', and
one, posterior and unpaired, the endoderm-disc (J2S). On the latter
an invagination of the blastoderm takes place, giving rise to a
small sac, the archenteron, which communicates with the exterior
by an aperture, the llastopore. By this process the embryo passes

Fio. 409. Early embryo of Astacus. BM mesoderm ; ES, endoderm disc ; K', head-lobes

TA, thoracico-abdominal rudiments. (From Lang's Comparative Anatomy, after Reichenbach.)

into the gastrula-stage, which, however, differs from the corre-
sponding stage in the types previously studied in the immense
quantity of food-yolk filling up the space (blastoccele) between
ectoderm and endoderm. Very soon the embryo becomes tri-
ploblastic, or three-layered, by the budding off of cells from the
endoderm in the neighbourhood of the blastopore : these accumu-
late between the ectoderm and endoderm, and constitute the
mesoderm.

Before long the blastopore closes, converting the archenteron
into a shut sac (Fig. 411, A) : the thoracico-abdominal rudiments
unite with one another, forming a well-marked oval elevation
(Fig. 410, TA), and three pairs of elevations appear between i
and the head-lobes. These are the rudiments of the first thr
pairs of appendages, the' antennules (ar), antennae (a.2,), an<^
dibles (m.)\ by their appearance the embryo passes into th

XI

PHYLUM ARTHROPODA

517

nauplius-stage, which in this case is passed through in the egg,
instead of being active and free-swimming as in Apus.

Between the bases of the antennules and antennae a pit appears,
which soon deepens and widens: it is the stomodceum (Fig. 411,
stdm.), and its aperture the mouth. A similar but narrower and
more cylindrical pit appears on the thoracico-abdominal rudiment :
it is the proctodceum (pcdm.), and its aperture the anus. For a
considerable time both stomodgeum and proctoda3um remain in
the condition of blind sacs, but after a time they open into the
archenteron, a complete enteric canal being thus constituted. In
"le meantime the endoderm cells lining the archenteron grow

FIG. 410,— Nauplius-stage of Astacus. A, (above) eye; A, (below) anus; aj. antennule ;
no. antenna ; G, cerebral ganglion ; gct^. anteimary ganglion ; gm, mandibular ganglion ;
I. labruni ; m. mandible ; TA, thoracico-abdominal rudiment. (From Lang's Comparative
Anatomy, after Reichenbach.)

outwards "in a radial direction, ingesting the yolk as they do so,
until they take the form of long columns, in contact by their outer
ends with the ectoderm (Fig. 411, B).

The thoracico-abdominal rudiment soon begins to increase
rapidly in length, but, being enclosed in the egg-membranes, it
grows not backwards but forwards, being in fact folded upon the
anterior part of the body in much the same way as the abdomen
of the adult during extreme flexion. Thus in Fig. 412 the ventral
surface of the head and anterior thoracic region faces the observer,
but the dorsal surface of the posterior thoracic and abdominal
regions : in order to bring the parts into their adult position, the
abdomen must be supposed to be lifted up and turned backwards.

518

ZOOLOGY

SECT.

In the meantime the post-mandibular appendages are formed
in regular order from before backwards : the eye-stalks appear
(Fig. 412, A\ as well as the labrum (/.) and a fold on each side
of the thorax, which is the rudiment of the carapace (ts), and
gradually extends dorsally until it meets with its fellow of the

ntes

fib

focd

ctbci

FIG. 411.— Sections of embryos of Astacus. A, Nauplius-stage (cf. Fig. 410) ; B, after develop-
ment of thoracic appendages (cf. Fig. 412). abd. abdomen ; an. anus ; br. brain ; ect. ectoderm ;
end. endoderm ; ent. enteron ; lit. heart ; tnes. mesoderm ; mes.' splanchnic layer of mesoderm ;
mth. mouth ; pcdm. proctodseum ; stdm. stomodfeum ; th. abd. thoracico-abdominal rudiment ;
v. nr. cd. ventral nerve cord. (From Korschelt and H eider, after Reichenbach.)

opposite side and covers in the cephalothorax. , The embryo now
consists of a nearly globular cephalothorax with a small abdomen
and a nearly complete set of appendages, all tucked in under the
cephalothorax and closely packed together within the egg-mem-
branes. In this condition the embryo is hatched, and for some

PHYLUM ARTHROPODA 519

.

time clings to the pleopods of the mother by means of the peculiarly
hooked chelae of its first pair of legs.

The development of the principal internal organs must be
referred to very briefly. From the ectoderm arise, no't only the
epidermis of the adult, but the epithelium of the gullet and
stomach and of the large intestine, the epithelium of the gills, the
nervous system, the vitreous cells and retinulse of the eyes, and
the epithelium of the auditory sac. From the endoderm arises
the epithelium of the small intestine and of the digestive glands,

FIG. 412. — Embryo of Astacus after development of thoracic appendages. A, eyes ; a\. an-
tennule ; 0%. antenna ; ab. abdomen ; g, archicerebrum and ganglion of antennule ; go, optic
ganglion ; I. labrum ; m. mandible ; mx\. mx^. maxilla? ; t. 1 — 8, thoracic appendages ;
t. telson ; ts, carapace. (From Lang's Comparative Anatomy.)

the latter being formed as tubular branching outgrowths of the
archenteron. The connective-tissues, the muscles, the vascular
system, the gonads, and perhaps the kidneys, are all of mesodermal
origin.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Crustacea are Arthropods in which the five l anterior seg-
ments are fused with the prostomium to form the head, while the
rest are usually divisible into two regions, the thorax and the
abdomen. More or fewer of the thoracic segments may be fused
with the head to form a cephalothorax. The head may bear a

1 Or four if the antenmilary region is counted as part of the prostomium.

520 ZOOLOGY SECT.

median eye, which frequently disappears in the adult, and a pair
of compound eyes, both belonging to the prostomial region : the
latter frequently become elevated on jointed eye-stalks. The
appendages of the head are (1) the antennules, which are usually
considered as belonging to the first metamere, but are perhaps
more correctly to be looked upon as prostomial ; (2) the antennae,
which are certainly post-oral or metameric appendages shifted for-
wards to a prae-oral position; (3) the mandibles or crushing jaws ; (4)
the first maxillae ; and (5) the second maxillae. The thoracic and
abdominal appendages are variously modified as jaws, legs, fins, or
accessory reproductive organs. With the exception of the anten-
nules, the appendages are typically biramous, consisting of a stem
or protopodite bearing two branches, the endopodite and exopodite.

The body is covered externally by a chitinous cuticle, which
becomes thickened and sometimes calcified in regions where no
movement is required, forming a series of hard parts or sclerites,
separated by flexible chitin : the whole chitinous cuticle thus
constitutes an exoskeleton. Typically there is one sclerite to each
metamere behind the head, and to each podomore in the append-
ages, but concrescence of sclerites frequently takes place. The
exoskeleton is produced into setae, which are hollow processes of
the cuticle, containing prolongations of the underlying epidermis.

Respiration takes place either by the general surface of the
body or by gills, which are hollow offshoots of the thoracic wall or
of the thoracic or abdominal limbs. The stomodaeum and proc-
todaeum form a considerable portion of the enteric canal, and are
lined with chitin : the mesenteron gives rise to digestiv^ glands.
The body-cavity is divided into compartments, most of which
contain blood and are portions of the vascular system : the true
coelome may be represented by compartments of the body-cavity
not containing blood and by the cavities of the reproductive organs.
There is a vascular system consisting of a contractile heart, formed
as a muscular dilatation of a dorsal vessel, and communicating by
valvular ostia with an enclosing pericardial sinus. The blood is
taken from the heart to the various organs by arteries, and is
returned to the pericardial sinus by sinuses and veins : the re-
spiratory organs are interposed in the returning current. The
renal organs are peculiarly modified nephridia, which may take
the form either of shell-glands opening on the second maxilla, or
of antennary (green) glands opening on the antenna.

The nervous system consists of a brain united by cesophageal
connectives with a ventral nerve-cord, formed of a double chain
of ganglia joined together by commissures and connectives. The
first three pairs of embryonic ganglia commonly unite to form
the brain, which is therefore a syn-cerebrum. The sexes are
separate or united : sexual dimorphism is common : partheno-
genesis frequently occurs. The sperms are usually non-motile :

PHYLUM ARTHROPODA 521

ie eggs are usually centrolecithal but may be telolecithal, or
almost alecithal. The muscles are striped, and there are no cilia.
Segmentation of the oosperm is usually superficial, but may be
complete or discoid. The embryo passes though a nauplius stage,
which may be a free-swimming larva or may be passed through
before hatching, and is characterised by the presence of three
pairs of appendages which become the antennules, antennae, and
mandibles of the adult.

The Crustacea are classified as follows : —

Sub-Class I. — Entomostraca,

Crustacea of comparatively simple organisation, and usually of
small, often almost microscopic size. The number of post-cephalic
segments is variable, and the appendages show comparatively
little differentiation. A large cephalic carapace is often present.
A considerable portion of the enteric canal is derived from the
mesenteron, and there is no gastric mill. The excretory organs
are shell-glands. There is a metamorphosis, and the larva usually
leaves the egg as a free-swimming nauplius.

ORDER 1. — PHYLLOPODA.

Entomostraca in which the body is distinctly segmented and
is covered by a cephalic carapace. The post-cephalic appendages
are leaf-like.

Sub- Order a. — Euphyllopoda.

Phyllopoda of considerable size, with 10-60 pairs of leaf-like
swimming-feet.

This group includes Apus and allied genera, such as Branchipus,
Artemia, and Estheria (Fig. 413).

Sub-Order 1). — Cladocera.

Small Phyllopoda with compressed body inclosed in a bivalved
carapace, and with four or five pairs of swimming-feet. The chief
organs of tocomotion are the large biramous antennae.
• Including Daplmia, Leptodora, &c. (Fig. 414).

ORDER 2. — OSTRACODA.

Small Entomostraca having the body inclosed in a carapace or
shell formed of articulated right and left valves. The body is
unsegmented and the abdomen rudimentary. There are only
seven pairs of appendages. The sperms exhibit motility after
reaching the female ducts.

Including Cypris, Cy 'there, &c. (Fig. 415).

522 ZOOLOGY SECT.

ORDER 3. — COPEPODA.

Entomostraca, mostly of small size, having an elongated body
distinctly segmented except in certain parasitic forms. The
carapace may extend over the first thoracic segment. The thorax
bears, in the free forms, four or five pairs of biramous appendages :
the abdomen is limbless. Many species are parasitic, and show
various stages of degeneration of structure.

Sub- Order a. — Eucopepoda.

Free or ectoparasitic Copepoda having biramous swimming-feet
and biting jaws, or, in the case of parasitic forms, a more or less
degenerate structure, and jaws often adapted for sucking. The eggs
undergo development in paired brood-pouches attached at the base
of the abdomen.

In this group are included (a) free-swimming forms, such as
Cyclops (Water-flea), and (ft) parasitic forms, or Fish -lice — e.g.
Ergasilus, Chondr acanthus, Lerncea (Figs. 416 and 417).

Sub-order b. — Branchiura.

Ectoparasitic Copepoda having compound eyes and a suctorial
mouth. The second maxillae form sucking-discs for attachment to
the host. The whole body is strongly depressed, and there are no
brood-sacs.

Including the carp-lice, Argulus (Fig. 418).

ORDER 4. — CIRRIPEDIA/

Imperfectly segmented marine Entomostraca, often of con-
siderable size, and either fixed during adult life or parasitic.
The sexes are united, and the sperms are motile.

Sub-order a. — Eucirripedia.

Fixed or parasitic Cirripedia in which the body is usually
enclosed in a fold of skin, strengthened by calcareous plates. The
abdomen is rudimentary. There are usually six pairs of biramous
appendages.

This group includes (a) fixed forms, such as Lepas (Barnacle)
zxi^Balanus (Acorn-shell), and (ft) parasites — e.g. Petrarca, Al-
cippe, Prcteolepas (Figs. 419 and 420).

Sub-order b. — Ehizocephala.

Parasitic Cirripedia in which the body has undergone extreme
degeneration, having no trace of segmentation or of appendages in
the adult condition. The juices of the host are absorbed through
long root-like processes.

Including Sacculina and Peltogaster (Fig. 421).

xi PHYLUM ARTHROPODA 523

Sub-Class II. — Malacostraca.

Highly-organised Crustacea, usually of considerable size, and
having, -except in one order, a thorax of eight, and an abdomen of
seven segments. The appendages are usually highly differentiated.
As a rule the mesenteron forms only a small portion of the adult
enteric canal, and there is a gastric mill. The renal organs are
antennary glands. The nauplius stage is usually passed through
in the egg, but there is a more or less complex metamorphosis.

ORDER 1. — PHYLLOCARIDA.

Small Malacostraca approaching the Entomostraca in structure.
The body is enclosed in a large bivalved cephalic carapace. The
thoracic feet are leaf-like, the abdominal feet biramo-us. The
abdomen has eight segments and a pair of caudal styles.

The principal genus is Ncbalia (Fig. 422).

ORDER 2. — SCHIZOPODA.

Small shrimp-like Malacostraca having the thorax more or less
completely covered by a soft carapace. The thoracic appendages
are all biramous. The eyes are stalked.

Including Mysis (Opossum-shrimp), Euphausia, &c. (Fig. 423).

ORDER 3. — DECAPODA.

Malacostraca in which all the thoracic segments are united with
the head to form a cephalothorax usually covered by a carapace.
The three anterior pairs of thoracic limbs are biramous foot-jaws,
the posterior five pairs are walking legs devoid of exopodites. The
third maxillipedes are leg-like. The eyes are stalked, the gills
thoracic.

Sub-order a. — Macrura.

The abdomen is usually larger than the cephalothorax, and is
commonly held in an extended position. There is usually a distinct
rostrum. The eyes are not enclosed in orbits. The antennules and
antennae are large, the antennules are not sunk in pits, and the
antennse have an exopodite or squame.

Including (a) swimming forms — Penceus and Palcemon (Prawns),
Crangon (Shrimp), Lucifer, &c. ; (/#) creeping forms — ffomarus
(Lobster), Astacus, Astacoides, Paranephrcps (Fresh-water Cray-
fishes), Palinurus (Rock-lobster), Scyllarus, &c. ; (7) anomalous
forms, approaching the Brachyura — Pagurus (Hermit-crab), Birgus
(Cocoa-nut crab) ffippa, &c. (Figs. 424 — 426).

Sub-order b.<* — Brachyura.

The abdomen is shorter than the cephalothorax and is per-
manently flexed under it. The eyes are enclosed in orbits or
tubular cavities of the carapace. The antennules and antennae are

524 ZOOLOGY SECT.

small : the bases of the antennules are sunk in pits of the carapace,
and the antenna has no exopodite. The third maxillipedes are
flattened and valve-like.

Including the true crabs, such as Cancer, Maia, Dromia, &c.

(Figs. 427 and 428).

ORDER 4. — STOMATOPODA.

Tlmracostraca of considerable size in which the three posterior
thoracic segments are not covered by the carapace. The abdomen
is very large. The five anterior thoracic limbs are maxillipedes,
and the second of them is very large and forms a defensive weapon :
the last three are small biramous legs. The gills are borne on the
abdominal segments. The eyes are stalked.

Including Squilla and Gonodactylus (Fig. 429).

ORDER 5. — CUMACEA.

Small shrimp-like Malacostraca having the first three or four
thoracic segments fused with the head, the rest free. The two
first pairs of thoracic appendages are maxillipedes, the remaining
six legs ; some of the latter are biramous. The eyes are sessile.

Including Cuma, Diasiylis, &c. (Fig. 430).

ORDER 6. — ARTHROSTRACA.

Malacostraca in which the first, sometimes also the second,
thoracic segment is fused with the head and bears maxillipedes :
the remaining seven are free and bear legs. The eyes are usually
sessile.

Sub-order a. — Amphipoda.

Arthrostraca in which the body is usually compressed or flattened
from side to side. The gills are borne on the thoracic ap-
pendages.

Including Gammarus (Fresh-water shrimp), Orchestia (Saiid-
hopper), Phronima, Gaprella, Cyamus (Whale-louse), &c. (Figs. 431
and 433).

Sub-order b. — Isopoda.

Arthrostraca in which the body is usually depressed or flattened
from above downwards. The gills are borne on the abdominal
appendages.

Including (a) free forms — Asellus, Oniscus (Wood-louse), Arma-
dillo (Pill-bug) ; (ft) parasitic forms — Cymothoc, Bcpyrus, &c. (Figs.
432 and 434).

Systematic Position of the Examples.

The genera Apus and Lepidurus belong to the family Apodidce,
sub-order Euyhyllopoda, order Phyllopoda, and sub-class Entomo-
straca.

xi PHYLUM ARTHROPODA 525

The indefinite number of segments, varying considerably in the
species of both genera, and the presence of a shell-gland place them
among the Entomostraca.

The leaf-like character of the swimming-feet is alone sufficient
to assign them to the Phyllopoda, and the large number (consider-
ably more than ten) of segments and swimming-feet decides their
position among the Euphyllopoda.

They are placed in the family Apodidse in virtue of the elongated
body with 40-60 pairs of swimming-feet, diminishing in size from
before backwards, and showing considerable differentiation ; and of
the elongated heart reaching to the twelfth post-cephalic segment.

Apus is distinguished by the absence of a post-anal plate, and
by elongated flagella (endites) to the first pair of thoracic feet : in
Lepidurus the post-anal plate is present, and the flagella of the first
thoracic feet are short.

Astacus fluviatilis is one of several species of the genus Astacus,
belonging to the family Potamobiidce, tribe Astacoidea, sub-order
Macrura, order Decapoda, and sub-class Malacostraca.

The possession of twenty-one segments — i.e. a prostomium, nine-
teen metameres, and a telson — places it among the Malacostraca :
the presence of five pairs of thoracic legs without exopodites, and
of thoracic gills, among the Decapoda.

The possession of a squame to the antenna, and of legs having all
seven podomeres distinct — the first three pairs chelate, and the first
pair greatly enlarged — determine its position in the tribe Astacoidea,
which includes all the fresh-water Crayfishes and the true Lobsters.
The family Potamobiidas is distinguished by having the podo-
branchiaB partly united to the epipodites, and by having append-
ages on the first abdominal segment of the male, and usually
on that of the female.

3. GENERAL ORGANISATION.

There is no class in the animal kingdom which presents so wide
a range of organisation as the Crustacea, or in which the devi-
ations in structure from the " type-form " are so striking and so
interesting from their obvious adaptation to the mode of life.

The most interesting modifications are those connected with
the external characters and the structure of the append-
ages. As we have seen, the body consists of a prostornium, a
variable number of metameres, and an anal segment. The first
five metameres — or four if the antennulary region is not reckoned
as a metamere — fuse with the prostomium to form a head, which,
as well as the anal segment, is homologous throughout the class.
On the other hand, there is no strict homology between the
various post-cephalic metameres in different forms until we come
to the Malacostraca, in which their number is constant.

526

ZOOLOGY

SECT.

There is considerable diversity of form among the Eupliyllopoda.
Apus has already been described. Branchipus (Fig. 413, 1) and
Artemia (the Brine-shrimp), are small shrimp-like forms, the
former living in fresh-water lakes, the latter in brine-pools ; they
have no carapace, and the eyes are raised on unjointed stalks. In
Limnetis (2), on the other hand, the carapace is large enough to
cover the whole body, and in Estheria (3) it takes the form of a
shell t formed of two parts or valves, united by a hinge, and resem-
bling the shell of a cockle or other bivalved mollusc. The limbs

anil

ov

1. Branchipus

ant. 2

3. E sl-heri a

FIG. 413.— Three Euphyllopoda. In 3, a is the shell, I the animal with one valve of the shell
removed ; anft. antennule ; ant2, antenna ; ht. heart ; m. adductor muscle ; md. mandible ;
or. f ovary _, a. unpaired process from head ; p, copulatory appendages ; sh.gl. shell-gland ;
t. testis. (After Gerstaecker.)

have the same general structure as those of Apus, but the
antennae are often of considerable size, and are sometimes modified
into prehensile organs.

In the Cladoccra, of which the common fresh-water Daphnia
(Fig. 414, 1) is a good example, there is a great reduction in size
(1-2 mm.), and a corresponding shortening of the body, by a
reduction in the number of metameres. Segmentation is very
imperfect, and the whole body is covered by a large folded
carapace. The abdomen is turned downwards and is in constant
movement, sweeping out any foreign particles which may have
made their way among the feet. Between the abdomen of the

PHYLUM ARTHROPODA

527

female and the posterior part of the carapace is a large brood-
pouch (br.p), in which the eggs are stored. The paired eyes
(E) have fused into a single organ, which exhibits a constant
trembling movement. The antennules (ant.l) are small, the
antennaB (ant.2) very large, biramous, and constitute the chief
organs of locomotion. The mandibles are large, the second
maxillaB absent in the adult, and there are five pairs of leaf-like
swimming-feet (/) on the thorax. The abdomen is devoid of
appendages. Many of the Cladocera have an extraordinarily

ant. 2

414. — Three Cladocera. ant.l, antennule ; ant. 2, antenna ; br. brain ; hr.p. brood-pouch ;
E. eye ; d.fil. digestive gland ; /. swimming-feet ; ht. heart ; md. mandible ; sh.gl. shell-
gland. (1 after Glaus, 2 and 3 after Gerstaecker.)

grotesque form (2, 3), owing to the peculiar shape of the head, the
immense antennae, and the great hump-like brood-pouch.

„

LThe Ostracoda are usually not more than 1-2 mm. in length,
d are found both in fresh and sea-water. One of the commonest
genera is Cypris, which occurs in immense numbers in stagnant
pools. Qythere is a common marine form.

The body (Fig. 415) is unsegmented, and is completely enclosed
in a carapace (A), the right and left halves of which are articu-
_ lated together along the dorsal edge so as to form a bivalved shell
(C), which may be variously ornamented or sculptured. The

528

ZOOLOGY

SECT.

valves are opened by the elasticity of a ligament, which passes
from one to another at the hinge, and are closed by a large
(i<!<fuctor muscle (m.), which passes transversely from valve to valve,
its insertions giving rise to markings on the shell (A, m.), often
of systematic value.

At the anterior end is a median eye (e), and in some forms
compound eyes are present as well. There are only seven pairs
of appendages. The antennules (ant.l) and antennae (ant. 2) are
large and uniramous. The mandible (md.) has a large leg-like palp
and a flabellum-like offshoot. The first maxilla (mx,l) also bears
a large plate resembling a flabellum of Apus. The last cephalic

FIG. 415. — A, external view of Cypris ; B, the same with the appendages exposed by the removal
of the left valve of the shell ; C, transverse section ; D, a single sperm. <ibd. abdomen ;
ant.l, antennule ; ant.3, antenna; d.gl. digestive gland; e. median eye; f.l,f.2, thoracic
feet; int. intestine ; m. adductor muscle; mil. mandible; mx.l, mx.2, maxillae; ov. ovary;
sh. shell ; t. testis. (After Gerstaecker.)

appendage (second maxilla, mx.2) is jaw-like in some forms
(Cypris) leg-like in others ( Cy there). The only thoracic appendag
are two pairs of slender legs ( f.l, f.2). The abdomen (dbd.)
devoid of appendages, and is terminated by strong setae.

ns

r

The diversity of form among the Copepoda is so great that it
will be advisable to consider separately the free-swimminj
Eucopepoda, the parasitic Eucopepoda, and the Branchiura.

The free-swimming Eucopepoda are well represented by th<
common water-tiea (Cyclops), found everywhere in fresh am
brackish water, and easily recognisable, in spite of its minul
size, by its elongated form, its rapid, jerky movements, and by th<
egg-sacs of the female.

PHYLUM ARTHROPODA

529

Cyclops (Fig. 416, 1) has been compared in form to a split pear,
the broad end being anterior, and the convex surface dorsal,
e first thoracic segment is fused with the head, and the

anl.Z

CLlt.f

2 . C a I o1 c a lanus

FIG. 41C. — la, female Cyclops, from the right side; b, dorsal view; C, antenna of male;

ID, swimming-foot. aJid.l, first abdominal segment; ant.l, aiitemiule ; ant.%, antenna;
c. th. cephalo-thorax ; e. median eye ; en. endopodite ; e.s. egg-sac ; ex. exopodite ; or. ovary ;
pr.l, />;•.,?, protopodite ; r. rostrum; s.f. swimming -feet ; th.2, th.6, thoracic segments. (After
Huxley, Gerstaecker, Hartog, and Giesbrecht.)

cephalo-thorax (c.tli.) thus formed is covered with a carapace pro-
duced in front into a short spine or rostrum (r), near the base
of which, on the dorsal surface, is the median eye (e). There are
five free thoracic segments : the last (th. 6) bears the genital
VOL. I M M

530 ZOOLOGY SECT.

aperture, and is fused in the female with the first abdominal
(aid. 1). There are four abdominal segments: the last bears the
dorsal anus (an), and a pair of caudal styles produced into plumed
setaB.

The antennules (ant. 1) are very large, and are the principal "
organs of locomotion. In the male they are modified (C.), by a
peculiar form of joint and long setae, as clasping organs, used
for holding the female during copulation. The antennae (ant. 2)
are comparatively short and uniramous. Mandibles and maxillae
are present, and the first four thoracic appendages bear biramous
swimming-feet (s.f, a), those of the right and left sides being
connected by transverse plates or couplers. The fifth thoracic
segment bears a pair of vestigial limbs : the abdominal segments
are limbless.

Some of the pelagic marine Eucopepoda (Fig. 416, 2) are re-
markable for their brilliant colours, and for the extraordinary
development of their setae, especially those of the caudal
styles.

The parasitic Eucopepoda, or Fish-lice, present a very interesting
series of modifications, illustrating the degeneration of structure
which so often accompanies parasitism. Ergasilus (Fig. 417, 1) is
found on the gills of the Bass (Morone labrax) ; it is readily recog-
nisable as a Copepod, but the appendages are greatly reduced, the
antennae modified into hooks for holding on to the host, and the eyes
absent. Anthosoma (2), found in the mouth of the Porbeagle Shark
(Lamna cornubicd), has recognisable appendages, but the form of
the body is much modified by the development of curious overlap-
ping lobes. Nicothoe (3), found on the gills of the Lobster, has
antennas and mouth-parts modified for suction : the abdomen is
normal, but the thorax is produced into huge lobes, which give
it a curiously deformed appearance. In ChondracantTius (4), the
various species of which are parasites on^.the gills of Bony Fishes,
there is, at the first glance, nothing to suggest that the animal
is a Crustacean, except the characteristic copepod egg-sacs : the
body is depressed, unsegmented, and produced into crinkled lobes,
and it requires careful examination to discover that antennules,
hooked antenna? (ant.2) — used for attachment — Lmandibles, maxillae,
and two pairs of legs (f.l, f.2) are present. The male (b) is of
higher organisation than the female, but of minute size — about TV
the length of its mate — and is permanently attached to her body,
close to the genital aperture (ft, M). In Lcrncea (7) and its allies
the body is vermiform with a curiously lobed anterior end : the
maxillae are adapted for piercing the skin of the host and sucking
its juices, and there are minute vestiges of feet. In Lesteira (o)
the degradation is even more marked : the female reaches a large
size — 70 mm. in length, excluding the egg-sacs — and is found
with the swollen head between the skin and flesh of a fish

XI

PHYLUM ARTHROPODA

531

(Gcnypterus blacodes), and the rest of the body hanging freely
into the water. Lastly, in Tracluliastes (6) the second maxillae

a.nt.2.

7. L e r n a e a

5. Le s f e i ra

6.Trachelia sres

417. — Various forms of parasitic Eucopepoda. 4a, female ; 4b, male. ant. 1, antennule ;
ant.8, antenna; e. median eye ; c.s. egg-sac ; /.;, f.:>, thoracic feet; M. male; mx.2, second
maxilla. (After Gerstaecker, Glaus, Cuvier, and G. M. Thompson.)

) are greatly enlarged, and form a characteristic organ of
attachment.

Artpihis (Fig. 418) is the most familiar example of the Branchiura, or Carp-
lice. It is an external parasite on fresh-water Fishes (Carp, Stickleback, &c. ), not

M M 2

532

ZOOLOGY

SECT-

permanently attached like the degenerate forms just described, but crawling
freely over the surface of the host. The body consists of an oval flattened
cephalo-thorax, and a small bilobed abdomen (a&. ). The mandibles and maxilla-
are piercing organs enclosed in a sucking-tube or proboscis (r. ), in front of which
is a median tube ending in a spine (st). The second maxilke are divided into
two portions, the anterior of which (kf.l) are modified into sucking-discs, by

FIG. 418.— Argulus foliaceus, young male. «j, antemmle ; «2. antenna; ab. abdomen;
1>\ — 64, thoracic feet ; <L digestive glands connected with intestine ; kfl, anterior or suctorial
feet ; kf2, posterior or leg-like portion of second maxilla; ; pa. paired eye ; r. rostrum ; sd, shell-
gland ; st. stylet ; ts. testis ; ua, median eye. (From Lang's Comparative Anatomy.)

which the parasite clings to the surface of its host, and there are four pairs of
swimming-feet (bl — b4). Alone among the Copepoda the Branchiura have 110
egg-sacs.

The most familiar examples of the Eucirripedia are the Barnacles
found on ships' bottoms, piles, &c., and the Acorn-shells or Sessile
Barnacles which occur in immense numbers on'rocks between tide-
marks in all parts of the world.

The common Barnacle (Lcpas anatifera) is attached by a long
stalk or peduncle (Fig. 419, fi^p)* coverecTwith a wrinkled skin, and
bearing at its distal end the body proper enclosed in a sort of

bi

PHYLUM ARTHROPODA

533

j

«

ivalved carapace, formed by a fold of the skin, and strengthened
by five calcareous plates. Of these one is median and dorsal, and
is called the carina (c); two are lateral and proximal, the scuta (s)'}
.d two lateral and distal, the terga (t). During life the carapace

s partly open, and from the ventrally placed aperture delicate
setose filaments are protruded and keep up a constant grasping
movement : these are the endo- and exopodites of the biramous
thoracic feet, of which there are six pairs. Removal of the carapace

hows the feet to be attached to a vermiform unsegmented body

or

3. — Lepas anatifera. A, the entire animal ; B, anatomy. «i, antennule ; c. carina ;
cd, cement gland ; I, digestive gland ; m. adductor muscle ; od, oviduct ; or. ovary ; p. (in B)
penis and (in A) peduncle ; s. scutum ; t. tergum and testis ; vd. vas deferens. (From Lang's
Comparative Anatomy, after Darwin and Claus.-)

(B), attached on the ventral aspect to the stalk and carapace by
its anterior end, while its posterior end is free and terminates in
a long filament, the penis (p), immediately dorsal to which is the
anus. The mouth is ventral and anterior, and is provided with a
pair of mandibles and two pairs of maxillae. There are no
tennse : at first sight the antennules appear to be absent, but a
reful examination shows the presence of a pair of minute
ructures (a) on the proximal or attached surface of the stalk,
and embedded in the cement by which the animal is fixed to its
support ; these are the antennules, and their position relatively to
the mandibles shows that the stalk is formed by an elongation of
the anterior region of the head.

534

ZOOLOGY

SECT.

The Sessile Barnacles or Acorn-shells (Balanus) have no stalk
(Fig. 420), the head-region being short and broad. The scuta (s)
and terga (t) support a valvular carapace, through the opening of
which the feet are protruded, and the whole animal is surrounded
by a sort of parapet (sk) formed of six calcareous pieces. One of
these, dorsal in position, is the carina, the others appear to be
represented by small ossicles developed on the peduncle of certain
stalked forms such as Pollicipcs.

Many of the Eucirripedia are parasitic. Some of these (Petrarca,
&c.), parasitic in Actinozoa, resemble the attached forms in
essential respects ; others (Alcippc), parasitic in the shells of
Molluscs and Cirripedes, have abdominal but no thoracic feet.

. sc
ad, \

m.

CD WQ

FIG. 420.— Balanus. A, external view; B, anatomy. «], antennules ; ad. adductor muscle;
m. muscles of scuta aud terga ; o, edge of parapet ; or. ovary ; ori. oviduct ; s. scutum ;
sA', parapet; t. tergum ; ico, female aperture. -(From Lang's Comparative A,uito»ui, after

Darwin.]

Protedepas, also parasitic on other Cirripedes, has a maggot-like,
segmented, limbless body, and a suctorial mouth.

The Khizocephala are represented by Sacculina (Fig. 421), parasitic
on Crabs, and Pdtogastcr on Hermit-Crabs. Both genera have the
appearance of an immense tumour (ks) on the abdomen of the host,
showing no sign of segmentation, no_ appendages, no mouth or
anus. From the attached end go off a number of delicate root-
like filaments, which extend through the body of the host and
absorb nutriment. Obviously degeneration is here as complete as
it can well be, and nothing but the developmental history of the
parasite (p. 553) would justify its inclusion among the Crustacea.

The most striking general character in the external features of
the Malacostraca is the limitation in the number of segments. The

XI

PHYLUM ARTHROPODA

535

head has the same composition as in the Entomostraca, but the
thorax is invariably formed of eight segments, and, except in the
Phyllocarida, the abdomen of six ordinary segments and a telson.

^j.. — Sacculini carcini, on abdomen of crab. In: branchial region of crab ; /, hepatic'
region; </, intestinal region; /•«, body of Peltogaster ; p. peduncle; iiitt, basilar membrane,
giving off root-like processes which are seen extending through the body of the host. (From
Lang's Comparative Anatouu/, after Uelage.)

The lirnbs are strikingly modified for the performance of various
ictions.

The Phyllocarida are interesting from the fact that they are
annectent or linking forms between the Phyllopoda and the
Copepoda on the one hand, and the higher Crustacea, particularly
the Schizopoda and Decapoda, on the other. The order contains
only three genera, the commonest of which, Nebalia (Fig. 422),
is a little shrimp-like marine Crustacean about 6-8 mm. in
length. The body is divisible into head, thorax, and abdomen,
all having the normal malacostracan number of segments, ex-
cept the abdomen, which is formed of eight segments, the
last bearing caudal styles — structures not found elsewhere in
the sub-class. There is a bivalved cephalic carapace (s), closed
by an adductor muscle (sm), and extending backwards to the
fourth abdominal segment : it is terminated in front by a
ovable rostrum (r).

536

ZOOLOGY

SECT.

The eyes (a) are large, compound, arid raised on movably articu-
lated stalks. The antennules (c^) and antennae («2) are large, the

mandibles (md.) have palps
(mt), and the exopodite of
the second maxilla (mxt)
has the form of a slender
filament which acts as a
" cleaning-foot " to keep the
cavity of the carapace free
from foreign bodies. There
are eight thoracic append-
ages (brf ), all of them leaf-
like, and recalling those of
Apus. The first four abdo-
minal appendages {pi — p4)
are large biramous swim-
ming-feet, like those of
Copepods; the fifth and
sixth (p5, p6) are small
and uniramous.

The Schisopoda (Fig. 423)
are small transparent,
shrimp-like forms, mostly
from 2-6 mm. in length.
They agree with the Crayfish
in the general form of the
body, in the union of the
head and thorax, in the
presence (except in Ana-
spidcs) of a carapace — which
may, however, leave some of
the posterior thoracic seg-
ments uncovered — and in the
number both of segments
and appendages, but present
several interesting charac-
ters indicating a lower grade
of organisation. One of the
most notable of these is the
absence of differentiation in
the thoracic appendages,
which have a leg-like and
not a leaf-like form, but
which are all alike, none of

them being modified into maxillipedes, except to a very slight
degree in some forms. Moreover the legs all possess exopodites

«, eye

FIG. 422.— Nebalia geoffroyi, mal

uj, antennule ; «2> antenna ; c, head; />//, thoracic
feet; <l, intestine; h. heart; kin, gizzard ; ,n<i.
mandible ; mt, mandibular palp ; mnt. exo-
podite of second maxilla ; pi — p±, pleopods ;
r. rostrum ; s, carapace ; x,n, adductor muscle ;
t. testis ; / — VIII, thoracic segments. (From
Lang's Comparative Anatomy, after Claus.)

XI

PHYLUM ARTHROPODA

537

(ex), thus retaining the primitive biramous or "split-footed"
form which is lost in the Decapoda. The first five pleopods
are large in the male, small in the female : the sixth is a uropod,

.— My Sis OCUlata. end. endopodite ; en. enopodite ; ot. otocyst. (After Gerstaecker.)

assists the telson in the formation of the characteristic
lalacostracan tail-fin : there is no trace of the entomostracan
caudal styles.

Amongst the Decapoda are included nearly all the largest and
most familiar Crustacea — the Prawns and Shrimps, Lobsters, Cray-
fishes, and Crabs. The cephalo-thorax is always completely covered
by the carapace. The three anterior pairs of thoracic appendages
are modified into maxillipedes, which retain the original biramous
character, but the five posterior pairs are enlarged, and form legs,
which are always — except as an individual variation — devoid of
exojtodites in the adult.

In the Prawns (Fig. 424, ./) the body is compressed, and the exo-
skeleton is not calcified. The abdomen is very large in proportion
to the cephalo-thorax, and has a peculiar bend close to its junction
with the thorax. The legs are very slender, used for swimming, not
walking ; and sometimes one pair, sometimes another, is enlarged
to form the chelipeds. The rostrum is large — sometimes longer
than the rest of the carapace — and both eye-stalks, antennae, and
legs may attain extraordinary dimensions.

The Lobsters and fresh-water Crayfishes agree with Astacus in
all essential details, but the sea Crayfishes (Pcdinurus) present some
striking modifications. There are no chela3, the legs all ending in
simple claws : the antennae are of immense size, and their proximal
segments atre fused with one another and with the carapace, quite
crowding out the epistoma : the rostrum is reduced, or even
vestigial, and the pleopods are very broad and fin-like. In Scyllarus

538

ZOOLOGY

SECT.

(Fig. 425) and its allies the body is broad and depressed, the bases
of the legs widely separated from one another by the broad
sterna, the antennae (ant. 2) short and plate-like, and the eye-stalks

2.Palaemon.

FK;. 424. — Shrimp (dorsal view), and Frawn (side view). (After Cuvier.)

} enclosed in socket-like grooves of the carapace. Most of
these characters show an approximation to what is found in
the Crabs.

PHYLUM ARTHROPODA

539

The Hermit-crabs (Pa-gums, &c., Fig. 426) are very strange!}7
lifted in relation with their peculiar mode of life. They are
hvays found inhabiting the empty shells of Gastropods (Whelks,
Periwinkles, &c.), the abdomen completely enclosed within the
shell and only the cephalothorax protruding. In correspondence
with this mode of protection, the abdomen is soft, having only
vestiges of terga (f) on the dorsal side, and its appendages are
more or less atrophied, except the sixth pair (up), which take the
form of pincers, and are used to hold on to the columella of the

-•

425.— Scyllarus arctus.

ant.l, antennule ; itiif.3, antenna ;
eye. (After Cuvier.).

FIG. 426.— Paeurus bernhardus. ch. chela of
first right leg ; l.fi, 1.5, fourth and fifth legs ;
t, abdominal terga ; v.p. uropods. (After Bell.)

shell. The fifth pair of legs (15) are much reduced, and in some
species one of the chelipeds is greatly enlarged and its chela (ch)
acts as an operculum, completely closing the mouth of the shell
when the animal is retracted. As the Hermit-Crab grows it
takes up its abode in larger and larger shells, sometimes killing

d removing piecemeal the original inhabitant.

Other Macrura, such as the Cocoa-nut Crab (Birgus), Hippo, &c.,
approach the Brachyura in the short, more or less permanently
flexed abdomen, but are clearly separated from them by the
structure of the head and its appendages.

540

ZOOLOGY

ECT.

In the Bracliyura, or true Crabs, we reach the highest degree of
specialisation known among the Crustacea. The cephalothorax
(Fig. 427) is always of great proportional breadth, and is frequently
much broader than long. The abdomen, on the other hand is greatly

B

CLTLl.1

FIG. 427.— Cancer pagurus. A, dorsal ; B, ventral aspect, ant.l, antennnle ; ant.2, antenna;
abd.l, abd.S, <iJ></.7, abdominal segments; £, eye-stalk ; l.l, l.Z, legs; ,,u^.J, third nuixilli-
pedes. (A, After Bell.)

reduced, its sternal region is uncalcified, and it lies permanently
flexed in a groove on the very broad thoracic sterna, so as to be
often quite hidden in a view from above. In correspondence with
this the ^leopods are much reduced, the male retaining only two
pairs as copulatory organs, the female four pairs for the attachment

XI

PHYLUM ARTHROPODA

541

of the eggs. The uropods are absent, so that there is no tail-fin.
The eye-stalks (^)are contained in orbits or sockets of the carapace,
which are so prolonged that the eyes appear to arise behind the

FIG. 428.— Typical Brachyura. (After Bell and de Haaii.)

antennules and antennae. Both pairs of feelers are small, and the
bases of the antennules are contained in sockets orfossettes. The
third maxillipedes (mavp.) are broad, flat, and valve-like, not leg-

542

ZOOLOGY

SECT.

like as in the Macrura. The first legs (l.l) form chelipeds often
of great size : the remaining legs generally end in simple claws,
but in the Swimming-crabs the distal segment (Fig. 428, 1)
in the fifth pair is flattened and forms a fin. The range of
variation in form, proportions, colour, markings, &c., among crabs
is very great (Fig. 428).

Unlike the Decapoda, the Stomatopoda form a very small order, .comprising a
few genera varying from the size of a Shrimp to that of a Lobster. Squilla (Fig. 429)
is the best known genus.

The abdomen (al — a7) is very large in proportion to the cephalothorax,
and the carapace (cth), which is thin and uncalcified, leaves the last three
thoracic segments ( VI — VIII) uncovered. The rostrum is movably articulated,
and covers the prostomial region, which is divided into two distinct segments,
the first bearing the large stalked eyes, the second the antennules. This
arrangement appears to support the view that the aiitemiulary region is a

FIG. 420.—

;. 420.— Squilla. al, antennule ; a2, antenna ; al—a7, abdominal segments ; br, gills ;
cephalothorax; p, copulatory organ; pi — po, pleopods; p6, uropods; VI — VIII,
thoracic segments ; 1 — S, thoracic appendages. .(From Lang's Comparative Anatomy.)

ills ; cth.
free

metamere distinct from the prostomium, but the division in question is absent
in the larva, and does not appear till the proper segmentation of the body is
established : probably it has a physiological meaning, and is connected with the
necessity of extreme mobility of the eyes and olfactory organs in an animal
which lives in a burrow with only the anterior end of the head exposed.

The antennule (al) has three flagella ; the antenna (a2) a single flagellum and
a very large exopodite. The first five pairs of thoracic limbs ' (1 — 5) are turned
forwards towards the mouth, and act as maxillipedes ; the second of these —
corresponding with the second maxillipede of Astacus — is very large (2), and
its distal segment is turned back and articulated to the penultimate segment
like the blade of a pocket-knife to the handle. In this way a very efficient
weapon called a xuh-chela is produced, both of the segments of which are pro-
duced into strong spines. The remaining three thoracic 'appendages (6 — S) are
slender legs provided with exopodites : the last of them has a styliform copu-
latory organ (p) developed from its proximal segment. The pleopods are large
and biramous : the first five (pi, p5) have gill-filaments (l>r) attached to their
plate-like exopodites : the sixth (2)6) form large uropods or lateral tail-lobes,
as in Astacus.

PHYLUM ARTHROPODA

The Cumacea are also a very small group : Diastylis (Fig. 430) is a good
example. They are little shrimp-like animals, differing from all the Malacostraca
previously considered in having poorly
developed, sessile eyes, sometimes fused
together, and in some genera altogether
absent. The carapace (cth) is so small
as to leave the five posterior segments
(th IV—VIII) uncovered. The first two
pairs of thoracic limbs are maxillipedes,
the last six legs : of these two or three
pairs have exopodites (ex).

In the Arthrostraca we come
once more to a very large and
important order, containing a
great number of genera and
species, many of them strangely
modified in correspondence with
special habits of life. The best
known examples of the Amphipoda
are the little Fresh-water Shrimp
(Grammarus, Fig. 431) and the
Sandhoppers (Talitrus, Orchcstia)
so common on the sea-shore. Of
the Isopoda very convenient ex-
amples are Asellus (Fig. 432),
common in fresh-water, and the
well-known Wood-lice or Slaters
(Oniscus, Fig. 434, 1). found under
any piece of wood, stone, &c.,
which has lain undisturbed on the
ground for a few weeks.

The body is usually compressed
or flattened from side to side in
Amphipods (Fig. 431), depressed
or flattened from above down-
wards in Isopods (Fig. 432). The
normal malacostracan number of
segments is present, but the first
thoracic segment is always united
with the head, so that the ap-
parent head is really an incom-
plete or partial cephalothorax
(c.th). In some genera (Tanais,
&c.) the second segment of the
thorax also unites with the head,
and such forms — sometimes in-
cluded under a distinct sub-order,
Anisopoda — form a transition to

FIG. 430.— Diastylis stygia. ai, an-

tennule ; a;?, antenna; ab.l — ub.7, ab-
dominal segments ; cth. cephalothorax ;
en. endopodite ; on, exopodite ; p.l,p. 6,
pleopods. (From Lang's Coiiipa,-nti.i:>:
Anatomy, after Sars.)

544

ZOOLOGY

SECT.

the other Malacostraca, and especially the Cumacea. The pos-
terior seven thoracic segments (th.£ — th.8) are free, and those
of the short abdomen are usually free in Amphipods (Fig. 431,
aid. 1-6), often more or less fused in Isopods (Fig. 432, aid). In
some Isopoda the thoracic segments are produced laterally into
large and prominent pleura.

The eyes (E) are compound and usually sessile : they are, how-
ever, stalked in some of the less specialised members of the order,
a circumstance which lends support to the view that the sessile
eyes have, in this particular group, arisen by the atrophy of eye-
stalks. The antennas (ant.%) as well as the antennules (ant.l)
are uniramous. The first pair of thoracic appendages (mscp) are
modified to form maxillipedes, which are sometimes united to-
gether in the middle line so as to form a sort of lower lip. The
remaining seven thoracic appendages take the form of legs (LI -1.7)

C-.'tft.

Ih.Z

th.8

FIG. 431. — Gammarus neglectus. aM.l — nbd.6, abdominal segments; ant.l, antennulc
ant. 2, antenna; cth. cephalothoi ax ; E. eye ; j. /. 1, first jumping foot; /. l—l. 7, legs
mxp. maxillipede ; os. oostegite ; ov. ova ; s.f.l, first swimming foot; tk.3—th.2,free thoraci
segments. (After Gerstaecker.)

which are usually arranged in two groups, four of them directe
forwards and three backwards, or vice versa. The legs end eithei
in simple claws or in large sub-chela3 : vestigial exopodites ai
present in some of the Anisopoda. In the female, certain oi
the legs bear flat plates, the oostegites (Fig. 431, os), probably modi-
fied epipodites, which enclose a brood-pouch for the reception oi
the eggs. In Amphipods the gills are also borne on the legs.

PHYLUM ARTHROPO

545

The abdominal appendages are very different in~the^Fwo orders.
In Amphipoda the first three are biramous swimming-feet (Fig.
431, sf), the last three peculiar stiff processes used for jumping
(/./). In Isopods more or fewer of the pleopods have broad plate-
like endo- and exopodites (Fig. 432, pl.3), the former thin and

B

;. 432.— Asellus aquaticus. A, dorsal; B, ventral view; ant.l, antennule ; ant.2, an-
tenna; bp, brood-pouch; c.th, cephalothorax ; E, eye; [l.l — 1.7, legs; pl.l—pl.7y pleopods;
th.2—th.8 ; free thoracic segments. (After Gerstaeckcr.)

vascular and acting as gills : the sixth pair (pl.G) are either leg-
like or aid in the formation of a tail-fin.

Interesting modifications occur in both sub-orders. Among the
Amphipoda, Phronima (Fig. 433, 1) is a marine form of glassy trans-
parency, the female of which inhabits a transparent barrel -like
structure — the test of a pelagic Tunicate — in which she brings up
her young. Caprella (3) is a singular creature in which the
abdomen is quite vestigial, and the rest of the body, as well as
the appendages, extremely slender. It creeps about on colonies
of Hydrozoa and Polyzoa, to the branches of which its own form
and colour are so closely assimilated as to 'render it difficult of
detection. The allied Cyamus (Whale-louse, 2) is parasitic on the
skin of whales : it also has a vestigial abdomen, but the body

VOL. I N N

540

ZOOLOGY

SECT.

—exceptionally among Amphipods — is broad and depressed and
the legs curiously swollen.

3. C a |a re I la
2. C y a m u s
FIG. 433.— AmphipOda. 3, <t, male ; /-, female. (After Gerstaecker, and Bate and Westwood.)

Among the Isopoda, one of the most interesting forms is the
common Wood-louse (Fig. 434, 1), which is almost unique among
Crustacea for its perfect adaptation to terrestrial life. The allied

2. Armadillo

3.Cy

4. Cry pl-oniscus

434. Isopoda. ./',«, entire animal; '*, posterior end with attached rnale(//f); Sta,

1>, adult female. (After Cuvier, Clans, and Gerstaecker.)

"Pill-bugs" (Arinadillidiuni , ..') have the habit of rolling them
selves up into a ball when disturbed. Cymotlwa and its allies are

larva

iem-

xi PHYLUM ARTHROPODA 547

large species (6-8 cm. in length) parasitic in the mouths of Fishes,
where they hold on to the mucous membrane with their short clawed
legs : their mouth-parts are often modified for sucking. In the Bopy-
rini, found in the gill-cavities of various Crustacea, parasitism is
accompanied by great degeneration and asymmetry (3), as well as
by a notable degree of sexual dimorphism, the males (3, b,m) being
very small and permanently attached to the bodies of the females.
Lastly, in Cryptoniscus, parasitic on Crabs, the adult female (^, b) has
no trace of crustacean organisation, and it is only by the study of
development that its true systematic position can be guessed.

With regard to the texture of the exo skeleton, there is every
gradation from the delicate polished cuticle of most Entomostraca,
Schizopods, &c., through the calcified but still flexible cuticle of
Astacus, to the thick, tuberculated, stony armour of many Crabs
(Fig. 428, 3) or the shelly pieces of Cirripedes. The exoskeleton is
secreted from a single-layered ectoderm, and undergoes periodical
moults or ecdyses. There is no transverse layer of muscle, and
the longitudinal layer is broken up into paired dorsal and ventral
bands. As a rule, each limb-segment is acted upon by two muscles :
the joints are nearly always hinge-joints.

The body-cavity consists of several chambers separated from
one another by partitions. In Palcemonetes, one of the Prawnsr
there is a median dorsal chamber enclosing the ophthalmic artery,
and not containing blood : it is probably a portion of the coelome
in the strict sense of the word. The cavities of the gonadsare also
coelomic, and the ducts by which they communicate with the
exterior are probably modified nephridia. In addition to these
cavities, there is a large central space, in which the enteric canal,
digestive glands, gonads, &c., lie ; paired lateral spaces containing
portions of the shell-gland ; spaces in the limbs ; and the pericardial
sinus, in which the heart lies. All these cavities contain blood, and
constitute a kind of secondary body-cavity, formed by the enlarge-
ment of blood-vessels, which have largely replaced the true ccelome.
Such a secondary or blood-containing body- cavity is called a
hcemoccele.

The enteric canal consists of a vertical gullet, an expanded
stomach, and a nearly straight horizontal intestine. In some of
the Cladocera the intestine is coiled, but this is quite exceptional.
In the Entomostraca, part or the whole of the stomach is formed
from the mesenteron, but in Malacostraca both gullet and stomach
are developed from the stomodaBum. A gastric mill is present in
Malacostraca, and a rudiment of such an apparatus occurs in
Ostracoda. The digestive glands are usually branched caeca formed
as offshoots of the mesenteron : in Arthrostraca (Fig. 435, I) they are
unbranched caeca extending into the abdomen : in Stomatopoda
they consist of ten metamerically arranged organs opening into
the intestine. In Amphipods there are intestinal caeca (ud) which

N N 2

548

ZOOLOGY

SECT.

may have an excretory function. So-called salivary glands, opening
on the labrum, have been found in several genera.

In most of the Entomostraca respiration takes place by the
general surface of the body, and the only respiratory organs are

Fi<;. 435. — Orchestia cavimana, male, a, eye ; «j, antennule ; a->, antenna ; <iod, anterior
aorta; <iop, posterior aorta; bm, ventral nerve cord; br, gills; C'+I, cephalothorax ; </<., v;
defereiis ; ed, rectum ; //, brain ; h, heart ; hd, intestine ; A;/', maxillipede ; /, digestive gland:
o;, gullet ; p 1 — p 7, abdominal segments ; sm, stomach ; w.l, intestinal ciecum ; vs. vcsicu
seminalis ; // — VIII. free thoracic segments. (From Lang's Ccni<jHt,-«t'n-r Aii.atouin. aft
Nebesky.)

the bracts and flabella of the appendages. In the stalked BarnacL
however, there are delicate processes attached to the feet, whicl
are supposed to be rudimentary gills. Amongst the Malacostraca,
also, the Phyllocarida and many Schizopoda have no specialised

PHYLUM ARTHROPODA

549

jspiratory organs, but other Schizopods possess tufted podobranchise
(Fig. 436) quite uncovered by the carapace. In the Decapoda the
gills may be either plume-like, as in Astacus and its allies, or the
delicate cylindrical gill-filaments may be replaced by flat plates, as
in Crabs and many Prawns. It is in this order only that we find

436. — Anterior portion of Euphausia pellucida, a Schizopod. a\, aiitennule ; ant.%, an-
tenna ; aii.l, first abdominal segment ; au, eye ; br.l — 8, podobranchia ; cth. cephalo thorax ; en.l,
en.3, endopodites of first two thoracic limbs ; en.l — en.6, exopodites of first six thoracic
limbs ; h. heart ; /, digestive gland ; -m, stomach ; or. ovary ; onl. oviduct ; I— VIII, protopo-
dites of thoracic limbs. (From Lang's Comparative Anatomy.)

the three types of gill described in Astacus and the examination
of numerous forms leads to the conclusion that the typical or
eoretical branchial formula for the group is as follows : —

the.

....

THORACIC T ! 1T '
SEGMENTS.

III.

IV.

V.

VI.

l+rp

VII.

VIII.

TOTAL.

Podobranchiae ...l+ep l+ep

l+ep

1-fcep

1 +rjL>

l + ep

l+ep

s'ts^

Arthrobranchise 2 2

2 2

2

2

2 2

16

Pleurobranchi;e 1 1

1

±+ep

1 1

1

1

I
± + ep

, 8

Total 4 + cp , 4 + cp

1 + ep

*+•

'•+»

•tr

32 + 8<3?

Actually, however, this formula never occii^, as there is always
more or less reduction in the number of gills. Palinurus has the
highest number known, viz., twenty-one, and in the Common Crab
the total number is nine. ^p

Many Crabs live on land, and their gills are enabled to discharge
their function in virtue of the moisture retained in the nearly
closed gill-chamber. In the Cocoa-nut Crab (Birgus) the upper

550

ZOOLOGY

SECT.

part of the gill-chamber is separated from the rest, and forms an
almost closed cavity into which vascular tufts project : it thus
functions as a true lung. Probably the inner surface of the gill-
cover or branchiostegite performs a respiratory function in the
Crayfishes.

In Amphipoda, also, the gills (Fig. 435, &r)^»a-etttgrowths of the
thoracic limbs : in Isopods they are the modified endopodites of the
second to the fifth pleopods: in Stomatopoda, gill-filaments (Fig.
429, br) spring from the exopodites of the first to the fifth pleopods.
Moreover many Crustacea perform rythmical contractions of the
intestine, taking in and expelling water: such anal respiration
is common among Entomostraca, and is especially noticeable in
Cyclops.

The heart is absent in many Copepods (including Cyclops), in
some Ostracoda (including Cypris), and in Cirripedia : it is an
elongated tube with several pairs of ostia in Euphyllopoda,
Leptostraca, Stomatopoda, and Arthrostraca (Fig. 435, h) ; in Clado-
cera and Decapoda it is shortened to an ovoid sac with one or more
pairs of ostia.

Excretory Organs. — In many larval Crustacea two pairs of
modified meso-nephridia are present, the antennary glands opening
on the bases of the antennae, and the sheU-
glands opening on the bases of the second
maxillae. But as development proceeds
one pair always atrophies, the shell-gland
alone being usually retained in the Ento-
mostraca, the antennary gland in the
Malacostraca. In the Stomatopoda, how-
ever, there is no antennary gland, and the
function of renal excretion may be dis-
charged by a pair of glandular tubes open-
ing into the rectum ; and in Amphipoda
a similar function is assigned to caeca
opening into the posterior end of the
mesenteron. In some of the Cirripedia
the shell-gland is described as opening
into one of the compartments of the
body-cavity like a typical nephridium.

The nervous system is always formed
on the ordinary arthropod type, as de-
scribed in Apus and Astacus, and the
chief variations it presents are connected
with the greater or less amount of con-
crescence of ganglia. In the sessile
Barnacles and in the Crabs (Fig. 437) this process reaches its
limit, the whole ventral nerve-cord being represented by a single
immense thoracic ganglion (by).

Fi<;. 437. — Nervous system of a
Crab (Maja squinado).

lit/, thoracic ganglion ; c<j.
c'onmiissimil ganglion; ;/,
brain ; //<, stomach ; *<•, oaso-
phageal connective ; *(/, vis-
coral nerves ; ?/, post-oeso-
phageal connective. (From
Lang's ('i>nifiii,-(il!i-< Anat-
Milne-Edwards.)

PHYLUM ARTHROPODA 551

The sense-organs are mostly of the same character as those of
the two examples. The median or nauplius-eye always occurs in
the larva, and can frequently be shown to exist in the adult of
even the higher groups (Decapoda). The Cirripedia and many
parasitic Copepods are eyeless in the adult, as also are certain
subterranean Malacostraca. Olfactory setae occur, as a rule, on the
antennules, and the auditory organs of Decapoda are open sacs in
the basal segment of the same appendages, but in Schizopoda occur
as closed otocysts (Fig. 423, ot) in the eridopodites of the uropods.

Reproduction. — In most Crustacea the sexes are separate, but
hermaphroditism occurs in some Phyllopods, in nearly all Cirripedes,
and in certain parasitic Isopods (Cymotlwci}. In the latter case the
animals are protandrous, male organs being developed at first, and
female organs at a later stage. In many Cirripedia minute com-
plemented males are found attached, like parasites, to the body of
the ordinary or hermaphrodite individual, the male organs of
which appear to be inadequate for the full discharge of the ferti-
lising function. Sexual dimorphism is almost universal, and
reaches its maximum in the parasitic Copepods and Isopods
already referred to.

The gonads are always a single pair of hollow organs discharg-
ing their products into a central cavity or lumen, whence they
pass directly into the gonoducts, and so to the exterior. The
gonads may be single or branched, and frequently there is more
or less concrescence between the glands of the right and left sides,
as in Astacus and Cyclops. The sperms vary greatly in form, and
are usually motionless : in Cirripedia, however, they are motile,
and in Ostracoda perform movements after reaching the female
ducts. In some Ostracoda they are about three times as long as the
animal itself (Fig. 415, D). In many Entomostraca reproduction is
parthenogenetic. In Daphnia, for instance, the animal reproduces
throughout the summer by parthenogenetic summer eggs, which
develop rapidly in the brood-pouch (Fig. 314, 1, lyr.p). In the
autumn winter eggs are produced, which are fertilised by the males :
they pass into the brood-pouch, a portion of which becomes speci-
ally modified and forms the &phippvuwi or saddle. At the next moult
the ephippium is detached and forms a sort of bivalved capsule, in
which the eggs remain in an inactive state during the winter,
developing in the following spring.

Development. — In some Crustacea segmentation is complete,
and a hollow blastula is formed : in others complete segmentation
is followed by an accumulation of yolk in the interior, resulting
in the formation of a superficial blastoderm as in Astacus :
in others, again, the egg is telolecithal, and the protoplasm,
accumulated at one pole, divides so as to form a disc of cells
which afterwards spreads over the whole yolk. But in most

552

ZOOLOGY

SECT.

cases the egg is centrolecithal and segmentation superficial as
in Astacus.

Development is always accompanied by more or less metamor-
phosis. In Euphyllopoda the young is hatched in the form of
a nauplius (Fig. 395, A), and further changes are of the same char-
acter as in Apus. In Cladocera development is direct, the naupliu^j-
stage being passed through in the egg, and the young hatched in
a form closely resembling the adult. In one of the Cladocera,
however, Leptodora (Fig. 414, 3), while development of the summer
eggs is indirect, the winter eggs give rise to free nauplii. In the
Ostracoda the nauplius is peculiar in having a bivalved shell, and
all three pairs of appendages uniramous. In all the Copepoda
there is a free nauplius, which, in the parasitic forms, leads a
free existence for a time, and then attaches itself to its particular
host and undergoes retrograde metamorphosis.

In the Cirripedia, also, there is a free nauplius, the body of which
is often produced into long spines. After several moults, the

ua

rf

FIG. 438.— Cypris-stage of iLepas fascicularis. ab. abdomen ; pa. paired eye; rf, thoracic
feet ; ua, unpaired eye ; 1, antennule. (From Lang's Comparative Anatomy, after Glaus.)

nauplius passes into a form called the Cypris-stage (Fig. 438),
characterised by the presence of a bivalved shell, like that of an
Ostracode : the antennules(^) also have become modified into organs
of adhesion by the development of the penultimate segment into a
disc, the antennas have disappeared, and six pairs of swimming-feet
like those of a Copepod have made their appearance : there are
paired compound eyes, and the shell is closed by an adductor
muscle. After leading a free existence for a time, the Cypris-
larva attaches itself by its antennules, aided by the secretion
of cement-glands, and becomes a pupa : the carina, terga, and
scuta appear beneath the shell, and within the skin of the mouth-
parts and legs of the pupa appear the corresponding appendages
of the adult. In Lepas the anterior region of the head grows out

PHYLUM ART1IROPODA 553

to a peduncle. The pupal integument is then thrown off, the
paired eyes disappear, and the adult form is assumed.

In Sacculina a still more extraordinary metamorphosis takes
place. The young is hatched as a nauplius, and passes into a
Cypris-stage. In this condition, after a brief free existence, it
attaches itself to the body of a young Crab, near the base of a seta.
The thorax with its appendages is thrown off, and the rest of the
body is converted into a rounded mass, from the anterior end of
which an arrow-like process is developed. This perforates the
cuticle of the host, and, through the communication thus formed,
the whole body of the parasite passes into the interior of the crab
and becomes surrounded by a new cuticle, the old cuticle being
left empty on the outside of the Crab's body. The Sacculina now
sends out root-like processes, grows immensely, and, pressing upon
the body-wall of the crab, causes atrophy of the tissues : this
' allows the now greatly-swollen parasite to project on the exterior
as the tumour-like adult described above (p. 535).

Amongst the Schizopoda the embryo of Euphausia leaves the egg
as a typical free-swimming nauplius : this passes into what is
called the protozocea-stage, distinguished by the possession of an
elongated, unsegmentecl abdomen without appendages. After suc-
cessive moults, the rest of the appendages appear, and the adult
form is assumed. In Mysis (Fig. 423) the nauplius is maggot-like,
and undergoes development in the brood-pouch, emerging in a
condition closely resembling the adult.

The development of the Decapoda presents a very interesting
series of modifications. In two genera of prawns (Penceus and
Lucifer) the embryo leaves the egg as a nauplius, and passes by
successive moults through a protozosea stage, a zccea-stage, with
segmented but limbless abdomen, and a Mysis or Schizopod-stage,
in which it resembles an adult Schizopod, having exopodites to all
the thoracic limbs.

In the Crabs the nauplius stage is passed through in the egg,
and the young is hatched in the form of a peculiarly modified
zosea (Fig. 439, A), with an immense cephalothorax produced into
spines, large stalked eyes, and a slender abdomen. This passes
by successive moults into the megalopa-stage (B), which resembles
an adult Macruran, having an extended abdomen with well-
developed pleopods. The megalopa passes by successive moults
into the adult form.

In the Lobster (Homarus) both nanplius- and zosea-stages are
passed through in the egg, and the embryo is hatched in the
mysis-stage with exopodites to all the thoracic limbs. In the
Rock-lobster (Palinurus) and its allies, the newly hatched young
.is a strangely modified Mysis-form called a Glass-Crab or Phyllo-
somo : it has broad, depressed cephalic and thoracic shields of

554

ZOOLOGY

SECT.

glassy transparency : the abdomen is very small and the legs
extremely long and biramous. Lastly, in the Fresh-water Cray-
fish the young resemble the adult in all but proportions and
certain unimportant details of structure. Thus in the series of
Decapoda we get a gradual abbreviation in development, stages
which are free larval forms in the lower types being hurried
through before hatching in the higher.

The larvae of Stomatopoda are grotesque little creatures with a
very large spiny carapace. In Amphipoda there is no free larval

FIG. 439.— Larva} of Crabs. A, Zotea-stage of Iftaja; B, Megalopa-stage of Portunua.
h. heart; it^— a6, abdominal segments; 1, antemmle ; ~\ antenna; I— VIII, thoracic append
ages. (From Lang's Comparaticc Anatoma, after Glaus.)

form, but in Isopoda the young leave the egg in the fori
curious maggot-like modification of the nauplius, which r
in the brood-pouch until it has attained the adult form.

form of a
remains

Ethology. — The Crustacea are remarkable for their very perfect
adaptation to the most various conditions of life : they occur in
fresh-water, in the sea, in brine-pools, in subterranean caves, and
on land : of the marine forms some are littoral, -some pelagic, some
abyssal, descending to over 3,000 fathoms. One species of Copepod,
Pontellinn mediterranea, may almost be considered as aerial : it
is described as taking long flying leaps out of the water, after the
manner of a Flying-fish. Some, like Lobsters, Crayfishes, &c., arc

sol

PHYLUM ARTHROPOD A 555

itary ; others, like Shrimps, are gregarious, occurring in immense
shoals. Most of them either prey on living animals or devour
carrion, but, as we have seen, the barnacles are fixed, and feed on
minute particles after the fashion of many of the lower animals,
and the members of more than one order are parasites remark-
able for their deviation from the typical structure of the class
and their adaptation to their peculiar mode of life. In size
they present almost every gradation from microscopic Water-fleas
to Crabs two feet across the carapace, or four feet from tip to
tip of legs.

As to geographical distribution, all the chief groups are cosmo-
politan, and it is only among the families, genera, and species that
matters of interest from this point of view are met with. Fossil
remains are known from very ancient periods. The oldest forms
are usually referred to the Phyllocarida, and occur from the Cam-
brian to the Trias. The shells of Ostracoda are also known from
the Cambrian upwards, and those of Cirripedia from the Silurian.
Arthrostraca are known from paloeozoic times, but are rare as
fossils : the earliest Macruran is a shrimp-like form from the
Devonian, while the highly differentiated Brachyura are not
known with absolute certainty until the Cretaceous period.

It was in the Crustacea that the recapitulation theory so often
alluded to was first worked out in detail. Embryology shows
that all Crustacea may be traced back in individual development
to the nauplius, upon which follows some kind of zoaea-stage, many
of the Entomostraca progressing no further. But in Malacostraca
the zosea is followed by the mysis-stage, which is permanent in
Schizopods, transient in Decapods. It was certainly a tempting
hypothesis that this series of forms represented as many ancestral
stages in the evolution of the class. But we have to remember
that all such free larvae are subject to the action of the struggle
for existence, and have no doubt been modified in accordance with
their own special needs and without reference either to their
ancestors or to the adult species into which they finally change.

Many Crustacea present instances of protective and aggres-
sivc^ characters, i.e., modifications in form, colour, &c., which serve
to conceal them from their enemies or from their prey. Probably
the ipost striking example is that of certain, crabs (Paramithrax),
which deliberately plant Sea-weeds, Sponges, Alcyonarians, Zoo-
phytes, &c., all over the carapace, and are thus perfectly concealed
except when in motion. Another Crab, a species of Dromia, carries
a relatively immense Ascidian or Sea-squirt on its back, and in
another species of the same genus the hinder legs are used to
hold umbrella-wise over the back a single valve of a bivalve
shell.

Several instances of commensalism occur in the class. The

550 ZOOLOGY SECT.

association of Hermit-crabs with sea-anemones has already been
referred to (p. 196) : another interesting example is the occurrence
of the little Pea-crab (Pinnotheres) in the mantle-cavity of Mussels.
Other Decapods are found in the intestines of Sea-urchins and
Holothurians, and one genus of Crab lives in a cavity in a Coral,
the aperture being only just sufficient to allow of a due supply
of food and water.

It is in Crustacea that we find the first indication of characters
the purpose of which appears to be their attractiveness to the
opposite sex. The immensely enlarged and highly coloured chelae
of some male crabs (Gela&imus, Fig. 428, 2) are said to be used for
attracting the female as well as for fighting. The sound-producing
organs of some Decapoda have probably also a sexual significance.
The Rock-lobster (Palinurus mdgaris) has a soft chitinous pad on
the antenna, which it rubs against a projecting keel on the sternal
region of the head, producing a peculiar creaking sound, and
Alphem, another Macruran, makes noises by clapping together the
fixed and movable fingers of its large chelae. The fact that these
sounds can be produced at the will of the animals seems to show
that they undoubtedly possess a sense of hearing, and that the
auditory sac is not merely an organ of the sense of direction.

Affinities and Mutual Relationships. — That the Crustacea
belong to the same general type of organisation as the articu-
lated worms is clear enough. The advance in structure is
shown in the reduction in number and in the differentiation
of the segments, and in the concrescence of those at the anterior
end to form a head ; in the hardening of the cuticle into sclerites
so as to form a jointed armour: in the jointing and mobility of
the limbs ; and in the differentiation of the dorsal vessel into a
heart by which the propulsion of the blood is alone performed..
The resemblance of the foliaceous limbs of Phyllopods to the
parapodia of the higher worms is so striking that one can hardly
believe it to be without significance. On the other hand, the
absence of transverse muscles and of cilia, the non-motile sperms,
and the replacement of the coelome by blood-spaces are funda-
mental points of difference from any known Chaetopod.

As to the mutual relations of the various orders, the Phyllopoda,
with their very generalised structure and parapod-like limbs, may
be taken as the base of the series. By a differentiation of the
post-cephalic limbs, and a reduction in the number of segments,
the phyllopod-type easily passes into that of the Phyllocarida.
These again lead to the Schizopoda, in which the segments are
fixed at the number occurring in all the higher Malacostraca, the
caudal styles are no longer present, and the first thoracic legs show
an indication of being modified into foot-jaws. From the Schizo-

PHYLUM ARTHROPODA

oo7

the Macrura are derivable by the differentiation of three
pairs of foot-jaws and the disappearance of the exopodites of the
legs. In the series of the Macrura we find, on passing from the
Prawns through such forms as Astacus, Palinurus, and Scyllarus,
a gradual shortening of the abdomen, accompanied by a broaden-
ing and flattening of the whole body. In Birgus, Hippa, &c., this
process goes a step further, and the abdomen becomes permanently
flexed under the cephalothorax, thus leading to the high degree of
specialisation found in the Crabs.

The Arthrostraca, Cumacea, and Stomatopoda may perhaps be
looked upon as derivatives of the Schizopod-type along distinct
lines of descent, the Arthrostraca showing the greatest amount of
specialisation, in virtue of the absence of carapace and of exopodites
(both present as vestiges in Anisopoda), and in the eyes being
sessile. The Ostracoda, Copepoda, and Cirripedia are best con-
ceived as derivatives, along separate lines, of an ancestral form
common to them and the Phyllopoda.

These relationships are expressed in the following diagram : —

Phyllofjoda
Oslracoda

Cofaefsoda

CirrijDedia

FIG. 440. — Diagram illustrating the mutual relationships of the orders of Crustacea

558

ZOOLOGY

SECT.

APPENDIX TO CRUSTACEA

Class TKILOBITA.

The Trilobita are extinct Arthropods peculiar to and characteristic of the
Palaeozoic rocks : they are specially abundant from the Upper Cambrian to the
Carboniferous.' They are often found in a wonderfully good state of preservation,
owing to the hard exoskeleton covering the dorsal surface : the greater part, of
the ventral region and the appendages were, however, very delicate, and are
preserved only in exceptionally favourable cases.

The body is depressed, more or less oval in outline, and divided into three
regions, the head (c.^k), the thorax (th), and the abdomen (p), all of which usually

''<

A

Fio. 441.— Dalmanites socialis, dorsal aspect; H, the same' rolled up; C, under-side of
head of Phacops fecundus. r..xA. cephalic shield; >•• eye ; f.c. fixed cheek ; /.s. frontal

suture ; <//. glabella ; ////•. labruni ; m»c. movable check ; p. pygidium ; pi. pleura ; *./•'/'•
sub-frontal plate ; tit. thorax. (After Gerstaecker.)

present an elevated median ridge and depressed lateral portions, whence the trilo-
bation generally characteristic of the group. The head is covered by a cara-
pace or cephalic shield (c.xh), the elevated median region of which, known as th(
t/lafic/fa ((jl), iisually presents three or four transverse grooves, probably indicat-
ing the presence of four or five segments. The lateral regions of the carapace
are divided by an oblique line of separation, the facial suture (f.s), into an innei
or mesial portion, the fixed cheek (f.c}, continuous with the glabella, and an
outer free portion, the mo cable, check (/».<•) : the latter bears the large paired
<'"in]M.und eye (e). In some cases there is an indication of a dorsal organ, like

PHYLUM ARTHROPODA

559

it of Apus, 011 the last cephalic segment. Ventrally the carapace is con-
med, as in Apus, into a sub-frontal plate (B, s.f.p], to the posterior edge of
which is attached a large labrum (Ibr).
The posterior angles of the carapace are
often produced into spines.

The thorax (th) is composed of a
variable number (2-29) of movably
articulated segments, which are com-
monly trilobed, consisting of a median
region or axis, and of lateral pleura (pi)
often produced backwards and down-
wards into spines. The abdomen is
covered by a caudal shield or pygidium
(ji), formed of a variable number of
fused segments. Owing to the mobility
of the thorax, the Trilobites were able
to roll themselves up like Wood-lice (B).

The appendages are very imperfectly
known. Quite recently a single pair of
antenna? (Fig. 442) has been shown to
exist in one species, probably attached
to the sub-frontal plate. Four pairs
of leg-like cephalic appendages have
been demonstrated, and the thorax
bears slender biramous legs consisting of
endo- and expedite, and bearing spiral
gills. Similar limbs are present on the
abdomen.

The larvae of several species of
Trilobites have been found in the fossil
state. In some of these the body con-
sists only of carapace and pygidium in
the youngest stages, and the thoracic
segments are subsequently intercalated
in regular order. In other species the
earliest stage has the form of a rounded
plate, the posterior portion, of which
elongates and segments to form the thorax and abdomen. Nothing is known
of the larval appendages, and none of the stages hitherto discovered can be
considered as iiauplii.

The precise systematic position of the Trilobites is uncertain, but their
ist affinities seem to be, on the whole, with the Phyllopoda.

-Fio. 441, bis.— Triarthrus beckii, dorsal
aspect, showing antennae and thoracic
abdominal

and abdominal appendages.
Bernard, after Beecher.)

(From

j

CLASS II.— ONYCHOPHO

The class Onychopbora comprises only the aberrant arthropod
genus Peripatus, which differs very widely in certain important
features of its organisation from all the rest of the Arthropoda,
and in some respects enables us to bridge over the interval
between the latter and some of the lower phyla, more particularly
the Annulata.

General external features. — Peripatus (Fig. 442) is a cater-
pillar-like animal of approximately cylindrical form, and not divided
into segments : it has a fairly well-marked head, and a series (14-42

560

ZOOLOGY

SECT .

according to the species) of pairs of short stumpy appendages.
The integument is thrown into a number of fine transverse
wrinkles, and is beset with numerous conical papillae, each
capped with a little chitinous spine. The head (Fig. 443) bears
a pair of antennae, a pair of eyes, a pair of jaws, and a pair of

KM:. 441'.— Peripatus cape nsis, lateral view. (From Balf our.)

short processes known as the oral papillae. The antenna are made
up of a number of short rings. The eyes are constructed some-
what after the model of the Chaetopod eye as described on p. 439.
On the surface of the oral papillae are situated the apertures of
a pair of glands — the slime glands. Each jaw is composed of

FIG. 443.— Ventral view of head of Peripatus capensis, with antennae, jaws, oral
and first pair of legs. (After Balfour.)

two curved, falciform, chitinous plates ; the\ lie at the sides of
the mouth enclosed by a circular lip. The jaws, as well as the
oral papillae, are developed as modified limbs.

The legs are not jointed, but rows of papilla? give them a ringed
appearance ; each consists of a proximal part and a small distal
part or foot, the latter terminating in a pair of horny claws.

PHYLUM ARTHROPODA

561

cm*

Lrn

The surface presents an elaborate pattern, which varies greatly
in different individuals, produced by minute mottlings of various
colours and tints, green, red, brown, and the arrangement of these
in stripes and bands.

Body wall and body cavity. — The wall of the body consists
of a cuticle, a layer of deric epithelium with an underlying
layer of fine fibres, a layer
of circularly arranged muscu-
lar fibres, a double layer of
diagonal fibres, and a layer
of longitudinal fibres divided
into a series of bundles. A
layer of ccelomic epithelium
lines the wall of the ccelome
and invests the contained
organs. Incomplete muscular
partitions divide the cavity
into a median and two lateral
compartments. As in the Ar-
thropoda in general, the body-
cavity is a hsemoccele, and is
filled with blood.

The enteric canal (Fig.
444) begins with a small
Imecal cavity enclosed by the
circular lip, and having on
its floor a slight prominence,
the tongue. This is followed
by a thick-walled pharynx
(phar.) leading to a narrow
oesophagus. The part which
follows, the mesenteron or
stomach-intestine, a wide,
somewhat thin-walled tube,
extends nearly to the pos-
terior end of the body. The
narrower rectum leads to an
anal aperture situated on the
last segment of the body.
A diverticulum leading back-
wards from the buccal cavity,
receives the secretion of two
long narrow tubular salivary
glands (sal. gld.).

Circulatory system.— The heart is an elongated tube run-
ning through nearly the entire length of the body. It presents a
number of pairs of ostia arranged segmentally — i.e. one opposite

VOL. i o o

- O f/ff

FIG. 444. — Dorsal view of the internal organs of
Peripatus. an. anus ; ant. antennae ; brn.
brain ; cox. gld. coxal gland of the seven-
teenth leg ; $ gen. male genital aperture ;
ne. co. nerve cord ; neph. nephridia ; phar.
pharynx ; sal. gld. salivary gland ; si. gld.
slime gland ; atom, stomach. (Combined
from Balfour.)

5(32 ZOOLOGY SECT.

each pair of legs. It is enclosed in a pericardial sinus imperfectly
cut off from the general body cavity by a longitudinal partition.
There are no other vessels with the exception of a median ventral
vessel.

The organs of respiration are delicate, unbranched or rarely
branched, tracheal tubes lined with a thin chitinous layer exhibiting
fine transverse striations. Groups of these open in little depres-
sions of the integument, the external openings of which are
known as the stigmata. The stigmata in some of the species are
distributed irregularly over the surface ; in others are arranged
in longitudinal rows. By means of these tubes air is conveyed to
all parts of the body.

A series of pairs of glands, the coxal glands (Fig. 444.
cox. gld.), lie in the lateral compartments of the body cavity, and
their ducts open on the lower surfaces of the legs. Their distri-
bution varies in the two sexes and in the different species : in
one species — P. edwardsii — they are only developed in the male.
A pair of larger glands — the slime glands (si. gld.) — opening at
the extremities of the oral papilla?, may be modified coxal glands :
the secretion of these is discharged in the form of a number of
fine viscid threads when the animal is irritated, and appears to
serve a defensive purpose.

The nervous system consists of a brain (brn.) situated in the
head, and of two longitudinal nerve cords (ne. co.) which run parallel
with one another throughout the body to the posterior end, where
they join together behind the anal aperture. A number of very
fine transverse commissures, more numerous than the segments,
(i.e. than the pairs of limbs) connect the two cords together to
form a ladder-like nervous system comparable to that of some of
the Flat Worms. The cords are very slightly swollen opposite
each pair of limbs : nerve cells cover them uniformly throughout
their entire length. The brain gives off nerves to the antenna1'.
The nerves to the jaws are given off just where the brain passes
into the longitudinal nerve cords.

The excretory organs are'nephridia (Fig. 445) of the type of
those of the Annulata, situated in pairs in the lateral compartments
of the body cavity, and opening on the lower surfaces of the legs at
their bases. Each nephridium consists of a wide funnel (tr.) with
fringed margins opening into the ccelome or into a closed sac ; a
looped tube (sg.) ; and a dilated terminal vesicle (eb.), situated close
to the external opening. The salivary glands and the reproductive
ducts are, as shown by the study of their development, specially
modified nephridia, as appear also to be a pair of glands — the ana
glands — opening close to the anus.

Reproductive organs.— Peripatus has the sexes distinct. I
the female there are two ovaries and two uteri, the latter in the
form of long curved tubes which unite behind in a median vagina

y

:

e

PHYLUM ARTHROPODA

563

opening on the exterior between the second last pair of legs

Connected with each uterus where it leaves the ovary are two

diverticula — the rcceptaculum seminis and receptaculwn ovornm. In

some species one or other of these

may be absent.

In the male there are two tubular

testes, each with a narrow vas effercns

opening by a funnel-like aperture into

a vcsicula seminalis ; this is followed

by a long, narrow, coiled vas defer ens.

The two vasa deferentia unite together

to form a median tube — the ductus

ejaculatorius — opening on the exterior,

like the vagina of the female, between

the second last pair of legs. The wall

of the proximal part of the ejaculatory

duct is glandular and secretes a sub-
stance forming complicated cases which

enclose masses of sperms to form

spermatophores.

Development. — The differences

between the different species of Peri-

patus as regards the segmentation and
the formation of the germinal layers,
as described by various observers, are
very considerable. All the species
are viviparous, but in some the egg, before the completion of
embryonic development, is enclosed in a well-formed shell, and
in certain cases the eggs may pass out to the exterior before the
emergence of the embryo. In some species the egg encloses a con-
siderable amount of food-yolk, in others the quantity of food-yolk
is small, and nutriment is obtained from the parent.

In P. novce-zcalandiw there is a superficial segmentation. The
first segmentation-nucleus is itself superficial, and segmentation
results in the development of a number of nuclei, each with its
island of protoplasm, which arrange themselves on what is destined
to become the dorsal side (Fig. 446 A), opposite the site of the
future blastopore, while some pass inwards to the central part of
the ovum. The peripheral nuclei multiply rapidly and grow round
the yolk so as to completely enclose it except on a small space
(blastopore) in the middle of the ventral side (E). There a thicken-
ing takes place, and an involution of the lips of the blastopore
results in a sort of invagination, the floor of the invagination
cavity being formed of yolk with scattered nuclei.

In another species — P. capensis — the segmentation has the
appearance of being total ; but the cells, though separated by
issures externally, are fused internally.

o o 2

FIG. 445. — A nephridium of P.
Edwardsii. tr, funnel ; sri,
looped canal ; eb, terminal vesi-
cle. (From Lang's Text Bool:}

564

ZOOLOGY

SECT.

In accordance with the smaller size of the ova and the rela-
tionship of the embryo with the wall of the uterus the American
species show a totally different mode of development. The eggs,
which are almost entirely devoid of yolk, undergo a total and
tolerably equal process of segmentation. Even at this stage the
embryo, which increases considerably in size, appears to receive
nutrient lymph from the uterine wall. When it has reached the
32-cell stage the embryo, according to one observer, consists of a
solid mass closely invested by the epithelium of the wall of the
uterus. It then becomes reduced in size and owing to exosmosis,
assumes the form of a disk placed in close apposition to one side
of the wall of the uterus. The embryo subsequently loses its

FIG. 44(5. — Two early stages in the development of Peripatus novce zealandiee. A, transverse
section of an ovum in which the yolk is nearly covered by the blastoderm (W.) ; B, transverse
section of an ovum in which the blastopore (blp.) is formed. (After Sheldon.)

flattened form and becomes somewhat vesicular, the cavity of the
vesicle opening into the cavity of the uterus. From its surface
are given off isolated cells which become applied in part to th<
wall of the uterus, and finally unite to form a complete envelop
(amniori) enclosing the embryo. The vesicle then becomes closed,
the embryo becomes raised from the surface of the uterine wall,
the part applied to the latter becoming narrowed so as to form
sort of stalk, at the base of which is a growth of cells term*
the placenta. Into close relation with this placenta comes
ring-shaped thickening of the uterine wall, the uterine placenta.

In P. capensis behind the elongated blastopore proliferation of
cells gives rise to an oval thickening. The mesoderm takes its
origin at this point and extends forwards in the form of two
germinal lands, one on the right of the blastopore and the other
on the left. These bands undergo a division into rudiments of

PHYLUM ARTHROPODA

565

segments — the division beginning in front. The lips of the blasto-
pore meanwhile become approximated, and fuse throughout the
greater part of their length, leaving only an anterior and a
posterior opening; these go to form the mouth and the anus

•V; 447. Three somewhat later stages in the development of Peripatus capensis, showing

the mode of closure of the blastopore and the appearance of the primitive segments. A, stage
in, which the blastopore (W.) has the form of an elongated slit ; B, stage in which the blastopore
is closing up in its middle part ; C, stage in which the blastopore has become closed up except
the anterior part which has gone to form the mouth (mo.), and the anus (an.) ; the whole embryo
has now become strongly curved towards the dorsal side. (After Balfour.)

respectively. The division into segments soon becomes well
marked. At the anterior end the head lobes'lbecome distinguish-
able. The body elongates, and the head and trunk become differ-
entiated. The limbs now arise as ventro-lateral outgrowths which
are developed from before backwards.

•")(;<; ZOOLOGY SECT.

Distribution. — The various species of Peripatus are all terres-
trial, and are found in damp localities, under bark, or dead timber,
or stones. Four species occur in South Africa, one in South
America and one of the West Indies, one in New Zealand, and
two in Australia.

Relationships. — Peripatus is the most primitive of existing
Arthropods, and presents some striking points of resemblance to the
Chaetopoda. The development is in the main arthropodan,
especially as regards the mode of segmentation (at least in the
forms with much food-yolk, which are probably the more primi-
tive), the mode of closure of the blastopore, and the mode of
development of the mesodermal strands. Arthropodan also are
the relatively large size of the brain and the presence of tracheae,
the character of the heart with its pairs of ostia, together with
the clawed appendages, and the jaws in the form of modified
limbs. The nephridia on the other hand, and their modification
in certain segments to form the gonoducts, which are ciliated
internally, are annulate in character, and in all probability the
slime glands and coxal glands correspond to the setigerous glands
of the Chaetopoda. The nervous system is peculiar, and is most
nearly paralleled among the Platyhelminthes and the Mollusca.
Also peculiar, and serving to distinguish Peripatus from the rest
of the Arthropoda, are the large number of stigmata with then-
irregular arrangement, the presence of only a single pair of jaws,
and the nature of the cuticle.

CLASS III.— MYRIAPODA

The class Myriapoda, including the Centipedes and the Milli-
pedes, consists of tracheate Arthropoda, which bear many features
of resemblance to the Insects. There is a distinct head, bearing
many-jointed antennas, a pair of eyes, and two or three pairs of
jaws; and a body, not distinguishable into regions, but consisting
of a number of similar segments, each bearing either one pair of
legs or two. A system of air-tubes or tracheae, similar to those of
Peripatus and the Insects, open by a series of stigmata, usually in
considerable numbers, on the sides or lower surfaces of the
segments.

A. — DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Myriapoda are tracheate Arthropoda in which there is a
head, bearing antennae and jaws, and a trunk made up of a number
of similar segments, provided, except the* last, with leg-like
appendages. Groups of ocelli take the place of the compound
eyes of Insects.

PHYLUM ARTHROPODA

507

ORDER 1. — SYMPHVLA.

Myriapoda in which there are not more than twelve leg-bearing
trunk segments ; a single pair of branching tracheae, the external
apertures of which are situated in the head ; and an unpaired
genital aperture on the fourth segment.
- This order includes only a single
genus, Scolopcndrclla (Fig. 448.)

ORDER 2. — CHILOPODA.

Myriapoda with a dorso-ventrally
compressed body, consisting of a con-
siderable number (15 to 173) of trunk
segments, each with a single pair of
legs, the first pair of legs forming poison
jaws (maxilHpedes). The unpaired
genital aperture is on the second last
segment.

This order includes the Centipedes
(Fig. 449) and Scutigera.

ORDER 3. — DIPLOPODA.

Myriapoda with a dorsally convex
body composed of a considerable num-
ber of segments, all of which, with the
exception of the first four, bear each
two pairs of legs. There are no
maxillipedes. The paired genital aper-
tures are situated between the third and fourth segments.

This order includes the Millipedes.

ORDER 4. — PAUROPODA.

Myriapoda with ten trunk segments and nine pairs of legs.
Antennae with several flagella. Tracheae not known. Paired
genital apertures situated at the bases of the second pair of legs.

*The order includes only a single genus, Pauropus (Fig. 451).
GENERAL ORGANISATION.

External features. — The head in the Myriapoda is as well
marked off as in an Insect ; it appears to be composed of about
four fused segments. The antennce consist sometimes of many,
sometimes of comparatively few segments. A pair of eyes,
situated on the dorsal surface of the head, consist of aggre-
gations of ocelli except in Scutigera, in which there are com-
pound eyes, differing, however, in their structure from those
of Insects.1 There is a movable lalrum, a pair of mandibles, and

1 See p. 596.

FIG. 448. — Scolopendrella

immaculata (From Leuck-
art, after Latzel.)

568

ZOOLOGY

SECT.

two pairs of maxillce. The mandibles have 110 palps : one or
both pairs of maxillae usually possess palps; the second pair of
maxillae are in some groups more or less united together.

The number of segments in the body varies from 12 to 173. In
the Millipedes the dorsal walls of the segments are very strongly

cutt

brn

shorn.

mal

cox.ap

Fir;, 449.— SCOlOpendra. (From

Cuvier's An'mml Kii'!/<!oin.)

FIG. 450.— Lithobius forficatus seen from the-
ventral side. ant. antenna?; &r». brain ; cox. dp.
coxae of appendages : ft. 15, fifteenth pair of legs ;
int. intestine ; mal. Malpighian tubes ; mxp,
rnaxillipedes ; ne. co. nerve cord ; as. oesophagus ;
stom. stomach. (From Leuckart.)

arched; in the Centipedes the segments are all dorso-ventrally
compressed, with distinct tergal and sternal shields separated
laterally by intervals of comparatively soft skin on which the
stigmata open. In the Chilopoda each segment bears a pair of
jointed legs ; of these the most anterior pair is extended forwards

::

PHYLUM ARTHROPODA

569

form a pair of poison jaws (maxillipedes), at the extremity of
the pointed terminal joint of which opens the duct of a poison
gland. In the Diplopoda each segment behind the fourth or fifth
bears two pairs of legs, the four or five most anterior having only
one pair each. In most of the Diplopoda the appendages of the
seventh segment are modified in the male to form copulatory
organs.

The integument and body wall do not differ widely from
those of Insects. Odoriferous glands are present on some of the
body segments in the Diplopoda, and open on the dorsal surface.
Scolopendrella possesses spinning glands.

The alimentary canal is straight, and is much simpler in
character than that of the Insecta. Connected with the mesen-
teron are a number of short hepatic caeca, and one or two pairs of
Malpigliian tubes, having a renal function, open into the beginning
of the hind gut.

The heart is greatly elongated, and consists of as many
chambers as there are segments in the body.

The respiratory system resembles that of Insects to be fully
dealt with later (p. 578, Fig. 459), consisting of air- tubes or
trachece. There is one pair of stigmata in each
segment in the Diplopoda, and the tracheae are
unbranched. In the Chilopoda the trachea?
are branched, and the number of stigmata is
in most cases less than the number of seg-
ments. In Scutigera, one of the stigmata is
unpaired and dorsal. In the Symphyla there
are only two stigmata, and these are situated
on the head.

The nervous system is, in accordance with
the form of the body, much less concentrated
than in the Insecta (see below, p. 594). In the
Chilopoda there is a double ganglion in each
segment of the trunk ; in the Diplopoda there
are two. The longitudinal commissures be-
tween the ganglia are usually distinct. The
ganglia of the first two segments unite to
form an infra- cesophageal ganglion from which
the nerves to the jaws are given off. A
sympathetic or visceral nervous system is
present as in Insects.

The sexes are always separate. The reproductive organs —
ovaries or testes — are usually paired, with paired ducts, having a
common terminal portion ; but in the Diplopoda there is an
unpaired ovary or testis with paired ducts, which jjemain distinct
throughout, and open by separate apertures. lofthe Chilopoda
the external aperture is situated on the penultimate segment ; in

FIG. 451.— Paur opus
huxleyi. (From
Leuckart, after Lat-
zel.)

570

ZOOLOGY

SECT.

the Diplopoda and Pauropoda the two apertures are usually
between the second and third segments of the body; in the
Symphyla the single aperture is situated between the legs of the
fourth segment.

The ovum, as in most Arthropods, contains a large quantity of
food-yolk. The centrally-placed segmentation-nucleus divides so
as to give rise to a number of nuclei, this division being accom-
panied by a division of the yolk into a number of masses, which,
however, are more numerous than the nuclei. The nuclei then, for
the most part, migrate to the surface, some being left behind in
the yolk. Those that reach the surface, surrounded each by its
little clump of protoplasm, become arranged into a continuous
superficial layer of cells — the blastoderm. On the surface of this

FIG. 4/2.— Two stages in the development of Strongylostoma, or.e of the Diplopoda.
A, early stage in the formation of the larva, whicn, however, exhibits distinct

B, larva immediately after hatching. (From Balfour, after Metschnikoff.)

segments

appears a thickening, and along the thickening is formed a groove
which may perhaps represent the blastopore, though the endoderm
is formed by direct modification of the cells in the interior of the
yolk. Stomodseum and proctodaaum are developed as invagina-
tions of the surface layer. The thickening of the blastoderm gives
rise to a germinal band in which rudiments of the segments soon
become recognisable. Larval membranes do not occur.

In some of the Diplopoda there is a metamorphosis, such as
has shortly to be described in the embryo Insect, and the larva
(Fig. 452, B) has a singular superficial resemblance to an Insect,
owing to the presence at first of only three pairs of appendages
on the anteriojitrunk region.

Fossil rempns of Myriapoda have been found in strata as far
back as the Devonian. The more ancient fossil forms are not

xi PHYLUM ARTHROPODA 571

capable of being grouped in the same orders as the living repre-
sentatives of the class, and are looked upon as constituting at
least two orders, the members of which are all extinct.

CLASS IV.— INSECTA.

The class of Insects, comprising the Cockroaches, Grass-hoppers,
Dragon-flies, House-flies, Butterflies, Beetles, and Bees, with their
many allies, though it is a very extensive one, including as it does
a larger number of species than any of the other classes of the
Arthropoda, is yet characterised by a remarkable degree of uni-
formity, no such extremes of modification occurring as are
observable within the class Crustacea.

Characteristic of all the members of the class is the presence of
three clearly-defined regions — the head, thorax, and abdomen.
There are present on the head, antennae, mandibles, and two pairs
of maxillae, the jaws being variously modified in the different
orders. All Insects have three pairs of thoracic legs, and most
have either one or two pairs of wings likewise borne on the thorax ;
the abdomen is not provided with paired appendages.

The organs of respiration are tracheae similar to those of the
M vriapoda.

The various systems of internal organs attain in all the higher
groups of Insects a very high grade of development. In most the
development is complicated by the occurrence of a strongly-marked
metamorphosis. Insects are terrestrial or aerial, only a few groups
living on the surface of fresh or salt water; but many are aquatic
throughout their larval condition.

Many groups of Insects are remarkable for the high grade of
their -intelligence as compared with the members of most classes of
the animal kingdom. This manifests itself mainly in a number
of instincts, often of a remarkable character, having to do with the
protection and rearing of the young — in some cases leading to the
formation of communities consisting of individuals of various
different kinds (workers, soldiers, sexual individuals) for mutual
support and protection.

K Ex AMPLE OF THE CLASS — THK COCKROACH (Periplaneta
(tmericana).

he Cockroach, familiarly known by the misleading title of
lack Beetle," is a common pest of kitchens, bakeries, and store-
rooms. It is nocturnal in its habits, rarely coming out of its
lurking-places in the day-time, and is almost nu^JBtt-ous in its
diet. It is a good example of the Insecta, not only <fl ; •ount of its
large size rendering it convenient for dissection, bSBso because

572

ZOOLOGY

of its generalised structure, which - makes it a fairly central
member of the class, devoid of any extreme modifications.

Three regions are very distinctly recognisable in the body of
the Cockroach. In front is the head, elongated vertically, bearing
the very long slender feelers and the large eyes, and contracted
behind to form a narrow neck. In the middle is the thorax,
consisting of three segments bearing the three pairs of legs and
the two pairs of wings. Behind is the abdomen, consisting of ten
segments covered over above by the wings. The entire surface
is covered by a chitinous cuticle, which is specially thickened on
the head, on certain parts of the thorax, and in the anterior pair
of wings.

The head consists of four parts — -the epicranium behind, com-
prising the region between and behind the eyes, and the clypeus,

FIG. 453.— Periplaneta americana. Male, dorsal view. Natural size.

or portion extending vertically downwards, with two latert
parts, the gence, in front. The eyes are a pair of reniform blacl
patches on the sides of the head ; each is seen when examined
with a lens to be divided into a number of minute hexagonal am
or facets, like those in the eye of the Crayfish. Borne in sockel
just below the eyes are throng, slender, highly mobile feelers o
antennae, each made up of SRrge number of short segments, the
first three being larger than the others. Internal to the base if
each antenna is a rounded white space — the fenestra — the nature
of which is not known, but which may be an abortive representa-
tive of the simple eyes or ocelli found in most Insects.

Movably articulated with the lower or ventral end of the
clypeus is jfltepad plate, the labrum or upper lip (Fig. 454, Ibr.}
overhang™ nil aperture of the mouth. Below the gena3 am
articulatinP ((rch the sides both of the epicranium and clypeus

XI

PHYLUM ARTHROPODA

573

and 455, man.},

JJIX.

a pair of stout mandibles (Fig.
which work horizon-

lly like those of the
yfish ; their inner

ges are divided
into a number of
teeth. Behind the
mandibles are a more
flexible pair of jaws
—the first pair of
maxillce (mx.1, max.1).
Each maxilla exhibits
a structure compar-
able to the funda-
mental type of the
appendages of the
Crayfish ; a basal part
W protopodite, consist-
ing of two segments
(podomeres), support-
ing an internal ramus
or endopodite, and an
external ramus or
exopodite. The former
consists of two parts :
an inner, pointed, hard
blade — the lacinia
(mi.), and an outer,
softer, more elongated — the galea (me.). The exopodite forms a
palp, the maxillary palp (pm.), consisting of five podomeres. Behind

• these are the second maxillae, which

€y are reducible to the same type, but

/Cf^v'^ which have their two basal seg-

i ti iK. ments (those of the protopodites)

united together in the middle line,
to form two median sclerites,
known respectively as mentum (m.)
and sitbmentt(,m(sm.), so that the two
appendages form a sort of lower lip
called the labium. The endopo-
rlites taken together constitute
what is termed the ligula ; each
is divided into two parts like the
endopodite of the first maxillae.
The exopodites form three-jointed

palps, the labial milvS (-pi.).
mi i

I he neck, or narrow region

FIG. 454. — Mouth parts of the Cockroach ; Ibr. labrum ;
md. mandible ; mxl. anterior pair of maxillse ; m. mentum ;
me. and mi,, outer and inner divisions of the second
pair of maxillae; pi. labrel palp. ; pm. maxillary palp;
st. stipes ; sm. submentum. .•(From Lang's Comparative
Anatomy.')

;' .r'eye gena

mandible ; max*., first pair of max-
-,,uu'-. second pair of maxillae

574 ZOOLOGY • SECT.

between the head proper and the thorax, is covered for the most
part by a thin flexible cuticle, but there are supporting it eight
thickened and hardened patches — the cervical sclcrites (cerr.).

Each of the three segments of the thorax — known respectively
as prothorax, mesothorax, and mctatlwrax — is covered over dorsally
by a chitinous plate — the tergum, and ventrally by another — the
sternum. The tergum and sternum of each segment are distinct
from one another, not united into a continuous sclerite as in the
Crayfish. The tergum of the prothorax is larger than that of the
other two segments, and overlaps the neck above. Attached to
the anterior border of the tergum of the mesothorax are the
anterior wings or elytra — a pair of thick opaque plates, which, in
their ordinary position, extend backwards over the abdomen to
some little distance beyond its extremity. Attached to the tergum
of the metathorax are the posterior wings — a pair of extremely
delicate membranous expansions, which, when at rest, are folded
up longitudinally, like a fan, under the elytra. Attached to the
sternum of each segment of the thorax is a pair of legs. Each leg
consists of a stout flattened proximal podomere or coxa ; a small
second, or trochanter; a third, the femur, similar to. the coxa but
narrower ; a fourth slender and spinose, the tibia ; and finally
the tarsus or foot, composed of six very short segments provided
ventrally with patches of setae to give adhesive power ; the last
segment (pulvillus) is armed in addition with a pair of claws.

Of the segments of the abdomen the most posterior are over-
lapped by those just in front. Each is enclosed in a dorsal tergum
and a ventral sternum, both of which are thinnish and flexible —
the terga and sterna of successive segments overlapping one another
from before backwards. The eighth and ninth terga are hidden
from view by being overlapped by the seventh. The tenth is
produced backwards into a thin flexible plate, tne posterior border
of which presents a deep notch : below this is the opening of the
anus, at the sides of which are a pair of small hard plates — the
podical plates ; at the sides of the tergum are a pair of many-
jointed palp-like bodies — the cerci. The sternum of the first
abdominal segment is rudimentary. In the male that of the ninth
bears a pair of short styles. In the female the sternum of the
seventh is very much more prominent than in the male. The
genital aperture is placed on the ventral aspect of the posterior
extremity of the abdomen beneath the anal opening.

When compared with the Crayfish, as regards the external
anatomy, the Cockroach is found to differ (1) in the arrangement
of the segments into regions ; (2) in the form and position of the
appendages. The head and thorax together correspond to the
cephalothorax of the Crayfish, but comprise fewer segments ; the
abdomen contains a larger number of segments. The single pair
of antennae probably correspond to the antennules of the Crayfish

PHYLUM ARTHROPODA

oyo

-the antennae of the latter not being represented. On this view
the homologies of the anterior appendages in the two animals may
K.-> rt~™.«oorwi jn the following table : —

COCKROACH.

Antennae.

Absent.

Mandibles.

First maxillae.

Second maxillae (labium).

First legs.

Second legs.

Third legs.

CRAYFISH.

Antennules.
Antennae.

Mandibles.
First maxilla?.
Second maxillae.
First maxillipedes.
Second maxillipedes.
Third maxillipedes.

;

add.c

Representatives of the five pairs of thoracic legs of the Crayfish
would thus appear to be absent in the Cockroach, and evanescent
rudiments, no traces of which remain in the adult, alone represent
in the latter the well-developed
abdominal appendages of the
former.

In the living Cockroach re-
spiratory movements are to
be observed, in which the ab-
domen becomes alternately ex-
panded and contracted ; these
movements bring about the
alternate inhalation and ex-
halation of air through certain
apertures — the stigmata — at
the sides of the body. Two
of these are situated on each
side of the thorax, one be-
tween the prothorax and meso-
thorax, and the other between
the mesothorax and the meta-
thorax. Eight occur on each
side in the abdomen between
the terga and sterna of the
segments. Just internal to
each spiracle the main trachea
into which it leads presents an
elastic ring or spiral, acting as
a valve for closing the passage.

The principal sets of muscles of the trunk of the cockroach

are (1) the longitudinal sternal muscles (Fig. 456, long, stern.),

which form a transversely segmented sheet, extending between

- adjoining sterna of the thorax and abdomen ; (2) oblique sternal

l-'ic. 4 :>i;.— Ventral portion of the muscular
system of the Cockroach. «</</. cox, ad-
ductor of coxa ; ab<l. cox. abductor of coxa ;
cxt. Jem. extensor of femur ; Ixt f, ,-». stem.
first tergo-stemal ; long, stern, longitudinal
sternal; ol>l. stern, oblique sternal. (After
Miall and Denny.)

576

ZOOLOGY

SECT.

muscles (oil. stern.), confined to the abdomen ; (3) and longitudinal
tergal muscles, best developed in the abdomen. The various
segments of the limbs are capable of being flexed or extended on
one another, as in the Crayfish, by the contractions of special
muscles. The wings are little used, the female cockroach being
incapable of flight, and the male not a strong flier, and accordingly
the wing muscles are not very strongly developed.

Between the body wall and the alimentary canal is a cavity
taking the place of the coelome, but in reality forming a specially

sected
lents ;

Fir,. 457. — Semi-diagrammatic view of the internal organs of a female Cockroach, dissect*
from the left side. The heart is not represented. . abd.l, abd.5 first and fifth abdominal segments ;
a/i anus ; ant. antennary nerve ; brn. brain ; cer. anal cercus ; CMC. caeca ; coll. colleterial
glands ; cr. crop ; gizz. gizzard ; yon. gonapophyses ; infr. gang, sub-cesophageal ganglion ;
int. intestine ; Ib. pip. labial palp ; l.ov. left ovary ; malp. malpighian tubes ; mx.plp. maxillary
palp. ; od, points to the external opening of the median oviduct (vagina) ; ces. oesophagus ;
i-.oc. right ovary; sal. gld. salivary glands ; sal. rcc. salivary receptacle (left); sal. du. salivary
ducts, indicating the point at which the median duct of the salivary glands unites with tl
median duct of the salivary receptacles ; spir. spiracles ; st. 7, sternum of the seventh se
ment ; te. 10, tergum of the tenth segment ; tkl, th^, £7*3, first, second, and third segments
the thorax.

developed part of the blood-vascular system (hsemoccele). This
is bounded externally by an irregular wall, formed of a mass of
polygonal cells constituting the fat body.

Digestive system. — The mouth opens into a buccal rcc if//
which receives the ducts of the salivary glands (Fig. 457, sal. gld.).
Each gland is divided into two lobes, each made up of numerous
ramifications. In close relation to each gland is an elongated
thin-walled sac — the salivary receptacle (sal. rec.). The duct given
off from the salivary receptacle joins that of the opposite side,
and the median duct thus formed is joined by a single duct (sal.

PHYLUM ARTHROPODA

577

FIG. 458.— Right salivaiy glands and salivary recep-
tacle of Cockroach. (After Miall and Denny.)

du.\ formed by the union of the two ducts of the salivary glands ;
the common duct thus formed opens into the buccal cavity

(Fig. 458). A chitinous fold
of the floor of the mouth
forms the lingua or tongue.
From the buccal cavity
there proceeds backwards
a narrow oesophagus (ces.),
which leads to an elongated
saccular dilatable sac — the
crop (cr.). On this there
follows the proventriculus
or gizzard (gizz.) — a pear-
shaped chamber with the
broad end directed for-
wards, its chitinous internal lining raised up into a number
of horny teeth. A narrow passage leads from this to the
chylific ventricle — a wide tube with glandular walls; from its
anterior end are given off eight
tubular hepatic coeca (hep. coz^) —
blind tubes somewhat narrower
than the chylific ventricle. The
point of junction of the chylific
ventricle with the intestine is
marked by the presence of very
numerous thread-like yellow ap-
pendages— the Malpighian tubes
(inalp.) — which are the renal
organs of the animal. The in-
testine (int.) terminates in a dilated
portion — the rectum (ret.) — the
walls of which are longitudinally
folded. Of the entire alimentary
canal only a small part — the
chylific ventricle — with the ap-
pended hepatic caeca, is of the
nature of a mesenteron, the region
in front being a stomodaaum, and
that behind a proctodaaum.

The heart is an elongated
tube, closed behind, open in front,
running along the middle line of
the abdomen and thorax, imme-
diately beneath the terga. In-
ternally the tube is divided into a number of chambers ; its
walls are perforated by a series of pairs of valvular apertures or
ostia. Running from the wall of the heart to the terga are a
VOL. i P P

FIG. 459.— Portion of a trachea of a Cater-
pillar. B, C, D, branches ; a, cellular
layer ; b, nuclei. (From Gegeubaur.)

578

ZOOLOGY

SECT,

series of segmentally-arranged fan-shaped . bundles of muscles —
the alary muscles (Fig. 484, m.).

Respiration takes place through the instrumentality of a
system of air-tubes or tracheae (Figs. 459 and 460), opening on the
surface at the stigmata, to which reference has already been made.
These tracheaB form a richly ramifying system extending to all
parts of the body. They possess a chitinous internal lining,
supported by means of a spirally-wound, fibre-like thickening.
By means of this system of air-tubes air is conveyed throughout

thor. 1

thor.Z

t/wr.S

FIG. 460.— Periplaneta. View of
the arrangement of the principal
trunks of the tracheal system.
(After Miall and Denny.)

FIG. 461.— Periplaneta. General view of th<
nervous system, aid, sixth abdominal
lion ; ant. antennary nerve ; br. brain ; conn.
cesophageal connective ; inf. sub-oasophagear
ganglion ; opt. optic nerve ; thor,1 thor,- thor,
first, second, .and third thoracic ganglia.
(After Miall and Denny.)

the body to all parts, and there is thus ensured the rapid and
complete oxygenation which the functional activity of the Insect
requires.

The nervous system consists of a "brain (Fig. 457, Irn., and
461, Jr.), a sub-cesophageal pair of ganglia (m/r. gang.), three
thoracic (Fig. 461, thor. 1, 2, and 3), and six abdominal pairs of a
ganglia, a system of connectives uniting the ganglia together, and
a series of nerves given off to the various parts of the body. The
brain consists of a bilobed mass of nerve-matter situated in the

PHYLUM ARTHROPODA

579

head, and divisible into two parts, anterior and posterior. From
the anterior part is given off on each side the optic nerve passing
to the eye to become expanded into an optic ganglion, and from
the posterior part the nerve to the antenna?. It is supported by
a chitinous framework — the tentorium. From the brain there run
backwards a pair of cesophageal connectives (conn.), passing, one on
each side of the oesophagus, downwards and backwards to the sub-
oesophageal ganglia. The latter, which are situated between the
submentum and oesophagus, give off a pair of connectives, passing
backwards to the first thoracic ganglia. From the sub-cesophageal
ganglia are given off the nerves to the labrum, the mandibles,
and both pairs of maxillae. The three paired thoracic and six
abdominal ganglia are connected together into a chain by a series
of double connectives ; the last pair of abdominal ganglia, situated
in the sixth segment of the abdomen, are larger than the others,
and supply the segments behind. A visceral nervous system, rami-
fying on the anterior part of the alimentary canal, is connected
with the two cesophageal connectives by two nerves, which join
above the oesophagus to form a median frontal ganglion.

The organs of special sense are the eyes, the antennae, and
the palpi. The eyes are compound — each being made up of a
large number of simple elements similar to those that go to make
up the eye of the Crayfish (see p. 513). ^ The antennae and palpi,
together with the anal cerci, act as organs of touch. In addition,
certain setaa on the antennae appear to hg,ye an olfactory function.

Reproductive organs. — In the male'the testes (Fig. 462, tst.),
are a pair of small bodies which lie in the fourth and fifth seg-
ments of the abdomen immediately below the terga. From these

stern? \

Fi<;. 4ic>.— Periplaneta. Male
reproductive organs, lateral
view. duct. ej. ductus ejacula-
torius with mushroom-shaped
gland ; stern. 7, sternum of
seventh segment of abdomen ;
f< /</. 7, tergum of the same
segment ; test, testis. (After
Miall and Denny.)

Fit;. 4(53.— Periplaneta. Female re-
productive organs, coll. gld. colleterial
glands ; od. oviducts ; or. ovaries.
(After Miall and Denny.) •

a pair of delicate tubes, the vasa deferentia, lead to the vesiculce
seminales, two tufts of whitish casca, which together constitute
what is known as the " mushroom-shaped gland " ; these open

p p 2

580

ZOOLOGY

SECT.

pen

into the anterior end of the ejaculatory duct (duct, ej.), an un-
paired tube with muscular walls opening on the exterior imme-
diately below the anus. Around the genital aperture are a series
of chitinous processes, the gonapophyses, which subserve copulation.
In the female there are two groups of ovarian tubes or ovarioles,
each group or ovary (Fig. 463, ov.) consisting of eight. The

tubules of each group are
united together anteriorly,
where they are connected by
a ligament to the dorsal
body-wall. Posteriorly each
group is connected with a
lateral oviduct (pd.). Each
ovarian tube has a beaded
appearance, owing to its con-
taining a row of ova, which
increase in size posteriorly.
The two oviducts unite to
open by a median aperture
on the sternal surface of the
eighth segment of the abdo-
men. A pair of unsymmetri-
cal sacs opening together in
the middle of the sternum
of the ninth segment con
stitute the spermatheea or
receptaculum seminis. A pair
of ramifying glandular tu
bules, the collet erial glands
(coll. gld.\ open behind th(
spermatheea. A series o
chitinous gonapophyaes, which
aid in depositing the eggs
are situated between the
female genital aperture am
the anus.

Development. — The eggs
are enclosed sixteen together

yk.c

FIG. 464. — A — D, successive stages in the seg-

mentation of the ovum of an Insect ; Mast. in homy Capsules, the Sub
blastoderm ; )>< ri. peripheral protoplasm ; sea. c -\ • -i • 1

segmentation cells ; ?/*-. yolk ; yk. c. yolk-cells. Stance OI WniCll IS Secreted

^romKorscheltandHeider.afterBlochmann.) the col]eterial glands. They

are laterally compressed, con-
cave on one side (the future ventral side), convex on the other (th(
future dorsal side). Each egg is enclosed in a thin egg-shell, or
chorion, with several small openings. The nucleus, originally plainly
distinguishable in the ovarian ovum, is no longer visible in the
egg when laid, owing to the accumulation of food-yolk. It is to b(

xi PHYLUM ARTHROPODA 581

inferred from what is known of other Insects that the nucleus with
a small quantity of protoplasm lies enclosed within the food-yolk.
It undergoes division (Fig. 464), and some of the resulting cells
travel to the surface, where they form an investing layer — the
blastoderm (blast.), — while others — the yolk cells (yk. c.\ — remain
scattered in the interior of the yolk.

On the ventral side there is soon formed a thickening of the
blastoderm, owing to the cells in this situation becoming columnar ;
this forms what is termed the ventral plate (Fig. 465). In front
this is wider (in the position of the future head) than it is behind.
It becomes divided by a number of narrow transverse lines which
indicate the boundaries of the future segments.

Rudiments of appendages appear on the head and thorax,
and a series also appear on the abdomen, which, however,
subsequently disappear. The segment on
which the rudiments of the antennae appear /^^

is at first post-oral in position, but subse- / \

quently becomes fused with a prse-oral seg-
ment (prostomium), so that the antennas i ^ ~
acquire their permanent prse-oral position
only secondarily. The prostomial segment,
the antennary segment, a segment devoid of
appendages, the segment bearing the rudi-
mentary mandibles, and those bearing the fe-^ j
two pairs of maxillae, all unite to form the
head of the adult. The ventral plate, which ' I
was superficially situated when first de-
veloped, becomes gradually sunk within the FlG< 465.-ventrai plate of
substance of the yolk, and thus becomes e"l^ryo, c°ckroach

«/ ' . with developing ap-

separated from the chonon by a layer of pendages. (After Mian

yolk. On this follows the appearance of the

larval membranes. On either side arises a

fold of the blastoderm (Fig. 466, amn.f.); and the two folds

gr.ow inwards, and eventually unite over the body of the embryo,

forming a complete two-layered covering for it. The outer layer

is termed serosa (ser.), the inner amnion 1 (amn.)

Along the middle of the ventral plate there soon appears a
groove — the germinal groove. This grows downwards, and forms
a tube, which becomes completely detached from the ectoderm.
The lumen of the tube becomes filled up with cells, and the solid
strand thus formed divides longitudinally into two parts — the
mesoderm bands. There is some doubt as to whether the endoderm
is also formed in the course of this invagination, or by modification
of the yolk-cells. Infoldings of the ectoderm at the anterior and

1 This term is derived from the Vertebrata, in which there is an analogous
membrane, occupying, however, a dorsal instead of a ventral position as regards
the body of the embryo.

582

ZOOLOGY

SECT.

Wast

ser

amn

amn.f

posterior ends of the embryo give rise to the stbmodseum and

proctodseum.

Each of the two mesoderm

gast -^^pgiP^ bands becomes divided trans-

A //^ .'•'•'• ^Q*?'-. :'."^\ versely into a series of seg-

ments, which become hollow,
and then become closely ap-
plied to one another, eventu-
ally coalescing, so that the
cavities of all of them unite
to form the body cavity of
the adult, the outer walls
becoming applied to the
ectoderm to form a somato-
pleure, or lamina consisting
of somatic layer of mesoderm
and of ectoderm ; the inner
becoming applied to the en-
doderm to form a splanchno-
pleure, or lamina consisting
of splanchnic layer of meso-
derm and of endoderm.

The ventral plate gradually
grows upwards at the sides,
and eventually its borders
meet and unite along the
dorsal middle line, the entire
yolk thus becoming enclosed
by it.

The ventral nerve-chain
is developed from a groove
of the ectoderm, bounded by
thickenings which become
detached from the surface
ectoderm, and form the chain
of ganglia. The brain is de-
veloped from a pair of ecto-
dermal thickenings. That
part which is developed in
the prostomial region -
the archi-cerebrum — becom es
united with that developed
in the following two seg-
ments to form the completed
brain or syn-ccrebrum.

It can hardly be said that

the Cockroach undergoes a metamorphosis, the young Insect when

blast

ect

amn sor

end

Wast

FIG. 466. — A — C, transverse sections through the
developing ovum of an Insect at successive
stages to show the mode of development of the
germinal layers and of the amnion. amn.
amnion ; amn. f. fold of the amnion ; amn. car.
cavity of the amnion ; Maxt. blastoderm cover-
ing the yolk ; ect. ectoderm ; end. endoderm ;
gast, invagination of ventral plate ; ser. serosa;
yk: yolk.

vent. pi. ventral plate ;
Korschelt and Heider.)

(After

XI

PHYLUM ARTHROPODA

583

it escapes from the egg differing from the adult only in its
smaller size, and in the absence of wings, which grow out sub-
sequently from the terga of the meso- and meta-thorax. Between
its hatching and its complete development the young Cockroach
undergoes no fewer than seven " moults " or changes of skin, in
which all the chitinous parts become thrown off and renewed.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Insecta are air-breathing Arthropoda, in which the body
consists of three well-marked regions — head, thorax, and abdomen ;
the head devoid of external segmentation, bearing compound eyes,
a pair of antennas situated on the prostomium, but post-oral in
origin, mandibles, and two pairs of maxillae ; the thorax of three
segments each bearing a pair of legs, and the second and third
usually wings ; the abdomen composed of a varying number of
segments (7— —11), which are devoid of appendages in the adult
condition. A liver is absent, but salivary glands are always
present. There is an elongated tubular heart, divided into eight
chambers, situated in the abdomen ; the vessels themselves are not
highly developed. The Insecta are, almost without exception, air-
breathers, and the organs of respiration take the form of branching
tubes, the tracheae, by means
of which air is conveyed to
all parts of the body. The
nervous system and sense-
organs are highly developed.
The excretory organs are a
number of blind tubes, the
Malpighian tubes, appended
to the intestine. The sexes
are separate ; development is
sometimes direct, more usually
complicated by a metamor-
phosis.

ORDER 1. — AFTER A (Collem-
bola and Thysanura).

Insecta in which the wings
are absent, and the surface
is covered either with scales

i - T-I , . FIG. 467.- Lepisma. (After Gu^rin

or narrs. Eyes are sometimes Percheron.)

absent ; sometimes there are

groups of ocelli; sometimes compound eyes. The segments of

the thorax are not fused together. Some progress by running

584

ZOOLOGY

SECT.

others by springing movements effected by a special springing
apparatus on the abdomen. Some have elongated, many-jointed

filaments or cerci at the extremity of the

abdomen. Development is direct.

This order includes the Spring-tails

(Podura, Fig. 468), and Silver-fish (Lepisma,

Fig. 467).

ORDER 2. — ORTHOPTERA.

Insects in which there are two pairs of
wings, of which, in most cases, the anterior
pair are hard and tough, and the posterior
pair delicate and transparent. The parts
of the mouth are masticatory The pro-
thorax is not united with the other seg-
ments of the thorax. Development is direct,
or there is a gradual and incomplete
metamorphosis.

This order includes Earwigs, Cockroaches, Stick- and Leaf-
insects, Grasshoppers and Locusts (Fig. 469).

FIG. 468.— Podura. (After
Guerin and Percheron.)

FIG. 469.- Loeusta. (From Cuvier's Fio.470.— Ephemera (May-fly) and

Animal Kingdom.) larva. (After Guerin and Percheron.)

.ORDER 3. — NEUROPTERA.

Insects with two pairs of netted membranous wings. The parts
of the mouth are adapted for biting. The prothorax is free from

XI

PHYLUM ARTHROPODA

585

the other segments of the thorax. The metamorphosis is some-
times complete, sometimes incomplete.

This order includes Termites (" White Ants "), May-flies r (Fig.
470), Dragon-flies, Ant-lions, Caddis-flies.

ORDER 4. — HEMIPTERA.

Insects in which wings are usually present, sometimes similar,
sometimes dissimilar, and in which there is a jointed suctorial

FIG. 471. — Aphis rosse and larva. (From Cuvier's Animal Kingdom.)

strum formed from the labium, enclosing the jaws in the form of
piercing organs. The prothorax is free from the other segments
of the thorax. The metamorphosis is incomplete.

FIG. 472.— Cicada. (After Gutrin and Percheron.)

This order includes Bugs, Water-bugs. Lice. Scale-insects, Plant-
lice (Fig. 471), Cicadas (Fig. 472).

ORDER 5, — DIPTERA.

Insects provided (except in the Fleas) with a single pair of
transparent membranous wings, representing the anterior pair of
other orders. The mouth parts are adapted for piercing and
sucking. The prothorax is fused with the other segments of the
thorax. There is a complete metamorphosis.

This order includes Fleas, Gnats and Mosquitoes (Fig. 473),

58H

ZOOLOGY

SECT.

House-flies and Blow-flies, Bot-flies (Fig. 474), Crane-flies and
"Daddy-long-legs."

FIG. 473. — Pulex (mosquito) and larva.
(After Guerin and Perclieron.)

FIG. 474.— Bot-fly of the horse (Gastro
philus equi). a, mature insect ; 6,
egg attached to a hair ; c, d, and e, stages
in the larval development. (After Brehm.)

ORDER 6. — LEPIDOPTERA.

Insects with both pairs of wings well developed and covered
with scales (modified hairs). The maxillae are modified to form

Fio. 475.— Butterfly (Pieris),with caterpillar and chrysalis stages. (After Guerin and Percheron.)

XI

PHYLUM ARTHROPODA

587

an elongated sucking tube, which is rolled up spirally ; the other
parts of the mouth are rudimentary, with the exception of the
labial palpi. The prothorax is fused with the mesothorax. The
letamorphosis is complete.
This order includes Butterflies (Fig. 475) and Moths.

ORDER 7. — COLEOPTERA.

Insects in which the anterior pair of wings take the form of
ard horny wing-cases, or elytra, which, when at rest, are folded
up along the back and cover over the
folded-up membranous posterior wings.
The prothorax is movable on the other
segments. The jaws are fully developed,
and adapted for biting and diewing. The
metamorphosis is complete.

This order includes the true Beetles
(Fig. 476).

ORDER 8. — HYMENOPTERA.

Insects in which both pairs of wings
are present and membranous. The mouth
parts are adapted both for biting and
licking. The prothorax is united with
the other segments of the thorax. There
is a complete metamorphosis.

Included in this order are Bees (Fig.
493) and Wasps, Ants (Fig. 494), Gall-flies,
Ichneumons.

FIG. 476.— Beetle (Crioceris)
with larva. (After Guerin
and Percheron.

Systematic Position of the Example.

The Cockroach is a member of the order Orthoptera and of the
sub-order Orthoptera genuina, which . comprises all the members of
the order with the exception of the aberrant group of the Ear- wigs
{sub-order Dermaptera). Of the Orthoptera genuina there are
three divisions, the Cursoria, to which the Cockroaches belong ;
the G-ressoria, comprising the Mantidce and Phasmidce, or Stick- and
Leaf-insects and their allies ; and the Saltatoria, including the
Grasshoppers, Locusts, and Crickets. The division Cursoria com-
prises the single family of the Cockroaches (Blattidce), characterised
by the deflexed head, the flat oval body, the large prothoracic
tergum, the long antennae, the three pairs of legs similar, with
large coxae entirely covering the sternal surface of the thorax, the
five-jointed tarsi, and the presence of anal cerci. Periplaneta
belongs to a section of the family distinguished from the rest by
the femora being spiny underneath, and by the valvular character
of the last sternum in the female.

588 ZOOLOGY SECT.

3. GENERAL ORGANISATION.

The exoskeleton of the Insecta (Fig. 477) consists of a chitinous
cuticle (cut.}, which varies in hardness and thickness in different In-
sects, and in different parts of the body of the same Insect, but is very
rarely calcined. Frequently it presents hexagonal markings ; some-
times it is perforated by numerous pores ; sometimes it is covered
with thin scales ; in many cases it is' developed into tactile hairs or
setce, which may be scattered over the
body, or may be located only on certain of
the appendages — the antennae, the maxil-
lary and labial palpi, and the tarsi of the
legs. In some glands are present in the
integument — odoriferous, honey-secreting,
or wax-formrhg glands ; poison glands are
present in connection with an abdominal
sting in certain insects ; spinning glands,
forming a silky material, are confined to

base, basement membrane ; the
cut. layers of the cuticle ; _..

epi. ' epidermis ; set. seta. I he head presents no , trace of seg-

mentation, but the history of its develop-
ment indicates that it may be looked upon

as composed of a prostomium and about five segments, intimately
united together. It varies a good deal in shape, but always presents
the regions that have already been described in the case of the
Cockroach. Sometimes the head is sunk within the anterior part of
the thorax ; sometimes it is free from the latter ; and there may be,
as in the Cockroach, a short narrow region or neck, covered with
soft skin, supported only by isolated cervical sclerites, intervening
between the two on the ventral aspect.

The three segments of the thorax — -pro-, meso-, and meta-thorax
— are usually firmly united together ; but in some Insects the pro-
thorax is movable upon the other segments : it is usually the
smallest of the three segments. In each the exoskeleton consists
of dorsal or tergal and ventral or sternal elements, sometimes
separate from one another laterally, sometimes united together in
such a way as to form complete rings round the segments. Laterally
projecting processes or pleura are sometimes developed.

The abdomen contains from seven to eleven segments, enclosed
in tergal and sternal shields. In some Insects the first abdominal
segment is united with the thorax so as to appear to belong to
the latter region.

The appendages of the head are four pairs, as in the Cock-
roach ; but a considerable variation is observable in the different
orders, especially as regards the jaws. In a few eyes are absent.
Most have large compound or faceted eyes, and many have simple
eyes or ocelli as well ; in a few groups the latter are alone present.

XI

PHYLUM ARTHROPODA

589

The antennae vary in shape in different groups, and sometimes
even in the sexes of the same species. They are sometimes taper-
ing, sometimes moniliform, sometimes club-shaped, sometimes
pectinate, sometimes plume-like. In addition to functioning as
tactile appendages they bear the olfactory setae. The mandibles
are always one-jointed, and differ from those of the Crustacea in
never being provided
with a palp. An arrange-
ment of the mouth-
parts adapted for biting
or chewing has already
been described in the
case of the Cockroach.
This type is character-
istic of the order Or-
thoptera, to which the
Cockroach belongs, and
a very similar type char-
acterises the Coleoptera.
In the Hymenoptera
(Fig. 478) the mouth
parts are adapted both
for biting and for licking
and sucking ; the mandi-
bles (md.) and maxillae
(inx\) are sharp and
lancet-like, the middle
part of the labium is
produced into a long
median tongue (ligula,
li.) at the sides of which
are a pair of accessory
tongues or paraglossce
(prg.). In the Hemip-
' tera there is a proboscis
formed from the labium
enclosing the stylet-
like mandibles and maxillae. In the Diptera (Fig. 479) the
mandibles (md.), usually not developed in the males, are biting or
piercing organs, while the basal parts of the labium form a pro-
boscis (mx2.) enclosing a spine or seta (hp.), which is a process
from the hypo-pharynx — a chitinous process on the roof of the
mouth, and sometimes stylet-like maxillae (mx1.). In the Lepidoptera
(Fig. 480) the mandibles are aborted in the adult, and the maxillae
are developed into elongated half-tubes, which when applied
together form a greatly elongated tube (sr.) capable of being
coiled up in a spiral manner under the head, the extremity

Li

FIG. 478.— A, mouth parts of the Honey-bee (Apis
mellifLca) ; B, the two pairs of maxilla? ; au, eye ;
a, antenna ; c. cardo ; ep. epipharynx ; Ibr. labrum ;
li. ligula ; m. mentum ; mm.mx1, first pair of maxilla? ;
md. mandible ; pin, palp of the first pair of maxilla? ;
PI'!/, paraglossa ; sm. submeiitum ; stm. stipes of the
first maxilla?. (From Lang.)

590

ZOOLOGY

SECT.

provided with hooks or spines for rupturing the nectaries of
flowers.

Appendages of the thorax. — Each of the segments of the
thorax bears a pair of five-jointed legs; the terminal section or
tarsus being made up of a number of short segments and ending
in a pair of claws, often with an adhesive pad or sucking disc
between them. In accordance with differences in the uses to
which they are put, considerable differences are observable in the
form of the legs in Different groups of Insects. In most they are
adapted for walking, and are long and slender ; in some they are

FIG, 479.— Mouth parts of the Diptera. A, of Tab anus ; B, of Culex. Lettering as in pre
ceding figure ; oc. ocellus. (From Lang.)

expanded to enable them to act as swimming paddles: in some
the first pair are prehensile, and develop a sub-chelate extremity ;
in others again the legs, or the first pair of them, are stout and
adapted for burrowing. In addition to the legs the meso- and
meta-thorax may each bear a pair of wings. The wings are thin
transparent expansions of the integument of the body, supported
by a system of branching ribs or nerviwes consisting of chitinous
material with branches of the tracheae, nerves, and tubular diver-
ticula of the body cavity. In most Lepidoptera the wings are
opaque, owing to their being covered with numerous overlapping
microscopic scales, to which the various colours of the wing are

XI

PHYLUM ARTHROPODA

591

due. In some Insects — e.g. Beetles and Orthoptera — the posterior
wings alone are delicate and membranous, the anterior pair being
converted into hard or tough cases — the elytra — which, when
folded up cover over and protect the delicate posterior wings.
In some Beetles the elytra are permanently united together along
the back of the Insect. In some Insects (Bugs) the anterior wings
are chitinous at the bases only. In the Diptera the anterior
wings alone are developed, the posterior being represented by
vestiges — the halteres or balancers. In the Strepsiptera, or Bee-
parasites, an aberrant
group of Neuroptera, on
the other hand, it is the
anterior pair that are
rudimentary. In some
Insects (Spring- tails, Lice,
Fleas) wings are entirely
absent in all stages. In
others again they are
present in one sex—
usually the male — and
absent in the other. In
the Aptera there is no
vestige whatever of wings
at any stage, and this,
taken in connection with
the simplicity of the
structure in other re-
spects, seems to indicate
that in these Insects we
have to do with the de-
scendants of a primitive
group in which wings
had not yet become de-
veloped.

The segments of the
abdomen ' are entirely
devoid of paired ap-
pendages in the adult condition (except in the Thysanura),
though vestiges of them may be present in the young at an
early stage. Each segment is enclosed in dorsal tergal and
ventral sternal plates, which usually remain separate laterally, but
may be united. At the extremity of the abdomen there are
frequently appendages which are perhaps of the nature of limbs,
having the function of stings, ovipositors, and genital processes.

Haemocoele. — The cavity intervening in an Insect between the
body wall and the various internal organs does not correspond
as already explained (p. 576) to the coelome of other groups ; but is

FIG. 480.— Mouth parts of the Lepidoptera. B, the
second maxillae. Lettering as in preceding figures.
pi. labial palp ; pm. palp of the anterior maxilla? ;
s<; sucking tube. (From Lang.)

\7F.~RftTTV

592

ZOOLOGY

SECT.

found when we study its mode of development, to be a hcemoceelc
— an extended part of the blood-vascular system. The coelome
is apparently represented only by the lumen of the reproductive
organs.

A fat body is always present, either in the larval condition or
throughout life. It consists of a mass of polygonal cells bounding
the haBmocoele externally. When young the cells are nucleated
and possess a protoplasmic body. At a later stage a fluid loaded

with minute granules takes the place
of the protoplasm, and crystals con-
taining uric acid are formed; these
crystals afterwards become absorbed ;
their appearance and subsequent ab-
sorption would seem to point to the
probability that the fat body is con-
cerned in the separating out of nitro-
genous waste matters, subsequently to
reach the exterior through the Mal-
pighian tubes.

Digestive system.— Some Insects
do not feed in the adult condition,
and when this is the case the mouth
may be absent, *as for example is the
case in the Day-flies (Ephemeridcc).
When a mouth is developed, as it is
in the vast majority of Insects, it is
situated on the lower aspect of the
head, bounded in front by the labrum,
and behind by the labium. It leads
into the buccal cavity, into which
open the ducts of a pair of salivary
glands, each of which often has asso-
ciated with it a thin-walled sac or
salivary receptacle. Also in the neigh-
bourhood of the mouth open, in such
larval Insects as spin a cocoon, the
ducts of a pair of spinning glands. A
projection of the roof of the mouth
cavity (epipharynx) is present in some
Insects; in others it is replaced by a projection from the floor,
the hypopharynx or lingua.

The alimentary canal is nearly always considerably longer than
the body; it is longer in vegetable-feeding than in carnivorous
forms. The mouth leads into a long, narrow passage — the oesophagus
(ce.) (Figs. 481 and 482) — which dilates behind into a crop (in.) for
the storage of food. The place of this in sucking Insects is taken
by a stalked sac, usually termed the sucking stomach. The essential

FIG. 481.— Digestive apparatus of a
a Beetle (Carabus auratus).

».</, anal glands ; rib, their muscu-
lar appendages ; cd, chylific ven-
tricle ; td, hind gut ; in, crop ;
'. I; head with mouth parts ;
ce. oesophagus ; pf. proventricu-
lus ; mi. Malpighian tubes. (From
Lang, afterDufour.)

uroc

PHYLUM ARTHROPODA

593

tftt

Too.

esses of digestion are carried on in an elongated chamber with
glandular walls — the chyle stomach or chylific ventricle (cd) — which
may be divided into several parts. Sometimes between the crop
and chyle stomach is intercalated a muscular- walled chamber, fre-
quently containing chitinous teeih,ih.Q proventriculus or gizzard (pr.).
Appended to the chylific ventricle at its anterior end are, in many
Insects, a number of tubular blind pouches, the hepatic cceca. At
its junction with
the small intestine,
or further back,
there open a num-
ber (from 2 to over
100) of narrow tubu-
lar appendages, the
Malpighian tubes
(vm.), which are the
organs of renal ex-
cretion. In the cases
in which the de-
velopment of the
alimentary canal has
been traced, it has
been found that the
Malpighian tubes
mark the point
where the mesen-
teron passes into the
proctpdaeum, and it
is assumed that this
holds good gene-
rally The lumen
of the tubes is
sometimes filled up
with cells. In some
insects, the Mal-
pighian tubes open
into a paired or
unpaired . sac — the
urinary bladder.

The intestine is usually elongated, its posterior portion (ed.) is
dilated to form a wide rectum (r), which opens on the exterior
by an anal aperture situated on the ventral side of the last
segment of the abdomen. Anal glands (ad.), producing an
odoriferous secretion, often open into the rectum.

The tracheal system (Fig. 482) communicates with the ex-
terior through a number of apertures — the 'stigmata (st.) — which
vary in the details of their arrangement in the different orders.

VOL. 1 Q Q

d

FIG. 482. — Nervous, tracheal and digestive systems of the
Honey-bee, a. antenna ; au, compound eye ; 6j, b2, bs,
the three pairs of legs ; cm, chylific ventricle ; ed, hind-gut ;
hm, honey stomach (crop) ; rd, rectal glands ; st. stigmata ;
tb, vesicle of tracheal system ; vm, malpighian vessels.
(From Lang's Comparative Anatomy.)

594

ZOOLOGY

SECT.

VF

VF

Ifc,

FIG. 483.— Thorax and anterior abdominal
segments of a larval Ephemerid with
tracheal gills. HF, hind wings ; trl, tr'2,
<y8, tracheal gills ; tl, tracheal longitu-
dinal trunks ; VF, fore wings. (From
Lang's Comparative Anatomy.)

They are always protected against the entry of foreign particles

by some means — either by being surrounded by special bundles

of hairs, or by being closed in
by a special sieve-like membrane.
In all cases they are capable of
being closed by muscular action.
In some Insects, mainly those
adapted for active flight, such as
the Hymenoptera, the tracheal
system is dilated in certain parts
of the body to form comparatively
large air sacs or air reservoirs (tb.).
In the aquatic larvae of some
Insects there is a series of soft
external, simple or divided, pro-
cesses— the tracheal gills (Figs.
483) — attached to the abdominal
segments, and richly supplied
with tracheae, which have no com-
munication with the exterior.

The blood-vascular system
is, in comparison with the other
systems of organs, not very

highly developed, the need of an elaborate system of vessels

being greatly diminished by the way in which all the tissues

and organs are supplied with oxygen

through the system of tracheae. The

blood is colourless or faintly yellowish or

greenish, and contains colourless cor-
puscles. A contractile dorsal vessel or

heart (Fig. 484) extends through the ab-
domen, immediately below the terga. Its

cavity is divided internally into a series

usually of eight chambers by a system

of valves. In its walls are a series of slits

or ostia, by which a communication is

effected between the internal cavity and

a surrounding pericardial sinus. In front

the heart gives origin to a main vessel, or

aorta (a), by means of which the blood is

conveyed throughout the body to enter a

system of sinuses in free communication

with the general body cavity, from the

various parts of which it finds its way

back to the pericardial sinus.

The nervous system (Figs. 482 and 485) is on the same general

plan as in the Crustacea. There is a double snpra-cesophagcal

. 484.— Heart of Cockchafer
(Melolontha). a, aorta ;
m, m, alary muscles. (From
Gegenbauer.)

PHYLUM ARTHROPODA

595

'inglion or brain, a sub-cesophageal ganglion, also double, and a series
of thoracic and abdominal pairs of ganglia, which are closely united
together in the middle line. The brain is relatively large in the
higher Insects, and is divided into several lobes. It gives off nerves
to the antennae and ocelli and the labrum, and on each side it gives
a large lobe — the optic ganglion — on which the compound eye
ts. A pair of oesophagcal connectives pass backwards on either
side of the mouth from the brain to the sub-cesophageal ganglia.
These connectives are very short, and, as a consequence, the brain
d sub-oesophageal ganglia are closely approximated. From the

uvy

~

B

us systems of four species of Diptera to illustrate various degrees of concentration,
mtrated nervous system of Chironomus plumosus with three thoracic, and

485. — Nervous syst
A, non-concer

six abdominal ganglia ; B, nervous system of Empis stercorea with two thoracic and five
abdominal ; C, nervous system of T abanus bovixvus, with one thoracic ganglion and with
the abdominal ganglia closely approximated ; D, nervous system of Sarcophaga carnaria,
with all the ganglia of the ventral chain united together with the exception of the sub-
oesophageal. (From Lang's Comparative Anatomy.)

latter there originate nerves to the appendages of the mouth — the
mandibles and two pairs of maxillae. There are sometimes three
pairs- of thoracic, and as many as eight of abdominal ganglia in
the adult insect ; but in many cases there is a greater or less
degree of concentration of the ventral ganglionic chain (Fig. 485),
and in some of the Diptera this reaches such an extreme that all
the ventral ganglia, with the exception of the sub-oesophageal, are
united into one continuous elongate mass. The Insects, like the
higher Crustacea, possess a visceral or sympathetic nervous system,
connected with the cesophageal connectives and passing backwards

I the oesophagus and crop.
The most highly-developed organs of special sense are the
.

59G

ZOOLOGY

SECT.

large compound eyes. The surface of the compound eye is
marked out, as in the case of the Crayfish, into a great number of
minute hexagonal facets, each of which represents one of the
elements (ommatidia) of the eye. Of these there may be as
many ;is 28,000 (dragon-fly). When the eye is examined in
section, each ommatidium is found to consist of a cornea-lens, the
outer surface of which forms the facet, a crystalline cone, and a
rhabdome. The crystalline cone is not always developed, its place
being taken in the eyes of some Insects by four crystal cells. The
rhabdome is an elongated rod. Beneath the rhabdomes is a
fenestrate memlrane, beneath which again is a dense plexus of
nerve-fibres. Nerve-fibres pass through the fenestrate membrane
and terminate in a delicate sheath which incloses each rhabdome,
the sheath, together with the nerves that end in it, constituting
the retinula. Pigment surrounds the crystalline cones and retinulae.
The ocelli, or simple eyes (Fig. 486), consist of a bi-convex
transparent thickening of the cuticle — the lens — and beneath it

of a group of specially
modified epidermal cells.
Of these, -some, situated
beneath the lens, form
a transparent mass, the
vitreous body ; another
set of elongated cells
being arranged to form
the retina.

The antennae and
palpi are the organs
of touch, and these
appendages seem to be
also the seat of the
olfactory sense. A num-
ber of minute processes

sometimes sunk in pits, and each having a special nerve-plate
connected with it, are regarded as being specially concerned
with this sense, and similar processes or pits on the maxillae
and the epipharynx, are perhaps connected with the sense of
taste.

Peculiar nerve-endings, supposed to be auditory, have been
found in the most various parts of the body. Each consists of a
ganglion-cell (Fig. 487, gz.) giving off a process which is inclosed
in an elongated tube, and which ends externally in a slender
rod (sc.). Groups of these are associated together to form the
auditory organ.

In certain Insects — the Fireflies and Glowworms — belonging to
the order Coleoptera, there occur organs — the luminous organs —
for the production of light.

FIG. 486. — Section through the ocellus of a -young
Dy tiscus larva ; ct. cuticle ; f/k, cells of the vitreous
body ; hy, epidermis ; I, cuticular lens ; no, optic
nerve ; re, retinal cells ; st, rods. (From Lang, after
Grenadier.)

XI

PHYLUM ARTHROPODA

597

SOUK
man
orde

FIG. 487. — Chordotonal (auditory) organ hi the tibia
of Isopteryx apicalis. My blood corpuscles ;
c, integument ; es, terminal fibrous strands at-
tached to the integument ; gz, nerve cells ; sc. ter-
minal rods ; tr. trachea. (From Lang, after v.
Graber.)

Sounds are emitted by many Insects, and are produced by a
variety of different means. Often the sound is the result of the
rubbing together of opposed rough surfaces of the integument.
The chirp of the Grass-
hopper, for example, is
produced by the rubbing
of the femur of the last
pair of legs over a series
of ridges in the anterior
wing, and that of the
Locust by the rubbing
against one another of the
roughened basal parts of
the first pair of wings.
In other cases the sound
results from the rapid
vibratory movement of the
wings; this is the case
with the buzzing of many
Diptera and Hymenoptera.
Again, the humming
unds characteristic of
y of the last-named

er are produced partly by the vibrations of the wings in flight,
partly by the vibration of leaf-like appendages in the tracheae, set
in motion by strong expiratory currents of air. The loud shrill
note of the Cicada is produced by the rapidly recurring contractions
of the fibres of a muscle inserted into a stiff chitinous membrane,
the result being a series of crackling sounds, which follow one
another so rapidly as to give rise to a continuous note.

Reproductive organs. — The sexes are always separate in
Insects, as in Arthropoda in general ; and the males and females
are very commonly distinguishable from one > another by various
modifications of form and of coloration. There are two ovaries
each of which consists of a greater or smaller number of narrow
tubes or ovarioles ; in each of these the ova are arranged in a single
row : the early stages in their formation being situated at the
anterior end, the more mature ova towards the posterior extremity.
Each group of ovarian tubes opens into a lateral oviduct, and the
two lateral oviducts, right and left (Fig. 488. A , od.), unite behind
to form a median oviduct or vagina (vg.\ which opens on the
second last segment of the abdomen. Connected with this median
oviduct, or opening close to it, are receptacula seminis (rs.) and
colleterial or cement glands (sd.). Usually there is a copulatory sac,
or bursa copulatrix (nva.). In the male the paired testes (J5, t.)
consists each of one or more long narrow tubes, which, when more
than one are present, unite into a vas deferens (B, vd.), the two

598

ZOOLOGY

SECT.

vasa deferentia uniting to form a median ejaculatory duct. A
vesicula seminalis is appended to each vas deferens or to the
ejaculatory duct. Accessory glands, opening into the vas deferens
or the ejaculatory duct, secrete cementing material for uniting the
sperms into masses, the spermatophores. In most instances the
eggs are laid shortly after their fertilisation, only a few forms, such
as the Aphides or Plant-lice, many Diptera, and some Coleaptera,
being viviparous. Some Insects, such as the Aphides and Bees
and Wasps, as well as some Lepidoptera and Neuroptera, present
us with the unusual phenomenon of parthenogenesis ; i.e. ova are
formed, as in ordinary female insects, in organs corresponding to

Fio. 488.— A, female ; B, male sexual apparatus of the Honey-bee; ad, accessory glands ; e?«,
common ejaculatory duct ; gd, poison glands ; gb, poison vesicle ; A's, bulb of the stinging
apparatus ; md, rectum, twisted back and cut off ; lira, accessory sac of the vagina (bursa
copulatrix) ; oil, oviduct ; p. penis ; rs. receptaculum seminis ; sd, colleterial gland ; t. testes ;
i-(j. vagina ; rd, sperm ducts. (From Lang's Comparative Anatomy.)

the ovary of the latter, and these are developed without fertilisa-
tion. In the case of the Aphides, an autumn generation of
completely developed males and females is followed by a spring
generation consisting entirely of females; these are both par-
thenogenetic and viviparous. In the Bees, the workers (imper-
fectly developed females) occasionally produce ova which, without
fertilisation, develop into drones (males). In one or two groups,
including the Scale-Insects (Coccidce) and Gall-Insects (Cynipidai),
males are never developed, so that reproduction is exclusively par-
thenogenetic. Pcedogenesis accompanies parthenogenesis in certain
Diptera ; i.e. the larvae produce ova and embryos without impreg-
nation.

The eggs when laid are protected from injury by a number of
methods ; they may be firmly fixed to the substratum, buried in

th(

PHYLUM ARTHROPODA

599

seg

blast

e earth, or laid in the interior of certain plants or even of
animals. The deposition of the eggs, by means of ovipositors, in
the leaves or other parts of plants gives rise to swellings — the
so-called galls, in the interior of which the young Insects live.
In the case of many ID sects the eggs are enclosed in a cocoon ; in
others they are enclosed in
gelatinous or waxy material.
The eggs are, for the most
part, of relatively considerable
size. In form they vary, but
the long oval prevails in most
instances. The ripe egg is
enclosed in two egg-mem-
branes— an inner, the vitel-
line membrane, produced by
the egg itself, and an outer,
the ckorion, formed from the
follicle cells. The chorion,
which usually exhibits a more
or less elaborate pattern, has
one or more apertures or
micropyles for the entry of
the sperm. The contents are
distinguishable into two layers
— a superficial, consisting of
protoplasm, and a central, of
nutrient yolk.

Development. — The seg-
mentation is usually of a type
already referred to (p. 551)
as very common among the
Crustacea, viz., the superfi-
cial segmentation. The actual
segmentation (Fig. 489) has
only been observed in the
case of certain Insects with
very little yolk; but there
can be very little doubt that
Jn ordinary forms with abund-
ant yolk the process is in
essence the same. The seg-
mentation-nucleus, originally situated near the middle of the
ovum, divides into a number of nuclei, and most of these migrate
towards the surface, and arrange themselves in the form of a
sphere almost parallel with the latter ; eventually they reach the
surface and coalesce with the peripheral protoplasm, which then
becomes divided into cell-areas corresponding with the nuclei.

"blast

yk.c

FIG. 489. — A — D, successive stages in the seg-
mentation of the ovum of an Insect ; blast.
blastoderm ; peri, peripheral protoplasm ;
seg. segmentation cells ; yk. yolk ; ykc. yolk-
cells. (From Korschelt and Heider, after
Blochmann.)

600

ZOOLOGY

SECT.

gast

vent.pl

Wast

ser

aitm

end

amn.f

The layer of cells thus formed constitutes the blastoderm. This

thickens along one side to
form the ventral plate, as
already described in the case
of the Cockroach (p. 581), and
the changes which this struc-
ture undergoes, together with
the mode of formation of the
appendages, are similar in
most members of the class.
The same holds good of the
formation of the amnion and
the development of the en-
doderm. In some cases there
is developed between the
serosa and the true amnion
a space filled with yolk, and
the ventral plate appears sunk
within the yolk. The nervous
system is developed from the
ectoderm in the manner in-
dicated in the account of the
development of the Cockroach
(p. 582). The tracheal system
is derived from a series of
pairs of segmentally arranged
ectodermal involutions (Fig.
492, st.)

Metamorphosis. — In some
instances the young Insect,
when it escapes from the
egg-membranes, has exactly
the form of the parent, ex-
cept that, as a rule, the
wings have not yet grown.
But in most cases there is a
metamorphosis. In some this
is comparatively slight and
gradual, the adult Insect dif-
fering from the larva only in
comparatively unimportant
points, and the segments and
appendages ,of the latter be-
coming directly converted int
those of the former. Such
metamorphosis, in which there

is no quiescent stage, is said to be incomplete. The term complete

blast

ect

amn

end

blast

FIG. 490. — A — C, transverse sections through the
developing ovum of an Insect at successive
stages to show the mode of development of
the germinal layers and of the amnion.
amn. amnion ; amn. f. fold of the amnion ;
amn. cav. cavity of the amnion ; blast, blasto-
derm covering the yolk ; ect. ectoderm ; end.
endoderm ; gast, invagination of ventral
plate ; ser. serosa ; vent. pi. ventral plate ;
yk. yolk. (After Korschelt and Heider.)

in

it

!

PHYLUM ARTHROPODA

601

applied to the metamorphosis of the majority of Insects, in
which the larva differs so completely from the imago, or perfect

'

fc

y-W

FiG. 491.— A— E, ventral view of five stages in the development of Hydrophilus j a and b,
points at which the blastopore first closes ; af. edge of the amnion fold ; a/', caudal fold ; af,
paired head fold ; an. antenna ; es, terminal segment ; g, pit-like invagination to form the
rudiment of the amnion cavity; k, procephalic lobes; •/•, groove-like medio-ventral in-
vagination ; .?, germinal bands covered by the amnion. (From Lang, after Heider.)

B

)2. — A and B, later stages of the embryo of Hydrophilus with the rudiments of the
extremities ; in B the abdominal appendages are visible ; a. anus ; an. antenna ; g, rudiment
of the ventral nerve chain ; -ni. mouth ; md. mandible ; mrf , first maxilla ; mx*, second
maxilla ; pi, p-2, p%, thoracic egs ; p±, p§, p?, pg, rudiments of the appendages of the first,
second, fourth," and sixth abdominal appendages ; st. stigmata ; vk, prostomium. (From
Lang, after Heider.)

Insect, in external form, the nature of the appendages, and the
internal organisation, that there is need of a quiescent or pupa

602 ZOOLOGY SECT.

stage, during which the whole animal, or a considerable part of it,
undergoes an entire transformation. The metamorphosis is com-
plete in the Diptera, Lepidoptera, Coleoptera, and Hymenoptera,
absent or incomplete in the other orders. In the most lowly
organised larvae (many Diptera) the body of the larva or " maggot "
is completely worm-like, without any appendages, and without
any distinct head. In other cases (Lepidoptera, &c.), there is a
distinct head ; the three thoracic segments have three pairs of
jointed legs, and the abdominal segments short unjointed ^ro-
hgs (Fig. 475). In most instances the larvae differ widely from the
adults in their food and mode of life ; very generally the jaws are
adapted for biting, even when the mouth of the adult is suctorial.
After a longer or shorter period passed in this larval condition, in
which it is usually active and very voracious, the young Insect
passes into a quiescent or pupa condition, during which it remains
passive, enclosed in a tough integument, while a more or less
complete reconstruction of the organs goes on, resulting in the
development of all the parts of the perfect Insect. The develop-
ment of the new parts takes place from certain patches of cells,
the imaginal discs, present in the larva.

In the Diptera the larva or maggot is sometimes completely
devoid of jaws. In some Diptera, however, the jaws are well
developed, and there is a distinct head. After frequent moultings
the maggot passes either into a quiescent or pupa stage inclosed
in a hard skin, or into the stage of an active aquatic pupa, which
swims about actively in water and may possess tracheal gills.

In the Lepidoptera the larvae (" caterpillars ") are worm-like, but
with well-developed jaws, three pairs of jointed thoracic legs, and
a number of unjointed stumpy abdominal legs. Lepidopterous
larvae are often brilliantly coloured, and are very active, and feed
with voracity, chiefly on leaves and other succulent parts of plants.
Eventually they spin a cocoon of a silky substance, inclosed within
which, and covered with a tough skin, they pass through a
quiescent or pupa condition — the condition of the chrysalis (Fig.
475). From the interior of this the imago subsequently emerges
with all the parts of the adult Insect fully formed.

In mode of life there is a very considerable difference between
different orders and families of Insects. Some are parasites in the
strict sense throughout life. This is the case, for instance, in the
Strepsiptera (Bee-parasites), the females of which live permanently
ensconced between the joints of the abdomen of their hosts. The
Lice and Bird-lice are external parasites throughout life ; Bugs and
Fleas, though not adhering to their hosts, are parasites as regards
their diet. Many Insects are parasites in the larval condition,
though free in the adult state. This holds good, for example, of
the larvae of the Ichneumons, which develop in the interior of the
bodies of other insect-larvae ; also of the larvae of the Bot-flies

PHYLUM ARTHROPODA

603

1;. 474), which inhabit the alimentary canal of mammalian hosts
rses, Oxen, Sheep, Rhinoceroses, Tapirs),
i accordance with the high grade of the structure of their
various systems of organs, Insects exhibit a correspondingly high
-degree of functional activity. The quantity of food consumed
assimilated is great in comparison with the bulk of the body,
the energy expended in muscular contractions is of very con-
erable amount. It is estimated that while the muscular force

sid(

!>;. 4U3.— Honey-bee (Apis mellifica). a, queen (perfect female); b, worker (imperfect
female), and c, drone (male). (After Brehm).

•exerted by a Horse bears a ratio of about 0*7 to its own weight
{reckoned as 1) the muscular force of an Insect bears a ratio to
its weight of from about 14 to about 23. Insects are also dis-
tinguished among the Invertebrata by the keenness of their
senses. The sense of sight is, as we should expect from the
elaborate character of the optic organs, the most highly developed,
many Insects having been shown by experiment to have a keen
sense of colour ; but a sense of smell, the seat of which is in the
antennas, can be shown to exist in a high degree, and the parts
about the mouth bear nerve-endings concerned in a well-developed
sense of taste. A sense of hearing does not appear to be universally
present, but is well marked in such forms as produce sounds. At
the same time Insects are remarkable for the instincts, often leading
to results of an elaborate character, which guide them in the pursuit

FJ.;. 494.— Red ant (Formica ruf a) ; male, worker, and female.

of food and the protection and rearing of their young. Among
the insects which are the most highly endowed in this respect are
home — the Ants, Bees, Wasps, and Termites — which live together
in organised associations or communities, the various individuals
composing which are distinguishable into sexual individuals, neuter
workers, and soldiers (Figs. 493 and 494), each specially or-
ganised for the part which it has to play in the economy of the
community.

604 ZOOLOGY SECT.

Distribution in time. — The earliest known fossil remains of
Insects have been found in rocks of Silurian age. A good many
fossil Insects have been found in the Devonian; but they only
become abundant in the Carboniferous. All the palaeozoic Insects
belong to a group which has been regarded as a distinct order,
and has been named the Palceodictyoptera. The members of this
group are characterised rather by the absence of the special
characteristics of any of the existing orders than by any positive
features of their own ; but different families of the order approxi-
mate to a certain extent towards the groups of living Insects.
Amongst them, for example, are forms representing the Cock-
roaches and the Phasmidae among the Orthoptera ; others repre-
senting the modern Day-flies among the Neuroptera ; others the
Coleoptera.

Of the existing orders the Neuroptera. Orthoptera, and Coleop-
tera are first found in the Trias ; the Hemiptera, Diptera, Hymen-
optera, and Lepidoptera in the Jurassic.

CLASS V.— ARACHNIDA.

The class Arachnida, comprising the Scorpions and Spiders, the
Mites and Ticks, the King-crabs, and a number of other families,
is a much less homogeneous group than the Insecta, approaching
the Crustacea in the variety which it presents in the arrangement
of the segments and their appendages. In most members of the
class, however, there is an anterior region of the body — the cephalo-
thorax — representing both head and thorax, and a posterior part,
or abdomen, which is typically composed of a number of distinct
segments ; in some cases cephalothorax and abdomen are amal^
mated. There are no antennas in the adult Arachnid, thou^
rudiments of them have been found in the larvae of some specie
The first pair of appendages of the cephalothorax (probably repr(
senting the antennae of the Crayfish) are the chelic'erce ; the secon(
are the pedipalpi, the representatives of the Crayfish's and Cock-
roach's mandibles. Behind these are four pairs of legs. Th(
organs of respiration are sometimes tracheae, similar to those oi
the Insects, sometimes book-lungs or sacs containing numerous
book-leaf-like plates: sometimes leaf-like external appendage
or gills.

1. EXAMPLE OF THE CLASS. — THE SCORPION (Euscorpio or

Buthus).

Scorpions are inhabitants of warm countries — the largest kin<
being found in tropical Africa and America. They are nocturnal
animals, remaining in holes and crevices during the day, and
issuing forth at night to hunt for their prey, which consists of

XI

PHYLUM ARTHROPODA

605

Spiders and Insects. These they seize with their pincer-claws and
sting to death with their caudal spine, afterwards sucking their

juices.

There are a number of different species of Scorpions, divided
into several genera, which differ from one another in comparatively
unimportant points, so that the following general description will
apply almost equally well to any of them.

External features. — A Scorpion (Fig. 495) has a long narrow
body, in superficial appearance not unlike that of a Crayfish.
There is a small cephalothoracic shield or carapace, covering over

IG. 495.— EuscorpiO. (From
Cuvier's Animal Kingdom.)

FIG. 406.— Scorpion. Ventral surface of the
cephalothorax and pne-abdomen. chd. cheli-
cerge ; op. operculum ; pect. pectines ; ped.
pedipalpi ; stig. stigmata.

•dorsally a short anterior region or cephalothorax. This is followed
by a long posterior region or abdomen, the terminal part of which
in the living animal is habitually carried over the back (Fig. 498),
•constituting the " tail," at the end of which the sting is placed. The
carapace bears a pair of large eyes about its middle, and several
pairs of smaller eyes on the antero-lateral margin. The anterior,
broader part of the abdomen, which is termed the prat-abdomen,
•consists of seven segments, each of which is enclosed in firm,
chitinous, dorsal and ventral plates, or terga and sterna. The
tergum and sternum of each segment are separated from one
another laterally by intervals of soft skin, except in the seventh,

606 ZOOLOGY SECT..

where they are united laterally for a longer or shorter distance.
The posterior, narrower part of the abdomen, known as the post-
abdomen, consists of five segments, each enclosed in a complete
investing ring of hard chitinous matter. Articulating with the
last segment of the post-abdomen is a terminal appendage, the
caudal spine or sting, swollen at the base and acutely pointed at
the apex, where open the ducts of two poison- glands. The anal
opening is situated on the ventral surface of the last segment of
the post-abdomen, immediately in front of the sting.

The aperture of the mouth, which is very small, is at the anterior
end of the cephalothorax on its ventral aspect ; a lobe which over-
hangs it in front is the labrum. On each side of the mouth is a
three-jointed appendage — the chelicera (Fig. 496. chel.) — which is
terminated by a chela. Behind these are the very\a,rgepincer~Glaws
or pedipalpi (ped.), each composed of six podomeres and terminating
in powerful chelae. The basal joint of each pedipalp has a process
which bites against the corresponding process of the other pedipalp,
these processes thus performing the function of jaws. Following
upon the pedipalpi are four pairs of walking legs, each composed
of seven podomeres, the last of which is provided with curved and
pointed horny claws. The basal segments of the first two pairs of
walking legs are modified so as to perform to some extent the
function of jaws.

All the six pairs of appendages hitherto described — the cheli-
cerse, the pedipalpi, and the four pairs of walking legs — belong
to the cephalothorax. The first segment of the prse-abdomen
(Fig. 496) has a narrow sternum, on which there is placed a soft
rounded median lobe divided by a cleft ; this is termed the genital
operculum (pp.) ; at its base is the opening of the genital duct. To
the sternum of the second segment of the prse-abdomen are attached
a pair of remarkable appendages of a comb-like shape — ihepectincs
(ped.) — each consisting of a stem, along the posterior margin of
which is a row of narrow processes, some-
what like the teeth of a comb ; the function
of these appendages is doubtful, but is prob-
ably sensory. The remainder of the seg-
ments of the prse-abdomen, and all those of
the post-abdomen, are devoid of appendages.
The sterna of the third, fourth, fifth, and
sixth segments of the praa-abdomen, which

PIO. 497.-Endosternite ^ ^^ br°ad' beai> 6ach a Pair °f obliqU6

of scorpion. (After slits — the stigmata (stig.) — leading into the

Lankester.) y V. *V &

pulmonary sacs.

In the interior of the cephalothorax, over

the nervous system, is a cartilaginous plate — the endosternite
(Fig. 497) — which serves to give attachment to muscles, and is
comparable to the cephalic apodeme of Apus (p. 491).

XI

PHYLUM ARTHROPODA

607

All the appendages of the Scorpion are post-oral in position, and
the most anterior — the chelicerae — are probably best regarded as
corresponding to the anternae of the Crayfish, the equivalent of
the Crayfish's antennules and of the antennae of the Cockroach not
being present. The pedipalpi would then be the homologues of
the mandibles of the Insect and the Crustacean.

CRAYFISH.

Antennules.
Antennae.
Mandibles.
First maxillae.
Second maxillae.
First maxillip'edes.
Second maxillipedes.
Third maxillipedes.

COCKROACH.

Antennae.
Absent.
Mandibles.
First maxillae.
Second maxillae.
First legs.
Second legs.
Third legs.

SCORPION.

Absent.
Chelicerae.
Pedipalpi.
First legs.
Second legs.
Third legs.
Fourth legs.

Digestive system. — The narrow mouth leads into a large
chamber with elastic walls, the pharynx ; this is capable of being
greatly dilated by the action of a number of radiating bundles of
muscular fibres, which run outwards from it to the walls of the
cephalothorax, the result of this being to cause suction through
the mouth, by which means the juices of the Scorpion's prey are
drawn in. A second dilatation, to which a narrow oesophagus
leads, receives the ducts of a pair of salivary glands (Fig. 499,
sal. gld.). Upon this follows the mesenteron (mcsent.), which is an
elongated, wide, straight tube, with glandular walls, corresponding
to the chylific ventricle of the Insect. Opening into the mesenteron
are five pairs of narrow tubes (Figs. 498 and 499, hep. du.) leading
into the substance of a large glandular body, usually termed the
liver (hep.), though its hepatic functions are doubtful. The proc-
todceum ( prod.) is a short, narrow passage ; into it there open two
delicate tubes — the Malpighian tubes (mal.) — which act as the
organs of renal excretion.

Circulatory organs. — An elongated tubular heart (Fig. 498,
hrt.) lies in the prae-abdomen enclosed in a pericardial sinus ; it is
divided internally into a series of eight chambers by transverse
partitions ; into each of these chambers the blood passes by a pair
of valvular apertures or ostia. The heart ends both in front and
behind in main arteries or anterior and posterior aortas (ant. art.,
post, art.) ; and a series of pairs of lateral arteries are given off from
the various chambers. The anterior aorta (truncus arteriosus) soon
bifurcates to form a pair of vessels which embrace between them
the oesophagus, and meet below in a median ventral trunk which
runs backwards above the nerve cord. The blood carried to the
various parts of the body by the arteries is gathered up into a
large ventral sinus from which it passes to the book-lungs. From

608

ZOOLOGY

SECT.

these it is carried by a series of veins to the pericardial sinus to
enter the heart through the ostia.

The organs of respiration in the Scorpions are in the form of
pulmonary sacs or book-lungs (pul.), the stigmata or external openings

of which have already been referred to. Each pulmonary sac is
a compressed chamber lined with a thin cuticle. The lining
membrane is raised up into numerous delicate laminae lying
parallel with one another like the leaves of a book. Into the

XI

PHYLUM ARTHROPOD A

609

chel

he/b du,

stonw-

-saigld-

mesent

numerous narrow spaces between the laminae the air penetrates,
and oxygenates the blood which enters the interior of the laminae

the ventral sinus.

A pair of coxal glands, situated near the base of the fifth pair
of appendages, are, in the embryo Scorpion, represented by tubes
which, like nephridia, effect a communication between the body-

; cavity and the exterior ; in the adult Scorpion the tube assumes

I the form of a closed gland, and its function is quite uncertain.

The nervous system is constructed on a plan which bears a

| considerable resemblance to that of the Crayfish and that of the

i Cockroach. There is a bilobed
cerebral ganglion or brain
(Fig. 498, brn.) from which
nerves are given off to the
eyes ; a nerve collar formed of a
pair of oesophageal connectives
unites ventrally in a sub-
cesophageal ganglion, forming
the anterior part of a ventral
nerve cord (ne. co.). The con-
nectives and sub-cesophageal
ganglion give rise to the nerves,
to the first six pairs of append-
ages, and to the operculum, the
pectines, and the two following
segments. The first ganglion
in the nerve cord appears in
the eleventh segment (reckon-
ing the cephalo-thorax as made
up of six) ; behind which a
ganglion appears regularly in
each segment as far back as the
fourth of the post-abdomen.

The organs of special
sense are the eyes and pectines.
The lateral eyes (Fig. 518) are

similar in character to the simple eyes or ocelli of Insects.
The two larger central eyes (Fig. 519) differ from them in
having the retinal cells arranged in groups as in the com-
pound eye, but resemble them in the presence of a single
cuticular lens.

Reproductive organs. — In the male the testes consist of two
pairs of longitudinal tubules united by cross branches. These
are connected with a median vas deferens, the terminal portion of
which, provided with accessory glands, is modified to form a double
penis ; its external opening is just behind the operculum as
already noticed. There is an unpaired ovary, which is made up of
VOL. i R R

mai.

root

FIG. 499. — Dorsal view of the internal organs
of Scorpion, chel. chelicerse ; hep. liver ;
hep. du. hepatic ducts ; mal. Malpighian
tubes ; mesent. mesenteron ; proct. procto-
damm; sal.gld. salivary glands; stomo. stomo-
dseum. (From Leuckart after Blanchard.)

610 .

ZOOLOGY

SECT.

three longitudinal tubules with transverse connecting branches :
the oviducts open on the operculum.

Scorpions are viviparous. The eggs, which are spherical or oval,
and in most species contain a large amount of food-yolk, lie in a
follicle formed of a diverticulum of the oviduct. Fertilisation
either takes place in the follicle or after the egg has escaped into
the oviduct. The further development takes place in the oviducts ;
and, when born, the young Scorpion differs from the parent very
little save in size.

Development. — The segmentation is of the type to which the
term discoidal is applied. On one side are formed a number of cells

FIG. 500.— Three surface views of the ventral
plate of a developing Scorpion. A,
, before the appearance of segments ; B, "after
five segments have become formed ; C, after
the appendages have begun to be formed.
(From Balfour, after Metschiiikoff.)

FIG. 501. — Embryo of Scorpion (Euscor
plus italicus), later stage, up.
II. — VI., abdominal appendages; ch.
chelicerte ; p. 1-4, legs ; m. mouth ;
ped. pedipalpi ; p. ab. post-abdomen,
(From Korschelt and Heider after
Metschiiikoff.)

in the form of a one-layered disc or cap, which gradually spreads over
the yolk. On this appears a thickening — the ventral plate (Fig. 500)
corresponding to that of the Insect. A longitudinal groove which
appears on the surface of this may be regarded as representing
an elongated blastopore (Fig. 500, A). The cells of the blastoderm
of the ventral plate become divisible into three layers — ectoderm,
endoderm, and mesoderm. The mesoderm becomes divided into a
series of masses which become hollowed out to form the primitive
segments (B) and their cavities. Embryonic membranes — serosa
and amnion — are formed as in the Insects. When about ten seg-
ments have become distinguishable, the rudiments of appendages
(Fig. 500 C, and Fig. 501) appear in the form of hollow processes
of the segments on either side of the middle line. Behind the

xi PHYLUM ARTHROPODA 611

rudiments of the thoracic limbs appear a series of six pairs of
abdominal appendages (ap. II. — VI.) ; the place of the first of these
is afterwards taken by the operculum ; the second develops into
the pectines. The four posterior pairs become aborted, though
they apparently have some relation to the development of the
book-lungs.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Arachnida are air-breathing Arthropoda, in which the body
is usually distinguishable into two regions — cephalothorax and
abdomen. The cephalothorax bears sessile, usually simple, eyes,
two pairs of jointed appendages — the chelicerse and pedipalpi — and
four pairs of legs. There are no antennae. The organs of respira-
tion, when present, are usually either tracheae or book-lungs, but in
the Xiphosura take the form of book-gills. Heart and vascular
system are usually present ; the heart is tubular, like that of the
Insects. The sexes are nearly always separate, and there is usually
no metamorphosis.

The class is divided into the following orders : —

ORDER 1. — SCORPIONIDA.

Arachnida in which the body consists of a continuous cephalo-
thorax and an abdomen, the latter consisting of an anterior
broader prse-abdomen of seven segments, and a posterior, narrower
post-abdomen of five, with a caudal spine in the form of a sting.
There are small chelate chelicerae and large chelate pedipalpi. A
pair of comb-like pectines occur on the second segment of the
prae-abdomen. The organs of respiration are four pairs of book-
lungs in the third, fourth, fifth and sixth segments of the prse-
abdomen.

This order includes the Scorpions.

ORDER 2. — PSEUDOSCORPIONIDA.

Arachnida in which there is a continuous cephalothorax, some-
times marked dorsally with two transverse grooves, and a broad
abdomen, not divided into prae- and post-abdomen, and not pro-
vided with a sting. The chelicerae are very small, the pedipalpi
similar to those of the Scorpions. The organs of respiration are

» system of tracheae. A pair of spinning glands are present.
This order includes the Book-scorpions (Fig. 502).
ORDER 3. — PEDIPALPIDA.

Arachnida in which the body consists of unsegmented cephalo
thorax and flattened abdomen of eleven to twelve segments.
The chelicerse are simple, the pedipalpi simple or chelate, the first

R R 2

612 ZOOLOGY SECT.

pair of legs terminate in a many-jointed flagellum. The organs
of respiration are two pairs of book-lungs on the second and third
segments of the abdomen.

This order includes the Scorpion-spiders (Fig. 503).

ORDER 4. — SOLPUGIDA,

Arachnida with three regions — head, thorax (of three segments),
and abdomen (of ten segments). The chelicerae are chelate ; the
pedipalpi elongated and leg-like. The organs of respiration are
tracheae.

This order includes Gfaleodes (Fig. 504).

ORDER 5. — PHALANGIDA.

Arachnida with an unsegmented cephalothorax, and an abdomen
of six segments. The chelicerae are chelate, the pedipalpi leg-like.
The organs of respiration are tracheae. No spinning glands are
developed.

This order includes the Harvest-men.

ORDER 6. — ARANEIDA.

Arachnida in which the body is composed of an undivided
cephalothorax and an unsegmented abdomen, which is usually soft
and rounded, and attached to the cephalothorax by a narrow neck.
The chelicerae are sub-chelate, with poison glands ; the pedipalpi
simple. The organs of respiration are book-lungs alone, or book-
lungs combined with tracheae.

This order comprises all the true Spiders (Fig. 505).

ORDER 7. — ACARIDA.

Arachnida in which the body exhibits no division into regions.
The mouth-parts are adapted either for biting or piercing and
sucking. The organs of respiration, when present, are in the
form of tracheae.

This order includes the Mites and Ticks (Figs. 508 and 509).

ORDER 8. — XIPHOSURA.

Arachnida in which the body consists of a cephalothorax,
covered over by a broad carapace, and an abdomen of seven
firmly united segments, with a long narrow tail-piece or telson.
The cephalothorax bears a pair of short chelate appendages
and five pairs of legs. The abdomen bears in front a pair of
united plate-like appendages, forming the operculum, followed
by five pairs of flat appendages overlapped by the operculum.
The organs of respiration are lamelliform gills attached to the
abdominal appendages.

This order includes the King-crabs (Limulus) (Fig. 510).

PHYLUM ARTHROPODA

613

ORDER 9. — EURYPTERIDA.

Arachnida with a relatively small cephalothorax, followed by
twelve free segments and a terminal, elongated, narrow telson.
There are a pair of prse-oral leg-like or chelate appendages and
four more leg-like appendages on the cephalothorax, the last
expanded to form swimming paddles. A broad operculum is
situated immediately behind the cepha-
lothorax. There are pairs of lamellate
appendages on certain of the anterior
free segments. The exoskeleton is char-
acteristically sculptured.

This order includes only a number of
extinct (Palaeozoic) forms of large size
(Fig. 512).

3. GENERAL ORGANISATION.

The external form in the Scorpionida
has already been sufficiently described.
Most nearly related to that order in this
respect are the Pseudoscorpionida or
Book-scorpions and their allies. In these
(Fig. 502) there is an unsegmented
cephalo-thorax, or the carapace is crossed by two transverse grooves
which may indicate segmental divisions. There is a broad abdomen
consisting of eleven, or more rarely ten, segments; the post-
abdomen is not represented, nor the caudal sting. The chelicerse
are small; the pedipalpi are large, and resemble those of the
Scorpions in their chelate form. Spinning glands are present.

FIG. 502.— Chelifer bravaisii.

2 — 6, second to sixth pairs of
appendages. (From Lang's
Comparative Anatomy.)

FTO. 503. — PhryxiUS (Pedipalpi). (From Cuvier's Animal Kingdom.)

The Pedipalpi, or Scorpion-spiders , (Fig. 503), are intermediate
in some of their external features between the Scorpions and the

614

ZOOLOGY

SECT.

Spiders. The abdomen is broad and marked out into a series of
eleven or twelve distinct segments ; in one of the genera of the
order there is a short post-abdomen formed of the last three
segments, with an elongated, many-jointed anal filament. The
chelicerse end in simple claws ; they are probably provided with
poison glands; the pedipalps are very long, either claw-like or

FIG. 504. — Galeodes dastuguei 9, natural size. 1 — 6, the six pairs of appendages
1, chelicerae ; 2, pedipalpi ; c, head ; th. thorax ; ab. abdomen. (From Lang, after Dufour.)

chelate ; the first pair of legs are very long and slender, their
terminal part made up of numerous short joints like antennae.
There are eight eyes on the carapace, two larger central, and six
smaller marginal.

The Solpugida (Fig. 504) have, at least superficially, the
appearance of being intermediate between the Insecta and the
other groups of Arachnida. The cephalothoracic region is divided

PHYLUM ARTHROPODA

615

by a constriction into two parts, head and thorax, the latter made

up of three segments. The chelicerse are chelate ; the pedipalpi

resemble the legs, and are used in

locomotion. The first pair of legs

are attached to the head. The

abdomen is distinctly segmented,

and there is no caudal appendage.

A pair of poison-glands open at

the bases of the chelicerae. There

are two simple eyes on the head.
In the true Spiders (Fig. 505)

the abdomen is rounded, unseg-

mented, and separated off from the

cephalothorax by a constriction.

~~he chelicerse (Fig. 506) are sub-
elate, and the duct of a large

poison gland opens at the ex-
tremity. The pedipalpi (Fig. 506, B) are elongated, and end in

simple extremities; in the male (Fig. 507) the terminal joint

ce

2

FIG. 505.— Spider Epeira diadexna).

hb

FIG. 506.— A, Chelicerae, and B, pedipalpi of female of Epeira diadexna. (After Leuckart.)

Is modified to serve for the reception and transference of the

sperms. At the extremity of the abdomen is the spinning

apparatus or arachnidium (Fig. 513,
arach.). This consists of four or six
elevations, the spinnerets, sometimes
jointed, probably derived from em-
bryonic rudiments of abdominal ap-
pendages. On the surfaces of these
open the numerous fine ducts of the
spinning glands (sp. glds.), secreting
the material of which the spider's
web is composed. The fine threads
of viscid secretion issuing from the
ducts harden on exposure to the air,

and are worked up into the web by means of the posterior legs.

There are six or eight eyes on the carapace.

In the spider-like Phalangida, or " Harvest-men," the cephalo-

PIG. 507. — Pedipalpi of male of
Epeira diadema. II. 111.
IV. V. podomeres ; bb, sac ; sph.
spiral tube. (After Leuckart.)

616

ZOOLOGY

SECT.

thorax is not constricted off
from the abdomen. The cheli-
cerse are chelate, the pedipalpi
short and leg-like, the legs
long and slender.

In the Acarida or Mites
and Ticks (Figs. 508 and 509)
the distinction into regions is
no longer recognisable. The
form of the mouth parts varies
somewhat in the different
families. Sometimes the basal
portions of the pedipalpi form
a sucking proboscis enclosing
the stylet-like chelicerse, modi-
fied to form piercing organs ;
sometimes these appendages
are claw-like or chelate. The
legs vary somewhat in shape in the different groups, according
as they are used for prehension, for creeping, for running, or

FIG. 508.— Itch mite (Sarcoptes scabisei).

(After Leuckart.)

FIG. 509.— Water mite. Trombidium fuliginosum, female, chel. chelicerse ; ped. pedipalpi

(After Leuckart.)

for swimming; they end usually in two claws, between which
there may be discs or stalked suckers.

XI

PHYLUM ARTHROPODA

617

In the Xiphosura or King-crabs (Fig. 510), the body consists of
two well-marked regions — cephalotliorax and abdomen. The former
is covered over by a wide, dorsally convex, sub-crescentic shield or
carapace, bearing two large compound, and two smaller simple, eyes.
The segments of the abdomen (seven in number) are united
together, being covered dorsally by a continuous abdominal cara-
pace. At the posterior end is attached a very long, narrow,
caudal spine. The anterior
appendages (Fig. 511) re-
semble those of the Scor-
pion. In front of the
mouth is a pair of short,
three-jointed, chelate ap-
pendages, the chelicerce (T),
at the sides of a labrum or
upper lip. Behind these
follow a series of five pairs
of legs, the bases of all of
which, with the exception
of the last, are covered
with spines, and have the
action of jaws, while the
extremities are for the
most part chelate. The
first pair of appendages of
the abdomen are flat plates,
which are united together
in the middle line and
together form the broad
operculum (operc.), over-
lapping all the posterior
appendages ; on its pos-
terior face are the two
genital apertures. The pos-
terior appendages, of which
there are five pairs, are
thin flat plates to which
the gills are attached ; each
of them is divided by
sutures into a small inner ramus or endopodite, and a larger
external ramus or exopodite. A labrum (rostrum) lies in front
of the mouth, and between the sixth pair of appendages is a
pair of processes, the chilaria.

In the Eurypterida (Fig. 512) there is a small cephalothorax
bearing a pair of large eyes and a pair of ocelli, and an elongated
segmented region containing twelve segments, followed by a
narrow pointed telson. There are usually five pairs of legs sur-

FIG. 510. — Limulus. Dorsal aspect.
(After Leuckart.)

618

ZOOLOGY

SECT.

:
;

rounding the mouth, and, with the exception of the first, toothed
at the bases in order to perform the functions of jaws ; the last
pair are stouter than the others and are expanded so as, apparently,
to assume the character of swimming paddles. Certain of the
more anterior of the free segments bear paired lamelliform
appendages which probably carried the branchia? as in the

Xiphosura. The exo-
skeleton is in many
cases elaborately
sculptured.

A cartilaginous in-
ternal endosternite
of the same nature
as that which has
been described as oc
curring in the Scor
pions is found in
Limulus and in cer-
tain Spiders, but
not in the other
groups.

Coxal glands,
similar to those that
have been described
in the Scorpion, oc
cur also in mos
Spiders, in the Sol
pugida and Phalan
gida, in some Acarid^
and the Xiphosura
In the Solpugida anc
Phalangida they oc
cur on the bases o
the last pair of legs
in the Araneida anc
Xiphosura, as in th
Scorpion, they are
found on the bases
of the fifth pair c
appendages.

Alimentary system. — The oesophagus (Fig. 513, CBS.) of th
Spiders is expanded behind into a special sucking stomach (suck, st.*
The mcscntcron (mesent.) gives off in the cephalothorax five pain
of narrow diverticula (cose.) which enter the bases of the pedipalp*
and legs ; in the abdomen it also gives off a number of caeca, which
branch and come in close relation with a mass of cells commonly
termed liver (hep.), though not known to have the function of that

a&d

teLs

FIG. 511. — Ventral view of Limulus. 1 — 6, appendages of
cephalothorax ; abd. abdomen ; ceph. cephalothorax ; operc.
operculum, behind which are seen the series of abdominal
appendages ; tels. caudal spine or telson. (After Leuckart.)

PHYLUM ARTHROPODA

619

:

organ. The rectum or proctodcewm (rect.) is dilated ; the dilated
portion (rect. ccec.) gives off two pairs of MalpigJiian tubes (mal.).

In the Pseudoscorpionida the mesenteron, which is bent into a
loop, gives off three diverticula; the proctodseum has also a
•diverticulum. In the Solpugida the mesenteron also gives off
diverticula ; the occurrence of Malpighian tubes is doubtful. In
the Acarida there are always diverticula, the number and arrange-
ment of which vary,
connected with the
mesenteron. There
.are usually two long
coiled Malpighian

<bes.
In the Xiphosura,
e mouth (Fig. 514,
mo.), which is situ-
ated some distance
behind the anterior
extremity of the
body, leads into a
.•suctorial pharynx, fol-
lowed by a stomach,
which opens into
the elongated mesen-
teron ; the procto-
•dseum, a short tube
with folded walls,
opens on the ex-
terior at the posterior
-extremity of the ab-
domen. Into the
mesenteron, as in the
Scorpion, open the
ducts of a large
gland, usually termed
the liver (/. liv.).

A heart is ab-
sent in all the Mites
with the exception

of one family. In the other Arachnida it is present, and has
the same general form as in the Scorpions, though always more
concentrated.

In the various orders the organs of respiration differ a good
•deal in their character. In the Pseudoscorpionida they take the
form of branching tracheae similar to those of Insects. In the
Pedipalpi there are two pulmonary sacs or book-lungs similar to
-those of the scorpions. In the Solpugida there is a system of

FIG. 512.— Eurypterus fischeri (Silurian).
(From Nicholson and Lyddeker.)

620

ZOOLOGY

SECT

tracheae. In the Spiders there are either four pulmonary sacs
(Fig. 515), or two pulmonary sacs and a system of tracheae (Fig. 516).

c ^3-g

iWl

ffi

S-8S.J

„>*: 9, x >

.apt

: .3 oc o to

Tracheae are present in the Phalangida and also in the majorit
of the Acarida. In the Xiphosura the organs of respiration are

PHYLUM ARTHROPODA

621

external appendages or gills (book-gills), in the shape of delicate
laminse attached to the abdominal appendages (Fig. 517).

The nervous system is, in most instances, more concentrated
than in the Scorpions. There may be one or two separate

hqb.du,

FIG. 514. — Diagrammatic view of a median longitudinal section of Limulus. abd.app. abdomina
appendages ; an. anus ; brn. brain ; chil. chilaria ; hep. du. opening of one of the hepatic
ducts ; ht. heart ; int. intestine ; 1. liv. liver ; mo. mouth ; ne. co. nerve cord ; oes. oesophagus ;
operc. operculum ; ids. telson ; ven. sinus, venous sinus ; 1-5, legs. (From Leuckart, partly
after Packard.)

abdominal ganglia behind the mass formed by the united cephalo-
thoracic and anterior abdominal (Pseudoscorpionida, Pedipalpida,
some Araneida, Solpugida, Phalangida). In most of the Araneida
and in the Acarida all the abdominal are united with all the

FIG. 515. — Book-lung of a Spider
(Zilla callopkylla). */.

stigma. (From Hertwig.)

FIG. 516.— Main branches of the tracheal
system of a Spider, st. stigma.
(From Hertwig, after Bertkau.)

cephalothoracic ganglia to form a single mass perforated by the
oesophagus, the part lying behind, which is much the larger,
representing the ventral nerve cord.

Sense organs. — Eyes are present in all except in some of the
Acarida. Their number and arrangement have been given with

622

ZOOLOGY

SECT,

the external characters of the groups. They are all (Fig. 518) of
the type of the ocelli or simple eyes of Insects, except the central

eyes of the Scorpions
(Fig. 519) and
pound eyes of

"K.

FIG. 517.— One of the book-gills of Linmlus, with
the appendage to which it is attached. (After
Lank ester.)

the com-
Limulus.

The former are intermedi-
ate in character between
ocelli and facetted eyes,
possessing the single cuti-
cular lens (lens) of the
ocellus, and resembling
the facetted eye in having
the retinal cells arranged
in groups corresponding
to ommatidia. Each re-
tinula, composed of five
cells, contains a thick axial
rod or rhabdome (rhdbd.).
In Limulus the com-
pound eye has a continu-
ous chitinous cornea-lens of the nature of a thickening of the
cuticle. This, though non-facetted, differs from the corresponding
part in the compound eye of the Scorpion in being produced
internally into a number of conical papillae, each of which lies
over one of the ornmatidea and may be looked upon as its lens.

A considerable variety is observable in the exact arrangement
of the parts of the re-
productive apparatus
in different groups of the
Arachnida. In general,
testes or ovaries are either
paired or (more rarely)
unpaired tubes, with
paired vasa deferentia or
oviducts, which unite in
a median duct opening
on the exterior by an
unpaired genital open-
ing. Viviparity is ex-
ceptional. In the Spiders
the ovaries (Fig. 113, ov.)
are two wide tubes, on
the surface of which
follicles project promi-
nently ; sometimes they

unite into a single circular ovary. Each ovary has a short oviduct,
or, when the ovary is single it has two, right and left ; these unite

FIG. 518. — Section of the lateral eye of Euscorpius
italicus. int. intermediate cells ; lens, cuticular
lens ; nerv. c. terminal nerve cells ; nerv. /. nerve
fibres of optic nerve ;• rhabd. rhabdomes. (After
Lankester and Bourne.)

PHYLUM ARTHROPODA

623

vitr

in a median vagina, which opens on the exterior by a median genital
aperture at the base of the abdomen. One, two, or three receptacula
seminis are present, and either open into the vagina or independ-
ently on the surface. In the male there are two elongated tubular
tcstes with two narrow, and often greatly coiled, efferent ducts, which
unite in a short median vas deferens, the aperture of which is on the
base of the abdomen between the stigmata of the first pair. The
pedipalpi of the male (Fig. 507) are modified to act as intro-
mittent organs ; the terminal segment is swollen, and contains a
twisted tube (sph.) into which the sperms from the reproductive
aperture are received in order to be transferred in the act of
copulation to the reproductive aperture of the female. The eggs
of spiders are laid in nests or cocoons, and are usually guarded
by the mother, some-
times carried about
by her.

In their mode of
life the Arachnida
present almost as
great a diversity as
the Insecta. Some
Acarida are parasites
throughout life. Most
of the other groups
of Arachnida are pre-
daceous — preying for
the most part on In-
sects or other Arach-
nids. To capture the
Insects which consti-
tute their food the
majority of Spiders
construct a web
formed of the threads secreted by the arachnidium. The
primary function of the threads formed from the secretion of
the spinning organ is to constitute the material for the manu-
facture of a cocoon for enclosing the eggs, and in some Arachnids
this is the sole purpose to which they are devoted. In others
there is added a nest for the protection of the eggs and of the
parent itself; this in many cases becomes a permanent lurking
place which the Spider inhabits at all seasons, and from which it
darts out to capture its prey ; in the Trap-door Spider, the nest has
a closely fitting hinged lid. In very many Spiders the secretion
is used mainly to form the web by means of which, the prey is
snared, with the addition frequently of a nest in which the Spider
lies in wait. A subsidiary function of the threads is to aid in
locomotion, the Spider being enabled by means of them to let

FIG. 519.— Section of the central eye of Euscorpius.
Letters as in preceding figure, pigm. cells containing
pigment ; vitr. vitreous body (a specialised part of the
ectoderm). (After Lankester and Bourne.)

624 ZOOLOGY SECT.

itself down safely from considerable heights, and even to float
in the air.

Some of the Mites, as already mentioned, are parasitic ; others
feed on various kinds of fresh or decaying animal or vegetable
substances. Most free Acarida are terrestrial ; some are aquatic.

The Xiphosura are marine, living at a depth of a few fathoms
in warm seas, burrowing in sand ; their food consists of various
kinds of marine Annelids.

Geological History. — The most ancient of the living groups of
the Arachriida are the Scorpions, which are represented in Silurian
rocks by various fossil forms not differing very widely from those
existing at the present day. The earliest known fossil Spiders
have been found in deposits of Carboniferous age ; and remains of
Pedipalpida occur in the same formation. In Tertiary deposits
there have been found representatives of all the principal groups
of living Arachnida.

The earliest fossil remains of Xiphosura that have been found,
occur in strata of the Triassic period. Other fossil species occur
in later formations. These are all nearly related to the living
species of Limulus. The Eurypterida, as already noted, are
entirely palaeozoic, ranging from the Lower Silurian rocks to the
Devonian.

APPENDIX TO THE ARACHNIDA.

THE PYCNOGONIDA, LINGUATULIDA, AND TARDIGRADA.

These three groups, though not in any way related to one another, and of
doubtful relationships to the Arachnida, are, as a matter of convenience, men-
tioned together here.

THE PYCNOGONIDA.

These are marine Spider-like Arthropods (Fig. 520) in which the body consists
of a cephalothorax composed of an anterior proboscis (.9), three head segments,
and one thoracic segment, followed by three free thoracic segments and a rudi-
mentary abdomen (ab.). The cephalothorax bears usually four simple eyes and
four pairs of appendages, one or both of the first two of which may be chelat
To these succeed a pair of usually ten-jointed ovigerous legs (3), and the first
of thoracic legs (4). The free thoracic segments bear lateral processes for tl
articulation of the remaining three pairs of legs. The rudimentary abdomen (ab.
is devoid of appendages.

Diverticula from the mesenteron penetrate for a considerable distance inl
the limbs. Malpighian vessels are absent. There is a heart with two or thn
pairs of ostia. Organs of respiration are absent. The nervous system consists
of brain, sub-oesophageal ganglia and three other ganglia. The testes in the mal
are partly, and the ovaries in the female either partly or completely, contain*
in the bases of the thoracic appendages on which they open. In the male 4-
cement glands are situated in the fourth joints of certain of the appendages
their secretion cements the eggs together into masses which are carried on th<
ovigerous legs of the male, and in one species on those of the female also.

A metamorphosis occurs in most cases. The larva usually has three pairs
appendages, so that it bears a superficial resemblance to a nauplius ; but the

PHYLUM ARTHROPODA

625

appendages are simple and in other respects the larva has no essential likeness
nauplius form. Additional segments with their appendages are formed

•Fio. 520. Nymphon hispidum. 1 — 7, appendages ; ab. abdomen ; s, proboscis.

(From Lang, after Hoek.)

behind the original three until the form of the adult is completed. Different
kinds of Pyciiogonids occur at different depths from between tidal limits to
considerable depths in the ocean. The larvae of the species of one genus are
internal parasites in certain hydroid
Zoophytes.

THE LlNGUATULIDA OR PENTASTOMIDA.

The Linguatulida (Fig. 521) are para-
sitic animals which, when superficially
examined, present little appearance of
affinity with the Arthropoda. The body
is completely worm-like, not divided
into regions, and presenting only a super-
ficial annulation, which in no way cor-
responds with division of the body into
segments. The sole representatives of
limbs are four hooks (hk.) at the sides
of the mouth. The muscular fibres are
striated. The alimentary canal is simple
and straight, and Malpighian tubes are
absent. Heart and organs of respira-
tion are wanting. The nervous system
is greatly reduced. A narrow nerve-
collar surrounds the oesophagus, present-
ing no brain, enlargement, and connected
behind with a single ventral nerve mass.
Organs of special sense are absent.

Some species of Pentastomum are in
the adult condition parasites in the
lungs of snakes. One species (Pentasto-
mum taenioides) inhabits certain cavities
— the frontal sinuses and maxillary antra
— connected with the nasal chambers,

acx.cp

FIG. 521.— Pentastomum tsenioides,

young female, an. anus ; gang, ganglion ;
Jtk. hooks ; mo. mouth ; oes. oesophagus ;
or. ovary; ovd. oviduct; rec. sem. re-
ceptaculum seminis ; sex. ap. sexual
aperture ; atom, stomach ; ut. uterus.
(After Leuckart.)

S S

626

ZOOLOGY

SECT.

in the Dog and Wolf. Its embryos escaping and falling on grass and other
herbage, which form the food of Hares and Rabbits, are taken up by the latter,
and perforating the wall of the alimentary canal, by means of a boring
apparatus composed of several chitinous pieces, lodge themselves in the liver,
where they become encysted and undergo a metamorphosis. Afterwards they
leave the cysts and move about. If it should be received into the mouth of a.
Dog (still contained probably in most cases in the tissues of the Hare or Rabbit)
the young Pentastomum may find its way to the frontal sinuses or maxillary
antra, there to undergo its final transformation into the adult form. The larva
possesses two pairs of short legs.

fit.

THE TARDIGRADA.

The Tardigrada ("bear animalcules ") are soft-skinned animals (Fig. 522) of
minute size, not exceeding a millimetre in length. The body is unsegmented and

not distinguishable into regions, except
that in some a slight constriction separates
off from the rest an anterior part or head.
The mouth is provided with a sucking
proboscis. There are four pairs of short
unjointed legs (I.— IV. ), the last of which
is terminal, and each is provided with
two or four claws. The mouth is -sur-
rounded by papilla? ; the buccal cavity
contains a pair of horny, sometimes partly
calcified, teeth (styl. ). The ducts of a pair
of salivary (?) glands (sali) open into the
cavity of the mouth ; there is a muscular
pharynx (ph.), a narrow resophagus, and
a wide mesenteron (stom. ) ; the anus is
sub-terminal, situated in front of the
last pair of limbs. A pair of tubes (mal.)
which open into the terminal part of the
intestine are perhaps representatives o
Malpighian tubes. The muscles are al
non-striated. There are no organs of re
spiration, and heart and blood-vessels are
likewise absent. There is a brain and a
ventral nerve-cord with four ganglia
Two eyespots situated at the anterior
end are the only representatives o
organs of special sense. The gonads in
both sexes are saccular, and open int(
the terminal part of the intestine. Seg
mentation is complete and regular. The
young animal at one stage has only twc
pairs of rudimentary legs, but develops
the full number before being hatched.
In the larva there is a head and four distinct segments.

Some of the Tardigrada live amongst damp moss, others in fresh or in salt
water.

FIG. 522.— Macrobiotus hufelandi.

I — IV, appendages ; bucc. buccal cavity ;
!/l'/. accessory gland ; mal. Malpighian
tube ; or. ovary ; rect. rectum ; sali.
salivary glands ; atom, stomach ; styl.
teeth. (From Hertwig's Lehrbuch,
after Greef and Plate.)

PHYLUM ARTHROPODA 627

N

RELATIONSHIPS OF THE AIR-BREATHING ARTHROPODA^

Notwithstanding the existence of some striking superficial
resemblances between the Arachnida and the Insecta, the evidence
afforded by anatomy and embryology points to the conclusion that
there is no direct genetic relationship between the two groups.
The occurrence in both of a peculiar form of respiratory organs,
the tracheae, seems at first sight to indicate such a relationship ;
but the evidence of an independent origin is so strong that it
must be supposed that the tracheae have been independently
developed in the two classes. The most important points of
difference are the separation of head and thorax in the Insecta,
the mode of development of the eyes, the presence in the Arachnida
of an extensive " liver " and (perhaps) the endodermal origin of
the Malpighian tubes in the latter class.

Resemblances between Limulus and the Scorpions are readily
apparent. In both there is a cephalothorax bearing six pairs
of appendages, together with two median and several lateral
eyes. The appendages in both are all originally post-oral, the
first pair becoming pra3-oral in course of growth, and the ganglia
belonging to it coalescing with the brain. The upper lip be-
tween the bases of these appendages is similarly developed in
both. The pair of processes situated behind the sixth pair of
appendages, which in Limulus form the chilaria, are represented
in the Scorpions by a small pentagonal plate in front of the oper-
culum. The abdomen of Limulus corresponds to the prae- and
post-abdomen of the Scorpion ; it contains only eight segments :
but there is evidence, from a comparison with certain fossil forms,
that the last segment represents several united metameres. A
certain amount of correspondence is also traceable in the append-
ages of the abdomen. In both the first pair form the operculum :
in the Scorpion the second pair form the pectines, while the rest
disappear ; in Limulus all persist as the lamelliform appendages to
which the book-gills are attached. In structure there is consider-
able similarity between the book-gills of Limulus and the book-
lungs of the Scorpion, but how far they are equivalent to one
another remains doubtful in view of the difference in their
position, the book-gills being attached to the dorsal surface of the
abdominal appendages and the book-lungs sunk within the
segments.

The presence in both of the large " liver," of a circum-oesophageal
artery, of a cartilaginous endosternite, and of a pair of coxal

s s 2

1 The Xiphosura and also the Pentastomida, though not air-breathing, are
I discussed here.

628 ZOOLOGY SECT.

glands on the basal joints of the appendages of the fifth pair, are
some of the points of correspondence in the internal anatomy.

While Linmlus is thus closely related to the Scorpions on the
one hand, it exhibits, on the other, indications of affinities with the
Trilobites, a group of extinct Arthropods probably finding their
nearest existing allies in the Phyllopod Crustacea (p. 559). This
resemblance to the Trilobites is most marked in the stage — the
trilobite stage — in which the young King-crab escapes from the
egg. Certain fossil representatives of the Xiphosura come still
nearer to the Trilobites than the adult Limulus, and thus increase
the probability that there is a genetic connection between the two
groups.

It seems probable that the air-breathing Arachnida were
derived through Limulus-like ancestors from the Crustacea, and
that the tracheaB were developed as modifications of the pulmonary
sacs, the latter having been originally derived from gills like those
of Limulus.

There is a very evident close relationship between the Myria-
poda and the Insecta. The Insects are more highly specialised,
and have their structure modified in adaptation to a special mode
of locomotion, but the resemblances in many respects are very
strong. One of the most striking points of difference is the
indefiniteness in the number of the segments in the Myriapoda,
and their constant and definite arrangement in the Insecta.
The well-defined thorax of the Insects is wanting in the Myriapods
in general, but certain of the segments following the head differ
from the rest in various respects, and might be looked upon a
constituting a thoracic region. The presence in both groups of a
sharply marked-off head bearing antennae and jaws is an importan
point of resemblance ; so is the absence in both of the voluminou.
" liver " of the Crustacea and Arachnida. The gap between th(
two classes is narrowed by two converging groups — the Symphyla
among the Myriapoda on the one hand, and the wingless and in
other respects primitive Aptera among the Insecta on the other.

While the Insecta thus appear to be nearly related to th(
Myriapoda, there are indications of relationship between the lattei
class and the Onychophora, and, through these, the ChaBtopoda
The elongated, homonomously segmented body, the well-defined
head with its antennas, the occurrence of similar appendages on all
the body segments, all point in this direction. Accordingly, instead
of placing the branchiate Arthropoda in one group and all tht
air-breathing forms in another, and deriving the latter from the
former, we should probably express more correctly the affinities
the various groups of Arthropods by some such scheme as that
expressed in the diagram (Fig. 523).

Here an intermediate link between Annelida and the existii

XI

PHYLUM ARTHROPODA

629

Arthropoda is supposed to have been constituted by hypothetical
primitive forms from which Peripatus, the Insecta, and the
Myriapoda are supposed to have been evolved in the one direction,
and the Crustacea, Eurypterida, Xiphosura, and air-breathing
" rachnida in the other.

On account mainly of general resemblances to the Spiders, the
Pycnogonida have frequently been grouped with the Arachnida,
and attempts have been made to homologise their appendages
with those of the Spiders and Scorpions. There is one pair more

A"ir- breaking
Arachnids

Insecra

Pyenogom'da
Crustacea

Tardigrade
Penhasfomida

Xibriosura
Eurybherida
Trilobifa

Primih've Arfhrobods

Annelida
FIG. 523. — Diagram to illustrate the affinities of the Arthropoda.

in the Pycnogonida ; and either the last pair would have to be set
down as corresponding to the vestigial first abdominal pair of the
ordinary Arachnida, or the ovigerous legs would have to be
reckoned, riot as independent appendages, but as parts of the
second pair, a view for which there is some ground. A close
relationship with the Arachnida, however, cannot be traced, and
their affinities are perhaps best expressed, as in the diagram, by
connecting them with the Arachnid branch of the Arthropod tree
at a point below that at which the air-breathing forms had become
developed from forms allied to the Xiphosura.

The position of the Pentastomida is a matter of uncertainty.
In the absence of organs of respiration and excretion, the only

630 . ZOOLOGY SECT, xi

feature in the adult which distinctly points to arthropod affinities
is the striated character of the muscular tissue. The presence of
two pairs of legs in the larva, however, is sufficient to confirm the
position of the group as aberrant and probably degenerate Arthro-
pods, while leaving it uncertain in what class they find their near-
est allies. The Tardigrada are still more aberrant in some respects.
They differ from Arthropods in general in the absence of external
segmentation in the adult state, in the simple unjointed character
of the appendages, in the absence of striation in the muscular
fibres, and in the absence of organs of respiration and circulation.
It is impossible to place them in any of the great classes, and they
are perhaps best looked upon as a special offset of the Arthropod
tree given off near the base.

SECTION XII
PHYLUM MOLLUSCA

THE Mollusca, like the Arthropoda, form one of the chief
divisions of the animal kingdom, both for diversity of organiza-
tion and for number of genera and species. They are sharply
distinguished from Arthropods by the absence of segmentation,
and by having, as a rule, an exoskeleton in the form of a shell,
usually external, sometimes internal. An enumeration of the
Classes of the Phylum will serve to give some ' notion of its
extent.

Glass 1. — PELECYPODA, including the bivalved Shell-fish, such
as Mussels, Cockles, Oysters, &c.

Class 2. — AMPHINEURA, including the Chitons and their allies.

Class 3. — GASTROPODA, including the univalved Shell-fish, such
as Periwinkles, Whelks, Snails, Slugs, &c.

Class 4. — SCAPHOPODA, including the Tooth-shells.

Class 5. — CEPHALOPODA, including the Cuttle-fishes, Squids,
Octopi, and Nautili.

CLASS I— PELECYPODA.

1. EXAMPLE OF THE CLASS — THE FRESH- WATER MUSSELS
(Anodonta and Unid).

Fresh -water Mussels are found in rivers and lakes in most parts
of the world. Anodonta cygnea, the Swan-mussel, is the commonest
species in England ; but the Pearl-mussel, Unio margaritifer is
found in mountain streams, and other species of the same genus
are universally distributed.

The Mussel (Fig. 524) is enclosed in a brown shell formed of
two separate halves or valves hinged together along one edge.
It lies on the bottom, partly buried in the mud or sand, with the
valves slightly gaping, and in the narrow cleft thus formed a

632 ZOOLOGY SECT.

delicate, semi-transparent substance (m.) is seen, the edge of the
mantle or pallium. The mantle really consists of separate halves
or lobes corresponding with the valves of the shell, but in the
position of rest the two lobes are so closely approximated as to
appear simply like a membrane uniting the valves. At one end,
however, the mantle projects between the valves in the form of
two short tubes, one (ex. sph.) smooth-walled, the other (in. sph.)
beset with delicate processes or fimbrice. By diffusing particles of
carmine or indigo in the water it can be seen that a current is
always passing in at the fimbriated tube, hence called the inhalant
siphon, and out at the smooth or exhalant siphon. Frequently a
semi-transparent, tongue-like body (ft.) is protruded between the
valves at the opposite side from the hinge and at the end furthest
from the siphons: this is the foot, by its means the animal is able
slowly to plough its way through the sand or mud. When the

ft-

FIG. 524.— Anodonta cygnea. The entire animal ; A, from the left side ; B, from the
posterior end; d. p. a. dorsal pallial aperture; ex. sph. exhalant siphon; ft. foot; in. sph.
inhalant siphon ; lg. ligament ; m. mantle ; urn. umbo. '(After Howes.)

Mussel is irritated the foot and siphons are withdrawn and the
valves tightly closed. In a dead animal, on the other hand, the
shell always gapes, and it can then be seen that each valve is
lined by the corresponding lobe of the mantle, that the exhalant
siphon is formed by the union -of the lobes above and below it,
and is thus an actual tube, but that the boundary of the inhalant
siphon facing the gape of the shell is simply formed by the
approximation of the mantle-lobes, so that this tube is a tem-
porary one.

The hinge of the shell is dorsal, the gape ventral, the end
bearing the siphons posterior, the end from which the foot is
protruded anterior: hence the valves and mantle-lobes are re-
spectively right and left.

In a dead and gaping Mussel the -general disposition of the
parts of the animal is readily seen. Th.fejnain part of the body

XII

PHYLUM MOLLUSCA

633

».

between the dorsal ends of the valves : it is produced in the
middle ventral line into the keel-like foot : and on each side,
between the foot and the corresponding mantle-lobe, are two deli-
cate, striated plates, the gills (Fig. 530). Thus the whole animal
has been compared to a book, the back being represented by the
hinge, the covers by the valves, the fly-leaves by the mantle-lobes,
the two first and the two last pages by the gills, and the re-
mainder of the leaves by the foot.

The Shell. — When the body of the mussel is removed from
the shell the two valves are seen to be united, along a straight

H.I

a-cui

FIG. 525.— Anodonta cygnea. A, interior of right valve; B, the animal removed from the
shell, a. ad. anterior adductor or its impression ; a. r. anterior retractor or its impression ;
d. g. digestive gland, seen through mantle ; ex. sph. exhalant siphon ; ft. foot ; gl. gills, seen
through mantle ; k. 1. hinge-line ; in. sph. inhalant siphon ; kd. kidney, seen through mantle ;
k. o. Keber's organ, seen through mantle; -HI. mantle; p.' ad. posterior adductor or its
impression ; pc. pericardium, seen through mantle ; pi. 1. pallial line ; pi. w. pallial^muscles ;
p. r. posterior retractor or its impression ; prc. protractor or its impression.

hinge-line (Fig. 525, A, h. l.\ by a tough, elastic substance, the
hinge-ligament (Figs.524 and 530,/^.) passing transversely from valve
xto valve. It is by the elasticity of this ligament that the shell is
opened : it is closed, as we shall see, by muscular action : hence
the mere relaxation of the muscles opens the shell. In Anodonta
the only junction between the two valves is afforded by the liga-
ment, but in Unio each is produced into strong projections and

(534

ZOOLOGY

SECT.

prc

pra

ridges, the hinge-teeth, separated by grooves or sockets, and so
arranged that the teeth .of one valve fit into the sockets of the
other.

The valves are marked externally by a series of concentric lines
(Fig. 524) parallel with the free edge or gape, and starting from a
swollen knob or elevation, the umbo (um.\ situated towards the
anterior edge of the hinge-line. These lines are lines of growth.
The shell is thickest at the umbo, which represents the part
first formed in the young animal, and new layers are deposited
under this original portion, as secretions from the mantle. As the
animal grows each layer projects beyond its predecessor, and in
this way successive outcrops are produced giving rise to the

markings in question. In
the region of the umbo
the shell is usually more
or less eroded by the
action of the carbonic acid
in the water.

The inner surface of the
shell also presents charac-
teristic markings (Fig.
525, A). Parallel with
the gape and at a short
distance from it is a deli-
cate streak (pi. I.) caused
by the insertion into the
shell of muscular fibres
from the edge of the
mantle : the streak is
hence called the pallial
line. Beneath the anterior
end of the hinge the
pallial line ends in an
oval mark, the anterior
which is inserted one of the
A similar but larger posterior

ep.s.

Fi<;. ;VJi;.— Vertical section of shell and mantle of
Anodonta. c. t. connective-tissue layer <>f
mantle: < />. 1, its outer epithelium; */'>..', its
inner epithelium ; n. nacreous layer of shell ;
•j>rr. pLjriustracum ; p/x prismatic layer. (After
Claus.)

adductor impression (a. ad.), into
muscles which close the shell.

adductor impression (p. ad.} lies beneath the posterior end of the
hinge. Two smaller markings in close relation with the anterior
adductor impression mark the origin of the anterior retractor (a.r.)
and of the protractor (prc.} of the foot : one connected with the
posterior adductor impression is that of the posterior retractor (p. r.)
of the foot. From all these impressions faint converging lines can
be traced to the umbo : they mark the gradual shifting of the
muscles during the growth of the animal.

The shell consists of three layers. Outside is a brown horn-like
layer, the periostracum (Fig. 526, prc.), composed of conchiolin, a
substance allied in composition to chitin. Beneath this is a

XII

PHYLUM MOLLUSCA 635

prismatic layer (prs.) formed of minute prisms of calcium car-
bonate separated by thin layers of conchiolin ; and, lastly, forming
the internal part of the shell is the nacre (n.), or " mother-of-pearl,"
formed of alternate layers of carbonate of lime and conchiolin
arranged parallel to the surface. The periostracum and the pris-
matic layer are secreted from the edge of the mantle only, the
pearly layer from the whole of its outer surface. The hinge
ligament is continuous with the periostracum, and is to be looked
upon simply as a median uncalcified portion of the shell, which is
therefore, in strictness, a single continuous structure.

By the removal of the shell the body of the animal (Fig. 525, B)
is seen to be elongated from before backwards, narrow from side
to side, produced on each side into a mantle-lobe (m.) and con-
tinued ventrally into a keel-like visceral mass (Fig. 527, v.m), which
passes below and in front into the foot (ft.). Thus each valve of
the shell is in contact with the dorso-lateral region of the body
of its own side together with the corresponding mantle-lobe, and
it is froir^ the epithelium (Fig. 526, ep.1) covering these parts that
the shell is formed as a cuticular secretion. The whole space
between the two mantle-lobes, containing the gills, visceral mass,
and foot, is called the mantle-cavity.

A single layer of epithelial cells covers the whole external sur-
face, i.e. the body proper, both surfaces of the mantle, the gills,
and foot ; that of the gills and of the inner surface of the mantle
(Fig. 526, ep.z) is ciliated. Beneath the epidermis come connective
and muscular tissue, which occupy nearly the whole of the interior
of the body not taken up by the viscera, the coelome being, as we
shall see, much reduced. The muscles are all unstriped, and are
arranged in distinct bands or sheets, many of them very large and
conspicuous. The largest are the anterior and posterior adductors
(Figs. 525 and 527, a. ad., p. ad.), great cylindrical muscles, pass-
ing transversely across the body and inserted at either end into
the valve.s of the shell, which are approximated by their con-
traction. Two muscles of much smalTer~~size- pass from the shell
to the foot, which they serve to draw back ; they are the anterior
(a. r.) and posterior (p. r.) retractors of the foot. A third foot-muscle
(prc.) arises from the shell, close to the anterior adductor, and has
its fibres spread fan-wise over the visceral mass, which it serves to
compress, thus forcing out the foot and acting as a protractor of
that organ. The substance of the foot itself consists of a complex
mass of fibres, the intrinsic muscles of the foot, many of which also
act as protractors. Lastly, all along the border of the mantle is a
row of delicate pallial muscles (Fig. 525, B, pi. m), which, by their
insertion into the shell, give rise to the pallial line already seen.

The coelome is reduced to a single ovoidal chamber, the peri-
cardium (Fig. 527, pc.}, lying in the dorsal region of the body and
containing the heart and part of the intestine ; it is lined by

636

ZOOLOGY

SECT.

ccelomic epithelium. In the remainder of the body the space
between the ectoderm and the viscera is filled by the muscles
and connective tissue.

Digestive organs. — The mouth (Fig. 527, mth.) lies in the
middle line, just below the anterior adductor. On each side of
it are two triangular flaps, the internal .(Lint, pip.) and external
(1. ext. pip.), labial palps; the external palps unite with one another
in front of the mouth, forming an upper lip ; the internal are.
similarly united behind the mouth, forming a lower lip ; both are
ciliated externally. The mouth leads by a short gullet (Fig. 528,
gul.) into a large stomach (st.), which receives the ducts (d.d.) of a
pair of irregular, dark-brown digestive glands (d.gl.). The intestine

ct.r

I. rrv rcl

a. ad.

FIG. 527.— Anodonta cygnea. The animal with most of the left mantle-lobe removed ;
a. anus ; a. ad. anterior adductor ; a. r. anterior retractor ; au. left auricle ; d. p. a. dorsal
pallial aperture; tx. *ph. exhalant siphon;/?, foot; in. sph. inhalant siphon; M. kidney;
1. ext. gl. left external gill ; 1. ext. pip. left external labial palp ; I. -int. <jl. left internal gill ;
1. int. pip. left internal labial palp ; 1. ui. cut edge of left mantle-lobe ; mth. mouth ; p. ad. pos-
terior adductor ; pc. pericardium ; p. r. posterior retractor ; prc. protractor ; ret. rectum ; /•. ni.
right mantle-lobe ; r. ventricle ; v. m. visceral mass.

(int.) is given off from the posterior end of the stomach, descends
into the visceral mass, where it is coiled upon itself, then ascends
parallel to its first portion, turns sharply backwards, arid proceeds,
as the rectum (ret.), through the pericardium, where it traverses the
ventricle of the heart, and above the posterior adductor, finally
discharging by the amis (a.) into the exhalant siphon, or cloaca.
The wall of the rectum is produced into a longitudinal ridge, or
typhlosole (ty.\ like that of the Earthworm, and two similar ridges
begin in the stomach and are continued into the first portion of
the intestihe. The stomach contains, at certain seasons of the
year, a gelatinous rod, the crystalline style.

The gills consist, as we have seen, of two plate-like bodies on
each side between the visceral mass and the mantle : we have thus

XII

PHYLUM MOLLUSCA

637

a right and a left outer (Fig. 527, /. cxt. gl.\ and a right and a left
inner gill (1. int.- gl.). Seen from the surface, each gill presents a
delicate double striation, being .marked by faint lines running
parallel with, and by more pronounced lines running at right
angles to, the long axis of the organ. Moreover, each gill is
double, being formed of two similar plates, the inner and outer
lamella, united with one another along the anterior, ventral,
and posterior edges of the gill, but free dorsally. The gill has thus
the form of a long and extremely narrow bag open above (Figs. 528?
529 and 530): its cavity, is subdivided by vertical bars of tissue^
the inter-lamellar junctions (i. /./.), which extend between the two

9"*

II

A.p.a.

in.sph

vjfil

W.I

FIG. 528. — Anodonta cygnea. Dissection from the left side. a. anus ; a. ad. anterior
adductor ; a. ao. anterior aorta ; a. v. ap. auriculo-ventricular aperture ; M. urinary bladder ;
c. pi. g>i. cerebro-pleural ganglion ; d. d. duct of digestive gland ; d. gl. digestive gland :
<i- /i. a. dorsal pallial aperture; ex. spli. exhalant siphon ; ft. foot; g. ap. genital aperture ;
gon. gonad ; gv.l. gullet ; i. 1. j. inter-lamellar junction ; in. spk. inhalant siphon ; int. intes-
tine ; kd. kidney ; in. mantle ; mth. mouth ; p. ao. posterior aorta ; p. ad. posterior adductor ;
p<'. pericardium; pd. gn. pedal ganglion; r. ap. renal aperture; r. an. right auricle;
ret. rectum ; r. p. a. reno-pericardial aperture ; st, stomach ; ty. typhlosole ; r. ventricle ;
r. tin. visceral ganglion ; 10. t. water tubes.

lamellae, and divide the intervening space into distinct compart-
ments or water-tubes (w. t.\ closed ventrally, but freely open along
the dorsal edge of the gill. The vertical striation of the gill is
due to the fact that each lamella is made up of a number of
close-set gilljilaments (/.): the longitudinal striation to the
circumstance that these filaments are connected by horizontal
bars, the inter-Jilamentar junctions (i. f. /.). At the thin free or
ventral edge of the gill the filaments of the two lamellae are con-
tinuous with one another, so that each gill has actually a single
set of V-shaped filaments, the outer limbs of which go to form the
outer lamella, their inner limbs the inner lamella. Between the

638

ZOOLOGY

SECT.

filaments, and bounded above and below by the inter-filamentar
junctions, are minute apertures, or ostia (as.), which lead from the
mantle-cavity through a more or less irregular series of cavities
into the interior of the water-tubes. The filaments themselves
are supported by chitinous rods (r.), and are covered with ciliated
epithelium, the large cilia (Fig. 529, D), of which produce a current

FIG. 529. — Anodonta cygnea. A, transverse section of outer ; B of inner gill; C, diagram
of gill-structure ; D, transverse section of gill filament ; b. c. blood-corpuscle ; b. r. blood-
vessels ; c/i. chitin ; /. branchial filaments ; ep. epithelium ; i. f. j. inter-filamentar junction ;
/'. /. inner lamella ; i. 1. j. inter-lamellar junction ; o. L outer lamella ; os. external ostiurn ;
os'. internal ostium ; r, chitinous rods ; ic. t. water tubes. (A, B, and D, after Peck.)

running from the exterior through the ostia into the water-tubes,
and finally escaping by the wide dorsal apertures of the latter,
The whole organ is traversed by bloodvessels (b. v.}.

The mode of attachment of the gills presents certain features of
importance. The outer lamella of the outer gill is attached along
its whole length to the mantle (Fig. 530) : the inner lamella of the
outer, and the outer lamella of the inner gill are attached

XII

PHYLUM MOLLUSCA

639

together to the sides of the visceral mass a little below the origin
of the mantle : the inner lamella of the inner gill is also attached
to the visceral mass in front (A), but is free further back (B). The
gills are longer than the visceral mass, and project behind it, below

rcb

ink

ic.. 530. — Anodonta cygnca. Three transverse sections, av. auricle ; 1>1. urinary bladder ;
< .<••'. ;//. external gill ; //. foot ; /. /. ./'. inter-lamellar junction ; int. intestine ; ii>t. <jl. internal
i.: ill ; M. kidney ; k. o. Keber's organ ; !(/. ligament; m. mantle ; p. ad. posterior adductor;
pc. pericardium ; ret. rectum ; s. Ijr. c. supra-branchial chamber ; sh. shell ; ty. typhlosole ;
r. gn. visceral ganglion. (After Howes, slightly altered.)

;he posterior adductor (C), as far as the posterior edge of the
mantle : in this region the inner lamellae of the right and left
inner gills are united with one another, and the dorsal edges
of all four gills constitute a horizontal partition between the pallial
cavity below and the exhalant chamber or cloaca above. Owing

640 ZOOLOGY SECT.

to this arrangement it will be seen that the water tubes all open
dorsally into a supra-branchial chamber (s.br. c.} continuous pos-
teriorly with the cloaca and thus opening on the exterior by the
exhalant siphon.

The physiological importance of the gills will now be obvious.
By the action of their cilia a current is produced which sets in
through the inhalant siphon into the pallial cavity, through the
ostia into the water tubes, into the supra- branchial chamber, and
out at the exhalant siphon. The in-going current carries with it
not only oxygen for the aeration of the blood, but also Diatoms,
Infusoria and other microscopic organisms, which ar% swept into
the mouth by the cilia covering the labial palps. The out-going
current carries with it the various products of excretion and the
faeces passed into the cloaca. The action of the gills in producing
the food-current is of more importance than their respiratory
function, which they share with the mantle.

The excretory organs are a single pair of curiously-modified
meso-nephridia, situated one on each side of the body just below the
pericardium. Each nephridium consists of two parts, a brown
spongy glandular portion or kidney (Fig. 528,7^.), and a thin- walled
non-glandular part or bladder (&/.). The two parts 'lie parallel to
one another, the bladder being placed dorsally and immediately
below the floor of the pericardium : they communicate with one
another posteriorly, while in front the kidney opens into the
pericardium (r. p. ap.), and the bladder on to the exterior by a
minute aperture (r. cvp.), situated between the inner gill and the
visceral mass. Thus the whole organ, often called after its dis-
coverer, the organ of Bojanus, is simply a tube bent upon itself,
opening at one end into the ccelome, and at the other on the
external surface of the body : it has thus the normal relations of
a nephridium. The epithelium of the bladder is ciliated, and
produces an outward current.

It seems probable that an excretory function is also discharged
by a large grandular mass of reddish-brown colour, called the
pericardial gland or Kelers organ (Fig. 530, B, k. 0.), It lies in the
anterior region of the body just in front of the pericardium, into
which it discharges.

The circulatory system is well developed. The heart lies in
the pericardium and consists of a single ventricle (Figs. 528, 530,
and 531, v.) and of right and left auricles (au.). The ventricle is
a muscular chamber which has the peculiarity of surrounding
the rectum (Figs. 528 and 530, B) : the auricles are thin-walled
chambers communicating with the ventricle by valvular apertures
opening towards the latter. From each end of the ventricle an
artery is given off, the anterior aorta (Fig. 528, a. ao.) passing
above, the posterior aorta (p. ao.} below the rectum. From the
aortae the blood passes into arteries (Fig. 531, art.1 art.2) which

xii PHYLUM MOLLUSCA 641

ramify all over the body, finally forming an extensive network of
vessels, many of which are devoid of proper walls, and have there-
fore the nature of sinuses. The returning blood passes into a
large longitudinal vein, the vena cava (v. c.) placed between the
nephridia, whence it is taken to the kidneys themselves (nph. v.\
thence by afferent branchial veins\af. br. v.) to the gills, and is finally
' returned by efferent branchial veins (ef. br. v.) to the auricles. The
mantle has a very extensive blood supply, and probably acts as the
chief respiratory organ : its blood (art.1) is returned directly to

CLfl2

FIG. 531. — Diagram of the circulatory system of Anodonta. Vessels containing aerated blood
red, non-aerted blue. a/, br. v. afferent branchial veins ; ao. aorta ; art. 1, artery to mantle ;
art. 3, artery to body generally ; au. auricle ; ef. br. v. efferent branchial veins ; nph. nephridial
veins ; pc. pericardium ; v. ventricle ; v. c. vena cava. The arrows show the direction of the
current.

the auricles without passing through either the kidneys or the
gills. The blood is colourless and contains leucocytes.

The nervous system is formed on a type quite different from
anything we have yet met with. On each side of the gullet is a
small cerebro-pleural ganglion (Fig. 528, c'pl. gn.) united with its
fellow of the opposite side by a nerve-cord, the cerebral commissure,
passing above the gullet. Each cerebro-pleural ganglion also gives
off a cord, the cerebro-pedal connective, which passes downwards
and backwards to a pedal ganglion (pd. gn.) situated at the junction
of the visceral mass with the foot : the two pedal ganglia are so
closely united as to form a single bilobed mass. . From each
cerebro-pleural ganglion there further proceeds a long cerebro-
visceral connective which passes directly backwards, through the
kidney, and ends in a visceral ganglion (v. gn.) placed on the ventral

VOL. I T T

642

ZOOLOGY

SECT.

ot

side of the posterior adductor muscle. The visceral, like the
pedal ganglia, are fused together. The cerebro-pleural ganglia
supply the labial palps and the anterior part of the mantle ; the
pedal the foot and its muscles ; the visceral the enteric canal, heart,
gills, and posterior portion of the mantle.

It will be seen that the cerebral commissures and cerebro-pedal
connectives together with the cerebro-pleural and pedal ganglia,
form a nerve ring which surrounds the gullet : the cerebro-pleural
ganglia may be looked upon as a supra-oesophageal nerve mass
corresponding with the brain of Annelids and Arthropods, and the
pedal ganglia as an infra-cesophageal mass representing the ventral
nerve cord.

Sensory organs are poorly developed, as might be expected in
an animal of such sedentary habits. In connection with each

visceral ganglion is a patch of
sensory epithelium forming
the so-called olfactory organ
or, better, ospkradium, the
function of which is apparently
to test the purity of the water
entering by the respiratory
current. Close to the pedal
ganglion a minute otocyst
(Fig. 532) is sometimes found,
the nerve of which is said to
spring from the cerebro-ped;
connective, being probabb
derived from the cerebra
ganglion. Sensory cells — prob-
ably tactile — also occur rounc1
the edge of the mantle, am
especially on the fimbrise oi
the inhalant siphon.
Reproductive organs. — The sexes are separate. The gonad*
(Fig. 528, gon.} are large, paired, racemose glands, occupying
considerable portion of the visceral mass amongst the coils oi
the intestine : the testis is white, the ovary reddish. The gon;
of each side has a short duct which opens (g. ap.} on the surface
of the visceral mass just in front of the renal aperture.

In the breeding season the eggs, extruded from the genital
aperture, pass into the suprabranchial chamber and so to th(
cloaca. There, in all probability, they are impregnated by sperm
introduced with the respiratory current. The oosperms are thei
passed into the cavities of the outer gills, which they distem
•enormously. Thus the outer gills act as brood-pouches, anc
in them the embryo develops into the peculiar larval form
presently to be described.

FIG. 532.— Otocyst of Anodonta. a, &, c, c',
cellular layers surrounding the otocyst ; ot.
otolith. (From the Cambridge Natural
History.)

XII

PHYLUM MOLLUSCA

643

mes

.mes

Development. — Segmentation of the oosperm is complete, but
unequal. A gastrula is formed by the invagination of the mega-
meres into the micro-
meres, but the archen- ;rk
teron (Fig. 533, ent.)
thus formed is quite
small and insignifi-
cant, and has no
physiological import-
ance until a late period
of larval life. Certain
of the cells of the
gastrula are budded
off into the blasto-
ccele, where they ac-
cumulate and form
the mesoderm (mes.).
At about the same
time a deep invagina-
tion (sd.) is formed,
which might easily be
mistaken for the ar-
chenteron, but is really a very characteristic molluscan organ,
the shell-gland : it marks the dorsal surface of the embryo. The
posterior end is distinguished by a tuft of long cilia.

The shell-gland becomes converted into a plate of long, cylin-

R. B.

jnes

FIG. 533.— Early embryo of Anodonta. e/i, vitelline
membrane; ent. archenteron ; m. micropyle ; mes. meso-
derm ; rk, polar cells ; sd, shell-gland ; sk, lateral cells ;
ic, cilia, (From Korschelt and Heider's Embryology.)

mes.

534.— Two later stages in the development of Anodonta. e;it. archenteron ; me*, meso-
derm ; s, shell ; sd, shell-gland ; so. sense-organs ; v:, cilia. (From Korschelt and Heider's
ology.)

drical cells (Fig. 534, sd:), from which an unpaired shell (s.) is
secreted. This is 'replaced before long by a bivalved shell of
triangular form, its ventral angles produced? into incurved hooks

m m^£

T T

644 ZOOLOGY SECT.

beset with spines (Fig. 535, sh). At the same time the body of the
larva, which has hitherto been an undivided mass projecting
between the two valves of the shell, becomes cleft from below
upwards, and thus divided into a single dorsally-placed body
proper, and paired — right and left — mantle-lobes. Upon the latter
peculiar brush-like sense-organs make their appearance, and on
the ventral surface of the body is formed a glandular pouch, which
secretes a long thread, the provisional byssus (/). The mesoderm
cells give rise to a single immense adductor muscle (sm), the fibres
of which extend from valve to valve.

The larva is now called a glochidium : it remains in the brood-
pouch, nourished by a secretion from the walls of the latter, and
entangled with its fellows by means of the byssus At this stage
the outer gill appears to be stuffed full of closely aggregated sand-
grains. Before long the larvas are ejected through the exhalant

,-sli.

FIG. 535.— A, advanced embryo of Anodonta. B, free glochidium ; J, provisional byssus ;
s, shell ; sh, hooks ; sm, adductor muscle ; so, sense-organs ; w, cilia. (From Korschelt and
Heider's Embi-yoloyy.)

siphon, and lie in masses on the bottom until they happen
to come in contact with a passing Stickleback or other fresh-water
Fish, when they fix themselves on some part of its body by means
of the hooked valves. The glochidia of Unio usually become
attached to the gills, those of Anodonta to the skin or the fins.
In this position they become encysted by an overgrowth of the
skin or mucous membrane of the host, and are nourished by its
juices absorbed through processes of the mantle. They thus lead
a truly ectoparasitic existence for about ten weeks.

While in this condition a metamorphosis takes place. The pro-
visional byssus and sense-organs disappear (Fig. 536), and imme-
diately posterior to the former an invagination, the stomodceum (m),
is formed, and soon communicates with the archenteron. The
posterior end of this cavity is in close contact with the ectoderm,
so that the anus is formed by a simple process of rupture, and
without the development of a proctodseum. The foot (fu) arises as
a median ventral elevation behind the mouth, and on each side of

XII

PHYLUM MOLLUSCA

645

it two papillae (&) appear, the rudiments of the gills. The larva is
now fitted for free" existence : it drops from its host, and gradually
assumes the adult form and mode of life.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Pelecypoda are bilaterally symmetrical, compressed Molluscs,
in which the mantle consists of paired right and left lobes, secret-
ing a bivalved cal-

R

/f,

B.

\

careous shell. There ^/ f

is no distinct head.
The ventral region of
the body is differenti-
ated into a muscular
foot, which is usu-
ally ploughshare- ox-
tongue - shaped : in
some cases there is a
byssus- gland posterior
to the foot, which se-
cretes a mass of horny
fibres, the lyssus, by
which the animal may
be permanently at-
tached. There are
usually two pairs of
gills, but the two gills
of each side are to be
looked upon as modi-
fications of a single
primitive gill or cteni-
dium : the chief func-
tion of the gills is the
production of a re-
spiratory and food-
carrying current of
water The body is
covered by a one-
layered epidermis,
which is ciliated on
the gills, and on the
inner surface of the
mantle. The muscular
system is well developed, the largest muscles being either one or
two adductors, which close the shell, and several bands connectedj
with the foot and byssus : the muscles are usually unstriped. The
ccelome is reduced to a dorsally-placed pericardium. The mouth

T T**

IV.

y

fu.

FIG. 536.— Three stages in the metamorphosis of Anoy
donta. d, enteric canal; /, provisional byssus; .Ac-
foot ; g, lateral pits ; A-, rudiments of gills ; m. mouth ;
sh. shell ; sm, adductor muscle ; so, sense-organs ; w
cilia. (From Korschelt and Heider's Embryology.)

646 ZOOLOGY SECT.

is bounded by two pairs of flat, triangular tentacles or labial palps
the cilia of which serve to carry food particles to the mouth : the
enteric canal is coiled, and is formed mainly from the mesenteron :
there are large paired digestive glands : the rectum passes through
the pericardium, usually perforates the ventricle, and ends above
the posterior adductor. The heart is contained within the peri-
cardium, and consists of a median ventricle, and of right and left
auricles : the blood, which is usually colourless, is taken from the
ventricle to the body by one or two aortse, and is returned partly
directly, partly by way of the renal organs and gills, to the auricles.
The renal organs are a single pair of meso-nephridia, which usually
open at one end into the pericardium, at the other on. the exterior.
The nervous system consists typically of four pairs of ganglia called
respectively cerebral, pleural, pedal, and visceral : the cerebral and
pleural of each side are usually fused into a single cerebro-pleural
ganglion. The chief sense-organs are otocysts and osphradia or
water-testing organs. The sexes are separate or united : there
are no accessory organs of reproduction. Development is accom-
panied by a metamorphosis, which usually includes a trochosphere
stage.

The classification of the Pelecypoda is as follows : —

ORDER 1. — PROTOBRANCHIA.

Pelecypoda, in which the gills take the form of a single pair of
plume-like organs or ctenidia, each with two rows of flattened
gill-filaments. The foot is not compressed, but has a flattened
ventral surface or sole upon which the animal creeps. There are
two adductor muscles.

This group includes only four genera — Nucida, Yoldia, Leda,
and Solenomya.

ORDER 2. — FILIBRANCHIA.

Pelecypoda, in which there are two pairs of plate-like gills
formed of distinct V-shaped filaments : interfilamentar junctions
are either absent or formed by groups of interlocking cilia :
interlamellar junctions are eitheip absenfr~~or nori- vascular. As a
rule there are two adductor muscles, but the anterior may be
greatly reduced or absent.

Including the Noah's ark shell (Area), Sea mussel (Mytilus),
Anomia, Trigonia, &c.

ORDER 3. — PSEUDO-LAMELLIBRANCHIA.

Pelecypoda, in which the gills are plaited so as to present
vertical folds: the interfilamentar junctions may be ciliary or
vascular: the interlamellar junctions vascular or non- vascular.

xii PHYLUM MOLLUSCA 647

There is a single large (posterior) adductor muscle. The shell is
frequently inequivalve.

Including — the Scallop (Pecten), Oyster (Ostrea), Pearl oyster
(Meleagrina), Lima, Pinna, &c.

ORDER 4. — EULAMELLIBRANCHIA.

Pelecypoda in which the gill-filaments are united by vascular
inter-filamentar and inter-lamellar junctions, firm, basket-like gills
being the result : the gills may be smooth or plaited. There are
two equal-sized adductor muscles.

Sub-order a. — Tntegripalliata.

Eulamellibranchia in which the siphons are small or absent,
and the pallial line on the shell is entire.

Including the Fresh-water Mussels (Anodonta and Unio).

Sub-order b. — Sinupalliata.

Eulamellibranchia in which the siphons are of considerable
size, and the pallial line is inflected to form a sinus.

Including the Cockle (Cardiuni), My a, Pholas, Teredo (Ship-
worm), Aspergillum, &c.

ORDER 5. — SEPTIBRANCHIA.

Pelecypoda in which the gills are reduced to a horizontal
muscular partition. There are two adductor muscles.
Including Poromya, Cuspidaria, &c.

P Systematic Position of the Examples.

Anodonta and Unio are two genera belonging to the family
Unionidae, sub-order Asiplwniata, order Eulamellibranchia.

Their complex basket-like gills are alone sufficient to place
them among the Eulamellibranchia. The incomplete ventral
siphon and the correlated entire pallial line (see p. 650) indicate
their position among the Asiplwniata. The regular shell, with
thick brown periostracum and large external ligament, the elon-
gated branchial or inhalant aperture, the long, compressed foot,
nd the absence of a byssus, place them among the Unionidae.
nodonta is distinguished from Unio by the absence of hinge-
eth.

3. GENERAL ORGANISATION.

The most important variations in structure in the present class
connected with modifications of the gills, the foot, the muscular

stem, and the siphons. With the structure of the muscles and
the siphons are correlated important variations in the shell

T Tf

-

648

ZOOLOGY

SECT,

which are of great systematic value, especially in cases where, as
with fossils, the shell is the only part available for examination.

In all the Protobranchia, some of the Filibranchia, such as
Area, and all the Eulamellibranchia and Septibranchia, there
are two almost equal-sized adductor muscles, as in Anodonta.
In many Filibranchs, such as the common Sea-mussel (Mytilus
edulis), the anterior adductor becomes greatly reduced and the
posterior correspondingly enlarged, and in another species of the
same genus (M. lotus) the anterior adductor has completely
atrophied, the function of closing the shell being performed by the

XIV

XV

IX

VI!

VI

FIG. 537.— Anatomy of Pecten. I, palpi; II, foot ; III, aperture of gonad into kidney; IV.
external renal aperture ; V, male portion of gonad ; VI, female portion ; VII, pallial eye ;:
VIII, visceral ganglion ; VIII', gill ; IX, anus ; X, striated iportion of adductor ; XI, smooth
portion; XII, retractor of foot; XIII, heart; XIV, liver; XV, stomach. (From Pelseneer's

MoUusques.)

great posterior adductor alone. In Anomia and in the Pseudo-
lamellibranchs there is a single immense adductor (Fig. 537 x, xi)
placed nearly in the middle of the greatly shortened body, and
known to represent the posterior add actor, both from the fact that
the rectum passes over it ; and from the circumstance that, in the
embryo Oyster, two' adductors are present, the anterior of which
atrophies, while the posterior enlarges to form the single muscle
of the adult.

These peculiarities in the muscular system bear their mark
upon the shell, in which impressions corresponding to the-

PHYLUM MOLLUSCA

649

Iductors are clearly marked on the inner surface (Fig. 538). The
whole class is, in fact, frequently classified on this basis, species
with equal-sized adductors (Protobranchs, some Filibranchs, and
all Eulamellibranchs and Septibranchs) being called Isomyaria (A),

. 538.— Left valves of A, My a: B, Modiola; C, Vulsella. The upper dotted line passes
through the hinge-lines, the lower connects the anterior and posterior adductor muscles.
(From the Cambridge Natural History.)

those with a large1 posterior and a reduced anterior adductor (most
Filibranchs) Heteromyaria (B), and those with large centrally
placed posterior and no anterior adductor (Pseudolamellibranchs
and Anomia among Filibranchs) Monomyaria (C).

In many forms, such as Nucula (Fig. 548), Ostrea, &c., the right
and left mantle-lobes are quite free from each other, so that there
are no siphons. In Anodonta and Unio, as we have seen, the two
lobes unite along the line of attachment of the gills, so as to
enclose a dorsal or exhalant siphon, a ventral or inhalant siphon
being formed simply by apposition of the lobes ventrally. In such
cases the pallial muscles in their neighbourhood act as retractors
of the short and imperfect tubes thus formed. In other species
a second concrescence of the mantle-lobes takes place so as to con-
vert the inhalant siphon into an actual circumscribed aperture or

FIG. 539.— Cardium edule. A, exhalant siphon ; B, inhalant siphon ; F, foot. (From the

Cambridge Natural History).

short tube. In the Sinupalliata the two siphons are prolonged
into distinct muscular tubes (Fig. 539, A. B) which, in the posi-
tion of extension, project beyond the posterior margin of the
shell, and may even be considerably longer than the body. Under

650

ZOOLOGY

SECT.

d.TTL

these circumstances the posterior pallial muscles become en-
larged to form retractors of the siphons, and the portion of the
pallial line from which they arise becomes, as it were, pushed
forwards so as to form a bay or pallial sinus (Fig. 540, p.sJ). Thus

the shells of species with well-
developed siphons are sinupal-
liate, or have an indented
pallial line, while those with
small or no siphons are in-
tegripalliate, or have an entire
pallial line. The larger the
siphons the stronger are their
muscles, and the deeper is the
pallial sinus : when very large
they cannot be completely
retracted, and the posterior
border of the shell then gapes
permanently. The siphons may
be separate (Fig. 541) or
united (Fig. 542). They are
specially adapted for species
of burrowing habits, which are
able to remain buried in the
mud or sand, the ends of the

siphons only being exposed for the supply of aerated water and
food, and even they can be instantly withdrawn in the event of
danger.

In addition to their union posteriorly to form the siphons, the
mantle-lobes may concresce to a greater or less extent along their
ventral border (Fig. 543), forming a more or less tubular invest-
ment for the body, and leaving an anterior pedal aperture for the

FIG. 540. — Venus gnidia, inner surface of
left valve ; al, anterior lateral tooth ; am,
anterior adductor impression ; c, cardinal
teeth ; I, ligament ; lu. lunule ; p, pallial
line ; p. I, posterior lateral tooth ; p. m, pos-
terior adductor impression ; p. s. pallial
sinus ; u. umbo. (From the Cambridge
Natural History.)

FIG. 541.— Scrobicularia piperata,in its natural position, partly buried in sand. A, exhalant
siphon ; B, inhalant siphon. (From the Cambridge Natural History.)

protrusion of the foot. Their anterior portions may also be united
to form a sort of hood.

To return to the shell, the muscular impressions and the pallial
line have already been referred to. As a general rule the right
and left valves are alike, or nearly so, the shell being therefore
equivalve. Each valve is inequilateral, being divided into unequal

PHYLUM MOLLUSCA

651

portions by a line drawn from the umbo to
the gape. It will be remembered that in
the Brachiopoda, the only other class of
bivalved animals, the precise opposite is
the case, the shell being equilateral and
inequivalved. Some Pseudolamellibranchs are,
however, nearly equilateral and markedly
inequivalved, such as the scallop (JPecten), and
the inequivalve character is still more marked
in the oyster, in which the right valve is
deeply concavo-convex and permanently at-
tached to a rock, while the left is flat and
forms a sort of lid. This condition of
things reaches its maximum in the extinct
Hippurites (Fig. 544, B.), in which the right
valve has the form of a long tube closed at
one end by the flat lid-like left valve. In
the extinct Requienia- (A) the left valve is
spirally coiled so that it resembles a snail-
shell, and its aperture is closed by the flat
lid-like right valve : in Dicer as, also extinct,
both valves are coiled.

The hinge-teeth (Fig. 540) vary indefinitely
in form and size or may be absent altogether :
the hinge-ligament is usually band-like, but
in Pecten takes the form of a cylindrical cord.
The variations in form, ornamentation, colour,
&c., among the many thousand known species of shell are too
numerous to mention ; but reference must be made to peculiar
lodifications found in certain burrowing forms. In Pholas, a

FIG. 542.— Solecurtus
strigillatus. «. af,
inhalant siphon ; s. ej\
exhalant siphon, the
two united at SS.
(From the C
Natural History.)

l:o

FIG. 543.— Diagram illustrating the various degrees of union of the mantle-lobes ; b. o, byssal
aperture ; /, foot ; s. a, exhalant siphon ; s. b, inhalant siphon ; 1, first point of union between
siphons ; 2, second, between inhalant siphon and foot ; 3, third, between byssa aperture and
foot. (From the Cambridge Natural History.)

,„

phonate genus which burrows in stone, the shell is weak and
brittle, and additional calcareous pieces are developed between

652

ZOOLOGY

SECT.

the two valves. In Teredo (Fig. 545), the so-called Ship-worm,
which causes great destruction by boring into piles, ships'-timbers,
&c., the valves (t\) remain very small and weak but movable, and
the general surface of the mantle secretes a continuous shelly
tube which lines the burrow. In
Aspergillum (Fig. 546) the valves
are completely fused to the tube,
the anterior end of which is closed
by a plate perforated with numerous
holes like the rose of a watering-
pot.

In Nucula, Area, &c., the foot
(Fig. 548, ft.) presents what may be
considered as its most primitive

FIG. 544.— Requienia ammonea; B, Hip-
purites cornu-vaccinum. a, right
valve ; /, point of fixation. (From the Cam-
Ih-iii'te Natural History.)

form, having a flat ventral surface
or sole upon which the animal creeps.
Far more common is the ploughshare-
like form we are already familiar
with in Anodonta and Unio, adapted
for slowly making its way through
sand or mud. In a few forms, e.g.
Trigonia and Cardium, it is bent
upon itself and is capable of being
l suddenly straightened so as to act as
a leaping organ : in Mytilus it is
cylindrical (Fig. 547, F): in the Oyster
it is absent. In addition to the anterior and posterior retractors
the foot is sometimes provided with a levator muscle (Fig. 548, /),
particularly well developed in Nucula and its allies.

Immediately posterior to the foot a byssus-gland is fre-
quently found : it secretes a silky substance in the form of
threads, which serve to anchor the animal permanently or

FIG. 545.— Teredo navalis, in

a piece of timber. P. pallets;
sx. siphons ; T. tube ; V. valve
of shell. (From the Cam />,•<</>!::
Natural History).

PHYLUM MOLLUSCA

653

;. 546.— Aspergillum.

(After Sowerby.)

temporarily. It is by means of the byssus that the Sea-mussel
(Mytilus) is attached to the rocks (Fig. 547, By) : in Pinna the
threads are fine enough to be woven in a
fabric. In Lima the threads of the byssus
are spun into a kind of nest in which the
animals lie protected, and in Vulsella the
gelatinous byssus forms a sheath within
which the entire shell, which is of a delicate-
character, can be retracted. In such forms
as Mytilus the muscles which ordinarily
serve to retract the foot are inserted mainly
into the byssus : the latter being fixed they
serve to rotate the animal in various direc-
tions or, in other words, act as adjust&rs.
It must be borne in mind that the definitive
byssus just described is not homologous
with the provisional byssus of Anodonta
(p. 644) which lies in front of the mouth.

The gills are found in their simplest form
in the Protobranchia (e.g. Nucula), where
they consist of a single plume-like organ
or ctenidium (Fig. 548, gl) on each side of the body. Each
ctenidium is of small size compared with the gills of Anodonta,
and is formed of a longitudinal axis, fixed at its anterior end and
free posteriorly, to which are attached two rows — an inner and
outer — of somewhat triangular gill-filaments, all perfectly free
from one another (Fig. 549, A).

In Amusium (B) the gill-filaments are much elongated and,
thread-like instead of
triangular. In the com-
mon Ark-shell (Area, C)
a great change is seen.
The gill-filaments are
delicate and somewhat
flattened threads, each
bent upon itself into
the form of an elongated
U, and therefore con-
sisting of a proximal or
fixed limb and a distal
or free limb. The flexure
takes place in such a
way that the free limb
is external in the outer
row of filaments, internal in the inner row. Adjacent filaments
are loosely united by groups of large interlocking cilia (Fig. 550),
laced at regular intervals, and in this way the outer and inner

FIG. 547.— Mytilus edulis, attached by byssus (By.)
to a piece of wood ; F. foot ; S, exhalant siphon.
(From the Cambridge Natural History.)

•654 ZOOLOGY SECT.

limbs of the filaments are respectively joined together, so as to
•convert each longitudinal row of U-shaped filaments into a double
plate, fairly coherent unless the ciliary junctions are forcibly
.separated. In this way the single ctenidium of Nucula has given
place to two plate-like gills, each formed of an outer and an
inner lamella : the inner lamella of the outer and the outer
lamella of the inner gill are united along their dorsal edges, the.
line of junction representing the axis of the ctenidium : the
outer "lamella of the outer and the inner lamella of the inner gill
-are free dorsal ly.

In Mytilus (Fig. 549, D) the gill is strengthened by the "develop-
ment of delicate non-vascular bars or inter-lamellar junctions

Son,

**

TIG. 548. — Nucula nucleus, the animal with' the left mantle-lobe removed ; a. ad. anterior
adductor; ar, anterior retractor ; ft. foot; gl. gill; r/on. gonad ; /. levator ; m. right lobe of
mantle ; mth. mouth ; p. ad. posterior adductor ; pip. palp ; p. r. posterior retractor. (After
Pelseneer.)

between the two limbs of each filament. In Lucina these
junctions are large and provided with blood-vessels, and vascular
bars of tissue, the inter-filamentar junctions, replace the ciliary
junctions of the lower forms. Thus by a regular series of grada-
tions the ctenidium is replaced by the complex double gill we are
already familiar with in Anodonta. In all the higher forms the
outer lamella of the outer gill concresces with the mantle and the
inner lamella of the inner gill with the visceral mass, while,
posterior to the latter, the inner lamellae of the right and left
inner gills unite with one another. The blood-vessels, which are
confined to the filaments in the simpler types, occur only in the
inter-filamentar and inter-lamellar junctions in the more complex
forms of gill. In the Septibranchia the gills are degenerate,
being represented by a horizontal muscular partition or septum

XII

PHYLUM MOLLUSCA

655

(Fig. 549, F, sep. and Fig. 551 IX), which divides the inhalant
and exhalant chambers from one another. Respiration in this
case is performed entirely by the internal face of the mantle.

Digestive Organs. — The mouth is anterior ; in forms with two
adductor muscles it is always placed immediately behind the
anterior adductor. It is bounded by two pairs of labial palps

FIG. 541*. — Half transverse sections of various Pelecypoda to show the chief kinds of gill.
A, Nucula; B, Amusium; C, Area; D, Mytilus; E. Anodonta; F, Poromya.

a. aperture in branchial septum ; b. v. blood vessel ; ft. foot ; i. f. inner row of filaments ;
i. g. inner gill ; i. 1. inner lamella ; i. 1. j. inter-lamellar junctions ; m. mantle ; o. f. outer row
of filaments; o. ;/. outer gill; o. 1. outer lamella; sep. branchial septum. (Modified from
Korschelt and Heider and Lang.)

which sometimes attain an immense size (Fig. 548) ; there is never
any trace of jaws or other masticatory apparatus. The convolutions
of the intestine are sometimes very complex. The crystalline
style either lies freely in the stomach and anterior part of the
intestine, or is contained in a coecal pouch of the stomach (Fig.
552), which may be prolonged into one of the lobes of the
mantle. The anterior end of the style, which projects into the
stomach, appears to be slowly dissolved by the digestive juice,

656

ZOOLOGY

SECT

forming a sort of cement to enclose the hard particles of the food
and prevent any harmful effect on the mucous membrane.

The excretory organs occur in their simplest form in the
Protobranchia, in which they have the forms of cylindrical curved

tubes or meso-nephridia (Fig. 553, vi),
opening at one end into the pericardium
and at the other on to the exterior ;
the whole nephridium is lined with
glandular epithelium, and has no com-
munication with its fellow of the oppo-
site side. In the higher forms the
organ becomes differentiated into a se-
creting ^portion or kidney, which becomes
very spongy in texture, and opens into
the pericardium, and a non-secretory
portion or bladder, which opens extern-
ally. Frequently there is a communi-
cation between the right and left
nephridia, and in some genera, such
as the Oyster, the organs become extensively branched.

Circulatory Organs. — The heart is usually perforated by
the rectum, but lies altogether above it in Nucula (Fig. 553, vii)
and some other genera ; the ordinary arrangement seems to have

FIG. 550.— Four gill-filaments of
Mytilus. c.j. ciliary junc-
tion ; /. filaments. (From the
-l<i<j<* Xntt>.i-al History.)

VIII

VII VI

IV

FIG. 551.— Dissection of Poromya. I, anterior palp ; II, foot ; III, lamella on branchial septum
IV, valve of branchial aperture ; IV, anal siphon ; V, posterior adductor ; VI, posterior
tractor of foot ; VII, heart ; VIII, ovary ; IX, branchial septum ; X, anterior adductor. (I
Pelseneer's Mollv.su c< *.)

been brought about by the heart becoming folded over the intes-
tine and united below. In the Oyster and some other forms tht
heart is below the rectum. Pores are often found on the surface
of the foot, and it has been asserted that through them the

657

I PHYLUM MOl^LUSCA

bernal water mixes with the blood ; this is, however, certainly
b the case : the blood system is everywhere closed.
The nervous system is found in its most primitive condition
in Nucula (Fig. 554). Instead of a single cerebro-pleural ganglion

VII VI

IX

VI

V2. — Sagittal section of part of enteric canal of Donax. 1, lower lip ; II, intestine ; III,
pyloric ccecum ; IV, crystal style : V. cuticle ; VI, stomach ; VII, gullet ; VIII, upper lip ;
IX, mouth. (From Pelseneer's J/o//cx'/(/, .,.)

iere are, on each side, distinct cerebral (xvi.) and pleural (I)
ganglia, each united by a connective with the pedal.

The most characteristic sense-organs are the otocysts and the
osphradia. The otocyst — auditory
or directive organ — is always
placed in the foot, just behind
the pedal ganglion, to which it
is connected by a nerve : the
latter probably has its origin in
the cerebral ganglion. The
otocysts are developed as in-
volutions of the ectoderm, but,
except in Nucula (Fig. 554,
x-xii), the connection with the
exterior is lost, and they be-
come shut sacs.

The osphradia — olfactory or
water-testing organs — are
patches of sensory epithelium
situated in immediate relation -with the visceral ganglia (Fig.
554, viii), from which they are usually said to be innervated.
There is, however, some reason for thinking that the osphradial

VOL. I ^— —tf-*^_

'-^B^X

V^ ov i^V

TY )]

IV

III

Fie. 558.— Diagram of Nucula. I, anterior
adductor ; II, foot ; III, renal aperture ;
IV, posterior adductor; V, anus; VI, kid-
ney ; VII, ventricle; VIII, pericardium;
IXygonad. (From Pelseneer.)

658

ZOOLOGY

SECT.

XVI

nerve actually springs from the cerebral ganglion. Patches of
sensory epithelium, very similar to the osphradia, and called
the abdominal sense-organs, occur, one on each side of the anus,
in Area and other asiphoniate forms, and a similar organ has

been described beside the retractor
muscles of the siphons in several
Siphoniata.

In a few instances eyes are present,
but never in what we are accustomed
to consider as the normal position for
such organs, at the anterior or head
end of the body. They occur, in fact,
in the only situation where they can be
of any use, namely, along the edge of
the mantle. The best known form in
which they occur is the common Scallop
(Pecten), which has a single row (Fig.
537, vii) all round the mantle border.
Each has a cornea (Fig. 555 1\ a cel-
lular (not cuticular) lens (#), a retina (5),
formed of cells, the inner ends of which
are modified into visual rods, and an
optic nerve (7), one branch of which
spreads over the front of the retina and
sends branches backwards to the visual
rods. In this pecularity, as well as in
the cellular lens, the eye of Pecten is
singularly like that of Vertebrates.
The pallial eyes of Pelecypoda are
probably to be looked upon as modified
tentacles.

Reproduction and Development.
— Most Pelecypoda are dioecious, but
several hermaphrodite forms are known.
Some of these, such as the Oyster, are
protandrous, the gonad producing first
sperms and afterwards ova : in others
part of the gonad serves as an ovary,
part as a testis, the two opening into a
common duct : in others again there is
a distinct ovary and testis on each side
opening by separate ducts. There are never any accessory organs
of reproduction, such as spermatheca, penis, etc. Fertilisation
frequently takes place in the water after the eggs are laid.
Segmentation is total but unequal, and the gastrula is formed either
by invagination or by epiboly. A shell -gland (Fig. 556 sd.) is
formed as an invagination of the dorsal surface, a stomodaBum (m)

VIII

VI

FIG. 554.— Nervous system and
auditory organs of Nucula.
I, pleural ganglion; II, pleuro-
pedal-connective ; 111, com-
mon connective from cerebral
and pleural to pedal ganglia ;
IV, auditory nerve ; V, pedal
ganglion ; VI, visceral gang-
lion ; VII, posterior pallial
nerve ; VIII, osphradium ;
IX, visceral connective ; X,
otocyst ; XI, canal of otocyst ;

XII, its external aperture ;

XIII, cerebro-pedal connec-
tive; XIV, anterior pallial
nerve ; XV, nerve to palps ;
XVI, cerebral ganglion. (From
Pelseneer.)

XII

PHYLUM MOLLUSCA

659

as an imagination of the ventral surface, and the larva of most
forms, unlike that of Anodonta or Unio, passes into a stage in
which it closely resembles the trochosphere of Worms (Fig. 556)
having a pre-oral and a post-oral circlet of cilia, a tuft of cilia
round the anus, and an apical tuft in the middle of the prostomium.
There is also an ectodermal thickening on the prostomium which
becomes the cerebral ganglion, and a similar ventral thickening

,;. 555. — Vertical section of eye of Pecten. 1, cornea; 2, lens; 3, external epithelium,
, blood-sinus ; 5, retina ; 6, pigmentary layer ; 7, optic nerve. (From Korschelt and Heider.)

rhich gives rise to the pedal ganglion and corresponds with the
idiment of the ventral nerve-cord in Worms. The pelecypod
trochosphere is, however, distinguished from the corresponding
stage in Worms by the presence of the shell-gland^ which soon
secretes a delicate unpaired shell. The prostomial region grows
it into a thickened retractile rim, bearing the pre-oral circlet
)f cilia, and called the velum (Fig. 557 vel.) : the larva at this stage
is distinguished as a veliger — a very characteristic molluscan,

u u 2

660

ZOOLOGY

SECT.

phase of development. The shell soon becomes "bivalved and
extends ventrally on each side, paired processes of the dorsal

E.

'r/C.

c.

D.

E.

Or.

FIG. 55(3. — Five stages in the development of Ostrea. a. anus ; /*/. blastopore ; m. mouth ;
ma, stomach ; mes. mesoderm ; rk, polar bodies ; s. shell ; sil, shell-gland ; sni, anterior adductor ;
ic, pre-oral circlet of cilia. (From Korschelt and Heider.)

region of the body accompanying it and forming the mantle-lobes.
A projection STOWS out from the ventral surface, between mouth

1 71

and anus, and becomes
the foot (Fig. 558 /), and
on the sides of the body
the gill-filaments (k) arise
as a row of delicate pro-
m cesses, at first simple, but

afterwards becoming bent
\ upon themselves so as to

A cm assume a V-shape. Eyes
are often present in the
larva at the base of the
ry " tt velum.

/ General Remarks.—

Although none of the
Pelecypoda are micro-
scopic, they present a con-
siderable range in size,
from the little fresh-water
Cyclas, about 1 cm. long
to the Giant Clam ( Tri-
dacna f/if/as) of the Indian and Pacific islands, which is sometimes
60 cm. (two feet) in length and 500 pounds in weight.

s.m.

nur.

Fie. :,',!.— Voliger larva of Ostrea. a. anus ; <?llt.
dorsal longitudinal muscle; /. liver; ,,>. mouth;
ma, stomach; *. shell; ,x,,t, adductor muscle; as,
hinge of shell ; ?d. velum ; cm. ventral longitu-
dinal muscle. (From Korschelt and Heider.)

PHYLUM MOLLUSCA

661

Many pelecypod shells are white or dull brown in colour, but
in several genera brilliant tints are the rule, the various species of
Scallop (Pecten} being specially remarkable in this respect. The
inner surface of the shell often exhibits beautiful iridescent tints,
noticeably in the so-called Pearl-oyster (Meleagrina) and the
Australian Trigonia. As far as is known, the colours are all what
are called " non-significant," i.e. are of no physiological or ethological
importance. In this connection the formation of pearls by some
species must be mentioned : they are deposits of nacre formed
round sand-grains or other foreign bodies, either between the
mantle and shell or in the soft parts. They are produced, amongst
other species, by the " Pearl-oyster " (Meleagrina margaritifera)

a//

•"it"

--V

FIG. 558. — Two embryos of Cyclas. a. anus ; by. byssus gland ; /. foot ; g. gonad ; I; gill ;
TO. mouth ; m+l. stomach and liver ; wr, edge of mantle ; n, kidney ; p. pericardium ; s'. un-
paired shell ; s". rudiment of paired shell ; sd, shell gland ; i-d, gullet ; ret. velar area. (From
Korschelt and Heider).

and by the Pearl-mussel (Unio margaritifera). Some species,
such as the common boring Pholas. are phopphnresppnt.

Most Pelecypc^^r"e~:s^a^giBh'irr habit, progressing only by slow
contractions of the foot, and some are permanently fixed, during
adult life, by the byssus. The Scallops, however, swim freely by
clapping the valves together. The Cockles (Cardium), Trigonia,
etc., jump by sudden movements of the foot, and the Razor-fish
(Soleri) jerks itself forward by suddenly withdrawing its foot and
thus ejecting water through the siphons. The only parasitic
genus is Mntovalva, found in the gullet of a Holothurian.

Pelecypoda are abundant both in fresh water and the sea ; the
marine forms are mainly littoral. None are pelagic or terrestrial.
They are very abundant in the fossil condition, occurring in all
formations from the Upper Cambrian upwards, and, owing to
their gregarious habits, frequently forming extensive deposits or
shell-beds. The oldest forms are all iso- or hetero-myarian the

662

ZOOLOGY

SECT.

monomyarian types (Pseudolamellibranchia) appear first in the
Carboniferous, and the Siphoniata not until the Triassic period.
The modern genus Area dates from the Upper Cambrian, and thus
furnishes as striking an example of a " persistent type " as some
of the Brachiopods.

There seems to be little doubt that the Protobranchia, and
especially Nucula, exhibit the most primitive type of pelecypod
organisation, as indicated by the plume-like gills with separate
filaments, the simple nephridia, and the distinct cerebral and
pleural ganglia ; absence of concrescence is always a mark of low
or generalised organisation. The Filibranchia with imperfectly
united gill-filaments come next, and are divisible into two groups,
isomyarian with equal-sized adductors and heteromyarian with
more or less atrophied anterior and proportionally enlarged posterior
adductor; the latter group is to be looked upon as the more
specialised, and leads to the Pseudolamellibranchia (monomyarian)
in which the anterior adductor disappears completely in the adult,
while the posterior is immensely enlarged and assumes a central
position. Similarly the isomyarian Filibranchia lead to the
Eulamellibranchia, which are equal-muscled, but have the gill-
filaments united into a complete basket-work. In the Eulamelli-
branchia, lastly, there is a gradual series of stages from compara-
tively generalised forms with free mantle-lobes up to the highly
specialised species with large siphons. That the Pseudolamelli-
branchia and the siphoniate Eulamellibranchia are to be looked
upon as the highest members of the class, is indicated, not only by
morphological evidence, but by their comparatively late appearance
in time.

SINUPALLIATE
EULAMELLIBRANCHIA

INTEGRIPALLIATE
EULAMELLIBRANCHIA

PSEUDO-LAMELLIBRANCHIA

HETEROMYARIAN
FILIBRANCHIA

ISOMYARIAN
FILIBRANCHIA

PROTOBRANCHIA

FIG. 559. — Diagram illustrating the mutual relationships of the Telecypoda.

xii PHYLUM MOLLUSCA 663

CLASS IL— AMPHINEURA.

The Amphineura are a class of marine Mollusca formerly
grouped with the Gastropoda, but now recognised as sufficiently
far removed from the latter to require separation as a distinct
class. The commonest, as well as the most highly organised, of
the Amphineura are the Chitons, a group of remarkably sluggish
Limpet-like Molluscs with a shell composed of eight pieces. The
other members of the class are lowly organised forms, comprising
the most primitive of the entire phylum, all of which are devoid
of a shell.

1. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Amphineura may be defined as bilaterally symmetrical,
more or less elongated Mollusca, with terminal mouth and anus,
either devoid of a shell, or possessing one which consists of eight
median valves. The mantle contains numerous spicules of carbon-
ate of lime, and is not divided into paired lateral lobes. The
•^ctenidia are either absent, or there is a single pair, or they
occur as a circlet round the anus, or as two lateral rows situated
between the edge of the mantle and the side of the foot. An
odontophore (vide infra) is sometimes present, sometimes absent.
The nervous system consists of two pairs of nerve-cords, pedal and
pallial, connected in front with a nerve-ring.
The class is divisible into two orders : —

ORDER 1. — PLACOPHORA.

Amphineura with a broad foot, and with a shell which consists
of eight transverse valves. There is a row of ctenidia on either side.
This order includes the Chitons.

ORDER 2. — APLACOPHORA.

Amphineura with an elongated body covered completely by the
mantle, without shell, but with calcareous spicules. There is no
foot, but generally a ventral longitudinal groove along which
usually runs a low ciliated ridge. In some there is a posterior
cavity containing a pair or a circlet of ctenidia.

This order includes Neomenia, Proneomenia, Chcetoderma, and
several allied genera.

2. GENERAL ORGANISATION.

External Features. — The Aplacophora are distinguished
by their worm-like body, • sometimes elongated and narrow and
capable of being coiled into a spiral, sometimes comparatively

664

ZOOLOGY

SECT.

FIG. 560.— Chsetoderma
nitidu.ru.rn. a. anus ;
in. mouth. (From the
C(tmtji'i<(<fi'. Sutural His-
tory.)

short and thick. In most instances there is little difference in
external appearance between the anterior and posterior ends. In
Chcetoderma (Fig. 560) alone is there a distinct head, separated off
from the body by a constriction, as well as
a posterior cloacal region which is similarly
marked off. A shell is completely absent.
The mantle covering the surface possesses
a cuticle, in the. substance or on the surface
of which are spicules of calcified material.
Along the middle of the ventral surface
runs, in most instances, a groove, in some
cases merely represented by a narrow strip
from which the cuticle and spicules are
absent. The ventral groove, when present,
usually contains a slight longitudinal ridge,
and this is all that in these primitive forms
represents the foot, an organ so highly de-
veloped in other Molluscs. In Chsetoderma
it is entirely absent. With the ventral
groove is connected in front an anterior
ciliated groove, while behind it is in direct
communication with the cavity of the cloaca.
In Proneomenia ctenidia are absent. In the remaining genera
there is either a pair or a circlet of gills situated in the cloaca
— a cavity at the posterior end of the body into which the anus
opens (Fig. 565).

In Chiton (Figs. 562 and 563) the body is dorso-ventrally com-
pressed, convex above, arid presents below a broad flat foot
(narrow in Chitonellus) which acts not only as an organ for effect-
ing creeping movements, but also as a sucker for enabling the
animal when at rest to adhere firmly, like a
Limpet, to the surface of a rock. The most
remarkable external feature of Chiton is the
presence on the dorsal surface of a calcareous
shell (Figs. 562 and 564) made up of no fewer
than eight transversely elongated pieces or
valves, arranged in a longitudinal row, arti-
culating together and partly overlapping one
another. These valves are primitive, being
direct developments of a series of shell-glands
of the embryo and not secondary structures
like the shells of other Molluscs. They are
sometimes partly, sometimes completely, covered over by the
mantle. Each valve consists of two very distinct layers, a more
superficial and a deeper, the latter formed of compact calcareous
substance, the former perforated by numerous vertical canals for the
lodgment of the sense-organs to be presently referred to. External

Fio. 561.— Neoxnenia
carinata. a. anus ;
///•. ventral groove ;
m. mouth. (From the
Ciiiniii-niiK', Natural
Hittory.)

PHYLUM MOLLUSCA

665

the valves the dorsal integument (mantle) of Chiton and its
Hies is usually beset with a number of horny or calcified tubercles

mo

manl-

5. 562.— Chiton spinosus, dorsal view.
(From the Cambridge Natural History.)

FIG. 563. — Chiton, ventral view. an. anus ;
cten. ctenidia ; ft. foot ; mant. mantle edge ;
mo. mouth. (After Pelseneer.)

and spicules. The mantle develops only very slight lateral flaps,
and under cover of these are a series of small ctenidia (Figs.
563 and 569, den) to the number of fourteen to eighty. The
mouth and anus are both median, situated
at the anterior and posterior extremities
respectively.

Alimentary System. — In the Apla-
cophora the mouth is usually a longi-
tudinal, rarely (ChaBtoderma) a transverse,
slit, situated ventrally near the anterior
extremity. There is a buccal cavity, with
in some cases an odontophore x (Fig. 565,
rad) in others a single chitinous tooth ;
sometimes teeth are entirely absent.
There are both salivary and buccal
glands. Very characteristic of the group
as compared with other Molluscs is
the presence of a straight intestine
devoid of coils. It has connected with
it either a single coecura or a double
row. In the Placophora the buccal
cavity always contains a well-developed
odontophore. The intestine is elongated

1 For a description of the structure of this
characteristic organ, see the account of Triton,
infra.

FIG. 564.— Chiton, valves of
shell. (From the Cambridge
Natural History.)

666

ZOOLOGY

SECT.

and coiled. There are salivary glands and a large paired liver
(Fig. 566, liv.)

Body-Cavity. — In the Aplacophora the interstices between
the organs and the body wall are filled with a form of connective
tissue with muscular fibres ; a vertical diaphragm (Fig. 565, clia.)
separates the posterior part of the body, containing the peri-
cardium (peri}, from the rest. In the Placophora the ccelome
(Fig. 566), is an extensive cavity, lined with a coelomic epithelium,
and divided into three completely separated parts, the pericardium,
the genital cavity, and the general body cavity.

If

Tbrn-

—cten,

fierv

FIG. 565.— Chsetoderma nitidulum, longitudinal section, an. anus ; Irn, brain ; m-r. gland-
ular coaca of mesenteron ; ilia, diaphragm separating off the posterior portion of the body
-mo. mouth ; peri, pericardium and heart ; rad. radula ; rect. rectum. (After Simroth.)

Vascular System. — The existence of a heart in the Aplaco-
phora is somewhat doubtful ; the organ which is supposed to be
of that nature is contained in a cavity termed the pericardium
(Figs. 565 and 568, peri).

In Chiton there is a well-developed heart (Fig. 566, ht.) consist-
ing of a median ventricle and two lateral auricles: The pericardial
cavity in which it lies is a space of considerable extent in the
posterior region of the body, below the two last valves of the
shell.

The Nervous System consists in the Aplacophora (Fig. 567,
A,B,C) of four longitudinal nerve cords — two pedal and twopallial.

XII

PHYLUM MOLLUSCA

667

peri

These are connected together by an oesophageal ring, thickened
dorsally into a single or double cerebral ganglion ; and in front of
this is a second, more slender stomato gastric nerve-ring with small
ganglia. The pedal
cords (v.v) may pre-
sent in front a pair
of ganglionic thick-
connected
imissure,
[er back
ry be a

F;G. 566.— Diagrammatic longitudinal section of Chiton,
specially intended to show the relations of the parts of the
ccelome, which are bordered with a thick line. an. anus ;
ent. enteric cavity ; ft. foot ; gon. gonad ; lid. head-lobes ;
lit. heart ; liv. liver ; mo. mouth ; neph. nephridium ; pen.
pericardial cavity. (From Simroth, after Haller.)

CO

enlarge-
nited by
res. The

palliaWords (LI) are
connected behind,
above the rectum,

by a commissure (p.c) which usually bears a median enlargement.
Sometimes a union takes place posteriorly between the cords of
the two pairs. There are no eyes, nor otocysts, nor tentacles.
Some have a sensory frontal lobe and a sensory pit or elevc^tion in
the middle line of the dorsal surface near the posterior end.

In the Placophora (Fig. 567, D) there is an oesophageal nerve-
ring consisting of a thicker dorsal cerebral portion not differentiated
into ganglia, and a thinner ventral l)uccal commissure. The cerebral
part sends off nerves to the labial palps, the lips, and the buccal

FIG. 567. — Nervous system of Amphineura, A, Proncomenia ; .-\B, Neomenia : C, Chcetoderma ',
D, Chiton, c, cerebral ganglia ; /, /, pallial cords ; pc, posterior commissure ; s, stomatogastric
commissure or ring, with ganglia ; v, v, pedal cord's. (From the Cambridge Natural History,
after Hubrecht.)

apparatus. Two pairs of longitudinal nerve cords, pedal and pallial,
are given off posteriorly. The former, which give off nerves to
the foot, are joined by numerous commissures passing beneath
the enteric canal. The latter, which send off nerves chiefly

ZOOLOGY

SECT.

yorv

rec,t

•peri-

to the mantle and the ctenidia, are united together by a supra-
rectal commissure at the posterior end of the body. Just behind
its origin each pallial cord gives off a slender visceral commissure,
which unites with its fellow of the opposite side : two small ganglia
lie in this visceral commissure near the middle line. The large
cords contain nerve-cells throughout their length.

The conspicuous organs of special sense, present on the head of
Gastropods (vide infra), a** absent in the Placophora, as in the
Aplacophora. A pair of processes situated in front, at t LjfeAk of

the mouth, have th
of labial palps. In
cavity there are
gustatory organs sup
nerves from the cere
missures, arid in fror
odontophore is a thickening
of the epithelium — the sub-
radular organ — containing
nerve - endings. Remarkable
sensory organs, the micrcesthetes
and the megalcestheies, lie in
the canals already mentioned
as occurring in the superficial
layer of the shell valves. The
megalassthetes may take the
form of eyes, with cornea, lens,
pigment layer with iris, and
retina ; in some cases the lens
in absent.

Reproductive and Renal
Organs. — In the Placophora
the sexes are distinct : in the
Aplacophora, with the excep-
tion of Chaetoderma, they are
united. In the Aplacophora
(Fig. 568), with the exception
of Chsetoderma, the gonads

The sexual products pass into the pericardia! cavity
and thence are carried to the exterior by a pair of ducts (which
may be of the nature of nephridia) opening into the cloaca.

In the Placophora (Fig. 569) there are two symmetrical
nephridia opening internally into the pericardium by a ciliated
funnel-like opening (n. peri. ap). and opening on the exterior
(neph. ap) between two of the posterior ctenidia. Each consists
of a looped main tube, into which open numerous minute
tubules which ramify among the viscera. The testis and ovary
(gon) are similar in appearance, differing only in colour when the

FIG. 568.— Neomenia carinata, reproduc-
tive organs ; cop. copulatory organs; </o,>.
gonads enclosed in extensions of the peri-
cardial cavity ; gonod. gonoducts ; peri, peri-
cardium ; rect. receptaculuru seminis. (From
Simroth, after Wiren.)

are paired.

PHYLUM MOLLUSCA

669

clen.

ucts are mature. Each is an impaired sac marked by a series
of slight lateral constrictions.

Little is known of the development of the Aplacophora. The
eggs undergo complete segmentation, and give rise to a gastrula
by invagination. This develops into a form of trochosphere
with a ciliated ring called the prototrocli or velum.

•The eggs of Chiton are fertilised in the mantle-cavity, where in
e species they are
until the
e fully de-
t first the
n is toler-
the ovum
divided into
A iproximately
blastomeres ;
the stage of
eight cells four on
one side are to be dis-
tinguished as larger
than the other four.
These two sets un-
dergo further divi-
sions, and arrange
themselves in such
a way as to form a
somewhat flattened
blastula, one side of
which (vegetal pole)
is composed of a com-
paratively small num-
ber of large cells.
Then follows the in-
vagination of the
cells of the vegetal
side and the resulting
formation of a gast-
rula. This soon be-
comes elongated in

the direction of the future long axis. Two endoderm cells
of specially large size in the neighbourhood of the blastopore,
with several others in their proximity, constitute the rudiments
of the mesoderm (Fig. 570, B, mes.) ; these pass into- the segmen-
tation-cavity, and speedily assume a bilateral arrangement.

Two rings of cells surrounding the embryo develop cilia (cil.\
and by the double circlet thus formed the larva becomes divided
into an anterior and a posterior region. The blastopore becomes

-gren.cip
n..peri.&p
n,eph.a.p

FIG. 569. — Chiton* nephridial and genital systems, an.
anus; cUn. ctenidia ; yen. ap. genital aperture ; gon. gonad ;
gonod. gonoduct ; mo. mouth ; neph.ap. nephridial aperture ;
n.peri. ap. aperture from nephridia to pericardium. (From
Simroth, after Haller and Lang.)

670

ZOOLOGY

SECT.

shifted from its original posterior position forwards on the ventral
surface, until it comes to be situated just behind the circlet of
cilia ; it becomes elongated, and an invagination of ectoderm round
its anterior end forms the mouth (mo.} and stomodseum. A ventral
diverticulum of this forms the rudiment of the radular sac (rd.). By
greater relative growth of the post-oral part the embryo assumes
the form of a pear ; and in this trochosphere stage with a pra3-oral
circlet and a bunch of cilia in the middle of the apical area,
it becomes free in the case of certain of the species, whilen
it remains enclosed in the egg up to a later stage of de.
As yet there is no anus, that aperture, with the proct
formed by invagination at a later stage. The apical
present in the early larva; but the rudiments of

B

cil

mese,nt

FIG. 570. — Chiton, development. A, general view of larva; B, section of early, C, of later
ti-ochosphere. calc. calcifications (rudiments of shell) ; cer. g. cerebral ganglion ; cil. ciliary ring ;
til. t. ciliary tuft at apical pole ; eye. eye ; ft. gl. foot-gland ; mes. mesoderm ; mesent. mesenteron ;
mo. mouth ; rd. radular sac ; sp. spines ; vis. g. visceral ganglion. (From Korschelt and Heider,
after Kowalewsky.)

ganglia (C, cer.g.\ which appear at the apical pole at a later stage,
probably represent it. Primitive nephridia, such as occur in
Annulate and many Molluscan trochospheres, are not present.

The post-oral region now becomes greatly elongated; the
mesoderm increases greatly in extent, and forms two well-defined
streaks, which afterwards become divided into parietal and visceral
layers with a ccelomic space between them. The post-oral part
of the embryo now presents an appearance resembling rudimentary
segmentation. This is due to the development of a series of
rudiments of the eight pieces of the shell (B, calc.), each of which
becomes formed independently after the fashion of the entire shell
of other Mollusca.

Ethology, Distribution, &c. — All the Amphineura are ma-
rine. The Placophora occur at all depths, though most abundant

xii PHYLUM MOLLUSCA 671

on the shores between tidal limits. The Aplacophora, on the
other hand, are rare in very shallow water, and absent alto-
gether from the littoral zone; some have been found at con-
siderable depths (down to 1,250 fathoms). The Placophora are
all vegetable feeders, their food consisting of minute algae
and diatoms. The Aplacophora subsist on small animals.
The Placophora when at rest adhere firmly to the surface
of a rock or a block of coral by means of the sucker-like foot.
Whejg. Jforcibly detached the animal curls itself up into a ball,
ily after a considerable time slowly extend itself again.
, movements are extremely sluggish. The Aplacophora
to fix themselves in this way ; many of them occur
mnd the stems of zoophytes, sometimes attached by a
rp viscid mucus.

^Aplacophora have no hard parts that would be recognisable
in the fossil condition ; but numerous fossil Placophora are known
from Silurian formations onwards. The valves of the Silurian
genera differ from those of recent forms in the absence of the
articulations.

CLASS III.— GASTROPODA.

The Gastropoda, including the Snails and Slugs, Limpets,
Whelks, Periwinkles, Sea-hares, and the like, are Mollusca in
which there is, as a rule, a shell composed of a single piece,
and in which the mantle is not divided into two lateral folds as
in the Pelecypoda. The body is inequilateral, owing to the
one-sided development of the -visceral mass. There is a well-
developed ventral foot, usually with a broad flat surface on which
the animal creeps. A head-region bearing eyes and tentacles is
distinguishable in front of the foot. The alimentary canal is
characterised by the presence in the buccal region of a peculiar
organ, the odontophore, present also in some of the Amphineura,
bearing rows of minute chitinous teeth. Plume-like ctenidia are
usually present. A metamorphosis occurs in the development,
during which the young Gastropod passes successively through
Trochosphere and Veliger stages. The majority of the families
of Gastropoda are marine, a few of these being pelagic ; but
some inhabit fresh water and others are terrestrial.

1 . .

Triton is a marine Gastropod living in shallow water, usually
close in shore. The species to which the following description
specially applies has a very wide range, from the English Channel
to the South Pacific, and occurs as a fossil as far back as the
Miocene. In most respects the English Whelk (Buccinum un-

tum) will be found to conform to the description.

EXAMPLE OF THE CLASS. — THE TRITON (Triton nodiferus).

dai

672

ZOOLOGY

SECT.

•shell.

The shell (Fig. 571) is a very hard and dense calcareous
structure, presenting no trace of division into the valves com-
posing the shell of the fresh-water Mussel, and lacking also
its bilateral symmetry. It is in the form of an elongated hollow
cone closely wound round a central axis. The apex of the cone
is the organic apex of the shell, corresponding to the umbo of
the fresh-water Mussel, and is the point from which the growth
of the shell has proceeded ; the base is represented by the

wide oblique opei
mouth or peristo1)
shell. Starting ;
apex along the
cavity of the spira
cone, in order to r<
mouth in an aduM
we have to pass completely
round the central axis five
times — i.e. the spiral con-
sists of five turns. In fol-
lowing the turns, the direc-
tion taken is to the right,
that is to say, the spiral
of the shell is a right-
handed or dcxtral one. The
axis (Fig. 572) is in the
shape of a twisted shelly
rod — the columella — con-
taining a narrow lumen ; it
is formed by the close union
of the axial portions of the
wall of the spiral. The
windings of the spiral are
marked on the outer sur-
face of the shell by a nar-
row impressed spiral line or
suture, parallel with which
are numerous fine ridges
and depressions — the lines
of growth; the increase in size of the shell takes place in
the direction of these lines, not at right angles to them as in
the shell of the fresh-water Mussel, and the lines that more
strictly correspond to the lines of growth of the latter are
excessively fine strife which run transversely to the stronger
lines. At certain points, usually three in a full-grown shell,
the spiral is interrupted by a transversely directed edge which
appears to overlap the succeeding portion; this edge marks
position which the mouth of the shell has occupied

FIG. 571.— Shell of Triton nodiferus.

Natural size.

the

PHYLUM MOLLUSCA

673

end

during regularly recurring periods of arrest of growth, probably
annual.

The mouth of the shell is bordered on the side turned away
from the columella by a prominent rim or outer lip of the
peristome ; this is produced at the extremity farthest from the apex
of the shell into a spout-like process — the siphonal process. The
prominent edge of the peristome is in relation to the dorsal
surface of the body of the

:he opposite side

jrominent edge,

lided off to form
inner lip ; a
ridges on_this
jowards the apical
the "animal in
drawing itself out after it
has become retracted into
the interior of the shell.
The outer lip is in rela-
tion to the dorsal surface
of the body of the animal,
the inner lip in relation
to the ventral surface ; the
siphonal process is for the
lodgment of a spout-like
process of the edge of the
mantle — the siphon.

When removed from the
water, or disturbed in any
other way, the animal be-
comes completely with-
drawn into the interior of
the shell, when the latter
is observed to become
closed by a plate — the oper-
culum (Fig. 573) — which
fits accurately across the
passage some distance in-
ternal to the peristome. The operculum is an oval plate of
chitinoid material hardened by calcareous deposits ; like the
shell itself, it exhibits lines of growth marking what has been
its edge at successive stages in the development of the shell.

The minute structure of the shell is in the main similar to that
of the fresh-water Mussel (p. 634). The outer surface of the
shell is covered with a thin layer of uncalcified chitinoid material
the periostracum ; beneath which is a thick prismatic layer, and,
lining the Jimer surface, a layer of nacre.

VOL. i x x

FIG. 572. — Longitudinal median section of the shell
of Triton iiodiferus.

074

ZOOLOGY

SECT.

FIG. 573. — Operculum of
Triton nodiferus.

External features of soft parts. — The Triton is able to
extend itself to a considerable degree beyond the mouth of the
shell ; but a portion of the body always remains concealed in the
interior, even when the animal has ex-
tended itself to its utmost, the body being,
like that of the fresh- water Mussel (and
of nearly all the Mollusca), organically
connected with the shell. In Triton the
connection is by means of a str« >i
— the columellar muscle (Fig.
— which extends from the coil]

side of the animal to the columella, into which it is ii
is by means of this muscle that the anterior portion oi
capable of being thrust out through the mouth of the
again be withdrawn.

If the Triton be examined in the extended condition (Fig. 574)
it will be found to present a distinct head, which bears dorsally
a pair of appendages — the tentacles (tent) — of a sub-cylindrical
shape, slightly compressed towards their bases, and narrowing
somewhat towards their free extremities; these are capable of
being extended and contracted, but not of being completely re-
tracted. Each bears on its outer side some little distance from
the base a prominent eye. At the anterior end of the head on
its ventral aspect is the opening of the mouth. When the animal

liv

siph

tent

rnesopcL

propd

;. 574.— Lateral view of the body of a female specimen of Triton nodiferus, removed from
the shell, moderately extended, col. m. columellar muscle ; coll. collar of mantle ; lie. liver ;
mant. eav. mantle cavity ; meso. mesopodium ; neph. nephridium ; op. operculum ; ov. ovary ;
prop, propodium; stom. stomach; tent, tentacles.

is feeding, an elongated cylindrical introvert (Fig. 575), comparable
to that of Sipunculus (p. 451), is extended forwards, bearing
the mouth at its anterior end; at other times the introvert is
completely involuted within the head and anterior portion of
the body. In the male on the right-hand side of the body, some
little distance behind the head, is a long narrow fleshy process,
broader at the base than at the free end, and deeply grooved

xii PHYLUM MOLLUSCA 675

longitudinally; this is the penis. Running back from its base
is a narrow groove with prominent lips — the sperm groove, con-
tinuous with that on the penis; in the female these parts are
not represented.

Foot. — On the side of the body (ventral) which the animal
applies to the surface of the ground when it extends from the
shell is a flat surface ^elongated in the antero-posterior direction;
the wall of the body in this region is composed of a^dense mass
«>f mJ^kir fibres: this is the principal part of the foot (pro-
11 mesopodium combined) ; the posterior portion (meta-
a thick process projecting behind this and bearing
tlum on its surface. The foot is highly contractile, and
i£ans of contractions passing over it in a succession of
Tons that the animal creeps
along, dragging after it the rest of
the body enclosed in the shell. In
the middle line of its flat surface,
nearer the anterior than the posterior
end, is a slit-like aperture leading
into a cavity lined with unicellular FIO. 575.— Diagram of the introvert of

, -, 7 7 7 / Triton, in longitudinal section,

tne peCLCU gland. as it appears when almost com-

T-amm'nrlAT' r\f fVio "KnrKr pletely extended. The black lines

remainder 01 tne body represent the waii of the ail-
has been removed from the Shell, mentary canal ; the cross-hatched
.... part the wall of the introvert ;
it IS found tO be twisted Up into a the dotted line marks the position

coil— the visceral spiral, correspond- i^ov7r?p±egsthr°ugh

.

ing to the spire of the shell within ™; «*• lumen of

which .it was lodged. This is unsym-
metrical, the axis of the spiral being
directed, not straight backwards, but backwards, upwards, and to
the right. The external asymmetry of the body is not strongly
marked in the part which is capable of being protruded from the
shell, but is still recognisable, and an examination of the internal
organs shows a marked excess of development on the left-hand
side, i.e. the side which corresponds with the longer outer side of
the spiral of the shell. The surface of the part of the animal
which is capable of being pushed out from the shell is covered
with a thick integument, which is darkly pigmented, except on
the lower surface of the foot. Over the visceral spiral the mantle
forms a thin, delicate, colourless layer. Anteriorly this becomes
thickened and pigmented, and at the posterior limit of the pro-
trusible part gives rise to a thickened ridge, the collar (Fig. 574,
cofl.), forming a semi-circle over the dorsal and lateral regions. In
the middle this is not in close contact with the body, but leaves a
large cleft leading into a very wide space extending backwards for
a considerable distance. This space, which is formed by an infold-
ing of the mantle, is termed the mantle or pallial cavity. In it are
to be found the ctenidium, the osphradium, and the anal, excretory;

x x 2

676 ZOOLOGY- SECT, xn

and reproductive apertures. The wall of the cavity is much
folded and plaited, and contains a quantity of glandular tissue,
the plaits being most numerous on the right-hand side in front
of the anus.

The ctenidium or gill (Fig. 576, den.} is closely applied to the
wall of the mantle-cavity to the left of the middle. It consists of
a main stem, with which are connected a row of delicate flexible
laminae set a& right angles to it : these are broadest in the middle,
becoming smaller towards the ends.

The osphradium (osph.) lies close to the ctenidium di|
hand side, i.e. nearer the middle of the body. It presenf
axis, connected with which are two rows of close -se1
lamellae placed at right angles to it. Like the ctenidil
closely applied to the wall of the mantle-cavity througl
length. The laminae of the osphradium are supplied with nerves
which ramify over them ; and its function seems to be to
ascertain the condition of the water that enters the mantle-cavity.

Digestive System. — The mouth, situated at the anterior end
of the introvert, leads into a large chamber with muscular walls,
the buccal cavity (buc.). At the sides .of the entrance to this cavity
the investing cuticle is thickened to form two distinct horny plates
—the jaws (Figs. 576, 577, and 578, jaw). The jaws are flexible,
and on examination under the microscope are found* to be com-
posed of numerous rows of minute bodies, the fli-nt'u-lcs; the
anterior edge is minutely denticulated. From the floor of the
cavity rises an elevation, the odontoplwre (Fig. 576, od., Fig. 577,
odont.), which is somewhat elongated in the direction of the long
axis of the body and compressed laterally. Over the summit of
the odontophore runs longitudinally a narrow strap-like body, the
radula or lingual ribbon (Figs. 577 and 578, rad.), beset with
numerous minute horny teeth arranged in transverse rows. Pos-
teriorly this toothed ribbon extends into a narrow curved pouch
—the radular sac (Fig. 576, rod. s, Fig. 578, rad. sac.) — extending
backwards from the posterior and lower aspect of the buccal
cavity. Anteriorly it does not extend beyond the odontophore
prominence. The latter contains cartilages (Fig. 578, cart.)
serving for the support of the whole apparatus, and is capable
of being extended, with the radula which it bears, through the
opening of the mouth by the contraction of sets of protractoi
muscular fibres. Inserted into the radula itself are sets of bane
of muscular fibres by which it can be drawn backwards and foi
wards over the odontophore as over a pulley, the effect being
rasping of any hard substance against wty&h it is pressed. The
entire buccal cavity is capable of being drawn forwards towai
the mouth opening, or backwards into the introvert by the coi
traction of strands of muscular fibres passing from its wall to th<
wall of the body.

-post.oes

FIG. 570. — Triton nodiferus. Dissection of the internal organs of a female, viewed from the
dorsal side. The roof of the mantle-cavity has been divided by a longitudinal incision and the
flaps laid out, that on the left bearing the ctenidium and osphradium, and that on the right the
rectum and terminal part of the oviduct. The muscular dorsal wall of the body and the
.introvert have been divided so as to bring into view the anterior part of the alimentary canal
and a portion of the nervous system. The buccal cavity has been tilted up and opened so as
to show the odontophore, and the oesophagus has been cut through near the anterior end.
A portion of the ventral wall of the crop has been removed so as to bring the internal
folds into view, and the interior of the nephridium with the contained portion of the
intestine has been exposed. The stomach is not seen, being hidden by the nephridium, and
the ovary is not represented, an. anus ; ant. aort. anterior aorta ; aur. auricle ; buc. buccal
cavity; c^r.lmc. con. cerebro-buccal connective j^cer. g. cerebral ganglia ; crop, crop; cten.
ctenidium ; uit. intestine ; >.(*'•, jaw ; /. buc. g. left buccal gangKon ; 1. sal.^l. Itfft saltvary
gland ; ,n./iJt. nephridium ; nep/t7 ap. nephridiat aperture ; od. odontophore'; ces. oesophagus ;
0».' anterior end of same, cut and turned aside; osph. osphradium; oxid. oviduct ; ^orid.'
terminal thick-walled portion of oviduct ; pletfr. g. pleural ganglion ; 2>ost. aort. posterior
aorta ; post. <?$. posterior oesophagus ; wul. s. radula sac ; net. rectum ; sal. du. salivary duct ;
K'<I>!<. si}ihnn ; xnj>m. g. supra-cesophageal visceral ganglion ; tent, tentacle; tent. n. tentacular
nerve ; t-ait. ventricle.

678

ZOOLOGY

SECT.

From the buccal cavity runs backwards a long narrow tube
with sacculated walls — the ccsophagus (Figs. 576 and 578, ccs.\

Posteriorly this opens into
a large ovoid sac — the crop
(Fig. 576, crop}. The outer

rad-

FIG. 577.— Triton nodiferus. Interior of the
buccal mass, from above, magnified. I. jaw, left
jaw ; odoiit. lateral surface of odontophore ; rad.
radula ; r. jaw. right jaw.

pears marked with numer-
ous close-set fine lines,
transverse or
direction, andj
cavity of thai
opened, it is jhl
these correspond^
ous delicate foil
extend to near the middle,
and almost completely
block up the lumen. On
either side of the crop is
a large gland — the salivary
gland (Fig. 576, /. sal, a/.,
Fig. 580, r. sal. gl.) — partly
composed of a compact glandular substance, partly of spongy
tissue in which the secretion collects. The two salivary glands
are unlike in size and shape, that on the left-hand side being
much longer than that on the right. Each has a narrow duct
(sal. die.) which runs forwards and inwards to the dorsal aspect
the oesophagus,
where the two
come into close
apposition, be-
coming embed-
ded in the wall
of the oesopha-
gus, along which
they run for-
wards to open
into the buccal
cavity.

From the crop
leads backwards

anrl +n tViA laff Fl°- 578 —Triton nodiferus. Diagrammatic longitudinal vei
tical section of buccal cavity; bod.- car. body cavity; cart, ca
a narrow Cylin- tilage of odontophore ; jcur, right jaw ; as. oesophagus ; ra

drical tube — the

boci. casts

CCLT't

radula ; /•<«,</. sac. radula sac.

posterior ces<>- •

phagus. On this follows a stomach (Fig. 574,s£om.) which is in th<
form of a U-shaped tube partly embedded in the substance <
the liver, the hepatic ducts from which open into it. The tubuh

xii PHYLUM MOLLUSCA 679

stomach is followed by a somewhat narrower tube — the intestine
(Fig. 576, int.). This enters the cavity of the nephridium, round
the interior of which it bends, and, leaving it at its right-hand
side, runs forwards in a straight course as the rectum (rect.)
embedded in the glandular wall of the mantle-cavity, to near
the anterior end, where it terminates in a short, freely projecting,
spout-like portion, with the anus (an.) at its extremity.

liver forms a mass of reddish-brown glandular follicles which

greater part of the bulk of the visceral coil.
liar system. — Close to the base of the ctenidium, behind
little to the right, is the heart, lodged, like that of the
[ter Mussel, in a cavity, the pericardium,, lined by a trans-
lembrane — the pericardial membrane. The heart consists
chambers, an auricle (Fig. 576, aur.) and a ventricle. The
auricle, which is the smaller of the two, is situated somewhat in
front of the ventricle, close to the ctenidium, from the main
central vessel of which it receives the blood. The ventricle (vent.)
is of somewhat pyramidal shape, but with the edges rounded
off. Its wall is extremely thick and muscular. Passing out from
the ventricle towards the right is a thick artery, which soon
divides into two, one running forwards, the other backwards — the
anterior (ant. aort.) and posterior (post, aort.) aot^tce. The former
is a very large trunk which runs forwards below the posterior
oesophagus, crop, and anterior oesophagus, giving off branches as
it goes, to the region of the head. The posterior aorta, narrower
than the anterior, passes into the visceral spiral, where it breaks
up into branches for the supply of the various parts. The blood-
system consists in large measure of sinuses, as in the fresh-water
Mussel, and the general course of the circulation is similar to
what has already been described in that Mollusc (p. 640).

Excretory System. — There is only one meso-nephridium
(neph), a large organ situated dorsally behind the pericardium.
It is a sac with thick, glandular, and highly vascular walls, the
inner surface of which is thrown into numerous complex folds.
In front it communicates directly by a large aperture (neph. ap)
with the mantle-cavity, and by a narrower passage with the
. pericardium.

The nervous system (Figs. 579 and 580) is more highly
elaborated than in the fresh-water Mussel. Two pairs of nerve-
ganglia — the cerebral (cer. g) and the pleural (pi. g) — lie close
together over the posterior part of the oesophagus, just where it
passes into the crop. The right and left ganglia of each pair are
fused together in the middle line, though separated by a con-
striction, and the ganglia of the two pairs are placed very close
together, though quite distinct. From each cerebral ganglion
there passes forwards a stout cerebro-luccal connective (cer. buc. con)
to a buccal ganglion situated on the posterior surface of the

680

ZOOLOGY

SECT.

bllC.g

l.ct&d.

. .r)7'.t.— Triton nodiferus. Nervous system, from the dorsal side. A /•.
buecal connective ; cer. .". cerelirul ganglion ; ro/. />. nerves to tlie coluinellar mu

iit/r/i. </

Infra-oesophagea] vism-al ganglion ; /. ni,<i. </. left abdominal ganglion ; /. hi\ ,>. left branchial
nerve ; /. />/•. ///. norvos to branchia and osphradium ; /. in<t,i.t. n. left mantle nerve; pi-<'. con
cerebro-pedal and pleuro-pedal connectives; ned. </. pedal ganglia; j>f. (/. pleural ganglion
,-. «l»i. .'/. i-ight abdominal ganglion ; r. br. ». right 'branchial nerve ; r. mant. n. right mantle
nerve; *<',/<'"• .'/• supra-intestinal visceral ganglion; rise. n. visceral nerve branches.

:

PHYLUM MOLLUSCA 681

ccal chamber. Also given off anteriorly from the cerebral
ganglia are optic nerves to the eye, and tentacular nerves to the
tentacles. From each cerebral ganglion passes downwards and
forwards a stout cerebro-pedal connective, and from each pleura!
ganglion a pleuro-pedal connective, to a large pair of closely-united
pedal <i<in<jlia (Figs. 579 and 580, peel g.) embedded in the
upper layers of the muscles of the foot, to which they give oft'
numerous nerves. The right pleural ganglion gives ofT behind a
.s7/vw/ corral connective, which bends across to the left,
, oesophagus, and, some distance to the left of the ali-
canal, expands into a triangular supra-intestinal visceral
(supra, g.), situated below the superficial layer of muscular
The left pleural ganglion gives off an infra-intestinal

FIG. 580. — Triton nodiferus. Lateral view of nerve-ganglia and related parts. Letters as in
Fig. 579 : in addition — ant. (to. anterior aorta ; cr. crop ; us. oesophagus ; sal. du. salivary
ducts ; r. sal. gl. salivary gland.

visceral connective, which passes obliquely backwards and to the
right, below the alimentary canal, to a ganglion situated a little to
the right of the middle line — the infra-intestinal visceral ganglion
(infra, g.). The supra-intestinal ganglion gives off a nerve which
runs towards the osphradium and ctenidium, which it supplies
with branch nerves, and unites with a stout mantle nerve (l.mant.n.\
which is given off from the left pleural ganglion. The right
pleural also gives off a stout connecting nerve to the infra-intes-
tinal ganglion. From the supra- and infra-intestinal ganglia the
left and right visceral connectives are continued backwards and
unite behind in the neighbourhood of the stomach ; each ends
in a triangular alidominal ganglion (I. dbd. g.; r. a~bd. g.}, and these
are joined by a transverse commissure, from which a number of
visceral nerves are given off. A remarkable torsion of the nerve
connectives is here to be observed, the two visceral connectives
becoming twisted into the form of the figure 8.

682

ZOOLOGY

SECT,

co

and

rcis

The organs of special sense of Triton, in addition to the
tentacles and the osphradium, which have been already referred
to, are the eyes and the otocysts. The eye (Fig. 581) is a rounded
invagination of the epidermis with an inner wall or retina (ret.}

composed of pigmented and sen-
sory cells. The latter (retinophores)
are elongated cells narrowed at
their central free end
duced at the opposite
to become continuous
fibres of the optic
former (retinulce) have
extremities much enla
surround the slender ends
retinophores. A layer of
rods (rds.) lies within the retina
proper. The outer wall is thin,
and, with the overlying epidermis,
forms a transparent cornea. In-
the interior of the eye is a clear
rounded lens (I.) of dense cuticular
matter secreted by the cells of
the retina ; this is surrounded
by a less dense body — the vitreous.

The sexes are distinct. There is a single gonad — ovary or testis
as the case may be — lodged in the visceral spiral. The sperm-duct
is a white tube, thickish and much convoluted where it leaves the
testis, narrower and straight distally ; it opens in front in the
mantle-cavity into the proximal end of the sperm-groove, which, as
already mentioned, runs forwards along the right side, and becomes
continuous with the groove traversing the penis. The oviduct
(Fig. 576, ovid.) is proximally a very delicate tube with colourless
transparent walls. This runs forwards to the right side of the
mantle-cavity, where it assumes the character of a stout tube
(ovid.') with thickened glandular walls, which passes forwards close
to and parallel with the rectum, and opens on the exterior near
the anus.

ret

31.— Triton. Section of the eye.
co. cornea ; ep. epidermis ; ?. lens ; o/it.
optic nerve ; rds. layers of rods (the line
is not continued far enough inwards) ;
ret. retina.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Gastropoda are unsymmetrical Mollusca, with a mantle
which is not divided into two lateral portions, and with a shell
which does not consist of two lateral valves, but of a single, un-
symmetrical, usually spirally coiled, valve, enclosing a visceral mass
of corresponding form. There are, typically, two plume-like
ctenidia enclosed in a mantle-cavity, out there may be only one ;
and in air-breathing forms ctenidia are not developed, respiration

PHYLUM MOLLUSCA 683"

:ing place through the wall of the mantle-cavity itself. A dis-
tinct head bearing eyes and tentacles is present in the majority.
The foot is situated behind the head, and usually has an extensive
flattened ventral surface. The buccal cavity contains an odonto-
phore. The nephridium is usually single. The nervous system
contains distinct cerebral and pleural, besides pedal, visceral, abdo-
minal, and buccal ganglia. The sexes are sometimes separate,
sometimes united. The larva passes through trochosphere and

Sub-Class I.— Streptoneura.

'ropoda in which the visceral commissures are twisted into
a fijlre of 8, and in which the sexes are distinct.

ORDER 1. — ASPIDOBRANCHIA.

Streptoneura with the nervous system but little concentrated :
the pedal ganglia produced into long cords with the anterior ends
< >f which the pleural ganglia are fused, the cerebral ganglia wide
apart, the osphradium little developed. The ctenidia, nearly
always present, are plume-like and free distally. The auricles-
and the nephridia are paired.

Sub-Order 1. — Dccoylossa.

Aspidobranchia in which the pleural ganglia are not connested
with the opposite visceral commissure. The eye is in the form of
an open pit, without lens. There are two osphradia, a single jawr
and no operculum. The visceral mass is conical.

This section includes the Limpets (Patellidce).

Sub-Order 2. — EJiipidoglossa.

Aspidobranchia in which each pleural ganglion is connected
with the opposite visceral commissure. The eye is a closed sac
and contains a lens. There is nearly always a single osphradium
(the primitive left), a pair of jaws, and two auricles in the heart.

This sub-order includes the Ear-shells (Haliotidce), Trochusr
Turbo, and others.

ORDER 2. — PECTINIBRANCHIA.

Streptoneura with a somewhat concentrated nervous system.
There is a single osphradium which is often pectinate. The
primarily left ctenidium and nephridium are alone developed.
The heart has a single auricle. The ctenidium consists of a
stem with a single row of lamellaa, attached throughout its length
to the wall of the mantle-cavity.

<>84 ZOOLOGY SECT.

'Sub-Order 1. — Platypoda.

Pectinibranchia with the foot flattened ventrally, at least in
front. Jaws are nearly always present.

This sub-order includes the Cowries, the Vermetes, the Tritons,
the Whelks, the Cones, and a number of other groups.

Sub-Order 2. — Hcteropoda.

Pelagic Pectinibranchia with the foot laterally compr
bearing, at least in the male, a ventral sucker. The visce^
and mantle form only a small part of the mass of the body,
are absent.

Sub-Class II,— Euthyneura,

Gastropoda in which the visceral commissures are not twisted
into a figure of 8, and in which the sexes are united.

ORDER 1. — OPISTHOBRANCHIA.

Marine Euthyneura with aquatic respiration, the auricle of the
heart usually posterior to the ventricle. The mantle-cavity, when
present, opens by a wide aperture.

Sub-Order 1 . — Tectibranchia.

Opisthobranchs provided in nearly all cases with a mantle and
a shell, nearly always with a true ctenidium, and an osphradium.

This section includes the Jiplysiidce, or Sea-hares; and several
•other families, including certain pelagic Gastropoda, some shell-
bearing, some shell-less, formerly regarded as constituting a
distinct class — the Pteropodn.

Sul>- Order 2 . — Nudibrancliia .

Opisthobranchs, which ^ire devoid of shell in the adult condition,
and have no true ctenidia or osphradium, respiration being carried
011 by means of secondary branchiae usually arranged in a circlet
around the anus or in rows on the dorsal surface, or laterally under
the edge of the mantle.

This sub-order includes Doris, Eolis, Tethys, and other shell-less
forms.

ORDER 2. — PULMONATA.

Euthyneura devoid of ctenidia, respiration being carried on
through the walls of the mantle-cavity, which has a narrow
contractile aperture.

This sub-order includes the Land Snails and Slugs.

xii

PHYLUM MOLLUSCA 685

3. GENERAL ORGANISATION.

Systematic Position of the Example.

Triton nodiferus is one of several species of the genus Triton,
which is the only member of the family Tritonidce, belonging to
the sub-order Platypoda. The family Tritonidae differs from
the other families of the sub-order in the possession of a 'pro-
boscis, of a well-developed, but not greatly elongated, siphon, and
of a short foot.

t

Eternal Features, Symmetry, &c. — All the Gastropoda
are GQ a greater or less extent asymmetrical. In the young animal
the mouth is situated at the anterior and the anus at the posterior
extremity of the body. But, as a result of one-sided growth, a
great distortion of the parts takes place, leading to a more or less
pronounced asymmetry. Usually it is the left side which grows a
good deal more rapidly than the right. On the right-hand side
the space between^ mouth and-anus increases relatively little, while
the left side develops rapidly. The result is the ^shifting of the
posterior parts — the anus, the cteniclia, and the nephridial openings,
together with the mantle- cavity in which these are enclosed — over
to the right side of the body, so that those parts come to be situ-
ated sometimes far forwards on that side, sometimes close behind,
sometimes even above (dorsal to) the mouth. Sometimes the
ctenidia, originally situated to the right and left of the anus,
retain this arrangement after the anus has become displaced for-
wards, only that the originally right ctenidium is now on the left
and the originally left on the right ;
but in a large number of Gastropoda
the originally right ctenidium aborts;
and, as will be more particularly de-
scribed later on, the same holds good
of the nephridia and their apertures.
This displacement of the anus takes
place pari passu with the great de-
velopment of the foot, and at the same
time with the outgrowth of the dorsal
region of the body into a great promin-
ence— the visceral prominence — con-
sequent on the great enlargement of FlG. 582._ Sheii of solarium per-
certain of the internal organs, more SJ^SS^oSftr^ifSwS
especially the liver and reproductive • History.)
organs. This visceral prominence is

asymmetrically developed, projecting towards the right side; some-
times, as in the limpets (Patella), it is simply conical ; usually it-
is coiled into a spiral of few or many turns enclosed' in the shell.

-686

ZOOLOGY

SECT.

The shell in the adult limpets (Patella and allied genera) is in
the form of a short cone. In most of the Gastropoda it is in the
shape of a spiral with the turns usually in close contact with
-one another, the inner walls of the turns coalescing to form an
axial, hollow or solid column — the columella. The portion of
the shell projecting inwards between the turns of the spiral
sometimes becomes absorbed. In certain cases, on the other hand,
the cavity of the apical portion of the spiral
may become cut off from the cavity,
rest of 'the shell by the formation of
verse partition, the animal then b
restricted to the basal portion. By
greater number of such spiral shellyare
dextral, i.e., if we begin at the apex of the
spiral, to reach the opening of the shell we
have to pass from left to right with the
columella always on our right-hand side : in
a few cases, however, the spiral is sinistral,
taking the opposite direction from that of
the ordinary dextral shell. The form of the
shell varies with the degree of obliquity with
which the whorls are set on the axis. When
the obliquity is very slight (Fig. 582) the
spiral is nearly flat ; when the obliquity is
great, an elongated tapering shell such as that
represented in Fig. 583 is the result. Some-
times the later whorls completely cover over
the earlier ones, so that the spiral form of
the shell is concealed. Sometimes only the
apical portion of the shell is spiral, the re-
mainder being a straight or sinuous cylinder.
The mouth of the shell has usually a promi-
nent margin or peristome, which is sometimes
entire and continuous, sometimes is broken
by a deep notch or a spout-like process
or canal, formed in connection with the de-
velopment of a spout-like prolongation, of the
mantle, the siphon, which lies in it. The mouth
of the shell in many Gastropoda is capable of
being closed by means of an operculum borne on the foot. In
some terrestrial forms in which an operculum is absent, the
opening may be closed up during winter by a layer of hardened
mucous matter to which the name of epipTiragm is applied. The
margin of the mantle in some cases bears a series of tentacles.
Lateral folds of the mantle are in some of the Gastropoda
(Fig. 584) reflected over the shell and may completely cover it.
In some cases these folds unite by their edge, so that the shell

FIG. 583.— Shell of
Terebra oculata.

XII

PHYLUM MOLLUSCA

687

comes to be enclosed in a complete sac of the mantle ; such
enclosed shells are
always imperfectly
developed and incap-
able of covering the
body. Thus in Aply-
sia and some other
Opisthobranchs the
shell is greatly re-
ducJtffc thin and
hor^Bind concealed
witlJJl the mantle,
while in the nudibranch (Fig. 585) members of the same sub-
order it is entirely absent. The shell is also completely ab-
sent in some of the pelagic forms
(Het&ro'poda, and Pteropoda); in others,
though present and external, it is too

-M

FIG. 584.— Cyprsea moneta (Cowrie). Showing the mantle,
provided with marginal tentacles, partly enveloping the
shell. SI. siphon ; M.M. mantle ; F. foot ; T', tentacles at the
edge of the mantle. (From Cooke, after Quoy and Gaimard.)

FIG. 585.— Doris (Archidoris)
tuberculata. a. anus ; l;\

branchiae; m, penis; /•/<. /•/<,
tentacles. (From the Ca,nh,-''<i>.i<
S< it i' i ~((l History. )

FIG. 586.— Car in aria mediterranea. a. anus;

br. branchia ; /. foot; i. intestine; m. mouth; p.
penis ; s. sucker ; sh. shell ; t. tentacles. (From the
Cambridge Natural History.)

PO

.small to enclose the animal (Fig. 586). In the slugs, among the
Pulmonata, the shell is vestigial and concealed by the mantle

(Fig. 587).

The foot varies in the
extent of its development
in the different families of
the class. It usually pre-
sents an elongated flat
ventral surface on which
the animal creeps by wave-
like contractions of the
muscular tissue. In the
typical Gastropods the foot
is usually distinguishable

middle

688

ZOOLOGY

SECT,

part or mesopodium, which is the most important, with a smaller
anterior propodium and posterior metapodium. In many burrowing
forms (Fig. 588) the propodium is well developed and sharply marked
off to act as a burrowing organ. In a few cases a pair of tentacles

FIG. 588.— Sigaretus Isevigatus, exemplifying great development of propodium (/v.), and
metapodium (met.), in a burrowing Gastropod. The shell has been removed. 1. liver ; s. ap.
aperture of proboscis ; t. t. tentacles. (From the C'amlrid'je Natural Jli^to,-//, after Quoy and
Gaimard.)

—the pedal tentacles — are situated at the anterior end of the foot :
still rarer is a pair of similar appendages at the posterior end.
The whole foot becomes reduced in the few Gastropods that remain
fixed. The metapodium very usually in the Streptoneura bears
a disc or stopper — the operculum — usually horny, rar,ely completely
calcined, more commonly horny with a thin
calcareous investment — by means of which
the aperture of the shell is closed when the
animal is retracted.

In some forms, such as the Sea-hares
(Aplysia) (Fig. 589), the foot develops a
pair of lateral lobes — the epipodia — which
act as fins ; and in the Pteropods (Fig. 590),
which are specially modified for a pelagic
existence, these constitute the largest part
of the foot. In the Heteropoda (Figs.
591, 592) which are also pelagic, the foot
is also modified to act as a swimming
organ. In one family of this sub-order
(Fig. 591) all three parts of the foot are
well-developed, the mesopodium bears a
sucker, and the metapodium an operculum ;
in the rest the mesopodium is alone well
developed and forms a laterally-compressed,
vertically-elongated fin.
A pedal gland is present in the majority: it is a simple or
branched invagination of the integument,4ined by mucus-secreting
cells. Very commonly, as in Triton, it opens on the exterior in
the middle line of the ventral surface of the foot.

Fio. 589.— Aplysia, dorsal
view, r, epipodia. (After
Keferstein.)

XII

PHYLUM MOLLUSCA

689

The Gastropoda have a well-marked head, separated from
the body by a constriction or neck. The mouth, situated at the
anterior end of the head on its ventral aspect, is in many instances

FIG. 590.— Shell-bearing Pteropoda. /./.fins; I. liver; o. ovary; sh. shell. (From Cooke

after Souleyet.)

provided with a protrusible proboscis or introvert, sometimes of
considerable length. On the dorsal surface of the head are a pair
of tentacles which vary a good deal in shape, but are usually
cylindrical or club-shaped. In most cases the eyes are situated on
tubercles at the bases of the tentacles, or elevated towards the

Fio, 501. — Atlanta peronii. a, cerebral ganglia; 6, pedal ganglia; e. eye; g. ctenidia ;
h. heart k. nephridium ; I. liver ; m. mouth ; o. ovary ; p. operculum ; t. male reproductive
gland.

middle ; but in the snails and slugs (Pulmonata) (Fig. 593) the
eyes are elevated on the extremities of a second, longer, pair of
tentacles (pc. tent) placed behind the first.

The mantle is usually developed into a fold — the mantle flap —

VOL. I Y Y

690

ZOOLOGY

SECT.

originally posterior, but subsequently becoming shifted round, in
the course of the displacement already referred to, to the right-
hand side. This covers over a cavity — the mantle cavity—

clcn

sh

FIG. 592.— Pterotrachea scutata. alt. alimentary canal ; cten. gills ; eye, eye ; fl. float ;
mo. mouth ; prob. proboscis ; repr. reproductive gland ; sh. shield covering a portion of the
dorsal surface ; su. sucker.

situated anteriorly, in which are situated the anal and nephridial
apertures and the ctenidia. The edges of the mantle-flap may
become united together in such a way as to form a chamber
opening on the exterior by a comparatively narrow opening.
In many of the Prosobranchia the edges of this aperture are
drawn out into a spout-like prolongation open ventrally — the
siphon — which lies in the corresponding prolongation of the peri-
stome of the shell, and serves as a channel for the ingress and egress

oc.tenl

lent

pulrrt

FIG. 593. — Helix nemoralis. an. anus ; gen. ap. genital aperture ; oc. tent, posterior eye-bearii
tentacles ; pulm. opening of pulmonary sac ; tent, anterior tentacles. (After Pelseneer.)

of water. In some Gastropods, however, there is no defmii
mantle-cavity, the anus, nephridial apertures, and ctenidi*
merely lying under cover of a comparatively slight ly-developec
lateral mantle-flap. Usually there is on the inner surface of th<
mantle a glandular area — the pallial mucus gland.

Respiratory Organs. — There are normally two ctenidia, one 01
the right side and the other on the left, contained in the mantle-

PHYLUM MOLLUSCA

691

cavity; but in the majority of the Streptoneura and bran'chiate
Euthyneura the primitively left (actually right) ctenidium alone is
retained. In those Gastropoda that possess two ctenidia, and in
many forms with only one, the axis of the ctenidium bears two
rows of compressed filaments, and is attached only towards its
base. But in the majority of those with one ctenidium there
is, as in Triton, only a single row of filaments retained, and the
organ is attached throughout its length.

In the Nudibranchs true ctenidia are absent, but their place
as birthing organs is taken by a number of secondary branchiae,
sometimes simple, sometimes branched or pinnate processes, which

ve

771.0

$.0-

FIG. 594.— Pleurophyllidia lineata.

from the ventral surface, a. anus :
br. secondary branchiae ; m. mouth ;
.«. o. sexual opening. (From the Cam-
bridge Natural History.)

FIG. 595.— Patella vulgata, seen from the ven-
tral side. /. foot ; <j. I. circlet of gill lamellse ;
m. e. edge of the mantte^ m u. attachment muscl4 ;
si. slits in the attachment muscle ; sh. shell ;
v. efferent branchial vessel ; v'. aorta ; ve. smaller
vessels. (From the Cambridge Natural History.)

are- distributed over the dorsal surface, as in JEolis, or, as in
Doris (Fig. 585), form a circlet surrounding the anus, or, as in
Pleurophyllidia (Fig. 594), a row on each side beneath • the
mantle-flap.

In the limpets (Patella and its allies) (Fig. 595) the true
ctenidia are represented only by a pair of vestiges, and respiration
is carried on by a number of secondary branchiae (g. I.) in the form
of lamellae situated between the short lateral fold of the mantle and
the foot. In the Pulmonata, and in some members of other groups,
ctenidia^ are absent, and the mantle-cavity, completely enclosed
except for a small rounded opening, has the function of a pul-
monary sac or lung (Fig. 596), its roof being richly supplied

Y Y 2

692

ZOOLOGY

SECT.

picl.ap

red

-vent

itr

neph

FIG. 596. — Pulmonary cavity and related parts in a slug
(Itimax). aort. aorta ; aur. auricle ; neph. iiephridium ;
peric. pericardium, laid open ; pul. ap. pulmonary aperture ;
pul. «. pulmonary vein with its ramifications ; rect. rectum ;
ur. ureter; vent, ventricle. (After Pelseneer.)

with blood-vessels : in the aquatic forms its function is appar-
ently as much hydrostatic as respiratory. In some of the Pulmon-
ata there is a return to a completely aquatic mode of respiration
accompanied by the development of secondary gills — vascular

processes of the
^ wall of the mantle-

cavity.

Near the base of
each ctenidium is
an elevation — the
osphradium— cor-
responding to the
body of that name
in other Mollusca
and having a similar
function.

Digestive Or-
gans. — In many
Streptoneura there
is a long introvert
capable of being
everted and re-
tracted, at the ex-
tremity of which the mouth is placed. A single curved horny
jaw lies on the roof of the buccal cavity in the Pulmonata; in
most Streptoneura (as in Triton) the place of this is taken by
two lateral pieces.

A characteristic feature of the alimentary canal of the Gastro-
poda, which, however, they share with some Amphineura and wit!
the Cephalopoda, is the possession of an odontophore and radula
& typical example of which has been described in that of Triton.
In the different groups differences are observable in the odontc
phore as regards the proportions of the parts, and the size, form,
and arrangement of the teeth. The arrangement of the alimentai
canal is similar to what has already been described in Triton, anc
.salivary glands and liver (hepato-pancreas) are always present.
The former may be tubular, but are usually .botryoidal. Th<
latter varies in its extent and the arrangement of its lobes
different forms.

In some Opisthobranchia the stomach contains a series "of teeth
which are sometimes sharp and chitinous, sometimes plate-like and
calcined. Frequently a special development of a cuticular lining
of the stomach forms a hard rod — the crystalline style — lodged ii
a coscum and comparable to the body of the same name in th<
Pelecypoda (p. 655). A pyloric coecum is frequently appended t(
the stomach. The intestine is long and thrown into folds in th(
vegetable-feeding forms, short and straight in the carnivorous.

XII

PHYLUM MOLLUSCA

093

In some cases, e.g., ffaliotis, it traverses the ventricle, in others
the pericardium ; in others it passes through the nephridium. In
Eolis (Nudibranchia) the stomach gives off a number of glandular
coeca which penetrate into the interior of the secondary branchia?
or cerata on the dorsal surface ; these coeca take the place of the
liver of other Gastropoda. In some of the Pectinibranchia there
is a peculiar adrectal gland situated at the side of the rectum
secreting a colourless fluid which in Murex and Purpura turns
purple on exposure to the air, and was anciently used as a dye—
the " Tyrian purple."

The heart is as in other molluscs enclosed in a special cavity —
the pericardium — a specialised part of the ccelome, communicating
with the cavity of the nephridia. It consists usually, as in
Triton, of two chambers — auricle and ventricle — but in some,
e.g., Haliotis, there are two auricles and a ventricle. In the
Opisthobranchia, as already mentioned, it lies in front of the
ctenidia ; in the Streptoneura at the side or behind. Given off
from the apex of the ventricle is a large vessel which soon bifur-
cates to form anterior and posterior aortae. These are the main
trunks of the arterial system, which is more highly developed than
in the Pelecypoda; the finest branches terminate in sinuses as in
the latter class.

The nervous system varies considerably in the different
groups in regard to the ar-
rangement of the ganglia
and their commissures.

In the majority the ar-
rangement is nearly that
which has been described as
occurring in Triton. There
is a pair of cerebral ganglia
usually closely united, but in
Patella (Fig. 597) widely
separated, placed over the
gullet, and giving off behind
a pair of nerve-cords — the
visceral nerve-cords — in the
course of which there is placed
laterally a pair of pleural

ganglia, and Which are United
4. * l,i i i • i • 7-

together behind in a median

abdominal ganglion (or two
as in Triton). In the course

of these visceral cords there is a pair of visceral ganglia. A
pair of pedal ganglia united • together by a transverse com-
missure and joined to the cerebral ganglia by connectives,
give off behind one or two pairs of pedal nerves as already

Y Y 2*

cer.g

FlG< 597._Nervous system of Patella% ^

cerebral ganglia ; want, n., mant. n. mantle
nerves; ospli. g. osphradial ganglia ; und. rt
pedal ganglia f pe<t. n. pedal nerve cords-

2*.* pleural gang&on. (After sPengei.)

694

ZOOLOGY

SECT.

mentioned. A pair of buccal ganglia are connected by slender
nerves with the cerebral. At the base of each osphradium is
usually a small osphradial ganglion connected by a slender nerve
with the visceral. In all the Streptoneura (Fig. 598), in accord-
ance with the displacement of the anus and the coiling of the

r.visc.com

r.ct&n.

l.clen

visc.com-

FIG, 598. — Diagrammatic representation of the displacement of the mantle-cavity and associatec
parts in the Gastropoda. Enteric canal blue, blood-system red. A, rej

symmetrical arrangement ; in B, C, I), are represented successive stages of displacement of
the mantle-cavity to the right and forwards ; in E, the anus and (primarily) right ctenidium
have passed the middle line. an. anus ; aort. aorta ; csr. g. cerebral ganglion ; /. cten. left
ctenidium ; /. cixc. com. left visceral commissure ; mant. mantle ; mo. mouth ; neph. up. nephri-
dial apertures ; pe<l. ;/. pedal ganglion ; pi., g. pleural ganglion ; /•. cten. light ctenidium ; r.
vise. com. right visceral commissure ; rixc. com. visceral commissures. (After Korschelt and
Heider.)

visceral mass, the visceral cords are twisted, as already descrii
in the case of Triton, into a figure of 8.

In Patella (Fig. 597) the pedal ganglia give origin to a paii
of elongated pedal nerve-cords. In Haliotis and Fissurella there
is a similar pair of pedal cords which are connected together by

PHYLUM MOLLUSGA

695

transverse commissures, and, in the latter genus, join together

posteriorly.

In the Euthyneura (Fig. 599) except in

Actceon and Chilina, the visceral cords are not

caught up in the twist of the visceral mass,

and the crossing of the cords does not occur.
In the Snails and other Pulmonata (Fig.

600) the ganglia of the nervous system are

more closely aggregated together. A pair

of cerebral ganglia overlie the oesophagus,

and below it is a mass of ganglia in which

are to be made out a pair of pedal and at

least two pairs of ganglia representing the

visceral and pleural. A pair of small buccal

ganglia are connected with the cerebral by

means of slender nerves.

The organs of special sense are the

eyes, the otocysts, and the osphradia. In

nearly all cases there are two cephalic eyes

(Fig. 601), the position of which has already

been referred to in the account given of the

external characters. In structure they are

the simplest in Patella (A), where each

consists of a pit-like depression, lined by

pigmented cells connected with nerve-
fibres. In the majority they have the

structure described in the case of Triton.

In certain species of Onchidium, a littoral Pulmonate, there are

numerous eyes of a simple type scattered over the dorsal surface.

In this case the optic
nerve pierces the retina
and the cells of the latter
have their free ends direct-
ed away from the centre of
the eye, as in Pecten (see
p. 658) and the Vertebrata,
instead of towards it, as in
other Mollusca. The in-
ternal cavity of the eye is
occupied by a refractive
body composed of a few
large transparent cells
The otocysts are usually
placed in close relation to

FIG. Goo.-xervous system of Limnseus (Puimo- the pedal ganglia, but are

nata). aba. ft. abdominal ganglion ; cer. g. cerebral 1 i . ° ° ,

ganglion; o»i>hf. <j. osphradial ganglion; ped. y. always innervated trom

geSUTlgli°n ; Pl' '* PlOUral gangli°n' (After the cerebral. An olfactory

I'm. .">:>!). — Xervous system
of Aplysia (Opistho-
branchia). a/itf. abdo-
minal ganglion ; cer. </.
cerebral ganglion ; oxj>/t.,:
(j. osphradial ganglion ;
1 1" i. ;/. pedal ganglion;
pi. g. pleural ganglion.
(After Spengel.)

696

ZOOLOGY

SECT.

organ is present in the shape of groups of cells, in which the
fibres of an olfactory nerve terminate, situated on the tentacles.

FIG. 601.— Eyes of Gastropoda. A, Patella; B, Trochus } C, Turbo; D, Murex.
ep. epidermis ; 1. lens ; op. n. optic nerve ; r. retina ; r. h. vitreous humour. (From the
Cambridge Natural History, after Helger.)

The osphradia are prominences, usually of simple form, situated
close to the base of the ctenidium. In many of the branchiate
Streptoneura (Fig. 602), as already mentioned in the case of Triton

(see p. 676, Fig. 576), the
right osphradium, which is
alone developed, assumes
the form of a pectinate
body with a central ridge,
on either side of which is
a row of close-set lateral
laminae, and is commonly
termed the parctbrancJiia
from its resemblance in
appearance to a gill. In
some cases it is of even more complicated shape than in Triton,
owing to the branching of the lateral ridges.

FIG. 602. — Transverse section of osphradium of
Murex. br. n. branch nerve passing to lamina ;
lam. laminae ; osphr. n. main, osphradial nerve.
(After Spengcl.)

XII

PHYLUM MOLLUSCA

697

The nephridia of the Gastropoda are dorsally placed glandular
tubes or chambers, which communicate internally with the peri-
cardium, and open on the exterior close to the anus, either
directly or through a duct — the ureter. Both right and left neph-
ridia may be present, though unequal in size, the one situated to

to

the
In a

rec.sem

the right of the anus being larger than that situated

left; or the former may alone be developed (j&uthyneura).

very limited number

of Gastropoda the

gonad opens into the

nephridium.

The sexes are separ-
ate in nearly all the
Streptoneura, united
in the Euthyneura.
Special gonoducts are
present, except in one
or two forms in which
the nephridia perform
that function. In the
unisexual forms the
reproductive apparatus
is of a comparatively
simple character, con-
sisting merely of a
racemose reproductive
organ, ovary or testis,
as the case may be,
situated dorsally in the
visceral spiral, with the
gonoduct opening far
forwards on the right-
hand side, and, in the
male a penis, which is
grooved longitudinally
and non-retractile. In
the hermaphrodite
forms, such as the
Pulmonata (Fig. 603),
on the other hand, a
considerable degree of complexity is observable. There is a
hermaphrodite gland (herm. gl., Fig. 604, A\ some of the follicles
of which produce ova, while others produce sperms, a convoluted
hermaphrodite duct (herm. d.\. an albumen^ gland, in which
the albumen of the relatively large eggs is formed, separate
oviduct and spermiduct leading to a common genital opening,
In addition there is a receptaculum seminis (i'cc. sem.) connected

r&c.s&m.ap NT^/Z

FIG. 603. — Reproductive organs of Helix, alb. gl. albumen
gland ; d. s. dart sac ; flag, flagellum of the penis ;
herm. <il. hermaphrodite gland ; kcrm. d. hermaphrodite
duct ; nmc. gl. mucous gland ; muc. gl. ap. apertures of
mucous glands into vestibule ; ovid. oviducal part of the
common duct ; orid. ap. aperture of oviduct into vesti-
bule ; pen. penis ; rtc. sem. receptaculum seminis ; rcc.
sem. ap. aperture of receptaculum seminis ; sp. d. sperm
duct ; sp. d'. spermiducal part of common duct. (-After
Pelseneer.)

698

ZOOLOGY

SECT.

FIG. 604. — Follicles of the hermaphrodite gland of the Gas-
tropoda. A, of Helix hortensis (Pulmonata) ; B, of
the Eolidac. a. a ova ; b. masses of sperms ; c. common
efferent duct. (From Gegenbaur.)

with the oviduct, a number of narrow accessory oviducal glands
(muc. #/.), frequently a gland termed prostate connected with the
spermiduct, an eversible sac — the sac. of the dart (d. s.) — contain-
ing a crystalline stylet, and a penis (pen.}, which is perforated by a

canal, and is capable
of being retracted by
a special muscle. The
duct may be simple
or may be incom-
pletely divided longi-
tudinally into two
parts. In the Pul-
monata the first part
(hermaphrodite duct
proper) is simple, and
serves for the passage
of both ova and
sperms ; the middle
part is incompletely
divided internally into
two passages, one serving as oviduct, the other as spermiduct. In
the distal part oviduct and spermiduct are completely separate.
Where the spermiduct enters the penis, there is given off a long,
slender, tapering diverticulum, the flagellum (flag.), in which the
sperms are made up into elongated masses or spermatophores.

Development. — The limpets (Patella) are exceptional in laying
the eggs one by one and unfertilised — impregnation taking place
in the water after they have been discharged. In almost all the
Gastropoda fertilisation is internal, and the eggs are laid in great
masses, embedded in jelly — each egg having its own hyaline
envelope. Very often the mass of spawn consisting of the jelly-
like substance, with the eggs embedded in it, attains a relatively
considerable' size. Inform it varies greatly: very often it is in
the shape of long strings which are cylindrical or band-like : some-
times several such strings are twisted together into a cord.
Sometimes the spawn is fixed to sea- weed or other objects ; some-
times it 'is unattached, and may float about freely. In the
Streptoneura (Fig. 605), instead of a jelly-like mass, the eggs are
enclosed in a firm parchment-like capsule, in which is contained,
in addition to the eggs, a quantity of an albuminous fluid, serving
to nourish the developing embryos. The shape of the capsule
(Fig. 605) varies greatly in the different genera ; sometimes it is
stalked, sometimes sessile ; in some cases there is a lid or oper-
culum, the opening of which permits of the escape of the embryos.
Very commonly large numbers of these capsules are aggregated
together, and usually they are attached to a rock or a sea-weed,
or similar object. In many cases only a limited number, some-

XII

PHYLUM MOLLUSCA

699

times only one, of the embryos contained in the capsule become
developed — the rest serving as nutriment for the survivors.

In the land Pulmonata each ovum is sometimes embedded in
gelatinous matter enclosed in a firmer envelope, and a number of
them are arranged in a string ; sometimes a large number are em-
bedded in a rounded gelatinous mass. Usually, as in Helix and other
genera, the outer layers of the albumen-like substance enclosing
the egg become toughened and impregnated with salts of lime,
so as to assume the character of a calcareous shell ; a number
of such eggs, which are of relatively considerable size, are laid
in holes excavated in the
earth.

In a few marine and
fresh- water Gastropoda the
ova undergo their develop-
ment in the body of the
mother, enclosed in an
enlargement of the ovi-
duct which serves as a
uterus.

The egg contains a con-
siderable quantity of food-
yolk, which may be evenly
distributed, or a clear pro-
toplasmic and an opaque
yolk-laden segment may
l>e distinguishable. There
is a fairly close agreement
throughout the class in
the nature of the segmen-
tation (Fig. 60G). In all
eases it is total, sometimes
equal at first, but soon
afterwards becoming un-
equal. The first four
blastomeres are usually
equal or nearly so; they

are so arranged that two of them are in contact in the middle,
and thus separate the other two : the line of contact of the
former pair becomes the transverse axis of the embryo.

From the four first-formed cells four small cells — micromeres —
become' constricted off, the larger cells being the megameres;
then four more micromeres are constricted off, and again the
same process is repeated. The embryo now consists of the four
megameres and twelve micromeres. The latter then increase by
division and form a cap of small cells (ectoderm) on the surface of
the megameres. The whole process, as will be noticed, has a

FIG. C05. — Forms of egg cases in Gastropoda. A and
J), Pyrula or Busycon J B, Conus ; C,
Voluta musica; E, Ampullaria. (From

iji. y<(ti'.i-<U History.)

700

ZOOLOGY

SECT.

remarkably close resemblance to the process of segmentation of
the ovum of a Polyclad as described on p. 257.

The megameres then give off internally four small endoderm
cells, and from one of these (endomesoderm cell) are formed two
primitive mesoderm cells, from which the cells of the mesoderm
are developed The megameres themselves eventually become
converted into endoderm cells. A segmentation-cavity is de-
veloped between the micromeres and the megameres, and the

jn.es

FIG. 606. — Diagram of the segmentation and formation of the germinal layers of the Gastropoda-
A and ^.lateral view; C—F, viewed from the animal pole; //, from the vegetal pole:.
6r, in optical section ; ect. ectoderm ; end. endoderm ; rues, mesoderm ; pol. polar bodies,
(After Korschelt and Heider.)

result is the formation of the blastula, one side of which (vegetal
pole) is greatly thickened owing to its consisting of the large
megameres, the opposite side (animal pole) being made up of the
micromeres. This may become a gastrula by epiboly or over-
growth of the ectoderm over the megameres, or, if the segmentation-
cavity is of considerable size, an invagination takes place.

The two larval stages, the trochosphere and the veliger, are
characteristic of the development of the Gastropoda. The former

PHYLUM MOLLUSCA

701

is most typically developed in Patella ; in other Gastropods it
undergoes more or less modification. In Patella (Fig. 607) there
is a ciliated blast ula (A) which has on one side the large megameres.
The latter become enclosed by the micromeres, and the foundation
of the mesoderm is laid in the manner already described. The

B

FIG. 607.— Earlier stages in the development of Patella. A, blastula ; B, beginning of endo-
dermal invagination ; C, completion of gastrula ; Z>, frontal section of somewhat later stage ;
ap. pi. apical plate ; bl. blastopore ; endm. endomesoderm cell ; end, endoderm ; mes. meso-
derm ; mesent. mesenteron ; pro to. prototroch ; sh. gl. shell-gland. (From Korschelt and
Heider, after Patten.)

blastopore is situated at the vegetal pole, destined to become
the hinder end of the larva. The blastopore soon changes its
position and extends forwards on the ventral side, and a ciliated
ring — the prototroch or future velum — -becomes formed. Subse-
quently the position of the blastopore becomes still further
shifted; it becomes U-shaped and then slit-like. It undergoes

702

ZOOLOGY

SECT.

elongation (Fig. 608, A) and eventually becomes partly closed up,
the closure taking place from behind forwards ; the most anterior
part remains open to form the mouth, or, perhaps more correctly,,
there is in the position of the anterior part a sinking-in of the
ectoderm, which pushes the blastopore inwards and forms the
rudiment of the stomodreum. The originally solid mass of
endoderm develops a lumen, and its cells become arranged to
form the enteric epithelium. From the posterior end, where
the mesoderm cells are situated, proceed two very regularly
formed mesoderm streaks (Fig. 608, B). On the dorsal surface
the shell-gland has already appeared as a pit lined by elongated
ectoderm cells ; on the surface of this appears the embryonic

FIG. 60S.— A and B, Trochospheres of Patella at different stages. In A are to be seen the
circular blastopore and the two foot-elevations ; in B the blastopore is drawn out, at the side&
of it are the two mesoderm bands. (From Korschelt and Heider, after Patten.)

shell. The rudiment of the foot (Fig. 608, A} appears at a re-
markably early stage as two protuberances lying on the ventral
side of the posterior end of the larva at the sides of the blastopore ;
these coalesce to form the median foot.

The larva (Fig. 609) has now assumed the trochosphere form. The
prae-oral part is large and convex, with an apical plate on which is
borne a bunch of long cilia, and near it two small ciliated elevations,
each consisting of a single cell. The prse-oral part of the larva
then becomes much flattened, and the apical plate (ap.pl.) increases
in size and importance. At the posterior end is a bunch of cilia
which are borne on two special large cells, the anal cells (an.c.\ The
embryonic shell becomes saucer-shaped. A slight ridge in the
neighbourhood of the shell represents the border of the mantle.
The mid-gut (mesenl) has become considerably widened : a

XII

PHYLUM MOLLUSCA

703

me sent

cin.c

diverticulum from it becomes recognisable, this afterwards opens
on the exterior to form the anus. A diverticulum of the fore-
gut (rad.) at the same time forms the rudiment of the radular
sac. The otolith-sacs appear as depressions of the ectoderm at
the sides of the
mouth : these grow
inwards and become
sac-like, subsequent-

ly lying at the sides JnillJJiUle^ ra.fl

of the foot, which
has meantime at-
tained a considerable
size.

The trochosphere
stage, which is so
well marked in the
case of Patella, oc-
curs in other Gastro-
pods,though,as a rule,
presenting modifica-
tions perhaps trace-
able to the enclosure
of the embyro in an
egg-shell and to the
presence of much
food-yolk. The his-
tory of the blastopore
is not the same in all
cases ; in some, the

mouth is developed from its anterior portion : in others the
stomodaeal invagination arises after its complete closure. In most
of the Gastropoda the prae-oral circlet or velum (Fig. 610, vel.)
undergoes a development not observable in the Pelecypod embryo,
and becomes greatly extended as a bilobed flap, the strong cilia
with which it is bordered rendering it a very efficient organ of
locomotion for the larva. With the full development 'of the velum
the larva passes into the Veliger stage (Fig. 610). In this stage
the shell (sh.) increases in size, loses its simple form, and begins
to develop a spiral. A cleft-like depression in the border of the
mantle on the right-hand side forms the rudiment of the mantle -
cavity in which, later, the gills are developed. The anus when
it first appears is symmetrically placed, but later becomes shifted
to the right side and forwards as well as dorsally. The foot (/.)
may attain a considerable development during the Veliger stage.
On its posterior and dorsal part appears the operculum. Two
little processes on the velar area develop into the tentacles (tent.),
and the eyes (cy-} appear at their bases. As the foot and other

FIG. 609.— Later trochosphere of larva of Patella in longi-
tudinal section, an. c. anal cells with cilia ; ap. pi. apical
plate ; /. foot ; mes. mesoderm cells ; mesent. mesenteron ;
mo. mouth ; rad. rudiment of radula sac ; sh. shell. (From
Korschelt and Heider, after Patten.)

704

ZOOLOGY

SECT.

organs advance in development the velum decreases in size and
gradually aborts. In some cases a portion of it persists as the
subtentacular lobes or labial tentacles in the neighbourhood of
the mouth.

In the Pulmonata the velum is not well developed, except in
Onchidium, though the trochosphere stage is well marked.

The young Gastropod is at first bilaterally symmetrical; the
prevailing asymmetry is the result of unequal growth of the two
sides of the body. In the majority of cases it is the left side that
grows more actively than the right, a result of which is that the
posterior parts — the anus and the parts surrounding it — are dis-
placed forwards towards the right, the space between the anus and

lent

-vel

terit

sh

FIG. 610.— Veliger stage of Vermetus. «/•. y. cerebral ganglia ; qi. eye ; f. foot ; mo. mouth ;
ot. otocyst ; sh. shell ; tent, tentacle ; vd; velum. (After Lacaze-Duthiers.)

the mouth on that side undergoing little or no increase in length.
In the Opisthobranchia and the Pulmonata the anus with the
mantle-cavity and its contents become displaced forwards to the
neighbourhood of the anterior end; in the Streptoneura the
anus, etc., in their displacement forward pass beyond the middle
line, one of the most striking effects of which is the crossing of the
pleuro-visceral commissures, already referred to (p. 694).

Ethology and Distribution.— Only a few aberrant families of
Gastropoda are parasites. Most are aquatic, all the most primitive
forms being inhabitants of the sea. Of the marine families the
majority move by creeping over the sea-bottom, some burrowing
in mud or sand, some in solid rock ; some are able to float in
.a reversed position, adhering to frothy mucus secreted by the

XII

PHYLUM MOLLUSCA

705

glands of the foot ; certain exceptional forms such as Vermetus
are fixed in the adult condition by the substance of the shell.
A few families — the Heteropoda and the Pteropoda — are specially
modified for a pelagic mode of existence, and swim through the
water by flapping movements of the lobes of the foot, which act
as fins. Gastropods are found at considerable depths — up to
nearly 3,000 fathoms — in the ocean. Many forms, however, are
inhabitants of fresh water, while many Pulmonata are terrestrial,
and occur even towards the summits of the highest mountains.

Fossil Gastropoda are known from almost the earliest fossil-
bearing rocks, and all the major divisions of the class are repre-
sented in formations of paleozoic age.

The mutual relationships of the various groups of Gastropoda
are shown in the following diagram (Fig. 611):—

Heferopoda

Pulmona^

Tech'branchia

Nudibranchia

Rhi^idoglossa

Docoglossa

Scajahofooda

FIG. 611. — Diagram to illustrate the relationships of the Gastropoda.

APPENDIX TO THE GASTROPODA.

A. CLASS IV.— SCAPHOPODA.

The Scaphopoda or Elephant's tusk shells are aberrant marine Molluscs
comprising only three genera — Dentalium, Siphonodentalium, and Pulsellum. The
body is elongated, so as to be
almost worm-like, with complete
bilateral symmetry. The mantle-
folds are almost completely united
to form a cylindrical tube en-
closed by the shell (Fig. 612),
which is in the form of a delicate,
curved tube, open at both ends
and wider at one end — the ante-
rior or oral — than at the other.

The foot (Fig. 613, /) is narrow, trilobed at the extremity, capable of being pro-
truded through the oral opening of the shell, and used for burrowing in sand.
The mouth is situated on a short oral proboscis, and is sometimes surrounded by
VOL. I Z Z

FIG. 612.— Dentalium, longitudinal section of
shell. (After Keferstein.)

706

ZOOLOGY

SECT.

lobed processes or pinnate palpi. Further back are a pair of tentaculiferous lobes,
each bearing a large number of filiform tentacles, which are probably respiratory
in function. The mouth leads into a buccal cavity
containing an odontophore. Connected with the mesen-
teroii is a large bilobed liver (I. ). The anus is situated
ventrally behind the base of the foot. The vascular
system is extremely simple, consisting of sinuses without
definite walls, and there is 110 distinct heart, though in
the neighbourhood of the rectum there is a specially
contractile part of the principal sinus. Two iiephridia
open near the anus, the right one acting as a goiioduct,
the left (k) entirely renal in function. The sexes are
distinct. There is an elongated unpaired goiiad ((/),
divided by lateral incisions into a number of lobes,
occupying all the posterior and dorsal parts of the body.
Anteriorly it narrows to form a duct opening into the
right nephridium.

The nervous system consists of paired cerebral, pleural,
pedal, and visceral ganglia ; the cerebral ganglia are
situated close together. There are no eyes or otocysts.

In the gastrula stage the embryo, which is provided
with cilia, becomes free. The ciliated cells are arranged
in a characteristic manner in three rows which, at first
situated close together about the middle of the body,
become shifted at a later stage nearer the apical pole,
and amalgamated into a broad band representing the
pree-oral circlet of other molluscaii larva? ; at the same
time a bunch of cilia previously developed at the apical
FIG. 613.— Dentalium, pole becomes more conspicuous, and a considerable part
a^rture 'of ma^tle^T °f the §eneral surface becomes covered with more
foot ; g. gonad ; k, neph- delicate cilia. The blastopore, at first terminal, becomes
ridium ; /. liver. (From shifted forwards on the ventral surface until it comes
S^SrSr ilSf. to be immediately behind the ciliated circlet. At its
Duthiers.) anterior end an imagination gives rise to the mouth an

stomoda?um.

The larva (Fig. 614) is now attaining the stage of a trochosphere, in whk
however, both apical plate and primitive nephridia are wanting. A shell-giant

FIG. 614.— Veliger of Dentalium. A, longitudinal section of a larva 14 hours old ; B, larv;
. of 37 hours ; C, longitudinal section of larva of 34 hours ; -/,*. mouth ; v. r. velum. (Fror
Cooke, after Kowalewsky.)

is developed, and soon the rudiment of the shell. The post-oral region, at firs
inconsiderable 'n size, soon undergoes an increase, until it forms eventually

PHYLUM MOLLUSCA

707

-mth

far the longest part of the body, while the pr;«-oral region almost completely
aborts. When the post-oral region has attained a certain size, there are
developed on it two lateral folds, the rudiments of the mantle (B), which grow
inwards towards the middle ventral line, and later on unite by their free
margins. The prse-oral circlet or velum changes its
form at first it is conical, later it becomes plate-like,
and then gradually becomes reduced, the larva sinking
to the bottom, and though still occasionally swim-
ming with the aid of the velum, coming to use the
foot as a creeping organ. The shell now increases
in size step by step with the growth of the mantle,
and bends round the body of the larva until its edges
meet and coalesce in the ventral median line. Later
it assumes the elongated conical form, curved towards <*

the dorsal side, characteristic of the adult. The foot
at the same time elongates and takes on the character-
istic three-lobed shape.

-salgl
-hue

int

int-

B. RHODOPE.

Rhodope (Fig. 614, bis) is a minute, elongated,
fusiform animal, ciliated externally with complete
(external) bilateral symmetry. There is no shell, but
within the body-wall, in the parenchyma between it
and the enteric canal, are numerous irregularly shaped,
calcareous spicules. There are no jaws or odonto-
phore. The enteric canal, which is a narrow tube,
consisting of buccal cavity, with salivary glands,
oesophagus^ mid-gut, with a coecum, and rectum,
opens in an anal aperture situated to the right of the
posterior extremity of the body. A liver is absent.
The central part of the nervous system consists of
two ganglia situated close together above the O3so-
phagus, and a single ganglion below. A pair of lateral
nerve cords run backwards from the posterior of the
two upper pairs of ganglia. There are a pair of eyes
and a pair of otocysts situated close to the posterior
upper ganglia.

The nephridial system consists of a chamber open-
ing on the right side in front of the anus ; into this
open nine or ten flask-shaped flame-cells similar to
those of the Flat-worms.

There are 110 blood-vessels, and specialised organs
of respiration are also absent.

The sexes are united. The gonads consist of about
twenty ventrally situated masses of cells, the anterioi
being ovaries and the posterior testes. There is a
common duct receiving the products of all the gonads :
anteriorly this divides into spermiduct and oviduct
with separate apertures situated on the right side, the
spermiduct with a muscular penis, the oviduct with a
receptaculum seminis and an accessory gland.

There is no metamorphosis, and the larva is not provided at any stage with
any representatives of either shell-gland or foot.

Though the occurrence of flame-cells is unique there can be little doubt that
Rhodope is best regarded as a degenerate member of the . Mollusca, and
probably finds its nearest relatives among the Gastropoda.

K;. OH, bi«. — Rhodope
veranii. General view.
The scattered curved
bodies are the spicules.
<J ap. male aperture ;
? up. female aperture ;
luc. buccal cavity ; Kr».
central nervous system ;
coec. coecum ; int. intes-
tine ; utt /i. mouth ; ov.
ovary ; snl. <//</. salivary
gland ; tt. testes. (After
von Graff.)

Z Z 2

708 ZOOLOGY SECT.

Class V.— CEPHALOPODA.

The Cephalopoda, including the Cuttle-fishes, Squids, Octopi, and
Nautili, are marine Mollusca of a high grade of organisation.
There is a very definitely-formed head, bearing a pair of highly-
developed eyes, and surrounded by the anterior portion of the foot,
modified into arms or tentacles. The body is bilaterally symmetrical.
The posterior part of the foot is modified to form a funnel leading
out from the large mantle-cavity. A shell is sometimes present,
sometimes absent. When present it is usually internal, but
sometimes external and in the Nautili is capable of containing
the body of the animal.

1. EXAMPLES OF THE GLASS.
i. THE CUTTLE-FISH (Sepia1).

Cuttle-fishes are marine Molluscs, which live usually at a depth
of a few fathoms, but often come into shallower water, and are
frequently caught in the trawl or the seine. The animal arrests
attention when compared with Unio or Triton by the strength,
and more particularly by the rapidity, of its movements ; by the
possession of a pair of eyes resembling in size and complexity
those of a Fish ; and by various other features, all pointing to
a higher grade of organisation than is attained by the members
of the classes of Mollusca dealt with in the preceding pages.

External Features.— The Cuttle-fish (Fig. 615) has a distinct
head, bearing ten long arms, and a pair of large, highly-developed
eyes. The head is connected with the body by a constricted
region or neck. The trunk is elongated and shield-shaped, the
base of the shield being towards the head. The long axes of
head and trunk are in line with one another. Not only the head,
but also the trunk, are completely equilateral, in which respect
there is a marked contrast to Triton ; and this symmetry extends
to most of the systems of internal organs. The free extremity of
the head bears the mouth, and is termed, accordingly, the oral
extremity, the opposite extremity, the apex of the shield-shaped
body, is the aboral end. The surfaces of the shield are anterior
or antero-dorsal and posterior or postero-ventral, its borders right
and left. The anterior surface is to be distinguished by its darker
colour, and by the firmness of the body-wall, due to the presence
in this position of a hard internal shell.

1 Most of the figures have reference to a common Australian species — S. c
— but the differences between the various species of the genus are slight and
unimportant, and the description given will apply fairly well to any other
species.

XII

«;

PHYLUM MOLLUSCA

709

The aperture of the mouth is surrounded by the bases of the
n anus. These are in pairs, situated to the right arid left of the
median plane. All of them, with the exception of the fourth pair
{the most anteriorly situated pair being reckoned as the first), are
stout at the base and taper towards the extremity. When extended

hey are about two-thirds of the

ength of the body. The outer
surface of each (i.e. that turned
away from the mouth) is strongly
convex, the inner flat, and beset
throughout its length with a
number of suckers (Fig. 616),
which are arranged in four longi-
tudinal rows. Each sucker is in
the form of a shallow cup. sup-
ported on a short, thick stalk (st.).
The lip of the cup is membran-
ous, and immediately within it
is a narrow, horny rim (dent.). In
the middle of the bottom of the
cup is a thick plug or piston (pi.),
which is capable of being drawn
downwards by the contraction of
muscular fibres inserted into it,
the effect, when the rim of the
sucker is in contact with some
solid object, being to create a
partial vacuum in the cavity,
resulting in firm adhesion, owing
to the pressure of the surround-
ing water. The fourth pair of
arms, usually known as the ten-
tacles, are comparatively long
and narrow, and provided with
suckers only towards their free
ends, which are somewhat
thickened and clublike. In the
male the fifth arm on the left
side presents a slight modifica-
tion, some of the suckers being

absent. This is an indication of a change termed Jiectocotylisation,
which, as will be pointed out in the general account of the class,
assumes in some cases a very remarkable character. From their
nerve-supply the arms of Sepia prove to be the equivalents of
the fore-foot of Triton, which here becomes extended to the region
of the head, and divided into a series of processes or. arms
encircling the mouth.

FIG. 615.— Sepia cultrata. Entire animal
viewed from the dorsal aspect.

710 ZOOLOGY SECT.

The trunk is covered over by the thick integument of the mantle,
which terminates toward the oral end in a ridge round the neck.
Anteriorly this ridge projects as a prominent rounded lobe under
cover of which the head can be partially retracted. Posteriorly it
forms the posterior lip of the opening of a large cavity bounded by
the mantle — the mantle-cavity — which extends along the entire
posterior face of the body almost to the apex. The wide cleft
between the oral edge of the mantle and the posterior surface of
the body is not the only aperture leading into the mantle-cavity.
On the oral side of this cleft is a large tube — the funnel (Fig. 621,
inf.) — opening on the exterior behind the neck, and internally
communicating by a wide aperture with the mantle-cavity. The
cleft is capable of being almost completely closed by che
apposition of a pair of oval projections (mant. cart.) of the
inner surface of the posterior mantle wall near its oral border,
and a pair of concave depressions (inf. cart.) on the opposite

s**f/* TTLU 9

Fie. 616. — Sucker of Sepia. A, oblique view of a sucker magnified; car. cavity; circ. mug.
circular muscle ; dent, denticulated border ; pi. muscular plug ; «£. stalk ; B, vertical section.
(After Vogt and Jung.)

(posterior) face of the funnel. The funnel is thus, under ordi-
nary circumstances, the main outlet of the mantle-cavity.
As such it not only carries to the exterior the effete water
of respiration, the faecal matters from the intestine, and the
products of the excretory and reproductive organs, but also
takes an important part in locomotion, the most important
movements of the Cuttle-fish, by which it darts rapidly through
the water in the direction of the aboral pointed end of the body,
being effected by rhythmical contractions of the muscular walls of
the mantle-cavity causing jets of water to be forced in the oral
direction through the funnel. The free passage of water inwards
through the funnel is prevented by the presence in its interior of
a flap-like valve opening outwards. The water required for re-
spiration and in locomotion is thus drawn in, not through the
tunnel, but through the partially-closed slit-like pallial aperture
previously referred to. The funnel seems, from the source of the

XII

PHYLUM MOLLUSCA

711

FIG. 617.— Shell of
Sepia cultrata,

posterior
Reduced.

view.

nerves which supply it, to be, like the arms, a specially modified

part of the foot ; it corresponds, perhaps, to the

metapodium of Triton.

Fringing each lateral margin of the body is^a '

thin muscular fold— the fin — which is used as a

swimming organ.

The anterior wall of the body exhibits, -as —

a! remedy mentioned, a -hard and resistent char-
acter owing to the presence of the internal

shell (Fig. 617). 'This is completely enclosed

in a sac of the mantle. Like the body itself,

it is bilaterally symmetrical. In shape it may

be described as leaf-like, with a rounded and

comparatively broad oral end, and a narrower

aboral, provided with a sharp, anteriorly-project-
ing spine. The posterior surface is convex, the

aiiterioxjcojiy_eJL towards its oral end, but deeply

concave aborally, and bounded laterally by thin

prominent wing-like ridges which converge to

meet at the aboral extremity. The main mass

of the shell consists of numerous, closely-
arranged, tkhr^mrrrme-oi^calcareous composition,

between- which are interspaces containing gas.

On the surface is a thin layer of chitinoid material,

thicker strips of similar composition run along the margins.

Oattle-fish will be observed to undergo frequent
changes of colour, and
blushes of different hues are
to be observed passing over
the surface. These are due
to the presence of numer-
ous contractile pigment-con-
taining cells or chromato-
phores (Fig. 618) situated
in the deeper layers of the
integument over the entire
surface. The chromatophores
have elastic walls, the con-
tracting tendency of which
is capable of being counter-
acted by the action of bundles

FIG. 618.— Chromatophore of Sepia, magnified. of UlUSCUlar fibres radiating

nuc. nuclei in wall of sac: pir/m. pigment; ^i+^royrlo frwm t>io wall nf

,-«,,!. „, ,<*., -:ldititiiiK strands of muscle. (After OUtWaiClS trOlYL tne Wall O

Yogt and Jung.) ^ne gac into the SUTTOUnding

tissues. When these radiat-
ing fibres are in action the wall of the chromatophore is drawn
outwards in different directions, and as a result its cavity is dilated,

rael.Tmis

712

ZOOLOGY

SECT.

and the pigment becomes more widely diffused. When the fibres
are relaxed the elasticity of the wall comes into play, and the
chromatophore contracts, the contained pigment thus assuming a
more condensed form. A peculiar iridescence which, in addition
to, the play of colours, is recognisable in the integument of Sepia,
is due to the presence of a number of cells, the iridocysts.

When the mantle -cavity is laid open (Fig. 621) there is seen
on each side of it one of the two plume-shaped ctcnidia (cten.).

In the middle line of the posterior
surface, close to the internal open-
ing of the funnel, is the anal aper-
ture (an.), situated at the oral ex-
tremity of a longitudinal tube —
the rectum. On either side of
the rectum is a much narrower
projecting tube with a terminal
opening — the nepliridial aperture
(neph.). On the left-hand side is
the opening of the spermiduct or
oviduct (ovid.) as the case may be.
In addition to the shell, which
is an important protective struc-
ture, and gives support to the
muscles of the fins, Sepia also has
a remarkably well developed in-
ternal skeleton composed of cartilage. An important part of
this — the cranial cartilage (Fig. 619) — protects the principal
nerve centres, encloses the auditory organ, and gives support
to the eyes. Other cartilages support the bases of the arms.
A thin shield-shaped plate — the nuchal cartilage (Fig. 620) —
lies on the posterior surface of the neck. The
pair of elevations on the posterior wall of the
funnel and the corresponding depressions on the
anterior surface of the body are borne each on
a thin plate of cartilage, and thin cartilages
support the bases of the fins.

Alimentary System. — The mouth is sur-
rounded by a thin peristcmial membrane, within
which is a circular lip beset with numerous
minute elevations. Lodged within the circular
lip is a pair of powerful horny jaws (Fig. 622,
Fig. 62B9jawl,jaw*-9 Fig. 624, /.; Fig. 626, ya
somewhat the ,appearance of the jaws of a parrot, with one,
the posterior, larger and more strongly bent than the other,
which it partly encloses. The mouth leads into a thick- wall<
buccal cavity, which contains an odontophore bearing numerous
minute horny teeth. The oesophagus (Figs. 623 and 624, ce.

FIG. 619.— Sepia cultrata, cranial car-
tilage seen from the posterior aspect,
the cavities of the otocysts exposed.
eye, position of eye indicated by dotted
line ; ot. otocyst ; pall. n. pallia! nerve ;
vise. n. visceral nerves.

Fir;. (i20. — Sepi<
cultrata, nuch;

cartilage.

These hav(

XII

PHYLUM MOLLUSCA

713

Fig. 626, aes.), following on the buccal cavity, is a narrow
straight tube, which runs between the halves of the liver towards
the aboral end of the body. It opens into a rounded thick- walled
stomach (st.), and, tdose to the pyloric aperture leading from the

621. — Sepia CUltrata, female ween from the posterior aspect, the wall of the mantle-cavity
divided along the middle line and the two flaps thus formed spread out so as to expose the
contents, ac. ni<I. accessory iiidamental glands ; an. anal aperture with its lateral appendages ;
/. membranous fold attaching the ctenidium to the wall of the mantle-cavity ; inf, external
opening of funnel ; inf. cart, infundibular cartilage ; ink .<t. ink-sac ; ink <1. ink-duct ; liy. liga-
mentous band which extends from the anterior wall of the mantle-cavity to the ovary, cut
across; liv. liver; 1. den. left ctenidium; I. neph. left nephridial aperture; I. nid. left
nidamental gland ; I. *t. g, left stellate ganglion ; -mant. curt, mantle cartilage ; mo. mouth;
HIM. neck muscles ; o<-. ovary ; odd. oviduct rect. rectum.

latter into the intestine opens a wide ccccum (c.). The alimentary
canal at this point bends sharply round upon itself, and the in-
testine runs nearly parallel with the oesophagus to open into the
mantle-cavity as already described.

A pair of glands (Fig. 624, s. g. ; Fig. 626, sal.), which are

714

ZOOLOGY

SECT.

commonly termed salivary, though their functional correspondence
with salivary glands has not been proved, are situated in the
head behind the cranial cartilage. The ducts of these two glands
run inwards and unite to form a median duct, which opens into
the buccal cavity. The name of liver (Fig. 624, //. ; Fig. 620, ///•.)
or digestive viand is given to a large brown glandular mass
which extends from the neighbourhood of the salivary glands

nearly to the aboral end of the
body. It consists of two com-
pletely independent right and
left portions, each of which has
a bile duct opening into the
cavity of the alimentary canal
opposite the point where stomach,
caecum, and intestine meet. Sur-
rounding the bile-ducts and
opening into them are masses of
minute vesicles (Fig. 624, l>. d.) ;
the secretion of these has the
property of converting starchy
matters into sugar; they some-
times, though without sufficient
reason, receive the name^ of
pancreas.

Immediately below the thin
integument of the anterior wall
of the mantle-cavity lies a
characteristic organ- — the ink-
sac (Fig. 626, ink s. ; Fig. 627).
This is a pear-shaped body, a
portion of the interior of which is
glandular and secretes a black
substance — the ink or sepia —
which collects in the main cavity
of the sac and is discharged by a
cylindrical duct opening into the
rectum close to the anal aperture.
When the Cuttle-fish is startled it discharges the ink, which,
mixing with the water in the mantle-cavity, is ejected through
the funnel as a black cloud, under cover of which the animal may
escape from a threatened attack.

Vascular System.— The heart (Figs. 025, 020, and 628) of
the Cuttle-fish consists of a ventricle' and two auricles. The
ventricle (vent.), which is divided into two lobes by a constric-
tion, is somewhat obliquely placed, but the rest of the vascular
system is almost completely equilateral. At its oral end the
ventricle gives off a large vessel — the oral aorta (awt.) ; aborally

Fi<;. 022.— Sepia officinalis, jaws. A,
in situ; B, removed and slightly en-
larged. (From the O//</,,v'«V/c Natural
History.)

XII

PHYLUM MOLLUSCA

715

it gives origin to a much smaller aboral aorta (aort'.), which
-bends over the ink-sac and supplies the aboral portions of
the body. The arteries which lead off from the aorta? com-
municate by their ultimate branches with a system of capil-
laries, and these with a system of sinuses and veins. A large
median vein, the vena cava (v. cav.), runs from the head to the
neighbourhood of the rectum, in front of which it bifurcates
to form to right and left afferent branchial veins (1. off. br. v.,
r. affilr. v.), each running through the cavity of the corresponding

JSti

FIG. 023.— Sepia, median section through
the buccal mass. //. l>t«\ buccal ganglia;
{/. stoiii. stomatogastric ganglia; trust, sup-
posed gustatory organ ; jatci, posterior
jaw ; ,/>«>•-, anterior jaw ; v. oesophagus ;
perist. peristomial membrane ; mi.!, radula.
(After Keferstein.)

FIG. 624.— Sepia officinalis, enteric
canal, a. anus ; b. d. one of the bile
ducts ; b. m. buccal mass ; c. caecum ;
i. ink-sac ; i. <L ink-duct ; j. jaws ; 1. 1.
liver lobes ; w. oesophagus ; p. pan-
creatic appendages ; r. rectum ; s. g.
salivary glands; st. stomach. (From
the (.'nnibfiiliii Sut i' rid History.)

renal organ to the base of the gill, where it is joined by veins from
the aboral region. At the base of the gill the afferent branchial
vein becomes dilated to form a contractile sac — the branchial heart
(/-. br. Jit.) — appended to which is a rounded body of a glandular
character — the (fj>j>< nf/ayc of the branchial heart. The afferent
branchial vein runs through the axis of the branchia giving off
branches as it goes. The blood is carried back to the ventricle by
a dilated contractile vessel, the auricle or efferent branchial vein
(/. car., r. aur.).

716

ZOOLOGY

SECT.

The coelome (Fig. 634) is a pouch of considerable size, divided
by a constriction into oral and aboral parts. The former is the
i_H' riccwdiutn or cavity in which the. heart is lodged ; it gives off
a pair of diverticula, right and left, each lodging the corresponding
branchial heart. The aboral part of the coelome forms the capsule
which encloses the ovary or testis.

r.a.bd.'V

le.

Fi<;. <;•_>">. — Sepia cultrata, imfe specimen seen from the posterior aspect, the mantle-cavity
opened as in Fig. 621, the posterior body-wall partly dissected off, so as to expose the organs
in the visceral sac, the ink-sac and duct removed, aort. main aorta ; aort.' aboral aorta :
a/>i>. appendage of left branchial heart ; we. caecum ; inf. cart, funnel cartilages ; li-c. liver ;
/. aliil. r. left abdominal vein ; /. aft; br. left afferent branchial vessel ; /. tun: left auricle ;
/. /</•. ht. left branchial heart; /. cttn. left ctenidium; /. st. g. left stellate ganglion; -mitdt.
furt. mantle cartilage ; pen. penis ; proxt. prostate ; r. ahil. v. right abdominal vein ; r. etui.
right ctenidium ; r. >•<„. ,,./,/,. appendages of right afferent branchial vessel; te. r. vein to
testis ; ra. valve of mantle ; rent, ventricle.,

- -The paired, plume-shaped ctenidium lies parallel with the long
axis of the body. It is attached throughout the greater part of il
length to the wall of the mantle-cavity by 'a thin muscular fol<
It consists of numerous pairs of delicate lamellae, the surface
which is increased by the presence of a complex system of foldm

XII

PHYLUM MOLLUSCA

Internally the lamellae are not completely in contact, an axial
.canal being left through which the water penetrates freely to all
parts of the gill. The blood carried to the gill by the afferent
branchial vessel passes in a system of minute branches through

. 620.— Sepia cultrata, lateral dissection of male. The left-hand half of the head has
been removed by an approximately median longitudinal section, the buccallfmass, however,
being left intact ; the funnel and the anterior and posterior walls of the mantle-cavity are
likewise bisected longitudinally. The left ctenidium with the left nephridial sac and
left branchial heart have been removed from their natural position and displaced backwards
so as to expose the other organs. The liver with its ducts and the pancreatic appendages
have been removed, but the position of the liver is indicated by a dotted line. app. appendage
of left branchial heart ; itort. aorta ; aort'. aboral aorta ; buc. buccal mass ; br, cart, section of
cartilage supporting the arms ; cer. g. cerebral ganglia ; giz. gizzard ; ink s. ink -sac ; inf,
funnel ; jaic, jaw ; t. aur. left auricle ; /. br. ht. left branchial heart ; L cten. left ctenidium ;
liv. position of liver ; /. neph. left iiephridial sac ; n. cart, nuchal cartilage ; m. oesophagus ;
of. cavity of otocyst laid open ; peiL g. section of pedal ganglion ; perist. peristomial mem-
brane ; post, c. abdominal vein ; r. aur. right auricle ; r. cten. right ctenidium ; rcct. rectum ;
•>>•"./. sulivary gland ; $h. shell ; st. stomach ; te. testis ; ra. valve of funnel ; r. car. vena cava ;
• rent, ventricle.

the lamellae, and is gathered up again into vessels which open into
the main efferent vessel leading to the auricle

Nervous system. — Though parts homologous with those of the
nervous system of Triton are recognisable in that of. Sepia their
proportions and arrangement indicate a higher grade of organisation.

718

ZOOLOGY

SECT.

The cerebral, pedal, and pleuro-visceral ganglia (Fig. 629), all of
relatively large size, are closely aggregated together around the
oesophagus, supported and protected by the cranial cartilage. The
cerebral ganglia (cer. g.) are fused together into a single rounded
mass, lodged in a hollow of the cranial cartilage, and covered

over anteriorly by a strong fibrous mem-
brane. Laterally are given off a pair of
short thick processes — the optic nerves
or optic stalks (opt. st.) — which expand
almost immediately into large masses —
the optic ganglia (opt. g.) — in immediate
contact with the eyes. At the sides
and posteriorly a pair of very thick com-
missural bands of nerve matter pass
round the oesophagus to unite with the
pedal and pleuro-visceral ganglia, which
lie behind. The pedal ganglia (Fig. 630)
are, like the cerebral, united into a
single mass; orally this is prolonged
forwards and expanded into a broad
mass from which the ten bmchieil nerves
(br. n.) are given off to the arms. The
pleuro-visceral ganglia, also united into
one, are in immediate contact with the
pedal behind the oesophagus.

Besides the optic nerves the cerebral
ganglia also give off a pair of slender
nerves which join a smaller pair of
closely united buccal ganglia (Fig. 629,
buc.), situated close to the buccal cavity
on the anterior aspect of the oesophagus.
The buccal ganglia again (which are
sometimes looked upon as separated por-
tions of the cerebral) are connected by
slender commissures with a pair of ganglia,
the stomatogastric (Fig. 623, #. stem.), also
closely united, situated on the posterior
aspect of the oesophagus. Besides the ten
brachial nerves, each of which, expand-
ing at the base of the arm into a
brachial ganglion, runs along the axis
of the arm to its extremity, the pedal ganglia also give off
nerves to the funnel, and also a pair to the otocysts; but the
latter are found, when their fibres are traced to their origin, to be
derived from the cerebral ganglia. The pleuro-visceral ganglia
give off two visceral nerves (Fig. 630, vise, n.) supplying the various
internal organs, one pair of branches, the brecnchieds, expanded into

• . 827. Sepia officinalis.

longitudinal section of ink-
sac. «. anus ; d. ink-duct ;
-/. </. ink-gland ; i. ,: cavity
of ink-sac ; o. orifice of ink-
gland ; /•. rectum ; sp. sphinc-
ter muscles. (From the Cam-

Girod.) "

abranchial f

PHYLUM MOLLUSCA

719

a branchial ganglion at the base of the ctenidium, and running along
its axis to its extremity. Two other ganglia of considerable size —

Fic.-:628. — Sepia cultrata, heart and main blood-vessels from the posterior aspect, (tort, aorta ;
ajo't't'. aboral aorta ; <i/>p. appendage of right branchial heart ; eff. l>r. r. right efferent branchial
vessel ; Ink «. artery to ink-sac ; ink. r. vein from ink-sac ; /. riff. br. r. left afferent branchial
vessel ; 1. aur. left auricle ; oc. v. deep ovarian vein ; ov. v'. superficial ovarian vein ; pall. v.
pallial vein ; r. abil. v. right abdominal vein ; r. aff. br. r. right afferent branchial vein ;
r. <•/< ii. right ctenidium ; /•. br. lit. right branchial heart ; v. car. vena cava ; ren. app. venous
appendages ; rent, ventricle.

FIG. 629.— Sepia cultrata, cephalic gang-
lia from the anterior aspect, ao. aorta ;
hue. buccal ganglion ; ctr. buc. con. cere-
bro-buocal connective ; cer. g. cerebral
ganglion ; opt. g. optic ganglion (removed
on the left side) ; opt. st. optic stalk ;
•/Mill. n. pallial nerve ; pi. g. pleural gang-
lion ; rise. n. visceral nerves.

•VLSC.7L

FIG. 630.— Sepia cultrata, anterior view
of pedal and pleuro-visceral ganglia after
removal of the cerebral and optic; //,-. re.
brachial nerves; conn, connectives be-
tween the cerebral and the pedal and
pleuro-visceral ganglia (cut across) ; <',?/! ,i.
nerve to funnel; pall. ,i_. pallial nerves;
rise. n. visceral nerves.

the visceral and the gastric — occur in the course of this system. The
pleuro-visceral ganglia also give off two very stout pallial nerves
(pall.n.) which run through the neck to the inner surface of the

720

ZOOLOGY

SECT.

mantle-cavity, where each expands into a large, flat, pallial or
stellate, ganglion (Fig. 621, l.st.g.) which is visible in front of
the ctenidium when the mantle-cavity is opened. From the
outer edge of this are given off a number of nerves supplying1
the various parts of the mantle.

The organs of special sense of the Cuttle-fish are much more
highly developed than those of Triton. The eyes (Fig. 631) are
supported by curved plates of cartilage connected with the cranial
cartilage. The significance of the various parts of the eye will not
be fully understood till the structure of that of the Yertebrata has

scl.carl

scl.carl

orb.ca.rl

IM<;. ('31. — Sepia, section > of eye. cil. proc. ciliary processes; corn, cornea ; ?'•>•. iris ; lens, lens ;
opt. ft. optic ganglion ; orb. cart, orbital cartilage ; nls. rods ; ret. retina ; set. cart, sclerotic
cartilage. (From Vogt and Jung, after Hensen.)

been studied. A transparent portion of the integument covering the
exposed face of the eye is termed the cornea (corn.). This is perfor-
ated by a minute aperture placing the internal cavity of the eye in
communication with the exterior. The eye-ball has a firm wall, or
sclerotic, strengthened by plates of cartilage' (scl. cart). Externally, i.e.,
on the side turned towards the surface of the head, this presents
a large opening — the pupil. The part of the sclerotic which im-
mediately bounds the pupil is termed the iris(ir.); it contains muscu-
lar fibres by whose action the size of the pupil can, to a limited
extent, be increased or diminished. Just internal to the iris and
projecting slightly through the pupil is the lens — a dense glassy-

xii PHYLUM MOLLUSCA 721

looking body of a spherical shape. The lens consists of two plano-
convex lenses in close apposition ; it is supported by an annular
process — the ciliary process (cil. proc) — projecting inwards from the
sclerotic. The lens with the ciliary process divides the cavity of
the eye into two cavities, a smaller outer — the cavity of the aque-
ous humour — containing water, and a larger inner, containing a
gelatinous substance — the vitreous humour. Over the wall of this
inner chamber extends the retina (ret), the sensitive part of the eye,
in which the optic nerve-fibres derived from the optic ganglion
terminate. The retina is of somewhat complicated structure,
consisting of a number of layers : of these that which immediately
bounds the internal cavity of the eye is a layer of short narrow
prismatic bodies — the layer of rods (rd), while the outermost is a
layer of optic nerve-fibres connected with the nerve-cells of the
optic ganglion on the one hand, and with the other elements of
the retina on the other.

In immediate contact with the eye, in addition to the optic
ganglion, is a large soft body of unknown function, the so-called
optic gland or white ~body. Bundles of muscular fibres bring about
limited movements of the eyeball in various directions. A pair
of integumentary folds of the character of eyelids are capable of
being drawn over the cornea.

The otocyst (Fig. 619), though not of such complicated structure
as the eye, is very much more highly developed than that of the
Pelecypoda or Gastropoda. The two otocysts are embedded
in the cartilage of the posterior portion of the cranium close
to the pleuro- visceral ganglion. The cavities of the two organs,
which are about 3 mm. in diameter, are separated by a median
cartilaginous septum. The inner surface is raised up into a
number of rounded and pear-shaped elevations. The surface is
lined with a flattened epithelium raised up on the posterior surface
into a ridge composed of large cylindrical cells provided at their
free extremities with short cilia, and produced
at their bases into processes continuous with
nerve-fibres derived from the otocyst nerve.
Enclosed in the cavity of the otocyst is a large
otolith (Fig. 632) of dense composition and
complicated form. The function of the otocysts
as organs of hearing is quite unproved; it
has been shown by experiment that their re-
moval leads to a loss of the power of co-
ordinating the movements in such a way as Fl( trat^
to maintain the equilibrium. highly magnified.

Supposed to be olfactory in function is a

pair of ciliated pits, which open by slits on the surface behind
each eye; among the ciliated cells lining the pit are numerous
narrow sensory cells connected at their bases with the fibres of

•VOL. I 3 A

722

ZOOLOGY

SECT.

a nerve derived from a small ganglion situated close to the
optic ganglion. A small elevation, covered with papillae, on the
floor of the buccal cavity, just in front of the odontophore, is
perhaps an organ of taste.

The excretory organs of Sepia (Figs. 633 and 634) are a pair of
thin-walled sacs, which open into the mantle-cavity by the con-
spicuous excretory apertures already described. On either side is
an aperture (ap.1) placing the cavity of the sac in communication

-v.cav

-ur

med.s

FIG. 633.— Sepia Officinalis, renal organs, abd. v. abdominal vein ; api, funnel-like opening •
from the pericardium ; np-. aperture of communication between the left and the niedi
nephridial sac; ink: s. r. ink-sac vein; med. s. median sac; pall. r. pallial vein ; ur. uret<
•r. ca,: vena cava; ven. app. venous appendages of the afferent branchial veins. (From V<
and Jung, after Grobben.)

with the pericardium. The right and left sacs communicate wit
one another anteriorly and posteriorly. From their posterior June
tion is given off a median diverticulum (Fig. 634, med. s), into
which the pancreatic follicles (pane) project. Through each ex-
cretory sac runs the corresponding afferent branchial vein, formed
by the bifurcation of the vena cava. Surrounding the vessel are
masses of glandular tissue (Fig. 633, ven. app); by whose agency the
process of renal excretion, the products of which, in the shape
urates, are to be detected in the internal cavity, is carried on.

.,

XII

PHYLUM MOLLUSCA

723

Reproductive system. — In the male the testis (Fig. 635, te)
forms a compact mass of minute tubules situated in the aboral
region of the body and enclosed in a capsule. The single kpermi-
duct (v. def) is a greatly convoluted tube which leads from the
cavity of the capsule towards the right ; it opens into an elon-
gated vesicula seminalis (ves.), to which is appended a glandular
body, the prostate (pr.). In the interior of the vesicula seminalis

FIG. 634. — Sepia officinal is, diagram of a median vertical section of a female specimen,|to
show the relations of the cavities, ap. aperture between the secondary body -cavity (peri-

Icardium) and the lateral nephridial sac ; br. lit. branchial heart ; inf. funnel ; ink: s. ink-sac ;
int. intestine ; hit. s. lateral nephridial sac ; liv. liver ; med. s. median nephridial sac ; or.
ovary ; or. ap. aperture leading from oviduct to secondary body-cavity ; pane, pancreatic-
appendages ; sh. shell ; st. stomach; v.r. ureter | rent, ventricle. (From Vogt and 'Jung, after
Grobben.)

the sperms are rolled up by the action of a system of grooves and
ridges into long narrow bundles of about 2 cm. in length, each
of which becomes enclosed by a chitinoid capsule of a narrow
cylindrical shape, forming a spermatopliore (Fig. 636, B) ; at one
end of the spermatophore is a complicated apparatus of the nature
of a spring for causing the rupture of the wall and the discharge
of the sperms. The vesicula seminalis expands into a wide sac —
the spermatoplioral sac or Necdham's sac (Fig. 635, sp. s) — in the

3 A 2

724

ZOOLOGY

SECT.

interior of which the spermatophores are stored. This opens into
the mantle-cavity by the aperture already described at the
extremity of the penis to the left of the middle line.

In the female the ovary (Fig. 621, ov) occupies a position exactly
corresponding to that of the testis in the male, and is enclosed in a
similar capsule, with the cavity of which the lumen of the oviduct
is continuous. An axial swelling bears numerous follicles, each
containing a single ovum at various stages of development, and sup-
ported on a long slender stalk. At the breeding season the ovary
becomes a compact mass of ova, which assume a polygonal shape
owing to mutual pressure. The oviduct (ovid) is a wide tube, opening,
as already described, into the mantle-cavity to the left of the rectum.

6e

FIG. 635.— Sepia, reproductive organs of male. ,r,i. penis; pr. prostate; sp. s. sperm-sac

resicula si

te. testis ; v. dcf. vas deferens ;

seminalis. (After Keferstein.)

Near its external opening are situated a pair of small accessory
glands (ac. nid). Occupying a conspicuous position on the anterior
wall of the mantle-cavity of the female is a pair of large flattened
glands, of somewhat oval outline, the nidamental glands (nid),
situated to the right and left of the ink duct. In the long axis of
each is a median canal, on either side of which is a range of
closely-set delicate lamellae ; the median canal opens into the
mantle-cavity by a slit bounded by a number of plaits situated at
the narrower oral end. The nidamental glands secrete the viscid

XII

PHYLUM MOLLUSCA

725

laterial by means of which the eggs when deposited adhere
together in masses. A glandular mass of unknown function,
known as the accessory nidamental glands,
lies at the sides and around the oral
ends of the nidamental glands proper.

ii. THE PEARLY NAUTILUS (Nautilus
pompilius}.

spr

The three living species of Nautilus,
of which N. pompilius is the best
known, are inhabitants of moderately
shallow water about the shores and
coral-reefs of the South Pacific, usually
creeping on the bottom at the depth of
a few fathoms, and probably never
coming voluntarily to the surface. The
body is enclosed in a calcareous, spirally-
coiled shell (Fig. 637), into which the
entire animal can be withdrawn for pro-
tection. The cavity of the shell is divided
by a system of septa into a series of
chambers, the last and largest of which,
opening widely on the exterior, alone
lodges the body of the animal. Between .
the animal and its shell there is a
direct organic connection through the
intermediation of a narrow tubular pro-
longation of the visceral region, which
perforates the entire series of the septa
to the apex of the spiral. This tube,
which is termed the siphuncle (si), has
its wall supported by scattered spicules of carbonate of lime;
but, in addition, as it passes through each septum, there is pro-
duced over it for some distance a shelly tube — the septal neck —
(s. ri) continuous with the substance of the septum. The apical or
initial chamber presents a small scar,- the cicatrix, which may
indicate the original presence of the larval shell, or protoconcfy
which has fallen off in the course of development.

When the shell of the Nautilus is compared with that of Triton
some points of resemblance, together with important points of
difference, will be at once recognised. In both the growth of the
shell has taken place in such a way as to produce a gradual and
regular increase in the width of the internal cavity, from the apex
to the mouth, the result being a form of shell which, if it were
straightened out, would be a long cone. In both the growth has
not taken place in a straight line, but in a spiral, and a spiral of

B

FIG. 630.— Sepia. A, sperms,
highly magnified ; B, sperrna-
tophore ; sp. mass of sperms ;
spr. spring apparatus by which
the wall of the spermatophore
is ruptured. (From Vogt and
Jung.)

726

ZOOLOGY

SECT.

so close a character that successive turns are in immediate contact
and their walls fused together. But in Nautilus all the turns of
the spiral are in the same plane ; the spiral, in other words, is a
flat one, as has already been found to be the case in certain of the
Gastropoda (p. 686), whereas in Triton the spiral is an elongated
helix : in other words, the spiral of Nautilus is that of a watch-
spring, that of Triton that of a corkscrew. The possession by
Nautilus of the series of septa marking the position which the
animal has occupied at successive stages in its growth is another
striking difference. Moreover the relations of the soft parts of
the shell are radically different in the two cases. In Triton the

s.n

FIG. (537. — Section of the shell of Nautilus pompilius, showing the septa (.?, s), the septal
necks (s. n., s. H.), the siphuncle, si. (represented by dotted lines), and the large .body-chamber
(c/i.). (From the Cambridge Natural History.)

body is attached to the shell by the columellar muscle; in
Nautilus the main organic connection is by means of the siphuncle,
for, though it is chiefly through the pressure exerted by two great
lateral masses of muscle that the Nautilus retains its hold of the
shell, the muscular fibres are not attached to the latter in the
same intimate way as those of the columellar muscle of Triton.
Again, while the curvature of the body of Triton with the en-
closing shell is towards the ventral side, in Nautilus ic is towards
the dorsal.

When the animal is removed from the shell it is found to
possess two regions, a distinct and relatively large, obtusely

PHYLUM MOLLUSCA

727

conical head, bearing eyes and a system of tentacles, and a
rounded sac-like trunk. Both head and trunk are very slightly
compressed, the direction of the compression being, as in Sepia,
from the antero-dorsal towards the postero-ventral side, and
are almost completely bilaterally symmetrical, only a very
slight disturbance of the symmetry being discernible. The
mouth, situated at the free extremity, is provided with a pair of

Jaws irtf

cart

rnant

cterv

rruis

. (538. — Nautilus pompilius, diagrammatic lateral view of a female specimen enclosed
in its shell, cart, cartilage ; cten. ctenidia ; hd. hood ; inj\ funnel ; jaics, jaws ; iMint. mantle ;
'tiiant'. dorsal mantle fold overlapping the coil of the shell ; mus. position of lateral mass of
muscle ; nid. nidameutal glands ; sept, first septum ; siph. siphuiicle. (After Keferstein.)

relatively enormous, partly calcined jaws (Fig. 638). Surrounding
the mouth is a series of bilaterally arranged lobes which represent
the fore-foot of other Molluscs. These are beset with numerous
slender tentacles, each provided with an elongated tubular sheath,
in the interior of which the greater part of the tentacle in the
retracted condition lies enclosed, only a small portion protruding.
Minute ring-like markings on the tentacle are due to the
presence of circular bands of muscular fibres. There are no

728

ZOOLOGY

SECT.

suckers ; but it is supposed that each tentacle represents not one
of the arms of Sepia, but one of the suckers. The tentacles are
arranged in two series, an outer and an inner. The outer, which
are borne on an annular muscular ridge of the fore-foot, are
nineteen on each side in both sexes. Anteriorly this muscular
ridge is thickened to form a massive lobe — the hood (M.) — in which
there is a concavity for the reception of the coil of the shell.
The hood bears two tentacles and has the appearance of being
composed of the immensely developed sheaths of these, completely
fused together in the middle line : on each side the enlarged
sheaths of a second pair of tentacles are closely applied to, though
not completely coalescent with, the hood, being separated from the
latter by a narrow groove. The hood, with these two enlarged
sheaths, is covered with a thickened tuberculated skin, and acts
after the manner of an operculum for protecting the tentacles
and other soft parts about the head. Altogether there are
forty-two tentacles of this outer series, including one situated on
the oral and another on the aboral side of each eye. The
tentacles of the inner series differ strikingly in number and
arrangement in the two sexes. In the female there are two inner
lateral lobes, right and left, quite symmetrically developed, and

each bearing twelve tentacles, and an
inner posterior lobe divided by a deep
median notch into two, each half bear-
ing fourteen tentacles. On the middle
of the oral surface of the latter, close
to the median notch, is an oval patch
raised up into numerous closely set
ridges. In some individuals, however,
the whole of this posterior lobe is re-
presented by a vestige (or rudiment)
with only slight indications of the
tentacles.

In the male the inner posterior
lobe with its ridged organ is only
represented by a median posterior
body consisting of two oval elevations
each divided into a number of folds.
The internal lateral lobes are greatly
modified, four of the tentacles on
either the right side or the left,
usually the latter, being modified to
form a structure termed the spadix
(Fig. 639), which is supposed to re-
present the hectocotylised arm of the male Sepi'a. It has the form
of a large compressed cone formed by the union of the enlarged
sheaths of three of the tentacles. The corresponding tentacles

V

FIG. 630.— Nautilus pompilius,

spadix of full-grown male, seen
from the outer side. 1, 2, 3, 4,
modified tentacles; 1 , withdrawn
into its sheath, its position and
shape indicated by the dotted
line ; 3, the flattened tentacle
with the rows of minute cavities ;
x, patch of modified integument.
Two-thirds of the natural size.
(After Haswell.)

e.

:

xii PHYLUM MOLLUSCA 729

themselves are in the adult male enormously thickened, and
the outer surface of the most posterior (3) is covered with regularly
arranged rows of minute pits. A fourth tentacle much smaller
than the others is closely applied to the outer surface of the
organ. In the internal lateral lobe, right or left as the case may
be, opposite that bearing the spadix, the latter is represented by
a group of four tentacles forming what is termed the anti-spa (Ji> .-.

A further difference between the male and the female with
regard to the foot is the presence in the latter, but not in the
former, on the inner surface of the outer
ridge, close to the inner posterior lobe on
either side, of an area thickly beset with
elicate membranous ridges.

On the posterior side of the head is a
unnel corresponding with that of Sepia,
but extending further forwards ; this, how-
ever, does not form a complete closed
tube, the edges of its right and left moieties
being simply in apposition posteriorly
without being united together. Near the
oral end is a large, somewhat triangular,
valve arranged like that of Sepia,

An internal skeleton of cartilage, as in
Sepia, protects "the nerve-centres and sup- FIG. wo.— Nautilus pom

*f , , P,I p ., /T-C ^A/k\ pilius, cartilaginous in-

ports the basal parts Of the foot (Fig. 640). ternal skeleton. (After

Mantle and Mantle-Cavity.— The

mantle is produced around the head into

a free flap, longer and looser than the mantle-flap of Sepia.
Dorsally this splits into two layers reflected over the convexity
of the shell which fits into a hollow behind the hood. Ventrally
and posteriorly the mantle encloses a large mantle-cavity
(Fig. 641) corresponding to that of Sepia. In this are lodged
two pairs of ctenidia (den.), having the same general structure
as the single pair present in Sepia. Between the bases of the
ctenidia of each side is a small knob-like elevation, the oral
ospliradium (ant. os.),1 and behind the bases of the more aborally
situated pair are two compressed bilobed projections, more or less
completely united in the middle so as to form a transverse
ridge ; these are the aboral osphradia (post. os.). In the middle
line of the mantle-cavity is the anus (an.), a large aperture with
minutely lobed margin, situated on a slight elevation, but by no

1 As in Sepia it is convenient to use the term oral for parts towards the
mouth end, and aboral for those situated towards the opposite extremity, the
same terms being also used to indicate relative position of different parts. The
relative position of the parts is, however, for the sake of simplicity given here
as they lie when the mantle-cavity is opened by turning back' its thin postero-
ventral wall.

730

ZOOLOGY

SECT. XII

means so prominent as in Sepia. On each side are two apertures,
the oral and aboral nepJiridial apertures (a. I, neph. ap.,p. I. neph. ap.,
p. r. neph. ap.), corresponding to the single pair of Sepia, but not
elevated on papillae. Close to each posterior nephridial aperture
is an opening — the mscero-pericarcUal (l.msc.ap.,r.msc.ap.) — lead-
ing into the viscero-pericardial section of the body-cavity ; these
are not represented in Sepia. In both sexes there are two repro-
ductive ducts, right and left ; but in both- the right alone appears
to be functional, and the left is much smaller. The opening of
the right spermiduct of the male (pen.) is situated on a cylindrical
prominence — the penis — placed close to the middle line. In the
female the nidamental glands are, as in Sepia, conspicuous objects,

jben.

FIG. 641.— Nautilus pompilius, interior of mantle-cavity of a male specimen with the
postero-ventral wall reflected, a. 1. neph. ap. oral left nephridial aperture ; an. anus ; cten.
ctenidia ; I. cJ ap. left reproductive aperture ; 1. ant. os. left oral osphradium ; I. vise. ap.
left viscero-pericardial aperture ; mant. flaps of mantle ; pen. penis ; p. 1. neph. ap. aboral
left nephridial aperture ; p. r. neph. ap. aboral right nephridial aperture ; post. os. aboral
osphradia ; r. vise. ap. right viscero-pericardial aperture.

when the mantle-cavity is exposed ; but they are mainly situated
on its posterior instead of its anterior wall.

Enteric Canal. — The mouth is surrounded by a peristomial
membrane beset with numerous papillae. There is a pair of
jaws (Fig. 64i2,jaiv) of similar shape to those of Sepia, but much
more powerful, and calcined towards the tips. The buccal mass
is a large rounded body with thick muscular walls. On the
floor of the contained cavity is a large and prominent odontophore
(odont.) with long and pointed curved teeth. In front of the
odontophore is a large bilobed soft prominence, the tongue (tong.).
Behind the odontophore, between it and the opening of the
oesophagus, are one large median and two lateral tongue-like pro-
minences beset with papillae ; on the inner surface of the latter

inl. /

atom.

. 042.— Nautilus pompilius, dissection of the internal organs of a male, from the left side. The
funnel and the hood have been divided by a longitudinal median section. A portion of the wall of the
buccal cavity has been removed to show the odontophore and the tongue, ace. gl. vesicula seminalis ;
tt/i. anus ; aort. oral aorta; aort'. posterior pallial artery; b. du. bile ducts; buc. n. buccal nerves; buc.
P<(i>. papillae of peristomial membrane ; c^r. <i. cerebral ganglion ; CIKC. caecum ; cr. crop ; lid. hood ; Inf.
funnel ; inf. n. infundibular nerve ; int.1 part of intestine- between stomach and caecum ; int.- part of
intestine following caecum ; jaw, larger (posterior) jaw ; /. ,,/f. /„: o. left efferent branchial vessels ; I. tent.
n'l. left internal tentacular lobe ; need. s. Needham'ssac ; odont. odontophore ; en' style passed from buccal
cavity into the opening of the oesophagus ; nx. (usuphagus ; olf. n. olfactory nerve; opt. n. optic nerve;
oto. otocyst ; pall. n. pallial nerves ; puL <j. pedal ganglion ; pi. <j. pleural ganglion ; /-. <.$'. lr. K-. right
efferent branchial vessel ; re.tr. retractor muscle of the buccal mass ; stom. stomach ; text, testis ; tong.
tongue -shaped elevation of the floor of the mouth ; m. valve of funnel ; rtn. c. vena cava ; cent, ventricle.

732 ZOOLOGY

SECT.

are apertures which may be the openings of salivary ducts, but
the existence of salivary glands has not been proved.

The oesophagus (oes.) becomes dilated aborally into a very
spacious crop (cr.) for the storage of the food, consisting of small
prawn-like Crustaceans and small Fishes broken up by the jaws
and radula. This opens into a rounded stomach (stom.) having
very much the appearance of the gizzard-like caecum of Sepia.
The intestine (int.) shortly after it leaves the stomach, develops
a rounded caecum (ccec.) with complexly folded walls, into which
the ducts of the liver open. The intestine does not pass
straight to the anus as in Sepia, but first bends round in a
short coil. The ink-sac and duct of Sepia are not represented.
There is a very large liver divided into four main portions or
lobes, each of which is made up of a number of lobules. The
bile-ducts (b. du.), opening as above mentioned into the caecum,
have a series of small diverticula which may represent the pan-
creatic appendages of Sepia.

Heart and Circulation.— The ventricle (Figs. 642 and 643
vent.) is a bilobed, transversely placed, muscular sac, very similar
to that of Sepia. On either side there open into it two auricles
or efferent branchial vessels (a. or.), one from each of the four
ctenidia. The ventricle gives off a large main aorta (aort.), which
passes to the head after giving off arteries to ' the stomach,
the crop, the liver, and the mantle. From the aboral surface of
the ventricle is given off a smaller artery, the lesser aorta, which
immediately bifurcates. One of its branches — the posterior pallial
artery (Fig. 642 bis, post. pall, a.) — passes to the area of the
mantle applied to the septum, bifurcates to supply this area,
and gives off a branch to the siphuncle. The other — anterior
pallial (ant. pall, a.) — after giving off arteries to the intestine
and rectum, and to the branchiaB and osphradia passes to the
muscular edge of the mantle, bifurcating anteriorly. Three genital
arteries (gen. a. 1,2, 3), supplying the various parts of the re-
productive apparatus, are likewise given off directly from the
ventricle.

A large vena cam (Figs. 642 and 643, ven. c.) occupies a position
corresponding closely with that of Sepia. It presents the re-
markable peculiarity of being in free communication by numerous
(valvular), apertures with the viscero-pericardial cavity of the
ccelome. At its aboral end it presents a dilatation from which
four afferent branchial veins (Fig. 643, a. 1. a/. /., p. I. a/., p. r. a/.}
r. ant, off.} — two right and two left — proceed to the corresponding
ctenidia, at the bases of which veins from the aboral region join
them. There are no branchial hearts.

The renal organs (Fig. 643) are, like the ctenidia and the afferent
and efferent vessels, four in number, instead of two as in Sepia.
Each renal sac (/. neph. s., r. ncpli. s., 1. post, nc.'p'h. s., r.post. nepli. s.)

PHYLUM MOLLUSCA

— reel

733

FIG. r.42 />/.*.— Nautilus pompilius (male), origin of pallial and genital arteries ; ant. pal. a.
anterior pallial artery ; iff. (>/•. <•. efferent branchial veins ; gen. a. 1, artery to vesieula
seminalis (r. .« ,/<.) ; <i< ,\. a. -2, testicular artery and its branches ; gen. a. 3, artery to pyriform
sac ; n. s. spermatophore sac ; i><>xi. //«//. a. posterior pallial artery ; pyr. pyriform sac : , • <•/.
rectum ; test, testis. (After Willey.)

L.n-eph..afj

Fio. 643. — Nautilus poxnpilius, renal sacs, with ctenidiaand other related parts, as seen from
the posterior aspect ; the boundaries of the four renal sacs represented by dotted lines.
(i. 1. aff. left oral afferent vessel ; cUn. right ctenidia ; /. ncph. s. left nephridial sac ;
/. nepk. ap. left oral nephridial aperture; I. post. /><_j>/<. ap. left aboral nephridial
aperture; 1. post. ncph. s.' left aboral nephridial sac; p. I. nff. left aboral afferent vessel;
p. r. aff. right aboral afferent vessel ; /•. n,it. «jt\ right oral afferent vessel ; /•. ant. aur. right
oral auricle; ren. app. renal appendages; ,•'. nepk. <(p. right nephridial aperture; r. pout,
neph. s. right aboral nephridial sac ; /•. v. up. right viscero-pericardial aperture ; ven. c. vena
cava ; vent, ventricle.

734 ZOOLOGY SECT.

opens into the mantle-cavity, as already stated, by an orifice
which is not drawn out into a tube. There is no communication
between the cavities of the different sacs, and thus no median
chamber as in Sepia. The cavities are found to contain phosphate
of lime. Into each projects, from the corresponding afferent
branchial vein, a compact rounded group of venous appendages
(ren. cipp.\ consisting of two symmetrical portions. Internal to
these, each afferent vein has connected with it a second group of
glandular appendages, which are cylindrical or club-like in form ;
they project, not into the nephridial sac, but into the viscero-peri-
cardial compartment of the ccelome. They have been compared
with the appendages of the branchial heart of Sepia, but differ in
their relations to the renal appendages.

Nervous system. — Nautilus differs strikingly from Sepia, and
resembles Chiton (p. 667) in the form assumed by the central
parts of the nervous system, distinct ganglia being absent. A very
thick nerve-collar, the posterior portion of which is double, surrounds
the oesophagus just behind the buccal mass. The anterior part of
the collar (Fig. 642, cer. g.) represents the cerebral ganglia, the
oral portion of the posterior part (pcd. g.) the pedal, the aboral
portion (pi. g.} the pleuro-visceral, while the lateral parts, not*
distinctly marked off from the rest, represent the cerebro-pedal and
cerebro-pleural connectives. From the cerebral "'ganglia" pass
nerves to the buccal mass, to the olfactory organs (plf. n.) the
otocysts, and a pair of very thick optic nerves (opt. n.) to the eyes.
The pedal ganglion gives off numerous nerves to the tentacles
and the funnel. The pleuro-visceral gives origin to pallial and
visceral nerves.

Sense Organs. — The otocysts are a pair of sacs embedded in
recesses close to the cerebral ganglia; each contains a number
of small fusiform otoliths. An olfactory function is ascribed to a
process having the appearance of a modified tentacle, situated on
the aboral side of the eye. Various parts connected with the foot
have also been supposed to be olfactory, but the marked sexual
differences which they present render this improbable. The
osphradia (p. 729) contain ganglion-cells and are undoubtedly
organs of special sense.

The eyes, situated at the sides of the head, are very large, but
extremely simple in structure, presenting a marked contrast to
those of Sepia, and scarcely comparable to those of any other
animal, with the exception perhaps of Patella (p. 694). Each is
of the shape of a saucer attached to the head by its convex side
through a short thick stalk, the mouth being closed in by a
slightly convex disc, with a circular aperture about its centre. A
slight, raised rim runs round close to the margin on the posterior
half, and a narrow groove runs inwards from this to the central
aperture. In the interior of the cup is neither lens, vitreous

humour, n

PHYLUM MOLLUSCA

735

r~.(jen, op

post aorl

tzst,

FIG. 644.— Nautilus pompilius, male repro-
ductive oi'gans. ace. vesicula seminalis ; cff. vets.
efferent branchial vessels ; I. gen. op. left genital
opening ; post.ao. posterior aorta ; pyr. pyriforni
appendage ; ?•. gen. op. right genital opening ;
sp. s. spermatophore sac; test, testis ; rent.
ventricle.

mmour, nor iris. The sea-water, passing in through the central
aperture, directly bathes the
retina, which is spread over
the interior in a thick layer.
Reproductive Organs —
The testis (Fig. 644, test.)
or ovary (Fig. 645, ov.\ like
that of Sepia, is single and
median, enclosed in -a special
sac towards the aboral end
of the body. The ducts are
paired in both sexes, but in
both the right alone appears
to be functional. In the
male a large glandular vesi-
cula seminalis,in which the
spermatophores are formed
(ace.) is connected with the
right duct, and this appears
to be represented on the
left-hand side by a vestige

—the so-called pyriform sac

(pyr.), situated close to the

ventricle. The distal part of the right duct dilates to form a

receptacle, the spermatophoral sac (sp. s.), and opens, nearly in

the middle line at
the end Of a pro_

minence — the penis
(Fig. 641, pen.). In
the female the right
oviduct has a glandu-
lar dilatation, which
is supposed to be
an albumen gland.
The ova are of large
size, greatly exceed-
ing those of Sepia
in dimensions, con-
taining a large pro-
portion of food-yolk.
Nidamental glands
are present, but are
mainly situated on
the posterior, instead

Fie. r,4:>.— Nautilus pompilius, female reproductive
organs, alb. albumen- gland ;/. nc,i. op. left genital open-
ing ; or. ovary ; pyr. pyriform appendage ; r. yen. op. right
genital opening; rent, ventricle. (After Lankester and
Bourne.)

of f]ie anterior

iii.

Wall 01 the mantle-

736 ZOOLOGY SECT.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Cephalopoda are bilaterally symmetrical Mollusca, which
have the propodium displaced forwards to the neighbourhood of
the mouth, and divided into a series of arms bearing suckers, or of
lobes bearing tentacles, while the metapodium forms a funnel for
the egress of water from the mantle-cavity. The visceral mass is
symmetrical and not coiled. The mantle encloses posteriorly and
ventral ly a large mantle-cavity, in which are situated the ctenidia
and the nephridial, reproductive, and anal apertures. The shell
may be absent or rudimentary ; when present and well developed,
it may be internal or external, undivided or divided internally by
septa into a series of chambers. There is an internal cartilaginous
skeleton, supporting and protecting the nerve-centres, and giving
attachment to muscles. The mouth is provided with a pair of horny
jaws, and an odontophore is present. In the majority there is an
ink-gland with a duct opening into the rectum. The ctenidia and
nephridia are either two or four in number. The nervous
system is highly developed ; and the principal nerve-ganglia are
aggregated together around the oesophagus. The sexes are
separate ; the segmentation of the ovum is meroblastic, and
there is no metamorphosis.

Sub-Class I. — Dibranchiata,

Cephalopoda in which the propodium assumes the character of
a circlet of either eight or ten arms bearing suckers, surrounding
the mouth. The funnel forms a complete tube. The shell is
usually internal ; when external its cavity is not divided by septa.
There are two ctenidia, two nephridia, and two branchio-cardiac
vessels or auricles. An ink-gland and duct are present.

ORDER 1. — DECAPODA.

Dibranchiata possessing ten arms, with stalked suckers, provi(
with horny rims, and with a well-developed internal shell.

This order includes the Cuttle-fishes, Squids, Spirula, and others,
as well as the extinct Belemnites.

ORDER 2. — OCTOPODA.

Dibranchiata provided with eight arms, the suckers on which
are sessile and devoid of horny rims : with or without slight vestiges
of an internal shell. An external shell, secreted by a specially-
modified pair of arms, is present in the female Argonaut only.

This order includes the Octopi and the Argonauts.

Sub-Class II. — Tetrabranchiata.

Cephalopoda, in which the propodium has the character of lobes
bearing numerous tentacles. The funnel does not form a com-

XII

PHYLUM MOLLUSC A

737

plete tube. There is an external, spiral, 'chambered shell. There
are four ctenidia, four nephridia, and four auricles. The ink-gland
is absent.

This sub-class includes only one living genus, Nautilus, but
the Ammonites and other extinct forms are usually referred to it.

Systematic Position of the Examples.

The genus Sepia is a member of the family Sepiidm of the order
Decapoda, which is distinguished from the seven other families of
the order by the combination of the following features : — The
body is compressed and comparatively broad ; the fins are narrow
and elongated ; the internal shell consists almost entirely of cal-
careous material.

Nautilus is the sole living representative of the sub-class Tetra-
branchiata.

3. GENERAL ORGANISATION.

The uniformity of structure among the Dibranchiate Cephalopoda
is very great, and, as already stated, Nautilus is the only living

PIG. 646. Octopus vulgaris. A, at rest ; B, in motion ; /. funnel, the arrow showing the

direction of the propelling current through the water. (From Cooke, after Merculiano.)

member of the Tetrabranchiata, so that comparatively little has to
l>e said to supplement the descriptions of these two examples.

External Features. — The general external shape differs very
little in the different members of the Dibranchiata : the body in
some is more elongated, in others, less ; the degree of compression

VOL. i 3 B

738

ZOOLOGY

SECT-

likewise varies. Fins may be absent, and the animal may progress
entirely by creeping with the aid of the long arms, or
by swimming by the movements of the arms, or under the
propulsion of a current of water forcibly ejected through the
funnel by the contraction of the muscular mantle. (Fig. 646.)
When fins are present they may not take the form of a continuous
lateral flap as in Sepia, but, more usually, are of the nature of
flattened lobes situated towards the aboral extremity of the body

(Fig. 647); in Cten-
jf t opteryx they have the

character of fringes of
filaments. The arms
vary in length and
proportions and in the
form and arrangement
of the suckers. Eight
arms are present in
the Octopoda and ten
in the Decapoda. In
the former group the
Argonauts (Fig. 648)
have, in the female,,
one of • the pairs of
arms (wa.), flattened
and expanded at the
extremities for the se-
cretion and support of
the shell (sh.). In the
Decapoda one of the
pairs of arms, the
fourth, is always speci-
ally modified, as in
Sepia, to act as pre-
hensile appendages or
tentacles capable of
being partly or entirely
retracted within cer-
tain sacs situated at

their bases. In nearly all one of the arms is specially modified
(or Jiectocotylised) to act as an intromittent organ. This modifi-
cation is only very slight in Sepia and confined to the base.
It is most marked in certain of the Octopoda (Fig. 649), in-
cluding the Argonauts. In the latter, before the breeding
season, the third arm in the male is found to be represented
by a rounded sac. This subsequently bursts and sets free the
elongated hectocotylised arm. Spermatophores are taken by
the arm from the genital opening, and in the act of copulation.

FIG. 647.— Loligo vulgaris.

horny internal

view ; B,

Kefersteiii.)

A, entire animal, dorsal
shell or pen. (From

i

XII

PHYLUM MOLLUSCA

739

the entire arm is detached, and left in the mantle-cavity of
the female. In other cases the arm is not detached. The
suckers are sometimes stalked, sometimes sessile, sometimes

FIG. t,4s.— Argon auta arpo, showing the relations of the animal to the shell in the living
state, the arrow showing the direction of movement. /. funnel ; in. mouth, with jaws project-
ing ; sh. shell, with arms as seen through it ; wa, webbed arm clasping the shell. ^(From
Cooke, after Lacaze-Duthiers.)

armed with hooks, sometimes replaced by hooks. In many cases
the arms are united by a web-like fold, the interbrachial mem-
brane (Fig. 650), which may reach nearly to 'their extremities.

In the Tetrabranchiata the series of groups of slender, ringed,
sheathed tentacles,- situated on lobes of the foot surrounding the

Fio 049 —Octopus lentus, male specimen, showing the structure of the hectocotylised'arm
(h. a.). (From Cooke, after Yen-ill.)

mouth, take the place of the arms, and suckers are not present.
In the males the spadix probably represents, functionally at least,
the hectocotylised arm of the Dibranchiata.

Nautilus is the only Cephalopod that has any head-appendages

SB 2

740

ZOOLOGY

SECT.

in addition to those formed by modification of foot ; they consist
of the two short tentacles situated on each side near the eye. In

all the Dibranchiata the funnel is
a complete tube. In the Nautilus,
on the other hand, as we have seen,
the folds which form the funnel
have their edges merely in apposi-
tion, and not united. A valve, such
as has been described in Sepia,
accurs in most Decapoda and in
Nautilus, but is absent in the
Octopoda. Ckromatophores, similar
to those of Sepia, are universal in
the Dibranchiata but absent in
Nautilus.

Shell.— The shell of Nautilus is
the most complete and yet in a
certain sense the most primitive.
As already stated, it is an external
shell of a spiral character, divided
internally by septa into a series of
chambers. The last of the cham-
bers is occupied by the body of the animal ; the 'rest are filled
with gas. Perforating the middle of all the septa in succession is

FIG. 650.— Amphitretus pelagicus,

an Octopod with the arms united by a
web. e. eyes ; /. funnel ; p. pouch in
the mantle. (From Cooke, after Hoyle.)

Fio. 651.— Shell of Spirula. A, outside view ; B, showing last chamber and position of
siphuncle ; C, in section, showing the septa and the course of the siphuncle ; D, shell broken
to show the convexity of the inner side of the septa ; E, portion of a septal neck. (After
Cooke.)

.

a spiral tube — the siphuncle — continuous with the centro-dorsal
region of the visceral prominence. In the course of its growth

XII

PHYLUM MOLLUSCA

741

the body of the Nautilus shifts forwards at intervals into a
newly formed chamber, and a new septum is formed closing the
latter off from the cavity last occupied.
Of existing Dibranehiata, Spirula alone
has a shell (Fig. 651) comparable to that
of Nautilus. The shell of Spirula is of
spiral form, the turns of the spiral, how-
ever, not being in close contact. In-
ternally it is divided into chambers by
a series of septa, and these are perforated
by a siphuncle. But the initial chamber
(protoconch) instead of being, like the
initial chamber in Nautilus, similar to
the others though smaller, is dilated into
a spherical shape, constricted off from the
succeeding chamber, and has passing
through it a tube — the prosiplwn — not
continuous with the siphuncle. Again,
as will be seen by comparing Figs. 638
and 652, the relation of the soft parts to
the shell is the reverse of what obtains
in Nautilus, the shell of Spirula curving
backwards, that of Nautilus forwards.
Moreover the shell of Spirula (Fig. 652)
is an internal structure, being almost
completely covered by the mantle.

The shell of the extinct Ammonites (Fig. 653), which are usually
referred to the Tetrabranchiata, resembles that of the Nautilus in
many respects, being a chambered spiral shell with a large terminal
chamber, and with a siphuncle. The chief external difference is

in the form of the sutures, or lines
of union of the edges of the septa
with the side wall of the shell ; these
are more or less complexly lobed, in-
stead of being entire as in Nautilus.
But in one important respect the
shell of an Ammonite differs from
that of Nautilus and approaches that
of the dibranchiate Spirula. At the
apex of the spiral is an initial cham-
ber or protoconch, which is dilated
and separated from the first of the
ordinary chambers by a constriction,
and has passing into it a prosiphon not continuous with the
siphuncle. The Ammonite was also characterised by the posses-
sion of a structure sometimes horny, sometimes calcareous, called
the aptychus, not represented in any existing form. The aptychus,

FIG. 652.— Spirula peronii,

lateral view ; d, terminal
sucker; /. funnel; s. s.i s.2
projecting portions of the
shell, the internal part of
which is indicated by dotted
lines. (From Cooke.)

FIG. 653.— An Ammonite (Ceratites
nodosus.)

742

ZOOLOGY

SECT.

pen

phr

which was composed of two parts, seems to have been of the
nature of an operculum for closing the mouth of the shell.
Young Ammonites, each with its aptychus,
have been found within the shell of the
parent, in which they must have remained
protected during their development.

In the ordinary decapod Dibranchiata
the shell may consist of three parts — a
horny pen or pro-ostraeum, a calcareous
guard, and a part termed the phragmocone.
The last, which alone represents the shell
of Spirula, has the form of a cone divided
internally by a series of septa perforated
by a siphuncle. These parts are most
completely developed in the extinct genus
Belemnites, in which the shell (Fig. 654)
consists of a straight, conical, chambered
phragmocone (phr.\ with a siphuncle, en-
closed in a calcareous sheath, the guard,
produced into a horny or calcareous plate,
the pro-ostracum (pen.). In Sepia the spine-
like projecting point represents the guard,
and the main substance of the shell is to

be looked upon as the pro-ostracum and phragmocone, the septa
of the latter being represented by the calcareous lamellae. In
Loligo (the Squids) the shell (Fig. 647, B) is long, narrow, and

FIG. 654.— Shell of a Belem
nite. gd. guard ; pen,
pro-ostracum ; phr. phrag-
mocone. (From Nicholson
and Lydekker's PahKonto-

Fio. C55.— Shell of Argonauto argo.

completely horny ; it corresponds to the pro-ostracum, the phrag-
mocone being entirely absent.

In Octopus the shell is represented only by a pair of rudiments
with which muscles are connected. In Argonauta there is no shell

•xii PHYLUM MOLLUSCA 743

in the male, but the female has an external shell (Fig. 655) of a re-
markable character. This is a delicate spiral structure, the internal
cavity of which is not divided into chambers. It is not secreted
l>y the mantle like the shells of other Mollusca, but by the surfaces
•of a pair of the arms ending in expanded disc-like extremities,
which become applied to its outer surface ; its chief function is to
•carry the eggs.

An internal cartilaginous skeleton is present not only in
Sepia and Nautilus, as already described, but in all the Cephalo-
poda. Such an internal skeleton occurs in other groups — some
'Chsetopoda (p. 433) and Arachnida (p. 618), but attains a much
more elaborate character in the present group than in any other
Invertebrates.

The plume-shaped gills, lodged in the mantle-cavity, are two
in all the Dibranchiata, as in Sepia. In the Tetrabranchiata there
•are four gills, similar in general character to those of the Di-
branchiata. Osphradia are present at the bases of the gills only
in the former sub-class.

The coelome in the Dibranchiata has the extent already
indicated (p. 715) in the case of Sepia, except that in the Octopoda
the oral part does not exist. In Nautilus it encloses, besides the
heart and gonad, the vena cava and a part of the glandular
appendages of the afferent branchial vessels. " In the Dibranchiata
the pericardial portion communicates with the nephridia ; in
Nautilus this communication is absent, but the coelome opens on
the exterior by two symmetrical orifices placed at the side of the
openings of the aboral nephridia.

Alimentary Organs. — Jaws similar to those of Sepia are
present in all the members of the class ; in Nautilus, instead of
being completely horny, they are partly calcified. Buccal cavity,
oesophagus, stomach, intestine, salivary glands, and liver are all
of the same general character throughout all the members of the
class. In some of the Dibranchiata, such as Octopus, there are
two pairs of salivary glands. In Nautilus the salivary glands are
absent, so far as known, the oesophagus is^ dilated to form a sort
of crop, and the stomach is gizzard-like. In that genus also the
ink-gland, general in the Dibranchiata, is absent, and there is a
€a3cal appendage to the intestine ; the liver is four-lobed, each
lobe having its duct. The so-called pancreas, described in Sepia,
is similarly developed in all the Dibranchiata, and is present also,
though only feebly developed, in the Tetrabranchiata.

Heart and vascular system are well developed in the
Cephalopoda, and their structure and arrangement closely corre-
spond with what has been described in Sepia, except that in
Tetrabranchiata there are, as already stated, in accordance with
the double number of gills, four auricles instead" of two, and
branchial hearts are absent.

744 ZOOLOGY - SECT..

Nervous system and sense organs. — The ganglia of the
central nervous system are in all closely aggregated together round
the oasophagus, as already stated to be the case in Sepia ; and the
general disposition is the same as that described. In Octopus the
ganglia are much less sharply marked off. In Nautilus, as
already stated, there is less concentration, and distinct ganglia are
absent. All the Dibranchiata possess highly developed c/A'*
similar to those of Sepia ; but in Nautilus the eyes are of a much
simpler character, each consisting of a sac opening on the exterior
by a small rounded aperture, lined internally by a two-layered
retina similar to that of Sepia, but without lens, vitreous humour.,
or cornea. In the embryo of the Dibranchiata, the eye passes
through a stage in which it is in the condition of an open cup
similar to the adult eye of Nautilus. Osphradia are present, as
already mentioned, only in the Tetrabranchiata ; but in both the
Dibranchiata and the Tetrabranchiata certain sensory processes
or depressions conjectured to possess an olfactory function are
developed on the head. Otocysts are universally present.

All the Dibranchiata have two nephridia similar in' character
to those of Sepia, and communicating with one another ; in Octopus
they are completely united. In the Tetrabranchiata there are
four nephridia, each opening on the exterior.

The sexes are distinct in all the Cephalopoda, and in addition
to the hectocotylised arm, there are frequently other external
differences between male and female. In all the Dibranchiata
the arrangement of the gonads and gonoducts is, as regards
general features, similar to what we find in Sepia. In Octopus,
however, there are two oviducts instead of one, and in one other
member of the Octopoda (Eledone moscliata) the same holds good
of the spermiducts.

Development. — The development of the Dibranchiata alone is
known. The eggs are very large, containing a relatively large
amount of food-yolk. They are usually laid in masses or strings
embedded in soft gelatinous, or tougher, more leathery substance,
usually attached to some foreign body ; in some cases each egg,
enclosed in its gelatinous sheath, is attached by a longer or shorter
stalk. A chorion or delicate transparent egg-membrane, in which
there is an aperture — the micropyle — immediately invests the
egg itself. In shape the egg is oval or spherical. The greater
part of the comparatively small quantity of protoplasm lies as
a disc-like elevation on the surface of the yolk on the side of
the egg at which the micropyle is situated. Continuous with this
germinal disc is a thin layer of protoplasm — the peripheral pvo
plasm — investing the entire ovum.

Segmentation (Figs. 656 and 657) is incomplete, being confined
to the germinal disc. At an early stage in the process of division,
the blastoderm exhibits a distinct bi-lateral symmetry. At first

XII

PHYLUM MOLLUSCA

745

dors

it consists of a single layer of cells — the ectoderm. Later on a
thickening is observable at the periphery, due to the development
of a second layer, which by
degrees extends inwards until
it comes to underlie the entire
ectoderm. This second layer
is commonly termed the meso-
derm, though it differs from
the middle layer of other em-
bryos in giving rise to the
enteric epithelium of the
mesenteron. The ectoderm
now extends rapidly, and even-
tually encloses the entire yolk.
Below the second layer extends
a thin stratum derived from
the peripheral protoplasm, con-

posl

vent

FIG. 656. — Segmenting ovum of LoligO.
(From Korschelt and Heider, after Watase.)

taining nuclei which appear to

have been previously scattered

through the yolk ; this is

termed the yolk epithelium (Fig. 658). It serves the purpose of

absorbing the yolk-substance for the benefit of the developing

/ * v^y •'•••' «v«'';*v*«'-*'/*\«'r«

* Jr\*^(i£i >I •;*•-«••.••.>'• *>_^

j/>fe

plc 657 Sepia blastoderm at a late stage of- segmentation. W. blastoderm ; yk. yolk. (From

Korschelt and Heider, after Vialleton.)

embryo. About the middle of the blastoderm appears a thickening
of a cap-like shape, the edges of which become raised above the

746

ZOOLOGY

SECT.

general level of the blastoderm ; this is the rudiment of the
mantle. On the surface of this is developed a depression which
subsequently forms a closed sac — the shell-gland (Fig. 659, sh. gl.).
Below the mantle — i.e. nearer the vegetal pole — appear two eleva-
tions each with a pit-4ike depression, the rudiments of the eyes ;
and still nearer the vegetal pole a series of paired elevations,
the rudiments of the arms.

After the complete enclosure of the yolk by the blastoderm, the
mouth (mo.} is developed as an oval depression between the rudi-
ments of the eyes. Immediately in front of the edge of the mantle
appear two short ridges, the beginnings of the gills (cten.), and a pair
of folds — the posterior funnel folds (post. f. f. ) — which are formed
between these and the eyes — are the first rudiments of the funnel ;

FIG. 658.— Sections through the edge of the blastoderm of Sepia at three successive stages
II. blastoderm ; yk. yolk ; yk. cp, yolk epithelium. (From Korschelt and Heider, afte
Vialleton.)

the greater part of which, however, is formed from a second pair
of folds — the anterior funnel folds (ant.f. f.) — developed further
forwards. Behind the anterior funnel folds appear two pit-like
depressions, which subsequently develop into the otocysts.

The elevations on which the eyes (eye) are situated become
more and more prominent. The eyes themselves are formed
from a part only of these elevations ; each is a pit which sub-
sequently becomes closed — to form a vesicle — the optic vesicle:
later an ingrowth of the ectoderm over this gives rise to
the lens.

The embryo covers only a part of the egg, and as it develops, it
withdraws itself more towards the animal pole, at which the
germinal disc was originally situated, a constriction, which soon
becomes very deep, separating it off from the rest of the egg ; the

XII

PHYLUM MOLLUSCA

747

latter, consisting of the greater part of the yolk enclosed in a thin
layer of blastoderm, forms a rounded appendage of the embryo — the
yolk-sac (yk. s.). The yolk-sac undergoes contractions, which are
due to the action of contractile cells in the thin mesoderm
lining it,rand by this means the yolk is forced into the interior of
the body^of the embryo.

iFio. 659. — Early stages in the development of IiOligO. A, stage at which the rudiments of the eyes
and of the shell-gland are first distinguishable ; B, later embryo from the oral side ; C and D,
from the anal side. ant. f. f. anterior funnel fold ; ar. rudiments of arms ; cten. ctenidia ;
eye, eye ; mo. mouth ; mant. rudiment of mantle ; ot. otocyst ; post. f. f. posterior funnel fold ;
$li. -jl. shell-gland ; yk. s. yolk-sac. (After Korschelt and Heider.)

The anus appears as an aperture situated on a little papilla — the
•anal papilla. A row of cilia, which are developed in the neighbour-
hood of the mouth in some forms, perhaps represent the velum
or prse-oral circlet of other molluscan embryos. The mantle now
increases in extent, and its margins become more prominent.
'The anterior funnel folds grow out, and become united in the

748

ZOOLOGY

SECT.

middle line ; and these, with the posterior folds, go to form the
completed funnel together with the " neck-muscles." For a time

FIG. 660.— Two later stages in the development of XiOligO, A, from the funnel side. B, obliquely
from above. Letters as in preceding figures ; ne. cart, nuchal cartilage (After Korschelt and
Heider.)

B

FIG. 661.— Two stages in the development of IiOligO. later than those represented in Fig. GOO.
From the anal or funnel side. Letters as in preceding figure ; in addition, ftn, fins. (After
Korschelt and Heider.)

the edges of the two folds which form the funnel remain free ;
eventually they coalesce to form a complete tube.

XII

PHYLUM MOLLUSCA

749

The edges of the mantle grow out into prominent folds to form
the mantle-cavity, into which the gills are drawn. Lateral out-
growths have already formed the rudiments of the fins. The
arms grow out into more and more prominent processes on which
the suckers become developed, the second pair — the prehensile
arms (ar. 2) — soon becoming distinguishable from the rest by its
greater length.

As the embryo increases in size, the yolk becomes gradually
absorbed, and the yolk-sac decreases in bulk, until, when the
embryo leaves the egg, it has almost completely disappeared.

FIG. 662.— Two late stages in the development of lioligo, seen from the funnel side. Letters as
in preceding figures. (After Korschelt and Heider.)

Distribution. — The Cephalopoda are all marine, and range from
tidal limits to a considerable depth. A large number are pelagic.
They are, nearly without exception, carnivorous. In length they
range from an inch or two to as much as fifty feet — the gigantic
members of the group such as Architeuthis, being by a long way
the largest of invertebrate animals. Like the other classes of
Mollusca they are most abundant in tropical and warm temperate
seas.

If the Ammonites are to be included among the Tetrabran-
chiata, that sub-class was most abundantly represented during

750 ZOOLOGY SECT.

the mesozoic period. The nautiloid Tetrabranchiata were
most abundant in the palaeozoic epoch, during which there-
lived a great variety of forms of this group, some having the
shell straight (Orthoceras), or curved (Phragmoceras), or in a flat
spiral with the turns not in contact, or in a helix, or a flat close
spiral (Nautilus, and others). The earliest representatives of the-
Nautiloids are found in rocks of Cambrian age ; they are com-
paratively scarce in the mesozoic epoch and in the tertiary ;
and are represented at the present day only by the genus^
Nautilus itself. The Ammonites are mainly mesozoic, the repre-
sentatives found in the earlier rocks (from the Upper Silurian
onwards) being few in number and simpler in structure than
the more typical later forms. The oldest fossil representatives.
of undoubted Dibranchiata belong to the extinct order of the
Belemnites, which flourished in the mesozoic period from the
Trias to the Cretaceous, and survived in scanty number into the
Tertiary. Unlike the Tetrabranchiata, the Dibranchiata would
appear to have reached their maximum at the present day.

The mutual relationship of the various groups of Cephalopoda
are indicated, as nearly as the information at oui disposal will
allow, in the following diagram (Fig. 663).

Nauh'loids
Ammonrtes

Decafioda

Belemnit-es

FIG. 663. — Diagram to illustrate the relationships of the groups of Cephalopoda.

GENERAL REMARKS ON THE MOLLUSCA.

The Mollusca, like the Arthropoda, form an extremely well-
defined Phylum, none of the adult members of which approach
the lower groups of animals in any marked degree. There are,
however, clear indications of affinity with " Worms," especially
in the frequent occurrence of a trochosphere stage in develop-
ment, in the presence of nephridia, and in the occurrence, in
Amphineura and some of the lower Gastropods, of a ladder-like
nervous system resembling that of some Turbellaria and of the

PHYLUM MOLLUSCA 751

most worm-like of Arthropods, Peripatus. Rhodope, moreover,
shows certain affinities with Flat-Worms. The head-kidneys or
primitive nephridia of the molluscan are practically identical
with those of the annelid trochosphere and are probably homo-
logous with the various types of nephridial tubes found in worms
from Platyhelminthes to ChaBtopoda. From developmental con-
siderations it appears, however, that the permanent renal organs
of Molluscs correspond not with the actual nephridia of Worms
but with their oviducts ; in other words, that they are meso- and
not ectonephridia.

The fact that there is usually a single pair of nephridia and of
ctenidia seems to indicate derivation from a group in which meta-
merism had not arisen ; the segmental arrangement of the shells
and gills of Placophora appears to be a specialised character of
that group and of no phylogenetic importance.

The lowest members of the phylum are undoubtedly the Proto-
branchia among Pelecypods and the Aplacophora among Amphi-
neura. The latter take the lowest rank in virtue of the absence of
both foot and shell, but the possession of an odontophore indicates
a comparatively high degree of specialisation. On the other hand,
while there is no indication of an odontophore, even in a rudi-
mentary condition, in the Pelecypoda, the foot and shell are well
developed even in Nucula and its allies. There is no actual
evidence to show that the foot and shell have been lost by degenera-
tion in the Aplacophora or the odontophore in Pelecypoda, and it
would appear, therefore, that the two groups are to be derived
independently from some primitive form.

The facts that the pelecypod shell, at its first appearance, is
univalve, that the foot of the Protobranchia is of the creeping type
and their ctenidia plume-like, suggest the derivation of the
class from a form resembling a simple type of Gastropod with no
odontophore and with undisturbed bilateral symmetry. The Amphi-
neura are also bilaterally symmetrical, with paired ctenidia, kidneys,
and auricles, and the fact that these organs are also paired in the
lower Gastropoda, seems to point to a common ancestor for Pele-
cypods, Amphineura, and Gastropods, which was bilaterally sym-
metrical, had a creeping foot, a simple shell, paired auricles,
kidneys, and gills, and no odontophore.

While the leading feature in the evolution of the Pelecypoda
has .been the splitting of the mantle into two halves and the
resulting bivalve shell, the most noticeable fact in that of Gastro-
poda, apart from the appearance of the odontophore, has been the
torsion of the visceral mass, producing a characteristic asymmetry.
In the Cephalopoda, on the other hand, the primitive bilateral
symmetry is retained, and the most characteristic special feature
of the group is the extraordinary modification of the foot into arms
or tentacles and funnel. The class is raised far above the remain-

752 ZOOLOGY SECT, xn

ing Mollusca by its wonderfully high organisation, especially in
the nervous system and the eye, and there is nothing to indicate
close relationship with any of the lower classes, beyond the general
conformity to the molluscan plan of organisation, and the presence
of an odontophore. The Cephalopods form, in fact, a singularly
isolated group. Palaeontology has not hitherto given any indica-
tion of their origin, and embryology is equally silent, the absence
of a free larva, and the profound modification in development
produced by the enormous mass of food-yolk sharply separating
them from all other members of the phylum.

INDEX

VOL. I 3 C

INDEX

All numbers refer to pages : words in italics are names of families, genera and
species: words in thick type are names of higher divisions: words in small
capitals are names of examples. Numbers in thick type are numbers of pages on
which there are figures : an asterisk after a number indicates a definition of the
term or of the group.

A Adamsia palliata, 195, 196

Adductor impressions, 633, 634, 650
Adductor muscles, 635, 648, 645, 646,

647, 649
Adhesive cells, of Hormiphora, 202 : of

Turbellaria, 248
Adipose tissue, 23*, 25
Adradius, 129
Adrectal gland, 693

144
143

188 : Addenda
J<'t/t/orea, 133

Affinities — See Relationships
A'galma, 151
Agamobium, 130

Aggressive characters, in Crustacea, 555
Air-sacs, of Insects, 593, 594
A/liertia, 309
Alciopidce, 447
Alcippe, 522, 534
Alcyonacea, 182*, 195
Alcyonaria, 182*, 187, 188, 197
Alcyonidium, 320
Alcyonium, 182, 190
Alecithal, 206*

Alimentary canal — See Digestive system
Alimentary system — See Digestive system
Alphem, 556
Alpine forms, 8

Alternation of generations — See Meta-
genesis

Alveolus, of Sea-urchin, 366
Ambulacral grooves, 347*
Ambulacral ossicles, 349
Ambulacral pores, 349
Ambulacral system, of Asterias, 350 :
Echinus, 367 : Sea-cucumber, 370, 371 :
of Antedon, 375 : of Echinodermata,
393

3 c 2

.BACTIXAL, 347*

Abdomen, of Apus, 489 ; Astacus, 498 ;
Periplaneta, 572, 574

Absorption, 32

Abyssal species, 8

Acanthocepliala, 275* : External charac-
ters, 290 : Body-wall, 290 : Body-
cavity, 290 : Proboscis, 290 : Vessels,
291 : Nervous system, 291 : Excretory
organs, 292 : Reproductive organs,
291, 292 : Development, 292

Acarida, 612*, 616, 618, 619, 620, 621

Achromatin, 15*

Aciculum, 405*

Acineta, 90, 91, 92

Acwla, .252

Acontia, 175, 188

Acorn-shells, 534

Actinal, 347*

Actinia, 180, 214

Actiniana, 181, 183, 185, 1SS

ActinoboliM, 84, 85

ActinodactyleUa, 244, 245, 251 1

Actinometra, 392

Actinomma asteracanthion, 60

Actinoxthryx sol, 55

Actinosphcerium, 56

Actinostome, 347*

Acfinotrocha, 330, 462

Actinpzoa, 118*: Example, 172: Dis-
tinctive characters and classification,
180 : Systematic position of example,
183 : General organisation, 183 : Bud-
ding, 184 : Structure of polypes, 185 :
" it^ric system, 188 : Fixed and free
>rms, 189 : Dimorphism, 189* : Skele-
m. 189 : Colour, 195 : Commensalism,
Distribution, 196

756

INDEX

Ammonites, 737, 741

Amnion, of Peripatu*, 564 : Perijrfaneta,
581 : Scorpion, 610

AMOSBA, 10, 11, 12, 13, 14: Pseudopods,
44 : Endosarc, 44 ; Ectosarc, 44 : Con-
tractile vacuole, 44 : Encystation, 44 :
Fission, 44 : Systematic position, 46

Amcebocytes, 352, 368

Amcebula, of Greyariiia, 77

Amphiblastula, 113", 114

Amphidiscs, 112

Amphilina, 267

Amphmeura, 631 : Distinctive characters
and classification, 663 : General organi-
sation, 663 : External features, 663,
664, 665 : Alimentary system, 665 :
Body-cavity, 666 : Vascular system,
666 : Nervous system, 666, 667 : Re-
productive and renal organs, 668 :
Development, 669 : Ethology, distribu-
tion, etc., 670

Amphipoda, 524*, 543, 544, 545, 547, 550

Amphiptyches, 247, 2(>7

Amphixtonuun, 242

Amphitrettis pelagicits, 749

Ampulla, 349, 350

A't»p/i//(iria, 699

Amuvium, 653, 655 .

Anal filament, 614

Anal glands, of Pert pat u*, 562

Anal respiration, 550

Anal spot, of Paramuecium, 81

Anatomy, 3*

Awjuillida, 283

Anisopoda, 543

Annulata, 403 : General remarks on, 481

Annuli, of Leech, 463

ANODONTA, 631 : Shell, 633 : Body, 635 :
Muscles, 635 : Coelome, 635 : Digestive
organs, 636 : Gills, 636 ; Excretory
organs, 640 : Circulatory system, 640 :
Nervous system, 641 : Sensory organs,
642 : Reproductive organs, 642 : De-
velopment, 643 : Systematic position,
647

Anodonta, 649, 652, (554, 655, 659

Anomia, 646, 648, 649

Antedon, 391

Anoplophyra, 85

ANTEDON ROSACEA, 373 : General external
features, 373 : Ossicles, 374 : Coelome,
374 : Enteric canal, 375 : Ambulacra!
system, 375 : Nervous system, 375 :
Blood-vascular system, 376 : Sacculi,
376 : Reproductive organs, 377 : Meta-
morphosis, 377 : Systematic position,
381 : Development,^ 398

Antenna, of AxfarH*, 502 — See also Ap-
pendages

Antennary gland, of Astacus, 509, 510

Antennary glands, 523, 55n

Antennule, of A$tacnv, 502 — See also
Appendages

ANTHENEA FLAVESCENS, 356, 357, 358

Anthomedusae, 131*, 133, 134

Anthophysa, 66

Anthojonia, 530, 531

Antimeres, 40

Antipatharia, 182*, 187, 188, 190, 197

Ant-lions, 585

Ants, 587, 603

Aorta— See Vascular system

Aphides, 598

Aphroditea, 435, 436

Apical plate, of Trochosphere, 298

Apical system of plates, 362, 365

Apia mellifica, 589, 603

Aplacophora,663, 664,666, 668, 669, 671

Aplysia, 687, 688, 695

Aph/xiidw, 684

Apoda (Holothuroidea), 380

Apopyle, 99, 101*, 108

Appendages, of Rotifera, 306 : Apn«,
487, Astacus, 500, 501 : Crustacea,
525 : Peripattix, 560 : Peripnetalff,,
572 : Insecta, 588 : Scorpion, 606

Aptera, 583, 591

Aptus rositj , 585

Aptychus, 741

APUS, 484 : External characters, 485 :
Appendages, 487, 488 : Body-wall,
489 : Muscular system, 490 : Digestive
organs, 490, 491 : Body-cavity, 491 :
Circulatory system, 491 : Respiration,
4!»2 : Renal organ, 492: Nervous system,
492, 494 : Organs of sense, 494 : Re-
productive organs, 495 : Development,
496 : Systematic position, 524

J////.S, 552

Apii* cancriformis, 485, 489

Apn* f/!ari(.(/i«, 487

Aquatic pupa, 602

Arachnida, 484, 604 : Example, 604 :
Distinctive characters and classifica-
tion, 611 : General organisation, 613 :
External form, 613, 619 : Endosternite,
618 : Coxal glands, 618 : Alimentary
system, 618 : Heart, 619 : Organs of
respiration, 619 : Nervous system,
621 : Sense-organs, 621 : Reproductive
apparatus, 622 : Mode of life, 623 :
Geological history, 624 : Appendix,
624

Arachnidium, 615

Araneida, 612*, 618, 621

Arbacia j)tmctulata, pedicellaria, 389

Area, 646, 648, 652, 653, 655, 658

Areella, 48

Archenteron, 21*

Archi-Annelida, 462, 482

Archi-cerebrum, 513

Archi-cerebrum, of Periplaneta, 5fc-2

ArcM-Chaetopoda, 427

ArcJtigetes, 247, 267

Architeuthis, 749

Aryiope, 338

INDEX

757

A ryonauta argo, 739, 742

Argonauts, 736, 738

Argulu*, 522, 531, 532

Aristotle's lantern, 386

Ark-shell, 653

Armadillidiuyn, 546

Armadillo, 524, 546

Armata, 455, 457, 458, 461

Artemia, 521, 526

Arthrobranchiae, 508

Arthropoda, Classes, 484; Affinities of
air-breathing, 627

Arthrostraca. 524*, 543, 547, 550

Articulata, 338* : Shell, 338

ASCARIS LUMBRICOIDES, 275 : External
characters, 275 : Digestive organs, 278,
279 : Ccelome, 279 : Excretory system,
279: Nervous system, 279, 280: Re-
productive organs, 279, 281 : Develop-
ment, 280 : Systematic position,
282

'/.s itt't/roi'enosa, 286, 287
primordiali* , 107

Ascon, 109

Asellus, 524, 543, 545

Asexual reproduction, 38 : in Amoeba,
44 : Euglena, 64 : Flagellata, 69 :
Choanoflagellata, 73 : Dinoflagellata,
74 : Sporozoa, 75 : Paramoecium, 81 :
Ciliata, 88 : Tentaculifera, 92 : Sponges,
112: Actinozoa, 184: Platyhelminthes,
263 : Bugula, 319 : Chaetopoda, 446

Asiphoniata, 647, 658

Aspqrgilium, 647, 652, 653

Aspidobranchia, 683

Aspidogaster, 264

AxiJanchna, 304, 305, 308

A xtacoides, 523

Astacus, 549

ASTACUS FLUVIATILIS, 498 : External
characters, 499 : Abdomen, 499 :
Thoracic region, 499 : Head, 499 :
Appendages, 500, 501 : Articulations,
.~><>3 : Body-wall, 504: Muscular system,
504, 505 : Digestive organs, 506 : Re-
spiratory organs, 507, 508 : Excretory
organs, 509, 510 : Circulatory organs,
509, 512 : Nervous system, 513 . Sen-
sory organs, 513 : Reproduction, 514,
515 : Development, 515 ; Systematic
position, 525

A Hfasiopsis, 66

ASTERIAS RUBENS, 346 : General external
features, 346, 347, 348: Transverse
section of an arm, 349 : Vascular and
nervous systems, 350 : Structure of the
disc, 351 : Body-wall and ccelome, 351 :
Digestive system, 353 : Ambulacra!
system, 354 : Ovoid gland, 354 : Re-
productive system, 356 : Systematic
position, 380

Axterina, development, 358, 359, 380,
361, 362

Asteroidea, Example, 346 : Development,
358 : Distinctive characters and classi-
fication, 378 : Apical system, 384 :
Modifications of form, 384 : Ccelome,
392 : Ambulacral system, 393 : Blood-
vascular system, 394 : Enteric canal,
394 : Nervous system, 395 : Repro-
ductive organs, 395 : Development,
397 : Ethology, 399

Axft'tm, 185, 194

Astrophyton, 387

Astrosphere, 16*

Atlanta peronii, 689

Atrochal, 445

Attraction-sphere, 16*

Auditory organs, 37

Aulactinium actinastrum, 60

Aulostoma, 476, 481

AURELIA AURITA, External character-
istics, 156, 157 : Digestive-cavity and
canal system, 158, 159: Cell-layers',
158 : Gonads, 158 : Muscular and
nervous systems, 160 : Sense-organs,
160 : Development and life-history,
160, 161, 162, 163: Systematic posi-
tion, 164

Auricle, of heart, 34

Auricles, of sea-urchin, 366

Auricularia, 372, 397, 398

AutoJytus corn ut it*, 446

Avicularium, 314, 315, 324

Axes, 40*

Axial fibre, 83

Axial sinus, 355

Axis-cylinder, 27, 28

B

B

>ALANTJS, 522, 534
Barnacles, 532, 548
Barrier reef, 197
Basal plate, of coral, 192*, 193
Bdelloida, 303*
Bdelloiira, 264
Bear-animalcules, 626
Bee-parasites, 602
Bees, 587, 589, 598, 603
Beetles, 587, 591
Belemnites, 736, 742
Benthos, 8
Berenice, 139
Beroe, 211 : Section, 214
Beroida, 208, 211
Bilateral symmetry, 39*, 41
Bile, 32

Bilharzia hcematobium, 265
Binomial nomenclature, 1*
Biology, 1*
Bionomics, 9
Bipaliwn, 240
Bipinnaria, 361, 397 '
Bird-lice, 602

758

INDEX

Bir(/ti«, 528, 54!)

Bivium, 348, 366 : of Sea-cucumber, 369

Black coral, 182

Blastoccele, 20*

Blastoidea, 380

Blastomeres, 2'i-

Blastopore, 21*

Blastosphere, 20*

Blastostyle, of Obelia, 119," 120: Septo-
Iina3, 140 : Porpita, 153

Blastula, 20*

Blatta—See Periplaneta

Blattidv, 587

Blood, 82

Blood-vascular system — See Vascular
system

Blood-vessels, 32

Blow-flies, 586

Blue coral, 182

Body-cavity — See Coelome

Body-wall, of Sea-anemone, 175 : Hormi-
phora, 202: Liver-fluke, 226: Platy-
helminthes, 248 : Nemertinea, 269 :
Atcaris, 276: Nematoda, 283: Ch^to-
gnatha, 293 : Brachionus miens, 300 :
Bugula, 316: Ectoprocta, 323: Magel-
lania, 335 : Asterias, 351 : Sea-cucum-
ber, 370 : Nereis, 408 : Earthworm,
419 : Cha3topoda, 434 : Sipuncuius, 451 :
Gephyrea, 456 : Hirudo, 467 : Apus,
489 : Astacus, 504 : Crustacea, 547 :
Peripatus, 561 : Myriapoda, 569 : In-
secta, 588

Bojanus, organs of, 640

Bolina hydatina, 211

Bone, 25*, 26

Bone-corpuscles, 25"'

Bondlia, 455, 456, 458, 460

Book-gills, 621

Book-lungs, 608, 619, 621

Book-scorpions, 611

Bopyrini, 546, 547

Bopyrus, 524

Bot-fly, 586

BothriocepJialus, 256, 259

Bothriocephalus latus, 265

Botryoidal tissue, of Leech, 467

Bougainvillea, 133, 134, 140

Brachial disc, of Discomedusa?, 171

Brachiolaria, 397

Brachionidce, 304*

Brachionux, 307

BRACHIOXUS RUBENS : External charac-
ters, 299, 300 : Body-wall, 300 : Di-
gestive organs, 300, 301 : Ctelome, 301 :
Excretory system, 301 : Nervous sys-
tem and sense-organs, 301 : Reproduc-
tion and development, 302 : Systematic
position, 304

Brachiopoda, 331 : Example, 321 : Dis-
tinctive characters and classification,
337 : Systematic position of example,
338: General organisation, 338 : Shell,

338, 339 : Peduncle, 339 : Lophophore,
339 : Muscular system, 339 : Enteric
canal, 340 : Heart, 340 : Nephridia, 340 :
Nervous system, 341 : Gonads, 341 :
Development, 341 : Distribution, 342

Brachyura, 523*, 539

Bract, 489

Brain, 3&^

Branckellion, 475, 476, 477, 481

Branchiaj, 33* : of Asterias, 347 : Sea-
urchin, 364 : Polychseta, 433, 434 :
Oligochseta, 434 : Branchellion, 475,
477 : Astacus, 507, 508 : Crustacea,
548 : Anodonta, 636, 637, 638 : Pelecy-
poda, 653: Triton, 676, 677: Gastro-
poda, 690

Branchial formula, of Astacus, &c., 549

Branchipus, 521, 526

Branchiura, 522*, 528

Brine-shrimp, 526

Brood-pouch, 527

Buccal cavity, 30

Buccinum* undcUum, 671

Budding, 38* — See Asexual reproduction

Budding, in' Turbellaria, 240

Buffori, 5

Bugs, 585, 591, 602

Bufjula, 320

BUGULA AVICITLABIA, 314, 315 : Bodv-

wall, 316 : Ccelome, 316 : Alimentary
canal, 317 ; Nervous system ; Repro-
ductive organs ; Development, 317,
318 : Systematic position, 320

Bw/ula piumosa, 317

Bmycon, 699

BUTHUS, 604 : External features, 605 :
Digestive system, 607, 608, 609 : Cir-
culatory organs, 607, 608 : Organs of
respiration, 608 : Nervous system, 608,
609 : Organs of special sense, 609 : Re-
productive organs, 609 : Development,
610

Butterflies, 587

Byssus, 645, 653

Byssus-gland, 645, 652

Byssus, provisional, 644

G

C

/ADDIS-FLIES, 585

Calcarea, 103% 111, 112, 115, 116
Calcareous spicules, of Sponges, (

111, 112

Calciferous glands, 421
Caltianira, 209
Callitiara, 139
Ccdoccddnus, 529
Calotte, 317

Calymna, of Radiolaria, 58
Calyptoblastea, 133*
Cambrian, 7
Canaliculre (bone), 25

INDEX

759

Canals, Haversian, 25, 26 : of Medusa,
125, 126

Canal system of Sponges, 108, 109

tfancer, .5243 540

Cannostomae, 164*, 169, 170, 171

Capillaries, 33*

Capillitium, of Mycetozoa, 62

Caprdla, 524, 545, 546

Carabus auratm, 592

Carapace, of Apns, 485 : of Astacus, 498 :
of Scorpion, 605

Carboniferous, 7

Cardiac sac, of Polychieta, 437

Cardium, 647, 652

Carina, of Cirripedia, 533

Carinaria mediterranea, 687

Cartilage, 24*, 25 : Hyaline, 24, 25 :
Fibrous, 24, 25 : Yellow elastic, 24 :
Calcified, 25

CABYOPHYLLJEUS, 247, 256

Caudal spine, 617

Caudal styles, of Apus, 485

Cell, animal, 14, 15*

Cell, 14, 15*, 17 : Forms, 21 : Ciliated,
21, 43 : Flagellate, 21, 43 : Amoeboid,
43 : Encysted, 43

Cell-colony, 50

Cell-division, 16,17

Cell-wall, 15

Cetlepora, 320

Cellulose, 14

Centipedes, 567, 568

Central capsule of Radiolaria, 58

Central nervous system of Medusae, 138

Centro-dorsal ossicle, of Antedon, 374

Centrolecithal, 207, 496

Centrosome, 15*

Cephalic apodeme, of Apus, 491 : of
Astacus, 499

Cephalopoda, 631, 708. Examples, 708,
725 : Distinctive characters and classi-
fication, 736 ; Systematic position of
the examples, 737 : General organisa-
tion, 737 ; External features, 737, 738,
739, 740 : Shell, 740, 741, 742 : Inter-
nal skeleton, 743: Gills, 743: Os-
phradia, 743 : Coelome, 743 : Ali-
mentary organs, 743 ; Heart and vas-
cular system, 743 : Nervous system
and sense-organs, 744 : Nephridia, 744 :
Development, 744, 745, 746, 747, 748,
749 : Distribution, &c., 749

Cephalo thorax, of Astacus, 498

Cerata, 693

Ceratites nodosus, 741

Ceratium, 74

Ceratosa, 116 '

Cercaria, 230*, 231, 259

Cerci, 574

Gericmthus, 187, 189

Cervical fold, of Apns, 485

Cervical groove, of Antaeus, 498

Cervical sclerites, 588

Cestida, 208*, 210

Cestoda, 238% 245, 246, 247, 253, 254
256

Cestode, development, 259, 260

Cestus i'< n< /v'.s, 210

Chcetoderma, 663, 664, 665, 666, 668

ChcetogaMa, 446

Chsetognatha, 275*, 293 : External cha
• racters, 293 ; Body-wall, 293 : Enteric
canal, 293, 294 : Ccelome, 294 : Ner-
vous system, 294 : Sensory organs,
295 : Reproduction, 295 : Develop-
ment, 295

Chcetonotus, 311

Chgetopoda, 403 : Examples, 404, 417 :
Distinctive characters and classifica-
tion, 426: Systematic position of ex--
am pies, 428 : General organisation,
429 : General form, 429, 430 : Para-
podia and setse, 430, 431 : Branchite,
433, 434 : Co?lome, 435 : Enteric canal,
435, 436 : Blood-vessels, 436 : Nervous
system, 437 : Organs of special sense,
438 ; Organs of excretion, 440 : Repro-
ductive organs, 441 : Development,
442 : Asexual reproduction, 446 : Mode
of life, &c., 447 : Appendix, 448

Chcetosomidce, 296, 297

Chalk, 55

Charybdwa marsupialis, 166, 168

Cheilostomata, 320

Chela?, 500*

Chelicer*, 606, 613, 615, 616, 617

Chelifer bramisii, 613

Chelipeds, 500*

Chilaria, 617

Chilopoda, 567, 568, 569

Chironomuft, 595

Chitin, 29

Chiton, 631, 663, 664, 665, 666, 667, 689,
670

CMamydomyxa, 48, 49, 50

CUor&midcK, 436

Chloragen cells, 421

Chlorophyll, 14, 64

Choanoflagellata, 65* : General structure,
72 : Collar, 72 : Colonies, 73 : Repro-
duction, 73

Chondracanthu8t 522, 530, 531

Chorion, of Cephalopoda, 744 : of Insects,
599

Choristida, 116

Chromatin, 15

Chromatophores, of Sepia. 711 : of
Cephalopoda, 740

Chrysalis, 602

Chylific ventricle, 577

Cicada, 585, 597

Cidaris, 393

Cilia, 21*

Ciliary flames, 253 : process, 720

Ciliata, 82* : Form of body, 83, 85, 86,
87, 88, 89 : Stalk, 83, 85, 87 : Arrange-

760

INDEX

merit of cilia, 83, 85 : Undulating
membranes, 83 : Meganucleus, 84 :
Micronuclei, 84 : Contractile vacuole,
84, 85 : Non-contractile vacnoles, 84 :
Trichocysts, 84 : Digestive apparatus,
86 : Skeleton, lorica, 86, 87 : Oper-
culum, 87 : Colonies, 88 : Reproduc-
tion, 88, 89 : Conjugation, 90

Ciliated chambers, 109

Cinclides, 175, 188

Circulation — See Vascular sj-stem

Circulatory system — See Vascular system

Cirri, 403*

Cirripedia, 522*, 534, 547, 550, 551, 552

Cistella, 339, 340, 341

Cladocera, 521, 526, 547
• Class, 4*

Classification, 3*, 5 : of Rhizopoda, 45 :
Mastigophora, 65 : Sporozoa, 76 : In-
fusoria, 82 : Porifera, 103 : Hydrozoa,
131 : Scyphozoa, 164 : Actinozoa, 181 :
Ctenophora, 207 : Platyhelminthes, 237 :
Nemertinea, 273 : Nematoda, 281 :
Rotifera, 303 : Polyzoa, 319 : Brachio-
poda, 337 ; Echinodermata, 377 :
Chnetopoda, 426 : Gephyrea, 454 :
Hirudinea, 474 : Crustacea, 519 : In-
secta, 583: Arachnida, 611: Pelecy-
poda, 645 : Amphineura, 663 : Gastro-
poda, 682 : Cephalopoda, 736

Clathrozoon, 136

Clathndina, 57

Clavatella, 135

Cleaning foot, 536

Clepsine, 475, 476, 477, 479

Cliona, 116

Clitellum, 417, 467

ClypeaMer *nl><1<-iir<-x*n*, 390

Clypeastridea, 379

Clypeus*, 572

Cnidoblast, 123*

Cnidocil, 124*

Coccid,.t>, 598

Coccidiidea, 76* : Characteristic features,
78

Coccidium, 78

Cockchafer, 594

Cockles, 631, 647, 661

Cockroach — See Periplaneta

Cockroaches, 584

Cocoa-nut crab, 549

Codonella, 85

Coelenterata, Classes, 118 : Relation-
ships, 213-216 : Appendix, 216 : Rela-
tionships to Sponges, 215

Coelome and body -cavity,, Of A«<-frri*,
279 : Nematoda, 284 : Acanthocephala,
290 : Choetognatha, 294 : Bwjiiln, 316 :
Endoprocta, 326 : Phoronis, 328 :
Mar/ellania, 336 : Asterias, 351 : Sea-
urchin, 3(iS : Sea-cucumber , 371 :
Antedon, 374 : Echinodermata, 392 :
Nereis, 406 : Clut-topoda, 435 : X/y, «•///-

452 : Apiis, 489 : Crustacea,
547 : Peripatits, 561 : Insecta-, 591 :
Anodonta, 635 : Amphineura, 668 :
Sepia, 715 : Cephalopoda, 743

Ccsloplana, 212, 266

Ccenenchyma, 194"

Ccenosarc, 121*

Coleoptera, 587, 589, 596, 598, 602, 605

Collar of choanoflagellata, 72

Collared cells, 98, 99, 100*

Collared monads, 73

Colleterial glands, 597

Collozoum, 59

COLOCHIRUS, 369 : General external fea-
tures, 369 : Structure of body-wall,

370 : Ambulacra! system, 370 : Nerve-
ring, 370 : Vascular ring, 370 : Coelome,

371 : Enteric canal, 371, 372 : Repro-
ductive organs, 372 : Development,

372 : Systematic position, 381
Colony, "38* : of Foraminifera, 50 :

Hefiozoa, 56 : Radiolaria, 59 : Flagel-
lata, 68 : Choanoflagellata, 73 : Ciliata,
88: Tentaculifera, 92: Obelia, 119:
Septolina?, 132 : Actinozoa, 183 : Poly-
zoa, 313: Bugula, 314: Ectoprocta,
321 : Endoprocta, 327

Colpoda, 89

Columella, of Coral, 192*, 193 : of Triton,
672

Comb-jellies, 118

Combs, of Hormipliora, 200, 201

Commensalism, in Sponges, 116 : Hy-
dractinia, 134 : Actinozoa, 195 :
Platyhelminthes, 264 : Chretopoda, 447 :
Crustacea, 555

Complemental males, 551

Conchiolin, 634

Condylostoma, 85

Cones, 684

Conjugation of Ciliata, 90 : of Flagellata,,
70 : of Paramosdum, 81

Connective tissue, 23* : Gelatinous, 23.
24 : Fibrous, 23 : Retiform, 23, 24

Connective tissue cells of Sponges, 102

Contractile vacuole, 11, 44, 79

Contractility of muscles, 26, 35

Conn*, 699

Conroltifa, 240, 25u

Copepoda, 522*, -V2S, 551, 552

Coral, Aporose, 194 : Black, 182 : Blue,
182 : Fossil, 197 : Organ pipe, 182 :
Perforate, 194 : Red, 182, 197 : Reef-
building, 197 : Stony, 182

Coral limestones, 197

Coral reefs, 197

Corallite, 192*

CoraUium, 184, 185, 190, 197

Corallum, 192*

Cordylopliora, 155

Cornea, 720

Corona, of Polyzoa, 317 : of Sea-urchin,
304

INDEX

761

Corpuscles, 28 : amoeboid, 28

Cortex, of Paramcecium. 79 : Spono-es,
102, 110

Coryomorpha, 137

Couplers, 530

Covered-budded Hydroids, 133*

Cowries, 684

Coxal glands, of Peripafus, 562 : of Scor-
pion, 609 ; of Arachnida, 618

Crabs, 537, 540, 541, 549, 553, 556

Crane-flies, 586

Grangon, 523, 538

Crania, 337, 339

Crayfish, 498

Crayfishes, 537, 550, 554

Cretaceous, 7

Crinoidea, example, 373 : Distinctive
characters and cjlassifi cation, 380 :
Modifications of form, 391 : Ccelome,
392 : Ambulacra] system, 393 : Blood-
vascular system, 394 : Enteric canal,
394 : Nervous system, 395 : Reproduc-
tive organs, 395 : Development, 398 :
Ethology, 399

Oriocens, 587

Crisia, 320

Cristate/la, 321, 323

Crop, 30

Crustacea, 484 : Example a, 484 : Ex- *
ample 6, 498 : Distinctive characters
and classification, 519 : Systematic
position of the examples, 524 : General
organisation, 525 : External characters
and structure of appendages, 525 :
Texture of the exoskeleton, 547 : Body-
cavity, 547 : Enteric canal, 547 : Res-
piratory organs, 548 : Heart, 550 :
Excretory organs, 550 : Nervous
system, 550 : Sense-organs, 551 : Re-
production, 551 : Development, 551 :
Ethology, 554 : Affinities and mutual
relationships, 556 : Appendix, 558

Crypt omonas, 66

Cryploniscus, 546, 547

Crystalline style, 636 : of 'Gastropoda,
692

Ctenaria, 139

Ctentfium, 645, 646, 653, 664, 665 : of
Triton, 676 : of Gastropoda, 693

Ctenophora, 118*, 198: Example, 198:
Distinctive characters and classifica-
tion, 207 : Systematic position of the
example, 208 : General organisation,
209, 210, 211 : Appendix, 212 : Re-
lationships, 214, 215

Ctenoplana, 212, 266

Ctenopteryx, 738

Ctenostomata, 32<r, 321

Cubomedusee, 104", 166

Cue H ma ria — See Colochirus

Culex, 590

Cuma, 524

i, 524*, 543

Cunarcha, 143
Cunina, 155
Cunina parasitica, 144
Cup-coral, 192
Cursor ia, 587
Cuticle, 29*

Cuttle-fish, 631, 708, 736
Cuvier, 3

Cuvieran organs, 394
Cyamm, 524, 545, 546
Cyclas, 660

Cyclop*, 522, 528, 529, 550, 551
Cyclostomata, 320
Cydippida, 208*, 209
Cymothoa, 551
Cymothoe, 524, 546
Cynipidx', 598
Cyprtea, 687

Cypris, 521, 527, 528, 550
Cypris stage, of Cirripedes, 552
Cyst, of Ama-ba, 44 : Ewjlena, 64
Cysticercoid, 260, 261
Cysticercus, 236, 261

Cystoflagellata, definition, 65 : Charac-
teristic features, 74
Cystoidea, 380
Cy there, 521, 527, 528
C}rtoplasm, 15

1)

ACTYLOPORES, of Millepora, 145

Stylaster, 147
Dactylozooids, 138*, of Millepora, 146

Stylaster i 147 : Halitstemma, 148, 149
Daddy long-legs, 586
Dahlia wartlet, 172, 173
Dallingeria, 66
Dalmanite.K wia'/i«, 558
Daphnia, 521, 526, 527
Darwin, 6
Dasychone, 438
Daughter-cell, 18
Daughter-chromosomes, 1 6
Daughter-cysts, 262
Daughter-nucleus, 17
Daughter-segments, 16
Day-flies, 592
Dead men's fingers, 182
Decapoda, 523*, 537, 548, 550, 551
Decapoda (Cephalopoda), 736*, 738, 740
Degeneration, in Copepoda, 530 : in Cir-

ripedia, 534 : in Isopoda, 547
Deiopea, 209, 210
/ >< ndroccelum — See Planaria
Dendrocometes, 71
DendrophyUia, 194
1>< adrosoma, 91, 92
DentaUum, 705, 706, 707
Derm (dermis), 2i)"':
Dermal branching of Echinoidea, 389
Dermal branchiw, 347, 393

762

INDEX

Dermal cortex, 102*, 110

Dermal pores, 347

Dermaptera, 587

Desmoscoletida, 296

J)e*moscolex, 297

Desmospongiae, 103*

Deutomerite of Gregatrina, 77

Development, of £ycon, 113, 114: Sponges,
115: Obelia, 129, 130: Septdinas, 141:
Trachylimv, 144 : Aurelia, 160, 161,
162 : Sea-anemone, 179 : Hormiphora,
204: Dinjemida, 217, 218 : Rhopalum,
219 : Planaria, 225 : Liver-Fluke, 229,
230: Tcttnia, 236: Platyhelminthes,
256 : Nemertinea, 272, 273 ; Ascaris,
280 : Nematoda, 286, 287 : Chaeto-
gnatha, 295 : Bracliionus rubens, 302 :
Rotifera, 309: Biujnla, 317: Ecto-
procta, 325: Phoronin, 329, 330:
Brachiopoda, 341 : Asterina, 358, 359 :
Sea-urchin, 368 : Sea-cucumber, 372 :
Antedon, 377, 398: Echinodermata,
396, 397 : Nereis, 413, 414, 415 : Lum-
bricus, 425, 426: Chaetopoda, 442:
Gephyrea, 459, 460 : Hirudo, 475 :
Hirudinea, 479 : A pus, 496 : Astacus,
515 : Crustacea, 551 : Peripatus, 562,
563, 564, 565 : Myriapoda, 570 : Ptri-
pIa-nefa,58Q: Scorpion, 610 : Anodonta,
643 : Pelecypoda, 658 : Amphineura,
668 : Gastropoda, 698 : Scaphopoda,
706 : Cephalopoda, 744, 755

Devonian, 7

Itiastyfa, 524, 543

JJibranchiata, 736*, 737, 738, 740, 743, 744

Diceras, 651

Dichyocysta, 86

Dicyema, 216, 217, 218

Dicyemidiv, 216, 217

Didinium, 85

DlDYMIUM DIFFORME, 61, 62

Didymogrctptus, 155

Differentiation, 21*

Difflugia, 48

Digenetica, 238*, 250, 253, 256

Digestion, intracellular, 31

Digestive glands, 31

Digestive system, 30

Digestive system, of Paramreritim, 86 :
Aurelia, 157, 158, 159 : Sea-anemone,
174, 175 : Hormiphora, 199, 200, 201 :
Planaria, 222, 223 : Liver-Fluke, 227 :
Platyhelminthes, 250, 251, 252: Ne-
mertinea, 269 : Ascdris, 278, 279 :
Nematoda, 283 : Cheetogndtha, 293 :
Brachi&vus ruben*, 300, 301 : Rotifera,
307 : Buynla, 315, 316 : Ectoprocta,

- 323 : Endoprocta, 326 : Phoroniy, 329 :
Magellaniq, 334 : Arenas, 353 : Sea-
urchin, 367 : Sea-cucumber, 371, 372 :
A ntedon, 375 : Echinodermata, 394 :
-V' n /.s, 400, 407 : Lwnbrirn*, 420 :
Chajtopoda, 436 : tiipunnduH, 452,

453 : Gephyrea, 457 : Hirudo, 468,
469 : Hirudinea, 476 : Apw, 489, 490 :
Abacus, 506: Crustacea, 547: Peri-
t>atux, 561 : Myriapoda, 569 : Peri-
'planet a, 576: Insecta, 592 : Scorpion,
607, 608 : Arachnida, 618 : Anodonta,
636 : Pelecypoda, 655 : Amphineura,
665 : Triton, 676, 677 : Gastropoda,
692: 'Scaphopoda, 706: Sepia, 712,
713, 714, 715 : Nautilus, 730, 731 :
Cephalopoda, 743

Dimorplia, 66, 67

Dimorphic zooids, of Ciliata, 89

Dimorphism, sexual, 38*

Dinobryon, 66

Dinoflagellata, definition, 65 : Charac-
teristic features, 73

Dinophilea, 310

Dinophilus, 482

Dicecious, 38

Diophrys, 83

Diphyes, 152, 153

Diplomita, 66

Diplonephridia, 439*

Diplopoda, 567, 569, 570

Diplozoon, 258

Diptera, 585, 589, 596, 598, 602, 605

Directive mesenteries, of Sea-anemone,
176

Discina, 337, 339

Discoidal segmentation, 610

Discomedusse, 164*, 169, 170

JDiscorbina, 53

Discovoma, 195, 197

Dissepiments, 192*

Distinctive characters — See Classification

DISTOMUM HEPATICUM, 226 : General
features, 226 : Body-wall, 226, 227 :
Digestive system, 227 : Water-vessels,
227 : Nervous system, 228 : Repro-
ductive organs, 228, 229 : Develop-
ment, 229, 230 : Systematic position,
238

Uistomum hepaticum, 265

D. rathousii, 265

D. sinense, 265

Distribution, 8: of Sponges, 113: Hydro-
corallinee, 147: Scyphozoa, 171 4 Ac-
tinozoa, 196 : Ctenophora, 211 : Platy- '
helminthes, 263 : Ectoprocta, 326 :
Brachiopoda, 342 : Gephyrea, 461 :
Hirudinea, 481 : Onychophora, 566 :
Pelecypoda, 661 : Amphineura, 670 :
Gastropoda, 704 : Cephalopoda, 749

Distribution, geographical, 8

Distribution, geological, 8 : of Sponges,
113: Corals, 197: Brachiopoda, 342:
Chretopoda, 447 : Insecta, 604 : Arach-
nida, 624 : Cephalopoda, 749

Distribution, vertical, 8

Dochmiua duodenoli*, 284

Docoylowa, 683

Doctrine of Descent, 6

INDEX

763

Dollinyeria, 66

Donax, 657

Doris, 684, 687, 691

Doris (Archtdoris) tuberculata, 687

Dorsal cirri, of Anfedoit, 373

Dorsal organ, of Apns, 4S5

Dorsal pores, of Earthworm, 418

JJraonwulus, 283, 289

Dragon flies, 585

Drepanidium, 78

Drilophaga, 309

Dromia, 524, 555

Dytixcus, 596

E.

I AH, 37
Ear- shells, 683
Earthworm — See
Earthworms, 403
Earwigs, 584
Ecc

Anabulacral system, 367 : Ovoid gland,

367 : Enteric canal, 367 : Ccelome, 368 :
Blood -vascular system. 368 : Repro-
ductive organs, 368 : Development,

368 : Systematic position, 381
Echiurua,W, 458, 459, 460
Ectoderm, 21* — See Body-wall.
Ectonephridia, 439*

Ectoprocta, 320*, 321 : Structure of
body-wall, 323 : Alimentary canal,

323 : Nervous system, 324 : Nephridia,

324 : Avicularia, 324 : Vibracula, 324 :
Reproduction and development, 325 :
Ethology and distribution, 326

Edu-ard«ia, 185, 187, 188, 189

Eggs — See Development

Eimerid, 78

Elasipoda, 379, 393

Eh ilone moschata, 744

Elephant's tusk shells, 705

Elk-horn coral, 145

Elytra, of Cockroach, 574 : Polychreta,

Holothuroidea-For references to this see under Echinoidea, p.

763.

uo4 : Ambulacral system, 393 : Blood-
vascular system, 394 : Enteric canal,
394 : Nervous system, 395 : Sexes, 395 :
Development and metamorphosis, 396 :
Echinopsedium, 396 : Ethology, &c.,

399 : Self -mutilation and regeneration,

400 : Affinities, 401

Hchinoidea; example, 363 : Distinctive
characters and classification, 379 :
Apical system, 384 : Modifications of
form, 387 : Dermal branchiae, 393 :
Stewart's organs, 393 : Ambulacral
system, 393 : Enteric canal, 394 : Ner-
vous system, 395 : Reproductive
organs, 395 : Development, 398 : Holo-
thuroidea, example, 369 : Distinc-
tive characters and classification, 379 :
Modifications of form, 390 : Ambulacral
system, 393 : Blood-vascular system,
394 : Enteric canal, 394 : Respiratory
trees, 394 : Cuvieran organs, 394 :
Nervous system, 395 : Reproductive
organs, 395 : Development and meta-
morphosis, 398 : Ethology, 399

Echinopcedium, 396

Echinorhynchn*, 289, 290, 291, 292

ECHINUS, 363 : General external features,
363, 364 : Corona, 364, 365 : Aristotle's
lantern, 366 : Nervous system, 367 :

-ii^, 3^0*, 326 : Vestibule,

326 : Nephridia, 326 : Cloaca, 327 :
Ganglion, 327 : Testes and ovaries,

327 : Foot-gland, 327 : Development,
327 : Relationships, 328

Endoskeleton, 29

Endosternite, 606, 618

Enteric canal — See Digestive system

Enteroccele, 359

Entomostraca, 521, 547, 548, 551

Entovalva, 661

Environment, 9*

Eocene, 7

Eolis, 684, 691, 693

Epeira diadema, 615

Ephelota, 91, 92

Ephemera, 584, 592

Ephippium, 551

Ephyra, 171

Ephyrula, of Aurelia, 161*, 162

Epiblast, 21*

Epicranium*, 572

Epidermis, 22, 29— See Body-wall

Epimerite of Gregarinida, 77

Epipharynx, 592

Epiphragm, 686

Epiphysis, of Sea-urchin, 366

Epipodite, 501*

Epipodium, 688

Epistoma, of Astacus, 499

762

INDEX

Dermal cortex, 102*, 110

Dermal pores, 347

])t nnaptera, 587

Desmoscolecichv, 296

Desmoscohx, 297

Desmospongiee, 103*

Deutomerite of Gfreffttrina, 77

Development, of &ycon, 113, 114: Sponges,
115: Obelia, 129, 130: SeptoUna*, 141:
Trachylimt, 144: Aurelia, 160, 161,
162 : Sea-anemone, 179 : Hormiphora,
204: Dici/tin'til,!-, 217, 218 : Khopalura,
219: Ptanaria,22o: Liver-Fluke, 229,
230: T<mia, 236: Platyhelminthes,
256: Nemertinea, 272, 273 ; Amiri*,
280 : Nematoda, 286, 287 : Chaeto-
gnatha, 295 : Brachionus rubens, 302 :
Rotifera, 309: Buyula, 317: Ecto-
procta, 325: Phoroni*, 329, 330:
Brachiopoda, 341 : Asterina, 358, 359 :
Sea-urchin, 368 : Sea-cucumber, 372 :
Antedon, 377, 398 : Echinodermata,
" ~"M 'IT ri', ,in (\\_<\_ 415 : Lum-

453 : Gephyrea, 457 : Hirudo, 468,
469 : Hirudinea, 476 : Apu«, 489, 490 :
Axtaciifi, 506 : Crustacea, 547 : Peri-
P'linx, 561 : Myriapoda, 569 : Peri-
plamta, 576: Insecta, 592 : Scorpion,
607,608: Arachnida, 618 : Anodontu,
636 : Pelecypoda, 655 : Amphineura,
665: Triton, 676, 677: Gastropoda,
692: 'Scaphopoda, 706: Sepia, 712,
713, 714, 715 : Nautilus, 730, 731 :
Cephalopoda, 743

Dinwrpha, 66, 67

Dimorphic zooids, of Ciliata, 89

Dimorphism, sexual, 38":;"

Dinobryon, 66

Dinoflagellata, definition, 65 : Charac-
teristic features, 73

Dinophilea, 310

Dinophilus, 482

Dice'cious, 38

Diophnjs, 83

Diphyes, 152, 153

Diplomita, 66

•

DiastylM, 524, 543

Dibranchiata, 736*, 737, 738, 740, 743, 744

Diceras, 651

Dichyocysta, 86

Dicyema, 216, 217, 218

Dicyemidw, 216, 217

Didinium, 85

DlDYMIUM DIFFORME, 61, 62

Didymograptus, 155

Differentiation, 21*

Diffiwjia, 48

Digenetica, 238*, 250, 253, 256

Digestion, intracellular, 31

Digestive glands, 31

Digestive system, 30

Digestive system, of Param<r<-ii(iii, 86 :
Aurelia, 157, 158, 159 : Sea-anemone,
174, 175 : Hormiphora, 199, 200, 201 :
Planaria, 222, 223 : Liver-Fluke, 227 :
Platyhelminthes, 250, 251, 252: Ne-
mertinea, 269: Jsm/v's, 278, 279:
Nematoda, 283 : Cluutognatlui, 293 :
Brachionus rnlx-H*, 300, 301 : Rotifera,
3U7 : /SH</n/n, 315, 316 : Ectoprocta,

• 323 : Endoproeta, 326 : Phormti*, 329 :
Maydlania, 334 : Axt.wia*, 353 : Sea-
urchin, 367 : Sea-cucumber, 371, 372 :
Aufcd.on, 375: Echinodermata, 394:
.V'/v/.s 40(5, 407: L-intiln'it'nx, 420:
Chaetopoda, 436 : tfijinnrnluH, 452,

Dissepiments, 192*

Distinctive characters — See Classification

DISTOMUM HEPATICUM, 226 : General
features, 226 : Body-wall, 226, 227 :
Digestive system, 227 : Water- vessels,
227 : Nervous system, 228 : Repro-
ductive organs, 228, 229 : Develop-
ment, 229, 230 : Systematic position,
238

Distomum hepatic urn, 265

D. rathousii, 265

D. xinense, 265

Distribution, 8: of Sponges, 113: Hydro-
corallinse, 147: Scyphozoa, 171 « Ac-
tinozoa, 196 : Ctenophora, 211 : Platy-
helminthes, 263 : Ectoprocta, 326 :
Brachiopoda, 342 : Gephyrea,, 461 :
Hirudinea, 481 : Onychophora, 5(ili :
Pelecypoda, 661 : Amphineura, 670 :
Gastropoda, 704 : Cephalopoda, 749

Distribution, geographical, 8

Distribution, geological, 8 : of Sponges,
113: Corals, 197: Brachiopoda, 342:
Chsetopoda, 447 : Insecta, 604 : Arach-
nida, 624 : Cephalopoda, 749

Distribution, vertical, 8
duodenal**, 284
, 683

Doctrine of Descent, 6

INDEX

763

Dollingeria, 66

Donax, 657

Doris, 684, 687, 691

Doris (Archidoris) tuberculata, 687

Dorsal cirri, oi An fdon, 373

Dorsal organ, of Apus, 485

Dorsal pores, of Earthworm, 418

Dracuncvlus, 283, 289

Dragon flies, 585

Drepanidium, 78

Drilophaga, 309

Dromia, 524, 555

Dytiscus, 596

E

E

IAR, 37

Ear- shells, 683

Earthworm — See Lumbricus

Earthworms, 403

Earwigs, 584

Ecclyses, of Apus, 497

Ecliinococcn*, 246, 231, 262

Echinocyamtt* />t/xi//ttx, 398

Echinodertdai, 296, 297

Echinodermata, 346 : Examples, 346,
363, 369, 373 : Distinctive characters
and classification, 377 : Systematic
position of examples, 380 : General or-
ganisation, 381 : General form and
symmetry, 381 : Systems of plates, 384 :
Modifications of form, 384 : Cuelome,
392 : Ambulacral system, 393 : Blood-
vascular system, 394 : Enteric canal,
394 : Nervous system, 395 : Sexes, 395 :
Development and metamorphosis, 396 :
Echinopredium, 396 : Ethology, &c.,

399 ; Self -mutilation and regeneration,

400 : Affinities, 401

Ichinoidea* example, 363 : Distinctive
characters and classification, 379 :
Apical system, 384 : Modifications of
form, 387 : Dermal branchiae, 393 :
Stewart's organs, 393 : Ambulacral
system, 393 : Enteric canal, 394 : Ner-
vous system, 395 : Reproductive
organs, 395 : Development, 398 : Holo-
thuroidea, example, 369 : Distinc-
tive characters and classification, 379 :
Modifications of form, 390 : Ambulacral
system, 393: Blood-vascular system,
394 : Enteric canal, 394 : Respiratory
trees, 394 : Cuvieran organs, 394":
Nervous system, 395 : Reproductive
organs, 395 : Development and meta-
morphosis, 398 : Ethology, 399

Echinopwdium, 396

Echirwrhynchus, 289, 290, 291, 292

ECHINUS, 363 : General external features,
363, 364 : Corona, 364, 365 : Aristotle's
lantern, 366 : Nervous system, 367 :

Arabulacral system, 367 : Ovoid gland,

367 : Enteric canal, 367 : Ccvlome, 368 :
Blood- vascular system. 368 : Repro-
ductive organs, 368 : Development,

368 : Systematic position, 381
E<-lnnni*, 456,458, 459, 460
Ectoderm, 21* — See Body-wall.
Ectonephridia, 439*

Ectoprocta, 320*, 321 : Structure of
body-wall, 323 : Alimentary canal,

323 : Nervous system, 324 : Nephridia,

324 : Avicularia, 324 : Vibraeula, 324 :
Reproduction and development, 325 :
Ethology and distribution, 326

E'lH-ardxia, 185, 187, 188, 189

Eggs — See Development

E "nutria, 78

Elasipoda, 379, 393

Eledone moxchata, 744

Elephant's tusk shells, 705

Elk-horn coral, 145

Elytra, of Cockroach, 574 : Polychasta,

432 : Coleoptera, 587, 591
Embryology, 3* — See Development
Empis, 595
Emulsions, 32
Endites, 488
Endoderm, 21*

Endoderm-disc, of Antaeus, 516
Endoderm lamella, 126, 128*
Endophragmal system, of Astacus,

499

Enclopodite, 500*
Endoprocta, 312, 320*, 326: Vestibule,

326 : Nephridia, 326 : Cloaca, 327 :
( Ganglion, 327 : Testes and ovaries,

327 : Foot-gland, 327 : Development,
327 : Relationships, 328

Endoskeleton, 29

Endosternite, 606, 618

Enteric canal — See Digestive system

Enteroco?le, 359

Entomostraca, 521, 547, 548, 551

Entomlm, 661

Environment, 9*

Eocene, 7

AW/*, 684, 691, 693

Epeira diadema, 615

Ephdota, 91, 92

Ephemera, 584, 592

Ephippium, 551

Epliyra, 171

Ephyrula, of Aurelia, 161*, 162

Epiblast, 21*

Epicranium*, 572

Epidermis, 22, 29— See Body-wall

Epimerite of Gregarinida, 77

Epi pharynx, 592

Epiphragm, 686

Epiphysis, of Sea-urchin, 366

Epipodite, 501*

Epipodium, 688

Epistoma, of Astacus, 499

INDEX

Epistome, 312, 322, 324, 328, 329

Epistylia plicatUis, 83, 85

Epitheca, 192, 193

Epithelia, 22" : Non-stratified, 22* :
Stratified, 22*

Epithelium, enteric, 31

Enjaxilux, 522, 530, 531

Errantia, 427

Esfhtria, 521, 526

Ethiopian, 9

Ethology, 9 : of Platyhelminthes, 263 :
Rotifera, 309 : Ectoprocta, 326 : Echi-
nodermata, 399 : Clwetopoda, 447 :
Hirudinea, 480 : Crustacea, 554 : In-
secta, 662 : Arachnida, 623 : Pelecy-
poda, 661 : Amphineura, 670 : Gastro-
poda, 704

Euchlanis, 304

Euchlom, 209

Eucirripedia, 522*, 532

Eucope, development, 141

Eucopepoda, 522*, 5L'8, 530

Eiit/(->t(f/-iin)>, 130

EUGLEXA VIRIDIS, 63, 64 : Systematic
position, 65

Eulamellibrancliiata, 647*, 64£, 662

Euphaitxiu, 523, 549, 553

Euphyllopoda, 521, 526, 550, 552

Enpftrfl://fl, 112

Eii2)onirttus, 445

Euryalida, 378

Eurypterida, 613*, 617, 619

EUSCORPIO — See BUTHUS

Euscorpius italicus, 610, 622, 623

Euspongia, 106

Euthyneura, 684, 691, 695, 697

Evolution, 6

Examples, of Rhizopoda, 44 : Mastigo-
phora, 63 : Sporozoa, 75 : Infusoria,
79: Porifera, 96: Hydrozoa, 118:
Scyphozoa, 156 : Actinozoa, 172 :
Ctenophora, 198 : Platyhelminthes,
222, 226, 231 : Nematoda, 275 :
Rotifera, 299 : Polyzoa, 314 : Brachio-
poda, 331 : Asteroidea, 346, 356 :
Echinoidea, 363 : Crinoidea, 373 :
Chaetopoda, 404, 417 : Gephyrea, 451 ;
Crustacea, 484, 498 : Insecta, 571 :
Arachnida, 604 : Pelecypoda, 631 :
Gastropoda, 671 : Cephalopoda, 708,
725

Excretion, 14, 35

Excretory system, of Septolina?, 138 :
I'/xiKtria, 223, 224: Liver-Fluke, 227 :
T'/ii/'t, 234: Platyhelminthes, 253:
Nemertinea, 270, 271 : Ascari*, 279 :
/•,'<'// innrh t/iH'/i /f*,292: lli'ttdi ion 11* rnl><: n*.
300, 301: Rotifera, 308: Ectoprocta,
324 : Endoprocta, 326 : Phoronis, 328 :
Mugillnnia, 337: Brachiopoda, 340:
Nereis, 411, 412: Lnitilin>-HK, 423:
Cli;i'to])oda, 440 : ti!piui>-n/iix, 4E3,
454: Gephyrea, 458: Hirndo, 47<",

471 : Hirudinea, 477 : Apus, 492 :
Astaait, 509, 510 : Crustacea, 550 :
I'< ripatns, 562: Peripfanefa, 577: In-
secta, 591, 592: Scorpion, 607:
Arachnida, 619 : Anodonta, 637, 640 :
Pelecypoda, 656 : Amphineura, 668,
669 : Triton, 679 : Gastropoda, 697 :
Scaphopoda, 706 : Sepia, 722 : Xautifii*.
732 : Cephalopoda, 744

Exhalant siphon, 632

Exites, 489

Exopodite, 500*

Exoskeleton, 29*— See Body- wall

External features, of Sycon, 96, 97 : of
Porifera, 105, 106, 107: Obelia, 119,
120: Anrdia, 156, 157: Sea anemone,
172, 173 : Actinozoa, 183 : Hbrmiphora,
198 : Planaria and Dendroccdum, 222 :
Liver-Fluke, 226: Tamia soUum, 231,
232 : Platyhelminthes, 239 : Nemer-
tinea, 268 : Ascaris luitibricoides, 275,
276 : Nematoda, 283 : EchinorhynchvH,
290: Chretognatha, 293: Brachionii*
rubens, 299, 300 : Rotifera, 304, 305,
306, 307 : Buyula aricnlaria, 314, 315 :
Ectoprocta, 321, 322, 323 : Endoprocta,
326: Phoronis, 328 : Magdlama, 331,
332: Brachiopoda, 338, 339 : Asterias
ritbens, 346, 347 : Anthenea fl((r(-*c(i<x,
356, 357 : Sea-urchin,- 363, 364 : Sea-
cucumber, 369 : Antedon roxacea, 373 :
Echinodermata, 381 : Asteroidea, 384 :
Ophiuroidea, 386 : Echinoidea, 387 :
Holothuroidea, 390 : Crinoidea, 391 :
Ifertix, 404: L-H)nl>rioi«, 417, 418:
Chaetopoda, 429: Sipuncti/ns IUH.IH*,
451: Gephyrea, 455: Archi-annelida,
462: Hinulo, 465, 466: Hirudinea,
476: Apux, 485: Antaeus, 498, 499:
Crustacea, 525, 547 : P<:r!)'>«fn«, 559,
560: Mvriapoda, 567: Peripfaneta,
571, 572": Insecta, 588 : Scorpion, 605 :
Arachnida, 613 : Anodonta, 631, 632:
Pelecypoda, 647 : Amphineura, 663,
664, "665: Triton, 671, 672, 673:
Gastropoda, 685 : Scaphopoda, 705 :
Sepia, 708, 709: XnutUn*, 725, 726,
727 : Cephalopoda, 737

Ex-umbrella, 124, 156*

Eyes, 37 : of Euglena, 64 : Medusa?,
'133: Planaria, 222 : Platyhelminthes,
253: NemertineaT 272: Nematoda,
285 : Chsetognatha, 2§o*: Brachionus,
302 : Rotifera, 308 : Dinophilus, 310 :
Brachiopoda, 341 : Asterias, 348 : Sea-
urchin, 365 : Nereis, 410, 411 : Cha-to-
poda, 438 : Hirudo, 472, 473 : Apus,
494, 495 : Astacus, 513 ; Crustacea,
551 : Periplaneta, 579 : Insecta, 596 :
Arachnida, 621, 622, 623 : Pelecypoda,
(15S, 659 : Chiton, <><>S : Triton, 682 :
Gastropoda, 695, 696 : Sepia, 720 :
Nautilus, 734

INDEX

765

JU ABRICIA, 438

Facial suture, 558*

Falciform young of Monocysti-y, 76

Fan/ly, 4*

Faf23*

Fat body, of Periplaneta, 576

Fauna, 8

Feather-star — See Anfedon. ro-yacea

Femur, of Cockroach, 574

Fenestrse, of Pe.riplaue.ta. 572

Fenja, 188, 189, Addenda

Ferment, 12

Fibres, nerve, 27, 28

Fibro-cartilage, 24*, 25

Filibranchiata, 646% 648, 649, 662

Fimbriffi, of Fresh-water Mussels, 632

Fire -flies, 596

Fission, 14, 38*, 44, 73

Fissurella, 694

Fixed cheek, 558*

Flabetlum, 183, 192, 193, 489

Flagellata, 05* : Cell-body, 65 : Flagella,
66, 67 : Bilateral symmetry, 67 : Modes
of nutrition, 67 : Skeleton, 68 : Colo-
nies, 68 : Asexual multiplication, 69 :
Sexual reproduction, 70

Flagellate canals, of Sponges, 98, 99, 100*,
109

Flagellate cells, 98, 99, 100*

Flagellula, 51, 62, 64

Flagellum, 21*

Flagellum, otAxtncitx, 5o2

Flame-cells, 224*, 253

Flat-worms — See Platyhelminthes

Fleas, 585, 586, 591, 6\>2

Floscularia, 303, 395, 306

F/itxfw, ,S2o

FoUiculimt, 85

Food vacuole of Paramoncium, 81

Foot, of Anodonfn, 632 : Pelec>ypoda,
052 : Amphmeura, 664, 665: Triton,
674, 675 : Sepia, 709, 710 : Nautilus,
727 : Cephalopoda, 738

Foot- gland, 327

Foraminifera, 46* : General structure, 50,
51 : Skeleton, 51, 52. 53 : Protoplasm,

54 : Dimorphism, 55 : Reproduction,

55 : Distribution, 55
Fnssettes. 541
Foxsulci, 197

Fresh-water Crayfish, 498
Fresh-water Mussel — See Anodonta
Fresh-water Sponges, 116
Fresh-water Worms, 403
Frondicularia, 53

Fulcrum (of Rotifera), 307
Fungia, 192
Funiculus, ,316, 323
Funnel, 748
Funnel folds, 746

ALEA, 573

Galeodes. 612, 614

Galls. 599

Gall-flies, 581

Gall- insects, 598

Gametes of Flagellata, 70

(.laiiiinarn*, 524, 543, 544

Gamobium, 130

Gastral cortex, 102*, 110

Gastric filaments, of Aurelia, 160

Gastric mill, 547

Gastric mill, of Astacus, 507

Gastric ostium, 101*

Gastric ridges, of Aurelia, 161

Gastrolith, 507

Gdstrophttus egid, 586

Gastropoda, Example, 671 : Distinctive
characters and classification, 682 :
Systematic position of example, 685 :
General organisation, 685 : External
features, symm^-y, &c., 685: Shell,
(ISO : Foot, 687 : Head, 689 : Mantle,
689 : Respiratory organs, 690 : Diges-
tive organs, 692 : Heart, 693 : Nervous
system, 693 : Organs of special sense,
695 : Nephridia, 697 : Reproduction,
697 : Development, 698 ; Ethology and
distribution, 704: Relationships/ 705 :
Appendix, 705

Gastropores, of Millepora, 145

Gastrotricha, 311, 312

Gastrula, 21, 3!)

Gastrulation, 20*

(•'< ftixi/'niis, 541, 556

Gemmules, 112

Gerue, 572*

Generic, 1

Genital operculum, 606

Genital system — SeeReproductive system

Geological distribution — See Distribu-
tion, geological

Gephyrea, 450 : Example, 451 : Dis-
tinctive characters and classification,
454 : Systematic position of the ex-
amples,455 : General organisation, 455 :
Body-wall, 456 : Coelome, 456 : Ali-
mentary canal, 457 : Vascular system,

457 : Nervous system, 457 : Nephridia,

458 : Reproductive organs, 458 : Sexual
dimorphism, 458 : Development, 45!) :
Distribution, affinities, &c., 461

Germinal bands, ofClepsine, 479 : of A /•/-

jn<fn*, 564

Germinal groove, 581
Germinal layers, 21*
Germinal spot, 18*, 28
Germinal vesicle, 18*, 28
Giant clam, 660
Giant fibres, 438
Giant nerve-cells, 438

766

INDEX

Gill-cover, of Astacu*, 49s

Gills, 33*— See Respiration

Gizzard, 30

Glabella, 558*

Glands, 22

Glands, Multicellular, 23* : Unicellular,

22 : Ducts, 23
Glass-crab, 553
Glass-rope sponge, 112, 116
Glenodinium, 74
Globigerina, 53, 54
Glochidium, (544
Glossocodon, 142
Glow-worms, 596
Glycera, 434
Glyceridce, 436
Gnathobase, 489
Gnathobdellida, 476, 480
Gnats. 586

Goblet-shaped bodies, 423
Goblet-shaped organs, of Leech, 473
Gonads — See Reproductive system
Gonotheca, of Obelia, 119", 120 «
Gordioidea, 2£2*, 283|£85
Gordiun, 283, 284, 286
Gorgonacea, 182, 191
(,'or'fonia, 184, 191
Granule glands, 254, 255
Graptolitbida, 132*, 154, 155
Grasshoppers, 584
Green gland, of Axtacnx, 509
Gregarina, 77
<,',•! (/'i/'iii'i b/aftrtnt/ti, 77

Gregcvrina dujardini, 77

Gregarina </i>juuf"t, 77

Gregarinida, characteristic features,

77

Gregarinida, definition, 76
(Iroinitt,, 51, 52
Guard-polypes, 138'"
(Juliet, 30 — See Digestive organs
diut'/n *«jiii< :itffif((, 240, 241
Gustatory organ, 37
Gymnoblastea, 133*
Gynmolaemata, 320

(.'//-/•nrfi*. 187

Gyrodactylidce, 243
Gyrodactylw, 243, 258

H

H

.ABITS — See Ethology
ffcemadipsct, 476, 481
H;enmtochrome, 49, 64

ir<i'i)1f.tfO'-orcit.-<. 67

Htemoccele, of Crustacea, 547

'/>'i.' /is, 561 : Insecta, 591
H: •inocyanin, 513
Haemoglobin, 28, 34

/In //jo//*/* ror«,x, 481 »
Halcaii!j><>i<l< x, 188

Peri-

Haliotitf, 693, 694
Halixtemma, 147
Halistemma tergestinum, 148
Halteres, 591
Hami)if/t'~i. 459
Hartea, 182, 183, 184, 190
Harvest-men, 612, 615
ffastigerina, 54
Haversian canals, 25*, 26
Head-lobes, of Astacuts, 516
Heart, 34

Heart— See Vascular system
Hectocotylisation, 709, 728
Heliozoa, 46* : General structure, 55 :
Colonies, 56 : Skeleton, 56, 57 : Re-
production, 58 : Conjugation
Heliopora, 182, 195, 196
Helix, 697, 699
Helix nemoralia, 690
Heinipneustes radiatu*, 390
Kemiptera, 585, 589, 605
Hepato-pancreas, 32
Hermaphrodite, 37x
Hermit-crabs, 539, 556
Hermit-crabs and Hydractinia, 134, 135
Hesionida, 436
Heterocoela, 103*
HeterocyemidcK, 216, 218
Heterogamy, 30*, 287
Heterogeny, 259", 266 •
Heteroinita, 67
Heteromyaria, 649
Heteropoda, 684, 687, 688
Heterotrichous, 83
Hexacanth embryo, of Timia, 236 :

Cestodes, 259
Hexactinellida, 103*, 116
H exact ini<v, 183, 187
Hexarthra, 304, 306, 307, 310
Hinge-ligament, 633
Hinge-line, 633
Hinge- teeth, 634, 651
Hii>jja, 523
Hippuritea, 651, 652

Hirudinea, 465 : Example, 465 : Distinc-
tive characters and classification, 474 :
General organisation, 476 : Form and
size, 476 : Proboscis, 476 : Enteric
canal, 476 : Blood-vessels, 477 : Respir-
atory organs, 477 : Nephridia, 477 :
Nervous system, 478 : Reproductive
organs, 478 : Development, 479 :
Habits, distribution, &c., 480
Hit'iido, 476, 477, 478
HIRUDO MEDICIXALIS, 465 : External
characters, 465, 466 : Body-wall, 467 :
Muscular system, 467, 468 : Alimentary
organs, 468, 469 : Excretory system,
470, 471 : Vascular system, 471, 472:
Nervous S3'stem, 472 : Sense-organs,
472, 473 : Reproductive organs, 473 :
Development, 474 : Systematic posi-
tion, 476

INDEX

767

HlRUDO QUIXQUESTEIATA — See H. MEDI-
CINALIS

Hirudo sangui&uga, 480
Histology, 3* «
Histrio&jWa*, 462, 463

• HiS^nytic, 49 ":;

Holothuria — See Sea-cucumber

Holothurian — See Colochirus

Holotrichous, 83

Holozoic, 49"

Homalogast&r, 242

Homoccela, 103"

Hoplonemertea, 270

HORMIPHORA PLUMOSA, 198 : External
characters, 198 : Enteric system, 199,
200, 201 : Cell-layers, 202 : Nervous
system, 203 : Sense-organs, 203 : Re-
productive organs, 203^ Development,
204, 205, 206, 207 : J^stematic posi-
tion, 208

House-flies, 586

Hyalonema, 112, 116 1

Hybrids, 2

Hydatids, 261, 262, 266

Hydatina, 304, 305, 306

Hydra, 136, 137, 142, 155

ffydrpctinia, 134, 135, 137, 155

Hydranths, of Obelia, 119*, 120,
122

Hydrocoele, 360

Hydrocorallina, 132*, 145, 155

Hydrocorallinae, distribution, 147

Hydroids, naked-budded, 133* : covered-
budded, 133"

Hydrophi/iix, 601

Hydrophyllia, of Halistemma, 149

Hydrotheca, of Obelia, 119*, 120

Hydrozoa, 118 : Example, 118 : General
structure and classification, 130: Alter-
nation of generations, 131 : General
remarks on, 155

Hydrnla, 129, 130, 141*, 144

Hymenoptera, 587, 589, 593, 594, 596,
602, 0<!5

Hypodermic impregnation, 309, 47^9 _

Hypopharynx, 589

Hypostome, of Obelia, 119*

Hypotrichous, 83

I

I

CHNEUMONS, 587

Ichthyophthiriw, 93

Idmonea, 320
Illoricata, 304*
Imaginal discs, 602
Impregnation, 18*, 19
Impregnation, hypodermic,

Inarticulata, 337* : Shell, 338

Incurrent canals, of Sponges, 98, 99,
108

Incus (of Rotifera), 307

Individual differences, in Nereis dumerilii,
413

Individual of the first order, 156*

Individual of the second order, 156*

Individual of the third order, 156"

Individual variations, 2, 104

Iiidividuation, in Hydrozoa, 155

Inermia, 455, 457, 461

Infundibula, of Aurelia, 161

Infusoria, Definition, 44: Example, 79 :
Classification, 82 : Systematic position
of example, 82

Infusoriform embryos, of Dicyemidie,
217, 218

Inhalant pores, 97, 108

Inhalant siphon, 632

Ink-sac, of Sepia, 714

Insecta, 484, 577: Example, 577: Dis-
tinctive characters and classification,
583 : Systematic position of the
example, 587 : General organisation,
588 : Exoskeleton, 588 : Head, 58S :
Thorax, 588 : Abdomen, 588 : Appen-
dages of head, 588 : Appendages of
thorax, 590 : Btaemocoele, 591 : Fat
body, 592 : Digestive system, 592 :
Malpighian tubes, 593 : Tracheal sys-
tem, 593 : Blood- vascular system, 5!)4 :
Nervous system, 594 : Organs of special
sense, 595 : Production of sounds, 597 :
Reproductive organs, 597 : Eggs, 598 :
Development, 599 : Metamorphosis,
600 : Mode of life, 602 : Distribution
in time, 604

Integripalliata, 647 *

Integripalliate, 650

Integument — See Body- wall

Inter-filamentar junctions, (537

Inter-lamellar junctions, 637

Interstitial cells, of Obelia, 122

Introvert, of Polyzoa, 322: Sipunciduat
451 : Gephyrea, 455 : Triton, 674 :
Gastropoda, 689

Imagination, 20*

Iris, 37, 720

Isomyaria, 649

Isopoda, 524*, 543, 544, 545, 551

Isopteryx, 597

Isopte.ryx apicalix, 597

CJ AMS, 30

Jelly-fish, Common (Aurelia aurita), 156
Jelly-fishes, 118
Jurassic, 7

768

INDEX

K

K

.ARYOKINESIS, 16, 17

Keber's organ, 640

Kidneys — See Excretory system

King-Crabs, 612, 617

Kingdom, 3

Klossia, 78

JLJABIAL PALPI, of Pelecypoda, 636

Labial palps, 573

Labium, 573

Labrum, 572

Labrum, of Axtaru*, 499 : of Scorpion,
606

Labyrinthula, 49

Labyrinthulidea, 46*, 48, 49

Lacinia, 573

La>-ri//naria, 85

Lacteals, 32

Lacuna? (bone), 25

La'jtua, 53

Lamellae (of bone), 25

Lam pet ia, 209

Land-snails, 684

Laomedea, 130

Lappets, of Sea-anemone, 174

Larva, of Desor, 273

Larval membranes, of Periplaneta, 581

Larval organ, 361

Lateral organs, of Nemerteans, 271

Latreiltia, 541

Laurentian, 7

Laurer's canal, 256

Leaf-insects, 584

Lola, 646

Lens, 37 — See Eye

Lepas, 522, 532/534, 552

Lepas anatifera, 533, 534

Lepidoptera, 586, 589, 591, 598, 603, 605

/,( /lidm-ti* — See Apus

L< /,'tilin-ii* l-irkii, 486

Lepisma, 583, 584

L< /itnchone, 438

Leptodificus, 74

L,/,foflora, 521, 527

Leptolinae, 131* : General structure, 132,
133, 134, 135 : Perisarc, 133 : Medusa?,
133, 134, 138 : Ccenosarc, 133 : Repro-
ductive zooids, 138: Development, 141

Leptomedusae, 131*, 133, 140

Lepto-itraca, 550

Lerncea, 522, 530, 531

Ltxfeira, 530, 531

Leucocytes, 28

Li ncodore, 196

Lice, 585, 591, 602

Liynla, 247, 267, 573, 589

Lima, 647, 653

Li max, 687, 692

Limjiceus, 695

Limnetis, "526 * •

Limnocnida, 155

Limnocodium, 155

Limpets, 671, 683, 685, 686,

Limulux, 612, 618, 621, 622, 627, 621

Lingua, 592

Lingual ribbon, 676

Linguatulida, 625

Liiir/ula, 337, 339, 340

Lithite, 125

Lithites, of Anrelia, 160*

Lithocircus annul aris, 58

Lithocysts, 125, 133

Liver, 32

Liver-Fluke — See Dittomum hepaticum

Liver-pancreas, 32

Lobata, 208*, 209, 210

Lobosa, 46*, 41^General structure, 46,
47* : Skeleto!

Lobsters, 537,

Locusta, 584

Locusts, 584

Lollyo, 738, 742, 745, 747, 748, 749

Lolif/o rutyaris, 738

Lophomonas, 84, 85 "}

Lophophore, 312, 322, 324, 333, 339

Lophopus, 324

Lorica, of Ciliata, 87 : of Flagellata, 68

Loricata, 304*

Loxosoma, 320, 326, 327

Lucernaria, 166, 172

Lucifer, 523, 553

Lucina, 654

Lumbricomorpha, 428

LOIBRICUS, 417 : General external fea-
tures, 417 : Body-wall, 419 : Sette, 417 :
Setigerous sacs, 420 : Enteric canal,
420 : Vascular system, 421 : Nervous
system, 422 : Organs of excretion, 423 :
Reproductive organs, 423, 424 : De-
velopment, 425 : Systematic position,
42S

Luminous organs, 596

Lung, 33* : of Scorpion, 608 : of Pulmo-
nata, 691

Jj ACROBDELLA VALDIVIANA, 476

Macrdbiotus hitfelandi, 626

Macrura, 523*, 537, 539

Mmii-i'jiora, 194

Madreporaria, 181*, 187, 185, 192, 193,
194, 197

Madrepores, 194, 195

Madreporic canal, '354, 367

Madreporite, of Starfish, 347 : of Sea-
urchin, 365

INDEX

769

MAGELLAXIA, Shell, 331, 332: Body, 333:
. Mantle-lobes, 333 ; Mantle -cavity, 333 :
Lophophore, 33J^K 335 : Food-
groove, 333if^Kive organs, 334 :
o : Muscular system, 335 :
: Blood system, 336 : Ex-
s, 337 : Nervous system,
337 : Reproductive organs, 337

Maia, 524

Maiacobdella, 274

Malacostraca, 523*, 535, 547, 548, 551

Malleus (of Rotifera), 307

Malpighiaii tiibes, 577

Malpighian tubes, of Scorpion, 607 : of
Arachnida, 619 : of Tardigrada, 626

Mandibles — See Appendages

Mantidce, 587

Mantle, of Anodonta, 632 : Pelecypoda,
649 : Amphineura, 664, 665 : Triton,
675 : Gastropoda, 689 : Scaphopoda,
705 : Sepia, 711 : XautA.*, 729

Manubrium, of Medusa, f5 : of Obelia,

119*, 120 m

Marginal lappets, of AiiWfia, 156*, 157

Marginal sense-organs, 125

Marginal tentacles, of Auretia, 156*, 157,

159

Marine Annelids, 403
Mastax of Rotifera, 307, 308
Mastigamceba, 66, 67
Mastigophora, 43* : Example, 63 : Clas-
sification, 65 : General organisation,
65, 66 : Systematic position of the
example, 65
Maturation, 18*, 19
Maxilla — See Appendages
Maxillary palp— See Appendages
Maxillipeds — See Appendages
May-flies, 585

Medulla of Paramcecium, 79
Medullary sheath, of Nerve-fibre, 27, 28
Medusa-buds, of Obelia, 119, 120
Medusa?, of Obelia, 124, 125
Megagametes, of Flayellata, 71
Megalwsthetes, 668
Megalopa stage, 553

Megameres, of Ctenophora, 205 : of Poly-
clad, 257

Meganucleus, of Ciliata, 84 : of Para-
mwitim, 79

Megaspores, of Radiolaria, 61

Megazooid, of Vorticella, 87, 89

Meleagrina, 647, 661

Meleayrina margaritifera, 661

Melicerta, 303, 305, 306

Melolontha, 594

Jiembranes, 22

Membranipora, 320

Mentum, 573

Meroblastic, 20*

Mesemy>ric filaments, of Flabelhtm,
I£l3 : ol Sea-anemone, 175

VOL. T

Mesenteries, of Sea-anemone, 174
Mesenteries, development of, in Sea-
anemones, 179, 180 : Arrangement in
Actinozoa, 185
Mesoblast, 21*
Mesoderm, 21*, 101, 102
Mesoderm bands, of Peri-planet a, 581
Mesoglcea, 101*, 102: of 0/>e//«, 121
Mesonephridia, 439*
Mesopodium, 675, 688
Mesothorax, 574
Mesotrochal, 445
Mesozoa, 216

Messmateism — See Commensalism
Metagenesis, 39*: of Obelia, 129: Lepto-
linse, 141 : Trachylinre, 144 : Aurelia,
161 : Liver-Fluke, 230 : Tcenia, 236 :
Platyhelminthes, 259, 266 : Nematoda,
286

Metameres, 41*, 403
Metamorphosis, of Trachylinse, 144 : Sea-
anemone, 180 : Platyhelminthes, 260 :
Nemertinea, 273 : Phoronis, 331 : A*-
terina, 361 : Antedon, 377 : Echino-
dermata, 396: Chastopoda, 445 : Ape*,
497 : Crustacea, 552 : Insecta, 600
Metamorphosis, retrogressive — See De-
generation
Metanaupliiis, 497
Metapodium, 675, 688
Metathorax, 574
Metazoa, 96
Micrsesthetes, 668
Microgametes, of Flagellata,v71
Microgromia, 50
Microhydra, 137, 155
Micromeres, of Ctenophora, 205
Micromeres, of Polyclad, 257
Micronucleus, of Ciliata, 84
Micronucleus, of Paramcecium, 79
Micropyle, 113

Micropyle, of Cephalopoda, 744
Microspores, of Radiolaria, 61
Microttomum, 263
Microzooid of Vorticelta, 87, 89
Miescher's corpuscles, 79
Miliola, 52
MiUepora, 145
Mtllepora alrirormv, 145
Millipedes, 567, 568
Minya*, 189
Miocene, 7
Mites, 612, 616, 619
Mitome, 15
Mitosis, 16*, 17
Modiola, 649
Mollusca, 631, 751 w
Molluscoida, 313
Molluscoida, mutual relationships of the

classes, 343
Monocystis agilis, 75
Monoecious, 37

.S i)

770

INDEX

Monogenetica, 238% 250, 252, 253, 254,
256

Monomyaria, 649

Monosiya, 72

Monotiis, 240

Monozoa, 238*

Morphology, 3

Morula, 20*

Mosquitoes, 586, 590

Mother-cyst, 262

Moths, 587

Mouth papillae, 347*

Movable cheek, 558*

Mulberry body, 20*

Miiller s larva, 258

MuHicUia, 84, 85

Multiple fission, of Euglena, 64

Murexy 693, 696

Muscle, striated, 26*, 27 : non-striated,
26,27

Muscle processes, Hydra, 136, 137

Muscles, 35

Muscular fibres of sponges, 102

Muscular system, of Aurelia, 160 : Sea-
anemone, 174, 175 : Magellania, 335 :
Brachiopoda, 339 : Hirudo, 467, 468 :
Apus, 489 : Astacus, 504, 505 : Crus-
tacea, 547: Periplaneta, 575: Ano-
donta, 635 : Pelecypoda, 648

Muscular tissue, 26*, 27

Mushroom coral, 192

Mussels, 631

Mj/a, 647 <

.My a arenaria, 649

Mycetozoa, 43* : Example, 61, 62 : Spo-
rangium, 62 : Capillitium, 62 : Spores,
62 : Flagellula, 62 : Plasmodiuin, 63 :
General remarks, 63 : Protomyxa, 63

Myomeres, of Apus, 490

Myriapoda, 484

Myriapoda, 566 : Distinctive characters
and classification, 566 : General organ-
isation, 567 : External features, 567

Myriothela, 137

Mysis, 523, 537, 553

Mytilus, 646, 652, 654, 655, 656

Mytihis tdulis, 648, 653

Mytilus latus, 648

Myxidium litberkuhnii, 78

Myxobolus mulleri, 78

Myxospongi*, 103*, 111, 115

Myxpsporidea, definition, 76 : Character-
istic features, 78

stoma, 448
-Myzostomida, 448

N.

N

ACRE, 635*

Naidomorpha, 428*
AW?, 446
Narcomedusae, 132*

Natural History, 1*

Nauplius, 496, 497^

Nausithrk, 169

Nautiloids, 750

NAUTILUS POMPILIUS. ShlJ^^'25, 726 :
External characters of sofHI^^ 726 :
Mantle and mantle-cavity,^^3^: En-
teric canal, 730 : Heart and circulation,
732 : Renal organs, 732 : Nervous
system, 734 : Sense-organs, 734 : Re-
productive organs, 735 : Systematic
position, 737

Nebalia, 535, 536

Neck, of cockroach, 573

Nectocalyx, 149*

Nekton, 8*

Nemathelminthes, 221, 275, 297

Nematocyst, 73, 79, 86, 123*

Nematoda, 275 : Example, 275, 281* :
External characters, 283 : Body-wall,
283 : Enteg| canal, 283 : Coelome,

284 : Nervo^l system, 285 : Eye-spots,

285 : Repro«ctive organs, 285 : De-
velopment, ^86, 287 : Life-historv,
286

Nematogene, 217*

Nematoidea, 282*, 284, 285

Nemertinea, 273* : General features,
268-271 : Alimentary canal, 269 :
Blood-vessels, 270, 271 : Excretory
vessels, 270, 271 : Nervous system,
270 : Lateral organs, 269, 271 : Eyes,
272 : Otocysts, 273 ; Reproductive
system, 273 ; Development, 273

Neo-crinoidea, 380*

Neomenia, 663, 664, 667, 668

Nephelis, 476, 477, 478, 481

Nephridiopore, 411, 418

Nephridium, 403, 411*, 412, 423, 440

Nephridium, provisional, 440

Nephrostome, 411, 423, 440

Nereidce, 428, 447

NEREIS, External features, 404 : Enteric
canal, 406 : Body-wall, 408 : Vascular
system, 409 : Nervous system, 409 :
Sense-organs, 410 : Excretory organs,
411 : Reproductive organs, 412 : Indi-
vidual variation, 413 : Development,
413 : Systematic position, 428

Nerve-cell, 27*, 28

Nerve-fibres, 27*, 28

Nervous system, 14, 29, 35* : Obelia,
123 : Leptolinre, 138 : Aurelia, 160 :
Tealia, 179 : Hormiphora, 203 : Planaria,
224 : Distomiim, 228 : Tcenia, 233 :
Platyhelminthes, 252 : Nemertinea,
270: Ascaris, 279: Nematoda, 285:
Acanthocephala, 291 : Chfetognatha,
294 : Srachionus, 301 : Rotifera, 308 :
Dinophilea, 310 : Gastrotricha, 312 :
Polyzoa, 316, 324 : Endoprocta, 326 :
Phoroni*, 329 : Magellania, 336, '337 :

INDEX

Brachiopoda, 341 : Starfish, 350 : Sta-
iirchin, 367 : Holothurian, 370: Antt-
don, 375 : Echinodermata, 394 : Nerti*,
409, 410 : Liniihrirn.y, 422 : Cha-topoda,
437 : Mvgostoinida, 448 : Sipunculun,
453 : Gephyrea, 457 : Archi-annelida,
464 : ffiritdo, 472 : Hirudinea, 478 :
Apux, 492 : Astacus, 513 : Crustacea,
550 : P<:i'ipatu«, 562 : Myriapoda, 569 :
Periplamta, 578 : Insects, 594, 595 :
Scorpion, 609 : Arachnida, 621 : Mns-
xds, 641 : Pelecypoda, 657 : Amphi-
neura, 666, 667 : Triton, 679 : Gastro-
poda, 693 : Scaphopoda, 706 : Rhodope,
707: Sepia, 717, 719: Natttihu, 734:
Cephalopoda, 744

Nervous tissue, 27

Neurilemma, 27*, 28

Neuropodium, 405*, 430

Neuroptera, 584*, 598

Nicothfr, 530, 531

Nidamental gland, 724*

Noah's Ark shell, 646

NoctUucvt, 74

Nodosuria, 53

Nomenclature, binomial, 1*

Non-Calcarea, 103*

Xottti*, 305

Notlwlca, 305

Uotommata iverneckii, 309

Notopodium, 405*, 430

Nuchal cartilage, 712

Nuclear membrane, 15*, 17

Nuclear sap, 16*

Nuclear spindle, 16*, 17

Nuclearia, 57

Nucleolus, 15*, 17

Nucleus, 11*, 15, 17

Nucula, 646, 652, 653, 654, 655, 656,
657, 658

Nudibranchia, 684*, 705

ffiummtuites, 53

NyHotheru*, 83, 85, 86

Ifymphon hiyjidum, 625

771

Oligochgeta, 4~2

Ommatidium, 494*, 495

Oitchidium, 695, 704

Oni«rn*, 543, 546

Onychophora, 484, 559, 629— See Peri-

patus

Ocecium, 314
Oosperm, 20*
Oostigite, 544*
Oostigopod, 489*
Opaliiui, 84, 89

Operculum (Gastropoda), 673*, 674, 688
Ophioglypha, 386
Ophiurida, 378*
Ophiuridea, 346, 378*, 382, 383, 384, 385,

386, 387, 392, 393, 394, 395, 398, 399,

400

Ophryodendron, 91
Ophryoylena, 85
Opisthobranchia, 684"
Opossum-Shrimp, 523
Orchtvtia, 543, 548
Order, 4*

Organic evolution — See Evolution
Organism, 1*
Organ-pipe Coral, 182
Organs, 29*
Orthonectidce, 218
Orthoptera, 584*
Orthoptera genuina, 587
Owardla, 108
0>icaria, 106

Osphradium, 642*, 657, 676, 695, 729, 743
Ostracoda, 521", 527
Ostrea, 647, 652, 656, 658, 660
Otocyst, 37*, 125*
Ovariole, 580*, 597
Ovary, 37*
Oviduct, 38*
Oviparous, 38*
Ovum, 18*, 28
Oxygen, oxidation, 34
Oxyurit, 285

0

'DELIA, General structure, 118, 120:
Microscopic structure, 121, 122: Me-
dusa?, 124, 125, 126: Comparison of
polype with medusa, 126, 127, 128 :
Reproduction, 129: Development, 129 :
Systematic position, 132

Octopoda, 736*, 738, 750

Octo2>ti«, 737, 739, 742

Off or chandra, 139

Odentophora, 676*, 678

(Esophagus— See Digestive system

Oikomona*, 66, 67

LACBYCHALINA, ill*

Paedogenesis, 39*, 211*, 266, 598
Payuru*, 539
Paloemon, 538
Palffimonetex, 547
Palaeocrinoidea, 380*
Palaeodictyoptera, 604
Palseo-echinoidea, 379*
Palaeonemertinea, 273*
PaLeontology, 6*
Palinurus, 537, 549, 553
Palinnrn* r«A/«rw, 556
Pallial line, 634*

772

INDEX

Pallium — See Mantle

Palpus. 40,1

Pcdudicella, 32o, 320

Palus. 192*

Palythoa, 116, 184

Pancreas, 32

Pancreatic juice. 32"'

PandoriiKi. 07. 08, 69

Papula, 393. 347"

Parabranchia. 090*

Paragaster, 1)7"""

Paraglossa, 589

Paragnatha, 488"; 506

Paramn-i-ii/n . 82

PARAMCECIU.M. 79, yJO, 81 : Systematic
position, 82

Param'ttltrux, 555

Paramitome, 15*

Paramylum, 04"

Parapodium, 403, 404*, 405, 430

Parenchyma, 222, 249

Parthenogenesis, 38*, 200

Parasitism, Protozoa. 90, 93

Parthenogonidium, 70*

Pnrthtuoi*, 541

Patello. (i85. 080, 691. 693, 694,,695, 696,
698, 701

PateHidtv, 683

Pft.iii-v/'O'/'i; 507"", 570

Pauropii*, 567

Paxilla, 380*

Peachia, 189

Pearl-oyster. 047

Pectin," 643, 648. 058, 659, 661

Pectinibranchia, 083*

Pedal gland, 088*

I'tflrifiou, 306, 307, 304, 491

Pedata, 379*

Pedicellaria, 348*, 357, 363, 383, 389

Pedicdlina, 320, 320. 327

Pedipalpida, 611*, 013, 019. 621

J'(-/arjia, 171

Pelagic, 8*

/'(•/tof/nxf'-r. 534

Pen, 7 12-

1'i-ii'i-n*. 553

Pennatula, 185, 186, 191, 195

Pennatulacea, 182*, 184, 191

Pentastomida — See Linguatulida

Pentastomiim t&nioide*, 625

Peptones, 32*

Perforate corals, 194*

Pericardial sinus, 509*

Perichondrium, 25*

Pericolpa, 167

Periosteum, 26*

Periostracum, 634*

PERU-ATI'S, External features, 559, 560:
Body-wall and body-cavity, 501 : En-
teric canal, 561 : Circulatory system,
561 : Organs of respiration, 562 : Coxal
and slime glands, 562 : Nervous

system, 562 : Nephridia, 562 : Repro-
ductive organs, 562 : Development,
563 : Distribution, 566 : Relationship.
566

PERIPLANETA AMERICANA, 571, 572 :
Head, 572 : Neck, 573 : Thorax, 574 :
Abdomen, 574 : Respiratory move-
ments, 575 : Muscles, 575 : Coelome,
576 : Digestive system, 576 : Renal
organs, 577 : Heart, 577 : Respiration,
578 : Nervous system, 578 : Organs of
special sense, 579 : Reproductive
organs, 579 : Development, 580 : Sys-
tematic position, 587

Perisarc, 121*

Peristaltic movements. 34*

Peristomium, 404*, 417. 432

Peromedusaj, 164*, 160, 167

Per-radius, 129

Petrarca, 534

Phaco2)$fecundu«, 558

Phseodium, 59

Phalangida, 012*, 015, 021

Phanerozonia, 378*

Pharynx — See Digestive system

Phaxmidir, 587

Philodina, 303, 305, 309

Phola*, 047, 061

Phoronida, 313. 328

Phoronis, 328, 329, 330, 343, 344

Phragmoceras, 750

Phragmocone, 742

Phronima, 545, 546

Phrynus, 613

Phyiactoleemata, 320, 321. 322. 323, 324.
325, 320, 343, 344

Phyllocarida, 523*, 535

Phyllopoda, 521*

Phyllosoma, 553

Phylogeny, 8*

Phylum, 5*, 41

Phyvafia, 151, 152

Physiology, 9*

Picris, 586

Pilema, 170

Pilidium, 272, 273

Pill-bug, 524

Pinna, 047, 653

Pinnothere*, 556

Piscicola, 476, 481

Placophora, 663*, 751, 665, 666, 667, 608,
071

PLAN ARIA, 222 : General features, 222,
226 : Digestive system, 222, 223 :
Water vessels, 223, 224 : Nervous
system, 223, 224 : Reproductive sys-
tem, 224, 225 : Systematic position,
238

Plankton, 8*

Planorbulwa, 53

Plant-lice, 5X5

Planula, 129*, 130, 141*, 101

INDEX

773

Plasmodiuin, 28*

Platomn isfwo'r<,um, 51

Platyhelmmthes, 221, 237": General
external features, 239 : Integument
and muscular layers, 248 : Parenchyma,
249 : Alimentary systems, 250, 251,
252 : Nervous system, 252 : Water
vascular system, 253 ; Reproductive
organs, 254, 256 : Development, 256,
257

Platypoda, 684*, 7<»5

Pleopod, 500*

P/t urobrOchiadce, 208

Pleurobranchia, 508*

Pleuron, 498*

Pttnrophyllidia, 691

Ploima, 304 , 305, 306

PlnriKitftri'.t, 138

Phi-nmtdla, 320, 322

Pluteus, 397, 398

Pneumatophore, 149'*

Podobranchia, 508

Podomere. 484*

Podophrya, 90, 91

Poditra, 584

Polar body, 18*, 19

Polian vesicle, 354*, 393

Pollicipes, 534

Polyarthra, 304, 305, 306

Polycdis, 240

Polycercus, 260

Polychseta, 427*, 432, 433, 435, 436, 438,
440, 443, 444, 446, 447, 44S

Polycladida, 237*, 240, 242, 248, 251, 252,
253, 255, 25G, 257

Polydora, 447

Polygordiidce, 462

Polyyordius lactettx, 463

Polygordiu* neapolitanttfy 462, 463, 464,
482

Polykriko*, 73, 74

Polymorphism, 131*

PolynGe, 440

Poly not exfeimata, 432

Poly not setoxissinia, 429, 431

Polynoidcs, 429

Polyceca, 72, 73

Poiyophthalmua, 438

Polype, 118*, 119

Polyphemus, 527

Polyphylitic, 266

P&lypodiuin, 155

Polyatometv, 243

Polyefomum, 243, 264

Polytrochal, 445*

Polyzoa, 313 : Example, 314, 319* : See
Eftoprocta and Endoprocta

Polyzoa (Cestoda), 238*

Pontdlina mediterranta, 554

Pontobdella, 475, 477, 478, 479, 481

Pore -membrane, 100*

Pores, inhalant, 97*

Porifera, Example, 9(5, 102* : General
form and mode of growth, 105 : Leading
modifications of structure, 107 : Skele-
ton, 111 : Reproduction, 112: Develop-
ment, 113 : Distribution, affinities, &c. ,
115, 215

Poromya, 647, 655, 656

Porpita, 153, 154

Port-hole — See Cinclis

Portuguese Man-of-War, 151

Port tiling 541, 554

Post-abdomen of Scorpion, 606

PotamobiidcK, 525

Poterion, 106

Prte- abdomen of Scorpion, 605

Prawns, 537, 538

Priapulidce, 455

Priapiilus, 456, 457

Primary axis, iO*

Prffneomenia, 663, 664, 667

Pronucleus, active and stationary, 82

Pronucleus, male and female, 18*, 19

Pro-ostracum, 742*

Prorocentrum, 73, 74

Prorodon, 85, 86

Proscolea, 237

Prosiphon, 741*

Prosobranchia — See Streptoneura

Prosopyle, 100*, 108

Prostomium, 404*, 417, 432

Protamctba, 47

Protandrous, 285

Protective characters, 555

Proteid, 15*

Proteolepas, 534

Proterosponyia, 72, 73, 116

Protobranchia, 646*, 648, 649, 653, 656,
662, 752

Protoconch, 725*, 741

Protodrilii*, 463, 482

Protohydra, 137

Protomerite, 77*

Protomyxa, 63

Protoplasm, 11*, 14, 15*

Protopodite, 496*, 500

Prototroch, 416*, 701

Protozoa, 43

Protozoan, 553

Proventriculus, 593

Px<immo<-l<'ina, 106

Pseudo-lamellibranchia, 646*, 662

Pseudopod, 10*

Pseudo-scorpionida, (ill*, 613, 619,
621

Pool-it*, 390

Pteropoda, 684, 687, 688, 689, 705

Pterotrachea, 690

Pulex, 586

Pulmonary sac, 691*, 692

Pulmonata, 684*, 689, 691, 705

Pulsellum, 705

Pupa (of Cirripedia), 552

774

INDEX

Pupa (of Insects), 502
Purpura, 693
Pycnogonida, 624, 620
Pygidium, 559*
Pymla, 699

Q

UADRULA, 47

E

R

UADIAL CANALS, 100*, 125

Radial Symmetry, 40*

Radiata, 401

Radii, orders of, 128,

Radiolaria, 46* : General structure, 58 :
Central capsule, 58 : Skeleton, 59 :
Colonial forms, 59 : Reproduction, 59 :
Symbiosis, 61

Radula, 676*, 678

Radular sac, 676*, 678

Rapliidiophrys, 57

Red coral, 182

Redia, 231, 259

Regularia, 379*

Relationships, of Protozoa, 93 : Sponges,
116: Cuelenterata, 213: Platyhel-
minthes, 267 : Nemathelminthes, 297 :
Rotifera, 309: Dinophilea, 311 : Poly-,
zoa, 328: Molluscoida, 343: Echino-
dermata, 401 : Gephyrea, 459 :
Annulata, 482 : Crustacea, 556 :
Air - breathing Arthropoda, 627 :
Pelecypoda, 662 : Gastropoda, 705 :
Cephalopoda, 750 : Mollusca, 751

Relationships, diagrams of : of Protozoa,
93 : Ccelenterata, 215 : Platyhel-
minthes, 267 : Echinodermata, 402 :
Annulata and Trochelminthes, 483 :
Crustacea, 557 : Arthropoda, 629 :
Pelecypoda, 662 : Gastropoda, 7< >"> :
Cephalopoda, 750

Renal organs— See Excretory system

Reproduction, Reproductive System, 14,
29, 37 : Amwba, 14, 44 : Foraminifera,
.">.") : Heliozoa, 58 : Radiolaria, 59 :
Mycetozoa, 62 : Englena, 64 : Flagel-
lata, 70 : Choanoflagellata, 73 : Dino-
flagellata, 74 : Cystoflagellata, 74 :
Monocystis, 75 : Gregarinida, 77 :
Paramveciiim, 81 : Ciliata, 88 : Ten-
taculifera, 92: Porifera, 113: Obc/ia,
129 : Leptolinas, 140 : Trachylinre, 144 :
Hydrocorallina, 147 : Siphonophora,
149, 152, 154: Auretia, 158: Tealia,
179 :'fformiphora, 203 : Mesozoa, 217,
219: PJanaria, 224, 225: Distvmum,
228, 229: 7W«, 234: Platyhel-
minthes, 254: Nemertinea, 273: Ax-

caris, 279 : Nematoda, 285 : Acantho-
cephala, 292 : Ch«tognatha, 295 :
Brachionns, 302 : Rotifera, 309 : Dino-
philei, 310 : Bucjula, 316 : Ectoprocta,
325 : Endoprocta, 327 : Phoronis, 329 :
MageUania, 337 : Brachiopoda, 341 :
Starfish, 356 : Sea-urchin, 368 : Holo-
tlinrkut, 372 : Antedon, 377 : Echino-
dermata, 394 : Nereis, 412 : Lumbricn*,
423, 424 : Chisetopoda, 441 : Myzosto-
mida, 449 : Sipunculus, 454 : Gephyrea,
458 : Archi-annelida,465 : Hirndo^l'^ :
Hirudinea, 478 : Apiis, 495 : Astaru*,
514 : Crustacea, 551 : Peripatns, 562 :
M}*riapoda, 569 : Periplaneta, 579 :
Insects, 597, 598 : Scorpion, 609 :
Arachnida, 622 : Mu^f, <)42 : Pelecy-
poda, 658 : Amphineura, 668, 669 :
Triton, 682 : Gastropoda, 697 : Scapho-
poda, 706 : Rhodope, 707 : Sepia, 723 :
Nautilus, 735 : Cephalopoda, 744

Sequienia, 651, 652

Reservoir (Euglena), 63

Respiration, 13*, 33

Respiratory organs, 33* : Starfish, 347 :
Sear-urchin, 364, 368 : Holothnruui.
372 : Chsetopoda, 433 : Hirudinea,
477: Apus, 492: Astac-us, 507, 508:
Crustacea, 548 : Peripatnx, 562 :
Myriapoda, 569 : Periplaneta, 575,
578 : Insects, 593 : Scorpion, 608 :
Arachnida, 619 : Mussel, 636, 638 :
Pelecypoda, 653 : Triton, 676 : '
tropoda, 690: Sejna, 716: XftutHiix,
729: Cephalopoda, 743

Respiratory trees, 372*, 394

Retina, 37*

Retinophore, 682*

Retinula, 494*, 495, 682

Rhabdites, 248

Rhabditis-form, 287

Khabdoccelida, 238*, 239, 240, 242, 248,
251, 252, 253, 254, 255, 256

Rhabdome, 494*, 495

Rhagon-type of Sponge., 109*

Rhipidodendron, 66, 6S

Bhipidoglossa, 683*, 7<»5

Rhizocephala, 522*, 534

Ehizopoda, 43* : Example, 44 : Clas-
sification and general organisation,
45

Rhizostomee, 164*, 169, 170, 213

Rhizote, 303*, 305, 306, 308

Rhodope, 707, 751

Rhombogene, 21 7 *

Rhopalnra, 218, 219

Rhynchobdellida, 475*, 476, 477, 478,
479

Rhynchodemii*, 240

Rhynchonella, 340

Rhynchota, 91

Rock-lobster, 523

INDEX

775

Rock systems, 7

Rocks, igneous and aqueous, 7*

1-iotalia, 52

Rotation, aense of, 37

Rotifer, 303

Rotifera, 229, 303* : External characters,
304 : Digestive organs, 307 : Excretory
system, 308 : Nervous system and
Sense-organs, 308 : Reproduction and
Development, 309 : Ethology, 309 :
Affinities, 309, 483

Round-worms, 275 — See Neurathelmin-
thes

Rugosa, 197*

IACCAMMIXA, 53
Saccocirrus, 438
Sacculina, 534, 535, 553
Srt'fitfft, 293, 294, 295
Saline? fa, 219, 220
Saliva, 32*
Salivary glands, 32*
Salivary receptacle, 576
SaJnviciiKi, 446
Salpiixja'cn, 72
Saltatoria, 5S7
Sand -hopper, 524
Sarcocystidea, 76*, 79
SarcocysPis, 79
Sarcolemma, 26*
Sarcophaga, 595
Sarcoptf-* xcfifiin-i, 616
S'.tr*ia, 135
Scale-insects, 585
Scallop, 647
Scaphopoda, 705
Schizonemertea, 274"
Schizopod-stage, 553
Schizopoda, 523*, 536
Scirtopoda, 304*, 300, 307
Sclerite, 520*
Scleroblast, 101*, 111
Scolex, 231
Scolopendra, 568
Scolopendrella, 567, 569
SCORPION — See BUTHUS.
Scorpionida, 611*, 613, 624
Scorpion -spiders, 612
S<'robint/<<>'ia piperata, 650
Snttiy;.rrf, 567, 5H<)
Scyllaru*, 537, 539
Scyphistoma — See Scyphula
Scyphozoa, 118, 156, 163*, 171
Scyphula, 161*, 162
Sea-anemones, 118, 181 — See Tealia
Sea-blubbers, 169

SEA-CUCUMBER, External features, 369 :
Structure of body- wall, 370 : Ambu-

lacral system, 370 : Nervous-vascular
systems, 370: Cceiome, 371: Enteric
canal, 372 : Reproductive organs, 372 :
Development, 372 : Systematic posi-
tion, 381

Sea-cucumbers, 346 — See Holothuroidea

Sea-fans, 182

Sea-firs (Sertularians), 133

Sea-hares, 684

Sea-lilies, 380

Sea-mats, 311— See Polyzoa

Sea-mussel, 646

Sea-pens, 183

SEA-URCHIX, External features, 363, 364,
365 : Corona, 364 : Aristotle's lantern,
366 : Nervous system, 367 : Ambu-
lacral system, 367 : Enteric canal, 367,
368 : Cceiome, 368 : Blood- vascular
system, 368 : Reproductive organs,
368 : Development, 368 : Systematic
position, 381

Sea-urchins, 346 — See Echinoidea

Secondarj- axis, 40*

Secretion, 22*

Sedentaria, 427*

Segment, 41

Segmental organ — See Nephridium

Segmentation of oosperm, 20*

Segmentation-cavity — See Blastocwle

Segmentation-nucleus. 19"

Seiton, 309

Selenaria, 320

Sefenariidce, 321, 324

Semostomse, 164*, 169, 213

Sense-organs, 37 : Obetia, 125 : Trachj'-
lin;v, 149: Aurelia, 160: Hormiphora,
198, 203 : Platyhelminthes, 253 : Ne-
mertinea, 271 :" Cha?tognatha, 2(.)5 :
Brachionus, 301 : Rotifera, 3< >8 :
Phoronis, 329 : Starfish, 348 : Sea-
urchin, 367 : Nereis, 410, 411 : 'Ghreto-
poda, 438 : Sipunculus, 454 : Hiri/flo,
472 : Hirudinea, 478 : Apn«, 494 :
Astacus, 513 : Crustacea, 551 : Peri-
patus, 560 : Periplaneta, 579 : Insects,
595, 596, 597 : Scorpion, 609 : Arach-
nida, 621 : Mussel, 642 : Pelecypoda,
657 : Amphineura, 668 : Triton, 674,
676, 682: Gastropoda, 695: fihodope,
707 : Sepia, 720 : Nautilm, 734 :
Cephalopoda, 743, 744— See also Eyes,
Auditor}- organs, Olfactory orgaiis,
Gustatory organs, Tactile organs,
Osphradia

SEPIA, External features, 708, 709 :
Shell, 711 : Chromatophores, 711 ;
Mantle - cavity, 712, 713: Internal
skeleton, 712 : Alimentary system,
712 : Ink-sac, 714 : Vascular system,
714 : Cceiome, 715 : Ctenidia, 716 :
Nervous system, 717 : Sensory organs,
720 : Excretory organs, 722 : Repro-

70

INDEX

ductive organs. 723 : Systematic posi-
tion, 737

S< jiin <•>!/< i-'(f>(, 708

Septal funnel, 101*

Septal neck, 725*

Septibranchia, 647*, 048, 054

Septum, 192*

Senofo«a,~32Q

Serosa, 581*

/'t, 434, 437, 447
uJ.n ' , 433

Sertulariana, 133

Seta, 405*, 419, 431

Seta, provisional, 416

Sctigerous sac, 405*, 420, 431

Sexual dimorphism, 38*

Sexual generation, 130

Shell, Magellania, 331,332 : Brachiopoda,
338, 339 : J/uW, 633 : Pelecypoda,
048 : Chiton, 004, 665 : Triton, 672,
673 : Gastropoda, 080 : Scaphopoda,
7o~>: Sepia, 111: Nautilu*, 725, 726:
Cephalopoda, 740

Shell-gland, 492*, 493, 550, 043*

Shelly loop, 332

Ship-worm, 047

Shrimp. 523 —-^Z>

Sicula, 155"

.SY</«/v; .'/<*, 688

Silicispongiae, 103*

Sinupalliata, 647*, 662

Siphonodentalium, 705

Siphonophora, 132*, 147, 148, 150, 152,
153, 154, 213

Siphonozooid, 189*

Siphons, inhalant and exhalant, 632*,

, 649

Siphuncle, 725*

SipunculidcBf 455

SIPUNCULUS XUDUS, General external
features, 451, 452 : Body-wall, 451 :
Gelome, 452 : Blood-vascular system,
452 : Alimentary canal, 452, 453 :
Nervous system, 453 : Nephridia and
gonads, 454 : Systematic position, 455

Skeleton, 29* : Lobosa, 47: Fdraminifera,
51 : Heliozoa, 56 : Radiolaria, 58 :
Mastigophora, 68 : Ciliata, 87 : Actino-
zoa, 189 : Sepia, 712 : Cephalopoda,
743— See also Shell and Body-wall

Skin, 29*— See Body-wall

Slugs, 684

Snails, 684

Solarium, 685

So/o-tli-'tl* */ri:l/.//'tfHH, 651

Solen, 661
Solenomya, 040
Solpugida, 012*, 614
SomatoMast, 414
Somatppleure, 582*
Spadella, 2<)3
ix, 728*

Spatangoidea, 379*. 383, 389, 390

Species, 1*, lo4':

Specific .name, 2*

Sperm, 18, 28*

Spermary, 37*

Spermatozoon — See Sperm

Spermiduct, .">S

Sphteridium, 303*, 383

Sphcerophrya, 91

Spicules, 30*

Spiders, 612

Spinning-glands, 592

Spirifera, 339

Spirolociilina, 53

Spirorbis, 448

Spirorbix l«jt"i*, 444

Spirula, 740, 741

Splanchnopleure, 582*

Sponyelia, 111

SpoiKjilla, 109

Spowjillidw, 112, 115

Spongin, 111*

Spongin-blasts, 112*

Sporangium (Mycetozoa), 62

Spore, 38*

Spore formation, 62, 75, 89

Sporocyst, 231, 258

Sporosac, 140*

Sporozoa, 44, 75, 70*

Squammulina, 51, 52

Squids, 736

Squilla, 542

STARFISH, External characters, 346, 347,
357 : Transverse action of arm, 349 :
Vascular and nervous systems, 3^50 :
Structure of disc, 351 : Body- wall and
ccelome, 351 : Digestive system, 353 :
Ambulacral system, 350, 354 : Ovoid
gland, 354 : Reproductive system, 356 :
Development, 358, 359, 360, 361 :
Systematic position, 380

Starfishes, 346 — See Asteroidea

Stauromedusae, 164*, 165, 213

Stentor, 83, 85

Sfc.phfinorerox, 303, 305, 306

St(;i'na«pi*, 432, 430, 440

Sf'.rii'iKpi* x/nn-osa, 433

Sternum, 498*

Stewart's organs, 393*

Stick-insects, 584

Stigma (Euglena), 04

Stigmata, 502, 575, 593

Stinging capsule— See Nematocyst

Stolon, 183*

Stomach — See Digestive system

Stomatogastric nerves, 422

Stomatopoda, *24*, 542

Stomidium, 188*

Stomodceum, 161*

Stone-canal, 354*

Stony corals, 118, 182

Strepsiptera, 002

INDEX

777

Streptoneura, (583", 688

Strobila, 233

Strongylocentrotus, 364 — See Sea-urchin

Strongylo8toma, 570

Stylarwides, 447

Stylaster, 146, 147

Stylonychia, 90

Sub-genital pit (Aurelia), 158"

Sub-genital portico, 171*

Sub-kingdom — See Phylum

Sub-radius, 129

Sub-umbrella, 125*

Succession of Life in time, 7

Sucker (Sepia), 709*, 710

Suctorial mouths (Rhizostomee), 170

Summer eggs, 551

Supporting lamella — See Mesogloea '

Swimming-bell — See Nectocalyx

Swimming-plate, 199, 200*

Sycantha, 103

Sycetta, 103

Sycettidce, 103

SYCOX : External characters, 96, 97 :

Microscopic structure, 99 : Systematic

position, 103 : Development, 113,

114

Sycon-type of sponge, 109*
Syflida, 436, 446, 447
Syl/i* ramosa, 446
Symmetry, 39*, 40 : Polype and Medusa,

128 : Tealia, 176, 179
Symphyla, 567*, 569
Synapta, 390, 399, 402
Synapticula, 192*
Syn-cerebrum, 513*
Syncoryne, 135
Syncrypta, 66
Syncytium, 99"

JL ABAS US, 590

Tabula, 143*, 192

Tactile organs, 37

Tfijnia ('(i')iitritx, 261

Tcenia crassicolli*, 265 . .

Tcenia echinococats, 246, 261, 262, 265

T<xnia mediocanellata, 265

Tfenia saginata, 265

Tit-ilia serrata, 265

>I\KMA SOLIUM : General features, 231,
232, 233 : Nervous system, 233 : Ex-
cretory organs, 234 : Reproductive
organs, 234 : Systematic position, 239

Tamiole, 161*

Tutifru*, 543

Tanai*, 543

Tape-worm — See Ta^nia and Cestoda

Tardigrada, 626, 629

TEALIA : P]xternal characters, 172, 173 :
Enteric system, 174, 175 : Muscular

.system, 173, 175 : Symmetry, 176 :
Microscopic structure/ 177, 178 : Ner-
vous system, 179 : Reproductive
organs, 173, 179: Development, 179,
180 : Systematic position, 183

Tectibranchia, 6S4", 7<'">

Telolecithal, 2(»7*

Telotrochal, 445*

Telson, 498*

Temnocephala, 244, 264

Temnoctphalw, 244

Tendon, 35*

Tentacles, 30*

Tentaculifera, 82* : Body and tentacles,
90: Nucleus, contractile vacuoles,
shell, colonies, reproduction, 92

Tentaculocyst, 143*, 160

Tentorium, 579

Terebella, 434

Terebellidtr, 433

Terebra, 686

Terebmtufa, 336, 338, 339

Terebrtotulidce, 338

Terebmfii/iitd* 343

Teredo, 647, 652

Tergum, 4<)s

Termites, 585, G< >3

Terssra, 165

Tethys, 684

Tetrabranchiata, 736", 739

Tetramita, 66

Tetrarhynchus, 246

T(ti-<i.«temma, 269

Thalassoplancta, 59

Theca, 192*

Thomson, J. Vaughan, 3

Thorax, of Apu*, 489: A*tacn«, 499':
Periplanetdk, 572, 574: Solpugida, 615

Thread- worms — See Nemathelminthes

Thyx'i'nozoon, 240

Thysanura, 5 S3

Ticks, 612— See Acarida

Tiedemann's vesicle, 354*, 393

Tintinnidinm, 85

Tissues, 21*

Tomo2)teri.<<, 447

Trachea?, 33*, 562, 577, 578, 593

Trachea! gills, 594*

TracheliaMes, 531

Trachelius, 85

Trachelomonas, 66

Trachylinae, 132* : General structure,

142 : Sense organs, 143 : ^Tentacles,

143 : Reproductive-organs,* 144 : De-
velopment, 144

Trachymedusse, 132*, 142, 143, 144 3

Trap-door Spider, 623

Tmpezia, 196

Trematoda, Example, 226, 238,*, 242,
243, 244, 245, 24S, 24U, 250, 251, 252,
253, 25*4, 250, 258, 259, 264, 265, 266,
267

778

INDEX

ax, 220
u* ln.ckt'i, 559
Trichina, 284, 288
Trichocyst, 80, 84
Trichoplax, 220
Trichostomata, 82

Tricladida, 237% 241, 248, 251, 252,
255

660

Trigger-hair — See Cnidocil

Trigonia, 646, 652, 661

TriloMta, 558, 559, 629

Trimorphism, 119*

TRITON XODIFERUS, 671 : Shell, 672, 673 :
External features of soft parts, 674 :
Foot, 675 : Visceral spirals, 675 : Man-
tle, 675: Ctenidium, 676: Osphradium,
676 : Digestive system, 676 : Vascular
system, 679 : Excretory system, 679 :
Nervous system, 679, 680 : Sensory
organs, 682 : Reproductive organs, 682 :
Systematic position, 685

Tritontdce, 685

Trivium, 348*

Trochelmintlies, 221, 298

Trorheta, 476

Trochophore — See Trochosphere

Tro-hotphwra, 304, 306, 307

Trochosphserida, 304*, 307

Trochosphere, 298

Twhnx, 683, 696

Trombidium fuliyinos n m, 616

Tube-feet, 348*, 363, 383

Tubiftx, 442

TubificidcK, 434, 447

T-uhipora, 186, 190, 195

Tnbti/trrM, 133, 135

Turbellaria, 237*, 239, 240, 248, 250,
251, 252, 253, 255, 263, 264, 266,
267

Turf >o. 683, 696

Typhlosole, 421*, 436

u

U:

MBO, 634"
Umbrella, 124*, 156
Undulating membrane, 83*
Unicellular, 18*
Unicellular gland, 22""
Unio, 631, 647— See Anodonta
Unto iii« r<i«rit iftr, 631
Unionida', 647
Unisexual, 38*
Urea, 35
Uric acid, 34
Urinary organs, 35*
UmatfOa, 326
Uropod, 500"
Uterus, 38*

ACUOLE, contractile, 11*, 79

Variation, individual, 2*, 104*

Variety, 2*, 105*

Vascular system, 32 : Nemertinea, 270 :
Acanthocephala, 291 : Phoronix, 329 :
M't'l'/f'tnia, 336: Brachiopoda, 340:
Starfish, 351, 355 : Holothurian, 370 :
A ntedou, 376 : Echinodermata, 394 :
Nereis, 409 : Lumbricus, 421 : Sipnii- -
culii*, 452 : Gephyrea, 457 : Archi-
annelida, 464: Hirudo, 471, 472:
Hirudinea, 477 : A pus, 491 : A$t.acu>s\~
509, 512: Crustacea, 550: Ptripafii*,
561 : Periplaneta, 577 : Insects, 594 :
Seorpion,Qffl : Arachnida, 619 : J/«*W,
640, 641 : Pelecypoda, 656 : Amphi-
neura, 666 : Triton, 679 : Gastropoda,
(593: Scaphopocla, 706: Sepia, 714,
719 : yantilnx, 732, 733 : Cephalopoda,
743

Vas deferens — See Spermiduct

Vaso-peritoneal vesicle, 396

Velarium, 167*

Vtltlla, 154

Veliger, 659*, 703*, 704

Velum, 125*, 126, 659*, 701

Ventral, 41*

Ventricle, 34*

Venn* < i u i<l in, 650

Venus' s Flower-basket, 112

Venus's Girdle, 210

Vermes, 221

Verjiietes, 684

Vermilia, 430

Vibracula, 321, 324*

Virgula, 155*

Visceral spiral, of Triton, 675

Vitelline glands— See Yolk-glands

Viviparous, 38*

Valuta, 699

Volvox, 67, 68, 70, 71

Vortex, 240

Vorticdla; 83, 84, 86.87, 88, 89

Vulsella, 649; 653 *

W

Magellania
Wandering cells, 102
Wasps, 587
Water-bugs, 585
Water-flea, 522
Water-pores, 375*, 393
Water-tubes, 37">'s, 393
Water-vascula ~iem, 222
Whale-louse, {
Wheel-aninm1 —See Kotifera

INDEX 779

'Whelks, 684 Yoldia, 646

(White substance of Schwann, 27* Yolk, 18*

[Winter eggs, -lol Yolk-glands, 222
iWood-louse, 524 — See Oni#-n*

; v Zoantharia, 181*, 214

! Xiphosura, 612*, 617, 618, 619, 621, Zoanthm, 183, 184, 187, 188
6°4 6°9 Zocecmm, 314

Zoo-geographical Regions, 8
Zooid, 38*
Zoology, 1*
Y Zoophyte, 118*

YZoothdmnium, 88
ELLOW CELLS— See Zooxanthetta. Zooxanthella, 61

Yellow elastic cartilage, 24* Zygote, 70*

END OF VOL. 1

9

RICHARD CLAY AND SONS, LIMITED, LONDON, AND BVSGAY.

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fl.F CALIFORNIA UBRJRLJF THE UNIVERSITY OF CALIFORNIA

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WAY 3 1 195' NKf 2 2

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