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THE GIFT OF
HENRY W. SAGE
1891
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ELEMENTARY
TEXT-BOOK OF ZOOLOGY.
PREFACE TO ‘SECOND EDITION,
(Ni this Edition it has been made possible to add about
50 new Figures as well as a short description of a
type of Rotifera. On the other hand, a careful revision
of the text and former figures has enabled me to keep the
volume to practically the same bulk as before without jhe
_ loss of any essential, parts. \
I have freely availed myself of the numerous criticisms
which have been offered, and desire to thank many friends
for their valuable aid in this respect, among whom I would
specially mention Professor W. C. McIntosh, Professor
Marcus Hartog, Dr H. Gadow, Dr Fraser Harris, Dr
E. W. G. Masterman, and Mr F. H. Marshall: to Professor
Cossar Ewart I am indebted for permission to reproduce a
figure (Fig. 330) from his work on the “ Development of
the Horse”: lastly, I have to thank my wife for the pre-
paration of a comprehensive Index.
A word of explanation upon the arrangement of the
subject-matter may be found useful. Part I. deals in
separate chapters with the general facts and principles of
the subject and its relationship to kindred sciences: in
Part II. the student is expected to study the types of each
group in the museum, or in the laboratory, as the case may
be, and then to proceed to the generalisations under each
phylum or class. This must ever be the natural way of
learning the subject, and has therefore been adopted here.
ARTHUR T. MASTERMAN.
New ScHooL, SCHOOL OF MEDICINE,
EDINBURGH,
PREFACE TO FIRST EDITION.
ha we may accept the hypothesis, generally acknowledged,
that efficiency of the few is attained only under the
stimulas of the inefficient many, no apology is needed for
another addition to the already numerous text-books in
existence. It is questionable whether it is possible to
provide the student with a book which can entirely take
the place of oral instruction, but it is intended in the
present work to provide the necessary accompaniment to
a well-ordered course of lectures and practical work.
Although there are still science “Schools” in existence in
which practical instruction is entirely neglected or relegated
to unqualified teachers, the importance of this branch of
education is being generally recognised: hence I have
written the discriptions of the types in this book, and in
the majority of cases have drawn the figures, with the
animals (or the parts of them) before me, in order that
the work may be found an aid to dissection as well as a
preparation for written examinations.
So far as is possible the scope of the work has been
largely modelled on the subject “Natural History,” as
interpreted in our Scottish Universities, and the method
of instruction by types has been adhered to as conducing
to the best results.
Ina volume of this kind which must necessarily hold in
view the necessities of examinations, there is a very definite
limit to the introduction of new features of classification
or even of new types, and a continual check has to be
applied to the inclination to add this or that new result,
PREFACE, vil
For example, the temptation to partition such time-honoured
institutions as the Ganotdei and some of the Orders of
Insects is almost irresistible. However, our whole system
of Zoological Classification is in such confusion that the
adoption of any particular scheme appears at present to be
purely arbitrary.
The thanks of the publishers and myself are due to
Messrs A. & C. Black, Messrs Cassell & Co., and Messrs
Methven & Co., to whom we are indebted for the use of a
number of the illustrations. I have also to record my
indebtedness to my friend and assistant, Mr R. A. Staig ;
he has not only contributed several of the illustrations, but,
in addition, his lengthy experience of zoological teaching
in the Edinburgh Medical School has been productive of
some valuable hints and suggestions. To Dr Ashworth
I am also under an obligation for kindly reading over the
portion relating to the Lobworm, which is largely illustrated
from his original work.
1 have also to thank Dr Traquair, F.R.S., for kind
permission to reproduce certain of the specimens in the
Edinburgh Museum of Science and Art.
Finally, I must express my thanks to Messrs E. & S.
Livingstone, who have met my wishes with respect to
illustrations and to the general scope of the work with a
rare liberality.
ARTHUR T. MASTERMAN.
New ScHOoOoL,
ScHOOL oF MEDICINE.
CONTENTS.
PREFACE ... j ee
List oF ILLUSTRATIONS
List oF PLATES...
List oF TABLES...
Part I.
CHAPTER I.
LIVING MATTER.
Physical Properties of Protoplasm
Chemical Properties of Protoplasm
Primary Vital Functions of Protoplasm
Secondary Vital Functions of Protoplasm
Food of Animals
Plants and Animals
Transfer of Energy
CHAPTER II.
COMPARATIVE PHYSIOLOGY.
Alimentary System aie i by ase
Motor System
Sense Organs
Excretory System
Vascular System ...
Nervous System ...
Skeletal System ...
Reproductive System
CHAPTER III.
COMPARATIVE MORPHOLOGY.
Animal Symmetry
Morphological Units
Structure and Function ...
Classification
ia
HOUDU ON AD
15
15
16
18
19
19
20
21
22
23
26
26
x CONTENTS.
CHAPTER IV.
HISTOLOGY.
Independent Cells
Dependent Cells ...
Structure of the Cell
CHAPTER. V.
GROWTH AND REPRODUCTION.
Asexual Reproduction
Sexual Reproduction
CHAPTER VI.
COMPARATIVE EMBRYOLOGY.
Larva and Embryo
Segmentation
Types of Larve ...
Metamorphosis
CHAPTER VIL
GEOGRAPHICAL DISTRIBUTION.
Physical Distribution
Aquatic Fauna
Terrestrial Fauna
/Erial Fauna ;
Topographical Distribution
Zoological Realms
Oceanic Islands ...
Discontinuous Distribution
CHAPTER VIII,
GEOLOGICAL DISTRIBUTION.
Fossils
Strata sos
Extinct Animals
3c
31
es
DD:
38
41
46
48
49
54
66
67
79
CONTENTS.
CHAPTER IX.
BIONOMICS,
Physical Relations
Coral Islands
Organic Relations a
Commensalism and Symbiosis ...
Endoparasitism mes
Protective Resemblance er mer.
CHAPTER X.
HEREDITY AND DESCENT.
Heredity and Variation ...
Evolution ... 2
Sexual Selection ...
Part II.
CHAPTER XI.
PROTOZOA.
Amoeba
Paramcecium
Vorticella ...
Gregarina ..
The Preinrod :
The Gymnomyxa and Cerkets
CHAPTER XII.
PORIFERA.
Sycandra ...
The Porifera
x1
80
81
82
103
107
xii CONTENTS.
CHAPTER XIII.
C@LENTERATA,
Hydra
Obelia
Actinia
Alcyonium
Aurelia
Cydippe :
The Ceelenterata ...
CHAPTER XIV.
PLATYVHELMINTHES, NEMATHELMINTHES
AND ROTIFERA.,
Distomum
Teenia ‘ vis
The Platyhelminthes
Hydatina ... s
The Rotifera
Ascaris ae
The Nemathelminthes
CHAPTER XV.
ARCHICG@LOMATA.
Asterias
Balanoglossus
Lophopus ...
Sagitta
Waldheimia
The Archiccelomata
CHAPTER XVI,
ANNULATA.
Polygordius
Arenicola ...
Hirudo
Lumbricus
III
117
121
125
127
131
133
137
144
142
151
152
152
155
156
161
166
168
168
170
179
181
190
198
CONTENTS.
CHAPTER XVII.
ANNULATA (continued).
Nephrops ...
Blatta
Peripatus ...
Epeira
The Annulata
CHAPTER XVIII.
MOLLUSCA.
Helix
Anodon
Sepia a
The Mollusca
CHAPTER XIX.
CHORDATA.
Ascidia
Amphioxus
CHAPTER XxX.
CHORDATA (continued).
Myxine, as a Type of Cyclostomata
Raia, as a Type of Pisces
Gadus
CHAPTER XXI.
CHORDATA (continued).
Rana, as a Type of Amphibia ...
CHAPTER XXII.
CHORDATA (continue.).
Columba, as a Type of Aves
xili
204
231
233
237
262
269
276
282
288
207
309
313
331
338
360
xiv CONTENTS.
CHAPTER XXIII.
CHORDATA (continued).
Lepus, as a Type of Mammalia
CHAPTER XXIV.
GENERAL FEATURES OF CHORDATA.
Phylum Chordata
Sub-Phylum Artriozoa ...
Sub-Phylum Vertebrata
General Features of Vertebrata
Nervous System ...
Sense-organs
Skeletal System ... fe
Blood-vascular System ...
Alimentary System
Urogenital System
Development
CHAPTER XXV.
CLASSES OF VERTEBRATA.
Cyclostomata
Pisces
Amphibia ...
Reptilia
Aves
CHAPTER XXVI.
GENERAL FEATURES OF MAMMALIA,
Skin
Hair
Teeth : sts
Brain and Nervous System
Circulatory System
Urogenital System
Skeleton ... :
Development
382
402
403
405
405
406
408
412
421
425
426
426
453
455
458
462
464
466
467
475
CONTENTS.
CHAPTER XXVII.
MAMMALTA (continued).
Prototheria
Metatheria
CHAPTER XXVIII.
MAMMALIA (continued)
The Eutheria cis 53 He
Horse, as Cursorial Type of Mammalia
Ox, " " "
Dog, " tt "
Cat, " t '
CHAPTER XXIX.
MAMMALIA (continued).
Sloth, as Arboreal Type of Mammalia
Mole, as Fossorial " "
Porpoise, as Aquatic " "
Bat, as Afrial m "
CHAPTER XXX.
MAMMALIA (continued).
Orders of Eutheria
CHAPTER XXXI.
GEOGRAPHICAL DISTRIBUTION OF MAMMALS.
Realms
Notogcea ...
Neogeea
XV
488
495
533
537
542
551
571
593
595
597
xvi CONTENTS.
Arctogoea ... sie ia 600
Madagascar Region 602
Ethiopian Region 603
Oriental Region ... 604
Holarctic Region 606,
Sonoran Region .. 607
Discontinuous Distribution 609
610
Mammalian Evolution ... es sie ore we eae
Index sice oe ee ae bik dia ade sn (OLS,
LIST OF ILLUSTRATIONS.
—=
Fig, Page | Fig. Page
1. Diagram of a Monodermic Or- 39. Transverse Section ofa Sycandra 104
ganism .. 24 | 40. Amphiblastula Larva of a Cel -
2. Diagram of a Didermic Or- careous Sponge . 106
ganism .. 24 4x. Ascetta Primordialis .. . 107
3 Diagram of a Tridermic Organ- 42. Transverse Section of an Ascon 107
ism, seen in cross section 25 | 43. Transverse Section of Part of
4. Ameeboid Cells 30 the Wall of a Leucon + 108
5. Flagellate Cells 30 | 44. Transverse Section of a Rhagon 109
6. Quiescent Cells 31 | 45. Hydra Viridis with Two Buds 411
7. Types of Epithelium 32 | 46. Transverse Section of Hydra .. 112
8. Connective Tissues 33 | 47. Portion of Body-wall of Hydra 113
g. Muscular Tissue .. 34 | 48. An Ectoderm Cell, Endoderm
to. Nervous Tissues .. 35 Cell, and a Nerve Cell .. 114
11. Diagram of a Cell 35 | 49. Development of the Nematocyst
12. Diagram of Mitosis 36 in Cnidoblast Cells .. 115
13. Diagram to illustrate Changes 50. Colony of Obelia Geniculata 117
of the Nucleus during Cell- 51. Colony of Obelia Geniculata .. 118
division .. 38 | se. A Medusa of Obelia 1Ig
14. Diagram to illustrate “Typical 53. Lateral View of a Medusa of
Conjugation .. 40 Obelia ++ 120
15. Diagram illustrating Nuclear 54. Actinia Mesembryanthemum wie ERT
Changes during pela Re- 55. Transverse Section through the
production aa ‘ » 43 Upper Part of a Young
16. Section of Blastula 50 Actinian el a8 +. 122
17. Section of Morula 50 | 56. Transverse Section through
18, Section of Gastrula 50 Lower Part of a eda
1g. Section of Planula : 51 Actinian : ss 123
20. The Origin of an Organ 53 | 57- Alcyonium Digitatum is 125
21. The Metamorphosis of the Silk: 58. View of Entire Colony with
worm Moth . 54 Tentacles Expanded 126
22, Diagram to illustrate Darwin’s 59. Aurelia Aurita . 127
Theory of Coral Reefs .. 72 | 60. Oral View of ‘Aurelia Aurita 128
23. Protective Resemblance 77 | 61. Median Longitudinal Section
24, The Leaf-butterfly of India 77 through the Inter-radial
25. Hypolimnas Missipus 78 Plane of Aurelia ‘ 129
26. An Example of peseetive Re- 62. Three Stages in Development
semblance 79 of Aurelia r P 130
27. Amoeba Proteus .. 85 63. Transverse Section ‘through
28, Amoeba Proteus 86 Upper Part of Scyphula
29. Parameecium é go Larva © eo a A 30)
30. Vorticella Nebulifera 93 | 64. Transverse Section through
31. Life-History of Gregarina ~ 95 Lower Part of Seypa
32. Types of Foraminiferan Shells 98 Larva... ‘ 3s 230
33. A Heliozoan et ai 99 | 65. Cydippe Plumosa_ 131
34. A Radiolarian roo | 66. Aborat View of Cydippe 132
35. A Living Foraminiferan too | 67. Adhesive Cells uf Cydippe 132
36. Acineta Tuberosa Expanded 68. Types of True Corals .. Mae E34
and Contracted ror | 69. Ventral View of Liver-Fluke .. 137
37- Sycandra Compressa_. 103 | 70. Transverse Section Hows the
38. Calcareous Triradiate Spicules Liver-Fluke_.. . 138
of Sycandra .. ro4 | 7x. Structure of Distomum . 139
xviii
Fig. Page
72. View of Liver-Fluke 140
73. Development of Distomum
Hepaticum 141
74. Sporocyst 142
75. A Redia .. 142
76. A Cercaria : 143
77. Cercaria and Distomum | 143
78. Tania Saginata 145
79. Head of Tzenia Solium - 146
80. Transverse Section of a Pro-
glottis of Tania - 146
81. Semi-diagrammatic View of a
Single Proglottis of a Tania 147
82. Proglottis of Tznia Saginata 147
83. Development of Tznia Solium 147
84. ‘‘Measly” Pork 148
85. Ventral View of Hydatina
Senta... I51
86. Dissection of Female Ascaris
Megalocephala from the
Dorsal Side af . 152
87. Diagrammatic Transverse Sec-
tion of Ascaris Megalocephala 153
88. Magnified View of ‘“‘ Trichi-
nosed” Pork - 155
89. Asterias Rubens 156
go. Transverse Section of the Arm
of Asterias Rubens 157
gx. Median Longitudinal Section
through the Starfish in the
Plane of its Symmetry iy ESS
92. Aboral Dissection of a Common
Starfish . 159
93. Diagram of the Water-Vascular
System of Common Starfish 160
94. Semi-Diagrammatic View of
Balanoglossus from the Dorsal
Surface .. -. 162
gs. Anatomy of Balanoglossus « 163
96. View of Entire Colony of
Lophopus 167
97: Ventral (A) and Dorsal (B) Shell
of Waldheimia Australis .. 169
98. A Brittlestar ‘ «172
99. A Common Sea-urchin 173
too. Diagram of Dorsal View of
Echinus . i - - 1736
ror. Echinus Microstoma |. 174
toz, View of Interior of Bisected
Sea-Urchin . «+ 174
103. The Rosy Feather Star | - 175
zoq. A Holothurian .. oat E75,
105. Polygordius Neapolitanus + 179
106. Transverse Section of Poly-
gordius . 180
107. Coronal Longitudinal | Section
of Polygordius 180
108. Lateral View of Front End of
Polygordius 181
1og. Lateral View (Left Side) of the
Lobworm ‘ - 182
rro. Dissection of Arenicola - 182
111. A Magnified View of a single
Gill-Segment of Arenicola .. 184
Fig.
r12.
a ey
. Second Dissection of Leech ..
. Transverse Section through the
. A Nephridium of the Leech °.
. Magnified View of Two Conse-
. The Common Earthworm...
. First Dissection of the Earth-
LIST OF ILLUSTRATIONS.
Page
Transverse Section of Arenicola 185
View of Nerve-ring and Brain
of Arenicola_ .. 187
. Section through the Otocyst of
Arenicola -. 188
. A Nephridium of Arenicola .. : 189
. The Medicinal Leech + 190
. Ventral View of the Leech 191
. First Dissection of Leech 192
Leech 193
. Dorsal View of the Anterior
End of a Leech
cutive Segments of the Leech
worm
. Second Dissection of the Earth-
worm
5 Transverse Section of an Earth-
worm in the Intestinal Region
128. A Nephridium of Lumbricus .. 202
z29. The Norway Lobster .. + 205
130. Lateral View of Norway
Lobster .. Bs »» 206
131. An Abdominal ” Segment of
Nephrops oe ++ 207
132. A Chela tf Nephrops se +» 209
133. A Chelate Leg of Nephrops . 209
134. A pee rset Leg of
Nephrop: 209
135. The Fi ret "Maxilipede (left) of
Nephrop: 210
136. The Second Maxillipede of
Nephrops +. 210
137, A Third Maxillipede of
Nephrops 210
138. A, First Maxilla, “and B, Second
Maxilla of Nephrops.. 21
139. The Antennule of Nephrops .. 212
140. The Mandible of Nephrops .. 212
141. Left Antenna of Nephrops 212
142. The First Pair of Swimmerets
f in Nephrops ae 2ES
143. The 2nd Swimmeret of
Nephrops as ee ++ 213
x44. A Typical Swimmeret of
Nephrops 213
. Section across the Abdomen of
. Lateral View of ‘Nephrops
. The Common Cockroach a
. The Mouth Appendages of the
. Transverse Section of Blatta |
. Lateral
. A Median Sagittal Section
through Nephrops .. 214
Nephrops
Common Cockroach
. Dissection of Cockroach from
the Dorsal Side 226
View of Peripatus
Capensis 5‘
Fig.
153-
154.
155.
156.
157.
158.
159.
160.
161.
162.
163.
164.
165.
166.
167.
168.
169.
170.
171.
172.
173.
174.
175.
176.
177.
178.
179-
180.
181.
182.
183.
184.
185.
186.
187.
188.
189.
Igo.
191,
192.
193:
19.
4. Dorsal View om the somes
LIST OF ILLUSTRATIONS.
Page
Peripatus
the Dorsal
A_ Dissection of
Capensis from
Surface ..
A Common Garden Spider
The Two First Pairs of Appen-
dages of Epeira Diademata .. 234
Longitudinal Sagittal Se
through Epeira Diademata .
Foot or Parapodium of a
Nereis..
The Life- ‘History of Cirripedia 242
Lateral View of Lepas
(Barnacle) 243
Lateral View of ‘Lepas Anati-
fera P «+ 243
A Zcea Larva of a Decapod + 244
Scolopendra Cingulata site
Centipede 245
Julus Terrestris (a Millipede) 245
The Life-History of the Com-
mon Cockchafer 247
Colorado Beetles 247
A Water-beetle .. 248
The Hive Bee 249
The Gall-fly 249
Tsetse Fly ee 250
Syrphus Pyrastri 250
Wheat Midge . 251.
The Daddy-Long- ‘legs or Crane:
Fly abe 252
The. Horse-Bot .. 252
The Cabbage White... 253
Demoiselle Dragon-Fly 254
Larva of Dragon-Fly .. 255
‘Uhe May-Fly.. Si 255
The Grasshopper A 250
A Group of Hemiptera 256
The Common Louse 257
The Rose Aphis 257
Mite causing Mange in the
Pig 259
The Harvestman
Lateral View of the Roman
Snail .. 262
First Dissection of Snail 264
Diagrammatic Median Sagittal
Section through. the head of
a Snail .. .. 265
Second Dissection of Snail |. 266
The Nervous System of the
Snail 267
Lateral View (Left) of Ano-
donta in Natural Position and
Feeding 270
Internal Siew of Right Shell
of Anodonta 271
View of Anodonta with Left
Mantle-Flap thrown back
Dissection of Anodonta from
Left Side 273
Dorsal View of Heart and
Pericardium of Anodonta .. 274
272
Cuttle . 276
Fig.
195.
196.
199.
201.
* 202,
205.
. Transverse
. Transverse
xix
Page
Ventral View of a Cuttle
Ventral View of — Sepia
Officinalis with Mantle-
Cavity cut open ne
. Dissection of Organs of Sepia
Officinalis from the Left Side
. Ventral View of Shell of Cuttle
Semi-diagrammatic View of
Heart, Gills and Excretory
Organs of Sepia Officinalis ..
. A Belemnite Restored .
Lateral View of a Nautilus in
its Shell .
2
Ammonites or Fossil Nautiloid
Cephalopoda
. Diagrammatic Median ‘Longi-
tudinal Section through an
Ascidian
. Oblique Section” through an
Ascidian
Development of an Ascidian .
. Transverse Section of Larva of
Ascidian as we we
Section through
Embryo of an Ascidian
. Chordula Larva of an Ascidian
. Development of an Ascidian
. Tailed Larva of an Ascidian
seen from the Right Side ..
. Transverse Section through the
Tail of an Ascidian Larva ..
. Lateral View of Amphioxus
Lanceolatus
. View of ‘Amphioxus from the
Right Side
. Transverse Section of “Amphi-
oxus behind the Atrium ,
Section through
Amphioxus in the Pharyngeal
Region ..
. Median Section of Brain of
Amphioxus..
. Oblique Section of Amphioxus
through the Pharyngeal
Region ..
. The Ppevelogriené of “Amphi-
oxus, as seen in Longitudinal
Section and Lateral View of
Larve
ee 3
. The Development of “Amphi-
oxus, as seen in Sections ..
. Transverse Sections through
Young Amphioxus ..
. Lateral View of Young Pelagic
Amphioxus at Commencement
- 277
293
294
294
301
of Larval Life . 307
222, Diagram of Young Pelagic
Amphioxus % 307
223. Lateral View of “Myxine
Glutinosa 309
224. Ventral Dissection of “Myxine
Glutinosa Ir
225. Median Sagittal “Section
through Myxine Glutinosa .. 312
XX
Fig.
226.
227.
228,
229.
230.
231.
232.
233.
234.
235.
236.
237.
238.
239.
240,
244.
247.
248.
253-
254.
. Diagram of
. Ventral View of Male-
LIST OF ILLUSTRATIONS.
Page
Jaws and Teeth of (A) Male
and (B) Female Skate ai BE
Diagram of Arterial System of
a Skate .. 316
Diagram of the Venous System
of a Skate :
Male Urogenital Organs of a
Skate ..
The Ear (Membranous Laby-
rinth) of the Skate ..
Dorsal View of Cranium of a
Skate 324
Lateral View of Skull of Skate
Dorsal View of Pectoral Girdle
and Fin of the Skate 327
Dorsal View of Pelvic Girdle
and Fins of the Skate «. 328
Lateral View of the Haddock
Dissection of Haddock from
the Left Side .. 333
Lateral View of Cod’s Skull :
The Right Pectoral Fin and
Girdle of the Cod with both
Pelvic Fins _ .. :
The Common Frog 338
Diagram of Venous System of
a Frog 4I
3
‘ Ventral View of the Female
Urogenital Organs of a Frog 343
. Diagram of Arterial System of
aFrog .. aa i ae 34a
the Truncus
Arteriosus of a Frog’s Heart 345
Dorsal View of Brain of Frog 346
. Ventral View of Frog’s Skull 348
. Dorsal View of Frog’s Skull .. 348
Dorsal View of Entire Frog’s
Skeleton. 349
Pectoral Girdle of Rana 351
. Fore-limb of Rana... 351
. Pelvic Girdle of Rana .. 352
. Hind-limb of Rana... 5 52
252.
3
Three Stages in Development
of Frog’s Egg 354
Sections of Frog Embryos a 355
The Structure of Frog’s
Embryo and Tadpole 356
. Young Tadpole distected from
the Ventral Side 357
. The Life History of the Com-
mon Frog = 358
. View of Respiratory Organs of
the Pigeon é 362
. Ventral View of the Sas
System of the Pigeon
. Ventral View of the ‘Arterial ”
System of the Pigeon 367
Uro-
genital Organs of the Pigeon 368
. A Cervical Vertebra of the
Pigeon .. . 3
. Latera! View “of Cervical
Vertebra of the Pigeon . 370
. A Rib of the Pigeon + 370
Fig.
264.
265.
266.
267.
268.
269.
270.
271.
290.
. Transverse
. Lateral View of a Lumbar
. Four Shanes in the Develop-
. Diagram of the Vertebrate Eye
Page
Ventral View of Sternum of
the Pigeon 372
The Pectoral Girdle “of the
Pigeon 372
The : Skeleton of a Bird's Wing 373
Left Leg of the Pigeon 374
Lateral View of Pelvis of the
Pigeon . 374
Diagram of a Fowl'’s ‘Egg at
Laying .. 375
Three Consecutive Stages of
the Blastoderm of a Chick in
Early Stages of Incubation 376
Section through a Chick’s Egg
at Various Stages 377
. View of the Area Pellucida of
a Chick’s Blastoderm of about
‘18 Hours 78
3
. View of Chick’s Blastoderm
about 24 Hours 378
. Cross-section through a Blasto-
derm of about 24 Hours - 379
Section of an
oa Chick of the Second
379
‘ ‘Diasraii of Developing Chick 380
. Permanent Dentition of the
Hare 384
. Female Urogenital Organs of
the Rabbit a 389
. Rabbit’s Brain .. 391
. A Median Longitudinal Section
through the Rabbit’s Brain .. 392
. Lateral View of Skull of the
Rabbit .. ++ 394
. Posterior View of Atlas Ver-
tebra of Rabbit at 305
. Lateral View of Axis Vertebra
of Rabbit oo 395
. Anterior View of a Cervical
Vertebra of Rabbit .. ++ 3905
. Lateral View of Thoracic Ver-
tebra of Rabbit — 396
. Anterior View of a Lumbar
Vertebra of Rabbit ..
Vertebra_of Rabbit 396
. Pectoral Girdle and Fore-limb
of the Rabbit .. 07
3
. Dorsal View of Left Manus of
Rabbit . 398
Bones of Pelvic Girdle and
Hind-Limb of Rabbit ++ 399
. Dorsal View of Left Pes of the
Rabbit
ment of the Vertebrate Brain 406
. Diagram of the Vertebrate
Brain... 407
. Diagrammatic Median’ Section
through a Vertebrate Brain
. Three Stages in the Develop-
ment of the Vertebrate Eye
409
Fig.
297-
298.
299.
300.
3or.
302.
303+
Embryo iis 6 sa
. The Parts of the Ccoelom in the
. The
. The River-Lamprey
. Tails of Fishes ..
. Fins of Fishes
. Diagram of the Typical Mam-
. The
4
. Three Early Stages in Develop-
. An
LIST OF ILLUSTRATIONS.
Page
Development of the Vertebrate
Ear oF if ts +. 41
A_Diagram of the Vertebrate
Ear Sf ‘a ve 412
Development of Vertebrate
Cranium Pe Da ve 405
Development of Vertebrate
Cranium 415
Diagram of Pentadactyle Limb 420
Development of the Vertebrate
eart .. wie fs os 421
Lateral Views of Anterior
Arterial System of Verte-
brates. 422
. The ‘Arterial Arches of Verte-
brates ..
5. Diagrammatic Transverse Sec-
tion of a Vertebrate Embryo
. Diagrammatic Transverse Sec-
tion through a later Vertebrate
424
Thoracic Cavity of a Mammal 424
Evolution of the Foetal
Membranes of Vertebrata .. 428
- 434
435,
ae oer 4S
‘Lateral View of Skull of
Rattlesnake 442
. Right Shoulder “Girdle of (a
Tortoise . 86
Skeleton of a Tortoise .. 444
ees View of a Crocodile’s
Skull
443
445
. Ventral View of Crocodile’s
Skull... =
. Archzopteryx
: Men ee of the Skull iat
n Ostrich
. The Kiwi.
. Section through the Skin of ig
Mammal
4st
454
. Diagrammatic Sections. illus-
trating the Bevelepnicnt of a
Hair... re nis
. A Rete Mirabile a6 a4
. Diagram of | Mammalian
Female Urogenital Organs . 467
. Three peer of Mammalian
Scapula .
5 Tater! Views of Crocodile’s
Pelvis, Pelvis of Prototheria,
and Pelvis of Eutheria
malian Fore- and. Hind-Limb
Mammalian Graafian
Follicle in the Ovary
ment of Rabbit 477
. Diagrams of the Foetal Mem-
branes of a Mammal .
Embryo Horse of Six
Weeks in its Membranes
Fig.
331-
332
333+
. Ventral
. Diagram of Hypsiprymnus (A
. Ventral
. Lateral View of Horse's Skull
. Ventral View of Skull of Horse
. Upper Jaw (left-half) of Foun
. The Foot Skeleton of
. Lateral View of Lion’s Skull .
. The Skull of the Dog from the
. Ventral View of Lion’s Skull
. The Permanent Teeth of the
xxi
Page
Six Different ‘l'ypes of Placenta 483
Diagram of the Voetal Mem-
branes of Echidna, as seen in
Cross-section ..
Ventral View of Male Uro-
genital Organs of Ornithor-
hynchus ag os +. 489
View of Pectoral
Girdle and Fore-Limb of
Ornithorhynchus ae 490
. Pelvis of Ornithorbynchus 490
491
. Fore (A) and Hind (B) Foot of
the Duckmole .. 492
488
- Duckmole
. Bones of Limbs of Ornithor-
hynchus
. Skull of Ornithorhynchus
. Diagram of — Phascolarctos
(Koala) Embryo and _ its
Fcetal Membranes
+ 493
» 494
496
Kangaroo) Embryo in_ its
Foetal Membranes... 496
. Diagram of Embryo of Pera-
meles with Fo:tal Membranes 497
. Lateral View of Skull of a
Young Kangaroo.
View of Skull
Kangaroo
+. 499
of
499
. Pelvic. Girdle of the Kangaroo 501
. Hind-foot of Kangaroo ++ 5OL
Jaws and Teeth of the Opossum 502
. Inner View of Left Ramus of
Lower Jaw of Amphilestes
Broderipi
. Posterior View of Lower Jaw
of Wombat
(A) and Old Horse (B)
. Stomach of a Ruminant 514
. The Right Manus of a Horse
. The Right Manus of an Ox
. Tibiofibula of a Horse ..
. Right Femur of a Home
. The Left Pes of an Ox.
. Right Pes of Horse ..
. The Manus of (A) the Tapir,
(B) the Rhinoceros ’and (C)
the Horse
the
Horse and Four of its
Ancestors 522
Right Side
Wolf
A Side View of a Cat’s Toe
with Retractile Claw
Entire Skeleton of the Ai or
Three-toed Sloth a
. Lateral View of Skull of Three-
Toed Sloth
Page
369. Manus of Three-Toed Sloth ze
370. Stomach of Sloth ++ 536
371. Jaws of Teeth of the Mole |. 538
372. Anterior View of Pectoral
Girdle and Limb of the Mole 539
373. Ventral View of Skeleton of
Mole... ‘ ‘ ae
374. The Common Porpoise
375. Section of Skull of Young
Dolphin oe Bad
376. Teeth of Porpoise ++ 545
377. Diagrammatic Section of
Stomach of Porpoise 54!
378. Lateral View of Pectoral Girdle
and Fin of a Porpoise 548
379. Female and Young of a Fox-
at zis Ha aa ae 1654
380. The Pectoral Girdle and Fore-
Limb of Pteropus .. 552
381. Lateral View of the Sternum
of a Fox-Bat .. 555
382. Tamandua Anteater 558
383. Lateral View of Skull of Ant-
eater .. aia sie + 559
Fig.
384.
385.
386.
387.
388.
389.
390.
391.
392.
393-
394-
395+
396.
397-
398.
LIST OF ILLUSTRATIONS.
Page
Lateral View of Sul of
Armadillo : 559
American Manatee 562
Lateral View of Skull of Daman 566
The Dasse 567
Surface Views “of 4 “ Single
Molar Tooth of (A) the «
African and (B) the Indian
Elephant - 569
The ineoieany Tapir is 572
The African Water- Chevrotain
Manus of Artiodactyla
Ventral View of Bear's Skull .
Feet of Bear seen from the
Upper Surface 582
Lateral View of | Skull of the
Aye-Aye 588
Lateral and Ventral Views of
Skull of Denpopsneeus
Nemceus 589
Front View of Skull of a
Gorilla 590
Bones of the Ankle and Foot
of Gorilla 591
Entire Skeleton of the Gorilla 592
LIST: OF PLATES.
——_@—____
First DIssECTION OF THE SKATE
SECOND " "
CRANIAL NERVES OF THE SKATE.
First DISSECTION OF THE FRoc .
SECOND ” "
THIRD " "
FourRTH " te
First DISSECTION OF THE iene
SECOND " "
THIRD " "
First DISSECTION OF THE Raawer
SECOND " "
THIRD " i
faces page 314
318
322
338
340
342
346
362
364
366
386
388
390
LIST OF TABLES.
Page
ANNELIDA. : ; : . 240
ARCHICGLOMATA f : ‘ 177
ARCTOG@AN REGIONS, MAMMALS OF . : . 608
ARTHROPODA . . ‘ : : . 61
CHORDATA _ , ‘ : . 401
CQ:LENTERATA . A : a : 3 . 136
CLASSIFICATION OF ANIMALS . z : _ 29
ECHINODERMATA : é Z : 3 . 176
MAMMALIA, ORDERS OF . A ‘ ‘ . 612
MOoL.usca 5 : : ‘ F = 287
PLATYHELMINTHES . r : ¢ 251
PORIFERA . é ‘ é . ‘ ‘ . 110
PROTOZOA . : : ‘ é ‘ - 102
VERTEBRATA. z * : 431
ZOOLOGY.
——_@——_
INTRODUCTION.
OOLOGY means, in its widest sense, the study of
animals. For the sake of convenience we may take
as our unit of study either the whole animal kingdom, a
single animal, or any intermediate group between these
two extremes. Let us first take the animal, or individual
organism, and notice how its study may be approached.
We can inquire into the manner in which the organism
is put together or constructed by an examination. of its
external appearance and by a dissection of its interior. This
study of structure is called MorpHotocy. It is often,
though somewhat unnaturally, divided into ANaTomy or
morphology of organs, and HisTroLocy or morphology of
celis and tissues. Our real knowledge of an organism would,
however, be very limited if we did not go on to inquire
the meaning of its structure and howit works. This study
of function is called PuysioLocy. Structure and function
go hand in hand throughout the constitution of the
organism, and it is impossible to study the one without
due consideration of the other.
The next important fact about an organism is its zz-
constancy in structure and function. The organism passes
through a definite sequence of changes from birth to death.
The greatest and most obvious changes are those which
occur during early life called development, and the study of
these is termed EmBryOLocy.
Embryology includes morphology and physiology of the
young, or more rapidly changing, organism. ;
In morphology, parts of an organism which have a
similar structure and structural relationship to other parts
are called homologous, whilst in physiology those parts which
M. i. 2
2 LIVLMULYYUE Liev.
perform a similar function are termed analogous. In many
cases one part may be both analogous and homologous
with another.
Again, if we take a number of structural characters in
an organism, these can be divided into iwherited and acguzred.
In the former case, the structure is such because of the
tendency in all organisms to resemble their parents ; in the
latter it is such because of the capacity of an organism to
adapt itself to its surroundings.
Hence the study of an organism resolves itself into the
following :—
ANATOMY.
HIsTo.ocy.
(Homology =similarity in structure. )
2. PHYSIOLOGY or study of function.
(Analogy = similarity in function.)
3. EMBRYOLOGY or study of the early history of an organism.
We now have to consider the relationship of an organism
to other organisms. The comparison of structure, function
and development can obviously be called Comparative
Anatomy, Comparative Physiology, and Comparative Em-
bryology respectively, but there are one or two other points
to notice.
If we take two closely allied organisms their structure
will show a certain degree of similarity or homology.
This similarity must in each case be due to one of two
causes. It is either due to the fact that the two organisms
are descended from a common ancestor, and therefore
inherited, or it is due to the two organisms having lived in
a similar environment, and thus acguived. The form of
homology in the first instance is termed homogeny, and
that in the latter homoplasy.*
Two brothers owe their similarity to homogenetic or
inherited homology, and two sailors owe their similarity in
uniform, gait and habits to homoplastic or acquired
homology. The distinction is clear when such a crude
example is given, but, if we assume the sailors to be
brothers, one would be in great doubt whether to ascribe
some similarities to one or to the other kind of homology.
1. MORPHOLOGY or study of form. <<
* The terms Palingenetic and Canogenetéc are often used in much the
same sense as homogeneti and homoplastic.
INTRODUCTION. 3
Let us now pass to the consideration of the animal
kingdom as an organic whole. We may here discern a
certain parallel to the organism. The study of the
structure of the animal kingdom as such means the
arrangement or distribution of animals on the world’s
surface, or, as it is usually termed, GrocrapuHicaL Dis-
TRIBUTION. The past history of animal life is in a
similar manner called GroLocicaL DIsTRIBUTION, whilst
physiology finds its parallel in the relationship of the
animal kingdom to the inorganic world, for which there
is no inclusive term.
We can at least see this structural parallelism :-—
ORGANISM.— ANIMAL KINGDOM.—
Morphology. Geographical Distribution.
Embryology. Geological Distribution.
By keeping this clearly in mind we are assisted in a
consideration of Distribution.
Part 1.
GENERAL ZOOLOGY.
>.
CHAPTER I.
LIVING MATTER.
ETURNING to the animal kingdom, we find that
there runs throughout it a presence of the primary
basis of life called protoplasm. The living part of all
organisms (animals and plants) consists of this substance,
So far as we know, protoplasm cannot, at least under
present conditions of the earth’s surface, arise spon-
taneously from less highly organised materials, although
it is one of the primary properties of protoplasm that it
can add to its bulk or grow by the aggregation to itself of
non-living materials.*
According to present views, the whole animal world
owes its origin to growth of some primeval protoplasm, and
the constituent organisms owe their being to the fact that
this growth is discontinuous.
The moving, thinking organism which we call a man
differs only in degree and not in kind from the isolated
and undifferentiated mass of protoplasm known as Amada.
Hence it is of primary importance that we should get a
clear idea of the physical, chemical and physiological pro-
perties of this basis of life, living protoplasm.
* This statement does not preclude the possibility of living matter
having been in the past evolved from non-living matter ; but of this ze
know absolutely nothing. ,
6 PROPERTIES OF PROTOPLASM.
Physical Properties of Protoplasm.—Protoplasm,
or living matter, is in itself usually colourless and trans-
parent. Its consistency varies considerably according to
the amount of contained water. There can be little doubt
that it is a physical as well as a chemical complex.
Differential staining and other methods reveal the existence
of a meshwork of more stable and less fluid substance,
sometimes termed sfongioplasm, and a more mobile and
less easily stained substance, sometimes termed hyaloplasm,
which permeates the interstices of the spongioplasm.
Scattered throughout the hyaloplasm is a number of
minute bodies, readily stained and of unknown com-
position. They are called microsomata and may be con-
nected with the nutrition of the more essential living parts
of the protoplasm, as they decrease and are absorbed when
the protoplasm is starved. This idea is often extended to
include the hyaloplasm, which is thus regarded as merely
a nutrient fluid bathing the primary living spongioplasm,
but there is little certainty regarding these points. It is
important to notice that at least three physical constituents
of protoplasm can be discerned, and that its mobility,
fluidity and reactivity are directly related to the amount
of contained water. A number of the physical phenomena
of protoplasm, such as its mobile movements and change
of shape, can be closely imitated by small isolated oil
drops and other devices.
Chemical Properties of Protoplasm.—It is very
generally accepted that protoplasm is not a definite
chemical substance but a complex of several. If it be a
single substance it must be of so great instability as to
break up into its constituents as soon as it is formed.
Analysis shows that protoplasm consists of a number of
substances called fro¢edds, which are sufficiently definite
to come within the power of chemical manipulation. They
may be the first decomposition-products of protoplasm
itself, or they may be the actual constituents of protoplasm.
In other words, protoplasm is either a physical or a chemical
aggregate of proteids.
Proteids are of very complex molecular composition, and are
known by definite chemical tests (such as the production of a violet
VITAL FUNCTIONS OF PROTOPLASM. 7
colour with copper sulphate and sodium hydrate, and a pink precipi-
tate on boiling with Millon’s reagent). They are divided into groups
according to their degrees of solubility. Common proteids are al-
bumens, albuminoids, and peptones. “The essential constituents of
proteids are the elements carbon, oxygen, nitrogen, hydrogen and
sulphur, the average percentage composition of albumen being—
Carbon about 53 per cent.
Oxygen mo 23 "
Nitrogen 1 15 u
Hydrogen u 7 "
Sulphur 4 2 "
Thus the physical and chemical evidence is in favour of
regarding living matter or protoplasm as an aggregate of
substances of high chemical constitution and of an unstable
nature.
Primary Vital Functions of Protoplasm.
1, ALIMENTATION,—Living matter has always, if in suit-
able surroundings, the property of aggregating to itself
foreign substances which are termed /vods, and thereby in-
creasing in bulk. The food is by necessity of an insoluble
or non-diffusible kind, and it has, before it is available for
absorption into the substance of the protoplasm, to undergo
a process of reduction to a soluble condition. This process
is known as digestion and has, by its nature, to be conducted
in the body of the organism.
2. MovemMENT.—Living protoplasm exhibits the power
to move, owing to its contractility. A drop of oil moves
according to the forces of gravity and capillarity, but an
organism can move in a definite direction in response to
other stimuli. The movement is essentially the same
throughout and consists of shortening of the organism, or
part of the organism, in one or more directions and a
corresponding lengthening in others. The movement
implies a loss of kinetic energy and the setting free of
heat.
3. SENSATION.—Protoplasm is zrrifable or capable of
responding to certain stimuli. The demonstration of this
fact lies in the preceding property of movement, for outside
our own consciousness we have no means of recognising
the effect of a stimulus except by its result in movement.
°
8 VITAL FUNCTIONS OF PROTOPLASM.
4. EXcrETION.—Movement implies a loss or expenditure
of energy which is furnished by the chemical decompost-
tion of protoplasm or its constituents, resulting in its
turn in the formation of waste products or excreta. These
products have to be removed, and, in the simplest organisms,
they are extruded at the limiting surface. The carbon of
proteids is removed in combination with oxygen, as
carbonic acid gas, and the hydrogen and oxygen as water.
For this purpose oxygen is taken into the interior of
the body. This form of excretion is often called Respira-
tion. It involves the introduction of oxygen and the
extrusion of carbonic acid gas. In addition, the nitrogen
and sulphur of the proteids leave the body, in combination
with other elements, as complex nitrogenous compounds,
such as urea. Thus the waste products are of two kinds,
non-nitrogenous and nitrogenous, removed by espiration
and WVitrogenous Excretion respectively.
The taking-in of oxygen during respiration should be
carefully distinguished from the ingestion of “ food,” as also
should excretion from the egestion of waste residue or
feeces. Ingestion and egestion are processes of alimenta-
tion, which itself is part of the building-up of fresh
protoplasm, whereas respiration and excretion are processes
essentially connected with the breaking-down or consump-
tion of protoplasm. A starving man will, unfortunately
for himself, continue to respire and excrete though the
alimentary function be in abeyance.
Secondary Vital Functions of Protoplasm.
1. GrowrH.—It is quite conceivable that protoplasm
might carry on the above functions in such a manner that
the waste and repair were exactly balanced, in which case
the original protoplasm would remain the ‘same in size
and other relations. This, however, is not the natural
state of matters. Given suitable conditions, an organism
will acquire a credit account with nature, and the result is
a continued production of fresh protoplasm and increase
in bulk or growth. In the case of living organisms growth
takes place by addition throughout the bulk of the body,
and is called growth by cutussusception to distinguish it
FOOD OF ANIMALS. 9
from increase in size of a non-living body (eg., a crystal),
which is merely an addition to the surface and is called
growth by accretion. We have seen that efficiency of the
vital functions depends upon the relationship of surface to
bulk in the organism, for alimentation and excretion depend
upon this proportion. But increase in bulk involves a
reduction of the proportion between surface and bulk to
the detriment of the former. Here we have a definite
limit to the bulk of an organism beyond which it cannot
go without further differentiation.
2. REPRODUCTION.—Further growth necessitates an
increase of surface by division of the organism. Division
results in the production of two organisms from the
former one, usually termed Reproduction. Reproduction
alternating with growth are the two vital phenomena which
result in life on this earth presenting itself as a series
of organisms or individuals, which have a common origin
in primeval protoplasm. This perpetual organic continuity
of protoplasm throughout the animal kingdom is a most
important principle in connection with the problems of
heredity and descent.
Food of Animals.—The foods of animals and their
nature have an important bearing on structure and function.
We may distinguish four kinds :—
I, PROTEIDS.—These form the most important foods. We have
already seen that they are highly organised, that they contain carbon,
hydrogen, oxygen, nitrogen and sulphur, and enter into the very com-
position of protoplasm. White-of-egg or albumen is a common example.
2. CARBOHYDRATES.—Carbohydrates differ in many respects from
proteids. Not the least is their chemical composition, into which
carbon, hydrogen and oxygen alone enter. Starches and sugars are
familiar examples.
3. Fars.—Fats are complex compounds of glycerine and some
fatty acid. They contain only carbon, oxygen and hydrogen. Dilute
alkalies decompose them into glycerine and soap.
4. MINERALS.—The minerals include water and numerous mineral
salts in solution, such as common salt and phosphates of lime.
The three first kinds of food are mostly, by the very nature of things,
insoluble, and the process of digestion consists essentially in reducing
them to a soluble state. If this occurred at the surface of the organism
the soluble substances would be largely lost, hence the insoluble food
has to be taken within the organism. Here we may say in a very
Io PLANTS AND ANIMALS.
general way that the insoluble proteids are converted into soluble
peptones, insoluble carbohydrates into sugars, and fats into soaps and
glycerine, though in some cases the fats are emulsified or broken into
minute particles which are then carried into the organism.
The next important point to notice is the constitution of
foods. Leaving out of consideration the minerals, which
are only of secondary importance, we find that the simplest
animal-foods are complex compounds of carbon, hydrogen
and oxygen, and that others have these elements with the
addition of nitrogen, sulphur and phosphorus. An animal is
incapable of building up its protoplasm from any simpler
products. It would be easy to supply an animal with mineral
salts alone, such as nitrates, sulphates and carbonates, con-
taining all the chemical elements in protoplasm, but they
would be of no practical use to the animal in the formation
of fresh protoplasm.
Plants and Animals.—On the other hand, it is
typical of plants that they can build up protoplasm from
such simple compounds as carbonic acid, water and mineral
salts, all of which are soluble and diffusible, either as gas
or liquid.
Hence the primary distinction between a plant and an
animal rests in the power of the former to perform the
synthesis of compounds containing carbon, hydrogen and
oxygen from carbonic acid and water. This power resides
in the presence of chlorophyll, a green colouring matter,
which under suitable conditions of warmth and sunlight
can effect the important synthesis. From this we can
derive the other differences between animals and plants.
The food of animals being solid, they require digestive
organs to bring it into a condition suitable for absorption.
Again, they require motor organs, for solids of this nature
are in isolated masses (plants and other animals) and must
be sought for.
The liquid and gaseous food of plants being already in
a condition for absorption (or assimilation) no alimentary
organs are required, and, being universally distributed, there
is no necessity for movement; the absence of movement
implies a low condition of the function of sensation.
We have already referred to the relationship between
the surface and the bulk of an animal, and in a typical
PLANTS AND ANIMALS. II
animal the demands of locomotion and alimentation are
best satished by a maximum bulk with minimum surface,
whereas In a plant the absorptive area, being mainly co-
extensive with the surface, the typical plant tends to attain
minimum bulk with maximum surface. With such a large
Proportion of surface there is no necessity for excretory
organs.
Lastly, from the difference in food it follows that a plant
can, from the simplest to the highest, protect its body in
a supporting membrane, usually of cellulose, whereas an
animal must always have a certain part of its surface
exposed to form an ingestive and egestive area. When, as
in low types. the ingestive area is co-extensive with the
surface (¢f Ameba), the difference in this respect from a
plant is very marked. We may tabulate these differences
as follows :—
PLANT.*
. Protoplasm has chlorophyll.
- Food liquid or gaseous.
. No alimentary organs nor
excretory ; motor and sen-
sory organs little developed.
. Form tending to maximum
surface with minimum bulk.
. Body completely clothed in
coat (cellulose).
. Are dependent on salts, car-
bonic acid gas, water and
sunlight. ‘
ANIMAL.#*
- No chlorophyll.
. Food solid, and mostly in-
soluble.
. Alimentary and_ excretory ;
motor and sensory organs
highly developed.
. Form tending to maximum
bulk with minimum surface.
. Body naked in lowest types,
partially enveloped in exo-
skeleton in higher.
. Live only upon plants or
other animals (highly or-
ganised food), and do not .
require sunlight or carbonic
acid.
The plant-nutrition is sometimes termed holophytic and animal-
nutrition is then known as hodozoic.
Transfer of Energy.—The movements of animals,
and the maintenance of a high temperature in the higher
* Fungi form an exception to 1 and 6 in the ‘‘P/ant” column,
whilst Hydra and a few other animals form an exception to I in the
“Animal” column. One or two plants are partial exceptions to 3.
12 ORGANISMS AND ENERGY.
animals, mean an enormous and ceaseless expenditure of
energy, and the question naturally arises, Whence is this
energy obtained ?
We find that the chemical decomposition of the
constituents of protoplasm, such as proteids, results in a
setting free of chemical energy. We have seen that pro-
teids and less complex carbohydrates are brought directly
into the body of the animal as food, so we are forced to
look beyond the animal itself for the source of energy.
On the other hand, these complex carbon compounds
are built up or manufactured by the plant from simple
constituents within it. In this building-up the same
amount of energy has to be supplied as is again set free
in movement and heat in the subsequent decomposition.
This building-up, or the chief part of it, is effected in the
plant by a process not fully understood, but certainly
requiring a supply of radiant energy from the sun’s rays.
Hence we are led to two important conclusions :—
1. The animal kingdom is entirely dependent (or parasitic) upon
the vegetable kingdom for all its energy.
2. The vegetable kingdom accumulates vast stores of energy in the
formation of complex chemical compounds, derived from the
radiant energy of the sun.
Organisms may be regarded as complex machines for
transmutation of energy. The work of plants is the
transmutation of kinetic (radiant) into chemical energy,
and that of animals is (like that of steam-engines) the
transmutation of chemical into kinetic energy.
We must therefore look to the sun as the sole source of
every movement, thought or impulse of the animal creation.
Plants and animals have the same essential living matter or
protoplasm, but with certain marked differences in form and
function. ‘These are more pronounced in the higher types,
but when the simplest living organisms are studied the
distinctions break down. Supposing the two kingdoms are
of common descent this state of affairs is to be expected.
We have thus passed in review the various physical,
chemical and vital properties of living matter, as found in
the organic world, and have noticed the main underlying
distinctions between the vital functions of plants and
animals.
LIFE. 13
The vital functions of organisms are :—
Primary. — 1. Alimentation.
2. Movement.
3. Sensation.
4. Excretion (and Secretion).
Secondary.—1. Growth.
2. Reproduction.
It cannot be too much insisted upon that these vital
functions are all exhibited by all living organisms from
highest to lowest.
If the secret of vital phenomena ever be revealed to the
future scientific investigator, the steps from 4ewéa to man
will appear as a mere nothing compared to the immeasurable
difference between living protoplasm and its non-living con-
stituent proteids.
We know life only by its effects, not in itself, and
the student should ever bear in mind that just as the
physicist has to assume the fundamental conceptions of
matter and motion, so the zoologist, the biologist and. the
physiologist have to start with the assumption of life and its
vital phenomena. The attempt to explain these premises
in each case is mere speculation.
14 COMPARATIVE PHYSIOLOGY.
CHAPTER II.
COMPARATIVE PHYSIOLOGY.
E have seen that every animal organism exhibits
the four primary functions of alimentation, move-
ment, sensation and excretion. In the lowest types these
functions are performed indifferently by all parts of the
body, but in all the higher types we find that one part of the
body becomes specially concerned with one function, another
part with another function, and so on. In every case all
the functions are represented in the single organism and
each part becomes dependent on the others for the execution
of the other functions. The parts concerned with each
function are usually called systems and the subsidiary parts
of these systems are termed organs. The following systems
are connected with the primary functions :—
Alimentation...(1) Alimentary system.
Movement...... (2) Alotor system (usually mezesceular system).
Sensation ...... (3) Sezse-organs.
(4) Respiratory system.
Excretion ... (5) Zxcretory system.
Inter-communication between the various parts is established by the
(6) Nervous system and (7) Circulatory system, whilst the function of
reproduction demands a separate (8) Reproductive system. Lastly, the
body is often supported and strengthened by the (9) Skeletal system.
The principle of gradual relegation of certain functions
to certain parts of the body is termed physiological division
of labour and proceeds hand in hand, throughout the animal
kingdom, with growing complexity of structure. This
principle can be best understood bya simile. In a primi-
tive human community each man hunts for himself, each
builds his fire, makes the clothes and weapons he may
require, and so on. In more advanced communities, how-
ever, there occurs a division of labour. One man does
nothing but make weapons whilst another perhaps builds
houses, and each of these depends upon the rest of the
community for his other necessities. The result is in-
creased efficiency of the whole at the expense of the
ALIMENTARY SYSTEM. 15
individuality of each unit, for the tailor soon loses the art
of making weapons, and wice versa. The greater the extent
to which the division of labour is carried the more pro-
nounced will be the individuality of the community. Ina
similar manner it will be seen that the lower animal types
with little physiological division of labour have little in-
dividuality and portions of them can survive when separated
from the parent, but the higher types have pronounced in-
dividuality and death ensues upon the disturbance of a
finely-balanced equilibrium of the parts.
1. The Alimentary System is perhaps the most
fundamental; the parts of which may be divided into :—
(1) INcEsTIvE SystEM.—The ingestive organs are those
connected with the seizure of food and its introduction into
the body. As the essential purpose of locomotion is the
obtaining of food they are closely allied to motor organs and
are often modified from them. The ingestive aperture is the
mouth, usually surrounded by organs for seizing or preparing
the food, ¢.g., jaws, teeth, tentacles, &c.
(2) DicestivE SystEM.—Digestive organs are more
directly concerned with the reduction of food into a soluble
and diffusible condition, There is usually a cavity, the
enteron or gastric cavity, in which digestion is effected, and
there are often digestive glands which secrete a digestive
fluid. This cavity is part of the a/mentary canal, occupying
the interior of the animal and opening to the exterior by the
mouth, or by mouth and anus.
(3) EcrstivE SystemM.—Egestive organs are concerned
with the removal of waste residue of the food. The egestive
aperture when present is called the azus; in higher types
it is usually at the posterior end of the body.
2. Motor System.—lIna motor system the property of
contractility is especially concentrated. The primary object
of movement is the obtaining of food, and in the case of
sedentary (fixed) animals the motor organs are employed, not
to move the animal to its food, but the food to the animal.
The two principal organs of movement are :—
(r) Cita AND FLacELLa.—These are vibratile processes
of protoplasm which, by striking the water or surrounding
fluid medium, cause either motion of the medium or that of
16 MOTOR ORGANS.
the animal as well. Cilia usually occur in great numbers
and are short. Flagella occur singly, or at most two or
three to each cell, and they not only lash the water in a
definite direction but often have a spiral motion.
In the lower animals, such as Protozoa, Porifera and
Cwlenterata, these organs usually act as motor organs or for
the purpose of obtaining food. In the higher animals
these functions are performed by muscles.
(2) MuscLEs.—A muscle is a specially contractile organ
which is either in the form of a straight line or a circle. In
the former the muscle, upon contracting, reduces the length
between the points; in the latter, contraction results in a
reduction of the diameter. Nearly all the lower Afetazoa,
sometimes called “‘ worms,” move by a system of circular and
longitudinal muscles and their alternate action upon the fluids
of the body, as more fully explained later (see Lobworm).
Above these, the other AZe¢azoa have the circular principle
mainly confined to the sphincter muscles, which close up
certain apertures, and to the muscles of the alimentary canal.
The great majority of their muscles are of the “long” or
straight-line type, which extend from one fixed point, called
the ovigzn, to another attached to the part intended to be
moved and called the zusertion. These muscles move a
definite system of levers and we can observe two great types.
In the one form (throughout the Arthropoda) the lever is
hollow and contains the muscle, and in the other (in the
Vertebrata) the lever is solid and the muscle is placed out-
side it. The former conduces to greater actual mechanical
advantage, but the latter has infinitely greater possibilities in
complexity and nicety of movement.
3. Sense - Organs. — Sense-organs are parts of an
organism in which is specially concentrated the property of
irritability. Quite far down in the animal scale, these sense-
organs become distinguished among themselves for response
to vibrations of a special wave-length. It is difficult for us
to appreciate any kind of senses other than our own. Our
eyes are sensitive to vibrations varying from 760 to A390*
*A=A millionth of a millimetre. The higher wave-length vibra-
tions give us the sensation which we call ‘‘ violet,” and the lowest we
call ‘“‘red” ; between them lie all the colours of the spectrum. A
mixture of all these wave-lengths we term light.
SENSE ORGANS. i
wave-length and the sensation so imparted we call sighz.
Lower vibrations than this we call heat, and we have, pro-
bably, sense-organs for the discernment of heat. Vibrations
of a still lower grade (from 40,000 to 30 per second) we per-
ceive by our ears and the sensation we term sound. Lastly,
the actual contact of particles upon a specially sensitive
surface gives us the closely allied senses of smell and taste.
Sight involves the perception of light or shade and also,
as a higher faculty, the discernment of actual images.
The former alone exists in a number of low animals and
the latter is only added when the organ of sight has the
addition of an optical apparatus, known as the dioptric
mechanism. It is highly probable that many animals
have organs for the perception of vibrations higher or
lower than those of sight and the “sense” thus produced
is quite inconceivable to us. It may differ from our senses
as widely as sight from hearing.
In the case of hearing much the same remarks hold.
There is little question that many aquatic animals have
organs of equilibrium or of motion which render them
cognisant of low mechanical vibrations of water produced
by the approach or proximity of a foreign object. In
certain land-animals (e¢g., Bats) there appears to be much
the same kind of faculty, which enables their possessors to
avoid objects without the aid of eye, ear or nose.
We may therefore divide sense-organs into three arbitrary
groups, as follows :—
1. High-vibration organs.— ' Roark
(1) Possible organs for perception of vibrations above
A760 wave-length.
(2) Eyes for as ae of vibration A760 to A390 wave-
(3) Possible ene for vibrations of lower frequency.
2. Low-vibration organs.— ar A.
(1) ‘ Auditory” organs for perception of vibrations above
40,000 per second.
(2) Auditory organs for perception of vibrations 40,000 to
30 per second. j
(3) Motion-organs for perception of vibrations 30 per
second to a single vibration.
3. Contact-organs — a :
(1) Olfactory organs for finely divided particles.
(2) Taste-organs for food.
(3) Touch-organs for mechanical contact.
M. 3
18 EXCRETORY ORGANS.
The structure of these sense-organs will be dealt with
in each type, but we may here note that they resemble
each other in consisting essentially of (1) @ modified sensory
epithelium or layer of cells, to which is added (2) a more or
less complex accessory apparatus. The epithelium is in
every case directly connected with part of the nervous
system, when this is present.
Excretory System.—Excretory organs are of several
types. We can usually recognise (1) an excretory surface
which by its secretory activity produces the waste products,
(2) a duct to the exterior often endowed with motor cells to
carry the waste products to the surface of the body, (3) a
reservoir for the accumulation of the waste products before
ejection. All these parts can be distinguished in the series
from the simple contractile vacuole to the flame-cell organs,
the nephridia and the kidneys.
The Respiratory organs are late in development. In the
lower animals, the surfaces of the body serve to effect the
interchange of oxygen and carbonic acid, but respiratory
organs, in the form of gills, arise from the worms onwards.
These gi//s are formed on the “plant” principle of maximum
(respiratory) surface and minimum bulk and are usually
formed from the outer surface of the body. They are
replaced in land-animals by air-breathing organs of quite
another type. Air is usually taken into the body sowards
the respiratory surface and pulmonary organs do not pro-
trude from the body. Air is so much more mobile than
water that the greatest economy is effected in this way.
Correlative Systems,—These four primary systems
are in intimate contact and relation with each other in the
lower types in which the functions are co-extensive with the
protoplasm of the body, but in the higher types the systems,
developed in each case in the most suitable parts of the
body, become removed from each other and systems of
correlation are necessary. The two most important of these
are the zevvous and vascular systems. The former isa system
of correlation between the sense-organs and the motor system,
whereas the vascular system connects all the others.
Vascular Systems.—The vascular system in the higher
animals is usually of two kinds—(1) the BLOOD VASCULAR
VASCULAR ORGANS. . 19
SYSTEM, which carries blood and is primarily a correlative
system between the motor and alimentary on the one hand,
and the respiratory and excretory on the other. Hence the
blood is primarily a respiratory and excretory fluid.
In most higher animals this system has a central organ
of propulsion, the earz, to ensure proper circulation. In
some cases, the heart drives the blood over the system, when
it is called systemic, whereas in the others it propels the
-blood directly to the respiratory organs, when it is known as
a respiratory heart. Occasionally we find that the heart
alternates in its action and it is then called reversible.
The blood-system arises as a system of sinuses or spaces
between the organs, in which condition it remains in the
lower types; in higher types definite walls are formed and
produce vessels. In those animals which possess a heart or
central circulatory organ, the vessels carrying blood away
from the heart are called arteries, those bringing blood to
the heart are vezns.
(2) THE C@ELOMIC SYSTEM, which usually carries a coe-
lomic fluid. This fluid is primarily zw¢rztive in function but
this function is often usurped by the blood-vascular system.
In the forms with a nutritive ccelom the fluid bathes the
muscles, gonads and skeletal system, and even in those cases
in which the nutritive function is largely transferred to the
blood, as in vertebrates, the ccelomic fluid (lymph) still acts
largely as a medium of exchange between the tissues and the
blood. Ccelomic hearts are not common, as the circulation
of the fluid is usually assured by the movements of the body,
but “‘lymph-hearts” are observable in the frog.
Nervous System.—In the lowest types, the protoplasm
of the body is alike irritable and contractile; but in the
-higher organisms, as seen above, the property of contractility
becomes concentrated in a motor system, and that of power
of transmitting impulses in the sense-organs, The latter are,
from their nature, bound to be situated peripherally, whilst
the position of the former is determined by the mechanical
principles of the body. Hence the necessity for a special
means of direct communication between the two systems.
The system which fulfils this condition is called the Nervous
System. It first appears as connecting strands or nerves
20 SKELETAL ORGANS.
running direct from sense-organs to muscles (or the motor
organs). In higher types, there appear nerve-cells with
connecting nerve-fibres, and the nerve-cells become aggre-
gated into masses called ganglia. The nerves become
differentiated into afferent (sensory) nerves, or those which
carry impulses to the ganglia, and efferent (motor) nerves,
which carry impulses from the ganglia to the muscles.
The brain is a specially differentiated mass of nerve-cells
often composed of several ganglia aggregated together. It
is usually at the anterior end in close contiguity to the
main sense-organs.
Skeletal System.—The skeletal system consists of
certain parts of the body which are formed by the secretory
activity of the protoplasm. These may be of three
principal kinds according to the material of which they
are composed :—
1. In a number of the lowest types seca is employed
in the formation of a skeletal system, but this substance is
confined to the Protozoa and Celenterata.
2. Calcareous matter is a very common skeletal material.
It occurs throughout the animal phyla, and is specially
important in the Vertebrata in which it enters into the
constitution of bone.
3. Horny matter or eratin is also very widespread.
Keratin is a complex nitrogenous chemical substance, thus
differing from the two former materials. Keratin, or its allies,
forms the main constituent of cuticles, horns, nails, hair,
hoofs, &c. os
Morphologically, these various skeletons may be divided
into exoskeletons and endoskeletons. The exoskeleton is
formed on the outside of the body and belongs to the
so-called zxztegumentary system. The endoskeleton is pro-
duced in the deeper tissues, usually in the middle layer or
mesoderm.
Lastly, a more or less consistent skeleton is composed
of certain modified tissues, such as cartilage or connective
tissue (see Chapter IV.).
It is not uncommon for many animals to employ foreing
bodies for protection, such as grains of sand or shells of
other animals.
REPRODUCTIVE ORGANS. 2l
_ In a general way, a skeleton performs three functions.
Firstly, it gives a general firmness or consistence to the whole
body which from its protoplasmic nature would otherwise
be mobile. Secondly, it protects the body from enemies,
physical or organic, and, thirdly, it provides a mechanical
system of levers through which the muscles can operate.
Reproductive System.—The vital phenomenon of
GRowTH does not become concentrated in one special sys-
tem of organs, though there are striking cases of differential
growth in many animals. In the case of REPRODUCTION it
is different, and a special reproductive system appears very
early in the animal kingdom. The subject of reproduction
is dealt with under Embryology, but we may note here that
reproductive organs usually have :—
1. The primary gonad, producing the germ-cells. The
male organ is called the ¢es¢is, the female the ovary.
2. Ducts leading to the exterior. The male duct is
called the vas deferens, the female the oviduct.
Further differentiations ensue as development becomes
more complex. Firstly, the eggs are supplied with yolk
and yolk-glands are often required. Secondly, the eggs
require protecting shells or capsules produced by she//-glands.
Thirdly, these additions require internal fertilisation, within
the oviduct, before the shell is added. ‘This means copula-
tion and a copulatory organ in the male, whilst there may
be a receptacle for the semen (veceptaculum seminis) in the
female. Lastly, the eggs and young may be retained for
some time within the oviduct of the female, in which case
the portion of the oviduct adapted for this purpose is the
uterus. Various accessory glands may become superadded.
We may tabulate the reproductive organs as follows :—
Male.....1. Production of spermatozoa......... Testis.
2. Transportation to exterior ......... Vas deferens.
3. Introduction of same into female, Penis.
Female.,.1. Production of eggs .........cceeeeeee Ovary.
2. Transportation of eggs to exterior, Oviduct. (glands.
3. Production of yolk and shell...... Yolk-glands and shell-
4. Reception of sperms ................ Receptaculum seminis.
5. Retention of egg and embryo...... Uterus.
22 COMPARATIVE MORPHOLOGY.
CHAPTER III.
COMPARATIVE MORPHOLOGY.
N studying the structure of an organism, we can re-
cognise two departments of morphology. In the
first we have to deal with the form assumed by the
organism or “‘ body-form,” the study of which is sometimes
termed Fro-morphology, and in the second we investigate
the internal construction of organisms. Ona first inspection
of typical examples of animals their body-form does not
appear to be referable to any definite plan. They do not
assume geometrical shapes, like a cube, or a cylinder, and so
on. Yet we can, especially by a study of the lower types,
find geometrical principles underlying their construction.
A like environment produces a similarity of structure in
response to it. For example, if an animal exposes two
sides to a similar environment, the structure of these two
sides will tend to be similar.
The manner in which the similar parts of an organism
are arranged is termed its symmetry.
Animal Symmetry.—Animals may be divided into
three groups according to the symmetry of their body :—
4 oe } (often termed radially symmetrical).
3. Plano-symmietric (bilaterally symmetrical).
1. CENTRO-SYMMETRIC animals have all their parts
arranged about a jozt in the centre of the body, hence
they are usually spherical or stellate. The only parts to be
distinguished are central and feripheral. This form of
symmetry is only found in the lowest aquatic animals (eg.,
Leadiolaria, Foraminifera, &c., and many eggs).
2, AXO-SYMMETRIC animals have their organs arranged
about an axis down the centre of the body and hence they
SYMMETRY. 23
tend to a cylindrical form. In this type we can distinguish
axtal and peripheral parts and the two ends of the main axis
can usually be recognised as the afex and the Jase. The
mouth is situated at the one end or apex often termed the
orad end, the base being known as the adoraZend. Examples
are found among the lowest animals (Protozoa, Porifera and
Calenterata) which are either sedentary (fixed by the aboral
end) or pelagic.
3. PLANO-syMMETRIC (bilaterally symmetric) animals
have their parts arranged about a central plane, which
usually lies in the long axis of the body. In these we
can determine an anterior and a posterior end, a dorsal
and a ventral surface, and a vight and “eft side. The
parts are either median or lateral. Nearly all the members
of the animal kingdom above the Cwlenterata conform
more or less closely to this type.
Certain organisms do not appear to conform to any of
these types. Ameba and some other low organisms have
no definite shape of body since they change their shape at
everymoment. They really belong to the centro-symmetric,
because, when encysted or subjected to a stimulus, they
assume the spherical shape. Other higher types, such as
the snail, show a distortion which destroys to some extent
the plano-symmetry underlying their general body-form.
The locomotion of animals usually conforms to their
symmetry. Most centro-symmetric animals rotate freely
about the centre, but do not move in a definite direction.
Axo-symmetric animals, if not sedentary, usually move in
the direction of the axis of symmetry, and plano-symmetric
animals usually move in the direction of the plane of
symmetry, with the anterior end forwards and usually with
the plane of symmetry vertical.
Morphological Units.—If we next proceed to in-
vestigate the actual constituent elements of organisms we
can discern a very definite unit which occurs throughout.
This unit is called a ce//. It is impossible to define a cell in
such a way as to include all the numerous forms and
modifications, but for our present purpose we must regard
it as a definite mass of protoplasm containing a nucleus,
and usually having more or less of a limiting cell-membrane.
24 STRUCTURAL UNITS.
The members of the lowest group or phylum, called
Protozoa, consist of single cells, or colonies of single cells,
whereas all the higher animals are multicellular or consist
of cell-aggregates, The study of cell-structure is Histology
(see Chapter IV.).
The second structural unit is the epzthelium (derm) or
layer of cells. A number of cells are aggregated together
and all perform the same common function. No animal
organism is entirely of this form, but many organs show
this stage very clearly.
The third unit is the éome or sac-like form in which the
layer of cells surrounds a common space (or ce/e) and forms
a complete organ separated from the parent-layer.
These three stages can be traced more or less clearly
in most organs and organisms. Their mutual relationship
may be made more clear by a comparison with a brick, a
wall, and a room, respectively.
Amongst multicellular animals we can distinguish three
important types according to their construction. The
simplest are those with a single layer or epithelium of
cells, called monodermic (or monoblastic).
Fig. 1.—DIAGRAM OF Fig. 2,—DIAGRAM OF
A MONODERMIC ORGANISM. A DIDERMIC ORGANISM.
This is a very simple condition found in only a few
types, such as the blastula larva and Volvox.* The single
epithelium of cells is called the archiderm, and may sut-
round a cavity called the archicwle. In the second type
the body is formed of two epithelia, when it is known as
* A low colonial protozoan belonging to the Mastigophora.
STRUCTURAL UNITS. 25
didermic (or diploblastic). The large phylum of Cwlenterata
is didermic. The outer layer is known as ectoderm, the
inner as exdoderm, and the space enclosed by them is the
archenteron. The space between the layers is usually filled
with a jelly-like substance called the mesoglea. The third
type is the ¢ridermic (or triploblastic). In it there can be
discerned, at least in early stages, three distinct primary
layers or epithelia. Nearly all the higher animals are
tridermic. In tridermic forms, the outer and inner layers
are called the ectoderm and entoderm, whilst the middle
Fig. 3.—DIAGRAM OF A TRIDERMIC ORGANISM, SEEN IN
CROSS SECTION.
Entoderm.
Mesoderm.
Ectoderm.
Note the somatic and splanchnic layers of mesoderm joined by dorsal and
ventral mesenteries ; the haemoccele is not seen as the mesoderm
is closely adherent to the other layers.
layer is the mesoderm. The space in the exfoderm is now
called the mesenteron. The ectoderm and entoderm are not
in contiguity, but there is always a more or less spacious
body-cavity or cavity of the body, which is the primary body-
cavity or avchicele (cf. monoblastic types). In this archiccele
is arranged the third layer or element. It may consist (in
the Archicela or Acelomata) of a mass of connective tissue,
muscle-cells and gonads formed of more or less isolated
cells (cystic) or layers (dermic). In this type the excretory
organs are of the type called flame-cell tubules (see Platy-
helminthes) which end internally in blind tubes or sacs. In
the second type (the Ca/omaza) the greater part of the meso-
derm is formed into a definite epithelium limiting a cavity
4
26 STRUCTURE AND FUNCTION.
or sac which is then termed the cwlom. This ccelom does
not usually fill the whole primary body-cavity for a large
part of the latter remains as the Aemocele or blood-space.
In the Calomata the typical excretory organs are excretory
tubules or xephridia which open directly into the cavity of
the coelom. The comparative size of the coelom and hzemo-
ceele varies greatly.
The actual connection, if any, between the two tridermic types is
not known. The archiccelic is evidently the simplest, but it is doubt-
ful whether in the evolution of the Ca/omata, the flame-cell tubules
become transformed into nephridia, whether they were merely replaced
by the latter in function and atrophied, or whether the ancestors of
Celomata never had flame-cell tubules.
In a general way, these three types correspond to the
three forms of symmetry; the monodermic organism is
centro-symmetric, the didermic usually axo-symmetric, and
the tridermic is in nearly all cases plano-symmetric.
Structure and Function.—Organs of the body are
of certain form and structure according to the functions
they perform. Hence there is a general similarity in the
form of the different systems referred to in the last chapter.
Nervous systems, for example, have certain striking resem-
blances throughout the whole animal kingdom, and so with
all other primary systems.
We can only notice here two important parts of this sub-
ject. Firstly, theré are many instances of loss of function.
This invariably leads to reduction or complete extinction
of the organ in question. The most endoparasitic animals,
such as tape-worms, undergo a complete loss of all ali-
mentary organs as they are not required. Again, in many
cases the organs persist as mere vestiges and are then
known as vestigial organs. Remarkable instances of these
are the hind limbs of whales, some of the jaws of the cray-
fish, and the splint-bones of the horse.
Other organs are just acquiring the function which is
raising them into importance and are still small. These
may be called rudimentary organs. Organs like everything
else in the world, have their rise, their culminating point, and
their fall. A ves¢¢géaZ organ is in the last phase of its history,
whilst a vzd/mentary organ is in the first. The electric organ
CLASSIFICATION 27
of the skate may be given as a possible example of a rudi-
mentary organ. Secondly, an organ may change its function
or, in other words, may lose its primary function but be
preserved and greatly modified by acquiring another function.
The skin-armour of placoid scales in sharks is not found as
such in higher vertebrates, except the few in the neighbour-
hood of the jaws, which form teeth. Again, the appendages
of the crayfish show every step in modification from the
primitive biramous swimming organ to the leg, jaw, or feeler,
in accordance with the various functions they have acquired,
Classification.—Hence we have seen that the animal
kingdom forms an ascending series of organisms of struc-
tural complexity, which is due to three kinds of gradations.
Firstly, animals show a gradation in symmetry from the
simple centro-symmetry to the complex plano-symmetry.
Secondly, they show a gradation in construction from simple
cells to many-layered individuals. Thirdly, they show a
gradation in structure due to the functional division of
labour. If these gradations were absolute we could form
no classification. It would be impossible to divide the
animal kingdom into groups if it presented a continuous
gradation in structural characters. The breaks in structural
sequence permit us to define certain animals and to separ-
ate them from certain others.
Whilst our classification is based primarily upon structural characters
there is an important reservation. We have seen in the introduction
that structural similarity is called omology and that there are two kinds
of homology, inherited and ‘acquired. The acquired homology is often
very difficult to distinguish from the inherited homology, but the ideal
classification to which all zoologists aspire is based purely upon inherited
homology or upon homogenetic characters ; if we place together in one
group a number of individuals because they have omogenous similarity
in structure, we shall by our definition be correlating animals which
are descended from a common stock. This is a satura classification,
for in it we strive to give expression to the natural relationships of the
animals. Let us take a very simple example. If we decide to put
in one group the animals which swim in the sea, have a tail-fin and
pectoral fins and are of a fish-like shape, we create a group containing
the whales and fishes. This is an artdficda/ classification, for further
examination shows that the whale agrees with land-mammals in nearly
all the most important mammalian characters and that its fish-like shape
is acgutred or due to adaptation to an aquatic life.
The determination of natural affinities is largely helped by the study
of embryology and of paleontology, but there is no exact criterion for
28 CLASSIFICATION.
recognising a natural affinity (for it is a relative term) and there is no
question that our classifications are still very unnatural.
All that can with present knowledge be done in classifi-
cation of the animal kingdom is to distinguish certain
large Phyla or branches, the members of which have certain
important structurai features in common. The funda-
mental distinction between unicellular and multicellular
animals enables us to separate the Protozoa from the rest,
which are termed J/efazoa. Hence we have two sub-
kingdoms, the Protozoa and Metazoa. The Protozoa have
two phyla, the Gymnomyxa and Corticata, and the Metazoa
several important phyla. The two lowest of these differ
from the rest by being typically axo-symmetric, retaining
the primary axis of the gastrula, whilst the rest are
primitively plano-symmetric about a plane at right angles
to the primary axis of symmetry. This important dis-
tinction is emphasised by the two divisions of Protaxonia
and #ilateralia, the latter being all tridermic.
The Phyla are divided into sub-phyla and classes, the
characters of which depend mainly upon general com-
munity of structural design. Finally, the classes are
further sub-divided into orders, families, and genera until
the species is reached.
The various groups are not in all cases exactly compar-
able, but the same order is always pursued in dividing
up a phylum.
The list here given includes all the more important
phyla which are dealt with in this work and their division
into classes.
It will be seen that, of the phyla of the Sz/ateralia, the first three,
or the Platyhelminthes, Rotifera, and Nemathelninthes, are of the Archz-
celic (or Acelomata) type, whereas the other four are Calomata.
(TABLE.
CLASSIFICATION.
29
an
see
Bao
PROTOZOA,
METAZOA.
PROTAXONIA,
BILATERALIA.
Phyla. Sub-Phyla.
re Gymnomyxa,
2. Corticata.
1. Porifera,
2. Coelenterata.
3. Platyhelminthes.
4. Rotifera,
5. Nemathel-
minthes.
6. Archi-Coelomata. | 1. Echinoder-
mata.
2, Archi-
chorda.
3. Brachio-
poda.
4. Polyzoa.
5 Chetog-
natha.
7. Annulata. 1. Annelida.
2. Arthro-
poda.
8. Mollusca.
g. Chordata. 1. Atriozoa,
2. Verte-
brata.
Classes. Type described.
1. Rhizopoda. Ameba.
2. Ciliata. Paramecium.
3. Mastigophora,
4. Acinetaria.
5. Sporozoa. Gregarina.
6, Calcarea, Sycandra.
7. Non-Calcarea,
8 Hydrozoa. Hydra (Obelia)
9. Scyphozoa. Actinia( Aurelia)
1o. Ctenophora. Cydippe.
1r. Trematoda. Distomum.
12. Cestoda, Tenia.
13. Turbellaria,
Hydatina,
14. Nematoda. Ascaris.
Asterias.
Balanoglossus,
Waldheimia.
Lophopus.
Sagitta.
1s. Archiannelida. | Polygordius.
16. Polycheta, Arenicola.
17. Oligocheta. Lumbricus,
18. Hirudinea, LfTirudo.
1g. Crustacea. Nephrops.
20. Insecta. Blatta.
2i. Protracheata. Peripatus.
22. Myriapoda.
23. Arachnida, Epeira.
24. Gastropoda. Helix.
25. Cephalopoda. Sepia.
26. Lamellibranch- | Azodon.
jata,
27. Urochorda. Ascidia.
28. Cephalochorda. | Amphioxus.
29. Cyclostomata. Myxine.
30. Pisces. Raia.
31. Amphibia. Rana.
32. Reptilia,
33. Aves. Columba.
34. Mammalia, Lepus.
30 HISTOLOGY.
CHAPTER IV.
HISTOLOGY.
ISTOLOGY is the study of cells. In the case of
the Protozoa this is the study of the whole organ-
ism; in the AMe¢azoa, of its constituent units.
Fig. 4.—AMGBOID CELLS.
I 2 3
1. Ameeba. z. Leucocyte of Frog. 3. Ovum of Hydra.
(After Howes.) = (After KLEINENBURG.)
Independent cells may occur in several character-
istic conditions. The principal are as follows :—
1. AMa@Boip.—These are cells resembling Amba, shape-
less, and showing movements by pseudopodia. A number
of Protozoa show this condition throughout the greater part
of their life. In the MZetazoa free amceboid cells occur with
great frequency They are usually termed /ewcocytes and
fulfil important functions, such as ingestion of bacteria.
Leucocytes of this nature occur in great numbers in human
blood.
Fig. 5.—FLAGELLATE CELLS.
ran
1. Spermatozoa. 2 Flagellate Protozoa. 3. Collared Cell of
Sponge.
EPITHELIA. 31
2, FLAGELLATE OR CiLiate.—These cells are found in
the protozoan classes Cv/iata and Mastigophora. The con-
tractility is concentrated in the cilia or flagella, and the rest
of the cell-body is often enveloped in a cell-membrane.
In Metazoa free flagellate cells occur in the case of the
male sexual elements or spermatozoa. Collared flagellate
cells occur in great numbers in Forifera, whilst ciliated cells
are commonly found in higher AZe/azoa, though not in the
free condition. (See below.)
3. QUIESCENT.—These are cells with no automatic
movement ; they are usually enveloped in a cell-membrane
which may assume the character of a cyst. They are
usually spherical, or nearly so. Encysted Protozoa always
Fig. 6.—QUIESCENT CELLS.
I 2
@
° ae
1. Encysted Amceba. 2, Human red 3. Ovum.
blood Corpuscles.
assume this character, and some Jow organisms are per-
manently in this phase. In JMetazoa free quiescent cells
occur in the case of the eggs or female sexual elements,
and in the “red corpuscles” of the blood. The former are
usually spherical or oval, the latter flattened.
Dependent cells of the A/efazoa are aggregated into
masses or surfaces which are termed ¢éssues. A tissue is
therefore an aggregate of cells which are alike in structure
and function,
We may recognise two sorts of tissue—(1) Tissues of
two dimensions or surface-tissues (Epithelia) ; (2) Tissues
of three dimensions or mass-tissues.
"1. EprrHEtia.—An epithelium is, in its simplest condi-
tion, of only one cell thick, but it has often several layers
superposed :—
(1) Ciliated epithelium is a common type, in which
each cell has its outer end or surface covered with
vibratile cilia. It is commonly found on the tentacles and
4
32 EPITHELIA.
Fig. 7.—TyPEs
gills of Archicwlomata, Annelida, and OF EPITHELIUM.
Mollusca. In some cases the outer limit-
ing surface or epithelium of the body is
composed of ciliated epithelium.
(2) Columnar epithelium,—The cells
are placed side by side in regular order,
usually deeper at right angles to the sur-
face than in other directions — in fact, Rane se apr nee of
like columns. Their upper or outer sur- le
face usually differs from the rest of the
cell and may be clear and hyaline, or
show striations, or it may be in an amee-
boid condition with minute pseudopodia.
It is a form of epithelium commonly lining
the alimentary canal.
(3) Squamous epithelium.—Each cell
is spread out into a flat, scale-like plate. Squamous Epithelium
Each touches its fellows at its edge, and Geetend
the whole forms a delicate limiting mem-
brane. Simple squamous epithelium forms
the outer limiting surface of sponges (pin-
nacocytes), and the inner peritoneal lining
endothelium of many higher types. In squamous Epithelium
the outer limiting surface of these latter (surface view).
the squamous epithelium is not simple but
stratified. The surface-cells only are flat-
tened, and these gradually pass downwards
to columnar, By cell-division the colum-
nar produce fresh squamous cells which
are lost at the surface by wear or otherwise.
Columnar Epithelium.
ame
(4) Glandular epithelium is a special
form of columnar epithelium. Glandular
secretion collects in the substance of the
cell and is then discharged at the surface.
(5) Lastly, there is Sensory epithelium,
in which the cells are specially modified
for sense-functions.
Sensory Epithelium.
These epithelia may often occur in a mixed condition.
Thus the endoderm of Hydra is an epithelial mixture of
MASS TISSUES. 33
flagellate, amoeboid and gland-cells, though possibly the
same cells may assume each of these forms. Again, a
ciliated glandular epithelium is very common, gland-cells
being interspersed amongst the ciliated cells.
2. Mass Tissues.—Of tissues in three dimensions, or
mass-tissues, we may distinguish the most important as—
(1) Connective tissues, (2) Muscular tissues, and (3)
Nervous tissues :—
Fig. 8. (1) Connective tissues——In these
Connective Tissuzs. the cells themselves usually become
(x and 3 after Howes). subservient to the substance around
or within them, which is secreted by
them— when outside the cells this is
termed the matvix. We can here
only notice the most important :—
(a) Fibrous connective tissue con-
sists of a matrix in which there are
intersecting elastic fibres. Certain
of its cells commonly secrete large
globules of fat and give rise to
adipose tissue.
(6) Chordoid tissue.—These cells
secrete in their substance a clear
fluid matrix which almost entirely
replaces the protoplasm, the nuclei
being squeezed to one side. The
whole forms a strong elastic sup-
porting tissue. It is a modified
glandular epithelium, and is best
known in the notochord of Verte-
brata.
(c) Cartilage.—In cartilage the
cells lie scattered in a dense mass
of secreted matrix, which may be
clear or hyaline, or may show a
fibrous structure.
Cartilage.
M,. 4
34 MASS TISSUES.
(@) Bone.—The cells
or bone-corpuscles form
a meshwork of finely
branched cells, anasto-
mosing in every direc-
tion, and the matrix con-
sists of concentric layers
or lamellee of calcareous
matter, producing a
hard, dense, supporting *
tissue.
Bone, more highly magnified.
(2) Muscular tissue.—The cells or fibres are aggregated
into masses, and each is usually elongated in the direction of
contraction. ' The property of contractility is concentrated
in them, and they may or may not show a cross striation.
In the higher types the whole cell is modified into a fibre,
but in Hydra, Ascaris, and other types, only a part of it is
so modified.
Fig. 9.—MUSCULAR TISSUE.
(After Howes)
x Transverse section of small muscle. z. Muscle-fibres,
(3) Vervous tissue.—The primary nervous elements are
nerve-cells. These are commonly stellate (multipolar), but
they may have only one or two branches (unipolar or
bipolar). The branches pass from the cells to muscles,
or to sensory epithelium, and they form nerve-fibres. A
number of nerve-fibres aggregated together and enclosed
in a sheath form a nerve.
STRUCTURE OF CELL.
Fig. 10.—NERvVoUus TISSUES.
z Transverse section of small nerve. 2. Multipolar nerve-cell.
3. Bipolar nerve-cell.
Structure of the Cell.—We may now pass from the
external form of a cell to its internal structure. Inside the
cellmembrane is the cytoplasm or cell-protoplasm. Lying
in the cytoplasm is the wwcleus surrounded by a delicate
Fig. 11.—DIAGRAM OF A CELL. (After CARNOY.)
Chromatin.
Nucleolus. Cytoplasm.
Nuclear
Membrane.
Centrosome.
nuclear membrane. ‘The nuclear substance is composed of
a clear fluid called nuclear sap and chromatin, so called
because of its staining properties, which is usually in the
form of a fine meshwork. There may also be one or more
rounded bodies, the zzcleolz. Near the nucleus there is
a clear rounded body, with radiating processes, called the
36 MITOSIS.
astrosphere; it contains in its centre a minute spot called a
centrosome. Theastrosphere and the chromatin appear to
play important parts in the process of cell-division.
A cell reproduces itself by binary fission (see Chapter
V.), and there are two types of cell-division, according to
the behaviour of the nucleus. In both types, the nucleus
first divides into two, the cytoplasm following. In the
direct or amitotic division the nucleus merely constricts into
two equal parts without special changes. In the éudirect or
mitotic division the nucleus undergoes division by m¢osis.
This is the most usual method of cell-division.
The changes, in a typical instance (see Fig. 12), may
be summarised as follows :—
1. The chromatin network breaks up into a number of chromo-
somes, usually elongated rods of chromatin.
2. The chromosomes split down the centre into halves, thus
doubling their number, and the astrosphere divides into two
parts which move to opposite ends of the cell.
3. The nuclear membrane, nucleoli, and nuclear sap disappear
and the chromosomes lie in the cytoplasm.
4. Half of the chromosomes migrate to one astrosphere and half
to the other, in the neighbourhood of which they are aggre-
gated into a nuclear network, and formed into fresh
nuclei.
5. The cytoplasm then divides into two, and cell-division is
complete.
Fig, 12.—D1aGRam oF Mitosis. (After FLEMMING. )
4. Loops migrate to each Centrosome.
5- Cell commences to divide. ,
6. Division complete. Re-formation of
Nuclei.
1. Chromatin Loops.
2. Loops split and Centrosome divided.
3. Centrosomes have diverged and loops
are at equator.
MITOSIS. 37
This is the egzal mitotic division, but in certain cases
a reducing division occurs. In a reducing division the
mitotic phenomena are much the same, but ¢he chromo-
somes do not divide into two, hence the resulting daughter
nuclet have only half the number of chromosomes of the parent
nucleus.
It is difficult to see the full meaning of mitosis, but
it has been interpreted as a process for ensuring the
equaé division of the chromatin. The astrospheres appear
to act as centres of attraction for the chromosomes, and
there can usually be discerned a nuclear spindle uniting
the rays of the two astrospheres, giving the whole the
appearance of a magnetic field.
The reducing division is characteristic of gonogenests, or
the production of the sexual elements. The primitive germ-
cell produces sperm-mother cells, or egg-mother cells, which
at the moment of division contain twice the number of
chromosomes. Two rapid reducing divisions then produce
four sperm cells in the male, or the mature ovum and polar
bodies in the female... Hence the mature ovum and the
spermatozoon have in their nuclei (¢ and 2 pronucleus) just
half the normal number of chromosomes. When fused
they produce a normal nucleus with the full number.
The student should compare this account carefully with
that given in Chapter V., page 42, and it will be clear that
the reducing divisions and the enumeration of the chromo-
somes lead to the same conclusion, namely, that the male
and female elements (spermatozoon and mature ovum) are,
morphologically speaking, merely half-cells produced by two
rapid divisions at the limit of growth instead of the normal
single division.
38 GROWTH AND REPRODUCTION.
CHAPTER V.
%
GROWTH AND REPRODUCTION.
HE process by which organisms give rise to fresh
_ generations is called Reproduction. There are two
main types of reproduction, the asexual and the sexual.
Fig. 13.—D1acraM To ILLusTRaATE CHANGES OF THE
Nuctevs (N) purtnNGc CELL-DivIsIon.
* N.
N grows to 2N.
Qn
2N divides into two.
Each grows to 2N.
Each divides into two.
2
Each product grows to2n.
2N
Each product divides into
N two.
&e.
aN
Asexual Reproduction.—In asexual reproduction a
single individual divides into two or more parts or portions.
When the individual splits into two parts, approximately
equal in bulk, the reproduction is called dary fission ;
when into many equal parts it is called multiple fission. If
the division is into two or more parts of which one is much
the larger, the process is distinguished as budding; the
lesser part is termed the dud, the larger is known as the
CONJUGATION. 39
parent. Buds are usually formed on the outside of the
parent but occasionally internal buds occur. In many cases
the buds may remain in organic contact with the parent,
when a compound organism or colony is produced.
Binary fission is the usual method of cell-reproduction
throughout the animal kingdom. In unicellular organisms,
such as Amada, the nucleus divides into two equal parts
with complex changes, called mitosis (see Chapter IV.), and
the cell follows suit. Each fresh cell then grows and, when
each nucleus and cell has reached the limit of growth, a
fresh binary fission takes place.
We may illustrate this process by a diagram (Fig. 13).
In this manner growth and reproduction alternate, and
the relationship of cell to nucleus, and of surface to bulk, is
maintained at the normal.
In a multicellular individual the constituent cells grow
and multiply in the same manner, and the same diagram
will serve if we recollect that the cells are aggregated into
one compound individual instead of becoming separate
organisms.
Returning to the unicellular organism, we might perhaps
suppose this cycle of alternate growth and reproduction
by binary fission to be capable of infinite repetition, but
such is not the case. After a certain number of repeti-
tions another process intervenes called Conjugation.
Conjugation consists of two series of events—(1) Pre-
paratory reduction of nuclei in two individuals, and (2)
Interchange of nuclear substance :—
1. Preparatory Reduction of the Nucleii—Two _indi-
viduals join together in such a way that their proto-
plasm is continuous. All activity is suspended and the
nucleus of each increases in bulk. Each nucleus then
divides into two and into four by binary fission.
2. Interchange of Nuclear Substance.—Each individual
now has four portions of the nucleus in its substance.
Two of these are absorbed and disappear, whilst one
from each individual moves across into the other individual,
and each of these migrants then fuses with the part
still left to form a compound nucleus. The individuals
40 CONJUGATION.
separate and growth, followed by binary fission, proceeds.
The whole process can be illustrated thus :—
Fig. 14.—DIAGRAM TO ILLUSTRATE TyPIcAL CONJUGATION,
Each nucleus grows to
double its bulk.
Each divides into two.
And into foar.
Transfer of one from
each.
Cell-divisions.
The process is not really quite so simple as here shown, for the
nucleus often grows to 4N and divides into eight, or it may be still
larger and divide still more, or only one half of the first division may
continue the divisions.
The essential part to notice is that whereas, zormally, there
ts a steady alternation of growth and binary fisston, in the
first stage of conjugation the nucleus ts reduced in size by at
least two divisions, following rapidly, before intermediate growth
can take place; in the second stage, the zormal bulk of the
nucleus ts restored by the addition of nuclear substance from
another individual. The fresh individuals produced by sub-
sequent binary fissions all have their nuclear material derived
from the two conjugating individuals. The cycle of pro-
tozoan individuals may be indicated thus :—
Conjugation (mixture of nuclear material).
Series of cell-divisions. Alternation of
growth and binary fission producing many
unicellular individuals.
SEXUAL REPRODUCTION. ‘41
Asexual reproduction is found most commonly in the
lower phyla of animals, but ce//s are produced asexually
throughout the whole kingdom.
In many instances, one or more asexual generations
may alternate with the sexual method. This phenomenon
is known as Alternation of generations or Metagenesis. It is
usually found in organisms whose life-history is very varied,
and involves such dangers at certain periods that a multi-
plication immediately prior thereto is necessary to the
continuance of the species (¢/ Parasitism, Chap. IX.).
Methods of Asexual Reproduction :—
A, FIssIon— Binary—two equal parts.
(or division into equal parts).
Multiple—many equal parts.
&B. BuppInc— Internal,
(or division into unequal parts).
External.
Sexual Reproduction.—It is characteristic of the
multicellular animals or Méefazoa that they reproduce
sexually, In sexual reproduction a portion of the parent
is liberated, as in asexual reproduction, and gives rise to a
fresh organism. The main differences are these :—(1) The
liberated portion is never more than a single cell (which is
called the sexual element) and is produced in special organs,
(2) This single cell completely fuses with another single
cell, liberated in the same fashion from another individual,
but differing in shape and structure. The fused cell so
produced divides into a multicellular individual by repeated
cell-division. Fhese processes are called respectively (1)
Maturation and (2) Fertilisation.
1. MaruraTion.—The essential reproductive organs
are called gonads and give rise to cells known as the
primitive germ-cells. The male element is produced in an
organ called the ¢es#is and the female element in an ovary.
In the case of the male, the male element or spevmatozoon
is produced by rapid increase to double its size of the
42 SEXUAL REPRODUCTION.
primitive germ cell, to form the sperm-mother cell, which
then divides rapidly by two divisions. ‘The mature sperma-
tozoon is usually an active organism with a head-portion
derived from the nucleus and a tail formed from the
protoplasm of the cell. The nucleus itself is often termed
the made pronucleus and is evidently one half of the original
nucleus of the primitive germ cell. In the case of the
female, the primitive germ cell grows to twice the bulk, to
form the egg-mother cell, and then divides into two, but they
are of very unequal size. The lesser is called the first polar
dédy and consists mainly of half the nucleus of the egg-
mother cell. Another division of the same kind produces a
second polar body consisting mainly of one half of the
original nucleus. These two polar bodies are seen for
some time resting on the exterior of the remaining portion,
which is known as the mature ovum or female element, its
nucleus being the female pronucleus. Eventually the polar
bodies atrophy.
The phenomenon of maturation consists in each case of
the production of the pronucleus, which is a half of the primi-
tive germ cell nucleus, but in the male the protoplasm is
also equally divided to form the tails of the male elements,
whereas in the female practically all the protoplasm is
aggregated to one of the half nuclei, and the others atrophy.
The explanation of this curious process will te easier
after we have taken a review of the following processes—
2, FERTILISATION.—The essential part of fertilisation
is the fusion of the male and female elements. The
spermatozoon embeds itself within the substance of the
ovum, the tail is absorbed, and the “head” or male pronucleus
fuses with the female pronucleus to form what is called the
segmentation-nucleus of the fertilised egg.
We may note that the one half of the segmentation
nucleus consists of male and the other half of female nuclear
material. The life of the new individual dates from the
formation of the fertilised ovum.*
* If we suppose that the fertilised ovum is an individual produced
by sexual reproduction, and that this by asexual reproduction gives rise
to the fresh individual, the adult metazoan, then there is a complete
alternation of generations in all metazoa, the sexual individual being
always a single cell.
SEXUAL REPRODUCTION. 43
After resting and growth of the segmentation-nucleus
a series’ of cell-divisions takes place called segmentation.
These cell-divisions continue throughout the life of the
individual, but the earlier and more evident divisions are
called segmentation.
We may illustrate the process of sexual reproduction
thus—
Fig. 15.—DIAGRAM ILLUSTRATING NUCLEAR CHANGES DURING
: “SEXUAL REFRODUCTION.
MALE. FEMALE.
Primitive germ cell grows
to form sperm-mother N N.
COL iced Sere eeisieieaters oe
Primitive germ cell grows
Sperm-mother cell divides to form egg-mother cell.
into two... ...... Egg-mother cell divides
into two.
“4 And four to form ovum
N and two polar bodies.
And into four to form four
spermatozoa .........+
wiz
Spermatozoon and ovum
fuse.
> Cell-divisions.
After hundreds of these cell-divisions, one N. becomes
the nucleus of a primitive germ-cell, increases to 2N. in the
mother cell, leaves the organism as ¥ in the pronucleus,
and the cycle recommences.
If this diagram be carefully studied it will be clear
that the process of maturation has for its object the forma-
tion of cells which have only half the usual nuclear element,
whereas fertilisation consists of the fusion of these halves to
form nuclei of normal proportions. Further, the dimorphism.
44 SECONDARY SEXUAL CHARACTERS.
or difference in structure between the sexual elements,
apparently confined to the protoplasm, promotes and ensures
the fusion of elements from two separate individuals. The
nutritive conditions of the male and female, with a deficiency
and an excessive proportion of protoplasm respectively, con-
duce to their mutual fusion and prevent fusion of the same
elements. The vazson ad’étre of this nuclear fusion appears
to lie in the fact that the nucleus is the carrier of hereditary
variations. Hence fertilisation ensures that every cell of the
new individual shall partake of the characters of at least two
antecedent organisms. This can only be effected by every
organism starting as a single cell.
As quite a secondary phenomenon we have what is
called dimorphism of the sexes. We have seen that the
sexual elements differ, the male element being the active
agent in reaching the female element, which itself is passive.
This physiological division of labour and consequent struc-
tural dissimilarity between the sexual elements is, in many
higher animals, reflected back to the reproductive organs of
the parent, producing male and female individuals or sexes.
If the sexual organs are found in separate individuals
the species is called deczous, if united in one individual the
species is described as hermaphrodite. Hermaphroditism is
of widespread occurrence throughout the animal kingdom,
but is rare in higher types.
In certain exceptional instances the female may produce
eggs which, without fertilisation, may develop into fresh
individuals. Such a phenomenon is termed parthenogenesis.
As a general rule, the sexual organs are the last to mature ;
hence the reproductive function: only takes place after all
development has ceased, but in certain rare instances a
larval form is known to attain maturity and reproduce itself.
The phenomenon is known as pcedogenesis. Axolotl is a
good example.
The differences in the structure and function of the
sexual organs are called grimary sexual characters, but in
those animals in which the fertilisation is not promiscuous
the sexes often show structural differences other than those
of the sexual organs. As examples we may cite the plum-
age of birds and the antlers of deer. These are called
secondary sexual characters.
CONJUGATION & SEXUAL REPRODUCTION. 45
We may illustrate the cycle of metazoan animals thus :—
Sexual Reproduction (fusion of nuclear material ).
Series of cell-divisions. Alternation of growth and,
binary fission producing a single multicellular
individual.
If we compare this with the cycle of protozoan animals
(page 40), the relation of conjugation to sexual reproduction
becomes clear.
We may tabulate the two phenomena as follows :—
CONJUGATION.
1. A preparatory process, con-
sisting of reduction of the
nuclear material by divisions
of the nuclei only.
2. Mutual interchange of nuclear
material between the two
individuals, and fusion of
the respective nuclei.
3. Separation of the two individ-
uals, followed by growth and
binary fission to form many
unicellular individuals.
SEXUAL REPRODUCTION.
1. Maturation of sexual ele-
ments, consisting of reduc-
tion of nuclear material by
divisions of nuclei and cells.
2. Fertilisation, consisting of the
passage of a/7 male element
over to the female element,
and the fusion of the two.
3. Division of fertilised ovum by
growth and binary fission to
form one multicellular indi-
vidual.
46 COMPARATIVE EMBRYOLOGY.
CHAPTER VI.
COMPARATIVE EMBRYOLOGY.
NTOGENY is the development or production of the
individual. The study of ontogeny is Embryology.
The individual dates its existence from the fertilised
ovum sexually produced from the male and female cell-
elements. From this and other considerations it will be
seen that there is no true ontogeny in the Protozoa or
unicellular animals, for they are produced asexually (by
binary or multiple fission) from their parents.
The first important fact about the ontogeny of the
Metazoa is that they commence life as a single cell, the fertilised
ovum. The second point to notice is that th7zs ovum, by rapid
alternation of growth (or increase in bulk) and asexual
reproduction (or binary fission), zs transformed into the
multicellular adult. The third is the differentiation of the
multicellular organism from a homogeneous cell-mass to a
heterogeneous structure, in accordance with the law of physi-
ological division of labour. This process, in the vast majority
of instances, takes place step for step along with the increase
in cells, because the individual requires to be a working
organism at every stage of its development. At any develop-
mental stage the organism, as when adult, has a definite
environment to which its structure and vital activities must
correspond or it would perish.
Larva and Embryo.—The environment of developing
organisms shows an infinite variety, but for purposes of
convenience we may distinguish at least two extremes. In
the first, called the Zarva/ type, the ovum, either at the
very outset or before development has proceeded far, is
freed from the parent and lives and fights for itself in the
outside world until, after many changes, it becomes an adult.
This type is common in L£chinodermata and occurs in
Amphioxus.
LARVA AND EMBRYO. 47
The larva is an immature organism functionally adapted
for its external environment at every stage. Very often the
larva passes through a succession of environments before
becoming adult, and the series is known as the ontogenetic
migration of the.species. For example, a cod is a ground-
feeder and lives at moderate depths near the sea-bottom, but
the egg and larva are pelagic, living in the surface-water of
the open sea. The larva migrates inshore to the shallows
before moving out to join its fellows, thus performing an
ontogenetic migration. At each stage its structure is adapted
for its particular environment ; whilst pelagic it is transpar-
ent, when inshore its coloration helps to hide it, and so on.
In the second, or Embryonic type, the developing ovum
is supplied with nourishment, in one form or another, from
the parent and is protected from the outside world by a
shell, or by the body of the parent, until all its earliest stages
are passed, when it leaves its protecting envelope more
or less like its parent.
In the ideal embryonic type cell-formation is completed
before differentiation commences, a condition nearly attained
in the embryo of some vertebrates. Asa general rule, the
lower and more primitive members of a marine phylum
develop by the larval method and the higher members of
marine phyla, together with nearly all terrestrial forms, have
an embryonic development.
The past descent of a group of animals is known as phylogeny, and
in nearly every known instance this past descent reveals a long change
of environment of the successive generations, or phylogenetic migration.
Thus it is usually held that our land amphibians, like the frog, are
descended from aquatic ancestors which must have gradually, as time
went on, migrated from the sea to fresh water and from fresh water
to marsh and eventually to dry land. Doubtless these ancestors were
fish-like in their characters at the epoch when they lived in the sea
and the rivers, but they gradually acquired amphibian characters as the
dry land was reached.
If we picture to ourselves the succession of individuals in this
instance we see that each must have passed through the same stage of
structure as its predecessor and then passed a little further on. Thus
the individual A was a fish and lived an aquatic existence. The in-
dividual B, its progeny, lives in the same surroundings, and by the
primary law of heredity he develops like his parent, but as he has taken
to slightly more air-breathing habits his structure adapts itself slightly
to this change of environment and traces of amphibian characters begin.
His progeny C will tend to resemble his parent and will pass through
the fish-structure of A to the partially amphibian structure of B. Hence
48 SEGMENTATION.
we see that it is only a variant of the /endency for offspring to resemble
their parents that the oztogeny (of an individual) ¢ends to be a rapid re-
capitulation of the phylogeny (of the group). This is termed the primary
Biogenetic Law of Recapitulation. The tendency can only be turned
into an actual fact in those (practically non-existent) cases in which the
ontogenetic migration exactly recapitulates the phylogenetic migration.
From these considerations it is easy to see that an embryonic de-
velopment never conforms to the law of recapitulation, for the environ-
ment of the embryo is at every stage quite different to that of the
corresponding phyletic stage.
A purely larval development may, in the impossible event of an
exactly similar sequence of environments (or migration), conform to the
law. An approximation only to this ideal can be attained and the want
of conformity results in this important truth, that a /arva at a certain
stage of its existence has a given number of its characters which are
palingenetic or resembling similar stages in the phylogeny, and others
which are canogenetic or developed in conformity with the new environ-
ment which has been adopted at that stage. (The palingenetic
characters owe their presence to heredity, the coenogenetic to adapta-
tion, using these terms as applied to the race, not to the individual.) In
the embryonic type the environment is so fundamentally changed that
the ccenogenetic characters usually outweigh the palingenetic and many
of the latter are completely obliterated.
Segmentation.—In larval and embryonic forms alike
there is the same necessity for the conversion of the uni-
cellular ovum into a multicellular organism. This is
attained by rapid cell-divisions or segmentation of the
ovum. In some embryonic types the multiplication is at
first confined to the nuclei, the cell-walls only appearing
later, but this is clearly only a retarded instance of seg-
mentation.
TYPES OF SEGMENTATION.—In many larval types the
ovum segments by a series of binary fissions into a hollow
(or occasionally solid) ball or sphere of cells. The
segments are termed 4/astomeres, are produced in multiples
of two, and are equal. This type is called total equal
segmentation, and occurs in eggs with little or no yolk,
usually termed a/ecithal eggs.
In the majority of developments, however, the egg has
an endowment of nutritive material from the parent, called
yolk, which is the beginning of the embryonic type.
This yolk enables the young form to dispense with the
necessity for ingestion of food. At the same time it affects
the segmentation. If the yolk were evenly distributed
throughout the egg, and not too abundantly, the only effect
SEGMENTATION. 49
of its presence would probably be a retardation of seg-
mentation, but it is usually either aggregated towards the
centre of the egg or in one hemisphere. Eggs with central
yolk are often called centro-dectthal, and those with polar
yolk are often called ¢e/o-lecithal,
In centro-lecithal eggs the segmentation is usually equal,
but the presence of the yolk retards or prevents the inner
part from segmenting; hence this type of segmentation is
called superficial (see Nephrops). In telo-lecithal eggs, if
the yolk be not too great in amount, it merely retards the
segmentation of the hemisphere in which it is situated and
we have a ‘total unequal segmentation (see Frog). In
many telo-lecithal eggs, however, the amount of yolk is so
enormous that it entirely prevents segmentation of the
part occupied by it and the cell-formation only proceeds at
one pole. This is called partial segmentation (see Chick).
There are numerous transitions and modifications of
these types.
SUMMARY :—
1. Equal segmentation (retaining centro-symmetry).
(1) Yotal Equal.— Found in eggs with no yolk (alecithal)
or with evenly distributed yolk.
(2) Partial Equal (superficial).—Found in eggs with yolk
aggregated symmetrically round the centre.
2. Unequal segmentation (showing axo-symmetry, one
pole of the egg differing from the other).
(1) Total Unequal.—Found in eggs with moderate
quantity of yolk, aggregated at one pole.
(2) Partial Unegual.— Found in eggs with a great
quantity of yolk.
Types of Larvze.—Several important larval types are
found in the Jefazoa. Many of them occur in several
groups and with sufficient persistency to indicate that
they represent phyletic stages. We may briefly note some
of the following :—
1. Monoblastic Larve.
1, THE BiastuLA.—The blastula larva is a hollow ball
of cells of one-cell thickness, it is usually free-swimming
and marine, and the cells bear either cilia or flagella,
M. 5
50 TYPES OF LARVAE.
by which it rotates freely about the centre. It represents
; the typical centro-symmetric and
ae aB monoblastic organism. The layer
SECTION OF BLASTULA. oF cells is called the archib/ast and
the internal cavity the archicele (or
blastocele). As the organism is com-
pletely centro-symmetric there can
be no division of labour between the
cells ; hence the blastula represents
the phyletic stage of a colonial protozoan rather than a true
metazoan.
2. MoruLta —The morula differs
from the blastula in having the in-
ternal cavity filled up with cells,
thus forming a solid ball or mul-
berry-mass, It is difficult to imagine
a living adult organism like a morula,
and it is probably a ccenogenetic
larva.
Fig. 17.
SECTION OF MoRULA.
2. Diploblastic Larve.
3. GASTRULA.—This is possibly the most widespread
and important larval form. It is typically of a “bell” shape,
varying from a ‘‘cup” to nearly a sphere or cylinder. Like
Fic. 18 the blastula it is usually a free-swim-
: ae ming marine larva. It has two layers
SECTION OF GASTRULA. OF calls—the outer, termed the ¢fv-
: blast, and the inner, the Aypod/ast.
The internal cavity is termed the
archenteron, and its opening to
the exterior is called the 4lastopore.
The epiblast cells are usually cili-
ca ated, and the larva is free-swimming,
astopore- with motion in a spiral direction
along the long axis through the blastopore. The gastrula
is the typical diploblastic axo-symmetric larva, with physio-
logical division of labour between the epiblast and hypoblast,
the latter being specially concerned with the function of ali-
mentation, the former with those of locomotion, sensation,
and excretion. Its body-plan is much the same as that of
living Calenterata. The gastrula is produced from the
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GASTRULATION. 51
blastula in several ways. The four most important are as
follows :—
(1) Archiblastic Invagination._-This method is common in
the typical (or free-swimming) larva. It consists
of the tucking-in of the whole of one hemisphere of
cells, very much as a hollow india-rubber ball when
punctured may be tucked in. The rim of the hemi-
sphere gradually narrows to form the blastopore.*
(2) Unipolar Ingresston.—Single cells at one spot or
pole of the blastula break away from the archiblast
and migrate inwards, arranging themselves later as
an inner layer, the pole of ingression afterwards
forming the blastopore.
(3) Multipolar Ingression.—Single cells at indefinite parts
of the whole archiblast break away and migrate in-
wards, arranging themselves as an inner layer, a
blastopore being acquired later as a perforation.
(4) Delamination.—Each archiblast cell divides into two
by tangential division and thus the one layer is
converted into two. A blastopore is then formed
as a perforation.
It is probable that multipolar ingression is the most
primitive of these methods of gastrula production and that
it leads, on the one hand, to the very ccenogenetic (or
embryonic) delamination, and, on the other, to unipolar
ingression and finally invagination.
Fig. 19. 4. THE PLanuta.—The planula
SECTION Or PLANULA. bears much the same relation to the
gastrula as does the morula to the
blastula. It is an oval larva, formed
by an outer layer of ciliated epi-
blastic cells, containing a solid mass
of hypoblast in its interior. It is
usually active, free-swimming, and
marine. It is found very commonly in Calenterata and is
a coenogenetic modification.
Hypoblast
Epiblast
* In embryonic developments with much yolk the epiblast cells may
grow gradually over the hypoblast cells, as the latter are too large to be
tucked into the former. This type of gastrz/atzon (formation of gastrula)
is termed efzbolic in contrast to the true invagination, often called embolic.
52 ORIGIN OF MESOBLAST.
3. Triploblastic Larve.
_There is great variety in the external form of the triplo-
blastic larvee and a description of each will be found in the
account of the phyla in which they occur. The most important
are:— Bipinnaria and Pluteus (Echinodermata), Tornaria
(Balanoglossus), Trochophore (Annelida and Mollusca), Naup-
lius (Crustacea), Chordula (Atriozoa), Tadpole (Amphibia).
The third layer or mesoblast develops from the hypoblast
in the same variety of manner as does the hypoblast from
the archiblast. Hence the mesoblast may arise by invag-
ination, ingression, or delamination.
With the origin of the mesoblast the diploblastic larva
becomes plano-symmetric; hence the mesoblast usually shows
a more or less paired arrangement. The hypoblast arises
by one invagination or by one ingrowth, but the mesoblast
arises by never less than two rudiments, which soon become
arranged laterally.
There is great variety in the details, but after the
mesoblast is established it mwsually shows the following
characters :—It consists of a more or less complex double
layer of cells, of which the outer layer lines the epiblast and
the inner covers the hypoblast. These two layers enclose a
spacious cavity called the ce/om, which usually is filled with
a nutrient fluid. The ccelom is not usually continuous but
it may be divided in the median plane by dorsal and
ventral mesenteries, which are double, and serve to support
the hypoblastic canal; or it may be divided up by lateral
mesenteries or septa running transversely to the long axis of
the organism. The mesoblastic walls later form the muscles,
skeletal tissue, gonads, and partly the excretory organs;
and the ccelom often communicates with the exterior by
paired canals called nephridia.*
The calom is therefore a cavity entirely surrounded by
mesoblast ; its walls give rise to the muscular, skeletal and
reproductive systems; and it usually communicates by
paired apertures or canals to the exterior.
It may arise in continuity with the cavity of the hypoblast or
archenteron which is obviously the case when the mesoblast
arises by invagination. This origin is called enxéerocalic.
* This should be compared with the types of structure in Chap. III.
ORIGIN OF ORGANS. 53
In other instances, it may arise as a split in a solid mass
of mesoblast which has been itself produced by delamination
or by polar ingression. This origin is called schzzocelic.
A third origin of the coelom is found in the case in which
the mesoderm arises by multipolar ingression. In this case
the ingressive cells arrange themselves in two layers to en-
close the ccelom, which is thus a transformed part of the
archicoele. Hence this origin is called archicelic,
The same methods of origin for the archenteron of
diploblastic larvee can be made out. It will, however, be
clearly seen that the origin of the layer itself (hypoblast or
Fig. 20.—THE ORIGIN OF AN ORGAN.
The upper row shows the cytic origin by single (dark) cells detached from
parent layer (light). The middle row “shows the dermic origin and the lower
the tomic,
mesoblast), and not that of the cavity, is the important con-
sideration. We must regard the primary layers of hypoblast
and mesoblast as ovgans, and as such they arise according
to circumstances in any of the ways in which an organ may
arise. These may be conveniently generalised as follows :—
1. Asa number of detached cells from the parent layer
(cydzc). These may be diffused or localised in their origin.
2, As a layer of cells or epithelium detached from the
parent layer (dermic).
3. As ahollow sac of cells invaginated from the parent
sac (tomic).
Organs originating in thes? ways from the three
primary layers form together the complex organisms found
54 METAMORPHOSIS. + >
in the animal kingdom. In a very general way, the organs
usually arise from the three primary layers as follows :—
1, Epipiast.— Epidermis.
Sense-organs and nervous system.
Excretory system (partly mesoblast).
2. Mrsopiast. —Muscular system.
Skeletal system.
Blood-vascular system.
Reproductive system.
3. Hyposiast.—Alimentary system.
Fig. 21.—THr METAMORPHOSIS OF THE SILK-worM MOTH.
The larva or caterpillar spins a cocoon and changes into a pupa (on the stem)
which later gives rise to the winged moth. Both sexes are shown.
Metamorphosis.—From the foregoing we see that
in the course of time any stage in the life-history of a species
may, to meet a special environment, be especially and cceno-
genetically developed until a larval stage is produced in
marked contrast to the adult.
In some instances this independent evolution of two
stages in the life of one organism has reached such a
climax that the adult stage can only be reached by an
METAMORPHOSIS. 55
entire reconstruction of the larva. The reconstructive
stage is known as a pupa (or pupal stage), and the whole
change is termed a metamorphosis.
The insects show us a complete series in the origin of
metamorphosis. One instance, of the silkworm moth, must
suffice; the caterpillar, or silkworm, is a worm-like larva
which lives often for a considerable time with all the
functions active except that of reproduction. It then
becomes transformed into a quiescent pupa and a number
of its organs are broken down and others constructed until,
finally, the perfect winged moth is set free.
Some instances of this avergent evolution of two stages in the life of
one individual have a deceptive likeness to the growth of a fresh indi-
vidual or generation upon the preceding one.
Summary.—
The individual commences life as a fertilised unicellular ovum.
By growth, cell-division and differentiation, it is converted into the
adult organism.
The early cell-division is called segmentation, which varies in type
according to the quantity and arrangement of the yolk.
Segmentation usually results in the production of a monoblastic
stage, with one primary layer, or archdblast.
The archiblast is converted into two primary layers, the epzblast
and hyfodblast, forming a diplodlastic stage.
The adult may remain at this stage or the third primary layer,
mesoblast, may be produced, forming a triploblastic organism.
Each primary layer then produces a series of ovgaxs in regular
sequence.
The primary layers and the other organs all arise by one of three
methods.
All or part of a development may be /arval or embryonzc.
In larval development, a divergent evolution of larva and adult
produces a metamorphosis.
56 GEOGRAPHICAL DISTRIBUTION.
CHAPTER VII.
GEOGRAPHICAL DISTRIBUTION.
HE distribution of animals may be divided into
distribution in time and in space. The former is
usually termed Grotocicat distribution, and in the latter
we may distinguish GEocRAPHICAL distribution, divided into
Physical and topographical.
Physical Distribution.—If we take note of the place
of animals in nature we see at once that some inhabit the
land and are zevrestrial, others again live in the sea or fresh-
water and are termed aguatic, and yet others are found
spending most of their life in the air, these being termed
@rial,
The aggregate of animals which are found in one of
these particular habitats is termed the fauna of the habitat,
just as that of plants constitutes the flora.
Hence we can distinguish three primary Aaditats of
animals, called the ‘ferrestrial, aquatic, and e@rial. The
fauna of any one of these may be very diverse and be made
up of animals differing widely from each other in many
respects, but still we shall be able to notice that connected
with each habitat there are certain main structural features
in the fauna, For example, all the erial types must have
some form of wings or organs of flight.
1. Aquatic Fauna.—In this fauna are included the
inhabitants of the ocean, of our seas, lakes, rivers, streams,
and ponds.
With such an enormous diversity of physical conditions,
there are few general features to be discerned. We may at
once divide it into (1) Marine and (2) Freshwater.
(1) Marine Fauns.—The importance of the marine
fauna can hardly be over-estimated. The ocean has been
AQUATIC FAUNA. 57
nature’s cradle, and in it still dwell numerous low types
of animals, which indicate to us the structural plan of the
earliest animals of our earth. ;
If we take the four groups which stand at the base of
animal creation, namely, the Protozoa, Porifera, Celenterata
and L£chinodermata, we find that the sea has an entire
monopoly of the Zchinodermata, a practical monopoly of
the Porifera, and an immense preponderance of the other
two. The same tale is told if we go on to the Polyzoa,
Brachiopoda, and Annelida. It is only when we come
to the Mollusca, Arthropoda and the Vertebrata, that a
considerable number of terrestrial and erial types make
their appearance,
There is evidence for believing that the ocean was
peopled with animal life for many ages before the dry land,
hence it is not surprising that a number of nature’s lowest
types still live on with little modification in the somewhat
similar environment.
Let us recollect that in the structural characters of
animals, both young and old, we have attempted to distin-
guish between the inherited and the acquired, the palin-
genetic and the ccenogenetic. In a precisely similar way
we may discern in marine fauna the palingenetic and the
ccenogenetic inhabitants. In the case of the great majority
of the lower phyla there is no reason to suppose that the sea
has ever been forsaken. The marine Protozoa, Porifera,
Calenterata and Echinodermata, have ever been marine,
but there are a number of marine birds, some marine
mammals (Cetacea and Strenia), and a few marine insects,
which are evidently descended from terrestrial ancestors
and have fallen from their high estate to once more rejoin
their more lowly organised relations in the ocean.
The marine fauna may be sub-divided into :—
Pelagic zone.
Neritic zone.
Abysmal zone.
(a) Pelagic Zone.—Of all the marine fauna, the pelagic
zone includes probably the most primitive types. They
consist of those animals which dwell at or near the surface of
the ocean far away from land. They belong mainly to the
58 FELAGIC ZONE.
Protozoa and the Calenterata, though there are a consider-
able number of Crustacea and fishes and a few representa-
tives of the Mollusca and Tunicata. It is very important
to notice that a great number, if not the majority, of the
neritic types pass the earlier part of their career in the
pelagic zone. Many have pelagic eggs, as, for example,
most fishes, Amphioxus, and a number of Crustacea and
worms, whilst still more have pelagic larva. Nearly all
the important larval types are pelagic, such as the different
kinds of echinoderm, ccelenterate, crustacean and annelid
larve. The blastula, planula, gastrula, trochophore, bi-
pinnaria, pluteus and nauplius are all typical of this zone.
All these perform an onfogenetic migration from shore to
pelagic water and back again, and the most natural inference
is that this is a repetition of a past phylogenetic migra-
tion when the neritic zone was peopled from the open sea.
Throughout the pelagic zone are countless myriads of
microscopic algeze which form the chief food-basis of the
animal life. Hence the food-supply, although of small in-
dividual dimensions, is inexhaustible, evenly diffused, and
easy of capture. Upon these organisms feed the multitudes
of Radiolaria and Foraminifera and swarms of Copepod
Crustacea. The smaller pelagic animals exhibit a perfect
translucency, the only means of concealment from foes in a
region suffused with light. The larger types, of too great a
bulk for this device (such as dolphins, mackerel, &c.), have
the dorsal part of the body of a sea-green or dark-bluish tint
and the ventral part a pearly-white.
We may note that the majority of pelagic organisms
have pelagic eggs and have no connection at any time
of their life with the neritic region. Some of the jelly-
fishes form a remarkable exception to this rule.
Pelagic organisms may be divided into two great groups,
according to their habits, often called the Plankton and
Nekton. These two rather cumbersome words merely mean
the floating and swimming forms respectively.
The Plankton are the lowest and simplest types, and
either drift passively or sustain themselves actively in the
water. Many have air-vesicles to render themselves buoyant
and the majority show axial symmetry (Cydippe and Aurelia
are examples).
NERITIC ZONE. 59
The Nekton swim about actively and determine their
own movements. They are, as a rule, higher and more
complex types and show plano-symmetry (Seda and Sagitta
are examples).
(0) Meritic Zone.—The neritic zone extends from high
tide-mark down to about 500 fathoms. It includes only
the animals found at or near the bottom, and is a zone
containing a rich variety of forms. It can be divided
into two well-defined sub-zones—(r) the littoral, and (2)
the katantic. The littoral sub-zone extends between ex-
treme tide-marks, It has a variety of animals capable of
exposure to great extremes of temperature, and often to
lack of water. Exploration of rock-pools gives one a very
good idea of its inhabitants. There are numerous Zchino-
dermata, Crustacea, Mollusca (especially gastropods), and
Annelida, whilst Calenterata and fishes are common.
We may divide neritic forms into two groups, according
to habits, as in the case of the pelagic. These are the
Nekton, as before, and the Benthos.
The Nekton are swimmers which usually feed upon the
Benthos, less commonly upon each other. They present
many modifications similar to those of the pelagic Nekton,
but can usually be distinguished from them. For example,
a pelagic fish can usually be at once distinguished from a
neritic fish.
The Benthos are a heterogeneous assemblage that live
on the sea-floor itself. We can discern the important group
of sedentary forms which corresponds to the Plankton of the
pelagic zone. They are fixed at one end to a foreign body,
and may have a tube or a burrow. They always show more
or less axial symmetry, and the higher types have a U- shaped
alimentary canal, mouth and anus opening away from the
point of fixation. They also frequently occur in colonies.
They belong to the Protozoa, Porifera, Calenterata, Echi-
nodermata, Polyzoa, Brachiopoda, Crustacea, Mollusca,
Annelida and Tunicata (Sycandra, Obelia, and Actinia,
are examples).
The second group creep or crawl over the surface of the
bottom, their weight being borne by it. These consist
principally of the creeping Mollusca and the crawling Crus-
tacea. These types are important, for they are the first to
60 ABYSMAL ZONE.
become adapted to locomotion over a hard surface, and to
support against gravity upon this surface. From types
which have been so adapted in the past originated all the
land animals, for the same problem in more pressing degree
has to be solved in them.
In marked contrast to these are the sedentary group,
which are never found on land and remain neritic.
The katantic sub-zone resembles in most respects the
littoral but there is great variety and diversity in so large
an area. This zone in a general way has the greater
proportion of our valuable food-fishes, together with great
numbers of bivalve and univalve Afol/usca and Crustacea.
Calenterata, such as corals and zoophytes, are in great
profusion and all the classes of marine fauna are well
represented (Vephrops and Raia are examples).
(c) Abysmal Zone—The Abysmal zone extends from 500
fathoms downwards to the greatest depths of the sea. The
physical conditions of this zone are unique. Below 500
fathoms it is practically certain that no light penetrates,
hence the abysmal zone, so far as natural light is concerned,
is in eternal darkness. The pressure increases rapidly with
the depth so that “at a depth of 2500 fathoms the pressure
is, roughly speaking, two-and-a-half tons per square inch.”
The greatest storms never affect this zone, hence there is
perpetual stillness. The temperature varies enormously
but is always lower than that of the surface-water, in many
cases very low indeed. This is probably due to extremely
slow but widespread polar currents which make their way
along the bottom towards the equator.
No plants can live in this zone for there is no sunlight,
but the pelagic life far above appears to shed downwards a
continual rain of shells and dead organisms. These former
are found in vast numbers in some parts of the ocean. The
floor consists of at least three important sediments called
ooze. The Red mud is found widely scattered in the
greatest depths. It contains the siliceous remains of Radio-
larian and diatom shells. Globigerina ooze occurs in
shallower water (2000 fathoms upwards) and is characterised
by numbers of calcareous shells of Glodigerina and other
foramintfera. Pteropod ooze appears to occur at depths of
about 1500 fathoms upwards in certain tropical regions. It
FRESHWATER FAUNA. 61
has far less lime than Glodigerina ooze as it contains siliceous
radiolarians and numerous pteropod shells. The abysmal
region is peopled by a fair number of species scattered
throughout the same phyla as are found in the neritic zone.
Indeed there is every indication that the deep-sea has been
gradually peopled from the neritic region by immigrants.
All the animals show more or less striking modifications.
All are carnivorous and many are phosphorescent. There
are numerous Crustacea which often attain enormous size.
Many of the large species are blind and of a light carmine
colour. The fishes, as at present known, are few in species,
all bony fishes or Ze/eostez, and of extraordinary appearance.
(2) FRESHWATER Fauna.—The freshwater fauna is very
diverse, as it includes dwellers in lakes, ponds, tivers and
streams. It shows clear indications of having been derived
from the Neritic zone of the marine fauna, though doubtless
in some cases terrestrial animals have reverted to the
freshwater.
The same divisions, into swimmers or Nekton, Plankton
and Benthos, can be made out, but the Plankton are very
few in number, including Protozoa and a few freshwater
Medusze. Amongst the swimmers we can notice two very
primitive orders of fishes, the Gazofdei and Dignoi, which
are confined to freshwater, apparently driven from the sea
by more specialised types.
Two important points should be noted. Firstly, a great
number of freshwater forms can meet the physical vicissi-
tudes of their habitat by encapsuling themselves and
remaining dormant for some time (¢.g., Ameba, Infusoria,
Rotifera, and Tardigrada). Secondly, the eggs and larve are
hardly ever of the floating or free-swimming types, and are
commonly protected by a hard capsule. As the rivers have
been the lines of migration, the eggs and larve would, if
floating, be borne back to the sea.
The primitive freshwater types are especially interesting
as leading along the path towards the terrestrial fauna.
2. Terrestrial Fauna.—The terrestrial fauna has evi-
dently been derived in the past from the aquatic. Only a few
phyla appear tu have effected this migration. Of these the
62 -ERIAL FAUNA.
Vertebrata and Arthropoda stand pre-eminent. In the first
we find the lowest class (/.e. fishes) is aquatic and mainly
marine; the amphibians are freshwater and terrestrial. The
reptiles still cling to the aquatic life but the majority
of birds and mammals are typically terrestrial or zerial.
In the Arthropoda, again, the Crustacea are typically
aquatic and are in many respects the lowest class, but the
Arachnida, and above all the Zusecta, are large and important
terrestrial classes. Other terrestrial animals belong to the
Mollusca (Gastropoda), Platyhelminthes, Nematoda and
Annelida. With the exception of these three lowest phyla,
all show special air-breathing respiratory organs, moisture is
supplied to the food by salivary glands, and iron replaces
copper in the blood. All are plano-symmetric (with very
few exceptions).
We may distinguish several subsidiary divisions :—
1. Cursorial (running). 3. Arboreal (tree-dwelling).
2. Fossorial (burrowing). 4. Reptant (creeping).
3. Afrial Fauna.—This fauna is still more select and
smaller in numbers than the last. Nearly all the birds, a
few mammals, one or two fishes, some extinct reptiles, and
any number of insects make up the group, They are mainly
characterised by extremely active bodies, paired “ wings”
as locomotor crgans, and highly-developed sense-organs.
They resort to the terrestrial or aquatic habitat for their
reproduction and they have themselves been derived from
terrestrial ancestors. (In one or two cases from aquatic.)
SUMMARISING, we may distinguish in the physical
distribution of animals certain habitats which involve special
physical conditions and are inhabited by special faunas.
Of these we can clearly distinguish—
1. Pelagic. 4. Freshwater.
2. Neritic. 5. Terrestrial.
3. Abysmal. 6. Azrial.
There is also evidence for believing that the general
trend of evolutional progress, the phylogenetic migration of
the animal kingdom, has teen from Pelagic to Neritic, from
Neritic to the Abysmal and the Freshwater. From the
Freshwater it has passed to the Terrestrial and thence to
ZOOLOGICAL REALMS. 63
the Afrial. This general conclusion is not vitiated by
the equally certain fact that there have also been cross-
migrations and back-migrations of certain types. Certain
mammals (whales) have obviously reverted to pelagic
habitat and some neritic types (land-crabs) have passed
directly to the terrestrial.
Topographical Distribution.—Just as the animal
kingdom is classified into phyla, classes and orders, so the
world’s surface is divided by zoologists into realms, regions,
and provinces, to emphasise degrees of difference in the
fauna. The same ideal of a natural classification is striven
after, and there is the same difficulty of distinguishing
between resemblances due to parallel evolution and those
due to genetic connection.
The limits of the realms, regions and provinces are
mainly defined by the presence or absence of certain
Mammalia, for, as will be seen later, they are specially
suitable for this purpose. Hence we need here merely
note the chief zoo-geographical realms and leave more
detailed consideration of them to the section dealing with
Mammata.
Zoological Realms—
1. Arcroc@a = N, America, Eurasia and Africa.
2. Neoc&a = S, America, W. Indies and part of
Central America.
3. Notocea = Australia, New Guinea, Polynesia,
New Zealand and certain Malay
Islands.
These three realms are divided into a number of import-
ant regions.
The Regions of Arctogcea are—
Hotarcric = Europe, N. Asia and N. America.
ORIENTAL = India and Further India.
. Ersropian = Africa (South of the Sahara).
. Mavacasy = Madagascar.
SONORAN = United States.
HOO wD
64 OCEANIC ISLANDS.
Oceanic Islands.—We have now to distinguish be-
tween the terrestrial and erial, for the distribution of
terrestrial types is profoundly modified by the present and
past distribution of land surface. Airial types, on the
other hand, are not affected by comparatively large straits
or channels.
This is well illustrated by the fauna of Oceanic [slands.
An oceanic island is an island which has been widely
separated from the mainland either from its very origin or
from a very remote date. Its fauna consists entirely of
immigrants from the adjacent mainland. Its truly terres-
trial fauna is usually small, consisting of small invertebrates,
reptiles, or mammals which may have effected the journey
in logs of wood or by other accidental means. On the
other hand its zerial fauna may be rich, for bats, birds
and insects can easily migrate across the water.
The most remarkable feature is that these serial types,
especially in small and widely-isolated islands, show a
tendency to give up their erial habits and become
terrestrial. Thus “wingless” birds and ‘“wingless’’ insects
are characteristic of oceanic islands. The explanation of
this will be clear after reading Chapter X., but we may only
indicate here that these wingless types are, in most instances,
assumed to be descended from winged ancestors, and that
the very wings which bore their ancestors to the island
would to them be a source of danger, their use involving
a risk of being blown out to sea. The entire absence
of terrestrial predatory forms removes one of the first
necessities for wings; hence the loss of wings resolves
itself into an adaptation to a very peculiar environment.
Discontinuous Distribution. — The consideration
of oceanic islands shows that there is no finality nor
permanency in the fauna of an area. There is the same
ceaseless change and succession of types as we find else-
where in nature. A particular species of animal will spread
slowly from one or more centres and reach a climax of
wide distribution, from which it will slowly recede till
extinction ensues. This extinction will not take place
in regular order, from the original centre outwards, but
will, in most instances, leave isolated remnants of the race
DISCONTINUOUS DISTRIBUTION. 65
in more or less separated areas. Thus is produced the
phenomenon known as “ discontinuous distribution.”
From the causes producing discontinuous distribution
it is evident that such a distribution will prevail amongst
primitive or vestigial types. The mudfishes were at one
time a widely scattered marine order of fishes, but at the
present day the only survivors are the vestiges of the race
which are found in various rivers. Amongst terrestrial
forms, one of the best instances is the archaic Peripatus,
which is found at the Cape, in New Zealand and Guiana,
but not in intermediate districts.
66 GEOLOGICAL DISTRIBUTION.
CHAPTER VIII.
GEOLOGICAL DISTRIBUTION.
“THE past history of animals might conceivably have
been a sealed book to man’s investigations but
fortunately the succession of organisms has left considerable
vestiges behind it. These vestiges, in a general way, are
termed /ossi/s, which are mostly found deposited in earth.
The surface of the earth for a slight but varying depth
consists of a loose soil, but below this there are layers or
strata, formed of various substances, such as limestone,
sandstone, coaland so on. These strata have been gradually
deposited in past ages by the action of natural forces. At
the present time the same process is going on. The dry
land is slowly being broken up by the action of rain, frost
and other agencies, and the finely divided remains are being
carried out to the sea by rivers. There the sediment in the
form of mud and sand is slowly deposited on the sea-floor.
All along the sea-coasts the waves are ceaselessly carrying
on the same work of destruction, the pebbles, sand and
mud being deposited out to sea. Hence the physical
agencies of wind, tide, rain and wave work to a common
end—the reduction of the earth’s surface to a dead level
which, if ever attained, will be some feet below the general
surface of the sea. At present there is a counteracting
force to the attainment of this in the elevation of the earth’s
surface by the active agencies in its interior. :
We must therefore conceive of the whole of the earth’s
surface as a shifting scene of land and water, upon which
the levelling and elevating agencies are constantly at work
in opposite directions. Should the elevating agencies, due
FOSSILS. 67
to the internal energy of the earth, be dissipated, as no
doubt they will in the far future, the dry land would
disappear for ever below the sea.
The products of destruction, in the form of mud, sand,
or silt, are deposited as strata, and in these are found the
organic remains we term /ossz/s. The commonest form of
fossil owes its existence to the power of organisms to
construct skeletons for their mechanical support in life.
These as we have seen are either calcareous, siliceous or
chitinous. They are shed in aquatic organisms into the
mud or sand and covered up by fresh deposits, or in the
case of land animals they may be carried out to sea or into
lakes by floods and other accidents.
In many cases, the skeletons only remain sufficiently
long for a cast of their shape to be taken, the fossil really
consisting of mineral matter but of precisely the same
shape as the original skeleton. Another way in which
fossils may be produced is by impressions. Soft sand
takes an exact impression of any body from a footmark to
a scratch, and in many instances these impressions have
been produced by the soft and perishable parts of an
organism. If mud or some fresh deposit differing from
the sand be then deposited in the impression a permanent
memorial of the organism is preserved in the rocks.
Skeletons and other remains of more recent date may
be found deposited in caves, peat-bogs and elsewhere, little
altered from their normal condition.
The strata of rocks can be arranged or classified by
careful study into a series corresponding with their succession
in time. They are thus divided into five primary groups,
called :—
I. Primordial. III. Secondary.
II. Primary. IV. Tertiary.
V. Quaternary.
These five groups are further subdivided into a number
of Systems. Each group evidently corresponds to a certain
lapse of time, during which it was produced, which is called
an Lyra, and each system represents a lapse of time called a
Period, These may be tabulated as follows :—
68 STRATA.
Approximate
*GROUP—(Era). SVYSTEM—(Period). thickness of Strata.
I. Archizoic. Cambrian.
Ordovician.
Silurian.
70,000 feet.
Il. Paleozoic. Devonian.
Carboniferous.
Permian.
42,000 feet.
III. Mesozoic. Triassic.
Jurassic.
Cretaceous.
15,000 feet.
IV. Cainozoic. Eocene.
Miocene.
Pliocene.
3000 feet.
SS OES ie
VV. Anthropozoic, Pleistocene.
Recent. } 600 feet.
This enormous thickness of about 130,000 feet represents
a vast duration of time and we can only compare one part
with another.
It has been estimated that at the present time the average
rate of deposition may be taken as about 1 foot in 1500
years. This would give us about 200,000,000 years, which
with corresponding periods of elevation might be 400,000,000
years. Such a calculation is really of practically no value as
there are many factors which might easily multiply the
figures.
The Archizoic group have strata in many cases modified
by heat and pressure and they are probably by no means
the first strata. In other words, the origin of animals is
antecedent to the Archizoic Era. Thus, the strata of this
era show Arthropoda, Echinodermata, Mollusca and other
phyla, all sharply differentiated as at the present day.
The geological record does not, therefore, help very
much in giving us the original ancestors of these phyla, but
it forms a very important guide with regard to the higher
animals. Thus, although fishes are found in the Silurian
system the other five orders of Vertebrata only occur there-
after. Hence there is always hope that the geological record
may assist us in tracing the descent of the higher vertebrate
* This table is taken from Heeckel’s ‘‘ History of Creation.”
GEOLOGICAL RECORD. 69
classes, and, in the case of AZammalia, the past history of
some orders has by this means been largely unravelled. If
the progress of evolution has been from lowest to highest
this is exactly the state of affairs we should expect to find,
We may tabulate the classes and the order of their
appearance in time.
Eocene...........
Jurassic...........
Triassic...........
Classes or
Phyla.
(Man.).. ....
Amphibia........
Reptilia
Birds-.cvscsee aes
Mammalia
Arthropoda ......
Mollusca .........
PISCESsieainsicren sas
Brachiopoda.....
Annulata ........
Porifera...........
Coelenterata .....
Echinodermata...
The geological record must therefore be regarded as
merely a last chapter of the history of creation, a chapter
with enormous imperfections and numerous omissions,
written in a language which is capable of many delineations
7O EXTINCT ANIMALS.
depending largely upon the imagination and ingenuity of
the reader.
Lastly, we may note that there are important animal
types which have their origin, and their end, within the
geological record. These are called extinct animals and their
study forms one of the most interesting chapters in zoology.*
Orders and classes which now are represented by a small
remnant ajypear to have flourished in the past in an
astonishing manner until they were replaced by other types.
The proximate agent causing their extinction may in many
cases be obscure, but it is evidently part of a general law
which ensures that the phylum, the class, the order, the
genus, and the species shall arise, flourish, and depart in
the same way as the individual.
* The study of extinct animals is often termed Paleontology, but it
is inseparable from Zoology.
BIONOMICS. mI
CHAPTER IX.
BIONOMICS.
HE term Bionomics is used to denote the study of
the. relationship of an organism to its environment,
in the widest sense. We may here briefly notice (1) The
relationship of an organism to the inorganic world, and
(2) The relationship of an organism to other organisms.
1. Physical Relations.—Many organisms live their
life and pass away, leaving very little, if any, direct material
impression on the world around them. Such may be illus-
trated by Amada or a jelly-fish. Others, again, have by
their resultant energy done a great deal in determining
the present physical condition of the earth. Amongst the
frotozoa there are the Radiolaria and Foraminifera. Their
countless numbers compensate for their microscopic size.
They secrete from the sea-water around them hard skele-
tons, some calcareous and others siliceous, which, on the
death of the animals, collect on the sea-floor in great
quantities. In Chapter VII. (page 60), on deep-sea fauna,
the “oozes” thus formed are alluded to. Whatever may be
the ultimate fate of these oozes, we know that large strata of
limestones, especially also chalk, are often made up almost
entirely of shells of Foraminifera.
Other rock-building forms are the sponges, echinoderms,
certain worms, Cvustacea,and Mollusca, all having calcareous.
skeletons which contribute to the formation of limestone.
rocks, consolidated under water, and then upheaved and
exposed. But the most important rock-builders are the
corals. The ceaseless, united energy of these animals has
resulted in the production of enormous structures, such as
the Great Barrier Reef, extending for more than 1000 miles
along the N.E. coast of Australia.
72 CORAL ISLANDS.
In pure water of a certain temperature the deposition of
lime by corals is very rapid. Coral Islands, or a/o/ds, are, as
a rule, nearly circular or horse-shoe shaped, the inner Jagoon
being shallow and communicating with the open sea by a
channel on the leéward side. There is usually deep water
off the island. Coral Reefs are small and skirt the shore of
an island, frequently as a long ridge parallel to the shore and
some distance from it when they are called Fringing Reefs,
or if they be large and a long way out from the shore
they are called Barrier Reefs. The water outside of a
barrier reef is often of great depth.
Fig. 22.—D1aGram To ILLUSTRATE DaRwin’s THEORY OF
CoraL REEFs.
Third level of sea
with Atoll
Second level of sea
with barrier reef.
First level of
_ sea with
fringing reef,
The slow subsidence of the land causes successive changes of sea-level.
The difficulty in accounting for the origin of coral
islands and coral reefs lies in the fact that the commonest
and best reef-builders do not find suitable conditions of
temperature below about 25 fathoms, whereas great depths
are found immediately outside atolls and barrier reefs. This
is overcome by various suppositions. That connected with
the name of Darwin assumes extensive subsidence of the
land, gradually converting a fringing reef round a peak
into an atoll, the process being so slow that the coral is
always built up to the surface, whereas that which passes
below the 25-fathom line ceases to grow, owing to death
of the animals. Other theories hold that a deeply-submerged
MUTUAL RELATIONSHIP OF ANIMALS. 73
peak can be built up to the 25-fathom line by a rain of
foraminiferan shells, assisted in many cases by deep-sea
corals, and that when once a coral colony is established
cn the summit, it can progress seawards on its own detritus
broken off and rolled down the slope. It may be taken for
granted that a coral colony growing in moderate depths will
reach the surface as a cup or small atoll, by the ordinary
laws regulating the growth of a sedentary organism.
Many marine organisms thus play an important part in
nature’s economy by the formation of chalks and lime-
stones. Others constitute powerful destructive agencies.
As examples we may cite the boring d/o/lusca which
tunnel through wood or rocks.
When we turn to terrestrial organisms, we find that
their efforts are quite as effective in modifying the surface
of the land, though usually they act indirectly through the
plant kingdom.
Earthworms have been shown to have an important
function in burrowing through the earth and passing it
through them. They are nature’s ploughs, and are cease-
lessly employed in bringing fresh soil to the surface, as can
be easily observed in an unrolled tennis-court. The lob-
worm (Avenicola) performs much the same function on the
seashore.
Insects, birds and mammals act on the physical
world mainly through plants. Birds are great distri-
butors of plant seeds, and thus conduce to supplying
oceanic islands and other districts with plants, which them-
selves alter the physical constitution of the islands. Grazing
cattle may denude a well-wooded district of its trees by
feeding on the young shoots, and the loss of forests may
alter the rainfall and other physical conditions, It has
been suggested that the Pampas of Argentina have thus
lost their primeval forests.
2. Organic Relations.—No organism can live with-
out having some action and reaction upon other organisms.
Animals, as we have seen, are either plant-eaters (herbivora)
or animal-eaters (carnivora). This connection, as regards
food, often leads to more permanent connection which is
known by different terms according to its intimacy.
74 MUTUAL RELATIONSHIP OF ANIMALS.
Animals of a similar structure or species often find
it advantageous to seek for food together, either for mutual
protection (herbivora) or for mutual support in attack
(carnivora). These are said to be gregarious.
In some cases, animals of a different kind are found in
partnership. Strange combinations of two or more animals
of divergent structure are found dwelling together. If
this partnership appears to be an equal one, with mutual
benefit accruing, it is termed Commensalism. A good
instance is found in the common hermit-crab, which has a
particular species of sea-anemone living upon its shell. If
one organism obtains all the benefit, then commensalism
shades off into Ectoparasitism: In many instances it is
impossible to decide between the two categories.
Sedentary marine organisms are nearly always intimately
connected. A cockle may havea hydroid zoophyte growing
upon it with Vorticel/a upon the hydroid zoophyte and a few
tunicates with small Folyzoa upon them. A tubicolous
worm may be fastened to the back of an oyster, with acorn-
barnacles covering its tube, andso on. In each case it is
impossible to decide how far commensalistic or ectoparasitic
proclivities predominate.
Ectoparasitism also gradates into the carnivorous habit. The lion
can hardly be termed an ectoparasite on the antelope, but the hagfish
has often had this appellation because it feeds on fish, and a leech is
another difficult instance.
If commensalism becomes still more intimate, and the
two organisms become inseparable in their vital processes,
the union is termed Syzdzoszes. Numerous instances of sym-
biosis occur. One of the best examples is that of radiolarians
and their partners the unicellular alge, termed yellow-cells.
The plant furnishes the oxygen required by the animal, and
itself uses the carbonic acid produced by the animal.
As in the case of the physical connection, so in this
organic union the partnership may be one-sided, in which
case it is termed Lxdoparasitism. In endoparasitism the
parasite depends for nutrition upon its host, living more or
less permanently in its body. Lastly, we can see that the
organic union of Ze individuals is termed a colony. which is
very common in Protozoa, Porifera and Cwlenterata, the
lowest phyla.
FPARASITISM. 75
We may classify the dwelling habits of animals into
Physical Partnership and Organic Partnership, thus :—
PHYSICAL. ORGANIC,
1, SIMILAR ORGANISMS,
Gregarious. Colonial.
2. DISSIMILAR ORGANISMS.
£qual—Commenaalistic. Symbiotic.
Onegual—Ectoparasitic. Endoparasitic.
Endoparasitism.—All animals which adopt the endo-
parasitic habit acquire certain features in common by
adaptation to their peculiar surroundings,
In following out these features we may divide endopara-
sites into two groups :—(1) Somatic and (2) Enteric.
1. SoMatic.—Somatic endoparasites live in the body of
‘their host, usually in the muscles or one of the organs, ¢.g.,
the liver. They feed upon the actual substance of the host,
and are therefore provided with a definite mouth and
alimentary canal. They may, in addition, be often provided
with locomotor organs. Their systems, which are most
modified, are the sensory, integumentary (skeletal), and
reproductive. Living inside their host, all sense-organs are
to them superfluous as they are removed from contact with
the outside world. In a similar manner all protecting
integuments, or exoskeletons, are superfluous. Many crus-
tacean parasites, whose free-swimming allies have a hard
calcareous skeleton, have a soft colourless skin. A loss of
colour is also usual. Lastly, an endoparasite requires well-
developed reproductive organs. Both sexes are usually
represented in one individual, owing to difficulties of fertilisa-
tion, and enormous numbers of eggs are also required. The
number of individuals in one host must be strictly limited,
or the host would perish and with it the parasites. Hence
the young are forced to seek fresh hosts, and the difficulties
and perils of the migration are such that a high fecundity
can alone counteract the danger of extinction. A common
device is the invasion of an intermediate host, which itself
forms an article of food to the original host. If the inter-
mediate host be not an article of the original host’s diet,
76 ENDOPARASITISM.
a further migration has to be instituted, which is re-
inforced bya second reproduction, causing metagenesis (cf
Distomum), It is also usual for the eggs to be provided
with yolk and a hard outside shell, to withstand the vicis-
situdes of the outside world. Thus a somatic parasite is
usualiy characterised by :—
(1) Loss of sense-organs.
(2) Loss of exoskeleton and pigment.
(3) Hypertrophy of reproductive organs.
2. ENTERIC.—An Enteric parasite may go considerably
further in its adaptation. It is usually resident in the enteron
or alimentary canal of its host, and is bathed on all sides by
soluble and diffusible proteids prepared for the use of the
host. Its alimentary organs are therefore superfluous and
atrophy, absorption taking place through the skin. The
intestine of higher animals has rhythmic (peristaltic) con-
tractions which tend to drive egestive products to the
exterior. Hence enteric parasites usually have organs of
fixation, such as hooks or suckers, to attach them to the
intestinal wall. All the characters of somatic parasites are
also shared by enteric, hence the adaptations of enteric
parasites read as follows :—
(1) Loss of sense-organs.
(2) Loss of skeleton and pigment.
(3) Loss of alimentary organs.
(4) Hypertrophy of reproductive organs.
(5) Acquirement of fixative organs.
Tenia and Gregarina are two good examples of highly
adaptive enteric parasites.
Protective Resemblance* and Mimicry.—One of
the most interesting sections of bionomics is the study of
these two phenomena. Protective resemblance comes under
the first heading above (physical relations), for it covers
the cases of resemblance between an animal and its sur-
roundings. In mimicry an animal shows a resemblance to
some other animal. In each case it is usually supposed that
the animal obtains a benefit or immunity from ever-watchful
foes by such resemblance. The simplest cases are those of
protective coloration, in which an animal has the power to
_ * Certain resemblances may be distinguished as aggressive rather than protec-
tive as they are meant to attract the prey or to put it off its guard.
PROTECTIVE RESEMBLANCE.
Fig. 23.—PROTECTIVE RESEMBLANCE.
Two examples of Baczl/us Rossii. A European stick-insect.
Fig. 24.—THE LEAF-BUTTERFLY OF INDIA.
(Callima inachis. )
On the left is an individual with wings closed ; on the right is another flying,
77
78 MIMICRY.
become of the same colour as its immediate surroundings.
As examples we may cite the chrysalides or pupz of many
butterflies, which .
Fig. 25.—HYPOLIMNAS MISSIPUS. may be any shade
of brown, golden or
green, according to
their surroundings.
The common frog,
the cuttle-fish,cham-
eleon, and many
fishes are familiar
examples. Protect-
ive coloration isalso
of almost universal
occurrence amongst
mammals. The out-
line of the body is
destroyed by spots
or stripes or there is
a uniform colour
like its surround-
ings. In othercases
there may be an
almost ludicrous re-
semblance to inani-
mate objects or parts
A. A male.
B. Same species but a female mimicking C.
C. Danais ss ae a noxious species unmolested of plants. We may
bird:
Png take as an example
the familiar Indian Cadiima. This butterfly has the upper
surface of the wings gorgeously coloured with yellow, white
and metallic blue. On the under surface there is a dull
brown pattern which closely resembles the dried leaf of a
common tree. When the butterfly settles the wings close,
and the sudden change from a bright colouring to a dull
leaf-like tint and shape serves to effectively remove it from
the vision of its pursuer. It should be noted that we have
here a,“‘contrast effect.” The more gorgeous the upper
surface the more sudden and effective is the change.
Hence the bright colours of the upper surface may
indirectly conduce to protection. Other insects imitate the
droppings of birds and thus obtain immunity.
°
MIMICRY. 79
An instance of mimicry is shown in Fig. 25. Certain
brightly coloured butterflies (Danazs) are of acrid taste
and hence secure immunity from foes. Other butterflies
(Aypotimnas), by closely imitating their coloration, share in
the same immunity although themselves not endowed with
the acrid taste. In this instance the mimicry is confined to
the female sex. Some common flies in a similar manner
mimic the colour and manner of wasps, and hence in-
directly make capital out of the wasp’s sting.
In a very general way coloration in the animal
kingdom is supposed to either secure concealment to its
possessor by harmony with its surroundings or immunity
from attack to its armed possessor by a warning display
of bright tints, but with our present knowledge there appear
to be numerous unexplained exceptions.
Fig. 26.—AN EXAMPLE OF PROTECTIVE RESEMBLANCE.
-
~
~
Thejcommon British Orange-tip Butterfly. The upper surface is white, with
a large orange patch on each wing. The mottled green and white under-surface
is seen in the figure.
80 HEREDITY AND DESCENT.
CHAPTER X.
HEREDITY AND DESCENT.
T is an everyday observation that an organism resembles
itsparents. This tendency to structural resemblance in
genetically-related forms constitutes the principle of Heredity.
On the other hand, it is equally evident that the offspring
is never identical in structure with either parent or even
its immediate ancestors ; there is always a structural diver-
gence which constitutes the principle of Variation. A
cursory inspection of a flock of sheep might fail to furnish
any individual variations, but a closer study would reveal
slight differences, which might enable one to discern the
particular sheep which had hereditary resemblances, or
were relatives of each other from others similarly con-
nected. A still more intimate acquaintance with them,
such as possessed by the shepherd, would lead to the
recognition of individual variations sufficiently definite to
identify each sheep at once by sight. In the ordinary
way the similarity due to heredity far outweighs the
dissimilarity due to variation. Hence a rabbit may
differ in small particulars, such as colour of hair or shape
of head, from its parents, but nevertheless it resembles them
in the vast preponderance of structural characters, which
we understand by the name ‘‘rabbit.” The individuals ex-
hibiting the small differences are often termed varzeties, those
exhibiting the more fundamental resemblances being termed
species. There is no real distinction between these two, as a
certain number of individuals of one species may form a
marked variety to which the appellation of a separate species
is a mere matter of opinion.
The subject of heredity is intricate and it is difficult to lay down
any general law governing the principle. Individuals having the same
parents differ widely from each other. Some varieties or races have
a much greater power of transmitting their structure to their offspring
EVOLUTION. 81
than others. These are sometimes termed re-foten/, and, as a general
rule, the male is probably pre-potent over the female. A peculiar form
of heredity of very doubtful occurrence is 7eZegony. If ashigh-bred
bitch have a litter to a mongrel it is a commonly accepted tradition
amongst breeders that future litters, although to high-bred dogs, will be
contaminated by mongrel characters. There is at present no definite
evidence for the occurrence of telegony.
But heredity is not confined to parents or others of the preceding
generation. Many structural characters are transmitted to the second
generation leaving the intermediate generation apparently unaffected.
Insanity is a remarkable instance of this.
Lastly, an individual may exhibit characters which resemble its more
remote ancestors. These characters are, of course, variations from the
point of view of the parents and are often termed a¢avistzc.
The term Reversion is also used to describe this phenomenon as well
as the wider return of a whole species to ancestral structure (see Colusa).
Evo.ution.—If we apply the principle of heredity to
the whole animal kingdom we are led to explain the
structural similarity of genera, orders, classes, and phyla as
due to a common descent from the same early types. The
process of descent with modification is called Lvolwtion,
and hence an evolutionist is one who holds that all living
animals are genetically connected in the past.
Assuming this to be the case, how can this descent or.
evolution have been effected? If we are able to show that
one species can, under certain conditions, evolve into
another, the same argument will apply to the higher grades,
such as genera, orders, &c. If there were no variation,
each generation of a species would by heredity be like its
predecessor and no structural change could be effected.
But we have seen that, owing to variation, the offspring
never quite resembles its parents, and it is evident that
if these differences could be accentuated and made per-
manent, an eventual transformation of the species could
be effected.
Darwin was the first to show that there are certain
conditions in nature which make this actually possible.
Starting with the first principle of variation, as stated
above, he went on to show that all animate nature tends to
reproduce itself at a far higher rate than the available means
of subsistence. This is the direct cause of the struggle for
existence. Every animal in nature has to struggle for its
very means of subsistence with other animals and above all
M. 7
82 SEXUAL SELECTION.
with its own kind. The inevitable result is the survival of
the fittest, t.e., those which are best adapted to their environ-
ment live and reproduce their kind, and the less fit die early.
The net result is a selection of the superabundant offspring
which, as it occurs throughout nature, has been termed
Natural Selection. Darwin’s theory of evolution by Natural
Selection therefore depends upon two main principles or
natural phenomena : -
1. Variation, or the structural differences between a parent and
its offspring.
2. The struggle for existence, due to production of offspring above
the means of subsistence.
The second principle acting upon the first #us¢ result in a
selection of the variations. All variations which tend to
higher efficiency are preserved and intensified through many
generations till a fresh species is produced.
Animals under domestication are not, as a rule, subjected
to a struggle for existence, and hence there is no natural
selection. Man has, however, persistently selected the
variations which appealed to his fancy, and by this a7tziczal
selection has been enabled to produce the numerous breeds
of dogs, horses, cattle, pigeons, rabbits, &c. In this case
the mental faculties of man perform the selective opera-
tion which is automatically effected in nature by the fierce
struggle for life.
It is questionable if these artificially produced ‘‘ breeds” are really
comparable to the natural ‘‘ species” for, if the breeds are left to them-
selves, rapid intercrossing results, in a few generations, in the
disappearance of the ‘‘ breed” characteristics and a reversion to the
primitive ancestors from which they were originally derived.
SEXUAL SELECTION.—In certain cases, especially among
the higher animals, the female individuals exercise a selective
faculty among the males. Contests of various kinds take
place among the males, and the successful competitors
alone pair with the females. This form of natural selection,
termed sexual selection, probably accounts for the production
of the secondary sexual characters referred to in Chapter V.
(See page 44.)
Let us apply the Darwinian theory of evolution to
the case of oceanic islands referred to in Chapter VII. (p. 64).
Suppose a number of winged insects have been blown by a
SEXUAL SELECTION. 83
high wind to a small oceanic island and have successfully
established themselves there. Among the variations pro-
duced in the fresh generations some will have larger and
better-developed wings than others. These will run more
risk of being blown to sea and perishing, whilst their wings,
being no longer required for spreading the species nor
for protection against terrestrial enemies, become a
positive handicap in the search for food. Ina few genera-
tions the variations with smaller wings will become pre-
dominant and eventually a wingless variety will be produced.
Again, we obtain from the same principles a plausible
explanation of the extraordinary phenomena of Protective
Resemblance and Mimicry referred to in Chapter IX.
(See page 78.)
An “accidental” variation causing an individual to bear
a faint resemblance to an inanimate object may be sufficient
to give it partial immunity from ever-watchful foes, and such
variations transmitted and accentuated may in time produce
these phenomena, which appear to imply such purposeful
resemblance.
The student should be careful to recognise that Natural Selection
is only a step, however important, in the explanation of evolution.
Zoologists are still groping in the dark with respect to the origin
and transmission of variations and the factors determining heredity.
The most important question pressing for solution is—Does Natural
Selection work through the experimental method of selecting from a
number of zuzdefinite variations, or are the variations produced in a
definite manner in response to the environmental needs? The only way
in which the variations can be definitely related to the environmental
needs is as follows :—During the life of an organism, especially during
its early stages, it is susceptible to external impressions which leave an
indelible mark upon its adult structure. Two individuals with the
same parents and the same hereditary tendencies may be subjected to
environments so dissimilar that they become structurally adapted in
different directions. These adaptations are called acguired characters
(see Introduction). If we assume that the offspring of these individuals
have the acquired characters transmitted to them, even in a modified
degree, then the acquired characters of one generation become the
hereditary characters of the next and the adaptation in nature has a
simple explanation. This theory of evolution involving the Trans.
mission of Acquired Characters is connected with the name of Lamarck.
The transmission of acquired characters has never yet been experiment-
ally demonstrated and has been strenuously denied by Weismann and
others. . Should such a process be indubitably proved to take place
in nature, natural selection would take a subordinate position as a
84 EVIDENCES FOR EVOLUTION.
factor in evolution. On the other hand, if hereditary variations are
all indefinite, and natural selection can only act when favourable
variations chance to occur, then this factor is all-important in causing
evolution. The difficulties in this assumption are—firstly, the enormous
time required by the theory of probabilities for the occurrence of
favourable variations; secondly, the inability of natural selection to
operate till the variations are sutficiently great to become of some vital
importance; and lastly, the necessary assumption that living matter
does not conform to Newton’s third law of motion, reactions in the
form of variations being produced with no correlation to action of the
environment.
EVIDENCES FOR EvoLUTION.—We may conclude this
chapter by mentioning a few of the chief evidences which
lead zoologists to believe in the evolution of the organic
world :—
1. The animal kingdom can be arranged in a series,
according to structure, which forms a more or less unbroken
gradation from lowest to highest.
2. Certain structures, called vestiges and rudiments, can
be best explained as examples of parts of an organism which
are either in their earliest or their last stages of evolution.
3. On an non-evolutional hypothesis the species should
and must form the lowest unchangeable unit, and yet it is so
variable that it is found quite impossible in any particular
case to define a species.
4. Series of fossil forms have in certain instances, e.g.,
the crocodile and horse, enabled scientists to actually re-
create all the stages in the evolution of the group.
5. Facts of geographical distribution, such as the fauna
of oceanic islands and discontinuous distribution, are un-
explainable by any other hypothesis.
85
Part 2.
——@ --—
CHAPTER XL
TYPES OF PROTOZOA.
AMCEBA. PARAMCECIUM. VORTICELLA. GREGARINA.
: IL.—AMCBA.
Sus-KINGDOM PROTOZOA.
PHYLUM GYMNOMYXA.
CLASS RUIZOPODA.
Fig. 27.—AM@&BA Proteus
(Magnified).
A, In the encysted state; B, C, D, different
shapes assumed ; 4, pseudopodia.
Ameeba Proteus is a microscopic organism com-
monly found in the mud of ponds and
streams. It varies considerably in size, the
average diameter being about 34 to zd, inch.
The whole body is of no definite outline, but looks like
an irregular transparent drop of semi-fluid jelly.
Size and
Habitat.
86 PROTOZOA.
Tf the outline of an Ameba be sketched every few
seconds and the series of drawings examined, it
will be seen that the shape has undergone con-
siderable change, and it will also probably be
found that the whole animal has moved somewhat from
its original position.
Further, this. peculiar change of shape is evidently due
to the pushing outwards of processes, which gradually
grow larger and larger. If a drop of treacle
be placed on a flat surface and the surface
gently inclined, the drop will progress in a similar fashion
External
Features.
Movements.
Fig. 28.—Ama@BaA Proteus (Magnified).
A large diatom is enveloped on the left. The largest sphere
is the contractile vacuole ; the smaller is the zucleus.
by processes thrust out in the direction of least resistance ;
but the treacle processes differ from those of Ameda,
because in the former the energy for such movement is
derived from without (the acceleration due to gravity),
and they are hence only in a downward direction,
whereas in the latter the energy is provided by the
chemical decomposition of the protoplasm itself, hence
their direction is determined by other factors and the
AMGEBA. 87
movements are effected in any direction on a flat surface.
Lastly, the treacle processes cannot be again withdrawn
unless the inclination of the surface be reversed, whereas
the processes of Ameba may be, and often are, withdrawn.
These processes are termed pseudopodia. If the flow of
protoplasm be maintained in one particular pseudopodium,
it results in a locomotion of the whole animal.
The body is not the same throughout in appearance and
structure. A thin superficial layer of more dense and clear
protoplasm, the ectop/asm, can be distinguished
from the more fluid interior formed of exdo-
plasm. In the endoplasm we can discern numerous
bodies. The majority of these are food-particles which lie
in small spaces, called /ood-vacuoles, but in addition we
can usually recognise the uclews and the contractile vacuole.
The zucleus lies loosely in the endoplasm and appears in
the living animal like a clear glassy sphere. It consists
of nucleoplasm differing somewhat in composition from
protoplasm.
_ The contractile vacuole is a large spherical space filled
with colourless fluid and always lies in or immediately
below the ectoplasm. It gradually expands in size and then
its walls suddenly contract. A temporary passage or duct
is formed through the ectoplasm to the exterior by which
the fluid is extruded. The same process is then repeated.
This contractile vacuole is usually interpreted as an
excretory organ for removing waste nitrogenous matters.
Lastly, there are scattered throughout the endoplasm minute
granules, the meaning of which is not known, small regular
crystals, and particles of debris such as sand grains.
If Ameba be subjected to a rise in temperature the
movements become more and more active, but when a
temperature of about 35°C. is reached the pseudopodia are
withdrawn, the animal assumes a contracted spherical shape,
and at about 40°C. it perishes. Ameda also reacts to chemical
and electrical stimuli, but in every case the whole protoplasm
reacts. In other words, there are no definite sense-organs
nor nervous system. :
We have already seen the method of locomotion by
pseudopodia. When Ameda in the course of its slow
Structural.
88 PROTOZOA.
peregrinations comes across one of the microscopic algse*
upon which it feeds, the protoplasm flows
round the alga which passes through the
ectoplasm into the endoplasm, the former closing up
behind it. This is the process of zugestion of food, and
with the alga is usually ingested a small drop of water
which constitutes the food-vacuole. In the endoplasm the
food is slowly digested ; its insoluble proteids are converted
into soluble and diffusible proteids which then pass into
the substance of the endoplasm, The cellulose walls of
the alga and the siliceous coats of some are not digestible,
and they are extruded or egested by the inverse process by
which they were ingested.
Two points are important. /7rs¢/y, ingestion may take
place at any point of the surface as Amba has no localised
mouth or ingestive aperture, and the same remark applies to
egestion and the azws or egestive aperture. ‘
Secondly, the food of Ameba appears to be confined to
the class called proteids which are themselves constituents
of protoplasm. It is said that Amedba cannot digest carbo-
hydrates or fats, hence it does not build up its protoplasm
from lower chemical constituents. Ameba cannot live
without free access to oxygen and it exhales carbonic acid.
As there is no definite respiratory organ the whole surface
of the animal must act in this capacity. The visible effect
of good feeding and equable surroundings
upon an Ameba is an increase in bulk—it
grows. When a certain size is attained, the nucleus
divides in two and then the protoplasm. Two equal-sized
individuals are produced from the one by Jd:nary fission
or splitting into two. The parent individual ends its life
at the moment of reproduction in giving rise to two fresh
individuals.
The process of conjugation (page 39) is said to take place
but it has not been fully followed in Ameéba.
Under unfavourable conditions, such as drought, Ameba
has the power of withdrawing its pseudopodia or becoming
spherical. The ectoplasm secretes a thin hyaline case or
Alimentative.
Reproductive.
* The food consists of diatoms, desmids, spores of alge and other
vegetable matters, but animal matter such as fragments of rotifers and
of Protozoa such as Avcella have also been observed in the endoplasm.
PARAMCGECIUM. 89
cyst. Under the protection of this cyst the excysted Ameba
lies dormant. All the active vital processes are suspended
and are only resumed in more favourable surroundings.
Such a cyst is termed a hypnocyst. Encysted Amebe are
doubtless transported from pond to pond by the wind or
other means.
Such is the simple structure and life-history of this
little animal. It may be taken as a type of the Sub-
kingdom PROTOZOA, for it is a single cell and its
vital activities are conducted within the limits of this cell.
It is a type of the phylum GYMNOMYXA (naked jelly),
for its protoplasm is freely exposed to the surrounding water.
The whole surface of the body performs the functions of
ingestion, egestion, sensation, and respiration. Lastly, it is
a type of the class Ruizopopa, for the protoplasm throws
out blunt pseudopodia.
II.—PARAMCECIUM.
Suxs-KINGDOM PROTOZOA.
PHYLUM CorTIcaTa.
Cass CILIATA.
ORDER HOLOTRICHA.
Paramcecium caudatum is a minute freshwater
animal which can be easily distinguished with the naked
eye. Its body is flexible but has a definite shape. It is
elongated, cylindrical, and rounded at each end. The
protoplasm in the interior of the animal is of a semi-fluid
consistency, like that of Ameba, but the definite shape is
maintained by a hardened outer part of the protoplasm,
called the cortex. ‘Vhis cortex should be carefully dis-
tinguished from the ectoplasm of Ameba, which is but
slightly differentiated from the endoplasm, and is. too
mobile to affect the shape of the body.
The cortex secretes on its outer surface a thin hyaline
cuticle, which is punctured by numerous minute holes.
Through these holes the cortical protoplasm protrudes in
the form of c/a, short vibratile hair-like processes which
contract forcibly in a definite direction.
go PROTOZOA.
Paramecium can be seen, by the naked eye, to move
with extreme rapidity through the water, and this move-
ment is performed by the uniform layer of cilia covering its
body. ‘There are also scattered all over the body a number
of ¢richocysts or little oval bodies which, upon stimulation,
Fig. 29.—PARAMG@CIUM CAUDATUM.
Lateral view of entire animal from right side. x 60. (Ad nat.)
Thread of Trichocyst.
Food-vacuole.
Micro-nucleus.
Macro-nucleus.
Contractile
Vacuole.
Trichocyst.
Cilia.
The anterior contractile vacuole is shown contracted into a star.
eject from their interior long processes or stings. Lara-
shiney macium not only has a definite shape, but as
* it also has definite organs we can distinguish
a symmetry in the arrangement of its parts. The animal,
in fact, is plano-symmetric, and has a dorsal and ventral
surface, two lateral surfaces and an anterior and a posterior
end (page 23).
Usually the anterior end is directed forwards in move-
ment, but, when required, the animal is quite capable of
“backing.” From about the middle third of the ventral
PARAMGCIUM. gt
surface there slopes backwards a shallow cone-shaped
depression, the vestébule. It is lined by
special cilia, which cause food-currents. At
its base or inner end is an opening into the inner fluid
protoplasm, which is the permanent mouth or ingestive
aperture. Food-particles, usually microscopic algee, are
driven by the ingestive cilia down the vestibule and
through the mouth into the interior of the body. Here
they are then digested. and their residua are egested,
but they appear to follow a definite course within the
organism, first towards the posterior end, then forwards
dorsally and backwards along the ventral surface, to
be eventually extruded or egested at a special spot just
behind the mouth. There appears to be no permanent
opening or anus, but the temporary anus is always formed
at the same spot.
Under the dorsal surface and lying towards the anterior
and posterior ends of the animal there are two contractile
vacuoles which do not differ essentially from the single one
found in Ameba. About the centre of the animal there lies
a large oval macro-nucleus. in close contact with which
there is a small mzcro-nucleus,
There are no definite organs of respiration, sense-organs,
nor nervous system. ‘
Growth in Paramecium is succeeded by binary fission
into two equal parts by an oblique division, but sooner or
later the process of conjugation must intervene in order
that life may be maintained.
We have already dealt with the general phenomenon of
conjugation (see Chapter V.). Paramecium proceeds
normally in conjugation. During this pro-
cess two individuals are in close contact
along their ventral surface, their protoplasm becoming
continuous through their mouths. The essential changes
are as follows :—
(1) In each individual the macro-nucleus breaks up
and disintegrates, to be thrown out or absorbed, and
the micro-nucleus grows rapidly and then divides by
two rapid divisions into four, two of these pieces being
absorbed. Thus by these processes the macro-nucleus
and micro-nucleus are now reduced to two fragments
Alimentative,
Conjugation.
92 PROTOZOA.
of a micro-nucleus. These. fragments are each one-fourth
part of the original overgrown micro-nucleus.
(2) In each individual one of the parts moves across
into the other individual and fuses with the remaining part
of that individual. Sometimes the migrating parts are
termed the male pronuclei, and the other two the female
pronuclet,
(3) Soon after this communication between the two
individuals becomes interrupted and they part. -In the
meanwhile the single-fused nucleus in each divides into two
and then into four, so that each individual has then four
nuclei.
(4) Two quarters pass to each end of the animal and
binary fission takes place. One quarter grows into a macro-
nucleus and the other remains a micro-nucleus. The result
is a pair of offspring with a macro-nucleus and a micro-
nucleus each
This account should be carefully compared with the
remarks in Chapter V. It will then be seen that the
presence of two kinds of nuclei is the principal factor
causing complication.
Paramecium isa type of the Sub-kingdom PROTOZOA,
for it is a single cell with all the vital activities confined
therein. It is a type of the phylum CORTICATA, for it
has a definite shape of the body due to a limiting cortex ;
this involves the important feature of a definite mouth.
In the CORTICATA it belongs to the class Citiata for
its locomotive organs are in the form of cilia. The cilia
are evenly distributed over the surface of the body, and
hence it is a member of the order Holotricha.
III.—VORTICELLA.
Vortzcella is a small freshwater and marine animal closely allied to
the last type, from which it chiefly differs in being sedentary or fixed.
It may affix itself to almost any foreign body, living or non-living. The
body of the animal is bell-shaped with a long stalk. Asin Paramecium
there is a cuticle and cortex. The cilia are confined to the rim of
the bell and produce vortex-currents by which food-particles are
brought to the mouth. The thickened ciliated rim is called the fer?-
stome, and immediately inside there runs a circular groove leading
down at one part into a funnel-shaped vestébu/e. The base of the
VORTICELLA. 93
pe opens into the inner protoplasm by a small aperture—the
mouth.
Under the upper surface there is a single contractile vacuole and
deeper down there appears a horseshoe-shaped macro-nucleus with a
small micro-nucleus. The feeding processes are very like those of
Paramecium as regards the course of the food and the temporary
anus.
Vorticella is, like most sedentary animals, axo-symmetric. The
stalk is straight when expanded, but on stimulation it contracts into
Fig. 30.—VORTICELLA NEBULIFERA
(Entire Colony Magnified).
C.V. Contractile Vacuole. A free-swimming individual
with two rings of cilia is seen on the right.
a spiral coil. The peristome is contracted and the whole bell becomes
spherical. As in Paramecium, there are no excretory nor respiratory
organs.
Pee niacin is by binary fission, the bell dividing down the centre.
In allied species the two fresh individuals remain on the same stalk and so
on for several generations. In these instances a ‘‘ colony” is produced,
but in Vorticella one of the individuals leaves the stalk soon after fission
and settles elsewhere. In such a case we may regard the migrating half
as the offspring and the other as the parent.
94 PROTOZOA.
At any time Vorticella is capable of breaking free from its stalk and
swimming away, and it can also encyst, in which condition it may, like
Ameba, experience considerable vicissitudes with impunity.
Conjugation is effected by one individual setting free by budding a
number of small buds which acquire a second band of cilia and swim away.
One of these settles upon another individual and interchange of
nuclear material is effected. The bud is said to then atrophy, the total
result being the transfer of nuclear material from one individual to
another. In this respect the conjugation of Vortccella more nearly re-
sembles the sexual reproduction of AZefazoa.
Vorticella belongs to the same class as Paramecium (Ciliata) but
to the order Perdtracha, the cilia being confined to 9 ring around the
mouth.
IV.—GREGARINA.
SuB-KINGDOM - PROTOZOA.
PHYLUM CORTICATA.
CLass SPOROZOA.
Gregarina blattarum is a small animal found in
part of the intestine (the mesenteron) of the common
Cockroach (B/atia). Hence it is an endopara-
site. Its body is elongated and has a definite
shape. In the protoplasm there can be discerned an outer
cortex which appears to be more or less contractile and an
inner more fluid medulla: The cortex secretes a thin cuticle
which envelopes the body. At one end, usually
regarded as the anterior end, the cuticle is
thickened into a cap with a rim of hooks, At about one-
third of the length of the body from the anterior end, the
cuticle extends a as thin septum or partition across the
protoplasm, dividing the body into an anterior protomerite
and a posterior deutomerite. In the medullary substance
of the deutomerite is an oval nucleus and occasionally there
can also be seen a small nucleolus.
There are no cilia nor pseudopodia and the animal can
progress only slowly by a creeping movement of the cortex.
There is no mouth nor anus, and no solid food
: passes into the body of the animal. Gvegarina
1s, from its habitat, surrounded on all sides by soluble and
diffusible proteids which have been prepared by its host, the
cockroach, for its own use. These are absorbed by G7e-
garina through the cuticle as required. There appears to
Habit.
Structural.
Alimentary.
GREGARINA. 95
be no contractile vacuole and no nervous nor respiratory
organs.
The cuticular cap serves to fix the animal to the wall
of the intestine in its young stages, but it is shed soon after
the attachment is lost. Conjugation takes
place but in a modified form. Two gregar-
ines become closely opposed to each other but do not fuse.
They together form a sphere which then becomes enveloped
Reproductive.
Fig. 31.—Lire-HisTory OF GREGARINA.
I (After BuTScHLI.)
Protomerite .
Capsule
Deutomerite
Cyst.
“Nucleus
Cortex
One
Individual.
Deutomerite.
Protomerite.
Other
Individual. Epithelial Cell.
x, The adult individual. 2, The cyst containing spores. 3, Asingle spore.
4, Two conjugating individuals. 5, Five stages in the intracellular
parasite, from left to right.
ina cyst. Under cover of this cyst the reproductive pro-
cess is effected, hence it is distinguished as a spovocyst from
the simply protective cyst (or Aypuocyst) of Amada. The
cyst is somewhat complex, for it has small tubular apertures
for the subsequent escape of the spores.
Inside the sporocyst the two gregarines break up by
multiple fission into a great number of small fragments or
spores, each of which secretes around itself a hard case.
Sometimes the conjugates separate and a single Gregarina
encysts and divides into spores.
96 : PROTOZOA.
The degenerate state of the conjugation appears to be of a similar
nature to the degenerate sexual process in certain low fungi, such as
Saprolegnia, in each probably an effect of parasitism. In each case
there is a sort of imitation of the real process although the essential
interchange of nuclear material is absent.
The sporocyst finally bursts and the coated spores are
set free out of the anus of the cockroach. Protected by
the hard coat these spores lie dormant till any
of them happen to be introduced with food
into the intestine of another cockroach. In this event the
spore-case bursts and its contents escape as a creeping
amceboid nucleated mass of protoplasm. This works its
way into the epithelial cells of the cockroach’s intestine
and there remains for some time. It is then termed an
intracellular parasite, living wwéthixn the epithelial cell.
Here it grows and assumes the elongated form and other
characters of the adult. Contemporaneously it gradually
protrudes from the cell into the lumen of the intestine,
still attached by the anterior end with its cap. Finally it
becomes detached and lives free in the lumen or cavity of
the intestine.
Life history.
We may note that there is a definite limit to the number
of gregarines which can dwell in one cockroach, and when
this limit is reached the gregarines would perish with their
host. Hence the gregarines and all endoparasites must
at some time, if the species is to be maintained, migrate
and by some means reach a fresh host. This is not
essentially different from a sheep moving to fresh pasture
after having exhausted the previous one, but in the former
case the probabilities of reaching the fresh scene of action
are infinitely less. The difficulties of the migration are
overcome in two ways :—Firstly, an enormous number of
the migrating units are produced just before the migration,
the number roughly corresponding to the probabilities of
survival; secondly, the migrating units are protected for
their hazardous journey by hard coats or cases. In these
respects the gregarine is typical of endoparasites. (See also
Parasitism, Chapter IX.)
PROTOZOA. 97
THE PROTOZOA.
We have seen in Chapter III. that the animal kingdom
can be naturally divided into two sub-kingdoms,
1. PROTOZOA.
2. METAZOA.
All the Protozoa are homologous with single cells. The
body of a Protozoan is a single cell, and all differentiations
take place within the cell, or are intra-cellular. For
example, the mouth of a Protozoan leads into the interior of
a cell and not, as in the AZefazoa, into a space between a
number of cells. The same consideration applies to every
other organ. This is sometimes emphasised by using the
terms cel/-mouth, cell-anus, &c.
In a number of sedentary Protozoa (cf. Vorticella) the
products of binary fission remain in organic continuity, and
form a “colony” of many individuals. The colony is
evidently a multicellular aggregate, but in the majority of
cases each cell retains all its vital functions of alimentation,
locomotion, sensation, and excretion. Hence there is little
or no united individuality of the aggregate, and it is
regarded as a colony of /rofozoa rather than a metazoan
individual. Ina few colonial Protozoa, such as Zootham-
nium, there is a physiological division of labour not affecting
the primary vital functions, but only between these and the
secondary reproductive function, Some of the individuals
of the colony have no mouth nor cilia, and are themselves
solely concerned with the production of reproductive
elements, depending for the exercise of vital functions upon
the other individuals. This is the nearest approach in
colonial Protozoa to the complete physiological dependence
of the constituent units of a metazoan.
The Protozoa must be regarded as the representation in
miniature of the metazoan type, showing us the possibilities
of adaptation with the single cell as a unit ; hence, although
the sub-kingdom only includes very small and apparently
unimportant animals, it must be regarded as having the
same morphological value as the Aezazoa.
M. | 8
98 PROTOZOA.
GYMNOMYXA AND CORTICATA.
The Protozoa fall into two fairly well-defined Phy/a, in
accordance with an important character. In the Gym-
nomyxa the body of the animal consists of naked protoplasm
which has no definite shape of itself. In many cases the
protoplasm has a shell to which it clings, inside or outside
of it, and under tonic contraction or when the vital
processes are dormant it assumes a spherical shape. The
nakedness of the protoplasm implies a very low differentia-
tion, the alimentary functions of ingestion and egestion being
co-extensive with the surface (cf Ameba). In the Corti-
cata the living organism assumes a definite shape, which is
maintained by a hardened cortex and often a cuticle as
well. The form of the body is not determined each
moment by the forces acting upon it, but a definite shape or
plan is assumed and adhered to for each species. A
definite mouth, definite egestive spot and definite motor
organs are involved. The Corticata are evidently a great
step in advance of the Gymnomyxa, from which apparently
they have been derived.
PHYLUM GYMNOMYXA.
Fig. 32,—TyPES OF FORAMINIFERAN SHELLS (After D’ORBIGNY)
ia
Uvigerina. Bulimina. Calcarina. Peneroplis. Planorbulina.
GYMNOMYXA. 99
Ameba is a type of the single class Ru1zopopa in which
there are pseudopodia, and of the order Zodosa with blunt
or lobose pseudopodia, but there are three other important
orders to.which we may briefly allude.
The Aedzozoa or sun-animalcules are usually spherical in shape; and
are found in freshwater. The pseudopodia are long rays usually stiff-
ened with an axial rod of silica. The central mass of protoplasm is
vacuolated, and some have a hollow perforated shell like those of the
next order, Nearly all are centro-symmetric.
Fig. 33.—A HE LI0z0an (Actinophrys sol).
The entire animal magnified. (Ad nat.)
Vacuole. Nucleus.
Pseudopodial
Ray.
Central Axis of.
Pseudopodium.
Note the central nucleus and stiffened pseudopodia.
The second order is that of the Xadiolaria. They are marine pelagic
organisms of microscopic size and have a siliceous skeleton of isolated
pieces called sfzcules, or a continuous perforated shell through the holes
of which the fine radiating pseudopodia protrude. The main mass of
protoplasm has .a thin capsule dividing it into central and peripheral
portions, and in the peripheral parts there are often found a number of
minute algoid bodies called yellow-cells. They live and multiply in close
organic unity with the radiolarian. Such a union is termed syzdzosds
(see Chapter IX.). Radiolarians are commonly centro-symmetric, but
some are axo-symmetric. Countless numbers of them live and die in
the pelagic water, and their shells and spicules cover the sea-floor at
great depths, constituting radiolarian ooze (Chapter IX.).
The third order, Foraminifera, also consists of a vast assemblage
of small pelagic organisms. They usually have a shell, made of
calcareous, arenaceous or chitinous material. It is often chambered,
100 PROTOZOA.
Fig. 34.-A RADIOLARIAN (Zhalassicola pelagica x 20).
(After HascKet.)
Note the radiate pseudopodia, the vacuolated protoplasm and the central capsule.
and the protoplasm consists of a main mass in and around it and
a fine anastomosing network of thin protoplasmic strands which serve
to entangle the food. The shells of these Foraminifera cover the
sea-floor in various regions, and similar shells form the main constitu-
ent of many chalk-strata. The pyramids of Egypt are built of
nummniulitic limestone which is an aggregate of Foraminiferan shells.
Hence, by virtue of their vast numbers and the imperishable nature
of their shells, the “oram7nifera are an important agency in the physical
changes of the earth’s surface.
Fig. 35.—A Livinc FORAMINIFERAN
(Miliola).
Protoplasmic
Processes.
Chambered Shell.
CORTICATA. IOI
PHYLUM CORTICATA.
The Corticata contain the important class Cruiata, of
which Paramecium and Vorticella are typical. They are
all active organisms, those like Pavamecdum moving rapidly
in pursuit of prey, whilst others like Vordice//a are themselves
fixed and use their cilia to bring food-particles to them.
They are divided into orders according to the arrangement
of the cilia.
The second class is that of the MasticopHora. They
are also small active organisms, often of very minute size.
They have only one, or sometimes two, long whip-like
processes which are called /age//a. The flagellum may be
situated at the posterior end and serve to drive the body
forwards, in which case it is called a pu/seZ/um, or it may be
at the anterior end and may draw the body after it, when it is
known as a ¢ractellum. he ¢ractellum may also by spiral
movements assist in bringing food to the mouth.
In one large section of these MasticopHora, often
placed in a class by themselves, the Choqno-Flagel/ata, the
ingestive action of the tractellum is supplemented by a
“collar” of protoplasm which surrounds the mouth and the
base of the tractellum. Colonial forms are common in
this class.
The ACINETARIA are a spe- Fig. 36.—AcINETA Tu-
cialised class of much the same BEROSA EXPANDED AND
general habit of life as the pre- CONTRACTED.
ceding classes, but there are no
cilia nor flagella. Their place is
taken by a number of fine pro-
cesses terminating in minute
suckers or adhesive discs with
which other Protozoa are caught
and their juices extracted. Most
are fixed and stalked, but some
are free and even parasitic. The
young are often actively ciliated,
and the whole class is probably
derived from ancestral CrLiaTa.
102
PROTOZOA.
The last class is that of the Sporozoa, the members of
which are endoparasitic.
They are found in nearly all the higher animals.
Gregarina is a type of the class.
Mono.
cystis is found in the seminal vesicles of the earthworm
and has a simpler body than Gregarina.
The young are
intra-cellular parasites within the sperm-cells.
The Coccidia are small Sporozoa of simple structure
which occur commonly in the liver of the rabbit and
elsewhere.
They may give rise to tumours and serious
pathological results.
SUB-KINGDOM PROTOZOA,
1 Unicellular or when multicellular the units are not mutually dependent.
z No true sexual reproduction, asexual by binary or multiple fission, preceded
by conjugation.
3. Mostly minute, marine or freshwater.
Puytum I.—Gymnomyxa.
Naked protoplasm with no
definite shape to body.
Class I.—Ru1zopopa.
Type—Ameba.
1. Locomotion by pseudo-
Ppodia.
z. No localised mouth,
diffuse ingestion.
3. Many have achambered,
calcareous, siliceous, or
arenaceous shell.
4. Reproduction mainly by
binary fission.
5. Floating or creeping,
marine or freshwater.
Puytum II,—Corrticata.
A cortex with definite
shape to body.
Class II.—Civiata.
Types— Paramecium ;
Vorticella.
Locomotion by cilia or
flagella.
Localised mouth.
No shell.
Reproduction usually by
binary fission.
Active, moving or seden-
tary, freshwater or
miarine,
Class III.—Sporozoa.
Type—Gregarina.
Little or no locomotion.
Hooks for fixation.
No mouth nor solid in-
gestion.
No shell.
ee re by multiple
fission with coated
spores.
Endoparasitic.
SYCANDRA. 103
CHAPTER XII.
IVPE OF PORIFERA.
SYCANDRA.
PHYLUM PORIFERA.
Cass CALCAREA.
Sycandra compressa isa small marine
sponge of a dull yellow tint, found fixed
to rocks or weeds between tide marks.
It is in shape like a flat-
tened flask and varying in
length up to 14 inch, It
is like all sponges axially symmetrical
(though the symmetry is often obscured),
hence we can distinguish merely a main
axis, a base, and an apex. The base is
fixed to a foreign body and the apex
has a large opening, the oscudum, which Fixed to sea-weed, with bud ,
leads into the interior of the sponge. parila
If the living Sycandra be watched carefully in a vessel of
water, it will easily be seen that currents of water are, with-
out intermission, pouring out of the osculum. Further
examination of the surface of the sponge would reveal an
immense number of extremely minute openings all over the
surface, into which the water perpetually flows. These are
termed the pores. ’
If a hand-section of the sponge be made it is seen
to be hollow, and the wall appears of even thickness all
round the central cavity. This cavity is called the para-
gastric cavity, opening through the osculum to the exterior.
The walls of the sponge are of a somewhat firm leathery
consistency and when boiled in potash the
animal matter is destroyed, leaving a residue
of numerous small spicules, transparent and
tri-radiate in shape. These hard spicules dissolve immedi-
ately, with effervescence, on the. application of any dilute
Fig. 37.—SYCANDRA
COMPRESSA.
Osculum.
Form and
Habits.
Internal
Structure.
104 PORIFERA.
Fig. 38—Catcarrous Trt- acid. They are found to be
RADIATE SPICULES OF calcareous in nature. They sup-
SycanpRA (Grantia). hort the wall of the sponge and
\ ‘form its skeleton. The further
structure of Sycandra must be
followed by prepared microscopic
sections or by teasing to pieces
and examination with the micro-
scope. A transverse section as
seen with low powers is shown
in Fig. 39. The wall here shows
Highly magnified.
Fig. 39.—TRANSVERSE SECTION OF A SYCANDRA (A Sycon).
yeuey syeyx”
SE Inhalent Canal.
The central cavity is the paragastric cavity.
a number of vadial canals, some of them with thick
walls and others with thin, The former open into
the paragastric cavity by small contracted apertures and
are called exhadent canals, whilst the latter open by the
pores to the exterior and are termed ¢whalent canals.
Further examination would show that the two sets of canals
SYCANDRA, 105
are incommunication with each other towards their inner
ends by minute cross-canals, sometimes called prosopyles.
The thickened appearance of the exhalent canal-walls is
due to the peculiar structure of the cells lining them.
These are arranged in a single layer, and they consist of
collared-flagellate cells, closely similar to those found in the
choano-flagellate Protozoa. The currents of water bearing
food-particles are due to the activity of these cells and their
flagella. They are termed choanocytes. The outside surface
ef the sponge is formed by flat irregular cells without
flagella, which are known as finnacocytes. Similar pinnaco-
cytes line the inhalent canals and the paragastric cavity.
The whole limiting surfaces of the sponge are therefore
formed either by a layer of pinnacocytés or of choanocytes.
The space enclosed by the limiting surfaces seems to be
filled with a semi-gelatinous matrix in which are numerous
scattered cells. Most of these are branched or amceboid in
appearance. Some surround and secrete spicules, one to
each cell; these are the sclerocy¢es and they are said to
periodically shed the spicules at the surface of the sponge.
Others are in some way connected with nutrition, and yet
others become ova and spermatozoa. These latter are the
gonocytes or sexual cells, whilst the former are phagocytes.
The alimentary processes of the sponge are not yet
certainly known. Food-particles can be seen to pass in
with the water at the pores and later the choano-
cytes are crowded with them. Further, these~
food-particles may be seen in the phagocytes in the interior
of the body. The choanocytes can withdraw their collars
and flagella and become amceboid,* and it is questionable
whether all the cells of the sponge are not capable on occa-
sion of becoming amceboid, though this may not be normal.
There are no definite excretory nor respiratory organs and
no sense-organs nor nervous system. A few cells round
some of the openings have been described as specially con-
tractile and have been termed myocytes or muscle-cells.
DEVELOPMENT.—Sycandra is dicecious, one sponge producing
spermatozoa and another ova. The ovum is an amceboid gonocyte
which protrudes into the lumen of an inhalent canal till it is fertilised
Alimentary.
* It is ‘more than probable” that the phagocytes are choanocytes which have
changed to the amceboid condition and migrated inwards.
106 PORIFERA.
by an incoming spermatozoon, after which it withdraws into the body
of the sponge and undergoes segmentation. .
The spermatozoa are produced from gonocytes apparently similar to
the female cells. A male gonocyte divides up into a great number of
spermatozoa which are discharged into the water.
The ovum segments totally and equally (Chapter V.) to produce a
hollow sphere of cells, each of which in some other sponges bears a
flagellum. This stage has been compared with the blastula larva of
other Afetazoa. The cells of one hemisphere then become more
numerous and acquire flagel/a, whilst those at the other hemisphere
remain few, large and granular. The larva escapes from the parent
and swims freely. This larva is only found in sponges and only in
certain of them ; it is called an amphiblastula. The granular cells then
grow round the flagellate cells, forming a sort of invagination of the
Fig. 40.—AMPHIBLASTULA LARVA OF A CALCAREOUS SPONGE.
(After ScHuULzE.)
Flagellate Cells.
ES lls.
Central Cavity. Granular Cells
latter, and the larva settles down by the free edge of the granular cells
upon a foreign body. A sort of metamorphosis then appears to take
place, the cells being largely reduced to an amceboid condition and
withdrawing their flagella. Ina manner little understood the amceboid
cells of the body of the sponge are produced between the two layers.
The nutritive granules in the outer layer are slowly consumed during
this process.
The osculum then opens at the apex, and pores are formed through
the sides. The inner layer then becomes flagellate. At this stage the
whole internal paragastric cavity is lined by flagellate cells. As soon as
the radial canals are produced the collared cells lining the paragastric
cavity become pinnacocytic, and the young sponge comes to resemble
its parent. The development is thus :—
. Total equal segmentation to blastula larva.
. Differentiation into amphiblastula.
. Invagination of flagellate half into granular half.
. Fixation and quiescent amceboid stage.
. Differentiation of ascon stage.
. Modification into sycon (sycandra).
Sycandra may also reproduce asexually by budding. A part of the
body-wall protrudes and acquires an osculum. It then separates from
its parent, or the bud may remain in connection with it and form a
colony.
aunip WN
Fig. 41.—ASscETTA
PRIMORDIALIS
(IL&cKEL).
A simple Ascon. Part of
body-wall is removed to show
Paragastric Cavity (X50).
PORIFERA. 107
PHYLUM PORIFERA.
The PoriFERA or Sponges are a
clearly-defined group. Their true re-
lationship to other AZefazoa is not clear.
They are evidently cell-aggregates with
a large amount of physiological divi-
sion of labour between the cells, and
as they also have sexual reproduction
they are undoubtedly A/efazoa. On
the other hand, the sponges have no
metazoan mouth nor anus — food is
ingested by the cell-mouths of the
choanocytes, so that ingestion, diges-
tion, and egestion are purely intra-
cellular. The cells are not aggregated
into tissues and division of labour pre-
vails more between individual cells
than between epithelia of these cells.
Hence sponges must be regarded as very
simple cell-aggregates, belonging to the
Metazoa. Most sponges are colonial,
the colonies being produced by bud-
ding. In many sponge-colonies the
number of oscula alone indicates the
theoretical number of individuals of
which the colony consists.
Fig. 42.—TRANSVERSE SECTION OF AN ASCON. (Diagrammatic.)
Pinnacocytes.
Choanocytes lining
Paragastric Cavity.
Pore.
108 PORIFERA.
In the phylum there can be discerned at least four
different types of sponges according to the distribution of
the choanocytes.
(1) Ascon type. In this simplest type the whole para-
gastric cavity is lined by choanocytes and there are no
radial canals (Fig. 39).
(2) Sycon type. The choanocytes are restricted to the
exhalent radial canals; inhalent canals and prosopyles are
present (Fig. 42).
(3) Leucon type. The choanocytes are restricted to a
number of secondary radial canals opening into the primary
radial canals (Fig. 43).
Fig. 43. TRANSVERSE SECTION OF PART OF THE WALL
oF A Leucon. (Diagrammatic).
Paragastric
Cavity.
i
einai, In-
R halent Canal.
“Secondary Exhal-
ent Canal, with
Choanocytes.
(4) Rhagon type. The secondary canals are contracted
into small round chambers and only open into the primary
radial canal by a number of exhalent canals or afopyles
(Fig. 44).
PORIFERA. 109
In these four types there will be noticed a progressive in-
crease in bulk of the body of the sponge and a progress of the
choanocytic areas from within outwards. Numerous transi-
tion types are found, and these types are distributed quite
indiscriminately throughout the classes or orders.
Fig. 44.—TRANSVERSE SECTION OF A RHAGON. ' (Diagrammatic. )
Sponges fall into two well-defined classes—(1) CALCAREA,
(2) Non-CALcaREA.
1. CALCAREA.—The Calcarea all have a_ calcareous
skeleton and the collared-cells are much larger than those of
the next class. They are usually Ascons and Sycons, and in
many points they are simpler and more primitive than the
Non-calcarea.
2. Non-Catcarga.—lIn these the skeleton consists of
ceratin (horny) fibres (with or without spicules), siliceous
spicules which may fuse, or there may be no skeleton. The
collared-cells are minute and the canal system is mostly
complex.
IIo PORIFERA.,
Luspongia has only horny fibres, hence its skeleton is
used for domestic purposes. Spongilla is a little fresh-
water sponge, found in lakes and rivers. In many, like
Luplectella (Venus’ flower-basket), the siliceous spicules
welded together make a beautiful network like spun glass.
The external form and habitat of Sponges have infinite variety. Very
few, like -lscef¢a, retain their simple axial symmetry. Large colonies
of indefinite shape are produced, in which the constituent individuals can
only be recognised by the number of oscula. A remarkable little
sponge (C/zova) forms burrows in oyster shells, and a great number
of sponges are commensalistic (see Chapter IX.). Chondrocladia shows a
remarkable protective resemblance to a bleached skeleton of a gadoid
fish, ¢.g., a cod, while others have more or less similitude to stones and
seaweeds.
PHYLUM PORIFERA.
1. Multicellular organisms, with physiological division
of labour between the cells. ya! Metazoa.
2. Sexual reproduction.
3. Axially symmetrical.
4. A central cavity (paragastric) with inhalent pores and exhalent
osculum.
5. A skeleton of calcareous, siliceous or fibrous nature.
6. Mostly marine and sedentary, forming colonies.
Class I.—CALCAREA. Class II.—Non-CALcCaREa.
Type—Sycandra. Type—LEuspongia.
1. Skeleton of calcareous spicules. | 1. Skeleton of siliceous spicules,
horny fibre or none.
2. Large collar-cells. 2. Small collar-cells.
3. Mainly Ascons and Sycons. 3. Complex systems of canals.
HYDRA. Ill
CHAPTER XIII.
TYPES OF C@LENTERATA.
HYDRA. OBELIA. ACTINIA, ALCYONIUM. AURELIA. CYDIPPE.
I.—HYDRA.
PHYLUM -
Crass
Fig. 45.—Hypra VIRIDIS
wirH Two Bups
(Magnified).
CaiLENTERATA.
Hyprozoa.
Hydra viridis is a small fresh-
water organism, which may attain
a length of one-half inch, but is
usually smaller. It is found in
ponds and streams attached to
water-weeds and is of a bright
green colour.
[Hydra fusca is the brown
species ; with the exception of the
absence of green chromatophores
it resembles the above. |
Hydra is axo-symmetric, hence
we can distinguish merely an oral
and an aboral end and axial and
peripheral parts. With the naked
eye it can be seen that
the body is an elon-
gated cylinder fixed at
the aboral end. At the oral end
there is a ring of tentacles, thin
processes which radiate in all
directions. In the centre of this
ring is a small raised part, the
peristome, wpon which is situated
the mouth.
We may notice at once that this
aperture, though usually termed
the mouth, functions both as a
mouth and an anus.
External
Characters.
112 CELENTERATA.
On agitation of the water, Hydra contracts its body and
tentacles till it becomes a round knob, but if left to itself it
will soon expand again to its normal condition.
Very often the body appears tu fork into two
parts each of which has a ring of tentacles.
One of these is a Jud which is destined later to drop off
the parent.
Interval
Structures.
Fig. 46.—TRANSVERSE SECTION OF Hypra (Magnified). (Ad zat.)
Mesogloea.
Ectoderm.—
Endoderm ~
Ccelenteron.
If the animal be killed and preserved and cut into trans-
verse sections, a low-power examination of such sections
reveals the fact that the whole body is a hollow sac, the
internal cavity being known as the cw/enteron. The wall of
the body is of two layers, the outer layer or ectoderm and the
inner or endoderm, between which is a thin supporting
lamella, the mesoglwa.
The ccelenteron may occasionally contain the bodies of
small animals which constitute the food of Hydra.
On examination with a higher power of the microscope
the endoderm cells prove to be arranged in
asingle layer and the cells themselves are
considerably larger than those of the ectoderm. Each cell
contains a nucleus and a number of small bodies scattered
through its protoplasm.
Histology.
HYDRA. 113
Fig. 47.—PorrIoNn OF Boby-waLL oF L[ypRA.
(Highly Magnified.) (dd maz.) |
Muscular
Process.
Interstitial Cells.
Cnidoblast.
Epithelial Cell.
__Vacuole.
Nucleus.
Chromatophores.
Mesogleea.
The chromatophores are numerous spherical bodies with
definite walls. They are bright green owing to the presence
of a green pigment called chlorophyll. This chlorophyll is
characteristic of the plant kingdom (see Chapter II.), and
some have regarded the chromatophores as symbiotic alge
living in the tissues of Aydva. The green tint of Hydra,
already noticed, is due to these bodies which are seen
through the transparent ectoderm. In about the centre of
the body the endoderm cells have one or more large
vacuoles, containing a clear fluid. The fluid is said to be
discharged into the ccelenteron and to be digestive in
function. Other bodies in the endoderm cells may be
recognised as particles of food. The inner ends of the
endoderm cells appear to have no cell-wall, and are either
produced into several flagella or into amceboid-like pseudo-
odia.
E The ectoderm cells are of two kinds, the larger efithelial
cells and smaller xéerstitial cells.
The epithelial cells are arranged in a single layer; each
has a definite cell-wall and a nucleus. In most of them the
inner end is produced into one or more processes, which
are not amceboid but show fine striation and appear to be
specially contractile. They are therefore known as muscular
M. 9
114 CE@LENTERATA.
processes. The muscular processes are pressed closely
against the mesogloea, to which their ends are probably
attached. Ina general way the processes run parallel to
the long axis of the animal though they are somewhat
indefinite in arrangement. Similar processes of the endo-
derm cells run in a circular direction, in a transverse plane.
Fig. 48.—AN EcTODERM CELL, ENDODERM CELL, AND A
NERVE CELL (After JICKELI) CONNECTED WITH
A NEMATOCYST.
Ectoderm Cell. Endoderm Cell.
Vacuole,
Muscular
Process.
__ Nucleus
Cnidoblast.
The interstitial cells lie at the base of the epithelial cells
between their tapering ends. They appear in sections as
simple rounded and nucleated cells. In the living animal
they may be ameeboid. All over the body, but especially
on the tentacles, the interstitial cells give rise to the
cnidoblasts. These grow outwards between the epithelial
cells till they reach the surface. They are large ovoid cells
which develop in their interior a cyst containing a long
thread with barbs at its base and a fluid. On stimulation
the cyst, or zematocyst, discharges the thread or sting which
has a paralysing effect on small animals.
Other interstitial cells accumulate in a mass to form
the germ-cells. The ¢est#is is a mass of these germ-cells
covered by epithelial cells and situated under
the tentacles. The ovary is a similar mass
towards the aboral end of the animal. The spermatozoa
are produced in great numbers by division of the germ-cells
Reproductive.
AVDRA. 115
Fig. 49.—DEVELOPMENT OF THE NEMATOCYST IN
CNIDOBLAST CELLS.
Rudiment of Nematocyst.
: Nucleus.
(——— Thread.
Cnidocil.
and are set free by rupture of the epithelial cells. The
ovum is produced by the growth and enlargement of a
single germ-cell in the ovary, which appears to grow at the
expense of the other germ-cells. It escapes by rupture of
the epithelial cells and is a creeping amceboid cell.
Both testes and ovaries are found in the same animal,
hence Hydra is hermaphrodite. The testes usually appear
and ripen prior to the ovaries, a condition known as
protandric.
ffydra has no definite respiratory, excretory, or sensory
organs, and there are no nervous* nor vascular systems.
Movement is effected by the contraction of the muscular
* Certain stellate ectoderm cells in connection with the cnidoblasts
have been described as nerve-cells. (See Fig. 48.)
116 CELENTERATA.
processes probably reacting with the elastic mesogloea.
The food is ingested and egested through the mouth which
thus functions as a mouth and an anus. In the ccelenteron
it is said to be digested partly by an zxter-cellular process,
consisting of the reduction to a soluble condition by the
digestive fluid discharged from the vacuoles of the endoderm,
and partly by an zvtra-cellular process, the particles of food
being taken into the endoderm-cells by their amceboid ends.
Hydra reproduces, not only by the sexual method, but
by the asexual process of budding. A bud is a simple
process of the body-wall which grows outwards, acquires a
mouth and tentacles and finally detaches itself from the
parent.
The amoeboid ovum containing a few scattered yolk
granules protrudes from the ectoderm of the parent and
is here fertilised. It loses its power of movement, becomes
spherical, and encysts. The cyst is secreted by the ovum
itself and the egg then falls from the parent and remains
dormant for several weeks. It segments by total equal
segmentation producing a blastula. Certain of the cells
then wander into the archiccele cavity to. form the hypoblast,
which is thus formed by multipolar ingression. Eventually
the diploblastic larva escapes from the cyst and elongates.
At one end the mouth is formed by rupture of the layers
and the other end becomes attached. The two embryonic
layers, epiblast and hypoblast, become the ectoderm and
endoderm and the archenteron becomes the ccelenteron.
The following special points in Hydra should be noted :—
. The two-layered body.
. The axial symmetry and sedentary habitat.
. The nematocysts and simple hermaphrodite sexual organs.
. The asexual reproduction by budding.
. The intra-cellular and inter-cellular modes of digestion.
. The protected development and formation of hypoblast by
multipolar ingression.
Hydra belongs to the phylum CCZELENTERATA
because its body is didermic or formed of two layers, and to
the class Hyprozoa because its mouth leads directly into a
stmple coelenteron.
AnBWN |
OBELId. 117
II._OBELIA.
PHYLUM C@:LENTERATA.
Cxass HypDROZzOA.
ORDER HypROMEDUSA.
OBELIA GENICULATA is « small marine organism, usually covering
seaweeds, such as the brown /amznaria, between tidemarks. It has
the appearance of a small plant and is hence often called a zoophyte. It
has a creeping basal portion from which there grow up main branches.
Fig. 50.—CoLoNY OF OBELIA GENICULATA.
(Natural size.)
Main Branch with Polypes.
Basal part of Colony.
Seaweed.
The main branch appears a zigzag, from each corner of which is pro-
duced a small branch. With a lens it can be seen that each branch
terminates in a swollen cup-shaped head or ¢heca, and, if the zoophyte
be alive and undisturbed, a ring of delicate tentacles will be seen
protruding from the theca.
Further examination shows that there are two separate structures—
the outer, hard and non-living part, and the inner, soft and living portion
of the zoophyte. The outer part is called the Zerzsarc, consisting of thin
translucent chitin. It forms the hollow axis terminating in the thecz
or cups. Inside the perisarc is a central protoplasmic axis, called the
cenosarc, which runs up to the thecze and here terminates in small
round bodies, having a ring of tentacles. These are the Aolypes which
conform closely in structure to Hydra. Each has a terminal mouth
inside the tentacles; each has a two-layered body-wall with nematocysts
and ccelenteron. They differ from AMydra in having the aboral end of
the body produced into a long central axis or cenosarc, and sections
show that this ccenosarc is similarly formed of two layers with a central
canal, the ccenosarcal canal, which communicates with the ccelenteron
of all the polypes.
118 C@LENTERATA.
Fig. 51.—COLONY OF OBELIA GENICULATA (Magnified).
‘ Sporosac.
fh Hf
Medusoid. — all
We may therefore regard Ode/ia as formed of a colony of individuals
like Hydra, organically connected by the ccenosarc. In this we are
OBELIA, 119
justified, as it first arises as a single polype individual which buds like
Hydra, but in this case the bud does not become detached. It remains
in continuity with the parent and later buds in its turn. Obelia is
therefore a hydroid (or hydra-like) colony produced by asexual budding.
The perisarc is secreted by the outer layer or ectoderm and is evidently
a necessity to a colonial form to give support.
Occasionally, at the base of the colony, there may be noticed large
ovoid masses completely enveloped in perisarc. These sforosacs contain
modified polypes which have’ no mouth nor tentacles and appear
cylindrical in outline. Later on the sporosac bursts and the modified
polypes are detached from the ccenosarc and become free. They
are then known as medusa.
Fig. 52,—A MEDUSA OF OBELIA.
Seen from the oral surface, magnified. (Ad nat.)
Velum. Sub-umbrellar cavity.
Ring-canal.
Sense-organ.
Radial Canal.
A medusa of Obeléa is bell-shaped with the opening downwards.
The cavity of the bell is known as the szb-umbrellar cavity and in its
centre there hangs the #zazubrzumz upon which the mouth opens. The
mouth leads into a ccelenteron which is continued down the wall of
the bell by four radial canals. These run to the rim of the bell to fall
into a ring-canal, passing completely round the rim. At each of the
four corners, at which the radial canals meet the ring-canal, there is
a sense-organ usually termed an otocyst. These otocysts are connected
by a double nerve-ring. They are probably balancing organs. The
opening of the bell is partially reduced by a-thin membrane or velum
projecting from the edge of the bell. Sections show that the medusa,
like the polype from which it is derived, consists of two layers, ectoderm
120 C@LENTERATA.
and endoderm, but the mesogloea is much thicker and forms the bulk
of the body. The radial and ring-canals_are produced from a continu-
ous ccelenteron by the squeezing together of the two layers of endoderm
in the intermediate parts.
The medusa moves through the water by contractions of the
“umbrella” or bell, which force water out of the sub-umbrellar cavity.
After some time there appear four swellings of the ectoderm lining the
sub-umbrellar cavity, overlying the four radial canals. These are the
gonads or reproductive organs, The medusa is dicecious, the sexes
being separate. The egg develops into a larva which swims to the
Fig. 53.—LaTERAL VIEW OF A MEDUSA OF OBELIA.
Magnified. (Ad nat.)
Mesogleea. Stomach,
‘un A
Sense-Organ.
Manubrium.
Tentacle.
Ring-Canal.
bottom, fixes itself and grows into a young hydroid polype. Thus
Obelia is an illustration of me¢ageneszs or alternation of generations,
the hydroid giving rise to a number of other hydroids, some of which
grow into medusz which in turn give rise to hydroid polypes by
sexual reproduction.
Obelia is also a remarkable instance of physiological division of
labour between the individuals of a colony producing nutritive hydroid
polypes and reproductive medusoids (cf Zoothamnium). The following
differences of Obelta from Hydra should be noted :—
1. Obelia is a compound animal or colony, produced by asexual
reproduction from a simple polype.
2. It has two phases: a sedentary hydroid and a free-swimming
medusoid.
3. It has a chitinous exoskeleton, the perisarc.
ACTINIA. 121
III.—ACTINIA.
PHYLUM CQ@LENTERATA.
Cuass SCYPHOZOA.
SuB-CLAss ACTINOZOA,
ORDER HEXACTINIA.
Fig. 54.—AcTINIA MESEMBRYANTHEMUM,
On the left is an expanded individual with viviparous young escaping from mouth.
On the right is a partially contracted specimen.
Actinia mesembryanthemum is a common marine
organism found between tide-marks. It is, at least ex-
ternally, axo-symmetric and cylindrical-in shape;
when expanded it may be about two inches
long. The base or aboral end is attached to
a foreign body, such as a rock, and the oral end has a
ring of numerous short ¢extac/es surrounding a flat peristome,
in the centre of which is situated the mouth. The exterior
of the body is smooth and of various shades of brown and
green, matching its surroundings The body often has
particles of sand and fragments of shell adhering to it,
which assist in hiding the animal. On stimulation the
tentacles are withdrawn into the peristome, and the whole
animal assumes a rounded and contracted form. So far
External
Features.
122 C@LENTERATA.
the general appearance closely resembles that of a very
large but short and broad Aydrva. An examination of the
mouth, however, will show that it is not circular like that
of Aydra but elongated in one direction, and at each
corner of the long axis there is a small groove called a
siphonoglyph. The walls of these grooves are ciliated and
water apparently passes down one groove and up the other,
even when the rest of the mouth is shut.
Fig. 55.—TRANSVERSE SECTION THROUGH THE UPPER PART OF
A Younc ACTINIAN.
Magnified. (After HerTwic and others.)
Directive Mesenteries.
Secondary
Mesenteries.
“AAR yeqdas-1ajuy
Ectoderm of = 4
Gullet. g &
g2
gs
meted
Gullet. “B
et
Ectoderm. Og
Mesoglcea. 58
OSS 7
Endoderm. Zee e
Secondary Mesenteries. Siphonoglyph.
Hence Actinia is not truly axo-symmetric like Hydra,
but is symmetric about two perpendicular planes, the one
parallel to the long axis of the mouth, the other
at right angles to it. This comparatively rare
form of symmetry is called d-plano-symmetry.
The interior of Actinia yields still more striking
differences. A transverse section through the
lower part shows that the internal cavity or
coelenteron is not simple like that of Hydra, but
is partially divided into a central gastric cavity and a
Symmetry.
Internal
Features,
ACTINIA. 123
number of peripheral cavities by a series of radial mesen-
teries or septa. A section through the upper part shows that
the peripheral cavities run up all round a central gullet or
esophagus derived from the ectoderm, The ccelenteron is
lined with endoderm throughout, but digestion appears to
be confined to the central gastric cavity, the peripheral
cavities being filled with a more or less nutritive fluid.
Fig. 56.—TRANSVERSE SECTION THROUGH LOWER PART OF A
YounG ACTINIAN.
Magnified. (After Hertwic and others.)
Gonad.:
Gastric Filaments of
Directive Mesentery. Gastric
Cavity.
Cavity between Directive Mesenteries.
The free ends of the mesenteries bear numerous gaséric
jilaments which assist the processes of digestion. On the
walls of the peripheral cavities are the muscles and the
reproductive organs or gozads.
The muscles consist of (1) a circular or sphincter muscle
running round a slight rim outside the tentacles. Con-
traction of the circular muscle causes the rim to tighten
over the retracted tentacles like the mouth of a bag. (2)
The longitudinal muscles which run down one special side
124 C@LENTERATA.
of the mesenteries. They originate at the aboral end and
are inserted in the peristome. On contraction they shorten
the animal. There are also diagonal or parietal muscles
across the lower corners, connected with the suction of the
base, and thin radial muscles on the mesenteries.
The mesenteries in a large Actimza are very numerous,
but in the young form there are only six pairs. Of these
the two pairs opposite the siphonoglyphs are called the
directive mesenteries and can be recognised by having the
muscles on their outer swzfaces. The muscles on the other
four pairs are opposite each other on the inner suzfaces of
each pair. The cavities within the pairs of mesenteries are
termed zztra-septal, those between the pairs are known as
inter-septal.
All six pairs join the gullet. The subsequent mesenteries
grow from the outer wall in pairs towards the centre. They
always have opposite muscles, never join the gullet, and
arise only in the inter-septal cavities. ‘They are known as
secondaries, tertiaries, guaternaries, and so on, and continue
to grow and increase in number throughout life.
The cellular structure of the anemone is somewhat in
advance of that of Hydra, The ectoderm contains nemato-
cysts, sensory cells and unicellular glands,
Scattered nerve-cells have also been described.
The mesoglcea is a thicker layer than in Hydra and passes
along the mesenteries. The endoderm contains, as in
ffydra, flagellate and ameeboid cells and also glandular
and possibly sensory célls.
Actinia reproduces both sexually and asexually. Buds
are periodically produced and shed. Our type is somewhat
exceptional in being viviparous, ze. the
young are developed in the radial cavities
and leave the parent by the mouth. Most of the group
have a free larval development with a p/anw/a larva.
The important point to notice in Actinia is the advance
in complexity upon Hydra. The perfect axial symmetry
of Hydra is replaced by a symmetry intermediate between
this and plano-symmetry, namely bi-plano-symmetry. We
can distinguish two ends with siphonoglyphs and two sides,
but we cannot distinguish Jetween the two ends. Some
allies have only one siphonoglyph and are plano-symmetric.
Histology.
Reproduction.
ALC YONIUM. 125
Again, the organs, such as muscles, gonads, and gastric
filaments, are much more definite. Thirdly, the ccelen-
teron is not simple but partially divided thus :—
A, Gastric cavity for digestion.
B, Radial cavities — nutritive and
vascular, walls form motor
(muscles), skeletal (mesen-
teries), and reproductive
(gonads) organs.
Lastly, the ectoderm is tucked in to form a gullet. At
the same time we may note the absence, as in Hydra, of
definite respiratory, excretory and blood-vascular organs.
C@:LENTERON<®
IV.—ALCYONIUM.
PHYLUM CQELENTERATA.
CLass ScYPHOZOA.
SuB-CLAss ACTINOZOA,
ORDER OCTACTINIA.
Fig. 57-—ALcYONIuM DiciTatum. (Ad nat.)
A B Gonad. Coenosarcal
Canal.
Longitudinal
Muscle.
Directive
Mesentery.
Body-wall of Polype.
A, Isolated spicules. B, A tangential section through the entire colony showing
the polypes in cross section.
Alcyonium digitatum (Dead Man’s Fingers) may be taken asa
type of the colonial Actznozoa. The colony may be fixed at its base to
a foreign body and branching like a coral, or it may grow closely
adherent to the tube of an annelid or other body. It is found most
126 C@LENTERATA.
plentifully in moderately deep water, and is often obtained attached to
the hooks of fishermen’s lines. It is of a dull fleshy hue, hence the
popular name. When the polypes or individuals are contracted it has
a slightly rough appearance which enhances its resemblance to its
gruesome appellation. When the polypes are expanded all over its
surface the colony is converted into a zoophyte of great beauty. Each
polype has eight feathered tentacles surrounding a central mouth. The
Fig. 58.—Virew oF ENTIRE COLONY WITH TENTACLES EXPANDED.
(After M‘InTosu.) (Magnified.)
body of the polype stands out from the general surface of the colony,
but on contraction is completely withdrawn. The general structural
principle of the interior of each polype is similar to that of Ac¢inia, but
there are only eight mesentertes, of which ¢wo only are directives, and
the muscles of the other mesenteries are a// on the same face ; there is
also only one szphonoglyph.
Fuither, the ccelenteron is continued aborally into a ccenosarcal canal
communicating with similar canals from the neighbouring polypes.
AURELIA. 127
The bulk of the colony is made up of ccenosarc, which contains a great
number of nodular calcareous spicules. These give a tough consistency
to the colony.
The colonial habit is found largely amongst the order Hexactinia,
to which Acfénéa belongs, but the arrangement of the mesenteries and
the feathered tentacles are characteristic of the order Octactinia.
V.—AURELIA.
PHYLUM CaELENTERATA.
Cass ScyPHOZOA.
Sus-Cuass ScyPHOMEDUS.
Fig. 59.—AURELIA AURITA.
Lateral view. About one-third natural size.
Aurelia aurita is a large medusa or “ jelly-fish ” about
the size and shape of a large saucer. It may be found
swimming in the sea in any numbers during late summer or
early autumn, supporting itself by rhythmic contractions of
128 CELENTERATA.
the body. It is perfectly transparent except for the four
gonads which are of a beautiful violet hue.
It differs in shape from the medusoids of Ode/a, for
it is flat, not bell-shaped. However, the general principles
of its construction are in many respects similar.
It is axially symmetrical and tetramerous, ze,
the peripheral parts are arranged in fours. The
mouth is four-cornered and opens ona short manubrium. It
is surrounded by four large oral tentacles which correspond
to the four corners of the mouth and are jer-radial.
The disc or umbrella is almost circular but slightly divided
External
Features.
Fig. 60.—ORAL VIEW OF AURELIA AURITA x $. (Ad nat.)
Per-radial Canal. Tentaculocyst. Ad-radial Canal.
Inter-radial _
Canal.
Oral
Tentacles.
genital Pit,
Mouth.
into eight lobes, the indentations between them being at the
four per-radii and the four zzfev-raditz. In each depression
or indentation there is situated a sense-organ or fentaculocyst
covered by a hood and having a pair of small processes or
lappets, one on each side. The whole border of the disc is
fringed by a great number of small tentacles and there is no
velum.
AURELIA. 129
The mouth passes by a short cesophagus into a gastric
cavity which is produced into four pockets in the inter-radii.
Each pocket contains on its oral wall a horse-
shoe-shaped goad, and near the middle a row
of gastric filaments which assist in digestion.
The gastric cavity is continued outwards towards the edge
of the disc by numerous vascular canals. The eight primary
branched canals are the four per-radial and the four inter-
radial. Between these there are the eight secondary .un-
branched canals or ad-radials. Allthe canals open into a ring-
canal round the edge of the disc. The gastric cavity and the
canals are ciliated. They are derived from the ccelenteron,
as'in Odelia. In the inter-radii, immediately below the
gonads, are four swb-genital pits, each opening on the oral
surface by a pore.
The mesogloea between the two layers is a thickened
jelly which in this case contains scattered cell-elements.
Internal
Features.
Fig. 61.—MEDIAN LONGITUDINAL SECTION THROUGH THE INTER-
RADIAL PLANE OF AURELIA. (Diagrammatic. )
Gastric Filaments.
Sub-genital Pit. | Stomach.
Gonad,
Inter-radial Canal.
Ring-canal.
Lappet.
Tentaculocyst.
Oral Tentacle. Mouth.
There is no nerve-ring, but there is a diffuse nerve-plexus
concentrated round the sense-organs or ‘¢entaculocysts.
These latter are complex and appear to unite the senses of
sight, hearing and smell in different parts.
DEVELOPMENT. — Aurelia is dicecious and the sexual elements
are discharged by the mouth. A free-swimming planula larva (Chapter
V.) settles down on rocks or weeds and forms .the Aydra-tuba, a minute
hydra-like individual. It is a two-layered sac, with a mouth at the
M. To
130
oral end, leading into a stomodzeum and ccelenteron.
and eight secondary tentacles soon appear, and the ccelenteron becomes
divided into a central gastric cavity and four peripheral cavities by four
inter-radial mesenteries or ¢enzole.
E
pi- Hypo-
Archenteron. blast. blast.
CE@LENTERATA.,
Fig. 62.--THREE STAGES IN DEVELOPMENT OF AURELIA.
(After G6TTE.)
or septal funnels, grow down the interior of these ~mesenteries.
polype has been termed a Scyphula, the presence of the teeniole and
stomodzum constituting a resemblance to the preceding type (Actzxza).
The Scyphula grows in length and by transverse fission it sets free
Fig. 63.—TRANSVERSE SECTION THROUGH UPPER ParT
OF SCYPHULA LARVA.
Gullet.
‘Radial Vascular
Cavity.
SS
~]
WZ
Fig. 64.—TRANSVERSE SECTION THROUGH LOWER PART
OF SCYPHULA LaRVA.
Tnter-zadial
Mesentery. Central Gastric
Cavity.
Radial Cavity.
The eight primary
Four hollow processes of epiblast,
CYDIPPE. 131
a number of free-swimming forms, called Zphyre. An Ephyra has
eight long arms, Zer-vadial and tnter-radial, down which are produced
the eight primary canals. The end of each arm is bifid, forming the
two lappets, between which is the tentaculocyst. By differential growth
the Zfhyra fills up the ad-radial depressions and becomes a young
Aurelia. The teeniole disappear, leaving only the gastric filaments,
whilst the bases of the septal funnels form the sub-genital pits.
Here we have a metagenesis, as in Ode/da, but the scyphula does not
form a true colony, abbreviating this stage by rapid transverse fission.
VI.—CYDIPPE.
PHYLUM Ca:LENTERATA.
Crass CTENOPHORA.
Cydippe is one of the most beautiful little organisms to be found in
the sea. It is pelagic and appears like an almost spherical transparent
ball of glass, usually about one-half inch in diameter. It feeds
voraciously on pelagic organisms, ¢.g., young fish. When alive it moves
Fig. 65.—CybIPPE PLUMOSA.
(After Cun.)
Tentacle with 2 Shy
Adhesive Cells, < 46
Row of Combs,
Longitudinal Canal.
with ceaseless activity and is iridescent with rainbow (interference)
colours. One axis is slightly longer than the other, at one end of which
(oral) is the mouth ; at the aboral end is a sense-organ. From oral
to aboral end there run eight meridional rows of rapidly moving coméds
which are formed by a row of cilia fused at their base. All the combs
strike in an oral-aboral direction and the result is a steady, fairly rapid
movement forward. Two long tentacles trail behind the animal and
give stability to its movements. They bear small branches which are
covered with spirally stalked adheszve cells. The tentacles are very
sensitive and can be completely retracted within a pair of sheaths
or pockets. The mouth leads into an ectodermal gwd/et which
passes into-a stomach. The stomach tapers towards the aboral
end and branches into four ducts which open symmetrically round
the aboral sense-organ.
132
C@LENTERATA.
Externally, Cydippe is bi-plano-symmetric, for the plane passing
through the tentacles and their sheaths, called the coronal plane, differs
from that perpendicular to it, or the sagittal plane. Both planes, however,
divide the animal into symmetric halves. The gullet is flattened and
Fig. 66.—ABORAL VIEW OF CyDIPPE. (After CHUN.)
Aboral Sense-organ.
Horizontal Canals.
Tentacle.
Longitudinal
Nerve.
elongated in the sagittal plane, as in Activa. Each of these planes
corresponds to two opposite per-radii. The stomach gives off, near the
gullet, four inter-radial canals which run horizontally outwards, each
Fig. 67.—ADHESIVE CELLS OF
CypIpre. (After HERTWIG.)
Highly Magnified.
Head.
Stalk.
5S
=
<
bifurcating into two ad-radials. Each
of these joins a long meridional canal
running from oral to aboral end just
below each row of the combs.
The aboral sense-organ consists of
a ciliated depression containing small
otoliths (cf Obelia) and probably
governs the equilibrium of. the ani-
mal. From it there pass eight
nerves down the eight rows of combs.
The ccelenteron is here partially
divided, as in Actinza, into a central
gastric cavity or stomach and peri-
pheral nutritive, or vascular cavities.
The muscles are not represented, but
the gonads (Cydifge is hermaphro-
dite) are situated on the walls of the
meridional canals. The mesoglcea is
enormously developed and forms the
main bulk of the organism, filling the
space between ectoderm and endo-
derm. Nematocysts are not found in Cydipfe but they have been
described in some members of the group.
CQ@LENTERATA. 133
Cydippe resembles Actinia in the presence of an ectodermal gullet
and of central and peripheral portions of the ccelenteron, but it differs
from all the preceding types in the possession of ‘‘ combs” of cilia.
PHYLUM CCELENTERATA.
The Phylum Ccelenterata is extensive and of great
zoological importance. The six types described above
(Z.e., Hydra, Obelia, Aurelia, Actinia, Alcyonium, Cydippe)
give a good general idea of its organisation and place in
nature,
They are mostly marine, all aquatic and all retain the
primary metazoan axis, about which they are usually
axo-symmetric though, as in the last two types, they may
progress to bi-plano-symmetry. They are usually either
sedentary or pelagic. R
In structure they are all formed of two epithelia (or
derms), an outer layer or ectoderm and an inner or endo-
derm, between which is a thin or thick mesoglea. This
two-layered condition has-been compared to that of the
typical diploblastic larva, the gastrula. The comparison is
as follows :—
GASTRULA,— Ca.ENTERATA.—
Epiklast. Ectoderm.
Hypoblast. Endoderm.
Archenteron. Ccelenteron.
Blastopore. Mouth. ©
Central axis. Primary axis.
We can divide CeZenferata into three classes :—
1. Hydrozoa.
2. Scyphozoa.
3. Ctenophora.
Crass I.— Hyprozoa. (Hydra and Odelta.)
In these animals the ccelenteron remains simple, the
axial symmetry is undisturbed and there is no ectodermal
gullet. They include hydra-like forms with only a hydroid
phase ; obelia-like zoophytes which have a hydroid and a
medusoid phase (though the medusoid may be degenerate) ;
and others (¢.g., Va/comedusa@) with only a medusoid phase.
C@LENTERATA.,
134
(After HaickeEL.)
Fig. 68.—Typres oF Trur CorALs,
CG@LENTERATA. 135
The Hydrocoralline are peculiar in having a massive
calcareous skeleton instead of the usual chitinous one and
for their very primitive little medusoids. Their calcareous
skeletons can be distinguished from the true corals by the
absence of sef¢a in the apertures left by the polypes. The
Siphonophora are floating pelagic colonies with little or no
skeleton but with remarkable division of labour, the members
of the colony being modified into a great variety of kinds.
Crass II.—ScypuHozoa. (Actinia and Aurelia.)
In these the ccelenteron at one time of their life is
divided into central (gastric) and peripheral (vascular)
cavities, and there is usually an ectodermal gullet. The
gastric cavity usually has gastric filaments and the gonads
are endodermal. Aurelia represents those types which have
hydroid and medusoid phases, but a number of other jelly-
fishes have only the medusoid phase. All these. form the
sub-class Scyphomeduse. The important forms with only
hydroid phase (¢g., Actinéa) form the sub-class Actinozoa.
Actinia, like Hydra, is solitary and without an exoskeleton,
but actinozoan colonies (like hydroid zoophytes) also occur.
The skeleton, usually ectodermal, is most commonly of
calcareous matter, and may assume vast proportions.
These colonial types are called cova/s and their skeletons
may be recognised by the presence of radial septa in the
holes formerly inhabited by the polypes. (Coral Islands,
see page 72.) The Actinozoa are divided into two important
orders, the Hexactinia and Octactinia, according to the
number of mesenteries and other structural features
mentioned in the types Actnta and Alcyonium.
Crass III.—CTENOPHORA.
The unique motor organs of this class tend to separate
them from the other two classes, but they are connected by
certain intermediate forms.
Cydippe is a very fair representative of the class.
They are typically free-swimming pelagic organisms of
carnivorous habits. Some (Cestum) become elongated in
one plane to form a long ribbon, or they may (Bevoé) form
a large bell by increase of the stomodzeum.
136
C@LENTERATA.
1, Metazoa with radial (axial) symmetry.
z. Body of two layers of cells, ectoderm and endoderm, enclosing one continuous
gastric cavity, which communicates to exterior by one opening, the mouth-
anus.*
An pw
formation).
Class I.—Hyprozoa.
Types—Hydra; Obelia;
(Tubularia).
1. Simple gastric cavity.
2. No ectodermal gullet.
3. Two phases, a free-
swimming medusoid
and sedentary hy-
droid.
MEDUSOID.
A velum.
Gonads ectodermal,
Four radial canals.
Simple sense-organs.
Nerve-rings.
HYDROID.
t When colonial,
usually has horny
perisarc.
2. Skeleton has no
septa.
Class II.—Scypnozoa.
Types—Actinia; Alcyon-
tum; Aurelia;
(Madrepora).
. Gastric cavity divided
by mesenteries into
central and peripher-
al cavities,
2. An ectodermal gullet.
3. Two phases, a_free-
swimming medusoid
and sedentary hy-
droid.
MEDUSOID.
No velum.
Gonads endodermal.
Many radial canals.
Tentacles modified in-
to complex sense-
__organs.
Diffuse nerve fibres.
HYDROID.
1. Whencolonial, has
calcareous skele-
ton.
2, Skeleton has septa.
PHYLUM CC@ELENTERATA,
. A structureless lamella, the mesogloea, between the two layers.
. Ectoderm cells bear nematocysts.
» Asexual reproduction by budding produces colonies.
. Aquatic and mostly marine, free-swimming, and sedentary (tending to coral
Class II1—CTENopPHoRA.
Types—Cydippe ; (Berot).
1. Gastric cavity consist-
ing of stomach and
gastro vascular
canals.
2. An ectodermal gullet.
3. One phase only, a free-
swimming, modified
from medusoid type.
4. Eight longitudinal
rows of cilia.
. Nematocysts rare,
. Single aboral sense-
organ.
an
* Usually termed the ‘‘ mouth.”
PLAT VHELMINTHES. 137
CHAPTER XIV.
PLATVHELMINTHES, ROTIFERA AND
NEMATHELMINTHES.
DISTOMUM. TANIA. HYDATINA, ASCARIS.
I._DISTOMUM.
PHYLUM PLATYHELMINTHES.
Cass TREMATODA.
Fig. 69.—VENTRAL View oF LIVER-FLUKE (Distomum hepaticum).
Natural size. (Ad nat.)
Anterior Sucker
Pharynx.
and Mouth. - a ae
(Esophagus. ; XK
Intestine.
Posterior"
Sucker.
A, Exterior. B, The Alimentary System.
Distomum hepaticum is the liver-fluke of the sheep.
It may grow considerably over one inch in length and shows
a flat leaf-like shape. It is plano-symmetric and flattened
dorso-ventrally.
’ It infests the liver and bile-ducts of the domestic sheep,
and causes the disease called “liver-rot” which is fatal to
great numbers of sheep. Obstruction of the bile-duct by
hundreds of these parasites causes inflammatory processes
and bleeding of the liver-tissues. General wasting and often
138 PLAT VHELMINTHES.
jaundice lead on to death. The life-history clearly shows
the reason why sheep incur this disease after grazing on
damp pastures.
At the anterior end is a blunt cone, at the tip of which
opens the mouth in the centre of a sucker.
The body tapers from two shoulders to a point.
In the mid-ventral line, about yjth of its total
length from the anterior end, is situated a second sucker.
The body is of a dull yellow colour and enveloped in
a thin cuticle, which forms hook-like pro-
cesses or sfinules pointing backwards and
scattered over the surface.
External
Features.
Integumentary.
Fig. 70.—TRANSVERSE SECTION THROUGH THE LIVER-FLUKE.
(Déstomune.)
(Somewhat diagrammatic.)
Longitudinal Vas Deferens.
Muscle. , Circular Muscles. Ovary. Ectoderm.
Parenchyma. | -
Cuticle.
Intestine. Excretory Nerve Cord.
Duct.
The animal creeps about slowly by muscular contrac-
tions. Under the cuticle are formed well-defined circular
and longitudinal layers of muscles. The mouth leads into
a sucking pharynx with muscular walls. This
opens backwards by a short wsophagus into a
large zntestine. The intestine forks into two main branches
which run back to the hind end of the body. On their
outer side they give off a great number of much-branched
processes which end blindly at the edge of the body.
Digestion appears to be purely intra-cellular. There is
no anus.
Alimentary.
DISTOMUM. 139
The excretory system consists of a median duct which
opens by a pore at the posterior end. It is connected with
innumerable branches which form a fine net-
work all over the body. Each branch eventually
terminates in a blind swelling, in the centre of which there
depends a fage/lum. The flickering motion of these flagella,
doubtless causing currents towards the exterior, bas given
the name of flame-cell excretory organs to the whole system.
There are no known sense-organs, but the nervous
system consists of a ring round the pharynx
with two lateral ganglia and a small ventral
ganglion. From the lateral ganglia are given off two
ventro-lateral nerves which pass to the hind end of the
body.
Excretory.
Nervous.
Fig. 71.— STRUCTURE OF DISTOMUM.
Ganglia. A,
Nerve Ring.
Right Lateral 4
cee Left Lateral Nerve.
ny As
} \ ae
Main Duct. . 2
V \d
Excretory Pore.
A, Nervous System. B, Excretory System,
The cavity of the body between the muscle layers and
the alimentary and reproductive organs is almost entirely
filled up with a mass of cells, arranged in
a mesh-work, to which the name of parenchyma
has been applied. Small cavities between these cells re-
present the primitive vascular cavity or hemoccele.
The reproductive organs are complex. D¢stomum is
hermaphrodite. The female organs consist of a branched
ovary on the right side of the animal, from
which there passes an ovarian duct. Two
large paired yolk-glands lie laterally and their viteldine ducts
Vascular.
Reproductive.
140 PLAT YVHELMINTHES.
meet to form a median vitelline duct. This runs forward
to meet the ovarian duct and from their junction there
passes a median dorsal tube to the exterior, the so-
called vagina. The junction is surrounded by a round
shell-gland which secretes the shells of the eggs, and the
united ducts lead towards the anterior end as a much-coiled
oviduct. This opens to the exterior in the median ventral
line between the two suckers.
Fig. 72.—View oF LIVER-FLUKE (Diéstomum).
Showing the Reproductive Organs. (After SomMER.)
Common
Genical Aperture.
Seminal
Vesicle.
Vas Deferens.
Ovary.
Oviduct.
Vitelline
Gland.
Left Testis.
Shell Gland.
Vas Deferens
(leading to
\ : Laurer’s Duct.
right testis).
Vitelline Duct.
The male organs consist of a pair of branched “estes,
one behind the other. The vasa deferentia from them
DISTOMUM. 14I
unite at the level of the posterior sucker to form a seminal
vesicle. In front of the seminal vesicle lies the protrusible
_ penis along which there runs an ejaculatory duct from the
former. Penis and seminal vesicle lie in a cavity called the
cirrus-sac, A small prostate-gland encircles the ejaculatory
duct. There may possibly be self-fertilisation.*
The small ovoid eggs (about +2, inch in length) accumu-
late in the oviduct and are enveloped in hard shells. They
ate discharged down the bile-duct into the
intestine and thence to the exterior. The
eggs, which are laid in the neighbourhocd of water, hatch
by detaching a circular cap and set free a small ciliated
larva not unlike an adult in shape.
Development.
Fig. 73.—DEVELOPMENT oF DisromMuM HEPATICUM.
A, Ciliated Larva; B, Sporocyst with contained embryos ; C, Limneus truncatulus
(natural size and magnified), the host of the Sporocyst.
It has an outer layer enclosing a solid mass of cells,
There are two small pigment spots which may serve as eyes.
It lives actively for a few hours, and if successful during that
time in finding a water-snail (Zimncus truncatulus ), it is said
to rotate rapidly with its pointed anterior end against the
body of the snail and bore its way therein. In the tissues
of the snail it loses its cilia and grows to about five times
* A small duct leads dorsally from the median vitelline duct to the exterior. It
is called Laurer’s duct, and its use is not definitely known.
142 PLAT VHELMINTAHES.
its length into a large two-layered sac
called the sfovocyst. ‘The inner layer
buds cells intotheinternal cavity, which
develop into organisms called vedie
through a mora and gastrula stage.
A vedia has an elongated body with
mouth at the anterior end, a pharynx
and simple intestine. Externally it has
a collar or thickened ridge round the
anterior end, behind which is a small
pore into the body-cavity, and a pair
of processes towards the hind end. It
also has excretory tubules. A redia
when developed bursts through the
brood-sac or sporocyst and eats its
way through the snail. Eventually it
produces, by budding of its internal
cells, a number of cevcari@ which are
young or larval flukes. The cercarza
escapes by the genital pore of the
redia and out of the snail into the
water. It has a rounded body and
vibratile tail. Two suckers, a mouth,
pharynx, and simple bilobed intestine
can be distinguished, and there is also
a flame-cell excretory system. The
surface is dotted with cystogenous cells
which produce the cyst. The cercaria
works its way to the edge of the pond
(the snail may be in grass already),
up a blade of grass or other plant
and there loses its tail, encysts and
remains dormant. Should the cyst
be introduced into the stomach of the
sheep the cercaria escapes, passes up
the bile-duct. and develops in a few
weeks into a young fluke.
We have to add that the sporo-
cyst may produce fresh sporocysts by
binary fission and that the redia may
give rise to fresh generations of redize.
Fig. 74.—SPOROCYST.
Sporocyst containing Rediz.
Fig. 75.—A Reva.
Young Redia. Notice the
mouth and alimentary canal,
and two lateral processes.
DISTOMUM. 143
Distomum is a type of the Digenous Trematoda, or those
with two or more generations in their life-cycle, which
alternate in their environment between two hosts, as in
Fig. 76.—A CERCARIA.
Fig. 77.—CERCARIA AND
DisToMuUM.
Encysted Cercaria.
(The structure can still be seen
through the cyst.)
nye
Young Distomum.
_ Notice tail, suckers, bilobed
intestine and dotted cysto-
genous cells.
Distomum. It also illustrates several adaptations due to
a somatic endoparasitic habit (see Chapter IX.). The high
fecundity, the complex sexual organs, and the absence of
sense-organs should be here noted.
144 PLAT VHELMINTHES.
Distomum belongs to the phylum PLATYHEL.-
MINTHES because of its flattened unsegmented body,
its simple alimentary canal with no anus, and its meso-
dermic parenchyma with flame-cell excretory organs. It
belongs to the class TREMATODA because of its parasitic
habit with suckers, thick hooked cuticle and complex sexual
organs.
II.—TANIA.
PHYLUM PLATYHELMINTHES.
- CLASS CESTODA.
Tenia solium is a common tape-worm inhabiting
the intestine of the human subject. It is of great length
Externay (Often _nine to ten feet) and flattened dorso-
ventrally. The anterior end is extremely small,
terminating in a knob called the head. The
body enlarges gradually backwards, and it is broadest at the
extreme hind-end. It is produced anteriorly into a process
or rostellum which bears a ring of (22-32) hooks and behind
them there are four large suckers. A little way behind the
head there appear transverse constrictions running across
the body. These get wider apart and deeper towards the
hind-end, and partially divide the body into a series of
sections known as proglottides. There may be about 850
proglottides, of which the broadest are about 3 inch across.
‘There is no mouth, no alimentary system, and no
sensory organs, but the. nervous and excretory systems
are well developed. There is a nerve-ring fn the
head with two lateral ganglia giving branches to
the suckers. There pass backwards from them a pair of
lateral nerves which run throughout the length of the body.
The excretory system has also a ring in the head and
four longitudinal ducts. The dorsal and ventral pair do
not proceed far, but the lateral ducts pass down the entire
length of the body just inside the nerves. In the posterior
part of each proglottis they are connected by a
transverse duct, and in the last proglottis this duct
opens medially through a contractile vesicle to the exterior.
Numerous secondary branches break up in the parenchyma
and terminate in “ flame-cells.”
Features.
Nervous.
Excretory.
TANTA. 145
There is a very thin cuticle and a rather indefinite layer
of ectoderm which merges into the parenchyma. In this tissue
are small calcareous bodies.
The muscles are arranged in a
transverse series and a scat-
tered longitudinal series out-
side it.
In the parenchyma are
found the complex repro-
ductive organs. Zenza, like
Distomum, is hermaphrodite,
and ‘the sexual
organs are re-
peated in each proglottis.
They mature gradually, hence
the front proglottides show
an earlier stage than the
hind ones. Those front pro-
glottides which show sexual
organs have male organs only.
The middle ones show both
sets of organs and the “ripe”
hind ones show a portion of.
the female organs only. The
common sexual opening is
found on the right side in
one proglottis, on the left in
another. The ¢estis is a
branched organ opening by a
vas deferens to a penis. The
paired ovaries lead by ovarian
ducts into a median ovéduct.
This oviduct first receives the
opening of the sperm - duct
and then passes through the £
shell-gland to the uterus. In @
the shell-gland it receives the #
vitelline duct from the yolk-
gland. The sperm-duct opens
at its other end into a seminal
receptacle, a chamber in which Selected portions from a single specimen,
M. II
Fig. 78.—Tania SAGINATA.
(After Leuckart.)
Reproductive.
146
the sperms are stored.
PLAT VHELMINTHES.
It communicates with the exterior
by a vagina, opening close to the penis.
Fig. 79.—HEAD of TANIA
SOLIUM.
(After LEuCKART.)
Proglot-
tides.
Note the ring of hooks on the
rostrum, the four suckers, and the
commencing proglottides.
.the exterior.
Eggs pass down the ovi-
duct, are fertilised by sperms
from the seminal vesicle,
receive yolk from the yolk-
glands and a shell from the
shell-gland, and then pass into
the uterus. Here they ac-
cumulate in enormous num-
bers, and a “‘ripe” proglottis
contains a large branching
uterus with eggs; the remain-
der of the sexual organs have
atrophied. The eggs are at
first surrounded by an oval
vitelline membrane filled with
albumen, but later this ruptures
and the egg has merely a thick
shell.
The ripe proglottides are
shed one by one and pass to
The eggs are set
free in millions on introduction
of the proglottis into the stomach of a pig. The embryo is
spherical and has three pairs of hooks.
By these, combined
Fig. 80.—TRANSVERSE SECTION OF A PROGLOTTIS OF TANIA.
(After SHIPLEY.)
Uterus.
Oviduct.
Ovary.
Testis.
Duct.
‘Excretory
Nerve Cord.
Longitudinal
Muscles,
Ectoderm.
TAENTA.,
Fig. 81.—SEMI-DIAGRAMMATIC VIEW OF A
SINGLE PROGLOTTIS OF A TANIA.
(Mainly after Leuckarr.)
Testis. Uterus,
Nerve Cord.
Excretory
Canal.
Seminal
Vesicle.
Common
Sexual
Aperture.
Seminal
Receptacle,
Ovary.
Yolk-gland. Shell-gland.
147
Fig. 82.—PRocLor-
TIS OF T-ENIA
SAGINATA.
(After LEuCKART.)
Branches of Uterus.
Fic. 83.—DEVELOPMENT OF TANIA SOLIUM.
(After LEucKART.)
I, The egg in its vesicular vitelline membrane and shell.
II, The free egg with
three pairs of haoks. III, The cystic stage, with developing head. IV, A later
stage of same. V, The bladder-worm, with head evaginated. VI, Young Tenia
from intestine of a rabbit.
148 PLAT VHELMINTHES.
with the muscular movements of the host, the embryo is
worked into the blood-vessels of the pig, along which it is
carried into the muscles. Here it loses its hooks and be-
comes a hollow vesicle or cyst. The wall of the oval cyst is
invaginated at one side and forms a pocket. On the wall of
the pocket are found suckers and hooks, and it is later
evaginated to form the cestoid worm. Pork containing such
cysts is known as “measly.” This is known as the cystic stage
or bladder-worm, and the cysts of Zenia solium were known
Fig. 84.—‘‘ MEas_y” Pork.
The oval bodies are cysts.
by the separate name of Cystzcercus cellulose before their
true nature was determined. The completed bladder-worm
shows a large bladder, depending from which is the ‘“‘ body”
of the worm. On being introduced, still alive, into the
human subject the bladder, and with it the greater part of
the body, is lost, and the head alone survives as a creeping
worm, fixes itself and grows into the tapeworm.
We may note that there is xo true metagenesis, since the
fission into proglottides can hardly be regarded as a method
TENTA, 149
of reproduction. In some allied forms, however, the cyst
or cystic stage produces several scolices, in which cases
metagenesis is evident.
Tenia solium is only found in man, and is chiefly
dangerous owing to a liability of the cystic stage being also
passed through in man, often in the brain.
Tenia is a striking instance of the effects of endoparasit-
ism, especially of the enteric type. (See Chapter IX.)
The life-history may be illustrated diagrammatically :—
Man.
Tenia
yer
Scolex Proglotts
IN
Bladder Worm
Cyst Embryos
Pig.
PHYLUM PLATYHELMINTHES.
The Platyhelminthes, or flat-worms, form a well-
defined group of the affinities of which little is known.
The two types given, Déstomum and Tenia, represent the
two “parasitic” classes of the phylum.
1. TREMATODA.—The 7Zyematoda are all parasites, the
Monogenea are mostly ectoparasites with one host, and the
Digenea are endoparasites with two hosts.
2. Cestopa.—The Cestoda illustrate enteric parasitism
with entire loss of alimentary canal. They usually alternate
between two hosts and show a cestoid and cystic stage.
Tenia saginata is a common type found in the ox and man.
It has no hooks and is larger than Tendéa solium. In these
the cystic stage has only one head and is called a cysticercus,
but in some, such as Zenia cenurus, alternating between the
dog (cestoid) and sheep (cystic), the cystic stage has many
heads and is called a Canurus. It produces “sheep-gid ”
150 PLATVHELMINTHES.
or ‘‘sturdy” by pressure on the brain. Another small tape-
worm of the dogs, Zienia echinococci, has enormous cysts,
with secondary cysts and many “heads” (Zchinococct), which
may occur in man, sheep or pigs.
The members of the third class, or Zurbellaria, are
not parasites, but are terrestrial, marine, or freshwater.
In a number of characters they resemble the other classes.
The body is usually flattened dorso-ventrally. There is
no anus. ‘There are neither vascular nor respiratory
organs. The excretory organs are of the ‘flame-cell”
type, there is a brain with two lateral nerves, and the
sexual organs are hermaphrodite and complex. On the
other hand, the Zwurbel/aria usually have simple sense-
organs and the body is usually ciliated.
In the Platyhelminthes we see a distinct advance in
structure when compared with the Calenterata. ‘The axial
or, at most, bi-plano-symmetry of the latter has given way
to plano-symmetry. The mesogloea of the Cwlenterata,
with or without a few scattered cells, has given place to a
definite mesoderm formed into muscles, gonads and
parenchyma, and drained by a definite excretory system.
PHYLUM PLATYHELMINTHES.
1. Three-layered metazoa with bilateral symmetry (plano-symmetry) and flat-
tened body.
2. Alimentary canal, when present, has no anus.
3. Mesoderm with no definite cavity and fills the space between skin and
alimentary canal (parenchyma).
4. Excretory organs consisting of ducts opening to exterior and blind branches
containing ‘‘ flame-cells.”
5. Nervous system usually with two lateral cords and an anterior paired brain.
6. No vascular nor respiratory systems.
7. Mostly hermaphrodite.
Class I. Class II. Class III.
CEsToDa. TREMATODA. TURBELLARIA,
Type—Tenia. Type—Distomum. Type—Mesostoma.
1. Elongated flat-worms | 1. Oval flat body with | 1. Oval flat body with
with thin cuticle. cuticle bearing fine cilia and rhabdites.
Hooks and suckers hooks and suckers.
on head.
z. No mouth nor alimen- | 2. Anterior mouth, race- | 2. Ventral mouth, simple
tary canal. mose intestine. or branched intestine.
3. Life-history of two | 3. Life-history often of | 3. Life-history simple.
phases—the _ cestoid two phases.
and the cystic.
4. No sense-organs. 4. No sense-organs. 4. Paired eyes.
5, Endoparasitic. 5. Endoparasitic or ecto- | 5. Free—aquatic and ter-
parasitic. restrial.
HYDATINA. : 151
IIL—HYDATINA.
PHYLUM ROTIFERA,
Hydatina senta is a small microscopic animal very commonly
found in freshwater ponds and streams. Its body is transparent and
elongated. At the blunt or ora/ end is a ciliated funnel-like depression,
the vestzbule, at the bottom of which is the mouth. The edge of the
vestibule is fringed with a band consisting of specially long cilia, which
is known as the céngudum. Further towards the centre of the vestibule
is a broken row of
longer cilia, called the Fig. 85.—VENTRAL Virw or HyYDATINA
trochus, whilst the SENTA x 40.
groove between trochus (After Pats).
and cingulum is raised "
into several lobes bear- sie Nel
ing styles. This com-
plex apparatus is often
called the wreath and
serves for locomotion
Trochus. Cingulum.
and for ingestion of Esophagus. Mastax
food. The aboral end |, Rien ‘
is tapering and termin- ‘Pao 7 ane—
fe . ys
ates in a bilobed foot Tubule.
endowed with a pair of
adhesive glands. The
body is enveloped in
a thin delicate cuticle Ovary.
covering a simple ecto-
derm. Themouth leads
into a mastax which
is a complex grinding
apparatus containing
chitinous teeth. From
this an esophagus is
continued into a large
digestive stomach fol- Adhesive Gland.
lowed by an zntestine.
The intestine terminates
in an anus, situated
not at the aboral end but on one surface, usually termed dorsal. Two
salivary glands open into the mastax, and two hepatic or, digestive
glands discharge their fluid into the stomach.
The alimentary canal hangs freely in the cavity of the body, which
is filled with colourless fluid. This body-cavity is traversed by connec-
tive tissue and muscle fibres, but has no ccelomic lining. Throughout
its course, laterally to the alimentary canal, is a pair of excretory
tubules which bear branches terminating in closed flame-cell sacs. Each
tubule opens behind into the urinary bladder with a single aperture to
the exterior near the anus, forming a cloaca. Anteriorly the two tubules
Yolk Gland.
152 PLATVHELMINTHES.
anastomose in front of the mouth. The brain is a large mass lying
dorsally to the mouth ; it supplies nerves to various parts of the body.
Ventral to the stomach is a single ovary with a large vitel/ine gland
and an ovzduct opening into the cloaca.
The male Hydatina is much smaller in size and has no alimentary
system,
PHYLUM ROTIFERA.
The Rotifera are an important phylum of common microscopic
animals. They are marine and freshwater in habit, and they may be
active, sedentary, tubicolous or ectoparasitic. They are interesting in
their diversity of external form, their sexual dimorphism (with small
and degenerate males), their summer and winter eggs and their power
of resisting drought. They are three-layered in structure, but they
have no ccelom; the cavity of the body is an archiccele and there are
no nephridia, excretion being conducted by flame-cell tubules. These
and other characters indicate a relationship to the Platyhelminthes.
Hydatina is fairly typical but for the exceptional absence of eyes or
other simple sense-organs. In other Rofzfera there is great diversity
in the form of the wreath and of the foot.
IV.—ASCARIS.
PHYLUM NEMATHELMINTHES.
CLASS NEMATODA.
Fig. 86.—DISSECTION OF FEMALE ASCARIS MEGALOCEPHALA
FROM THE DorsAaL SIDE. (dd mat.)
Excretory Duct.
(Esophagus.
Intestine,
Oral
Papilla.
Ascaris megalocephala is a large nematode worm
found commonly in the stomach of the horse. It is usually
known as the ‘‘maw-worm.” The body is long and
cylindrical, tapering at each end. The female may be one
foot or more in length; the male is usually less. In addition,
ASCARIS. 153
the hind-end of the male is slightly coiled, and has a pair of
Bxterna) ™inute bristles ur aza/ sete protruding from the
Features, 202! aperture. The body is of a whitish
“colour, and it has four lines (a dorsal and
ventral and two lateral) along its surface.
Fig. 87.—DIAGRAMMATIC TRANSVERSE SECTION OF ASCARIS
MEGALOCEPHALA. (fd nal.)
Dorsal Nerve Cord.
Cuticle.,
Ectoderm,,
Muscle Cell.
Ts
Excretory
Duct,
Oviduct.
Uterus. Ventral Nerve Cord..
The mouth is at the anterior end, surrounded by a dorsal
and two lateral lips which bear sense-papille, probably
tactile. The anus is a little distance from the posterior end
on the ventral surface. Some little way behind the mouth
is a minute median ventral pore, the excretory opening.
On dissection we find a definite body-cavity* in which
lie freely the alimentary and reproductive organs: The
mouth leads into a pharynx with muscular walls lined by
chitin. The suctorial pharynx opens into a long dufestine
terminating in the azus. The intestinal epithelium is said
to secrete on both surfaces a delicate cuticle.
* This cavity may be a modified coelom or may be an archiccele : the structure
of the excretory organs points to the latter.
154 PLAT VHELMINTHES.
The outer surface of the body is covered by a thick
cuticle, underneath which is a layer of
ectoderm in which the cell-walls are said
to be absent. This ectoderm is thickened in the mid-
dorsal, mid-ventral, and the two lateral lines, corresponding
to the four external lines. Below the ectoderm is a single
layer of longitudinal muscle-cells, divided into four sections
by the four ridges of ectoderm. Each muscle-
cell has an outer muscular part with longi-
tudinal striation and an inner protoplasmic part with a
nucleus. As in Aydra, only a portion of the muscle-cell is
differentiated into contractile tissue.
The nervous and excretory systems are best seen in
sections. The former consists of a nerve-ring round the front
of the pharynx which is thickened dorsally and
ventrally. Six small nerves run forwards and
six others run backwards. Of these the four lateral soon
become very thin, but the dorsal and ventral run the whole
length of the body, embedded in the ectodermal ridges.
They are connected by alternate lateral commissures.
In the lateral ectodermal ridges there runs a pair of
excretory ducts which apparently end blindly behind, but
meet in front to open by the median ventral
pore a little behind the mouth.
The male sexual organs consist of a single long coiled
tube. The blind and tapering end forms the /es¢is, the
middle part the vas deferens, and the lower part swells out
to form the seminal vesicle. This opens bya small duct into
the intestine close to the anus. A small se¢a/ Stand secretes
the anal sete. The female organs consist of a pair of long
coiled tubes. The inner part of each forms the ovary, the
middle portion the ovéduct which swells out to form the
uterus. The two uteri join in a common vagina to open
to the exterior by a median ventral opening towards the
anterior end of the body.
The fecundity is enormous, many thousands of fertilised
and encapsuled eggs being discharged daily from the uteri.
These pass out of the body of the host, but
; their subsequent history is unknown. It is
said that, as in the case of liver-rot, the maw-worm is
acquired by feeding on damp pasture.
Integumentary.
Motor.
Nervous.
Excretory.
Development.
~
NEMATHELMINTHES. 15s
Ascaris is a fair type of the numerous NEmatTopa or
threadworms which infest.the body of the higher animals.
Its characters belonging to the class are the unsegmented
body, the absence of appendages and the characters of the
body-wall, excretory and reproductive systems.
PHYLUM NEMATHELMINTHES.
This phylum contains a great number of free and parasitic
worms forming the class Vematoda, and another divergent
class of parasitic worm called Acanthocephala. The latter
have a hooked anterior process and no alimentary canal,
showing a complete adaptation to enteric parasitism, but the
former show few similar modifications. Some appear to be
only occasionally parasitic and some are entirely free.
Trichina spiralis infests the pig, the muscles of this animal
with encapsuled Zyichina being known as ¢vichinosed pork.
Introduced into the human subject, they give rise to ¢richin-
osis. The Fi/aride also produce dangerous diseases, and the
eel-worms or Anguillulide cause great destruction to crops.
Fig. 88.—MAGNIFIED VIEW OF ‘‘ TRICHINOSED” PoRK. THE
NEMATODE WoRMS (TRICHINA SPIRALIS) ARE SEEN
ENVELOPED, IN THEIR CAPSULES.
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156 ARCHICELOMATA.
CHAPTER XV.
ARCHICG@LOMATA.
ASTERIAS, BALANOGLOSSUS. LOPHOPUS. SAGITTA. WALDHEIMIA.
I.—ASTERIAS.
PHYLUM ARCHICELOMATA.
Sus-PHYLUM ECHINODERMATA.
CLass ASTEROIDEA.
Fig. 89.—ASTERIAS RUBENS x }.
On the left the oral surface is seen with the five ambulacral grooves and
tube-feet ; on the right is the aboral surface with the madreporite
between the two lower arms.
Asterias rubens (the starfish) is one of the commonest
marine littoral animals. The body is of a dull yellow-red
colour, flattened and produced into five equal-sized arms.
This gives an external appearance of axial
symmetry, but it will be seen that it is really
plano-symmetric. For purposes of description,
the five axes of the arms are termed radii and the five
radii between them are called the znter-radit. In the
External
Features.
ASTERIAS. 157
middle of the upper surface is a minute aperture, the
anus, and placed eccentrically in one inter-radius is a round,
slightly convex, perforated plate called the madreporite.
The line drawn through anus and madreporite and
continued on either side gives the direction of the perpen-
dicular plane of symmetry. The whole upper surface of the
body is covered with a scattered mass of ossicles, calcareous
nodules which can be seen and felt ¢hvough the extremely
thin and delicate ectoderm.
Fig. 90.—TRANSVERSE SECTION OF THE ARM OF
ASTERIAS RuBENS. (Diagrammatic.)
Adambulacral Ossicle.
: Ambulacral Ossicle. -
Radial Nerve.
Radial Blood-vessel.
Ampulla.
The general cavity is the hypogastric ccelom.
On the under or oral side of the starfish the mouth is
situated in the centre. Along the centre of each arm there
runs a deep groove, the ambulacral groove. This is filled
with two double rows of ambulacra or tube-feet, a number of
small processes terminating in suckers. They form the
locomotive organs of the animal. If the double rows be
pressed aside from the middle line, there can be seen a
delicate vadiaZ nerve in the middle of the groove. Further,
if the tube-feet be cut away it will be noticed that they
158 ARCHIC@LOMATA.
really emerge from small holes between the ambulacral
ossicles arranged in a row on each side. These ossicles
form the walls of the grooves. At the edges of the grooves
are rows of calcareous spines, and a few extra large ones,
the oral spines, project inter-radially or towards the mouth.
At the tip of each arm is a small eye-spot.
If the upper surface be entirely removed the alimentary
organs are brought into view. The mouth leads into a
spacious cardiac part of the stomach which is
radially lobed. A constriction leads into the
pyloric portion which is pentagonal in shape, the angles pro-
jecting radially. From each angle there runs a duct which
Alimentary.
Fig. 91.—MEDIAN LONGITUDINAL SECTION THROUGH THE
STARFISH IN THE PLANE OF ITS SYMMETRY.
Madreporite..
Stone Canal. ‘bs
} e Pedicellaria.
Cardiac part
of Stomach.’
ak oe
os * Ambulacral Ossicles.
*. Water Vascular Vessel.
Axial Sinus.“ 7 Radial Nerve.
, yy y
Ovoid Gland. Water Vascular Ring.
(If exactly median the section would cut the median mesentery and not the gonad.)
bifurcates into two long pyloric glands in each arm. The
intestine is very short and has a pair of small branched
anal glands. It will easily be seen that the pyloric glands
are attached to the aboral wall by paired mesenteries,
and that the cardiac part of the stomach is attached
to the oral wall by ze¢vactor muscles. The cardiac portion is
often protruded (e.g., into oyster-shells) and prey is obtained
in this way. The pyloric glands are said to be digestive
in function and the anal glands mainly excretory.
.The ccelom is spacious and is cut into during dissection.
It is divided into several separate parts. The
most important is (1) the water-vascular system
(or ambulacral system). This is a part of the ccelom in
which is concentrated the motor function found elsewhere
Vascular.
ASTERIAS. 159
Fig. 92.--ABORAL DISSECTION OF A COMMON STARFISH. (4d zat.)
Ovoid Gland.
Pyloric Part
of Stomach. H
Madreporite.
Ampulle.
Cardiac Part
of Stomach.
Anus,
Gonads.
Anal Glands.
Pyloric Glands.
Tube Feet.
The lower right arm is turned over to show the oral surface ; the lower left arm has its
aboral wall removed and the upper left arm has the pyloric glands removed.
160 ARCHICELOMATA.
(¢.g., Arenicola) as a general feature of the whole ccelom. It
consists mainly of a ring-canal round the mouth, five radial
vessels just below the heads of the ambulacral ossicles,
and a single inter-radial s/one-canal running in the median
mesentery to the madreporite, through which it communi-
cates with the exterior. Each radial canal gives off paired
Fig. 93.—DIAGRAM OF THE WATER-VASCULAR SYSTEM
OF THE COMMON STARFISH.
(Altered from GEGENBAUR.)
q 3 Madreporite.
Stone Canal.
Radial Canal.
C ) Ring Canal.
Ampulla.
Tube-foot,—_ >
lateral canals which lead to the round vesicles or ampulle
seen as a double row on the inside of the ambulacral ossicles.
These ampullze communicate with the tube-feet, which, as
noticed before, protrude into the ambulacral groove, between
the ambulacral ossicles. The walls of this system are mus-
cular, and it works as a hydraulic system by means of the
tube-feet.
The rest of the ccelom is formed by (2) a large and spacious cavity,
the hypogastric cavity, surrounding the lower part of the stomach,
produced into the arms and forming a median mesentery in the
madreporic inter-radius. (3) Aboral to this, lying on the stomach and
produced into each arm along the aboral surface of the pyloric glands, is
the epigastric cavity. Its walls form, with those of the hypogastric
cavity, the two mesenteries along each hepatic caecum. The hypo-
gastric cavity is produced along the oral surface of each arm, inside
the radial nerve, to form a pair of perihzemal cavities. The inner walls of
these cavities form a median mesentery, in which is contained the radial
BALANOGLOSSUS. 161
blood-vessel. (4) Alongside of the stone-canal and opening into the
madreporite is a long but small cavity called the axzal sinus. Part of
its wall appears to form the so-called ovoed gland.
The walls of the ccelom form muscles and the paired
gonads which are situated inter-radially, opening dorsally
to the exterior by fine pores. Scattered over the aboral
surface are pores through which the ccelomic wall protrudes,
as small vesicles or dranchie. The blood-vascular system
of the starfish is represented, as in Badanoglossus, by sinuses
in the mesenteries and possibly by a central heart.
The radial nerves are connected with a nerve-ring round
the mouth. Throughout its course the nervous system is
an integral part of the ectoderm. An aboral nervous
system has also been described.
The eggs are fertilised in the water ; segmentation is total and equal,
producing a blastula and gastrula larva. The gastrula is further dif-
ferentiated into a free-swimming pelagic Azpénnaria.
Development. This larva has a pre-oral and a post-oral ciliated band
coiling over the surface of the body, and is serfectly
plano-symmetric. Its mesoderm is early segmented into five principal
parts, one pre-oral and two post-oral pairs. The adult grows like a
large wart on the left side of the larva. ;
This development is important, for it shows that the apparently axo-
symmetric Echinodermata are descended from plano-symmetric forms,
with an archimeric segmentation like that of the other Avchicelomata.
II._BALANOGLOSSUS.
PHYLUM ARCHICCELOMATA (p. 170).
Sus-PHYLUM ARCHICHORDA (p. I7I).
Balanoglossus is a long worm-like animal of a bur-
rowing habit. It has the body divided into three segments.
The anterior, or proboscis, lies in front of the
mouth (or is pre-oral) and can be expanded or
contracted at the will of the animal. The second
segment or collar encircles the body immediately behind
the mouth which is in the median ventral line. The third -
part or ¢runk is long and forms the remainder of the body.
At its extreme end opens the azus. In the constricted
neck between proboscis and collar there open dorsally
two small pores, the proboscis pores, which lead into the
cavity of the proboscis. A similar pair of pores (the
M. 12
External
Features.
162 ARCHICGLOMATA.
collar pores) lead out from the collar at its hind end.
In the front region of the trunk, opening dorso-laterally
into a long dorsal groove, there are two rows of small
- slits which open downwards into the alimentary canal.
They are numerous and are known as the pharyngeal
clefts. Tying outside the pharyngeal clefts, and also con-
tinued backwards behind them, are two rows of genital
pores.
Fig. 94.—SEMI-DIAGRAMMATIC VIEW OF BALANOGLOSSUS
FROM THE DoRSAL SURFACE.
Proboscis.
Collar.
Trunk,
Hepatic Glands.
Openings of
onads.
The ectoderm consists of a simple ciliated epithelium
with unicellular mucous glands.
The mouth leads into an elongated pharynx. The
extreme anterior wall of this pharynx is pushed forwards
into the proboscis as a diverticulum, the szd-
BUMRET: a oural gland.* The epithelial cells, forming
the wall of this organ and that of the anterior part of
the pharynx, are metamorphosed into chordoid tissue (see
Chapter XXIV.). Their protoplasm is almost entirely re-
placed by vacuoles with walls and scattered nuclei, amongst
which there may be small glands. This structure converts
* This organ is also known as the ‘‘ notochord” or ‘‘ stomochord.”
BALANOGLOSSUS.
Fig. 95.—ANATOMY OF BALANOGLOsSUS (Diagrammatic).
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163
Proboscis
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Subneural
Ventral [Gland.
Mesentery.
Central
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Mesentery.
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Ventral Nerve Cord.
A, Median sagittal section. B, Transverse section of proboscis. C, Transverse
D, Transverse section of trunk.
section of collar.
164 ARCHICELOMATA.
the pharyngeal wall into a stiff body. Hence the subneural
gland serves to support the proboscis, and the pharyngeal
wall with the mouth are permanently rigid and open.
The posterior portion of the pharynx becomes almost
divided into a dorsal and ventral part, each of which is
ciliated. The pharyngeal clefts open into the dorsal por-
tion, which is often called the respiratory part, the ventral
being distinguished as the nutritive portion. Water, mud
and nutritive particles pass in at the mouth through the
pharynx. The water is said to pass out dorsally by the
pharyngeal clefts The mud and food pass ventrally into
the intestine in which digestion is effected. In many
species numerous fepfatic glands open dorsally into the
intestine. The intestine runs to the end of the trunk and
terminates in the anus.
The external segments of the body are found to corre-
spond to cavities of the mesoderm. The proboscis has a
single coelomic cavity, opening behind by a
single or, in some cases, two proboscis-pores.
The walls of the cavity form complex muscles for the
movements of the proboscis.
Colom.
In the collar there are paired ccelomic cavities separated
by dorsal and ventral mesenteries which suspend the
pharynx. Each opens to the exterior by a collar-pore.
The trunk also has a pair of cavities with dorsal and ventral
mesenteries. The walls form a well-developed system of
longitudinal muscles and paired gonads. The trunk-cavities
are produced forwards into the collar by two dorsal (or
perthemal) and’ two lateral (or seripharyngeal) processes.
The ccelom is filled with a network of connective tissue.
The nervous system consists of a central nerve-mass in
the dorsal region of the collar, a nerve-ring
round the hind end of the collar, and a median
dorsal and median ventral nerve along the trunk. Nerves
pass forwards on to the proboscis.
Except in the dorsal collar-region, the nerves are still
parts of the ectoderm and are connected in all directions
by a diffuse nerve-plexus.
The blood-vascular system consists of sinuses or vessels
between the constituent layers of the dorsal and ventral
Nervous.
BALANOGLOSSUS. 165
mesenteries — the dorsal and ventral vessels. ‘lhe dorsal
vessel runs forward to the ‘central sinus” which
lies just over the subneural gland — partly
surrounded by the contractile pericardium. In
its course through the trunk the dorsal vessel receives
efferent branchials from the pharyngeal slits. The blood
appears to leave the central sinus and pass forwards to a
paired glomerulus. This is a glandular excretory organ
formed from the wall of the proboscis-ccelom. From this
it finds its way into the ventral vessel round the pharynx.
The ventral vessel takes the blood back to the body.
The course of the blood is thus as follows :—
Pe vessel
Central sinus a
{ Gils System
Blood Vas-
cular,
Excrefory organ
“Saal vessel
Development.—The sexes are separate and the sexual products
are shed into the water. There are two types of development. In
one there is a larval development with a free-swimming pelagic larva
called Yornarda; in the other there is a demersal larva with direct
development. The main facts in the direct development are as
follows :— :
1. Total equal segmentation to form blastula larva.
2. Invagination to form a ciliated gastrula which escapes from
egg-membrane.
3. The hypoblast gives off five archenteric pouches to form the
five segments of the mesoderm, one pre-oral and two pairs of
post-oral segments. The exterior of the elongated larva
becomes marked off into proboscis, collar and trunk.
4. Mouth and anus are broken through, the latter at the same spot
as the closed blastopore.
5. The subneural gland grows forward from the front end of the
pharynx and pharyngeal clefts appear.
The larva Zornaria is a transparent pelagic form with three complex
ciliated bands, a pre-oral, a post-oral, and a peri-anal. The last is
motor and the two former are mainly trophic. It has a complete
alimentary system from the first and differs mainly from the demersal
larva in the large size of the heemoccele, the small mesodermic segments,
the early formation of the pericardium and the presence of eye-spots.
The pharynx of Yornaria appears to have paired chordoid areas
(pleurochords) found in adult allies.
166 ARCHICELOMATA.
Balanoglossus is an important type of the Archicalomata
and is additionally interesting as having in its anatomy a
foreshadowing of certain organs found in the Chordata (see
Chordata).
It belongs to the Archicalomata, chiefly because of its
archimeric segmentation of the mesoderm and the Azchi-
chorda from the presence of chordoid structures and pharyn-
geal clefts.
III._LOPHOPUS.
PHYLUM ARCHICCELOMATA (p. 170).
SuB-PHYLUM BrYOZOA (p. 177).
Lophopus is a small freshwater organism common in rivers and
streams. It is a colony of individuals or polypes which are embedded
in a common gelatinous investment. The whole colony executes slow
creeping movements.
Each individual has a crown of tentacles or /ophophore which is in
the form of a horseshoe. It bears a row of tentacles the whole way
round the edge of the horseshoe, the row on the outer or convex edge
being continuous round the ends with that on the inner or concave edge.
A web unites the bases of the tentacles. In the centre of the horse-
shoe, between the rows of the tentacles, is situated the mouth. This is
overhung by a small flap or process, the egzstome, between it and the
inner row of tentacles. In the concavity of the lophophore, and hence
outside the tentacles, opens the anus. When undisturbed, the animal
spreads the tentacles apart and the cilia covering them cause currents
with food-particles to pass towards the mouth. _—On stimulation the
polype retracts itself and the tentacles are withdrawn. os
In the interior of the animal we find that the mouth leads to an
cesophagus and a lobular stomach, from which the intestine runs forward
to the anus. The whole alimentary canal is therefore flexed into a U.
The ectoderm is simple and secretes the gelatinous investment.
Between it and the alimentary canal is the spacious ccelom lined by
mesoderm. In the epistome is a pre-oral portion, the epistomial
cavity, which communicates on either side with a lophophoral cavity
produced out into each arm of the horseshoe and separated from the
spacious trunk-cavity by a ¢ransverse septum. The trunk-cavity is lined
by a thin layer of mesoderm which extends over the alimentary canal
and inside the ectoderm. At the aboral end it is differentiated into
circular muscles called the parietal muscles. These on contraction com-
press the coelomic fluid and force the oral part of the polype upwards.
From the base of the stomach there runs a band or fznzculus which
attaches the alimentary canal to the aboral end and a refractor muscle runs
beside it. The mesoderm upon the funiculus gives rise to testes and
ovaries and the animal is hermaphrodite. The main nerve-ganglion
LOPHOPUS. 167
lies between mouth and anus just under the epistome. It gives off
a ring round the cesophagus and other branches.
In allied species the trunk-ccelom opens by paired ciliated canals
to the exterior near the anus.
Fig. 96.—View oF ENTIRE CoLony oF LoPHorus.
(After ALLMAN.)
Lophopus is an annual and dies down on the approach of winter.
It produces, however, before this event, a number of encapsuled_ buds
called statoblasts which give rise to a fresh colony in spring. These
are not found in its marine allies. (See page 61.) The colony is
produced by asexual budding from a single individual, hence metagenesis
occurs as in hydroid zoophytes.
168 ARCHICGELOMATA.,
Many bryozoan colonies have a close superficial resemblance to the
hydroid colonies, hence it should be noted that the bryozoan polype is
a far more highly organised animal than the hydroid. The possession
by the former of mesoderm and a ccelom and a definite nervous system
may be specially emphasised.
IV.—SAGITTA.
PHYLUM ARCHICGLOMATA (p. 170).
SuB-PHYLUM CHA&TOGNATHA (p. 177).
Sagitta, the arrow-worm, is a free-swimming pelagic animal of
elongated body and may be about 3{ inch in length. It is one of the
simplest and best types of the pelagic zekton. Its body is of a glassy
transparency, cylindrical in transverse section and perfectly plano-
symmetric. The anterior end is formed into a head, with mouth
surrounded by tufts of sete or bristles which act as jaws. The
posterior end bears a bifid caudal or tail-fin and two pairs of lateral
fins protrude from the body.
Three parts of the body can be distinguished. The head, the
elongated zvunk and the faz/. The mouth leads into a pharynx, which
passes into a simple intestine, terminating in an anus, situated ventrally
between trunk and tail. Corresponding with the three segments are
the three mesodermic segments. The head segments have their walls
largely modified into jaw-muscles; the trunk segments also form
dorsal and ventral longitudinal muscles and a pair of spacious coelomic
cavities. The walls of these form dorsal and ventral mesenteries
supporting the intestine. In the tail the segments are very similar,
but as there is no intestine in this part the mesentery is continuous and
median.
In the trunk the ccelomic walls form paired lateral ovaries; in the
tail they form ¢estes. Each of these lead, by paired oviducts and vasa
deferentia respectively, to the exterior near the anus. The animal is
therefore hermaphrodite. Transverse septa are found between the
three segments.
The nervous system consists of a dorsal brain in the head with paired
connectives round the neck to a large subcesophageal mass on the
ventral surface of the trunk. The brain supplies nerves to a pair of
simple eyes on the head and certain sense-papille.
Sagitta reproduces only sexually. The eggs and larve are pelagic
and transparent, though demersal eggs and larvee are known in the
sub-phylum.
V.—WALDHEIMIA.
PHYLUM ARCHICCELOMATA (p. 170).
Sus-PHYLUM BRACHIOPODA (p. 177).
Waldheimia is a small marine organism enveloped in two shells.
They are roughly circular in outline and convex externally. The so-
called ventral shell is produced behind the other or dorsal into a
bee be
WALDHEIMI4. 169
process. Through a hole in this process there projects a peduncle
which fastens the animal to a foreign body, such asa rock. A side
view of the two shells recalls the appearance of a Roman lamp, with
the peduncle as a wick ; hence the Brachzopoda are sometimes termed
Fig. 97.—VENTRAL (A) AND Dorsar (B) SHELL or Waldheimia
Australis, (After DAVIDsON. )
a, Adductor. a, Adductor.
a', Apex. 2, Loop.
6, Ventral Adjustor. Pp, Septum.
c, Divaricator. 7 s, Socket.
J, Foramen.
o, Peduncular Muscles.
t, Teeth.
Lamp-shells. The two shells are hinged upon each other at the posterior
end (towards the peduncle) and can be widely opened anteriorly. The
shells and the animal are plano-symmetric, about a perpendicular plane
passing through the middle line of each shell. (The bivalve Mollusca
are plano-symmetric, about a plane passing Je¢ween the shells, which
are therefore zzght and eft, not dorsal and ventral. )
Inside the dried dorsal shell can be seen a complex calcareous
skeleton in the form of a twisted loop. The growth of the shell is like
that of bivalves. The cavity inside the shells is lined by a soft double
flap of the body called the mule, enclosing the manéle-cavity. Its
edge is fringed with sete.
The most conspicuous part of the body is the lophophore, which
consists of a pair of coiled arms carrying a great number of ciliated
tentacles. A ridge lying dorsal to the mouth, the epzstome, is continued
round the lophophoral arms. The mouth leads into a short cesophagus,
a swollen stomach, and a short intestine which ends blindly. There is
a pair of large racemose digestive glands, with ducts leading into the
stomach. The ccelom is spacious, and the same parts of the mesoderm
170 ARCHICELOMATA.
can be recognised as in other drchicalomata. Thus there is an un-
paired epistomial cavity, a pair of lopbophoral cavities and a large
trunk-cavity partially divided up by a ventral mesentery and certain
bands. The trunk-cavity opens by paired tubes or zepfhridia into the
mantle-cavity. Its walls also form the muscles and the gonads. The
muscles are numerous and well developed, mainly for moving the
shells and peduncle. The gonads and the trunk-cavity spread into
the mantle.
There seems a somewhat indefinite blood-system with a contractile
heart situated dorsal to the stomach. The nervous: system is a ring
round the cesophagus with dorsal brain and ventral subcesophageal
ganglia, which latter are the larger. Numerous nerves supply the parts
of the body.
The sexes are separate and the development is larval. Most
brachiopod larvee are pelagic and have three segments.
PHYLUM ARCHICCQZLOMATA.
The five preceding types represent the five most impor-
tant divisions of this diverse phylum. The phylum includes
the most primitive and simplest representatives of the truly
coelomate animals. They are usually described as wn-
segmented, but there can be discerned in them, with more or
less clearness, a primitive avchimeric segmentation into three
parts. The first is anterior to the mouth or pre-oral, and
the other two are post-oral. They may be called the
protomere, mesomere, and opisthomere. They are probably
represented in the segmented worms by the fvostomium, the
peristomium, and the rest of the body respectively ; hence
these differ from the Archicwlomata in having the opistho-
mere divided into a great number of segments or mefa-
meres, oy metamerically segmented.
In addition the Archicelomata usually have a nervous
system, often in continuity with the ectoderm, a dorsal
brain, an cesophageal nerve-ring and usually a ventral pair
of ganglia. The ccelom retains its primitive relationships
and any of the segments may have ciliated tubes to the
exterior. The vascular system, if present at all, is a series
of heemoccelic sinuses and the archimeric heart is, if present,
dorsal to the alimentary canal.
All have more or less primitive methods of feeding; they
are mostly pelagic, sedentary or burrowing, and are modified
accordingly.
ARCHICG@LOMAT-. 171
In their development there is, in the majority of cases, an
equal segmentation, a blastula, gastrula and a pelagic larva ;
in fact, a typically larval development throughout. The
ie in most cases arises by pouches from the hypo-
ast.
Sus-PoyLum I.—ArcHicHoRDA. — Balanoglossus is the
burrowing vermiform representative of this sub-phylum, but
there are also sedentary relatives. They have special interest
as they appear to be allied to the ancestors of the metamer-
ically segmented Chordata. Thus the sub-phylum shows a
dorsal nervous system, pharyngeal clefts and chordoid por-
tions of the pharynx. This relationship will be mentioned
in the Chordata.
Cephalodiscus is a small deep-sea form which lives in
sedentary communities. There is only a single pair of
pharyngeal clefts and two pleurochords. Others approxi-
mate in habits to the Folyzoa.
Sup-Puytum II.—EcHInopERMaAtA.—As¢erias is a fair
representative of this large sub-phylum. They all show
plano-symmetric larvae, which go through a metamorphosis
into the adult, Their special features are the great develop-
ment of a mesodermic calcareous skeleton and a modification
of part of the ccelom into a water-vascular (or ambulacral)
system. The larve show the archimeric segmentation of the
Archicelomata. The peculiar axial symmetry is usually
supposed to be due to a sedentary or fixed habit in the
past. Like most Archicalomata, they are well represented
in early epochs.
There are five classes of the Echinodermata :—
Crass I.—Asteroidea, of which Asferias is a type.
Ciass II. — Ophiurordea (the brittlestars).—These are five-rayed,
but the arms are almost entirely filled by the enlarged ossicles
and are sharply distinguished from the central disc. The greater
flexibility of the arms enables them to be largely used as motor
organs and the tube-feet are correspondingly reduced. There is no
anus and the madreporite appears to have become shifted to the
ventral surface. They differ in several other points from the
Asteroidea. ’
172
ARCHICELOMATA.
Fig, 98.—A BritrLesrar (Natural size).
ANA fi a
este We
View of oral surface showing mouth and genital pores. Notice the jointed arms.
Ciass III.—2£chinotdea (Sea-urchins).—These are spherical or oval
in shape, and the calcareous skeleton forms a continuous mass of
plates bearing spines. The anus opens at one pole and is sur-
rounded by five gemztal plates which are inter-radial. One forms
the madreporite and a genital opening is situated oneach. Between
these there lie the smaller ocz/ars. They bear the simple eyes and
are radials. From these ten plates there run down orally ten
double rows of plates. Those below the oculars are called the
radials or ambulacrals as they bear rows of tube-feet. Those
between them are zx¢er-radza/s or antambulacrals.
The mouth opens in the middle of a buccal membrane and is
armed with five teeth. These are borne by a beautiful calcareous
structure often called Avrdstotle’s lantern.
ARCHICELOMATA. 173
Fig. 99.—A ComMMon SEA-URCHIN ( Echinus
Microstoma).
Natural size (After WyviLte THomson).
The animal is seen from the aboral surface, from the left
half of which the spines have been‘removed. The plates can be
identitied from the next figure. - ‘ pete
Fig. 100.—D1acGRraM OF Dorsat Virw or EcHINus
SHOWING THE PLATES.
Genital Plate.
Madreporite. Anus.
Ocular Plate.
Antambulacral Plates.
Ambulacral Plates.
In the sand-dwelling types, or Heart-urchins, such as Spatangus,
there are no Jantern nor teeth, and the body becomes plano-
symmetric.
174 ARCHICG@LOMATA.
Fig. 101.—ViEw oF Echinus Microstoma.
(After WyviLtE THomson.)
A, Internal View of the Skeleton showing Aristotle’s Lantern in position.
B, Aristotle’s Lantern or Dental Pyramid.
Crass IV.—Crinoidea. —The Crinoids have five jointed arms which
bifurcate at the base, forming ten. Each has a number of pinne or
small processes containing the gonads. The crinoids are fixed by
a long stalk or axis to the sea-floor either throughout life or for the
earlier part of. it. They are known as the stone-lilies and are
mostly deep-sea forms.
Fig. 102.,—ViIEW OF INTERIOR OF BISECTED SEA-URCHIN
(Echinus Lividus ).
Note the long coiled intestine suspended to the body-wall by mesentery.
@, Gullet. a, Anus.
2, First coil of intestine. ca, Ocular plate.
m, A jaw-muscle. z, Intestine.
“po, Cut end of a radial vessel. Z, Second coil of intestine.
s, Part of the dental pyramid. ?, Radial water-vascular vessel.
v, Ovary.
Crass V.—Holothuroidea (the Sea-Cucumbers).—These have the body
elongated in an oral-aboral direction, in some cases simulating a
‘¢worm.” The ambulacra run in five rows down the sides of the
body and, in addition, there is a ring of branching tentacles round
the mouth. They have scattered calcareous spicules in the body-
wall which give it a tough but flexible consistency.
ARCHICELOMATA. 175
Fig. 103.—THE Rosy FEATHERSTAR (Antedon Rosacea ).
_ Natural size. View of oral surface. The ten arms are pinnate, and from
their bases pass five ciliated grooves to the mouth in the centre of the disc. The
anus is situated on an inter-radial papilla (seen below the mouth in the figure).
Fig. 104.—A HoLoTHuRIAN (Cucumaria Planci) x 2.
Note the oral tentacles and the elongated body with five rows of tube-feet.
ARCHIC@LOMATA.
176
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ARCHICELOMATA. “77
Sus-Puy_um III.—Bryozoa.—The Bryozoa are marine
and freshwater colonial forms. They are all practically seden-
tary and in many types there is great physiological division
of labour, some polypes degenerating into mere vibratile
processes or snapping pincers.
The class Phylactolemata (with horseshoe-shaped lopho-
phore) are all freshwater types and the polypes are better
developed.
The Gymnolemata (with circular lophophore) are
nearly all marine and have more modified polypes.
Their skeletons may be calcareous or chitinous and are,
as in the case of the hydroid zoophytes, constructed upon
the same principles of branching as plants.
Sup-PHyLum IV. —Cuetocnatua.—This is a small
group for Sagitfa and its allies. Sagétta is practically
typical of the sub-phylum. It is important, showing the
possibilities in the Avchicwlomata of an active progressive
type.
Sup-PHyLum V.—BracHiopopa.— These are like Wald.
heimia, all two-shelled, and are divided into the hinged and
those without hinges. They are like the rest of the Archi-
celomata of ancient origin and some types, such as Zinguda,
with a long peduncle used as a motor organ, appear to have
remained constant in structure from the earliest geological
times.
PHYLUM ARCHICCELOMATA.
1. Ccelomate tridermic metazoa with plano-symmetry.
2. No metameric segmentation, but a distinct archimeric segmenta-
tion into three parts.
3. Coelom well developed and divided more or less into parts
corresponding: with the segmentation.
4. Nervous system simple, with dorsal brain, circumcesophageal
ring and cords, often retaining its connection with the ectoderm.
5. A simple blood-vascular system of heemoccelic sinuses.
6. Usually have a free larval pelagic development.
7. Mostly marine and pelagic, sedentary or burrowing.
M. ; 13
ARCHICGLOMATA.
178
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ANNULATA.,
CHAPTER XVI.
ANNULATA.
POLYGORDIUS. ARENICOLA. HIRUDO. LUMBRICUS.,
I.—POLYGORDIUS.
Fig. 105 —PoLyGorpIUs
NEAPOLITANUS.
Puyium - ANNULATA (p. 237).
SuB-PHYLUM - ANNELIDA (p. 238).
Crass - ARCHIANNELIDA (p. 239).
Polygordius neapolitanus is a small
delicate marine (13% inch) worm, of elon-
gated body, found in the sand at moderate
depths. It is of a pale pink hue. At the
anterior end are a pair of small tentacles,
the prostomial tentacles, covered with fine
sete. They are parts of the Jrostomium,
a lobular process lying anterior and dorsal
to the south. Immediately behind the
mouth is the eristomium, a large and
well-defined segment. The rest of the
body is divided externally and internally
into a series of segments. The terminal
or anal segment is swollen and bears the
anus. On its broadest part this segment
bears a ring of papilla. The mouth leads
into an cesophagus which continues as a
simple intestine terminating in the anus.
The ectoderm is a simple epithelium with
unicellular glands ; it secretes a thin cutécle.
Below it the mesoderm forms a well-
developed system of longitudinal muscles.
Inside them is the delicate ccelomic
epithelium, lining a spacious ccelom. This
is divided by dorsal and ventral mesen-
teries, in which are simple blood-vessels,
and by transverse septa between each
segment. Each segmental part of the
ccelom opens by simple paired tubes (or
nephridia) to the exterior. The nervous
system is still part of the ectoderm. It
consists of « brain in the prostomium,
179
Prostomial Tentacles.
Anus,
Entire animal seen from dor-
sal aspect X 5.
FRrateont.)
(After
180 ANNULATA.
Fig. 106.—TRANSVERSE SECTION OF PoLyGoKDIUS, SEMI-
DIAGRAMMATIC. (After FRAIVON’.)
Dorsal Blood-vessel.
Intestine.
Oblique Septum
bearing the
Gonad.
Nerve-cord. Ventral Blood-vessel.
a ring round the cesophagus and a ventral nerve-cord, swollen into
ganglia in each segment. Immediately behind the prostomium and
placed laterally are a pair of ciliated pits which are probably sensory.
Fig. 107.—CORONAL LONGITUDINAL SECTION OF
PoLtycorpius (Highly Magnified).
Intestine.
Circular Blood-
___vesse!
Y Nephridium.
Segment. Nephridiopore.
Segment.
The sexes are separate and the testes and ovaries arise on two diagonal
muscular bands intersecting the ccelom.
ARENICOLA. 181
Fig. 108.—LaTERAL Virw oF Front Enp or
PoLtycorpDius. (After FRAIPONT.)
Ciliated
Peristomium. Pit. Prostomium,
1
Mouth.
The sexual products escape by rupture of the body-wall. The
following points in the development may be noted :—
. Fertilisation external.
. Total equal segmentation to form a dlastila.
. Invagination to form gastrida and closure of the blastopore.
. Elongation of the larva and invagination of anterior oesophagus
and posterior hind-gut, forming mouth and anus.
. Production of ¢vochophore, with three bands of cilia, pre-oral,
post-oral and peri-anal, paired larval ‘‘ kidneys” consisting
of branching blind tubes opening externally, an apical plate,
with pigment spot, and mesoblastic pole-cells laterally to the
hind-gut. ;
6. Elongation and growth of the posterior region to form the body
(except prostomium and peristomium):of the worm. Splitting
and segmentation of the mesoblastic bands to form the ccelom,
whilst the walls form muscles. Formation of nerve-cord from
epiblast cells.
7. Loss of ciliated bands and pelagic habit. Growth of prostomial
tentacles and nephridia. The young worm assumes the creeping
burrowing habit of adult.
wm BWNHe
II.—ARENICOLA.
PHyYI.UM ANNULATA (page 237).
Sus-PHYLUM ANNELIDA (page 238).
Cass POLYCHATA (page 239).
Arenicola marina is a worm, usually about 8 inches
long, found very commonly burrowing in the sand between
Hapits, ‘ide-marks. Its burrow is U-shaped, and from
one to two feet in depth. At one end the sand
is thrown up at the surface in small coils or casts} which
have been ejected from the animal, whilst at the other end
is a conical depression in the sand, below which rests the
head of the worm. The burrow is lined with a mucous layer
secreted by the skin.
182 ANNULATA.
Fig. 109. —LaTERAL Vitw (LEFT Fig. 110.—DISSECTION OF ARENI-
SIDE) OF THE LoBworm ( Aren- COLA. “(Ad nat.)
tcola Marina). (Ad nat.)
.Pharynx.
First Mesentery;
Peri-
stomium Septal Pocket.
jum.
. i. Nephridium.
ll
:
(Esophageal
Nephridiopore. Gland
Ventricle.__
5 Auricle., \
Neuropodium.
Efferent
6 Dorsal. nee”
Lateral. Branchial.
Ventral.
Subintestinal. -
e
j Gills.
9
Io
me.
12
z 3 a
#{ - Seventh Efferent
14 Branchial.
Seventh Afferent
Branchial.
Dorsal Blood-vessel.
First Posterior
Mesentery.
GHITTLLD Anus.
The pharynx is protruded. The body-wall is cut down the median
dorsal line and pinned down on each side.
ARENICOLA. 183
The animal is plano-symmetric, with a mouth at the
anterior and an anus at the posterior end. The pharynx
is protrusible and is used by the animal for
“rolling” sand and food into the alimentary
canal. In the living animal its action may
often be seen. It is covered with rough papille on its
front part and with hooks further back.
The body is differentiated by its structure into three
portions :—
(1) The anterior region with appendages but no gills.
(2) The middle part with gills and appendages.
(3) The posterior region or tail with neither gills nor appendages.
External
Features.
The whole body is marked off by a great number of
rings or annul, but these should be carefully distinguished
from the far less numerous mefameric segments. In the
greater part of the body there are five annuli to each
segment. The number of segments, at least in (1) and (2),
can be counted by enumeration of the appendages or the
gills.
The class of Polycheta has each segment typically pro-
vided with a pair of lateral appendages, called feet or
parapodia, and each parapodium usually has two parts—a
dorsal portion or nofopodium and a ventral part or euro-
podium. Each part bears a tuft of sete or bristles. In the
active, swimming allies of Avenicola these feet are well
developed, but in burrowing forms they tend to become
reduced in size. Thus in Avenicola the notopodium is
reduced to a small process with sete, and the neuropodium
to a long pad with a single row of short hook-shaped sete.
Arenicola has nineteen pairs of these appendages, and they
are all similar except for the reduced size of the xeuro-
podium in the first few segments. In the anterior region (1)
the mouth is overhung by a small dorsal process, the pvos-
tomium, and immediately behind this is the peristomium
which differs from the true segments in having no
appendages. Then follow six true segments, each having
appendages, and the last three of which have, just above
the zeuropodium, a minute excretory pore or xephridiopore.
The middle portion (2) has thirteen segments, each
having a pair of appendages, and the first three also have
nephridiopores. All bear gills (or dranchie) which project
184 ANNULATA.
dorso-laterally and are beautifully branched delicate organs
of respiration. Through their thin walls the blood and
outside water interchange carbonic acid and oxygen. The
posterior part (3) consists of a great number (about 30) of
Fig. 111.—A MaGNIFIED GILL-SEGMENT OF ARENICOLA.
(After AsHworTH and GAMBLE.)
Notopodial Sete.
Branches of Gills. ¢
f
Annulus.
Nephridiopore. Torus Uncinatus
(Neuropodium).
compressed segments, on none of which are there any
appendages or gills.
The body is covered by a fine but definite
cuticle secreted by the simple underlying ecto-
derm in which there are unicellular glands.
If the animal be cut open down the median dorsal line
the whole internal anatomy is exposed, for Avendcola is a
ccelomate animal and all the internal organs lie in contact
with the ccelom.
The ccelom is spacious, as in Polygordius, but it is not
completely divided up into compartments. The front part
is divided by three transverse sef/a2,* between the
first, second, third and fourth segments, which
Integu-
mentary.
Colom.
* The first septum has a pair of hollow septal pockets which are muscular, and
probably assist in protrusion of the pharynx. "
ARENICOLA. 185
hold the front part of the alimentary canal in position.
There is no dorsal nor ventral mesentery, but in the “tail”
region, from segment 20 backwards, there are regular trans-
verse ‘septa. Hence the alimentary canal, from segment 4
to segment 19 inclusive, is free to move in the ccelom.
The pharynx leads into a_long esophagus which widens
out into the stomach. Just before the commencement of
the latter there is a pair of esophageal glands
or pockets, the lumen of which opens into the
cesophagus. They are probably digestive glands. The
ay Alimentary.
Fig. 112,—TRANSVERSE SECTION OF ARENICOLA (Diagrammatic).
Dorsal Blood-vessel receiving Efferent Branchial.
Gill (1-6).
Cay
Branchial.
Efferent 7
Branchial. . Oblique
Muscle.
Afferent :
Branchial. #” ~- Longitudinal
Muscle.
Circular Muscles
é ; - Co with Ectoderm
Ventral Vessel.’ on at. re and Cuticle
ge GP outside,
On the left is seen the arrangement of the first six branchial segments ; on the right the last seven.
stomach is wide, and is covered with yelJore-cel/s, intersected
by a network or plexus of blood-vessels. At about the level
of the seventh pair of gills the stomach passes into the
intestine which terminates in an anus. ;
Arenicola is perpetually employed in passing quantities
of sand and food-particles through its alimentary canal,
digesting the latter and egesting the former.
186 ANNULATA.
The blood-system is complex. ‘The blood is respiratory
in function and is said to contain hemoglobin, giving it a
red colour. The vessels lie between the coelomic
epithelium and the alimentary canal or the body-
wall, as the case may be. Along the whole
length of the alimentary canal runs a median dorsal
vessel in which the blood runs forwards. It supplies
branches to the alimentary canal throughout its course,
and it receives rated blood from the Jast seven pairs
of gills by paired efferent branchials. Below the ali-
mentary canal, but hanging free from it, runs the median
ventral vessel. Its chief branches are thirteen pairs of
afferent branchials taking blood #o the gills and some to the
nephridia. In this vessel the blood flows backwards, and
it drains the regions of the alimentary canal supplied by
the dorsal vessel. At the commencement of the stomach
there are a pair of hearts. Each is two-chambered, consist-
ing of an auricle and a ventricle. On contraction of the
ventricles on each side the blood from the heart is driven
into the ventral vessel.
Over the stomach is a plexus of vessels, of which we may
discern the two fosterior lateral vessels and two sudbintes-
tinals in the ventral wall of the stomach.
The subintestinals receive blood from the first six pairs
of gills by six efferent branchials on each side. The
subintestinals communicate through small vessels with
the posterior laterals, which carry the blood forwards
and, together with paired wsophageals on the cesophagus,
fall on each side into the auricle of the heart. On con-
traction of the heart the blood is driven through the
ventricles and thence into the ventral vessel. We may
summarise this rather complex arrangement by a diagram—
Blood Vas-
cular.
Anterior Posterior
Dorsal
f, 7 Gills
: Post”
Ant ater 28!" lat im a:
{ oe rer rnlesr ;
Gills
Ventral
ARENICOLA. 187
It should be noticed (1) that the hearts are paired, two-
chambered and independent in action; (2) that the hearts
are not on the main outer circle formed by the dorsal and
ventral vessels, but merely force blood into this current ; (3)
that the hearts pump blood to the gills and the body, and
hence they are not definitely either respiratory or systemic.
Fig. 113.-- Virw oF NERVE-RING AND BRAIN OF
ARENICOLA.
(After ASHworTH and GAMBLE.)
Note the Césophagus cut across leaving a hole round which the nerve-ring connects
the brain with the ventral nerve-cord.
The nervous system in Avenicola consists of a paired
brain in the prostomium, a ring round the pharynx in the
peristomium and a ventral nerve cord running
in the median ventral wall of the body. There
are no ganglia. Arenicola has no eyes but is endowed with
Nervous.
188 ANNULATA.
a pait of ofocysts. These are situated laterally on the peris-
tomium and are supplied by nerves from the
brain. They consist of spherical sacs communi-
cating with the exterior by fine ducts. The cells lining the
Sensory.
Fig. 114.—SECTION THROUGH THE OTOCysT OF ARENICOLA.
(After ASHWORTH and GAMBLE.)
External Aperture.
Leta Rap
ial
Epithelium.
sac are sensory and the cavity contains a number of loose
concretions or ofo/iths which appear to be sand-grains.
On the prostomium is a ciliated pit called the nuchal-
organ, a probable sense-organ allied to the paired ciliated
pits of Polygordius. Arenicola has no prostomial tentacles,
but the prostomium is produced into two odes which are
also probably sensory.
The muscular system is well developed and consists of
a circular layer under the ectoderm and a longi-
tudinal layer inside it. There are also diagonal
fibres running from the lateral lines to the mid-ventral line.
Nearly all the animals usually called ‘‘ worms ” move by the system
of circular and longitudinal muscles. The body of the ‘‘ worm” which
contains coelomic fluid acts much like an elongated bladder or sausage-
skin filled with fluid. When the circular muscles contract they press on
the coelomic fluid which forces out each end and hence elongates the
worm, reducing its calibre. When the longitudinal muscles contract
the body is shortened, the coelomic fluid at the same time forcing the
walls outwards. Hence the alternate movements of the two muscular
Muscular.
ARENICOLA. 189
series enable the animal to lengthen or shorten its body, like a concer-
tina. But this by itself would not effect movement of the whole body.
Further provision is usually found in hooks, suckers, or setae, which
catch the ground or surrounding medium and prevent movement in ove
direction, usually backwards. By this means all the expansion or
lengthening and shortening or contraction must take place forwards and
progress is therefore rapid. We may emphasise in this arrangement the
~ motor function performed by the ccelom and its fluid, which may be
conipared with the more specialised condition in the proboscis of
Balanoglossus and the ambulacral system of Aséerias.
The excretory organs consist of six pairs of nephridia (in
segments 4 to 9 inclusive). These are wide tubes which
open into the ccelom by large funnels or
nephrostomes and to the exterior by small ne-
phridiopores. A patch of coelomic epithelium covering each
Excretory.
Fig. 115.—A NEPHRIDIUM OF ARENICOLA.
(After ASHworTH and GamBLE.)
Gonadial
Filaments. Excretory
Portion.
Nephridiopore.
4
>
Vesicle or Bladder.
of the nephridia, except the first, gives rise to the sexual
elements, which lie free in the nutritive
coelomic fluid and pass to the exterior by the
nephridia. Avenzcola is dicecious and breeds in spring
and summer. It develops by a free larval form allied to
the trochophore.
Reproductive.
[ TABLE,
190 ANNULATA.
ARENICOLA.
SEGMENTS, APPENDAGES, Beene Pees APERTURES, GILLS,
Prostomium. Brain. Mouth.
Peristomium1 Nerve ring. Otocyst.
2| Parapodia.
"W
" Nephridium | Nephridio-
5 " a 2 pore I
6 " " 3 1 2
7: " " 4 " 3
8 " " 5 " 4 |Gillr
9 " 1 « 6 " 5 rae
sfe) " uM! 6 n 3
II " u 4
12 " Nerve cord. u 5
13 ty u 6
14 " "7
15 an n 8
16 " u 9
17 " i ce)
18 " wiI
19 tt uwiI2
20 " uI3
Tail, &c. Anus.
III.—HIRUDO.
PHYLUM - ANNULATA (p. 237).
Sus-PHYLUM ANNELIDA (p: 238).
Crass - HIRUDINEA (p. 239).
Fig. 116.—THE MEDICINAL LEECH
(Hirudo medicinalis ),
The more tapering end is anterior. The centre, individual
is seen in the action of swimming.
AIRUDO. 191
The common leech (Aivudo medicinalis) is an elon-
gated, slightly-flattened worm of freshwater habitat. It is
found commonly in ponds and is usually of a dark greenish
Hapits, nt with yel-
low lines and pig pr7.—VewrraL Vinw oF THE
spots. The ventral sur- precy. (Natural size.) (Ad nat.)
face is darker, sometimes
black. Like Avenicola, it
is plano-symmetric. The
mouth is at the anterior
end in the Nephridiopore.
centre of a
sucker. It is
triangular and armed with
three chitinous teeth. At
the hind end is a large Aperture? ~|
posterior sucker and the
anus is situated dorsal to
It.
As in Avenicola, we
can see a great number
of annuli or rings, of
which about five are
contained in each true
segment, but there is a
marked absence of gills
and appendages. Airudo
breathes by the skin and
the suckers take the place
of appendages in loco-
motion. The segments
can be made out by the
presence on the first an-
nulus in each of paired
rows of sense-organs. In
this way there can be
found twenty-six seg-
ments, and development
indicates that the posterior sucker is formed from seven
segments fused together. Hence the body of the leech
consists of thirty-three segments.
Mouth.
External
Features.
Aperture 6 —~
192 ANNULATAL.
Fig. 118.—Fikxst DissecrIOoN oF Fig. 119.—SEconp DIssECTION
Lreecu (Hirudo). or LEECH (47rudo).
(Ad nat.) (Ad nat.)
Brain.
2
ae th
3 »
we S
au :
tp
(Esopha
Crop with eleven pairs of pouches.
Nephridia.
a —
[s]
5 g
é ES
. 2 ag
od 3 a=
5 cc 2m
3 - 33
a
ae Ag
a ne
oH ew
is >
oe
Ds
Sa
‘Ed
Z
fo}
a
The body-wall is cut open along the The alimentary canal is picked out, and
mid dorsal line and pinned back. the nervous, blood-vascular, genital
and excretory organs are exposed.
ATRUDO. 193
Fig. 120.—TRANSVERSE SECTION THROUGH THE LEECH
(Atrudo medicinalis). (Mainly after BoURNE.).
Dorsal Sinus.
Crop cut in
three places. Dorso-
Botryoidal ventral Muscle.
Tissue,
Circular
Muscles.
Muscles.
Lateral
Blood-vessel.
Nephridium.
Bladder of Lt
Nephri- Nephridium,
die, is Lobe of Vas Deferens. °
‘estis Lobe o: e ,
Nephridium. “Nheawe _ \Testicular Coelom,
: Ventral Testis.
Cord. Si
inus,
On the last annulus of each segment, from segment 6
to 22 inclusive, there open on the ventro-lateral line
paired nephridiopores.
On the first five segments one
Fig. 121.—DorsaL View of the sense-organs on each side
OF THE ANTERIOR END jg enlarged into an eye; thus
OF A LEECH. there are five pairs of eyes. In
Ser Wee “a tary, the median ventral
egument@ry’ line the unpaired
male genital aperture is found in
segment 10 and the female in
segment 11. The body is flexible
and is enveloped in a thin cuticle
secreted by the simple ectoderm.
The mouth leads into a sucking
pharynx, the walls of which contain
unicellular salivary glands and can
be pulled out by
radiating muscles.
A short wsophagus connects it with
the crop. This is a thin-walled but spacious sac which
: 14
The small dots are the sense-
organs, of which five pairs are Alimentary.
large and formed into eyes.
194 ANNULATA.
extends laterally as eleven pairs of pockets, of which the
eleventh is the longest. The crop opens into a small
bilobed stomach and from this the zxfestine passes to the
anus.
Fig. 122.—A NEPHRIDIUM OF THE LEECH.
(After Bourne.)
is)
»
an! am,
“eg
a, The funnel ; Z, the nephridiopore ; k, the bladder ; c—A, various parts of the
excretory portion. The cross lines AB, CD, EF and GH indicate the cross-sections
shown on the left.
The three jaws rasp a hole in the skin of the victim;
the pharynx sucks the blood and passes it into the crop
(the saliva is said to delay clotting of the blood). From
ATRUDO. 195
the crop the blood is slowly passed into the stomach and
digested as required.
The student will have noticed already in cutting open
the leech (by a dorsal incision) that the interior is very
different from that of Avenicola. There is no spacious
coelomic cavity; the organs have to be picked
out and the skin peeled off. This is due to
the fact that the ccelom is filled up almost entirely by
Colom
Fig. 123. MAGNIFIED VIEW OF Two CONSECUTIVE SEGMENTS
OF THE LEECH (10th and 11th). (Ad nat.)
Nerve Cord in
Ventral Sinus.
Penis pulled in
from exterior.
Vas
Deferens.
connective tissue, the only parts of this cavity which are left
being (1) a median dorsal sinus or space, (2) a median
ventral sinus surrounding the nerve-cord, and (3) a few
small spaces lying immediately over the testes.
The blood-vascular system is complex and the blood-
vessels have definite walls. There are two
main J/ateral vessels which give off num-
erous smaller branches. These communicate with a
Blood-Vascular.
196 ANNULATA.
complex system of fine vessels called the botryotdal tissue.
Through the medium of this tissue the blood-vascular and
ccelomic systems are said to communicate.
The nervous system is on the annelid plan, but somewhat
concentrated. A double brain lies on the pharynx, from
which there passes a pair of commissures round
the pharynx, meeting below to form the double
ventral nerve-cord along the body. On this cord there
are twenty-three ganglia which are segmentally arranged.
The first is larger than the rest and forms a subcesophageal
mass consisting probably of five fused ganglia. The last,
supplying the sucker, is said to contain seven fused ganglia,
the sucker itself being supposed to represent seven fused
segments.
The muscles consist of powerful circular and longitudinal
series and a number of dorso-ventral muscles as well. The
leech not only progresses by its suckers, but
it can swim rapidly by undulating motion
of the whole body. (See Fig. 116.) ;
There are seventeen pairs of nephridia found in segments
6 to 22 inclusive. A typical nephridum has (1) an internal
branched but c¢/osed funnel which rests in a
small cavity of the ccelom, (2) a much-coiled
excretory portion with a ciliated duct, and (3) a bladder or
vesicle opening to the exterior by the nephridiopore. The
first four nephridia are without the funnel.
The leech is hermaphrodite and the male organs are
segmented. They consist of nine pairs of zestes in segments
12 to 20 inclusive. Each opens inwards by a vas efferens
into a vas deferens on each side. ‘These are coiled into
epididymes in segment ro and then unite and pass down the
penis; at the base of the penis is a small Jrvostate gland.
The female organs are in segment 11 and are formed by
paired ovdsacs, containing in their cavities the ovaries, which
unite to form an oviduct swollen at its distal end into a
vagina and opening to the exterior on the same segment.
Fertilisation is mutual and the eggs are laid in a cocoon,
usually in damp places. They have yolk and pursue an
embryonic development.
Nervous.
Muscular.
Excretory.
AITRUDO, 197
LEECH.
SEGMENTS. ence epee ne eae SENSE-ORGANS., | APERTURES.
Prostomium | Brain Mouth.
I | Eyes
2 Sub- nu 2
3 | ; esophageal “3
4 mass. " 4
5 a)
6 |Ganglion 1 Nephridium1 Sense-organs.
7 " 2 2 "
8 " 3 3 W
9 " 4 4 "
Io " 5 5 " é ap.
II " 6 6 Ovisacs. " Q ap.
12 " 7 7| i Testis. "
13 " 8 8 2 " u
14 tt" 9 9| 3 tt "
15 n Io Io] 4 " "
16 " Il I1| 5 a a
17 n I2 I2| 6 " "
18 u 13 13| 7 " "
19 " 14 14/8 " "
20 " 15 I " "
21 " 16 I 2 "
22 " 17 17 "
23 Nn 18 "
24 " 19 "
25 " 20 "
26 " 21 "
27 " 22
28
a Posterior
3I ganglion
32 (23)
33 Anus.
198 ANNULATA,
IV.—LUMBRICUS.
PHYLUM - ANNULATA (p. 237)-
SuB-PHYIUM ANNELIDA (p. 238).
Crass OLIGOCHATA (p. 239).
Fig. 124.—THE CoMMON EARTHWORM
(Lumbricus terrestris ).
The darker end is anterior.
The common earthworm has a shape and appearance
familiar to all. A full-grown specimen may be eight or
Aapivs nine inches long. The anterior end is usually
"of a dark purple colour which becomes lighter
further down the body to a dull pink. The animal lives in
self-constructed burrows in the earth, though at times it
emerges from these and creeps on the surface.
LUMBRICUS. 199
Like all other Aznelida, the earthworm is plano-sym-
metric, though the absence of appendages makes this less
evident than in other classes.
The body is constricted throughout into a series of
about 150 segments, but there are no annuli. The segments,
from about 29 to 35,* have a swollen appearance
mectibee anda yellowish colour. They form the cdcted/um.
‘ The mouth is at the anterior end, overhung by
a prostomium and bordered by the Zertstomium. At the
extreme posterior end is the anzs.
As in the leech, there are neither gills nor appendages.
If the body of the worm be drawn through the fingers from
tail to head it will rasp with some resistance to the fingers.
This is due to the presence of minute se¢e which are found
oneach segment. The setee are in pairs and are arranged in
two ventral and two lateral rows. Each segment, therefore,
has eight sete. They naturally project backwards and aid
the locomotion of the worm in the same way as the appen-
dages of Avenicola. In the mid-dorsal line is a row of median
dorsal pores, occurring between each segment from about
the gth backwards and communicating with the celom. A
pair of minute nephridiopores open on the ventral surface
of each segment (except the first two), but they are too
small to be recognised without the aid of a lens. On the
15th segment there is a pair of ventral openings with tumid
lips, the male genzfa/ openings, and on the segment (14th)
in front are the two female genital apertures.. Between seg-
ments 9, 10 and 11 there are the two paired openings of
the spermathece.
The body is covered by a cuticle with simple ectoderm,
forming a flexible but firm envelope. Scattered
throughout the ectoderm are numerous uni-
cellular glands, specially abundant in the region
of the clitellum.
There are no eyes nor otocysts, but the prostomium has
sense-organs for perception of contact and per-
haps of taste. The alimentary canal is exposed
by making a median dorsal incision along the body of the
worm. ‘The mouth passes into the muscular pharynx, from
External
Integu-
mentary.
Sensory.
* There is great variation in the position of the clitellum.
200 ANNULATA.
Fig. 125.—Firsr DissrcTION OF Fig. 126.—SECOND DIssECTION oF
THE EARTHWORM. (4d nat.) THE EARTHWORM. (Ad xat. )
Pharyngeal
Muscles.
cal PNephridia.
Lateral Hearts.
ag |: Nerve Cord.
|_Spermatheca.
Csophageal
Gl.
Seminal Vesicle.
lands. _4 y
=
Dorsal Blood-
vessel. 7
onic (~~ Ovary.
| Vas Deferens.
~)“Oviduct.
Gizzard.,__|
Intestine.
The body is cut open along the dorsal line The alimentary canal is cut through and
and the body-walls pinned down. removed, exposing nervous, excretory and
genital organs
LUMBRICUS. 201
which a long thin wsephagus passes back to the thin-walled
crop. Upon the cesophagus are three pairs of pouches, the
two hinder pair being known as calciferous
glands. The crop leads into the muscular
gizzard, which has a small opening into the long zu/estine.
A fold of the dorsal wall, called the zypAlosule, projects into
the lumen of the intestine. The alimentary canal is held in
position by a complete series of ¢ransverse septa dividing the
coelom into compartments. Each septum has an aperture
by which ccelomic fluid can pass from one segment to the
other.
Alimentary.
Fig. 127.—TRANSVERSE SECTION OF AN EARTHWORM IN THE
INTESTINAL REGION. (Semi-diagrammatic. )
Yellow Cells. Dorsal Pore.
Ectoderm. ‘A Dorsal Blood-vessel.
Cuticle.
Circular
Muscles,
2 ae
Setze. ae
Longitudinal 4 1 k
Muscles, eo} Wind ¥6
aed
Nerve Cord. Ventral Blood-vessel.
Under the ectoderm is a layer of connective tissue
beneath which lies a series of circular muscles,
Inside this there is a longitudinal series. These
function very much as in Avenicola.
The ccelomic fluid is nutritive and the celom is spacious.
The ccelomic wall covering the intestine is thickened into a
mass of excretory yellow cells. The blood-vascular system
Blood. 18 easy to see, for the blood (mainly respiratory) :
Vaienian. 1 bright red from the presence of hemoglobin
(cf Arenicola). There is a main dorsal vessel
running forwards and a ventral backwards. These are con-
nected by numerous civcudar vessels, of which six at the
Muscular.
202 ANNULATA.
anterior end are specially contractile and form the /aeral
hearts. The dorsal vessel is also contractile.
The drain lies in the prostomium, with a xerve-ring
ee round the pharynx and a double nerve-chain
"along the ventral surface. The nerve-chain has
double ganglia in each segment.
The xephridia are numerous, a pair occurring in nearly
every segment. They are complex coiled tubes with an
eeretory. internal funnel, a coiled excretory part and a
* small bladder or vesicle leading to the exterior.
The funnel always opens into the coelomic compartment in
front of the one in which the bulk of the nephridium lies.
Fig. 128.—A NEPHRIDIUM OF LUMBRICUS.
Nephrostome.
\
a B
8 we
4 " §
2 j g
First 4 s
Thine &
walled
Portion. First
Thin-
walled
Second Portion.
‘Loop.
Third So Te SE Pies
Muscular Sai PR og — ple Sag .
Portion. External
SRR Aperture.
Respiration appears to be carried on by the skin. The
worm is hermaphrodite. The male organs consist of two
pairs of ¢es¢es lying in the roth and r1th seg-
ments. They are enveloped in the large branch-
ing seminal vesicles, from each of which there passes a vas
deferens backwards to open on the 15th segment. The
pair of ovaries lie in the 13th segment, and the ov¢ducts, with
funnels opening into the same segment, run through the
septum and open to the exterior on the 14th segment.
Two pairs of sfermathece or small spherical sacs open
between 9 and 10 and between 10 and 11 on each side.
The eggs are laid in cocoons and undergo an embryonic
development. The cocoons are secreted by the clitellum.
Reproductive.
LUMBRICUS. 203
LUMBRICUS (Ist twenty segments).
NERV EXCRETORY | REPRODUCTIVE
SEGMENTS. | APPENDAGES.| Sv0 Ae SEE CREA. APERTURES.
Prostomium. | Brain. Mouth.
Peristomium I Nerve-ring.
2 Setee, Ganglion. Nephridio-
3 " " Pat pores
4 ” " Nephridium 1 (same as
5 " " " nephridia).
6 " " "
7h " " "
8 " n" nW
9 u " " Spermatheca.
Spermathecal.
Io " " " Sperm. Testis.
Spermathecal.
1 " " " Testis. Dorsal pore.
I2 " tt " "
.13 " " " Ovary. "
14 " " " Oviducts. tt
15 " " u "
16 " " " 9 ap. wu
17 " " " 6 ap. u
18 it " tt "
19 " " tt
20 1 " " u
&c.
(For the general characters of the Sub-Phylum Annelida,
see page 238.)
204 ANNULATA.
CHAPTER XVII.
ANNULA TA— Continued.
NEPHROPS. BLATTA. PERIPATUS. EPEIRA.
I.—NEPHROPS.*
PHYLUM ANNULATA (p. 237).
SuB-PHYLUM ARTHROPODA (p. 240).
CLass CRUSTACEA (p. 241).
‘he Norway Lobster (JVephrops norvegicus) is a very
common kind of lobster found, amongst other places, in the
wapits. Firth of Forth. It is caught in great numbers
"in the trawl] and is apparently gregarious in its
habits. It is a ground-feeder and is fond of shell-fish, but
will eat almost any marine animal of a sufficiently small
size. It is rather smaller than the common lobster, and
is at once distinguished by its pale yellow and red colour
and its more angular outline.
The body is perfectly plano-symmetric and is encased in
a hard calcareous exoskeleton. As in the Aznelida, the
whole body is enveloped in a chitinous cuticle
secreted by the underlying ectoderm (or epi-
dermis), but this cuticle is greatly thickened
over certain areas, and is, in addition, converted into a hard
plate by the deposition of calcareous matter in the chitin.
The hard plates are called sc/erites, and the soft cuticular
parts between them which make movements possible are
called the arthrodial membranes.
We can distinguish the body and the appendages, as in
many Annelida. In the body the largest sclerite is the
carapace. This rests like a saddle on the anterior
half (or more) of the body. The front end is
produced into a sharp rostrum, and on either
side it hangs down as a lateral branchial plate. The
branchial plate can be broken off, and the gills are then
exposed in the branchial chamber which is plainly only a
Integu-
mentary.
External
Features.
* The following description, except in the case of the gills, will apply equally
well for the crayfish (Astacus), the lobster (Homarus), or the shrimp (Crangon), and
with very little modification for the crab (Carcinus).
NEPHROPS. 205
Fig. 129.—THE Norway Losster (Mephrops norvegicus ).
Dorsalzaspect. (Ad nat.)
_ Chela,
Antennule.
wer end Leg.
3rd "
4th
sth
6th
Telson.
206 ANNULATA.
part of the exterior partially shut in. In many Crustacea
the part of the body here covered by the carapace is divided
into head and thorax, and in Wephrops the line of junction
is shown by a cervical suture passing down laterally and
obliquely from the mid-dorsal line. ;
Fig. 130.—LATERAL VIEW OF Norway LOBSTER (Right) x 3.
(Ad nat.)
Branchial Cervical
rst Abdominal Segment. Plate.
N
: Ne 7 :
¢ oe t \ a ;
/ Swimmeret. wy nt 4 ere ee
| Telson. } ) \\ " 3rd Maxillipede.
6th Abdominal Segment. a ist Leg.
The third portion of the body, behind the carapace, is
called the abdomen. In this region there are several
sclerites movable on each other and they are found to
correspond with the metameric segments of the body.
Each abdominal sclerite is roughly in the form of a ring.
The dorsal part is called the ¢exgon, the ventral narrow
part is the s¢ernon, and from the junction there hangs down
laterally the pleuron. Just inside the pleuron there is an
appendage on each side. Each abdominal sclerite is
fastened to its fellows in front and behind by dorsal and
ventral arthrodial membranes and laterally by a pair of ball-
and-socket joints, which allow of movement only around
the axis through them. The sclerites overlap dorsally those
behind them. The last (7th) abdominal sclerite is flattened
and bears no appendages: it is called the ze/son. . The appen-
dages of the abdominal sclerites are termed szwzmmerets for
they are mainly used for swimming. They consist typically
of a basal piece, the proftopodite, bearing two paddles, the outer
NEPHROPS. 207
one called the exopfodite and the inner termed the endopodite.
Hence they are termed dcramous appendages. The ab-
domen of /Vephrops, therefore, resembles that of a polychzete
annelid in that it is divided into a number of segments, each
of which bears a pair of biramous swimming appendages.
In the part in front of the abdomen the segmentation
cannot be traced by the sclerites for they are united, at
least dorsally and laterally, into one sclerite, but the appen-
dages still enable us to determine the number of segments
which have become fused. From these we find that the
Fig. 131.—AN ABDOMINAL SEGMENT OF NEPHROPS x 3.
(Ad nat.)
Tergon.
Articular Facet.
- Pleuron.
Sternon. Arthrodial Membrane.
thorax consists of eight segments and the head of five,
which, with seven abdominal, gives a total of twenty seg-
ments. The telson having no appendages, there are
only nineteen pairs of appendages.
Glancing at the thoracic and cephalic (head) appendages,
we see that there are four pairs of legs preceded by a pair
‘of pincers ; these are succeeded by a pair of foot-jaws, inside
which there are no less than five more pairs of jaws; and,
lastly, in front of the mouth there are two pairs of feelers.
We can recognise at once that the appendages have altered
considerably in form and function if they all were at one time
of a common type. The evidence of development and of
comparative anatomy leads us to suppose that the ancestors
of lobsters had simple biramous appendages to each segment.
All were used as swimming organs, but when walking on the
208 ANNULATA.
sea-floor became the vogue, the swimmerets in the neigh-
bourhood of the centre of gravity became modified for bear-
ing the weight of the body. In this case the endopodites
evidently form the main axis of support, being nearest the
perpendicular through the centre of gravity, and the
exopodites, being superfluous, disappear. A “leg” or achela
therefore consists of protopodite and especially endopodite.
Both parts become jointed for further movement, so the
protopodite acquires two sclerites and the endopodite five.
On the other hand, the appendages near the mouth
naturally take part in the ingestion of food. In this the
basal part or profopodize, being nearest the mouth, becomes
the gzathobase or jaw-element. Hence the jaw-elements
always consist largely of protopodite, the endopodite and
exopodite becoming subsidiary. These three axioms should
be held in mind :—
1. Swimming organs at the hind end, retaining their prim-
ary functions, retain. the primitive biramous condition with
equal endopodite and exopodite.*
2. Walking organs, round the centre of gravity, lose the
exopodite and have a large and complex endopodite.
3. Eating organs, round the mouth, lose the exopodite
and reduce the endopodite, but have a large protopodite.
4. Tactile organs, at the anterior end, may be specially
modified, with or without reduction.
5. Organs between these, with ill-defined functions, such
as foot-jaws, ay retain all the parts more or less modified.
(It will be found convenient to remove the appendages
from the last leg forwards as the jaws overlie each other
forwards.)
If the four legs be removed we can at once contrast them.
The two first have pincers at their ends, or are chelate;
whereas the two last are non-chelate. Each has two joints
to the protopodite and five to the endopodite. This
completes the last leg, but the three in front of it bear
a long hairy pad called an efipodite, and attached to its
base is a filamentous g#//. If the specimen be a male,
the genital aperture will be found on the basal joint of
the last leg, whereas, if a female, the genital aperture
* In a few cases, as in the sixth abdominal, the exofodite is the larger.
NEPHROBS. 209
will be on the basal joint of the anti-penultimate leg.
In front of the legs are the che/a, or pincers, usually asym-
metric, as one is modified for cutting and the other for
crushing. They are evidently large legs and do not differ
essentially from a typical leg.
Fig, 132.—A CHELA or NepHrops (9th appendage). (4a mat. )
Fig. 133.-—A CHELATE LEG or NrePHROPS
(roth or 11th appendage). (Ad nat.)
Ae
Dactylopodite.
Propodite.
Meropodite.'
Coxopodite. 4
Basipodite.
Gill.
Fig. 134. A Non-CHELATE LEG OF
NEPHROPS (12th appendage).
(Ad nat.)
Epipodite.
In front of the chele are three pairs of maxtllipedes
or foot-jaws. The second maxillipede may be examined
first. It has a leg-like five-jointed endopodite, a two-jointed
protopodite and a long filamentous exopodite Finally
there is a long epipodite, but no gill.* The third maxilli-
pede is like it, but can be easily distinguished by the serrated
* In Astacus there is a gill on this appendage.
M. 15
210 ANNULATA.
(or toothed ) edge on the basal joint of the endopodite and
by the presence of a gill. The first maxillipede, on the other
7 Fig. 136.—THE SECOND MAXILLI-
Fig. 135.--THE First MAXILLI- PEDE OF NEPHROPS
PEDE (left) oF NEPHROPS.
Enlarged. (Ad nat.)
Enlarged. (Ad nat.)
Endopodite.
Endopodite.
Exopodite.
Protopodite Exopodite.
forming az °
Jaw.
&
uo}
z.
uel
fs)
a.
=
2
Vestigial Gill (2).
Two joints of
Protopodite.
Epipodite.
Note the jaw formed of protopodite
and the two-jointed endopodite.
Fig. 137-—A THIRD MAXILLIPEDE OF NEpHRops. (Ad zai.)
Exopodite.
Gill,
Endopodite.
Protopodite, Epipodite.
hand, has a small twoyointed endopodite and the protopodite
is produced into a jaw.
NEPHROPS. 211
These eight appendages complete the thorax.
Still passing forwards on to the head we find two small
foliaceous maxzlle: The second maxilla (the first to be
removed) has a guadrifid jaw-like protopodite, a thin
unjointed endopodite and a scoop-shaped epipodite (called
the scaphognathite). The exopodite may possibly be repre-
sented by a small process.
The first maxilla is the smallest of all the appendages.
It has a bifid jaw or protopodite and a small unjointed
endopodite.
Fig. 138.—A, Frrst MaxILLa, AND B, SECOND MAXILLA OF
Nepurops. (Ad nat.)
B
—
Endopodite.
i) Epipodite.
aw.)
ie
Quadrifid# _ Exopodite.
Protopodite c
Protopodite.
J
Endopodite.
Epipodite.
The mouth is guarded by a pair of powerful biting
jaws, formed by the protopodite of the mandddles, the little
endopodite being three-jointed and forming the pal~. Pass-
ing in front of the mouth we reach the large axtennae. On the
ventral surface of the basal protopodite of these appendages
is an aperture, the excretory pore. The endopodite is pro-
duced into a long tactile feeler, and the exopodite forms a
small semi-circular scale. The antennule has a small aper-
ture in the protopodite leading to the otocyst. It has no
exopodite, and the endopodite is formed into two fila-
mentous feelers.
If we now return to the swimmerets we find that they
are not all alike. The first swimmeret has, in the female,
only one “paddle” (or the endopodite, borne on a small
protopodite), whereas, in the male, the protopodite alone
212 ANNULATA.
Fig. 139. —THE ANTENNULE OF Fig. 1441—Lrrt ANTEN-
Nepurors. (Ad nat. ) NA OF NEPHROPS.
(Ad nat.)
g
#
Beg
Bs
ae
Aperture of
Otocyst.
Fig. 140.—THE MANDIBLE
oF NEPHROPS x 2. | __Endopodite.
(Ad nat.)
Scale
Palp or Endopodite.
Protopodite.
Excretory Pore.
(Exopodite).
Tendon.
Ventral view.
NEPHROPS. 213
remains as a long, grooved spike, which apparently acts as
an accessory organ of reproduction.
In the female the next four are normal, but in the male
the second one has a process of the protopodite which
gives the whole appendage the appearance of being tri-
ramous. The other three are normal.
Fig. 142.—TuE First Pair oF
SWIMMERETS IN NEPHROPS
(3). (Ad nat.)
Fig. 144.—A Tv-
PICAL SwIiM-
MERET OF
NEPHROPS.
(Ad nat.)
Fig. 143.—THE 2ND
SWIMMERET OF
NeEPHROPS ( ¢ ).
(Ad nat.)
Protopodite.
Protopodite.
Spine.
Protopodite.
Endopodite.
Exopodite.
Endopodite.
Note the spine on the
Protopodite. eS
In both sexes the sixth swimmerets are of large size, the
exopodite being jointed. ‘These “paddles,” together with
the median /e/son, form the tail which, on flexion of the
abdomen, strikes the water forwards resulting in a rapid
backward motion of the whole body.*
At least four senses can be recognised in Wephrops. (1)
The eyes are paired and situated just below
the rostrum upon eye-stalks. They are called
compound eyes.
Compound eyes are characteristic of Arthropoda and
have throughout the group a characteristic structure. They
are called compound because they consist of an aggregate
of elements called ommatidia, each of which has its own
Exopodite.
Sensory.
* Lhe swimmerets in Astacus have the paddles (or exopodite and ,endopodite)
differentiated into a basal unjointed and an upper filamentous portion. In Carcinus
the swimmerets are vestigial ( g) or very reduced (@ ).
5
214 ANNULATA.
Fig. 145.—A MEDIAN SAGITTAL SECTION THROUGH NEPHROPS
(Semi-diagrammiatic).
Extensor Muscle.
Dorsal
Abdominal Artery.
Flexor
Muscle.
Intestine,
_-Sternal Artery.
Gonad.
Digestive
Gland.
Ophthalmic
Ventral Artery.
Pyloric part
Stomach.
Endophragmal
Skeleton.
Stomach.
SY
Sub-cesophageal ~ :
Ganglia. \
Mandibular
Mouth. i Muscle.
Brain.
Green Gland.
Duct of Green Gland.
ES
\e
PI
<4
of
Cardiac part of
NEPHROPS. 215
complete optical apparatus. The ommatidia are arranged
radially, converging to the centre of the eye towards the
optic ganglion, and their outer ends are covered by a
thickened cuticle divided into facets. Each ommatidium
or eye-element consists of (2) an outer layer of cells which
secrete a long, lens-like body, the crystalline cone; (6) an
inner layer of cells, called vefixuZe, which secrete in their
common inner space the rabdomes, or rod-like bodies.
From these there pass fine nerves to the optic ganglion,
which in its turn communicates with the brain. The
crystalline cones form the dioptic apparatus, and the
retinule and rhabdomes are the sensory apparatus. Between
the ommatidia, cells loaded with pigment grow up from the
connective-tissue layers below. They serve to isolate the
ommatidia and shut out cross-rays.
(2) The otocysts consist of paired hollow cavities in the
base of the antennule. Each communicates with the ex-
terior by a minute aperture. The cavity contains a few
sand-grains, and its wall has sensitive ‘‘hairs” projecting
into the cavity, supplied by fibres from the antennulary
nerve. (3) A number of the “hairs” on the antennule
are sensory and are said to have an olfactory sense. (4)
Crustacea, with a hard exoskeleton, can hardly have the
tactile sense distributed all over the surface like some other
animals, but they have numerous sensory or ¢actile hairs.
These should be carefully distinguished, on the one hand,
from mammalian hairs, and, on the other, from annelid sete.
The seta is a cuticular bristle formed of chitin throughout,
but the lobster’s “hair” consists of a delicate cuticle on the
surface and a living protoplasmic axis connected by sensory
nerve to the nerve cord.
The mouth, as we have seen, passes from the antero-
ventral mid-line past the mandibles through a short wsoph-
agus into the spacious stomach. ‘This is divided
by a constriction into- two parts, the so-called
cardiac and pyloric chambers. The pyloric chamber leads
into a short mesenteron, into which open the paired ducts
from a large digestive gland, and thence into an intestine to
the anus on the ventral surface of the telson.
_ Development teaches us that the whole of the alimentary
canal, except the mesenteron, arises from ectoderm, and, in
Alimentary.
216 ANNULATA.
accordance therewith, it is lined with a chitinous cuticle.
In addition the cuticle in the stomach has a number of
hard sclerites which form the gastric mil. This apparatus
has a median tooth and two lateral teeth worked by power-
ful muscles. Further, the aperture between the cardiac
and pyloric portions is guarded by strainers, or small pro-
cesses, covered with “hairs.” Digestion of the food is
apparently confined to the region of the mesenteron.
The sclerites of the lobster are moved by a complex series
of muscles lying inside the body or limbs. There are two
series of muscles—(1) the flexors which by con-
traction bend the abdomen or the limb; (2) the
extensors which straighten it. In the limbs, at least, these
are attached to the arthrodial membranes by tendons, but in
some cases to the edge of the sclerite. A cross-section of
the abdomen shows the powerful flexors, the contractions of
which bend the tail and drive the lobster backwards through
the water, and above them the much thinner exdensors.
The anterior flexors are attached in the thorax to the
endophragmal skeleton, which consists of parts of the ecto-
derm with cuticle; these have grown in from the ventral
surface during development. Hence the endophragmal
skeleton does not constitute an exdoskeleton.
‘The lobster can swim gently forwards by the action of
the swimmerets, it can creep in any direction by its legs and
it can shoot rapidly backwards by contraction of the tail.
The skeleton being an exoskeleton, it has already been
noticed in the external features. We need only emphasise
the tucking of the ectoderm into the stomach
and into the ventral region of the thorax, the
sclerites in each case forming the gastric mill and the endo-
phragmal skeleton.
The vascular systems of the lobster are in a peculiar
condition. In the Annelida and Archicelomata we could
distinguish two vascular systems. The larger and
more spacious, contained z7¢him the mesoderm,
was called the ccelom and was mainly nutritive and motor;
the smaller consisted of fissures and small sinuses lying detween
the three primary layers, was called the blood-vascular or
hemocecelic system, and was usually respiratory and ex-
cretory.
Muscular.
Skeletal.
Vascular.
NEPHROPS. 217
In the Arthropoda this condition is reversed. The
coelom is reduced to a few small spaces z¢hin the meso-
derm, such.as the cavity of the gonads and of
green gland, is either indifferent or excretory,
and has lost its motor function, whereas the blood-vascular
or heemoccelic system is spacious and forms the main cavities
Colom.
Fig. 146.—SECTION ACROSS THE ABDOMEN OF
NEPHROPS x 2. (Ad nat.)
Dorsal Blood-Vessel. Intestine.
Extensor Muscle.
Nee Cor d. Ventral Blood-Vessel.
> Exopodite }
Endopodite
Swimmeret.
of the body. It is divided by a median horizontal pericardial
septum in the thorax into an upper cavity, the
pericardial sinus, and a lower cavity which forms
the general body-cavity. The Jody-cavity of the
lobster is therefore purely heemoccelic, mostly a large venous
cavity, but partly a small arterial* pericardial cavity. The
heart lies dorsally in the pericardial sinus, with which tt com-
municates by six valves; on contraction it drives the blood
forwards and backwards by main arteries. Forwards there
is a median ophthalmic artery, paired antennary and
Blood-
Vascular.
* Arterial in containing zrated blood; structurally it is a part of the venous
system,
218 ANNULATA.,
paired hepatic arteries. Posteriorly the heart gives off a
dorsal abdominal backwards, and a sterna/ artery down-
wards which, on reaching the ventral surface, divides
into a ventral thoracic forwards and an abdominal back-
wards.
All these arteries supply the organs with pure blood,
and the impure venous blood accumulates in the cavity of
the body whence it passes out to each gill by an afferent
branchial. After zxration in the gills, it is collected by
efferent branchials and passed by branchio.cardiac canals up
the sides of the thorax into the pericardial sinus.
The heart of the lobster is thus sys¢emzc, and the course
of the blood is as follows :—
Hearl
Arferial body eavily
~
\pericerdial
System Gills *"°"*
Venous body cavny
The special point to notice is the hemoccelic body-
cavity converting the venous system into a number of large
sinuses or spaces, the arterial vessels alone having definite
walls.
The nervous system is constructed on essentially the same
plan as that of the Annelida, but there are more concentra-
tions of the ganglia. If the lobster were a simple
annelid we might expect to find a dorsal brain
over the anterior end of the alimentary canal, a ring round
it to the ventral surface, and a double nerve-chain to the
hind end, with double ganglia in each segment; but in reality
matters are rather different. The Jrvazm of the lobster has
the true brain portion supplying sensory nerves to the eyes,
but, in addition, it has the two next pairs of ganglia belonging
to the antennules and antenne fused with it. The-anten-
nules and antenne are really post-oral appendages, but they
move forward in development to the adult position in front
Nervous.
NEPHROPS. 219
of the mouth. The ganglia corresponding to them follow
suit and fuse with the primary brain to form one mass.
In a similar manner the ganglia of the next six append-
ages, which are all jaws or foot-jaws, fuse to form one large
sub-esophageal mass. After this follow the five thoracic
gangha of the chelz and the legs, and, lastly, the s¢x abdo-
minal gangha. The last supplies both the sixth segment
and the telson, and thus may be two (6th and 7th) fused
ganglia.
In this way the primitive chain of twenty ventral ganglia
is reduced in number by fusions at each end till twelve only
remain.
The lobster excretes nitrogenous waste products by a pair
of green glands in the head, which may be a pair of much
modified nephridia. Each consists of a complex
excretory tube leading to a bladder, which opens
to the exterior by the excretory pore on the ventral side of
Excretory.
Fig. 147.—LaTERAL View or NepHrops. (Ad nat.)
Pleurobranch,
3rd
Maxillipede.
Chela.
With branchial plate removed. Behind the scaphognathite are the epipodites
of the first two foot-jaws, then follow the five podobranchs and their epipodites.
The arthrobranchs and three of the pleurobranchs are hidden.
the antenna. The excretory products are thus discharged
into the stream of water emerging from the branchial chamber
(see below). We may notice that the tubes have no internal
nephrostome, for there is no ccelomic body-cavity into which
they can open.
220 ANNULATA.
There are nineteen pairs of gills. They are situated
along the sides of the thorax and are protected by the
branchial plate of the carapace. The branchial
chamber so formed communicates freely with
the exterior between the legs and at the hind end, but the
principal aperture (the cervical canal) lies at the front end
and opens beside the mouth.
In this there lies the scaphognathite of the 2nd maxilla
which is said to bale or scoop the water ow¢ of the branchial
chamber, fresh water coming in from behind and between
the legs. Each gill consists of a central axis with lateral
branches, covered with a very thin cuticle, ectoderm and
mesodermic layer. In its interior the blood circulates from
afferent to efferent branchials.
Five of the gills are fixed to the bases of the third
maxillipede, chela and first three legs. They are termed
podobranchs. To the arthrodial membrane of each of
the same appendages is attached a pair of small arthro-
branchs ; whilst higher up, on the side-wall of the thorax,
are found four large pleurobranchs, which are supposed
to correspond to the four last segments.* The epipodites lie
between each set of gills in each segment and force the in-
coming water to pass the whole way up the gills instead of
taking a short cut to the cervical canal.
It is possible that the primitive arrangement was that
of aSpodobranch, two arthrobranchs and a pleurobranch to
each segment of the thorax, making a total of thirty-two,
but this number persists only in numbers 5, 6 and
7, where the thorax is broadest. The cavity has
become narrower in front and behind, hence the last leg
loses its podobranch and its two arthrobranchs, and the
pleurobranchs all disappear in front of number 5: so also
do the arthrobranchs and podobranchs in the first two seg-
ments. With a loss of ten gills in front and three behind,
the thirty-two is reduced to nineteen. This will be clear
after an inspection of the diagram.t
Respiratory.
* It is probable that @// the gills arise on the basal joint of the thoracic appen-
dages, but the pleurobranchs and arthrobranchs migrate during development to their
final positions.
+ In Astacus there is a podobranch on the second maxillipede and one arthro-
branch above it, but only the last pleurobranch remains; thus it possesses only 18
pairs of gills, z.e., podobranchs (6), arthrobranchs (z1) and pleurobranchs (1).
NEPHROPS. 221
The testes are a pair of organs lying in the dorsal part
of the thorax. They lead by paired tubes, the vasa
deferentia, continuous with the testicular cavity,
to the exterior on the last leg. The ovaries are
also paired, and in a similar way lead to the exterior by paired.
tubes, the ovéducts, on the anti-penultimate leg.
The eggs are shed in great numbers and adhere to the
swimmerets of the female. In this condition the female
is known as a “ berried” lobster, and swimming is at that
time impracticable. The male discharges the male element
upon the eggs and development takes place within the egg-
membrane.
The full development of the Norway lobster has not
been followed, but its close ally the crayfish has been well
studied.
The chief points of special importance in the develop-
ment are as follows :—
1. The egg has a large amount of yolk arranged
symmetrically and the segmentation is equal and super-
ficial. (See page 49.)
2. Invagination takes place at one spot, resulting in a
sac of endoderm pushing into the yolk, the blastopore
closing.
3. The endoderm cells ingest the yolk within themselves
and thus come to lie close under the ectoderm.
4. From the middle line (future ventral surface)
the ectoderm invaginates to form stomodzeum and procto-
deum, which open into the archenteron and form the
gullet and stomach and the intestine respectively.
5. Paired thickenings of the ventral surface form the
head, the thorax and abdomen and the paired appendages.
6. The first three pairs of appendages to appear are the
antennules, antennz and mandibles, the embryo at this
stage being somewhat comparable to the zauplius larva of
some other Crustacea. (See page 242.)
. 7. The paired appendages then appear gradually in
order backwards and the young crayfish hatches, with a
cephalothorax distended dorsally with yolk.
Reproductive.
[ TABLE.
222 ANNULATA.
NEPHROPS.
SEGMENTS. APPENDAGES. NERVOUS SYSTEM. APERTURES. GILLS.
u Ea 3
Prostomium. Primary 3 a 5
brain. e ele
1 | Antennule. Brain. | Otocyst.
d 2 | Antenna. Excretory pore.
3+ 3 Mandible. - Mouth.
|] 4 | 1st Maxilla. sg
5|2nd uy a 5p as
6 | ist Maxillipede Aas
7 2nd " R
4 8} 3rd u 8 t|2
gJ 9 | Chela. Gangl. 1. 1|2
= | 10 | 1st Leg. n 2 I/2/1
& ] rr | and un nu 3. | 9 ap. r]2]/1
12 3rd " in 4. 2 a ee” ae 6
\ 13 4th "1 " 5. | 6 ap. I
14 | 1st Swimmeret a 6:
d 15 2nd tt tt 7.
2] 16 | 3rd " " 8.
84 17 | 4th " " 9.
ae 18 5th " n IO.
< 19 6th " }e t. 1
e ost, gangl. IT. | anus.
(Telson. )
II.—BLATTA.
PHYLUM - ANNULATA (page 237).
SuB-PHYLUM ARTHROPODA (page 240).
Cass INSECTA (page 246).
The common cockroach—Alatta ( Periplaneta Orientalis s)
—is of a dark brown colour except when young and is
usually about one inch in length. The American species
(Blatta Americana) is considerably larger and is thus pre-
ferable for dissection.
The male cockroach is winged and the female has no.
wings.” The cockroach is found most frequently in places
with a high temperature, such as kitchens,
laundries, or bake-houses, hence it is typically
terrestrial. It is an omnivorous feeder and thrives in
Habits.
* Both sexes are winged in the case of B. Americana.
BLATTA. 223
confinement upon bread. The body is plano-symmetric,
and is encased in a hard exoskeleton. This consists of
a chitinous cuticle secreted by the ectoderm, but it differs
Int from that of the lobster in the absence of
egu- zi
saan calcareous matter. Hence the exo-skeleton Is
¥ tough and somewhat flexible, but not nearly
so hard and thick as that of the latter. We can still
Fig. 148.—THE Common Cocxroacu (Blatta Orientalis ).
Natural Size.
A, Male with wings expanded ; B, Female with vestigial wings ; C, Wingless young.
distinguish chitinous sclerites united by softer arthrodial
membranes.
The body is divided into three parts—the head, the thorax
and the abdomen. Of these the head is not segmented, the
‘thorax is partially so, but the abdomen is as
estate’ clearly segmented as in the lobster. The head
‘ bears one pair of long antenmne@ at the anterior
end, and close to them is a pair of compound eyes, not
External
224 ANNULATA.
differing essentially from those of the lobster. The mouth
lies on the ventral surface of the head and is surrounded by
a /abrum or upper lip anteriorly. Itis a flat plate formed
of the head shield, produced downwards, and is in no
way related to an appendage. Posteriorly, the mouth is
bounded by a /adium or lower lip, formed by the fusion
Fig. 149.— THE MouTH APPENDAGES OF THE
CoMMON COCKROACH x 9. (dd zat.)
Mandible.
Maxillary Palp.
Joints of
Protopodite.
Labial Palp.
The mandibles are above the first maxille in the middle and
the labium (2nd maxillz) below.
across the middle line of a pair of appendages, the second
maxille. Between labrum and labium and Jateral to the
mouth lie a pair of mandibles, hard-toothed crushing organs
with no palp, and a pair of first maxille. The first maxille,
when dissected out, show a two-jointed basal portion (proto-
podite) which bears a double endopodite, the inner part of
BLATTA. 225
which is the éacénia or blade and the outer the ga/ea or
hood, and a long jointed exopfodite usually known as the
maxillary palpb. The second maxille closely resemble the
first maxillze in structure, but the /adcal palps (or exopodites)
are smaller and the protopodites are fused across the
middle line, as noticed above, the two appendages forming
the /abium. The head is joined by a neck with small
sclerites to the thorax. The thorax has three segments,
called the prothorax, the mesothorax and the metathorax.
These are freely movable. Each has a pair of /egs on the
ventral surface, hence the cockroach has three pairs of
legs. Each has a basal piece or coxa, a small ¢vochanter, a
long femur and tibia, and a six-jointed ¢avsus terminating in
two claws. On the dorsal surface of the male the mesothorax
‘bears a pair of leathery wings (sometimes termed ¢dytra),
and the metathorax carries a pair of membranous wings.
The abdominal segments, like those of the lobster, are
movable, and each has a ¢ergon and sternon. ‘There are
ten abdominal segments. The terga overlap each other,
and the 7th comfpietely overlaps the small 8th and 9th;
hence one can only count eight (1 to 7 and 1o).
The last or roth is notched, and bears laterally a pair of
many-jointed anal cerc?. Of the nine sterna, the first isa
mere rudiment and the gth in the male bears a pair of
small s¢yées. In the female, the 7th is boat-shaped and
envelopes the sterna behind it which are adapted for sexual
functions. Hence in the female only seven sterna can be
made out externally. At the hind end of the body the anus
opens and below it is the opening of the genital organs.
There are no excretory pores, but the respiratory organs or
tracheze open by ten paired apertures, the s#gmata. Two of
these open laterally between the thoracic segments, and the
other eight lie between the terga and sterna of each of the
first eight abdominal segments. Air is inhaled and exhaled
through these stigmata by a rhythmic lengthening and
shortening of the segments upon each other (caused by
tergal and sternal muscles).
The external features show a marked contrast to those of the
lobster. The principal differences are (1) the presence of only one pair
of antennze ; (2) only three pairs of thoracic appendages ; (3) the absence
of abdominal appendages (except, perhaps, the anal cerci); (4) the
M. 16
226 ANNULATA.
presence of s¢igyzata and absence of gills ; and (5) the terminal position
of the genital aperture.
The antennz are tactile and, like the antennules of the
lobster, they are also said to possess odfactory
hairs. The palpi are also tactile. ‘The eyes have
been already referred to.
Sensory.
Fig. 150.—DISSECTION OF COCKROACH FROM THE DorsaL SIpkn.
(Ad nat. )
Salivary Bladder.
Brain.
ist Thoracic
anglion.
ist Thoracic
Stigma.
2nd Thoracic
1st Abdominal
Nerve Ganglion.
Gizzard,
we ,
Stigmata. { f
Ventral Abdo-
minal Muscles. ~q .Hepatic Czca.
ig as - \ } p Malpighian Tubules.
Rectum.
Ovarioles.
The body-wall is removed and the alimentary canal pulled over to the right. The
ventral tracheal system is seen as white tubes leading from the stigmata.
If the terga be gently cut off or freed by a scalpel the
principal organs of the body are exposed. The mouth
passes into a duccal cavity provided with a hard
chitinous tongue. The paired salivary glands,
with saivary bladder, open by ducts into this part. Thence
a delicate wsophagus passes gradually into a large and spacious
crop. At the hind end of the crop is the small thick-walled
gizzard, provided with six chitinous ¢eeth and strainers, as in
the lobster.
Alimentary.
BLATTA. 227
The cavity of the gizzard is continued into that of the
mesenteron, a comparatively short tube which leads into a
still shorter and narrower intestine, terminating in a
vesicular vectum. At the front end of the mesenteron are
eight (or nine) hepatic ceca or hollow glandular processes,
and at its hind end are six tufts of extremely fine long
processes, called the malpighian tubules. They constitute
the excretory organs of the cockroach. The rectum has six
longitudinal folds. As in the lobster, the mesenteron alone
is formed from endoderm, and absorption is confined to it.
The parts in front and behind are formed of ectoderm and
are lined by chitin. The digestive fluid from the ceca
is said to pass forwards into the crop where it is mixed
with the food. Here the food is digested or reduced to
a soluble condition. The gizzard then relaxes and allows
the digested food to pass on into the mesenteron, in
which absorption is effected. The most important dif
ferences in the alimentary system from that of the lobster
are (1) the presence of salivary glands (connected with the
terrestrial habit); (2) the division of the “stomach” into a
large storage crop and a small gizzard ; and (3) the presence
of excretory organs opening into the alimentary canal.
The cockroach has a complex system of muscles. In
the abdomen the dorsal and ventral abdominal muscles are
little modified. They serve to execute the
respiratory movements, not to flex the abdomen.
In the thorax the muscles are broken up into special limb
muscles, moving the legs and wing-muscles for flight. The
alary muscles run as a triangular band from the tergon of
each segment towards the heart, spreading out under the
pericardial septum and meeting its fellow below the heart.
They may serve to move the pericardial septum.
As in the lobster, the muscles are attached to the exo-
skeleton but there is no endophragmal skeleton. The cavity
of the body is largely filled up by the corpus adiposum or
fat body, a mass of fat cells.
Bined: The heart is a long delicate tube running in
the median dorsal line of the thorax and abdo-
men. It lies just under the terga. In each
segment (three thoracic and ten abdominal) it opens by
paired valves or ostia into the pericardial cavity surround-
Muscular.
Vascular,
228 ANNULATA.
ing it. On contraction of the heart the blood is driven
forwards along the dorsal aorta, which terminates near
the brain in a funnel opening into the body-cavity. The
body-cavity is, therefore, a blood-space or heemoccele in
which the blood bathes all the tissues and eventually finds
its way back to the heart. Immediately under the heart
the pericardial septum stretches across the body-cavity,
partially dividing it into a dorsal pericardial sinus and a
ventral main cavity. The septum is a fenestrated mem-
brane, being perforated by numerous apertures.
Fig. 151.—TRANSVERSE SECTION OF BLATTA,
(Semi-diagram matic. )
Heart.
Dorsal Branch of Dorsal Muscles.
Trachea. Alary Muscles.
Tergon. /, Pericardial
“a Septum.
ventral Muscle.
Gizzard with
Sternon. Teeth.
Ventral
Muscles. . Hepatic Caca.
Nerve-cord. “ Body-cavity (a blood-space).
Passing through the anterior portion of the abdonien.
The brain lies in the head dorsal to the cesophagus. It
has a paired anterior lobe which supplies the
eyes and a posterior giving nerves to the an-
tenne. A ring round the cesophagus is completed by a’sub-
cesophageal mass, composed of three pairs of fused ganglia,
belonging to the mandibular, maxillary and labial segments.
This is followed by a double ventral nerve-chain with three
thoracic ganglia and six abdominal.
Nervous.
The cockroach has a nervous system much like that of the lobster.
As in the latter, we can recognise certain fusions. If we start with a
brain and a chain with ganglia to each segment we get a total of
five cephalic (of which the second has no appendages), three thoracic
and ten abdominal ganglia, or eighteen in all. These are reduced to
ten by the fusion of the first two to the brain, the fusion of the next
BLATTA. 229
three to form the subcesophageal mass, and the fusion of the last five to
form the terminal abdominal ganglion. As in the lobster, the first two
ganglia move forwards to the brain and the jaw-ganglia fuse together.
In the cockroach there is no trace of excretory glands
opening at the base of any of the -appendages.
A different kind of excretory organ is found in
the Alalpighian tubules described above.
‘The ¢rachee are tubes, lined with chitin, thickened in a
spiral, and passing inwards from the stigmata. They branch
all over the body and pass into the wings. They
appear in dissection like delicate silver tubes. In
a general way, there pass inwards from each stigma a dorsal,
a ventral and a splanchnic branch. The dorsal branches
anastomose beside the heart, the ventral anastomose near
the nerve-cord, and the splanchnic branch all over the vis-
cera. The first stigma sends two large paired trachez
forwards to supply the head.
The male organs consist of a pair of minute des¢es in the
dorsal part of the middle of the abdomen. ‘They lead by
small vasa deferentia into a “ mushroom-shaped
gland,” a tufted organ which is really a paired
vesiculum seminalis ; an ejaculatory duct passes from these to
the exterior.
In the female each of the paired ovaries consists of eight
long tubes or ovarioles. They unite to form a pair of
ovtducts which open together on the 8th sternon. The
short united portion is sometimes called the uterus. A
pair of branched colleterial glands open into the uterus and a
small sac or sfermatheca opens on the gth sternon. In
both sexes there are gonapophyses or paired sclerites, modi-
fied to assist reproduction—in the male for copulation, in the
female for deposition of the ova. In the female the 8th
and oth sterna are telescoped within the large 7th,
producing a genital pouch, It should be noted that the
female opening, as in the lobster, is two segments anterior
to that of the male.
The eggs are laid in a capsule (one from each ovariole, making
sixteen in all) formed by the co/letertal giand;. They have much yolk,
and the segmentation is equal and superficial. A ventral
Development. A/afe is produced by a thickening of the cellular layer.
This is invaginated, the walls meeting above and forming
an anmion, a remarkable protective membrane, found also in land
Excretory.
Respiratory.
Reproductive.
230 ANNULATA.
vertebrates. Some points of special interest in the subsequent develop-
ment are the presence of a segment between that of the antennze and
that of the mandibles, and the presence of abdominal appendages which
disappear later. These seem to point to the cockroaches having sup-
pressed a head segment, probably corresponding to that bearing the
antennze in the lobster, and to their having in a similar way lost a
number of abdominal appendages.
The mesoblast is present in the embryo as paired somites containing
ceelomic cavities, separate from the hemoccele or blood-space, part of
which forms the heart. In later development, however, the mesoblast
walls break up to form the muscles, connective tissue, gonads and
walls of the heart; the cavities of the somites then become continuous
with the hemoccele. Thus there is no true perivisceral ccelom in the
cockroach, a condition agreeing with other Arthropoda.
The young cockroach only differs from the adult by an
absence of wings, and it grows gradually into the adult, pass-
ing through periodic ecdyses or shedding of its integument.
Hence the cockroach is ametabolic, or developing without
metamorphosis.
BLATTA.
SEGMENTS. APPENDAGES. NERVOUS SYSTEM. APERTURES.
Prostomium Primary
: brain.
I | Antenne. Brain.
wp 2 Mouth
8 Mandibles. :
= : 9 hee Subresophageal
Bikeqd mass.
4 6 | 1st Leg. Ganglion 1 ,
&
tat 7 and u " 2 Suga
a 8 3rd " " 3
9 : ¥ i
10 " 5
ju ‘i 6 ;
a 12 " 7:
a} 13 n 8 :
6 "
3 a n Gap
“4 16 Posterior " "
17 ganglion 9
18 | Anal cerci. Anas. P
PERIPATUS. 231
III.—PERIPATUS.
PHYLUM ANNULATA (p. 237).
Sus-PHYLUM ARTHROPODA (p. 240).
Crass PROTRACHEATA (p. 244).
Fig. 152.—LaTERAL VIEW OF PERIPATUS CAPENSIS.
(After BALFour.)
Note the thick antennz on the head, the long soft body with seventeen pairs of
soft ringed legs, and the oral papilla at the sides of the mouth.
Peripatus capensis is a small worm-like animal. The female
may be 2% inches in length and the male slightly smaller. The body
is of a warm olive-green hue, shading off to light brown on the ventral
surface. It is usually to be found hiding under stones or in the crevices
of rocks, and occurs on Table Mountain.
The anterior end bears a pair of thick antenne. Extending down
either side of the body and protruding ventrally are seventeen pairs of
stumpy legs terminating in two claws.
The mouth is on the under side of the head or anterior end and is
covered laterally by a pair of oral papilla, on which are the openings
of the slime glands, They are apparently the first pair of post-oral
appendages. Inside the mouth is a pair of chitinous jaws. At the
hind end opens the azzs which also has a pair of anal papilla, probably
the last pair of appendages. At the base of each leg, on the inner side,
there is a nephridiopore. Immediately below the anus is the gezdfal
aperture.
The animal is strictly plano-symmetric. The body is soft and the
cuticle is not thickened into sclerites, but there are a number of soft
papillze all over the surface which bear cuticular spines. Under the
cuticle is a simple ectoderm covering the muscles.
The antennz are tactile and there is a pair of sémple eyes at the base of
the antenne. The mouth, with its chitinous jaws, leads into a pharynx,
into which there opens a large pair of salzvary glands, said to be a
modified pair of nephridia. A short cesophagus continues into a
spacious but simple stomach. Quite at the hind end of the body the
short zfesténe leads to the anus. The whole alimentary canal, as in
the cockroach, lies in the cavity of the body and there are no mesen-
teries. Immediately below the ectoderm there is a thick layer of
circular muscles, internally to which there is a series of longitudinal
muscles, more or less broken up into dorsal, ventral, and lateral bands,
232 ANNULATA.
In addition, there are oblique bands running from the sides to the mid-
ventral line.
Peripatus has little more skeleton than the Annelida, the scattered
cuticular spines forming the nearest approach to an exoskeleton.
Fig. 153.—A DIssECTION OF PERIPATUS CAPENSIS FROM
THE DoRSAL SURFACE.
(After BALFour.)
Tentacle. -
Oral Papilla.
Brain.
Slime Gland. Salivary
Gland
:
Feet. 4 Intestine.
————
=o
SSeS
FF
=—=
Nerve Cord ==
(paired). . ec ¢
Last Crural
Glnd: Nephridium.
The ccelom is not present as a body-cavity, but is in the adult only
represented by the cavities of the gonads and those of the nephridia.
The actual body-cavity is a venous blood-space which thus contains blood
and belongs to the blood-vascular system. Hence, as in the lobster
and the cockroach, it leads directly into the dorsal heart by paired
ostia or valves. The heart is itself a long tube lying dorsally, extending
nearly the length of the body. It is surrounded by a fericardial sinus,
as in the lobster.
EPEIRA. 233
The brain over the pharynx supplies the eyes and antenne. A
nerve-ring round the cesophagus unites it with the ventral nerve-chain.
The two cords of this chain are widely apart and are connected by cross
strands. At the hind end they communicate over the intestine. There
are ninéteen pairs of ganglia upon the cords, supplying the jaws, oral
papillze and the seventeen pairs of legs. There are seventeen pairs of
nephridia (and the pair of salivary glands belonging to the segment of
the oral papillae). Each hasa bladder or vesicle, leading to the exterior
at the inner base of the leg, a coiled excretory portion and an internal
nephrostome which opens into a small closed ccelomic space.
Peripatus breathes by tracheze opening to the exterior by stigmata.
Their arrangement is indefinite, though some are arranged in rows.
The sexes are separate. The male organs are a pair of testes lying
over the stomach, leading to the genital pore by paired vasa deferentia.
The ovary is unpaired and leads to the.exterior by paired oviducts
which are swollen to form wert, The development takes place in the
uterus, hence Peripatus Capens?s is viviparous.
IV.—EPEIRA.
PHYLUM _7 ANNULATA (p. 237).
SuB-PHYLUM mn - ARTHROPODA (p. 240).
Crass ARACHNIDA (p. 258).
Fig. 154.—A COMMON GARDEN SPIDER
(Epetra diademata).
Resting in the centre of its web. _ Dorsal aspect and about
natural size.
Epeira diademata is one of the commonest of our British
spiders. The figure is about the natural size of the female ;
234 ANNULATA.
the male is smaller and of more delicate build. The colour
varies considerably in shades of brown, but is
always mottled in blotches and irregular mark-
ings of white. The most characteristic of these is a
T-shaped white mark on the abdomen, followed by two or
more large white dots. The legs are barred.
LEpeira diademata lives in the centre of its vertical web.
usually head downwards. The web is commonly suspended
between branches of a shrub.
The body is constricted by a ‘‘waist” into an anterior
smaller part called the cephalothorax, and a large posterior
ze globose part, the addomen. Neither part shows
Xternal Sic agielin iat :
any external indications of segmentation and
the abdomen is soft to the touch. The
abdomen bears no appendages but the cephalothorax
has six pairs. The anterior of these are called the
chelicere. They are two-jointed, and the distal joint is
in the form of a sharp curved stylet connected with a
. poison gland in the proximal joint.
ane rl sapere Oat The second pair are the pedipalpi
DAGES OF Eprrra Dia- Or feelers; they appear like a pair
DEMATA. (Ad nat.) of short legs and really function as
Magnified. arms. The basal joint is formed
into a kind of jaw and the terminal
joint in the adult male is modified
into a swollen ‘“palpal organ” for
transferring the sperms into the
seminal receptacle of the female.
The next four appendages are /egs,
many - jointed and covered with
numerous hairs.
a\, LE The spider, therefore, differs from
Pedipalpi. the insect in having no pre-oral ap-
epieocaen) foes acne ane Te
pedipalpiwith long palps, ako S€SSing four pairs of legs instead of
non-chelate. three.
The mouth is a minute ventral aperture between the two
cheliceree and the anus is at the tip of the abdomen.
Immediately in front of the anus is a swollen process which
is found to consist of four papille or spinnerets, at the
tip of each of which there is a great number of minute
Habits.
Features.
EPEITRA, 235
apertures. These communicate with the spinning glands
lying in the abdomen, a complex series of glands which pro-
duce threads of various kinds, according to the requirements
of the spider. Further forward on the ventral line of the
abdomen opens the genital aperture and on either side of
it the single pair of stigmata leading into the pulmonary
sacs. Lastly, just in front of the spinnerets there is a small
median aperture leading into four ¢rachee.
The integument of the spider consists of a thin cuticle
over the abdomen, thickened in the cephalothorax. The
Thiceuisisataey, whole surface is more or less covered with
’ fine hairs which extend down to the tips of
the legs. The dorsal anterior surface of the cephalothorax
Fig. 156.—LONGITUDINAL SAGITTAL SECTION THROUGH EPEIRA
DiaDEMATA (2). (Semi-diagrammatic, after LEUCKART.)
Heart. Digestive
Gland.
Dorsal
Blood-vessel.
Lecce Eye. Mapighian
Tubules.
Cloaca,
Cloacal
Aperture.
on od
3g oes ae
Mouth. is zg B = b Spinning Glands.
Czcum of Stomach. a S aad | Ovary.
7) Ss Genital Aperture.
is smooth and bears six eyes which are of the simple
type. Four are arranged in a small square and the other
two laterally.
The mouth leads up a small tubular pZarynx and a short
esophagus into the large “sucking” stomach. The walls of
this organ can be drawn outwards by strong
muscles, causing powerful suction. The true
stomach is small and expands into long ceca which end
blindly towards the bases of the legs. The intestine
is narrow and leads through the “waist” into the
abdomen. Here it swells into a sac, receiving the
Alimentary.
236 ANNULATA.
ducts of a large digestive gland and then is continued
as the vectum into the cloacal sac. The spider kills
its prey by its poison cheliceree, bites it open with the
cutting bases of the pedipalpi, and sucks its juices by
means of its sucking stomach, The juices are stored in
the stomach and its czeca and digested and absorbed in
the intestine. We may note the absence of any crushing
gastric mill, so characteristic of the lobster and cockroach,
Again, we can observe a certain resemblance in the ali-
mentary system of the spider to that of the leech, due to a
similar method of feeding.
The muscular system is much broken up into limb
muscles and other special muscles, and it is
difficult to recognise much trace of the annelid
and protracheate arrangement.
The coelom has much the same relationship as in the
Jnsecta—that is, it is inferred that the perivisceral part is
not represented. There is a pair of small and
degenerate coxal glands which in some young
spiders open by a duct at the bases of the legs. These
are held to be vestigial excretory organs of the nephridial
type, and in the young scorpion they are said to have
internal openings into the ccelom.
The heart is a long dorsal tube surrounded by a fer-
cardial sinus into which it opens by three pairs of ostia.
The heart is continued forward into main arteries which
finally open into the venous sinuses composing the body-
cavity. Some of these communicate with the pulmonary
sacs and thence pass to the heart. The pulmonary sacs are
therefore in the same position in the blood circuit as are
the gills of the lobster and similarly the heart is systemic.
The nervous system is concentrated in the cephalo-
thorax. It consists of a brain above the pharynx, supplying
the eyes and the chelicerz, and connected by a nerve-ring
with a ventral nerve-mass formed of at least
five pairs of fused ganglia. From it nerves are
given off to the pedipalpi, the legs and the abdominal
organs. pera shows a great degree of nerve-concentra-
tion and in this respect differs from some Arachnida.
The vestigial excretory organs, or coxal glands, have
already been alluded to. The functional organs are four
Muscular.
Coelom.
Nervous.
EPETRA. 237
long coiled malpighian tubules opening into the cloaca. In
addition to possessing two kinds of excretory
organs, the spider also has two kinds of respira-
tory organs. The two pulmonary sacs are situated in the
antero-ventral part of the abdomen and consist
of large chambers -containing a number of flat
horizontal /amelle with thin walls. Stigmata put their
cavities in communication with the exterior. There are in
addition four trachez opening, as stated, by a ventral
aperture in front of the spinnerets. They do not differ
essentially from the trachez of the insects. Hence the
spider has two sets of breathing organs, pulmonary sacs
and trachez.
The ovaries are paired tubes uniting to form ovéducts
which open into a median uéerus. The uterus opens into
the genital pouch, into which also open
two seminal receptacles. The pouch is
provided with a kind of gonapophysis, called the epzgy-
nium. The ¢estes are sitnple tubes with vasa deferentia
uniting into a spferm-sac with a median aperture just behind
the stigmata.
The eggs are laid in holes and corners during the autumn,
and are often enveloped in silky cocoons. ‘They have a
: large amount of yolk, and the development
is embryonic. They hatch in the spring,
the young spider differing but little from its parent. The
spiders form the order Avaneina of the class Arachnida.
(For General Characters of Sub-Phylum Arthropoda,
see page 240).
Excretory.
Respiratory.
Reproductive.
Development.
PHYLUM ANNULATA.
The Axnulata form one of the three great phyla of the
Metazoa. They are typically elongated plano-symmetric
animals. They always have three primary layers, the meso-
derm filling more or less of the space between the ectoderm
and endoderm. The whole body is segmented or made up
of a number of segments or metameres, in which many
organs are repeated. In the lower types there can be dis-
tinguished a pre-oral part, in front of the mouth, called the
prostomium, and a segment immediately behind the mouth
238 ANNULATA.
called the Zeristomium which differs in many respects from
the segments behind it. In the great majority each segment
carries a pair of appendages which may be parapodia,
legs, jaws, and so on.
The nervous system consists of a dorsal brain in the
prostomium, a circumoral ring round the front end of the
gut and a double ventral nerve-chain with or without
ganglia.
The heart, when present, is dorsal to the alimentary canal
and may show traces of segmentation.
There are never true “shells,” as in the A/ollusca, but
the body is enclosed in a thin cuticle or a thickened
cuticular exoskeleton.
The phylum is divided into two sub-phyla :—(1) ANNE-
Lipa and (2) ARTHROPODA.
Sus-PHYLUM I.—ANNELIDA.
The Annelida is the sub-phylum of segmented worms,
and in anatomical characters it is sufficiently definite. The
most diagnostic characters of the sub-phylum are (1) the
metameric segmentation. The body has a great number of
segments, usually preceded by a prostomium and a peri-
stomium. The nervous, blood-vascular, ccelomic and
excretory systems are mostly repeated in the segments.
(2) The nervous system always consists of a dorsal brain
in or near the prostomium, a nerve-ring in the peri-
stomium, and a long ventral chain, usually more or less
segmented and showing a double origin. (3) The muscular
system and chief method of locomotion are quite character-
istic. The circular and longitudinal muscles, contained
in a tough, flexible body-wall, work in conjunction with
external organs (sete, suckers) and with the internal vas-
cular coelomic fluids in the way described for Avenicola. (4)
Highly developed nephridia are not confined to the sub-
phylum, but are very characteristic of it.
The four classes are intimately connected by inter-
mediate types but can hardly be further approximated.*
Crass I.—ARCHIANNELIDA. From the type Poly-
gordius it can be seen that this class contains the simplest
* The Polycheta and Oligocheta are often placed together as Chetopoda, with
the presence of setaze in comnion.
ANNELIDA. 239
and most primitive of the Annelida, as is shown by the
ectodermal nervous system, the persistence of radial septa
and longitudinal mesenteries, the simple nephridia and the
absence of appendages. It contains two or three other small
worms.
Cxass II.—Potycuata. This class has a great number
and variety of types. Many live in tubes and burrows and
the anterior end bears a mass of tentacles and gills, whilst
the free-swimming forms often have a great development
of lateral appendages which are in many cases used for
swimming. They are called Polychaeta because they usually
have great numbers of setze.
Fig. 157.—FooT oR PARAPODIUM OF A NEREIS. (Ad nat.)
Dorsal Cirrus.
Notopodium.
Acicula.
Ventral Cirrus. Neuropodium.
Crass III.—Ouicocuata. As in Lumdbricus, the body
is usually without appendages or gills and has only com-
paratively few sete. They are usually divided into the
mud dwelling (freshwater) forms and the terrestrial. Their
hermaphrodite and complex sexual organs and protected
embryonic development are characteristics.
Crass 1V.—HirupinzEa. In many respects this class
resembles the last, especially in the absence of appendages,
the hermaphrodite sexual organs and the development.
It is, however, clearly characterised by the reduced con-
dition of the coelom and its continuity with the blood-
vascular system, by the suckers and the mode of life.
The most important features of the sub-phylum and the
classes are summarised in the subjoined table :—
240
WB NOH
ANNULATA.
SUB-PHYLUM ANNELIDA.
. Metameric segmentation.
nerve-chain with ganglia.
Nan +
Class I.
ARCHIANNELIDA
Lype—Polygordius.
1. No sete on body.
2. Prostomial tenta-
cles, but no bran-
chia.
3. Dicecious.
4. Larval develop-
ment.
5. Marine.
. Paired lateral appendages often present.
Class IT.
PoOLYCHATA,
Lypes—Arenicola
and Nereis.
Many sete on para-
podia.
Usually _ branchiz,
cirri and tentacles.
Dicecious.
Indirect larval devel-
opment (Trocho-
phore).
Marine, free-swim-
ming or sedentary.
- Ceelomate Metazoa with bilateral symmetry (plano-symmetry).
. Muscles are arranged in definite circular and longitudinal layers.
Excretory organs are paired nephridia (many).
Class IIT.
OLicocuata.
Lype—Lumobricus.
No parapodia and
few seta.
No branchiz, cirri
or tentacles.
Hermaphrodite.
Direct development.
Freshwater or terres-
trial.
. Nervous system is a brain above oesophagus, a circumoral ring and double ventral
. A vascular system of vessels or sinuses and perivesceral caelom is usually present.
Class IV.
Hirvpinea (Disco-
PHORA).
Type—Hirudo,
A pair of suckers and
no parapodia.
No branchie, cirri
or tentacles,
Hermaphrodite,
Ccelom reduced to a
dorsal and ventral
sinus and other
smaller parts,which
communicate with
the vascularsystem.
Gonads have separate
ducts to exterior.
Free freshwater or
marine, partially
ectoparasitic.
Susp-PHyLuM II.—AR1THROPODA.
In the ARTHROPODA the body, as a rule, is enclosed in
a thickened cuticular exoskeleton, which may or may not
be further strengthened by calcareous particles. The
paired appendages undergo a similar modification, pro-
ducing jointed limbs. These are bent towards the ventral
surface and serve to support the body. These appendages
show far more adaptive modification into jaws, legs and
feelers than in the lower sub-phylum. In many of the
higher types of Avthropoda the body and its parts become
compressed into a compact form, losing the elongated
CRUSTACEA. 241
character and disguising the segmentation. The simple
annelid eyes are replaced by the compound eyes.
In the mesodermic organs there are important modi-
fications from the annelid type. The simple circular and
longitudinal muscles of the body-wall become largely broken
up into segmental muscles and limb-muscles, At the same
time the perivisceral part of the ccelom is replaced by the
enormously developed hzmoccele or blood-space, the actual
body-cavity of an arthropod being a venous blood-space
communicating directly with the heart. The paired
nephridia or excretory organs are replaced gradually
within the sub-phylum by excretory organs of another type.
The nephridia are still present in Perzpatus, but the coxal
glands of Avachnida, and the shell-gland and green-gland of
Crustacea, are usually supposed to be much modified
nephridial organs. Malpighian tubules appear in Jusecéa,
Arachnida and Myriapoda, Lastly, a centrolecithal type
of segmentation appears to be characteristic of the Arthro-
poda.
The Arthropoda have five classes— (1) Crustacea, (2)
Protracheata, (3) Myriapoda, (4) Insecta, and (5) Arach-
nida.
Crass I.—CRUuUSTACEA.
The Crustacea are typically aquatic and breathe by gills.
They have two pairs of antenne or feelers on the head.
The first five segments are aggregated together into one
mass, termed the head, and a number of the other segments
may form a thorax and abdomen. The appendages are
typically biramous and used for swimming, but more or
fewer are modified into legs and jaws. The Crustacea are typi-—
cally marine and the lower marine types have a free nauplius
larva. This larva is pelagic and has a dorsal shield} an
unpaired eye and three pairs of swimming appendages
round the mouth. The first is uniramous and becomes
the antennules; the second and third are biramous and form
the antenne and mandibles. The nauplius, like the trocho-
phore, grows into the adult by elongation of the hind-end of
the body and production of fresh segments. In the higher
Crustacea, with much yolk in the egg, a stage comparable
to the nauplius is passed through in the egg.
M. 17
242 ANNULATA.
If we trace the class from the lowest to the highest, we
can notice a general advance in size and complexity of the
body, in reduction and consolidation of the segments, and
in the gradual adoption of embryonic development.
There are two sub-classes—(1) Entomostraca and (2)
Malacostraca.
Susp-CLass I.—ENTOMOSTRACA.
These are nearly all small and simple Crustacea. There
is great variety in the number of the segments. The excre-
tory organ (shell-gland) is situated on the second maxille,
and there is never a gastric mill. The Lxtomostraca
develop by a free-swimming xaupiius larva.
Fig. 158.—THE LIFE-HISTORY OF CIRRIPEDIA.
1. Nauplius larva of Balanus. Ventral 2. A rather later larval stage of
view. Note three pairs of swimming Chthamalus. The posterior region is
appendages, the last two being biramous __ elongating.
and the median simple eye.
3. Cypris larva of Lepas. Just fixed by
its anterior end (antennz) to a piece of
wood, Note the six pairs of biramous
appendages and the enveloping shield.
The Phyllopoda have foliaceous or leaf-like appendages.
Some are small and are known as water-fleas. Daphnia
is a very common freshwater type. Apus is a large phyl-
lopod with a head-shield covering most of the body. The
CRUSTACEA. 243
Ostracoda have the exoskeleton formed into a pair of lateral
shells resembling those of bivalve Mollusca. They show
a very degenerate condition of the body. Cyfris is a
common freshwater type. The Cofepoda are an immense
assemblage of marine and freshwater crustaceans, usually
of small size. They play the part in marine life of the
insects on land. Great numbers are pelagic and form the
staple food of larval fish. Cyc/ops is a common little ‘‘water-
flea” found in ponds. Many Cofefoda are parasites and are
so modified in form and shape that their crustacean affinities
would hardly be recognised except for the early develop-
ment. Much the same remark applies to the Cirripedia,
of which the barnacles and acorn-shells are important types.
The barnacle (Zegas) has a long stalk which is usually
affixed to a floating log, the hull of a ship, &c. The body
Fig. 159.—LATERAL VIEW OF Fig. 160.—LATERAL VIEW OF
Lepas (BARNACLE). (Natural size.) Lrepas ANATIIERA.
(Ad nat.) (Ad nat.)
Scutum. 3g Bens
fe
as
4 5
A Genital
Aperture.
4
g
q
3
Carina.
With right shell removed showing anima
lying in mantle cavity.
is enclosed in five calcareous shells, and there are six pairs
of legs which are covered with processes. Their perpetual
movement serves to supply the animal with microscopic
food. The acorn-barnacle has no stalk and is enclosed in
a conical outer shell in addition to the movable shells.
In each case the young start life as xaupus larve, and
pass through the stage of a free-swimming crustacean which
fixes itself to a foreign body and becomes a sessile adult.
244 ANNULATA.
Sus-Ciass II.—Ma.Lacostraca.
The Malacostraca include the higher types of Crustacea.
The body usually consists of twenty segments and the
appendages are much modified. The excretory organ, the
antennary gland, opens on the second antennze and there
is usually a gastric mill. The nauplius larva is of rare
occurrence, the early development being embryonic.
The order Arthrostraca comprises Crustacea with sessile
eyes, and with not more than two thoracic segments fused
with the head. The freshwater shrimps, sand-hoppers, and
the terrestrial woodlouse (Oxzscus) are good examples.
The Decapoda form the most important order of Madacos-
traca. The head and thorax are enveloped in a carapace
and there are five pairs of legs (including chele). The
eyes are stalked. They include the lobsters, shrimps and
prawns, the crabs and hermit-crabs.
The crabs have the
Fig. 161.—A Z@a Larva OF A’ abdomen reduced and
Decapop. (Lateral view.) tucked forward on the
under side of the thorax.
The appendages are
closely similar to those
of the lobsters, but the
nerve-ganglia are more
consolidated. The her-
WNW ( mit-crabs have a long,
OY Wh) em Componne soft abdomen, which
Jd they protect in a shell.
The shell is usually a
disused whelk-shell or
that of some smaller
gastropod. The chelz
Rostrum: are of different sizes,
adapted to the spiral of
the shell. The appen-
Note the paired eyes, the spines, abdomen without dages on the abdomen
appendages, and gills with no gill-cover. are ves tigial.
Dorsal Spine.
Abdomen.
Crass II.—PROTRACHEATA.
Peripatus constitutes, not only the type, but the sole
order of this class.
MYRIAPODA. 245
Cuass JIJ.—Myriapopa.
The Myriapoda resemble most nearly the /zsecta. Like
them, they breathe by trachez, excrete by malpighian
Fig. 162.—SCOLOPENDRA CINGULATA (A CENTIPEDE).
Note head with antennz, segmented body and a single pair of jointed
* legs to each segment.
tubules and have one pair of antennz. They differ from
them in having no definite thorax nor abdomen; the body
Fig. 163.—JuLus Trerrestris (A. MILLIPEDE).
Note head with antennz, the very numerous segments, and two legs to each
segment. On the left is seen an individual coiled up.
246 ANNULATA.
consists of a series of separate segments, each having one
(or two) pair of jointed legs Scolopendra is typical of the
carnivorous order of Chilopoda (Centipedes). The other
order, Chilognatha, is herbivorous and a common example
is the millipede (/wlus terrestris). The millipedes chiefly
differ from centipedes by the more cylindrical body, two
pairs of legs in each segment and the forward position of
the genital aperture.
Crass IV.—InsEcta.
In the Jusecta the body is sharply defined into three
parts—the head, thorax and abdomen. The head consists of
five segments and carries one pair of antenne@ and three
pairs of jaws. The thorax has three segments and bears
three pairs of legs. It may also carry two pairs of wings.
The abdomen is jointed and has about zex segments with
no appendages. There are no true gills and respiration is
effected by trachee. Excretion is by malpighian tubules
and there is usually a metamorphosis.
Insects are mainly terrestrial and rial. The cockroach
is typical in all features except the absence of a meta-
morphosis.
Amidst a multitude of adaptive modifications, the insects
conform to a remarkable extent to the general characters of
the class. They are divided into orders by (1) the adap-
tations connected with the mouth-parts or jaws, (2) the
condition and structure of the wings, and (3) the degree of
metamorphosis.
The largest and economically the most important orders
are those with a full metamorphosis. The youngoneis hatched
as a /arva which is usually more or less worm-like. The
larva passes through a quiescent pupal stage of varying
duration, and is then set free as the zmago or perfect insect.
ORDER I.— Coleoptera (Beetles).
The beetles have a complete metamorphosis, the mouth
parts, like those of the cockroach, are of the biting type,
and the first pair of wings are modified into hard edy¢ra or
wing-covers.
INSECTA. 247
Fig. 164.—Tue Lire-HIsToRY OF THE COMMON COCKCHAFER
(Melolontha vulgaris).
as
‘
4 I
f
Na uh
ae
t
;
ik
The underground larva is seen in the middle, the pupa to the left, and the male is
emerging on the right. The female is flying, showing elytra and wings,
A very typical and common beetle is the cockchafer
which works havoc upon vegetable life throughout its career.
The eggs are laid in the soil and the larvee feed upon the
roots of grass or almost any herbaceous plant. After about
four years of larval and pupal life, the beetle emerges in early
summer and commences its depredations upon the leaves
of trees. The larva of some click-beetles is called a
“wire-worm” and does great harm to crops. The Colorado
Fig. 165.—COLORADO BEETLES (Chrysomela decemlineata ).
Ly
NG
=
«t, Eggs on the under surface of the leaf; 4, c, d, various stages in the larva ;
é, pupa—the upper is the ventral view, the lewer the dorsal.
248 ANNULATA.
or potato-beetle works untold mischief in potato fields, the
larva feeding upon the leaves. The whole development is
accomplished in four weeks and the fecundity is very high.
Other interesting beetles are the burying-beetles which bury
the bodies of small animals as food for their larve, the
useful “‘lady-birds ” which feed on green aphides, and the
various water-beetles which have aquatic larve.
Fig. 166.—A WATER-BEETLE (Dytiscus marginalis ).
aA, The aquatic larva with soft body.
OrvER II.—Hymenopeera.
The metamorphosis is complete, the mouth parts are
modified for biting and licking and there are two pairs of
membranous wings. There is no one popular name for
the Aymenoptera, but they include the Ants, Bees, Wasps
and Gall-flies. The “biting and licking” mouth parts are
well illustrated by those of the bee. The mandibles are of
the biting and crushing type, and the first maxillz form a
pair of semi-cylindrical tubes enveloping the labium. The
maxillary palps are vestigial The labial palps are long
and the end of the labium is produced into a long flexible
hairy “tongue” or Zigwla. It can be withdrawn inside the
basal part of the labium. The maxille form a suctorial
cylinder and the ligula serves to lick honey and pollen.
The Hymenoptera are of special interest from their social
habits. Ants, bees and wasps of many species live in
communities in which there is structural and physiological
division of labour. In the case of the bees there can be
distinguished the males or drones, the female or queens,
« LNSECTA. 249
Fig. 167.—Tue Hive Bree (Apis mellifica).
Drone ¢. Queen ¢?. Worker or Neuter.
Three sorts of individuals.
and the workers, which are sterilised females. In the ants
the workers have no wings.
The gall-fly lays its eggs on plants and the “gall” is
produced by the plant around the egg. The insect, escapes
from the “gall” by a small hole. The ichneumon-flies
Fig. 168.—Tur GALL-FLy (Cynips guercus-foliz).
The galls are shown on the left, the interior of a gall on the right, and the
: perfect fly below,
are of economic value from their habit of laying eggs in
caterpillars of certain Lepidoptera. The larvee feed on the
substance of the caterpillar and eventually kill it. The
saw-flies have larvee somewhat like caterpillars but with more
legs. They are sometimes called “false” caterpillars and
250 ANNULATA
infest turnips. Many of the Aymenopiera have a sting at
the hind end of the abdomen. ‘This is modified from the
ovipositor which in its turn is comparable with the gonapo-
physes of the cockroach. In the saw-flies the ovipositor is
in the form of a pair of saws which are used for perforating
holes in twigs, in which the eggs are deposited.
OrvDER ILI.—Dzipeera.
Fig. 169.—TsETSE FLY (Glossina morsttans) X 3.
The deadly African fly.
The Dipéera have a full metamorphosis, the mouth parts
adapted for “ piercing and sucking,” and there is a single
pair of membranous wings. . The hind wings are reduced
to a pair of small Aalteres or balancers, processes with
knobs. They comprise the Flies, Gnats and Fleas.
Fig. 170.—SYRPHUS PYRASTRI.
A fly (A) whose larva (B) feeds upon the green aphis ; C is the pupa.
INSECTA. 251
The “ piercing and sucking” mouth parts are well shown
in the gad-fly (Zadanus). The upper lip (4adrum) mandibles
and maxille are lengthened and produced into sharp stylets,
whilst the labium is produced into a long hairy proboscis
with two terminal lobes. In gnats the “piercing” stylets
are best developed, whilst in flies, such as the house-fly, the
‘sucking ” proboscis is large and the stylets are small.
Fig. 171.—WHEAYT MIDGE (Cectdomya tritict).
A, Larva in wheat-flower; B, larva in grain; C, larva; D, fly.
252 ANNULATA.
The gnats have aquatic larve, the eggs being laid on the
surface of the water. The Hessian Fly and the Wheat
Midge both belong to the same genus, and both are
destructive to crops, the larva feeding on the leaves or
flowers. The common “ daddy-long-legs” or crane-fly has
Fig. 172.—Tur Dappy-LONG-LEGS OR CRANE-FLY (77pzla oleracea),
Male and also larva on the left, the female and the pupa on the right.
a larva which feeds underground on the roots of grass.
The bot-flies have a peculiar life-history. The common
“horse-bot” lays its small white eggs on the hair of the
horse. The larva is found in the stomach of the horse
and may give rise to serious inflammation. Other ‘‘ bots”
live in the nasal cavity of the sheep or under the skin of
A, Egg on a horse-hair ; C and B, larve; D, pupa case; and E, the fly.
INSECTA. 253
the ox. The fleas form a modified type of the Diptera,
with the wings reduced to mere rudiments, a loss of motor
organs characteristic of parasites.
OrvER IV.—Lepidoptera.
The metamorphosis is complete. The mouth parts are
adapted for ‘‘sucking,” and there are two pairs of large
opaque wings which are covered with minute scales. In
this order are included the “butterflies” and “moths.”
The mouth parts are much modified. The mandibles are
mere vestiges, and the maxilla are produced into a long
NRE
Female depositing eggs, larva (caterpillar), and pupa,
spirally-coiled ‘ proboscis,” composed of two half-cylinders
apposed together. The labium is small and bears a pair
of fairly large labial palps ; the maxillary palps are vestigial.
In use the proboscis is uncoiled and thrust into flowers,
nectar being sucked up its interior.
The wings are covered with minute scales of varying
shape which are easily rubbed off when the membranous
wing is exposed. As a general rule butterflies or moths
have bright colours on the upper surface of the wings, and
sombre protective colours below (cf Chap. IX.).
The larva is a ‘‘caterpillar” which often executes great
destruction amongst plant-life.
254 ANNULATA.
The ‘Cabbage White” lays its eggs on cabbages and
turnips which the larvee devour. A great number of the
night-flying moths have underground caterpillars which do
damage to crops.
The relationship of Zefidoptera to flowers and the cor-
related structural modifications in each are full of interest.
In a general way, the flowers employ Lepidoptera to carry
pollen, and so fertilise and attract them by a supply of
nectar.
ORDER V.—LVeuroptera.
Fig. 175.—DEMOISELLE DRAGON FLy
(Agrion puella ).
Notice the Nervured Wings.
The Weuroptera have biting mouth parts and two pairs
of membranous wings, usually of equal size and covered
with a network of veins. The metamorphosis is usually
incomplete, but in many cases is complete.
The most important of the Meuroptera are the Dragon-
flies, with an incomplete metamorphosis and an aquatic
larva with a movable labium like a hand; the May-flies,
also with aquatic larvee, the fly only living a few hours; the
Caddis-flies, the aquatic larvee of which protect themselves
in cases of twigs or stones and pass through a complete
metamorphosis with a pupal stage; lastly, the Ant-lions, the
larval stage of which digs traps for ants.
INSECTA. ass
Fig. 176.—THE STAGES OF DRAGON-FLY.
Larva of Dragon-fly catching prey (a larval May-fly) by the labium, On right’
the perfect insect is emerging.
Fig. 177.-THe May-FLy (Zphemera vulgata ).
ORDER VI.— Orthopeera.
The mouth parts are of the “biting” type; the first pair
of wings are chitinous and form covers for the second pair
which are membranous. The metamorphosis is incomplete
or absent.
256 ANNULATA.
Our type, the cockroach, belongs to this order and with
it is a remarkable series of forms, of which we can merely
mention the most important. The earwigs have the gona-
pophysis formed into pincers and live mostly in flowers,
The grasshoppers and locusts are large types with powerful
hind legs; in tropical countries great devastation is caused
Fig. 178. —THE GRASSHOPPER (Locusta
viridissima ).
by swarms of the migratory locust. The mole-cricket has
the habits and many of the structural peculiarities of the
mole. The stick- and leaf-insects exhibit remarkable pro-
tective resemblance.
Fig. 179.—A Group OF HEMIPTERA (WATER-
BuGs) in natural surroundings.
On the left is the long Ravatra lnearis; on the right
are two Water-Scorpions (Neda cinerea); and in the
centre is the Water-Boatman (Notonecta glauca),
INSECTA. 257
ORDER VII.— Hemiptera.
In this order there is great variety in the wings, which
are often absent, but the mouth parts are typically
“sucking,” the labium forming a long sucking “ rostrum,”
and the metamorphosis is incomplete. The Hemzptera
are mostly either aquatic insects or dwell on plants and
Fig. 180.—THE Common Louse (Pedicudus),.
«, Natural size; 4, magnified; c, a leg ; d, hair with ‘‘nits” or eggs; e, ditto
magnified. A degenerate Hemipterous insect. oa
Fig. 181.—THE Rose ApHIs.
suck their juices. Of the aquatic types the “ water-
scorpion” has the first pair of legs modified into kind of.
maxillipedes; the water-boatman swims at the surface on its
back, the hind legs imitating a pair of oars. Of the
terrestrial type the green aphis is peculiar in reproducing
parthenogenetically during the summer, and in secreting a
juice of which ants are very fond.
OrpbER VIII.—Apeera.
A few small insects comprise this order, their mouth
parts are biting, they have no wings and no metamorphosis.
M. 18
258 ANNULATA.
In addition, the segments of the thorax are free. They are
probably the most primitive of insects. The common silver-
fish (Zepisma) is a good example.
Cuiass V.—ARACHNIDA.
The spider is not so typical of the Arachnida as is the
cockroach of the Znsecta. The Arachnida are a more
primitive class and the various orders are more divergent
in structure than those of the Jwsecta.
As a class they are distinguished by the absence of pre-
oral appendages or antennz, by the division of the body
into cephalothorax and abdomen, or no division. They
resemble the insects in the common presence of trachez,
in the malpighian excretory organs, and in absence of
appendages on the abdomen. The four pairs of walking
legs are usual and the presence of coxal glands in several of
the orders is important.
Of the many and divergent orders we can here only refer
to three.
ORDER I.—Scorpionida.
The scorpions are large arachnids. They have six
pairs of appendages on the cephalothorax, as in spiders,
but the first two pairs form small and large chelez (called
chelicere and pedipalpi) respectively, the other four being
the walking legs. The abdomen is segmented, the first
seven segments being much larger than the last five. The
sternon of the first segment has a pair of genital apertures.
The second bears a pair of fectines or combs, probably
tactile in function, and the next four have diagonal slits on
their ventral surface, the s#/gmata, leading into the lung-
books. The seventh segment has no appendages nor
apertures. The five last are elongated and form the tail,
terminating in a post-anal spine. At the base of the spine
is a poison-gland, a duct from which passes up a groove
along the sting The scorpion agrees with the spider in
the possession of simple eyes, coxal glands and the general
structure of its body, but its nervous system is less con-
centrated.
ORDER II.—Araneina.
The spiders are a widely distributed and successful order
of Arachnida. They prey naturally upon insects which
ARACHNIDA. 259
they either hunt or catch by webs. One group, with
four pulmonary sacs, contains large hairy hunting spiders.
Some build small tunnels with trap-doors. The other
group, with only two pulmonary sacs, contains all the
common web-spiders. One species (Argyroneta) lives
under water in a web diving-bell.
Female spiders, as a rule, are larger and more powerful
than. males.
OrDER ILI.—Acarvina.
The mites are small animals with soft globose body in
which there is no distinction of cephalothorax or abdomen,
and no trace of segmentation. They have four pairs of.
legs and the chelicerze and pedipalpi are used for piercing
Fig. 182.—MITE CAUSING MANGE IN THE
Pic (Sarcoptes scabit) x 120.
Ventral view. Note the chelicerz, pedipalpi and four pairs of legs.
260 ANNULATA.
and sucking. The best known are skin-parasites (or ecto-
parasites) upon various animals. The type shown is a
mange-mite which tunnels in the skin of the domestic
animals, and gives rise to the painful “itch” or skin mange.
Other Arachnida are the little long-legged “ harvest-
men,” the book-scorpions and certain parasites. Lastly,
there is an interesting animal, the king-crab (Limulus),
which lives in mud of shallow seas in the Oriental region.
It breathes by gill-books and has a large cephalothoracic
shield, six pairs of chelate appendages and a long post-
anal spine. It appears to be an aquatic Arachnid of very
primitive character.
Fig. 183.—THE HARVESTMAN
(Phalangium cornutum). Magnified.
(TABLE.
261
ARTHROPODA.
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262 MOLLUSCA.
CHAPTER XVIII.
MOLLUSCA.,
HELIX. ANODON. SEPIA.
I.—_ HELIX.
PHYLUM MOoLtusca (p. 282).
CLass GASTROPODA (p. 283).
Fig. 184.LaTERAL VIEW OF THE ROMAN SNAIL (Heléx pomatia).
Natural size.
Ocular
Tentacle.
Note_the creeping”foot, spiral shell and head with tentacles.
Helix pomatia (the edible or Roman snail) is slightly
larger than Helix aspersa (the garden snail) and more con-
venient for dissection. It does not differ in
essential features. The body is of a dark
greenish-slate colour, and the shell of a uniform pale drab.
The snail is a vegetable-eater and mostly nocturnal in its
habits. It hibernates in the winter, when it closes up the
Habits." 2perture of the shell by an epiphragm of chalky
j "™ matter and hardened mucus. It is in most
features a plano-symmetric’ animal but its symmetry is in
Colour.
HELIX. 263
part destroyed by the twisting of the portion contained
in the shell and consequent loss of some organs and
distortion of others.
The whole ventral surface is expanded into a flat muscular
creeping organ or foo¢, and in the mid-dorsal region is the
shell, containing a part of the body called the
visceral hump. The whole body is soft, and has
no cuticular exoskeleton as in the Arthropoda,
nor is there any trace of metameric segmentation.
The shell is a right-handed spiral. Its central axis is
called the columella, with a hollow cavity, the umbilicus, in
its centre. The apex of the shell represents its first formed
portion or mucleuvs. The shell consists of three layers, the
outer ¢hitinous and coloured part, the middle white calcareous
layer, and the inner thin smooth zacreous layer. Round its
edge may be seen the cod/ar or thickened edge of the mantle
which secretes the shell. The anterior end of the body
forms the Head, which bears two pairs of retractile tentacles,
the upper of which carry a terminal eye. Just below the
head is the mouth, with a chitinous upper jaw and a pair of
soft lateral lips. Below the head and above the foot is the
wide opening of the fedal gland which secretes the slime on
which the foot creeps. On the right side of the head is a
small opening, the genital aperture. Towards the right end
of the collar is a large opening, the pulmonary aperture,
leading into the pulmonary chamber, a space below the
mantle. Close to this aperture are the avws and the
excretory pore. All the four external apertures last men-
tioned are therefore asymmetric and on the right side only.
If the shell be broken off carefully the visceral hump is
exposed. ‘The lowest inch or so of the coil will be seen to
be formed of a soft membranous man¢/e in which there are
numerous pulmonary veins. Air is taken through the pul-
monary aperture into the pulmonary chamber, hence the
mantle forms the respiratory organ of the snail. In this
respect it differs from the great majority of Gastropoda, which
are aquatic and breathe by gills under the mantle.
If the thickened edge of the mantle (co//ar) be cut away
from its line of fusion with the dorsal wall of the body, and
the cut be carried up the inner spiral just below the rectum
(seen as a white tube running down to the azus), the mantle
External
Features.
264 MOLLUSCA,
flap can be reflected over to the left and the true dorsal
surface of the body or diaphragm exposed. The pulmonary
chamber is then seen to be triangular in shape, bounded by
the collar in front, the vascular mantle above and the
diaphragm below. Along its outer edge, emerging at the
Fig. 185.—First DissEcTION OF SNAIL (Helix pomatia).
. (Ad nat.)
Ocular Tentacle.
Pulmonary
Aperture.
* Excretory
Diaphragm.
Rectum
Visceral Hump.
Auricle of
Heart.
Kidney.
The snail is pinned out on its ventral surface, and the mantle is cut free from the
body by a cut along the collar and round the spiral. Note the pulmonary veins in
the mantle leading to the auricle of the heart, which passes to the ventricle and thence
by an aorta to the body.
* The excretory pore is really within the pulmonary aperture.
HELIX. 265
posterior angle from the upper coils, is the long tubular
rectum terminating in the anus.
Just inside this is a fine tube, the wrefer, leading from
the excretory pore backwards to the &dzey. This is a large
lobular dark-brown organ lying at the posterior
angle of the cavity where the mantle joins the
body. On its inner anterior side is an oval space with thin
walls, the pericardium. ‘The kidney opens by a small aper-
Blood. ure into the pericardium, which is a part of the
Vascular, Colom, the kidney being regarded as a large
* specialised nephridium. Inside the pericardium
lies the eart, a two-chambered organ. The thin-walled
Excretory.
Fig. 186.—DIAGRAMMATIC MEDIAN SAGITTAL SECTION
THROUGH THE HEAD OF A SNAIL.
(In part after Howes.)
Ocular Tentacle.
Ocular Nerve.
Brain.
Salivary Duct.
(Esophagus.
Odontophore,
Chitinous Upper
Jaw.
Opening of Pedal Gland. Retractor Muscle.
Cartilage, Root of Odontophore.
auricle receives blood from the pulmonary veins and pumps
it into the ventricle. This, on contraction, propels the blood
along a main aorfa, which passes into the body and divides
into anterior and posterior arteries. The venous system
consists of large lacunze or spaces around the organs. As in
the Arthropoda, the body-cavity is a hemoccele or venous
blood-space, the ccelom being only represented by the
pericardium and possibly other parts. As in the lobster,
the heart is systemic and receives blood from the breathing
organs.
266 MOLLUSCA.
Fig. 187.—SECOND DISSECTION OF SNAIL (Helix pomatia). (Ad nat. )
= g Spermatheca.
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Flagellum.
Vas
Columellar
Muscles.
Salivary
Gland.
Intestine.
Stomach,
Rectum. Tebes of
Digestive Gland.
The body wall is cut open along the mid-dorsal line and up the spiral. The spiral
lobe of the digestive gland is cut through and thrown over to the right along with all
the reproductive organs. The alimentary organs are released and thrown over to the
left, the nervous system and columellar muscles remaining in their normal position.
HELIX, 267
‘If the dorsal surface of the body be now cut open by a
median incision, the alimentary, reproductive and nervous
systems are all exposed, and may be easily dis-
sected out. If the alimentary organs be moved
over to the left and the reproductive to the right, the
appearance of Fig. 187 is produced.
The mouth leads into a large muscular buccal mass. It
contains the odontophore (or radula), an important molluscan
Alimentary.
Fig. 188.—THE NERVOUS SYSTEM OF THE SNAIL.
Removed entire, and viewed from the dorsal side. (After Howes.)
Buccal Nerve
Cerebral
Ganglion. ——~
Nerve to
Otocyst. __
Pleural.
Visceral. ~ a i Otocyst.
organ. ‘The odontophore is a long ribbon bearing in-
numerable rows of little chitinous teeth. It grows from
a root posteriorly as it is worn away anteriorly, and lies over
a buccal cartilage moved by muscles. The snail employs it
like a rasping tongue. On the dorsal side of the buccal
mass, just over the odontophore, open a pair of salivary
ducts leading from salivary glands covering the stomach.
The esophagus leads from the posterior end of the buccal
268 MOLLUSCA.
mass to the séomach—which is a dilatation of the alimentary
tube—and is continued onwards as the cztestine. A little
way up the coil the intestine bends on itself and receives the
ducts of a four-lobed digestive gland. The fourth lobe
occupies the top spiral of the shell. After another bend the
intestine ends in the rectum.
The nervous system consists of ganglia and connectives,
but the ganglia are to a large extent concentrated. The
brain (or cerebral ganglia) lies dorsal to the
cesophagus* and is joined by two connectives
on each side to the ventral nerve-mass. ‘This is formed
of three pairs of partially-fused ganglia, the pedal, pleural
and visceral. The brain supplies the eyes and otocysts and
the buccal mass, whilst the ventral nerve-mass
sends long nerves to all parts of the body.
In the substance of the ventral nerve-mass is a pair of
otocysts, supplied, as stated, by nerves from the brain.
The snail is hermaphrodite and the reproductive organs
are complex. ‘The genital organ or ovotestis is a small white
branching organ situated in the spiral lobe of
the digestive gland. From it the genztal duct
passes as a coiled white tube down beside the columella.
Here it swells into a wide common duct, and receives the
opening of the large aldumen gland. The common duct has
its internal lumen gradually divided into male (é ) and female
(?) parts bya septum. Eventually these two diverge as the
thin vas deferens and the thicker oviduct. The vas deferens,
after bending on its course, terminates in a large protrusible
penis. At the base of the penis there is a retractor muscle
running across to the left side of the body and a long hollow
tube or flagellum. The oviduct receives the spermathecal
duct, running backwards beside the common duct to ter-
minate in the round sfermatheca near the upper end of the
latter.t The oviduct leads into the vagina. Two branched
mucus glands then open into this, and it ends at the genital
pore beside the male opening. Just at the opening lies a
large muscular organ, the dart-sac, in the lumen of which
there often is found a calcareous dart.
Nervous.
Sensory.
Reproductive.
_* Occasionally the buccal mass is withdrawn through the nerve-ring and the
brain is then found lying in front of the former.
+ In Helix aspersa the spermathecal duct has a long flagellum.
ANODONTA., 269
Hence we have— Ovotestis
genital duct
albumen gland
common duct
) | |, Q
oviduct
vas deferens
spermatheca and duct
flagellum
| mucus glands
penis
vagina
dart-sac
The functions of these organs, so far as known, are :—
The snail is a frotandric hermaphrodite, z.e., the male organs
become mature first and the female after. The ovotestis gives rise to
spermatozoa, which pass down the genital duct, the common duct and
the vas deferens into the flagellum. Here they are aggregated into a
rod-like mass, the spermophore. During this process darts are.secreted
in the dart-sac and forcibly ejected from the genital pore into the skin of
other snails. This is followed by copulation, when the sperms are intro-
duced by the penis of one snail into the base of the spermathecal duct
of another. They pass up into the spermatheca and are there retained.
The ovotestis next produces eggs which pass down the genital duct
to the head of the common duct. The sperms then leave the sperma-
theca, make their way down the spermathecal duct and back again up
the oviduct and common duct, at the upper end of which they fertilise
the eggs. Albumen is then added to the eggs from the albumen gland,
and they pass down to the vagzza. Here they are covered with mucus
from the mucus glands and are discharged to the exterior. In some
species they are contained in calcareous shells. The eggs are laid in
damp earth and the development is embryonic, the young newly-hatched
snail differing little from its parent.
II.—ANODONTA.
PHYLUM MOoLLusca (p. 282).,
Cass LAMELLIBRANCHIATA (p. 284).
Anodonta cygnea* (the freshwater mussel) is a con-
venient example of the large and important class
of Lamellibranchiata, or bivalve molluscs. A
full-grown individual may be as long as five inches. The
Habits.
* This description also applies to Anodonta anatina.
270 MOLLUSCA.
whole body is completely enclosed n a pair of large oval
shells which, unlike the shells of Brachiopoda, are lateral.
The animal is found half buried in the mud of ponds and
streams. The shells are of a dark brownish-black colour
and composed of the same three layers as in
the snail. On the dorsal side they move against
each other by a Aimge, and they can be opened
by the contraction of an elastic /igament, just outside the
hinge Just above the hinge is a small first-formed part
External
Features.
5 > yee
Fig. 189.—LATERAL ViEW (LEFT) OF ANODONTA IN NATURAL
POSITION AND FEEDING.
(Mainly after Howes.)
Exhalent Current.
Exhalent Tube.
‘Water current
passing into
Inhalent
-— Aperture.
Umbo.
Lines of Growth.
Level of Mud.
called the wmébo. On the inside, the dried shells have
several scars caused by the attachment of parts of the
body. A little way inside and parallel to the ventral
edge of the shell is a line called the pallial line, caused
by the edge of the mantle, or pallium.. At the anterior
end of the pallial line is a large oval scar produced by
the anterior adductor muscle; and at the posterior end is
ANODONTA. 271
a similar scar of the posterior adductor. The adductors
run across from shell to shell, and their contraction draws
the shells together. Inside each adductor scar is a smaller
round scar, caused by the anterior and posterior re¢ractors,
which serve to draw the foot into the shell. Lastly, near
the anterior adductor scar is a small protractor scar, the
muscle serving to draw the foot forward. The attachments.
of the muscles shift outwards and downwards as the shells
grow.
Fig. 190.—INTERNAL VIEW OF RIGHT SHELL OF ANODONTA.
(Ad nat.) .
Umbo.
Hinge.
Anterior Retractor. f
Posterior
Retractor.
Anterior
Adductor.
Posterior
Adductor.
Pallial Line.
Protractor.
When the shells are forced open they expose a large
mantle-cavity. This is bounded dorsally by the body of the
animal and laterally by the lateral mantle folds ; ventrally
it is widely open to the exterior, except when the shells are
shut. The mantle-flaps line the inner surface of the shells,
which they secrete. The free edges are pressed together,
except at the posterior end, where they diverge to form a
large znhalent opening, then meet, and again diverge to form
the smaller exhalent opening.
In the centre of the mantle-cavity a large muscular foot
depends downwards and on occasion it can be protruded
aenweiy outwards between the shells. Embedded in the
* foot, near the pedal ganglia, are the otocysts, but
Anodonta has no eyes. There.is a pair of osphradia or
sense-organs of an olfactory nature at the base of the gills,
innervated from the visceral ganglia.
272 MOLLUSCA.
On either side of the foot there hang the gzd/-damella, or
ctentdia. These are lamellze on each side, formed in each
case by a gi//-plate folded on itself, the outer
gill-plate outwards and the inner inwards. The
gill-plates are themselves composed of a number of gi//-
Jfilaments, which hang perpendicularly in a single row from
a horizontal axis which is fused with the body-wall.
Respiratory.
Fig. 191 VIEW oF ANODONTA WITH LEFT MANTLE-FLAP THROWN
Back. (Ad nat.)
_ Left Mantle Flap.
Outer Gill.
Inner Gill.
Anterior Adductor.
Exhalent
Chamber.
Inhalent
Aperture.
Outer Gill.
Renal Inner Gill.
Foot. Labial Genital Aperture.
f Palp. Aperture.
A ctenidium therefore consists of a medium axis with
two rows of gill-filaments, each row forming a gill-plate. In
Anodonta. but not in all Lamellibranchiata, these gill-plates
are bent double to form in each case two gill-lamellz. In
addition, the filaments and the gill-lamelle have fused with
their fellows and thus form a network of filaments. The
whole are ciliated and cause currents of water and food-
particles to pass into the mantle-cavity by the inhalent
aperture. . The free edges of the upturned gill-plates are
fused to the body-wall, and thus shut off outer and inner
f
ANODONTA, 273
supra-branchial chambers from the mantle-cavity below.
Posteriorly these lead into the exhalent chamber. ‘The water
appears to pass between the gill-filaments directly into the
supra-branchial and exhalent chambers, erating the blood
in the gill-filaments in its course. The food-particles appear
to pass forward to the mouth, which is situated just under
the anterior adductor muscle. They are assisted by a pair
of flat triangular /adca/ palps in each side. From this it is
seen that the ctenidia serve the two purposes of alimentation
Fig. 192.—DIssECTION OF ANODONTA FROM LEFT SIDE
(Slightly Diagrammatic).
Aperture of Kidney into Pericardium.
Digestive Gland. Dorsal
Aperture of Digestive | Artery.
Gland into Stomach.
Ventricle of |
Heart. Pericardium,
*ra}21Q)
Intestine,
Anterior
Adductor. Dorsal
Canal
38
mol
i=}
<
Mouth. 3
ao
Cerebral ied 5
anglion, Ay
ao
a
Poste.ior
Adductor.
Visceral Ganglion.
Pedal Ganglion. Intestine. Kidney.
Gonad.
(food ingestion) and of respiration. They appear to be
derived from organs of the same nature as the gills of other
molluscs. ee
The mouth leads into a short esophagus passing into a
globular stomach, into which open the ducts of a digestive
gland. From the stomach the long zu/estine de-
scends into the base of the foot, and after com-
plex coils it again ascends to the dorsal region and passes
backwards over the posterior adductor muscle to open by an
anus into the exhalent aperture. We may note the entire
absence of “head,” buccal mass and odontophore.
M 19
Alimentary.
274 MOLLUSCA.
We have already referred to the adductor muscles for
closing the shells and the protractors and retractors of the
foot. The main substance of the foot is mus-
cular and it is thrust out ventrally at the will of
the animal, acting as a burrowing organ.
The heart is situated dorsally and is three-chambered.
The median ventricle envelops the intestine and passes for-
= wards and backwards into main arteries. It is
ood- é : : A :
wacesie fed by paired lateral auricles which open into it
by valves. They receive blood from the ctenidia.
The heart and this part of the intestine lie in a spacious
cavity, the pericardium, which is coelomic in origin. The
Motor.
Fig. 193.—DorsaL VIEW oF HEART AND PERICARDIUM
OF ANODONTA. (dd zat.)
Aperture of Kidney.
Anterior Artery.
Intestine.
Ventricle.
Auricle.
Posterior Artery under Intestine.
venous system, as in the snail, is lacunar, and formed of
sinuses and cavities in the body. A large median sinus
below the pericardium feeds the ctenidia. Hence the blood-
vascular system closely resembles that of the snail; the chief
difference is the paired condition of the auricles (like that of
the shells).
The brain, situated laterally to the mouth, consists of a pair
of cerebral gangla joined forwards by a connec-
tive. From the brain there run paired connec-
tives to the peda/ ganglia in the anterior part of the foot, and
Nervous.
ANODONTA. 275
to the visceral ganglia situated immediately below the pos-
terior adductor muscle. There is here less concentration
than in the snail, the pedal and visceral loops being very
long and wide.
Immediately under the pericardium lhe the paired Azdneys.
They consist of tubes bent upon themselves. Each has
an internal opening into the anterior end of
the pericardium, which passes into the lower
excretory part or kidney. From the posterior end of each
kidney a ureter passes forward between it and the peri-
cardium to open into the inner supra-branchial chamber,
and thence to the exterior. These tubes may be regarded
as two specialised nephridia. The walls of the pericardium
also have excretory cells, which are known as the pericardial
glands (organ of Keber).
Anodonta is dicecious. The ¢estis or ovary is a diffuse
paired organ lying below the kidneys. The
paired genital duct (oviduct or vas deferens)
passes up and opens just below the excretory pore on each
side.
The eggs are shed into the supra-branchial chamber,
where they are fertilised and develop into glochidia, or
small two-shelled larval forms, which differ in
many respects from their parents. They leave
the parent by the exhalent aperture. A little dorsal to the
exhalent aperture, the two mantle-edges again diverge to
form a small slit-like aperture. This is connected by a
median canal above the intestine with the exhalent cham-
ber, and embryos have been observed escaping by it. The
glochidium is said to be parasitic upon certain fish, and
undergoes a metamorphosis into the adult.
The general likeness of Anodonta to the snail will be
apparent. The plano-symmetry is, however, more perfect,
shown in the paired shells, kidneys, auricles, gills, &c. The
absence of buccal mass, odontophore and eyes, and the
immense development of the ctenidia (which, present in
most Gastropoda, are absent in the snail) are the chief points
of distinction.
Excretory.
Reproductive.
Development.
276 MOLLUSCA.
III.—SEPIA.
PHYLUM MOLLUSCA (p. 282).
Crass CEPHALOPODA (p. 284).
Sepia officinalis is a large mollusc, often nearly a foot
in length. It is found commonly round our coasts, though
more abundant in the south. It lives a free, roaming, pelagic
life, and is a voracious flesh-eater. Its dried shell is often
found cast up on the shore. The animal consists of a head
and Jody. The body is flattened and shield-shaped, with a
lateral expansion or fin along each
Fig. 194.—DorsaL View edge. The head has ten tentacles,
OF THE COMMON CUTILE of which the fourth pair are as long
(Sepia tia as the body and bear a pad of
A suckers at the end. The other
eight have four rows of small
suckers on their inner surface. A
dead “‘cuttle” appears of a dull
white colour with patches of drab,
but in life there is a beautiful play
of colour and light over the whole
surface of the body. This is
caused by a number of chromato-
phores or pigment cells which are
actively contractile, and hence can
alter their extent. The result is
an ever-changing colour and irid-
escence. In a general way, the
upper surface of the body is of a
dark brown hue. It is horizontally
striped inst irregular bands of white and the fins are
similarly dotted with white. The dorsal surface of the
head is also brown. The tentacles and the whole under-
surface are pearly white. Sepia is plano-symmetric to a
marked degree, and there is no trace of torsion as in the
snail. The mouth is situated between the tentacles and
is armed by a pair of powerful horny jaws or beaks, not
unlike those of some parrots in size and appearance. The
head is connected to the ‘body by a constricted mech,
around which hangs the front edge of the mantle.
SEPIA, 277
On each side of the head is a large simple eye; although
of the simple type the eye is complex in structure. It has
all the more important parts of the vertebrate
eye, such as cornea, lens, iris, vitreous humour
and retina, and is supplied by large optic nerves from the
brain. Just behind each eye is a
ciliated olfactory pit, and near the Fig. 195.—Venrrat Virw
brain is a pair of large ofocysts, OF A CUTTLE (Sepia
Asis to be expected from its free &cenatis) x }.
active life, large size and com- /
plexity of structure, the ‘“cuttle”
has sense-organs far in advance of
those found in any other Mollusca.
The mantle fuses on the lower
surface to enclose a large mantle-
cavity which is blind behind but
opens widely at the neck. Just in
front of this open-
ing lies the szphon,
a tube which opens by a large
funnel behind into the mantle-
cavity, and by a small aperture
forwards under the head. The
hind edges of the siphon are so
arranged that water expelled from Note the ten arms with suckers,
the mantle-cavity passes through t! the mouth between them Tn
the siphon, but water inhaled “Po” immediately behind it
passes in between the edges of the siphon and the mantle.
By muscular contraction the animal forcibly ejects
water from the mantle-cavity through the siphon, and in
this manner drives itself backwards through the water.
If the mantle be cut open along the mid-ventral line and
thrown back, the interior of the mantle-cavity is exposed.
The two most conspicuous organs are a pair of large feathery
ctenidia, consisting of a median axis and lateral branches.
They are purely respiratory. In the middle line of the
body the rectum may be seen running forwards and ter-
minating in the amws. A little further backwards open the
paired excretory pores and the unpaired genital pore on the
left side. As in the mussel and the snail, the mantle-cavity -
is evidently a part of the external surface of the body.
Sensory.
Respiratory.
278 MOLLUSCA.
The mouth leads through the jaws into a buccal chamber
which contains a rasping odontophore of essentially the same
nature as that of the snail. A duct from a
pair of salivary glands opens into the buccal
chamber. The esophagus leads back some way to the
stomach, a large rounded sac.* From close to the junction
of stomach and cesophagus the intestine passes forwards
and downwards to the anus, and a small saccular cecum
opens at the same point. Here also open the paired ducts
Alimentary.
Fig. 196.—VENTRAL VIEW OF SEPIA OFFICINALIS WITH
MANTLE-Cavity Cut OPEN. (4d nat.)
-Ctenidium.
Nephridiopore.
, Genital Aperture.
Mantle-flap.
from the two digestive glands, large masses lying right and
left. The ducts are covered with masses of pancreatic ceca.
Close to the anus the intestine receives the duct from a
large zzk-gland. ‘The ink or seféa is ejected with the water
from the mantle and forms a dark cloud, behind which the
animal can beat a retreat.
The prey is seized by the tentacles with their adhesive
suckers and is torn to pieces by the horny jaws and the
odontophore. The flesh is passed down the cesophagus
SEPIA, 279
into the stomach, in which it is mixed with the digestive
juices from the digestive gland and pancreatic czeca.
It may be noticed that the anus is not at the hind end
of the body, but the intestine is bent forwards along the
under surface till the whole alimentary canal is U-shaped,
with a ventral flexure.
Fig. 197.—DISSECTION OF ORGANS OF SEPIA OFFICINALIS FROM
THE LertT Sipr. (Semi-diagrammatic.) (dd at.)
Esophagus, Buccal Glands. ,
Anterior Aorta, | Intestine.
Buccal Mass. i Digestive Gland.
a . .
a
oO
3
&
8
]
n
Horny Jaws.
Anterior Vein.
Anus.
Ink Gland.
Ctenidium. Posterior Aorta.
Auricle. Mantle Cavity.
In its natural position, the cuttle rests suspended in the
water near the surface with the body horizontal, the tentacles
Motor. hanging loosely downwards, the two long ones
* being coiled up inside the others. A forward.
swimming motion is caused by undulations of the two lateral
fins. A powerful backward jerk is produced by forcible
ejection of water through the siphon. There are special
muscles for moving the tentacles and the eyes.
Fig. 198.—VENTRAL VIEW OF SHELL OF CUTTLE.
The inner part is calcareous, outer horny.
280 MOLLUSCA.
Along the upper surface the mantle-edges meet and
completely enclose the shell, which is therefore invisible
externally. If the mantle be slit the shell may
be removed. It consists of a long ovate mass of
chitin with a calcareous portion on its under surface, thickened
posteriorly. Hence only the two outer layers of the typical
molluscan shell are represented.
But, in addition, Sefza has an important internal skeleton
of cartilage. ‘This forms a cranium enclosing the brain and
the otocysts and bearing a, remarkable resemblance to the
cranium of a vertebrate. Other cartilages support the fins
and the tentacles.
Skeletal.
Fig. 199.—SEMI-DIAGRAMMATIC VIEW OF HEART, GILLS AND
EXcRETORY ORGANS OF SEPIA OFFICINALIS.
Anterior Aorta.
Anterior Vein.
Ventricle.
Auricle.
Nephridiopore.
Efferent
Branchial.
Afferent Branchial.
Branchial Heart.
Excretory Cells,
Nephridial Sac.
Pericardium.
Posterior Aorta, Posterior Branchial Vein.
The ccelom is fairly well developed and to a large extent
retains its perivisceral or motor function. The anterior
portion surrounds the heart and the “ branchial”
hearts and is usually known as the fericardium,
and the posterior part contains the ovary. Two small aper-
tures lead from the front end of the ccelom into the paired
kidneys, and at the hind end a similar opening leads into
the oviduct. (¢).
Celom,
SEPIA. 281
The blood vascular system is highly developed. The
heart lies below the intestine (if the intestine were bent
Blog. Pack into a straight line it would be in the
Vascular, Usual dorsal position) and consists of a ventricle
“and two auricles. The auricles receive blood
from the ctenidia by the efferent branchials and drive it into
the ventricle. From the ventricle it passes forwards and
backwards by anterior and posterior aorta.
The veins are largely sinuses but are rather more
definite than in other molluscs. A main vein, the vena
cava, runs along the mid-ventral line from the head to the
level of the anus, where it divides into two afferent branchials
going out to the ctenidia. At the base of the ctenidia each
afferent branchial swells into a dvanchial heart or contractile
bulb, which also receives an aédominal vein from the hind
region and on contraction drives the blood up the ctenidium.
The heart of the cuttle, like that of our preceding types, is
therefore systemic, but in addition there is a pair of special
respiratory or branchial hearts.
The brain is a large mass lying over the cesophagus and
protected by the cranial cartilage. It supplies nerves to the
eyes and the otocysts. Connections run round
the cesophagus to a ventral nerve-mass which,
as in the snail, consists of several ganglia. The feda/ and
pleurovisceral may be distinguished. Nerves from the pedal
supply the ten tentacles and the siphon. For this and
other reasons derived from embryology we are led to regard
the tentacles and the siphon as together representing the foot
of the other Jo//usca. We have seen that the intestine and
excretory pores have moved forwards along the mid-ventral
line and the foot, divided into tentacles, has moved forwards,
like the appendages of the lobster, to surround the mouth.
As in the lobster, the ventral surface of the body is bent
upwards anteriorly. There are two large stellate ganglia
on the lateral walls of the mantle-cavity, connected by pallial
nerves to the pleurovisceral ganglia.
There is a pair of large tubular Azdmeys which open
internally into the pericardium and externally to the
exterior as described. They envelop the
afferent branchial and abdominal veins, and
their walls consist of thickened excretory cells.
Nervous,
Excretory.
282 MOLLUSCA.
The cuttle is dicecious. The ovary is enveloped in an
ovisac and lies at the extreme hind dorsal end of the body.
The single oviduct leads to the exterior on
the left side of the mantle-cavity. There are
paired xidamental glands which secrete a sticky mass for
fixing the eggs. The Zes//s lies in a similar position to the
ovary and is enclosed in a testicular sac continuous with
a vas deferens which swells into a seminal vesicle, receives the
ducts of two prostate glands and opens along a penis into
the mantle-cavity.
The eggs are laid on weeds in masses. They are black
and like small grapes in appearance. There
is much yolk and the development is em-
bryonic, with no larva.
Reproductive,
Development.
PHYLUM MOLLUSCA.
The Mollusca are the second great division of the Metazoa.
Their external body-form may be very diverse but they
always have a fundamental plano-symmetry. Typically
tridermic or triploblastic, the majority have a persistent
ceelom, though there may be traced the same general
tendency to a reduction of the perivisceral motor part, and a
reciprocal expansion of the heemoccele or venous-spaces. A
portion, however, remains as the pericardium. and it typically
communicates with the exterior by two specialised nephridia.
The gonadial part of the ccelom in some cases still com-
municates with the pericardial. There is no trace of the
metameric segmentation which is so marked a feature of the
Annulata, though traces of archimeric segmentation persist.
The nervous system consists of dorsal brain, a nerve-ring
and at least two other pairs of ganglia below the alimentary
canal. Compound eyes are never found, but the simple eye
sometimes reaches a high state of perfection. The blood-
vascular system is usually well developed, the arteries being
nearly always definite vessels. The heart is typically three-
chambered, a median ventricle and paired lateral auricles,
and is always dorsal and systemic.
The body itself is always soft and has no exoskeleton,
but there is usually a dorsal expansion called the mantle
which secretes a three-layered shell, either single or double.
GASTROPODA. 283
Similarly, part of the ventral surface is expanded into a
separate muscular organ called the /oot. ‘his is usually
concerned with locomotion, but in the Cephalopoda the hind
part only assists locomotion, the front part becoming
modified into ingestive organs (cf legs of Arthropoda).
In all but the Lame/hbranchiata the buccal cavity contains
a peculiar toothed tongue or odontophore. The gills are
typically one pair of ctenidia, usually enveloped by the
mantle.
The Mollusca are sometimes divided into two sub-phyla,
the Lamellibranchiata being contrasted with the other two
classes, but these also are so divergent that it is convenient
to keep them apart.
The Mollusca do not invade the land with such success
as the Annulata. Only one class, Gastropoda, has terrestrial
representatives in the slugs and snails, and these are not
completely adapted for terrestrial life, for they revel in wet
and can only progress on a wet surface.
The development of the phylum is very divergent. As
in the Aznulata, the lower marine types have larve, the
pelagic ‘rochophore being a specially important type.
Again, as in Amnulata, the terrestrial forms. and the
highest marine forms (Cephalopoda) have eggs with quantities
of yolk and an embryonic development.
-Cxiass I.—GASTROPODA.
Gastropoda are divided into two important sub-classes.
The /sop/eura are few in number and small, but they are
interesting from their worm-like character and the absence
of the torsion of other Gastropoda. Chiton is one of the
commonest types. A species about one inch long occurs
round our coast. It has several dorsal shige: and the gills
are also repeated.
The Anisopleura comprise all the rest of the Gastropoda.
They have a trace of more or less dorsal torsion, supposed
to be the effect of a spiral shell. In most this also involves
the loss or reduction of one gill and one nephridium.
The order of /u/monata stands rather apart owing to the
adaptation to rial respiration and the loss of gills. It
comprises the snails and the slugs. The rest are marine or
284 MOLLUSCA.
freshwater. Some, the sea-slugs or Wudibranchs, lose their
shells and have an external approximation to plano-
symmetry. Others are adapted for a pelagic life, they are
usually transparent, and the shells if present are thin
and pellucid. The foot is usually reduced. but may form
a swimming organ. The great majority of the sub-class,
however, creep on the sea-floor and may be carnivorous
scavengers, ¢g., whelks, or herbivorous, ¢g., periwinkles.
The shells of such types as the
Fig. 200.—A BELEMNITE 7; a
Rusrorsp. (AfterOwen.) impets and earshells (fa/iotis) are
; fi not spirally twisted.
Crass II.—LAMELLIBRANCHIATA.
The bivalve AZod/usca are usually
completely enveloped in the paired
shells. The ctenidia have been
enormously developed and serve to
feed the animal. They are mostly
burrowing types, all aquatic, and
most are marine. ‘They illustrate
degrees in degeneration, the oyster
entirely losing its foot. The scallop
(Pecten) moves actively through the
water by snapping its shells together.
Teredo is a worm-like form with
very small shells which bores its
way through wood. Cockles and
mussels are other common species.
Cuass III.—CEPHALOPODA.
min
In these the molluscan plan
reaches its highest level.
Sepia is a very fair type of the
class. They are all active free-
g, Eight hooked tentacles (the Swimming forms, with the fore part
oe oe) fees aes. are of the foot produced into tentacles,
part containing the shell; 4 the hind part into a siphon, and
hh: 3; @, ink-sac; é
Eohon (or funnel. «the organs are plano-symmetric.
=
K<<
_
eke
CEPHALOPODA. - 285
The order Zetrabranchiata contains the pearly nautilus
(Nautilus) and a number of extinct allies. The nephridia
and ctenidia are reduplicated, hence there are two pairs.
The tentacles have no suckers and there is a large external
shell. The shell of the pearly nautilus is chambered. The
animal inhabits the last chamber. A median hole through
each septum transmits a long process of the body called
the s¢phuncle.
Fig. 201.—LATERAL VIEW OF A NAUTILUS IN ITS SHELL.
(After OWEN.)
Note the hollow chambered shell and the numerous short tentacles.
0, eye; g, siphuncle ; ¢, tentacles; 7, mantle; Z, hood ; e, siphon.
The Dibranchiata contains the cuttles, squids and the
octopus. In all there are two ctenidia and nephridia and
the shell is either internal or absent. Octopus has only
eight tentacles and no shell. The paper Nautilus (A7gonauta)
also has only eight arms, and the female secretes a thin delicate
shell. It is used to carry the eggs and is unchambered.
The ammonites are fossil forms allied to Vautelus, whilst
the belemnites are fossil Dibranchiata. They occur in great
286 MOLLUSCA.
numbers in the mesozoic strata. The tentacles in the
belemnites had little hooks as well as suckers. The actual
term “ belemnite”’ is applied usually to the fossil shell only.
Some of the Cephalopoda reach an enormous size, and
their whole organisation represents the highest point attained
outside the phylum of the Chordata.
Fig. 202.—AMMONITES oR FossIL NAUTILOID CEPHALOPODA.
[TABLE.
287
MOLLUSCA,
“rT
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*SUISIMY
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‘aroydojuopo ou puv proy on *f ‘atoydoyuopo uy *€ ‘azoydojuopo uy
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(Csnjydyy ) vzuopoup—agh 7 “wrgas—agh 7 "(wenuiz9INg) xya~T—agh fp
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‘Jzeay o1maysds TesIop YM waysks efnoseA pedopaaap-]faa y “8
‘sansstwwoo Aq peutof ‘ersues [ersosta pue yeinatd ‘tepad
pened ‘snSeydoseo ay} r0A0 (er[3ueS yesqerso) ureiq jo sisisuoo wiayshs snoAlan *2
‘s0eds-poojq snousa ® st Ay1Av0-Apoq
ay} pu “Jole}x9 ayy 91 etprydau paired Aq Surpesy utojeoo oni} & st UWMNIpIvollag "9
“eIpIuajd JO s[fIs jo med a1our 10 auQ °S
«JOOJ,, Tepnosnur ay} sursoy xed [wayusn “+
“S[[@YS SIOUI IO BUO Sojadoas aTJUBU 9y} ‘Apoq jo yzred TesIoq “E
‘sosepuedde jnoyim Apoq yos pue pojuauZasuq ‘2
‘(Aqpensn) Anjewuuids [erayelig & YA vozmzapyy ayewoqeo7)
‘VOSNTION WO TAHA
288 CHORDATA.
CHAPTER XIX.
CHORDATA.
ASCIDIA. AMPHIOXUS.
I.—ASCIDIA.
PHYLUM CHORDATA (p. 403).
SuB-PHYLUM ATRIOZOA (p. 404).
Cass TUNICATA (or UROCHORDA) (p. 405).
Ascidia mentula is a small sac-like marine animal, of
which common examples may be one inch in length. It
occurs in great numbers at moderate depths,
adhering to shells and other foreign bodies, thus
belonging to the sedentary types. The shape
is roughly cylindrical and the colour is usually of some dull
neutral tint. The aboral end is fixed and the oral end
terminates in a round aperture usually termed the mouth.
A little way down one side there is another opening called
the atriopore.
The plane passing through the two apertures and divid-
ing the body into equal parts, is the median plane, about
semitnenty which several of the organs are plano-symmetric.
* Hence, like the Echinodermata, the ascidian has
an underlying bilateral or plano-symmetry disguised by a
more superficial approach to axial symmetry. The surface
of the body is smooth and devoid of special features.
If Ascidia be watched in the living condition it can
be seen that currents of water and food-particles pass into
the interior by the mouth, whilst a current of water emerges
by the atriopore.
As in the cases of Sycandra and Anodonta, the exhalent
current is devoid of food-particles, which are similarly retained
for the use of the animal..
On being disturbed the living animal can contract its
body to a considerable extent, and water is then forcibly
expelled through the mouth. This habit, occurring in indi-
viduals left dry by the tide, has given rise to the popular
name of ‘“sea-squirt,” applied to ascidians in general.
Colour and
Form.
ASCIDIA, 289
An incision will reveal at once that the body of the
animal is enveloped in a thick ¢es¢ (or /vnzc), out of which
taberael the animal may be removed entire, like a bean
out of its pod. ‘The test is thick and of a
semi-transparent, gelatinous appearance. It
is produced chiefly by the layer of underlying ectoderm,
Features,
Fig. 203. —DIAGRAMMATIC MEDIAN LONGITUDINAL SECTION
THROUGH AN ASCIDIAN.
Mouth.
|
&
"eo
S
a
oO
v 4 a
i 3 +-——~ Buccal Cavity.
Subneural 4 5
Gland. ra :
Atriopore. AN i Peripharyn-
r > = 2) geal Groove.
oN 7 a Pharyngeal
6 ae ai \ Clefts. 2
= a
Intestine. qo i -— Endostyle,
Genital Fy
Duct.
4 ~~~ Pharyngeal part
Atrium. of Ventral P
os Vessel.
Dorsal ;
Blood-vessel.™ re
Test, - \% 4
(Cuticle). * ase Heart.
Mantle X\
(Ectoderm). a
ps: Gonad
Stomach. Intestinal part of Ventral Vessel.
The right half has been removed.
andfconsists of a hyaline basis containing ce//ulose (a
material mainly confined to the plant-kingdom), through
which are scattered a number of cells. Below the
M. 20
290 CHORDATA.
test is a single-layered ectoderm,* covering fairly well-
developed J/ongitudinal and circular layers of muscles. The
test may therefore be regarded as a modified and thickened
form of cuticle produced from the ectoderm.
On cutting open the body-wall the course of the alimen-
tary canal can be made out. The mouth leads into a buccal
sigs cavity, which is short, and expands into the enor-
entary. 3
mous pharynx. Between the two is a row of
small zentacles. The pharynx extends nearly throughout the
length of the body and forms a large sac, the lateral walls of
which are perforated by rows of innumerable small slits, or
stigmata.
These are evidently clefts in the side-walls of the pharynx, but are
not exactly the same as the pharyngeal clefts of the Chordata. They
are produced from the less numerous true pharyngeal clefts of the larva
by secondary division of the latter.
The pharynx is surrounded on all sides except the mid-
ventral line by the atrium, a large spacious cavity into which
open the stigmata. It leads to the exterior by the atriopore.
Along the mid-ventral line of the pharynx is a grooved
ridge, the endostyle, formed of ciliated and glandular cells.
At the oral end of the pharynx it is continuous with the
peripharyngeal grooves, which pass up each side of the
pharynx just behind the tentacles. The two peripharyngeal
grooves meet in the mid-dorsal line, and are produced back-
wards along the mid-dorsal line of the pharynx as the ef7-
branchial groove, the edges of which hang down as the dorsal
lamina. This groove terminates at the dorsal posterior corner
of the pharynx, where a small wsophagus leads into a sac-like
stomach. ‘This is continued by a bent dzdestine to the anus,
opening into the atrium. The greater part of the alimentary
canal is ciliated.
The outstanding feature of this system is the pharynx, with
its numerous clefts and its system of grooves. The endostyle
secretes mucus, which is driven forward by the ciliated cells,
up the peripharyngeal grooves and back along the epi-
branchial groove. The mucus strands appear to form a
complex meshwork of glutinous threads hanging across the
cavity of the pharynx, the ultimate fate of which is to be
carried into the stomach through the cesophagus. The cilia
* Often termed the Mantle.
ASCIDIA. 291
covering the inner surface of the pharynx cause the currents
of water already referred to; but, whilst the water itself is
carried through the stigmata into the atrium and thence to
the exterior, the food-particles become entangled in the
mucus and are transferred through the cesophagus into the
stomach. The pharyngeal walls between the
stigmata carry blood-vessels, and the constant
stream of water over them serves to erate the blood.
Thus the pharynx of the Ascidian, like the ctenidia of
Anodonta, functions for alimentation as well as respiration,
though it should be carefully noted that in the former
the alimentation is the original primitive function, the
respiration being acquired later; whereas the reverse
holds in Axodonta, ctenidia being originally respiratory
organs.
Respiratory.
Fig. 204.—OBLIQUE SECTION THROUGH AN ASCIDIAN. (Ad nat.)
Brain.
Subneural Gland.
Longitudinal
Muscle.
Dorsal Blood-vessel.
Dorsal Lamina.
Pharyngeal
Wall.
Pharyngeal
Test Cavity.
est.
Endostyle.
Heart.
The circular muscles are scattered throughout the body-
wall, but mainly concentrated as large sphincter rings around
Muscular, ‘the, mouth and atriopore. Similarly the longi-
: tudinal muscles are best developed in relation
to the two external apertures. The circular muscles serve
to close the apertures and the longitudinal to contract the
whole body.
The ccelom is not present as a definite perivisceral space,
but the blood-vascular system is not difficult to follow. It
consists of a dorsal and a ventral vessel, connected by vessels
and sinuses. The dorsal vessel runs above the epibranchial
292 CHORDATA.
groove, and has paired dranchials leading down the pharyn-
geal walls into the ventral vessel which lies immediately
below the endostyle. ‘lhe dorsal vessel runs back to the
stomach and intestine, over which it breaks up into sinuses.
The ventral vessel also runs back to these sinuses; but in
its course, just after leaving the pharynx, it is modified into
a simple contractile heart. ‘Ihe heart has a single chamber
and is clearly ventral in position. It contracts rhythmically,
driving the blood forwards to the pharynx for a certain
number of beats, and then, reversing its action, drives it
backwards to the viscera; hence it is alternately systemic
and respiratory. For this reason it is impossible to speak
of arteries or veins.
In the accompanying diagram the heart is shown in its
respiratory phase, during which the dorsal vessel may be
directly compared with the dorsal aorta of Vertebrata, the
ventral vessel with the ventral aorta and the part of the
ventral vessel between the heart and the system with the
main subintestinal vein. A reversible heart such as this
also occurs in some allies of Ba/anoglossus.
ll vessel Se
System
Pharynx a
soe vessel<— Hearf
The main nerve-ganglion or brain lies dorsally between
the mouth and atriopore, just under the ectoderm, and gives
off fine branches to the muscles. A main nerve-
trunk runs back dorsally to the stomach. Under
the brain lies a swéneural gland, which communicates by a
duct with the front part of the pharynx just inside the ring
of tentacles. It may possibly be an excretory organ.
No special sense-organs are recognisable, though the
papilla around the mouth apparently function for testing
the quality of the incurrent water.
Excretory products are said to accumulate in solid
masses in parts of the body, and to be extruded
only on the death of the individual. No
definite excretory organs have been described.
Nervous.
Excretory.
ASCIDIA. 293
Ascidia is hermaphrodite. The ¢esfes and ovaries are
simple paired sacs lying over the stomach
and leading by separate ducts into the atrium.
The main interest attaching to Ascdra is involved in its
development. If the anatomical account has been carefully
Reproductive.
Fig. 205.—DEVELOPMENT OF AN ASCIDIAN.
(After KowALEvsk1.)
Blastula. Gastrula.
Blastoccele.
Hypoblast. -
Blastopore.
——
Ts
a
re
mecdGae
sine
Neural Tube.
get
oe
5
Peeodisn:
xh
The various stages are shown in median section.
followed, it will have been noticed that Ascdza differs from
nearly all the preceding types in having the
nervous system confined to the dorsal region of
the body, in possessing xumerous paired slits in the wall of the
pharynx, and in the presence of a ventral heart, which is (in-
termittently) respiratory, involving a backward current in the
dorsal vessel and a forward in the ventral. ‘These are all
characters in which the Zumcata resemble the other members
of the important phylum C/ordata. In addition, in the
structure of the pharynx, the method of feeding and the
Development,
294 CHORDATA.
presence of the atrium, Ascidéa can be directly compared
with the other class of the Azrtozoa.
The conclusions drawn from these characters have, how-
ever, an ample corroboration in the development.
The eggs are laid into the atrium, in which they are
fertilised and pass their early stages. Later, the larva is
free-swimming and pelagic. i
The segmentation is total and nearly equal, producing a
blastula which is invaginated to form a gastruda. The
gastrula elongates, and the blastopore comes to lie in a
postero-dorsal position in relation to the adult axes. From
Fig. 206.—TRANSVERSE SECTION THROUGH EMBRYO OF AN
ASCIDIAN.
(After DELAGE.)
Neural Groove.
Norochord: Mesoblastic
Sac.
Epiblast.
Archenteron.
the blastopore forwards to the anterior end of the gastrula
the median dorsal line of cells.becomes the dorsal nervous
system, which is at first dermic, but it is transformed into a
long dorsal nerve-tube by invagination -proceeding from
behind forwards. The front end of the tube, called the
neuropore, is open, and the posterior end, leading through the
blastopore into the archenteron, is known as the neurenteric
canal. Meanwhile the hypoblast has been developing.
The hypoblastic cells lying in the mid-dorsal line immedi-
ately below the neural tube become pinched off from the
rest to form a long rod-like body, the zotochord. Laterally
to this organ are paired pouchings of the hypoblast which
give rise to the mesob/ast or third embryonic layer. Their
lumen is soon lost, and the mesoblast comes to lie as a pair
of lateral masses of cells between epiblast and hypoblast.
We now have the ¢ypical chordate larva or Chordula, con-
sisting of an elongated body, with a long dorsal nerve-tube,
opening anteriorly to the exterior, posteriorly into the arch-
enteron, a median dorsal notochord separated from the
ASCIDIA, 295
Fig. 207.—TRANSVERSE SECTION OF LARVA OF ASCIDIAN.
(After Van BENEDEN and JuLin.)
Nerve Cord.
Mesoblastic Sac,
hypoblast, and a pair of lateral mesoblastic masses more or
less broken up.
This larva is characteristic
of the Chordata though only
found as a larva in the
Atriozoa, being represented
by an embryonic stage in
Vertebrata. The further
development of the Ascidian
diverges from that of the next
class. The larva becomes
divided into a body and a
tail, nearly all the notochord
and mesoblast being carried.
back into the tail (hence
Urochorda), whilst the tail
Fig. 208. —CHORDULA LARVA
OF AN ASCIDIAN.
(After KowALEVSKI1.)
Neuropore.
: _-Neural Tube.
Dorsal view.
Fig. 209.—AN ASCIDIAN TADPOLE.
Brain.
Neural Tube.
Mesenteron.
Notochord.
A median section.
296 CHORDATA.
part of the enteron remains as a mere cord of cells. In
the trunk the enteron becomes modified into pharynx,
stomach and intestine and acquires a mouth. The front
end of the neural tube becomes a hollow brain in which
are formed a median otocyst and eye.
At the front end, below the mouth, are formed pafilla,
and two lateral pits sink in from the epiblast covering the
trunk to form the paired a¢vium. The anus then opens into
the left atrium and pharyngeal clefts open into each. Below
the enteron the heart is formed from mesoblast. Meanwhile
the tail acquires dorsal and ventral median fins, the noto-
chordal cells form a strong elastic median axis, and the
notochord, and mesoblast cells form longitudinal muscles.
Fig. 210.—TAILED LARVA OF AN ASCIDIAN SEEN FROM
THE RIGHT SIDE. (Altered from SEELIGER. )
Atriopore. Brain with Eye
and Ear. Neuropore.
Remains of Caudal . L
Intestine. Intestine.
5
\
Papilla,
Pharyngeal
Clefts.
Endostyle in Wall
of Pharynx.
In this manner the tail is converted into an efficient loco-
motor organ by which the larva can move rapidly through
the water. It is often known as the ascidian tadpole, and
is evidently a chordate type of comparatively high structure.
After a period of free life the ascidian tadpole fixes itself
by its papillae to a rock or other object, and is then con-
verted into the adult ascidian by a process of retrogressive
metamorphosis, z.e., a metamorphosis involving simplification
in structure.
The sense-organs atrophy, together with the main part of
the brain and nerve-tube, the notochord and _tail-muscles
AMPHIOX US 207
break up, the tail is resorbed, Fig. 211.—TRANsveRsE Strc-
and the trunk rotates through TION THROUGH THE TAIL
nearly 180° upon its papillee. OF AN ASCIDIAN Larva.
In this way the active, sensi- (anereaivecey
tive, highly- organised “ tad-
pole” is reduced to a quiescent,
sedentary, vegetative ascidian.
In Chapter VI. it is ex-
plained that ontogeny may in \ Nerve Lube.
many cases be interpreted as
a repetition of phylogeny.
This principle applied to the (
case in hand leads us to the — ypotochord.
conclusion that the ascidians
are descended from active,
free-swimming, highly - organ-
ised Chordata which have
degenerated on the adoption
of a sedentary habit.
Median Fin.
‘Caudal
Hypoblast.
IL—AMPHIOXUS.
PHYLUM CHORDATA (p. 403).
SuB-PHYLUM ATRIOZOA (p. 404).
Ciass CEPHALOCHORDA (p. 405).
Fig. 212,—LATERAL VIEW OF AMPHIOXUS LANCEOLATUS x 3.
(Ad nat.)
Myomere Muscles. Dorsal Fin.
Tail.
“EID [WI
Anal Fin. Atriopore. Metapleural Fold.
Amphioxus lanceolatus (the Lancelet) is a small
marine organism about one to one-and-a-half inches in
length. It is of elongated, fish-like shape, tapering at each
298 CHORDATA.
end. It is flattened laterally, and the whole body is plano-
symmetric. It is of a milky white, semi-transparent
appearance, and a number of the organs may be seen
through the skin in the living animal.
Amphioxus lives in moderate depths near the sandy
bottom. It may swim about actively or may lie on one
side upon the sand, or on occasion it may
bury all but the anterior part of its body in
the sand and there remain in a resting condition.
There are no definite external divisions of the body, but
the anterior part, to about the level of the mouth, is some-
times termed the ead and the posterior quarter of the
body is often referred to as the Zazv.
The anterior end forms a snout or rostrum, just below
which is the mouth, surrounded by a ring of oval cirri or
External ‘emtacles. Along the mid-dorsal line is a
Seuiancs median unpaired dorsal fin which is continuous
"behind with a caudal fin. The caudal fin is
continued round the tip of the tail and forwards along
the ventral surface for about a quarter of the length of the
body as an anal fin.
The tail of the animal runs symmetrically down the
centre of the caudal fin, hence Amphioxus is said to have
a protocercal tail. (See Pisces.)
At the anterior termination of the anal fin there is a
median ventral aperture, the a¢riopore, and anterior to this,
as far forwards as the mouth, there is a pair of ventro-lateral
flaps of the body, called the metapleural folds. On the left
side of the body, at the base of the caudal fin, there opens
a minute aperture, the azws.
The whole body is enveloped in a thin, transparent skin
formed of a single layer of ectodermal cells,
which secrete on their outer surface a deli-
cate cuticle. Sensory cells are scattered throughout the skin.
The mouth, surrounded by its oral cirri. leads into a
buccal cavity. The posterior wall of this cavity is formed
by the velum, a thin septum with a central
Beene snedinee leading into the pene The aper-
ture is surrounded by velar tentacles which protrude inwards.
The pharynx is a large, spacious chamber extending about
2 of the length of the body. In general structure it
Habits.
Integumentary.
AMPHIOXUS. 299
resembles the pharynx Fig. 213.—VIEw oF AMPHIOXUS FROM
of Ascidia. Its internal
walls are mostly ciliated.
The exdostyle extends
along the median ven-
tral line, joined by
peripharyngeal bands to
a median dorsal ef7-
branchial groove. The
lateral walls of the
pharynx are perforated
by a great number of
pharyrigeal clefts which
run diagonally back-
wards as long slits.
These pharyngeal clefts
are twice as numerous
as those of the larva,
each of the latter be-
coming divided longi-
tudinally into two by
a long tongue-bar of the
pharyngeal wall growing
downwards from above.
The same method
of feeding as in Ascidia
is adopted. The water
and food-particles are
brought into the phar-
ynx, and the latter are
entangled in strands of
mucus which are even-
tually carried into the
intestine at the hind
end. The water is
driven through the
pharyngeal clefts into
the atrium, a spacious
cavity which, as in
Ascidia, surrounds the
pharynx. In Amphioxus,
THE RicutT SipE. (Ad zat.)
Metapleural
Fold.
‘Branchial
Caudal Fin.
300 CHORDATA.
however, the atrium is not continued round the dorsal
line of the pharynx. In Ascidia it was the mid-ventral
portion which was incomplete. The atrium is continued
backwards behind the pharynx and along the intestine
until it terminates in the atriopore, through which the water
has exit.
Fig. 214.—-TRANSVERSE SECTION THROUGH AMPHIOXUS IN THE
PHARYNGEAL REGION.
(After LANKESTER, Boveri and others.)
h 3) Dorsal Fin-skeleton.
Nerve Cord. \
Notochord.
_.Myomeric
Muscle.
be.
ll
Perivisceral
Ceelom.
.Pharynx leading
into Atrial
Cavity through
Metapleural Cavity. - Clefts.
Endostyle. Metapleural
Ventral Blood-vessel. ‘old.
The dark shading is the connective tissue and the light outside is the simple
ectoderm. The myomeres and pharyngeal clefts are cut across as they run
diagonally. The section is taken across C D in Fig. 213.
Lying on the right side of the pharynx in the atrium is
a long hollow sac, the “vez, which opens into the alimentary
canal at the junction of pharynx and intestine. ‘The zz/es-
tine is produced backwards as a long tube to the anus.
The muscular system is well developed. The longitudinal
muscles consist of a dorsal longitudinal system of muscles,
aistoy called the myomeric muscles, extending through-
‘out the length of the body. The muscle-fibres
extend between numerous connective-tissue septa which are
AMPHIOXUS. 301
arranged in V-shaped bars. These characteristic Vs can be
seen through the skin in the living animal. Contraction of
the myomeric muscles moves the tail from side to side, driving
the animal forwards. The ventral longitudinal muscles ex-
tend from the region of the mouth to the atrium.
The most important skeletal organ is the notochord. It
extends as a long cylindrical elastic rod from one end of the
a body to the other. Hence at the anterior end
eletal. . : :
it passes forwards to the tip of the rostrum, in
front of the brain. It consists of chordoid tissue and is
enveloped in a mesoblastic sheath. Amphioxus burrows
Fig. 215. TRANSVERSE SECTION OF AMPHIOXUS BEHIND
THE ATRIUM. (4d nat.)
Dorsal Aorta.
Coelom.
Subintestinal
Vein.
Anal Fin
(Skeleton).
The dark shading is the connective tissue and the light outside is the simple ecto-
derm. The section is taken across E F in Fig. arz.
with its rostrum and the notochord apparently gives it
the necessary solidity. It also assists the motor muscles
by its elasticity.
Around the notochord and nervous system and between
the myomeric muscles is a continuous mass of mesoblastic
connective-tissue, which at the bases of the dorsal and anal
fins forms a row of fiz-rays.
302 CHORDATA.
The cirri and the side-walls of the pharynx between the
pharyngeal clefts are supported by skeletal rods or bars.
The coelom is well developed, a perivisceral cavity ex-
tending round the intestine and forming a dorsal mesentery
behind the atriopore ; but forwards its relations
are obscured by the presence of the atrium. Its
dorsal part lies above the atrium and communicates down
the primary pharyngeal bars with the ventral part lying below
the endostyle.
‘The blood system is not unlike that of Ascidia. A dorsal
aorta or artery extends throughout the body. In the
pharyngeal region it is paired and receives nu-
merous efferent branchials from the walls of the
pharynx. The ventral vessel is a vein and is
interrupted at the liver in which it breaks up into small
capillaries. The part behind the liver is the sudbcntestinal
vein. The part running forwards from the liver is the portal
vein, which runs to the pharynx, on the ventral surface of
which it is continued as the branchial artery, giving off
paired afferent branchials. ‘The afferent and efferent bran-
chials really form continuous aortic arches. There is no
heart but the bases of the afferent branchials are contractile.
The arrangement by which the venous blood is supplied
direct to the liver instead of passing directly forwards is
called the Hepatic-portal system and is characteristic of
Vertebrata.
It should be noted that, as there is no true heart, the terms ‘‘ artery”
and ‘‘vein” are not morphologically accurate, but are applied to the
vessels which correspond in structure and function with those of the
higher Chordata.
The course of the blood is as follows :—
a Dorsal daria
°
TS _Branchial Veno a oe ub-infesfin
i Ss al
<—— <Live
arlery vein
Vascular,
Blood-
Vascular,
The nervous system lies immediately dorsal to the
notochord ; it consists of a long tube, the front
portion of which forms a small drain and the
rest the spinal cord.
Nervous.
AMPHIOXUS. 303
The brain has a single ventricle or cavity and two pairs
of cranial nerves. The spinal cord gives off paired spinal
nerves. The dorsa/ nerves are sensory, the ventral are
motor, and they do not join.
Fig. 216.—MEDIAN SECTION OF BRAIN OF AMPHIOXUS.
(After Kurrrer.)
Neural Tube. Ganglion Cells.
Unpaired
Olfactory
Lobe.
Pigment Spot.
Infundibulum. Cerebral Vesicle.
The front wall of the brain has a simple unpaired mass
of pigment, probably a very simple eye. Over
the brain there is a pit or depression. called
the olfactory pit.
Sensory.
Fig. 217.—OBLIQUE SECTION oF AMPHIOXUS THROUGH THE
PHARYNGEAL REGION.
Notochord.
Dorsal Blood-vessel
paired).
Nephridium.
Ceelomic Canal.
B
ranchial Blood-
vessel." ==
. ‘
Branchial Blood- Branchial Bar.
vessel.
Branchial Bar,
Skeletal Plate.
Ventral Blood-vessel. Endostylar Coelom.
A secondary or tongue-bar is cut through on the left, a primary bar on the
right. The section passes along A B in Fig. 213. :
The ccelom leads to the exterior by numerous nephridia
which open into the atrium. They are in the pharyngeal
region and open over each tongue-bar. There
is also a large pair of atvio-cxlomic funnels \ead-
ing from the ccelom into the atrium.
Excretory.
304 CHORDATA.
Amphiorus is dicecious. The gonads lie as a paired
lateral row of organs just inside the ventral
longitudinal muscles. They are said to have
no ducts and to burst into the atrium when ripe.
Reproductive.
Fig. 218.—THE DEVELOPMENT OF AMPHIOXUS AS SEEN IN LonGI-
TUDINAL SECTION AND LATERAL VIEW or LARVé.
Notochordal
Hypoblast.
Blastopore.
Notcchord. Neuropore.
Blastopore {
Neurenteric
saqTUIOS
o1se[qose yl
Neural Tube. Neuropore.
Notochord.
Anterior Meso-
blastic Sac.
1 Collar-sac, followed by
8 the Mesoblastic Somites.
A, Blastula. B, Gastrula. C, Completed Gastrula. D, Commencing Neural Tube.
E and F, Lateral view ’ of later chordula stages. (After HATSCHEK.)
AMPHIOXUS. 395
Fig. 219.—THE DEVELOPMENT OF AMPHIOXUS AS SEEN IN
SECTIONS.
Neural Groove. Neural Tube. Neural Tube.
Notochord.
‘OBG OTISE|qOsoT
‘eg dIISE[qOSaTL
Chorda.
,Notochord.
; Mesoblastic
g Somite.
w
rst
p=} Arch-
a enteron.
a ¢
8 §
vo ~
a 3
rat
o
a
v
Eo
4
Pre-oral Sacs.
Neuropore.
Pre-oral SSO
Mesoblastic Sac. EY
ee
es
oI
R
te
= secu
9 sean
Collar-sac. IAA
SS a
S|
: eae
Mesoblastic pees
Somites.
Trunk- sacs. Neural Tube,
picarentsay
‘anal,
HO :
._, (A Through D 1-2 in Fig. 218; B through E 3-4, and C through E 5-6 in
Fig. 218; D through F 7-8 in Fig. 218; E through H 9-10 in Fig. 219 ; and F through
‘posterior part of Fig. 221.) :
A shows the neural groove and developing mesoblastic sacs; B and C show
the neural tube ; D shows the completed mesoblastic sacs; E shows the notochord
completely formed; and F shows the formation of myotome from dorsal part of
mesoblast and-perivisceral ccelom from ventral. (After HATSCHEK.)
G, Longitudinal horizontal section of an Amphioxus larva. (After M‘Brip4.)
H, Horizontal longitudinal section through advanced chordula larva of A mphi-
oxus. (After Hatscuex.)
M. 21
306 CHORDATA.
Development.—The eggs are shed through the atriopore to the
exterior, where they are fertilised. Segmentation is total and equal and
results in a blastula which in its turn is converted into a gastrula by
archiblastic invagination. The gastrula then elongates, the blastopore
taking up a postero-dorsal position.
The epiblast then invaginates along the mid-dorsal line to form a
nerve-tube and the hypoblast gives rise to a median dorsal notochord
and paired lateral mesoblastic sacs. In this manner is produced a
chordula larva practically similar to that of Ascidia. The main dis-
tinction lies in the origin of the mesoblast. Instead of a single pair of
somites which rapidly become a pair of solid mesoblastic masses, event-
ually breaking up into scattered cells, there are in Amphioxus a great
number of somites, each of which has a definite ccelomic cavity. It is
Fig. 220.—TRANSVERSE SECTIONS THROUGH YOUNG AMPHIOXUS,
SHOWING DEVELOPING ATRIUM.
(After LANKESTER and WILLEY, Bovenrt, and others.)
Mesoblastic
Sheath of
Notochord,
Sclerotome.
Myotome
Sclerotome.
Gonotome.
Perivisceral’ -
Ccelom. ug
6
De
g8
[fS)
Metapleural =
Cavity. 3)
a Atrium.
Atrium.
Ventral Muscle.
said that one pair of pre-oral somites arise from the anterior end of the
archenteron, a second pair behind these, called the col/ar-sacs, and a
third pair at the posterior end laterally to the blastopore. The pre-
oral pair form the head-cavity (right) of the larva and the pre-oral pit
(left). Each of the collar-somites divides into a dorsal portion, which
forms the first myomere muscle, and a ventral part forming the meta-
pleural cavity. Lastly, the posterior somites divide up to form a great
number of mesoblastic somites: so far as is known, they alone are
found in Asczdia.
The three pairs evidently correspond to the three archimeric seg-
ments of Balanoglossus and the other Archicelomata, and the metameric
segmentation of the Chordata is clearly produced by secondary segmen-
tation of the posterior segment or opisthomere.
AMPHIOXUS. 307
The fully-formed chordula larva of Amphioxus thus consists of an
elongated body with a hollow dorsal nerve-tube opening anteriorly by a
neuropore and posteriorly by the neurenteric canal, or persistent blasto-
pore, into the archenteron. Below the nerve-tube is a long dorsal
notochord and below this the spacious archenteron. Laterally, between
the archenteron and the epiblast lies a row of mesoblastic somites,
hollow sacs of mesoblast.
Fig, 221.—LATERAL VIEW OF YOUNG PELAGIC AMPHIOXUS AT COMMENCE-
MENT OF LARVAL LIFE x 120. (After HarscHEK.)
Pharynx.
Neuropore. Mouth. |,
il
First
Pharyngeal Cleft. Notochord. Nerve Cord. Anus.
ope.
Note head cavity (with dotted walls) and the thick-walled pre-oral pit in front of pharynx.
A little before this stage the embryonic period comes to an end and
the chordula larva is set_free from the egg-membrane, swimming in the
water by means of the flagella of the epiblast cells. It still, however,
subsists upon the diffuse yolk-granules scattered throughout the cells.
Fig. 222.—DIAGRAM OF YOUNG PELAGIC AMPHIOXUS TO SHOW THE
DIVISIONS OF MESOBLAST AND Cc@&Lom. (After M‘BRIDE.)
A ae
Right Head
Cavity. | J
ae Myotomes.
Collar-sac. Perivisceral Ccelom.
Elongation of the hind end of the body produces a larva much core
like Amphioxus in shape; at the same time the notochord grows
forwards to the extreme front end of the body. The neurenteric canal
closes and the mouth and anus open, the former at first on the left side.
The mesoblastic somites have grown downwards round the arch-
enteron and each has divided into a dorsal and a ventral part. The
308 CHORDATA.
ventral parts unite together to form the continuous ccelom and the
dorsal parts divide into three portions, the sclerotome, myotome and
gonotome, which give rise to the connective tissue, myomere muscles
and the gonads respectively.
The larva lives for about three months in pelagic water and then moves
to the sandy bottom. During this period the rows of pharyngeal clefts
appear as paired apertures, and the atrium arises as a mid-ventral
ingrowth of epiblast which pushes the ccelom before it and comes to lie
around the pharynx. It will be noticed that up to the production of the
chordula larva the development is closely similar to that of Asc¢dia.
MYVXINE. 309
CHAPTER XxX.
CHORDATA — (Continued).
MYXINE, RAIA.
I—MYXINE.
PHYLUM CHORDATA (p. 403).
SuB-PHYLUM VERTEBRATA* (p. 406).
CLASS CYCLOSTOMATA (p. 433).
Fig. 223.—LATERAL VIEW OF MYXINE
GLUTINOSA x 4. (4d nat.)
eas
£ f ,
Fe
a
a3 ‘
a 3 4 Branchial
a I 4 Aperture.
a
4 ;
5
a
Tail Fin.
Myxine glutinosa (the hag-fish) is a small eel-like
animal occurring off our coasts. Its body is elongated and
cylindrical, about 1 foot long. The hind end or tail is
slightly flattened laterally, and is encircled by a simple
caudal fin which is part of a continuous median fin
running dorsally and ventrally for some distance along
the body. ‘The skin is usually of a pale dull-pink hue and
is intensely slimy. The slime is secreted by a lateral row of
slime-glands which pour out enormous quantities of the
adhesive material. ‘The front end tapers to a snout, below
__* The members of this sub-phylum are often called ‘‘ Vertebrates” in contrast
with the rest of the animal kingdom, termed ‘‘ Invertebrates.”
310 CHORDATA.
which is the mouth, surrounded by four pairs of small buccal
cirri. Above the mouth is a small median aperture usually
termed the xasal opening. It leads into a tube, the
pituitary sac, which also has an internal opening into the
pharynx.
The single olfactory sac opens into the pituitary sac near
its external aperture. The mouth is situated at the base of
a suctorial buccal funnel, on the dorsal wall of
which is a large median horny tooth. Other
horny teeth are attached to a large tongue which is moved
by enormous muscles.
Myxtne is an active carnivorous animal and often devours
fish caught on the lines. It is indeed frequently so caught
itself. ‘Ihe edges of the buccal funnel are said to form a
sucker, and the movements of the tongue serve to rasp the
flesh of its victim. The mouth passes into a pharynx
continued backwards into:a wsophagus. The pharynx
has six pairs of lateral openings which pass outwards
into large branchial sacs (or gill-pouches) containing the
gills. From each of these a canal leads back-
wards. Those of each side unite to open by a
single branchial aperture situated ventro-laterally. Behind
the last branchial sac the cesophagus has a duct on the left
side (the wsophageo-cutaneous duct) leading directly to the left
branchial aperture. When the mouth is being employed
the respiratory current passes through the pituitary sac to
the gills. The cesophagus expands into an intestine which
receives a dile-duct from a simple dilobed liver. ‘There is a
small gall-bladder. The intestine terminates ventrally in an
anus,
The xotochord consists of a skeletal chordoid rod running
from below the mid-brain to the tip of the tail. It is sur-
Skeletay, YOUnded by a thick sheath. A membranous
* sheath also surrounds the nerve-cord. There
is no trace of a vertebral column. Cartilage is found in
rings supporting the ‘‘nasal passage,” and the buccal cirri
are supported by cartilages. Under the brain there lies a
ventral cartilaginous portion of a cranium, completed dor-
sally by membrane. A trace of visceral arches may be
represented by a sudocular bar and other cartilaginous
structures connected with it, united with the cranium.
Alimentary.
Respiratory.
MYXINE. 311
A small cartilage near the cesophageo-cutaneous duct
represents a complex branchial basket found in the lampreys.
Lastly, a large Zingual cartilage supports the tongue.
Myxine is unique amongst the Vertebrata in having no
trace of vertebree.
The coelom is spacious, and consists of a pericardium
around the heart
Ceelom. Fig. 224. —VENTRAL DISSECTION
- and an abdominal OF Myxi1NE GLUTINOSA TO
cavity surrounding the intes- SHOW THE GILI,- POUCHES
tine which communicates x4. (Ad nat.)
with the exterior by an
abdominal pore. BuccalCirri
The Heart is like that of
a fish. It is two-chambered,
with one auricle and a
ventricle. It lies on the
ventral surface
of the cesoph-
agus, and drives
blood by a branchial artery
diverging to the six gill-
pouches by six afferent
branchials. Six efferent
branchials unite dorsally to
form a dorsal aorta. These
branchials pass between the
gill-pouches and each sup-
plies blood to the adjacent
surfaces of two gill-pouches.
Blood-
Vascular,
Gill-Pouches.
Efferent Ducts of Gill-Pouches.
The venous system con- g
sists of paired jugulars and og
cardinals uniting in a sinus Bog
venosus and thence to the raga
heart. xg &
The éraiz is small and a
simple. There are paired
cerebral hemi-
spheres, the cere-
bellum is very small and the
optic nerves do not cross. There are zen cranial nerves, as
in fishes, but there is no sympathetic system.
Nervous,
CEsophageo-cutaneous Duct.
Nasal Aperture
312 CHORDATA.
The median olfactory sac has been noted. The paired
eyes are present but scarcely functional, whilst
the paired auditory sacs are small and contain
only one semicircular canal.
The &édneys are paired organs lying in the dorsal part of
the ceelom. They consist of coiled tubules communicating
cowenttul: with the exterior by paired ureters beside the
anus. The gonad is simple and produces
sperms in the young individual and eggs at a later period.
Hence M@yxine is a protandric hermaphrodite. There are
no genital ducts.
Sensory.
Fig. 225.—MEDIAN SAGITTAL SECTION THROUGH MYXINE
Guutinosa x 4. (dd nat.)
Nasal Sac. Internal Nas. Opening of
Brain. | Gill-Pouch. Nerve Cord. Notochord.
(Esophagus. h /
ff :
To
Heart. Liver. Intestine.
ngue,
Gill-Pouch. |
Branchial Artery.
The eggs are large, oval, and have much yolk. They
are enveloped in capsules which have hooks at
each end and are usually found embedded in
slime. The development is unknown.
Myxine shows a number of very primitive vertebrate
features, of which we may specially note the absence of
paired fins, vertebral column, pancreas, spleen, sympathetic
nerves and genital ducts. The unpaired nasal sac, the
peculiar pituitary pouch with its internal aperture, the ear
with a single semi-circular canal, and the hermaphrodite
condition are also unique characters amongst Vertebrata.
With the lampreys it constitutes the class Cyclostomata, a
name emphasising the suctorial condition of the mouth.
Development.
RATA. 313
II.—RAIA.
PHYLUM = CHORDATA (p. 403).
Sus-PHYLUM VERTEBRATA (p. 406).
Cass Pisces (p. 435).
Raia radiata* is one of the commonest of the British
skates, and is perhaps the most convenient as regards size.
The other species only differ in size, colour, general shape
of the body and other small structural features.
The skate lives near the bottom at moderate depths,
and is carnivorous in diet. In shape its body is rhomboidal,
Externay With a long tail depending from one angle. The
body is flattened dorso-ventrally, the two large
side-flaps being made up of the enormously
expanded jectoral and fairly large pelvic fins. Attached
Features.
Fig. 226.—JAwWs AND TEETH OF (A) MALE AND
(B) FEMALE SKATE.
serum
to the pelvic fins in the male is a pair of large c/aspers.
These fins represent the front and hind-limbs respectively
of the higher Vertebrata.
The pointed anterior end is termed the rostrum.
* The following. description will apply for the Common British species, Rusa
maculata and Raia batis. i g
314 CHORDATA.
On the dorsal or upper surface we can notice the paired
eyes a little way behind the rostrum, and behind them is a
pair of oval apertures called the sfzracles, for through them
passes the water employed in respiration.
The skin is slimy, owing to the secretion of numerous
slime-glands, and dotted over its surface are numerous
placeid scales. Each scale consists of a hard base bearing
a sharp spine. Towards the tip of the tail is a pair of
small dorsal fins, and the caudal fin surrounds the end of
the tail. The upper lobe of the caudal fin is larger than
the lower and contains the true end of the tail. Sucha
shape of tail is called heterocercal (see Pisces).
Ventrally we can recognise the median transverse mouth
with a pair of grooves (07o-nasa/) passing forwards from
each angle to the olfactory or nasal openings. The jaws
bear numerous rows of small placoid scales which act as
teeth. Posterior to each angle of the mouth, and slightly
outwards, there is on each side a row of five diagonal
slits leading into the pharynx. These are the pharyngeal
clefts or gill-sits. Behind the last gill-slit and running
across the ventral line is the coracoid bar, which can be felt
through the skin. At the base of the tail is a conspicuous
median aperture, the cloacal aperture, and close to its
posterior border is a pair of minute slits, the abdominal
pores, which lead indirectly into the abdominal cavity. In
front of the cloacal aperture, and crossing the ventral line,
may be felt the hard pudic bar. Scattered over the skin,
especially on the ventral surface, are numerous fine apertures
of sensory tubes. The tubes are full of a gelatinous material
which may be squeezed out of the apertures.
If the mouth be forced open it will be seen to
lead into a spacious pharynx, into which open dorsally the
two spiracles and laterally the five pharyngeal clefts on each
side. Posteriorly it can be seen to taper into an @sophagus.
If a gill-slit be opened up it will be seen to pass as a
short canal into the pharynx. On both anterior and
posterior wall of this canal are a great number of branchial
filaments constituting the gz//s. In them the blood is only
separated from the water by a thin membrane and eration
is effected. On the wall of the spiracle may be noticed a
pseudobranch or vestigial gill. The water passes in by the
Alimentary.
Plate I.—FirstT DISSECTION OF THE SKATE.
Thyroid. —
Branchial Artery.
N
\ om,
(Ad nat.)
_~ Coraco-mandibular Muscle
,
Ventricle of Heart. ty,
A Sg,
Duodeno-hepatic Mesentery
h Bile-duct,
wit!
Right Lobe
of Liver.
Ceeliac Arte
Tleum, with
Spiral Valve.
Pancreas.
Portal Vein and
a,
X thrown forward.
Oro-nasal Groove.
Gill supplied by first
Afferent Branchial
SBIR
&
ominal Pore.
The ventral body-wall has been removed completely from the abdominal region,
exposing the alimentary system in the abdominal cavity.
have been spread out and the stomach slightly drawn away to the left.
The lobes of the liver
Anteriorly
the skin has been removed from the region between mouth and coracoid bar and the
afferent branchial system dissected out on the left side only.
The heart is lying in
its natural position in the pericardium. The veins are blue, arteries red.
RAIA, 315
spiracle and out over the gills. | Breathing is therefore
independent of the mouth.
If the skate be laid on its dorsal surface, and the skin
and underlying muscle be removed in the area lying between
the coracoid and pubic bars, the spacious abdominal cavity
will be exposed. In it lie freely the other portions of the
alimentary system. The esophagus entering the abdomen
at the front end leads-into the large stomach, which is U-
shaped and inclined to the left. It is mainly hidden by the
spreading trilobed gland, the “ver, which is attached at the
anterior part of the abdomen, but each lobe hangs free
posteriorly. From the opposite end of the stomach to the
cesophageal opening there arises the zzfestine, a tube of
varying calibre passing down to the cloacal aperture. Its
first portion, the dwodenum, is short and leads into the very
wide z/ewm containing in its interior a sfzval valve. The
last portion is the vectwm, narrower than the intestine proper
and with no spiral valve. It opens into the cHaca.
This alimentary canal has three important glands which
open into it. The “Aver has already been referred to.
Between its median and right lobes is a gad/-bladder, from
which there passes a long Jdz/e-duct to open into the duo-
denum. -The bile, secreted by the liver, is stored in the
gall-bladder and periodically discharged into the duodenum
by the bile-duct. Near the duodenum is a bilobed whitish
organ of moderate size, called the pancreas. It secretes
a digestive fluid which is discharged by a short pancreatic
duct into the commencement of the ileum on its dorsal
side. Lastly, near the termination of the rectum is a small
oval rectal gland opening into the rectum; it may be excretory.
Before leaving the alimentary system, the sp/een, a dark-
red ductless gland near the stomach, should be observed.
Note also the Jorta/ vez, a large vein draining the stomach,
intestine, spleen and pancreas, and passing forwards to the
liver; and the celiac artery which supplies the stomach,
duodenum and liver with arterial blood.
If the skin and muscles be removed from the area
between the mouth and the coracoid bar, the gericardial
cavity will be exposed. It communicates with
the abdominal cavity by a pair of small canals,
and the two cavities compose the perivisceral ccelom. They:
Vascular.
316 CHORDATA.
are lined by a peritoneum or thin membrane and contain a
colourless coelomic fluid. ‘The ccelom communicates by the
abdominal pores with the exterior. The peritoneum sur-
rounds all the organs in the abdominal cavity, and the
Fig. 227.—D1aGRAM OF ARTERIAL SYSTEM OF A SKATE.
Seen from Ventral Surface.
External
Carotid.
Internal Carotid.
Anterior
Innominate.
Hyoidean.
Posterior
I Innominate.
zZ
ES)
Gills.
Subclavian.
Efferent
Branchials
Hepatic. Be
‘Ventral Aorta.
Duodenal.
Gastric.
F Ceeliac.
Anterior.
Mesenteric.
Genital.
Posterior.
Mesenteric.
Renals.
Tliac.
Caudal.
The afferent branchial system is shaded darker than the rest.
cesophagus and rectum are suspended from its dorsal wall
by a mesentery, formed of two folds of peritoneum apposed ;
the part of the alimentary canal between these two has no
mesentery and lies freely in the cavity. Another fold, the
omentum, contains the bile-duct and portal vein.
RATA. 317
The feart lies in the pericardium. It has two chambers
a thick-walled ven¢ricle and a larger thin-walled auricle lying
dorsal to it. The ventricle leads forwards
out of the pericardium as the conus arteriosus
containing valves, beyond which it is continued
as the dvanchial artery.* The auricle receives blood from
a thin-walled triangular séwws venosus formed from the
swollen termination of the main veins.
The branchial.artery gives off a pair of posterior in-
nominates, which trifurcate into three afferent branchials
supplying the three posterior pairs of gills. The branchial
artery runs forward and terminates just behind the lower jaw,
near a small ductless thyroid gland. Here it diverges into two
anterior innominates, each of which bifurcates into two afferent
branchials supplying the first two pairs of gills : this comprises
the afferent branchial system. On contraction of the ven-
tricle of the heart the blood passes forward to the gills,
hence the skate’s heart is purely vespzratory. If the coracoid
bar be now carefully removed, the sinus venosus will be seen
to run downwards and outwards on each side to the fre-
caval sinus, which communicates with a spacious /epatic
sinus in connection with the liver and receives a jugular
vein from the head, a /azeral vein (formed of a Ze/vic from
the pelvic fin and a érachial from the pectoral fin) and
a cardinat vein from the posterior part of the body and
kidneys. A median cauda/ vein from the tail diverges into
a pair of renal portals to the kidneys, in which the veins
break up into capillaries. A skate has therefore a renal
portal system as well as a hepatic portal.
If the ventral wall of the pharynx and the skin of the roof
of the pharynx be removed, the efferent branchial system is
exposed (Plate II). It consists of five efferent branchials
leading from the gills towards the middle line and back-
wards. The two first unite into one, as also do the two
last ; hence three arteries are produced which then unite to
form the dorsal aorta. From the first efferent branchial on
each side runs forward a carotid, dividing into ¢xternal
carotid to the brain and external carotid to the head. The
dorsal aorta gives off paired szbclavians to the pectoral fins,
and is continued along the dorsal line under the vertebral
* Often termed the Ventral Aorta.
Blood-
Vascular.
318 CHORDATA.
column till it terminates in the caudaZ It gives off a median
celiac to the stomach, a superior mesenteric to the spleen,
pancreas and intestine, paired vena/s to the kidneys and
paired z/acs to the pelvic fins. These arteries are fully
exposed by removal of the coracoid bar and, if necessary,
the pubic bar.
We may note some special features of the blood-vascular
system of the skate which are also typical of the class. The
blood-vascular system can be divided into the arterial and
the venous system as in all Vertebrata, but the venous system
Fig 228.—DIAGRAM OF THE VENOUS SYSTEM OF A SKATE.
Sinus
Venosus.
Hepatic Sinus. Jugular.
Brachial.
Hepatics. :
Cardinal. q
Genital.
~ Renal.
Lateral.
Renals.
Caudal.
is chiefly composed of wide sinuses (also a common condi-
tion of invertebrates). The arterial system has two distinct
parts separated by the capillaries of the gills. The ventral
or afferent branchial system carries venous blood forwards to
the gills; the dorsal or efferent branchial system carries
blood mostly éackwards to the system. In the venous
system the capillaries of the liver and those of the kidney
both break up the continuity of the sinuses, forming a
hepatic-portal and renal-portal system respectively.
Plate II.—SEconpD DISSECTION OF THE SKATE (2). (Ad nat.)
Upper Jaw.
Carotid.
hoy
tst Gill Cleft. a ertebral.
and "
3rd u q
Innominate formed
by rst and 2nd Efferent
4th wr = Branchials.
3rd Efferent Branchial.
5th " Innominate formed
by 4th and sth Efferent
Branchials,
viduct.
Subclavian. aan
Coeliac.
z : Ovi 1 Gland.
Anterior Mesenteric. ducal Gland,
Dorsal Aorta.
Kidney.
Pubic Bar.
Urinary Papilla.
Apertures of Oviducts, /
Cloaca/
as Pore.
Showing the efferent branchial and main arterial systems and the urogenital
systems. The coracoid and pubic bars are cut through, the heart and alimentary
canal, together with the floor of the buccal cavity, have been removed,
RATA. 319
The chief organs now remaining in the abdominal cavity
belong to the urogenital system. ‘The excretory and repro-
Urogenital ductive organs of vertebrates are so intimately
* connected that they are usually described in this
way as one system.
In the male, the Zestes are two large pale brown organs
lying in the abdominal cavity and suspended by a dorsal
mesentery towards their front end.
Fig. 229.—MaLE UROGENITAL ORGANS OF A SKATE. (Ad nat.)
Mesonephros (Epididymis).
Vasa Efferentia.
“Mesonephric
uct.
Metanephros
(Kidney).
Seminal Vesicle.
‘Sperm-sac.
Urogenital Papilla. ~ Opening of Ureters into
Urogenital Sinus.
Cloacal Aperture.
Each testis gives off from its anterior end a long coiled
tube, the vas deferens, which passes along on either side of
the dorsal middle line to open posteriorly into the uzo-
genital sinus. Connected with its posterior end is the
sperm-sac. The anterior half of the vas deferens is sur-
rounded by an epididymis, which is said to be the persistent
mesonephros, and the posterior half is in close contact with
the surface of the kidney.
The kidneys are pazrved elongated reddish bodies lying
above the abdominal cavity, and they can be dissected by
320 CHORDATA...
removing the dorsal peritoneum. Each has a fine duct, the
ureter, leading from its inner lower border posteriorly to open
into the urogenital sinus. This sinus opens into the cloaca
by a small papilla. ;
As already noted, the male skate has a pair of claspers,
long firm organs strengthened by cartilages developed in
connection with the pelvic fins. They are deeply grooved
and have a large clasper-gland which opens into the groove
by a duct.
- Each testis discharges its sperms into its vas deferens
and thence into the sperm-sac in which they are mixed with
a secretion ; they then pass out of the cloacal aperture and
down the clasper-grooves.
In the female, the ovaries are paired and occupy the
same position as the testes. They often contain large
partially ripe ova. The oviducts are paired tubes of large
size leading the whole length of the abdominal cavity. At
the anterior end they open by a common aperture zzfo the
abdominal cavity, and posteriorly each opens into the cloaca.
The anterior part is called the Fa//opian tube which is thin-
walled and of small calibre; the posterior part, sometimes
called the u¢erine portion, is thick-walled and wide; at the
junction of these two parts isa large ovdducal gland. (There
is a vestige of the epididymis.) The urinary organs do not
differ essentially from those of the male.
The eggs on ripening are shed free into the abdominal
cavity, and thence pass down the oviducts. They are
fertilised in the Fallopian tubes and the oviducal gland then
secretes around them the egg-capsule or purse; they are
laid singly. through the cloacal aperture.
If the skate be now turned upon its ventral surface, and
the skin removed from the head region, as far out as the
gills and backwards, the following structures can
ee be recognised (Plate III). In the centre is the
"cranium, the dorsal cartilaginous wall of which
may be carefully removed, when it will be seen to possess
a large central cavity containing the drazm, a pair of anterior
cavities of the olfactory capsules and a pair of posterior cavi-
ties, those of the auditory capsules. Between these and the
olfactory capsules are the eyes. Hence the side of the head
in the skate bears three pairs of sense-organs, olfactory sacs,
RATA. 321
eyes and auditory sacs. The further structure of these organs
will be referred to later. Lying farther out on each side
opposite the eyes is a large oval mandibular muscle. Its
front end nearly meets the olfactory capsule and its hind
border approaches the auditory capsule. Lastly, the sAzracle
lies slightly in front of the auditory sac.
Returning to the brain, we notice the large cerebrum at
the anterior end which is produced forward as a pair of
long offactory lobes to the olfactory capsules. Behind the
cerebrum is the narrow ¢halamencephalon produced dorsally
into a small gizneal body and ventrally into a process called
the infundibulum. From its ventral surface originate the
pair of optic nerves to the eyes. The cerebrum and thala-
mencephalon form the fore-brain with the two first cranial
nerves—I, olfactory and II, optic.
The paired optic lobes then succeed. They form the
mid-brain and give off the third cranial nerves or oculomotor
(to the eye-muscles) from their ventral surface, and the
fourth or ¢vochiear (to a single eye-muscle) from their dorsal
surface. Behind them is the /znd-brain formed of a large
cerebellum which has a large anterior lobe partially covering
the optic lobes and a posterior lobe covering the medulla
oblongata. The medulla oblongata has a thin dorsal wall
and is continued backwards into the spinal cord which
passes posteriorly to the tail. From its lateral walls there
arise the fifth (trigeminal), sixth (abducens), seventh (faciad),
eighth (auditory), ninth (glossopharyngeal) and tenth (vagus)
cranial.nerves. They can be seen passing out of the cranial
capsule by foramina and their subsequent distribution has
now to be followed.
The eye is held in position and moved in the orbit by
six eye-muscles which originate in the cartilaginous orbit
and are inserted in the sclerotic of the eye. At the
anterior end are the ob/iguus superior and inferior radiating
from one point of origin and posteriorly are the four vecti
muscles. These radiate from one point and are easily
identified as the rectus superior, inferior, internus and
externus.* Without further dissection we can recognise the
* The names of the last two have no meaning in an animal like the skate with
lateral eyes, but have been passed down from human anatomy. :
M. 22
322 CHORDATA.
obliguus superior with its trochlear nerve, and the rectus
superior, externus and internus muscles. The rectus ex-
ternus is supplied by the minute sixth (abducens) nerve
which is not easily seen. The other four are supplied by
the third or oculomotor. (For list of these muscles see
page 410.)
Running over the bases of the upper eye-muscles is a
long nerve called the ophthalmicus superficialis. It enters
the orbit posteriorly and leaves it anteriorly to pass for-
wards to the rostrum. It is a compound nerve formed of
the fifth and seventh. Entering the orbit and leaving it
by the same foramina-as this nerve. is another, called the
ophthalmicus profundus. It, however, lies below the three
upper eye-muscles (¢e., rectus superior, obliquus superior,
and rectus internus), though well above the other eye-
muscles and the optic nerve.
A very little dissection between the auditory capsule and
the mandibular muscle will reveal a large nerve, the yo-
mandibular, an important branch of the seventh nerve
which can be traced to ampullz or sensory tubes in the
skin and backwards to the front of the auditory capsule.
It gives off a large external mandibular round the outer
folds of the mandibular muscle and other branches which
are the recurrent facial, internal mandibular,* facial proper
and hyotdean.
If the eye be now carefully removed by cutting the
eye-muscles and optic stalk and the orbit be cleared,
a number of deeper nerves are brought into view. The
outer buccal (VII.) is a large branch easily found lying
between the olfactory capsule and the mandibular muscle.
It runs across the floor of the orbit and outwards to
ampulle. Very deep in the orbit, below the ophthalmicus
profundus, lies the zamer buccal (VII.) passing to the roof
of the mouth. In the angle formed by the two buccals lie
the maxillary (V.) to the upper jaw and the mandibular (V.)
to the lower jaw.
Lastly, in front of the spiracle is a palatine (VII.) with a
branch, the prespivacular (VII.).t
* This nerve is also sometimes termed the chorda tympani.
+ These lie very deep on the actual roof of the mouth.
Plate III.—THE CraniaL NERVES OF THE SKATE. (Ad nat.)
te
" ms e
sea eo... \ Se, _— Ophthalm
Olfactor: fae OL FACTORY Superfic
Outer . po MAPSULE Ophthal.
Buccal. * 5 a » Profur
Mandibular» 3
ms a ‘ea
: | .
Inner a Y é
Buccal ‘ aut ao
= [INT “MANDE »
i! ge gReC" g 2 MUSCLE.
‘ if ;
Maxillary. My = ad
Pre- = : / d —_
spiracular 3 id
Ay
: ma
” : é lyoidean
Bs be ane P "\Facial Proper.
Palatine? Cae i ng
ae a | Ye, “Internal Mandib
5 ty 7
Cut End of > se ‘R i
Ophthalmics V. and VIL 3 Sree
Optic Lobes. :
Cerebellu “~~ Glossoph:
a sopharyngeal.
Medull
~ ~ast Branchial.
Neural Ridge—
—~2nd "
Lateral Ridge.
_._Lateral Line.
isceral.
On the right the eye is seen 7” sitz with its muscles: the skin has been removed,
exposing the mandibular muscle, and the olfactory capsule has been opened to show
the olfactory lobe. The hyo-mandibular has been dissected out with its branches ;
further back the jugular sinus has been cut open showing the glossopharyngeal and
vagus. In the centre the roof of the cranium has been removed to expose the brain.
ee ve the eye has been cut away showing the deeper nerves on the floor of
the orbit.
The v. or trigeminal nerve is red, the vii. or facial is blue. The x. or vagus is
es |
RATA.
323
The auditory nerve is simple and short and passes to
the auditory capsule.
If the jugular sinus be cut
open throughout its length
the glossopharyngeal ( IX.)
and vagus (X.) nerves will
easily be exposed. The 1Xth
is simple and passes from
behind the auditory capsule
to the first gill-slit.
The vagus (X.) has four
branchial branches to the
four last gill-slits, a Ja¢eral
Zine branch under the skin
and a wrsceral branch which
passes to the heart and
stomach.
The spinal cord gives
off paired spinal nerves, the
first fifteen (or 15 to 18) of
Fig. 230.— THE Ear (Mrem-
BRANOUS LABYRINTH) OF THE
SKATE (Diagrammatic).
Semi-circular Canal.
\
Utriculus,
Horizontal
Semi-circular
Canal.
Sacculus,
Note that there is no middle or outer
ear, and that the inner ear communicates
by a duct to the exterior,
which join together to form the brachial plexus, passing to
the pectoral fin.
We may summarise the cranial nerves as follows :—
FORE-BRAIN.
I. Olfactory.
II. Optic.
MID-BRAIN.
III. Oculomotor.
IV. Trochlear.
HIND BRAIN.
V. Trigeminal.
1. Part of ophthalmicus tke
superficialis.
2, Ophthalmicus profun- | 2
dus. 3. Outer buccal,*
3. Maxillary. 4.
4. Mandibular. 5. Palatine
VI. Abducens.
VII. Facial.
Part of ophthalmicus
superficialis.
. Hyomandibular.
Inner buccal.t
spiracular).
VIII. Auditory.
IX. Glossopharyngeal.
X. Vagus.
1. Four branchials,
2. Lateral line.
(and pre-
3. Visceral.
* The branches in ifa/ic type disappear in Vertebrata above the fishes, besides
parts of other branches.
+ The maxillary anast--moses to some extent with the inner buccal herve, but
whether fibres of V. actually supply the ampulla at the termination of the inner
buccal is doubtful.
324 CHORDATA.
We have already referred to the exoskeleton of scales
and teeth, but a more extensive endoskeleton has to be
noticed. This is entirely formed in the meso-
blast and consists of connective tissue (or mem-
branous tissue) and cartilage. The connective tissue binds
all the organs together and may be directly compared with
that of Amphioxus. A gentle heat serves to disintegrate
this tissue and enables us to easily isolate the firmer and
Skeletal.
Fig. 231.—DorsaL VIEW OF CRANIUM OF A SKATE. (4d nat.)
Rostrum.
Anterior Fontanelle.
Orbito-nasal Foramen.
Olfactory Capsule.
Posterior Fontanelle.
Orbit.
‘Auditory Capsule.
Auditory Aperture. _ Foramen Magnum.
more consolidated cartilage. - In certain. parts the cartilage
is hardened by the deposition of calcareous matter, a fore-
shadowing of the “ bone ” of other forms.
For purposes of description we may divide the cartila-
ginous skeleton into (1) Axial and (2) Peripheral (appendic-
ular). The Axial is divided into (1) Cranium, (2) Visceral
arches and (3) Vertebral column; and the Peripheral into
(1) Pectoral and (2) Pelvic elements.
1. AxtaL.—The Cranium is an elongated hollow case
enclosing a spacious cavity in which lies the brain. At the
RAIA. 325
anterior end it is produced into a pointed rostrum and at the
posterior end is a large hole, the foramen magnum, leading
into the cranial cavity. On either side of the foramen
magnum is a large occipital condyle or facet. At the base of
the rostrum on each side is an olfactory capsule. opening
Fig. 232.-LATERAL VIEW OF SKULL OF SKATE (Natural Size).
(Ad nat.) :
ae
Rostrum,
Ligament.
Palatoquadrate.
Mandible.___ Nasal Capsule.
Optic Foramen.
™ Orbit.
Spiracular
Cartilage.
Foramen for
rar ee V.and VII.
ce
eee 2 Audit
f 1st Branchial. Capeule.
“and Branchial..
ard Branchial., s
4th Branchial.
3th Branchial.
ventrally.and containing the nasal sac. On either side of
the posterior region is a large auditory capsule containing
the auditory sac. The lateral walls of the cranium bound
the orbit and have several foramina for transmission of the
cranial nerves. °
326 CHORDATA.
The cavity of each auditory capsule opens by a small
aperture on the dorsal surface. The dorsal wall of the
cranium is incomplete and the two large openings are known
as the anterior and posterior fontanelles.
‘Lhe Visceral Arches form the jaws and the supporting bars
of the gill region (cf page 417). The principal parts are
(t) The paired Ayomandibular cartilage, fastened to the
auditory region of the cranium; (2) The paired padato-
guadrate cartilage, bound to the distal end of the hyo-
mandibular. Each has near the hyomandibular a convex
condyle to which is articulated the mandibular cartilage.
The two palatoquadrate cartilages form the upper jaw
and the mandibular cartilages form the lower jaw. Each
is covered by the placoid scales forming ‘teeth.
Behind the jaws and attached to the hyomandibular is a
long jointed Ayord cartilage. Behind this are five branchial
cartilages on each side, which are joined together ventrally
by a median plate of cartilage. They support the gills.
The palatoquadrate and mandibular form the first visceral arch bent
upon itself, the hyomandibular and hyoid form the second visceral aich
and the branchials are the third to seventh visceral arches.
Tn the skate the first two visceral arches, mandibular and hyoid, are
only loosely attached to the cranium, but in the higher types a shzd/ is
formed by the fusion of the cranium and these two arches, which latter
form the facza/ portion of the skull.
The vertebral column consists of a row of axial cartilages
from the cranium to the tip of the tail.
The anterior vertebral plate is a long cartilage which
articulates anteriorly with the two occipital condyles and pos-
teriorly with the free vertebre. It has a dorsal (or neural)
ridge and two lateral ridges, and is pierced by the neural canal
for transmission of the spinal cord. Behind the vertebral plate
the vertebral column consists of a series of centra, with a
hollow facet at each end (amphiccelous). Each has a pair of
dorsal xewval processes and lateral transverse processes (bear-
ing small vzbs). These five cartilages are intimately connected
and lie together below the spinal cord. The neural arch
over the cord is completed by xeura/ spines lying directly
dorsal to the centrum and lateral zxterneural plates. In
the caudal (or tail) portion there are added a pair of hemal
processes to each centrum and a femal spine. They may
RATA, 327
possibly be homologous with the transverse processes and
ribs of the trunk-portion, respectively.
2. PERIPHERAL.—(1) The anterior or pectoral element
consists of a pectoral girdle and pectoral fin. The girdle is
a single piece of cartilage which has a ventral ‘“coracoid
bar” across the ventral middle line, and expanded lateral
Fig. 233.—DorsaL VIEW oF PECTORAL GIRDLE AND FIN OF THE
SKATE. (4d zat.)
Fin-rays.
Propterygium,
Coracoid
Fontanelle., | Mesopterygium.
oracoid.
Anterior Facet.
Middle Facet.
Posterior Facet.
Scapula,
Scapular Fontanelle.
Metapterygium. .-
portions. Each of these is pierced by three foramina and
bears three glenoid facets. Dorsal to these a portion, the
Scapular cartilage, is bent towards the middle line and
attached to the vertebral column by ligament. Articulated
328 CHORDATA.
to the facets are the three dasa/ elements of the fin, called
the propterygium, mesopterygium and metapterygium, the
first and last extending forwards and backwards. Each
bears on its outer border a row of numerous fiz-rays which
are jointed.
The posterior elements are the pelvic girdle and pelvic fin.
The girdle consists of a ventral pudic dar at each end of
which is a small dorsal z/iac process, an anterior prepubic
Process and a pair of acetabular facets. To the posterior of
Fig. 234.—DorsaL Vizw oF PELVIC GIRDLE AND FINS OF THE
SKATE. (4d nat.)
Pubic Bar. | Prepubic Process.
First Fin-ray.
Iliac Process. Second and Third
in-rays.
Metapterygium.
Distal Joint of \
Basipterygium.
these is articulated the metapterygium, bearing a number of
Jin-rays. To the anterior facet is attached the thickened
first fin-ray.
In its complete adaptation to an aquatic habitat, with gills
or gill-slits, its paired fins with fin-rays, its two-chambered
respiratory heart, and its sensory ampull, the skate is a type
of its class Pisces.
In its cartilaginous skeleton, its placoid scales, heterocercal
tail, spiral intestine and cloaca, the form of its urogenital
organs and its embryonic development, it is typical of the
order Elasmobranchit.
_ Its dorso-ventrally compressed body and reduced dorsal
fin are typical of the Bazoidel, a group comprising the rays:
and skates. Minor features determine its family and genus.
RATA. 329
Development.—The skate lays its eggs in the autumn and the
young are hatched in early spring.
The eggs are large yellow spheres which break away from the ovary
into the abdominal cavity. Thence they pass into the Fallopian tubes
by their internal openings. The male skate is said'to thrust the claspers
into the cloaca and the base of the oviducts of the female, and to, dis-
charge sperms down the grooves of the claspers into the oviduct. The
sperms then appear to pass up the oviduct and to fertilise the egg
in the Fallopian tube. After fertilisation the egg passes down to the
oviducal gland in which is secreted an enveloping egg-case or ‘‘ purse.”
The eggs contained in these purses are deposited two at a time in
moderately deep water, usually amongst dark seaweed. The ‘‘ purse”
is of a tough consistency and a dark greenish-black colour. It is
flattened and has long processes at the four corners. The ‘‘ purse” has
the edges of its two walls at one end lying loosely against each other,
allowing free egress but making ingress impossible. In this purse the
egg develops slowly, and the young skate on emergence is practically
a diminutive adult. During all this period it is sustained by the maternal
“‘ yolk,” hence the skate has a purely embryonic development and only
a lecithal type of nutrition (see page 427).
Segmentation.—The segmentation is meroblastic, z.¢., the proto-
plasm is largely aggregated to one pole of the large egg, and there segments
or divides into a multicellular disc or cap called the blastoderm. Therest
of the protoplasm with few nuclei is scattered throughout the yolk.
These nuclei divide and are gradually added to the blastoderm during
development. At completion of segmentation the blastoderm has an
outer layer or epithelium of cells which represents the efzd/ast and an
inner mass which, with the rest of the egg, forms the hyfob/ast.
Gastrulation.—One part of the rim of the blastoderm can soon
be distinguished by its greater thickness and is called the embryonic
vim. This represents the future hind end of the embryo, and immedi-
ately below it the blastoderm-cells commence to be invaginated, forming
an archenteron. Hence this rim is comparable to the dorsal edge of
the blastopore in Amphioxus.
Two separate processes now take place contemporaneously. Firstly,
the whole blastoderm commences to envelop the lower yolk-cells by
increase of cells at the rim, partly by cells added from the yolk-mass,
and partly by division of the blastoderm-cells. This enveloping process
does not take place equally all round the edge of the blastoderm or the
last point of meeting would be the lower pole, but the embryonic rim
does not progress over the yolk ; hence the rest of the rim grows over,
and the whole rim gradually closes in immediately behind the blastopore.
If it be recollected that the edge of the blastoderm is the line of
junction of the epiblast and the hypoblast, it is clear that the growth
of the former over the yolk-mass is a modified and retarded form of
archiblastic invagination, which is called epzbolic.
‘The process is so slow that at the same time the embryo becomes
differentiated in the middle: line forwards’ from the embryonte rim.
The nervous system arises along this region as a median dorsal
medullary groove which, by the upgrowth and meeting above of its
edges.or medullary folds, becomes converted into a complete tube.
330 CHORDATA.
The folds meet in the middle of the embryo and anteriorly, but are
open posteriorly till the blastopore is nearly closed; then they meet behind
it and so produce a neurenteric canal. The anterior end of the nerve
tube swells to become the brain and the eyes and parts of the brain
arise as described in the general account for vertebrates (see page
406).
Immediately below this nerve tube the hypoblast cells in the middle
line become differentiated into a notochord, and laterally the hypoblast
also becomes differentiated into a pair of cell-plates which form the
mesoblast. The topographical relationships of the neural tube, the
notochord and mesoblastic plates are therefore much the same as in
Amphioxus, but the last two arise as solid masses of cells, not as hollow
outgrowths. At the embryonic rim the nerve tube, notochord and
mesoblast are all merged into a growing mass of cells.
The embryo then becomes folded off from the rest of the blastoderm
until it is only connected therewith by a small stalk called the yolk-
sac stalk, The whole developing organism is then clearly defined into
the emdryo and its yolk-sac, attached to each other by a short stalk.
The wall of the yolk-sac and the embryo are alike produced from the
blastoderm, and we may make matters clearer at once by explaining
that the yolk-sac is really a huge enlargement of the abdoininal wall of
the embryo. Over its surface there ramify vzted/ine arteries and veins
which serve to absorb nourishment for the embryo.
The mesoblastic plates now grow round ventrally inside the epiblast
to enclose the yolk-mass. They divide into a dorsal portion which
splits up into a series of protovertebre lying on either side of the noto-
chord and a ventral portion which forms the /ateral plate. A split
occurs between the cells of the lateral plate and forms the ccelom,
which is thus schizoccelic. This split extends completely round the
yolk-mass, dividing the mesoblast of the lateral plate into an outer
somatic layer under the epiblast and an inner splanchnic layer resting
on the hypoblast and yolk-mass.
Thus the extra-embryonic part, which we called the yolk-sac, now
consists of an outer layer of epiblast and mesoblast which we may term
the serosa (or serous membrane) and an inner layer of mesoblast and
hypoblast enveloping the yolk and called the yolk-sac proper. These
two embryonic (or foetal) membranes are separated by a cavity (the
extra-embryonic ccelom) which is continuous through the stalk into the
embryonic ccelom.
It is evident that the serosa is merely the much distended body-
wall and the yolk-sac proper a similarly distended part of the gut-wall.
The protovertebre give rise to the vertebra] column and myomere
muscles,
The gill-slits appear at the side of the neck, and from them there
soon protrude a number of long, delicate gill-filaments, the external
gills which are lost before hatching, their bases alone persisting as
the permanent gills.
The organs in general arise much as narrated in the general vertebrate
account (see pages 405-430).
In comparing this development with that of Amphioxus much assist-
ance will be rendered by study of the frog, which in the amount of yolk
GADUS. 331
and the consequent modification in development is in an intermediate
position. The three types will be compared after the frog has been
dealt with (see page 358).
III.—GADUS.
PHYLUM - CHORDATA (p. 402).
SuB-PHYLUM VERTEBRATA (p. 405).
Cass PISCEs (p. 434).
ORDER TELEOSTOMI (p. 437).
The haddock (Gadus eglefinus) is one of the commonest and best
known of our British fishes. It is described here as a type of the order
Teleostomé or bony fishes. The haddock is a smaller fish than the cod
but larger than the whiting; all three belong to the
oe ane large family of Gadide. It frequents the deeper offshore
Habits, :
water and is a ground-feeder upon small Crustacea,
Mollusca and Annelida. The freshly-caught haddock is of a beautiful
colour. The ventral surface is « pearly-white which gradates up
Fig. 235.—LATERAL View oF THE Happock (Gadus aglefinus) x %.
(Ad nat.)
Lateral Line.
i and Dorsal Fin.
J ae 3rd Dorsal Fin. Caudal Fin,
eZ u
ist Dorsal Fin. Zp
“end Anal Fin.
Yes
Operculum. // Won.
\ \
Pectoral Fin. Pelvic Anus. | Aper. ist Anal Fin.
Fin. Genital Aperture.
each side into a metallic violet darkest along the dorsal surface.
Along each side is a thin black line, the /ateral dine, extending from
the head backwards to the tail. Just below the anterior part of this
line there is on each side a black spot of pigment. The eyes are
silvery and black. The whole body is enclothed in an
ae investing coat of delicate overlapping cycloid scales,
* developed in the dermis and carrying no spines. The
skin is extremely slimy, as in the skate. :
At the anterior end of the head is a large gaping mouth armed with
upper and lower rows of teeth. Below the chin is a small sensitive
332 CHORDATA.
papilla called the da7éel. Above the mouth and quite free from it are
two small openings on each side. These are the wares,
Respiratory. each nasal sac having an amtertor and a posterior nas
opening directly to the exterior. There is no external
opening of the ear. At the hind-end of the head there is on each side a
movable plate formed of several bones, called the operculum. If this
be raised it exposes the four pairs of ¢¢//s, consisting of long rows of gill-
filaments, with large clefts between them, leading into the pharynx. In
front of the gills on the first cleft is a vestigial gill, the pseudobranch.
The gills of the haddock appear very different from those of the skate,
but they are developed ina similar manner. In the skate the clefts are
narrow, the filaments short and the body-wall between the clefts broad.
In the haddock the clefts are wide, the filaments long and the inter-
mediate body-wall reduced to a minimum. In addition the gills are
covered over by an operculum.
The skate takes water in at the spiracle and passes it out by the gill-
clefts, but the haddock normally takes water in at the mouth and passes
it out through the gill-clefts, the operculum being opened and shut by
special muscles.
Just behind the operculum_and situated laterally are the large
pectoral fins. Ventrally and slightly forwards are the paired Ze/vic fins.
In many Ze/eostomi the pelvic fins are far back, as in the skate, but in the
Gadide they are often jugu/ar (on the neck) in position, moving forwards
during development. The larval haddock has, in addition to these fins,
a continuous median fin stretching along the dorsal surface round the
tail and forwards to the azzs on the ventral side (cf A/yxzne). In later
life this fin breaks up into three dorsals, a caudal and two anals, by
differential growth and atrophy of the intermediate parts. The tail-fin
is symmetrical, the dorsal and ventral halves being equal, but the end of
body. bends up into the dorsal half, hence the tail is Aomocercal (see Pisces,
p- 435). All the fins have the same structure, consisting of a delicate
double fold of membrane supported on a series of elastic skeletal dermal
Jjin-rays. Just in front of the first anal fin is a small cloacal depression
into which open three apertures. The anterior is the avws, the inter-
mediate the genztul aperture and the posterior the urénary aperture.
If the skin be carefully dissected off one side there can be noticed fine
superficial nerves supplying the lateral line and the fins. They arise
mainly from the Vth and Xth cranial nerves. Below these the whole
lateral wall of the body is formed of diagonal myomere
Muscular. muscles, separated by connective-tissue myocommata (cf.
Amphioxus). From a little way behind the anus the rest
of the body backwards, usually known as the /az/, is composed almost
entirely of these myomere muscles. Their alternate contractions serve
to move the ‘‘tail” and caudal fin and thus propel the body. This
method of locomotion is similar to that of 4mphioxus and is also found
in many Zlasmobranchii : the skate itself has adopted a different method
of progression by the pectoral fins, which in the haddock merely act as
balancing, steering and stopping organs. __
The pertvisceral cavity may now be opened up by 2 median ventral
incision from chin to anus. The cavity is completely divided into two
parts, the anterior fericardial cavity and the' posterior abdominal cavity.
GADUS. 333
The heart lies in the former and the alimentary canal and other organs
in the latter. The somatic layer of peritoneum is deeply
Alimentary. pigmented, forming a black wall to the cavity, whilst the
splanchnic forms a glistening transparent membrane sur-
rounding the alimentary canal and forming a dorsal mesentery.
The teeth have already been mentioned : they are borne on the pre-
maxilla and dentary and a small inner patch on the vomer (cf Frog).
If the jaws be pulled open and the pharynx examined, the five lateral
gill-clefts can be noticed and in addition a large paired patch of teeth on
its dorsal surface, borne on the superior pharyngeal bones or upper
bones of the branchial arch. Inimediately ventral to these are a pair
of patches of teeth on the inferior pharyngeal bones, representing the
Fig. 236.—DIssECTION OF HADDOCK FROM THE LEFT SIDE. (4d ma?.)
Closed Duct of Air-bladder.
Kidney.
Liver. Body-wall.
Air-bladder.
Kidney.
Urinary
| Bladder.
Ureter.
Gills,
Heart, | |. Anus, Genital Aperture.
e | Gonad.
(Esophagus. Rectum.
Gall- . “a vi
bladder. Pyloric Stomach. Liver. Spleen. -
Ceca. :
The left abdominal wall has been cut and thrown back dorsally, and the intestine and
stomach have been pulled out ventrally. 7
fifth branchial arches. These teeth, working upon each other, form a re-
markable subsidiary pair of jaws for propelling food down the esophagus:
into the stomach. In all cases the teeth are merely Aaplodant, z.e:, they:
are sharp conical points which sieze prey but are not used for mastication,
The stomach is large and bent onitself. It is continued into a duodenum,
at the commencement of which there opens a number of long czecal tubes
called the pyloric ceca. They are said to secrete a digestive juice and.
have been compared tentatively to a pancreas. The /ver is a large bi-;
lobed organ with a gall-bladder from ‘which thére passes a single bile-’
duct to open into the duodenum. The duodenum is continued into the
ileum which is long and coiled and terminates in the anus. Its hind
334 CHORDATA.
portion is sometimes distinguished as the rectum. The lumen is simple
and has no spiral valve.
From the dorsal wall of the cesophagus there is produced a solid~
cord of connective tissue, which is connected at its distal end with a
large and spacious az7-bladder lying immediately above the abdominal
cavity. It is filled with gases and its walls have a dense vascular supply.
This air-bladder is used as a hydrostatic apparatus and is not found in
demersal fish (those habitually frequenting the bottom). In many, e¢.,
the herring, the connecting cord is a duct putting its cavity into communi-
cation with that of the cesophagus. It always arises in the young as a
diverticulum of the alimentary canal.
In the mesentery above the ileum is a small red sp/eex. Dorsally
to the abdominal cavity and to the air-bladder lies a pair of elongated
kidneys of a dark-red colour. They are thin in the
Excretory. region above the air-bladder, but swell out anteriorly
immediately behind the head into large bulbous organs,
and also posteriorly where they give off an unpaired zreter passing down
to the zrzzary aperture. It swells into a small urinary bladder near
the aperture. These kidneys are said to be mesonephric in origin.
_ The heart is smaller in proportion than in the skate. It has two
chambers, an azricle and a ventricle. The former is fed from a thin-
BI walled szzzas venosus, and the latter leads forwards as the
000- branchial artery. There is no valvul teri
Vascular, 27@”chtal artery. ere is no valvular conus arteriosus,
as in the skate, its vestige being seen in a single pair of
valves ; there is a swollen base to the branchial artery sometimes dis-
tinguished as the dzdbus arteriosus. ‘The branchial artery gives off four
paired afferent branchials to the gills which give fine branches to the
gill-filaments. The blood after eration is collected by four pairs of
efferent branchzals in the roof of the mouth, which are difficult to follow.
The efferent branchials of each side unite to form a vessel often termed
the epibranchial artery. Anteriorly each epibranchial is continued
forwards to meet its fellow across the base of the skull, completing
the so-called cephalic cercle. Each gives off a carotid to the head.
Posteriorly each epibranchial converges towards the middle line, and
gives off a subclavian artery to the Zectoral fin. They then unite to
form the dorsal aorta, which runs backwards immediately below the
vertebral column. It can be seen between the kidneys on removal of
the air-bladder. Posterior to the abdominal cavity it divides into the
caudal artery, supplying the tail-muscles and the veszczar artery to the
urinary bladder and anal fin. The dorsal aorta gives off numerous
renals to the kidneys throughout its course. From the right epi-
branchial anterior to the origin of the subclavian there arises a pair of
median w7sceral arteries. The anterior of these supplies the pyloric
ceca, and the posterior, sometimes known as the ce/éaco-mesenteric, gives
branches to the stomach, intestine, air-bladder, spleen and gonads.
The venous system is difficult to follow except in injected specimens.
It consists of paired Zrecavals leading out from the sinus venosus,
which give off jugudars forwards and cardinals backwards. There are no
lateral veins. The cardinals run in the kidneys and receive numerous
renal veins. The caudal vein is large and runs forwards immediately
below the caudal artery. At the level of the posterior portion of the
GADUS., 335
kidneys it divides into two renal portals, as in the skate ; the left renal
portal breaks up into capillaries in the left kidney, but the right is
usually continuous forwards with the right cardinal vein. A branch
from the caudal vein, the ves¢cudar vein, runs ventralwards and joins
the mesenteric branch of the Zorta/ vein.
The portal system is well developed and consists of a mesenteric
vein from the intestine, a splenic from the spleen and a branch from
the air-bladder leading to the liver. The blood from the liver is carried
by paired Aefazzc veins into the sinus venosus.
Fig. 237.—LATERAL ViEw or Cop’s SKULL.
4., Branchiostegal Rays. 2,, Lacrymal. .0., Preoperculum.
c.4., Ceratohyal. mm, Maxilla. g-, Quadrate.
d, Dentary. 2., Nasal. 5.0., Supraoccipital.
#., Frontal. o, Operculum. s.0., Suboperculum
4.m., Hyomandibular. p.s., Parasphenoid., (lower reference).
z.0., Interoperculum. p.m., Premaxilla.
The vascular system shows a peculiar asymmetry of both the arterial
and venous systems, and a marked tendency to anastomosis of certain
outlying vessels, seen also in the bird. Both the veins and arteries are
remarkably small compared with the size of the fish, and there is a very
small quantity of blood.
Skeletal The skeleton of the cod is in marked contrast to that
"of the skate in consisting almost entirely of bone.
AXxIAL.—The [skull is formed of a cranium, mainly bone, and a
series of bony visceral arches. The cranium is formed of (1) an occipital
336 CHORDATA.
ring posteriorly, consisting of a supraoccipital, paired exoccipitals and
a bastoccipital ; (2) the offc bones surrounding the auditory capsule
and lying. immediately in front of the occipitals. There are five otic
bones—frootic, epiotic, opisthotec, pterotic and sphenotic. On the two
last is a large facet for the hyomandibular bone. (3) At the front end of
the cranium are a series of bones arising in connection with the nasal
capsules, the dorsal zasa/s, median mesethmozd and lateral ectethmords..
(4) Between these and the occipital and auditory region the dorsal surface
of the cranium is completed by small farzeza/s in front of the supra-
occipital and large fronta/s covering the orbits. In front of the otic
bones there is a pair of small al/sphenozds in the orbit. (5) The basal
axis of the cranium is formed, anteriorly to the basioccipital, by the
long farasphenoid and vomer, the latter bearing teeth and lying below
Fig. 238.—THE RicHT PecroraL FIN AND GIRDLE OF THE CoD
WITH BOTH PELVIC FINs. (Ad xat.)
A Post-temporal.
Supraclavicle.
Scapula.
Clavicle.
Postclavicle. . Coracoid,
Basipterygium, _,
phores.
Dermal Fin-rays.
Distal Pterygiophores.
Proximal Pterygiophores
(Brachial Ossicles).
Pterygio-
Fin-rays.
the mésethmoid. . (6) Lastly, in connection with the orbit is a chain of
orbital bones, of which the anterior and largest is known as the dacrymal.
To this cranium are loosely attached a number of bones belonging
to visceral arches. -Anteriorly are paired premaxzl/@ bearing teeth, and
maxille without teeth. These are supposed to be connected with the
labial cartilages of the skate.
The first or mandibular arch ossifies into the pa/atines attached to
the ectethmoids, the pterygozds with meso- and metapterygoids, and the
guadrates which form the upper half (or palatoquadrate chain) and the
articular, angular and dentary forming the lower half or mandible.
An articulation is formed between the articular and the quadrate. _
The second or hyoid arch consists in its upper half of a hyomandi-
bulax attached to the otic region, bearing four opercular bones on its
GADUS. 337
posterior border (the preopercular, opercular, subopercular and inter-
opercular), and joined to the quadrate by a small symplectic. Its lower
half forms a chain of Ayo¢d bones which carry on their posterior surface
seven branchtostegal rays.
The four branchial arches consist of pharyngo-, ept-, cerato- and
hypobranchials, united below by the daszbranchials. The pharyngo-
branchials fuse to form the superior pharyngeal bones already noticed,
and the Ceratobranchials of the fifth arch form the inferior pharyngeal
bones.
The vertebral column consists of a large number of amphicalous
vertebre. The anterior are termed addomznal and the posterior are
caudal, All the vertebree have complete neural arches and neural
spines. Most of the abdominal have also transverse processes, which
bear » pair of xzds and a pair of more dorsally placed so-called zxder-
muscular bones. In the caudal vertebrae the transverse processes meet
below and form a complete Aema/ arch. The median fins are sup-
ported on dermal fin-rays, which rest on short pterygdophores and inter-
spinous bones.
APPENDICULAR.—The pectoral girdle is attached to the otic region of
the skull by the supratemporal bone. There are three clavicular bones,
the supraclavicle, clavicle and postclavicle. A small scapula and
coracoid complete the girdle ; they bear on their posterior border four
small brachial ossicles (or pterygiophores), which in their turn bear the
numerous ectoral fin-rays. The pelvic girdle is absent, but there is a
large basipterygium on each side which carries the pelvic fin-rays.
The brain is small and differs from that of the skate,
Nervous chiefly in the large optic Zobes and small cerebral hemt-
System, spheres. On the other hand, the cerebellum is equally
well developed.
The gonads are simple, paired hollow sacs opening
Reproductive.by short genital ducts to the exterior. They lie in the
abdominal cavity.
There is as great a contrast to the skate in the development as in the
anatomy. The haddock lays several million eggs which are of small
size, perfectly transparent and buoyant. Fertilisation is
Development, external and the eggs are pelagic. There is a consider-
able amount of yolk and segmentation is meroblastic.
The young haddock is hatched as a transparent larva, with a large yolk-
sac depending from its ventral surface. After a time the young fish
absorbs its yolk and feeds on pelagic organisms ; still later it takes to a
ground-feeding habit. :
The haddock is a type of the order 7¢/costom# or bony-fishes, which is
usually contained in the class Pisces, with the lasmobranchit and
some smaller orders. It is, however, evident that the two types are
widely divergent in numerous structural characters.
338 CHORDATA.
CHAPTER XXI.
CHORDATA—( Continued.)
IV.—RANA.
PHYLUM CHORDATA (p. 402).
SuB-PHYLUM VERTEBRATA (p. 405).
Cass AMPHIBIA (p. 439).
Fig. 239.—THE CoMMON FROG (Rana temporaria).
(Natural Size.)
cago
aoe OSE ag tie
Note the large mouth, and tympanum behind the eye, long hind-limbs with webbed
toes and pigmented skin.
Rana temporaria is the common British frog of
universally familiar appearance. A slightly larger form,
Rana esculenta, or the Edible Frog, common upon the
Continent, is often preferred for dissection, but the
description here given will suffice for either species.
The frog is a water-loving terrestrial animal. In loco-
motion it is equally at home in water or on land. In the
early morning and early evening, when dew and damp are
frequent, it becomes active in the pursuit of insects, worms
(4d nal.)
Plate IV.—Firsr DissEcTION oF FRoc.
Mylo-hyoid
—
~Pectoralis
|__- Muscles.
noc FBS
Toa fT pee,
|) (== S en
suinjdag
‘aposnyy, stjvulUOpqY sn99 yy
|
“UIDA [BIYORIG | ‘aposnyAT snutayxq sunbyqg
“UID A SnosUuvyns-o/Nasu FAT
The skin is cut by a median ventral incision and pinned back. On the right the
pectoralis muscle is mostly removed to show the course of the subclavian vein
dividing into brachial and musculo cutaneous
RANA. 339
and other small animals, but retires through the day into
water or rocky holes. In the winter the frog hibernates in
pond-mud or in holes.
The Head is set upon the ¢runhk with no neck and the
latter carries two conspicuous pairs of limbs. ‘The mouth,
when open, is a wide gaping fissure, literally
extending ‘‘from ear to ear.” At the tip of the
“nose” is a pair of small external nares leading
into the olfactory or nasal sacs. Further back are the eyes,
and a little behind and below them are the tympanic mem-
branes of the ears or auditory organs. These are covered
with skin and appear as round surfaces. The front limbs
have four digits, the thumb being absent. The ind limbs
have five long toes or digits with a web stretched between
them. The male Rana temporaria in the breeding season
has a thickened callosity on the first digit of each fore-limb.
Dorsal to the junction of the hind limbs and the trunk is a
single cloacal aperture.
The whole body is enclothed in a loose moist skin, with
an entire absence of scales, hairs, or other exoskeleton.
There are abundant skin-glands’ which serve
to keep the skin moist. Under the skin are
numerous blood-vessels which enable the skin
to assist in the function of respiration.
The skin has scattered pigment of various colours, and
the frog has the power to adapt its general coloration to its
surroundings fairly rapidly. If the jaws be widely opened
the duccal cavity is exposed. The fongue is forked, free
behind and fastened at the front end; it can be shot out
with great rapidity for catching insects.*' The lower jaw has
no teeth, but a row of delicate teeth lines the upper. In
addition there-are in the roof of the mouth two patches of
small vomerine teeth, so-called because they are on the
vomer bones. ;
Just behind these teeth are paired zzdernal nares leading
to the exterior by the nasal sacs and the external nares.
They serve for the introduction of air. Further back, near
the angles of the jaw, is found on each side a widely-open
passage, the Eustachian tube, leading almost at once into the
External
Features.
Integu-
mentary.
* The tongue is protruded by the pressure of lymph forced into its interior by the
contraction of muscles, such as the mylohyoid.
340 CHORDATA.
tympanic cavity. Further back still, on the ventral surface,
is a small median longitudinal slit, called the g/o/t7s, leading
into the lungs. Lastly, the wide wsophagus leads down to
the stomach.
If the skin be cut open along the mid-ventral line from
chin to cloaca it will be noticed that its looseness is due to
a large subcutaneous lymph-space which forms a sort of lymph-
jacket between the skin and the body-muscles (see Plate IV.).
Emerging from the region of the “armpit” can be seen a
large vein, the subclavian, dividing into a brachial coming
down from the fore-limb, and a large musculo-cutaneous,
which arises by a mass of small veins covering the inner
surface of the skin. This vein brings zrated blood back
from the skin to the heart.
Extending across from one mandible to the other is a
peculiar loose muscle, the mylohyoid. Further back the
sternum may be felt in the mid-ventral line, from the hind
end of which to the pelvis there runs a muscular band, the
rectus abdominalis. Inthe middle line of this muscle can
be seen a dark line caused by the underlying anterior
abdominal vein.
A median incision can now be made from chin to pelvis
through the mylohyoid muscle, the sternum and the rectus
muscle (to one side of the anterior abdominal
vein). The body-cavity thus opened up has much
the same relationship as that of the skate (see Plate V.).
The abdominal cavity extends forwards to the level of the
cesophagus and backwards to the pelvis. The much
smaller pericardial cavity surrounds the heart and is. com-
pletely separated from the abdominal cavity. As in the
skate, the organs are suspended in folds of peritoneum
which form dorsal mesenteries.
The cesophagus enters the abdominal cavity anteriorly,
and soon swells into a stomach towards the left side. It is
dinkeauary covered by a large two-lobed “ver with a
‘ roundish gal/bladder. The stomach leads
into a duodenum into which there falls a bile-duct leading
down from the gall-bladder. Around the bile-duct is a
branched whitish gland, the pancreas, which opens by ducts
into it. The rest of the small intestine, called the z/ewm, is long,
of small calibre and coiled. It passes into a wide but short
Colom.
Plate V.—SEconD DIssECTION OF THE Froc. (4d nat.)
Hypoglossal Nerve.
Lingual Vein.
Lingual Artery, peg" , Mandibular.
. Internal
uy
Jugular,
Thymus_
fp)
\ ff -. -- Brachial Vein.
y — ----Sub-scapular.
tae fj ae s A 4 ‘a _ ..Musculo-
cutaneous.
Ant. Wall of Pe:
7 visceral Cavit
~~~... Rt. Lobe of Liver
of ¥ is if tei : *
iin , per |, —eall-biadder.
( A=. y __ Stomach (below i
Right Lobe of Liver.__
lies the Pancrea
and Bile-duct).
wee ae Q ba aha = _. _ Anterior Abdominal
Teum-~ a Vein:
: : ____ Urinary Bladder.
Spleen.
Rectum”
The ventral body wall is cut open by a median incision from chin to vent through the
sternum. The pectoral girdle is completely removed and the body wall pinned out. The
mylo-hyoid muscle is removed on the left and the anterior venous system is removed on the
right. | The branches of the portal vein can be identified by the organs to which they run.
The veins are blue, arteries red.
RANA. 341
rectum which opens into the cloaca. From its ventral wall,
close to the cloacal aperture, is a large bilobed w7znary
bladder.
Close to the pancreas, and near the head of the rectum,
is a round reddish sf/een, one of the ductless glands..
At the extreme front end of the abdominal ‘cavity there
lies dorsally a pair of Zzmgs. Each rests loosely in the cavity,
but is attached anteriorly to the cesophagus.
If the lungs be inflated by a blowpipe through
the glottis they will be seen to consist of hollow sacs of
great elasticity. When punctured they return to their former
small bulk and soft condition.
Respiratory.
Fig. 240.—DIAGRAM OF VENOUS SYSTEM OF A FROG.
(Ventral view.)
External Jugular.
Lingual, Sinus Venosus,
Mandibular. | | Precaval.
Internal Jugular. q \y :
* Subscapular. Subclavian.
Innominate.
Musculo-
cutaneous.
Portal.
Gastric,
Mesenteric.
Branches.
Pelvic. Renal Portal.
Renal Portal.
Sciatic.
Femoral.
, 6
The anterior abdominal is unlabelled, but is seen running forwards from the
two pelvics to the portal.
The frog fills its buccal cavity with air through the nares,
and then pushes upwards the floor of the cavity with its
hyoid cartilage (or lingual plate). This forces the air down
to the lungs and effects izspiration. The-air is expired by
the elastic walls of the lungs.* We may notice. that the
* The expiration may be assisted by contraction of the abdominal muscles upon
the viscera.
342 CHORDATA.
lungs and the urinary bladder belong morphologically but
not physiologically to the alimentary system.
The Zeart lies ventrally to the cesophagus, enveloped in
its pericardium. Its structure will be con-
sidered later. The veins lie superficially to
the arteries and consist of an anterior and
a posterior system. The anterior system is paired; the
posterior is in great part single.
Anteriorly a small “gual vein from the tongue is seen
to unite with a mandibular from the lower jaw to form the
external jugular. This runs backwards to join with the
large subclavian already seen, which runs along the anterior
wall of the abdominal cavity. As already noticed, the sub-
clavian is made up of the dvachial and the musculo-cutaneous.
The area between external jugular and subclavian is drained
by a small but deep vein, the zz#ominate, which is formed
of the znternal jugular emerging from the brain and the sué-
scapular from the dorsal region. The innominate joins the
external jugular and subclavian, the three uniting to form
the precaval vein, which passes backwards and inwards to
fall into the s¢zus venosus dorsal to the heart.
In the posterior system the portal vein can be seen
coming from the stomach, spleen, pancreas and duodenum,
and falling into the liver; it constitutes the Aepatic-portal
system. Just before it enters the liver it receives the
anterior abdominal vein-already noted.
If the alimentary canal be now carefully removed by
cutting through the rectum and through the cesophagus,
the £idneys are exposed and the rest of the venous system
is clearly distinguished (see Plate VI.). The /emora/veins are
large veins leading up from the legs. Before entering the ab-
dominal cavity each divides into a pelvic and a renal portal.
The former comes up to meet its fellow and the two form the
anterior abdominal to the liver. The renal portal proceeds
forwards, receives a sciatic from the inner side of the leg, and
breaks up along the outer border of the kidney; hence the
frog has a well-developed renal-portal system as well as
a hepatic-portal. The blood from the large hind-limbs
must pass either through the kidney by the renal portal, or
‘through the liver by the anterior abdominal before reaching
the heart. Between the kidneys is a large postcaval which
Blood-Vascu-
lar. Venous,
Plate VI.—THIRD DISSECTION OF FROG.
Internal Jugular.
Sub-scapula
Post-caval Vein :
Testis with Vasa
Efferentia.
Adrenal Body.
Renal Vein.. v es s ; 3 Kidney.
Dorso-lumbar.
-Renal Portal.
Urinary Bladder. XG
Openings of Ureters
into Cloaca
Sciatic.
Femoral.
Pelvic. Anterior Abdominal.
i, rhe alimentary canal is cut away together with liver, pancreas and right lung.
The heart is thrown forwards and the left lung pulled outwards. The cloaca is slit
open. The veins are all coloured blue and the arteries red.
RANA. 343
receives blood from the kidneys by vewa/s and from the
genital organs. It then passes forwards through the liver,
which it drains by paired Aepatics, and discharges itself
into the sinus venosus.
The two lungs have separate pulmonary veins which fall
together into the left auricle of the heart.
In the frog there are no paired cardinal veins* as in the
skate, their function being executed by the unpaired post-
caval. On the other hand, the presence of a renal-portal
system is a feature of both types.
Fig. 241.—VENTRAL VIEW OF THE FEMALE UROGENITAL
ORGANS OF A Froc. = (dd nat.)
Internal Opening of
Oviduct.
Oviduct.
Corpus
Adiposum.
Ovary.
Adrenal.
“(Jterine” Portion
of Oviduct.
Aperture of Ureter. ,
Aperture of Oviduct.
Ureter.
Cloaca.
The cloaca is slit open to show openings of ureters and oviducts.
The &zdneys are long, red bodies lying in the dorsal
wall of the abdominal cavity. Each has a thin, yellow,
adrenal body on its ventral face. <A ureter
Urogenital. eaves the outer posterior border of each kidney
to open separately into the cloaca just dorsal to the opening
of the urinary bladder already noticed.
In the male the estes are oval, light-yellow bodies lying
ventral to the anterior part of thekidneys and attached to them
by peritoneum. A number of fine tubules, the vasa effer-
entia, pass from the testes into the kidney, through which they
eventually communicate with the ureters. These, therefore,
* They are present in the Urodela.
344. CHORDATA.
function as vasa deferentia and have a small prostate gland
attached to them.* To the front end of the testes are
attached a number of branching /at-bodies (or corpora adt-
posa). In the female the ovaries are large dark organs,
suspended by dorsal mesenteries near the kidneys. The
oviducts are long, coiled, paired tubes running throughout
the length of the abdominal cavity. They open behind -into
Fig. 242,DIAGRAM OF ARTERIAL SYSTEM OF A FROG.
(Ventral view.)
Lingual.
aN Carotid Gland.
Carotid.
Cutaneous.
Cutaneous.
Brachial. Pulmonary.
Systemic Arch.
Arteriosus.
Cceliaco-
mesenteric.
Dorsal Aorta. D— Ceeliac.
Kidney.
the cloaca, in front into the abdominal cavity by separate
funnel-shaped apertures just in front of the lungs. Their
lower portions are wide and saccular and are sometimes
called the uterine portions of the oviduct. The walls of the
upper portion are glandular and secrete albumen. The eggs,
when ripe, are discharged into the abdominal cavity and pass
* In Rana esculenta the prostate gland is absent, but the ureters are swollen for
a part of their course.
RANA. 348
down the oviducts, where they receive a coat of albumen
and accumulate in the uterine part.
Bisoavaced If the urogenital organs and the anterior
lar. Arterial, Venous system be now carefully removed the
arterial system can be completely exposed.
The Zeart is three-chambered, consisting of a ventricle
and two auricles. The r7ght auricle receives venous blood
from the sinus venosus and the /eft auricle receives arterial
blood. from the pulmonary veins. Both auricles, on con-
traction, drive their contents through valves into the
ventricle. From the ventricle there runs forwards between
the auricles a ¢runcus arteriosus which first diverges into two,
Fig. 243.—DIAGRAM OF THE TRUNCUS ARTERIOSUS
OF A Froc’s HEART.
2 _» Carotid.
AE Systemic.
A Pulmonary.
*S
and each of these divides into three, arterial arches. The
anterior, called the carotid arch, passes up to a swollen
carotid gland and divides into a “ngual and carotid artery
to the head. The second or systemic turns backwards,
gives off a brachial artery to the forelimb, and meets its
fellow dorsally to the liver to form the single dorsal aorta.
The dorsal aorta gives off a large celiaco-mesenteric to the
liver, stomach and other viscera, veza/s to the kidneys, and
eventually divides into two zH#acs to the hind-legs. The
third arch, or pudmocutaneous, divides into cutaneous to the
skin and pu/monary to the lungs.
The ¢runcus arteriosus has a long valve running up its lower part
which is arranged in such a way that certain portions of the blood pass
up certain arches. The auricles discharge venous and arterial blood
respectively into the ventricle, and in the ordinary way these would
346 CHORDATA.
completely mix and every organ would on contraction of the ventricle
be supplied with mixed blood. On the other hand, greater efficiency
would be attained if the arterial blood could be sent to the tissues
generally and venous blood to the lungs, and this is practically the
case. The ventricle contracts rapidly after the auricles, before the
blood from the latter has had time to mix, and hence the first portion of
the blood leaving the ventricle is nearly all venous, because the opening
of the truncus inclines to the right. This passes up the wide passage
to the pulmonary arches, and only when these are comparatively full
does the next portion of mixed blood diverge up the smaller aperture
to the top of the truncus arteriosus. Here it passes up the wide open-
ings of the two systemic arches, whilst only the last and most arterial
portion reaches the small aperture to the carotids, ensuring a supply of
pure blood to the brain.
Fig. 244.—DorsaL View OF BRAIN OF FROG.
— Olfactory Nerve.
Olfactory Lobe.
Cerebral
Hemisphere.
Pineal Stalk. Optic Thalami.
Optic Lobes.
Cerebellum.
4th Ventricle. Medulla
Oblongata.
The spinal nerves are clearly seen lying in the dorsal wall
of the abdominal cavity. The first spinal, called the Aypo-
glossal, lies ventrally to the tongue, and can be
seen on removal of the mylohyoid muscle. It
joins the spinal cord between the first two vertebre.
Nervous.
Plate VII.—FourTH DIssEcTION OF THE FROG. (Ad nat.)
Hypoglossal.
\~ Glosso-pharyngeal.
BA Lingual.
i\ Carotid Gland.
Brachial
Spinal 2." e ‘ ae Cutaneous.
Plexus
Spinal 3.
Ceeliac.
c Sympathetic.
Spinal 4.
Spinal 5 ~ |
‘Dorsal Aorta,
Spinal 6.- 4
ss \
“Spinal 7.
Sciatic at
Plexus} Spinal 8.-~ a
Spinal 9.
Spinal 10. x P
Showing IXth and Xth cranial nerves, the spinal nerves, sympathetic nerves,
and arterial system. After the third dissection, the kidneys, veins, and liver are
carefully removed and the heart is reflected over to the right. The sympathetic and
vagus are only shown on the left. The latter is unlabelled but is seen emerging from
between the glossopharyngeal and the hypoglossal and passing to the heart and lung,
another branch passing behind the lung down the cesophagus.
RANA. 347
The ‘second and third unite to form a brachial plexus to
the fore-limb ; the fourth, fifth and sixth pass to the body-
muscles ; the seventh, eighth and ninth unite to form the
sciatic plexus continued into the hind-limb. The tenth is a
small spinal beside the wrosty/e. On either side of the aorta
is a thin pigmented nerve-chain with ganglia, called the syz-
pathetic system. From each ganglion a connection passes
to each spinal nerve. Forward, the sympathetic chain ter-
minates in the Gasserian ganglion of the fifth cranial nerve.
The ten cranial nerves are essentially like those of the
skate, though smaller and more difficult to follow, and the
fifth, seventh and tenth nerves are much simpler. The fifth
has three main branches—the ophthalmicus, the maxillary
and mandibular. The sevénth has only two main branches
—the palatine and the hyomandibular. The vagus has no
branchial branches, but supplies the larynx, lungs, heart
and stomach.
At the sides of the vertebree are a number of masses of calcareous
matter called cakareous bodies. They have a curious developmental
connection with the ear.
‘The brain may be seen by removing the dorsal bones of
the cranium. It is small and has a very small cerebellum.
The various parts are in one horizontal axis and do not
overlap each other.
The spinal cord passes down the vertebral column, as in
the skate, and terminates in the uvostyle.
The frog has no exoskeleton. The endoskeleton can,
as in the skate, be divided into axial and peripheral parts.
Skel The axial is composed of a skull and vertebral
etal. : 3
column. The skull is composed of the cranium
and the first two visceral arches, mostly joined together.
The first important difference from the skull of the skate
is the presence of bones in addition to the cartilaginous por-
tion. Some of these bones are formed in dermal membrane
and sink on to the cranium; these are called membrane-
bones. The others are formed in the cartilage, or rather
they replace the cartilage which is destroyed as they grow.
These are termed carttlage-bones. The cartilage may be seen
extending between the bones, or the membrane-bones may
be removed, in which case the true extent of the cartilaginous
cranium can be clearly seen. The actual cranium is small
348 CHORDATA.
and lies between the two orbits. Its front end is surrounded
by a girdle bone, the sphenethmoid. Its walls are formed of
cartilage, in the roof of which are a large anterior fontanelle
Fig. 245.—VENTRAL VIEW OF Froc’s SKULL.
Cartilage black, Bones white.
Vomer. Premaxilla.
Maxilla.
Sphenethmoid,
Parasphenoid. .
Pterygoid.
Quadrate.
Prootic. Exoccipital. Columella.
and a small pair of posterior fontanelles, as in the skate.
The fontanelles are not seen, as they are covered up by a
pair of large membrane-bones, the frontoparietals. ying
Fig. 246.—DorsAL View oF Froc’s SKULL.
Cartilage black, Bones white.
Premaxilla.
Maxilla. External Nas.
Nasal.
2 Sphenethmoid.
Pt d. °P!
er Frontoparietal.
S iy
sree Quadratojugal.
RAS %
Quadrate.
Exoccipital. Prootic.
under the cranium is a long dagger-shaped bone, the para-
sphenoid. At the hind-end of the cranium is a pair of bones,
the exoccipitals, each of which bears an occipital condyle.
Anteriorly the cranial cartilage is continuous with the
RANA, 349
cartilaginous nasal capsules and posteriorly with the audi-
tory capsules. At the front end of the latter are the prootic
bones and on the nasal capsules are the zasa/s. On the
ventral face of the nasal capsules are the vomers.
In the prepared cartilaginous skull, a large cartilaginous
bar (the swdorbital bar) can be seen running backwards
from the nasal region outside the eyes to meet a similar bar
projecting from the auditory region. Here it protrudes out-
wards as the guadrate cartilage which bears the mandible.
The whole represents the palatoquadrate bar of the skate.
In the natural condition it is covered up by a number of
Fig. 247.—DorsaL VIEW OF ENTIRE FRoG’s SKELETON.
(Natural Size.) (Ad zat.)
|
Wh ip Phalanges.
" Metatarsals.
Proximal tarsals,
350 CHORDATA.
bones which thus belong to the visceral arches. In con-
nection with these visceral arches are the premaxilie and
maxille forming the upper jaws and carrying a single row
of small teeth. From the hind-end of the maxilla is a small
bone (the guadratojugal) which unites behind with the
quadrate.
In connection with the suborbital bar are paired fa/a-
tines anteriorly and paired pzerygoids of a triradiate shape.
On the dorsal side a pair of T-shaped sgwamosals overlie the
quadrate cartilages. A rod-like cartilage runs from the tym-
panum to the auditory capsule; it is called the columella
and probably corresponds to the hyomandibular of the skate.
The lower jaw, as in the skate, is formed of the mandibular
cartilage, but it has also three bones. It bears no teeth.
A large plate of cartilage, the Zngual plate (or hyoid
cartilage), rests below the tongue ; it has two long anterior
cornua, which are attached to the auditory capsule, and the
posterior cornua which are shorter. It is the Ayoid cartilage,
with perhaps a single pair of branchial arches (fosterior
cornua).
At first sight there is little in common between this skull and that of
the skate. If, however, we carefully follow the following modifications
which have probably taken place, the comparison is easier. Let us
suppose that the palatoquadrate cartilages of the skate become fused on
to the nasal region anteriorly and to the auditory region posteriorly, and
that further these cartilages are bent out laterally so that they lie no
longer under the cranium but round the outer border of the eyes. A
condition is thus produced closely similar to the cartilaginous cranium
of the frog. The cartilage is then replaced by bone in parts, producing
the CARTILAGE-BONES, sphenethmoid, prootics and exoccipitals. Lastly,
this skull is covered up by a number of MEMBRANE-BONES, paired /ronto-
parietals and nasals above, parasphenoid and vomers below, and a
number of others, palatines, pterygoids,.squamosals, quadratojugals,
premaxille and maxzlle, in connection with the visceral arches.
The vertebral column in the frog consists of nine free
vertebree and a uvostyle. The vertebre are ossified and
form rings. The first or atlas is a simple ring with two
facets for articulation with the skull. The second to seventh
are proccelous, ze., they articulate with each other by a
concavity in front and a convexity behind. The main
portion of the vertebra is called a centrum and above this is
a neural arch covering over the spinal cord. A large lateral
process on each side consists of a ¢vansverse process which
RANA, 351
bears a small cartilaginous 74. The eighth is like the
preceding in general structure but is amphicalous, t.e., it has
a concave articular surface ateach end. The xnth vertebra
has large transverse processes to which is attached the pelvis.
Hence this vertebra is called the sacrum. It is biconvex,
with a convexity at each end of the centrum. The uvostyle
is a long bone formed of at least three fused vertebrae. It is
hollow for part of its length and contains the posterior end
of the spinal cord.
Fig. 248.—PrecroraL GIRDLE oF Rana.
Episternum.
Clavicle. Suprascapula.
Omosternum.
Coracoid.
Sternum.
Xiphisternum.
View with dorsal parts bent downwards. Bone is black and cartilage dotted.
The presence of this urostyle, the single sacral vertebra
and the small number of vertebre are the important
peculiarities of the vertebral column. The vestigial ribs
are also to be noted.
Fig. 249.—FORE-LIMB OF Rana.
Phalanges of
Digits.
Metacarpals.
Note fusion of radius and ulna and absence of pollex, a metacarpal only remaining.
The peripheral (or appendicular) skeleton consists of
the two limb-girdles and limbs. These are constructed on
the pentadactyle type (see page 420). The shoulder-girdle
(pectoral girdle) consists of paired clavicles and coracoids
352
Fig. 250.—PELVIC
llium.
CHORDATA.
GIRDLE OF RANA.
(Lateral view.)
Fig. 251.—HINb-LimB oF Rana.
({ Phalanges of
Digits.
Metatarsals,
Prehallux.
Calcaneum.
Astragalus.
Tibiofibula.
Note elongation of tarsal bones, fusion of
tibia and fibula and presence of six digits.
Pubis.
which meet in the mid-ventral
line and there bear an omo-
sternum in front and a xiphi-
sternum behind. They meet
laterally with the scapula or
dorsal element and form the
glenoid cavity for articulation
of the limb. The scapula
carries on its dorsal border a
suprascapular cartilage. The
fore-limb has two peculiarities.
The radius and ulna are united
into one bone and there is no
pre-axial digit or thumb.
The pelvic girdle has a very
long forwardly-directed chum
articulating with the sacral
vertebra. The pubes and
ischia are welded into a disc-
shaped mass with a concavity
on each side, the acetabulum.
The frog’s pelvic girdle is
peculiar in the great length
of the ilium and the solid
nature of the pelvis. The
hind-limb is greatly elongated.
As in the front-limb, the “a
and fibula are fused. In
addition, the two proximal
tarsal bones, the astragalus
and calcaneum, are elongated
into long bones. On _ the
RANA. 353
inner side of the first digit is a rudimentary sixth digit, the
prehallux.
Development.—The eggs of the frog are small black spheres,
about 75-inch in diameter, shed into the abdominal cavity from the
ovary. They pass forwards into the oviducts and thence to the exterior.
As they pass down the oviduct they are enveloped in a glassy albu-
minous matter, which, after deposition, swells up by absorption of water
into a firm jelly, serving to protect'the egg.
The eggs are fertilised outside the body by the sperms of the male
shed over them. The early part of the development is embryonic
(usually about the first fortnight). During this time the nutrition is
lecithal (yolk). The later part is larval and the nutrition is herbivor-
ous. The larvee are termed tadpoles and show striking resemblances
to fish in their general organisation. After about two months of this
larval existence a metamorphosis occurs. Great changes in most of the
organs result in the production of the young frog and the assumption
of a terrestrial carnivorous existence.
The Embryonic Period.—The egg is telolecithal, z.¢., the yolk
is aggregated towards one half of the egg, which is lighter in colour,
the other half being covered with black pigment. The first two divisions
of segmentation are parallel to the axis of symmetry, and hence divide
the egg into equal quadrants, but the third divides it into unequal halves,
producing four large and four small octants. In further segmentation
the cells in the pigmented half are produced more rapidly and are
smaller than those in the yolk-half.
Hence the segmentation is total but unequal, producing a modified
blastuda, in which one half has. few-large hypoblast cells and the other
has many small epiblast cells. Such a blastula is converted into a
didermic embryo or modified gastrula, not by archiblastic invagination,
but by egzboly or gradual extension of the epiblast over the hypoblast.
This overgrowth is not effécted all round the edge of the epiblastic
portion ; but at one spot, the future hind-end of the embryo, there is
formed a slight split between the two layers, extending into the hypo-
blast as the commencing archenteron. This spot, the embryonic rim,
evidently represents the dorsal edge of the blastopore in Amphioxus.
Elsewhere, especially at the opposite side, the pigmented epiblast is
seen to slowly envelop the hypoblast, till eventually there only remains
a small hole just below the embryonic rim which we may recognise as
the dlastopore. The epiblast cells are said not to actually grow over the
hypoblast, but to be continually increased in number and extent by
actual conversion of the light hypoblast cells into small pigmented
epiblast cells. The final result is the same, z.¢., that the whole egg
becomes dzdermic (or diploblastic), an outer layer of epiblast enveloping
an inner of hypoblast. The small blastopore eventually closes into a
small longitudinal slit called the sremztzve groove. Meanwhile the
archenteric cavity has extended inwards by a splitting of the hypoblast
cells, and the blastoccele or segmentation cavity disappears in front of it
at the anterior end.
M. 24
354 CHORDATA.
The embryo is now directly comparable to the gastrula of Amphi-
oxus, with the reservation that the hypoblast cells containing yolk are
heaped up in the floor of the archenteron.
The neural tube is now formed in the mid-dorsal line by the up-
growth and fusion of two neural folds. These extend backwards
Fig. 252.—TuHRrEE STAGES IN DEVELOPMENT OF FRoc’s Ecc.
Small Cells.
Archiccele,
A
Large Cells
with Yolk.
Archiccele.
B
Commencing
Archenteron.
Cc
Blastopore.¢
Edge of advan-
cing Epiblast. ——~ “
A, The Modified Blastula. B, Commencing Gastrulation.
C, The Modified Gastrula nearly formed.
to envelop the primitive groove, and convert the blastopore into
a neurenteric canal, as in Amphioxus. Meanwhile the ‘mid-dorsal
cells of the hypoblast become cut off as a sofdd rod of cells, forming
the notochord, and dorso-laterally two sheets of mesoblast are simi-
larly cut off from the hypoblast, their separation taking place slightly
before that of the notochord (as in Amphzoxus). The differentiation
RANA. 355
takes place from before backwards, hence at the embryonic rim the
epiblast, mesoblast and hypoblast are all merged into one common
mass of cells.
Fig. 253.—SEcTIONS OF Froc Emsryos.
Notochord,
A
Blastopore. Hypoblast.
Neural Groove.
Notochord. Mesoblast.
Mesoblast.
B
Hypoblast. ’
Nerve Tube.
Notochord.,
Ceelom.,
Mesenteron.
Yolk Cells.
A, Longitudinal Median Section through a Frog’s Embryo at a late Gastrula
stage, B, Transverse Section through Frog's Embryo (after Hertwic). C, Trans-
verse Section of Young Tadpole (ad xai.).
The mesoblast plates divide into a dorsal vertebral plate, segmented
into protovertebree and a ventral dateral plate. The lateral plates grow
downwards on either side, and a coelomic cavity arises in each by a
splitting between the cells, the mesoblast then forming an outer somatic
layer and an inner splanchnic.
356 CHORDATA.
The body is now distinctly elongated and compressed slightly from
side to side.
This stage may (except for the closure of the neuroproe and preco-
cious formation of the brain) be compared to the chordula larva of the
Fig. 254.—THE STRUCTURE OF FRoc’s EMBRYO AND TADPOLE.
Head.
bse.
tA)
Crp
ry B
Tort DD
¢ Stomodzum.
Anus etre 2
oo oe
Second Gill.
First Gill. | Ear.
Eye.
Anus.
Notochord. Pronephric Duct.
Neurenteric
Canal.
nus.
Yolk-cells. Liver. Heart.
A Longitudinal Median Section through an Embryo of Frog X 20 (after
MarsHatt). Note the dorsal nerve-tube swollen into a brain anteriorly, the noto-
chord below it, and the mass of yolk-cells on the ventral wall of the archenteron.
RB, Side (right) View of just-hatched Tadpole of Frog x 6 (ad at.). C, Median
Longitudinal Section through a newly-hatched Frog’s Tadpole (chiefly after
MARSHALL).
Alriozoa. As in the case of the gastrula, the chief difference is in the
presence of a large mass of yolk containing hypoblast on the ventral
wall of the archenteron.
RANA. 357
The step to the newly-hatched tadpole is not great.
The exterior shows the long tail already formed, the developing eyes
and ears on the head, the two pair of external gills (soon followed by a
third), and, on the ventral surface of the head, a large sucker The
openings of the stomodeum and anus are also seen. The former is
blind, but the latter is already an aperture leading into the gut. In
longitudinal section we may notice the commencing liver as a ventral
diverticulum of the gut, and in front of it the simple tubular heart.
The pronephros is present as three ciliated funnels on each side, leading
by a paired archinephric duct on each side to the hind-end of the gut.
Fig. 255.—YouNG TADPOLE DISSECTED FROM THE VENTRAL
SIDE.
(Mainly after MarsHact.)
Mouth.
Afferent
A Branchials,
wi 5 ity.
Opercular INC | taut
Cavity.: \ } :
creak ercular Aperture.
eart. Glomerulus.
Pronephros. a
; : Mesonephros.
Pronephric Duct. :
Aorta, vy) — Intestine.
Hind-Limb.
Cloacal
Aperture.
A day or two after hatching the mouth opens,” with horny jaws; the
yolk has been used up and the tadpole feeds upon small water-plants.
At the same time the four gill-clefts open and internal gills are formed
on their walls. The external gills then atrophy. The gill-slits be-
come covered over. by a fold of skin or ofercat/um on each side. A
small opercular aperture remains on the left side, but none on the right.
.The tail is provided with a dorsal and ventral median fin, and the tadpole
swims actively by its action. During this stage, whith lasts for about
six weeks from hatching, the tadpole has an internal organisation like
a fish. The two-chambered heart drives blood by afferent: branchials
‘to the four gills. The limbs developas small buds, the fore-limbs- in
the opercular chamber and the hind-limbs beside the anus. The
mesonephros arises as a set of small tubules which join the pronephric
duct and gradually replace the pronephros. The lungs then develop
and become functional. i
The tadpoles frequently come to the surface and take air into the
lungs. Thus is instituted a stage comparable to the Dzpzoz, in which
both forms of breathing are functional. At about two months this
358 CHORDATA.
stage culminates in the metamorphosis. The gills atrophy, the gill-
slits close up, the intestine shortens, the aortic arches assume the
adult condition and the tail commences to be absorbed. The animal
now begins to leave the water by degrees till, when the tail has com-
pletely disappeared and the limbs are completed, it becomes a terres-
tuial frog.
COMPARISON OF AMPHIOXUS, SKATE AND FRoG.—We now have
to compare the early stages of the skate, the frog and the Amphioxus,
Fig. 256.—THE Lire History OF THE Comvuon Froc.
Showing the egg and larval or tadpole stages. (For description see Text.)
The frog passes beyond the fish-stage to the amphibian, but in its early
stages, with external fertilisation, short embryonic period and small
amount of yolk, it is a nearer approximation to Amphioxus than is
the skate.
Blastula.—In Amphioxus the segmentation is total and equal
and produces a (nearly) centro-symmetric blastula with equal cells. In
the frog there is some yolk aggregated in the future hypoblast cells,
RANA. 359
hence the segmentation is total and unequal, the hypoblast half being
retarded. A modified blastula is, however, still produced. In the
skate the bulk of yolk is so enormous that only the epiblast cells and a
portion of the hypoblast can segment. Hence the segmentation is only
partial or meroblastic, producing, not a true blastula, but a cap or
blastoderm resting on a mass of unsegmented hypoblast and yolk.
Gastrula.—The yolk in the frog is already sufficient to prevent the
normal archiblastic invagination, and as the hypoblast is too bulky to
be tucked into the epiblast, the epiblast perforce extends round the
hypoblast, producing finally a small blastopore in the same position as
that of 4mphioxus, viz., at the postero-dorsal part of the embryo. In
the skate this is carried still further, and the rim of the epiblast has to
extend so far round and over the enormous mass of yolk that the
embryo differentiates during the process. The final result is the same
as before, the epiblast eventually meeting round a small blastopore at
the posterior end of the embryo.
But in both the frog and the skate it is doubtful how far the archen-
teron is produced by true invagination. Probably in both cases it
arises mostly by a split in the hypoblast. In the frog the archenteron
is largely filled by a ventral mass of yolk-cells, but in the skate this is
*so enormous that it protrudes as a large separate mass of the body.
Chordula.—The frog embryo, a day or two before hatching, as has
been seen, can be directly compared with the chordula larva, but here
again there are modifications. The notochord is not folded off, but
arises as a solid mass, and the mesoderm no longer arises as a paired
series of pouches with cavities in continuity with that of the gut, but as
solid masses, with cavities produced later by splitting.
In the skate much the same as in the frog occurs, if we consider the
embryonic portion only. We may note that both the frog and skate
appear to have at least the main part of the mesoblast formed from two
posterior sheets or plates comparable to the posterior sacs of Amphioxus
and of Ascidia. Again, in Amphioxus the whole of these sacs divide
into mesoblastic somites, the ventral parts of which fuse later to form
the perivisceral ccelom, the dorsal parts remaining segmented. On the
other hand, in the skate and frog the ventral part or /ateral plate is never
segmented, but splits at once to form the perivisceral ccelom, only the
dorsal part or vertebral plate being segmented, as in Amphioxus. The
result in all three types is the same though brought about in a different
manner.
Foetal Membranes.—In the frog the yolk distends the abdomen,
but is not sufficient to cause the formation of a complete yolk-sac. In
the skate, however, the yolk is so abundant that the embryo cannot
possibly be built up to include the yolk, and the latter has to be held in
a special sac. The outer wall of this sac is the serous membrane, a
continuation of the body-wall, and the inner is the yolk-sac proper,
4 continuation of the gut-wall. Hence we see that in the skate the
abundance of yolk (and lecithal nutrition) has caused the formation of
two extra-embryonic fetal membranes, the serous membrane and the
yolk-sac membrane. In all the Ammzofa not only are these two present,
but two more, the amnion and the allantois, are superadded.
360 CHORDATA.
CHAPTER XXII.
CHORDATA—( Continued.)
V.—COLUMBA.
PHYLUM CHORDATA (p. 402).
SuB-PHYLUM VERTEBRATA (p. 405).
CLAss - AVES (p. 447).
Columba livia (the common Pigeon) is a type of con-
venient size for illustrating the important class of Aves or
birds. It shares with the next type, the rabbit, the fate of
domestication by man. As explained in Chapter X., a
careful selection of suitable varieties by man has led to the,
production of numerous breeds, such as fantails, pouters,
jacobins, &c., which, especially in external characters, may
differ remarkably from each other. If a number of these
breeds be left together and allowed to breed promiscuously,
the offspring rapidly reverts to the common wild pigeon from
which they have all been derived. Our description will apply
to any domestic pigeon.
The ead is well separated from the ¢runk by a long
and flexible eck and at the hind-end of the trunk there is
a small and stumpy ¢az/. The deak is formed of horny
material covering both jaws. At its base is a small
pair of external nares, often surrounded by a sensitive
swollen patch of skin called the ceve. The eyes are large and
have upper and lower eyelids. In addition, there is a thin
membranous eyelid which can be drawn from the anterior
angle transversely across the eyeball. It is called the
nictating membrane.
A little way behind and below the eye is a round hole or
aperture leading into a tube, the external auditory meatus.
This passes in for some distance and terminates in the drum
or tympanum. Hence the tympanum in the bird is not at
the surface, as in the frog, but is sunk to the base of a
meatus or canal. The mouth opens between the jaws
into a buccal cavity, on the floor of which is a pointed
COLUMBA. 361
tongue. Immediately behind the tongue is a median slit,
the glottis. Dorsally lie the two znternal nares, and behind
them is a single Eustachian aperture which soon diverges
into the two Eustachian canals to the ears. At the hind-
end below the tail is a single median cloacal aperture. The
fore-limbs are formed into wimgs and the hind-limbs form
the gs. There are four toes terminating in claws.
The whole body, with the exception of the beak and the
lower part of the legs, is completely enveloped in a coat of
eathers. A feather, structurally as well as
ari oes i is an organ sud pei Nothing
quite like it is found in any group outside the birds,
A feather arises from the epidermis and remains attached
to the skin by its base. If the feathers be plucked or
pulled out of their epidermic pits, it is seen that they are
attached only on certain areas of the skin called pzevyla, the
portions of bare skin between them being called afferia.
The skin itself is dry and powdery, and there is an
absence of the numerous skin-glands found in the frog, with
the exception of the large green-g/and at the base of the tail.
This involves the “ preening” of the feathers by the bird, in
which process the greasy secretion from the gland is spread
by the bird’s beak over each feather.
The largest feathers are found in the wing and tail and
form the quill- or flightfeathers. The central axis of the
feather is hollow in its lower part, called the gu¢//. Open-
ing into the hollow cavity is a small aperture at the base,
called the znferior umbilicus ; and at the distal end of the
quill region is a smaller superior umbilicus.* Above the
quill the axis is extended as the solid shaft, bearing on either
side the vaze. The vane or flattened part is formed of a
great number of parallel dards attached basally to the shaft
and laterally to each other by small interlocking processes
or barbules.
The quills on the wing are called vemiges and those of
tail are rectrices. A remex usually is more tapering, and has
the vane very unequal in size in comparison with a rectrix,
* This peculiar structure is explained by the development of the feather from a
single tube, of which the part above the superior umbilicus splits longitudinally and
spreads out to form the vane and shaft, leaving the quill to open to the exterior by
the superior umbilicus.
362 CHORDATA.
The smaller feathers are called coverts and contour
feathers, according to their size and structure. The /i-
plumes are still smaller feathers, resembling hairs, with a
thin shaft terminating in a very small vane. They can be
seen still attached to the skin after plucking. The scales
on the legs and claws are epidermic and closely similar to
those found in the reptiles.
After plucking, the skin may be removed from the
ventral surface by a median incision from head to cloacal
aperture (see Plate VIII.). The greater part of the body
Fig. 257.—VIEW OF RESPIRATORY ORGANS OF THE
Pigeon. (Slightly Diagrammatic.)
Trachea.
Clavicular-Sac.
Syrinx.
Bronchus.
Opening of
Bronchus into
Air-Sac,
Opening of
Bronchus into
Posterior
Air-Sac.
_ Anterior
Abdominal.
--—— Posterior
Abdominal.
The median sac is the interclavicular. -
is seen to be occupied by the “ breast,” a mass of muscles
lying on the large sternum. The central ee? (or carina)
of the sternum may be seen in the middle line. The
pectoral muscle can be cut away from its point of origin
along the sternum and clavicle, and thence forward. It
is inserted into the large deltoid ridge of the humerus.
It is evident that on contraction this muscle will depress
the wing. Under it lies the swdclavian muscle, originating
from the sternum and passing upwards by a tendon which
can be followed through a foramen in the shoulder-girdle,
called the foramen triosseum, on to the upper side of the
Plate VIII.—First DissEcTION OF PIGEON. (Ad zat.)
Tendon of Sub-clavius
passing to Dorsal Surface
of Humerus.
Crop.,
Humerus...
Pectoralis
Major.
Head of
Clavicle.
y. Humerus.
Brachial Artery
Corac and Vein,
Pectoralis gg ti ; Brachialis.
Major Muscle. ‘
Sub-clavius.
Cut Edge of Origin of
Pectoralis Major.
Pubis.
Cloaca.
The skin and feathers are removed from the whole ventral surface. On the left
of the pigeon the fectoralis mayor has been cut away from its origin along the carina
and posterior border of the sternum and thrown forwards, It is still seen attached,
COLUMBA. 363
humerus. By this arrangement a contraction of the sub-
clavian results in raising the wing. The tendon runs along
beside the large coracoid bone, and on the outer side of this
bone originates a small triangular muscle called the coraco-
brachialis, the tendon of which is inserted in the head of the
humerus. It apparently helps in depressing the wing.
The large keel of the sternum is developed in response to
the necessity for a large area of attachment for the ‘‘ flight-
muscles.” In birds which do not fly the keel is absent.
The sternum may now be removed by cutting round its
edge posteriorly and laterally, and the abdominal cavity
may be opened by a median ventral incision.
The air-sacs should be noticed, large cavities with thin
walls. They are nine in number, and communicate with
the lungs (v.z.). Some are also produced into the interior
of the bones, such as the humerus. Three pairs of them lie
behind each other in a row on each side of the viscera, from
which they are separated by an obiigue septum.
Down the neck may be noticed two long tubes, one
stiffened with bony rings, the ¢rachea, and the other soft,
which is the esophagus. The trachea can be
traced down to the thorax where it passes
dorsal to the heart. The esophagus expands into a large
thin-walled sac, the ¢vog, from which it passes into the
body-cavity dorsal to the heart and terminates in a
glandular stomach. The stomach opens directly into a
large round gzzard with very thick muscular walls. The
first loop of the small intestine is, as in other types, termed
the duodenum, and in its loop there rests a whitish pancreas.
The “ver is bilobed and lies over. the gizzard; it has swo
bile-ducts. The left opens into the proximal loop of the
duodenum and the right into the distal; the left is thick
and short but the right is longer and more delicate. The
pancreas has no less than three ducts which open into
the distal loop of the duodenum. The rest of the smal
intestine is coiled and of considerable length. It ends
in a short rectum, and at the junction below the two
is a pair of small pockets, the rectal ceca. The rectum
opens into the cloaca.
The alimentary system presents some peculiar characters.
All modern birds like the pigeon have no teeth, though
Alimentary.
364 CHORDATA.
they are present in certain fossils. The large crop is used
for the storage of quantities of grain. The pigeon has
many enemies and has to fill its crop when the occasion
presents itself. In the crop the food is partially softened,
and is passed gradually into the stomach which secretes
a digestive fluid. It is then passed into the gizzard in
which it is ground and crushed to pieces. There are always
present in the gizzard a number of small fragments of stones
which, churned together with the food by the muscular walls
of the stomach, reduce the grain to small pieces. Thence
they pass into the duodenum and ileum in which absorption
is effected. It will be seen that there is no gall-bladder in
the pigeon, but this is present in closely allied birds.
The ccelom is mainly represented by the large abdominal
cavity and the smaller pericardial cavity around
the heart. The two cavities are, as in the frog,
completely separated from each other.
The alimentary canal is suspended by a dorsal mesentery
in which run the blood-vessels, as in most vertebrates. A
median ventral mesentery attaching the liver to the sternum
is termed the fakiform ligament.
The heart is proportionately very large; it les imme-
diately in front of the liver, and is four-chambered. The
single ventricle of the lower types is here divided
into two by a septum. Hence there is a left
ventricle communicating with the left auricle
and a right ventricle communicating with the right auricle.
The supply of blood to the auricles is similar to that of
the frog, z.e., venous blood from the system comes back
to the right auricle and arterial blood from the lungs
comes back to the left auricle. On contraction each
auricle empties its blood into the ventricle of the same
side through the auriculo-ventricular valves. On con-
traction of the ventricles the left sends its blood to the
system and the right to the lungs. Hence the two currents
are quite apart throughout their course, and the right side
of the heart acts as a respiratory heart, the left side per-
forming the part of a systemic heart.
If a section be made across the posterior half of the
heart, the two ventricles will be seen. The left ventricular
cavity is small and has very thick walls; the right is
Colom.
Blood-
Vascular.
Plate IX.—SECOND DISSECTION OF THE PIGEON. (Ad zat.)
Post-caval entering
Right Auricle. Heart.
Pulmonary Veins
entering Left Auricle.
_ Left Lobe of Liver.
Proventriculus
(Spleen to Right).
Portal Vein. f : | Oe
: eis Mg : Epigastric.
Right Bile Duct.
Left Bile Duct.
Tleum
ancreas (with Three Ducts).
Posterior-mesenteric.
Cloacal Aperture.
The sternum is removed by lateral cuts through the ribs, the coracoids and
clavicles. The liver and heart are both thrown forwards, and the duodenum and
omentum are thrown to the left, the ileum to the right. In the mesentery are seen
the two bile ducts (white) the portal vein (blue), and the cceliac artery (red).
COLUMBA. 365
crescentic and has a thin outer wall, its inner wall being
formed of the thick wall of the left ventricle.
The venous system consists of two complete parts—(r)
the two pulmonary veins which are short and lead directly
from the lungs to the d/¢ auricle, and (2) the systemic
system which leads into the right auricle. There is no
Fig. 258.—VENTRAL VIEW OF THE VENOUS SysTEM OF
THE PIGEON. (Ad nat.)
Vertebral. External Jugular.
Brachial.
y— Pectoral.
Precaval. Aperture to Right
Auricle,
Left Lobe of Liver.
Postcaval.
Right Lobe of Liver.
Anterior Lobe
of Kidney.
Renals.
Portal.
Gastric. 4 . 7 B- Femoral.
. Middle Lobe of
Anterior Kidney.
Mesenteric.
B‘Sciatic.
Posterior Lobe
of Kidney.
— Internal Iliac.
Posterior Mesenteric.
Renals.
Caudal.
The lobes of the liver are drained by hepatic veins, and the left hepatic receives a
long epigastric from the omentum, seen hanging down the centre.
Sinus venosus, but three large veins converge together to.
open into the auricle. Two are paired and anterior and
are called the pvecavals; the other one is median and
posterior, called the ostcaval.
The precavals are formed of three large veins, the
jugular from the head and neck, the érachial from the
wing and the fectora/ from the flight-muscles. The two
366 CHORDATA.
jugulars anastomose together below the tongue. The post-
caval can be traced backwards through the liver where it
receives paired epatics. A little way behind the liver it
diverges into two lac veins. The portal vein may be seen
passing to the liver from the stomach and intestine. Its
most posterior branch, the posterior mesenteric, anastomoses
with the systemic system (see below). The portal has the
same relationships as in the skate and frog, but there is no
anterior abdominal.
The epigastric vein is said to represent this vein. It drains the
omentum, a fatty fold of peritoneum, and runs forward to join the
left hepatic vein.
If the rectum be cut through and the intestine carefully
removed, the veins and arteries in the abdominal region will
be easily seen (see Plate IX.) They are in relation to the two
large three-lobed kidneys, lying in a hollow of the pelvis.
From the tail there emerges a small caudal vein which
bifurcates into two renal portals diverging right and left
towards the kidneys. Each receives an internal iliac and
then passes through the kidney. Between the second and
third lobe of the kidney, the renal portal receives the seéatic
and between the first and second it receives a large femoral.
The femoral and sciatic then form the zac which receives
a venal from the kidney, and then unites with its fellow to
form the gostcaval. Hence the iliacs and renal portals
form a complete “renal cycle” running left and right from
caudal to postcaval.
At the point of junction of caudals and renal portals
there runs forward beside the rectum a large median vein,
the posterior mesenteric. It joins the portal anteriorly.
The arterial system consists, like the venous, of two
parts. (1) The right ventricle gives off a trunk which
immediately bifurcates into two pulmonary arteries going to
the lungs. These correspond to the third
arterial arches (pulmocutaneous) of the frog.
(2) The systemic system—the left ventricle
gives off a main trunk which divides into three. Two
are paired and anterior; they are called the zanominate
arteries and divide into carotid to the head and sué-
clavian which itself divides into drachial and pectoral.
The third bends over to the right and passes dorsal to the
Respiratory
System.
Plate X.—THIRD DISSECTION OF PIGEON (?) TO SHOW THE
BLOOD-VASCULAR AND UROGENITAL SYSTEMS.
if
The colours of the heart are physialogical, those of ¢he arteries and veins are morphological).
Right Jugular->——— _-~Carotid Artery.
Tracheal Ring.
Right Carotid: AEsophagus.
Jugular Vein.
Brachial Arter
Brachial Vein§
Pectoral Artery?
Pectoral Vein!
Post-caval... : ‘Funnel of Oviduct.
Hepatic Veins.
Epigastric.
Renal Portal Vein”
Posterior-mesenteric
Ureter:~ Vein.
o
Vestigial Right Oviduct”
i Internal Iliac.
The cesophagusjand cloaca have been cut through and the alimentary canal and
appended organs have been removed. Note specially the tri-lobed kidneys with
ureters, the single left ovary and oviduct, the four-chambered heart, the right
systemic arch, the ‘‘renal cycle,” and the posterior-mesenteric vein.
yaae a
nie AUSF Ula he Sh
lnc posverior-mesenteric, and the
s1vw wwe renal portal.)
COLUMBA. 367
heart. Here it bends into the middle line and proceeds to
the hind-end of the body as the dorsal adrta. Its main
branches are celiac, anterior mesenteric, paired renals,
Jemorals, sciatics and internal tlacs, and it terminates in
the tail as the caudal artery.
Fig. 259.—VENTRAL VIEW OF THE ARTERIAL SYSTEM
OF THE PIGEON. (Ad nat.)
GH
Systemic Arch.-—~
\ Innominate (left).
LS.
Hire
Dorsal Aorta. Anterior
Mesenteric.
re Renal.
A Femoral
Renal.
Sciatic.
Renal.
Internal Iliac. Internal Iliac.
Posterior .
Caudal. Mesenteric.
The anterior arterial system is peculiar in lying super-
ficially to the venous system. Apart from the four-
chambered heart, which is shared by mammals, the blood-
vascular system of the pigeon is chiefly remarkable for
the very high temperature of the blood, the systemic arch
persisting only on the right, and the large size of the
pectoral arteries and veins.
If the heart be now removed, the trachea can be traced
throughout its length till it bifurcates into the two bronchi.
At its front-end is a /axynx which, however, is not an organ
for producing sounds in the bird. The trachea is distended
368 CHORDATA.
by small bony rings; those of the bronchi, except the first,
are of cartilage. At the junction of the trachea and bronchi
is the syzinx, the true organ of voice in the birds. The
bronchus passes into the lung and there branches. Its
branches emerge from the lung to open into the air-sacs
already noticed. The lungs themselves are dense, rather
small, and closely pressed against the ribs. They lie dorsal
to the ccelom and their ventral face only is covered by peri-
torieum. The air taken into the lungs can pass freely into
the air-sacs. The bird respires in a different manner to the
frog. The air is drawn through the lungs into the air-sacs
and is expelled forcibly again by the movements of the
body-muscles. The lungs themselves have only a small
respiratory surface, correlated with the free current of air
through them.
The general form and
Fig. 260.—VENTRAL VIEW OF MALE position of the Azdneys
UROGENITAL ORGANS OF THE
PIGEON. (Ad nat.)
have been already de-
scribed. A small ureter
«4.7 passes from
testis, Tem the ventral
face of each kidney
backwards into the
cloaca. There is no
urinary bladder.
In the male the zestes
are paired and situated
just in front of, and
Swollen Beret ‘between, the kidneys.
; They are oval, white
bodies, and each gives
off a fine, twisted tube,
the vas deferens, passing backwards into the cloaca.
In the female the single left ovary lies between the
anterior lobes of the kidneys. It is fastened by a dorsal
mesentery and usually contains eggs of various sizes. The
left oviduct is a large, coiled tube with an internal funnel
near the ovary. It opens posteriorly into the cloaca.
There is a vestige of the vight oviduct.
The brain is easily exposed by scraping off the dorsal
surface of the skull. The usual parts are all present and
Vas Deferens.
Ureter..
Cloaca.
COLUMBA. 369
the special points to notice are as follows:—(1) The
cerebral hemispheres are large and reach the
cerebellum posteriorly, hence the optic lobes
are /ateral in position. (2) The whole of the brain lies
behind a line drawn through the eyes. (3) The olfactory
lobes are very small and poorly developed.
The skeleton of the pigeon is as remarkably
modified as is the rest of its anatomy. °
In the skull we may note the complete fusion of the
cranial and some of the facial bones, leaving no sutures.
The upper beak is supported chiefly by the premaxille and
by the maxi//e, the thin juga/ joining the maxille with the
quadrate posteriorly. Further, towards the middle ventral
line the two fadatines pass back from the maxille to meet
the perygoids which pass outwards and backwards to join the
Nervous.
Skeletal.
Fig. 261.—A CERVICAL VERTEBRA OF THE PIGEON. (4d nat.)
Neural Crest.
Neural Canal.
\ Vertebrarterial
anal.
Cervical Rib.
Heteroccelous Facet.
guadrates. ach quadrate has a condyle for articulation with
the mandible bearing the lower beak ; they are freely movable
upon the skull. All the other bones are fused.
The orbits are very large and are separated by a thin
septum only partially ossified, the zxéerorbital septum. Its
ventral edge, under the palatopterygoid junction, is thick-
ened and forms the rostrum.
The septum is said to be formed of the mesethmoid and presphenoid
of the rabbit, whilst the rostrum is supposed to be homologous with the
anterior part of the frog’s parasphenoid, the posterior part of which is
represented by the paired dasztemforals ventral to the dasisphenoid.
There are three ear-bones but the fro-ot/c alone remains free, the
others fusing with the occipital bones.
There is a single occipital condyle on the basisphenoid
and the mandible is ossified into five bones.
M, 25
370 CHORDATA.
The vertebral column consists of a great number of
vertebrae which are known as cervical, or neck-vertebre,
thoracic, lumbar, sacral and caudal. Of these the cervical
are numerous, forming a very flexible neck; the thoracic
Fig. 262. — LATERAL VIEW OF CERVICAL VERTEBRA OF
THE PIGEON. (Ad nat.)
Posterior Zygapophysis. Anterior Zygapophysis.
Cervical Rib.
Heteroccelous Articulation. Vertebrarterial Canal.
are largely fused together and rigid, while a great number
of the caudal are also fused.
The fourteen cervicals have (except the first two) cervical
ribs fused on to them, and as the ribs have two heads their
fusion with the vertebra forms a canal on each side, called
Fig. 263.—A RIB OF THE PIGEON. (dd nat.)
(Slightly magnified).
Tuberculum. Uncinate
Process.
Capitulum.
Vertebral Part.
Sternal Part.
the vertebrarterial canal because it transmits the vertebral
artery. The vertebrae are called /eterocelous to describe
their peculiar articulations with each other, which are convex
in one direction and concave the other, like a saddle. The
COLUMBA. 371
five thoracic vertebree bear ribs which articulate distally with
the sternum. Each rib has a longer vertebral part and a
shorter external part, and the former has two heads arti-
culating with its vertebra. The first four have short
uncinate processes. The capitulum of the rib articulates
with the centrum of the vertebra and the suderculum with
the transverse process. The first three thoracic vertebree
are fused, the fourth is free, while the fifth is involved in
the sacrum.
In the young bird there are five free lumbar vertebra
and then ¢wo sacral to which the ilium is attached, but as
development proceeds the ilium grows forwards and becomes
attached to all the lumbar and to the fifth thoracic. Simi-
larly there are in the young bird fifteen free caudal vertebree,
and the ilium gradually grows backwards and fuses with five
of these. Of the other ten the last four fuse together to
form the pygostyle.
This means that the young bird presents us with a
reptilian-like condition of the vertebral column in which
all the vertebrz are free. They consist of—
Ceivicall jchecndsean decane suatis 14
TPROTACIC 2 x..g2ec0s ehsieose ane 5
Lia bak 2.4.4 ncodacsiendsensions on 5
Sacral: gaianccanrenebsou star 2
Catal. aed Soaked deseo. 15
The modifications then take place as age advances.
1. The first three thoracic become ankylosed or fused.
2. The last four caudal become ankylosed to form the
pygostyle. 3. The ilium grows forwards and fuses with all
the lumbar and the last thoracic, and backwards to include
five caudals.
Cervical. Thoracic. Lumbar. Sacral. Caudal.
14 [3] #rtr1+5+24+5+6+ [4]
ta por
These fusions are supposed to be a recapitulation of
similar modifications which have taken place gradually in
the descent of birds from reptiles and in adaptation to the
gradual adoption of flight and bipedal progression. It will
be remembered that a similar fusion of vertebrae into an
anterior vertebral plate is found in the skate, in which the
372 CHORDATA. '
front-limb is greatly developed, and in the caudal vertebra
of the frog (urostyle), in which the hind-limbs are enlarged.
Fig. 264.—VENTRAL VIEW oF STERNUM OF THE PIGEON.
(Ad nat.)
Manubrium.
Carina.
Coracoid Groove.
Costal Ridge.
Xiphoid
Process.
Posterior Xiphoid
Process.
Fontanelle.
Fig. 265.—THE PECTORAL GIRDLE OF THE PIGEON.
(Ad nat.)
Scapula,
Left Clavicle.
aay
Right Clavicle.
Coracoid.
Episternum.
The shoulder-girdle is formed of three elements, the
clavicle, coracoid and scapula. Of these the clavicle is slender
and joined to its fellow by fusion with an episternum. The
COLUMBA. 373
compound bone so produced is called the furcula; the
coracoid is very large and powerful and the scapula is long
and flat. The coracoid and scapula form the glenotd cavity
between them, and on the inner side the three bones border
the foramen triosseum. The coracoids rest upon the front
end of the enormous szernwm, their ends being fastened in
its coracoid grooves. Projecting ventrally is the large keel
or carina and laterally there is a costal process, followed by
an indented costal ridge, to which the distal ends of the ribs
are attached.
The fore-limb has a short and powerful Aumerus, a thick
ulna and a rather more slight vadzus, followed by a pair of
proximal carpal bones. These are succeeded by a single
Fig. 266.—THE SKELETON OF a BirD’s WING. (4d nat.)
Carpo-metacarpus. _— Carpal. Radius. Humerus.
First
Third Metacarpal.
Third Digit.
ist Phalanx
of
Second Digit.
end
Phalanx.
compound bone, the development of which shows it to be
composed of the distal carpals and three metacarpals fused
together. It is hence termed the carpo-metacarpus. It
bears a first digit with a single phalanx, a second .digit
with two large phalanges and a third with one small one.
Hence the two peculiarities of the bird’s forearm are the
fusion of distal carpals and metacarpals into one bone and
the loss of the two last digits.
To the first digit is attached the a/a spuria, a miniature
wing. To the hind-border of the second and third digits and
the carpo-metacarpus are attached the twelve primary quill-
feathers, and to the ulna are attached the twelve secondary
quill-feathers.
374 CHORDATA.
Fig. 267.—Lzrr Lec or THE Picron. (Ad nat.)
Femur. Head.
Trochanter.
Tibiotarsus,
II.
Fig. 268.—LaTERAL VIEW OF PELVIS OF THE PIGEON, (Ad nat.)
(Slightly magnified. )
Ischiatic Foramen.
Tlium.
Acetabulum. Ischium.
The pelvis has a long cium which, as already seen, is
attached to a large number of vertebrae. The round
acetabular cavity, which is incompletely ossified, is about
COLUMBA. 375
half-way along its ventral border. Posterior to it is a
triangular ¢schiwm with a large oval foramen (the éschiatic
foramen). From its anterior border there runs backwards
beside the edge of the ischium a long pudis. There is
no symphysis. Just above and posterior to the acetabulum
is a small facet, the axtitvochanter, which articulates with the
trochanter of the femur.
The hind-limb has a short femur, a small and vestigial
fibula, but a large ##bia to which are fused the proximal
tarsal bones, hence it is known as the “#dzofarsus. This is
followed by another compound bone, consisting of the
Fig. 269.—DIAGRAM OF A Fowl’s Ecc aT LAYING.
(After ALLEN THOMSON.)
Blastoderm,
Plug of White Yolk.
Air-Chamber.
Outer Egg-
Membrane
Inner Egg-
Membrane. Vitelline Membrane.
distal tarsals and three metatarsals, which is known as the
tarsometatarsus. It forks into three processes at its distal
end, each of which bears a digit. A small bone on its
inner side is the first metatarsal, which bears the first digit.
The number of phalanges increases outwards from two to
five. The pigeon has therefore no fifth or outer toe, and
the first is opposable to the other three.
In the hind-limb there can be recognised the same two
features as in the front-limb, “2, the reduction in the
number of digits and the fusion of tarsals and metatarsals.
In the hind-limb there is, however, a further fusion of the
376 CHORDATA.
proximal tarsals to the tibia. The foot moves in the birds
upon an zntertarsal joint, the movement being between the
two rows of tarsals.
Development (Ga//us).—The true ovum of the fowl is a large yellow
sphere enclosed in a delicate vitelline membrane. It is usually termed
the ‘‘yolk” of an ‘‘egg.” It is fertilised at the top of the Fallopian
tube and passes slowly down the oviduct, developing as it goes, so that
a laid ‘‘egg” has already developed for about eighteen hours. As it »
passes down the oviduct albumen is added to it from glands of the
oviduct, and this is twisted by rotation of the ovum into two cords at
the ends of the ovum (chalaze). Further down a double egg-mem-
brane and a shell are added and the egg is then laid.
Segmentation is, as in the skate, meroblastic and produces a small
blastoderm resting on the yolk. On laying, the reduction of tem-
perature causes development to cease, and in the natural condition
it is not resumed till the full complement of eggs has been produced
and the hen commences to “sit.”
Fig. 270.—THREE CONSECUTIVE STAGES OF THE BLASTODERM
OF A CHICK IN EARLY STAGES OF INCUBATION.
(After KoLier.)
Area Opaca.
Area
Pellucida.
Blastopore.
The blastopore is seen in the first to be crescentic, and is gradually converted
by differential growth into a longitudinal groove which closes
to form the primitive groove.
If sections of the blastoderm be made it will be found, as in the
skate, to consist of two layers, epiblast and hypoblast, and a segmenta-
tion cavity between them. At the future hind-end, as in the skate, is a
thickened rim, immediately behind which a crescentic hole passes into
a cavity, the subgerminal (or, possibly, the archenteric cavity). As
in the skate, the epiblastic edge of the blastoderm extends gradually
round and envelops the yolk by epiboly, but in this case the extension
is on all sides, and hence the final closure is effected at the distal
pole (opposite to the embryo). In the future posterior region of the
embryo the epiblast and hypoblast remain in continuity; hence the
epiblast does not actually extend backwards at this point, but it
sweeps round each side, converts the crescentic groove into a longi-
tudinal one and completes an even edge beyond it. By the third
COLUMBA. 377
day the edge of the epiblast has reached the equator and eventually
completes the enclosure by the sixteenth or seventeenth day.
Fig. 271.—SECTION THROUGH A CuHICcK’s Ecc
AT VARIOUS STAGES.
(After Duvat.)
Segmentation Cavity.
Hypoblast. Epiblast. | Archenteron.
Blastopore. CPE
0 Ons Onc}
Eee? 8 Heke
ER GH eIO ES
~ Hypoblast.
Mesoblast. Leg
Edge of Soe Oh
Blastoderm. Primitive Groove. Epiblast.
eA rere os
Archenteron. Mesoblast.
8 Hy,
Yolk-nuclei. blast.
In each case only the blastodermic pole is shown, the large mass of yolk being
cut off below. A, Section through the egg at blastula stage. B, Longitudinal
median section of the unincubated egg at the gastrula stage, C, Cross-section
through the blastopore of same stage. D, The same further forward.
The crescentic groove, we already showed, was comparable to the
blastopore, and, after conversion into a longitudinal groove, it is known
as the primitive groove. The cells on either side of it are thickened
378 CHORDATA.
because, as in the lip of the blastopore (cf Amphioxus and Frog), the
three layers are there continuous, This thickening gives rise to an
opacity called the primiteve streak.
Fig. 272,.-VIEW OF THE AREA PELLUCIDA OF A CHICK’s
BLASTODERM OF ABOUT 18 Hours.
(After BALFourR.)
Amniotic Head-fold.
Neural Groove.
Primitive Groove
and Streak.
Fig. 273.—ViEW OF CHICK’s BLASTODERM ABOUT 24 HOURS.
(After Duvat.)
Pro-amnion. Head.
Area Opaca.
Vitelline Vein.
Mesoblastic
Lateral Sheet.
Protovertebre.
Neural Groove.
Primitive Groove
and Streak.
In a similar manner the whole rim of the blastoderm has a thicker
layer of cells than the middle and gives rise to an opacity. Hence the
rim is called the avea opaca and the centre the area pellucida. These
COLUMBA. 379
are optical distinctions and there is no real morphological distinction
between the two areas.
Fig. 274.—CROsSs-SECTION THROUGH A BLASTODERM OF
ABOUT 24 HOURS.
Epiblast.
Hypoblast. Mesoblast.
Yolk. Archenteron. Notochord.
A shows the whole blastoderm lying on the yolk. B shows the median
part only more highly magnified.
Fig. 275. TRANSVERSE SECTION OF AN EmpBryo CHICK
OF THE SECOND Day. (Ad nat.)
(Slightly diagrammatic.)
Nerve Cord. Protovertebra.
Notochord. Amniotic Fold.
Lateral Fold of
mnion,
Extra-embryonic
Ccelom.
Mesoblast.
Epiblast.
Serosa.
Hypoblast of Yolk-sac.
Splanchnic Mesoblast.
The first appearance of the embryo is the neural tube which arises
immediately in front of the primitive streak. Paired neural folds grow
up to form a neural tube and eventually enclose the primitive streak.
380 CHORDATA.
The mesoblast is differentiated from the hypoblast as paired sheets
of cells which grow from the primitive streak forwards in two wings.
They give rise dorsally to protovertebre along the sides .of the neural
tube, and ventrally they slowly follow in the track of the epiblastic rim
round the yolk. In the median dorsal line a rod of hypoblast cells
forms the notochord.
Fig. 276.—D1acram OF DEVELOPING CHICK.
Amniotic Fold.
Embryo.
Notochord.
Mesoderm.
Rim of Rim of
Blastoderm. Yolk. Blastoderm.
Amnion. Amniotic Cavity.
Allantois.
Wall of Yolk-sac.
A, The blastoderm is gbout 2-sths round the yolk. _B, It is about 3-4ths
round. C, Phe blastoderm has nearly enveloped the yolk.
For later stage see E on page 428.
At the end of the first day the embryo has about half-a-dozen proto-
vertebrze, an open neural tube, a blastoderm extending about #-inch in
diameter and mesoblast growing out under the epiblast. In front of
the anterior end the mesoblast sheets do not meet till late, hence here
the blastoderm is only two-layered. This area is sometimes called the
COLUMBA. 381
pro-amnion. On the second day there arises a fold of the blastoderm
in front of the embryo, called the head-fold of the amnzon. Similar
lateral folds and a tail-fold all meet above and fuse together. The
inner portions of the fold form the amzzon, completely enveloping the
embryo in a sac, and the outer portions are part of the serous membrane.
The amnion by its formation is clearly lined with epiblast and covered
with mesoblast. It contains a fluid gawor amnzz and envelops the
embryo till hatching. The mesoblast has already split into somatic and
splanchnic layers before the formation of the amnion. As this split is
continued downwards round the yolk-sac, it divides the wall of the yolk-
sac into serous membrane and inner yolk-sac membrane. The amnion
is completely formed on the fourth day, but the serous and yolk-sac
membranes are not completely separated till about the seventeenth day.
The embryo becomes pinched off from its yolk-sac in much the same
way as in the skate, and the general origin of the organs is much as
described in the general account of the Vertebrata,
The last foetal membrane to appear is the allantois. Traces of it
occur on the second day, but it grows out from the embryo on the
fourth and fifth days. It is a median ventral diverticulum of the hind-
gut and hence is lined with hypoblast covered with mesoblast. It
spreads between the amnion and the dorsal wall of the serous membrane.
Its walls are covered with branches of an allantoic artery and vein and
it acts as a breathing organ, its cavity serving as a urinary bladder. It
has been compared with the urinary bladder of the frog. The yolk-sac
membrane also has yitelline arteries and veins which serve to absorb
the yolk. In the later stages, the yolk-sac also absorbs the albumen,
apparently through the serous membrane. On the twenty-first day the
yolk-sac is absorbed, the chick breaks its way first into the air-chamber
and inflates its lungs, and then breaks its shell. It ruptures the amnion
and the remains of the allantois adhere to the inner surface of the shell.
We may note that the development of the chick, like that of the
skate, is purely embryonic, with a lecithal and albuminal nutrition. In
contrast with the skate and frog, we note the incubation by the mother
and the presence of amnion and allantois.
382 CHORDATA,
CHAPTER XXIII.
CHORDATA—( Continued.)
VI._LEPUS.
PHYLUM CHORDATA (p. 402).
SUB-PHYLUM VERTEBRATA (p. 405).
Crass MamMALIA (p. 453).
Lepus cuniculus (the Common Rabbit) is a type of
the more highly organised and commoner mammals. Its
general appearance and habits are too well known to necessi-
tate much description. Of a habit partially terrestrial and
partially fossorial or burrowing, the rabbit is little specialised
though one of the most successful and dominant of mam-
mals. In nature it is gregarious and of high fecundity. In
these respects, and in the burrowing habits, it differs from
its close ally the hare (Zefus timidus). Except when run-
ning it is plantigrade, i.e, places the whole foot upon the
ground. :
We can readily recognise a head, neck, body and tail.
The whole body is coloured a dull greenish-brown which
External armonises closely with its usual surroundings,
Pogtueds but the under-surface of the tail is white, the
* under-surface of the body having a tendency
to assume the same colour. It has been suggested that the
white tail, so conspicuous when the rabbit runs or disappears
down its burrow, is useful as a “ danger signal” to the other
members of the community that it is time to be moving.
The mouth is at the anterior end of the head, and is
bounded by soft lips which cover a single row of teeth.
The paired external nares open above the mouth, and
laterally to them are long sensitive bristles or vidvissa.
Further back are the large paired eyes, facing laterally,
which are guarded, as in the pigeon, by three eye-lids
LEPUS. 383
Behind the eyes are the large so-called ‘“ ears,” or more
properly pzzne. At the base of the pinna is the opening of
the external auditory meatus which leads, as in the pigeon,
a short way into the tympanum. The pinna is movable and
serves to collect and concentrate the sound.
The limbs closely resemble each other; but the fore-
limb has five claws, the hind-limb four. At the base of the
tail is the avus, and in front of this opening is the urogenital
aperture, either in the female a simple opening, the vulva,
or in the male an opening situated at the end of a fens, at
the base of which are the ¢es¢es situated in scrotal sacs. In
neither sex is there a cloaca.
The whole body is clothed in “ fur,” which consists of a
dense mass of hair. A hair is an epidermic structure pecu-
liar to mammals; it grows from a follicle
and is provided with glands (sebaceous glands)
at its base (see page 455). The secretion of the glands keeps
the hair flexible and moist. The fur forms a remarkable
protection, for a warm-blooded animal like the rabbit, against
changes of temperature. Like the frog, the rabbit has a
great number of glands in its skin. These are known as
the sudorific glands and excrete water and salts in the form
of “sweat.” Large serinzal glands are also found near the
anus secreting an offensive liquid.
But the most remarkable skin-glands of the rabbit are
the mammary glands. These are modified from sebaceous
glands and secrete “milk.” They open by ducts to the
exterior upon mamme or teats and are intermittently active
for the nourishment of the young. In the rabbit the teats
are in two ventral rows upon the hinder portion of the body
or abdomen.
The skin may now be removed by a median ventral
incision from chin to anus, the mammary glands—at the
right season—being observed as yellowish glandular patches
on the inside of the skin.
A median ventral incision of the muscular wall of the
body, as far forwards as the hind-border of the sternum,
exposes the large abdominal cavity. The anterior end of
this cavity is formed of a large septum or diaphragm, partly
muscular and partly membranous: through it emerge the
cesophagus and main blood-vessels: in front of it lies
Integumentary.
384 CHORDATA.
another cavity (the thoracic cavity) containing, as will be
seen, the heart and lungs.
The buccal cavity can be exposed by cutting one man-
dible. The ¢ongue is large and mobile, and behind its base
is the glottis covered by a flap, the ¢figdotiis.
Almentary: ‘The internal nares open very far back, almost
over the glottis. This is due to the formation of a
palate or secondary roof to the buccal cavity which shuts
off a complete nasal chamber, at the hind-end of which
open the two Lustachian apertures.
Fig. 277.—PERMANENT DENTITION OF THE HARE
(Lepus timidus).
Note the long incisors, four above and two below, and the cheek-teeth -_
Into the mouth there open the ducts of four pairs of
salivary glands—the parotid, below the ear; the infra-orbital,
below the eye; the susmaxillary, between the mandibles ;
and the sublingual gland, under the.tongue. ‘These secrete
saliva which is mixed with the food by mastication and has
a digestive action on certain foods.
At the anterior end of the jaws is a pair (upper and
lower) of large sharp-edged incisor teeth. These have hard
enamel mainly on the outer surface and are kept sharp by
wearing upon each other. They grow throughout life as
LEPUS. 385
fast as they are worn away by use. Just behind the upper
incisors is a pair of little peg-like second incisors. Behind
the incisors is a part of the jaws with no teeth, forming a
space or dastema, and further back is a row of six flat teeth
on each side of each jaw. These are the molar teeth
with flattened ridges which serve to crush and masticate
the food (various vegetables). The cheeks can be pushed
together across the diastema ; and in this way the incisors
may be used on occasion for gnawing without the products
passing into the cesophagus.
This peculiar type of dentition is characteristic of the
order Rodentia to which the rabbit belongs.
The esophagus (see Plate XI.) passes down the neck as a
soft tube and emerges through the diaphragm, opening into
the large stomach towards the left side. The duodenum forms
the usual loop, in which is a diffuse pancreas with a single
pancreatic duct passing into the distal limb of the duodenum.
The liver is very large and has five lobes. Partially em-
bedded in it is the gadl-b/adder, from which there passes a
bile-duct opening into the proximal limb of the duodenum.
After the duodenum, the z/ewm forms an enormously long
(8 feet) and coiled tube of small calibre. It terminates in
the sacculus rotundus, a swollen sac which opens distally into
the cecum. The cecum is a blind tube of large calibre which
terminates in a small process, the vermiform appendix. It
is continued, in the opposite direction, into the colon with
sacculated walls and is about 18 inches long. It gradually
loses its sacculation and passes into the rectum, a thin-walled
tube about two feet long terminating in the anus.
The large size of the caecum (about two feet long) and
great length of the intestine are usually correlated with a
herbivorous diet. .
The duodenum and ileum are the two portions of the
small intestine, the colon and rectum forming the “large
intestine.” The sp/en is, as in other types, a dark-red body
lying near the pancreas and beside the stomach.
The portal vein, as in preceding types, should be noticed
before removal of the alimentary canal. It is formed of a
“ienogastric from the stomach and spleen, a duodenal and
anterior and posterior mesenterics. The organs drained by
the portal are supplied with arterial blood by the celiac,
M. 26
386 CHORDATA.
anterior mesenteric and posterior mesenteric arteries, which
should be identified (see page 388)
The cesophagus may then be cut through near the dia-
phragm and the rectum near the anus, and if the mesentery
be carefully cut through the whole alimentary system may
be removed and unravelled. The thoracic cavity should
now be opened by cutting through the ribs on either side
and between the diaphragm and the sternum. The cavity
is almost entirely filled by the two lungs and the heart. The
trachea can be traced down the neck (see Plate XIII.).
Just where it emerges from the buccal cavity there is a
cartilaginous /arynx which forms the organ of voice. It
is formed of thyroid and cricoid cartilages modified from
branchial arches in the embryo (see page 417). The trachea
throughout its course is distended by cartilaginous rings. It
passes into the thoracic cavity anteriorly and divides into
two bronchi which lead to the lungs in which they branch.
(These are best seen on removal of the blood-vessels.)
The Jungs are of a bright-red colour, spongy, and lying
quite free in the cavity around them. The left lung has two
Respiratory lobes, the right has four. Each lung is envel-
* oped by a layer of peritoneum called the /eura,
which has the same relationship to the lung as has the
pericardium to the heart. The outer layer of the pleura is
pushed against the ribs and the inner adheres to the lung.
Between the two is the pleural cavity, which is practically
squeezed out of existence in the living animal by the ex-
pansion of the lungs. Between the two pleura is a space,
the mediastinal space, nearly filled by the heart and peri-
cardium.
Hence the perivisceral coelom in the rabbit is divided
into no less than four separate parts—the pericardial
cavity, two pleural cavities and the abdominal
cavity. Between the last and the other three
is the diaphragm. The diaphragm is innervated by a pair
of phrenic nerves arising from the fourth spinal nerve in
the neck. They may be easily seen passing down between
heart and lungs. The capacity of the thorax is increased by
raising of the ribs, caused by contraction of the intercostal
muscles and by the lowering of the diaphragm. Air is
in this way inspired or drawn into the lungs. Expiration
Colom.
Plate XI.—First DissEcTIon oF Ragsir. (Ad nat.)
. Vermiform
Appendix.
_ Lieno-gastric Vein.
Czecum,
~-Duodenum.
_Pancreas
Bile-duct. . -—
Pancrea
Portal. —"~
Duct.
Stomach.
~ Sacculus
Rotund
Spleen.
~
“-Tleum,
~ Colon.
“Rectum.
The skin is reflected fron chin to anus; the abdominal wall is cut open and
reflected ; the ventral wall of the thorax is cut away; the intestine is partially freed
from its mesentery and thrown over to the right; and the lobes of the liver are
thrown forwards. (The lobes of the liver are R.C., Right Central; C.A., Caudate ;
L.C., Left Central; L.L., Left Lateral; and S., Spigelian. The red arteries are
branches of the dorsal aorta lying deep, the more anterior is the coeliac forking into
hepatic and gastric, the other is the anterior mesenteric; the blue veins are all
branches of the portal.)
LEPUS. 387
is more passive. The elastic lungs contract, the ribs fall
and the diaphragm rises.
Anterior to the heart and lying over the great blood-vessels
is the ¢hymus, a ductless gland which must be removed to
expose the blood-vessels.
The ear? is four-chambered, as in the pigeon, and is but
slightly larger. It differs but little from that of
the latter. The three auriculo-ventricular valves
on the right side are called ¢rvicuspid valves and
the two on the left side are called the mtral valves.
The venous system, as in the two last types, has definite
vessels or veins and consists of three parts. We have
already noticed the epaticportal system. The pulmonary
system consists of two pulmonary veins leading from the
lungs and opening directly into the left auricle. The
systemic system consists of three main veins opening into
the vight auricle. Two are paired and anterior, and are
known as the fprecavals, and the third is posterior and
median, known as the postcaval.
The venous blood from the superficial part of the head
is removed by the anterior and postertor facial veins which
unite behind the mandible to form the external jugular
vein. This passes down the neck, at the base of which it
receives a small znéernal jugular coming from the brain and
a vertebral, It then unites with the subclavian, a large
vein mainly formed of a continuation of the brachial vein
(of the fore-limb), and the two form the precaval which
passes into the thoracic cavity in front of the ribs.* The
right precaval only has an azygos vein passing backwards
beside the vertebral column and said to be a vestige of the
right cardinal vein of lower types.t
The postcaval can be traced backwards through the
diaphragm. It commences in the pelvic region by the
union of two internal tliacs, and then receives two femorals
from the legs, two 2//0-/umbars from the back, genztals, renals,
dorso-lumbars, hepatics and phrenics from the genital organs,
kidneys, dorsal muscles, liver and diaphragm respectively.
Blood-
Vascular.
* The skin has two cutaneous veins not unlike those of the frog in position. The
anterior arises from the subclavian and the posterior from the femoral. Both are
enormously distended in the female when the mammary glands are active.
+ The thoracic wall is drained by two small veins, the axtertor intercostal and
the internal mammary falling into the precaval on each side,
388 CHORDATA.
In comparing this arrangement with that of the pigeon, we
notice the absence of renal portals and a different relation-
ship of the posterior veins caused by the backward extension
of the kidneys in the latter. Hence there is nothing in the
rabbit approaching the ‘‘renal cycle” of the pigeon.
The arterial system has two parts. The pulmonary
system consists of a pair of large but short pulmonary
arteries leading from the right ventricle to the lungs. The
left pulmonary artery is connected with the dorsal aorta by
a transverse vessel, the ductus arteriosus. (Plate XIII.)
It is only functional in the embryo, becoming a solid band
in the adult.
The systemic system consists of two main arteries which
separate soon after emergence from the left ventricle. The
vight innominate runs forwards and outwards, and divides
into the right carotid to the head and right subclavian to
the fore-limb and shoulder. The aorta bends forwards and
outwards to the left, and gives off a eft carotid to the head,
then a left swéclavian, and is continued backwards to the
left of the mid-dorsal line through the diaphragm which
it supplies by a small phrenic artery breaking up on the
surface of the diaphragm. It lies dorsal to the postcaval
to the hind-end of the body where it comes round and lies
ventral to it. It gives off a median unpaired ce/ac to liver,
stomach and spleen, anterior mesenteric to the intestine and
pancreas and posterior mesenteric to the rectum, paired renads
and genitals to kidneys and genital organs, and then divides
into two common iliacs which give off zio-lumbars and in
turn bifurcate into femoral and internal tac. The persist-
ence of the 4/t aortic arch alone instead of the right, as in
the pigeon, should be noted. Again, there is only one
(right) innominate artery, the left carotid and subclavian
communicating directly with the dorsal aorta instead of
forming a separate left innominate, as in the pigeon. It
will be remembered that in the frog the subclavians (or
brachials) come off from the aortic arch on each side, so
that the rabbit must be regarded as the more primitive in
having only one innominate. However, the arrangement
of carotids and subclavians varies very much throughout
the AZammata. Lastly, the very close correspondence
of the arterial and venous system is striking. With the
Plate XII.—Srconp DIssEcTION oF Raspit. (+a za/.)
ight Auricle.
Right Ventricle.
External Jugular..
Post-caval ’
Phreni
Pulmonary Vein. :-~ . Phrenic.
(Esophagus with
agus Nerve, --~ yi
Azygos. Liver.
Dorsal Aorta. Diaphragm.
i. Cieliac
(cut).
~ Liver.
Anterior Mesenteric- ~
(cut).
—-~-Dorso-lumbar A:
and Vein.
Left Adrenal ~Kidney, supplied by
‘Ureter.
Posterior Mesenteric—~
cut).
Femorals.~_ ~ Genitals
(cut).
Spermatic
Cord with Genitals...
Caput Epididymis...
- JHo-lumbars.
__.... Urinary B
~ Internal
Vas Deferens. -.._
Uterus Masc
Testis, -~
Prostate.
Cauda Epididymis. —~~
ne Cowper’ 's Gland.
Penis.
Perineal Gland.
Anus.
The alimentary system is removed by a cnt through the cesophagus and th
rectum, the liver and diaphragm are deflarted over to the left. The pelvis is cu
left. Forwards the heart is ben
1 is pulled | outwards
LEPUS. 389
exception of the difference caused by the hepatic-portal
system, the vein and artery to each organ are in close
-contact and agree in distribution.
The ¢horacic duct is the main vessel of the lymphatic
system; it discharges into the left precaval and runs
backwards beside the dorsal aorta.
Fig, 278.—FEMALE UROGENITAL ORGANS OF THE Rabbit.
(Ad nat.)
Right Kidney. — %
Left Kidney.
Ureter, Ovary.
Z
5
Fallopian Tube.
Uterus.
Uterus.
Vagina.
Cowper’s Gland, a, Urinary Bladder.
Rectum: = Vestibule.
Rectal Gland.
= Clitoris.
Perineal Gland. A
t
Anus, Vulva.
The lower part is twisted to show lateral view ; the upper part is a ventral view.
The &cdneys lie in the dorsal region of the abdomen, the
right further forward than the left. From each there runs
back a delicate ureter opening into the base of
the large thin-walled urinary bladder. ‘ From
the bladder there runs backwards a urethra.
In the male the /es¢es are pale bodies lying in the scrotal
sacs. These communicate with the abdominal cavity by
Urogenital.
390 CHORDATA.
inguinal canals. Through each of these canals there passes
a spermatic cord with genital artery and vein to the testis from
the lumbar region. The testis is partially surrounded by
an epididymis consisting of coiled tubes, the vasa efferentia,
which unite to form the vas deferens. This duct leaves the
scrotal sac by the inguinal canal and passes up round the
ureter of the same side, then backwards to open into the
urethra. The testes in the young rabbit occupy the normal
position in the neighbourhood of the kidneys, but by a
process called the descensus testiculorum they pass down-
wards and into the scrotal sacs.
The swollen base of the two vasa deferentia, as they
enter the urethra, is often termed the wéerus masculinus. In
the same position are the prostate glands opening into the
urethra, and posterior to them are a pair of Cowjer’s glands.
The urethra passes along the posterior surface of the fevzs,
which is formed of vascular erectile tissue.
In the female the ovaries are small oval bodies attached
by mesentery to the dorsal abdominal wall. The ovzducts
are paired tubes of the same size. Each has three parts,
viz::—the anterior portion or Fallopian tube, of small calibre,
and opening into the abdominal cavity by a large funnel ;
the middle portion or uterus, which has thick muscular
walls and is used for the retention of the young during
gestation ; the third portion or vagina, which in the rabbit
is fused with its fellow, resulting in a single wide vagina, at
the anterior end of which opens the os of each uterus, and
posteriorly it leads into the urethra which is, in the female,
known as the vestibule, There are Cowper's glands, as in
the male, and a vestigial penis called the chtorvis. The
opening to the exterior is called the vulva.
The brain may be isolated by careful removal of the roof
of the skull. It is chiefly remarkable for very large cerebral
hemispheres which are connected across by a
large band called the corpus callosum, for the
lateral expansion and coiling of the cerebellum, for the
division of the optic lobes into four, called the corpora
guadrigemina, and for the presence of fwe/ve cranial nerves,
the spinal accessory and hypoglossal being added to the ten
of the skate.
Nervous.
HAE dade iRY Wisosncit1UN OF THORAX AND NECK OF A
RABBIT FROM THE VENTRAL SIDE. (Ad xat.)
Anterior Facial.
Internal SR
Hypoglossal,
Posterior Facial.
External Caro
Thyroid Cartilage (L
Depressor. a
. Anterior Larynge:
Vagus,
Thyroid |
Phrenic,
_Cricoid Cartila;
(Larynx).
Recurrent
Laryngeal.
Carotid.
_—--»~ Jugu
Subclavian, ~
Innominate, - .. Subclavia
*. Subclavian.
“=. Ductus
Arter
Systemic Arc
~..Phrer
Ne
The ventral wall of the thorax is removed, the heart is thrown over to the
rabbit’s right, and the left lung is also drawn over to the right under the left phrenic
nerve. The sympathetic nerve and internal jugular veins are omitted in order not
hae . OA eta inl ’ branches have been removed.
Da se te seer ss . dns are all blue and the arteries
L
LEPUS. 391
A description of the nerves cannot be entered into
here, but a few of the more important are to be seen in the
neck. (Plate XIII.) In this region we have already noticed
the carotid arteries, the internal and external jugular veins,
the oesophagus, trachea and phrenic veins. Just internal to
the phrenic nerve and close beside the carotid artery runs
Fig. 279.—RABBIT’s BRAIN.
A, Dorsal View.
Olfactory Lobe.
Position of
CorpusCallosum.
Cerebral
Hemisphere.
Corpora
Pineal Body, Quadrigemina.
Flocculus of
Cerebellum.
Medulla Oblongata.
Olfactory Lobe.
Cerebrum. Infundibulum.
Crura Cerebri.
Hind-Brain.
Medulla
Oblongata.
B, Ventral View.
the vagus (or tenth cranial). It has a slight ganglionic
swelling, just opposite the larynx, and here gives off two
branches—the anterior laryngeal, which runs into the larynx,
and the depressor, which is a long delicate nerve running
backwards to the heart dorsally to the carotid. The vagus
is continued backwards into the thoracic cavity and along
the cesophagus to the stomach. It gives off the recurrent
392 CHORDATA.
laryngeal, a peculiar nerve which, on the right side, loops
round the subclavian artery and, on the left side, passes
round the ductus arteriosus. In each case it goes forwards
beside the trachea to the larynx.
The sympathetic can be followed as a ganglionated cord
between the vagus and its depressor branch, and the spiza/
accessory and hypoglossal can also be recognised supplying
certain of the neck-muscles,
280.—A MEDIAN LONGITUDINAL SECTION THROUGH THE RaBBIT’s BRAIN.
(Mainly after MARSHALL.)
Cerebral Hemisphere.
Middle Commissure.
Pineal Body.
sellum. Corpus
Callosum.
Fifth
Ventricle
ae Se Olfactory
Lobe.
Infundibulum. Optic Nerve. Anterior _
Foramen of Munro. Commissure.
Fourth Ventricle.
The skeleton of the rabbit can be divided, as in preced-
ing types, into axial and peripheral portions. It consists in
Skeletal, the adult chiefly of bone and it presents the
* remarkable feature of eiphyses. An epiphysis
is a cap of bone which, up to a certain age, can be detached
from the main portion of the bone as it is united to it
merely by cartilage. The meaning of these epiphyses will
be pointed out later (see page 413).
Skull_—The skull has mainly persistent sutures. The
characters of the teeth have already been noticed. The
important mammalian features are the heterodont (incisors,
LEPUS. 393
canines and molars) and thecodont (in sockets) teeth,
axial, the bony palate formed of bony expansions of
* the maxillze and palatines meeting in the middle
line, two occipital condyles upon the exoccipitals, the
suspension of the lower jaw by the squamosal, the single
bone of the lower jaw and the three auditory ossicles.
The large size of the nasal chambers and nasal bones,
the incomplete ossification of some bones, such as maxillee
and occipitals, the confluence of orbit and temporal fossa,
and, above all, the character of the dentition, are features of
the order Rodentia, whilst the presence of small second upper
incisors and other lesser features are characteristic of the
sub-order containing rabbits and hares.
The cranium of the skate could be recognised as formed
of the cranium proper and the cartilages of the three
sense-capsules. Similarly the bones of the rabbit’s skull
can be correlated with the cranium proper and the sense-
capsules.
The former are arranged more or less in rings around
the cranial cavity which facilitates their recognition.
The occipital ring is the most posterior. In many skulls
it may be completely detached from the others. It is
formed of a supraoccipital, paired exoccipitals and a dbast-
occipital,
The sphenoid ring is formed of paired parietals above
(and a small znterparietal), paired alisphenoids laterally and
a basisphenotd.
The presphenoid ring has a pair of frontals above, a pair
of orbitosphenoids laterally and a presphenoid below.
The ethmoid ring has a pair of nasals above, a mes-
ethmoid below, which lies between the two nasal chambers
and broadens out posteriorly to form the cribriform plate,
and a perforated bony septum between the cranial and
nasal cavities.
The bones of the sense-capsules are closely united with
those of the cranium proper. The auditory bones lie
between the occipital and sphenoid rings. ‘They consist
of a feriotic containing the inner ear and produced into a
prominent mastoid process, and the ¢ympanic which is swollen
into a hollow auditory dw//a and produced upwards there-
from as a bony auditory meatus. The eyes are embedded
304 CHORDATA.
in orbits which are mainly formed by the adsphenords, orbito-
sphenowds, frontals and other bones, but the anterior corner
is completed by a Zacrymal bone which develops especially in
connection with the eye.
The nasal capsules have several bones which are thin and
coiled in order to present a large surface. They are called
the ¢urdinals and are attached to the ethmoid, nasal and
maxilla. Hence they are called ethmo-, naso- and maxillo-
turbinals.
Fig. 281.—LATERAL VIEW OF SKULL OF THE RassBit. (dd nat.)
Lacrymal.
Alisphenoid. | Orbitosphenoid.
Frontal. Maxilla.
Squamosal. j
Parietal.
‘ Nasal.
Interparietal.
Supra-
occipital. Pre-
‘ maxilla,
Periotic.
Tympanic.
Mastoid
Process.
Paroccipital .
Process. Second
Incisor.
4%
Periotic. “
Pterygoid. ’
Malar.
These all form the cranium, and to them are added
a number of bones which arise in connection with the first
two visceral arches and form the main part of the facial
region. The premaxil/a and maxilla form all the anterior
region of the skull below the nasal chamber. Above the
maxille the small vomers are found. They are hidden by
the palatine processes of the maxille. The alatines and
plerygoids lie in the roof of the mouth. The sguamosal is a
large and important bone which lies between the auditory
bones and the sphenoid ring ; it has a glenoid cavity for the
LEPUS. 395
condyle of the lower jaw and is joined under the orbit to
the maxilla by a small jugal (or malar). This bony bar is
called the zygomatic arch.
The mandible or lower. jaw is in one piece or ramus on
each side. It has a condyle for articulation with the skull,
an angde at its posterior end and a coronoid process produced
upwards in front of the condyle.
The Ayoid consists of a central piece and two pairs of
cornua, as in the frog.
Fig. 282. —PosTERIOR VIEW Fig. 283.—LATERAL VIEW OF
OF ATLAS VERTEBRA OF AXIS VERTEBRA OF RaB-
RapBit. (Ad nat.) Bir. (Ad nat.)
Vertebrarterial Canal.
\
Neural
Spine:
Transverse Process.
Ligament,
Post. Zygapophysis.
oH
33
eV
eo
on
oA
eo)
Fig. 284.—ANTERIOR VIEW OF A CERVICAL VERTEBRA
OF RasBiT. (Ad nat.)
Cervical Rib,
Vertebrarterial Canal.
Lastly, in the middle ear is a chain of three ear-ossicles,
the malleus, incus and stapes. The malleus is attached to
the inner surface of the tympanum and the stapes to the
Jenestra ovalts of the inner ear.
The vertebral column consists of cervical, thoracic, lumbar,
sacral and caudal vertebre.
There are seven cervicals, as in nearly all mammals. The
first is the aé/as with two lateral wing-like cervical ribs, a
396 CHORDATA.
small centrum and two hollow facets for the occipital
condyles of the skull. The second or axis has a peg-like
odontoid process which belongs by origin to the atlas. The
other five have low neural spines and short centra. All the
cervical vertebrae have vertebrarterial canals, produced by
fusion of cervical ribs, as in the pigeon.
The thoracic vertebra are twelve. All have long neural
spines. ‘The rib has in each case a capitulum articulating
. between the centra of
Fig. 285.—LaATERAL VIEW io. weutebes: Gad a
oF THORACIC VERTEBRA OF RABBIT. : -
(Ad nat.) tuberculum axticulating
with the transverse
process of the hind-
most of the two verte-
Pree bre (see page 418).
pee: lhe a ee ae
in the sternum, which
is divided into a num-
Facet. —~ Weg eb ber of joints or s¢erne-
bre. The anterior end
is known as the manudbrium and the posterior end as the
xiphisternum.
”“
Neural Spine.
Fig. 286.—ANTERIOR VIEW OF Fig. 287.—LATERAL VIEW OF
A LUMBAR VERTEBRA OF A LUMBAR VERTEBRA OF
-RaBBIT. (4d nat.) RanBit. (4d zat.)
Metapophysis,
Metapophysis. f
Post. Zygapophysis.
Prezygapophysis.
, Transverse Process.
Articular Facet.
Articular Hemal Hamal Transverse
Facet. Spine. Spine. Process.
The lumbar vertebre are sever in number ‘They have
large transverse processes which slope forwards and down-
wards. The neural spines are smaller than in the dorsal
and there is a mid-ventral process or hypapophysis.
LEPUS. 307
The sacral vertebre are two. They are ankylosed
together and are firmly joined to the ilium.
The caudal vertebre vary in number up to fwenty. The
first few are ankylosed to the sacral vertebree; the rest
gradually become simpler till they are mere rods of bone
representing the centra only.
Compared with that of the pigeon, the vertebral column
of the rabbit exhibits far less adaptive modification, espe-
cially in the direction of fusions. With the exception of
Hig. 288.—PrcToraL GIRDLE AND FORE-LIMB OF THE
Rassit. (4d nat.)
; B Head.
Olecranon.
Spine.
Radius.
Acromion.
e
Supratrochlear Foramen,
Coracoid Process.
A, Scapula. B, Humerus. C, Radius and Ulna.
the first few caudals, there is no fusion of vertebra, a con-
dition probably due to the multiplicity of movement involved
in the varied life of the rabbit.
The pectoral girdle consists of a small vestigial clavicle
connecting the sternum with the second element or scapula
This is a large, triangular, flat bone with a glenoid cavity at
one angle. Down the centre of one surface is a ridge or
spine, culminating towards the glenoid cavity in an acromion
process which usually has a backwardly projecting part
398 CHORDATA.
or metacromion. The anterior or coracoid border of the
scapula is continuous with a coracoid process projecting
inwards. It represents a vestigial portion of the precoracoid
bone.
The humerus has a
Fig. 289.—Dorsat Virw or Lert large ead and _ two
MANus oF Rappir. (Ad nat.) prominent /uderosities.
(Slightly magnified). Distally it moves in the
z trochlea or articular sur-
face of the fore-arm,
above which is a small
supratrochlear foramen.
The radius and ulna
are distinct but closely
united to allow of no
pronation. The fore-
limb is permanently
supinated. The ulna is
produced back beyond
the radius to form the
olecranon process.
The carpal bones are
nine, Closely bound to-
gether by ligament. The
proximal carpals consist
of scaphoid, lunare and
cuneiform, and a small
sesamoid (pisiform),
together with a small
centrale. ‘The distal
carpals are the ¢vapezzum,
trapesoid, magnum and
unciform.
There are five meta-
carpals, each bearing a
digit. The first digit has two phalanges and the others
three.
The pelvic girdle has large za which run dackwards to
the acetabulum. The pubes are united ventrally to the
tschia, thus enclosing on each side a large obturator foramen.
The symphysis is pubic only.
Os
Magnum.
Unciform.
Cuneiform.
Ulna.
LEPUS. 399
The femur is long and has three trochanters, the ¢hi7d
trochanter being on the outer side. The ¢dza is also long
and is fused with the d/a, though the proximal end of the
latter is separate for part of its course.
The ¢arsus consists of a condylar astragalus articulating
with the distal end of the tibia, a long cadcaneum produced
Fig. 290.—BoNES OF PELVIC GIRDLE AND HIND-LIMB
oF RazpBit. (Ad nat.)
Great Third
Head. Trochanter, Trochanter.
lium.
Fibula.
Attachment
to Sacrum,
Little Trochanter.
Acetabulum.
Obturator
Foramen.
Ischium.
A Condylar Groove. B Cc
A, Pelvic ‘seen in ventral view. B, Femur or proximal limb-bone.
C, Distal limb-bones, tibia and fibula. :
backwards to form the /ee/, a small xavicular in front of
the astragalus, and a distal row of three bones. The
internal cuneiform (see page 420) is apparently fused with the
second metatarsal, hut the middle and external cuneiform
and the cuvdoid are distinct.
The first metatarsal and digit are absent, but the other
Jour are long, and each bears a three-jointed digit.
400 CHORDATA.
The front-limb of the rabbit shows a primitive condition
by the presence of a distinct centrale in the wrist, but the
hind-limb is specialised in the loss of fibula and first digit.
The third trochanter, however, appears to be an archaic
character.
Fig. 291.—DorsaL VIEW OF LErr Pes or THE RasBiIT. (Ad zat.)
(Slightly magnified. )
Terminal
Phalanx.
end Phalanx.
ist Phalanx.
SHUR Internal
: Cuneiform.
Navicular. me : Middle
Cuneiform.
External
Astragalus. zs
2 Cuneiform.
Calcaneum.
Development.—The rabbit is like nearly all mammals, viviparous.
Its period of gestation is thirty days and several young are produced at
a birth. The placenta is discotdal and dectduate. (For details of
mammalian development see Chapter XX VI.)
401
CHORDATA.
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402 CHORDATA.
CHAPTER XXIV.
GENERAL FEATURES OF CHORDATA.
PHYLUM CHORDATA.
The Phylum Chordata is in many respects the most
important of the whole animal kingdom and contains an
infinite variety of types from Zunicata to Man. It has five
leading structural characteristics which are present through-
out the group at one time in the life of each individual.
(1) A hollow dorsal nerve-tube, the anterior end of which
is hypertrophied to form the brain. It arises from the
epiblast.
(2) Lhe primary skeletal axis or notochord, an elastic rod
of chordoid tissue lying under the nervous system and
arising from the hypoblast.
(3) Paired pharyngeal clefts formed from protrusions of
the hypoblast in the anterior region of the alimentary canal.
(4) A metameric segmentation of the mesoblast, obscure
only in the lowest class. :
(5) A ventral heart or contractile circulatory organ (which
may be multiple, as in Amphioxus), and a particular course
of the blood-system, z.¢., forwards ventrally and backwards
dorsally.
All the other phyla differ from Chordata in these
characters and they are often contrasted with them as
NNon- Chordata.
It will be remembered that certain of the Celenterata present gastro-
vascular pouches which appear to be incipient coelomic pouches. In
the functions performed by their walls and in their hypoblastic origin
they agree with the latter, but they are not completely separated from
the gastric cavity and hence are not regarded as forming a third layer
or mesoderm. Ina similar way certain of the Mon-Chordata, namely,
a class of the 4rchicelomata, called Archichorda (or Hemichorda), show
several ‘of the chordate characters in an incipient stage. The type of
Archichorda described (2.e., Balanoglossus) shows a series of pharyngeal
clefts not essentially differing from those of Amphioxus, and these are
ATRIOZOA., 403
also present in another member of the class. In addition, there is a
dorsal nervous system, partially tubular, but there is no brain, and the
whole nervous system is still in structural continuity with the epiblast (or
ectoderm). Lastly, there are certain portions of the endoderm (or
hypoblast), the epithelial cells of which undergo a modification into
chordoid tissue histologically similar to that of the notochord.
In Balanoglossus, a pre-oral part called the stomochord (the ‘‘ noto-
chord” of some writers), the whole anterior wall of the pharynx, and
an area in the intestine (ygochord) (and in its allies a pair of
pharyngeal diverticula, called plewrochords) are of this nature. ence
the Archichorda resemble the true Chordata in having pharyngeal
clefts, a dorsally-situated though simpler nervous system, and incipient
chordoid structures.
In the other two features they differ from the Chordata, 7.¢., they
have no true metameric segmentation and no ventral heart. The
circulation is usually forwards dorsally, but one member of the
Archichorda has a reversible circulation like the 7wzdcata.
The Chordata fall very naturally into sub-phyla, Azriozoa
and Vertebrata.
SUB-PHYLUM I.—ATRIOZOA.
The AZviozoa are more lowly organised than the Verte-
brata. The pharyngeal clefts are multiplied and the pharynx
is specialised into a huge sac (or sieve) for obtaining food,
with a complex apparatus of dorsal and ventral grooves and
gland-cells. The water separated from the food-particles
passes into a spacious a¢rium which arises from the epiblast.
(Hence the name of the group). The notochord is never
replaced by any other axial skeleton, and at most is sur-
rounded by a membranous sheath. The brain has only a
single internal cavity or vesicle, and the eye is single and of
simple structure.
The development is external to the parent, purely larval
(except for the very earliest stages), and there is a gastrula
larva followed by the chorduda larva.
The sub-phylum is entirely marine and mainly pelagic
or sedentary.
It contains two classes—r1. Tunicata (UROCHORDA) ;
2, CEPHALOCHORDA.
Cuiass I.—TuNIcATA.
Ascidia was the type of this class and is representative of
the simple sedentary Zunicata.
404. CHORDATA.
They chiefly differ from the Cepha/ochorda in the simple
and doubtfully segmented nature of the mesoderm, involving
an absence of nephridial excretory organs and of peri-
visceral ceelom. ‘Their real relationship to the other class
is shown most clearly by the structure of the larval form
rom the chordula onwards. In some respects the larval
ascidian attains a higher level of chordate structure than
Amphioxus.
Like most sedentary forms the Zumicata show a tendency
to reproduction by budding, and to its natural corollary, the
formation of colonies. These colonial types are called
compound ascidians, the individuals being usually embedded
in a common test, and sharing a common atrial cavity.
Most are sedentary, but some (e.g., Pyrosoma) are pelagic.
This compound form is a large bell-shaped organism with a
huge atrial cavity in its interior. It is strongly phosphores-
cent. Amongst other pelagic forms are Appendicularia,
remarkable for retaining its notochord down the centre of a
vibratile tail throughout life and possessing a number of
other simple features and Sa/éa, which shows a well-marked
metagenesis or alternation of generations.
Crass II.—CEPHALOCHORDA.
This class contains only Amphioxus and a few other
genera which do not differ essentially from it. Hence the
characters of the class are those of the type. We may
specially notice as differences from Vertebrata the produc-
tion of the notochord to the extreme anterior end of the
body, the absence of paired sense-organs, of a median heart
and of jaws, the different method of feeding therein involved,
and the whole structure of the pharynx and atrium.
On the other hand it approaches the Vertebrata nearer
than do the Zunicata, in the structure of the mesoderm,
highly developed into segmented myomere muscles, a peri-
visceral coelom and numerous nephridial excretory organs,
the definite direction of circulation in the blood-vascular
organs, and the clear indication of a hepatic-portal system.
In a very general way the method of locomotion is
vertebrate and the method of feeding atriozoan.,
VERTEBRATA. 40
SUB-PHYLUM II.—VERTEBRATA.
The Vertebrata have been illustrated by no less than seven
types taken from the six classes. They show a remarkable
gradation in structure, which has only one break involved
in passing from aquatic to terrestrial habitat.
The general: characters of the Vertebrata separating them
from the Azriogoa are as follows :—
1. A complex skin or external covering to the body.
2. A brain with three primary vesicles.
3. Three pairs of cephalic sense-organs.
4. The notochord surrounded, and in most cases
replaced, by a mesoblastic skeleton of cartilage, and in
higher types, of bone.
5. The presence of ingestivé organs, in the form of jaws
or teeth, in correlation with which the pharyngeal clefts are
purely respiratory (gill-slits) and the endostylar apparatus
becomes vestigial.
6. In all but the lowest class there are two pairs of paired
limbs and a series of cartilaginous visceral arches.
ORGANS OF VERTEBRATA.
We may now briefly review the chief organs of Vertebrata.
Skin.—The shiz is formed of two distinct parts termed
the epidermis and dermis. The epidermis is formed of a
basal epithelium resting upon the dermis, which represents
the primary epiblastic layer of the embryo and of a mass
of cells above it which have been produced by prolifera-
tion. This mass can be defined as consisting of a lower
portion of growing cells, called the mucous Jayer, and an
upper superficial layer of compressed horny cells, called the
corneous layer.
The dermis is derived from the mesoblast and is formed
of connective tissue and muscle intersected by nerves and
blood-vessels.
There are usually skin-glands formed from the mucous
layer, and there is commonly an exoskelefon consisting of
local productions of horny material, such as scales, claws,
horns, feathers, or hairs.
406 CHORDATA.
Nervous System.—The drain arises as a swelling of
the anterior portion of the dorsal nerve-tube, the posterior
portion remaining as the spinal cord. The single swelling
soon becomes constricted into three primary vesicles called
the fore-brain, mid-brain, and hind-brain. The fore-brain
then gives off the two optic vesicles as described below, and
constricts into two secondary vesicles called the cerebrum and
the thalamencephalon. The mid brain remains simple and
Fig. 292.,—Four STAGES IN THE DEVELOPMENT OF THE
VERTEBRATE BRAIN.
Neuropore.
< ec
%
2
oQ
=
<o =
, 3.
ao
Q
5
v Ps eels — 2
: Spinal Cord.
I,, Fore-brain. IL., Mid-brain. IIJ., Hind-brain.
A, A tube with opening at each end. 1, Cerebrum.
B, A swollen brain at the anterior end. 2, Thalamencephalon.
C, Formation of the three primary vesicles. 3, Optic Lobes.
D, Formation of the five secondary vesicles. 4, Cerebellum.
5, Medulla.
gives rise to the optic Jobes, and the hind-brain forms the
cerebellum and medulla oblongata. Hence the brain has now
five parts in succession, z.e., cerebrum, thalamencephalon,
optic lobes, cerebellum and medulla oblongata. The
original cavity of the brain remains to a large extent in
these parts. The cavities in each half of the cerebrum are
known as the /ateral ventricles, each communicating by a
Joramen of Munro with that of the thalamencephalon or
the ¢hird ventricle, and that of the medulla oblongata or the
VERTEBRATA. 407
fourth ventricle. The part in the optic lobes becomes con-
stricted into a small canal or z#er leading from third to
fourth ventricles.
Fig. 293.—DIAGRAM OF THE
VERTEBRATE BRAIN.
(Mainly after Hux.ey.)
Cerebrum. Pineal Body.
Optic
Lobe. Cerebellum. Spinal Cord.
Crura
Cerebri.
The dorsal wall of the thala-
mencephalon is produced into a
process called the prneal body,
which, in some cases, shows
evidence of being a vestigial eye.
The ventral wall is also produced
into a process called the infundt-
bulum, coming into relation with
the pituitary body (v.i.); the
lateral walls become thickened
and form the optic thalami.
Thus the brain becomes a com-
plex organ consisting of a linear
series of specialised portions ;
but a further complication takes
-place in the flexure of one part
upon another. In the highest
types (mammals) the brain is
twice flexed upon itself and its
+ et
Fourth \
Notochord.
Ventricle.
Fig. 294. — DIAGRAM-
MATIC MEDIAN SECTION
THROUGH A VERTEBRATE
BRAIN, SHOWING THE
VENTRICLES,
Lateral _
Ventricle.
Third Ventricle.
Iter.
Fourth Ventricle.
origin from a single tube is thus disguised. .
From the brain there arise at least ten pairs of cranial
nerves which are remarkably constant in their relationship.
The fore-brain gives rise to the olfactory (I.) and optic
(II.), the mid-brain to the oculomotor (III.) and trochlear
(IV.), and the hind-brain to the trigeminal (V.), abducens
408 CHORDATA.
(VI.), facial (VII.), auditory (VIII.), glossopharyngeal (IX.)
and vagus (X.). In Amniota two more are added — the
spinal accessory (XI.) and hypoglossal (XII.).
From the spinal cord there arises a series of spinal nerves,
each of which has a dorsal (sensory) and ventral (motor)
root, the two uniting soon after emergence from the spinal
cord.
Sense-Organs.—The first sense-organs or olfactory
organs arise as a pair (single in Cyclostomata) of epiblastic
pits at the anterior end of the head. They form the
olfactory sacs with a sensory epithelium. The fore-brain in
development grows out in a pair of olfactory lobes which
Fig. 295.—THREE STAGES IN THE DEVELOPMENT OF THE
VERTEBRATE EYE,
Fore-brain. Secondary Optic Vesicle.
: Epiblast. : Optic Vesicle.
_ Optic. Lens. Optic Stalk.
Vesicle.
Mid-brain.
Primary Optic Vesicle. R
Pigment Layer of Retina.
Sensory Layer of Retina.
Lens. @ Optic Stalk.
Epiblast.
ChoroidjFissure. Secondary Optic Vesicle.
come into intimate contact with the sensory epithelium by
means of the olfactory nerves. The lobes may be of great
length, as in the skate. In the Amaio¢a the surface of the
olfactory sacs is kept perpetually moist by gland-cells, and
they acquire internal openings or zuéernal nares into the
buccal cavity. They then form a passage for the current of
respiratory air.
The second sense-organs or eyes arise from three sources.
The fore-brain grows out laterally into two primary optic
vesicles towards the skin. These take the form of a round
VERTEBRATA. 409
vesicle connected with the fore-brain by a narrow stalk,
called the optic stack. The outer half of the vesicle then
becomes pressed in, like an invaginating blastula, and the
rim so produced gradually constricts to a small aperture,
like the blastopore of a gastrula. Hence the sac is now a
two-layered optic cup, like a gastrula, and contains a cavity,
the posterior chamber of the eye. The outer layer becomes
the pigment-dayer, and the inner becomes the sensory-dayer, of
the retina. Meanwhile, the epiblast on the lateral wall
of the head opposite the optic cup invaginates a small
Fig. 296.—DIAGRAM OF THE VERTEBRATE EYE.
(Seen in median section.)
Lens.
Vitreous
Humor.
Conjunctiva.
Cornea.
Aqueous
Humor.’
Tris.
Blind Spot.
Sheath of Optic
erve,
Retina. i
Sclerotic. Choroid. Optic Nerve.
vesicle, which becomes the Zens of the eye and fills up the
small aperture of the optic cup.
The sensory cells of the retina send out nervous pro-
cesses, which grow along the optic stalk and eventually
reach the brain where they end in the optic lobes.
These processes arise from the ends of the retinal cells
which are nearest the posterior chamber; and the actual
sensory elements, called rods and cones, arise from their
deeper ends towards the pigment-layer. Hence the light
has to pass through the nervous layer to reach the sensory
410 CHORDATA.
layer, a peculiarity of the vertebrate eye. If it be recollected
that the brain is invaginated from the dorsal epiblast and
the eye is an invaginated part of the brain, it will be clear
that the rods and cones really lie on the morphological outer
surface, the normal situation for sensory elements.
The third element of the eye is mesoblastic ; it consists
of a choroid coat carrying blood-vessels and partially cover-
ing the lens as the 77s, and the sclerotic, a hard cartilaginous
capsule enveloping the eye. In front of the lens it is trans-
parent and forms the cornea, the anterior chamber being
formed between it and the lens.
To this we must add the eye-mmuscles which are inserted
in the sclerotic and serve to move the eye. They have been
noticed in the skate and do not differ essentially in any
type.
Obliquus superior innervated by 4th nerve.
Obliquus inferior innervated by 3rd nerve.
Rectus superior innervated by 3rd nerve.
Rectus inferior innervated by 3rd nerve.
Rectus internus innervated by 3rd nerve.
Rectus externus innervated by 6th nerve.
Accessory organs, such as eyelids and lacrymal glands, are
added in terrestrial types.
The third sense-organs, or auditory sacs, appear to be a
single much hypertrophied pair of /ateral-line sense-organs,
organs which were noticed in the skate but are not found as
such in terrestrial Vertebrata. The auditory sacs arise as
paired pits of the epiblast, far back on the head. Each pit
swells out as an auditory sac, its connection with the epiblast
becoming constricted into a thin duct, the agueductus vestibul.
The walls of the sac then grow out into three (one in
Myxine).semi-circular canals, long tubes which run in a semi-
circle in three separate planes and open at each end into the
sac. Their bases are swollen into ampulle, to which the
8th nerve gives off numerous branches. The sac itself is
now known as the vestibule. In many fishes, eg., the skate,
its cavity remains connected with the exterior by the ague-
ductus vestibuli, In the skate this zzmer ear (or membranous
abyrinth) lies close to the hyomandibular cartilage, near
which is the spiracle. Vibrations of the water may be trans-
mitted through the hyomandibular to the inner ear.
VERTEBRATA. 4it
In the frog and higher types the auditory sac becomes
constricted into two portions called the wériculus and the
sacculus. The utriculus gives rise to the semi-circular canals,
and the sacculus to a coiled cochlea. The agueductus vestt-
éuf remains closed and is known as the ductus endolym-
phaticus.
Fig. 297,—DEVELOPMENT OF THE VERTEBRATE Ear.
Auditory Vesicle. B Duct.
Utriculus,
A
Sacculus.
Cc Semi-circular Canal.
Aqueductus Semi-circular
Vestibuli, — Canal.
| | Ampulla,
Utriculus.
‘Lagena.
Sacculus.
A Epiblastic invagination. B, Division into superior and inferior parts.
C, The ear as in the skate (cK Fig. 230, p. 323).
But the most important modification is involved in the
formation of the médd/e ear. The cleft corresponding to the
spiracle of the skate appears to be modified into a tube,
closed at the surface in the frog by a membrane or tym-
panum, but still opening into the throat by the ustachian
aperture. The hyomandibular appears to become the
columella which leads from the tympanum to the inner ear,
and transmits the vibrations of the air thereto.
412 CHORDATA.
In the pigeon a further complication is involved in the
formation of the outer ear, represented by an external audi-
tory meatus leading from the exterior to the tympanum ;
and, lastly, in the rabbit, the Azza is added.
In the mammals the columella appears to be represented
by three auditory ossicles, as noticed in the rabbit.
These three sense-organs, their accessories, and the brain
mark’ out the head of the Vertebrata.
Fig. 298.—A DIAGRAM OF THE VERTEBRATE EAr.
Semi-circular
Canals.
,Ductus Endo-
lymphaticus.
External Audi-
tory Meatus. y
vo
as) Bony Labyrinth.
Tympanum. /f & DB 3g Perilymph.
7 ES
@ & 8
m@ O09
wm
2 ees
Tee
55 28
Ae a?
The whole diagram represents the ear of the rabbit (except that only one
ear-ossicle is indicated) ; all to the right of AA represents the ear of the pigeon ;
to the right of BB represents the frog with middle and inner ear only; and the
ear of the skate is represented by the part to the right of CC, forming the inner
ear only.
Skeletal Organs.—The skeleton of Vertebrata shows
a succession of three kinds, which replace each other in time
throughout the classes and in the development of the higher
VERTEBRATA. 413
individuals. These are the membranous, the cartilaginous
and the bony. All arise from the mesoblast: the first is
continuous, the second is largely segmented and the third is
completely segmented.
In Myxine we find the membranous skeleton enveloping
the notochord or primary chordate skeletal axis and the
nerve-cord, and continued into the septa between the
myomeres. There is little progress here beyond Amphioxus.
Cartilaginous nodules in the vertebral column, a cartila-
ginous cranium, and other parts appear in the lampreys, and
a more or less complete cartilaginous skeleton is found in
the skate. .
In bony fishes and in the Am=nzota the cartilage becomes
supplemented and eventually replaced by a bony skeleton.
Bone is produced by the secretory activity of certain
cells called osteoblasts, and bones are known as membrane-
bones or cartilage-bones, according to their origin. The
membrane-bone is produced at once in the connective or
membranous tissue, whereas the cartilage-bone is preceded
by cartilage which has to be removed piecemeal as the bone
is produced. The distinction is merely arbitrary, and is
somewhat the same as the difference between building a
roof with single slates in situ (cartilage-bone) and construct-
ing an entire roof (as do many primitive peoples at the
present day), and then lifting it into position (membrane-
bone). The latter is, in each case, the more primitive
method. The final result in each kind of bone is the same,
and the two kinds cannot be structurally distinguished.
Complete ossification is usually effected fairly late in
life, mainly because cartilage can grow more readily than
bone. In nearly all the ammata most of the bones have
separate caps or epiphyses at each end, probably to allow
free use of a formed joint in the early stages, whilst the
parts between the epiphyses and the main bone are still
growing cartilage. In late life the epiphyses usually fuse on
to the main bone.
The replacement of cartilage by bone is effected from
certain centres, called centres of ossification, and the history
of these throws light upon many obscure points in the
skeletal structure. The simplest plan for the ossification of a
long bone would be to institute a single centre of ossification,
414 CHORDATA.
say in the mechanical centre, and thence to form bone to
either end. But bone is a harder and'more resistant substance
than cartilage. and it is often more to the advantage of the
organism that the parts which are subjected to special strain
should first be ossified. Hence the ends of the long bones,
which form the joints, and very often other parts, such as
trochanters and tuberosities acting as points of attachment
for muscles or tendons, have separate centres of ossification.
When the cartilage ceases to grow, then the ossification
proceeding from each centre, the bony elements meet and a
single complete bone results. Jf, as in mammals, the bony
elements are separated for a long time bya thin layer of
growing cartilage, then the elements are separated in the
dried skeleton by “sutures” and may fall apart. Hence
the caps or epiphyses already referred to. But the final
result is a single bone of the same size and shape as the
cartilage.
In many cases the single piece of cartilage may be re-
placed permanently by two or more bones with a joint
between them. Cartilage is elastic, and a piece of cartilage
may therefore “ give” to certain strains, by virtue of its elas-
ticity, sufficiently to dispense with the necessity for a joint.
Bone, however, is far more rigid, and hence a single elastic
cartilage, such as the palatoquadrate bar, is replaced by at
least three bones—the palatine, pterygoid and quadrate—
which are more or less movable on each other. The replace-
ment of the hyomandibular cartilage of lower types by three
(or four) ossicles of the ear is probably another instance.
The skeleton can be conveniently considered under two
heads :—1. The axial skeleton, skull and vertebre. 2. The
appendicular skeleton, limbs and limb-girdles.
AxtaL.—The skull has a double origin, being really
formed of two parts which are almost entirely distinct in
the fishes. These are (1) cranium; (2) the visceral arches.
The cranium arises essentially as a protecting mass to
the underlying brain, and the visceral arches arise primarily
as strengthening bars between the branchial clefts. The
first two of these arches alone take any part in the formation
of the skull.
(1) The Cranium.—tIn the earliest stages the brain is
enclosed on all sides by a membranous sheath which also
VERTEBRATA. 415
envelops the three pairs of vertebrate sense-organs. The
notochord runs in the ventral wall of this membranous
cranium as far as the mid-brain, terminating behind the
infundibulum (Fig. 293, page 407). The first cartilages
Fig. 299.—DEVELOPMENT OF VERTEBRATE CRANIUM.
Dorsal View of Embryo.
Nasal Sac.
Trabecule. ) Rye
Parachordals.
Auditory Sac.
Notochord. FI
!
Fig. 300.—DEVELOPMENT OF VERTEBRATE CRANIUM.
(Later stage.)
Internasal
Septum.
Nasal Capsule.
Pituitary Fossa.
Basis Cranii.
Capsule.
Basis Cranii.
Notochord.
to appear are two pairs of plates alongside of the noto-
chord. The first pair extends forwards on either side
of the infundibulum as the ¢vadecude, the hinder pair,
416 CHORDATA.
or parachordals, soon fuse above and below the noto-
chord to form the dasés craniz. The trabecule then meet
in front under the fore-brain to form a median plate,
called the ethmo-nasal septum. All three pairs of sense-
organs now acquire cartilaginous sense-capsules, which, with
the exception of that of the eye (or sclerotic) fuse on to the
primitive cartilaginous cranium, or dasal plate, formed by
the trabeculez and parachordals. The basal plate then
grows up on either side to enclose the brain. The edges
meet dorsally in the occipital region and also forwards in
the ethmoid region. Thus is formed the cartilaginous or
chondro-cranium. ;
In the sharks and skates this condition remains, but
in Amphibia a number of bones are added, and in the
higher classes the bones almost completely replace the
cartilage, forming a complete osteo-cranium. This osteo-
cranium is produced partly by membrane-bones, which sink
in, and partly by cartilage-bones. The bones of the osteo-
cranium are arranged more or less in rings, a system which
gave rise, in the hands of Goethe, Oken and Owen, to the
beautiful vertebral theory of the skull. The hindmost ring
is the occipital, with a dasioccipital, two exoccipitals and a
supraoccipital. The second ring is the sphenoid, with a
basisphenoid, two alisphenoids and a pair of parietals. In
front of this is the presphenoid ring, with presphenoid (at
base), paired ordbitosphenoids and a pair of frontals. The
ethmoid ring completes the front-end with a mesethmoid
and zasads. Between the occipital and sphenoid rings are
the periotic, a bony capsule of the ear,* and the large
sguamosal.t Connected with the orbit, and lying at the
anterior corner of it, is the small /acryma/.{ Lastly, immedi-
ately below the mesethmoid, in the roof of the mouth, are
the vomers, unpaired in mammals, and the farasphenoid.
We have already seen that the skull is, in the course of its develop-
ment, preformed in membrane, and the greater part of it in cartilage.
The cartilage is then gradually replaced by bone, a stronger and harder
substance, by the process of ossification described above. If ossification
* The periotic may be represented by as many as five separate otic bones, as in
the cod (p. 336).
+ The “temporal” bone of human anatomy is the fused periotic, tympanic and
squamosal.
{ The lacrymal is one of a series of circumorbital bones (cf cod).
=
VERTEBRATA. 417
were to commence at one part of the skull, say the hind-end, and work
forwards, the one part of the skull would become ossified too soon to
allow the necessary growth in size, and the rest would ossify too late to
form an efficient protective cranium. Hence ossification begins at
various points simultaneously. These points are called centres of ossifi-
cation, and their position is determined by mechanical conditions.
Radiating in all directions from these centres, each bone is gradually
produced until it comes to touch its fellow. Hence the ossification of
the skull is effected.by ‘‘piecework,”’ divided amongst the centres of
ossification ; and in the cranium, in which a general protective function
is requisite, the ‘‘ pieces” are divided fairly accurately into successive
rings, each of which is again subdivided into three or four. Thus we
seek to explain the ‘‘segmental” formation of the bony skull as due
rather to an orderly mechanical, method of producing an osseous cranium
from cartilage, than as indicating a primary origin of the skull from
vertebrae. ‘The method of ossification of a vertebra is due to a similar
cause.
If all the cartilage becomes ossified, a continuous bony cranium is
the result, incapable of further increase in size; but in most Ammnzota
the bones remain for a long time (until late in life) separated by a thin
layer of growing cartilage which leaves a ‘‘suture” in the dry skull.
‘This enables every bone to continue increasing in size and with them
the entire cranium. ;
(2) Zhe Visceral Arches.—The first two cartilaginous
visceral arches of the fishes are called the mandibular and
the Ayoid. Each has an upper and lower half on each side.
The upper half of the mandibular arch is called the padazo-
guadrate bar and the lower the mandible, and the two are
bent upon each other to form upper and lower jaw. The upper
half of the hyoid arch is the Ayomandibular cartilage which
is attached to the otic or ear-region of the skull; the lower
is the Ayoid cartilage. These visceral arches are attached by
ligament to the cranium in the lower types, but in the higher
the bones which replace them form the very important facial
part of the skull.. The palatoguadrate cartilage is replaced
by the palatines, pierygoids and guadrates, and, in addition,
the premaxille, maxille and jugals are added in connection
with it. The mandibular cartilage is replaced by the
mandible and the hyomandibular cartilage by the hyo-
mandibular bone.
The succeeding arches are called branchial arches. There
are five in the skate, four in Zé/eostomi, and in all fishes they
serve as a support to the gills and walls of the pharynx. In
the Ammniofa they mainly disappear. The first branchial
usually remains in part as the posterior cornu of the hyoid,
M. 28
418 CHORDATA.
the second and third form the ¢yroid, and probably the
arytenoid cartilages of the larynx.
THe VERTEBR#.-—-A typical vertebra consists of a
centrum or main axis, above which is a bony xeural arch
covering in the spinal cord. It is often surmounted by
a more or less prominent median neural spine. From each
side of the neural arch there usually protrudes a lateral
process known as the ¢ransverse process. In the anterior
part of the vertebral column the vertebra usually bears a
rib, which is articulated to the centrum by its head or
capitulum and to the transverse process by its tuderculum.
The rib may, however, become completely fused on to the
vertebra (cervical), or it may be attached only to the
transverse process, or may become fused with the transverse
process (lumbar and sacral). In the region behind the
sacrum there is often a hemal arch, but in mammals this is
only found in a few types in the form of chevron bones
which articulate Jefween the vertebre. In dAZammatia the
centra have epiphyses or caps of bone, and these are usually
flat, though they may be opisthoccelous in some of the
cervicals. At the front and hind-end are anterior and
posterior zygapophyses which serve as articulations between
the vertebree.
The vertebre are usually divided into—(1) cervical,
(2) thoracic, (3) lumbar, (4) sacral and (5) caudal. The
cervicals are defined as lying between the skull and the
first thoracic, or the first vertebra that has a pair of ribs
which meet the sternum. The thoracic vertebre bear ribs
which meet the sternum. In all the higher Vertebrata the
sternum is formed from the fusion of the distal ends of the
ribs.
Development.— The embryo has a notochord, around which is
formed a continuous mesoblastic membranous or skeletogenous sheath.
This sheath extends dorsally round the neural tube ( cranium).
Paired masses of cartilage then appear above and below in the sheath.
Their bases fuse across from side to side and dorso-ventrally to form the
cartilaginous centrum, and the dorsal arches grow up round the spinal
cord to form the neural arch. Ossification then takes place, there
being usually several centres of ossification.
APPENDICULAR, — All Vertebrata above Cyclostomata
(and exceptions) have two pairs of limbs and limb-girdles,
an anterior or pectoral and a posterior or pelvic.
VERTEBRATA. 419
The girdles in a general way usually present three parts,
cartilaginous in lower types, bony in higher. These are the
following :—
PECTORAL GIRDLE, PELVIC GIRDLE,
Antero-ventral. Precoracoid. Pubis.
Postero-ventral. Coracoid. Ischium.
Dorsal. Scapula. Tlium.
Cavity for articulation Glenoid. Acetabulum.
of the limb.
Although these two girdles can be thus directly com-
pared, they become very dissimilar in higher types. The
pectoral girdle has a membrane-bone, the clavicle, which
replaces the precoracoid. This joins the sternum, and
hence the pectoral girdle becomes connected ventrally with
the axial skeleton; but in the pelvic girdle the dorsal
element or ilium becomes attached to the vertebral column,
forming a sacrum, and the pubes and ischia tend to fuse
ventrally.
The limbs are in most Pisces formed on the type called
ichthyopterygium, consisting of one or more basal pieces
(f. Skate) bearing a row of distal elements or fiz-rays. (See
fisces.) In the other Vertebvata the limbs are of the type
called a cheiropterygium or pentadactyle limb. In describ-
ing this type we may first explain the following terms :—
Both limbs in their supposed primitive position hang
down on either side of the body, and if we draw an
imaginary axis down the centre of the limb, certain parts
of the limb are nearer the head, these being termed jve-
axial, whereas those nearest to the hind-end of the animal
are called postaxial, Again, the part of the limb which is
closest to the body is termed the proximal end and the
part furthest away the dzsta/ end, and generally any point
described as dista/ lies further out than one called proximal.
The typical cheiropterygium has a single proximal limb-
bone, called in the fore-limb the Aumerus, in the hind-limb
the femur. Then follow two distal limb-bones in each case ;
the preaxial is the vadzus in the fore-limb, the “za in the
hind-limb; whilst the postaxial are respectively the wna
and the jidu/a. The small bones which follow are the
carpalia or wrist-bones and the ¢arsafa or ankle-bones.
fo) CHORDATA.
he proximal carpal bones are the radiale, intermedium
id ulnare, together with the centrale (which in some cases
is paired), whereas the proxi-
Fig. 301.—D1aGRaM OF mal tarsals are tibiale, inter-
eee medium and fibulare, and also
cha acentrale. In each limb there
follow five distal carpalia and
tarsalia, each of which bears
a metacarpal or metatarsal,
followed by the phalanges of
the fingers or toes. The primi-
tive position of the limbs is
not retained, but they become
altered. Firstly, they are bent
into a Z-shape by a bend
downwards between the proxi-
mal bones and the distal and
a bend upwards between the
latter and the carpals or tarsals.
The limbs still protrude out-
wards at right angles to the
body, a condition still pre-
served in many reptiles with
shuffling gait. Secondly, the
upper joint (knee or elbow)
becomes deflected
ga 10N ’ inwards towards
cr ense rans the body, . the knee
Ge! oo ; Jorwards and the
A ee elbow Jbackwards,
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DISTAL EXTREMITY part of the limb are
therefore both bent
VERTEBRATA, 421
forwards through the same angle, but in the forelimb the
elbow is bent backwards and the foot forwards, resulting in
a twisting of the two distal limb-bones (radius and udna).
Thus-is produced the important movement of fronation.
In a great number of mammals which use their fore-limbs
almost entirely for progression the bones are permanently
pronated, but in others the radius rotates and allows of
supination and pronation at the desire of the animal.
Blood-vascular system.—The heart arises* as a
contractile portion of the ventral vessel running forwards to
the gills. It soon be-
comes constricted into
Fig. 302.—DEVELOPMENT OF THE
an anterior ventricle VERTEBRATE HEART.
and a posterior auricle. Heart.
It then becomes bent a
upon itself in an §, ——
hence theauricle comes
to lie dorsally, and fin- Ventricle. Pericardium.
ally anteriorly, to the Conus Arteriosus.
ventricle. Accessory ? Eo es
to this two-chambered a
heart in the fishes are Artery. — Auricle. Sinus Venosus.
the sinus venosus or
5 Auricle. Sinus Venosus.
dilated part of the
veins opening into the Cc
auricle, and the conus — Branchial
arteriosus. or valved Artery:
portion of the ventral Gans: ia
aorta leaving the ven-
tricle. This heart is
entirely systemic. In
the mud-fishes and Amphibia the auricle becomes divided
into two by a median septum, the left auricle receiving
blood from the lungs only. In the pigeon and rabbit the
ventricle also is divided by a median septum, and then the
respiratory and systemic currents are completely divided,
the right side of the four-chambered heart acting as a
respiratory heart and the left as a systemic.
A, A swelling on ventral vessel. __B, Constriction
into chambers. C, Twisting into an S.
* The heart in many cases has a double rudiment in the embryo.
422 CHORDATA.
In the skate the blood from the heart passes by |
ventral aorta to the gills by five afferent branchials, anc
thence by five efferent branchials to the dorsal aorta
There are in fishes never less than four branchials
When the gills are lost in terrestrial animals the afferent
become directly continuous with the efferents, and th
arches so formed are called arterial arches.
Fig. 303.— LATERAL VIEWS OF ANTERIOR ARTERIAL SYSTEM O
VERTEBRATES.
Carotid. Efferent Branchials. Dorsal Aorta.
£\Xs L
A
z ——
Afferent Afferent Branchial Artery.
Branchials. Branchials.
4 Dorsal Aorta.
Carotid. 2 3 4 Dorsal Aorta,
Cc
3
g
7 S) Systemic 2 3 4 Pulm
TH 32 3 4 Afferent Branchials.
A, Skate. B, Teleostome. C, Frog.
In the frog there are four arterial arches at an early stage
but later the first remains as the carotid arch, the secon
persists as the sys¢emc, the third is said to atrophy, and th
fourth forms the pu/monary. The connections betwee
these arches persist as membranous vestiges called ductu
Botalui.
In reptiles much the same arrangement holds, but i
birds the left systemic is lost, whilst in mammals the rigl
atrophies.
In the venous system the principal change is the replaci
ment in vertebrates above fishes of the paved cardinals b
VERTEBRATA, 423
the unpaired postcaval, We have already noticed in the
rabbit, as in all mammals, that the right cardinal persists as
an azygos vein (page 387).
Fig. 304.—THE ARTERIAL ARCHES OF VERTEBRATES,
Ventral view.
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A, Frog and reptiles. B, Birds. C, Mammals.
Ccelom.—The ccelom or primary cavity of the meso-
derm arises throughout the Vertebrata as a schizocele, the
mesoblast splitting into somatic and splanchnic layers.
Fig. 305.—DIAGRAMMATIC TRANSVERSE SECTION OF A VERTEBRATE
EMBRYO.
(Mainly after VAN WIJHE.)
Nerve Cord.
Myotome.
Notochord...
Aorta.
Pronephros..
Perivisceral
‘celom.
Intestine.
Nevertheless it has the same relationships as in the etero-
celic celom of Amphioxus. As in the latter, the myoccele (or
24 CHORDATA,
avity of the myotome) early disappears and the ventral
lement alone persists as a continuous perivisceral cavity.
Fig. 306.—DIAGRAMMATIC TRANSVERSE SECTION THROUGH A
LATER VERTEBRATE EMBRYO.
(Mainly after VAN W1JHE.)
Nerve Cord,
er,
Mesonephric
Tubules,
Pronephric Duct.
Intestine. y
Perivisceral Ccelom.
Fig. 307.—THE PARTS OF THE Ca:LOM IN THE THORACIC
CAVITY OF A MAMMAL.
(After WIEDERSHEIM.)
Vertebra.
-
Somatic Layer
of Pleura.
Trachea, isceral Layer
Lung. Yi; of Pleura.
( B
A ‘ %
Lung.
‘ t Ny
mae < ae Heart.
Somatic Layer Visceral Layer Somatic Layer _ Visceral Layer
of Pleura. of Pleura. of Pericardium. of Pericardium.
A, The lungs. B, Cross-section of the thorax.
n the cavity lie the heart and all the alimentary organs.
‘ach is surrounded by a splanchnic layer of the ccelomic
VERTEBRATA. 425
lining (or peritoneum) which in most cases forms a mesentery
dorsally where the splanchnic layer joins the somatic. The
perivisceral cavity becomes divided into pericardial and
abdominal cavities, and in mammals there is a further
separation of two pleural cavities.
Alimentary System.—The most outstanding feature
of the vertebrate alimentary system is the presence of paired
pharyngeal clefts which arise as hypoblastic pockets, growing
out into contact with the epiblast and then opening to the
exterior. In fishes these pharyngeal clefts function as gill-
slits, the hypoblastic epithelium growing out into gill-fila-
ments. The first pharyngeal cleft appears in the skate to
have already lost its branchial function, and serves only as
a spiracle or aperture for introduction of water. In many
fishes the mouth is used for this purpose and the first cleft
is then given up.
In the terrestrial Vertebrata the first pharyngeal cleft
persists as the Eustachian canal and middle ear whilst all
the others atrophy. They are found more or less distinctly
in the embryo, but are merely vestigial organs.
In the mid-ventral line of the pharynx in vertebrate embryos there
arises a groove having the same relationships as the endostyle of Atrdozoa.
As development proceeds, however, it becomes completely separated
from the pharynx and gives rise to the ¢hyrodd gland. The thymzts also
appears to arise by several rudiments in connection with the gill-slits.
The extreme anterior part of the alimentary canal is formed by an
epiblastic ingrowth called the stomodeum ; this gives off a dorsal diver-
ticulum called the yfophysis which may be homologous with the
subneural gland of the Zznzcata. Its distal end becomes detached and,
coming into close relationship with the infundibulum of the brain, forms
the petuctary body.
The alimentary canal is in its earliest condition a simple
tube, but certain parts, such.as the pharynx and stomach,
develop by rapid growth into large sac-like swellings. The
Jungs, in terrestrial forms, arise as a single ventral diverti-
culum of the cesophagus which forks into two, and each
becomes distended into a sac. The sac becomes the lung
and the connecting stalk persists as the trachea and bronchi.
Behind the stomach the intestine buds out a ventral diverti-
culum which forms the liver, its stalk becoming the bile-duct;
426 CHORDATA.
and the pancreas arises from several dorsal processes in the
same region. The essential epithelium of the gland in each
case arises in this way, the bulk of the organ being composed
of mesoblastic connective tissue and blood-vessels.
Urogenital organs.—The urinary organs show a suc-
cession in the group of three separate series—the pronephros,
mesonephros and metanephros.
The pronephros is always situated far forward in the
ceelom. It is functional in AZyxine and in the tadpole of the
frog. It consists typically of three or more paired tubules
opening by funnels into the ccelom and leading to the
exterlor by a paired lateral pronephric duct. The meso-
nephros arises behind the pronephros and replaces it in de-
velopment. It is formed of a number of tubules arising
from the ccelom and becoming connected with the pro-
nephric duct. The duct then splits into two, one of which
remains functional in the female as the Afi//erian duct or
oviduct, and the other becomes the Wolffian or mesonephric
duct, functioning in the female as a ureter, in the male as a
ureter and as a vas deferens. It is enabled to do this by
certain of the mesonephric tubules growing out towards the
testes, becoming connected with them and forming the vasa
oferentia. The other mesonephric funnels close in adult
life. This condition is found in the frog.
In the skate and in Ammniota the mefanephros arises as a
set of tubules posterior to the mesonephros. They become
connected to the cloaca by ureters, and the mesonephros
then atrophies so far as the excretory function is concerned.
It persists in the male rabbit as the epzdidymis. In the
metanephros the tubules have no funnels. The exact
meaning of this successive replacement of one kind of
excretory organ by another throughout the sub-phylum is
unknown.
Development.—The types of development already out-
lined are very diverse, but it is possible to trace a phyletic
sequence from one to the other.
In young forms, even more than in adults, because the
reproductive element is not present, the nutritive conditions
are the secret of the structural modifications, and we can
discern in the vertebrate series no less than five different
VERTEBRATA. 427
forms of nutrition in regular sequence. They are as
follows :—
1. FREE OR LarvaL Nutrition.—This is found at a
very early stage in Amphioxus and later in fishes and
Amphibia. In it the larva or young form catches its
own food with mouth and ingestive organs. It is practically
the only mode of nutrition adopted by Amphioxus.
2. YOLK or LeciTHAL Nutrition.—The young form
is supplied by the parent with an inert mass of yolk or
fatty material, and whilst the yolk lasts it is mainly enclosed
in the egg-membrane and is known as an embryo instead
of a larva. The yolk is stored primarily in the alimentary
canal which causes the latter to protrude as a large bag
or sac called the yo/k-sac. In certain fishes and Amphibia
the lecithal form of nutrition is succeeded directly by the
larval nutrition, the mouth and other ingestive organs be-
coming functional at the completion of yolk-absorption.
In other words, the young frog, for example, is supplied
with yolk till shortly after hatching, when the mouth opens
and a vegetable diet is then resorted to.
The lecithal form of nutrition culminates in elasmobranch
fishes, in Sauropsida and in Monotremata amongst mammals.
Like the larval nutrition, it is entirely given up in the rest
of the Mammalia.
3. ALBUMINAL NutTRITION.—In Amp/ibia, such asthe frog,
the egg itself is surrounded by a clear hyaline mass of an
albuminous substance which swells up after oviposition and
serves as a protection to the embryo. It does not appear
in the frog to be used as nutriment. but in the Sauropsida,
e.g. Chick, the same material surrounds the true egg as a
mass of albumen between it and the shell. As in the frog,
this material is produced by a series of glands in the lower
part of the oviduct. Here, however, the albumen is not
“required for protection as this function is performed by the
shell, but it is absorbed by the embryo towards the later
days of incubation when the lecithal nutrition is terminating.
Little is known about the absorption of this albumen. The.
serosa may play some part, but the basal part of the yolk-sac,
in contact with it, is said to become the absorbing area, and
the nutriment would thus find its way to the embryo through
the medium of the yolk-sac. Little is known concerning
428 CHORDATA.
Fig. 308.—TuHr EvoLuTion oF THE Fa@:TAL MEMBRANES
OF VERTEBRATA.
‘Body-wall, future Serosa,
Yolk, Yolk-sac Wall.
Embryo.
we’ Amniotic Fold.
\ Extra-
Alimentary Canal.
Yolk in Yolk-sac.
Hypoblast of
Yolk-sac.
Extra-embryonic
Ceelom.
Mesoblast of -
. Yolk-sac.
§ § Mesoblast. ~~
5 Epiblast. ~ ga
wn Yolk-sac.
Embryo. D
iniotic
Amniotic Canal. E Allantois.
Cavity. , on g
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ss Oe
Ped a
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Yolk-sac. Yolk-sac.
A, Stage of the Frog with only small Yolk-sac; B, Stage of the Skate ;
>, Stage of Developing Amnion; D, Stage as in many Reptilia, the amnion and serosa are
not completely separated (cf also MonotremaTA); E, A Typical Sauropsid.
Epiblast is represented white, mesoblast black and hypoblast dotted.
VERTEBRATA. 429
the albuminal nutrition of the mammals, though in the
Metatheria, at least, it appears to be an important factor in
the nourishment of the young, and in many Luwtheria, in
which the lecithal nutrition is entirely replaced, it probably
plays an important réle. There are numerous glands of
the oviduct, uterine glands, which probably secrete the
albumen. The albuminal nutrition is therefore the second
form of nutriment supplied by the parent to the embryo.
4. LacrgeaL Nutrition. — This is the production of
“milk” by mammary glands. The “milk” is elaborated -by
skin-glands and is supplied, not to the embryo, but to the
young animal after birth; hence no special organ beyond the
mouth and alimentary canal is needed. Traces of this form
of nutrition are found in the Sauropsida (pigeon’s “ milk”)
and in the Prototheria, but it attains its greatest develop-
ment in the AMezatheria, in which it follows very closely
upon the albuminal nutrition. It is found usually in the
Eutheria, but is in them being replaced by the last method
of nutrition.
5. H@mat Nutrition.—In this form the young animal
feeds directly upon the blood of the mother by absorption
through the blood-vessels of the yolk-sac and of the allantois.
The maternal vessels form with those of these two organs a
complex vascular organ called a placenta. The yolk-sac
placenta is found in MMe¢atheria and, exceptionally, the
allantoic, but neither is sufficiently elaborated to replace to
any extent the lacteal; whereas in the Hwtherta this heemal
nutrition is much the most important, though preceded by
an albuminal and succeeded by a lacteal.
Foetal Membranes.—The distension of the ventral
wall of the body by an accumulation of yolk produces a
large sac-like protuberance of the intestine, called the yo/k-
sac, covered by the distended body-wall forming the serous
membrane or serosa. In the Ammnio¢a other two foetal mem-
branes are found. The amnion is a protective membrane
produced from the serosa and similarly formed of epiblast
.and somatic mesoblast, whilst the a//anfois is a median
ventral process of the intestine and is similarly formed of
hypoblast and splanchnic mesoblast. In Sauropsida the
allantois acts as a urinary bladder and a respiratory organ,
430 CHORDATA.
whilst in mammals (g.v.) it takes part in the formation of
the placenta.
Conclusion.—It is clear that the study of comparative
anatomy and of development throughout the sub-phylum of
Vertebrata can scarcely be over-estimated as a means of in-
terpreting the complex and often puzzling structure of the
highest vertebrates.
Certain organs appear to have retained the same function
throughout, such as the brain and heart, and we may only
trace the lines of growing complexity from a simple tube to
the intricate mechanism of such organs as found in man.
But others show a still more remarkable history, involving a
change of function, which in some instances may almost be
regarded as loss of function (though it is daring to assume
that an organ can be structurally existent after a// function
has disappeared). We may recall our teeth traced back to
placoid scales, the thyroid and thymus to glandular organs
of the atriozoan pharynx, the inner ear to one of a series of
aquatic sense-organs, the middle ear to one of a series of
visceral clefts and the jaws and the ear-ossicles to parts of a
segmented series of visceral arches.
These and numerous other instances of the same kind
teach us that a true knowledge of anatomy can only be
obtained by a due appreciation of what we have been as
well as what we are.
(TABLE,
431
VERTEBRATA.
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{(equaseyd) stoqueye
SsaQUyNU pue Tore
“qeudazuT WORRsTTIE.F
“ainqeiedura}
WHE jW1esU0D pe
SOAIOU [RIIRID DATaaN TL,
*sa[oisso Aroypne
29141 ‘fesowenbs
Aq papuadsns ‘adard
auo yo gqIpuey
“yuopoday}
pue juopoiajay
‘eqixeuraid mo yea]
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9uog Ajuyeur u032[94S
“years
-ysod & ‘az uo ATUO
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parsqureyo-ino0o gy
vazeyed Auoq
YA seieu yeusezT
“uy UeIpaul ON
“suesio
-ASUaS SUT] ]219I2] ONT
“userydeip e pue squat
Aq ayjeaig -ssun]
‘squiy ayA}oepejueg
*(spueys Areur
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Spueys urys pure sirezy
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yeurpre> = Jo1103s0d
pue s01zozue ‘saysie
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poisqueys-omM TL
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OU JIM sovs jeseN
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432 CHORDATA.
CHAPTER XXV.
CLASSES OF VERTEBRATA.
The Vertebrata are naturally divided into the aquatic
or predominantly aquatic types called Axamnia and the
typically air-breathing terrestrial forms called Amuzota.
The names are derived from the absence and presence
respectively of an enveloping foetal membrane called the
amnion.
In addition the Axamuza always possess, at one time in
their life, fins, gills and lateral line sense-organs.
The Azxamnia have three classes—
I. CYCLOSTOMATA.
2. PISCES.
3. AMPHIBIA.
The Ammnioza also have three classes —
4. REPTILIA.
5. AVES.
6. MAMMALIA.
Crass I.—CyYCLOSTOMATA.
The Cyclostomata were at one time, like Amphioxus,
included in the fishes, but the important differences from
the latter necessitate a separate class. In many respects
they are the most primitive of all the Vertebrata, whilst, as
must of necessity be the case, they also exhibit a degree of
specialisation.
In their external appearance they approach the fishes,
especially the eels and other elongated types, but the entire
absence of limbs is remarkable. There is no evidence that
Cyclostomata ever possessed these organs. Again, they re-
semble fishes in the presence of lateral sense-organs, a median
fin with fin-rays, and in their method of respiration by gills
situated upon gill-slits. There are no jaws nor other free
visceral arches, the deficiency being supplemented by a
CYCLOSTOMATA. 433
suctorial mouth and by a branchial basket-work. There is
no vertebral column, and at most an incomplete cartilaginous
cranium. In accordance with this the notochord and its
thickened sheath form the skeletal axis throughout life.
The olfactory sac is single and does not open directly
to the exterior, but into a long Actuctary sac formed of the
enlarged Aypophysis. The hypophysis usually opens into the
stomodzeum from which it originates, but in the course of
development (in the lamprey) a large upper lip is formed
behind the opening of the hypophysis, and pushed out to
such an extent as to carry the base of the hypophysis on to
the dorsal surface of the head. As we have seen the
hypophysis acquires an internal opening into the pharynx in
Myxine but not in the Lampreys. The olfactory capsule ‘is
free from the cranium.
The auditory organ never has three semi-circular canals.
The brain shows a very small cerebellum and is of small
proportionate size. The optic nerves do not cross, and
there is no sympathetic nervous system nor spleen.
In some forms the pronephros persists throughout life,
the tubules opening into the pericardium. There are no
genital ducts, the sexual elements leaving the coelom by
pores.
ORDER I.— Fetromyzontes.
The Lampreys are active free-swimming forms with pre-
datory habits. They have dorsal fins, well-developed eyes,
and have two semi-circular canals to the ear. The pituitary
sac is blind. The skeleton is a slight advance upon that of
Myxine as there are paired lateral nodules of cartilage
representing vertebree. There is also a complete branchial
skeleton, but no buccal cirri nor cartilages.
The seven gill-pouches open separately to the exterior
laterally, and internally they all open into a respiratory tube
which communicates anteriorly with the cesophagus. The
intestine contains a spiral valve. The sexes are distinct.
The Lampreys are widely distributed in the sea and in
fresh water. They develop by an early embryonic stage
and later larve. The larva is known as Ammocetes. It
differs from the adult in several important particulars, e.¢., the
M. 29
434 CHORDATA.
eyes are rudimentary, and the gill-pouches open directly
into the cesophagus.
OrvER II.—Myxinoider.
The Hag-fish (AZyr’ne) has been described. The other
genus is the large Bde/ostoma which has separate external
branchial openings. Both are marine.
>
Fig. 309.—THE River-LAMPREY (Letromyzon
fluviatilis) x %.
—
Note the single median fin and tail, the sucker and the ventro-
lateral row of branchial openings behind the head.
A small fossil from the Devonian (Paleospondylus) has
some claims to be regarded as a fossil Cyclostome.
Cxiass II.—Pisces.
The fishes are much more heterogenous in structure than
thé last class. They have paired fins and median fins sup-
ported on fin-rays. The median fin may be perfectly con-
tinuous, with dorsal, ventral and caudal portions, as in the
PISCES. 435
sand-eel, or it may be broken up into numerous dorsal fins,
a caudal and numerous anal fins. The caudal fin may be
one of three kinds. The simplest, found in larval fishes, is
the protocercal. In this the “tail” or prolongation of the
body lies symmetrically in the centre of a symmetrical fin.
In the Aeterocercal caudal fin an anal fin is added ventrally,
so that the whole caudal fin thus formed is asymmetric with
Fig. 310.—TAILs OF FISHES.
Vertebral
Column,
Dorsal
Part.
Ventral Part.
A, Protocercal. B, Heterocercal. C, Homocercal.
a large dorsal portion into which the “tail” is continued.
This is found in most sharks. The Zomocercal caudal fin
is itself symmetrical, the ventral portion being of the same
size as the dorsal, but the “tail” is bent up into the dorsal
half, showing that this type has a secondarily acquired
symmetry through the heterocercal stage. Most Zeleostom:
have this typeZof tail.
436 CHORDATA.
The paired fins show similar modifications. The archi-
pterygium is found in the Dzpxoi and consists of a median
axis with symmetrical lateral rays (¢f the protocercal tail).
The ichthyopterygium consists of one or more basal parts
bearing secondary rays only on the outer border. In the
pectoral fin there are commonly three primary basal pieces
(of. skate) and in the pelvic only one.
Fishes are usually covered or protected with scales, of
which there are usually distinguished four kinds. The
placoid scale has a bony base and bears a spine, usually
Fig. 311.—FINS OF FISHES.
Basipterygium. |
A, Archipterygium. ” B, Uniserial type of ichthypterygium,
pelvic fin of Skate.
found in Elasmobranchit ; the cycloid is a flat circular plate
arising in the dermis: with its ally, the ctezoid, which has a
toothed edge, it is found chiefly in the Zeéostei, Lastly,
the ganoid scale is a hard rhomboidal plate closely apposed
to its neighbours and occurring in the certain archaic fishes
of the Teleostomi.
All fishes have gills borne upon four or more gill-slits.
The slits may be widely apart and opening separately, as
in Elasmobranchit, or they may be close together and
covered by a flap or operculum, as in the other orders.
PISCES. 437
There is an extensive system of lateral line sense organs
innervated chiefly by the Vth, VlIth, and Xth cranial
nerves. All fishes have a well-developed vertebral column
and visceral arches; the first arch is always modified into
upper and lower jaws. As in Amphioxus and the Cyclo-
stomata, the greater portions of the body and the longi-
tudinal muscles form the organs of locomotion. In the
great majority of fishes the heart is two-chambered and
respiratory, and the posterior venous system consists of
paired cardinal veins or sinuses.
Development is mainly embryonic, the egg containing
much yolk, but a free larval form is found in many.
ORDER I.—TZeleostomt.
The Zeleostomi are'called the “bony fishes” because
their skeleton is almost entirely formed of bone. Hence
a cod’s skull is a very different structure from that of a
skate, for it consists of a great number of bones which,
to a large extent, fall apart when the connective tissue is
destroyed by boiling.
The Zeleostomi usually have cycloid, ganoid or ctenoid
scales, but some have none. The tail is nearly always
homocercal. The genital organs communicate with the
exterior by genital ducts in both sexes, and the genital
and anal apertures are separate, hence there is no cloaca.
The gills are enveloped in a bony operculum. In con-
nection with the cesophagus many have a large air-bladder
lying under the vertebral column. In the brain the cere-
brum is very small; the optic lobes and the cerebellum are
large. The kidneys are purely mesonephric. Development
is embryonic in its earlier stages, but the young are hatched
as larvae with a large dependent yolk-sac. The eggs in
many marine types are pelagic, but in the freshwater forms
and some marine they are demersal or deposited on the
bottom : they are small and numerous.
The Zeleostomi show a peculiar combination of structural
characters, some, such as the ossification of the skeleton
and the absence of a cloaca, placing them above the
Elasmobranchit, whilst the condition of the urinary organs
and the brain are at a decidedly lower level.
438 CHORDATA.
They are world-wide in distribution, and are freshwater,
pelagic, littoral, katantic and abysmal in habitat.
They are divided into two unequal divisions — the
Crossopterygit, mainly extinct but including olypterus of
the Nile, and the Actinopterygii which embrace the
remainder. These are in their turn divided into three
sub-orders :—
1. The Chondrostei are mainly extinct types, together
with the sturgeon of sub-arctic regions and one or two
species found in North America. Their skeleton is
cartilaginous.
2. The Aolostei include the bony pike (Lepzdosteus )
of North America and several extinct forms. The skeleton
is osseous and there is a spiral valve in the intestine.
3. The Zeleostei * constitute an immense number of well-
known fishes. Their skeleton is osseous, they usually have
horny (cycloid or ctenoid) scales, they have no conus
arteriosus to the heart and no spiral valve in the intestine.
Their principal groups are as follows :—
1. Physostom? (air-bladder communicating with the cesophagus),
most freshwater fishes and common marine forms, such as
herring, sprat, eels.
2. Anacanthint (air-bladder closed, the fins are soft), comprising
cod, haddock and the flat-fish. (In the flat-fish the air-
bladder is absent.) :
3. Acanthopteri (fin-rays are spiny, air-bladder closed), including
perch, mackerel, gurnard.
4. Plectognathi and 5. Lophobranchit, two small groups with very
specialised members. The Plectognathi usually have a
hard bony exoskeleton and few powerful teeth with certain
bones of the jaw fused. The Lophobranchii have tufted
gills and may assume peculiar shape and habits; they
include the pipe-fishes and the sea-horses.
OrvDER Il.—Zlasmobranchit.
This order includes the Sharks and Skates. Their tail
is heterocercal, the scales are placoid. The gill-slits are
* The archaic freshwater types like the sturgeon, bony-pike, and Polypterus
all the extant Teeostomi, except Teleostez) used to be combined in an order called
Ganoidei, but their genetic relationships to the Teleosted are now recognised.
AMPHIBIA. 439
separate and widely apart with no operculum. The skeleton
is cartilaginous and the palatoquadrate is free from the
cranium. There is a spiral valve to the intestine. A cloaca
is present and the kidney is mainly a metanephros.
Development is purely embryonic, the egg has much yolk
and the young is not hatched till like the adult.
The Lvasmobranchit are marine. The sharks are mostly
pelagic and the skates and rays mainly littoral or katantic.
OrverR III.—olocephal.
A small order formed to contain Chimera (the King of the
Herrings) and its allies. They resemble the last order in
their cartilaginous skeleton and some other structural features,
but differ in having an operculum covering the gill-slits, a
protocercal tail and no cloaca. The palatoquadrate (upper
jaw) is completely fused to the cranium.
The few genera are widely scattered, one being a deep-
sea type.
ORDER 1V.—Dzpnoz.
The Dzpnoi or mud-fishes differ from the other orders
of fishes in the possession of true lungs in addition to their
gills, and in the partial division of the auricle into two, thus
producing a three-chambered heart; the nasal sacs have
internal nares. The paired fins are archipterygia, ze, a
central axis with rays on each side; the caudal fin is pro-
tocercal. There is a spiral valve and a cloaca and the
skeleton.is partly cartilaginous and partly bony.
Like the more primitive of the Zeleostomi, the Dipnoi
-are freshwater forms and have a discontinuous distribution.
Ceratodus is found in Australia, Profopterus in the Nile and
Lepidosiven in the Amazon.
Cuass III.—AMPHIBIA.
The Amp/ibia form a transition class from the two pre-
ceding to the three following terrestrial classes. The frog is
about the most terrestrial of all the class. Gills, median
fins and lateral line sense-organs are found throughout life
440 CHORDATA.
in some, only in the larval stages in others. The paired
limbs are pentadactyle. Lungs are present in the adult,
and the nasal sacs have internal nares through which air is
supplied to the lungs. The heart is three-cchambered and
there is a postcaval vein replacing the cardinals. The
skeleton is partly cartilaginous. There is always a cloaca.
The eggs are fertilised externally and there is usually a
metamorphosis.
The order Anwura includes the frogs and toads, with no
tail and with no gills in the adult. The Urode/a, such as the
newts and salamanders, retain their tail and aquatic habits
throughout life ; whilst others, such as Proteus (a blind form
found in the subterranean caves of Austria), retain also their
gills. Hence the gilled Urodela, the Urodela which lose
their gills and the Aura form a complete series illustrating
the changes from an aquatic to a terrestrial life.
There is also a small third order of Gymnophiona with
no limbs.
Cxiass IV.—REPTILIA.
The Reptilia are a class of animals very definitely marked
off by structural features at the present day, but the fossil
forms show a gradation into Amphibia and Mammalia; and
some of these even exhibit characters approximating to those
of birds. During the secondary epoch, especially in the
Trias, the reptilian was the dominant vertebrate type,
and, as such, exhibited as wide adaptive modification as
do the dominant mammals of the present day. Large rep-
tiles ruled the sea, the land and the air, and some attained
an enormous size. Since then the efzi/ia have declined in
numbers and in size, and only five comparatively small
orders remain.
These all differ from the Amphzbia in never at any time
in their life possessing gills, fins, or lateral sense-organs, in
having an embryonic development involving internal fer-
tilisation and usually an oviparous habit. The embryo is
enveloped in a foetal membrane called the amnion, and has
also a large excretory and respiratory organ, the allantois.
(These foetal membranes, as well as the yolk-sac already
REPTILIA. 441
noticed in the skate, have been more fully described in
the chick, p. 380.)
Again, the reptiles have twelve cranial nerves, the spinal
accessory and hypoglossal being added to the ten of
Amphibia. and the skeleton is much more completely ossi-
fied than is the case in the latter. The body is usually
protected in an exoskeleton of scales or scutes, which is
either purely epidermic and cuticular in nature, or is dermal
and formed of bony tissue
In the skeleton the reptiles have the typical pentadactyle limbs and
the ankle-joint is intertarsal. The shoulder-girdle usually has clavicles
and episternum as well as the three primary bones—the precoracoid,
coracoid and scapula. In the pelvic-girdle the ilium usually fuses with
two sacral vertebree and there are usually epipubic bones. There are
often a number of membrane-bones called abdominal ribs. In the skull
the quadrate suspends the lower jaw which is composed of several
bones ; the teeth are polyphyodont and homodont and are attached to
the surface of the bone (acrodont) or at the side (pleurodont), and they
may occur on the palatines, pterygoids and vomers, as well as the
premaxillz, maxilla and dentary. The skull has a single occipital
condyle, formed largely by the basioccipital but partly by the ex-
occipitals, and the facial portion of the skull is much larger and broader
than the cranial. There is often a peculiar ¢ransverse bone connecting
the maxilla and the pterygoid. There is only one ear-ossicle, the
columella auris.
Most of the reptiles resemble the amphibians in the
three-chambered heart, the three complete aortic arches and
the condition of the circulatory system.
ORDER I.—Rhynchocephalia.
Sphenodon (or the New Zealand Lizard) is a lizard-like
animal, found in New Zealand, possessing a series of
primitive structural peculiarities which lead zoologists to
place it in an order by itself. The principal of these are the
amphiccelous vertebre, the presence of intercentral elements
between the vertebree and of teeth on the palatines and
vomers (young).
OrDER II.—Lacertilia.
The lizards have an exoskeleton of horny epidermic
scales which are periodically shed. Most have two pairs of
walking limbs and a long tail. The teeth are either fused
442 CHORDATA.
to the upper surface of the jaw (acrodont), or to the lateral
surface (pleurodont). The lizards are distinguished from
their nearest allies, the snakes, by the almost universal pres-
ence of four limbs, by the bones of the skull being immov-
able, and the mandibular rami being fused together. They
also have eyelids. Lizards are widely distributed, but found
in most profusion in equatorial regions. The common
slow-worm (with no limbs) and the sand lizard are British
examples. :
Fig. 312.-LATERAL VIEW OF SKULL OF
RATTLESNAKE (Crotalus).
Note the freely movable quadrate with pterygoid
continued into small palatine in front and joined to the
maxilla by a long transverse bone. Maxilla bears the fang.
OrDER II].—Ophidia.
The snakes have an exoskeleton of epidermic scales.
They have no limbs, but progress by a movement of ventral
scales, to the inner surface of which the distal ends of the
numerous ribs are attached. Hence there is no sternum.
The vertebrze usually have extra articular facets (zygosphene
and zygantrium). The eyes have no eyelids. But in ad-
dition a unique method of locomotion, the snakes exhibit
a peculiar method of feeding. The quadrate is loosely
hinged on the skull, and the maxillz, palatines and pterygoids
are all freely movable. In addition, the mandibular rami
REPTILIA, 443
are loosely united by ligament. Hence the snakes have an
enormous “‘ gape,” and can ‘‘swallow” entire animals which
exceed their own diameter. All Op/idia are carnivorous.
The non-poisonous groups usually have two rows of Jong
recurved teeth on the maxillz and the palatines and ptery-
goids respectively. Between these rows fits the row of teeth
on the mandible. In the poisonous group the maxilla is
freely hinged, and bears a single large fang or grooved tooth
connected with the poison-gland, a modified salivary gland.
There are also a few teeth on the pterygoids, palatines and
mandibles. On closing its jaw, the snake’s maxilla with its
Fig. 313-—RIGHT SHOULDER GIRDLE OF A TORTOISE.
fi.) Scapula.
; <\ Precoracoid.
f Glenoid.
fang is swung” up into the roof of the mouth by the automatic
movement of the quadrate, pterygoid, transverse bone and
maxilla.
The snakes may therefore be said to exhibit extreme
specialisation for a unique method of locomotion, involving
loss of limbs and limb-girdles, and for an equally remarkable
method of feeding.
ORDER LV.—Chelonia.
The Chelonia comprise the tortoises and turtles. They
have an exoskeleton of horny epidermic plates, to which is
added an underlying dermal layer of bony scutes. The whole
444 CHORDATA.
body is enveloped in this hard protective case, which is
formed of a dorsal carapace and a ventral plastron, joined
together laterally. The carapace usually has a median row
of neural plates resting over the neural region, a lateral row
of costal plates modified from the ribs, and a distal row of
marginal plates. The plastron seems to be made up of a
Fig. 314.—SKELETON OF A TORTOISE.
The plastron has been cut away on the right side of the body and thrown over
to the animal’s left. Note the carapace and plastron, flexible cervical and caudal,
but ankylosed dorsal, vertebrae.
pair of c/avicles and an episternum anteriorly and a num-
ber of abdominal ribs posteriorly. The “case” of the
Chelonia is evidently to a large extent composed of a number
of pre-existing structures. Inside the case the vertebra, as
might be supposed, are vestigial, with the exception of the
cervicals and caudals. All four limbs are present and
REPTILIA. 445
protrude from between the carapace and plastron. The
skull shows the bones all immovably fixed. There are no
teeth, their functions being performed by sharp horny ridges.
All Chelonia are herbivorous.
The tortoises are terrestrial and have a convex carapace.
The turtles are aquatic and the carapace is more flat, often
comparatively soft or leathery in texture. Many turtles are
truly pelagic.
Fig. 315.—DoRSAL VIEW OF A CROCODILE’S SKULLx}. (4d nat.)
Premasilla }-—External Nares.
Mazxilla.
Nasal. J
Lacrymal.
... Prefrontal.
Orbit.
Jugal. —.Frontal.
Post-
Lateral frontal.
Temporal.
Fossa.
Quadrato- &
jugal.
Squamosal,
Quadrate. a!
a 2
So oc fa
al
gd_> 2
2888. 8
dO 26
oy Bc) aa
ge 8
ZS
n fe)
Note the pitted bones, the wide ‘‘ gape” from the two quadrates,
and the pre- and post-frontal bones.
ORDER V.— Crocodilta.
The crocodiles are in many respects the most highly
organised of reptiles. They have an exoskeleton of bony
scutes covered by epidermic scales. Both limbs are
present and the skull-bones are immovable. .
446 CHORDATA.
The crocodiles resemble the AZammatvia in the following
characters :—
The teeth are thecodont (in sockets) and confined
to maxilla, premaxilla and dentary.
2. The maxilla and palatine form a bony palate.
3. The heart is four-chambered, a septum dividing the
ventricle into two.
4. There is an incomplete diaphragm.
Fig. 316.—VENTRAL VIEW OF CROCODILE’s SKULLx4. (Ad nat.)
Palatine.
Transverse.
Quadrato-
6) jugal.
“Quadrate.
Pterygoid..--
Internal Nas.
Note the thecodont teeth in single row on premaxilla and maxilla, the back-
ward position of the internal nas, the transverse bone, single
occipital condyle and quadrate-suspensorium.
True crocodiles are found in the rivers of tropical Africa
and in central America. The aé/igators are found in the
Southern States, West Indies and South America. The
gavials are small Crocodilia found in the Ganges and its
tributaries,
REPTILIA. 447
Among the enormous number of extinct Repiz/ia we may
here merely notice a few.
The Pterodactyles (order Prerosauria) were winged rep-
tiles, with the wing formed of a membrane stretched from
the enormously elongated fifth digit. They had a skull
somewhat like that of a bird, but with teeth.
The Lchthyosauria were large fish-lizards with long tail
and the limbs modified into flippers. The skull had a
rostrum like that of the porpoise. Jchthyosaurus is a
common example.
The Dinosauria were large terrestrial reptiles, some of
which show structural features resembling birds. Lewanodon
is perhaps the best known.
Lastly, the Zheromorpha appear to be reptiles showing
remarkable resemblances to mammals, especially in the
.heterodont dentition; some of this group also point to
relationships with fossil Amphrbza.
Crass V.—AVES,
Birds are closely allied to the reptiles in their structure,
but they are so completely adapted for an erial habit that
there is no difficulty in at once distinguishing them. They
resemble the reptiles, especially in their skeletal structure,
the similar bones of the skull, the suspension of the
mandible by the quadrate, the many elements of the
mandible, the single ear-bone or columella and the absence
of epiphyses. In addition, they have the same oviparous
habit, with meroblastic segmentation, and the same fcetal
membranes. These and other similarities are sometimes em-
phasised by the grouping of the two classes together under
the head of Sauropsida.
On the other hand, the birds show the following adapta-
tions to an serial habit. The fore-limbs are not used for
terrestrial locomotion, as in reptiles, but are formed into
wings, the method of formation involving the entire loss
of the two postaxial digits and a great reduction of the
preaxial. Probably at first each digit had its separate tuft
of flight feathers or a/z/a, but in all modern birds the alula
of the first digit alone remains, those of the second and
third combining with the flight-feathers of the ulna to form
450 CHORDATA.
Fig. 317.—ARCHOPTERYX x 4.
(From cast of Berlin Specimen
in the Edinburgh Museum
of Science and Art.)
Pilititee
Note the teeth, the free metacarpals,
the three clawed digits, the ab-
eominat ribs and the elongated
tall.
AVES. 451
ORDER I.—Ravre. :
These are nearly all large birds which all show a degen-
eration of the wings, in some cases to mere vestiges, in cor-
relation to which the carina of the sternum is lost, giving it
Fig. 318.—VENTRAL SURFACE OF THE SKULL OF AN OSTRICH x $4.
(Ad nal.)
Premaxilla.
Premaxilla,
Rostrum.
Nasal.
Maxilla.
Palatine Process
of Maxilla.
Vomer.
Rostrum.
Jugal.
.Palatine.
Pterygoid.
¢ Quadrate
Basisphenoid.
sa Foramen
‘ondyle. Magnum.
Note the absence of teeth, single occipital condyle and quadrate-suspensorium.
arounded appearance. The hind-limbs are always large and
powerful. These birds exhibit certain structural features
which may be regarded as of a primitive nature. The
feathers have no hooked barbules, and, as a rule, they are
evenly scattered over the surface of the body. The bones
of the skull are mostly still separated by sutures, and the
452 CHORDATA.
quadrate has only a single articulation with the skull. The
Ratite illustrate discontinuous distribution. The ostrich
(Struthio) is found in Africa and South-Western Asia. It
has only two toes—a large fourth and a small fifth. The
American ostrich (ea) has three toes and is found in
South America. The Cassowary (Casuarius) and Emu
(Dromeus) are found in the Australian region and the small
Kiwi (Apzeryx) in New Zealand. On this latter island are
also found the remains of the recently extinct Moas (Din-
ornis), huge wingless birds. Others have been found in
Madagascar.
an
Fig. 319.—THE Kiwi (Apteryx) x
‘A wingless ” bird of New Zealand. The wings are vestigial
and hidden below the feathers.
OrvDER II.—Carinate.
The Carinate comprise the remainder of modern birds.
Considering the enormous number of species and wide dis-
tribution, they present remarkably few structural differences
which are available for classification. In one or two fossils,
such as the Cretaceous Hesperornis and Lchthyornis, teeth still
survive.
They are classified by reference to the arrangement of
the feathers, the structure of the skull and of the alimentary
organs.
MAMMALIA 453
CHAPTER XXVI.
GENERAL FEATURES OF MAMMALTA.
Crass VI.—Mammatia.
The Mammalia are the last class of the Vertebrata,
and as they indubitably stand at the head of the animal
kingdom, both structurally and intellectually, they will be
specially treated here. Special emphasis is laid upon
the skeleton, because the skeleton of a vertebrate is always
a permanent embodiment of the part played by its former
possessor in the arena of life.
Sxin.— The skin of mammals conforms to that of
vertebrates in general, hence two layers of the epidermis
can be distinguished—the outer horny layer or stratum
corneum and the inner mucous layer or stratum mucosum.
The base of the mucous layer which rests upon the
dermis consists of a single layer of epithelial cells, the
basal epithelium, which by tangential divisions (parallel to
the surface) are perpetually giving rise to more cells
in layers above them. The lower of these cells are
still living and protoplasmic, but those nearer the surface
have undergone a comification, by which the proto-
plasm is replaced by horn or ceratim. The cells thus
cornified are no longer living, but are continually being
shed in detail upon the surface. Thus the whole surface
of the mammal is enveloped in a thin, flexible layer of
ceratin, the corneous layer, produced by the underlying
mucous layer of living protoplasmic cells.- The dermis,
as in other vertebrates, consists of a dense mass of con-
nective tissue, blood-vessels, nerves, muscles, fat and skin-
glands. With the first three we are not here concerned,
but one of the essential features of the class Mammala
is the development of the three latter. The muscle is
present beneath the skin, connecting it tightly with the
body below, as a thin sheet known as the panniculus
454 CHORDATA.
carnosus, whilst the fat is concentrated as a layer at the
base of the dermis, called the panniculus adiposus. This
layer is enormously developed in most aquatic mammals
Fig. 320.—SECTION THROUGH THE SKIN OF A MAMMAL.
x
p
on
ans
—_—— +-F
—
=
ima
. dacs oa
3 See
. De eel
as sie
=
a, Say
ic}
g
3
-
—_
ero, fs
3
i MBUDORIFIC Sree =.
RV DORIFLE GLAND, ; 25
pi
VAs ge
tC 8
y
=]
Cs
-§2.
ih Set
2G
g
(whales and seals). The skin-glands really belong by origin
to the stratum mucosum of the epidermis and arise from it
in the embryo; but as development proceeds they protrude
downwards into the dermis and become much coiled in
MAMMALIA, 485
order to increase the secretory surface. Their connection
with the epidermis is, however, retained by the ducts which
pass outwards, their cavities opening freely to the exterior
on the surface of the skin. The great development of skin-
glands is a marked feature of the Mammatia.
We may distinguish two different kinds—(1) the sudorific
or sweat-glands and (2) the sebaceous glands.
1. The sudorific glands are developed by local ingrowth
of the basal epithelium of the mucous layer. They lie
deep in the dermis and excrete water, with inorganic salts
in solution (sweat), discharged freely on to the surface of
the skin. ‘he sudorific glands are of the tubular type,
coiled and unbranched.
2. The sebaceous glands are also produced from the
basal epithelium of the mucous layer, but are only de-
veloped in connection with hair-pits or follicles. Sebaceous
glands are usually of the acinous or branching type, and
they secrete sebacin, a fatty substance, the primary function .
of which is to lubricate the hair. They also differ from the
sudorific glands in being xecrobiotic, ¢.e., the sebacin is pro-
duced from dead cells.
Hair.—A hair is a structure found only in the Mammalia
and it can only very doubtfully be compared with feathers
or epidermic scales. It is essentially epidermic and its first
trace in development is a small process or hair-germ formed
from the mucous layer. This protrudes inwards into the
dermis and elongates rapidly. Its base then becomes
pushed into a pit within which the dermis protrudes, and at
the apex of this pit the basal epithelium gives rise by rapid
growth to a central axis of cells. The basal pit becomes the
dermal papilla and the medullary axis gives rise later to the
medulla of the hair. Around the medulla, between it and
the basal epithelium, a thin layer or cylinder of the mucous
layer becomes cornified, produced above the end of the
medulla up to the surface of the corneous layer. Later on
this cylinder divides into two so that a cylindrical cavity is
produced. This cavity becomes continuous with the
.exterior and terminates above the papilla. It differentiates
the whole follicle:into a hair in the centre and the 7oot-
sheaths around it. The basal epithelium, next the dermis,
454 CHORDATA.
carnosus, whilst the fat is concentrated as a layer at the
base of the dermis, called the panniculus adiposus. This
layer is enormously developed in most aquatic mammals
Fig. 320.—SECTION THROUGH THE SKIN OF A MAMMAL.
AuI0 FY]
aART
snoony
aAeT
A ese,
a.
sunyayaid
=| ~~
s ee
AA ane!
4 Ss B
5° a ig
ee — Se ats 3
8a e
Se
w- (g ea ee
3s - a il
—S ssupoRiFie Se ee
» AND. 3
o Beer
a a
n
as]
(z}
5
-#5
3
ae
a
a
(whales and seals). The skin-glands really belong by origin
to the stratum mucosum of the epidermis and arise from it
in the embryo; but as development proceeds they protrude
downwards into the dermis and become much coiled in
MAMMALIA, 488
order to increase the secretory surface. Their connection
with the epidermis is, however, retained by the ducts which
pass outwards, their cavities opening freely to the exterior
on the surface of the skin. The great development of skin-
glands is a marked feature of the Mammatia.
We may distinguish two different kinds—(1) the sudorific
or sweat-glands and (2) the sebaceous glands.
1. The sudorific glands are developed by local ingrowth
of the basal epithelium of the mucous layer. They lie
deep in the dermis and excrete water, with inorganic salts
in solution (sweat), discharged freely on to the surface of
the skin. ‘The sudorific glands are of the tubular type, ~
coiled and unbranched.
2. The sebaceous glands are also produced from the
basal epithelium of the mucous layer, but are only de-
veloped in connection with hair-pits or follicles. Sebaceous
glands are usually of the acinous or branching type, and
they secrete sebaciz, a fatty substance, the primary function -
of which is to lubricate the hair. They also differ from the
sudorific glands in being wecrobiotic, z.e., the sebacin is pro-
duced from dead cells.
Harr.—A hair is a structure found only in the Mammalia
and it can only very doubtfully be compared with feathers
or epidermic scales. It is essentially epidermic and its first
trace in development is a small process or hair-germ formed
from the mucous layer. This protrudes inwards into the
dermis and elongates rapidly. Its base then becomes
pushed into a pit within which the dermis protrudes, and at
the apex of this pit the basal epithelium gives rise by rapid
growth to a central axis of cells. The basal pit becomes the
dermal papilla and the medullary axis gives rise later to the
medulla of the hair. Around the medulla, between it and
the basal epithelium, a thin layer or cylinder of the mucous
layer becomes cornified, produced above the end of the
medulla up to the surface of the corneous layer. Later on
this cylinder divides into two so that a cylindrical cavity is
produced. This cavity becomes continuous with the
.exterior and terminates above the papilla. It differentiates
the whole follicle into a hair in the centre and the 7oot-
sheaths around it. The basal epithelium, next the dermis,
456 CHORDATA.
forms the outer root-sheath ; the mucous layer inside it is
known as the zz#er root-sheath and upon its surface is pro-
duced the sheath-cuticle. In a similar manner the hair
itself has a central medulla produced from the basal
epithelium, a cortex around it, formed by the mucous layer,
Fig. 321.—DIAGRAMMATIC SECTIONS ILLUSTRATING THE
DEVELOPMENT OF A HAIR.
€ D
(ROANEOUS
4 pvcous
ae a
ee | {
(amin GERM,
Se yee, [ epitHELUn
a BASAL @PITHELI
mer
~~
: OyreR RooT-sHEATH—
fi ANNER ROOT
\
} mene
OUTER ROOT swe gf
f
smeneiesmnrn Ls
Re
Wer
SHEATH CUTUCLE 5
A, The hair-germ. _B, Formation of papilla and axis. C, Formation of cuticular
cylinder. D, Splitting of cuticular cylinder and formation of hair.
E, Transverse section of D.
and outside this a thin Aair-cuticle. The cortex becomes
eventually transformed into ceratin and usually carries the
pigments which give hairs their peculiar coloration, whilst
the medulla becomes a spongy network of cells which often
MAMMALIA. 457
have air-vacuoles amongst them, giving rise in many cases to
white hair.
The dermis surrounding the hair-germ is gradually differentiated
into a follicle. Its base protrudes into the hair-papilla, forming the
dermal papilla with blood-vessels, and the rest forms a more or less dis-
tinct dermic coat outside the root-sheaths. It may have an inner circular
layer of connective tissue and an outer longitudinal. Lastly, muscles
called the arrectores pild are attached to the coat and serve to erect
the hair.
Hairs differ very much in structure and texture and in some aquatic
mammials they are almost entirely absent.
Other epidermic structures of the same nature, z.e., localised corni-
fications of the epidermis, are nails, claws, spines or bristles, horns, and
even scales, as in the pangolins; horny teeth occur in the duckmole
and the Szrenza.
Hairs form a very efficient and light covering for the body and are
a protection mainly from climatic conditions. Indirectly, however, they
constitute an important protection from foes, as they are nearly always
of » colour in harmony with surroundings. Thus it is asserted that
because of their stripes the tiger and zebra in natural surroundings are
difficult to discern, whilst the spots of the axis deer are said to
exactly simulate the lights and shades formed by the sun shining
through leaves. The white colour of arctic animals is another example,
and a still more remarkable instance is that of seasonal coloration,
found in temperate regions. In many of the fur animals, such as the
polecat, weasel and ermine, the hair is of a brownish or black shade,
except in winter, when it becomes a pure white. In many marsupials
(Metatheria) the stripes are confined to the hind-quarters, as these parts
are most exposed when the animals are curled up asleep, and from this
direction an enemy can easily approach unseen.
Mammary GLAnps.—The mammary glands are of uni-
versal occurrence throughout the Mammalia. They are
skin-glands, usually situated on the ventral or lower surface
of the animal, and their secretion (milk) is used for the
nourishment of the young. Whilst thus physiologically
distinct, they do not appear morphologically to be organs
sui generis. In the Monotremata the mammary glands are
modified from sudorific or sweat-glands, so that the
“milk” in these forms is sweat and is said not to differ
essentially in composition from this excretory product: in
Metatheria the mammary glands are said to be a mixture
of sudorific and sebaceous glands; whilst in the higher
mammals (Zutheria) they consist purely of sebaceous glands
and the milk becomes a highly nutritive product. The
mammz or teats form a like series, as there are none
458 CHORDATA.
in the Monotremata, temporary teats only in the Mefatheria
and permanent ones in the Eutheria.
The evolution of mammary glands probably commenced before
that of the viviparous habit. We can see how the ventral surface of
the parent lent itself first for the incubation of the eggs and later for the
tending of the young. The desire of the young for fluid would naturally
be satisfied by the local glands, and if we may suppose that the duties
of incubation and nurture were shared by both sexes, we can to some
extent understand how the males of many mammals still have mamme
and functionless mammary glands. After the viviparous habit was
developed the male, divested of his share in the incubation, would
gradually give up the mammary function as well.
The mamme in modern mammals vary much in posi-
tion, though all situated upon the ventral surface. They
may be pectoral, axillary, abdominal or inguinal according
to their position on the breast, under the armpit, along the
abdomen and in the groin respectively.
TrEETH.—The teeth are well developed in most mammals,
though some types, such as whales, ant-eaters, Monotremata
and others, appear to have lost them. The characters of
mammalian teeth may be summed up in the expressions—
thecodont, heterodont, diphyodont , to which we may add a
limitation to a single row on the premaxillze and maxillz
above and to the dentary below. In a ¢hecodon? dentition
the teeth are held in definite sockets in the bone; a Aetero-
dont dentition is one in which the teeth differ markedly
amongst themselves in size and shape; and, lastly, in a
diphyodont dentition there are two sets of teeth succeeding
one another in the life of the individual. [We may recall
that the teeth of most reptiles are fused to the bone
(acrodont or pleurodont), they are usually of the same size
(homodont), there are several series of teeth (polyphyodont),
and the teeth often occur upon the palatines, pterygoids or
vomers in addition to the premaxille and maxilla. The
crocodile, as in other anatomical features, approaches the
mammal in having one row of thecodont teeth which are
slightly heterodont. ]
DEVELOPMENT OF A TooTH.—A typical mammalian tooth arises
from an examel-organ consisting at first of a protrusion of the mucous
layer of the epidermis downwards into the dermis. This becomes
pushed in on the under side and the dermis thus protrudes into it
as a small ‘‘dentine-germ.” The mucous epithelium, bordering the
MAMMALIA. 459 —
dermis, commences to be modified by calcification into a hard dense
enamel, whilst a layer of odontoblasts or cells of the dermis becomes
active and gives rise on its outer side, near the enamel, to a bony
dentine less dense than the enamel. In the centre the formation of
dentine does not take place, so that a pulp-cavity remains. In the great
majority of teeth this cavity becomes constricted and nearly closed and
no further production of tooth-substance takes place; but in teeth which
grow from persistent pulps, or continue to grow throughout life, the
pulp-cavity remains widely open and the enamel-germ and odonto-
blasts continue to produce fresh enamel and dentine respectively.
To the teeth of many mammals is added a third substance called
cement. This surrounds the dentine at the base of the tooth or lies
between the enamel-crests on the upper surface of the tooth. It is
produced by the dermis. As development proceeds the tooth forces
its way to the surface and later its base becomes surrounded by bone,
forming the socket.
In most flesh-eating animals the enamel remains intact throughout life,
but in vegetable-eaters the crown of the tooth, especially in the case of the
molars, becomes worn away, and as the cement and dentine wear more
rapidly than the enamel, the latter forms ridges which assist in mastica-
tion. We may note in ‘this typical development of a tooth that it isa
joint production of epidermis and dermis. The development is in
essential features similar to that of a placoid scale (Elasmobranch
fishes) and it is usual to regard the two structures as homologous.
In the great majority of mammals the teeth are hetero-
dont, z.e., differ markedly in shape and size in the different
parts of the jaws. It is found impossible to directly com-
pare the teeth of the same shape throughout the class as
this would be a very unnatural grouping and would lead to
confusion. For the determination of dental homologies we
have to resort to other means. In the upper jaw the teeth
are borne upon premaxilla in front and maxilla behind. All
the teeth borne upon the former are called zxcisors. This
name is given to them because, as a rule, they are chisel-
shaped. They may, however, be of a very different shape,
and their homology depends not upon their shape but upon
their position on the premaxilla. The tooth immediately
behind the suture between premaxilla and maxilla is known
as the canine tooth because it is typically developed in dogs.
It is usually a long single-rooted fang, but is often absent
or of a different shape. The remainder of the teeth on the
maxilla are called molars because they are mostly for grind-
ing or cutting food ; they are usually many-cusped and have
several roots. Some of the molars are further distinguished
from the rest as premolars (see next page).
460 CHORDATA.
In the lower jaw the teeth are all upon one bone, so that
the only criterion for distinguishing the kinds of teeth is by
their position relative to the upper teeth.
An incisor tooth is hence a tooth borne by the pre-
maxilla or by the mandible exactly opposite it. A canine
tooth is a tooth borne by the maxilla immediately con-
tiguous to the suture between premaxilla and maxilla, or
by the mandible immediately opposite to and biting im-
mediately in front of it.
Succession or TeeTH —In the majority of mammals
there is a modification of the polyphyodont arrangement of
reptiles and the succession isreduced to two (diphyodont). The
first series of teeth is known as the Zacteal or deciduous series,
which are sooner or later replaced by absorption of their
roots and the pushing-up of the permanent series from below.
The incisors and canines usually correspond in number in
each series, but the deciduous molars are not so numerous
as the permanent ones. Thus there results a distinction
between the permanent molars, the front ones only being
preceded by deciduous molars. The former are known as
premolars and the latter as molars proper.
There are many exceptions to the diphyodont condition,
and even in typical forms there is often a retardation in
appearance of the hind-molars (cf wisdom-teeth) which
simulates the beginning or the vestige of another series.
A dental formula is often used as a symbol of the dentition of a
mammal.
The evolution of a dental formula may be illustrated as follows :—
DENTAL FoRMULA OF MAN.
. 2—2 . ITI 2-2 ooo!
1. Lncisors —, canines —, premolars —, molars ”-— = 32.
2-2 I-1t 2—2 3-3
The four figures mean right and left half of upper and lower jaw.
2. 2.3, 04, pm. 3, m 2.
Here the names are represented by initials, and it is recognised that
in the greater proportion the right and left half are similar ; and, lastly,
a knowledge of simple arithmetic is assumed and the total is omitted.
2123
3: 2123"
As the teeth are always quoted from in front backwards, the initials
-are superfluous and a very short, compact symbol is the final product.
As already indicated, the structure of the teeth and that of
the limbs form the two most diagnostic features, so that the
MAMMALIA. 461
importance of correctly interpreting the dentition ofa mammal
as far as possible at sight can hardly be over-estimated.
The incisor and canine teeth remain more or less siniple throughout
the majority of the Aammatta. In Ayrax, Galeofithecus and some
rodents the incisors have their edges indented to form small cones, but
these are exceptional. Again, the canines may resemble premolars in
shape and may have more than one root (Pliohyrax, Erinaceus). The
cheek-teeth or premolars and molars show infinite variety in shape and
size according to the uses to which they are put.
The complex types are derived from the more simple by the forma-
tion of cusps or tubercles which may fuse to form ridges and crests.
We may notice a few of the more important changes :—
1. It is usually assumed that the earliest mammals had a homodont
dentition like that of reptiles, each tooth being a simple cone. Those
of the upper jaw fitted between those of the lower jaw, forning a ‘‘rat-
trap” ‘arrangement, very efficient for seizing prey, but of little use for
purposes of mastication. This first type is called a Aaplodon¢ dentition
and is still found in the toothed whales (see Porpoise)..
2. The next differentiation is the origin of small secondary cones
upon the sides of each tooth, typically one on each side of the primary
cone, though the whole margin may be serrated. These secondary”
cones move upon those of the next tooth and considerably add to the
‘‘tearing” and rending capacity of the teeth. Typically there is one
cone on each side of the primary one, hence this type is known as the
triconodont dentition. In the upper jaw the primary cone is known as
the protocone, the anterior secondary one as the paracone and the
posterior as the metacone. Those of the lower jaw are known as
protoconid, paraconid and metaconid.
3. In the next type the secondary cones move out of the same line
as the main cone, those of the lower jaw moving inwards and those of
the upper jaw outwards, The three cones or tubercles are now
arranged in a triangle. The dental surface has no less than three
interlocking rows of tubercles, the outer formed by the paracones and
metacones, the middle by the protoconids, and the inner by the proto-
cones with the paraconids and metaconids. This type is known as the
tritubercular and is a very important one. It occurs in many modern
mammals with little modification, such as certain /wsecttvora and Car-
nivora, and is also very general amongst the mesozoic metatherian
mammals and inodern Polyprotodontia.
From the tritubercular type onwards we may trace three series. In
one there is specialisation for a true carnivorous type producing the
secodont or cutting dentition. In this the cones become connected by
ridges which retain a sharp edge, acting as cutting organs. (Carndvora.)
In the second the cones remain blunt and increase considerably
in number. In later life their surfaces are ground away and there
may further be important fusions forming blunt ridges. This is the
bunodont series, found in herbivorous and omnivorous mammals.
In the third the general tubercular character is retained though
other cones may be added. This is probably to be traced to the reten-
tion of a similar mode of nutrition and the examples are naturally to be
462 CHORDATA,
found in the 4vsectzvora. In both jaws there arises posterior to the pro-
tocone (or protoconid) a fourth cone called the Aypocone (or hypoconid).
All four become regularly arranged, giving a quadruple row of trap-like
tubercles. This type is called the guadritubercular, often complicated
by further smaller tubercles forming a mudtdtubercular arrangement.
The quadritubercular condition is well seen in the hedgehog.
In both the other series a hypocone also arises and in the lower
jaw it may be double.
Thus a quadritubercular condition is produced in the bunodont
series by a similar development of a hypocone and hypoconid. The
hypoconid is developed in such a position that il moves up into the
depression in the primary triangle of the upper jaw, whereas the hypo-
cone is, like the protocone, between the primary triangles of the lower
jaw. The consequence is that, when a lateral motion is given to the
lower jaw and transverse columns are formed by fusion across of the
tubercles, the upper jaw has a normal fusion of protocone with paracone,
and metacone with hypocone, but the lower jaw has a fusion of proto-
conid and metaconid to form the anterior transverse ridge, whilst the
posterior is formed by the hypoconid and subsidiary cones, the para-
conid disappearing altogether. Thus is produced the dzlophodont type
with two transverse ridges, those of the upper jaw alternating with
those of the lower, This important type is found in kangaroos and
in tapirs and forms the starting point of the perissodactyle series.
Further differentiation of the grinding molars is in the direction of
complex foldings which tend to increase the number and extent of
enamel-ridges. (See Horse and Ox.)
Summary.—In the cheek-teeth of mammalia we can distinguish
the following series :—
1. Haplociont—single series of simple conical teeth (Odoztoce/z).
2. Triconodont—single series of teeth with three cusps or cones
(Triassic Jetatheria).
3. Tritubercular—series with three cusps, usually with two in
different position from the other, the whole forming a triple series
(Triassic Metatheria).
4- Quadri- and multitubercular—series with four or more cusps
forming four or more series and retaining typical (insectivorous) char-
acters (/usectivora).
5. Secodont series, with cusps united by sharp ridges and often
increased in number—carnivorous (Carnivora).
6. Bunodont series, with cusps separated and often increased in
number, blunt and crushing—omnivorous or herbivorous. (Szde,
Urside, Primates. )
7. Bilophodont and other types, increase of tubercles, transverse
and longitudinal ridges formed by fusion, complex folding and the
crowns worn flat during life—herbivorous (Ungulata, Nodentia).
Brain AND NERVOUS SystEM.—The characters dis-
tinguishing the brain of mammals from that of the other
Vertebrata are not so striking as one would perhaps be led
to assume, considering that mammals largely owe their
MAMMALIA. 463
supremacy to development of the mental faculties. The
brain develops in a typical vertebrate manner, and we may
here merely note the following characteristics :—
1. The cerebral hemispheres are large and encroach
backwards over the thalamencephalon and the optic lobes.
In the higher types their surface becomes much convoluted
and they cover the cerebellum.
2. The cerebral hemispheres are united across the
middle line by the corpus callosum.
3. The optic lobes become divided to form four, the
corpora quadrigemina.
4. One of the most striking characters of the mammalian
brain is the great increase in proportionate size that has
taken place in comparison with the brain of extinct forms.
The brain of the Eocene mammals was far smaller in pro-
portion to the total bulk than that of modern forms. This
is probably due to the fact that since that epoch the race
has not been so much to the strong as to the “cunning.”
In the same way, if we compare the weight of a mammal’s brain
with the total weight of the body, we find that there are three impor-
tant laws.
Firstly, in equally organised animals the relative weight of brain
decreases with increase in size. Thus the smallest animals tend to have
proportionately heavier brains. The relative brain-weight of a cat is
given as z4,, whereas that of a tiger is ¢tg. On account of this law,
we find that the relative brain-weight of man (#5) is exceeded by that
‘of the lesser shrew (345) and the whiskered bat (75).
Again, the relative brain-weight increases very rapidly in proportion
to the organisation of the animal and in animals of equal size it varies
with the organisation.
Thus we may cite from Dubois the following equal-sized species:—
Siamang (Simiide) . {
Budlug (Cercopithecidze) Oey A,
Civet- Cats. cocsinivce aceneaonnes Carnivora,....... eu
Javan Pangolin,.... Edentata, ..........66655 4
If the effect of the varying size of mammals be eliminated, a table
showing degree of ‘‘cephalisation” can be formed, and this agrees
generally with the recognised succession of the mammalian orders, the
Metatheria, Edentata, Rodents and Insectivora taking the lowest
places, followed by Ungulata, Cetacea, Carnivora and lower monkeys,
and, lastly, anthropoid apes and man.
Thirdly, taking extinct mammals into account, it would appear that
in mammals of similar size and bodily organisation the relative brain-
weight increases with the time, as we have seen that the greatest
advance from Eocene times has been cerebral.
464 CHORDATA.
Bioop-VascuLar SysTeM.—The heart and circulatory
system do not show any great adaptation throughout the
class. The heart is always four-chambered and the systemic
arch is only found on the left. In various regions of the
body there are developed fine meshworks of blood-vessels
termed retia mirabilia. These are found at the bases of
the limbs in many arboreal animals which have to hang
from boughs, in which case they appear to counteract the
Fig. 322.—A RETE MIRABILE.
A, Ge.eral appearance. B, Cross-section of the blood-vessels. C, Anastomosis of
smaller and larger vessels, (After MuRIE.)
retarding effects of gravity upon the circulation (¢& Sloth).
They also occur in whales, possibly for the storage of arterial
blood to allow of a long sojourn under water.
RESPIRATOkY SysTEM.—The lungs lie freely in the
thoracic cavity, being completely surrounded by the pleura,
and respiration is effected by the ribs and _ intercostal
muscles, supplemented by the diaphragm, as in the rabbit.
The diaphragm is foreshadowed in types like the crocodile,
but it is typically a mammalian organ in its perfect condition.
MAMMALIA, 465
The temperature of reptiles is directly dependent upon
that of their surroundings, but that of mammals and birds
is constant—that is to say, the heat-producing agencies of
the body are so adjusted that the body-temperature is
maintained at a certain mean average. That of birds is
much higher than that of mammals, and for this reason the
body-temperature of birds is sometimes described as hot
and that of mammals as warm. ‘The special point, however,
is in each case the constancy of the temperature, whatever
the environment. In this respect, as in many others, the
Prototheria and Metatheria approach the reptilian condition.
ALIMENTARY SysTEM.—The same general plan of ali-
mentary system holds throughout, though certain changes
are found in correlation to special methods of feeding. A
number, such as the anteaters, pangolins and Zchidna, have
an elongated protrusible tongue and highly developed
salivary glands, the saliva being used to make the tongue
sticky, by which means the ants and other insects may be
readily caught.
The stomach is more or less simple in some forms but
extremely complex in others. The complexity is of two
kinds. The first is its division into two or more chambers
which are easily visible externally and the second involves
the distribution of the glands. A more or less prominent
part of the stomach which immediately succeeds the
cesophagus has an entire absence of glands and is lined
only by stratified epithelium. The whole stomach is of
this nature in Ornithorhynchus. Again, this area is followed
typically by an area containing cardiac glands, another
containing fundus glands. and, lastly, by the hinder
portion containing pyloric glands. The fundus glands may
often be absent.
The first division of the stomach is effected by a con-
striction dividing it into cardiac and pyloric chambers, as in
certain rodents. In most cases the cardiac portion has no
glands, whilst cardiac and pyloric glands are found in the
pyloric portion. In others, as the porpoise (p. 546), there
are three chambers, consisting of a non-glandular cardiac
part, a second chamber with cardiac glands and a small third
and fourth with pyloric glands. In the Ruminants there’
M. 31
466 CHORDATA.
are typically four chambers, of which the two first are non-
glandular, as is also the third (see Ruminantia, page 514).
It is difficult to find any general law regulating the amount
of complexity of the stomach. In a very wide sense, the
carnivorous animals have the simpler and the herbivorous
have the more complex stomach, but there are many excep-
tions to these, such as the whales.
The intestine is usually long in the herbivorous mammals
and comparatively short in carnivorous, and the same applies
especially to the caecum which may be entirely absent in
certain Carnivora.
Lastly, we may notice that in the great majority of
mammals the anus opens to the exterior independently of
the urogenital sinus, no cloaca being present.
UROGENITAL SysTEM.—The urogenital organs show a
transition series as the viviparous habit is acquired and
elaborated. In the oviparous JZonotremata the oviducts are
like those of reptiles, simple throughout and opening
separately into the urogenital sinus. In the higher types
the oviduct becomes differentiated into (1) the upper part
or Fallopian tube, (2) the middle part or wéerus and (3) the
lower part or vagina. At the same time there takes place
a fusion of the two oviducts in the middle line. In the
majority of the A/etatheria there is little or no fusion, so
that there are two uteri and two vagine, but in the Eutheria
the two vagine are always fused into one. Lastly, in all the
higher Zutheria the two uteri are more or less fused into
one, transition forms giving rise to the types of uterus called
bicornuate and bi-bipartite.
In the male there is a corresponding progress in the evolution of the
penis and the urogenital system generally. It is evident that the
viviparous habit requires a complete internal fertilisation, even more than
in the terrestrial oviparous forms. The penis in the Sauropsida is
merely the specialised ventral wall of the cloaca, which is only partially
protrusible ; on its dorsal surface is a groove, the penial urethra. In the
Monotremata the penial urethra has become a tube along the dorsal sur-
face of the penis, which, however, communicates freely behind with the
cloaca as well as with the urogenital sinus. In the Marsupdalia the
urogenital sinus and the penial urethra are continuous and completely
apart from the rectum, but the distal end of the penis is still surrounded
by the same sphincter muscle as the anus (cf female), whereas in the
Eutheria the penis is perfectly distinct and free from the anus (the space
between the two being the perinzeum) and is more complex in other ways
MAMMALIA. 467
than that of the lower types. The main point to notice is the gradual
separation of rectum and urogenital canal from a common cloaca, a
process akin to that seen in the female.
SKELETON.—The skeleton in Mammalia is almost en-
tirely bony, but the bones mostly have efzphyses. These,
as already explained in the general features of Vertebrata
(page 413), are produced by the persistence of a thin layer
of unossified cartilage during life.
Fig. 323.—DIAGRAM OF MAMMALIAN FEMALE UROGENITAL
ORGANS.
0) FALLOPIAN
i, The Prototheria. 2, The Metatheria. 3, The Eutherian bipartite uterus. 4, The
Eutherian bicornuate uterus. 5, The Eutherian simple uterus.
The general characters of the mammalian skull have
been noticed in the rabbit. The facial and cranial por-
tions are completely joined together. Specially interesting
are the parts connected with the suspensorium and the ear-
ossicles.
In Sauropsida the quadrate suspends the mandible, but
in the mammals the squamosal bone grows down to meet
468 CHORDATA.
the dentary and forms a fresh articulation, so that the
quadrate is no longer necessary for this function, and passes
backwards to form the ¢ympantc bone which surrounds the
outer part of the ear.*
The lower jaw also appears to consist of a single bone
on each side.
In this connection we may note that the squamosal
articulation has the condyle on the movable part, whereas
the quadrate articulation of Sauropsida has the condyle
on the quadrate or immovable part. The first has a
mechanical advantage which may partially account for the
substitution.
Other special points we may note in the skull of the
mammal are these:—The skull is suspended to the first
vertebra by two condyles borne on the two exoccipitals.
The maxille and palatines meet their fellows across the roof
of the mouth to form a bony palate, so that the nasal
cavity only communicates with the buccal cavity by small
naso-palatine foramina in front and by the internal nares
behind. The maxilla and squamosal are connected across
under the orbit by the malar or jugal, forming a bridge of
bone called the zygomatic arch (or suborbital bar). Ridges
for the insertion of muscles may be formed, such as the
sagittal crest along the median dorsal line and the occipital
crest at right angles to it in the occipital region. These are
best developed when a heavy “bite” is required. The
tympanic bone very commonly expands into a swollen bulla
tympani below the ear, enclosing the tympanic cavity or
middle ear.
THE VERTEBR#.—The cervical vertebre are usually
seven in numbert and are distinguished from all other
vertebree by having a pair of lateral foramina as well as the
large central one. These are known as the vertebrarterial
canals, because the vertebral artery runs through them.
They are formed by the cervical rib, with its head forked
into capitulum and tuberculum, becoming fused on to the
* This is one of several views as to the fate of the quadrate in mammals. Many
hold that it forms the incus.
+ Exceptions are found in the Zdendata and Sivenia. Bradypus has eight or
nine, 7% dua eight, Cholapus and Manatius six.
MAMMALIA. 469
vertebra and hence enveloping the artery in a complete
an ring. (Camels form a remarkable exception to this
rule.
The thoracic vertebree bear the functional ribs. They
may be known in mammals by the articular half-facet on
the centrum for the capitulum of the rib. As a rule in
mammals the capitula of the ribs articulate detween the
vertebrae (¢f chevron-bones), hence the half-facet. The
transverse process also has a facet for the tuberculum. In
many thoracic vertebree the neural spines are very long.
The thoracic vary in number throughout the orders.
The lumbar vertebree approximate at the anterior end
to the thoracic in character, but they have no free ribs.
The ribs are fused on to the transverse processes, thus
producing large flat lateral wings which are usually known
as “transverse processes.” The neural spines are never
long in the lumbar vertebree.
The sacrum is formed of two primary sacral vertebre
which are firmly welded together and to the ilium. They
also contain rib-elements in the short transverse processes
(still seen in crocodiles). There are usually in Lutheria one
or more caudal vertebree more or less welded into the
sacrum.
The caudal vertebree vary enormously amongst mammals
in size and number, just as the “tail” also varies. They
are usually more or less simple rod-shaped bodies. In the
aquatic forms, such as Svvenza and Cefacea, the tail is hyper-
trophied and the vertebrz, as also in some terrestrial forms,
é.g., Kangaroo, bear chevron-bones or'ventral arches articulat-
ing between the centra. Ina good number of mammals the
tail forms a valuable accessory limb, more especially in the
arboreal types. The muscles of the prehensile tail are
strengthened and the end of the tail is wound round a
bough sufficiently firm to bear the weight of the animal,
thus freeing the limbs for other purposes. The forests of
South America present us with a remarkable abundance of
forms with prehensile tails, some examples being the spider-
monkeys, tree-porcupines, tree anteaters, opossum-rats and
opossums.
In some Axthropoidea the tail is vestigial, reduced to
half-a-dozen fused vertebra called the coccyx, which no
470 CHORDATA.
longer protrudes from the surface as a “tail” but may
even occasionally become fused to the sacrum.
Tue Lime-GirDLEs.—The girdle of the fore-limb or
pectoral arch closely approximates to the reptilian type in
the Afonotremata, but becomes more specialised in the
Marsupialia and Eutheria.
In the Monotremata the coracoids are large and meet
the sternum. They bear on their inner border a pair of
precoracoids. There is also a T-shaped episternum. In
the Metatheria and Eutheria the coracoids atrophy, as also
the precoracoids. Amongst other vestiges of these bones
there is a process upon the. scapula, the coracoid process,
which is said to be the distal end of the precoracoid, the
true coracoid being represented by a small bone taking part
in the formation of the glenoid cavity.
Hence in nearly all mam-
Fig. 324.—THREE TyPEs OF mals the scapula alone is
MAMMALIAN SCAPULA. left to bear the fore-limb,
Cee especially as in a great num-
ber the clavicle atrophies.
The scapula is correspond-
ingly highly developed. It
is a large, triangular-shaped,
flattened bone, with a bony
ridge down its outer surface
called the spzme, terminating
ot z in a free process, the acro-
A, Cursorial. _B, Aquatic or natatorial. heal Me which the distal end
ae "C, ‘Arboreal. ‘of the clavicle is attached,
when present.
In the running types (Ungulata), in which the limb has
little diversity of movement, the clavicles go and the scapula
is long and tapering, with short suprascapular border. In
the climbing types (Primates), with varied movements of
the forelimb, the scapula is an approximation to an equi-
lateral triangle. whilst in the swimming types (whales, seals)
the scapula is broadened out, shortened lengthwise, with
long suprascapular border. The spine is pushed forwards,
so that the postscapular fossa is very large and the pre-
scapular fossa is small.
MAMMALIA. 47
The pelvic arch in mammals is fairly constant in its
structure. The ilium always becomes firmly attached to
one or more of the vertebrz. It always slopes backwards
from its junction with the sacrum to the acetabulum,
whereas the acetabulum is usually immediately below the
ilium in reptiles. The Monotremaza in this respect approxi-
mate to the reptiles, the angle between the axis of the ilium
and that of the sacrum being less acute.
Fig. 325.—LATERAL VIEWS OF—A, CROCODILE’S PELVIS;
B, PELVIS OF PROTOTHERIA; AND C, THAT
OF EUTHERIA.
a 6 IMQlium. Ilium @ 36 llum, a@ &
f
Ischium.
Epipubis. Pubis. Epipubis,
a= Perpendicular axis through acetabulum, 4= Perpendicular axis through sacrum.
In mammals the pubes unite with the ischia on each
side and thus enclose a large hole or foramen, the od¢urator
JSoramen. In most the pubes meet across the middle line
to form a symphysis pubis and the ischia also meet to form
an ischial symphysis, but in several types (¢g., man) the
ischia no longer meet across the middle line, the pubes
forming the whole symphysis. A small acetabular bone is
also very generally present and usually fuses with one of
the other elements.
In Metatheria and Prototheria there is a pair of epipubic
bones running forwards from the pubes, which serve, at least
in the former, for support of the pouch. Similar epipubic
bones are found in certain reptiles (¢.g., Watteria).
STERNUM AND Rips.—The sternum in mammals arises
from the fusion of the distal extremities of the ribs and is
usually segmented into a series of joints or so-called
472 CHORDATA.
“sternebree.” The anterior end is called the manubrium
and the posterior end is the xphoid process. The ribs are
many in number and articulate by a capitulum Jdefween the
vertebrae and a tuberculum on the transverse process. This
peculiar articulation of the ribs is explained thus:—In
certain fossil reptiles the vertebree are double; each has a
centrum and an intercentrum which are equal in size. The
rib articulates primarily with the intercentrum by its
capitulum. In extant reptiles the intercentrum disappears
and the rib acquires a secondary connection (the tuber-
culum) with the transverse process ; the capitular attachment
may then, in some cases, be given up. In mammals the rib
also acquires a secondary connection with the transverse pro-
cess, but although the intercentrum disappears, as in modern
reptiles, the capitular attachment still remains at the spot
between the centra at which the intercentrum has dis-
appeared. ’
The intercentra are represented in mammals by the zzter-
vertebral discs which are only very rarely (cf Mole) ossified.
The cervical ribs are completely fused on to the vertebree
and are no longer recognisable as such. The ribs in
Mammalia have an important function in connection with
respiration. They are moved upon the vertebre by the
intercostal muscles. When the ribs are raised the cubic
capacity of the thorax increases and inspiration takes place,
conversely when they are depressed. This action is sup-
plemented by the movements of the diaphragm forming the
posterior wall of the thorax.
The thorax can be enlarged in two ways. In the dog,
horse and most quadrupeds the ribs are much bent, and
they move forward in such a way that the “narrow” chest
of these animals enlarges laterally, whereas in man the
sternum is raised and pushed outwards, so that the chest
is, in this case, expanded vertically.
:Limgs.—In the mammalian fore-limb the three proximal
carpals are known as scaphoid, lunare and cuneiform, the
centrale is often absent, and the distalia are known as
trapezium, trapezoid, os magnum and unciform, the last
being the fourth and fifth distal bones fused. There is very
often another bone, the fisform, usually attached to the
MAMMALIA. 4733
postaxial border of the cuneiform. It may be a sesamoid
or possibly a carpal bone. In the hind-limb the proximal
tarsals are always three in number, the radiale and inter-
medium are fused to form astragalus and the fibulare is
Fig. 326.—DIAGRAM OF THE TYPICAL MAMMALIAN FORE- AND
Hrnp-Lims,
--HUMERUS -~ -FEMUA
| “RADIUS -TIBIA
1
Lc ULNA LoFIBuLa
,
LUNARE.
SCAPHOID,
N
7CUNEIFORM, ASTRAGALNS. Wa =CALCANEUM
,
ca 7 CUBOID,
-
Mp, CUNESFO, {\ ) 7 GHT.CUNEIFORM.
-
- %
,UNCIFORM. NAVICULARY *
,
int. CUNELFORR
OS MAGNUN,
TRAPEZOB fs
TRAPEZIUM
METACARPALS.
Note fusion of fourth and fifth distalia and limited number of phalanges.
known as the cadcaneum. The centrale persists on the pre-
axial side as the zaviculare. The distalia are the internal,
middle and external cuneiform and the cuboid, the last being
the fourth and fifth distalia fused together. In mammals
the main joint of the hind-limb and foot is between the
crus or tibiofibula and the proximal tarsals, hence it is a
474 CHORDATA.
crurotarsal joint, whereas in reptiles and birds the main
joint is an intertarsal joint.
In both limbs of mammals the number of phalanges is
normally two in the first digit and three in each of the
others.
In the various orders we shall notice that there may
occur fusions of certain bones, loss of others and modifica-
tions of others, but when once this type be learnt and
retained in one’s mind, there is no difficulty in interpreting
aright the most modified mammalian limb.
As general rules for the identification of the bones we may lay down
the following (see Fig. 326) :—
1. Humerus and Femur.—The proximal limb-bones (Aumerus and
Jemur) are long bones and have az articular condyle at each end.
Towards their proximal ends they have a ‘‘ ball” which moves in the
socket of the limb-girdle, and two or more processes called /zcberostties
(humerus) or ¢vochanters (femur). At the distal extremity they both
have a sigmoid condyle or ‘¢vochlea. The humerus may be distin-
guished from the femur by its large shallow condyle, whereas the femur
has a rounder condyle raised on a ‘‘ neck.” The humerus usually has
a conspicuous deltoid ridge on its preaxial border. The proximal
end of the humerus (or femur) can always be distinguished from the
distal by the condylar or ball-and-socket joint in the former and the
sigmoid or é2/atera/ joint in the latter.
The humerus often has a small foramen on the inner or postaxial
side of the sigmoid condyle termed the entepicondylar foramen. It
seems to have occurred very generally amongst Eocene mammals, such
as Condylarthra, Tillodontia and Typotheria, and is very generally
found amongst Metatheria, Edentata, some Carnivora, most /nsectivora,
Lemuroidea and Cebide.
This foramen should be carefully distinguished from the supra-
trochlear foramen in the median line above the trochlea and produced
by incomplete ossification.
The ¢hird trochanter of the femur has much the same interest as the
entepicondylar foramen. It is on the postaxial border (cf Horse)
and is present in Condylarthra, Tillodontia, Typotheria, Creodonta and
other extinct types. It also occurs in Dasyfodide, Orycteropodide,
many Rodentia, most Iusectivora, in Pertssodactyla and (small) in
Hyracoidea.
z. Distal limb-bones.—The distal limb-bones have a hollow artz-
cular facet at each end when they are fully developed. At the
proximal extremity they receive the condylar ends of the proximal
limb-bones ; at the distal end they receive the condyles of the proximal
carpals.
The two most important bones are the preaxial (or the radius and
zibza), and the ulna and fibula are, in a great number of cases, merely
vestiges fused on to their respective preaxial bones, forming a single
MAMMALIA, 475
radioulna or tibiofibula. The proximal ends of the radius and ulna
both take part in the formation of the facet or sigmoid notch in which
the condyle of the humerus moves, and the ulna is always produced
backwards as an olecranon process for the insertion of the triceps muscle.
This olecranon part of the ulna remains in cases where the ulna
atrophies, hence the radioulna or ulna has its facet deep and not
quite at the proximal end of the bone. Distally both radius and ulna
have shallow facets for articulation with the proximal carpals.
Tibia and Fibula.—The proximal ends of the tibia and fibula both
usually take part in the formation of the shallow facet of the knee-joint
upon which moves the distal end of the femur.
Their proximal end therefore has a shallow facet which is a¢ the
extreme end, Notice that of the two girdle-joints ; that of the fore-limb
or the glenoid joint is less deep than that of the hind-limb or the
acetabulum, But in the case of the limb-joint that of the fore-limb or
elbow-joint is much deeper than that of the knee-joint. By keeping
these points in mind there should be no difficulty in recognising a
radioulna from a tibiofibula or an ulna from a tibia.
Manus and Pes,—The wrist-bones or carpus and the ankle-bones
or tarsus require special study to be distinguished one by one, but the
astragalus and calcaneum are always fairly characteristic, the former
bearing a well-developed sigmoid head for the tibia and the latter being
produced into the heel in which is inserted the tendon of Achilles.
The metacarpals and metatarsals are remarkably developed in the
Ongulata. In correlation with a reduction in the number of the toes,
those remaining are correspondingly increased in size, forming the
cannon bones of the horse and ox. These have the appearance of the
true long-bones of the limbs, but they may at once be recognised by
having a hollow facet at one end (proximal) and a bilateral condyle at
the other end (distal).
DEVELOPMENT.—In studying mammalian development
we have to keep in mind that the larval and lecithal nutri-
tions have been given up and that there is a succession of
three forms of nutrition—the albuminal, the hemal and
the lacteal. The Prototheria are oviparous, 7.e., the young
are discharged from the body as eggs surrounded by a shell,
and further development takes place outside the body of the
parent; but the great majority of the Mammalia are vivi-
parous, z.¢, the young are retained during early stages in
a special part of the oviduct, called the uterus, and are
“born” later.
MATURATION AND PRODUCTION oF THE Ovum.—The
eggs arise in the ovaries which are paired. The outer epi-
thelial layer of the ovary is the germinal epithelium, and
from it the eggs sink into the underlying connective tissue
476 ' CHORDATA.
surrounded by a mass of follicle-cells which are usually
regarded as nutritive. These cells increase in number and
the whole follicle grows rapidly. A split occurs between
them, so that in a fully-formed ‘“ Graafian follicle” the
ovum lies towards the centre surrounded by certain of the
follicle-cells. A large cavity separates them from the outer
layer of follicle-cells which form the outer tunic of the follicle
and the two layers are connected by strands.
When ripe, the follicle bursts and discharges the ovum at
the surface of the ovary, whence it passes into the oviduct
through its fimbriated opening. The ripe egg has a hyaline
membrane around it, the zoza radiata; and inside this
there is the delicate vitelline membrane. The mammalian
Fig. 327.—THE MAMMALIAN GRAAFIAN FOLLICLE IN THE OVARY.
Central Cavity.
Outer Layer
of Follicle
Cells.
Nucleus.
A, Early stage. B, Later.
egg so produced is always of minute size, often about ‘1 mm.
in diameter (about the same as Amphioxus). Maturation is
effected by the extrusion of two polar bodies and fertilisa-
tion takes place high up in the Fallopian tube.
SEGMENTATION.—The egg immediately commences to
segment whilst it passes down the Fallopian tube. There is
no yolk and the segmentation is total and nearly equal.
The first division is into two blastomeres, of which one is
very slightly the smaller. Each divides into two and then
into four. The larger cells then become tucked inside
the smaller, which on their part divide more rapidly and
spread round them. Thus there is produced a stage
in which the larger or hypoblast cells are enclosed on every
MAMMALIA. 477
side by the smaller or epiblast, except at one pole, the
blastopore. It is difficult to withhold from this stage a
homology with the gastrula of lower types, such as Amphi-
oxus. It is sometimes called a “ metagastrula.” The
embryo has now reached the uterus and then commences a
remarkable process. The blastopore closes, and the whole
Fig. 328.—TuREE EARLY Staces IN DEVELOPMENT
oF Rassir. (After VAN BENEDEN.)
VITELLINE BLasTorONe
eumanne
nN
N JEPIDAST
1, The metagastrula; 2, the commencement of the rapid enlargement of
the egg; 3, the fully-formed blastocyst.
embryo increases rapidly in size. The epiblast-cells become
flat and continue to divide, keeping pace with the great
increase in size, but the hypoblast-cells remain in a small
heap at the blastoporic pole. Thus is produced the so-
called d/astocys?, its large cavity filled with a colourless fluid.
The hypoblast-cells may now increase and commence to
spread round the inner surface of the vesicle, or, as in the
‘hedgehog, they may split to form an internal cavity or sac
and then expand. In either case the same result is attained
478 CHORDATA.
Fig. 329.—DIAGRAMS OF THE Fa:raL MEMBRANES OF A
MAMMAL.
Epiblastic Disc,
ae
clase ras
Hypoblastic Disc.
Epiblast. Extra-
<i _Hypoblast. f embryonic.
Fluid in Yolk-sac.
A, The diploblastic embryo of a mammal in section.
Amniotic
Folds. | Epiblast of Embryo.
aivi y Hypoblast of Embryo.
'Villi of Serosa,
Fluid in Yolk-sac.
B, A later stage with serosa villi and a developing amnion,
MAMMALIA, 479
annie Cavity.
Villi of Serosa,
!
i
Serosa.
Cc
Yolk-sac.
Fluid in
Yolk-sac,
Villus of Serosa.
Embryo.
Allantois. Extra-
embryonic
Allantoic Villus. €celom.
~Yolk-sac
Villus.
Prokalymma.
Epiblast is white, Mesoblast black, Hypoblast dotted.
—the blastocyst becomes two-layered or diploblastic through-
out. Both epiblast and hypoblast form thin-walled spheres,
with a disc or cap of cells at the blastoporic pole. The
hypoblast remains for the present in this condition, but the
epiblast divides in a variety of ways to give rise to the
480 CHORDATA.
embryonic epiblast, the amnion and the serosa. In perhaps
the simplest (pig, rabbit) the disc sinks in towards the under-
lying hypoblast and the walls coming up on either side as
folds meet above and fuse. The disc then becomes the
embryonic epiblast, the inner walls of the folds become the
amnion and the outer form part of the sevosa. Thus the
central disc is solely the embryonic epiblast and the rest of
the epiblast or extra-embryonic epiblast forms the serosa and
the amnion. In a similar manner the hypoblast-disc forms
the embryonic hypoblast and the remainder, the extra-
embryonic hypoblast, forms the yolk-sac only.
The embryo is formed from the epiblastic and hypo-
blastic discs, the former bending over and surrounding the
latter. The hypoblast also bends up to form the alimentary
canal, and both epiblast and hypoblast become nipped off
from the amnion and yolk-sac, respectively, by folds. The
mesoblast arises between these layers around a primitive
streak at the blastoporic pole, and the organs arise from the
three layers very muchas in the chick. We may here merely
recall the fact that the epiblast gives rise to epidermis,
nervous system and stomodzeum ; the hypoblast to the epi-
thelium of the alimentary canal and appended glands and
organs; and the mesoblast to the muscles, skeleton, con-
nective tissue and blood-vascular system.
The mesoblast later grows outwards from the embryo to
cover the embryonic membranes, creeping out as a sheet
over the surface of the amnion and yolk-sac and eventually
reaches the serosa. The outer layer of mesoblast now
invests the amnion and the upper part of the serosa,
whilst the inner layer covers the upper half of the yolk-
sac. At the edge the two layers meet and extend as an
unsplit sheet of mesoblast still further down between the
yolk-sac and the serosa. Further down still the serosa and
yolk-sac are still closely apposed and there is no mesoblast.
Hence the blastocyst wall is now formed (1) at its upper
half by a wall of epiblast and a single layer of mesoblast,
the completed serosa ; whilst (2) below the equator there is
a broad zone consisting of the epiblast of serosa, a double
layer of mesoblast and a layer of hypoblast (yolk-sac), all
in close contact; and (3) the lower pole or cap consisting
of epiblast (serosa) and hypoblast (yolk-sac). This is an
MAMAMALTA. 481
exceedingly characteristic stage in most mammals and it is
also present in the chick; but whilst in the latter the
mesoblast extends to the lower pole and then splits all
round to form a completed serosa and yolk-sac, each with
its mesoblast wall, in the mammal the mesoblast remains at
this stage throughout feetal life.
The lower disc (3) forms the prokalymma or absorptive
disc for albuminous nutrition, the zone (2) forms later the
yolk-sac placenta for hemal nutrition and the upper half (1)
will undergo further changes. Whilst this development has
been going on within the blastocyst, the serosa has been
pushing out processes which come in contact with the wall
of the uterus and moor the blastocyst to the uterus. They
may in some mammals extend all over the surface and
seem in some cases to assist in absorption of nutritive fluid ;
hence this serosa, without its mesoblastic sheath, has been
termed the “ ¢vophoblast.” In others they form a girdle, or
they may be concentrated at one part.
In the region of the prokalymma both epiblast and
hypoblast become modified into thickened active layers,
probably to subserve albuminal nutrition. Meanwhile from
the hind-gut of the embryo there arises in the mid-ventral
line a small outgrowth, which grows rapidly and pushes out
into the space between serosa, amnion and yolk-sac. As it
is a production of the gut-wall, it has from its first origin az
inner wall of hypoblast and an outer wall of mesoblast. It
is known as the a//antois and soon spreads over the dorso-
posterior part of the embryo, coming to lie in close con-
tact'with the serosa in this region. In the chick it grows
till it covers practically the upper half of the blastocyst-wall
or serosa, and in Prototheria it occupies the whole right
half of the cavity. (See below.) The mesoblast of the
allantois and that of the yolk-sac now develop complete
systems of arteries and veins, the former being the a//antoic
arteries and veins and the latter the wze//ine.
The vitelline blood-vessels ramify all over the placental
zone, and vascular villi or processes are thrust out into the
serous villi, coming into intimate contact with the uterine
blood-system. Thus is formed the true yolk-sac placenta
and a hemal nutrition, which rapidly replaces in function
the prokalymma and its albuminal nutrition.
M. 32
482 CHORDATA.
The yolk-sac placenta is a functional organ in the Meta-
theria. In them the zonal placental area extends upwards
till it covers the greater part of the upper half of the
blastocyst and probably largely replaces the prokalymma
at the lower pole. The allantois in A/etatheria degenerates
and eventually loses its connection with the serosa, though
in certain forms it may remain attached over a small disc-
like area and form, indeed, a true allantoic placenta.
In the Z£utheria this state of affairs is carried still
further, and the allantois spreads over a large area of the
serosa, throws out villi and forms a large allantoic placenta.
The yolk-sac in these forms degenerates; it eventually
loses its connection with the serosa and lies as a small
vestige beside the allantoic stalk. Indeed, it is questionable
how far in Zutheria the true yolk-sac placenta is formed,
for the allantois is developed at a very early stage and tends
to become functional as the organ of hemal nutrition,
whilst the prokalymma is still functional. In many Eutheria
the allantois lines the whole inner surface of the serosa in
late stages, just as the yolk-sac tends to do in the case of
the Metatheria (of. Figs. 331 and 342).
The allantoic placenta attains a far higher standard of
perfection than the yolk-sac placenta. In shape we have
seen that it originates as a sac or disc (dscotdal) from which
it may spread over the equator to form the dome-shaped ; the
villi may then disappear at the pole and produce the zonary,
or the spreading may extend to the other pole and form the
diffuse, a modified form of which is the cotyledonary in which
the villi are aggregated into tufts.
Again, the villi may remain more or less simple processes
protruding into the maternal tissues, so that at birth they
can be withdrawn from their pockets, leaving the maternal
tissues intact, or they may become extremely complex and
branching and so inextricably interwoven with the maternal
tissues that parts of the latter have to be shed at birth. The
former type of placenta is termed on-deciduatze and the latter
deciduate. ‘The only other alternative is for the embryo to
leave its share of the placenta (allantois) behind at birth.
This occurs (Perameles) and the remains of the allantois
are absorbed by the maternal tissues. This type has been
termed contra-deciduate. .
MAMMALIA. 483
Fig. 330.—ANn EmBryo Horse oF SIx WEEKS IN ITs MEMBRANES.
(After Ewart.)
tied eee
hiotowte
va
‘Note the reduced yolk-sac (y.s.), the enormously distended allantois (ad/.) forming a diffuse
placenta, the prokalymma (a.c.), and the villi of the serosa (#.g.). “The amnion (a#.) is black.
Fic. 331.—S1x DIFFERENT TYPES OF PLACENTA.
E
A, Discoidal. B, Dome-shaped. C, Zonary. D, Diffuse. E, Cotyledonary.
F, Metadiscoidal.
484 CHORDATA.
It must be remembered that these types of placenta,
depending for distinction upon the degree of quantitative
or qualitative production of the villi, are gradational.
The amnion remains as a thin membrane enveloping the
embryo and containing the Zguor amanii, a colourless indif-
ferent fluid. Its walls are said to contract rhythmically and
rock the embryo. At birth the amnion is ruptured and its
remains are thrown off with the placenta.
The yolk-sac, as before stated, never contains yolk and,
after the prokalymma has ceased to be functional, it either
shrivels and folds up at the lower pole or its outer wall, the
prokalymma, is shed, and the inner wall remains as a
vascular membrane.
In this general account there are a series of striking
differences from the lower types (the Sauropsida), and yet at
the same time. there is a great degree of similitude. Here
are the same four foetal organs, the serosa, the amnion, the
allantois‘and the yolk-sac (or umbilical vesicle), but their
origin and function are different. The actual development
of the embryo and its organs is very similar, although it
commences much later.
The main differences are as follows :—
MAMMAL,
1. The egg is minute (g4,> in.
diam. in rabbit) with little or no yolk
and segmentation is total and equal.
2, The development of the em-
bryois very slow, but that of the
membranes is rapid, hence the ser-
osa, amnion and yolk-sac are formed
at first without their mesoblastic
sheaths.
3. The serosa early becomes an
attaching organ, possibly also nutri-
tive, and the yolk-sac never contains
yolk, but it becomes an organ for
interchange of blood, hence a nutri-
tive, excretory and respiratory organ,
to be replaced in the Eutheria by
the allantois similarly modified.
SAUROPSIDA.
1. The egg is large (about 1 inch
diam. in chick), has a mass of yolk
and segmentation is partial.
2. The development of embryo
commences first and that of mem-
branes is slower, hence the mem-
branes have their mesoblastic
sheaths from their inception.
3. The serosa remains simple but
is covered by a porous shell, the
yolk-sac contains plentiful yolk, and
the allantois is a respiratory organ,
its cavity forming an excretory re-
servoir.
How are we to explain these important differences ?
The lowest mammals or A/onotremata have large eggs, with
MAMMALIA, 485
shells and yolk ; and the structure of the foetal membranes,
so far as is known, does not essentially differ from that in
Sauropsida. This fact and others appear to justify zoologists
in assuming that the present-day mammals are descended
from ancestors which in these respects resemble the mono-
tremes. In other words, the change from an oviparous to
a viviparous habit is supposed to account for the differences
in structure and function. We must assume that gradually
the egg was retained for a longer period before being laid.
The serosa then became an organ of attachment to retain
the egg, the shell having become superfluous. The embryo
was thus nourished by albumen from the uterine glands.
Thus was instituted a habit of ovoviviparity in which the
young was hatched inside the mother. The interchange of
blood-elements between the blood-vessels of the widely dis-
tended yolk-sac and the enveloping maternal tissue was
inevitable, and the yolk being no longer required it com-
menced to atrophy. Thus the metatherian condition is
reached in which the yolk-sac placenta is functional and the
allantois becomes vestigial.
If, however, the allantoic arteries and veins, as well as
the vitelline, become connected with the uterus, the same
atrophy of yolk results, and the allantois eventually replaces
the yolk sac as a placental organ. To its former function of
respiration is therefore added that of nutrition.
The removal of the yolk explains the reversion to a total
equal segmentation and the formation of a ‘“‘ metagastrula,”
whereas the enormous increase in size of the egg on entry
into the uterus may be explained as being due to the
necessity for the egg being of the same large size as it
originally was when there was much yolk, the large surface
being required both for absorption and mechanical attach-
ment.
We may briefly summarise the development of a mammal
as follows :—
1. Discharge of ovum from Graafian follicle of ovary and
passage into Fallopian tube.
2. Maturation and fertilisation in Fallopian tube, followed
by total equal segmentation and invagination of hypoblast to
form metagastrula.
486 CHORDATA.
3. Closure of blastopore and entry of ovum into uterus,
accompanied by rapid increase of embryo to form blasto-
cyst.
4. Division of epiblast into embryonic disc and extra-
embryonic part, which afterwards forms amnion and serosa ;
and growth of hypoblast round inside of serosa, the disc
forming embryonic hypoblast, the vesicular wall the yolk-sac.
5. Attachment of serosa by villi to the uterine wall.
6. Addition of mesoblastic covering to yolk-sac, growth
of allantois and growth of yolk-sac villi to form yolk-sac
placenta.
4. Growth of allantoic villi into the uterine tissues and
attendant changes, producing the true allantoic placenta.
Atrophy of yolk-sac.
8. Birth of embryo by rupture of serosa and amnion,
followed by shedding of after-birth or placenta. Termination
of uterine gestation.
9. Commencement of mammary gestation.
CLASSIFICATION OF MAMMALIA,
Mammalia have, as we believe, been descended from
amphibio-reptiles in the past, so those mammals which still
present us with reptilian characters must take the lowest
place. Of these we find that two small mammals, the duck-
mole and the porcupine anteater, differ from all other
mammals in having an oviparous habit, so we are con-
strained to emphasise this fact by putting them into a sub-
order by themselves, called Protofheria (first quadrupeds) or
Ornithodelphia. This distinction is further corroborated by
numerous anatomical characters. The extant sub-class Pro-
totheria have but one order, the Monotremata. All the
other mammals are viviparous, but almost the whole of the
indigenous mammals of Australia and a few allies in America
show a simpler condition of the reproductive organs and
along with this a much less pronounced viviparous habit.
The young are born at a very early stage and there is, in all
but a single exception, no true allantoic placenta. These
and other features enable us to divide the “ marsupial”
animals from the rest into the sub-class Me¢atheria, all the
higher forms being known as Eutheria.
MAMMALIA. 487
We thus divide the class Mammalia into three sub-
classes :—(1) Prototheria, (2) Metatheria and (3) Eutheria.
The three sub-classes are divided into orders and sub-
orders. The extinct forms are in z/alics :-—
<«f ORDERS. SUB-ORDERS.
Z| 1, Monotremata. Duckmole, Echidna.
5 | 2. Allotheria, Microlestes, Plagiaulax.
5
4
aN
. -
s 3. Polyprotodon- Banded anteater, bandicoot.
SI tia Tasmanian wolf, opossum.
< K halanger, wom
& | 4. Diprotodontia. aaa SAE Sh WOM:
a at.
=z \
Be
( 5. Edentata. I. Xenarthra. Anteater, sloth, armadillo,
2. Nomarthra. Aard-vark, pangclin.
6, Sirenia, Manatee, dugong.
7. Rodentia. t. Duplicidentata. Rabbits, hares, and picas
2, Simplicidentata. | Porcupine, guinea-pig, chin-
chilla, squirrel, beaver,
rat, mouse, pouched-rat.
8. Tillodontia. Tillotherium,
g. Ungulata. 1. Hyracoidea. Hyrax.
: 2. Amblypoda. Coryphodon.
< 3. Proboscidea, Elephant.
% 4. Condylarthra. Phenacodus.
a q 5. Perissodactyla. Tapir, rhinoceros, horse.
S 6. Artiodactyla. Pig, hippopotamus, camel,
a ox, sheep, deer.
to, Cetacea. 1. Odontoceti, Porpoise, dolphin, killer.
2, Mystacoceti, Whale.
rz. Carnivora, 1. Fissipedia. Bear, badger, weasel,” doe
hyzena, civet, lion, cat.
z. Pinnipedia, Seal, sea-lion, walrus.
12, Insectivora. 1. Insectivora Vera. Mole, hedgehog, shrew.
2. Dermoptera, ‘Flying Lemur.”
13. Chiroptera. 1. Microchiroptera. Bats.
2, Macrochiroptera, Fruit-bats,
14. Primates. Lemuroidea. Lemurs.
Anthropoidea. Monkeys"and man
488 CHORDATA.
CHAPTER XXVII.
THE MAMMALIA.
Sub-Class I.—Prototheria.
The Prototheria have only one living order, though there
are reasons for believing that certain extinct forms of
mammals may belong to this sub-class. They constitute
the order AVotheria, whilst the living types comprise the
order Monotremata.
Their great importance consists in the fact that they are
the lowest types of mammals and in many respects they
form a transition in
Fig. 332.—DIAGRAM OF THE structure to the rep-
Fa@TAL MEMBRANES OF EcHIDNA 4s tiles. Like most lowly
SEEN IN CROSS-SECTION, and primitive forms,
|, Serosa.. they also have a num-
Yolk-sac. / Allantois. — ber of very specialised
features superposed up-
on their generalised
organisation.
We have already
seen that the division
into sub-classes is
based upon the mode
of reproduction and
on the comparative
structure of the repro-
ductive organs. The
features of the sub-
class are therefore as
follows :—
The yolk-sac is on the embryo’s left and
the allantois on the right. - I. REPRODUCTION.
—It was not till 1884
that the egg-laying propensities at these mammals were
definitely discovered. The eggs are much larger than
MAMMALIA. 489
those of other mammals, have a tough flexible shell and
a large quantity of yolk. The segmentation is meroblastic,
like. that of reptiles, and the foetal membranes are well
developed, as in Sauropsida, the yolk-sac functions for store
of nourishment and the allantois for respiration. The
Fig. 333.—VENTRAL VIEW OF MALE UROGENITAL ORGANS
OF ORNITHORHYNCHUS. (Ad zat.)
Ureter. Kidney.
Epididymis,
Vas Deferens,
Urogenital
Sinus,
Rectum,
Cloaca.
amnion does not appear to completely separate from the
serosa, hence the yolk-sac takes up the left and the allantois
the right half of the egg-cavity, instead of ventral and dorsal,
respectively, as in the chick.
2. UROGENITAL OrGaNs.-—The base of the oviducts is
swollen into a so-called “uterine” part which probably
490 CHORDATA.
secretes the shell. They have no distinction into Fallopian
tube, uterus and vagina, and they open separately into the
urogenital sinus.
Fig. 334. VENTRAL VIEW OF PECTORAL GIRDLE AND
Fore-LiMB OF ORNITHORHYNCHUS. (Ad zat.)
Episternum,
Clavicles.
|
| Scapula,
ig
;
4 Humerus. _
Radius.
|
|
‘Ulna.
3. CLoaca.—The urogenital canal and the alimentary
canal have a common passage called the cloaca which opens
by a single aperture to the exterior, the cloacal aperture
(hence Afonotremata).
Fig. 335.—PELVIS OF ORNITHORHYNCHUS x 4. (Ad nat.)
Sacrum. Tlium.
Epipubic. ~-=
Pubis.—@ . / : Acetabulum.
Obturator Fora- 2 i i
men.
Ischium.”
g : 1
A Epipubis. B Ischium.
A, Ventral view. B, Lateral view.
4. SKELETON. — Shoulder-girdle has complete precora-
coids, coracoids and episternum. The scapula is bent
forward and the spine is at the anterior border, not down
the middle.
MAMMALIA. 491
Other skeletal peculiarities must be noted, The pelvis
bears a pair of efipudbic bones similar to those of the MJeta-
theria, and at least in Echidna the acetabulum is incompletely
ossified. The cervical ribs are incompletely fused on to the
cervical vertebra, and the dorso-lumbar vertebrze have no
Fig. 336—DuUCKMOLE (Ornithorhynchus anatinus).
(From Goutp’s Mammals of Australia.)
epiphyses, or only traces of them. The cranial bones anky-
lose early, obliterating all sutures,* and the rami of the
mandible are free. There is an entepicondylar foramen in
the humerus.
The temperature of the body is low and inconstant. In
all these features the Profotheria show a low grade of struc-
ture approximating to the reptilian. type.
* The young Ornithorhynchus is said to possess Are- and fost-frontal bones.
492 CHORDATA.
OrvDER I.—Jonotremata.
There are two families in this order—(1) Ornithorhyn-
chide and (2) Echidnidee, closely allied in many ways.
Ornithorhynchus anatinus, or the duckmole, is found in the
Australian region. Its
Fig. 337.—Fore (A) anpD Hinp (B) general appearance may
Foot oF THE DUCKMOLE. be seen from the figure.
The body, usually about
18 inches long, is cover-
ed with dense, soft,
brownish hair, and the
head has a remarkable
pair of horny “ beaks.”
The eyes are small and,
as in most aquatic forms,
there is no external ear.
Both pairs of limbs have
five digits with claws and
a “web” or membrane
is present in the front
limb, none in the hind.
The tail is flat, and in
old specimens the hair
is absent from its lower
surface. In habits the
duckmole is ‘ fossorial”
and ‘‘aquatic.” It swims
freely and lives in deep
burrows in river-banks.
At the end of its burrow
it constructs a nest in
which it lays its eggs.
There are no teats and
f the mamma lands
Note the bees ce eee as poison-spur are modifie cu Se fic
glands. The teeth are
only present in the young and adolescent forms and
appear to be worn away early, when they are replaced
by the familiar horny pads or “cornules” found in most
skulls. The teeth are only molars and few in number, eight
MAMMAL A, 493
to ten in all. They appear to have two main cusps and
smaller ‘‘crenulations,” the cusps lying externally in the
lower jaw and internally in the upper. The beaks are borne
upon bony processes of the premaxilla and the skull of the
duckmole is at once recognised by the peculiar ‘“ beak-
shape ” of the facial region together with the hard cornules.
The male Ornithorhynchus has a “ spur” on the inside of
the hind-foot which is traversed by a canal continuous with
the duct of a gland situated over the thigh. It is probably
Fig. 338.—Bones oF LimBs OF ORNITHORHYNCHUS. (dd nat.)
Distal end
Tibia. of Fibula. Femur.
i '
Entepicondylar Foramen.
Radius. Ulna.
Humerus.
Hind-limb above. _ Fore-limb below.
a poison-apparatus and is supposed to be functional early in
the breeding season. It is rudimentary in the young female.
The skeleton of the limbs shows powerful ridges and crests
on humerus and femur, and the fibula has a projecting
process beyond the knee-joint which gives it a deceptive
resemblance to an ulna.
Echidna and the allied genus Proechidna, the “ Porcu-
pine anteaters,” are, like Ornithorhynchus, confined to the
Australian region, Proechidna to New Guinea. Lchidna
may be about 16 to 18 inches long, with a fat compact
494 CHORDATA.
body, covered not only with thick fur but with strong
pointed spines scattered amongst the hair. The general
colour is brownish and the spines are usually yellowish.
The facial part of the head is produced into a long tubular
rostrum. The eyes aresmall. The limbs each have five
toes and in the typical species all are clawed. The tongue
is long and protrusible. There is a small poison-spur on
the hind-limb. The tail is almost absent. The animal is
fossorial and anteating in its habits and can burrow rapidly.
It is said not to make a nest but to carry its egg, which
has a thin horny shell, in a temporary pouch. The mam-
mary glands are like those of Ornithorhynchus. The skull
Fig. 339.—SKULL OF ORNITHORHYNCHUS x 3.
Maxilla. -%
Intermaxillary. Ag — Symphysis.
Horny Pad.
Condyle.
Foramen Magnum.
A, Ventral view of skull. B, The mandibles from above.
of Echidna, in its modifications for ant-diet, is rather like
that of the true anteaters. We may note (1) the absence of
teeth ; (2) the great elongation of the facial region; (3) the
degeneration of the lower jaw or mandible. The functions
of teeth and lower jaw have largely been usurped by a long
adhesive tongue.
OrnerR IIl.—AUlotheria.
These consist of a series of small extinct mammals
(Plagiaulax, Microlestes), chiefly known to us by their
mandibles or lower jaws and their teeth. They occur in the
MAMMALIA, 495
mesozoic period from the Trias onwards and have doubtful
claims to be regarded as Prototheria. These claims rest
chiefly upon the resemblance of their molar teeth to those
of Ornithorhynchus. They have, however, large incisors,
one pair being much larger than the rest. The heterodont
condition is therefore already present. Still more doubtful
are the supposed vestiges of a coracoid and episternum. It
is obvious that nothing is known of the soft parts, but if
their skeleton were shown to agree closely with that of the
Monotremata there would be reasons for assuming that
they probably also possessed the three first features of the
Prototheria.
Sub-Class II.—Metatheria.
The Metatheria have two living orders, the Diprotodontia
and the Polyprotodontia. They may be said to present
at least five important sub-class characters :—
1. They are viviparous but have a very short period of
uterine gestation, during which a yolk-sac placenta is
present and an allantoic placenta only exceptionally.
2. The oviducts are divided into three parts—(x) Fallo-
pian tube, (2) uterus, (3) vagina, and there is no fusion
between the oviducts except at the lower part of the
vagina.
3. Urogenital sinus and rectum open separately to the
exterior, though surrounded by the same sphincter muscle.
4. Amongst numerous skeletal peculiarities we may note
the presence of epipubic bones and of only one deciduous
tooth on each side of each jaw.
5. The temperature is more constant thanZin Proto-
theria, but is lower than in Eutheria.
The condition of the placenta has been described. The
allantois is obviously in a degenerate condition in the
majority of Metatheria (of. Hypsoprymnus), but in forms like
Phascolarctos it is normal and possibly performs its primary
function of respiration.
Recently, however, the discovery of an allantoic placenta
in Perameles has shown us that at least one metatherian has
advanced to the foetal condition of the Eutheria. The
structure of this placenta would seem to have certain
496 CHORDATA.
Fig. 340. —DIAGRAM OF PHASCOLARCTOs (Koala) EMBRYO AND
ITS F&TAL MEMBRANES.
(Modified from SEmon).
Yolk-sac.
Allantois.
Yolk-sac.
Edge of
Mesoderm.
Prokalymma.
Note the yolk-sac villi, but none to the allantois.
Fig. 341.—DIAGRAM OF HypsopRyMNus (A KANGAROO) EMBRYO
IN ITS Fara MEMBRANES. (Modified from SEMON.)
"Yolk-sac
Yolk-sac. = Villi.
Prokalymma. Va
Note the degenerate allantois lying freely.
MAMMALIA, 497
features which might indicate an independent evolution of
the allantoic placenta within the group.
The epipubic bones have the same relations as those of
the Prototheria, and the exact significance of the tooth-
succession is not yet decided. The known facts are as
follows :—
The majority of the AZe¢atheria retain the one set of teeth
throughout life, with the single exception of the third upper
and lower tooth on each side behind the canine, hence
Fig. 342.—DIAGRAM OF EMBRYO OF PERAMELES WITH
Fa@tTaL MEMBRANES,
(After Hitt.)
Yolk-sac Villi.
Edge of Allantoic Placenta. >
re \ wh \
a,
Allantois with Villi. yo
Edge of
True Allantoic~..
Placenta
“Sr
Yolk-sac.
© we! Gee
Prokalymma.
Note the allantoic villi.
termed the third premolar. This tooth usually resembles
the teeth behind it rather than those in front, and at some
time (earlier or later according to the species) it falls out
and is replaced by a permanent tooth. ;
The next fact to note is the later discovery of a series of
tooth-germs in the front of the jaw, which never cut the
gum but are absorbed after reaching a certain stage. The
deciduous premolar is said to rise in connection with these,
and the most reasonable view seems to be to regard these
M 33
498 CHORDATA.
germs and the deciduous premolar together as the lacteal
or deciduous series and the replacing premolar, together
with the other functional teeth, as the permanent series.
The difference between the Metatheria and Eutheria in
their dentition would then resolve itself into one of degree
only, the former having reduced their lacteal dentition till
only vestiges of all but the last remain. This reduction
might be correlated with the great development of the
lacteal nutrition involving a sucking mouth and loss of
function for teeth till a later period in life.
Other structural features of the Metatheria are as
follows :—
There is a prolonged period of mammary gestation,
during the early part of which the young are fed by the con-
traction of muscles over the mammary glands, the milk being
injected down the throat of the young. In a large number
of the Metatheria a fold of the abdominal integument
envelops the young, forming a pouch or nlarsupium. The
teats are long and are always abdominal in position.
The brain is small in proportional size and has a large
anterior commissure but a small corpus callosum, as in
Prototheria. "The skull of a metatherian may be known by
the following peculiarities, of which the majority are usually
present :—
1. The angle of the mandible is inflected. (See Fig. 349.)
2. The lacrymal foramen is outside the orbit.
3. The malar extends backwards to the glenoid cavity.
4. The bony palate is incomplete.
The inflected mandibular angle is probably a trace of the modifica-
tion by which the quadrate bone has become the tympanic, the malar
probably in early types extending back behind the squamosal to the
quadrate (see ear-ossicles). The lacrymal foramen was probably
primitively outside the orbit, and the complete bony palate is a mam-
malian character, its incompleteness hence indicating an early type.
These skeletal features may be illustrated by taking the
kangaroo as a type of the Ale¢atheria.
THE Kancaroo (Macropus).
The kangaroo belongs to the order Dzprotodontia or
herbivorous section of the AMefatheria.
MAMMALIA 499
The skull is seen in side view in Fig. 343.
Notice speci-
ally the continuation of the malar to the glenoid cavity, and
Fig. 343. -LATERAL VIEW OF SKULL OF A YOUNG KANGAROO.
(Ad nat.)
Lacrymal Foramen.
Incisor
Teeth. %
Lower
Incisor,
First Molar.
Angle of Mandible. |
Paroccipital Process,
Note the dentition with only two lower incisors, no canines and five cheek-teeth.
Also the Metatherian characters.
the situation of the lacrymal
foramen outside the orbit,
the incomplete ossification
of the palate and the in-
flected angle of the lower
jaw.
The dentition is pecu-
lar. There are three upper
incisors, flat and chisel-
shaped, then a space or
diastema in which there are
no teeth. The canines are
absent, though occasionally
present in some kangaroos,
and there are five cheek-
teeth. But the true dental
formula of a karigaroo is
3.0.2.4, So that there should
be séx cheek-teeth in all.
Fig. 344.—VENTRAL VIEW OF
SKULL OF KANGAROO x }.
(Ad nat.)
Incisor Teeth.
Ltt
in Palate.
The reason for the discrepancy is that the first premolar
drops out at the same time that the second or last premolar
500 CHORDATA.
replaces its antecedent “milk” tooth, so that only one
premolar persists.
Another peculiar feature is that a younger kangaroo with
the milk premolar not yet replaced also has only five teeth
in all, because the last molar does not appear till the first
premolar has dropped out.
Thus, although the old kangaroo has only five back
teeth, of which the first is the second premolar and the
other four are the molars, the dental formula of the species
1s 3.0.2.4, because another premolar has been lost during
life in front of the remaining teeth. The lower jaw has
one long incisor on each side which has a cutting edge
down each side. The two rami of the mandible are
bound by ligament only, which permits a movement of
one ramus upon the other. When the two posterior ends
of the rami are approximated the incisor teeth diverge
and cut any substances between them and the upper
incisors of each side. On divergence of the two pos-
terior ends the two incisors come together like the blades
of a pair of scissors and sever any substances lying between
them. Hence the kangaroo differs from the sheep and
horse in cu¢ting its forage rather than breaking it. There
are no canines and the premolars and molars resemble
those of the upper jaw. The inflected angle is another
metatherian character.
In the vertebrae the chief feature to notice is the presence
of chevron-bones in the tail. These hang down under the
vertebrae and are usually present only in those mammals
which have a highly-developed tail. The fore-limb is small
and has five complete digits with claws. The shoulder-
girdle is closely similar to that of the Zutheria, the coracoid
and precoracoid elements being only represented by
vestiges. The hind-limb usually has only four digits, the
hallux or big toe being lost. Of the remainder, the fourth
is very large and strong, with a powerful claw; the fifth is
smaller and the second and third are reduced to attenu-
ated remnants. These two are united together in one
flap of skin from which the two little claws protrude. This
very peculiar condition is known as syxdactylism. It is not
a true metatherian character, as it is only found in the
Diprotodontia and one family of the Polyprotodontia
MAMMALIA.
(Peramelide). It is,
however, confined to
these and not found in
the Lutheria.
The tibia and fibula
are very long and the
femur is short but
powerful. The pelvis
shows well the large
epipubic bones, found
not only in Metatheria
but in Prototheria.
The foot of the
kangaroo is modified
for rapid locomotion,
mainly by jumping, and
the toes are corres-
pondingly reduced. In
501
Fig. 345.—PELVic GIRDLE OF THE
KANGAROO x f.
(dd nat.)
Epipubic. ~~
Obturater
Foramen..,
Fig. 346.—HInp-roor or KANGAROO
x 3. (Ad nat.)
Calcaneum,
Metatarsals.
Note the absence of the hallux, the large fourth
toe and the syndactylic second and third.
some. respects it is not
unlike that of the two-
toed ostrich. The ves-
tigial second and third
toes take no part in
locomotion but may be
useful for scratching the
fur.
Orver I.
Polyprotodontia,
The Polyprotodontia
are so-called because
they have a large num-
ber of front or incisor
teeth. There are always
more than three pairs
of incisors in the upper
jaw, usually four or five;
hence the skull of a
polyprotodont can, apart
from other metatherian
502 CHORDATA.
characters, be recognised at once by the presence of more
than three pairs of upper incisors. The canines are large
and prominent and the molars are cusped. In other words,
the Polyprotodontia have a typical carnivorous dentition, and
all are flesh- or insect-eaters. They are aquatic, cursorial,
fossorial, or arboreal.
Family I.—Didelphidz comprise the Ofossums, found in the
warmer regions of America. They are usually ‘‘true arboreal” and
hence have an opposable hallux or big toe, the other four toes being
nearly equal and each bearing a claw. The Yapock, however, is
aquatic and has webbed feet. The opossums vary in size and colora-
tion and there is a large number of species.
Fig. 347._JAWs AND TEETH OF THE OpposuM (Didelphys).
Note the five upper and four lower incisors, long canines, sharp cusped molars
with four true molars (234). An essentially carnivorous dentition.
4134.
Family II.—Dasyuridz comprise a number of carnivorous and
insectivorous animals found in the Australian region. They vary in
size from the Tasmanian wolf ( 7hylacinus) to the little mouse-like
Phascogale. The Tasmanian ‘‘devil” (Sarcophilus) has the fossorial
habits of the badger, and the Dasyures (Lasyurus ) are much like small
civets. The Banded Anteater (Wyrmecobius) has a great number of
small teeth and it has no pouch.
Family IIJ].—Peramelidz comprise a number of small animals,
the Bandicoots, found only in the Australian region. They are ‘‘ small-
flesh” eaters (worms, insects and occasionally vegetable diet).
They are interesting for two structural features, viz., the presence of
an allantoic placenta and the syndactylic condition of the hind-foot (see
Diprotodontia). The ‘‘ Native Rabbit” ( Peraga/e ) is fossorial.
Family IV.—Notoryctidz is made for the curious metatherian
mole (Motoryctes). A true fossorial type found in the sandy districts
of centrai Australia. Its structure is adapted for rapid bunowing
and in this respect shows a likeness to the fossorial armadillos and
to the mole.
MAMMALIA. 503
DisTRIBUTION.—The Family of the Opossums is found
extending throughout the American continent, except the
extreme north. The other three families are found in
Australia or the Australian district, including Tasmania and
New Guinea.
This present-day distribution of Polyprotodontia differs
from that of the past. There are a large number of meso-
zoic mammals found widely scattered in Britain, Europe,
United States and elsewhere, which, mainly in their
dental character, seem to resemble the modern Polyproto-
dontia (especially Myrmecobius). These appear to indicate
that the distribution of the Polyprotodontia was in these early
times much wider than at present (cf Diprotodontia).
Fig. 348.—INNER VIEW OF Lerr Ramus or Lower Jaw
oF AMPHILESTES BRODERIPI.
(From Flower and LyppDEker, after OWEN.)
P ”
€\12394 6 Bil 2
1\al \
owe Dg :
——
From the Stonesfield Slate.
OrvER II.—D¢protodontia.
The Diprotodontia are essentially herbivorous, and hence
they have few chisel-shaped incisors, never exceeding #
and in some cases being reduced to }. The incisors
of the lower jaw never exceed one pair, hence the name of
the order. The lower canines are always lost, and often the
upper molars have not the sharp cusps of the Polyproto-
dontia but have blunt tubercles more suited for crushing
vegetable food. The limbs vary in character, but they
always have the syndactylic hind-foot described in the
kangaroo. (This feature is also found in the Peramefide.)
Family I.—Macropodidz.—A large family of kangaroos and
their allies. The kangaroo has been used as a type of metatherian
skeleton. The hind-limbs and tail are enormously developed for
504 CHORDATA.
jumping. At rest the kangaroo places the whole foot on the ground.
All are strictly herbivorous and the stomach is complex, the front part
being sacculated and containing the cesophageal and cardiac glands. The
true kangaroos and wallabies are cursorial, playing the part of antelopes
or deer in the districts they frequent. The rat-kangaroos are smaller,
ae and partially fossorial. Others, the tree-kangaroos, are
arboreal,
Family II.—Phalangeridz.—A family containing a great number
of small arboreal animals. They are usually woolly and often have a
prehensile tail. In addition, a number of them have a flap of skin or
patagium which enables them to ‘“‘sail” from tree to tree (incidental
zrial). From these habits it is not surprising to find five toes all
present on each limb and the hallux opposable to the other four.
These phalangers approach more nearly the Polyprotodontza, especially
as they have additional small functionless incisors in the lower jaw, and
their diet is by no means strictly herbivorous. The common koala
(Phascolarctos) and the flying squirrels (Petaztrzs) should be noted.
Family IIJ.—Phascolomyidz.—A very small family, consisting
of about three species of wombats. The wombat (Phascolomys) is a
small bear-like terrestrial or partially fossorial animal. All five digits
are retained on both limbs and the syndactylism is not very pronounced.
But the peculiar dentition is the great feature of this form. Just as
similar external conditions cause a resemblance of the Tasmanian wolf
( Thylacinus) to the dog, or Notoryctes to a mole, so here we have a
metatherian repetition of the eutherian rodent. There is one pair of
incisors in each jaw; they grow
Fig. 349.—PosTERIOR VIEW OF from persistent pulps and have
Lower Jaw oF Womsat. enamel only on the front surface.
There are no canines and there
is a large space or diastema
between the incisors and the
‘* cheek-teeth.”” These are five
in number, one premolar and
four molars; a is the formula.
Family IV.—Epanorthide.
—Another small family which
contains a remarkable little ani-
mal, the selva or opossum-rat
Showing inflected angle. (Cenolestes). The selvas have
recently been found alive in S.
America though they were supposed to be extinct. They are dipro-
todont in their lower jaw, but the teeth of the upper jaw more nearly
resemble certain of the Polyprotodontia. They differ from the rest of the
Diprotodontia in not having a syndactylous foot, though doubtful traces
of syndactylism in some of their fossil allies have been stated to exist.
DisTRIBUTION OF DIPROTODONTIA.—The families of the
kangaroos, phalangers and wombats, in fact nearly all the
MAMMALIA, 505
Diprotodontia, are confined to the Australian region; but
the selvas, occurring as they do in South America, form a
remarkable exception. We have already seen that the Po/y-
protodontia have three families in the Australian region and
one in America, and the same is now known to be the case
in the Dérotodontia. The possible explanations of this
distribution will be given in the chapter on Geographical
Distribution. We may here note that syndactylism, or the
curious union of the second and third hind-toes, occurs in
one family of the Polyprotodontia and in three families of the
Diprotodontia, but that all these families are found in the
Australian region.
The Diprotodontia do not appear in the past to have had
a much wider distribution than at present, though there are
one or two extinct forms which are found in the same regions
as their modern relatives.
Diprotodon was a large rhinoceros-like animal of Pleis-
tocene times. It is intermediate in structural characters
between the kangaroos and the phalangers. Zhy/acoleo was
another large phalangeroid type, and Phascolonus from
Queensland was a large tapir-like form of wombat. These
types would lead us to suppose that the Diprotodontia of
Australia attained considerable dimensions in the past,
and the absence of diprotodont remains outside the Aus-
tralian area seems to point to an evolution of these herbivorous
animals from polyprotodonts within that area, especially as
the Australian remains do not date further back than the
Pleistocene. In South America, however, the selvas and
the fossil Epanorthus extend back to the mid-tertiary epoch,
perhaps indicating that the diprotodont type was evolved in
this region at an earlier period than in Australia, but was
never so successful for want of isolation from eutherian
types.
506 CHORDATA.
CHAPTER XXVIII.
MAMMA LIA—( Continued. )
Sub-Class III.— Eutheria.
TYPES 2 AND 3, HORSE AND OX; 4 AND 55 DOG AND CAT.
The Zu¢heria mark the culminating point in mammalian
structure and, as might be supposed, the members of this
sub-class show the greatest diversity of adaptive modifica-
tions. Asa general rule we may say that the hemal form
of embryonic nutrition is highly developed, the chief organ
forming the heemal placenta being the allantois. The yolk-
sac placenta, when formed at all, is merely a transitory
structure of little functional significance. Further advances
upon the metatherian type are found in the reproductive
organs. The urogenital and anal openings are, as a rule,
quite distinct, the perineum separating the two orifices:
this is especially evident in the male. The lowest part of
the oviducts, the vagine, are always fused together and with
few exceptions the second or uterine portion shows also
varying grades of fusion, such as the bicornuate and
bipartite uterus: this fusion of the uteri probably has
partly to do with a reduction in the fecundity. Correlated
with the high development of a hzmal gestation there is a
tendency to a reduction in the period of lacteal gestation,
though the mammary glands are still well developed and the
mamme are permanent.
In the skeleton there are important features. The teeth
are typically diphyodont and heterodont, and it is usual to
derive the very numerous modifications from the typical
dentition of 3443. This typical dentition is, indeed, only
found in very few types. of which perhaps the pig is the best
known ; but the assumption of loss of certain teeth in some,
MAMMALIA 507
and of multiplication of molars in others by secondary divi-
sion, makes it possible to derive the more aberrant types.
Of these we may instance the Adentata and Cefacea as
differing widely from the type. The typical dentition indi-
cates two very important differences from the Metatheria.
Firstly, the incisors are never more than three on each side
and, secondly, the molars are not more than three. We have
seen that four incisors and four molars are the rule in the
Polyprotodontia and that four molars are usual in the Dzpro-
todontia. Lastly, we may call to mind the peculiar condition
of the deciduous or milk-dentition in the Mefatheria. A
complete milk series (diphyodont) is the rule in Lu¢heria.
Turning to the rest of the skeleton we find that, as in
Metatheria, the coracoid element of the shoulder-girdle is
reduced to a mere vestige; the coracoid process of the
scapula, and the episternum is absent as a separate bone.
In the pelvic girdle there are no epipubic bones.
The temperature of Zu¢heria is higher than that of either
Metatheria or Prototheria and is also more constant ; that
is to say, the temperature of the body varies only within
narrow limits whatever the temperature of the surroundings.
This is only another instance of the higher type having its
internal economy adjusted in such a way as to be inde-
pendent of the immediate surroundings. The individual
variations within the sub-class are from about 35°C. to
40°C.
Modern £u¢heria have not only an important structural
distinction in their brain from that of the other sub-classes,
such as the great development of the corpus callosum and a
corresponding reduction in the anterior commissure, but
also an advance in the type of brain. It usually forms a
greater proportion of the bulk of the body, the cerebrum
gradually assuming more and more comparative importance
as the higher orders are reached. Thus the cerebrum comes
to completely overlie not only the optic lobes but the cere-
bellum as well, and its surface becomes folded into numerous
convolutions. Apparently the earlier fossil forms (ég.,
Eocene) had far smaller brains in proportion, and a
gradual increase in size and complexity of the brain there-
fore appears to be one of the most important lines along
which mammals have progressed. The exact significance of
508 CHORDATA.
this fact is not quite clear, but as the brain is the special
centre regulating interaction between the organism and its
environment, it is probably the structural expression of the
increasing “complexity” of life now followed by higher
organisms. (See page 462).
At the present day the Zutheria are tolerably sharply
differentiated into orders, but the energy of palaeontologists
has in recent times unearthed a number of transition forms
which, whilst adding enormously to the difficulties in the
way of a “natural” classification, enable us to trace the
descent of the greater number of our modern types.
Adaptive modification is very conspicuous in the
Lutheria, and, as elsewhere, it has taken place to a large
extent independently of genetic connection. At the same
time we find in several cases that the two are parallel. Thus
the orders Sirenta and Cefacea are entirely aquatic or nata-
torial, the Chiroptera are entirely erial, and as a rule the
Primates are arboreal, though only of the “transition”
group, whilst the true cursorial are mainly in the Ungulata.
Again, we find as a general rule that the lower or more
primitive types affect the primitive terrestrial, arboreal, or
fossorial habitats usually with nocturnal proclivities. In-
stances of this may be seen in the Zdentata, [nsectivora, a
number of Rodentia and the most generalised of the
Carnivora. —
We may illustrate the structure of the Eutheria by a
short study of the following types :—
T. Rabbit oe ccvescica various Primitive terrestrial.
2-3. Horse and Ox,............ Cursorial.
4-5. Dog and Cat, ............. Transition cursorial.
6: ‘Sloth: owsses- essere ian ceseee Arboreal.
y's. MOLGS way oseroneonees nes ean te ane Fossorial.
8. Porpoise, ....Natatorial.
Gi: TBAty cin estaeeehaenacs ant es ferial.
1. The Rabbit has already been described.
1. The Primitive Terrestrial Types. — Hedgehog, Shrew and
_ Bear.—We must suppose that the first mammals were small generalised
terrestrial mammals, with tubercular teeth and omnivorous diet, inclining
to insects and ‘‘small-flesh.” They merge into the incidental arboreal,
fossorial and cursorial forms: some of the /isectzvora of the present
day probably give us an approximate semblance of them. They were
.
ry
MAMMALIA. 509
possibly able on occasion to scratch or burrow, to run and climb.
From this it will be seen that the later mammals have become special-
ised in varying degrees for special habitats, the five principal of which
we will notice.
A study of man will show that he does not agree with the true or
specialised group of any of these types, but that he would really fall
into the incidental, if not the transition, group of all but the erial. To
this adaptability to all environments without a corresponding modifica-
tion involving loss of organs and specialisation, man probably owes his
position at the head of the mammalian world. In other words, as the
environment is ever inconstant and specialisation means a modification
for one particular temporary form of environment, it also means certain
extinction of the type, sooner or later. True evolutionary progress is
effected by an acquired reactivity to a varzety of environmental surround-
ings and not by an adaptation to a “‘ special” environment, which checks
further progress and culminates in extinction of the type.
2 and 3.—THE Horse (Zguus caballus) and Ox (Bos
taurus).—CURSORIAL.
The horse and ox represent two culminating points in
the evolution of the large herbivorous cursorial type, the
former belonging to the sub-order ertssodactyla and the
latter to the Artodactyla, which together comprise the order
Ungulata or hoofed animals.
Both the horse and ox stand high on their four legs and
walk only on their toes (digitigrade). In each case the
legs and neck are long. As they obtain their food from the
level of the ground, or graze, the elongation of the neck and
head must keep pace with that of the limbs. They are
surrounded in natural conditions by the carnivorous types,
the large “cats” and the “dogs,” and they are endowed
with keen senses. The sense of hearing is assisted by the
large. external ear or pinna which can be turned in any
direction to catch the sound. That of sight is mainly
assisted by the long neck which adds considerably to the
field of vision. The sense of smell is also highly developed.
The effects of these developments in the Ungulafa will be
seen in corresponding modifications of the carnivorous types
(see Dog and Cat).
- Both-types are covered with dense hair which is particu-
larly long upon the tail. This organ is mainly used for
protection against the attacks of certain flies. ‘The horses
and their close allies, the zebras and asses (£guide),
Sees iets Suagese
510 CHORDATA.
frequent high, open, grassy plains, their limbs being adapted
for fleet movements over hard ground. Even in a domestic
state the horse shows a peculiar aversion to trusting itself to
soft or boggy ground. On the other hand, the ox family
(Bovide) is at home upon any’ grassy pasture, whether in
forest glades or in rocky districts. The food is in each type
much the same and the long soft lips assist greatly in
obtaining it. We shall see below, however, that the method
of feeding or dealing with the food is different, involving
certain differences in the structure of the stomach.
Fig. 350.—LATERAL VIEW OF HorsE’s SKULL.
The right mandible has been removed. (Ad nat.)
Frontal. Lacrymal. Nasal.
Symphysis.
Note complete orbit, large nasals and lacrymals, and predominant facial regions,
In habits both families are, as a rule, gregarious, congre-
gating in herds. This habit conduces to mutual protection,
and is made possible by the wide expanses of pasture at
present existent on the earth’s surface, though it involves
more or less extensive periodic migrations from place to
place.
Bearing in mind both the points of similarity and of
difference in the habits of the two types, we can pass to
MAMMALIA. 511
the skeleton. Let us first glance at the skulls of the two
types. Notice in both the large development of the facial
region in comparison with the cranial. This is due not
only to the large maxilla, but also to the part taken by the
jugal and the lacrymal in the formation of this region.
Fig. 351.—VENTRAL VIEW OF SKULL or Horse
(Zgeus Caballusx%). (Ad nat.)
-- Incisors.
Canine,
Hard Palate —... 3 Molars (three
Premolars_ in
front of them).
Internal Nares.,.~
Pterygoid.....
T: ic.
SRE ne 'Glenoid Cavity.
c
Occipital Condyle. Foramen Magnum.
Hence the small size of the orbit, which is also completely
bounded by bone,* a postorbital process of the frontal
descending to meet the zygomatic arch below,
In both types the mandibles are large and heavy, ex-
panded behind into broad strong plates, evidently built
* This complete closure of the orbit is not effected in the lower Perissodactyla
(¢.g., Rhinoceros and Tapir), nor in the more primitive Artodactyla (e.g., Pig).
512 CHORDATA.
for long-continued and powerful masticatory movements.
In the same way the molar teeth in both are ridged or
tuberculated, the ridges being worn down very early in life,
exposing the dentine. The parts between the enamel crests
are filled up with cement. The enamel being harder than
either dentine or cement, it always forms rough ridges with
complex outline, on the inner side of which rests the
dentine, on the outer the cement. In both the horse and
the ox the crowns of the molar teeth are much elongated,
forming the type called hypsodont. This condition, like the
bony orbit, has been developed within the two sub-orders,
many of the less specialised members of each order having
short crowned or érachydont molars.
In this way the row of molars forms a crushing mill
which is capable of reducing to a pulp the most siliceous
of grasses, and the size of which largely accounts for the
prominent facial region. The molar series is separated by
a more or less prominent space or diastema from the front
teeth, indicating a separation in function between the two
series. The condyle of the mandible is transversely cylin-
drical, and allows of some lateral but little backward
motion, owing to the presence of a postglenoid process
of the squamosal.
‘Apart from these general resemblances, the differences
are sufficiently striking. Firstly, we notice that the skull of
the ox bears a pair of large bony processes or cores upon
the frontal bones, which form the basis of support for the
long hollow horns in which they are encased in the living
animal. These horns, assisted usually by the speed of the
animal, form the organs of defence, or even offence, of the
large family to which the ox belongs (Bovide), whilst frontal
organs of one kind or another (antlers, &c.) are largely
found in the Artiodactyla ; there is no trace of them in the
horse, which trusts to its speed, or on occasion to its kick-
ing powers, for defence.
Less conspicuous distinctions in the skulls are the much
larger nasals and the presence of an alisphenoid canal
(through which runs the main branch of the external carotid
artery) in the horse. These two features are small and
may appear unimportant, but they serve to distinguish the
two large sub-orders of the Artiodactyla and Perissodactyla.
MAMMALIA. 513
Turning to the dentition there are sufficiently obvious
differences. In the ox there are no incisors nor canines
in the upper jaw, their place being taken by a horny pad.
In the mandible there are three pairs of chisel-shaped
incisors and a pair of canines which resemble incisors in
shape and size. In the horse, on the other hand, there are
three pairs of incisors in both upper and lower jaw which
are of a peculiar shape. They have their terminal surface
pushed in as a deep pit, partially filled with cement. On
being worn flat the surface of the tooth presents two con-
centric circles of enamel, the inner circle becoming narrower
with age. The canines
are small and pointed Fig, 352, Upper Jaw (LEFT-HALF)
and are only rarely pre-op Younc (A) AND OLD Horsk (B).
sent in the female.
As regards the molar
series we have seen that
there are considerable
resemblances in the two
types, and in each there
are six functional teeth
on each side, of which
three are premolars and sth
three are molars. Here i) ly
the resemblances end. te
In most horses there is, eS)
at least in theadolescent vi
stage, a very small first
premolar in each upper
jaw, which usually falls out at maturity. Thus the full
dentition of a young horse may be given as #4, but that of
a mature mare is #33. The dentition of the ox is <3,
Though the patterns of the enamel in the molars have
a general resemblance, a little study shows that they are
derived from different types. The horse starts from the
simple bilophodont type, found in the tapir, consisting of
a pair of transverse ridges: this is further complicated, as
in the rhinoceros, by a junction of the two ridges and by
their bending into a crescentic outline: in the horse these
ridges are still further twisted, the multiplication of enamel
‘tidges being the end in view.
M. 34
514 CHORDATA.
The ox, on the other hand, starts from the bunodont
type found in the pig, with four principal crowns. These
do not unite transversely, but each independently becomes
crescentic, producing the se/enodont or crescentic type of
molar. The crescents may unite longitudinally but not
transversely.
The study of the teeth of these two types shows that
in this respect the ox is more specialised than the horse,
a conclusion which agrees with the comparative structure
of the stomach. The stomach of the horse is fairly simple ;
it is at most constricted into cardiac and pyloric portions,
whereas that of the ox has four distinct parts or chambers.
Fig. 353-—STOMACH OF A RUMINANT, SHOWING INTERNAL
STRUCTURE.
(FLoweErR and LypDEKER.)
a, (Esophagus; 4, Rumen (paunch); c, Reticulum (honeycomb) ; d, Psalterium
soul (many-plies) ; e, Abomasum (reed); 7 Duodenum.
The rumen (or paunch) is a large and capacious sac for
storage of food; the veticudum (or honeycomb bag) is a
small globose sac with reticulate walls: following this
is the psalterium (or many-plies) with folded walls, suc-.
ceeded by the abomasum (or reed) which is the true
digestive stomach. The food is cropped and swallowed,
passing down to the paunch, in which it is stored. After
feeding, the animal retires to a secure retreat or at least
comes to rest, and the food is passed by the reticulum
up the cesophagus into the mouth. Here the process of
MAMMALIA. 515
mastication or rumination is effected by the molar teeth.
The chewed food is then passed down to the psalterium
and the abomasum where digestion commences. The
horse, on the other hand, masticates his food at the time
of feeding, and there is in this case no rumination or
“chewing the cud.” The rest of the alimentary canal is
very similar in both types, the caecum being large and
the intestine long, characters usually found in herbivorous
animals.
Returning to the rest of the skeleton we find that the
vertebral column is of the same general type, the cervical
vertebra especially being markedly ofisthocelous. The axis
vertebra has a crescentic odontoid process, another feature
in which. the horse and the ox converge, though the more
primitive forms of each sub-order differ in having simple
conical odontoid processes.
The dorso-lumbar vertebrae are ménefeen in number in
the ox, but ¢wenty-three in the horse. In a similar manner
the ox has usually twelve to fifteen pairs of ribs, whilst the
horse has from eighteen to nineteen pairs. The ribs of the
ox are usually flatter and broader. In both types the front
dorsal vertebrae bear very long neural spines, to which is
attached the elastic ligament (Agamentum nuche) running
forward along the cervical vertebree to the skull and sup-
porting the weight of the head.
The difference in the number of dorso-lumbar vertebrae
is probably due to the shifting of the pelvis further forward
in the ox than in the horse, in its turn connected with the
greater proportionate “‘ pushing ” power of the ox.
Now let us turn to the limbs and limb-girdles. In both
the same plan prevails. The scapula is elongated and
narrow, of the cursorial type, and the clavicle is absent ; it
is not required in animals in which the limbs are not moved
inwards to the middle line and would indeed be a source of
danger when, as in jumping, the weight of the body is
‘thrown on to the fore-limb. The pelvis is of the same
general type in each, with large ilia fusing not only with the
primitive sacral vertebree, but with three or four others in
addition. The limbs have in the cursorial type to perform
a great uniformity of movement, and by reduction and
fusion from the pentadactyle type they approximate to the
516 CHORDATA.
condition of a simple jointed lever. Thus in each case the
ulna and fibula tend to disappear, their remains or vestiges
being seen along the border of the radius and tibia respec-
tively. The carpal bones are reduced to six in each case,
and the tarsals to five or six in the horse and to four in
the ox. In addition, the two rows are firmly interlocked
Fig. 354.—Tue Ricut Manus Fig. 355.—Tue RicuT Manus
or A Horse. (Ad nat.) OF AN Oxx 4. (Ad nat.)
Anterior View x }. Cuneiform. Lunare.
ne one Pisiform. Seacihotl,
sissies kg Scaphoid.
5 Si Magnum.
Unciform. fe! qian at Magnu
4 Unciform.
Os Magnum. |.
v
it F Metacarpal 5.
Third 4
Mctacarpal. od
Cannon-bone.
Phalanx tr.
Phalanx 2.
Phalanx of
Digit 4.
* Phalanx 3.
and lie alternately with each other (dip/arthrous) to prevent
all lateral twisting. The mode of locomotion is digitigrade,
the toes alone touching the ground, and the metacarpals and
metatarsals are reduced in number and elongated. The
terminal phalanx or phalanges bear horny hoofs.
With all these general resemblances we can note such
important differences that it is an easy matter to distinguish
MAMMALIA, 517
all the limb bones of the two types. The humerus of the
ox has a very prominent great tuberosity which bends over
the condyle as a hook-shaped process and the bicipital
groove is single; in the horse there is a double bicipital
groove and the great tuberosity is simple. The ulna of
the ox extends down the side of the radius for the whole
distance, whereas that of the horse has fused on to the
Fig. 356.—-TIBIOFIBULA OF A Fig. 357.—Ricut FEMuR oF
Horse x %. (Ad nat.) A Horse. (dd nat.)
A, Anterior View. Anterior View x 4.
B, View of Distal Extremity.
Great
Trochanter.
Cnemial Crest.
Lesser
Trochanter.
Third
‘Trochanter.
radius more completely, and can be traced only at most
about half-way down. In the carpus the suture separating
the os magnum and the unciform is in the middle line,
whereas in the horse the magnum is much larger than the
unciform and the dividing suture is towards the outer side.
This is directly connected with the important difference in
the manus. Both have been evolved from a pentadactyle
type, but the ox has lost the first digit or pollex, followed
518 CHORDATA.
by a great and equal reduction of the second and fifth
digits, leaving the third and fourth of equal size. Meta-
carpals three and four fuse together to form the so-called
“cannon-bone,” which still, however, bears distally the two
functional digits (three and four) and two small vestigial
digits (two and five). The former carry the two paired
Fig. 358.—Tue Lerr PEs Fig. 359.—RiGHT Pes oF Horse
OF AN OX x }. (Zquus Caballus x 3).
(Ad nat.) (Ad nat.)
Calcaneum.
~Calcaneum,
Astragalus.
: Cuboid.
Naviculo-
cuboid. Cuneiform.
c b 3rd Metatarsal.
annon bone
(Metatarsals
3 and 4).
Digit: }
hoofs, which appear superficially like a “‘cloven” hoof, and
the latter also bear small hoofs or horny nodules. In the
horse the same derivation from the pentadactyle type can be
traced, but the weight is borne predominantly on the third
digit. Thus the first and fifth disappear altogether, the
MAMMALIA. 519
second and fourth digits also go, though their metacarpals
remain as the two splint-bones down the hinder borders of
the large and elongated third metacarpal or “ cannon-bone,”
which bears the third digit and the single hoof. Just as in
origin the cannon-bone of the ox is formed of two meta-
carpals and that of the horse is one, so they can be imme-
diately distinguished by the double hinge-joint at the distal
extremity of the former and the single hinge-joint on that
of the latter.
In the hind-limb the femur is recognised by the presence
in the horse of a ¢hird trochanter on its outer border, and
the tibiofibula or tibia, carrying the fused remnant of the
fibula, will be seen in the ox to have three articular facets
at its distal extremity. The two larger articulate with the
astragalus, as in the horse, but the small outer one articu-
lates with a small condyle on the calcaneum. The astragalus
in the horse has a flat facet for the navicular below it, but
that of the ox has a hinge-joint with the naviculo-cuboid
bone below it, which gives ita double appearance, a hinge-
condyle at each end. In other words, the horse has only
a crurotarsal joint, as in most mammals, but the ox has a
certain amount of intertarsal movement as well as the
crurotarsal. Of the distal tarsals the navicular and cuboid
fuse across in the ox to form a naviculo-cuboid, whereas in
the horse the navicular commonly fuses with the ecto-
cuneiform below it, or remains distinct, but never fuses with
the cuboid. There is usually a small middle cuneiform in
the horse over the inner splint bone (digit two). The
digits of the hind-foot are modified in a closely similar way
to those of the fore-foot. :
The metacarpal ‘‘cannon-bone” of the ox is distinguished from
the metatarsal by the much shallower median groove in the former,
and the metacarpal ‘‘cannon-bone” of the horse is flattened from the
front behind, whereas the metatarsal is round in cross section.
If it be remembered that the horse’s limb is formed from
hypertrophy of one digit and the bones in the main axis
above it, whereas that of the ox is really bilateral or formed
from two digits and the bones above them, which are only in
later geological times fusing together to form one, it is easy
to account satisfactorily for the persistent calcaneo-fibular
joint, for the fusion across the middle line of navicular and
520 CHORDATA.
cuboid, for the “‘ double” astragalus, and for the fused third
and fourth metapodials.
The numerous structural resemblances and differences
in the horse and the ox we may sum up as follows :—
1. Resemblances of the two types which are due to
descent from a common mammalian ungulate ancestor.
These are characters of ordinal rank or the distinctive
characters of the order Ungulata. The most important
are the presence of a dentition adapted for a vegetable diet,
heterodont and diphyodont; the commencing adaptation
of the limbs for terrestrial locomotion with claws tending to
assume the condition of hoofs ; little or no clavicle.
2. Resemblances due to evolution on similar lines since
the divergence from a common stock. Of these we may
instance (1) The assumption of a digitigrade locomotion
and reduction in number of toes. (2) The interlocking of
carpal and tarsal bones (diplarthrous) connected with the
increasing size and rapidity of movement on harder ground.
(3) The expansion of the facial region, correlated with the
increased size of molar teeth, and the completion of bony
orbit. (4) The conversion of brachydont teeth into hypso-
dont, the increased complexity of the enamel ridges and the
addition of cement.
3. Differences due to evolution on somewhat distinct
lines since divergence from the common ancestor. The
principal of these are (1) The modelling of the limbs in the
horse, on the one-toe principle, the main axis passing down
the third toe, and in the ox, on the two-toe principle, the
main axis passing down between the third and fourth toe.
(2) The formation in the ox of bony frontal organs (horns
and horn-cores) for defence and their absence in the horse.
(3) The different method of feeding involving a more com-
plex stomach and loss of upper incisors in the ox. (4) The
- different principle upon which the complex molars are
evolved. (5) Other peculiarities, such as the presence of
alisphenoid canal, of twenty-three dorso-lumbar vertebre,
and of broad nasals in the horse. (1), (4) and (5) are
characters of subordinal value, as they are distinctive of the
Artiodactyla and Perissodactyla.
Before leaving these two important types we may inquire—How do
we know that they have been descended in the past from a common
MAMMALIA, 521
ancestor which had five toes, was plantigrade, and had other primitive
characters? The proofs are several.
Firstly, we note that the horse has splint bones or vestiges of meta-
podials, 2 and 4, and that the ox has two complete though small
vestigial digits, the second and fifth, making four in all. As the
pentadactyle limb is the only type from which all mammalian limbs
can be derived by a supposition of fusions and reductions having taken
place in the course of evolution, it is legitimate to infer that these forms
have degraded from this type and lost four and three functional toes
respectively.
Fig. 360.—THE Manus or (A) THE Tarir; (B), THE RHINOCEROS
AND (C) THE HORSE
(After FLowEr.)
Ulna.
Note the alternate carpal bones and the predominant third digit in each, but the
gradual reduction in the other digits.
Secondly, the types which are most kindred in structure to the horse
and the ox, z.¢., the other Perissodactyla and Artiodactyla respectively,
arrange themselves in two series, thus :—
Perissodactyla— Artiodactyla—
‘Tapir. Pig. :
Rhinoceros. Chevrotain.
Horse. Ox.
In all three of the first series the third toe is the largest and strongest,
but whilst the tapir has four toes touching the ground (in the fore
522 CHORDATA.
limbs), the rhinoceros has three and the horse has one. Hence the
conclusion is irresistible that the tapir, haunting the soft ground of
forests, has remained at the four-toed stage ; the rhinoceros has pro-
gressed slightly further and given up its fifth toe; and the horse,
frequenting drier, harder ground and moving more rapidly, has lost all
but the third or middle toe. The same lesson is taught by the other
series in which the third and fourth toes are of equal size. Here the
pig has four toes, all touching the ground, though the second and fifth
are smaller and shorter than the others. The chevrotain and ox show
a further reduction of these two toes, and the camel (in this respect the
last of the series) has lost all trace of them and has only the third and
fourth. (See Fig. 391, page 577.)
We have seen that the same series can be traced in the teeth, the
simple bilophodont teeth and nearly complete dentition (244%) of the
Fig. 361.—TuHe Foot SKELETON OF THE HORSE AND
Four OF ITS ANCESTORS.
(From Marsn.)
a b c
Showing Gradual Reduction of Outer Toes and Increase of the Middle Toe.
ua, Pachynolophus (Eocene); b, Anchitheriuim (Early Miocene); c, Anchitherium
(Late Miocene); d, Hipparion (Pliocene); e, Equus (Pleistocene).
tapir leading through the rhinoceros to the horse, whilst the simple
bunodont molars of the pig, with its full dentition of $444, leads through
the chevrotains, with no upper incisors but still with canines, to the
very specialised condition of the ox. A similar gradation can be made
out in other structural features, such as the loss of fibula and ulna and
fusion of tarsal bones.
Thirdly, there is the direct evidence furnished by fossil forms. In
the case of the horse and the ox the series is practically complete.
We cannot do more here than merely enumerate the known ancestors
of the horse. Fossil remains of the horse itself are found no further
back than the Pliocene in Europe, or possibly the Miocene in India.
Hipparion, as large as a donkey, and with three toes, is found in the
MAMMALIA, 523
Pliocene and Upper Miocene ; .4zchétherium, an animal about the size
of a sheep, with three functional toes (like the rhinoceros), is found in
the late Miocene; whilst a similar form in the early Miocene shows the
vestige of the fifth toe as a small metapodial splint-bone. Pachynolophus
of the Upper Eocene and Ayracotherium of the Lower Eocene were
still smaller (about the size of a hare), and in the front-limb they had
four toes (2, 3, 4 and 5) and three on the hind—in fact, resembling in
this feature the tapirs; one species of Pachynolophus shows a vestige
of the first digit in the presence of a splint-like metacarpal. These
types also show the changes in other structural features, such as the
teeth. (See Fig. 361.)
In the New World the same series has been made out and carried
back even further to the little Phesacodus of the Eocene, with five per-
fect plantigrade digits and a complete dentition, which, with its allies,
forms a meeting point of the modern Ungulata.
A very interesting point is the separate series of the New and Old
World, and it has been maintained with much reason that the horse
was independently evolved in the two hemispheres.
In the case of the ox a similar” series can be made out, true Bovide
dating back to the Upper Miocene, whilst forms allied to the pigs and
chevrotains go back to the Eocene. On this point we may quole
Flower and Lyddeker :—‘‘ The primitive Artiodactyles, with the typical
number (44) of incisor, canine and molar teeth, brachydont molars,
conical odontoid process, four distinct toes on each foot, with meta-
podial and all carpal bones distinct, no frontal appendages, and (in all
probability) simple stomach and diffused placenta, were separated at a
very early period into Bunodonts and Selenodonts, although there is
evidence of intermediate forms showing a complete transition from the
one modification to the other. These and other fossil forms so com-
pletely connect the four groups-——Suina, Tylopoda, Tragulina and
Pecora—into which the existing members of the sub-order have become
divided, that in a general classification embracing both living and ex-
tinct forms these divisions cannot be maintained.”
4 and 5.—THE Doc (Canis familiaris) and Cat (felis
domesticus).—TRANSITION CURSORIAL TYPES.
The dog and cat are examples of mammals which, whilst
having fully adopted the quadrupedal terrestrial mode of
life, have retained the varied use of their limbs in other
directions to such an extent that these limbs do not show
complete adaptation to a cursorial habit.
Both belong to the large and important order of Carn-
vora, which, in the most typical representatives, feed upon
the flesh of other mammals. This is usually the case with
both the dog and cat, but the latter, like the whole
family of Fedde, is in this respect the most typical of all
524 CHORDATA.
the Carnivora. It is a commonplace observation that a
dog may be fed indefinitely upon vegetable food and not
suffer in health, a diet hardly suitable for a cat.
The habits of the two animals, in a state of nature and
when domesticated, are full of interest. The dog tribe, as
a rule, hunts in packs (though the fox and a few others are
exceptions). He also hunts by scent and sight and relies
upon dogged persistent pursuit to catch his prey. When
run down the victim is torn to pieces by combined action.
Hence a dog will bark when on the trail, as the advantage
this gives in assisting his companions more than compen-
sates for warning the prey. Again, the dog is typically a
“long-winded,” enduring animal, and his fore-paws are fully
engaged with running, so that he attacks with the mouth
alone.
On the other hand, the cat is, as a family, solitary, or
hunts in pairs, and obtains his prey by stealth and sur-
prise. Lurking in the regions frequented by the victims
he seizes them unawares. If a fleet-footed animal be
attacked and missed it is usually not pursued for any
distance. Thus the “cats” are quiet, are proverbially soft-
footed and hunt in silence. The structure of their foot is
described below, but we may point out here that the “pads”
are usually softer than those of the dog, and the claws are
in walking withdrawn over the tops of the toes, partly for
preserving their sharpness and partly, no doubt, to pre-
vent noise. Though powerfully built, the cats are mostly
“short-winded” and incapable of sustained exertion. When
caught, the prey is killed not only by the teeth but by
the claws, which are then protruded. Thus the fore-paws
of a cat are not nearly so exclusively cursorial organs as
those of a dog.
Other habits necessarily follow from these. A “cat”
frequenting the haunt of victims with a high sense of smell
must be scrupulously clean—the feeces have to be buried
and the fur must be periodically cleaned. With a dog there
is no such necessity, and, indeed, presence of uncleanly
habits has probably proved of use in nature as a means of
communication for keeping the packs together. Many of
these habits are retained in our domestic friends, though
apparently of little use to them now.
MAMMALIA. 525
Keeping these points in mind, we may glance at the
anatomy of the two types.
Placing the two skulls before us, we note their features
in common as follows:—In each the incisor teeth are
small and pointed and are never more than 3, a distinc-
tion from Polyprotodontia ; the canines are long, powerful
and pointed ; and the premolars and molars have sharp-
edged cusps, with an absence of the flat grinding -surfaces
seen in the herbivorous types. In both there is a specially
large cusped tooth in upper and lower jaw which is called
the ‘‘carnassial” tooth, usually said to be used for breaking
slippery bones. The. glenoid cavity of the squamosal is a
Fig. 362,—LaTERAL View or Lion’s SKULL x 4. (Ad nat.)
Canine Tooth.
Zygomatic Arch. Auditory Bulla.
Carnassial Tooth, : ===} Postglenoid Process.
transverse groove, and into this there fits the cylindrical
condyle of the mandible. Owing to this arrangement the
mandible can only move in a perpendicular plane. Imme-
diately behind the glenoid cavity is a wide process of the
squamosal, called the postglenoid process, which prevents
all backward horizontal motion of the mandible.
On the cranial surface there are at least two large bony
crests—the sagittal crest along the middle dorsal line and
the occipital crest from side to side at the junction of
parietals and occipitals. These form the surfaces of origin
for the large jaw-muscles (¢emporalis) which pass down in
526 CHORDATA,
the temporal fossa to be inserted in the mandible, the
coronoid process of which is large. In this region we also
observe the strong and widely protruding zygomatic arch.
The tympanic bone is expanded into a large bulbous swel-
ling or tympanic bulla. If the inside of the cranium be
viewed through the foramen magnum, a bony septum or
tentorium will be seen which protrudes between cerebrum
and cerebellum.
Compared with the horse and ox the cranial part of the
skull is larger and longer in comparison with the facial
portion, the orbits face forwards and, in dried skulls, are
confluent with the temporal fossze.
Fig. 363.—THE SKULL OF THE DOG FROM THE RIGHT SIDE.
(From FLower and LyDDEKER.)
74 ay
\
%
We saw that the facial portion of the Ungulata (Horse
and Ox) was long, partly at least to provide room for the
long row of grinding molars. In the dog and cat the pro-
portion between cranial and facial part is altered from at
least two causes. Firstly, the brain is proportionately larger
and more highly developed, hunting being a more intel-
lectual pursuit than grazing ; and, secondly, the mechanical
necessities for a powerful “bite” demand a shortened jaw.
(See below.) a.
The features given above are typical of the skull of the
higher Carnivora, and they are mostly referable, directly or
indirectly, to the carnivorous habit.
MAMMALIA, 527
The skull of the dog has a dental formula of $342, and
it has thus two molars (one on each side) short of the full
typical Eutherian dentition. In this and in many other
respects the dog is the more generalised type of the two
(ff. diet).
The cat has a dentition of 3131, hence there has been
a great reduction in the number of teeth, especially as the
upper molar is also merely a vestige. In this case, however,
as in the dog, the last premolar of the upper jaw and first
Fi
ee
g. 364.—VENTRAL VIEW OF LIOoN’s SKULL x ¥.
Note the large round tympanic bulla, the wide zygomatic arch, the shortened
facial region and small number of cheek-teeth. Dental formula—3.1.3.1.
molar of the lower jaw are the carnassial, hence it is easy to
observe which teeth have disappeared. Correlated with the
reduction in number of the teeth is the shortening up of the
jaws, involving a still further reduction of the facial region.
If the mandible be regarded as a lever (of the third order),
the “‘ weight” will act at the level of the canines, the fulcrum
is at the glenoid cavity and the ‘‘ power” at the insertion of
the jaw-muscles, a little in front of the glenoid cavity.
Hence the simplest way of increasing the “power” is to
move the “weight” nearer the “fulcrum,” or, in other
528 CHORDATA.
words, to shorten the whole mandible. This shortening is
carried to an extreme in the cat and gives the face of the
animal its peculiar “‘ round” appearance.
As smaller anatomical differences which are valuable in
classification, note the alisphenoid canal in the dog but
none in the cat, and the larger auditory bulla in the latter,
inside which there is a more complete bony septum.
Fig. 365.—THE PERMANENT TEETH OF THE WOLF. (Nat. size.)
j .
Note sniall pointed incisors, large canines and cusped molars. The large
fourth upper premolar bites on the large first lower molar
and both are the carnassial teeth.
The vertebral column of the dog and cat call for little
mention. Both have the same number of vertebre, cervical
7, dorsal 13, lumbar 7, sacral 3, caudal 18-22. The dorso-
lumbar are 20, compared with 19 in the ox and 23 in the
horse. They have very little, if any, tendency to the opistho-
ceelous condition of the horse and ox. The tail is usually
long and flexible and is put to a variety of purposes.
MAMMALIA, 529
The ribs in the cat and dog form a compact thorax
which, however, is remarkably narrow from side to side.
The explanation of this peculiarity will be found in Chapter
XXVI. (Sternum and Ribs).
The limbs are fairly long and about equally developed.
They resemble each other (dog and cat) far more closely
than do those of the horse and ox. Both types are digiti-
grade and unguiculate (little claws or unguicule on each
toe). There are five toes in the front-limb and four in the
hind, the hallux being the only aborted toe. In each the
under-surface of the toes has a series of “pads” or callosities,
consisting of a large middle one and a row of smaller ones.
Coming to details, the scapula of the dog is slightly
elongated, but broad, with about equal prescapular and
postscapular fosse. It is distinctly a “transition” type.
There is no clavicle, except for an occasional minute trace.
The humerus is curved and there is a large supratrochlear
foramen. The radius and ulna are both developed but
immovably fixed together. The carpus has the scaphoid
and lunare united to form one bone, the scapholunar (a
carnivore feature), hence there are, with the pisiform, only
seven carpal bones. The pollex is shorter than the other
four toes and does not reach the ground. Hence the animal
really walks on four toes in fore- and hind-limb. The last
phalanges bear small blunt claws which are not retractile.
In the hind-limb and girdle the pelvis is not unlike the
ungulate type, but the ilium has not two “angles” or pro-
cesses, as in the horse and ox, as the “angle of the croup” is
very small. The femur is long and curved, the tibia and fibula
are also proportionately long, and the latter is complete
though thin. There is no reduction of the tarsal bones, and,
as stated above, there are four functional toes. The hallux is
often represented by a metatarsal bone, and may, as in the
“dew-claw” of domestic dogs, be present as a small digit.
The claws resemble those of the front-limb. In the cat
there is a clavicle which is reduced in part and connected
only by cartilage to the scapula and the sternum. The
scapula has a metacromion barely present in the dog. The
humerus is similar to that of the dog but proportionately
longer. There is no supratrochlear foramen, but there is
an entepicondylar foramen on the inner side. The radius
M, 35
530 CHORDATA.
and ulna are like those of the dog but proportionately
longer. The carpal bones and manus are very similar, but
the terminal phalanges of the digits can be withdrawn, with
their sharp claws, over the penultimate phalanx.
In the hind-limb and
Fig. 366.—A SIDE View oF a CaT’s girdle we may note again
Tor witH Rerracrite Craw. the greater length of limb
but a general similarity to
the dog. There are the
same retractile claws as
in the forelimb. As in
the dog, the hallux is re-
presented by a vestigial
metatarsal.
The stomach of these
carnivorous types is al-
ways simple and there is
a small cecum. In the
cat the tongue is armed
with rasping horny papillee
which assist the teeth in
“stripping” bones. The
intestine is always very
short. The brain is well
convoluted and the senses
are highly developed. The
external pinnze of the ear
are large and triangular-
On left the claw is retracted by the ligament shaped. Most of the
Mincdaadthedweseet «© eats” ave lone and
sensitive hairs or vibrissze
on each side of the snout, useful in nocturnal prowls, as
fine organs of touch.
We may trace the same three series of features as
were pointed out in the horse and ox, ze, (1) Resem-
blances due to descent from a common ancestral species ;
(2) Resemblances due to similar modifications since that
time ; (3) Differences due to divergent modifications since
that time.
1. The Carnivora appear to be descended from
very generalised mammals which combined many of the
(After T. J. Parker.)
MAMMALIA. 531
characters of the earliest Ungulata and Jnsectivora. The
dog has very nearly the typical Eutherian dentition, and
the bears, show the primitive plantigrade mode of pro-
gression. The extinct Cveodonta have more generalised
characters. “The scaphoid and lunare were not fused,
the feet were plantigrade and pentadactyle, the femur had
a third trochanter, and some of them appear to- have had
3 molars, thus completing the typical dentition. The
formation of the teeth was, however, distinctly carni-
vorous, with large canines and cutting-molars, though
“carnassials” were not so distinctly defined. Hence we
may with some certainty suppose that the earliest carni-
vore was plantigrade, pentadactyle, teeth ‘carnivorous ”
and diphyodont, formula $143, probably scaphoid and
lunare fused.
2. Since the divergence from a common ancestor,
each has changed by the adoption of a digitigrade pro-
gression, loss of hallux, and reduction of pollex, develop-
ment of cranial crests and temporal fossa, loss of last
upper molar.
3. At the same time the two have diverged into sepa-
rate families by the greater “carnivorous” progress of the
cat, involving shorter jaws, less teeth, retractile claws and
other differences noticed above.
The differences between dog and cat are of the family
grade, or little beyond, but those between horse and ox are
subordinal and therefore greater.
The carnivorous diet largely releases a mammal from
distributional limits of temperature, as its food is cosmo-
politan; hence the Camde are universally distributed
(except in oceanic islands), whilst the Fedde are found
everywhere, except in Madagascar, Notogeea and oceanic
islands.
The Fedde, representing the culminating point of the
Carnivora, must be regarded as one of the most successful
types of the /ammalia. They are pre-eminent for physical
and intellectual strength, great “ slimness” and alertness,
for an absolute disregard of the feelings, and the power
and will to profit by the toil and mishaps, of others.
Such traits carry all the elements of success.
532 CHORDATA.
CURSORIAL ADAPTATION.
This is not so strongly marked as some of the-others, as it mer
into the primitive terrestrial. As examples, we may take marsuy
dog, dog, pig, ox, sheep, rhinoceros, horse, and other Ungulata.
1. Incidental group: wrséde or bears, mustelide or weasel fam:
viverride ox civets. They are not well marked off from the primit
terrestrial, but can on
occasion move rapidly
on hard ground. They
show an incipient raising
of the body upon the
toes, a leading feature of
the cursorial type.
z. Transition group:
dog, feide(or cat family), eg
pig, rhinoceros, tapir, €
kangaroo. This shows ¢
an increasingacquirement
of the ability to move fast
over hard ground either
to catch or to escape.
The digitigrade mode of
locomotion is acquired
and the claws in many
cases commence to form
hoofs. Here also com-
mences a reduction in
the number of the toes
to four or three.
3. True cursorial :
horse, sheep, ox, deer,
&c. These types show
the ultimate cursorial
modification. The toes
_are reduced to two, pair-
ed, or one unpaired, and
bear hoofs. The clavicles
tend to disappear. The
carpus and tarsus are
reduced and many of
the elements fuse. The
proximal elements be-
come dovetailed into the
distal (diplarthrous) as a
palliative against the tor-
sion due to rapid loco-
motion on hard ground.
All these three cursoria
types are herbivorous.
MAMMALTA, 533
CHAPTER XXIX.
6. SLOTH. 7. MOLE. 8. PORPOISE. 9. BAT.
VI.—THE StotH (Bradypus tridactylus) — Arboreal.
The sloths are truly arboreal mammals, being very com-
pletely adapted to a tree life. The hair is shaggy and
its neutral tint is much in harmony with its surroundings,
the more so in those species which cultivate the growth of a
green alga upon the hair. The external ears are reduced,
probably for easy passage through boughs, and the tail is
vestigial. The fore-limbs are longer than the hind, as one
tendency of the arboreal habit is the greater use of the fore-
limbs. The sloths are herbivorous, feeding solely upon the
leaves of trees, and they belong to one of the lowest orders
of the Lutheria, namely, the Edentata. We shall therefore
expect to find in them certain “ edentate” characters, others
due to a herbivorous diet, and yet others correlated with the
arboreal habit.
The skull has several peculiarities. The zygomatic arch
is not complete, as the jugal does not reach back to the
squamosal, and the premaxille -are nearly absent, which
assists in reducing the facial part of the skull in proportion
to the cranial. The incisor teeth (on premaxilla and oppo-
site it) are absent, and so most probably are the canines
(though in the two-toed sloth the first tooth is long and
pointed like a canine). There are five stump-like homodont
teeth in the upper jaw and four in the lower jaw. These
grow from persistent pulps, as they are worn away and have
no enamel. An outer layer of cement envelops the hard
thin coat of dentine, which in its turn encloses a softer vaso-
dentine. In use the same principle is involved as in other
herbivorous types, the hard dentine here playing the part of
enamel and forming the slowly-wearing ridges between cement
and vasodentine. So far as is known there is no milk
dentition. It is difficult to say how far these peculiar dental
characters are due to degeneration from a higher Eutherian
type, or how far they are due to a primitive condition.
534 CHORDATA.
The cervical vertebree are nine in number, an exception
to the very general rule of seven in mammals. On the
other hand, the two two-toed species have seven and six
respectively. This anomaly may be probably connected
with the low organisation of the Zdenfata.. The same varia-
tion is seen in the dorso-lumbar vertebre. Our species has
usually nineteen to twenty, with fifteen to seventeen pairs of
ribs, but the two-toed species may have twenty-seven, with
twenty-four pairs of ribs. The neural spines all slope back-
wards and are not arranged about a centre of motion as in
the cursorial types. The pelvis fuses with at least six
vertebrze and the caudal vertebre are vestigial.
Fig. 368.—LATERAL VIEW OF SKULL OF THREE-
ToED Story. (GBradypus tridactylus.)
Note the peg-like molar teeth, the short muzzle and the forked
malar bone.
It is in the limbs and limb-girdles that the arboreal
adaptation is most marked. The scapula is triangular, of
the climbing type. The coracoid process sometimes forms a
distinct bone, but is always large, and the clavicle is attached
to it. The arm-bones are very long and slender and the
radius and ulna are both present, the radius being capable of
rotation over the ulna (in supination and pronation). This
movement is usually developed in arboreal or even transition
arboreal types, as the variety of movement involved in such
MAMMALIA. 535
a habitat demands it. In the cursorial types we have
seen that this movement is given up, the bones being per-
manently crossed or even fused. On the other hand, the
arboreal habit, like the cursorial, does not entail differential
use of the digits, and there is a corresponding reduction
in their number and complexity. As in the cursorial
types, it is the digits near the central axis that alone re-
main. Our type has lost
the first and fifth digits, | Fig. 369.—Manus or THREE-
and the other three are long ToED SLoTH (Bradypus
and curved, each being tridactylus).
armed with a long curved
claw. The digits are in-
capable of independent
motion and are largely
enveloped in one fold of
skin. In fact, the hand is
reduced to the condition
of a triple hook, fit only
for the function of suspen-
sion from the boughs of
trees. [The two-toed sloth
has, in addition, lost its
fourth digit, and the tree
anteater (Cycloturus) has
gone a stage further, the
third digit (cf horse) having
a very large claw and the
second a: smaller one, the Note the three long recurved claws, the
other digits being lost. | fusion of the first phalanges and the meta-
The metacarpals and the ‘mals io one one, the fasion of se:
proximal phalanges are 0s magnun. The unciform is round the
. : corner, making six carpal bones instead of
fused together into one the usnaleight.
bone, and with them are
joined the vestigial metacarpals of digits one and five.
The carpal bones are quite immovable, and the scaphoid
is fused with the trapezium, as also is the os magnum with
the trapezoid.
This modification allows the sloth to hang from the
boughs of trees without any muscular effort, and, indeed,
it is said so to hang after death. -At the same time, it
536 CHORDATA.
renders the animal quite unfit for terrestrial locomotion
(f- bat and whale), the horse being equally unfitted for
climbing or burrowing.
The pelvis is much smaller than in the terrestrial types,
the comparison in this respect with its near ally, the ground
sloth (Megatherium), being very striking. The hind-limb is
similar in general characters to the fore-limb. The femur
is long and slender; the tibia and fibula are of about equal
size and allow the foot to face inwards. The foot, like the
hand, is elongated and has the same condition of the digits,
the second, third and fourth alone remaining. The tarsus
shows the same fusion and reduction of elements as the
carpus, all the bones but the astragalus becoming fused
Fig. 370.—STOMACH OF SLOTH.
Note the complex folds and the two-chambered condition.
together in many individuals. As in the case of the meta-
carpals, so here the three metatarsals are fused together into
one bone. The fibula has lost its connection with the
calcaneum, but articulates with the astragalus.
At the base of the limbs the brachial arteries break up
into networks of vessels, known as vefia mirabilia, an adap-
tation apparently serving to overcome the effects of gravity
upon the circulation of the limbs (see page 464). The
mamme are pectoral, a position common amongst arboreal
and zerial types.
The stomach of the sloth, as in most herbivorous
animals, is complex, consisting of at least two chambers,
each of which has an appendix or caecum.
MAMMALIA. 537
ARBOREAL TYPE.
These all dwell typically in trees. As examples we may cite—the
marten, polecat, lemur, monkey, pangolin, opossum, tree-shrew, squirrel,
tree hyrax, tree anteater, phalangers, sloths.
1. The “incidental” group: marten, polecat, pangolin, squirrels.
In these types a terrestrial life in forests is indulged in and the animals
can walk with ease on the earth, but resort to the trees for food or
shelter. The limbs usually commence to show a ‘‘climbing” form,
the claws are sharp and the animal ‘hangs on” to the tree by this
means.
2. The ‘‘transition” group: monkeys, lemurs, opossums. Here
the “tree” and ‘‘ground” habit are both indulged in, but the arboreal
adaptations are marked. The first digit becomes opposable to the other
four to form a climbing ‘‘grip.”” The limb-bones are all retained and
partake of the ‘‘climbing” characters.
3. The true arboreal type: sloth, tree anteater. In these the
arboreal habit is predominant. The claws are permanently curved for
hanging to the boughs and the number of digits tends to be reduced to
three or two. Retia mirabilia are usually present to allow of free
circulation in the vertically placed limbs. Both insectivorous and
herbivorous diets are found.
Like the-cursorial type, the arboreal is evidently derived from the
primitive terrestrial and its incidental group has given rise to the eerial.
VIIL—Mo te (Zalpa europea).—FossoriaL TYPE.
The mole is the commonest and best known of the
true fossorial or burrowing types and its anatomy is an
object-lesson in adaptation. Externally we note the elon-
gated cylindrical body, clothed in fine short fur which will
lie with equal facility in either direction. There are no
external ears, and the eyes are extremely minute, lying deep
in the fur. The snout is pointed and the tail is small and
stump-like. The skull is long and tapers to the front end,
which is strengthened by the forward projection of the
mesethmoid. The teeth are numerous and in many cases
they are forty-four in number, corresponding to the typical
eutherian dentition of 34-43. The mole belongs to the
Insectivora, an order the members of which typically prey
upon small invertebrate animals, such as insects and worms.
This is naturally a more primitive mammalian diet than
mammalian flesh or even grass or fruits, so it is not
surprising to find that the /zsectivora illustrate in their
dentition a type usually regarded as of early origin, the
538 CHORDATA.
mole being no exception. The incisors are small and
chisel-shaped, the canines somewhat prominent in the
upper jaw, but more like incisors in the lower, in which
the first premolar resembles a canine. The premolars
as a whole are simple and conical, and the molars are
tuberculate, having sharp conical cusps adapted for tearing
and crushing rather than grinding. These teeth are pre-
ceded by a complete milk-dentition.
The vertebral column is a strong axis, and the constituent
vertebree are articulated together by very “strong surfaces.”
The dorso-lumbar vertebrz are usually nineteen, a common
mammalian number. The pelvis is attached to six vertebree.
Fig. 371.—JAWS OF TEETH OF THE MOLE x 3.
Note the tubercular molars and the incisor-like lower canine followed by
acaniniform premolar. Dental formula $}43.
Between the dorso-lumbar vertebrae are small extra bones,
sometimes called ‘“intercentra,” represented in most mam-
mals by mere discs of cartilage (intervertebral discs). The
ribs are well formed and taper off rapidly forwards. At the
front end of the sternum there is a large and conspicuous
“ presternum,” which in great part appears to represent the
episternum as found in AZonotremata. The first pair of ribs
are strong and short and support the base of the episternum.
At the front end just under the throat the episternum forks,
and to it is attached on each side a short strong cylinder of
bone. This bone is usually termed the clavicle, but as it is
ossified partly from membrane (clavicle) and partly from
MAMMALIA. 539
cartilage, it probably represents the clavicle and precoracoid
joined in one. Its distal end forms an attachment for the
humerus and it is also joined by a ligament to the scapula.
The scapula is long and narrow and assists, as usual, in
bearing the humerus. The bony connection of “clavicle,”
episternum, ribs and vertebral column, assisted by the
scapula, forms a solid fulcrum for the fore-limb. The
humerus is quite unique. Short and stout, it is expanded
into lateral crests and processes. The radius and ulna are
also short and stout and the olecranon is long, increasing
the mechanical advantage of the extensor muscles.
Fig. 372,ANTERIOR VIEW OF PECTORAL GIRDLE AND LIMB
or THE MOLE. ;
Note the shortened limb, powerful clavicle and humerus and
broad scoop-like manus.
The carpal bones are compact and the whole manus is
broad and flat. There are five short digits with strong
claws. Inside the first digit is a falciform bone which
some authorities regard as a prepollex or sixth digit. What-
ever its homology, it assists greatly in adding to the
“expanse” of the hand. The movement of digging, like
those of swimming and flying, involves a great development
of the pectoral muscles; and in correlation with this there
is a median keel or ridge on the sternum at their point of
origin (¢f bat and bird). All the above structural features
point to the burrowing function of the fore-limbs. With
regard to the forward extension of the episternum, and with it
540 CHORDATA.
Fig. 373.—VENTRAL VIEW OF SKELETON OF MOLE x %,
(From Flower and LyDDEKER.)
eh, Articulation of humerus with cZ., clavicle; s.4., ditto with scapula; e.c.,
external condyle of humerus; 7, femur; _/%., fibula;_/c., falciform bone; 4, humerus ;
z2., left ilium ; 7.4., ramus of ilium and pubes; /.d., ridge of latissimus dorsi muscle ;
2.t,, lesser trochanter ; #., manubrium terni; 4.77., ridge of pectoralis major muscle ;
ft, pectineal ridge; 7d., first rib; s., plantar sesamoid of hind-limb; 4., tibia.
MAMMALIA. 541
of the fore-limb, Flower and Lyddeker remark : “ The fore-
limbs are thus brought opposite the sides of the neck, and
from this position a three-fold advantage is derived: in the
first place, as this is the narrowest part of the body, they
add but little to the general width, which, if increased, would
lessen the power of movement in a confined space ; secondly,
this position allows of a longer fore-limb than would other-
wise be possible and so increases its power; and, thirdly,
although the entire limb is relatively very short, its anterior
position enables the animal, when burrowing, to thrust the
claws so far forward as to be in a line with the end of the
muzzle, the importance of which is evident. Posteriorly the
hind-limbs are similarly removed out of the way by approxi-
mation of the hip-joints to the centre of the body.” The
pelvis is bent inwards towards the middle line in the
acetabular region and there is no pelvic symphysis.
The hind-limbs are not so abnormal as the fore-limbs,
burrowing being effected only by the latter. The lower half
of the fibula is fused with the tibia. There are five clawed
toes and the animal is plantigrade.
We may notice that the mole is a type not only extremely
specialised for one habitat, but, like the sloth, it has certain
primitive characters which have persisted from early times.
We already mentioned that the more primitive types have,
as a rule, survived in arboreal or fossorial habitats: in the
mole we recognise primitive characters in the form and
number of the teeth, in the “‘intercentra,” the episternum
and possibly the prepollex.
FossoRIAL ADAPTATION.
The fossorial type is fo be derived directly from the primitive
terrestrial and like the others is found in varying degrees. We may
take the following as examples :—Zchzdna, badger, anteater, armadillo,
aard-vark, rabbit, bandicoot, marmot, prairie-dog, mole.
As in the other types, we may take three groups :—
1. Incidental group: Zchidua, anteater, Prote/es, banded ant-
eater (Myrmecobius). This consists of animals which prey upon
earth-loving insects, such as ants. The limbs show strong claws on each
digit, and in most cases the tongue and salivary gland are modified for
ingestion of ants, or at least the teeth show an approximation to the
insectivorous type or a degeneration from a carnivorous type (Proteles ).
They are really little modified from the primitive terrestrial group.
542 CHORDATA.
2. Transition group: armadillo, aark-vark, bandicoot, rabbit,
marmot, prairie-dog, &c., show a well-developed burrowing habit
and a domicile underground, although their food is still in most
cases obtained terrestrially. The claws are well developed, and in
many cases the fore-limbs are shortened and the ridges or crests for
the limb-muscles are prominent. The terrestrial habit is still well
in evidence, however, and the necessity for speed above ground limits
the limb-modification.
3. True fossorial: mole, golden mole, Notoryctes. In this type
the food is subterranean and the habit is completely fossorial. The
sense of sight is vestigial, hearing and smell being hypertrophied ; the
fur is reversible, lying evenly either forwards or backwards, and the
limbs are essentially fossorial. The body is cylindrical, and the fore-
limbs are shortened up with powerful keeled sternum and tuberosities,
digits strong and spreading, with strong claws. In some respects the
structure resembles that of the swimmers, the motion being somewhat
similar. Insects and other ‘‘small flesh” form the diet, a truly
herbivorous fossorial mammal being unknown.
VIIL—TueE Porpoise (Phocena communis).—NATATORIAL
OR AQUATIC.
The porpoise belongs to the order Ce/acea (see page 578)
and the sub-order Odontoceti (or toothed whales). Its
general appearance is familiar. It may be anything up to
five feet in length and is fish-like in shape, 7.e., the body is
more or less circular in cross-section and is thickest just a
little anterior to the middle, from which it tapers gradually
to the tail, more abruptly to the head. It is dark greyish-
green on the upper half of the body and head, on the tail
and fins, and a pearly-white on the under surface. The
surface of the body is smooth and oily and there is no hair.
The mouth, with wide gape, is at the front end of the head,
the eyes are lateral and small with no lacrymal glands, whilst
the external ears are absent. A minute pin-hole leads from
the exterior to the tympanum on each side, and at the top of
the head is a single crescentic nostril which is open or closed
as required. About a quarter of its length from the head
the paired fins are seen protruding ventro-laterally, formed
from much modified fore-limbs. Behind the middle line
here is a median dorsal fin, and the tail is modified into
a bilateral or symmetrical tail-fin, the “flukes” of which
lie horizontally. Thus we find that externally in form,
colour, reduction of ears and loss of hair, the porpoise is
MAMMALIA. 543
perfectly adapted for its marine life. It is essentially
gregarious, living in herds or “schools,” and haunts the
pelagic water, #.e., at or near the surface of the open sea,
though not usually found far from land. Along with the
rest of the Ce¢acea, it was for long regarded as a fish till the
researches of Cuvier revealed its true relationships.
The diet of the porpoise is fish, the pelagic species,
such as mackerel, herrings and pilchards, being the usual
victims.
Fig. 374. THE CoMMON PorpolsE (Phocena communis).
(From Flower and Lyppexer.)
Passing to the internal characters we note the absence,
or practical absence, of salivary glands. The primary
function of lacrymal glands is to supply moisture for the
surface of the eye, that of the salivary glands to supply
moisture to the food: hence the absence of both in aquatic
animals. Under the tough skin we find a very dense thick
layer of fatty tissue or “‘blubber,” which is really the
enormously hypertrophied panniculus adiposus. The por-
poise has dispensed with its outer coating of hair to
produce less friction and consequently greater speed, hence
the warmth of the body is retained by “ blubber.”
544. CHORDATA.
The skull is of very peculiar shape and construction.
The cranial part is almost globose in shape, and the facial
is long, flat and tapering, forming the so-called ‘“ rostrum.”’
It is not difficult to get at least some insight into the
reasons underlying these peculiarities. If we recall the
Fig. 375.—SECTION OF SKULL OF YOUNG DOLPHIN (Globdocephalus mela.
(After FLOWER.)
Wa IP Pr
Pi, Palatine. Per, Internal nares. Pt, Pterygoid. PS, Presphenoid. BS, Basisphi
iid. AS, Alisphenoid. Vo,Vomer. Ax, Maxilla. Px, Premaxilla. MZ, Mesethmo
tm, External nares, Wa, Nasal. JP, Interparietal. Fr, Frontal. a, Parietal. S
jupraoccipital. £20, Exoccipital. BO, Basioccipital. Sg, Squamosal. Per, Periot
A, Hyoid. cd, Condyle. a, angle. s, Symphysis.
facial part of any of our terrestrial animals we find that
it subserves two functions. The under-surface and lower
part of it, forming the buccal chamber, is connected with
the alimentary function; it bears the teeth and forms the
palate. The upper part contains the large and complex
nasal chambers, access to which is obtained by the
MAMMALIA, 545
anterior nares at the front end of the facial region. The
nasal chambers serve the dual functions of smell and of
respiration. The length of the nasal chambers and the
distance between the anterior and posterior nares, combined
with the great exposed surface of the turbinals, ensure the
activity of the olfactory sense.
In the Ce¢acea the erial olfactory sense is of little or no
use, whilst a rapid and easy passage of air to the lungs is
essential. Hence the anterior nares have progressed back-
wards till they come to lie vertically over the internal nares,
and the nasal “chamber” of terrestrial types, with its
complex turbinals, has been converted into a simple pair of
short passages, with no turbinals, leading directly downwards
to the glottis. In terrestrial types the roof of the nasal
chamber is formed by the nasals and partly the frontals.
Here the nasals and frontals are pushed backwards before
the retiring nostrils. The frontals squeeze the parietals to
the sides and meet the supraoccipital, whilst the nasals are
pressed against the front wall of the cranial cavity. Hence
the “rostrum” represents only the ventral or alimentary
part of the mammalian facial region, consisting solely of
the premaxille—which follow the nostrils backwards and
become very elongated—the maxille, mesethmoid and the
vomer.
The maxillz, premaxille and mandibles bear a single
row of small teeth, very numerous and all of the same size
‘ (homodont). Each tooth has a single root, and in the
porpoise is scoop-shaped
and raised on a short Fig. 376.—TEETH OF PORPOISE x 2.
base. (In the dolphin
each is a simple conical
point.) There are usual-
ly about twenty-five on
each side, upper and
‘lower jaws, and as they
are homodont we can
use no dental formula
but $2 (dolphin $$ to
$9), There is no suc-
cession (monophyodont), but there are said to be traces of a
second or permanent dentition which is only transitory and
M. : 36
(From Flower and LyDDEKER.)
546 CHORDATA.
never replaces the functional teeth ; hence these latter are
often regarded as the milk-teeth (f. Afetatheria). The homo-
dont condition is correlated with the function of seizing and
retaining small slippery fish which are not masticated but
“ bolted ” entire, as in the case of sharks and other fish. The
immense number of teeth is a great morphological difficulty,
especially if we assume that the Cefacea are descended from
terrestrial Eutheria, with teeth approximating in number
Fig. 377.—DIAGRAMMATIC SECTION OF STOMACH OF PORPOISE.
(After FLOWER and LYDDEKER.)
h, Bile-duct. g, Duodenum. / Pylorus. ¢, Pyloric Portion. c, Middle
Portion. 4, Cardiac Part. a, Gésophagus.
to the typical dentition of 3343. It has been suggested
that the tritubercular or multitubercular carnivorous type
of tooth of these ancestors, losing its tearing and crushing
function, became split up into three or more single-cusped
elements which would give at least $4423 or 25. The
rudimentary molar teeth of the ystacoceti or toothless
whales are said to break up in this way into simple elements.
MAMMALIA, 547
The molar teeth of elephants consist of many (up to twenty-
four) successive ridges, each with its roots. Hence it is
possible, by an appeal to the principle of cusp-multiplica-
tion followed by separation, to suggest an origin for the
great number, as well as the simple structure of the ceta-
ceous teeth.
Behind the mouth the larynx and glottis are produced
from the ventral wall of the cesophagus upwards as a long
cylinder into the base of the internal nostrils, a striking
adaptation which enables the porpoise to open its mouth
under water and even to swallow whilst breathing. A
similar modification is found in young Me/atheria, in this
case enabling them to breathe and swallow milk at the same
time.
Returning to the porpoise we find that it possesses a
complex stomach, a rare possession for a flesh-eater. The
first and largest chamber is a storage sac with no glands,
probably a mere dilatation of the cesophagus: this is
followed by a smaller receptacle with fundus (tubular)
glands and folded walls: a very small globular third
compartment passes into a long vermiform fourth part
which has pyloric glands and leads into the duodenum.
Reverting to the skeleton, we find the cervical vertebree,
seven in number, are short and fused together. A flexible
neck, far from being a necessity, is rather a drawback to an
aquatic animal, rigidity of the anterior end being imperative
for rapid locomotion. The dorso-lumbar vertebre are hard
to define for there is no sacrum, but between the first
caudal and the last cervical there are about twenty-seven in
number, the first thirteen, as in most mammals, bearing
ribs. The transverse processes are prominent, as also are’
the neural spines. The former arise from the side of the
centrum in the last lumbar, but higher and higher up on the
neural arch as one proceeds forwards. The round disc-
shaped epiphyses are very conspicuous. The hindermost
of these dorso-lumbar are probably the former sacral verte-
bree, but as the ilia have atrophied there is no certainty.
The caudal vertebre are numerous (30-31) and, as in
the kangaroo, bear paired chevron bones on their under
surface. It is usually assumed that the caudals commence
with the chevron bones, The fore-limb and girdle are
548 CHORDATA.
unique. _Clavicles are absent, but the scapula is large, flat
and broadened into a fan-shape. The prescapular fossa is
very small, the spine being bent forwards (aquatic type).
The humerus moves freely on the scapula, but this is the
only possible movement of the limb. The humerus 1s very
short and stout and bears two equally short flattened bones,
the radius and ulna. Six small carpal bones follow carry-
ing five digits. The digits are peculiar in having a greater
number of phalanges than is usual for mammals (2.3.3.3.3.)-
This feature has formed a puzzle to morphologists ; a pos-
sible explanation of their multiplication is the formation of
supplementary phalanges from the epiphyses of the others
Fig. 378.—LATERAL VIEW OF PECTORAL GIRDLE AND FIN
OF A PORPOISE x 4. (Ad nat.)
Scapula.
:Acromion.
Coracoid.
Distal Carpals. | | Uina Humerus.
Proximal Carpals. Radius.
to meet the demand for increased surface. Some of the
phalanges present the anomalous feature of an epiphyses at
each end. The whole limb is firmly welded together by
fibrous tissue and little or no motion is possible at elbow or
wrist: indeed, in old specimens, the limb-bones are anky-
losed together. The shortening of the limb is due to the
same cause as in the mole, z.¢., the need for a short, quick,
powerful stroke.
The hind-limbs have entirely disappeared. leaving no
trace, and the pelvis is represented only by a pair of small
bones which represent the ischia. In terrestrial mammals
the ischia form a support for the cavernous bodies of the
penis and these small ischia of the porpoise perform a like
MAMMALIA. 549
function ; it is probably to this subsidiary function that
they owe their preservation.
The dorsal fin and the tail, except for its central vertebral
axis, have no osseous support like that of the paired fin, but
are stiffened by strong dense fibrous tissue.
The heart in Cefacea is large, and there are underlying
the vertebral column a number of fine vessels, or retia
mirabilia, which may assist the -animal in keeping under
water for long periods (see page 464).
It is sometimes asked, How do we know the porpoise
(Cetacea) to be a mammal? And again, How is a porpoise
adapted for an aquatic habit? If we divide the structural
facts of the porpoise into (1) resemblances to other mam-
mals and into (2) adaptive characters, the questions will be
answered. Of the first category we have only to refer to
Table on page 431 and it will be found that the porpoise
agrees with all the twelve mammalian characters there
enumerated with the reservations of no hair, no hind-
limbs and homodont teeth. Again, it conforms to no
one character of the second class (fishes).
Of adaptations to an aquatic habitat we may specially
note :—
1. Fish-like shape, with dorso-ventral coloration.
2. Loss of hair and external ears and formation of
“ blubber.”
3. Fore-limbs formed into fins, bind-limbs lost and tail
forming a fin.
4. Homodont dentition (fish diet).
5. Modification of nostrils to form vertical blow-hole and
prolongation of larynx.
6. Retia mirabilia.
7. Loss of salivary and lacrymal glands.
AQUATIC ADAPTATION.
A large number of Mammatia frequent the water either tempor-
arily or permanently, and the degree of aquatic habit marks the degree
of adaptation. We may cite the following—hippopotamus, water-
voles, the yapock (Chéronectes), river-shrew (Potamogale), otter, sea-
otter, walrus, sea-lions and seals, manatee and dugong, whales, porpoises
and dolphins. These may be studied from this point of view in the
following order :—
550 CHORDATA.
I. The incidental group.—Leaving out of consideration the hippo-
potamus, water-vole, musquash and other mammals which frequent
water but do not show marked adaptations thereto, we have the duck-
mole, coypu, yapock, desman, river-shrew, otter and beaver. These all
swim actively in the water and the toes are often united with a web of
skin which enables the limbs to act as paddles. In addition, the tail
is usually modified. In many cases its hair is lost and it is scaly and
flat (cf duckmole, beaver, river-shrew). They are all freshwater river
animals, and the majority are also fossorial, living in holes, so that the
claws remain long and powerful. They are fairly at home on land and
retain their hair.
2. The transition group.—The sea otter (/#/ra) carries us on to
the second group of the walrus, sea-lion and seals. | Here the body is
fish-like and the limbs are modified into true paddles, the front-limbs
forming the steering paddles and the hind-limbs the motor paddles.
The terrestrial habit is more and more forsaken. The walrus and sea-
lion can still place the sole of their hind-limb on the ground and can
walk clumsily. They come ashore to breed. The seal has progressed
further. The hind-limbs are permanently bent backwards for swimming
and the external ears have disappeared. In all this group, however, the
hair remains as fur all over the body.
3. The true aquatic.—The Svrenia or manatee and dugong and
the Cetacea remain. They are fish-like in shape, the fore-limbs are
formed into paddles and the hind-limbs have disappeared altogether
as the motor paddle is formed by the tail. In this respect they carry
on the adaptation of group I rather than group 2, which form their
motor paddle from the hind-limbs. The hair is almost entirely lost and
the pinna of the ear is lost. The claws, reduced in group 2, are lost
here. The blood-system has networks of blood-vessels, called retia
mirabilia, to allow of ‘‘ holding the breath” under water.
The Cefacea are further adapted than the Sivexia. They become so
fish-like in form that they were for a long time supposed to be fish.
Many have the dark upper-surface and light under-surface characteristic
of fish (dolphin, porpoise). The front-limbs are very shortened for a
sharp quick stroke, and the phalanges are increased in number from the
normal mammalian type. The nostrils open on the top of the head
and in many there is a dorsal fin. A flexible neck is no longer required
and the cervical vertebrae fuse into one mass. Salivary glands for
moistening food tend to disappear. There are special adaptations to a
fish diet (homodont dentition), as in Odontoceti, and to a plankton diet
(pelagic animals), as in Mystacocetz.
We may trace the evolution of aquatic forms from the resort of
fossorial types to the soft ground in the neighbourhood of rivers, then
to the acquirement of aquatic food, either fish or water-weeds. The
river leads to the river mouth (Szvevza) and this to the open sea. The
Pinnipedia, however, may have taken to the sea direct from a polar-
bear-like habit.
In all, the mammalian type has its teeth modified for the fish diet
and the limbs and tail modified for the fish mode of locomotion, sharp
short strokes with a large surface being the end to be attained.
MAMMALI41. 581
IX.—TuHE Bar (Pteropus edulis). The Arial type.
The Fox-bat belongs to the order Chiroptera and to the
sub-order Megachiroptera. The other sub-order of the
Microchiroptera includes the small British bats. As a rule,
Fig. 379. FEMALE AND YOUNG OF A Fox-Bat
(Xantharpyia collaris).
(From ScLaTER, Proc. Zool. Soc., 1870.)
the Megachiropiera are fruit-eating and the Microchiroptera
feed upon insects. As shown below, this is correlated with
the greater adaptation to flight in the latter. However,
the fox-bat is here taken as a type because its greater size
facilitates an examination of its anatomy.
552 CHORDATA.
The fox-bat may have an expanse of five feet across the
wings. The head is not unlike that of a small fox, with a
sharp intelligent look about the eyes. The external ears are
large and the sense of hearing is acute. The body is
covered with fine thick hair which is woolly round the
neck. The whole appearance of the animal is totally unlike
that of any other mammals outside the order, owing to the
presence of an enormous pair of membranous wings (though
Fig. 380.—THE FEcToRAL GIRDLE AND Fore-LimB oF PTEROPUS.
(Ad nat.)
Clavicle.
Scapula.
Humerus. Ulna.
an approximation to this condition is found in the colugo).
The wing, as is clearly seen in the skeleton, is formed by the
fore-limb, upon which is stretched the membrane. The con-
cavity of the elbow is filled with a small antebrachial mem-
brane. The hind-limbs are very small and armed with claws.
The patagium extends from the fore-limb down to the ankles,
being attached to the sides of the body, and a slight inter-
femoral membrane stretches across between the hind-limbs.
These three membranes form the patagium, consisting
MAMMALIA. 553
throughout of a double fold of skin. There is no external
tail. In the Microchiroptera the tail is well developed and
forms an axial support for the interfemoral membrane.
By this means the latter group are able to turn rapidly in
the air in pursuit of insects,
The sense of touch is remarkably developed in bats,
some families having a pair of peculiar organs, the “nose
leaf,” on the snout. It consists of an irregular cutaneous
expansion, supplied by the fifth nerve, and apparently
enables the animal to be cognisant of variations in vibra-
tions of the air caused by objects in close proximity. In
a great number of bats the ear-pinna is also enormously
developed, though not excessively so in our type.
The bat has lost almost all power of terrestrial locomo-
tion and at best can shuffle clumsily along the ground.
This is due to the great reduction of the hind-limbs and
especially to the fact that the knees, in connection with the
support of the patagium, are bent backwards like the elbows,
making them unfit for walking. The “wings” are also
quite unsuited for the same purpose. The hind-limbs are
used for grasping boughs, and the bat thus hangs suspended
head downwards, often enveloped in its patagia. We have
already noticed that the zrial types have been evolved from
the arboreal, and in this respect the MMegachiroptera are
less specialised than the Aficrochiroptera, as their food and
resting-place are arboreal.
The bats were for a long time regarded as birds, or at
least not recognised as true mammals. There is, however,
if possible, less difficulty in noting their mammalian affinities
than in the case of the porpoise. A reference to the two
columns of Aves and Mammalia in Table, page 431, will
make this quite clear. The generally accepted view regards
them as modified Jnsectivora.
The skull is very variable in general form and structure
throughout the group. The fox-bat has a fairly even set
of teeth, well defined into incisors, canines and molars, the
canines being slightly the longest. There are only two
incisors in each jaw, a common condition in bats, though
the lower jaw may have as many as three. The molars
and premolars have blunt crowns and are 3. No bats
have more than $ or 33.
554 CHORDATA.
Thus the fox-bat has a dental formula of 2-3-3-3, a
considerable reduction in number from that of the typical
eutherian. The number agrees closely with the fruit-eating
or frugivorous Primates, the marmosets having 3-4 3-4.
The cervical vertebree are small and compressed and
carry very small neural spines. The thoraco-lumbar ver-
tebree, bearing fourteen pairs of ribs, are seventeen in
number. They are all set in one curve, have few processes,
very little motion on each other, and are not infrequently,
as in birds, largely ankylosed or fused together. In each
case rigidity of the central axis is a necessity.
The caudal and sacral vertebre are fused together.
The thoracic cavity is spacious and the ribs are compact.
The sternum has a prominent median keel, which is largest
on the presternum but is continued as a series of smaller
keels on each sternebra. The scapula is large and trian-
gular and is firmly connected with the presternum by the
clavicles. These are stout and curved though not shortened,
as in the mole. The fore-limb is enormously long and the
bones are slender. The ulna is vestigial, like that of the
horse, but the radius is very long. It bears six small carpal
bones and five digits. The pollex is short and free from
the wing’; it bears a claw. The other four digits are enor-
mously elongated and serve when separated to extend the
wing-membrane. The second digit terminates in a claw,
but the others end in tapering phalanges.
It is instructive to compare this wing with that of the bird. We
see at once that the same ends are attained by a different method.
The main axis of support is the fore-limb in each case, supplemented
in the birds by the reduced digits. The lateral axes are formed in
the bird by the shafts of true feathers and in the bat by the digits.
Lastly, the vanes of the feathers serve the same mechanical purpose as
the patagium of the bat. The sternal carina is found in each, as
an attachment for the pectoral or ‘‘ flight” muscles, but whereas the
fulcrum of the fore-limb is attached to the sternum mainly by the
coracoid, supplemented by the clavicle in the bird—the bat having,
as a mammal, practically lost its coracoid in early times, has to rely
upon the clavicle alone.
We may recollect that the mole has a keeled sternum
and a strong bony junction of scapula to presternum.
These are alike due to hypertrophy of the pectoral muscles,
in its turn connected with excessive use of the fore-limb,
MAMMALIA. 555
but in the mole the fore-limb is shortened, not length-
ened, as the medium upon which the work is done is
solid earth, not air.
The pelvis of the bat is produced backwards and there
is usually no pelvic symphysis. The hind-limbs are small.
The fibula is a small splint-like bone down the side of the
tibia. There are five toes, of which the first is slightly
_ the smallest: they bear curved claws.
As in the mole and porpoise, the adap- Fig. 381. — Larera.
tation has resulted in increase of the VIEW oF THE STER-
fore-limb and reduction of the hind- NUM or A Fox-BaT
limb. (Pteropus).
The stomach of the fox-bat is simple,
though the pyloric portion is produced
into a process.
The mammz of the bat are pec-
toral and paired. The fecundity is low,
as is natural when the parent has to
carry the family about with her, cling-
ing to her under-surface. Such a
position of the young may account for
the pectoral mamme.
The adaptation of the bat to an
zrial habit may be summarised as
follows :—(1) Fore-limbs and four digits
elongated, supporting “wing mem-
brane.” (2) Hind-limbs bent back-
wards and assisting to support ‘“ wing
membrane” and “‘interfemoral mem-
brane.” (3) Keeled sternum, connected
by large clavicle to scapula. (4) Partial
ankylosis of dorso-lumbar vertebrae. (5)
Great development of hearing and of
‘‘motion-sense.” (6) Small fecundity and pectoral mamme.*
Note discontinuous keel.
AERIAL ADAPTATION.
The eerial types are modified from the arboreal, ayshort step only
intervening between the two. As examples we may instance the flying
‘squirrels ” and phalangers of Australia (Czscws, Petaurus), the true
* Probably connected with arboreal habit which preceded that of flight (c/,
Primates).
556 CHORDATA,
flying squirrels (Pteromys), Anomalurus, the, colugo or flying lemur
(Galopithecus) and, lastly, the bats. We may divide these into three
groups :—
1. The incidental group. —The phalangers, marsupial squirrels
and true flying squirrels. All are arboreal and still adapted thereto.
“Flying” is to them merely incidental, as is swimming to the first aquatic
group. They have a thin fold of skin or patagiwm which stretches from
fore-limbs to hind-limbs and acts asa parachute. In all, the tail is bushy
and not only acts as a balancing organ in jumping but as a steering
organ in flight. The spreading of this patagium is an easy addition to
the long jumps from bough to bough performed by their ‘‘ non-flying ”
allies.
2. Transition type.—The colugo or flying lemur. In this the pata-
gium extends further between the tail and the hind-limbs. The animal
appears to have more direct means of steering itself, and flight is less
‘incidental ” and more evenly balanced in the life of the animal with
the arboreal habit. The limb-bones are long and slender to allow of a
larger patagial surface.
3. True zrial.—The bats. These are the culminating group of the
zrial types. Here the zrial habit becomes predominant. The patagial
surface becomes further extended, especially that part of it which can be
voluntarily moved in the neighbourhood of the forelimb. The fore-
limb and the digits are greatly elongated, forming axes for support of
the patagium, the pectoral muscles are employed for movement and a
keel on the sternum is the result. In one group of bats (Pteropodidz)
two digits retain their claws and in the rest only one, the thumb. In
the former the diet is still arboreal (fruits), but in the latter it is strictly
zerial (insects). As, however, insects are not confined to the air we do
not find a specially marked peculiarity in the teeth. The adaptations
to flight are, therefore, mainly to be found in the locomotor organs.
MAMMALIA. 557
CHAPTER XXX,
MAMMALIA —( Continued).
Sub-Class III.—Eutheria.
ORDER V.—Z£dentata.
The general anatomical characters of the sloth have
been already described, the animal being taken as a com-
pletely arboreal type (page 533). The order appears to
occupy the lowest place in the sub-class and its members
have great diversity of habits. They are either arboreal
and herbivorous or semi-fossorial and insect-eating. The
body is clothed in hair, in one family supplemented by
bony plates. Both pairs of limbs are well developed and
armed with claws. The digits may vary from 2 to 5.
The teeth are always either simple and homodont, with
persistent pulps, and with few exceptions monophyodont
or they are absent altogether. They are usually also
deficient in enamel. and the incisors and canines always
are absent. The order shows remarkable diversity in the
structure of the placenta and in specialisations of teeth,
limbs and body-covering. It is divided into two sub-
orders which are widely separated, both structurally and
geographically.
SUB-ORDER I.—XENARTHRA.
In this sub-order are contained at least four families.
In them the uterus is simple and the placenta is dis-
coidal or dome-shaped and deciduatee Mammez are
usually two and pectoral. The vertebrae usually have
558 CHORDATA.
extra articular processes. The sub-order is entirely re-
stricted to the Neogeean realm (South America).
Family I.—Bradypodidz or Sloths.—Purely arboreal, leaf-eating
animals ; Bradypus has been described. We may recall (1) the adapta-
tion to arboreal habit ; (2) the low eutherian characters shown in a bi-
partite uterus, occasional presence of a complete coracoid and varying
number of cervical vertebrae. They are found only in forests of South
America.
Family II.—Megatheriidee or ground sloths.—Extinct terrestial
forms, occurring backwards from the Pleistocene. They are closely allied
to the sloths, but show certain resemblances to the anteaters. They
'Fig. 382, —TAMANDUA ANTEATER (Tamandua tetradactyla. )
(From Proc. Soc., 1871., PL, xxti1.)
were apparently huge hairy monsters, that fed upon leaves of trees.
Megatherium walked upon the outer side of the feet, on pads covering
the fifth digit of the front-limb and the fourth and fifth of the hind-limb.
The second, third and fourth digits of the front-limb and the third of the
hind-limb were armed with huge claws. JZylodon was another well-
known form which may possibly still survive in parts of South America.
Family III. —Myrmecophagidz or Anteaters. — These show a
similar adaptation to anteating to that already noticed in Echidna.
There are no teeth, the mandible is rudimentary, facial region tapering
and terminating in a small round mouth. The tongue is very long
and copiously supplied with saliva from the large submaxillary glands.
The tail is usually long and in the tree-anteaters is prehensile. The
MAMMALIA. 559
third toe of the manus is always large and bears a large claw. The
great anteater is purely terrestrial; the Tamandua and Two-toed anteater
(Cycloturus) are arboreal.
Fig. 383.—LATERAL VIEW OF SKULL OF ANT-EATER. (4d zat.)
Maxilla, Nasal. « Frontal. Parietal. Supraoccipital.
Premaxilla.
Auditory
Meatus.
Occipital
Condyle.
Lacrymal, Malar. Alisphenoid, Squamosal.
Note the absence of teeth, the elongated jaws and incomplete zygomatic arch.
Family IV.—Dasypodidze or Armadillos.—They are unique amongst
mammals in having the head_and body enveloped in bony dermal scutes
covered with horny epidermis. In the typical forms there can be
distinguished a cephalic plate over the head, a large pectoral and pelvic,
covering respectively the fore and hind part of the body, and a number
of rings between them. The tail is also enveloped in a series of rings.
The ventral surface is usually soft and hairy and the habit of rolling-up
Fig. 384.—LaTERAL VIEW OF SKULL OF ARMADILLO.
‘
Note the absence of incisors and canines, the numerous cheek-teeth, the long
snout with small premaxille.
in a balliscommon. Armadillos are largely insectivorous and have a
long sticky tongue with large submaxillary glands. On the other hand,
they have « large number of simple teeth which in many cases are
diphyodont. They are mostly fossorial and the toes are armed with
strong claws. The genus Zolyfeutes, in which the rolling-up is best
perfected, is said not to burrow. They vary in size from the little
Pichiciago of 6 inches to the great Armadillo of three feet. The largest
560 CHORDATA.
and most specialised of the Armadillos, known as the Glyptodonts, are
extinct forms found in the Pleistocene. The body was enveloped in
one huge shield into which the head could be retracted. The vertebral
column is ankylosed together, the shield preventing free movement
(cf. tortoise).
From this brief description of the Xexarthra, it will be
seen that the present forms are only the remains of an
extensive group of mammals which once held a dominant
position in the Neogzan realm. Why such powerful
creatures as Megatherium and Glyptodon have disappeared
is a question that has puzzled many. All we can say is
that a type, like an individual, has a limited part to play on
the stage of organic evolution, determined by the relation-
ship of an organism to its environment.
SUB-ORDER II.—NOMARTHRA.
This small sub-order contains two families which are
doubtfully related to each other. They are terrestrial or
arboreal and feed on ‘‘ants” or termites. Hence in them
is found the same elongated snout, small mouth, long
mobile tongue and large salivary glands, as in the ant-
eaters and Echidna. The uterus is bicornuate, or there
are two uteri, and the placenta is non-deciduate and diffuse
(or zonary, modified from the diffuse). There are no extra
articular processes on the vertebree.
Family I.—Orycteropodidz.—Aard-varks or earth pigs. The
aard-vark of South Africa is a nocturnal and partially fossorial animal.
Its body is sparsely covered with hairs. It is plantigrade with four and
five toes all armed with strong claws. The teeth are unique in structure
amongst mammals. They grow from persistent pulps, gradually pushing
forward in a manner similar to that found in the elephant and the
kangaroo. There are usually five on each side in use at the same time
and about ten in all. All but the three last are preceded by a milk set,
which are absorbed before cutting the gum. This appears to indicate
premolars and molars and a possible degeneration from a higher type
of heterodont dentition.
Family II.—Manidz or Pangolins.—The pangolins are elongated,
terrestrial, fossorial animals: many can climb trees. They have the
body clothed in a series of large overlapping scales of horny epidermic
origin. On the under-surface there is usually hair only. The tail is long
and protected in a similar manner to the body. Like the Armadillos,
they can usually roll themselves into a ball. The skull, especially in the
jaw region, is modified for the ‘‘termite-eating” habit, as in the anteaters:
MAMMALIA. 561
thus there are no teeth, the mandible is much reduced, the jaws are
long and tapering, the mouth small and the tongue long and mobile.
The limbs are short and the claws long and powerful. The Pangolins
are found in East Africa and in the Oriental region (India), and comprise
one genus.
Like the Xenxarthra, the Nomarthra are very low types
of Lutheria, which affect an arboreal or fossorial habit.
They are confined to Arctogcea, just as the Xenarthra are
confined to Neogcoea,
ORDER VI.—Sivenia.
The Svenia are aquatic herbivorous animals known as
the Manatees and Dugongs, or sometimes collectively as
the Sea-cows. They live either in rivers or at the river-
mouth, and, although well adapted for aquatic habit, they
do not quite reach the same stage in this direction as the
Cetacea. As in tke latter, the body is more or less fish-
like with tapering tail ending in a horizontal “ fluke,” there
is little or no hair and no pinna to the ear, the fore-limbs are
in the form of flippers and the hind-limbs are absent. The
valvular external nares open far back towards the top of the
head, resulting in the formation of a rostrum, and there are
retia mirabilia in parts of the body. In all these anatomical
features the Sivenia are like the Cefacea, but here the resem-
blance ends. The cervical vertebree are never fused to-
gether, the teeth are neither absent nor homodont and the
food consists of aquatic weeds. The flippers usually have no
more than the normal number of phalanges* (2.3.3.3.3.) and
the joints of the fore-limb are largely functional, as the flipper
is used not only for swimming but for assisting food to the
mouth, and in some cases possibly in holding the young.
In comparing these characters with the porpoise, it will
clearly be seen that the Szvenza have not progressed quite
so far in adaptation as the Ce¢acea. Other special points in
the anatomy show that there is no true genetic connection
between the two orders.
In Stvenia there is the same tendency to disappearance
of the front teeth as we have noticed in the Ldentata.
The manatees have no functional incisors nor canines
* Rarely four,
M. 37
562 CHORDATA.
and the male dugongs have a single pair of tusk-like upper
incisors. The place of front teeth is taken by hard horny
pads upon the rostrum and mandible. The molar teeth
have a pair of transverse ridges, like those of the tapir,
and they succeed each other in series, as in the elephant,
armadillos and kangaroo. The extant forms are apparently
monophyodont. The stomach is fairly complex, with at
least two chambers, and the intestine is long. The
placental characters are not fully known, but the dugongs
have a zonary placenta which is non-deciduate. The
mamme are paired and pectoral in position. At the
present day the order is limited to a zone between 30° N.
and 30° S. of the equator.
Fig. 385.—AMERICAN MANATEE (Manatus Americanus) from life.
(From Flower and LYDDEKER.)
Family I.—Manatidz or Manatees.—Three species found in the
rivers falling into the Atlantic basin. They are peculiar in having only
six cervical vertebrae. Beneath the horny pads of the jaws are vesti-
gial incisor teeth # and the molars may be as many as }}.
Family IJ.—Halicoridz.—The Dugongs are larger and are found
in the Red Sea, Indian Ocean and Northern Australia. The males
have incisor tusks which are vestigial in the female. The molars do
not exceed 8. They are more marine than the Manatees.
Family III.—Rhytinide.—The Rhytina or Steller’s sea-cow
was a large sirenian (25 feet) formerly found in the district of Behring
Island. It was finally exterminated at the hand of man in 1768. This
species had no teeth, their places being supplied by horny pads.
Certain fossil forms, such as Halitherium (Miocene), show
us that the sirenians were abundant at that epoch and even
MAMMALIA. 563
to the Eocene. Haditherium also had a diphyodont dentition
and the pelvic-girdle and hind-limb were not so reduced as
in present-day species. The Svvenéa are usually regarded as
having been derived from very generalised terrestrial herbi-
vores, approximating to the lowest Ungu/ata, but there is little
direct evidence at present for such a view. They are a
primitive and much modified order, in these respects resem-
bling the two preceding orders, and though there is no
question that they are descended from terrestrial eutherian
mammals, little more can be said.
ORDER VII.— Rodentia.
The rabbit has already been described as a typical
mammal, and, except in respect to their peculiar dentition,
the Rodentia, as a whole, are a group with habits and
structure which apparently approximate to those of the
primitive Eutherian Mammaha. ‘Thus they are all of small
size, mainly terrestrial, though some are arboreal, usually
plantigrade, with little or no reduction in the number of
toes, each of which carries a scratching claw. The orbit is
never completely encircled by bone, the clavicles are always
present though often reduced, and there is often a third
trochanter.
But the most distinctive character of the order is the
dentition. The canines are always absent and the incisors
are reduced in the majority of cases to two in each jaw.
These grow perpetually from persistent pulps, and as the
enamel or hardest portion of the tooth is only present on
the outer surface, the wear of upper and lower teeth on
each other produces a sharp chisel-like edge. These teeth
are used, in the majority of cases, for other purposes in
addition to that of obtaining food. The teeth are succeeded
by a large space or dastema and a number of premolars
and molars, which are often reduced from the $% of the
rabbit to 2-3, or even, in exceptional cases, to f+.
The molars vary much in character, but are always flat
and worn on the surface, exposing complex enamel-ridges.
In order that the incisors may have free play, the condyle is
freely movable in the glenoid cavity and there is no post-
glenoid process.
564 CHORDATA.
This peculiar dentition is not confined to the Rodentia ; two persistent
permanently-sharp incisors of a large size, with corresponding reduction
or loss of the others, appear to have been evolved in several independent
series of Mammalia. In present-day forms, the wombat (Fig. 349)
amongst Diprotodontia, the aye-aye (Fig. 394) amongst the lemurs, and
Hyrax (Hyracoidea) of the Ungulata (Fig. 386), all have essentially
the same adaptation, whilst the single pair of persistent incisors of the
elephants may also be recalled.
In extinct types, the important orders of 77//odontia and Typotheria
have a somewhat similar arrangement, the former being often regarded
as transition types between Rodentia, Carnivora and Ungulata.
All Rodentia are herbivorous and usually have a long
intestine and large czecum.
The brain is of a low type, proportionately small; the
cerebrum is little convoluted and too small to reach back-
wards over the cerebellum.
The uterus is often double, as in the rabbit, or is widely
bicornuate, and there is usually a high fecundity. The
placenta is discoidal and deciduate.
From these and other characters the Rodentia occupy a
low place amongst utheria, but apparently their adapted
dentition has enabled them to become the most widely
distributed and abundant mammalian order. ‘Their present
day headquarters appear to be the Neogcean realm
(South America) in which there occur enormous numbers,
including the Capybara or largest existing rodent.
The order can be traced back to the Upper Eocene,
below which it is more or less merged into the earliest
Ungulata.
The hares, rabbits (Zeporide) and the picas (Lagomyida)
are placed in a sub-order, Duplicidentata, characterised by
more or less enamel on the inner surface of the incisors, the
presence of a small inner pair of incisors in the upper jaw,
a tendency to a larger number of molars and the descent
of the testes into a scrotal sac. They are confined to the
Arctogcean realm.
The rest of the Rodentia form the large sub-order Sim-
plicidentata, with only one pair of upper incisors, having
enamel only on their outer surfaces; the molar teeth
tend to become reduced in number and the testes are
mainly abdominal. They are of world-wide distribution
and include the /ystricomorpha or porcupine-like forms
MAMMALIA. 565
(porcupines, guinea-pigs and capybara); the Alyomorpha
or mouse-like forms (rats, mice, and voles); and the
Sciuromorpha or squirrel-like forms (squirrels, marmots
and beavers).
The beavers are confined to Arctogcea and the Aystrico-
morpha are most abundant in Neogcea.
OrDER VIII.— Ungulata.
The order Ungudata has four living sub-orders which’
are sharply distinguished from each other and from other
orders. The labours of paleontologists have brought
to light a number of extinct forms which are evidently
allied to the living Ungulata, though in most cases they
show, as is to be expected, a number of characters in
common with the more primitive members of other orders.
Hence the order has been gradually widened till it now
contains such a variety of types that they have few special
features incommon. In a general way they are all herbivor-
ous and adapted for walking upon land on all four limbs.
The teeth are heterodont and the canines are, as a rule,
not longer than the incisors or molars, in many cases resem-
bling in appearance either of these latter, or they may be
altogether absent. The premolars and molars are large and
flat, adapted for grinding and crushing rather than cutting.
The dentition is diphyodont and the first or milk-series
remains functiorial for a long time, largely assisting the
permanent series in their long and arduous duties. The
lower types have the typical eutherian dentition of $343, but
this is considerably changed in the more specialised forms.
The limbs are devoted in this order solely to terrestrial
locomotion, with its single series of motions. Hence the
clavicles are nearly always absent and the ulna and fibula
reduced in the higher types. The carpal and tarsal bones
remain serial only in the lower types, becoming alternately
interlocked in the higher. There is a tendency throughout
the order for a reduction in the number of toes, the third
alone or third and fourth persisting in the higher forms.
The typical mammalian claws at the end of the digits
usually become converted into unguz, or hoofs, presenting a
flat surface to the ground. There can, along with these
566 CHORDATA.
progressive changes, be noticed the gradual assumption of a
digitigrade method of walking from the primitive planti-
grade. Special allusion has been made to most of these
points in dealing with the horse and ox. The intestine is
always long, the uterus is usually of the bicornuate type and
the placenta is non-deciduate and either zonary, diffuse, or
cotyledonary.
SUB-ORDER I.—CONDYLARTHRA.
The members of this sub-order are all extinct and they
represent the very lowest point of the ungulate stock. They
have the typical eutherian dentition of 344% and the molars
Fig. 386.—LATERAL VIEW OF SKULL OF DAMAN
(Ayrax syriacus).
Note the rodent-like incisors, absence of canines and long row of seven
grinding premolars and molars. The malar bone is seen to extend
back to the glencid cavity.
were of simple brachydont structure. The limbs were planti-
grade, with five toes, and the carpal and tarsal bones were
serial. The fibula and ulna were not reduced, though the
latter had already lost its connection with the calcaneum.
The femur had a third trochanter, as in modern Perissodactyla.
The tail was long. The humerus, contrary to that of other
Ungulata, had an entepicondylar foramen, resembling that
of Carnivora. The toes appear to have borne blunt claws
rather than hoofs. Phenacodus is the best known genus
to which the modern horse, and hence Perissodactyla, can
MAMMALIA. 567
be traced by a continuous series of forms. Periptychus is
also regarded by many as being at or near the point of
origin of modern Artiodacty/a. On the other hand, many
of the Condylarthra show structural resemblances to the
Hyracoidea especially in the serial carpal bones. Thus
they form the point of convergence for at least three of the
four modern sub-orders. They are all rather small animals
and are found in the Lower Pliocene of Europe and North
America. \
Fig. 387.—THE Dasse (Ayrax capensis).
(From Flower and LyppDEkeEr.)
\ %
enon ya teas IN
pt
we
SUB-ORDER II.—HYRACOIDEA.
This is a small modern sub-order, comprising a few furry
rodent-like animals of the genus Ayrax (Procavia and
Dendrohyrax) and a third extinct genus, Phohyrax. The
first pair of upper incisors grow from persistent pulps as in
rodents, and the others are absent (the second pair being
rudimentary in the young). There are two pairs of incisors
in the lower jaw. Canines’ are absent, but the molars and
premolars are complete, all tending to resemble each other.
The enamel of the molars is folded, the pattern most nearly
resembling that found in the rhinoceros.
568 CHORDATA.
The feet are plantigrade, but the toes have become
reduced in number: there are four on the front foot and
three behind, the third (middle) being the largest. The
carpals and tarsals are serial, as in Condylarthra. The
fibula is complete and has acquired an articulation with
the astragalus.
The stomach is slightly constricted into two chambers
and there is a fairly large ceecum. There are also two,
peculiar, paired, conical czeca attached to the large intestine,
which are not known to occur in any other mammals. The
placenta is said to be zonary and deciduate. Myrax
inhabits rocky grounds and extends from Syria to Cape
Colony; in the latter place it is known as the “ dasse,” or
“klip das,” in the former as the daman. Dendrohyrax is
arboreal and is found in East Africa.
The recent discovery of Plohyrax in the Lower Pliocene
of Europe (Samos) has added yet more interest to these
extraordinary little animals. PZohyrax, known only by the
skull, was larger than AHyrax and more generalised. Thus,
in addition to the large median incisors there were also
two smaller ones and a canine the latter in shape resembling
a premolar. Hence, in the upper jaw at least, the dentition
was 3.1.4.3. The premolars differed somewhat from the
molars. Judging by the peculiar position of the anterior
and posterior nares and the orbits, PZohyrax was probably
amphibious, if not completely aquatic. As already indi-
cated, the Myracoidea are probably an offshoot from a
condylarthrous type which have retained many primitive
characters.
SUB-ORDER III.—PROBOSCIDEA.
The elephants differ from the other Ungulata so much
that they have to be placed at the least in a sub-order apart.
The most important anatomical characters are these :—
The nose produced into a long proboscis or trunk; one pair
of upper incisors forming long tusks; molar and premolar
teeth large and polylophodont, showing horizontal succes-
sion; fibula and ulna complete; the carpals and tarsals serial
and five toes present; placenta zonary and non-deciduate
and mammee pectoral. .
MAMMALIA. 569
_ The proboscis forms a “limb” capable of almost any
diversity of movement and function. Its presence and use
involves a shortening of the neck and a raising of the
occipital crest of the skull: This is effected by the growth
of a mass of bone, lightened by a number of enclosed air
sinuses. In this manner the muscles for raising the head,
inserted in the occipital region of the skull, obtain sufficient
leverage to support the weight of the trunk and tusks.
These latter are true incisors, though during development
they move from the premaxillary to the maxillary region.
Fig. 388.—SurFACE VIEWS OF A SINGLE MoLaR TOOTH OF
(A) THE AFRICAN AND (B) THE INDIAN ELEPHANT.
Note the polylophodont enamel ridges in each, worn by attrition into flat crests.
They have a tip of enamel which is soon worn off and
the tusk then consists of dense ivory or dentine.
The molar teeth consist of a vestigial first premolar, only
found occasionally, and six others, or making a normal
dentition of 1933, but each tooth is of enormous size and
they succeed each other in horizontal succession, only two
being generally in use at the same time. There appears to
be no milk-series, hence the Proboscidea are monophyodont.
Each tooth is polylophodont, ze., with many transverse
ridges. If we start with the multitubercular tooth and
gradually form a number of transverse ridges by union
570 CHORDATA.
across the tooth, we produce a molar not unlike that of
some fossil Aastodons. The ridges are then filled up by
the addition of cement, and further deepening of the valleys
and multiplication of the ridges would produce the tooth of
the elephant. The worn surface presents crests of enamel,
between which are alternate layers of dentine and cement.
It should be specially noted that the elephant’s molar is
produced from the simple brachydont multitubercular type
by a similar and parallel series of processes to those in the
ox and horse, consisting of (1) multiplication of enamel
crests ; (2) heightening of the tooth to allow for wear; (3)
addition of cement.
The limbs in elephants show primitive characters.
Although the clavicles are lost and the femur has no third
trochanter, the radius and ulna are quite distinct and per-
manently crossed and the fibula is well formed, articulating
with the caleaneum. The animal is practically plantigrade
and moves slowly; the carpus and tarsus are not twisted
nor interlocked to form alternate rows, but are serial. Each
toe has a small broad hoof, the weight of the body being
borne on the sole or pad of the foot. Elephants are strictly
herbivorous, feeding principally on the leaves of trees, such
as the mimosa. Their stomach is simple and there is a
large caecum. :
Family I.—Elephantidz.—The modern elephants are found in the
Oriental and Ethiopian regions. The molars of the African Elephant
have diamond-shaped ridges, the ears are larger and both sexes have
tusks. The Mammoth (Ziephas primigentus) flourished in recent
times in Europe, N. Asia and parts of America. It had a woolly
coat, enormous curved tusks and broad deep molars. Other fossil
elephants of the Pliocene and Pleistocene connect modern elephants
with the mastodons. These had large straight tusks and in some the
molars were tubercular. In many there was a small pair of lower incisors.
Mastodons first occur in the middle Miocene and extend throughout
Pliocene in Europe and into the Pleistocene in N. America. They
are important, as they clearly show us the lines along which the
elephants have been evolved from a primitive ungulate stock.
Family II.—Dinotheridz.—In Dénotherium, an elephant-like
animal of the Miocene and Pliocene, the /ower incisors hung down-
wards below the chin as a pair of long tusks. The molars were
bilophodont or trilophodont and had no horizontal, but a regular
vertical, succession.
MAMMALIA. 571
SUB-ORDER IV.—PERISSODACTYLA.
A good deal has already been said concerning the
FPerissodactyla in the chapter upon the Horse and Ox, in
which this sub-order is contrasted with that of the Avzo-
dactyla (page 509).
The main structural features of the sub-order are as
follows :— (1) The molar teeth are bilophodont, or with
complex crowns derived from the bilophodont condition.
(2) Dorso-lumbar vertebrz, usually twenty-three in number.
(3) The femur has a third trochanter. (4) In the skull the
nasals are large and there is an alisphenoid canal. (5) The
carpus and tarsus are alternate and the toes are never more
than four, mostly three or one, but in all cases the main
axis of support passes through tibia, astragalus, navicular,
and third toe. (6) Stomach simple. (7) Diffuse placenta
and mammee inguinal.
The molar teeth pass, in the group, from the simple
brachydont bilophodont condition (derived, as shown, page
462, from the tubercular type) to the complex hypsodont
type with cement added.
The third trochanter is preserved in this group from the
early condylarthrous ancestors, and the disappearance of
the toes can be traced upwards within the group. No
modern Perissodactyla have five toes, but the tapir has four
in the fore-foot, the pollex being lost, the rhinoceros has
three and the horse merely the one. The main axis passing
through tibia, astragalus, navicular and third toe, it naturally
follows that the fibula is reduced or at least loses its articu-
lation with the calcaneum, and the astragalus has nearly all
its distal articular surface attached to the navicular. In the
front-limb the os magnum becomes more and more pro-
minent as the third toe usurps the functions of the others.
In the simplicity of the stomach and the diffuse placenta
the Perissodactyla appear to present more primitive char-
acters than the Artiodactyla. (As has been noticed, there
has been a great deal of parallel evolution in these two sub-
orders. The common characters thus acquired form a basis
for the institution of the group Ungulata Vera containing
these two sub-orders, in contrast with the three preceding
sub-orders as Sub-Ungulata. Such a classification, based
upon parallel evolution, must, however, be unnatural.)
572 CHORDATA.
Family I.—Tapiridae.—These interesting animals, the tapirs, are
found in swampy forest districts of Brazil and of Malay. Hence
they form an instance of discontinuous distribution of a family. They
form the base of the present-day Perdssodactyla as they have } toes and
the teeth are bilophodont and brachydont. The upper molars show.an
external ridge connecting the two transverse ridges, thus approaching
the rhinoceroses; the incisors are of average length and the third
lower one resembles a canine. The dental formula is $443. There is
a very slight proboscis or trunk. They feed upon the leaves and young
shoots of trees.
Fig. 389.—Tue AMERICAN Tapir (Zapirus Americanis).
(From Fiower and LyDDEKER.)
Tapirs occur in Europe and Asia in the Miocene strata, thus explain-
ing the discontinuous distribution in this instance by a dying-out of the
intermediate portions of a once widely and continuously distributed form.
Family 1I.—Rhinocerotidz.—The rhinoceroses form a transition
family between the tapirs and horses. They are found in forest regions
of the Ethiopian and Oriental regions. They can move rapidly on fairly
hard ground and have three toes and hoofs on each foot. The teeth
are slightly more complex than the typical bilophodont. The two
transverse ridges are curved backwards, forming crescentoid ridges,
whilst they are connected externally by a longitudinal ridge. The
MAMMALIA, , 573
crowns are still low and there is little or no cement. The incisors are
few and rudimentary and the upper canines are absent. The nasals
bear an unpaired ‘‘horn” of purely epidermic origin and having no
horn core. In the two-horned species the second and smaller horn is
carried on the frontals: this species is African. The upper lip is long
and prehensile and the skin is very thick and hard with little hair. The
food consists of herbage and leaves of trees.
Family III].—Equidz.—Little need here be said of this family (see
Horse). The horses are essentially graminivorous inhabitants of hard
upland plains. The teeth are hypsodont and the crowns are extremely
complex, though to be derived from the bilophodont type. Cement
fills up the spaces between the ridges. The third toe alone remains
and bears a hoof, the second and fourth metapodials being represented
by two splint bones.
The pedigree of the horse can be traced from Condylarthra (Phen-
acodus). (See page §23.) The fossil ancestors of the horse are hard
to classify as they are gradational, but the Paleotheritde is a family
often constituted for Palgeotherium, Anchitherium and other forms,
which, as a rule, were at about the level of the rhinoceros in the
structure of their teeth and toes. The earlier types of the Zocene,
such as Pachynolophus and its allies, form the family Lophiodontide.
They have still more generalised characters and connect the Perdsso-
dactyla with Condylarthra.
Thus this sub-order Perissodactyla forms a remarkable field for the
study of evolution. One important point we may notice before leaving
it. The tapirs and rhinoceroses take in many structural points a lower
level than many forms which have perished. For example, Hipparion
was a horse-like type of the Pliocene, which certainly comes within the
range of the Zyuzde, and the question often arises—How is it that
these lower forms (tapir and rhinoceros) have survived and ‘‘ higher”
have become extinct? Put more generally, the question becomes—
How is it that primitive animals still survive contemporaneously with
the higher types? Leaving. out of count special explanations applying
to cases like the Australian Ae¢atheria, the general explanation is :—
(1) Species survive only so long as they are in structural harmony with
their environment. (2) Environments change rapidly, but ‘ancient ”
environments exist at the present day as well as ‘modern or up-to-
date” environments.
Hence the widely-scattered tapirs of the Miocene are now found only
in the low-lying swampy forest land for which their structure is suited 5
in the regions where now the open grassy plains have become predom-
inant the tapir died out, to be replaced by horse-like types more suited
to the changed surroundings. The soft ground and the arboreal diet are
complimentary to the numerous toes and the simple teeth of the tapir,
whilst the hard level ground and siliceous grass calls forth the limb
with single axis and the deep, complex, cemented teeth of the horse.
In response to the environmental factors which have changed, such
as the presence of large Carnzvora, these primitive types have also
evolved horns (rhinoceros), or incisor tusks (elephant), or have adopted
an arboreal or fossorial habit (yrax).
Bs
574 CHORDATA.
Hence we find that gradational adaptive structure in living forms is
mainly due to ‘ gradational” environments, and that in fossil forms it
is due to gradual change of environment.
SUB-ORDER V.—ARTIODACTYLA.
The Artiodactyla form a large branch or assemblage of
Ungulata, which in many respects show parallel evolution to
the Perissodactyla. They follow, however, rather different
lines:—(1) The molar teeth are bunodont or selenodont.
(z) The dorso-lumbar vertebrae are nineteen. (3) The
femur has no third trochanter. (4) No alisphenoid canal
and small nasals. (5) The carpus and tarsus are alternate
and the toes are four or two; the main axis is between the
third and fourth toes. (6) The stomach may be simple or
complex and the placenta diffuse or cotyledonary.
One division of the Artiodactyla retain the bunodont
teeth (Bunodonta), only multiplying the number of the
tubercles, whilst the other division (Se/enodonta) have the
tubercles twisted into crescents or curves and worn down,
thus producing the selenodont type. As in the Pevisso-
dactyla, there is the addition of cement and the heightening
of the crowns.
The femur appears to have lost its third trochanter very
early in the history of this sub-order. The toes show the
same gradational reduction as in Perissodacty/a, but on a
different plan. The third and fourth toes are always equal
and larger than the second and fifth. These latter are
hoofed and touch the ground in pigs, but are greatly re-
duced in sheep and oxen and disappear altogether in the
camel. It follows from the main axis passing between the
third and fourth toes that the cuboid and ectocuneiform
tend to be more or less equally developed, and that the
astragalus articulates equally with the cuboid and navicular
whilst the fibula, or its distal end, still remains in articula-
tion with the calcaneum. The cuboid often fuses across
the middle line with the navicular.
Family I.—Hippopotamidz.—The Hippopotamus is confined to
the rivers of Africa. Its canines and incisors are large and grow from
persistent roots. The molars are of a slightly modified bunodont type
and each tubercle wears into a three-lobed crown. The stomach is
complex and the diet herbivorous. All four toes (first is absent) are
MAMAIALIA. 575
present, the hoofs are not compressed and there is no fusion of the
metapodials. In Pliocene and Pleistocene times the Auppopolamide
were found throughout Eurasia.
Family II.—Suide.—The pigs have « bunodont dentition with
many tubercles which, when worn, form irregular crowns. The canines
grow from persistent pulps and form tusks. The dental formula is
Fig. 390.—THE AFRICAN WATER-CHEVROTAIN
(Dorcatheritim aguaticun).
(From FLower and LyDDEKER.)
” typical $144. The stomach is simple and the diet omnivorous. All
four toes are present, but the second and fifth are shortened up and the
hoofs of the third and fourth are compressed into the middle line, forming
the ‘‘cloven hoof.” The metapodials and tarsal bones are, however, not
yet fused and the ulna and fibula are still unreduced. The placenta
is diffuse. The typical pigs are confined to the old world, but the
peccaries (Dicotyles) are found in South America; they differ in
dentition from the true pigs.
“ Family I1I.—Tragulide.—This is a small family of little Ungulata
called the chevrotains. In dentition they most nearly resemble the Pecora
576 CHORDATA.
as there are no upper incisors. There is, however, a pair of well-
developed upper canines. The molars are selenodont. The stomach
is complex, Jacking only the manyplies of the Pecora. The ecto-
cuneiform, navicular and cuboid bones fuse in one, and in most the
third and fourth metapodials fuse together. The chevrotains resemble
the Swzde in having a diffuse placenta and in the presence of a complete
fibula, whilst in one genus, Dorcatherium, the third and fourth meta-
podials are not fused. The chevrotains ( 7ragzlus) are found in the
forests of the Oriental Region and the water-chevrotain ( Dorcatherium )
is found in West Africa.
They are an interesting family, showing anatomical characters partly
resembling the Swzde and partly the Pecora. In the complete fusion
of distal tarsal bones they go beyond both these families. Dovcathertum
is found in the Miocene and Pliocene of Europe and India.
Family IV.—Camelidz.—The camels form with the American
llamas and their allies a natural family. They have three pairs of
upper incisor teeth in the young, but all except the third incisor are lost
later. Canines are present and the molars are typically selenodont.
The loss of the two pairs of upper incisors foreshadows the condition
found in the Pecora. The stomach has only two compartments
corresponding to the first and fourth of the Pecora. The tarsal and
carpal bones are distinct and separate, but the third and fourth meta-
podials are fused to form a ‘‘cannon bone.” The third and fourth
toes are alone present and the weight is borne upon pads under
the penultimate phalanges; the small nail-like hoofs do not touch
the ground. The placenta is diffuse. The camels are indigenous to
Western and Central Asia. In South America are found the closely
allied and similarly domesticated Hama (Auchenia) and the alpaca,
with their wild relatives, the guanaco and vicufia. They inhabit
mountainous regions and are domesticated for their wool.
Family V.—Pecora.—The ecora are the most important family of
Ungulata, comprising deer, antelopes, sheep, oxen and the giraffe.
They have the following characters in common, with isolated exceptions.
The upper incisors and canines are lost and replaced byahard pad. The
molar teeth are selenodont and show every gradation from brachydont
to hypsodont types. The stomach is complex, with four compartments
(see Ox, page 514). The cuboid and navicular bones are fused and the
third and fourth metapodials are fused to form the ‘‘ cannon bone.”
There are usually only traces of the second and fifth toes. The fibula
is completely fused to the tibia and the ulna to the radius. Most early
fossil Pecora and a few modern types (musk-deer) have no processes of
any kind on the head, but the majority of modern forms have paired
tony processes attached to the frontal bones. These may be small and
permanently covered with hair, as in the giraffe, or they may when
complete consist of naked bone and are then known as antlers, as in
deer: these antlers are shed annually. Lastly, the bony core may
form a central support for a hollow ‘‘ horn” of epidermic structure. The
horn is never (except in the American Prongbuck) shed and grows
perpetually from the base. The young deer has no frontal processes,
MAMMALIA. 577
Fig. 391.—Manus OF ARTIODACTYLA. (Ad nat.)
A Ulna. B Ulna. Cc Ulna,
Line of Fusion of Metacarpals 3 and 4.
Line of Fusion of Metacarpals 3 and 4.
A, The Pig, third and fourth metacarpals are free and there are four functional
toes. B, Dorcatherium, closely resembling the pig. C. Tragudus, with third and
fourth metacarpals fused, second and fifth still entire. D, Deer. E, Sheep, and F,
Camel, showing gradual disappearance of second and fifth toes and of ulna.
578 CHORDATA.
but these gradually arise as small protuberances covered with hair or
“velvet.” When the antlers are full-grown the ‘‘ velvet” isrubbed off
by the deer by friction against trees or other objects until the bony
antler alone remains. The branches of the antler are called ‘‘ tynes,”
and in those species with many tynes the number of these increases every
year. Antlers are usually confined to the male sex.
The musk-deer (A/oschus) and the water-deer (Hydropotes) have no
antlers in either sex, but, on the other hand, they retain the upper canine
teeth as long sharp tusks. '
In all the Pecora the placenta is cotyledonary, a specialised derivative
of the diffuse.
The true deer are not found in the Ethiopian region, their place
being taken by the ‘“‘horned” antelopes. To this region are confined
the giraffes. The sheep, oxen and goats are more or less northern
forms, the north temperate regions of Eurasia and N. America being
their headquarters.
The above five families of Artiodactyla are intimately
connected by numerous fossil forms.
ORDER XI.—Cefacea.
The porpoise has been described as a typical aquatic
mammal and it also serves as a type of the order Cezacea.
Under the heading of the porpoise we have noticed the
adaptations to an aquatic life which constitute the main
peculiarities of the Cefacea. These consist of the follow-
ing :—
1. Fish-like shape, with dorso-ventral coloration.
2. Loss of hair and external ears and formation of
“ blubber.”
3. Fore-limbs formed into fins, hind-limbs lost and tail
forming a fin.
4. Homodont dentition (fish diet).
5. Modification of nostrils to form vertical blow-hole and
prolongation of larynx.
6. Retia mirabilia.
4. Loss of salivary and lacrymal glands.
In addition, we may note the well-convoluted cerebrum
of the brain and the abdominal testes. The stomach is
usually somewhat complex, though the whole order is essen-
tially carnivorous—an important distinction from the Sivenia.
The uterus is bicornuate and the placenta, like that of many
Ungulata, is diffuse and non-deciduate.
MAMMALTA, 579
The Ce¢acea are usually gregarious and are widely dis-
tributed marine mammals. They are divided into two sub-
orders, the Odontoceti and the Mystacocett, which are widely
apart,
SUB-ORDER I.—ODONTOCETI.
The Odontoceti (toothed-whales) comprise the families
of the sperm-whales (Physeteride), the gangetic dolphins
(Platanistide) and the dolphins (Delphinide). They have
_a great number of homodont monophyodont teeth. They
are more adapted to aquatic habits than the AZpstacoceté in
one or two respects, such as the entire loss of the olfactory
organ and the formation of a single external nas.
The Physeteride are large predaceous marine forms,
such as the sperm-whale. The Platandstide comprise the
estuarine or freshwater river-dolphins, such as the blind-
dolphin of the Ganges, The large family of the Dehinide
includes the dolphins and porpoises of European seas, the
narwhal (AZonodon) of Arctic seas, with a single twisted tusk
formed of a left upper incisor, and the “ killers” (Orca).
SUB-ORDER II.—MYSTACOCETI.
The Afystacoceti (baleen-whales) have teeth only in the
embryonic young, which never become functional. They
are replaced by a row of baleen-plates suspended from the
upper-jaws, forming the so-called “whalebone.” Their
edges are frayed and they act as a sieve for separation of
the food from the water. The head, especially the facial
portion, is enormously developed, and the rami of the lower
jaw are only connected by ligament. The whales feed upon
small pelagic organisms, such as pteropods and certain
Crustacea. The buccal cavity is huge and becomes filled with
sea-water containing such pelagic organisms. The former is
then driven out between the baleen-plates by elevation of
the tongue, the latter being retained and swallowed. In
Mystacoceti the ribs are attached to the transverse processes
of the vertebree only, and only one pair meet the small
sternum, features which give the baleen-whales a greater
freedom of respiration than the Odontocet.
On the other hand, the external nares are paired and
partially covered by the nasal bones and there is a distinct
580 CHORDATA.
olfactory organ. In these respects the AM/ystacoceti are not
so completely adapted to aquatic habit as the Odontocett.
OrDER XII.— Carnivora.
The dog and cat have been taken as types of the order
Carnivora. They really represent the highest of the Carni-
vora, and the characters of the order are somewhat wider
than those deduced from these two types. As in the case
of the Ungulata, they present a series in which certain
structural characters graduate from one end to the other.
They have chiefly to be distinguished from the Zusectivora
and, in a more remote degree, from the Ungu/ata.
The great majority are carnivorous or flesh-eaters and
are terrestrial cursorial types. They have usually at least
four toes, which are armed with claws or unguiculz, never
hoofs or unguee, as the limbs are nearly always called upon
to perform other duties than locomotion.
The diet reflects itself in the dentition. They are always
diphyodont and may have a large number of teeth. The
teeth never have persistent pulps, the canines are always
prominent, long and pointed; the incisors are usually 3,
small and pointed, and the molars are usually cusped with
cutting edges, often tritubercular. The enamel is usually
little worn and there is no cement.
There is always a more or less prominent postglenoid
process of the squamosal, preventing backward motion of
the mandible, and the condyle is transversely elongated ;
these modifications being connected with the “grip” as
described in the “ Cat” and “ Dog.”
The stomach is simple and the intestine comparatively
short, with a short or simple cecum. The uterus is bi-
cornuate and the placenta zonary and deciduate.
Other skeletal characters to be noticed are the almost
entire absence of the clavicle, the complete condition of
radius, ulna, tibia and fibula, the fusion of the scaphoid and
lunare bones into a scapholunar and the common occur-
rence of an entepicondylar foramen (in the humerus).
All the Carnivora show a well-convoluted cerebrum
which partially covers the cerebellum.
As in the case of several orders, the Carnivora are
sharply divided into two sub-orders, differing mainly in their
MAMMALIA. 581
habits and the structural modifications involved. The sub-
order Sissipedia are terrestrial and the /innifedia are
aquatic.
SUB-ORDER 1.—-FISSIPEDIA.
The Fissipedia (or Carnivora Vera) have always the full
complement of incisors (3), and one of the cheek-teeth in
each jaw is formed by the carnassial tooth (see page 525).
The limbs are formed for terrestrial locomotion and, as in
the typical pentadactyle limb, have the third digit as long as,
or longer than, the rest.
The present day /issifedia can be divided into the
A luroidea, Cynoidea and Arctoidea, having affinities with
the cats, dogs and bears respectively.
The -£luroidea are the most specialised. Their teeth
are reduced in number and the skull is shortened. They
are nearly all digitigrade. The characters of the auditory
region are found to form a useful distinction between these
and the other two divisions. Thus in the 4/urordea the
auditory bulla is large, divided into two by an internal bony
septum and partially covered externally by the paroccipital
process of the exoccipital bone.
Family 1.—Felidae.—The Fede comprise the true cats, with re-
tractile claws. Amongst them are the lion and leopard of the Ethiopian
and Oriental regions, the jaguar of Neogcea, the tiger of Asia, the puma
of America and the wild-cats and lynxes of Europe.
Family 2.—Viverridae.—The Viverride comprise the civets and
mongooses, found only in Arctogoea. They have more teeth than the
Fehide and non-retractile claws.
Family 3.—Protelidae.—The Protelide consist of a single genus
(Proteles), the aard-wolf of South Africa, a nocturnal burrowing animal
of degenerate necrophagous habits.
Family 4.—Hyenidae.—Lastly, the Hyenide comprise the hyzenas
of Arctogcea, with more teeth than the Fedde, but with no septum to
the auditory bulla.
The Cynoidea have a larger number of teeth (3443) and
longer jaws than the -@/wroidea, in correlation with which
they are less strictly carnivorous. ‘There is only a trace of
an auditory septum and the paroccipital process does not
overlap the bulla. They are mostly digitigrade but never
582 CHORDATA.
Fic. 392.—VENTRAL VIEW OF BEAR’s SKULL x Y.
Note the flattened tympanic bulla, the long palate and the broad molars. The
second premolar has been lost.
b.ty., Tympanic bulla; 0.c., occipital condyle ; 7a., auditory meatus ;
£., glenoid cavity; 7, jugal; 4, paroccipital process.
Fig. 393-—FEET OF BEAR SEEN FROM THE UPPER SURFACE x }.
Note the flat broad sole or palm, the scapholunar bone and the five
complete digits in each limb.
MAMMALIA. 583
have retractile claws. The toes are usually 5, The single
family of the Canzde is of world-wide distribution and com-
prises the dogs, foxes, wolves and jackals.
The Arvctoidea have, like the dogs, a large number ot
teeth (often 3342). They are largely omnivorous and the
molars are tuberculated with crowns worn to a flat surface.
The auditory septum is absent and the bulla itself is
flattened. The paroccipital process is quite free from it
and projects downwards, as in other orders. All are either
plantigrade or semi-plantigrade and there is the full com-
plement of taes.
Family 1.—Ursidae.—The largest forms are the Urside or bears,
which are found everywhere except in Notogcea and the Ethiopian
region.
Family 2.—Procyonidae.—The Procyonide are a small family of
fox-like animals, such as the American raccoons and coatis and the
panda of the Oriental region.
Family 3.—Mustelidae.—The third family, M/ustelide, have a small
number of molars (4) and comprise the otter, the skunk of America,
the badger of the Palearctic region, and a series of small fur-animals,
such as the marten, sable and weasel.
SUB-ORDER II.—PINNIPEDIA.
The sub-order Prnnipedia have the limbs adapted for
aquatic locomotion. The fore-limbs, as in Ce¢acea, form the
paddles or flippers, but the hind-limbs are not aborted but
reflected back to form a double “tail,” the true tail being
correspondingly reduced. They still retain their hair and,
to a large extent, their power of terrestrial progression. All
the digits are retained and the first and fifth of the hind-
limb are longer than the rest, forming a strong edge to the
flipper. Between the digits is suspended a web. The claws
of the hind-limb, when present, are situated on the upper
surface of the digits and do not reach to their ends. The
teeth vary considerably, but the incisor dentition is never
complete and there is no carnassial tooth.
Family 1.—Otariidae.—The eared-seals or sea-lions (Ofariida )
are the most terrestrial. They can place the sole of the hind-limb upon
the ground and thus shuffle along. They are piscivorous in diet and
congregate in herds at the breeding season. Their fur, with the longer
hair removed, furnishes the ‘‘sealskin ” of commerce.
584 CHORDATA.
Family 2.—Trichechidz.—The walruses ( 77ichechide) are Arctic
and of large size. The teeth are blunt and reduced in number, the
adult dentition being #428. The canines are long, forming the tusks :
they grow for some time from persistent pulps. The condition of the
teeth is correlated with the molluscan diet. As in the sea-lion, the
walrus can use its hind-limbs for terrestrial locomotion.
Family 3.—Phocidz.—The seals ( Phoctde) have no pinnz to the
ears and the hind-limbs are permanently bent backwards. Hence the
seals are more exclusively aquatic than the preceding families. The
teeth are of the typical carnivorous type, with cusped ridged molars.
OrvDER XIII.—Jnsectivora.
The mole is a member of this order and has been
described as illustrating the fossorial or burrowing habit.
As implied in the name, the /ysectivora are all feeders upon
insects, worms and other small Znvertebrata. This diet
must of necessity be much more primitive than that of the
Carnivora or the Ungulata, for the invertebrate animals are
antecedent in time to the warm-blooded animals which
constitute the food of the former and to the grasses
devoured by the latter. Hence the Jusectivora appear to
retain many dental features in common with the early
Eocene mammals. Their small size and general habits are
also usually of the primitive terrestrial type, though as in
all primitive groups certain members are very specialised
for particular habits. They are all diphyodont and hetero.
dont, the molars are usually sharp-cusped and of the tri-
or quadri-tubercular types. On the whole, the dentition
most resembles that of certain Carnivora, but the canines
are never sO prominent as in this order. The typical
Eutherian dentition of 3443 is common. In external
appearance a number are closely similar to the Rodentia,
but they never possess the peculiar incisor teeth of this
order. There are always more than two pairs of incisors
on each side of the lower jaw and they do not grow from
persistent pulps. The dental characters of Jvsectivora and
Rodentia are therefore quite distinct.
In the limbs the Zzsectivora are little modified from the
mammalian type. There are five digits on each limb and
they are plantigrade ; in these respects they differ from a
great number of Carnivora, but in addition they nearly all
MAMMALIA. 585
have a well-developed pair of clavicles, bones which are
absent or vestigial in the latter order. Other generalised
features are the presence, in some, of ossified intervertebral
discs (see Mole), and of an episternum and the frequent
occurrence of an entepicondylar foramen and a third
trochanter. The placenta, like that of the Rodenfra, is
discoidal and deciduate.
Many of the Jmsectivora are fossorial or arboreal, but
most are terrestrial They are widely distributed throughout
the Arctogoean realm, but are absent from Neogcea and
Notogeea. In both these realms their place in nature is
occupied by insectivorous Polyprotodontia.
SUB-ORDER I.—DERMOPTERA.
The sub-order Dermoptera is constituted for the
remarkable so-called “‘flying-lemur” (Galeopithecus) of
the Malay Islands. It has a large patagium stretched
from the neck to the fore-limb, between the fingers laterally
to each hind-limb and thence to the tail. It is arboreal
and uses its patagium for “gliding” from tree to tree in
much the same way as Australian phalangers and the flying-
squirrels.
Its structural peculiarities are chiefly as follows :—The
lower incisor teeth are deeply pectinated or cleft and the
second upper incisor and the canine have double roots, the
tibia and fibula are distinct, and there is an intertarsal joint
to allow of the hind-foot being rotated inwards for climbing.
The mamme are axillary.
SUB-ORDER II.--INSECTIVORA VERA.
The sub-order Jnsectivora Vera comprises the re-
mainder of the order, including the moles (Za/a) found
in the temperate parts of Eurasia, the hedgehogs (Z7inaceus),
with great numbers of spines in addition, confined to
Europe, Asia and Africa, the shrews (Sorex) of the Hol-
arctic region, closely resembling mice in external appearance,
the tree-shrews (Zupaia) of the Oriental region and the
jumping-shrews (AMacroscelides) of the Ethiopian region. In
all these five families the molar teeth are multi- or quadri-
tubercular, presenting a broad crown. The other four
586 CHORDATA.
families are the water-shrew (Potamogale) and the golden-
moles (Chrysochloris) of the Ethiopian region, the tenrec
(Centetes) of Madagascar and the mole-like Solenodon of
West Indies (strictly speaking, comprised in the Neogoean
realm). These families retain the more primitive trituber-
cular teeth with a V-shaped cutting edge.
ORDER XIV.— Chiroptera.
The fox-bat has been used as an illustration of the
Chiroptera. They are evidently closely allied to the
Lnsectivora but have the fore-limbs modified for flight, the
test of the skeleton also undergoing important modifications
which have been noticed under the type. They resemble
the Znsectivora in their simple brain (the cerebrum having
few conyolutions and not extending over the cerebellum), in
the abdominal testes and in the discoidal and deciduate
placenta.
SUB-ORDER I.—MICROCHIROPTERA.
The sub-order Microchiroptera comprises a number of
smaller insect-eating bats, with cusped molars and with
greater adaptation for flight than the other sub-order, as
shown by the presence of a claw on the first digit only
and the part taken by the tail in the formation of the
interfemoral membrane (see page 553). The common
British bats and the South American vampires belong to
this sub-order.
SUB-ORDER II.—MEGACHIROPTERA.
The sub-order Megachiroptera comprises the large
frugivorous bats typically represented by the Preropodide.
They have flat cuspidate or comparatively smooth molars,
a claw on the first two digits of the manus and an inter-
femoral membrane free from the tail. The Preropodide
have a peculiar distribution, being found in Australia, the
Oriental region and Madagascar.
ORDER XV.—Pyvimates.
The Primates stand at the head of the orders of AZam-
matia and of the animal kingdom. They are essentially
MAMMALIA. 587
generalised and belong to the transition arboreal group.
The possibilities of movement in the pentadactyle limb and
vertebrate skeleton are seen in this order at their maximum.
Many of the order are omnivorous, though a frugivorous or
insectivorous diet is common. The incisors are usually
reduced to $ and may be 4; they are commonly chisel-shaped.
The canines are mostly longer than the incisors and nearly
always present. The cheek-teeth are usually quadrituber-
culate and have flat grinding crowns.
In the limbs the five digits are usually all present and
the hallux is with one exception opposable to the other
toes (arboreal). The claws have a tendency to become
flattened into nails. The radius, ulna, tibia and fibula are
all complete and the full movement of supination and pro-
nation is retained. For similar reasons the clavicle is always
well developed and there is little or no fusion of the tarsal or
carpal bones. ‘Terrestrial locomotion is plantigrade.
The orbits tend to face forwards instead of laterally
and they are always complete.
The brain is highly developed, the cerebrum being much
convoluted and covering the cerebellum. Its proportion to
the body is very high (see page 463).
The placenta is either diffuse and non-deciduate or
metadiscoidal and deciduate.
The Primates are, like a good many other preceding
orders, sharply divided into two sub-orders, z.e., the Lemur-
oidea and Anthropoidea.
SUB-ORDER I.—LEMUROIDEA.
The Lemuroidea unquestionably rank lower than the
other sub-order. They are more quadrupedal and in
Eocene strata they appear to gradate into the Jusectivora.
They differ from the Avthropoidea in the invariable
presence of all five digits, in the lengthened facial region
of the skull, the orbit being only separated from the tem-
poral fossa by a (postorbital) bar of bone, not a partition,
and the lacrymal foramen being outside the orbit, in the
lower type of brain with smaller and less-convoluted cere-
brum, in the possession of a diffuse, or dome-shaped, non-
deciduate placenta and somewhat bicornuate uterus.
588 CHORDATA.
Family 1.—Lemuridz.—The true lemurs. Found in Madagascar,
Africa and the Oriental region.
Family 2.—Tarsiida.—Comprising only the peculiar little Zarszzs
of the Malay Islands. Its incisors are?. The proximal tarsal bones are
elongated and two of the hind-digits are clawed.
Family 3.—Chiromyidz.—Another aberrant lemur, known as the
Aye-Aye. It is found in Madagascar, nocturnal and arboreal. It has a
rodent-like dentition with incisors growing from persistent pulps. Its
dental formula is 44%. All the digits are clawed but the hallux which
bears a nail. The third digit of the manus is very long.
Fig. 394.—LATERAL VIEW OF SKULL OF THE
AYE-AYE (Chetromys ).
Note the rodent-like incisors. Dental formula 1943,
Distribution of the Lemuroidea.—The chief feature of
the distribution of lemurs is their extraordinary abundance
in Madagascar. (For an account of this, see page 602.)
SUB-ORDER II.—ANTHROPOIDEA.
The Anthropoidea are advanced types of Primates. They
have a tendency to loss of the pollex; the facial portion
of the skull tends to recede below the cranial and the
orbits look more forwards than those of the Lemuroidea,
being also completely separated from the temporal fosse by
a bony partition. The lacrymal foramen in all Anthropoidea
opeas inside the orbit. ‘The brain is of a higher type, the
MAM AIA LIA, 589
cerebrum being large, well convoluted and covering the
cerebellum. The placenta is metadiscoidal (see page 482)
and deciduate and the uterus is simple.
Fig. 395.—LaTERAL AND VENTRAL VIEWS OF SKULL OF
SEMNOPITHECUS NEMc&us.
(After Dr BLAINVILLE.)
Frontal. Parietal.
Note the shortened facial and expanded cranial regions, the dentition 3}34,
the bony auditory meatus, the suture between frontal and squamosal.
Family 1.—-Hapalida.—The Marmosets of Neogcea (S. America).
They are the most quadrupedal of the Anthropotdea, ~ As an exception
590 CHORDATA.
to the rest of the Przmates they have only two molars. Their dental
formula is 3433. All the digits except the hallux are clawed and the
pollex is present but not opposable to the other digits. They have a
long, bushy tail and are strictly arboreal.
Family 2,—Cebidz.—The American Monkeys. They are confined
to Neogcea, are strictly arboreal and often have prehensile taik.
Their dental formula is 343, hence they differ from the Hapalide in
having an additional molar. They also have a pollex to a large
extent opposable. They include the Spider-monkeys and Capuchins.
Fig. 396.—FRONT VIEW OF SKULL OF A GORILLA.
Note forward position of the complete orbits, the (vertical) sagittal crest,
the two incisors (2) and the rather longer canines.
2.2.
Family 3.—Cercopithecidz.—All this family of Monkeys is found
in the Old World, mainly in the Oriental and Ethiopian regions.
The tail is not prehensile but is often of great length. There are
usually brightly coloured ischial callosities. The pollex, if present at
all, is always opposable, and the front-limbs are always markedly
shorter than the hind-limbs. The dentition is 212%. All the best
known monkeys belong to this family, including the baboons (Cyzo-
cephalus) of Africa, which are not arboreal but frequent rocky regions
in communities, and the familiar Macaques (AZacacus) of Asia.
Family 4.—Simiidz.—The family of Anthropoid Apes. They
are all found in the Old World and comprise the Gorilla: and Chim-
-panzee of equatorial Africa, the Orang of Borneo and the Gibbons of
the Oriental region. They mostly have no tail; there are never ischial
callosities. The pollex is always opposable and the front-limbs always
exceed the hind-limbs in length. The dentition is 3433.
MAMMALIA. 591
The two, families of Old World monkeys differ so markedly from
the two New World families that there is great probability of their
having been independently evolved. The chief differences are as
follows—In the skull the New World monkeys always have three pre-
molars and three or two (marmosets) molars, an auditory bulla with no
bony auditory meatus; the alisphenoid is suturally united with the
parietal to the exclusion of the squamosal from the frontal. In the Old
World monkeys there are always only two premolars and three molars,
there is no auditory bulla, but there is a bony auditory meatus and the
squamosal has a sutural connection with the frontal.
Fig. 397,—BoNES OF THE ANKLE AND Foot oF GorILLa.
Note the opposable hallux and shortness of the “‘ instep.”
Family 5.—Hominidee.—Man is now usually regarded as forming a
zoological family of the Primates. He differs anatomically from the
other families in the very high development of the brain, in the great
proportionate length of the hind-limbs, the non-opposable hallux, the
curvature of the spine and other minor features correlated with an
upright gait. His dentition is 343§, but differs from that of all monkeys
in having an even series of teeth with no diastema.
Distribution of the Anthropoidea—The occurrence of
two differing series of monkeys in the Old and New World
592 CHORDATA,
respectively has already been noticed. The last family is of
course at the present day cosmopolitan. A fragmentary
fossil from the East Indies, called Anthropopithecus erectus, is
said to be a link between-man and the anthropoid apes, the
Fig. 398.—ENTIRE SKELETON OF THE GORILLA.
(De BLaInviLie.)
ae
A):
AG
Note opposable hallux and long fore-limbs.
chief evidence being based upon the cranial capacity and
relative brain-weight. Fossil Azthvopoidea are found as far
back as early Miocene, but they still have their “family”
characters.
MAMMALIA, 503
CHAPTER XXXI.
GEOGRAPHICAL DISTRIBUTION OF
MAMMALIA.
AMMALS, with the exception of the erial (Cirvop-
Lh tera) and aquatic (S¢venza and Cetacea) types, lend
themselves specially to the solution of geographical problems,
because, as a rule, a strait of water of a few miles (twenty or
so) forms an effective physical barrier to their migratory
progress. Hence the first important fact of mammalian
distribution is their entire absence from (1) all oceanic
islands, z.e., from islands raised above the level of the sea
by volcanic agency or by the growth of coral; and (2)
all islands which were separated from the mainland at a
date antecedent to the evolution of mammals (e.¢., New
Zealand).
Leaving these islands out of consideration, we find that
there is great diversity in the occurrence of Mammata in
certain districts. This diversity, like that of organic struc-
ture, must be primarily due to diversity in the physical
environment. ,
It must be remembered that certain mammals are adapted
for certain habitats. Thus arboreal forms are confined to
forest lands, others to the open plains, and so on. The par-
ticular kind of habitat affected by a mammal is called its
station, and as these natural conditions recur throughout all
the large regions, they do not affect the general problems of
geographical distribution. As an example, if we say that
marmosets are characteristically found in South America, we
do not mean to imply that they occur in the open “ pampas ”
of the Argentine, but that having a forest s¢azzon they usually
occur in the forests of South America.
Coming to the prime physical factors which govern the
spreading or distribution of mammals, we find that they act
through one of the two primary functions of locomotion and
food.
M. 39
594 GEOGRAPHICAL DISTRIBUTION
In the case of locomotion, certain mountain ranges offer
effective barriers to certain mammals, the physical difficulties
being impassable. Again, a comparatively narrow strait
of water may act as an effective barrier to the great majority
of mammals.
As regards food, the whole mammalian class is either
directly or indirectly dependent upon vegetable food and
the great determining factor in the distribution of plants is
temperature. It is probable that the direct effect of tem-
perature upon mammals is not very potent, as their hairy
covering with its possible variations allows of great latitude,
but the indirect effect through plants is very marked. Thus
many mountain ranges act as barriers more by virtue of their
great altitude than by mechanical difficulties, and ranges
parallel to isothermals.are more effective than those in other
directions. Were there no other physical elements of
diversity than temperature, it is probable that the herbivorous
mammals would be evenly distributed in zones, according to
the isothermals or lines of equal temperature.
Deserts may act, through absence of food and water, as
effective barriers, as, for example, in the case of the Sahara.
The difficulties are multiplied when we recollect that
these factors of water-isolation, rock-isolation, and sand-
isolation are. like all physical phenomena only transitory, and
therefore act only for certain periods The present distri-
bution of mammals cannot be satisfactorily explained by an
appeal to the present isolative agencies, just as the present
environmental factors of an organism will not account for
its structure. In other words, the fauna of a given area
is determined, firstly, by its past physical history and,
secondly, by its present physical condition. Hence we
must, in dealing with the characteristic fauna of the great
realms, take into consideration their past as well as their
present.
Throughout the Triassic and Jurassic the reptiles were
the dominant group, and certain of these, the Anomodontia
(with heterodont teeth), appear to be closely allied to the
amphibio-reptilian-like ancestors of the mammals. It is in
the higher strata of the Triassic that the 4/otheria (Proto-
theria) first make their appearance, together with certain
types which may be Polyprotodontia (Metatheria). The
OF MAMMALIA. 505
Allotheria can be traced through the Jurassic and possibly
into the Eocene (Tertiary). The Polyprotodontia can also
be traced through Jurassic and Cretaceous into the Ter-
tiary. Through the Tertiaries can be traced certain of the
Metatheria (Didelphide) to the present day and the
eutherian types first occur in the Eocene. In the Eocene
strata there are abundant remains of many £utheria, in
marked contrast to their absence in the Cretaceous.
Let us now glance at the present distribution of mam-
mals. The geographical world is usually divided up into
three zoological realms :—
t. Notoca@a—comprising Australia, New Guinea, Poly-
nesia and New Zealand and certain of the Malay
Islands.
2. NEoGa@Aa—comprising South America, West: Indies
and part of Central America,
3. ARcroc@Ha—North America, Eurasia and Africa.
This is a very unequal division of the world’s surface,
but is justified by the quality of the faunistic differences in
each region.
1. Noroca@a.—In this realm we may, from a mammalian
point of view, leave out of consideration New Zealand and
Polynesia, for, with the exception of bats and a rodent or
two, they have no mammals. The realm has an entire
monopoly of one sub-class of mammals, the Prototheria. Of
the AZefatheria, it contains all the order Diprotodontia (with
one exceptional family in South America) and four out of
the five families of Polyprotodontia. Of the third sub-class,
Liutheria, there are extremely few representatives. There
are seven genera of Kodentia (Muridz) and the dingo or
native dog, together with many bats, and a pig in New
Guinea. 7
Notogcea is essentially a Prototherian and Metatherian
world. Here the Me¢atheria reach an extraordinary diversity
in structure and show adaptations closely resembling those
met with in the Lutheria elsewhere. The realm gradates to
the south-east of Asia by a series of islands of the Austro-
Malay region, and here the characters of Notogcea and
Arctogcea merge more or less sharply into each other. The
596 GEOGRAPHICAL DISTRIBUTION
line called Wadlace’s line dividing the two realms passes
northwards between Bali and Lombok, between Borneo and
Celebes and eastwards of the Philippines. Celebes has,
however, some claims to be regarded as belonging to
Arctogeoea.
We may naturally ask—What is the meaning of this
faunistic character of the realm Notogcea? How has this
realm come to be a sort of “preserve” for the two archaic
sub-classes to the almost entire exclusion of the third and
more highly organised sub-class? The past history of
Notogcea does not help us much, for its geological strata
have not been sufficiently investigated. Remains of the
modern Monotremes are found in the Pleistocene of
Australia. The same is true of the Ae¢atheria, including
the large extinct forms of Dzprotodon and Thylacoleo.
Further back than the Pleistocene, or at least the Pliocene,
we know nothing of the fossils of Notogcea. On the other
hand, we find that remains of Mefatheria occur in Europe
and North America from the Triassic through Jurassic
to Cretaceous. Indeed, Europe and North America,
and possibly Asia by inference, were largely peopled by
Metatheria during the secondary period; but, we have as
yet no evidence of Eutherian mammals during this
period. Hence in these important respects the fauna of
Notogcea at the present day resembles that of Eurasia in
the secondary period. The inference is that at that period
Notogoea (Australia and New Guinea) was connected by
land with the Eurasian continent, and further, that this land
connection was broken at the dawn of the Tertiary epoch
before the Eutherian mammals were evolved. ‘The connec-
tion having never since been restored, the Afe¢atheria in
Notogcea have been free to become modified and adapted
into their numerous types now existing. The few Eutheria
now existing in Notogcea have on this assumption effected a
crossing either in canoes (man and probably the native
dingo), or timber (the M/uride), or by flight (the bats). Man
has more lately introduced the Eutherian rabbit and sheep,
besides other types, and the rabbit at least appears to be
making up rapidly for the time lost since the early Tertiary,
during which it has been excluded from the district. If
we may regard the AMotheria as true Prototheria, we may
OF MAMMALIA. 597
assume much the same history of events as above to have
occurred in the secondary strata of Europe, America and
Africa, and possibly in the Eocene of America. ‘The
Monotremata have, however, had to compete with the
Metatheria even in Notogcea and have only survived in
small numbers.
It is well to note that the regions of Notogcea form more
or less of a gradation. (1) Polynesia has practically an
oceanic fauna and there are no mammals but bats. (2)
New Zealand is equally bare of mammals but has more
reptiles and birds. (3) Australia has Prototheria and
Metatheria with a few incidental Eutheria. (4) Austro-
Malaysia has more Lutheria, ¢.g., the pig, and approximates
to Eurasia in faunistic character. The best line of separa-
tion between Notogcea and the rest of the world would
pass to the east of Celebes, but the demarcation is merely
arbitrary. The chief point about the mammalian fauna of
Notogcea is that it essentially belongs to the two lowest sub-
classes to the almost entire exclusion of the third. The
assumption in explanation is that Notogoea has been isolated
from the rest of the world before the evolution and spread
of the last sub-class, but not before the two lowest sub-
classes had spread downwards from the north.
Extant Mammalia of Notogeea.
SUB-CLASS, ORDER, FAMILIES,
Prototheria Monotremata 2 (Duckmoles, Echidne.)
Metatheria Diprotodontia 3 (Kangaroos, wombats,
phalangers. )
Poly protodontia 3 1c (Dasyurus, bandicoots,
; marsupial-moles. )
Eutheria Rodentia I 7 Species.
Carnivora rr
Ungulata IT
Chiroptera s Large number.
2. Nroca@a.—This realm contains the remainder of the
sub-class Ae¢atheria, not found in Wotogwa. These are the
opossum-rats and the opossums, representatives respectively
of the orders Dzprotodontia and FPolyprotodontia. Of the
Eutheria there is no lack in point of numbers. It has a
monopoly of one sub-order, the Xezarthra, or sloths, anteaters,
598 GEOGRAPHICAL DISTRIBUTION
and armadillos, whilst there are also an enormous number
of Rodentia, especially of the sub-order Aystricomorpha, in-
cluding the porcupines, squirrels, chinchillas, cavies and
agoutis. On the other hand, the Ungulata are very few.
A few deer, tapirs, llamas and alpacas, and _peccaries
representing four families, make the total list. The Carnz-
vora are fairly represented, though, except for the raccoon
family, not by many special types. The jaguar and puma
represent the larger cats, whilst there are several of the dog
family. There is but one bear and these are no civets
nor hyzenas ; a few “ weasels,” such as the skunk and otter,
and several peculiar raccoons, such as the coati and kin-
kajou, give a complete general list. The Zusectivora and
Lemuroidea are entirely absent (save the Solenodontide of
West Indies), but three families of bats are found, of
which one, the vampires, is confined to the realm. The
Anthropoidea are represented by two families, the mar-
mosets and spider-monkeys.
Such then are the general characters of the realm.
Of this large assemblage we may note which are confin-
ed to the realm, for upon this largely depends the claim
for such an important distinction. Of the Aetatheria,
the Diprotodontia are elsewhere confined to Notogcea, but
the Polyprotodontia are still found in North America.
Of the Rodentia, Neogoea has four peculiar families,
forming the majority of the Aystricomorpha. In the
Ungulata, one family, the peccaries, is peculiar to the
realm. In the Carnivora there are no peculiar families
but a number of peculiar genera. The vampires are only
found in this realm, as also are the marmosets and spider-
monkeys.
‘It is well to note that the absence of many types is as
much a feature of the realm as the presence of others. The
most striking of these deficiencies are perhaps the sub-class
Prototheria, the order Jnsectivora, the sub-orders Momarthra
and Lemuroidea, Proboscidea and Hyracoidea, and the im-
portant families of Viverrid@ (civets), Bovide (oxen, sheep,
antelopes), Suzde (pigs), Eguide (horses), Pteropodide (fox-
bats) and Rkinolophide (horse-shoe bats), Cercopithecide and
Simitde (old-world monkeys).
OF MAMMALIA. 599
But one of the most extraordinary discoveries with regard to this
realm is the fact that it has had a great past history. The fossils teach
us, firstly, that the Xexarthra at one time were so numerous and attained
such large dimensions as to form quite the leading feature of the realm.
The giant ground-sloth or Megatherium and its near ally the AZylodon
are found in the Pleistocene and Recent, whilst the equally ponderous
Glyptodons or giant armadillos occurred at about the same period.
These Edentata as a group appear to have extended back at least as far
as the Miocene, if not the Oligocene, and at present we have no good
evidence that Edentata have ever occurred in other parts of the world,
with the reservation that one or two types appear to have made their
way into North America during the Miocene epoch, just as some arma-
dillos have done at the present time.
: The second lesson learnt from the fossil beds is that the peccaries,
vicufias, guanacos, deer and tapirs which now form the very sparse
tepresentatives of the great order Ungulata, and the Carnivora are all
comparatively recent immigrants from the North, no trace of any of
them occurring below the Pliocene of Neogcea, though abundant remains
are found in North America. On the other hand, there appears to have
been a very rich ungulate fauna during the past, though they in their
turn may have originated in the North and migrated southwards. How-
ever that may be, the horse flourished here in Pleistocene times, as also
the Mastodon, both probably northern immigrants. In addition, there
were from the Miocene onwards an enormous number of strange ungu-
lates, some like rhinoceroses in size and other features. At least four
entirely peculiar sub-orders of the Ungulata have to be instituted to
hold these extinct forms.
The Carnivora, for the same reasons as stated above, appear to
have been comparatively recent immigrants from the North, like the
peccaries and others. We therefore have a considerable light thrown
upon the past history of Neogcea which enables us, at any rate to some
extent, to explain its peculiarities at the present day. Put succinctly
the history of events appears to have been as follows :—The land-union
between North and South America appears to have been of recent date,
and from some unknown time up to at least the close of the Miocene
epoch, the two continents were separated by the sea. South America
then had its peculiar fauna of abundant Zdentata and Ungulata,
differing from any other part of the world, but upon the establishment
of the land connection between the two continents the Neogoean
realm was flooded with up-to-date immigrants from the north. Vicufias
and guanacos, ‘‘cats” and “dogs” (Feléde and Canide), raccoons and
skunks, deer, horses, peccaries and mastodons, opossums and many
rodents rapidly spread over the land and may have contributed consider-
ably to the extermination of many of the indigenous types. Through
the Pliocene and Pleistocene this hybrid fauna flourished until all the
larger types were for some unknown reason exterminated and the present
fauna remained.
But we still have the indigenous fauna of Neogcea and cannot help
attempting to trace its origin. Whence arose all the primitive Ungulates,
the Edentates, hystricomorphous rodents, the monkeys and the mar-
supials other than opossums (opossum-rats and fossil allies in the
600 GEOGRAPHICAL DISTRIBUTION
Miocene). Of this we Zvow nothing, but it has been suggested that a
land-connection with Australia would account for the marsupials, that
a similar junction with Africa would give monkeys, hystricomorphous
rodents and the selenodonts, and, lastly, that an early connection with
North America would give the Vngu/ata from primitive allies in the Eocene
there. But these are all surmises and none of the evidence pro or con
can be here given. Of this we may be fairly certain that Neogoea has
a remarkably primitive “indigenous” fauna of primitive Eutherian
animals belonging to the Edentata, Rodentia, early Ungulata and low
Anthropoid types, the greater number of which have perished, that
these have been enabled to survive and to reach a climax of adaptation
owing to an isolation of the realm up to nearly the commencement of
the pliocene epoch, and that subsequent connection led to an introduc-
tion of a northern fauna of higher Eutherian types.
We may say that the peculiar isolation of Notogcea and of Neogcea.
have furnished us with an example of the evolutionary possibilities of
the Metatherta and of the Edentata respectively, taken in the former
case from the prevalent fauna of the early dawn of the Tertiary epoch
and in the latter of some slightly later date.
Extant Mammalia of Neogeea.
SUB-CLASS. ORDER. FAMILIES,
Metatheria. Diprotodontia. 1 (Opossum-rats).
Polyprotodontia. 1 (Opossums).
Eutheria. Edentata. 3 (Sloths, anteaters, armadillos).
Rodentia. 9 (Squirrels, beavers, cavies, por-
cupines). .
Carnivora. 5 (Jaguars, pumas, coatis
raccoons).
Ungulata. 4 (Peccaries).
Insectivora. 1 (Selenodonis).
Chiroptera. 3 (Vampire-bats),
Anthropoidea. 2 (Marmosets, Spider-Monkeys).
3. ARcroceéa.—This third zoological realm comprises a
vast extent of land, including nearly all North America,
Europe, Asia and Africa. It has very distinctive faunistic
characters, separating it from the other two realms. Taking
the present-day fauna first, we find that there are no Proto-
theria, and only one family of M@etatheria (Opossums). On
the other hand, the Lu¢heria are abundant and of great
diversity. It has a monopoly of the sub-order Momarthra
(Aard-varks and Pangolins) ; of Rodentia it has the families
of Beavers (Castorida), the Jerboas (Dipodide) and the Picas
(Lagomyide) to itself, and abundant representatives of other
families, suchas /uride, the hystricomorphous Neogcean types
being the most conspicuous absentees. Of Carnivora the
hyzenas and civets and earth-pig (Proteide) are confined to
OF MAMMALIA. 601
the realm, whilst all the other families are present. Of
Ungulata all the families of Perissodactyla and Artiodactyla
are found, and all are confined to the realm except the pigs,
tapirs, camels and deer. In addition, the two sub-orders of
the elephants and hyraces are only found here. Of Jnsecti-
vora, the realm has almost a monopoly, one family alone
(the Solenodontide of West Indies) being found outside its
borders. The lemurs are also confined to the realm, as
are three of the anthropoid families.
Thus the realm has a practical monopoly of the order
Lnsectivora and of the large order Ungulata (except four
families), including the whole of the two sub-orders Pro-
boscidea and Ayracoidea, of the sub-orders Momarthra and
Lemuroidea, three families of the cosmopolitan Carnivora
and three (of five) of the Anthropoidea, besides a great
number of rodent families. On the other hand, the
absence of Prototheria, and all the families of AMetalheria
but one, is equally diagnostic.
The past history of Arctogcea shows that in the secondary epoch its
fauna was remarkably uniform, not only as regards reptiles but in
mammals. Of these the Prototheria were represented by the Al/otheria
occurring in Europe, North-America and Africa, and the Aetatheria
by numerous small Polyprotodontia from North America and Europe.
No evidence of Zutherza in this realm (or indeed anywhere else) has yet
been forthcoming from secondary strata, and we have already seen that
at this period (Jurassic and Cretaceous) Notogcea was in direct con-
nection with this realm, as probably was Neogcea as well. Thus all the
realms probably had much the same reptilian and early mammalian
fauna. At the base of the Eocene, there appear early Lemuroidea and
very primitive Carnivora (Creodonta) and Ungulata (Condylartha) ;
all were very generalised with simple tritubercular teeth and penta-
dactyle limbs. During the Eocene the greater number of the orders
make their first appearance, together with numerous types now extinct,
and at the commencement of this period, the Metatherian types dis-
appear, with the exception of the opossums. Hence the Arctogeean ~
realm assumed its general diagnostic characters in early Tertiary times
and has continued onwards to differentiate into several important regions.
Apparently it has by later communication given ofits types considerably
to Neogcea and to some extent (incidentally) to Notogcea, but has received
from them very little except perhaps a few Zdentata from the former.
Arctogcea can be divided into five regions, as follows :—
(1) Madagascar and adjacent islands; (2) Ethiopian, or
Africa south of the Sahara; (3) Oriental—India, southern
India and Malay; (4) Holarctic—the rest of Asia, Europe
602 GEOGRAPHICAL DISTRIBUTION
and North America north of (5); (5) Sonoran—roughly
corresponding to greater part of United States.
1. MADAGASCAR REGION, comprising Madagascar, Mau-
ritius, Bourbon, Rodriguez, Seychelles and Cornova Islands.
—The mammalian fauna of Madagascar is so remarkable
that it has strong claims for being placed in a region apart
from Africa. The most striking feature is the huge quantity
and variety of lemurs, representing three families and nearly
forty known species. The allied order of JLusectivora 1s
represented by a large and unique family, the Cenfecide, in
addition to a probable immigrant, the musk-shrew, and one
potamogale. A cat-like carnivore (Crypfoprocta) and a
number of mongooses represent the Carnivora, all belonging
to the civet family (Viverride). There are in the case of
Notogcea about seven species of the cosmopolitan AMuride,
of the Rodentia, and the list is completed by the bush-pig.
We may note also the fox-bats (Pferopus) and an extinct
Lippopotamus.
Lemurs, insectivores, carnivores and rodents occur on
the mainland of Africa, but none of the genera found in
Madagascar. Indeed, the only genera common to the two
regions are the bush-pig and hippopotamus and the musk-
shrew. The latter was probably introduced at a later date,
and the two former probably introduced themselves by swim-
ming, possibly at a date when the strait was of narrower
dimensions than now.
Madagascar has the monopoly of the ‘ollowing families :
—The Chiromyide (Aye-Aye) and Censetide (Tenrecs), and
by some authorities the “ Foussa” (C7yptoprocta) is placed
in a family by itself.
Almost as strange as these inhabitants is the entire
absence of all the characteristic African mammals, the large
Ungulata and Carnivora.
The usually accepted explanation of these peculiarities is the as-
sumption that Madagascar has been isolated from the mainland of
Africa from early Miocene or upper Oligocene. In the Oligocene the
lemurs flourished in Europe, as also the civets; and a separation effected
at this period might easily isolate a sample of these two groups, together
with the primitive Zsectevora, whilst the modern Ungulata and Carnt-
vora of Africa would not by then have reached that region. Hence the
history of Madagascar is a more recent repetition on a smaller scale of
the history of Notogoea. Occurring later, it merely serves to preserve
OF MAMMALIA. 603
a few families or at least the greater part of an order, instead of nearly
two whole sub-classes.
We may here allude to the hypothetical continent of LEMuRIA.
Apart from the distribution of the fox-bats and a peculiar civet, the
evidence for the former existence of this continent connecting Mada-
gascar with India and Further India is based largely upon the resem-
blances in amphibians, land-tortoises, birds and molluscs.
The presence of lemurs in Malay has led to the supposition that one
feature of this continent was an abundance of this type, hence the name.
Geographical evidence for the same is found in the constitution of the
Seychelles, which, unlike oceanic islands, are formed of granitic rocks
of the primary period.
This sunken continent, if it existed at all, would appear to have
scarcely survived into the Tertiary period, so that it can hardly be said
to come into Eutherian mammalian times, and we have seen that the
lemurs can be accounted for in another less hypothetical way.
2. Eraiopian Recion. —The Ethiopian region com-
prises the continent of Africa south of the Tropic of Cancer.
The area is much more isolated zoologically than geogra-
phically, for the Sahara Desert extends across its northern
part, and has probably since the Cretaceous epoch formed
an effectual barrier to mammalian migrations, which have
hence been confined to the Nile basin on the east side.
This region has four sub-divisions differing in physical
characters, the pasture lands south of the Sahara, the Sahara
desert itself with sparse fauna, the equatorial forests, and the
area south of these.
It has a wonderfully rich mammalian fauna, though it is
for the most part being rapidly exterminated. It has of
course no representative of the two lowest sub-classes, but
possesses in the aard-varks and pangolins two families of
the very low order Zdentata. Rodents are plentiful,
including squirrels, Anomalurus (a peculiar flying squirrel),
a large number of the ubiquitous J/uride, jerboas, cape
jumping hares, whilst the hystricomorphous types are re-
presented by the octodonts, which we have already met
with in Neogcea. But the most remarkable feature is the
abundance of Ungulata; elephants and dasses, hippopo-
tamuses, water chevrotains, bush-pigs and wart-hogs, giraffes,
rhinoceroses, zebras and quaggas, and lastly antelopes of
every description. Every family of this great order is
represented except the Camelide and Tapiride. Of the
abundant Carnivora we may note the lion and leopard,
604 GEOGRAPHICAL DISTRIBUTION
civets, mongooses, the aard-wolf, hyenas, jackals, foxes, and
ratels. Of Jusectivora there are the jumping-shrews, golden
moles, a few hedge-hogs and shrews, the river-shrews. Of
the Primates there are one family of lemurs, the gorillas
and chimpanzee, and a great number of smaller monkeys
of the family Cercopithecide, and including the baboons.
Of this great and heterogeneous assemblage there is a
large number peculiar to the region. No order is of this
category, but there are the following families :—The aard-
varks, the dasses (partly in Syria), the Anomalurida, giraffes,
hippopotamuses, aard-wolf (Prote/ide). golden moles and
jumping-shrews ; and of lesser groups, the zebras and pre-
dominance of the antelopes. Again, we may note the
absence of the bears, tapirs, camels and deer and poor
representation of the A/us/elide@ ; and of lesser groups, sheep
and goats and wolves.
The palzontological history of Africa during the Tertiary period has
yet to be worked out, but the evidence of the faunistic characters of
Madagascar on the one hand and of the Oriental and Holarctic regions
on the other, lead us to suppose that there is a remarkable parallelism in
the history of Ethiopia to that of Neogcea. As in the latter case, we can
recognise an indigenous fauna of Africa flourishing during the Eocene
_.and Oligocene periods, of which we havea kind of sample in Madagascar
at the present day. Certainly, lemurs, civets and primitive types of
Jusectivora abounded. During the Miocene, or possibly later, Mada-
gascar became separated from the mainland and subsequently there com-
menced a great immigration from the Oriental and partly the Holarctic
regions, probably by the north-east district, of the rhinoceroses, hippo-
potamuses, giraffes, water -chevrotains, large ‘‘ cats,” hyenas and
monkeys. The evidence for this is based partly upon the great present-
day resemblance between the mammals of the Ethiopian and Oriental
region and also upon the fossil remains of these types found in Greece,
Persia and India, dating from Miocene. Thus here again we may
trace the irruption of a more primitive fauna during early Oligocene into
Africa from the north and later, probably during early Pliocene, a
second immigration southwards of more modern types. It is usually
assumed that, during the interregnum between these two migrations,
Ethiopia was isolated by sea from the north, but this assumption
scarcely appears to be absolutely necessary though quite probable.
3. OrtENTAL REGION.—The Oriental region comprises
India, Further India, Southern China and Malay, up to the
line of east of Celebes. As a whole, this region most
resembles the Ethiopian, mainly owing to the late migration
of Oriental types at a comparatively late date into the latter.
OF MAMMALIA. 605
The principal mammalian fauna is as follows :—The pan-
golins represent the Zden¢ata, the aard-varks being absent
at the present day. The Ungu/ata are rich in numbers and
types, and elephants, tapirs, rhinoceroses, pigs, chevrotains,
deer, antelopes and buffaloes are amongst the most impor-
tant. The absentees are the sub-order of AHyracoidea and
the families of Camelde, Giraffide and Hippopotamide.
Of Rodents, the squirrel-family and rat-family are abundant,
besides a few hystricomorphous types. There are great
numbers of the cat-family, the tiger, lion, leopards and
tiger-cats being representative. The civet-family is as
abundant as in Africa. The striped hyena, wolves, jackals,
black bears, sloth-bears, the panda (Procyonide), and ratels
complete the commoner carnivores. Of Jmsectivora, the
flying lemur (Gadeopithecus) is confined to Malay. Tree-
shrews, hedgehogs and musk-shrews are found within the
region, though we may note the absence of moles and
shrews. Two families of the lemurs are represented, the
peculiar Zarsius being confined to Malay. The same two
families of monkeys are found as in Ethiopia. The Szmiide
are represented by the orang of Borneo and the gibbons
of Assam and Malay, and the Cercopithecide by great num-
bers which mainly differ generically from the Ethiopian.
In passing over this list we find that the Oriental region
is not so faunistically distinct as the Ethiopian. Whilst the
latter has the monopoly of at least eight families, the
Oriental has not more than three, namely, the 7upaiide,
Tarsude and Galeopithecide, though the two latter really
rank as sub-orders.
Whilst the Ethiopian region was distinguished by a
marked absence of bears, tapirs, deer, wolves and few pigs,
these are all found in the Oriental region, the deer and pigs
in abundance. On the other hand, both regions agree in
small representation of Mustehde and in almost entire
absence of sheep, goats, moles and shrews, features which
are in marked contrast to the Holarctic region.
We have already seen that Ethiopia probably owes its faunistic
similarity to the Oriental region to a migration from the latter to
the former, and ‘‘during the Pliocene, India, at least, could not
have been distinguished as a region from Ethiopia as it exists at the
present day, and even in the Pleistocene the connection between the
606 GEOGRAPHICAL DISTRIBUTION
faunas of the two areas was much mote intimate than it is now.” *
Why the giraffe, hippopotamus and other Ethiopian types died out
altogether in the Oriental region we do not know.
4. Hoxarctic Recion.—The Holarctic region cor-
responds to North America, Europe, Northern Africa and
all Asia not included in the Oriental. This vast area
appears to have sufficient community of ‘fauna to comprise
one region. It has characteristically large numbers of
Bovide, especially sheep, goats-and oxen, the deer, camels
and pigs being also present (Dasses occur in Syria). Of
rodents, the squirrels, beavers, AZuride, picas, rabbits and
hares. In the Carnivora there are abundance of bears and
Mustelide (weasels, polecats, martens, wolverenes, otters,
skunks and badgers), whilst the /é/ide are poorly repre-
sented by the lynxes and other forms, as also are the civet-
family by mongooses and genets, the Canzd@ by wolves and
foxes. Of the Zzsectivora, the moles, hedgehogs and shrews
are all common, and in bats only the Mfrcrochiroptera are
found, except for those inhabiting the Pyramids. The only
Primates are the baboons of Gibraltar.
There is hence a marked absence of a great number of
large Ungulates, Carnivora, and of the 2dentata, lemurs
and monkeys, in comparison with the other regions. Two
typical families of rodents, the beaverst and picas, are
confined to the region, and the camels are not found else-
where in Arctogcea. The walruses (Z7ichechide) are also
peculiar to the region. The moles and shrews are very
characteristic and are found only to a small extent outside
the region.
At first sight it appears anomalous to separate Africa and
Madagascar into regions and to unite Eurasia and North
America into one region, but the large number of identical
or closely allied species occurring in these two continents
compel us to adopt such a classification.
As regards the past history of the region we have already referred to
the widely scattered Mesozoic Polyfrotodontia and to the lemurs of a
later date. But as late as the Pleistocene epoch the mammals of the
Holarctic region resembled those of the Ethiopian and Oriental far more
nearly than at the present day. For example, there are well-authenti-
cated remains from the Pleistocene of Europe, of the macaque monkeys,
* Lyddeker. Geo. History of Mammals, page 288.
t Also found_in Sonoran.
OF MAMMALTA. 607
elephants, several species of rhinoceros, hippopotamus, hyzenas and lions.
Mixed up with these in a remarkable manner are the remains of
northern forms like the wolverene, arctic fox, northern vole and reindeer,
From this it follows that the past history ofthe Holarctic region is to
a large extent an epitome ofthe faunas found in the other several regions
(leaving out of consideration the Sonoran). In early Oligocene of Europe
we find the lemurs and civets, now so characteristic of the Madagascar
region, and later on in the Pliocene and Pleistocene, the fauna with its
early aard-varks, elephants, hippopotamuses, and other early ungulates
approximated to the present-day fauna of Ethiopia and to the present
and early past of the Oriental. As most of those occur in the Miocene of
India, it is probable that they migrated thence to Europe, either directly
or through northern Africa.
The resemblances in the faunas of North America and northern
Eurasia are usually explained as being due to a land-connection across
Behring sea, for which there is muchevidence. Thisserved to cause an
approximation in faunas between the northern parts, leaving the Sonoran
ah and the Medissarenea district more or less distinct from each
other
5. Sonoran ReEcion.—This, comprising practically the
United States of America, has been constituted as a separate
region mainly because it is a transition zone between Hol-
arctic and Neogcea, though it has some peculiar types of
its own. Of Neogcean types, we may note the armadillos,
opossums, peccaries and some Procyonide, whilst the skunks
and other Mustelide, the marmots and the pouched rats,
form Holarctic types. The most typital family of the
region is the American prongbuck (Azflocapra) which
has deciduous horns. This species also extends partly
into Canada.
We may add here the names of some of the most char-
acteristic mammals found at the present day in the regions
of Arctogcea :—
[TABLE.
608
GEOGRAPHICAL DISTRIBUTION
Typical Mammalian Fauna of Arctogcean Regions.
I. MADAGASCAR, z. ETHIOPIAN. 3. ORIENTAL, 4. HOLARCTIC. 5. SONORAN.
Lemurs. Aard-varks. Pangolins. Beavers. Opossums.
Tenrecs. Pangolins. Elephants. Picas. Armadillos.
River shrews. Cape jumping- Tapirs. Hares and Pouched rats.
hares. Rabbits.
Civets. Octodonts. Pigs. Voles. Prairie marmots.
Mongooses. Dasses. Deer. Marmots. Peccaries.
Elephants. Antelopes. Dasses. Deer.
Rhinoceroses. Chevrotains. Pigs. Prong buck.
Zebras. Lion &leopard. Deer. Bears.
Hippopotami. Tiger. Sheep & goats. Skunks.
Giraffes. Civets. Bisons. Raccoons.
Antelopes. Mongooses. Musk-ox. Shrews.
Water- Hyzenas. Wild-cats. Moles.
chevrotains.
Lion & leopard. Jackals. Lynxes.
Civets. Wolves. Walruses.
Mongooses. Foxes. Wolves.
Aard-wolf. Bears. Foxes.
Hyzenas, Ratels. Bears.
lackals. ‘Raccoons.’ Skunks.
Ratels, ‘Flying lemur.’ Sea-otters.
Jumping- Lemurs. Wolverines.
shrews.
Martens.
Golden-moles. Orang. Weasels
Gibbons.
River-shrews. Many smaller Shrews.
monkeys.
Lemurs. Moles
Gorilla Hedgehogs
Chimpanzee.
Nunierous
Cercopithe-
cide.
The Orders of each Realm.
NOTOGEA. NEOGEA. ARCTOGGA,
Monotremata
Diprotodontia Diprotodontia
_ _Polyprotodontia Polyprotodontia Edentata
¥ ( Rodentia Edentata Rodentia
{ Carnivora Rodentia Carnivora
g& (Ungulata Carnivora Ungulata _
Ungulata Insectivora
Primates Primates
OF MAMMALIA. 609
In conclusion, we may touch upon a few special points.
The first of these is the phenomenon. of discontinuous
distribution (cf. p. 64). All mammalian species are found in
continuous or contiguous areas, but the different species of
a genus may in certain instances occur in widely separated
areas A good example, usually quoted, is that of the
tapirs, which are found in Malay and South America. Dis-
continuous distribution of a family is also fairly common ;
we may instance the Zvaguid@e or Chevrotains of Africa
and India. Of a discontinuous order, we may instance the
Diprotodontia, which has one family (opossum-rats) in
America and the rest in Australia, and a similar case in the
Polyprotodontia, with the opossums in South America. An
instance of much the same kind is the distribution of the
Primates, the lemurs being found in Madagascar and Africa,
on the one hand, and in Further India and Malay, on the
other, and the Axshropoidea occurring in America, Africa
and India.
There are two possible explanations of this phenomenon.
The first is based on the assumption that the discontinuity
is fundamental and that the genera, families, or orders have
been separately evolved from the same earlier ancestors,
their resemblances being due to parallel or convergent
evolution. As an instance of this we may quote the Anthro-
potdea. It is quite conceivable that the New World monkeys
and those of the Old World have been separately evolved
from primitive types which were not monkeys. The
same applies to the Diprotodontia, which may have been
separately evolved from Polyprotodontia. There is very
strong evidence for supposing that horses and rhinoceroses
were independently evolved from primitive ungulates in each
hemisphere.
Without entering into the question of the polyphyletism
of the class Mammata—by which we mean the separate
evolution of mammalian types from pre-existent amphibio-
reptiles—we may note that this very highly differentiated
class would lend itself more than any other to the phenom-
enon of parallel evolution. Rodentia are specially distin-
guished as an order by their peculiar incisor-dentition, yet
the same modification is found in the Dasse (Hyracoidea),
the Aye-Aye (Lemuroidea), the Wombat (Diprotodontia) and
M. 40
610 GEOGRAPHICAL DISTRIBUTION
the Zoxodontia. In other words, there is no real distinction
between adaptive and genetic characters.
The second explanation assumes that the two discon-
tinuous types were at one time continuous, and that the
intermediate members have now died out. Upon ‘the evo-
lution of a successful type it naturally spreads in every
suitable direction, and later, when the type has had its
day and becomes replaced by others, it dies out first in
the central areas where competition is fiercest but may
linger on in more remote parts. There is no doubt that
this is the actual course of events in many cases. Thus we
find traces of tapirs in Europe, India and North America.
Remains of lemurs are also found in Europe and North
America.
The second point is with regard to the course of evolu-
tion. There is much evidence for assuming that the northern
hemisphere has been the scene of early mammalian evolu-
tion, and that a succession of mammalian types have radiated,
especially southwards, from this centre. The Prototherian
wave reached the southern limit in Australia, where it still
lingers. The Metatherian wave appears to have spread down
to Australia, Africa and South America. Extinguished in
Africa, it still lingers im America and has reached and passed
its climax in Australia. A third wave consists of the Eden-
tata, the lowest of Eutheria. These also reached their
zenith in South America, where they still linger. Yet a
fourth wave, of more recent date, of the lemur type, lingers
in the outlying parts of South East Asia (Malay) and reaches
a climax in the isolated region of Madagascar. Finally,
the most “up-to-date” types of Ungulata, Carnivora and
Rodentia are either at their world-wide zenith or have not
yet reached the outlying regions and extend mainly over the
Holarctic region. °
Lastly, we may recall the instances we have had of
“oceanic” islands, like New Zealand, with no indigenous
mammals. By gradation we are led through types like
Madagascar, Ceylon, and others which have a fauna differing
in degree from that of the adjoining continent, till finally we
reach islands, such as Britain, which have a fauna usually
approximating closely to that of the mainland, though often
differing in quantity. Geological history usually gives us
OF MAMMALIA. 611
evidence that these ‘‘ Continental” islands have been only
recently separated from the mainland, and a sufficient time
has not elapsed for the mammalian fauna to diverge from
the parent stock. In the case of Britain, for example, it is
generally accepted that in Pleistocene times the North Sea
was dry land, thus accounting for the identity of fauna at
that time between Britain and the Continent. The extinc-
tion in Britain of many continental types has not yet, been
explained, though of course the wolf, beaver, wild boar and
brown bear have been exterminated by man, by whose agency
have also been introduced the rabbit, brown and black rats
and fallow deer.
In fact, the faunistic character of an island or a continent,
like the structure of an organism, is a complex relationship
in space, the facts of which are easily attainable by observa-
tion. The explanation of the facts in each case is obscure,
depending upon the relationship in time, a factor in which
the investigating unit is too severely limited to permit of
anything beyond the slowest progress.
612 THE PRINCIPAL FEATURES
THE PRINCIPAL FEA’
EDENTATA. SIRENIA. RopENTIA. Uncul
LCC E in teseancesdiint No incisors nor No incisors nor Incisors 2 or |. Often 1
canines, molars | canines, or a |} growing from | incisol
simple with no | single pair grow- persistent pulps, | C@?ines,
enamelor absent, | ing from persis- | pocanines,molars | C!S°FS 4 ]
grow from persis- | tent pulps,molars |} with flat complex | be $.4.
tent pulps, mono- | few or absent. ridges. 1 ‘J
phyodont. euweys.
complex
TEUMO Sa ca disalesavies Plantigrade or Fore-limbs Plantigrade or | Plantig
prehensile. paddles, hind- | subplantigrade. digitigra:
. limbs absent.
Digits usually Digits 5. Digits & usually. Digits
& but reduced to mostly h
$ in arboreal.
Clavicles. No clavicles. _ Clavicles some- Nocla
times absent.
Placenta........ Discoidal or Zonary (non- | Discoidal (de- Zonary.
dome-shaped | deciduate). ciduate). or cotyl
(deciduate), dif- (non-deci
fuse or zonary
(non-deciduate).
LL AGEP eeessecparissi oe Fossorial or Aquatic, Herbi- Terrestrial, Terre
Arboreal, In- | vorous. Fossorial, Arbor- | Herbivor
sectivorous or eal, Herbivorous.
Herbivorous.
Distribution.... Neogean Rivers of At- Cosmopolitan, Widei
realm and Ethio- | lanticand Indian | mainly Neogoean | goean ar
pian and Oriental | Oceans 30° N. | realm. gean rea
regions.
to 30° S
OF EUTHERIAN ORDERS.
OF EUTHERIAN ORDERS.
613
CETACEA.
No teeth or
varying number
of homodont
teeth.
CARNIVORA.
Incisors 3 and
pointed, canines
large t molars
secodont (cut-
ting ridges),
INSECTIVORA,
Incisors gtoz
and pointed.
Canines small,
tubercular molars
CHIROPTERA.
Incisors 2, can-
ines small, molars
tubercular
or grooved.
PRIMATES.
is 2
Incisors 2
chisél-shaped,
canines moderate,
molars tuber-
cular.
Fore-limbs
paddles, hind-
limbs absent.
_ Digits $(hyper-
phalangic).
No clavicles.
Plantigrade and
digitigrade (pad-
dles).
.
Digits 4 usually
(aquatic, 4).
Noclavicles (or
vestigial).
Plantigrade.
Digits
ole
Usually clavi-
cles.
Fore-limbs
wings, hind-
limbs prehensile.
Digits § (claw
only on 1 or on
x and 2 of fore-
limb).
Large clavicles.
Plantigrade or
prehensile. ,
Digits $ (or 4),
Large clavicles.
Diffuse (non- Zonary (decid- |~ Discoidal (de- Discoidal Diffuse (non-
deciduate). uate) ciduate). (deciduate). deciduate) or
metadiscoidal
(deciduate).
Aquatic, Pisci- Terrestrial, Terrestrial, fErial, _ Frugi- Arboreal or
vorous.
Carnivorous (or
Fossorial or Ar-
vorous or Insecti-
Terrestrial,
Aquatic and | boreal, Insecti- | vorous. Frugivorous or
Piscivorous). vorous. Omnivorous.
Cosmopolitan. Cosmopolitan, Arctogean Cosmopolitan. | * Neogoea and
few in Neogcea | realm (except Arctogoea,
and Notogcea.
Solenodon)
AARD-VARK
Aard-wolf
Abdominal pores
" riks
Abducens nerve -
Abomasum -
Acanthocephala -
Acanthopteri
Acarina
Acetabulum
Acineta
Acinetaria -
Acipenser
Acorn-barnacle
Acrodont teeth
Acromion
Actinia
Actinophrys
Actinozoa, -
Adambulacrals
Adhesive cells
Adrenal body of rabbit
f®luroidea - -
4Erial adaptation of mammals
Afferent branchials, skate
Air-bladder of fishes -
Air-sacs of birds -
Albumen gland of snail
u of bird’s egg
nu of frog’s egg
Albuminal nutrition of Verte-
brata
Alcyonium -
Alecithal
Alimentary system
Alimentary system of verte-
brates -
" u of mammals
Alisphenoid canal
Allantois -
Allantoic placenta
Alligators
Allotheria
Alpaca -
Alternation of generations
Alula -
Amblypoda
Ambulacra - :
Ambulacral ossicles
Ametabolic :
Amitosis
Ammoccetes
Ammonites
Ammnion
Amniota
Ameoeba
Amphibia
Amphiblastula
Amphilestes -
Amphineura (see Isoplenra)
Amphioxus - -
Amphioxus, development of
Ampullee of starfish
Anacanthini
Anal cerci of cockroach
Anal glands, starfish
616
Anamnia
Anapophyses
Anchitherium
Anguillulidee
Animals and plants
Anisopleura
Annelida
Annulata
Anodonta
Anomalurus
Ant-eaters
Antedon
Antennz
Antennules-
Anthropoid Apes
Anthropoidea
Anthropopithecus
Anthropozoic group (Era)
Antelope
Antlers
Ants -
Ant-lions
Anura -
Apes -
Aphides
Apis - -
Appendicular skeleton of
Vertebrata
Appendicularia -
Appendix vermiformis
Aptera
Apteria
Apteryx
Apus -
Aquatic adaptation of mam-
mals : -
Aqueductus Vestibuli
Aqueous humor of eye
Arachnida - 2
Arboreal types of mammals -
Archeeopteryx
Archzeornithes
Archenteron
Archiannelida
Archichorda
Archiccelomata
Archiccele
Archiblast -
Archipterygium of fishes
Archizoic group (Era) -
INDEX.
PAGE
432 | Arctogcea, mammals of
396 | Arctogcean regions
573 | Arctoidea
155 | Areaopaca, chick
10 | Areapellucida, chick
283 | Arenicola
238 | Argonauta
237 | Argyroneta
269 | Aristotle’s lantern
603 | Armadillos-
558 | Arterial arches of Verte-
175. brata
212 | Arterial system - -
212 | Arthrobranchs of crayfish
590 | Arthropoda, classes of-
588 " general charac-
592 ters of
68 | Arthrostraca
578 | Artiodactyla
576 | Ascaris
248 | Ascetta
254 | Ascidia
440 | Ascon type (of sponge)
590 | Asexual reproduction -
257 | Astacus
249 | Asterias
Asteroidea
418 | Astragalus
404 | Atavisms or reversions
385 | Atlas vertebra, rabbit -
257 | Atrial cavity (atrium) -
361 | Atrioccelomic funnels -
452 | Atriopore - * 288,
242 | Atriozoa
Auchenia
549 | Auditory capsules of cranium
411 | Auditory nerve
409 | Auditory sacs, Vertebrata
258 | Aurelia -
537 u life history of -
449 | Auricularia (of Holothuria) -
449 | Aves - -
133 | Axial sinus, starfish
238 | Axial skeleton
171 | Axis vertebra, rabbit
170 | Axolotl
50 | Aye-Aye
50
435 | BaBoons
68 | Badger
Balena
Balanoglossus
Balanus
Baleen
Bandicoot
Barbule
Barnacle
Bats - -
Bat, as zrial type of mammal
Bdellostoma
Bears -
Beaver
Bees
Beetles
Belemnites -
Beroé -
Bilateral symmetry
Bilophodont
Bionomics -
Bipinnaria
Birds -
Blastocyst
Blastoderm
Blastomere -
Blastopore -
Blastula (Blastosphere)
Blatta ~
Blood
Blood-vascular
Vertebrata
" of mammals
Body- -cavity
Bone -
Bones of vertebrate skull
" limbs
system of
Bony pike -
Book scorpions
Bot-fly -
Brachiopoda
Brachydont
Brady podidze
Bradypus_ -
Brain of Vertebrata
Branchial arches -
Branchial plate, Nephrops
Branchiostegal rays
Brittle-stars
Bryozoa -
Buccal mass, snail
Budding
INDEN.
Bugs
Bulbus arteriosus
Bulla -
Bunodont
Butterflies
CaBBAGE WHITE
Caddis flies
Cainozoic Era (group)
Calcaneum
Calcarea
Calciferous glands,
worm
Cambrian period (system)
Camelidz
Canidee
Canis -
Cannon-bone
Capitulum of rib
Capybara - -
Carboniferous system eueHOS)
Carinatee -
Carnassial teeth -
Carnivora
Carpalia
Cartilage
" bones
Cassowary
Cat
Caterpillar
Caviidee (guinea Pigs, &e. )
Cebidze
Cecidomya
Cells -
u structure of
Cellulose in Tunicata
Centetes
Centipedes - -
Centrale of rabbit
Centrolecithal
Centrosomes
Cephalochorda
Cephalodiscus
Cephalopoda
Ceratodus
Cercariz -
Cercopithecidze
Cestoda
Cestum Veneris -
Cetacea
earth-
618 INDEX.
PAGE PAGE
Cheetognatha 177 | Ccenolestes 504
" type of 168 | Ccenurus 149
Chzetopoda 238 | Coleoptera - 246
Chameleon 78 | Colorado beetle 247
Cheiropterygium 419 | Columba 360
Chela, crayfish 209 | Columella of vertebrate ear - 412
Chelicerze 234, 258 | Commensalism 74
Chelonia 443 | Compound eyes 213
Chevron bones 418 | Condylarthra 566
Chevrotain 575 | Conjugation, dimorphic 40
Chilognatha 245 " of Parameecium - I
Chilopoda 245 " of Vorticella 94
Chimeera 439 | Connective tissue 33
Chimpanzee 590 | Contractile vacuoles 87, 91
Chiromys 588 | Conus arteriosus - 421
Chiroptera - 586 " " of skate 317
Chitin (Keratin) 20 | Copepoda 243
Chiton 283 | Coracoid 419
Chlorophyll 11 | Corals 134
Choanocyte 105 | Cornule” - 493
Choanoflagellata 101 | Corpora adiposa 344
Chondrocladia 110 | Corpus callosum 463
Chondrocranium 416 | Corticata 101
Chordata and Non-chordata 402 | Cotyledonary placenta 482
Chordoid tissue 33, 162 | Cowper’s glands - 390
Chromatin 35 | Coxal glands - 258
Chrysalis (see Pupa. 246 " u of scorpion 258
Chrysochloris 586 " » of spider 236
Cilia 15 |} Crab 244
Ciliata lor | Crane-fly - 252
Circulatory system 18 ; Cranial nerves of Vertebrata 407
Cirripedia - 243 " «of skate 321
Classification of Animals 29 | Cranium of Vertebrata 415
Clavicle 419 | Creodonta - 531
Cliona 110 | Cretaceous period 69
Clitoris 390 " birds 452
Cloaca of Vertebrata 339 | Cricket 256
Cnidoblasts 115 | Crinoidea 174
Cnidocil 115 | Crocodilia - 445
Coccidia 102 | Crossopterygii 438
Cochlea 412 | Crotalus - 442
Cockle 284 | Crura cerebri 407
Cockchafer 247 | Crural glands 232
Cockroach - 222 | Cruro-tarsal joint, mammals 474
Cocoon 202 | Crustacea 241
Cod 331 | Ctenidia - 283
Cod’s skull 335 | Ctenoid scale 430
Ccelenterata 133 | Ctenophora 135
Ceelom (see Body-cavity ; 26 | Cuboid 473
Ccelomata and Ccelenterata- 26 ' Cucumaria - 175
INDEX.
PAGE
Cuneiforms 473
Cursorial adaptation in mam-
mals 532
Cuttle 276
Cycloid scale 436
Cyclops 243
Cyclostomata 432
Cycloturus - 559
Cydippe_ - 131
Cynocephalus 590
Cynoidea 581
Cynips 249
Cypris 243
Cysticercus - 149
Cysts of Protozoa 89, 95
Cytoplasm 35
DADDY-LONG-LEGS 252
Daphnia - 242
Dart sac of snail 268
Dasse 567
Dasypodidze 559
Dasypus 559
Dasyuride - 502
Dasyurus 502
Dead-man’s-fingers 125
Decapoda 244
Deer - 576
Delamination 51
Delphinus 579
Dental formulz - 460
Dentition, Acrodont 441
" Bilophodont 462
" Bunodont 462
" Diphyodont 460
" Haplodont - 462
" Homodont - 441
" Heterodont 460
" Lacteal 460
" Multitubercular - 462
" Pleurodont 441
" Polylophodont 569
" Polyphyodont 460
" Secodont 462
" Triconodont 461
" Tritubercular - 461
" of Mammals 460
Dendrohyrax - 568
Dermis of Vertebrata 405
Dermoptera 585
619
PAGE
Descent 80
nu of Horse 522
Development of—
Amphioxus 304
Anodonta 275
Ascidia 293
Balanoglossus 165
chick 376
crayfish 221
- Crustacea 241
earthworm 201
Echinodermata 161
eye 408
feather 361
frog - 353
haddock 337
hair - 456
Hydra 116
Insects 246
Mammalian teeth 458
Mammals 475
Peripatus - 233
Petromyzon 433
placenta 48t
Reptilia 440
skate 329
skull 416
Sponges 105
Tunicata 293
Vertebrata : 426
Devonian System (Period) 68
Diaphragm 386
Dibranchiata 285
Dicotyles 575
Didelphidze 502
Didelphys 502
Diffuse Placenta - 482
Digenea- 149
Digestive System . 15
Dimorphism of sexes - 44
Dinornis 452
Dinosauria 447
Dinotherium 571
Diphycercal : 435
Diphyodont dentition - 460
Diploblastic 25, 50
Dipnoi 439
Diprotodon 505
Diprotodontia 503
Diptera 250
620
Discoidal segmentation
" Placenta
Distomum - -
" Life History
Distribution, geographical
" geological
Division of labour
Dog
Dolphin
Dome-shaped DPlacenta
Dorcatherium
Dragon-flies
Dromeeus
Duckmole
Ductus arteriosus of rabbit -
" botalii
Dugong
Duplicidentata
Ear OF VERTEBRATA
Eared-seals -
Ear-shell (Haliotis)
Earth-pig
Earthworm
Earwigs
Ecdysis or moulting of Ar-
thropods
Echidna
Echinococcus
Echinodermata
Echinoidea
Echinus
Ectoderm
Edentata -
Efferent branchial system of
skate -
Egg of fowl
Elasmobranchii
Elephantidze
Elephas
Embryology
Emu -
Endoderm
Endophragmal
crayfish
Endostyle of Tunicata
Enteroccele
Entomostraca
Epanorthidze
Epeira
skeleton,
INDEN.
PAGE PAGE
49 | Ephemera 255
483 | Ephyra 131
137 | Epiblast 50
141 | Epidermis of Vertebrata 405
56 | Epididymis 426
66 | Epiphragn of snail 262
14 | Epiphyses 467
523 | Epipubic bones 471
579 | Episternum 470
482 | Epithelial tissue - 31
575 | Equidee 573
254 | Equus 509
452 | Erinaceus 535
491 | Ethiopian region 63
388 " mammals of- 608
422 | Ethmoid ring of skull 393
562 | Euplectella 110
564 | Euspongia 110
Eustachian tube - 4Il
411 | Eutheria 506
583 | Evolution 81
284 | Excretory systemof animals 18
560 | Exoskeleton of Arthropoda 238
198 | Exoskeleton of Vertebrata 405
256 | Extinct reptiles 447
Extinction of animals 70
230 | Eyes of Arthropoda 215
493 u of Vertebrata 408
150 " " develop-
171 ment of 404
172 | Eye-muscles 410
172
25 | FactiAL NERVE 408
557 | Falciform ligament 364
Fallopian tube 466
317 | Feather-stars 175
375 | Feathers, development of 361
438 " of pigeon 361
570 | Felidae 581
570 | Felis 523
46 | Femur - 474
452 | Fenestrated membrane 228
‘25 | Fertilisation 42
Fibula 474
216 | Filaria - 155
290 | Filaments, gastric 129
52 | Fins - 436
242 | Fin-rays 301, 328
504 | Fishes 434
233 | Fishes, abysmal 60
Fission .
Fissipedia
Flagella
Flagellum of snail
Flame-cell excretory organs
Fleas -
Flies
Flight-feathers
Flying lemur
Foetal membranes of binils -
of Manmals
of Vertebrata
tt W
uw ii
Follicle cells
Follicle, Graafian
Foot of Anodonta
u of Cephalopoda -
u of Gasteropoda
nu of Mollusca
Foramen of Munro
un _ triosseum
Foraminifera
Fossils - -
Fossorial adaptation in mam-
mals
Fox
Fox-bat
Freshwater mussel
Frog
Frontals
Functions, change of
" of Animals -
GabDus
Galeopithecus
Gall-fly
Galls on plants
Ganglion
Ganoid scales
Ganoidei_ -
Garden spider
Gastric filaments
Gastric mill, crayfish
Gastropoda
Gastrula 50,
Gastrus ¢
Gavials
Geographical distdiudion
Geological distribution -
Geological record
Germ cells - -
INDEX
PAGE
41 | Gibbon
581 | Gill-slits
15 | Gills (see Respiratory system)
268 | Giraffe -
139 | Gizzard of cockroach
253 n of earthworm
251 n of pigeon
361 | Globigerina
585 | Glochidium of Anodonta
381 | Glossina
478 | Glossopharyngeal nerve
428 | Gnats
476 | Goat
476 | Gonads = Reproductive
274 organs
284 | Gorilla -
283 | Graafian follicle -
282 | Grantia
406 | Grasshopper
373 | Green gland
roo | Gregarina
67 | Guanaco
Gymnolemata
541 | Gymnomyxa
583 | Gymnophiona
551
269 | Hapbock -
338 | Heemal nutrition of Ver te-
416 brata
27 | Hemoccele -
14 | Heemoglobin of iblbed 201,
Hag
331 | Hair
585 | Halicore
249 | Haliotis
249 | Halitherium
20 | Hapale
436 | Hapalidze
437 | Hare
233 | Harvestmen
129 | Hatteria (Sphenodon)
216 | Heart of Vertebrata
283 | Hedgehog
133 | Heliozoa
252 | Helix
446 | Hemichorda larenichiarda
56 | Elemiptera -
66 | Heredity
69 | Hermaphroditism
41 | Hesperornis
622 INDEX.
PAGE
Hessian fly 252
Heterocercal 435
Heteroccelous - 3609
Heterodont dentition 458
Hexactinia 135
Hipparion 522
Hippopotamus 574
Hirudinea 239
Hirudo 19!
Histology 30
Holoblastic segmentation
(total) 49
Holocephali 439
Holophytic II
Holothuroidea 174
Holozoic II
Holotricha - 92
Homarus 204
Homo 591
Homocercal : 435
Homodont dentition 441
Homology of organs “I
Homoplastic = - 2
Honeycomb 514
Horns 576
Horse 506
Humerus 473
Hyzena 581
Hydatina 151
Hydra Ili
n development of 116
Hydra-tuba of Aurelia 129
Hydrocorallinze 135
Hydroid colonies 117
Hydromedusze 120
Hydrozoa 133
Hymenoptera - 248
Hyoid arch of Vertebrata 417
Hyomandibular arch 417
Hypoblast - 50
Hypsoprymnus 496
Hypophysis 425
Hyracoidea 567
Hyrax 567
Hystricomorpha - 564
ICHTHYOPTERYGIUM OF
FISHES 435
Ichthyornis 452
Ichthyosauria 447
Iguanodon -
Tum -
Incisor
Infundibulum — -
Ink-gland of Cephalopoda
Insecta
Insectivora
Interorbital septum
Intervertebral discs
Intracellular digestion
Invagination
Ischium
Isopleura
Iter
JACKAL
Jaguar
Javan Pangolin
Jellyfish = -
Jugal -
Julus -
KANGAROO -
Karyokinesis (Mitosis)
Keber’s organ -
Keratin —-
Kidney of Vertebrata -
King-crab - -
King of the herrings
Kiwi -
Koala
LABYRINTH OF Ear-
Lacertilia
Lachrymal gland
Lacteal dentition
Lagomys
Lamellibranchiata
Lamprey
Lamp shells
Lancelet
Lateral line system
Laurer’s duct
Leech
Lemuridz
Lemuroidea
Leopard
Lepas
Lepidoptera
Lepidosiren
Lepidosteus
Lepisma
Lepus
Leucon type of sponge
Lice
Limbs and girdles of Verte-
brata-
Limnzeus
Limpet
Limulus
Lingula -
Lion -
Littoral life
Liver fluke
Lizards
Llama
Lobosa
Lobster
Lobworm
Locust
Lophobranchii
Lophophore
Lophopus
Lumbricus -
Lung-books of scorpion
Lungs -
Lymph
u hearts
Macacus
Macronucleus
Macropodidee
Macropus
Madreporite
Malacostraca
Malar or jugal -
Malpighian tubules of cock-
roach
Mammalia - -
nu Geographical distri
bution of -
Mammary glands
Mammoth -
Manatee -
Mandible of crayfish -
Mandibular arch of Verte-
brata
Manidz
Manis
Mantle of Mollusca
INDEX.
PAGE
Manubrium of sternum
| Medusa
Manyplies
Marmosets -
Marmot
Marsupials (Metathesies
Marten
Mastigophora
Mastodon
Maxillee of crayfish -
Maxillipedes of crayfish
May-flies
Medulla
Medusze
Medusoids
Megachiroptera
Megatherium
Membrane bones
Meroblastic segmentation
(partial) : 49,
Mesenteries of sea-anemone
Mesenteron
Mesoderm -
Mesogloea
Mesonephros
Mesopterygium
Metacarpals
“Metagenesis, or alternation
of generations
Metamere
Metanephros
Metapleural fold
Metapterygium
Metatarsals
Metatheria
Metazoa -
n and Protozoa -
Microchiroptera -
Micronucleus
Miliola
Milk
Millipedes
Mites -
Mitosis
Moa 7
Molars
Mole -
n cricket
Mollusca
Monkeys
624
Monocystis-
Monogenea
Monotremata
Morphology
Morula -
Moths
Mud fishes
Miillerian duct
Muscle -
Muscular tissue
Mussel, freshwater
Mustelidze
Myomeres
Myotomes
Myriapoda -
Myrmecobius
Myrmecophagidze
Mystacoceti
Myxine
Myxinoidei
NNARCOMEDUSE -
Narwhal
Nasal capsule
Natural selection
Nauplius
Nautilus
Nekton
Nematocysts
Nematoda -
Nemathelminthes
Neornithes -
Neogoea - : :
" Mammals of -
Nephrops
Nereis
Nerve tissue
Nerves, Cranial,
brata
Nervous system
Neuropodium
Neuroptera
Newts
Nictitating
eye
Non-calcearea-
of Verte-
of
membrane
Notochord, Origin of
‘i Sheath of -
Notogcea
" Mammals of
INDEX.
PAGE
102 | Notopodium
149 | Notoryctes
492 | Nucleus” -
22 | Nudibranchs
50 | Nummulites
253
439 | OBELIA
426 | Octopus
16 | Oculomotor nerve
34 | Odontoceti - -
269 | Odontophore of Cephatopeds
583 u of Snail -
300 | Olfactory lobes
424 n Nerves -
245 | Oligocheeta
502 | Omentum
558 | Oniscus
579 | Ontogeny
309 | Ooze, Atlantic
434 | Operculum of Teleostei
Ophidia
133 | Ophiuroidea
579 | Opisthoccelous
415 | Opossums
82 | Optic lobes
241 nn nerves
285 « thalami
39°| Orang
114 | Organs
155 " origin of
155 » rudimentary
449 | Oriental region -
63 1" Mammals of
597 | Ornithorhynchus
204 | Orthoptera -
239 | Orycteropus
304 | Osculum
Osphradium of Mollasea
408 | Ossicles of ear
19 | Ostracoda
239 | Ostrich
254 | Otaria
440 | Otocysts
Otter - -
360 | Oviducal gland of skate
109 | Oviduct of Vertebrata
418 | Ovotestis of snail
413 | Ovum, the -
63 maturation of the
595 | Oyster
PALONTOLOGY
Palato - pterygo - quadrate
cartilage -
Palp
Panda
Pangolin
Panniculus adiposus
" carnosus
Parachordals of skull
Paramcecium
Parapodia
Parasitism -
Parasphenoid
Parenchyma
Paroccipital process
Parthenogenesis -
Paunch
Peccaries
Pecora - -
Pectines of scorpion
Pectoral girdle
Pedipalpi
Pelagic life
Pelvic girdle
Peragale
Peramelidze
Pericardium
Peripatus
Periptychus
Perissodactyla
Periwinkle -
Petaurus
Petromyzon
Phalangeridz
Phalanges
Phascolarctos
Phascolomyidze
Phascolonus
Phenacodus
Phoczena
Phocidze
Phylogeny
Physeteridze
Physiology - }
Physical relations of animals
Physostomi
Pigs
Pigeon
Pineal body
Pinnacocyte
M.
INDEX.
PAGE
70 | Pinnipedia
Pipe-fishes - -
417 | Pisces (see Fishes)
211 | Pisiform
583 | Pituitary body
560 | Placenta of Mammals - -
453 | Placentation, classification of
453 | Placoid scales
“416 | Plankton -
89 | Plants and animals
239 | Planula larva
75 | Plastron
416 | Platanista
139 | Platyhelminthes -
499 | Plectognathi = - -
44 | Pleurobranchs of crayfish
514 | Pleurodont teeth
575 | Pluteus larva
576 | Pneumogastric nerve (see
258 Vagus) - -
419 | Podobranchs of crayfis
258 | Polar bodies
57 | Polycheeta -
419 | Polygordius
501 | Polyprotodontia
502 | Polypterus -
425 | Polyzoa (see Bryozoa) -
231 | Porcupine
567 | Porifera
571 | Porpoise
284 | Prawn
504 | Preen gland of pigeon
434 | Primary vesicles of brain
504 | Primates
420'| Primitive streak -
504 | Proboscidea
504 | Proccelous -
505 | Procyonidz
566 | Proechidna-
542 | Proglottis
584 | Pronephros
48 | Propterygium
579 | Protandric - -
14 | Protective resemblance
71 | Protelidz
438 | Proteus
575 | Protocercal
360 | Protogynous
407 | Protoplasm
105 | Protopterus
41
626
Prototheria -
Protracheata
Protozoa
Psalterium -
Pterodactyles
Pteropods
Pteropus
Pterosauria
Pulmonata -
Puma
Pupa
Pygostyle
QUADRATE
RABBIT
Raccoon
Radial symmetry
Radiolaria
Ratitee
Rattlesnake -
Recapitulation, law of
Rectal gland
Redia
Reed -
Reproduction
Reproductive aan
Reptilia
Respiration in andinals
Respiratory system
Retia mirabilia
Reticulum
Retina
Reversion
Rhagon
Rhea -
Rhinoceros
Rhizopoda -
Rhytina
Ribs of Vertebrata
Rodentia -
Rotatoria = Rotifera
Rudimentary organs
Rumen - - *
Ruminants (Pecora)
Rumination
INDEX.
PAGE
488 | SAcRUM
244 | Sagitta
97 | Salamander
514 | Salpa
447 | Sauropsida -
60 | Scales of fishes
551 of reptiles
447 | Scapula
283 | Schizoccele
581 | Sciuromorpha
246 | Sclerotic
371 | Scolopendra
Scorpion
Scutes -
ay Scyphomedusze
Scyphozoa -
382 | Scyphula
583 | Sea-anemone
22 1 COWS
99 1 cucumbers
313 un lion -
338 w squirts (w2de Tieteatal
565 n urchin
451 | Seals - -
442 | Sebaceous glands
48 | Segmentation of ovum
315 | Selenodonta : -
142 | Semicircular canals of ear
514 | Sense organs
38 | Sepia -
21 | Sexual selection -
440 | Sexual reproduction
8 | Shell of—
18 Anodonta
464 Argonauta
514 Chiton
409 Helix
81 Nautilus
10g | Shrews
452 | Shrimp
572 | Simia
99 | Simiidze
562 | Simplicidentata -
418 | Siphon of Cephalopoda
563 | Siphuncle of Nautilus -
152 | Sirenia
26 | Skate -
514 u development of
576 | Skeletal system
514 | Skin
INDEX. 627
PAGE PAGE
Skull - 416 | TADPOLE oF FRoc 356
Skunk 583 | Tenia - 144
Sloths 558 | Tails of fishes 435
Sloth, as arboreal type 533 | Talpa 537
Snail (see Helix) 262 | Tamandua - 558
Snakes 442 | Tape-worms 144
Species 80 | Tapir- 572
Spermatozoa 43 | Tarsius 558
Sphenethmoid 348 | Tarsus 420
Sphenodon 441 | Tarso-metatarsus 375
Sphincter muscles 16 | Tasmanian wolf - 502
Spicule 99, 104, 125 Teeth of mammals 460
Spiders Teleostei - 438
Spider, as a type of Ar ach- Teleostomi 437
nida - 233 | Telolecithal - 49
Spider Monkeys - 590 | Temperature of birds - 465
Spinal nerves 408 | Temporal (see Squamosal) 416
Spiracle of skate 314 | Tenrec - - 586
Spiracular cartilage 325 | Tentaculocysts 128
Spiral valve - 315 | Teredo - 284
Sponge, development of a 106 | Terrestrial fauna 61
Sponges 107 | Test of Ascidian 289
Spongilla 110 | Tetrabranchiata 285
Sporocyst 142 | Thalamencephalon 406
Sporozoa 102 | Thaldssicola - 100
Squamosal - 416 | Thread cells=Stinging cells 115
Squirrels 565 | Thread-worms (Nematoda)- 155
Starfish 156 | Thylacinus 502
Steller’s sea-cow - 562 | Thymus 425
Sternum of Vertebrata 418 | Thyroid 425
" Mammals - 471 | Tibia - 419
Stigmata 228, 258, 290 | Tibio-tarsus 375
Stinging cells 114 | Tiger - 581
Stone canal of starfish 160 | Tissues 31
Struggle for existence - 82 | Toads 440
Struthio 452 | Tornaria 165
Sturgeon - - - 437 | Tortoises - 444
Sub-neural — gland of Trabeculz of skull 415
Ascidian - 292 | Trachez 229
Sudorific glands - 455 | Tragulidee 575
Suidz - 575 | Tragulus 576
Sun animalcules - 99 | Trematoda - 149
Sus - 575 | Trichechidze 584
Swim- bladder (air- Binder) 2 437 | Trichina 155
Sycandra - 103 | Trichocysts go
Sycon type (of sponge) 108 | Trochanter 474.
Symbiosis. 74 | Trochlear nerve - 407
Symmetry of animals 22 | Trochophore 181
Sympathetic nervous system 347 | Truncus arteriosus 345
Syrinx 368 | Tunicata —>~ 403
628
Turbellaria
Turtles
Tympanic bulla
Typhlosole
ULNA
Umbo
Uncinate processes
Ungulata
Ureters
Urogenital ducts
Urochorda
Urodela
Urostyle
Ursidee
Uterus -
Utriculus of ear
VACUOLES, contractile
" food -
Vagina
Vagus nerve
Vampire bat
Variation
Vascular system -
Venous system (see Vascular
system) -
Ventricles of brain
" of heart
Venus flower-basket
nu girdle
Vermiform appendix
Vertebra, parts of a
Vertebral column
Vertebrata
Vestigial structures
Vicufia -
Visceral arches
INDEX.
PAGE PAGE
150 yilseods humor 409
445 iverra 581
468 | Vole 565
201 | Volvox 24
Vorticella - 92
474
27° | WaLDHEIMIA 168
a Wallace’s line 596
5 : Walrus 584.
o é Wasps 249
pa Water boatman 256
"scorpion - 256
ae Water - vascular system of
=e starfish 160
5 6a Weasel 583
sn z Whales - - 579
4 ie animalcule (Rotifera) 152
ing of bat - 552
a7 u of birds 361, 373
a nu of insect 225
pas Wingless birds 64
586 Wolf - — - ae 583
85 Wolffian duct 426
aS XANTHARPYIA 557
18
407 redid Cris 99
21 olk 48
Ene Yolk-sac - 429
I 3 8 " placenta 429, 482
395
418 | ZEBRA 608
418 | Zoea 24.
405 | Zoological realms ra
26 | Zoophytes 117
576 | Zygapophyses - 418
417 | Zygomatic arch - 468
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