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OUTLINES OF ZOOLOGY
OUTLINES OF
ZLOOLOGY
BY
ry
Ke ARTHUR THOMSON, M.A.
REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN 3
JOINT-AUTHOR OF ‘‘ THE EVOLUTION OF SEX”;
‘THE STUDY OF ANIMAL LIFE,” ‘THE SCIENCE OF LIFE,”
‘‘THE PROGRESS OF SCIENCE,” ‘“‘ HEREDITY,”
** DARWINISM AND HUMAN LIFE,” ETC.
AUTHOR OF
FIFTH EDITION, REVISED, WITH q20 ILLUSTRATIONS
NEW YORK
D. APPLETON AND COMPANY
1916
107
PREFACE TO THE SIXTH EDITION
—>—
THis book is intended to serve as a Manual which
students of Zoology may use in the lecture room, museum,
and laboratory, and as an accompaniment to several well-
known works, cited in the Appendix, most of which follow
other modes of treatment.
To numerous authorities I acknowledge an: obvious
indebtedness, a detailed recognition of which would be
out of place in a book of this kind. I must, however,
acknowledge that in the preparation of a previous edition
I had throughout the able assistance of Miss Marion
Newbigin, D.Sc., and I have also been aided by sugges-
tions from various kindly critics, especially Professor
W. C. M‘Intosh, Professor P. J. White, the late Dr.
Ramsay Traquair, Dr. John Beard, the late Mr. J. G.
Goodchild, Dr. Arthur Masterman, Dr. John Rennie,
Dr. W. D. Henderson, Mr. E. S. Russell, Mr. W. P.
Pycraft, Mr. C. Tate Regan, and Professor H. J. Fleure.
For most of the figures I am indebted to my artist
friends, Mr. William Smith, Miss Florence Newbigin,
vi PREFACE TO THE SIXTH EDITION.
Miss E. M. Shinnie, and the late Mr. George Davidson.
In almost every. case the illustrations have been derived
from original memoirs and works of reference, or drawn
from specimens.
° J. A. T.
THE UNIVERSITY,
ABERDEEN, March 1914,
CONTENTS.
° —+—
GENERAL
CHAPTER I ‘
PAGE
GENERAL SURVEY OF THE ANIMAL KINGDOM 5 « FE
CHAPTER II
PHYSIOLOGY c , ‘ i . é * 2
CHAPTER III
MORPHOLOGY. . 7 . : . » 34
CHAPTER IV
EMBRYOLOGY . : A . . ' » 52
CHAPTER V
PALLONTOLOGY . . . . . » 97
CHAPTER VI
DocTRINE OF DESCENT . ‘ a i " . 84
vill CONTENTS.
INVERTEBRATES
CHAPTER VII
PROTOZOA .
CHAPTER VIII
SPONGES . F “ : ‘ . :
CHAPTER IX
COLENTERA . ‘ P : * .
. CHAPTER X
UNSEGMENTED ‘‘ WorRMS” i P F %
CHAPTER XI
ANNELIDS ' . .
CHAPTER XII
ECHINODERMS . ‘
CHAPTER XIII
ARTHROPODA . .
CHAPTER XIV -
ONYCHOPHORA OR PROYOTRACHEATA, Myrioropa,
INSECTA ; ‘ ‘i ‘ %
CHAPTER XV
ARACHNOIDEA AND PALAOSTRACA . . .
'
CHAPTER XVI
MOoLuuscs . ‘ ‘ ‘ ‘ ql
PAGE
88
124
- 137
179
209
« 280
AND
318
363
+ 380
HEMICHORDA
TSC HORS
CEPHALOCHORDA
STRUCTURE AND
-CYCLOSTOMATA
FISHES .
AMPHIBIA
REPTILES.
BIRDS ‘
MAMMALS
DEVELOPMENT OF VERTEBRATES
CONTENTS.
VERTEBRATES
CHAPTER XVII
CHAPTER XVIII
CHAPTER XIX
CHAPTER XX
CHAPTER XXI
CHAPTER XXII
CHAPTER XXIII
CHAPTER XXIV
CHAPTER XXV
CHAPTER XXVI
ix
PAGE
434
443
459
473.
516
529
578
610
647
692
x CONTENTS.
GENERAL
CHAPTER XXVII
DIsTRIBUTION
CHAPTER XXVIII
THEORY OF EVOLUTION.
APPENDIX ON BOOKS
INDEX
LIST OF ILLUSTRATIONS
© ON ONAWNHNG
. Fertilisation in Ascaris megalocephala—after Boveri .
. Modes of Segmentation : : : ‘ ;
. Life history of a coral, Monoxenia darwinii—from Haeckel .
. Embryos—(1) of bird; (2) of man—after His. The latter
—>—
. Duckmole (Ornithorhynchus). z
/’henacodus, a primitive extinct Mammal—after Cope
Extinct moa and modern kiwi—-after Carus Sterne
. Crocodiles . é
Salamander, an Amphibian
. Queensland dipnoan (Ceratodus)
. Alancelet, Amphioxus—after Haeckel
. Ascidian or sea-squirt—after Haeckel
. Cephalopod (paper nautilus, female) . : : :
. Fresh-water crayfish (Astacus), a Crustacean—atter Huxley .
« @, Caterpillar; 4, pupa; c, butterfly . . :
. Spider . .
. Crinoid or feather-star
. Earthworm ‘i ‘ ;
. Bladderworm stage of a Cestode—after Leuckart 3
. Sea-anemones on back of hermit-crab—after Andres . ‘
. Fossil Foraminifera (Nummulites) in limestone—after Zittel .
. Diagrammatic expression of classification in a genealogical
tree. B indicates possible position of Balanoglossus.
D of Dipnoi, S of Sphenodon or Hatteria x %
. Diagram of Vertebrates
. Diagram of Invertebrates 7
. Diagram of cell structure—after Wilson
. Structure of the cell—after Carnoy
. Fertilised ovum of Ascarzs—after Boveri
. Diagram of cell division—after Boveri te
. ‘Karyokinesis—after Flemming é . .
. Diagrammatic expression of alternation of generations
. Diagram of ovum, showing diffuse yolk granules.
. Forms of spermatozoa (not drawn to scale) . :
. Diagram of maturation and fertilisation. (From “Evolution
of Sex”) . F :
about twenty-seven days old F :
a)
>
a
a
WOON ANU WW N
LIST OF ILLUSTRATIONS.
- Gradual transitions between Paludina neumayré (a), the
oldest form, and Paludina hernesd (7)—from Neumayr .
. Life history of Amaba
. Actinophrys sol (Sun- animalcule)—after Grenacher
. Polystonella, megalospheric form, with large central chamber
(AZ) and one nucleus (4/)—after Lister
Polystomella, microspheric form, with large central chamber
(c.c.), numerous nuclei (), bridges of- protoplasm
between chambers (8)—after Lister a ‘
. Paramecium—after Biitschli .
. Conjugation of Paramecium aurelia — four stages—after
Maupas .
. Diagrammatic expression of process of conjugation i in Para-
mectum aurelia—after Maupas . .
. Vorticella—after Biitschli , . : ‘
. Volvox globator—after Cohn . i 3 .
. Life history of MWonocystzs—after Biitschli a .
. Life history of Gregarina—after Biitschli . .
. End-to-end union of Gregarines—after Frenzel -
. Life history of Coccidium—after Schaudinn. . . .
. Diagram of Protomyxa aurantiaca—after Haeckel . .
. Formation of shell in a simple Foraminifer—after Dreyer
. A Foraminifer (Polystomel/a) showing shell and pseudopodia
—after Schultze .
. A pelagic Foraminifer—Hastigerina. (Globigerina) Murrayi
after Brady
. Optical section of a Radiolarian (Actinomma)—after Haeckel
. Glossina palpalis, tse-tse fly ‘i
. Trypanosoma gambiense
. A colonial flagellate Infusorian—Proterospongia haechelii—
after Saville Kent .
. Simple sponge (Ascetta primoriialis)—afier F Haeckel ss
. A sponge colony :
. Sponge spicules
. Section of a sponge—after FE. Schulze F
. Diagram showing types of canal system—after Korschelt and
Heider. The flagellate regions are dark throughout, the
mesogloea is dotted, the arrows show the direction of the
currents. All the figures represent cross-sections through
the wall . :
. Development of Sycandra raphanus—after F, E. Schulze
. Diagrammatic representation of mag susan of Oscarella
/obularis—after Heider .
. A, Young ae i a Whitman. “RB, Female Orthonectid
(Khopalura giardii)—after Julin .
. Salinella—after Frenzel
. Diagram of Ccelenterate structure, endoderm ‘darker through:
out
. Colony of Hydractinia on back of a Buccinum shell tenanted
by a hermit-crab . ‘ . . : .
93-
- Diagram of a typical Hydrozoon polyp—after Allman
. Hydra hanging from water-weed—after Greene é
. Minute structure of Aydra—after T. J. Parker and sebes
. Development of ydra—after Brauer
. Bougainvillea—after Allman
. Structure of a Medusoid—after Allman
. Surface view of Aurelia—from Romanes
. Vertical section of Aure/a—after Claus ;
. Diagram.of life history of Aure/ia—after Haeckel .
. Lucernaria—after Korotneff
. Diagram of Lucernaria—after Allman
. External appearance of Tealia crassicornis. .
. Vertical section of a sea-anemone—after Andres
. Section through -sea-anemone (across arrow in Figure 79)—
. Alcyonarian Spicules
. Diagram of a gymnoblastic Hydrotd—after Allman .
. Graptolites ; ;
. Hydroids—after Hincks :
. Campanularian Hydroid—after Allman .
. Diagram: of a Ctenophore—after Chun
. Hydroctena. A medusoid with hints of Ctenophore structure
LIST OF ILLUSTRATIONS.
after Andres
. Z, Diagrammatic section of Zoantharian ; re of Alcyonarian
—after Chun
. The formation of a coral- shell (Astroides)—after Pfurtscheller
. Structure of Antipatharians .
Diagrams of Types of Alcyonarla—after Hickson
Coralium rubrum, a corner of a Ss Lacaze-
Duthiers
Commensalism of sea-anemones and hermit-crab_ . .
934. Portions of excretory system of flat-worms a .
94.
95.
Diagram of Turbellarian—after Lang i E -
Structure of liver fluke—after Sommer. From ventral surface.
The branched gut’(g.) and the lateral nerve (/.7.) are
shown to the left, the branches of the excretory vessel
(e.v.) to the right -
. Reproductive organs of liver ‘fluke—after Sommer : .
. Life history of liver fluke—after Thomas. : .
. Diagram of life cycle of liver fluke -
Diagram of reproductive organs in Cestode joint—Constructed
from Leuckart . .
. Life history of Zenda solium—after Leuckart . ‘
. Diagram: of life history of Zaxza solium
. Diagrams of bladder-worms . ,
. Diagrammatic longitudinal section of a Nuedtoas (Amphi-
porus lactifloreus), dorsal view—after M‘Intosh
. Transverse section of the Nemertean Drepanophorus latus
—after Birger .
. Transverse section of a simple Nemertean (Car inella)—after
Biirger . . : . . . .
Xilf
PAGE
141
143
145
148:
150
ISI
153
155
150
158.
158
160-
161
162.
163.
165
166.
167
168.
169:
170
17I
172
173
174
175
177
178
181
184
185
187
189:
192
193
194
196:
197
198
19%
LIST OF ILLUSTRATIONS.
. Cross section through Ascarés . .
. Illustrating the structure of a Nematode ,
. Trichine in muscle, about to be encapsuled—after Leuckart
. Trichine in muscle, encapsuled. There may be 12,000 in
a gramme of pig’s muscle—after Leuckart
. Earthworms. . . .
. Anterior region of earthworm—after Hering A
. Transverse section of earthworm. . .
. Reproductive organs of earthworm—after Hering .
. Stages in the development of earthworm—after Wilson F
. Arenicola marina
. Anterior part of nervous system in Arenicola—after Vogt
and Yung ‘i ‘
. Dissection of lob-worm from “dorsal surface ‘
. Cross-section of Arenzcola—after Cosmovici
. Development of Polygordzus—after Fraipont . .
. Parapodium of ‘‘Heteronereis” of MVerezs pelagica—after
Ehlers . .
. Free-living Polycheete (Nereis cultrifera) 5
. Transverse section of leech—after Bourne F
. Alimentary system of leech—after Moquin- -Tandon . 7 ‘
. Dissection of leech—after Bourne . : 5 ‘
. A nephridium of leech—after Bourne . :
. Development of Sagitta—after O. Hertwig. Illustrating
formation of a body cavity by pockets from the archen-
teron ; also the early separation of reproductive cells.
. Actinotrocha or larva of Phoronds—after Masterman
. Phoronis
. Diagram of an Ectoproctous "Polyzoon (Plumatella)
. Interior of Brachiopod shell, showing calcareous support for
the ‘‘arms ”’—afler Davidson .
. Pluteus larva of Ophiuroid, with rudiment of adult—after
Johannes Miiller
. Star-fish 5 . .
. Alimentary system of star-fish—after Miiller ‘and Troschel .
. Diagrammatic cross-section of star-fish arm—after Ludwig .
. Ventral surface of disc of an oes (Ophiothrix pica
—after Gegenbaur ji .
. Apical disc of sea-urchin : . .
. Dissection of sea-urchin 3 F .
. Spicules of Holothurians—after Semon .
. Dissection of Holothurian (Holothuria tubulosa) from the
ventral surface .
. Diagrammatic vertical section through disc and base of one
of the arms of Antedon rosacea—after Milnes Marshall .
. Stages in development of Echinoderms—after Selenka
. Appendages of Norway lobster :
. Section of compound eye of A/ysés vulgaris—after Grenacher
. Longitudinal section of lobster, showing some of the organs
. Male reproductive organs of crayfish—after Huxley
. Mouth appendages of cockroach—after Dufour
. Transverse section of insect—after Packard .
. Head and mouth parts of bee—after Cheshire
. Nervous system of bee—after Cheshire 2
. Food canal of bee—in part after Cheshire
. Hive-bees and the cells in which they develop
. Mouth-parts of mosquito—from Nuttall and Shipley
. Young may-fly or ephemerid—after Eaton .
. Diagrams of insect embryo—after Korschelt and Heider
. Life histories of insects
. Life history of the silk-moth ‘(Bombyx mori)
. Development of blow-fly ne erythrocaphala) —ateer
LIST OF ILLUSTRATIONS.
. Female reproductive organs of crayfish—after Siskow :
- Section through the egg of, Astacus aftér the completion of
segmentation—after, Reichenbach
. Longitudinal section of later embryo of Astacus—after
Reichenbach
. Section through cephalothorax ofa crab—after Pearson
. Dorsal aspect of swimming crab (Portunusy
. Dorsal aspect of shore crab (Carcinus)
. Ventral aspect of female shore crab .
- Dorsal surface of. Apus cancriformis — from “Bronn’s
_ Thierreic! .
. Daphnia
Cypris
. Cypris, side view, after removal of o1 one valve—after “Zenker,
. Cyclops type .
. Acorn-shell (Balanus tintinnabulum)—after Darwin
. Development of sail china (Not drawn to
scale). . ‘ . 2
. Nebatia—after Sars... . .
. Anaspides—after Calman. 5 ‘ :
. An Amphipod (Caprella linearis) . ; . ‘
. Hermit-crab withdrawn from its shell. The anterior ap-
«, pendages are broken off ‘ A : .
. Mysis flexuosa, from side
. Nervous system of shore-crab ( Carcinus manas)—afier Bethe
. Zozea.of common shore-crab (Carcinus menas)—after Faxon
. External form of Peripatus—after Balfour .
. Dissection of Peripatus—after Balfour
. Embryos of Peripatus capensis, showing closure of blasto-
pore and curvature of i ala Korschelt and Heider
. A millipede . . a . . .
. Acentipede . . 3 .
. Female cockroach (P. ortentalis) ane :
Male cockroach (P. orzentalzs) 1
Ventral aspect of male cockroach with the wings: extended.
An imaginary median line has been inserted
. Leg of cockroach,
Thomson Lowne
5
XV.
PAGE
293
294
295
296
297
297
298
299
300
xvi
FIG.
188,
189.
190.
191.
192.
193.
. 194.
195.
196.
197.
198.
199.
200.
201.
202.
203.
204.
205.
206.
207.
208,
209.
210,
2m.
212.
213.
214.
215.
216,
217
218,
219.
220,
221.
222,
223.
224,
225.
226.
227).
228,
229.
LIST OF ILLUSTRATIONS.
Mosquito—from Nuttall and Shipley :
Anurida maritima (after Imms), one of the primitive wing-
less Collembola s . . .
Acerentomon, a very primitive. insect .
Scorpion. f ‘ : . .
Garden spider
Dissection of Mygale from the ventral surface—after Cuvier
Section of lung-book—after Macleod A
Follicle-mite (greatly enlarged)
Itch-mite (Sarcoptes scabiez) (greatly “enlarged)
Tick (Zxodes réduvius, female), dorsal surface (after Wheler),
showing the oval shield. .
Tick (Zxodes riduvius, nie ventral surface—after
Wheler . i ; . 7
Limulus or King-crab i 5 :
Young Lzmulus—after Walcott
Trilobite (Cosocephadétes)—after Barrande .
Vertical cross-section of a Trilobite (Calymene)—after
Walcott .
Sea-spider (Pycnogonum littorale), from the dorsal surface .
Male of Nymphon—after Sars é P 3 .
Ideal mollusc—after Ray Lankester . . .
Stages in molluscan development . 5 . .
Roman snail (Helix pomatza) i
Vertical section of the shell of a species of Helix .
Dissection of snail
Reproductive organs of Helix pomatia—after Meisenheimer
Snail (Helix pomatia), laying its eggs—after Meisenheimer
Diagram of larva of Paludina—after Erlanger
The fresh-water mussel ( Uzo) f .
Structure of Axodonta—after Rankin . z
Development of Anodonta—after Goette
Side view of Sepza—after Jatta ‘
External appearance of a cuttlefish (Lotigo) .
Diagram of the structure of Sef7a.—Mainly after Pelseneer |
Diagram of circulatory and excretory systems in a i as
like Sepia—after Pelseneer
Male of Avgonauta (after Jatta), showing ‘« hectocotylus ”
arm ; compare Fig. 9 of female .
Bunch of Sepia eggs attached to plant—after Lieto 3
Common buckie (Buccinum undatum) ‘
Bivalve (Panopea norvegica), showing siphons
Nudibranch (Dendronotus arborescens), showing dorsal out-
growths forming adaptive gills .
Ventral surface of Patella vulgata—after Forbes and Hanley
Chiton—after Prétre :
Dorsal view of nervous system of Chiton—after Pelseneer .
Anatomy of Chiton .
A Pteropod (Cymbulia peronii), showing the wing- ‘like ex-
pansions (pteropodial lobes) of the mid-foot . s
LIST OF ILLUSTRATIONS.
FIG.
230. Stages in molluscan development . .
231. Proneomenta. Nervous system—from Hubrecht | .
232. Section of shell of Nautilus—after Lendenfeld . .
233. The Pearly Nautilus (Mautdlus pompelius)—after Owen
234. Male of Balanoglossus (Dolichoglossus) howalevskit—after
Bateson.
235. Transverse section through. gill- -slit region of Piychodera
minuta—after Spengel .:
236. Direct development of Dolichoglossus—after Bateson
237. Tornaria larva, from the side—after Spengel _—_—.
238. Piece of a colony of Cephalodiscus, showing the tubes in-
habited by the animals—after Ridewood i
239. An individual Cephalodiscus—after Ridewood ?
240. Dissection of Ascidian—after Herdman . P
241. Diagram of Ascidian—after Herdman
242. Young embryo of Ascidian ( Clavelina)—after Van Beneden
and Julin 7
243. Embryo of Clavelina—modified after Seliger ‘ .
244. “Nurse” of Doliolum miilleri—after Uljanin :
245. Sexual individual of Dololum miilleri—after a é
246. Diagram of Salpa africana . .
247. Anatomy of Appendicularia—after Herdman E
248. Lateral view of Amphioxus—after Ray Lankester . ,
249. Transverse section through phenees region of Amphioxus
—after Ray Lankester .
250. Development of atrial chamber ‘in Amphioxus—alter
Lankester and Willey .
251 and 252. The nephridia of ‘Amphioxus—after Boveri 2
253. Early stages in the development of Amphioxus—after
_ Hatschek
254. Sections through embryos of Amphioxus, to illustrate de-
velopment of body cavity .
254A. Portions of excretory systems of Phyllodoce and Api
oxus—after Goodrich
255.°Transverse section through an Elasmobranch "embryo
(diagrammatic)—after Ziegler . 4
ase and 257. Ideal fore and hind limb—after Gegenbaur :
258. Partial section of brain of young Vertebrate. c
259. Vertical section of the ne eye in an embryo of Spheno-
don—after Dendy
260. Diagram of parts of the brain in Vertebrates — after
Gaskell . a . 5 :
261. Diagrammatic section of spinal cord
262. Diagram of spinal cord of man, thoracic region— after John-
ston ;
263. Diagram showing the ear and related parts in a young cat .
264. Diagram of the eye .
265. Development of the eye—after Balfour and Hertwig
266. Origin of lungs, oe and Meret in the chick—after
Goette . . . : ‘
xviii LIST OF ILLUSTRATIONS.
FIG,
267. Section through a young newt . : '
268. Blood corpuscles of Vertebrates .
269. Diagram of circulation—after Leunis
270. Development of excretory system of Vertebrate—in part
after Boveri . . i .
271. Urogenital system . . .
272. Mammalian ovum—after Hertwig
273. Median longitudinal section of anterior end of Myxine—
after Retzius and Parker r . :
274. Respiratory system of hag, from ventral surface.
275. Bdellostoma stouti (Californian hag), aiitiriaiie in sheath of
mucus—after Bashford Dean
276. The lamprey (Petromyzon marinus)
277. Longitudinal vertical section of anterior "end of larval
lamprey—after Balfour .
278. Restored skeleton of Palaospondylus gunni—after Traquair
279. Pterichthys milleré. Lateral view—restored by Traquair"
280. Under surface of skull and arches of skate—after W. K.
Parker . 7 :
281. Side view of skate’s skull—after W. K. Parker F ‘
282, Skeleton of skate—from a preparation . .
283. Dissection of nerves of skate ~
284. Side view of chief cranial nerves of Blasmobranchs—sightly
modified from Cossar Ewart. ‘ :
285. Spiral valve of skate—after T. J. Parker .
. Upper part of the dorsal aorta in the skate—after Monro
. Heart and adjacent vessels of skate—in part after Monro .
. Urogenital organs of male skate . :
. Urogenital organs of female skate—in part after Monro ‘
. Elasmobranch development—after Balfour .
Embryo dogfish in egg-case (‘* Mermaid’s purse °) which
has been cut open to show contents
. The haddock .
. External characters of a Teleostean—a carp (Cxprinus
carpio)—after Leunis
. Caudal vertebra of haddock .
. Disarticulated skull of cod—from Edinburgh Museum of
Science and Art
. Pectoral girdle and fin of cod—from Edinburgh Museum of
Science and Art
Diagram of a Teleostean gill i in section . ‘
. Diagram of Teleostean circulation—after Nuhn_ .
. The early development of the salmon ‘i ‘. ri
. Development of eel—after Smit . j . :
. Young skate—from Beard .
. Lateral view of dogfish (Scyd/éum catulus) . ‘:
. Outline of Acanthodes sulcatus—after Traquair .
. Larva of Polypterus (after Budgett), 14 inch in length
. Sturgeon (Aczpenser sturio) ’. . . P
. The goldfish (Cyprinus auratus) . . i z
FIG.
307-
308.
309.
310.
311.
312.
313.
314.
315.
316.
317.
318.
319.
320,
321.
322,
323.
324.
325.
326.
327.
328,
329.
330.
331.
332.
333+
334+
335+
37
338.
339:
340.
341.
342.
343-
LIST OF ILLUSTRATIONS,
Lepidosiren (after Graham Kerr), showing (22.7) pectoral
fin and the tufted pelvic fin (Pv.f,) of the mature male
Skeleton of Ceratodus fin—from Gegenbaur .
Head region of Protopferus—from W. N, Parker .
Larva of Protopterus—after Budgett . .
Larva of Lepédosiren—after Graham Kerr . :
The edible frog (Rana esculenta) j i
Vertebral column and pelvic girdle of bull- -frog A
Skull of frog—upper and lower surface—after W. K. Parker
Skeleton of frog. The half of the pectoral girdle, and fore-
and hind-limb of the right side are not shown . .
Pectoral girdle of Rana esculenta—after Ecker ,
Side view of frog’s pelvis—after Ecker . F
Brain of frog—after Wiedersheim . . . .
Nervous system of frog—after Ecker . . .
Arterial system of frog . : : . ’
Venous system of frog
Urogenital system of male edible frog—after Ecker .
Urogenital system of female frog—after Ecker .
Division of frog’s ovum—after Ecker
Section of frog embryo—after Ziegler’s model and Marshall
Dissection of tadpole—after Milnes Marshall and Bles ‘
Life history of a frog—after Brehm . i ‘
Cecilian (/chthyophis) with eggs—after Sarasin ‘
External appearance of tortoise
Skull of turtle és F
Carapace of tortoise
Pectoral girdle of a Chelonian
Internal view of bones of the plastron of the Greek tortoise
Scales on ventral surface of plastron of Greek tortoise
Infernal view of skeleton of tortoise . ‘
Dissection of chelonian heart—after Huxley . .
Heart and associated vessels of tortoise—after Nuhn .
Hyoid apparatus of a chelonian % f
Lateral view of brain of Hatteria punctata—after Osawa
Hatleria or Sphenodon—after Hayek - .
Side view of skull of Lacerta—after W. K. Parker . 3
Heart and associated vessels of a lizard—after Nuhn P
Lung of Chameleo vulgaris, showing air-sacs—after Wieders-
heim. . . . :
344. Anterior view of python’ s vertebra r ‘ ‘
345. Posterior view of python’s vertebra . ‘ ‘
346. Snake’s head—after Nuhn .. - .
347. Skull of grass-snake—from W. K. Parker . :
348. Lower surface of skull of a young crocodile . . :
349. Cervical vertebra of crocodile . . . .
350. Crocodile’s skull from dorsal surface . .
351. Pectoral girdle of-crocodile . : .
352. Half of the pelvic girdle of a young crocodile . :
353. Origin of amnion and allantois—after Balfour °
PAGE
XX
FIG.
LIST OF ILLUSTRATIONS.
3534. Vertical section through backbone and ribs of a Chelonian
381.
and a Mammal—in part after Jaekel
. Position of organs in a bird—after Selenka . :
. Fore-limb and hind-limb compared . . .
. Diagrammatic section of young bird—after Gadow . .
. A falcon :
. Young bearded griffin (Gypiictus barbatus)—after Nitzsch .
. Young feather and filoplume—after Nitzsch 7
. Types of feathers . . a 2 e
. Parts of a feather—after Nitzsch
. Entire skeleton of condor, showing the relative positions of
the chief bones .
. Disarticulation of bird’s skull—after Gadow. Membrane
bones shaded :
Under surface of gull’s skull : ‘
. Wing of dove i : “ 2 :
. Side view of pelvis of cassowary é : . .
- Bones of hind-limb of eagle . 7 . : .
. Brain of pigeon . .
. Diagrammatic section of cloaca of male bird—after Gadow .
. Heart and arterial system of pigeon . . ‘ .
- Heart and venous system of pigeon . . .
. Female urogenital organs of pigeon. : .
- Male urogenital organs of pigeon. : : .
. Pectoral girdle and sternum of swan
. Position of wings in pigeon at maximum ‘elevation—from
Marey . F ‘ ‘
. Wings coming down—from Marey .
. Wings completely depressed—from Marey . :
. Stages in development of chick—after Marshall
. Diagrammatic section of egg—after Allen Thomson .
. Diagrammatic section of embryo—after Kennel
Hesperornts—after Marsh . is ‘
381A. Fore and hind limbs of rabbit
382.
393:
394.
. Side view of rabbit’s skull . . .
. Dorsal view of rabbit’s skull , . .
. Under surface of rabbit’s skull i ‘i i
» Skull of capybara . - .
- Dorsal view of rabbit’s brain
. Under surface of rabbit's brain—after Krause
. Diagram of ceecum in rabbit «
. Duodenum of rabbit—from Krause, in part after Claude
Diagram of skull bones (partly after Flower and Weber),
the membrane bones shaded.
Bernard ‘ z a ,
. Circulatory system of the rabbit
Vertical section through rabbit’s head—from a section, with
help from Parker’s Zootomy and Krause F .
Urogenital organs of male rabbit. : . .
Urogenital organs of female rabbit . ‘ . .
LIST OF ILLUSTRATIONS.
FIG.
395. Segmentation of rabbit’s ovum—after Van Beneden :
396. Development of hedgehog. Three early sists alist
Hubrecht
397. Embryo of Perameles with its foetal membranes—after Hill”
398. Two stages in segmented ovum of hedgehog—after Hubrecht
399. Development of foetal membranes—after Hertwig . -
400, Diagram of foetal membranes—after Turner FA
401, View of embryo, with its foetal membranes—after Kennel .
402. Pectoral girdle of Echidna . . : : :
403. Pelvis of Echidna . q : 4 . :
404. Lower jaw of kangaroo 5 é :
405. Foot of young kangaroo.
406. Side view of sheep’s skull, with roots of back teeth exposed
407. Stomach of sheep—from Leunis a‘ ‘ ‘ ‘
408. Side view of lower part of pony’s fore-leg . z 5
409. Side view of ankle and foot of horse . .
410. Side view of horse’s skull, roots of teeth exposed . .
411. Feet of horse and its predecessors—from Neumayr -
412. Left fore-limb of Balenoptera .
413. Fore-limb of whale (Afegaptera longimana)—alter Struthers
414. Pelvis and hind-limb of Greenland whale alee eas
Struthers 7
415. Vertebra, rib, and sternum of Balenoptera—from specimen
in Anatomical Museum, Edinburgh . . .
416. Skull of tiger, lateral view . * : i 2
417. Lower surface of dog's skull 3 : .
418. Skull of Orang-Utan . F . . .
419. Skull of gorilla ; . ‘ A , j
420. Skeleton of male gorilla . . , . .
BIRDS.
Placentals.
MAMMALS. Marsupials.
SIMPLEST ANIMALS.
Flying Birds. Running Birds. Monotremes.
uw :
ie) :
a Snakes. Lizards. REPTILES. Crocodiles. Tortoises.
a
is Dipnoi. AMPHIBIANS.
= Bony Fishes. Newt. Frog.
2 FISHES.
a) “Ganoids.”” CYCLOSTOMES.
> Elasmobranchs, Lamprey. Hag-fish.
<
: LANCELETS. TUNICATES.
=
| BALANOGLOSSUS.
e Cuttle-fishes. a
© [Insects. Arachnids. ANNELIDS. Gasteropods. 4
>
Myriopods. MOLLUSCS. N
Peripatus. : °
Bivalves. >
ARTHROPODS. c
oa Crustaceans. “WORMS.” Feather-stars.
i Brittle-stars.
HH Star-fish.
<q
% ECHINODERMS.
ial
a) UNSEGMENTED J] Sea-urchins.
i WORMS. Sea-cucumbers,
[24
&
>
Z, Ctenophores. Jelly-fish. Sea-anemones. Corals.
Loe!
CCQELENTERA.
Medusoids and Hydroids.
SPONGES.
Infusorians. Rhizopods. Sporozoa,
VOZ
-OLOU
OUTLINES OF ZOOLOGY
CHAPTER I
GENERAL SURVEY OF THE ANIMAL
KINGDOM
In beginning the study of Zoology, it is natural and useful
to try to get a bird’s-eye view of the “ Animal Kingdom.”
Without this, one is apt to miss the plan in studying the
details. But the survey can be of little service unless the
student has the actual animals in his mind’s eye.
VERTEBRATES, OR BACKBONED ANIMALS
Mammals.—We begin our survey with the animals which
are anatomically most like man—the monkeys. But
neither we nor the monkeys are separated by any structural
gulf from the other four-limbed, hair-bearing animals, to
which Lamarck gave the name of Mammals. For although
there are many different types of Mammals—such as
monkeys and men; horses, cattle, and other hoofed quad-
rupeds ; cats, dogs, and bears ; rats, mice, and other rodents ;
hedgehogs, shrews, and moles, and so on—the common
possession of certain characters unites them all in one
class, readily distinguishable from Birds and Reptiles.
These distinctive characters include the milk-giving of
the mother mammals, the growth of hair on the skin, the
general presence of convolutions on the front part of the
brain, the occurrence of a muscular partition or diaphragm
between the chest and the abdomen, and so on, as we shall
I
2 GENERAL SURVEY OF THE ANIMAL KINGDOM.
afterwards notice in detail. Most mammals are suited for
life on land, but diverse types, such as seals, whales, and sea-
cows, have taken to the water. In another direction the
bats are markedly adapted for aerial life.
Among the mammalian characteristics of great import-
ance are those which relate to the bearing of young, and
even a brief consideration of these shows that some
mammals are distinguished from others by differences
deeper than those which separate whales from carnivores,
or rodents from bats. These deep differences may be
stated briefly as follows:—(a) Before birth most young
mammals are very closely united (by a complex structure
Fic. 1.—Duckmole ( Ornzthorhynchus).
called the placenta) to the mothers who bear them. (4) But
this close connection between mother and unborn young
is of rare occurrence, or only hinted at, in the pouched
animals or Marsupials, which bring forth their young in a
peculiarly helpless condition, as it were prematurely, and in
most cases place them in an external pouch, within which
they are sheltered and nourished. (c) In the Australian
duckmole and its two relatives, the placental connection is
quite absent, for these animals lay eggs as birds and most
reptiles do. These differences and others relating to
structure warrant the division of Mammals into three sub-
classes :—
(a) Eutheria, Monodelphia, or Placentals—those in which there is
a close (placental) union between the unborn embryo and its
mother, e.g. Ungulates, Carnivores, Monkeys.
BIRDS. 3
(4) Metatheria, Didelphia, or Marsupials—the prematurely bearing,
usually pouch-possessing kangaroos, opossums, etc.
{c) Prototheria, Ornithodelphia, or Monotremes—the egg-laying
duckmole (Ornzthorhynchus), Echidna, and Proechidna.
aS
Fic. 2.—Phenacodus, a primitive extinct Mammal.—After Cope.
Birds.— There can be
no hesitation as to the
class which ranks next to
Mammals. For Birds are
in most respects as highly
developed as Mammals,
though in a different direc-
tion. They are character-
ised by their feathers and
wings, and many other
adaptations for flight, by
their high temperature,
by the frequent spongi-
ness and hollowness of
their bones, by the tend-
ency to fusion in many
parts of the skeleton,
by the absence of teeth
in modern forms, by the
fixedness of the lungs
and their association with
‘numerous air sacs, and so on.
Fic. 3.—Extinct moa and modern
er eed
kiwi.—After Carus Sterne.
But here again different grades must be distinguished—(1) There is
the vast majority—the flying birds, with a breast-bone keel or carina, to
4 GENERAL SURVEY OF THE ANIMAL KINGDOM.
which the muscles used in flight are in part attached (Carinatz) ; (2)
there is the small minority of running birds (ostriches, emu, cassowary,
kiwi, and extinct moa), with wings incapable of flight, and with no keel
(Ratitee) ; and (3) there is an extinct type, Avcheopteryx, with markedly
reptilian affinities,
Reptiles—There are no close relationships between
Birds and Mammals, but the old-fashioned Monotremes
have some markedly reptilian features, and so have some
aberrant living birds, such as the Hoatzin and the Tinamou.
Moreover, when we consider the extinct Mammals and
Birds, we perceive other resemblances linking the two
highest classes to the Reptiles.
Fic. 4,.—Crocodiles.
Reptiles do not form a compact class, but rather an
assemblage of classes. In other words, the types of Reptile
differ much more widely from one another than do the
types of Bird or Mammal. Nowadays there are five dis-
tinct types:—the crocodilians, the unique New Zealand,
“lizard” (Sphenodon), the lizards proper, the snakes, and the
tortoises. But the number of types is greatly increased
when we take account of the entirely extinct saurians, who
had their golden age in the inconceivably distant past.
The Reptiles which we know nowadays are scaly-skinned
animals; they resemble Birds and Mammals in having
during embryonic life two important “foetal membranes”
(the amnion and the allantois), and in never having gills ;
they differ from them in being “cold-blooded,” and in
many other ways.
AMPHIBIANS. 5
Amphibians.—The Amphibians, such as frogs and newts,
were once regarded—e.g. by Cuvier—as naked Reptiles,
but a more accurate classification has linked them rather to
the Fishes. Thus Huxley grouped Birds and Reptiles
together as Sauropsida ; Amphibians and Fishes together as
Ichthyopsida—for reasons which will be afterwards stated.
Amphibians mark the transition from aquatic life, habitual
Fic. 5.—Salamander, an Amphibian.
among Fishes, to terrestrial life, habitual among Reptiles ;
for while almost all Amphibians have gills—in their youth
at least—all the adults have lungs, and some retain the gills
as well. In having limbs which are fingered and toed, and
thus very different from fins, they resemble Reptiles. But
the two foetal membranes characteristic of the embryonic life
of higher Vertebrates are not present in Amphibian embryos,
and the general absence of an exoskeleton in modern forms
is noteworthy.
Fishes.—The members of this class are as markedly
adapted to life in the water as birds to life in the air. The
very muscular posterior region of the body usually forms
Fic. 6.—Queensland dipnoan (Ceratodus).
the locomotor organ, and we say that a fish swims by
bending and straightening its tail. The limbs have the
form of paired fins—that is, they are limbs without digits.
There are also unpaired median fins supported by fin rays.
All have permanent gills borne by bony or gristly arches.
6 GENERAL SURVEY OF THE ANIMAL KINGDOM.
There is an exoskeleton of scales, and the skin also bears
numerous glandular cells and sensory structures.
In many ways Fishes are allied to Amphibians, especially
if we include among Fishes three peculiar forms, known as
Dipnoi, which show the beginning of a three-chambered
heart, and have a lung as well as gills. Ordinary Fishes
have a two-chambered heart, containing only impure blood,
which is driven to the gills, whence, purified, it passes
directly to the body.
Apart from the divergent Dipnoi, there are two great orders of
Fishes—the cartilaginous Elasmobranchs, such as shark and skate ;
and the Teleosteans or bony fishes, such as cod, herring, salmon, eel,
and sole. There are several smaller orders of great importance, some
of which, é.g. the sturgeons, are often called ‘* Ganoids.””
Primitive Vertebrates.—Under this title we include—
(1) the Roundmouths or Cyclostomata; (2) the lancelets
or Cephalochorda; (3) the Tunicates, some of which are
See Ses
SSSR WW
Ay uice,
LE
Fic. 7.—A lancelet, Amphioxus.—After Haeckel.
called sea-squirts; and (4), with much hesitation, several
strange forms, especially Balanoglossus, which exhibit
structures suggestive of affihity with Vertebrates.
The Cyclostomata, represented by the lamprey (Pe/vo-
myzon) and the hag (AZyxine), and some other forms,
probably including an interesting fossil known as FPalgo-
spondylus, are sometimes ranked with fishes under the title
Marsipobranchii. But they have no definitely developed
jaws, no paired fins, no scales, and are in other ways more
primitive.
The lancelets or Cephalochorda are even simpler in their
general structure (see Fig. 7). Thus there is an absence
of limbs, skull, jaws, well-defined brain, heart, and some
other structures. The vertebral column is represented by
an unsegmented (or unvertebrated) rod, called the noto-
chord, which in higher animals (except Cyclostomes and
some fishes) is a transitory embryonic organ afterwards
replaced by the backbone.
PRIMITIVE VERTEBRATES. y)
The Tunicata or Urochorda are remarkable forms, the
majority of which degenerate after larval life (Fig. 8).
In the larvee of all, and in a few adults which are neither
peculiarly specialised nor degenerate, we recognise some of
the fundamental characters of Vertebrates. Thus there is a
dorsal supporting axis (or notochord) in the tail region, a
dorsal nervous system, gill - clefts
opening from the pharynx to the
exterior, a simple ventral heart, and
so on.
Of Balanoglossus and its allies
(Hemichorda or Enteropneusta) it is
still difficult to speak with confidence.
The possession of gill-clefts, the
dorsal position of an important part
of the nervous system, the occurrence
of a short supporting structure on
the anterior dorsal surface of the
pharynx, and other features, have led
many to place them at the base of
the Vertebrate series.
Characteristics of Vertebrata.—At
this stage, having reached the base of the .
Vertebrate series, we may seek to define a Fic. 8.—Ascidian or
Vertebrate animal, and to contrast it with sea - squirt, — After
Invertebrate forms. Haeckel.
The distinction is a very old one, for
even Aristotle distinguished mammals, birds, reptiles, amphibians,
and fishes as ‘‘ blood-holding,” from cuttle-fish, shell-bearing animals,
crustaceans, insects, etc., which he regarded as ‘‘ bloodless.” He was,
indeed, mistaken about the bloodlessness, but the distinctiveness of the
higher animals first mentioned has been recognised by all subsequent
naturalists, though it was first precisely expressed in 1797 by Lamarck.
Yet it is no longer possible to draw a boundary line between Verte-
brates and Invertebrates with that firmness of hand which characterised
the early or, indeed, the pre-Darwinian classifications. We now
know—(1) that Fishes and Cyclostomata do not form the base of the
Vertebrate series, for the lancelets and the Tunicates must also be in-
cluded in the Vertebrate alliance ; (2) that Balanoglossus, Cephalodiscus,
and some other forms, have several Vertebrate-like characteristics ;
(3) that some of the Invertebrates, especially the Chzetopod worms,
show some hints of affinities with Vertebrates. The limits of the
Vertebrate alliance have been widened, and though the recognition of
their characteristics has become more definite, not less so, the apartness
of the sub-kingdom has disappeared.
8 GENERAL SURVEY OF THE ANIMAL KINGDOM.
It does not matter much whether we retain the familiar title Verte
brata, or adopt that of Chordata, provided that we recognise—(1) that
it is among Fishes first that separate vertebral bodies appear in the
supporting dorsal axis of the body ; (2) that, as a characterdstzc, the
backbone is less important than the notochord, which precedes it in
the history alike of the race and of the individual. Nor need we
.object to the popular title backboned, if we recognise that the adjective
“bony ” is first applicable among Fishes, and not to all of these.
The essential characters of Vertebrates may be summed up in the
following table, where they are contrasted, somewhat negatively, with
what is true of Invertebrates :—
‘BACKBONELESS,” INVERTEBRATE
or Non-CHORDATE.
‘“BaCKBONED,” VERTEBRATE
oR CHORDATE.
If there is a nerve-cord, it is ventral,
No internal dorsal axis.
No gill-slits.
The eye is usually derived directly from
the skin.
The heart, if present, is dorsal.
The central nervous system—brain and
spinal cord—is dorsal and tubular.
There is a dorsal supporting axis or
notochord, which is in most cases
replaced by a backbone.
Gill-slits or visceral clefts open from the
sides of the pharynx to the exterior.
In fishes, and at least young amphi-
bians, they are associated with gills,
and are useful in respiration; in
higher forms they are transitory and
functionless, except when modified
into other structures.
The essential parts of the eye are formed
by an outgrowth from the brain.
The heart is ventral.
INVERTEBRATES, OR BACKBONELESS ANIMALS
Molluscs.—If we take the concentration of the nervous
system as a useful criterion, the highest backboneless
animals are the Molluscs. This series of forms includes
Bivalves, such as cockle and mussel, oyster and clam;
Gasteropods, such as snail and slug, periwinkle and whelk ;
Cephalopods, such as octopus and pearly nautilus.
Unlike Vertebrates, and such Invertebrates as Insects
and Crustaceans, Molluscs are without segments and
without appendages. A muscular protrusion of the ventral
surface, known as the “foot,” serves in the majority as ap
organ of locomotion. In most cases a single or double
fold of skin, called the “ mantle,” makes a protective shell.
The nervous system has three chief pairs of nerve centres
or ganglia. In many cases there are very characteristic
free-swimming larval stages.
ARTHROPODS, 9
Fic. g.—Cephalopod (paper nautilus, female).
Arthropods. — This large series includes Crustaceans,
Myriopods, Insects, Spiders, and other forms, which have
segmented bilaterally symmetrical bodies and jointed
Fic. 10.—Fresh-water crayfish Fic. 11.—a, Caterpillar ;
(Astacus), a Crustacean,— 6, pupa; ¢, butterfly.
After Huxley.
10 GENERAL SURVEY OF THE ANIMAL KINGDOM.
appendages. The skin produces an external, not-living
cuticle, the organic part of which is a substance called
chitin, associated in Crustaceans with carbonate of
lime. The nervous system con-
sists of a dorsal brain, connected,
by a nerve-ring around the
gullet, with a ventral chain of
ganglia.
Echinoderms. This is a well-
defined series, including star-fishes,
brittle-stars, sea-urchins, sea-cucum-
bers, and feather - stars. The
symmetry of the adult is usually
radial, though that of the larva is
Fic. 12.—Spider. bilateral. A peculiar system, known
as the water-vascular system, is
characteristic, and is turned to various uses, as in
locomotion and respiration. There is a marked tend-
ency to deposition of lime in the tissues. The develop-
ment is strangely circuitous or “indirect.”
Segmented ‘‘worms.”
—It is hopeless at
present to arrange with
any definiteness those
heterogeneous forms to
which the title ‘worm ”
is given. For this title
is little more than a
name for a_ shape,
assumed by animals of
varied nature who be-
gan to move head
foremost and to acquire
sides. There is no
class of “worms,” but
an assemblage—a mob Fic. 13.—Crinoid or feather-star.
—not yet reduced to
order. It seems useful, however, to separate those which
are ringed or segmented from those which are unsegmented.
The former are often called Annelids, and include two chief
classes :—
UNSEGMENTED ‘‘WORMS.” 1s
(1) Cheetopoda or Bristle-footed worms, ¢.g. earthworm.
and lob-worm ; and (2) Hirudinea or Leeches.
ee
Tf, Wa
Ge?
Unsegmented ‘“worms.”—These differ from the higher
“worms” in the absence of true segments and appendages,
and resemble them in their bilateral symmetry. There is
a motley lot :—the free-living Turbellarians or Planarians ;
the parasitic Trematodes or Flukes ; the parasitic Cestodes.
or Tape-worms; the Nemer-
teans or Ribbon-worms; the
frequently parasitic Nematodes
or Thread-worms; and several
smaller classes.
As to some other groups,
such as the sea-mats (Polyzoa
or Bryozoa), the lamp-shells
(Brachiopoda), the worm-like
Sipunculids, and the wheel-
animalcules or Rotifers, we
must confess that they are still a, Early stage with head inverted.
incert@ seats. &, Later stage with head everted.
But the general fact is not
without interest, that in the midst of the well-defined
classes of Invertebrates there lies, as it were, a pool from
which many streams of life have flowed; for among the
heterogeneous “worms” we may find in diverse types.
affinities with Arthropods, Molluscs, Echinoderms, and.
even Vertebrates.
Contrast of Coelomate and Ccelenterate.—At this stage
we may notice that in all the above forms the typical symmetry is.
bilateral (in Echinoderms, the superficial radial symmetry belongs
only to the adults); that in most types a body cavity or coelom 1s.
developed ; that the embryo consists of three germinal layers (external:
Fic. 15.—Bladderworm stage
ofa Cestode.—After Leuckart.
12 GENERAL SURVEY OF THE ANIMAL KINGDOM.
‘ectoderm or epiblast, internal endoderm or hypoblast lining the gut,
and a median mesoderm or mesoblast lining the body cavity). In the
next two classes (Ccelentera and Sponges) the conditions are different,
-as may be expressed in the following table :—
Sponces AND CG&LENTERA. HIGHER ANIMALS (C@&LOMATA).
There is no body cavity. There is but | There is a body cavity or cwlom be-
one cavity, that of the food canal. tween the food canal and the body-
wall. But this is often incipient, or
degenerate.
Except in ctenophores, there is no | There is a distinct middle layer of cells
definite middle layer of cells (meso- (mesoderm) between the external
derm), but rather a middle jelly ectoderm and the internal endo-
(mesogloea), and the embryo is derm. The embryo is triploblastic.
diploblastic.
The radial symmetry of the gastrula | The adults are usually bilateral, in some
embyro is usually retained in the cases asymmetrical, in echinoderms
adult, and the ‘longitudinal (oral- superficially radial.
aboral) axis of the adult corresponds
to the long axis of the gastrula.
Coelentera.—This series includes jelly-fishes, sea-anemones,
corals, zoophytes, and the like, most of which are equipped
FIG. 16.—Sea-anemones on back of hermit-crab,
—After Andres.
with stinging cells, by means of which they paralyse their
prey. All but a few are marine. The body may be a
tubular polyp, or a more or less bell-like “ medusoid,” and
PORIFERA. 13
in some cases the two forms are included in one life cycle.
Budding is very common, and many of the sedentary forms.
—‘‘corals””—have shells of lime.
Porifera.—Sponges, or Porifera, are the simplest many-
celled animals. In the simplest forms, the body is a
tubular, two-layered sac, with numerous inhalant pores by
which water passes in, with a central cavity lined by cells
bearing lashes or flagella, and with an exhalant aperture.
But budding, folding, and other complications arise, and
there is almost always a skeleton, calcareous, siliceous, or
“horny.” Apart from one family (Spongillidze), all sponges.
are marine.
Contrast of Metazoa and Protozoa.—aAll the animals hitherto-
mentioned have Jdodzes built up of many cells, but there are other.
animals, each of which consists of a single cell. These simplest animals-
are called Protozoa.
Every animal hitherto mentioned, from mammal or bird to sponge,
develops, when reproduction takes its usual course, from « fertilised.
egg-cell. This egg-cell or ovum divides and redivides, and the
daughter cells cohere and are differentiated to form a ‘‘ body.” But
the Protozoa form no ‘‘ body”; they remain (with few exceptions)
single cells, and when they divide, the daughter cells almost invariably
go apart as independent organisms.
Here, then, is the greatest gulf which we have hitherto noticed—
that between multicellular animals (Metazoa) and unicellular animals.
(Protozoa). But the gulf was bridged, and traces of the bridge remain.
For—(a) there are a few Protozoa which form loose colonies of cells,
and (4) there are a few multicellular animals of great simplicity.
Protozoa.— The Pro-
tozoa remain single cells,
with few exceptions. Thus
they form no “body”;
and necessarily, therefore,
they have no organs in
the ordinary sense. They
illustrate the deginnings of
sexual reproduction, and ‘N
they are Met subject to Fic, 17.—Fossil Foraminifera
natural death in the same (Nummulites) in limestone. —
degree as Metazoa are. After Zittel.
The series includes—
tA cell may be defined as a unit corpuscle or unit area of living
matter, typically controlled by a single nuclcus.
14 GENERAL SURVEY OF THE ANIMAL KINGDOM.
(a) Rhizopods, with outflowing threads or processes of living mattey,
é.g. the chalk-forming Foraminifera (Fig. 17).
(6) Infusorians, with actively moving lashes of living matter.
(c) Sporozoa, parasitic forms, usually without either lashes or out-
flowing processes.
Note on Classification.
We always group together in our mind those impressions which
are like one another. In this lies the beginning of all classification,
whether that of the child, the savage, or the zoologist. For there are
many possible classifications, varying according to their purpose,
according to the points of similarity which have been selected as
‘important. Thus we may classify animals according to their habitats
or their diet, without taking any thought of their structure.
But a strictly zoological classification is one which seeks to show the
blood-relationships of animals, to group together those whose affinities
are shown by their being like one another in architecture or structure.
It must, therefore, be based on the results of comparative anatomy—
technically speaking, on ‘‘ homologies,” z.e. resemblances in funda-
mental structure and in mode of development. Whales must not be
ranked with fishes, nor bats with birds.
To a classification based on structural resemblances, two corrobora-
tions are of value, from embryology and from paleontology. On the
-one hand, the development of the forms in question must be studied :
thus no one dreamed that a Tunicate was a Vertebrate until its life-
history was worked out. On the other hand, the past history must be
inquired into : thus the affinity between Birds and Reptiles is confirmed
by a knowledge of the extinct forms.
In classification it is convenient to recognise certain grades or degrees
of resemblance, which are spoken of as species, genera, families, orders,
‘classes, and so on.
To give an illustration, all the tigers are said to form the species
Felis tégrés, of the genus Fes, in the family Felide, in the order
Carnivora, within the class Mammalia. The resemblances of all tigers
are exceedingly close ; well marked, but not so close, are the resem-
blances between tigers, lions, jaguars, pumas, cats, etc., which form
the genus eé/is ; broader still are the resemblances between all members
of the cat family Felidz ; still wider those between cats, dogs, bears,
and seals, which form the order Carnivora; and Jastly, there are the
general resemblances of structure which bind Mammals together in
contrast to Birds or Reptiles, though all are included in the series or
phylum Vertebrata.
It must be understood that the real things are the individual animals,
and that a species includes all those individuals who resemble one
another so closely that we feel we need a specific name applicable to
them all. And as resemblances which seem important to one naturalist
may seem trivial to others, there are often wide differences of opinion as
to the number of species which a genus contains.
But while no rigid definition can be given of a species, certain
common-sense considerations should be borne in mind :—
CLASSIFICATION. 15
1. No naturalist now believes, as Linnzeus did, in the fixity of species ;
swe believe, on the contrary, that one form has given rise to another.
At the same time, the common characteristics on the strength of which
we deem it warrantable to give a name to a group of individuals, must
BIRDS SOF
Snakes Ligand, coylians
a Ss S cero?
Rants
Fishes i. Amphibian
= é 78
ancelet ’ cycloto™
B wna
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‘ / motu Z A Syerons
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-- fs} a D F
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we = Ter astaceans
iS
@eflenfdra
Mesozoa~y, A Spon ges,
(nfusonians —, ; w_Gregarines
do.
Prrotoza Plants.
per ee
Fic. 18.—Diagrammatic expression of classification in a
genealogical tree. B indicates possible position of Balano-
glossus, D of Dipnoi, S of Sphenodon or Hatteria.
not be markedly fluctuating. The specific characters should exhibit
a certain degree of constancy from one generation to another. 3
2. Sometimes a minute character, such as the shape of a tooth or the
marking of a scale, is so constantly characteristic of a group of indi-
viduals that it may be safely used as the index of more important
16 GENERAL SURVEY OF THE ANIMAL KINGDOM.
characters. On the other hand, che distinction between one species and
another, should always be greater than any difference between the
members of a family (using the word family here to mean the progeny
of a pair). For no one would divide mankind into species according to
the colour of eyes or hair, as this might lead to the absurd conclusion
that two brothers belonged to different species. Thus it is often doubly
unsatisfactory when a species is established on the strength of a single
specimen—(a) because the constancy of the specific character is undeter-
mined ; (4) because the variations within the limits of the family have
not been observed. Indeed, it has happened that one species has been
made out of a male, and another out of its mate.
3. Although cases are known where members of different species
have paired and brought forth fertile hybrids, this is not usual. Zhe
members of a spectes are fertile inter se, but not usually with members
of other species. In fact, the distinctness of species has largely depended
on a restriction of the range of fertility.
TABULAR SURVEY.—(for Future Reference)
METAZOA CHORDATA
Eutheria. Bd
Mamma.ta. Metatheria. Marsupials. aa
Prototheria. Monotremes. Oviparous.) & &
Carinate. Keeled flying birds. ‘
Aves Qdontolcz. Extinct toothed birds.
= Ratite. Keel-less running birds.
Extinct reptile-like birds.
Crocodilia. Crocodiles and alligators. |
Ophidia. Snakes.
Lacertilia. Lizards.
Rhynchocephalia. Sphenodon.
Chelonia. Tortoises and turtles.
Extinct Classes.
Anura. Tail-less frogs and toads.
Urodela. Tailed newts.
Gymnophiona, e.g. Cecilia.
Labyrinthodonts and other extinct
Amphibians. c
{eles Mud-fishes.
Sauropsida.
REPTILia.
Gnathostomata
(z.e. jawed).
Craniota
(with skulls).
AMPHIBIA,
Ichthyopsida.
Teleostomi. Bony fishes, etc.
Elasmobranchii. Cartilaginous fishes.
Hag-fish (A/yxine), and Lamprey
(Petromyzon).
CErHALOCHORDA. Amfphioxus. }
Pisces.
CycLOsTOMATA. {
Urocuorpa. Tunicates.
Hemicuorpa. Balanoglossus, Cephalodiscus
TABULAR SURVEY OF CHIEF CLASSES. 17
METAZOA NON-CHORDATA
Gasteropoda. Snails.
Lamellibranchiata, Bivalves.
‘Two smaller classes :—Scaphopoda and Solenogastres
Cephalopoda. Cuttle-fishes.
Mo ttusca.
Insecta. ae
Myriopoda. , Centipedes and millipedes.
Prototracheata. Pertpatus.
Crustacea.
Palzostraca :—Trilobites, Eurypterids, and King-crabs.
Some smaller classes.
Arachnoidea. Spiders, scorpions, mites
ARTHROPODA.
Crinoidea. Feather-stars. (Cystoids and Blastoids, éxtinct.)
Ophiuroidea. _ Brittle-stars.
Asteroidea, Star-fishes.
Echinoidea. Sea-urchins.
Holothuroidea. Sea-cucumbers.
EcHINODERMA.
Annelids or
Discophora. Leeches. Annulata.
{ Chetopoda. Bristle worms. }
Some smaller classes.
( Brachiopoda. Lamp-shells.
| Potyzc, e.g. Sea-mat (Flustra).
Sipunculoidea, eg. Sipunculus.
Worn: Nematoda. Thread-worms.
Acanthocephala.
Nemertea. Ribbon-worms.
Rotifera. Wheel-animalcules.
Cestoda. Tape-worms.
{Brematoaa. Flukes. Platyhelminthes.
\ (Turbellaria. Planarians.
Ctenophora, ¢.g. Beroé. ‘
Actinozoa or Anthozoa. Sea-anemones. Alcyonarians and re-
CGLENTERA. lated corals.
Scyphomedusz or Acraspeda. Je
Hydrozoa. Zoophytes and medisoids.
PoRIFERA. Sponges. Calcareous and non-calcareous.
PROTOZOA
Inrusorta. Ruizopopa. SPporozoa.
Simplest forms of animal life.
VERTEBRATES.
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CHAPTER it
THE FUNCTIONS OF ANIMALS
(PHYSIOLOGY)
Most animals live an active life, in great part ruled by the
two motives of love and hunger in their widest sense ; they
are busy finding food, avoiding enemies, wooing mates,
making homes, and tending the young. These and other
forms of activity depend upon internal changes within the
body. Thus the movements of all but the very simplest
animals are due to the activity of contractile parts known
as muscles, which are controlled by nervous centres and by
impulse-conducting fibres, and the energy involved in these
movements, and in most other vital activities, is supplied
by the oxidation or combustion of the complex carbon-
compounds which form a substantial part of the various
organs.
The work done means expenditure of energy, and is
followed by exhaustion (muscular, nervous, etc.), so that
the necessity for fresh supplies of energy is obvious. This
recuperation is obtained through food, but before this can
restore the exhausted parts to their normal state, or keep
them from becoming, in any marked degree, exhausted, it
must be rendered soluble, diffused throughout the body,
and so chemically altered that it is readily incorporated
into the animal’s substance. In other words, it has to be
digested. A fresh supply of oxygen and a removal of waste
are also obviously essential to continued activity.
We may say, then, that there are f2o master activities in
the animal body, those of muscular and those of nervous
parts. To these the other internal activities—digestion,
respiration, excretion, and the like—are subsidiary.
LIVING AND NOT LIVING. 21
Besides the more or less constantly recurrent activities or
functions, there are the processes of growth and repro-
duction. When income exceeds expenditure in a young
animal, growth goes on, and the inherited qualities of the
organism are more and more perfectly developed. At the
limit of growth, when the animal has reached “ maturity,”
it normally reproduces—that is to say, liberates either parts
of itself.or special germ-cells which give rise to new
individuals.
Living and not living.—Although no one is wise enough
to tell completely what is meant by the simple word alive, it
is safe to say that active life involves the following facts :—
(a) The living organism grows at the expense of material
different from itself, while the crystal—one of the few not-
living things which can be said to grow—increases only at
the expense of material chemically the same as itself.
(4) The living organism is subject to ceaseless chemical
change (metabolism), and yet it has the power of retaining
its integrity, of remaining more or less the same for prolonged
periods. The physical basis of life invariably includes com-
plex compounds known as gvoteids, built up chiefly of
Carbon, Hydrogen, Oxygen, and Nitrogen, and these are
continually being broken down and made anew.
(c) The living organism resembles an engine, in being a
material system adapted to transform matter and energy
from one form to another; but it is a self-stoking, and,
within limits, a self-repairing engine, and it is able to do
what no engine can effect, namely, reproduce. From a
physical standpoint it differs from an inanimate system in
this, that the transfer of energy into it is attended with
effects conducive to further transfer and retardative of
dissipation, while the very opposite is true of an inanimate
system,
(d) A living creature is a more or less perfect :ntegrate,
it has a unzfied behaviour, it gives effectzve response to
external stimuli.
(e) A living organism exhibits five everyday activities—
contractility (the power of movement), irritability (the
power of feeling in the wide sense), nutrition or utilisation
of food, respiration, and excretion, besides the periodic
activities of growth and reproduction.
22 THE FUNCTIONS OF ANIMALS.
Division of labour.—All the ordinary functions of life
are exhibited by the simple unicellular animals or Protozoa.
Thus the Amceba moves by contracting its living substance,
draws back sensitively from hurtful influences, engulfs and
digests food, gets rid of waste, and absorbs oxygen.
But all these activities occur in the Amceba within’ the
compass of a unit mass of living matter—a single cell,
physiologically complete in itself.
In all other animals, from Sponges onwards, there is a
“body ” consisting of hundreds of unit areas or cells. A cell
is a unified area of living matter almost always with a definite
centre or nucleus. It is impossible for these cells to remain
the same, for as they increase in number they become
diversely related to the outer world, to food, to one another,
and soon. Division of labour, consequent on diversity of
conditions, is thus established in the organism. In some
cells one kind of activity predominates, in others a second, in
others athird. And this division of labour is associated with
that complication of structure which we call differentiation.
Thus in the fresh-water Aydra, which is one of the
simplest many-celled animals, the units are arranged in
two layers, and form a tubular body. Those of the outer
layer are protective, nervous, and muscular; those of the
inner layer absorb and digest the food, and are also muscular.
In worms and higher organisms, there is a middle layer
in addition to the other two, and this middle layer becomes,
for instance, predominantly muscular. Moreover, the units
or cells are not only arranged in strands or tissues, each
with a predominant function, but become compacted into
well-defined parts or organs. None the less should we
remember that each cell remains a living unit, and that, in
addition to its principal activity, it usually retains others of
a subsidiary character.
Plants and animals.—Before we give a sketch of the
chief functions in a higher animal, let us briefly consider the
resemblances and differences between plants and animals.
(a) Resemblance in function.—The life of plants is
essentially like that of animals, as has been recognised since
Claude Bernard wrote his famous book, Phénomenes de la
vie communs aux animaux et aux végétaus, The beech-
tree feeds and grows, digests and breathes, as really as does
PLANTS AND ANIMALS. 23
the squirrel on its branches. In regard to none of the main
functions (except excretion) is there any essential difference.
Many simple plants swim about actively ; young shoots and
roots also move; and there are many cases in which even
the full-grown parts of plants exhibit movement. Moreover,
the tendrils of climbers, the leaves of the sensitive plant, the
tentacles of the sundew, the stamens of the rock-rose, the
stigma of the musk, and many other plant structures exhibit
marked sensitiveness.
(2) Resemblance in structure.—The simplest plants (Pro-
tophyta), like the simplest animals (Protozoa), are single cells ;
the higher plants (Metaphyta) and higher animals (Metazoa)
are built-up of cells and various modifications of cells. In
short, all organisms have a cellular structure. This general
conclusion is part of the Cell Theory or Cell Doctrine
1838
: (c) ne in development.— When we trace the
beech-tree back to the beginning of its life, we find that it
arises from a unit element or egg-cell, which is fertilised by
intimate union with a male element derived from the pollen-
grain. When we trace the squirrel back to the beginning
of its life, we find that it also arises from a unit element or
egg-cell, which is fertilised by intimate union with a male
cell or spermatozoon. Thus all the many-celled plants and
animals begin as fertilised egg-cells, except in cases of
virgin birth (parthenogenesis) or of asexual reproduction.
From the egg-cell, which divides and redivides after fertilisa-
tion, the body of the plant or animal is built up by con-
tinued division, arrangement, and modification of cells.
Contrasts—But while there is no absolute distinction
between plants and animals, they represent divergent
branches of a V-shaped tree of life. It is easy to distinguish
extremes like bird and daisy, less easy to contrast sponge
and mushroom, well-nigh impossible to decide whether
some very simple forms, which Haeckel called “‘ Protists,”
have a bias towards plants or towards animals. We cannot
do more than state average distinctions. The food which
most plants absorb is cruder or chemically simpler than that
which animals are able to utilise. Thus most plants derive
the carbon they require from the carbon dioxide of the air,
while only a few (green) animals have this power; all the
24 THE FUNCTIONS OF ANIMALS.
others depend for their carbon supplies on the sugar, starch,
and fat already made by other animals, or by plants. As
regards nitrogen, most plants take this from nitrates and
the like, absorbed along with water by the roots; whereas
animals obtain their nitrogenous supplies from the complex
proteids formed within other organisms. Most plants,
therefore, feed at a lower chemical level than do animals,
and it is characteristic of them that, in the reduction of
carbon dioxide, and in the manufacture of starch and
proteids, the kinetic energy of sunlight is transformed by
the living matter into the pctential chemical energy of
complex foodstuffs. Animals, on the other hand, get their
food ready made; they take the pounds which plants have,
as it were, accumulated in pence, and they spend them.
For it is characteristic of animals that they convert the
potential chemical energy of foodstuffs into the kinetic
energy of locomotion and other activities. In short, the
great distinction—an average one at best—is that most
animals are more active than most plants,
Chief functions of the animal body.—We have seen that
there are two master activities in animals, those of muscular
and of nervous structures, and that the other vital processes,
always excepting growth and reproduction, are subservient
to these. Let us now consider the various functions, as
they occur in some higher organism, such as man.
Nervous activities—Life has been described as consisting
of action and reaction between the organism and its en-
vironment, and it is evident that an animal must in some
way become aware of surrounding influences.
The unit in nervous reaction in any highly organised
animal is the vefex. It requires three structures, a receptor
(end-organ), a conductor, and an effector (muscle). The
conductor consists of two or more nerve cells or neurones
which span the distance between receptor and effector by
means of their long processes. Stimulation of the receptor
causes a nervous impulse to be transmitted along the
conductor to set the effector in action, The whole nervous
system is essentially a connected series of such reflex-arcs,
all intricately joined up with one another.
There are two chief kinds of stimuli which are transmitted
to the central nervous system—stimuli from without the
23
CHARACTERISTICS OF PLANTS AND ANIMALS.
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26 THE FUNCTIONS OF ANIMALS.
body, which make the organism aware of changes in its
environment; and stimuli from within the body, which
make it aware of the dispositions of its organs, eg. the
stimuli transmitted by the afferent nerves of the muscles,
tendons, etc.
The chief functions of the nervous system are, then, to
make the animal aware of its environment and to co-
ordinate and integrate all its bodily functions and activities.
As we ascend in the scale, we find that in addition the
brain possesses, to an increasing extent, the power of
correlating present and past experiences, and of originating
or inhibiting action in accordance with this correlation.
In whatever part there is activity, there is necessarily waste of
complex substances and some degree of exhaustion; and it is
interesting to notice, aS a triumph of histological technique, that
Hodge, Gustav Mann, and others have succeeded in demonstrating in
nerve cells the structural results (cellular collapse, etc.) of fatigue, and
that in such diverse types as bee, frog, bird, and dog.
Muscular activity—The movements of a unicellular
animal are due to the contractility of the living matter, or
of special parts of the cell, such as lashes or cilia. In
sponges specially contractile cells begin to appear; in most
higher animals such cells are aggregated to form the muscles.
A piece of typical muscle consists of numerous fine
transparent tubes or fibres, each invested by a sheath or
sarcolemma, while the whole muscle is surrounded by
connective tissue. It usually runs from one part of the
skeleton to another, and is fastened to the skeleton by
tendons or sinews. It is stimulated by motor nerves, and
is richly supplied with blood. ,
When a muscle contracts, usually under a stimulus
propagated along a motor nerve, there is of course a
change of shape—it becomes shorter and broader. The
source of the energy expended in work done is the
“chemical explosion” which occurs in the fibres, for the
oxygen stored up (intramolecularly) in the muscle enters
into rapid union with carbon compounds. Heat, CO,, and
water are produced as the result of this combustion, and
lactic acid is also formed as a by-product. Besides the
chemical change and the change of shape, there are also
CHIEF FUNCTIONS OF THE ANIMAL BODY. 27
changes of “electric potential” associated with each con-
traction. Beside muscular movement we must rank ciliary,
amoeboid, and epithelial movement. Under the last head-
ing are included active non-amceboid contractions and
expansions of covering cells.
Digestion.—The energy expended in work or in growth
is balanced by the energy of the food-stuffs :—proteids,
carbohydrates, fats, water, and salts, in varying pro-
portions.
In some of the lower animals, such as sponges, the food
particles are engulfed by certain cells with which they come
in contact, and digested within these cells (éxtracellular
digestion). In most cases, however, the food is digested
within the food canal, by ferments made by the secretory
cells of the gut or of associated glands. The peculiarity
of these ferments is that a small quantity can act upon
a large mass of material without itself undergoing any
apparent change. However digestion be effected, it means
dissolving the food and making it diffusible. In a higher
vertebrate there are many steps.
(a) The first ferment to affect the food, masticated by the teeth and
moistened by the saliva, is the Atyalim of the salivary juice, which
changes starch into sugar. The juice is formed or secreted by various
salivary glands around the mouth.
(6) The food is swallowed, and passes down the gullet to the stomach,
where it is mixed with the gastric juice secreted by glands situated in
the walls. These walls are also muscular, and their contractions churn
the food and mix it with the juice. In the juice there is some free
hydrochloric acid and a ferment called pepsin: these act together in
turning proteids into peptones. The juice has also a slight solvent
effect on fat, and the acid on the carbohydrates.
(c) The semi-digested food, as it passes from the stomach into the
small intestine, is called chyme, and on this other juices act. Of these
the most important is the secretion of the pancreas, which contains
various ‘ferments, ¢.g. trypsin, and affects all the different kinds of
organic food. It continues the work of the stomach, changing proteids
into peptones and peptones into much simpler compounds such as
amino-acids; it continues the work of the salivary juice, changing
starch into sugar; it also emulsifies the fat, dividing the globules into
extremely small drops, which it tends to saponify or split into fatty
acids and glycerine.
(d) Into the beginning of the small intestine the bile from the liver
also flows, but it is not of great digestive importance, being rather
of the nature of a waste product. It seems to have a slight solvent,
emulsifying, and saponifying action on the fats; in some animals it is
28 THE FUNCTIONS OF ANIMALS.
said to have slight power of converting starch into sugar; by its
alkalinity it helps the action of the trypsin of the pancreas (which,
unlike pepsin, acts in an alkaline fluid) ; it affects cell membranes, so
that they allow the passage of small drops of fat and oil; and it is said
to have various other qualities.
(e) In addition to the liver and the pancreas, there are on the walls
of the small intestine a great number of small glands, which secrete a
juice which seconds the pancreatic juice. The digested material is
in part absorbed into the blood, and the mass of food, still being
digested, is passed along the small intestine by means of the muscular
contraction of the walls known as peristaltic action. It reaches the
large intestine, and its reaction is now distinctly acid by reason of
the acid fermentation of the contents. The walls of the large intestine
contain glands similar to those of the small intestine, and the digestive
processes are completed, while absorption of water also goes on; so
that by the time the mass has reached the rectum, it is semi-solid,
and is known as feeces. These contain the indigestible and un-
digested remnants of the food and the useless products of the chemical
digestive processes.
Absorption.— But the food must not only be rendered
soluble and diffusible, it must be carried to the different
parts of the body, and there incorporated into the hungry
cells. It is carried by the blood stream, and in part also
by what are called lymph vessels, which contain a clear
fluid resembling blood mus red blood corpuscles.
Absorption begins in the stomach by direct osmosis into the capillaries
or fine branches of blood vessels in its walls, and_a similar absorption,
especially of water, takes place along the whole of the digestive tract.
But lining the intestines there are delicate projections called villi;
they contain capillaries belonging to the portal system (blood vessels
going to the liver), and small vessels known as lacteals connected
with lymph spaces in the wall of the intestine. The lacteals lead into
a longitudinal lymph vessel or thoracic duct, which opens into the
junction of the left jugular and left subclavian veins at the root‘of the
neck. The contents of the duct in a fasting animal are clear; after a
meal they become milky ; the change is due to the matters discharged
into it by the lacteals. It is probable that nearly all the fat of a meal
is absorbed from the intestines by the lacteals, but it is not certain in
what measure, if at all, this is true of the other dissolved foodstuffs ;
the greater part certainly passes into the capillaries of the portal
system, which are contained in the villi. The digested proteid,
chiefly in the form of amino-acids, passes into the blood of the portal
vein, either directly or through the intermediary of leucocytes, which
flock to the intestine when proteid food is being digested.
Function of the liver—We now know the fate of the
fats, and of the proteids of the food, and the manner in
CHIEF FUNCTIONS OF THE ANIMAL BODY. 29
which they pass into the blood; but we must follow the
starchy material, or carbohydrates, a little further. ‘The
starch, we know, is converted into sugar, and this, with the
sugar of the food, passes into the capillaries of the villi,
and is carried to the liver. During digestion there is an
increase of sugar in the blood vessel going to the liver from
the intestine—that is, in the portal vein—but no increase
in the hepatic veins, the vessels leaving the liver. The
increase must therefore be retained in that organ, and we
recognise as one of the functions of the liver the regulation
of the amount of sugar in the blood. There is no special
organ for the regulation of the amount of fat; the drops
pass through the walls of the capillaries, and are stored in
connective tissue cells.
All the products of digestion, except the fat, pass through
the liver, which receives everything before it is allowed to
pass into the general circulation. Thus many poisons,
such as metals, are. arrested by the liver, and various
harmful substances which are formed in the course of
digestion are changed by the liver into harmless com-
pounds. The excess of sugar, we have already noted, is
stored in the liver. It is synthesised there into a substance
called glycogen, which can be readily retransformed into
sugar according to the needs of the system. Glycogen is
stored in the muscles also, and forms an‘ important part of
the fuel for the supply of muscular energy and of the
warmth of the body. Thus, if an animal be subjected to
a low temperature, the glycogen of the liver disappears
just as it does during the performance of muscular work.
Another of the many functions of the liver is that in it
nitrogenous waste products begin to be prepared for their
final elimination by the kidneys.
Respiration.—There is another most important foodstuff
to be noticed, namely, the oxygen which is absorbed from
the air by the lungs. We may picture a lung as an elastic
sponge-work of air chambers, with innumerable blood
capillaries in the walls, enclosed in an air-tight box, the
chest, the size of which constantly and rhythmically varies.
When we take in a breath, the size of the chest is increased
the air pressure within is lowered, and the air from without
rushes down the windpipe until the pressure is equalised.
30 THE FUNCTIONS OF ANIMALS.
The oxygen of this air combines with a substance called
hemoglobin, contained in the red corpuscles of the blood,
and is thus carried to all parts of the body. From the
blood it passes to the tissues usually through the medium
of the lymph. It is used in the tissues for oxidation.
The carbon dioxide formed as a waste product is ab-
sorbed by the serum of the blood, or enters in part into
loose chemical combination with its salts, and so in time
reaches the lungs. But as the partial pressure of the
carbonic acid in the air is lower than it is in the serum,
the gas escapes from the latter into the air chambers of
the lungs. When the size of the chest is decreased, the
pressure is increased, and the gas escapes by the mouth or
nose until the pressure is equalised.
Excretion.—We have seen that the blood carries the
digested food to the various parts of the body, and that it is
also the carrier of oxygen and of the waste carbon dioxide.
But there is much waste resulting from tissue changes,
which is not gaseous. It is cast into the blood stream by
the tissues, and has to be got rid of in some way. This is
effected by the kidneys, which are really filters introduced
into the blood stream. But they are the most marvellous
filters imaginable, and give us a good example of the
intricacy of life processes. For the kidneys not only take
out of the blood all the waste products that result from
the metabolism of proteids and contain nitrogen, they also
maintain the composition of the blood at its normal,
rejecting any stuffs that vary from that normal, either
qualitatively or quantitatively, doing this work according to
laws quite different from the simple ones of diffusion or
solubility: thus sugar and urea are about equally soluble,
and yet the sugar is kept in the body, while the urea is cast
out. Even substances as insoluble as resins are removed
from the blood by the living cells of the kidneys.
A considerable quantity of water, and traces of salts, fats,
etc., leave the body by the skin, but its chief use is to
protect, and to regulate the temperature by variations in
the size of its blood vessels.
This completes our sketch—(a) of the process by which
the food becomes available for the organism as fuel for the
maintenance of its life energies, and (4) of the removal of
MODERN CONCEPTION OF PROTOPLASM. 31
He waste products which are formed as the ashes of
life.
There are indeed some organs which we have not
mentioned, such as the spleen, which seems to be an area
for the multiplication of red blood corpuscles (fishes,
newts, embryo-mammals) or for the destruction of worn-
out corpuscles (mammals), and the thyroid gland, which
seems to have to do with keeping the blood at a certain
standard of efficiency; but what we have said is perhaps
enough to convey a general idea of the processes of life
in a higher animal.
In conclusion, it is perhaps useful to remark that whén in the
course of further studies the student meets with organs which are called
by the same name as those found in man or in Mammals, as, for
example, the “‘liver” of the Molluscs, he must be careful not to
suppose that the function of such a ‘‘ liver” is the same as in Mammals,
for comparatively little investigation into the physiology of the lower
types of animal life has as yet been made. At the same time, he must
clearly recognise that the great internal activities are in a general way
the same in all animals; thus respiration, whether accomplished by
skin, or gills, or air-tubes, or lungs, by help of the red pigment (hzemo-
globin) of the blood, or of some pigment which is not red, or occurring
without the presence of any blood at all, always means that oxygen is
absorbed almost like a kind of food by the tissues, and that the carbon
dioxide which results from the oxidation of part of the material of the
tissues is removed.
MopERN CONCEPTION OF PROTOPLASM
The activities of animals are ultimately due to physical
and chemical changes associated with the living matter or
protoplasm. This is a mere truism. We do not know
the nature of this living matter; perhaps our most certain
knowledge of it is, that in our brains its activity is
associated with consciousness.
When more is known in regard to the chemistry and
physics of living matter, it may be possible to bring vital
phenomena more into line with the changes which are
observed in inorganic things. At present, however, it is
idle to deny that vital phenomena are things apart. Not
even the simplest of them can be explained in terms of
chemistry and physics. Even the passage of digested food
from the gut to the blood vessels is more than ordinary
32 THE FUNCTIONS OF ANIMALS.
physical osmosis; it is modified by the fact that the cells
are living. :
But though we cannot analyse living matter, nor thoroughly
explain the changes by which the material of the body
breaks down or is built up, we can trace, by chemical
analysis, how food passes through various transformations
till it becomes a usable part of the living body, and we can
also catch some of the waste products formed when muscles
_or other parts are active.
What is known in regard to the structure of protoplasm does not
help the physiologist very much. The microscopists discover an in-
tricate structure which pervades cach unit of living matter, but no
physiologist dreams of explaining the life of a cell in terms of its
microscopically visible structure.
One general idea, however, the study of structure has suggested,
which the conclusions of physiologists corroborate. This idea is—that
a cell consists of a relatively stable living framework, and of a changeful
content enclosed by it.
Now, many physiologists regard the framework as the genuine living
protoplasm, and the content as the material upon which it acts. ‘‘The
framework is the acting part, which lives, and is stable ; the content is
the acted-on part, which has never lived, and is labile, that is,—in a
state of metabolism or chemical transformation.” This view naturally
leads those who adopt it to regard protoplasm as a sort of ferment
acting on less complex material which is brought to it, which forms
the really changeful part of each cell.
Somewhat different, however, is another idea,—that the protoplasm
is itself the seat of constant change ; that it is constantly being unmade
and remade. On the one hand, more or less crude food passes into
life by an ascending series of assimilative or constructive chemical
changes, with each of which the material becomes molecularly more
complex and more unstable. On the other hand, the protoplasm, as
it becomes active or a source of energy, breaks down in a descending
series of disruptive or destructive chemical changes ending in waste
products. °
The former view, which considers protoplasm as a sort of ferment,
restricts the metabolism to the material on which the protoplasm acts.
The second view regards protoplasm as the climax or central term of
the constructive and disruptive metabolism.
It is highly probable that there is no one substance which should be
called protoplasm, but that vital phenomena depend upon the inter-
actions of several complex substances. As Verworn says, ‘‘ The life-
process consists in the metabolism of proteids.”
Generalising from his studies on colour sensation, Professor Hering
was led to regard all life as an alternation of two kinds of activity,
both induced hy stimulus, the one tending to storage, construction,
assimilation of snaterial, the other tending to explosion, disruption,
disassimilation.
MODERN CONCEPTION OF PROTOPLASM. 33
Generalising from his studies on nervous activities, Professor Gaskell
was led to regard all life as an alternation of two processes, one of
them a running down or disruption (katabolism), the other a winding
up or construction (anabolism).
All physiologists are agreed that in life there is a twofold process of
waste and répair, of discharge and restitution, of activity and recuper-
ative rest. But there is no certainty as to the precise nature of this
twofold process. ;
CHAPTER 11]
THE ELEMENTS OF STRUCTURE
(MorpHotocy)
ANIMALS may be studied alive or dead, in regard to their
activities or in regard to their parts. We may ask how they
live, or what they are made of; we may investigate their
functions or their structure. The study of life, activity,
function, is physiology; the study of parts, architecture,
structure, is morphology.
The first task of the morphologist is to describe structure
(descriptive anatomy); the second is to compare the parts
of one animal with those of another (comparative anatomy) ;
the third is to try to state the “principles of morphology,”
or the laws of vital. architecture.
But just as the physiologist investigates life or activity at
different levels, passing from his study of the animal as a
unity with certain habits, to consider it as an engine of
organs, a web of tissues, a city of cells, and a whirlpool of
living matter; so the morphologist has to investigate the
form of the whole animal, then in succession its organs,
their component tissues, their component cells, and finally,
the structure of protoplasm itself. The tasks of morphology
and of physiology are parallel.
Morphology thus includes not only the description of ex-
ternal form, not only the anatomy of organs, but also that
minute anatomy of tissues and cells and protoplasm which
we call histology. Moreover, there is no real difference
between studying fossil animals which died and were buried
countless years ago, and dissecting a modern frog. The
‘anatomical paleontologist is also a student of morphology.
FORM AND SYMMETRY. 35
Finally, as the greater part of embryology consists in study-
ing the anatomy and histology of an organism at various
stages of its development, the work of the embryologist
is also in the main marphological, though he has also
to inform us, if he can, about the physiology of develop-
ment.
Morphology has been defined by Geddes as “the study
of all the statical aspects of organisms,” in contrast to
physiology, which is concerned with their vital dynamics.
In this chapter we shall follow the historical development
of morphology, and work from the outside inwards.
I, Form and symmetry.—The form of an animal is due
to the interaction of two variables—the protoplasmic
material which composes the organism, and the environ-
ment which plays upon it. In fact, an animal takes definite
form just as a mineral does: in both the shape is determined
’ by the nature of the stuff and by the surrounding influences.
Activity, or function, also affects form; but function is
merely action and reaction between the animal and its
surroundings.
As regards symmetry, animals may be distinguished
as—(a) radially symmetrical; (4) bilaterally symmetrical ;
(c) asymmetrical.
In a radially symmetrical animal, such as a jelly-fish, the body can
be halved by a number of vertical planes—it is symmetrical around a
median vertical axis. That is, it is the same all round, and has no
right or left side. In a bilaterally symmetrical body, such as a
worm’s, there is but one plane through which the body can be halved.
In an asymmetrical animal, such as a snail, accurate halving is im-
possible. ,
Radial symmetry is illustrated by simple Sponges, most Ccelentera,
and by many adu/t Echinoderms. As it is the rule in the two lowest
classes of Metazoa, and as it is characteristic of the very common
embryonic stage known as the gastrula (an oval or thimble-shaped sac
consisting of two layers of cells), it is probably more primitive than
the bilateral symmetry characteristic of most animals above Ccelentera.
Radial symmetry seems best suited for sedentary life, or for aimless
floating and drifting. Bilateral symmetry probably arose as it became
advantageous for animals to move energetically and in definite direc-
tions, to pursue their prey, avoid their enemies, and seek their mates.
The formation of a ‘‘brain” is correlated with the habit of moving
head foremost. Among many-celled animals, some worm type prob-
ably deserves the credit of beginning the profitable habit of moving
head foremost. Had some one not taken this step, we should never
have known our right hand from our left.
36 THE ELEMENTS OF STRUCTURE.
II. Organs.—We give this name to any well-defined
part of an animal, such as: heart or brain. The word sug-
gests a piece of mechanism; but the animal is more than
a complex engine, and many organs have several different
activities to which their visible structure gives little clue.
Differentiation and integration of organs——\When we
review the animal series, or study the development of an
individual, we see that organs appear gradually. The
gastrula cavity—the future stomach—is the first acquisition,
though some would make out that it was primitively a
brood-chamber. To begin with, it is a simple sac, but it
soon becomes complicated by digestive and other out-
growths. The progress of the individual, and of the race,
is from apparent simplicity to obvious complexity. We
also notice that before definite nervous organs appear
there is diffuse irritability, before definite muscular organs
appear there is diffuse contractility, and so on. In other
words, functions come before organs. The attainment of
organs implies specialisation of parts, or concentration of
functions in particular areas of the body.
If we contrast a frog with Aydra, one of the great facts in
regard to the evolution of organs is illustrated. Among the
living units which make up a frog, there is much more
division of labour than there is among those of Hydra. An
excised representative sample of Aydra will reproduce the
whole animal, but this is not true of the frog. The struc-
tural result of this physiological division of labour is difer-
entiation. The animal, or part of it, becomes more complex,
more heterogeneous.
If we contrast a bird and a sponge, another great fact in
regard to the evolution of organs is illustrated. The bird is
more of a unity than a sponge; its parts are more closely
knit together and more adequately subordinated to the life
of the whole. This kind of progress is called zntegration.
Differentiation involves the acquisition of new parts and
powers, these are consolidated and harmonised as the
animal becomes more integrated.
Correlation of organs.—It is of the very nature of an
organism that its parts should be mutually dependent. The
organs are all partners in the business of life, and if one
member changes, others also are affected. This is especially
ORGANS. 37
true of certain organs which have developed and evolved
together, and are knit by close physiological bonds. Thus
the circulatory and respiratory systems, the muscular and
the skeletal systems, the brain and the sense organs, are
very closely united, and they are said to be correlated. A
variation, for better or worse, in one system often brings
about a correlated variation in another, though we cannot
always trace the physiological connection.
Homologous organs.—Organs which arise from the same
primitive layer of the embryo (see Chapter IV.) have some-
thing in common. But when a number of organs arise in
the same way, from the same embryonic material, and are
at first fashioned on the same plan, they have still more in
common. Nor will this fundamental sameness be affected
though the final shape and use of the various organs be very
different. We call organs which are thus structurally and
developmentally similar, omologous. Thus the nineteen
pairs of appendages on a crayfish are all homologous; the
three pairs of “jaws” in an insect are homologous with the
insect’s legs ; and it is also true that the fore-leg of a frog,
the wing of a bird, the flipper of a whale, the arm of a man,
are all homologous. The wing of a bird and the arm of
man exhibit the same chief bones, blood vessels, muscles,
and nerves, and they begin to develop in the same way ;
they are homologous but not analogous. The wing of a bird
and the wing of an insect, which resemble one another in
being organs of flight, are not the least alike in structure ;
they are analogous but not homologous. Yet two organs
may be doth homologous and analogous, e.g. the wing of a
bird and the wing of a bat, for both are fore-limbs, and
both are organs of flight. Sometimes two organs or two
organisms—deeply different in structure—have a marked
superficial resemblance, simply because both have arisen
in relation to similar conditions of life. Thus a burrow-
ing amphibian, a burrowing lizard, and a burrowing snake
resemble one another in being limbless, but this ‘“ conver-
gence,” or “homoplasty,” of form does not indicate any
relationship between them.
Change of function.—Division of labour involves restric-
tion of functions in the several parts of an animal, and no
higher Metazoa could have arisen if all the cells had
38 THE ELEMENTS OF STRUCTURE.
remained with the many-sided qualities of Amcebe. Yet
we must avoid thinking about organs as if they were
necessarily active in one way only. For many organs, e.g.
the liver, have several very distinct functions. In addition
to the main function of an organ, there are often secondary
functions ; thus the wings of an insect may be respiratory
as well as locomotor, and part of the food canal of Tunicates
and Amphioxus is almost wholly subservient to respiration.
Moreover, in organs which are not very highly specialised,
it seems as if the component elements retained a consider-
able degree of individuality, so that in course of time what
was a secondary function may become the primary one.
Thus Dohrn, who especially emphasised this idea of
function change, says: ‘Every function is the resultant of
several components, of which one is the chief or primary
function, while the others are subsidiary or secondary.
The diminution of the chief function and the accession of a
secondary function changes the total function ; the secondary
function becomes gradually the chief one; the result is the
modification of the organ.” The contraction of a muscle is
always accompanied by electric changes, and in the electric
organs of fishes we see the electric changes in the modified
muscular tissue composing the organs becoming more
important than the contractility. The structure known as
the allantois is an unimportant bladder in the frog, in Birds
and Reptiles it forms a foetal membrane (chiefly respiratory)
around the embryo, and in most Mammals it forms part of
the placenta which effects vital connection between off-
spring and mother. ;
Substitution of organs.—The idea of several changes of
function in the evolution of an organ, suggests another of
not less importance which has been emphasised by Kleinen-
berg. An illustration will explain it. In the early stages
of all vertebrate embryos, the supporting axial skeleton is
the notochord,—a rod developed along the dorsal wall of
the gut. From Fishes onwards, this embryonic axis is
gradually replaced in development by the vertebral column
or backbone; the notochord does not become the back-
bone, but is replaced by it. It is a temporary structure,
around which the vertebral column is constructed, as a tall
chimney may be built around an internal scaffolding of
ORGANS, 39
wood. Yet it remains as the sole axial skeleton in
Amphioxus, persists in great part in hag and lamprey, but
becomes less and less persistent in Fishes and higher
Vertebrates, as its substitute, the backbone, develops more
perfectly. Now, what is the relation between the notochord
and its substitute the backbone, seeing that the former does
not become the latter? Kleinenberg’s suggestion is that
the notochord supplies the stimulus, the necessary condi-
tion, for the formation of the backbone. Of course we
require to know more about the way in which an old-
fashioned structure may stimulate the growth of its future
substitute, but the general idea of one organ leading on to
another is suggestive. It is consistent with our general
conception of development—that each stage supplies the
necessary stimulus for the next step; it also helps us to
understand more clearly how new structures, too incipient
to be of use, may persist, and why old structures should
linger though they have only a transitory importance.
Rudimentary organs.—In many animals there are struc-
tures which attain no complete development, which are
rudimentary in comparison with those of related forms, and
seem retrogressive when compared with their promise in
embryonic life. But it is necessary to distinguish various
kinds of rudimentary structures. (a) As a pathological
variation, probably due to some germinal. defect, or to the
insufficient nutrition of the embryo, the heart of a mammal
is sometimes incompletely formed. Other organs may be
similarly spoilt in the making. They illustrate arrested
development. (6) Some animals lose, in the course of their
life, many of the prominent characteristics of their larval
life ; thus parasitic crustaceans at first free-living, and sessile
sea-squirts at first free-swimming, always undergo degenera-
tion, which can be seen in each lifetime. (¢) But the little
kiwi of New Zealand, with mere apologies for wings,
and many cave fishes and cave crustaceans with slight
hints of eyes, illustrate degeneration, which has taken such
a hold of the animals that the young stages also are degener-
ate. The retrogression cannot be seen in each lifetime,
evident as it is when we compare these degenerate forms with
probable ancestors. (d) But among “rudimentary organs ”
we also include structures somewhat different, e.g. the gill-
40 THE ELEMENTS OF STRUCTURE
clefts which persist in embryonic reptiles, birds, and
mammals, though most of them serve no obvious purpose,
or the embryonic teeth of whalebone whales. These are
“vestigial structures,” traces of ancestral history, and in-
telligible on no other theory. The gill-clefts are used for
respiration in all vertebrates below reptiles; the ancestors
of whalebone whales doubtless had functional teeth.
Classification of organs.—We may arrange the various parts of the
body physiologically, according to their share in the life. Thus some
parts have most to do with the ex¢ermal/ relations of the animals ; such
as locomotor, prehensile, food-receiving, protective, aggressive, and
copulatory organs. Of zz¢ernal parts, the skeletal structures are passive ;
the nervous, muscular, and glandular parts are active. The repro-
ductive organs are distinct from all the rest. They are conveniently
called ‘‘ gonads,” which is a better term than reproductive glands.
For by a gland we mean an organ which secretes, whose cells produce
and liberate some definite chemical substance, such as a digestive
ferment ; whereas the gonads are organs where there is periodic multi-
plication of certain cells, kept apart from the specialisation character-
istic of most of the ‘‘body cells” or ‘‘somatic” cells, It is true,
however, that an accessory glandular function is often associated with
the gonads.
Another classification of organs is embryological, z.e. according to the
embryonic layer from which the various parts arise. Thus the outer
layer of the embryo (the ectoderm or epiblast) forms in the adult—(1)
the outer skin or epidermis ; (2) the nervous system ; (3) much at least
of the sense organs: the inner layer of the embryo (the endoderm or
hypoblast) forms at least an important part (the ‘‘ mid-gut ”) of the food
canal, and the basis of outgrowths (lungs, liver, pancreas, etc.) which
may arise therefrom, and also the notochord of Vertebrates: the middle
layer of the embryo (the mesoderm or mesoblast) forms skeleton,
connective swathings, muscle, lining of body-cavity, etc.
III. Tissues.—Zoological anatomists, of whom Cuvier
may be taken as a type, analyse animals into their com-
ponent organs, and discover the homologies between one
animal and another. But as early as 1801, Bichat had
published his Anatomie générale, in which he carried the
analysis further, showing that the organs were composed of
tissues, contractile, nervous, glandular, etc. In 1838-39,
Schwann and Schleiden formulated the “cell theory,” in
which was stated the result of yet deeper analysis—that all
organisms have a cel/wlay structure and origin. The
simplest animals (Protozoa) are typically single cells or unit
masses of living matter; as such all animals begin; but all,
TISSUES. 41
except the simplest, consist of hundreds of these cells united
into more or less homogeneous companies (tissues), which
may be compacted, as we have seen, into organs. If we
think of the organism as a great city of cells, the tissues
represent streets (like some of those in Leipzig), in each of
which some one kind of function or industry predominates.
The student should read the introductory chapters in one
of the numerous works on histology, so as to gain a general
idea of the characters of the different tissues.
There are four great kinds,—epithelial, connective,
muscular, and nervous.
(a) Epdthelial tissue is illustrated by the external layer of the skin
(epidermis), the internal (endothelial) lining of the food canal and its
outgrowths, the lining of the body cavity, etc. ; by the early arrange-
ments of cells in all embryos; and by the simplest Metazoa, such as
Hydra, whose tubular body is formed by two layers of epithelium.
Embryologically and historically, epithelium is the most primitive kind
of tissue. It may be single layered or. stratified; its cells may be
columnar, scale-like, or otherwise. The cells may be close together,
or separated by intercellular spaces, and they are often connected by
bridges of living matter. Nor are the functions of epithelium less
-diverse than its forms, for it may be ciliated (effecting locomotion,
food-wafting, etc.), or sensitive (and as such forming sense organs), or
glandular (liberating certain products or even the whole contents of its
cells), or pigmented (and thus associated with respiration, excretion,
and protection), or covered externally with 4 sweated-off cuticle,
susceptible of many modifications (especially of protective value).
(6) Connective tissue.—This term includes too many different kinds
of things to mean much. It represents a sort of histological lumber-
room.
The embryologists help us a little, for they have shown that almost
all forms of connective tissue are derived from the mesoderm or middle
layer of the embryo. As this mesoderm usually arises in the form of
outgrowths from the gut, or from (‘‘ mesenchyme”) cells liberated at
an early stage from either (?) of the two other layers of the embryo
(ectoderm or endoderm), we inay say that connective tissue is primarily
derived from epithelium.
The general function of ‘‘ connective tissue” is to enswathe, to bind,
and to support, but the forms assumed are very various. :
The cells may be without any intercellular ‘‘ mortar” or matrix.
They may be laden with fat or with pigment.
In other cases the cells of the connective tissue lic in a matrix,
which they secrete, or into which they in part die away. Sometimes
the matrix becomes secondarily invaded by cells. The connective cells
are very often irregular in outline, and give off, in most cases, fine
processes, which traverse the matrix as a network. ‘They may secrete
long fibres, as in the various kinds of fibrous tissue. The fibrous tissue
42 THE ELEMENTS OF STRUCTURE.
ot tendons and the different kinds of gristle or cartilage illustrate
connective tissue with much matrix. Cartilage is sometimes hardened
by the deposition of lime salts in its substance, and then has a slight
resemblance to another kind of ‘‘ connective tissue”—bone. But bone,
which is restricted to Vertebrate animals, is quite different from the
cartilage which it often succeeds and replaces. It is made by strands
or layers of special bone-forming cells (osteoblasts), which may rest on
a cartilage foundation, or may be quite independent. These osteoblasts
form the bone matrix, and some of them are involved in it, and become
the permanent bone cells. These have numerous radiating branches,
and are arranged in concentric layers, usually around a cavity or a blood
vessel. (There are no blood vessels in cartilage.) The matrix becomes
very rich in lime salts (especially phosphate); and the cartilage
foundation, if there was one, is quite destroyed by the new formation.
Here we may also note two important fluid tissues, the floating
corpuscles or cells of the blood, and those of the body cavity or
‘* perivisceral” fluid, which is often abundant and important in back-
boneless animals.
(c) Muscular tzssue.—The single-celled Ameba moves by flowing out
on one side and drawing in its substance on another. It is diffusely
contractile, and it has also sensitive, digestive, and other functions.
In Aydra and some other Ccelentera the bases of some of the epithelial
_cells which form the outer and inner layers are prolonged into con-
tractile roots. Here, then, we have cells of which a special part
discharges a contractile or muscular function, while the other parts
retain other powers.
In other Ccelentera the muscular cells are still directly connected with
the epithelium, but become more and more exclusively contractile. In
all other animals the muscular tissue is derived from the mesoderm,
which, as we have already mentioned, is not distinctly present in
Coelentera. In the majority, the muscle cells arise on the walls of the
body cavity, and their origin may often at least be described as epithelial.
But in other cases the muscles arise from those wandering ‘‘ mesen-
chyme” cells to which we have already referred.
Smooth or unstriped muscle fibres are elongated contractile cells,
externally homogeneous in appearance. They are especially abundant
in sluggish animals, e.g. Molluscs, and occur in the walls of the gut,
bladder, and blood vessels of Vertebrates. They are less perfectly
differentiated than striped muscle fibres, and usually contract more
slowly.
A striped muscle fibre is a cell the greater part of which is modified
into a set of parallel longitudinal fibrils, with alternating ‘‘clear and
dark ” transverse stripes. A residue of unmodified cell substance, with
a nucleus or with many, is often to be observed on the side of the fibre,
and a slight sheath or sarcolemma forms the ‘‘cell wall.” Many
muscle fibres closely combined, and wrapped in a sheath of connective
tissue, form a muscle, which, as every one knows, can contract with
extreme rapidity when stimulated by a nervous impulse.
(d@) Nervous tissue.—Beginning again with the Ameba, we recognise
that it is diffusely sensitive, and that a stimulus can pass from one part
of the cell to another. '
TISSUES 43
In some Ccelentera a few of the external cells seem to combine
contractile and nervous functions. Therefore they are sometimes called
** neuro-muscular.”
But in Hydra there are superficial sensory cells, whose basal pro-
longations are connected either directly with contractile cells, or with
deeper ganglion-cells, some of which give off motor processes to the
contractile cells.
In sea-anemones and some other Ccelentera there is a more sharply
defined division of labour. Superficial sensory cells are connected
with subjacent nerve- or ganglion-cells, from which fibres pass to the
contractile elements.
In higher animals the sensory cells are mostly integrated into sense
organs, the ganglionic cells into ganglia, while the delicate fibres.
which form the connections between sensory cells and ganglionic cells,
and between the latter and muscles, are represented by well-developed
nerves.
So far as we know, nervous tissue always arises from the outer or
ectodermic layer of the embryo, as we would expect from the fact that
this is the layer which, in the course of history, has been most directly
subjected to external stimulus.
Let-us consider first the ganglionic cells’ which receive stimuli and
shunt them, which regulate the whole life of the organism, and are the
physical conditions of ‘‘ spontaneous” activity and intelligence. They
are of very varied shape, but consist always of a cell-body which gives
off one or more processes. One of these processes is long, branches
very sparingly, and is known as the axis-cylinder. There are usually
present other processes which ramify like the branches of a tree and
are called dendrites. The cell-body contains a nucleus, distinct
granules, and a network of fine fibrils. The nervous system is built up
of such ‘‘neurones.” In the ganglia they are supported and held
apart by much-branched neuroglia cells.
In all but a few of the simplest Metazoa, the nerve fibres (axis-
cylinders) are surrounded by a sheath called the neurilemma, said to be
formed by adjacent connective tissue. Several nerve fibres may com-
bine to form a nerve, but each still remains ensheathed in its neuri-
lemma while fibrous sheaths bind the nerve fibres together. In Verte-
brate animals each nerve fibre usually has in addition a medullary
sheath. But even in the higher Vertebrates, ‘‘non-medullated” or
simply contoured nerve fibres are found in the sympathetic and olfactory
nerves, and this simpler type alone occurs in hag, lamprey, ‘and
lancelet, as well as in all the Invertebrates with distinct nerves.
A nerve fibre contains numerous fibrils like those seen within a
ganglion cell. These are regarded by some-as the essential elements in
conducting stimuli, while others maintain that the essential part is the
less compact, sometimes well-nigh fluid stuff between the fibrils, or that
the fibrils are but the walls of tubes within which the essentially nervous
stuff lies.
The nerve fibres arise as prolongations of the ganglion cells,
which extend themselves in the embryo like Amoebze sending out
pseudopodia.
44 THE ELEMENTS OF STRUCTURE.
IV. Cells.—In discussing tissues, it was necessary to
refer to the component cells. Let us now consider the
chief characteristics of these elements.
A cell is a unit mass or area of living matter usually with
a nucleus. Most of the simplest animals and plants
(Protozoa and Protophyta) are single cells; eggs and male
elements are single cells; in multicellular organisms the
cells are combined into tissues and organs.
Most cells are too
small to be distinguished
except through lenses;
many Protozoa, ¢.g. large
Ameebee, are just visible
to our unaided eyes; the
chalk -forming Foramin-
ifera are single cells, whose
shells are often as ‘large
as pin-heads, and some of
the extinct kinds were as
big as half-crowns (see
Fig. 17); the bast cells
of plants may extend for
i several inches; the largest
Fic. 21.—Diagram of cell structure. animal cells are eggs dis-
atte WAG Rs tended with yolk.
Pi. Plastids in cytoplasm. The typical and primi-
cc. Centrosome.
ate Beclealis tive form of cell is a
‘W, Nucleus sphere—a shape naturally
ct. General cytoplasm. assumed by a complex
V. Vacuole. f
Gr. Granules. coherent substance situ-
ated in a medium different
from itself. Most egg-cells and many Protozoa retain this
primitive form, but the internal and external conditions of
life (such as nutrition and pressure) often evolve other
shapes,—oval, rectangular, flattened, thread-like, stellate,
and so on.
As to the structure of a cell, we may distinguish (see
Fig. 21)—
(a) The general cell substance or cytoplasm, which con-
sists partly of genuinely living stuff or protoplasm, and
partly of complex materials not really living (metaplasm) ;
CELLS. 45
(4) A specialised nucleus, with a complex, structure,
and important functions ; :
(c) One or more specialised bodies called central
corpuscles or centrosomes, which seem to be centres of
activity during cell division ; :
(d) A cell wall, which occurs in very varied form, or may
be entirely absent.
(a) As to the cell substance, it often appears at first sight
almost homogeneous, but higher magnification shows con-
siderable structural complexity. It is certainly not like
white of egg, but shows a reticular, fibrillar, or vacuolar
structure. It is usually slightly fluid, but it may be firm
and compact in passive cells. It is usually translucent, but
there are often obscuring granules of different kinds.
In thinking of the cell substance or cytoplasm, we
distinguish the genuinely living protoplasm, which may be
a mixture of proteids, from other materials of simpler
chemical composition, such as carbohydrates, fats, pigments,
etc. Some of these may be nutritive materials in process
of elaboration into more complex substances; others are
disruptive products of the metabolism.
(6) As to the nucleus, one at least is present in almost
every cell. It used to be said that some very simple
animals, which Haeckel called Monera, had no nuclei, but in
many cases the nuclei have now been demonstrated. In
other cases, e.g. some Infusorians, the nuclear material seems
to be diffused in the cell substance. The red blood cells
-of Mammals seem to be distinctly nucleated in their early
stages, though there is no nucleus in those which are full
grown.
The nucleus is a very important part of the cell, but it is
not yet possible to define precisely what its importance is.
In fertilisation an essential process is the union of the
nucleus of the spermatozoon or male cell with the nucleus
of the ovum or female cell (Fig. 23). In cell division the
nucleus certainly plays an essential part. Cells bereft of
their nuclei die, or live for a while a crippled life. Accord-
ing to some,. the nucleus is important in connection with
the nutrition of the cell; according to others, it is of special
importance in connection with the respiration of the cell.
It is certain that there are complex actions and reactions’
46 THE ELEMENTS OF STRUCTURE.
between the living matter of the nucleus and that of the
cytoplasm.’ Cytoplasm and nucleoplasm form a “cell firm,”
potent in their co-operation. In many cells it has been
shown that fragments or extensions of the nucleus pass into
the cytoplasm, forming what is called a “ chromidial appar-
atus,” which seems to be of much functional importance.
The nucleus often lies within a little
nest in the midst of the cell substance,
but it may shift its position from one
part of the cell to another. It has a
definite margin, but this. may be lost,
e.g. before cell division begins. Inter-
nally, it is anything but homogeneous
(see Fig. 22); at any rate, homogeneous
nuclei are rare. Twisted strands or
Fic. 22, —Structure tubes of “linin” bear a more stainable
of the cell.—After material called “chromatin,” and when
Carnoy. the cell is preparing to divide the
W, Nucleus with chro- strands assume the form of a definite
matin coil; note pro-
toplasmic reticulum. number of separable rods or loops or
granules, the “chromosomes.” Sur-
rounding the linin and chromatin is the nuclear sap.
Sometimes a linin thread shows a row of minute chromatin bodies
{microsomata), like jewel-stones embedded on a belt. Weismann
maintains that the chromosomes or idants of the germ-cells are the
vehicles of the heritable qualities. He has made a hypothetical scheme,
according to which the chromosomes or zdazés are built up of zds, and
the ids of determinants, and the determinants of dzophors.
Many nuclei also contain little round bodies or nucleoli,
or sometimes a single nucleolus. The term is applied
somewhat vaguely to little aggregations of chromatin, and
more properly to vacuole-like bodies, in which some believe
that the waste products of the nucleus are collected.
(c) As to the centrosomes, it may be noted that when an
animal cell divides, these bodies play an important part.
The chromatin elements of the nucleus are divided, and
separate to form the two daughter nuclei. In this separa-
tion extremely fine “archoplasmic” threads pass from the
centrosomes to the chromosomes. The centrosomes are
therefore regarded as “division organs,” or as “dynamic
centres.” They also occur, in most cases singly, in resting
CELLS. 47
cells, and it seems likely that they are present in most
animal cells, at least in those which retain the power of
division.
(d) As to the cell wall, it seemed
of much moment to the earlier
histologists, who often spoke of
cells as little bags or boxes. It
is, however, the least important
part of the cell. In plant cells
there is usually a very distinct
wall, consisting of cellulose. This 1G. 23.—Fertilised ovum of
is a product, not a part, of Ascarts.—After Boveri.
chr., Chromatin elements, two
the protoplasm, though some from ovum nucleus and two
protoplasm may be intimately ee Bielend ) Gee
Fy . ‘* . rom whic
associated with it as long as its “archoplasmic” threads
growth continues. In animal dadinte, wartly Sods elivom:
a me:
cells there is rarely a very distinct
wall chemically distinguishable from the living matter
itself. But the margin is often different from the in-
terior, and a slight wall may be formed by a superficial
compacting of the threads of the cell network, or by a
physical alteration of the cell substance, comparable to the
formation of a skin on cooling
porridge. In other cases, especi-
ally in cells which are not very
active, such as ova and encysted
Protozoa, a more definite sheath -
is formed around the cell sub-
stance. Again, animal cells may
secrete a superficial “cuticle,” e.g.
the chitin formed by the ectoderm
cells in Insects, Crustaceans, and
Fic. 24.—Diagram of cell other Arthropods.
division.—After Boveri. In animals, as well as in plants,
chr. Chromosomes forming adjacent cells are often linked
an cquatorial' plate; ¢s by intercellular bridges of living
matter, which may be paths for
the passage of materials or of disturbances from cell to cell.
In many cases, ¢.g. of gelatinous tissue, a matrix arises out-
side of and between the cells, as an exoplasmic product.
In regard to cell division, the most important facts are the
48 THE ELEMENTS OF STRUCTURE.
following :—There is a striking similarity in most cases, and
the nucleus plays an essential part in the process. The
dividing nucleus usually passes through a series of complex
changes known as karyokinesis or mitosis, and these are
much the same everywhere, though different kinds of cells
have their specific peculiarities. Occasionally, however,
both in Protozoa and Metazoa, the nucleus divides by
simple constriction (direct or amitotic division). This is a
quicker process than the other, and occurs especially when
there is rapid growth or frequent replacement of cells.
Another departure from the ordinary scheme is seen when
the nucleus shows a multiple division, while the cell
remains undivided. This occurs normally in some marrow
cells.
The eventful changes of karyokinesis are as follows :—
(a) The resdeng stage of the nucleus shows a network or complete
coil of filaments (chromatin elements) (Fig. 22).
(4) First stage.—As division begins, the membrane separating
the nucleus from the cell substance disappears, and the
chromatin elements are seen as a tangled or broken coil
(Fig. 25, 1).
(c) Astroid stage.—The chromatin elements bend into looped
pieces (or chromosomes), which are disposed in a star, lying
flat at the equator of the cell, the free ends of the U-shaped
loops being directed outwards. Meanwhile a centrosome
has appeared and divided into two separating halves,
between which a spindle of fine achromatin threads is
formed. This seems to form (at least part of) what is
called the nuclear spindle. The centrosomes separate until
one lies at each pole of the cell, surrounded by radiating
‘archoplasmic” threads which become attached to the
chromosomes (Fig. 25, 2). J
(d) Diviston and separation of the loops.—Each of the loops
which make up the star divides /ongztudinally into two,
and each half separates from its neighbour. They lie at
first near the equator of the cell, but they are apparently
drawn, or driven, to the opposite poles (Fig. 25, 2-4).
(e) Déastrocd.—The single star thus forms two daughter stars,
which separate farther and farther from one another towards
the opposite poles of the cell, remaining connected, how-
ever, by delicate threads (Fig. 25, 3-5).
(7) Each daughter star is reconstituted into a coil or network for
each daughter cell, for the cell substance has been con-
stricted meanwhile at right angles to the transverse axis of
the spindle. The halves separate in the case of Protozoa,
but in most other cases, e.g. growing embryos, they remain
adjacent, with a slight wall between them (Fig: 25, 6).
i CELLS. 49
(g) Each daughter nucleus then passes into the normal resting
phase. The spindle disappears, and the centrosomes may
also vanish.
The essential fact is the exact partition of the nuclear material
between the two daughter cells :
Flemming gives the following summary of karyokinesis :—
MOTHER NUCLEUS DauGHTER NUCLEUS
(progressive changes). (regressive changes).
a Resting stage. Resting stage. z
& Coil. Coil.
w ¢ Astroid. Diastroid. é
» d Division of Astroid and its loops ——>
(Prophases) (Metakinesis) (Anaphases).
Fic. 25.—Karyokinesis.—After Flemming.
1. Coil stage of nucleus ; ¢.c., central corpuscle.
2. Division of chromatin elements into U-shaped loops, and longitudinal
splitting of these (astroid stage). ‘
, 4. Recession of chromatin elements from the equator of the cell
(diastroid). ,
5. Nuclear spindle, with chromatin elements at each pole, and
achromatin threads between.
6. Division of the cell completed.
Besides the ordinary indirect division just described, the
net result of which is that each of the two daughter cells
gets an equal number of chromosomes, a precise half of
each of the chromosomes in the original cell, there is
another kind of cell division (meiotic or reducing division)
which occurs only in the maturation of the ovum and
4 .
50 THE ELEMENTS OF STRUCTURE. oe
spermatozoon, and has for its net result the reduction of the
number of chromosomes to a half of the normal number.
We are far from being able to give even an approximate
account of the “mechanism” of cell division. The whole
process is vital, and cannot, at present at least, be re-
described in terms of matter and motion.
On the other hand, Leuckart, Spencer, and Alexander
James have given a general rationale of cell division. Why
do not cells grow much larger? why do they almost always
divide at a definite limit of growth? ‘The answer is as
follows :—Suppose a young cell has doubled its original
volume, that means that there is twice as much living
matter to be kept alive. But the living matter is fed,
aerated, purified through its surface, which, in growing
spherical cells, for instance, only increases as the square
of the radius, while the mass increases as the cube. The
surface growth always lags behind the increase of mass.
Therefore, when the cell has, let us say, quadrupled its
original volume, but by no means quadrupled its surface,
difficulties set in, waste begins to gain on repair, anabolism
loses some of its ascendancy over katabolism. At the limit
of growth the cell divides, halving its mass and gaining new
surface. It is true that the surface may be increased by out-
flowing processes, just as that of leaves by many lobes; and
division may occur before the limit of growth is reached,
but, as a general rationale, applicable to organs and bodies
as well as to cells, the suggestion above outlined is very
helpful. The ratio of the amount of nuclear material in
the cell to the amount of cytoplasmic. material seems also
to have a determining influence upon cell division (R.
Hertwig).
Protoplasm. —Morphological as well as physiological
analysis passes from the organism as a whole to its organs,
thence to the tissues, thence to the cells, and finally to the
protoplasm itself. But although we may define protoplasm
as genuinely living matter—as “the physical basis of life”
—we cannot definitely say how much or what part of an
Ameeba, or an ovum, or any other cell, is really protoplasm.
We are able to make negative statements,.eg, the yolk of
an egg is not protoplasm, but we cannot make positive
‘statements, or say, This is protoplasm, and nought else.
PROTOPLASM. 51
Thus what is spoken of as the structure of protoplasm is
really the structure of the cytoplasm. It is often.specifically
different in different cases.
In regard to this structure, we know that it is very complex, but we
are not sure of much more. For different experts see different appear-
ances, even in the same cells.
Thus some, ¢.g. Frommann, describe a network or reticulum, with
less stable material in the meshes; others, ¢.g. Flemming, describe
a manifold coil of fibrils; and others, ¢.g. Biitschli, describe a foam-
like or vacuolar structure. It seems likely that the structure is different
at different times, or in different cells.
Professor Biitschli’s belief that the cytoplasm has a vacuolar structure
is corroborated by his interesting experiments on microscopic foams.
Finely powdered potassium carbonate is mixed with olive oil which has
been previously heated to a temperature of 50°-60° C., an acid from the
oil splits up the potassium carbonate, liberates. carbon dioxide, and forins
an extremely fine emulsion. Drops of this show a structure not unlike
that of cytoplasm, exhibit movements and streamings not unlike those
of Amcebze, and are, in short, mimic cells. Just as a working model
may help us to understand the circulation, so these oil-emulsion drops |
may help us to understand the living cell, by bringing the strictly vital
phenomena into greater prominence.
Tt cannot be said, however, that subsequent research has corroborated
the conclusion that cytoplasm has, in general, a vacuolar structure.
There is increasing evidence of specific architectural organisation in
different kinds of cells, and of the significance of infinitely small
bodies—the plastosomes—which are included in the general cyto-
plasmic matrix, and appear to be the vehicles of particular properties
or formative potencies.
What is certain is that the cell-substance is not homogene-
ous like white-of-egg, but very heterogeneous and intricate.
CHAPTER IV
THE REPRODUCTION AND LIFE HISTORY OF
ANIMALS
I. REPRODUCTION
In the higher animals the beginnings of individual life are
hidden, within the womb in Mammals, within the egg-shell
in Birds. It is natural, therefore, that early preoccupation
with those higher forms should have hindered the recogni-
tion of what seems to us so evident, that almost every
animal arises from an egg-cell or ovum which has been
fertilised by a male cell or spermatozoon. The exceptions
to this fact are those organisms which multiply by buds or
detached overgrowths, and those which arise from an egg-
cell which requires no fertilisation. Thus Hydra may form
a separable bud, much as a rose-bush sends out a sucker ;
thus drone-bees “have a mother, but no father,” for they
arise from parthenogenetic eggs which are not fertilised.
Sexual reproduction.—There is apt to be a lack of clear-
ness in regard to sexual reproduction, because the process
which we describe by that phrase is a complex result of
evolution. It involves two distinct facts—(a) the liberation
of special germ cells from which new individuals arise ; (4)
the union or amphimixis of two different kinds of germ
cells, ova and spermatozoa, which come to nothing unless
they unite. Furthermore, these dimorphic reproductive
cells are produced by two different kinds of individuals
(females and males), or from different organs of one
individual, or at different times within the same organ
(hermaphroditism).
It is conceivable that organisms might have gone on
REPRODUCTION. 53
multiplying asexually, by detaching overgrown portions of
themselves which had sufficient vitality to develop into
complete forms. But a more economical method is the
liberation of special germ cells, in which the qualities of the
organism are inherent. This is the primary characteristic
of sexual, as opposed to asexual, multiplication.
It is also conceivable that organisms might have remained
approximately like one another in constitution, and at all
times very nearly the same, and that they might have
liberated similar germ cells capable of immediate develop-
ment. Such a race would have illustrated the one charac-
teristic of sexual reproduction, the liberation of special germ
cells; but it would have been without that other character-
istic of sexual reproduction—the amphimixis or fertilisation
of dimorphic germ cells, usually produced by different
organs in one individual or by distinct male and female
individuals.
Liberation of special germ cells,x—One must think of
this as an economical improvement on the method of start-
ing a new life by asexual overgrowth or by the liberation of
buds. Asexual reproduction, as Spencer and Haeckel point
out, is a mode of growth in which the bud, or whatever it is,
becomes distinct or discontinuous from the parent. The
buds of a sponge, of a coral, of a sea-mat, or of many
Tunicates, remain attached to the parent. If there be a
keen struggle for subsistence, this may be disadvantageous ;
but in some cases, doubtless, the colonial life which results
is a source of strength. In the case of Aydra, however,
the buds are set adrift ; the same is true of not a few worms.
This liberation of buds takes us nearer the sexual process
of liberating special germ cells. But unless the organism
is in very favourable nutritive conditions, in which over-
growth is natural, the liberation of buds is an expensive way
of continuing the life of a species. Not only so, but we
can hardly think of budding even as a possibility, in very
complex organisms, like snails or birds, in which there is
much division of labour. Moreover, the peculiarity of true
germ cells is that they do not share in building up the “ body,”
and that they retain an organisation continuous in quality
with ‘the ‘original germ cell from which the parent arose;
they are thus not very liable to be tainted by the mishaps
54 REPRODUCTION AND LIFE HISTORY.
which may befall the “body” which bears them. And,
finally, in the mixture of two units of living matter which
have had different histories, an opportunity for new permuta-
tions and combinations, in other words, for variation, 18
supplied. Thus it is not surprising to find that the asexual
method of liberating buds has been replaced in most
animals by the more economical and advantageous process
of sexual reproduction.
SumMMARY OF MopEs OF REPRODUCTION
A. In Single-celled Animals (Protozoa)
(1) The almost mechanical rupture of an amoeboid cell, which has
become too large for physiological equilibrium.
(2) The discharge of numerous superficial buds at once (e.g. Arcella
and Pelomyxa). 5 *
(3) The formation of one bud at a time (very common).
(4) The ordinary division into two daughter cells at the limit of
growth.
(5) Repeated divisions within limited time and within limited space’
(a cyst). This results in what is called spore-formation (e.9.
in Sporozoa).
B. Ln Many-celled Animals (Metazoa) ,
(Asexual)
(a) The separation of a clump of body cells, e.g. from the surface of
some Sponges. (A crude form of budding.)
(4) The formation of definite buds which may or may not be set free.
(c) Various forms of fission and fragmentation.
(Sexual)
The liberation of special reproductive or germ cells, which have
not taken part in the formation of the body, and which retain
the essential qualities of the original germ cell from which the '
parent arose. These special germ cells—the ova and sperma-
tozoa—are normally united in fertilisation, but some animals
have (parthenogenetic) ova which develop without being
fertilised.
Evolution of sex.—A further problem is to account for
the two facts—(a) that most animals are either males or
temales, the former liberating actively motile male elements
or spermatozoa, the latter ‘forming and usually liberating
more passive egg cells or ova; and (4) that these two
EVOLUTION OF SEX, 55
different kinds of reproductive-cells usually come to nothing
unless they combine. “te Bs
The problem is partly solved by a clear statement of the
facts. Let us begin with those interesting organisms which
are on the border line between Protozoa and Metazoa,
the colonial Infusorians, of which Volvox is a type. The
adults are balls of cells, and the component units are con-
nected by protoplasmic bridges. From’such a ball of cells
reproductive units are sometimes set adrift, and these divide
to form other individuals without more ado. In other con-
ditions, however, when nutrition is checked, a less direct
mode of reproduction occurs. Some of the cells become
large, well-fed elements, or ova; others, less successful,
divide into many minute units or spermatozoa. The large
cells are fertilised by the small. - Hete we see the formation
of dimorphic reproductive cells in different parts of the
same organism. But we may also find Volvex balls in
which only ova are being made, and others, with only
spermatozoa. The former seem to be more vegetative and
nutritive than the latter; we call them female and male
organisms respectively ; we are at the foundation of the
differences between the two sexes.
All through the animal series, from active Infusorians and
passive Gregarines to feverish Birds and more sluggish
Reptiles, we read antitheses between activity and passivity,
between lavish expenditure of energy and a habit of storing.
The ratio between disruptive (Aafabolic) processes and con-
structive (azabolic) processes in the protoplasmic metabolism
varies from type. to type. It may be that the contrast
between the sexes is another expression of this fundamental
alternative of variation.
Stages in the history of fertilisation. —While it is not difficult
to see the advantage of fertilisation as a process which helps to sustain
the standard or average of a species and as a source of new variations,
we can at present do little more than indicate various forms in which
the process occurs,
(a) Formation of Plasmodia, the flowing together of numerous feeble
cells, as seen in the life-history of those very simple Protozoa
called Proteomyxa, ¢.¢. Protomyxa, and Mycetozoa, ¢.g. flowers
of tan (4 thaliwm septicum), .
(8) Multiple conjugation, in which more than two cells unite and fuse
together temporarily, as in some Sporozoa and in the sun-
animalcule.(Actinospherium). a wa
. 56 REPRODUCTION AND LIFE HISTORY.
(¢) Ordinary conjugation, in which two similar cells unite, with
fusion of their nuclei, observed in Sporozoa, Heliozoa, Flagel-
lates, and Rhizopods. In ciliated Infusorians, the conjugation
may be merely a temporary union, during which nuclear elements
are interchanged,
(d) Dimorphie conjugation, in which two cells different from one
another fuse into one, a process well illustrated in Vorticella
and related Infusorians, where a small, active, free-swimming
(we may say, male) cell unites with a fixed individual of normal
size, which may fairly be called female (see Fig. 42 and Fig. 47).
(e) Fertilisation, in which a spermatozoon liberated from a Metazoon
unites intimately with an ovum, usually liberated from another
individual of the same species.
Divergent modes of sexual reproduction.—(2) Herm-
aphroditism is the combination of male and female sexual
functions in varying degrees within one organism. It may
be demonstrable in early life only, and disappear as male-
ness or femaleness predominates in the adult. It may
occur as a casualty or as a reversion; or it may be normal
in the adult, e.g. in some Sponges and Ccelentera, in many
“worms,” such as earthworm and leech, in barnacles and
acorn-shells, in one species of oyster, in the snail, and in
many other Bivalves and Gastropods, in Tunicates and in
the hag-fish. In most cases, though these animals are
bisexual, they produce ova at one period and spermatozoa
at another (dichogamy). It rarely occurs (e.g. in some
parasitic worms) that the ova of a hermaphrodite are
fertilised by the sperms of the same animal (autogamy).
Certain facts, such as the occurrence of hermaphrodite
organs as a transitory stage in the development of the
embryos of many unisexual animals (e.g. frog and bird),
suggest that hermaphroditism is a primitive condition, and
that the unisexual condition of permanent maleness or
femaleness is a secondary differentiation. Other facts, such
as the hermaphroditism of many parasites, where cross-
fertilisation would be difficult, suggest that the bisexual
condition may have arisen as a secondary adaptation. It
seems likely that there is both primitive and secondary
hermaphroditism.
(4) Parthenogenesis, as we know it, is a degenerate form
ofsexuals reproduction, in which ova produced by female
organism develop without being fertilised by male elements.
It is well illustrated by Rotifers, in which fertilisation is the
MODES OF SEXUAL REPRODUCTION. 57
exception (in some genera males have never been found),
by many small Crustaceans whose males are absent for a
season; by Aphides, from among which males may be
absent for the summer (or in artificial conditions for several
years) without affecting the rapid succession of female
generations ; by the production of drones in the bee-hive
from eggs which are never fertilised.
(c) Alternation of generations. A fixed asexual hydroid
or zoophyte often buds off and liberates sexual medusoids
or swimming-bells, whose
fertilised ova develop in- ee, ee
to embryos which become I .
fixed and grow into hydroids 4 tor or
(Fig. 71, p. 150). This is
the simplest illustration of
alternation of generations, Grow
which may be defined as .p ©
the alternate occurrence in a ar"
one life-cycle of two (or more)
different forms differently ¥ic. 26.—Diagrammatic expression
produced (Fig. 26): of alternation of generations.
The liver-fluke (Distomum 1. Hydromeduse,
ov. Fertilised ovum gives rise to an
hepaticum) of the sheep asexual form 4, which, by bud-
produces eggs which, when fae Gane Be one Cee
fertilised, grow into embryos. medusz, A is represented by
Within the latter, certain eo ee
cells (which might be called 7 pertlioet ovum gives rise to
spores) grow into numerous paae e see Nad i, from
Dp special spore-like cells pro-
ee Ee = ae a eae the sexual
orm. ithin these e fluke (5).
same process is repeated,
and finally the larvee thus produced grow (in certain con-
ditions) into sexual flukes (Fig. 98, p. 189). In this case,
reproduction by special cells, like undifferentiated precocious
ova, alternates with reproduction by ordinary fertilised egg-
cells. So, too, the vegetative sexless “fern-plant” gives rise
to special spore cells, which develop into an inconspicuous
bisexual “ prothallus,” from the fertilised egg-cell of which
a “fern-plant” springs.
Various kinds of alternation are seen in the life-cycle of
the fresh-water sponge, in the stages of the jelly-fish Auzelza,
58 REPRODUCTION AND LIFE HISTORY.
in the history of some “worms” and Tunicates. They
illustrate a rhythm, between asexual and sexual multiplica-
tion, between parthenogenetic and normal sexual reproduc-
tion, between vegetative and animal life, between a relatively
“anabolic” and a relatively “‘katabolic” preponderance.
II. EMBRYOLOGY
Egg cell or ovum.—Apart from cases of asexual repro-
duction and parthenogenesis, every multicellular animal
begins life as an egg cell with which a male cell or sperma-
tozoon has entered into intimate union. :
The most important characteristic of the reproductive
cells, whether male or
female, is that they
retain the essential
qualities of the fer-
_tilised ovum from
which the parent
animal ‘was devel-
oped.
The ovum has the
usual characters of a
cell; its substance is
traversed by a fine
protoplasmic net-
work ; its nucleus or
germinal vesicle con-
Fic. 27.—Diagram of ovum, showing diffuse
yolk granules. tains the usual chro-
g.v., Germinal vesicle or nucleus ; chv., chromatin matin elements 3 it
elements. has often a store of
reserve material or
yolk, and a distinct sheath representing a cell wall
(Fig. 27). . ;
In Sponges the ova are well-nourished cells in the middle
stratum of the body; in Ccelentera they seem to arise in
connection with either outer or inner layer (ectoderm or
endoderm) ; in all other animals they arise in connection
with the middle layer or mesoderm, usually on an area of.
the epithelium lining the body ‘cavity.. In lower animals
they often arise somewhat diffusely ; in higher animals their.
EMBRYOLOGY. 59
formation is restricted to distinct regions, and usually to
definite organs—the ovaries.
The young ovum is often amceboid, and that of Hydra
retains this character for some time (Fig. 70, p. 148). The
ovum grows at the expense of adjacent cells, or by absorb-
ing material which is contributed by special yolk glands or
supplied by the vascular fluid of the body.
The yolk or nutritive capital may be small in amount,
and distributed uniformly in the cell, as in the ova of
Mammals, earthworm, starfish, and sponge; or it may be
more abundant, sinking towards one pole as in the egg of
the frog, or accumulated in the centre as in the eggs of
Insects and Crustaceans; or it may be very copious, dwarf-
ing the formative protoplasm, as in the eggs of Birds,
Reptiles, and most Fishes (Fig. 31).
Round the egg there are often sheaths or envelopes of
various kinds—(a) made by the ovum itself, and then very
delicate (e.g. the vitelline membrane); (4) formed by ad-
jacent cells (e.g. the follicular envelope) ; or (¢) formed by
special glands or glandular cells in the walls of the oviducts
(e.g. the “shells” of many eggs). The envelope is often
firm, as in the chitinous coat around the eggs of many
Insects, and in these cases we find a minute aperture
(micropyle) or several of them through which’ the sperma-
tozoon can enter. The hard calcareous shells round the
eggs of Birds and Tortoises, or the mermaid’s purse en-
closing the egg of a skate, are of course formed after
fertilisation. Egg-shells must be distinguished from egg
capsules or cocoons, ¢.g. of the earthworm, in which several
eggs are wrapped up together.
Male cell or spermatozoon.—This is a much smaller
and usually a much more active cell than the ovum.
In its minute size, locomotor energy, and persistent
vitality, it resembles a flagellate Monad, while the ovum is
comparable to an Amoeba or to one of the more encysted
Protozoa.
A spermatozoon has usually three distinct parts: the
essential ‘“‘head,” consisting mainly of nucleus, and the
mobile “tail,” which is often fibrillated, and a small middle
portion between head and tail, which is said to be the
bearer of the centrosome. The spermatozoa of Thread-
60 REPRODUCTION AND LIFE HISTORY.
worms and most Crustaceans are sluggish, and inclined to
be ameeboid (Fig. 28 (6, 7)).
Both ova and spermatozoa are true cells, and they are
complementary, but the spermatozoon has a longer history
behind it (Fig. 29). The homologue of the ovum is the
mother sperm cell or spermatogonium. This segments as
the ovum does, but the cells into which it divides have
little coherence. They go apart, and become spermatozoa.
There is often a resemblance between the different ways
in which a mother sperm cell divides and the various kinds
of segmentation in a fertilised ovum.. In most cases the
75
Fic. 28.—Forms of spermatozoa (not drawn to scale).
z1and 2. Immature and mature spermatozoa of snail; 3. of bird;
4. of man (4., head; 7., middle portion ; ¢., tail); 5. of sala-
mander, with vibratile fringe (4); 6. of Ascaris, slightly
ameeboid with cap (c); 7. of crayfish.
spermatogonium divides into spermatocytes, which usually
divide again into spermatids or young spermatozoa.
Maturation of ovum.—When the egg-cell attains its
definite size or limit of growth, it bursts from the ovary or
from its place of formation, and in favourable conditions
meets either within or outside the body with a spermatozoon
from another animal. Before the union between ovum and
spermatozoon is effected, generally indeed before it has
begun, the nucleus or germinal vesicle of the ovum moves
to the periphery and divides twice. This division results in
the formation and extrusion of two minute cells or polar
bodies, which come to nothing, though they may linger for
MATURATION OF OVUM. 61
atime in the precincts of the ovum, and may even divide.
The second division follows the first without the intey-
vention of the “resting stage” which usually succeeds a
nuclear division. In most cases the division which forms
the first polar body is a reducing or meiotic division, the
number of chromosomes being reduced to half the number
characteristic of the cells of the body. The extrusion of
polar globules and the associated reduction is almost
universal in the history of ova, but in most parthenogenetic
ova only one polar body is formed, and there is no reduc-
tion in the number of chromosomes. In some other cases
B
Fic. 29.—Diagram of maturation and fertilisation.
(From Zvolution of Sex.)
A. .Primitive sex cell, supposed to be ameeboid.
B. Ovum; C. formation of first polar body (1. 4.2.); D. formation
of second polar body (2. 4.4.).
B’, Mother sperm cell; C’. the same divided (sperm-morula).
2’. Ball of immature spermatozoa: sA., liberated spermatozoa. __
£. Process of fertilisation; #. approach of male and female nuclei
within the ovum.
the parthenogenetic ovum passes through the meiotic
phase and forms two polar bodies. The second of these,
however, is not liberated, but remains within the ovum and
re-uniting with the reduced nucleus restores the normal
number of chromosomes.
Reducing or Meiotic Division.—In each kind of animal
there is a definite number of chromosomes, say , in each
of the body-cells. In the ripe germ-cells, however, there is
half the normal number, 2, so that when spermatozoon and
ovum unite in fertilisation the normal number is restored.
In the history of the germ-cells, therefore, in one way or
another, at one stage or another, the number of chromo-
62 REPRODUCTION AND LIFE HISTORY.
somes undergoes reduction to half the normal. In eps
cases this reduction comes about through a “ heterotypic ”
meiotic division. We give a condensed account of what
happens in a large number of cases.
The germ-cells grow relatively large ; the nuclear material takes the
form of a definite number of chromatin loops; at a certain stage it is
seen that the number is half what it was in more immature stages of the
germ-cells, and half what it is in the somatic cells of the species under
consideration. If the normal number be z, it is reduced to 4. There
has been 2207 of chromosomes.
The chromatin-loops contract away from the nuclear membrane
(synapsis) ; the chromatin granules divide so that each loop appears
doubly-beaded ; the ends of each loop are separated, and there are now
n bodies with chromatin, each equivalent to a chromosome.
The ends of the loops move apart, and, with or without a second
synapsis, they change in shape, unite end to end, and form 2 twin-
bodies or gemini, sometimes rod-like, sometimes like two brackets,
sometimes like four dots.
The nuclear membrane disappears, the = meiotic gemini are set free,
they become arranged on the division-spindle at right angles to the
equatorial plane (not flat as in ordinary karyokinesis), with their axes
parallel to the axis of the spindle. They halve as if transversely,
separating into two parts which go to the two poles of the spindle.
Thus each daughter-cell has 7 chromosomes.
In the case of the ovum the meiotic division usually occurs in the
formation of the first polar body, so that it and the reduced nucleus of
the ovum have each 7 chromosomes. There is no further reduction in
the formation of the second polar body, which involves an ordinary
equation-division. The first polar body often divides into two. Thus
the result is one viable cell (the mature ovum) and three non-viable
cells (the polar Lodies), each with 2 = chromosomes.
In the spermatogenesis or production of spermatozoa the meiotic
division is usually the second-last. A ‘“‘mother-sperm cell” or
spermatogonium divides into spermatocytes with z chromosomes, each
of these divides into 2 spermatocytes with = chromosomes, and these
again divide into spermatocytes which differentiate into spermatozoa.
The result is that from each of the penultimate generation of spermato-
cytes there arise four spermatozoa, each with = chromosomes. Thus
there is a close parallelism in the maturation process in the two sexes,
That the fertilisation of the ovum restores the number to the normal
is obvious.
Part of the significance of the long circuitous process of meiotic
division is that it affords opportunity for fresh permutations and
combinations of hereditary qualities, for it seems probable that the
chromosomes are the bearers of these..
FERTILISATION. 63
It is important to understand that in ordinary mitosis or cell-division,
each daughter-cell gets an absolutely similar half of each chromosome
of the mother-cell, whereas in meiotic division the daughter-cells get
dissimilar halves.
’ A very important fact, discovered by Farmer, Moore, and Walker, is
that the meiotic phase occurs among the cells of malignant growths
(cancer). ‘*Through the action of one or several different causes at
present unknown, certain cells of the soma, passing out of co-ordination,
go through the meiotic phase and produce a number of generations of
cells that live upon the parent organism in a parasitic manner.”
Fertilisation.—In the seventeenth and eighteenth cen-
turies, some naturalists, nicknamed “ ovists,” believed that
the ovum was all-important, only needing the sperm’s
awakening touch to begin unfolding the miniature model
which it contained. Others, nicknamed “animalculists,”
were equally confident that the sperm was essential, though
it required to be fed by the ovum. Even after it was
recognised that both kinds of reproductive elements were
essential, many thought that their, actual contact was un-
necessary, that fertilisation might be effected by an aura
seminalis. Though spermatozoa were distinctly seen by
Hamm and Leeuwenhoek in 1679, their actual union with
ova was not observed till 1843, when Martin Barry detected
it in the rabbit. i
Of the many facts which we now know about fertilisation,
the following are the most important :—
(1) Apart from the occurrence of parthenogenesis in a
few of the lower animals, an ovum begins to divide only
after a spermatozoon has united with it. After one sper-
matozoon has entered the ovum, the latter ceases to be
receptive, and other spermatozoa are excluded. If, as
rarely happens, several spermatozoa effect an entrance into
the ovum, the result is usually some abnormality. It is
said, however, that the entrance of numerous spermatozoa
(polyspermy) is frequent in insects and Elasmobranch
fishes. fans
(2) The union of spermatozoon and ovum is very
intimate ; the nucleu$ of the spermatozoon and the reduced
nucleus of the ovum approach one another, combining to
form a unified nucleus. ;
(3) The ovum centrosome disappears before fertilisation,
and it is a centrosome introduced by the spermatozoon that
64 REPRODUCTION AND LIFE HISTORY.
divides into the two which play an important réle in the
cleavage or segmentation of the fertilised ovum.
(4) When the combined or segmentation nucleus begins
the process of development by dividing, each of the two
daughter nuclei which result consists partly of material
derived from the sperm nucleus, partly of. material derived
from the ovum nucleus. In other words, the union is
Fic. 30.—Fertilisation in Ascarzs megalocephala,
—After Boveri.
1. Spermatozoon (s.) entering ovum, which contains reduced nucleus
(1), having given off two polar bodies (4.4. 1 and 2).
2. Sperm nucleus (the upper), and ovum nucleus (JV), each with two
chromatin elements or idants, and with centrosomes (c.s.).
3. Centrosomes (c.s.) with ‘‘archoplasmic” threads radiating outwards
in part to the chromosomes of the two approximated nuclei.
4. Segmentation spindle before first cleavage.
orderly as well as intimate, and the subsequent division is
so exact, that the qualities marvellously inherent in the
sperm nucleus (those of the male parent), and in the ovum
nucleus (those of the mother animal), are diffused through-
out the body of the offspring, and persist in its reproductive
cells,
(5) Some eggs, eg. of sea-urchins, can be artificially
induced to develop without fertilisation (by being immersed
for a couple of hours in a mixture of sea water and solution
SEGMENTATION. 65
of Magnesium chloride, and by other means). It seems,
therefore, justifiable and useful to distinguish in ordinary
fertilisation, (a) the mingling of the hereditary qualities of
the two parents, and (4) an exciting or liberating stimulus
which induces the ovum to divide. It should be noted
that the chromosomes of the spermatozoon do not fuse with
the chromosomes of the ovum when fertilisation occurs.
There is some evidence for the view that they remain
distinct from one another until maturation again takes
place, and one theory of the reduction in the number of
chromosomes which takes place at maturation, is that it
involves the fusion in pairs of the paternal chromosomes
with the maternal.
In some insects there is an accessory chromosome
present in one half of the spermatozoa. It has been
interpreted as an element whose presence or absence
determines whether the.offspring is to be male or female.
Segmentation.—The different modes of division exhibited
by fertilised egg-cells depend in great measure on the
quantity and disposition of the passive and nutritive yolk
material, which is often called deutoplasm, in contrast to
the active and formative protoplasm. The pole of the ovum
at which the formative protoplasm lies, and at which the
spermatozoon enters, is often called the animal pole; the
other, towards which the heavier yolk tends to sink, is called
the vegetative pole. In the floating ova of some fish, how-
ever, the yolk is uppermost, and the embryonic area
lowest.
In contrasting the chief modes of segmentation, it
should be recognised that they are all connected by
gradations.
A. COMPLETE Divis1on—Holoblastic Segmentation
(1) Eggs with little and diffuse yolk material divide completely into
approximately equal cells,
[or, Ova which are alecithal (z.e. without yolk) undergo approxi-
mately equal holoblastic segmentation].
This is illustrated in most Sponges, most Ccelentera (Figs.
31 (1) and 32), some ‘‘ Worms,” most Echinoderms, some
Molluscs, all Tunicates, Amphioxus, and most Mammals.
{2) Eggs with considerable yolk material accumulated towards one
pole, divide completely, but into unequal cells,
5
66
REPRODUCTION AND LIFE HISTORY.
Fic. 31.—Modes of Segmentation.
x. Ovum, with little yolk, segments totally and equally into a
blastosphere, ¢.g. Hydra, sponge, sea-urchin.
2. Ovum, with a considerable amount of yolk (y.) at lower pole,
segments totally but unequally, e.g. frog ; (y.s.) larger yolk-
laden cells.
3. Ovum, with much yolk (y.) at lower pole, segments partially and!
discoidally, forming blastoderm (42), e.g. bird, most fishes.
4, Ovum, with central yolk, (y.) segments partially and peripherally,
e.g. most Arthropods
BLASTULA AND GASTRULA. 67
[or, Ova with a considerable amount of deutoplasm lying towards.
one ‘ag (telolecithal), undergo unequal holoblastic segmenta-
tion].
This is illustrated in some Sponges, some Ccelentera (e.g.
Ctenophora), some ‘‘ Worms,” many Molluscs, the lamp-
rey, Ganoid Fishes, Dipnoi, Amphibians (Fig. 31 (2)).
B, ParTiaL DivistoN—Meroblastic Segmentation
(3) Eggs with a large quantity of yolk on which the formative
protoplasm lies as a small disc at one pole, divide partially,
and in discoidal fashion,
[or, Ova which are telolecithal, and have a large quantity of
deutoplasm, undergo merobiastic and discoidal segmentation].
This is illustrated in all Cuttle-fishes, all Elasmobranch and
Teleostean Fishes, all Reptiles and Birds (Fig. 31 (3)),
and also in the Monotremes or lowest Mammals.
(4) Eggs with a considerable quantity of yolk accumulated in a
central core and surrounded by the formative protoplasm,,.
divide partially, and superficially or peripherally,
{or, Ova which are centrolecithal undergo meroblastic and super-
ficial segmentation].
This is illustrated by most Arthropods (Fig. 31 (4)), and’
by them alone.
Blastosphere and morula.—The result of the division is.
usually a ball of cells. But when the yolk is very abundant
(3), a disc of cells—a discoidal blastoderm—is formed at
one pole of the mass of nutritive material, which it gradually
surrounds.
As the cells divide and redivide, they often leave a large
central cavity—the segmentation cavity—and a hollow ball
of cells—a blastosphere or blastula—results.
But if the so-called ‘segmentation cavity” be very small
or absent,.a solid ball of cells or morula, like the fruit of
bramble or mulberry, results.
Gastrula.—The next great step in development is the
establishment of the two primary germinal layers, the outer
ectoderm and the inner endoderm, or the epiblast and the
hypoblast.
One hemisphere of the hollow ball of cells may be appar-
ently dimpled into the other, as we might dimple an india-
rubber ball which had a hole init. Thus out of a hollow
ball of cells, a two-layered sac is formed—a gastrula formed
by invagination or emébolé (Fig. 32). The mouth of the
gastrula is called the blastopore, its cavity the archenteron.
68 REPRODUCTION AND LIFE HISTORY.
But where the ball of cells is practically a solid morula,
the apparent in-dimpling cannot occur in the fashion de-
scribed above. Yet in these cases the two-layered gastrula
Fic. 32.—Life history of a coral, Monoxenia darwiniz,
—From Haeckel.
A, B, Ovum. C, Division into two. D, four-cell stage. E, Blas-
tula. F, Free-swimming blastula with cilia. , Section of
blastula. H, Beginuing of invagination. I, Section of com-
pleted gastrula, showing ectoderm, endoderm, and archenteron
, Free-swimming ciliated gastrula.
ORIGIN OF ORGANS. 69
is still formed. The smaller, less yolk-laden cells, towards
the animal pole, gradually grow round the Jarger yolk-con-
taining cells, and a gastrula is formed by overgrowth or
epibole.
In various ways the ectoderm and the endoderm are
established, either by some form of gastrulation, or by some
other process, such as that called delamination (see p. 163).
Mesoderm.—We are not yet able to make general state-
ments of much value in regard to the origin of the middle
germinal layer—the mesoderm or mesoblast. In Sponges
and Ccelentera it is not a distinct layer except in Cteno-
phora, being usually represented by a gelatinous material
(mesoglea) which appears between ectoderm and endoderm,
and into which cells wander from these two layers. In the
other Metazoa, the middle layer may arise from a few
primary mesoblasts or cells which appear at an early stage
between the ectoderm and endoderm (eg. in the earth-
worm’s development); or from numerous “ mesenchyme”
immigrant cells, which are separated from the walls of the
blastula or gastrula (e.g. in the development of Echino-
derms); or as celom pouches—outgrowths from the en-
dodermic lining of the gastrula cavity (eg. in Sagé?tta,
Balanoglossus, Amphioxus); or by combinations of these
and other modes of origin. The mesoderm lies or comes
to lie between ectoderm and endoderm, and it lines the
body cavity, one layer of mesoderm (parietal or somatic)
clinging to the ectodermic external wall, the other (visceral
or splanchnic) cleaving to the endodermic gut and its
outgrowths.
Origin of organs.—From the outer ectoderm and inner
endoderm, those organs arise which are consonant with the
position of these two layers, thus nervous system from the
ectoderm, digestive gut from the endoderm. The middle
layer, which begins to be developed in ‘“ Worms,” assumes
some of the functions, e.g. contractility, which in Sponges
and Ccelentera are possessed by ectoderm and endoderm,
the only two layers distinctly represented in these classes.
In a backboned animal the embryological origin of the
organs is as follows :—
(a) From the ectoderm or epiblast arise the epidermis
and epidermic outgrowths, the nervous system, the
“7O REPRODUCTION AND LIFE HISTORY.
most essential parts of the sense organs, infoldings
at either end of the gut (fore-gut or stomodeum
and hind-gut or proctodzeum).
(b) From the endoderm or hypoblast arise the mid-gut
(mesenteron) and the foundations of its out-
growths (e.g. the lungs, liver, allantois, etc., of
higher Vertebrates), also the axial rod or noto-
chord.
(c) From the mesoderm or mesoblast arise all other struc-
tures, ¢.g. dermis, muscles, connective tissue, bony
skeleton, the lining of the body cavity, and the
vascular system. ‘This layer aids in the formation
of organs originated by the other two. With it
the reproductive organs are associated. Con-
nective tissues, vascular system, and unstriped
muscles are formed by mesenchyme cells which
are budded off from the true mesoderm.
Physiological embryology.—Of the physiological conditions of develop-
ment we know relatively little. To investigate them is one of the
tasks of the future. Why does the fertilised egg-cell divide, how does
the yolk affect segmentation, what are the conditions of the infolding
Fic. 33.-—Embryos—(1) of bird ; (2) of man.—After His.
The latter about twenty-seven days old.
y.s., Yolk-sac ; A2., placenta.
which forms the endoderm, and of the outfolding which makes the
coelom pouches ; in short, what are the immediate conditions of each
step in the familiar process by which, out of apparent simplicity, cbvious
complexity arises?
Generalisations.—(1) Zhe ovum theory or cell theory.—
All many-celled animals, produced by sexual reproduction,
GENERALISATIONS. zr
begin at the beginning again. “The Metazoa begin where
the Protozoa leave off”—as single cells. Fertilisation does
not make the egg cell double; there is only a more com-
plex and more vital nucleus than before. All development
takes place by the division of this fertilised egg-cell and its
descendant cells.
(2) Zhe gastrea theory.—As a two-layered gastrula stage
occurs, though sometimes disguised by the presence of much
yolk, in the development of the majority of animals, Haeckel
concluded that it represents the individual’s recapitulation
of an ancestral stage. He suggested that the simplest stable,
many-celled animal was like a gastrula, and this hypo-
thetical ancestor of all Metazoa he called a gastrea. The
gastrula is, on this view, the individual animal’s recapitula-
tion of the ancestral gastreea. Rival suggestions have been
made: perhaps the original Metazoa were balls of cells like
Volvox (Fig. 43), with a central cavity in which repro-
ductive cells lay; perhaps they were like the planula larvie
of some Ccelentera—two-layered, externally ciliated, oval
forms without a mouth.
(3) The idea of recapitulation.—It is a matter of experi-
ence that we recapitulate in some measure the history of
our ancestors. Embryologists have made this fact most
vivid, by showing that the individual animal develops along
a path the stations of which correspond to some extent
with the steps of ancestral history.
(1) The simplest animals are single | (1) The first stage of development
cells (Protozoa). is a single cell (fertilised
(2) The next simplest are balls of ovum).
cells (¢.g. Volvox). (2) The next is a ball of cells
(3) The next simplest are two- (blastula or morula).
layered sacs of cells (e.g. | (3) The next is a two-layered sac
Hydra). of cells (gastrula).
Von Baer, one of the pioneer embryologists, acknow-
ledged that, with several very young embryos of higher
Vertebrates before him, he could not tell one from the
other. Progress in development, he said, was from a
general to a special type. In its earliest stage every
organism has a great number of characters in common
with other organisms in their earliest stages; at each
successive stage the series of embryos which it resembles
72 REPRODUCTION AND LIFE HISTORY.
is narrowed. The rabbit begins like a Protozoon as a
single cell; after a while it may be compared to the
young stage of a very simple vertebrate; afterwards, to
the young stage of a reptile; afterwards, to the young
stage of almost any mammal; afterwards, to the young
stage of almost any rodent; eventually it becomes un-
mistakably a young rabbit.
Herbert Spencer expressed the same idea, by saying that
the progress of development is from homogeneous to
heterogeneous, through steps in which the individual
history is parallel to that of the race. But Haeckel has
illustrated the idea more vividly, and summed it up more
tersely, than any other naturalist. His ‘fundamental
biogenetic law” reads: “Ontogeny, or the development
of the individual, is a shortened recapitulation of phylogeny,
or the evolution of the race.”
It is hardly necessary to say that the young mammal is
never like a worm, or a fish, or a reptile. It is at most
like the embryonic stages of these, and it may also be
noticed that, as our knowledge is becoming more intimate,
the individual peculiarities of different embryos are be-
coming more evident. But this need not lead us to deny
the general resemblance.
Moreover, the individual life history is much shortened
compared with that of the race. Not merely does the one
take place in days, while the other has progressed through
ages, but stages are often skipped, and short cuts are dis-
covered. And again, many young animals, especially those
“larvee” which are very unlike their parents, often exhibit
characters which are secondary adaptations to modes of
life of which their ancestors had probably no experience.
In short, the individual’s recapitulation of racial history is
general, but not precise. It is seen rather in the stages
in the development of organs (organogenesis) than in the
development of the organism as a whole.
(4) Organic continuity between generations.—Heredity.—
Everyone knows that like tends to beget like, that offspring
resemble their parents and their ancestors. Not only
are the general characteristics reproduced, but minute
features, idiosyncrasies, and pathological conditions, inborn
in the parents, may recur in the offspring.
HEREDITY 73
At an early stage in the development of the embryo the
future reproductive cells of the organism are often dis-
tinguishable from those which are forming the body.
These, the somatic cells, develop in manifold variety, and,
as division of labour is established, they lose their likeness
to the fertilised ovum of which they are the descendants.
The future reproductive cells, on the other hand, are not
implicated in the formation of the “body,” but, remaining
virtually unchanged, continue the protoplasmic tradition.
unaltered, and are thus able to start an offspring which
will resemble the parent, because it is made of the same
protoplasmic material, and develops under similar con-
ditions.
An early isolation of reproductive cells, directly con-
tinuous and therefore presumably identical with the original
ovum, has been observed in the development of some
“worm types” —(Sagitéa, Thread-worms, Leeches, Polyzoa),
and of some Arthropods (e.g. Aoima among Crustaceans,
Chironomus among Insects, Phalangidee among Spiders),
in Micrometrus aggregatus among Teleostean fishes, and
with less distinctness in some other animals. A cell which
will give rise to the germ-cells can be recognised in the
gastrula stage of Cyclops, and in the very first segmentation
stages of the thread-worm Ascaris.
In many cases, however, the reproductive cells are not
recognisable until a relatively late stage in development,
after differentiation has made considerable progress.
Weismann gets over this difficulty by supposing that the
continuity is sustained by a specific nuclear substance—
the germ-plasm—which remains unaltered in spite of the
differentiation in the body. It is perhaps enough to say
that, as all the cells are descendants of the fertilised ovum,
the reproductive cells are those which retain intact the
qualities of that fertilised ovum, and that this is the reason
why they are able to develop into offspring like the
parent.
Finally, it may be noticed in connection with heredity,
that there is great doubt to what extent the “body” can
definitely influence its own reproductive cells. Animals
acquire individual bodily peculiarities in the course of
their life, as the result of what they do or refrain from
74 REPRODUCTION AND LIFE HISTORY’.
doing, or as dints from external forces. The “body” is
thus changed, but there is much doubt whether the repro-
ductive cells within the “body” are affected specifically by
such changes. Weismann denies the transmissibility of
any characters except those inherent in the fertilised egg-
cell, and therefore denies that the influences of function
and environment are, or have been, of direct importance
in the evolution of many-celled animals. Such influences
affect the dody, and produce what are technically called
““ modifications,” but these modifications do not affect the
reproductive cells—at least not in a specific representative
way. Therefore modifications are not likely to be trans-
mitted, and there seems no good evidence to show that
they are. Many of the most authoritative biologists are at
present of this opinion. On the other hand, many still
maintain that profound changes due to function or environ-
ment may saturate through the organism, and affect the
seproductive cells in such a way that the changes or
modifications in question are in some measure transmitted
to the next generation. The question remains under dis-
‘cussion, but the probabilities are strongly against the
transmissibility of acquired characters.
It is important to try to distinguish different modes of
hereditary resemblance. The characters of the two parents
may be d/ended in the offspring, or those of one parent
may find predominant expression (exclusive inheritance), or
the characters of one parent may be expressed in one part
of the offspring and those of the other parent in another
(particulate inheritance).
Another important inquiry is into the share that the
various ancestors have ox an average in forming any indi-
vidual inheritance. The inheritance of an animal repro-
duced in the ordinary way is always dual, partly maternal
and partly paternal, but ¢zrvovgh the parents there come
contributions from grandparents, etc. Galton’s Law of
Ancestral Inheritance states that ‘‘The two parents con-
tribute between them, on the average, one half of the total
heritage; the four grandparents, one quarter; the eight
‘great-grandparents, one eighth, and so on.”
Another generalisation of great interest is Mendel’s Law,
which seems to apply to certain cases, ¢.g. peas, stocks,
HEREDITY 75
mice, and rabbits. In its simplest expression the law may
be stated as follows :—If A be a well-established, pure-bred
variety with a certain character, e.g. of stature or colour,
and # be another well-established variety in which the
corresponding character is different, and if A and & are
crossed, the hybrid offspring (4) will usually resemble one
of the parents in the particular distinguishing character.
The character which finds expression is called the dominant ;
the character which remains latent in the hybrids is called
the recessive. Now, if the hybrids are bred together, their
descendants will be of two kinds, some like the dominant
grandparent, some like the recessive grandparent. When
those like the recessive grandparent are in-bred, they yield
only recessives. When those like the dominant grand-
parent are in-bred, some yield pure dominants only—that
is, forms which if in-bred yield only dominants ; but others
yield apparent dominants like the original hybrid—that is,
with the power of throwing off when in-bred more pure
dominants, more pure recessives, and more apparent
dominants like the original hybrid. The results tend to
be always in the proportion 14+2A (B)+1B, as regards
the two contrasted characters of A and B.
Two diagrams (after T. H. Morgan and R. C. Punnett)
may make the matter clearer.
A B
A (B)
io" s Re
1A 2A (B) 1B
\
76 REPRODUCTION AND LIFE HISTORY.
: D x R
D I
vi
a %
oe “te
6 D(R) R
= # a Sy
y,
e £ i & *
vil D D(R) R R
Heredity may be defined as the relation of genetic
continuity between successive generations, and inheritance
as all that the organism is or has to start with in virtue
of this hereditary relation. Development is the expression
or realisation of the heritable qualities. which have their
physical basis in the germ cells, and it presupposes an ap-
propriate environment of nutrition and “liberating stimuli,”
—‘ nurture” in the widest sense. What the organism
becomes is the resultant of two components, inherited
“nature” and external “nurture.”
CHAPTER V
PAST HISTORY OF ANIMALS
(PALZONTOLOGY)
In the two preceding chapters we have noticed two of the
great records of the history of animal life,—that preserved
in observable structures, and the modified recapitulation
discernible in individual development ; in this we turn to
the third—the geological record. In the early days of the
Evolution theory the modern science of Embryology was
still in its infancy, and could furnish few arguments, and it
was the opponents of the new theory rather than its sup-
porters who appealed to Paleontology. They asserted that
the palzontological facts refused to lend the support which
the theory demanded. To their attacks the evolutionists
usually replied by pointing out that the geological record
was very incomplete. The numerous investigations which
have since been carried on on all sides now show con-
clusively that it was imperfection rather of knowledge than
of the record which produced the negative results. We
must, however, still acknowledge that, except in a few
cases, there is but little certainty as to the precise pedi-
gree of living animals, and seek for reasons to explain
this.
“Imperfection of the geological record.”—If we re-
member the rule of modern Geology, that the past is to
be interpreted by the aid of the present, there can be no
difficulty in realising that the chances against the preserva-
tion of any given animal are very great. Many are destroyed
by other living creatures, or obliterated by chemical agencies.
Except in rare instances, only hard parts, such as bones,
78 PAST HISTORY OF ANIMALS.
teeth, and shells, are likely to be preserved, and this at once
greatly limits the evidential value of fossils. The primitive
forms of life would almost certainly be without hard parts,
and have left no trace behind them. A number of ex-
tremely interesting forms, such as many worms and the
Ascidians, are, for the same reason, almost unrepresented
in the rocks. Finally, we cannot suppose that such an
external structure as a shell can always be an exact index of
the animal within.
After fossilisation has taken place, the rock with its con-
tents may be entirely destroyed by subsequent denudation,
or so altered by metamorphic changes that all trace of
organic life disappears. Of those fossils which have been
preserved only a small percentage are available, for vast
areas of fossiliferous rocks are covered over by later deposits,
or now lie below the sea or in areas which have not yet
been explored.
With all these causes operating against the likelihood of
preservation, and of finding those forms that may have been
preserved, it is little wonder if the geological record is
incomplete; but such as it is, it is in general agreement
with what the other evidence, theoretical and actual, leads
us to expect as to the relative age of the great types of
animal life. Further, those specially favourable cases which
have been completely worked out have yielded results which
strongly support the general theory.
Probabilities of ‘‘ fossils.’?—But it will be useful to note the
probabilities of a good representation of extinct forms in the various
classes of animals. Thus among the Protozoa the Infusoria have no
very hard parts, and have therefore almost no chance of preservation,
and the same may be said of forms like Amcebee ; while the Foramin-
ifera and the Radiolaria, having hard structures of lime or silica, have
been well preserved. The flinty Sponges are well represented by their
spicules and skeletons, Of the Coelentera, except an extinct order
known as Graptolites, only the various forms of coral had any parts
readily capable of preservation, and remains of these are very abundant
in the rocks of many ancient seas. But, strange as it may seem, some
beautiful vestiges of jelly-fish have been discovered.
Of the great series of ‘‘ worms,” only the tube-makers have left
actual remains; the others are known only by their tracks, while of
any that may have lived on the land there is no evidence.
The Echinoderms, because of their hard parts, are well represented
in all their orders, except the Holothurians, where the calcareous
structures characteristic of the class are at a minimum.
“ PALAZEONTOLOGICAL SERIES.” 79
The Crustacea, being mostly aquatic, and in virtue of their hard.
shells, are fossilised in great numbers.
The Arachnida and the Insects, owing to their air-breathing habit,.
are chiefly represented by chance individuals that have been drowned,
or enclosed within tree-stumps and amber.
The Molluscs and Brachiopods are perhaps better preserved tham
any other animals, since nearly all of them are possessed of a shell
specially suitable for preservation.
Among the Vertebrates some of the lowest are without scales, teeth,
or bony skeleton ; such forms have therefore left almost no traces.
Fishes, which are usually furnished with a firm outer covering, or
with a bony internal skeleton, or with both, are well represented.
The primitive Amphibians were furnished with an exoskeleton of
bony plates, and are fairly numerous as fossils. ‘The bones and teeth
of the others have been fossilised, though more rarely. Of some the
only record is their footprints.
The traces of Reptilia depend upon the habits of the various orders,.
those living in water being oftenest preserved, but the strange flying
Reptiles have also left many skeletons behind them.
Of the Birds, the wingless ones are best represented, and then those-
that lived near seas, estuaries, or lakes.
The history of Mammals is very imperfect, for most of them were
terrestrial. But the discoveries of Marsh, Cope, and others show how
much may be found by careful search. The aquatic Mammals are-
fairly well preserved.
“Paleontological series.”—In spite of the imperfection
of the “geological record,” in spite of the conditions un-
favourable to the preservation of many kinds of animals, it
is sometimes possible to trace a whole series of extinct forms.
through progressive changes. Thus a series of fossilised
fresh-water snails (Planorbis) has been worked out; the
extremes are very different, but the intermediate forms link
them indissolubly by a marvellously gradual series of transi-
_tions. The same fact is well illustrated by another series of
fresh-water snails (Paludina, Fig. 34), and not less strikingly
among those extinct Cuttle-fishes which are known as.
Ammonites, and have perfectly preserved shells. Similarly,
though less perfectly, the modern crocodiles are linked by
many intermediate forms to their extinct ancestors, for it is-
impossible not to call them by that name. In short, as
knowledge increases, the evidence from Paleontology
becomes more and more complete.
In a general way it is true that the simpler animals pre-
cede the more complex in history as they do in structural
rank, but the fact that all the great Invertebrate groups are
80 PAST HISTORY OF ANIMALS
represented in the oldest distinctly stratified and fossiliferous
rocks—the Cambrian system—shows that this correspond-
ence is only roughly true. To account for this, we must
remember that almost the whole mass of the oldest rocks,
known as Archean or Pre-Cambrian, has been so pro-
foundly altered, that, as a rule, only masses of marble and
‘carbonaceous material are left to indicate that forms of life
-existed when these rocks were laid down. Careful searching
in Pre-Cambrian beds has revealed the presence of several
Molluscs, a Eurypterid, and a fragment of Trilobite. There
are also “annelid tracks” indicative of life.
Fic. 34.—Gradual transitions between Paludina
neumayrzt (a), the oldest form, and Paludina
hernest (j).—From Neumayr.
Extinction of types.—Some animals, such as some of
the lamp-shells or Brachiopods, have persisted from almost
the oldest ages till now, and most fossilised animals have
modern representatives which we believe to be their actual
‘descendants. That a species should disappear need not
surprise us, if we believe in the “transformation” of one
species into another. The disappearance is more apparent
than real: the species lives on in its modified descendants,
“different species” though they be.
But, on the other hand, there are not a few fossil animals
which have become wholly extinct, having apparently left
no direct descendants. Such are the Graptolites, the
ancient Trilobites, their allies the Eurypterids, two classes
EXTINCTION OF TYPES. 81
of Echinoderms (Cystoids and Blastoids), many giant
Reptiles, and some Mammals.
It is almost certain that there has been no sudden
extinction of any animal type. There is no evidence of
universal cataclysm, though local floods, earthquakes, and
volcanic eruptions occurred in the past, as they do still,
with disastrous results to fauna and flora. In many cases
the waning away of an order, or even of a class of animals,
may be associated with the appearance of some formidable
new competitors ; thus cuttle-fish would tend to exterminate
Trilobites, just as man is rapidly and often inexcusably
annihilating many kinds of beasts and birds. Apart from
the struggle with competitors, it is conceivable that some
stereotyped animals were unable to accommodate them-
selves to changes in their surroundings, and also that some
fell victims to their own constitutions, becoming too large,
too sluggish, too calcareous,—in short, too extreme.
Appearance of animats tn time.—Such tables as those given here are
apt to be misleading, in that they convey the impression that the great
types of structure have appeared suddenly. It must be noted that any
apparent abruptness is merely due to incompleteness of knowledge or
inaccuracy of expression. The table is a mere list of a few important
historical events, but one miust fully realise that they are not isolated
facts, that the present lay hidden in the past and has gradually grown
out of it. Of the relative length of the periods represented here we
know almost nothing, and we are also ignorant of the earliest ages in
which life began. But the general result is clear. We find that in the
Cambrian rocks, before Fishes appeared, the great Invertebrate classes
were represented, though as yet but feebly. As we pass upwards they
increase in number and in differentiation, Again, Fishes precede
Amphibians, Amphibians are historically older than: Reptiles, and many
types of Reptiles are much older than Birds. In short, in the course
of the ages life has been slowly creeping upwards.
[TaBLes.
82
PAST HISTORY OF ANIMALS.
Quaternary or
Post-Tertiary.
Pliocene. ¢
coe santecee aces Bases ans sees Beale oesienteess I >| eee] Cree
RS : a 3 3 g Ea g ()
8 Miocene. 4 a 2 = z &
<3 2 ek i :
raf
< G ee! fey wee 9 cece ee . oe € ween % ed . = er es
5 < Modern
Eocene. Types Placentals.
Cretaceous, cious rien Modern rogened and
steans. | Types. orms:
A
RS gala armenia es eieanens ee Corie) Career (ee ee eee Ger civtd piseinjwieies oa o%
as ;
8 : J) Marsupials
« 8 Jurassic. ee and Mono-
§ g pleryx." tremes (?)
GY wscseuising Gace pamanead es Paces: Pees eeerer i onbatradanad oostenonennen Laie mahae neon
Triassic. Few primi-
tive types.
Permian.
“i Laby-
Carboniferous. tint
donts,
Devonian or Old ae ;
Red Sandstone. Dipnoi.
MS viecuors i pcennedle uxpaauaens Paseeneh ntenee yeaa o18s~enpa ies Redeeen ee a Beat g Ahoy Eye :
8
8 Ganoids
8 ‘Naxians and
s Silurian. Elasino-
$ branch
Q i) Seren Serine i ier rr i a ei cr eae
8 .
& Ordovician.
x es Pee ee) e! Ce es oe naee
Representa-
tives of all
. the chief
Cambrian. lasses. GE
Inverte-
brates.
Pre-Cambrian
or Archzan.
APPEARANCE IN TIME. 83
Coelentera. Echinoderma. Arthropoda. Cephalopoda.
—~ ———, os
EET T
Quaternary or Post-
Tertiary.
Pliocene.
Sepia and recent forms.
Miocene.
Tertiary or
Cainozotc.
Eocene. ,
Cretaceous.
&
sy : a A .
ere eee eee eee PSP E Eee (ree pee eee 2 A fs vee ac oe le
AX : al 41-30-37 oF" os Sg] 8
SHS Sere] SiS] SVS) | TELE
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sy gee a
Triassic. 2
3
ee
ra
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Permian. 2
Carboniferous.
a
; 7
aS a|..2 &
fcr VO | OCC) Co Ge en ee Oe
Sy a ioe] mt
iS fi i} a =
& Devonian or Ol -z ; g
< Old Red Sandstone. ec c|
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8 Silurian, : .|A
S LG 8
& 5 =
g . 3 nee ay . 3 . . saenee
7 fr g
Ordovician. so} f° a
5
Cambrian.
Pre-Cambrian or
Archean.
CHAPTER VI
THE DOCTRINE OF DESCENT
Wuen we ask, as we are bound to ask, how the living plants
and animals that we know have come to be what they are—
very numerous, very diverse, very beautiful, marvellous in
their adaptations, harmonious in their parts and qualities,
and approximately stable from generation to generation—
we may possibly receive three answers. According to one,
the plants and animals that we know have always been as
they are; but this is at once contradicted by the record in
the rocks, which contain the remains of successive sets of
plants and animials very different from those which now live
upon the earth. According to another, each successive
fauna and flora was destroyed by mundane cataclysms, to
be replaced in due season by new creations, by new forms
of life which arose after a fashion of which the human mind
can form no conception. Of such cataclysms there is no
evidence, and if it be enough to postulate one creation, we
need not assume a dozen. ‘The third answer is, that the
present is the child of the past in all things: that the plants
and animals now existing arose by a natural evolution from
simpler pre-existing forms of life, these from still simpler,
and so on back to a simplicity of life such as that now
represented by the very lowest organisms.
This third theory is really an old one; it is merely man’s
application of his idea of human history to the world around
him. It was maintained with much concreteness and
power by Buffon (1749), by Erasmus Darwin (1794), and
by Lamarck (1801). Yet in spite of the labours of these
thoughtful naturalists and of many others, the general idea
of the natural descent of organisms from simpler ancestors
was not received with favour until Darwin, in his Origin
EVIDENCES OF EVOLUTION. -85
of Species (1859), made it current intellectual coin. By’
his work, and by that of Spencer, Wallace, Haeckel, Huxley,
and many others, the doctrine of descent, the general fact
of evolution, has been established, and is now all but
universally recognised. ;
The chief arguments which. Darwin and others have
elaborated in support of the doctrine of descent, according
to which organisms have been naturally evolved from simpler
forms of life, may be ranked under three heads—(a) struc-
tural, (4) physiological, (¢) historical.
Evidences of evolution.—(a) Structural.—Some say that
there are over a million living animals of different species.
In any case, there are many myriads. These species are
linked together by varieties which make strict severance
often impossible ; they can be rationally arranged in general
orders, families, and classes, between which there are not a
few remarkable connecting links ; there is a gradual increase
of complexity from the Protozoa upwards along various
lines of organisation; it is possible to rank them all ona
hypothetical genealogical tree (Fig. 18). A little practica,
experience makes one feel that the facts of classification
favour the idea of common descent.
Throughout vast series of animals we find in different
guise essentially the same parts twisted into most diverse
forms for different uses, but yet referable to the same funda-
mental type. It is difficult to understand this “adherence
to type,” this “homology” of organs, except on the theory
of natural relationship.
There are many rudiméntary organs in animals, especially
in the higher animals, which remain very slightly developed,
and which often disappear without having served any
apparent purpose. Such are the “gill-slits” or “ visceral-
clefts” in Reptiles, Birds, and Mammals, the teeth of young
whalebone whales, the pineal body (a rudimentary eye) in
Vertebrates. Only on the theory that they are vestiges of
structures which were of use in ancestors are these rudiments
intelligible. They are relics of past history, comparable, as
Darwin said, to the unpronounced letters in many words.
(2) Physiological —Observation shows that animals are to
some extent plastic. In natural conditions they usually
exhibit some measure of changefulness from generation to
8G THE DOCTRINE OF DESCENT.
generation. This is especially the case if one section of a
species be in any way isolated from the rest, or if the animals
be subjected in the course of their wanderings to novel
conditions of life.
The evidence from domesticated animals is very convinc-
ing. By careful interbreeding of varieties which pleased his
fancy or suited his purpose, man has produced numerous
breeds of horses, cattle, sheep, and dogs, which are often
distinguished from one another by structural differences
more profound than those which separate two natural
species. In great measure, however, domestic breeds are
fertile with one another, while different species rarely are.
The numerous and very diverse breeds of domestic pigeons,
which are all derived from the rock-dove (Columba Jivia),
vividly illustrate the plasticity or variability of organisms.
It sometimes happens that the offspring of an animal
resemble not so much the parent as some other form be-
lieved or known to be ancestral. Thus a blue pigeon like
the ancestral Co/umba livia may be hatched in the dovecot.
Such reversions are not readily intelligible except on the
theory of descent.
(c) Historical—Among the extinct animals disentombed
from the rocks, many form series by which those now
existing can be linked back to simpler ancestors. Thus
the ancient history of horses, crocodiles, and cuttle-fish is
known with a degree of completeness which makes it almost
certain that the simpler extinct forms were in reality the
ancestors of those which now live. Moreover, that many
connecting links have been discovered in the rocks, and
that the higher animals appear gradually in successive
periods of the earth’s history, are strong corroborations of
the theory.
It is less easy to state in a few words how the facts of
geographical distribution, or the history of the diffusion of
animals from centres where the presumed ancestral forms
are or were most at home, favour the doctrine of descent.
The individual life history of an animal—often strangely
circuitous or indirect—is interpretable as a modified re-
capitulation of the probable history of the race.
Such, in merest outline, is the nature of the evidence
which leads us to conclude that the various forms of life
EVIDENCES OF EVOLUTION. 87
have descended or have been evolved from simpler ancestors,
and these from still simpler, and so on, back to the mist of
life’s beginnings. None of the evidence is logically demon-
strative; we accept the evolution idea because it is a
plausible interpretation which is applicable to many orders
of facts, and is contradicted by none.
In accepting the evolutionist interpretation naturalists are
unanimous; but in regard to the manner in which the
modification of species or the general ascent of life has
been brought about, there is much difference of opinion.
The fact of evolution is admitted; debate goes on with
regard to the factors (see Chapter XXVIII).
CHAPTER VII
PHYLUM PROTOZOA—THE SIMPLEST
ANIMALS
CuieFr DIvIsions
Ruizopops : Classes—Loposa, HELIOZOA, FORAMINIFERA, RaDIO-
LARIA, etc.
INFUSORIANS: Classes —FLAGELLATA, CILIATA, ACINETARIA,
etc.
SpoRroz0a: SEVERAL CLASSES.
THE Protozoa are the simplest animals, and they are of
peculiar interest on this account. They throw light upon
the beginnings of organic structure and vital activity, and
they give us hints as to the nature of the first forms of life,
of which we can know nothing directly. Almost all the
Protozoa are single cells, unit masses of living matter; and
in virtue of their simplicity, they are in some measure
exempt from natural death, which is “the price paid fora
body.” In their variety they exhibit, as it were, a natural
analysis of the higher animals, which are built up of many
diverse cells,
GENERAL CHARACTERS
The Protozoa, the simplest and most primitive animals,
are usually very small single cells. Most of them feed on
small plants or on other Protozoa, or on dtbris, and not a
Jew are parasitic. Most of them live in water, but many can
endure dryness for some time. In one series (Rhizopods)
the living matter is without any rind, and flows out in more
or less changeful threads and lobes, by the movements of which
the animals engulf their food and glide along. The others
AMGBA. 89
have a definite rind, which in a large number (Infusorians)
bears motile cilia or flagella, but in the others (Sporozoa)
is usually without locomotor structures. But these three
phases—ameboid, ciliate or flagellate, and encysted—may
occur in the life history of one form; and the three main
lines of evolution—Rhizopods, Infusorians, and Sporozoa—are
marked by the predominant occurrence of the ameboid, ciliate
or flagellate, and encysted phase of cell life. Many have a
skeletal framework,—of lime, flint, or other material,—while
within the cell there ts a special kernel or nucleus, or there
may be several. There are also other less constant structures.
A Protozoon multiplies by dividing into two daughter units,
or into a large number; and two individuals often unite,
temporarily or permanently, in conjugation, which is analogous
to the union of ovum and spermatozoon in higher animals.
A few types, instead of remaining single cells, form by
dtuiston or budding loose colontes, taking a step, as it were,
towards the Metazoa, but never forming differentiated tissues.
first Type of Protozoa—AM@BA
Ameba, a type of Rhizopods, especially of those in which
the outflowing processes of living matter (pseudopodia) are
blunt and finger-like (Lobosa).
Description.—Ameba proteus and some other species are
found in the mud of ponds; 4. ¢erricola occurs in damp
earth. Some are just large enough to be seen with the
unaided eye. The diameter is often about one-hundredth
of aninch. Each is a unified corpuscle of living matter,
and glides over the surface of stone and plant by protruding
and retracting the pseudopodia. As they move the shape
constantly changes, whence the old (1755) name of “ Proteus
animalcule.” Round the margin, which may show an
apparent radial striation, the cell substance is firmer and
clearer than it is in the interior, where it is more fluid, but
contains very abundant granules, some of which are of
a proteid, and others of a fatty nature. In the centre of
the cell lies the usually single nucleus. The food consists
of minute Alga, such as diatcms, or of vegetable débris.
There is reason also to suspect cannibalism. The food
is surrounded by the finger-like processes, and engulfed
90 PHYLUM PROTOZOA——-THE SIMPLEST ANIMALS,
along with drops of water, which form food vacuoles in the
cell substance, Into these vacuoles digestive ferments flow.
After the digestible parts of the food have been absorbed,
the undigested residue is got rid of at any point of the
protoplasm. One or more contractile vacuoles are visible
in the cell substance. They have an excretory function,
and serve to get rid of the finer waste products.
Life history.k—In favourable nutritive conditions the
Ameeba grows. At the limit of growth it reproduces by
dividing into two. In disadvantageous conditions, such as
drought, it may become globular, and, secreting a cell wall
or cyst, lie dormant for a time. The cyst wall is said to be
chitinoid. With the return of favourable conditions the
Fic. 35.—Life history of Ame@éa.
«. Ameeba with pseudopodia ; ., nucleus; c.v., contractile vacuole. ~. Division
intwo. 3. Encystation. 4. Escape of Ameeba from its cyst.
Ameeba revives, and, bursting from the cyst with renewed
energy, recommences the cell-cycle. The conjugation of
two Amoebe has been observed, and spore - formation
occasionally occurs.
Second Type of Protozoa—ACTINOPHRYS
The Sun-animalcule, Actinophrys sol, is a type of: the
Heliozoa.
Description. — Like most other Heliozoa, Actinophrys
lives in fresh water, floating about or rolling over the
bottom. It is spherical and minute, measuring at most
o’o5 mm. in diameter. Long stiff pseudopodia radiate
out from the body. A clear axial filament runs up each
pseudopodium, and the small organisms on which Actino-
ACTINOPHRYS. gl
phrys feeds are paralysed when they come in contact with
the pseudopodia.
The body consists of ectoplasm and endoplasm. The
ectoplasm is a thick external layer closely packed with
large vacuoles, which are non-contractile and contain a
clear fluid. But food yacuoles are formed as in other
Protozoa, and there is also: a single contractile vacuole.
The endoplasm forms the central mass. It is not
vacuolated, and contains the large, centrally placed nucleus.
Life history. —
An Actinophrys may
withdraw its pseudo-
podia and divide
into two, with or
without the forma-
tion of a cyst. A
number of — indi-
viduals may unite
for a time by the
ectoplasm _ alone,
and separate with-
out any nuclear
fusion having taken
place (plastogamy).
But Schaudinn has
described a ttue Fic. 36.—Acténophrys sol (Sun-animalcule).
Sexual process - After Grenacher.
which ‘olfers, an > elbagig feed camila; g, coment
interesting analogy Poni
to the processes of maturation and fertilisation in the
higher animals. © : ;
A number of individuals become joined up in a common
gelatinous cyst. Each loses its pseudopodia and forms a
membranous cyst. These cysts become associated in pairs.
The nucleus of each ‘cyst divides mitotically.and a polar
body is extruded from each, after which the nucleus
returns to the resting condition. The cysts now fuse in
pairs, with complete arid intimate union of their nuclei
and cell-bodies. The zygote so formed rests for a short
period, then divides up into two daughter cysts from which
emerge two new individuals of Actinophrys,
92 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
In the allied genus Actinospherium, with very numerous
nuclei, there is a strange and complicated formation and
fusion of cysts within a single individual.
Third Type of Protozoa—POLysTOMELLA
Polystomella (see Fig. 50) is a type of Foraminifera with
a calcareous perforate shell or test.
Description.— Polystomella crispa is common on the
shore, especially among Zostera. It looks like a miniature
of an Ammonite shell, and Foraminifera were indeed
classified by the older naturalists with the Ammonites.
The test forms a close spiral with beautifully chiselled
surface; only the last whorl is visible from the outside.
The test is made up of a series of chambers which com-
municate with one another and with the exterior by fine
pores. Granular protoplasm fills up the chambers and
forms also a thin layer on the outside. Long slender
pseudopodia issue from the openings in the test and are
given off also by the external protoplasmic layer. They
frequently branch and anastomose with one another, and
their granular protoplasm exhibits marked streaming
movements. The pseudopodia serve to catch and en-
tangle the diatoms and Infusoria on which the Foraminifer
feeds.
Like many other Foraminifera, Polystomella shows
a remarkable dimorphism. It occurs in two forms,
outwardly indistinguishable, but differing in internal struc-
ture. In the megalospheric form the central chamber is
large (a megalosphere), and there is a single large
nucleus, placed about the middle of the series of
chambers; in the microspheric form the central chamber
is small (a microsphere), being about one-tenth of the
diameter of the megalosphere, and there are numerous
small nuclei. The megalospheric individuals are about
thirty times as numerous as the microspheric indi-
viduals,
Life history.—The microspheric form has its nuclei
replaced by chromidia (chromatin bodies detached from
the nuclei into the protoplasm). These chromidia form
the centres of amceboid nucleated spores which leave the
POLYSTOMELLA. 93
shell or are liberated by the protoplasm creeping out and
forming a halo of anastomosing threads round the deserted
test. The spores secrete
a shell and grow into the
typical megalospheric forms.
When the megalospheric
form is about to reproduce,
its nucleus disintegrates and
is replaced by numerous
scattered nuclei formed
around chromidia.
protoplasm segregates into
little masses, each centred
in a nucleus. Each of
these nuclei divides by
mitosis into two, then into
four, and the division of
the nucleus is followed
by the division of the
protoplasmic mass,
The-
so that hosts of
e provided with flagella, swim out
Fic. 37.—Polystomella, megalo-
spheric form, with large central
chamber (JZ) and one nucleus
(W).—After Lister.
tiny cells are
Fic. 38.—Polystomella,
central chamber (c.c.),
microspheric form, with small
umerous nuclei (V7), bridges of
protoplasm between chambers (B).—After Lister.
94 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
into the water, leaving behind them the empty test, and
there conjugate in pairs, not with one another but with
similar “gametes” from another megalospheric individual.
The “zygote” so formed becomes the initial chamber of
a microspheric individual. In a more direct way—by
fission — the megalospheric individual may give rise to
another like itself. There is therefore in this complex life
history of Polystomella an alternation between a sexual and
an asexual generation.
Fourth Type of Protozoa—PaRAM@CIUM
Paramecium, a type of ciliated Infusorians, especially
of those which are uniformly covered with short cilia
(Holotricha).
Fic. 39.—Paramecium.—After Biitschli.
ad. Adult form, showing cilia, ‘‘ mouth,” contractile vacuoles, ete.
div. Transverse division.
con. Conjugation.
Description.—Specimens of Paramecium may be readily
and abundantly obtained by leaving fragments of hay to
soak for some days in a glass of water. A few individuals
have been lying dormant about the plant; they revive and
multiply with extraordinary rapidity. They are also
PARAMGCIUM. 95
abundant in most stagnant pools, and are just visible
when a test-tube containing them is held between the
eye and the light. Their food consists of small vegetable
particles. : ;
The form is a long oval, with the blunter end in front;
Fic. 40.—Conjugation of Paramecium aurelia—four
stages. —After Maupas.
1. Shows macronucleus (/V) and two micronuclei (z) in each ot
the two conjugates.
z. Shows breaking up of macronucleus, and multiplication of
micronuclei to eight. 2
3+ Shows the fertilisation in progress; the macronucleus is
vanishing. :
4. Shows a single (fertilised) micronucleus in each conjugate.
the outer portion of the cell substance is differentiated into
a dense rind or cortex, with a delicate external cuticle,
perforated by cilia. There is a definite opening, the so-
called mouth, which serves for the ingestion of food
particles ; and there is also a particular anal spot posterior
Fic. 41.—Diagrammatic expression of process
of ‘conjugation in Paramecium aurelia.
—After Maupas.
A. The two micronuclei enlarge.
B. Each divides into two.
C. Eight micronuclei are formed.
D. Seven disappear ; one (darkened) divides into two.
E. An interchange and fusion occurs, and the con-
jugates separate.
F. The fertilised micronucleus divides into two.
G. Each conjugate begins to divide, the micronucleus
of each half dividing into two, one of which
becomes the macronucleus, while the others form
the two normal micronuclei. The top line repre-
sents four individuals, each with a macronucleus
and two micronuclei.
to the mouth, from which undigested residues are got rid of.
The surface is covered with cilia, in regular longitudinal
rows; these serve both for locomotion and for driving
food particles towards the mouth. Among the cilia there
are small cavities in the cortex, in which lie fine protrusible.
96 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
threads (‘“trichocysts”). These, though parts of a cell,
suggest the thread cells of Ccelentera, and are probably
of the nature of weapons. The cortical layer is contractile,
and is distinctly fibrillated. In the substance of the cell
lie two nuclei, the smaller “micronucleus” lying by the
side of the larger ‘‘ macronucleus.” Food vacuoles occur
asin the Ameba. There are two contractile vacuoles, from
which fine canals radiate into the surrounding protoplasm ;
these discharge into the vacuole, which then bursts to the
exterior.
Life history.—Growth is followed by obliquely transverse
division into two (Fig. 39, @v.). One half includes the
“mouth,” the other has to make one. As well as this
simple fission, a process of transient conjugation also
occurs. -Two individuals approach one another closely,
the two nuclei of each break up, an exchange of pieces
of the micronucleus takes place; the two then separate,
each to reconstruct its two nuclei (Fig. 40). This process
is necessary for the continued health of the species.
The details of the conjugating process have been worked out with
great care by Maupas and others. They differ slightly in different
species; what occurs in P. aure/éa is summarised diagrammatically
in Fig. 41,
The micronuclear elements are represented by two minute bodies.
As conjugation begins, these separate themselves from the macronucleus.
The macronucleus degenerates, and each micronucleus increases in
size (A). Each divides into two (B); another division raises their
number to eight (C) ; seven of these seem to be absorbed and disappear,
the remaining eighth divides again into what may be called the male
and female elements (D) ; for mutual fertilisation now occurs(E). After
this exchange has been accomplished, the Infusorians separate, and
nuclear reconstruction begins. The fertilised micronucleus divides into
two (F), and each half divides again (G), so that there are four in each
cell. Two of these form the macronuclei of the two daughter-cells
into which the Infusorian proceeds to divide (H); the other two form
the micronuclei, but before another division occurs each has again
divided. Thus each daughter-cell contains a macronucleus and two
micronuclei.
Fifth Type of Protozoa—VorTicELLA
Vorticella, or the bell-animalcule, is a type of those
ciliated Infusorians in which the cilia are restricted to a
region round the mouth (Peritricha).
VORTICELLA. 97
Description.—Groups of Vorticella, or of the compound
form Carchesium, grow on the stems of fresh-water plants,
and are sometimes readily visible to the unaided eye as
white fringes. In Vorticella each individual suggests an
inverted bell with a long flexible handle. The base of the
stalk is moored to the water-weed, the bell swings in the
QE
Let
KA Tt a
WAC RR lia:
Ny ue oO
Fic. 42.— Vorticella.—After Biitschli.
2. Structure. JV., Macronucleus; ., micronucleus; ¢.v., con-
tractile vacuole ; 7z., mouth; /v., food vacuole; v., vestibule.
2. Encysted individual. 3. Division.
4. Separation of a free-swimming unit—the result of a division.
5. Formation of eight minute units (#g-).
6. Conjugation of microzooid (#zg.) with one of normal size -
water, now jerking out to the full length of its tether, and
again cowering down with the stalk contracted into a close
and delicate spiral. In Carchestum the stalk is branched,
and each branch terminates in a bell. Up the stalk there
runs, in a slightly wavy curve, a contractile filament, which,
in shortening, gives the non-contractile sheath a spiral form.
-This contractile filament, under a high power, may exhibit
a fine striation, (A similar striated structure is seen in
7
98 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
some Amcebee, Gregarines, spermatozoa, etc., and of a much
coarser type in striped muscle fibres. It seems to be some
structural adaptation to contractility.) The bell has a
thickened margin, and within this lies a disc-like lid; in
a depression on the left side, between the margin and the
disc, there is an opening, the mouth, which leads by a
distinct passage into the cell. On the side of this passage
there is a weak spot, the potential anus, by which useless
débris is passed out. The cilia are arranged so as to waft
food particles into the mouth and down the passage.
There is a large and horseshoe-shaped macronucleus, and
a small micronucleus. Food vacuoles and contractile
vacuoles are present as usual.
Sometimes a Vorticella bell jerks itself off its stalk and
swims about; in other conditions it may form a temporary
cyst; normally, the cilia are very active, and the move-
ments of the stalk frequent and rapid. Multiplication may
take place by longitudinal fission—a bell divides into
similar halves; one of these acquires a basal circlet of cilia
and goes free, ultimately becoming fixed. Or the division
may be unequal, and one, or as many as eight, microzooids
may be set free. These swim away by means of the
posterior girdle of cilia, and each may conjugate with an
individual of normal size. In this case a small active cell
(like a spermatozoon) fuses intimately with a larger passive
cell, which may be compared to an ovum.
Sixth Type of Protozoa—VoLvox
Volvox is a type of flagellate Infusorians, especially of
those with flagella of equal size.
Volvox is found, not very commonly, in fresh-water pools,
and is usually classed by botanists as a green Alga. It
consists of numerous biflagellate individuals, connected by
fine protoplasmic bridges, and embedded in a gelatinous
matrix, from which their flagella project, the whole forming
a hollow, spherical, actively motile colony. In V. globator
the average number of individuals is about 10,000; in
V. aureus or minor, 500-1000. The individual cells are
stellate or amceboid in VM. globator, more spherical in V.
aureus; each contains a nucleus and a contractile vacuole.
VOLVOX. 99
At the anterior hyaline end, where the flagella are inserted,
there is a pigment spot; the rest of the cell is green, owing
to the presence of chlorophyll corpuscles. In consequence
of the presence of these, Volvox is holophytic, z.e. it feeds
as a plant does and builds up starch granules.
In its method of reproduction Vo/vox is of much biological interest
and importance. As Klein, one of its best describers, says, it is am
Fic. 43.—Volvox globator.—After Cohn.
a, Balls of sperms ; 4, immature ova ; c, ripe ova.
epitome of the evolution of sex. Some of the colonies are asexual.
In these a limited number of cells possess the power of dividing up to-
form little clusters of cells; these clusters escape from the envelope of
the parent colony, and form new free-swimming colonies. In other
colonies there are special reproductive cells, which may be called ova,
and spermatozoa. :
In V. globator the two kinds of reproductive cells are usually formed
in the same colony, the formation of spermatozoa generally preceding
that of the ova, Technically the colony may then be described as a
protandrous hermaphrodite.
too PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
In V. aureus the colony is oftenest unisexual or dicecious, z.e. either
male or female. But it may be moncecious or hermaphrodite, and is
then generally protogynous, z.e. producing eggs first.
Whether in a hermaphrodite or in a unisexual colony, the sex cells
appear among the ordinary vegetative units ; the ova are distinguishable
by their larger size, the ‘‘sperm mother cells” divide rapidly and form
numerous (32-100 or more) slender spermatozoa, each with two cilia.
In V. globator their bundles may break up within the parent colony ;
or, as always occurs in V. aureus, they may escape intact, and swim
about in the water. In any case, an ovum is fertilised by a spermato-
zoon, and, after a period of encystation and rest, segments to form a
new colony. Occasionally, however, this organism, so remarkable a
condensation of reproductive possibilities, may produce ova which
develop parthenogenetically.
Here, then, we have an organism, on the border line between plant
and animal life, just across the line which separates the unicellular from
the multicellular, illustrating the beginning of that important distinc-
tion between somatic or body cells and reproductzve cells, and occurring
in asexual, hermaphrodite, and unisexual phases. Klein records no less
than twenty-four different forms of V. aureus from the purely vegetative
and asexual to the parthenogenetic, for there may be almost entirely
male colonies, almost entirely female colonies, and other interesting
transitional stages. Klein has also succeeded to some extent in showing
that the occurrence of the various reproductive types depends on outside
influences,
Seventh Type of Protozoa—Mownocystis
Monocystis, a type of Sporozoa in which the cell is zot
divided into two parts by a partition.
Description.—Two species (AZ agilis and MZ. magna)
infest the male reproductive organs of the earthworm almost
constantly. The full-grown adults are visible to the naked
eye. They are usually flattened worm-like cells, but the
shape alters considerably during the sluggish movements.
There is a definite contractile rind, which is sometimes
fibrillated, and a more fluid medullary substance, in which
the large nucleus floats. In one species there is an anterior
projection which resembles the cap of Gregarina, otherwise
unrepresented in AZonocystis. As in Gregarina, and many
_ other parasitic forms, a contractile vacuole is absent.
Life history.—The young form of JZ. agitis is parasitic
within one of the sperm mother cells of the earthworm. It
grows, and becomes free from the cell as a trophozoite. In
the free stage, two individuals may unite in the curious
MONOCYSTIS. 101
end-to-end manner observed also in Gregarina. In repro-
duction two individuals (gametocytes) become associated
inside a common cyst. The nucleus of each divides up
repeatedly, and the daughter nuclei migrate to the surface of
the cell, where each becomes surrounded by a little mass of
protoplasm. Each of the gametocytes has thus given rise
to a number of gametes, while there remains over a mass
of residual protoplasm which has not been used up during
this process. The wall between the two gametocytes now
breaks down and the gametes conjugate in pairs, forming
zygotes. It is probable that of each pair of conjugating
gametes one is derived from each gametocyte. Each zygote
Fic. 44.—Life history of Monocystis.—After Biitschli.
1, Young Gregarine lying within a sperm mother cell of earthworm.
2. Association of two Gregarines within a cyst, ready to form gametes.
3. Numerous spore-cases (pc. pseudonavicelle) within a cyst.
4. A spore-case with eight spores (s.) and a residual core (74).
secretes a membrane and becomes a spore-case. The
nucleus divides up, and eight elongated spores are formed
round a residual core. The spore-case now takes its typical
shape and is known as a pseudonavicella. The spores are
considerably larger than those of Grvegarina. Eventually, in
the alimentary canal of another earthworm the cyst bursts,
the spore-cases are extruded, the spores emerge from their
firm chitinoid cases. The young spore (sporozoite) is like
a bent spindle (falciform), and seems next door to being
flagellate. It bores into a mother sperm cell, and from this
it afterwards passes as an adult into the cavity of the
seminal vesicles. Intracellular parasitism and copious food
naturally act as checks to activity, and the adult is sluggish.
The allies of Monocystis occur chiefly in “Worms,”
Tunicates, and Arthropods; none are known in Vertebrates.
102. PHYLUM PROTOZOA—THE SIMPLEST ANIMALS
Eighth Type of Protozoa—GREGARINA
Gregarina, a type of Sporozoa in which the cell is divided
into two regions by a partition.
Description.—Various species occur in the intestine of
the lobster, cockroach, and other Arthropods. When young
they are intracellular parasites, but later they become free in
the gut. They feed by absorbing diffusible foodstuffs, such
Fic. 45.—Life history of Gregarina.—After Biitschli.
x. Young forms (a, 4, c) emerging from intestinal cells (é.c.);3 .7.,
nucleus of intestinal cell.
2. Two forms conjugating (G. dlattarum).
3. Spore formation within a cyst.
4. Adult with deciduous head-cap (c.c.), and a cuticular partition
dividing the cell into an anterior part (A) and a posterior part
(B); ., the nucleus.
5. A spore within its spore-case (sf.c.).
as peptones and carbohydrates, from their hosts, and store
up glycogen within themselves. In many the size is about
one-tenth of an inch. There is a firm cuticle of ‘“ proto-
elastin,” which grows inwards so as to divide the cell into
a larger nucleated posterior region and a smaller anterior
region, and also, in the young stage, forms a small anterior
cap. The cell substance is divided into a firmer cortical
layer and a more fluid central substance. The protoplasm
often presents a delicate fibrillar appearance, suggesting
that of striated muscle. The nucleus is very distinct, but
there are no vacuoles. We may associate the absence of
GREGARINA—COCCIDIUM SCHUBERGI 103
locomotor processes, “mouth,” and contractile vacuoles,
as well as the thickness of the cuticle and the general
passivity, with the parasitic habit of the Gregarines. It is
not clearly understood how these and other intestinal
parasites have become habituated to resist the action of
digestive juices.
Life history.—The young Gregarine is
parasitic in one of the lining cells of the
gut; it grows, and, leaving the cell, re-
mains for a time still attached to it by
the cap (Fig. 45, a, 4,c); later this is cast
off, and the individual becomes free in the
gut, while still increasing in size. Two or
more individuals attach themselves together
end to end, but the meaning of this is
obscure. Encystation occurs, involving a
single unit or two together. ‘The details
of spore-formation are similar to those in
Monocystis. All the protoplasm is not always
used up in forming the spores, but a residue
may remain, which forms a network of
threads supporting the spores. The cyst is
sometimes (as in G. blattarum) complex, ,
with “ducts” serving for the exit of the
spores, each of which is surrounded by a = - Bnd
firm case. Eventually the cyst bursts, the ect Sieh
spore-cases are liberated, and from within of Gregarines,
each of these eight spores emerge to be- —After Fren-
come cellular parasites. The adult of G. el.
(Porospora) gigantea is sometimes three-
quarters of an inch in length—enormous for a Protozoon.
Ninth Type of Protozoa-—-CocciD1UM SCHUBERGI
Coccidia are intracellular parasitic Sporozoa, attacking
mainly the epithelial cells of the gut or associated organs.
They are found chiefly in insects, myriopods, molluscs, and
vertebrates.
Coccidium schubergt infests the intestinal epithelium of
the centipede Lithobius forficatus. The adult is a minute
104 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
oval or spherical cell with a nucleus. It lives a quiescent
life within the host cell, growing and absorbing nourishment
until the resources of the cell are exhausted.
Life history.—The coccidium enters the host cell as a
minute sickle-shaped body, pointed at the anterior end, and
Fic. 47.—Life history of Coccidium.—After Schaudinn.
1. Sporozoite; 2. Sporozoite entering a cell and becoming a trophozoite ;
3-4. Schizont, forming merozoites; 5. Merozoites entering another
cell; 6°. Merozoite forming macrogamete; 6%. Merozoite forming
microgametes ; 7. Free microgamete ; 8-9. Fertilisation of macrogamete
by microgamete; 10. Zygote within odcyst; tx. Formation of spores
within odcyst ; 12. Spores forming sporozoites.
more blunt posteriorly. This is the spovozoite stage of the
life history; it is liberated from a cyst (odcyst) when the
latter is swallowed by the centipede in its food. When
freed in the gut the sporozoite progresses by forward gliding
movements, alternating these by flexions, bending itself like
COCCIDIUM. 105
a bow and straightening out again. When about to enter
an epithelial cell it presses the anterior end through the cell
wall and wriggles its way in. Once within the cell in which
development is to proceed, its movements gradually cease,
but it may pass through several cells before coming to rest.
Within the host cell the coccidium—now in the ¢rophozotte
stage—becomes oval in form, and in about twenty-four hours
has reached full size and has exhausted the host cell
contents. This is the completion of the trophozoite period,
and the parasite now enters the schizon¢t stage, where its
nucleus divides into a number of daughter nuclei. These
arrange themselves around the periphery of the cell, whilst
the protoplasm breaks up to form along with them bodies
of a shape similar to the sporozoites. There are important
structural differences, however, apart from the difference in
origin. The parasites, now known as merozoites, rupture
the host cell, move in the gut cavity after the manner of the
sporozoites, enter fresh epithelial cells, and repeat the fore-
going cycle until ultimately the greater part of the gut
epithelium is destroyed. In about five days, however,
owing perhaps to the failing capacity of the host to nourish,
the limit of asexual reproductivity is reached, and the
parasite now enters upon a spore-forming stage. Certain
merozoites grow more slowly than the others, and instead ot
becoming schizonts give rise to elements of two types, viz.
microgametes, slender cells bearing a flagellum at each end,
which are male, and macrogametes, larger bean-shaped cells,
which are female. The latter after maturation free them-
selves from the host cell, and in the cavity of the gut are
fertilised by a male element. After fertilisation, a trans-
parent membrane forms around the zygote (fertilised cell).
This membrane in the first instance serves to exclude all
microgametes after the first, and later, becoming very tough
and resistant, forms a pretecting envelope or odcys¢. After
the odcyst is formed the parasite may pass from the host to
the exterior or remain for some time longer within it. The
nucleus cf the zygote within the odcyst now divides into
four, around which the protoplasm aggregates itself to form
the speres. There are thus four spores within a cyst.
Each spore divides, forming two sporozoites, which on the
arrival of the odcyst in the gut of a fresh host are liberated, _
106 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
and attacking the lining epithelium recommence the life
history.
GENERAL CLASSIFICATION OF PROTOZOA
Since the Protozoa are unicellular organisms (except the
few which form loose colonies), their classification should
be harmonious with that of the cells in a higher animal.
This is so. Thus (a) the Rhizopods, in which the living
matter flows out in changeful threads or “‘ pseudopodia,” as
CLASSIFICATION OF PROTOZOA
{CorTICATA.) (GYMNOMYXA.) (CORTICATA.)
Predominantly F Predominant]
ciliated and ey encysted nat
active. passive.
INFUSORIANS. RHIZOPODS. SPOROZOA.
ACINETARIA. RADIOLARIA.
FORAMINIFERA.
CILIATA. SPOROZOA
LABYRINTHULIDEA,
RHYNCHOFLAGELLATA
HELI0z0A. OR
DINOFLAGELLATA.
LOBOSA.
GREGARINES.
FLAGELLATA.
PROTEOMYXA and MyYcETOzOA,
PRIMITIVE FoRMS.
in the common Ameba, are comparable with the white
blood corpuscles or leucocytes, many young ova, and other
“ameboid” cells of higher animals; (4) the Infusorians,
which have a definite rind and bear motile lashes (cilia
or flagella), e.g. the common Paramecium, may be likened
to the cells of cé//ated epithelium, or to the active sperma-
tozoa of higher animals; (c¢) the parasitic Sporozoa, which
SYSTEMATIC SURVEY. 107
have a rind and no motile processes or outflowings, may
be compared to degenerate muscle cells, or to mature ova,
or to “‘excysted” passive cells in higher animals.
This comparison has been worked out by Professor Geddes, who also
points out that the classification represents the three physiological
possibilities—(a) the amoeboid units, neither very active nor very passive,
form a median compromise; (4) the ciliated Infusorians, which are
usually smaller, show the result of a relative predominance of expendi-
ture; (c) the encysted Gregarines represent an extreme of sluggish
passivity. \
But, as Geddes and others have shown, the cells of a higher animal
often pass from one phase to another,—the young amceboid ovum
accumulating yolk becomes encysted, the ciliated cells of the windpipe
may, to our discomfort, sink into amceboid forms. The same is true of
the Protozoa ; thus in various conditions the ciliated or flagellate unit
may become encysted or amceboid, while in some of the simplest forms,
such as Protomyxa, there isa ‘‘ cell-cycle ” in which all the phases occur
in one life history.
SYSTEMATIC SURVEY
A. Primitive forms.—Under this heading may be included two
classes : (1) the Proteomyxa, primitive, insufficiently known forms often
without a nucleus, though nuclear material may be present in the form
of scattered granules (chromidia), and (2) the Mycetozoa, organisms
with somewhat complex fructifications, often classed as plants allied
Fic, 48.—Diagram of Protomyxa aurantiaca.—After Haeckel.
x. Encysted; 2. Dividing into spores; 3. Escape of spores, at first
flagellate, then amceboid ; 4. Plasmodium, formed from fusion of
small amoebz.
to Fungi. As examples of the Proteomyxa, we have the interesting
Protomyxa in four phases: (a) encysted and breaking up into spores,
which (4) are briefly flagellate, (c) sink into amceboid forms, and (d)
flow together into a composite ‘‘ plasmodium ” ; Vampyrella, parasitic
on fresh-water Algz ; and many others.
The Mycetozoa are well illustrated by Fudigo or 4thalium septicum,
“¢ flowers of tan,” found in summer as a large plasmodium on the bark
of the tan-yard. The coated spores are formed in little capsules which
rise from the surface of the plasmodium. The spores may be first
flagellate, then amceboid, or amoeboid from the first ; the characteristic
plasmodium is formed by the fusion of the amcebee,
108 PHLYUM PROTOZOA—THE SIMPLEST ANIMALS.
B. Predominantly amceboid Protozoa.—Rhizopoda.—The
simplest Rhizovods generally resemble Amada, and are ranked in the
class (3) Lobosa, They may reproduce simply by division, as does
Ameba itself, or may liberate several buds at once (Avcel/a), or form
spores which conjugate (Pelomyxa). Warious forms, such as A7cella,
are furnished with a shell.
(4) The Labyrinthulidea are represented by forms like Labyrznthula
on Algee, and Chlamydomyxa on bog-moss, which consist of a mass of
protoplasm spread out into a network, and of numerous spindle-shaped
units, which travel continually up and down the threads of the living
net.
Fic. 49.—Formation of shell in a simple Foraminifer.
—After Dreyer.
In A the shell has one chamber ; B, C, and D show the formation
of asecond. Note outflowing psuedopodia and the enclosure of
the shell by a thin Jayer of protoplasm; note also the nucleus
in the central protoplasm.
As (5) Heliozoa are classified the sun-animalcules (Actenospherium,
Actinophrys sol), and others, in which there are stiff processes radiating
froma spherical body. Reproduction may be by division or by spore
formation ; skeletal structures may be represented by spicules.
The (6) Foraminifera or Reticularia include an interesting series
of shelled forms in which the peripheral protoplasm forms branching
interlacing threads. A few simple forms occur in fresh water ; the great
majority occur on the floor of the sea at varying depths; some
SYSTEMATIC SURVEY. 109
families are abundantly represented on the surface. The shell is
usually calcareous, more rarely arenaceous or chitinous. There is
sometimes dimorphism. Multiplication occurs by fission, or by the
formation of swarm-spores (amoeboid or flagellate). Foraminifera are
common as fossils from Silurian rocks onwards, and at the present day
are very important in the formation of calcareous ooze ; in this respect
Globigerina, with a chambered shell, is especially important. Species
of Gromia are found in both fresh and salt water; Halphysema, a
form utilising sponge-spicules to cover itself, was once mistaken for a
minute sponge.
Most kinds of chalk consist mainly of the shells of Foraminifera
Fic. 50.—A Foraminifer (Polystomella) showing shell and
pseudopodia.—After Schultze.
accumulated on the floor of ancient seas; Mummudlztes (Fig. 17) and
related fossil forms were as large as shillings or half-crowns.
More complex are the (7) Radiolaria, which are divided by a chitinoid
membrane into an inner central capsule (with one or more nuclei), and
an outer portion, gelatinous and vacuolated, giving off radiating thread-
like pseudopodia, which very rarely interlace. There is usually a
skeleton in the form of a siliceous lattice-work or regularly disposed
spicules outside the central capsule, but in some cases the shell is
formed of a horn-like substance called acanthin, which is probably a
complex silicate. Radiolarians multiply by fission, which sometimes
includes a halving of the skeleton, and by spores, which in some cases
are dimorphic. Most Radiolarians include unicellular Algz (yellow
110 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
cells), with which they live in intimate mutual partnership (symbiosis).
Most Radiolarians float on the surface of the sea; others live below
the surface at varying depths; and some are abyssal. They are
abundant as fossils, and of much importance in the formation of the
ooze of great depths.
Examples.—Thalassicola, Eucyrttdium, and the colonial Collozoum
and Spherozoum,
Fic. 51.—A pelagic Foraminifer— Hastigerina (Globigerina)
murrayt.—After Brady.
Note central shell, projecting calcareous spines with a protoplasmic
axis ; also fine curved pseudopodia and vacuolated protoplasm.
C. Predominantly active forms (ciliate and flagellate),
generally called Infusorians.—Protozoa, with a definite rind and
with 1-3 undulating flagella, are included as (8) Flagellata, a very
large group, among which are such familiar forms as the common
Euglena of ponds ; the Monads; Volvox, a colonial form ; Codosdga, a
colony in which the individual cells are furnished with a collar (Choano-
flagellata), The Hzemoflagellata are important blood parasites, generally
called Trypanosomes (see p. 121),
SYSTEMATIC SURVEY, I1t
Modified flagellate torms are included in the groups Dinoflagellata
and Cystoflagellata, in both of which there are two flagella, differ-
ently placed in the two cases. In the first are included Pertdindum and
Ceratium ; in the latter, the large phosphorescent Moctdluca. They
form an important part of the plankton of lakes and sea.
As (9) Ciliata are included a very large number of forms, more or
less closely resembling Paramecium or Vorticella, and very abundant
in infusions ; some, such as Ofaina, in the intestine of the frog, are
parasitic,
As specially modified Ciliata are included (10) Acinetaria, highly
specialised forms, ciliated when young, but usually furnished when adult
Fic. 52.—Optical section of a Radiolarian (Acténomma).
—After Haeckel.
a, Nucleus; 4, wall of central capsule; c, siliceous shell within
nucleus ; cl, middle shell within central capsule ; ¢?, outer shell
in extra-capsular substance. Four radial spicules hold the
three spherical shells together.
with suctorial tentacles, They are fixed in adult life, and feed on other
Protozoa. As examples may be given Acéneta ; Dendrosoma, forming.
branched colonies; and Oshryodendron, without suctorial tentacles.
Some, like Spherophrya, are minute and parasitic.
D. Predominantly encysted Protozoa,—Sporozoa.—Forms.
like Gregarina and Monocystds are included in a group of the (11)
Sporozoa, the Gregarinida in the strict sense. They are parasites in
the gut or body cavity of many Invertebrates, especially Arthropods.
Cocctdium is a type of the Coccidiidea, which are intracellular parasites.
occurring in Arthropods, Molluscs, and Vertebrates. A very im-
portant group, with a life cycle essentially similar to that of the
Coccidiidea, are the Hzemosporidia, which are parasitic in the red
blood corpuscles of Vertebrates. The malaria parasites belong to this
112, PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
group. In many of the Hzemosporidia 1 part of the life cycle takes
place in an intermediate host, usually a mosquito or a tick.
Other groups of the Sporozoa are the Myxosporidia, with peculiar
nematocyst-like organs (Invertebrates and cold-blooded Vertebrates),
and the Sarcosporidia, which are found inside the striped muscles of
warm-blooded Vertebrates.
GENERAL NOTES ON THE FUNCTIONS OF PROTOZOA
Movement.—The simplest form of movement is that
termed amceboid, as illustrated by an Amada. In ordinary
conditions it is continually changing its shape, putting forth
blunt lobes and drawing others in. With this is usually
associated a streaming movement of the granules. A more
defined contraction, like that of a muscle cell, is illustrated
in the contractile filament of the stalk of Vortzce//a and similar
Infusorians ; and not less definite are the movements of cilia
and flagella, by means of which most Infusorians travel
swiftly through the water. Cilia in movement are bent and
straightened alternately ; while flagella, which are usually
single mobile threads, exhibit lashing movements to and fro,
or, more often, are held stretched out in front, and by a
curious rotatory movement draw the cell along. They are
then more aptly termed ¢vacte//a. It seems probable that
cilia and flagella consist of an elastic core surrounded by
a sheath, which may be uniformly contractile, or may have
one contractile band, or two opposite contractile bands, and
so on.
Considered generally, the movements are of two kinds: either (1) re-
flex, z.e. responses to external stimulus, as when the Protozoon moves
towards a nutritive substance ; or (2) automatic, z.e. such movements as
appear to originate from within, without our being able to point to the
immediate stimulus, e.g. the rhythmical pulsations of contractile
vacuoles. Actively moving Protozoa usually show the following motor
reaction to stimulus:—they move backward, turn over on one side
structurally defined, and then move forward again.
Sensitiveness.—The Amceba is sensitive to external influ-
ences, It shrinks from strong light and obnoxious materials ;
it moves towards nutritive substances. This sensitiveness
is, so far as we know, diffuse—a property of the whole of
the cell substance; but the pigment spots of some forms
are specialised regions.
FUNCTIONS OF PROTOZOA, ~~ °° 113
Many Protozoa well illustrate a strange sensitiveness to the physical
and chemical stimuli of objects or substances with which they are not
in contact. Thus the simple amoeboid Vampyrella will, from a con-
siderable distance, creep directly towards the nutritive substance of
an Alga, and the plasmodium of a Myxomycete will move towards a
decoction of dead leaves, and away from a solution of salt. The same
sensitiveness, technically termed chemotaxis, is seen when micro-
organisms move towards nutritive media or away from others; when the
spermatozoon (of plant or animal) seeks the ovum, or when the phago-
cytes (wandering amceboid cells) of a Metazoon crowd towards an in-
truding parasite or some irritant particle.
Nutrition.—The Amada expends energy as it lives and
moves ; it regains energy by eating and digesting food
particles. Most of the free Protozoa live in-this manner
upon solid food particles; a few, such as Volvox, in virtue
of their chlorophyll, are holophytic, ze. they feed like plants ;
the parasitic forms usually absorb soluble and diffusible
substances from their hosts.,
Respiration.—Oxygen is simply taken up by the general
protoplasm from the surrounding medium, into which the
waste carbonic acid is again passed. The bubbles which
enter with the food particles assist in respiration. In
parasitic forms the method of respiration must be the same
as that of the tissue cells of the host.
Excretion.—Of the details of this process little is certainly
known, but the contractile vacuoles are, without doubt,
primitive excretory appliances. In the more specialised
forms they appear to drain the cell substance by means of
fine radiating canals, and then to burst to the exterior.
Uric acid and urates are said to be demonstrable as waste
products. BS get
Colour.—Pigments are not infrequently present in the Protozoa.
We have already noticed the presence of chlorophyll in some forms ;
with Radiolarians the so-called ‘‘yellow cells” are found almost
constantly associated. Each of these cells consists of protoplasm,
surrounded by a cell wall, and containing a nucleus. The protoplasm
is impregnated with chlorophyll, the green colour of which is obscured
by a yellow pigment. Starch is also present. The cells multiply by
fission, and-continue to live after isolation from the protoplasm of the
Radiolarian. All these facts point to the conclusior that the cells
are symbiotic Algze, so-called Zoochlorelle. According to some, the
“chlorophyll corpuscles” seen in the primitive Avcherina, in some
flagellate forms, as Huglena, and in many Ciliata, as Stentor, Stylo-
nichia, one species of Paramacium, Volvox and the. allied forms, are
114 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
also symbiotic Algee, which have lost the power of independent exist-
ence. The evidence for this is, however, insufficient, and this explana-
tion will not apply in cases like that of Vortzcella viridis, where the
green colouring matter is uniformly distributed through the protoplasm.
In many cases there is, besides the chlorophyll, a brown pigment,
identical with the datomin of Diatoms. In many of the Flagellata
there are one or more bright .pigment spots at the anterior end of the
cell; these may be specially sensitive areas. In some of the simpler
Gregarines the medullary protoplasm is coloured with pigment which is
apparently a derivative of the hemoglobin of the host.
Psychical life.—Protozoa often behave in a way which
suggests control, but it should be noted that cut-off
fragments sometimes behave just as effectively as the
intact units. Verworn has decided, after much labour,
that the Protozoa do not exhibit what even the most
generous could call intelligence; but this is no reason why
he or any other evolutionist should doubt that they have in’
them the indefinable rudiments of mind. Jennings has
shown that the behaviour of some Infusorians corresponds
to what may be called the method of trial and error; they
“try” one kind of response after another until, in some
cases, they give the effective answer.
GENERAL NOTES ON THE STRUCTURE OF PROTOZOA
The Protozoa are sometimes called “structureless,” but
they are only so relatively. For though they have not
stomachs, hearts, and kidneys, as Ehrenberg supposed, they
are not like drops of white of egg.
The cell substance consists of a living network or foam,
in the meshes or vacuoles of which there is looser material.
Included with the latter are granules, some of which are
food fragments in process of digestion, or waste products in
process of excretion.
The cell substance includes one or more nuclei, special-
ised bodies which are essential to the life and multiplication
of the unit. In the Protozoa there are several conditions
under which the nucleus may exist :—
(1) In some adult forms, and in many spores or young forms, no
definite nucleus has yet been discovered. It is, however, unnecessary
to preserve the term ‘‘ Monera” for such simple forms, as it is probable
that nuclear material does exist in the form at granules.
NOTES ON THE STRUCTURE OF PROTOZOA. 115
(2) In the majority of cases, notably in the Sporozoa, the nucleus
is single, often large, and placed centrally. From a consideration of the
cells of Metazoa we may call this the typical case.
_(3) In many of the Ciliata, e.g. Paramecium, there are two dimorphic
nuclei. There is a large oblong nucleus, and beside it a smaller
spherical one. he
(4) In some Ciliata the macronucleus exists in the form of powder
scattered through the protoplasm, ¢.g. in Ofalinopsds. The granules
may collect to form a compact nucleus when fission is about to take
place.
(5) In Ofalina, from the intestine of the frog, and a few other forms,
there are very numerous nuclei, arranged in a symmetrical manner in
the cell substance. In some cases these isolated nuclei have been
observed to unite to form one large nucleus just before binary fission
takes. place. Of these various cases the diffuse condition is apparently
very primitive. ,
The nucleus, when stained and examined under high powers, is
observed to be complex in structure. It consists of a nuclear network,
or a coil of chromatin threads. Karyokinesis has been observed in
some cases.
While we cannot at present define the physiological import of the
nucleus, we must recognise its importance. Thus Bruno Hofer has
shown that when an Amada is cut in two, the part with the nucleus
lives and grows normally, while the part without any nucleus sooner or
later dies ; and Balbiani has observed that in the case of Infusorians cut
into pieces, those parts which have nuclei survive, while if no nucleus is
present in the fragment, the wound may remain unhealed, and death
ensues.
The outer part of the cell substance (‘ectoplasm”) is
often clearer and less granular than the inner part (‘‘ endo-
plasm”). In corticate Protozoa there is a more definite
rind or thickened margin of cell substance. Outside this
there may be a “cuticle” distinct from the living matter,
sometimes consisting of chitin, or gelatin, or rarely of
cellulose. The cuticle may form a cyst, which is either a
protection during drought, or a sheath within which the
unit proceeds to divide into numerous spores. Moreover,
the cuticle may become the basis of a shell formed from
foreign particles, or made by the animal itself of lime, flint,
or organic material.
In the cell substance there may be bubbles of water taken
in with food particles (food vacuoles), contractile vacuoles,
fibres which seem to be specially contractile (in Gregarines),
spicules of flint or threads of horn-like material, which may
build up a connected framework, and the pigments raealdy
mentioned.
116 PHYLUM PROTOZOA——-THE SIMPLEST ANIMALS.
. REPRODUCTION OF PROTOZOA
Growth and reproduction are on a different plane from
the other functions. Growth occurs when income exceeds
expenditure, and when constructive or anabolic processes
are in the ascendant. Reproduction occurs at the limit of
growth, or sometimes in disadvantageous conditions.
As it is by cell division that all embryos are formed from the egg, and
all growth is effected, the beginnings of this process are of much interest.
(a) Some very simple Protozoa seem to reproduce by what looks like
the rupture of outlying parts of the cell substance. (6) The production
of a small bud from a parent cell is not uncommon, and some Rhizo-
pods (é.g. rcella, Pelomyxa) give off many buds at once. (c) Com-
moner, however, is the definite and orderly process by which a unit
divides into two—ordinary cell division. (d) Finally, if many divisions
occur in rapid succession or contemporaneously, and usually within a
cyst enclosing the parent cell, z.e. in narrowly limited time and space,
the result is the formation of a considerable number of small units or
spores. In the great majority of cases, each result of division is seen
to include part of the parent nucleus.
A many-celled animal multiplies in most cases by
liberating reproductive cells—ova and spermatozoa —
different from the somatic cells which make up the “ body.”
A Protozoon multiplies by dividing wholly into daughter
cells. This difference between Metazoa and Protozoa in
their modes of multiplication is a consequence of the
difference between multicellular and unicellular life. Each
part of a divided Protozoon is able to live on, and will
itself divide after a time, whereas the liberated spermatozoa
and ova of a higher animal die unless they unite.
By sexual reproduction we mean—(a) the liberation of
special reproductive cells from a “body,” and (4) the
fertilisation of ova by spermatozoa. As Protozoa have
no “body”—though the beginnings of one are seen in
the colonial forms—they cannot be said to exhibit sexual
reproduction in the first sense (a), yet many of them
(especially the Sporozoa) give origin by division to special
reproductive cells. And although many Protozoa can live
on, dividing and multiplying, for prolonged periods without
the occurrence of anything like fertilisation, processes
corresponding to fertilisation are of general occurrence.
For in many of the Protozoa there occurs at intervals a
process of “conjugation” in which two individuals unite
REPRODUCTION OF PROTOZOA. 117
either permanently or'temporarily. This is an incipiently
sexual process; it is the azalogue of the fertilisation of an
ovum by a spermatozoon. In many cases, moreover,
there is a difference between the two conjugates, analogous
to the difference between ovum and spermatozoon.
(1) It is one of the recurrent phases in the life history of some of the
simplest Protozoa (Proteomyxa and Mycetozoa) (see p. 107), that a
number of amceboid units flow together into a composite mass, which
has been called a ‘‘ plasmodium.” ;
(2) It is known that more than two individual Sporozoa and other
forms occasionally unite. To this the term ‘‘ multiple conjugation ”
has been applied.
(3) Commonest, however, is’ the union of two apparently similar
individuals, either permanently, so that the two fuse into one, or
temporarily, so that an exchange of material is effected. Permanent
conjugation has been observed in several Rhizopods, Infusorians, and
Sporozoa. ‘Temporary conjugation is well known in not a few ciliated
Infusorians, and it is possible that a curious end-to-end union of certain
Sporozoa is of the same nature, or it may be of the nature of a
‘‘plasmodium ” formation. The formation of small spores (gametes)
which conjugate is not uncommon.
(4) There are some cases where one of the conjugating individuals
is larger and less active than the other. Thus in Vorézcella, a small
free-swimming form unites and fuses completely with a stalked indivi-
dual of normal size. This ‘‘dimorphic conjugation” is evidently
analogous to the fertilisation of a passive: ovum by an active sper-
matozoon. In Volwox this is even more obvious, for the small: and
active cells, both in shape and method of formation, recall the sper-
matozoa of higher fornis. :
Significance of Conjugation.—The precise interpretation of
conjugation is uncertain. We may regard it as a mutual rejuvenescence,
each unit supplying some substances or qualities which the other lacks ;
or we may regard it rather as a process by which the average character
of the species is sustained, peculiarities or pathological variations of one
individual being counteracted by other characters in the neighbour
(apparently no near relation) with which it conjugates ; or we may see
in it a source of variation as the result of new combinations among
the essential hereditary substances. The researches of M. Maupas have
thrown much light on the facts, and some of his results deserve summary.
It has been often alleged that the subsequent dividing is’ accelerated
by conjugation ; but Maupas finds that this,is by no means the case.
The reverse in fact is true. While a pair of Infusorians (Onychodromus
grandis) were engaged in conjugation, a single individual had,. by
ordinary asexual division, given rise to a family of from forty thousand
to fifty thousand individuals. Moreover, the intense internal changes
preparatory to conjugation, and the general inertia during subsequent
reconstruction, not only involve loss of time, but expose the Infusorians
to great risk. Conjugation seems to involve danger and death rather
than to conduce to multiplication and birth.
118 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
The riddle was, in part at least, solved by a long series of careful
observations. In November 1885, M. Maupas isolated an Infusorian
(Stylonichia pustulata), and observed its generations till March 1886,
By that time there had been two hundred and fifteen generations pro-
duced by ordinary division, and since these lowly organisms do not
conjugate with near relatives, there had been no conjugation.
What was the result? At the date referred to, the family was
observed to have exhausted itself. The members were being born old
and debilitated. The asexual division came to a standstill, and the
powers of nutrition were lost.
Meanwhile, before the generations had exhausted themselves, several
of the individuals had been restored to their natural conditions, where
they conjugated with unrelated forms of the same species. One of
these was again isolated, and watched for five months. In this case,
up till the one hundred and thirtieth generation, it was found that on
removal to fresh conditions the organisms were capable of conjugating
with unrelated forms. Later this power was lost, and at the one
hundred and eightieth generation the individuals of the same family
were observed making vain attempts to conjugate with each
other. j
We thus see that without normal conjugation the whole family
becomes senile, degenerates both morphologicaliy and physiologically.
Morphologically, the individuals decrease in size, until they measure
only a quarter of their original proportions, the micronucleus atrophies
completely or partially, the chromatin of the macronucleus gradually
disappears, other internal structures also degenerate. Physiologically,
the powers of nutrition, division, and conjugation come to a standstill,
and this senile decay of the isolated individuals or family inevitably
ends in death.
The general conclusion is evident. Sexual union in those Infusorians,
dangerous, perhaps, for the individual life, and a loss of time so far as
immediate multiplication is concerned, is absolutely necessary for the
species. The life runs in strictly limited cycles of asexual division.
Conjugation with allied forms must occur, else the whole life ebbs.
Without it, the Protozoa, which some have called ‘‘immortal,” die a
natural death. Conjugation is the necessary condition of their eternal
youth.
It must be noted, however, that some subsequent investigators have
watched over two hundred asexual generations of ciliated Infusorians
without seeing the slightest trace of senile degeneration. Calkins has
cultivated Paramecium for over six hundred generations without
conjugation by giving beef extract when degeneration threatened to set
in. The same result was obtained by stimulating with alcohol,
strychnine, etc.
Ecology. — Many Protozoa raise organic débris once more
into the circle of life, and many form part of the food
of higher animals. Thus those pelagic Foraminifera and
Radiolarians, which sink dying to the great oceanic depths,
form along with more substantial débris the fundamental
REPRODUCTION OF PROTOZOA. TI9
food supply in that plantless world. Fundamental, since it
is plain that the deep-sea animals cannot all be living on
one another.
Almost every kind of nutritive relation occurs among the
Protozoa. Predatory life is well illustrated by most In-
fusorians, and thoroughgoing parasitism by the Sporozoa ;
Opatina in the rectum of the frog may serve as a type of
those which feed on decaying débris, and Volvox of those
which are holophytic. Radiolarians, with their partner
Algee, exhibit the mutual benefits of symbiosis, the plants
utilising the carbon dioxide of their transparent bearers, the
animals being aérated by the oxygen which the plants give
off in sunlight, and probably nourished by the carbohydrates
which they build up. Some of the parasitic forms, especially
among the Sporozoa, are fatally injurious to higher animals.
Though Protozoa may be seriously infected by Bacteria,
by Acinefa parasites, by some fungi, like Chytridiwm, etc.,
fatal infection is rare, because of the power of intracellular
digestion which most Protozoa possess. “The parasite,”
Metchnikoff says, “makes its onslaught by secreting toxic
or solvent substances, and defends itself by paralysing the
digestive and expulsive activity of its host; while the latter
exercises a deleterious influence on the aggressor by digest-
ing it and turning it out of the body, and defends itself by
the secretions with which it surrounds itself.” With this
struggle should be compared that between phagocytes and
Bacteria in most multicellular animals.
History.—Of animals so small and delicate as Protozoa, we do not
expect to find distinct relics in the much-battered ancient rocks. But
there are hints of Foraminifer shells even in the Cambrian ; more than
hints in the Silurian and Devonian ; and an abundant representation in
rocks of the Carboniferous and several subsequent epochs. The shells
of calcareous Foraminifera form an important part of chalk deposits.
There seem at least to be sufficient relics to warrant Neumayr’s
generalisation in regard to Foraminifera, that the earliest had shells
of irregularly agglutinated particles (Astrorhizidze), that these were
succeeded by forms with regularly agglutinated shells, exhibiting types
of architecture which were subsequently expressed in lime.
Relics of siliceous Radiolarian shells are also known from Silurian
strata onwards, with, perhaps, the exception of the Devonian. Best
known are those which form the later Tertiary deposits of Barbados
earth, from which Ehrenberg described no fewer than two hundred and
seventy-eight species. ~
120 PHYLUM. PROTOZOA—THE SIMPLEST ANIMALS.
Protozoa and Disease.—The discoveries of recent years
have shown that the study of Protozoa is an inquiry
of great practical importance. Numerous Protozoa—
representing the main divisions of the group—are known
at some stage of their life history to be parasitic in the
human body or in domestic animals. Some of them
are associated with serious and fatal diseases. Thus,
Amaba (Entameba) histolytica causes an inflammation
of the intestinal mucous membrane and liver abscesses.
Several flagellates of the genus Zrypanosoma are serious
Fic. 53.—Glossina palpalis, tse-tse fly.
parasites of the blood affecting man, horses, cattle,
camels, and other domestic animals in both the old
and new worlds. TZ7ypanosoma gambiense (Fig. 54) is
the parasite causing the fatal “sleeping sickness,” a human
disease disseminated by tse-tse fly, Glossina palpalis, in
Africa (Fig. 53). In the fully formed Trypanosome, the
flagellum is expanded into an undulating membrane
which extends down the edge of the cell. In this
membrane there are eight fine contractile threads or
myonemes, which are connected at the lower end with
a modified nucleus called the blepharoplast. The so-
PROTOZOA AND DISEASE. 121
called Leishman-Donovan body, the parasite of dum-dum
fever or splenomegaly, a disease occurring in India and
Africa, has recently been shown to be a stage in the
life history of a flagellate protozoon. Of great import-
‘ance, also, is the family of Spirochztes, one of which,
Treponema (Spirochete) pallidum, is the organism which
causes syphilis. Another highly important genus is Fivo-
plasma (Babesia), a sporozoon. These are blood parasites,
Fic. 54.
2. Trypanosoma gambiense, showing nucleus, blepharoplast,
and flagellum.
2and 3. Individuals undergoing longitudinal fission.
4. Leucocyte engulfing a trypanosome.
causing Texas fever in cattle and analogous diseases in
horse, sheep, dog, and possibly man also. The parasite
of Texas fever is transmitted through two generations
of ticks. Lastly may be mentioned the parasites of
‘malaria, JLaverania and Plasmodium, whose compli-
‘cated life histories in mosquito and man are now well
‘known. j ,
General zoological interest.—The Protozoa illustrate, in
122 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS.
free and single life, forms and functions like those of the
cells which compose the many-celled animals. T 'ypically,
they show great structural or morphological simplicity, but
great physiological complexity. Within its single cell the
Protozoon discharges all the usual functions, while in a
higher animal distinct sets of cells have been specialised for
various activities, and each cell has usually one function
dominant over the others. The Metazoan cells, in acquiring
an increased power of doing one thing, have lost the
Protozoan power of doing many things.
The Protozoa remain at the level represented by the
reproductive cells of higher forms, and are comparable to
reproductive cells which have not formed bodies. In the
sexual colonies of Volvox, however, we see the beginning of
that difference between reproductive cells and body cells
which has become so characteristic of Metazoa. The
Protozoa are self-recuperative, and in normal conditions
they. are not so liable to “natural death” as are many-celled
animals. Weismann and others maintain that they are
physically immortal.
They illustrate—(a) the beginnings of reproduction, from
mere breakage to definite division, either into two, as in
fission, or in limited time and space into many units, as in
the formation of spores within a cyst; (4) the beginnings of
fertilisation, from ‘‘ the flowing together of exhausted cells”
and multiple conjugation, to the specialised sexual union
of some Infusorians, Heliozoa, Sporozoa, etc..—where two
individuals become closely united; along with this, the
beginnings of maturation, as shown in the formation of
polar nuclei in some Heliozoa, Sporozoa, Flagellata, and
Lobosa ; (¢) the beginnings of sex, in the difference of size
and of constitution sometimes observed between two con-
jugating units (e.g. in Coccidium); (d) the beginnings of
many-celled animals, in the associated groups or colonies
which occur in several of the Protozoan classes. These
colonies show a gradation in complexity. Raphidiophrys
and other Heliozoa form loose colonies, which arise by the
want of separation of the products of fission. Among the
Radiolarians there are several colonial forms; in these the
individuals are united by their extra-capsular protoplasm,
but are all equivalent. In /voterospongia the cells show
GENERAL ZOOLOGICAL INTEREST. 123
considerabie morphological distinctiveness; some are
flagellate, some amoeboid, some encysted and _ spore-
forming. Again, in Volvox, as we noticed above, the cells
of the colonies show a distinction into nutritive and repro-
ductive units.
Fic. 55.—A colonial flagellate Infusorian— Proterospongia
haeckeliz.—After Saville Kent.
There are about qo flagellate individuals. a, nucleus ; 4, contractile
vacuole; c¢, amoeboid unit in gelatinous matrix ; 3 @, division
‘ of an amoeboid unit; e, flagellate units with collars contracted ;
J, hyaline outer membranes ; g, unit forming spores.
Lastly, in their antithesis of passivity and activity, con-
structive and destructive preponderance, anabolism and
katabolism, the Protozoa illustrate the phases of the cell-
cycle, and so furnish a key to the variation of higher
animals.
CHAPTER VIII
PHYLUM PORIFERA—SPONGES
Class I. CALCAREA.
Class 11. HEXACTINELLIDA.
Class III. DEMOSPONGIA.
SPONGES seem to have been the first animals to attain
marked success in the formation of a “body.” For though
their details are often complex, their essential structure is
simpler than the average of any other class of Metazoa, and
some of the simplest forms do not rise high above the level
of the gastrula embryo. A “body” has been gained, but
it shows relatively little division of labour or unified life ; it
is a community of cells imperfectly integrated. The cells
of the body show an arrangement in two distinct layers,
which is one of the most essential characters of the
Metazoa. There are no definite organs, and the tissues
are, as it were, in the making. Sponges are passive,
vegetative animals, and do not seem to have led on to
anything higher; but they are successful in the struggle
for existence, and are strong in numbers alike of species
and of individuals.
GENERAL CHARACTERS
Sponges are diploblastic (two-layered) Metaszoa, the middle
stratum of cells, the mesoglea, not attaining to the definiteness
of a proper mesoderm. There is no celom or body cavity.
The longitudinal axts of the body corresponds to that of the
embryo; tn other words, the general symmetry of the
gastrula ts retained. In these three characters the Sponges
\
STRUCTURE OF SPONGES. 125
agree with the Celentera, and differ from higher (trt-
ploblastic and celomate) Metazoa.
The body varies greatly in shape, even within the same
species. It ts traversed by canals, through °
which currents of water bear food in-
wards and waste outwards. Numerous
minute pores on the surface open into
afferent canals, leading into a cavity or
cavities lined by flagellate cells, many or
all of which have a goblet shape with a
delicate collar through which the flag-
ellum rises (‘‘choanocytes”). To the
activity of the flagella the all-important
water currents are due. The internal
cavity may be a simple tube, or it may
have radially outgrowing chambers, or tt
may be represented by branched spaces,
Jrom which afferent canals lead to the
exterior. When there is a distinct central Fic. 56.—Simple
cavity there ts usually but one large sponge (Ascetta
exhalant aperture (osculum), but in other hig age 1 oe
cases there are many exhalant apertures. ae
A delicate outer layer covers the body, Nee aie aie
and is perhaps continued into the affer- uaeaite pores in the
ent canals. Beneath the covering layer ~*~
there is in all but the simplest forms a mass of cells (the
mesoglea) which may be very varied in its composition. Thus
there are scleroblasts making the skeleton of lime, flint, or
Spongin ; amebotd cells or phago-
cytes, important in digestion and
excretion , reproductive cells, and
other elements.
This median mass of cells ts
traversed by the afferent canals
and by the diverticula of the
central cavity or the branches of
poecre the original central cavity, lined
Fic. 57.—A sponge colony. Sy flagellate cells, It is difficult
to call this cavity or system of
cavities the gut or enteron, or to call the layer which lines it
the endoderm, or the outer covering layer the ectoderm. In
ERO
126
PHYLUM PORIFERA—SPONGES.
fact, the sponges are very different Jrom other Metazoa, and
represent a cul de sac in evolution.
Budding is very common,
and in a few cases buds are
set adrift. Both hermaph-
rodite and unisexual forms
occur. The sexually-produced
embryo ts almost always
developed within the mes-
oglea, and leaves the sponge
as a ciliated larva. With
the exception of one family,
Fic. 58.—Sponge spicules.
1, Monaxon ; 2, triod; 3, triaxon; 4,
tetraxon 3 5, anchor 2 6, polyaxon ;
7, a kind of amphidisc,
all are marine.
Description of a simple
sponge. — A
sponge, such as Ascetfa, is
very simple
a hollow vase, moored at
one end to rock or seaweed, with a large exhalant aperture
at the opposite pole, and with
numerous minute inhalant pores
penetrating the walls. In the
calcareous sponges, the pores are
minute perforations in single cells
(porocytes).
The walls consist of—(1) a flat
covering layer; (2) a mesogloea
containing triradiate calcareous
spicules, phagocytes, and reproduc-
tive elements; and (3) a layer lining
the central cavity, and composed of
collared flagellate cells, like some
of the monad Infusorians (cf. Fig.
55):
More complicated forms.—But a
description of a simple sponge like
Ascetta conveys little idea of the
structure of a complex form such as
the bath-sponge (Zuspongia). Let us
consider the origin of complications.
Fic. 59.— Section of a
sponge.—After F. E.
Schulze.
Showing inhalant
canals,
flagellate chambers, a
gastrula forming in the
mesogloea, etc.
(a) Sponges—long regarded as plants—are plant-like in
being sedentary and passive.
They seem also to feed
STRUCTURE OF SPONGES. 127
easily and well. Like plants, they form buds, the outcome
of surplus nourishment,
a rose-bush, often ac-
quire some apparent
independence, and: the
sponge looks like many
vases, not like one.
Moreover, as they grow
these buds may fuse,
like the branches of a
tree tied closely to-
gether. Thus the struc-
ture becomes more in-
tricate.
(6) In the simple
sponge the cavity of the
vase is completely lined
by the collared flagellate
cells (Ascon type). But
the inner layer may grow
out into radial chambers
to which the choan-
ocytes are restricted
(Sycon type), and the
walls of these may also
be folded into side aisles
(Leucon type). The out-
growing of the inner
layer into the mesogloea
may be continued even
further, and the cells
may become pavement-
like except in the
minute flagellate cham-
bers, where alone the
characteristic choano-
cytes are retained (see
Fig. 60).
It may be that the
characteristic folding or
‘These buds, like the suckers of
Fic. 60.— Diagram showing types of
canal system. — After Korschelt and
Heider. The flagellate regions are
dark throughout, the mesogloea is
dotted, the arrows show the direction
of the currents. All the figures re-
present cross-sections through the wall.
Simple Ascon type (Zc., outer layer; Az.,
inner layer; /Zg., mesoglcea).
Se type, with flagellate radial chambers
ree
the main radial chambers.
Still more complex type, with small flagel-
late chambers (/.ch.).
A.
B.
C. Leucon ‘type, with flagellate side, aisles on
D.
outgrowth of the inner layer is
necessitated by the fact that the component cells are better
128 PHYLUM PORIFERA—SPONGES.
nourished and multiply more rapidly than ‘those of the
outer’ layer.
(c) By infoldings of the outer layer and a subjacent
sheath of mesoglcea—subdermal spaces may be formed;
an outer cortex may be distinctly differentiated from the
internal region in which the flagellate chambers occur; the
pores may collect into sieve-like areas, which open into
dome-like cavities; these and many other complications
are common.
(d) The covering layer usually consists of flat epithelium,
but flask-shaped cells have also been observed (Bidder).
It may be folded inwards, as we have noticed, and, accord-
ing to some, it also lines the inhalant or afferent canals in
whole or in part. In a few cases, e.g. Oscarella lobularis,
it is ciliated, and its cells may also exhibit contractility, as
around the osculum of Ascetfa clathrus, though the con-
tractile elements usually belong to the mesoglcea.
The inner layer consists typically of collared flagellate cells,
but in the more complex sponges these are replaced, except
in the flagellate chambers, by flat epithelial cells, with or
without flagella.
The mesoglea contains very varied elements, and illus-
trates the beginnings of different kinds of tissue. Thus
there are migrant amceboid cells (phagocytes); irregular
connective tissue cells; spindle-shaped connective tissue
cells, united into fibrous strands; contractile cells, e.g.
those forming a sphincter around the oscula of some forms,
such as Pachymatisma; skeleton-making cells; pigment-
containing cells; supposed nerve cells, projecting on the
surface, and believed to be connected internally with
multipolar (ganglion?) cells; and lastly, the reproductive
cells,
(e) The skeleton consists of calcareous or siliceous
spicules, or of spongin fibres, or of combinations of the
two last. A calcareous spicule is formed of calcite, with a
slight sheath and core of organic matter; a siliceous spicule
is formed of colloid silica or opal; the spongin is chemically -
somewhat like silk. Uniradiate, biradiate, triradiate, quadri-
radiate, sexradiate, and multiradiate spicules occur, and
they are effective in keeping the meshes open and in giving
the body architectural stability. In every pole scaffolding
ORDINARY FUNCTIONS. 129
we see, as it were, huge hexactinellid spicules, spliced to-
gether with ropé. It is convenient to distinguish the large
macroscleres from the small microscleres. Each spicule
begins to. be formed by one or more “scleroblasts,” and
may be speculatively regarded as an organised intra-
cellular excretion. ‘ During its growth,” Professor Sollas
says, “the spicule slowly passes from the interior to the
exterior of the sponge, and is finally (in at least some
sponges—Geodia, Stelletta) cast out as an effete product.”
The fibres of spongin are formed as the secretions of
mesogloea cells, known as spongioblasts.
Ordinary functions.—Excepting the fresh-water Spong-
illide, all sponges are marine, occurring from between
tide marks to great depths. After embryonic life is
past, they live moored to rocks, shells, seaweeds, and
the like. Their motor activity is almost completely
restricted to the lashing movements of the flagella, the
migrations of the phagocytes, and the contraction of
muscular mesogloeal cells, especially around the exhalant
apertures. In the closure of the inhalant pores, sponges
show sensitiveness to injurious influences, but how far this
is localised in specialised cells is uncertain.
The most important fact in the life of a sponge is that
which Robert Grant first observed—that currents of water
pass gently in by the inhalant pores, and more forcibly
out by the exhalant aperture or apertures. This may be
demonstrated by adding powdered carmine to the water.
‘The instreaming currents of water bear dissolved air and
supplies of food, such as Infusorians, Diatoms, and particles
of organic débris. The outflowing current carries away
waste. When a sponge is fed with readily recognisable
substances, such as carmine or milk, and afterwards
sectioned, the grains or globules may be found—(a) in the
collared flagellate cells; (4) in the adjacent phagocytes of
the mesogloea ; (c) in the phagocytes surrounding the sub-
dermal spaces, if these exist. It is uncertain whether the
epithelium of the subdermal spaces or the flagellate lining
of the deeper cavities is the more important area of absorp-
tion, but it is certain that the phagocytes play an important
part in engulfing and transporting particles, in digesting
those which are useful, and in getting rid of the useless.
9
130° PHYLUM PORIFERA—SPON GES.
In an extract of several sponges, Krukenberg found a
(tryptic) digestive ferment, probably formed within the
phagocytes, but digestion is wholly intracellular.
Many sponges contain much pigment; thus the lipo-
chrome pigment zoonerythrin (familiar in lobsters) is
common. Some pigments, such as floridine, may help in
respiration. The green pigment of the fresh-water sponge
is closely analogous, if not identical, with chlorophyll, and
probably renders some measure of holophytic nutrition
possible.
Reproduction.—If a sponge be cut into pieces, these may
regenerate the whole—a fact which illustrates the relatively
undifferentiated state of the sponge body. It is possible
that fission may sometimes occur naturally.
The frequent budding is merely a kind of continuous
growth, but when buds are set adrift, as sometimes happens,
we have discontinuous growth or asexual reproduction.
In the fresh-water Spongillidze there is a peculiar mode of reproduc-
tion by statoblasts or gemmules, A number of mesoglceal cells occur
in a clump, some forming an internal mass, others a complex protective
capsule, with capstan-like spicules, known as amphidiscs. According
to W. Marshall, the life history is as follows: In autumn the sponge
suffers from the cold and the scarcity of food, and dies away. But
throughout the moribund parent gemniules are formed. These survive
the winter, and in April or May they float away from the dead parent,
and develop into new sponges. Some become short-lived males, others
more stable females. The ova produced by the latter, and fertilised
by spermatozoa from the former, develop into a summer generation of
sponges, which, in turn, die away in autumn, and give rise to gemmules.
The life history thus illustrates what is called alternation of genera-
tions. Interpreted from a utilitarian point of view, the formation
of gemmules is a life-saving expedient. As Professor Sollas says,
“the gemmules serve primarily a protective purpose, ensuring the
persistence of the race, while as a secondary function they serve for
dispersal.”
All sponges produce sex cells, which seem to arise from
amoeboid mesoglcea cells retaining an embryonic character.
In the case of the ovum, the amoeboid cell increases in
size, and passes into a resting stage; in the case of the
male elements, the amceboid cell divides into a spherical
cluster of numerous minute spermatozoa. The similar
origin of the ova and spermatozoa is of interest. Most
sponges are unisexual, but many are hermaphrodite. In
DEVELOPMENT. 131
the latter case, however, either the production of ova or the
production of spermatozoa usually preponderates, probably
in dependence upon nutritive conditions.
Development.—lIt is not surprising to find that there is
great variety of development in the lowest class of Metazoa ;
it seems almost as if numerous experiments had been made,
none attended with progressive success.
The minute ovum, without
any protective membrane, usu-
ally lies near one of the canals,
and is fertilised by a spermato-
zoon borne to it by the water.
It exhibits a certain power of
migration, as in some Hydroids.
Previous to fertilisation, the usual
extrusion of polar bodies has
been observed in a few cases,
and is doubtless general. Seg-
mentation is total and usually
equal, and results in a spherical
or oval embryo more or less
flagellate. This leaves the parent
sponge, swims about for a time,
then settles down, and undergoes
a larval metamorphosis often
difficult to understand. It is
peculiarly difficult to bring the
history of the germinal layers in
sponges into line with that in
other Metazoa.
Fic. 61.—Development of Sycandra
raphanus.—After F. E. Schulze.
x. Ovum.
2. Section of 16-cell stage.
3. Blastula with 8 granular cells (gxc.) at
lower pole.
4. Free-swimming amphiblastula, with
-upper hemisphere’ of flagellate cells
(fc.), and lower hemisphere of granu-
lar cells (gv.c.).
5. Gastrula stage settled down. c., outer
layer; Zw, inner layer; 42., closing
blastopore ; a@7.Z., mooring, amceboid
processes. ,
132 PHYLUM PORIFERA—SPONGES.
(a) In the small calcareous sponge Sycandra raphanus (Fig. 61), as
described by F. E. Schulze, the segmentation results in a hollow ball
of cells—the J/astula. A few cells at the lower pole remain large,
and are filled with nutritive granules; the other cells divide rapidly
and become small, clear, columnar, and flagellate. The large granular
cells become invaginated, forming what is called a “‘ pseado-gastrula.”
This leaves the parent, and the cells forming the lower hemisphere
of the embryo become rounded and
non-flagellate. The embryo swims for
a time actively, but the flagellate cells
of the upper hemisphere are invaginated
into or overgrown by the large granular
cells, and thus what is generally called
the gastruda stage results. This soon
settles down, on rock or seaweed, with
the blastopore or gastrula mouth down-
wards, and is moored by amceboid
processes from the granular cells, which
likewise obliterate the blastopore. The
granular cells lose their granules, for
the larva is not yet feeding; the now
internal flagella disappear in the absence
of the stimulating water; a mesogloea
with spicules begins to be formed
between the inner and outer layer,
probably by migrants from the latter.
But this disadvantageous state of affairs
cannot last. Pores open through the
walls, the entrance of water enables the
inner cells to récover their flagella, and
an exhalant aperture is ruptured at the
upper pole.
Fic. 62.—Diagrammatic re-
presentation of development
of Oscarella lobularis. —
After Heider.
Bl, Free-swimming blastula with
flagella; G., gastrula settled
down.
Next figure shows folding of inner
layer (Fx.); £c., outer layer.
Lowest figure shows radial cham-
bers (2.C.); Mesoglea (AZg.);
inhalant pore (/.); exhalant
osculum (0.).
hemispherical gastrula, which settles mouth downwards.
The young sponge is now
in an Ascon stage, from which, by
the outgrowth of the inner layer into
radial chambers, it passes into the
permanent Sycon form, grows into a
cylinder, and becomes differentiated in
detail (Fig. 61).
(6) In Oscarella (Halisarca) lobularis
(Fig. 62), a sponge without any skeleton,
the ovum segments equally into a
blastula, which is flagellate all over.
This free-swimming stage may’ be in-
vaginated from either pole to form a
Pores, an
osculum, and the mesogloea are formed as before, and the inner layer
becomes folded into flagellate chambers.
The main features of sponge embryology are thus summarised by
Minchin :—
“I, The larva is composed of three classes of cell-elements: (1)
Columnar flagellated cells, forming the outer covering or localised at
CLASSIFICATION. 133
the anterior pole ; (2) rounded, more or less amoeboid elements, rarely
flagellated, forming the inner mass or aggregated at the posterior
pole ; and (3) the archzeocytes, usually scattered in the inner mass,
and often represented by undifferentiated blastomeres... .
“TI, The larva fixes and undergoes a metamorphosis whereby the
flagellated cells become placed in the interior, while the cells of the
inner mass come to surround them completely.
“JIT. (1) The flagellated cells of the larva become the choanocytes
of the adult (gastral layer), acquiring a collar; . . . (2) the inner mass
_gives rise to the dermal layer in its entirety; . . . (3) the archzeocytes
become the wandering cells of the adult, from which the reproductive
cells arise.”
It is interesting to note that the primitive germ-cells are early set
apart.
Classification. __
Class I.—Catcarea. With skeleton of calcareous spicules :—
Grade I.—Homoccela. — Continuous’ internal layer of collared
flagellate cells, e.g. Ascetta, Leucosolenia.
Grade II.—Heteroccela.—Collared flagellate cells restricted to
radial tubes or chambers, ¢.g. Sycou (Grantia),
Class II.—HEXACTINELLIDA, or Triaxonia, with sexradiate siliceous
spicules (triaxons). The members live chiefly in deep water,
e.g. Wenus Flower-Basket (Zzp/ectella) and the Glass-Rope
Sponge (Ayalonema).
Class III. —Demospongiz. Skeleton of siliceous spicules, but never
triaxons, or of spongin fibres, or of spongin fibres and siliceous
spicules, or absent.
Grade I.—Tetraxonida, typically with tetraxon spicules, e.g.
Pachymatisma, Tetzlla.
Grade II.—Monaxonida, with monaxon spicules, sometimes with
spongin in addition, ¢.g. Mermaid’s Gloves (Cha/ina oculata),
Crumb-of-Bread Sponge (Halzchondria or Amorphina pantcea),
Fresh-Water Sponge (Spongzl/a).
Grade III.—Ceratosa, ‘‘ horny” sponges with or without spicules,
e.g. the Bath-Sponge (Zuspongia).
Grade IV.—Myxospongida, without any skeleton, eg. Halisarca
and Oscarella.
A very remarkable form called /er/za seems to have both a siliceous
and a calcareous skeleton.
History.—Sponges, as one would expect, date back almost to the
beginning of the geological record. Thus the siliceous Protospongia
occurs in Cambrian rocks, and in the next series—the Silurian—the
main groups are already represented. From that time till now they
have continued to abound and vary.
The division between calcareous and siliceous sponges goes deep
down to the very roots of the phylum, and the siliceous branch must
have divided very early into Triaxonia and Tetraxonia.
Ccology.— Sponges are living thickets in which many
small animals play hide-and-seek. Many of the associa-
134 PHYLUM PORIFERA—SPONGES.
tions are harmless, but some burrowing worms do the
sponges much damage. The spicules and a frequently
strong taste or odour doubtless save sponges from being
more molested than they are; the numerous phagocytes
wage successful war with intruding micro-organisms. Some
sponges, such as C/iona on oyster-shells, are borers, and
others smother forms of life as passive as themselves.
Several crabs, such as Dromia, are masked by growths of
sponge on their shells, and the free transport is doubtless
advantageous to the sponge till the crab casts its shell.
A compact orange-coloured sponge (Suderites domuncula)
of peculiar odour often grows round a whelk-shell tenanted
by a hermit-crab, and gradually dissolves the shell-substance.
Within several sponges minute Algz live, like the “yellow
cells” of Radiolarians, in mutual partnership or symbiosis.
One of the cuttlefishes, Ross¢a glaucopis, puts its eggs care-
fully into pockets in the substance of a siliceous sponge.
Finally, sponges deserve mention as factors in human
civilisation.
General zoological position. — Sponges form the first
successful class of Metazoa. They illustrate the beginnings
of a “body,” and the beginnings of tissues. Along with
the Ccelentera, they differ markedly from the triploblastic,
Ccelomate Metazoa, which do not retain the radial
symmetry of the gastrula. In their germinal layers and
in their internal cavity they differ so much from Ccelentera
and all other Metazoa, that they must be regarded as on
a by-road of evolution. This has been emphasised by
Professor Ray Lankester in the term ‘‘ Parazoa” ; he speaks
of them as a sterile stock.
Their origin is wrapped in obscurity; it may be that
they are the non-progressive descendants of primitive
gastrula-like ancestors with a sluggish constitution. The
presence of choanocytes suggests a relationship with certain
of the flagellate Protozoa (Choanoflagellata), and Protero-
spongia (Fig. 55) may possibly be regarded as a connecting
link.
InceRT& SEpDis. MEsozoA
The title Mesozoa was applied by Van Beneden to some simple
orginisms which appear to occupy a very humble position in the
.MESOZOA. 135
Metazoan series. He regarded them as intermediate between Protozoa
and Metazoa; but others have remarked on their resemblance to
Platyhelminthes, and especially to the sporocysts of certain Flukes.
They may perhaps be regarded as precociously reproductive sporocysts.
It will be enough here merely to notice four types :—
1. Dicyemidz (type Dzcyema) occur as parasites in Cephalopods ;
Fic. 63.—A. Young Décyema.— Fic. 64.—Salinella. —
After Whitman. 3B. Female After Frenzel.
Orthonectid (Ahopalura giar- Pee en eee
dzz).—-After Julin. = stron e ‘ pape
terior.
¢., Ectoderm ; e7., inner endoderm cell
with nucleus (z.); and embryo (e77.).
Note the segmentation and the
fibrillation supposed to be muscular.
z. Transverse section.
the body, consists of a ciliated outer layer, enclosing a single multi-
nucleate inner cell, within which egg-like germs develop, apparently
without fertilisation, into dimorphic embryos (see Fig. 63, A).
2. Orthonectide (type RAopalura) occur as parasites in Turbellarians,
Brittle-stars, and Nemerteans; the body is slightly ringed, and con-
sists of a ciliated outer layer, a subjacent sheath of contractile fibres,
and an internal mass of cells, among which ova and spermatozoa
appear. ‘The sexes are separate and dimorphic (see Fig. 63, B).
*
\
136 MESOZOA.
3. Professor F. E. Schulze discovered a small marine organism —
Trichoplax adherens—in the form of a thin, ¢hvee-layered, externally
ciliated plate ; and Monticelli records a similar form under the title
Treptoplax adherens. But Trichoplax is now said to be the planula
of the Hydromedusan Eleuthera,
4. Professor J. Frenzel discovered. in brine solutions a minute
Turbellarian-like organism—Salénella salve—whose body consists of
one layer of cells (Fig. 64). There is an anterior mouth, a ciliated
food canal, and a posterior anus. The ventral surface is finely ciliated,
the other cells bear short bristles. The animal reproduces by trans-
verse fission, but conjugation and encystation also occur.
It must be confessed that some corroborative evidence in regard to
this peculiarly simple animal is much to be desired.
CHAPTER IX
PHYLUM CQ@Q:LENTERA
Class 1. HYDROZOA. Class 3. ANTHOZOA or
Hydroids and ACTINOZOA.
Medusoids. Sea-anemones,
Class 2. SCYPHOMEDUS or Madrepore-corals,
ACRASPEDA. Alcyonarians, etc.
Jelly-fishes. Class 4. CTENOPHORA.
Tue Ccelentera—including zoophytes, swimming-bells, jelly-
fish, sea-anemones, Alcyoriarians, corals, and the like—form
a very large series of Accelomate Metazoa, 7.e. multicellular
animals without a body cavity. Their simplest forms are
not much above the level of the simplest sponges, but the
series has been more progressive. Thus many illustrate
the beginnings of definite organs. In their variety they
seem almost to exhaust the possibilities of radial symmetry,
and some types (¢g. Ctenophora) may be regarded as
pioneers of the yet more progressive bilateral ‘“ worms.”
Many are very vegetative, deserving the old name of
zoophytes (which should rather be read backwards —
Phytozoa), and in their budded colonies afford interesting
illustrations of co-operation and division of labour. With
the exception of three or four fresh-water forms like Hydra,
all are marine.
GENERAL CHARACTERS
The Calentera are almost always radially symmetrical
animals in which the primary long axis of the gastrula
becomes the long axis of the adult. There is no body cavity,
or ceelom, distinct from the digestive cavity (enteron) and its
outgrowths. In the lower members of the phylum, the
138 PHYLUM CELENTERA.
primary opening of this cavity becomes the mouth of the adult,
but in the more specialised types there ts an (ectodermic) oral
invagination, which forms a gullet-tube or stomodeum.
Between the ectoderm and endoderm of the body wall there
7s a supporting layer, or mesoglea, often of jelly-like con-
sistency, In Ctenophora, however, a more definite mesoderm
zs established at an early stage in development. In the
simplest cases the mesoglea is a secretion quite devoid of cells,
but secondary cells may migrate into tt from the endoderm.
Stinging cells of varying complexity are almost always present,
but in most of the Ctenophora their place ts taken by adhesive
cells,
The Celentera exhibit two types of structure—polypotd
and medusoid—which recur in modified forms throughout the
group, and may be both present in the course of one life
history, when they illustrate the phenomenon of alternation of
generations, or metagenesis. The more primitive type is the
sessile tubular polyp, which, at its simplest, may be com-
pared toa gastrula fixed by one end, and furnished with a
crown of tentacles round the central aperture of the other pole.
The other derived form, which has. become specialised in
various directions, 1s the active medusoid or jelly-fish type.
In several divisions the formation of a calcareous “ skeleton”
by the polypoid type results in the production of “corals.”
Multiplication by budding ts common, and often results in the
JSormation of colonies, some of which show considerable adivt-
sion of labour.
The preservation of the primary axts, the absence of true
mesoderm and of a ceelom, are often said to distinguish
Calentera and Sponges from the other Metazoa (Celomata),
but the results of recent researches on the nature of the
mesoderm seem to rob this distinction of part of its precision.
GENERAL SURVEY
The Ccelentera or ‘Stinging animals” include a large
number of familiar and beautiful forms. The graceful
zoophytes which fringe shells and stones, and the tiny
transparent bells which float in the pools ; the sea-anemones
which cluster in the nooks of the rocks, and the active jelly-
fish which swim on the waves, are but different expressions
GENERAL SURVEY. 139
of the antithesis between sedentary polypoid and active
medusoid types which is characteristic of the phylum. The
delicate iridescent globes, which represent the class
Ctenophora, illustrate the climax of activity, and have no
hint of a sedentary phase. :
In our preliminary survey of the series, we may begin
with the little fresh-water Hydra (Fig. 68), which is often
Ne
Fic. 65.—Wiagram of Ccelenterate structure, endoderm
darker throughout.
1. To left, shows longitudinal section of Hydra; to right, of
sea-anemone, g., gut; g., incipient gullet.
z. To left, shows cross-section of Hydra; to right, of sea-
anemone, in the region of the gullet.
3 To left, shows vertical section of Craspedote Medusoid
(with velum); to right, of Acraspedote Medusa, with-
out velum. g., gut; gZ, gullet.
Note anatomical correspondence of the polypoid and medu-
soid forms.
to be found attached to the stems and leaves of water
plants. The structure here is extremely simple, but the
simplicity is probably due to degeneration. In favourable
conditions the polyp may give off daughter buds, which
remain for a time attached to the parent, and then separate
as independent polyps. The bud itself, before leaving
the parent, may also bud, so that three generations are
present. If we picture this process of gemmation, but with
140 PHYLUM C@LENTERA.,
imperfect separation of the units, continued indefinitely, we
can understand the formation of hydroid colonies, such as
the zoophytes. In such cases the colony is usually sup-
ported by an organic sheath (Jerisarc) of varying complexity.
But the members of such a colony do not usually remain
similar and equivalent. In Mydractinia, for example, which
often grows on a Gastropod shell tenanted by a hermit-
crab, the colony consists of polyps of varied structure and
function. Some of the polyps are nutritive “persons,”
like Hydra in appearance ; some are reproductive “ persons,”
with rudimentary tentacles, with or without a mouth; others
Fic. 66.—Colony ot Hydractenza on back of a Buccinum
shell tenanted by a hermit-crab.
are long, slender, mobile, sensitive, ften abundantly fur-
nished with stinging cells; while he little protecting
spines at the base of the colony may perhaps be abortive
“persons.” All these polyps are united by connecting
canals at the base. Thus Aydractinia exhibits polymorphism
among the members of the colony, and a tendency towards
more or less division of labour is common in the Ccelentera.
In most hydroid colonies the division of labour only
amounts to dimorphism; there are reproductive “ persons,”
different from the ordinary polyps. These are in many
cases sessile and mouthless, or they may after a time
GENERAL SURVEY. 141
become detached and float away as delicate, pulsating
swimming-bells. These swimming-bells are male and
female, they give rise to male and female elements, and so
to embryos, which, after a time, settle down and form new
zoophyte colonies. This is an instance of alternation of
generations.
_ Again, just as the predominance of passivity is exhibited
in Aydractinia and some zoophytes, where the active:
swimming -bell stage is
left out of the life history,
so the predominance of
activity is exhibited in
the permanent medus-
oids, e.g. Geryonia, where
the sedentary hydroid
stage is omitted, and the
embryo becomes at once
medusoid. Finally, the
medusoids _ themselves
may become colonial,
and we have active float-
ing colonies, like those
of the Portuguese man-
of-war, which show, on a
different plane, as much
polymorphism as Aydrac-
asi Fic. 67.—Diagram of a typical
The same general con- Hydrozoon polyp.—After Allman.
clusions apply tothe jelly- 5c, Ectoderm; £W., endoderm; C., the
fish and sea-anemones. cavity of the gut (coelenteron); G., a re-
The jellyfish present a prosuctive tnd: 7g tenadies #4. hype
strong resemblance to
the medusoids, but are distinguished from them by their
usually greater size, as well as by greater complexity and
several anatomical differences. It is in accordance with
this increased complexity that the alternation of active and
passive forms, though as real, is less obvious. But even
here we find one type (Zéedagia) always locomotor, another
(Auzelia) whose early life is sedentary, and others (Lu-
cernarians) which in their adult life are predominantly
passive, and attach themselves by a stalk.
142 PHYLUM C@LENTERA.
The sea-anemones and their numerous allies may be
regarded as bearing a relation to the jelly-fish, somewhat
similar to that which the hydroid polyps bear to the
swimming-bells (Fig. 65). They are, however, much more
complicated in structure than the hydroids. Solitary forms
are much commoner than in the hydroids, but the colonial
type is nevertheless very frequent. The colonies may be
supported by an organic framework only, but very commonly:
there is a tendency to accumulate lime in the tissues, which
results in the formation of “corals.” It should be noted,
however, that various quite distinct polypoid types may
form “corals.” Thus, while the most important reef-building
corals are included in the Anthozoa, the Millepore-corals
are hydroids.
Finally, as the corals are predominantly passive, so there
is a climax of activity in the Ctenophores, which move by
cilia united into combs, and often shine with that ‘ phos-
phorescence” which is an expression of the intensity of life
in many active animals.
As to diet, many of the larger forms, ¢.g. sea-anemones
and jelly-fish, are able to engulf booty of considerable size ;
the active Ctenophores are carnivorous, attaching them-
selves by adhesive cells to one another, or to other small
animals; but most Ccelentera feed on small organisms, in
seizing and killing which the tentacles and stinging cells
are actively used.
Stinging cells or cnidoblasts are so characteristic of Ccelentera that
they deserve particular notice. They occur, in all Coelentera except
the Ctenophores, and even there they have been detected in Zuchlora
rubra. They also occur in some Turbellarian worms, and in the
papillz of A®olid nudibranchs amongst molluscs; but it has been
shown that these animals obtain their nematocysts from the Ccelentera
on which they feed. Each cnidoblast contains a capsule or nemato-
cyst, which encloses a coiled lasso lying in an irritant gelatinous
substance. The nematocyst fills most of the cell, but there is a nucleus,
etc., besides. At the distal end there may be a trigger-like cnidocil or
a fringe of bristles, etc. At the proximal end there may be fixing
processes. In some Anthozoa the coiled lasso is simply ruptured out,
but in most cases it is evaginated. The basal part of the lasso is
often stronger than the rest, and may bear stilets; spirally arranged
roughnesses and bristles are also frequent on the thread itself. The
explosion of the cnidoblast is believed to be due to an entrance of
water, which causes the gelatinous substance to swell up. According
to others, the cnidoblast contracts as a whole. The action of the
TYPES OF CELENTERA—HYDRA. 143
threads is mechanical and chemical. They fix, e.g. by the stilets, into
the victim, and the secretion poisons the wound, paralysing or killing
small animals, and sometimes acting as a solvent. Many seem to be
prehensile threads rather than weapons,
.TyPES OF C@LENTERA
first Type—Hynra, a simple representative of the
Class Hyprozoa
General life. —The genus 7ydra—cosmopolitan, like many
other small fresh-water animals—is represented by several
species, e.g. the green Hydra viridis, the brownish H. oligactis
or fusca, and the orange ZH. vulgarts
or grisea, widely distributed in fresh
water. They are among the simplest
of Ccelentera, for the body is but a
two-layered tube, with a crown of
(6-10) hollow tentacles around the
mouth, and with no organs except
those concerned in reproduction.
The body is usually fixed by its base
to some aquatic plant, often to the l
lower surface of a duckweed. It @ff
may measure 4~4 inch in length, but
it is as thin as a needle, and contracts ue Prien Saree
into a minute knob. — After Greene,
The animal sways its. body and
tentacles in the water, and it can also :
loosen its base, lift itself up by its tentacles, stand on
its head, or creep by looping movements. According
to some observers, its movements are helped by fine
pointed pseudopodia protruded from the ectoderm cells
of the tentacles and base, and by threads ejected from
large cylindrical stinging cells. Usually, however, the Hydra
prefers a quiet life. It feeds on small animals, which are
paralysed or killed by stinging cells on the tentacles, and
are swept into the tubular cavity of the body by the action
of flagella on the internal cells. Sometimes animals as
large as water-fleas (e.g. Daphnia) are caught, and the
ffydra may sometimes be seen struggling fiercely with
a small Annelid worm (Zwudzfex). Tpfusorians (Euplotes,
ov., Ovary; Z., testes.
144 PHYLUM C@LENTERA.
etc.) are often seen wandering to and fro on the surface
of the Hydra, but these wonted visitors do not provoke the
stinging cells to action.
So simple is Hydra, that a cut-off fragment may grow
into an entire animal. Thus the Hydra may be multiplied
by being cut in pieces. The two conditions of a fragment
regenerating a whole are—(1) that the fragment be not too
small, and (2) that it bea fair sample of the various kinds
of cells in the body. Thus neither a little corner off the
base nor the tip of a tentacle will grow into a new Aydra.
If the animal be turned inside out (a delicate operation),
the status guo is soon restored. The Abbé Trembley, who
first made this experiment, thought that the out-turned
endoderm assumed the characters of the ectoderm, and
that the inturned ectoderm assumed the characters of
endoderm. But this is not the case. Either the animal
rapidly rights itself by turning outside in, or,. if this be
prevented, the inturned ectoderm disappears internally,
and by growing over the out-turned endoderm, from the
lips downwards, restores the normal state.
In favourable nutritive conditions, the Hydra forms buds,
and on these a second generation of buds may be developed.
A check to nutrition or some other influence causes the
buds to be set ‘adrift. Sometimes a Aydra divides across
the middle, and each half grows into a complete polyp in
afew days. Besides these asexual modes of multiplication,
the usual sexual reproduction occurs. ;
General structure.—The tubular body consists of two
layers of cells, ze. the animal is diploblastic. The cavity
is the gut, and it is continued into the hollow tentacles.
These, when fully extended, may be much longer than the
body. The mouth is slightly raised on a disc or hypostome.
Of the two layers of cells, the outer or ectoderm is trans-
parent, the inner or endoderm usually contains abundant
pigment. On the tentacles especially, even with low power,
one can see numerous clumps of clear stinging cells. The
male organs appear as ectodermic protuberances a short
distance below the bases of the tentacles; the ovary, with
a single ovum, is a larger bulging farther down. Both male
and female organs may occur on the same animal, either
at one time or at different times, but often they occur on
TYPES OF C@LENTERA—AHVDRA. 145
different individuals. Abundant food favours the develop-
ment of female forms ; when food is scarce males are more
abundant. The buds have the same structure as the parent
body ; in origin they appear to be mainly due to multiplica-
tion of interstitial cells.
Minute structure.—The outer layer or ectoderm includes the
following different kinds of cells :—
that
b
Fic. 69.—Minute structure of Hydra.—After T. J. Parker and Jickeli.
A. Ect., ectoderm ; #g., mesogleeal plate 5 s¢.c. stinging cell; Zxd., endo-
derm with flagella and amceboid processes.
B. 2.c., nerve cell, and st.c., stinging cell.
C. Stinging cell with ejected thread; ., nucleus.
D. Mesoglceal plate (#g.) with contractile roots resting on it.
E. m.c., muscular cell with contractile roots, ¢c.7.
(1) Large covering or epithelial cells, within or between some of
which lie the stinging cells. The epithelial cells are somewhat conical,
broader externally than internally, and in the interspaces lie interstitial
cells. By certain methods, a thin shred can be peeled off the external
surface of the ectoderm cells. This is a cztzcle, z.e. a pellicle no
longer living, produced by the underlying cells.
(Ia) Many of these large cells have contractile basal processes, or
roots, running parallel to the long axis of the body, and lying on a
10
146 PHYLUM CELENTERA.
middle lamina which separates ectoderm from endoderm (Fig. 69, E).
The cells themselves are contractile, but there is special contractility
in the roots. Like the muscle cells of higher animals, they contract
under certain stimuli, and are often called ‘‘neuro-muscular.” But the
presence of special nerve cells shows that even in Aydra there is a
differentiation of the two functions of contractility and irritability.
(2) Stinging cells or cnidoblasts occur abundantly on the upper parts
of the body, especially on the tentacles. Each contains a protrusible
nematocyst. This consists of a sac, the neck of which is doubled in
as a pouch, usually bearing internal barbs, and prolonged into a long,
hollow, spirally coiled filament or lasso. This lasso is bathed in a
fluid, presumably poisonous. On its free surface the stinging cell usually
bears a delicate trigger hair or cnidocil. Under stimulus, whether
directly from the outside or from a nerve cell, the cnidoblast explodes
and the nematocyst is thrown out. With the help of the barbs they
penetrate through even a chitinous membrane, and the secreted fluid
has a solvent action. The victim is held fast and drawn closer.
Besides the ordinary stinging cells, there are others of small size which
coil into a spiral after explosion.
(3) There is to the inner aspect of the covering cells a network of
ganglion cells and nerve processes. More superficially there are
minute sensory cells, some of them connected by fine fibres with the
ganglion cells.
(4) Small interstitial or indifferent units fill up chinks in the ecto-
derm, and seem to grow into reproductive, stinging, and other cells.
(5) Granular glandular cells on the basal disc or ‘‘ foot” probably
secrete a glutinous substance. They are also said to put out pseudo-
podia, and so move the animal slowly.
The endoderm is less varied. Its cells are pigmented, often
vacuolated, and most of them are either flagellate or amceboid. The
pigment bodies in A. wirédzs are like the chlorophyll corpuscles of
plants ; it seems almost certain that they are unicellular Algae. When
a green Hydra liberates an egg while kept in the dark, that egg gives
rise to a white Hydra, which is supposed to imply that the partner
Algze do not migrate into the egg when there is no light. In the other
species of Hydra, the pigment is quite different from chlorophyll.
The active lashing of the flagella causes currents which waft food in
and waste out. If some small animal, stung by the tentacles, is thus
wafted in, it may be directly engulfed by the amceboid processes of
some of the cells, and it has been noticed that the same cell may be at
one time flagellate and at another time amceboid (cf. the cell-cycle,
p- 107). After this direct absorption the food is digested within the
cells, and while some of the dark granules seen in those cells may be
decomposed pigment bodies, others seem to be particles of indigestible
débris. Thus Aydra illustrates what is called intracellular digestion,
such as occurs in Sponges, some other Ccelentera, and some simple
“worms.” But experiments show that some of the food may be
digested in the gut cavity, and subsequently absorbed. Thus it seems
that both intracellular and extracellular digestion occur.
Some of the endoderm cells have muscular roots like those of the
ectoderm. They lie on the inner side of the middle lamina, in a trans-
TYPES OF C@&LENTERA—AVDRA. 147
verse or circular direction. A few cells near the mouth and base are
described as glandular, and the presence of a few stinging cells has
been recorded, though some suggest that the last are discharged ecto-
dermic nematocysts which have been swallowed.
The middle lamina, representing the mesogloea, is a thin homogene-
ous plate, bearing on its outer and inner surfaces the muscular roots of
ectodermic and endodermic cells (Fig. 69, D).
It is historically interesting to notice the important step which was
made when, in 1849, Huxley definitely compared the outer and inner
layers of the Coelentera with the epiblast and hypoblast which embry-
ologists were beginning to demonstrate in the development of higher
animals, Not long afterwards, Allman applied to the two layers of
hydroids the terms ectoderm and endoderm.
Tie division of labour among the cells of Hydra is not very strict,
but already the essential characteristics of ectoderm and endoderm are
evident. We may summarise these as follows, comparing them with
the characteristics of epiblast and hypoblast in higher animals :—
OutTER LAYER.
MrippLe Layer.
INNER Laver.
In Hydra the ectoderm
forms— ,
Covering cells, stinging
cells, nerve cells, muscle
cells, etc.
None in Hydra, apart
from the middle lamella.
In Hydra the endoderm
forms—
Digestive cells lining
the food canal, and also
muscle cells, etc.
The embryonic epiblast
of higher animals grows
into epidermis, nervous
system, and essential parts
The mesoblast of higher
animals becomes muscu-
lar, connective, and skele-
tal tissue.
The embryonic hypo-
blast of higher animals
always lines the digestive
part of the food canal.
of sense organs.
The reproductive organs.—(a) From nests of repeatedly dividing
interstitial cells, several (I-20) simple male organs or testes are formed.
Each consists merely of a clump of male elements or spermatozoa,
bounded by the distended ectoderm. Through this the spermatozoa
are extruded at intervals, and one may fertilise the ovum of the Aydra,
In other words, self-fertilisation, which is very rare among animals,
may occur. The spermatozoon is a motile cell, with a minute cylin-
drical ‘‘head” consisting of nucleus, a more minute middle-piece, and
a long thread-like vibratile tail (Fig. 70, 1).
(4) Usually there is but one female organ or ovary, but in A. fusca
as many as eight have sometimes been observed. The ovary arises, like
the testes, from a nest of interstitial cells, in the centre of which, distinct
from the start, the single ovum lies. In rare cases in A. viridis,
H. fusca, and H. grisea there are two ova; in . diecéa there may be
several.
Development.—The ovum of Hydra is the successful central cell
in the ovary. It is at first amceboid, and becomes more and more
rich at the expense of its neighbours. Their remains (perhaps nuclei)
148 PHYLUM C@LENTERA.
accumulate within the ovum as “yolk spherules” or ‘‘ pseudo-cells.”
Some yolk-granules, formed within the ovum, may coalesce in ‘‘ pseudo-
cells” of another type. With increase of size the ovum changes its
form from amceboid to cake-like, and from that to spherical. Around
the spherical ovum a gelatinous sheath is formed. When the limit of
growth is reached, the nucleus or germinal vesicle divides twice, and
two polar bodies are extruded at the distal pole. There are twelve
chromosomes to begin with, and by the reduction division in forming the
first polar body, the number is reduced to six. Thereafter the ectoderm
of the parent Aydra yields to the increasing strain put upon it, and
Fic. 70.—Development of Aydra.—After Brauer.
1. Sf., spermatozoa. 3
2. Amceboid ovum; g.v., germinal vesicle or nucleus; y.s., yolk
spherules.
3. Ovum with lobed envelope (sz.) around it.
4. Ovum protruding ; ., the nucleus ; ecf., the ruptured ectoderm ;
end., the endoderm.
5. Section of blastosphere—Zct., ectoderm;. Znd., endoderm—
being formed.
6. Section of larva. Zct., ectoderm; Exd., endoderm; g.c., gut
cavity: sk., ruptured envelopes.
ruptures, allowing the ovum to protrude. By abroad base it still remains,
however, attached to the parent, and in this state it is fertilised, the
spermatozoon entering by the distal pole (Fig. 70, 4).
The segmentation which follows is total and equal, and results in
the formation of a blastosphere (Fig. 70, 5). By inwandering, or by
division of the cells of the blastosphere, an internal endoderm is formed,
and this formation takes place on all sides. In a word, it is multipolar.
The segmentation cavity of the blastosphere is thus filled up, and the
two layers become differentiated from one another,
The outer or ectodermic layer forms—(a) an external “ chitinoid ”
shell of several layers; (4) an internal membrane, homogeneous, thin,
TYPES OF C@LENTERA—A MEDUSOID. 149
and elastic ; and (c) the future ectoderm of the adult. In Aydra fusca
the egg is separated from the parent before the shell is formed, and is
fastened by its gelatinous sheath to aquatic plants; in A. werddés and
1. grisea the egg falls off after the outer shell has been formed. In
all species the separation from the parent appears to be followed by a
period of quiescence lastirfg from one to two months. It is probable
that this resting-stage is carried by wind and birds from one water basin
to another.
Within the shell differentiation at length recommences, but it pro-
ceeds slowly. Interstitial cells arise in the ectoderm; a middle
lamella is formed ; a gastric cavity begins to appear in the midst of the
endoderm. Thereafter the shell bursts, and development proceeds
more rapidly. The embryo elongates, acquires a mouth by rupture at
the distal (sometimes called vegetative) pole. The inner sheath is also
lost, and the young “ydra fixes itself and begins to live as its parent or
parents did.
Forms like Hydra.—Even simpler than Hydra is Protohydra,
without tentacles, occurring both in the sea and in fresh water. An
American fresh-water form (Mécrohydra ryder?) is known to liberate
free-swimming medusoids. A fresh-water Medusoid Limnocodium was
found in the Victoria Regia tanks in the Botanic Gardens, Regent’s.
Park, London. Its native habitat is unknown. Another species,
L. kawaiz, has been found in the Jantszekiang in China, 1000 miles
from its mouth, A related form, Limnocndda, occurs in Lakes Tangan-
yika and Victoria Nyanza, and in the river Niger. A strange simple
polype—Lolypodium—has been found as a parasite on the eggs of
sturgeons. Further details in regard to all these forms are much
wanted,
Second Type of CELENTERA.—A Medusoid.
Class HypRozoa
Hydra is too simple to be thoroughly typical of the
Hydrozoa. The class includes the hydroid colonies or
zoophytes, which may be compared to Aydre with many
buds, and also free medusoid forms, which may be (a)
liberated members of a hydroid colony, or (4) independent
organisms. Besides these there are complex colonies of
medusoid forms (Siphonophora).
The hydroid type, except in minor details, usually
resembles Hydra. In some cases the tentacles are solid,
instead of hollow as in Hydra, and they may be arranged in
two circles,—an outer and an inner (¢.g. Zudularia). In
some of the hydroid colonies, notably the Millepores and
Aydractinia, the polyps are very dissimilar to one another,
and have become specialised for the performance of different
functions.
150 PHYLUM C@LENTERA.
The medusoid type is like an inflated hydroid adapted
for swimming. It is bell-shaped, and down the middle of
the bell hangs a prolongation—the manubrium—which
terminates in the mouth. Around the margin of the bell
there is a little shelf, the velum or craspedon, which projects
inwards, and is furnished with muscle cells. The margin of
Fic. 71.—Bougainvillea.—After Allman.
A. Asmall piece of a hydroid colony.
p-, Perisarc ; #., medusoid bud; 4., hydranth or polyp head.
B. A medusoid ; #a., manubrium; ~.c., radial canal; s., sense-
organ.
the bell also bears tentacles, usually hollow, and abundantly
furnished with stinging cells (Fig. 65, 3).
On the convex surface of the bell the ectoderm forms
simply an epithelial layer; on the concave surface it is
differentiated into muscle cells on the velum, the manu-
brium, and the tentacles, nerve cells at the base of the
velum, and stinging cells on the tentacles. The endoderm
is ciliated ; it lines the food canal, and extends also into the
TYPES OF C@&LENTERA—A MEDUSOID. 151
tentacles. The mesogloea forms a thickened jelly, present
more especially on the convex (ex-umbrellar) surface.
The mouth opens into the canal of the manubrium, which
leads to the central cavity of the dome. With this a varying
number of unbranched radial canals communicate; these
open into a marginal circular vessel, which communicates
with the cavities of the tentacles. A plate of endoderm lies
in the mesogloea between the radial canals. Digestion is
intracellular, and probabiy goes
on throughout the whole of this
‘gastro-vascular” system.
The movements of the bell
are caused by the contractions
of the ectodermic muscle cells.
The nervous system consists
of a double ring of nerve fibres
around the margin of the bell.
With these are associated gang-
lionic cells, which apparently
control the muscular contrac-
tions. Sete rr
IG. 72.— structure of a
a miter bet ot ey eee M alse —After ee
base of the tentacles (Ocellatz), Ae T ernen ee. coded
or in the form of “auditory” ference canal; G., gonad; &.C.,
. ix ahe radial canal; 4WV., endoderm;
vesicles developed as pits in the | ZC., ectoderm; 4/G’, mesoglea.
velum (Vesiculatz).
The reproductive organs develop either in the manu-
brium or on the radial canals. The products always (?)
ripen in the ectoderm, and often seem to arise there; but
Weismann and others have shown that the reproductive
cells of a medusoid derived from a hydroid, or of the
reduced and fixed reproductive persons of many hydroids,
have considerable powers of migration, and may originate
(sometimes in the endoderm) in the hydroid colony at
some distance from the place where they are matured within
the medusoid bud. The sexes are usually separate. The
commonest kind of free-swimming larva is the planula, which
is oval, ciliated, and diploblastic, devoid of an opening, and
usually without a central cavity. In the case of those
medusoids which arise as liberated sexual members of
152 PHYLUM C@LENTERA.
a fixed asexual hydroid colony, the planula settles down,
loses its cilia, buds out tentacles, and develops into a new
hydroid. ;
In many hydroid colonies, as has been already noticed,
the sexual members are not set free, but remain as buds
attached to the parent. These fixed “gonophores” show
many stages of degeneration ; some, notably in the floating
colonies of Siphonophora, differ little structurally from true
medusoids, while others, as in Wydractinia, are simply small
closed sacs enclosing the genital products (Fig. 87).
Third Type of CELENTERA.—The common Jelly-fish
—Aurelia aurita. Class SCVPHOMEDUSZ
This Medusa is almost cosmopolitan, and in the summer
months occurs abundantly around the British coasts. It
swims by pulsating its disc, and also drifts along at rest
without any pulsations. They often occur in great shoals,
and hundreds may be seen stranded on a small area of flat
sandy beach. The glassy disc usually measures about four
inches in diameter, but may be twice as large. The jelly-
fish feeds on small animals, such as copepod crustaceans,
which are entangled and stung to death by the long lips.
External appearance.—The animal consists of a gela-
tinous disc, slightly convex on its upper (ex-umbrellar)
surface, and bearing on the centre of the other (sub-
umbrellar) surface a four-cornered mouth, with four long
much-frilled lips. The circumference of the disc is fringed
by numerous short hollow tentacles, by little lappets, and
by a continuation of the sub-umbrella forming a delicate
flap or velarium. Conspicuously bright are the four re-
productive organs, which lie towards the under surface.
Nor is it difficult to see the numerous canals which
radiate from the central stomach across the disc, the eight
marginal sense organs, and the muscle strands on the lower
surface (Fig. 73).
The three layers.—The ectoderm which covers the
external surface bears stinging cells, sensory and nerve cells,
and muscle cells. The ectoderm seems also to be invagin-
ated to form the gullet or stomodeum. The endoderm
lines the digestive cavity, is continued out into its radiating
TYPES OF C@LENTERA—AURELIA AURITA. 153
canals, and is ciliated throughout. The mesogloea is a
gelatinous coagulation containing wandering amoeboid cells
from the endoderm. The whole animal is very watery ;
indeed, the solid parts amount to not more than 10 per
cent. of the total weight. Yet some jelly-fish (species of
Rhopilema) are used as food in Japan!
Nervous system.—The nervous system consists—(a) of a
special area of nervous epithelium, associated with each of
the eight sense organs, and (4) of numerous much-elongated
bipolar ganglion cells lying beneath the epithelium on the
under surface of the disc. This condition should be con-
trasted with the double
nerve-ring in Craspedote
medusoids, but too much
must not be made of the
contrast, for a nerve-ring
is described in Cubo-
medusz, one of the orders
of Acraspedote jelly-fish.
In Aurelia the sense organs
are less differentiated than
in many other jelly-fish.
Each of the eight organs,
protected in a marginal
niche, consists of a pig- Fy, 73.—Surface view of Aureloa,—
mented spot, a club-shaped From Romanes.
projection with numerous Showing four genital pockets in centre,
6c ; m4 much branched radial canals, eight peri-
calcareous otoliths -_ pheral niches for sense organs, and peri-
its cells, and a couple of _ pheral tentacles.
apparently sensitive pits or
grooves. The sense organs arise as modifications of
tentacles, and are often called “‘tentaculocysts” or “rho-
palia.” Their cavities are in free communication with
branches of the radial canals.
Muscular system.—Between the plexus of nerve cells.
and the sub-umbrellar mesogloea there are cross-striped
muscle fibres, each of which has a large portion of non-
contractile cell substance attached to it. They lie in ring-
like bundles, and by their contractions the medusa moves.
Unstriped muscle fibres are found about the tentacles and
lips.
154 PHYLUM CQ@LENTERA.
Alimentary system.—The four corners of the mouth are
extended as four much-frilled lips, each with a ciliated
groove and stinging cells, and with an axis of mesoglcea.
They exhibit considerable mobility. Their crumpled and
mobile bases surround and almost conceal the mouth. A
short gullet or ‘‘manubrium” connects the mouth with the
digestive cavity in the centre of the disc. From this central
chamber sixteen gastro-vascular canals of approximately
equal calibre radiate to the circumference, where they open
into a circular canal, with which the hollow tentacles are
connected. Eight of the radial canals are straight, but the
other eight are branched, and thus in an adult Aurelia the
total number of canals is large. These canals are really due
to a partial obliteration of the gastric cavity by a fusion of
its ex-umbrellar and sub-umbrellar walls along definite lines.
They are all lined by ciliated endoderm.
Where the gullet passes into the central digestive cavity,
there are four strong pillars of thickened sub-umbrellar
material. Beside these pillars, there are four patches
where the sub-umbrellar surface remains thin. These are
the gastro-genital membranes, lined internally by germinal
epithelium (Fig. 74, &.).
To the inside of these genital organs, within the digestive
cavity, are four groups of mobile gastric filaments (g.f,, Fig.
74), which are very characteristic of jelly-fish. In appear-
ance these are very similar to the small tentacles of the
margin, and, like them, are hollow. ‘They are covered with
endoderm—with ciliated, glandular, muscular, and stinging
cells.
The body is mapped out into regions by the following convention :
The first tentacles to appear in the larva are four in number, and
correspond to the four angles of the mouth; the radii on which they
appear are called ‘‘perradial,” marked by the four lips. Half-way
between these, four ‘‘interradials” are then developed, marked by the
gonads and gastric filaments. Then eight ‘‘adradials” may follow,
between perradii and interradii, marked by the eight unbranched
radial canals.
Reproductive system.—The sexes are separate. The
reproductive organs—ovaries or testes—consist of plaited
ridges of germinal epithelium, situated on the four patches
already mentioned, within sacs which are derived from and
TYPES OF C@ELENTERA—AURELIA AURITA. 155
communicate with the floor of the gastric cavity. They
are of a reddish violet colour, and at first of a horseshoe
shape, with the closed part of the curve directed outwards.
Afterwards the ridges become circular, and surround the
walls of the sacs in which they lie. But the sub-umbrellar
surface is modified beneath each genital sac in such a way
that the sac comes to lie in a sub-genital cavity com-
municating with the exterior (g.4., Fig. 74). The con-
tractions of the umbrella produce a rhythmic movement of
the water which enters the sub-genital cavities, and this
constant renewal of the water suggests some respiratory
significance for the sacs. The genital sacs containing the
plaited ridges of germinal
epithelium communicate
with the gastric cavity
only, while the sub-genital
cavities containing water
and enveloping the geni-
tal sacs communicate with
the exterior only.
The ova and = sper- Fic. 74.—Vertical section of Aurelia,—
matozoa pass from the After Claus.
frills of germinal epi- mM, Mau st., stomach ; Tbs radial canal;
‘ ‘ +) Teproductive organs; g./., gastric
thelium into the sacs, filaments; g.Z., sub-genital cavity; 2,
and thence into the gas- "aging entices cs, sense organs
tric cavity. They find
exit by the mouth, but young embryos may be found
swimming in the gastro-vascular canals, and also within the
shelter of the long lips.
Variations.—The jelly-fish often exhibits variations, i.e.
inborn changes of germinal origin which result in the
organism being different from the norm or average of its
species. It is normally tetrapartite, but sexpartite, penta-
partite, and, more rarely, tripartite forms occur; and the
detailed variations are manifold.
Life history of Aurelia.—The fertilised ovum divides completely,
but not quite equally, to form a blastosphere, with a very narrow slit-like
cavity. From the larger-celled hemisphere, single cells migrate into
the cavity, and fill this up with a solid mass of endoderm. The
archenteron arises as a central cleft in this cell mass, and opens
to the exterior temporarily by the primitive mouth. During these
156 PHYLUM C@ELENTERA.
processes the embryo elongates, the outer cells become ciliated, and
the mouth closes. Thus the embryo becomes a free-swimming oval
planula,
After a short period of free life, this planula settles down on a
stone or seaweed, attaching itself by the pole where the mouth formerly
opened, Ata very early stage the mesoglcea appears between the two
layers. At the free pole an ectodermic invagination next occurs, an
opening breaks through at its lower end, and thus a gullet lined with
ectoderm is formed, which hangs freely in the general cavity. During
this process there are formed first two and then four diverticula of the
Fic. 75.—Diagram of life history of Aure/ia.—After
Haeckel.
1. Free-swimming embryo ; 2-6, various stages of Hydra-tuhba ;
7, 8, Strobila stage; 9, liberation of Ephyre; ro, 11,
growth of Ephyra into Meduse.
general cavity, which are arranged round the gullet above, and open
freely into the digestive cavity below. In the gullet region these are
separated by broad septa, which are continued into the lower region of
the body as four interradial ridges or teeniolee. The tentacles bud out
from the region of the mouth, the first four corresponding in position to
the four pouches. Interradially above the four septa, four narrow
funnel-shaped invaginations arise ; these are produced by the ingrowth
of ectoderm, which then forms the muscle fibres which run down the
teeniolze (contrast the ezdodermic muscles of Anthozoa). In contrasting
this development with that of the hydroid polyp, Goette specially
TYPES OF CELENTERA—AURELIA AURITA. 157
emphasises the fact that the radial symmetry is first indicated by the
gut pockets, and the tentacles are late in development. Goette
describes a quite similar process of development_in certain sea-
anemones, and claims to have found there rudiments of septal pockets
and ectodermal muscles, thus confirming his view of the intimate
relation between the Anthozoa and Scyphomedusz.
The larva now forms a ‘‘ Hydra-tuba” or ‘‘Scyphistoma”; it is
about an eighth of an inch in height. By lateral budding, or by the
formation of creeping stolons, it may givé rise to larve like itself.
The gradual widening of the central cavity renders the gullet tube
less obvious, and results in an increasing resemblance to the medusa
type.
In late autumn, however, a more fundamental change occurs in the
history of the Hydra-tuba. (a) Occasionally, as has been observed by
Haeckel, the Scyphistoma becomes detached and converted into a free-
swimming Ephyra, which in turn becomes a jelly-fish. (4) Sometimes,
in unfavourable conditions, 4 furrow appears round the upper region of
the Scyphistoma, the upper portion is converted into an Ephyra, and
floats away, while the lower portion re-forms its oral region by regenera-
tion, and produces another Ephyra. (c) In ordinary conditions the
Scyphistoma elongates, and displays a succession of annular constric-
tions. This stage, often compared to a pile of discs or saucers, is
called a Strobila. Each disc is separated off in its turn as a free-
swimming Ephyra, which becomes a jelly-fish. The still undivided
basal portion may rest for a time, and then undergo further con-
striction. This is probably an abbreviation of the primitive mode of
development,
In the conversion of the Scyphistoma into the Ephyre, the diverticula
coalesce into a general cavity, the entrances to the septal invaginations
probably persist as the sub-genital pits, the gastric filaments sprout out
from the remains of the septa, and so mark the place where the ecto-
dermal gullet passed into the endodermal cavity. :
The first Ephyra differs from those which come after it in bearing the
original tentacles of the Hydra-tuba. From its margin eight bifid lobes
grow out, each embracing the base of a perradial or interradial tentacle.
The bases of these eight tentacles become the sense organs or rhopalia.
The other eight adradial tentacles atrophy. On the Ephyre which
follow there are at first no tentacles, only the eight bifid marginal lobes
which bear the sense organs in their niches.
This development illustrates alternation of generations, From the
fertilised ovum a fixed asexual Scyphistoma results. This grows into a
Strobila, from which transverse buds or Ephyree are liberated. Each
of these grows into a sexual jelly-fish, producing ova or spermatozoa.
Relatives of Aurelia.—The Meduse, or true jelly-fish, include
forms which agree with the Anthozoa in relative complexity of
structure as compared with Hydrozoa, and in the possession of
an ectodermal gullet, but differ in possessing ectodermal septal
muscles and in some histological features. If Goette’s discovery of
rudimentary ectodermal muscles in the larve of certain sea-anemones
be confirmed, however, it would greatly increase the probability of
a close relationship between the two sets. Among the Scyphomedusz
158 PHYLUM CE@LENTERA.
closely allied to Auveléa some, e.g. Pelagéa, have a direct development
without the intervention of Scyphistoma or Strobila stages, but this
may occur exceptionally in Azrelia, Cyanea is often very large,
Fic. 76.—Lucernarta.—After Korotneff.
“‘it may measure 74 ft. across the bell, with tentacles 120 ft. long.”
Chrysaora is hermaphrodite, and has diffuse sperm sacs even upon
the arms. In the Rhizostome, eg. Cass¢opeca and Pilema, the
Fic. 77.—Diagram of Lucernaria.—
After Allman,
C., Cavity of gut (ccelenteron); #, gastric fila-
ments; /7., hypostome; G., gonad; 7., tentacle;
¢ ¢., circumference canal.
mouth is obliterated, and replaced by numerous small pores on the
four double arms. Lzcernaria and its allies are interesting sessile
forms which have been compared to sexual Scyphistomas, that is, are
regarded as persistently larval forms,
TYPES OF C@LENTERA—A SEA-ANEMONE.
159
Contrast between Medusoids (Hydromeduse) and
Medusa (Scyphomedusa)
Mepusorps. (CRASPEDOTA.)
Mepusm&. (ACRASPEDA.)
The majority are small ‘‘swimming-
”
A flap or velum (craspedon) projects in-
wards from the margin of the bell.
No teeniole, nor gastric filaments.
A double nerve-ring around the margin.
Naked sense organs either optic or audi-
tory. hey are usually derived
from the skin, but the auditory sacs
may be modified tentacles.
Reproductive organs on the radial canals
or by the side of the manubrium.
The reproductive cells are usually
derived from the ectoderm.
With the exception of the Trachy-
medusa, all arise as the liberated
reproductive persons of hydroid
colonies.
Many are large “ jelly-fish.”
No velum. (The velarium of Aurelia
is a mere fringe, very inconspicuous
in the adult, and not inturned.)
In the Scyphistoma there are four
teniole, from part of which the
gastric filaments of the adult grow.
Eight separate nervous centres be-
side the sense organs, and a sub-
umbrellar nervous plexus. :
Sense organs are modified tentacles,
-and probably have almost always
a triple function. They are usually
protected by a hood.
Reproductive organs in special pockets
on the floor of the gastric cavity.
The reproductive cells arise in the
endoderm.
Have no connection with hydroids, but
may have a small sedentary polyp
stage (or Scyphistoma) in the course
of thei life history.
Probably more nearly related to
Anthozoa than to Hydrozoa.
Fourth Type of CELENTERA.—A Sea-Anemone, such as
Tealta crassicornis. Class ANTHOZOA
Most sea-anemones live fixed to the rocks about low-
water mark. All these fixed forms have a distinct basal
disc, and may, like Zealia crassicornis, be half buried in
sand and gravel; others, without a basal disc, are loosely
inserted in the sand, e.g. Edwardsia and Certanthus. All
are able to shift their positions by short stages. Some
reef-anemones (Cvadactis) can crawl about on their
tentacles. They feed on small animals — molluscs,
crustaceans, worms—which are caught and stung by the
tentacles. Many» depend on minute organisms; others
may be seen trying to engulf molluscs decidedly too
large for them. A few anemones, without pigment or with
little, have symbiotic Algz in their endoderm cells; the
bright pigments of many others seem to help in respiration.
Besides the sexual reproduction (in which the young are
160 PHYLUM C@LENTERA.
sometimes developed within the parent), some sea-anemones
also multiply asexually by detaching portions from near the
base, and fission occurs in a few forms.
External appearance of a fixed Anemone. — The
cylindrical body is fixed by a broad base; it bears whorls
of hollow tentacles around the oral disc; the mouth is
usually a longitudinal slit. The tentacles are contracted
when the animal is irritated, and the whole body can be
much reduced in size. Just below the margin of the oral
disc there is a powerful sphincter muscle; this contracts,
Fic, 78.—External appearance of Zealia crassicornis.
and pulls together the body wall over the mouth and
retracted tentacles. Water may pass out gently or
otherwise by a pore at the tip of each tentacle, and long
white threads, richly covered with stinging cells, can be
ejected in many anemones through the walls of the body
(Fig. 79).
General structure.—- The Anthozoon polyp differs
markedly from the Hydroid polyp—not only because an
invagination from the oral disc inwards has formed a gullet
tube, which hangs down into the general cavity, but also
because a number of partitions or mesenteries extend from
the body wall towards this gullet. Some of the partitions
are “complete,” ze. they reach the gullet; others are “in-
TYPES OF C@LENTERA—A.SEA-ANEMONE. 161
complete,” ze. do not extend so far inwards. The complete
mesenteries are attached to the oral disc above, to the side
of the gullet, and to the base, and all the mesenteries are
ingrowths of the body wall. The cavity of the anemone
is thus divided into a number (some multiple of six) of.
radial chambers. These are in communication at the base,
so that food particles
from the gullet may pass
into any of the chambers
between the partitions.
Moreover, each partition
is perforated, not far from
the mouth, by a pore,
besides which there is
often another nearer the
body wall. The tentacles
are continuous with the
cavities between the mes-
enteries, and thus all the
parts of the body are in
communication. The
mouth is usually a longi-
tudinal slit, and its two
corners are often richly
ciliated. The gullet is
marked with longitudinal
grooves, two of which,
the ‘“siphonoglyphes,”
correspond to the corners
of the mouth, and are z., Tentacles; o., mouth; @s., cesophagus;
especially broad and c.,¢c’., apertures through a mesentery; 4.,a.,
= acontia; g., genital organs on mesentery;
deep. Along these two ro, meen Ene filaments ; 7.2., longitudinal
grooves, and by these two muscles; s., primary septum or mesentery ;
. s’., secondary septum; s”., tertiary septum 3
corners, food particles 2,’ basal disc. :
usually pass in; but in
some, one side is an incurrent, the other an excurrent
channel. Occasionally only one corner of the mouth
and side of the gullet is thus modified. The gullet
often extends far down into the cavity of the anemone. It
admits of a certain-amount of extrusion. The mesenteries
bear—(a) mesenteric filaments; (4) retractor muscles; (c)
II
Fic. 79.—Vertical section, of a sea-
anemone.—After Andres,
162 PHYLUM C@LENTERA.
ridges of reproductive cells, almost always either ova or
spermatozoa, rarely both; and (d) in some cases offensive
threads or acontia. The mesenteric filaments seem to be
closely applied to the food, and perhaps secrete digestive
‘juice. Intracellular digestion also occurs. Sea-anemones
have no sense organs; the sapphire beads, which are so
well seen at the bases of the outermost tentacles of the
common Actinia mesembryanthemum, are batteries of
stinging cells. The nervous system is uncentralised, and
consists of superficial sen-
sory cells connected with a
plexus of sub- epithelial
ganglion cells.
The layers of the body.—
The ectoderm which clothes the
‘ exterior is continued down the
inside of the gullet. The endo-
derm lines the whole of the
internal cavity, including mes-
enteries and tentacles. The
mesogloea is a supporting plate
between these two layers, and
forms a basis for their cells.
The ectoderm consists of
ciliated, sensory, stinging, and
glandular cells, and also of sub-
epithelial muscle and ganglion
Fic.
80. — Section
anemone (across arrow in Figure
79).—After Andres.
A, B, Directive septa; mf, mesenteric
through _ sea-
filaments; g., genital organs; 7..,
longitudinal muscles; s., primary sep-
tum ; s’:, secondary septum ; s”., tertiary
septum. The arrow enters between two
primary septa (an intra-septal cavity),
and passes out between two tertiary
cells based on the mesogloea, but
mainly restricted to the circum-
oral region.
The endoderm consists mainly
of flagellate cells, with muscle
septa. fibres at their roots. These form
the chief muscle bands of the
wall, the mesenteries, and the gullet. Nor are glandular and even
sensory cells wanting in the endoderm.
The mesenteries.—In sea-anemones and nearly related Anthozoa,
twelve primary mesenteries are first formed. These are grouped in
pairs, and the cavity between the members of a pair is called intra-
septal, in contrast to the inter-septal cavities between adjacent pairs.
In these inter-septal chambers other mesenteries afterwards appeat in
pairs. Two pairs of mesenteries, however, differ from all the rest—those,
namely, which are attached to.the two corners of the mouth and to the
corresponding grooves of the gullet. These two pairs of mesenteries
are called ‘‘ directive,” and they divide the animal into bilaterally sym-
metrica ‘halves. Anatomically, a pair of directive mesenteries differs
from the other paired mesenteries, because the retractor muscles, which
TYPES OF C@LENTERA—A SEA-ANEMONE. 163
extend in a vertical ridge along them, are turned away from one another,
and run on the inter-septal surfaces, whereas in the other mesenteries
the retractor muscles run on the intra-septal surface—those of a pair
facing one another. The arrangement of these muscles is of great im-
portance in classifying Anthozoa. It is possible that the mesenteries
are homologous with the teeniolee of jelly-fish, and the mesenteric with
the gastric filaments.
From the above description it will be noticed that the funda-
mental radial symmetry of the Ccelentera has here become profoundly
modified. |
Development.—Comparatively little is known in regard to the early
stages of development in sea-anemones. From the fertilised ovum a
blastosphere may result which by invagination becomes a gastrula. In
KD
oH
5
A
Fic. 81.—Z, Diagrammatic section of Zoantharian ; 4, of
Alcyonarian.—After Chun.
The line S-S in Z is through the siphonoglyphes (a), the line
7-T passes through two inter-septal spaces. The retractor
muscles are represented by dark thickenings on the mesen-
teries—all on one (the ventral) side in the Alcyonarian. The
line S-S in A represents the axis of symmetry. °
some cases the ovum segments into a solid morula; this becomes a
free planula, in which a cylindrical depression at one pole forms
the mouth and gullet. Or the two layers may be established by
a process known as delamination, in which a single layer of cells is
divided into an inner endodermic and an outer ectodermic layer.
According to Goette, the development is in essentials the same as that
of the Hydra-tuba. The larva of Cerianthids is for a time pelagic, and
used to be recognised as a distinct genus, Avachnactis.
Related forms.—The sea-anemones are classified in the sub-class
Anthozoa or Actinozoa, and along with many corals are distinguished
as Zoantharia or Hexacoralla from the Alcyonaria or Octocoralla, like
Alcyonium and the related forms. This contrast is not very satis-
factory, but it rests on such distinctions as the following :—
164
PHYLUM C@LENTERA.
ANTHOZOA OR ACTINOZOA
ZOANTHARIA, HEXACORALLA, ¢.g.
SEA-ANEMONE.
ALcYoNARIA, OCTOCORALLA, 6.2%
Drap-MEn’s-FINGERS.
Many are simple, many colonial.
The polyps of a colony may give rise
to others directly by fission or
budding.
Tentacles usually simple, usually some
multiple of six, often dissimilar.
Mesenteries usually some multiple of
six, complete and incomplete.
Retractor muscles never as in Alcyo-
naria.
Two gullet grooves or siphonoglyphes,
or only one.
No dimorphism.
Calcareous skeleton, if present, is derived
from the basal ectoderm.
Examples.
Sea-anemones—eg. Tealia and
Actinia.
Madrepore corals, many of them reef-
building.
Antipatharians. An aberrant Anti-
patharian, Dendrobrachia fallax,
has e7ght feathered tentacles,
All colonial, except a small family in-
cluding Monoxenia and Haimea.
The polyps of a colony give rise to
others not directly, but through
stolons or solenia.
Tentacles eight, feathered, uniform.
Mesenteries eight, complete.
Retractor muscles always on one (ven-
tral) side of each mesentery (see
Fig. 81).
One (ventral) gullet groove (siphono-
glyplfe or sulcus), or none.
Frequent dimorphism among members
of a colony.
There are usually calcareous spicules (of
ectodermic origin) in the mesoglcea.
Examples.
Alcyonium (Dead-men’s-fingers), with
diffuse spicules of lime.
Tubipora (Organ- pipe coral), with
spicules fused into tubes and trans-
verse platforms.
Corallium rubrum (Red coral), with an
axis of fused spicules.
Pennatula (Sea-pen), a free phosphor-
escent colony, witha ‘‘horny” axis,
possibly endodermic.
ZOANTHARIA
The Zoantharia include many orders, ¢.g. the primi-
tive Cerianthidea (Cerianthus, etc.) and Edwardsiidea
(Zdwardsia), the Actiniidea (including the typical sea-
anemones and the Madreporaria), and the divergent Anti-
pathidea.
Making of a typical coral.—Although the term “ coral”
is applied to many different Ccelenterate types with
substantial calcareous skeletons, e.g. to Millepores which
are Hydrozoa, and to “blue corals” and “red corals”
which are Alcyonarians, the corals par excellence are the
Madreporarians. They form the coral rock and “coral
islands” found in many parts of the globe, but rarely north
or south of a belt extending 30° on each side of the
equator, and rarely below the 4o-fathom line.
ZOANTHARIA. 165
In a general way a Madrepore polyp is like a sea-anemone
in structure, and the “coral” it forms is its external shell
rather than its skeleton. It is altogether a product of the
ectoderm. From one polyp others usually arise by budding
or by division, e.g. Astr@a and Madrepora and Lophohelia
(North Sea), but there are solitary forms such as Pungia
and Caryophyllia (British).
The first part of the “shell” to be formed is the dasal
plate between the ectoderm of the base and the substratum.
DIN" {!
Ne
'
‘
,
We
»
Fic. 82.—The formation of a coral shell (Astroides).—
After Pfurtscheller.
st., Stomodzeum ; 7zs., mesentery ; s., calcareous septum ; &., basal plate.
On this plate a number of radially arranged vertical ridges
(septa or cnemes) are then formed, and as they grow in
height they push the ectoderm of the base up before them
(see Fig. 82). An external wall or ¢heca is then formed,
partly by the fusion of the outer margins of the septa and
partly by a circular upgrowth from the basal plate. This
theca pushes the body wall before it, as the septa pushed
the base. Sometimes a second external wall or efztheca is
formed outside of and concentric with the theca. By the
coalescence of septa in the central line a colume//a or median
166 PHYLUM C@LENTERA.
pillar may be formed. ‘The outer wall of the theca may
bear vertical ridges or cost, and these may be connected
with neighbouring coste of other polyps by horizontal
shelves or dissepiments. Both septa and costz correspond
to intermesenteric spaces. (See Shipley’s Zoology of the
Invertebrata, pp. 68-71.)
ANTIPATHARIANS
Usually arborescent, sometimes whip-like colonies, of wide distribu-
tion in most seas, often called ‘‘black corals.” A spinose hollow
horny axis is covered with coenenchyma and regularly arranged polyps,
ae
wees
Fic. 83.—Structure of Antipatharians.
1. A group of polyps—J/., mouth ; ¢., tentacles.
2. Axis without polyps and ccenenchyma, covered with spines
S;
3. Vertical section of a polyp—A., axis; ¢., tentacle; g., gullet ;
m., mesentery ; 0., ovary ; #2.., mesenteric filaments.
4. Cross section of a polyp—ZC., ectoderm; /., mesoglcea ;
EN., endoderm ; G., gullet; 17S., mesenteries.
without any trace of spicules. A polyp is usually oval in section, with
its long diameter in the line of the axis, and its gullet elongated at
right angles to this. There are usually six simple non-retractile
tentacles, ten mesenteries, and two ill-defined siphonoglyphes. The
mesenteries are without muscle-banners. The two longest, running at
right angles to the elongated stomodeum, bear gonads. The develop-
ment is unknown.
ALCYONARIA. 167
Examples :—
Antipathes (arborescent).
Cirripathes (whip-like).
Leiopathes (with twelve mesenteries).
Dendrobrachia (with eight pinnate retractile
tentacles). :
ALCYONARIA
In the Alcyonarian polyp there are al-
ways eight fzmmate tentacles and eight
mesenteries attached to the stomodzum
or gullet. There is one longitudinal
at
Fic. 84.—Diagrams of Types of Alcyonaria.—After Hickson.
Types of Alcyonaria :—I. Of Stolonifera ; II. of Aleyonacea; III. of Axifera;
IV. of Stelechotokea.
ciliated groove (siphonoglyphe or sz/cus) in the stomodzeum
168 PHYLUM C@LENTERA.
(ventrally). The mesenteries bear retractor muscles, all
situated on the sulcar aspect (see Fig. 81), and each
mesentery bears a mesenterial filament. The two dorsal
(asulcar) mesenteries are long, ciliated, and non-glandular ;
they are respiratory in function and cause an upward
current, that in the sulcus being downward. Many Ale
cyonarians are dimorphic, having in addition to the typical
polyps (autozooids) dwarf siphonozooids, with suppressed
\
Fic. 85.—Corallium rubrum, a corner of a colony.—
After Lacaze-Duthiers.
A,, Anthocodia or retractile portion of a polyp; 7.2., com-
pletely retracted polyp, with the verruca or calyx portion
left protruding ; C., coenenchyma; 7., pinnate tentacles.
tentacles, strongly developed sulcus, no mesenteric fila-
ments, and often ill-developed mesenteries. Their function
is to drive currents of water through the canal systems of
the colony, and they are sometimes reproductive as well.
With the exception of one small family of solitary forms
(Haimeide), the Alcyonarians form colonies which are in
various ways supported by spicules, or by spicules and an
axis. The spicules, which take the most diverse forms,
seem to be begun at least by ectodermic cells (a pair to
ALCYONARIA, ; 169
each spicule), but they usually pass into the mesoglcea.
The nematocysts are usually small. A number of Alcyon-
arjians are viviparous ; the embryo is usually a planula.
Colonies are formed’ in different ways. (1) A parent polyp gives
off hollow stolons or so/enza, which bud off new polyps, and the whole
forms a spreading network or flat plate, 2g. Clavularia, a type of
Stolonifera (Fig. 84, I.).
(2) The polyps may be crowded together so as to form bundles
raised on a stalk, or lobose fleshy growths with the polyps projecting
on the surface of a dense mesogloeal mass honeycombed by solenia, 4g.
Xenia and Alcyonium, types of Alcyonacea (Fig. 84, II.).
(3) Or the colony may raise itself in the water by forming 2
Fic. 86,—Alcyonarian spicules.
common upright coenenchyma, in which the polyps are embedded,
and the medullary part of which may form a substantial axis of
cemented spicules, ¢.g. Corallium, a type of Pseudaxonia.
(4) Or the vertical extension of the colony may be effected by
a horny secretion from the polyps, which comes to form an axis,
really outside of the polyps though encrusted by them. This axis may
be purely horny or in part calcareous, e.g. Gorgonda and Acanella, types
of Axifera (Fig. 84, III.).
(5) Fifthly, the vertical extension may be due to a great elongation
of a single primary polyp which gives off solenia bearing numerous
secondary polyps, e.g. Pennatula, a type of Stelechotokea (cf. Fig.
84, IV.).
An altogether aberrant type is represented by the blue coral
(Heliopora) and its extinct relatives (Hedoltes, etc...
170 PHYLUM C@ELENTERA.
GENERAL SURVEY OF CQ@ELENTERA
Before we proceed to the systematic survey, we may contrast the
essential structural features of the four classes of Ccelentera.
I. In the Elydrozoa or Hydromeduse there is no inturned ectodermic
gullet or stomodzum; there are no partitions or mesenteries; there
are no special digestive organs; in the body wall the ectodermic
muscles are mostly longitudinal and the endodermic muscles circular ;
the sex cells are usually produced in the ectoderm; there is very
frequently a combination of polypoid and medusoid phases in the life
history ; the circumference of the medusoid bears a muscular velum of
ectoderm and mesoglcea ; there is no calcareous ‘secretion (except in
Millepores). :
II. In the Scyphomeduse there is an inturned ectodermic gullet or
stomodzeum ; there are hints of mesenteries ; there are special digestive
filaments ; the sex cells are endodermic; there is no velum; there is
often a non-sexual sedentary stage ; there is no calcareous secretion.
III. In the Anthozoa there is an
inturned ectodermic gullet or stom-
odzeum ; there are distinct mesenteries
or partitions from body wall to gullet
wall; there are often digestive fila-
ments; in the body wall the ecto-
dermic muscles are circular (except
in Cerianthus), and the endodermic
muscles longitudinal; the sex cells
are endodermic ; there is no medusoid
hase.
IV. The Ctenophora are very di-
vergent and apart from the other
classes, ¢.g. in rarely having any
stinging cells, and in having a well-
defined mesoblast,
SYSTEMATIC SURVEY
lass I. HyvpRozoa
Fic. 87.—Diagram of a gymno- c : oa
blastic Hydroid.—After All- Solitary polyps like Hydra, hydroid
man.
a., Stem; 4,, root 5 ¢., gut cavity; @.,
Peete (dark); @., Rarer
., horny perisarc; g, ra-like
Unaraon* Chydranths pom the
same, contracted; 4., hypostome
bearing mouth; &%., sac-like repro-
ductive bud (sporosac); ., a
modified hydranth (blastostyle)
bearing sporosacs; 2, medusoid
“* person.” i
colonies or zoophytes with medusoid
reproductive buds, medusoids without
sedentary stages, colonies of modified
medusoids.
1. Order Hydromedusze. — Simple
or colonial forms in which the sexu-
ally reproductive persons are either
liberated as free-swimming medusoids
or are sessile gonophores.
SYSTEMATIC SURVE Y—HYDROZOA. 172
(2) Hydrophora,—Two types are included here. The first includes
the Tubularians, Aydvactinza, and other forms in which the polyps are
not enclosed in the protective perisarc which often surrounds the colony
(gymnoblastic), and in which the free medusoid forms, when present,
have their genital organs placed in the wall of the manubrium
(Anthomedusz), and are furnished with ‘
ocelli placed at the base of the tentacles,
ffydra and its allies may be included here.
An unattached marine hydroid—Ayfolytus
peregrinus—has been described, and as it
bore gonophores it was obviously mature,
which is doubtful as regards two other
unattached forms, Protohydra leuckartiz and
Halermita cumulans, which may turn out
to be larval. The hydroid stages of Pelago-
hydra and Margelopsés are free-swimming.
Examples :—
Syncoryne sarstz, the free medusoid of
which is called Sarsza tubulosa.
Bougatnvillea ramosa \iberates the
medusoid Margelis ramosa.
Cordylophora lacustris and Tubularia
larynx have sessile gonophores or
sporosacs.
The second type includes Campanularians
and Sertularians along one line; Halecids
and Plumularians along another line. The
protective perisarc surrounding the colony
is continued into little cups (hydrothecz)
enclosing the polyps (calyptoblastic). These <
hydrothecze are stalked in Campanularians,
sessile in Sertularians and Plumularians.
The free medusoids have their gonads placed
in the course of the radial canals (Lepto-
medusze), and are either ‘‘ocellate” or
“* vesiculate.”
Examples :—
Plumularia, with hydrothece on one
side of the branches, and Sertudlaria,
AI.
with hydrothecze on both sides of the dL.
branches.
The Campanularian Odelia geniculata ic. 88.-—Graptolites.
liberates the medusoid Odea gent- I. Monograptus.
culata. II.’ Diplograptus.
(2) EHydrocorallinze. —Colonial torms which
suggest the Hydractiniz in their polymorphism and division of labour,
but are distinguished by their power of taking up lime, and so forming
‘*corals.” The colonies are complex and divergent, the reproductive
persons are either sessile gonophores or simple medusoids. A/¢//egora,
Stylaster.
(c) Trachymedusze.—These exist as a rule only in the medusoid form,
172: PHYLUM C@LENTERA.
Fic. 89.—Hydroids.—After Hincks.
I, Tubularia, II A. Piece of Sertularia. II B. A fragment enlarged,
showing sessile hydrothece (/7.) on both. sides of the twigs. IIT A.
Plumularia. III B. A fragment enlarged, showing hydrothece (H.)
on one side of each twig, an epillary penothees (G.) and minute nemato-
phores. IVA. Campanularian. B. A fragment enlarged, showing
stalked hydrothece (H.), a gonotheca (G.); C., the coenenchyma; P.,
the perisarc ; S., a stalk.
SYSTEMATIC SURVEY—SCYPHOMEDUSA. 173
and are divided into two groups, Trachomeduse and Narcomeduse,
according to the position of the gonads. The fresh-water medusz
Limnocodium and Limno--
cntda may possibly belong by?
to this group. \\y f far
Geryonia, Carmarina, - Pies
Ey]
Cunina, Aeginopsis.
2. Order Siphonophora.
—Free-swimming colonies
of modified medusoid per-
sons (medusomes), with
much division of labour.
Physalia _ (Portuguese
man-of-war), Dephyes, Vel-
ella, Porpita.
Incerte ‘sedis. Grapto-
lites.—Extinct unattached
colonies with a rod-like
axis found in Upper
Cambrian, Ordovician, and
Silurian systems, The
colony is usually linear,
and consists of cup-shaped
hydrothecze borne on one,
two, or four sides of the
solid axis (wz~gula). Each
opens into a common
median canal. At the
proximal free end there
is a- minute triangular
or dagger-shaped body
—the szcu/a—which re-
presents the embryonic
skeleton. Some repro-
ductive bodies or gon-
angia have been found.
-The animals were prob-
Fic. 90.—Campanularian Hydroid.—
After Allman.
ably free-swimming in #H., Hydrotheca or polyp-cup; AY; hy-
muddy seas, and of a dranth, or polyp-head; G., gonotheca,
Hyd. d 0 enclosing a reproductive polyp producing
ydromedusan nature, medusoid buds; J/., a liberated medu-
soid ; S7., basal stolon.
Class II. ScypHoMEDUS (= Acraspeda)
Jelly-fish with gastric filaments, sub-genital pits, and no velum—
(1) Lucernarize.—Sedentary forms. Lucernaria, Haliclystus, and
Depastrum.
(2) Discomedusee.—Active forms, often with complicated life
history. Aurelia, Pelagia, Cyanea, Rhizostoma.
174 PHYLUM C@LENTERA.
(3) Cubomedusze.—Forms with broad pseudo-velum, and other
peculiar features. Charybdea.
(4) Peromeduse.—Forms with four inter-radial tentaculocysts
only. Pericolpa.
Class III. AntHozoa (= Actinozoa)
Polypoid forms with well-developed gullet and septa, and circumoral
tentacles.
(1) Zoantharia or Hexacoralla.
(a) Actiniaria. Sea-anemones. Actinia, Anemonia, Tealia,
Cerianthus.
(4) Madreporaria. Stone or reef corals.
Astrea, Madrepora, Fungia, Meandrina,
(c) Antipatharia. ‘‘ Horny” black corals. <Antépathes.
(2) Alcyonaria or Octocoralla. :
Alcyonium (Dead-men’s-fingers), Zwbzgora (Organ-pipe
coral), Coraliium (Red coral), Gorgonta, Pennatula (Sea-
pen), AZonoxenza (non-colonial).
Class IV. CreENopHORA
Delicate free-swimming organisms, generally globular in form,
moving by means of eight meridional rows of ciliated plates, or comb-
M
Fic. 91.—-Diagram of a Ctenophore.—After Chun.
#., Mouth; S., sensory organ; 7., tentacle cut short; SH.,
pouch of tentacle; C., ciliated combs; ., funnel or central canal 5
SV., paragastric canal running parallel with stomodeum; G.,
other canals of the gut; ., one of the meridional canals, bear-
ing gonads, running under the bands of ciliated combs,
HISTORY OF C@LENTERA 175
like combinations of cilia. The stinging cells are almost always
replaced by ‘‘ adhesive cells.” The mouth is at one pole, and leads
into an ectodermic gullet. The gastric cavity is usually much branched.
The mesenchyme is very well developed, and includes muscular and
connective cells. At the aboral pole there is a sensory organ, including
an “‘otolith,” which seems of use in steering. Here, also, there are
two excretory apertures. Except in Beroé and its near relatives, there
are two retractile tentacles. All are hermaphrodite. The development
Fic. 92.—Aydroctena. A medusoid with suggestions:
of Ctenophore structure, but a true medusoid none the less.
aé.p., Aboral sensory organ; 7., retractile tentacle;
v., velum ; JZ., mouth; S7Z7., stomach.
1
is direct. They are pelagic, very active in habit, carnivorous in diet,
and often phosphorescent. According- to some, they lead on to
Polyclad worms, especially through Cztenoplana and Celoplana, two
curious flattened forms which crawl like Planarians. Mortensen’s
remarkable sessile 77a/fed/a corroborates this affinity.
Examples :— ° :
(a) With tentacles, Cydpfe and the ribbon-shaped Venus’ Girdle
(Cestum venerts). (6) Without tentacles, Beroé.
History of Ceelentera.—Of corals, as we would expect, the rocks
preserve a faithful record, and we know, for instance, that in the
older (Palzeozoic) strata they were represented by many types. We
often talk of the imperfection of the geological record, and rightly, for
much of the library has been burned, many of the volumes are torn,
whole chapters are wanting, and many pages are blurred. But this
imperfect record sometimes surprises us, as in the quite distinct remains
of ancient jelly-fish, which animals, as we know them now, are appar-
ently little more than animated sea-water. We should also grasp the
conception, with which Lyell first impressed the world, of the uniformity
of natural processes throughout the long history of the earth. Thus in
connection with Coelentera we learn that there were great coral reefs in
176 PHYLUM C@LENTERA.
the incalculably distant past, just as there are coral reefs still. So in
the Cambrian rocks, which are next to the oldest, there are on sandy
slabs markings exactly like those which are now left for a few hours
when a large jelly-fish stranded on the flat beach slowly melts away.
On the other hand, some forms of life which lived long ago seem to
have been very different from any that now remain, as is well shown
by the abundant Graptolite fossils, which, though probably Ccelentera,
do not fit well into any-of the modern classes.
As to the pedigree of the Ccelentera, the facts of individual life
history, and the scientific imagination of naturalists, help us to construct
a genealogical tree—a hypothetical statement of the case. Thus it
seems very likely that the ancestral many-celled animals—ancestral to
Sponges, Coelentera, and all the rest—were small two-layered tubular
or oval forms. The many-celled animals must have begun as clusters
of cells; the question is, what sort of clusters—spheres of one layer of
cells, or mouthless ovals, or little discs of cells, or two-layered thimble-
like sacs? Possibly there were many forms, but Haeckel and other
naturalists were led to fix their attention especially on the two-layered
sac or gastru/a, because this form keeps continually cropping up as an
embryonic stage in the life history of animals, whether sponge or coral,
earthworm or starfish, mollusc or even vertebrate, and also because this
is virtually the form which is exhibited by the simplest sponges
(Ascons), the simplest Ccelentera (Hydra), and even by the simplest
“worms” (Turbellarians).
If we begin in our survey with such a gastrula-like ancestor, the
probabilities are certainly in favour of the supposition that it was a free-
swimming organism. A gradual perfecting of the locomotor character-
istics might yield the two medusoid types of which we have already
spoken. But we know that the common jelly-fish Aurelia has a
prolonged larval stage which is sedentary, vegetative, and prone to bud.
If we suppose with W. K. Brooks that many forms, less constitutionally
active than others, relapsed into this sedentary state, with postponed
sexuality, and with a preponderant tendency to bud, we can understand
how polyps arose, a these of two types, one nearer the jelly-fish and
Lucernarians and leading on to sea-anemones and corals, the other
nearer the swimming-bell type and leading on to a terminus in Hydra.
It is certainly suggestive that we have jelly-fish wholly free (Pelagia),
jelly-fish with a sedentary larval life (Azre/éa), jelly-fish predominantly
passive (Zucernaria), and related polyps (Sea-anemones, etc.), which
only occasionally rise into free activity; while in the other series we
have medusoid types always free (Trachymedusze), others which are
liberated from (Campanularian and Tubularian) sedentary hydroids,
other (Sertularian and Plumularian) zoophytes whose buds though often
medusoid-like are not set free, and finally Hydra, which, though it
may creep on its side, or walk on its head, is predominantly a sedentary
animal, without any youthful free-swimming stage,
Ccology.—The Ccelentera are almost all marine. In
resh water we find the common Aydra, the minute A/icro-
hydra without tentacles, the strange olypodium, which in
G@COLOGY. 177
early life is parasitic on sturgeons’ eggs, the compound
Cordylophora, occurring in canals and in brackish water,
and the fresh-water Medusoids (Limmnocodium and
Limnocnida). Most of the active swimmers are pelagic, but
there are also a few active forms in deep water. Many
polyps anchor upon the shells of other animals, which they
sometimes mask, and there are most interesting constant
Fic. 93.—Commensalism of sea-anemones and hermit-crab,
partnerships between hermit-crabs and sea-anemones, eg.
between Eupagurus prideauxtt and Adamsia palljata,
The hermit-crab is masked by the sea-anemone, and may
be protected by its stinging powers; the sea-anemone is
carried about by the hermit-crab, and may get crumbs from
its abundantly supplied table. This illustrates a mutually
12
178 PHYLUM C@LENTERA.
beneficial external partnership or commensalism. In some
other animals it may degenerate into parasitism (see Fig.
93).
Another kind of partnership is illustrated by many sea-
anemones and Alcyonarians. Minute unicellular Alge
(Zoochlorellz) live within the cells of the animals in close
physiological partnership with them (symbiosis).
A spatial partnership in which one animal finds habitual shelter within
or near another is not infrequent ; ¢.g. small horse-mackerels (Carangidz)
swimming in shelter of large jelly-fish; a small fish (Amphzprion
bzcinctus) inside a giant sea-anemone (Crambactzs arabica) which has
a diameter of a foot ; another fish (/zerasfer) that goes in and out of
the hind-gut of Holothurians ; another that lives among the very long
hair-like spines of the Red Sea rock-urchin (Diadema saxatile); and
another (Afpogonichthys strombz) that spends part of its time in the
mantle cavity of the large sea-snail (Stvombus gigas) of the Bahamas.
The quaint little hydroid Zar sabellarum lives at the mouth of the
tubes of the worm Sade//a; another hydroid (Sty/actds mznoz) grows
all over the skin of a rock-perch (A@énous) from the Indian Ocean ;
Stylactis vermicola was found on the back of the worm Aphrodite at
the great depth of 2900 fathoms,
A Fic. 93A.
A., a minute portion of the branched excretory system of a Plathelminth, showing
longitudinal duct (/), with cilia (C.), its branches (/7 and ///), and the terminal
flame-cells ZV); B., one of the characteristic hollow flame-cells, leading into
the excretory tubule (1), showing the.long cilia (2), the excretory globules (3), the
nucleus (4), and pseudopodia-like processes (5) passing among adjacent cells.
CHAPTER X
UNSEGMENTED “WORMS”
PHYLUM PLATYHELMINTHES:
Chief Classes—Turbellaria, Trematoda, Cestoda.
PHYLUM NEMERTEA.
PHyLum NEMATHELMINTHES :
Chief Classes—Nematoda, Nematomorpha, Acanthocephala.
THE title “worms” is hardly justifiable except as a con-
venient name for a shape. The animals to which the
name is applied form a heterogeneous mob, including
about a dozen classes whose relationships are’ imperfectly
known.
It is likely that certain “‘ worms” were the first animals
definitely to abandon the more primitive radial symmetry,
to begin moving with one part of the body always in front,
to acquire head and sides. And if one end of the body
constantly experienced the first impressions of external
objects, it seems plausible that sensitive and nervous cells
would be most developed in that much-stimulated, over-
educated head region. But a brain arises from the
insinking of ectodermic cells, and its beginning in the
cerebral ganglion of the simplest “worms” is thus in part
explained.
Worm types begin the series of ¢7iploblastic celomate
animals, z.e. of those which have a well-defined mesoderm,
and a ccelom or body cavity lined with mesoderm and
distinct from the gut. It must be noted, however, that the
appearance of a well-developed ccelom and mesoderm is
very gradual; thus there is practically no ccelom in the
Platyhelminthes, and the mesoderm is sometimes not more
definite than in Ctenophora.
130 UNSEGMENTED “WORMS.”
Puy_LuM PLATYHELMINTHES
The Platyhelminthes or flat-worms tnclude three chief classes
—Turbellarians, Trematodes, and Cestodes—which form a
velated series. The body ts flattened from above downwards ;
the mesoderm forms a compact mass of cells or parenchyma
without a definite celom; there ts the beginning of a head-
brain ; the excretory system consists of a pair of lateral canals,
gluing off many branches, whose twigs end in peculiar “flame-
cells” ; almost all are hermaphrodite.
There is no doubt that the three classes, Turbellarians or
Planarians, Trematodes or Flukes, and Cestodes or Tape-
worms, are related to one another. A fourth class of
Temnocephalids must also be admitted. It is interesting
to notice that the Turbellarians and Temnocephalids are
free-living, except in the case of a few marine Turbellarians
which have taken to parasitism; that the Trematodes are
all parasitic, either external hangers-on (ectoparasites) or
internal boarders (endoparasites); and that the Cestodes
are altogether endoparasitic. It is probable that the flukes
and tape-worms arose from Turbellarian-like ancestors which
adopted parasitic habits. Attention must be directed to
the flame-cells which are characteristic of Platyhelminthes.
Each terminal twig of a branch of an excretory canal
leads into a large hollow cell, from the base of which
a bunch of cilia—with rapid movements suggesting a
flickering flame — projects into the cavity towards the
lumen of the twig. .
Class TURBELLARIA, Planarians, etc.
Turbellarians are unsegmented “worms,” usually leaf-like,
living in fresh, brackish, or salt water, or in moist earth.
Almost all are carnivorous, a few are parasitic. They re-
present the beginning of definite bilateral symmetry.
The ectoderm is ciliated, often glandular, often with peculiar
rod-like bodies (rhabdites) which may be discharged on irrita-
tion, A pair of ganglia in the anterior region give off
lateral nerve-cords, and there are usually simple sense organs.
The food canal has a protrusible muscular pharynx, ts often
branched, and ts always blind. There are no special
TURBELLARIA. 185
respiratory or circulatory organs, the body cavity is not
represented, unless it be by intercellular lacune in the
parenthyma; the excretory system usually consists of two
longitudinal canals, whose branches end internally tn flame-
cells. The Turbellarians are almost always hermaphrodite ;
and the reproductive organs usually show some division of
Fic. 94.—Diagram of Turbellarian.—After Lang.
C., Cerebral ganglia; £., eye; 7., tentacle; PH., pharynx; MJo., mouth;
M., male aperture; /., female aperture; the ovaries and testes are
branched organs on both sides, represented by dots.
labour, e.g. in the development of a yolk gland, which may
have arisen as an over-nourished (hypertrophied) part of the
ovary. The eggs are usually enclosed in shells or cocoons,
and the development may include a metamorphosis. Some
forms multiply by fission. There seem to be affinities between
Turbellaria and Calentera, especially the Ctenophora.
182 UNSEGMENTED “WORMS.”
The Turbellarian worms form an exceedingly interesting group; they
are often beautiful, and the ciliated ectoderm and well-developed
muscles enable them to move with singular grace. Although the
bilateral symmetry and the distinction of anterior and posterior ends is
quite marked, the ‘‘ mouth” or single opening of the food canal is often
near the middle of the ventral surface. The anterior region is usually
furnished with tactile processes. The shape of the body in the aquatic
forms ‘is flattened and leaf-like, as in the delicate Lef/oflana, the
“living film” found on the shore-rocks. Fresh-water forms are
usually smal] and often minute, but those living in the sea may attain
a length of six inches, though most are small. Land Planarians are
elongated and more worm-like in shape; they may measure a foot or
more in length, and are most abundant in tropical countries. Some,
like Planaria, have so much regenerative capacity that half a dozen or
more may be produced by cutting one into pieces.
Classification.—
Order 1. Rhabdoccelida—small fresh-water and marine forms.
The food canal is very slightly branched, or quite straight, or
blocked.
Rhabdoccela. With straight intestine, e.g. Azcrostoma, a fresh-
water genus. It is first male and then female (protandrous
hermaphrodite) ; it forms temporarily united asexual chains,
sometimes of sixteen individuals, suggesting the origin of a
segmented type. Grafilla and Anoplodium are parasitic on
Gastropods. Among the Vorticide allied to Graffi/la we may
notice Provortex teliing in Tellina and a related form in the
cockle.
Alloioccela. With irregular cacca on the gut, e.g. Alostoma.
All marine except one from Swiss lakes (Plagiostoma
lemanz) and Bothrioplana.
Accela. Without intestine, ¢.2. Covoluta, which contains green
cells, regarded by some as symbiotic Algae. Marine.
Order 2. Tricladida. Elongated flat ‘‘ Planarians” with
three main branches from the gut, eg. Planaria and
Dendrocelum (fresh-water), the former sometimes dividing
transversely ; Polycelts nigra, a common fresh-water form ;
Gunda (Procerodes) segmentata (marine), showing hints of
internal segmentation ; Geodesmus and Bipalium (in damp
earth); Bzpalium kewense is an import often found in
Britain.
Order 3. Polycladida. Large leaf-like marine ‘‘ Planarians,”
with numerous intestinal branches diverging from a central
stomach, e.g. Leptoplana (not uncommon on the seashore),
Thysanozoon,
Class TEMNOCEPHALOIDEA
The Temnocephalids are flattened forms, eg. Temmnocephala,
found clinging to fresh-water animals, especially Crustaceans ;
there is a lerge ventral sucker ; the epidermis is a nucleated
TREMATODA. ~ 183
syncytium (z.e. without distinct demarcation into cells) which
secretes a-thick cuticle, contains rhabdites, and rarely bears
cilia. The class seems to be intermediate between Rhab-
doceelid Turbellaria and Trematodes,
Class TREMATODA. Flukes, etc.
The Trematodes are leaf-like, or sometimes cylindrical
external or internal parasites. With their parasitic life may
be associated the absence of cilia on the surface of the adults,
the thick “cuticle,” the presence of attaching suckers (occasion-
ally with hooks), and the rarity of sense organs. After
embryontc life the ectoderm degenerates, ceases to be distinctly
cellular, and sinks inwards. Jt is likely that they have
arisen from free Turbellarian-like ancestors, and they resemble
the Turbellarians in being unsegmented, in having anterior
ganglia, from which nerves pass backward and forward, in
the rudimentary nature of the body cavity, in the ramifying
system of fine excretory canals, in the hermaphrodite and
usually complex reproductive system. The excretory and
nervous systems are, however, more complex than those of
Turbellaria. The alimentary canal is usually forked,
often much branched, and always ends blindly. In many
cases the animals are self-impregnating, but cross-
fertilisation also cecurs. The development of the external
parasites is usually direct, of the internal parasites usually
indirect, involving alternation of generations. They occur
on ovr in al] sorts of Vertebrates, but those which have
an indirect development, and require two hosts to complete
their life-cycle, often pass part of their life in some
Invertebrate.
Type, Zhe Liver Fluke (Distomum hepaticum)
The adult fluke lives as a parasite in the liver and bile
ducts of the sheep, causing “‘liver-rot.” Unlike most
flukes, it has many occasional hosts,—it sometimes occurs
in cattle, horses, deer, camel, antelopes, goat, pig, beaver,
squirrel, kangaroo, and rarely in man. The animal is flat,
oval, and leaf-like, almost an inch in length by half an
inch across the broadest part, reddish brown to greyish
yellow in colour. As the word Distomum suggests, there
184 UNSEGMENTED ‘“‘WORMS.”
are two suckers—an anterior, perforated by the mouth;
a second, imperforate, a little farther back on the mid-
ventral line.
Fic. 95.—Structure of liver fluke.—After
Sommer. From ventral surface. The
branched gut (g.) and the lateral
nerve (/.z.) are shown to the left, the
branches of the excretory vessel (¢.v.)
to the right.
m., Mouth; 4/., pharynx; g., lateral head
ganglion; v.s., ventral sucker; ¢.s., position
of cirrus sac. An arrow indicates the ex-
cretory aperture,
Thence they are expelled by an ejaculatory duct, which le
» muscular protrusible penis. The retracted penis an
There is a muscular
pharynx and a_ blind
alimentary canal which
sends branches through-
out the body. The
food is the d/ood sucked
from the liver of the
host. From a ganglion-
ated collar round the
pharynx, nerves go for-
ward and_ backward;
of those which run back-
ward, the two lateral are
most important. Al-
though the larva has
eye spots to start with,
there are no_ sense
organs in the adult.
The body cavity is not
represented unless it be
by minute intercellular
spaces in the body par-
enchyma. Into these
there open the internal
ciliated ends of much-
branched excretory
tubes, which unite pos-
teriorly in a_ terminal
vesicle opening to the
exterior.
The reproductive system is.
hermaphrodite and complex.
From much-branched testes,
spermatozoa pass by a pair of
ducts (vasa deferentia) into
a seminal vesicle lying in
front of the ventral sucker.
ane through
the seminal
TREMATODA. 185
vesicle lie in a space or ‘‘cirrus sac” between the ventral sucker and
the external male genital aperture. The ovary is also branched, but
Fic. 96.—Reproductive organs of liver fluke.
—After Sommer.
J. Female aperture. ov. Ovary (dark).
s.z. Seminal vesicle. ut, Uterus.
y.gl. Diffuse yolk glands. e.s. Cirrus sac.
sh.g. Shell gland. p. Penis.
v.d. Vas deferens. m. Mouth.
7. Testes (anterior). g. Anterior lobes of gut.
less so than the testes. The ova pass from its tubes into an ovarian
duct. Nutritive cells are gathered from very diffuse yolk glands,
collected in a reservoir, and pass by a duct into the end of the afore-
186 UNSEGMENTED “WORMS.®
said ovarian duct. At the junction of the yolk duct and the ovarian
duct there is a shell gland, which secretes the ‘‘horny” shells of the
eggs, and from near the junction a fine canal (the Laurer-Stieda canal) y
seems to pass direct to the exterior, opening on the dorsal surface.
The meaning of this is still somewhat uncertain. In some flukes it is
said to be a copulatory duct ; in others it is regarded as a safety valve
for overflowing products. From the junction of the ovarian duct and
the duct from the yolk reservoir, the eggs (now furnished with yolk
cells, accompanied by spermatozoa, and encased in shells) pass into a
wide convoluted median tube, the oviduct or uterus, which opens to
the exterior at the base of the penis. Self-fertilisation is probably
normal, but in some related forms cross-fertilisation has been observed.
Life history.—The fertilised and segmented eggs pass in
large numbers from the bile duct of the sheep to the
intestine, and thence to the exterior. A single fluke may
produce about ©50,000 embryos, which illustrates the
prolific reproduction often associated with the luxurious
conditions of parasitism, and almost essential to the con-
tinuance of species whose life cycles are full of risks.
Outside of the host, but still within the egg-case, the
embryo develops for a few weeks, and eventually escapes at
one end of the shell. Those which are not deposited in
or beside pools of water soon die. The free embryo,
known as a miracidium, is conical in form, covered with
cilia, provided with two eye-spots, and actively locomotor.
By means of its cilia it swims actively in the water for some
hours, but its sole chance of life depends on its meeting
a small amphibious water-snail (Zimmneus truncatulus or
minutus), into which it bores. In an epidemic among
horses and cattle in the Hawaiian Islands, the host was
L. oahuensis; in the Sandwich Islands the host is
L. peregra, in Victoria Bulimus tenuistriatus. This
diversity of host, also remarkable in the aduli, is very
unusual. Within the snail, ¢.g. in the pulmonary chamber,
the embryo becomes passive, loses its cilia, increases in
size, and becomes a sforocyst. The sporocyst is a hollow
sac, with a slightly muscular wall and with the beginnings
of an excretory system. Sometimes this sporocyst divides
transversely (Fig. 97 (4)).
Within the sporocyst a few cells behave like partheno-
genetic ova. Each segments into a ball of cells or morula,
which is invaginated into a gastrula, and grows into another
form of larva—the vedia. These rediz burst out of the
LIVER FLUKE. 187
‘Org
1]
es g
1°)
x
10,
AJ
ie
S25
K}
onezagsa!
Reece
el
Fic. 97.—Life history of liver fluke.—After Thomas.
x. Developing embryo in egg-case; 2. free-swimming -ciliated embryo;
3. sporocyst; 3a. shell of Lismaaus truncatulus; 4. division of sporo-
cyst; §. sporocyst with redie forming within it; 6. redia with more
rediz forming within it; 7. tailed cercaria; 8. young fluke.
188 UNSEGMENTED “ WORMS.”
sporocyst, and migrate into the liver or some other organ.
Each sporocyst usually forms at a time 5-8 rediaz; each
of these forms 8-12 more redize; and each of these forms
14-20 cercariz. In the winter a sporocyst may give rise
to cercariz directly. A redia is a cylindrical organism
with a short alimentary canal, excretory canals with “ flame
cells,” and a pair of blunt locomotor processes posteriorly.
A cercaria has a bifurcated gut, two suckers, a locomotor
tail, and the beginnings of gonads (Fig. 97 (6)).
The cercarize emerge from the rediz, wriggle out of the
snail, pass into the water, and after swimming for a short
time, moor themselves to stems of damp grass. There
they lose their tails and become encysted. If the encysted
cercaria on the grass stem be eaten bya sheep, the cyst
is dissolved in the stomach, and the young fluke makes
its way up the bile duct and its tributaries. In about six
weeks it grows into the adult sexual fluke.
It will be noted that the sporocyst is the modified embryo, but that
it has the power of giving rise asexually to redie. These develop,
however, from special cells of the sporocyst, which we may compare to
spores or to precociously developed parthenogenetic ova. Though the
reproduction is asexual, it is not comparable to budding or division.
The same power is possessed by the redize, and there are thus several
(at least two) asexual generations between the embryo and the adult.
The disease of liver-rot in sheep is common and disastrous. It has
been known to destroy a million sheep in one year in Britain alone; and
in the winter 1879-80 the mortality attributed to fluke disease was
estimated at three millions. It is especially common after wet seasons,
and in damp districts.
Classification.—Order 1. Heterocotylea, with a posterior ad-
hesive organ, often with a pair of accessory suckers beside the mouth.
Most are ectoparasitic. The development is direct and associated with
one host (monogenetic).
e.g. Polystomum integerrimum, This form with many suckers
is often found in the bladder of the frog. It attaches
itself in its youth to the gills of tadpoles, passes thence
through the food canal to the bladder, where it develops
slowly for years.
Gyrodactylus, found on the gills and fins of fresh - water
fishes. It is viviparous, but the embryo, before it is
extruded, itself contains an embryo, and this in turn
another, so that three generations of embryos are re-
presented simultaneously.
Diplozoon paradoxum consists of two individuals united.
The single embryo (Dzforfa) is at first free-swimming,
LIVER FLUKE. 189
Ne
REDIAE SPOROCYST
Fic. 98.—Diagram of life cycle of liver fluke.
Upper quadrant, adult in sheep; £%., pharynx; s., sucker; g., gut. Right
quadrant, free-swimming larva with eye-spots (¢). Lower quadrant,
sporocyst and rediz in water-snail; & , redia within sporocyst or within
redia; g., gut in redia; C., cercarie in redia. Left quadrant, free cer-
caria; g., gut; s., sucker; 7., tail. ’
190 UNSEGMENTED “WORMS,”
but becomes a parasite on the gills of a minnow, and
there two individuals unite very closely and permanently.
Tristomum, with three suckers, on some marine fishes.
Order 2. Aspidocotylea, with a large sucker occupying most
of the ventral surface. Development is direct, and there is one host.
e.g. Aspidogaster in Molluscs.
Order 3. Malacotylea, with never more than two suckers. The
development is indirect and requires two hosts, the adult usually
frequenting the gut of a vertebrate.
e.g. Distomum, with numerous species. Sch¢stosomum ( Belharzia)
Aematobium, a parasite of man, widely distributed in Africa,
e.g. in Egypt. It occurs in the portal vein, the blood vessels
of the bladder, large intestine, etc., causing inflammation,
heematuria, stone, etc. The sexes are separate, and the male
(about half an inch in length) carries the more thread-like
female (about an inch in length) inserted in a groove or gynze-
cophoric canal. Man is probably infected through impure
water, but the intermediate host is still unknown. The
embryos are passed out in the urine.
Monostomum, with one sucker; adult in ducks, young in
fresh-water snail, Planorbis.
The relationships of the Trematodes are on the one hand with the
free-living Turbellarians, on the other hand with the parasitic
Cestodes.
Class CEstopa. Tape-worms
The Cestodes ave internal parasites, whose life history
includes a bladder-worm (proscolex) and a tape-worm (strobila)
stage, the former in a Vertebrate or Invertebrate host, the
latter (with one exception) in a Vertebrate. In a few cases
the body ts unsegmented, e.g. Archigetes and Caryophylleus,
with one set of gonads ; in a few others, e.g. Ligula, there
ts a serial repetition of gonads without distinct segmentation
of the body ; in most cases, e.g. Tenia and Bothriocephalus,
the body of the tape-worm forms a chain of numerous joints or
proglottides, cach with a set of gonads. Thus the class in-
cludes transitions from unsegmented to segmented forms, but
the latter are imperfectly integrated. The general form of
the body ts tape-like and bilaterally symmetrical, with anterior
hooks, grooves, or suckers ensuring attachment to the gut of the
host. The body wall consists of a cuticle and a well-innervated
epidermis, within which there ts parenchymatous connective
tissue, often with cortical deposits of lime, and at least two sets
(longitudinal and transverse) of unstriped muscles. The
CESTODA. 191
nervous system consists of two or more longitudinal nerve-
strands and anterior commissures ; there are no special sense
organs. There ts no alimentary system; the parasite
Jtoating in the digested food of its host absorbs soluble
material by its general surface. There is no vascular nor
respiratory system, and a body cavity is represented merely by
irregular spaces in the solid parenchymatous tissue. In some
of these spaces there are “flame-cells,” which lie at the ends of
the fine branches of longitudinal excretory tubes, which are
united in a ring in the head, are connected transversely at
each joint, and open terminally by one or more pores. All
tape-worms are hermaphrodite, and most, if not all, are
probably self-fertilising. The male reproductive organs in-
clude diffuse testes, a vas deferehs, and a protrusible terminal
cirrus. The female organs include a pair of ovaries, yolk
glands, a shell gland, a vagina by which spermatozoa enter, a
receptacle for storing spermatozoa, and a uterus in which the
ova develop. The embryo develops within another host into a
proscolex or bladder-worm stage, which forms a “head” or
scolex. When the host of the bladder-worm is eaten by the
jinal host, the scolex develops into an adult sexual tape-worm.
With the conditions of endoparasitic life may be associated the
occurrence of fixing organs, the absence of sense organs, the low
though somewhat complex character of the nervous system, the
entire absence of a food canal, and the prolific reproduction.
Life history of Tenia solium.—This is one of the most
frequent of the tape-worms infesting man. In its adult state
it is often many feet in length, and is attached by its “head”
to the wall of the intestine. The head bears four suckers
and a crown of hooks, and buds off a long chain of joints,
which develop complex reproductive organs as they get
shunted farther and farther from the head. The last of the
joints or froglottides is liberated (singly or along with
others), and passes down the intestine of its host to the
exterior. It has some power of muscular contraction and
of movement, and it is distended with little embryos within
firm egg-shells. When the proglottis ruptures, these are
set free.
In certain circumstances, the embryos, within their firmly
resistant egg-shells, may be swallowed by the omnivorous
pig. Within its alimentary canal the egg-shells are dis-
192 UNSEGMENTED “WORMS.”
solved, and embryos (hexacanths) bearing six anterior
hooks are liberated. They bore their way from the in-
testine into the muscles or other structures, and there
encyst. They lose their hooks, increase in size, and
Fic, 99.—Diagram of reproductive organs in Cestode joint.
—Constructed from Leuckart.
+, Ovary, with short oviduct; w¢., ‘‘uterus”; #., diffuse testes;
sh.g. shell gland ; y.g.,yolk gland ; v.d.,vas deferens; v., vagina 5
7S., receptaculum seminis; Ze., longitudinal excretory ducts ;
z.e., transverse bridges connecting these.
The dotted lines above and below represent the anterior and
posterior borders of the proglottis. Note that the so-called uterus
is blind ; it opens to the exterior in a few tape-worms, and is
perhaps the homologue of the Laurer-Stieda canal of Trematodes.
become passive, vegetative, asexual “bladder-worms.” A
bud from the wall of the bladder or froscolex grows into
the cavity of the same, and forms the future “head” or
scolex. This is afterwards everted, and then the bladder-
TANIA SOLIUM. 193
worm consists of a small head attached by a short neck to
a relatively large bladder.
Fic. 100.—Life history of Zenza solium.—After Leuckart.
x. Six-hooked embryo in egg-case; 2. proscolex or bladder-worm
stage, with invaginated head ; 3. bladder-worm with evaginated
head ; 4. enlarged head of adult, showing suckers and hooks ;
5. general view of the tape-worm, from small head and thin
neck to the ripe joints; 6. a ripe joint or proglottis with
branched uterus (cf. Fig. 99); all other organs are now lost.
When man unwittingly eats ‘“‘ measly ” pork—that is, pork
infested with bladder-worms—an opportunity for further
13
194 UNSEGMENTED “WORMS.”
development is afforded. The bladder is lost, and is of no
importance, but the “head ” or scolex fixes itself to the wall
of the intestine. There it is copiously and richly nourished,
and buds off asexually a chain of joints.
As these joints are pushed by younger interpolated buds
farther and farther from the head, they become sexually
Fic. 101.—Diagram of life history of Zenda soldum.
First chapter: Tapeworm in man; #., head; PR., proglottides. Second
chapter: Free proglottis and egg-cases; #7¢., uterus; ¢.a., genital aper-
ture; embryo within the egg-case. Third chapter: Within the inter-
mediate host, the pig; A., hexacanth embryo ; 4.sc., proscolex or bladder-
worm; 2., muscle of pig; sc., scolex or head.
mature. The ova are fertilised, apparently by spermatozoa
from the same joint; the joint becomes distended with
developing embryos. These ripe joints are liberated, the
embryos are set free by rupture, and the vicious circle may
recommence. Happily, however, the chances are many
millions to one against the embryo becoming an adult.
The above history is true, mez¢atis mutandis, for many other tape-
worms. The embryo grows into a proscolex or bladder, which buds off
CESTODA. 195
a scolex or head, which, in another host, buds off the chain of proglotdzdes.
As it is virtually the same animal throughout, the life history does not
include an ‘‘alternation of generations.” It is doubtful, however, what
term should be applied to those cases in which the bladder-worm
(Cenurus and Echinococcus) forms not one head only but many, each
of which is capable of becoming an adult tape-worm. The only known
exception to the fact that sexual tape-worms are parasites of Vertebrates
is Archigetes sieboldiz, a simple cestode which is sexual within the small
fresh-water oligocheet Tud¢fex rivulorum.
Representative Life Histories.
ADULT, SEXUAL, OR TAPE-WORM
STAGE.
Non-SEXuAL, PROSCOLEX, OR BLADDER-
WORM STAGE.
1. Tenia solium, in man, with four
suckers and many hooks.
2. Tenia saginataor mediocanellata,
in man, with four suckers, but no hooks.
3. Bothriocephalus latus, in man,
with two lateral suckers, but no hooks,
with less distinct separation of the pro-
glottides than in Zenia. It may be
ir yards in length,
4. Tania (Echinococcifer) echino-
coccus, in dog. Very small, with three
joints.
5. Tenia cenurus, in dog.
6. Tenia serrata, in dog.
7. Tenia cucumerina, in cat.
1. Cysticercus cellulose, in muscles of
the pig.
2. Bladder-worm in cattle.
3. Theciliated,free-swimming embryo
becomes a parasite in the pike, trout,
burbot, etc., but without a distinct
bladder-like stage.
4. Echinococcus veterinorum, in
sheep, cattle, pigs, etc., and some-
times in man, producing brood cap-
sules, which give rise to many “‘ heads.”
5. Canurus cerebralis causing sturdie
or staggers in sheep, with numerous
“heads.” Also in cattle, goat, horse,
etc.
6. Cysticercus pisiformis, in rabbit.
7. Cysticercus fasciolaris, in mouse.
Zoologically the cestodes are interesting, on account of their life
histories, the degeneration associated with their parasitism, the pre-
valence of self-impregnation, and the complexity of the reproductive
organs. Practically they are of importance as parasites of man and
domestic animals.
Classification.—The class Cestoda includes a number of families :—
Cestodariidze. No joints, one set of gonads.
eg. Archigetes, Caryophylleus, Amphilina, Gyrocotyle.
Bothriocephalide. Two weak flat suckers ; genital openings usually
on the flat surfaces.
e.g. Bothriocephalus ; Ligula, with no suckers or joints but with
serial gonads.
Tetrarhynchide. With four protrusible proboscides armed with
hooks, parasites of fishes, Found also in Sefza.
e.g. Tetrarhynchys. The finest pearls in the Ceylon pearl oyster
are formed round a larval Zetrarhynchus.
196 UNSEGMENTED “WORMS.”
Tetraphyllidee. With four very mobile suckers.
e.g. Echeneibothrium, Phyllobothrium.
Teeniide. With four suckers, often with apical hooks, with marginal
genital apertures.
eg. Tenia.
GENERAL NOTE on Ptaty-
HELMINTHES
The four classes Turbellaria,
Trematoda, Cestoda, and Temno-
cephaloidea, constitute the Platy-
helminthes or Flat-worms — an
interesting group, because its mem-
bers illustrate so well the progressive
degeneration associated with increas-
ing parasitism, and also because of
the relatively great simplicity. The
four classes are nearly related, for
forms like Zemmnocephala connect
Turbellaria and Trematoda, and the
H “‘monozoic” Cestodes like <Archd-
getes, Amphilina, Caryophylleus,
and Gyrocotyle connect Trematoda
sit and Cestoda. It is probable that
both Cestodes and Trematodes arose
from a Turbellarian stock.
Among the most striking of the
Platyhelminth characters are the
nature of the excretory and repro-
ductive organs and the condition of
the mesoderm. The excretory system,
with its longitudinal trunks, its
ramifying canals, and ‘‘flame-cells,”
is characteristic. The reproductive
organs are complex, show division
Fic. 102.—Diagrams of of labour, and are furnished with
bladder-worms. ducts of their own, unconnected with
L. The ordinary Cysticercus type the excretory system—a condition
with one head (/.). not common elsewhere. The pres-
Il. The Cente type, with many ence of shells around the eggs is
eads. | = S
igh Bie eiieokeeens ened) with another point of interest. It be
many heads, and with’ brood comes of great importance to the
capsules producing many parasitic flukes and tape-worms, but
heads. occurs also in the free - living
‘ Turbellaria. The formation of yolk
cells from a specialised part of the ovary (yolk gland) is also note-
worthy. There is no true body-cavity, the space between gut and
body-wall being filled with a packing tissue; the absence of an anus
is also important, the two characters taken together being held to
indicate affinity with the Ctenophora.
NEMERTEA.
Class NEMERTEA. Nemertines
197
The ribbon-worms or Nemertines are interesting in many
ways, ¢.g. in being the simplest animals to have an open
gut, a closed blood-system, and, occa-
sionally, hemoglobin ; in having some
very peculiar structures, notably a pro-
trusible proboscis and ciliated head
slits; in being in many cases extra-
ordinarily extensile and liable to break
into pieces.
The Nemertines are worm-like ant-
mals, unsegmented and generally elongate
in form; they are almost all marine,
and most, if not all, are carnivorous.
The ectoderm is ciliated. There is a
remarkable retractile proboscis, uncon-:
nected with the alimentary canal, and
Jorming a tactile organ or a weapon.
The nervous system consists of a brain,
a commissure round the proboscis, and
two lateral nerve-cords ; in connection
with the brain there ts a pair of ciliated
pits. The gut terminates in a posterior
anus, and ts furnished with lateral
pockets. |There is no body cavity in the
adult, but the closed vascular system ts
probably of cwelomic origin. The ex-
cretory system is apparently of the
Platyhelminth type. The sexes ave usu-
ally separate and the organs simple. The
development is in some cases direct, while
in others there is a peculiar pelagic larva.
Fig. 103.—Diagrammatic longitudinal section
of a Nemertean (Amphzporus lactifloreus),
dorsal view.—After M‘Intosh.
#.£-, Proboscis pore; 4., brain giving off the lateral
nerve-cords (.); #o., cesophageal pocket; %., pro-
boscis lying within its sheath 3. s¢., stilet of proboscis ;
m., retractor muscles of proboscis; g., gut shown in
outline at the sides of the proboscis; ¢, the three
main longitudinal blood vessels, which unite both
anteriorly and posteriorly.
(i
FP
Bs po
[PS \
i
B n
bff XJ
Key
3
3
;
BF
H p
04
‘
e
aN
Vye
Vi
s;
a)
3c
Sie
FS
Ks
6
ot
al 3
“
7
ki
Nya
R
¥y
al :
198 UNSEGMENTED “WORMS.”
GENERAL ACCOUNT OF NEMERTEA
In appearance most Nemertines are ribbon- or thread-like, and the
cross-section is generally a flattened cylinder. They show a greater
diversity of size than any other ‘‘ worms,”—from a Lz7eus, 12 or more
feet in length (25 metres has been recorded for an extended Lzvevs
Zongisstmus), to the pelagic Pelagonemertes, which is under an inch.
The, colours are often bright, and tend to resemble those of the sur-
roundings. The ectoderm is covered with numerous short cilia, and
FIG. 104.—Transverse section of the Nemertean Drepanophorus latus..
—After Birger.
@.x., Dorsal or proboscis nerve; P.s., proboscis sheath; P.c., proboscis
cavity; P.s’., sac of proboscis cavity; d.v.2., dorso-ventral muscles ;
c.m., circular muscles ; Z.7., longitudinal muscles ; Z.7., lateral nerve
with branches; P., parenchyma; g., gut; Zv.., lateral blood vessel,
beside which lies an excretory vessel ; #.4., excretory pore; d.v’., dorsal
blood vessel; ZZ. epidermis.
many of its cells are also glandular, secreting the mucus, which often:
forms a tube around the animal, or is exuded in movement. Beneath
the epidermis there is a parenchyma, consisting in part of connective-
tissue, and often in part gelatinous. The body is remarkably con-
tractile, and in some cases the spasms result in breakage. The muscles.
are circular and longitudinal, and often also diagonal. The fibres are
striped. In the adult there is no distinct coelom, the space between:
the gut and the body wall being filled up with gelatinous connective
tissue. In the larvae, however, a body cavity may be seen, either as an.
archiccele, z.e. the persistent segmentation cavity (Zzweus obscurus), on
GENERAL ACCOUNT OF NEMERTEA. 199
as a schizoccele, z.e. a space formed by the cleavage of the mesoderm
into two layers (Pilddiam-larve). In the adult only the blood spaces
and the cavity of the proboscis sheath are coelomic. The nervous
system consists of 2 brain generally four-lobed,—the two lobes of each
side being closely united and connected with those on the other side by
a commissure above and by another below the proboscis cavity. From
the lower lobes two longitudinal nerve-stems run along the sides, and
are sometimes united posteriorly above the anus (Fig. 104, 2.7.). In
some forms there is in addition a dorso-median nerve, and sometimes a
ventro-median nerve.
On each side of the head there is a ciliated pit communicating with
the exterior through an open slit or groove, and communicating inter-
nally either with the brain itself or with adjacent nervous tissue. In
those cases in which the development has been studied, these so-called
lateral organs arise from
epiblastic insinkings and
cesophageal outgrowths, In
the most primitive genus,
Carinella, they are absent,
except in one species. It
has been suggested that they
conduce to the respiration of
the brain, which is rich in
hemoglobin, and they have
even been compared with
gill-slits. In some forms the
groove through which they
open to the exterior is rhyth-
mically contractile. It has
also been suggested that they
are sensory. Apart from ‘ :
these organs, Nenfertines are, din, Doral neve ic mobs cavity
Wery. sensitive, and an many. filing, cadssless BUM, dorso- ventral by
this is associated with a super- — diagonal muscles; Zv., lateral blood vessel.
ficial nerve plexus. Tactile
papillae and patches are often present; eyes and eye spots are
general ; and in some there are otocyst-sacs. Apart from the cephalic
slits, the head also bears sensory pits and grooves and terminal
sensory spots. In some there is a pair of lateral sense organs
in the (anterior) nephridial region. The mouth is ventral, and leads
into a plaited glandular fore-gut or cesophagus, which is followed by a
straight, ciliated mid-gut (stomach and intestine), usually with regularly
arranged lateral caeca. Between the cxeca run transverse muscle parti-
tions. The anus is in most cases terminal. In a cavity along the dorsal
median line there lies the remarkable proboscis. It is protruded and
retracted through an opening above, or, in a few cases, within the
_mouth. It arises as an invagination from infront, and is a muscular, °
very richly innervated tube lined with glandular epithelium, sometimes
protruded with such force that it separates from the body, and then
often retains its vitality for a long time, as if it were itself a worm. It
has been compared in its retracted state to a glove-finger drawn in by
Fic. 105.—Transverse section of a simple
Nemertean (Carznella).—After Biirger.
200 UNSEGMENTED “WORMS.”
two threads attached to its tip, the threads being retractor muscles.
But in front of the attachment of the retractor muscles there is a non-
eversible glandular region which secretes an irritant fluid. In many
cases there are stilets at the tip of the eversible portion, and if these be
absent, there are stinging cells or adhesive papille. There is a hint of
a similar structure in some Turbellarians, and the organ may be inter-
preted as one which was originally tactile, but which has become
secondarily aggressive. It is protruded by the muscular contraction of
the walls of the proboscis sheath, which forms a closed cavity surround-
ing the proboscis, and containing a fluid with corpuscles (Fig. 103).
In the majority there are three longitudinal blood vessels or spaces,
a median and two laterals, which unite anteriorly and posteriorly, and
also communicate by numerous transverse branches. The vessels or
spaces are remnants of a ccelom. The blood is a colourless fluid,
sometimes at least with nucleated elliptical corpuscles in which heemo-
globin may be present. ;
The excretory system usually consists of two coiled ciliated canals
opening in the anterior region by a varying number of ducts. They are
said to divide up internally into numerous fine branches ending in flame-
cells, or in blind ampullee embedded in the walls of the blood vessels.
The sexes are usually separate, and the reproductive organs are
always simple. A few species (of Geonemertes and Prosadenophorus) are
hermaphrodite, and some species of 7etrastemma are protandrous. The
organs consist of siniple sacs, arranged in a series on each side between
the intestinal caeca, and communicating with the exterior by fine pores.
The ova are often laid in gelatinous tubes, and are probably fertilised
shortly before or at the time of expulsion. In three or four forms
(Prosorhochmus, a fresh-water Zetrastemma, a species of Linews) known
to be viviparous, the fertilisation must, of course, be internal.
Segmentation is total and almost always equal ; a complete or partial
gastrula is formed, and development may be direct or indirect.
In Cerebratulus, etc., the larva is adapted for pelagic life, and is
known as the Pilidium. ‘‘In external shape it resembles a helmet with
spike and ear lobes, the spike being a strong and long flagellum or a
tuft of long cilia, the ear lobes lateral ciliated appendages” (Hubrecht).
Out of this, somewhat abruptly, the adult form arises.
Relationships.—The Nemertines are probably nearly
related to Turbellaria, but show some very distinct marks
of advance. Of these, the most noticeable are the presence
of an anus, of a closed vascular system, of a coelom at least
in the larva. The presence of flame-cells in connection
with the excretory system confirms the idea of Platyhelminth
affinities; but it is to be noticed that the reproductive
system is strikingly different. Professor Hubrecht has~
suggested that Nemertines exhibit affinities with Verte-
brates, comparing proboscis sheath with notochord, and
so forth.
CLASSIFICATION AND HABITS OF NEMERTEA. 201
Classification. ;
Order Protonemertini. Brain and lateral nerves outside the muscular
layers ; mouth behind brain ; no stilets.
Carinella, Hubrechtia.
Order Mesonemertini. Lateral nerves in the muscular layer ; mouth
behind brain ; no stilets.
Carinoma, Cephalothrix.
Order Metanemertini. Mouth in front of brain, usually opening
along with proboscis; usually with stilets; lateral nerves
internal to the muscular layers; usually with an intestinal
cecum.
e.g. Amphiporus, Drepanophorus, Tetrastemma.
An isolated form, A/alacobdella, parasitic in bivalves,
has a posterior sucker, a coiled intestine, and other
peculiarities.
Order Heteronemertini. Mouth behind brain; no stilets; three
layers of muscle, the outermost and innermost longitudinal ;
lateral nerves outside circular muscular layer.
eg. Lineus, Cerebratilus.
Habits.—Most Nemertines are marine, creeping about
in the mud, under stones, among seaweed, and the like;
many, ¢.g. Cerebratulus, are able to swim; Felagonemertes
and Planktonemertes are \eaf-like hyaline forms of pelagic
habit ; two or three species of Prostoma live in fresh water ;
seven species of Geonemertes are terrestrial; MJalacoddella
and a few others live in the mantle-cavity of marine
bivalves, and some others are found as commensals in
Ascidians; Cephalothrix galathee destroys the eggs of
its host—the crustacean Galathea. Most seem to be
carnivorous, eating annelids, molluscs, and even small
crustaceans. Many break readily into pieces when irritated,
and some are able to regenerate what they lose in this way.
The fresh-water Prostoma lumbricoides forms a protective
cyst of mucous threads in unfavourable conditions, and
Tetrastemma dorsale often does the same along stems of
the hydroid Tubularia.
PuyLtumM NEMATOHELMINTHES
Class Nematoda, e.g. Ascaride.
Class Nematomorpha, Gordiide.
Class Acanthocephala, e.g. Echinorhynchus.
202 UNSEGMENTED “WORMS.”
Class NemMatopa. Thread-worms, Hair-worms, etc.
The Nematodes are unsegmented, more or less thread-like
“worms,” some of which are free-living and others parasitic.
The body is covered by a cuticle, often thick, often subject to
moulting ; the muscular system consists of elongated muscle-
cells arranged longitudinally, and usually leaving two free
“lateral lines.” From a nerve-ring around the gullet, six
nerves go forwards and six backwards. The alimentary canal
is usually well developed, has mouth and anus, and is divided
into three regions. Vascular and respiratory systems are
unrepresented ; the cavity of the body is not celomic, the
remarkable excretory system consists of two lateral canals
opening anteriorly by a single pore. The sexes are usually
separate and the reproductive organs simple ; there 1s distinct
sexual dimorphism. The life history ts often intricate. There
are many remarkable features such as the sluggish ameboid
spermatozoa, the absence of cilia, and the absence of migratory
phagocytes.
Type, Ascaris megalocephala, the Round-worm
of the horse
This round-worm occurs in the small intestine of the
horse, while other species similarly infest man, ox, pig,
etc. The body is cylindrical in cross-section and tapering
at each end. The colour is dead-white, the absence of
pigment being very characteristic of Nematodes. Some of
the small thread-worms, e.g. Trichostrongylus pergracilis in
the ceeca of the grouse, are quite transparent and almost
invisible when alive. At the anterior end is the mouth,
furnished with three lips bearing sense papille ; the anus is
posterior and ventral. The male is smaller than the female,
and has a recurved tail furnished with two horny spines and
numerous sense papillz. It is usually about seven inches
long, while the female may be as much as seventeen.
(2) Most externally there is a thick chitinoid cuticle,
perhaps of protective value. With its presence may be
associated the scarcity of cutaneous glands, and the absence
of cilia. (4) Beneath this is the sub-cuticula or epidermis,
thickened along four longitudinal lines—median dorsal,
ASCARIS. 203
ventral, and lateral—and consisting of a protoplasmic matrix.
without distinct cell-limits. Except at the tail-end the
nuclei are confined to the longitudinal lines, and are most
numerous laterally. The epidermis makes and remakes.
the cuticle, which is periodically moulted. (¢) Beneath the
epidermis is a layer of remarkable muscle cells, lying in
groups defined by the lines mentioned above. Many of
the Nematodes are very
agile.
Around the pharynx there
is a nerve-ring from which
six nerves run forwards and
six backwards. One runs
along the median dorsal
line—a unique position in
an Invertebrate. Here and
there on the ring and on the
nerves there are ganglionic
cells, but there is but little
aggregation of these into
ganglia. Sense organs are
represented by the papillze
already mentioned.
As the food consists of
juices from a living host,
it is not surprising to find py, 106, Cross-section through
that the alimentary canal Ascaris.
has but a narrow cavity. azx., Dorsal nEEvE aie non-contractile
. a portion of muscle cells; ¢., cuticle; ¢.
It consists of three parts epidermis; 2.2, lateral line; 0.5, ex.
a fore-gut or cesophagus, erStory. vessel Me, cee portion of
: . s 3 Utes tra 3
lined bythe inturned cuticle, Qury; uz, uterus;g, gue
a mid-gut or mesenteron of
endodermic origin, and a usually short hind-gut or rectum
lined by the cuticle. When the external cuticle is shed, so
is that of the fore-gut and hind-gut (cf. Crayfish).
There is a distinct space between gut and body wall, but it is lined
externally by the muscle cells, internally by the endoderm of the gut,
which has no mesoblastic coat; the space is therefore not strictly
ccelomic. It contains a clear fluid, which probably discharges some of
the functions of blood. There are xo free amaboid phagocytes.
Embedded in each lateral line there is a longitudinal canal. These
204
UNSEGMENTED “WORMS.”
unite anteriorly, and open in a ventral excretory pore near the head.
They seem to be associated internally with fixed phagocytic cells.
In the species discussed there are four giant branched cells situated
PY,
Fic. 107.—Diagram of
the structure of a
male Nematode.
M.,mouth; G.,cesophagus;
GA., nerve ring; B., bulb
atlowerend of fore-gut; G.,
mesenteron; S/., spine
with sheath; 4., anus;
D., ejaculatory duct; VS.,
seminal vesicle; 7., testis;
ET,, longitudinal excre-
tory tube, cut short; 2P.,
excretory pore.
anteriorly, which are especially connected with
taking up waste particles. The relation of
this excretory system to that of other In-
vertebrates is unknown.
The sexes are separate. In the
male the testis is unpaired—a coiled
tube gradually differentiating into vas
deferens, seminal vesicle, and ejacu-
latory duct. The genital aperture is
close to the anus. The spermatozoa
have not the typical form, and are
sluggish. In the female the ovary is a
paired tube, which passes gradually into
an oviduct and a uterus at each side,
and a short unpaired vagina. The
genital aperture is ventral and anterior.
The ova meet the spermatozoa at
the junction of uterus and oviduct.
Segmentation is total, and results in
the formation first of a blastula and
then of a gastrula. The germ-cells are
distinguishable very early from the
body-cells. Blastopore and archen-
teron are obliterated, the mid-gut
arising as a secondary splitting between
two rows of endoderm cells. The eggs
pass out of the gut of the host and
probably hatch in water, and are thus
re-introduced. No intermediate host
has yet been found.
The Nematoda form an important group,
interesting both on account of their parasitism
and on account of their peculiarly isolated
zoological position. Though parasitism is
exceedingly common, many are free - living
for at least a part of the life cycle, and feed on putrefying organic
matter.
Again, although the number of individuals which may infest
‘one host shows how successful the parasitism is, yet Nematodes exhibit
few of the ordinary adaptations to a parasitic life, and there is no sharp
structural line of demarcation between free and parasitic forms, Among
LIFE HISTORIES. 205.
histological peculiarities, the absence of cilia—paralleled elsewhere:
only among the Arthropods—the nature of: the muscle cells, the con-
dition of the sub-cuticular layer, are to be noticed. Among the grosser
structural peculiarities, the nature of the excretory system, of the body
cavity, and of the nervous system, are worthy of special note. Sense-
organs are never well developed, but in the free-living forms simple:
eyes may occur. The alimentary canal is usually completely developed,
but may, as in Spherularia, be degenerate. Of the relationships
nothing is known.
Lire HIsToRIES
I. The embryo grows directly into the adult, and both live in fresh.
or salt water, damp earth, and rotting plants—Enoplide, e.g.
Enoplus.
2. The larvee are free in the earth, the sexual adults are parasitic im
plants, or in Vertebrate animals, e.g. Zylenchus scandens, «a
common parasite on cereals; Strongylus and Dochmius im
man.
3. The sexual adults are free, the larve are parasitic in insects,
e.g. Mermis. The fertilised females of Spherularia bombt
pass from the earth into the body-cavity of humble-bee and
wasp, whence their larvze bore into the intestine and eventually-
emerge.
. The larvee are parasitic in one animal, the sexual adults in another
which feeds on the first. Thus O//zdanus passes from mouse:
to cat, Czcullanus from Cyclops to perch.
as
There are other life histories, and many degrees of parasitism. The
most remarkable form is Axgiostomum (or Ascaris or Leptodera)
nigrovenosum, In damp earth males and females occur, the progeny of
which pass into the lungs of frogs and toads. There they mature into.
hermaphrodite animals (the only example among Nematodes), which
produce first spermatozoa and then ova. They are self-impregnating,
and the young pass out into the earth as males or females. Here there-
is alternation of generations: and a somewhat similar story might be
told of Rhabdonema strongyloides from the intestine of man, and
Leptodera appendiculata from the snail.
There are several quaint reproductive abnormalities, thus—the female-
Spherularia bombz, which gets into the body cavity of the humble-bee,
has a prolapsed uterus, larger than itself; the male of 7rechodes crassi-~
cauda passes into the uterus of the female.
[TaBLE.
206
UNSEGMENTED “‘ WORMS.”
Some of the most Important Forms Parasitic in Man.
cotdes, Maw-worm
(common).
[A.mystax,com-
mon in dogs and
-| cats, has also been
found in man.]
intestine.
ment has shown that
infection results if
the eggs (with their
outer envelope en-
tire) are swallowed
along with vegetable
food or otherwise.
RESULT ON
Name. PosiTIon. History. Host
Ascaris lumbri- Usually in small Repeated experi- Commonest in
children; rarely
dangerous, unless
very numerous, or
wandering into
other parts of the
body, ¢.g. respira-
tory tract, bile
duct, vermiform
appendix. Like
others, it may
puncture the wall
of the gut and
liberate pathogenic
bacteria.
Oxyuris vermi-
cularis (common).
Trichocephalus
dispar or trichi-
urus, the whip-
worm (common).
From stomach to
rectum, mostly in
colon.
From food
water.
or
Rarely more than
discomfort.
Colon ; more
rarely appendix
and small intestine.
(Australia, China,
India, Egypt, and
Brazil).
(80-100 mm.) in
lymphatic glands,
embryos in blood.
Males rare (30-45
mm.).
quito.
Anchylostomum | Small intestine. The larvae seem] Ulceration, he-
duodenale (widely to live freely in the | morrhage, and dan-
distributed). F _, | earth. Infection by | gerous anemia. It
Rhabdonema| Associated with | ingestion or cutan- | was common in the
strongyloides. Anchylostomum. | eously. workersattheMont
Cenis Tunnel, and
has occurred in
Cornish mines.
Filaria bancrofti| Mature female} Larve in a mos-
Elephantiasis
and hematuria.
Dracunculus (Fil-
The female is 1-6
Larve in a Cy-
Subcutaneous
tine; embryos, pro-
duced rapidly and
viviparously, find
their way to
muscles, and be-
come encysted.
to man.
aria) medinensis ft. long, encysts | clops. abscesses.
(Guinea-worm), in| beneath skin, es-
Arabia, Egypt, | pecially of back or
Abyssinia, etc. legs. Male rarely
seen.
Trichina (Tri-| Becomessexually| From | ‘‘trichi-| Inflammatory pro-
chinella) spiralis. | mature in the intes- | nosed” pig’s muscle
cesses, often fatal,
are brought about
by the migration
of the young worms
from intestine to
muscles.
TRICHINA. 207
Trichina,—The formidable Trichina deserves fuller notice. It is
best known as a parasite in man, pig, and rat, but occurs also in
hedgehog, fox, marten, dog, cat, rabbit, ox, and horse, The sexual
forms live in the intestine, the female about 3 mm. in length, the male
less than half as long. After impregnation the female brings forth
numerous embryos viviparously, sixty to eighty at a time, and altogether
about 1500. These are produced in the wall of the intestine, or in the
adjacent lymphatic spaces. Most of them find their way into lymph
and blood vessels, and are swept by the blood stream to the muscles ;
occasionally some seem to migrate actively, boring their way especially
through connective tissue. The migration causes inflammation and
fever. In or between the muscle fibres they grow, coil themselves
Fic. 108.—Trichinz in muscle, Fic. 109.—Trichinz in muscle,
about to be encapsuled.— encapsuled. There may be
After Leuckart. 12,000 in a gramme of pig’s
muscle. —After Leuckart.
spirally, and become encysted within a sheath, at first membranous
and afterwards calcareous (Figs. 108 and 109). The cyst is partly due
to the muscle, and partly to the parasite. The infected muscle fibre
degenerates. In these cysts, which may be sometimes counted in
millions, the young Trichine remain passive, unless the flesh of their
host be eaten by another,—pig eating rat, man eating pig. In the
alimentary canal of the new host the capsule is dissolved, the embryos
are set free, and become in two or three days reproductive. The male
seems to die after copulation.
Among the numerous other parasitic Nematodes the following may
be noted :—The giant palisade worm (Zustrongylus gigas) occurs in the
renal region of domestic animals, etc. ; the female may be 3 ft. long.
The armed palisade worm (Strongylus armatus) occurs in the intestine
208 UNSEGMENTED “WORMS.”
and intestinal arteries of horse, causing aneurisms, colic, ete. The
young forms are swallowed from stagnant water, bore from gut into
arteries, become adult, return to gut, copulate and multiply. Various
other species of S¢rongylus occur in sheep, cattle, etc. Of the genus
Ascaris alone, over 200 species have been found in all types of Verte-
brates ;—A. megalocephala in horses, A. /umbricordes in man, A. mystax
in cats and dogs. Syngamus trachealis occurs in the trachea of birds,
causing ‘‘gapes,” ¢.g. in poultry and pheasants. It pierces the wall
of the trachea, and ‘‘ actually clenches the teeth with which its mouth
is provided in the tracheal rings.” A remarkable large form, /chthyo-
nema grayz, is found inside sea-urchins. Various species of Zylenchus,
especially 7. devastatrix and T. scandens (or T. triticz), destroy cereal
and other crops. Various species of Heterodera (especially H. schachtet
and #. radiczcola) infest the roots of many cultivated plants, e.g. turnip,
radish, cabbage.
Class NEMATOMORPHA
The Gordiide (e.g. Gordius aguaticus—the horse-hair worm) are so
different from true Nematodes that they must be ranked in a separate
class. There are no lateral lines. Three nerve-strands lie close
together in the mid-ventral line. In the adult Gordéus the mouth is
shut and the food canal is partly degenerate. The adult Gordiide
usually live freely in fresh water; larval forms occur in aquatic
molluscs, young insects, etc. ; later stages usually occur in carnivorous
insects, whence they emerge to become adult in the water. One form,
Nectonema agile, is marine.
Class ACANTHOCEPHALA
For a few genera, of which the best known is Achznorhynchus,
whose larvee live in Arthropods, and the adults in Vertebrates, a
special class, ACANTHOCEPHALA, has been established. They may be
placed beside Nematodes, but the relationship does not seem to be
very close. Mouth and gut are absent. The anterior end bears a
protrusible hooked proboscis used in boring in the intestinal wall of the
host. In the minute swellings at the ends of the two much-branched
excretory organs of Z. gégas, there are ciliated cells,—the only case
known among Nematohelminthes.
LEchinorhynchus proteus of pike, minnow, trout, etc., larva in the
Amphipod Gammarus pulex.
4 angustatus of perch, larva in the Isopod Ased/us
aguaticus,
ve moniliformis of rat, etc., larva in larval beetles
(Blaps).
of gigas of pig, larva in grubs of cockchafer, etc.
CHAPTER XI
PHYLUM ANNELIDA
Chief Classes—CuH&TOPODA, DISCOPHORA
THe Annelids or Annulata are segmented “worms,” in
most of which the segmentation of the body ts visible exter-
nally. The head usually consists of a pre-oral “ prostomium”
and a post-oral peristomium. The body wall has several
layers. of muscles, and many, eg. Chetopods, have sete
embedded in the skin. In most, there is a well-developed
calom, communicating with the exterior by patred nephridia.
The nervous system consists typically of two dorsal cerebral
ganglia, a commissural ring round the gullet, and a ventral
ganglionated chain. The gonads arise on the calomic ept-
thelium. Not infrequently the nephridia function also as
genital ducts. The development may be direct or indirect,
and if indirect it usually includes a larval Trochosphere stage.
In habit, form, and structure the Annelids exhibit much
diversity. The Chztopods, represented on the one hand
by the familiar earthworm, and on the other by the marine
worms, best illustrate the typical Annelid structure. With
these, however, may be included the aberrant Echiuride,
e.g. Echiurus and Bonellia. A few primitive forms (Archi-
Annelida), and the Myzostomata (parasitic on Crinoids),
may also be appended to the Cheetopod class. The leeches
(Discophora) are probably Annelids which have diverged
in consequence of a peculiar half-parasitic habit. Finally,
some zoologists include Sagit#fa (Chzetognatha) in this series
as an Annelid with three segments, and also the Rotifers
(Rotatoria), whose adult form somewhat resembles the
‘Trochosphere larvee of many Annelids.
14
210 PHYLUM ANNELIDA.
Annelids with Bristles
Segmented animals with sete developed in little skin-sacs,
either on a uniform body wall or on special locomotor pro-
trusions known as parapodia. The segments, indicated
externally by rings, are often marked internally by parti-
tions running across the body cavity, which is usually well
developed. The nervous system generally consists of a double
ventral chain of ganglia, connected with a pair of dorsal
cerebral ganglia by a ring round the beginning of the gut.
Two excretory tubes or nephridia are typically present in
each segment, and they or their modifications may also
function as reproductive ducts. The reproductive elements
are formed on the lining membrane of the body cavity. The
development ts either direct or with a metamorphosis.
The two chief orders of this class may be contrasted :—
Class CHATOPODA.
OxicocHa&TA, e.g. Earthworm.
Potycu ata, e.g. Nereis.
With no parapodia, and with relatively
few seta.
Without any “jaw” apparatus in the
pharynx.
Head not highly developed. No tent-
acles or cirri. Gills in a few forms.
With complex hermaphrodite reproduc-
tive organs, limited in number and
definitely localised.
Development direct.
Living in fresh water or in the soil.
With parapodiaand with very numerous
seta.
The pharynx is often armed with
“jaws.”
The head is much more developed, and
bears tentacles and cirri. Gills are
often present.
Sexes usually separate, and reproduc-
tive organs simple.
A metamorphosis in development.
Marine, with two or three exceptions.
Type of OLticocHa#Ta. The Earthworm (Lumbricus)
Habits.—Earthworms eat their way through the ground,
and form definite burrows, which they often make more
comfortable by a lining of leaves. The earth swallowed by
the burrowers is reduced to powder in the gut, and, robbed
of some of its decaying vegetable matter, is discharged on
the surface as the familiar “‘worm-castings.” By the
burrowing the earth is loosened, and ways are opened for
plant-roots and rain-drops; the internal bruising reduces
mineral matter ‘to more useful form; while, in covering
the surface with earth brought up from beneath, the earth-
EARTHWORM. 211
worms have been ploughers before the plough. Darwin
calculated that there were on an average over 53,000
earthworms in an acre of garden ground, that ro tons of
Fic, 110,—Earthworms,
soil per acre pass annually through their bodies, and that
they cover the surface with earth at the rate of 3 in. in
fifteen years. He was therefore led to the conclusion that
earthworms have been the great soil- makers, or, more
212 PHYLUM ANNELIDA.
precisely, that the formation of vegetable mould was mainly
to be placed to their credit.
Though without eyes, earthworms are sensitive to light
and persistently avoid it, remaining underground during
the day, unless rain floods their burrows, and reserving
their active life for the night. Then, prompted by “love”
and hunger, they roam about on the surface, leaving on
the moist roadway the trails which we see in the morning.
More cautiously, however, they often remain with their tails
fixed in their holes, while with the rest of their body they
move slowly round and round. The nocturnal peregrina-
tions, the labour of eating and burrowing, the transport of
leaves to their holes, the collection of little stones to pro-
tect the entrance to the burrows, include most of the
activities of earthworms, except as regards pairing and egg-
laying, of which something will afterwards be said. When
an earthworm is halved with the spade, it does not neces-
sarily die, for the head portion may grow a new tail, while
a Gecapitated worm may even grow a new head and brain.
Phagocytes help as usual in the regeneration. ‘The earth-
worm is much persecuted by numerous enemies, ¢.g. centi-
pedes, moles, and birds. The male reproductive organs
are always infested by unicellular parasites—Gregarines of
the genus Monocystis ; and minute thread-worms (Lelodera
pellio) usually occur in the nephridia and body cavity, and
often in the ventral blood vessels.
Form and external characters.—The earthworm is often
about 6 in. long, with a pointed head end, and a cylindrical body
rather flattened posteriorly. The successive rings seen on the surface
mark true segments. The mouth is overarched by a small lobe called
the prostomium, and the food canal terminates at the blunt posterior
end. The skin is covered bya thin transparent cuticle, traversed by
two sets of fine lines, which break up the light and produce a slight
iridescence. On a region extending from the 31st to the 38th ring,
the skin of mature worms is swollen and glandular, forming the
clitellum or saddle, which helps the worms as they unite in pairs, and
also forms the slimy stuff which hardens into cocoons. The middle
line of the back is marked by a special redness of the skin. On the
sides and ventral surface we feel and see four rows of tiny bristles or
setae, which project from little sacs, are worked by muscles, and assist
in locomotion. These bristles are fixed like pins into the ground, at
times so firmly that even a bird finds it difficult to pull the worm
from its hole. As each of the four longitudinal rows is double, there
are obviously eight bristles to each ring. On the skin of the ventral
EARTHWORM. 213
surface there are not a few special apertures, which should be looked
for on a full-grown worm ; but careful examination of several specimens
is usually necessary. Almost always plain on the 15th ring are the
two swollen lips of the male ducts,
less distinct on the 14th are the
apertures of the oviducts through
which the eggs pass, while on each
side, between segments 9 and Io,
1o and 1r, are the openings of two
receptacula seminis or spermathecze
into which male elements from
another earthworm pass, and from
which they again pass out to fertilise
the eggs of the earthworm when
these are laid. Each segment
contains a pair of excretory tubes,
which have minute ventral-lateral
apertures, while on the middle line
of the back, between the rings,
there are minute pores, through
which fluid from the body cavity
may exude on to the skin.
Fic. 111.—Anterior region of
earthworm,—After Hering.
Skin and bristles. — The ee the eight setze (s.) on each segment.
. : : S., Spots between g-10, 10-11,
thin cuticle is produced by indicate openings of receptacula
the cells which lie beneath, Siti nd openings of oviduct
and is perforated by the aper- _deferentia on segment x,
tures previously mentioned.
The epidermis clothing the worm is a single layer of cells,
of which most are simply supporting or covering elements,
while many are slightly modified, as glandular or mucous
cells, and as nervous cells. As the latter are connected
with afferent fibres which enter the nerve-cord, the skin is
diffusely sensitive. In a few species the skin is slightly
phosphorescent. The chitinous bristles, which are longest
on the genital segments, are much curved, and lie in small
sacs of the skin, in which they can be replaced after
breakage.
Muscular system and body cavity. — The earthworm
moves by the contraction of muscle cells, which are
arranged in circular hoops and longitudinal bands under-
neath the skin. The special muscles above the mouth
and pharynx have considerable powers of grasping, while
less obvious muscular elements occur in the wall of the
gut, in the partitions which run internally between the
214 PHYLUM ANNELIDA.
segments, and on the outermost portions of the excretory
tubes.
Unlike the leech, the earthworm has a very distinct body
cavity, through the middle of which the gut extends, and
across which run the partitions or septa incompletely
separating successive segments. In this cavity there is
some fluid with cellular elements, of which the most
numerous are yellow cells detached from the walls of the
gut. Possible communications with the exterior are by
the dorsal pores, and also by the excretory tubes, which
open internally into the cavities of the segments.
Nervous system.—Along the middle ventral line lies a
chain of nerve-centres or ganglia, really double from first
to last, but compactly united into what to unaided eyes
seems a single cord. As the segments are very short, the
limits of the successive pairs of ganglia are not very evident,
especially in the anterior region, but they are plain enough
on a small portion of the cord examined with the micro-
scope, when it may also be seen that each of the pairs of
ganglia gives off nerves to the walls of the body. Anteriorly,
just behind the mouth, the halves of the cord diverge and
ascend, forming a ring round the pharynx. They unite
above in two dorsal or cerebral ganglia, which are situated
in the peristomium or first ring, and not, as in Polychetes,
in the prostomium. These form the earthworm’s “ brain,”
and give off nerves to the adjacent pre-oral lobe or pro-
stomium, on which are numerous sensitive cells. These,
coming in contact with many things, doubtless receive
impressions, which are transmitted by the associated nerves
to the “brain.” As Mr. Darwin observed that earthworms
seized hold of leaves in the most expeditious fashion, taking
the sharp twin leaves of the Scotch fir by their united base,
we may credit the earthworms with some power of profiting
by experience; moreover, as they deal deftly with leaves
of which they have no previous experience, we may even
grant them a modicum of intelligence. From the nerve-
collar uniting the dorsal ganglia with the first pair on the
ventral cord, nerves are given off to the pharynx and gut,
forming what is called a “visceral system.” The earth-
worm has no special sense organs, but there are abundant
sensitive cells, especially on the head end. By them the
EARTHWORM. 215
animal is made aware of the differences between light and
darkness, aid of the approaching tread of human feet, not
to speak of the hostile advances of a hungry blackbird.
The sense of smell is also developed. The afferent or
sensory nerve fibres from the nervous cells of the skin enter
the nerve-cord and bifurcate into longitudinal branches,
which end freely in the nearest ganglia. In this the earth-
worm’s nervous system suggests that of Vertebrates.
The nerve cells, instead of being confined to special centres or
ganglia, as they are in Arthropods, also occur diffusely along with the
nerve fibres throughout the course of the cord. Along the dorsal
surface of the nerve-cord there run three peculiar tubular “giant
fibres,” with firm walls and clear contents. They are probably
comparable to the medullated nerve fibres of Vertebrates.
Alimentary system.—Earthworms eat the soil for the sake
of the plant débris which it may contain, and also because
one of the modes of burrowing involves swallowing the
earth. In eating they are greatly helped by the muscular
nature of the pharynx; from it the soil passes down the
gullet or cesophagus, first into a swollen crop, then into a
strong-walled grinding gizzard, and finally through a long
digestive and absorptive stomach-intestine. There are
three pairs of cesophageal glands. Canals from the posterior
two pairs open into the anterior pair, and thus into the
gullet. Their contents are limy, and perhaps counteract
the acidity of the decaying vegetable matter. It may be
that they are in part excretory; or it may be that they
serve to fix some of the carbon dioxide formed by the
animal. The long intestine has its internal surface
increased by a dorsal fold, which projects inwards along
the whole length. In this “typhlosole,” and over the outer
surface of the gut, there are crowded yellow cells.
There is no warrant for calling the yellow cells hepatic or digestive.
Structurally they are pigmented cells of the peritoneal epithelium, which
here, as in most other animals, lines the body cavity and covers the
gut. As to their function, they absorb particles from the intestine,
and go free into the body cavity, whence, as they break up, their
débris may pass out by the excretory tubes. When a worm has been
made to eat powdered carmine, the passage of these useless particles
from gut to yellow cells, from yellow cells to body cavity, and thence
out by the excretory tubes, can be traced. The amceboid cells of the
body cavity fluid act as phagocytes. Various ferments have been
216 PHYLUM ANNELIDA.
detected in the gut, a diastatic ferment turning the starchy food into
sugars, and others—peptic and tryptic—not less important. The wall
of the stomach-intestine from without inwards, as may be traced in
sections, is made up of pigmented peritoneum, muscles, capillaries,
and an internal ciliated epithelium. In the other parts of the gut the
innermost lining is not ciliated, but covered with a cuticle.
Vascular system.—The fluid of the blood is coloured
red with hemoglobin, and contains small corpuscles. Along
1)
WH ey
oR hel
eX Ex,
Ss
ae
Ail
CA
es
WED
FiG. 112,—Transverse section of earthworm.
A., Cuticle; B., epidermis; C.JZ., circular muscles; 2.1, longitudinal
muscles ; D., aseta; C., coelom;_¥C., yellow cells; F., typhlosole ;
V.V., supra-neural blood vessel; S./., sub-neural vessel ; D.V., dorsal
vessel; /., peritoneum; £., cavity of gut; G, endodermic lining of gut;
WV., part of a nephridium ; &., opening of a ne hridium ; 7., the nerve-
cord; /., a nerve given off; ., giant fibres in the nerve-cord.
the median dorsal line of the gut a prominent blood vessel
extends, another (supra-neural) runs along the upper surface
of the nerve-cord, another (infra-neural) along the under
surface, while two small latero-neurals pass along each side
of this same cord. All these longitudinal vessels, of which
the first three are most important, are parallel with one
another ; the first three meet in an anterior network on the
EARTHWORM. 217
pharynx ; the dorsal and the supra-neural are linked together
in the region of the gullet by five or six pairs of contractile
vessels or “hearts.”
‘ Respiration is effected by the distribution of blood on
the general surface of the skin.
Excretory system.—There is a pair of nephridia in each
segment except the first four. Each opens internally into
the segment in front of that on which its other end opens
to the exterior. They remove little particles from the body
cavity, and get finer waste products from the associated
blood vessels. Nephridia occur in many animals, in most
young Vertebrates as well as among Invertebrates, but they
are never seen more clearly than in the earthworm. When
a nephridium is carefully removed, along with a part of
the septum through which it passes, and examined under
the microscope, the following three parts are seen :—
(a) An internal ciliated funnel; (4) a trebly coiled ciliated
tube, at first transparent, then glandular and granular; and
(c) a muscular duct opening to the exterior. Minute par-
ticles swept into the ciliated funnel pass down the ciliated
coils of the tube, and out by the muscular part which opens
just outside of the ventral bristles. The coiled tube con-
sists in part at least of a series of intracellular cavities, that
is to say, it runs through the middle of the cells which
compose it; the external muscular portion arises from an
invagination of skin.
Reproductive system.—Like all Oligochetes, the earth-
worm is hermaphrodite and the organs complex. The
complexity is produced by the specialisation of certain of
the nephridia to form genital ducts and accessory organs
and by the presence of chambers (seminal vesicles) con-
nected with the testes, formed by the shutting off of portions
of the body cavity.
The organs in the earthworm are difficult to dissect, and
differ considerably in old and young specimens.
(a) The Male Organs consist of two pairs of testes, three
pairs of seminal vesicles, and paired vasa deferentia.
(1) The testes, flattened lobed bodies, about ,/5 in. in size,
arise from proliferations of the peritoneal lining of the body
cavity, and are invested by a delicate membrane derived
therefrom; they lie near the nerve-cord, attached to the
218 PHYLUM ANNELIDA.
posterior surfaces of the septa between segments 9-10 and
10-11. They are minute, translucent, and difficult to see.
In immature worms they lie exposed in the body cavity; in
mature worms they are concealed by the great development
of the seminal vesicles.
(2) The seminal vesicles are much-lobed structures,
exceedingly prominent in dissection. Small and laterally
placed in young worms, in the adult the anterior two
Fic. 113.—Reproductive organs of earthworm.—
After Hering.
N., Nerve cord; 7., anterior testes; S., sacs of sete; 2.S.,
receptacula seminis; 5,4, seminal funnels; v.d., vas deferens ;
ovd., oviduct; ov., ovary; sv, seminal’ vesicles cut open;
Vill. -XV., segments.
pairs fuse in the middle line and cover the anterior pair
of testes and its ducts, while the posterior pair similarly
conceals the second pair of testes with its ducts. Into the
seminal vesicles mother sperm cells from the testes pass,
and there divide up to form spermatozoa.
Development shows that the seminal vesicles arise as
outgrowths of the septa of segments 9-12, and that their
lumen is a portion of the body cavity. This is of importance,
EARTHWORM. 219
for in Polycheetes the genital products mature in the general
body cavity, just as the spermatozoa in the earthworm
mature in the seminal vesicles.
(3) From the seminal vesicles the spermatozoa are carried
to the exterior by means of the vasa deferentia. The in-
ternal openings of these are large and funnel-shaped, and
are concealed by the seminal vesicles. Each of the four
funnels opens into a duct, and the two ducts unite at each
side to form the two elongated vasa deferentia, which pass
backwards to open externally on the 15th segment.
(6) The Female Organs consist of two ovaries and two
oviducts, each of which has a side receptacle for the eggs.
(1) The two ovaries are small bodies situated near the
nerve-cord on the septum between segments 12-13. Each
is pear-shaped, the stalk of the pear being a string of ripe
ova. They are more readily seen than the testes.
(2) The two oviducts open internally on the anterior
face of the septum between 13-14, and externally on the
ventral surface of segment 14. Into the wide ciliated in-
ternal mouths, which lie opposite the ovaries, the ripe eggs
pass.
(3) The egg-sac or receptaculum ovorum, near the internai
mouth of each oviduct, is a posterior diverticulum of the
septum between segments 13-14. Within it a few mature
ova are stored.
(c) Two pairs of receptacula seminis or spermathecze
receive spermatozoa from another earthworm, and liberate
them to fertilise the eggs of this one. They are white
globular sacs, opening in the grooves between segments
g-to and 10-11, and probably, like the genital ducts, arise
from modified nephridia. According to some, these sper-
mathece not only receive and store spermatozoa, but make
them into packets or. spermatophores. Others say that the
glands of the clitellum make these packets. At any rate,
minute thread-like packets of spermatozoa are formed, and
a pair of them may often be seen adhering to the skin of
the earthworm about the saddle region.
When two worms unite sexually, they lie apposed in
opposite directions, the head of the one towards the tail of
the other. What happens is that the spermatozoa of the
one pass into the receptacula of the other.
220 PHYLUM ANNELIDA.
When the eggs of an earth-
worm are liberated, they are
surrounded by a sheath of
gelatinous stuff, believed to
be secreted by the saddle.
As this is peeled off towards
the head, spermatophores are
also enclosed.
Development. — Many
cocoons are made about the same
time, and each contains numerous
ova, and also packets of sperms,
so that fertilisation takes place
outside the body. These cocoons
are. buried in the earth a few
inches below the surface. They
measure about a quarter of an inch
in length.
The favourite time for egg-
laying is during the spring and
summer, though it may be con-
tinued throughout the whole year.
The earthworm of the dungheap
(Z. fetidus) makes this a habit,
induced probably by the warmth
of its environment.
Of the many ova in the cocoon
of ZL. derrestris, only one comes
to maturity, while in Z. fetidus a
few, and in Z. communzs two may
do so. But in the last species
Fic. 114.—Stages in the develop-
ment of earthworm. — After
Wilson.
1. Two-celled stage; 4.c., polar bodies.
2. Blastula; JZ, a primary mesoblast.
3. Gastrula stage; Zc., ectoderm or
epiblast ; £7., endoderm or hypo-
blast,-in process of being covered
by the small ectoderm cells. Note
the widely open blastopore; JZ,
mesoblast cells.
4. Longitudinal section in late gastrula
stage, showing germ-bands; ec,
ectoderm; ex., endoderm; JZ,
mouth; sz, stomodeum; wm,
primary mesoblasts; Vé., neuro-
blasts; #c., nerve-cord; JV., ne-
phridioblasts ; ms., mesoderm
bands; xpc., incipient nephridia.
EARTHWORM. 221
the two embryos are often twins formed from one ovum, separation
taking place at the gastrula stage.
The whole process of growth, until leaving the egg, lasts from two
to three weeks, the time varying, however, with the temperature.
The ovum is surrounded by a vitelline membrane, and is laden with
yolk granules. Segmentation is slightly unequal (Fig. 114 (1)), and
exhibits considerable variation even within the limits of a species.
In about twenty-four hours a nearly spherical, one-layered blasto-
sphere or blastula is formed. It consists of only about thirteen cells.
During the next twenty-four hours the cells increase in number rapidly,
but the blastula remains one-layered. Two cells lying together do not
take part in this division ; they are rather larger than the rest, and their
inner ends project into the cavity, and are soon cut off as daughter-cells.
Gradually the large cells still undergoing division begin to sink in, and
at last are quite included in the cavity (Fig. 114 (2)). Thus there arise
two parallel rows of cells within the blastula, and these define the
longitudinal axis of the embryo. This is the beginning of the mesoblast
which forms all the muscles of the trunk, and which thus takes origin
from two primary mesoblasts.
After five to six pairs of secondary mesoblasts have been formed, the
blastula begins to flatten, and to elongate, becoming an oval disc. The
cells of the lower surface become clearer, and the hypoblast is thus
defined. The cells of the upper surface are smaller, and become very
much flattened ; they compose the epiblast. The mesoblasts lie side
by side near one end, forming two rows extending forwards and down-
wards, but divergent, because of the flattening of the blastula. The
hypoblast now becomes concave, and thus the blastopore arises,
occupying the whole of the lower surface (Fig. 114 (3)). The sides close
in and the blastopore becomes a slit, which further closes from behind
forwards, leaving only a small opening—the future mouth. During
these processes the cells at the anterior tip of the blastopore, which
will give rise to the pre-oral lobe, undergo no change, but the mesoblast
has been active.
As gastrulation proceeds, the mesoblast rows grow forwards and
upwards, until they come near each other above the anterior tip of the
blastopore, while their middle portions are carried downwards until they
lie on the ventral surface. Over them the epiblast is thickened in two
bands. Two longitudinal rows of epiblast cells near the anterior end,
and ending behind in large cells, sink in just as the primary mesoblasts
did. The thickening now extends ventrally until the two bands meet
and, passing into the blastopore, form the stomodeum. Even before
this the embryo has begun to swallow the albumen in which it floats.
There are now two lateral bands of cells called the germ bands,
composed of three layers (Fig. 114 (4)); outside is the thickened epi-
blast, next the rows of cells which sank in, and innermost the meso-
blast rows. The mesoblast rows have met in the middle line by
dividing and widening out into a pair of flattened plates, but they still
end behind in the two primary mesoblasts. Ccelomic cavities develop
in the plates, and the anterior ends meet above the mouth. The
epiblastic rows which sank in (there were eight of them, four on each
‘side of the median line, and each ending in a large mother-cell) go on
.
222 PHYLUM ANNELIDA.
growing. The mother-cells are apparently carried backwards as the
embryo lengthens, leaving a trail of daughter-cells behind them. The
cells so formed also divide, the embryo rapidly lengthening and finally
becoming vermiform. Of the eight rows the innermost on each side
(neuroblasts) give rise to the nervous system, the next two rows on
either side (nephridioblasts) form parts of the nephridia (Fig. 114 (4)),
while of the fourth row nothing definite is known. Each row, ending
behind in a single cell, widens out and deepens as it is traced forwards.
The neural and mesoblastic rows can be traced round the mouth, and
help to form the prostomium ; the others fade away at the sides of the
stomodzeum.
Let us sum up this complex history :—
la (a) The original outer layer
becomes the epidermis.
6) The secondary inner strat-
Epiblast um consists of neuroblasts
or which form the nervous
ectoderm. system, of nephridioblasts
which form parts of the
nephridia, and of lateral
cells of unknown function.
Two-layered
Fertilised Plastecehesr _gastrula | Mesoblast
ovum. blastula with primitive oe
‘ mesoblasts. inesodeem Muscle.
formed from Blood vessels. a
the division of | Ler parts of nephridia.
the primitive Reproductive organs.
“*mesoblasts.”
Mypeblest Lining of
endoderm. mid-gut.
Type of Potycua#ta. The Lob-worm (Avenicola
marina)
Habits.—On the flat sandy beach uncovered at low tide,
the ‘“‘castings” of the lob-worm or lug-worm are very
numerous. There the fishermen seek the worms for bait,
and have to dig quickly, for the burrowers retreat one to
two feet into the sand. The burrows are curved tubes,
lined by a yellowish green secretion from the animal’s
epidermis, and the surrounding sand is often discoloured by
some change which the secretion effects on the iron oxides
and other constituents. The tubes are at first vertical,
afterwards oblique or horizontal, and then turn vertically
upwards again.
The lob-worm burrows like the earthworm, not only
forcing the anterior part of its body onwards, but eating the
ARENICOLA. 223
sand for the sake of the organic particles and small organisms
which it contains. The sandy castings, which pass from
the end of the food canal, and are got rid of at the mouth
of the tube, fall into spiral coils. It has been calculated
that in a year the average volume of sand per acre thus
brought up in castings is about 1900 tons, representing a
layer of 13 in. spread out over the surface. This work,
comparable to that of earthworms, tends to cleanse the sand
and to reduce it toa finer powder. When getting rid of the
casting, the worm lies with its tail upwards and its head
downwards, or with its body bent like a bow; when the
tide comes in, the mouth may protrude at the other end of
the U-shaped tube. The worms that live between tide-
marks seem to differ in many respects (as to colour, gills,
Fic. 115.—<Aventcola marina.
Entire animal viewed slightly from left side. Note anterior mouth;
sete on anterior region; sete and gills on median region 3
thinner tail region often longer than shown.
habits, and sexual maturity) from those which occur in the
Laminarian zone, which is only uncovered at low spring-tides.
External appearance.—-The lob-worm varies in length
from 8 to 16 inches, and at its thickest part is about
half an inch in diameter. There are three regions in the
body: (2) The anterior seven segments, of which all but
the first have bristles; (4) the middle region of thirteen
segments, with both gills and bristles; (c) the thinner
posterior part of variable length, without either gills or
bristles, and with an inconstant number of segments (up
to about thirty). In the very front there is a head-lobe or
prostomium, but there are no tentacles or eyes. Anteriorly
a soft proboscis is often protruded from the gut. The
anus is terminal.
224 PHYLUM ANNELIDA.
Skin, muscles, and appendages.—-Each segment is
marked by about four superficial rings. ‘he epidermis
is pigmented and secretes mucus, and is divided into
numerous polygonal areas, separated by shallow grooves.
Beneath the epidermis is a sheath of circular muscles, and
then a layer of longitudinal muscles. Besides these there
are (from the middle of the gullet to the beginning of the
tail) thin oblique muscles arising from the sides of the
nerve-cord, and dividing the body cavity longitudinally into
a central and two lateral compartments. Other muscles
control the prostomium, the proboscis, and the bristles.
Unlike many of the marine Annelids, Avenicola has very
rudimentary appendages. This reduction of appendages
must be associated with the animal’s mode of life; it
occurs also in many tube-inhabiting worms. Neither the
prostomium nor the first segment show any trace of
appendages, but the next nineteen have rudiments. The
dorsal part (notopodial) consists of a tuft of bristles, whose
bases are enclosed in a sac ;—the ventral part (neuropodial),
separated by a short interval, bears several hooks.
Nervous system.—This is in its general features like
that of the earthworm, but ganglia are
not developed. In the ventral nerve-
cord, the ring round the gullet, and
the slight cerebral enlargement which
represents a brain, nerve cells occur
diffusely scattered among the nerve
fibres. Along the dorsal surface of the
nerve-cord, in the branchial region,
. there are two “giant fibres” like those
Fic. 116.— Anterior - 5
part of nervous sys- 19 the earthworm; anteriorly and
tem in Avendcola.— posteriorly there is only one.
After Vogt and Yung.
¢., Cerebral part on dorsal The prostomial lobes are diffusely sensory,
surface; @.r., cesoph- and bear also two ciliated, probably olfactory,
ageal ring ; ss gullet; pits—the ‘‘ nuchal organs.” Otherwise sense
Sordi ee ice nerves organs are represented only by a pair of oto-
ot., otocyst. cyst sacs (Fig. 116), one on each side of the
cesophageal nerve-ring. These sacs, like those
which occur in many other Invertebrates, seem to have to do rather with
the direction of the animal’s movements than with hearing. Professor
Ehlers notes an interesting series: In 4. claparedi’ there are simply
two open grooves ; in A. marina the sacs have open necks, and contain
ARENICOLA, 225
foreign particles; in A. eraubid and A, antillensis the sacs are closed,
and contain intrinsic otoliths of lime.
CI
-
LV
Mase Y
{1
Fic. 117.—Dissection of lob-worm from dorsal surface.
m., Opening of retracted buccal cavity; @., gullet; g/’., diverticula
on first diaphragm ; g?’., oesophageal glands; d@., dorsal blood
vessels ; ef1., first efferent branchial; g., stomach intestine ; 75.,
sixth nephridium; ¢/13., thirteenth efferent branchial; @/!%.,
thirteenth afferent branchial; @., anus; a@/1., first afferent
branchial; %., heart of left side.
15
226 PHYLUM ANNELIDA.
Food canal.—(1) The buccal cavity is protrusible as a
“proboscis” or introvert, which grips the sand, and bears
internal papilla with chitinous tips. The protrusion is due
to the pressure of the ccelomic fluid, while special muscles
bring about retraction. (2) The gullet has smooth walls,
and bears a posterior pair of glands, which secrete a
yellowish fluid, probably digestive. (3) The gastric region,
from the heart to the twelfth or thirteenth notopodium, is
covered with yellow cells and many blood vessels, and has a
Fic. 118.—Cross-section ot Avenzcola.—After Cosmovici.
E., Epidermis; c.#z., circular muscles ; ¢.7., longitudinal muscles ;
b.c., body cavity; g/., gill; s., seta; 2.f., nephridial pore;
a.6x., afferent branchial; ¢.47., efferent branchial; ., ventral
nerve-cord, with blood vessels above; d@.z., dorsal vessel; /.v.,
lateral vessel; s.z.v., sub-intestinal vessels; v.v., ventral vessel ;
Sy Bute
median-ventral ciliated groove. (4) The intestinal region is
much folded, ‘“‘in a concertina-like manner,” by the caudal
septa, and is full of sand, from which the nutritive matter
has been absorbed. The anus is at the very end.
Body cavity.x—This is spacious, except in the tail
region, and contains a viscous coelomic fluid. Anteriorly
there are three transverse, partly muscular, septa or
diaphragms which moor the gullet. The first of these
diaphragms bears a pair of small pouches. Behind the
ARENICOLA. 229
third diaphragm the gut swings freely until the beginning
of the tail region, in which there are many septa.
Vascular system.—The blood has a bright red colour, and is rich
in hemoglobin. It flows in a very elaborate system of blood vessels, in
regard to the details of which there is still some uncertainty. ‘here is
along the whole mid-dorsal line of the gut a contractile dorsal vessel,
which carries blood forwards from the seven posterior gills, etc.
Connected with this by capillaries, there is below the gut an equally
long, feebly contractile ventral vessel, which carries blood backwards
to gills, nephridia, etc. Around the gastric region of the gut there is an
elaborate plexus of blood vessels, which communicate by. two lateral
vessels with the paired heart. There are also two sub-intestinal vessels
between the ventral vessel and the gut; these lead through the plexus
into the lateral gastric vessels, and thus into the hearts. These organs
lie just behind the cesophageal glands, and consist on each side—(a) of
a thin-walled auricle, an expansion of the lateral gastric vessel ; and (4)
of a muscular ventricle, which drives the blood into the ventral vessel.
Like the sub-intestinals, the dorsal vessel communicates with the heart
only indirectly through the gastric plexus. The ventricle contains a
spongy “‘ cardiac body,” which probably prevents regurgitation from the
ventral vessel.
From the ventral vessel arise afferent branchial vessels to gills,
nephridia, etc. From the seven posterior gills efferent branches enter
the dorsal vessel; while those from the six anterior gills join the sub-
intestinals. Each efferent vessel gives off a branch to the skin, while
the dorsal and sub-intestinal vessels give off numerous branches to the
gastric plexus on the gut.
Respiratory system.—There are thirteen pairs of gills,
on the seventh to the nineteenth bristle-bearing segments.
Each is a tuft of hollow thread-like branches, through the
thin walls of which the red blood shines. The afferent
branches to the gills all come from the ventral vessel; the
first six efferent vessels from the gills open into the sub-
intestinals ; the posterior seven open into the dorsal vessel.
As the papillz on the proboscis are hollow and contain
vessels, they are doubtless of respiratory significance.
Indeed, the gills may be regarded as exaggerated papille.
Excretory and reproductive systems.—In the anterior
region, in segments 4-9, there are six pairs of nephridia,
of which the foremost seems in process of degeneration.
Each consists of three parts—a funnel opening into the
body cavity, a glandular portion, and a bladder com-
municating with the exterior.
The sexes are separate and similar. The reproductive
organs are very simple, and arise by proliferation of the
228 PHYLUM ANNELIDA.
peritoneal membrane beside the blood vessels supplying
the funnels of the nephridia. The reproductive cells are
liberated into the body cavity, and there matured. They
pass out by the nephridia, and may be temporarily stored
in the bladder portions of all but the first. Little is known
in regard to the development, beyond the fact that the
young are for a time free-swimming pelagic forms.
Development of Polychzta.—As an example of the development
of the marine Chetopods, we may take Hzfomatus, which has been
investigated by Hatschek. Here segmentation is complete, but some-
what unequal, and results in the formation of a blastula, with its upper
hemisphere composed of small (ectodermic) cells, and the lower of large
(endodermic) cells. Among these latter are two spherical cells—the
primitive mesoblasts. Invagination takes place in the usual way to
form a gastrula; the primitive mesoblasts divide and form mesoblastic
bands. During these processes the external form has altered con-
siderably. The apical (aboral) region of the gastrula becomes tilted
forward, an ectodermic invagination arises posteriorly, and, uniting with
the archenteron, produces hind-gut and anus, while a similar insinking
anteriorly, in the region of the blastopore, forms fore-gut and mouth,
The larval gut so formed has a distinct ventral curve. Cilia appear on
the surface at an early stage, and now form a distinct pre-oral ring, and
also a less constant post-oral ring. At the apex of the pre-oral region
an ectodermic thickening takes place; this gives rise to an apical
ganglion, with which sensory structures are often associated. The
mesodermic bands give rise to muscle cells, used in swimming, and also
to the ‘‘ head kidneys ”—a pair of larval excretory tubes. The larva so
formed is a typical Trochosphere, such as occurs in the great majority of
Polycheeta, in a more or less modified guise in many other worm-types,
and also in Molluscs. Its chief characters are the following :—
(1) There is a prominent pre-oral region, with an apical ganglion and
a ring of cilia.
(2) The gut has a distinct ventral curve, and a threefold origin.
(3) The larval body cavity is simply the persistent segmentation
cavity, and in it posteriorly lie the primitive mesoblasts.
The Trochosphere is a free-swimming pelagic larva, which, among
worms, corresponds largely to the future head region of the adult. Its
metamorphosis into the adult probably takes place in the most primitive
fashion in the little worm folygordius. We shall therefore follow it
there (Fig. 119).
In the larva, which is a typical Trochosphere, the first sign of
segmentation appears in the bands of mesoblast. These become divided
into successive segments, while at the same time the posterior region of
the larva elongates greatly, carrying the larval gut backwards with it.
Meanwhile a cavity appears in each of the mesoblastic segments.
These cavities, taken together, form the adult body cavity; the outer
and inner walls form the somatic and splanchnic layers; the posterior
and anterior walls of adjacent segments fuse to form the septa of the
adult worm ; the inner (splanchnic) walls of the primitive segments on
v
ic osssaanmas Beebe sesesestiniy 5
Fic. 119.—Development of Polygordius.—After Fraipont.
a., Mother sperm cell; 4., ¢., sperm morule; ¢., spermatozoa.
1. Ovum with large nucleus ; 2. two-cell stage; 3. four-cell stage; 4. blastosphere ;
5. gastrula; ac., archenteron; 6. closure of gastrula mouth or blastopore ;
7. formation of stomodzeum (sz.), and proctodzeum (/7.), which invaginate to meet
archenteron (ac.); 8. complete gut formed; 9. elongation of larva; af. sf.,
apical spot; céZ., ciliated ring; 2eph., primitive nephridia ; 10. formation of’
posterior segments ; 11. form of adult Polygordius.
230 PHYLUM ANNELIDA.
each side fuse above and below the gut to form the dorsal and ventral
supporting mesenteries of the gut. The head region is at first dispro-
portionately large, but later, by an independent process of growth,
becomes reduced. The larva abandons its pelagic life, and becomes
adult.
Comparing the development of Polycheta with this, we find that the
Trochosphere is often modified, and that segmentation tends constantly
to appear at an earlier stage. As a further step in the same direction,
we may note that in some Polychzeta the Trochosphere stage is no
longer recognisable as such.
GENERAL SURVEY OF THE CLASS CHETOPODA
I. Oligocheeta.—The general characters may be gathered from
the description of the earthworm, but it is to be noticed that the earth-
worms are specialised forms, and that the fresh-water Oligochetes are
of much simpler structure. The most essential distinction from the
Polychzeta is to be found in the complex reproductive organs. The
absence of gills, though general, is not universal, for a few fresh-water
forms, such as Dero and Branchzura, possess gills of simple structure,
while the West African Alma mnzlotica has more complex branched
retractile gills. Among other characters may be noticed the tendency
to variation in the structure of the excretory system. In all, with the
exception of olosoma, certain of the nephridia are modified to serve
as genital ducts, while in the Megascolicide the nephridia tend to be
reduced to a mass of minute tubules ramifying over the inner surface
of the body wall. In general the Oligochztes, however, show more
uniformity of structure than their marine allies.
They may be divided into two main groups—(1) the Microdrili,
and (2) the Megadrili. The first group includes the small aquatic
forms; of these most familiar are Tudbéfex rivulorum, often found
in the mud of brooks, and the species of Mazs, remarkable for their
power of asexual budding. Some Microdrili live between tide-marks.
The leech-like Branchzobdella, which is parasitic on the gills of the
fresh-water crayfish, is a somewhat aberrant member of the group.
The Megadrili include the larger Oligochzetes, mostly living in earth,
and commonly designated as “earthworms.” The largest form is a
Tasmanian species (M@egascolides gippslandicus), measuring about 6 ft.
in length.
II. *Dalyohets,— As contrasted with the more or less subterranean
earth- and mud-worms, the marine Polychzta have a richer develop-
ment of external structures and a more complex life history. The
external appearance is greatly modified by the relative degree of
development of the parapodia, which are lateral outgrowths typically
functioning as walking ‘‘legs,” or as swimming organs. A para-
podium, when fully developed, is divisible into a ventral neuropodium
and a dorsal notopodium. Each of these is bilobed, bears a tactile
process or cirrus, and is fringed with firm bristles or setae, Within the
substance of each lobe is embedded a stout needle-shaped ‘‘ aciculum,”
GENERAL SURVEY OF THE CLASS CHAETOPODA. 231
which functions as an internal skeleton, both by giving support and by
serving as an attachment for muscles. With the notopodium, further,
true gills containing prolongations of the body cavity are often associ-
ated. Such typical parapodia occur especially in the active free-living
forms like Wereds and its allies, but in the order in general the parapodia
show much variation, and may be almost suppressed, as in Avenzcola.
Parapodia are absent from the ‘‘ prostomium,” and are rarely fully
developed on the first true segment or peristomium. In both cases,
however, tactile cirri and tentacles are often present. The prostomium
varies greatly in development and structure, and is of great systematic
importance ; it is frequently furnished with eyes and other sense organs,
—;
=]
Fic. 120,—Parapodium of ‘ Heteronereis ” of
Nereis pelagica.—After Ehlers.
I, 2, 3, 4, the leaf-like outgrowths ; cl., notopodial cirrus; c2.,
neuropodial cirrus ; al., 2%, acicula or supporting bristles
of notopodium and neuropodium; s., sete.
but these may also occur in other regions of the body. Apart from the
parapodia, the shape and appearance of the body are most affected by
the condition of the septa. In the active free-living forms (Errantia)
these are usually present throughout the body, and gave a characteristic
worm-like appearance. In burrowing and tubicolous forms (Sedentaria),
the septa tend to be suppressed. Their absence facilitates burrowing,
by permitting free movement of the coelomic fluid, and is often associ-
ated with a division of the body into regions, and a loss of the typical
uniform shape (cf. Arenzcola).
With regard to internal organs, the gut is frequently branched and of
large calibre. In some cases (Capitellidz) it possesses an accessory
232 PHYLUM ANNELIDA.
communicating tube (Nebendarm), which is of interest, because it has
been compared to the notochord of Vertebrates. There is typically a
pair of nepbridia in each segment, but they are often reduced in
number. They may open into the ccelome by a ciliated funnel or
nephrostome, or end in a group of solenocytes, which are comparable
to the flame-cells of Flatworms (see Fig. 254A). With the nephridia
there are often associated ciliated ‘‘ ccelomoducts,’’ which typically
open to the exterior and into the coelome: They often combine with
the nephridia. Though the sexes are usually separate, there are a few
hermaphrodite forms. There is a metamorphosis in development,
Fic. 121.—Free-living Polycheete (Mereds cultrifera).
Note, as compared with Avenicola, the absence of gills, and the
well-developed parapodia, which are absent from the peris-
tomium (e.), or first true segment. The prostomium bears
eyes (¢.), the small tentacles (¢.), and the large palps (/.);
c., the four paired cirri, borne by the peristomium}; a., the anus,
with two long cirri.
and some interesting peculiarities occur in regard to reproduction.
Thus several species of the common genus /Vere7s, when sexually
mature, have the body divided into two regions—a posterior region
containing the ova or sperms, and an anterior unmodified asexual
region. The posterior region is distinguished by the structure of its
parapodia, which become converted into broad, flattened swimming
organs, and there is sexual dimorphism. Worms of this peculiar
type were long described as a distinct genus under the name of
‘* Fleteronereis,” and even yet the subject is imperfectly understood,
for there is from unknown causes much variation as regards the
extent of the modification. A complete change of habit at the spawn-
GENERAL SURVEY OF THE CLASS CH‘ETOPODA. 233
ing season is probably common here as elsewhere in marine Invertebrates.
In the Syllidee a phenomenon occurs similar to the formation of a
“« Heteronereis,” but a process of fission may result in the division of
the modified form into an anterior asexual zooid and a posterior sexual
one. In Myrianidaa long chain of sexual zooids is formed. In this
way a regular alternation of sexual and asexual generations may arise.
Some Polychzeta dwell habitually within tubes, others are at least at
times active and free-living. The latter have usually well-developed
parapodia and sense organs, the anterior part of the gut may be
furnished with strong jaws, the body is more or less uniform, and the
worms are carnivorous. These forms are all included in the sub-order
Nereidiformia, which embraces such familiar animals as the common
sea-mouse (Aphrodite), with its mass of iridescent bristles covering the
sides of the body, the species of Merezs and Nephthys, so common
under stones on the shore, and others equally remarkable for beauty of
colour. The bright colours may be due to the iridescent cuticle or to
pigments. There are a few transparent pelagic forms, e.g. Zomopteris.
The sedentary forms lead a sluggish life within various kinds of
tubes—limy, sandy, papery, or gelatinous. They are not nearly
related, but possess in common certain adaptive characters, such as the
aggregation of gills, cirri, tentacles, and sense organs to the anterior
exposed part of the body; the reduction of the parapodia, often used
solely for clambering in the tube; the absence of ‘‘ jaws,” and the
habit of feeding on minute Algze or other substances suspended in
water. Among these are included SerZz/a, which forms twisted limy
tubes outside shells and other marine objects ; the aberrant Sade//aria,
which often builds reefs of porous rock formed of the aggregated sandy
tubes; the common Zeredella or Lanice conchilega, with its tubes of
glued sand particles; and the strange phosphorescent Chetopterus,
found in deep water, within its yellow parchment-like tube.
III. Echiuridee.—In holes in the rocks on the warmer coasts of
Europe there lives a curious ‘‘ worm ”—Bonellia viridis, of a beautiful
green colour, with a globular body, and a long, grooved, anteriorly
forked, pre-oral protrusion. Such, at least, is the female ; but the
male is microscopic in size, lives in or on his mate, and is exceedingly
degenerate. His gut is without mouth and anus, the surface is covered
with cilia, and the body cavity almost obliterated. Related to Bonediza,
but of less anomalous shape, are a few other forms, like Zhalassema
and Echiurus.
In all, the body in the adult shows mere traces of segmentation ;
parapodia, cirri, and gills are absent, but, except in the degenerate
males, a few setee are always present. The most characteristic structure
is the elongated solid proboscis, which has the mouth at its base. The
nervous system consists of a gullet-ring and a ventral cord, but the latter
is unsegmented, and there is no brain. The gut is coiled, and bears a
curious adjacent tube known as the ‘‘collateral intestine,” and » pair
of excretory ‘‘anal vesicles,” opening from gut to body cavity, and
formed in development from nephridia. The anus is terminal, there
is a closed vascular system, and one to four pairs of nephridia. The
sexes are separate, the reproductive elements are formed on the walls
of the body cavity, and are shed into it.
234 PHYLUM ANNELIDA.
There is a metamorphosis in development, but the nature of the larva
differs markedly in the different genera. In Ech¢éwrus and Thalassema
it bears a striking resemblance to a Trochosphere. Thus there is a
well-developed pre-oral lobe with an apical sense organ, and pre-oral
and post-oral bands of cilia. ‘‘ Head kidneys” or provisional nephridia
occur, and the post-oral region shows distinct segmentation, the
segments being marked externally by rings of cilia. As development
proceeds, all trace of segmentation is lost. In Bonediza the larva shows.
no trace of segmentation, and is Turbellarian-like ; owing to a premature
arrest of development, the male remains at this level throughout life.
Appendix (1) to Chetopoda
Primitive Forms. ARcCHI-CHATOPODA or
ARCHI-ANNELIDA
There are a few, small, simple, marine worms, with some Annelid or
Cheetopod characters, which are sometimes supposed to be ancestral
forms. Thus Dzzophzlus is a minute Planarian-like animal found among
Alge. In the young at least the body is distinctly segmented, but
there are no bristles, gills, or tentacles. There are circling bands
of cilia. The nervous system consists of a brain and two widely
separated ventral ganglionated cords, but it remains in contact with the
epidermis.
More distinctly Annelid are the marine worms Polygordius, Proto-
drtlus, Saccocirrus, and Histriodrilus.
The small body is segmented and uniform; there are no sete,
parapodia, cirri, or gills, but the head bears a few tentacles ; the pre-
oral region is small, and the segment around the mouth is large ; the
very simple nervous system is retained in the epidermis.
Polygordius (Fig. 119 (11)) is a thin worm, an inch or more in length,
living at slight depths in sand or fine gravel, often along with the
lancelet. It has two tentacles, a few external cilia about the mouth in
a pair of head-pits, and sometimes on the body; it moves like a worm,
but has no bristles. It feeds like an earthworm, or sometimes more
discriminatingly on unicellular organisms. The females are usually
larger than the males, and in some species break up at sexual maturity.
The development includes a metamorphosis, and the larva are
ciliated, free-swimming, light-loving, surface animals, feeding on
minute pelagic organisms, seeking the depths as age advances.
According to some, the larva represents a primitive unsegmented
ancestral Annelid, with medusoid affinities ; according to others, the
larval characteristics are adaptive to the mode of life, and without
historic importance.
Protodrilus is even smaller than Polygordius, with more cilia, mobile
tentacles, and two fixing lobes on the posterior extremity ; the move-
ments are Turbellarian-like, the reproductive organs hermaphrodite,
the development direct. ¢strdodrzlus is parasitic on the eggs of the
lobster, and its affinities are doubtful. ‘
AIRUDINEA OR DISCOPHORA. 235
Appendix (2) to Chetopoda
Parasitic and Degenerate Chetopods. Myzostomata
The remarkable forms (/yzostoma) included in this small class, live
parasitically on feather-stars, on which they form galls. They are
regarded as divergent offshoots from primitive Annelids, the larval form
showing some distinctly Chaetopod characters. The minute disc-like
body is unsegmented, and bears five pairs of parapodia, each with a
grappling hook, with which four pairs of suckers usually alternate.
There are also abundant cirri. The skin is thick, the body muscular,
the nervous system is concentrated in a ganglionic mass, which encircles
the gullet, and gives off abundant branches. There is « protrusible
proboscis and a branched gut; the mouth and anus are ventral. The
ova arise in the reduced body cavity, and pass by three meandering
oviducts “> the anal aperture. The testes are paired, branched, and
ventral, with associated ducts, which open anteriorly on the side of
the body.
The series are united, but there is marked protandry. The very
young forms, originally described as ‘‘dwarf males,” contain sperma-
tozoa, and are often carried on the back of the mother ; as they grow
older they become hermaphrodite, and later the power of forming
spermatozoa is lost and the animals become female.
It must’ be allowed, however, that all would not agree with the above
summary. Thus Beard says: ‘‘The various kinds of parasitism
presented by the numerous species of J/yzostoma, have led in some
cases to the preservation of the males, in others to their extinction, in
yet others to their conversion into hermaphrodites.” He distinguishes—
1. Purely dicecious forms with small males, ¢.g. JZ. pulvinar.
2. Hermaphrodite forms and true males, which remain males, ¢.g.
M. glabrum.
3. Hermaphrodite forms and males, which, retaining their positions
on the hermaphrodites, afterwards become female, e.g. JZ. alatum.
4. Hermaphrodite forms, in which the males have lost their dorsal
position, and have either become extinct or converted into
protandric hermaphrodites, e.g. AZ. cérriferum.
Class Hirupingea or DiscopHoRA. Leeches
This class includes forms in which the body is oval and
flattened, usually devoid of sete or gills, and marked ex-
ternally by vings which are much more numerous than the
true segments. The body cavity ts much reduced and broken
up (except in Acanthobdella), and may communicate indirectly
with the well-developed vascular system. The nephridia are
numerous and segmentally arranged. There are usually two
suckers, one at each end of the body, the anterior being formed
236 PHYLUM ANNELIDA.
by the mouth. Almost all are hermaphrodite,—the male
organs are numerous and segmentally arranged, and special
genital ducts are present. The genital openings are median.
The development is direct. Most live in fresh water or on
land, but a few are marine.
Type, the Medicinal Leech (Airudo medicinalis)
Habits.—This is the commonest and most familiar of
leeches, once so constantly used in the practice of medicine
that leech became synonymous with physician. It lives in
ponds and sluggish streams, and though not common in
Britain, is abundant on the Continent, where leech farms,
formerly of importance, are still to be seen. Leeches feed
on the blood of fishes, frogs, and the like, and are still
caught in the old fashion on the bare legs of the callous
collector. As animals are naturally averse to blood-letting
and hard to catch, leeches make the most of their
opportunities. They gorge themselves with blood, and
digest it slowly for many months, it may be, indeed, for
a year. Watched in a glass jar, the leech is seen to move
by alternately fixing and loosening its oral and posterior
suckers, and, on some slight provocation, it will swim
about actively and gracefully. At times it casts off from
its skin thin transparent shreds of cuticle,—a process
which, in natural conditions, usually occurs after a heavy
meal, when the animal, as if in indigestion, spasmodically
‘contracts its body, or rubs itself on the stems of water-
plants. Numerous eggs are laid together in cocoons in
the damp earth near the edge of the pool. Thence, after
a direct development, the young leeches emerge and make
for the water.
External features.—The leech is usually from 2 to 6 inches
in length, amd appears cylindrical or strap-like, according to its state
of contraction. The slimy body shows over one hundred skin-rings ;
its dorsal surface is beautifully marked with longitudinal pigmented
bands, while the ventral surface is mottled irregularly ; the suctorial
mouth is readily distinguished from the unperforated hind sucker, above
which, on the dorsal surface, the alimentary canal may be seen to end.
According to Whitman’s precise investigations, there are 102 skin-
rings and 26 somites or true segments. The hind sucker is supposed
to consist of 7 fused segments, making the total number 33.
MEDICINAL LEECH. 237
These segments may be recognised externally by conspicuous.
pigment spots (‘‘segmental papillee”), which in the middle region of
the body occur on every fifth ring. In type, therefore, five rings.
correspond to a segment, but at either end of the body the number of
rings is abbreviated. In the head region there is a pair of ‘‘eyes”
on the Ist, 2nd, 3rd, 5th, and 8th rings; these are homologous with
‘*segmental papille,” and therefore in this region eight rings corre-
spond to five segments.
The penis is protruded on the middle véntral line between rings 30:
and 31; the aperture of the female duct lies five rings farther back.
Also on the ventral surface there are seventeen pairs of small lateral
apertures, through which a whitish fluid may be squeezed—the openings
of the excretory organs. The skin of segments 9-11 is especially
glandular, and forms the so-called clitellum or saddle, the secretion.
of which forms the cocoon for the eggs.
Skin.— Most externally lies the cuticle—a product of the:
epidermis—periodically shed, as we have already noticed.
In this shedding some of the genuine epidermis cells are-
also thrown off. These are somewhat hammer-like units,.
with the heads turned outwards, while the spaces between.
the thick handles contain pigment and the fine branches.
of blood vessels. As the latter come very near the surface,
a respiratory absorption of oxygen and outward passage of
carbon dioxide is readily effected. Opening between the
epidermal elements, but really situated much deeper, are
numerous long-necked, flask-shaped glandular cells, secret-:
ing the mucus so abundant on the skin. Underneath the
epidermis there is much connective tissue, besides yellow
and green, brown and black pigment.
Muscular system and body cavity. — The muscular
system consists of spindle-shaped cells arranged externally
in circular bands like the hoops of a barrel, internally
in longitudinal strands like staves. Besides these there
are numerous muscle bundles running diagonally through
the body, or from dorsal to ventral surface, and there are
other muscles associated with the lips, jaws, and pharynx.
The body cavity, though distinct in the embryo, is almost
obliterated in the adult leech, where the predominant con-
nective tissue has filled up nearly every chink.
Nervous system and sense organs.—The nervous system:
mainly consists of a pair of dorsal ganglia lying above the
pharynx, and of a double nerve-cord, with twenty-three:
ganglia, lying along the middle ventral line. The dorsal (or
238 PHYLUM ANNELIDA.
supra-cesophageal) ganglia are connected with the most
anterior (or sub-cesophageal) pair on the ventral chain, by
a narrow nerve-ring surrounding the beginning of the gut.
The sub-cesophageal ganglia represent about five pairs of
ganglia fused together. From the dorsal ganglia nerves
proceed to the “eyes” and anterior sense spots ; from the
ventral centres the general body is innervated. Special
Fic. 122.—Transverve section of leech.—After Bourne.
c., Cuticle; ¢., epidermis; c.7., dermis and outer muscles (circular
and oblique); 2.., longitudinal muscles (the peculiar connective
tissue is hardly indicated); ».#., radial muscles; Zv., lateral
blood vessel; d@.s., dorsal sinus; vs., ventral sinus enclosing
nerve-cord (x.); g., median part of crop, with lateral pockets (.)3
z., testis ; £, nephridial funnels; v.@., vas deferens.
nerves from the dorsal ganglia supply the alimentary canal,
forming what is called a visceral system.
The sense organs of the leech are ten so-called “ eyes,”
besides numerous sense spots usually occurring on every
fifth skin-ring. The eyes are arranged round the edge of
the mouth, and look like little black spots. Microscopic
<xamination shows them to be definite cups, surrounded by
MEDICINAL LEECH. 239
connective tissue with black pigment, and containing clear,
strongly refracting cells, each in connection with a fibre of
the optic nerve.
It has been shown (Whitman) that the eyes of leeches are serially
homologous with the segmental sense organs. At the one extreme
there are purely tactile organs, at the other extreme there are purely
visual organs, and between these there are
compound sense organs, in part tactile and
in part visual—a series which is full of
suggestiveness in regard to the evolution of
sense organs (cf. the series of sensitive setz
in the crayfish). The visual organs of the
leech are not able to form images of external
objects, but the animals are exquisitely
sensitive to alterations of light.
Alimentary system.— When the
leech has firmly fastened itself to its
prey by the hind sucker, it brings its
muscular mouth into action, pressing
the lips tightly on the skin, and pro-
truding three chitinous tooth-plates
which lie within. Each of these
tooth-plates is worked by muscles,
and is like a semicircular saw, for the
edge bears from 60 to 100 small
teeth. Rapidly these saws cut a
triangular wound, whence the flowing
blood is sucked into the muscular
pharynx. The process may be ob-
served and felt by allowing a hungry GS
leech to fasten on the arm. As the Fic. 123. — Alimentary
blood passes down the pharynx, it is system of leech. —After
influenced by the secretion of glandu- _ Moain-Tandon.
lar cells which lie among the muscles He ae 2 ae eeons
of the seventh, eighth, and ninth Doe yy SERNA thy
segments, and exude a ferment which ” .
prevents the usual clotting. The blood greedily sucked in
gradually fills the next region of the gut—the crop—which
bears on each side eleven storing pockets. These become
wider and more capacious towards the hind end, the largest
terminal pair forming two great sacs on each side of the
comparatively narrow posterior part of the gut. As all the
240 PHYLUM ANNELIDA.
pockets point more or less backwards, it is evident that a
leech to be emptied of the blood which it has sucked must
be pressed from behind forwards. The pockets filled, the
leech drops off its victim,
seeks to retire into more
private life, and digests
at leisure. The digestion
does not take place in the
pockets, but in a small area
just above the beginning
of the terminal part or rec-
tum. This rectum, running
between the two last
pockets, is separable from
the true stomach just
mentioned by a closing or
sphincter muscle. It ends
in a dorsal anus above the
hind sucker.
Vascular system.—Two
main lateral vessels run
longitudinally, one on each
side of the body. They
are connected with one
another by looping vessels,
give off numerous branches
which riddle the spongy
body, and have a definite
muscular coat. On the
dorsal surface and vent-
rally around the nerve-cord
are two lacunar spaces,
which are really portions
Fic. 124. —Dissection of leech.
—After Bourne.
c.g., Cerebral ganglia; 4., penis; s.v.
is opposite the seminal vesicle; ov.,
ovary; uz., uterus; 7.d., vas de-
ferens ; 2.4.v., lateral blood vessel ;
T.4, fourth testis ; 2.v., nephridial
vesicle: V.17, last nephridium ;
G.19, nineteenth pair of ganglia;
v.¢., nerve-cord.
MEDICINAL LEECH. 241
of the true body cavity, and not parts of the vascular
system. With those and similar spaces, however, the
blood vessels are connected by means of a secondarily
developed series of canals, roughly corresponding to the
lymphatic vessels of Vertebrates. The blood is red, and
contains colourless floating cells of diverse form.
Excretory ‘system.— There are seventeen pairs ot
excretory tubules or nephridia, from the second to the
eighteenth segment inclusive. These open laterally on the
as me
pears
O26 one Ua ooua\'e
Se ie wn ee,
eo
Fic, 125.—A nephridium of leech.—After Bourne.
#., Internal terminal funnel; C., glandular coil covered ‘with blood
vessels; V., external terminal vesicle.
ventral surface, voiding the waste products extracted from
the blood vessels which-cover their walls. From the
seventh to the seventeenth, each nephridium ends internally
in a ciliated “ cauliflower lobe,”-corresponding to the funnel
of Oligocheeta, and enclosed in a blood space, apparently
part of the reduced ccelom. In the first nine of these
funnel-bearing nephridia the terminal lobe lies close upon
and dorsal to a testis, but there is no morphological
meaning in this approximation. Each consists of two
16
242 PHYLUM ANNELIDA.
parts, a twisted horseshoe-shaped glandular region, where
the actual excretory function is discharged, and a spherical,
internally ciliated bladder opening to the exterior. Within
the latter there is a whitish fluid with waste crystals.
Reproductive system.— The leech, like many other
Invertebrates, is hermaphrodite, containing both male and
female reproductive organs. The essential male organs or
testes are diffuse, being represented by nine pairs, lying on
each side of the nerve-cord in the middle region of the
body. Each is a firm globular body, within which mother
sperm cells divide into balls of sperms. The spermatozoa
pass from each testis by a short canal leading into a wavy
longitudinal vas deferens. This duct, followed towards
the head, forms a coil (so-called seminal vesicle) as it
approaches the ejaculatory organ or penis. From the coil
on each side the sperms pass into a swollen sac at the base
of the penis, where, by the viscid secretion of special
(‘‘prostate”) glands, they are glued together into packets
or spermatophores. These pass up the narrow canal of
the muscular penis, and leave the body on the middle
ventral line between rings 30 and 31, when they are
transferred in copulation to the female duct of another
leech.
The female organs are more compact. The two small
tubular and coiled ovaries are enclosed in spherical vesicles,
the walls of which are continued as two oviducts, which
unite together in a convoluted common duct. This is
surrounded by a mass of glandular cells, which exude a
glairy fluid into the duct. Finally, the duct leads into a
relatively large muscular sac—the “ uterus”—which opens
through a sphincter muscle on the middle ventral line
between rings 35 and 36.
The favourite breeding-time is in spring. Two leeches
inseminate one another, uniting in reverse positions, so
that the penis of each enters the uterus of the other.
Spermatophores are passed from one to the other, and the
contained sperms may remain for a‘ long time within the
uterus, or, liberated from their packets, may work their way
up the female duct, meeting the eggs at some point, or
reaching them even in the ovaries. |The development is
direct.
GENERAL NOTES ON LEECHES. 243
GENERAL NoTEes oN LEECHES
The leeches constitute a relatively small class, whose structure has
“been insufficiently worked out. The presence of suckers, the parasitic
habit, the reduction of the body cavity, have led many naturalists to
associate them with Flat-worms, but all recent work goes to emphasise
their affinity with Annelids, especially Oligochzetes. In leeches setz
are absent, except in Acamthobdella, which has paired segmentally
arranged bristles in the anterior region ; but it is to be noted that they
are absent in some Oligochetes. As in Oligocheetes, gills are usually
absent, but occur in Branchellion. The condition of the body cavity
affords one of the most striking contrasts to Oligochetes; but in
Acanthobdella the adult has a typical Annelid ccelom divided into
regions by septa. In others, in spite of the large amount of connective
tissue in the adult, there are distinct traces of segmental septa. In
Hirudo the reduction is carried so far that the ccelom is represented
merely by canals without trace of septa. In all cases, however,
development shows that the reduction is secondary, and that in the
embryo there is a true Annelid body cavity unconnected with. the
vascular system. The condition of the alimentary canal affords a basis
for classification, for in one set the anterior region is protrusible, and in
the other it is not, but is furnished with jaws or tooth-plates. The
jaws are interesting, because they are absent from Oligochzetes, except
in a few forms, like Branchiobdella; the jawed leeches are more
specialised than those without these structures,
With regard to the nephridia, in Clepszze, which has a fairly well-
developed body cavity, there is a direct communication between ccelom
and nephridia by means of a ciliated funnel of typical Annelid form.
Where the ccelom is much reduced, as in Azrudo, the funnel is repre-
sented by the blind ciliated ‘‘ cauliflower lobe.” In the reproductive
system, apart from the numerous male organs, the leeches differ from
the Oligochzetes in the apparent continuity of the organs and ducts ;
but in the case of the ovaries, at least, the connection is secondary. In
the processes of fertilisation and egg-laying, in the formation of a
cocoon, and in the development, the two groups show marked
resemblance.
Most leeches are worm-like aquatic animals, with blood-sucking
propensities ; but some live in moist soil, and others keep to the open
surface, while the parasitic ‘‘ vampire” habit, familiarly illustrated by
the apothecary’s ancient panacea, is in many cases replaced by
carnivorous habits and predatory life. The medicinal leech (Arado)
is typical of the majority, for it lives in ponds and marshes, and sucks
the blood of snails, fishes, frogs, or of larger available victims. . The
giant leech (Aacrobdella valdiviana), said to measure 24 ft. in length,
though this is very doubtful, is subterranean and carnivorous ; while the
wiry land-leeches (Hemadipsa, etc.), of Ceylon and other parts of the
244 PHYLUM ANNELIDA.
East, move very rapidly along the ground, fasten on to the legs of
man or beast, and gorge themselves with blood. The hungry horse-
leeches are species of Memopis, greedily suctorial, though the teeth,
which occur in two rows, are too small and irregular to be useful in
medicinal blood-letting ; but the name is also applied to species of the
common genus Az/ostoma, which are carnivorous in habit. Other
common leeches are species of Wephelzs, predaceous forms with indis-
criminating appetites, and the little C/epsize, also common in our
ponds, notable for carrying its young about on its ventral surface.
Several marine forms prey upon fishes and other animals, e.g. the
‘¢skate-sucker ” (Pontobdella muricata), with a leathery skin rough with
knobs. This form lays velvety eggs in empty mollusc shells, and
mounts guard over them for more than a hundred days. The remark-
able Branchellion on the Torpedo has eleven pairs of leaf-like res-
piratory plates on the sides of its body, and so has the related
Ozobranchus jantseanus, a parasite of a river turtle in the Jantsekiang.
One of the strangest habitat is that of Lophoddel/a, on the lips and
jaws of the crocodile.
Classification. —
Family 1. Rhynchobdellide, in which the fore part of the
pharynx can be protruded as a proboscis. There is an anterior
as well as a posterior sucker. The blood plasma is colourless.
The ova are large and rich in yolk; the embryos are hatched
at an advanced stage, and soon leave the cocoon, which contains
no albuminous fluid.
2g. Clepsine, Pontobdella, Branchellion.
Family 2. Gnathobdellidz, in which there is no proboscis, but
the pharynx usually bears three tooth-plates. The mouth is
suctorial. The blood plasma is red. The ova are small
and without much yolk; the embryos are hatched at an
early stage, and swim about in the nutritive albuminous fluid
of the cocoon. ;
2g. Hirudo, Hemopis, Hemadipsa, Aulostoma, Nephelis.
Family 3. Acanthobdellide. By itself is the Siberian fish parasite
Acanthobdella, which has rows of setz on the first five segments,
a spacious ccelom, and other peculiarities,
Appendix (1) to Annelid Sertes
Class CHa@aToGNaTHA. Arrow-worms
There are two little pelagic ‘‘ worms,” Sagz¢/a and Séadel/a, which
are so different from all others, that they have been placed in a class
by themselves, It is possible to regard them as Annelids with three
segments,
The translucent body, which may be nearly 3 in. long, but is
usually much less, has three distinct regions, —a head bearing a ventral
mouth with spines and bristles (whence the name Chetognatha), a
median region with lateral fins, and « trowel-like tail. The nervous
system consists of a supra-cesophageal ganglion in the head, a sub-
ROTATORIA. 245
cesophageal about the middle of the body, long commissures between
them, and numerous nerves from both; it retains its primitive con-
nection with the epidermis. There are two eyes and various patches
of sensitive cells. The food canal is complete and simple, and lies in a
spacious ciliated body cavity. Corresponding to the external divisions,
the cavities-of the head, body, and tail are distinct, being separated
from one another by septa; a longitudinal mesentery supports the gut
and divides the cavities into lateral halves,
There is no vascular system, nor are there any certain nephridia. It
is possible that the latter may be represented by the genital ducts.
_ The animals are hermaphrodite, and the simple reproductive organs
lie near one another posteriorly. The two ovaries project into the
body cavity, and their ducts open laterally where body and tail meet. .
The two testes project into the cavity of the tail ; and their ducts have
Fic. 126.—Development of Sagztfa.—After O. Hertwig.
Illustrating formation of a body cavity by pockets
from the archenteron; also the early separation of
reproductive cells.
£c., Ectoderm; £z., endoderm; ac., archenteron; &., repro-
ductive cells; 42, blastopore; ¢.g., ccelom pouches; ™.,
mouth; x. section of gastrula; 2 and 3. origin of celom
pouches.
internal ciliated funnels, and open on the tail. Two reproductive cells
are set apart at 4 very early stage, and each divides into the rudiment
of an ovary and ofa testis. The eggs undergo complete segmentation ;
a gastrula is formed by the invagination of the blastula; the body
cavity arises, in enteroccelic fashion, as two pockets from the arch-
enteron. The young forms are like the adults.
Appendix (2) to Annelid Sertes
Class Rotatoria. Rotifers
Rotifers are beautiful minute animals, abundant in fresh water, also
found in damp moss, and in the sea. They owe their name and the
old-fashioned title of wheel-animalcules to the fact that the rapid move-
246 PHYLUM ANNELIDA.
ments of cilia on their anterior end produce the appearance of a rotating
wheel. The food seems to consist of small organisms and particles
caught in the whirlpool made by the lashing cilia. The little animals_
are tenacious of life, and can survive prolonged drought. If they are
left dry for long, however, they die, though the ova may survive and
subsequently develop.
The body is usually microscopic, and is sometimes (¢.g. in Melicerta
and Foscularéa) sheltered within an external tube. There is no
internal segmentation, but there are sometimes external rings, and the
attaching outgrowth or ‘‘ foot” is sometimes segmented. The anterior
nd bears, on a retractile ridge, the ciliated ring or ‘‘ trochal apparatus.”
The nervous system is a single dorsal ganglion with a few nerves.
-\n unpaired eye and some tufts of sensory hairs are usually present.
The food canal extends along the body in a well-developed ‘‘ ccelom,”
and the fore-gut contains a mill, in which two complex hammers beat
upon an anvil, The canal ends posteriorly on the dorsal surface
between the body and the foot, and, as the terminal portion also
receives the excretory canals and the oviduct, it is called a
cloaca.
There is no vascular system, but a nephridial tube of a primitive type
lies on each side of the body, and opens posteriorly into the cloaca.
The sexes are separate ; the reproductive organs are simple. Except
in the marine parasite Sezson, in Rhinops vitrea, and two or three other
forms, the males are dwarfed and degenerate, destitute even of a true
food canal, and often ‘‘little more than perambulating bags of
spermatozoa.” In many cases the sexual union (effected by a
penis) seems to be ineffective, and there is no doubt that many, if not
most, Rotifers are pathenogenetic. No males have as yet been found
in Philodina, Rotifer, Callidina, or Adineta. The females lay three
different kinds of eggs, according to their conditions and constitution—
either small ova, which become males, or thin-shelled ‘‘ summer ova,”
or thick-shelled ‘‘ resting or winter ova,” the two last developing into
females. The so-called winter eggs may occur at any season, and
seem usually to have been fertilised. Many species, however, are
viviparous. We include the Rotifers beside the Annelids proper, be-
cause it seems possible to regard them as derived from ancestors
somewhat like Annelid larvze.
Rotifers living in fixed tubes or envelopes,—Melicerta, Floscularia,
Stephanoceros.
Free Rotifers,—Notommata, Hydatina, Brachionus,
Parasitic on the marine crustacean Mebalza,—Seison. .
Pedalion occupies a unique position ; it has hints of appendages and
a peculiar jumping motion,
At this stage it may be mentioned that there are several sets of
small worm-like animals of which we know very little. It is quite
possible that some of them, may become of great interest to the
systematic zoologist, but we do not yet understand what places in
the system they should occupy. Moreover, as they are small, un-
familiar, and unknown to myself, I shall simply refer to more complete
works for an account of the Gasterotricha, Echinoderidze, Demosco-
lecidze, and Cheetosomide.
SIPUNCULIDS AND PRIAPULIDS. 247
Appendix (3) to Annelid Series
A. Class SIPUNCULIDA, e.g. Sipunculus, and
’ B. Class PRiaPuLIDA, e.g. Priapulus
These two classes were formerly ‘united with the Echiuride as
Gephyrea, but it is improbable that the three are nearly related. The
Echiuridee are apparently modified Cheetopods, while the position of -
the Sipunculide and Priapulide is quite uncertain. :
Both include marine worms, living in the sand or mud upon which
they feed, having unsegmented bodies with a capacious body cavity,
and an anterior protrusible proboscis or introvert, which is moved ‘by
special retractor muscles, and bears the mouth at its tip. In most other
respects the two classes differ markedly from one another,
In the Sipunculids, the large introvert terminates in a hollow
tentacular fringe, within the cavity of which closed blood vessels run.
The gut is much coiled, and the anus is dorsal and anterior. A nervous
system with a distinct brain, a gullet-ring, and a ventral cord is present,
but the ventral cord is unsegmented. Peculiar ciliated vesicles or
“urns” arise in some Sipunculids as buds from the blood vessels,
and many swim freely in the body cavity. By collecting and agglutinat-
ing particles they help to purify the coelomic fluid. Large nephridia or
brown tubes, usually two in number, occur in the anterior region, and
function also as genital ducts. The sexes are separate except in
Phascolosoma minutum, and the reproductive cells develop on the lining
of the body cavity. In the development, which includes a meta-
morphosis, several peculiarities are observable, tending to show that
the animals are not primitive. The larva of Sdpunculus is sometimes
compared to a trochosphere, but differs from a typical trochosphere,
notably in the total absence of segmentation, of ‘‘head kidneys,” of a
pre-oral band of cilia, as well as in the position of mouth and anus, and
the slight development of the pre-oral lobe.
The class includes eleven genera, which are widely distributed; many
of the species are large and conspicuous. It should be noticed that
while Sipunculids are typically without trace of sete, some genera,
e.g. Phascolosoma, have distinct hooks on the introvert.
The Priapulidee include two genera—Priapulus and Halicryptus, both
almost entirely confined to the northern hemisphere. They have no
tentacles, no vascular system, no brown tubes, and no brain. The gut
is straight, or has a single loop; the anus is posterior. A gullet-ring
and ventral nerve-cord are present as in Szpaculius, but retain their
primitive connection with the epidermis. There are complex genital
ducts opening by a pore on each side of the anus, which in the young
are connected with an excretory system of the Platyhelminth type,
while in the adult they are overgrown and concealed by the repro-
ductive cells, The development is unknown. In Prdagulus there is.a
peculiar respiratory (?) appendage at the posterior end of the body.
248 PHYLUM ANNELIDA.
Appendix (4) to Annelid Series
Under the old term Molluscoidea are sometimes included the three
classes—Phoronoidea, Polyzoa or Bryozoa, and Brachiopoda.
The Molluscoidea are characterised by the presence of a true
ceelom, formed in development by the folding off of pouches from
the archenteron, and by the shortening of the dorsal region of the
body, which results in the close approximation of mouth and anus.
The mouth is typically furnished with ciliated tentacles, and is often
overhung by an epistome; both tentacles and epistome, when present,
contain spaces which are part of the body cavity. Except in the
Ectoprocta among Polyzoa, two or four nephridia are present, and
serve also as genital ducts. There is always a metamorphosis in
development, and the larvee are peculiar.
The development is in most cases in-
sufficiently known, and it is probable
that further knowledge of it will remove
these sets of animals from their apparently
anomalous position.
Class PHORONOIDEA.
This class was erected for the genus
Phoronis, which has been associated both
with the Gephyrea and with Polyzoa.
Another genus, Phoronopsis, from the
Cape, has been recently established. It
has been proposed to associate these
two genera, along with Cephalodiscus
and Rhdbdopleura, with the Hemi-
chorda, on account of certain Chordate
affinities, said to be exhibited by the
larva,
The genus Phoronis includes a few
species of small marine ‘‘ worms,” social
in habit, and found enclosed in fixed
leathery tubes often encrusted with foreign
particles. Each individual is furnished
with a horseshoe-shaped crown of tent-
Fic. 127.—Actinotrocha or
larva of Phoronis.—After
Masterman.
The mouth is overhung by the
prominent pre-oral hood; the
anus is at the other endof the
body. Behind the mouth is a
ring of ciliated tentacles.
SP., the nerve ganglion in the
hood; V.G., the nerve gan-
glion of the region called collar
region by Masterman; ClV&.,
nerve-ring- at base of tentacles.
acles, which are hollow and supported
by an internal skeleton. The nervous
system lies in the ectoderm—a very
primitive character, and consists of a
ring round the mouth, and of a cord
down the left side of the body. An
interesting point is the presence of a
closed vascular system with nucleated
POLYZOA. 249
red cells. The body cavity is well developed, and is divided into
chambers. The sexes are united;.and the larva, known as Actino
h VAN
an
),
,
yy
Fic. 128.—Phoronis, much enlarged.
TR., Trunk; TZ., tentacles; 7U., tube.
trocha, undergoes a remarkable metamorphosis in the course of its
conversion into the adult.
Class PoLyzoa or BRyYOzoA
As usually defined, the class includes two sub-classes, the Ectoprocta
and the Entoprocta, but it seems almost certain that these are distinct
classes.
The Ectoprocta include fresh-water and marine forms, in which the
anus is outside the basis of the tentacles. The nervous system is repre-
sented by a ganglion placed between the mouth and anus. There isa
body cavity. There is no vascular system. Nephridiaareabsent. All
are colonial and ‘bud very freely; the marine forms show considerable
division of labour among the members of the colony.
250 PHYLUM ANNELIDA.
‘
(a) Tentacles in a crescent—Fresh water, Crdstatella, Lophopus, etc.
(6) Tentacles in a circle—Marine, except Pa/udicella ; Flustra, the
common sea-mat ; Membranzgora, encrusting seaweed, etc. ; Ced/epora,
very calcareous; Alcyonzdium, gelatinous.
The Entoprocta include the colonial Pedicellina, with « few allied
genera, also the non-colonial ZLoxosoma, in which the buds separate
as soon as they are formed. All the forms are stalked and minute.
The anus is included within the tentacular circle. In the meta-
Fic, 129.—Diagram of an Ectoproctous Polyzoon
(Plumatella),
Z., Lophophore; P//., pharynx; A., anus; S., stomach;
M., retractor muscle ; /., funiculus, a cord of mesodermic
tissue; O., cells that form ‘‘statoblast” buds; B., an
ordinary bud; Z., epistome over the mouth; 7., tent-
acles; S%., outer wall of zocecium.
morphosis of Pedicediina there is an elongation of the dorsal region of
the body, and a consequent approximation of the mouth and anus on
the shortened ventral surface. There is no apparent body cavity in the
adult, and the mesoderm arises from two primitive mesoblasts. The
nephridia are anterior, minute, and do not serve as genital ducts, but
resemble the ‘‘head kidneys” of Annelid trochospheres. They are said
to terminate in flame-cells like those of Platyhelminths. In all these
three respects the Entoprocta differ from the Ectoprocta, and from the
Molluscoidea generally.
BRACHIOPODA, 251
Class BRACHIOPODA
The Brachiopods or Lamp-shells are quaint marine animals, once
very numerous, but now decadent, The body is enveloped dorsally and
ventrally by two folds of skin or mantle; these secrete a shell, usually
of lime, but sometimes organic. The development of this shell has
apparently modified both the position and the relations of. the organs.
There is no real resemblance between a Brachiopod shell and that of a
bivalve Mollusc, except that both consist of two valves, In Brachiopods
these lie dorsally and ventrally; in Lamellibranchs they are lateral ;
moreover, in Brachiopods the ventral
valve is usually the larger. It is hardly
necessary to say that the Brachiopod
organism is not the least like a Mollusc.
A considerable part of the space
between the valves of the shell is filled
up by two long ‘‘arms,” which are
coiled in a spiral, and often supported
by a calcareous skeleton. These arise
in development from the specialisation
of a horseshoe-shaped ‘‘lophophore,”
such as is characteristic of the Polyzoa.
The mouth is placed between the arms,
and opens into the ciliated food canal.
This may end blindly, or may be
furnished with an anus placed near Fic. 130.—Interior of
the mouth ; in Cyrazza the anus is dorsal Brachiopod shell, showing
and posterior. The muscular system calcareous support for the
is well developed, the shell being both “arms.” —After Davidson.
opened and closed by means of muscles.
There is a nerve-ring round the gullet, with a slight brain and an
inferior ganglion. Sensory structures in many cases perforate the
valves. Above the gut lies the heart, which is connected with blood
vessels. Two (or more rarely four) nephridia open. near the mouth,
and serve also as genital ducts. The posterior region of the body often
forms a stalk by which the shell is moored, but in many this stalk is
absent, and the animal is directly attached to the substratum. The
sexes are sometimes separate, but perhaps some are hermaphrodite.
There is a metamorphosis in the development, and the larvee resemble,
in some respects, those of Polyzoa.
The Brachiopods date from the earliest known fossiliferous rocks, and
had their maximum representation in the Ordovician and Silurian,
CHAPTER XII
PHYLUM ECHINODERMA
Class 1. HOLOTHUROIDEA. Sea-cucumbers.
>, 2. ECHINOIDEA. Sea-urchins. I SuB-PHYLUM
5, 3. ASTEROIDEA. Starfishes. J ELEUTHEROZOA.
>» 4. OPHIUROIDEA. Brittle-stars.
>, 5. CRINOIDEA. Feather-stars,
», 6 EDRIOASTEROIDEA. Extinct. SuB-PHYLUM
3» 7 BLASTOIDEA. Extinct. PELMATOZOA.
>, 8 CYSTIDEA. Extinct.
In contrast to the “worms,” the Echinoderms form a well-
defined series. They may be described as sluggish marine
animals, generally with superficially radial symmetry, with a
tendency to form limy skeletons. The radial symmetry led
the older zoologists to place the Echinoderma near Ccelen-
tera, but there seems to be no real affinity. Moreover, the
larval Echinoderm is bilateral in its symmetry. It seems
likely that the Echinoderms represent an offshoot of some
“worm” stock. As in Ccelentera, the nervous system
shows a marked absence of -centralisation, which may be
connected with the absence-of a definite head region, and
this again with the sedentary or sluggish habit.
GENERAL CHARACTERS
The Echinoderms are celomate marine animals in which
the bilateral symmetry of the larva is replaced in the adult
by more or less marked radial symmetry. In addition to
the dominant radial symmetry, the adults show to a varying
extent a tendency towards the bilateral type, but this is never
the same as that of the larva, nor is it equivalent in the
different forms. Lime is always deposited in the mesodermic
GENERAL NOTES ON STRUCTURE. 253
tissues (mesenchyme), and in consequence there is frequently a
very complete skeleton. From the primitive gut of the larva,
pouches grow out to form the usually spacious celom and the
characteristic water vascular system (hydrocel), which may
have locomotor or respiratory functions.or both. The branches
of this system, together with the nerves, exhibit in most cases
a typical five-rayed arrangement. In addition to the water
vascular system, there ts an ill-defined lacunar system of blood
vessels, In the hemal vessels, water vessels, and celom,
there are abundant migratory amebocytes. Well-defined
excretory organs are absent. Gonads arise on the lining of
the body cavity, and are radt-
ately disposed except in Holo-
thurians. The sexes are almost
always separate. There ts
usually a striking circuitous-
ness or indirectness in develop-
ment. The larve are almost
always free-swimming, and
exhibit a metamorphosis. The
diet ts vegetarian (most sea-
urchins), or carnivorous (star-
Jishes), or consists of the organic
particles found in sand and
mud, the. Holothurians in par-
ticular practising this worm-
like mode of nutrition. .
Most Echinoderms have toa ana oh a Aral
remarkable extent the power After Johannes Miiller.
of casting off and regenerating .
portions of their body. This power is probably one of their
means of defence, but they often mutilate themselves as a
consequence of unfavourable conditions of life. This self-
mutilation, or autotomy, seems to be reflex, and not voluntary.
GENERAL NoTES ON STRUCTURE
The Echinoderma, in spite of the numerous fossil representatives,
form an exceedingly well-defined group, showing no close relation to
any other, and exhibiting certain striking peculiarities. The skeleton
is generally well developed; in Holothurians it consists of isolated
spicules, but elsewhere of a series of plates which may be firmly united
254 PHYLUM ECHINODERMA.
together, as in most sea-urchins, or may be capable of movement upon
one another. Apart from the skeleton proper, lime may appear in
almost any of the organs of the body. With this deep-seated tendency
to form skeletal substance may perhaps be associated the sluggish habit
of the majority, and the absence of definite excretory organs, Except
in Holothurians, where the calcareous plates are diffusely scattered, the
parts of the skeleton show much regularity of arrangement. The
primitive skeleton is believed to have consisted of two series of plates,
constituting respectively the oral and apical systems. These, especially
the latter, were of much importance in the formation of the skeleton of
the extinct Blastoids and Cystoids, but in modern Echinoderms they
are absent or unimportant, and are functionally replaced by accessory
plates, such as those which form the ‘‘test” of sea-urchins, The oral
system consists of five plates surrounding the mouth, and in living forms
it is fully developed only among Crinoids. The apical system in the
Pelmatozoa typically forms a cup or calyx enclosing the viscera, and
consists of a central plate to which a stalk may be attached, and three
sets of plates arranged around this, five infra-basals, five basals, and five
radials. In the larva of Aztedon this apical system is fully represented,
except that the infra-basals are reduced to three, but in other Crinoids
and in the adult Azzedon there tends to be reduction. Among other
Echinoderms the apical system is best represented among sea-urchins,
where there are often five basals (the genitals) around the anus. The
‘‘oculars” seem to correspond to the ‘‘terminals” at the tips of
starfish arms. In Ophiuroids the apical system is sometimes re-
presented both by basal and radial plates, but often only by radials ;
in starfishes it is typically absent in the adult, though more or less
clearly shown in the larva.
The other most striking characteristic of Echinoderms is the peculiar
water vascular system. This arises in development from the ccelom,
and consists typically of the following parts:—An external opening or
madreporite opens into a canal with calcified walls, called the stone
canal; this opens into a ring canal around the mouth, which has often
connected with it little vesicles and glandular bodies; the ring canal
opens into five radial canals which run in the radii of the body, and
give off branches to the protrusible tube-feet which project on the
surface of the body, and may be furnished with suckers; the radial
canals are also often connected with internal reservoirs or ampull.
The tube-feet are very characteristic, and have different functions in
the different classes. In Asteroids, in most Holothurians, and in part
in Echinoids, they are primarily locomotor ; in Ophiuroids, in Crinoids,
and in part in Echinoids, they are respiratory, tactile, or used for food-
catching. But there is great variety of structure and functions; thus in
many Holothurians the tube-feet are represented only by a ring of
tentacles around the mouth,
Class ASTEROIDEA. Starfishes
Star-like or pentagonal Echinoderms more or less flattened
at right angles. to the main axis of the body ; usually with
ASTEROIDEA. 255
well-defined simple arms containing the gonads and prolonga-
“tions of the gut, and with a ventral ambulacral groove
supported by paired ossicles and bearing the tube-feet.; with
regularly disposed calcareous, often spinous, plates on the skin ;
with an external madreporite (occasionally multiple), always
on the uppey surface of the disc in living forms; with a
mouth at the centre of the lower surface, and usually with an
anus at the opposite pole.
Description of a Starfish.
The description applies especially to the common five-
rayed starfish (Asterias or Asteracanthion rubens). It is
often seen in shore pools exposed at low water, but its
haunts are on the floor of the sea at greater depths. There
it moves about sluggishly by means of its tube-feet.
Each of the five arms bears a deep ventral groove in
which the tube-feet are lodged. The mouth is in the
middle of the ventral surface, the food canal ends about
the centre of the dorsal disc. With this flat, five-rayed
form, the 11-13 rayed sun-star (So/as/er), the pincushion-
like Porania, and the flat pentagonal Padmipes, should be
contrasted. Between two of the arms lies the perforated
madreporic plate, thus defining the d/vium, while the three
other arms constitute the, ¢vivdum. |
The body is covered by a ciliated ectoderm, beneath
which lies a mesodermic layer. In association with the
latter there is developed on the ventral surface of each arm
a double series of sloping plates. These meet dorsally, like
rafters, in the middle line of the arm, forming an elongated
shed. The rafter-like plates are called ambulacral ossicles ;
the groove which they bound lodges the nerve-cord, the
water vessel, and the tube-feet of each arm.
In association with the outer mesodermic layer of the
integument, numerous smaller plates are developed, e.g. the
adambulacrals, which articulate with the outer lower ends of
ambulacrals. The dorsal surface bears a network of little
ossicles, and many of these bear spines. Peculiarly modi-
fied spines, known as pedicellariz, look like snapping
scissor-blades mounted on a single soft handle. They
256 PHYLUM ECHINODERMA.
have been seen gripping Alge and the like, and probably
keep the surface of the star-fish clean. .
A starfish is not very muscular, but it often bends its
arms upwards by means of a muscular layer in the body
wall. . Other muscles affect the size of the ventral grooves,
and muscular elements also occur on the protrusible part
of the stomach, and in connection with the water vascular
system.
GEE
we
Rey
is
iN
ea
Fic. 132.—Starfish.
I. Ventral surface; 4.4, tube-feet extended; a.g., the ambulacral
groove with the tube-feet retracted; 2., the mouth. II. Dorsal
surface, showing the position of the madrepore (J7.); the two
adjacent arms form the bivium.
Underneath the ciliated ectoderm lies a network of
nerve fibrils, with some ganglion cells. But besides these
diffuse clements there is a pentagon around the mouth,
and a nerve along each arm. The system is not separable
from the skin. Ganglion cells are developed also on certain
parts of the wall of the ccelom.
A red eye spot, sensitive to light, lies on the terminal
ossicle at the tip of each arm, and is usually upturned. It
is a modified tentacle, bearing numerous little cups, lined
ASTEROIDEA. 257
by sensitive and pigmented cells, containing clear fluid, and
covered by cuticle. The skin is diffusely sensitive. The
term#nal tube-foot of each ray seems to be olfactory.
The starfish may be found with part of its stomach
extruded over young oysters and other bivalves. This
protrusible portion of the stomach is glandular and saccu-
FIG. 133.—Alimentary system of starfish. —After
Miiller and Troschel.
The dorsal surface has been removed ; the digestive czeca and the
stomach are shown.
lated, and bulges slightly towards the arms; it is followed
by an upper portion, giving off five branches, each of which
divides into two large digestive ceeca,—a pair in each arm
(Fig. 133). These glands are comparable to a pancreas;
their secretion contains three ferments, which convert
proteids into peptones, starch into sugar, and break up fats
into fatty acids and glycerine. From the short tubular
17
258 PHYLUM ECHINODERMA.
intestine between the stomach and the almost central dorsal
anus two little outgrowths are given off, perhaps homologous
with the “respiratory trees” of Holothurians (Fig. 139, 7.2).
Some parts of the food canal are ciliated.
The ccelom is distinct, though not much of it is left
unoccupied either in the disc or in the arms. It is lined
by ciliated epithelium, and contains a fluid with amceboid
cells. A few of these have a pigment which probably aids
in respiration; others are phagocytes, which get rid of
injurious particles through the “skin-gills”; others con-
tinue the work of digestion.
When a starfish is crawling up the side of a rock, scores
of tube-feet are protruded from the ventral groove of each
arm; these become long and tense, and their sucker-like
terminal discs are pressed against the hard surface. There
they are fixed, and towards them the starfish is gently
lifted. The protrusion is effected by the internal: injection
of fluid into the tube-feet; the fixing is due to the pro-
duction of a vacuum between the ends of the tube-feet
and the rock.
As to the course of the fluid, it is convenient to begin with the
madreporic plate, which lies between the bases of two of the arms (the
bivium). This plate is a complex calcareous sieve, with numerous
perforating canals and external pores. It may be compared to the rose
of a watering-can, but the holes are much more numerous, and lead
into small canals, which converge into a main ciliated canal, the stone
canal. This, as usual, opens into a ring canal around the mouth.
The ring canal gives off nine glandular bodies (Tiedemann’s bodies),
and, five radial tubes, one for each of the arms. Considerations of
symmetry suggest that there should be ten glandular bodies, but in the
inter-radius containing the stone canal there is only one. In many
starfishes there are five or ten little reservoirs (Polian vesicles) opening
into the circumoral ring, but in Asterzas rubens these are hardly dis-
tinguishable from the first ampullee of the radial vessels. These run
along the arms, and lie in the ambulacral groove beneath the shelter
of the rafter-like ossicles. From them branches are given off to the
bases of the tube-feet, but from each of these bases a canal ascends
between each pair of ambulacral ossicles, and expands into an ampulla
or reservoir on the dorsal or more internal side (see Fig. 134). The
fluid in the system may pass from the radial vessels into the tube-feet,
and from the tube-feet it can flow back, not into the radial vessel, but
into the ampullz. There are muscles on the walls of the tube-feet,
ampulla, and vessels. At the end of each arm there is a long unpaired
tube-foot, which seems to act asa tactile tentacle, and has also olfactory
significance.
ASTEROIDEA. 259
With regard to the vascular system there is considerable uncertainty
There is probably no definite vascular system at all. The organ de-
scribed asa heart is really the ‘‘ genital stolon.” There is a ‘‘ pseud-
heemal sinus” surrounding the stone canal, leading into a circum-
cesophageal ring, which gives off a vessel along each ray.
From the dorsal surface and sides of a starfish in a pool,
numerous transparent processes may be seen hanging out
into the water. They are the simplest possible respiratory
structures, contractile outgrowths of the skin with cavities
L, Td.
a bv.
I,
Fic. 134.—Diagrammatic cross-section of starfish arm.—
After Ludwig.
#., Radial nerve; J.v., radial blood vessel according to Ludwig,
septum in pseud-hezmal vessel according to others; w.v.,
radial water vessel; az., ampulla; 7, tube-foot; g.c., a
pyloric caecum cut across ; s.f., a calcareous spine; g., askin-
gill; Zac., spaces in the wall; go., ova in ovary ; @.0., ambu-
Tacral ossicle.
continuous with the ccelom, and are called ‘“skin-gills.”
It is likely that pigmented cells of the body cavity fluid act
like rudimentary red blood corpuscles; the water vascular
system may help in aeration; and the whole body is, of
course, continually washed with water.
The “skin-gills” are said to have an excretory function ;
for phagocytes, bearing waste, seem to traverse their walls.
It may also be that excretion is somehow concerned in
260 PHYLUM ECHINODERMA.
forming the carbonate of lime skeleton, but facts are
wanting. ,
The sexes are separate, and they are like one another,
both externally and internally. The gonads develop periodi-
cally, and lie in pairs in each arm. Each is branched like
an elongated bunch of grapes, and is surrounded by a
“blood sinus.” Each has a separate duct, which opens
on a porous plate, between the bases of the arms on the
dorsal surface. In Asterina gibbosa, however, the eggs are
extruded ventrally. In the same species there is an in-
teresting sexual variability: many are first males and then
females (protandric), others are simply hermaphrodites,
others seem exclusively of one sex. The eggs of starfishes
are fertilised in the water, and the free-swimming larva is
known as a Bipinnaria or as a Brachiolaria.
Other Starfishes
Parental care is incipient among Asteroids. A species of
Asterias has been seen sheltering its young within its arms:
there is a definite brood-pouch in the form of a sort of tent
on the dorsal surface of Preraster.
Many Asteroids break very readily, or throw off their
arms when these are seized. The lost parts are slowly
regenerated, and strange forms are often found in process
of regrowth. Thus the “comet form” of starfish occurs
when a separated arm proceeds to grow the other four.
There are many deep-sea forms, such as the ophiuroid-
like Brisinga, the widely-distributed Aymenaster, and the
blue Porcellenaster ceruleus; but the majority occur in
water of no great depth.
Asteroidea first occur in Silurian strata.
Classification.—
Order I. Phanerozonia. With strongly developed marginal
plates, the upper and lower marginals in contact ; with skin-
gills restricted to the dorsal (abactinal) surface; with broad
ambulacral plates; with prominent adambulacrals in the peri-
stome, with pedicellariz sessile (if present), with two rows of
tube-feet. ;
e.g. Astropecten, Luidia, Porania, Asterina, Palmipes.
Order II. Cryptozonia, - With indistinct or rudimentary marginal
plates in the adults, often with intermediate plates between the
OPHIUROIDEA, 261
upper and lower marginals, with skin-gills not restricted to the
dorsal (abactinal). surface, with narrow ambulacral plates, with
ambulacrals or adambulacrals prominent in the peristome, with
pedicellariz sessile or stalked (if present), often with apparently
four rows of tube-feet.
e.g. Asterias, Solaster, Henricia, Brisinga.
Class OpHiuRoIDEA. Brittle-stars, e.g. Ophiopholis
aculeata ~
Echinoderms with a stellate flattened body, nearly related
to starfishes, but usually differing from them in having the
arms (sometimes branched) sharply marked off from the
Fic. 135.—Ventral surface of disc of an Ophiuroid
(Ophiothrex fragilis).—After Gegenbaur.
&, Openings.of genital pockets or burse; 7., mouth; v., ventral
plates of arms; s/., spines of arms ; ¢/, tube-feet—at the right side
these are represented as retracted; 0., the openings through-which
they are protruded ; #., plates around mouth bearing the so-called
teeth; one of these plates is perforated, and functions as the
madreporite. :
central disc, no ambulacral groove on the ventral surface of
the arms, the digestive organs and gonads restricted to the
disc, and the madreporite ventral. There is no anus. There
are deep respiratory clefts on the dise at the insertion of the
262 PHYLUM ECHINODERMA.
arms. They agree with starfishes in being free, in having
radially disposed gonads, in having the tube-feet restricted to
the under surface, and in other features.
The body of a brittle-star differs from that of a star-
fish in the abruptness with which the arms spring from
the central disc (cf. Brisinga). These arms are muscular,
and useful in wriggling and clambering; they do not con-
tain outgrowths of the gut, nor reproductive organs.
Moreover, there is no ambulacral groove, and the tube-feet
which project on the sides are usually very small. They
are often of locomotor service, adhering even to vertical
surfaces, but in many cases they seem to be only sensory.
Each segment of the arm includes a central “vertebral
ossicle,” with four plates forming a tube round about it.
There is a complete oral skeleton. .The madreporic plate
is situated on the ventral surface, usually on one of the
plates around the mouth. The food canal ends blindly.
Some brittle-stars have small luminescent glands, e.g.
Amphiura squamata. The reproductive organs lie in
pairs between the arms, and open into pockets or bursze
formed from inturnings of the skin, which communicate
with the exterior by slits opening’ at the bases of the arms.
Water currents pass in and out of these pockets, which
probably have both respiratory and excretory functions.
The free-swimming larva is a luteus, very like that of
Echinoids (see Fig. 131).
Ophiuroids are first found in Silurian strata.
The Ophiuroids are usually classified according to the characters of
their ossicles and covering plates. Some common genera are Ophiothrix,
Ophiocoma, Ophiopholis, Ophiura. In the deep-water Astrophyton and
Gorgonocephalus the arms are repeatedly branched. In <Astronyx
Jovenz, often caught in the trawl off the north coast of Britain, the disc
is relatively large and soft and the arms very long. In the extinct
Lysophiuree there is an ambulacral groove.
Class EcuinoipEA. Sea-Urchins, e.g. the common
Lchinus esculentus
Lchinoderms with the body covered by rows of plates,
usually in vertical series and forming an inflexible test; the
shape of the majority approaches a sphere, but some are pin-
ECHINOIDEA. 263
cushion-like, flat, or obviously bilateral; the test ts covered
with spines which vary greatly in length and thickness in the
different types’; the locomotor and respiratory tube-feet usually
extend from the peristome to near the aboral pole; there is
often a well-developed system of apical plates ; the mouth is at
the lower pole, the anus either at the aboral pole or in the
posterior inter-radius ; the gonads are unpaired, five in
number, and inter-radial.
Description of the Common Sea-Urchin.
Most sea-urchins live off rocky coasts, and not a few
shelter themselves sluggishly in holes. They move by
Fic. 136.—Apical disc of sea-urchin
In the centre is the periproct bearing the anus; around it are five
genital or inter-radial plates (g.), one of which is modified as the
madreporite (7.); beyond these five ocular or radial plates (0.) ;
z.y.a., an inter-radial or inter-ambulacral area, with spines
only ; ~.@., a radial or ambulacral area, with spines and open-
ings for tube-feet.
means of their tube-feet and spines, and seem to feed on
“acorn-shells” and other small sedentary animals, some
seaweeds, and the organic matter found in ‘mud and other
264 PHYLUM ECHINODERMA.
deposits. After the perils of youth are past, the larger
forms have few formidable enemies.
The hard and prickly body is more or less spherical.
The food canal begins in the middle of the lower surface ;
it ends at the opposite pole in the middle of an apical disc,
formed in the young animal of a central plate surrounded
by five “‘ocular” and five “genital” plates. .In the adult
the central plate is no longer distinct. Each has a hole
for the protrusion of a sensitive tube-foot; the genitals
bear the apertures of the genital ducts, and one also bears
the perforated madreporic plate. From pole to pole run
ten meridians of calcareous plates, which fit one another
firmly ; five of these (in a line with the ocular plates) are
known as ambulacral areas, for through their plates the
locomotor tube-feet are extruded; the five others (in a line
with the genital plates) are called inter-ambulacral areas,
and bear spines, not tube-feet. Altogether, therefore, there
are ten meridians, and each meridian area has a double
row of plates. On the dry shell from which. the spines
have been scraped, the ambulacral plates are seen to be
perforated by small pores, three pairs or so to each plate.
Through each pair of pores a tube-foot is connected with
an internal ampulla. In the starfish the ambulacral areas
are wholly ventral, and the apical area seen on the dorsal
surface of the young forms is not demonstrable in the adult.
On the shell there are obviously many spines, most
abundant on the inter-ambulacral areas. Their bases fit
over ball-like knobs, and are moved upon these by muscles.
But besides these, there are modified spines—(a) several
kinds of pedicellarize, with three snapping blades on a soft
stalk, and sometimes with apical glands; and (4) small
globular sphzeridia, which show some structural resem-
blances to otocysts. It is said that, like true otocysts,
they are concerned with the perception of direction of
motion. New spines and pedicellarie can be grown to
replace those that are shed in unwholesome conditions or
rubbed off by accident. This is the only marked regenera-
tion in sea-urchins.
In front of the mouth project the tips of five teeth, which
move against one another, grasping and grinding small
particles. They are fixed in five large sockets or pyramids,
SEA-URCHIN. 263
and along with five stout “ braces” (rotulz) and five curved
“compasses” (radii) form “ Aristotle’s lantern,” a complex
masticating apparatus, of whose history we know little. It
surrounds the pharynx, and is swayed about and otherwise
moved by muscles, many of which are attached to five
beams which project inward from the margin of the shell
and form a “girdle” round about the mouth.
Fic. 137.—Dissection of sea-urchin.
M. at the lower pole is the mouth; . at the upper pole is the madreporic
plate; 7.7., one of the large tentacular tube-feet around the mouth;
S.G., a skin-gill; SZ., a standard or perignath; AZ., an alveolus;
R.V., a radial vessel, with ampulla (4.); intestine (/#z.) fixed by
jassenterlesy P., a pedicellaria; G., a gonad: SP., spines; 7.F,
tube-feet.
As in other Echinoderms, the skeleton of lime is meso-
dermic. The shell is covered externally by a delicate
ciliated ectoderm, beneath which, in a thin layer of. con-
nective tissue, there is a network of nerve fibres, and some
ganglion cells. Internally, there is another thin layér of
connective tissue, and a ciliated epithelium lining the
body cavity. The skeleton grows by the formation of
266 PHYLUM ECRINODERMA,
new plates around the apical disc, and also by the indi-
vidual increase of each. In a few forms the shell retains
some plasticity.
The nervous system consists of a ring around the mouth,
of radial branches running up each ambulacral area, and
of the superficial network. Tube-feet, sphzridia, pedi-
cellarize, and spines are all under nervous control, and each
radial nerve ends in the “‘eye specks” of the apical ‘‘ ocular
plates.” It is probable that all the tube-feet are sensory,
and this is certainly the main function of ten which lie
near the mouth.
The alimentary canal passes through Aristotle’s lantern,
and the intestinal portion lies in two and a half coils
around the inside of the shell, to which it is moored by
mesenteries. It contains fine gravel, sand, and some
organic débris. It ends near the centre of the apical disc,
whence the pedicellarize have been seen removing the feeces.
The spacious body cavity is lined by ciliated epithelium,
and contains a “perivisceral” fluid, whose corpuscles have
a respiratory pigment (echinochrome). When the fluid of
a perfectly fresh sea-urchin is emptied out, the contained
corpuscles unite in plasmodia, forming composite amceboid
clots (cf. Protomyxa, etc.).
The madreporic plate communicates with a membranous
stone canal (calcareous in C7daris) which runs downwards
into a circular vessel near the upper end of the lantern.
This gives off five inter-radial transparent “ Polian” vesicles
and five radial vessels, which run down the sides of the
lantern and up each ambulacral area. Each radial vessel
gives off numerous lateral branches, which communicate
with the internal ampulle and thence with the external
tube-feet. When the tube-feet are made tense with fluid,
they extend far beyond the limit of the spines, and are
attached to the surface of the rock over which the sea-
urchin slowly drags itself. The sucker at the tip of each
tube-foot bears a rosette of small calcareous plates; indeed,
there is hardly any part of an Echinoderm in which lime
may not be deposited. Before bending upwards from
the base of the lantern, each radial vessel gives off a branch
to two large tentacle-like tube-feet without attaching discs.
The five pairs lie near the mouth, and are sensitive.
ECHINOIDEA. 26)
The blood vascular system is not readily traced, and there is un-
certainty as to many points. A “dorsal or axial organ” lies beside the
stone canal, and seems to be connected with a ‘‘genital ring” and
with a circular vessel around the gullet. There js also a ‘“‘ pseud-
hzemal” system consisting of « circum-cesophageal sinus with radial
branches. The fluid cannot be distinguished from that of the body
cavity ; it contains corpuscles, some of which are pigmented.
On the area round about the mouth there are ten hollow
outgrowths, which resemble the skin-gills of starfishes.
There are also five large vesicles at the top of the lantern
(Stewart’s organs”) which may function as internal gills.
As already mentioned, the pigmented cells of the body
cavity fluid seem able to absorb oxygen. There is no
doubt that the water vascular system plays a very important
part in respiration. It probably also aids in excretion.
The sexes are separate, and indistinguishable externally.
Five large branched yellow-brown ovaries or rose-white
testes lie inter-radially under the apex of the shell, and
open by separate ducts on the five genital plates. In
spring the apical disc may be seen covered with orange
ova or milky-white spermatozoa.
The eggs are fertilised externally by sperms wafted from
adjacent sea-urchins, and the free-swimming larva is called
a Pluteus.
Classification.—
The class may be divided into three sub-classes or groups of orders.
Sub-Class I. Regularia Endobranchiata. Mouth and anus at opposite
poles; the anus surrounded by the apical system of plates if these
are developed ; no external gills.
e.g. the somewhat primitive Czdarzs.
Sub-Class II. Regularia Ectobranchiata. Mouth and anus at opposite
poles; a double circle of apical plates surrounds the anus ; there
are external gills,
e.g. the common genera Zchzzus, Giese Dualtes Arbacia.
‘The Echinothurine have flexible tests and powerful muscles.
e.g. Asthenosoma and Phormosoma.
Sub-Class III. Irregularia. The anus lies outside the apical system
of plates in the posterior inter-radius.
e.g. the heart-urchins, Spatangus and Echinocardium, without
lanterns. In the related Achénoneus there is a lantern in the
young forms, It is interesting to contrast the large massive
Clypeaster with the minute EZchinocyanrus pusillus, common
_in the stomach of cod-fishes.
268 PHYLUM ECHINODERMA.
Class HoLtorHuROIDEA. Sea-Cucumbers
Cylindrical or worm-like Echinoderms, elongated in the
direction of the main axis, with more or less tendency to
bilateral symmetry, with a usually soft or leathery skin, with
irregularly scattered microscopic calcareous bodies, with a
terminal mouth surrounded by tentacles, with a posterior anus,
with or without tube-feet, with no external madreporite, with
a muscular body wall.
The Holothurians do not at first sight suggest the other
Echinoderms, for they are like plump worms, and the
Fic. 138.—Spicules of Holothurians.—After Semon.
The series 7-6 shows stages in the development of an anchor and a plate
in a Synapta. The series A-Z# shows stages in the development of
a wheel in CArridota, a Synaptid.
calcareous skeleton is not prominent. But closer examina-
tion shows the characteristic pentamerous symmetry, and
the occurrence of calcareous plates in the skin. These
seem to be absent in the unique pelagic Pelagothuria.
Holothurians occur in most seas, from slight to very
great depths. Their food consists of small animals, and of
organic particles from the sand. Some of them catch these
in their waving tentacles, which are then plunged into the
pharynx. The muscles of a captured Holothurian often
over-contract and eject the viscera at the ends or through
HOLOTHUROIDEA. 266
a side rupture; in this way the animal may sometimes
escape, and the viscera can be regrown.
In Synapta the rupture of the body takes place very rapidly, and is
probably defensive, the anterior portion re-forming a complete individual.
In some forms of Cucumaria planci the body divides by stricture,.
torsion, or stretching into two or three equivalent parts, each of which
may regenerate the whole. In this case the autotomy seems to be
reproductive.
The worm-like body is often regular in form, with five
equidistant longitudinal bands, along which tube-feet emerge.
But three of these “ambulacral areas” may be approxi-
mated on a flattened ventral sole, leaving two on the
convex dorsal surface, and there are other modifications of
form. In many cases the tube-feet are modified into
pointed papillee.
The body wall is tough and muscular, consisting of
epidermis, dermis, and circular muscles, and there are
paired longitudinal muscles along each radius. A skeleton
is represented by scales, plates, wheels, and anchors of lime
scattered in the skin, and by plates around the gullet and
on a few other regions.
The nervous system consists of a circumoral ring in
which the five radial nerves running in the ambulacral areas
unite, and from which nerves to the tentacles arise. The
ring and the radial nerves are sunk below the skin.
Ccelomic nervous tissue is developed on the perihzmal
canals, Sense organs are represented by the tentacles,
which sometimes have “‘ear-sacs” at their bases, and by
tactile processes on the dorsal surface of some of the
creeping forms.
From the terminal or ventral mouth, surrounded by five,
ten, or more tentacles, the food canal coils to the opposite
pole. ‘There it expands in a cloacal chamber sometimes
contractile, and from this are given off in many forms a
pair of much-branched “respiratory trees,” which extend
forward in the body cavity. These “trees” are supplied
with water by means of the rhythmic contractions of the
cloaca. They are respiratory, hydrostatic, and excretory.
The body fluid sometimes contains a red pigment like
hemoglobin. Arising from the base of the respiratory trees
in some Holothurians there are the remarkable “Cuvierian
270 PHYLUM ECHINODERMA.
organs,” which emit white conical bodies from the cloaca
when the animal is irritated. The bodies remain adherent
by their bases, are greatly elongated by internal fluid
pressure into sticky tubes which break off. They will
adhere to almost everything but the Holothurian itself.
Those Holothurians, e.g. Holothuria nigra, in which the
organs are well developed are often called “cotton-
spinners,” on account of the dense mass of viscid substance
which they eject. A little fish, Averasfer, introduces itself
—tail first—into the cloaca of several Holothurians, and
lives three as an innocent commensal.
The water-vascular system shows many peculiarities. In what, by
analogy with the other classes, may be described as the primitive
condition, there is « ring canal round the mouth communicating with
the exterior by a stone canal, with one or more Polian vesicles hanging
in the body cavity, and with five radial canals. The radial canals, as
in starfishes and sea-urchins, are connected with internal ampullee and
external tube-feet. The anterior tube-feet are greatly enlarged and
modified to form the tentacles which encircle the mouth. It is, how-
ever, only rarely that the water-vascular system exhibits this primitive
condition. In most cases the stone canal loses its original connection
with the exterior and opens merely into the body cavity ; often it is
represented by numerous small canals, hanging freely in the body
cavity (Fig. 139, s¢.). Certain of the tube-feet are always modified to
form tentacles, and these may, as in Syzafta, be the only representatives
of the tube-feet. In regard to the function and degree of development
of these, there is indeed much variation.
The blood-vascular system consists of a circum-cesophageal ring and
vessels to the alimentary canal and the gonads. The system is in great
part lacunar. There is also a pseud-hamal system.
The sexes are usually separate. The reproductive organs
do not exhibit radial symmetry, and are branched tubes
which open within or just outside the circle of tentacles.
Like other internal organs of Holothurians, they are often
very brightly coloured. ‘The larva is, in most cases, what
is known as an Awricularia. Sometimes, however, the
larval stage is skipped, as in Cucumaria crocea and Psolus
ephippifer, where the eggs and young are attached to the
back of the mother. In C. cuvata the eggs and young are
sheltered on the ventral surface; in C. parva in a shallow
ventral insinking; in C. /evigata there is an invaginated
ventral brood-pouch ; in Chiridota contorta the young are
sheltered in the genital tubules; in Sywapta vivifara and
fi
Bs
>
=a
Ce
Fic. 139.—Dissection of Holothurian (Holothuria tubulosa) from
the ventral surface.
#., Tentacles surrounding the mouth; 44, scattered tube-feet of
ventral surface ; ¢., calcareous ring surrounding the food canal ;
a., ampulle of tentacles (modified tube-feet); ~., circular vessel
surrounding the gullet, giving off the branched stone canal (s#.),
the single Polian vesicle (o.), and the five radial canals (7c.),
which run forwards, pass through the calcareous ring, and then
curve outwards to run on the surface of the longitudinal
muscles (¢.7.) along the radial areas. Of the five longitudinal
muscles, one only is marked. gl., The gut cut through at the
beginning of the first loop; 2., the mesentery which attaches
the gut to the body wall, showing the course of the gut; g2.,
the other end of the gut ; c/., the cloaca bound down by muscles ;
an., the anus ; 7.4., the right respiratory tree—the left is cut short
mle to its origin; ov., the ovary. The blood vessels are not
shown.
272 PHYLUM ECHINODERMA.
some others the body cavity serves as a brood-pouch. This
illustrates how the same result may be reached in a great
variety of ways.
The calcareous plates of Holothurians are found as far
back as Carboniferous strata.
As “trepang” or “béche-de-mer,” the Holothurians of
the Pacific form an important article of commerce, being
regarded as a delicacy by the Chinese.
Classification.—
Order 1. Actinopoda. The radial water vessels are associated with
external tentacles, tube-feet, and ambulacral papilla, but the
tube-feet and papilla may be absent. There are several
families, e.g. the deep-sea Elasipoda, markedly bilateral, almost
always flattened ventrally, often with an external pore for the
stone canal, e.g. Hipzdia and Kolga; the Aspidochirote, e.g.
Holothuria and Stzchopus, and Dendrochirote, e.g. Cucumaria,
Thyone, Psolus, with tube-feet as well as tentacles; the Molpa-
diidze with tentacles only, eg. Molpadia; the Pelagothuriidze
containing the free-swimming Pelagothuria.
Order 2. Paractinopoda or Apoda. The only external outgrowths ot
the water-vascular system are the pinnate tentacles around the
mouth. One family, Synaptide, e.g. Syzapta and Chzridota.
There are no tube-feet or respiratory trees or Cuvierian organs.
The calcareous bodies are usually beautiful anchors and plates.
Many are hermaphrodite.
Class Crinoipea. Feather-stars
Usually stalked forms, with five jointed, often branched
arms (“brachia”), growing out from a central cup or
“theca,” and bearing pinnules; the arms arise from a
‘orresponding number of thecal plates or “radials,” below
which there is a circlet of alternating “‘basals,” often with
“infra-basals” alternating again with them; below the
“Bbasals” or “infra-basals” there is usually a jointed stem
anchored to the substratum by “ cirri.”
The feather-stars or sea-lilies differ from other Echino-
derms in being fixed permanently or temporarily by a jointed
stalk. The modern Comatulids, eg. the rosy feather-star
(Comatula or Antedon rosacea) leave their stalk at a certain
stage in life; but the other Crinoids, e.g. Pentacrinus, are
permanently stalked, like almost all the extinct stone-lilies
or encrinites, once so abundant. Most of them live in deep
CRINOIDEA. 273
water, and many in the great abysses. An anchorage is
found on-rocks and stones, or in the soft mud, and great
numbers grow together—a bed of sea-lilies.s The free
Comatulids swim gracefully by bending and straightening
their arms, and they have grappling “cirri” on the aboral
side, where the relinquished stalk was attached. By these
cirri they moor themselves temporarily. Small organisms—
Diatoms, Protozoa, minute Crustaceans—are wafted down
ciliated grooves on the arms to the central mouth, which
is of course on the upturned surface. Some members of
“Il
vil i Hl
“
«
i om
i Pye
Fic. 140.—Diagrammatic vertical section through disc and
base of one of the arms of Axmtedon rosacea.—After
Milnes Marshall.
The section is inter-radial on the left, radial on the right. 74., Cili-
ated openings in body wall; %., sub-epithelial ambulacral nerve ;
Z., water-vascular canal; %, tentac'e; ~, mouth; s., intestine ;
&, central plexus, with ‘‘chambered organ” at its base; f,
ceelom ; #1.-R3,, radial plates ; B., brachial plates ; ~., muscle ;
a,, axial nerve-cord; @., central capsule; C.D., centro-dorsal
plate; Z., cirri; ¢., nerve branches from central capsule to cirri.
the class, eg. Comatula, are infested by minute parasitic
“worms” (Myzostomata) allied to Chzetopods, which form
galls on the arms. A lost arm can be replaced, and even
the visceral mass may be regenerated completely within a
few weeks after it has been lost. It has been suggested that
the occasional expulsion of the visceral sac frees the Crinoid
from parasites (Dendy).
The animal consists of (I) a cup or calyx, (2) an oral disc forming the
lid of this cup, (3) the radiating ‘‘ arms,” and (4) the stalk supporting
the whole. The lowest part of the cup is supported by a pentagonal
18
274 PHYLUM ECHINODERMA
**centro-dorsal” ossicle, bearing the cirri; this conceals the coalesced
‘*basals” of the larva; above this are three tiers of ‘‘ radials,” whence
spring the ‘‘ brachials” of the arms.
The oral disc, turned upwards,. is supported by plates. Here the
anus also is situated. The arms usually branch in dichotomous fashion,
and thus ten, twenty, or more may arise from the original five. But the
growing point continues to fork dichotomously, like the leaf of many
ferns, and as each altcrnate fork remains short, a double series of lateral
‘*pinnules” results. The arms are supported by calcareous plates. The
stalk usually consists of numerous joints, especially in extinct forms, in
some of which it measured over fifty feet in length. Except in Holopus,
Hyocrinus, and in the stalked stage of Antedon, the stalk bears lateral
cirri.
The nervous system consists (a) of a circumoral ring with ambulacral
nerves, and (4) of axial coelomic nerves up the ossicles on the opposite
side of each arm and connected with » peculiar ‘‘ chambered organ”
in the interior of the centro-dorsal plate.
Apart from the superficial epithelium, there are no sensory structures.
The ciliated food canal descends from the mouth into the cup, and
curves up again to the anus, which is on a papilla. The last part of
the gut is expanded to form an anal tube, which during life is in con-
stant movement, and has apparently a respiratory function. From the
cup, where the body cavity is in great part filled with connective tissue
and organs, four coelomic canals extend into each of the arms. They
communicate at the apices of the arms and pinnules, and currents pass
up one and down the other.
The blood-vascular system consists of a circumoral ring, which is
connected with a radial vessel under each ambulacral nerve, and with a
circum-cesophageal plexus.
The water-vascular system consists as usual of a circumoral ring and
radial vessels, but in several respects it shows remarkable modification.
The madreporite of other forms is represented by fine pores which open
from the surface of the calyx directly into the body cavity, and which
may be very numerous ; there are said to be 1500 in Aztedon rosacea.
By these pores water enters the body cavity, and from it enters the
numerous stone canals which hang from the ring freely in the body
cavity, and open into it near the pore canals. There are no Polian
vesicles or ampullz, the tube-feet are small, are arranged in groups of
three, and are connected by delicate canals with the radial vessels.
Certain of them form tentacles around the mouth, and these are supplied
by canals coming off directly from the ring canal.
The sexes are separate. The reproductive organs extend as tubular
strands from the disc along the arms, but are rarely functional except
in the pzznules, from each of which the elements burst out by one duct
in females, by one or two fine canals in males.
The oval ciliated larva of Amtedon, the only one known, is less
peculiar than that of other Echinoderms,
There are about 400 living species in twelve genera, but about 1500
species in 200 genera are known from the rocks. The class is obviously
decadent. It is represented in the Cambrian, and attained its maximum
development in Silurian, Devonian, and Carboniferous times.
DEVELOPMENT OF ECHINODERMS. 275
The recent forms include the stalked Pentacrinus, Rhizocrinus, etc.,
and the free Comatulids, which pass through a stalked Pentacrinus
stage, e.g. Antedon.
Class EDRIOASTEROIDEA. Wholly extinct
These extinct Pelmatozoa had a sac-like theca of an indefinite number
of irregular plates, with a mouth in the centre of the upper surface,
with at most a short stalk. Ordovician, Silurian, and Devonian.
‘* They are alone among Pelmatozoa in presenting a type of ambulacrum
from-which the holothurian, stellerid, and echinoid types may readily
be derived” (F. A. Bather).
Class BLastorpEa. Wholly extinct
The Blastoids are first found in the upper Silurian, later than Cystoids
and Crinoids; they had their golden age in the Carboniferous and
Devonian times, but then disappeared. Their body was ovate, with
five ambulacral areas, with each groove of which jointed pinnules were
associated.
Class CystipgEa. Wholly extinct
The Cystidea are first found in the Lower Silurian rocks, had their
golden age in Upper Silurian times, and died out in the Carboniferous
period. Their body was ovate or globular, sessile or shortly stalked,
covered with polygonal plates often irregularly arranged.
DEVELOPMENT OF ECHINODERMS
The ovum undergoes total segmentation, and a hollow
ball of cells or blastosphere results. A typical gastrula is
formed by invagination.
The mesoblast has a twofold origin: (a) from ‘“ mesen-
chyme” cells, which immigrate from the invaginated endo-
derm into the segmentation cavity ; (4) from the outgrowing
of one or more ccelom pouches (vaso-peritoneal vesicles)
from the gastrula cavity or archenteron. From these
vesicles the body cavity and the rudiments of the water-
vascular system arise.
The larva is, first of all, a slightly modified, diffusely
ciliated gastrula. In Holothuroids, Echinoids, Asteroids,
and Ophiuroids, it becomes quaintly modified by the
outgrowth of external processes, and the formation of
276 PHYLUM ECHINODERMA.
special ciliated bands. These are at first simply pre-oral
and pre-anal rings, but they become drawn out along
variously disposed and shaped processes. The larva of
Crinoids (of Anfedon) is not so divergent. In all cases
the bilateral symmetry is preserved.
The larva does not grow directly into the adult. On the
contrary, the adult arises, for the most part, from new
growth within the larva on one side. The arms or pro-
cesses peculiar to the larva are absorbed or in part thrown
off. Only in a few forms which have brood-chambers or
Fic. 141.—Stages in development of Echinoderms.—After Selenka.
1. Section of blastula of Synapta digitata (Holothuroid), with a hint of
gastrulation. 2. Section of gastrula of 7o.xopneustes brevispinosus (sea-
urchin) ; ec., ectoderm ; ez., endoderm ; 7., segmentation cavity with
mesenchyme cells init. 3. Section of larva of Asterina gibbosa (star-
fish) ; BZ., blastopore ; g., archenteron; v.Z., vaso-peritoneal vesicle ;
yr. and Z., right and left sides.
are viviparous is the development direct, and without free-
swimming larvee.
The celebrated comparative anatomist and physiologist, Johannes
Miiller, was the first to show that the various types of Echinoderm
larvee might be derived from one fundamental form.
‘This fundamental type is an elongated, oval, or pear-shaped larva,
which is somewhat flattened on its ventral side. It has arisen from a
gastrula whose blastopore has become the anus, while the archenteron
is bent towards the ventral surface, where it communicates by the larval
mouth with the exterior. Besides these two apertures, the larva has a
third, namely, the dorsal pore of the water-vascular system. The cilia,
with which the larva was at first uniformly covered, partly disappear,
a persist only in restricted regions or ciliated bands” (Korschelt and
eider).
Crinoids.—The simplest Echinoderm larva is that of Azedon, a
somewhat modified oval, with five transverse rings of cilia (the most
277
RELATIONSHIPS OF ECHINODERMA.
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‘uorsa1 Teolde ayy Japun at]
suvsio aatjonpoiderayy,
‘quswWasuele pafel-aay & y1qiyxs
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aya jo aseq ayy seau uedo sayy
! Aytazo Apog ay3 UI saqn} peyouriq
are suesio aaljonpoider ayy
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pur ‘sapejuay Aloyendsar
are jaaj-aqny ayy, ‘saiod
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34} YUM syeuvd perlaaas
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‘Uys Ie(MIsnut YSnor e& YIM ‘IIT
sreaq 4[eIS paquiol Arerod | woy ‘ostp yeuoSejuad | -ajeyjays Jo ‘jeuoSeyuad | :pauayzeg 10 ‘padeys-jreay
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‘VACIONIND *VACIOUNIHAO “vadIOuaLsy “VACIONIHOD
-UIOM pue pazesuoya SI Apog ayy,
| "VAdCIOUNHLO TOF
SWUACGONIHOY AO SASSVID LNVLXY AZAIA AHL NAAMLAL SLSVYLNOO AWOS
278 PHYLUM ECHINODERMA.
anterior is less distinct), and a posterior terminal tuft. Eventually the
posterior end is elongated to form, in the pentacrinoid stage, an attach-
ing stalk, which is afterwards absorbed. As all the extinct Crinoids
are permanently stalked, there is here an instance of Recapitulation.
Holothuroids.—The larva of Holothuroids (an Aurécularza) is much
quainter. Its diffuse cilia are succeeded by a wavy longitudinal band,
which in the fuga stage breaks into transverse rings, usually five in
number. ‘The pre-oral region becomes large.
Asterotds,—Nearest the Auriculariéa is the larva of starfishes, which
has the same enlarged pre-oral region. There are ¢wo ciliated bands,
of which the ad-oral is smaller, the ad-ana] much larger. They are
extended peripherally by the development of soft bilateral arms, and
such a larva is known as a Szpznnaréa. But another larval form in
Asteroids is the Avrachzolarta stage, in which three warty arms are
formed at the anterior dorsal end, independently of the ciliated bands.
Ophiuroids and Echinotds.—In the Pluteus larve (Fig. 131) char-
acteristic of these classes the pre-oral region remains small, while the
post-anal region becomes large. There is one undulating ciliated
band, the course of which is much modified by the growth of six long
arms, with temporary calcareous supports. This quaint form is often
‘compared to a six-legged easel.
The development of these larval forms into the adult is very intricate.
The adult is a new formation within the larva, retaining the water-
vascular system and mid-gut, but absorbing or rejecting the provisional
larval structures. As certain parts are broken down, others are built
up, chiefly through the agency of the wandering amceboid cells of the
mesenchyme. The first steps in the upbuilding of the adult, and
especially of its skeleton, are to some extent parallel in the five classes.
One of the most important changes is that from bilateral to radial
symmetry. In connection with this, it has been conjectured that the
primitive ancestor was bilaterally symmetrical, and that the radiate
symmetry was acquired by early sessile or sedentary Echinoderms, such
as the Cystoids. As we have already seen, the adults in the different
classes tend to acquire an independent and secondary bilateral symmetry.
It is very difficult to compare the Echinoderm larva, even in their
simplest form, with those of other animals. The nearest type is perhaps
the Tornaria of Balanoglossus, but it again is very unique. One
naturally tries to compare the Echinoderm larva with the Trochosphere
of Annelids, but the differences are very marked. One of the most
marked of these is the absence of the apical sense organ, so charac-
teristic of the Trochosphere. The fact that this is represented in the
larva of Axtedon is regarded by many naturalists as a point of much
importance.
RELATIONSHIPS OF ECHINODERMA
The Echinoderms form an exceedingly well-defined phylum, but
the Holothurians especially show how many of the significant char-
acters may be lost. In that class we see how the power of forming a
calcareous skeleton, the characteristic tube-feet, and the greater part of
RELATIONSHIPS OF ECHINODERMA. 279
the peculiar water-vascular system, may all disappear ; it is conceivable
that further modification of the same kind might eliminate all the dis-
tinctively Echinoderm characters, and produce an organism whose
systematic position would be very difficult to determine. This is
important, because, as we have already seen, there are many ‘‘ worm-
like” types of whose affinities we know nothing. That some of these
are related to Echinoderms has been often suggested.
It is conceivable that Holothurians of the worm-like Syxag¢a type
are nearest the primitive stock of Echinoderma. But there are strong
arguments in favour of the view that the free forms, the Eleutherozoa,
have been derived from attached Pelmatozoic ancestors. The extinct
Edrioasteroidea are in some ways intermediate between the Cystidea
and the Eleutherozoa.
CHAPTER X11
PHYLUM ARTHROPODA
Chief Classes—CRUSTACEA, PROTOTRACHEATA, MyRIOPODA,
InsEcTA, ARACHNOIDEA, PALAOSTRACA
More than half the known species of animals are included
in the Arthropod phylum, for of insects alone there are said
to be more species than of all other animals taken together.
The Arthropods are in some ways like Annelids—in the
bilateral symmetry; in the division of the body into suc-
cessive segments, some or all of which bear appendages ; in
the plan of the nervous system; and so on. Furthermore,
Peripatus, which has air-tubes or tracheze somewhat similar
to those of Myriopods and Insects, has nephridia like those
of some Annelids ; and the biramose appendages of a simple
Crustacean like Afus may be compared with the parapodia
of an Annelid.
It is difficult to discern the relationships of the various
classes included in the Arthropod phylum. Crustaceans,
most of which are aquatic and breathe by gills, are often
opposed to the Prototracheata, Myriopods, Insects, and
Arachnoids, most of which are terrestrial or aerial, and
breathe by trachez, or possible modifications of these.
Three divergent groups—the King-crabs (Zimu/us), and
the extinct Eurypterids and Trilobites—may be conveniently
referred to a separate class—Palzostraca.
General Characteristics of Arthropods (to which primitive,
parasitic, and degenerate forms present exceptions)
The body ts bilaterally symmetrical, and consists of numer-
ous segments variously grouped, Several or all of the segments
CRAYFISH. 281
bear paired jointed appendages variously modified. The
cuticle is chitinous. Ciltated epithelium is almost always
absent. The dorsal brain is connected by a ring round the
gullet with a double chain of ventral ganglia, Above the food
canal lies the heart. The true or primitive celom is always
small in the adult; the apparent body cavity is of secondary
origin, and has ina great part a blood-carrying or vascular
Junction. The sexes are almost always separate, the reproduc-
tive organs and ducts are usually paired. There is often
some metamorphosis in the course of development. In habit
the Arthropods are predominantly active.
Class CRUSTACEA
General Characteristics of Crustaceans (to which primitive,
parasitic, and degenerate forms offer exceptions)
With few exceptions, e.g. land-crabs, wood-lice, and sand-
hoppers, Crustaceans live in water. They breathe by gills or
cutaneously, The head carries two pairs of antenne in
addition to other appendages, e.g. at least three pairs of
jaws; the thorax, sometimes distinct from, and sometimes
Jused to the head, bears various kinds of limbs ; the abdomen
zs usually segmented, and often has appendages. The
typical appendage consists of two branches and a basal
portion, to which gills may be attached. To the chitin of the
cuticle, carbonate of lime ts added.
A Type of CRUSTACEA. The fresh-water Crayfish
(Astacus fluviatilis)
(Most of the following description will apply also to the Lobster
(Homarus), to the Rock Lobster (Paiimurus), and to the Norway
Lobster (Wephrops norvegicus), often called a crayfish.)
Mode of life.—The fresh-water crayfish lives in streams,
and burrows in the banks. It is not found in Scotland, but
occurs here and there in England and Ireland, and is
common on the Continent. It is not found in districts
where the water contains little lime. The food is very
varied—from roots to water-rats ; cannibalism also occurs.
The animals swim backwards by powerful tail strokes, or
282 PHYLUM ARTHROPODA.
creep forwards on their “ walking legs.” Their life is toler-
ably secure, but the frequent moultings during adolescence
are expensive and hazardous. When hatched the young are
like miniature adults ; for a time they cling beneath the tail
of the mother.
External appearance.—The head and thorax are covered
by a continuous (cephalothoracic) shield; the abdomen
shows obviously distinct segments movable upon one
another. As indicated by the appendages, there are three
groups of segments or metameres—five in the head, eight
in the thorax, six in the abdomen, as well as an unpaired
piece or telson on which the food canal ends. Each of the
mineteen segments bears a pair of appendages. Among
other external characters may be noticed the stalked
movable eyes, the two pairs of feelers, the mouth with six
pairs of appendages crowded round it, and the gills under
the side flaps of the thorax.
(1) The external shell or cuticle, composed of
various strata of chitin, coloured with pig-
ments, hardened with lime salts ;
‘The Bopy WALL } (2) The ectoderm, epidermis, or hypodermis,
consists of— which makes and remakes the cuticle ;
(3) An internal connective tissue layer or dermis,
with pigment, blood vessels, and nerves.
Internal to this lie the muscles.
Between the rings and at the joints the cuticle contains
no lime, and is therefore pliable. It is a layer not in itself
living or cellular, made by the underlying living skin. As it
cannot expand, it has to be moulted periodically as long as
the animal continues to grow. The old husk becomes
thinner, a new one is formed beneath it, a split occurs
across the back just behind the shield, the animal with-
‘draws its cephalothorax and then its abdomen, and an
empty but complete shell is left behind. The moulting is
preceded by an accumulation of glycogen in the tissues, and
this is probably utilised in the rapid growth which intervenes
‘between the casting of the old and the hardening of the
new shell.
How thorough the ecdysis or cuticle-casting is, may be appreciated
from the fact that the covering of the eyes, the hairs of the ears, the
lining of the fore-gut and hind-gut, the gastric mill, and the tendinous
CRAYFISH. 283
inward prolongations of the cuticle to which some of the muscles are
attached, are all got rid ofand renewed. The moults occur in the warm
months, eight times in the first year, five times in the second, thrice
in the third, after which the male moults twice, the female once a
year, till the uncertain limit of growth is reached. It is not clearly
known in what form the animals procure the carbonate of lime which is
deposited in the chitinous cuticle, but Irvine’s experiments have shown
that a carbonate of lime shell could be formed by crabs even when the
slight quantity of carbonate of lime in sea-water was replaced by the
chloride. Moulting is an expensive and exhausting process, and great
mortality is associated with the process itself or with the defenceless
state which follows. It is the necessary tax attendant on the
advantage of armature. Inequalities in the legs are usually due to
losses sustained in combat, but these are gradually repaired by new
growth.
The surface of the body bears setze or bristles of various
kinds. These have their roots in the epidermis, and are
made anew at each moult. There are simple glands
beneath the gill-flaps, and on the abdomen of the female
there are cement glands, the viscid secretion of which
serves to attach the eggs.
Appendages.—The limbs of a Crustacean usually exhibit
considerable diversity ; in different regions of the body they
are adapted for different work; yet all have the same
typical structure, and begin to develop in the same way.
In other words, they are serially homologous organs, illus-
trating division of labour. Typically each consists of a
basal piece or pvofopodite, and two jointed branches rising
from this—an internal exdopodite and an external exopodite ;
but in many the outer branch disappears.
The protopodite has usually two joints—a basal or proximal coxo-
podite, and a distal basipodite; the five joints which the endopodite
frequently exhibits are named from below upwards—ischio-, mero-,
carpo-, pro-, dactylo-podites—details of some use in the comparison
and identification of species.
The stalked eyes are not included in the above list, since their develop-
ment is not like that of the other appendages; but cases where an
excised eye has been replaced by an antenniform structure suggest that
the eye-sta/k may be of the nature of an appendage.
With many of the thoracic appendages, gills, plate-like epipodites,
and sete are associated.
‘ It is interesting to connect the structure of the appendages with their
functions. Thus it may be seen that the great paddles are fully spread
when the crayfish drives itself backwards with a stroke of its tail, while
in straightening again the paddles are drawn inwards, and the outer
joint of the exopodite bends in such a way that the friction is reduced.
284
PHYLUM. ARTHROPODA.
THE APPENDAGES OF THE CRAYFISH
Head
(s)
Thorax
(8).
Abdomen
(6).
No.
™
NAME.
FuncTION,
STRUCTURE.
Antennules (pre-
oral ?).
Antenne
oral ?).
Mandibles.
(pre-
ust Maxillee.
and Maxille.
Tactile, olfactory,
with ear-sac at
base.
Tactile, opening of
kidney at base.
Masticatory.
i
Produces respira-
tory current.
Two branches, but probably
not homologous with endo-
podite and exopodite.
Small exopodite.
Four joints, of which three
form the palp (endopodite
and upper joint of proto-
podite).
Thin single-jointed protopo-
dite, small endopodite, no
exopodite.
Thin protopodite, filamen-
tous endopodite; the
“baler” is formed from
the epipodite, probably
along with the exopodite.
ist Maxillipedes
(foot-jaws).
Walking Legs
(chelate).
2nd Maxillipedes.
3rd Maxillipedes.
Forceps (chelate).
Masticatory.
Fighting, seizing.
Walking.
Genital opening in
female.
Genital opening in
male,
Thin protopodite, small en-
dopodite, large exopodite.
Two- jointed protopodite,
five - jointed endopodite,
long exopodite.
Two - jointed protopodite, |
large five-jointed eidopo-
dite with strong teeth on
its ischiopodite, slender
exopodite.
No exopodite. In the claw
the last joint bites against
a prolongation of the
second last.
Without chela.
”
Modified swim-
merets in male;
in female, rudi-
mentary,
Modified swim-
merets in male,
Swimmerets.
”
.
Great paddles.
normal in female.
Serve in the male
as canals for the
seminal fluid.
oars, and carry
the eggs in the
female.
Important in swim-
ming.
ee slightly like
[Protopodite and endopodite
form acanal; no exopodite.
All the three parts,
Fic. 142.— Appendages of Norway lobster.
£x., Exopodite : Ex., endopodite ; protopodite dark throughout ; #/., epipodite.
1. Antennule—Z., position of ear; 2. antenna—X.., opening of kidney; 3. mand-
ible—P., palp; 4. first maxilla; 5. second maxilla—B., baler ; 6. first maxilli-
pede ; 7. second maxillipede ; 8. third maxillipede—the basal joint of the proto-
poditeis called coxopodite, the next basipodite ; the five joints of the endopodite
are called—ischiopodite (z.) ; meropodite (7z.) ; carpopodite (¢.) ; propodite (z.) ;
dactylorodite (d.) ; 9. forceps—(7) coxopodite ; (6) basipodite, the joints of the
endopodite are numbered ; 10-13. walking legs; 14. modified male appendage;
15-18. small swimmerets ; 19. large paddles.
286 PHYLUM ARTHROPODA.
{t is likely that some of the crowded mouth-parts, e.g. the first
maxillz, are almost functionless. The hard toothed knob which forms
the greater part of the mandible is obviously weil adapted to its crush-
ing work.
In connection with the skeleton, the student should also notice the
beak (rostrum) projecting between the eyes; the triangular area
(éfzs¢oma) in front of the mouth, and the slight upper and lower lips ;
and the lateral flaps of the body wall which project the gills. Each
posterior segment consists of a dorsal arch (¢ergzmz), side flaps (pleura),
a ventral bar (sternum), while the little piece between the A/euron and
the socket of the limb is dignified by the name of efzmeron. The
hindmost piece (e/son), on which the food canal ends ventrally, is
regarded by some as a distinct segment. The most difficult fact to
understand clearly, is that the cuticle of certain appendages (e.g.
the mandibles), and of the ventral region of the thorax, is folded inwards,
forming chitinous ‘‘ tendons” or insertions for muscles, and, above all,
constituting the complex, apparently, but not really, internal,
‘“‘endophragmal” skeleton of the thorax, protecting the ventral nerve-
cord and venous blood sinus.
Muscular system.—The muscles are white bundles of
fibres, which on minute examination show clearly that trans-
verse striping which is always well marked in rapidly con-
tracting elements. The muscles are inserted on the inner
surface of the cuticle, or on its internal foldings (apodemata).
The most important sets are—(1) the dorsal extensors or
straighteners of the tail; (2) the twisted ventral muscles,
most of which are flexors or benders of the tail, which have
harder work, and are much larger than their opponents ;
(3) those moving the appendages; (4) the bands which
work the gastric mill.
Nervous system.—The supra-cesophageal nerve-centres
or ganglia, forming the brain, have been shunted far forward
by the growth of the pre-oral region. We thus understand
how the nerve-ring round the gullet, connecting the brain
with the ventral chain of twelve paired ganglia, is so wide.
The dorsal or supra-cesophageal ganglia are three-lobed,
and give off nerves to eyes, antennules, antennz, and food
canal, besides the commissures to the sub-cesophageal
centres. They act as a true brain.
The sub-cesophageal ganglia, the first and largest of the
ventral dozen, innervate the six pairs of appendages about
the mouth. There are other five ganglia in the thorax, and
six more in the abdomen.
Though the ganglia of each pair are in contact, the
CRA VFISH. 287
ventral chain is double, and at one place, between the fourth
and fifth ganglia, an artery (sternal) passes between the two:
halves of the cord. From each pair of ganglia nerves are
given off to appendages and muscles, and apart from the
brain these minor centres are able to control the individual
movements of the limbs. In the thoracic region the cord is
well protected by the cuticular archway already referred to.
From the brain, and from the
commissure between it and the
sub-cesophageal ganglia, nerves
are given off to the food canal,
forming a complex visceral or
stomato-gastric system, Simi-
larly, from the last ganglia of the
ventral chain, nerves go to the
hind-gut. If the brain be regard-
ed as the fusion of two pairs of
ganglia, as the development sug-
gests, and the sub-cesophageal as
composed of six fused pairs, ther
these, along with the eleven other
pairs of the ventral chain, give
a total of nineteen nerve-centres,
—a pair for each pair of append-
ages, ‘
Sensory system.—A skin ‘
clothed with chitin is not Fic. 143.—Section of compound eye
likely to be in itself very of Adyses vulgards.—After Gren-
sensitive, but some of the cher. ae
sete are, and some Ob- “Jdllings in the course ‘of the® optic
servers describe a perl- nerve; 7.5. the nerve fibrils passing up
to the retinule; +/4., the rhabdoms;
pheral plexus of nerve-cells ve., elements of retinule; 4., band of
beneath the epidermis. ements ca crete ee sacle
The sete are not mere
outgrowths of the cuticle, but are continuous with the
living epidermis beneath ; and though some are only fringes,
both experiment and histological examination show that
others are ¢actdle.
On the under surface of the outer fork of the antennules
there are special innervated sete, which have a smelling
function.
Other specialised setee have sunk into a sac at the base
of the antennules, and are spoken of as auditory. The sac
288 PHYLUM ARTHROPODA,
opens by a bristle-guarded slit on the inner upper corner of
the expanded basal joint, and contains a gelatinous fluid
and small “ otoliths,” which appear to be foreign particles.
This “ear” seems to be an equilibrating organ, connected
with directing the animal’s movements. In some other
Crustaceans the auditory hairs are lodged in an open de-
pression; this has become an open sac in the crayfish, a
closed bag in the crab. Small sete on the upper lip of
the mouth have been said to have a tasting function.
The stalked eyes, which used to be regarded as append-
ages, arise in development from what are called “ procephalic
lobes” on the head. They are compound eyes—that is,
they consist of a multitude of elements, each of which is
structurally complete in itself. On the outside there is a
cuticular cornea, divided into square facets, one for each
of the optic elements; beneath this lie, as in other parts
of the body, the nucleated epidermal cells. Then follows a
focussing layer, consisting of many crystalline cones. Each
crystalline cone is composed of four crystalline cells, which
taper internally, and externally secrete a firm crystalline
body. The bases of the crystalline cones are surrounded
by the retinula cells. Each retinula consists of five
elongated cells arranged about a central axis. Distally,
this axis is formed by the crystalline cone, proximally by
a little rod or rhabdom. The rhibdom consists of four
little red rods closely apposed together, and connected by a
nerve-fibre with the optic ganglion, which lies at the end of
the optic nerve. The proximal ends of the retinal cells are
deeply pigmented. Thus each element consists of corneal
facet, crystalline cone, and retinula, and the retinula consists
of internal rhabdom and external retinula cells. Between
the individual optic elements lie some pigment cells. The
retinule image is erect, not inverted as in the eyes of
Vertebrates.
Alimentary system.—The food canal consists of three
distinct parts—a fore-gut or stomodaum developed by an
intucking from the anterior end of the embryo, a hind-gut
or proctodzeum similarly invaginated from the posterior end,
and a mid-gut or mesenteron, which represents the original
cavity of the gastrula.
The mouth has been shunted backwards from the anterior
CRAYFISH. 289
end of the body, so that the antennules and antennz lie far
in front of it. The fore-gut, which is lined by a chitinous
cuticle, includes a short “gullet,” on the walls of which there
are small glands, hypothetically called ‘‘salivary,” and a
capacious gizzard, which is distinctly divided into two
regions.
In the anterior (cardiac) region there is 4 complex mill; in the
posterior (pyloric) region there is a sieve of numerous hairs. The mill
Fic. 144.—Longitudinal section of lobster, showing some
of the organs.
#., Heart; AO., ophthalmic artery; a@a., antennary artery; ah.
hepatic artery ; S7Z., sternal artery; SA., superior abdominal
artery; J7G., mid-gut ; DG., digestive gland; AG., hind-gut ;
£x., extensor muscles of the tail; #2, flexor muscles of the tail:
IA., inferior abdominal artery; G., gizzard ; C., cerebral ganglia
P., pericardium ; 7., testes.
is very complex ; there are supporting ‘‘ossicles” on the walls with
external muscles attached 1o them, and internally projecting teeth which
clash together and grind the food. Three of the teeth are conspicuous ;
. a median dorsal tooth is brought into contact with two large laterals.
On each side of the anterior part of the gizzard there are two limy
discs or gastroliths, which are broken up before moulting, and though
quite inadequate to supply sufficient carbonate of lime for the new
skeleton, seem to have some relation to this process. The occurrence
of chitinous cuticle, setee, teeth, and gastroliths in the gizzard, is
intelligible when the origin of the fore-gut is remembered, and so is the
dismantled state of this region when moulting occurs.
The mid-gut is very short, but outgrowths from it form
19
290 PHYLUM ARTHROPODA.
the large and complex digestive gland. The mid-gut, here
as always, is the digestive and absorptive region, but both
processes are carried on to a large extent in the digestive
gland, which communicates with the mid-gut by two wide
ducts. It is roughly three-lobed at both sides, and consists
of an aggregated mass of czeca, closely compacted together.
The gland is more than a “liver,” more even than a
“hepatopancreas.” It absorbs peptones and sugar; like
the Vertebrate liver, it makes glycogen; its digestive
juices are comparable to those of the pancreas and the
stomach of higher animals. The hind-gut is long and
straight. It is lined by a chitinous cuticle, as its origin
suggests. There are a few minute glands on its walls.
Body cavity.—The space between the gut and the body
wall is for the most part filled up by the muscles and the
organs, but there are interspaces left which contain a fluid
with amceboid cells. These interspaces seem to represent
enlarged blood sinuses (a heemoccele), rather than a true body
cavity or coelom. One of the spaces forms the blood-con-
taining pericardium, or chamber in which the heart lies.
Vascular system.— Within this non-muscular pericardium,
and moored to it by thin muscular strands, lies the six-sided
heart, which receives pure blood from the gills (wa the
pericardium) and drives it to the body.
The arterial system is well developed. Anteriorly, the
heart gives off a median (ophthalmic) artery to the eyes and
antennules, a pair of (antennary) arteries to the antennz,
and a pair to the digestive gland (hepatic). Posteriorly
there issues a single vessel, which at once divides into a
superior abdominal, running along the dorsal surface, and
a sternal, which goes vertically through the body. This
sternal passes between the connectives joining the fourth and
fifth ventral ganglia, and then divides into an anterior and
posterior abdominal branch. All these arteries are con-
tinued into capillaries.
From the tissues the venous blood is gathered up in
channels, which are not sufficiently defined to be called veins.
It is collected in a ventral venous sinus, and passes into the
gills. Thence, purified by exposure on the water-washed
surfaces, it returns by six vessels on each side to the peri-
cardium. From this it enters the heart by six large and
CRAYFISH. 291
several smaller apertures, which admit of entrance but not
of exit.
The blood contains amceboid cells, and the fluid or
plasma includes a respiratory pigment, hemocyanin (bluish
when oxidised, colourless when deoxidised), and a lipochrome
pigment, called zoonerythrin. Both of these are common
in other Crustaceans.
Respiratory system.—Twenty gills—vascular outgrowths
of the body wall—lie on each side of the thorax, sheltered
by the flaps of the shield. A current of water from behind
forwards is kept up by the activity of the baling portion,
or scaphognathite, of the second maxilla. Venous blood
enters the gills from the ventral sinus, and purified blood
leaves them by the six channels leading to the pericardium.
Observed superficially, the gills look somewhat like
feathers with plump barbs, but their structure is much more
complex.. The most important fact is that they present a
large surface to the purifying water, while both the stem
and the filaments which spring from it contain an outer
canal continuous with the venous sinus, and an inner canal
communicating with the channels which lead back to the
pericardium and heart.
Three sets of gills are distinguishable. To the basal joints of the
six appendages, from the second maxillipede to the fourth large limb
inclusive, the podobranchs are attached. They come off with the
appendages when these are pulled carefully away, and each of them
- bears, in addition to the feathery portion, a simple lamina or epepodzie.
The membranes between the basal joints of the appendages and the
body, from the second maxillipede to the fourth large limb inclusive,
bear a second set, the avthrobranchs, which have no epipodites. In
connection with the second maxillipede there is a single arthrobranch ;
in connection with each of the five following appendages there are two ;
so that there are eleven arthrobranchs altogether. There remain three
pleurobranchs, one on the epimeron of the fifth large limb, and two
others quite rudimentary on the two preceding segments. ‘The bases
of the podobranchs bear long setze.
In Nephrops, the podobranchs are represented by a small rudiment
on the second maxillipede, and by five well-developed gills on the next
five appendages; there are eleven arthrobranchs, the most anterior
being small; and there are four large pleurobranchs.
Excretory system.—A kidney or “green gland” lies
behind the base of each antenna, and its opening is marked
by a conspicuous knob on the basal joint of that appendage.
292 PHYLUM ARTHROPODA.
Each kidney consists of a dorsal sac communicating: with
the exterior, and of a ventral coiled tube which forms the
proper renal organ. The latter is supplied with blood from
the antennary and abdominal arteries, and forms as waste
products uric acid and greenish guanin. Each kidney may
be regarded as homologous with a nephridium.
The crayfish has also, near
the gills, small branchial glands
which excrete carcinuric acid
from the blood, and also help
in phagocytosis, that important
process in which wandering
ameeboid cells resist infection
and help to repair injuries (cf.
possible function of thymus in
Fishes). In not a few inverte-
brates there are scattered groups
of excretory cells or nephrocytes,
and it seems that the endothelial
cells of the lymphatic vessels
and renal capillaries in tadpoles
have a similar function.
Reproductive organs.—
The male crayfish is distin-
guished from the female by
his slightly slimmer build,
of crayfish.—After Huxley. and by the peculiar modi-
4, Testes; vd., vas deferens; va’., open- fication of the first two pairs
ing of vas deferens on last walking leg. of abdominal appendages.
In both sexes the gonads
are three-lobed, and communicate with the exterior by
paired ducts.
The testes consist of two anterior lobes lying beneath
and in front of the heart, and of a median lobe extending
backwards. Each lobe consists of many tubules, within
which the spermatozoa develop. From the junction of
each of the anterior lobes with the median lobe, a genital
duct or vas deferens is given off. This has a long coiled
course, is in part glandular, and ends in a short muscular
portion opening on the last thoracic limb. The spermatozoa
are at first disc-like cells; they give off on all sides long
pointed processes like those of a Heliozoon, and remain
very sluggish. The seminal fluid is milky in appearance,
Fic. 145.—Male reproductive organs
CRAYFISH. 293
and becomes thicker in its passage through the genital
ducts. It is possible that the genital ducts represent
modified nephridia, and that the cavities of the gonads
are coelomic.
The ovaries are like the testes, but more compact. The
eggs are liberated into the cavity of the organ, and pass out
by short thick oviducts opening on the second pair of
walking legs. As they are laid they seem to be coated with
the secretion of the cement glands of the abdomen, and the
mother keeps her tail bent till the eggs are glued to the
small swimmerets.
Fic. 146.—Female reproductive organs of crayfish, —
After Suckow.
ov., Ovaries ; ov’., fused posterior part ; od., oviduct ; vz., female
aperture on the second walking leg.
Before this, however, sexual union has occurred. The
male seizes the female with his great claws, throws her on
her back, and deposits the seminal fluid on the ventral
surface of the abdomen. The fluid flows down the canal
formed by his first abdominal appendages, and these seem
to be kept clear by the movements of the next pair, which
are also modified. On the abdomen of the female the
agglutinated spermatozoa doubtless remain until the eggs
are laid, when fertilisation. in the strict sense is achieved.
The Development has been very fully worked out, and is of interest
in being direct, without the metamorphosis so common among the
294 PHYLUM ARTHROPODA.
Arthropoda. . The spherical ovum is surrounded by a cuticular vitelline
membrane, and contains a considerable quantity of yolk. After ferti-
lisation the segmentation nucleus divides in the usual way into two,
four, eight, and so on, but this nuclear division is not followed by
division of the plasma. Eventually the nuclei, each surrounded by a
small amount of protoplasm, approach the surface of the egg and
arrange themselves regularly round it. The peripheral protoplasm then
segments round these nuclei, and thus we have a central core of un-
segmented yolk enveloped by a peripheral sphere of rapidly dividing
Fic. 147.—Section through the egg of Aséacus after the com-
pletion of segmentation.—After Reichenbach.
st., Stalk of the egg ; c/., chorion envelope ; /., peripheral blastoderm
within which are the yolk pyramids (dark).
cells. In the central yolk, free nuclei are frequently found ; these are
the so-called yolk nuclei. Such a type of segmentation is called peri-
pheral or centrolecithal, and is very characteristic of Arthropod eggs.
Overa particular region of the segmented egg, known as the ‘‘ ventral
plate,” the cells begin to thicken ; at this region an invagination occurs,
which represents the gastrula. At the anterior lip of the blastopore the
mesoderm appears, being many-celled from the first. Soon the blasto-
pore closes ; the cavity of the gastrula thus becomes a closed sac—the
future mid-gut. The cells of this archenteron take up the core of yolk
CRAYFISH. 295
into themselves in 4 way which early suggests their future digestive
function. On the surface of the. egg there have already appeared
ectodermic thickenings,—the so-called eye-folds,—rudiments of the
appendages, and of the thoracic and abdominal regions.
In the later stages invaginations of the ectoderm form the fore- and
hind-gut, which grow inward from opposite ends to meet the endo-
dermic mid-gut. The ear-sac and the greater part of the gills have
-also an ectodermic origin. From the mid-gut the digestive gland is
budded out. The heart, the blood vessels, blood, and muscles are due
to the mesoderm.
Fic. 148.—Longitudinal section of later embryo of
Astacus.—After Reichenbach.
£c., Ectoderm ; #z., mesoderm cells; ¢.g., cerebral ganglia; s¢.,
stomodeum; A., anus; 7., telson; g., ventral ganglia; s.s.,
sternal sinus ; Ad., proctodeum; %., heart ; #zg., mid-gut ; yolk
pyramids dark. ‘
As usual, the nervous system arises from an ectodermic thickening.
The eyé arises partly from the optic ganglia of the ‘‘ brain,” partly from
the ‘‘ eye-folds,” and partly from the epidermis.
When the young crayfishes are hatched from the egg-shells, they still
cling to these, and thereby to the swimmerets of the mother. In most
respects they are like the adults, but the cephalothorax is convex and
relatively large, the rostrum is bent down between the eyes, the tips of
the.claws are incurved and serve for firm attachment, and there are
other slight differences. The noteworthy fact is that the development
is completed within the egg-case, and that it is continuous without
metamorphosis. The shortened life history of the crayfish is interesting
296 PHYLUM ARTHROPODA.
in relation to its fresh-water habitat, where the risks of being swept
away by currents are obviously great; but it must also be remem-
bered that the tendency to abbreviate development is a general one.
There is some maternal care in the crayfish, for the young are said
sometimes to return to the mother after a short exploration on their
own account.
THE CRAB
It is instructive to contrast the crab-type with that of the crayfish or
lobster. The cephalothorax is broadened by a hollow extension of the
gill-covering (branchiostegite) region. The abdomen is greatly re-
duced, with a soft sternal region, and is bent permanently upwards
and forwards in a groove in the thoracic sterna. In the male there are
only two pairs of abdominal limbs, which have a reproductive function ;
in the female there are four pairs, which carry the eggs.
Fic. 149.—Section through cephalothorax of a crab, —
After Pearson.
#7., Heart; Te., extension of the tergum ; raeea sternum ; PL., pleuron;
T., tendons; rst W.L., insertion of first walking leg; Bv., gill in gill-
chamber; g., gut; d.a., descending artery; A., afferent branchial ;
£., efferent branchial.
The eye-stalks lie in sockets of the carapace; the bases of the
reflexed antennules are also in sockets; the antennze are short and
straight.
The third maxillipedes are broad and flat and form a kind of oper-
culum over the five preceding pairs of appendages. The great claws
are relatively very large, the other thoracic legs are non-chelate, and
in the swimming crabs, ¢.g. Portunus (see Fig. 150), the fifth pair of
thoracic legs have their last joint adapted as a paddle.
As to the soft parts, there is a noteworthy change in the nervous
system. From the cerebral ganglia a pair of cesophageal commissures
extend to a large ganglionated mass sheltered by the endosternal
skeleton. It is composed of numerous pairs of ganglia fused together,
and gives off nerves to maxillze, maxillipedes, and thoracic limbs. It
is perforated by the sternal artery. The cesophageal commissures are
united by a transverse commissure just behind the gullet, and in front
of this cross junction there are two small ganglia giving off nerves to
the mandibles. On the lower surface of the anterior part of the
gizzard there are two small gastric ganglia innervated from the cerebrals.
7.N
\
h
4)
4
M7 |
Y
bs AN
Fic. 150.—Dorsal aspect of swimming crab (Portunzs).
&., Paddle; Ada., abdomen; A1.,antennules ; 42., antennz ; Z., eyes ; /., forceps
Fic. 151.—Dorsal aspect of shore crab (Carcznus).
Abd., Abdomen; A}, antennules; A%., antenne; Z., eyes; ., forceps.
298 PHYLUM ARTHROPODA.
When the-branchial chamber is opened the large pyramidal gills are
seen, also the long sword-shaped epipodite (flabellum) of the first
maxillipede which seems to help the “baler,” the smaller and mobile
epipodites borne by the second and third maxillipedes, and the broad
Fic. 152.— Ventral aspect of female shore crab.
Aéd., Abdomen ; #x#., third maxillipede.
scaphognathite of the second maxilla which bales the water forwards
and outwards,
It must be clearly understood that the branchial chamber is entirely
outside of the body, being formed by the lateral extension of a hollow
reduplicature from the tergal region.
The large gizzard, the enormous greyish-yellow hepatopancreas, the
transparent pericardium, and other organs are readily seen.
SystEmaTIC SURVEY OF THE Ciass CRUSTACEA -
(1) Entomostraca, lower forms.
They are usually small and simple.
The number of segments and ap-
pendages is very diverse.
The larva is generally hatched as a
simple unsegmented Mauplius.
There is no gastric mill.
The excretory organ is associated
with the second maxillz.
(2) Malacostraca, higher forms.
They are usually larger and more
complex.
The head consists of 5, the thorax.
of 8, the abdomen of 6 (7 in
Leptostraca) segments.
The larva is usually higher than a
Naupiius.
There is often a gastric mill.
The excretory organ is usually
associated with the antennz, but
maxillary glands may be present
in the larvee, and may even per-
sist in adults.
ENTOMOSTRACA.
First Sub-Class.
ENTOMOSTRACA
299
Order 1. Phyllopoda,—In these at least four pairs of leaf-like swimming
feet bear respiratory plates.
and is protected by a shield-like or bivalve shell.
are without palps, and the maxille are rudimentary.
(a) Branchiopoda.
or more) foliaceous append-
ages with respiratory plates.
The shell is rarely absent,
usually shield-like or bi-
valved. The heart is a long
dorsal vessel with numerous
openings. The eggsare able
to survive prolonged desicca-
tion in the mud.
Branchipus, a beautifully
coloured _ fresh - water
form, with hardly any
shell.
Artemia. Brine - shrimps.
Periodically partheno-
genetic. By gradually
changing the salinity
of the water, Schmanke-
witsch was able, in the
course of several gen-.
erations, to modify 4.
salina into A. mil-
hauseniz, and vice versa.
Artemia fertilts is one
of the four animals
known to occur in the
dense waters of Salt
Lake.
Apus, an archaic fresh-
water form with a large
dorsal shield.
The body is generally well segmented,
The mandibles
The body has numerous segments and (10-20.
Fic. 153.—Dorsal surface of Apus
cancriformts. —From Bronn’s.
Thierretch.
In the anterior region are the two com-
pound eyes, and behind them the
simple unpaired eye. The whip-like
outgrowths of the first thoracic ap-
pendage project laterally.
Afus is over an inch in length, a giant among Entomostraca. It has
an almost world-wide distribution.
numerous and mostly leaf-like.
The appendages are very
They may be regarded as.
representing a primitive type of Crustacean limb. Professor
Ray Lankester enumerates them as follows :—
I. Antenna,
Pre-oral. 2. Second antenna.
Oral.
4. Maxilla.
3. Mandible.
5. Maxillipede.
(This is sometimes absent, and
apparently always in certain species, )
300 PHYLUM ARTHROPODA.
6. First thoracic foot (leg-like).
7-16. Other ten thoracic feet (swimmers).
The 16th in the female carries an egg-sac or brood-
chamber. There are eleven thoracic rings on the body.
Abdominal 17-68. Fifty-two abdominal feet, to which there corre-
(Post-genital). | spond only seventeen rings on the body.
The large dorsal shield is not attached to the segments behind the
one bearing the maxillipedes. Many of the thin limbs doubtless
function as gills. The genital apertures are on the sixteenth
appendages. The anus is on the last segment of the body.
Thoracic
(Pregenital).
Fic. 154.—Daphnia,
£., Eye; A.®, second antenna; A.1, first antenna; de., digestive cxca; SLs;
shell gland; go., gonad; 4., heart in. pericardium ; o., ovum; L.p.,
brood-pouch; sf., spine; /, furca; s., setae; Aé., rudimentary abdomen ;
z., caudal fork; g., gut; 745, thoracic limbs.
There is a pair of ventral ganglia to each pair of limbs; the ventral
nerve-cords are widely apart; and the cephalic ganglion is
remarkably isolated. There is periodic parthenogenesis.
ENTOMOSTRACA. 30r
(6) Cladocera. Small laterally compressed ‘‘ water-fleas,” with few
and somewhat indistinct segments. The shell is usually bivalved,
and the head often projects freely from it. The second antennie
are large, two-branched, swimming appendages, and there are
4-6 pairs of other swimming organs. The heart is a little sac
with one pair of openings. An excretory organ (the shell or
maxillary gland) opens in the region of the second maxillz. It
is the Entomostracan equivalent of the antennary green gland
of Malacostraca. The males are usually smaller and much rarer
than the females. The latter have a brood-chamber between
the shell and the back. Within this many broods are hatched
throughout the summer. Periodic parthenogenesis (of the
“summer ova”) is very common. ‘‘ Winter eggs,” which
require fertilisation, are set adrift in a part of the shell modified
to form a protective cradle or ephippium.
Daphnia, Moina, Sida, Polyphemus, Leptodora, and many
other ‘‘ water-fleas,” are extraordinarily abundant in fresh
water, and form part of the food of many fishes. A few
occur in brackish and salt water.
In Daghnia the appendages are:—antennules, antennz,
mandibles, first maxille, second maxille (disappearing in the
larva), and five thoracic limbs. The abdomen is turned down-
wards and forwards, and shows three segments and a telson.
Order 2. Ostracoda.—Small Crustaceans, usually laterally compressed,
with an indistinctly segmented or unsegmented body, rudimentary
abdomen. and bivalve shell, There are only seven pairs of
~qules, antennz, mandibles, first maxillze,
é two pairs of thoracic limbs. Parthenogenesis-
is often prolonged.
Examples.—Cygris (fresh water), Cyprédina (marine).
Fic. 155.—Cypris.
M., Marks of adductor muscle; £., eye seen through the shell (S/.);
A.J, first antennz ; A.2, second antenne ; F., thoraciclegs.
302 PHYLUM ARTHROPODA.
Fic. 156.—Cypris, side view, after removal of one valve.—
After Zenker. :
z., Eye; A.J, first antennz ; 4.2, second antenne; AZ.V., mandibles ;
mx.1, first maxilla ; 72.2, second maxilla; /.7, £2, thoracic legs 5
Aé., rudimentary abdomen,
Fic. 157.—Cyclops type.
JA,, first antenna; // A., second antenna; OV., ovary; R.S.,
receptaculum seminis; OS., ovisac; /., caudal fork.
ENTOMOSTRACA. 303
Order 3. Copepoda.—Elongated Crustaceans, usually with distinct seg-
ments. There is no dorsal shell. There are five pairs of biramose
thoracic appendages, but the last may be rudimentary or absent.
The abdomen is without limbs, and of its five segments the first
two are sometimes united. The females carry the eggs in external
ovisacs. Most Copepods move very actively in the water, jerking
themselves rapidly by means of their thoracic legs, or swim more
gently by means of their second antennz. Many are ecto-parasitic,
especially on fishes (‘‘fish-lice”), and are often very degenerate.
The free-living Copepods form an important part of the food-
supply of fishes.
Cyclops, free and exceedingly prolific in fresh water. Its
appendages are:—antennules, antennz, mandibles, first
maxille, second maxille, four pairs of flattened biramous
thoracic legs united across the middle with those of the
opposite side, another rudimentary pair, and probably the
genital valve. Cetochzlus, Calanus, free and abundant in
the sea. In Chondracanthus, as in many other cases, the
parasitic females carry the pigmy males attached to their
body. Caligus, a very common genus of “‘fish-lice.’”’” In
the carp-lice (Arvgu/us) the mouth is a sucker with sharp
stilets and the second maxillze form adhesive discs.
Lernea, Penella, etc. The adult females are parasitic, and
almost worm-like. The males and the young are free.
Order 4. Cirripedia.—Barnacles and acorn-shells, and some allied
degenerate parasites.
Marine Crustaceans, which in adult life are fixed head down-
wards. The body is indistinctly segmented, and is enveloped
in a fold of skin, usually with calcareous plates. The anterior
antennz are involved in the attachment; the posterior pair
are rudimentary. The oral appendages are small, and in part
atrophied. In most there are six (or less frequently four)
pairs of two-branched thoracic feet, which sweep food particles
into the depressed mouth. The abdomen is rudimentary.
There is no heart. The sexes are usually combined, but
dimorphic unisexual forms alsos occur. The hermaphrodite
individuals occasionally carry pigmy or ‘‘complemental”
males. The spermatozoa are mobile, which is unusual
among Crustacea.
Lepas, the ship-barnacle, is as an adult attached to floating logs and
ship-bottoms. The anterior end by which the animal fixes itself is
drawn out into a long flexible stalk, containing a cement gland, the
ovaries, etc., and involving in its formation the first pair of antennee and:
the front lobe of the head. The second antennz are lost in larval life.
The mouth region bears a pair of small mandibles and two pairs of
small maxillz,—the last pair united into a lower lip. The thorax has
six pairs of two-branched appendages, and from the end of the rudi-
mentary abdomen a long penis projects. At the base of this lies the
anus. Around the body there is a fold of skin, and from this arise five
calcareous’ plates, an unpaired dorsal carina, two scuta right and left
304 PHYLUM ARTHROPODA.
anteriorly, two ¢evga at the free posterior end. The nervous system
consists of a brain, an oesophageal ring, and a ventral chain of five or
more ganglia. There is a fused pair of rudimentary eyes. No special
circulatory or respiratory organs are known. Two excretory (?) tubes
lead from (coelomic) cavities to the base of the second maxillze, and are
probably comparable with shell glands and with nephridia. There is a
complete food canal and a large digestive gland. Beside the latter lie
the branched testes, whose vasa deferentia unite in an ejaculatory duct
in the penis. From the much-branched ovaries in the stalk, the ovi-
ducts pass to the first thoracic legs, where they open into a cement-
making sac, opening to the exterior. The eggs are found in flat cakes
between the external fold of skin and the body.
Fic. 158.—Acorn-shell (Bdlanus tintinnabulum).
—After Darwin.
T., tergum ; CR., thoracic legs; F., outer shell in section ; D., aper-
ture of oviduct; #., mantle cavity; ¥., depressor muscle of
tergum; AX., antenne; OV., ovary; G., depressor of scutum;
#., oviduct ; AM., adductor muscle of scuta; S., scutum.
The life history. Nauplius larve escape from the egg-cases, and,
after moulting several times, become like little Cyprids. The first
pair of appendages become suctorial, and, after a period of free-
swimming, the young barnacle settles down on some floating object,
mooring itself by means of the antennary suckers, and becoming firmly
glued by the secretion of the cement glands. During the settling and
the associated metamorphosis, the young barnacle fasts, living on a
store of fat previously accumulated. Many important changes occur,
the valved shell is developed, and the adult form is gradually assumed.
The food consists of small animals, which are swept to the mouth by
the waving of the curled legs. Growth is somewhat rapid, but the
usual ecdysis is much restricted, except in one genus. Neither the
valves, nor the uniting membranes, nor the envelope of the stalk, are
‘marks. It may be ‘described,
ENTOMOSTRACA. 305
moulted, though disintegrated portions may be removed in flakes and
renewed by fresh formations.
In the allied genus Scalpellum,
some are like Lepas, hermaph-
rodites, without complementary
males (Sc. dalanozdes); others
are hermaphrodite, with comple-
mentary males (Sc. v2llosum) ;
and others are unisexual, but
the males are minute and para-
sitic (Sc. regezm).
Balanus, the acorn-shell, en-
crusts the rocks in great numbers
between high and low water
in Huxley’s graphic words, as a
crustacean fixed by its head,
and kicking the food into its
mouth with its legs. The body
is surrounded, as in Leas, by
a fold of skin, which forms a
rampart of six or more cal-
careous plates, and a fourfold
lid, consisting of two scu¢a and
two ¢erga. When covered by
the tide, the animal protrudes
and retracts between the valves
of the shell six pairs of curl-like
thoracic legs. The structure
of the acorn-shell is in the main
like that ‘of the barnacle, but
there is no stalk.
The life history also is similar.
A Nauplius is hatched. It has
the usual three pairs of legs, an
unpaired eye, and a delicate
dorsal shield. It moults several
times, grows larger, and ac-
quires a firmer shield, a longer
spined tail, and stronger legs.
Then it passes into a Cyprds
stage, with two side eyes, six
pairs of swimming legs, a bi-
valve shell, and other organs.
Fic. 159.—Development of Saccudina.
As it exerts itself much but does ae Delage. (Not drawn to
not feed, it is notunnatural that °° €.)
it sHould sink down as if in 4., Sereswimnting Tapplius, with tees
: . . pairs of appendages ; B., pupa stage; C.,
ek d re ees gout protruding from the abdomen of a
? crab.
by the secretion of the cement
gland. Some of the structures, ¢.g, the bivalve-shell, are lost; new
20
306 PHYLUM ARTHROPODA.
structures appear, e.g. the characteristic Cirriped legs and the shell.
Throughout this period, which Darwin called the ‘‘ pupa stage,” there is
external quiescence, and the young creature continues to fast. The skin
of the pupa moults off; the adult structures and habits are gradually
assumed, At frequent periods of continued growth the lining of the shell
and the cuticle of the legs are shed. In spring these glassy cast coats
are exceedingly common in the sea. Acorn-shells feed on small marine
animals. They fix themselves not to rocks only, but also to shells,
floating objects, and even to whales and other animals.
On the ventral surface of the abdomen of crabs, Sacczlina, one of the
most degenerate of all parasites, is often found. Its history has
been beautifully worked out by Professor Delage. It is in shape an
ovoid sac, and is attached about the middle of a segment. On the
lower surface of the sac there is a cloacal aperture, opening into a large.
brood-chamber, usually distended with eggs contained in chitinous
tubes. The brood-chamber surrounds the central ‘‘ visceral mass,”’
consisting of a nerve ganglion, a cement gland which secretes the egg-
cases, and the hermaphrodite reproductive organs; of digestive or
vascular systems there is no trace. The parasite is attached by a
peduncle, dividing up into numerous ‘‘ roots,” which ramify within the
body of the crab, and by them the Saccz/7a obtains nutrition and
gets rid of its waste products; it is practically an exdopfaraszte. The
larvee leave the brood-chamber as Nauplii; they moult rapidly and
become Cyprid larvae. These fix themselves by their antennz to
young crabs, at the uncalcified membrane round the base of large
bristles. The thorax and abdomen are cast off; the structures within
the head region contract; eyes, tendons, pigment, the remaining yolk
and the carapace, are lost; a little sac remains, which passes into the
interior of the crab. It reaches the abdomen, and, as it approaches
maturity, the integuments of the crab are dissolved beneath it, and
the sac-like body protrudes. It appears to live for three years, during
which time the growth of its host is arrested, and no moult occurs,
Forms allied to Sacculina are grouped together as Rhizocephala.
One of them—Sesarmaxenos—occurs on a fresh-water crab, Sesarma,
in the Andamans; all the rest are marine.
Second Sub-Class. ManacosTRaca
Series I. Leptostraca, Division Phyllocarida.
Marine Crustaceans of great systematic interest, retaining in many
ways the simplicity of ancestral forms, and linking Malacostraca and
Entomostraca. The most important genus is Vebal/za.
A bivalve shell covers the whole of the lank body, except the last
four abdominal segments ; the head is free from the thorax ; the eight
segments of the thorax are free from one another, and the plate-like
appendages resemble those of Phyllopods; the abdomen has seven
segments and a telson with two forks ; the elongated heart extends into
the abdomen, and has seven pairs of lateral apertures or ostia. There
are both antennary and maxillary excretory organs. Veda/éa and its
MALACOSTRACA. 307
congeners are probably related to certain ancient fossil forms from
Paleozoic strata, e.g. Hymenocaris from the Cambrian.
Fic, 160.—Nebalia.—Alter Sars.
SH., Shell; 4.2, first antenne; A.2, second antenne; 7H., 8 thoracic
limbs; 4é.4, 44.6, fourth and sixth abdominal limbs.
Fic. 161.—Anaspedes.—After Calman.
A.s, A.2, antenne; £x., rudimentary exopodite ; G., respiratory lamina
PR.7, PR.S, seventh and eighth thoracic limbs or pereiopods; PL.s,
2, 6, first, second, and sixth abdominal limbs or pleopods.
308 PHYLUM ARTHROPODA.
Series IJ. Eumalacostraca.
Division 1. Syncarida, the order Anaspidacea, primitive fresh-water
forms, without a carapace; with the eight thoracic segments all
distinct (Azzasfzdes), or with the first one fixed to the head (Koonunga) ;
with stalked eyes in Anaspides, sessile eyes in Koonunga; with
lamellar branchize on the thoracic legs, whose slender exopodites are
also respiratory.
Division 2. Peracarida, with a carapace that leaves at least four of
the thoracic segments free, with the first thoracic segment always fused
to the head, with usually sessile eyes, with a brood-pouch on the
thoracic appendages of the female, with an elongated heart, with
direct development. Numerous orders including :—the pelagic Mysi-
dacea (formerly united with Euphausiacea
as Schizopods), e.g. dZysis; the pelagic
and deep-water Cumacea, e.g. Cuma and
Diastylzs ; the Isopods, with dorso-ventral
flattening of the body, a posterior heart,
and respiratory organs on the abdominal
limbs, ¢.g. the terrestrial wood-lice (Por-
cellio, Oniscus, etc.), which show minute
trachea-like respiratory tubes in the abdom-
inal limbs, and corresponding forms on the
shore (e.g. Ligza, Jdotea) ; the Amphipods,
with lateral flattening of the body, an
anterior heart, and respiratory organs
usually on the thoracic limbs, e.g. Gam-
marus locusta in the shore pools, G. pulex
in fresh water, and sandhoppers like
Talitrus and Orchestia; the ‘‘no body”
crabs, Caprella; Phronima, living inside
the glassy case of the free-swimming
Tunicate Pyrosoma.
: : Division 3. Hoplocarida, with a carapace
Fic. 162.—An Amphipod that leaves et feast four of the thoracie
(Caprella linearis), segments free, with stalked eyes, with the
The two anterior thoracic seg- eggs carried in a chamber formed by the
Be dan we cee maxillipedes, with an elongated heart,
duced and without append- 2nd with a complicated metamorphosis.
ages; the fourth and fifth Order :—Stomatopods, e.g. Sguzl/a, with
thoracic segments bear only the second maxillipedes forming very large
respiratory plates. raptorial organs.
Division 4. Eucarida, with a cephalo-thoracic shield uniting the
head and thorax segments; with stalked eyes; with a saccular heart ;
with eggs attached to the abdominal endopodites; with spherical
spermatozoa showing peculiar radiating pseudopodia; usually with a
complex metamorphosis.
Order 1. Euphausiacea :—shrimp-like surface and deep-water forms,
with biramous thoracic limbs as in Mysids, e.g. Huphausza.
Order 2. Decapoda:—with the three anterior thoracic limbs
turned forward as maxillipedes, with the other thoracic limbs almost
always uniramous, ;
MALACOSTRACA., 309
Sub-order Macrura.—Abdomen long. Homarus (lobster) ; Meph-
vops (Norway lobster, sea crayfish); As¢acus (fresh-water crayfish) ;
Palinurus (rock lobster), whose larva was long known as the glass-
crab (Phyllosoma) ; Peneus, a shrimp which passes through Nauplius, .
Zoza, and Mysis stages ; Lucéfer and Sergestes are also hatched at a
stage antecedent to the Zoza 3; Crangon vulgaris (the British shrimp) ;
Palemon, Pandalus, Hippolyte (prawns); Galathea (with the abdomen
Fic. 163.—Hermit-crab withdrawn from its shell.
The anterior appendages are broken off.
hd., Head; ¢., thorax ; aéd., abdomen.
bent forwards) ; Pagurus, Eupagurus (hermit-crabs) ; Bzrgus latro (the
terrestrial robber or palm-crab), in which the upper part of the gill-
cavity is shut off to form a “‘lung,”.the walls having numerous
vascular plaits. ;
Sub-order Brachyura. — Abdomen short, and bent under the
thorax. It is narrow in the male, and does not usually bear more
than two pairs of appendages; it is broader in the female, and
bears four paired appendages. The ventral ganglia have fused
310 PHYLUM ARTHROPODA.
into. an oval mass. Cancer (edible crab); Carcénus manas (shore-
crab); Portunus (swimming-crab); Dromia (often covered by a
sponge).; Pinnotheres (living inside bivalves); Ze/phusa (a fresh-
water crab); Gecarcinus (land-crabs, only visiting the sea at the
breeding season).
History.—Fossil Crustaceans are found in Cambrian strata, but the
highest forms (Decapoda) were not firmly established till the Tertiary
period. Some of the genera, e.g. the Branchiopod Zs¢herda, living from
Devonian ages till now, are remarkably persistent and successful. How
the class arose we do not know ; itis probable that types like Anaspzdes
and Nebalza give us trustworthy hints as to the ancestors of the higher
Crustaceans ; it is likely that the Phyllopods, e.g. Afus, bear a similar
relation to the whole series ; the Copepods also retain some primitive
ELK:
Niet
Fic. 164.—Mysis flexuosa, from side.
&., Brood-pouch borne on posterior thoracic limbs ; 0., otocyst
in tail. Note eight pairs of similar biramose thoracic feet.
a last two thoracic segments are not covered by the
shield,
characteristics; but it is difficult to say anything definite as to the
more remote ancestry.
We naturally think of a segmented worm-type as a plausible starting-
point for Crustaceans, and it is not difficult to imagine how a develop-
ment of cuticular chitin would tend to produce a flexibly jointed limb
out of an unjointed parapodium ; how the mouth might be shunted a
little backwards, and two appendages and ganglia a little forwards ;
and how division of labour would result in the differentiation of
distinct regions,
GENERAL NOTES ON CRUSTACEANS
Of a class that includes animals so diverse as crabs,
lobsters, shrimps, ‘ beach-fleas,” ‘“ wood-lice,” barnacles,
acorn-shells, and “‘ water-fleas,” it is difficult to state general
GENERAL NOTES ON CRUSTACEANS. 3Ir
characteristics, other than those facts of structure which
we have already summarised.
Admitting the parasitism of many Crustaceans, and the
sedentary life of barnacles and acorn-shells, we must still
allow that great activity characterises the class. With this
may be connected the brilliant colouring, the power of
colour change, and the phosphorescence of many forms.
Except in the case of
a few primitive and
degenerate forms, the
Crustacea are all seg-
mented. In this, in
the presence of hollow
jointed appendages, in
the reduction -of the
coelom, and in their firm
chitinous cuticle, the
Crustacea resemble other
Arthropeds ; as special
characteristics we notice
the two pairs of antennz,
the presence of Carbdon-
ate of lime in the cuticle,
and the nature of the
respiratory organs
—these, with few excep-
i i Fic, 165.—Nervous system of shore-crab
ee sea usnrtes (Carcinus menas).—After Bethe.
2 br., The supra-cesophageal mass; g., gullet
While these characters surrounded. by gv, the gullet ring; mt. , the
remain constant through- sub-cesophageal mass representing a fusion of
S the thoracic ganglia of the crayfish, and
out the group, there is giving off merc to the lint behind i it :
1 j 7 a short strand representing the abdominal
aly almost infinite variety ganglia of the crayfish. a1., antennules ;
in detail. In regard a@?., antenna ; é., eye.
to the segmentation of
the body, we notice that, apart from the general tendency
to reduction which is so marked in many parasitic forms,
the higher forms as compared with the lower show marked
specialisation. In the primitive Phyllopods the body con-
sists of a large but varying number of segments, remarkably
uniform in structure. The higher Crustacea, on the other
hand, are characterised by their relatively few but constant
312 PHYLUM ARTHROPODA.
segments, which exhibit marked division of labour; a
comparison of Mebalia, Mysts, Euphausia, Penaeus,
WVephrops, will make this plain. The same gradual process
of specialisation is observable in the appendages. Typically
consisting of a basal piece and two branches, the append-
ages, like the parapodia of Annelids, are primitively organs
of locomotion, usually adapted as swimming organs. In
Phyllopods the great majority of the appendages remain
permanently at this level. Ita ice_ that in the
Naupli i d in free-swimming copepods,
ig antenna themselves aie—suumming Organs, Just as,
however, in the Annelid head the locomo tion of
the parapodia becomes subordinated to the sensory one,
so also in Crustacea the anterior appendages of the head
become specialised as sense organs. Again, the append-
ages in connection with the mouth become modified in
connection with alimentation, and the further processes of
specialisation which differentiate the regions of the body
are reflected in the appendages of these regions. It is this
specialisation of certain appendages to function as mastica-
tory organs which especially characterises Arthropods as
compared with Annelids.
In the nervous system there is always a certain amount
of fusion of ganglia—these never being so numerous as the
segments—but the fusion is more marked in the more
specialised forms. In the Crabs the ventral chain is repre-
sented by a lobed ganglionic mass in the thorax, connected
with a mere rudiment, which corresponds to the abdominal
portion of the cord in the crayfish (Fig. 165). Sense
organs are usually well developed, and are not confined
to the head region; thus many Mysids have “auditory”
organs in the tail (Fig. 164). Dhemelimentanmeneanal
runs straight throughout the body; it consists of fore-gut,
id-gut, and hind-gut. e-gut and hind are
GER OaET POSLA Pagina ae ectoderm, and are
always large, especially in ostraca. In the higher
Malacostraca the fore-gut is furnished with a gastric mill:
The mid-gut or archentéfon is always short, barns con
nected with it diverticula which form the so-called hepato-
pancreas. In the Entomostraca there is usually only a
single pair of outgrowths; in Mysids, Cumacea, and larval
GENERAL NOTES ON CRUSTACEANS. 313
‘Decapods there are three pairs; a process of rapid growth
and branching converts these into the compact digestive
gland of the adult Decapods. In connection with the
posterior end of the mid-gut in Amphipods and some
others, there is a pair of blind tubes functioning as excretory
organs, and presenting an interesting similarity to the
Malpighian tubes of insects, which, however, are in con-
nection with the hind-gut. The body cavity is never lage
being mainly filled up with muscles and organs, and, as in
In the blood, hemocyanin is the commonest pigment, ut
is not universal. respiration is carried on in many
different ways. In the simple forms it may be merely by
the general surface, but in the majority of cases, certain
portions of the limbs, or outgrowths of the limbs, constitute
definite respiratory organs, often specialised to form gills..
In the excretory system the numerous nephridia of Annelids
are absent. The typical excsesery—ergans of the Entomos-
traca are the “‘shell glands”—paired coiled tubes opening
on the second maxilla; of the Malacostraca, the antennary
glands exemplified by the green glands of the crayfish.
The genital ducts are possibly modified nephridia.
There are many peculiarities connected with reproduc-
tion—thus parthenogenesis for prolonged periods is common
among “water-fleas” ; _herma hroditism is frequent, occur-
often complicated_by the simultaneous existence o y
often very diverse. The spermatozoa are often exceptional i in
being very slightly motile. Some appendages are often
modified for copulation or for carrying the eggs.
Development.—The ova of most Crustacea show con-
siderable similarity to those of Astacus, and the segmenta-
tion is typically of the kind already described. But while
this is the most typical case for Crustacean, and, indeed,
for Arthropod development, it is possible, within the limits
of the class Crustacea, to trace out a complete series, in
which the first term is a segmentation of the complete
and equal type, like that of a worm, and the last the
purely peripheral. In the same way, though gastrulation
is usually much disguised, there are many modes, from
314 PHYLUM ARTHROPODA.
an invagination of the simplest embolic type (Zucifer), and
through the condition described for Asfacus, to the forma-
tion of endoderm by the ingrowth of a solid plug of cells.
Compared with Astacus, however, the most important
point we have to notice is the frequent occurrence of a very
striking metamorphosis in the life history. In other words,
the larva hatched from the egg is rarely like the parent, and
only acquires the adult
characters after a series
of profound changes. In
some cases (WWVebalia,
Mysis) a metamorphosis
takes place within the
egg-cases, and in the few
forms in which develop-
ment seems to be direct,
slight traces of meta-
morphosis are found.
Almost all the lower
Crustaceans and some
higher forms, e.g.
Euphausia and Peneus,
are hatched in a Nauplius
stage. In the remaining
cases the Nauplius stage
is indicated within the egg
by the moulting of a larval
cuticle (as in Astacus),
The Nauplius is char-
acterised by a typically
Fic. 166.—Zozea ot common shore-crab
(Carcinus menas).—After Faxon. rounded. Dad, and. by
a pakeacy the presence of three
e appendages are numbered ; ¢., gills; :
z., alimentary canal, pairs of appendages,
F which are the only
obvious indications of segmentation. The first pair
of appendages; are unbranched, and bear larval sense
organs, the next two are biramose swimming organs.
There is an unpaired median eye, but no heart, and
frequently no hind-gut. The three pairs of appendages
become the first and second pairs of antenne and
the mandibles of the adult. The head region of the
f
GENERAL NOTES ON CRUSTACEANS. 315
Nauplius becomes the head region of the adult; the
posterior region also persists; the new growth of segments.
and appendages takes place (with numerous moultings) in
the region between these.
The second important form of larva is the Zozea, which
has all the appendages on to the last maxillipedes inclusive,
a segmented abdomen, and two lateral compound eyes,
in addition to the unpaired one of the Nauplius stage.
Most Decapoda are hatched in the Zozea stage.
(a) The crayfish (Astacus) is hatched almost as a miniature adult.
The development is therefore very direct in this case,
(4) The lobster (Homarus) is hatched in a AZysds stage, in which the
thoracic limbs are two-branched and used for swimming. After
some moults it acquires adult characters. |
(c) Crabs are hatched in the Zoga form, and pass with moults through
a Megalopa stage, with the abdomen in a line with the cephalo-
thorax. The abdomen is stibsequently tucked in under the
thorax. :
(d) Penaeus (a kind of shrimp) is hatched as a Mazplius, becomes a
Zoea, then a Myszs, then an adult. Its relative Luczfer starts
as a Meta-Nauplius with rudiments of three more appendages
than the Nauplius. Another related form, Sevgestes, is hatched
as a Protozoea, with a cephalothoracic shield and an unseg-
mentedabdomen. Thus there are two grades between Nauplius.
and Zoza.
Three facts must be borne in mind in thinking over the life histories.
of crayfish, lobster, crab, and Pexeus: (1) There is a general tendency
to abbreviate development, and this is of more importance when meta-
morphosis is expensive and full of risks ; (2) there is no doubt that larve ,
exhibit characters which are related to their own life rather than to that
of the adult ; (3) it is a gezera/ truth, that in its individual development
the organism recapitulates to some extent the evolution of the race, that
ontogeny tends to recapitulate phylogeny. But while there can be no.
doubt that the metamorphosis of these Crustaceans is to some extent
interpretable as a recapitulation of the racial history,—for there were
unsegmented animals before segmented forms arose, and the Zoea stage
is antecedent to the A/yszs, etc.,—yet it does not follow that ancestral
Crustaceans were like Nauplii. On the contrary, the Vauzplcus must be
regarded as a larval reversion to a type much simpler than the ancestral
Crustacean.
C&cology.—Most Crustaceans are carnivorous and pred-
atory; others feed on dead creatures and organic débris in
the water ; a.§4inOrifgdepend upon plants. Many of the
smaller forms play a very important part in the economy of
nature—in the circulation of matter—for while they feed on
316 PHYLUM ARTHROPODA.
animalcules and débris, they are themselves the food of
larger animals such as fishes. ;
Parasitism occurs in ovetZe0 species, in various degrees,
and, of course, with varied results. Most of the parasites
keep to the outside of the host (e.g. fish-lice), and suck
nourishment by their mouths; the Rhizocephala (e.g.
eater, weend_zamifving—absorptive roots through the
ody of the host. Sometimes the parasitism 1s temporary
( Areulus); sometimes only the females are parasitic (e.g. in
Lernea). The parasites tend to lose appendages, segmen-
tation, sense organs, etc., but the reproductive organs
become more fertile. The hosts, e.g. crabs, infested by
Rhizocephala, are sometimes materially affected, and even
rendered incapable of reproducing.
Some Crustaceans live not as parasites, but as commensals
with other animals, doing them no harm, though sharing their
food. Thus there is a constant partnership between some
hermit-crabs and sea-anemones (Fig. 16). The hermit-crab is
concealed and protector vy ane Sea anemons—thelaiter is
camied about by the Crustacean, and gets fragments_of
food.
Masking is also common, especially among crabs. Some
will cut the tunic off a sea-squirt and throw it over their own
shoulders. Many attain a mask more passively, for they are
covered with hydroids and sponges, which settle on the
shell. There is no doubt, however, that some actively
mask themselves, for besides those known to use the
‘Tunicate cloak, others have been seen planting seaweeds
on their backs. The protective advantage of masking both
in offence and defence is very obvious. .
The intelligence of crabs and some of the higher Crus-
taceans is well developed. Maternal care is frequent.
Fighting is very common. Many will “voluntarily” part
with a leg to save themselves from their enemies. The
loss of limbs is readily repaired.
Deep-sea Crustaceans are very abundant, and often
remarkable “for their colossal size, their bizarre forms,
and brilliant red colouring”; in many cases, they are
brilliantly phosphorescent. Yet more abundant are the
pelagic Crustaceans (especially Entomostraca and Mysids) ;
they are often transparent except the eyes, often
G@COLOGY. 317
brightly coloured or phosphorescent. Many Crustaceans
live on the shore, and play a notable part in the struggle
for existence which is so keen in that densely crowded
region. The lower Crustaceans are abundantly repre-
‘sented in fresh water, in pools, streams, and lakes. A
few Crustaceans, such as wood-lice and land-crabs, are
terrestrial, and some blind forms occur in caves.
CHAPTER XIV
PHYLUM ARTHROPODA—(continued)
Classes (continued)—ONYCHOPHORA or PROTOTRACHEATA ;
Myriopopa; and INSECTA
‘THESE three classes form a series of which winged insects
are the climax. The type Levipatus is archaic, and links
the series to the Annelids: the Myriopods lead on to the
primitive wingless insects. All breathe by trachese—tubes
which carry air to the organs of the body—and all have
antenne ; hence they are often united under the title
‘Tracheata Antennata.
First Class of Tracheata Antennata.—ONYCHOPHORA or
PROTOTRACHEATA
GENERAL CHARACTERS
The body ts worm-like in form, soft-skinned, and without
external segmentation.
The appendages are—a pair of prominent pre-oral antenna,
a pair of jaws in the mouth, a pair of slime-secreting oral
papilla, which development shows to be true appendages,
numerous pairs of short, imperfectly jointed legs, each with
two claws, and a pair of anal papille, which are rudt-
mentary appendages. The legs contain peculiar (crural)
glands.
Respiration is effected by numerous unbranched trachee
with openings irregularly scattered. The heart ts an elongated
dorsal vessel with valvular ostia, There ts a series of
nephridia in the legs. The halves of the ventral nerve-cord
are widely separate. All are viviparous.
ONYCHOPHORA OR PROTOTRACHEATA. 319
In its possession of trachee and nephridia this type is an
interesting connecting link; in many ways it seems to be an
old-fashioned survivor of an archaic stock. There are about
half a dozen genera very widely distributed.
The Onychophora are very beautiful animals. Prof.
Sedgwick says: “The exquisite sensitiveness and continu-
ally changing form of the antehnz, the well-rounded plump
body, the eyes set like small diamonds on the side of the
head, the delicate feet, and, above all, the
rich colouring and velvety texture of the
skin, all combine to give these animals an
aspect of quite exceptional beauty.” They
are shy and nocturnal, with a great dislike to
light. They seek out damp places under
leaves and among rotting wood. They feed
on insects, which they catch by the ejection
of slime from the oral papille. The slime
is also squirted out when they are irritated.
To their shy habits their persistence is
possibly in part due. They are able to
move quickly, somewhat after the fashion of
millipedes, especially like Scolopendrella.
They have been seen to climb up vertical
glass plates. When at rest or irritated they
coil up in a circle.
; Fic. 167.—Ex-
Like some other archaic types, ¢.g. Dipnoi, the ternal form of
Onychophora have a very wide range of distribution, Peripatus,—
which may be briefly indicated :—Ferdpatus (tropi- After Balfour.
cal America and tropical Africa) ; Hoperipatus (Indo- N 5
Malay) ; Perzpatoddes and Ooperdpatis (Australasia) ; “gapleleee
Opesthopatus (Chili and South Africa) ; Paragpertpatus
(New Britain); Perzpatopszs (Central Africa).
A more Detailed Account of Peripatus
Form.—The body suggests an Annelid or a caterpillar, but, apart
from the appendages, there is no external segmentation. There is a
clear dorso-median line. Over the soft skin are numerous minute warts
with small bristles. The mouth is ventral and anterior; the anus
terminal and posterior.
Appendages.—The first are the large, ringed antennz ; then follow
the sickle-like jaws in the mouth cavity; a little farther back are two
oral papillee from which slime is exuded. Then there are the 14-42
stump-like legs, each with two terminal chitinous claws.
320 PHYLUM ARTHROPODA.
Skin.—The chitinous cuticle, ordinarily thick in Arthropods, is
delicate. It is subject to moulting. -The epidermis is a single layer of
cells. Beneath it there is a dermis. i
Muscular system.—Externally there is a layer of circular muscles ;
within this lies a double layer of diagonal fibres ; internally there are
strong longitudinal bundles. Finally, in connection with this internal
layer, there are fibres which divide the apparent body cavity into a
median and two lateral compartments, The median includes heart,
gut, slime glands, reproductive organs; the laterals include the nerve-
cords and salivary glands; the legs contain nephridia and coxal or
crural vesicles. Striped, rapidly contracting muscles are characteristic
of Arthropods, but in Perdpatus the muscles are unstriped, excepting
those which work the jaws and are perhaps the most active. The true
ccelom is represented in the embryo by the cavities of the mesoderm
segments, which give origin to the muscular system.
Nervous system.—The dorsal brain is connected by an ceso-
phageal ring with the two- widely separate latero-ventral nerve-cords.
These are connected transversely by numerous commissures, are slightly
swollen opposite each pair of legs, to which they give off nerves, and
are united posteriorly over the anus. There are only hints of ganglia,
but there is a continuous layer of ganglionic cells. The brain is very
homogeneous, simpler than that of most Insects. Sense organs are
represented by two simple eyes on the top of the head. These are
most like the eyes of some marine Annelids.
Alimentary canal.—Round about the mouth papillae seem to
have fused to form a ‘‘ mouth cavity,” which includes the mandibles, «
median pad or tongue, and the opening of the mouth proper. The
mouth leads into a muscular pharynx, into which opens the common
duct of two large salivary glands, which extend far back along the body.
Mouth, pharynx, and short cesophagus are lined by a chitinous cuticle,
like that of the exterior. The long endodermic digestive region or mid-
gut extends from the second leg nearly to the end of the body. . Its
walls are plaited. Finally, there is a short rectum or proctodeum,
lined by a chitinous cuticle.
Circulatory system.—The dorsal blood vessel forms a long con-
tractile heart. It lies within a pericardial space, and receives blood
by segmentally arranged apertures with valves. The circulation is
mostly in ill-defined spaces in the apparent body cavity or ‘*hzmo-
coele.”
Respiratory system.—Very long and fine unbranched trachez are
widely distributed in the body ; a number open together to the exterior
in flask-like depressions. These openings or stigmata are irregularly,
distributed.
Excretory system.—aA pair of nephridia lie in each segment.
Each consists of an internal mesodermic terminal funnel, a looped canal,
and a wide vesicle which opens near the base of each leg, the two last
parts being invaginations of the ectoderm. Nephridia are not known in
any other Tracheate. The salivary glands and the genital ducts seem to
be modified nephridia. It may be noted, too, that the same is perhaps.
true of the ‘‘coxal glands” of ZLzwewlus and of the antennary glands
of Crustaceans,
ONYCHOPHORA OR PROTOTRACHEAT. 321
Coxal or crural glands lie in the legs and open to the exterior. They
can be in part evaginated, and they probably help in respiration. In
the male of P. capgenszs the last pair are very long (Fig. 168, a.g.). The
large mucous glands, which pour forth slime from the oral papillz, are
regarded as modified crural glands.
eproductive system.—(a) Female (of P. edwardszz).—From the
two ovaries, which are surrounded by one connective tissue sheath, and
arise, as usual, from the coelomic epithelium, the ova pass by two long
ducts leading to a common terminal vagina opening between the second
last legs. These ducts are for the most part uteri, but on what may be
called the oviduct portions adjoining the ovaries, there are two pairs of
pouches—a pair of receptacula seminis (for storing the spermatozoa
slg.
|
>: Use
sid. 4 08 vw.| co ewe
wae ph. Z s.0.17] cl
at, : A A_A oh Dr
tat taal Suited, p
5.0.4 i
5.0.5 oe
“06.60 eT ag as
Fur Fa
Fic. 168.—Dissection of Perdpatus.—After Balfour.
az,, Antenne; o7.4,, oral papille; c.g., cerebral ganglia; sé.d.,
duct of slime gland (s/.g.); 5.0.8, eighth segmental organ or
nephridium 3 v.c., ventral nerve connected by transverse com-
missures (co.) with its fellow; s.o.77, seventeenth nephridium ;
£.0., genital aperture ; A., anus; 4.d.c., posterior commissure ;
f.17, seventeenth appendage ; a.g., last crural gland—that of
the opposite side is marked v.g.; 7.1, 7.2, first and second
legs; 0¢.co., esophageal nerve commissure ; ve., cesophagus ;
~A., pharynx—the remainder of the gut is removed.
received during copulation), and a pair of receptacula ovorum for storing
fertilised eggs.
The eggs are hatched in the uteri, and all stages are there to be found
in regular order. The young embryos seem to be connected to the wall
of the uterus by what has been called « ‘‘ placenta,” so suggestive is it
of mammalian gestation. The older embryos lose this ‘‘ placenta,” but
each lies constricted off from its neighbours. When born the young
resemble the parents except in size and colour. In P. capemszs the
period of gestation is thirteen months.
(4) Male (of P. edwardsiz7).—The male elements are produced in
small testes, pass thence into two seminal vesicles, and onwards by two
vasa deferentia into a long single ejaculatory duct, which opens in front
?
21 1
322 PHYLUM ARTHROPODA.
of the anus. In the ejaculatory duct the spermatozoa, which are
thread-like, are made into spermatophores which are attached to
the female. It is uncertain how the spermatozoa get into the female.
Fertilisation is ovarian. :
While it is characteristic of Arthropods, in which chitin is so pre-
dominant, that ciliated epithelium is absent, it seems that in Perzpatus,
which is much less chitinous than the others, ciliated cells occur in
some parts of the reproductive ducts,
Development.—There is some variety of development in different
species. Thus there is much yolk in the ovum of P. xove zealandie,
extremely little in that of P. capenszs.
In P. capensis the ‘‘segmentation”
is remarkable, for true cleavage of
cells does not occur. The fully
‘segmented ” ovum does not exhibit
the usual cell limits. It is a proto-
plastic mass—or syncytium—with
many nuclei. Even when the body
is formed, the continuity of cells
persists, nor does the adult lack
traces of it. To Prof. Sedgwick this
singular fact suggested the theory
that the Metazoa may have begun
as multinucleate Infusorian-like ani-
mals,
The gut appears from a fusion of
vacuoles within the multinucleated
mass, and a gastrula stage is thus
established. A very interesting
feature is that the blastopore or
mouth of the gastrula is first elon-
gated, then dumb-bell shaped, then
closed except at the two ends which
é form the mouth and the anus.
In the ova of P. nove zealandie,
Fic. 169.—Embryos of Perépatws which have much yolk, a superficial
capensis, showing closure of multiplication of nuclei forms a sort
blastopore and curvature of of blastoderm, which spreads over
embryo.—After Korschelt and almost the entire ovum. The seg-
Heider. mentation in this case has been called
a., Anus} 82., blastopore; #., mouth; centrolecithal (the type characteristic
DeSoy Faracive segments; w., zone of of Arthropods), but it is again
proliferation. true that for a long time the cells do
not exist as well-defined units. It
has been said, indeed, that ‘‘the embryo is formed by » process of
crystallising out 27 széw from a mass of yolk, among which is » proto-
plasmic reticulum containing nuclei.”
Zoological positi »n.—The synthetic characters of Pertpatus and
ils allies may be thus summarised :—
MYRIOPODA. 323
ANNELID CHARACTERISTICS.
Segmentally arranged nephridia
as in Chaetopods.
The muscular ensheathing of the
body.
The cilia in the genital ducts.
Less important are the stump-like
hollow legs and the simple
eyes.
ARTHROPOD AND TRACHEATE
CHARACTERISTICS.
The presence of tracheze,
The nature of the (a tube
with paired ostia communicat-
ing with a pericardium) and
the lacunar circulation.
The modification of appendages
uth organs.
The form of the salivary glands.
The smallness of the genuine
ceelom; the cavity of the
body is heemoccelic.
The Onychophora differ from other Tracheata Antennata in the
simplicity and diffuseness of the trachez, in having only one pair of
jaws, in the absence of external segmentation, in the nature of the
body wall, and so forth.
The ladder-like character of the ventral nervous system (cf. primitive
Molluscs, Phyllopod Crustaceans, and Nemerteans) is probably primi-
tive. That salivary glands and genital ducts are homologous with
nephridia is a fact of much morphological interest. It is possible that
the slime glands are modifications of crural glands, and that the latter
are homologous with the parapodial glands of some Annelids. It is
not certain that the antennz, jaws, and oral papille of Per¢patus
precisely correspond to the antennz, mandibles, and first maxille of
Insects.
Our general conclusion is that Perdpatus is an archaic type, a sur-
vivor of forms which were ancestral to Tracheata and closely related to
Annelids.
Second Class of Tracheata Antennata.—MyRriopopa.
Centipedes and Millipedes
The centipedes and millipedes, which are grouped
together in the class Myriopoda, are usually elongated,
somewhat vermiform animals, with a distinct head and a
very uniform segmented trunk. The head bears eyes
(groups of eye-spotsAngt)¥compound eyes like those of
insects, except in Scutigera), jomted antenne, and two or
three pairs of jaws. The segments of the trunk bear six- or
seven-jointed legs with terminal claws, very similar through-
out. The nervous system, the_tr t the ex-
cretory tubiles, etc., are like those of Insects, Tt cannot
324 PHYLUM ARTHROPODA.
be said that the centipedes (Ciidggaga) and the millipedes
(Diplopoda) are very closely related to one another, and
-therearetwo other distinct orders, Symphyla and Pauropoda.
The resemblances are in part resemblances of convergence,
not of genuine affinity. Simple wingless insects, known as
Collembola and Thysanura, are closely approached by such
Fic. 170.—A millipede. Fic. 171.—A centipede
Myriopods as Scolopendredla ; and it is likely that Myriopods
and Insects are divergent branches from a common
stock.
Centipedes and millipedes are characteristically texrestr#l.
Most are very shy animals, lurking in dark places and
avoiding the light, but it is interesting to note that at
least two Myriopods—Geophilus submarinus and Linotenia
maritima—occur on British coasts.
INSECTA, 325
MYRIOPODA
CENTIPEDES. MILLIPEDEs.
CHILOPODA, DipLopopa (or CHILOGNATHA).
Carnivorous. Vegetarian.
Harmless.
popes:
ody usually flat.
One pair of appendages to
each segment. The stigmata do
not correspond in number to the
segments; they often occur on
alternate segments.
Many-jointed antennz.
Toothed cutting mandibles.
| Two pairs of maxille, usually
with palps.
Body cylindrical.
By the imperfect separation of
the segments, all but the first three
behind the head seem to have two
pairs of appendages each, and also
two paired ganglia, and two pairs
of stigmata (tracheal openings).
Seven-jointed antenne. .
Broad masticating mandibles.
A pair of maxille fused in a
broad plate, usually four-lobed.
No poison claws.
| he first pair of legs modified:
as poison claws. —
ee
A single genital aperture on the
| second last segment.
Examples. —Scolopendra,
Lithobius,
Geophilas.
teriorly.
Examples.— /zlus.
Polyxenus.
Glomeris.
More
Genital apertures open ap- i,
In the order Symphyla (Scolofendrella) there are not more than
twelve segments, and there is only one pair of trachez, which open on
the head. Scolopendrella is in several ways like the primitive insects
known as Thysanura. In the order Pauropoda (Fauropus), there are
ten segments, and the antennz are branched.
Third Class of Tracheata Antennata.—INSECTA
Insects occupy a position among the backboneless
animals like that of birds among the Vertebrates. The
typical members of both classes have wings and the power
of true flight, richly aerated bodies, and highly developed
respiratory, nervous, and sensory organs. Both are very
active and brightly coloured. They show parallel differ-
ences between the sexes, and great wealth of species within
a narrow range.
326 PHYLUM ARTHROPODA.
GENERAL CHARACTERS
Like other Arthropods, Insects have segmented Ladies, jointed
__legs, chitinous armature, and a vertwat-chain of ganglia linked
to a dorsal brain. Compared with Peripatus and Myrtopods,
adult insects show concentration of the body segments, decs&éease
in the number and ucrease in the quality of the appendages,
and wings tn the great majority. SoS
Insects are teryestrial and aerial, and xanely aguatie—
animals ; usually winged as adults, breathing by means of
Le
aa
RS
Lia
ZL
Fic. 172.—-Female cockroach Fic. 173.—Male cockroach
(P. orientalis). (P. ortentalds),
trachea, and often with a metamorphosis tm the gourse gf their »
life history. <> arpeed 43 FEMS
Th, — ;
Atiddebdgmen. The head bears a pair of gre-oral antenge,
and th : endages ; the thorax bears a pair
of legs on each of its three segments, and, typically, a pair of
wings on each of the posterior two; theabdomen has no
appenda dimentary modifications of these be re-
presented by stings, ovipositors, ett. aaa
First Type of Insects, Periplaneta (or Blatta).—
The Cockroacu
Habits.—The cockroaches in Britain are immigrants
from the East (P. orientalis), or from America (P. americana).
COCKROACH.
EXTERNAL CHARACTERS
REGIon.
APPENDAGES,
OTHER STRUCTURES.
The head is ver-
tically elongated
and separated
from the thorax
by a neck.
The insect’s
head seems to
consist of seven
fused segments—
ocular,antennary,
intercalary, man-
dibular, maxillu-
lar, maxillary, and
labial.
The thorax con-
sists of three seg-
ments—
% prothorax,
4) mesothorax,
Q metathorax.
Each segment
is bounded by a
dorsal tergum
and ventral ster-
num.)
The abdomen
consists of 10
(or x1) distinct
segments, with
terga and sterna
as in the thorax.
The first sternum
is rudimentary in
both sexes, and
in the female the
eighth and ninth
segments are con-
cealed by the
large seventh.
1. The antenna (probably homologous with
appendages), long, slender, many-jointed,
tactile.
2. A pair of stout toothed mandibles work-
ing sideways.
3. The first maxille, each consisting—
(a) of a basal piece or protopodite with two
joints : a basal cardo, a distal stipes ;
(2) of a double endopodite borne by the
basal piece, and consisting of an inner
lacinia and a softer outer galea ;
(c) of an exopodite or maxillary palp also
borne by the basal piece, and consist-
ing of five joints.
4. The second pair of maxilla, fused to-
gether as the ‘‘labium,” consisting—(a@) of a
fused basal piece or protopodite with two
joints: a basal sub-mentum, a smaller distal
mentum; on each side this protopodite
ears—
(4) a double endopodite (ligula) consisting
of an inner lacinia and an outer
paraglossa ;
(c) an exopodite or labial palp, consisting
of three joints.
(a) First pair of legs.
(4) Second pair of legs.
(c) Third‘pair of legs. Each leg consists
of many joints —a basal expanded
“coxa” with a small ‘‘ trochanter” at
its distal end, a “‘femur,” a ‘‘tibia,”
a six-jointed tarsus or foot ending ina
pair of claws (Fig. 175).
Two cigar-shaped tactile anal cerci, at-
tached under the edges of the last tergum,
are possibly relics of the last abdominal
appendages.
The ninth sternum of the male bears a
pair of styles, possibly relics of appendages.
Both sexes have complex hard structures
(gonapophyses) beside the genital apertures.
They are possibly relics of appendages.
The large black compound |
eyes.
The “ upper lip” or labrum, in
front of the mouth.
The white oval patches near
the bases of the antennz, pos-
sibly sensory.
In some primitive insects a
minute pair of appendages,
known as maxillulz, occurs be- |
tween the mandibles and the
first maxille.
(4) Apair of wing-covers (modi-
fied wings), rudimentary in
female of P. orientalis.
(c)A_ pair of membranous
wings, sometimes used in
flight, folded when not in
use, absent in female of |
P. orientalis.
Between the segments of the
thorax are two pairs of respira-
tory apertures or stigmata.
A pair of stigmata occur be-
tween the edges of the terga and
sterna in the first eight abdo- |,
minal segments.
The anus is terminal, beneath
the tenth tergum of the abdo-
men ; a pair of “‘ podical plates” |
lie beside it.
The genital aperture is on the
eighth segment in the female, |
behind the ninth sternum in the
male.
The opening of the sperma-
theca—the female’s receptacle
for spermatozoa —lies on the
ninth sternum of the abdomen.
328 PHYLUM ARTHROPODA,
They are omnivorous in their diet, active in their habits,
hiding during the day and feeding at night. They are
ancient insects, for related forms occurred in Silurian ages ;
they are average types, neither very simple nor very highly
specialised. Their position is among the Orthoptera, in the
same order as locusts and grasshoppers. The hatched young
are like miniatures of the adults, except that wings are
absent. If there are wings, they appear at the last moult,
when the cockroach becomes sexually mature.
Fic. 174.—Ventral aspect of male cockroach with the wings extended.
An imaginary median line has been inserted.
A., antenne; £., eye; P.7., prothorax; W1, first pair of wings ;
W*, second pair of wings; C., cercus; St., style; Co., coxa;
Tr., trochanter; #., femur; 772., tibia; 7a., tarsus.
Skin.—There is an external chitinous cuticle and a
subjacent cellular Jayer—the epidermis or hypodermis—
from which the cuticle is formed. The newly hatched
cockroaches are white, the adults are dark brown.
Moulting, which involves a casting of the cuticle, of the
internal lining of the trachez, etc., occurs some seven times
before the cockroach attains in its fifth year to maturity.
The muscles which move the appendages, and produce
COCKROACH.
329
the abdominal movements essential to respiration, are
markedly cross striped. They
are in many cases attached
to special tendons, which
arise as cuticular invagina-
tions, and are lost and re-
placed at each moult.
Nervous system.—A pair
of supra-cesophageal or cere-
bral ganglia lie united in the
head. As a brain they receive
impressions by antennary and
optic nerves. By means of a
paired commissure surround-
ing the gullet, they are con-
nected with a double ventral
Fic. 175.—Leg of cockroach.
¢., Broad expanded coxa; ¢~, troch-
anter; f, femur; ¢z., tibia; ¢a., six-
jointed tarsus with terminal claws
and adhesive cushions.
chain of ten ganglia. Of these, the first or sub-cesopha-
geal pair are large, and give off nerves to the mouth-parts,
Fic. 176.—Moutn appendages of cock-
roach.—After Dufour,
I, Aln., mandibles ; II. first maxilla ; C., cardo ;
Sz, stipes; Z., lacinia; G., galea; mx p.,
maxillary palp; III. second maxille or la-
bium; S7., submentum; #., mentum; Z.,
lacinize ; Bo paraglossa 3 2, poy labial palp.
etc. ; from each of the
ganglia of the thorax
and the abdomen
nerves are given off
to adjacent parts.
There are three pairs
of ganglia in the
thorax, and six in the
abdomen, of which the
last is the largest.
From the cesophageal
commissures visceral
nerves are given off to
the gullet, crop, and
gizzard. Besides the
large compound eyes,
there are other sensory
structures —some of
the setee on the skin,
the maxilla (to some
extent organs of oo
the antennse (tactile and olfactory), the anal cerci (tactile
and possibly the oval white patches on the head.
330 PHYLUM ARTHROPODA.
Alimentary system.—(1) The fore-gut (stomodzum) is
lined by a chitinous cuticle continuous with that of the outer
surface of the body. It includes—(a) the buccal or mouth
cavity, in which there is a tongue-like ridge, and into
which there opens the duct of the salivary glands ; (0) the
narrow gullet or cesophagus ; (c) the swollen crop; (@) the
gizzard, with muscular walls, six hard cuticular teeth, and
some bristly pads.
There is a pair of diffuse salivary glands on each side of the crop, and
between each pair of glands a salivary receptacle. The ducts of the
two salivary glands on each side unite; the two ducts thus formed
combine in a median duct, and this unites with another median duct
formed from the union of the ducts of the receptacles. The common
duct opens into the mouth.
(2) The mid-gut (mesenteron) is lined by endoderm. It
Fic. 177.—Transverse section of insect.—After Packard.
‘., Heart; g., gut; #., nerve-cord; sz., stigma; ¢r., trachea; w., wing;
J, femur of leg.
is short and narrow, and with its anterior end seven or
eight club-shaped digestive (pancreatic) outgrowths are
connected.
(3) The hind-gut (proctodzum) is lined by a chitinous
cuticle. It is convoluted and divided into narrow ileum,
wider colon, and dilated rectum with six internal ridges.
Respiratory system.—The tracheal tubes, which have
ten pairs of lateral apertures or stigmata, ramify throughout
the body, and have a spirally thickened chitinous lining.
Circulatory system.—The chambered heart lies along
the mid-dorsal line of abdomen and thorax. It receives
blood by lateral valvular apertures from the surrounding
COCKROACH.
338
pericardial space, and drives it forwards by a slender aorta.
The blood circulates, however, within ill-defined spaces in
the body.
Excretory system.
There are sixty or so fine (Mal-
pighian) tubules, which rise in six bundles from the begin-
ning of the ileum, and twine through the “fatty body”
and in the abdominal cavity. The absence of nephridia
in insects has been already mentioned.
REPRODUCTIVE SYSTEM
OF THE MALE.
OF THE FEMALE.
The testes are paired organs, sur-
rounded by the fatty body
below the 5th and 6th ab-
dominal terga. They atrophy
in the adult.
From the testes, two narrow ducts
or vasa deferentia lead to two
seminal vesicles.
These seminal vesicles (the
‘*mushroom - shaped gland”)
open into the top of the ejacu-
latory duct.
This duct opens between the gth
and ioth sterna. Beside the
aperture, there are copulatory
structures (gonapophyses).
With the ejaculatory duct a
gland is associated.
Large branched tubular glands
secrete a volatile alkaline sub-
stance, with a strong mousy
odour, probably offensive to
enemies,
The ovaries are paired organs, in
the posterior abdominal region,
each consisting of eight ovarian
tubes. These are bead-like
strings of ova at various stages
of ripeness,
From the ovarian tubes of each
side eight eggs pass at a time |,
into a short wide oviduct.
The two oviducts unite and open
in a median aperture on the
8th abdominal sternum. Be-
side the aperture are hard
structures (gonapophyses)
which help in the egg-laying.
On the gth abdominal sternite a
pair of arborescent glands pour
out their cementing secretion
by two apertures. The sper-
matheca is a paired sac opening
between the 8th and the gth
abdominal sternum.
Sixteen ova, one from each ovarian tube, are usually
enclosed within each egg-capsule.
The latter is formed
from the partly calcareous secretion of the arborescent.
332 PHYLUM ARTHROPODA.
glands. Each egg is enclosed in an oval shell, in which
there are several little holes (micropyles), through one of
which a spermatozoon enters. Spermatozoa, from the store
within the spermatheca, are included in the egg-capsule.
At an early stage in development some cells associated with the
mesoderm are set apart as reproductive cells, and originally these have
a segmental arrangement as in Annelids; at a later stage other meso-
derm cells join these, some forming ova, others epithelial cells around
the latter. The distinction between truly reproductive cells and
associated epithelial cells, which is said to be late of appearing in some
of the higher insects, is established at a very early stage in the
cockroach,
Second Type of Insects.—The British Hive-BEE
(Apis mellifica)
This is a much more highly specialised type than the
‘cockroach. It belongs to the order Hymenoptera.
Habits.—The Hive-Bee (Apis mellifica) is a native of
this country, and is the species most commonly found
domesticated. It is the only British representative of the
genus 4s, and exhibits, in its most fully developed form,
the social life which is foreshadowed among the Humble-
Bees. As a consequence of this social life, there is much
division of labour, which expresses itself alike in habit and
in structure. The males (drones) take no part in the work
of the colony, and are wholly reproductive ; the females
include the queen-bees and the workers. In the workers,
which perform all the work of the hive, the reproductive
organs are normally abortive and functionless. In the
queens, of which there is but one adult to each hive, the
enormous development of the reproductive organs seems to
act as a check upon the brain and other organs, which
are less developed than in the workers. The workers are
further divisible into nurses, which are young and do not
leave the hive, being occupied with the care of the larvee,
and the older foraging bees, which gather food for the whole
colony.
In consideying the relation between the life of the Hive-
Bee and that of many allied forms (Boméus, etc.), it is
important to notice that the habit of laying up stores of
food material for the winter enables the colony, and not
BRITISH HIVE-BEE. 333
merely an individual, to survive, and must thus have greatly
assisted in the evolution of sociality.
External features.—The body shows the usual division into head,
thorax, and abdomen, and varies considerably in the three different
types, being smallest in the workers. It is entirely covered with hairs,
some of which are sensitive, while others are used in pollen-gathering, etc.
The head bears antennz, -
which are composed of a long #
basal and numerous smaller
joints. They are marvellously
sensitive, serving to communi-
cate impressions, and also con-
taining organs of special sense.
A pair of compound eyes, largest
in the drones, and three median
ocelli, are also present in the
head region. Of the other
appendages of the head, the
mandibles are in the workers
very powerful, and used for
many purposes connected with
comb - building. In the first
maxille the maxillary palps are
aborted, and the appendage con-
sists of an undivided lamina at
each side, borne on a basal piece
consisting as usual of’ stipes
and cardo. The second pair of
maxillee form as usual the labium
or so-called lower lip, and are
much modified. The united
basal joints form the mentum
and sub-mentum. From the
mentum at either side springs Fic. 178.—Head and mouth parts
the long labial palp, which re- of bee.—After Cheshire.
presents the outer fork of the a., Antenna; ., mandible; g., labrum or
typical appendage. The endo- epipharynx; #7x.., rudiment of maxil-
podite at each side is divided iy Pale gig apne tenes ies
into two parts, but the inner two The parabltees ie couvecied berwees thie
(lacinize) are united, much elon- basal portions of the labial palps and
gated, and form the tongue or _ the ligula.
ligula of the bee. The outer
halves form the paraglossee, which are closely apposed to the base of
the ligula. It is the great elongation of the ligula and labial palps
which especially fits the bee for nectar-gathering. The three structures
can be closely apposed to one another, and then form an air-tight tube,
up which, by the action of the stomach, nectar is sucked. In many
of our British bees the ligula is much shorter, and more or less trowel-
like in shape, and is then used largely, as in wasps, in the operation of
plastering the nest. In such cases the bee can only suck those flowers
334 PHYLUM ARTHROPODA.
in which the nectar is superficial, The hive-bees and humble-bees, on
the other hand, are specially modified to enable them to extract nectar
from tubular flowers. When not in use, the elongated mouth-parts are
folded back upon themselves, not coiled as in butterflies and moths,
where there is even greater elongation.
In the queen and in the drone the mouth-parts are shorter, and are
not used in honey-gathering.
The thoracic appendages consist as usual of three pairs of legs, which
have the usual parts. On the first leg, at the junction of the tibia and
the first tarsal joint, there is a complicated mechanism which is em-
ployed in cleaning the antennz ; this is present in all three forms, and
varies with the size of the antenne. In the workers the third leg is
remarkably modified for pollen-gathering purposes. The first tarsal
joint bears regular rows of stiff straight hairs on which the pollen grains
are collected ; they are borne to the hive in the pollen basket, placed
at the back of the tibia, and furnished with numerous hairs. In queen
and drone these special arrangements of hairs are absent.
The second and third thoracic segments bear each a pair of wings.
These are largest in the drones and relatively smallest in the queen,
who flies but seldom. At the base of each wing there is a respiratory
spiracle.
In the adult queen and worker, the abdomen is divided into six
segments; in the drone, into seven. There are no abdominal appen-
dages. On the ventral surface in the worker, but not in the queen or
drone, there are four pairs of wax pockets or glands, which secrete
_the wax, which, after mastication with saliva, is employed in building
the combs. The abdomen also bears in queen and worker five pairs of
spiracles, but in the drone, on account of the additional segment, there
are six pairs. The total number of spiracles is thus fourteen for queen
and worker, and sixteen for the drone. The posterior region of the
abdomen bears the complicated sting. In the worker this consists of a
hard incomplete sheath, which envelops two barbed darts. The poison
flows down a channel lying between the darts and the sheath. Ramify-
ing through the abdomen are found the two slender coiled tubes which
constitute the poison gland. At the posterior end of the body these
unite and open into a large poison sac. When a bee uses its sting, the
chitinous sheath first pierces the skin, and then the wound is deepened
by the barbed and pointed darts, while at the same time poison is
steadily pumped down the channel mentioned above, and pours out by
minute openings at the bases of the darts. The poison contains formic
acid, and is fatal to the bee if directly introduced into its blood.
Associated with the sting there are a pair of delicate tactile palps. In
the queen the sting is curved and more powerful, but it is apparently
only used in combat with a rival. In the worker, the sting, and with it
a portion of the gut, is usually lost after use, and, in consequence,
death ensues; the queen, on the other hand, can withdraw her sting
from the wound with considerable ease. The sting is really an
ovipositor adapted to a new function. Naturally, therefore, there is no
trace of it in the drones.
Nervous system,—In the adult this exhibits considerable
BRITISH HIVE-BEE. 335
fusion of parts. The supra-cesophageal ganglia are very
large, and send large lateral extensions to the compound
eyes. This “brain” is best developed in the active
workers. The sub-cesophageal mass is formed by the
fusion of three pairs of ganglia. In the thorax there are
two pairs of ganglia, of which the second supplies the wings
and the two last pairs of legs. In the worker there are five
Fic. 179.—Nervous system of bee.—After Cheshire.
A, of larva, B, ofadult. a, Antenna; wx., maxilla; 7., mandible ;
w., origin of wing ; 1-5, abdominal ganglia.
pairs of abdominal ganglia, but in the queen and drone
only four. The sense organs are the simple and compound
eyes, and the antenne, which are furnished with numerous
sensitive structures.
Alimentary system.—The csophagus is a narrow tube
which runs down the thoracic region. In the abdominal
region it expands into the crop or honey-sac. The crop
opens by a complicated orifice, with a remarkable stopper
336 PHYLUM ARTHROPODA.
arrangement, into the digestive region or chyle stomach,
which is separated by a pylorus from the coiled small
intestine. The inner wall of the small intestine bears
numerous rows of chitinous teeth set in longitudinal ridges,
and is perforated by the apertures of the excretory tubules.
At the junction of the small with the large intestine there
are six brownish plates, perhaps functioning as valves.
Inconnection with the anterior
region of the gut there is a very
complicated series of glands.
First we have, in the workers
only, on either side of the head,
a long coiled gland which is
intracellular in type. Itis largest
in the so-called ‘‘ nurses” which
feed the young, and diminishes
in size later. According to Mr.
Cheshire, this gland secretes a
nitrogenous fluid which is fur-
nished to all the larva in their
early stages, but is supplied to
the future queen during the
whole of the feeding period, and
also during the period of egg-
laying ; this secretion was form-
erly termed ‘‘royal jelly.” In
addition to this pair of glands,
there are in the worker three
other gland systems. Of these,
the second and third pairs have
a common central outlet on the
mentum, and secrete the saliva,
which is plentifully mixed with
the nectar during suction. The
fourth pair is small, and the
ducts open just within the mand-
Fic. 180.—Food canal of bee,—In ible. The last three pairs of
part after Cheshire. glands are found also in drone
and queen.
mx., Maxilla; @., antenna; ¢., eye; sg,
salivary glands; o¢., cesophagus; 4.5., ‘ re
honey-sac; s., stopper; c.s., chylifie | The method of feeding in
stomach ; 7.2, Malpighian tubules; s.2., the bee di :
small intestine; 42, large intestine; : e bee differs considerably
st., sting. in the three types. In the
; worker, the honey sucked
up from flowers is mixed with saliva, passes down the gullet
into the crop, thence by the opening of the “stomach
BRITISH HIVE-BEE. 337
mouth” it may reach the true stomach and so be digested,
or may be carried in the crop to the hive, and there
emptied into the cells by regurgitation. The pollen, which
is frequently mixed with the honey, is separated from the
latter by means of the stomach mouth, and is digested.
Before impregnation, the queen, like the worker, feeds on
pollen and honey; after it, she is always fed by the
Fic. 181.—Hive-bees and the cells in which they develop.
D., Drone cells; W., worker cells; Q., queen cell, open and closed ;
., drone; w., worker; g., queen.
attendant workers. The drones, like the young workers,
avail themselves of the general food-supply of the colony,
and do not themselves collect honey.
Other systems.—The respiratory system is represented
by the ramifying tracheal tubes. They open to the ex-
terior by the lateral spiracles, which can be completely
closed. In connection with the trachee there are large
air-sacs.
The circulatory system is in essentials the same as that
2?
338 PHYLUM ARTHROPODA.
of. the cockroach. The blood contains a few nucleated
amceboid corpuscles.
The excretory system consists of numerous fine Mal-
pighian tubules which open into the small intestine.
Reproductive system.—In the drone the reproductive
organs consist of a pair of testes, each furnished with a
narrow vas deferens, expanding at its distal end into a
seminal vesicle. The seminal vesicles open into the ejacu-
latory duct, and at their junction a large paired mucus gland
opens. When maturity is reached, the testes diminish in
size, while the spermatozoa accumulate in the terminal
expanded part of the ejaculatory duct, and there become
aggregated into a compact spermatophore. With the ter-
minal portion of the male duct copulatory organs are
associated.
Mating takes place only once in the life of the queen,
and is followed by the death of the drone.
In the queen the large ovaries occupy considerable space in the
abdominal region. As usual, each consists of numerous (100-150)
ovarian tubes, containing ova in various stages of maturity. The
ovarian tubes open into the right and left oviducts, which again unite
to form the common oviduct. With the anterior portion of the common
duct the globular spermatheca is associated. In connection with it
there is a gland corresponding to the mucus gland of the male. The
oviduct terminates in a copulatory pouch.
Previous to laying, the eggs are fertilised by sperms set free from
the spermatheca. In the case of drone eggs, this liberation of sper-
matozoa does not take place, and the eggs in consequence are partheno-
genetic. Queens which have never mated, or which have exhausted
their stock of male elements, habitually lay drone eggs, but those which
are laying abundant fertilised eggs at times also lay unfertilised eggs.
This withholding of spermatozoa is said to be ‘‘ voluntary,” and
related to the needs of the colony, but the physiological reason is
unknown,
The workers possess female organs similar in type to those of the
queen, but of an extremely rudimentary nature.
The eggs are laid singly in the cells of the comb, at the rate of
about two per minute, for weeks together. They are of the usual
insect type. According to the size of the cell in which it is deposited,
and the food with which it is furnished, the fertilised ovum develops
into a worker or into a queen. The development takes place within
the cell, and includes a complete metamorphosis.
GENERAL NOTES ON INSECTS. 339
CLASSIFICATION OF INSECTS
I. Primitive wingless insects, Apterygota or Aptera, including
Thysanura, e.g. Machilis, Campodea, Lepisma; Collembola,
Springtails, e.g. Podura, Smynthurus.
II. Winged insects, Pterygota (in some degenerate forms the wings
have been lost).
A. With mouth-parts usually adapted throughout life for biting
(Menognathous), with no metamorphosis (Ametabolic) or
with incomplete metamorphosis (Hemimetabolic).
e.g. Orthoptera (cockroach, locust, cricket, etc.) ;
Corrodentia (Termites, bird-lice); Odonata
(Dragon-flies) ; Ephemerida (May-flies); and
Dermaptera (Earwigs).
B. With mouth-parts adapted in the main as suctorial
organs (Menorhynchous), usually with no metamorphosis
(Ametabolic).
e.g. Rhynchota or Hemiptera, ¢.¢. Phylloxera, aphides,
coccus insects; Cicadas; bugs; water-scor-
pions, lice.
C. With complete metamorphosis (Holometabolic), with
mouth-parts always adapted for biting (Menognathous),
or adapted at first for biting and afterwards for sucking
(Metagnathous).
e.g. Coleoptera (beetles); Diptera (two-winged flies) ;
Lepidoptera (butterflies and moths); Hymen-
optera (ants, bees, and wasps).
GENERAL Notes ON INSECTS
The main characteristics of insects have already been
described in the two types chosen, but we here revise them
in general terms.
Form.—The body of an adult insect may be divided into
three distinct regions :—
1. The head, probably consisting of seven fused segments.
2. The median thorax, divided into pro-, meso-, and meta-thoracic
segments, each with » pair of legs, the last two often with
wings.
3. The abdomen, usually with ten to eleven segments, withnever
—inorethan.izansformed traces of appendages.
Within these limits there is great variety of form, e.g. the long
dragon-fly with its large outspread wings, the compact cockchafer, the
thin-waisted wasps and long-bodied butterflies, the house-fly and
cricket, the large moths and beetles, and the almost invisible insect
parasites,
340 PHYLUM ARTHROPODA.
Appendages.—Insects feel their way, test food, and
apparently communicate impressions to one another, by
means of the antenne. Then follow the mandibles, first
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Fic, 182.—Mouth-parts of mosquito.—After Nuttall and Shipley.
4., labium ; 7.Z., maxillary palps; cé., clypeus; cs., head scales.
a., Antennz ; Zre., labrum and epipharynx ; 7., mandibles ; 4., hypopharynx 3 #x., first maxillz ;
maxillz, and second maxille, on the head; the three pairs
of legs on the thorax; and sometimes vestiges of legs on
the abdomen.
GENERAL NOTES ON INSECTS. 341
It was a step of some importance in morphology when Savigny
showed that the three pairs of anpendages about the mouth are
homologous with the other appendages, z.¢. are masticatory legs.
(1) Farthest forward lie two mandzb/es, the biting and cutting jaws.
These are single-jointed, and thus differ from the organs of the same
name in the crayfish, which bear a three-jointed palp in addition to the
hard basal part. In those insects which suck and do not bite, e.g.
adult butterflies, the mandibles are reduced.
(2) Next in order is the first pazr of maxdllg. Each maxilla consists
of a basal piece (protopodite), an inner fork (endopodite), and an outer
fork (exopodite). The entomologists divide the protopodite into a
lower joint, the cavdo, and an upper, the s¢zZes ; the endopodite into an
internal Jacézza and an external galea; while the exopodite is called
the maxzllary palp.
(3) The last pair of oral appendages or second maxille are partially
fused, and form what is called the /adcum. The lower and upper
joints of their fused protopodites are called sadmentum and mentum ;
the endopodites on each side are double, as in the first maxille, and
consist of internal /acénza and external Zaraglossa ; the exopodites are
called the labzal palps.
The three pairs of thoracic legs consist of many joints, are usually
clawed and hairy at their tips, and differ greatly according to their uses,
as may be seen by comparing, for instance, the hairy feet by aid of
which the fly runs up the smooth window-pane, the muscular limbs of
grasshoppers, the lank length of those which characterise ‘‘ daddy-long-
legs,” the bees’ legs with their pollen baskets, the oars of water-beetles.
Wings.—These arise as flattened hollow sacs, which grow
out from the two posterior segments of the thorax. They are
moved by muscles, and traversed by “ veins” or ‘‘ nervures,”
which include air-tubes, nerves, and vessel-like continuations
of the body cavity. Most insects have two pairs, but many
sluggish females and parasites, like lice and fleas, have lost
them. On the other hand, there is no reason to believe
that the very simplest wingless insects, known as Collembola
and Thysanura, ever had wings.
There are many interesting differences in regard to wings in the
various orders of Insects. Thus in beetles the front pair form wing-
covers or elytra; in the little bee parasites—Strepsiptera—they are
twisted rudiments ; in flies the posterior pair are small knobbed stalks
(halteres or balancers); in bees the wings on each side are hooked
together. When the insect is at rest, the wings are usually folded neatly
on the back ; but dragon-flies and others keep them expanded ; butter-
flies raise them like a single sail on the back; moths keep them flat.
Many wings bear small scales or hairs, and are often brightly coloured.
It is well known that the colours also vary with sex, climate, and
surroundings. Most interesting are those cases in which the colours of
an insect harmonise exactly with those of its habitat, or make it a
mimetic copy of some more successfully protected neighbour.
342 PHYLUM ARTHROPODA.
As to the origin of wings, it may be mentioned that in many cases
they are of some use in respiration as well as in locomotion, and the
theory seems plausible that wings were originally respiratory outgrowths,
which by and by became useful for aerial locomotion. New organs
seem often to have arisen by the predominance of some new function
in organs which had some prior significance. Moreover, we can fancy
that an increase in respiratory efficiency brought about by the out-
growths in question would quicken the whole life, and would tend to
raise insects into the air, just as terrestrial insects can be made to frisk
and jump when placed in a vessel with relatively more oxygen than
there is in the atmosphere. Finally, we
must note that the aquatic larve of some
insects, e.g. may-flies, have a series of
respiratory outgrowths from the sides of
the abdomen, the so-called ‘‘ tracheal
gills,” which in origin and appearance are
like young wings (Fig. 183).
Insects excel in locomotion.
“They walk, run, and jump with
the quadrupeds ; they fly with the
birds ; they glide with the serpents,
and they swim with the fish.” They
beat the elastic air with their wings,
and though there cannot be so
much complexity of movement as
in birds where the individual
feathers move, the insect wing is
no rigid plate, and its up-and-down
/ x motions are complex. They can
Fic. 183.-Young may-fly soar rapidly, but their lightness
ercphemsnds ice Eaton. often makes horizontal steering
sno peatngahonta them difficult. The wind often helps
as well as hinders them; thus
the insects which fly in and out of the windows of
express trains are probably in part sucked along. Marey
calculates the approximate number of wing strokes per
second at 330 for the fly, 240 for the humble-bee, 190 for
the hive-bee, r10 for the wasp, 28 for the dragon-fly, 9 for a
butterfly. For short distances a bee can outfly a pigeon.
Skin.—As in other Arthropods, the epidermis (or hypo-
dermis) of Insects forms a firm cuticle of chitin, which in
the exigencies of growth has sometimes to be moulted.
This cuticle is often finely marked, so that the animal seems
iridescent ; and there are many different kinds of scales,
oe
GENERAL NOTES ON INSECTS. 343
hairs, and spines. Chitin is not favourable to the develop-
ment of skin glands. Most insects have “salivary glands”
opening in or near the mouth. Bees have wax-making
glands opening on the abdomen; aphides have glandular
tubes ; not a few have poison bags; and many larvz besides
silkworms have organs from which are exuded the threads
of which a cocoon is made.
Muscular system.—In very active animals like Insects,
we of course find a highly developed set of rapidly contract-
ing striped muscles. These work the wings, the legs, and
the jaws. The resulting movements have this further
significance, that they help in the respiratory interchange of
gases, and in the circulation of the blood.
Nervous system.—As in other Arthropods, the nervous
system consists—(a) of a dorsal brain or supra-cesophageal
ganglionic mass, and (4) of a double ventral nerve-cord with
a number of paired ganglia, of which the most anterior (the
sub-cesophageal) are linked to the brain by a ring com-
missure around the gullet; and (c) of nerves given off from
the various ganglia to the sense organs, the alimentary canal,
and the other organs. In many of the higher insects the
ganglia of the ventral nerve-card are in some degree con-
centrated, and in the adults are usually more centralised
than in the larvee.
Sensory structures.—Animals so much alive as Insects,
and in surroundings so stimulating as many of them enjoy,
have naturally highly-developed sense organs.
Two compound eyes are present on the head of all adults
except the primitive Collembola, the degenerate lice, the
likewise parasitic fleas, and blind insects which live in caves
or other dark places. Each eye contains a large number of
similar elements, in each of which we can distinguish—(1) a
cuticular or corneal facet; (2) a glassy lens-like portion ; (3)
a retinal portion in association with which are fibres from
the optic nerve; and there are also pigmented cells between
the elements.
In addition to the compound eyes, simple eyes or ocelli
are present in the adults of many insects, e.g. ants, bees,
and wasps; they occur without the accompaniment of com-
pound eyes in Collembola, lice, and fleas, and they are
usually the only eyes possessed by larve. They have only
344 PHLVYUM ARTHROPODA.
one lens (monomeniscous), whereas the compound forms
have many lenses (polymeniscous). In the simple eye each
retinal unit is a single cell, of which the distal part is unpig-
mented. In the compound eye the recinal unit consists
of six cells around an axis. The stricture of ocelli varies
greatly, and their use is very uncertain.
Auditory (or chordotonal) organs have been found in all orders of
Insects (except as yet the Thysanoptera), and occur both in the larve
and in the adults. Their essential structure is as follows :—A nerve ends
in a centre or ganglion near the skin; some of the cells of this ganglion
" grow out into long sensitive rods enclosed in a tiny sheath ; the rods are
directly or indirectly connected with the epidermis above them. ‘‘ They
are found in groups of 2-200 in various parts of the body, antennz,
palps, legs, wings, in the halteres of Diptera, and upon the dorsal aspect
oftheabdomen.” Quite different from these, and occurring in flies alone,
on the hind end of the larva, or at the base of the adult’s feelers, are
little bags with fluid in which clear globules float.
In addition to the ‘‘eyes” and ‘‘ears,” there are innervated hairs
(tactile, tasting, olfactory) on the antenne and mouth-parts of many
insects. Not a few insects seem to possess a diffuse or dermatoptic
sense, by which, for instance, they can, when blinded, find their way
out of a dark box.
Many Insects produce sounds. We hear the whirr of rapidly moving
wings in flies; the buzz of leaf-like structures near the openings of the
air-tubes in many Hymenoptera ; the scraping of legs against wing ribs
in grasshoppers; the chirping of male crickets, which rub one wing
against its neighbour; the piping of male Cicadas, which have a
complex musical instrument; the voice of the death’s-head moth,
which expels air forcibly from its mouth. The death-watch taps with
its head on wooden objects, as if knocking at the door behind which
his mate may be hidden. In some cases the sounds are simply auto-
matic reflexes of activity; in many cases they serve as alluring love
calls ; and they may also serve as expressions of fear and anger, or as
warning alarms.
In the case of hive-bees there is definite evidence of a sense of direc-
tion. They return straight to the hive from a distance of over a mile,
even when they have been blinded and robbed of antenne, even when
they have been carried afield in a box.
Alimentary system.—The diet of Insects is very varied.
Some, such as locusts, are vegetarian, and destroy our
crops; others are carnivorous (we need not specify the
homceopathist’s leech), and suck the blood of living victims,
or devour the dead; the bees flit in search of nectar from
flower to flower, while the ant-lion lurks in his pit of sand
for any unwary stumbler; the termites gnaw decaying
wood; some ants keep aphides as cows (‘“vacce formi-
carum,” Linnzeus called them), whose sweet juices they
GENERAL NOTES ON INSECTS. 345
lick; and a great number of larve devour the flesh and
vegetables in which they are hatched.
Many modifications of mouth organs, and of the ali-
mentary canal, are associated with the way in which the
insect feeds.
The alimentary canal consists of fore-gut, mid-gut, and
hind-gut, but in many cases it seems very doubtful if the
mid-gut has its typically endodermic character.
The fore-gut conducts food, and includes mouth cavity,
pharynx, and cesophagus, the latter being often swollen into
a storing crop, or continued into a muscular gizzard with
grinding plates of chitin.
The mid-gut is digestive and absorptive, often bearing a
number of glandular outgrowths or ceca, and varies in
length (in beetles at least) in inverse proportion to the
nutritive and digestible quality of the food.
The hind-gut is said to be partly absorptive, but is chiefly
a conducting intestine, often coiled and terminally expanded
into a rectum with which glands are frequently associated.
In association with the alimentary canal are various glands :—
(a) The salivary glands, which open in or near the mouth. They
are usually paired on each side, and provided with a
reservoir. They arise as invaginations of the ectoderm
near the mouth. Their secretion is mainly diastatic in
function, z.¢. it changes starchy material into sugar by
means of a ferment. Along with these may be ranked
the ‘spinning glands” of caterpillars, etc., which also
open at the mouth. They secrete material which hardens
into the threads used for the cocoon.
(6) From the beginning of the mid-gut blind outgrowths sometimes
arise (in some Orthoptera, etc.), which are apparently
digestive. They are sometimes called pyloric caca. In
other cases (some beetles) there may be more numerous and
smaller glandular outgrowths resembling villi in appearance.
Respiratory system.—The body of an insect is traversed
by a system of air-tubes (trachez), which open laterally by
special apertures (stigmata), and by means of numerous
branches conduct the air to all the recesses of the tissues.
In animals which breathe by gills or lungs the blood is
-carried to the air; in insects the air permeates the whole
body. But how does the air pass in and out? In part, no
doubt, there is a slow diffusion ; in part the movements of
the wings and legs will help; but there are also special
346 PHYLUM ARTHROPODA.
expiratory muscles. We see their action when we watch a
drone-fly panting on a flower. Inspiration is passive, as in
birds, and depends on the elasticity of the skin and of the
tracheal walls ; expiration is active, and depends upon these
muscles. They are chiefly situated in the abdomen, but in
some beetles (at least) they are also present in the metathorax.
The tracheze seem to arise as tubular ingrowths of skin,
and, primitively, each segment probably contained a distinct
pair ; but their number has been reduced, and they are often
in part connected into a system. With the doubtful excep-
tion of one of the primitive Collembola, and the certain
exception of caterpillars, no insects have any tracheal
openings in the head region. There are rarely more than
two pairs in the thorax; there are often six to eight pairs in
the abdomen ; the maximum total is ten pairs. Each trachea
is kept tense throughout the greater part of its course by
internal chitinous thickenings, which apparently have a
spirai course. The branches of the trachez penetrate
into all the organs of the body, carrying oxygen to every
part. The very efficient respiration of insects must be kept
in mind in an appreciation of the general activity of their life.
As the conditions of larval life are often different from those of the
adult insects, the mode of respiration may also differ in details.
In insects without marked metamorphosis, and even in some beetles
in which the metamorphosis is complete, the young insect and the adult
both breathe by tracheze with open stigmata. Both are said to be
‘“holopneustic.”
When the larvee live in water, the tracheal system is closed, other-
wise the creatures would drown. This closed condition is termed
‘“apneustic.” These larvee (of dragon-flies, may-flies, and some others)
breathe by ‘‘ tracheal gills” (see Fig. 183)—little wing-like outgrowths
from the sides of the abdomen, rich in trachese—or by tracheal folds
within the rectum, in and out of which water flows. In either case,
an interchange of gases between the tracheze and the water takes place.
In adult aerial life the trachez of the body acquire stigmata, and the
insect becomes ‘‘holopneustic.”
In most insects with complete metamorphosis, the larva (e.g. cater-
pillar or grub) has closed stigmata on the last two segments of the
thorax (those which will bear wings), but there is a pair of open
stigmata on the prothorax. In the adult the reverse is the case.
There are some other modifications—for instance, what obtains in the
parasitic larvee of some flies, e.g. gadflies. In these the stigmata are
open only at the end of the body. In all cases, however, the stigmata
of the adult are already present as rudiments in the larva, though they
may not open till adolescence is over.
CIRCULATORY SYSTEM. 34)
Circulatory system.—As the respiratory system is very
efficient, air passing into the inmost recesses of the body,
it is natural that the blood-vascular system should not be
highly developed. Within a dorsal part of the body cavity,
known as the pericardium, the heart lies, swayed by special
muscles. It is a long tube, usually confined to the abdomen,
and with eight chambers, with paired valvular openings on
its sides, through which blood enters from the pericardium.
The blood is driven forwards, the posterior end of the heart
being closed, and there is usually an anterior aorta or main
blood vessel. But, for the most part, the blood circulates
in spaces within what is commonly called the body cavity.
Such a circulation is often described as lacunar. The blood
may be colourless, yellow, red, or even greenish, and, in
some cases, hemoglobin, the characteristic blood pigment of
Vertebrates, has been detected. The cells of the blood are
amoeboid.
Body cavity.—It is necessary to distinguish the primitive ccelom
from the apparent body cavity of the adult. In discussing the develop-
ment of Peripatus, Sedgwick notes the following characteristics of a
true coelom :—It isa cavity which—(1) does not communicate with the
vascular system ; (2)does communicate by nephridial pores with the
exterior ; (3) has the reproductive elements developed on its lining;
(4) develops either as one or more diverticula from the primitive
enteron (or gut), or as a space or spaces in the unsegmented or
segmented mesoderm. Now, in Arthropods the apparent body cavity
of the adult is not a true ccelom: it consists of a set of secondarily
derived vascular spaces ; it has been called a pseudoccel or a heemoccel.
The true ccelom of Arthropods is very much restricted in the adult.
The apparent body cavity in which the organs lie, and in which
the blood circulates, is well developed in Insects. It includes, zter
alia, a peculiar fatty tissue, which seems to be a store of reserve
material, which is especially large in young insects before metamorphosis,
and is also interesting as one of the seats of ‘‘ phosphorescence.”
Excretory system.—Although no structures certainly
homologous with nephridia have yet been demonstrated in
Insects, the excretory system is well developed. From the
hind-gut (proctodzeum), and therefore of ectodermic origin,
arise fine tubes, or in some cases solid threads, which extend
into the apparent body cavity. Their number varies from
two (in some Lepidoptera, for instance) to one hundred and
fifty (in the bee). They twine about the organs in the
abdominal cavity, and their excretory significance is inferred
from the fact that they contain uric acid.
g
348 PHYLUM ARTHROPODA.
Reproductive system.—Among Insects the sexes are
always separate and often different in appearance. The
males are more active, smaller, and more brightly coloured
than the females. Darwin referred the greater decorative-
ness of the males to the sexual selection exercised by the
females. The handsomer variations succeeded in courtship
better than their rivals. Wallace referred the greater plain-
ness of females to the elimination of the disadvantageously
conspicuous in the course of natural selection. There may
be truth in both views, but both require to be supplemented
by the consideration, in part accepted by Wallace, that the
“secondary sexual characters” of both sexes are the natural
and necessary expressions of their respectively dominant
constitutions.
The organs consist of :—
MALE.
FEMALE.
The paired testes, usually formed
of many small tubes.
Two ducts (vasa deferentia) con-
ducting spermatozoa (perhaps
in part comparab’e to neph-
ridia).
An unpaired terminal and ejacula-
tory duct, paired and with two
apertures in Ephemeridsonly ;
sometimes formed by aunion of
the vasa deferentia, sometimes
by an external invagination
meeting the vasa deferentia.
From the vasa deferentia or from
the ejaculatory duct, opens a
paired or unpaired seminal
vesicle for spermatozoa,
Various accessory glands, whose
secretion sometimes unites the
spermatozoa into packets or
spermatophores.
Sometimes a copulatory penis.
Often external hard pieces.
The paired ovaries, usually formed
ofmanysmall tubes(ovarioles).
Two ducts (oviducts) conducting
the ova (perhaps in part com-
parable to nephridia).
An unpaired terminal region or
vagina, paired and with two
apertures in Ephemerids;
usually formed from an ex-
ternal invagination meeting
the united ends ofthe oviducts.
Near or from the vagina, opens
a receptaculum seminis for
storing spermatozoa received
froma male during copulation.
Various accessory glands, ¢.g. those
which secrete the material sur-
rounding the eggs.
Sometimesa special bursacopula-
trix in the vagina.
Often external hard pieces, e.g.,
ovipositor.
SOME PECULIARITIES IN REPRODUCTION. 349
Some peculiarities in reproduction.—Many Insects, such as
aphides, silk-moth, and queen-bee, are exceedingly prolific. The
queen termite lays thousands of eggs, ‘‘at the rate of about sixty per
minute” !
The store of spermatozoa received by the female, and kept within
the receptaculum seminis, often lasts for a long time,—for two or three
years in some queen-bees,
Parthenogenesis, or the development of ova which are unfertilised,
occurs normally, for a variable number of generations, in two Lepidop-
tera and one beetle, in some coccus insects and aphides, and in certain
saw-flies and gall-wasps. It occurs casually in the silk-moth and several
other Lepidoptera, seasonally in aphides, in larval life in some flies
(Afiastor, Chironomus), and partially or ‘‘ voluntarily ” when the queen-
bee lays eggs which become drones.
A few insects hatch their young within the body, or are “‘ viviparous.”
This is the case with parthenogenetic summer aphides, a few flies, the
little bee parasites Strepsiptera, a few beetles and cockroaches,
Development of the ovum.—The tubes which compose
the ovaries and lead into the oviducts begin as thin fila-
ments, the ends of which are usually connected on each
side. These thin filaments consist of indifferent germinal
cells, all of them potential ova, and of mesodermic epithelial
cells, which form the ovarian tubes, etc., and are connected
anteriorly to the pericardial wall.
But in most cases only a minority of these cells be-
come ova, the others become nutritive cells which are
absorbed by the ova, and follicle cells which line the
walls of the ovarian tubes and help to furnish the egg-
shells.
There may be, indeed, ovarian tubes without nutritive
cells (e.g. in Orthoptera), and then each tube is simply a
bead-like row of ova, which become larger and larger
as they recede from the thin terminal filaments and ap-
proach the oviducts. In other cases the bead-like row
consists of ova alternating with clumps of nutritive cells
(e.g. in Hymenoptera and Lepidoptera). In other cases
the nutritive cells mostly remain in the terminal region,
but their products pass down to the receding ova.
As there are numerous ovarian tubes in each ovary,
and as the same process of oogenesis is going on in each,
numerous eggs are ready for liberation at the same time,
and are simultaneously discharged into the oviduct of each
side.
The eggs are large and contain much yolk. In relatively
350 PHYLUM ARTHROPODA,
few cases yolk is almost absent, as, for example, in the
summer eggs of the Aphides, which are hatched within the
body, and in some forms where the young are endoparasitic.
The ovum is surrounded by a vitelline membrane, and also
by a firm chitinous shell, secreted by the follicular cells,
which is often sculptured in a characteristic manner. ‘This
shell is pierced by one or more minute holes (micropyles).
Through a micropyle the spermatozoon finds entrance,
Fic. 184. Diagrams of Insect embryo.—After Korschelt and Heider.
A transverse section before the union of the amnion folds, and a
longitudinal median section after the union of the folds. a.,
Anterior end of blastoderm; Z., posterior end of blastoderm;
af., in the left-hand figure, the beginning of the amnion fold ;
am., amnion; @.c., amniotic cavity; s., serosa; ¢c., ectoderm;
2., lower germinal layer; y., yolk. The amniotic cavity marks
me ae ventral region of the embryo, so that the yolk mass
lorsal,
sometimes (as in the cockroach) after moving round and
round the shell in varying orbits.
The ripe egg usually consists of a central yolk-containing mass, sur-
rounded by a thin sheath of protoplasm. As is usual in Arthropods,
the segmentation is peripheral or centrolecithal. The central nucleus
divides up into several nuclei, which, being united by protoplasmic
cords, form fora time a central syncytium. Later, these nuclei emigrate
into the peripheral protoplasm, which segments around them ; thus.a
peripheral layer of similar epithelial cells is formed. Some of the nuclei
DEVELOPMENT OF THE OVUM. 351
may be left behind in the central yolk to form the yolk nuclei, or, what
is probably the more primitive condition, these are formed by subse-
quent immigration from the blastoderm.
The next process is the appearance of differentiation among the similar
cells of the blastoderm. Over a special area—the ventral plate—(cf.
Astacus) the cells increase in number and become cylindrical in shape ;
over the rest of the egg the cells flatten out and become much thinner.
In the middle of the ventral plate a slight groove is formed by rapid
multiplication of the cylindrical cells. This represents the disguised
gastrulation, the open roof of the groove being the much-elongated
blastopore. The surrounding cylindrical cells unite over this open roof,
the groove usually flattens out, and thus we have formed a two-layered
germinal streak which spreads forwards and backwards over the egg,
and early exhibits externally transverse division into segments. The
upper layer is the ectoderm ; the lower includes the rudiments of both
mesoderm and endoderm.
Meanwhile another very important event has taken place. We saw
that while the cells of the ventral plate increased in depth, the remain-
ing cells flattened out laterally; at the point where the two kinds of
cells unite, on either side of the ventral plate, a double fold arises. The
two folds unite over the surface of the ventral plate, forming a mem-
branous arch over it. The internal fold is called ‘‘ amniotic,” the
outer ‘‘serous,” from their resemblance to the similar envelopes in the
embryos of higher vertebrates. The folds take no direct part in the
development of the embryo.
We must now return to the germinal streak. The gastrula groove
may persist as a tube after closure of the blastopore, but it is usually
compressed by the ectoderm, or never exists as a distinct cavity. The
greater part of the lower stratum of the germinal streak consists of
mesoderm. This becomes divided into successive segments at each
side, each containing a primitive coelomic cavity, perhaps continuous
with the gastrula cavity. The endoderm arises as paired clusters of
cells, found only at the anterior and posterior ends of the primitive
streak. These clusters increase rapidly and form long endodermal
streaks, which curve downwards so as to enclose the yolk. The streaks
meet and fuse, first ventrally and later dorsally, thus constituting the
mid-gut. The yolk nuclei previously mentioned have meanwhile
increased rapidly, forming yolk cells which absorb the yolk. These
cells are included in the endodermic mid-gut, and there break up. As
the endoderm grows round the yolk, it is accompanied by a layer
(splanchnic) of the mesoblast. Fore- and hind- gut are formed by
invaginations which fuse with the mid-gut.
In the later stages of development the primitive coelomic pouches
lose their cross partitions, become filled with mesenchyme cells, and
practically obliterated. The body cavity of the adult is formed by the
appearance of lacunze amid the cells of the mesenchyme.
The trachez arise as segmentally repeated invaginations of the ecto-
derm. The openings of the invaginations form the stigmata. From
the hind-gut arise the Malpighian tubules, which are therefore ecto-
dermic. The development of the other organs is similar to that of
the Crustacea.
352 PHYLUM ARTHROPODA.
In summarising the development of Insecta, one must
specially note the peripheral segmentation, the formation of
the two-layered germinal streak, the presence of an over-
arching blastodermic fold, the segmentation of the meso-
derm, and the formation of the mid-gut by the union of
endodermic bands. :
Metamorphosis of Insects.—(1) In the lowest Insects,
namely, in the old-fashioned wingless Thysanura and
Collembola, the hatched young are miniatures of the adults.
By gradual growth, and after several moultings, they attain
adult size.
Similarly, the newly hatched earwigs, young of cock-
roaches and locusts, of lice, aphides, termites, and bugs, are
very like the parents, except that they are sexually immature,
and that there are no wings, which indeed are absent from
some of the adults.
These insects are called ametabolic, te. they have no
marked change or metamorphosis.
(2) In cicadas there are slight but most instructive
differences between larve and adults. The adults live
among herbage, the young on the ground, and the diversity
of habit has associated differences of structure, as in
the burrowing fore-legs of the larva. Moreover, the larva
acquires the characters of an adult after a quiescent period
of pupation.
The differences between larva and adult are more striking
in may-flies, dragon-flies, and the related Plecoptera (eg.
ferla), for in these the larva are aquatic, with closed
respiratory apertures, and with tracheal gills or folds, while
the adults are winged and aerial, and breathe by open
tracheze.
These insects are called Lemimetabolic, z.e. they have a
partial or incomplete metamorphosis.
(3) Very different is the life history of all other sets of
Insects—ant-lions, caddis-flies, flies, fleas, butterflies and
moths, beetles, ants, and bees. From the egg there is
hatched a larva (maggot, grub, or caterpillar), which lives a
life very different from the adult, and is altogether unlike
it in form. The larva feeds voraciously, grows, rests, and
moults. Having accumulated a rich store of reserve
material in its “fatty body,” it finally becomes for some
Fic. 185.—Life histories of Insects.
L., P., and A., larva, pupa, and adult respectively of water-beetle (Dytiscus
marginalis); 1, p., @., larva,‘pupa, and adult of blue-bottle fly (Musca
vomitoria); 1.1, 2.1, 4.1, larva, pupa, and adult of Cossus ligniperda,
23
384 PHYLUM ARTHROPODA.
time quiescent, as a pupa, nymph, or chrysalis, often within
the shelter of a cocoon. During this period there are great
transformations ; wings bud out, appendages of the adult
pattern are formed, reconstruction of other organs is
effected. Finally, out of the pupal husk emerges a
miniature winged insect of the adult or imago type.
These insects are called holometabolic, i.e. they exhibit a
complete metamorphosis.
Two kinds of larvee occur among insects. (a) In many
ametabolic and hemimetabolic forms the larva is somewhat
like one of the lowly Thysanuran insects (Campodea), and is
A 6
Fic. 186.—Life history of the silk-moth (Bombyx mor).
A, caterpillar ; B, pupa; C, imago; the cocoon is cut open to
show the pupa lying within. In the caterpillar note the three
pairs of true legs in the anterior region, and the four pairs
of pro-legs in the posterior region.
therefore called campodeiform. It has the regions of the
body well defined, three pairs of locomotor thoracic limbs,
and mouth-parts adapted for suction. (4) The other type
is worm-like or eruciform, ¢.g. the caterpillars of Lepidoptera
(Fig. 186, A), with distinct head and limbs; the more modified
grubs of bees, etc., with distinct head, but without limbs ;
and the degenerate maggots of flies (Fig. 187, A), etc., not
only limbless, but with an ill-defined head. A typical cater-
pillar has a cylindrical body often “hairy,” a distinct hard
head, simple eyes, small antennze, mouth-parts suited for
biting, three pairs of jointed clawed thoracic limbs (corre-
METAMORPHOSIS OF INSECTS. 355
sponding to those of the butterfly), and four ot five pairs of
unjointed clasping abdominal “ pro-legs.” Other abdominal
appendages are known on the larve of other insects, and
even in the embryos of some whose larve are campodei-
form. These facts make it likely that the primitive form
had many legs.
The larvee of Insects vary enormously in habit and in structure, and
exhibit numerous adaptations to conditions of life very different from
‘hose of the parent. Thus caterpillars, which are usually plump and
Fic. 187.—Development of blow-fly (Calliphora erythrocephala),
—After Thompson Lowne.
The lower figure (A) shows the adult larva (maggot). Note, as
compared with the caterpillar, the absence of appendages,
except those about the mouth; %., the large hooks connected
with the maxille ; J/., pro-legs.
The upper figure (B) shows the pronymph removed from the pupa-
case. In the abdominal region the imaginal discs are shown ;
2., rudiments of legs; w., of wings.
tense, so that a peck from a bird’s bill may cause them to bleed to death,
even if no immediate destruction befall them, are protectively adapted
in many different ways. Their colours are often changed in harmony
with those of their surroundings; some palatable forms are saved by
their superficial resemblance to those which are nauseous ; a few strike
“ terrifying attitudes” ; while others are like pieces of plants.
Internal metamorphosis.—In Insects with no marked
metamorphosis, or with merely an incomplete one, the
organs of the larve develop gradually into those of the
adult. But in Insects with complete metamorphosis there
356 PHYLUM ARTHROPODA.
is a marvellous internal reconstruction during the later
larval, and especially during the quiescent pupal stage.
The more specialised larval organs are disrupted, their
débris being used in building new structures. In some
cases, such as flies, phagocytes play a very important part
in this metamorphosis; in other cases there is no true
phagocytosis. Parts of larval organs which have not been
highly specialised form the foundations of new adult
structures. Of special importance are certain ingrowths
of the. larval skin (the epi- or hypo- dermis) which form
what are called “imaginal discs,” ze. embryonic or
germinal areas, from which arise the wings, legs, etc., of
the adult insect. The reconstruction is very thorough ;
most of the musculature, much of the tracheal system, part
of the mid-gut, etc., are gradually replaced by the corre-
sponding organs of the adult. There is first a disruptive
process of histolysis, and then a reconstructive process of
histogenesis. Yet in most cases the disruption and
replacement of organs is very gradual.
CEcology.—The average insect is active, but between
orders (¢.g. ants, bees, and wasps versus aphides, coccus
insects, and bugs), between nearly related families, between
the sexes (e.g. male and female cochineal insect), between
caterpillar and pupa, we read the constantly recurrent
antithesis between activity and passivity.
The average length of life is short. Queen-bees of five
years, queen-ants aged thirteen, are rare exceptions. In
many cases death follows as the rapid nemesis of repro-
duction. But though the adult life is often very short,
the total life may be of considerable length, as in some
Ephemerids, which in their adult life of winged love-making
may be literally the flies of a day, while their aquatic larval
stages may have lived for two years or more.
The relation between the annual appearance of certain
insects and that of the plants which they visit, the habits
of hibernation in the adult or larval state, the occasional
“dimorphism” between winter and summer broods of
butterflies, should be noticed.
The prolific multiplication of many insects may lead to
local and periodic increase in their numbers, but great
increase is limited by the food-supply and the weather, by
GCOLOGY, 357
the warfare between insects of different kinds, by the
numerous insects parasitic on others, by the appetite of
higher animals,—fishes, frogs, ant-eaters, insectivores, and,
above all, birds.
There is a great variety of protective adaptation. The
young of caddis-flies are partially masked by their external
cases of pebbles and fragments of stem; many caterpillars
and adult insects harmonise with the colour of their environ-
ment; leaf-insects, ‘walking sticks,” moss-insects, scale-
insects, have a precise resemblance to external objects
which must often save them; a humming-bird moth may
resemble a humming-bird; many palatable insects and
larvee have a mimetic resemblance to others which are
nauseous or otherwise little likely to be meddled with.
Many insects may be saved by their hard chitinous armour,
by their disgusting odour or taste, by their deterrent
discharges of repulsive formic acid, etc., by simulation of
death, by active resistance with effective weapons.
Many flowers depend for cross-fertilisation upon insects,
which carry the pollen from one to another. Many insects
depend for food on the nectar and pollen of flowers. Thus
many flowers and insects are mutually dependent. But
many insects injure plants, and many plants exhibit
structures which tend to save them from attack. On the
other hand, there may be “partnerships” between insects
and ‘plants—as in the “myrmecophilous” (ant - loving)
plants, which shelter a bodyguard of ants, by whom they
are saved from unwelcome visitors. And again, the
formation of galls by some insects which lay their’ eggs
in plants, and the insect-catching proclivities of some
carnivorous plants, should be remembered.
Most insects are terrestrial and aerial; the majority live
in warm and temperate countries, but they are represented
almost everywhere, even above the snow-line, in arctic
regions, in caves. Even on the sea the Challenger
explorers found the pelagic Halobates, a genus of bugs.
The distribution of insects is mainly limited by food-
supplies and climate, for their powers of flight are often
great, and their opportunities of passive dispersal by the
wind, floating logs, etc., are by no means slight.
Many insects are more or less parasitic, either externally
358 PHYLUM ARTHROPODA.
as adults, eg. fleas, lice, bird-lice, plant-lice, etc., or in-
ternally as larve, eg. the maggots of bot-flies in sheep,
and a great number of borers within plants.
We need only mention Hessian-fly, phylloxera, Colorado
beetle, weevils, locusts, to suggest many more which are of
much economic importance as injurious insects. On the
other hand, our indebtedness to hive-bee and silk-moth, to
cochineal and lac insects, to those which destroy injurious
insects, and to those which carry pollen from flower to
flower, is obvious.
Finally, we must at least mention that in ants, —
Ly
ra iy Vi Tos
id )
a
Fic. 188.—Mosquito,—After Nuttall and Shipley.
wasps, and termites we find illustration of various grades of
social life, and marvellous exhibitions of instinctive skill as
well as some intelligence.
INSECTS AND DISEASE
As carriers of disease-germs insects play a very im-
portant part. The réle of flies as mechanical distributors
of anthrax, plague, and other bacterial diseases has been
clearly proved. Besides carrying bacilli upon their bodies
and leaving them on wounds or food, they also swallow
germs, and subsequently deposit them in their excreta in
similar situations. Undoubtedly, however, the most serious
cases are those of the blood-sucking Diptera which act as
PEDIGREE, 359
hosts as well as carriers of disease-producing parasites.
The gnats or mosquitoes (Culicide) are perhaps the most
important in this respect. Human malaria is conveyed by
at least twelve different species of mosquito, of which those
belonging to the genus Axopheles have the widest dis-
tribution. Anopheles maculipennis occurs all over Europe,
in many parts of Africa, North America, and India,
and in all these countries it carries malaria (see Fig. 182).
Proteosoma, the malaria parasite of birds, is carried by a
Culex, a related genus. The unknown parasite of yellow
fever is transmitted by the bite of another mosquito,
Stegomyia fasciata, It occurs in all parts of the world
between the parallels 40° N. and S. “It is a most vicious
biter both by day and night, and breeds in small artificial
collections of water, such as barrels, puddles, cisterns, and
even in such small receptacles as sardine tins” (Theobald).
Culex fatigans' and C. pipiens act as carriers of Filaria
bancrofti or F. sanguinis hominis nocturna, the parasite of
the human disease filariasis. The African Tsetse flies,
Glossina palpalis (Fig. 53) and G. morsitans, convey the
parasites (Trypanosomes) of sleeping sickness and
Nagana respectively. The latter disease, which is com-
municable to horses, cattle, goats, sheep, and other
domesticated animals, is probably also
conveyed by other species of Tsetse
flies. In general, one may say that
wild animals, which appear to be un-
affected by the parasites which they
contain, are the source of the fatal
infection of new-comers.
PEDIGREE
Insects must have appeared relatively
early, for remains of a cockroach-like
form have been found even in Silurian
strata. The higher forms with complete Fic. 189.—- Anurida
metamorphosis appear much later (e.g. i date ga oe
beetles in the Carboniferous ages); but it primitive wingless
seems that the Palaeozoic insects were Collembola.
360
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ORDERS OF INSECTS.
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362 PHYLUM ARTHROPODA.
mostly generalised types, prophetic of rather than referable
to the modern orders.
As to the pedigree of insects, the wingless Collembola
and Thysanura are doubtless primitive. In /votopteron,
for instance, there are appendages on the first four seg-
ments of the abdomen, and the genital apertures are paired.
Similarly, Aceventomon is a little blind creature, without
antenne, without cerci, without stigmata, with suctorial
mouth-parts, with eleven abdominal segments, with a
peculiar anal segment, with an unpaired genital aperture
Fic. 190.—Acerentomon, a very primitive insect.
H, Head; TH.1, TH.?, TH.%, terga of thoracic segments ; 7, 2, 7, the
thoracic legs; 4.1, 4.2, 4.3, A.4, abdominal appendages; P.A.,
eighth abdominal tergum ; G., genital aperture ; AP.P., post-anal
appendix,
on the eleventh urosternite. For Acerentomon, Acerentulus,
and Zosentomon (with stigmata) the special order Protura
has been proposed. These and similar primitive forms
lead us back to some of the less specialised Myriopods
(e.g. Scolopendrella), back further to the level represented
by Leripatus, which helps to link the Tracheate to the
Annelid series.
But though the primitive wingless insects, the simple
types of Myriopods, and /erpatus, represent ascending
steps in evolution, what the actual path has been we do
not know.
CHAPTER XV
PHYLUM ARTHROPODA—(continued)
Classes ARACHNOIDEA (Spiders, Scorpions, Mites, etc.)
and PaL#ostTraca (King-crabs, Eurypterids, Trilobites)
Tue class Arachnoidea is far from being a coherent unity.
Its subdivisions are numerous and diverse, and a statement
of general characters is consequently difficult.
The anterior segments, about seven in number, are usually
Jused into a cephalothorax, with six pairs of appendages.
The most anterior of these 5 sible may be turned in front-
of the mouth, but there are al antenne as in [nsects.
The first two pairs of appendages (cheliceree and_pedipalps)
generally have to do with seizing and holding the food ; the
pthers are walking legs. But although six pairs occur in
most, there may be more or less.
but-not. always, without..appendages ; it may be segmented or
unsegmented ; tt is generally distinct from, but may be fused
to the cephalothorax. A plate-like internal skeleton, called
the endosternite, ts often present. The elaborate compound
eyes of Insects are notrepuescuted, the-eyes-being almost always
spixation may be by tubular trachea, or..by lung:
books (chambered trachee?), or by both, or cutaneous, and
many would include the branchiate Paleostraca along with
Arachnoidea. In the tracheate forms there are never more
than four pairs of stigmata. Within all or some of the legs
lie coxtaluglands,. perhaps comparable to nephridia. An
elongated dorsal heart usually lies in the abdomen. The
position of the genital aperture or apertures is usually on one
of the anterior abdominal segments, All have, separate sexes,
In most cases the newly hatched young are essentially like the
adults—that 1s to say, there is no metamorphosis,
364 PHYLUM ARTHROPODA.
Order 1. SCORPIONIDE
Scorpions are elongated Arachnoids, restricted to warm
countries, lurking under stones or in holes during the day,
but active at night. The Scorpio afer of the East Indies
attains a length of 6 inches, but most are much smaller.
They feed on insects, spiders, and other smail animals.
‘The “tail,” with the venomous sting at its tip, is usually
curved over the anterior part
of the body, and can reach
forward to kill the prey caught
by the anterior appendages, or
can be suddenly straightened
to strike backwards. When
man is-stung, the poison seems
to act chiefly on the red blood
corpuscles, and, though never
or very rarely fatal, may cause
much pain. It has been said
that scorpions commit suicide
when surrounded by fire or
otherwise fatally threatened,
but it has been answered that
they do not sting themselves,
that they could not if they
would, and that, even if they
could, the poison would have
Fic. 191.—Scorpion. no effect!
ch. Chelicere; Af. pedipalps; 0, The body is divided into—
genital operculum; /., pectines;
Ss. stigma of a lung-book on the (1) a cephalothorax or “ pro-
seinen ey See AP BS soma’ of six gepments, Whose
terga fuse into a carapace,
and (2) an abdomen, which includes a broad seven-
segmented ‘“‘mesosoma,” and a narrow five-segmented
“metasoma.” At the end of the latter there is a post-anal
curved spine or “telson,” containing a paired, compressible
poison gland opening at the sharp tip. There is a strong
cuticle of chitin, and also an interesting internal piece of
skeleton (the endosternite), partly chitinoid, but also
SCORPIONS. 365
resembling fibro-cartilage, which lies in the cephalothorax
above the nerve-cord, and serves for the insertion of
muscles,
The appendages are—
1. Small, three-jointed, chelate chelicerze or falces just above the
mouth, used in holding prey.
2. Large, six-jointed, chelate pedipalps. These seize the prey;
their basal joints help in mastication, and in some cases they produce
rasping sounds.
3-6. Four pairs of seven-jointed, non-chelate walking legs. The
basal joints of the first two pairs help in connection with the mouth.
Apparently equivalent to a first pair of abdominal appendages is a
small notched plate or operculum which covers or bears the genital
aperture or apertures. — ;
Apparently of the nature of appendages are the comb-like, probably
tactile, pectines on the second abdominal segment.
Six other pairs of abdominal appendages are present in the embryo,
but they abort.
The nervous system consists of a dorsal brain, a ring round the
gullet, and a ventral nerve-cord. The eyes are innervated from the
brain, the first six appendages from the collar and the sub-cesophageal
ganglion. Behind the latter there are seven ventral ganglia in the
eleventh to seventeenth segments inclusive. There are in scorpions
two to six pairs of eyes placed on the carapace. The lateral eyes are
simpler than the median pair, and the type is in some ways inter-
mediate between the simple eye and the compound eye. There is, as
in ocelli, a single crystalline-lens-like portion, below which there are
a few groups of retinal cells. Each group has five cells, and the outer
part of each cell is pigmented. There is no crystalline cone.
Scorpions seize small animals with their pedipalps, hold them close
to the small mouth by their chelicerze, sting them if need be, and suck
their blood and juices. The pharynx serves as a suction pump; a
narrow gullet leads to a slight enlargement, into which a pair of
salivary glands open; from the narrow mid-gut several large digestive
outgrowths arise; the hind-gut ends in a ventral anus beneath the
base of the sting. The narrowness of the gut may be associated with
the fluid nature of the food. The so-called Malpighian tubes of Bzthzs
europeus are really the ducts of the liver.
The cavity of the body is for the most part filled up with organs,
muscles, and connective tissue. A pair of coxal glands, perhaps
excretory and nephridial, but apparently closed in the adult, lie near
the base of the third pair of walking legs. It is stated that in the
embryo they open into the body cavity by internal funnels.
The blood contains amceboid corpuscles and the respiratory pigment
hemocyanin. An eight-chambered heart, within «a pericardium, lies
along the back of the mesosoma. It gives off lateral arteries from the
posterior end of each of its chambers, is continued backwards in a
posterior aorta, and forwards in an anterior aorta. The latter supplies
the head and divides into two branches, encircling the gullet and
366 PHYLUM ARTHROPODA.
reuniting in a ventral artery above the nerve-cord. From capillaries
the blood is gathered into a ventral venous sinus, is purified in the
lung-books, and thence returns by veins to the pericardium, finding
its way by valved lateral openings (ostia) into the anterior end of each
heart-chamber.
On the ninth to twelfth segments lie slit-like stigmata, the openings
of four pairs of lung-books. Each lung-book is like a little purse with
numerous (over a hundred) compartments. Air fills the much-divided
cavity, and blood circulates in the lamellz or partitions.
The testes consist of two pairs of longitudinal tubes, united by cross
bridges ; the vas deferens, with a terminal copulatory modification,
Opens under the operculum on the first abdominal segment. The
ovary consists of three longitudinal tubes, united by cross ducts, and
two oviducts open on the under surface of the operculum.
Fertilisation is internal; the ova begin their development in the
ovary, and complete it in the oviduct. The segmentation is discoidal,
the ova are hatched within the mother. The young, thus born ‘ vivi-
parously,” are like miniatures of the adults, and adhere for some time’
after birth to the body of the mother. ,
In Euscorpio etalicus there is abundant yolk in the ovum ; in Scorpio
there is little; but the embryo of the latter seems to eat the terminal
part of the ovarian tube in which it develops. In the embryo of
Opisthophthalmus there are peculiar horn-like outgrowths, possibly
absorptive in function.
The race of scorpions is of very ancient origin, for one.
has been found in Silurian strata, and others nearly resem-
bling those now alive are found in the Carboniferous.
In many ways, eg. in their appendages, endosternite,
and coxal glands, the scorpions link the Arachnoids to the
King-crabs, and thus to the Trilobites.
Order 2. PSEUDOSCORPIONID&. ‘‘ Book-Scorpions,” eg.
Chelifer, Chernes
Minute animals, most abundant in warm climates, under bark, in
books, under the wing-covers of insects, etc. They are like miniature
scorpions, but without the long tail and sting. Their food probably '
consists of the juices of insects. There is a cephalothorax with six .
pairs of appendages ; the chelicerze are minute and chelate, with |
openings of spinning glands, the pedipalps like those of scorpions.
The abdomen is broad, with ten to eleven segments. They breathe |
by tubular trachez.
Order 3. PEDIPALPI. ‘‘ Whip-Scorpions,” e.g. Zhelyphonus, Phrynus
Small animals, found in warm countries. There is a cephalothorax
with six pairs of appendages ; the abdomen is depressed, well-defined
SPIDERS. 367
from the thorax, and has eleven to twelve segments. The chelicerz
are simply clawed, but are poisonous ; the pedipalps are simply clawed
or else truly chelate. The first pair of limbs are like antenne.
Respiration is by two pairs of abdominal lung-sacs. In Thelyphonus
there is a long terminal whip.
Order 4. PHALANGID (or OPILIONINA). ‘‘ Harvest-men,” e.g.
Phalangium
The small, spider-like ‘‘harvest-men” are noted for their extremely
long legs, by which they stalk slowly along, avoiding the glare of day.
The broad six-segmented abdomen is not constricted off from the
unsegmented cephalothorax ; the chelicerze are chelate ; the pedipalps
are like legs. Respiration is by tubular trachee. The harvest-men
do not trouble us, but feed on small insects.
Order 5. SOLPUGID or SOLIFUGA, ¢g. Galeodes or Solpuga
Active, pugnacious, venomous, nocturnal animals, found in the
wariner parts of the earth, The head and abdomen are distinct from
the thorax. The thorax has three segments, the abdomen nine or ten.
The chelicetz are large and chelate, the pedipalps like long‘legs. The
respiration is by means of tubular tracheze. The presence of distinct
segments on the thorax is remarkable.
Several other small orders of Arachnids must be recognised, e.g.
Palpigradi for a very interesting minute form, Kezenza, with the last
two joints of the cephalothorax free, and with an abdomen of eleven
segments ending in a long-jointed whip.
Order 6. ARANEID. Spiders
Spiders are found almost everywhere upon the earth,
and a few are at home in fresh water, eg. Avgyronefa, and
on the seashore, eg. Désis, Desidiopsis. Most of them
live on the juices of insects, and many form webs in which
their victims are snared. They may be divided, accord-
ing to habit, into the wanderers who spin little, and the
sedentary forms who spin much. -
The body of a spider is very distinctly divided into two
parts: the cephalothorax and the abdomen, connected by
a narrow waist. The chitinous cuticle varies in hardness, ,
hairiness, and colouring; it has, as usual, to be moulted
as the spider grows. Thus the young garden spider moults.
eight times in its first year.
There are six pairs of appendages—
1. The two-jointed chelicerze or falces, whose terminal joint or fang
368 PHYLUM ARTHROPODA.
bends down on the basal joint in ‘‘sub-chelate” fashion, and is per-
forated by the duct of a poison gland.
2. The leg-like, usually six-jointed, non-chelate pedipalps, whose
basal joint helps in mastication, while the terminal joint in the male
expands as a reservoir for the spermatozoa and serves as a copulatory
organ.
ae Four pairs of terminally clawed 7-jointed walking legs. The
most anterior pair are much used as feelers. The spinnerets at the
end of the abdomen are modified abdominal legs. Besides these the
embryo has four pairs of abdominal appendages which abort.
Fic. 192,—Garden spider.
I., Female garden spider; II., end view of head of the same
showing the simple eyes, the poison fangs (ch.), and the
pedipalps (A.); III., pe end of body showing two pairs
of spinnerets (s#.), with anus above.
The nervous system is of the usual Arthropod type, but
shows much centralisation. Thus the ventral ganglia are
fused into one large centre in the cephalothorax (see Fig.
193), a condition comparable to that in crabs. There
are two or three rows of simple eyes on the cephalothorax,
whose focal distance is very short, spiders trusting most
to their exquisite sense of touch, by which they discriminate
SPIDERS.
the various vibrations on a web line.
369
The senses of smell,
hearing, and taste are also present, but little is known in
regard .to the organs.
Body cavity, endosternite, and coxal glands generally
resemble those of scorpions.
The spider usually sucks the
blood and juices of its prey,
and behind the gullet lies
a powerfully suctorial region,
strengthened by chitinous
plates, and worked by muscles.
From the small mid-gut arise
five pairs of long ceca, a pair
running forwards and a pair
passing into the bases of each -
pair of legs, and then back
again. These ceca sometimes
anastomose. Farther back the
mid-gut gives off numerous
digestive outgrowths, which fill
a large part of the abdomen.
Their secretion digests pro-
teids. Terminally there is a
large cloaca, and where the
intestine joins this, four much-
branchedexcretory Malpighian
tubes are given off, which are
said to be endodermal in
origin.
A three-chambered heart,
containing colourless blood,
lies within a pericardium near
the dorsal surface of the
abdomen. It gives off an
anterior and a posterior aorta
and lateral vessels; and the
Fic. 193.—Dissection. of AZygale
from the ventral surface. —After
Cuvier.
1, Chelicerze; 2, pedipalps cut short ;
3-6, walking legs; g.1, large thoracic
ganglion; g.2, ganglion at base of
abdomen; c.#., chambered trachez
or lung-books—at the left side the
anterior is cut open to show the
lamellee (2.); 2., muscle of abdomen;
stl and sz.2, stigmata of lung-books ;
ov., ovary ; Sf., spinnerets.
circulation corresponds in general to that of the scorpion.
In a few forms (Tetrapneumones) respiration is effected
by four “lung-books,” e.g. in the large bird-catching AZygale
(Fig. 193).
two lung-books, and tubular trachez in addition.
24
In the vast majority (Dipneumones) there are
The
370 PHYLUM ARTHROPODA.
stigmata of the lung-books lie on the anterior ventral surface
of the abdomen; the trachez open posteriorly near the
spinnerets, or just behind the opening of the lung-books, or
at both places.
The spinnerets (4-6) lie just in front of the anus. They
are movable and perforated by numerous (often many
hundred) tubes or “spinning spools,” each of which is
connected with a compressible gland secreting silk. There
are various kinds of glands; both the amount and the
nature of the secretion are under control. The spinnercts
are transformed abdominal appendages (a new organ from
an old—as is so often the case); and the glands are
ectodermic invaginations.
Many spiders have at the
base of their spinnerets. a
transverse surface or cribrel-
um perforated by spinning
tubes, and from this they
comb out a peculiar curled
silk with the help of a row
of stiff bristles or calamistrum
on each posterior leg.
The males are usually
Fic. 194 —Section of lung-book. smaller and often more
“Hitter Map IRou brightly coloured than their
d., Dorsal; v., ventral; 2., lamella; ., mates. From the paired
posterior; @., anterior; d.c., dorsal 7 ,
chamber; x., posterior wall; s¢., testes, in the anterior part
stemes fg one of the interlamellar of the abdomen, two vasa
deferentia pass to a com-
mon aperture beside the openings of the lung-books.
From the paired ovary two oviducts likewise arise and open
into a uterus, whose external aperture is surrounded in the
mature female by a complex genital armature or epigynium.
Here also in most females are the openings of two recep-
tacula seminis, in which the sperms received from a male are
stored, and from which they pass by a pair of internal ducts
to the oviducts, there to fertilise the ova. The sperms of
the male, after emission, may be stored up in the last joint
of the palps. The ova are usually surrounded by silken
cocoons, which are carried about by the mother or carefully
hidden in nooks or nests. There is no metamorphosis ‘but
SPIDERS. 371
spiders at birth are often very different in details from their
later stages.
Spinning.—Muscular compression of the glands causes a flow of
liquid silk through the fine spools of the spinnerets. The extremely
thin filaments from each spinneret unite into a thread, and the thread
of one spinneret is often combined with that from the others. In this”
way a compound thread of exquisite fineness, though rivalled by a
quartz-fibre, is produced; but two or four separate threads are often
exuded at the same time. Before beginning to ‘‘spin,” the spider
often presses the spinnerets against the surface to which the thread is
to adhere, and draws the filaments out by slowly moving away. Often,
however, the filaments ooze out quite apart from any attachment. The
legs are also much used in extending and guiding the thread, and some
spiders have, as has been mentioned, a special comb (calamistrum).
One of the most important ways in which the secreted threads are
used is in forming a web. The common garden spider (Zfezra) makes
a web which is a beautiful work of unconscious art, and very effective
as a snare for insects. The spider first forms ‘‘ foundation lines”
around the selected area; it then swings across the area with the first
‘* yay,” which it fixes firmly; another and another is formed, all inter-
secting in one centre. Thirdly, it starts from the centre, and moves
from ray to ray in a long wide spiral gradually outwards, leaving a
strong spiral thread as it goes. Fourthly, the spider moves in a closer
spiral from the circumference inwards, biting away the former spiral,
replacing it by another, which is viscid and adhesive. It is to this that
the web chiefly owes its power of catching insects which light there.
There is usually a special thread running to the adjacent hole or nest,
and the spider feels rather than sees when a victim is caught.
The spun threads are used in many other ways. They line the nest,
and form cocoons for the eggs. They often trail behind the spiders as
they creep; they greatly assist locomotion, and are used in marvellous
feats of climbing. Small and young spiders often stand on tiptoe on
the top of a fence, secrete a parachute of threads, and allow them-
selves to be borne by the wind. The fallen threads are known as
gossamer. :
The distribution of spiders, e.g. on islands, does not appear to be much
affected by the absence of wings. Many young forms are aeronauts,
and many are carried about by the wind apart from ballooning.
Courtship.—The males are often much smaller than the females.
The disproportion is sometimes stich as would be observed if a man
6 ft. high and 150 Ib. in weight were to marry a giantess 76-90
ft. high, 200,000 Ib. in weight. The smallness of the males may be
due to the fact that they are males; others say that the smaller the
males are, the less likely they are to be caught by their frequently
ferocious mates.
The males are often more brilliantly coloured than the females.
Wallace spoke of the brilliancy of males-as due to their greater
vitality, and xeferred the relative plainness of the females to their
greater need for protection. Darwin referred the greater decorative-
ness of males to the fact that those which varied in this direction found
372 PHYLUM ARTHROPODA.
favour in the eyes of their mates, were consequently more successful
in reproduction, and thus tended to entail brilliancy on their male
successors. The careful researches of Prof. and Mrs. Peckham greatly
strengthen the position of those who believe in the efficacy of sexual
selection. Inthe Zvolution of Sex it has been suggested that sexual
selection may help to establish the brilliancy of males, and that natural
selection may help to keep the females plain, but that the decorative
and other differences between the sexes are primarily associated with
the more fundamental qualities of maleness and femaleness.
Classification of Spiders
1 Tetrapneumones or Mygalomorpha, with four lung-books and
no trachez ; the fangs of the cheliceree move vertically,
parallel to each other, e.g.—
Mygale, a large lurking spider which has been known to
kill small birds, but usually eats insects; Atypus, Cleniza,
and others make neat trap-door nests.
z. Dipneumones or Arachnomorpha, with two lung-books and
trachece as well; the fangs of the chclicereze move somewhat
horizontally toward each other.
The web-spinners, eg. Hfezra; wolf-spiders, e.g. Lycosa,
Tarantula, the latter with poisonous qualities which have
been much exaggerated ; jumping spiders or Attide, e.g.
Altus salticus. The common house spider is Tegenarza
domestica; the commonest garden spider is petra
diademata. Agyroneta aguatica fills an aquatic silken
nest with bubbles of air caught at the surface.
Order 7. ACARINA. Mites and Ticks
Mites are minute Arachnoids inclined to parasitism. They occur in
the earth, or in water, salt and fresh, or on animals and plants, They
feed on the organisms they infest or upon organic débris.
The abdomen is fused with the cephalothorax, but there is sometimes
a clear boundary line; both are unsegmented except in Of¢/oacarus,
which has a segmented abdomen. According to the mode of life, the
mouth-parts are adapted for biting or for piercing and sucking.
Respiration may be simply through the skin; in the majority there are
tracheze with two stigmata, A heart seems usually absent, but it is
present in Gamasus. Many of the young have only three pairs of legs
when hatched, but soon gain another pair. When some mites are
starved or desiccated, and to some extent die, certain cells in the body
unite within a cyst, and are able in favourable conditions to regrow the
animal.
Examples—
(a) Without tracheze. Cheese- mite (Zyroglyphus). Itch - mite
(Fig. 196) (Savcoptes scabtec), causing “itch” in man; S.
MITES AND TICKS. 373
canis, causing ‘‘mange” in dogs. Follicle-mite (Demodex
Solliculorum), common in the hair follicles of man and domestic
animals (Fig. 195). Gall-mites (Phytoptids), forming dimples
and pouches on plants.
(4 With tracheze. Harvest - mites (7rombidium), whose minute
hexapod larvee are troublesome parasites in summer on
’ Fic. 195.—Follicle-mite Fic. 196.—Itch-mite (Sarcopies scabiet)
(greatly enlarged). (greatly enlarged).
insects, many mammals, and man. The so-called ‘red
spider” (Zétvanychus teleartus) spins webs, and lives
' socially under leaves. Water-mites, e.g. Hydrachna on
water-beetles, and Azar on gills of fresh-water mussels.
Beetle-mites (Gamasus), often found on carrion beetles.
There is a common red mite on the shore-rocks, known as
Molgus (Bdella) littoralis.
Ticks (Ixodidee, etc.) are the largest Acarina. They show a movable
“‘capitulum ” bearing serrated cutting chelicerze and strong four-jointed
pedipalps. They are responsible for spreading the germs of some
diseases affecting man and beast, eg. human ‘‘tick-fever” on the
374 PHYLUM ARTHROPODA.
Congo, spread by Ornithedoros moubata ; a spirocheet disease in, Diiry,
borne by Argas reflexus and A. persicus ; Texas fever or “‘red wy ‘em in
cattle, carried by Boophzlus annulatus. The common sheep, ic in
Bi
io
\t
|
Fic, 197.—Tick (Jxodes riduvius, Fic. 198.--Tick (/avdes " LUTUS,
female), dorsal surface, showing the female), ventral surface -\_Aftey
oval shield (.S/.).—After Wheler. Wheler. an
H., Hypopharynx ; P., palp; Z./., L.ZV. R., Rostrum; P., palp; Go cent
first and fourth leg. mee Pe , , aperture ; ST. stigma ; ae
.
<
Britain is /xodes ricinus. It may be noted that mites have been fourg
inside human tumours, and there are many facts suggesting that some 5¢
the small Acarines may share in spreading disease germs. Eve,
_Demodex may play its part. \
Aberrant Orders or Classes (
i
Order LINGUATULIDA or PENTASTOMIDA, ¢.g. Pentastomum
teniotdes
This strange animal is parasitic in the nasal and frontal cavities, etc., ‘
of the dog and wolf. It is worm-like in form, externally ringed, :
without any oral appendages, but with two pairs of movable hooks near '
the mouth. The muscles are striated. The alimentary canal is very ;
simple, without Malpighian tubes. A narrow circumcesophageal nerve- !
ring, without a brain, is connected with a single ventral ganglion.
There are no sense organs nor trachese, nor is there any heart. The
sexes are separate ; the males smaller than the female.
Embryos within egg-cases pass from the nostrils of the dog. If they { {
happen to be swallowed by a rabbit or a hare, or it may be some other
mammal, the embryos hatch in the gut and penetrate to liver or \
THE KING-CRAB. 375
lung. There they éncyst, moult, and undergo metamorphosis.
The final larval form has two pairs of short legs, and has been
compared to a larval mite. Liberated from its encystment, it moves
about within its host, but will not become adult or sexual unless its host
be eaten by dog or wolf. There are a few other species occurring in
Reptiles, Apes, and even man, but their history is not adequately
known, and the systematic position is very uncertain. There is very
little reason for ranking them along with Arachnoids.
Order TARDIGRADA. Water-bears or Sloth-animalcules,
eg. Macrobiotus
Microscopic animals, sometimes found about the damp moss of
swamps or even in the roof-gutters of houses. Some occur in fresh
water, others in the sea. The unsegmented body is somewhat worm-
like, with four pairs of unjointed clawed limbs like little stumps, with
mouth-parts resembling those of some mites, and adapted for piercing
and sucking. The muscles are unstriped. There is no abdomen.
There is a food canal, a brain, and a ventral chain of four ganglia,
sometimes even a pair of simple eyes, but no respiratory or vascular
organs. The sexes are separate ; the males rarer and smaller.
The terrestrial Tardigrada, even as adults, have great powers
of successfully resisting desiccation, but sometimes only the eggs do so,
developing rapidly when favourable conditions return. There is very
little reason for ranking them along with Arachnoids. Perhaps, as the
seta-like ‘‘ claws” and the cirri of some types suggest, they are nearer
to Annelids. ; ;
Class PALHOSTRACA
The three following orders, Xiphosura, Eurypterina, and
Trilobita, may be united under this title. They live .or
lived in water, and have or had gills in association with the
limbs. The recently discovered antennz of Trilobites,
together with the markedly biramose character of some of
their limbs, suggest an affinity with Crustacea, but, on the
other hand, the affinities of the Xiphosura seem to be
distinctly Arachnoid.
Order 1. XIPHOSURA
There is one living genus, the King-crab or Horseshoe-
crab (Limulus).
The King-crab lives at slight depths off the muddy or
sandy shores of the sheltered bays and estuaries of North
America, from Maine to Florida, in the West Indies, and
also on the Molucca Islands, etc., in the far East. The
376
PHYLUM ARTHROPODA.
body consists of a vaulted cephalothorax shaped like a
horseshoe, and an almost hexagonal abdomen ending in a
long spine.
Burrowing in the sand, Zému/us arches its
body at the joint between cephalothorax and abdomen, and
pushes forward with legs and spine.
It may also walk
about under water, and even rise a little from the bottom.
Fic. 199.—Lémudlus or King-crab.
ch., Chelicerz ; of., operculum ;
@., anus.
It is a hardy animal, able to
survive exposure on the shore,
or even some freshening of
the water. Its food consists
chiefly of worms.
The King-crab is interesting in
its structure and habits and also
because it is the only living repre-
sentative of an old race,
The hard, horseshoe - shaped,
chitinous cephalothoracic shield is
vaulted, but the internal cavity is
much smaller than one would at
first sight suppose ; the well-defined
abdomen shows some hint of being _
divisible into meso-and meta-soma;
the long sharp spine is (like the
scorpion’s sting) a post-anal telson.
On the concave under-surface of
the cephalothorax there are six
(or seven) pairs of limbs, as in
spiders and scorpions—
(t) A little pair of three-
jointed chelicerze in front
of and bent towards the
mouth.
(2) A pair of pedipalps lateral
to the mouth.
(3-6) Four pairs of walking
legs, the bases of which
surround the mouth, and
help in mastication. Be-
hind these, still on the cephalothorax, there is a pair of small
appendages called chilaria,
Then follows on the abdomen a double ‘‘ operculum” with the
genital apertures on its posterior surface,
Under the operculum lie five pairs of flat plates bearing remark-
able respiratory organs (‘‘gill-books”).
These appendages
show hints of the exopodite and endopodite structure character-
istic of Crustaceans.
Each ‘‘ gill-book” looks like a much-plaited gill, or like a book with
EURVPTE RINA—TRILOBITA. 377
over a hundred hollow leaves. The leaf-like folds are externally
washed by the water, and within them the blood flows. The leaves
of the gill-books are often compared to the leaves of the insunk lung-
books of scorpions.
Spawning occurs in the spring and summer months. The ova
and spermatozoa are deposited in hollows near high-water mark.
Some of the early stages of development present considerable resem-
blance to corresponding stages in the ‘scorpion. In the larve,
both cephalothorax and abdomen show signs of segmentation, but
this disappears. The spine is represented only by a very short
plate, and the larva presents a striking superficial resemblance to a
Trilobite. ,
It seems likely that Limulus is linked to the extinct Eurypterids by
some fossil forms known as Hemi-
aspidee, e.g. Hemiaspis, Bélinurus.
Order 2. EURYPTERINA (=Mero-
stomata or Gigantostraca), eg.
Lurypterus
Large extinct forms found from
Cambrian to Carboniferous strata.
The body is divided into head, thorax,
and abdomen. The head is small
and unsegmented. The thorax is
composed of six distinct segments,
the abdomen of six with a terminal
telson. On the head are borne six
pairs of appendages .of varying shape,
two lateral compound eyes, and two .
median ocelli. On the ventral surface F1G. 200.—Young L2mulus.—
of the thorax there are five pairs of After Walcott.
gills covered by flat plates, of which
the most anterior pair are very large, and form the so-called operculum
(cf. Ldveulus), The surface of the body was covered with scales.
Some of the Eurypterids reached a length of 6 ft. The oldest
Merostomes are referred by Walcott to a sub-order Limulava somewhat
divergent from other Eurypterids.
This order is sometimes placed near the Crustacea, but the general
opinion is that they are linked through Zzmzlus to Arachnoids.
Order 3. TRiLopiTA. Trilobites, e.g. Calymene, Phacops,
Asaphus
Extinct forms chiefly found in Cambrian and Ordovician strata, but
extending up to the Carboniferous. The body as found is divisible
into three parts—the unsegmented head shield, often prolonged back-
wards at the angles; the flexible thorax of a varying number of
segments; the unsegmented abdomen or pygidium. A median
longitudinal ridge, or rachis, divides the body into three longitudinal
portions.
378 PHYLUM ARTHROPODA.
Traces of limbs are only rarely preserved. In the head region there
Fic. 201.—Trilobite (Conoceph-
alttes).— After Barrande.
h.s., Head shield ; 4/., pleura of
thoracic region; Ay., pygidium.
are four pairs, apparently simple.
Antennz have been recently found
in this region. The thorax and
abdomen are furnished with biram-
ose appendages, with long-jointed
endopodite, short exopodite, and a
gill (or epipodite ?) of varying shape.
In the abdominal region the gills
were perhaps rudimentary.
Trilobites are often found rolled
up in a way that reminds one of
some wood-lice. So abundant are
they in some rocks that even their
development has been studied with
some success.
The limbs seem to be more like
those of Crustaceans than those of
Arachnoids, and the occurrence of
antenne, observed by Linnzus
(1759), and recently corroborated,
accentuates the reseniblance. The
affinities with Zzmzlus, according
to the views of other authorities,
justify the association of Trilobites
and Arachnoids. A compromise
may be perhaps effected by regard-
ing the Trilobites as an offshoot from a stock ancestral to both
Arachnoids and Crustaceans.
Fic. 202,—Vertical cross-section of a Trilobite (Calymene).
—After Walcott.
z., Intestine; s., shield ; Z., endopodite; ¢., exopodite; 4., epipodial parts.
Incerte Sedis
Class PYCNOGONIDA, PANTOPODA, or PODOSOMATA
Marine Arthropods, sometimes called sea-spiders. They may be
ranked between Crustaceans and Arachnoids.. Many climb about
PANTOPODA OR PYCNOGONIDA.
379
seaweeds and hydroids near the shore, but some live at great depths.
The body consists of an anterior proboscis, cephalothoracic region
with three fused and three free segments, and an unsegmented rudi-
mentary abdomen. Four some-
what primitive eyes on an anterior
hillock, are nearer to the eyes of
Arachnoids than to those of any
other class. There are typically
seven pairs of appendages. The
first are short and chelate, but
may be absent in the adult.
The next two are small and
slender, and are often absent in
the adult female ; the second pair
may also be absent in the male,
but the third in the males of all
genera carries the eggs. The
last four pairs of appendages are
always present, and form the
walking legs. Into them, and
Fic. 203.—Sea-spider (Pycnogonum
littorale), from the dorsal surface.
into the chelicerze when these are present, out-growths of the mid-gut
extend. The sexes are separate» The larvee are at first unsegmented,
with three pairs of appendages,
Fic. 204.—Male of Nymphon.—After Sars.
PR., Proboscis; CH., chelophores; P., pedipalps; Z., eggs carried on
ovigerous legs; A., rudimentary abdomen.
Examples.—Lycnogonum, Nymphon, A mmothea. In Pentanymphon
and Decolopoda there is.an extra pair of long walking legs.
CHAPTER XVI
PHYLUM MOLLUSCA
Classes :—1. GASTEROPODA, ¢.g. Snails, 2. SOLENOGASTRES—A small
class of doubtful worm-like forms, e.g. Meomcenia, 3. SCAPHO-
PoDA—A small class, e.g. Dentalium. 4. LAMELLIBRANCHIATA
—Bivalves. 5. CEPHALOPODA—Cuttle-fiskes.
THE series of Molluscs is in many ways contrasted with
that of Arthropods; thus the body of the Mollusc is un-
segmented, and there are no appendages. The general
habit of life is also very different, for, although there are
active Molluscs and sluggish Arthropods, it is true as an
average statement that Molluscs are sluggish and Arthro-
pods are active. In the frequent presence of a trochosphere
larva, in the nerve-ring around the gullet, and in some other
features, Molluscs resemble Annelids, but it is probable
that they took their origin from a still lower level.
GENERAL CHARACTERS
Molluscs are unsegmented and without appendages. The
symmetry ts fundamentally bilateral, but this is lost in most
Gasteropods. The “foot”—a muscular protrusion of the
ventral surface—ts very characteristic , it usually serves for
locomotion, but is much modified according to habit. Typically,
a projecting dorsal fold of the body-wall forms a mantle, or
pallium (Fig. 205, ¢.), which often secretes a single or bilobed
shell covering the viscera, and roofs in a space—the mantle
cavity—within which lie the gills. But both mantle and shell
may be absent. There are three chief pairs of ganglia—cere-
brals, pedals, and pleurals—with connecting circum-esophageal
commissures, and there ts also a visceral nervous system con-
GENERAL CHARACTERS. 381
sisting typically of (a) a loop connecting the two pleurals and
provided with two visceral ganglia, and (b) a stomato-gastrie
ope f zl J? gab,
Fic, 205.—Ideal mollusc.—After Ray Lankester.
m., Mouth ; g.c., cerebral ganglia; c., edges of mantle skirt; z.g.,
duct of right lobe of digestive gland ; s., pericardial cavity ; /,
edges of shell-sac; w., ventricle of heart; %., nephridium 3 az.,
anus ; #., posterior part of the foot ; 2., opening of nephridium ;
&., genital aperture; g.ad., abdominal ganglion on visceral
loop ; g.v., visceral ganglion ; 2.2., left lobe of digestive gland ;
B., foot ; g.fe., pedal ganglion; g.A/. pleural ganglion.
loop connecting the cerebrals below the gullet and provided
with two buccal ganglia (Fig. 205). Lxcept in Lamelli-
branchs, in which the head region is degenerate, there is in the
Fic. 206.—Stages in molluscan development.
D, Larva of Heteropod (after Gegenbaur); sk., shell covering
visceral hump; v., velum; 7, foot.
E, Larva of Atlanta (after Gegenbaur); v., velum; s%., shell;
J, foot ; of., operculum.
mouth a chitinous ritbon or radula, usually bearing numerous
small teeth, and moved by special muscles, the whole structure -
being known as the odontophore. There is much unstriped
muscle, but the more rapidly contracting muscles have cross-
382 PHVLUM MOLLUSCA.,
striped fibres, or fibres with unstriped fibrils twisted in a
spiral. A portion of the true body cavity or celom usually
persists as the pericardium at least (Fig. 205, S.), and
communicates with the exterior through the nephridium or
nephridia. The rest of the cavity of the body is hemoceltc.
The vascular system is almost always well developed, but
part of the circulation ts in most cases lacunar, the heart
typically consists of a ventricle and two auricles. Respiratory
organs are most typically represented by gills or ctentdia,
consisting of an axts attached to the body and bearing lamella,
but the gills may have simpler forms, or may be absent, and
in the terrestrial snails the mantle cavity 1s adapted for
aerial respiration. At the base of the gills there is generally
an olfactory organ or ‘osphradium. The sexes are separate
or united. There are two common larval stages, — the
Trochosphere, which resembles the same stage in some
Annelids, and the more characteristic Veliger (Fig. 206) ;
but the development ts often direct.
First Type of Motiusca. The Snail (/e/ix), one of the
terrestrial (pulmonate) Gasteropods
Habits.—The common garden snail (47. aspersa), or the
larger edible snail (A fomatia), which is rare in England
SONS
AI
AH
0
Fic. 207.—Roman snail (//elix pomatia).
Note shell covering visceral hump; Z.a¢., pulmonary aperture
(including anus and opening of ureter); 7, the foot; g.ap.,
genital aperture ; #., mouth ; ¢., eye on long horn; s.4., one of
short horns.
but abundant on the Continent, serves as a convenient type
of this large genus of land-snails. They are thoroughly
THE SNAIL. 383
terrestrial animals, breathing air directly through a pulmon-
ary chamber, and drowning (slowly) when immersed in
water. Their food consists of leaves and other parts of
plants, but they sometimes indulge in strange vagaries of
appetite. They are hermaphrodite, but there is always
cross-fertilisation. The breeding time is spring, and the
eggs are laid in the ground. In winter snails bury them-
selves, usually in companies, cement the mcuths of their
shells with hardened mucus and a little lime, and fall into a
state of ‘latent life,” in which the heart beats feebly. They
have been known to remain dormant for years.
Fic. 208.—Vertical section of the shell of a species
of Helix, :
™., Mouth of shell; A., apex; C., columella.
General appearance.—A snail actively creeping shows a
well-developed head, with two pairs of retractile horns or
tentacles, of which the longer and posterior bear eyes. The
foot, by the muscular contraction of which the animal
creeps, is very large ; it leaves behind it a trail of mucus.
The viscera protrude, as if ruptured, in a dorsal hump,
which is spirally coiled and protected by the spiral shell.
On slight provocation the animal retracts itself within its
shell, a process which drives air from the mantle cavity, and
thus helps indirectly in respiration. Around the mouth of
the shell is a very thick mantle margin or collar, by which
the continued growth of the shell is secured. On the right
384 PHYLUM MOLLUSCA.
side of the expanded animal, close to the anterior edge of
the shell, there is a large aperture through which air passes
into and out of the mantle cavity. Within the same
aperture is the terminal opening of the ureter. The food
canal ends slightly below and to the right of the pulmonary
aperture. All the three openings are close together. The
anterior termination of ureter and food canal is one of the
results of the twisting of the visceral mass forwards to the
right. But still farther forward, at the end of a slight groove
which runs along the right side of the neck, indeed quite
close to the mouth, is the genital aperture. Lastly, an
opening just beneath the mouth leads into the large mucus
gland of the foot.
Shell.—The right-handed spiral shell is a cuticular product made
and periodically enlarged by the collar. Chemically it consists of
carbonate of lime and an organic basis (conchin). The outermost
layer is coloured, without lime, and easily rubbed off; the median
layer is thickest, and looks like porcelain; the innermost layer is
pearly. The twisted cavity of the shell is continuous, and the viscera
extend to the uppermost and oldest part,
As the shell is made, the inner walls of the coils form a central
pillar (columella), as’ on a staircase, to which the animal is bound by
a strong (columellar) muscle. Many Gasteropods bear on the foot a
lid or operculum, of conchin or of lime, which closes the mouth of the
shell. In Ae/¢x there is none; the ‘‘epiphragm” with which the
shell is sealed in winter consists of hardened mucus, plus phosphate
and a smaller quantity of carbonate of lime. It is formed very quickly
from the collar region when cold weather sets in, has no organic
connection with the animal, such as binds the operculum to the foot of
the whelk, and is loosened off in the mildness of spring.
Sinistral shells, with left-handed spiral, occasionally occur as
variations. The shell, held with its summit towards the observer, has
its aperture to the left. The internal organs are inverted, and at the
start there is a reversal of the cleavage planes of the egg.
Appearance after the shell is removed.—If the shell is
removed carefully, so that nothing is broken except the
columellar muscle, many structures can be seen without
any dissection. The skin of the head and foot should
be contrasted—(a) with the thick collar of the mantle;
() with the mantle itself, which forms the loose roof of
the pulmonary chamber ; (c) with the exceedingly delicate,
much-stretched, and always protected skin of the visceral
hump. The mantle is a downgrowth of the skin of this
dorsal region. It is peculiar in the snail, in that its margin
MUSCULAR AND NERVOUS SYSTEMS. 385
(the collar) is fused to the body-wall. The result is to
form a respiratory cavity, which is as much outside the
body as is the gill-chamber of the crayfish. It is important
to realise that the great rupture-like hump of viscera on the
dorsal surface has been coiled spirally, and that there is
the yet deeper torsion forward to the right.
A great part of the hump consists of the greenish brown
digestive gland, in which the bluish intestine coils; behind
the mantle chamber, on the right, lies the triangular and
greyish kidney ; the whitish reproductive organ lies in the
second last and third last coil of the spiral.
Skin.—This varies greatly in thickness. It consists of
a single-layered epidermis and a more complex dermis,
including connective tissue and muscle fibres. There are
numerous cells from which mucus, pigment, and lime are
secreted; those forming pigment and lime are especially
abundant on the collar, where they contribute to the growth
of the shell.
Muscular system.—Among the important muscles are—
(a) those of the foot; (4) those which retract the animal
into its shell, and are in part attached to the columella ;
(c) those which work the radula in the mouth; (d) the
retractors of the horns; and (e) the retractor of the penis.
The muscle fibres usually appear unstriated. There is
much connective tissue, some of the cells of which contain
glycogen, pigment, and lime.
Nervous system.—This is concentrated in a ring around
the gullet. Careful examination shows that this ring con-
sists dorsally of a pair of cerebral ganglia, connected ventrally
with a pair of pedals and a pair of pleuro-viscerals, which, .
according to some authorities, have a median abdominal
ganglion lying between them.
The cerebrals give off nerves to the head, eg. to the
mouth, tentacles, and otocysts, and also two nerves which
run to small buccal ganglia, lying beneath the junction of
gullet and buccal mass. The pedals give off nerves to
the foot; the pleuro-viscerals to the mantle and posterior
organs.
Sense organs.—An eye, innervated from the brain, is situated on
one side of the tip of each of the two long horns _It is a cup invaginated
from the epidermis, lined posteriorly by a single layer of pigmented and
25
386 PHYLUM MOLLUSCA.
non-pigmented retinal cells, filled with u clear vitreous body perhaps
equivalent to a lens, closed in front by a transparent ‘‘cornea,” and
strengthened all round by a firm “sclerotic.” How much a snail sees
we do not know, but it detects quick movements. Though the eye is
by no means very simple, the snail soon makes another if the original
be lost, and this process of regeneration has been known to occur
twenty times in succession.
The otocysts appear as two small white spots on the pedal ganglia.
Each is a sac of connective tissue, lined by epithelium which is said to
be ciliated in one region, containing a fluid and a variable number of
oval otoliths of lime, and innervated by a delicate nerve from the cere-
bral ganglia.
Though no osphradium or smelling-patch, comparable to that which
occurs at the base of the gills in most Molluscs, has been discovered in
Helix, the snail is repelled or attracted by odours; it shrinks from tur-
pentine, it smells strawberries from afar. This sense of smell seems to
be located in the horns, for a dishorned snail has none. The tips of
both pairs of horns bear sensory cells connected with ganglionic tissue
and nerve-fibres within.
Other sensory cells, probably of use in tasting, lie on the lips; and
there are many others, which may be called tactile, on the sides of the
foot, and on various parts of the body. In short, the snail is diffusely
sensitive.
Alimentary system.—lIn cutting a piece of leaf, the snail
uses two instruments—the crescentic jaw-plate on the roof
of the mouth, and the toothed ribbon or radula on the floor.
This radula is like a flexible file——a short and broad strip
of membrane, bearing several longitudinal rows of minute
chitinoid teeth. It rests on a cartilaginous pad on the floor
of the mouth cavity, and is moved (backwards and forwards,
and up and down) in a curve by protractor and retractor
muscles. The whole apparatus, including teeth, mem-
brane, and pad, is called the odontophore. The radula
wears away anteriorly, but is added to posteriorly within a
radula sac which projects from the floor of the buccal cavity.
Its action on leaves may be compared very roughly to that
of a file, but its movements within the mouth also produce
a kind of suction which draws food particles inwards. In
this suction the muscular lips and the cilia in the mouth
cavity assist.
The ducts of two large salivary glands open on the
dorsal surface of the buccal cavity, and there are numerous
distinct glandular cells close to the entrance of the two
ducts. The salivary glands are large lobed structures, and
extend far backward on the crop. They consist of hundreds
VASCULAR SYSTEM. 387
of glandular cells or unicellular glands, which secrete a clear
fluid. This travels up the ducts, and is forced, in part
at least, by muscular compression, into the buccal cavity.
While some say that this fluid converts starch into sugar
(after the usual fashion of saliva), other authorities deny
that it has any effect upon the food. Similar glands are
found in all Gasteropods, while they are entirely absent in
Lamellibranchs. In some boring Gasteropods the secretion
contains 2-4 per cent. of free sulphuric acid.
The gullet extends backward from the buccal cavity, and
expands into a storing-crop; this is followed by a small
stomach surrounded by the digestive gland; thence the
intestine extends, and, after coiling in the visceral hump,
passes forward to end on the right side anteriorly beside
the respiratory aperture. The digestive tract is muscular,
and in part ciliated internally.
A large part of the visceral spiral is occupied by the so-
called “‘liver.” This gland has two lobes, each of which
opens by a duct into the stomach. The left lobe is again
imperfectly divided into three. Besides producing juices
which digest all kinds of food, the gland makes glycogen,
stores phosphate of lime, and contains a greenish pigment.
It is thus more than a “liver,” more even than a ‘‘hepato-
pancreas,” it is a complex digestive gland, producing several
digestive ferments.. The phosphate of lime may possibly
be used to form the autumnal epiphragm.
Vascular system.—The blood contains some colourless
amoeboid cells, and a respiratory pigment called hzemo-
cyanin, which gives the oxidised blood a blue tint, and is
very common among Molluscs.
The heart, with a ventricle and a single auricle, lies in a
pericardial chamber on the dorsal surface, to the left side,
behind the mantle cavity. The average number of pulsa-
tions in Gasteropods is about one hundred per minute, but
in the hibernating snail the beating is scarcely perceptible.
From the ventricle: pure blood flows by cephalic and
visceral arteries to the head, foot, and body, passes into
fine ramifications of these arteries, and thence into spaces
among the tissues. From these the blood is collected in
larger venous spaces, and eventually in a pulmonary sinus
around the mantle cavity, on the roof of which there is a
388 PHYLUM MOLLUSCA.
network of vessels. There the blood is purified. Most of
it returns directly to the auricle by a large pulmonary vein,
but some passes first through the kidney.
Respiratory system.—Most Gasteropods, ¢.g. the dog-
whelk (Purpura), the buckie (Buccinum), the periwinkle
(Littorina), breathe by gills covered by the mantle. The
snail being entirely terrestrial, has a pulmonary or lung
cavity, formed by the mantle fold. On the roof of this
cavity the blood vessels are spread out. Air passes into and
out of the pulmonary chamber by the respiratory aperture.
When the animal is retracted within its shell, the freshening
of the air in the pulmonary chamber takes place by slow
diffusion, but when the snail extends itself at full length,
the chamber is rapidly filled with air, and it is even more
rapidly emptied when the body is withdrawn into the shell.
Excretory system.—There is a single triangular greyish
kidney behind the pulmonary chamber, between the heart
and the rectum. It is a sac with plaited walls, and excretes
nitrogenous waste products, which pass out by a long ureter
running along the right side of the pulmonary chamber, and
opening close beside the anus. There are two sources of
blood supply to the kidney —(a) from the pulmonary
chamber, and (4) from the heart by a renal artery. As in
most other Molluscs, the kidney communicates by a small
aperture with that part of the ccelom which forms the
pericardial sac. Thus, as in earthworm, lobworm, etc., the
coelom has a nephridial connection with the exterior.
Reproductive system.—The snail is hermaphrodite, and
its reproductive organs exhibit much division of labour.
(2) The essential reproductive organ (the ovofestis) is a
whitish body near the apex of the visceral spire. It consists
of numerous cylindrical follicles, in each of which both ova
and spermatozoa are formed, but not at the same time.
(6) A much-convoluted hermaphrodite duct of a white
colour conducts the sex cells from the ovotestis, and leads
to the base of a large yellowish albumen gland.
(c) This tongue-shaped albumen gland varies in size with
the age and sexual state of the snail. It forms gelatinous
proteid material, which envelops and probably nourishes
the ova.
(2) The ova and spermatozoa pass from the hermaphrodite
REPRODUCTIVE SYSTEM. 389
duct towards the head along a common duct, but not at the
same time. Moreover, their paths are different, for the
portion of the duct down which the ova travel is much
plaited, while the path which the spermatozoa follow is a
Fic. 209.—Dissection of snail.
T., Short horn; 77., long horn with eye; WV., cerebral ganglia; S.G.,
salivary glands on the crop; #., foot; M., columellar muscle; .C.,
visceral coil; O.7., ovotestis; V., ventricle of heart; R. rectum; U.,
ureter; B.V., blood vessels returning to the auricle from the mantle;
A., pulmonary aperture ; J7A., edge of the mantle.
less prominent groove, incompletely separated from the
other. Both paths are glandular, and the glands on the
male side are often called prostatic.
(e) At the base of this common duct, a distinct vas
deferens diverges to the left and leads into a muscular fezs,
390 PHYLUM MOLLUSCA.
which can be protruded at the single genital aperture and
retracted by a special muscle. Before the vas deferens
enters the penis, a long process or flagel/um is given off.
It is like the lash of a whip, and is as long as the common
duct. Its secretion is used in forming a sperm-packet or
spermatophore of a large number of spermatozoa, which are
OT
D.S
Fic. 210.—Reproductive organs of Helix pomatia.—
After Meisenheimer,
O.T., Ovotestis ; H.D., hermaphrodite duct; 4.G., albumen gland; F.D.,
female side of common duct; J7.D., male side of common duct; O.,
oviduct; &.S., receptaculum seminis; JZ.G., mucus glands; D.3S.,
dart-sac; V.D., vas deferens; FL., flagellum; P., penis; AZ., retractor
muscle of penis ; A/., genital aperture.
compacted together at the time of sexual union partly in
the flagellum, partly in the penis. Thé spermatophore is
transferred by the penis into the genital aperture of another
snail.
(f) Continued from the oviducal side of the common
duct, there is a separate ciliated ovéduct. This has a short
course, and ends in the common genital aperture. Before
REPRODUCTIVE SYSTEM. 391
it reaches this, however, the oviduct is associated with two
structures. The first of these is a long process, as long as
the common duct beside which it runs, in appearance
suggesting the flagellum, but expanding at its free end into
a globular sac—the veceptaculum seminis or spermatheca.
In Helix aspersa a long slender diverticulum is given off
from the duct of the receptaculum. This is also occasionally
seen in Helix pomatia. A spermatophore from another
Fic. 211.—Snail (He/zx pomatza) laying its eggs. —
After Meisenheimer.
snail passes into the receptaculum, and is there dissolved
after some days, liberating hundreds of spermatozoa. By
these spermatozoa the ova of the snail are fertilised. It
seems likely that the place of fertilisation is in a small
diverticulum at the upper end of the oviducal side of the
common duct, whither the spermatozoa are said to find
: their way. The second structure associated with the female
duct is a conspicuous mucus gland, formed of two sets of
finger-like processes. The secretion is very abundant
during copulation, and as it contains not a little lime, it is
possible that it may form the calcareous shells of the eggs.
392 PHYLUM MOLLUSCA.
It seems to serve as a lubricant which facilitates the
expulsion of a calcareous dart and the copulation.
(g) Finally, between the entrance of oviduct and penis
into the terminal aperture there lies a firm cylindrical
structure, larger than the penis and with muscular walls. It
is the Cupid’s Dart Sac, and contains a pointed calcareous
arrow (spiculum amoris), which is jerked out previous to
copulation. The dart is sometimes found adhering to the
foot of a snail, and after copulation the sack is empty, soon,
however, to be refilled.
When two snails pair, the genital apertures are dilated, the
protruded penis of one
is inserted into the
aperture of the other,
and the spermatophore
of each snail is trans-
ferred to the recepta-
culum of the other.
The large eggs are laid
in the earth in June
and July. Each is sur-
Fic. 212.—Diagram of larva of Pala. rounded by gelatinous
dina.—After Erlanger. material acquired in the
Ec., Ectoderm; £x., endoderm 5 v, velum, oviduct and by an elastic
with cilia; g., gut-cavity; S.c., segmenta-
tion cavity; c.Z., coelom pocket from gut; but calcareous shell.
bi.g., blastopore groove closed, except at Segmentation is total
6l., which becomes the anus. The origin »
of ‘the mesoderm from a gut-pocket has as but slightly unequal. AS
yebooly Been ilespabed 1 in Paludina among the snail is a terrestrial
Gasleropod, there is no
trochosphere larva, nor more than a slight hint of the char-
acteristic Molluscan velum. A miniature adult is hatched
in about three weeks. The study of development may be
more profitably followed in the pond-snail Zimncaus, where
gastrula, trochosphere, and veliger can be readily seen.
Second Type of Mottusca. The Fresh-water Mussel
(Anodonta cygnea), one of the Lamellibranchiata
Habit.—The fresh-water mussel lives in rivers and ponds.
It lies with its head end buried in the mud, or moves
slowly along by means of its ploughshare-like foot. Its food
FRESH-WATER MUSSEL. 393
consists of minute plants and animals, which are wafted in
at the posterior end by the currents produced by the cili-
ated gills. What is noted here in regard to Anodonta will
also apply, for the most part, to Unio and other fresh-water
mussels.
External appearance.—The bivalve is 4 to 6 in. long;
its valves are equal and united in a dorsal hinge by an
elastic ligament, an uncalcified part of the shell; on the
ventral surface when the valves gape the foot protrudes ; the
anterior end is rounded, the posterior end is more pointed,
and it is there that the water currents flow in (ventrally)
and out (dorsally). In bivalves the ligament is generally
posterior to the dorsal knob or wmso—the oldest part of the
shell—and the umbo generally points towards the anterior
end. The greenish brown soft (“horny”) layer of the shell
is often worn away near the umbo on each side, and then
-displays the median layer of lime. This is called prismatic,
since the lime salts are deposited in prisms, transversely
varicose or striated, like those which form the enamel
of our teeth. Internally there is a pearly layer. Lines
of growth on the shell mark the position of the margin
in former years, the newest part being obviously at the
edge.
The shell is a cuticular structure, ze. it is made by the
epidermis of the mantle. It consists, as in the snail, of
calcium carbonate plus conchiolin or conchin. Thus the
composition of a Pinna shell is:—Lime salts, 89°2 ; organic
matrix, 1°3; water, 9°5.
Internal appearance.—When the right half of the shell
is folded back, the anterior and posterior closing muscles
having been carefully cut close to the gently raised valve,
the mantle folds are seen lining the shell, and forming
posteriorly the ventral inhalant and dorsal exhalant lips.
The ventral lips have papillary processes. Internal to the
mantle there are two gill-plates on each side; projecting
from between these is the foot, muscular ventrally, softer
dorsally ; the median dorsal pericardium is just beneath the
ligament; the ventricle shines through its walls, and the
dark-coloured kidneys are*seen through its floor. Below
the anterior adductor muscle is the large mouth, bordered
beneath by two lip processes (labial palps) on each side.
pl.
Vv
Fic. 213.—The fresh-water mussel (U/20).
The uppermost figure represents the bivalve in motion in the mud
with protruded foot (/.) ; note inhalant and exhalant apertures,
The middle figure shows the inside of the shell(left valve). The
lower figure shows the outside (right valve). «#., The umbo;
Z., the ligament; ¢.4., lateral téeth; @.a., anterior adductor
mark ; a.7,, mark of protractor of the foot; 4.2. pallial line ;
~-@., posterior adductor mark ; 4,.~., mark of posterior retractor
of the foot ;4g., a line of growth; 4., anterior (the blunter
end); P., posterior; V., ventral.
ar
FRESH-WATER MUSSEL, 395
These resemble the gills in appearance, and are probably
modified portions of the gills. The anus is above the
posterior closing muscle. The whole space between the
two mantle flaps is called the mantle cavity, and it is divided
by a slight partition at the bases of the gills into a large
ventral infra-branchial chamber and a small dorsal supra-
branchial chamber which ends at the exhalant orifice.
On the surface of the valves of the shell a few small
pearls may be seen; they are formed by the enclosure of
some minute grains of sand in the prismatic layer. There
are two teeth in front of the umbo in Uzio, but not in
Anodonta. The following muscles are inserted on the shell,
and leave impressions :—
(a) The anterior adductor.
(4) The posterior adduetor.
(c) The anterior retractor of the foot continues with (a).
(d) The protractor of the foot a little below (a).
(e) The posterior retractor of the foot continues with (4).
As the shell grows, the insertion of the muscles and the attachment ot
the mantle change, and the traces of this shifting are visible.
Skin.—There is much ciliated epithelium about Azodonta,
especially on the internal surface of the mantle, on the gills,
and on the labial palps; and little pieces cut from an
animal incompletely dead (e.g. from the oyster swallowed
half-alive) have by means of their cilia a slight power of
motion. The skin of the foot is not ciliated but glandular ;
on the mantle edge sensitive and glandular cells are abund-
ant, but usually in inverse ratio to one another.
Muscular system.—The shell is closed and kept closed
by the action of the two adductor muscles. When these
are relaxed under nervous control, the elasticity of the hinge
ligament opens the valves. The foot is a muscular protru-
sion of the ventral surface, under the control of three
muscles—a retractor and a protractor anteriorly, and a
posterior retractor. Its upper portion contains some coils
of gut and the reproductive organs ; its lower region is very
muscular. The protrusion or extension of this locomotor
organ is mainly due to an inflow of blood, which is pre-
vented from returning by the contraction. of a sphincter
muscle round the veins. In moving, the animal literally
ploughs its way along the bottom of the pond or river pool,
306 PHYLUM MOLLUSCA.
and leaves a furrow in its track. The muscle fibres, as in
the snail, are mainly of the slowly contracting non-striped
sort, but those of the adductor and of the heart show
oblique cross-striping. In that part of the adductor muscle
of Pecten (and some other bivalves) that effects the rapid
closing of the valves, and hence the swimming, the muscle-
fibres are transversely cross-striped, and the same is true of
those found in the margin of the mobile mantle. There is
here therefore a good instance of the connection between
striation and rapidity of contraction and relaxation.
Nervous system.—There are three pairs of nerve-
centres :— :
(a) Cerebro-pleural ganglia, lying above the mouth on
each side on the tendon of the anterior retractor
of the foot, connected to one another by a
commissure, connected to the two other pairs
of ganglia (4) and (c), by long paired connect-
ives, and giving off some nerves to mantle,
palps, etc.
(2) Pedal ganglia, lying close together about the
middle of the foot, united by connectives to (a),
giving off nerves to the foot, and having beside
them two small ear-sacs, each with a calcareous
otolith, and with a nerve said to be derived
from the cerebral ganglion.
(c) Visceral ganglia (also called parieto-splanchnic or
osphradial), lying below the posterior adductor,
connected to (a) by two long connectives, and
giving off nerves to mantle, muscles, etc., and
to a patch of “smelling cells” (osphradium) at
the bases of the gills.
Sense organs.—Unlike not a few bivalves, which have
hundreds of “eyes” on the mantle margin, Azodonta has
no trace of any. The ear-sac, originally derived from a skin-
pit, is sunk deeply within the foot, and is of doubtful use.
The ‘‘smelling patch” or “ osphradium” at the base of the
gills has perhaps water-testing qualities. There are also
“tactile” cells about the mantle, labial palps, ete.
Alimentary system.—The mouth lies between the
anterior adductor and the foot, and beside it lie the ciliated,
vascular, and sensitive labial palps, two on each side, which
ALIMENTARY SYSTEM. 397
waft food into the mouth. It opens immediately into the
gullet, for the pharynx of other Molluscs, with all its
associated structures, is absent in Lamellibranchs. The
short wide gullet leads into a large stomach surrounded by
a paired digestive gland. Part of the food digested by
Fic. 214.—Structure of Azodonta.—After Rankin.
a.a., Anterior adductor; ¢..g., cerebro-pleural ganglia; sz,
stomach; v., ventricle, with an auricle opening into it; 4,
kidney, above which is the posterior retractor of the foot;
vy, rectum ending above posterior adductor; wg., visceral
ganglia with connectives (in black) from cerebro-pleurals ; ¢.,
gut coiling in foot ; Z.g., pedal ganglia in foot, where also are
seen branches of the anterior aorta and the reproductive organs ;
2.g., labial palps behind mouth. At the posterior end the ex-
halant (upper) and inhalant (lower) apertures are seen. ~
these juices in the stomach is compacted in autumn into a
“ crystalline style ””—a mass of reserve foodstuffs, and similar
but less solid material is found in the intestine. On this
supply the mussel tides over the winter. The intestine,
which has in part a folded wall like that of the earthworm,
coils about in the foot, ascends to the pericardium, passes
398 PHYLUM MOLLUSCA,
through the ventricle of the heart, and ends above the
posterior adductor at the exhalant orifice.
Vascular system.—The heart lies in the middle line on
the dorsal surface, within a portion of the body cavity called
the pericardium, and consists of a muscular ventricle which
has grown round the gut and drives blood to the body,
and of two transparent auricles—one on each side of the
ventricle—which receive blood returning from the gills and
mantle. In bivalves the heart-beats average about twenty
per minute, much less than in Gasteropods. The colour-
less blood passes from the ventricle by an anterior and a
posterior artery ; flows into ill-defined channels ; is collected
in a “vena cava” beneath the floor of the pericardium ;
passes thence through the kidneys, where it loses nitrogenous
waste, to the gills, where it loses carbonic acid and gains
oxygen; and returns finally by the auricles to the ventricle.
The blood from the mantle, however, returns directly to the
auricles without passing through kidneys or gills, but
probably freed from its waste none the less. The so-called
“organ of Keber” consists of “ pericardial glands” on the
epithelium of the pericardial cavity. They seem to be
connected with excretion. Many of the cells lining the
blood channels secrete glycogen, the principal product of
the Vertebrate liver.
Respiratory system.—Lying between the mantle flaps
and the foot there are on each side two large gill-plates,
whence the title Lamellibranch. They are richly ciliated ;
their internal structure is like complex trellis-work ; their
cavities communicate with the supra-branchial chamber.
As in many other Molluscs, the gills or ctenidia are not
merely surfaces on which blood is purified by the washing
water-currents (a respiratory function), but some of their
many cilia waft food-particles to the mouth (a nutritive
function), and in the females the outer gill-plate shelters
and nourishes the young larve (a reproductive function).
The water may pass ¢hrough the gills to the supra-branchial
chamber and thence out again, or over the gills to the
mouth, and thence into the supra-branchial chamber. It is
likely that the mantle has no small share in the respiration.
In many cases, e.g. Lutraria elliptica, the posterior end of
the mantle gives origin to a contractile respiratory siphon, a
REPRODUCTIVE ORGANS. 309
double tube, the upper half of which is expiratory and the
lower half inspiratory. A cross-section shows a cuticular
investment of conchin, a layer of epidermis, a narrow zone
of circular muscle-fibres, a thick zone of longitudinal muscle-
fibres, a narrow zone of circular muscle-fibres, an internal
epithelium, and the two canals. The white circular muscle-
fibres are unstriped; the longitudinal muscle-fibres, which
are greyish yellow, show a lozenge-shaped marking as in the
more opaque fibres of the adductor muscles.
The precise structure and attachment of the gill-plates is complex,
but it is important to understand the following facts:—(a) A’ cross
section of the two gill-plates on one side has the form of a W, one half
of which is the outer, the other the inner gill-plate ; (4) each of these
- gill-plates consists of a united series of gill filaments, which descend
from the centre of the W and then bend up again; (c) adjacent fila-
ments are bound together by fusions and bridges both horizontal and
vertical, so that each gill-plate becomes like a complex piece of basket
work ; (Z) both gill-plates begin by the downward growth of filaments
from a longitudinal * ‘ctenidial axis,” the position of which on cross-
section is at the median apex of the W; (ce) this mode of origin, and the
much leéss complex gills of other bivalves, lead one to believe that there
is on each side one gill consisting of two gill-plates formed from a series
of united and reflected gill filaments. On the gills there are often
parasitic mites (Uzonccola or Atax ypsilophorus).
Excretory system.—The paired kidney, which used to
be called the “organ of Bojanus,” lies beneath the floor of
the pericardium. Each half is a nephridium bent upon
itself, with the loop posterior, the two ends anterior. The
lower part of this bent tube is the true kidney; it is dark
in colour, spongy in texture, and excretes guanin and other
nitrogenous waste from the blood which passes through it.
It has an internal opening into the pericardium, which thus
communicates indirectly with the exterior. The upper part
of the bent tube, lying next the floor of the pericardium,
is merely a ureter. It conveys waste products from the
glandular part to the exterior, and opens anteriorly just
under the place where the inner gill-plate is attached to the
visceral mass. As already mentioned, the “ pericardial
glands” probably aid in excretion, and possibly the same
may be said of the mantle.
The reproductive organs.—These lie in the upper part
of. the foot, adjacent to the digestive gland. Ovaries and
400 PHYLUM MOLLUSCA.
testes occur in different animals, and the two sexes are
distinguishable, though not very distinctly, by the greater
whiteness of the testes and by slight differences in the shells.
The females are easily known when the larva begin to
accumulate in crowds in the outer gill-plates. The repro-
ductive organs are branched and large; there are no
accessory structures; the genital aperture lies on each side
under that of the ureter.
The ova pass from the ovaries in the foot, and appear to
be moved to the exhalant region, whence, however, they do
not escape, but are crowded backward .till they pass into
the cavity of the outer gill-plate. At some stage they are
fertilised by spermatozoa drawn in by the water currents,
though it is difficult to believe that this is entirely a matter
of chance. Development takes place within the external
gill-plate, and the larve feed for some time on mucus
secreted by the gill.
Development and life history.—The development of Asodonta
differs in certain details from that of most bivalves, perhaps in adapta-
tion to fresh-water conditions. Moreover, a temporary parasitism of
the larva has complicated the later stages.
The egg-cell is surrounded by a vitelline membrane, and attached to
the wall of the ovary by a minute stalk, the insertion of which is marked
on the liberated ovum by an aperture or micropyle, through which the
spermatozoon enters.
Segmentation is total but unequal. A number of small clear yolkless
cells are rapidly divided off from a large yolk-containing portion, which
is slower in dividing. Eventually a hollow ball of cells or blastosphere
results (Fig. 215).
On the posterior dorsal region a number of large opaque cells form
an internally convex plate,—the beginning of the future shell-sac. A
pair of large cells are intruded into the central cavity, and begin the
mesoderm.
On the under surface posteriorly there is a slight protrusion of ciliated
cells forming a ciliated disc. In front of this, at an unusually late stage,
an invagination establishes the archenteron, and the embryo becomes a
gastrula (see Fig. 215).
The shell-sac forms an embryonic shell, and many of the mesoderm
cells combine in an adductor muscle. The mouth of the gastrula closes,
and a definite mouth is subsequently formed by an ectodermic invagina-
tion. Gradually a larva peculiar to fresh-water mussels, and known as
a Glochidium, is built up.
The Glochidium has two triangular, delicate, and porous shell sed
each with a spiny incurved tooth on its free edge. The valves clap
together by the action of the adductor muscle. The mantle lobes are
very small, and their margins bear on each side three or four patches of
DEVELOPMENT AND LIFE HISTORY. 401
sensory cells. The foot is not yet developed, but from the position
which it will afterwards occupy there hang long attaching threads of
‘*byssus,” which moor the larva. If it manage to anchor itself on the
tail, fins, or gills of a fish, the Glochidium shuts its valves and fixes
itself more securely, and is soon surrounded by a pathological growth of
- its host’s skin.
In this parasitic stage a remarkable metamorphosis occurs. The
sensory or tactile patches not unnaturally disappear; the ‘‘ byssus”
Fic. 215.—Development of Azodonta.—After Goette.
x. Section of blastosphere. s.d., Shell gland; ¢.d., ciliated disc; e.,
beginning of ectodermic invagination, Note mesoderm cells in
the cavity.
2. Later stage. 2., Mesoderm.
3. Embryonic shell has appeared.
4. Glochidium larva; note byssus threads, and teeth on shell
valves.
and the embryonic ‘‘ byssus glands” vanish, but a true byssus gland
(which remains quite rudimentary in Anodonta) appears; the single
adductor atrophies, and is replaced by two; the foot and the gills
make their appearance ; the embryonic mantle lobes increase greatly,
or are replaced by fresh growths ; and the permanent shell begins to be
made.
After -this metamorphosis, when the larva has virtually become a
miniature adult, no longer so liable to be swept away, it drops from its
temporary host to the bottom of the pond or river pool. :
26
402 PHYLUM MOLLUSCA.
Third Type of Mortusca. The Common Cuttlefish
(Sepia officinalis), one of the Dibranchiate Cephalopods
Habits.—This common cuttlefish is widely distributed,
especially in warmer seas like the Mediterranean. Unlike
Octopus, which usually lurks passively, Seféa is an active
swimmer; it moves head foremost by working the fins
which fringe the body, or it jerks itself energetically back-
wards by the outgush of water through the funnel. It likes
the light, and is sometimes attracted by lanterns. The
beautiful colours change according to external conditions
and internal emotions; and a plentiful discharge of ink
Fic. 216.—Side view of Sesza.—After Jatta.
often covers, its retreat from an enemy. Its food includes
fish, other molluscs, and crabs. In spring the female
attaches her encapsuled eggs to seaweeds and other
objects, and often comes fatally near the shore in so doing.
The cuttles are caught for food and bait. The “cuttle
bone” and the pigment of the ink-bag are sometimes
utilised by man.
External appearance.—A large Seféa measures about
to in. in length and 4 to 5§ in breadth; the body, fringed
by a fin, is shaped like a shield, the broad end of which
bears a narrowed head, with eight short and two long
sucker-bearing arms. Besides the diffuse pigment cells,
there are bands across the “back.” The large eyes, the
parrot-beak-like jaws protruding from the mouth, the spout-
like funnel on the neck, and the mantle cavity, are con-
spicuous. Beside the eyes are the small olfactory pits;
CUTTLEFISA. 403
within the mantle cavity lie the anus and the openings of
the nephridia and genital duct. :
The true orientation of the different regions in Sedza is
not obvious. If the “arms” surrounding the mouth be
divided portions of the anterior part of the “foot,” the
ventral surface is that on which the animal rests when we
make it stand on its head. We can fancy how the “foot”
of a snail might grow forward and surround the mouth, so
as to bring that into the middle of the sole. Then the
visceral mass has been elongated in an oblique dorso-
posterior direction, so that the tip of the shield, directed
forward when the cuttle jerks itself away from us, represents
in anatomical strictness the dorsal surface tilted backwards.
(As above noticed, the animal may also swim with foot and
mouth in front.) The side of lighter colour, marked by the
mantle cavity and the siphon or funnel, is postertor and
slightly ventral; the banded and more convex side, on which
the cerebral ganglia lie in the head region, and on which
the shell lies concealed in the visceral region, is anterior
and slightly dorsal.
Skin.—There are numerous actively changeful pigment
cells or chromatophores lying in the connective tissue
beneath the epidermis. Each cell is expanded by the
contraction of muscular cells which radiate from it, and
-contracts when these relax. It is probable that these
chromatophore cells have some protoplasmic spontaneity
of their own, but the controlling muscular elements are
also affected by nervous impulses from the central ganglia.
As the cells dilate or contract, the pigment is diffused or
concentrated, and the colours change. The animal’s beauty
is further enhanced by numerous “‘iridocysts” or modified
connective tissue cells, with fine markings which cause
iridescence.
Muscular system.—The cuttlefish is very muscular,
notably about the arms, the mantle flap, and the jaws.
Many of the muscles show double oblique striping. The
animal seizes its prey by throwing out its two long arms,
which are often entirely retracted within pouches. With
great force it jerks itself backwards by contracting the
mantle cavity, and making the water gush out through the
pedal funnel. This mode of locomotion is very quaint.
404 PHYLUM MOLLUSCA.
At one time the mantle cavity is wide, and you can thrust
your fingers into its gape; when about to contract, this
gape is closed bya strange double hook-and-eye arrange-
ment; contraction occurs, and the water, no longer free to
leave as it entered, gushes out by the funnel, the base of
which is within the mantle cavity. The suckers on the
arms are muscular cups, borne’ on little stalks (unstalked
in Octopus, etc.) well innervated, and able to grip
with a tenacity which in
giantcuttlefishis dangerous
even to men. The inner
edge of the cup margin is
supported by a chitinoid
ring bearing small teeth.
Each cup acts as a sucker,
in a fashion which has
many analogues, for a
retractor muscle increases
the size of the cavity
after the margin has been
applied to some object.
The external pressure is
then greater than that
within the cup, and the
little teeth keep the attach-
ment from slipping.
It seems likely that the
arms represent a_ pro-
podium, and the siphon
a mesopodium, and a
Fic. 217.—External appearance of Valve within the siphon
a cuttlefish (Zo/igo), has been compared to a
metapodium.
Skeletal system.—An internal skeleton is represented by
supporting cartilaginous plates in various parts of the body,
especially—(a) in the head, round about the brain, arching
over the eyes, enclosing the “‘ears”; (4) at the bases of the
arms; (c) as a crescent on the neck; (d) at the hook-and-
eye arrangement of the mantle flap; (e) along the fringing
fins. Ramified “stellate” cells lie in the structureless
transparent matrix of the cartilage.
NERVOUS SYSTEM. 405
On the shore one often finds the “cuttle bone” or sepio-
staire, which is sometimes given to cage birds to peck at
for lime, or ysed for polishing and other purposes. It lies
on the dorsal side of the animal, covered over by the mantle
sac. In outline it is somewhat ellipsoidal, thinned at the
edges like a flint axe-head, and with curved markings which
indicate lines of growth. In the very young Sepa it con-
sists wholly of the organic basis conchiolin, but to this lime
is added from the walls of the sac. Between the plates
of lime there is gas, and though the structure may give
the cuttle some stability, it is probably of more use as a
float.
Internal appearance.—When the mantle flap is cut open
and reflected, the two plume-like gills are seen, and the
lower end of the siphon. The dark outline of the ink-bag,
foilowed along towards the head, leads our eyes to the end
of the food canal. Near this are the external apertures of
the two kidneys and of the genital duct. On each side of
the base of the funnel lies a very large and unmistakable
“stellate” ganglion. Removing the skin as carefully as
possible over the whole visceral region between the gills,
and taking precautions not to burst the ink-sac, we see the
median heart, the saccular kidneys, contractile structures or
branchial hearts at the base of each gill, and the essential
reproductive organs near the apex of the visceral mass.
Disturbing the arrangement of these organs, we can follow
the food canal, with its stomach, digestive gland, etc.
Nervous system.—Three pairs of ganglia surround the
gullet,—cerebral on the dorsal and anterior side, pedal and
pleuro-visceral on the ventral and posterior side (Fig. 218),
but lying so close together that their boundaries are defined
with difficulty. All are well protected by the investing
cartilages. —
The cerebral ganglia are three-lobed, and are connected anteriorly by
two commissures with a ‘‘supra-pharyngeal” ganglion, which gives off
nerves to the mouth and lips, and is connected also with an ‘‘ infra-
pharyngeal” ganglion. The cerebral ganglia are also connected by
short double commissures with the pedals and pleuro-viscerals on the
ventral side of the gullet. The pedal ganglia at each side are in part
divided into two,—one half forming the brachial ganglion which sends
nerves to the arms, the other the infundibular which supplies the
funnel.
406 PHYLUM MOLLUSCA.
The following chief nerves arise from the central system :—
(1) The very thick optic nerves are given off from the commissures
between cerebrals and pleuro-viscerals, and lead to a large
optic ganglion at the base of each eye. is
(2) Ten nerves to the ‘‘arms” are given off by the pedal ganglion,
and this is one of the reasons which have led most morph-
ologists to regard these arms as portions of the ‘‘ foot.”
(3) Two large nerves from the more ventral portion of the pleuro-
visceral ganglia form a visceral loop, and give off many
branches to the gills and other organs. From the pleural
portion arise two mantle nerves, each of which ends in a
large stellate ganglion.
Sense organs.—The eyes are large and efficient. They present a
striking resemblance to those of Vertebrates, and, as they are not ‘‘ brain
eyes,” they illustrate how superficially similar structures may be
developed in different ways and in divergent groups. In cuttlefishes
the eyes lie on the sides of the head, protected in part by the cartilage
surrounding the brain, and in part by cartilages on their own walls.
The eye is a sensitive cup arising in great part from the skin. Its
internal lining is a complex retina, on the posterdor surface of which the
nerves from the optic ganglion are distributed. It seems likely that the
Cephalopod retina corresponds only to the rods and cones (the sensory
part) of the Vertebrate retina. In the cavity of the cup there is a clear
vitreous humour. :
The mouth of the cup is closed by a lens, supported by a ‘ciliary
body.” The lens seems to be formed in two parts—an outer and an
inner plano-convex lens. The pupil in front of it is fringed by a con-
tractile iris.
The outer wall of the optic cup is ensheathed by a strong supporting
layer—the sclerotic, which is in part strengthened by cartilage, covered
by a silvery membrane, and continued into the iris.
In front of the eye there is a transparent cornea, and the skin also
forms protecting lids,
Round about the optic ganglion there is a strange ‘‘ white body,”
which seems to be a fatty cushion on which the eye rests,
The two ear-sacs, containing a spherical otolith and a fluid, sometimes
with calcareous particles, are enclosed in part of the head cartilage,
close to the pedal ganglia. The nerves seem to come from the pedals,
but it is said that their fibres can be traced up to the cerebrals.
A ciliated ‘‘ olfactory sac” lies behind each eye, and is innervated
from a special ganglion near the optic. There are no osphradia of the
usual type.
Finally, there are tactile or otherwise sensitive cells on various parts
of the body, especially-about the arms,
Apart from sight altogether, an octopus can find a dead fish at a
distance of over a yard in a few minutes, and even slight movements in
the water are detected.
In many Decapods there are luminous organs, usually on the ventral
surface in diverse positions, and often buried. They may serve as
recognition-marks or as search-lights. They may be glandular or
ALIMENTARY SYSTEM. 407
non-glandular, and those of the second type are often somewhat
eye-like, with pigment layer, reflector, lens, and diaphragm, or with
some of these structures. .
Alimentary system.—The cuttlefish eats food which
requires tearing and chewing, and this is effected by the
chitinous jaws worked by strong muscles, and by the
toothed radula moving on a muscular cushion. The mouth
lies in the midst of the arms, bordered by a circular lip, and
opens into a large pharynx or
buccal cavity (cf. the snail). The p-Z
narrow gullet passes through the Ch
ganglionic mass, and leads into st,
the globular stomach, lying near 4
the dorsal end of the body. The
stomach is followed by a cecum
or pyloric sac, and the intestine
curves headwards again, to end
far forward in the mantle cavity.
There do not seem to be any
glands on the walls of the food g
canal; the stomach has a hard = §
cuticle ; the digestion which takes
place there must therefore be
due to the digestive juices of the
glandular annexes. Of these the
Fic. 218.—Diagram of the
structure of Sepza.— Mainly
most important is usually called
the liver; it is bilobed, and lies
in front of the stomach, attached
to the cesophagus. Its two ducts
conduct the digestive juice to
the region where the stomach,
“pyloric sac,” and intestine
meet; and these ducts are
fringed by numerous vascular
and glandular appendages, which
are called “ pancreatic,”
after Pelseneer.
a., Eight short arms around mouth ;
Z.a., one of the two long arms;
b., beak of the mouth; c.g., cere-
bral ganglia, with commissures
to the others; £., eye; 2,
gullet; ag., digestive gland (the
“salivary glands” are not repre-
sented); sz., stomach; @., anus}
she, shell-sac with sepiostaire }
&, kidney; &., reproductive
organ; 4r.., branchial heart ;
fy a zill; 2.6., ink-bag; 712.¢.,
mantle cavity; “, funnel.
and arise from the wall of the
unpaired portion of the nephridia.
Far forward, in front
of the large digestive gland, lie two small white glands
on each side of the gullet, with ducts which open into the
mouth (cf. the “salivary glands” of the snail). A diastatic
ferment has been proved in the salivary secretion of
408 PHYLUM MOLLUSCA.
Cephalopods, but that of Octopus has a poisonous, paralys-
ing effect on the crabs, etc., which are bitten, and also a
peptonising action. At ‘the other end of the food canal,
the ink-sac, full of black pigment, probably of the nature
of waste products, opens into the rectum close to the anus.
This ink-sac is a much enlarged anal gland; for, while
most of the bag is made of connective tissue and some
muscle fibres, a distinct gland is constricted off at the
closed end, and the neck is also glandular. Beside the
anus are two pointed papillee. ;
Vascular system.—The blood of Sega is bluish, owing
to the presence of hemocyanin in the serum; the blood
cells are colourless and ameeboid. The median but some-
what oblique ventricle of the heart drives the blood forward
and backward to all parts of the body. It reaches the
tissues by capillaries, and apparently also by lacunar spaces.
The venous blood of the head region is collected in an
annular sinus round the basis of the arms, and passes
towards the heart by a large vena cava, which divides into
two branchial veins, covered by spongy outgrowths of the
nephridia. Joined by other vessels from the apical region
of the viscera, each branchial vein enters a ‘‘ branchial
heart” at the base of each gill. The branchial heart is
contractile, and drives the venous blood through the gills,
whence, purified, it returns by two contractile auricles into
the ventricle. There are valves preventing back-flow from
the ventricle to the auricles, or from the arteries to the
ventricle. Beside each branchial heart lies an enigmatical
glandular structure known as a “ pericardial gland,” possibly
an excretory or incipiently excretory organ. The course
of the blood differs from that in the mussel and snail
in this, that none returns to the heart except from the
respiratory organs. In the nephridial outgrowths around
the branchial veins the interesting parasite Dicyema is found.
Respiratory system.—The blood is purified by being
exposed on the two feather-like gills which are attached
within the water-washed mantle cavity. The water pene-
trates them very thoroughly; the course of the blood is
intricate. At the base of the gills there is some glandular
tissue, which those impatient with enigmas have credited
with blood-making powers.
EXCRETORY SYSTEM. 409
Excretory system.—The excretory system is difficult to dissect and
to explain. On each side of the anus there is a little papilla, through
which uric acid and other waste products ooze out into the mantle
cavity, and so into the water. A bristle inserted into either of these
two papillz leads into a large sac—the nephridial sac. But the two
sacs are united by two bridges, and they give off an unpaired dorsal
elongation, which extends as far back as the reproductive organs.
The dorsal wall of each nephridial sac becomes intimately associated
with the branchial veins, and follows their outlines faithfully. It is
likely that waste material passes from the blood through the spongy
appendices into the nephridial sacs.
Fic, 219.—Diagram of circulatory and excretory systems
in a Decapod-like Sefza.—After Pelseneer.
x, Gill; 2, renal sac; 3, afferent branchial vessel; 4, branchial heart ;
5, abdominal vein; 6, heart; 7, viscero-pericardial sac (body
cavity); 8, genital organ; 9, posterior aorta; 10, “auricle” ; 11,
glandular appendix of branchial heart; 12, renal appendices of
branchial vein; 13, external aperture of kidney; 14, vena cava;
15, anterior aorta; 16, bifurcation of vena cava; 17, reno-pericardial
aperture, E
Into the terminal portion of each nephridial sac, a little below its
aperture at the urinary papilla, there opens by a ciliated funnel another
sac, which is virtually the body cavity. It surrounds the heart and
other organs, and is often called the viscero-pericardial cavity.
Through the kidneys or nephridial sacs it is in communication with the
exterior. Associated with the branchial hearts there are numerous
diminutive cells which contain ammoniacal salts, phosphates, pigment,
etc. ; these waste products are probably passed into the blood and got
rid off by the kidneys, just as, in 2 Vertebrate, the urea formed in the
liver passes by the blood to the kidneys. In Invertebrates there
is often this co-operation between ‘‘closed. kidneys” and ‘‘ open
kidneys.”
410 PHYLUM MOLLUSCA.
Reproductive system.—The sexes are separate, but there
is not much external difference between them, though the
males are usually smaller, less rounded dorsally, and have
slightly longer arms. When mature, the male is easily
known by a strange modification on his fifth left arm. The
essential reproductive organs are unpaired, and lie in the
body cavity towards the apex of the visceral mass.
The testis—an oval yellowish organ—lies freely in a peritoneal sac,
near the apex of the visceral mass. From this sac the spermatozoa pass
along a closely twisted duct—the vas deferens. This expands into a
twofold ‘‘ seminal vesicle,” and gives off two blind outgrowths, of which
one is called the ‘‘ prostate.” The physiological interest of these parts
is that within them the spermatozoa begin to be arranged in packets.
In this form they are found within the next region, the spermatophore
sac, which opens to the exterior to the left of the anus. Each spermato-
7 Phore is like an automatically explosive
bomb; within the transparent shell
there lies a bag of spermatozoa, and a
complex spring-like arrangement. Even
on the scalpel or slide these strange but
efficient bombs will explode. The
liberated spermatozoa are of the usual
type.
The ovary—a large, rounded white
organ—lies freely in a peritoneal sac
near the apex of the visceral mass.
From this sac the eggs pass along a
short direct oviduct, which opens into
the mantle cavity to the left of the anus.
Associated with the oviduct, and pouring
viscid secretion into it, are two large
‘‘nidamental glands,” of foliated struc-
. ture. Close beside these are accessory
glands, of a reddish or yellowish colour,
with a median and two lateral lobes ;
while at the very end of the oviduct are
two more glands. All seem to contribute
to the external equipment of the egg.
The en es pass from the
genital duct of the male to the fifth
left arm, which becomes’ covered
with them and quaintly modified.
This modification of one of the arms
is usual among cuttlefish; indeed,
Fic. 220.— Male of Argo- in some, e.g. Argonazita and Trem-
nauta (after Jatta), show- octopus, the modified arm, with its
ing ‘‘hectocotylus” arm; load of spermatozoa, is discharged
compare Fig. 9 of female. bodily into the mantle cavity of the
GENERAL NOTES ON MOLLUSCS. 4it
female. There its discoverers described it’ as a parasitic worm,
“* Hfectocotylus.” The lost arm is afterwards regenerated. In Sepia,
however, the modified arm is not discharged, but is simply thrust into
‘the mantle cavity of the female. The spermatophores probably enter
the oviduct, and burst there.
Fic, 221.—Bunch of Sefza eggs attached to plant.—After Jatta.
The eggs, when laid, are enclosed within separate black capsules
containing gelatinous stuff, but the stalks of the capsules are united, so
that a bunch of ‘‘ sea-grapes ” results.
GENERAL NoTEs on Mo.Luuscs
From the description of these three types a general idea
of the structure of Mollusca may be obtained, but it should
be noted—(r) that all the three types are specialised ; (2)
that two small classes, the Solenogastres and the Scaphopoda,
are unrepresented in the descriptions; (3) that in the three
classes to which the types belong there is much diversity
of structure, this being especially true of the large and
heterogeneous class of Gasteropods.
In surveying the structure of the whole group, it is con-
venient to begin with the most striking of the external
characters—the absence or presence of a well-developed
head region.
In the Lamellibranchs or Pelecypoda the head is absent,
and along with it the tentacles, the radula, and the
412 PHYLUM MOLLUSCA.
pharynx with all its associated structures. Elsewhere a head
region, usually furnished with tentacles and eyes, and con-
taining within it a pharynx and radula, is always present.
Best developed in Gasteropods and Cephalopods, the head
region may elsewhere be represented, as in Dentalium,
merely by a buccal tube fringed with tentacles.’ Apart from
Lamellibranchs, the radula is characteristic and, with few
exceptions, universal.
Almost as important is the condition of the characteristic
Molluscan foot. Primitively this had the form of a ventral
creeping sole, as shown, for example, in its simplest
Fic. 222.—Common buckie (Buccénum undatum),
e., Eye; s., respiratory siphon ; ¢., operculum ; 7, foot.
condition, in C/zton (Fig. 228). This condition is retained
in many Gasteropods, and in the simplest Lamellibranchs,
like Solenomya. In most Lamellibranchs, however, in
adaptation to a more or less passive life in the sand, the
foot became wedge-shaped, and the characteristic byssus
gland, which secretes attaching threads, is developed. In
the Cephalopods the foot became greatly modified, and in
those related to Sefza a portion of it is specialised as the
funnel—the main organ of active locomotion. That the
condition of the foot cannot in itself be emp‘oyed as a basis
of classification is, however, obvious, when its differences
within the limits of a class are considered. Thus it is
obsolete in the pelagic Phy//irhoé among Gasteropods, in
GENERAL NOTES ON MOLLUSCS. 413
the sedentary oyster among Lamellibranchs ; in the pelagic
Pteropods part of it forms lateral wing-like lobes used in
swimming, while in Janthina, which has a similar habit, its
chief use is to secrete a “float” to which the egg-capsules
are attached. In various Lamellibranchs, and in Dextalium,
itis modified as a conical boring organ.
The mantle is another important Molluscan structure,
and as it secretes the shell, the shape of the latter is of
course determined by it. Primitively the mantle is repre-
sented by a uniform downgrowth of skin from the dorsal
surface, surrounding the ventral foot, and secreting a dorsal
cap-shaped shell. Such a simple condition occurs in the
limpet. In the Lamellibranchs, with the lateral flattening
Fic. 223.—Bivalve (Panopea norvegica), showing siphons.
e., Exhalant aperture ; z., inhalant aperture.
of the body, the mantle becomes divided into right and left
halves, and the shell becomes two-valved. In most Lamelli-
branchs the mantle is prolonged into two tubes or siphons,
through which the water of respiration enters and leaves the
mantle cavity. A similar but unpaired siphon is found in
many Gasteropods. In Scaphopoda the mantle folds fuse
ventrally to form a continuous tube. In most Gasteropods
the mantle skirt is retained, and secretes a spiral shell, as
well as enclosing a space in which the gills lie; in
some, both mantle and shell are absent. In the snail
and its allies (Pulmonata), the mantle forms the
pulmonary chamber, which opens to the exterior by a
small aperture. In Cephalopoda the mantle skirt is well
‘developed and muscular, and, besides sheltering the gills, is
of much importance in locomotion.
414 PHYLUM MOLLUSCA.
Typically the Mollusca are bilaterally symmetrical
animals, and this symmetry is marked in the Solenogastres,
the Lamellibranchiata, and occurs to a less extent in the
Cephalopoda (cf. the unpaired genital organs). In most
Gasteropoda it is completely lost. This seems to be in
some way associated with the dorsal displacement of the
viscera in Gasteropods to form the (usually coiled) visceral
hump. In Cephalopods there is a somewhat similar dis-
Fic. 224.—Nudibranch (Dendronotus arborescens), showing
dorsal outgrowths forming adaptive gills.
placement in a postero-dorsal direction, in Lamellibranchs
in a ventral direction, but in neither case is it so marked as
in Gasteropods.
The characters of the internal organs of Mollusca must
be inferred from the description of the types, but the nature
of the respiratory organs may be briefly noted. Typically,
these consist of two feathery gills, or ctenidia, with an axis
attached to the body and bearing a double row of lamellae.
These are sheltered beneath the mantle, and bear at their
bases two osphradia or smelling patches. Gills of this
typical form occur in Cuttles (4Vawéi/us has four), in the
simplest Gasteropods (but many other Gasteropods have a
simple unpaired gill), and in the lowest Lamellibranchs
(Solenomya, Nucula, etc.). The respiratory organs in other
Mollusca show much diversity when compared with this
primitive type. Thus the gills may be totally suppressed
and the mantle may directly take on a respiratory function.
This occurs in many marine Gasteropods, for example, in
GENERAL NOTES ON MOLLUSCS. 415
the common limpet (fate//a) (Fig. 225), as well as in
terrestrial forms like the snail, where the mantle cavity
forms the pulmonary chamber. Even in Lamellibranchs,
where the gills are present in much modified form, it is
probable that the mantle has much importance in respira-
tion, the gills being perhaps of most importance in connec-
tion with nutrition, and as brood-chambers. In those
Gasteropods in which the gills are suppressed, there are
often special respiratory organs (“adaptive gills”), such as
the circle of plumes around the anus in Doris and its allies
(Fig. 224). The osphradia are
absent in Cephalopods, except
in Nautilus, and one at least
is usually suppressed in Gas-
teropods.
Shell.—On the dorsal surface
of almost every mollusc em-
bryo there is a little shell-sac
in which an embryonic shell is
begun; the adult shell, how-
ever, is always started and
increased by the mantle. Like
other cuticular products, it has
an organic basis (conchiolin or - deed ,
conchin), along | with which Pie, ant
carbonate of lime is associated. 3,4 Hatley.
There is a thin outer “horny 7 Note simple eyes at base of tentacles,
layer, a, thick median “pris- ~ mouth, median foot, and vascular
matic” see oi a Seale. mantle replacing the
an internal mother-of-pear
layer, which may be divided into two strata by a clear
intermediate layer, well seen in the fresh-water mussel,
Margaritana margaritifera.
My. Irvine’s experiments at Granton Marine Station suggest that the
lime salt originally absorbed is not the carbonate (of which there is a
scant supply in sea-water), but the sulphate (which is abundant), and
that the internal transformation from sulphate to carbonate is perhaps
associated with the diffuse decomposition of nitrogenous waste products.
Thus carbonate of ammonia, which seems to occur abundantly in the
mantle of perfectly fresh mussels, would, with calcium sulphate, yield
carbonate of lime and ammonium sulphate. One cannot suppose that
shell-making is expressible in a chemical reaction of this simplicity, but it
416 PHYLUM MOLLUSCA.
is reasonable to inquire how far shell-making may express a primitive mode
of excretion to which a secondary significance has come to be attached.
Pearls are formed in sacs of the external epithelium of the mantle,
sometimes around a centre of a periostracum-like substance, sometimes
around the larva of a Trematode or Cestode. They are to be dis-
tinguished from concretions formed around an intruded irritant particle.
The latter do not show the characteristic lamination of pearls. Some
pearl-like structures are fixed to the shell; true pearls are free. While
some investigators insist on the parasitic origin of pearls, others are
equally emphatic in declaring that they may arise independently. But
all are agreed that they are pathological products.
Larve.—In their life history most Molluscs pass through
two larval stages. The first of these is a pear-shaped or
barrel-shaped form, with a curved gut, and with a ring
of cilia in front of the mouth. It is a “trochosphere,”
such as that occurring in the development of many
“worms.”
Soon, however, the trochosphere grows into a yet more
efficiently locomotor form—the veliger. Its head bears a
ciliated area or “‘velum,” often produced into retractile
lobes ; its body already shows the beginning of “foot” and
mantle ; on the dorsal surface lies the little embryonic shell
gland (Fig. 206).
But although trochosphere and veliger occur in the
development of most forms, they do not in any of the
three types which we have particularly described,—not in
Anodonta, partly because it is a fresh-water animal, with a
peculiarly adhesive larva of its own; not in /e/ix, partly
because it is terrestrial; and not in Sega, partly because
the eggs are rich in yolk.
CLASSIFICATION OF MOLLUSCA
Leaving aside the difficult Solenogastres, which may not be Molluscs
at all, we may rank as lowest the Isopleura, bilaterally symmetrical
Gasteropods with many primitive characters. Some of these forms, like
Chiton, are probably not far removed from the primitive Mollusca.
From primitive forms, related perhaps to Chzfon, Mollusca have
diverged in two directions. In Gasteropoda, Scaphopoda, and
Cephalopods, the head region becomes well developed, and the radula
present in the primitive Isopleura is retained, except in rare cases, such
as one of the species of Hudéma, a semi-parasite. These three classes
are therefore often placed together as Glossophora or Odontophora,
in contrast to the Lamellibranchiata (Lipocephala or Acephala),
GASTEROPODA. 417.
where the radula has disappeared, and the head region remains un-
developed. As already seen, however, the lowest Lamellibranchs
have a flattened creeping foot and simple feathery gills, in these respects
resembling Gasteropods. There is also much reason to believe that the’
Scaphopota arose from a stem common to them and the lowest Gastero-
pods, which are central unspecialised forms. The Cephalopoda are
the most highly specialised of all the Mollusca, and in their existing
forms at least not nearly related to the other classes.
Class I. GASTEROPODA
Molluscs with a usually well-developed head region with
tentacles and odontophore. The foot is usually a flat median
sole on which the animal creeps; it ts often divided into pro-,
meso-, and meta-podium. Most are unsymmetrical, but there
ts a primitive bilateral symmetry in. Isopleura and a secondary
superficial bilateral symmetry in some pelagic forms such as
Fleteropods. The manile or covering of the visceral sac usually
Jorms a well-marked fold or flap where the visceral sac joins
the head and foot, and thus encloses a mantle cavity. In most
cases the shell ts a single piece; in Chitons there are eight
pieces; in many cases the shell ts rudimentary or absent.
There is usually a trochosphere and veliger larva, except in
terrestrial forms.
Sub-class I. GasTEROPODA ISOPLEURA
The [sopleura are marine Gasteropods more or less elongated
in form, with bilateral symmetry. The symmetry ts not only
seen in the form of the body, but in the numerous ctenidia, the
paired nephridia, auricles, and genital ducts. The shell con-
sists of eight pieces. The mouth ts anterior ; the anal and”
nephridial apertures are posterior. The mantle, which bears
cuticular spicules, covers at least a great part of the body.
The nervous system consists of a cerebral commissure and
two paired longitudinal cords (pedal and visceral), with
ganglionic cells but at most very slightly developed ganglia,
which run the whole length of the body. Of these paired cords
the pedals are connected by numerous cross-commissures, and
the viscerals or pallials are united posteriorly by a commissure
above the rectum. The bilateral symmetry ts shown internally,
27
418 PHYLUM MOLLUSCA.
eg.
Fic. 226.—Chiton,—
After Prétre,
Fic. 227.—Dorsal view
of nervous system of
Acanthochiton.—After
Pelseneer.
x, Upper buccal commissure ;
2, upper buccal ganglion ;
3, stomatogastric commis-
sure; 4, labial commissure;
5, sub-radular ganglia; 6,
anterior pedal commissure }
7, pedal nerve with pallio-
pedal connections; 8,
supra-rectal pallial com-
missure ; 9, pallial nerve;
10, anastomosis of branches
ofpedalnerves; r1,stomato-
gastric ganglia; 12, ceso-
phageal nerves; 13,cerebral ;
cominissure,
in the paired nephridia, auricles, and genital ducts.
The class, is of ancient origin,
dating from the Silurian, There
is one order—Polyplacophora, ©.
Chiton. -
The Isopleura or Polyplacophora are
represented on British coasts by several
species of Chzton, sluggish, usually vege-
tarian, animals, occurring from the shore
to great depths. The foot is generally as
long as the body; the mantle covers the
back and bears eight shell-plates (Fig.
226), perforated, in many cases at least,
“by numerous sensory organs, which are
in part optic; numerous gills lie in a
regular row along a groove on each side
between the mantle and the foot.
In most cases the eight shell-plates are
jointed..on one another, and the animal
can roll itself up. The uncovered parts
of the mantle bear spicules. Ganglia, in
the strict sense, are scarcely developed,
but there is a supra-cesophageal gangli-
onic commissure from which the visceral
and pedal cords extend backwards along
the whole length of the body. There are
no special sense organs on the head,
which is but slightly differentiated; but
the pallial sense organs are usually numer-
ous and varied. A twisted gut runs
through the body, surrounded by a diffuse
digestive gland. There is a radula in the
mouth. The heart is median and pos-
terior, and consists of a ventricle and
two to eight auricles. There are two
symmetrical nephridia opening posteriorly,
and consisting of much-branched tubes,
The sexes are separate; a single repro-
ductive organ extends dorsally between
gut and aorta almost the whole length
of the body; the genital ducts are paired
and open posteriorly in front of the
excretory apertures. The ova, with
chitinous spiny shells, are usually re-
tained for some time by the female be-
tween the mantle and the gills. The
.segmentation is holoblastic, and a gastrula
is formed by invagination.
GASTEROPODA. 419
/ -_
UTR
Fic. 228.—Anatomy of Chzton.
A, ventral surface (after Cuvier), B, dorsal view of alimentary
canal (after Lankester). C, genital and excretory organs from
dorsal surface (after Lang and Haller, diagrammatic). 7.,
mouth ;a@., anus ; 6v., numerous simple gills ; 7, foot ; 4., buccal
mass ; 4., liver 3 z., intestine ; ao., aorta; v., ventricle of heart ;
ya and Za., right and left auricles ; ov., ovary ; od., oviduct ;
od'., opening of oviduct ; ., part of nephridium, represented in
black throughout ; ~o., external opening of nephridium; Z.,
outline of pericardium.
Sub-class II. GasTEROPODA ANISOPLEURA, 6.2.
Snail, Whelk, Limpet
In these more or less asymmetrical Gasteropods, the head
region, which is well developed, remains symmetrical, and so
does the foot, which ts typically a flat creeping organ. But
the visceral mass_or hump, with its mantle fold, is more or less
twisted forwards and to the right. Thus the pallial, anal,
nephridial, and genttal apertures usually lie on the right side,
more or less anteriorly. A further asymmetry is shown by
the twisting of the morphologically right gill to the left side,
while the original left gill ts usually lost. Similarly, one of
the nephridia, probably that which is morphologically the left,
tends to disappear, and in most cases only one persists—
topographically on the left side. The main torsion must be
distinguished from the spiral twisting which the visceral
hump often exhibits, and from the frequently associated spiral
coiling of the untvalve shell, Moreover, a superficial secondary
bilateral symmetry tends to be acquired by free-swimming
Jorms, e.g. Heteropods. “There are never more than two gills
420 PHYLUM MOLLUSCA.
of the ctenidium type. The shell is usually in one piece ; but
it is sometimes rudimentary or absent. The foot usually
contains a mucus gland, and tends to be divided into three
regions—the pro-, meso-, and meta-podium. There ts a singl:
reproductive organ and genital duct.
Branch A. STREPTONEURA
In the torsion of the body one limb of the visceral loop crosses the
other in a figure 8.
Order 1, ZYGOBRANCHIATA
The atrophy of the primitively left-side gills and nephridia is not
carried out, or only partially, e.g. Halzotds (ear-shell) ; Azsszzella (key-
hole limpet) ; Pated/a (limpet).
Order 2, AZYGOBRANCHIATA
The originally left gill and the originally left nephridium have been
lost. Heart with single auricle, one gill, one nephridium ; operculum
present. : Fa
Periwinkle (Lzttorina), buckie (Buccznum, Fig. 222), dog-whelk
(Purpura), Ianthina, and the majority of the marine Gasteropods
with coiled shells, together with some fresh-water forms. The
pelagic Heteropods are also included here :—Ad¢/anéa, shell well
developed ; Cardnaria,
-with small shell; P/ezo-
trachea, with no shell.
Branch B. EUTHYNEURA
The visceral loop does not
share in the torsion of the
visceral hump.
Order 3.
OPISTHOBRANCHIATA
The visceral loop is euthy-
neural, as in snails; the
single auricle lies behind the
ventricle; the shell and
mantle are often absent.
A. Tectibranchiata. A
shell is present,
but may be rudi-
mentary; there is
Fic. 229.—A_ Ptleropod (Cymbulia a well-developed
perontt), showing the wing-like expan- mantle fold and
sions (pteropodial lobes) of the mid-foot. a single gill, e.g.
MODE OF LIFE. 421
Bulla, Aplysta, Dolabella, Umbrella. The Tectibranchiata
also include the Pteropoda, the winged snails or sea-butter-
flies, which have become much modified for pelagic life.
They have a secondarily acquired bilateral symmetry, and-
swim by two large lateral lobes of the foot. They often
swim actively in shoals, and occur in all seas. They afford
tood for whales, etc., and the shells of some are abundant in
the ooze. They include—
(a) Thecosomata, with mantle fold and shell, diet of
minute animal or vegetable organisms, closely related
to Bulla and its allies.
Examples.—Hyalea, Cymbulia.
(4) Gymnosomata, without mantle fold or shell in the
adult. Closely allied to Aflys¢a and its allies.
Actively carnivorous, ¢.g. Clio, Pueumoderma,
B. Nudibranchiata. Shell, mantle fold, and true gill are absent ;
various forms of ‘‘ adaptive gills” may be present, or there
may be no special respiratory organs, ¢.g. sea-slugs, Dords,
Lolis, Dendronotus (Fig. 224).
Order 4. PULMONATA
The visceral loop is short and untwisted, gills are absent, and the
mantle cavity functions as a lung ; all are hermaphrodite, e.g. the snail
(Helix); the grey slug (Zzmax) ; the black slug (dréon) ; fresh-water
snails, such as Lemnea, Planorbis, and Ancylus.
Mode of life.—From the number of diverse types which
the class includes, it is evident that few general statements
can be made about the life of Gasteropods. We are safe in
saying, however, that though the majority are sluggish when
compared with Cephalopods, they are active when compared
with Lamellibranchs. ;
The locomotion effected by the contractions of the
muscular foot is usually a leisurely creeping, but there are
many gradations between the activity of Heteropods in
open-sea, the gliding of fresh-water snails (Zzmuca) foot
upwards across the surface of the pool, the explorations of
the periwinkles on the sand of the shore, and the extreme
passivity of limpets (/a¢e//a), which move only for short
distances at a time from their resting-places on the rocks.
The number of terrestrial snails and slugs, breathing the
air directly by means of a pulmonary chamber, is estimated
at over 6000 living species, while the aquatic Gasteropods
are reckoned at about 10,000, most of which are marine.
422
PHYLUM MOLLUSCA.
Of this myriad, about gooo are streptoneural, the relatively
small minority are euthyneural Opisthobranchs and Nudi-
branchs, with light shells or none.
The Heteropods and
some Opisthobranchs live in the open sea; the great
majority of aquatic Gasteropods frequent the shore and
the sea bottom at relatively slight depths; the deep-sea
forms are comparatively few.
Gasteropods rarely feed at such a low level as bivalves do
Fic. 230, —Stages in mol-
luscan development.
A., Blastula of limpet (after
Patten). £, Gastrula_ of
Paludina vivipara (after
Tonniges); v., beginning of
velum; a@vc., archenteron ;
m., mesoderm cells. C,
later stage of the same; w,
velum; 7#., mouth inva-
gination; a7c., _archen-
teron; a., anus; f, begin-
ning of foot; sh.g., shell
gland.
—indeed, some of them are fond of
eating bivalves. Most Prosobranchs
(streptoneural), with a respiratory
siphon and a shell notch in which
this lies, are carnivorous, e.g. the
buckies (Buccinum) and “dog-whelks”
(Purpura); on the other hand, those
without this siphon, and with an
unnotched shell mouth, feed on
plants, ¢g. the seaweed-eating peri-
winkles (Litforina). Most land
snails and slugs are vegetarian.
Many Gasteropods, both marine and
terrestrial, are voracious and indis-
criminate in their meals; others are
as markedly specialists or epicures.
Some marine forms partial to Echino-
derms have a salivary secretion of
dilute sulphuric acid, which changes
the carbonate of lime in the starfish
into the more brittle and readily
pulverised sulphate. About ten
genera are parasitic on or in Echino-
derms, e.g. Stylifer, Turtonia, Thyca,
and the extremely degenerate Ex/o-
concha, within the Holothurian Syz-
afta. Some species of Lu/ima also
live a semi-parasitic life on certain
Echinoderms.
Life history.—The eggs of Gasteropods are usually small,
without much yolk, but surrounded by a jelly, the surface
of which often hardens.
is an egg-shell of lime.
In the snail and some others there
LIFE HISTORY—@GCOLOGY. 423
Sexual union occurs between hermaphrodites as well as
between separate sexes, and fertilisation is effected inside
the genital duct. Development sometimes proceeds within
the parent, but in most cases the fertilised eggs are laid in
gelatinous clumps, or within special capsules. The free-
swimming Janthina carries the eggs in capsules attached to
a large raft-like float towed by the foot. On the shore
one often finds numerous egg-capsules of the “buckie”
(Buccinum undatum) united in a ball about the size of an
orange. Under the ledges of ‘rock are many little vases or :
cups, the egg-capsules of the dog- -whelk (Purpura lapillus).
In the buckie and whelk, and in some other forms, there is
a struggle for existence—an infant cannibalism—in the
cradle, for out of the numerous embryos in each capsule
only a few reach maturity,—those that get the start eating
the others as they develop.
The development is usually simple and iynical In other
words, segmentation is total though often unequal ; gastrula-
tion is embolic or epibolic according to the amount; of yolk
present ; the gastrula becomes a trochosphere, and later a
veliger (Fig. 230).
Past history.—As the earth has grown older the Gasteropods have
increased in numbers. A few have been disinterred from the Cambrian
rocks ; thence onwards they increasé. Most of the Paleozoic genera
are now quite extinct, but many modern families trace their genealogy
to the Cretaceous period. Those with respiratory siphons were hardly,
if at all, represented in Paleozoic Agee, and the terrestrial'air-breathers —
are comparatively modern. 3
CEcology.— As voracious animals, with irresistible
raspers, Gasteropods commit many atrocities in the
struggle for existence, and decimate many plants. Professor
Stahl shows, however, that there are more than a dozen
different ways in which plants are saved from snails,—by
crystals, acids, ferments, etc.; in short, by constitutional
characteristics sufficiently important to determine survival
in the course of natural selection or elimination. As food
and: bait, many Gasteropods are very useful; their shells
have supplied tools and utensils and objects of delight ; the
juices of Purpura and Murex furnished the Tyrian purple,
more charming than all aniline.
mer)
424 PHYLUM MOLLUSCA.
Class II. SOLENOGASTRES
The members of this class are worm-like animals, in which the
mantle envelops the whole body and bears numerous spicules, but no
shell. It is somewhat doubtful if they are Molluscs at all. There are
two families—Neomeniidze and Cheetodermidee.
Of Neomeniide, six genera are known, e.g. Meomenia and Pro-
neomenta, They have a longitudinal
pedal groove, an intestine without
distinct digestive gland, two neph-
cbc wea ridia with a common aperture, and
ae ip hermaphrodite reproductive organs.
VY, The Cheetodermidz, represented by
EF one genus Chetoderma, are cylin-
ry drical in form, without a pedal groove,
rae ot OO with a radula bearing one tooth, with
1 a distinct digestive gland, and with
TY, two nephridia opening separately into
rT a posterior cavity, which also contains
rt: two gills. The sexes are separate.
—1-T
mA Class III. ScaPHOPODA
a a Very different in many respects from
7 26. Gasteropoda are the Scaph opoda, of
ma which Deztalzum (Elephant’s tooth-
Esl > shell) is the commonest genus. They
| g are apparently related to the Zeugo-
me branchiate Gasteropods, and also to
fo the simplest Bivalves. They burrow
wa we in the sand at considerable depth off
wz the coasts of many countries. The
a w p es mantle has originally two folds, which
fuse ventrally, and the shell becomes
cylindrical, like an elephant’s tusk.
Fic. 231.—Proncomenia, Ner- It is open at both ends. The larger
vous system.—From Hubrecht. OPEnns (directed downwards in the
es Coccbeal arigtist sie gout fueueli sand) is anterior, the concave side of
a'p.g., anterior pedal; ",A.g., pos- the shell is dorsal. The mouth opens
terior pedal; A.z.g., posterior, vis- at the end of a short buccal tube,
cerals ; sZ., sublingual connectives; at the base of which is a circle of
ees ,,cerebro-pedal connective; #e. ciliated tentacles. The foot is long
ongitudinal pedal nerves ; /a., long- ~. . 2
ftadinal lateral nerves: with three small terminal lobes. It
is used in slow creeping, and is pro-
truded at the anterior opening. There are cerebral and pleural ganglia
near one another in the head, pedal ganglia in the foot, and a long
untwisted visceral loop with olfactory ganglia near the posterior anus.
Sense organs are represented by otocysts beside the pedal ganglia.
There is an odontophore with a simple radula, The food consists of
minute animals. There isa much reduced heart, and colourless blood
circulates in the body cavity. There are two nephridial apertures, one
LAMELLIBRANCHIATA. 425
on each side of the anus ; and two nephridia. The sexes are separate ;
the reproductive organ is simple and dorsal in position; the elements
pass out by the right nephridium. The gastrula is succeeded by a free-
swimming stage, in which there is a hint of a velum and a rudimentary
shell gland.
Examples.—Dentalium, Entalium. About forty widely distributed
species are known. Dentalium entale occurs off British coasts.
The genus occurs as a fossil from Devonian strata onward.
Class IV. LAMELLIBRANCHIATA or BIVALVES
(Synxonyms—Acephala, Conchifera, Pelecypoda,
Lipocephala, etc.)
Examples.—Cockles, Mussels, Clams, and Oysters
Lamellibranchs are bilaterally symmetrical Molluscs, in
which the body is compressed from side to side and the foot
move or less ploughshare-like. The head (or prostomium)
region remains undeveloped, and without tentacles ; radula,
horny jaws, and salivary glands are absent, but there ts a
pair of labial palps on each side of the mouth. The mantle
skirt ts divided into two flaps, which secrete the two valves of
the shell, now lateral instead of dorsal in position. The
values are united by a dorsal elastic ligament, and closed by
two transverse adductor muscles or by one. Internal bilateral
symmetry is marked by the paired nature and disposition of
the nephridia, auricles, gills, digestive gland, and reproductive
organs. The gills (ctenidia) consist of numerous gill filaments,
which typically grow together into large plates (hence the title
Lamellibranch). There are usually three pairs of ganglia:
(a) cerebropleurals in the head ; (b) pedals in the foot; (c)
viscerals at the posterior end of the body. The heart consists
of a ventricle and two auricles, and is surrounded by a
pericardium which is coelomic in origin, and communicates
with the exterior by means of the two nephridia. Repro-
ductive organs are always simple, and the sexes are usually
separate. The typical development includes trochosphere and
veliger stages. Most Lamellibranchs feed on microscopic
organisms and particles ; the distribution is very wide, both
in salt and fresh water ; the general habit ts sedentary or
sluggish.
426 PHYLUM MOLLUYSCA.:
Classification.—That of Pelseneer is based on the structure of
the gills.
Order 1. PROTORRANCHIA.—There are two simple posterior gills,
quite similar to those of Zeugobranchs ; the foot has a flattened creeping
surface ; the pleural and cerebral ganglia are distinct, e.g. Mucula,
Solenontya.
Order 2. FILIBRANCHIA.—The gill filaments are greatly elongated
and reflected, so that they consist of an ascending and a descending limb,
e.g. Arca (Noah’s-ark shell), AZy¢zdus (edible mussel), AZodzo/a (horse-
mussel).
Order 3. PsEUpO-LLAMELLIBRANCHIA.—The successive gill filaments
are loosely connected together to form gill-plates/ ¢.g. Pecten (scallop),
Ostrea (oyster).
Order 4. EULAMELLIBRANCHIA.—The separate filaments are no
longer discernible ; the gills form double flattened plates. The great
majority of Bivalves are included here, eg. Anodonta, Venus, Pholas
(a boring form), JZya.
GENERAL NoTES ON LAMELLIBRANCHS
Structure.—The organs which show most variety in. bivalves are
the foot, the gills, the adductor muscles, and the mantle skirt. The
foot shows much diversity in size and shape; the pedal gland of
Gasteropods is often represented by a ‘‘ byssus” gland, which secretes
attaching threads, well seen in the edible mussel (J/y2z/us). The gills
show a series of gradations, from a slight: interlocking of separate gill
filaments to the formation, by complicated processes of ‘‘ concrescence,”
of plate-like structures such as those of Avxodonta. These processes
are more closely related to the method of nutrition than of respiration,
which, indeed, is probably largely performed by the mantle skirt. The
mantle skirt is often united to a greater or less extent inferiorly, and is
often prolonged and specialised posteriorly to form exhalant and inhalant
“siphons” (Fig. 223). These siphons sometimes attain a considerable
length ; they occur especially in forms such as AZya, which live buried
in sand or mud, or which burrow in wood or stone, e.g. Pholas._ The
diversities in the adductor muscles afford one basis for classification.
We may associate with the sluggish habits and sedentary life of
bivalves—(1) the undeveloped state of the head region ; (2) the largeness
of the plate-like gills, which waft food-particles to the mouth; and (3)
the thick limy shells. We may reasonably associate these and other
facts of structure (e.g. the rarity of anterior eyes, biting or rasping
organs) with the conditions of life.
In some Lamellibranchs, ¢.g. Mytilidee, small eyes occur at the base
of the most anterior filament of the inner gill-plate; in some other
cases they are present in the larva, but not in the adult.
Habit.—Most bivalves, as every one knows, live in the sea, and
their range extends from the sand of the shore to great depths. They
occur in all parts of the world, though only a few forms, like the edible
mussel (AZytz/us edulis), can be called cosmopolitan. Some, such as
oysters, can be accustomed to brackish water. ‘The fresh-water forms
may have found that habitat in two ways—(a) a few may have crept
GENERAL NOTES ON LAMELLIBRANCHS. 427,
slowly up from estuary to river, from river to lake; Dvrezssensia poly-
morpha has been carried on the bottom of ships from the Black Sea to
the rivers and canals of Northern Europe; and it is likely that aquatic
birds have assisted in distributing little bivalves like Cyc/as ; (4) on the
other hand, it is more probable that the fresh-water mussels (U/7zo,
Anodonta, etc.) are relics of a fauna which inhabited former inland
seas, of which some lakes are the freshened residues.
Between the active Zzma and Pecten, which swim by moving their
shell valves and mantle flaps, and the entirely quiescent oyster, which
has virtually ‘no foot, there are many degrees of passivity, but most
incline towards the oyster’s habit. Of course, there is much internal
activity, especially of ciliated cells, even in the most obviously sluggish.
The cockle (Cardizum) uses its bent foot to take small jumps on the
sand ; the razor-fish (Soe) not only bores in the sand, but may swim
backwards by squirting out water from within the mantle cavity ; many
(e.g. Leredo, Pholas, Lithodomus, Xylophaga) bore holes in stone or
wood ; in the great majority the foot is used for slow creeping motion.
The food consists of Diatoms and other Algze, Infusorians and other
Protozoa, minute Crustaceans and organic particles, which the cilia of
the gills and palps sweep towards the mouth. The bivalves are them-
selves eaten by worms, starfishes, gasteropods, fishes, birds, and even
mammals.
Several commensal bivalves (Montacutidz) are known,—Montacuta
on heart-urchins, Z7zova/va in the gullet of Synaptids, Sczoderetza on a
sea-urchin, and Jousseaumdella on a Sipunculid.
Life history.—The eggs are sometimes laid in the water, either
freely or in attached capsules, or they are fertilised by spermatozoa
drawn in with the inhaled water, and are subsequently sheltered within
the’ body during part of the development. In the Unionidz the
embryos are retained within the cavities of the outer gills; in Cyclas
and Pzszdium there are special brood-chambers at the base of the gills.
In Cyc/as the embryos are nourished by the maternal epithelial cells.
Seginentation is always unequal ; a gastrula may be formed by invagina-
tion or by overgrowth, the two cases being connected by a series of
gradations. A trochosphere stage is more or ‘less clearly indicated,
being most obvious in cases where the eggs'are laid in the water.
The free-swimming trochosphere becomes a veliger, and this is
modified into the adult. The fresh-water forms, with the exception
of Dredssensia polymorpha, in which the habit is recently acquired, do
not possess free-swimming larvze ; this must be regarded as an adapta-
tion.
Past history of bivalves.—Even in Cambrian rocks, which we
may call the second oldest, a few bivalves have been discovered ; in the
Upper Silurian they become abundant, and never fall off in numbers.
Those with one closing muscle to the shell seem to have appeared after
those which have two such muscles. Those which, from the shell
markings, seem to have had an extension of the mantle into a pro-
trusible tube or siphon, were also of later origin’ The present fresh-
water forms were late of appearing. Of all the fossil forms the most
remarkable are large twisted shells, called Azppurdtes (Rudistze), whose
remains are often very abundant in deposits of the chalk period.
428 PHYLUM MOLLUSCA.
Class V. CepHALopopa. Cuttlefish
Examples.— Sepia, Octopus (Polypus), Loligo, Nautilus
The Cephalopods, are bilaterally symmetrical and free-
swimming. The head is surrounded by numerous “ arms”
bearing tentacles or suckers. These arms seem to be equivalent
to processes of the margin of the foot. Another portion of the
foot forms a partial or complete tube—the “siphon” or
“ funnel” —through which water ts forcibly expelled from the
mantle cavity.. The muscular mantle flap which shelters
well-developed plumose gills is posterior in position; the
visceral hump shows no trace of spiral coiling, but is elongated
in a direction anatomically dorsal and posterior, though it
may point forwards when the animal propels itself through
the water. Except in the pearly Nautilus, the shell of
modern forms has been enclosed by the mantle, and ts, in most
cases, only hinted at. There is a very distinct head region,
jurnished with eyes and other sensitive structures, and the
mouth has strong beak-like jaws, as also a well-developed
radula, The nervous system shows. considerable specialisa-
tion; the chief gangla are concentrated in the head, and
sheltered by cartilage. The true body cavity, pericardium of
other Molluscs, ts usually well developed, and frequently
surrounds the chief organs. Except in the Nautilus, it com-
municates with the exterior by the nephridia. The nephridia
are disposed on the walls of the afferent branchials.
The vascular system is well developed, and, except in the
Nautilus, there are accessory branchial hearts. The sexes are
separate. The gonad ts in a celomic sac and not directly
continuous with the gonoduct. The ovum undergoes incom-
plete segmentation. Development is direct. ln habit,
Cephalopods are predominantly active and predatory ; in diet,
carnivorous.
The shells of the pearly Nautilus are common on the
shores of warm seas, but the animals are much less familiar.
The Nautilus creeps or swims gently along the bottom at
no great depth, and its appearance on the surface, “ floating
like a tortoiseshell cat,” is probably the result of storms.
It is called “pearly” on account of the appearance of the
CEPHALOPODA. 429
innermost layer of the shell. This is exposed after the soft
organic stratum and the median porcellanous layer which
bears bands of colour have been worn away, or dissolved
in a dolphin’s stomach, or artificially treated with acid.
The beautiful shell is a spiral in one plane, divided into
a set of chambers, in the last of which the animal lives,
while the others contain gas. The young creature inhabits
a tiny shell curved like a horn; it grows too big for this,
and proceeds to enlarge its dwelling, meanwhile drawing
itself forward from the older part, and forming a door of
lime behind it. This process is repeated again and again; -
as an addition is made in front,
the animal draws itself forward
a little, and shuts off a part of
the chamber in which it has
been living. All the compart-
ments are in communication by
a median tube of skin—the
siphuncle—which is in part cal-
careous. ,
It has been suggested ‘that
“each septum shutting off an
air-containing chamber is formed
during a period of quiescence,
probably after the reproductive
act, when the visceral mass’ of
the Nautilus may be slightly
shrunk, and gas is secreted from Z Sree Satie may
the dorsal integument so as to “Fe Ee ee eld.
fill up the space previously
occupied by the animal.”
There can be no confusion between the beautiful shell of
the cuttlefish called the paper Nautilus (Avgonauta argo)
and that of our type. For it is only the female Argonaut
which bears a shell; it is not chambered, and is a shelter
for the eggs—a cradle, not a house. It seems to be
formed by two of the arms.
It is instructive to compare the Nautilus shell with that
of some Gasteropods, for there also chambers are occasion-
ally, formed. But these arise from secondary.alterations of
an originally continuous spiral. The Gasteropod shell is
430 PHYLUM MOLLUSCA.
usually unsymmetrical, and the foot (ventral) is turned
towards the internal curve of the coil, while in Nautilus
the dorsal part of the animal is towards the internal
surface of the chamber.
Fic. 233.—The Pearly Nautilus (Mazte/us pompilius).
—After Owen.
The shell is represented in section, but the animal is not dissected.
Part of the mantle has been removed. c¢., Last or body
chamber, separated by a septum (se.) from the compartment
behind ; s., the siphuncle traversing all the compartments ; 7z.,
the portion of the mantle which is reflected over the shell; 4.,
the hood ; ¢., the eye with its opening to the exterior; 7, the
lobes which bear the sheathed tentacles (#.); sz., the incomplete
sipaons mit, the shell muscle ; ., the position of the nidamental
gland.
There are only about half a dozen living species of
NVautilus, but there are many hundred fossils of this and
allied genera. This list is usually swelled by the addition
of the extinct Ammonites, but there are some reasons for
| CEPHALOPODA.
431
suspecting that these belong to the Dibranchiate section
of Cephalopods.
The following table states the chief points of distinction
between Mautilus and the other series of Cephalopods :—
CEPHALOPODA
TETRABRANCHIATA (Wautdlus).
D1BRANCHIATA (Sepia, Octopus, etc.)
Allextinct except one genus—Nautilus ;
the extinct forms are usually ranked
as Nautiloid and Ammonoid.
Shell external, chambered, straight or
bent or spirally coiled. That in which
Nautilus lives has been described,
with its siphuncle, gas-containing
compartments, etc.
The part of the foot surrounding the
mouth bears a large number of lobes,
which.carry tentacles in little sheaths,
but no suckers.
The two mid-lobes of the foot form a
siphon, but they are not fused into a
tube, .,
The eye is without a lens, and is bathed
internally by sea-water, which enters
by a small pinhole aperture. There
are two
patches at the bases of the gills.
phridia; two genital ducts (the left
rudimentary). 4
The ccelom sac -(pericardium) opens
directly to the exterior by two aper-
tures.
The heart has two pairs of auricles, and
there are no branchial hearts.
No ink-bag. No salivary glands.
“‘osphradia”” or smelling |
“Two pairs of gills; two pairs of ne-
;, Eledone moschata; in others an un-
Numerous living genera, ranked as
Decapods or Octopods; along with
the former the extinct Belemnites are
included.
No living Dibranchiate lives in a shell.
The shell was internal even in the
extinct Belemnites, and in modern
forms it occurs in various degrees of
degeneration (cf. Spzrula, Sepia,
Loligo), or is quite absent (Octopoda).
The part of the foot surrounding the
mouth is divided into ten or eight
arms, which carry suckers, stalked in
Decapods, sessile in Octopods.
The two mid-lobes of the foot fuse to
form a completely closed tubular
siphon or funnel.
The covering of the eye may be per-
forated, but the mouth of the retinal
cup is closed by alens, There are no
osphradia, though there may be
“olfactory pits” behind the eyes.
One pair of gills; one pair of nephridial
sacs; two oviducts in Octopoda and
Oigopsida; two vasa deferentia in
. paired genital duct.
The ccelom opens into the nephridia
by two pores, and thus to the ex-
terior.
The heart has two:auricles, and there
are branchial hearts.
Usually with an ink-bag.
Salivary
glands.
CLASSIFICATION OF CEPHALOPODA.
Order I. Tetrabranchiata (see Table).
Family I. Nautilide.
Nautilus alone alive; but a great
series of fossil forms, Orthoceras— Trochoceras.
Family II. Ammonitide.
All extinct, but with shells well
preserved, so that long series can be studied. They
furnish striking evidence of progressive evolution in
definite directions, e.g. Bactrztes, Ceratites, Baculites,
Turrilites, Heteroceras, and the whole series of genera
formerly classed as Ammonttes.
432 PHYLUM MOLLUSCA.
Order II. Dibranchiata (see Table).
-Sub-Order Decapoda. Eight shorter and two longer arms.
Suckers stalked and strengthened by a strong ring.
Large eyes with a horizontal lid. Body elongated, with
lateral fins. Mantle margin with a cartilaginous ‘‘hook-
and-eye” arrangement.’ Some sort of internal ‘‘ shell,”
enclosed by upgrowths of the mantle.
With calcareous internal ‘‘shell.” Sfzvula; extinct Bel-
emnites ; Sepia,
With organic internal ‘ shell.”
(a) Eyes with closed cornea, Myopsida, e.g. Loligo.
(4) Eyes with open cornea, Oigopsida, ¢.g. Ommeastrephes.
Sub-Order Octopoda. Eight arms only. Suckers sessile
without horny ring. Small eyes with sphincter-like .
lid. Body short and rounded. No ‘‘hook-and-eye”
arrangement. No ‘‘shell,” except in the female
Argonauta,
eg. Octopus (Polypus), Eledone (Moschites), Argo-
naula, Ctrroteuthts (with cirri on the arms and
no radula).
The classification given above is that usually adopted, but it is not
certain that the Ammonites should be included in the Tetrabranchiata.
The Nautiloids began in the Cambrian and died out at the end of
the Paleozoic period, except the Orthoceras and MNautclus-like types.
The genus Vauti/us appeared in the Cretaceous. The Ammonite series
lasted from the Silurian to the early Tertiary.
The Cephalopods are the most specialised of the
Molluscs, and present much diversity of type. They swim
freely in the sea, or creep sluggishly among the rocks.
They are voracious eaters, and devour very diverse kinds
of animals, their parrot-like jaws and powerful odontophore,
as well as the numerous suckers, rendering them formid-
able adversaries. Many live at considerable depths, and
their chief foes are the toothed whales, some of which, like
the sperm whale (Pfyseter), and the bottlenose (Ayperoo-
don), subsist almost entirely on cuttles. Some deep-sea
forms have highly evolved luminous organs.
Shells of Cephalopods.—A chambered external shell, serving as
a house, is present in Mazedéz/us alone among living Cephalopods.
Most of the extinct forms had large and efficient shells of very
diverse shape, some straight like Orthoceras, or coiled, with chambers
separated by complex septa, as in the Ammonites. In the majority
of shells of the Ammonitid series, the septa between the chambers
are convex towards the aperture (the opposite in the Nautilus); the
siphuncle is marginal or ventral; the septal necks of the siphuncle
project forwards (not backwards as in the Nautilus); there is
CEPHALOPODA. 433
an initial chamber or protoconch at the apex of the spiral (per-
haps represented by a cicatrix in the Nautilus); the suture lines
marking the chambers tend to be lobed. There is often a single or
paired ** Aptychus,” perhaps of the nature of an operculum. Most
of the modern forms seem to be more active than their ancestors, and
their shells have degenerated. But the line of degeneration is still
debated. In Nautilus, although the animal lives within the shell, the
mantle fold is for some distance reflected over it; in the other series
of Cephalopods this process has gone farther, and, where a shell is
present, it is enclosed within the mantle fold, and is much reduced in
size. In the extinct Belemnites the internal shell was straight and
chambered, but almost concealed by secondary deposits of lime,
secreted by the walls of the shell-sac, and forming the ‘‘ guard” or
rostrum. The conical chambered shell, with a siphuncle, is known
as the phragmacone. It is produced anteriorly into a gladius or pro-
ostracum. In the extinct Spirudérostra the shell was spiral and
mostly internal; it has a guard. In Spzrz/a the shell can be caught
sight of in the young animal, but it becomes surrounded by the
secondary mantle folds that form the mantle-sac.. It is a spiral
chambered shell, with a ventral siphon. Its relation to the dorsal and
ventral surface of the animal is the opposite of that of the Mazdédlus.
The shell is inside the animal; in Maztd/us the animal is inside the
shell. It seems that Spzru/a is a swift swimmer at great depths ;
though the empty shells are often cast ashore, the creature itself is rarely
seen. In Sefza, the narrowed tip of the ‘‘bone” probably represents
the remains of the phragmacone; the bulk of the ‘‘ bone” probably
corresponds to the pro-ostracum in the Belemnites. Besides lime
there is chitin in the ‘‘Sepia-bone.” In Lo/zgo there is no deposit of
lime, an organic chitinous pen only being left. In Octopus there is no
trace of shell at all, and no mantle-pocket, save a trace, in the very
young animal,
23
CHAPTER XVII
PHYLUM CHORDATA
SUB-PHYLUM HEMICH ORDA
UNnbeER the title Hemichorda are included a number of
interesting types which seem to have affinities with Verte-
brates. These affinities are clearest in certain worm-
like animals with distinct gill-clefts, eg. Balanoglossus
and tychodera, which form the class Enteropneusta.
Perhaps allied to these are two peculiar types,—Rhabdo-
pleura and Cephalodiscus, which may be united in the class
Pterobranchia. Very doubtfully in this alliance is Phoronis.
GENERAL CHARACTERS OF ENTEROPNEUSTA
The worm-like body has three regions—a pre-oral “ pro-
boscis,” a “collar” around and behind the mouth, and a
trunk, the anterior part of which bears gill-slits. A dorsal
and in part tubular nerve-cord arises from the ectoderm along
the middle line, and ts connected, by a ring round the pharynx,
with a ventral cord. In the skin, which is covered with
ciliated ectoderm, there 1s also a nerve plexus. From the
anterior region of the gut a diverticulum grows forward for
@ short distance, becomes a firm support for the proboscis, and:
7s often called the “notochord.” The gill-slits open dorsally,
are very numerous, and increase in number during life. The
mesoblast is formed by the outgrowth of five caelom pouches
Jrom the -archenteron. An unpaired anterior pouch forms
the pre-oral or proboscis cavity of the adult; there are two
collar cavities and two trunk cavities.
There are about 30 species in 9 genera, ¢.g. Balanoglossus,
BALANOGLOSSUS. 435
Dotichoglossus, Piychodera, Schizocardium, and Glandiceps.
They are very widely, though locally, distributed, and most
occur in the littoral area.
DESCRIPTION OF BALANOGLOSSUS
Form and habitat.—The species which form this genus
are worm-like marine animals, burrowing in sand and mud
in almost all seas. They vary in length from about 1 in. to
over 6 in., and are brightly coloured and have a peculiar
odour, like that of iodoform. ‘The sexes are distinct, and
are marked externally
by slight differences in
colour. The body con-
sists of a prominent
turgid and muscular
“proboscis,” a firm
“collar,” a region with
gill-slits, and, finally, a
long, soft, slightly coiled
portion.
Skin and muscles.—
The epidermis is ciliated,
and exudes abundant %=
mucus from unicellular Fic. 234.—Male of Balanoglossus (Do-
glands. With theaddi- “choglossus) kowalevskit.—After Bate-
tion of grains of sand, S ;
the mucus sometimes Nos. t"Gperculum behind the collar; then the
forms a tube round the _ region with gill-slits; zs., testes ; a., anus.
body. Some species are
phosphorescent. The muscular system is best developed
about the proboscis and collar, which are used in leisurely
locomotion through the soft sand. There are external
circular and internal radial and longitudinal muscles. The
fibres are unstriped. There is great regenerative capacity.
Nervous system.—The dorsal nerve-cord is most de-
veloped in the collar, but is continued along the whole
length. It arises as a longitudinal groove of ectoderm and
often remains tubular, like a typical spinal cord. The
dorsal nerve-cord is connected by a band round the collar
with a ventral nerve. There is also a nervous plexus
Mo., Mouth; o/.,
436 SUB-PHYLUM HEMICHORDA.
beneath the epidermis. There are no special sense organs
in the adult. In the larvae of some species there are two
eye-spots.
Alimentary system.—The permanently open mouth is
on the ventral surface between the proboscis and the collar.
Sand seems to pass into it during the wriggling movements
of the animal, which are greatly aided by the turgidity of
the proboscis and collar. The pharynx is divided into a
dorsal and ventral region, of which the former is respiratory
(Fig. 235, g.1), and connected with the exterior by many
gill-slits, while the latter is nutritive (Fig. 235, g.), and
conveys the food particles onwards. Behind the region
with gill-slits, the gut has a dorsal and a ventral ciliated
groove, and bears, throughout the anterior part of its course,
numerous glandular sacculations, which can be detected
through the skin. The anus is terminal. The animal eats
its way through the sand, and derives its food from the
nutritive particles and small organisms therein contained.
Skeletal system.—The skeletal system is represented by
the “notochord,” which lies in the proboscis, and arises,
like the notochord of indubitable Vertebrates, as a diverti-
culum from the dorsal wall of the gut in the collar region.
Beneath the notochord there is a chitinous ‘proboscis
skeleton.” The septa between the gill-slits are supported
by chitinous “forked primary” bars; and each slit, at first
circular, is split into a V-shape by the growth downwards
of a double rod of chitin called a “tongue bar”; the whole
is suggestive of Amphioxus.
The body cavity.—The body cavity consists of five
distinct parts, all of which are lined by mesoderm, and
arise as pouches from the archenteron. (a) There is first
the unpaired cavity of the proboscis, which communicates
with the exterior by a dorsal pore at the' base of the pro-
boscis next the collar. (d) In the collar region there are
two small paired coelomic cavities, from which two funnels
open to the exterior. Both these cavities and that of the
proboscis tend to be obliterated by growth of connective
tissue. (c) Two other cavities extend along the posterior
region of the body, to some extent separated by the dorsal
and ventral mesentery which moors the intestine. In these
there is a body cavity fluid with cells.
BALANOGLOSSUS. 437
Respiratory and vascular systems.—The respirator}
system consists of many pairs of ciliated gill-slits. They
open dorsally by minute pores behind the collar. In
development they begin as a pair, increase in number from
in front backwards, and they go on increasing long after
gs an
Uv.Y. un,
Fic. 23 speereide digs section through gill-slit region of
Ptychodera minuta.—After Spengel.
The section, somewhat diagrammatic, shows a gill-slit (g.s.) to left,
and a septum between two slits to the right; @.z., dorsal
nerve; d.z., dorsal vessel; v.7., ventral nerve; v.v., * ventral
vessel; £., hutritive part of gut; g-1, respiratory part of gut;
¢., lateral coelomic spaces; ¢.77., longitudinal muscles; 2.,
reproductive organs. As the gill-slits are oblique, the whole of
one could not be seen in a single cross-section.
the adult structure has been attained. Water passes in by
the mouth and out-by the gill-slits, where it washes branches
of the dorsal blood vessel.
There is a main dorsal blood vessel, which, at its anterior
end, forms a heart lying adove the notochord, and below
a closed contractile dilatation, sometimes called the “ peri-
cardium.” Beside the latter there is a paired ‘“ proboscis
gland,” formed from the ccelomic epithelium. There is a
ventral vessel beneath the gut; and numerous smaller
vessels. The almost colourless blood flows forwards
dorsally, backwards ventrally. This system should be
contrasted with that of Amphioxus.
Excretory and reproductive systems.—No nephridia are
438 SUB-PHYLUM HEMICHORDA.
known, but from the region of the collar two ciliated
funnels open to the exterior, and the enigmatical proboscis
gland is possibly excretory.
The sexes are separate. A number of simple paired
genital organs lie dorsally in a series on each side of the
body cavity in and behind the region with gill-slits
(Fig. 235, &.). They open by minute dorsal pores.
Development.—The eggs are fertilised outside of the
body. Segmentation is complete and approximately equal ;
Fic. 236.—Direct development of Dolichoglossus.—After Bateson.
The mesoderm is represented by the broken dark line.
In the upper row, from the left—
Section of blastula ; beginning of gastrulation, Zzd., endoderm ;
section of gastrula, 4/., blastopore; Ac., Archenteron; S.c.,
segmentation cavity ; closure of blastopore, outgrowth of five
ccelom pouches (/¥/.).
In the lower row, from the left—
Longitudinal section, showing the five parts of the body cavity
(bc.1, bc.2, bc.3) or ccelom.
Cross-section, C.V.S., central nervous system ; Vch., notochord ;
bc.2, body cavity in collar region.
Section at a later stage, D.d.v., dorsal blood vessel.
a blastosphere results; this is invaginated in the normal
fashion, and becomes a gastrula.
The development may be direct without a larval stage,
as in Dolichoglossus howalevskii, or indirect with a Zornaria
larva, as in Balanoglossus biminiensis,
In the direct development the blastopore of the gastrula narrows and
closes; the external surface of the gastrula becomes ciliated; the
DEVELOPMENT. 439
endoderm lies as an independent closed sac within the ectoderm,
Meanwhile the embryo has become or is becoming free from the thir
egg envelope, and begins to move about at the bottom in shallow
water. It elongates and becomes more worm-like ; there is an anterior
tuft and'a posterior ring of cilia; the primitive gut forms five coelomic
pouches ; a mouth and an anus are perforated; there seem to be no
fore-gut nor hind-gut invaginations. Two gill-slits appear; the regions
of the body are defined at a very early stage.
In the indirect development, there is a Tornaria larva, at first bell-
shaped. A ventral mouth opens into the curved gut, which is furnished
with a posterior terminal anus. A ‘‘dorsal pore” leads into a thin-
walled sac which becomes the proboscis
cavity of the adult. There are external
bands of cilia, something like those of
an Echinoderm larva, and also an apical
sensory plate (like that of many Annelid
trochospheres), with. two eye - spots.
The Tornaria is a,pelagic form. During
its period of free pelagic life it gradually
loses its distinctive bands of cilia, be-
comes diffusely ciliated, acquires a pro-
boscis and two gill-slits, and thus ap-
proaches the form already described.
It is elongated in the post-oral region,
and becomes a creeping form. The
Tornaria must be regarded as the more
primitive larval form; the temporary
absence of mouth and anus in the other
type is probably an adaptation acquired
after the pelagic habit was lost.
Johannes Miiller, ranked the Tornaria
larva, whose adult form was not then
known, beside the larvz of Echinoderms,
and the resemblance has been recently
emphasised by Willey. The ciliated
bands of the Tornaria resemble those
of Echinoderm larvee, but this is only
a superficial characteristic. The an-
terior pouch, which forms the cavity
a
Fic. 237.— Tornaria larva,
from’ the side. — After
Spengel.
M., mouth; g.,- gut; «., anus;
h., heart; g., pore entering
proboscis cavity ; ¢.., anal ring
of cilia: s.¢.7., secondary anal
ring. he dark wavy line in-
dicates the margin of the lobes
of the larval body with their
bands of cilia. Note also the
apical spot with cilia and sense
organ,
of the proboscis and communicates
with the exterior, has also been com-
_ pared with the beginning of the water-vascular system in Echinoderms,
and it is true that in both several independent ccelom pouches
grow out from the primitive gut. The anterior body cavity in Balano-
glossus communicates with the exterior by a pore, which becomes the
proboscis-pore of the adult, and this has been compared with the
water-pore, or outlet of the water-vascular system of Echinoderms,
which similarly opens from an anterior enteroccel to the exterior. On
the other hand, the presence of an apical plate—a structure almost
invariably absent in Echinoderms—suggests an affinity with an Annelid
trochosphere. ‘
440 SUB-PHYLUM HEMICHORDA.
Affinities with Vertebrates (especially emphasised by Bateson).
(1) ‘* Motochord.”—A dorsal outgrowth from the anterior region
of the gut grows forward for a short distance into the pro-
boscis, and becomes a solid supporting rod (Fig. 236, Vcz.).
It may be compared with the notochord of Vertebrates,
which also arises dorsally from the gut. But it lies delow
the main dorsal blood vessel, is of very limited extent, and
may be merely an analogue of the notochord.
(2) Gell-séits.’—Numerous gill-slits (Fig. 234) open from the
anterior region of the gut to the exterior, and are separated
from one another by skeletal bars, which in some ways
vesemble the framework of the respiratory pharynx in
Amphioxus, There are, however, many differences in
detail, —thus the slits open dorsally, not laterally; the
skeletal bars are differently disposed; the blood supply is
different.
(3) ‘* Dorsal nerve-cord.”—A dorsal median insinking (Fig. 235,
d.n.) of ectoderm, especially strong in the region of the
collar, may be compared with the medullary canal of Verte-
brates. But it must be noticed that there is also a ventral
nerve-cord (Fig. 235, v.7.).
(4) ‘* Zhe celom.”—The development of five enteroccelic pouches
is very suggestive of affinities with Amphioxus,
Affinities with Annelids (after Spengel).
The larva (Tornaria) (Fig. 237) may be regarded as a modified
Trochosphere, but this points at most to a far-off common
stock. Moreover, the nephridia, usually present in the
Trochosphere, are unrepresented in the Tornaria.
The heart lies, as in some Annelids, dorsal to the gut, not
ventral as in Vertebrates; the dorsal vessel carries blood
forwards, the ventral backwards, as is usual in Annelids.
But the double nervous system is essentially different from
that of Annelids; and the gill-slits are unrepresented there,
though Salensky has described cesophageal pockets opening to
the exterior in four Annelid types—Polygordius, Saccoctrrus,
Spio fuliginosus, and Polydora cornuta. In the last there are
five pairs in the larva, and two persist. If there be a
relationship between Enteropneusta and Annelids, it must
be a very distant one, perhaps restricted to origin from some
common stock. :
Class PTEROBRANCHIA. (1) Cephalodiscus
Cephalodiscus dodecaiopaus was dredged by the Challenger in the
Magellan Straits. Others are known from Japan, the Malay Archi-
pelago, South Africa, and the Antarctic. It was at first described by
M‘Intosh as a divergent Polyzoon, but the researches of Harmer point
to relationship with Balanoglossus.
PTEROBRANCHIA.
The minute individuals are associated
together within a gelatinous investment ;
the colony may attain a size of 9 in.
by 6 in. The gut is curved, the anus
being beside the mouth, beneath which
are 4-6 pairs of arms with ciliated tent-
acles. These two characters, formerly
supposed to indicate Polyzoan affinities,
may perhaps be adaptations to the sedent-
ary life. With Balanoglossus this type has
been compared, on account of the possession
Fic. 239.—An individual Cephalo-
discus.—After Ridewood.
b., Buds: sé#., stolon; go., to the left,
bulging of the body caused by the
gonad ; ga., to the right, bulging of the
body caused by the stomach ; J.s., pos-
terior lobe of buccal shield; ~.2., a red
line on the buccal shield; J.s., dark
edge of the buccal shield ; 22. tentacular
plumes.
Fic, 238.—Piece of a colony
of Cephalodiscus, showing
the tubes inhabited by the
animals. — After Ridewood.
of the following characters :—(a)
The body is divided into three
regions, which correspond to the
proboscis, collar, and trunk of
Balanoglossus; this is especially
obvious in the young bud; (4)
each of the three regions contains
a coelomic cavity, the most anterior
being single, while the other two
are divided by a median par-
tition; (c) the anterior pre-oral
cavity’ opens to the exterior by
two pores (cf. proboscis pore
of Balanoglossus); (d) the collar
region is also furnished with two
collar-pores; (2) in the collar
region the dorsal nervous system
is also placed, and is continued
to some extent into the proboscis ;
(f) beneath the nervous system
lies a diverticulum from the gut,
which extends towards the pro-
boscis region; this has been
compared to the ‘‘ notochord”
of Balanoglossus ; (g) the anterior’
region of the gut is perforated by
442 SUB-PHVLUM HEMICHORDA.
,
a pair of lateral gill-slits. The gonads lie between anus and pharynx
Buds are given off from a lateral stalk.
(2) Rhabdopleura
This genus is found at considerable depths in the North Sea and
Atlantic. Like Cephalodiscus, the individuals are minute and stalked,
and occur in a colony; in this case, however, they remain attached to
one another by a common stolon, instead of being united only by an
investment. The proboscis or buccal shield makes a thin annulated
tube within which the polyp moves up and down. In the head region
there are two hollow lateral arms bearing numerous ciliated tentacles,
which have a skeletal support. The gut, as in Cephalodéscus, has a
U-shaped curvature and an anterior diverticulum (‘‘ notochord”).
There are five coelomic cavities, and two collar-pores. There are no
gill-slits,
CHAPTER XVIII
PHYLUM CHORDATA
SUB-PHYLUM UROCHORDA or TUNICATA
(Ascipians, SEA-SQUIRTS, ETC.)
Tue Tunicates are remarkable animals, which seem to
stumble on the border line between Invertebrates and
Vertebrates. They were classified with Polyzoa and
Brachiopoda as Molluscoidea, until, in 1866, Kowalevsky
described the development of a simple Ascidian, and
correlated it, step by step, with that of Amphioxus. He
showed that the /arval Ascidian has a dorsal nerve-cord,
a notochord in the tail region, gill-slits opening from the
pharynx to the exterior, and an eye developing from the
brain. It is true that in most cases the promise of youth
is unfulfilled ; the active larva settles down to a sedentary
life, loses tail and notochord, nerve-cord and eye, and
becomes strangely deformed. Nevertheless we must now
class Tunicates along with the Chordates. Of their possible
relations to simpler forms nothing definite is known.
GENERAL CHARACTERS
The Tunicates ave marine Chordata, but the chordate
characteristics—dorsal tubular nervous system, notochord,
gill-slits, and brain eye—are in most cases discernible only
in the free-swimming larval stages. They usually degenerate
in the course of their development, and the adults, which are
in most cases sedentary, tend to diverge very widely from the
Vertebrate type. Thus the nervous system 1s generally re-
duced to a single ganglion placed above the pharynx. The
444 SUB-PHYLUM UROCHORDA OR TUNICATA.
body is invested by a thickened cuticular test, which contains
cellulose. The relatively large pharynx 1s perforated by twe
(tn Larvacea), or (in the majority) by numerous ciliated gill-
slits, and is surrounded to a greater or less extent by a
peribranchial chamber, which communicates with the exterior
by a special dorsal (atrial) opening. The ventral heart ts
simple and tubular, and there is a pertodic reversal in the
direction of the blood current. Nephridia are absent, and the
renal organs have no ducts. All are hermaphrodite. There
7s usually a metamorphosis in development. Colonies are
Jrequently formed.
Type of Tunicata—a simple Ascidian (Ascdta mentula)
An adult Ascédia is an irregular oval of 3 to 4 in. in
length ; one end is attached to stones or weed; the other,
more tapering, bears the 8-lobed mouth; close beside this,
on the morphological dorsal surface, lies the 6-lobed ex-
halant or atrial aperture. During life, water is constantly
being drawn in by the mouth and passed out by the atrial
opening. If irritated, the animal may drive a jet of water
with considerable force from both apertures, whence the
name “sea-squirt.”
Test.—The whole body is clothed in a thick test, some-
times called a tunic, though this name is more frequently
applied to the underlying body wall. From this body wall
the test can be readily removed, the two being unattached
except at one spot, where blood vessels pass into the test,
and also to a less degree at the two openings. To begin
with, this test is a true cuticle, produced by secretory
prolongations of the ectoderm cells; but soon after its
formation mesenchyme cells migrate into it, and give rise
to patches of connective tissue cells. These cells apparently
retain throughout life some phagocytic importance. In
Ascidia outgrowths of the body wall with prolongations of
blood channels enter the test, ramifying in all directions.
In some Ascidians this is carried further, so that the test
becomes an important accessory organ of respiration. The
test consists in great part of a carbohydrate identical with
the cellulose of plants. This “cellulose” or “tunicin” is
common throughout the group, but the relative amount
ASCIDIA. 445
produced varies markedly in the different forms. In some
forms the “test ”-cells make calcareous spicules.
. In. ap.
Fic. 240.—Dissection ot Ascidian.—After Herdman.
Jn. ap., Inhalant aperture; 7., test, cut away below to show mus-
cular layer, pharynx, etc.; Zx., endostyle or ventral groove
of pharynx. Note removal of pharynx to show, on the other—
the left—side, stomach (S7.), intestine (with fold seen at inci-
sion), and reproductive organs (G.); H., opening of pharynx
into cesophagus; G.D., genital duct; 4., anus; C/., cloacal
chamber; £x. ap., exhalant aperture; Gz., lies above the
ganglion, which is seen between the two apertures ; beneath it
is the sub-neural gland and its duct. :
Body wall and muscular system.—The body wall, mantle,
or tunic, disclosed by peeling off the test, is a structure of
considerable complexity. Its outer surface is covered by a
446 SUB-PHYLUM UROCHORDA OR TUNICATA,
single layer of ectoderm cells, which secrete the test.
Beneath these there lies a gelatinous matrix containing
numerous connective tissue cells, blood-carrying spaces,
muscle cells forming slender fibres, and so on.
A true coelom has been described in some embryos, but
it is afterwards almost suppressed, being represented at
most by the pericardium and small lacunar spaces. The
apparent body cavity of the Ascidian—the space between
gut and body wall—is, as we shall see, lined throughout by
ectoderm.
The muscular system is not well developed. The muscle
cells are much elongated and unstriped ; they are aggregated
into fibres of varying thickness, which form an irregular net-
work on the right side of the body, while they are virtually
absent on the left. Special sets of fibres form sphincters
round the apertures.
Alimentary and respiratory systems.—The mouth opens
into a short stomodzum, separated from the branchial
sac itself by a sphincter muscle, whose posterior border
is furnished with numerous simple elongated tentacles.
Behind this lies a ciliated peripharyngeal groove. In the
living animal the tentacles form a sort of sieve over the
opening of the branchial sac. This sac is morphologically
the pharynx, and extends almost to the posterior end of the
body. It is separated from the mantle by a space whose
dimensions vary greatly in the different regions of the body.
This space is the peribranchial chamber, which is formed
from the ectoderm, and communicates with the exterior by
the atrial opening, and with the branchial sac by innumer-
able slits). The remainder of the alimentary canal lies on
the left side of the body, between pharynx and mantle, and
consists of a short cesophagus leading from the pharynx to
the fusiform stomach, and of an intestine which describes
an S-shaped curve, and then crosses the atrial chamber, to
end in an anus iying beneath the exhalant opening. The
absorbing surface of the intestine is increased by a marked
infolding, corresponding to the typhlosole of the earthworm.
A mass of tubules connected by a duct with the cavity of
the stomach is possibly a digestive gland.
The structure of the pharynx is exceedingly complex, for
it has a double function—respiratory and nutritive. More-
NERVOUS SYSTEM AND SENSE ORGANS. 447
over, the breathing organs of sedentary animals tend to be
elaborate. The water which enters by the branchial aper-
ture is not only used in. respiration, but brings with it
the minute food particles. Similarly, the outgoing current
carries with it the water used in respiration, the undigested
residue of the food, and the spermatozoa and ova. The
water of respiration passes from the pharynx through its
numerous gill openings to the peribranchial chamber, and
so to the exterior. On its way it purifies the blood in the
vessels running in the complex framework of the pharynx
wall. The water-current is produced and maintained by
the action of the ciliated cells lining the gill-slits, and its
force necessitates special arrangements to prevent the food
particles being swept out before they have entered the
digestive region of the gut. In this connection there is a
longitudinal glandular groove or endostyle along the ventral
surface of the pharynx, and a ciliated fold on its dorsal—
‘the regions being defined by the nerve ganglion. According
to Willey, the minute alge and the like of the food are
entangled in the abundant mucus secreted by the ventral
groove or endostyle, and are swept forward in a cord of
slime, until at the anterior end of the endostyle they reach
the ciliated peripharyngeal groove, whose two halves sur-
round the pharynx, and unite’ to form the dorsal lamina or
fold. The food particles passing round the peripharyngeal
groove are swept backwards by the cilia of the dorsal
lamina until they reach the cesophageal opening. In many
Ascidians the dorsal lamina is replaced by a series of pro-
cesses—the dorsal languets, which may be sensory, as well
as food-wafting structures.
Nervous system and sense organs.—In the adult both of
these show marked degeneration. In the larva there is a
slightly developed brain continued into a dorsal nerve-cord,
and having connected with it a median eye and an otocyst.
The two latter are completely absent in the adult, and the
nervous system consists merely of a ganglionic mass lying
between the two apertures, giving off a few nerves forwards
and backwards.
A structure of doubtful utility, but of considerable morphological
interest, is the small sub-neural gland which lies beneath the ganglion,
and communicates by a ciliated duct with the pharynx. The opening’
448 SUB-PHYLUM UROCHORDA OR TUNICATA.
is usually complex, and forms the so-called dorsal tubercle, which
is very distinct on the wall of the pharynx. It lies at the point
where the two halves of the ciliated groove, or peripharyngeal band,
already described, converge dorsally to form the dorsal lamina. In
Ascidia the sub-neural organ is ventral to the brain, and partly
RU
STULANRUNLAN
A LAN
a:
Fic. 241.—Diagram of Ascidian.—After Herdman.
The arrows indicate the two openings; the dark border the test.
Ph., Pharynx, with gill-slits; G., reproductive organs; H.,
heart, with blood vessels; G.D., genital ducts; &., rectum,
ending in cloacal chamber. Surrounding the pharynx the
peribranchial cavity is shown.
glandular in character, and so it is in many; in some cases, however,
it is dorsal in position, and its glandular portion is reduced to nil,
It is probable that the sub-neural gland and its duct correspond to
the olfactory pit of Amphzoxus, and perhaps to the hypophysis of
Vertebrates.
VASCULAR AND REPRODUCTIVE SYSTEMS 449
It is probable that the pigment spots between the lobes of the
apertures, the tentacles in the branchial siphon, and the dorsal lamina,
or its representatives, the languets, have some sensory function. '
Vascular system.—The simple tubular heart lies in a
pericardial space at the ventral side of the lower end of the
pharynx. In development, two diverticula grow out from
the pharynx; these meet and fuse, forming the pericardium.
The heart arises as an invagination from its dorsal wall, and
is thus endodermal in origin, and probably not homologous
with the heart of the other Vertebrates. A periodical
reversal of the direction of the waves of contraction is
discernible in the heart ; for a certain number of beats the
blood is driven upwards, and then the direction is reversed.
This same reversal also occurs in Phoronis.
According to Herdman, the ventro-dorsal contractions occasion the
following circulation :—The blood, which is spread out on the walls of
the pharynx in vessels lying between the slits, collects into one large
(branchio-cardiac) vessel, which, after receiving a vessel from the test,
enters the ventral end of the heart. From the dorsal end it is poured
into a great (cardio-visceral) trunk, which sends one branch to the
test, and then breaks up among the viscera. From the visceral lacunze
the blood is again collected (in a branchio-visceral) to be distributed
to the branchial sac. At the reversal of the contractions this circulation
is also reversed. The reversal occurs every couple of minutes or so.
The blood is very colourless, but usually contains a few pigmented
corpuscles.
Excretory system.—In the loop of the intestine there
lies a mass of clear yesicles containing uric acid and other
waste products. This, therefore, seems to be a renal organ,
lout there is no duct. Bacteria are usually found in the
vesicles, and their activity may make diffusion easier. It is
interesting to find such a plant-like method of storing up,
instead of eliminating, waste products in these very passive
animals. It has been suggested that the sub-neural gland
may have some-renal function.
Reproductive system.—Tunicates are hermaphrodite.
The reproductive organs (Fig. 240, G.) are very simple, and
lie in the loop of the intestine. The ovary is the larger,
and contains a cavity into which the ova are set free, and
from which they pass outwards along an oviduct which
opens into the cloacal chamber. The testis surrounds the
ovary, and is mature at a different time (dichogamy) ; its
29
450 SUB-PHYLUM UROCHORDA OR TUNICATA.
duct runs by the side of the oviduct. In some forms, where
the gonads are near the cloaca, there are no ducts. The
ova are surrounded by follicular cells, and probably fertilised
in the cloaca.
Development.—The fertilised ovum divides completely and almost
equally. The spherical blastosphere becomes slightly flattened, and
ultimately forms a two-layered gastrula,
Along the dorsal median line of the gastrula the ectoderm cells form
the medullary groove, the sides of which arch together and form a
canal—the medullary canal. This opens anteriorly to the exterior by
Fic. 242.—Young embryo of Ascidian (C/aveléna).—After
Van Beneden and Julin,
NP., Neuropore; NC., neural canal; WCH., notochord; Z.,
ectoderm ; J7., mesoderm; A., archenteron.
the neuropore, and posteriorly communicates with the archenteron by
the neurenteric canal. :
With regard to the origin of mesoblast and notochord, there is more
difficulty. Both originate from the endoderm in the region of the
blastopore, and for a time grow forward together. The notochord lies
in its usual position on the roof of the gut, from a specialisation of
which it arises; but its forward extension is limilted,—it never extends
into the anterior region, and in the posterior region—the future tail—it
increases at the expense of the primitive gut, whose lumen it obliterates.
The mesoderm, on the other hand, extends right forward, and becomes
divided into two regions—a posterior, ultimately forming the muscula-
ture of the tail, and an anterior, giving rise to the blood, connective
tissues. body muscles, excretory and genital organs. According to Van
GENERAL NOTES ON TUNICATA. 451
Beneden and Julin, the mesoderm primarily originates in the form of
two pockets, which grow out from the gut, as in. Amphioxus, and
whose cavity is the true ccelom. According to the majority of investi-
gators, it originates as solid blocks of cells, and the body cavity is
only represented by spaces produced by the subsequent separation of
these cells.
The further processes of development result in the formation of
a tadpole-like larva, with dorsal nervous system, notochord in the
tail region, and well-developed sense organs. Two ectodermal in-
vaginations form the original double peribranchial chamber, and
small diverticula from the pharynx meet these and form the first
gill- slits,” :
For some kours the larva enjoys a free-swimming life, using its tail
as an organ of locomotion. Then it fixes itself by papillee on its head,
& /P
Fic, 243.—Embryo of Clavelina.— Modified after Seeliger.
J#-, Fixing papilla; ef£, ectodermic fold; c.g., ciliated groove;
en., endostyle; s.o., cerebral vesicle with sense organs; g's.,
gill-slits ; May nerve-cord beginning to degenerate; ch., noto-
chord; g., gut curving upwards towards atrial opening. The
atrial invagination is marked bya dotted line ; the mouth and
atrial opening are indicated by arrows.
and begins almost immediately to degenerate. The tail shrinks and
disappears, being consumed by phagocytes. The nerve-cord is lost,
and with it the larval sense organs, while simultaneously a change of
axis results in the adult relation of parts. The peribranchial chamber
becomes greatly enlarged, and its two openings fuse together to form the
single atrial aperture of the adult. The gill-slits increase greatly in
number, the increase being due both to the formation of new slits and
to the division of those first formed, and the whole animal under-
goes 2 metamorphosis which is one of the mast signal instances of
degeneration.
GENERAL NOTES ON TUNICATA
The description of Ascidia given above is, in its general
outlines, applicable to all the simple Ascidians, which are
452 SUB-PHYLUM UROCHORDA OR TUNICATA.
abundantly represented on British coasts. As contrasted
with this type, we have in other members of the class most
remarkable diversity in structure, habit, and life history.
The simple Ascidians are usually sedentary, growing fixed
to stones, shells, or weed, and are widely distributed, occur-
ring on or near the coasts of all seas. With the exception
of the so-called social Ascidians (e.g. Clavelina), they do not
reproduce by budding, but are often gregarious, great
masses being found together.
To the compound Ascidians (e.g. Botryllus) those simple
forms are linked by Clavelina, where each individual is
surrounded by its own test, but is united to its fellows by a
common blood system. In the compound Ascidians, on the
other hand, many individuals are enveloped in a common
test, and all like C/avelina possess the power of reproducing
asexually by budding. There is, however, no doubt that
the so-called compound Ascidians are an artificial group,
whose members diverge widely in structure, though all dis-
play the two characters mentioned.
Some of the compound Ascidians are not fixed, but form
floating colonies. These forms lead up to the beautiful
Pyrosoma or phosphorescent, fire-flame, where the whole
colony with its numerous individuals swims as one creature.
All these belong to the Ascidian series, and display
interesting diversity in their methods of development.
The simplest case is that already described for Ascidia,
where the tailed larva gives rise to a sexual adult without
any power of budding. This occurs in almost all simple
Ascidians, but even here there are indications of possible
complication. ‘Thus, on the one hand, in some, eg. Aol
gula, there is a tendency towards abbreviation—the larval
stage being suppressed, while, on the other, the adult
acquires the power of reproducing asexually, eg. C/avelina.
Both processes are carried further in the compound
Ascidians. In these the eggs have usually a considerable
amount of yolk, and development takes place either in the
atrial cavity of the mother, or in special brood-pouches. In
consequence, the development, especially in the early stages,
shows considerable modification, although the larval stage
is quite distinct. Again, the tailed larva develops into an
adult which has no sexua! organs, but forms a colony by
GENERAL NOTES ON TUNICATA. 453
budding. The individuals of the colony then give rise to
eggs and so to larvae. The development thus includes a
distinct alternation of generations.
Budding takes place in many different ways in the com-
pound Ascidians. In one set (the Diplosomide) the tailed
larva is precociously reproductive, giving rise to buds before
undergoing metamorphosis. This forms an_ interesting
transition to the condition seen in /yrosoma, where the
fertilised egg gives rise to a rudimentary larva (cyathozooid),
from which a young colony of four individuals arises by
budding. These individuals again bud, until a Jarge colony
is formed, the members of which become sexual. The ova
are few in number, a statement which is generally true for
the pelagic Tunicates, as contrasted with sedentary forms.
' While the Ascidians in the narrow sense include all the
more typical Tunicates, there are two other sets, few in
number both as regards genera and species, but of great
theoretic importance.
The one set includes the free-swimming genera Sa/pa and
Dotiolum, together with the aberrant deep-water genus
Octacnemus ; the other, a few active free-swimming forms,
which exhibit throughout life many of the characteristics of
the larval Ascidian. Of these, Appendicularia is the most
familiar type.
Both Sapa and Doléolum are pelagic in habit, and differ markedly in
structure from the Ascidians. The body is fusiform (Sa/ga) or barrel-
shaped (Do/olum), and wholly or partially encircled by definite muscle
bands, which replace the scattered fibres of the Ascidians, The mouth
is at one end of the body, and the atrial aperture at the other; the
animals swim by forcing the water out of the peribranchial chamber
posteriorly. Many of the most marked signs of specialisation in the
Ascidians are here absent. Thus the test may be, as in Dolzolem, very
thin and devoid of cells, and the branchial sac is relatively simple in
structure ; the cilia on its walls are never so important in producing the
respiratory current as in the Ascidians, and the gill-slits may be few in
number, or, as in Sa/pa, may be represented by two large holes in the
walls of the pharynx. Further, the hermaphroditism is modified by the
occurrence of very marked protogyny, and the ova are never numerous
—in Sa/pa each sexual individual usually produces only one.
On the other hand, the development exhibits marked alternation of
generations, both solitary and colonial forms being included in one life
history.
In Doliobin the fertilised egg gives rise to a tailed larva, which
develops into an asexual ‘‘nurse,” possessing the power of budding (cf.
SUB-PHYLUM UROCHORDA OR TUNICATA.
454
Compound Ascidians). The ventral stolon of the nurse gives rise to a
P QO : M Do
I i { _FTE cl
c =| 10). : Nf 4 -E
=F a
En F
Hoe D
Fic. 244.—‘‘ Nurse” of Doliolum miiller?.—After Uljanin.
I, Inhalant, E., exhalant aperture; C., ciliated band round
peas (P.); En., endastyle ; O., ‘‘ otocyst” ; N., nerve-gang-
ion; H., heart ; C&., cesophageal opening; D., stomach; A.,
anus; Cl., cloaca; DO., dorsal organ; M., muscle bands.
number of primitive buds, which migrate over the body until they reach
a dorsal outgrowth, apparently well supplied with blood. Here they
N M
P B
—
OE
En H G
Fic. 245.—Sexual individual of Doliolum miller. —
After Uljanin.
G., gonads ; B., gill-slits ; other letters as before.
reference line points to the stomach.
The unlettered
fix themselves and divide up to form three series of buds—two lateral
and one median. All these buds develop into individuals belonging to
GENERAL NOTES ON TUNICATA. 455
the sexual generation, but only a few become truly sexual. The two
.. lateral series develop into nutritive forms, which supply the nurse with
food. The nurse itself loses its alimentary and respiratory organs, and
becomes a mere organ of locomotion. The median buds develop into
‘* foster mothers,” which ultimately go free, bearing with them other
buds destined to develop into the solitary sexual forms. In these, first
ova and then spermatozoa are produced, which start the life cycle afresh.
It is thus obvious that there is considerable division of labour in the
sexual form, accompanied by polymorphism ; the whole process presents
some curious analogies to the conditions seen in the Ccelentera,
In Sala the single egg is fertilised within the body of the mother,
and becomes attached to the wall of the peribranchial chamber. Here
the developing egg is nourished by means of a “placenta,” and the
development is in consequence much abbreviated, the tailed larva not
being represented. This embryo gives rise to a solitary ‘‘nurse” form,
ana aon
SAU UY \
Fic. 246.—Diagram of Salpa africana.
o.a., Oral aperture ; @.¢., dorsal tubercle; Ze., tentacle; g., ganglion; .,
muscle bands; ady., atrium; &.v., blood-vessel; az., anus; a@.a.,
exhalant aperture; v.7., visceral nucleus; 4., heart; s¢., stolon; 2.2,
dorsal lamina; Z., endostyle; s.2.g., sub-neural gland; 2%., pharynx ;
p.p.., peri-pharyngeal band.
which by budding produces a chain of embryos. This chain is set free,
jts members become sexual, and, either while still united or after
separation, give rise to the eggs which develop into the nurse form,
The remaining order of Tunicates includes minute simplified forms
like Appendicularia, also pelagic in habitat, but without any power of
budding, and never forming colonies. These forms have a distinct tail,
which is bent at an angle to the body, and is the main-organ of locomo-
tion. The mouth is at the anterior end; the anus, which is distinct
from the atrial openings, is at the root of the tail. These atrial openings
lie slightly behind the anus, and are merely small ectodermic invagina-
tions communicating with the two gill-slits of the pharynx. They
correspond to the similar invaginations in the Ascidian larva. The test
may form a large investing ‘‘ house,” but it does not contain cells, and
is periodically cast and renewed. The important points as regards
internal structure are the presence of the notochord throughout life, and
the structure of the nervous system. The latter consists of a lobed
456 SUB-PHYLUM UROCHORDA OR TUNICATA.
ganglionic mass above the mouth, and a dorsal nerve-cord extending
backward from this into the tail, where it is furnished with other
ganglia. In connection with the cerebral ganglion there is a pigment
spot, an otocyst (auditory ?), and a tubular process communicating with
the pharynx, and corresponding to the sub-neural gland and the ciliated
duct of other Tunicates. We have already noted the simple structure
of the pharynx, which has but two gill-slits communicating directly with
the exterior. The same simplicity of structure is observable in the
heart, which is without any associated vessels. The hermaphrodite
7?
oe 78
- ae ot iz
ov
—
br
ors SO
st 2x
if ieee
. Nat - : end
app
les ee
<n va et!
a 72
72R'
Fic. 247.—Anatomy of Appendicularia.—After
Ferdman.
5.0.5 Sense organ ; 47., branchial aperture ; a/., dorsal tubercle ; of.,
otocyst; #.g., nerve ganglion; Ag., peripharyngeal band ; ,
nerve-cord ; @., cesopbagus ; sf., stomach; ov., ovary; /es.,
testes; 7., intestine ; 4., heart; #., urochord, cut at w’.5 2.¢’.,
mg"., nerve ganglia of tail; ., muscle band ; af¢., tail cut
theaugh a., anus ; @z., one of the atrial apertures ; evd., endo-
style.
reproductive organs lie posteriorly, and open to the exterior by a
very fine duct on the dorsal surface. As contrasted with Sa/sa and
Dotiolum, the animals are protandrous, and not protogynous. The
development is unknown.
Classification.—
Order 1. LARVACEA
Free-swimming, pelagic, and solitary forms provided with a large
locomotor tail containing a notochord. The pharynx opens to the
CLASSIFICATION. 457
exterior by two ventral ciliated slits, and there is no peribranchial
chamber. The nervous system extends into the tail region. A
relatively large cuticular ‘‘ house” is formed as a secretion round the
animal ; it is periodically cast off and rapidly replaced. The house acts
as a most efficient filtering apparatus for capturing minute diatoms and
protozoa upon which the animal feeds. The Larvacea or Appendicu-
larians are of special interest because they show little or no degeneration,
and retain throughout life the chordate characters which other Tunicates
lose during metamorphosis. <Appendicularia, Otkopleura, Fritillaria,
Megalocercus, Kowalevskia,
Order 2. ASCIDIACEA
Ascidians which may be fixed or free, simple or colonial, but which
in the adult have no tail and no trace of notochord. There is a large
branchial sac opening by many slits into the peribranchial chamber,
which communicates with the exterior by a single opening. There is
apermanent and well-developed cuticular test into which cells from
the body migrate. Many have the power of budding, and there is
frequently alternation of generations.
Sub-order 1. Ascidize Simplices. Solitary fixed forms which rarely
bud; when colonial, each individual has a separate test. As-
ctdta, Phallusia, Czona.
Sub-order 2. Ascidiz Composite. Fixed Ascidians which repro-
duce by gemmation, the individuals being embedded in a
common investing mass. Sotryllus, Polyclinum.
Sub-order 3. Ascidize Luciz. Free-swimming Ascidians which re--
produce by gemmation to form a colony, having the shape ot
a hollow cylinder, open at one end. There is one genus,
Pyrosoma, widely represented, especially in tropical seas.
They are brilliantly phosphorescent, and some attain a length
of twelve feet.
Order 3. THALIACEA
Free-swimming pelagic forms, which may be either single or
“social,” and in the adult are never provided with tail or notochord.
The muscles are in the form of distinct circular bands, which effect
locomotion by squirting out the water from the body. The test,
which may be well or ill developed, is always transparent. The life
history exhibits distinct alternation of generations, and there is some-
times polymorphism.
(a) Cyclomyaria. Muscle bands form complete rings. Dololum,
Anchinia.
(4) Hemimyaria. Muscle bands are in the form of incomplete rings.
Salpa, Octacnemus.
RELATIONSHIPS
The questions as to the origin of the Tunicates and the relations of
the orders are too difficult to be discussed here, but we may note that
458 SUB-PHYLUM UROCHORDA OR TUNICATA.
there are two possible views as to the position of Appendicularia and
its allies). They may be regarded as the slightly modified descendants
of the primitive Tunicates, from which the Ascidians have diverged in
the direction of degeneration, or as prematurely sexual larve derived
from an already degraded Ascidian-like form. Both views have had
supporters, and the one adopted materially affects the general method
of regarding the group.
In any case the Larvacea retain persistently a number of characters
which were probably possessed by the primitive Tunicata.
There are several resemblances between Tunicates and Lancelcts
(see the next chapter), e.g. the relatively large respiratory pharynx and
the peribranchial cavity, but this probably does not mean more than
that both groups aiose from a common stock of primitive chordate
animals
CHAPTER XIX
PHYLUM CHORDATA
SUB-PHYLUM CEPHALOCHORDA
(Synonyms—ACRANIA, LEPTOCARDI, PHARYNGOBRANCHII)
Tuis small sub-phylum includes about sixteen species,
popularly known as lancelets. The type represents an
offshoot from the primitive Vertebrate stock, lost, it is to
be feared, for ever; but while some authorities regard it as
a pioneer-type and as a far-off prophecy of a fish, others
hold it to be degenerate—a “weed in the Vertebrate
garden.” It is possible that both views are right, and that
the lancelet is a somewhat degenerate pioneer.
GENERAL CHARACTERS
There is a dorsal tubular nerve-cord, but no well-definea
brain region. The notochord is persistent and unsegmented ;
it is surrounded by a continuous sheath, and projects in a
unique manner in front of the anterior end of the nerve-cord.
Ln the adult the gill-slits are very numerous, and open into an
atrial or pertbranchial cavity. The body wall is built up of
over fifty myotomes. From Fishes, the lancelets are widely
removed by the absence of limbs, skull, jaws, differentiated
brain, sympathetic nervous system, eye, ear, definite heart,
spleen, and genital ducts. There are numerous separate
nephridia. The gonads are numerous and arranged seg-
mentally. The larval form is strangely asymmetrical and
the larval period is prolonged. The species have a wide
distribution, like many old-fashioned animals. They occur
near the coasts in warm and temperate seas, are sluggish in
habit, and feed on microscopic organisms or organic particles,
460 SUB-PHVLUM CEPHALOCHORDA.
Amphioxus lanceolatus, the best-known species
Mode of life.—The lancelets are fond of lying in the
sand in water about two fathoms deep, with only the fringed
aperture of the mouth projecting. ‘They feed on diatoms
and other small organisms, which are sucked into the
mouth. At times, especially in the evening, the adults start
up and swim about, but they are never so active as the
larve. The early embryo is pelagic. It is of interest to
note that along with lancelets, specimens of .the Annelid
Ophelia are often obtained ; they closely resemble lancelets,
not only in shape and size, but also in the way they burrow
and swint.
Form.—The body, between 14 and 2 in. in length, is
pointed at both ends, as the names suggest. The living
Fic. 248.—Lateral view of Amphzoxus.—After Ray Lankester.
The notochord runs from tip to tip. :
z., Tentacular cirri ; G., reproductive organs ; a.g., atriopore ;
@., position of anus ; 40 and 62, indicate number of myotomes.
animal is translucent, with a faint flesh colour, and is much
plumper than a spirit specimen. The muscles are arranged
in sixty-two segments or myotomes. There are three un-
paired apertures—(a) the median, ventral, pre-oral hood
overarching the true mouth, and fringed with tentacle-like
cirri; (4) the atriopore in myotome thirty-six, giving exit to
the water which enters by the mouth; (¢) the anus, ventral
and slightly to the left, behind the atriopore, but some
distance from the posterior end of the body. Along the
back there is a median fin, which is continued around the
tail, and along the ventral surface as far as the atriopore.
In front of this region the ventral surface is flattened, and
fringed on either side by a slight fin-like “metapleural”
fold. These folds are continuations downwards of the walls
of the atrial or branchial chamber, which extends from
AMPHIOXUS 461
behind the mouth to the atriopore, and into which the gill-
slits of the pharynx open in the adult.
Skin.—The epidermis consists of a single layer of cylin-
drical cells. Some of them project slightly from the surface,
Ph.
alr,
Fic. 249.—Transverse section through pharyngeal region of
Amphroxus.—Alter Ray Lankester.
sf.c., Spinal cord; c/., notochord, beneath which lie the two dorsal
aorte; mz., myotome; a.c.f, atrio-ccelomic funnel, opening
into sub-chordal coelom; C., cecum; G., a genital sac with
ova; 7zp., metapleural fold; @/~., atrial cavity; P%., pharynx,
with dorsal and ventral grooves, and bars between gill-stits,
Note in the primary bars and in the ventral groove the small
cozlomic spaces. The ectoderm ‘is dark throughout.
and are connected at the base with nerve-fibres. These are
sensory cells, and may be compared to the cells of the
lateral line in fishes and tadpoles. Here, however, they
are scattered over the surface of the body, though especially
462 SUB-PHVLUM CEPHALOCHORDA
abundant on the buccal cirri. The epidermis lies upon a
thin layer of clear cutis.
Beneath this there is a layer of fine tubes, which unite in a longitudinal
canal running along each metapleural fold. These metapleural canals
are said by some to arise in development by a splitting of an originally
solid mass (schizoccelic); but it seems more probable that they are
morphologically portions of the true coelom—ventro-lateral extensions
of the ‘‘ collar-ccelom ” (enteroccelic).
Skeleton.—This is slightly developed, for there is not
only no bone, but the material is not even definitely
cartilaginous. It may be called “ chordoid ” tissue.
(a) The notochord runs from tip to tip. It consists of
vacuolated cells, and the supporting power is probably due
to their turgidity, as in many vegetable structures. Its
anterior extension beyond the end ofthe nerve-cord is
particularly characteristic.
(2) The pharynx is supported by chitinoid bars, which
border the numerous gill-slits. There is also a series of
paired plates underlying the mid-ventral groove.
(c) The margin of the pre-oral hood contains a supporting
ring, segmented into about two dozen pieces, each of which
sends a process into the adjacent cirrus.
Connective tissue—The sheath which envelops the
notochord and is continued round the nerve-cord, the septa
of connective tissue (myocommas) which divide the muscle
segments, and the numerous “fin rays” which support the
dorsal and ventral fins, may be noticed here.
Muscular system.—The sixty-two muscle segments, myo-
tomes, or myomeres, are dovetailed into one another like a
succession of V-shaped plates, and are particularly strong
dorsally. These produce the side-to-side wriggling move-
ments by which the animal swims. On the ventral surface,
between the mouth and the atriopore, there is a transverse
set of fibres, which help to drive out the water from the atrial
cavity. Other muscles occur in the region of the mouth,
and elsewhere. Most, if not all, of the fibres are striated.
Nervous system.—The dorsal nerve-cord is shorter than
the notochord, and has no definite brain. In the anterior
region, however, there is some differentiation in minute
structure, and the central canal widens out to form the so-
ALIMENTARY AND RESPIRATORY SYSTEMS. 463
called cerebral vesicle, which in the larva communicates
with the exterior by a pore (the neuropore). From the
nerve-cord there arise two sets of nerves, dorsal and ventral.
Of these the two anterior pairs of dorsal nerves, which go to
the pre-oral hood, are called cranial, and do not correspond
to the myotomes. Behind these a pair of dorsal nerves
arise at each myotome, but, as is the case with most of the
other segmentally arranged parts of the lancelet, the
members of a pair are not directly opposite to one another.
The ventral nerves are absent in the region of the two first
pairs of dorsals, and behind this they divide up into many
minute fibres just as they leave the nerve-cord. The two
sets of nerves are compared respectively to the single-rooted
sensory dorsal nerves, and to the many-rooted motor ventral
nerves of higher Vertebrates. But the dorsal nerves of
Amphioxus supply the transverse muscles as well as the
skin, so that they must be partly motor. Furthermore,
there is no connection between the two sets, and the dorsal
nerves have no ganglia, except in so far as these are repre-
sented by aggregations of nerve nuclei. Nor are there any
sympathetic ganglia.
The nervous system of the lancelet is thus very divergent from what
is typical for Vertebrates :—(1) A brain is almost undeveloped ; (2) the
ventral roots far outnumber the dorsal roots; (3) the two sets of roots
do not unite ; (4) the dorsal nerves are partly motor; (5) there are no
spinal ganglia ; (6) there are no sympathetic ganglia.
The anterior region of the nerve-cord exhibits some histological dis-
tinctiveness ; and with it the following structures are associated :—
(a) Slightly to the left side there is a ciliated pit, often called
olfactory. It arises from an ectodermic invagination in the position of
the neuropore or original anterior opening of the nerve-cord. Below
this there is a minute diverticulum from the front dorsal wall of the
nerve-cord.
(6) At the end of the nerve-cord there is » pigment spot, sometimes
called an eye-spot. There are no true eyes, but numerous regularly
arranged pigment spots on each side of the spinal cord appear to he
optic.
(c) On the roof of the mouth there opens a small sac, the pre-oral pit,
which may have a tasting or smelling function.
It is likely that the most important sensory structures of the adult are
the sensitive cells of the epidermis. The feeble development of sense
organs may be associated with the almost sedentary habit.
Alimentary and respiratory systems.—The true mouth
lies within the projecting pre-oral hood. It is surrounded
464
SUB-PHYLUM CEPHALOCHORDA.
by a membrane called the velum,
and is fringed by twelve velar
tentacles, which must not be
confused with the external cirri.
In the larva the hood is absent,
and the mouth is flush with the
surface.
The mouth opens into the
pharynx, which, like it, is richly
ciliated. The pharynx, like that
of Tunicates, and indeed of
Fishes also, is modified for re-
spiration (Fig. 249, 2%.) Its
walls are perforated by numerous
gill-slits on each side, and be-
tween these lie supporting bars
alternately split and unsplit at
their lower ends.
Along the mid-dorsal and mid-
ventral lines there are grooves,
respectively called hyper- and
hypo- branchial. The latter is
comparable to the endostyle of
Ascidians, by which name it is
often called. As in Ascidians,
Fic. 250.—Development of atrial cham-
ber in Amphioxus.—After Lankester
and Willey.
In I. the metapleural folds are seen sending
a slight projection inwards. In 11, the two
projections have united and enclose a small
space (A7.), which is the rudiment of the
atrial chamber. In III. this space is enlarg-
ing at the expense of the ccelom, which it
pushes up before it. A comparison of this
figure with the cross-section of the adult
(Fig. 249) will show the relation of ccelom
and atrial chamber.
FR., celomic space within dorsal fin; AZ.,
gut; S., coelomic space of metapleural fold ;
4A7P., metapleural fold; SA7., projection
which forms floor of atrial chamber; AQ.,
aorta; £&.C., celom; S$./.V., sub-intes-
tinal vein; 4., nerve-cord; S//., sheath of
notochord; JZ¥., myotome; C., remains
of myocel; AZ., atrial chamber. ‘The
dotted line indicates the mesodermic wall
of the ccelom.
BODY CAVITY. 468
anterior part of the pharynx.
The water-current which enters the mouth is, as in,
Tunicates, connected both with respiration and nutrition.
The food particles, entangled in mucus, are said to pass
backwards along the hyperpharyngeal groove; the water
passes down the pharynx, through its numerous gill-slits
to the atrial chamber, and so to the exterior by the single
atriopore. In the larva the gill-slits are few in number, and
open directly to the exterior; in the adult they are con-
cealed by the atrial chamber, and have greatly increased in
number ; there may be more than 100 pairs. The water-
currents are kept up by the cilia, probably assisted by the
transverse muscles.
The first sign of the development of the atrial chamber is the appear-
ance of two lateral folds on the body wall, which form the metapleural
folds of the adult. On their inner apposed, but not united, surfaces,
two ridges appear. These grow towards one another and unite, leaving
only the atriopore open. Thus the floor of the atrial chamber (Fig.
250, II.) is produced. The chamber, as first formed, is a tube with a
very small lumen, but, secondarily, it becomes enlarged, and extending
upwards and inwards, constricts the ccelom, until it comes almost to
surround the gut. The atrium eventually becomes a cavity, crescent-
shaped in cross-section, surrounding the pharynx and extending back-
wards as a blind pouch on the right side of the intestine. At the same
time, the metapleural folds increase in size until they assume the adult
appearance (Fig. 250, III.). During these processes the originally few
gill-slits have been increasing in number, both by the addition of new
slits and by the division of those first formed. The division is effected
by the downward growth of a secondary bar or tongue-bar in the
middle of each slit. The primary bars differ from these tongue-bars
in being split at their lower ends, in enclosing a coelomic space, and
in some other respects.
The pharynx opens into the intestinal region of the gut,
which is straight and simple. Near its commencement a
pouch-like “liver” or cecum (Fig. 249, C.) arises, and
extends forwards on the right side of the pharynx. The
anus is some distance from the end of the body (cf. Fishes) ;
in the larva it is close to the caudal fin.
Body cavity.—This can only be understood when its development
is studied (see Fig. 254). It is a fine example of what is called the
enterocelic mode of origin. From the archenteron of the embryo a
hollow ridge grows out on each side, and becomes almost at once
30
466 SUB-PHYLUM CEPHALOCHORDA.
Fics. 251 and 252.—The nephridia of 4mphioxus.—After Boveri.
Both figures are lateral views of the upper region of the pharynx, the
body wall being removed. In the upper figure the atrial
chamber is laid completely open by the removal of its outer
CIRCULATORY SYSTEM. 467
segmented into a serjes of small sacs. These lie one behind the other,
and lose all connection with the gut. Each ultimately divides into two
—a dorsal thick-walled portion, and a ventral thin-walled portion.
The dorsal portions form the body musculature, and retain their seg-
mentation. Their cavity, the myoccel, persists to some extent in the
adult, forming the system of lymph spaces and canals which lie below
the cutis. In the ventral portions the septa disappear, and the enclosed
spaces, bounded by somatic mesoderm and splanchnic mesoderm, unite to
form the ‘‘splanchnoccel” which surrounds the gut. In the adult this
space is reduced anteriorly to small spaces and coelomic canals, by the
development of the atrial chamber (see Figs. 249 and 250). The
coelomic spaces and canals contain coagulable fluid, and represent the
lymphatic system of higher forms.
Besides the main trunk portion of the coelom, there is an anterior
portion, which is separated off from the very front uf the gut, and is then
divided into two cavities. The right becomes the cavity of the snout
in the larva, but is almost obliterated in the adult. The left becomes
the pre-oral pit. This anterior coelom pouch may correspond to the
head coelom of Balanoglossus, and to the bilobed head cavity which lies
beneath the eyes of fishes, and forms most of the eye muscles.
Thirdly, there is a pair of pouches, which form the first pair of
muscle segments, and are continued out into the atrial folds. These
may correspond to the collar ccelom of Balanoglossus (MacBride).
Two brown canals or atrio-ccelomic funnels discovered by Professor
E. Ray Lankester open into the dorsal part of the atrium about the
level of the junction between pharynx and intestine, while their
anterior ends project into the dorso-pharyngeal coelom about the 27th
myotome. They are probably diverticula of the atrium.
Circulatory system.—The blood is colourless, with a
few amceboid cells. There is no definite heart, but the
branchial artery is rhythmically contractile.
This branchial artery lies in the portion of the body cavity which is
enclosed by the endostyle, and is the anterior continuation of a large
hepatic vein from the cecum. From the branchial artery a series of
smaller vessels arise, which pass up the primary gill-bars, and also
supply the tongue-bars. These unite on the dorsal surface of the
wall, which is cut through along its line of insertion. The result
is to show that the chamber is prolonged dorsally into a series of
bays (.), which lie on the surface of the tongue-bars (¢.4.). Into
these bays each of the nephridia (z.) opens by a pore (0.), while
they also project internally by blind funnels (/), fringed by very
large solenocytes (c.). The bays are separated by ridges (d.),
formed by a downgrowth of the walls of the ccelom over the
primary bars (2.2.). y., Amyotome ; sy., one of the synapticula
connecting the pharyngeal bars.
The lower figure is a more superficial view, to show the blood vessels
which form an anastomosing plexus (¢.) over the walls of the
nephridia (#ph.). d., Dorsal aorta; ¢c@., ccelomic space within
primary bar; 4.v., blood vessel of secondary bar; #., cut edge
of the wall of the atrial chamber ; other letters as before.
468 SUB-PHVYLUM CEPHALOCHORDA.
pharynx to form the right and left dorsal aorte, which join at the
hinder end of the pharynx to form a single vessel running backward
over the intestine, and breaking up into capillaries on its wall. From
the right dorsal aorta there arises a complex of vessels supplying the
anterior region. From the capillaries of the intestine the blood is
collected in a sub-intestinal vein, which again breaks up in the
cecum. The circle is completed by the capillaries which form the
hepatic vein. The course of the circulation is essentially that of a
Vertebrate. °
Excretory system.—Boveri has described an elaborate system of
about ninety pairs of #ephridia lying in the dorso-lateral wall of the
pharynx. ‘They are short tubules, with a single opening into the
atrial cavity.’ On the inner aspect there are a number of blind funnels
projecting into the body cavity. On these funnels are set a number
of solenocytes (like those on the nephridium of some Polycheetes),
which are long tubular cells (Fig. 251, ¢.), closed above by a knob con-
taining the nucleus, from which hangs down a long flagellum. The
vessels of the primary gill-bars and of the tongue-bars form an anasto-
mosing vascular plexus, called a glomerulus,'over the tubules. In
number the tubules correspond to the primary gill-clefts, and are
therefore in origin segmental structures. They are regarded by their
discoverer as equivalent to the pronephric tubules of Vertebrates.
They develop from the mesoderm.
Reproductive system.—The sexes are separate and similar.
The organs are very simple, and without ducts. They form
twenty-six pairs of horseshoe-shaped sacs, lying along the
inner wall of the atrial cavity in segments ten to thirty-five
on each side (Fig. 248, G.). Each lies in a “genital
chamber” formed in development by constriction from the
cavity of the myotome.
In the mature female the ovaries are large and con-
spicuous ; the oya burst_into the atrial. cavity, whence they
pass out by the atriopore.
The testes are like the ovaries; the spermatozoa burst
into the atrial cavity, and pass out by the atriopore. The
eggs are fertilised in the surrounding water.
Development.—The fertilised ovum is about 4% in. in
diameter. ‘The segmentation is complete and almost equal
(Fig. 253). The first cleavage is vertical, and divides the
ovum into two equal parts; the second is also vertical,
along a meridional plane at right angles to the first, and
the result is four equal cells. The third cleavage is equa-
torial, and gives rise to four larger cells (or macromeres)
below or towards the vegetative pole, and to four smaller
cells (or micromeres) above or towards the animal pole.
DEVELOPMENT. _ 469
The blastosphere, which is the final result of segmentation,
invaginates to form a gastrula.
Along the mid-dorsal line of the gastrula the ectoderm
cells sink in slightly so as to form a groove. This is the
medullary groove, which here follows an unusual coursé of
development. Instead of immediately closing to form a
canal, the groove sinks inwards, and the lateral ectoderm
grows over it before closing takes place. Later, the groove
forms the medullary tube, which opens posteriorly into the
Ly
iT}
[sfejstass
7] SS
Tne
fe]
£7]
coll
on 2
Fic. 253.—Early stages in the development of Amfhioxus.
—After Hatschek. -
x. Ovum with germinal vesicle; 2 four-cell stage; 3. external
appearance of blastula; 4. blastula in section; 5. beginning of
gastrula stage ; 6. section of completed gastrula.
gut by a “neurenteric canal,” and to the exterior by the
anterior neuropore (Fig. 254).
The cavity of the gastrula—the archenteron—becomes
the gut of the adult, and gives rise to the coelomic pouches.
The notochord arises along the mid-dorsal line of the
archenteron ; its forward extension is secondary.
‘During the early part of larval life the ectoderm cells,
including those forming the medullary canal, are ciliated.
At this stage the larva is much more active than the adult.
The later larvee are more sedentary, lying much on the
470 SUB-PHYLUM CEPHALOCHORDA.
right side, and they are strongly asymmetrical. The mouth
is placed at the left side; the gill-slits of one side appear
considerably before those of the other; the primitive seg-
ments of one side are not opposite those of the other, and
SToTST Srey
efefe
=
val
aS
Fic. 254.—Sections through embryos of Amphioxus, to
illustrate development of body cavity.
On the upper line, three longitudinal sections ; on the lower line,
three transverse sections. ec., Ectoderm; ex., endoderm ; a.,
archenteron ; 4.s., primitive segments (protovertebra); 7.c.,
nerve-cord ; Z., posterior end; #g., neuropore; we.c., neuren-
teric canal; 2.4., medullary or neural plate ; c#., notochord 3
ef., splanchnocee]—above it is the myoceel.
soon. By the process known as the “symmetrisation” of
the larva, the apparent symmetry of the adult is produced.
The adult position of the anus and of the olfactory pit,
both to the left side, and the position of the unpaired liver
diverticulum, show how partial this process is.
RELATIONS OF AMPHIOXUS AND TUNICATES, 471
Experimental embryology.—As an. illustration of experimental em-
bryology, and of the developmental potentiality of the early segmentation
cells, reference may be made to the experiments of Professor E. B,
Wilson.
By shaking the water in which the two-celled stages floated, Professor
Wilson separated the two cells, and the result was two quite separate
and independent twins of half the normal size. Each of the isolated
cells segments (ke a normal ovum, and gives origin, through blastula
and gastrula stages, to a half-sized metameric larva.
If the shaking has separated the two first segmentation cells incom-
pletely, double embryos—like Siamese twins—result, and also form
short-lived (twenty-four hours) segmented larvee. ;
Similar experiments with the four-celled stages succeeded, though
development never continued long after the first appearance’ of meta-
merism. Complete isolation of the four cells resulted in four dwarf
blastulee, gastrulze, and even larve. Separation into two pairs of cells
resulted in two half-sized embryos. Incomplete separation resulted in
one of three types—(a) double embryos, (4) triple embryos—one twice
the size of the other two—and (c¢) quadruple embryos, each a quarter
size. é
Isolated blastomeres of the eight-celled stage never formed gastrule.,
Flat plates, curved plates, even one-eighth size blastulz were formed,
but none seemed capable of full development.
Thus a unit from the four-cell stage may form an embryo, but a unit
from ‘the eight-cell stage does not. For various reasons it seems likely
that this is due’ to qualitative limitations, not merely to the fact’ that
the units of the eight-cell stage are smaller. For although the separated
cells of the eight-cell stage have considerable vitality, and swim about
actively, the difference between macromeres and micromeres has by this
time been established ; in fact, the cells have begun to be specialised,
and have no longer the primitive completeness, the absence of differentia-
tion, which explains the developmental potentiality of the separated units
of the two-celled or four-celled stages.
Somewhat similar experiments have been made by other investigators
on the developing ova of Ascidians, sea-urchins, etc. Specialisation of
segmentation cells appears to occur at different times in different animals,
but it is illogical to infer the absence of specialisation from the fact that
any of the first four blastomeres, let us say, can produce an entire embryo.
For specialised cells may retain a power of regeneration.
RELATIONS OF AMPHIOXUS AND TUNICATES
The above account of Amphioxus will in its details
recall to the student the description of Tunicates. It is
indeed remarkable that the resemblance should be so much
stronger in minor anatomical points than in broad outline,
but this is in part explained by the very marked degenera-
tion displayed by the adult Ascidians,
472 SUB-PHYLUM CEPHALOCHORDA.
The following important resemblances should be noticed :
—In both cases the walls of the pharynx are perforated by
numerous slits, which open, not directly to the exterior, but
into an atrial or peribranchial chamber, formed from the
ectoderm, and with a single external aperture. In both, the
pharynx has a distinct ventral glandular endostyle, and a
dorsal fold (Tunicates) or groove (Amphioxus), connected
anteriorly to the endostyle by means of a ciliated band.
On the other hand, the Ascidians differ from the lancelets
in many ways, e.g. the sessile habit, the presence of the
test, of a heart, and of genital ducts; the absence of seg-
mentation, of nephridia, and any trace of ccelom in the
adult; the U-shaped alimentary canal; the power of budding,
so common in sedentary animals; and the hermaphroditism.
The detailed study of development yields similar series
of facts— marked resemblances coupled with marked
differences ; among the latter, the absence in Ascidians of
the segmented ccelomic pouches of lancelets is especially
noteworthy. It is probable that Lancelets and Tunicates
are descended from a common primitive chordate ancestry.
In strict usage the name <Amphioxus should be replaced by
Branchiostoma, and another genus, Asymmetron, with uniserial (right)
gonads and asymmetrical metapleura, should be recognised.
Fic. 25§4A.—Small portions of excretory organs of Amphzoxus (A),
and the Polychete, Phyl/odoce (B).—After Goodrich.
S., Solenocyte; ¥., nucleus; /*Z.., flagellum; 7., tube; ZC., excretory canal.
CHAPTER XX
STRUCTURE AND DEVELOPMENT OF
VERTEBRATA
Tue obvious distinction between higher and lower animals,
between the backboned and the backboneless, was to some
extent recognised by Aristotle over two thousand years ago.
Yet it was not till 1797 that the line of separation was
drawn with firmness—by Lamarck.
But the doctrine of descent—the idea of organic evolu-
tion—which Darwin made current intellectual coin in 1859,
suggested inquiry into the apparently abrupt apartness ot
the group of Vertebrates.
The inquiry bore fruit in 1866, when the Russian
naturalist Kowalevsky worked out the development of the
Vertebrate characteristics of Amphioxus, correlated this
with the development of Ascidians, and discovered the
pharyngeal gill-slits of Balanoglossus. ‘Thus the apparent
apartness of the Vertebrata was annulled.
GENERAL CHARACTERS
Vertebrates are celomate Metazoa,with a segmental arrange-
ment of parts. The central nervous system lies in the dorsal
median line, and ts tubular in its origin. A skeletal rod or noto-
chord, formed as an outgrowth along the dorsal median line of
the primitive gut, is always present in the embryo at least, but
tends to be replaced by a mesodermic axial segmented skeleton—
the backbone. Pharyngeal gili-slits, which may or may not per-
sist in adult life, are always developed, but above Amphibians
they are restricted to embryonic life, ave not directly functional,
and have no associated gill-lamelle. The heartis ventral, The
eve begins its development as an outgrowth from the brain,
STRUCTURE OF VERTEBRATA.
474
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GENERAL CLASSIFICATION. 475
General Classification of Phylum Chordata
{
aj Carinatze i
ass J} Odontolcz (extinct).
Birps, {ei (running). Class Mammats,
Saururz (extinct). : ‘ ‘
3. Eutheria, Placentalia, Monodel-
A Crocodilia (crocodiles, phia: the higher placental °
etc.). mammals.
Ophidia (snakes). : Seed :
Lacertilia (lizards, | 2, Metatheria, Marsupialia, Didel-
Cl etc.). phia: Kangaroos, etc. ; young
Rep oS Rhynchocephalia— born precociously, usually nur-
| wp SEMtiLes. Sphenodon. tured in pouches.
3 2 Chelonia (tortoises,
2; etc.). s
4 a Extinct Reptiles —|S-1. Prototheria, | Monotremata, Or-
3 é . (maity orders). nithodelphia: oviparous, Ornz-
an thorhynchus and Echidna.
aziz ~- —-
< 7 Sauropsida. Mammalia.
z\oO .
< in Amniota, embryos with amnion and allantois.
OT§
= YE] Class ;
5 {2 | Fisues.—e.g. Dipnoi(double-breath- Class
= Oo ing nud-fishes). —|S-AMPHIBIANS.—
A, Teleostomi (modern bony Anura (tailless frogs, etc.).
By fishes and ‘‘ Ganoids”). Urodela (tailed newts, etc:).
> Elasmobranchii (skate, Gymnophiona (worm-like Ca-
a shark, etc.). cilia, etc.).
Extinct Stegocephali (Lady:
rinthodon, etc.).
Ichthyopsida (fishes and amphibians).
Class Hypostomata (extinct).
Class Cyc.osromata (Round Mouths), without true jaws.
Myxine, hag-fish. Petromyzon, lamprey.
F Salpa type.
Sus-Puytum | Ascidian type (sea-
Urocuorpba or squirts). «
TUNICATA. Afppendicularia (lay-
val type persistent).
Sus-PHyLuM CEPHALOCHORDA.—A miphi-
oxus or Lancelet.
Surviving offshoots of ancestral Vertebrates.
Puytum HemicHorpa or ENTEROpNEuSTA (offshoots of incipient Vertebrates ?).
Balanoglossus, etc.; probably Cephalodiscus ; possibly Rhabdopleura,
Ancestry of Vertebrates
It is not at present possible to trace the path along which Vertebrates
have evolved, though our faith in the doctrine of evolution—as a modal
476 STRUCTURE OF VERTEBRATA.
theory of origins—leads us to believe that Vertebrates arose from forms
which were not Vertebrates.
But even when we recognise that Amphzoxus is a Vertebrate very
simple in its generad features, and that the Tunicata, especially in their
youth, are Vertebrates, we must admit that these are specialised not
very primitive types.
The Enteropneusta carry us a little farther back. For, while many
of their alleged Vertebrate characteristics are debatable, one cannot
gainsay, for instance, the possession of pharyngeal gill-slits. But the
affinities of the Enteropneusta with Invertebrate types are quite
obscure.
We have, in fact, to acknowledge that the pedigree of Vertebrates
remains unknown, though alleged affinities have been discovered
among Annelids, Nemerteans, Arachnids, Crustaceans, Palzostraca,
etc. There is almost no great class of Invertebrate Metazoa whose
characters have not been ingeniously interpreted so as to reveal affinities
with Vertebrates. It will be enough to select one illustration.
Annelid affinities.—Dohrn, Semper, Beard, and others maintain that
Annelids have affinities with Vertebrates.
(t) Both Annelids and Vertebrates are segmented animals.
(2) The segmental nephridia of Annelids correspond to the primi-
tive kidney-tubes of a Vertebrate embryo.
(3) The ventral nerve-cord of Annelids may be compared (in
altered position) to the dorsal nerve-cord of Vertebrates.
Both cords are bilateral, and it is possible that the tubular
character of the spinal cord and brain is the necessary
result of its mode of development, and without much
morphological importance.
(4) Segmentally arranged ganglia about the appendages of some
Cheetopod worms may correspond to the branchial and
lateral sense organs of Ichthyopsida, and the ganglia asso-
ciated with some of the nerves from the brain.
(5) The formation of the oral part of the pituitary body is
suggestive of the way in which the mouth of Annelids is
sometimes formed. Perhaps the pituitary body represents
an old lost mouth and its ancient innervation,
To minor points, such as the red blood and well-developed body
cavity of many Annelids little importance can be attached.
STRUCTURE AND DEVELOPMENT OF VERTEBRATES
Having separately discussed the Hemichorda, Urochorda,
and Cephalochorda, we propose in this chapter to discuss
the general structure of Craniata and the development of
some of the important organs.
Skin.—This forms a continuous covering over the surface
of the body, serves as a protection to the underlying tissues,
in some instances retains its primitive respiratory sig-
THE SKIN. 477
nificance, and is frequently concerned in the excretion of
waste and the regulation of the body temperature. As one
or other of its many functions predominates, there are cor-
responding structural modifications. One function which
we find oftenest emphasised, at the expense of the others,
is that of protection, and yet the extinct G/yptodon, the
sluggish Chelonia, the decadent ‘“‘ Ganoids,” seem to indicate
that this, in itself, or in its correlated variations, is not con-
ducive to the continuance of the species.
The skin includes— é
(a) The epidermis, usually
in several layers, the
outer, ‘‘ horny” stratum
corneum, the inner ac-
tively growing stratum
Malpighii, or mucosum ;
both derived from the
ectoderm or epiblast of
the embryo.
(6) The dermis, cutis,
corium, or under-skin,
derived from the meso-
derm or mesoblast of the
embryo.
From the epidermis are de-
rived feathers, hairs, and some
kinds of scales. The dermis,
as is natural when we consider
its origin from the mesoblast
(mesenchyme) or vascular layer, :
assists in nourishing these Fic. 255.—Section through Elasmo-
epidermic structures. In the branch embryo.—Ziegler.
case of feathers and the scales C.,nervecord; WV., notochord; AU., aorta}.
of Reptiles, the dermic papilla gut; V7 subintestina! vei MC es
is of primary importance, but fin; C., celom: U., segmental duct;
in the case of hairs it arises %.-M., myotome; MP., muscle plate; ~
late and is always small. SK., skeletogenous cells around noto-
Fiowi the dermis are derived “Hott ) 2E= Setoderm :
the bony shields of armadillos, and a few related mammals, the bony
scutes of crocodiles and some other reptiles, and the scales of most
bony fishes. This again is readily explained by the fact that the
mesoblast is also the skeletal layer of the embryo. The ordinary teeth
of Vertebrates, as well as the superficial or skin-teeth of gristly fishes,
are largely formed-from the dermis, but are usually covered by a thin
coating of ectodermic enamel.
The mesoderm is divided in the embryo into (1) a series of dorsal
segments or somites, with a transient cavity (the myoccel), and (2) an
unsegmented ventral portion or ‘‘lateral plate.” The dorsal part
478 STRUCTURE OF VERTEBRATA.
gives rise to the myotomes forming all the segmented muscles, to out-
growths into the limbs, to the cutis or dermis, to a sheath round the
notochord, etc. The ventral part gives rise to the splanchnic or visceral
muscles (usually unstriped), to the coelomic epithelium, etc.
Skeletal system.—Apart from the exoskeleton of skin-
teeth, scutes, shields, etc., the skeleton consists of the
following parts :—
The skull and its associated “ arches.”
(2) Axial The backbone and associated ribs.
Skeleton. (The notochord is transitory except
in the simplest Vertebrates.)
(4) Appendicular {Fore limbs, and pectoral girdle.
Skeleton. (Hind limbs, and pelvic girdle.
Skull.—The notochord grows forward anteriorly as far
as that region of the brain known as the optic thalami.
Around notochord and brain the mesenchyme forms a
continuous sheath, which is the foundation of the skull.
As in the case of the notochordal sheath of the trunk
region, so also here cartilage is formed in the primitive
membranous cranium. The first cartilages to appear are
the two parachordals, which lie on the lower surface of the
head at the sides of the notochord, and the two trabeculee
lying in front. The parachordals grow round and above
the notochord, producing the basilar plate, while the trabe-
cule unite in front to form the ethmoid plate. The
continuance of the process of cartilage formation, together
with the addition of cartilaginous nasal capsules in front
and auditory capsules behind, completes the formation of
the primitive cartilaginous brain-box or chondrocranium of
the lower Vertebrates.
Also connected with the head region, and of great import-
ance, are the visceral or gill arches which loop around the
pharynx on either side, and separate the primitive gill-clefts.
At the time when cartilage begins to be formed in the
membranous cranium, the arches also become chondrified,
and at the same time divided into segments.
Of these arches there are never more than nine. The
most anterior is the mandibular arch which bounds the
mouth, the second the Ayord; these two are of great
importance in the development of the skull. The others,
in Fishes and at least young Amphibians, bound open gill-
SKULL. 479
slits and support the pharynx; above Amphibians, they are
less completely developed.
In the Elasmobranch fishes, the mandibular and hyoid arches do not
form any direct part of the cartilaginous brain-case, but in the Tele-
osteans and thence onwards, the cartilages or bones arising in connection
with the mandibular and upper part of the hyoid arches contribute
directly to the formation of the skull. The hyoid proper,. or lower
part of the hyoid arch, forms the skeleton supporting the' tongue.
Cartilages arising in the lower part of the third visceral arch assist
in the formation of the hyoid bones of the higher Vertebrates, and parts
of two other arches appear to help in forming the laryngeal skeleton of
Mammals,
The mandibular arch in Elasmobranchs and frogs divides into a lower
portion—Meckel’s cartilage—which forms the lower jaw or its basis,
while from the upper portion a bud grows forward, the palato-pterygo-
quadrate cartilage, which forms the upper jaw in shark and skate, and
has a closer union with the skull in the frog. In higher Vertebrates
the lower portion of the mandibular always forms the basis of the lower
jaw, a quadrate element is segmented off from the upper part, but the
palato-pterygoid part seems to arise more independently. The hyoid
arch also divides into a lower portion, the hyoid proper, and an upper
portion, the hyo-mandibular, which may connect the jaws with the skull,
or from Amphibians onwards may be more remarkably displaced and
modified as a columella or stapes connected with the ear.
Returning now to the brain-box itself, we must notice
another complication,—the development of ‘ membrane”
bones. If we examine the skull of the skate, we find that
the brain lies within a cartilaginous capsule ; but this is not
entirely closed, spaces (the ‘fontanelles) being left in the
roof, which during life are covered only by the tough skin
with its numerous “dermal denticles.” In the sturgeon,
again, the small skin-teeth are replaced by stout bony plates
covering over the cartilaginous capsule. From such super-
ficial bony plates it is supposed that the “membrane”
bones, or ossifications in membrane, which form so import-
ant an element in the skull of the higher Vertebrate, have
originated.
In some bony fishes, notably the salmon, we find the brain enclosed
in a double capsule. Inside there is a cartilaginous brain-case in which
what are called centres of ossification have appeared, and upon this a
layer of membrane bones is placed, which can be readily removed with-
out injury to the cartilage beneath. In general, however, we must
recognise that, with the appearance of membrane bones, two changes
tend to occur,—first, the cartilaginous cranium tends to be reduced and
to exhibit considerable openings; second, in the remaining cartilage
centres of ossification appear, and we thus have ‘‘ cartilage” bones
480 STRUCTURE OF VERTEBRATA.
SUMMARY OF THE DEVELOPMENT OF THE SKULL
Onicin. REsuLTs.
ELEMENTS.
I. Parachordals
and _ trabeculze,
aided in some,
cases by the end
of the notochord.
Their precise
relations, ¢.g.to
the notochord,
are unknown.
Occipital region, with four bones—basi-occi-
ital, two ex-occipitals, and a supra-occipital
lin part). The basi-occipital is distinct only in
Reptiles, Birds, and Mammals.
Sphenoidal and ethmoidal region, with basi-
sphenoid and pre-sphenoid (present only in
Reptiles, Birds, and Mammals), paired ali-
sphenoids and orbitosphenoids, the inter-orbital
septum, the lateral or ectoethmoids, the inter-
nasal septum.
II. Sensecapsules.
(a) Nasal.
(4) Auditory.
From cartilage
surrounding the
ectodermic pits
which form the
foundation of
nose and ear.
,
(a) Unite with ethmoidal region.
(4) May give origin to five bones—pro-,
sphen-, pter-, epi-, and opisth- otics, or to the
single periotic of Mammals,
III. Arches,
(2) Mandibular.
(4) Hyoid arch.
These arches,
like those which
follow them, are
supports of the
pharynx, lying
between primit-
ive or persistent
gill-slits.
(2) Upper part = palato-pterygo-quadrate
cartilage of Elasmobranchs, palatine, pterygoid,
and quadrate bones in the higher Vertebrates,
but in Mammals the quadrate is believed by
many to become the incus of the middle ear.
Lower part = Meckel’s cartilage—the basis of
the lower jaw in all animals ; the part next the
quadrate becomes the articular bone, which in
Mammals is believed by many to become the
malleus of the middle ear,
(4) Upper part or hyo-mandibular=the ‘‘sus-
pensorium”’ cartilage of Elasmobranchs, the
hyo-mandibular and symplectic of -Teleosteans,
the columella auris of Amphibians, Reptiles,
and Birds, the stapes of the Mammal’s ear.
Lower part=the hyoid proper (cartilage or
bone).
IV. Investing
membrane bones,
(az) From the
roof of the skull.
(4) On the floor
of the skull, z.e.
from the roof of
the mouth.
(c) On the sides
of the skull,
(d) On upper
jaw
(e) On lower jaw.
Originally of
the nature of
external bony
plates, tooth
structures, and
the like,
(2) Parietals, frontals, nasals, ete.
(4) Vomer, parasphenoid, ete.
(c) Lachrymal, squamosal, orbitals, ete.
(d) Premaxilla, maxilla, jugal, and quadrato-
jugal (in part).
(e) Dentary, splenial, angular, supra-angular,
coronoid. Some recognise also a gonial, often
fusing with the articular.
‘VERTEBRAL COLUMN. 481
formed. Further, in spite of the developmental differences, the mem-
brane and cartilage bones become closely united to one another, or
even fused, and there is thus formed ‘‘a firm, closed, bony receptacle
of mixed origin,” as exemplified by the skull of any of the higher
Vertebrates.
We may thus say that in the evolution of the skull there
is first a cartilaginous capsule, that this becomes invested
to a greater or less extent by dermal ossifications, and that
finally the dermal bones lose their superficial position, and,
fusing with the ossified remainder of the cartilaginous
cranium, form a complete bony capsule. In Cyclostomes
and Elasmobranchs the brain-box is wholly cartilaginous ;
above Elasmobranchs the cartilage is more or less thoroughly
replaced or covered by bones. In the individual, develop-
ment there is a parallel progress.
The segmentation of the head, in contradistinction to the unseg-
mented skull, is expressed, although indistinctly, by the muscle seg-
ments and by the nerves supplying these, perhaps also by the lateral
sense organs, the ganglia, and the arches.
There are three pro-otic head-segments (pre-mandibular, mandibular,
and hyoid), which correspond to the orbital region, their walls forming
the six eye-muscles. Behind the auditory capsule there are ten or
eleven head-segments.
Vertebral column.—A dorsal skeletal axis is character-
istic of Vertebrata, and its usefulness is evident. It gives
coherent strength to the body; it is usually associated very
closely with a skull, with limb girdles, and with ribs; it
affords stable insertion to muscles; its dorsal parts usually
form a protective arch around the spinal cord.
To understand this skeletal axis, we must distinguish
clearly between the notochord and the backbone.
The notochord is the first skeletal structure to appear in
the embryo. It arises as an axial differentiation of endo-
derm along the dorsal wall of the embryonic gut or
archenteron beneath the nerve-cord. ‘The backbone, which
in most Vertebrates replaces the notochord, has a meso-
blastic origin. It develops as the substitute of the noto-
chord, and not from it, but from a skeletogenous sheath
surrounding it.
According to Kleinenberg, the notochord supplies the
necessary growth stimulus for the rise of its substitute, the
backbone.
31
482 STRUCTURE OF VERTEBRATA.
A vertebra generally consists of sevzral more or less
independent parts: the substantial centrum; the neural
arches which form a tube for the spinal cord, and are
crowned by a neural spine; the transverse processes which
project laterally, and the articular processes.
The ribs which support the body wall usually articulate
with the transverse processes, or with the transverse pro-
cesses and centra.
Amphibians are the first to show a breast-bone or sternum. It
arises from two cartilaginous rods in a tendinous region on the ventral
wall of the thorax. The sternum of some Reptiles, and of all Birds
and Mammals, arises from a cartilaginous tract uniting the ventral
ends of a number of ribs.
Limbs and girdles.—The pectoral girdle consists of a
dorsal scapula, a ventral coracoid, and a forward growing
membrane-bone, the clavicle or collar-bone.
According to Broom, frogs and some primitive Reptiles show a
coracoid and a pre-coracoid ; lizards and birds only a pre-coracoid ;
the Monotremes a coracoid and « pre-coracoid; other mammals a
coracoid only.
The pelvic or hip girdle consists of a dorsal iliac portion,
a ventral posterior ischiac portion, with the articulation for
the limb between them, and of a ventral, usually anterior,
pubic portion.
The fore limb—from Amphibians onwards—consists of a
humerus articulating with the girdle, a lower arm composed
of radius and ulna lying side by side, a wrist or carpus of
several elements, a “hand” with metacarpal bones in the
“palm,” and with fingers composed of several phalanges.
The hind limb—from Amphibians onwards—-consists
of a femur articulating with the girdle, a lower leg com-
posed of a tibia and fibula lying side by side, an “ankle”
region or tarsus of several elements, a foot with metatarsal
bones in the “sole,” and with toes composed of several
phalanges.
In Fishes the limbs are fins, z.e. without digits.
Distinct from the other bones are a few little sesamoids
of occasional occurrence, ¢.g. the knee-pan or patella. They
develop in connection with the tendons of muscles.
Nervous system.—This includes—(a) the central nervous
system, consisting of brain and spinal cord; (0) the peri-
NERVOUS SYSTEM—BRAIN. 483
pheral system, consisting of spinal and cranial nerves ; and
(c) the sympathetic nervous system.
The central nervous system first appears as a superficial
groove along the mid-dorsal line of the embryo. The sides
of this ectodermic groove meet, and, uniting, convert the
medullary groove into the medullary canal. The greater
Fics. 256 and 257,—Ideal fore and hind limb.—After Gegenbaur.
H., Humerus; R., radius; U., ulna; ~., radiale; w’., ulnare; ¢., inter-
medium; ¢., centrale; 1-5, carpalia bearing the corresponding
digits with metacarpals (zc.) and phalanges (£%.).
SJ, Femur ; 72., tibia ; 72., fibula ; z., intermedium ; 4., tibiale (astragalus);
JS» fibulare (os calcis); ¢., centrale; 1-5, tarsalia bearing the corre-
sponding digits with metatarsals (#¢.) and phalanges (f/.).
part of this canal forms the spinal cord; the anterior
portion of it is specialised as the brain. There is at first
a posterior connection between the neural canal and the
primitive gut of the embryo ; when this is lost the cavity of
the neural tube still persists as a little ciliated canal in the
centre of the cord, and as the internal cavity of the brain.
Brain.—At an early stage, even before the closing-in
484 STRUCTURE OF VERTEBRATA.
process is completed, certain portions of the anterior region
of the medullary canal grow more rapidly than others, and
form the three primary brain vesicles. By further processes
of growth and constriction, these three form the five regions
of the adult brain.
When first formed the brain vesicles lie in a straight line, but asa
consequence, probably, of their rapid and unequal growth, this condition
is soon lost, and a marked cranial flexure is produced. In the lower
forms, e.g. Cyclostomata, the flexure is slight, and is corrected later,
but in the higher types it is very distinct, and causes the marked over-
lapping of parts so obvious in the adult.
Fic. 258.—Partial section of a Vertebrate brain (diagrammatic).
OLF., Olfactory lobe ; CH., cerebral hemispheres ; C., wall of cerebrum
cut to show ventricle, behind this the figure is that of a median sec-
tion; PA., parietal organ arising from thalamencephalon; //.,
pineal organ; /VF., infundibulum descending from thalamen-
cephalon; #., hypophysis; OL., optic lobes; C4., cerebellum ;
CPL., choroid plexus on roof of fourth ventricle ; AZO., floor of the
medulla oblongata ; CC., central canal of spinal cord.
We must now follow the metamorphosis of the primary
brain vesicles.
The first vesicle gives rise anteriorly to the cerebral hemi-
spheres, while the remainder forms the region of the optic
thalami or thalamencephalon.
The cerebral hemispheres (prosencephalon or fore-brain)
are exceedingly important. They predominate more and
more as we ascend in the scale of Vertebrates, and become
more and more the seat of intelligence. Except in a few
cases, the prosencephalon is divided into two parts—
the cerebral hemispheres—which contain cavities known as
the lateral ventricles. The two hemispheres are united by
PITUITARY BODY—PINEAL BODY. 485
bridges or commissures, which have considerable classifica-
tory importance. With the anterior region of the hemi-
spheres olfactory lobes are associated.
In Cyclostomata, ‘‘ Ganoids,” and Teleosteans, the fore-brain has no
nervous roof, but is covered by an epithelial pallium which resembles
what is called the choroid plexus of the third ventricle in higher Verte-
brates. This choroid plexus is a thin epithelium, with blood vessels in
it. But in Elasmobranchs, Dipnoi, and Amphibians the basal parts of
the fore-brain have grown upwards to form a nervous roof, and this
persists in higher Vertebrates.
The optic thalami (thalamencephalon or tween-brain)
form the second region of the adult brain. Hence arise
the optic outgrowths, which form the optic nerves and
some of the most essential parts of the eyes. The
original cavity persists as the third ventricle of the brain ;
the thin roof gives off the dorsal pineal outgrowth or epi-
physis, and, uniting with the pia mater, or vascular brain
membrane, forms a choroid plexus; the lateral walls
become much thickened (optic thalami); the thin floor
gives off a slight ventral evagination, or infundibulum,
which bears the enigmatical pituitary body or hypo-
physis. The infundibulum also bears in most Teleosts
a peculiar posterior saccus vasculosus, which seems
to be a sense organ. It is not developed except in
Fishes.
The pituitary body.—This is derived partly from a downgrowth
from the thalamencephalon and partly from an upgrowth from the roof
of the mouth. The two parts unile to form a complex little organ,
whose morphological nature is very puzzling. It produces an internal
secretion of importance, and a pathological state of the organ is
associated in man with certain diseases, e.g. acromegaly.
The pineal body.—The dorsal upgrowth from’ the roof of the
thalamencephalon is represented, though to a varying extent, in all
Vertebrates. It consists of two parts, a pineal organ or epiphysis
proper, and a parietal organ, which arises as a rule from the epiphysis
but may have an independent origin in front of it. It is probable that
they were originally right and left members of a pair. The parietal
organ may become atrophied, but in some cases, especially in Reptiles,
it 1s terminally differentiated into a little body known as the pineal
body. This was entirely an enigma until De Graaf discovered its eye-
like structure in Anxguzs, and Baldwin Spencer securely confirmed this
in the New Zealand “‘ lizard” (Sphenodox), where the pineal body
shows distinct traces of a retina. In /etromyzon both the epiphysis
486 STRUCTURE OF VERTEBRATA.
and the parietal organ show an eye-like structure, most marked in the
case of the epiphysis.
Fic. 259.—Vertical section of the
pineal eye in an embryo of Spheno-
don.—After Dendy.
E., Epidermis; D., dermis; Z., lens; /.W.,
inner wall of the eye; O.W. outer wall of
the eye; PA.N., parietal nerve; PA.S.,
parietal stalk; C., cartilage.
of the parietal organ) receives a
nerve from a ‘‘ parietal centre”
near the base, but independent
of the epiphysis; this nerve is
transitory in Amguts, more or
less persistent in /gwana. Above
Reptiles the pineal stalk is relatively
short, and its terminal portion is
glandular. Among mammals the
epiphysis is absent in the dugong
and some Cetaceans; the pineal
body is absent in Dasypus and the
dolphin.
The significance of the pineal
body is uncertain. According to
some, its primitive function is that
of an unpaired, median, upward-
looking eye—a function retained
only in the Reptiles mentioned
above, the organ having elsewhere
undergone (independent) degenera-
tion. It may be, however, that the
optic function is not primitive, but
the result of a secondary transforma-
tion,
In Elasmobranchs the pineal
process (epiphysis) is very
long, and, perforating the
skull, terminates below the
skin in 4 closed vesicle. In
the young frog it also comes
to the surface above the skull,
but degenerates in adoles-
‘cence. In Sphenodon the
stalk passes through the skull
by the ‘parietal foramen,”
so that the ‘‘eye” itself,
developed from the parietal
organ, lies close beneath the
skin, the scales of which
in this region are specialised
and transparent. In Jgwana,
Anguis, Lacerta, etc., the
epiphysis loses connection
with the ‘‘eye” portion;
and it is also to be noticed
that in Amguzs and Jguana
the pineal body (on the end
Fic. 260.—Diagram of the parts
of the brain in Vertebrates. —
After Gaskell.
c.4., Cerebral hemispheres; c.f2.,
choroid pisses; o.th., optic thal-
ami; 0.4, optic lobes; ¢é., cere-
bellum; ¢.f2, choroid plexus;
4M.0., medulla oblongata; S.C.,
spinal cord.
THE BRAIN 487
The second primary vesicle of the brain forms the third
region, that of the optic lobes (mesencephalon or mid-brain)
in the adult brain. The floor and lateral walls form the
thickened crura cerebri; the roof becomes the two optic lobes,
which are hollow in almost all Vertebrates. In Mammals
a transverse furrow divides each optic lobe into two (corpora
quadrigemina). The cavity of the vesicle becomes much
contracted, and forms the narrow iter or aqueduct of Sylvius,
a canal connecting the third ventricle with the fourth.
The third primary vesicle gives rise to the metencephalon,
or hind-brain, or region of the cerebellum, and to the
myelencephalon, or after-brain, or region of the medulla
oblongata.
In the metencephalon the roof develops greatly, and
gives rise to the cerebellum, which often has lateral lobes,
and overlaps the next region. In the higher forms the
floor forms a strong band of transverse fibres—the pons
Varolii.
From the region of the medulla oblongata most of the
cranial nerves are given off. Here the roof, partly over-
lapped by the cerebellum, degenerates, becoming thin and
epithelial, the cavity—called the fourth ventricle—is con-
tinuous with the canal of the spinal cord.
Summary
(1) Cerebral hemispheres, prosencephalon, or
fore-brain. Note commissures, olfactory
lobes and nerves, and first and second
First Embryonic | ventricles.
Vesicle. (2) Optic thalami, thalamencephalon, or tween-
brain. Note—(a) optic, (4) pineal, (c)
pituitary outgrowths, and the third ven-
tricle.
(3) Optic lobes, mesencephalon, or mid-brain.
Note crura cerebri, and the aqueduct of
Sylvius.
Median Embryonic |
| (4) Cerebellum,-metencephalon, or hind-brain.
Vesicle.
Note pons Varolii.
(5) Medulla oblongata, myelencephalon, or
after-brain. Note rudimentary roof,
fourth ventricle, and origin of most of
the cranial nerves.
Third Embryonic
Vesicle.
483 STRUCTURE OF VERTEBRATA,
Enswathing the brain and spinal cord, and following its irregularities,
is a delicate membrane—the pia mater—rich in blood vessels, which
supply the nervous system, Outside this, in higher Vertebrates, there
is another membrane—the arachnoid—which does not follow the minor
irregularities of the brain so carefully as does the pia mater. Thirdly,
a firm membrane—the dura mater—lines the brain-case, and is
continued down the spinal canal. In lower Vertebrates the dura
mater is double throughout ; in higher Vertebrates it is double only in
the region of the spinal cord, where the outer part lines the bony
tunnel, while the inner ensheaths the cord itself. In Fishes the brain-
case is much larger than the brain, and a large lymph space lies
between the dura and the pia mater.
An understanding of the relations of the different regions will be
facilitated by a study of the following table, which Dr. Gadow gives in
his great work on Birds in Bronn’s Thierreich :—
REGION, FLoor. SIDES. Roor. Cavity.
Spinalcord.| | Anterior grey} White and] Posteriorcom-| Central canal.
and white com- | grey substance. | missure.
missure.
: Myelen- ‘ Epithelium of | Posterior part of
cephalon. Medulla oblongata. choroid plexus. | fourth ventricle.
Meten- Commissural Pedunculi of | Cerebellum. Anterior part of
cephalon. | part. crura cerebri, fourth ventricle.
Mesen- Crura cerebri. Cortex of] Anterior com-|, Aqueduct of Syl-
cephalon. optic lobes. missure, velum | vius and _ lateral
of Sylvius. extensions.
Thalamen-| Infundibulum, Inner part of | Epiphysis ani) Third ventricle.
cephalon. | hypophysis, | optic lobes and | epithelium — of
chiasma, optic thalami. | choroid plexus.
Corpus callo-
sum.
Anterior com-
missure.
Prosen- Corpus stria- Lateral ven-
cephalon. Kiana : tricles.
amina ter- . é
finalis, Cerebral hemispheres.
Olfactory
lobes.
SPINAL CORD—CRANIAL NERVES. 489
Spinal cord.—After the formation of the brain vesicles,
the remainder of the medullary canal forms the spinal
cord.
The canal is for a time continuous posteriorly with the
food canal beneath, so that a >-shaped tube results. The
connection between them is called the neurenteric canal
(Fig. 254, 7e.c.), and though it is only temporary, its frequent
occurrence is of much interest.
The wall of the medullary canal becomes very much
thickened, the roof and floor grow less rapidly, and thus
the cord is marked by ventral and dorsal longitudinal
furrows. At the same time, the.canal itself is constricted,
and persists in the fully-formed structure only as a minute
canal lined by ciliated epithelium, and continuous with the
cavity of the brain. *
In the cord it is usually easy to distinguish an external region of
white matter, composed of medullated nerve-fibres, and an internal
region of grey matter, containing ganglionic cells and non-medullated
fibres.
The arrangement of the grey matter, together with the longitudinal
fissures, give the cord a distinct bilateral symmetry, which is sometimes
obvious at a very early stage.
The brain substance is also composed of grey and white matter, but
there, at any rate in higher forms, the arrangement is very complicated,
Cranial nerves. — The origin and distribution of the
cranial nerves may be summarised as follows :—
[TaBLE,
490 STRUCTURE OF VERTEBRATA.
Name. OriGIN. DistTRIBUTION. Noves.
x. Olfactory. s.* - Front of fore- | Olfactory organ. Quite Jer se.
rain.
z. Optic. s Opticthalami.| Eye. Quite fer se.
3. Oculomotor or
ciliary. 92.*
4. Pathetic or
trochlear. 7,
. Trigeminal.
s. and m.
6. Abducens. mz.
7. Facial.
s. and 7.
8. Auditory. S.
g. Glossopharyn-
geal.
s. and 7.
Vagus or Pneu-
mogastric.
s. and a.
Io.
Floor of mid
brain.
From pos-
terior part of
optic lobes.
Medulla ob-
longata.
All the muscles of
a eye but two,
Superiar oblique
muscle of the eye.
me Pptinatnne te
ee Maxillary to
the upper jaw, etc. s.
(3) Mandibular to
lower jaw, lips, etc.
m. and s.
External rectus of
eye.
(x) Hyoidean and
spiracular. 1
(2) Palatine.
(3) Buccal, facial,
and auditory.
Ear.
First gill arch.
Posterior gills and
arches, lungs, heart,
gut, and body
generally.
They cross before
they enter the brain,
and generally unite
at their intersection.
A ciliary ganglion
at roots.
Perhaps belongs to
5, as a ventral root,
Gasserian ganglion
at roots.
The ophthalmicus
profundus, often in-
cluded with 5, is pro-
bably the dorsal com-
ponent of 3.
Perhaps belongs to
7,asaventral branch.
Ganglia at the
roots of 7 and 8.
Apparently a com-
plex, including the
elements of four or
five nerves.
In a Vertebrates there are two others, the spinal accessory (11).and the hypo-
lossal (12:
S The fourth or pathetic nerve is peculiar among motor nerves in that it appears to
arise from the extreme dorsal summit of the brain, between the mid- and hind- brain,
from the region known as the “‘ valve of Vieussens.” In Fishes the seventh nerve is
mainly a nerve of special sense ; in higher Vertebrates it bas lost most of its sensory
branches, and become chiefly motor.
* The letter s. is a contraction for sensory or afferent, z.¢. transmitting impulse
froma sensitive area to the centre 5 and WM. is a contraction for motor or e! erent, Le.
transmitting impulses from the centre to the body.
There is much uncertainty in regard to the morphological value of
the various cranial nerves, but the following conclusions may be
stated :—
(1) Like the spinal nerves, the cranial nerves are primarily seg-
mental, and there are probably about seven of them,—three pro-otic
and four metotic. The olfactory and optic nerves are quite by
themselves and not segmental.
(2) Like the spinal nerves, the cranial nerves have primarily two
roots,—a dorsal and a ventral, but the ventral roots do not join the
SPINAL NERVES. 491
dorsals, which have a more superficial course and include numerous
motor fibres (correlated with the great development of visceral
musculature in the head).
(3) The pre-mandibular primitive segment (I.) was probably supplied
by the oculomotor (ventral) and the ophthalmicus profundus (dorsal).
The mandibular primitive segment (II.) was probably supplied by
the pathetic (ventral) and the trigeminal (dorsal).
The hyoid primitive segment (III.) was probably supplied by the
abducens (ventral) and the facial (dorsal). The auditory, glosso-
pharyngeal, and vagus nerves have no ventral roots.
Spinal nerves.—Each spinal nerve has two roots—a
dorsal, posterior, or sensory, and a ventral, anterior, or
motor. These arise separately and independently, but
Fic. 261.—Diagrammatic section of spinal cord.
@4-, Posterior fissure; g.c., posterior column of white
matter; @.f.s., dorsal, posterior, sensory or afferent
root ; g-, ganglion ; ¥.a.12., ventral, anterior, motor or
efferent root; c.#., compound spinal nerve with
branches; s.g., sympathetic ganglion; @.c., anterior
column—the anterior fissure is exaggerated; g.c.,
ganglion cells ; g.22., grey matter ; z.7., white matter.
combine in the vicinity of the cord to form a single nerve.
The dorsal root exhibits at an early period a large ganglionic
swelling—the spinal ganglion ; the ventral root is apparently
non-ganglionated. Moreover, the dorsal root has typically
a single origin (as in the cranial nerves), while that of the
ventral root is often multiple.
The dorsal roots are outgrowths of a continuous ridge or crest along
the median dorsal line of the cord. As the cord grows the nerve roots
of each side become separated. They shift sidewards and downwards
to the sides of the cord. The ventral roots are later in arising; they
spring as outgrowths from the latero-ventral angle of the cord.
According to most authorities, the sympathetic ganglia are offshoots
from the same rudiment as that from which the dorsal ganglia arise,
492 STRUCTURE OF VERTEBRATA.
They are usually connected in a chain, which is linked anteriorly to
cranial nerves. They are also connected by fine fibres with the ventral
roots, They give off nerves to blood vessels and viscera,
Fic, 262,—Diagram of spinal cord of man, thoracic
region.—After Johnston,
S.S., Somatic sensory; V..S., visceral sensory ; S.J7., somatic motor ;
V.M.and U.M., visceral motor; d@.r., dorsal root; U.&., ventral
root.
Sense organs.—The ectoderm or epiblast gives origin to
the essential parts of the sense organs. The Vertebrate
eye is formed in great part as an outgrowth from the brain,
but as the brain is itself an involution of epiblast, the eye
may be also referred to external nerve-cells.
Branchial sense organs.—In many Fishes and Amphib-
ians there are lateral sense organs which form the “lateral
lines,” while others lie in the head, and were in all likeli-
hood primitively connected with gill-clefts. In Sauropsida
and Mammals these branchial sense organs are no longer
distinct as such.
The nose.—lIt is possible that the sensory pits of skin
which form the nasal sacs were originally two branchial
sense organs. They are lined by epithelium in great part
sensory, and innervated by the olfactory nerves. In Fishes
the nasal sacs remain blind posteriorly, but there is a
peculiar condition in Dipnoi, where the grooves from
SENSE ORGANS. 493
Pd
anterior nares to mouth are arched over and open
posteriorly into the front of the mouth. In Amphibians,
and in all the higher Vertebrates, the nasal chambers open
posteriorly into the mouth, and serve for the entrance of
air. The peculiar nostril of hag-fish and lamprey is referred
to in the chapter on Cyclostomata.
The ear in Invertebrates develops as a simple invagina-
tion of the ectoderm, forming a little sac, which may become
entirely detached from the epidermis, or may retain its
primitive connection; so in Vertebrates, at an early stage,
an insinking forms the auditory pit. In some. Fishes
(Servanus, salmon) and Amphibians a common ectodermic
thickening seems to form the rudiment from which the ear,
the lateral line, and a pre-auditory sensory patch are
derived. The auditory sac sinks farther in, and the ori-
ginally wide opening to the exterior becomes a long narrow
tube. In Elasmobranchs, which exhibit many primitive
features, this condition is usually retained in the adult; in
other Vertebrates the tube loses its. connection with the
exterior, and becomes a blind prolongation of the inner
ear—the aqueductus vestibuli, or ductus endolymphaticus.
In Anura the ductus endolymphaticus gives rise to a long
sac dorsal to the spinal cord giving off outgrowths in which
the “ calcareous bodies” lie.
The auditory vesicle, at first merely a simple sac, soon
becomes very complicated. It divides into two chambers,
the larger utriculus and the smaller sacculus. From the
utriculus three semicircular canals are given off, except in
the lamprey and hag, which have two and one respectively.
From the sacculus an outgrowth called the cochlea or
lagena originates ; it is little more than a small hollow knob
in Fishes and Amphibians, but becomes large and im-
portant in Sauropsida and Mammals.
As this differentiation of the parts of the internal ear takes place, the
lining epithelium also becomes differentiated into flattened covering cells
and sensory auditory cells. The auditory cells are arranged in patches
to which branches of the auditory nerve are distributed. With these
sensory patches calcareous concretions (otoliths) are associated, except
in the cochlea of Mammals.
The fact that lime salts are often deposited in the skin, and that the
ear-sac arises as an insinking of epiblast, may perhaps shed some light
on the origin of otoliths.
494 STRUCTURE OF VERTEBRATA.
The parts which we have so far considered constitute together the
membranous labyrinth of the ear. Round about them the mesoblast
(mesenchyme) forms a two-layered envelope. Its inner layer disin«
tegrates to produce a fluid, the perilymph, which bathes the whole
outer surface of the membranous labyrinth. Its outer layer forms a
firm case, the cartilaginous or bony labyrinth, surrounding the internal
ear. The membranous labyrinth itself contains another fluid, the
endolymph.
With regard to the function of the parts of the ear, the semicircular
canals are believed by many to be concerned with the appreciation of a
Fic. 263.—Diagram showing the ear and related parts
in a young cat.
P., Pinna; Sg., squamosal: £.A.M., external auditory meatus; 7.,
tympanum; 47., malleus; /., incus; S7., stapes abutting on foramen
ovale; Z., bulla of tympanic bone; Se., a septum in the bulla; £.7.,
eustachian tube leading from the tympanic cavity to the back of the
mouth; B.O., basi-occipital: C., cochlea; S., sacculus; U., utriculus ;
D.£., ductus endolymphaticus; .V., auditory nerve; S.C., semi-
circular canal; PZ., periotic bone.
change in the direction or velocity of movement. Tow far the ears of
Invertebrates (¢.g. Crustacea and Mollusca) are adapted for any function
except this, is still doubtful, and we can hardly see that any other
would be of much use to purely aquatic animals. It seems likely at
any rate that the primitive function of the ear was the perception of
vibrations, and that from this both the sense of hearing and the sense
of equilibration have been differentiated.
It is in accordance with the facts mentioned above that we rarely
find in Fishes any special path by which impressions of sound may
travel from the external world to the ear. In Amphibians and higher
Vertebrates, however, the ear has sunk farther into the recesses of the
skull, and a special path for the sound is present. In Elasmobranchs,
SENSE ORGANS. 495
the spiracle, or first gill-cleft, is situated in the vicinity of the ear; in
higher forms, according to many authors, this first gill-cleft is metamor-
phosed into the conducting apparatus of the ear. In development, a
depression beneath the closed gill-cleft unites with an outgrowth from
the pharynx, and thus forms the tympanic cavity, which communicates
with the back of the mouth by the Eustachian tube. The tympanic
cavity is closed externally by the drum or tympanum, which may be
flush with the surface, as in the frog, or may lie at the end of a narrow
passage, which in many Mammals is furnished externally with a projec-
tion or pinna. In Amphibia and Sauropsida the tympanic cavity is
traversed by a bony rod—the columella, which extends from the drum
to the fenestra ovalis, a little aperture in the wail of the bony labyrinth.
In Mammals this is replaced by a chain of three ossicles, an outermost
malleus, a median incus, an internal stapes. ;
The homologies of these
ossicles are still uncertain.
One interpretation has
been stated on p. 480; the
following is Hertwig’s :—
Malleus = Articular +
angular elements of
Meckel’s cartilage.
Incus = Palato-quad-
rate of lower Verte-
brates.
Stapes of Mammals
has a double origin,
being formed from
the upper part of
hyoid arch + an ossi-
fication from the
wall of the ear cap-
sule = (wholly?) col-
umella of Birds,
Reptiles, and Am-
phibians. Fic. 264.—Diagram of the eye.
. C., Cornea; @.4., aqueous humour; ¢.é,, ciliary
The eye.—There is body; 4, lens; 7, iris; Sc., sclerotic; Ch.,
2 3 choroid; #., retina; v.4., vitreous humour;
no eye in Amphioxus, y.sp., yellow spot; #., optic nerve.
it is rarely more than
larval in Tunicates, it is rudimentary in Myxine and in
the young lamprey. In higher forms the eye is always
present, though occasionally degenerate, ¢.g. in fishes from
caves or from the deep sea. It is hidden under the
skin in Proteus, an amphibian cave-dweller, and in the
subterranean amphibians like Cac/za, very small in a few
snakes and lizards, and its nerves are abortive in the
mole.
496 STRUCTURE OF VERTEBRATA.
The adult eye is more or less globular, and its walls con:
sist of several distinct layers. The innermost layer bound-
ing the posterior part of the globe is the sensitive retina,
innetvated by fine branches from the optic nerve. It may
be compared to the nervous matter of the brain, from which,
indeed, it arises. Outside of the retina is a pigmented
epithelium, and outside of this a vascular membrane;
Fic. 265.—Development of the eye.—After Balfour and
Hertwig.
x. Section through first embryonic vesicle, showing outgrowth of
optic vesicles (of.v.) to meet the skin; 7.., thalamencephalon ;
G., the gut. :
2-4. Sections illustrating the formation of the lens (2) from the
skin, and the modification of the optic vesicle into an optic
cup; &., retina; v.4., vitreous humour.
5. External aspect of embryonic eye; Z., lens.
together these are often called the choroid. The vascular
part may be compared to the pia mater covering the brain,
and like it is derived from mesoblast. Outside of the
choroid is a protective layer or sclerotic, comparable to,
and continuous with, the dura mater covering the brain,
and also mesoblastic in origin. Occupying the front of
the globe is the crystalline lens, a clear ball derived directly
SENSE ORGANS. 497
from the skin. It is fringed in front by a pigmented and
muscular ring—the iris, which is for the most part a
continuation of the choroid. The space enclosed by the
iris in front of the lens is called the pupil. Protecting and
closing the front of the eye is the firm cornea continuous
with the sclerotic, and covered externally by the con-
junctiva—a delicate epithelium continuous with the
epidermis. Between the cornea and the iris is a lymph
space containing aqueous humour, while the inner chamber
behind the lens contains a clear jelly—the vitreous humour.
The lens is moored by “ciliary processes” of the choroid,
and its shape is alterable by the action of accommodating
ciliary muscles arranged in a circle at the junction of iris
and sclerotic. In many Reptiles, and in Birds, a vascular
fold, called the pecten, projects from the back of the eye
into the vitreous humour. A similar fold in Fishes
(processus falciformis) ends ina knot-like structure in the lens.
Itacts as an “‘accommodator.” The retina isa very complex
structure, with several layers of cells, partly supporting and
partly nervous; the layer next the vitreous humour consists
of nerve-fibres, while that farthest from the rays of light and
next the pigment epithelium consists of sensitive rods and
cones. The region where the optic nerve enters, and
whence the fibres spread, is called the blind spot, and near
this there lies the most sensitive region—the yellow spot,
with its fovea centralis, where all the layers of the retina
have thinned off except the cones.
Among the extrinsic structures must be noted the six muscles which
move the eyeball, the upper and lower eyelids, which are often very
slightly developed, and the third eyelid or nictitating membrane.
Above Fishes there is a lachrymal gland associated with the upper lid,
and a Harderian gland associated with the nictitating membrane. In
Mammals there are also Meibomian glands. The secretions of all these
glands keep the surface of the eye moist.
While the medullary groove is still open, the eyes arise
from the first vesicle of the brain as hollow outgrowths or
primary optic vesicles. Each grows till it reaches the skin,
which forms a thickened involution in front of it. This
afterwards becomes the compact lens. Meantime it sinks
inwards, and the optic vesicle becomes invaginated to form
a double-walled optic cup. The two walls fuse, and the
32
498 STRUCTURE OF VERTEBRATA.
one next the cavity of the cup becomes the retina, while the
outer forms the pigmented epithelium and the muscles of
the iris. Meanwhile, surrounding mesoblast has insinuated
itself past the lens into the cavity of the optic cup, there
forming the vitreous humour, while externally the mesoblast
also forms the vascular choroid, the firm often cartilaginous
sclerotic, the inner layer of the cornea, etc. Along the
thinned stalk of the optic cup the optic nerve is developed.
Its protective sheath is continuous with the sclerotic of the
eye and the dura mater of the brain. As the nerves enter
the optic thalami, they cross one another in a chiasma, and
their fibres usually interlace as they cross.
Alimentary system.— The alimentary tract exhibits
much division of labour, for not only are there parts suited
for the passage, digestion, and absorption of the food, but
there are numerous outgrowths, eg. lungs and allantois,
which have nothing to do with the main function of the
food canal.
By far the greater part of the food canal is lined by
endoderm or hypoblast, and is derived from the original
cavity of the gastrula—the primitive gut or archenteron.
This is the mid-gut or mesenteron. But the mouth cavity
is lined by ectoderm, invaginated from in front to meet the
mid-gut. This region is the fore-gut or stomodzum.
Finally, there is usually a slight posterior invagination of
ectoderm, forming the anus. ‘This is the hind-gut or
proctodzum, but it is practically absent in Vertebrates.
Associated with the mouth cavity or stomodzum are—(a) teeth
(ectodermic rudiments of enamel combined with a mesodermic papilla
which forms dentine or ivory); (4) from Amphibians onwards special
salivary glands; (c) a tongue,—a glandular and sensitive outgrowth
from the floor. The tongue develops as a fold of mucous membrane
in front of the hyoid, and afterwards becomes increased by growth of
connective tissue, etc. In larval Amphibians muscle strands find their
way into it, and Gegenbaur suggested that their original function was
to compress the glands. As they gained strength they became able
for a new function, that of moving the tongue. In all higher animals
(above Fishes), the nasal sac opens posteriorly into the mouth; in
some Reptiles and Birds, and in all Mammals, the cavity of the
mouth is divided by a palate into an upper nasal and lower buccal
portion, ;
The origin of the oral aperture is’ uncertain, In Tunicates it is
formed by an ectodermic insinking which meets the archenteron ; in
ALIMENTARY SYSTEM. 499
Amphioxus it seems to arise as a pore in an ectodermic disc ; in other
cases it is a simple ectodermic invagination ; or it may owe its origin to
the coalescence of an anterior pair of gill-clefts innervated by the fifth
nerve. If the last interpretation be true, its origin illustrates that
‘change of function which has been a frequent occurrence in evolution.
But if the mouth arose from a pair of gill-clefts, and in some cases it
actually has a paired origin, then there must have been an older mouth
to start with. Thus Beard in his brilliant morphological studies dis-
tinguishes between ‘the old mouth and the new.” The new mouth
is supposed to have resulted, as Dohrn suggested, from a pair of gill-
clefts; the old mouth was an antecedent stomodzum, of which the
so-called nose of AZyxine and the oral hypophysis of higher forms may
be vestiges. This theory harmonises with the observations of Kleinen-
berg on the development of the mouth in some Annelids (Zopado-
rhynchus), in which the larval stomodzeum is replaced by a paired
ectodermic invagination.
The mouth cavity leads into the pharynx, on whose walls
there are the gill-clefts. Of these the maximum number is
eight, except in Amphioxus. If we exclude the hypo-
thetical clefts, such as those possibly represented by the
mouth, the first pair form the spiracles—well seen in skates.
In the position of the spiracles the Eustachian tubes of
higher Vertebrates develop. In front of the spiracle there
is sometimes a spiracular cartilage, which Dohrn dignifies as
a distinct arch. The other gill-clefts are associated with
gills in Fishes and Amphibians, while in Sauropsida and
Mammals, in which there are no gills, four “visceral” clefts
persist as practically functionless vestigial structures. In
some cases their openings are very evanescent. The clefts
are bordered by the branchial arches, and supplied by blood
vessels and nerves.
With the anterior part of the alimentary canal two
strange structures are associated—the thyroid and the
thymus.
The ¢hyroid gland arises as a diverticulum from the ventral wall of
the pharynx. It may be single (as in some Mammals), or bilobed (as
in Birds), or double (as in some Mammals and Amphibians), or diffuse
{as in Bony Fishes). Only in the larval lamprey does it retain its
original connection with the pharynx, and is then a true gut-gland.
As to its morphological nature, its mode of origin suggests com-
parison with the hypobranchial groove in Amphioxus and the endostyle
of Ascidians.
Almost the only light which has been cast on the physiological nature
of the thyroid is from the pathological side. Goitre and Derbyshire
neck are associated with an enlargement and diseased state of this
500 STRUCTURE OF VERTEBRATA.
organ, and myxcedema with its degeneration or absence. As injection
of extract of sheep’s thyroid, or even eating this organ, alleviates myx-
cedema, it is concluded that the thyroid must have some specific effect
on the large quantity of blood which flows through it. It is probably
safe to say that the thyroid aids in keeping the blood at a certain
standard of health, through some specific secretion.
The ¢hymus arises as a dorsal endodermic thickening where the
outgrowths which form the gill-clefts meet the ectoderm. It may
‘be associated with a variable number of clefts—seven in the shark
Heptanchus, five in the skate, four in Teleosteans, three in the lizard,
one in the chick, and one (the third) in Mammals. In the young
lamprey there are said to be no fewer than twenty-eight thymus rudi-
ments. In Mammals it often seems to degenerate after youth. In the
rabbit it has its maximum weight in the fourth month, and thereafter
begins to be rapidly reduced. As it has from its first origin a distinct
lymphoid nature, and apparently forms leucocytes, it has been inter-
preted (Beard) as a structure adapted for the phagocytic protection of
the gills from bacteria, parasites, and the effects of injury. If this be
so, we can understand its diminishing importance in Sauropsida and
Mammalia, where its place may be to some extent taken by the palatal
and pharyngeal tonsilsy which are believed by some (Stohr, Killian,
Gulland) to have a similar phagocytic function.
The pharynx leads into the gullet or cesophagus, which is
a conducting tube, and this into the digestive stomach,
which is followed by the diges-
tive, absorptive, conducting
intestine, ending in the rectum
and anus.
From the cesophagus the air-
or swim- bladder of most Fishes,
and the lungs of higher Verte-
brates, grow out. The air-
bladder usually lies dorsally and
is almost always single; the
lungs lie ventrally and are
double, though connected with
the gullet by a single tube.
The beginning of the intes-
tine gives origin to the liver,
saa which regulates the composition
ss er ee - "t. of the blood and secretes bile,
chick. —After Goette. and to the pancreas, which
a itera coe is shaded ; the endo- secretes oe juices. The
erm dark. ancrea: i
ég., One of the lungs; S¢., stomach ; P s has often a multiple
Z., liver ; 4., pancreas, rudiment.
ALIMENTARY SYSTEM. 501
From the hindmost region of the gut, the allantois
grows out in all animals from Amphibians onwards. In
Amphibians it is represented by a cloacal bladder ; in the
higher Vertebrates it is a vascular foetal membrane con-
cerned with the respiration or nutrition of the i or
both,
Fic. 267. Beri through a young newt.
c.2,, Connective tissue ; Z., epidermis; D., dermis; S.C. spinal cord; J7.,
muscle; JV., notochord Sh, mesodermic sheath of notochord; K.,
kidney; 2, lung; S 5 spleen 3 3 ST., stomach; Pe., peritoneum 3 L.,
liver; a@., duct of the pancreas (P); G.B., gall-bladder ; Bex dorsal
aorta. .
Cilia are very common on the lining of the intestine in
Invertebrates, but.they are much rarer in Vertebrates. Yet
as they occur in Amphioxus, lampreys, many fishes, Proto-
pterus, some Amphibians, and in embryonic Mammals, it
502 STRUCTURE OF VERTEBRATA.
seems not unlikely that the alimentary tract was originally a
ciliated tube.
At the posterior end an ectodermic invagination or proctodzeum meets
the closed archenteron, and at the junction the two epithelial layers
give way, so that an open tube is formed.
The formation of the anus does not take place close to the posterior
end of the primitive gut, but at a point some short distance iri front of
this. In consequence the so-called post-anal gut is formed. This is
continuous with the neurenteric canal, and so communicates with the
neural canal. The post-anal gut attains in Elasmobranchs a relatively
considerable length. It has been very frequently found in Vertebrates,
and is probably of universal occurrence. After a longer or shorter
period it becomes completely atrophied, and with it the communication
between neural and alimentary canals is completely destroyed. In
some Fishes and Amphibians the anus is formed directly from the
blastopore. om
Speculative.—The primitive gut was probably a smooth straight
tube, but the rapid multiplication of well-nourished cells would tend
to its increase in diameter and in length. But on increase in both
directions the slower growth of the general body would impose limita-
tions, and in this we may find the immediate growth-condition deter-
mining the origin of folds, crypts, czeca, and coils, which would be
justified by the increase of absorptive and digestive surface. There
are regular longitudinal folds in AZyxzne, cross-folds traversing these
would form crypts, which may be exaggerated into the pyloric caeca of
Teleosteans and Ganoids, while other modifications would give rise to:
‘*spiral valves” and the like. In the same way it may be suggested
that the numerous important outgrowths of the mid-gut, such as lungs,
iver, pancreas, and allantois, so thoroughly justified by their usefulness,
may at first have been due to necessary conditions of growth—to the
high nutrition, rapid growth, and rapid multiplication of the endoderm.
It may be noted that in the development of the Amphibian Mecdurus,
there are hints of more numerous endodermic diverticula (Platt). It is.
also said that the hypochorda—a transitory structure—arising below and
subsequent to the notochord, is in part due to a series of dorsal out-
growths from the gut (Stdhr). Even the notochord, which arises as.
a median dorsal fold, may be speculatively compared to a typhlosole—
folded outwards instead of inwards. The future elaboration of the
organs which arise as outgrowths of the gut would, however,
depend on many factors, such as their correlation with other parts.
of the body, and would at each step be affected as usual by natural
selection.
[TaBLE.
ALIMENTARY SYSTEM—BODY CAVITY. 503
ALIMENTARY SySTEM.—SUMMARY
REGION OF THE GuT. OuTGROWTHs. ASSOCIATED STRUCTURES.
Mouth cavity, Oral part of the} Teeth.
or Stomodzum, hypophysis. Salivary glands. i
or Fore-gut, Tongue. :
originating as an ectodermic
invagination.
Pharynx, gullet or ceso-| Thyroid\and the| With the several out-
phagus, stomach, small in-}| Thymus f gill-clefts. | growths the surrounding me-
testine, large intestine, and| Air bladder; lungs. | soderm becomes associated,
rectum ;=the mesenteron or| Liver. often to a great extent.
mid-gut, originating from} Pancreas. Note also the origin of
the cavity of the gastrula, Allantois. the notochord as an axial
the archenteron or primitive] The pancreas is | differentiation of cells along
gut; lined by endoderm. usually the result of | the mid-dorsal line of the
two ventral © out- | embryonic gut.
growths and a dorsal
one. In Cyclostomes
Anal region, and Elasmobranchsit | In some Fishes, all Amphi-
or Proctodzum, seems to have but bians, all Sauropsida, and
or Hind-gut, one rudiment; in the | the Prototherian Mammals,
originating as an ectodermic | Sturgeon four. the terminal part of the
invagination. gut is a cloaca or common
chamber, into which the
rectum, the urinary, and the
genital ducts open.
Body cavity.—In Amphioxus the ccelom arises as pouches
from the archenteron (enzerocelic). In the other Vertebrates,
owing to modified processes of development, probably first
arising from the presence of much yolk, solid cell masses
grow out in place of hollow sacs, but the cavities which
appear later, apparently by splitting of the cell mass
(schizocelic), are in reality the retarded cavities of true
coelom-pouches. A dorsal segmented portion (protoverte-
bree) becomes separated off from a ventral unsegmented
portion (Fig. 255). It is this ventral portion which forms
the body cavity of the adult. In the adult it is divided
into an anterior pericardial and a posterior peritoneal
portion.
The body cavity may form part of one or all of the following systems :
—(1) excretory, voiding waste by abdominal pores or by nephrostomes ;
(2) reproductive, receiving the liberated genital elements; and (3)
lymphatic, receiving transudations from visceral and abdominal organs.
504 STRUCTURE OF VERTEBRATA.
It is probably never quite closed, but may communicate with the
exterior by abdominal pores (or through nephrostomes) opening into
the renal system. Both occur together in some Elasmobranchs, but
they are usually mutually exclusive. In the higher Teleostei, in some
Saurians, and in Mammals, there are neither abdominal pores nor
nephrostomes, but only openings (stomata) into the lymphatic
system.
Vascular system.--From Cyclostomata onwards the
blood fluid contains red cor-
puscles, z¢. cells coloured
with heemoglobin—a pigment
which readily forms a loose
union with oxygen, and bears
it from the exterior (through
gills or lungs) to the tissues.
These pigmented cells are
usually oval and nucleated.
In all Mammals except
Camelidz they are circular.
Moreover, the full-grown red
corpuscles of Mammals have
no visible nuclei. The blood
fluid also contains uncoloured
Fic. 268.—Blood corpuscles.
x. Amphibian, seen on the flat, oval,
bi-convex disc (nucleated); 2, am-
phibian, in profile; 3, mammalian
(non-nucleated), circular, bi-concave
disc; 4, mammalian, in profile; 5,
camel’s (non- nucleated), oval; 6,
mud-fish (Lepidosiren) in_ section,
like Amphibian ; 7, Lepidosiren, seen
on the flat ; 8, an amoeboid leucocyte
with lobed nucleus and large gran-
ules; 9, a leucocyte with non-lobed
nucleus and minute granules; 10, a
leucocyte dividing into two; 11, a
flat amoeboid corpuscle or blood
platelet
inequality of pressure which makes the blood flow.
nucleated amceboid cells, the
white corpuscles or leuco-
cytes, of much physiological
importance. Some of them,
specialised as phagocytes,
form “a body-guard,”. at-
tacking and destroying micro-
organisms within the body.
The heart receives blood
from veins, and drives it forth
through arteries. Its contrac-
tions in great part cause the
It lies
in a special part of the body cavity known as the peri-
cardium, and develops from a single (sub-pharyngeal)
vessel in Cyclostomata, Fishes, and Amphibians, from a
pair in Reptiles, Birds, and Mammals.
The receiving region of the heart is formed by an auricle
or by two auricles; thence the blood passes into the
VASCULAR SYSTEM 505
muscular ventricle or ventricles, and is driven outwards.
Except in adult Birds and Mammals, the veins from the
body enter the auricle (or the right auricle if there are two)
by a porch known as the sinus venosus. In Fishes (except
Teleosteans) and in Amphibians the blood passes from the
ventricle into a valved conus arteriosus, which seems to be
a continuation of the ventricle. In Teleosteans there is a
superficially similar structure, but without valves and non-
contractile, and apparently developed from the aorta, not
from the ventricle; it is called the bulbus arteriosus, and
may occur along with the conus arteriosus in other Fishes.
In Vertebrates higher than Amphibians there is no distinct
conus.
In Cyclostomata, and in all Fishes except Dipnoi, the heart has one
auricle and one ventricle, and contains only impure blood, which it
receives from the body and drives to the gills, whence purified it flows
to the body.
In Dipnoi the heart is incipiently three-chambered.
In Amphibians the heart has two auricles and a ventricle. The right
auricle always receives venous or impure blood from the body, the left
always receives arterial or pure blood from the lungs. The single
ventricle of the amphibian heart drives the blood to the body and to
the lungs. ;
In all Reptiles, except Crocodilia, the heart has two auricles and an
incompletely divided ventricle. The partition in the ventricle secures
that much of the venous blood is sent to the lungs; indeed, the heart,
though possessing only three chambers, works almost as if it had
four.
In Crocodilia there are two auricles and two ventricles. But the
dorsal aorta, which supplies the posterior parts of the body, is formed
from the union of two aortic arches, one from each ventricle. Therefore
it contains mixed blood.
In Birds and Mammals the heart has two auricles and two ventricles,
and ome aortic arch supplies the body with wholly pure blood. his
aortic arch always arises from the left ventricle, but in Birds it curves
over the right bronchus, z.e. is a right aortic arch, and in Mammals
over the left, ze. is a left aortic arch. Impure blood from the body
enters the right auricle, passes into the right ventricle, is driven to the
lungs, returns purified to the left auricle, enters the left ventricle, and is
driven to the body. ;
The arterial system of a fish consists of a ventral aorta continued
forwards from the heart, of a number of afferent vessels diffusing the
impure blood on the gills, and of efferent vessels collecting the purified
blood into a dorsal aorta.
In the embryo of higher Vertebrates the same arrangement persists,
though there are no gills beyond Amphibians. From a ventral arterial
stem arches arise, which are connected so as to form the roots of the
506 STRUCTURE OF VERTEBRATA.
dorsal aorta. This aorta gives off vessels to the body, while in embry-
onic life it sends important vitelline arteries to the yolk, and (in
Reptiles, Birds, and Mammals) equally important allantoic arteries
to the allantois.
Returning to the arterial system of a fish, we must consider the
arches more carefully, and compare them with those of Sauropsida
and Mammals, where they are no longer connected with functional
gill-clefts, and also with those of Amphibians, where the complications
due to lungs, etc., begin (see the following Table).
SUMMARY AS TO AORTIC ARCHES
SAUROPSIDA AND
FISHES. AMPHIBIANS. MAMMALS.
(a) Mandibular aortic} Aborts, or is not} At most merely em-
arch usually aborts; | developed. bryonic.
there isa persistent
trace in Elasmo-
branchs (spiracular
artery).
(6) Hyoid aortic arch | Aborts. At most merely em-
aborts, or is rudi- bryonic.
mentary.
(c) Ist branchial. Carotid. Carotid.
(d@) 2nd branchial. Systemic arches, | Systemic. Onlythe right |
unite to form} persists in Birds; only
dorsal aorta, the left in Mammals.
(e) 3rd branchial. Rudimentary or | Disappears.
disappears in
most forms.
(f) 4th branchial (gives | Pulmonary. The pulmonary.
off artery to “lung”
of Dipnoi).
The important features in the development of the venous system are
as follows :—
(a) In the embryo the vitelline veins bring back blood from the
yolk-sac, at first directly to the heart, and later to the liver.
Into these veins, blood returned from the intestine is poured
in increasing quantity by other veins. In the adult these
persist to form the hepatic portal system, by means of which
blood from the stomach and intestine is carried to the liver,
and not directly to the heart.
VASCULAR SYSTEM. 507
(6) At an early stage in development the blood is brought back from
the anterior region by the superior cardinal veins, from the
posterior region by the inferior cardinals. The two cardinals
on each side unite to form the short transverse ductus Cuvieri,
the two ducts entering the
sinus venosus of the heart.
In Fishes the superior car-
dinals persist, the inferior
cardinals bring back blood
from the kidneys, and also
to some extent, by means
of their union with the
caudal vein, from the pos-
terior region of the body.
In some cases this union
with the caudal is only in-
direct, through the medium
of the kidney (Elasmo-
branchs); in this way the
renal portal system is con-
stituted. In higher Verte-
brates, before development
is completed, the superior
cardinals are replaced by
the superior venze cave
(into which the superior
cardinals open as external
jugulars). The inferior car-
dinals at first return blood
from the Wolffian bodies
and the posterior region ;
later they atrophy, and are
replaced by an unpaired
inferior vena cava which
eo agli peg Fic. 269.—Diagram of circulation.
from the liver (hepatics), —After Leunis,
and from the hind-limbs ~.2., Right auricle receiving superior vena
except when there is a @y% {0 aoe ences a, ae
renal portal system. The monary artery to lungs (Z.); .v., right
azygos vein of Mammals pulmonary vein; 4da., left auricle;
is a persistent remnant 4., left ventricle; @o., aortic arch;
fi i : d.ao., dorsal aorta giving off arteries to
of the inferior cardinals. liver (dé.), to gut (g.), to body (B.);
(c) In Amphibia 4 vein known —4o.v., portal veins; 4.v., hepatic vein.
as the epigastric (anterior
abdominal) carries blood from the hind-limbs into the hepatic
portal system. This vein also receives blood from the allantoic
bladder, a fact which is of great theoretical importance. In
all higher Vertebrates in embryonic life, the blood from the
allantois passes through the liver, and to a greater or less
extent into its capillaries, on its way to the heart. In
aoa
dao.
508 STRUCTURE OF VERTEBRATA.
Reptiles the allantoic veins persist throughout life as the
epigastric vein or veins. In Birds and Mammals, on the
other hand, they atrophy completely at the close of foetal
life. In Birds, however, a vein is developed which connects
the veins coming from the posterior region with the allantoic
veins; this persists when the remainder of the allantoic veins
atrophy, and thus in Birds as in Amphibia there is a con-
nection between the components of the inferior vena cava
and the portal system. In Mammals no such connection occurs.
According to many authorities, the vascular system is de-
veloped in the mesoblast from the hollowing out of strands
of cells, the outer cells forming the walls of the vessels, the
inner forming the constituents of the blood. The heart, with
the exception of its endothelial lining, is a tubular de-
velopment of the splanchnic mesoderm. :
Associated with the vascular system is the spleen, which
‘appears to be an area for the multiplication or destruction
-of blood corpuscles.
The lymphatic system, developed in mesoblastic spaces,
is a special part of the vascular system. It consists of fine
‘tubes which end blindly in the tissues and drain off fluids,
of larger vessels which the tubes combine to form, and
which open into veins. The lymph vessels contain amce-
boid cells, and have associated lymphatic glands in which
these lymphocytes are produced.
Respiratory system.—In Balanoglossus, Tunicates, and
Amphioxus, the walls of the pharynx bear slits, between
‘which the blood is exposed in superficial blood vessels to
the purifying and oxygenating influence of the water.
In Cyclostomata, Fishes, all young and some adult Am-
‘phibians, there are not only clefts on the walls of the
pharynx, but gills associated with these. On the large
‘surface of the feathery or plaited gills, the blood is exposed
_and purified.
In Reptiles, Birds, and Mammals, traces of gill-clefts
occur in the embryos, but without lamellz or respiratory
function. In the embryo the blood is purified, as will be
explained afterwards, by aid of the foetal sac known as the
allantois; and after birth the animals breathe by lungs.
All adult Amphibians also have lungs, to which the lung or
‘swim-bladder of Dipnoi is physiologically equivalent.
" The gill-clefts arise as outgrowths of the endodermic gut
which meet the ectoderm and open. The ventral paired
EXCRETORY SYSTEM. 50
lungs arise from an outgrowth of the gut, as does also.
the swim-bladder of many Fishes, though it usually lies
on the dorsal surface, has rarely more than a hydrostatic
function, and usually has a blood supply different from
that of the lungs. In Dipnoi and some “Ganoids” it is
supplied by a pulmonary artery arising from the sixth aortic
arch. There is probably a homology between lung and
swim-bladder.
Excretory system.—The development of this is always compli-
cated. In the embryos of Vertebrates at an early stage there are always.
traces of a pronephros, or so-called head-kidney. This is perhaps seen:
in its most primitive condition in Amphioxus, where, as already de-
scribed, there is a series of tubules, segmentally arranged, opening on
the one side into the body cavity by several flame-cells, and on the
other into the atrial chamber, ze. the exterior. On the surface of
each tubule # vessel connecting the sub-intestinal vein with the dorsal
aorta forms a vascular plexus—the so-called glomus. Such a con-
dition of parts is never in its entirety found in the Craniata. There
the tubules open not directly to the exterior, but into a longitudinal
pronephric or segmental duct, and they are usually few. in number ;.
but in their segmental arrangement, as shown by the blood supply,.
and in the presence of glomera, they agree entirely with those of 4m-
phioxus. In connection with the glomera, it may be noted that while
the blood supply usually comes directly from the dorsal aorta, it has been.
shown by Paul Mayer and Riickert that in the embryos of Selachians
connecting vessels occur between the dorsal aorta and the sub-intestinal
vein, which form rudimentary networks on the tubules of the pronephros.
This shows a very striking correspondence with the conditions seen in.
Amphioxus.
The pronephros develops from the parietal mesoblast at the junction.
of the muscle segments and the unsegmented body cavity (see Fig. 270)
in the anterior region, and varies greatly in its degree of development.
In AGyxine and Bédellostoma it persists in adult life, though apparently,
at least in part, in a degenerate condition, and is said to be the functional
excretory organ of the little (degenerate ?) fish /zerasfer and some other
Bony Fishes. In most Bony Fishes, and in Amphibia, it is merely a
larval organ, but is then large and important. In Elasmobranchs and
Amniota, except Crocodiles and Turtles, it is from the first rudimentary
and functionless.
The origin of the segmental or pronephric duct is still undetermined.
It usually arises from the mesoblast, in some cases growing backwards.
directly from the rudiment of the pronephros, while in others the sur-
rounding mesoblast takes an important part in its formation ; in Elasmo-
branchs, in Mammals, and in the chick, a connection with the epiblast
has been described by various observers. Riickert is of opinion that it
originally arose by the fusion of the outer ends of the pronephric
tubules, and that the occasional connection with the ectoderm indicates.
the position of former excretory pores (cf. Amphioxus).
510
STRUCTURE OF VERTEBRATA.
At a late period in those types in
which the pronephros is a functional
larval organ, but much earlier in the
higher Vertebrates, another series
of tubules is differentiated from the
mesoblast, and, acquiring a con-
nection’ with the segmental duct,
constitutes the mesonephros, or mid-
kidney. The tubules arise usually,
though not invariably, nearer the
posterior end of the body than the
pronephros, and are formed from
the portion of the mesoblast which
connects the muscle segment and
the lateral plate (see Fig. 270).
Below the Amniota the mesonephros
forms the permanent excretory
organ. In higher forms another
series of nephridial tubules arises
still farther back in the body, and
forms the metanephros, or perma-
nent kidney. The mesonephric
and metanephric tubules resemble
each other closely, but the relation
of the former to the pronephros
is still a debated point. When
fully developed, a mesonephric
tubule consists of—(1) an internal
ciliated funnel (nephrostome), which
opens into the body cavity, but
is only rarely represented; (2) a
Fic. 270.—Development of excre-
tory system of Vertebrate.—In
part after Boveri.
In I. the primitive segments are not
separated off from the lateral plate,
and the pronephros (/.) is seen arising
from the lower part of the primitive
segment. In II. the pronephros is com-
pletely separated off from the primi-
tive segment and lateral plate. In
III. the origin of the mesonephric
tubules is seen. They arise from the
upper part of the lateral plate, which
is now completely separated from the
primitive segment, and curving round
the pronephric duct come tg open into
it.
w.c., nerve cord; zch., notochord; Amn.,
pronephros; g., gut; /.s., primitive
segment; es., mesonephric tubule;
gn.d., pronephric duct; 4.c., body
cavity; @o., aorta; szv., sub-intestinal
vein, with vessel to the aorta.
SUPRARENAL BODIES. 511
small cavity (Malpighian capsule) derived from the ccelom, and con-
taining a mass of capillaries which project into the cavity of the
tubule ; and (3) a coiled tube in part excretory, in part a conducting
canal for the waste filtered from the blood. The metanephric
tubules have a quite similar structure, but the nephrostome is never
present. :
In all Vertebrates the primitive nephridia open into a
pair of longitudinal ducts, developed like the nephridia as
special portions of the ccelom. These ducts open into the
end of the gut. According to their connections with the
nephridia these longitudinal ducts are called pronephric,
mesonephric, or metanephric ducts, and they are also called
segmental ducts. In Elasmobranch fishes a Miillerian duct
is separated off from in front backwards from the
longitudinal duct and forms the oviduct of the female, a
rudiment in the male. After the separation of the Miillerian
duct, the longitudinal duct (now called mesonephric or
Wolffian) forms in the male the vas deferens and also
receives the tubes from the permanent kidney (mesonephros).
Tn the female the Wolffian duct has this last function. In
general it may be said that the original longitudinal duct
becomes the vas deferens in the male Vertebrate, and that
another duct—the Miillerian—whose development is obscure
except in Elasmobranchs, forms the oviduct. The meta-
nephric duct, developed in part from the hinder end of the
mesonephric duct, is the ureter of the permanent kidney in
Amniota. ,
Suprarenal bodies.—These are found in most Vertebrates near
the reproductive organs and kidneys. They seem to increase in
importance as we ascend the series. Typically, each shows a dis-
tinction into a cortical and a medullary zone. It is usually asserted
that-these two areas have a different origin, the medullary region being
derived from the sympathetic nervous system, the cortex from the coelomic
epithelium. There is much evidence (morphological and physiological)
that the suprarenals of Elasmobranchs correspond to the medullary part
in Mammals, while the interrenals of Elasmobranchs and the suprarenals
of Teleosts and Ganoids correspond to the cortical portion in Mammals.
With regard to function, there is some uncertainty. The suprarenal
bodies are relatively very large in embryonic life, but fail to maintain
their primitively rapid rate of growth. A substance, adrenalin, can be
extracted from them which has a remarkable action upon the parts
innervated by the sympathetic system, producing on injection the same
effects as stimulation of the sympathetic would have, é.g. constriction
of the arterioles, and consequent heightening of the blood pressure,
STRUCTURE OF VERTEBRATA.
512
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REPRODUCTIVE SYSTEM. 513
Reproductive system.—The ovaries and testes are
developed from a ridge formed by a part of the epithelium
lining the abdominal cavity, this ridge constituting the
so-called germinal epithelium.
In the male the proliferating germinal epithelium is
divided by embryonic connective tissue into numerous
follicles. The cells of the follicles form seminal mother-
cells, which, by their ultimate divisions, give rise to sper-
matozoa. From the mesonephros, tubules grow out to the
embryonic testes; these form the collecting tubes of the
organs and open into the Wolffian duct, the vas deferens
of the adult. :
In the female the ovary is similarly divided up into
follicles. In this case, however, differentiation sets in
among the originally equivalent cells of the follicle. One
cell in each follicle is more successful than its neighbours,
which are sacrificed to form an envelope of follicular cells
around the single large ovum cell. The ova are usually
shed into the body cavity, and pass thence to the exterior
by the Miillerian ducts or oviducts.
“In many cases, between the follicular cells and the ovum there is 4
membrane, the zona radiata, which is traversed by fine pores, and, in
consequence, has a striated appearance ; other egg membranes, more or
less transitory in nature, also occur. In the lower Vertebrates the layer
of follicle cells is single, but in Mammals (except in Monotremes) it is
multiple, and a quantity of clear fluid accumulates between the cells
and the ovum. The whole forms a ‘‘ Graafian follicle,” which bursts
when the ovum is liberated.
Before fertilisation takes place, the ovum undergoes a process of
maturation, during which extrusion of polar bodies typically occurs ;
the technical difficulties in the way of the definite observation of this
fact are, however, often very great. The ovaare fertilised outside the
body in Cyclostomata, Ganoids, Teleosteans, Dipnoi, and tailless
Amphibians ; internally in the other Vertebrates.
Hermaphroditism occurs as a normal state in Tunicata, most of which
are first functionally female and then male (protogynous) ; in AZyxine
(g.v.), which is first male and then female (protandrous); in some
species of the Teleostean genera Chrysophrys and Serranus, of which
the latter is regularly self-fertilising ; and in a solitary Batrachian. It
occurs casually in some Selachians, in the sturgeon, in about a score ot
Teleosteans, ¢.g. cod, in various Amphibians, and more rarely in
Amniota. There are also embryological facts which suggest that the
embryos of higher Vertebrates pass through a state of hermaphroditism
before the unisexual condition is reached. On these grounds it has
often been suggested that the original Vertebrate animals were
hermaphrodite. .
33
514 STRUCTURE OF VERTEBRATA.
The quantity of yolk present in the egg varies very greatly in
Vertebrates, and its presence or absence exercises a profound influence
upon the processes of development. Following Hertwig, we may notice
that the presence of yolk has both a physiological and a morphological
effect. Physiologically, the presence of a store of nutriment enables the
developmental process to be carried on uninterruptedly, and the period
of independent life to be postponed until more or less complexity of
organisation has been attained. Morphologically, the yolk acts as a
check to the activity of the protoplasm, and by substituting an
embryonic mode of nutrition for that for which the adult organism
is fitted, tends to prevent a speedy establishment of the adult form.
When much yolk is present, it usually forms a hernia-like yolk-sac,
hanging down from the embryonic gut. Asa further consequence, we
may notice the tendency to the production of embryonic organs useful
only during embryonic life. We must consider the formation of an
organic connection between mother
and unborn young as a further step
in the same direction as the acqui-
sition of yolk. This is hinted at in
some Fishes and Reptiles, but cul-
minates in the placental Mammals,
It may be looked at in two differ-
ent ways. On the one hand, the
diversion of the nourishment from
the ovary, during the period of
gestation, tends to starve the remain-
ing ovarian ova, and this check to
Fic. 272.—Mammalian ovum.— owt is further prolonged during
‘After Hertwig. actation (Ryder); on the other
: hand, the chance of survival is
ov., Ovum; /,, follicular capsule; /z., much increased, and the maternal
follicle cells ; fc., follicle cells form- ° 26 Daan f ‘
ing discus proligerus; £2, cavity Sacrifice finds its justification in
occupied by liquor folliculi. the increased specialisation of the
offspring.
In accordance with the effect of the presence of yolk as noted above,
we find that segmentation is total (holoblastic) in the ova of the lam-
prey, the sturgeon, Ceratodus, Amphibians, and all Mammals except the
Monotremes. In the ova of Elasmobranchs, Teleosteans, Reptiles,
Birds, and Monotremes, the activity of the protoplasm is not sufficient
to overcome the inertia of the yolk, and segmentation is partial
(meroblastic). .
Similarly we find that a gastrula is formed, in part at least, by dis-
tinct invagination in the development of the lamprey, the sturgeon,
and Amphibians (though the occurrence of invagination has been denied
for the frog); it is more modified in Teleosteans and Elasmobranchs,
whose ova have more yolk; it is much disguised in Sauropsida and
Mammals.
Most Vertebrates lay eggs in which the young are hatched
outside of the body, and to all these forms the term ovi-
REPRODUCTIVE SYSTEM. 515
parous is applied. In some sharks, a few Teleosteans, some
tailed Amphibians, a few lizards and snakes, the young are
hatched before they leave the body of the mother. To
these cases the awkward term ovo-viviparous is applied, but
there is no real distinction between this mode of birth and
that called oviparous, and both may occur in one animal
(e.g. in the grass-snake) in different conditions. In the
placental Mammals there is a close organic connection
between the unborn young and the mother, and the
parturition in this case is usually called wvigarous. But all
the three terms are bad.
CHAPTER XXI
PHYLUM CHORDATA
SUB-PHYLUM CRANIATA
CLass CYCLOSTOMATA
(Synonym, MARSIPOBRANCHII)
Tue hag (A@yxine), the lamprey (Petromyzon), and a few
others like them, differ in so many ways from Fishes, that
they must be ranked in a distinct class. They represent an
archaic type, whose interest has been enhanced by the
discovery of Paleospondylus in the Old Red Sandstone.
GENERAL CHARACTERS
Unlike all higher Vertebrates (Gnathostomata), the
Cyclostomata have round suctorial mouths, without distinctly
developed jaws. They are also without paired fins and
without scales. Their respiratory system consists of paired
gill-pouches, to which the term Marsipobranch refers. The
body is vermiform, the unpaired fins have no true fin-rays.
In the extant forms the skeleton is wholly cartilaginous, and
the notochord persists unconstricted. The nasal organ ts
unpaired, there is no sympathetic nervous system, no conus
arteriosus, no distinct pancreas, no spleen, no genital ducts,
and the segmental duct persists as such. Their geographical
distribution ts wide.
First Type. M€dyxine—The Hag
The glutinous hag (AZyxine glutinosa) is not uncommon
off the coasts of Britain and Scandinavia, the Atlantic
coast of America, etc. It lives in the mud at depths of
MYXINE, 517
40 to 300 fathoms. It often lies buried with only the
nostril protruding from the mud, but it can swim gracefully
and rapidly in eel-like fashion in search of prey. : It eats
the bait off the fisherman’s long: lines, and it also enters and
devours the cod, etc., which have been caught on the hooks.
According to some, the hag also bores its way into free-
swimming fishes, but the evidence is not satisfactory. Ac-
cording to Mr. J. T. Cunningham, the young animals are
hermaphrodite, containing immature ova and ripe sper-
matozoa, while older forms produce ova only. If the same
form is first functionally a male and afterwards functionally
a female, the term “protandrous hermaphroditism” is
justified, and Nansen corroborated Cunningham’s dis-
covery, which is, however, disputed by Bashford Dean. A
somewhat similar “ protandrous” hermaphroditism is known
elsewhere, eg. in the Nemertean Stichostemma eilhardit, in
the aberrant AZyzostoma, and in the crustacean Cymothoide.
Hag are said to spawnin lateautumn. Of the development
and early history nothing is known.
Porm, skin, and muscular system.—The body is eel-
like; measuring 15 to 24 in. in the adult. The colour is
pinkish, the red blood shining through an unpigmented
skin. There is a slight median fin around the tail; beside
the mouth and nostril there are four pairs of sensitive
barbules. There are no paired fins. The cloacal opening
is near the posterior end of the body.
The skin is scaleless, and rich in goblet cells, which
secrete mucus. There is also a double row of glandular
pits, partly embedded in muscle, and arranged segmentally
on each side of the ventral surface along its entire length.
Each opens by a distinct pore, and so much mucus is rapidly
secreted that the ancients said the hag “could turn water
into glue.” This makes the hag difficult to grip, and its
function is doubtless in part protective. The mucus chiefly
consists of strange spiral threads which uncoil when ejected
from the sacs.
The zigzag muscle segments or myomeres are traceable.
The rasping teeth are worked by a powerful muscular
structure, sometimes called a “tongue.” A section
of this shows a strong muscular cylinder surrounding a
cartilage.
518 CYCLOSTOMATA.
The skeleton.—The skeleton is wholly cartilaginous.
The notochord persists unsegmented within a firm sheath,
the skull is a simple unroofed trough, jaws are not
distinctly developed, there is only a hint of the complicated
basket-work which supports the gill-pouches of the lamprey ;
but the tongue, the barbules, etc., are supported by cartila-
ginous rods. The tail is protocercal.
Nervous system.—The brain has the usual parts, but
the cerebrum and cerebellum are little more than rudiment-
ary: It is much compressed, with practical obliteration
of the ventricles. The fore-brain seems to agree with that
of “ Ganoids” and Teleosteans in having a non-nervous roof.
Fic. 273.—Median longitudinal section of anterior region of
Myxine.—After Retzius and Parker.
&., Barbule; 4, nasal aperture ; V7., nasal tube with rings of cartilage ;
NC., nasal capsule; BR., brain; SC., spinal cord; ., notochord ; G.,
gut; 7., cartilage of “tongue”; M7U., muscle; WTT., posterior part of
nasalsac; 7.,atooth plate; A77., median tooth on roof of mouth (/Z.).
The spinal cord is somewhat flattened, and is sheltered simply
by fibrous tissue. Throughout at least a portion of the cord
there are two dorsal roots for each ventral root. The union
of dorsal and ventral roots is only partial, and there is no
sympathetic system. There is no lateral line system.
The eye is without lens, cornea, iris, or muscles, and is
hidden beneath the skin; the optic nerves do not cross
until they enter the brain; the ear has only one semi-
circular canal, The single nasal sac (with paired folds of
olfactory epithelium in Adedlostoma, an American relative)
opens dorsally at the apex of the head, and. communicates
posteriorly with the pharynx by a naso-palatine duct. It
MYVXINE, 519
may be, as in the lamprey, a combination of olfactory and
pituitary involutions. The absence of pigment and sensory
structures in the skin, and the simple state of the eye and
ear, may be partly associated with the hag’s mode of life.
It seems probable that the simplicity is primitive rather
than degenerate.
Alimentary system. — The
mouth is suctorial. There is a
median tooth above, and two rows
of teeth are borne on each side of
the muscular “tongue.” These
teeth are entirely “horny,” but
sharp. Into the mouth, just in
front of a fringed velum which
separates it from the pharynx, the
nasal, or, as some would say, the
naso-pituitary, sac opens. Thus
water passes from the nostril into
the pharynx. It may be, as Beard
suggests, that this passage is‘a per-
sistent “old mouth,” the palzeo-
stoma of Kupffer. From the gullet
open six respiratory pouches, each
of which has an efferent tube, and
the six efferent tubes of each side
unite in a common exhalant ori-
fice. The gut is straight and Fig. 274,—Respiratory sys-
uniform, with wavy longitudinal tem of hag, from ventral
ridges internally, with a two-lobed _ surface.
liver and a gall-bladder, but with- 4 Barbules; m., mouth opening.
out the usual pancreas. The ¢2, first eill-pouch cat open
anus lies within an integumentary {fshowjpterna) lamella se #.%
cloacal chamber. canal of first gill-pouch ; ts
Respiratory system.— Water {tosinon calen geen
may enter by the nasal sac or
by the mouth. It passes into the pharynx, down the
gullet, into the six pairs of respiratory pouches and
their efferent tubes, and leaves the body by the single
aperture at each side. The respiratory pouches have
much-plaited internal walls, on which the blood vessels are
spread out. On the left side, behind the sixth pouch, a
520 CYCLOSTOMATA.
tube (the cesophago-cutaneous duct) opens from the
cesophagus to the exhalant aperture. Perhaps some water
enters by it in inspiration.
Vascular system.—The blood contains the usual ame-
boid leucocytes and red blood corpuscles, elliptical in
form (circular in the lamprey). It is collected from the
body in anterior and posterior cardinals, passes through
a sinus venosus into the auricle of the heart, thence to
the ventricle, thence along a ventral aorta, which gives off
vessels to the respiratory pouches. From these the purified
blood passes dorsalwards in efferent branchial vessels, which
unite posteriorly, to form the dorsal aorta, while from the
most anterior a branch goes to the head. The portal vein
has a contractile sinus which drives blood through the
liver. The pericardium is in free communication with the
general body cavity. es
Excretory system.—The segmental pronephric ducts persist,
and give off short lateral tubules, metamerically arranged, ending in
globular malpighian capsules. The pronephros is functional in the
young form, and at least part of it persists throughout life, e.g. in a
lymphoid structure beside the pericardium.
The ducts end by separate pores on a papilla within the integument-
ary cloaca,
Reproductive system.—//yxine is a protandrous herma-
phrodite, spermatozoa being formed at an early period,
and ova afterwards. The reproductive organ is simple,
unpaired, and moored by a median dorsal fold of peri-
toneum. Owing to the large size of the ova, the ovary is
very conspicuous in full-grown forms. When the ova are
freed from the ovary, they pass into the body cavity. Each
has an oval horny membrane, with a circlet of knobbed
processes at each end. By these they become entangled
together. There are no genital ducts, but just above the
anus there is a large genital pore opening from the body
cavity into the integumentary cloaca. The development
is still unknown.
Besides Myxcne glutinosa, two other species are known—one from
Japan, another from the Magellan Straits. The southern JZ, australis
lives in shallow water close by the shore, but the others live in deep
water. The genus Bdellostoma, from the Pacific coasts America,
off the Cape of Good Hope, etc., is nearly allied.
PETROMYZON.
The best-known species,
Bdellostoma dombeyz, resembles
the hag in many ways. It lives
at the bottom of the sea, at
depths of a hundred fathoms or
more, and is often found inside
caught halibut, etc. The gill-
pouches have separate openings,
and are extraordinarily. variable
in number, from six to fourteen
on either side—a variability per-
haps pointing to ancestral reduc-
tion from a larger number (cf.
Amphioxus). Large eggs are
laid on a shelly or rocky bottom,
become connected by polar
hooks in chains or clusters, are
fertilised after deposition, and
exhibit merdblastic discoidal
segmentation and direct devel-
opment. Ayers’ experiments
show that the removal of one or
both ears in this form does not
materially affect equilibration.
SECOND TYPE
Letromyzon—The Lamprey
There are three British
species—the sea lamprey
(Petromyzon marinus), over
3 ft. in length; the river
lampern (Pf. ftuviatilis),
nearly 2 ft. long; and the
small lampern or “stone-
grig” (P. planeri). They
eat worms, small crustace-
ans, insect larve, dead
animals, etc. ; but they also
attach themselves to living
fishes, and scrape holes in
their skin. As their names
suggest, they also fix their
mouths to stones, and some
draw these together into
nests.
521
ss
a.
Teh 0,
Fic. 275.—Bdellostoma stout? (Cali-
fornian hag), enveloped in sheath
of mucus.—After Bashford Dean.
4., Barbules ¢.,eyes m. mucus; eg., eggs.
522 CYCLOSTOMATA.
The spawning takes place in spring, usually far up rivers.
Before laying the eggs, the lamprey seems to fast (cf.
i,
yf
Fic. 276.—The lamprey (Petromyzon marinus).
I, The entire animal ; note the seven gill-slits of which the first is
marked g.s., the nostril #., and the unpaired median fins.
II. Ventral pee of the head ; 2.2. pest teeth; 22, lower
teeth; ., the piston in the mouth.
Uppers surface of the
head ; z., the nostril with the pineal Bi be
‘ind it ; ¢., the eye.
salmon, Protopterus, frog), and its muscles undergo a
granular degeneration (cf. Protopterus, tadpole, etc.). Soon
PETROMYZON. 523
after spawning the adults of both sexes die. For reproduc-
tion is often the beginning of death as well as of life, though
in higher animals the nemesis may be slow. The young
are in many ways unlike the parents, and after 2-4 years.
pass through a striking metamorphosis. To the larve
before metamorphosis the old name Ammocetes is applied.
Form, skin, and muscles.—The body is eel-like, with
two unpaired dorsal fins, and another round the tail.
The skin is scaleless, slimy, and pigmented. Its structure,
like that of AZyxine, is complex. Sensory structures occur
on the head and along the sides, and form a lateral line
system.
Fic. 277,.—Longitudinal vertical section of anterior end
-of larval lamprey.—After Balfour.
m., Mouth; th., thyroid ; ¢.., one of the gill-pouches ; v.a0., ven-
tral aorta; 4., heart; V., notochord; S.C., spinal cord; £.,
auditory. vesicle ; cb., cerebellum; 4.4, pineal body; c.z.,
cerebral hemispheres ; off, olfactory invo, ution
The muscle segments or myomeres are well marked.
The suctorial mouth and the rasping “tongue” are very
muscular.
The skeleton.—The skeleton is wholly cartilaginous.
The notochord persists unsegmented, but its firm sheath
forms rudimentary neural arches. The skull is imperfectly
roofed. There are no distinct jaws, but a cartilaginous ring
supports the lips of the mouth. There is a complex basket-
work around the gill-pouches, but it is zof likely that its
elements correspond to visceral arches. Endoskeletal
cartilaginous rods, not comparable to the dermal fin-rays
of fishes, support the dorsal and caudal fins, and other
skeletal parts occur about the “tongue.” The caudal end
of the notochord is quite straight.
$24 CYCLOSTOMATA.
Nervous system.—The brain has the usual parts, but is
small and simple; the roof of the fore-brain is composed
of non-nervous epithelium; there is a distinct pineal
body, with hints of an eye; the oral part of the hypo-
physis is developed from in front of the mouth, and
becomes closely connected with the involution of epiblast
which forms the nostril. A unique peculiarity in the brain
is that the middle part of the roof of the ztev is simply
epithelial. The spinal cord is flattened; the dorsal and
ventral roots of the spinal nerves alternate and do not
unite ; there is no sympathetic system.
Though the larva sometimes receives the name of “ nine-
eyes ”—which expresses a popular estimate of the branchial
apertures—it is blind, for the eyes are rudimentary and
hidden. In the adult they rise to the surface, and are
fairly well developed. The optic nerves do not cross until
they enter the brain. The ear has only two semicircular
canals instead of the usual three. The single nasal sac
does not open posteriorly into the mouth as it does in
Myxine ; though prolonged backwards it ends blindly. Its
external opening is at first ventral, but is shunted dorsally
and posteriorly.
Alimentary system.—The oral funnel, at the base of
which the mouth lies, has numerous horny teeth. It is
applied to the lamprey’s victim, and adheres like a vacuum
sucker ; the toothed “tongue” works like a piston; both
flesh and blood are thus obtained. From the floor of the
pharynx an endostylar groove is constricted off to form
the thyroid.
From the gullet of the young larva seven gill-pouches
open directly to the exterior; in the adult this larval gullet
becomes wholly a respiratory tube. It is closed pos-
teriorly, and opens anteriorly into the gullet of the adult,
which is a new structure. At the junction of the respira-
tory tube with the gullet of the adult lie two flaps or
vela.
The rest of the gut is straight and simple, with a single-
lobed liver, but with only a hint of a pancreas. The gall-
bladder and bile-duct disappear-in the adult, and the whole
intestine is partially atrophied. There is a slight spiral
fold in the intestine.
REPRODUCTIVE SYSTEM 525
Respiratory, vascular, and excretory systems.—Seven
gill-pouches with plaited walls open directly to the exterior”
on each side, and communicate indirectly with the gullet.
Water enters the pouches partly v/a the mouth, partly
by the external apertures (spiracula), and the movements
of the branchial basket and of the tongue-piston aid greatly
in the process. In the larva there is an eighth most’
anterior pouch which does not open to the surface. It
corresponds to the spiracle of Elasmobranchs. With each
of the seven open pouches in the larva four thymus
rudiments are associated.
The vascular system is essentially the same as in the
hag. The red blood cells are biconcave, circular, nucleated
discs. ; ;
The segmental or pronephric ducts persist as ureters,
and are connected with lateral mesonephric tubules forming
a kidney more complicated than that of the hag. The
pronephros, which is functional in the larva, entirely dis-
appears. The ureters unite terminally in a urogenital sinus
(not present in the hag), into which there open two genital
pores from the body cavity. The sinus opens, like the
anus, into an integumentary cloacal chamber.
Reproductive system.—The sexes are separate, but ova
sometimes occur in the testes. The reproductive organ is
elongated, unpaired, and moored by a median dorsal
mesentery. There are no genital ducts. The ova and
spermatozoa are liberatéd into the body cavity, and pass
by two genital pores (true abdominal pores) into the uro-
genital sinus, and thence to the exterior. In the male
there is an ejaculatory structure, or so-called “penis.”
There are many more males than females.
Development of P. planeri.—The ripe ovum has a considerable
quantity of yolk, but segmentation is total though slightly unequal. A
blastosphere is succeeded by a gastrula. The blastopore persists as
the anus of the animal, and there is no neurenteric canal.
The formation of the central nervous system is peculiar, for the sides.
of the epiblastic infolding remain in contact instead of forming an open.
medullary canal.
In the head region, where the gut is not surrounded by yolk-cells,
the mesoblast is formed from hollow folds in ‘‘ enteroccelic” fashion ;
but in the trunk region the cushions of hypoblastic yolk-cells change
gradually into mesoblast, and acquirea coelom cavity in ‘‘ schizoccelic’”
526 CYCLOSTOMATA.
fashion. Thus the two main ways in which a body cavity arises—
(a) from ccelom pouches of the archenteron, (4) from a splitting of solid
mesoblast rudiments—are here combined. :
Metamorphosis of Lampreys.—The larve live wallowing in
the sand or mud of streams, and feed on minute animals. Those of 2.
planeri are so unlike the adults that they were once referred to a dis-
tinct genus Avmocetes, and though a Strasburg fisherman, Baldner, is
said to have discovered their true nature about two hundred years ago,
the fact was overlooked until August Miiller traced the metamorphosis
in 1856. Inthe small lampern the change to the adult state is some-
times postponed until the autumn of the fourth or fifth year, when it
completes itself rapidly. Less is known about the metamorphosis of
the other species.
In the Ammocetes, or larva before metamorphosis, the head is small,
the dorsal fin is continuous, the upper lip is semicircular, the lower lip
is small and separate, the mouth is toothless and not suctorial, the
brain is long and narrow, the eyes are half made and hidden beneath
the skin; the future gullet, as distinguished from the respiratory tube,
is not yet developed. -
Contrast between Hag and Lamprey
Hac (Myxine).
Lamprey (Petvomyzon).
Exclusively marine.
The fin is confined to the tail.
Numerous large glands in the com-
plex, slimy skin.
Mouth with barbules, no lips, few
teeth.
Skull without any roof. .
Skeletal system less developed than
in the lamprey. Only a hint of a
branchial basket.
Cerebrum and cerebellum rudiment-
ary.
Eyes hidden and rudimentary.
Ear with one semicircular canal.
Nasal sac opens posteriorly into the
mouth cavity.
Six pairs of gill-pouches, opening
directly into the gullet, less directly to
the exterior.
Longitudinal ridges in the intestine.
No urogenital sinus; one genital
pore.
Ova large and oval, with attaching
threads; meroblastic in Bdellostoma.
Development unknown in Myxine;
direct in Bdellostoma.
In rivers and seas.
Two unpaired dorsal fins.
Sensory structures in the complex,
slimy, pigmented skin.
No barbules (except in the larva),
but lips, and many teeth. .
Skull very imperfectly roofed.
Hints of vertebral arches.
Cartilaginous basket- work around
gill-pouches.
All the usual parts of the brain are
distinct.
Eyes hidden and retarded in the
larva, exposed and complete in adult.
Ear with two semicircular canals.
Nasal sac ends blindly.
Seven pairs of gill-pouches, opening
directly to the exterior, less directly
into the adult gullet.
A slight spiral fold in the intestine.
A urogenital sinus, and two genital
pores.
Ova small and
blastic.
Development with metamorphosis.
spherical; holo-
PALAZZOSPONDYLUS. 527
Lampreys are distributed in the rivers and seas of north and south
temperate regions. They are often used as food. Besides Petromyzon
there are several related genera, e.g. Mordacia and Geotria, from the
coasts of Chili and Australia, and /chthyomyzon, from the west coast
of N. America. Certain structures called ‘‘conodonts,” from very
ancient (Silurian) strata, have been interpreted as teeth of lampreys
or hags.
Palzospondylus gunni.— Under
this title Dr. Traquair has de-
scribed a very remarkable fossil
form from the Old Red Sandstone
of Caithness. He'speaks of it
as a “strange relic of early verte-
brate life.”
It is a dainty little creature,
somewhat tadpole-like at first
sight, usually under an inch in
length. The following characters
point strongly to its affinities with
Cyclostomata :—
(1) ‘The skull is apparently formed
of calcified cartilage, and devoid of
discrete ossifications.” An anterior part
is comparable:to the trabecular and
palatal region of a lamprey’s skull;
a posterior part is comparable to the
parachordal region and auditory cap-
sules.
(2) **There is a median opening or
ring, surrounded with cirri, and presum-
ably nasal, in the front of the head”
(Fig. 278, ‘n.)s
(3) ‘*There are neither jaws nor
limbs.”
(4) “The rays which support the
caudal fin expansion, apparently spring-
ing from the neural and hzemal arches,
are dichotomised (at least the neural
ones), as are the corresponding rods in
the lamprey.”
Just behind the head lie two small
oblong plates (Fig. 278, x.), closely
apposed to the commencement of the
vertebral column, one on each side.
The notochordal sheath is calcified in
the form of ring-shaped or hollow verte-
Fic.278.—Restored skeleton
of Palaospondylus gunni.
—After Traquair.
d.c., Cirti of dorsal margin 3 Zc.,
long lateral cirri; v.c., cirri of
ventral margin ; 2., nasal ring ;
“.p., anterior ‘trabeculo- -palatine
part of cranium; 4., anterior
depression or fenestra ; 3 C-, pos-
terior depression or fenestra ; 3
@., lobe divided off from anterior
part; #.@., posterior or para-
chordal part of cranium; .,
post-occipital plates,
528 HYPOSTOMATA.
bral centra with neural arches. . Towards the tail the arches are
produced into slender neural spines, opposite which are shorter
heemal ones. :
Class HyPOSTOMATA or OSTRACODERMI
Extinct forms without jaws, without a segmented axial skeleton in
the trunk, without any trace of girdles, with complex dermal armature,
Fic. 279.—Pterichthys miller?. Lateral view. —Restored by Traquair.
with a head shield; Silurian’and Devonian, e.g. Pteraspzs and Cepha-
laspis, both without paired limbs; and Pterichthys and Bothriolepi's,
with strange armoured paddles (probably not limbs in the ordinary
sense) fixed to the antero-lateral angles of the body-shield. Their
systematic position is very doubtful. They are the oldest known
Vertebrates, :
CHAPTER XXII
Ciass PISCES—FISHES
Sub-Class I. ELASMOBRANCHII :-— .
Order Plagiostomi (skates and sharks).
Order Holocéphali (Chimera and Callorhynchus).
Several extinct orders, e.g. Acanthodei.
Sub-Class II. TELEOSTOMI :—
Order Crossopterygii (Polypterus),
Order Chondrostei, ¢.g. sturgeon.
Order Holostei, ¢.g. bony pike.
Order Teleostei, the great majority of living fishes,
Sub-Class III. DipNor :—
Ceratodus, Protopterus, and Lepidosiren, and many extinct forms..
Fisues form the first markedly successful-class of Verte-
brates. For though the Tunicates are numerous, most of
them are degenerate; the level attained by the lancelets.
is represented by, at most, two or three closely related.
genera; and the Cyclostomes are also few in number.
In the possession of a vertebrate axis and central nervous.
system, in the general integration of their structure, and in.
their great fecundity, Fishes have an easy pre-eminence.
over their Invertebrate inferiors. With their typically
wedge-like bodies, supple muscular tails, fin-like limbs, and.
the like—they are well adapted to the medium in which
they live.
Their success may be read in the immense number of
individuals, species, and genera, not only now, but in the:
past; in the geological record which shows how the
cartilaginous Elasmobranchs have persisted strongly from.
Silurian ages, or how the mysterious decadence of the.
“Ganoids” has been followed by a yet richer predomin-
ance of the modern Bony Fishes; and, furthermore, im
34
530 PISCES—FISHES.
the plasticity with which many types appear to have
assumed particular specialisations, such as the lungs of
Dipnoi, which point forward to the epoch-making transition
from water to dry land.
GENERAL CHARACTERS
Fishes ave aquatic Vertebrates, breathing by gills,—vascular
outgrowths of the pharynx, bordering gill-clefts and supported
by gill-arches. In Dipnot a single or double outgrowth from
the gut—the air- or swim- bladder—functions as a lung, air
being inspired at the surface of the water. In most Teleo-
stomes the same structure ts present, but though occasionally of
some use in respiration, ts typically hydrostatic.
Two pairs of non-digitate limbs, i.e. in the form of fins,
ave usually present, and there are also unpaired median fins,
supported by dermal fin-rays (dermotrichia). There are two
chief types of paired fin, but no hint of the pentadactyl type of
higher Vertebrates. In Dipnoi, and in some extinct forms,
the fin has a median segmented axis, which (e.g. Ceratodus)
bears on each side a series of radial pieces. In other fishes
the radials diverge outwards on one side from several basal
pieces, and there ts no median axis.
The skin usually bears numerous scales, mainly or wholly
due to the dermis, but covered by a layer of epidermis, which
may produce enamel. They vary greatly in form and texture,
are suppressed in electric fishes, and rudimentary in eels and
some other forms. Numerous glandular cells occur in the
skin, but these are not compacted into multicellular glands,
except in Dipnot and a few poisonous fishes. The skin also
bears sensory structures, usually aggregated on the head, and
arranged in one or more “lateral lines” along the trunk.
There are no muscular elements in the dermis. The muscle
segments or myotomes persist as such in adult life.
In many the gut ends in a cloaca, in others a distinct anus
lies in front of the genital and urinary aperture, or apertures.
The nostrils are paired, and do not communicate with the
mouth by posterior nares, they are exclusively olfactory
organs. There is no tympanic cavity or tympanum, or ear-
ossicles.
The heart is two-chambered, and contains only venous
THE SKATE. 531
blood, except in the Dipnot, where it shows hints of becoming
three-chambered, and receives pure blood from the lungs as
well as impure blood from the body. Apart from the Dipnot,
the heart has a single auricle receiving impure blood from the
body, and a ventricle which drives this through a ventral
aorta to the gills, whence the purified blood flows to the head
and by a dorsal aorta to the body. In addition to the two
essential chambers of the heart, there is a sinus venosus,
which serves as a porch to the ‘auricle, and there is often a
muscular conus arteriosus in front of the ventricle, or a
Sulbus arteriosus at the base of the ventral aorta. Except
in Dipnot, there is no vein which resembles what is known
in all higher Vertebrates as the inferior vena cava, i.e. a
single vessel receiving hepatic veins from the liver, renal veins
Srom the kidneys, and others from the posterior region. Its
place is taken by paired posterior cardinals. The kidney ts
usually a persistent mesonephros.
There ts no distinct indication of an outgrowth from the
find end of the gut comparable to that which forms the
bladder of Amphibians or the allantois of higher Vertebrates.
Most fishes lay eggs which are fertilised in the water.
Compared with Cyclostomes, the true fishes show a distinct
advance. Thus we may note—the jaws formed from the
first visceral arch, the limbs, the dermal exoskeleton of scales,
the frequent occurrence of bone, the true teeth, the paired
nostrils, the three semicircular canals, the renal-portal
system, the spleen, and the genital ducts.
First type of Fisues. The Skate (#aj2)—one of the
Elasmobranchii
The smooth skate (2. datis), the thornback (2. clavata),
and the ray (2. maculata), and other species, are common
off British coasts. They are very voracious fishes, and
live on the bottom at considerable depths.
External characters.—The body is flattened from above
downwards or dorso-ventrally, unlike that of the bony flat-
fishes, such as plaice and flounder, which are flattened
from side to side. The skate rests on its ventral surface,
the flounder on its side. The triangular snout, the broad
pectoral fins, the long tail with small unpaired fins, are
532 PISCES—FISHES.
obvious features. On the dorsal surface the skin is pig-
mented and studded with placoid scales; on the top of
the skull there are two unroofed areas or fontanelles ;
numerous jointed radials support the pectoral fins. Behind
the lidless eyes are the spiracles—the first of the obvious:
gill-slits, opening dorsally, containing a rudimentary gill,
and communicating posteriorly with the mouth cavity.
On the ventral surface are seen the sensory mucus canals,
the transverse mouth, and the nostrils incompletely separated
from it, as if in double harelip, the five pairs of gill aper-
tures, the cloacal aperture and two abdominal pores beside
it. Pectoral and pelvic girdles support the fore- and hind-
fins. In the male the hind-fins are in part modified into
complex copulatory “ claspers.”
The skin.—On the dorsal pigmented surface, embedded
in the dermis, there are many “skin-teeth,” or ‘dermal
denticles,” or “placoid scales.” Each is based in bone,
cored with dentine or ivory, tipped with enamel. The
enamel is mainly, if not wholly, due to the ectoderm
(epidermis), the rest to the mesoderm (dermis) ; the whole
arises as a skin papilla. The enamel is practically in-
organic, the cells having been replaced by lime-salts ;
dentine has 34 per cent. of organic matter (apart from
water); the bone is more obvious cellular tissue. On the
ventral unpigmented or less pigmented surface there are
numerous mucus canals or jelly tubes, sensory in function.
Some are also present on the dorsal aspect, especially
about the head. Most of the slime exudes from glandular
goblet cells in the epidermis.
Muscular system.~—In the posterior part of the body
and in the tail, the segmental arrangement of the muscles
may be recognised. The large muscles which work the
jaws are noteworthy. Professor Cossar Ewart has described
a small electric organ in the tail region of Raja datis and
R. clavata, apparently too small to be of any use, probably
incipient rather than vestigial.
Electric organs are best developed in two Teleostean fishes—a S.
American eel (Gymnotus) and an African Siluroid (Ala/apterurus), and
in the Elasmobranch Zorpedo, In Gymnotus they lie ventrally along
the tail, in Malapterurus they extend as a sheath around the body,
and in Torpedo they lie on each side of the head, between the gills and
the anterior part of the pectoral fin. In other cases where they are
THE SKELETON. 533
slightly developed (certain Elasmobranchs and Teleosteans), they lie
in the tail. Separated from one another by connective tissue partitions
are numerous ‘‘electric plates,” which consist of strangely modified
muscle substance and numerous nerve-endings. The electric discharge
is very distinct in the three forms noted above, and is controlled in
some measure at least by the animal.
The skeleton.—The skeleton is for the most part cartil-
-aginous, but here and there ossification has begun, as a
crust over many parts, more deeply in the vertebre, teeth,
and scales.
The vertebral column consists of an anterior plate not divided into
vertebrze, and of a posterior series of distinct vertebrae. Each of these
has a biconcave or amphiccelous centrum. From each side of the
centrum a transverse process projects outwards, and bears a minute
hint of a rib. From the dorsal surface of each centrum two neural
processes arise. Between each two vertebrae there is at each side
a broad interneural plate, which not only fills what would be a gap
between the neural processes and the slightly developed neural spine,
but also links the vertebrz together. In the caudal vertebree, what
seem to be the transverse processes are directed downwards to fofm
a heemal arch enclosing the caudal artery and vein. In the lozenge-
shaped spaces between the vertebrz lie gelatinous remains of the
notochord.
In Selachians and Dipnoi amoeboid cartilage cells from the arcualia
(paired nodules of cartilage in the mesenchyme or embryonic connec-
tive tissue outside the sheath of the notochord, which form neural and
hheemal arches) migrate into the sheath of the notochord and convert
it into a cylinder of cartilage (segmented into centra in Selachians),
This is called a chordacentrous vertebral column. In Teleostomes and
higher Vertebrates, the expanded bases of the arcualia fuse to form
cartilaginous (eventually bony) centra, outside the sheath of the noto-
chord. This is called an arczcentrous vertebral column.
The skull is a cartilaginous case, with a spacious cavity
for the brain, a large posterior aperture or foramen magnum
through which the spinal cord passes, two condyles working
on the end of the vertebral plate, a large ear capsule on
each side posteriorly, a similar nasal capsule on each side
anteriorly, a long rostrum in front, two fontanelles on the
roof. Compared with the skull of a cod or of a higher
Vertebrate, that of a skate is simple; it is not ossified, nor
divided into distinct regions, nor has it anything corre-
sponding to the investing membrane bones, which in higher
animals are added to the original foundations of the skull,
nor do the visceral arches in the skate take part in forming
the skull, which arises, as usual, from parachordals, trab-
ecule, sense capsules, etc.
534 PISCES——FISHES.
The visceral arches are primitively supports for the
N
\\
an
Sz, i)
wy
Za
2D
ln. A. br J. V4
Fic. 280.—Under surface of skull and arches of
skate.—After W. K. Parker.
41, First labial cartilage; 2., rostrum; ¢., trabecular
region ; #.c., nasal capsule; @.0., antorbital cartilage ;
2tg-s palato-pterygo-quadrate ; M.c., Meckel’s car-
tilage; .%., hyo-mandibular; 4.47.1-5, hypobran-
chials ; ¢.47.5, fifth ceratobranchial ; ¢.4,., cerato-hyal ;
4.2-4, labial cartilages.
wall of the anterior part of the food canal, but the first two
of them are much modified in connection with the jaws.
THE SKELETON. 535
The upper jaw of the skate is a strong transverse bar,
formed from the union of two _palato-pterygo-quadrate
cartilages. The lower jaw is a similar bar formed from the
union of two Meckel’s cartilages.
From the ear capsule to the articulation of upper and
lower jaw there extends on each side a club-shaped cartilage,
which connects the jaws with the skull, known as the hyo-
mandibular or suspensorium. It is the upper half of the
second arch. Attached to it is a slender four-jointed rod—
the lower half of the hyoid arch.
Fic, 281. —Side view of skate’s skull.
—After W. K. Parker.
éx., First labial cartilage; 2.c., nasal capsule; @.o., antorbital;
p.pt.g., palato-pterygo-quadrate; .c., Meckel’s cartilage;
h.m., hyo-mandibular; ¢.4., epi-hyal; ¢.4., cerato-hyal; 4./.,
hypo-hyal ; %.67.1-5, ypobranchials ; ¢.d., ceratobranchial ;
e.6r., epibranchial ; 4.47.1., first prebranchial ; 2.4., inter-hyal ;
m.pt., meta-pterygoid ; 2, 5, 7, foramina of exit of the corre-
sponding nerves,
Then follow five -branchial arches, each primarily four-
jointed, forming the framework of the gill-bearing region.
Of less importance are the labial cartilages about each
nasal capsule, an antorbital cartilage uniting the nasal
capsule with the end of the pectoral fin, and a spiracular
or meta-pterygoid cartilage supporting the rudimentary gill
in the spiracle.
The pectoral girdle forms an almost complete hoop of
536 PISCES——FISHES.
eeeaagg Ge”
\<3
Lp
Fic. 282,—Skeleton of skate. —From a preparation.
In the skull notice the anterior rostrum, the nasal capsules (t.0.)
with the antorbital cartilages projecting laterally; the palato-
pterygo-quadrate cartilage (f.g.) or upper jaw; Meckel’s
THE SKELETON. 537
cartilage attached dorsally to the crest of the vertebral plate.
The ventral region is distinguished as the coracoid, and is
separated from the dorsal or scapular region by three facets,
to which the three basal pieces of the pectoral fin are fixed.
A separated portion of the girdle forms the supra-scapula,
which connects the scapula with the crest of the vertebral
plate.
Of the three basal pieces of the fin, the anterior or
propterygium and the posterior or metapterygium are
jarge, the median or mesopterygium is small. All bear
jointed radials, which are parts of the endoskeleton; a
few radials articulate directly with the shoulder-girdle (see
Fig. 282). The true fin-rays, comparable to the dermal
rays in the fins of Bony Fishes, are represented by “horny”
(ox, more strictly, elastoidin) fibres. These are intercellular
products of mesoderm (mesenchyme) cells.
-The pelvic girdle is simpler than the pectoral, and is not
fixed to the vertebral column. Its dorsal region is pro-
longed into an iliac process, while anteriorly a prepubic
process projects from the ventral (pubic) bar. The girdle
bears two articulating facets, to the posterior of which the
strong basal piece or metapterygium of the hind-limb is
attached. From this, and from the anterior facet of the
girdle, the jointed radials proceed. The claspers of the
males are closely connected with the posterior part of the
hind-limb, and ‘have a complex cartilaginous skeleton and
an associated gland.
The brain.—The brain (see p. 483) has the following
parts :—
i. The fused cerebral hemispheres or prosencephalon, with a
nervous roof, and without ventricles. :
cartilage (JZ.) forming the lower jaw; and the hyo-mandibular
(4.m.) which suspends the jaws to the skull. A little farther
back are seen the five branchial arches and the anterior hyoid
arch ; 4.47., the fifth hypobranchial ; v.f/., the vertebral plate.
At the right is seen the skeleton of the paired ‘fins, at the left
the surface of the skin with the sensory tubes (s.4.); sc., the
scapular region of the shoulder-girdle, with the scapular
fontanelle ; c., the coracoid region; 2.f2., the ‘anterior basal
cartilage or pro-pterygium ; 7./., the meso-pterygium ; 77.f4.,
the meta-pterygium—all three bear jointed radials, while a few,
as shown here, articulate directly with the shoulder-girdle ;
pu., pubic bar of pelvic girdle ; s¢., stomach; s.v., spiral valve
of intestine.
538 PISCES—FfISHES.
c
t Py
BRANCHIALS
Fic, 283.—Dissection of nerves of skate.
CH., Cerebral hemispheres; O.TH., optic thalami; OL., optic
lobes ; M., medulla; 4V., posterior part of cerebellum, covering
CRANIAL NERVES. 539
2. The thalamencephalon or region of the optic thalami, with a
thread-like pineal body above, infundibulum and pituitary
body below, thinly roofed third ventricle within.
3. The mesencephalon or mid-brain, with the optic lobes above,
the crura cerebri below, the iter passing between.
4. The cerebellum, with an anterior and a posterior lobe, both
marked by ridges and grooves.
5. The medulla oblongata, with thin vascular roof, with dorso-
lateral extensions called “‘ restiform bodies.”
The region beneath the thalamencephalon bears—(a) two ovoid inferior
lobes ; (6) the infundibulum, which carries the pituitary body; and (c)a
thin-walled three-lobed saccus vasculosus, situated between the pituitary
body and the inferior lobes.
Cranial nerves.!—Owing to the flat form of the skate
and its frequently large size, the dissection of the cranial
nerves is perhaps easier than in any other Vertebrate.
Expecting practical verification, we shall describe their
distribution in some detail, following in regard to certain
points the investigations of Professor Cossar Ewart.
I. The o/factory, rising from the olfactory lobes of the
cerebral hemispheres, extend to the nostrils, and
there expand in olfactory bulbs, which give off
small nerves to the nostrils.
II. The oftic, leaving the region of the optic thalami,
cross in an optic chiasma, and extend to the
retina of the eye.
III. The oculomotor or ciliary, arising from the crura
cerebri, near the mid-ventral line, supply four of
the six muscles of the eye. There is a ciliary
ganglion in connection with III and also with
the ganglion of the ophthalmicus profundus.
1] have to acknowledge indebtedness to Dr. Beard for his kindness
in helping me to state the distribution of these nerves.
fourth ventricle; OB., olfactory bulb ; OC., olfactory capsule ;
SO.,.superior oblique muscle ; E., eye; SR., superior rectus;
ER., external rectus ; SO.VII., superficial ophthalmic branch
of VII. ; SO.V., superficial ophthalmic branch of V. ; OP., oph-
thalmicus profundus; A.C., auditory capsule; B.Pl., brachial
plexus ; R.F., recurrent facial ; C.T., chorda tympani; F.P.,
facial proper ; Hy., hyoidean ; Hyomn., hyomandibular ; E.M.,
external mandibular ; M.M., mandibular muscle ; Sp., spiracle ;
P.sp., prespiracular ; Pl., palatine ; O.B., outer buccal ; Mn.,
mandibular; Mx., maxillary; 1.B., inner buccal; L., lateral
branch of X.; Py., pyloric branch; C., cardiac branch.
7540 PISCES— FISHES,
IV. The pathetic or trochlear are small nerves emerging
dorsally from between the mid- and hind- brain,
and supplying the superior oblique muscles of
the eye.
VY. The trigeminal, or nerve of the “mouth-cleft,”
arising from the medulla oblongata (as do all
that follow), has a (Gasserian) ganglion on its
root, and three main branches—the sensory
maxillary, which unites with the inner buccal
of VII.; the motor mandibular, which inner-
vates the muscles of the jaws; and the sensory
superficial ophthalmic (or orbitonasal), which
runs over the eye to the snout, closely united
(inside the same sheath) with a similar branch
of VII. ;
Parallel to these superficial ophthalmics, internal
to and above the inner buccal of VII., there is
a ganglionated ophthalmicus profundus, which
sends branches to the eyeball, snout, etc.
VI. The abducens, a slender nerve, arising near the
mid-ventral line, adjacent to V. and VIII., and
hidden beneath the former, supplies the external
rectus muscle of the eye.
WII. The facial, the nerve of the spiracular cleft,
supplies all the five groups of ampullz on the
head, and has numerous branches.
1. The ophthalmicus superficialis runs over and past
the eye, in intimate association with the similar
branch of V., and supplies ampulla on the
snout.
2. The inner buccal runs under the eye, through the
nasal capsule, to inner buccal ampulle. The -
outer buccal runs under the eye, external to the
olfactory capsule, to outer buccal ampullee.
3. The large hyomandibular runs directly outwards
behind the spiracle to hyoid ampulle. It gives
off minor hyoidean nerves.
4. The external mandibular runs behind and outside
of the mandibular muscle to mandibular ampulle,
and is a branch of the hyo-mandibular.
5. The palatine descends in front of the spiracle to the
roof of the mouth. Close beside it there is a
prespiracular.
CRANIAL NERVES. 54D
6. The ‘‘ facial proper,”’ apparently os from 3,
supplies the muscles of the hyoid arch
7. The ‘‘chorda tympani,” apparently arising from 3,.
runs under the spiracle to the inner side of the-
jaw.
With the loss of the sensory ampullee, the seventh:
nerve of higher Vertebrates becomes restricted to:
the last three branches (5, 6, and 7).
A recurrent branch of the facial also runs externah
to the auditory capsule to IX., and is equivalent.
to Jacobson’s anastomosis in higher forms.
VIII. The auditory, arising just behind VIL, is the nerve:
of the ear.
IX. The glossopharyngeal, the most typical of all, is the
nerve of the first functional gill-cleft. Its root
passes through the floor of the auditory capsule,
and bears a ganglion above the cleft. Its.
branches, as named by Beard, are :—
I. Peal to the muscles of the first branchial!
arch ;
2. Pree-branchial, arches over the cleft and runs along.
its front wall;
3. Intestinal or visceral, to the pharynx ;
4. Supra-branchial or dorsal, to a few sense organs om
the mid-dorsal line of the head.
X. The vagus, apparently made up of several cranial
nerves, has numerous roots, and divides into six
main ganglionated portions, which supply the
four posterior clefts and arches, the posterior
jelly-tubes, and the heart and stomach. It thus.
consists of :—~
1. Ganglionated roots with nerves to the clefts and’
arches (2 to 5 inclusive), with post-branchial,
pree-branchial, and pharyngeal branches as in IX.
2. A ganglionated root, arising in front of all the
others, from which arises the lateral branch
innervating all the posterior sensory tubes.
3. From the fourth branchial branch arises the gang-
lionated intestinal which innervates the heart and
the stomach.
The spinal cord lies in the ee neural archwad!
above the vertebral column, is divided by deep dorsal any
ventral fissures, and gives off numerous spinal nerves,.
formed as usual from the union of dorsal (sensory) and
542 PISCES—-FISHES.
ventral (motor) roots. The first sixteen or eighteen nerves
form the brachial plexus, which supplies the pectoral fin.
The sympathetic system consists of a longitudinal gang-
lionated cord along each side of the vertebral column.
of ch ch Ino
Fic. 284.—Side view of chief cranial nerves of Elasmobranchs,
—-Slightly modified from Cossar Ewart.
olf., Over olfactory nerve; ch., over cerebral hemispheres ; cd., over
cerebellum ; 7z.0., over medulla oblongata; 7., mouth; mx.,
maxillary branch of 5; 2.5, mandibular branch of 5 3 2.7,
mandibular branch of seventh nerve ; @.1~5, groups of ampulla ;
0.8.5, superficial ophthalmic of § ; 0.f., ophthalmicus profundus ;
0.8.7, superficial ophthalmic of 7; JV., nostril; 3, oculomotor ;
e.g., Ciliary ganglion; 5, trigeminal; 7.4., inner buccal; 0.d., +
outer buccal ; 74., buccal of 7; 2., palata: of 7; sf., spiracle ;
ch., chorda tympani; 7.47., hyomandibular of 7; 8, auditory;
£., ear; 9, glossopharyngeal ; 10, roots of vagus ; ¢.10, lateral
nerve of vagus ; 7.10, intestinal nerve of vagus ; 1’-s’, gill-clefts.
Sense organs.—
(a) The eyes (see p. 495). The iris has a fringed upper margin.
(6) The ears (see p. 493). The vestibule is connected with the sur-
face by a delicate canal—the aqueductus vestibuli—a remnant
of the original invagination. A small part of the wall of the
auditory capsule is covered only by the skin, forming a kind of
tympanum. Within the vestibule are calcareous otolithic par-
ticles surrounded by a jelly.
(c) The nasal sacs are cup-like cavities with plaited walls.
(d@) The sensory tubes are best seen on the ventral surface, where
they lie just under the skin. At their internal ends lie ampullee,
containing sensory cells. At their outer ends there are pores.
It is probable that they are organs partly of touch, and partly
of ‘‘ chemical sense.”
Alimentary system.—The mouth is a transverse aperture ;
the teeth borne by the jaws are numerous, and those worn
away in front are replaced by fresh ones from behind ; naso-
ALIMENTARY SYSTEM. 543
buccal grooves connect the nostrils with the corners of the
mouth ; the spiracles, which open dorsally behind the eyes,
communicate with the buccal cavity; from the gullet five
gill-clefts open ventrally on each side. The stomach, lying
to the left, is bent upon itself; the large brownish liver is
trilobed, and has an associated gall-bJadder, from which the ©
bile-duct extends to-the duodenum—the part of the gut
immediately succeeding the stomach ; the whitish pancreas
lies at the end of the duodenal loop, and its duct opens
opposite the bile-duct. The intestine is exceedingly short,
but it contains an internal spiral fold—which greatly
increases the absorptive surface. ;
The development of this spiral intestine is of
general interest. The well-nourished gut grows
quickly, but its increase in calibre is hindered by
the peritoneal mesodermic sheath, and the growth
is expressed in an internal invagination or fold.
But as the growth continues in length as well as
in calibre, and as the gut is fixed at both ends,
twisting or coiling or both must result. In
Mammals, for instance, the result is a coiled in-
testine. But in Elasmobranch fishes the coiling
or twisting takes place w2thzz the peritoneal sheath,
not along with it. In the case of the skate and
some other Elasmobranchs, close twisting occurs,
and the so-called spiral valve is mainly due to the
fusion of the walls of adjacent twists.
A small “rectal gland” of unknown ces hh
significance arises as a vascular diverti- —After T. J.
culum from the end of the gut. The end Parker.
of the gullet and the anterior portion of the
stomach and the rectum are supported by folds of peri-
toneum,—the membrane which lines the body cavity; the
rest of the gut lies freely. Rectum, ureters, and genital ducts
all communicate with the. exterior through the common
terminal chamber or cloaca. An abdominal pore opens on
each side of the cloacal aperture, and puts the body cavity
in direct communication with the exterior. Excepting
mouth cavity and cloaca, the gut is lined by endoderm.
Respiratory system.—The first apparent gill-clefts—the
spiracles—open dorsally behind the eyes. Each contains
a rudimentary gill on the anterior wall, supported by a
544 PISCES—FISHES.
spiracular cartilage. Through the spiracles water may enter
or leave the mouth.
There are other five pairs of gill-clefts, separated by com-
plete partitions (Elasmobranch), and with ventral apertures.
The first is bounded anteriorly by the hyoid arch, posteriorly
by the first branchial arch. The hyoid bears branchial
filaments on its posterior surface ; the first four branchials
bear gill filaments on both surfaces; the fifth branchial
bears none. Each set of branchial filaments is called a half-
gill; and as the first four branchial arches bear a half-gill on
Fic. 286.—Upper part of the dorsal aorta in the skate.
—After Monro.
@.a., Dorsal aorta; ¢., coeliac artery; #., superior mesenteric;
s.cl., subclavian ; ¢.d., efferent branchial vessels, three formed
from the union of nine; w., vertebral ; c., carotid.
each side, and the hyoid arch a half on its posterior surface,
there are four and a half gills in all. There is no operculum
or gill cover.
Circulatory system.—The impure blood from the body
enters the heart by a bow-shaped sinus venosus, opening
into a large thin-walled auricle. Thence through a bivalved
aperture the blood passes into the smaller muscular ventricle,
and from this it is driven through a contractile conus
arteriosus, with three longitudinal rows of five valves, into
the ventral aorta.
CIRCULATORY SYSTEM. 545
The ventral aorta gives off a pair of posterior innominate arteries,
which take blood to the three posterior gills, and a pair of anterior
innominate arteries, which supply the anterior gill and the hyoid half-
gill on each side.
The purified blood passes from each half-gill by an efferent branchiah
artery. To begin with, there are nine of these on each side, but by
union they are reduced first to four and then to three efferent trunks,
which combine to form the dorsal aorta.
From the efferent branchial of the hyoid arch a carotid arises, which:
divides into internal and external branches supplying the brain: and
head. The two internal carotids unite, and pass through a small hole:
‘i ae
Fic. 287.—-Heart and adjacent vessels of skate.—In part
after Monro.
v., Ventricle; ¢.@., conus arteriosus ; #.t., posterior innominate ;
Ua, ventral aorta ; @.7., anterior innominate ; 7’4., thyroid;
M., ‘mouth ; a, auricles S.%., sinus venosus 3 S.C, precaval
sinus or sinus of Cuvier ; "he Sey hepatic. sinus ; j., jugular; dn,
brachials ; cdi, cardinal ; efg., epigastric.
on the ventral surface of the skull. Just after the first and second mair
efferent branches have united, a vertebral is given off, which passes.
through a hole in the vertebral plate to the spinal cord and brain.
The dorsal aorta gives off—(1) a subclavian to each pectoral fin ; (2) a.
coeliac to the stomach, duodenum, and liver; (3) a superior mesen-
teric to the intestine, pancreas, and spleen; (4) spermatic arteries to
the reproductive organs; (§) an inferior mesenteric to the rectum ;
(6) renal arteries to the kidneys; (7) arteries to the pelvic fins. It
ends in the caudal artery.
At each end of the bow-shaped sinus venosus there is a precaval
sinus. This receives venous blood as follows :—(a) from the head by
35
546 PISCES——FISHES.,
a jugular vein ; (4) from the liver by a hepatic sinus, which runs from
one precaval sinus to the other like the string of the bow; (c) froma
large posterior cardinal sinus (between the reproductive organs) by
a cardinal vein on each side; (@) from the hind-fin by an epigastric,
with which brachials from the fore-limb unite anteriorly. The great
cardinal sinus receives blood from the hind-limbs, the kidneys, and
other posterior parts.
Blood asses zo the liver (a) from the cceliac artery, and (4) by
portal veins from the intestine (the hepatic portal system) ; blood /eaves
the liver by hepatic veins which enter
the hepatic sinus.
Blood fasses znto the kidneys (a)
from the renal arteries, and (4) by
renal portal veins from the caudal,
pelvic, and lumbar regions (the renal
portal system); blood eaves the
kidneys by posterior cardinal veins,
which enter the cardinal sinus.
Into the precaval sinus there also
opens the lymphatic trunk.
The heart lies in a_ pericardial
cavity, which is connected with the
abdominal cavity by two fine canals,
and is an anterior part of the ccelom.
The blood contains, as usual, red
blood corpuscles and leucocytes.
The dark red spleen lies in the
curve of the stomach. The red
thyroid gland lies just in front of
the anterior end of the ventral aorta.
The thymus is represented by a
whitish body dorsal to each of the
first four gill-clefts. | Each begins as
a patch of endoderm, and this is
invaded by migratory mesenchyme
cells which multiply as lymphocytes.
Fic. 288.—Urogenital organs
of male skate. E :
is Teter why, wolaiagiiiaY wt, xcretory and reproductive
vas. deferens + °K idney § aoe systems.—The dark red kid-
ee tec ainey CA, eloace, neys lie far back on each
side of the vertebral column.
They are developed from the hind part of the mesonephros.
Several tubes from each kidney combine to form a ureter.
The two ureters of the male open into the urogenital
sinus, whence the waste products pass out by the cloaca;
in the female they open into little bladders,—the dilated
ends of the Wolffian ducts,—and thence by a common
aperture into the cloaca. _
EXCRETORY AND REPRODUCTIVE SYSTEMS. 547
The segmental duct of each side divides into Wolffian
and Miillerian ducts. The Wolffian duct becomes in the
male the vas deferens, in the female it is an unimportant
Wolffian duct; the Miillerian duct becomes in the female
the oviduct, in the male it is a mere rudiment.
The muscles and other organs of Elasmobranchs retain
considerable quantities of nitrogenous waste products.
There can be no doubt
that the body cavity helps
in excretion, and gets rid
of waste through the two
abdominal pores. In some
Elasmobranchs these are
replaced by openings (neph-
rostomes) into the kidney.
Occasionally there are both
nephrostomes and abdomi-
nal pores.
pon ae
The male organs or
testes lie on each side
of the cardinal sinus,
moored by a fold of,
peritoneum. Sperma-
tozoa pass from the
testis by vasa effer-
entia into a tube sur-
rounded anteriorly by
epididymis. The tube
of the epididymis is
continued into the F!G. 289.—Urogenital organs. of female
vas deferens, which bee nee A nee page
a . : @g., Aperture of united oviducts; W.D, olffan
is dilated posteriorly “uct; ov, ovary; O.D.G., oviducal gland ;
info @ seminal vee ai. Sivan deter ied
icle and an adjacent kidney (arrow from base of oviduct into cloaca).
sperm - sac. Finally, ;
the two vasa deferentia open into the urogenital sinus,
whence the spermatozoa pass into the cloaca. Thence, in
copulation, they pass into the complex “claspers”.of the
male, which are said to be inserted into. the cloaca of the
female..
The female organs or ovaries lie on each side of the car-
$99 7» 2 2.2.
548 PISCES—FISHES.
dinal sinus, moored by a fold of peritoneum. In young
skates they are like the young testes, but in the adults they
are covered with large Graafian follicles, each containing an
ovum. The ripe ova burst into the body cavity, and enter
the single aperture of the oviducts, which are united an-
teriorly just behind the heart. About the middle of each
oviduct there is a large oviducal gland, which secretes the
horny “purse”; the elastic lower portions open into the
cloaca.
_ Development.— The ripe
ovum which bursts from the
ovary is a large sphere,
mostly of yolk, with the for-
mative protoplasm concen-
trated at one pole.
The formation of polar
bodies (maturation) takes
place at an early stage.
Fertilisation occurs in the
upper part of the oviduct.
Some observers have de-
scribed the occurrence of
polyspermy.
As the ovum descends farther, it
Fic.290.—Elasmobranch develop-
ment.—After Balfour.
Uppermost figure shows blastoderm at
an early stage. £f., Epiblast; sg.c.,
segmentation cavity; ., yolk-nuclei.
Middle figure shows the invagination
which forms the gut. .x., Blastopore ;
&-, archenteron. Mesoderm dark.
Lowest figure, a longitudinal section at
a later stage. Zf., epiblast; ~.c.,
neural canal; #e.c., neurenteric canal ;
is surrounded first by albuminous
material, and then by the four-
cornered ‘‘mermaid’s purse” se-
creted by the walls of the oviducal
gland. This purse is composed of
&-y gut; #., notochord. Mesoderm
keratin—a common skeletal sub-
ark.
stance which occurs for instance
in hair and nails. Its corners
are produced into long elastic tendrils, which may twine round’
seaweed, and thus moor the egg. Rocked by the waves, the
embryo develops, and the young skate leaves the purse at one end.
Development is very slow, and takes perhaps the greater part of a year.
The egg-case of some sharks, e.g. the Port Jackson shark (Cestracton
philippz), has elastic spiral fringes, and is found securely wedged among
the rocks ; that of a neighbour species (C. ga/eatzs) has reduced spirals.
ending in a couple of tendrils, which may be go in. in length, and
serve very effectively to entangle the egg among seaweed.
The segmentation is meroblastic, being confined to the
disc of formative protoplasm. From the edge of the
DEVELOPMENT, 549
blastoderm, or segmented area, some nuclei (so-called
“merocytes”) are formed in the outer part of the subjacent
yolk (Fig. 290, ~.). It seems most probable that these
are hypoblast elements which assist in the preparation of
the yolk for absorption, and eventually degenerate in the
empty external yolk-sac.
At the close of segmen-
tation the blastoderm is a
lens-shaped disc with two:
strata of cells. It is thicker
at one end—where the em-
bryo begins to be formed.
‘Towards the other end, be-
tween the blastoderm and
the yolk, lies a segmentation
cavity (Fig. 290, 5g.c.).
At, the embryonic end
the outer layer or epiblast
undergoes a slight invagina-
tion (Fig. 290, x.), beginning
to form the roof of the
future gut (g.); in other
words, establishing the hypo-
blast. This inflected arc of
the blastoderm corresponds
to the blastopore or mouth een F ire
of the _gast tula, which is Gua et hich
much disguised by the pres- has been cut open to show con-
ence of a large quantity of tents.
yolk. As the invagination eg. ‘‘External” gills; dA, dorsal fin
proceeds, the segmentation {14s 2, yelksacs at, Salk of yolk
cavity is obliterated. The case By mene Ot niet it is ed 2
floor of the gut is formed by ee ee ee
infolding of the lateral walls.
Along the mid-dorsal line of the epiblast a medullary
groove appears—the beginning of the central nervous
system. Its sides afterwards arch towards one another, and
meet to form a medullary canal (Fig. 290, #.c.). A posterior
communication between this dorsal nervous tube above and
the ventral alimentary tube persists for some time as the
neurenteric canal (Fig. 290, 7e.c.).
550 PISCES—FISHES.
The mesoblast arises as two lateral plates, one on each
side of the medullary groove. The plates seem to arise as
a pair of solid outgrowths from the wall of the gut. They
are afterwards divided into segments. Between the meso-
blast plates, along the mid-dorsal line of the gut, the
notochord is established (Fig. 290, 7.).
Besides the internal establishment and differentiation of
layers, there are two important processes,—(a) the growth
of the blastoderm around the yolk, (4) the folding off of
the embryo from the yolk. The result of the two processes
is that the yolk is enclosed in a yolk-sac, with which the
embryo is finally connected only by a thin stalk—the
umbilical cord.
The history of the yolk is briefly as follows:—It is accumulated
by the ovum from neighbouring cells, and from the vascular fluid ; it
is partly prepared for absorption by the merocytes or yolk-nuclei ; it is
at first absorbed by the blood vessels of the yolk-sac ; at a later stage,
absorption by blood vessels becomes less and less important, and the
yolk passes inside the embryo and into the gut, where it is digested.
Then the yolk-sac, empty of all but merocytes, degenerates, shrivels,
and disappears.
Second type of Fisoes. The Haddock (Gadus eglefinus)
—A type of Teleosteans with closed swim-bladder
(Physoclysti).
Form and external features.—The elongated wedge-like
form is well adapted for rapid swimming. The lower jaw
bears a short barbule,—long in the cod (G. morrhua),
absent in the adult whiting (G. merlangus). The nostrils,
situated near the end of the snout, have double apertures.
The eyes are lidless, but covered with transparent skin.
Over the gill chamber and the four gills lies the operculum,
supported by several bones. Distinct from one another,
but closely adjacent, are the anal, genital, and urinary
apertures,—named in order from before backwards. Along
the sides of the body runs the dark lateral line containing
sensory cells. There are three dorsal and two anal fins,
and an apparently symmetrical tail fin.
Skin.—The small scales are developed in the dermis,
and consist of flexible structureless bone (vitrodentin).
THE HADDOCK. 55?
Their free margin is even, a characteristic to which
the term cycloid is applied, in contrast to ctenoid, which
n.@., Nasal apertures (double on each side); @. 7.1, df.2, 2,/.3, dorsal
unpaired fins ; c.f, the caudal fin of the homocercal tail.
&., Barbule; 9f., operculum covering the four gills; 47.7., con-
tinuation of the gill-cover forming the branchiostegal mem-
brane ; Av. /, pelvic fin(=hind-limb)—note its jugular position
in front of J.f£, the pectoral fin (=fore-limb).
a., Anus; g, genital aperture; #., urinary aperture; @./.1, @,/.2,
unpaired anal fins. :
describes those scales which have a notched or comb-like
Fic. 293.—External characters of a Teleostéan—
a carp (Cyprinus carpio).—After Leunis.
R., Dorsal unpaired fin; S., homocercal caudal fin; A., anal fin; ~
B., B., pectoral and pelvic paired fins. Note also the lateral
line and barbule.
free margin. Over the scales extends a delicate partially
pigmented epidermis.
Appendages.—The pectoral fins are attached to the
552 PISCES— FISHES.
shoulder-girdle just behind the branchial aperture. The
pelvic or ventral fins, attached to what is at most a rudiment
of the pelvic girdle, lie below and slightly in front of the
pectorals—far from the normal position of hind-limbs.
Muscular system.—The main muscles of the body are
disposed in segments,—myotomes or myomeres, separated
by partitions of connective tissue. The effective swimming
-organ is the posterior body and the tail, as contrasted with
the pectoral fins in the skate.
Skeleton.—The vertebral column consists of biconcave
‘or amphiccelous bony vertebrae, and is divided into two
regions only, caudal and pre-caudal. The
spaces between the vertebre are filled by
the remains of the notochord. Each cen-
trum in the trunk region bears superior
neural processes, uniting in a neural arch
crowned by a neural spine, and transverse
processes projecting from each side. Artic-
ulated to the distal ends of the transverse
processes are the downward curving ribs,
and also more delicate intermuscular bones
which curve upwards. In the caudal verte-
bre (Fig. 294), the centra (¢.) bear not only
superior neural processes (7.@.), but also
inferior heemal processes (4.a.); they are of
course without ribs.
Fic.294.—Cau- At the end of the vertebral column lies
dal vertebra q fan-shaped hypural bone which helps to
ofhaddock. support the tail, and is developed from an
hee pee enlarged hemal arch. The fin-rays are
et jointed flexible rods, which in the dorsal and
. anal fins are attached to the ends of inter-
spinous bones alternating with the neural and hzemal spines,
and connected with them by fibrous tissue.
The skull includes the following bones, which may be
‘grouped in the following regions (the membrane bones in
italics) :—
(2) Around the foramen magnum: basi-occipital, two ex-occipitals,
and a supra-occipital.
(4) Along the roof: sepra-occipital, paréetals, frontals, mesethmoid,
nasals. Beneath the parzeta/s lie the alisphenoids.
SKELETON. 553
{c) Along the floor: basi-occipital, Jarasphenotd, vomers.
(@) Around the ear on each side: sphenotic, pterotic, and épiotic
(above), prootic and opisthotic (beneath).
4e) In front of and around the orbit: parethmoid, Jachrymal,
orbitgls.
Thus the haddock’s skull shows in two respects an ad-
Fic. 295.—Disarticulated skull of cod.
S.O., Supra-occipital ; Pa., parietal; 77, frontal; 47.Z., meseth-
moid; WV., nasal; P.#., parethmoid; Oz, otics; #.O., ex-occi-
pital; B.O., basi-occipital; Pa.S., parasphenoid; V., vomer;
L., lachrymal; ord., orbitals; A.4Z., hyomandibular; S.,
symplectic; Q., quadrate; Pz., pterygoid ; w.pz., metaptery-
goid; ms.f¢., mesopterygoid; PZ, palatine; AZx., maxilla;
Pmy., premaxilla; Av., articular; Am., angular; D., dentary;
u.h., urohyal ; 2.4., hypohyal; ¢.4., ceratohyal ; ¢f.4., epihyal ;
i.h., interhyal; Of., opercular; S.of., sub-opercular; z.op.,
inter-opercular ; 2.0%., pree-opercular.
vance upon that of the skate: first, in the ossification of the
primitive cartilage ; and second, in the addition of membrane
bones. Of the latter, the parietals and frontals cover over
the spaces which in the skate form the fontanelles.
554 PISCES—FISHES.
The first or mandibular arch is believed by many to form Meckel’s °
cartilage beneath, and the palato-pterygo-quadrate cartilage above.
Meckel’s cartilage becomes the foundation of the lower jaw, and bears
a large tooth-bearing membrane bone—the dentary, a small corner
bone—the angular, while the articular element is a cartilage bone.
Of the bones associated with the upper part, the palatine lies in front,
the quadrate articulates with the lower jaw; while between palatine
and quadrate lie the pterygoid, the mesopterygoid, and the meta-
pterygoid.
The second or hyoid arch is believed by many to form the hyo-
Fic. 296.—Pectoral girdle and fin of cod.
fr., Fin-rays ; 8.0., brachial ossicles; cor., coracoid; se., scapula;
ed., clavicle; .cZ., post-clavicle ; s.cZ., supra-clavicle ; 4.2., post-
temporal.
mandibular and the symplectic above, and various hyoid bones beneath.
The hyomandibular, and its inferior segment the symplectic, connect
the quadrate with the side of the skull. Of the six hyal bones, the
largest and most important is the ceratohyal, which bears seven long
branchiostegal rays. It is important to note that the bones formed
in connection with these arches do not yet form an integral part of the
skull.
The toothed premaxilla forms the upper part of the gape, while the
maxilla which articulates dorsally with the vomer, and nearly reaches
the quadrate posteriorly, does not enter into the gape. Both are mem-
brane bones.
NERVOUS SYSTEM. 555-
In the opercular fold are four membrane bones.
_ There are four pairs of complete branchial arches, which are divided
into various parts. Of these the most interesting are the two superior
pharyngeal bones, which lie in the roof of the pharynx and bear teeth,
and are formed by the coalescence of the dorsal elements of the arches.
Their teeth bite against those of the inferior pharyngeal bones,, which
lie on the floor of the pharynx, and represent the fifth branchial arches.
The limbs and girdles.x~The dermal rays of the pectoral fin are
attached to four small brachial ossicles ; these articulate with a dorsal
scapula and a more ventral coracoid ; both of these are attached to the:
inner face of a large clavicle or cleithrum, which almost meets its fellow
in the mid-ventral. line of the- throat. From the clavicle a slender:
post-clavicle extends backwards and downwards; while a stout supra-
clavicle extends from the dorsal end of the clavicle upwards to
articulate with a forked post-temporal, which articulates with the back
of the skull. It must not be assumed that the elements of this girdle
are directly comparable to those of a higher Vertebrate, although the
nomenclature is the same.
The pelvic girdle seems to be absent, as in almost all Teleostomes,
but its place is taken by a thin plate of bone, apparently due to a
fusion of some basal elements of the pelvic fins.
Nervous System.—The relatively undifferentiated fore-
brain with defective cortical region, the thalamencephalon
with its inferior lobes and infundibulum, the large optic
lobes, the tongue-shaped cerebellum which conceals most
of the medulla oblongata, have their usual general relations.
Each of the olfactory nerves is at first double; their bulb-
like terminations lie far from the brain behind the nasal
sacs. The large optic nerves cross one another without
Jusion at a slight distance from their origin ; otherwise the
nerves generally resemble those of the skate.
The eyes are large but lidless; the small nasal sacs with
plaited walls have double anterior apertures ; the vestibule
of the ear contains a large solid otolith, and another very
small one in a posterior chamber. The dark lateral line,
covered over by modified scales, lodges sensory cells, and
is innervated by a branch of the vagus.
Alimentary system.—Teeth are borne by the premaxille,
the vomer, and the superior pharyngeal bones above, by
the dentaries and the inferior pharyngeal bones beneath.
There are no salivary glands, no spiracles, nor posterior
nares. A small non-muscular tongue is supported by a
ventral part of the hyoid arch. Five gill-clefts open from
the pharynx; their inner margins are fringed by horny gill-
556 PISCES—FISHES.
rakers attached to the branchial arches and serving as
‘strainers; they prevent the food from being swept out
with the respiratory current. The gullet leads into a
curved stomach ; at the junction of stomach and duodenum
numerous tubular pyloric czeca are given off; into the duo-
denum opens the bile-duct from the gall-bladder and liver;
the coiled intestine passes gradually into the rectum, which
has an aperture apart from those of the genital and urinary
ducts. There is no spiral valve, and
there are no abdominal pores. A
pancreas is absent ; perhaps the py-
loric ceeca take its place. (In some
Teleosteans the pancreas, apparently
absent, is combined with the liver.)
The peritoneum is darkly pigmented.
Respiratory system.—Water that
passes in by the mouth may pass
out by the gill-clefts; the branchial
chamber is also washed by water
which passes both in and out under
the operculum. The gill-filaments
borne on the four anterior branchial
arches are long triangular processes,
whose free ends form a double row.
As there are no partitions between
the five gill-clefts, the filaments pro-
ject freely into the cavity covered by
the operculum. On the internal
surface of the operculum lies a red
patch, the pseudobranch or rudi-
Section ofa mentary hyoidean gill. Inspiration
Ba aay . and taking food into the mouth
G.F,, Gill-filament; A., artery are associated with the retraction of
Gengua nog) eR imgaues the hyoid apparatus ; expiration and
swallowing are associated with the
protraction of the hyoid arch. The usual retractor of the
lower jaw is absent in Teleosts, and the lowering of the lower
jaw comes about automatically in the retraction of the hyoid
arch and the raising of the operculum,—in short in the
inspiratory phase. A large and quaint parasitic copepod—
Lernea branchialis—is often found with its head deeply
CIRCULATORY SYSTEM.
557
buried in the tissues of the gills and head. Many related
forms are common on fishes.
The swim-bladder lies along the
dorsal wall of the abdomen; the
duct which originally connected it
with the gut has been closed. The
dorsal wall of. the bladder is so
thin that the kidneys and vertebrae
are seen through it; the ventral
wall is thick, and bears anteriorly
a large vascular network or
rele mirabile, which receives
blood from the mesenteric artery
and returns blood to the portal
vein.
Circulatory system.—The heart
lies within a pericardial chamber,
separated by a partition from
the abdominal cavity. The blood
from the body and liver enters the
heart by the sinus venosus, passes
into the thin-walled auricle, and
thence to the muscular ventricle.
From the ventricle it is driven up
the ventral aorta, the base of which
forms a .white non-contractile
bulbus arteriosus.
The ventral aorta gives off, on
each side, four afferent branchial
vessels to the gills. Thence the
blood is collected by four efferent
trunks, which unite on each side
In an epibranchial artery. The
two epibranchials are united pos-
teriorly to form the dorsal aorta,
while anteriorly they give off the
carotids, which are united by a
transverse vessel closing the
“ cephalic circle.”
Blood enters the sinus venosus
veins, and by hepatics from the liver.
Fic. 298.—Diagram of
Teleostean circulation.—
After Nuhn. ‘
A., auricle; V., ventricle; .a.,.
bulbus arteriosus ; v.@., ventral
aorta; a.dr., afferent branch-
ials; ¢.d7., efferent branchials;
e.¢., cephalic circle; ¢., caro-
tids; 4.c.v., anterior cardinal’
veins; P.C.V., posterior car-
dinal veins; d.c., ductus
Cuvieri; d@.a., dorsal aorta;.
¢.v., caudal vein; ¢.@., caudal
artery; K., kidney.
by two vertical precaval
Each precaval vein is.
558 PISCES—FISHES,
formed from an anterior cardinal from the head and a
posterior cardinal from the body. The posterior cardinals
extend along the kidneys, and are continuous with the caudal
vein, but the middle part of the left cardinal is obliterated.
The circulation of the blood seems to be helped, in some
fishes at least, by the respiratory movements and by the
muscular contractions in swimming.
Excretory system.—The kidneys are very long bodies,
extending above the swim-bladder under the vertebral
‘column. The largest parts lie just in front of and just
behind the swim-bladder. From the posterior part an
unpaired ureter extends to the urinary aperture, before
reaching which it gives off a small bilobed bladder.
‘The pronephros degenerates; the functional kidney is a
mesonephros.
Reproductive system.—The testes are long lobed organs,
conspicuous in mature males at the breeding season; there
is no epididymis. The ovaries of the female are more
compact sacs, more posterior in position.
Two vasa deferentia combine in a single canal. The
likewise single oviduct is continuous with the cavity of the
-ovaries. The genital aperture in either sex is in front of,
but very close to, that of the ureter. The oviducts of
most Teleosts seem to be backward extensions of the
‘ovarian sacs, but they may be disguised Miillerian ducts.
In salmonids the eggs are shed into the coelom, and escape
by a pair of pores opening together behind the anus.
Development.—The ova of the haddock, like those of
‘other Teleosteans, contain a considerable quantity of yolk,
.are fertilised after they have been laid, and undergo
meroblastic segmentation. The eggs float, ze. are pelagic ;
while those of the herring sink, ze. are dimersal.
At one pole of a transparent sphere of yolk lies a disc of formative
protoplasm of a light terra-cotta colour. The ovum is surrounded by a
firm vilelline membrane. After fertilisation the formative disc divides
first into two, then into four, then into many cells, which form the
blastoderm. From the edge of the blastoderm certain yolk-nuclei or
periblast-nuclei are formed, which afterwards have some importance.
At the end of segmentation the blastoderm lies in the form of a doubly
‘convex lens in a shallow concavity of the yolk.
The blastoderm extends for some distance laterally over the yolk ;
‘the central part raises itself, and thus forms a closed segmentation
Fic. 299.—The early development of the salmon.
x, The fertilised egg ; 2, the egg just before hatching ; 3, the newly
hatched salmon ; 4 and 5, the larval salmon nourished from yolk-
sac (y.s.) which is diminishing while the fish is increasing in
size; 6, the salmon with yolk absorbed (about six weeks old).
The small figures to the right indicate the actual sizes.
560 PISCES—FISHES,
cavity ; one radius of the blastoderm becomes thicker than the rest, and
forms the first hint of the embryo ; an inward growth from the edge of
the blastoderm forms an invaginated layer—the dorsal hypoblast or roof
of the gut; the periblast forms the floor of the gut, and afterwards aids.
the mesoblast, which appears between epiblast and hypoblast; the
medullary canal is formed as usual in the dorsal epiblast. It is likely
that the edge of the blastoderm represents the blastopore or mouth of
the gastrula, much disguised by the presence of yolk.
The newly hatched larva is still mouthless, and lives for awhile om
the residue of yolk, which, by its buoyancy, causes the young fish to be
suspended in the water back downwards.
GENERAL NOTES ON THE FUNCTIONS, HABITS, AND
Lire Histories oF FISHES
Movement.—A fish may well compare with a bird in its mastery
of the medium in which it lives. Thus a salmon travels at the rate of
about eight yards in a second, or over sixteen miles an hour. The
motion depends mainly on the powerful muscles which produce the
lateral strokes of the tail and posterior part of the body. It may be
roughly compared to the motion of a boat propelled by an oar from the
stern, So energetic are the strokes that a fish is often able to leap
from the water to a considerable height. In some cases undulating
movements of the unpaired fins, and even the rapid backward outrush
of water from under the gill-cover, seem io help in movement. The
paired fins are chiefly used in ascending and descending, in steering and
balancing. The large pectoral fins of the flying-fish (Dactylopterus and
Exocetus) are used rather as parachutes than as wings during the long
skimming leaps.
Shape in relation to habit.—The characteristic form of the
body, as seen in herring or trout, is an elongated laterally compressed
spindle, thinning off behind. like a wedge. In most cases the trunk
passes quite gradually into head and tail. This torpedo-like form is.
well adapted for rapid progression. Flat-fishes, whether flattened
from above downwards, like the skate, or from side to side like the
plaice and sole, usually live more or less on the bottom; eel-like
forms often wallow in the mud, or creep in and out of crevices ;.
globe-fishes, like Dzodon and Zetrodon, often float passively,
Colour.—The colours of fishes are often very bright. They
depend partly on the presence of pigment cells in the skin, partly on
the physical structure of the scales. The common silvery colout is.
due to small crystals of guanin in the skin. In many cases the colours
of the male are brighter than those of his mate, as in the gemmeous
dragonet (Caddonymus lyra) and the stickleback (Gasterosteus), and
this is especially true at the breeding season. The colours of many
fishes change with their surroundings. In the plaice and some others.
the change is rapid. Surrounding colour affects the eye, the influence
passes from eye to brain, and from the brain down the sympathetic
nervous system, thence by peripheral nerves to the skin, where the
GENERAL NOTES ON FISHES. 561
distribution of the pigment granules in the cells is altered. In shallow
and clear water this power of colour-change may be protective, but an
appreciation of the protective value of colouring demands careful
attention to the habits and habitat of the fishes, to the nature of the
ig in which they live, and to the enemies which are likely to attack
them.
Food.—The food of Fishes is very diverse — from Protozoa to
Cetaceans. Sharks and many others are voraciously carnivorous ;
many engulf worms, crustaceans, insects, molluscs, or other fishes;
others browse on seaweeds, or swallow mud for the sake of the living
and dead organisms which it contains, Their appetite is often
enormous, and cases are known (4g. Chéasmodon niger) where a fish
has swallowed another larger than its own normal size. Many fishes
follow their food by sight ; many by a diffuse sensitiveness, to which it
is difficult to give a name; a few, it would seem, by a localised sense
of smell. It is important to realise that fishes depend very largely on
small crustaceans, and these again on unicellular plants and animals,
Just as we may say that all flesh is grass, so we may say that all fish is
Diatom.
Senses, etc.—Fishes do not seem to have much sense of taste or of
smell, but diffuse sensitiveness to touch, chemical stimuli, etc., is well
developed, especially on the head and along the lateral line. Though
there is no drum, and the ear is deeply buried, a few seem to hear.
Some experiments suggest that the semicircular canals of the fish’s ear
are indispensable in the direction or equilibration of movement. The
sense of sight is, on the whole, well developed, and many have
“¢ darkness eyes.” As to the intellectual powers of their small brains
we know little, but many show quickness in perceiving friends or foes,
a few give evidence of memory, and many of their instincts are complex.
At the breeding season there is sometimes an elaborate expression of
excitement, well seen in the stickleback.
Reproduction. — Hermaphroditism is constant in some bony
fishes, e.g. Chrysophrys auratus (dichogamous), and three species
of Serranus (autogamous); almost constant in Pagellus mormyrus ;
very frequent in Box salga and Charax puntazzo; and exceptional
in over a score of fishes, such as sturgeon, cod, herring, pike, and
carp. The simplicity of the genital organs and their ducts may
in part explain why casual hermaphroditism is more frequent in
Fishes than in higher Vertebrates. In many cases the males are
smaller, brighter, and less numerous than the females. Courtship
is illustrated by the sticklebacks (Gasterosteus, etc.), the paradise-
fish (Macropodus), and others; and many male fishes fight with
their rivals. ‘
Most fishes lay eggs which are fertilised and develop outside of the
body. They may be extruded on gravelly ground, or sown broadcast
in the water. Sturgeon, salmon, and some others ascend rivers for
spawning purposes, while the eels descend to the sea, In the case of
trout, Barfurth has observed that the absence of suitable spawning
ground may cause the fish to retain its ova. This results in ovarian
disease, and in an inferior brood next season. Except in Elasmo-
branchs, the ova are relatively small, and large numbers are usually
36
562 PISCES—FISHES.
laid at once. In Elasmobranchs the egg is large, and in the oviparous
genera it is enclosed in a ‘‘ mermaid’s purse.”
Most sharks and a few Teleosteans, ¢.g. Sebastes marinus, Zoarces
viviparus, are viviparous, the eggs being hatched in the lower part of
the oviduct in sharks, in the ovary or oviduct in Teleosteans. In two
viviparous sharks (A/ustelus levis and Carcharias glaucus) there is a
union between the yolk-sac and the wall of the oviduct, to be com-
pared with a similar occurrence in two lizards, and with the yolk-sac
placenta of some Mammals.
As to fertilisation, the usual process is that the male deposits
spermatozoa or ‘‘ milt” upon the laid eggs or ‘‘ spawn,” but fertilisation
is of course internal when the eggs are enveloped in a firm sheath, or
when they are hatched within the mother.
Most fishes have a great number of offspring, and parental care is
proportionately little. Moreover, the conditions of their life are not
suited for the development of that virtue. When it is exhibited, it is
usually by the males,—e.g. by the sea-horse (Azpfocampus) and the
pipe-fish (Syzgnathws), which hatch the eggs in external pouches, and
‘*the male of some species of Arcus, who carries the ova about with
him in his capacious pharynx.” The female of Aspredo carries the eggs
on the under surface of the body until they are hatched, much in the
same way as the Surinam toad bears her progeny on her back ; while
in Solenostoma a pouch for the eggs is formed by the ventral fins and
skin. At least a dozen kinds of fishes make nests, of which the most
familiar illustration is that of the male stickleback, who twines grass
stems and water-weeds together, glueing them by mucus threads exuded
as semi-pathological products from the kidneys, which are compressed
by the enlarged male organs.
Fishes have a less definite limit of growth than most other Vertebrates,
and it is rare for a fish to exhibit any of the senile changes associated
with old age in other Vertebrates. But surroundings and nutrition
affect their size and colour very markedly. Some, such as the flounder,
seem almost equally at home in fresh or salt water, but many are
sensitive to changes of medium. Many can endure prolonged fasting,
and some may survive being frozen stiff. Lowered temperature may
induce torpor, as seen in the winter sleep of the pike, while in the dry
season of hot countries the mud-fishes, the Siluroids, and others, encyst
themselves in the mud, and remain for a long time in a state of ‘‘ latent
life,’
Life histories.—The life histories of fishes form the subject of an
endless chapter, of which we can only give a few illustrations. We
know how the lusty salmon return from the sea to the possibly safer
rivers, and after a period of fasting deposit their eggs and milt on the
gravelly bed of the stream. A similar migration is true of the
sturgeon.
In great contrast to these cases is the life history of the eel, the
mystery of which has been at least partially removed. From the
inland ponds and river-stretches the female eels migrate on autumn
nights seawards, meet their mates lower down the rivers, and descend
to very deep water in the sea (250 fathoms or more). There the eggs
are laid, and there in all probability the parents die. Thence the
GENERAL NOTES ON FISHES. 563
transparent larvee (Leptocephalz) rise to the surface and are for a year
or so pelagic. From the open sea the young eels or elvers migrate up
the streams in a marvellous procession or eel-fare, the females ap-
parently going farther inland than the males,
Inter-relations.—Commensalism is illustrated by some small
fishes which shelter inside large sea-anemones, and by Fverasfer, which
goes in and out of sea-cucumbers and medusz. On the outside or about
the gills of Fishes, parasitic Crustaceans (fish-lice) are often found ;
various Flukes are also common external parasites, and many Cestodes
in bladder-worm or tape-worm stage infest the viscera. The immature
stages of Bothriocephalus latus occur in pike and burbot ; a remarkable
TY
PT PUN UL LTO
TT ANNA} AIN DION
Fic. 300.—Development of eel.—After Smit.
Change from Leptocephalus shape (I.) to “‘ Elver” shape (V.).
hydroid (Polypodium) is parasitic on the eggs of a sturgeon; the young
of the fresh-water mussel are temporarily parasitic on the stickleback ;
and the young of the Bitterling (Rhodeus amarus) live for a time
within the gills of fresh-water mussels.
Distribution in space.—There are about 2300 species of fresh-
water fishes, three or four Dipnoi, about thirty ‘‘ Ganoids,” and the rest
Teleosteans, over a half being included in the two families of carps
(Cyprinidee) and cat-fishes (Siluridz),
Among marine fishes, about 3500 species frequent the coasts, rarely
descending below 300 fathoms. A much smaller number, including
many sharks, live and usually breed in the open sea. About 100
genera have been recorded from great depths.
In regard to the last, Dr. Giinther has shown that in forms living at
564 PISCES— FISHES.
depths from 80 to 200 fathoms, the eyes tend to be larger than usual,
as if to make the most of the scanty light ; beyond the 200-fathom line
small-eyed forms occur with highly developed organs of touch, and
large-eyed forms which have no such organs, but perhaps follow the
gleams of ‘‘ phosphorescent” organs; finally, in the greatest depths
some forms occur with rudimentary eyes. Many of these abyssal fishes
are phosphorescent; the colouring is usually simple, mostly blackish
or silvery ; the skin exudes much mucus; the skeleton tends to be light
and brittle; the forms are often very quaint; the diet is necessarily
carnivorous.
GENERAL NOTES ON THE STRUCTURE OF FISHES
Fins.—Along the dorsal and ventral median line of some fishes,
e.g. flounder, there is a continuous fin—a fold of skin with dermal
fin-rays (dermotrichia) and deeper skeletal supports (somactids),
In the embryos of many fishes the same continuous fringe is seen,
while the adults have only isolated median fins. There is no doubt
that these isolated median fins—of which there may be two dorsals,
a caudal, and an anal or ventral—arise, or have arisen, from a modifica-
tion of a once continuous fin.
Now, the paired fins, which correspond to limbs, often resemble
unpaired fins in their general structure, and in their mode of origin.
It is possible that the paired fins may have arisen by a localisation of
two once continuous lateral folds. According to another theory, the
origin of paired fins is to be found in the visceral arches.
The paired fins are supported by dermic fin-rays (dermmotrichia)
and by endoskeletal pieces (somactéds or radials), some of which are
articulated to the girdles and are then called dasa/za. Two main types
of fish fin are distinguishable—(a) that best illustrated among living
fishes by Ceratodus, in which a median jointed axis bears on each side
a series of radial rays—a form often called an archipterygium ; and (4)
the commoner type, in which the radials arise on one side of the basal
pieces (an ichthyopterygium). In the bony fishes the support of the
fin beyond the base seems mainly due to dermal rays.
Tail.—In Dipnoi and a few Teleosteans, e.g. the eels, the vertebral
column runs straight to the tip of the tail, dividing it into two equal
parts. This perfectly symmetrical condition is called diphycercal or
protocercal. ;
In Elasmobranchs, Holocephali, cartilaginous and many extinct
** Ganoids,” the vertebral column is bent dorsally at the end of the tail,
and the ventral part of the caudal fin is smaller than, and at some little
distance from, the upper part. This asymmetrical condition is called
heterocercal.
In most Teleostei, and in extant bony ‘‘Ganoids,” the end of the
vertebral column is also bent upwards, but the apex atrophies, and, by
the disproportionate development of rays on the ventral side, an
apparent symmetry is produced. The vertebral column usually ends
in a urostyle,—the undivided ossified sheath of the notochord. Most
of the fin really lies to the ventral side of this. The condition is
termed homocercal.
GENERAL NOTES ON STRUCTURE OF FISHES. 565
The effect of a stroke with the heterocercal tail is to force the anterior
region downwards, and thus the heterocercal tail in fish is associated
with a ventral mouth and the habit of ground-feeding. The movement
of the homocercal tail, on the other hand, drives the body straight
forwards, and is associated with a terminal mouth. _
Scales.—(1) In Elasmobranchs the scales (placoid) have the form of
skin-teeth (dermal denticles), tipped with enamel, cored with dentine,
and based with bone sunk in the dermis, They arise from skin papillee,
the (ectodermic) epidermis forming the enamel, the (mesodermic) dermis
forming the rest. In other fishes the scales are almost wholly dermic,
in marked contrast to those of Reptiies.
(2) ‘*Ganoid” scales, as in Lepzdosteus, are plates of bone with an
enamel-like covering called ganoin. .
(3) In most Teleosts the scales are relatively soft dermic plates
of thin bone. In the sturgeon and many Teleosts the scales are
substantial bony plates. The typical ‘‘soft” Teleost scales are called
cycloid or ctenoid, as their free margins projecting from sacs in the
dermis are entire or notched. The concentric rings on the scales
indicate periods of growth, like the rings on a tree stem, and it is
possible in some cases to tell the age of a fish from its scales, as also
from the otoliths in the ear when these have a layered structure.
_ The scales “of Elasmobranchs are homologous with teeth, and a
number may fuse into'a plate just as teeth often do.
~ Swim-bladder.—The swim-bladder of fishes is one of the
numerous outgrowths of: the gut. It is absent in Elasmobranchs and
some Teleosteans, such as most flat-fish, and it forms the lung of
Dipnoi. Unlike a lung, it opens dorsally into the gut, except in
Dipnoi and the Ganoid olypterus, where the aperture is ventral.
The original duct communicating with the gut may remain open, as in
Physostomatous Teleosteans, or it may be closed, as in Physoclystous
Teleosteans. The bladder is usually single, but it is double in
Protopterus, Lepidosiren, and Polypterus.
In regard to the use of the swim-bladder, there is still considerable
uncertainty. Where it is abundantly supplied with impure or partially
purified blood, as in Dipnoi, Polygterus, and Ama, and where the gas
within is periodically emptied arid renewed, it is doubtless respiratory.
But what of other cases, where its supply of blood is arterial, and what
especially where it is entirely closed? In such cases it is usual to speak
of its function as hydrostatic.
In greater detail the function of the air-bladder is—(1) to render the
fish, bulk for bulk, of the same weight as the medium in which it lives ;
moreover (2), the volume of the contained gas varies with increased
secretion and absorption, and seems to adjust itself to different external
pressures as the fish descends or ascends, There is sometimes a well-
developed gas-gland with a rich blood-supply on the inner wall of the
bladder. (3) In many fishes the bladder may help indirectly in
respiration by storing the superabundance of oxygen introduced into
the blood by the gills. (4) There is in several Teleosteans a remarkable
connection between the swim-bladder and the ear, sometimes by an
anterior process of the bladder, as in the herring and perch-like fishes,
sometimes by 4 chain of bones, as in Siluride. This has suggested
566 PISCES—FISHES.
the view that the connection serves to make the fish aware of the
varying tensions of gas in the bladder, due to the varying hydrostatic
pressure.
CLASSIFICATION OF FISHES
Sub-Class I. ELASMOBRANCHII
Cartilaginous Fishes,
e.g. Sharks and Skates
Voracious carnivorous fishes, with cartilaginous skeleton,
placoid scales, usually heterocercal tails, “claspers” on
Fic. 301.—Young skate.—From Beard.
The yolk-sac has been cut off, the yolk-stalk is
left. 1., Mouth; o/.0., nostril, e.g. ‘external
gills”; a., cloaca; c¢., claspers.
the pelvic fins of the
males. Except in Holo-
cephali there 1s no cover
over the (5-7) gill-aper-
tures; anterior to these
there is often a spiracle
—the first gill-cleft—
with a rudimentary gill.
The gill-clefts are
separated by complete
septa, and the gill-
filaments are attached
throughout their length
to the septa. The mouth
extends transversely on
the under side of the
head. The nostrils are
also ventral. There is
no air-bladder. A spiral
fold extends along the
internal wall of the large
intestine. Into the ter-
minal chamber (or clo-
aca) of the gut the genital
and urinary ducts also
open. The ventricle of
the heart has a con-
tractile conus arteriosus.
Fertilisation is internal.
The ova are few and
large, z.e. with much yolk.
ELASMOBRANCHII, 567
Large egg-purses are common, but some Elasmobranchs
are viviparous. ‘The embryos have gill-filaments projecting
out of the gill-clefts, so-called external gills. They are
really elongated internal gills. Elasmobranchs retain more
embryonic features, e.g. the naso-buccal groove and auditory
opening, than other fishes.
Order 1. PLAGIOSTOMI or SELACHII
With transverse ventral mouth, pre-oral rostrum, uniserial paired
fins, claspers, heterocercal tail, usually five pairs of open gill-clefts.
Subdivisions.—(1) The older Selachoidei, with approximately
cylindrical bodies and lateral gill-openings, as in shark and dogfish ;
(2) the more modified Batoidei, with flattened bodies, ventral gill-
openings, and pectoral fins joined to the head, as in skates or rays.
Mustelus, Carcharias, Squalus, Torpedo, Acanthias, and others, are
Fic. 302.—Lateral view of dogfish (Scy/zum catulus).
Note ventral mouth with naso-buccal groove, heterocercal tail, and
unpaired fins. gs., Gill-slits; Zc., pectoral fins; 4v., pelvic
fins.
viviparous ; Raja, Scylléum, Cestracton, and others, are oviparous. In
most species of AM/ustelus there is a placenta-like connection between
the yolk-sac of the embryo and the uterus of the mother. In several
viviparous genera long filaments are developed from the inner surface
of the uterus which secrete a nutritive fluid. In some cases the
nutriment seems to be afforded by degeneration of the uterine wall.
In Acanthias vulgaris there is no nutritive material, and the young
are unattached. ‘This is intermediate between oviparous and specialised
placental conditions. Zygena has a peculiar hammer-like head expan-
sion; Se/ache reaches a length of 40 ft. ; Przstés has the snout prolonged”
in a tooth-bearing saw ; Zorgedo has a powerful electric organ. The
Greenland Shark (ZLemargus borealis) is unique in having small eggs,
without egg-cases, perhaps fertilised in the water. In the eel-like
deep-water Japanese Shark (Ch/amydoselachus) the mouth is anterior,
the nostrils lateral, the vertebral column is imperfectly segmented, _
there is a slight opercular fold, and there are six pairs of gill-openings
and arches. In the large viviparous Notidanide, e.g. Hexanchus (six ,
568 PISCES—FISHES.
gills) and Heptanchus (seven gills), the mouth is almost inferior, the
vertebral column is imperfectly segmented with persistent notochord.
History.—The Elasmobranchs appear in the Upper Silurian, are
very abundant from the Carboniferous onwards, but are now greatly
outnumbered by the Bony Fishes. An increasing calcification of the
axial skeleton is traceable through the ages, and in some of the
ancient forms the exoskeleton was greatly developed, often including
long spines or ichthyodorulites firmly fixed on the dorsal fins or on
the neck.
Order 2, HOLOCEPHALI
The Holocephali are represented by the sea-cat or Chimera from
northern seas, and Callorhynchus from the south. There is a fold or
operculum covering the (4) gill-clefts and leaving only one external
opening on each side; there is no spiracle; the vertebral column is
unsegmented ; the upper jaw is fused to the cartilaginous skull, and
thus the hyoid does not help in its suspension (azéostylic) ; the skin is
naked except in the young, which have some dorsal placoid spines.
There is a urogenital aperture separate from the anus. In general
the Holocephali most nearly resemble Plagiostomi, but they have many
affinities with Dipnoi, ¢.g. in the autostylic skull. :
Teeth (of Ptychodus, Rhynchodus, etc.), which have been referred to
Chimeroids, occur in Devonian rocks, and some at least of the
detached spines of Carboniferous age may have belonged to fishes
of this order. Undoubted Mesozoic Chimeroids are Sgualoraja,
Myriacanthus, Chimeropsis, Ischyodus, etc., while others, including
the recent genus Chimera, are found in strata of Tertiary age. The
other recent genus, Callorhynchus, is also represented by a Cretaceous
species, C. hector?.
EXTINCT ORDERS
Order 3. PLEUROPTERYGII
Devonian, Carboniferous, and Permian. Forms with unconstricted
notochord, heterocercal tail, terminal mouth, paired fins with unseg-
mented parallel radials. C/adoselache.
Order 4. ICHTHYOTOMI
Lower Carboniferous to Permian. Forms with unconstricted noto-
chord, diphycercal tail, and pectoral fins with a segmented axis of
basals bearing biserial radials. Plewracanthus,
Order 5. ACANTHODEI
Another interesting extinct group, whose position was for long a
matter of dispute, but which is now usually placed near Elasmobranchii,
is that of the Acanthodei. These flourished principally in Devonian
times, but lived on through the Carboniferous to the Lower Permian.
TELEOSTOMI. 509
They are usually rather small fishes, with minute rhomboidal shagreen-
like scales, and a strong spine in front of each fin, except the caudal.
In some genera (Parexus, Climatius) there are two rows of small
intermediate spines between the proper pectorals and the pelvics.
d
FIG. 303.—Outline of Acanthodes sulcatus.—After Traquair.
é., Pectoral fins; v., pelvics.; a., anal; d., dorsal.
Sub-Class II. TELEoOsToMI
Fishes with more or less ossified skeletons, especially as
regards skull, jaws, operculum, and -pectoral girdle. The
skull is hyostylic, the jaws being supported by the hyoman-
dibular. The pelvic girdles are usually rudimentary or
absent. The mouth is usually terminal; the scales are in
the majority soft and cycloid. There is always a gill-
cover; the inter-branchial septa are much reduced; the
gill-filaments project freely from the gill-arches. There is
usually a swim-bladder. There are no claspers, no naso-
buccal grooves; there is no cloaca. The fore-brain has
a non-nervous roof. The ova are small and numerous,
usually meroblastic, sometimes holoblastic. Fertilisation is
usually external.
Order 1. CROSSOPTERYGII
Ancient forms with pectoral fins obtusely lobate and uniserial or
acutely lobate and biserial; with scales and dermal skull bones often
covered with enamel-like ganoin ; with a pair of jugular plates between
the rami of the lower jaw. All are extinct except Polypterus and
Calamoichthys from African rivers. Examples, Osteolepis (Lower
Devonian), Holoptychius (Devonian), Megalichthys (Carboniferous),
In Polypterus, the body is covered with rhombic ganoid scales;
there are numerous dorsal fins; the tail is diphycercal; the pectoral
fin has three basal pieces as in Elasmobranchs, then two rows of
radials, and then the dermal fin-rays or dermotrichia; the air-bladder
is double and is used in respiration, its duct opens ventrally into the
pharynx ; the young form has an external gill on the operculum; the
570 PISCES—FISHES.
oral part of the hypophysis retains its opening into the mouth, The
genus Calamoichthys has very similar characters, but no pelvic fins.
These two forms may almost be called living fossils.
AT gs
ee Se PAGS ad
nn
mama
Fic. 304.—Larva of Polypterus (after Budgett), 14 inch in length,
e.g., Large external gill of the hyoid arch; Pc., pectoral fins; Pv., pelvic
fins. ‘The larva is drawn in a very characteristic attitude.
The following three orders are often grouped as Actino-
pterygii, with the following characters. The paired fins are
never lobate, they have short basal pieces, and are mainly
supported by dermal fin-rays.
Order 2, CHONDROSTEI—Wwith cartilaginous
internal skeleton
Living examples :—Sturgeon (Acipenser), Polyodon, Sca-
phirhynchus.
Fic. 305.—Sturgeon (Aczpenser sturio).
Note the elongated snout, the barbules bounding the ventral mouth,
the operculum covering the gills, the rows of bony scutes, the
markedly heterocercal tail.
Extinct examples :— Chetrolepis, Pal@oniscus, Chondrosteus.
In the sturgeon (Acépenser) the skin bears five rows of large bony
scutes; the tail is heterocercal; the notochord is unsegmented. A
snout, with pendent barbules, extends in front of the ventral mouth,
which is rounded and toothless. Sturgeons feed on other fishes,
TELEOSTE, 578
which they swallow whole. They are the largest fresh-water fishes,
for A. sturdo may attain a length of 18 ft. and a weight of 600 lb.,
while the 4. uso of Southern Russia may measure 25 {t. and weigh
nearly 3000 lb, !' Most of the species are found both in the sea and in
rivers or lakes. The roes or ovaries form caviare; the gelatinous
internal layer of the swim-bladder is used as isinglass.
The genus Scaphirhynchus is represented in Asia and the United
States ; Polyodon or Spatularia spatula is the paddle-fish or spoon-bill
of the Mississippi.
Order 3. Ho.ostE1 !—with bony skeleton
Living examples :—Lepidosteus and Amia.
Extinct examples :—Lepidotus, Pycnodus, Aspidorhynchus.
The N. American bony pike—Zefidosteus—is covered
with rows of “ganoid” scales; the whole skeleton is well
ossified, and the vertebral bodies are opisthoccelous; the
swim-bladder is like a lung in structure, and to some
degree in function. The bow-fin, Ama calva, frequenting
still waters in the United States, has a similar lung-like
swim-bladder. Its scales are similar to those of a Teleost.
Order 4. TELEostTe1. The “ Bony Fishes”
This order includes most of the fishes now alive.
Though comparatively modern fishes, they are older than
was formerly supposed, as several Jurassic genera (Zhrissops,
Leptolepis, etc.), which used to be classed as “ Ganoids,” 4
must be considered as actual Clupeoids, or herring-like
Teleostei. It is, however, not until the Upper Cretaceous
and Tertiary epochs that they assume among fishes that
overwhelming preponderance in numbers which they possess
at the present day. The physostomous type of Teleostean
is the most ancient, and probably stands in a continuous
genetic line with the Holostei.
The skeleton is well ossified, with numerous investing
bones on the skull, others in the operculum, and on the
shoulder-girdle. There is always a supra-occipital in the
1 The term ‘ Ganoids,” which we abandon, is often used to include
Crossopterygii, Chondrostei, and Holostei. Though they agree in
having « conus arteriosus with many valves, as opposed to the
Teleostean bulbus, an optic chiasma, as opposed to the decussate
condition in Teleosts, and an intestinal spiral valve which is absent in
Teleosts, they do not seem to form a natural division.
572 PISCES—FISHES.
skull. The tail is sometimes quite symmetrical or
diphycercal, but in most cases it is heterocercal at first,
and acquires a secondary symmetry termed homocercal ;
for while the end of the notochord in the young forms is
bent upwards as usual, the subsequent development of rays
produces an apparent symmetry. The scales are in most
cases relatively soft. The roof of the fore-brain is without
nervous matter. The optic nerves are remarkable, because
they cross one another without interlacing (decussate).
The partitions between the gill-clefts disappear ; so, instead
of the pouches seen in Elasmobranchs, there is, on each
side, one branchial chamber, covered over by an opercular
fold. Into this chamber the comb-like gills, borne by
the branchial arches, project freely. There is usually a
Wtedaaaes
Ny Be)
a
Fic. 306.—The goldfish (Cyprinus auratus).
rudimentary gill or pseudobranch associated with the hyoid.
There is no spiracle. In most, a swim-bladder is developed
from the dorsal side of the gullet. The duct of the swim-
bladder may remain open (Physostomous), as in herring,
salmon, and carp; or it may be closed (Physoclystous), as in
perch and cod. There is no spiral valve in the intestine,
and the food canal ends in front of, and separate from, the
genital and urinary apertures or aperture. The base of
the ventral aorta is swollen into a non-contractile bulbus
arteriosus, but there is no conus, unless very exceptionally,
as in Butirinus. A remarkable peculiarity is that the
gonads are usually continuous with their ducts. The ova
are numerous, usually small and fertilised in the water.
The segmentation is meroblastic, and there is usually a
distinct larval stage.
DIPNOL 573
The Teleosts include the great majority of living fishes, which
are classified in thirteen sub-orders and numerous families, ¢.g.
Clupeidee (herrings); Salmonide (salmon, trout); Cyprinidze
(carps) ; Murzenidze (eels); Esocidee (pike); Gasterosteide (stickle-
backs) ; Syngnathidze (pipe-fish and sea-horses) ; Gadidze (cod-fishes) ;
Percidee (perch) ; Scombridze (mackerels) ; Pleuronectidse (flat-fishes) 5.
Cottidee (bull-heads) ; Triglidee (gurnards) ; Lophiidz (anglers) ;
Tetrodontidz (globe-fishes).
Sub-Class III. Drpnor. ‘ Mud-Fishes”
Fishes with a lung—the modified swim-bladder—as well
as gills ; the paired fins are of the archipterygium type, with
a long segmented axis, sometimes bearing a series of lateral
pieces on each. side, with overlapping cycloid scales, with’
multicellular skin-glands, with a diphycercal tail. The
notochord persists, and its sheath is unsegmented ; the skull
is autostylic and is largely a persistent chondrocranium with
the addition of some membrane bones; there are large
compound grinding teeth. The external nares are on the
ventral surface of the snout, or even within the upper lip,
and the arching over of the nasal grooves leads to the
formation of separate internal nares. The heart is
incipiently three-chambered, containing mixed blood, with
a spiral conus arteriosus with numerous valves; there is a
vein resembling the inferior vena cava of higher vertebrates.
There is a spiral valve in the intestine. The eggs are large
and exhibit total unequal segmentation, as in Amphibians.
The Dipnoi, whose name means double breathers, are
now represented by three genera—Ceratodus, from two.
rivers of Queensland; /votopierus, from certain African
rivers, e.g. the Gambia; and Lefidosiven, from the Amazons.
The wide distribution is noteworthy.
They are very ancient forms, for Ceratodus existed in
Triassic and Jurassic times (though no _ post-Jurassic
remains are known). There were also undoubted Dipnoi
far back in Paleozoic times, such as Dypterus and
Phaneropleuron of the Devonian, Cienodus and Uronemus
of the Carboniferous.
The living Dipnoi are probably the survivors of an
archaic. group; in their teeth and autostylic skull they
resemble Holocephali; in their fins and air-bladder. they
574 PISCES—FISHES.
recall Crossopterygii; in their cartilaginous skeleton
and persistent notochord they are
primitive ; in their lung, heart, in-
ferior vena cava, multicellular skin-
glands, and eggs they approach
Amphibians.
The Dipnoi are physiologically
transitional between Fishes and
Amphibians, having, for instance,
acquired lungs while retaining gills,
but it does not follow that they
are morphologically transitional.
They are intermediate, but that is
not to say that they are ¢Ze connect-
ing links.
Ceratodus.— The genus Cera-
todus is abundantly represented by
fossils in the Mesozoic beds of
Europe, America, Asia, and Aus-
tralia, but the living animal is now
limited to the basins of the Burnett
and Mary rivers of Queensland
(see Fig. 6). Like that other old-
fashioned animal the duckmole,
Ceratodus frequents the still, deep
places of the river’s bed, the
so-called ‘water-holes.” At the
bottom of these it lies sluggishly,
occasionally rising to the surface
to gulp in air. Its diet was for-
merly supposed to be exclusively
vegetarian, but Semon holds that
it crops the luxuriant vegetation of
the river-banks only for the sake
of the associated animal life—
larvee and eggs of insects, worms,
molluscs, amphibians, and _ fishes.
Though Ceratodus is quite unable
to live out of water, its air-breath-
ing powers enable it to exist in
water which is laden with sand or rotten vegetable matter.
ie:
\)
"
Fic. 307. —Lepidosiren (after Graham Kerr), showing (7¢,/.) pectoral fin and the
tufted pelvic fin (Pv.f.) of the mature male.
PROTOPTERUS. 575
Ceratodus sometimes attains a length of 6 ft. The body is elongated
and compressed, and bears a continuous vertical fin. The. paired fins
are trowel-like. There are five gill-clefts, four internal gills, and a
hyoid half-gill. There are no external gills.
The swim-bladder or lung issingle. It is supplied with blood from
the fourth branchial arches, as is the swim-bladder of Polypterus
and Ama. It arises ventrally, but lies
dorsally, and is divided into compart-
ments.
The auricle of the heart has a
dorsal fibrous ridge hinting at a divi-
sion, A similar incomplete septum
occurs in the ventricle, and the sinus
venosus is divided into a left pulmonary
and a right systemic portion. The conus
arteriosus is peculiarly twisted, and
contains a short longitudinal spiral
valve and numerous large ‘‘ pocket”
(or ‘* Ganoid ”) valves.
Protopterus. — This mud-fish
lives in the Gambia, Quilimane,
and some other African rivers.
It is mainly but not exclusively
carnivorous, and attains a length
of 2 to 3 ft. or more. It has
extraordinary vitality, surviving
severe wounds, long fasting, and
desiccation. It appears to be
most active at night, and to prefer
shallow water, swimming rapidly
with powerful tail-strokes, or
“walking” slowly along the bottom
with its filamentous fins moving
alternately on each side, somewhat
like the legs of a newt. At short Fic. 308.—Skeleton of Cera-
intervals it comes to the surface “sen fin.—From Gegen-
to take mouthfuls of air, which 0 asics» eadiale:
passes out again through the 7", ‘fasal pies,
opercular aperture.
As the dry season approaches, Protogerus burrows into the earth to
a depth of about 18 in., coils itself up, and secretes abundant mucus
from its skin glands. This secretion forms a cocoon or capsule, with
adherent earth externally, with moist slime internally, and with a lid,
on which there is always a small aperture. Thus encapsuled, the
576 PISCES—FISHES,
animal may remain dormant for many months, ¢.g, from August to
December. The air seems to pass directly from the mouth of the
burrow, through the aperture of the capsule-lid (which is produced
Fic, 309.—Head region of Protopterus.—From W. N. Parker.
” sat, Sensory tubes ; Z.2., lateral line ;.¢.d7., external gills ; fc.2.,
pectoral fin ; of., operculum.
inwards in a short pipe) to the nostrils, and thence to the lungs.
The nourishment appears to be derived from a store of fat deposited
in the lymphoid tissue around the reproductive organs and kidneys,
and among the lateral muscles of the tail (cf. fatty bodies in
Fic. 310.—Larva of Protopterus.—After Budgett.
e.g. external gills; Pe., pectoral fin; Pv., pelvic fin.
caterpillars, amphibians, etc.). Moreover, some of the muscles are
replaced by fat, and others undergo a pathological granular degenera-
tion (cf. lamprey). To a certain extent, therefore, the dormant
animal lives on its own tail. It is probable that leucocytes aid in the
absorption and transportation of the degenerated muscles (cf. tadpoles):
LEPIDOSIREN. 577
These capsules, with the surrounding earth, have often been transported
from Africa to Northern Europe, without injury to the dormant fish
within, The fish makes .a nest which is guarded by the male. The
larvze have four pairs of external gills, and a crescentic sucker like that
of an Amphibian tadpole.
Fic. 311.—Larva of Lepidostren.—After Graham Kerr.
Lepidosiren.—This mud-fish from the Amazons has an
eel-shaped body, with a continuous vertical fin. The limbs
are reduced to the axis only. There is a well-developed
septum in the auricle, an all but complete septum in the
ventricle, and a complete septum in the conus. The lung
is double. The eggs are laid in burrows, and the male
remains curled up beside them. The young are hatched
with external gills.
CHAPTER XXIII
Ciass AMPHIBIA
Order I. STEGOCEPHALI (extinct).
», II. GYMNOPHIONA or APODA (a small order).
», III, URODELA or CAUDATA, ¢.g. Newts and Salamanders.
», IV. ANURA or EcauDATA, ¢.g. Frogs and Toads.
AMPHIBIANS made the transition from aquatic to terrestrial
life. But almost all have lagged near the water. Certain
acquisitions, such as lungs and a three-chambered heart,
incipient in the Dipnoi, are here firmly established. As
regards bodily size, the Amphibian race has dwindled since
the days of its prime, but it seems to have been progressive,
for some of its members show affinities with Reptiles.
GENERAL CHARACTERS
Amphibia are Vertebrates in which the visceral arches of
the larva almost always bear gills, which may be retained
throughout life, though the adults normally possess functional
lungs. Whence it follows that the nostrils, through which the
air enters, must open into the mouth. When limbs ave
present, they have distinct digits. . The unpaired fins, fre-
guently present both in larve and adults, are without fin-rays.
In existing forms there ts rarely any exoskeleton, but some
extinct forms had an armour of bony plates. The skull has
two occipital condyles. The heart is three-chambered, with
two auricles and a ventricle,—and a conus arteriosus. The
gut ends in a cloaca, into which the ducts from kidneys and
reproductive organs also open. A bladder, growing out from ~
the hind region of the gut, ts probably homologous with the
allantots of the embryos of higher Vertebrates. The ova are
small, numerous, usually pigmented, and with yolk towards
one pole. They are almost always laid in water; the seg-
‘AMPHIBIANS. 579
mentation is holoblastic, but unequal,
metamorphosis in development,
Huxley was the first to recognise the affinities between Fishes and._
Amphibians, and to unite the two classes under the title Ichthyopsida.
Of the characters common to the two classes, the following are
important : Gill-slits are functional in respiration, but in Amphibians
they may disappear after larval life, the Eustachian tube excepted ;
gills are always present, but they may be restricted to the larval stages
in Amphibians ; in fishes and larval Amphibians a single ventral aorta
leaves the heart; there is no amnion, and at most a homologue of
the allantois (in Amphibians); there are only. ten pairs of cranial
nerves ; there are lateral sensory structures, such as the ‘‘ branchial
sense organs” and those of the ‘‘ lateral line,” but these may be dim-
inished in the adults; unpaired fins are almost always represented, but
may not persist in the adult life; there is a functional pronephros in
early stages. ;
From the higher Vertebrates or Amniota the Ichthyopsida are clearly
distinguished by the presence of gills (in youth at least) and by the
absence of amnion and functional allantois. For though the bladder of
Amphibians may be homologous with an allantoic outgrowth, it does
not function as such, ze. it does not aid in the respiration or the
nutrition of the embryo. ‘
_ Itis more difficult to distinguish between Fishes and Amphibians, more
_ especially if we include the Dipnoi in the former class. The most obvious
differences are the absence of fin-rays and the development of fingers and
toes. In the following table the two classes are contrasted :—
There is usually a
FISHES.
AMPHIBIANS.
Gills persist throughout life.
The swim-bladder functions as a lung
in Dipnoi and less markedly in
some ‘‘Ganoids,” but in most cases
its respiratory significance is slight.
The heart is two-chambered (incipiently
three-chambered in Dipnoi). There
is no inferior vena cava, except in
Dipnoi.
The limbs are fins.
The unpaired fins are supported by fin-
rays (dermotrichia).
The skull has, in most cases, one
occipital condyle.
There is usually an exoskeleton of scales
or scutes.
There are no true posterior nares.
There is no certain homologue of the
allantois.
Gills may disappear as the adult form
is attained.
Lungs are always developed in the
adults.- It is doubtful whether
they are directly comparable with
the swim-bladder.
The heart has three chambers. There
is an inferior vena cava, and paired
posterior cardinals are seen only in
the larva.
The limbs have digits.
There are no fin-rays.
There are two occipital condyles. A
columella runs from the tympanum
to a fenestra ovalis in the ear
capsule.
There is no exoskeleton, except in a
few cases, and in extinct forms.
There are posterior nares opening into
the cavity of the mouth.
The cloacal bladder seems to be the
homologue of the allantois ~
580 AMPHIBIA.
THE FROG AS A TYPE OF AMPHIBIANS
The common British frog (Rana temporaria) and the
frequently imported continental species (A. esculenta) agree
in essential features.
Though aquatic in youth, they often live in dry places,
hiding in great drought, reappearing when the rain returns.
Every one knows how they sit with humped back, how they
leap, how they swim. They feed on living insects and slugs.
Fic. 312,—The edible frog (Rana esculenta).
These are caught by the large viscid tongue, which, being
fixed in front of the mouth and free behind, can be jerked
out to some distance, and with even greater rapidity re-
tracted. When a frog is breathing, the nostrils are alternately
opened and closed, the under side of the throat is
rhythmically expanded and compressed, the mouth re-
mains shut meanwhile. The males trumpet in the
early spring to their feebly responsive mates. In our
British species the pairing takes place soon after; the
young are familiarly known as tadpoles, and a notable
metainorphosis takes place. In winter the frogs hiber-
THE FROG 581
nate—buried in the mud of the ditches and ponds, mouth
shut, nose shut, eyes shut—and breathe through their skin.
Form and external features.—The absence of neck and
tail, the short fore-limbs almost without thumbs, the longer
hind-limbs with five webbed nailless toes and with a long
ankle region, the apparent hump-back where the hip-girdle
is linked to the vertebral column. There is a very rudi-
mentary thumb, and there is a horny knob at the base of
the hallux or “great toe.” At pairing time the skin of the
first finger is modified in the males into a rough cushion,
darkly coloured in 2. temporaria.
The wide mouth, the valvular nostrils, the protruding
eyes, the upper eyelid thick, pigmented, and slightly mov-
able, the lower rudimentary and immovable, the third
eyelid or nictitating membrane semi-transparent and moving
very freely, the circular drum of the ear, the slightly dorsal
cloacal aperture.
Skin.—The smooth, moist skin is loosely attached at
intervals to the muscles by bands of connective tissue,
which form the boundaries of over a score of lymph-sacs.
These contain fluid partly absorbed through the skin, and
open into the veins by two pairs of lymph-hearts. The skin
consists of a two-layered (ectodermic) epidermis, and an
internal (mesodermic) dermis. The transparent outer layer
of the epidermis is shed periodically, and swallowed by the
frog. The dermis differs markedly from that of a fish, for-
there is no exoskeleton, though this was present in the
extinct Labyrinthodonts; there are multicellular glands,
whose secretion keeps the skin moist and is in part
poisonous; and there is a stratum of unstriped muscle
fibres. Pigment cells occur in the dermis, and’ some
extend between the cells of the epidermis. The colour
changes a little according to the state of these cells, the
protoplasm expanding and contracting partly through the
direct influence of light and moisture on the skin, partly by
a more complex reflex action in which the eyes, the brain,
and the sympathetic nervous system are all implicated. In
the larval salamander the pigment cell seems to contract
and expand as a whole, but this is not usually the case.
There are cutaneous blood vessels, by means of which the
frog can, to a certain extent, breathe by its skin. The
582 AMPHIBIA,
tadpole has sensory cells in distinct lateral lines, but of this
regularity the adult retains little trace, though it has many
nerve-endings and “touch-spots” in various parts of its skin.
The axial skeleton.—The vertebral column consists of
nine vertebra, and an unsegmented urostyle or coccyx.
The first vertebra bears two facets for the two condyles of
the skull, and an odontoid process which lies between the
condyles. It has no transverse processes, and its arch is
incompletely ossified. Each of the
next six has an anteriorly concave or
proccelous centrum, a neural arch sur-
rounding the spinal cord, a transverse
process from each side of the base of
the arch, an anterior and a posterior
pair of articular processes, and a short
. neural spine. The eighth vertebra has
a biconcave or amphiccelous centrum.
The ninth is convex in front, with two
convex tubercles behind, and _ bears
large: transverse processes with which
the hip-girdle articulates. The uro-
style, formed by the fusion of several
vertebre, has anteriorly a dorsal
arch enclosing a prolongation of the
spinal cord; but both arch and
nerve-cord soon disappear posteriorly.
FIG. 313. — Vertebral The notochord, around which the
column and pelvic vertebral column has developed, is
girdle of bull-frog. finally represented only by the ves-
4p., Transverse processes tiges in the centra of the verte-
of sacral vertebra; /2,
ilium; U., urostyle ; Fe., bree.
ea 5 4seh., ischiae ~The skull consists — (a) of the
persistent parts of the original car-
tilaginous brain-box or chondrocranium, developed, as in
the skate, from parachordals and trabecule, plus nasal
and auditory capsules ; (4) of ossifications of parts of
the chondrocranium, cartilage bones; (c) of membrane
or investing bones; and (d) of associated visceral arches
Two ex-occipitals bounding the foramen magnum and forming the
condyles, two pro-otics or ossifications of the original auditory capsule,
THE AXIAL SKELETON.
583.
and an unpaired sphenethmoid forming the front of the brain-case, are
cartilage bones.
or jugals are also cartilage bones.
Probably the slendet rods known as quadrato-jugals:
Two parieto-frontals and two nasals above, a paired vomer and an
unpaired dagger-shaped parasphenoid beneath, and two. lateral
hammer-shaped squamosals (para-
quadrates) are membrane bones.
There is no basisphenoid ossifica-
tion.
To these are added the small
premaxillze in the very front of the
skull, and the long maxillze on each
side. The quadrato-jugal connects
the maxille with a minute nodule
which represents the quadrate
bone.
On the roof of the mouth, ex-
tending from the quadrate forwards
to near the vomers, are the
triradiate pterygoids, while at
right angles to the anterior end
of the parasphenoid’ and behind
the vomers are the palatines.
Each half of the lower jaw,
based on Meckel’s cartilage, con-
sists of three pieces,—the largest
an articular angulo-splenial, out-
side this a thin dentary, and
anteriorly uniting with its fellow
a minute mentomeckelian.
A delicate rod—the columella
auris— extends from the tympanum
to the fenestra ovalis in the inter-
nal capsule of the ear. According
to Parker, it represents the upper
part of the hyoid arch, the lower
portion of which forms the car-
tilaginous ‘or partially ossified
hyoid plate, which lies in the floor
of the mouth and is produced into
two anterior and two posterior
cornua.
The teeth are borne by the pre-
maxillze, maxillee, and vomers.
There is no parietal foramen,
but in the Labyrinthodonts it is
always distinct.
Fic. 314.—Skull of frog—upper
and lower surface.—After W. K.
Parker.
Upper surface—
Puix., premaxilla ; 1V., nasal; J7., max-
illa; Sy., squamosal; Q.j., quadrato-
jugal ; ¢.0., ex-occipitals ; PA, parieto-
frontals ; S#2.Z., sphenethmoid ; P.O.,
pro-otic,
Lower surface—
Pmzx., premaxilla; JZ, maxilla; Q.7.,
quadrato-jugal; @., quadite Pt,
pterygoid; Ps., parasphenoid; P.O.,
pro-otic; SJ.Z., sphenethmoid; Pd,
palatine ; ., voiner ; ¢., columella.
The cartilage which bears the quadrate at its lower end, and runs
between pterygoid and squamosal, connecting the articulation of the
lower jaw with the side of the skull at the auditory capsule, is called
the suspensorium,
In Elasmobranchs the hyomandibular is the sus-
584 AMPHIBIA.
pensorium ; in Teleosteans the name is applied to the hyomandibular
and symplectic ; in Sauropsida the quadrate occasionally gets the same
confusing title.
When the lower jaw is connected with the skull wholly by elements
of the hyoid arch, as in most Elasmobranchs and Ganoids, and all
Teleosteans, the term hyostylic is used. When the connection is due
to a quadrate element only, as in Amphibia and Sauropsida, it is
called autostylic. When there is both a hyoid and a quadrate element,
as in Lepzdosteus among Ganoids, or a hyoid and a palato-quadrate, as
in Cestracton among Elasmobranchs and perhaps also in Holocephali,
the term amphistylic is used. Finally, it may be noted here that in
Mammals the lower jaw articulates with the squamosal.
The first or mandibular arch gives origin inferiorly to Meckel’s
cartilage, which forms the basis and persistent core of the lower
jaw, and superiorly to the palato-pterygo-quadrate cartilage which
is represented in the adult by the minute quadrate bone, by the
suspensorial cartilage, and by other cartilages which are invested
by the pterygoid and palatine bones.
The second or hyoid arch gives origin inferiorly to the hyoid plate ;
superiorly, according to Parker, to the columella.
Of the four posterior branchial arches, there are in the adult some
persistent remnants, ¢.g. in the larynx.
The limbs and girdles.—The shoulder-girdle consists of
a dorsal portion—the scapula and the partially cartilagi-
nous supra-scapula, and of: a ventral portion—the coracoid
and the pre-coracoid. With the latter, according to most
authorities, a thin clavicle is associated. The glenoid
cavity, with which the humerus articulates, is formed by the
junction of scapula and coracoid.
Between the median ends of the coracoids lie two fused
cartilaginous epicoracoids, behind which is a bony part of
the sternum, prolonged posteriorly into a notched cartila-
ginous xiphisternum. Anteriorly lies a bony portion called
the omosternum, which is prolonged forwards into an epi-
sternum cartilage. This sternum does not arise like that of
higher Vertebrates, from a fusion of the ventral ends of ribs.
Indeed, there are no ribs in the frog, unless they be minute
rudiments at the ends of the transverse processes.
The true frogs (Ranidz) have what is called a firmdsternal pectoral
arch, in which precoracoid and coracoid nearly abut on the middle line,
and are only narrowly separated by the epicoracoids. In toads, tree-frogs,
etc., the arch is arcéferal, the precoracoid and coracoid being widely
separated medianly, and connected by a large arched epicoracoid, over-
lapping its fellow.
The skeleton of the fore-limb consists of an upper arm
Fic. 315.—Skeleton of frog. The half of the pectoral girdle, and
fore- and hind- limb of the right side are not shown.
gmx., premaxilla; mx., maxilla; ., nasal; s4%., sphenethmoid ;
PS, Pparieto-frontal; P.O., pecs 24., pterygoid; g.7.,
a
quadrato-jugal ; sg., squamosal; Q., quadrate; ¢., columella
auris ; 4., atlas ; ¢.g., transverse process ; S.7., sacral vertebra 3
U., urostyle ; S.sc., supra scapula; H., humerus; 2.U., radio
ulna; C%., carpals ; Mc., metacarpals ; /2,, ilium ; Zs., ischium ;
f., femur; 7./., tibio-fibula; Ca., calcaneum; As., astra-
galus; C., calcar; .4¢., metatarsals.
586. AMPHIBIA.
or humerus, a fore-arm in which the inner radius and the
outer ulna are fused, a wrist or carpus including two
Fic. 316.—Pectoral girdle of Rana esculenta.
—After Ecker.
The cartilaginous parts are dotted. Z%., Episternum ; 07., omo-
sternum; £%.c., epicoracoids ; sz., sternum $ Bey xiphisternum 3
cl., clavicle with underlying precoracoid cartilage 3 3 €0., Cora
coid ; 3 Se., scapula; S.sc., supra-scapula; GZ, glenoid cavity
for humerus.
proximal and three distal elements, and a central piece
wedged in between them, five metacarpal bones, of which
the first—corresponding to the absent thumb—is very
Fic. 317.—Side view of frog’s pelvis. —After Ecker.
i2., Uium; Zs., ischium ; Pd., pubis; 4c., acetabulum.
small, and four fingers, of which the two innermost have
two joints or phalanges, while the two others have three.
The pelvic girdle is shaped like a V, or like a pair of
tongs. The ends are cartilaginous and articulate with the
THE LIMBS AND GIRDLES. 587
expanded transverse processes of the ninth or sacral
vertebra. Each limb of the V is an ilium; the united
posterior part consists of a fused pair of ischia, and a ventral
cartilaginous pubic portion. Ilium, ischium, and pubis unite
in bounding the deep socket or acetabulum with which the
femur articulates. :
The skeleton of the hind-limb consists of a thigh bone or
femur, a lower leg formed from the united tibia and fibula,
an ankle region or tarsus including two long proximal
elements—the astragalus or tibiale and the calcaneum or
Fic. 318.—Brain of frog.—After Wiedersheim.
I. DorsAL AspEcT.—o.2., Olfactory ‘lobes; ¢.4., cerebral hemi-
spheres; P., pineal body, rising from region of optic thalami ;
op.i., optic lobes ; cd., rudimentary cerebellum; JZ.0., medulla
oblongata.
IL. Ventrat Asrect.—The numbers indicate the origins of the
nerves. ch., Optic chiasma; Z.c., tuber cinereum (infundib-
ulum); Z., hypophysis.
Ill. Horizontal SECTION.—Zz. 4y I and 2, lateral ventricles of
cerebrum ; 7.72. foramen of Monro; V., 3 and 4, third and
fourth ventricles ; Aq., cavities of optic lobes and aqueduct of
Sylvius from third to fourth ventricle.
fibulare—and three imperfectly ossified distal elements, five
métatarsal bones, and’ five toes.- The first toe or hallux
has two phalanges, the second also two, the third three, the
fourth four, the fifth three, and, finally, outside the hallux
there is a “calcar,” which looks like an extra toe, and con-
sists of three pieces. The astragalus is in line with the first
toe. The long bones of the skeleton show readily separable
calcified terminal caps.
588 AMPHIBIA.
Muscular system.—The muscles are enswathed in con-
nective tissue. They consist of bundles of striated fibres,
and at their ends or at one of them they are usually con-
tinued into’ tendons,
which are more or
less directly attached
to parts of the skele-
ton.
For an account of
the musculature of
Vertebrate types,
the student is re-
ferred to the
guides to practical
work cited in the
Appendix.
Nervous system.—
The brain, covered
with a darkly pig-
mented pia mater,
has the usual five
parts.
The elongated
cerebral hemi-
spheres have
“olfactory lobes”
in front of them,
and are con-
nected by an-
terior and
ByMbne eof
A
Fic. 319.—Nervous system of frog.—After posterior com-
Ecker. ee missures, and
1-10, The cranial nerves ; oc., eyes; cvd., in front of 7
optic chiasma; /o., optic’ tract’; syit., sympa- by a hint of a
thetic system; #zsf., spinal cord; sé., spinal oF corpus cal-
nerves.
losum ” (?).
The thalamencephalon gives origin dorsally to a pineal
outgrowth. The pineal body lies outside the skull in
the tadpole, but is partially atrophied in the adult, so
that little more than the stalk is left. On the ventral
side will be seen the chiasma or interlaced crossing of
the optic nerves, and a tongue-shaped mass (the tuber
SENSE ORGANS. 589
cinereum or infundibulum), to which the pituitary body
_ is attached. :
The optic lobes, a pair of oval bodies, between and
below which is the iter.
The cerebellum, a very narrow transverse band.
The medulla oblongata, on the roof of which the pia
mater forms a very vascular “ choroid plexus.”
The cavities of the brain and the canal of the spinal cord
are in the adult lined by ciliated epithelium. _
The cranial nerves are, as usual, on each side the
following :—
(1) Olfactory, from the olfactory lobe to the nose ;
(2) Optic, crossing and interlacing with its fellow ;
(3) Oculomotor, to four muscles of the eye ;
(4) Pathetic, to the superior oblique eye muscle ;
(5) Trigeminal, with ophthalmic, maxillary, and mandibular branches ;
(6) Abducens, to the external rectus eye muscle ;
(7) Facial, arising along with the auditory, with a ganglion uniting
with the Gasserian ganglion of,the trigeminal, with a palatine
branch to the roof of the mouth, and a hyoid branch to the
lower jaw ;
(8) Auditory, to the ear ;
(9) Glossopharyngeal, to the tongue and some of its muscles; with
a ganglion which unites with that of the tenth ;
(10) Vagus, with branches to lungs, heart, stomach, etc.
The spinal cord gives origin to ten pairs.of spinal nerves, and is
swollen at the origin of those which go to the limbs. Around the
union of the anterior and posterior roots lie sacs with crystals of
carbonate of lime.
The sympathetic system consists of about ten pairs of ganglia—(a)
united by branches to the spinal nerves; (4) united to one another by
longitudinal trunks which accompany the dorsal aorta and the systemic
arches, and end anteriorly in the Gasserian ganglion; (c) giving off
branches to the heart, the aorta, and the viscera in the pelvic region.
Sense organs.—The eyes project on the top of the head
and on the roof the mouth. There is a third eyelid.
The transparent cornea in front, the firm sclerotic surround-
ing the eyeball, and the sheath of the optic nerve, are as
usual continuous. The next layer includes the vascular
and pigmented choroid and the brilliant iris. Internally is
the sensitive retina, while vitreous humour fills the cavity
behind the lens.
The internal ears have the usual parts, and lie within the
auditory capsules, which are in great part bounded by the
590 _ AMPHIBIA.
pro-otics. Connecting the fenestra ovalis of the ear with
the tympanic membrane, which is flush with the skin, there
is a delicate bony rod—the columella. This lies in the
Eustachian tube, which opens into the mouth at the corner
of the gape.
The nostrils open into small nasal cavities, with folded
walls of sensitive membrane; the posterior nares open into
the front of the mouth.
There are taste papilla on the tongue, and touch-spots
on the skin.
Alimentary system.—The frog feeds in great part on
insects, which it catches dexterously with its tongue. This
is fixed in front and loose behind. There are teeth on the
premaxille, maxilla, and vomers. Into the cavity of the
mouth the nasal sacs open anteriorly, and the Eustachian
tubes posteriorly. The males of Rana esculenta have a pair
of resonating sacs which open into the mouth cavity at the
angle of the jaw, and are dilated during croaking. The
tongue bears numerous taste papille. Behind the tongue
on the floor of the mouth is the glottis, the opening of the
short larynx which leads to the lungs. The larynx is sup-
ported by two arytenoid cartilages, and also by a ring; with
the arytenoids the vocal cords are closely associated. The
lungs lie so near the mouth that laryngeal, tracheal, and
bronchial regions are hardly distinguishable. On the floor
of the mouth is the hyoid cartilage, which serves for the
insertion of muscles to tongue, etc.
Of the (4) gill-clefts which are borne on the walls of the
pharynx in the tadpole, there are no distinct traces in the
adult. The lungs develop as outgrowths from the gullet.
The gullet leads into a tubular stomach, which is not
sharply separated from it. There is a pyloric constriction
dividing the stomach from the duodenum, or first part of
the small intestine. After several coils the small intestine
opens into the wider large intestine or rectum, which enters
the cloaca.
The liver has a right and a left lobe, the latter again sub-
divided. ‘The gall-bladder lies between the right and left
lobes; bile flows into it from the liver by a number of
hepatic ducts, which are continued onwards to the duodenum
ina common bile-duct. The pancreas lies in the mesentery
VASCULAR SYSTEM. 591
“between stomach and duodenum, and its secretion enters
the distal portion of the bile-duct. The bladder is a ventral
-outgrowth of the cloaca, has no connection with the ureters,
and seems to bé homologous with the allantois of Reptiles,
Birds, and Mammals. ,
Vascular system.—The heart, enclosed in a pericardium,
is three-chambered, consisting of a muscular conical ven-
tricle, which drives the blood to the body and the lungs, of
a thin-walled right auricle receiving impure blood from the
body, and of a thin-walled left auricle receiving purified
blood fromthe lungs. From each of the auricles blood
enters the ventricle. The two superior vene cave which
bring back blood from the anterior regions of the body, and
the inferior: vena cava which brings back blood from the
posterior parts, unite on the. dorsal surface of the heart in a
thin-walled sinus venosus, which serves as a porch to the
right auricle. From the ventricle the blood is driven up a
truncus arteriosus, which is at first single (the py/angium)
and then multiple (the syzangium).
Thus we may distinguish five regions in the heart,—the ventricle,
the right auricle, the left auricle, the sinus venosus, and the truncus
arteriosus. The sinus venosus is the hindmost, the truncus arteriosus
the most anterior part. The opening of the pylangium into the
ventricle is guarded by two semilunar valves ; the cavity of the pylangium
is incompletely divided by a longitudinal valve; there are also valves
separating pylangium from synangium, and in the cavity of the latter.
The complex mechanism is interesting because it determines the course
of the blood: leaving the ventricle. The truncus arteriosus corresponds,
in part at least, to the conus arteriosus of many fishes.
As the heart continues to live after the frog is really dead, its contrac-
tions can be readily observed. The sinus venosus contracts first, then
the two auricles simultaneously, and finally the ventricle. Although
the ventricle receives both impure and pure blood, the structural ar- -
rangements are such that most of the impure blood jis driven to the
lungs, the purest blood to the head, and somewhat mixed blood to
the body.
The blood contains in its fluid plasma—(a) the oval
“red” corpuscles, with a definite rind, a distinct nucleus,
and the pigment hemoglobin; (4) white corpuscles or
leucocytes, like small amcebe in form and movements ;
(c) very minute bodies, usually colourless and variable in
shape. When the blood clots, the plasma becomes a
colourless serum, traversed by coagulated fibrin filaments,
592 AMPHIBIA,
the red corpuscles often arrange themselves in rows, and
the white corpuscles are entangled in the coagulated shreds.
When the web of a living frog is examined under the micro-
scope, it will be seen that the flow of blood is most rapid in
Fic. 320,—Arterial system of frog.
Z, Lingual; c., carotid; s., systemic; cz., cutaneous; Z., pulmon-
ary; v., occipito-vertebral; 47, brachial; c.m#., cceliaco-
mesenteric ; ~., renal ; 7/., common iliacs ; 4., hemorrhoidal.
the arteries, more sluggish in the veins, most sluggish in the
capillaries or fine branches which connect the arteries and
the veins, The red corpuscles are swept along most rapidly,
and are often deformed by pressure; the leucocytes tend to
ARTERIAL SYSTEM. 593
‘cling to the walls of the capillaries, and may indeed pass
through them (diapedesis).
The arterial system.—Each branch of the truncus arteri-
osus is triple, and divides into three arches :—
Fic. 321.—Venous system of frog. -
m., 1., Mandibular and lingual ; ¢.7., external jugular 3 2.7., internal
jugular ; scf., subscapular ; z7., innominate ; sc/., subclavian;
ér., brachial; m.c., musculo-cutaneous; %.v., hepatic vein;
4.p., hepatic portal; @.a., anterior abdominal; ~4., renal-
portal ; 4.v., pelvic; sc., sciatic ; 4, femoral ; z.v.¢., inferior vena
Cava; ¢., cardiac vein.
I. The carotid arch, the most anterior, corresponding to
the first efferent branchial of the tadpole, gives off—
A lingual artery to the tongue ;
A carotid artery, which bears near the origin of the lingual a
spongy swelling (the ‘‘carotid gland”), and gives off an
38
594 AMPHIBIA.
external carotid to the mouth and the orbit, and an internal
carotid to the brain.
II. The systemic arch, the median one of the three,
corresponding to the second efferent branchial in the
tadpole, gives off—
The laryngeal artery to the larynx ;
The cesophageal to the cesophagus ;
The occipito-vertebral to the head and vertebral column ;
The subclavian or brachial to the fore-limb.
From the left aortic arch, just as it unites with its fellow
of the other side to form the dorsal aorta, or from the’
beginning of the dorsal aorta, there is given off the cceliaco-
mesenteric to the stomach, intestine, liver, and spleen.
Farther back the dorsal aorta gives off—
The renal arteries to the kidneys, and the genital arteries to the
' reproductive organs; |
The inferior mesenteric to the large intestine.
Then it divides into two iliacs, each of which supplies the bladder
(hypogastric), the ventral body wall (epigastric), and the leg (sciatic).
III. The pulmocutaneous arch, the most posterior,
corresponding to the fourth efferent branchial in the
tadpole, gives off—
the cutaneous artery to the skin,
and the pulmonary artery to the lungs.
The venous system.—I. Each superior vena cava is
formed from the union of three veins, and each of these
three is formed from two smaller vessels.
External ace from the mouth and tongue.
jugular. Mandibular from the lower jaw.
Internal jugular from the inside of the skull.
Superior | Innominate. 4; Subscapular from the back of the arm and
vena cava. the shoulder.
Brachial from the arm.
Subclavian. + Musculo-cutaneous from the skin and sides
\ , of the body.
”
II. The inferior vena cava begins between the kidneys,
and ends in the sinus venosus.. Its components are as
follows :—
Inferior
Genital veins from the reproductive organs,
vena cava.
Efferent renal veins from the kidneys.
Efferent hepatic veins from the liver.
LYMPHATIC SYSTEM. 595
The renal portal system, by which venous blood from
the posterior region filters through the kidneys on its way
back to the heart, is as follows on each side :—
A posterior branch of the femoral vein from the
hind-limb forms the renal portal vein, which
receives the sciatic from the back of the leg, and
the dorso-lumbar veins from the dorsal wall of
the body, and oviducal veins in the female.
Renal portal
system.
The anterior branch of the femoral vein is called the
pelvic, and unites with its fellow of the opposite side, and
gives origin to a median vein which runs to the liver—the
anterior abdominal, By means of an anastomosing branch,
the anterior branch of the femoral is also connected to the
sciatic.
The hepatic portal system, by which venous blood from
the posterior region and from the gut passes through the
liver on its way back to the heart, is as follows :—
Anterior abdominal vein, from the union of the
se two pelvics, receiving tributaries from the
Hepatic portal bladder, ventral body wall, and _ truncus
system. arteriosus.
Hepatic portal vein, from the union of veins from
the stomach, intestine, and spleen.
III. The pulmonary veins, which bring back purified
blood from the lungs, unite just before they enter the left
auricle. There are numerous valves in the veins of the
frog.
Lymphatic system,.—The lymph is a colourless fluid, like blood
without red corpuscles. It is found m the spaces between the loose
skin and the subjacent muscles, in the pleuro-peritoneal cavity in which
heart, lungs, and other organs lie, in ‘a sub-vertebral sinus extending
afong the backbone, and in special lymphatic vessels which pass fatty
materials absorbed from the intestine’ into the venous system, There
are two pairs of contractile ‘‘lymph’ hearts” at two regions where the
lymphatic system communicates with the veins. A pair lie near the
posterior end of the urostyle; the other two lie between the transverse
processes of the: third and fourth vertebrae, Their pulsations can be
seen on the back of the living frog.
Mechanism of the heart.—The right half of the ventricle,
being nearer the right auricle, contains more impure blood, and it is
from the right side of the ventricle that the truncus arteriosus arises,
The middle of the ventricular cavity contains mixed blood. The
left corner contains pure blood received from the pulmonary veins.
596 AMPHIBIA,
The various valves and the conditions of pressure are such that the
venous blood passes by the pulmonary artery to the lungs, the next
quantum of blood enters the systemic arches, and the nearly pure
arterial blood from the left side of the ventricle passes into the carotids.
To understand the mechanism, it is necessary to consult some book
with a complete anatomical decenipten, especially Gaupp’s edition of
Ecker and Wiedersheim’s Anatomie des Frosches (1899).
Spleen, thyroid, and thymus.—The spleen is a small red
organ lying in the mesentery near the beginning of the large intestine.
The thyroid is represented by two little bodies near the roots of the
aortic arches. The thymus, perhaps originally associated with the
gill-clefts, lies on each side just behind the angle of the lower jaw.
Respiratory system.—The larval frog breathes at first
through its skin, then by gills. The adult frog breathes
chiefly by its lungs, but some cutaneous respiration is still
retained, for even without its lungs a frog may live for
some time, and it does not use them when hibernating.
The lungs arise as outgrowths of the cesophageal region
of the gut, and are connected with the back of the mouth
by a short laryngo-tracheal tube, whose slit-like aperture is
the glottis. Each lung is a transparent oval sac, with
muscle fibres in its walls. The cavity is lessened by the
spongy nature of the internal walls, which form numerous
little chambers bearing the fine branches of blood vessels.
In respiration the mouth is kept shut, and air passes in
and out through the nostrils. A frog will die of asphyxia
if its mouth be artificially kept open for a considerable
time. When the floor of the mouth is lowered, and the
buccal cavity thus increased, air passes in. When the
nostrils and the opening of the gullet are shut, and the
floor of the mouth at the same time raised, air is forced
through the glottis into the lungs. When the pressure on
the lungs is relaxed, and when the muscles of the sides of
the body contract, the air passes out. e
Excretory system.—The paired kidneys are elongated
organs situated dorsally and posteriorly beside the urostyle.
The waste products which they filter out of the blood pass
backward by two ureters which open separately on the
dorsal wall of the cloaca, and are not directly connected
with the bladder. The ureter or Wolffian duct is seen as
a white line along the outer side of each kidney; in the
male it functions also as the duct of the testis. On the
ventral surface of each kidney is a longitudinal yellowish
REPRODUCTIVE SYSTEM. >» 597
streak, the adrenal gland, and little spots mark ciliated
apertures or nephrostomes, which remain as communica-
tions between the abdominal cavity and the renal veins,
though they are originally connected with: the urinary
tubules. There are also, as in higher Vertebrates, open-
ings from the abdominal cavity into the lymphatic system.
female frog.—After Ecker.
f4b., Fatty bodies; v.c., vena cava; ovd., Opening of oviduct ; ov., ovary ;
T., testis; K., kidney; w.d., Wol- /.4., fatty body; X., kidney; U4, .
ffian duct ; c/., cloaca; B., bladder. uterus; Uyv., opening of ureters into
x cloaca (cZ.), in front of the openings
of the oviducts.
Reproductive system.—The males are distinguishable
from the females by the swollen cushions on the first fingers.
At the breeding season in spring, they trumpet to their
mates. The male clasps the female with his fore-limbs,
and retains his hold for several days, fertilising the ova as
they pass out into the water.
The paired testes are oval yellowish bodies lying in front
of the kidneys; the spermatozoa pass by vasa efferentia
598. AMPHIBIA,
through the anterior part of the kidney into the Wolffian
duct, which functions both as a ureter and as a vas deferens.
In the male of &. esculenta the vas deferens is.dilated for
some distance after leaving the kidney; in &. temporaria
it bears on the outer side near the cloaca a dilated glandular
mass or ‘‘seminal vesicle.” In the males, rudiments of the
Miillerian ducts are sometimes seen. In the male toad a
small rudimentary ovary, known as Bidder’s organ, occurs
at the anterior end of the testis.
The paired ovaries when mature are large plaited organs,
bearing numerous follicles or sacs containing the pigmented
ova. The spawn laid by a single frog may consist of several
thousand eggs. The ripe ova are liberated into the body
cavity, and moved anteriorly towards the heart, near which
the oviducts open. The movement of the ova is mainly
due to the action of peritoneal ciliated cells, which converge
towards the mouths of the oviducts, but partly to muscular
contraction, including the beating of the heart. The
oviducts are long convoluted tubes, anteriorly thin-walled
and straight, then glandular and coiled, terminally thin-
walled and dilated. In the median part the ova are
surrounded with jelly; the terminal uterine parts open on
the dorsal wall of the cloaca. In the females the Wolffian
ducts act solely as ureters. Attached to the anterior end of
the reproductive organs are yellow, lobed, ‘fatty bodies,”
largest in the males. It has been suggested that they
contain stores of reserve material, which is absorbed at
certain seasons. They seem to be fatty degenerations of
the anterior part of the genital ridges. The head kidney or
pronephros persists for some time in the embryo, but event-
ually degenerates. It does not seem to have anything to
do with the fatty bodies.
Development of the frog.—The ripe ovum exhibits
“polar differentiation”; its upper portion is deeply pig-
mented, the lower has no pigment and contains much yolk.
This yolk-containing hemisphere is’ the heavier, and conse-
quently is always the lower half of the egg, however this may
be turned about. Round the ovum there is a delicate
vitelline membrane, and this is again surrounded by a gela-
tinous investment which swells up in water. The formation
of polar bodies takes place before the liberation of the eggs.
* DEVELOPMENT OF THE FROG. 599
The spheres of jelly preserve the eggs and embryos from
friction, prevent their being eaten by most birds, appear to
be distasteful to Gammarids, and often enclose in their
interspaces groups of green Algz, which help in aeration.
The spheres may also be of use in relation to the absorption
and radiation of heat.
Fertilisation occurs immediately after the eggs are laid.
The spermatozoa, which exhibit the usual features of male
elements, work their way through the gelatinous envelopes,
and one fertilises each ovum.
The first cleavage is vertical, and divides the ovum into
a right and a left half. If one of these two cells be punc-
Fic. 324.—Division of frog’s ovum.—After Ecker.
The numbers indicate the number of cells or blastomeres.
tured, and the ovum be kept still, the other half will,
according to Roux, form a one-sided half-embryo. At
a certain stage Roux’s half-embryo regenerated the missing
half, usually by re-vitalising the remains of the cell which
was punctured. If the ovum be shaken about after punctur-
ing, a readjustment of material is effected, and a half-sized
embryo is formed (Morgan). The second cleavage is also
vertical, and at right angles to the first, dividing an anterior
from a posterior half. The third cleavage is equatorial, at
right angles to the first two, dividing the dorsal region from
the ventral.
The segmentation is total but unequal, and results in the
formation of a ball of cells, those of the upper hemisphere
being smaller and more numerous than the yolk-laden cells
600 AMPHIBIA, 2
below. Within there is a small segmentation cavity. Since
the presence of yolk acts as a check on the activity of the
protoplasm, we can understand why the smaller cells continue
to divide much more rapidly than the large yolk-containing
cells, and so how the smaller epiblastic cells gradually
spread over the egg, covering in the larger ones. At one
point, where upper and lower cells meet, a groove is
formed. This groove represents the dorsal lip of the
blastopore. It becomes crescentic and moves as a whole
down over the large yolk-cells. Invagination of the small
cells of the upper hemisphere goes on rapidly all round this
crescentic groove, and the archenteron is thus formed. The
horns of the cres-
cent meetata point
near the lower pole
of the egg to form
the ventral lip of
the blastopore.
The _ blastopore
now becomes re-
duced, by the in-
growing of its mar-
gins, to a small
Fic, 325.—Longitudinal vertical section of . a
frog embryo, shortly before closure of blasto- = C1 cular area which
pore.—After Ziegler’s model and Marshall. appears white, the
FB., fore-brain; EC., ectoderm; N., notochord; SC. j
canal of spinal cord ; NVE., neurenteric canal; B. colour being due
blastopore; J7., mesoderm cells; ¥., Yolk-laden toa plug of yolk-
cells; 4ZV., mesonteron; /., beginning of pituitary cells which almost
invagination.
obliterates its
opening. The whole egg now rotates backwards through
a little more than a right angle, so that the blastopore is
carried up into the position previously occupied by the first
trace of its dorsal lip. The blastopore now marks the
posterior end of the embryo. The archenteron has by this
time greatly enlarged, and has pushed the segmentation cavity
almost out of existence. The embryo elongates slightly, but
the mass of yolk-laden cells which lie on the floor of the gut
prevents the body acquiring at once the fish-like shape.
Along the mid-dorsal line the usual neural plate forms the
medullary canal. At the posterior end this communicates
with the archenteron for a time by the neurenteric canal.
DEVELOPMENT OF THE FROG,
601
Internally, a differentiation of hypoblast forms the notochord
along the mid-dorsal line of the
archenteron, At each side of
this lie masses of mesoblast which
have been split off from the hypo-
blast. Each of these divides into
the primitive segments (proto-
vertebre) above, and the un-
segmented lateral plates below.
The lateral plates split into two
layers, the splanchnic or inner
investing the gut, the somatic or
outer layer being applied to the
epiblast ; the space between the
two layers is the body cavity.
The body now becomes dis-
tinctly divided into regions, the
eyes bud out from the brain, a
rudiment of the gills appears,
and the larva, still within its
gelatinous case, exhibits peculiar
lashing movements of the tail.
~ Eventually, about a fortnight
after the eggs are laid, the larva
escapes from the surrounding
jelly and swims in the water. At
this stage and for some time the
ectoderm is ciliated. There is a
cloacal opening, but the mouth is
not yet more thana dimple. A
glandular crescent, often mis-
named a sucker, lies on the
under surface of the head, and
secretes a sticky slime, by means
of which the tadpole attaches
itself to foreign objects. The
protruding gills soon become
branched. -There are three of
them -on each side, the first the
largest. They are covered with
Fic. 326. — Dissection of
tadpole. — After Milnes
Marshall and Bles.
DL., Lower lip; #., ventricle of
heart; DZ., cesophagus; 1VA.,
head kidney; A.; aorta; K.,
kidney; AU., ureter; DO.,
cloaca; ZLH., hind-limb; XV.,
opening of ureter into cloaca >.
GR., genifal ridge; GF., fatty
body; L/., fore-limb; OG., gills ;
a, epidermis ; 4, dermis.
ectoderm, and are borne on the outside of the first
602 AMPHIBIA.
three branchial arches. The mouth, which has pre-
viously been merely a blind pit, opens into the gut,
the gut itself lengthens rapidly, and becomes coiled
like a watch-spring; the larve feed eagerly on vegetable
matter and increase in size. The glandular crescent forms .
two small discs, which gradually disappear as the power
of locomotion increases. About the time when the mouth
is opened, four gill clefts open from the pharynx to the
exterior.
A second period, the true tadpole stage, now begins.
A skin-fold or operculum covers the external gills,
which then atrophy, and are replaced by “internal”
gills developed on the ventral halves of four branchial
arches. These gills, though called internal, are covered
with ectoderm Wke their predecessors, and are com-
parable not to ordinary fish-gills, but to the external
gills of Polypterus, Protopterus, and Lepidosiren. The
mouth acquires horny jaws, and the fleshy lips bear
horny papilla. By the continued growth of the opercular
fold the gillchambers are closed, with the exception of
a single exhalant aperture on the left side. Through
this opening, the water which is taken in by the mouth
in respiration passes outwards, having washed the gills’
on its way.
In the third period the rudiments of the limbs appear.
The fore-limbs are concealed within the gill-chambers, and
so are not obvious until later; but the hind-legs may be
watched in the progress of development from small papillz
to the complete limb.
The lungs are developed as outgrowths from the ceso-
phagus, even before hatching, but grow very slowly.
After the appearance of the hind-legs, the larvee come to
the surface of the water to breathe, showing that the lungs
are now to some extent functional. At this stage the
tadpoles, now about two months old, are at the level of
Dipnoi.
The changes in the relations of the blood vessels, which
accompany the successive changes in the methods of
respiration, and render these possible, are somewhat com-
plicated.
When respiration is by the gills only, the circulation
DEVELOPMENT OF THE FROG. 603
is essentially that of a fish. From the two-chambered
heart the blood is driven by afferent branchials to the .
gills ; from these it collects in efferent vessels which
unite on each side to form two aorte. The aorta’*send
arteries to the head, and passing backwards unite to
form the single dorsal aorta which supplies the body.
For a time there are two dorsal aorte. When the first
set of gills is replaced by the second set, new gill-
capillaries are developed, but the circulation remains
the same. As in Cevatodus, a pulmonary artery arises
from the fourth efferent branchial. At the time when
the hind-legs begin to be developed, a direct com-
munication is established between afferent and efferent
branchial vessels, so that blood can pass from the heart
to the dorsal aorta without going through the gills.
As the pulmonary circulation becomes increasingly
important, the single auricle of the heart becomes.
divided into two by a septum, and the pulmonary veins
are established. At the time of the metamorphosis an
increasing quantity of blood avoids the gills in the manner
indicated above, and these, being thrown out of con-
nection with the rest of the body, soon atrophy, while
the lungs become the important respiratory organs. The
fate of the various branchial arteries is shown in the table
on the following page.
The tadpole has by this time grown large and strong,
feeding in great part on water-weeds. Now it seems to
fast, but the tail,.which begins to break up internally,
furnishes, with the help of phagocytes, some nourishment
to other parts of the body. The habit becomes less
active, the structural adaptations to the aquatic life
disappear. “The horny jaws are thrown off; the large
frilled lips shrink up; the mouth loses its rounded
suctorial form and becomes much wider; the tongue,
previously small, increases considerably in size; the eyes,
which as yet have been beneath the skin, become
exposed ; the fore-limbs appear, the left one being
pushed through the spout-like opening of the branchial
chamber, and the right one forcing its way through
the opercular fold, in which it leaves a ragged hole”
(Marshall).
604 AMPHIBIA,
!
SKELETAL CLEFTs. Aortic ARCHES AorTic ARCHES
ARCHES. : IN THE Empryo. IN THE ADULT.
2 ;
Mandibular. ay Late in develop- | Only a trace per-
ment vessels sists.
appear which re-
present a modifi-
cation of those of
a branchial arch.
Eustachian tube.
Hyoid. The arch is repre- | Disappears en-
sented in a less tirely.
modified form.
First cleft.
Carotid arch.
F.rst branchial. First branchial arch.
Second cleft.
Second branchial. Second i Systemic arch.
Third cleft.
Third branchial. Third a Atrophies.
Fourth cleft.
Fourth branchial. Fourth a Pulmo-cutaneous.
While these changes are in progress, and as the supply of
food afforded by the tail begins to be exhausted, the tadpole
recovers its appetite, but is now exclusively carnivorous,
feeding on any available animal matter, or even on its
fellows. The change is not, however, so great as it seems,
for even at a very early stage animal food is eagerly
devoured.
With the change of diet, the abdomen shrinks, stomach
and liver enlarge, the intestine becomes relatively narrower
and shorter. The- tail shortens moré and more, and as it
does so the disinclination for a purely aquatic life seems to
increase. Eventually it is completely absorbed, the hind-
Iimbs lengthen, and the conversion into a frog is completed.
In the reduction of the tail the epidermis thickens and is partly cast,
partly dissolved ; the muscles break up, and their substance undergoes
intracellular digestion or is dissolved in the body juices; the notochord
is repeatedly bent on itself and is also disrupted; the same is true of
nervous system and blood vessels. It is a pathological process which
has become normal. Some credit the phagocytes with playing a very
important part in the reduction of the tail; but others restrict their
function to engulfing solid particles, such as pigment granules, and say
that most of the material degenerates until it becomes almost liquid,
when it passes directly into the vascular fluid.
In many respects the development of the tadpole is very
interesting, especially because it is a modified recapitulation
CLASSIFICATION OF AMPHIBIA. 605
of that transition from aquatic to aerial respiration which
must have marked one of the most momentous epochs in
the evolution of Vertebrates.
Fic. 327.—Life history of a frog.—After Brehm.
1-3, Developing ova; 4, newly hatched forms hanging to water-
weeds; 5, 6, stages with external gills; 7-10, tadpoles during
emergence of limbs ; 11, tadpoles with both pairs of limbs appa-
rent ; 12, metamorphosis to frog.
CLASSIFICATION OF AMPHIBIA
Order ANURA or ECAUDATA
The adults have no tail or external gills or open gill-clefts, There
are always four limbs. z
Sub-order Phaneroglossa.—Tongue present; the Eustachian tubes
open separately into the pharynx. :
Series A. Arcifera (see p. 584), ¢.g. the toothless toads (Bz/fo) ;
the tree-frogs (Hy/a), with adhésive glandular discs
on the ends of the digits; the obstetric frog (Alyzes) ;
Bombinator, Pelobates, and others.
Series B, Firmisternia (see -p. 584), the frogs proper (Ranidz),
eg. the gtass-frog (R. temporaria), the edible
frog (R. esculenta), the N. American bull-fiog
(R. catesbiana), sometimes 8 in. in length, and
with a sonorous croak.
606 AMPHIBIA,
Sub-order Aglossa.—Tongueless; the Eustachian tubes have a
common median aperture into the pharynx. The Surinam
toad (Pipa americana), and the allied African genus
Xenopus,
Order URODELA or CAUDATA
The tail persists in adult life ; the larval gills and gill-slits may also
persist ; the limbs are weak when compared with those of Anura, and
the hind pair may be absent.
Family 1. Amphiumide.—The N. American Amphiuma, with two
pairs of rudimentary legs, with a slit persisting in adult life
as a remnant ofthe gilled state; Cryptobranchus maximus,
the largest living Amphibian, found in Japan and Thibet,
attains a length of over 3 ft. : }
Family 2. Salamandridse.—Sal/amandra maculosa and S. atra, both
European, both viviparous; the usually oviparous newts
—Tritton or Molge—of which Triton alpestris becomes
sexually mature while still larval (sedogenesis). Desmognathus
Jusca, the common /umgless water salamander of the United
States, lays its eggs in a wreath which the female twines
round its body. The N. American Ammdblystoma, with its
sometimes persistent larval form the Axolotl, formerly thought
to be a different species.
Family 3. Proteide.—With persistent gills. Several species of
Proteus inhabit the caves of Carinthia and Dalmatia. There
are two pairs of limbs. The eyes are degenerate-and the skin
white, as we should expect in cave-animals. Two species of
Necturus (or Menobranchus) occur in N. American rivers and
lakes.
Family 4. Sirenidee.—Two extant genera, Szven and Pseudobranchus,
both N. American, both with persistent gills, and only
anterior limbs. Papillae in the lower dermic layer in Szrez,
hidden by looser superficial dermis and epidermis, look like
vestiges of ancestral scales,
Order GYMNOPHIONA or APODA
Worm-like or snake-like forms, subterranean in habit; without
1imbs or girdles; with extremely short tail; with dermic calcified
scales concealed in transverse rows in the skin ;.in at least some forms
(Aypogeophis) external gills are present in the very young stages, but
disappear before hatching ; there may be no larval stage ; if there is, the
respiration is pulmonary, There are many other striking peculiarities :
—the eyes are small, covered up, and functionless; there is no
tympanum or tympanic cavity; there is « peculiar protrusible
tentacle in a pit behind the nostril; there are only two pairs of
aortic arches (systemic and pulmonary). The notochord is largely
persistent ; the vertebre are amphiccelous ; the frontals are distinct
LIFE OF AMPHIBIANS. 607
from the parietals; the palatines are fused with the maxille. The
eggs are large and meroblastic. They are altogether peculiar archaic
Amphibians. Examples :—Cacd/¢éa (S. America) ; Zchthyophis (Ceylon,
India, Malay); Aypogeophis (E. Africa); Siphonops, without scales
(America).
Order STEGOCEPHALI
Extinct forms, occurring from Carboniferous to Triassic strata,
The earliest known digitate animals,
Dermal armour is present, the teeth are frequently folded in a
complex manner (Labyrinthodonts). MJastodonsaurus, Dendrerpeton,
Archegosaurus, Branchiosaurus. :
/LIFE. OF AMPHIBIANS
Most Amphibians live in or near fresh-water ponds, swamps, and
marshes. They are fatally sensitive to salt. Even those adults which
have lost all trace of gills are
usually fond of water. The tree-
toads, such as Ay/a, are usually
arboreal in habit, while the
Gymnophiona and some toads
are subterranean.
The black salamander (Sa/a-
mandra atra) of the Alps lives
where pools of water are scarce,
and instead of bringing forth
gilled young, as its relative the
spotted salamander (S. mzaczlosa)
does, bears them as_ lung-
breathers, and only a pair at a
time. The unborn young have os .
gills which are pressed against Fic. 328.—Cecilian (Zchthyophis)
the vascular wall of the uterus. with eggs. —After Sarasin.
It is said that the respiration (and :
nutrition) of the young is helped by crowds of red blood corpuscles
which are discharged from the walls of the uterus; the débris of
unsuccessful eggs and embryos seems also to be used for food.
Species of Hylodes, such as A. martinicensis of the West Indian
Islands, live in regions where there are few pools. In such cases the
development is completed within the egg-case, and a lung-breathing
tailed larva is hatched in about fourteen days. a
In some Mexican and N. American lakes there is an interesting
amphibian known as Amélystoma or Siredon. It has two forms—one
losing its gills (Ambystoma), the other retaining them (Axolotl). Both
these forms reproduce, and both may occur in the same lake. Formerly
they were referred to different genera. But the fact that some
Axolotls kept in the Jardin des Plantes in Paris lost their gills when
their surroundings were allowed to become less moist than usual, led
608 AMPHIBIA.
naturalists to recognise that the two forms were but different phases of
one species. It has been shown repeatedly that a gilled Axolotl
may be transformed into a form without gills ; and this metamorphosis
seems to occur constantly in one of the Rocky Mountain lakes.
Abundant food and moisture favour the persistence of the Axolotl stage.
Amphibians are very defenceless, but their colours often conceal
them. Not a few have considerable power of colour-change. The
secretion of the'skin is often nauseous, and therefore protective. Ina few
cases, such as Ceratophrys dorsata, there is a bony shield on the back
made of a number of small pieces arising as ossifications of the inner
stratum of the dermis and of the subcutaneous connective tissue. It is
interesting to notice the occurrence of numerous hair-like filaments on the
sides and thighs of the males of a Kamerun frog (Astylosternus robustus).
Many Amphibians liye alone, but they usually congregate at the
breeding seasons, when the amorous males often croak noisily.. Alike
in their love and their hunger, they are most active in the twilight.
Their food usually consists of worms, insects, slugs, and other small
animals, but some of the larval forms are for a time vegetarian in diet.
They are able to survive. prolonged fasting, and many hibernate in
the mud. Though the familiar tales of ‘‘ toads within stones” are for
the most part inaccurate, there is no doubt that both frogs and toads
can survive prolonged imprisonment. Besides having great vital
tenacity, Amphibians have considerable power of repairing injuries to
the tail or limbs.
Although the life of Amphibians seems to have on an average a low
potential, even the most sluggish wake up in connection with re-
production. The males often differ from their mates in size and colour.
Some of their parental habits seem like strange experiments.
Thus in the Surinam toad (Pifa americana) the large eggs are
fertilised internally and placed by the everted cloaca of the female upon
the back, the male apparently helping in the process. The skin
becomes much changed—doubtless in response to the strange irritation
—and each fertilised ovum sinks into a little pocket, which is closed by
a gelatinous lid. In these pockets the embryos develop, perhaps ab-
sorbing some nutritive material from the skin. They are hatched as
miniature adults. In Mototrema the female has a dorsal pouch of skin
opening posteriorly, and within this tadpoles are hatched. In Rhzvo-
derma darwinzi the male carries the ova in his capacious croaking-sacs.
In the case of the obstetric toad (Alytes obstetricans), not uncommon in
some parts of the Continent, the male carries the strings of ova on his
back and about his hind-legs, buries himself in damp earth until the
development of the embryos is approaching completion, then plunges
into a pool, where he is freed from his living burden.
In the Anura the ova are fertilised by the male as they leave the
oviduct ; in most Urodela fertilisation is internal, sometimes by approxi-
mation of cloace, sometimes by means of complex spermatophores
which the male deposits in the water close to the female.
The eggs of the frog are laid in masses, each being surrounded by a
globe of jelly ; those of the toad are laid in long strings ; those of newts
are fixed singly to water-plants; those of some tree-toads, such as
Hylodes, are \aid on or under leaves in moist places.
LIFE OF AMPHIBIANS. 609
There are about goo living species of Amphibia, most of them tail-
less. All are averse to salt water, hence their absence from almost all
oceanic islands. The anura are well-nigh cosmopolitan; the Urodela
are almost limited to the temperate parts of the northern hemisphere.
History.—lIt is likely that Amphibians were derived from a Piscine
stock related to. the Dipnoi and perhaps also to the Crossopterygians.
The Stegocephali were the first pentadactyl animals (Lower Carboni-
ferous). Of living forms, the Gymnophiona are more old-fashioned
than the others. The modern types gradually appear in Tertiary
times. Some of the extinct forms were gigantic.
Huxley emphasised the following affinities between Amphibians and
Mammals :—The Amphibia, like Mammals, have two condyles on the
skull; the pectoral girdle of Mammals is as much amphibian as it is
sauropsidian ; the mammalian carpus is directly reducible to that of
Amphibians. In Amphibians only does the articular element of the
mandibular arch remain cartilaginous; the quadrate ossification is
small, and the squamosal extends down over it to the osseous elements
of the mandible, thus affording easy transition to the mammalian con-
dition of these parts. But Mammals are, on the whole, more nearly
related to Reptiles.
There are some remarkable affinities between the Stegocephali and
some of the extinct Reptiles, such as the Anomodonts, which in their
turn have affinities with Mammals.
39
CHAPTER XXIV
Ciass REPTILIA
CHELONIA. RHYNCHOCEPHALIA. LACERTILIA. OPHIDIA.
CrocopiniaA. Many ExTINcT ORDERS
THE diverse animals—Tortoises, Lizards, Snakes, Croco-
dilians, etc.—which are classed together as Reptiles, are
the modern representatives of those Vertebrates which first
became independent of the water, and began to possess the
dry land. While almost all Amphibians spend at least their
youth in the water, breathing by gills, this is not necessary
for Reptiles, in which embryonic respiration is secured by
a vascular foetal membrane known as the allantois. As in
still higher Vertebrates, gill-slits are present in the embryos ;
but they are not functional, and are without gills. Reptiles
seem to form among Vertebrates a great central assemblage,
like “ worms” among Invertebrates, more like a number of
classes than a single class, exhibiting close affinities with
Birds and Mammals, and more distant affinities with
Amphibians.
Reptiles, Birds, and Mammals are distinguished, as
Amniota, from Amphibians and Fishes, which are called
Anamnia, the terms referring to the presence or absence of
a protective foetal membrane—the amnion—with which
another, the allantois, is always associated. Among other
common characters the following may be noted :—the
generally terrestrial habit, the absence of gills, the absence
of a conus arteriosus, the breaking ‘up of the ventral aorta,
the presence of twelve cranial nerves, the importance of the
hyo-mandibular gill-cleft.
ICHTHYVOPSIDA, SAUROPSIDA, AND MAMMALIA. 611
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612
REPTILIA.
Some of the main contrasts between living Reptiles and
Birds are summarised in the following table :—
REPTILES.
Birps.
The exoskeleton consists of horny
epidermal scales, sometimes augmented
by bony dermal scutes. ,
The centra of the vertebrae are rarely
like those of birds.
When there is a sacrum, its vertebrze
(usually two in number) have large ex-
panded ribs with the ends of which the
ilia articulate.
The cartilaginous sternum may be-
come bony, but is not replaced by
membrane bones, unless perhaps in
Pterodactyls.
When there is an interclavicle or epi-
sternum, it remains distinct from the
clavicle and sternum.
The hand has more than three digits,
and at least the three radial digits are
clawed.
In living reptiles the ilia are prolonged
farther behind than in front of the aceta-
bulum ; the pubes slope downward and
forward; there are usually pubic and
ischiac symphyses.
There are often five toes; the tarsals
and the metatarsals remain distinct.
At least two aortic arches persist ;
only the Crocodilia have a structurally
four-chambered heart; more or less
mixed blood always goes to the pos-
terior body.
The body has approximately the tem-
perature of the surrounding medium.
The optic lobes lie on the upper
surface of the brain.
There is an outer covering of feathers,
and though there may be a few scales,
there are never scutes.
‘The centra of the cervical vertebra
| have usually a saddle-shaped terminal
curvature.
The two sacral vertebre have no
expanded ribs, they fuse with others
to form a long composite ‘‘synsacrum.”
The cartilaginous sternum is replaced
by membrane bone from several centres.
When there is an interclavicle, it is
confluent with the clavicles.
The hand has not more than three
digits, and at most two digits are
clawed. The fore-limbs are modified
as wings; some carpals fuse with the
fused metacarpals.
The ilia are greatly prolonged in front
of the acetabulum, the inner wall of
which is membranous. The pubes slope
backwards, parallel with the ischia;
only in Struthio is there a pubic
symphysis, only in Rhea is there an
ischiac one.
There are not more than four toes;
the proximal tarsals unite with the
tibia, forming a tibio-tarsus; the first
metatarsal if present is free, but the
three others are fused to one another
and to the distal tarsals, forming a
tarso-metatarsus.
There is but one aortic arch, to the
right; the heart is four-chambered ;
the blood sent to the body is purely
arterial.
The body temperature is high and
almost constant.
The optic lobes lie on the sides of the
brain.
The lungs have associated air-sacs.
The sutures between the bones of the
skull are usually obliterated at an early
stage.
The right ovary atrophies.
CHELONIA. 613
Order CuEtonia. Tortoises and Turtles
GENERAL CHARACTERS.—TZhe broad trunk is encased in
bones which form a dorsal and a ventral shield, within the
Fic. 330.—Skull of turtle.
§.0O., supra-occipital; PAR., parietal; #R., frontal; P.F., pre-
frontal; PO.F., post-frontal; SQ., squamosal; PALX., pre-
maxilla; M2X., maxilla; /., jugal; Q./., quadrato-jugal ; Q.,
quadrate; D., dentary; AW., angular; AR., articular; S.,
surangular.
shelter of which the head and nech, tail and limbs, can be
more or less retracted. The dorsal carapace ts usually formed
614 REPTILIA.
from—(a) the flattened neural spines (plus dermal scutes) ;
(b) expanded and more or less coalesced ribs (plus costal
dermal scutes); (c) a series of dermal marginal scutes around
the outer edge. In the Athece the dorsal vertebra and ribs
are not fused to the dermal plates which form the carapace.
The ventral shield or plastron ts formed of nine or so dermal
bones. There ts no sternum.
Overlapping, but not corresponding to the bony plates, there
are (except in Trionychia and Athece) epidermic horny plates
of “tortoise shell,” which, though very hard, are not without
sensitiveness, numerous nerves ending upon them.
The quadrate ts immovably untted
with the skull. There is only a
lower temporal arcade. The jaws
are covered by a horny sheath, and
are without teeth, though hints of
these have been seen in some em-
bryos. There is a single anterior
nasal opening. The scapular arch
is internal to the ribs. The limbs
are pentadactyl, but often in the
form of paddles.
The average life of Chelonians is
sluggish. Perhaps this is in part
due to the way in which the ribs
are lost in the carapace, for this
Fic, 331.—Carapace of yyust tend to make respiration less
tortoise. “active. The lungs are divided into
The dark contours ae these of a mumber of compartments.
Citas siesta of theme: . Tae slamcal aperture is usually
which have been removed. longitudinal, never transverse, the
copulatory organ is unpaired.
All are oviparous. The eggs have firm, usually calcareous,
shells.
Some Peculiarities in the Skeleton of Chelonia
The (10) dorsal vertebrae are without transverse or articular processes,
and along with the ribs are for the most part immovably fused in the
carapace. The tail and neck are the only flexible regions. There are
two sacral vertebrae.
The greater part of the dorsal shield is due to a coalescence of eight
ribs with eight costal plates derived from the dermis,
SKELETON OF CHELONIA. 615
Similarly, the median pieces are the result of fusion between median
dermal bones and the neural spines of the vertebre. The plastron
usually consists of nine dermal bones, and the three anterior pieces
perhaps represent clavicles and interclavicle (or episternum).
The eight cervical vertebrse have at most little rudiments of ribs, are
remarkably varied as regards their articular faces, and give the neck
many possibilities of motion, There are no lumbar vertebree.
The bones of the skull are immovably united; there is only a lower
temporal arcade, formed by jugal and quadrato-jugal; there are no
ossified alisphenoids, but downward prolongations of the large parietals
Fic. 332.—Pectoral girdle of a Chelonian.
G., Glenoid cavity; SC., scapula; P.C., procoracoid fused
to the scapula; C., coracoid; Z.C., epicoracoid cartil-
age; L., ligament. : ‘
take their place ; neither presphenoid nor orbitosphenoids are ossified ;
there are no distinct nasal bones in modern Chelonians, their place
being taken by the prefrontals ; the premaxille are very small; there
are no teeth. ; :
There is no sternum. The pectoral girdle on each side consists of a
ventral coracoid and a dorsal scapula attached to the carapace. The
‘ scapula bears an anterior process of large size, usually regarded as a
‘* precoracoid” or procoracoid. ;
The pelvic girdle consists of dorsal ilia attached to the carapace,
posterior ischia, and anterior pubes, with pre-pubic processes and an
epi-pubic cartilage. There is a pubic and an ischiac symphysis.
The girdles originally lie in front of, or behind the.ribs, but are over-
arched by the carapace in the course of its development.
616 REPTILIA.
Some Peculiarities in the Organs of Chelonia
In Chelonians and in all higher animals except serpents, there are
twelve cranial*nerves, for, in addition to the usual ten, a spinal
accessory to cervical muscles, and a hypoglossal to the tongue, are
ranked as the eleventh and twelfth.
The gullet of the turtle shows in great development what is hinted at
in others, long horny papillae pointing downwards ; it is probable that
these help to tear up the food (seaweed in the case of the turtle).
<p irfee Wee
GENS
Fic. 333.—Internal view of the plastron of the
Greek tortoise.
EP., Epiplastron (clavicle?); ENT, entoplastron (inter-
clavicle?); AYO., hyoplastron; H¥PO., hypo-
plastron; X/P/H1J., xiphiplastron.
The heart is three-chambered, but an incomplete septum divides the
ventricle into « right portion, from which the pulmonary arteries and
the left aortic arch arise, and a left portion, from which the right aortic
arch issues. From the right aortic arch, which contains more pure
blood than the left, the carotid and subclavian arteries are given off.
ae left aortic arch gives off the cceliac artery before it unites with the
right.
The lungs are attached to the, dorsal wall of the thorax, and have
only a ventral investment of peritoneum; each is divided into a series
CLASSIFICATION OF CHELONIA. 617
of compartments into which branches of the bronchus open. There is
a slight muscular “‘ diaphragm.” The filling and emptying of the lungs
is helped by the protrusion and retraction of the head and legs, but
there are also “‘ swallowing movements.” There are 1fo vocal chords,
but there is sometimes a feeble voice.
In the males, the kidney, the epididymis, and the testes lie adjacent
Fic. 334.—Scales on ventral surface of plastron of
: Greek tortoise.
G., Gular; H., humeral; P., pectoral; Ad., abdominal ;
f., femoral; A., anal; 4Z., marginal.
to one another on each side. The males have a grooved penis attached
to the anterior wall of the cloaca. There is a urinary bladder.
Classification of Chelonia
J. ATHEC&. Vertebrze and ribs free from carapace. Skull
without descending processes from parietals.
Sphargide, leathery-skinned turtles, with flexible carapace. Sphargis
(Dermatochelys) cortacea, the only living species, the largest modern
618
Chelonian, sometimes measuring 6 ft. in length.
REPTILIA.
It is widely, but
now sparsely, distributed in intertropical seas, and is said to be herbi-
vorous.
Fic. 335.—Internal view of tortoise skeleton.
#7, humerus; SC., scapula running dorsally; PC.,
recoracoid; C., coracoid; EC., epicoracoid carti-
age; P., pubis; /Z., ilium running dorsally to
sacral vertebrz ; /S.,ischium; DV., dorsal verte-
bre fused in-carapace; &., head of a rib; CEV.,
cervical vertebre free; CA V., caudal vertebre free.
World, but not in Australia. In
diet they are vegetarian. The
common tortoise (Zestudo greca)
and the nearly exterminated giant
tortoises of the Mascarene and
Galapagos Islands are good repre-
sentatives. The latter may reach
the age of 150 years.
Order RHYNCHOCEPHALIA
The only dving repre-
sentative of this “class” is
the New Zealand “Lizard”
or Tuatara — Hatteria
( Sphenodon) punctata. Lizard-
II. THECOPHORA.
Dorsal vertebrze and
ribs fused in the
carapace. Parietals
prolonged down-
wards. - Including
the following and
other families :—
Chelonide, marine
turtles, with fin-like feet,
and partially ossified
carapace. They occur in
intertropical seas, and
bury their soft-shelled
eggs on sandy shores.
The green turtle (Chelone
virides) is much esteemed
as food; the hawk’s-bill
turtle (Caretta imbricata)
furnishes much of the
commercial tortoise-shell,
Testudinidz, land tor-
toises, with convex per-
fectly ossified carapace.
and feet adapted for
walking. They are found
in the warmer regions of
both the Old and the New
FIG. 336.-—Dissection of Chelonian
heart.—After Huxley.
7.v., Right half of ventricle ; S., septum ;
4v., left half of ventricle; ~a@., right
auricle; Za., left auricle; d.ao., left
aortic arch; ».a@o., right aortic arch;
?.@., pulmonary arch.
RHYNCHOCEPHALIA, 619
like in appearance, it measures from one to two feet
in length, has a compressed crested tail, is dull olive-green
spotted with yellow above and whitish below.+ It is now
rare, but is preserved in some small islands off the New
Zealand coast. It lives in holes among the rocks or in
ee burrows, feeds on small animals, and is nocturnal in
abit.
Fic. 337-—Heart and associated vessels of tortoise.—After Nuhn.
y.a., Right auricle; superior vene cave (s.v.c.) and inferior vena
cava (z.v.c.) enter it. 7.v., Right half of ventricle ; pulmonary
arteries (#.a.) and left aortic arch (2.aa.) leave it ; ced., coeliac;
d.ao., dorsal aorta. Z.a., Left auricle; #.v., pulmonary veins
enter it. Zz., Left half of ventricle; right aortic arch (7.a0.),
giving off carotids (¢c.) and subclavians (s.c/.). 4s
The skull, unlike that of any lizard, has an ossified quadrato-jugal,
and therefore a complete infra-temporal arcade; the quadrate is firmly
united to’ pterygoid, squamosal, and quadrato-jugal; the pterygoids
meet the vomer and separate the palatines; there are teeth on the
palatine in a single longitudinal row, parallel with those on maxilla
and mandible, and the three sets seem to wear one another away;
there is also a single tooth on each side of a kind of beak formed by’
the premaxille ; the’nares are divided. i
The vertebree are amphiccelous or biconcave, as in geckos among
lizards and in many extinct Reptiles. Some of the ribs bear uncinate
620 REPTILIA.
processes, as in Birds ; as in crocodiles, there are numerous ‘‘ abdominal
ribs,” ossifications in the subcutaneous fibrous tissue of the abdomen.
The anterior end of the ‘‘ plastron” thus formed overlaps the posterior
end of the sternum. The inner ends of the clavicle rest on a median
episternum (interclavicle). ; .
The pineal or parietal eye, which reaches the skin on the top of the
head, is less degenerate than in other animals, retaining, for instance,
distinct traces of a complex retina,
Near the living Sphenodon,
the Permian Palgohatteria, the
Triassic Myperodapedon, and
some other important types may
be ranked. Along with these
may be included the remarkable
Proterosaurus from the Pérmian,
though Seeley establishes for it
a special order—Proterosauria,
as distinguished from Rhyncho-
“cephalia. According to Baur,
quoted by Nicholson and
Lydekker, ‘‘the Rhyncho-
cephalia, together with the
Proterosauria, to which they
are closely allied, are certainly
the most generalised group of
all Reptiles, and come nearest,
in many respects, to that order
of Reptiles from which all others.
took their origin.”
Order LacerTILia
Lizards
GENERAL CHARACTERS.
—The body ts usually well
stoniant covered with scales. In
BH., Body of the hyoid (basihyal) ; mast, both fore- and hind-
#., representing another part of the limbs are developed ana
pyle orehs Ae? Beco: = Dear clawed digits, but either
arch; P.C., posterior cot repre- pair or both pairs may be
senting the second branchial arch. absent. The pec toral and
pelvic girdles are always present, in rudiment at least.
There is a sternum and a V-shaped episternum. Unlike
snakes, lizards have non-expansible mouths. The maxilla,,
palatines, and plerygoids are fixed, and there is usually
a mandibular symphysis. There are almost always
Fic. 338.—Hyoid apparatus of a
LACERTILIA. 621
movable eyelids and external ear-openings. The teeth
are fused to the edge or to the ridge of the jaws, never
planted in sockets. The tongue, broad and short in some,
e.g. Geckos and Iguanas, long and terminally clubbed
in Chameleons, is oftenest a narrow bifid organ of touch.
9 10 “
Fic. 339.—Lateral view of brain of Hatteria punctata,
—After Osawa.
1-12, Cranial nerves; 4.¢., parietal eye; 4.g., pineal gland ; 0., 0 tic
lobe 5 Coy cerebellum; v., fourth ventricle 3 z., infundibulum
and pituitary body.
The opening of the cloaca ts transverse. There ts a urinary
bladder, corresponding to that of the frog, and a double pents.
Most ave oviparous, but in a few the eggs are hatched within
the body. They are usually active, agile animals, beautifully
and often protec- -
tively coloured.
The tail is readily
thrown off by a
veflex action ; lost
tails and even legs
may be regenerated. z
The food generally Fic. 340.—Hatteria or Sphenodon.— After
consists of insects, Hayek.
worms, and other small animals, but some prey upon larger
animals, and others are vegetarian. Most are terrestrial,
some arboreal, a few semi-aguatic, and there ts one marine
form. Lizards are most abundant in the tropus, and are
absent from very cold regions.
622 REPTILIA.
DESCRIPTION OF A LIZARD AS A TYPE OF REPTILES
The following description applies especially to the long-
tailed green lizard (Lacerta viridis), found abundantly in
Jersey, but, except in minor points, it will be found to apply
equally to the small British grey lizard (Lacerta agz/is) and
to the viviparous lizard (Lacerta vivipara) :—
Form and external features.—The depressed head is
separated from the body by a distinct neck, but the
posterior region of the body passes gradually into the
long tail, which is often mutilated in captured specimens.
Both fore- and hind-limbs are present, and both are fur-
nished with five clawed digits. Of the apertures of the
body, the large mouth is terminal, the external nares are
close to the end of the snout, and the cloacal. aperture is
a considerable transverse opening placed at the root of the
tail. There is no external ear, but the tympanic membrane
at either side is slightly depressed below the level of the
skin of the head. The eyes are furnished with both upper
and lower eyelids, and also with a nictitating membrane.
Skin.—As contrasted with that of the frog, the skin has
a distinct exoskeleton of epidermic scales, the external
covering of which is shed from time to time. In the head
region these exhibit a definite arrangement characteristic
of the species. With the presence of an exoskeleton we
must associate the absence of the numerous cutaneous
glands of the frog. Peculiar tubular ingrowths of epidermis
form a row of so-called “femoral glands,” which open by
pores on the ventral surface of the thigh. Their product
{débris of epidermic cells) is most obvious in the male at
pairing time. The structure of the skin is very similar to
that of the frog. Pigment is deposited here also in two
layers, of which the outer is greenish, the inner black.
Over the parietal foramen on the top of the skull the black
pigment is absent, the green only feebly represented ; in
this region, therefore, the skin is almost transparent. In
moulting—which means casting off the outermost portions
of the scales—there is a distension of the blood vessels and
a great increase of blood pressure.
Many lizards, such as the Chameeleons, exhibit in a remarkable degree
the power of rapidly changing the colour of their skin, This is due to
SKELETON OF LIZARD. 623
the fact that the protoplasm of the pigment cells contracts or expands
under nervous control. The change of colour is sometimes advan-
tageously protective, but it seems often to be merely a reflex symptom
of the nervous condition of the animals. ;
In many cases, ¢.g. in some of the skinks, in Anguzs, Heloderma,
there are minute dermal ossifications beneath the scales.
Skeleton.—The backbone consists of a variable number
of vertebree, and is divisible into cervical, dorsal, lumbar,
sacral, and caudal regions. Except the atlas and the last
caudal, all the vertebrae are proccelous, as in all living
Lacertilians except Geckos, where they are amphiccelous.
The atlas consists of three separate pieces; its centrum ossifies as
usual as the odontoid process of the axis. There are two sacral verte-
bree with large expanded sacral ribs. To the ventral surfaces of many
of the caudal vertebrze Y-shaped ‘‘chevron” bones are attached.
Across the centre of the caudal vertebra there extends a median
unossified zone ; it is in this region that separation takes place when a
startled lizard loses its tail.
The ribs are numerous, but only five reach the sternum.
The skull is well ossified, but in the region of the nares,
in the interorbital septum, etc., the primitive cartilaginous
brain-box persists. On the dorsal surface the bones exhibit
numerous impressions made by the epidermic scales, which
render it difficult to distinguish: the true sutures of the
bones. As in Reptiles in general, the brain-case is small
in comparison with the skull, and is largely covered by
investing bones, between some of which are spaces or fossz.
Two fused parietals with the rounded median ‘‘ parietal foramen,”
two frontals, and the two nasals, are the most important constituents of
the roof of the skull. Anteriorly, the’ premaxillee appear between the
nasals, while posteriorly the sickle-shaped squamosal is attached by a
suture to the parietal, and is overlapped by one of the two small supra-
temporal bones. The orbit is roofed by a series of small bones, of
which the anterior and posterior are respectively known as pre- and post-
frontal. :
On the floor of the adult skull there is a large basal bone, composed
of fused occipital and sphenoidal elements, and continued forward as a
slender bar (parasphenoid). This bone gives off two stout processes,
the basipterygoid processes, which articulate with the pterygoids. Each
pterygoid is connected posteriorly with the quadrate bone of the corre-
sponding side, and anteriorly with the palatine. From the union of
pterygoid and palatine, a stout os transversum or transpalatine extends
outwards to the maxilla. In front of the palatines lie the small vomers,
which, in their turn, articulate with the premaxilla and maxilla, both of
* Seg
624 REPTILIA.
which are furnished with small pointed teeth. In the posterior region
of the skull we have still to notice the large ex-occipitals with which the
opisthotics are fused, and which are continued into the conspicuous
parotic processes. The lateral walls of the brain-case are largely formed
by the paired pro-otics. Internally, an important bone, the epipterygoid
or “columella” (not to be confounded with the columella or stapes of
the ear), extends from the pro-otic to the pterygoid. The orbit is
Fic. 341.—Side view of skull of Zacerta.—After W. K. Parker.
px., Premaxilla; x., maxilla; 2, lachrymal; 7., jugal; ¢.42.,
transpalatine ; efg., epipterygoid 3 Ag, pterygoid; dfg., basi-
pterygoid; d.0., basioccipital; g., quadrate; oc.c., occipital
condyle ; sg., squamosal ; v.0., pro-otic ; A¢.0., postorbital ; s¢.z,
st.2, supratemporals ; Zs., presphenoid: (the optic nerve is seen
issuing in front of the end of the reference line); Z.¢., mes-
ethmoid ; s.oJ,, supraorbitals; 4%, prefrontal ;, 2., nasal ; a~.,
articular ; ag., angular ; sag., surangular; cv., coronary; @.,
dentary.
bounded posteriorly and inferiorly by the jugals. There is no ossified
quadrato-jugal, and thus the lateral temporal fossa is open below in the
dried skull (contrast Ha¢terza). The other fossce of the dried skull are
the supra-temporal on the upper surface, and the posterior-temporal on
the posterior surface.
Each half of the lower jaw is composed of six bones, which use in
the adult. The two rami are sutured to one another in front.
VASCULAR . SYSTEM. 625
Limbs. and girdles.—In the shoulder- girdle, the flat
coracoids, with an anterior precoracoid region, articulate
with the sternum, which is represented by a cartilaginous
plate of rhomboidal shape. Over it projects the long limb
of the T-shaped episternum or interclavicle (a membrane
bone), which, at the sides, is continued outwards by the
curved clavicles, The remaining elements are the scapula,
which are continuous with the cartilaginous supra-scapule.
The fore-limbs have the usual parts.. In the carpus all
the typical nine bones are represented, and there is in
addition an accessory “ pisiform” bone.
In the pelvic girdle, ilium, pubis, and ischium are repre-
sented as usual ; there are both pubic and ischiac symphyses.
In the tarsus the fibulare and tibiale are united, and the
distal row consists of only two bones.
Nervous system.—The brain consists of the usual parts.
The cerebellum is small and only partially overlaps the
fourth ventricle. In the region of the thalamus the epiphysis
is distinct and conspicuous, but in the adult the pineal body
is quite separated from it, and lies in its connective tissue
capsule below the skin.
Alimentary system.—Small pointed teeth are present on
the maxilla, premaxille, palatines, and on the lower jaw.
They are fixed without sockets inside the edge of the jaw-
bones (pleurodont) ; in many Lacertilians they are implanted
along the ridge (acrodont). Salivary glands occur on the
floor of the mouth cavity. The narrow gullet passes
gradually into the muscular stomach, which again passes
into the coiled small intestine. Near the commencement
of the large intestine there isa small cecum. A voluminous
liver, with a gall-bladder embedded in it, and a pancreas,
are present as usual.
Embedded in the mesentery below the stomach lies the
rounded spleen. A whitish thyroid gland lies on the ventral
surface of the trachea a short distance in front of the heart.
Vascular system.—The heart is completely enveloped by
the pericardium, and is three-chambered, consisting of two
thin-walled auricles and a muscular ventricle. From the
ventral surface of the ventricle arises the conspicuous
truncus arteriosus, which is formed by the bases of the
aortic arches, and exhibits a division into two parts. From
40
626 REPTILIA.
the more ventral part arises the left aortic arch, which
curves round to the left side, first giving off a short connect-
ing vessel (ductus Botallii) to the carotid arch. From the
other division of the truncus arteriosus, a great arterial
trunk arises, and this gives off the right aortic arch and the
right and left carotid arches. The right aortic arch sends a
ductus Botallit to the carotid arch of the right side, and then
curves round the heart to join the left arch, the two thus
2 av
Tic. 342.—Heart and associated vessels of a lizard.
—After Nuhn.
A., Right auricle; jugulars (/.), subclavians (Sc.v.), and inferior
vena cava (/.V.C.) enter it, V., ventricle; ¢., truncus arteri-
osus ; 1, first aortic arch giving off carotids; 2, second aortic
arch; Z.a., pulmonary artery; Sc.a., subclavian artery ; Ao.,
dorsal aorta. A.1, left auricle; pulmonary veins (4,.v.) enter
it. In the lizard described, the left jugular is not developed.
forming the dorsal aorta. The carotid arches supply the head
region with blood. From the base of the truncus arteriosus,
the right and left pulmonary arteries also arise (Fig. 342).
From the right aortic arch as it curves round, arise the right and left
subclavian arteries, which carry blood to the fore-limbs. A cceliaco-
mesenteric artery arises from the dorsal aorta and supplies the viscera.
Smaller vessels are also given off to the genital organs, etc., and then at
the anterior end of the kidneys the aorta divides into two femoral
arteries, which break up into a network of small vessels, supplying hind-
RESPIRATORY SYSTEM 627
limbs and kidneys, and finally, at the posterior end of the kidneys,
reunite to form the caudal artery, which runs down the tail.
The blood from the anterior region of the body is returned to the
heart by the right and left precaval veins or superior venze cavee. The
right precaval is formed by the junction of external and internal
jugulars with the subclavian vein ; on the left side the jugular is absent.
From the posterior region of the body, blood is brought back by the
postcaval vein or inferior vena cava. The three great veins open into a
thin-walled sinus venosus, which opens into the right auricle.
The postcaval is formed by the union of two veins which run along
the genital organs, and receive renal veins from the kidneys. In pass-
ing through’the liver the postcaval receives important hepatic veins.
From the tail region the blood is brought back by a caudal, which
bifurcates in the region of the kidneys into two pelvics. The pelvic
veins give off renal-portals to the kidneys, and receive the femoral and
sciatic veins from the hind-limbs. They then unite to form the
Fic. 343.—Lung of Chamaleo vulgaris, showing air-sacs.
—After Wiedersheim.
epigastric or anterior abdominal, which carries blood to the liver.
Except through the medium of the renal-portal system, there is no
connection between the anterior abdominal and the postcaval. To the
liver, blood is carried as usual from the stomach, etc., by the portal
vein. ; .
From the lungs blood is brought to the left auricle by the pulmonary
veins.
There is, as usual, a lymphatic system, including a pair of lymph hearts.
Respiratory system. — The lungs are elongated oval
structures which taper away posteriorly. The mouth does
not, as in the frog, play an important part in the respiratory
movements. In some lizards (Chameleon and Geckos) the
lungs are prolonged in air-sacs, suggesting those of Birds
(Fig. 343). . ioe
Excretory system. — The paired kidneys lie in the
extreme posterior region of the abdominal cavity, and
628 REPTILIA
extend a little farther back than the level of the cloaca.
Each is furnished with a very short ureter. In the male the
ureters unite with the vasa deferentia; in the female they
open separately into the cloaca. Into the cloaca opens also
a large thin-walled “urinary bladder”; this is a remnant of
the foetal allantois, and has no functional connection with
excretion. The urine is semi-solid, and consists largely of
uric acid.
Reproductive system.—In the male the testes are two
white oval bodies suspended in a dorsal fold of mesentery.
Along the inner surface of each runs the epididymis, which
receives the vasa efferentia, and is continuous posteriorly
with the vas deferens. The two vasa deferentia, after
receiving the ureters, open by small papillee into the cloaca.
In connection with the cloaca there is a pair of eversible_
copulatory organs, postero-lateral in position.
In the female the ovaries occupy a similar position to
that of the testes in the male. The oviducts open far
forward by wide ciliated funnels; as they pass backward
they show a gradual increase in cross-section, but there is
no line of demarcation between oviducal and uterine
portions. Posteriorly, the oviducts open into the cloaca.
The right reproductive organ (ends to be larger and in front of the
left. In many of the males the Wolffian body is well developed.
Viviparous, or what is clumsily called ovo-viviparous, parturition is
well illustrated by Lacerta vivipara, Anguts fragilis, Seps, etc., but
most lay eggs with more or less calcareous shells. In Zrachydosaurus
and Cyc/odus the embryo seems to absorb food from the wall of the
uterus. It is likely that Lacertilians existed in Permian ages, but their
remains are not numerous before the Tertiary strata.
Many instructive illustrations of evolutionary change are afforded by
lizards. Thus there are numerous gradations in the reduction of the
limbs, from a decrease in the toes to entire absence of limbs. The
diverse forms of tongue and the varied positions of the teeth are also
connected by gradations. From the variations of the wall-lizard
(Lacerta muralis), Eimer elaborated most of his theory of evolution.
Some Families of Lacertilia
In the Geckos (Geckonidze) the vertebrze are biconcave or amphi-
ccelous, the tongue is short and fleshy, the eyelids are rudimentary, the
tecth are pleurodont, the toes bear numerous plaits, by means of which
they adhere to smooth surfaces, e.g. Platydactylus. :
OPHIDIA, 629
The Agamas (Agamidz) are acrodont lizards common in the Eastern
hemisphere. Examples.—4Agama; Draco, with the skin extended on
long prolongations of five or six posterior ribs; Chlamydosaurus, an
Australian lizard, with a large scaled frill around the neck ; Moloch,
another Australian form bristling with sharp spikes.
Iguanas (Iguanide) are pleurodont lizards, represented in the
warmer parts of the New World. Examples.—/guana, an arboreal
lizard, with a large distensible dewlap ; Amélyrhynchus or Oreocephalus
cristatus, a marine lizard confined to the Galapagos Islands; Axo/zs,
the American chameleon, with powers of rapid colour-change ;
Phrynosoma, the American ‘‘horned toad,” with numerous horny
scales, and a collar of sharp spines suggesting in miniature that of
some of the extinct Reptiles.
The slow-worms (Anguidze) are limbless lizards, with serpentine
- body, long tail, rudimentary girdles and sternum. The British Anguzs
fragzlzs is not blind or poisonous, as popularly asserted ; the tail breaks
readily ; the young are hatched within the mother.
The poisonous Mexican and Arizona lizards (Heloderma horridum
and A. suspectum) are over a foot in length, and are covered with
bead-like scales,
The Varanide are large carnivorous forms, most at home in Africa, but
represented also in Asia and Australia. The Monitor of the Nile, Vaxanus
niloticus, 5 or 6 ft. long, destroys eggs and young of Crocodiles.
The Amphisbzenide are degenerate subterranean lizards, without
limbs, with rudimentary girdles, with no sternum, with small covered
eyes, with hardly any scales.
The Lacertidz are Old World pleurodont lizards, such as Pseudopus
(Europe and 8. Asia) and Lacerta virddds, the green lizard of Jersey and
S. Europe.
The Chameleons (Chameleontide) are very divergent lizards,
mostly African. There is one genus, Chameleo. The head and the
body are compressed ; the scales are minute; the eyes are very large
and separately movable, with circular eyelids pierced by a hole; the
tympanum is hidden; the tongue is club-shaped and viscid; the
digits are divided into two sets, and well adapted for prehension ; the
tail is prehensile; the power of colour-change is remarkably
developed.
The Chameleons exhibit numerous anatomical peculiarities. As in
the Amphisbeenas, there is no epipterygoid. The pterygoid does not
directly articulate with the quadrate, which is ankylosed to the adjacent
bones of the skull.
Order Opuipia. Serpents or Snakes
The elongated limbless form of snakes seems at first sight
almost enough to define this order from other Reptiles, but
it must be carefully noticed that there are limbless Lizards,
limbless Amphibians, and limbless Fishes, which resemble
snakes in shape though they are very different in internal
630 REPTILIA.
structure. For the external shape is in great part an adapta-
tion to the mode of life, to the habit of creeping through
crevices or among obstacles. But the limblessness of
serpents is not a merely superficial abortion; there is no
pectoral girdle nor sternum, and never more than a hint of
a pelvis.
GENERAL CHARACTERS.— Zhe skin ts covered with scales,
and the outermost epidermal layer ts periodically shed in
a continuous slough.
There are never any hints of anterior appendages, girdles,
Fic. 344.—-Anterior view of Fie. 345.—Posterior view of
Python’s vertebra. Python’s vertebra.
N.SP., neural spine ; ZS., ZYZor ZA., zygantrum, a double cavity
sphene (a projecting wedge); PR.Z., for the zygosphene; P7.Z., post-
pre-zygapophysis (mooth articular zygapophysis (smooth articular
surface seen from above); &., surface seen from below); 7.P.,
articulation-surface for a rib; HY., transverse process ; C., centrum.
hypapophysis.
sternum, or episternum,; but in pythons, boas, and a few
others, there are rudiments of a pelvis, and even small clawed
structures which represent hind-legs.
The mouth is expansible; maxilla, palatines, pterygotds,
and guadrates are movable; and the rami of the mandible
are connected only by elastic ligament. The teeth are fused to
the jaws ; there are no movable eyelids. Snakes have no
external ear openings nor drum, nor tympanic cavity, nor
Eustachian tube. The nostrils le near the tip of the
head,
GENERAL NOTES ON SNAKES. 631
The bifid, mobile, retractile tongue ts a specialised organ of
touch. tn the mouth there is often a poison gland, which is a
specialised salivary gland.
There are many peculiarities in the skeleton. The numerous
vertebre are all procelous.
The brain has only ten nerves.
The heart is three-chambered, the ventricular septum being
incomplete, as in all Reptiles except Crocodilians.
There is a transverse cloacal aperture. In the males a
double saccular and spiny copulatory organ ts eversible from
the cloaca.
Snakes ave widely distributed, but are most abundant in
the tropics.
ri
Fic. 346.—Snake’s head.—After Nuhn.
dv., Poison fangs; ., sheath of fang; 2. tongue ;. 77., muscles ot
tongue.
General notes on snakes, — Snakes, . especially when
poisonous, are often brightly coloured. The scales on the
head form large plates, and those on the ventral surface are
transverse shields. In many cases there are odoriferous
glands near the cloacal aperture.
The muscular system is very highly developed, and the
limbless serpent, Owen says, “can outclimb the monkey,
outswim the fish, outleap the zebra, outwrestle the athlete,
and crush the tiger.”
The vertebrae are very numerous, some pythons having
four hundred; they are proccelous, and are distinguishable
only into a pre-caudal and caudal series.
All the pre caudal vertebree except the first—the atlas—
have associated ribs, which are movably articulated, and
632 REPTILLA
used as limbs in locomotion, being attached to the large
ventral scales which grip the ground. In the caudal region
the transverse processes, which are elsewhere very small,
take the place of ribs.
One of the most distinctive characteristics of the skull
is the mobility of some of the bones. Many of the Ophidians
swallow animals which are larger than the normal size of
the mouth and throat. The mobility of the skull bones is
an adaptation to this habit. Thus the rami of the mandible
are united by an elastic ligament; the quadrates and the
squamosals are also movable, forming ‘‘a kind of jointed
lever, the straightening of which permits of the separation
of the mandibles from the base of the skull.” The nasal
region may also be movable. On the other hand, the
bones of the brain-case proper are firmly united. The
premaxille are very small and rarely bear teeth; the
palatines are usually connected with the maxille by trans-
verse bones, and through the pterygoids with the movable
quadrates.
Teeth, fused to the bones which bear them, occur on the
dentaries beneath, and above on the maxillz, palatines,
and pterygoids, and very rarely on the premaxille. The
fang-like teeth of venomous serpents are borne by the
maxillz, and are fewin number. Each fang has a groove
or canal down which the poison flows. When the functional
fangs are broken, they are replaced by reserve fangs which
lie behind them. In the egg-eating African Dasypeltis the
teeth are rudimentary, but the inferior spines of some of
the anterior vertebree project on the dorsal wall of the
gullet, and serve to break the egg-shells.
When a venomous snake strikes, the mandible is lowered,
the distal end of the quadrate is thrust forward (this pushes
forward the pterygoid), the pterygo-palatine joint is bent, the
maxilla is rotated on its lachrymal joint, the fangs borne by
the maxilla are erected into a vertical position, the poison
gland is compressed by a muscle, and the venom is forced
through the fang.
Some of the peculiarities in the internal organs of Ophidia
may be connected with the elongated and narrow shape of
the body. Thus one lung, usually the left, is always smaller
than its neighbour, or only one is developed; the liver
GENERAL NOTES ON SNAKES. 633
Fic. 347.—Skull of grass-snake.—From W. K. Parker.
A, Dorsal surface—fx., premaxilla; #zx., maxilla ; a#., external nostril ; #., nasal ; o/.,
nasal cartilages ; 4, prefronto-lachrymal ; ., parietal ; 7., frontal; da., palatine;
z.pa., transpalatine; Jg., pterygoid; f70., pro-otic; ef., epiotic; of., opisthotic;
so., supraoccipital ; ¢0., exoccipital ; av., articular ; s.ag., surangular ; ag., angular ;
d.,dentary ; g., quadrate; sz.,squamosal. B, Ventral surface—fx., premaxilla; o.,
nasal cartilage; #z., maxilla ; v., vomer; Za., palatine; 4., parasphenoid; 7, frontal ;
d/.,prefrontal; Zg., pterygoid; ds. ,basisphenoid; ads., alisphenoid; 5.0., basioccipital;
oc.c., occipital condyle ; ¢o., exoccipital ; g., quadrate ; av., articular ; ag., angular ;
S.ag., surangular ; ¢v., coronary ; sf., splenial ; @.,dentary ; of., opisthotic region,
634 REPTILIA,
is much elongated; the kidneys are not opposite one
another.
The poison is useful in defence, and in killing the prey,
which is always swallowed whole. It is interesting to notice
a recent discovery, requiring amplification, that the bile of
a poisonous snake is an antidote to its venom.
The British adder (Peas berus) is viviparous, and so are a few
others. The great majority are oviparous, but confinement and
abnormal conditions may make oviparous forms, like the Boa con-
strictor and the British grass-snake (7vopedonotus natrix), viviparous.
The female python incubates its eggs.
Many Ophidians become lethargic during extremes of temperature,
or after a heavy meal.
Snakes are especially abundant in the tropics, but occur in most
parts of the world. They are absent from many islands; thus there
are none in New Zealand, and we all know that there are no snakes
in Iceland. Most are terrestrial, but not a few readily take to the
water, and there are many habitual sea-serpents.
The serpent still bites the heel of progressive man, the number of
deaths from snake-bite in India alone amounting to many thousands
yearly.
True Ophidians first occur in Tertiary strata.
Some Examples of Ophidia
Typhlopide. The lowest and most divergent Ophidians, occurring
in most of the warmer parts of the earth, generally smaller than
earthworms, usually subterranean burrowers, with eyes hidden
under scales, with a non-distensible mouth, with teeth restricted
either to the upper or to the lower jaw. ‘‘The palatine bones
meet, or nearly meet, in the base of the skull, and their long axes
are transverse ; there is no transverse bone; the pterygoids are not -
connected with the quadrates.” The quadrate articulates with the
pro-otic, for there is no squamosal.
Example.— 7yphlops, very widely distributed.
In other Ophidians the palatines are widely separated, and their
long axes are longitudinal ; there are transverse bones connecting
palatines and maxille; the pterygoids are connected with the
quadrates.
In innocuous snakes the poison gland is not developed as such; the
maxillary teeth are not grooved.
Examples.—The British smooth snake (Coronel/la levis); the
British grass-snake (Zropidonotus natrix); the Pythons ;
the Boas, of which the Anaconda (Boa murina) (30 feet) is
the largest living Ophidian.
In venomous snakes some of the maxillary teeth are grooved, and in
the most venomous the groove becomes a canal open at both ends, '
CROCODILIA. 635
Examples. — Cobras, Maja tripudians (Indian), Naja haje
(African); the Hamadryad (Ophzophagus elaps), eating
other snakes; Coral-snakes (Z/aps, etc.); Sea-snakes
(Aydrophis, etc.), with paddle-shaped tails. The British
adder (Pelias berus); the rattlesnake (Crotalus), with a rattle
formed chiefly from epidermic remnants of successive slough-
ings ; the African Puff-adder (Clotho arzetans).
Order Crocopitia. Crocodiles, Alligators; Gavials
GENERAL CHARACTERS.— The
Crocodilians are carnivorous
fresh-water reptiles of large size,
now represented by a few genera,
e.g. Crocodilus, Alligator, and
Gavialis.
The skin bears epidermic scales,
underneath some of which there
are dermis bones or scutes.
The tail ts laterally compressed,
and assists in swimming.
Teeth occur in distinct sockets
in the premaxille, maxille, and
dentaries.
In modern Crocodilians, almost
all the vertebre are procelous.
The skull has many charac-
teristic features, such as the union
of maxille, palatines, and ptery-
goids in the middle line on the
roof of the mouth, and the conse-
guent shunting of the posterior
nares tothe very back of the
mouth.
-
C
Fic. 348.—Lower surface of
skull of a young crocodile.
p-mx., Premaxilla; #ex., maxilla;
pal., palatine; 0.2, os trans-
versum ; #7., pterygoid ; 7., jugal ;
Q7.,quadrato-jugal ; Q., quadrate 5
p.#., posterior nares ; c., condyle.
Some of the ribs have double articulating heads, and bear
small uncinate processes ; transverse ossifications form so-
called abdominal ribs.
The heart is four-chambered; a muscular diaphragm
partially separates the thoracic from the abdominal cavity.
The cloaca has a longitudinal opening. The males have
a grooved pents.
636 REPTILIA.
The Crocodilians ave oviparous. The eggs have firm cal-
careous shells, and are laid in holes in the ground.
Skeletal system.—Numerous transverse rows of sculptured bony
plates or scutes, ossified in the dermis, form a dorsal shield. On the
ventral surface the scutes are absent, except in some alligators, in
which they are partially ossified. But besides and above the scutes,
there are horny epidermic scales like those in other Reptiles. The hide
is often used as leather.
‘ The vertebral column
= consists of distinct cer-
/ vical, dorsal, lumbar,
sacral, and caudal verte-
bree, all proccelous except
the first two cervicals,
the two sacrals, and the
first caudal. In most
of the pre-cretaceous
Crocodilians, however,
the vertebree were amphi-
coelous. The centra of
the vertebree are united
by fibro-cartilages, and
the sutures between the
neural arch and the cen-
trum persist at least for
a long time. Chevron
bones are formed beneath
the centra of many of
the caudal vertebrz.
Many of the ribs have
two heads — capitulum
and tubercle—by which
they articulate with the
vertebree. From seven
to nine of the anterior
Fic. 349.—Cervical vertebra of crocodile. dorsal ribs are connected
7.S., Neural spine; P.A., posterior articular pro- with the sternum by
cess; A.A., anterior articular process; C.R., sternal ribs, and from
cervical rib; C., proccelous centrum, several of these anterior
ribs cartilaginous or par-
tially ossified uncinate processes project backwards. The so-called
abdominal ribs have nothing to do with ribs, but are ossifications in
the fibrous tissue which lies under the skin and above the muscles.
They form seven transverse series, each composed of several ossicles.
As to the skull, there is an interorbital septum with large alisphenoids ;
the presphenoid and orbitosphenoids are at best incompletely ossified ;
all the bones are firmly united by persistent sutures; both upper
and lower temporal arcades are completely ossified ; the maxille, the
palatines, and the pterygoids meet in the middle line of the roof of
SKELETAL SYSTEM. 637
the mouth, covering the vomers, and determining the position of the
posterior nares—at the very back cf the mouth ; an os transversum or
transpalatine extends between the maxilla and the junction of palatine
and pterygoid; a postorbital rod (epipterygoid or columella) is formed
by a downward process of the postfrontal meeting an upward process
from the jugal ; the quadrate is large and immovable ; there are large
Fic. 350.—Crocodile’s skull from dorsal surface.
p.mx., Premaxilla; mzx., maxilla; 2, lachrymal; 47.4, prefron-
tal; j., jugal; 2.4, postfrontal; g.7., quadrato-jugal; ¢.,
uadrate; sg., squamosal ; Ja., parietal; e.¢., epipterygoid 5
ZR frontal; 4z., pterygoid (on lower surface); 0.¢., os trans-
versum (on lower surface); #., nasal.
parotic processes ; the tympanic cavity is completely bounded by bone s
the teeth, which are borne by premaxillze, maxillee, and dentaries, are
lodged in distinct cavities ; beside and eventually beneath the teeth lie
reserve ‘‘ germs” of others.
Each ramus of the mandible consists, as in most Reptiles, of a
cartilage bone—the articular— working on the quadrate, and five
638 REPTILIA.
membrane bones — dentary, splenial, coronoid, angular, and sur-
angular.
The hyoid region is very simple.
The pectoral arch includes a dorsal scapula and a ventral coracoid
(with a characteristic foramen); there are no clavicles; the epicoracoids
are thin strips between the ventral ends of the coracoids and the front
of the sternum; there is an episternum ; the fore-limb is well though
not strongly developed ; there are five digits, webbed and clawed.
In the pelvic arch, large ilia are united to the strong ribs of the two
sacral vertebre: the pubes, or more strictly the epipubes, slope for-
ward and inward, and have a cartilaginous symphysis ; the ischia slope
Fic. 351.—Pectoral girdle of crocodile.
sc., Scapula ; gi.c., glenoid cavity; co., coracoid; c.f, coracoid
foramen; z.cd., episternum.
backward and have a symphysis ; ilia and ischia form almost the whole
of the acetabulum, a small part being occupied by the true pubes. The
hind-limbs bear four digits, webbed and clawed.
Organs of Crocodilians.—The Crocodilians are seen to best
advantage in the water, swimming by powerful tail-strokes. The limbs
are too weak for very effective locomotion on land, the body drags
on the ground, and the animals are stiff-necked. Although many,
especially in their youth, feed on fishes and small animals, the larger
forms lurk by the edge of the water, lying in wait for mammals of
considerable size. These they grasp in their extremely powerful jaws,
and drown by holding them under water. If the dead booty cannot
be readily torn, it is often buried and left until it begins to rot. In
connection with their way of feeding, we should nolice several
peculiarities of structure; the nostrils are at the upper end of the
snout, and the eyes and ears are also near the upper surface, so that
the Crocodilians can breathe, see, and hear while the body is alto-
gether immersed except the upper surface of the head; the nostrils can
ORGANS OF CROCODILIANS. 639
be closed by valves, and the eyes by transparent third eyelids, and
the ears by movable flaps, so that the head can be comfortably im-
mersed ; a flat tongue is fixed to the floor of the mouth, and the
cavity of the mouth is bounded behind by two soft transverse mem-
branes, which, meeting when the reptile is drowning its prey, pre-
vent water rushing down the gullet ; the posterior opening of the nostrils
is situated at the very back of the mouth, and when the booty is being
drowned, the Crocodilian keeps the tip of its snout above water, the
glottis is pushed forward to meet the posterior nares, a complete channel
for the passage of air is thus established, and respiration can go on un-
impeded. For their shore work the Crocodilians prefer the darkness,
but they often float basking in the sun, with only the tip of the snout
and the ridge of the back éxposed.
Glands with a secretion which smells like musk are usually developed
on the margin of the lower jaw, at the side of the cloacal aperture,
and on the posterior margins of the dorsal scutes. The musky odour
is very strong during the pairing
season, and when the animals are
attacked. ;
In connection with the muscular
system, the presence of what is
often called an incipient diaphragm
between the thoracic and the ab-
dominal cavity is of interest.
The brain seems very small in
relation to the size of the skull.
The eyes are provided with a
third eyelid, as in most Reptiles,
Birds, and Mammals; there are
large lachrymal glands, but there
is no special deceitfulness about
“*crocodile’s tears.” Fic. 352.—Half of the pelvic
The ears open by horizontal girdle of a young crocodile.
slits, over which lies a flap of
skin; three Eustachian passages—
one median and one on each side—
open into the mouth behind the posterior nares.
The nostrils alsa. can be closed, and, as we have already noticed,
their internal opening lies at the back of the mouth.
The stomach suggests a ‘bird’s gizzard, for it has strong muscular
walls, and its pyloric end is twisted upward so as to lie near the cardiac
art.
The heart is four-chambered, the septum between the ventricles being
complete, as in Birds and Mammals. Butas the dorsal aorta is formed
from the union of a left aortic arch containing venous blood, and a
right aortic arch containing arterial blood, the blood which is driven to
many parts of the body is ‘‘mixed blood,” ze. blood partly venous,
partly arterial, with some of its red blood corpuscles carrying heemo-
globin and others oxyhzmoglobin. At the roots of the two aortic
arches there is a minute communication between them—the foramen
Panizze.
7i,, lium; af, acetabulum; /s.,
ischium ; P., pubis or epipubis.
€40 REPTILIA. -
Into the right auricle venous blood is brought by the two superior
venze cavee and by the inferior vena cava. The blood passes through
a valved aperture into the right ventricle, and is driven thence—(a) by
the pulmonary artery to either lung, or (4) by the left aortic arch to
the body. From this left aortic arch, before it unites with its fellow
on the right to form the dorsal aorta, is given off the great cceliac
artery. The anterior viscera thus receive wholly venous blood from
the heart.
The blood driven to the lungs is purified there, and returns by pul-
monary veins to the left auricle. Thence it passes through a valved
aperture into the left ventricle. Thence it is driven into the right aortic
arch, From this the carotids to the head and the subclavians to the
fore-limbs are given off. These parts of the body thus receive wholly
arterial blood from the heart.
The venous blood returning from the posterior regions may pass
through the kidneys in a renal-portal system, and thence into the
inferior vena cava; or it may pass through the liver in a hepatic-portal
system, and thence by hepatic veins into the inferior vena cava ; or some
of it may pass directly into the inferior vena cava. The renal-portal
veins arise from a transverse vessel uniting the two branches of the
caudal, but the latter are also continued forward as lateral epigastrics
which enter the liver.
The temperature of the blood is not above that of the surrounding
medium.
In regard to the respiratory system, we should notice that the lungs
are invested by pleural sacs, as is the case in Mammals.
The ureters of the kidneys, the vasa deferentia from the testes in the
male, the oviducts from the ovaries in the female, open into the cloaca,
which has a longitudinal opening. The penis is on the anterior surface
of the cloaca,
The eggs, which in size are like those of geese, have a thin calcareous
shell, are buried in excavated hollows, and, warmed by the sun, hatch
without incubation.
Of one species of crocodile it is known that the mother opens up the
nest when the young, ready to be hatched, are heard to cry from
within the eggs. The mothers take some care of the young, which
require to be defended even from the appetite of the males,
Crocodiles are relatively sluggish, and fond of basking passively,
sometimes hiding in the mud during the hot season. They are remark:
able for the long continuance of growth, which does not seem to have
so definite a limit as in most other animals.
Classification of Crocodilia
(a) The true Crocodiles, of the genus Crocodilus, occur in Africa,
Southern Asia, tropical Australia, Central America, and the West
Indies.
The Indian crocodile (C. porosus) may measure about 18 ft. in
length, and even larger forms have been recorded. The sacred African
CLASSIFICATION OF CROCODILIA. 641
crocodile (C. wzlgarzs) is still formidably common in some of the
fresh waters of tropical Africa.
The eggs and the young are often eaten by 2 mammal called the
Ichneumon, and by a species of lizard. The adults have few enemies
except man. They seem to live in friendly partnership with little
birds (Pluvianus egypticus), which remove parasites from the body, and
in their familiarity almost justify the account which Herodotus gives of
their cleaning the reptile’s teeth.
(4) The Alligators, of the genus A//igator, are, with the exception of
one Chinese species, confined to N. and S. America. In N. America,
A. mississippiensis, in S. America A. sclerops, is common.
(c) The Gavials or Gharials, of the genus Gavzalzs, are distinguished
by their long narrow snout. In the Ganges and its tributaries, G.
gangeticus, said to attain a length of 20 ft., is common. They feed
chiefly on fishes. ‘‘ Old males have a large cartilaginous hump on the
extremity of the snout, containing a small cavity for the retention
of the air, by which means these individuals are enabled to remain
under water for a longer time than females or young.”
DIFFERENCES BETWEEN CROCODILES, ALLIGATORS,
AND GAVIALS
ALLIGATORS. CRocoDILEs. GaVIALs.
The head is short and} Longer. The snout is very long.
broad.
First, and fourth lower| The first bites into a| First and fourth iower
teeth bite into pits in the| pit; the fourth into aj teeth bite into grooves in
upper jaw. groove. the upper jaw.
The union of the two; Not beyond the eighth. The union extends at
rami of the lower jaw does least to the fourteenth.
not extend beyond the
fifth tooth.
The nasal bones form| As in thealligator. The nasal bones do not
part of the nasal aperture. form part of the nasal
aperture.
The teeth are very un-| Unequal. Almost equal.
equal.
The scutes on the neck Sometimes distinct, Continuous.
are distinct from those on | sometimes continuous.
the back.
History of Crocodilians.—These giant reptiles form a decadent
stock. Fossil forms are found in Triassic strata (e.g. Belodon, Para-
suchus, and Stagonolepis) ; their remains are abundant in Jurassic rocks.
In Cretaceous strata, crocodilians with proccelous vertebree first occur,
the pre-Cretaceous forms having centra of the amphiccelous type.
The oldest crocodilians have the posterior nares situated farther for-
ward, behind the palatines. Huxley has worked out an “almost
unbroken” series from the ancient Triassic crocodilians down to those
of to-day.
41
642 REPTILIA.
Development of Reptiles
The ovum contains much yolk, at one pole of which there is a small
quantity of formative protoplasm surrounding the germinal vesicle.
The segmentation is necessarily meroblastic and discoidal, as in
Birds.
The segmented area or blastoderm, originally at one pole, gradually
grows round the yolk. The central region of the dorsal blastoderm
is separated from the yolk by a
shallow space filled with fluid, and
is clearer than the rest of the blas-
toderm. In this central region or
area pellucida, the germinal layers and
subsequently the parts of the embryo
are established, while the rest of the
blastoderm — the area opaca — simply
forms a sac round the yolk. One of
the first signs of development is the
appearance of a thickened band of cells
extending forward in the middle line from
the posterior margin of the area pellucida.
This band is called the primitive streak,
and seems to represent a fusion of the two
edges of the blastoderm behind the future
embryonic region. The embryo develops
in front of the primitive streak, and one
of the first signs of its development is the
formation of a primitive or medullary
groove in a line with the primitive streak.
As development proceeds, folds appear
around the embryo, constricting it off
from the subjacent yolk or yolk-sac.
It is with Reptiles that the series of
higher Vertebrates or Amniota begins.
Fic. 353.—Origin of amnion and
allantois. —After Balfour,
1. Rise of amniotic folds (@.£) around embryo (¢) ;
pp.) pleuro-peritoneal cavity ; 4, yolk.
z. Further growth of amniotic folds (a) over
enibryo and around yolk.
3. Fusion of amniotic folds above embryo ; a.f.,
amnion proper ; s.z.7#., subzonal membrane ;
ys., yolk-sac.
4. Outgrowth of allantois (a/.); amniotic cavity
(a.c.); #., head end; z., tail end.
5. Complete enclosure and reduction of -yolk-sac
(y.s.)5 $.2.22., subzonal membrane; a.J.,
amnion proper; aé., allantois; &, gut of
embryo.
DEVELOPMENT OF REPTILES. 643
It is here that the foetal membranes known as amnion and allantois
are first formed.
(2) The Amnion.—At an early stage in development the head end of
the embryo seems to sink into the subjacent yolk. A semilunar fold of
the blastoderm, including epiblast and mesoblast, rises up in front.
Similar folds appear laterally. All the folds increase in size, arch up-
wards, and unite above, forming a dome over the embryo. Each of
these folds is double ; the inner limbs unite to form ‘‘ the true amnion” ;
the outer limbs unite to form ‘‘ the false amnion,” ‘‘ serous membrane,”
or subzonal membrane. The cavity bounded by the true amnion
contains an amniotic fluid bathing the outer surface of the embryo;
the cavity between the true and the false amnion is lined by mesoblast,
and is continuous with the pleuro-peritoneal or body cavity of the
embryo. The amniotic folds extend not only over the embryo, but
ventrally around the yolk-sac, which they completely invest.
(4) The Allantocs.—While the amnion is being formed, a sac grows out
from the hind end of the embryonic gut. This is the allantois lined
internally by hypoblast, externally by mesoblast. It rapidly insinuates
itself between the two limbs of the amnion, eventually surrounding both
embryo and yolk-sac.
The amnion is a protective membrane, forming a kind of water-bag
around the embryo.
The allantoic sac is vascular, and has respiratory and perhaps also
some yolk-absorbing functions. It seems to be homologous with the
outgrowth which forms the cloacal bladder of Amphibians; it has been
called ‘‘a precociously developed urinary bladder.”
Before the amnion is developed, the heavy head end of the embryo
has already sunk into a depression (in Lizards, Chelonians, Birds (?),
and Mammals), and is surrounded by a modification of the head fold
termed the pro-amnion. This does not include any mesoblast, and is
afterwards replaced by the amnion.
Hints of a placenta before Mammals.—As will be explained after-
wards, the placenta, which characterises most Mammals, is an organic
connection between mother and unborn young. Its embryonic part is
chiefly formed from a union of the serous or subzonal membrane and
the allantois, but in some cases the yolk-sac and the subzonal membrane
form a provisional placenta. The placenta establishes a vital union
between the embryo and the mother.
Now it is interesting to notice that there are some hints of placental
connection in animals which are much lower than Mammals. In
some species of Afzste/us and Carchardas there is a connection between
the yolk-sac and the wall of the uterus; in the Teleostean Anableps
the yolk-sac has small absorbing outgrowths or villi; in Zrachydosaurus
and Cyclodus among Lizards, the vascular yolk-sac is separated from
the wall of the uterus ‘‘ only by the porous and friable rudiment of the
egg-shell ; in CZemmys among Chelonians, there is an absorbing pro-
trusion of the foetal membranes. In Birds also, small villi of the yolk-
sac absorb yolk, and others on the allantois absorb albumen.” (See
A. C. Haddon’s Embryology.)
644 REPTILIA.
Extinct Reptiles
The first known occurrence of fossil Reptiles is in Permian
strata; in the Trias most of the orders or classes are repre-
sented; while the “golden age” of the group was un-
doubtedly during Jurassic and Cretaceous times.
Some of the modern Reptiles are linked by a series of
fine gradations to very ancient progenitors,—the Crocodiles
of to-day lead back to those of the Trias, the New Zealand
Hatteria to the Triassic Rhynchocephalia ; but we have no
example of a Reptilian genus which has persisted from age
to age as Ceratodus has done among Fishes. Among the
fossil forms we find “generalised” types, which exhibit
affinities with groups which in our classification of recent
forms may be very widely separated.
The following types of extinct reptiles seem to have
entirely disappeared :—
Theromorpha.—Lizard-like terrestrial animals with limbs adapted
for walking, found in the Permian and Trias. The group shows a
remarkable combination of reptilian and mammalian characters. In
illustration of veptz/éan characters we may note the pineal foramen, the
complex lower jaw, usually articulating with a firmly fixed quadrate, the
usual presence of pre- and post- frontals. Mammalian features are
illustrated in some types by the differentiation of the teeth into incisors,
canines, and molars; by a single temporal arcade like a zygomatic
arch; by the way the limbs raise the body off the ground; by the union
of the pelvic bones into an os innominatum (pubes and ischia forming a
stout ventral symphysis); by the reduction of the quadrate; by the
share the squamosal may take in forming the articulation for the
lower jaw.
Examples.—FParetosaurus, Dicynodon, Elginia.
Plesiosaurta.—Amphibious and marine reptiles represented from the
Trias to the Chalk, without exoskeleton, usually with a long neck and
short tail. The skull has a single broad temporal arcade, pterygoids
meeting in the middle line, fixed quadrates, and a pineal foramen.
There are strongly developed pectoral and pelvic girdles. The limbs
vary; in the earlier, more generalised, forms they are adapted for
walking on land; but in the more specialised types they are modified
into’ powerful paddles, like those of Chelonia. The nearest affinities
are with the Chelonia. Vothosaurus had limbs adapted for progression
on land; leszosaurus (40 ft. in length) and Pléosaurus were carni-
vorous forms adapted to an aquatic life.
Ichthyosauria. — Large marine carnivorous Reptiles, represented
from the Trias to the Chalk, with tapering body like that of a shark,
large dorsal and caudal fins, and two pairs of paddle-like limbs. In
EXTINCT REPTILES. 645
the paddle the number of digits may be more than five, and the
phalanges of each digit are often very numerous. The pectoral arch
consists of coracoids, scapule, clavicles, and a T-shaped episternum,
but there is no sternum. The skull has a long tapering rostrum, large
orbits, « large parietal foramen, and usually sharp conical teeth in a
continuous groove. The vertebree are deeply amphiccelous. There
was no dermal armour. The length of the body is sometimes 30 to
40 ft. Some species were viviparous.
Examples.— Jchthyosaurus, Ophthalmosaurus.
Pythonomorpha.—These strange Cretaceous Reptiles should prob-
ably be placed near the Lacertilia and the Rhynchocephalia. They
are specially characterised by the enormous elongation of the body,
which sometimes reached a length of 75 to 80 ft. The skull is like
that of the Monitor among the lizards, but, according to Cope, it also
presents affinities with snakes. The body is snake-like, but there are
two well-developed pairs of limbs, forming swimming-paddles. All
were carnivorous and marine ; the distribution was cosmopolitan.
Examples.—Aosasaurus, Clidastes, Liodon, Dolichosaurus.
Dinosauria.—Terrestrial Reptiles, ranging from the Trias to the
Chalk, often very large, and, like Marsupials, specialised in various
directions. They were long-necked and long-tailed forms, some bipedal,
some quadrupedal. The skull has a superior and an inferior temporal
arcade, a fixed quadrate, teeth in sockets, and confined to the margins
of the jaws. They exhibit many points of resemblance to Crocodiles
and Rhynchocephalia on the one side and to Birds on the other. The
pelvis and hind-limbs are particularly avian, eg. in the tendency to
form a tibio-tarsus. Arontosaurus, a gigantic, herbivorous form, nearly
60 ft. in length, was probably amphibious. A¢/antosaurus was even
larger, the femur measuring over 6 ft. in length. Compsoguathus,
Iguanodon, and Camptosaurus are examples of the ‘‘bird-footed”
herbivorous Dinosaurs. Compsognathus only reached a length of 2 ft.,
and hopped on its hind-legs like a bird. Jguwanodon habitually walked
on its hind-limbs, and, like several others, had hollow bones ; it reached
a height of 15 ft. Of the carnivorous Dinosaurs, Megalosaurus is a
good type. The limbs were furnished with powerful claws, and the
teeth show much specialisation.
Pterosauria ox Pterodactyls,—F lying Reptiles, represented from the
Lower Jurassic to the Upper Chalk, exhibiting many points of resem-
blance to Carinate Birds, but still distinctly Reptilian in type. They
resemble birds especially in some features of the skull and pectoral
girdle, but they differ markedly in their vertebral column, pelvis, and
organ of flight. An expansion of the skin seems to have been stretched
on the much-elongated outermost finger, and to have extended back-
wards to the hind-legs and the tail. The long bones are hollow. The
sternum is keeled, and teeth are often present on the margin of both
jaws. . There is both a superior and an inferior temporal arcade. The
quadrate is fixed. Some were no larger than sparrows, but others—
the giants with which the race ended—had in some cases a spread of
wing of nearly 20 ft. It is probable that the resemblances of these
forms to Birds indicate similar habits, and not any close true affinity.
Examples.—Prerodactylus, Rhaniphorhynchus, Pteranodon,
646 REPTILIA.
RELATIONSHIPS
Reptiles, in their widest sense, form a central assemblage
among Vertebrates. Some of the extinct forms exhibit
affinities with Amphibians, others with Birds, others again
with Mammals. Though we cannot with certainty point to
any of the extinct types as directly ancestral to Birds or
Mammals, it seems likely that the ancestors of both were
derived from the plastic Reptilian stock,
N.SP
Fic. 3534.—Vertical section through backbone and
ribs of Chelonian (I.) and Mammal (II.).—In
part after Jaekel.
N.SP., neural spine; NV.SC., neural scute; 7., tubercle of rib;
C.SC., costal scute over rib (7.); CA., capitulum of rib;
7.P., transverse process; CZ., centrum; WVA., cavity of
neural canal. In the Chelonian the tubercle abuts against
the flattened neural spine, and the capitulum against the
transverse process. In the Mammal, the tubercle articu-
lates with the transverse process and the capitulum with
the centrum.
CHAPTER XXV
Crass AVES—BIRDS
I, Sub-class ARCHAORNITHES (or Saururce); extinct Archaeopteryx.
II. Sub-class NEORNITHES.
1. Division Ratite. ‘* Running Birds.” Ostrich, etc.
2. Division Odontolee. Extinct Hesperornis.
3. Division Carinate. ‘Flying Birds” with keeled sternum.
Birps share with Mammals the rank of the highest Verte-
brates. For although Mammals excel in brain development,
and in the closer organic connection between mother and
unborn young, it must be allowed that as regards muscles
and skeleton, heart and lungs, indeed most of their struc-
ture, the two classes are almost equally differentiated. They
are not, however, in any way nearly related, but represent
quite divergent lines of evolution. They are related to one
another indirectly, since they have in all probability a
common ancestry among Reptiles.
Like Insects among Invertebrates, so Birds among
Vertebrates are pre-eminently creatures of the air, and the
analogies between these two widely separated classes are
many, é.g. as regards power of flight, elaborate respiratory
system, bright colouring, sexual dimorphism, preferential
mating, and parental instincts. The high body temperature
of Birds, exceeding that of all other animals, is a physio-
logical index of their rapid metabolism or intense activity.
Compared with lower Vertebrates, Birds show a marked
increase of emotional life, as seen in their affection for their
mates, in their care of the young, and in the joyousness of
their mood, often bursting forth in song,
648 BIRDS.
GENERAL CHARACTERS OF BIRDS
Warm-blooded, oviparous, feathered bipeds.
The fore-limbs are modified as wings, generally capable of
fight ; the neck ts long and the tail is short, except in the
extinct Archeeopteryx.
The epidermic exoskeleton is represented by the feathers,
which are usually arranged in definite feather tracts (pterylia),
with bare patches between, and also by scales on the legs
similar to those of reptiles. Almost the only skin gland ts
an oil or preen gland, lying dorsally at the root of the tail.
The pectoral muscles used in flight are generally large ;
in many there is a muscular gizzard ; there ts no diaphragm
comparable to that of Mammals,
In the brain, which fills the large cranial cavity, the
predominance of the basal parts of cerebrum and cerebellum
has resulted in displacing the optic lobes to the sides. The
spinal cord ts at an angle to the medulla oblongata, not in a
line with it as in lower Vertebrates.
The nostrils are often surrounded by a sensitive cere; there
ts never more than a very rudimentary pinna outside the
external auditory meatus ; the connection between tympanum
and inner ear ts by means of a columella; the eyeball is
strengthened by sclerotic ossicles; there is a well-developed
third eyelid, and a large nutritive and secretory pecten.
There are no epiphyses on the bones. Many bones contain
prolongations of the air-sacs connected with the lungs. When
a long bone contains an air-sac, there ts little or no marrow.
The curvature of the vertebral centra, especially in the cervical
vegion, viewed from in front, is typically concave from side
to side, and convex from above downwards (heterocelous),
but other shapes occur, e.g. opisthocelous in thoracic region
of gulls and penguins. The cervical vertebra have small ribs,
Jused in most cases with the transverse processes. The
thoracic vertebra tend to fuse; and numerous vertebra (one
to three dorsals, all the lumbars, and some caudals) fuse with
the two or three true sacrals. The terminal vertebre usually
fuse as a ploughshare bone.
In most birds the bones of the brain-case fuse very early,
the sutures being obliterated. Only the lower jaw, the
quadrate, the columella, and hyoid are always movable, but
GENERAL CHARACTERS OF BIRDS. 649
the pterygoids usually articulate freely with the basisphenoid,
the lachrymals may remain free, and there may be a joint in
the beak at the end of the premaxille. There is but one
condyle. A membrane bone called the basitemporal covers
the basisphenoid. There is an interorbital septum formed
from presphenoid and mesethmoid. The otic bones fuse with
adjacent bones and with one another about the same time.
In modern birds there are no teeth, but the jaws are covered
C g pr.
Fic. 354.—Position of organs in a bird.—After Selenka.
n., Nostrils; ¢7., trachea; cv., crop; %., heart; s¢., sternum ; p7.,
proventriculus ; ¢., gizzard; Css ceca : p + pygostyle ; pu. » pelvis;
&., kidney ; Z., lung.
by horny sheaths. The premaxille are large, and form most
of the beak. There is a complete infra-temporal arcade
formed by a delicate jugal and quadratojugal reaching back
to the des The supra-temporal arcade is usually
incomplete, but in some cases a process of the sqguamosal joins
a postorbital process of the frontal. The lower jaw consists
on each side of five membrane bones and a cartilage bone—
the articular—which works on the quadrate. Many of the
skull bones have a spongy texture, due to cavities filled with
air from: the nasal and Eustachian tubes.
650 BIRDS.
There ts a well-developed sternum, generally with a keel,
with a separate centre of ossification, to which the pectoral
muscles are in part attached. The strong coracoids reach
and articulate with the sternum. In flying birds the clavicles
Fic. 355.—Fore-limb and hind-limb compared.
H., Humerus; &., radius; U., ulna; ~, radiale; ~., ulnare; C., distal
carpals united to carpo-metacarpus; CC., the whole carpal region;
MC./., metacarpal of the thumb; /., phalanx of the thumb; A7C.//.,
second metacarpus; //., second digit; J¢C.///., third metacarpus ;
177, third digit, , femur; 7.7\, tibio-tarsus; 77, fibula; Pz.,
proximal tarsals united to lower end of tibia;-d¢., distal tarsals united
to upper end of tarso-metatarsus (7.77.); 7., entire tarsal region;
MT.1., first metatarsal, free ; /.-/V., toes.
are usually well developed, and connected by an tnterclavicle,
which may be connected with the apex of the sternum. The
fore-limb has not more than three digits (I., I1., and III.), the
three metacarpals are fused (except in Archeopteryx), and
GENERAL CHARACTERS OF BIRDS. 651
there are only two separate carpals, the others fusing with the
metacarpals, and thus forming a carpo-metacarpus. The
thumb ts often clawed, the second digit rarely.
The ilia of the pelvis may be firmly fused to the complex
sacrum ; the acetabulum is incompletely ossified; the pubes
are directed backwards parallel to the tschia. There is no
pubic symphysis except in the African ostrich (Struthio), and
no ischiac symphysis except in the American ostrich (Rhea).
in the hind-limb the fibula ts incomplete, and more or less
united to the tibia, the proximal tarsal bones are united to
the distal end of the tibia (which ts
therefore calied a tibio-tarsus), the others
being united to the proximal end of three
united metatarsals (which thus form a
tarso-metatarsus). As in Reptiles, the
ankle-joint ts therefore intertarsal. The
maximum number of toes is four, of
which the first ts the hallux: if there be
four, the metatarsal of the hallux ts free
from the other three fused metatarsals ;
uf there are only three, the hallux has been Fre: 946) —Disgrim-
suppressed. . matic section of
In regard to the alimentary system, the young bird.—After
absence of teeth, the frequent occurrence of Gadow.
a crop and a gizzard, the usual shortness n., Spinal cord, v., ver-
of the large intestine, the presence of a ect Mgt irean,
spl
liver; G., gut; som.
cloaca, may be noted. (ottea ate be
The heart is four-chambered ; the single (dotted), splanchnic
aortic arch curves to the right side; only [ay Oh, mespvlasts
the pulmonary artery rises from the right productive organ; K.,
ventricle ; the two valves between the right
auricle and the right ventricle are in part muscular , there ts
no renal portal system , the red blood corpuscles are oval and
nucleated ; the blood temperature is from 2°-14° F. higher
than that of Mammals.
The non-expansible lungs are fixed to the dorsal wall of
the thorax , the bronchial tubes expand in irregular branches
in the lungs ; the ends of some of these branches are continued
into surrounding air-sacs ; these may be continued into the
bones, and end in minute air-spaces. The trachea has bony
rings, a larynx (without vocal chords) at its upper end, and a
652 . BIRDS.
syrinx or song-box (with vocal chords) at the origin of the
ne
Fic. 357.—A falcon.
mn., Mandible; C., cere; V., nostril; Z.C., ear covert; #h.7.,
thumb wing ; C., wing coverts; D., dorsal coverts; S., second-
aries; P., primaries; #., rectrices; A., ankle; A74, tarso-
metatarsus; /., first toe.
bronchi. Lxpiration ts the more active part of the respiratory
process.
GENERAL CHARACTERS OF BIRDS. 653
_ The (metanephric) kidneys are three-lobed, and lie embedded
in the pelvis » the ureters open into the cloaca; there is no
bladder ; the urine is semi-solid, and consists chiefly of urates.
Water must be mainly got rid of by evaporation from the
walls of the air-sacs and atr-passages.
The testes lie beside the kidneys ; the vasa deferentia run
Fic. 358.—Young bearded griffin (Gypaétes barbatus).—After Nitzsch.
Showing the feather-tracts or pteryle, for instance those on the breast
(PT.). £., ear; P., web or propatagium; 7H., thumb; PR., bases
- of primary feathers; S., bases of secondary feathers; B.S., bare streak
without pteryle; CL., cloaca; F., bases of rectrices or tail feathers.
outside the ureters, and open into the middle region of the
cloaca. The right ovary atrophies, the right oviduct ts rudt-
mentary. There ts rarely any copulatory organ, but it ts
large in ostriches, ducks, geese, and some other birds.
The eggs have much yolk and hard calcareous shells. The
segmentation is meroblastic and discoidal. The allantois is
chiefly respiratory, though it helps in absorbing the nutritive
654 BIRDS.
substance of the egg, and acts as a receptacle for the embryo’s
waste products.
Tue Piczon (Columba) as A TYPE OF BiRDS
The numerous varieties of domesticated pigeon (pouter,
fantail, tumbler, etc.) are all descended
i from the rock-dove, Columba livia, and
afford vivid illustrations of variation, and
\ hy of the results of artificial selection.
Certain variations, e.g. in beak or tail,
crop up, we know not how; and similar
forms are bred together until a new breed
is established.
External characters.— The form of
the body, well suited for rapid flight,
i Be Cc ceases to be graceful when stripped of
_ its feathers. The cere above the nos-
trils, the third eyelid in the anterior
upper corner of the orbit, the external
opening of the ear concealed by the
feathers, the preen gland on the dorsal
surface at the root of the tail, and the
cloacal aperture, are external features
easily recognised.
The feathers most important in flight
are the twenty-three remiges of the
wing, divided into eleven primaries
borne by the metacarpals and phalanges
Fie, -459,-Atter of the two fingers, and twelve secondaries
“Nitzach, borne by the ulna. Twelve tail feathers
A. Filoplume. B., very OF rectrices serve as a brake, and help
young feather within g little in steering. A distinct tuft of
its sheath (s%.) ; c., the 5
core of dermis; 2., the feathers borne by the thumb is called
barbs. C., the same, the bastard wing. Covering the bases
of the large feathers are the coverts,
—wing-coverts and tail-coverts,—which belong to the
series of contour feathers which give shape to the whole
body. In the pigeon there are no true down-feathers or
plumules, but among the ordinary contour feathers or penne
there are little hair-like feathers (filoplumes) with only a few
THE PIGEON AS A TYPE OF BIRDS. 655
terminal barbs. In herons and some other birds some of
the down-feathers are covered with dusty powder (powder-
down) formed from the brittle ends of the barbs. Apart
from their use in flight, the feathers, being bad conductors
of heat, serve to sustain the high temperature of the bird.
There is usually pigment in feathers, and the coloration
thus produced is often enhanced by structural peculiarities
of texture and surface. In perfectly white feathers the
whiteness is due to gas-bubbles.
Any one of the large feathers consists of an axis or scapus, divided
into a lower hollow portion—the calamus or quill, and an upper solid
portion—the rachis, which forms the axis of the vane. This vane con-
sists of parallel rows of lateral barbs, linked to one another by barbules,
which may be joined to one another by microscopic hooklets. In the
running birds the barbs are free. The quill is fixed in a pit or follicle
of the skin, from which muscle fibres pass to the feather and effect in-
dividual movement. At the base of the quill there is a little hole—
the inferior umbilicus—through which a nutritive papilla of dermis is
continued into the growing feather. At the base of the vane there is
a little chink—the superior umbilicus—but this has no importance,
except that parasites sometimes enter by it. Close to this region,
however, in many birds, a tuft or branch arises, called the aftershaft.
In the Emu and Cassowary the aftershaft is so long that each feather
seems double.
A feather begins as a papilla of skin, but the whole is formed from
the cornification of the inner layer of the epidermis. The papillz
rarely occur all over the skin (¢.g. penguin), but are usually disposed
along definite feather-tracts. . Each papilla consists externally of epi-
dermis and internally of dermis, and becomes surrounded at the foot
by a moat, which deepens to form the feather-follicle in which the
base of the quill is sunk. The epidermis has two layers—(a) an outer
stratum corneum, which in the developing feather forms merely a pro-
tective external sheath, and (4) an inner stratum Malpighii, which
becomes cornified and forms the whole feather. The process by which
this cylinder of cells becomes horny is remarkable; in the upper part
ridges are formed, which separate from one another as a set of barbs,
the lower part remaining intact as the quill. When we pull off the
horny sheath of a young feather, we disclose a set of barbs lying almost
- parallel with one another, yet slightly divergent. The central pair
predominate, and fuse to form the rachis; their neighbours gradually
become the lateral barbs. The external sheath falls off; the core
of dermis is wholly nutritive, and disappears as the feather ceases to
row.
On the four toes and on the base of the legs there are horny epidermic
scales, the presence of which reminds us of the affinities between Birds
and Reptiles. Thetoesare always clawed. The thumb of Birds is often
clawed; the second digit very rarely. Only in the embryo of the
ostrich (Struthio) is the third digit clawed. The beak is covered by
656
BIRDS.
Fic, 360.—Types of feathers.
feather in sheath (s%.).
eon—C., calamus ;
(4.$.).
ortion
Sy
3, Covert of heron showing aftershaft
ftershaft; F., rachis; V., vane.
bilicus (S.%.), pith (P.); #., filoplume.
4S.
bilicus (7.2. ), superior um
al
ing f
pig!
feather oF
ferior um
2, Develop
, Secondary,
quill showing in!
Down.
4
of
Ds
MUSCULAR SYSTEM. 657
a horny sheath, which is annually moulted in the puffin. A moulting
of claws occurs in the grouse. The dermis is thin and vascular, and
1s rich in tactile nerve-endings or Pacinian corpuscles, especially
abundant in the cere. The only skin gland—the preen gland—secretes
an oily fluid, which some birds use in preening their feathers. It is
absent in the ostrich, emu, cassowary, and in a few Carinate birds.
Fic. 361.— Parts of a feather.—After Nitzsch.
Z., Four barbs (8.) bearing anterior harbules (4.B8.) and posterior barbules
(P.BB.); Lf., six barbs (B.) in section, showing interlocking of barbules ;
fff, anterior barbule with barbicels (#.).
Muscular system.—The largest breast muscle (pectoralis
major) arises from the sternum and its keel, and from the
clavicle, is inserted on the ventral surface of the humerus,
and depresses the wing. The smaller but longer pectoralis
minor or subclavian, exposed when the large one is reflected,
raises the wing. It arises from the keel and sides of the
sternum, and is continued over the shoulder (through the
foramen triosseum, which serves as a pulley) to its insertion
on the dorsal surface of the humerus. Arising chiefly from
the coracoid, but in part from the sternum, and inserted
on the humerus, is a small coraco-brachialis, which helps
a little in depressing the wing. There are several yet
smaller muscles. ,
Interesting also is the mechanism of perching. When the bird sits
on its perch, the toes clasp this tightly. The flexor tendons of the
42
Fic, 362.—Entire skeleton of condor, showing the relative positions
of the chief bones.
SKELETON. 659
toes (perforati muscles) are stretched automatically when the ankle is
bent in perching. In some birds, an ambiens muscle, inserted on the
front of the pubis, is continued down the anterior side of the femur,
and its tendon, bending round the knee to the opposite side of the tibia,
is inferiorly connected with the tendon of the flexor of the second or
third toe, or with the third and fourth. It has nothing to do with
bending the first toe, and its importance has been exaggerated. The
bending of the toes is mainly due to the perforati muscles.
In connection with the muscular system, it may also be ‘noted that
the walls of the gizzard consist of thick muscles radiating around
tendinous discs. Two small sterno-tracheal muscles ascend from
sternum to trachea, and are apt to be confused in dissection with the
carotid arteries. Complex muscles are associated with the song-box.
Fic. 363.—Disarticulation of bird’s skull.—After Gadow.
Membrane bones shaded.
B.Oc., basioccipital; £.Oc., exoccipital; S.Oc., supraoccipital ;
Pa., parietal; F., frontal; Wa., nasal; f., premaxilla; JZ,
maxilla; /z., jugal; Q7., quadrato-jugal; Qu., quadrate ; Ze.,
periotic; Sg., squamosal; AS., alisphenoid; B.S., basi-
sphenoid; O.S., orbito-sphenoid; Px.5f%., presphenoid ; zo.,
vomer; zOS., interorbital septum; £., ethmoid; Se., nasal
septum; De., dentary; S%., splenial; Az., angular; Sa., sur-
angular; A~., articular; 47K., Meckel's cartilage.
Skeleton.—The skeleton of birds is lightly built, with
much strength and surface for its weight, on the hollow
girder principle. The texture of the bone is often very
spongy, and air-sacs from the lungs may be continued into
many of the bones, which are then more or less completely
destitute of marrow in adult life. In the pigeon, most of
the bones, except those of the tail, forearm, hand, and
hind-limb, contain air-spaces. Another general character is
the marked tendency to fusion of bones, as seen in the skull,
dorsal vertebrae, sacral vertebrae, ploughshare bone, carpo-
metacarpus, and tarso-metatarsus.
The vertebral column is divided into five regions—cer-
660 BIRDS.
vical, thoracic, lumbar, sacral, and caudal. The mobile
neck consists of fourteen cervical vertebre ; from the third.
to the twelfth these bear short ribs fused to the centra and
transverse processes ;: the thirteenth and fourteenth have
them free and well developed, but not reaching the sternum.
-Of the thoracic vertebra, namely, those whose ribs reach
the sternum, the anterior three are fused to one another,
while the fourth is free. The complex sacral region consists
of the fifth thoracic (with free ribs reaching the sternum),
five or six lumbars, two sacrals, and five caudals, all fused.
Lastly, there are six free caudals ending in a pygostyle or
ploughshare bone,—a fusion of about four vertebre (cf.
coccyx in man). This bone serves as a base for the
rectrices.
A cervical vertebra shows on the anterior surface of the
centrum a distinctive curvature, described as saddle-shaped
or heteroccelous. It is concave from side to side, convex
from above downwards. Posteriorly the curvatures are, of
course, the reverse.
The ribs have two heads—a capitulum articulating with
a centrum, a tubercle articulating with a transverse process.
The ventral part of the rib, which reaches the sternum, is
called the sternal rib, and is joined at an angle to the dorsal
part, which articulates with a vertebra. In Birds the sternal
ribs are always bony ; in Mammals they are usually cartila-
ginous. On the posterior surface of each of the first four
thoracic ribs there is an uncinate process, absent only in
the 5. American screamers (Palamedez).
The skull has a rounded cranial cavity, large orbits, and
a narrow beak, which is mostly composed of the premaxillz.
All the bones are fixed except the quadrate, lower jaw,
columella, and hyoid. The surface is polished ; the sutures
are obliterated very early in life.
The back part of the skull is formed by the basioccipital,
the two exoccipitals, and the supraoccipital, surrounding
the foramen magnum. The basioccipital forms most of the
single condyle.
‘The roof of the skull is formed from the paired parietals,
frontals, and nasals, the last being small and in part super-
seded by the upward extension of the premaxillz.
The line of the upper jaw consists of premaxilla, small
SKELETON. 661:
maxilla, jugal, and quadrato-jugal, the last abutting on the
movable quadrate.
Of the membrane
bones on the side of the
skull, the lachrymal in
front of the orbit, and
the squamosal between
the quadrate and the
parietal, are the most
important.
On the roof of the
mouth, the basisphenoid,
which lies just in front
of the basioccipital, is
covered over by a mem-
brane bone—the basi-
temporal. In front of
this is a sharp “basi-
sphenoid rostrum” or
parasphenoid, also a
membrane bone. Artic-
ulating with the quadrate
and with the rostrum
are the pterygoids; in
front of these lie the
palatines. The vomer
is vestigial. The bony
front of the palate is
formed from inward ex-
tensions of the pre-
maxille and maxilla.
The interorbital septum
is formed chiefly from Fic. 364.—Under surface of gull’s
the mesethmoid, but also skull. —From Royal Scottish Museum,
from the presphenoid. Edinburgh.
From the tympanum to «., cae 4.¢., basitemporal; 4.s., basi-
: sphenoidal rostrum; 4f., pterygoid; pa.
the inner ear extends the palatine; z., vomer 3 bmx, premaxilla; aise,
rod-like columella. The mala | fs iagel getty uatmaeriaial yey
lower jaw originally con- 9%
sists of four membrane bones—dentary, splenial, angular, and
surangular ; and one cartilage bone—the articular... The
662 BIRDS.
hyoid consists of a flat “body,” with anterior and posterior
“horns,” the latter derived from the first branchial arch.
The pectoral girdle consists of sabre-like scapulz extend-
ing dorsally over the ribs, of stout coracoids sloping ventrally
.
’
adius 3 #., ulna; ¢., carpals
primary feathers.
Wing of dove.
iPS
feathers ; ~
Fic. 365.
4., Humerus; s.f, secondary
mc., Carpo-metacarpus
and articulating with the sternum, of the clavicles which are
united by the interclavicle to form the merrythought or
furcula. The opening left where the upper ends of the
clavicles touch the scapula and coracoid is called the fora-
men triosseum.
SKELETON.
663
_ The sternum bears a .conspicuous keel, is produced
laterally and posteriorly into two xiphoid processes, and
Fic. 366.—Side view of pelvis of cassowary,
Z2., ium ; /sch., ischium ; Pé., pubis; Ac., acetabulum.
bears articular surfaces for the cora-
coids anteriorly, for the sternal ribs
laterally.
The skeleton of the wing includes
the stout humerus, the separate radius
and ulna (the latter the larger), two
free carpals, a carpo-metacarpus of
a i o one another
, oc cvensents, and three
digits—the thumb with one joint, the
first finger with two joints, the second
with one. In adaptation to flight, the
wing of a bird has much less flexibility
of parts than the arm of a Mammal.
The radius and ulna do not move upon
each other.
The pelvic girdle consists of dorsal
ilia fused to the complex sacral region,
of ischia sloping backwards, and of
pubes running parallel to the ischia.
The incomplete ossification of the
acetabulum and the absence of ventral
symphyses are noteworthy.
The hind-limb consists of a short
stout femur, a tibia to which the
proximal tarsals (astragalus and os
calcis) are fused (forming a tibio-tar-
sus), an incomplete fibula joined to
Fic. 367.—Bones of
hind-limb of eagle.
J, Femur ; ¢.2.,tibio-tarsus ;
Jo., fibula; a@., ankle-
joint; 7.¢., tarso-meta-
tarsus ; 77.2’., first meta-
tarsal (free).
664 BIRDS.
the tibia, three metatarsals fused to one another and to
the distal tarsals (forming the tarso-metatarsus), a free first
metatarsal, and, finally, the four toes. The first, turned
backwards, has two phalanges, the second three, the third
four, and the fourth five.
Nervous system.—In contrast to the brain of crocodiles
and other Reptiles, the brain of the pigeon and other
Birds fills the cranial cavity. The cerebral hemispheres
Fic. 368.—Brain of pigeon (I. dorsal, II. ventral, III. lateral aspects).
OLF.L., Olfactory lobes ; C.H., cerebral hemispheres; PB., pineal body ;
OL., optic lobes; CB., cerebellum; FZ., flocculus or lateral exten-
sion of cerebellum; 47.0., medulla oblongata; P/7., pituitary body
at end of infundibulum (/VF.); O.V., optic nerves crossing in. the
chiasma.
are large and smooth. Their roof is thin, their main mass
consists of the large corpora striata which bulge into the
ventricles. They meet the cerebellum and throw the solid
optic lobes to the sides. The olfactory’ lobes are very
small (cf. deficient sense of smell). Between the cerebral
hemispheres and the cerebellum, the pineal body rises to
the surface, and a slight posterior separation of the
hemispheres will disclose the region of the optic thalami.
The large cerebellum is ridged transversely and divided
into a median lobe and two small lateral flocculi. The
ALIMENTARY SYSTEM. 665
curvature of the brain is well marked in the adult; thus
the medulla is quite hidden by, and descends almost
vertically from, the cerebellum.
There are as usual twelve cranial nerves.
In connection with the spinal cord, the brachial plexus of nerves
to the forearm, and the sacral plexus to the leg, should be noticed.
In the lumbar region the halves of the cords diverge for a short ,
distance, forming a wide space—the rhomboidal sinus—roofed only
by membrane. The cervical part of the sympathetic nervougy system
is double on each side.
Sense organs.—The sense of smell is not well developed
in Birds. The nostrils are longitudinal slits overhung by
the swollen, more or Icss tactile, cere. Apart from the
cere, there is only a diffuse sense of touch, and the sense
of taste is also slightly developed.
The sense of hearing is acute. Externally the ear is
marked by an open tube—the external auditory meatus ;
the aperture of which lies behind the eye, concealed
beneath the feathers. Within the tube, a little beneath
the surface, lies the drum or tympanum; connecting this
with the fenestra ovalis of the inner ear is the columella;
the tympanic chamber is continued past the ear as the
Eustachian tube, which unites with that of the opposite
side, and opens into the mouth cavity in front of the
basisphenoid bone. The cochlea, or curved protuberance
of the sacculus, which is incipient in Amphibians, and
larger in Reptiles, is yet more marked in Birds.
The eye has an upper, a lower, and a third eyelid or
nictitating membrane. The last is frequently twitched
across the eye, and helps: to keep the front clean; it is
present in many Reptiles and most Mammals. The
front of the sclerotic protrudes in a rounded cone, and is
strengthened by a ring of little bones. Into the vitreous
humour a vascular pigmented pecten protrudes from the
‘region of the blind spot where the optic nerve enters.
Birds have remarkable powers of optic accommodation. .
Alimentary system.—The jaws are ensheathed in horn,
and this sheath takes the place of teeth, and is sometimes
ridged, as in ducks. It is interesting to notice that this
horny beak was absent in some of the extinct toothed
birds. In modern birds there are no hints of teeth,
666 BIRDS.
except that “a dental ridge” (see Mammals) has been
detected in some embryos. A narrow tongue lies in the
floor of the mouth; it is unimportant in the pigeon, but
is often useful, as in parrots, woodpeckers, and hummihg-
birds. Associated with the tongue there are numerous
glands. On the roof of the mouth lie the posterior nares,
and behind them the single aperture of the Eustachian
tubes. The gullet expands into a thin-walled, bilobed,
non-glandular crop, in which the hurriedly swallowed
seeds are stored and softened. Especially at the breed-
ing season, the cells lining the crop degenerate, and
form “pigeon’s milk,” which both sexes give to the
young birds.
From the crop the food canal is continued into the
glandular part of the stomach (the proventriculus), where
gastric juice is secreted from large glands.
Beneath the proventriculus is the gizzard, in which.the
food is ground. The walls are very muscular, the fibres
radiating from two tendinous discs; the internal surface is
lined by a hard, horny epithelium; and within the cavity
are small stones which the bird has swallowed. In hawks
and fish-eating birds the gizzard region is, naturally enough,
soft. The pyloric opening, from the gizzard into the
duodenum, is very near the cardiac opening from the
proventriculus into the gizzard.
In the fold of the long duodenum lies the pancreas with
three ducts, whose number points to the triple origin of
the pancreatic rudiment in the embryo. Into the same
region open two bile-ducts from the two-lobed liver, which
is without a gall-bladder in the common pigeon, though
this is present in some birds, and even in some species of
pigeon.
The small intestine is long; the large ‘intestine very short,—not
more than a rectum two inches in length: At the junction of the
small and the large intestine there are two short cca. In some birds,
‘e.g. the fowl, these are of considerable length; in the ostrich they are
very long ; there are three in many ducks and birds of prey ; there is
only one in some fish-eating birds; in hornbills, parakeets, etc., they
are absent.
The cloaca has three divisions (see Fig. 369),—an upper part into
which the rectum opens, a median patt into which the ureters and the
genital ducts open, and a posterior region (proctodzeum), opening into
MUSCULAR SYSTEM. 667
which from the dorsal surface is « sac of obscure function, the bursa
Fabricii, which usually disappears during adolescence. It is at first
a blood-forming organ, but often becomes a mass of fibrous connective
tissue. t
Vascular system.—The relatively large four-chambered
heart, the complete separation of arterial and venous blood,
the single aortic arch bending over to the ght side, and
the hot blood (about 38° C., 100° F.), are important
characteristics. The heart-beats are more rapid in birds
than in other Vertebrates, being about 120 per minute
when the bird is at rest, and far more when it is
flying.
The impure blood returned by the
ven cave to the right auricle passes
into the right ventricle through the cd.
auriculo-ventricular valve (which has y,
two muscular flaps without chorde
tendinez or papillary muscles). From x a
pd..
the right ventricle it is driven to the |. \
lungs. From the lungs the purified is
blood returns to the left auricle, and ;
passes through two membranous valves Fic. 369.—Diagram-
(with chorde tendinez and papillary age uae be
muscles) into the left ventricle. After Gadow. —
“Thence it is driven through the .7 Upper region of clo-
arterial trunk into the carotids, the aca into which rectum
subclavians, and the dorsal aorta. ae
The bases of the aortic and pul- te, ae ae
monary trunks are guarded by three side; _d., posterior re-
gion into which bursa
semilunar valves. From the capil- Patvicii (7) opens.
laries the impure blood is .collected
anteriorly in two superior venz cave (precavals), and
posteriorly in an inferior vena cava (postcaval), composed
of veins from hind-legs and kidneys, and receiving as it
approaches the heart the hepatic veins from the liver.
The right auricle of the heart is larger than the left ; the right ventricle
has thin walls, and partly surrounds the more muscular left ventricle.
The muscular right auriculo-ventricular valve does not quite encircle
the opening from the auricle, an imperfect differentiation which recurs
in the Monotreme Mammals.
The arterial system consists of the following vessels (Fig. 370) :—
668
Sy >,
J
iL \ Ragll
X\
c Rep
Fic. 370.—Teart and arterial system of pigeon.
R.A., right auricle ; R.V., right ventricle; Z.V,, left ventricle; Z.A., left auricle;
P.V., pulmonary veins; P., pectoral artery; Br., brachial artery ; C., carotid
artery ; D.A., dorsal aorta ; CZ., coeliac; A./., anterior mesenteric ; R., renals ;
4", femoral; Sc., sciatic; /Z., iliac ; 4.m., posterior mesenteric ; C., caudal,
VASCULAR SYSTEM, 669
Fic. 371.—Heart and venous system of pigeon.
&.A.,Right auricle ; 2. ., right ventricle ; Z.V.,left ventricle ; Z.A.,
left auricle; P.., pulmonary veins ; P.A., pulmonary arteries ;
J., jugular; Bv., brachial; P., pectoral; H.V., hepatic; £.2.,
epigastric; /.V.C., inferior vena cava; C.//., coccygeo-mesen-
teric; /.V., iliac; #., femoral; &., renal; Sc., sciatic; Hy.,
hypogastric or ‘‘ renal-portal ” ; z.2/., internal iliac; C., caudal.
670 BIRDS.
.
(a) The arterial trunk, as it rises from the heart, gives off on each
side an innominate artery. Each innominate gives off a carotid
and a subclavian, and the subclavian immediately divides into
a brachial to the arm and a pectoral to the breast muscles.
(6) The dorsal aorta, formed by a continuation of the arterial trunk
bending round on the right side, gives off coeliac, mesenteric,
renal, femoral, sciatic, iliac, and other arteries.
(c) The pulmonary arteries carry impure blood from right ventricle
to lungs.
The venous system consists of the following vessels (Fig. 371) :—
(a) Two superior venze cavee, each formed from the union of
jugulars from the head, a brachial from the arm, and a pectoral
from the breast.
(4) The inferior vena cava is formed from the junction of two iliac
veins just in front of the kidneys. Each of these iliacs results
from the union of a femoral from the leg, an efferent renal
from the kidney, and a ‘‘renal-portal,” or hypogastric, which
passes upwards through the kidney. To understand this hypo-
gastric, it is convenient to begin at the tail. A short caudal
vein divides anteriorly into right and left branches, each of
which receives an internal iliac from the sides of the pelvic
region. Thus the hypogastric is formed at each side, and
this, passing upwards through the kidney, receives the sciatic,
and finally joins with the femoral and with the renal.
(c) The pulmonary veins carry pure blood from lungs to left auricle.
The hepatic portal system is as usual,—mesenteric veins from the
intestine combine in portal veins; the blood filters through the liver,
and is collected in hepatic veins, which unite with the anterior end of
the inferior vena cava.
A hint of a renal-portal system is represented by small branches, which
the femorals give off to the kidney.
From the transverse vein formed between the two hypogastrics or by
the division of the caudal vein, a coccygeo-mesenteric arises, which
receives vessels from the cloaca and large intestine, and is continued
along the mesentery to join the hepatic portal system.
As there are rarely any valves in the hypogastric veins, the blood
from the viscera and hind-limbs can pass freely either through the iliac
veins and thence to the inferior vena cava, or through the coccygeo-
mesenteric vein to the hepatic portal system.
The epigastric vein of the bird takes blood from the fat-laden sheet
or great omentum which covers the abdominal viscera. It leads not
into the liver, but into one of the hepatic veins. -
Associated with the blood-vascular system there is a
lymphatic system with a few lymphatic glands.
The spleen lies on the right side of the proventriculus, the
paired thyroid lies beside the origin of the carotids, and a
paired thymus is found in young birds in the neck region.
Small yellowish (suprarenal) glands lie on the front part of
the kidneys.
RESPIRATORY SYSTEM. 675
Respiratory system.—The ‘important facts are,—that
there is no true diaphragm; that some of the bronchial
branches in the lungs are continued into adjacent air-sacs ;
that expiration is a more active process than inspiration.
The nostrils lie at the base of the beak overlapped by
the cere. Only in the kiwi are they at the tip of the beak.
The glottis behind the root of the tongue leads into the
trachea, which has a voiceless larynx at its anterior end, and
a syrinx, with.vocal chords, at its base. The trachea is
strengthened by bony rings, and is moved by two sterno-
tracheal muscles from the sternum. The bronchial tubes
branch irregularly, in a kind of tree-like fashion, in the
lungs and end in very fine air-tubes. These lie attached
to the dorsal wall of the thorax, indented by the ribs, and
covered with pleural (peritoneal) membrane on their ventral
surface’ only.
Around the lungs, and connected with the ends of the
main bronchial branches, are the nine air-sacs. In order
from behind forwards, lie the abdominals, the posterior
thoracics, the anterior thoracics, the cervicals, and the inter-
clavicular in the middle line in front. The interclavicular
sac is in connection with both lungs, and is continued into
two axillary sacs in the arm-pits. The anterior and posterior
air-sacs are continuous with air-spaces in the bones. In
the resting bird the sternum rises and falls; in the flying
birds the thoracic region compresses the lungs and air-sacs ;
in either case, expiration is the more active part of the
respiratory process.
Excretory system.—The kidneys are three-lobed, and
lie embedded in the pelvis. They receive blood from the
dorsal aorta by renal arteries, and the filtered blood leaves
them by renal veins which unite with femorals and renal
portals to form the iliacs, or, we may almost say, the inferior
vena cava. But the kidney also receives a little venous
blood from branches of the femoral veins. Thus, there
is just a hint of a renal-portal system, which does not
occur in Mammals. The kidneys are metanephric in
origin.
The waste products, consisting for the most part of urates, pass im
semi-solid form down the ureters into the median compartment of the
cloaca,
672 BIRDS.
In front of each kidney, at the base of the iliac vein, there lies a
suprarenal body.
Reproductive system.—The testes lie in front of the
kidneys. Like the ovary, they increase in size at the breed-
ing season, and dwindle afterwards ; the sexual period in
Fic. 372.—Female urogenital Fic. 373.—Male urogenital
organs of pigeon. organs of pigeon.
K., Kidney with three lobes ; w., ureter ; T., testes; V7., base of inferior vena
cl., cloaca; ov., ovary; od., oviduct ; cava; S.R., suprarenal bodies; K.,
/-t., funnel at end of oviduct ; 7.7.0d., kidneys with three lobes (x, 2, 3);
rudimentary right oviduct. u., weter; v.d., vas deferens; vs.,
seminal vesicle ; c/., cloaca.
birds being much more narrowly limited than in most other
Vertebrates.
The spermatozoa pass from the testis into a vas deferens,
which lies to the outside of the corresponding ureter. The
vasa deferentia, slightly convoluted when full of sperms, and
with a posterior swelling or seminal vesicle, open separately
into the median compartment of the cloaca.
In the adult pigeon, and in most birds, there is only one
HABITS AND FUNCTIONS OF BIRDS. 673
ovary ; that of the right side usually atrophies early in life.
The right oviduct is represented by a small rudiment close
to the cloaca.
The ovary is covered with follicles containing ova at
various stages of ripeness. As these ova become dilated
with yolk and otherwise mature, they burst from the ovary,
and are caught by the dilated end of the oviduct which
opens into the ccelom. The first part of the oviduct is a
funnel or ostium tubze, which grips the ovum and probably
form the thin (chalaziferous) layer of dense albumin next
the yolk. The second part is the albumin-secreting
portion, which forms dense albumin. The third portion,
called the isthmus, makes the shell-membrane and more
albumin. The fourth region, badly called the uterus,
makes the shell and 30-40 per cent. of the albumin, which
passes by diffusion through the shell-membtane before the
shell is formed. The fifth portion, called the vagina,’ is
very muscular and expels the egg. It has only unicellular
glands which perhaps secrete the external cuticle of the
shell, plus pigment.
A section through the oviduct shows—a peritoneal invest-
ment, longitudinal muscles, connective tissue with blood
vessels, circular muscles, connective tissue, a thick layer
of convoluted branched tubular glands except in the funnel
and the vagina, and most internally ciliated epithelium,
except in the anterior part of the funnel.
In sexual union the cloaca of the male is closely apposed
to that of the female; only in a few cases (in ducks and
geese, Crax, Tinamus, and in the Ratite) is there a
copulatory organ. The eggs are incubated by the parents
for a fortnight, a high temperature of about q4o° C. being
sustained throughout.
HaBITs AND FUNCTIONS OF BIRDS.
Flight.—As birds are characteristically flying animals, many ot
their peculiarities may be interpreted in adaptation to this mode of
motion.
(a) Shape and general structure of the body.—The resistance offered
by the air to the passage of a body through it depends in part on the
shape of the body, and the boat-like shape of the bird is such that it
43
674 BIRDS.
offers relatively little resistance. The attachment of the wings high
up on the thorax, the high position of such light organs as lungs and
air-sacs, the low position of the heavy muscles, the sternum, and the
digestive organs, the consequently low centre of gravity, are also
structural facts of importance. But it must be remembered. that the
frictional resistance of the air is slight.
(4) The muscles of flight.—The pectoralis major brings the wing
downward, forward, and backward, keeping the bird up and carrying
it onward. As it has
most work to do, it is
by far the largest. The
pectoralis minor razses
the wing for the next
stroke. There are others
of minor importance.
On an average these
muscles weigh about one-
sixth of the whole bird,
nearly one-half in some
pigeons. Buffon noted
that eagles disappeared -
from sight in about three
minutes, and a common
rate of flight is about fifty
feet per second. In mi-
gration many birds fly at
a rate of over 100 miles
an hour.
(c) The skeleton.—The
rigidity of the dorsal part
of the backbone, due to
fusion of vertebrze, is of
advantage in affording a
firm fulcrum for the wing-
a strokes, while the arched
; clavicles (meeting in an
Fic. 374.—Pectoral girdle and sternum _interclavicle and often
of Bewick’s swan. fused in front to the
A part of carina removed shows peculiar loop of sternum) and the strong
trachea (¢r.); cl., clavicle; cor., coracoid; sc,, Coracoids (which articu-
scapula ; g/., glenoid cavity for head of humerus; late with the sternum)
%., parts of sternal ribs. are adapted to resist the
inward pressure of the
down-stroke. As the keel of the breast-bone serves in part for the in-
sertion of the two chief muscles, its size bears some proportion to the
strength of flight. It is absent in the running birds, such as the
ostriches, and has degenerated in the New Zealand parrot (S¢rzngops),
which has ceased to fly and taken to burrowing.
(ad) Air-sacs and air-spaces.—The lungs of birds open into a number
of air-sacs, which have a larger cubic content than the lungs, and in
many cases these air-sacs are continued into the bones, among the
FLIGHT. 675
viscera, and even under the skin, From a broken bone it is possible
to inflate the air-sacs, and through a broken bone a bird with choked
windpipe may for a time breathe. The whole system of air-containing
cavities is continuous, except in the case of the skull bones, whose
spaces receive air from the nasal and Eustachian tubes. The air must
lessen the specific gravity of the bird, but a few mouthfuls of food are
sufficient to counteract the lightening. Moreover, in many small
birds of powerful flight, all the large bones, or all except the humerus,
contain marrow, and are therefore’ not. ‘* pneumatic” ; and the hornbill,
Fic. 375.—Position of wings in pigeon at maximum elevation.
From Marey. ;
which has no great power of flight, is one of the most pneumatic of
birds. It is certain that in ordinary flight the lightest of birds has
to keep itself from falling by constant effort. The air-sacs increase
the bird’s respiratory content, secure more perfect aeration of the
lungs, and assist in internal perspiration, thus helping in the regulation
of the body temperature.
To carry the weight of the bird, the wings strike vertically ; to carry
the bird onwards, they strike obliquely. Sometimes the direction.of the
stroke is more vertical, and then the bird mounts upward ; sometimes
it is more oblique, and then the bird speeds onwards; usually both
676 BIRDS,
directions are combined. The raising of the wing after each stroke
requires relatively little effort, the resistance to be overcome being
very slight. In steering, the feathers of the tail often bear to the
wings a relation comparable to that between rudder and sail.
Fic. 376.—Wings coming down.—From Marey.
Modes of flight,—There are three chief modes of flight :-—
1. By gliding or skimming, during which the bird has its wings
spread, but does not flap them, depending for its movement on the
ms
Fic. 377.— Wings completely depressed.—From Marey.
velocity acquired by previous strokes, by descending from a higher
to a lower level, or by the wind. This may be readily observed in
gull and heron, in a pigeon gliding from its loft to the ground, or in
a falcon swooping upon its quarry.
SONG—COURTSHIP—WNESTS. 677
2. By active strokes of the wings, in which the wings move down-
ward and forward, backward and upward, in a complex curve. This
is of course the commonest mode of flight,
3. By sailing or soaring with motionless spread wings, in which the
bird does. not necessarily lose in velocity, or in vertical position, as is
the case in gliding. It is illustrated by such birds as crow, falcon,
stork, albatross, and has been observed only when there was wind.
Song of birds.—Singing is a natural expression of emotional
intensity. It is richest at the breeding season, and is always best and
often solely developed in the males. But song in any excellence is the
gift of comparatively few birds, though nearly all have a voice of some
sort, often so characteristic that the species may be recognised by its
call. The parrot and the jackdaw, and others, can be taught to
pronounce articulate words; and the power of imitation‘is widespread
among birds, which are notorious plagiarists. This power of imitation
is important in relation to the general theory of instinct, for the song of
all birds is probably in great part imitative, though to a limited extent
inherited. Young birds taken away from their nests when very young,
so that they have hardly heard the voices of their kind, may utter the
characteristic note of the species, but they sing the song imperfectly.
Many birds, apart from those who have been educated, have ‘‘ words,”
expressing pleasure, pain, sense of danger, presence of food, and the
like. But there is a difference between uttering words and having a
language, which implies the expression of a judgment.
Courtship.—Birds usually pair in the springtime, but there are many
exceptions. A few, ¢.g. some of the birds of prey, live alone except at
the pairing time; others, notably the doves, always live together in
pairs ; many, such as rooks, parrots, and cranes, are sociable, gregarious
birds. A few, like the fowls, are polygamous; the cuckoo is poly-
androus.
In most cases, however, birds pair, and the mates are true to one
another for a season, The pairing is often preceded by a courtship, in
which the more decorative, more vocal males win their desired mates,
being, according to Darwin, chosen by them. Darwin attributed the
captivating characteristics of the males, well seen in peacocks and birds
of paradise, or as regards musical powers in most of our own British
songsters, to the sexual selection exercised by the females ; for if the
more decorative or the more melodious males always got the preference
in courtship, the qualities which contributed to their success would tend
to predominate in the race. He believed, moreover, that characteristics
of male parents were entailed on male offspring. Wallace regarded the
differences between males and females in another way, arguing that in
the course of natural selection the more conspicuous females had been
eliminated, brightness being disadvantageous during incubation. It
seems likely enough that both conclusions are to some extent true,
while there is much to be said in favour of a deeper explanation, to
which Wallace inclines, that the secondary differences between the sexes
are correlated with the fundamental constitutional differences involved
in maleness and femaleness.
Nests.— Alter pairing, the work of nest-building is begun. Almost
all birds build nests ; the well-known habit is a characteristic expression
678 BIRDS.
of their parental care. Other creatures, indeed, such as sticklebacks
among Fishes, and squirrels among Mammals, besides numerous Insects,
build nests, but the habit is most perfectly developed among Birds.
As is well known, each species has its own peculiar style of nest, and
builds it of special materials, Generally the nest is solitary, hidden in
some private nook. The perfection of art which is reached by some
birds in the making of their nests is marvellous ; they use their bills and
their feet, and smooth the inside by twisting round and round. Usually
the hen does most of the work, but her mate sometimes helps, both in
building the nests and in hatching the young.
The nest is a cradle rather than a house, for its chief use is to secure
an approximately constant warmth for the young which are being
formed within the eggs; and to afford protection for the helpless
fledglings. At the same time, the nest secures the comfort of the
parent-bird during the days and nights of brooding.
The variety of nests may be illustrated by mentioning the burrowed
nests of sand-martins and kingfishers, the ground-nests of game-birds
and gulls, the mud-nests of house- martin and flamingo, the holes
which the woodpecker fashions in the tree-stem, the platforms built by
doves and eagles, storks and cranes, the basket-nests of most singing-
birds, the structures delicately woven by the goldfinch, bullfinch, and
humming-birds, the sewed nest of the tailor-bird, the mossy nests of
the wrens, the edible nest of the Col/ocalia, which is chiefly composed
of mucin secreted by the salivary glands.
Eggs of birds.—When the nest is finished, the eggs are ready to
be laid. After they are laid, the patience of brooding begins. With
the great care that Birds take of their young we may associate the
comparatively small number of the eggs; but there are probably other
reasons why the number of offspring decreases as animals become more
highly evolved.
The size of the egg usually bears some relation to the size of the bird.
Of European birds, the swans have the largest eggs, the golden-crested
wrens the smallest. It is said that the egg of the extinct J/oa some-
times measured 9 in. in breadth and 12 in. in length; while that
of the extinct 4pyornds held over two gallons, some six times as
much as_an ostrich’s egg, or a hundred and fifly times as much as
afowl’s. Yet the size of the egg is only generally proportional to that
of the bird ; for, while the cuckoo is much larger than the lark, the eggs
of the two are about the same size; and while the guillemot and the
raven are almost of equal size, the eggs of the former are in volume
about ten times larger than those of the latter. The eggs of birds
whose young are rapidly hatched and soon leave the nests are large.
Professor Newton remarks that ‘‘the number of eggs to be covered at
one time seems also to have some relation to their size,” while from
what one notices in the poultry-yard, and from a comparison of the
habits of different birds, it seems probable that a highly nutritive,
sluggish bird will have larger eggs than a bird of more active habit and
sparser diet.
The shell of the egg is often very beautifully coloured ; there is a
predominant tint upon which are spots, streaks, and blotches of varied
colour and disposition, so that the egg is almost always characteristic of
DIET. 679
the species. Pigments related to those of the blood and the bile are
deposited while the shell is being formed in the lower part of the
oviduct. As the eggs may move before the pigments are fixed, blotch-
ings and markings naturally result. The coloration is often protectively
harmonious with that of the surroundings. Thus eggs laid almost on
the ground are often brownish like the soil, those laid near the seashore
often look very like stones, while conspicuous eggs are usually found
in covered nests.
Some newly hatched young are naked, blind, and helpless, and have
to be carefully fed by their parents until they are fully fledged. This
is true of the thrush and of many other song-birds. Others are born
covered with down, but still helpless; while a few, like the chicks,
are able to run about and feed themselves a few hours after they leave
the egg. Those which require to be fed and brooded over are called
Altrices ; those which are at once able to feed themselves are called
Preecoces.
Moulting.—Every year birds lose their old feathers. This
moulting generally takes place after the fatigue of the breeding: season,
but in the case of the swallows, and the diurnal birds of prey and
some others, the moult is in mid-winter. The process is comparable
to the casting of scales in Reptiles, and to the shedding of hair in
Mammals. Feathers are so easily injured that the advantage of the
annual renewal is evident, especially when it takes place just before
the time at which it may be necessary to set forth on a long migratory
flight.
Sh moulting, the feathers fall out and are replaced gradually, but
sometimes they are shed so rapidly that the bird is left very bare ; thus
moulting geese, ducks, and rails lose all their quills at once and are
unable to fly. There are many birds that moult, more or less com-
pletely, more than once a year; thus the garden warbler sheds its
feathers twice. The males of many birds assume special decorations
after a partial or complete moult before the time of pairing (ruff, knot,
golden plover). The ptarmigan changes its dress three times in the
year; after the breeding season the plumage becomes grey; as the
‘winter sets in it grows white, and suited to the surrounding snow ; in
the spring, the season of courtship, the mottled brown wedding robes
are put on.
Diet.—The food of birds varies greatly, not only in different kinds,
but also at different seasons. Many are herbivorous, feeding on the
soft green parts of plants, and in these birds the intestine is long.
Some confine themselves to grain, and these have large crops and strong
grinding gizzards, while those which combine cereals and insects have
in most cases no crop. A few sip honey, and may even help in the
cross-fertilisation of flowers ; those that feed on fruits play an important
part in the dissemination of seeds; those that devour insects are of
great service toman. In fruit-eating and insectivorous birds the crop
is usually small, and the gizzard only slightly muscular. But many
birds feed on worms, molluscs, fishes, and small mammals; in these
the glandular part of the stomach is more developed than the muscular
part. The nature of the stomach in the Shetland gull changes twice
680 BIRDS.
a year, as the bird changes a summer diet of grain and seeds for a
winter diet of fish, and wzce versa. In the case of canaries, bullfinches,
parrots, etc., it has been noted that the food influences the colouring of
the plumage. ;
Migration of birds.—Migration remains in no small degree a
zoological mystery. On certain points we need more facts, and -even
where facts are abundant we but imperfectly understand them. Let us
first state some of the outstanding facts.
1. Most birds seem to be more or less migratory, but the range
differs greatly. It is said that the dotterel may sup on the North
African steppe and breakfast next morning on the Arctic tundra; and
although the alleged rate may not be demonstrable, there is no doubt
that a distance of about 2000 miles is traversed by this bird and by
many others. In the tropics, on the other hand, the migration may
simply be from valley to hillside.
2. Observers in temperate countries long ago noticed that the birds
they saw might be grouped in reference to their migrations. Thus
(a) some arrive in spring from the South, remain to breed, and leave for
the South in autumn, e.g. swallow and cuckoo in Britain; (4) some
arrive in autumn, chiefly from the North, stay throughout the winter,
and fly northwards again in spring, e.g. the fieldfare and the redwing in
Britain ; (c) some—the ‘‘ birds of passage”—are seen only for a short
time twice a year on their way to colder or warmer countries in spring
or autumn, ¢.g. sandpipers ; and (d@) some seem to deserve, the name of
“residents,” but really exhibit a partial migration, such as the song-
thrush and redbreast in Britain. In Europe the spring migration is
on the whole northwards and north-eastwards, in autumn southwards
and south-eastwards, but the paths are great curves.
3. There is a striking regularity in the advent and departure of many
of the migrants. In spite of the immense distances which many of our
immigrants travel, and in spite of unpropitious weather, they are often
punctual within a day or two to their average time of arrival for many
years. Similarly some birds, such as the swifts, are hardly less precise
in Jeaving our shores.
4. It has been proved in a few cases that individual birds may find
their way back to where they made their nest in previous years. Not
less marvellous is the security with which the flight from country to
country is continued in darkness, at great heights, and over the track-
less sea. At the same time it must be noticed that the mortality
during migration is very great.
Having stated a few of the outstanding facts, let us note some of the
interpretations and suggestions which help us to understand them.
The impulse to migrate is instinctive; but it is likely that there are
always immediate causes which prompt the instinct, such as scarcity
of food, the shortening daylight, and the increasing cold in the case of
many birds which leave us in autumn. It is more difficult to recognise
the immediate causes prompting their return. In leaving Britain the
young birds usually fly first ; in returning, the sexual adults lead the
way.
It seems likely that the origin of the migrating habit is wrapped up
with the history of climates, and we can understand how the setting in
DEVELOPMENT OF THE CHICK. 68x
of glacial conditions from the north would gradually force birds, century
by century, to a longer flight southwards. And if the climatic condi-
tions limit the area of safe and comfortable breeding to one country (the
more northerly), and the possibility of food during winter to another
country (the more southerly), we can understand, with Wallace, ‘‘ that
those birds which do not leave the breeding area at the proper season
will suffer, and ultimately become extinct ; which will also be the fate
of those which do not leave the feeding area at the proper time.” In
short, given environmental changes of climate on the one hand, and a
measure of plasticity and initiative on the part of the organism, the instinct
of migrating would be perfected in the course of natural elimination.
But while this view is so far satisfactory, it leaves us face to face with
the problem how birds migrate as safely and surely as they do on their
pathless way. For to point out that the merciless elimination which
continually goes on keeps up the standard of racial fitness, leaves us
still wondering how any became fit at all. ‘
One welcomes therefore any suggestion as to the manner in which
birds learn or have learned to find their way. The power has been
compared to the ‘‘ homing ” faculty of some pigeons, but most believe
that pigeons are guided largely by noticing landmarks, which could
hardly be done over 10,000 miles of land, and obviously not over 1090
miles of sea, or during the night. Some have urged that birds follow
river valleys, the lines of old ‘‘land bridges” connecting continents,
the roll of the waves, and so forth, but the difficulty remains of flight by
night and at very great heights. Attractive is the suggestion that birds
are guided by what may be called a ‘‘ tradition” based on experience ;
those guide well one year who have followed well in previous years.
But many young birds fly apart from their parents, and some birds do
not fly in flocks at all. Moreover, it is difficult to understand how the
experience could be gained except by sight, which in many cases is
excluded by the darkness. In face of these difficulties, many authorities,
such as Professor Newton, have been led to believe that birds have, in
an unusual degree, ‘‘a sense of direction.” :
DEVELOPMENT OF THE CHICK
The ovayian ovum of the hen is a large spherical body, consisting
chietly of yolk, but exhibiting at one region a disc of formative proto-
plasm with a large nucleus. The ripe ovarian egg is surrounded by a
vitelline capsule, mainly due to the follicular theca in which it is formed.
There is an innermost non-cellular membrane, then an epithelium, then
a connective tissue outer membrane. The ripening of the egg is
accompanied by the disappearance of the nuclear membrane, and also
by the formation of polar bodies; but the details of the process are
obscure... : , ;
Either before it leaves the ovary, or in the upper part of the oviduct,
the egg is fertilised by a spermatozoon. During its passage down the
oviduct it undergoes two sets of changes. On the one hand it is sur-
rounded by various envelopes added to the delicate vitelline membrane
682 BIRDS.
Fic. 378.—Stages in development of chick, —After Marshall.
1. Segmentation, superficial view of blastoderm,
2. Vertical section of blastoderm, Z%., Epiblast ; Zc., lower layer
of cells; s.g.c., sub-germinal cavity; y., yolk.
DEVELOPMENT OF THE CHICK. 683
with which it is already invested ; on the other hand, segmentation goes
on rapidly in the formative area. :
The fully formed and laid egg is surrounded by a firm porous shell of
carbonate of lime, and beneath this there is a double shell membrane,
the two layers of which are separated at the broad end of the shell to
form an air-chamber. This chamber grows larger as development pro-
ceeds, and is of some importance in connection with respiration, as an
intermediate region between the embryo and the external medium.
Beneath the shell membranes lies the albumen, or ‘‘ white of egg,”
which is secreted by the thin-walled region of the oviduct ; in it lie two
spirally-twisted cords or chalazze, produced by the rotation of the egg in
the oviduct. Within the enveloping
albumen lies the ovum proper, with
its enormous mass of yolk. The
yolk is not homogeneous, but con-
sists of two substances, known re-
spectively as white and yellow yolk.
The white yolk forms a central flask-
shaped mass, and occurs also as thin
concentric layers in the yellow yolk.
The minimum temperature at
which a hen’s egg will develop nor-
mally is 28°C. If the temperature , : .
fall below this, development stops. F16. 379.—Diagrammatic section
In early stages the interruption may of egg.—After Allen Thomson.
last for days without fatal’ results, g.v., Position of germinal vesicle;
though always with a tendency to #¢ Rie ener ccm yolk a
induce subsequent abnormalities. rite”) ch, chalaze
Towards the end of incubation more oy
than a day’s cooling is usually quite fatai.
On the upper surface of the yolk, in whatever position the egg be
held, lies the segmented blastoderm, whose exact origin we must con-
sider more precisely.
As we have seen, yolk is to be regarded as an inert and passive sub-
stance. In the hen’s egg we have an increased specialisation along
the line indicated by the egg of the frog. For there is a small patch
of formative protoplasm at one pole, and a large aggregate of yolk
composing the remainder of the egg. In consequence, the activity of
3. Diagrammatic surface view. .f., Area pellucida; a.o, area
opaca; #.p., neural groove ; Z.s., primitive streak ; 47., meso-
blast spreading over yolk.
4. Diagrammatic surface view at later stage. a.., Area pellucida ;
@.0., area opaca; m.s., mesoblast segments; #.s., primitive
streak. The dark border shows the spreading of the mesoblast
over the yolk.
s. Cross-section. s.c.,Spinal cord ; s.g., rudiment of spinal ganglia ;
N., notochord; #2.g., mesoblastic plates; A., aorta; Am.,
amnion fold; ¢., coelom or pleuro-peritoneal cavity.
6. Embryo. Cé., Cerebellum; #., ear; H., heart; ., fore-limb ;
4.1, hind-limb; y.s., stalk of cut-off yolk-sac; AZ, allantois ;
E., eye; C., cerebrum. On the dorsal surface the mesoblastic
somites are indicated.
684 BIRDS.
the protoplasm is unable to overcome the inertia of the yolk, and
segmentation is meroblastic and discoidal (cf. Elasmobranchs),
In the protoplasm of the egg horizontal and vertical furrows appear
in rapid succession, The result, as exhibited by vertical sections, is
to produce an upper epithelial layer of cells, separated by a small
space from larger, more irregular cells, which are still in connection
with the yolk on which they lie. At the circular border of the
germinal disc the two sets of cells are continuous. According to some
authorities, this stage represents the blastula, the upper layer of cells
corresponding to the cells of the animal pole in the frog, the lower
with the enormous mass of yolk on which they lie to the cells of the
vegetative pole, the space to the segmentation cavity.
At the next stage there appears a crescent-shaped groove. In this
region there is an ingrowth of cells, which probably represents a
modified process of gastrulation, and results in the obliteration of the
segmentation cavity, and the formation of a ‘‘ sub-germinal” cavity or
archenteron. The floor of the sub-germinal cavity is formed by the yolk,
in which, by a process of supplementary cleavage, yollk-nuclei appear.
This condition is that attained when the egg is laid. On surface view
we see a central ill-defined ‘“‘pellucid area.” This, which becomes
much more distinct during the early hours of incubation, is the area of
the blastoderm which overlies the sub-germinal cavity, and is contrasted
with the surrounding ‘‘ opaque area,” which lies directly on the yolk.
At the posterior region of the opaque area, as alveady noted, there is
the crescentic groove, where the outer and inner layers are continuous.
After the commencement of incubation, the blastoderm spreads
rapidly over the yolk, chiefly by the extension of the area opaca; the
area pellucida meanwhile elongates and becomes oval.
Another important change which also occurs in the early hours of
incubation is the conversion of the transverse crescentic groove into the
longitudinal primitive streak. The precise meaning of this change is
difficult and uncertain, but there seems no doubt that the primitive
streak represents the anterior lip of the blastopore of the frog. It runs
down the centre of the area pellucida and is marked by a central furrow,
the primitive groove. At its sides two wings of cells are obvious ; these
soon spread out laterally and anteriorly, and constitute the mesoblast.
All three layers of the embryo are connected at the sides of the
primitive streak, as at the margin of the blastopore in the frog.
In the region in front of the primitive streak, a row of hypoblast
cells becomes differentiated to form the notochord. At its sides the
sheets of mesoblastic cells split into an inner or splanchnic layer, and an
outer or somatic layer. A little later the mesoblast divides into the
segmentally arranged mesoblastic somites, lying at the sides of the noto-
chord, and the unsegmented lateral plate, whose outer and inner walls
form the corresponding boundaries of the ccelom.
At the time when the notochord has appeared internally, the external
epiblast becomes differentiated to form the medullary groove, which
gives rise in the usual way to the medullary canal. The folds at first
diverge posteriorly on either side of the primitive streak, but as the
union travels backwards, this is included in the medullary canal, and so
disappears.
DEVELOPMENT OF THE CHICK. 685
During the course of the second day the embryo seems to sink
farther into the yolk, while both anteriorly and posteriorly double folds,
known respectively as the head and tail folds, rise up. In the course
of their development the embryo becomes completely ‘‘folded off”
from the yolk. At a slightly later stage, side folds also appear ; all the
folds now consist of a double layer of somatic mesoderm covered by
epiblast. The folds meet above the back of the embryo and coalesce.
The inner layer forms the true amnion, the outer the false amnion or
subzonal membrane. Into the space between the amniotic folds, a
diverticulum from the posterior region of the gui, the allantois, grows
out,
Before the end of the first day,
blood vessels begin to be developed
in the extra-embryonic region of
the blastoderm. These form the
beginning of the vitelline vessels,
which are of great importance in the
early stages of development, and
have probably at first some respira-.
tory importance. As development
proceeds, the allantois increases
greatly, and, fusing with the sub-
zonal membrane, approaches close
to the egg-shéell. It hasa large blood
supply, and functions as an organ of
respiration ;- in addition it absorbs
the white of egg, thus serving as an
organ of nutrition; it also receives
deposits . apes thus oe Fic. 380.—Diagrammatic section
in connection with excretion. thi at
We have spoken of the ‘‘ folding oo ni eer eee
off” of the embryo; asa result of Di Mallsneend walltevalksactaie
this, the embryo is attached by a ““Zut‘of embryo: aZ, al, inner and
relatively narrow stalk to the large outer wall of the allantois; aw,
yolk-sac, over which the blastoderm ee proper ube ee line
is now slowly spreading. Inthisre- S00." bats ie te Mee eikonal
spect the embryo strongly resembles membrane; /. is placed within the
that of the dogfish ; it differs from extra-embryonic body cavity into
the latter in the presence of the which the allantois grows.
overarching amniotic folds, and
in, the respiratory allantois, which functionally replaces the protruding
gills of the young dogfish. In the young tadpole the yolk lies heaped
up on the floor of the gut, and causes a certain amount of distortion.
In the chick, as in the embryo dogfish, the amount of yolk is so great
that it forms a hernia-like protrusion of the gut, and only at a very late
stage is the greatly reduced sac withdrawn into the body cavity, after
which the dermal and intestinal umbilical openings are closed.
The chick embryo never exhibits any trace of gills, but the gill-clefts
perforate the pharynx. The embryonic organ of respiration is the
allantois, but that arrangement of aortic arches by means of which in
the tadpole blood is carried to the gills is repeated here.
686 BIRDS.
About the twentieth day the beak, which has a hard “‘ tooth” on the
tip, perforates the membranes of the air-chamber, and the air, rushing
in, expands the hitherto functionless lungs. At the same time import-
ant changes occur in the circulatory system, ‘‘the umbilicus becomes
completely closed, the allantois shrivels up, and the chick, piercing
the broad end of the shell with repeated blows of its beak, steps out
into the world.”
CLASSIFICATION OF BIRDS
I. Sub-Class ARCHAORNITHES or SAURUR4. Ancient extinct birds,
connecting Birds and Reptiles
The oldest known bird is Archeopteryx, two specimens of which
have been found in the Solenhofen Lithographic Stone (Upper Jurassic)
of Bavaria. ‘‘The stone is so fine-grained that, besides the bones of
the wings, the furculum or merrythought, the pelvis, the legs, and the
tail, we have actually casts or impressions on the stone (made when it
was as yet only soft mud) of all the feathers of the wings, and of the
tail.”,—Nicholson and Lydekker.
This link between Birds and Reptiles seems to have been a land bird
about the size of acrow. The skull is like that of a typical bird. The
upper jaw shows thirteen pairs of conical teeth, the lower about three
pairs. They are embedded in sockets. Each of the twenty vertebrze
of the long tail bears a pair of lateral rectrices—a unique arrangement.
There is no pygostyle. The vertebrae seem to have been either
amphiccelous or with flat ends; the ribs are very slender, without
uncinate processes ; there seem to have been ‘‘abdominal ribs” ; the
sternum is not clearly known; there is a U-shaped furcula. The
metacarpals seem to have remained separate; the first finger has
two phalanges, the second three, the third three or four, and all are
clawed. There is a tarso-metatarsus and four toes, as in the pigeon.
II. Sub-Class NEORNITHES
The metacarpals are fused. The second finger is the longest, and
the third is reduced. Only in Opzsthocomus are the three digits of the
fore-limb clawed; in most cases claws are confined to the thumbs.
Caudal vertebrz are apparently not more than thirteen in number.
1. Division RaTIT#. Running Birds with raft-like unkeeled
breast-bone
The African ostrich (S¢vuthzo) is represented by two or three species,
at home in the plains and deserts of Africa, and notable for their size,
swiftness of foot, and beauty. There are but two toes, the third and
the fourth, with stunted nails. There are no clavicles. The pubes
form a ventral symphysis. The enormous size of rectum and czca is a
unique character. The ostrich is polygamous, and at the breeding
season the hens lay the eggs, at intervals, in a hollow dug out in the
sand by the male. The eggs are incubated by both parents, but
especially by the cock.
CLASSIFICATION OF BIRDS. 687
The American ostrich (hea) is represented by three species in the
S. American Pampas. In the Rhea there are three toes, all clawed,
and the ischia form a ventral symphysis. There are no clavicles,
Only here among Ratitee is there a well-developed syrinx. The cxeca
are large. The male excavates a shallow nest in the ground, and
there, surrounded by a few leaves and grasses, the numerous eggs are
usually laid. It seems that the male bird alone hatches the eggs.
Single eggs are often laid here and there on the plains, but these are
not incubated.
The Emu (Dromeus) is represented by two species in Australian
deserts and plains. The fore-limb is greatly reduced, the feathers have
long aftershafts. Nearly related are the Cassowaries (Caswarzus) living
in the Austral-Malayan region, eight species in the Papuan Islands, one
in N.-E. Australia, and one in Ceram. They live in the forests and
scrub. The fore-limb is very small, with the shafts of the wing feathers
reduced to spines; the ordinary feathers have long aftershafts. On
the top of the skull there is a horny helmet, covering a core of light
spongy bone ; this protects the bent head as the bird rushes through the
scrub. There are three toes, the inner one with a long sharp claw—a
formidable weapon. In Emu and Cassowary the clavicles are repre-
sented by separate rudiments and the czeca are small.
The Kiwi (4f¢eryx) forms a very distinct genus of Ratitee, represented
by four species, restricted to New Zealand. It is not larger than a
hen, and has simple hair-like or bristle-like feathers, a long bill and
terminal nostrils, a very rudimentary wing and no clavicles, and no
distinct tail feathers. There are four clawed toes. The czeca are large.
It is a nocturnal bird, swift and noiseless in its movements, feeding in
great part on earthworms. The egg is very large for the size of the bird.
Among the extinct Ratite are the gigantic Moas (Dzwornis), which
seem to have been exterminated in New Zealand in comparatively recent
times. The fore-limbs were almost completely reduced, the hind-legs
were very large, and some forms attained a height of 10 ft. or even more,
Another recently lost order of giant birds is represented by remains
of pyornds found in Madagascar. Some of these indicate birds as
large as ostriches, but eggs have been found holding six times as much
as that of an ostrich. ;
We may think of the Ratite, according to W. K. Parker, as ‘‘ over-
grown, degenerate birds that were once on the right road for becoming
flying fowl, but through greediness and idleness never reached the
‘ goal,’—went back, indeed, and lost their sternal keel, and almost lost
their unexercised wings.”
2. Division ODONTOLC&. Represented by Hesperornds from N.
American Cretaceous strata, somewhat like a swimming ostrich,
with sharp teeth sunk in a groove, with saddle-shaped cervical
vertebrze as in modern birds, with a rudimentary fore-limb, but
with a powerful swimming leg. In an English representative—
Enaliornis—the vertebrae are chiefly biconcave. These extinct
birds have many Ratite skeletal characters, and they have also
interesting resemblances to some old-fashioned living Carinate,
notably the divers (Colymbidz).
688 BIRDS
Fic. 381.—Hesperornis.— After Marsh.
ST., Sternum; CO., coracoid; CL., clavicle; ., rudimentary humerus 5
SC., scapula; P., pectineal pubic process; /7Z., ilium; /S., ischium ;
P.P., post-pubis; C.7., crest of tibia; /, fibula; 7.7., base of tibio-
tarsus; 7.4/.7., tarso-metatarsus.
RATITZ, AND CARINAT&, 689
3. Division CARINAT&. Flying birds with a keeled breast-bone
Apart from the extinct types of Carinate, such as /chthyornds (with
teeth and biconcave vertebree), and the large Tertiary Odontopleryx,
with tooth-like pegs of bone on its jaws, there seem to be over 11,000
living species, These may be grouped in twenty-one orders, such as
Passeres (thrushes, etc.), Accipitres (hawks, etc.), Columbze (doves),
Gallinee (pheasants, etc.), Gavize (gulls, etc.), Psittaci (parrots). Of
the twenty-one orders only three are unrepresented in Britain.
The old classification of birds into snatchers, perchers, climbers,
scratchers, stilt-walkers, and swimmers was interesting and suggestive,
but an arrangement of this sort is bound to be misleading, since birds
of very different structure may have very similar habits.
It may be of interest to contrast the two divisions of living birds.
SOME CONTRASTS BETWEEN MODERN RATITA AND
MODERN CARINATA
RATITA,
CARINATA,
Running Birds, with wings more
or less degenerate and unused in
flight, witha keelless raft-like breast-
bone.
The skull is dromzognathous,
z.e. the vomer is interposed between
the palatines, the pterygoids, and
the basisphenoidal rostrum.
The sutures in the skull remain
for a long time distinct. The
quadrate articulates with the skull
by a single head.
The long axes of the adjacent
portions of the scapula and coracoid
lie almost in the same line, or form
a very obtuse angle, and the two
bones are fused.
The clavicles are small or absent.
The ilium and ischium are not
united behind, except in old Rheas
and Emus, No pygostyle.
The feathers of the adult have
free barbs, the Larbules have no
hooks. There is no oil gland,
except in the kiwi. There are no
regularly arranged pterylz.
The male has a penis.
The young are always przeccces,
Vlying Birds, with wings almost
always well exercised in flight, with
a keeled breast-bone,
(The keel is rudimentary in the
New Zealand parrot Strinmgops, in
the exterminated Dodo (Dédus),
and in the extinct Ag/ornis—one of
the rails. The penguins do not fly
at all; the Tinamou, the Hoatzin,
and some other birds, fly very little )
Except in the Tinamous, the
skull is never dromzeognathous,
z.e. the vomer is not fused with
the neighbouring bones of the
palate, and the palatines articulate
with the basisphenoidal rostrum.
The sutures in the skull almost
always disappear very early. The
quadrate articulates by a double
head.
The scapula and coracoid meet
almost at right angles, and are
connected with one another by
ligament only.
The clavicles are in most cases
very well developed.
The ilium and ischium unite,
enclosing a sciatic foramen. Usually
a pygostyle, ,
‘The barbs of the. feathers are
generally united, the barbules have
hooks. There is usually an oil
gland.
The male has rarely a penis.
The young may be precoces or
altrices.
44
?
690 BIRDS.
Pedigree. — Birds have many structural affinities with
Reptiles ; some of the ancient Dinosaurs present approxi-
mations to Birds; the extinct flying Pterodactyls show that
it was possible for flight to be developed among Reptiles ;
the oldest bird — Archaeopteryx —is in many ways a
connecting link between the two classes; and the develop-
ment of some Birds reveals many remarkable resemblances
with that of Reptiles,—therefore, with the strength of the
general argument for evolution to corroborate us, we
conclude that Birds evolved from a Reptile stock.
It is likely, then, that Birds arose from an ancient Saurian
stock, but by what steps and under what impulses we do
not know. To some it seems enough to say that the
evolution was accomplished gradually in the course of
natural selection by the fostering of fit variations and the
elimination of the disadvantageous ; to others it seems that
the incipient birds were “fevered representatives of reptiles,
progressing in the direction of greater and greater con-
stitutional activity”; but both these suggestions leave
much in the dark, leave us still to “wonder how the
slow, cold-blooded, scaly beast ever became transformed
into the quick, hot-blooded, feathered bird, the joy of
creation.”
Fic. 3814.—Fore-limb and shoulder-girdle (I.) and
hind-limb (II.) of rabbit.
SC., Scapula; A., acromion; ., metacromion process; .,-humerus ;
O., olecranon process; U., ulna; &., radius; C., carpals; A/C.,
metacarpals; D., five digits; #., femur; P., patella; F/., fibula;
T., tibia; OC., os calcis; AS., astragalus; D7., distal ‘tarsals;
MT., metatarsals; D., four digits.
CHAPTER XAVI
Crass MAMMALIA
1, PROTOTHERIA; 2. METATHERIA; 3. EUTHERIA
Birps and Mammals have evolved along very different
lines, Birds possessing the air and Mammals the earth, and
it is difficult to say that either class is the higher. But
apart from the fact, which prejudices us, that man himself
is zoologically included among Mammals, this class is
superior to Birds in two ways—in brain development, and
in the relation between mother and offspring. In most
Mammals there is a prolonged organic connection between
the mother and the unborn young, which may have been,
as Robert Chambers suggested, one of the conditions of
progress. It is also characteristic of Mammals that the
young are nourished after birth by their mother’s milk, and
it has been suggested that the usually prolonged infancy
was one of the factors in the evolution of the humaner
feelings. It is certain at least that the carefulness and
sacrifice of the mothers has been one factor in the survival
and success of Mammals, and we may find in the term
Mammalia, which Linneeus first applied to the class, a hint
of the idea that in the evolution of this class the mothers
led the way.
GENERAL SURVEY OF MAMMALS
There are three grades of Mammalian evolution :—
A. The duckmole (Ornithorhynchus) and the spiny
ant-eaters (Zchidna and Proechidna) difler very markedly
from all other Mammals. The young are hatched outside
GENERAL SURVEY OF MAMMALS. 693
of the body; in other words, the mothers are oviparous.
The brain is poorly developed when compared with that
of other Mammals. Some of the characteristics of the
skeleton, etc., suggest Reptilian affinities. To this small
sub-class the titles Prototheria, Ornithodelphia, and Mono-
tremata are applied.
8B. The kangaroos and bandicoots, phalangers and
opossums, and the like, form the second sub-class. In
these the young are born prematurely after a short gestation,
during which the organic connection between the mother
and the young is comparatively slight. Most female
Marsupials have an external pouch or marsupium, to which
the tender young are transferred, and within which they are
nourished and protected for some time. Moreover, the
brains even of the most intelligent Marsupials are not so
well developed as those of higher Mammals. To this
heterogeneous sub-class the titles Metatheria, Didelphia, and
Marsupialia are applied.
C. In all the other Mammals there is a well-developed
allantoic placenta uniting the unborn young to the mother,
while in Marsupials this is only known in Perameles, where
it is of relatively little importance. It is among these
placental Mammals that the brain begins to be much con-
voluted,—as it were, wrinkled with thought. To this
sub-class the titles Eutheria, Placentalia, and Monodelphia
are applied.
Among the extant orders of placental Mammals the Edentata and
the archaic Sirenia stand very much apart. The rest may be pro-
visionally grouped in three sets, perhaps representing three main lines.
of evolution.
On one side we place the great series of hoofed animals or Ungulates,.
including—(a) those with an even number of toes (Artiodactyla), such
as pigs, hippopotamus, camels, cattle, and deer ; (4) those with an odd
number of toes (Perissodactyla), such as tapir, rhinoceros, and horse ;
(c) the elephants (Proboscidea) ; (d) the Hyraxes (Hyracoidea). And
not far from the Ungulates it seems legitimate to rank (a) the whales and
dolphins (Cetacea), and (4) the rabbits and hares, rats and mice, etc.
(Rodentia).
On the other side we place the great series of Carnivora, such as cats,
dogs, bears, and seals. Beside these may be ranked the Insectivora,
such as hedgehog, mole, and shrew, and the bats or Chiroptera, which
seem to be specialised Insectivores.
In the middle we place the series which, beginning with the Lemurs,
leads through various grades of monkeys to a climax in man.
694 MAMMALIA,
But it must be carefully noted that these orders are often linked by
extinct types. Thus, to take one instance only, it is believed by some
that the extinct Phenacodus has affinities with Ungulates, Carnivores,
and Lemurs,
GENERAL CHARACTERS OF MAMMALS
All Mammals are quadrupeds, except the Cetaceans and
Strenians, in which the hind-limbs have disappeared, leaving
at most internal vestiges. There is generally a distinct neck
between the head and the trunk, and the vertebral column ts,
in most cases, prolonged into a tail.
Fairs are never entirely absent. In most they form a thick
covering, but.they are scanty in Sirenians and in the hippo-
potamus, and almost absent in Cetaceans, in which they are
sometimes restricted to early stages in life. The skin has
abundant sebaceous and sudorific glands. In the female,
milk-giving or mammary glands develop as specialisations
of sebaceous glands, except in Monotremes, where they are
nearer the sudorific type.
A complete muscular partition or diaphragm separates the
chest cavity, containing the heart and lungs, from the abdominal
cavity, and is of great importance in respiration.
The vertebra and long bones have terminal ossifications or
epiphyses, absent or very rudimentary, however, in the vertebra
of Monotremes and Sirenia. The centra of the vertebrae have
generally flat or slightly rounded faces, and there are usually
seven cervical vertebra.’
The bones of the skull are firmly united by sutures, which
generally persist. Only the lower jaw, the ear ossicles, and
the hyoid are movable. There are two occipital condyles, as in
Amphibians? The lower jaw on each side consists, in adult
life, of a single bone which works on the squamosal; the
1Jn the Manatee there are, however, only six; the pangolin ands
has sometimes eight; and it is often said that the two-toed sloth
(Cholepus hoffmannt) has only six, and the three-toed sloth (Bradypus
tridactylus) nine ; but in the case of the sloths there is apparently con-
siderable variation. It will be noticed that these deviations from type
occur only in the case of the two most old-fashioned orders of Eutherian
Mammals.
2 It may be noted, however, that for various reasons, ¢.g. that some
Birds and Reptiles are not very clearly single-condyled, morphologists
no longer attach so much importance to this character as they once did.
GENERAL CHARACTERS OF MAMMALS. 695
guadrate which intervenes in Sauropsida has disappeared, or
has been shunted to become one of the ear ossicles. For it ts
a plausible theory of the three ossicles—malleus, incus, and
stapes—which connect the drum with the inner ear, that they
correspond respectively to the articular, guadrate, and columella
or hyo-mandibular of other Vertebrates The otic bones fuse
with each other to form a compact pertotic. A bony palate,
Jormed from premaxille, maxilla, and. palatines, separates
the buccal cavity from the nasal passages. In most cases there
are teeth, borne in sockets by the premaxille, maxilla, and
mandible.
Except in Monotremes, the coracoid ts represented by a
small process from the scapula, and sometimes by a small
ossification, forming part of the glenoid cavity in which the
head of the humerus works. The sternum includes—(a) a
presternum, with which the clavicles (if well developed)
articulate ; (b) a mesosternum divided into segments, with
which the sternal parts of the ribs articulate; and (c) a
xiphisternum, often cartilaginous. There are generally two
sacral vertebra, but several caudals, and more rarely a
lumbar, may be fused to these. The ilio-sacral articulation
is in front of the acetabulum. The ventral symphysis ts
usually vestricted to the pubes, but.in some Insectivores and
Bats these do not meet. xcept in Echidna, the acetabulum
is completely ossified, and there is often a special acetabular
bone. The ankle joint ts cruro-tarsal.
The cerebral hemispheres have usually a convoluted surface,
and always cover the optic thalamit and the optic lobes (now
jourfold corpora quadrigemina), and in higher forms the
cerebellum aswell. The commissural system is well developed,
being especially represented by a large corpus callosum, except
in Monotremes and Marsupials, in which the anterior com-
missure ts large and the corpus callosum absent or very small.
There 1s also an important set of longitudinal fibres called the
Jjornix.
Except in Monotremes, in which there is a cloaca, the food
canal ends separately from the urogenital aperture.
The heart ts four-chambered, and the temperature of the
blood is high, though lower than that of Birds. There is but
1 There are many other theories as to the quadrate, e.g. that it forms
the malleus,
696 MAMMALIA.
one aortic trunk, which curves over the left bronchus. The
red blood corpuscles are, when fully formed, non-nucleated,
and appear as slightly biconcave discs, circular in outline,
except in the Camelide, where they are elliptical, There is
no renal-portal system.
Mammals are warm-blooded or stenothermal, i.e. their
body-temperature does not change with that of the surrounding
medium, In this they agree with Birds, and differ from other
Vertebrates, which are cold-blooded or potkilothermal,
Fic. 382.—Diagram of skull bones (partly after Flower and
Weber), the membrane bones shaded.
BO., Basioccipital; £O., exoccipital; C., condyle; SO.,
supraoccipital; Par., parietal; 7, frontal; Wa., nasal; Pyzx.,
premaxilla; A7Z., mesethmoid; Z., lachrymal; 7z., turbinal;
PS.,_presphenoid; OS., orbitosphenoid; AS., alisphenoid;
BS., vasisphenoid ; SQ., squamosal; P., periotic; 7., tympanic};
Pi, palatine; Pt, pterygoid; JAZx., maxilla; Jz., jugal; 7.4,
tympano-hyal; S.H., stylo-hyal; £.H/., epi-hyal; C.H., cerato-
hyal; &.7/., basi-hyal; 72.H., thyro-hyal.
The lungs are invested by pleural sacs, and lie freely
in the chest cavity. Within the lungs the bronchial tubes
Jork repeatedly into finer and finer branches. At the top
of the trachea there is a complex larynx with the vocal
chords.
The kidneys are generally compact and rounded bodies ;
the ureters open into the bladder, except in Monotremes, in
which they enter a urogenital sinus. Except in Monotremes,
the outlet or urethra of the bladder unites in the male with
the genital duct, to form a urogenital canal; in the female,
THE RABBIT AS A TYPE OF MAMMALS. — 697
except in Monotremes and a few other cases, the urethra and
the genital duct open into a common vestibule.
In the more primitive Mammals the testes lie in the
abdomen ; in the majority they descend permanently (or in a
Jew cases temporarily) into a single or paired scrotal sac,
lying, except in Marsupials, behind the penis.
The ovaries are small, Except in Monotremes, the genital
ducts of the female are differentiated into—(a) Fallopian tubes,
which catch the ova as they burst from the ovaries ; (b) a
uterine portion in which the young develop ; and (c) a vaginal
portion ending in the urogenital aperture. In Monotremes
the two ducts are simple, and open separately into the cloaca ;
in Marsupials there are two utert and two vagine ; in
Eutherian Mammals the uterine regions ave more or less
united, and the vaginal regions are always completely fused.
In Monotremes the eggs are large and rich in yolk, in all
others they are small and almost yolkless. In the ovary each
ovum lies embedded in a nest of cells, within a swelling or
Graajian follicle, which eventually bursts and liberates the
egg-cell. In Monotremes the segmentation, as might be
expected, 1s meroblastic » in other cases tt is holoblastic. As in
Sauropsida, there are two fetal membranes—the amnion and
the allantots, both of which share in forming the placenta of
the Placental Mammals. In Marsupials the allantois ts
usually small and degenerate.
The Monotremes are ovtparous; the Marsupials bring
jorth their young prematurely after a short gestation, but a
true allantotc placenta may be represented, as in Perameles ;
the Eutherian Mammals have a longer gestation, during
which the young are vitally connected to the wall of the uterus
by means of the placenta, which is always well developed, and
of great importance in the nutrition of the embryo. y
Ln all Mammals the young are for a longer or shorter
period dependent upon the milk secreted by the mammary
glands of the mother; in Marsupials this dependence is
especially marked.
THE RABBIT AS A TYPE OF MamMMaLs
The rabbit (Lepus cuniculus) is a familiar representative of
the Rodent order, to which rats and mice, voles and beavers,
698 MAMMALIA.
lemmings and marmots, also belong. Like the hare (Lepus
timidus) and other species of the same genus, and like the
Picas or tailless hares (Zagomys), the rabbit has two pairs of
incisors in the upper jaw, while other Rodents have a single
pair. Therefore the genera Lepus and Lagumys are some-
times ranked as Duplicidentata, in contrast to all other
Rodents (Simplicidentata).
With the rabbit’s mode of life all are familiar. It is herb-
ivorous, and often leaves softer food for the succulent bark
of young trees; it is gregarious and a burrower; it is very
prolific, frequently breeding four times in a year. It is said
to live, in normal conditions, seven or eight years. The
rabbit seems to have had its original home in the western
Mediterranean region, but it has spread widely throughout
Europe, and is now abundant in regions, such as the High-
lands of Scotland, in which, a few generations ago, it was rare.
Introduced into Australia and New Zealand, it has multiplied
exceedingly, and has become a scourge. There are many
varieties of rabbit, some in isolated regions perhaps illustrat-
ing the effect of segregation in fostering divergent types.
According to Darwin, the rabbits introduced early in the
fifteenth century into Porto Santo, an island near Madeira,
are now represented by a dwarf race of about half the normal
size, and these are said to be incapable of breeding with the
ordinary forms. But the varieties with which we are familiar
in the breeds of tame rabbits illustrate variation under
domestication and the efficacy of artificial selection.
External appearance.—The head bears long external
ears, which are freely movable. The black patch at the
tip of the ears in the hare is either absent or very small in
the wild rabbit. This external ear is characteristic of most
Mammals, and collects the sound like an ear-trumpet. In
the rabbit it is longitudinally folded, thin and soft towards
its tip, firm and cartilaginous at its base. The eyes have
two eyelids with few eyelashes, and a third eyelid or nicti-
tating membrane—a white fold of skin—in the anterior upper
corner. This third eyelid, which also occurs in Reptiles
and Birds, is present in most Mammals, and is of use in
cleaning the cornea. It is absent in Cetaceans, where the
front of the eye is bathed by the water, and it is rudimentary
in man and monkeys, where its absence is compensated for
SKIN AND MUSCLES. 699
by the habitual winking of the upper eyelid. The nostrils
are two slits at the end of the snout, and are connected
with the mouth by a “hare-lip” cleft in the middle of the
upper lip. In front of the mouth are seen the chisel-edged
incisors, a pajr on the mandibles, and two pairs on the pre-
maxille—the smaller pair hidden behind the larger pair.
The first milk incisors above and below never cut the gum,
but are absorbed before birth; the second milk incisors
above (there are none below) are functional, but are shed
about the third week of extra-uterine life; the same is true
of the milk premolars. Into the toothless gap or diastema
between the front and back teeth the hairy skin of the lips
projects into the mouth. This generally occurs in Rodents,
and is said to prevent the inedible substances which they
gnaw from passing backwards to the gullet. On the sides
’ of the snout, and about the eyes, there are tactile hairs or
vibrissze.
The plump trunk is separated from the head by a short
neck. ‘The tail is very short, but in the scampering wild
rabbit it is conspicuous as a white tuft, which some
naturalists interpret as a directive signal. Beneath the base
of the tail the food canal ends, and beside the anus are the
openings of the perineal glands, whose secretion has a char-
acteristic odour. In front of the anus is the urogenital
aperture,—in the male at the end of an ensheathed penis,
in the female a slit or vulva, with an anterior process or
clitoris—the homologue of the penis. Beside the penis in
the male lie the scrotal sacs, into which the testes descend
when the rabbit becomes sexually mature. Along the
ventral surface of the thorax and abdomen in the female
there are four or five pairs of small teats or mamme.
The limbs have clawed digits, five on the fore-feet, four
on the hind-feet ; they are very hairy.
Skin and muscles.—The skin is thickly covered with
hair, and has the usual sebaceous and sudorific glands,
besides special glands, such as the perineal glands beside
the anus, the glands of the eyelids, the lachrymal glands,
and the mammary glands developed in the females.
Between the skin and the subjacent muscles there is a
layer of fatty tissue, known as the panniculus adiposus ; it
is usually present in Mammals, but absent in the common
700 MAMMALIA.
hare; it forms the blubber of whales. Beneath the skin
is a thin sheet of muscle (the panniculus carnosus), by
means of which the skin can be twitched, as in horses,
etc., and when this is removed with the skin, many of the
muscles of head and neck, limbs and trunk, are disclosed
(see Parker’s Zootomy).
Skeleton.—The bones, like those of other Vertebrates,
are developed either as replacements of pre-existent cartil-
ages, or independent of any such preformations, but in all
cases through the agency of active periosteal membranes.
By themselves, however, must be ranked little sesamoid
bones, which are developed within tendons and near joints,
notably, for instance, the patella or knee-pan. There is no
bony exoskeleton in any mammals except the armadillos,
unless we rank the teeth, which develop in connection with
the skin of the jaws, as in a sense exoskeletal.
The vertebree may be grouped in five sets :—cervical
(seven in number), thoracic (with well-developed] ribs),
lumbar (without ribs), sacral (fused to support the pelvis),
and caudal. The faces of the centra are more or less flat,
and between adjacent vertebrze there are intervertebral discs
of fibro-cartilage. A vestige of the notochord is found in
Mammals in the gelatinous nucleus pulposus in the centre
of the intervertebral discs.
The first vertebra or atlas is ring-like, its neural canal
being very large, its centrum unrepresented except by the
odontoid process, which fuses to the second vertebra. The
tring is divided transversely by a ligament, through the
upper part the spinal cord passes, into the lower the odon-
toid process projects. The transverse processes are very
broad ; the articular surfaces for the two condyles of the
skull are large and deep.
The second vertebra or axis has a broad flat centrum
produced in front in the odontoid process. The neural
spine forms a prominent crest, the transverse processes are
small, the anterior articular surfaces are large.
A typical lumbar vertebra will show the centrum and its
epiphyses, the neural arch and neural spine, the transverse
processes, the anterior and posterior articular processes ort
zygapophyses, the median ventral hypapophysis, the small
anapophyses from the neural arch below the posterior
SKELETON. 7OL
zygapophyses, below the anapophyses the posterior inter-
vertebral notches—passages through which the spinal nerves
pass out, and anteriorly a similar pair of notches. There are
twelve or thirteen pairs of ribs which support the wall of the
thorax and aid in the mechanism of respiration. The first
seven pairs articulate with the breast-bone, the eighth and
ninth are connected to the ribs in front, the others are free.
Any one of the first seven or more typical ribs consists of
two parts, a vertebral portion articulating with a vertebra, an
imperfectly ossified sternal portion connecting the end of
Fic. 383.—Side view of rabbit’s skull.
Pitx., Premaxilla; WVa., nasal; Fr., frontal; Pa., parietal; Sg.,
squamosal; S.O., supraoccipital ; Per, periotic; 7., tympanic
(the reference line points to the bony external auditory meatus,
beneath it lies the inflated bulla); O., paroccipital process.
the vertebral portion with the sternum. Each of the first
nine ribs has a double head—the capitulum articulating
with the centrum of the corresponding vertebra, and partly
with that of the one in front, the tubercle articulating with
the transverse process of the corresponding vertebra. The
posterior ribs have no tubercles, and the capitular articula-
tions are restricted to the corresponding vertebre.
The sternum is a narrow jointed plate, with a large keeled
preesternum or manubrium, then five segments composing
the mesosternum, then a posterior xiphisternum ending in
cartilage.
702 MAMMALIA,
The sul consists, as in all the higher Vertebrates, of two
sets of bones,—cartilage bones preformed in the cartilage of
the original gristly brain-box and its associated arches, and
membrane bones developing in the investing membrane and
not preformed in cartilage. (The names of the membrane
bones are printed in italics.)
We have already noticed the chief characteristics of the
mammalian skull, such as the usual persistence of sutures,
the two condyles, the bony
palate, the fusion of the
periotic bones, the articu-
lation of the mandible with
the squamosal, the fusion
of the parts of each ramus
of themandibleintoa single
bone in the adult, and the
three ossicles of the ear.
The foramen magnum is
bounded by the basioccipital
beneath, the exoccipitals on the
sides, the supraoccipital above.
The exoccipitals form most of
the occipital condyles, but the
basioccipital contributes a small
part. In many Mammals the
exoccipitals alone form the con-
dyles. From each exoccipital a
paroccipital process descends,
and is applied to the tympanic
bulla—a dilatation at the base
of the ¢yspanzc bone which pro-
tects the external auditory tube,
Along the roof of the skull
lie the supraoccipital, the zxder-
rabbit’s skull. ; :
W., Anterior nostril; PAZX., premaxilla; MA., ia tage ue vel a the fron-
nasal; FR., anterior part of frontal; JLX., tals, and the zasaés.
posterior part of maxilla; /., anterior part On the very front of the skull
etugsl Pepe supraorbital eee oh on are the premaxdlle, bearing the
tal; -» posterior part of frontal; //., ++ ‘ -
posterior end of jugal protruding below. oO IneIsOr teeth, Behind each pre
matic portion of squamosal (Z.SQ.); PA., maxilla is a maxilla, bearing
parietal; AM., external auditory meatus; the premolars and molars; be-
SO., supraoccipital ; /P., interparietal ; SQ., hind this, along the z gomatic
squamosal, 2 § ek)
or temporal arch projecting be-
neath the orbit, is the jugaZ or malar, which unites posteriorly with
the sguamosal. This zygomatic arch bridges over the deep temporal
fossa behind the orbit, and serves for the insertion of muscles, and its
‘*squamoso-maxillary” structure occurs outside of Mammalia in the
Fic. 384.— Upper surface of
SKELETON. 703
Anomodont reptiles only, The fact that in Rodents the madar does
not form part of the face is of considerable systematic importance. The
squamosals form a great part of the posterior side walls of the skull, and
articulate with the pardetals, frontals, orbitosphenoids, and alisphenoids.
At the posterior end of the zygomatic arch is the longitudinally elon-
gated glenoid fossa in which
the mandible moves back-
wards and forwards.
In connection with the floor
of the skull and the roof of
the mouth, there lie from be-
hind forwards the following
components: — The median
basioccipital; the median
basisphenoid, which lodges
the pituitary body in « dorsal
depression called the sella
turcica; the paired alisphen-
oids fused to the sides of the
basisphenoid ; the median pre-
sphenoid, which forms the i
lower margin of the optic PT. i
foramen between the two’ * |
orbits; the paired orbitosphen- G
oids, fused to the presphenoid, \
sutured to the alisphenoids
and sguamosals, and surround- TB
ing the optic foramen; the
vertical pterygoids attached A
at the junction of basisphenoid
and alisphenoids; the partly
vertical padatines, united above :
to the presphenoid and behind Fyg, 38¢,Under surface of rabbit’s
to the plerygodds and alisphen- skull.
oids, separating the posterior inc. I., First incisors; Jc. /7., second in-
nasal passages from the orbits, “"Cjsorss PALX., premaxilla: PA. PICY., pala.
and uniting to a slight extent tal process of premaxilla; JZX., maxilla;
in front to form the posterior are ne 32.5 aieomalic anne ee
. asisphenoid ; /., posterior part of jugal;
part of the bony apeeira the BO., basioccipital ; PAR.OCC., paroccipital
median vertical mesethmoid _ process of exoccipital ; SOC., supraoccipital ;
cartilage extending in front C., one of the cendsless Se ae audi:
1 i tory meatus ; .» tympanic bulla; .
of the presphenoid, Separating glenoid fossa ; Pr: pterygoid. :
the two nasal cavities, pos- : aes
teriorly ossified and expanded into the sieve-like cribriform plates
through the apertures of which the branches of the olfactory nerves
pass to the nose; the paired vomers along the ventral edge of the
mesethmoid ; and lastly, the anterior bony palate (formed from inward
extensions of maxz//le and premaxitle), which in the rabbit is very
incomplete. a ;
Wedged in between the occipitals, the sgzamosals, and the bones of
the basisphenoid region, there is on each side a periotic bone surround-
1
'AMMALIA,
704
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proiseu “zy forucdurAy “ZS jesowenbs “@g {prousydsonqio “CQ { prouaydsiye “Pp fyesul “/ tyewAagoey ‘7
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ervqAdes jo []NYS—9sl “OIA
SKELETON. 705
ing the internal ear. It ossifies from three centres in the cartilaginous
auditory capsule, and consists of a dense petrous portion enclosing the
essential part of the ear and a more external porous mastoid portion
which. is produced downwards into a mastoid process in front of the
paroccipital process. From each periotic a ¢ympanic bone extends
outwards, swollen basally into a tympanic bulla in which the tympanum
or drum of the ear is stretched, and continued around the external
auditory meatus. From an aperture between the ¢ympanzc and the
periotic the Eustachian tube passes to the pharynx. Stretching from
the tympanum to the fenestra ovalis of the inner’ ear is the chain of
minute ear ossicles, the three links of which—malleus, incus, and stapes
—possibly correspond respectively to the articular, the quadrate, and
hyo-mandibular or columella of most other Vertebrates.
The orbits are bounded anteriorly by the /achryma/s and the maxilla,
and above by the frozéals. The interorbital septum is formed above
and behind by the orbito-sphenoids, below by the presphenoid.
Associated with the olfactory chambers are the zasal/s above, the
vomers beneath, the mesethmoid in the median line, while internally
there are several thin scroll-like turbinal bones. As special characters
of the skull should be noted the incomplete ossification of certain of the
bones, ¢.g. of the maxz//a, and the development of slender rod-like
processes from some of them, e.g. the sgwamosal, which help to keep
the parts of the skull firmly connected.
The lower jaw or mandzble consists in adult -life of a single bone
or ramus on each side, but this is formed around Meckel’s cartilage
from several centres of ossification. Its condyle works on the
sguamosal.
_ The hyoid lies between the rami of the mandzble, in the back of
the mouth, and consists of a median ‘‘ body,” and two pairs of horns
or cornua extending backwards.
\
The appendicular skeleton consists of the bones of the
limbs.and. the girdles. ;
The pectoral girdle, which supports the fore-limbs, and
is itself attached by muscles and ligaments to the vertebral
column, virtually consists of one bone—the scapula—on
each side. For in all Mammals, except Monotremes, the
coracoid is vestigial. It is represented by an “epicoracoid”
feces overhanging the edge of the glenoid cavity in which
the head of the humerus works, and there is also in
some cases a small independent ossification (coracoid or
metacoracoid) on the ventral surface of the glenoid cavity.
The clavicle is much reduced in the rabbit, being only
about an inch in length and very slender. The triangular
scapula has a prominent external ridge or spine, continued
ventrally into an_acromion with a long metacromion pro-
cess. The scapula is usually strong and the clavicle is
45
706 AMIAMMALIA.
as a rule present in Mammals which grasp or climb or
burrow.
The fore-limb consists of an upper arm or humerus, a
forearm of two bones—the radius and the ulna, a wrist or
carpus, five palm-bones or metacarpals, and five digits with
joints or phalanges.
The head of the humerus works in the glenoid cavity formed by the
scapula.
When the arm of a mammal is directed outwards at right angles to
the body, with the palm vertical and the thumb uppermost, the thumb
and the radius are in a preaxial position, the little finger and the ulna
are in a postaxial position. But in the normal position of the limb in
most mammals, the radius and the ulna cross one another in the fore-
arm, so that the preaxial radius is externab at the upper end, internal
at the lower end. The hand is borne by the expanded end of the
radius.
The typical mammalian wrist or carpus consists of two rows of bones
with a central bone between the two rows. In the rabbit all the bones
—nine in number—are present, viz. :—
First Ulnare or Intermedium or Radiale or
Row Cuneiform. Lunar. Scaphoid.
Centrale,
Carpale 5 and 4 Carpale 3 Carpale 2 Carpale 1
— P or or or or
OM Unciform. Os magnum. Trapezoid. Trapezium
In Mammals the fourth and fifth carpals are never represented by
two distinct bones; the centrale is often absent. In the tendons of
the flexor muscles there are often two sesamoid bones, of which the
ulnar is called the pisiform. :
In the rabbit there are five metacarpal bones and five digits, each
with three phalanges, except the thumb or pollex, which-has but-two. -
The pelvic girdle is articulated to the backbone, and
bears externally a cup-like socket or acetabulum in which
the head of the thigh-bone works. Each half of the girdle
—forming what is called the innominate bone—really con-
sists of three bones, which meet in the acetabulum. The
dorsal bone or ilium, which corresponds to the scapula,
articulates with the sacral vertebree ; the pubis—the anterior
of the two lower bones— unites with its fellow on the
opposite side in the pubic symphysis; the two ischia,
which correspond to the coracoids, extend backwards,
separated from the pubes by the large obturater foramen,
NERVOUS SYSTEM. : 707
and expand into posterior tuberosities. The ischia of
Mammals may touch one another ventrally, but do not
fuse in a symphysis; the pubic symphysis is almost invari-
ably present. Only in Cetacea and Sirenia is the pelvis
markedly rudimentary.
The hind-leg consists of a thigh or femur, a lower leg
with two bones—the tibia and the fibula, an ankle or
tarsus, the sole-bones or metatarsals, the toes with several
joints or phalanges.
The head of the femur works in the acetabulum of the pelvis. Near
the head are several processes or trochanters, serving for the insertion
of muscles; in the rabbit there are three—the great trochanter, the
lesser trochanter, and the third trochanter. :
In front of the knee there is a sesamoid bone—the knee-pan or
patella—and posteriorly there are smaller fabellz.
In the lower leg, the tibia, which corresponds to the radius, is pre-
axial, and ia the normal position interior ; the fibula, which corresponds
to the ulna, is postaxial, and in the normal position exterior. There
is no crossing of bones as in the forearm. In the rabbit the fibula is
slender, and is fused distally with the tibia.
In the mammalian tarsus there are two rows of bones, and a central
bone interposed between the two rows on the inner or tibial side. .
First } Calcaneum Astragalus
Row Jf, or Fibulare. or Tibiale.
Centrale
or Navicular,
Tarsalia 5 and 4 Tarsale 3 Tarsale 2 Tarsale 1
SECOND =Cuboid. or or or
Row External Middle Internal
Cuneiform. Cuneiform. Cuneiform.
In the rabbit the first cuneiform and the corresponding hallux are
wanting. There are thus only four metatarsals and digits. Each digit
has three phalanges, and ends in a claw.
Nervous system.—The brain has the usual five parts—
cerebral hemispheres, optic thalami, optic lobes, cerebellum,
and medulla oblongata, but the cerebral hemispheres cover
the next two parts, and the cerebellum conceals the medulla.
Of the brain membranes, the dura mater lines the cranial
cavity, projecting longitudinally between the cerebral hemi-
spheres, and transversely between the latter and the cere-
bellum, while the vascular pia mater invests the brain
closely. There are the usual twelve pairs of cranial
nerves. The spinal cord gives off the usual spinal nerves,
708 : MAMMALIA.
and there is a sympathetic system as in most other
Vertebrates.
The cerebral hemispheres of the rabbit are very slightly convoluted,
and they leave the cerebellum quite uncovered. They are connected
Fic. 387.—Dorsal view of Fic. 388.—Under surface of rab-
rabbit’s brain. bit’s brain. —After Krause.
off.t., Olfactory lobes; ¢.%., cere- olf.l., Olfactory lobes ; 0.7., olfactory tract;
bral hemispheres; 0./., optic J-t., frontal lobe of cerebral hemisphere ;
lobes (corpora quadrigemina) ; ch, optic chiasma; é.c., infundibulum ;
cb., median part of cerebellum ; ¢.m., Corpus mammillare; 3, root of
Fl., flocculus of cerebellum; oculomotor; 4, root of pathetic; 5,
10, root of the tenth or vagus root of trigeminal ; 6, root of abducens 3
nerve; s.¢., spinal cord. 7-8, roots of facial and auditory: F/.,
flocculus of cerebellum ; 9, root of glosso-
pharyngeal ; 10, roots of vagus; H., r2th,
or hypoglossal ; 4.v., pons Varolii.
transversely by a broad bridge—the corpus callosum, and beneath this
there is a longitudinal band of fibres—the fornix. The corpus callosum
is readily disclosed by gently separating the hemispheres. The outer
-wall and floor of the anterior part of the cavity or ventricle of each
NERVOUS SYSTEM. 709
hemisphere is formed by a thick mass, called the corpus striatum, and
the internal cavity is lessened by a prominent convex ridge, called the
hippocampus major. The ventricles of the cerebrum communicate
with the third ventricle, between the optic thalami, by a small
aperture, called the foramen of Monro. In front of the hemispheres
two club-shaped olfactory lobes project. The thin cortical layer of
the cerebrum consists of grey (ganglionic) matter, and so does the
thick corpus striatum, while the central part consists of white matter
(nerve fibres),
The thalamencephalon is entirely hidden, but gives origin as usual
to the dorsal epiphysis, ending in a pineal body, which lies on the
surface between the cerebrum and cerebellum, and to the ventral
infundibulum, at the end of which the pituitary body lies, lodged in a
fossa of the basisphenoid. Immediately in front of the infundibulum
the optic nerves cross in a chiasma, from which optic tracts can be
traced to the optic lobes. Immediately behind the infundibulum lies
a rounded elevation, called the mammillary body. ‘ Anteriorly, on the
ventral surface of each side of the thaiamencephalon, there is a rounded
swelling, called the corpus geniculatum. The roof of the third
ventricle is formed by a thin membrane or velum, with a plexus of
blood vessels. In the anterior wall of the third ventricle lies the small
anterior commissure; across the third ventricle the large middle com-
missure runs; in the roof of the hind part of the ventricle lies a small
posterior commissure.
The optic lobes are fourfold—corpora quadrigemina. They are in
large part covered by the cerebrum. Between them runs the iter con-
necting the third ventricle and the fourth. The floor of this passage is
formed by the thick crura cerebri which connect the medulla with the
cerebrum.
The cerebellum has a median and two lateral lobes (with accessory
flocculi), and is marked by numerous folds, mostly transverse. The
two sides are connected ventrally by the pons Varolii, lying across the
anterior ventral surface of the medulla.
The medulla oblongata lies beneath and behind the cerebellum, and
is continued into the spinal cord. The cavity of the fourth ventricle is
roofed by a thin membrane or velum, above which lies the cerebellum.
‘ On the ventral surface the medulla is marked by a deep fissure, bordered
by two narrow bands or ventral pyramids. :
The spinal cord presents its usual appearance, with its dorsal sensory
nerve-roots with ganglia, its ventral motor nerve-roots apparently with-
out ganglia, and the spinal nerves formed from the union of these. The
ganglia of the adjacent sympathetic system perhaps belong to the ventral
roots of the spinal nerves.
A large number of nerves pass down the neck. Of these the follow-
ing are most important :—
I. The eleventh cranial nerve or spinal accessory, leaving the skull
with the ninth and tenth, and distributed to the muscles of the
neck.
2. The twelfth cranial nerve or hypoglossal, lying at first close to
the ninth, tenth, and eleventh, turning, however, to the muscles
of the tongue.
710 MAMMALIA,
3. The tenth cranial nerve, the pneumogastric or vagus, lies outside
the carotid artery, and gives off a superior laryngeal to the
- larynx with a depressor branch to the heart, an inferior or
recurrent laryngeal, which loops round the subclavian artery
and runs forward to the larynx, and other branches to the heart,
lungs, and gullet.
4. The cervical part of the sympathetic, lying alongside of the
trachea, with two ganglia.
5. The great auricular, a branch of the third spinal nerve, running to
the outer ear.
6. The phrenic nerve, a branch of the fourth cervical nerve, with
branch from the fifth and sometimes from the sixth, runs along
the backbone to the diaphragm.
For details as to these nerves, the student should consult the practical
manuals of Marshall and Hurst and of Parker.
As to the sense organs little need be said, for their general structure
is like that of other Vertebrates, while the detailed peculiarities are
beyond our present scope.
The third eyelid is well developed. The lachrymal gland (absent in
Cetacea) lies under the upper lid, and the lids are kept moist by the
secretion of Harderian and Meibomian glands. The external ear or
pinna is conspicuously large. The cochlea of the inner ear is large.and
spirally twisted. The nostrils are externally connected with the mouth
by a characteristic cleft lip. The tongue bears numerous papillee with
taste bulbs, The long hairs or vibrissze on the snout are tactile.
Alimentary system.—In connection with the cavity of
the mouth we notice the characteristic dentition, the hairy
pad of skin intruded in the gap between incisors and pre-
molars, the long and narrow, in part bony, palate separating
the nasal from the buccal cavity, the muscular tongue with
taste papille, the glottis leading into the windpipe, the
bilobed epiglottis guarding the opening, the paired apertures
of the Eustachian tubes opening into the posterior nasal
passage, the end of this passage above the glottis, and the
beginning of the pharynx. The organs of Jacobson are
paired tubular bodies, vascular and richly innervated, lying
enclosed in bone in the front of the nasal chamber, and
communicating with the nostrils above, and on the
other hand with the mouth by two naso-palatine canals
which open behind the posterior incisors. Opening into
the mouth and conducting the salivary juice, whose
ferment alters the starchy parts of the food, are the ducts
of four pairs of salivary glands. The parotid, which is
largest, lies between the external ear-chamber and the
angle of the mandible; the infra-orbital lies below and in ~
ALIMENTARY SYSTEM. vai
front of the eye; the submaxillary lies between the angles
of the mandible ; the small sublinguals lie along the inner
side of each ramus of the mandible.
The pharynx passes into the gullet, and that leads through
the diaphragm to the expanded stoinach, which is dilated
at its upper or cardiac end, and narrows to the curved
pyloric end. Partly covering the stomach is the large liver. -
The first. portion of the intestine, which is called the
duodenum, receives the bile duct, and has the pancreas in
its folds. Then follows the much-coiled small intestine,
measuring many feet in length. It should be noted that, in
adaptation to the more slowly digestible food and the larger
amount of indigestible residue,
the intestine of herbivorous
mammals is relatively longer
than that of carnivorous types.
The lower end of the small
intestine is expanded into a
sacculus rotundus. Here the
large ceecum—a blind diverti-
culum—is given off; it ends
in a finger-like vermiform ap-
pendix. Its proximal end is
continuous with the colon or . os
% ‘ s.2., Small intestine; s.7., sacculus
first part of the large intestine, rotundus; co/., sacculated colon ;
the beginning of which is much {> ,cipeums a vermiform
sacculated. The large intestine
narrows into the long rectum, in which lie little feecal
pellets. On the last two inches of the rectum there are
paired yellowish glands. Beside the anus are two bare
patches of skin, with the openings of the ducts of the
perineal glands, whose secretion has a characteristic and
strong odour.
The liver is attached to the diaphragm by a fold of peri-
toneum—the glistening membrane which lines the abdomi-
nal cavity. In the liver there are five lobes. From these
lobes the bile is collected by hepatic ducts into a common
bile duct, which is also connected to the gall-bladder by the
cystic duct.
The very diffuse pancreas lies in the mesentery of
the duodenal loop. Its secretion is gathered by several
Fic. 389.—Diagram of
caecum in rabbit.
712 MAMMALIA.
tubes into the pancreatic duct which opens into the
duodenum.
The mesentery, which supports the alimentary canal, is a
double layer of peritoneum reflected from the dorsal abdo-
minal wall.
The dark red spleen lies behind the stomach. In the
mesentery, not far from the top of the right kidney, lie a
pair of coeliac ganglia, which receive nerves from the thoracic
sympathetic system, and give
off branches to the gut.
Vascular system. — The
blood of Mammals contains,
as in other Vertebrates, red
blood corpuscles (erythro-
cytes) and white blood cor-
puscles (leucocytes), but the
former are non-nucleated
except in their young stages.
It is probable that the nuclear
material becomes _ diffused
through the cell. They appear
as slightly biconcave circular
discs (elliptical in Camelide),
but many good observers
describe spherical or cup-
oe ee ie shaped or bell-shaped red
ral ih Ce ote blood corpuscles. It is not
Claude Bernard. certain how far these shapes
P., Pyloric end of stomach; g.d., gall- Aare normal. The four-cham-
bladder with bile duct and hepatic bere heart lies in the thoracic
ducts; .d., pancreatic duct. ;
cavity between the lungs. It
is surrounded by a thin pericardium, and immediately in
front of it there lies the soft thymus, which is larger in the
young than in the adult animal.
By two superior vene cave, and by the inferior vena
cava, the venous blood collected from the body enters the
right auricle. Thence the blood passes into the right
ventricle through a crescentic opening, bordered by a
threefold (tricuspid) membranous valve (worked by chordz
tendinez attached to papillary muscles projecting from the
wall of the ventricle).
VASCULAR SYSTEM. 713
The right ventricle is not so muscular as the left,
which it partly surrounds. By its contraction the blood
is driven into the pulmonary trunk, whose orifice is
guarded by three semilunar valves. During contraction
the tricuspid valves are pressed together, so that no
regurgitation into the right auricle can take place.
The pulmonary trunk -
divides into two pulmonary a
arteries, which branch | into
capillaries on the walls of the
lungs. There the red biood
corpuscles gain oxygen, and
the blood is freed from much
Fic. 391.—Circulatory system of
the rabbit.
(a) Letters to right—
e.c. External carotid.
z.c. Internal carotid.
ej. External jugular.
scl.a, Subclavian artery.
scl.y. Subclavian vein.
g.a. Pulmonary artery (cut short).
p.v. Pulmonary vein.
L.A. Left auricle.
L.V. Left ventricle.
d.ao. Dorsal aorta.
hv. Hepatic veins.
c. Ceeliac artery.
a.m, Anterior mesenteric,
5.7.6. Suprarenal body.
ir.a. Left renal artery.
Zr.v. Left renal vein.
&. Kidney.
p.m. Posterior mesenteric artery
(inadvertently shown as if
paired).
spm. Spermatic arteries and veins.
c.zl.a. Common iliac artery.
() Tetters to left—
pf. and a,f. Posterior and anterior
facial.
ej. External jugular vein,
z.j. Internal jugular.
R.Sct. Right subclavian artery.
S.V.C. Superior vena cava.
R.A. Right auricle. :
R.V. Right ventricle.
J.V.C. Inferior vena cava,
v.r.a. Right renal artery.
yr.v. Right renal vein.
s.7.6, Suprarenal body.
sfm. Spermatic arteries and veins,
2.2, Ilio-lumbar vein.
fv. Femoral vein.
1.21.v. Internal iliac veins,
714 MAMMALIA.
of the carbonic acid gas which it has borne away from the
tissues. The purified blood returns to the heart by two
pulmonary veins, which unite as they enter the left auricle.
From the left auricle the pure blood passes into the left
ventricle through a funnel-like opening, bordered by a
(mitral) valve with two membranous flaps, with chordz
tendineze and musculi papillares as on the right side, but
the muscles here are larger.
The left ventricle receives the pure blood and drives it
to the body. During contraction the mitral valve is closed,
so that no blood can flow back into the auricle. The
blood leaves the left ventricle by an aortic trunk, whose
base is guarded by three semilunar valves, just above
which coronary arteries arise from the aortic trunk and
supply the heart itself.
The aortic trunk bends over to the left, and passes
backward under the backbone, dividing near the pelvis
into two common iliac arteries, which supply the hind-
legs and posterior parts. The chief blood vessels may be
grouped as follows :—
The aortic trunk
gives off the innominate artery,
which divides into (a) the right subclavian, continued as the
brachial to the fore-limb, but giving
off the vertebral to the spinal cord
and brain, and the internal mam-
mary to the ventral wall of the
thorax ;
(4) the right carotid, running along the
trachea, dividing into the right
internal carotid to the brain, and
the right external carotid to the
head and face ;
(c) the left carotid, with a similar course ;
thereafter the aorta gives off—
the left subclavian artery, which branches like the right ;
the cceliac artery to the liver, stomach, and spleen ;
the anterior mesenteric to the pancreas and intestine ;
the renal arteries to the kidneys ;
the single posterior mesenteric to the rectum ;
the paired spermatic or ovarian arteries to the reproductive
organs ;
the lumbar arteries to the posterior body walls.
The aorta is continued terminally in the median sacral artery to the
tail, and laterally in the common iliacs, which form the femorals of
RESPIRATORY SYSTEM. 715
the hind-legs, and give off in the abdomen several branches to the
abdominal walls, the pelvic cavity, the bladder, and the uterus,
The two superior venz cave bring blood’ from the head, neck,
thorax, and fore-limbs. Each is formed from the union of—
a subclavian from the shoulder and fore-limb,
an external jugular from the face and ear,
an internal jugular from the brain,
an anterior intercostal from the spaces between the anterior ribs,
an internal mammary from the ventral wall of the thorax ;
and the right superior vena cava also receives an azygos cardinal vein,
which runs along the mid-dorsal line and collects blood from the
posterior intercostal spaces.
The inferior vena cava is a large median vein lying beside the aorta
beneath the backbone. Anteriorly it is embedded in the liver, and
receives the hepatic veins. Thence it passes through the diaphragm
into the right auricle. Posteriorly the inferior vena cava has the
following components :— :
Internal iliacs from the back of the thighs, forming by their union
the beginning of the inferior vena cava ;
femoral veins from the inner borders of the thighs, continued into
external iliacs which open into the inferior vena cava ;
paired ilio-lumbars from the posterior abdominal walls ;
spermatic or ovarian veins from the reproductive organs ;
renal veins from the kidneys.
There is no renal-portal system.
The food which has been digested—rendered soluble and diffusible—
passes from the food canal into the vascular system by two paths—
(a) All except the fatty material is absorbed by veins from the
stomach and intestine. These unite in a main trunk, the
portal vein. The components of the portal vein are—the lieno-
gastric from the stomach (and also from the spleen), the
duodenal from the duodenum (and also from the pancreas),
the anterior mesenteric from the intestine, the posterior
mesenteric from .the rectum. The portal vein breaks up
into branches in the liver, whence the modified blood passes
by hepatic veins into the inferior vena cava.
(6) The fat passes through the intestinal villi into the lymphatic
vessels, which combine to form a thoracic duct, which runs
forward and opens into the left subclavian vein at its junction
with the left external jugular. Here and there lie lymphatic
glands.
Respiratory system.—The lungs are pink, spongy bodies,
lying in the thorax, connected with the exterior by the
bronchial tubes and the trachea, and with the heart by
blood vessels. The pleural membrane which invests the
surface of the lungs is reflected from the sides of the
thoracic cavity. When the lungs expand, the pleural
cavity—between the two folds of pleural membrane—is
716 MAMMALIA.
almost obliterated. The thoracic cavity is separated from
the abdominal cavity by a partly muscular diaphragm,
which is supplied by two phrenic nerves, arising from the
fourth cervical spinal nerves. By its contraction the
diaphragm alters the size of the thoracic cavity, and thus
W190
Fic. 392. —Vertical section through rabbit’s head.—From a section,
with help from Parker’s Zoofomy and Krause.
fmx., Premaxilla with incisors ; ,.e., part of mesethmoid partition ;
Lb. maxillary turbinals; ¢.¢., ethmoidal turbinal ; .e., part
of mesethmoid ; off, olfactory lobe of cerebrum; /s., pre-
sphenoid; c.c., position of corpus callosum ; és., basisphenoid
with depression for pituitary body; cé., cerebellum ; 4.0., basi-
occipital ; s.c., spinal cord ; 7.f., nasal passage ; gullet; Urs
trachea; eAg., epiglottis 5 svzx., submaxillary y glane ;
sf, sublingual salivary gland} 7., tongue; A/., transverse
portion of palatine ; m., anterior end of mandible.
shares in the mechanism of respiration. At the top of
the trachea lies the complex larynx, the seat of the voice
in Mammals.
Anteriorly the larynx is supported on its sides and beneath by the
thyroid cartilage ; behind this lies the ring-like cricoid ; dorsally to the
cricoid are two small triangular arytenoids,
EXCRETORY SYSTEM—REPRODUCTIVE ORGANS 717
Within the larynx there are stretched membranous bands—the vocal
chords. Beside the larynx is the paired thyroid gland.
Excretory .system.—This includes the blood-filtering
kidneys, their ducts the ureters, and a reservoir or bladder,
into which these open. The kidneys and their ducts are
formed from the metanephros and metanephric ducts of
the embryo. The bladder arises as a diverticulum from
the hind end of the gut, being in fact. a remnant of the
intra-embryonic part of the allantois. It loses its con-
nection with the gut, and the ureters which originally
opened into the rectum follow the bladder and open
into it.
The kidneys are dark red ovoid bodies lying on the
dorsal wall of the abdomen; the one on the left is farther
down than that on the right, because of the position of
the stomach on the left side. When a kidney is dissected,
a marked difference is seen between the superficial cortical’
part and the deeper medullary substance. On papille or
pyramids in the very centre the coiled excretory tubules
open, and empty the water and waste products into the
“pelvis” or mouth of the ureter.
The ureters run backward along the dorsal wall of the
abdomen, and open into the bladder, a thin-walled sac
lying in front of the pelvic girdle.
In front of each kidney lies a yellow suprarenal body.
Reproductive organs.—(a) Male.—The testes arise on
the dorsal abdominal wall near the kidney, but as the
rabbit becomes sexually mature, they are loosened from
their original attachment, and pass out on the ventral
surface, as if by a normal rupture, into the scrotal sac.
A spermatic cord, consisting of an artery, a vein, and a
little connective tissue, runs from the abdomen to the
testis.
The testis is attached to the base of the scrotal sac, and
is bordered by a mass of convoluted tubes—the epididymis
—consisting of the caput epididymis anteriorly, the larger
cauda epididymis posteriorly, and a narrow band between
them. ‘The cauda epididymis is connected to the scrotal
sac by a short cord or gubernaculum. ;
Througt the tubes a tae epididymis (the modified meso-
nephros) the spermatozoa developed in the testis are
718 MAMMALIA.
collected into the vas deferens (the modified Wolffian
duct), which arises from the cauda epididymis, ascends to
Fic. 393.— Urogenital organs of Fic. 394.— Urogenital organs of
male rabbit. female rabbit.
K., Kidney; U., ureter ; B2., bladder ; &., Kidney; U., ureter; O., ovary;
7., testis; s.c., spermatic cord 5 J’.¢, Fallopian tube; Qvd., ovi-
cph.cp., caput epididymis; ca.cp., duct; U¢., uterus; V., vagina;
cauda epididymis; Sc., scrotal Bl, vladder; Ve., vestibule or
., one of the lobes of the female urethra; U.G., — uro-
23 ¢.g., Cowper's glands; genital aperture; A., anus.
py. perineal glands; Un, Bladder and vestibule are cut
urethra ; ¢.¢., Corpus cavernosum 5 Open.
P., penis.
the abdomen, loops round the ureter, and, passing dorsally
to the bladder, opens beside its fellow into a median sac
REPRODUCTIVE ORGANS. 719
called the uterus masculinus. In many Mammals, paired
diverticula, known as seminal vesicles, are connected with
the ends of the vasa deferentia, but they are not developed
in the rabbit.
The uterus masculinus is the homologue of the vagina in
the female, and seems to arise from the Miillerian ducts.
It opens into the urethra, which runs backwards from the
bladder, and the urogenital canal thus formed is continued
through the penis. ‘
Beside the uterus masculinus and the vasa deferentia,
there are lobed prostate glands opening by several ducts
into the urogenital canal. Behind the prostate, on the
dorsal wall of the urogenital canal, lie two Cowper’s
glands.
The penis projects in front of the anus behind the pubic
symphysis, has vascular dorsal walls (corpus spongiosum),
stiff ventral walls (corpora cavernosa), and. is invested by a
loose sheath of skin—the prepuce. At the side of the penis
lie two perineal glands.
(3) Female.—The ovaries are small oval bodies about
three-quarters of an inch in length, attached behind the
kidneys to the dorsal abdominal wall, exhibiting on their
surface several clear projections or Graafian follicles, each of
which encloses an ovum.
The ova, when mature, burst from the ovaries, and are
caught by the adjacent anterior openings of the oviducts.
The oviducts are modified Miillerian ducts, differentiated
into three regions. The anterior portion or Fallopian tube
is narrow, slightly convoluted, with a funnel-shaped, fimbri-
ated mouth lying close to the ovary. The median portion
or uterus is the region in which the fertilised ova become
attached and develop. In the rabbit the uterine regions of
the two oviducts are distinct, forming what is called a double
uterus. In most Mammals the uterine regions of the ovi-
ducts coalesce, forming a bicornuate or a single uterus,
according to the completeness of the fusion. In all Mam-
‘mals above Marsupials the posterior parts of the two
oviducts unite in a median tube—the vagina.
The vagina unites with the neck of the bladder, and forms
the wide but short urogenital canal or vestibule, which.
opens at the vulva, ventral to the anus. On the ventral wall
720 MAMMALIA.
of the vestibule lies the clitoris, a small rod-like body—the
homologue of the penis. On the dorsal wall lie two small
Cowper’s glands, and there are also perineal glands as in
the male.
The fertilised egg develops within the uterus, and in the
rabbit, as in all Eutherian Mammals, the allantois of the
embryo becomes intimately connected with the wall of the
uterus to form the vascular placenta, the organ by means of
which the nutrition and respiration of the embryo are pro-
vided for. In the rabbit, and in other Rodents, there is,
before the development of the allantoic placenta, a pro-
visional yolk-sac placenta—a structure of similar function
but of much less morphological complexity. The details
of the placentation of Mammals will be considered later.
NoTES ON COMPARATIVE ANATOMY OF MAMMALS
Skin.—This consists of a superficial epidermis (ecto-
dermic) and of a subjacent mesodermic dermis or cutis.
The most characteristic modification of the mammalian
epidermis is the hair. Each hair arises from the cornifica-
tion of an ingrowing epidermic papilla of the Malpighian
stratum of the epidermis, surrounded at its base by a moat-
like follicle, and nourished during growth by a vascular
projection of the dermis.
Each’ hair consists of a spongy central part and a denser cortex, but
there are many diversities of form and structure, such as short fur and
long tresses, the soft wool of sheep and the bristles of pigs, the spines of
hedgehog, porcupine, and Zchzdua, the cilia of the eyelids and the
tactile vibrissve of the lips and cheeks.
The hair keeps the animal dry and warm ; in the practically hairless
Cetacea the layer of fat or blubber underneath the skin also serves to
sustain the temperature of the body. Like feathers, hairs die away
and are cast off, being replaced by fresh growths.
Among other tegumentary structures are the scales which
occur along with hairs on the pangolins (AZanzs); the
scales on the tails of rats and beavers and some other forms ;
the thickened skin-pads or callosities on the ischia of apes,
the breast of camels, the legs of horses; the nails, claws, or
hoofs which ensheath the ends of the digits in all Mammals
except Cetaceans. Unique is the armature of the armadillos,
SKIN 721
for it consists of bony plates developed in the dermis,
overlaid by epidermic scales. The median solid horns of
the rhinoceros are epidermic outgrowths, comparable to
exaggerated warts; the paired horns of the Ruminants con-
sist of epidermic sheaths covering outgrowths of the frontal
bones, but extending far beyond these; the antlers of
stags are outgrowths of the frontal bones, are cast and re-
grown each year, and are possessed by the males only,
except in the reindeer.
The skin of Mammals, unlike that of Birds, is rich in
glands. Sebaceous glands are always associated with the
hair follicles, and sudorific or sweat glands are scattered
over the skin.
Specialised glands are also very common, especially those which
secrete some strongly odoriferous stuff, scenting which the animals
recognise their fellows, their mates, or their young. Often they are most
developed in the males, and their activity increases at the pairing
season.
Among the numerous special glands may be noted those which are
connected with a perforated spur on the hind-legs of male Monotremes,
the sub-orbital glands of antelopes and deer, the anal glands of
carnivores, the perineal glands of the civet, the preputial glands of the
musk-deer and beaver, the inter-digital glands of the sheep.
Most characteristic, however, are the mammary glands,
functional in female Mammals after parturition. They
seem to be specialisations of sebaceous glands, except in
Monotremes, in which they are nearer the sudorific type.
They-consist of branching tubes opening by one or several
apertures on the skin.
In Monotremes the simple glands, compressed by
muscles, open by many pores on.a bare patch of’ skin.
This is depressed into a slight cup, from which the young
lick the milk. In Marsupials the glands open by teats or
mamme, generally hidden within a marsupium ; and again
the action of surrounding muscles forces the milk into the
mouths of the young, which do not seem to be able to suck.
An anterior prolongation of the larynx to meet the posterior
nares establishes a complete air passage, and enables the
young to continue breathing while they are being fed. In
Cetacea the milk ducts are dilated into large reservoirs, the
contents of which can be rapidly injected into the mouth of
722 MAMMALIA.
the young. In all other Mammals the young suck the
milk from the mamme.
Dentition.—The teeth of Mammals are developed in the
gum or soft tissue which covers the borders of the pre-
maxilla, maxille, and mandibles. As in other animals,
they are in part of epidermic, in part of dermic origin. In
the course of their development their bases are usually
enclosed in sockets formed in the subjacent bones.
In most teeth there are three or four different kinds of
tissue. The greater part consists of dentine or ivory (of
which about a third is organic matter); outside of this
there is a layer of very hard glistening exame/ (practically
inorganic) ; in the interior there is a cavity which in grow-
ing teeth contains a gelatinous tissue or pu/p supplied by
blood vessels and by branches of the fifth: nerve, and con-
tributing to the increase of the dentine ; lastly, around the
narrowed bases or roots of the tooth, or between the folds
of the enamel if these have been developed, there is a bone-
like tissue called the cvusta petrosa or cement.
The development of teeth begins with the formation of
a dental ridge, an invagination of the ectodermic epithelium.
From this ridge a number of bud-like ‘enamel germs ” are
next differentiated. Beneath each germ a papilla of the
vascular mesodermic dermis is defined off as the “dentine
germ.” The crown of this papilla becomes hard, and the
ossification proceeds downwards and inwards, while above
the dentine crown the enamel begins to form a hard cap.
Meantime the tissue around the base of the tooth papilla
becomes differentiated into an enclosing follicle or sac,
from the inner layer of which the cement is developed.
.The papilla forms the pulp,—consisting of connective tissue,
with blood vessels and nerves, and an enveloping zone of
dentine-forming cells or odontoblasts.
The base of a tooth may remain unconstricted, and the core of pulp
may persist. Such a tooth goes on growing, its growth usually keeping
pace with the rate at which the apex is worn away with use, and it is
described as ‘‘rootless” and ‘‘ with persistent pulp.” The incisors ot
Rodents and of elephants illustrate this condition.
In the development of most teeth, however, the base is narrowed
and prolonged into a root or several roots which become firmly fixed
in the socket. Through a minute aperture at the end of the root,
blood vessels and nerves still enter the pulp-cavity and keep the tooth
DENTITION. 723
alive, but, as the limit of growth is reached, the residue of soft pulp tends
to disappear. ;
The two most marked characteristics of the teeth of Mammals are
that they are typically Aeterodont—that is, different from one another in
form and function—and that the succession is practically reduced to
two sets, a condition described as diphyodont as contrasted with the
polyphyodont condition seen in Fishes and Reptiles, where the suc-
cession is practically unlimited.
As exceptions, there are cases like that of the dolphins, where the
teeth are uniform or. homodont and very numerous. This, however,
is not a primitive but a secondarily acquired condition.
In the typical dentition of Mammals there are forty-four
permanent teeth, eleven on each side above and below; but
it is rare in the Eutherian Mammals to find the full number
developed, and the dentitions of the Marsupials, of the
Edentates, and of the Cetacea cannot be reduced to this
type. The eleven on each of the upper jaws may be divided
in the typical case into four sets. Most anteriorly, associated
with the premaxilla, are three simple, single-rooted teeth,
usually adapted for cutting or seizing. These are called
incisors. Posteriorly there are crushing or grinding teeth,
whose crowns bear cusps or cones, or are variously ridged,
and which have two or more roots associated with the
maxilla. But of these grinders the last three occur as one
set, having no calcified successors, or, as others maintain,
having no milk predecessors. They are therefore dis-
tinguished, as true molars, from.the four more anterior and
often simpler premolars, which usually occur in two sets,
the milk set being replaced by a permanent set. In many
cases, however, the first premolar seems to be only once
represented, Finally, the tooth just behind the incisors—
that is to say, immediately posterior to the suture between
premaxilla and maxilla—is distinguished as the canine, and
is often long and sharp.
This classification of teeth is in great part one of convenience: thus
the distinction between incisors and grinding teeth is anatomical, that
between molars and premolars refers to the history of these teeth ; the
connection between the teeth and the subjacent bones is a secondary
matter; there is often little to differentiate canine from premolar.
Moreover, the teeth of the lower jaw, which is a single bone on each
side, cannot be so certainly classified as those of the upper jaw. Here
the lower canine is defined as the tooth which bites in front of the
upper, and the incisors as the teeth in front of this tooth.
724 MAMMALIA.
The typical mammalian dentition already referred to may be expressed
as follows :—
Incisors 33, canines = premolars 4—4, molars 3—3 = 227" = total, 445
3-3 4- 3° «I-11
or, using initial fevers
303 0 Ia 4-4 3-3_
i. Spt por PM mas
or, recognising that the right and left side are almost invariably identical,
and omitting the initial letters— ar
The formulz for the adult dentition of some representative Mammals
are the parent —
Opossum = be +, Thylacine * = 4) Kangaroo 25 ots 4) Wombat nae , Pig == 3143 , Camel eas,
413 = ay 3143" 3123
Sheep 33 ser Horse ele ey Rabbit on Cat on Dog 374?, Bear 247, Seal 3247,
3143” gran’ 3143 35 43° 214r’
cient ‘ et , Marmoset = 2732 2, New ar Monkey 2793, Old World Monkey >
Man 2123.
2123
It is interesting to note the relation in particular cases between the
diet and the form of the teeth. Thus the dolphins, which feed on fish
and swallow them whole, have numerous, almost uniform, sharp,
recurved, conical teeth, well suited to take a firm grasp of the ‘slippery
and struggling booty. Toa slight extent the same piscivorous dentition
may be seen in seals, In the more strictly carnivorous Mammals the
incisors are small, the canines are long and sharp, piercing the prey
with a deathful grip, while the back teeth have more or less knife-like
edges, which sever flesh and bone. In typical insectivorous Mammals
the upper and lower incisors meet precisely, ‘‘so as readily to secure
small active prey: quick to elude capture but powerless to resist when
once seized,” while the crowns ‘of the molars bear many sharp points.
Herbivorous Mammals have front teeth suited for cropping the herbage
or gnawing parts of plants, the canines are small or absent, the molars
have broad grinding crowns with transverse ridges. In omnivorous:
Mammals the incisors are suited for cutting; the canines are often
formidable weapons in the male sex; the molars have crowns raised
into rounded tubercles.
A primitive form of tooth with three cusps in one plane is called
triconodont ; when the three cusps form a triangle, the tooth is called
tritubercular ; when the crown has a number of blunt or pointed cusps,
it is called bunodont ; when the cusps run into ridges, the term /ophodont
is used ; when the cusps form a crescent, the tooth is called selenodont ;
when there is a long crown with the neck (the junction region between
crown and root) deep in the socket, the tooth is called Aypsodont ;
when there is a short crown with the neck at the surface of the gum, the
term drachyodont is used.
Development and placentation.—The ova of Mammals,
except Monotremes, are small; even those of the Whales are
DEVELOPMENT AND PLACENTATION. 725
“no larger than fern seed.” They are formed from germinal
epithelium, the cells of which grow inwards in clustered
masses into the connective tissue or stroma of the ovary.
In each cluster one cell pre-
dominates over its neigh-
bours ; it becomes an ovum;
the others invest and nourish
it, and are called follicle cells.
Tn the middle of each clump
or Graafian follicle a cavity
is formed containing fluid,
and into this cavity the follicle
cells immediately surrounding
the ovum project, forming
what is called the dzscus pro-
ligerus (see Fig. 272, p. 514).
When mature, the ovum
protrudes on the surface of
the ovary, and is liberated
by the bursting of the Graafian
follicle. Ovulation may occur
spontaneously — as in man,
monkeys, horse, cattle, pig,
dogs; or after sexual union—
as in rabbit, guinéa-pig, mouse,
and cat. An ingrowth of
epithelial cells surrounding
the follicle develops into a
glandular body called the
corpus luteum. Its secretion
is believed to be very import-
ant—influencing the prepara-
tion of the uterus, the early Hie Soy Berwenbaion wt
nutrition of the embryo, and rabbit’s ovum.—After Van
the multiplication of cells in Beneden.
the milk glands. It seems 'e.¢., External cells (epiblast) ; z.c.,
that the ovary, besides produc- _iuternal cells (hypoblast); 2.2,
ing ova, is a gland whose in-
ternal secretion, passing into the blood, induces, directly or
indirectly, the phenomena of heat and menstruation, and
influences the uterus during pregnancy,
ic
726 MAMMALIA.
The spermatozoa are formed from germinal epithelizm
in the testes. The primitive male cells or spermatogonia
give rise by division to daughter cells
or spermatocytes, which, with or
without further division, form sperma-
tozoa.
The homologue of the ovum is the:
spermatogonium or mother sperm cell,
but the physiological equivalent of
the ovum is the spermatozoon.
The ovum, having burst from the
ovary, is immediately caught by the
fimbriated mouth of the Fallopian
tube, and begins to pass down the
oviduct. There it is met by ascend-
ing spermatozoa, received by the
female as the result of sexual union,
and is fertilised. One of the sperma-
tozoa enters the ovum, and sperm
nucleus unites with ovum nucleus in
an intimate and orderly manner.
The connection between embryo
and mother.—(a) The lowest Mam-
mals, the Duckmole (Ornithorhyn-
chus) and the Portupine Ant-Eater
(Echidna), resemble Birds and most
Fic. 396. — Develop- Reptiles in bringing forth their young
ment of hedgehog. as eggs, ze. in being oviparous. The
Three early stages.— : ;
After Habrecht’ eggs are large, with a considerable
creepiest oO of yolk, and after fertilisa-
“hypoblast; the disc and tion divide partially, z.e. exhibit mero-
fo ane ail ones blastic segmentation like the eggs of
from trophoblast; the disc Birds and Reptiles. The tunic formed,
Cr) tha Wastin Tound about them in the Graafian
p blastodermic : A :
tile Pe, Mie follicles of the ovary consists, as in
re advances a . . -
Tr, trophoblast; 29., Birds and Reptiles, of a single layer
disc of formative epiblast; of cells. Development begins in the
Bu., blastodermic vesicle 5 . :
H., hypob'ast. oviducts, but the eggs are in no
way attached to the wall. They are
laid in a nest by the Duckmole; in the Echidna they are
hatched in a slight, periodically developed, external pouch.
CONNECTION BETWEEN EMBRYO AND MOTHER. 727
(2) In the Marsupials the embryo is born prematurely
after a short gestation. It is very small and helpless. Till
recently it was believed that during its intra-uterine life it
was either not attached to the wall of the uterus at all, or
only to a slight extent by a yolk-sac placenta. It is now
known, however, that, in Perameles at least, there is not
only an efficient yolk-sac placenta, but a distinct, though
small, allantoic placenta. The general absence of a
an be
Fic. 397.—Embryo of 2erameles with its foetal membranes.
—After Hill.
am., True amnion; ad, allantois; @/.s., allantoic stalk; y.c.,
cavity of yolk-sac ; chy chorion or false amnion; s.¢., sinus
See 6.c., extra-embryonic body cavity; v.0., vascular
omphalopleura, or area of non-separation between yolk- -sac wall
and chorion, constituting the yolk -sac placenta, The endoderm
is dotted throughout. Note the large size of the yolk-sac, and
the sinking of the embryo into it.
placenta in Marsupials, and the small size of the allantois,
must therefore be ascribed to degeneration, and not toa
primitive condition. The presence of a yolk-sac placenta
in Marsupials is not in itself of great importance, for a
connection between the yolk-sac of the embryo and the
wall of the oviduct exists in two Elasmobranch fishes and in
two lizards, but the similarity between the allantoic placenta
of Perameles and that of the Eutheria seems to point in-
disputably to a common origin for, the two structures.
728 MAMMALIA
(c) In the Eutherian Mammals, although a temporary
yolk-sac placenta may occur, there is always a well-
developed and exceedingly important allantoic placenta,
which is the main organ for the nutrition of the embryo.
The placenta, in rough physiological language, is a double
vascular sponge, partly embryonic, partly maternal, by
means of which the blood of the mother nourishes and
purifies that of the embryo. It is formed by the inter-
locking of foetal and maternal tissue.
In giving an account of the placentation of the Eutheria,.
we shall mainly follow Hubrecht in his account of the
: placentation of the hedge-
hog, which is at once a
simple and central type.
Before doing so, it may
be well to note briefly
certain facts in regard to
the early development of
the egg. In Eutheria,
segmentation is holoblastic
and yolk is absent, but the
process of development is
ee, F nteq Very different from a simple
TiC: Gram of hedgehog. © After case like that of Amphi:
Hubrecht. oxus. In the latter, all the
Ep., Epiblast ; Ay., hypoblast. cells of the blastosphere
form part of the embryo;
in the former, only a few take a direct part in the process ;
the remainder form the wall of the embryonic sac or
blastocyst, from which the yolkless yolk-sac or umbilical
vesicle is later developed. A process of folding-off of the
embryo occurs therefore in Mammals as in Birds and
Reptiles, the chief difference being that, roughly speaking,
in the former the yolk-sac has a cellular wall from the first,
in the latter the germinal layers slowly spread over the yolk
as development proceeds.
Bearing these facts in mind, let us then seek to define
the embryonic and maternal structures which are associated
with placentation. (1) At a very early stage the divided
ovum of the hedgehog consists of a sac of cells, an outer
layer, epiblastic or ectodermic, enclosing another aggregate
CONNECTION BETWEEN EMBRYO AND MOTHER. 729
—the future inner layer, endoderm or hypoblast (Fig. 398, I.).
(2) The epiblast divides into an embryonic disc, which will
form the epidermis, nervous
system, etc., of the embryo,
and an external layer, the wall
of the embryonic sac or blasto-
cyst, with which the disc retains
a slight connection until the
protective amnion is formed.
In the outer epiblastic wall
lacunee develop, which are
bathed by the maternal blood,
and the pillars of tissue
between the lacune grow out
into villi, which aid in this
earliest’ connection between
mother and offspring. Long
before. any vascular area or
foetal placenta is developed,
the outer epiblastic wall has
the above nutritive function,
and deserves its name of
trophoblast (Fig. 396, 77r.).
(3) The hypoblast or inner
mass, which is at first a solid
aggregate of cells (Fig. 395,
zc), becomes a sac, as a
morula may become a blasto-
sphere. The upper part
Fic. 399.—Development of foetal
membranes.—After Hertwig.
Uppermost figure shows up-growth and
down-growth of amnion folds. £.,
Embryo; a.f, amnion fold; @.1, amnion
proper ; 2.2, subzonal membrane; g., the
gut; y., umbilical vesicle or yolk-sac.
The dotted line represents mesoderm ;
the dark, hypoblast. The second figure
shows origin of allantois, and the,amnion
folds have met. The third figure shows
increase of allantois (a/.); the dwindling
yolk-sac (ys.); @c., amniotic cavity ;
sz.m., subzonal membrane. The fourth
figure shows the embryo apart from its
membranes: #., mouth; @., anus. Note
umbilical connection with yolk-sac.
730 MAMMALIA.
of this sac forms the lining of the incipient gut, while the
lower portion, following the contour of the blastocyst wall,
forms the lining of the umbilical vesicle (cf. the Chick).
From this vesicle or yolk-sac the embryo becomes folded
off, and the connection between the two is narrowed, just
as in the chick, into a canal—the vitelline duct, which is
part of the “umbilical cord,” entering the embryo at the
future navel. (4) Between the epiblast and the hypoblast
of the embryo, the
mesoblast develops,
splitting into an
outer, parietal, or
somatic, and an
inner, visceral, or
splanchnic layer.
The cavity between
these is the incipient
body cavity. A
double fold of so-
matic mesoblast,
carrying with it a
j single sheet of epi-
== blast, rises up round
Fic. 400.—Diagram of fcetal memlnanes about the embryo,
—AfterTumer. arching over it to
“the dark i mésdblasts OP), uaiheat verte a, form the amnion,
nes ® Te ee aeiors Over the embryo
Batty? a, allantois : ah ‘nay be here faben ie the folds of amnion
Hypize does Wot chew that tae amelon fie conse, Meee A, Cupola,
of hoth epiblast and mesoblast. and the inner layers
: of the double fold
unite to form the “amnion proper,” while the outer 1ayers
also unite to form a layer lying internally to the epiblastic
blastocyst wall,—and termed by Sir William Turner she
subzonal membrane. The folds of amnion are continued, as
the diagram shows, ventrally as well as dorsally, so that the
subzonal membrane surrounds the embryo beneath the
blastocyst wall, while a splanchnic layer of mesoblast grows
round about the hypoblastic yolk-sac. The space between
the two layers of mesoblast is continuous with the body
cavity of the embryo. The epiblastic outer wall or tropho-
a
cc~e
‘t
a
CONNECTION BETWEEN EMBRYO AND MOTHER. 731
blast, and the mesoblastic subzonal membrane, are included
in Hubrecht’s term—diplotrophoblast. (5) From the hind-
wall of the gut there grows out a hypoblastic sac, the
allantois, insinuating itself and spreading out in the space
between the two layers of mesoblast. As an outgrowth of
the gut, homologous with the bladder of the frog, the
allantois is lined by hypoblast or endoderm, but it is
covered externally by a layer of mesoblast, which it bears
with it as it grows. In all placental Mammals, the allantois,
which becomes richly vascular, unites with the subzonal
membrane, and therefore with the external epiblast as well,
to form the foetal: part of the placenta, with outgrowing
vascular processes or villi, which fit into corresponding
depressions or crypts on the wall of the uterus. To the
mesoblastic wall of the allantois, plus the subzonal membrane,
the term “chorion” is sometimes applied ; but as the word
has been used in many different senses, its abandonment is
almost imperative. The complex union of allantois with
diplotrophoblast Hubrecht calls the aliantoidean trophoblast.
(6) But in the hedgehog, rabbit, and some other Eutherian
types, as well as in certain Marsupials, there is a mode ot
embryonic nutrition between that attained by the epiblastic
trophoblast and that affected by the final placenta. The wall
‘of the yolk-sac, hypoblastic internally, mesoblastic externally,
unites with the subzonal membrane, and becomes the seat of
villous processes, which through the external epiblast are con-
nected with the uterine wall. Thus is formed what Hubrecht
calls an omphaloidean trophoblast or yolk-sac placenta. In
connection with this yolk-sac placenta it will be recollected
that the yolk-sac, here as in the Bird, is a vascular structure
well fitted for a placental function. In the Bird and in
most Mammals, however, the splitting of the mesoblast as
it follows the contour of the yolk-sac forms a space—the
extra-embryonic body cavity—between the yolk-sac and the
subzonal membrane. When a yolk-sac placenta is developed,
the splitting of the mesoblast is retarded, so that the
vascular yolk-sac comes to lie close under the subzonal
membrane. This is especially well seen in Perameles (see
Fig. 397), and is of much importance in the formation of
an efficient yolk-sac placenta. ,
(7) The embryo lay at first in a groove of the uterine wall,
732 MAMMALIA.
moored by the preliminary blastocyst villi, which are as it
were pathfinders for those subsequently developed from
yolk-sac and allantoic regions. At the point of attachment
the mucous lining of the uterus ceases to be glandular, and
becomes much more vascular. As the embryo becomes
fixed, the blastocyst almost eating its way in, the outer
epithelium degenerates and disappears ; below this the next
layer of the mucous membrane becomes spongy and exhibits
unique blood spaces, forming what Hubrecht calls the
trophospongia; below this there is the vascular and vitally
active remainder of the mucosa, less modified than the
above-mentioned sponge; below this again there are the
muscular and other elements of the uterine wall, with which
we are not now concerned. The most important fact to
emphasise is, that the maternal blood in the spaces of the
spongy outer layer of the mucous membrane directly bathes
the foetal tissue represented by the trophoblast. By the
activity of the trophoblast cells, the nutritive and respiratory
advantages of the maternal blood are secured for the villi of
the allantois and yolk-sac. It ought also to be mentioned
that, mainly by a folding of the uterine wall, the hedgehog
embryo is virtually enclosed in a maternal sheath, homo-
logous with a fold called the decidua reflexa in human em-
bryology,and analogous with a similar capsule in the rabbit.
To sum up—
1. At an early stage a wall of epiblast encloses an aggregate of
hypoblast (Figs. 395, 396, I., 398).
2. The epiblast divides into an embryonic disc and an‘outer blasto-
cyst wall, with fixing and nutritive functions,—the trophoblast
(Fig. 396, I. and II.).
3. The hypoblast becomes a sac, of which the upper portion lines
the gut, while the lower part forms the yolk-sac (Fig. 396, ITT.).
4. The mesoblast divides into somatic and splanchnic layers; a
double fold of the somatic layer (along with a slight sheet of
epiblast) forms the amnion, of which the outer limbs unite as
the subzonal membrane, and form, along with the external
epiblast, the diplotrophoblast. The splanchnic layer of the
mesoblast is continued round the yolk-sac (Fig. 399).
5. The allantois grows out from the hind region of the gut, being
lined internally by hypoblast, externally by splanchnic meso-
blast. The allantois plus the diplotrophoblast always forms the
true placenta (Fig. 400).
6. Part of the yolk-sac wall, uniting with the diplotrophoblast, also
forms an efficient but temporary placenta.
CONNECTION BETWEEN EMBRYO AND MOTHER. 733
7. At the area of fixing, the uterine epithelium degenerates, the
glands disappear, vascularity increases. The outer part of the
modified mucous membrane (or decidua) becomes a spongy
tissue, with spaces filled with maternal blood. This maternal
blood bathes the trophoblast, which is intermediate between it
and the placental villi. '
The three modes of embryonic nutrition are as follows :—
(a) At first the maternal blood bathes the lacunz in the epiblastic
outer wall—the trophoblast with its preliminary pathfinding villi. |
Fic. 401.—View of embryo, with its foetal membranes.
—After Kennel.
am., Amnion proper; @., dwindled yolk-sac; a/., allantois; aZ.,
subzonal membrane; z., 2’., villi. Outside the subzonal mem-
brane there is the delicate ectodermic trophoblast (s.ci.).
(4) An efficient yolk-sac placenta functions for a time, but decreases
and shrivels as the final allantoidean placenta develops. The
maternal blood in the spaces of the outer layer of the mucous
laver of the uterus bathes the trophoblast. Thus it comes into
indirect connection with the vascular villi from the region where
the yolk-sac wall unites with the diplotrophoblast. This yolk-
sac placenta is well seen in Insectivora, Chiroptera, Rodentia,
the horse, ‘etc., and seems to be to some extent developed in all
Mammals (except Monotremes) as yet examined.
(c) The final placenta is allantoidean. :
In the above description the yolk-sac placenta has been emphasised
on account of its comparative importance, but it must be clearly under-
734 MAMMALIA,
stood that the allantoic placenta is often the only one well developed,
and is always of supreme importance in reference to the nutrition of
the embryo.
From the comparative standpoint the most important variations in
regard to the placenta are—first, the method of distribution of the villi
on the surface of the allantois ; and second, the extent of the connection
between maternal and foetal tissues. Where the connection is very
intimate, parts of the maternal tissue come away at birth, and the
placenta is said to bedeciduate. Where there is a less close interlocking,
the foetal villi are simply withdrawn from the maternal crypts, and the
placenta is indeciduate. In Perameles, and toa less extent in the mole
(Tal~a), not only is there no loss of maternal tissue, but part—in
Perameles the greater part—of the foetal portion of the placenta is
absorbed 77% sztu by maternal leucocytes, a condition described by
Hubrecht as contra-deciduate. The distinction between the deciduate
and indeciduate forms is not perfectly sharp, and Hubrecht prefers the
older terms, Caducous and Non-Caducous.
THE CUSTOMARY CLASSIFICATION OF PLACENTATION
Meta-Discotdal.—Villi, at first scattered, are Homo and
restricted to a disc. f Monkeys.
Around the embryo the maternal
mucous membrane forms a capsule
Caducous ; (decidua reflexa), also seen in hedge-
or hog.
Deciduate.
(Vascular Rodentia.
parts of Insectivora (in the mole inde-
maternal ( Discotdal.—Villi on a circular} Ciduate and in part contra-
placenta cake-lilce disc: deciduate)and Chiroptera.
come Most Edentata,
away [nets (contra - decidu-
at birth.) ate).
Carnivora. .
Elephants and Hyrax.
Oryeteropus and Dasypus
among Edentata.
Dugong (wholly or in great
part non-deciduate).
Non-Caducous / Cotyledonary.—Villi in patches. Ruminants.
Zonary.—Villi on a partial
or complete girdle
round the embryo.
or
Indeciduate.
(Maternal Lemurs.
part of Most Ungulates, except
placenta does | Dzfuse.—Scattered Villi.; Ruminants.
not come away Cetacea.
at birth.) Manis among Edentata.
GENERAL LIFE OF MAMMALS. 735
There is some uncertainty as to the primitive form of the placenta,
but the fact that it is discoidal in Pevameles seems to confirm Balfour’s
view that this form must be placed lowest.
The formation of the allantoic placenta in Perameles is in essentials
the same as in Eutherian Mammals, but in details there are some
striking differences. The most noteworthy of these is, perhaps, that the
cells of the uterine epithelium, instead of disappearing at an early stage,
as in Eutherian Mammals, proliferate greatly, lose their cell outlines,
and by the increase of the nuclei form what is known as a syncytium.
Later this syncytial layer becomes highly vascular, and forms the
maternal portion of the placenta, whereas, as already seen, in Eutheria
it is the uterine mucosa which forms the maternal part of the placenta.
Into the vascular syncytium the allantoic capillaries grow down, until
ultimately maternal and foetal vessels are separated merely by their
endothelial walls and a mere trace of syncytial protoplasm. The
connection between the yolk-sac wall and the uterus is effected in a
similar manner.
GENERAL LirE oF MamMMALs
Most Mammals live on dry land. The bats, however,
have the power of flight, and various forms are able to
take long swooping leaps from tree to tree. Thus there are
“flying phalangers,” such as Fefaurus, among Marsupials ;
“flying squirrels,” such as Pteromys, among Rodents;
“flying lemurs” (Gadeopithecus), allied to Insectivores. Not
a few are aquatic,—all the Cetaceans, the two Sirenians, and
the Pinniped Carnivores, such as seals and walruses ; while
water-voles, beavers, otters, polar bear, and many others
are also at home in the water. Burrowers are well repre-
sented’ by moles and rabbits; arboreal forms by squirrels
and monkeys. ;
As to diet, man, many monkeys, the pigs, and many others,
may be called omnivorous ;: kangaroos, hoofed animals, and
most rodents are herbivorous; the Echidna, the ant-eaters,
hedgehogs and shrews, and most bats, are insectivorous ;
most of the Carnivora are carnivorous; dolphins and seals
feed chiefly on fishes; but in most cases the diet varies not
a little with the available food-supply.
The struggle for existence among Mammals is sometimes
keen among fellows of the same kind; thus the brown rat
(Mus decumanus) tends to drive away the black rat (AZ
rattus); but stress, due to over-population, is sometimes
mitigated by migration, as in the case of the lemmings. The
struggle seems to be keener between foes of different kinds,
736 MAMMALIA.
between carnivores and herbivores, between birds of prey
and small mammals; but combination for mutual defence
often mitigates the intensity of the conflict. Teeth and
claws, hoofs and horns, are the chief weapons, while the
scales of pangolins, the bony shields of armadillos, the spines
of hedgehogs and porcupines, and the thick hide of the
rhinoceros, may be regarded as protective armature. In
keeping their foothold some Mammals are helped by the
harmony between their colouring and that of their surround-
ings; thus the white Arctic fox and hare are inconspicuous
on the snow, the striped tiger is hidden in the jungle, and
many tawny animals harmonise with the sandy background
of the desert.
The majority of Mammals are gregarious; witness the
herds of herbivores, the cities of the prairie-dogs, the packs
of wolves, the schools of porpoises, the bands of monkeys.
Combinations for attack and for defence are common;
sentinels are posted and social conventions are respected ;
such migrations as those of the lemming and reindeer are
characteristically social. In the beaver village and, among
monkeys there is combination in work, and their communal
life seems prophetic of that sociality which is distinctively
human.
Among Birds, mates are won by beauty of song and
plumage ; Mammals not less characteristically woo by force.
Rival males fight with one another, and are usually larger
and stronger than their mates. The antlers of male deer,
the tusk of the male narwhal, the large canine teeth of boars,
illustrate secondary sexual characters useful as weapons.
But manes and beards, bright colours and odoriferous
glands, are often more developed in the males than in the
females, and may be of advantage in the rough mammalian
courtship. At the breeding season a remarkable organic
reaction often affects the animal: the timid hare becomes a
fierce combatant, and love is often stronger than hunger.
The courtship of Mammals is usually like a storm—violent
but passing; for, after pairing, the males return to their
ordinary life and the females become maternal. Some
monkeys are faithfully monogamous; and exceptional pairs,
such as beavers and some antelopes, remain constant year
after year; but this is not the way of the majority.
GENERAL LIFE OF MAMMALS. 737
The duckmole lays eggs and brings up her young in the
shelter of the burrow; the Echidna has a temporary pouch.
In Marsupials the time of gestation is very short, and there
is rarely a true placental union between the unborn young
andthe mother. The new-born Marsupials are very helpless,
and are in most cases transferred to an external pouch or
marsupium, within which they are nurtured. In Eutherian
Mammals the gestation usually lasts much longer than in
Marsupials,—its duration varying to some extent with the
rank in the mammalian series ; but there are great differences
in the condition of the young at birth. “In those forms,”
Sir W. H. Flower says, ‘‘ which habitually live in holes, like
many Rodents, the young are always very helpless at birth ;
and the same is also true of many of the Carnivora, which
are well able to defend their young from attack. In the
great order of Ungulates or Hoofed Mammals, where in the
majority of cases defence from foes depends upon fleetness
of foot, or upon hugé corporeal bulk, the young are born in
a very highly developed condition, and are able almost at
once to run by the side of the parent. This state of relative
maturity at birth reaches its highest development in
the Cetacea, where it is evidently associated with the
peculiar conditions under which these aninfals pass their
existence.”
The maternal sacrifice involved in the placental union
between the mother and her “foetal parasite,” in the pro-
longed gestation, in the nourishment of the young on
milk, and in the frequently brave defence of the young
against attack, has been rewarded in the success of the
mammalian race, and has been justified in the course of
natural selection. But it is important to recognise that the
maternal sacrifice—whatever its origin may have been—
expresses a subordination of self-preserving to species-
maintaining. Thus other-regarding as well as self-regarding
activities have been factors in evolution.
Pedigree.—The origin of Mammals remains obscure, but there
is much to be said for their affiliation to some ancient Reptilian stock,
such as the Anomodontia (especially the Theriodontia).
In several features the Monotremes link the Mammals to living
Reptiles, ¢.g. the structure of the pectoral girdle, the cloaca, the
condition of the genital ducts, the relatively large ova with meroblastic
segmentation, but it is out of the question to think of any cf the
47
738 MAMMALIA,
living types of Reptiles as near the direct line of Mammalian
pedigree.
In ‘‘ Anomodontia” there are so many mammalian features in the
skeleton that in spite of the complex lower jaw articulating with a
fixed quadrate, the presence of an os transversum, pre- and post-
frontals, etc., some have doubted whether they should be ranked as
Reptiles at all. We may note that they were purely terrestrial
animals (of large size) with limbs lifting the body high off the ground,
that the squamosal sometimes descends far down outside the quadrate
and may share in the articulation for the lower jaw, that the quadrate is
often small, that there is a single temporal arcade comparable to the
mammalian zygomatic arch, that the teeth are heterodont, that the
pelvic bones unite in an os innominatum with a continuous ischiac
symphysis, that the scapula often has a spine, that the occipital condyle
may be double, that there is a beginning of reduction and consolidation
of skull bones, and so on.
But it may quite well be that the Anomodontia are not in the direct
line of Mammalian ancestry, but represent a side-branch from transi-
tional forms connecting Reptiles and Mammals.
The student should look back to the characters common to the
Amniota (Reptiles, Birds, and Mammals), e.g. the presence of amnion
and allantois, the absence of gills, etc., for these indicate a close
alliance far apart from Ichthyopsida, and it seems therefore unprofitable
to look for the roots of the Mammalian stock so low down as among
Amphibians.
Nevertheless, amid so much uncertainty, we may recall the facts that
in Amphibians we find two occipital condyles, a reduced quadrate, a
somewhat mammalian carpus, holoblastic ova, and so on.
The oldest Mammalian fossils are from Triassic strata, but they throw
little or no light on pedigree, partly perhaps because they are few and
fragmentary, partly also because they seem already specialised forms.
They are often grouped together as Allotheria or Multituberculata and
placed near the Monotremes.
In the Jurassic period there are more of the dubious Allotheria,
e.g. Plagiaulax, some ‘‘triconodont” Marsupials, e.g. Zrzconodon and
Amphilestes, and the Trituberculata, e.g. Amphitherium, some of which
suggest primitive Insectivora. There are few Cretaceous fossil remains
of Mammals, but some of them suggest that the orders of Eutheria
were incipient.
In the earliest Eocene strata, Mammals related to modern types begin
to be abundant, but we cannot do more than notice two points—(a)
there were some generalised types, e.g. Creodonts and Condylarthra,
with relationships to several extant orders ; (4) that the early forms were
mostly small animals with small brains, pentadactyle, with 44 teeth
including small canines and bunodont molars.
Professor Osborn has suggested that there were two main lines of
mammalian evolution—(a) the ‘‘ Mesoplacentalia,” e.g. Amblypoda,
Coryphodontia, Dinocerata, Tillodontia, and many Condylarthra and
Creodonts, in which the brain remained small and unspecialised, which
died out in the Miocene (unless the Marsupials, Insectivores, and
Lemurs represent their descendants), and (0) the successful lines of
SURVEY OF THE ORDERS OF MAMMALIA. 739
‘“Cenoplacentalia,” which made, so to speak, a fresh start, with a
premium on brains, and led to most of the modern types. In almost
every case, it may be said that an order begins with small repre-
sentatives, and that the giant forms almost always indicate the end
of a race.
SyvSTEMATIC SURVEY OF THE ORDERS OF MAMMALIA
I. Sub-class PROTOTHERIA or ORNITHODELPHIA, Orders
Monotremata, and (?) Allotheria or
Multituberculata.
II. 5 METATHERIA or DIDELPHIA, Orders Poly-
protodontia and Diprotodontia.
ITI. on EUTHERIA or MoONODELPHIA.
Orders of EUTHERIA.
1. Xenarthra.
2. Nomarthra.
3. Sirenia.
4. Ungulata.
Artiodactyla. ;
Perissodactyla. } Ungulata tics
Hyracoidea.
Proboscidea.
Extinct sub-orders.
5. Cetacea.
Mystacoceti—baleen cetaceans.
Archzeoceti—(extinct types).
Odontoceti—toothed cetaceans,
6. Rodentia.
Simplicidentata,
Duplicidentata,
4. Carnivora.
8. Pinnipedia.
g. Insectivora.
10. Galeopithecide or Dermaptera.
tx. Chiroptera.
Megachiroptera.
Microchiroptera.
12, Prosimize or Lemuroidea.
13. Anthropoidea.
\ “ Edentates.”
\ = Primates.
740 MAMMALIA.
Sub-Class PROTOTHERIA (Syz. ORNITHODELPHIA),
Orders Monotremata and (?) Allotheria
The Monotremes include the duckmole (Ornithorhynchus
anatinus), the spiny ant-eater (Echidna aculeata), and a
third form resembling Zchidna, but often referred to a
distinct genus as Proechidna. These are the lowest
Mammals, very different from all the rest, and they exhibit
affinities with Reptiles.
The duckmole is found in the rivers of Australia and
Tasmania; Zchidna in Australia, Tasmania, and New
Guinea; Proechidna in New
Guinea.
In Ornithorhynchus the skin
is covered with soft fur; in
Echidna and Proechidna there
are spines among the hairs.
The mammary glands in the
female Ornithorhynchus open
on a flat patch; in Zchidua,
in a depressed area around
which a temporary pouch seems
to be developed. There are
no distinct mamme.
: p The vertebral centra have
sc., Scapula; cé., clavicle; z.cé., : : -
prosternum or “‘interclavicle” ; co., weak epiphyses in Ornithorhyn-
coracid of metacoracid:, ¢ce chus, and apparently none in
Pecotemm, ° “Echidna. In the duckmole the
post-sacral vertebrze are stronger
than the pre-sacral. The skull is smooth and polished as in
Birds, for the sutures disappear. The rami of the lower jaw
do not unite in front, have no ascending process, and have a
slightly inflected angle. In Ornithorhynchus there are true
mammalian teeth, but only in the young; in ZAchidua none
are present. Cervical ribs remain distinct for a time at least ;
the odontoid process of the second vertebra is for a long
time free from the centrum. Except on the atlas of
Echidna, the cervical vertebrae are without zygapophyses
or articular processes. The (meta-) coracoids reach the
sternum; there are also large precoracoids (often called
epicoracoids, but homologous with the precoracoids of many
Fic. 402.—Pectoral girdle
of Echidna.
741
SOB-CLASSES OF MAMMALS.
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742 MAMMALIA.
Reptiles and Amphibians) and a T-shaped prosternum
{sometimes called interclavicle), on which the inner ends of
the clavicles rest, the outer ends abutting on the acromion
of the scapule. In Ornithorhynchus the ischia form a long
ventral symphysis; in Zchidna the acetabulum socket for
the femur is incompletely ossified (reminding one of Birds,
though it is only a secondary peculiarity); the pubes bear
epipubic bones, as in Marsupials. On the side of the
tarsus, in the duckmole, there is a spur perforated by the
duct of a gland. This spur is well developed in the males,
but rudimentary in the females. The male Zchidna has a
similar but smaller spur. The fibula
has a proximal process like an ole-
cranon.
The brain is smooth in the duck-
mole, convoluted in £chidna; the
cerebellum is not covered by the
cerebrum, there is a large anterior
commissure, and the corpus cal-
losum is rudimentary or absent.
Fic. 403.—Pelvis of The food canal ends in a cloaca.
iin The right auriculo-ventricular valve
‘Lo te ee in Ornithorhynchus is partly muscular
of obturator foramen ag in Birds, while in other Mammals
between ischium and pubis -, -
). it is membranous and worked by
papillary muscles attached to it by
tendon-like cords (chord tendinez). The temperature
of the blood is about 25°-28° C., and is noteworthy in
being unusually variable. In fact, the Monotremes are
imperfectly warm blooded.
The ureters open, not into the bladder, but into the
urogenital canal, as they do in the embryos of higher
Mammals,
The testes remain in the abdomen. The left ovary is
larger than the right, as in Birds. The vasa deferentia
open separately into the urogenital canal. So in the
female do the oviducts, and these have no fringed fim-
briated apertures nor distinct uterine region. The penis is
attached to the ventral wall of the cloaca, and the uro-
genital canal communicates both with the cloaca and with
the canal of the penis. ‘The whole structure resembles in
METATHERIA, DIDELPHIA, OR MARSUPIALIA. 743
many ways the copulatory organ of certain Reptiles and
Birds.
The ova are large, with abundant yolk, and undergo
meroblastic segmentation. The Prototheria are oviparous.
The duckmole, or duck-billed platypus, lives beside lakes and rivers.
It swims by means of its fore-limbs, which are webbed as well as
clawed ; it grubs for aquatic insects, crustaceans, and worms, in the
mud at the bottom of the water or among the floating weeds. It
collects small animals in its cheek pouches, and chews them at leisure
with its eight horny jaw-plates. It makes long burrows in the banks,
often with two openings, one above, one under the water. The animal
is shy, and dives swiftly when alarmed. When about to sleep, it rolls
itself into a ball. In the recesses of the burrows the eggs are laid, two
atatime. The egg measures about three-quarters of an inch in length,
and is enclosed in a flexible white shell, through which the young
animal has to break its way.
The full-grown duckmole measures from 18 to 20 in. in length; the
male slightly exceeds his mate. The fur is short and soft, dark brown
above, lighter beneath. The jaws are flattened like the bill of a duck,
and covered with naked skin, which forms a soft, sensitive collar around
the region where the bill joins the rest of the skull. The eyes are very
small. There is a well-developed but inconspicuous pinna ; the nostrils
lie near the end of the upper part of the bill. The tail is flat.
True teeth, three on each jaw above and below, are calcified, last for
about a year, and are then lost, being replaced by horny plates, two on
each jaw, above and below. The spur. borne on the heel seems to be
sometimes used as a weapon, and as it persists only in the males, is
perhaps useful in contests between rivals.
Echidna and Proechidna live in rocky regions, are mainly nocturnal
in habit, and burrow rapidly, legs foremost. They feed on ants, which
are caught on the rapidly mobile, slender, viscid tongue. No traces of
teeth have as yet been seen.
Strong spines occur thickly in Echédua, more sparsely in Proechidua
among the hairs. The snout is prolonged into a slender tube. There
is a distinct pinna about an inch Jong. The limbs bear five toes, two
of which in Proechidna are often without claws and somewhat rudi-
mentary. In Echidna the eggs seem to be hatched in a temporarily
developed pouch, which is apparently comparable to a much-expanded
mamma of the type seen in the cow.
The Allotheria or Multituberculata include small extinct Mammals
{from Triassic to Eocene) with multituberculate molars, e.g. Plagiaulax,
Microlestes, Tritylodon. They are often classed with the Marsupials,
Sub-Class METATHERIA, DIDELPHIA, or MARSUPIALIA
With the exception of the American opossums, and
a littleknown mouse-like animal (Cexolestes) from S.
America, all the Marsupials now alive are natives of
744 MAMMALIA.
Australasia. But fossil remains found in Europe and
America show that they once had a wide range. As
there are no higher Mammals indisputably indigenous to
Australasia, it seems as if the insulation of that region had
occurred after the Marsupials had gained possession, but
before higher mammalian competitors had arrived. Thus
saved and insulated, the Marsupials have evolved in many
different directions.
The brain is less developed than in Eutherian Mammals,
for the convolutions are simple or absent, the anterior com-
missure is large, the corpus callosum is practically absent.
In the skeleton there are several peculiarities: thus the
angle of the lower jaw is more or less inflected, except in
the genus Zursipes; the jugal reaches far back to share in
making the glenoid cavity ; there is practically only one set
of teeth; there are more incisors above than below (except
in the wombat), and the number of incisors sometimes
exceeds three on each side. There are usually epipubic or
marsupial bones in front of the pubic symphysis. These
have no connection with the marsupium, as is evident from
the fact that they occur in both sexes; they are sesamoids
developed in the inner tendon of the external oblique
muscle of the abdomen.
The teeth cannot be readily reduced to the typical Eutherian formula.
According to recent research, the milk set is degenerate, and is usually
represented only by the last premolar, which in most cases cuts the
gum, and is for a time functional. The other teeth correspond to
the permanent set of the Eutheria. According to another view, the
functional teeth are milk-teeth. In living Marsupials there seems to
be a suppression of what in typical placentals would be called the
second premolar.
A common sphincter muscle surrounds the anus and the
urogenital aperture, and in the majority of cases the
anus lies so much within the urogenital sinus that the
arrangement may be described as cloacal. The scrotal sac
containing the testes lies in front of the penis—a unique
position. The genital ducts of the females are often
separate throughout, so that there are two uteri and two
vaginze. But the bent proximal parts of the vagine some-
times fuse and form a caecum, which, according to the
degree of fusion, may be a single tube or divided by a
CLASSIFICATION OF MARSUFPIALS. 745
partition. Moreover, in Bennett’s kangaroo, the cecum
opens independently into the sinus between the apertures
of the distal portions of the vaginz, and forms the so-called
third vagina. In Ferameles, although such a median
passage does not exist in the young female, it is formed by
a process of rupture at the period of parturition. The true
vaginee are apparently too narrow for the passage of the
embryos.
The allantois in Perameles, as already seen, forms a true allantoic
placenta; in Phascolarctos it fuses with the subzonal membrane,
becomes highly vascular, and functions as an embryonic respiratory
organ, but does not unite with the uterine wall; in all other Mar-
supials, so far as is known, it is small, only projects slightly into the
extra-embryonic body cavity, and is apparently functionless. Accord-
Fic. 404.—Lower jaw of kangaroo.
a., Inflected angle ; 7., single incisox.
ing to Hill, the condition seen in Perameles is primitive, and the other
Marsupials show degeneration. The wall of the umbilical vesicle or
yolk-sac is highly vascular, and may unite with the uterine wall tc form
a yolk-sac placenta. :
The gestation is short, only lasting a fortnight in the
opossum, about five weeks in the kangaroo; whereas that
of the mare, for instance, is about eleven months. Except
in some opossums, there is a marsupial pouch, usually with a
forward-directed aperture. Within this pouch are the teats,
and here the delicate young are nurtured after birth. As
they are unable to suck, the milk is forced down their
throat, the mammary gland being compressed by the
cremaster muscle which covers it. Vague vestiges of a
marsupium are said to be visible in some Placentals.
Classification of Marsupials.—The Marsupials are divided
into two sub-orders, each of which contains four families. The two
746 MAMMALIA,
sets are defined by the characters of the teeth, which are, of course,
adapted to habit. In the members of the first sub-order the incisors
are numerous (not less than 4), small, and almost equal in size ; while
the canines are large, and the molars furnished with sharp cusps. The
whole dentition presents a striking resemblance to that of the Eutherian
Carnivores. To this group the name Polyprotodontia is applied, and
the forms included in it are typically carnivorous or insectivorous. The
ceecum is absent or very small.
In the remaining families the incisors are usually 3 in number, and
those above are of unequal size, the centre ones being largest. The
canines are usually small or absent; the molars are furnished with
blunt tubercles, or transverse ridges. To these typically herbivorous
forms the name Diprotodont is applied ; they are more highly special-
ised than the Polyprotodonts, and are more modern.
A, POLYPROTODONTIA
1, Family Didelphyidee.—American opossums, distributed from the
United States to Patagonia, arboreal in habit, usually carnivorous
or insectivorous in diet. The limbs have five clawed digits ;
the hallux is opposable. The tail is generally long, and often
prehensile. The stomach is simple; the cecum small. The
pouch is generally absent, but the young are often carried on
the back of the mother, their tails coiled round hers. Dentition,
5134
eG
Examples: The Virginian or crab-eating opossum (Didel¢hys
marsupialis), with a pouch ; the woolly opossum (D. /azigera) ;
the aquatic Yapock (Chzronectes), which feeds on fish and
smaller water animals.
2. Family Dasyuridee.—Carnivorous or insectivorous Marsupials. The
limbs have clawed digits, five in front, four or five behind. The
canines are generally large. The stomach is simple; there is
no czecum.
Examples.—The Tasmanian wolf (Zhy/acinus), of dog-like form,
dentition a and the Dasyure (Dasyzrus), civet-like, den-
tition Pl are specialised as carnivores. The members of the
genus Phascogale are small and insectivorous. The banded
ant-eater (AZyrmecobius) of W. and S. Australia, 2 somewhat
squirrel-like animal, has a long thread-like protrusible tongue,
and more teeth than any other Marsupial, oe It differs
markedly from the other members of the family.
3. Family Notoryctida.—This family has been erected for the mole-
like Marsupial (Motoryctes typhlops), found in the sandy deserts
of S. Australia. It lives underground, is a rapid burrower, and
in its rudimentary eyes, keeled sternum, and some other respects,
DIPROTODONTIA. 747
markedly resembles the Cape golden mole. It is thus a good
illustration of ‘‘ convergence,” ze, the appearance of similar
characters in forms not nearly related, apparently in indirect
response to similar conditions of life.
4. Family Peramelide.—The burrowing bandicoots, all small in size,
insectivorous or omnivorous in diet. In the fore-feet two or
three of the middle toes are well developed and clawed, the
others being rudimentary ; in the hind-feet the hallux is small or
absent, the second and third toes are very slender and united
in the same fold of skin, the fourth toe is very large, the fifth
smaller,—the whole foot suggesting that of the kangaroo. The
‘stomach is simple; the czecum not large. Clavicles are absent
in the adult but present in the foetus. Dentition, ae
Examples.—The true bandicoot (Perameles), remarkable for its
allantoic placenta; the native rabbit (Peragale lagotis); the
rat-like Cheropus.
B. DIPROTODONTIA
1. Family Epanorthide.—The selvas, a family of S. American forms,
till recently believed to be entirely extinct. The existing forms
are included in the genus Camodestes, with two species, They
are remarkable in having the upper jaw of the polyprotodont
type, and the lower distinctly diprotodont ; and also in having
all the digits of the hind-foot free, whereas in all other living
Diprotodonts certain of these are united by skin (syndactylous).
They are probably primitive forms, and their presence in S.
America is highly important. There seems little doubt that
the Diprotodonts have been evolved in the Australian area from
a primitive widely-spread polyprotodont stock. If, therefore, the
Epanorthidee are really allied to the Diprotodomits, their exist-
ence in S. America seems to indicate a former connection
between that continent and Australia.
2. Family Phascolomyide.—The wombats, terrestrial, vegetarian,
nocturnal Marsupials, somewhat bear-like in appearance. The
dentition is rodent-like, we the teeth have persistent pulps, the
incisors are chisel-edged, there being no enamel except in front.
In the embryo, however, there are four upper incisors, of which
the first persists, and five lower incisors, of which the third
persists. The fore-feet have five distinct toes, with strong nails ;
the hind-feet have a small nailless hallux, the second, third, and
fourth toes partly united by skin, the fifth distinct. The tail is
very short. The stomach is simple ; the czecum very short.
There is but one living genus—Phascolomys, with three
species.
3. Family Phalangeridee.—Small woolly arboreal nocturnal Marsupials,
with vegetarian or mixed diet. The fore-feet have five distinct
toes ; the hind-feet have a large, nailless, opposable hallux, the
748 MAMMALIA.
second and third toes are narrow and bound together by skin,
the fourth and fifth free. The tail is generally long and pre-
hensile. The stomach is simple, the caecum usually large.
Average dental formula, 23: 34,
Ty 0, 0-2, 3-4
Examples.—The grey Cuscus (Phalanger orientalis); Tarsipes, a
small mouse-like animal which feeds on honey, and is remark-
able in having no inflection of the angle of the mandible and
no cecum ; the flying phalangers (Pefaurus), with a parachute
of skin extending from the little finger to the ankle ; the Koala,
or ‘‘native bear” (Phascolarctos cin-
ereus), a relatively large form, about
2 ft. in length. An extinct form,
Thylacoleo, of the late Tertiary period
of Australia, is interesting in its extra-
ordinary dentition, the functional teeth
being reduced to large front incisors and
the third premolars, both adapted for
sharp cutting.
4. Family Macropodidee. — Kangaroos, herbi-
vorous terrestrial Marsupials. Denti-
tion, 222274,
7 1, 0, 2 4
and suited for cropping herbage. The
hind-legs are usually larger than the
fore-legs, and the animals move by leaps.
Examples. — The true kangaroos, ¢.g.
Macrogus ; the rat-kangaroos or potoroos
(Potorous) ; the genus Hypsiprymnodon,
with a foot approaching that of the
Phalangers.
The incisors are sharp,
Fic. 405.—Foot of The true kangaroos, belonging to the genus
young kangaroo. = Afacropus, include the largest living Marsupials ;
2, 3, Small syndactylous but within the genus there is much difference in
toes; 4, large fourth sjze,
toe; 5, fifth toe. The grey kangaroo (JZ. giganteus) lives on
the grassy plains of Eastern Australia and
Tasmania, and is as tall as a man ; the Wallabies, at home in the bush,
are smaller, and some are no bigger than rabbits,
The hind-limbs seem disproportionately long, and are well suited for
rapid bounding, The long tail, carried horizontally, helps to balance
the stooping body as the animal leaps, and it gives additional stability
to the erect pose. The fore-limbs sometimes come to the ground when
the animal is feeding, and in the largest species they are strong enough
to throttle a man.
The fore-limbs bear five clawed digits; the hind-feet have only four.
The hallux is absent; the fourth toe is very long; the fifth is about
half as large ; the third and second are too slender to be useful for
more than scratching, and are bound together by the skin (syndactylous).
The length of the hind-limb is due to the tibia and fibula, and to the
EDENTATES. 749
foot. The clavicles and fore-arm are well developed. The epipubic or
marsupial bones are large.
The kangaroos feed on herbage, and are often hunted down on
account of the damage which they do to pastures and crops. The
sharp incisors are suited for cropping the grass and herbs, which the
ridged and tuberculated molars crush.
As the kangaroos are exclusively herbivorous, it is not surprising to
. , find that the stomach is large and complex, with numerous saccules on
its walls. The whole gut is long, and there is a well-developed coecum.
Numerous fossil forms related to the kangaroos are found in Australia,
some considerably larger than the existing forms. The gigantic Dzpro-
todon austral’s, which was as large as a rhinoceros, is related both to
the kangaroos and the phalangers. Except the S. American forms
already mentioned, Diprotodont marsupials are unknown, either living
or fossil, outside the Australian area. Forms related to the Polypro-
todonts are, on the other hand, common as fossils in both Europe and
America. In S. America, further, fossil marsupials related to the
Dasyuridze occur ; and as these are not known elsewhere, their presence
affords a further confirmation of the view that Australia and Patagonia
were once connected.
Sub-Class EUTHERIA
EDENTATES
The Edentates include a number of very distinct types,
which require at least two orders—(a) the New World sloths,
ant-eaters, and armadillos ; (4) the Old World pangolins and
aard-varks. The modern forms are specialised survivors of
waning and probably primitive stocks, and they show many
interesting protective peculiarities of structure and habit
which secure their persistence. Thus some are arboreal,
others are burrowers, and many are covered with strong
armature.
While the existing sloths, ant-eaters, and armadillos are not nearly
related to one another, the numerous fossil Edentates found in S.
America connect them to a common stock. It is otherwise, however,
with the pangolins and the aard-varks. Some authorities separate
them (as Nomarthra or Effodientia) from the American Edentates
(Xenarthra) ; but according to others there is little evidence that the
pangolins and aard-varks are related to each other. In view of the
uncertainty, it will be readily understood that few ‘‘ general characters”
of Edentates can be given. Almost the only common characters of
Edentates concern the dentition. Functional teeth may be absent,
but the ant-eaters (Myrmecophagidee) are the only forms which still
appear strictly edentulous. When present, the teeth are uniform,
usually simple, without roots, and with persistent pulp. They are
never present in the very front of the mouth, and they have not
750 MAMMALIA.
more than hints of enamel. Till recently the dentition was de-
scribed as monophyodont, but there is evidence of two sets in 7a¢usia,
Orycteropus, Dasypus, and others. It is the milk set which dis-
appears.
A common frimitive character is the persistence of the testes in the
abdominal cavity.
The placenta shows much diversity, but the reproductive phenomena
are imperfectly known. In the sloths and ant-eaters the placenta is,
usually described as dome-shaped ; but according to some authorities
this is merely a stage in the growth of a placenta, which is at first poly-
cotyledonary, and later discoidal. The discoidal deciduate type appears
again in the armadillos, but in Dasyfus among them it is said to be
zonary. In the pangolins it is diffuse and indeciduate ; in Orycteropus,
apparently by a suppression of the polar villi of a diffuse type, it is
zonary, and doubtfully deciduate.
Order XENARTHRA
1. Bradypodidaee—Sloths.—The three-toed sloths (Bradypus) and the
two-toed sloths (Cholwpzs) are restricted to the forests of S.
and Central America, They are the most arboreal of mammals,
passing their whole life among the branches, to which they
bang, and along which they move back downwards. They are
solitary, nocturnal, vegetarian animals, sluggish, as their name
suggests, and with a very firm grip of life. Their shaggy hides
harmonise with the mosses and lichens on the branches, and
the protective resemblance is increased by the presence of a
green alga on the hair. Their food consists of leaves and
shoots and fruits.
The body is covered with coarse shaggy hair; the head is rounded,
and bears very small external ears; the fore-limbs are longer than the
hind-limbs, and the two or three digits are bound together by skin, and
have long claws; the tail is rudimentary.
Concerning the skeleton we may note the rootless, unenamelled,
peg-like teeth, the incomplete zygomatic arch with a descending process
from the jugal, the presence of clavicles, the rod-like appearance of the
embryonic stapes, the occurrence of nine cervical vertebra in Bradypus,
of six in Cholepus (but see p. 694). The adult Bradypus has some-
times a separate coracoid or epicoracoid.
As in most herbivorous animals, the stomach is complex, but there is
no ceecum. In the limbs the main blood vessels break up into numerous
parallel branches. The uterus is simple; the vagina seems to be
originally divided by a median partition; the placenta is deciduate,
and changes in shape during development. One young one is born at
a time.
2. Megatheriidee or Ground Sloths—extinct forms of large size,
intermediate between the sloths and the ant-eaters. Their
remains are found in Pleistocene deposits in N. and S. America.
Megathertum exceeded the rhinoceros in size. Near the Mega-
theriidze the recently exterminated or still living Meomylodon
may be included.
NOMARTHRA. 751
3. Myrmecophagide—the Ant-eaters, hairy animals, without even
traces of teeth, with long thread-like protrusible tongue, viscid
with the secretion of greatly enlarged submaxillary glands.
One form, Alyrmecophaga jubata, is terrestrial ; the others,
belonging to the genera Zamandua and Cycloturus, are
arboreal. All feed on insects. All are Neotropical. The
skull is long ; the third finger is greatly developed, the others
are small; the pes has four or five almost equal clawed toes ;
the clavicles are rudimentary ; the tail is long and sometimes
prehensile. The brain is well convoluted. The uterus is
simple: the placenta is dome-like or discoidal.
4. Dasypodidee—the Armadillos, all S. American except 7atusia
novemcincta, which extends as far north as Texas. They are
nocturnal, omnivorous animals, able to run and burrow rapidly.
They are unique among living Mammals in having a dermal
armature of bony scutes united into shields and rings, and
covered by horny epidermis. The teeth are numerous, simple,
and of persistent growth. Clavicles are well developed. The
digits have strong claws or nails. The brain has large olfactory
lobes; the cerebral hemispheres have few convolutions.
The tongue is long and protrusible, and the submaxillary
glands are large. The stomach is simple. The uterus is
simple; the placenta is discoidal and deciduate, except in
Dasypus.
Examples.—Dasypus, Chlamydophorus, Tatusia.
5. Glyptodontidae—extinct Pleistocene types, mostly S. American,
but represented in Mexico and Texas. The body was often
huge, and was covered by a solid carapace of great strength.
Order NOMARTHRA
1. Manide—the Ethiopian and Oriental Pangolins, covered dorsally
with overlapping horny scales, They are terrestrial, burrow-
ing animals, but sometimes climb trees. They usually feed
on termites. Teeth are rudimentary, the tongue is long
and protrusible. The uterus is bicornuate; the placenta
diffuse and indeciduate. There is one extant genus,
Manis.
2. Orycteropodidee—the Ethiopian Aard-varks, represented by two.
species of Orycteropus, ranging from S. Africa to Egypt. They
are shy, nocturnal animals, living in burrows, feeding on
termites. There are numerous complex teeth, differing in
structure from those of any other known Mammal. The
skin bears scanty bristles.) The mouth is tubular, and the
tongue is narrow and protrusible. The digits bear nails
suited for digging. The uterus is bicornuate, the placenta
broadly zonary. The relation to the other Edentates, or,
indeed, to other Mammals, is uncertain,
752 MAMMALIA,
Order SirENIA. Sea-Cows
A small decadent order of sluggish, aquatic, vegetarian
Mammals, in no direct way connected with Cetaceans, to
which they have some superficial resemblance (convergence).
There are two living genera, — Halcore (Dugong) and
Manatus (Manatee), and one was recently exterminated
(Rhytina).
The Sirenia are sluggish, with massive heavy bones, a
plump body, some oil, and sparse hair on the thick tough
skin. In adaptation to aquatic life, they have a fish-like
form, a powerful tail with a “ caudal fin,” no external trace
of hind-limbs, flipper-like fore-limbs, no external ear, valved
nostrils at the end of the snout, networks (retia mirabilia)
in the arteries (useful in prolonged immersion). They are
herbivorous, feeding on alge and estuarine plants; and,
like others of similar habit, have a chambered stomach, a
long intestine, and a caecum.
They are primitive, and with this fact may be associated
the abdominal testes, the absence of distinct epiphyses on
the vertebree (cf. Prototheria), and the small brain with few
convolutions.
The paddle-shaped fore-limbs have, at most, rudimentary
nails ; the digits have never more than three phalanges, and
the elbow and wrist joints are distinctly movable, whereas
in the Cetacea the fore-limbs are more or less stiff from the
shoulder. There are no clavicles. The skull is not like
that of Cetaceans. The nasals are, at most, rudimentary.
There are no canine teeth. There are chevron bones below
the tail. There are no hind-limbs. The pelvis is rudi-
mentary, and there is no sacrum. In the extinct Au/i-
thertum there was a vestigial femur.
The small eyes have imperfect eyelids, but have a nicti-
tating membrane. In the mouth there are horny crushing
plates. The ventricles are separated by a cleft. The
uterus is bicornuate. Two teats lie behind the armpits.
The placenta of the dugong is zonary, wholly or in great
part non-deciduate. The placenta of the manatee has not
yet been investigated.
UNGULATA.
753
MANATEE (Manatus).
Ducone (Halicore).
Neck vertebrze reduced to six.
Abortive incisors (¥) in both sexes.
Molars (}}) six or so at a time,
uniform, with square enamelled
crowns, and tuberculated trans-
verse grinding ridges. ;
Premaxillze almost straight.
Tail rounded.
Rudimentary nails on fingers.
Czecum divided. :
M, australis and M. senegalensis
live in the mouths of great rivers
which flow into the’ tropical
Atlantic.
The usual seven neck vertebrae,
Two tusk-lke incisors persist in
the inale. ;
Molars (# or $, 2 or 3 at a time),
primitive, with persistent pulps
and no enamel.
Premaxillze crooked downwards.
Deeply notched tail. ~
Nailless digits.
Thick and single caecum.
H, tabernaculi, E. African coast
and Red Sea ; #. dugong, Indian
and Pacific Oceans, eastward
from the home of the last species
to the Philippines; A. australis,
E. and N. Australia,
The genus RAytina was toothless, with a slightly crooked snout,
small head and arms, and thick naked skin.. Steller’s sea-cow (7.
stelleri)—the only known species, from the North Pacific—seems to
have been exterminated about 1768. The tertiary Halétherdum had
traces of hind-limbs,
Order UNGULATA
Hoofed Animals— Artiodactyla, Perissodactyla, Hyra-
coidea, Proboscidea, and extinct sub-orders.
This large and somewhat heterogeneous order in-
cludes pigs, hippopotamus, camels, cattle, deer, tapirs,
rhinoceros, horses, hyrax, elephants, and some other distinct
types.
They are terrestrial, and for the most part herbivorous
animals. Their digits generally end in hoofs or at least in
broad flat nails. In the adults of the modern types there
are no clavicles. The teeth are diverse, the milk set in part
persistent until the animal attains maturity.
In the typical Ungulates the feet are never plantigrade.
In modern types there are never more than four functional
toes. The os magnum of the carpus articulates freely with
the scaphoid. The brain is well convoluted. The testes
descend into a scrotum. The uterus is bicornuate. The
48
754
MAMMALIA.
placenta is (a) indeciduate, and diffuse or cotyledonary ;
or (4) deciduate and zonary.
Ungulata Vera: ARTIODACTYLA and PERISSODACTYLA
ARTIODACTYLA—PIGS, CAMELS,
CHEVROTAINS, AND RUMINANTS.
PERISSODACTYLA— TAPIRS,
RHINOCEROS, Horsxs.
The third and fourth digits of each
foot are equally developed, and
the line: halving the foot runs
between them.
The premolars and molars are
usually different, but generally
bunodont or selenodont.
‘There are nineteen dorso-lumbar
vertebree.
The femur has no third trochanter.
‘The astragalus has always equal
articular facets for the navicular
and for the cuboid.. The cal-
caneum has an articular facet for
the fibula, if that bone is fully
developed.
The stomach tends to be complex,
and the caecum is small.
The mamme are few and inguinal,
or numerous and abdominal.
The placenta is diffuse or cotyle-
donary.
There are often bony outgrowths
from the frontals.
There is no alisphenoid canal.
The third digit occupies the middle
of the foot, is largest, and is
symmetrical on itself, so that the
line halving the foot bisects the
third digit.
The premolars
molars,
resemble the
There are almost always twenty-
three dorso-lumbar vertebrae.
The femur has a third trochanter.
The astragalus has a large facet
for the navicular, a small facet
for the cuboid. The calcaneum
.does not articulate with the
lower end of the fibula (except
Macrauchenia).
The stomach is always simple, and
the caecum is large.
The mammez are always inguinal.
The placenta is always diffuse.
There are never bony outgrowths
from the frontals,
There is an alisphenoid canal
transmitting the external carotid
artery.
Sub-Order ArtiopactyLa. Even-toed Ungulates
_ Pigs and Hippopotamus (Suina), Camels (Tylopoda),
Chevrotains (Tragulina), and Ruminants (Pecora) like
Cattle and Deer.
The general characters of this sub-order have been stated
above in contrast to those of Perissodactyla. The equal
development of the third and fourth digits, the fact that the
premolars have a single lobe while the molars have two, the
ARTIODACTYLA, 755
nature of the tarsal bones, the tendency that ‘the stomach
has to be complex (as in Camels and Ruminants), are im-
portant characteristics. There are others of less obvious
importance, such as the absence of the alisphenoid canal,
which in Perissodactyla encloses the external carotid artery
as it passes along the alisphenoid.
There are primitive extinct Artiodactyla which connect
the four modern groups—Suina, Tylopoda, Tragulina,
and Pecora. Thus they unite the bunodont types, such
as pigs, with cone-like tubercles on the crowns of the
molars, and the selenodont types, such as cattle, with the
tubercles expanded from before backwards, and curved in
crescents.
Group 1. Suina—hippopotamus, pigs, and peccaries. The molars are
bunodont ; the third and fourth metacarpals and metatarsals are
not completely fused as ‘‘ cannon bones.”
Hippopotamide.—Huge African mammals, included in the single
‘ genus Hippopotamus. They spend the day in the rivers and
lakes, swimming and diving well, but usually remaining concealed.
At night they come on land and browse on grass and herbage.
The skin is extremely thick, with a few hairs restricted to the
snout, head, neck, and tail. There are four toes on each foot, all
reaching the ground. The rootless incisors continue growing ;
so do the large curved canines; the dental formula is =3+*43
4 1-3, 143°
The stomach ‘has three,chambers ; there is no ceecum.
Suidee.—The Old World boars and pigs, ‘characterised by the mobile
snout and terminal nostrils. There are four well-developed
digits on the narrow feet, but the second and fifth do not reach
the ground in walking. The incisors are rooted; the upper
canine curves outwards or upwards. The stomach is almost
simple, but has more or less of a cardiac pouch and several
short blind saccules ; there is a cecum.
Examples.—Sus, eee Babirusa, Som the male with remarkable
canines, the upper pair growing upwards from their base
through the skin, arching backwards as far as the forehead,
and sometimes forwards and downwards again, the lower pair
with a more or less parallel course; Phacocherus, the wart-hog.
Dicotylide.—The New World peccaries (Décoty/es), with a snout
like that of pigs, with four toes on the fore-feet, and three behind.
The incisors are rooted, the upper canines are directed down-
wards, the dental formula is 83, The stomach is complex, and
3133 Plex,
there is a caecum.
Group 2.—Tylopoda, comprising the family Camelidze—the camels
of the Old’ World and the llamas of S. America. The limbs
756 MAMMALIA.
are long, with only the third and fourth digits developed; the
two metacarpals and metatarsals are united for the greater part
of their length, but there is a deep distal cleft; the tips of the
digits have very incomplete hoofs, and the animals walk on a
broad pad of skin surrounding the middle phalanges. The
femur is long and vertical, and the knee is low down. Of the
three upper incisors only one persists in adult life, as an isolated
sharp tooth, those of the lower jaw are long and slope forwards.
There are canines both above and below. The molars are
gy
Fic. 406.—Side view of sheep’s skull.
PMX., Premaxilla; JZX., maxilla; VA., nasal; /., Jugal; Z., lachrymal;
FR., frontal; PA., parietal; SQ., squamosal; CO., condyle; PP.,
paroccipital process.
Camelidz are unique among Mammals in having oval instead of
circular red blood corpuscles. The placenta is diffuse.
Examples.—Camelus, 8, the Arabian camel (C. dromedarius)
has a dorsal hump of fat, the Bactrian camel (C. dactrianus)
has two humps. The camel has a very small area of visible
perspiration on the back of the neck, and seems to have a
somewhat variable body-temperature, two associated facts which
may be adapted to conserving the animal’s water-supply in arid
countries. The genus Azchenia, fae includes the llama,
alpaca, huanaco, and vicugna of S. America, smaller forms
than the camels, and without humps.
Group 3.—Tragulina or Chevrotains, small animals, ‘‘ intermediate
in their structure between the deer, the camels, and the pigs.”
There are four complete toes on each foot, but the second and
fifth are slender; the third and fourth metacarpals and meta-
ARTIODACTYLA. 757
tarsals are fused in Zragulus, free in the other genus Dor-
catherium ; the fibula is complete. There are no upper incisors,
the upper canines are long and pointed, especially in the males ;
the lower canines are like incisors; the dental formula is
pee The Chevrotains ruminate, and the stomach is divided
into three chambers, the many-plies being .rudimentary. The
placenta is diffuse. The Chevrotains are often confusedly
associated with the musk-deer (AZoschus), with which they have
no special affinities.
Species of Zragu/us (smallest among living Ungulates) occur in
Indo-Malaya, India, and Ceylon; one species of Dorcatherdum,
of aquatic pig-like habits, is found on the west coast of Africa.
Group 4.— Pecora or Cctylophora — the true ruminants, including
deer, giraffes, cattle, and sheep. Only the third and fourth
Fic. 407.—Stomach of sheep.—From Leunis.
a, Esophagus ; c, rumen or paunch; d, reticulum or honeycomb-
bag ; ¢, psalterium or many-plies; 4, abomasum or reed ; 4,
beginning of duodenum.
digits are complete, the fused third and fourth metacarpals and
metatarsals form ‘‘cannon bones.” In the embryos of ox and
sheep, the second and fifth metacarpals and metatarsals are also
represented; the second metacarpal and fifth metatarsal are
unstable and soon disappear; small traces of the fifth metacarpal
and second metatarsal persist. The fibula is represented by 2
small nodular bone articulating with the lower end of the tibia,
and forming the external malleolus. There may be in addition
a rudiment of the proximal end attached to the upper part of
the tibia, but the two parts are never united. Paired outgrowths
of the frontal bones are common, capped with horny sheaths in
the Bovide, deciduous and restricted to the males in almost
all Cervide. There are no upper incisors, and rarely upper
canines; there are three pairs of lower incisors, which bite
against the hardened gum above ; and the lower canine resembles
and is in the same series as the incisors; the typical dent’tion is
= The stomach has four distinct compartments. The placenta
758 MAMMALIA.
is cotyledonary, the villi occurring on a number of distinct
patches. .
The process of rumination or chewing the cud cannot be understood
without considering the complex stomach. It is divided into four
chambers,—the paunch or rumen, the honeycomb-bag or reticulum, the
many-plies or psalterium, the reed or abomasum. The swallowed food
passes into the capacious paunch, the walls of which are beset with
close-set villi resembling velvet pile. After the food has been softened
in the paunch, it is regurgitated into the mouth, where it is chewed
over again and mixed with more saliva. Swallowed a second time,
the food passes not into the paunch, but along a muscular groove on
the upper wall of the globular honeycomb-bag into the third chamber
or many-plies. The honeycomb-bag owes its name to the hexagonal
pattern formed by the mucous membrane on its walls. The many-
plies or psalterium is a filter, its lining membrane being raised into
numerous leaf-like folds covered with papille. Along these the food
passes to the reed, which secretes the gastric juice. The first three
chambers are strictly oesophageal, not stomachic.
Cervidee—the widely distributed deer, absent only from the Ethiopian
and Australian regions. The second and fifth digits are usually
represented, often along with the distal parts of the correspond-
ing metacarpals and metatarsals. The upper canines are usually
present in both sexes. The horns, if present, are antlers, de-
ciduous, and usually confined to the males. .In the reindeer,
they are possessed. by both sexes. They are outgrowths of the
frontal bones, are covered during growth by vascular skin—the
velvet—and attain each year to a certain limit of growth. After
the breeding season the blood supply ceases, the velvet dies off,
and an annular absorption occurs near the base. Then the
antlers are shed, leaving a stump, from which a fresh but larger
growth takes place in the next year. The earliest (Lower
Miocene) deer had no antlers, thus resembling young stags of
the first year ; the Middle Miocene deer had simple antlers, with
not more than two branches, thus resembling two-year-old stags.
Thus there is a parallelism between the history of the race and
the individual development.
Examples.—Cervas, most Old World deer; Rangéfer, the rein-
deer; ‘Adces, the elk or moose; Capreolus, the roe-deer ;
Hydropotes, the water-deer, without antlers; A/oschus, the
musk-deer, without antlers, with long sharp upper canines
and large musk glands in the males.
Giraffidee, represented by the giraffe (Grraffa camelopardalis), a tall
Ethiopian animal, notable for its enormously elongated cervical
vertebra, and for its long limbs. It is gregarious in its habits,
and feeds on the leaves of trees. The lateral digits are entirely
absent. The dental formula is ae In both sexes there are
on the forehead short erect prominences, over the union of
parietals and frontals, which arise from two distinct centres of
ossification, but afterwards fuse with the skull. In front of
these there is a median protuberance. The Okapi (Ofafia),
| PERISSODACTYLA 759
from a West African forest, has a shorter neck, and the horns
are on the frontals. It links the giraffe to. the extinct
Palaotragus, —
Antilocapridee, represented solely by the prongbuck (Avédlocapra
americana), a North American animal, with most of the char-
acteristics of Bovide. The horny sheath bears one branch, and
is periodically detached from the bony core.
Bovidze, the hollow-horned Ruminants, widely distributed throughout
the world, but without indigenous representatives in Australia,
South or Central America. The second and fifth digits may be
completely absent, but are often represented by minute hoofs and
supporting nodules of bone. The frontal appendages, if present,
consist of a solid bony core growing from the frontal, and a much
longer sheath of horn, which grows at the base as it is worn away
at the tip. They are not deciduous, and are usually present in
both sexes, though larger in'the males.
Examples.—Antelope, Gazella, Capra, Ovis, Bos.
Sub-Order PERISSODACTYLA
Horses, Tapirs, Rhinoceros, and their extinct allies.
The middle or third digit of fore- and hind- feet is larger
than the others, and symmetrical on itself. It may be the
only complete digit, as in the horse, or it may be accom-
panied by the second and the fourth, and in the fore-foot of
tapirs and some extinct forms by the fifth, digit. No modern
forms have any trace of the first digit. The astragalus
has a pulley-like surface above for articulation with the
tibia; its distal surface is flattened and unites to a much
greater exterit with the navicular than with the cuboid.
The last-named bone is of less importance than in the
Artiodactyla. The calcaneum does not articulate with
the lower or distal extremity of the fibula. The femur
has a third trochanter or process for the insertion of
muscles. There are usually twenty-three dorso-lumbar
vertebrae. .
As to the dentition, the premolars and molars form a
continuous series, with broad transversely ridged crowns,
the last premolars often very like the molars.
The stomach is simple; the czecum is large; there is no
gall-bladder.
The mamme are inguinal; the placenta is diffuse and
non-deciduate.
760
MAMMALIA.
Fic. 408, —Side view of lower
part of pony’s fore-leg.
h., Distal end of humerus; z.,
olecranon process of ulna; ~,
radius ; sc., scaphoid ; 2, lunar ;
c., cuneiform ; 7., os magnum ;
un., unciform; £., pisiform3
mic.4, splint of fourth metacar-
pal; 7c.3, third metacarpal ;
s., sesamoid ; 3, 2, 3, phalanges
of third digit.
cub,
Fic. 409.—Side view of ankle
and foot of horse.
a., Astragalus; ¢., calcaneum 3
m, navicular; e¢.c., external
cuneiform ; czé., cuboid ; 7.3,
third metatarsal ; 22.4, splint
of fourth metatarsal ; s., sesa-
moid; A. 1-3, phalanges of
third digit.
PERISSODACTYLA. 761
Families of Perissodactyla
- Family Tapiridee.—In the Tapirs (Zagzrus) there are four digits in
the manus, but the third finger is still practically median, as the
fifth digit scarcely reaches the ground. The hind-foot has three
digits. The dentition of the genus is ee The orbit and
temporal fossa are continuous. The nose and upper lip form a
short proboscis. The thick skin has but scanty hair. In habit,
the tapirs are shy and nocturnal, fond of forests and water,
feeding on tender shoots and leaves. The distribution is some-
what remarkable, for four species live in Central and South
America, while a fifth is Malayan. The genus- was once
widespread, but has survived in these two far-separated regions.
Fic. 410.—Side view of horse's skull.
P., Parietal; FR., frontal; WA., nasal; PALX., premaxilla; JX., maxilla;
J., jugal; Z., lachrymal; SQ., squamosal; PP., paroccipitai process ;
CO., condyle; CA., canine.
Family Equide.—In the modern horses (Zgzus) there is on each .
foot one functional digit—the third, with splints representing the
metacarpals and ‘metatarsals of the second and fourth. Professor
Cossar Ewart has demonstrated in the embryo of the horse the
rudiments of the three phalanges of the second and fourth digits.
The vestigial phalanges of these digits subsequently fuse with
one another and with the respective metacarpals or metatarsals,
forming ‘‘‘ buttons” at the end of the splints. The ulna and
fibula are incomplete, but the former is quite complete in the
foetus. The dentition is oe but the first premolar is rudi-
mentary, and soon lost in both sexes, and the canines are rarely
present in the mare. The orbit is complete.
The modern horses are connected by a very complete series of forms
with ancestral Eocene types, but it is not clearly proved that these
forms were actually in the line of descent of the genus Zguus. The
progress shows an increase of size, a diminution in the number of
762 MAMMALIA.
digits, an increased folding of the back teeth, and other differentiations,
The Eocene Phenacodus is regarded by some as near the origin of the
stock, it had five complete digits on each foot; Ayracotherium and
Systemodon had only four functional digits in the manus ; Amchithertam
from the Miocene, an animal about the size of a sheep, had three digits,
or three and a rudiment; Azppotherium and Protohippus from the
Pliocene were as large as donkeys, and show a marked diminution of
the second and fourth digits; in the Pliocene also, the modern forms
appeared.
The living species are the horses (Agus caballus), apparently
originating in Asia, domesticated in prehistoric times, artificially selected
. 411.—Feet of horse and its predecessors. —
From Neumayr. :
x, Palzotherium ; 2, Anchitherium ; 3, Hippotherium ; 4, Equus.
into many breeds, sometimes reverting to wildness, as in the case of
those imported into America and Australia by European settlers ; the
wild horse of Central Asia (2. prsevalskez) ; the donkey (Z. aszuus) of
African origin ; the wild asses of Africa and Asia; the striped African
species—the zebras and the (exterminated) quagga.
Family Rhinocerotide.—There is now but one genus, RAznoceres,
species of which occur in Africa and in some parts of India and
Indo-Malaya. They are large, heavy Ungulates, shy and noc-
turnal, fond of wallowing in water or mud, feeding on herbage,
shoots, and leaves. The skin is very thick, with scanty hair.
One or two median horns grow xs huge warts from the snout
and forehead. The dentition is very variable, but the back
teeth re are almost uniform; there are no upper canines, but
y
HYVRACOIDEA. 763
sometimes a large lower pair; there are a few incisors, but these
are often small and deciduous.
There are several entirely extinct families of Perissodactyla, such as—
Lophiodontidee (Eocene), e. eg. Lophiodon, Hyracotherium, Sys-
temodon,—a family perhaps ancestral to most of the modern
Perissodactyla.
Palotheriidze (Eocene to Miocene), eg. Paleotherium and
Anchitherium. :
Other remarkable types—Lamébdotherium, Chalicotherium, Titano-
therium, of elephantine size, and the specialised Afacrauchenia
—are referred to distinct families.
Sub-Order HyracorDEa
An isolated order of small Rodent-like Ungulates, repre-
sented by Ayrax (Procavia) and Dendrohyrax, living in
rocky regions and on trees in Africa and Syria. The
species (14) are adept climbers.
The upper incisors have persistent pulps, and are curved
as in Rodents, but they are sharply pointed, not chisel-
edged. The outer lower incisors are straight, and have
trilobed crowns. There are no canines in the second set,
but the upper milk canine sometimes persists; and there
is a wide space between incisors and premolars. The back
teeth are very uniform, and like those of Perissodactyla.
The milk dentition is 34, the permanent is 5. Ayrax is
one of the few Mammals in which the first premolar is a
replacing tooth. The jugal forms part of the glenoid cavity
(cf. Marsupials).
In the fore-feet the thumb is rudimentary, the ‘little
finger is smaller than the median three, which are almost
equal. In the hind-feet, which are like miniatures of
those of the rhinoceros, the hallux is absent, ‘and the
fifth toe is rudimentary. There are no hoofs in the
strict sense. There are no clavicles. The tail is very
short.
The brain is like that of Ungulates. The stomach is
divided into two parts by a constriction. In addition to
the short but broad’ cecum, there are two supplemental
ceca lower down on the. intestine. The testes are
abdominal. Of the mamme, four are on the groin and
two are axillary. The placenta is zonary, as in the Pro-
boscidea and Carnivora. A few extinct forms are known.
764 MAMMALIA.
Sub-Order PROBOSCIDEA
The sub-order is now represented by two species of
elephant (Z/ephas). They occupy a somewhat isolated
position, though distinctly Ungulates. As regards skull,
proboscis, and teeth, they are highly specialised, but their
limbs are of a generalised type.
The elephants are confined to the Ethiopian and Oriental
regions. They feed on leaves, young branches, and herbage,
By means of the mobile proboscis they gather their food,
and they drink by filling the proboscis and then ejecting the
water into the mouth.
The proboscis is a muscular extension of the nose, and
bears the nostrils at its tip. The skin is strong, and the
hair somewhat scanty.
In the limbs, radius and ulna, tibia and fibula, are
quite distinct ; the radius and ulna are fixed in a crossed
position; owing to the length of the humerus, and yet
more of the femur, and the vertical position in which
they are carried, elbow and knee are lower than usual,
and the gait is peculiar; the carpal and tarsal bones have
flat surfaces; the feet are broad, and bear five hoofed
toes embedded in a common integument. There are no
clavicles.
The skull is very large, being adapted to support the
proboscis and tusks, and to afford a broad insertion for
the large muscles. In most of the bones there is during
growth an extraordinary development of air-spaces, which
communicate with the nasal passages. The supra-occipital
is very large ; the nasal bones are very short ; the zygomatic
arch is slender and straight, its anterior part is formed by
the maxilla, for the elephant differs from the typical
Ungulates in the fact that the jugal merely forms the
median part of the zygoma, and does not extend on to the
face. The lachrymal is also small, and placed almost
entirely within the orbit (cf. the Rabbit).
The dentition is unique. The two upper incisors or
tusks are mainly composed of solid ivory; the enamel is
restricted to the apex, and soon wears off. As the tusks
grow, their roots sink through the premaxillaz into the
maxilla, There are mo canines nor premolars. The
SEVERAL EXTINCT SUB-ORDERS. 765
molars are very large, and the enamel is very much plaited,
forming a series of transverse ridges enclosing the dentine,
and united to one another by cement. Thus on the worn
tooth there are numerous successive layers of enamel,
dentine, and cement. Extinct forms show transitions
between this complex type and the hoise’s tooth. In a
lifetime there may be six molar teeth on each side of each
jaw, but of these only one, or portions of two, can find
Space at a time. The series gradually moves forward as
the front parts are worn away and cast out.
The brain is highly developed.
The stomach is simple, and there is a large caecum,
There are two superior venz cave entering the right
auricle. The kidneys have several lobes, separated by
muscular partitions.
The testes remain abdominal in position.
There are two pectoral mammee ; the uterus is bicornuate ;
the placenta is non-deciduate and zonary.
Elephas, es now represented by the Indian Elephant (£. zzdzcus),
with parallel folds of enamel on the molars, and ears of moderate size,
and the African Elephant (2. afrdcanus), with lozenge-shaped folds of
enamel, and very large ears.
The mammoth (Z. przmigenzus) belonged to the Pleistocene period,
and had a wide geographical- range, occurring, for instance, in
Britain.
The genus Mastodon is represented by fossil remains in Miocene,
Pliocene, and even in Pleistocene strata, in Europe, India, and America.
The molar teeth show transitions between those of elephants and those
of other Ungulates.
In Dinotherium, found in Miocene and Pliocene strata in Europe
and Asia, the lower jaw bore an enormous pair of tusks projecting
vertically downwards, and all the back teeth seem to have been in use
at the same time.
SEVERAL EXTINCT SUB-ORDERS
Although we cannot describe the following remarkable types, it is
important to notice their existence, for they serve to impress us with
the original connectedness of what are now separate orders.
The huge Amblypoda, in Eocene formations in America and Europe,
had usually remarkable protuberances on the top of the skull, a very
small brain, large upper canines, especially in the males, and six back
teeth. :
Example.— Uintatherium (Dinoceras), with no upper incisors.
Some Tertiary American forms, e.g. Zoxodon and Nesodon, varying
766 MAMMALIA,
in size from that of a sheep to that of a rhinoceros, form the sub-order
Toxodontia.
Cope includes a number of generalised Eocene Ungulates under
the title Condylarthra. Some ‘seem ancestral to the Perissodactyla
and Artiodactyla; some suggest a union of: ancestral Ungulates and
ancestral Carnivores. The genus /erdptychus may be regarded as
an ancestral Bunodont, and Phenacodus as near the origin of the
horse stock. But Phenacodus is so generalised that Cope suggested
affinities between it and not only Ungulates, but also Carnivores and
Lemurs. : F
From the Eocene of N. America, Marsh disentombed a group
of animals which he called Tillodontia, eg. Z2l/otherium, which
seem to combine the characters of the Ungulata, Rodentia, and
Carnivora.
Few orders of Mammals are of more interest to the palzeon-
tologist than the Ungulates, Not only are fossil representatives
numerous, but their usually large size, and the fact that the teeth
are frequently an index 'of general structure, makes the determina-
tion of affinities much easier than in most cases. In consequence,
problems like that of the origin of the horse, or the relations of the
different proboscidians, have been worked out with a completeness
rare elsewhere. ;
Order CETACEA
The Cetaceans, including whales and dolphins and
their numerous relatives, are aquatic mammals of fish-like
form.
The spindle-shaped body has no distinct neck between
the relatively large head and the trunk, and tapers to a
notched tail, horizontally flattened into flukes. The fore-
limbs are paddle-like flippers, and there are no external
hints of hind-limbs beyond mere button-like knobs in some
embryos. Most forms have a median dorsal fin. Hairs
are generally absent, though a few bristles may persist near
the mouth. The thick layer of fat or blubber beneath
the skin retains the warmth of the body, and compensates
for the absence of hair. In one of the dolphins dermal
ossicles occur, a fact which has suggested the idea that the
toothed whales may have had mailed ancestors. Traces
of dermal armour have also been found in the extinct
Zeuglodonts.
The spindle shape, the absence of external ears, the
absence of an eye-cleansing nictitating membrane, the dorsal
position and valvular aperture of the single or double nostril,
the sponginess of the bones, the retia mirabilia storing
CETACEA. 767
arterial blood in different parts of the body, may be asso-
ciated with the aquatic life.
The cervical vertebree are thin, and more or less fused.
There is no union of vertebree to form a sacrum, for the
Fic. 412.—Left fore-limb of Fic. 413.—Fore-limb
Balenoptera. of whale (A/egaptera
Sce., Scapula with spine (s4.); 4, longimana).—After
humerus; #., radius; U., ulna; Struthers. ,
C., carpals embedded in matrix ;
Mc., metacarpals; PA., phal-
anges.
hind-limbs are at most very rudimentary. Under the
caudal vertebre there are wedge-shaped chevron bones.
The brain-case is almost spherical; the supraoccipital
meets the frontals and shuts out the parietals from the roof
of the skull; the frontals arch over the orbit; the snout or
rostrum of the skull is composed of premaxillz, maxillz,
768 MAMMALIA
and vomer, and of the mesethmoid cartilage. The periotic
in whales is an exceedingly dense bone, and is of interest
because it is the only part of the skeleton found at great
depths on the floor of the ocean, and is often preserved as
a fossil.
There are at least rudiments of two sets of teeth, as
in other Mammals, but in baleen whales only the teeth of
the milk set are calcified, and they come withal to nothing,
being to some extent replaced by the horny baleen-plates
developed on the palate. In toothed whales the two sets
are said by Kiikenthal to fuse, but the usual interpretation
is that the functional teeth belong to the milk set. It
is possible that the simple, homodont, conical teeth of
Odontoceti have resulted from a splitting of more complex
cusped teeth. No clavicles are developed. The bones of
the forelimb are flattened, and, except at the shoulder,
articular surfaces are not developed, so that the limbs form
stiff paddles. The carpals are fixed in a fibrous matrix,
tend to be rudimentary, and are often unossified. They
cannot be readily compared with the members of the
typical mammalian carpus. In the absence of true joints,
a slight flexibility is given by the absence of ossification.
There are four or five nailless digits, of which the second
and third, and sometimes the first, may have more than
the usual number of phalanges (see Fig. 413), a peculiarity
possibly due to a duplication and separation of epiphyses.
The pelvis may exhibit a rudimentary ischium, with small
vestiges of femur and tibia.
The rounded brain is relatively large, with well-convoluted
cerebral hemispheres.
As to the alimentary system,—salivary glands are: rudi-
mentary or absent, the stomach is chambered, the intestine
has rarely a caecum, the liver is but slightly lobed, there is
no gall-bladder.
The heart is often cleft between the ventricles. Both
arteries and veins tend to form retia mirabilia.
The larynx is elongated, so that it meets the posterior
nares, and forms a continuous canal, down which air passes
from nostrils to. lungs. The inspiration and expiration
occur at longer intervals than in terrestrial mammals. The
water-vapour expelled along with the air from the lungs
CETACEA. 769
‘condenses into a cloud, which is sometimes increased by
an accidental puff of, spray.
The kidneys are lobulated. The testes are abdominal.
There are no seminal vesicles. The uterus is bicornuate.
The placenta is non-deciduate and diffuse. The two
mamme lie in depressions beside the genital aperture, and
Fic. 414.—Pelvis and hind-limb
of Greenland whale (Balena).
—After Struthers,
P., Pelvis; #., femur; 7., tibia.
the milk is squeezed from special reservoirs into the mouth
of the young. Usually a single young one is born at a time,
and there are never more than two.
All are carnivorous; but, while many feed on small
pelagic animals, other swallow cuttles and fish, and Orca
attacks other Cetaceans and: seals. Most are gregarious,
and live in schools or herds.
[TaBLz.
49
770
MAMMALIA,
CONTRASTS BETWEEN THE TWO SUB-ORDERS OF
LIVING CETACEANS
MystTacocetTI or BALZNOIDEA,
baleen Cetaceans.
OponToceET! or DELPHINOIDEA,
toothed Cetaceans.
The teeth are absorbed before birth,
Whalebone or baleen-plates develop as
processes from the palate.
The skull is symmetrical.
The nasals roof the anterior nasal pas-
sages, which are directed upwards
and forwards.
The maxilla does not overlap the orbital
process of the frontal:
The lachrymal is small, and distinct
from the jugal.
| The tympanic is ankylosed to the peri-
otic.
The rami of the mandible are arched out-
wards, and have no true symphysis.
All the ribs articulate only with the
transverse processes of the vertebra,
the capitulum being imperfect.
The sternum is a single piece, and arti-
culates with a single pair of ribs ; the
sternal ribs are not ossified.
The external nostrils are separate.
The olfactory organ is distinctly de-
veloped.
There is a short cecum.
Examples.—
The right-whale (Balena), the
hump-back (Megaptera), the
rorqual (Batenopiera).
The teeth persist after birth, and are
generally numerous and functional.
There is no baleen.
The skull on its upper surface is more
or less asymmetrical.
The nasals, always small, do not roof
the anterior nasal passages, which are
directed upwards and backwards.
The maxilla covers most of the orbital
process of the frontal.
The lachrymal is fused to the jugal, or
is large, and helps to roof the orbit.
The tympanic is not ankylosed to the
periotic.
The rami of the mandible are straight,
and form a symphysis.
Several anterior 2-headed ribs articulate
by capitula with the centra.
The sternum has usually several seg-
ments, with which several usually
ossified sternal ribs articulate.
The nostrils unite in a single blow-hole
on the top of the head.
The olfactory organ is rudimentary or
absent.
There is no cacum, except in Plata-
nista.
Examples.—
The sperm-whale (Physeter), the
dolphin (Delphinus), the por-
poise (Phocena), the ‘ Gram-
pus” (Orca), the Ca’ing-whale
(Globicephalus), Grampus, the
narwhal (Monodon), with an
enormous tusk in the male.
RODENTIA. 77%
The two sub-orders of living Cetaceans—the Mystacoceti, without
functional teeth, but with baleen-plates on the palate, and the Odonto-
‘ceti, with functional teeth and without baleen, do not seem closely
related, and it may be that many of their resemblances are due to
convergence. The toothed whales seem to be the older stock.
The Odontoceti have probably arisen from the Zeuglodonts and
these from the Creodonts. Like the Sirenia, the Cetacea appearéd in
the Lower Eocene and evolved very rapidly, attaining full adaptation
by the Mid-Eocene. They found the seas clear of the great Reptiles,
and the change from walking to floating led to many readjustments of
Fic. 415.—Vertebra, rib, and sternum of Balenoptera.—
From specimen in Anatomical Museum, Edinburgh.
C., Centrum; 2.@., neural arch ; #.s5/., neural spine ; 4.2., transverse
process; #., rib; S%., sternum.
a thoroughgoing sort. It is interesting to notice that flippers must
have arisen several times independently—in Ichthyosaurs (post-Triassic),
Plesiosaurs, Cetacea (twice ?), Sirenia, and Pinnipedia,
Order RopENTIA
Rodents are represented in all parts of the world, and by
more species than any other order of Mammals. Most of
them are small and terrestrial. They are typically vegetarian,
and gnaw their food in a characteristic way.
Thedentition is quite distinctive. The incisors are chisel-
edged, for, as the enamel is either restricted to the front or
is at most thin-posteriorly, the back part wears away more
772 MAMMALIA.
rapidly. The incisors are rootless, growing from persistent
pulps, and the same is sometimes true of the bunodont or
lophodont back teeth. There is never more than a pair of
lower incisors, and in most cases the upper jaw has only a
pair. There are no canines, and the skin projects as a
hairy pad into the mouth through the gap between incisors
and premolars.
The feet are plantigrade or semi-plantigrade, generally with
five clawed or slightly hoofed digits. Clavicles, though often
rudimentary, are generally present. The scapula has usually
a long acromion process, sometimes with a metacromion.
The condyle of the mandible (and the corresponding
articular surface for it) is usually elongated, and the jaw
moves backward and forward. The mandible has an
abruptly narrowed and rounded symphysis, and a very large
angular portion. The orbits are confluent with the
temporal fosse. The zygomatic arch is complete, but the
jugal is restricted to the middle of it. The premaxille are
large, the palatines small. There is generally a distinct
interparietal bone. The tympanic bulle are always de-
teloped, and often large.
The cerebral hemispheres are almost without convolu-
vions, and leave the cerebellum uncovered. The skin is
generally thin, and the panniculus carnosus but slightly
developed. The intestine has a large caecum, except in
Myoxide. Special anal or perineal or other glands secreting
odoriferous substances are frequent.
The testes are inguinal or abdominal; only in the hares
and rabbits do they completely descend into scrotal sacs.
The mamme are on the abdomen, or on the abdomen
and thorax. The uterus is double or very markedly
bicornuate. There is a provisional yolk-sac placenta; the
allantoic placenta is discoidal and deciduate.
The Rodents are very widely distributed, but are most abundant in
S. America, where they form a very characteristic part of the fauna.
Out of seventeen existing families, nine are represented there, and four
are peculiar to it.
The Rodents are divided into four sub-orders :—
1. Sciuromorpha.—Squirrels (Scéurus), marmots (Arctomys),-
prairie-dogs (Cynomys), and beavers (Castor). P
z. Myomorpha.—Rats and mice (4/us), voles (Arvzcola), lemmings
(Myodes), and jerboas (Dipus).
CARNIVORA. 773
3. Hystricomorpha.—Porcupines (Hystrix), agoutis (Dasyprocta),
guinea-pigs (Cavza), and the S. American capybara (Hydro-
cherus), the largest living Rodent, measuring about 4 ft. in
length.
4. Lagomorpha.—Hares and rabbits (Zefzs), and the picas or
tailless hares (Lagomys), with incisors 2,
_ In the first three sub-orders there is only a single pair of upper
incisors, and the three may be united as Simplicidentata, in contrast
with the Duplicidentata, where there are two pairs. Only in the Jatter
does the enamel extend to the posterior surface of the incisors, which
are also peculiar (in this order) in having well-developed milk pre-
decessors.
Fic. 416.—Skull of tiger, lateral view.
px., Premaxilla ; #x., maxilla. Note the insertion of upper canine
(c.1) just behind the suture line, and the fact that the lower
canine (c.2) bites in front of it. 2a., Nasals; Za., lachrymal
bone with foramen; /7., frontal; a., parietal; so., supra-
occipital ; fa., paroccipital process; az., auditory aperture (the
reference line crosses the inflated bulla) ; sg., zygomatic process
of squamosal; @., angle of lower jaw; jz., jugal; ca.,
carnassial tooth of upper jaw; co., coronoid process of lower
jaw.
Order CARNIVORA
This order includes lions and tigers, foxes and dogs, bears
and otters, etc.
Most of the Carnivora feed on animal food, and the most
typical forms prey upon other animals and devour their
774 MAMMALIA.
warm flesh. Most are bold and fierce animals, with keen
senses and quick intelligence, and often much beauty of
form and marking.
Fic. 417.—Lower surface of dog’s skull.
0.¢c., Occipital condyle; B.0., basioccipital ; 7., tympanic bulla;
m.c., postglenoid process Behind fossa for condyle of mandible 3
B.S., basisphenoid ; P.S., base of presphenoid ; /’., vomer 3
Af.2, second molar; A/.1, "first molar ; 7.1-4, premolars, the
4th the large carnassial; c., canine; /.1-3, incisors
Parts,
premaxilla; #x., maxilla; Pad., palatine; /., jugal; A.S.
alisphenoid ; Pz., pterygoid ; Sq., squamosal (the reference line
points to the glenoid fossa).
Almost all have well-developed claws; there are never
fewer than four toes. The teeth are always rooted, except
in the case of the tusks of the walrus; the canines are
CARNIVORA. 775
strong and sharp; some of the back teeth are generally
sharp, and specially adapted for cutting.
There are generally strong occipital and sagittal crests for
the insertion of muscles of neck and jaw. The glenoid
fossa for the articulation of the lower jaw is deeply concave,
and bounded by a large postglenoid process, the result
being that the lower jaw can only move up and down.
This is important, as it minimises the risk of any failure
of grip in seizing living prey. The muscles of the lower
jaw are very strongly developed, and with this may be
associated the strength and the protrusion of the zygomatic
arch in the more specialised types. The widening of this
arch has prevented the formation of a frontal bridge behind
the orbit, so that the orbit is confluent with the temporal
fossa. There is a strongly developed and ossified tentorium
descending between cerebrum and cerebellum. The tym-
panic bullee are in most cases large.
The clavicles are incomplete or absent (an important
contrast with all Insectivora except Potamogale) ; the radius.
and ulna are always distinct; the fibula is slender but dis-
tinct. The scaphoid and lunar bones are fused.
The cerebrum is well convoluted, and the cerebellum is.
more or less covered by the cerebrum.
The stomach is always simple; the czecum is absent, or
short, or simple ; the colon is not sacculated.
There are no vesicule seminales. The uterus is bicor-
nuate) The mamme are abdominal. The placenta is
deciduate and zonary.
‘ Representatives of Carnivora are found in all parts of the
world.
The true Carnivores are for the most part terrestrial. The incisors
are almost always 3, the canines are usually large; one of the
back teeth is modified as a trenchant carnassial or sectorial.
The digits generally have sharp claws, which may be retractile.
Within the sub-order there are three sections—®luroidea,
Cynoidea, and Arctoidea—represented respectively by cat,
dog, and bear, but these types are connected by extinct
forms. :
In retractile claws, the last phalanx of the digit with its attached
claw is drawn back into a sheath on the outer side of the middle
phalanx in the fore-foot, on the upper side in the hind-foot. When
the animal is at rest or is walking, the claw is retained in this bent
776
MAMMALIA.
position by an elastic ligament, and is in this way protected from
wear.
protruded.
When the animal straightens the phalanges, the claws are
4
(1) ZLUROIDEA,
| e.g. cat, civet, hyzena.
(2) CYNOIDEA,
e.g. dog, fox, wolf, jackal.
(3) ARCTOIDEA,
eg. bear, otter,
| Digitigrade.
. tgs 3131
Typical dentition, are
The tympanic bulla is
much dilated,
rounded, and _ thin-
walled, and is divided
into two chambers
by an internal septum
(exceptin Hyzenidz).
The paroccipital pro-
cess of the exocci-
pital is applied to the
hinder part of the
tympanic bulla,
The czecum is small,
rarely absent.
Digitigrade,
Typical dentition, an
The tympanic bulla is
dilated, but the in-
ternal septum is
rudimentary.
The paroccipital pro-
cess is in contact
with the bulla, but
it is prominent.
The czecum is some-
times short and
simple, sometimes
long and peculiarly
folded,
Plantigrade or sub-
plantigrade.
Typical dentition, 2*47.
ae "3143
The tympanic bulla is
often depressed, and
there is no hint of an
internal septum.
The paroccipital pro-
cess is quite apart
from the bulla.
The czecum is absent.
Digitigrade animals walk on their toes only; plantigrade forms res
the whole sole of the foot on the ground; but between these condition
there are all possible gradations.
Many Carnivores are sub-plantigrade
often when at rest applying the whole of the sole to the ground, br
keeping the heel raised to a greater or less extent when walking.
(1) ZELUROIDEA—Cat-like Carnivores
Family Felid, including the most specialised forms.
The canine
are large, the molars are reduced to ‘, the carnassials are th
last premolars above (with a three-lobed blade), and th
molars beneath (with a two-lobed blade).
The tuberculate
upper molars are very small, and of little if any use i
mastication.
The skull is generally rounded, the zygomat'
arches are wide and strong, and the tympanic bulle are Jarg
and smooth. The limbs are digitigrade, the claws retractile
CARNIVORA., 777
There is no alisphenoid canal. The dentition of the typicay
genus Felzs is or The cats are the most specialised of all Car-
nivores, and are exclusively adapted for a flesh diet. The
sharp claws and pointed canines form powerful offensive
weapons ; the cusped cheek-teeth and rasping tongue are em-
ployed to separate the flesh from the bones of the prey.
Examples.—The lion (els /eo), in Africa, Mesopotamia, Persia,
N.-W. India; the tiger (7. tagris), widely distributed
in Asia; the leopard (/. pardus), in Africa, India, Ceylon,
Sumatra, Borneo, etc.; the wild cat (#. catus); the
Caffre cat (F. caffra) of Africa and S. Asia, venerated and
mummified by the Egyptians, perhaps ancestral to the
domestic cat.
A high degree of specialisation for carnivorous habit is well
illustrated by the sabre-toothed tigers (M/acherodus) of Tertiary
ages, whose serrated upper canines were sometimes 7 in. long.
Family Viverride—Old World forms, such as civets (Vverra), of
Africa and India; genets (Genetta), of S. Europe, Africa, and
S.-W. Asia; ichneumons or mongooses (Herfestes), in Spain,
Africa; India, Indo-Malaya.
Family Proteleide—represented by Proteles cristatus, the hyzna-
like aard-wolf of S. Africa.
Family Hyzenidze—represented by the genus Ayena, found in
Africa and S. Asia. The tympanic bulla is not divided by a
septum.
(2) CynorpEA—Dog-like Carnivores
Family Canidee—including forms intermediate between the cats and
the bears. The dentition is more generalised than in the
Felidze, its usual formula is ae Within the tympanic bulla
there is only a rudimentary septum. The paroccipital process
in contact with the bulla is prominent. The czcum is either
short and simple or long and peculiarly folded upon itself.
Examples. —The genus Cazzs has representatives in all parts
of the world,—the wolves (C. /upus, etc.), the jackals
(C. aureus, mesomelas, etc.), the domestic dogs (C. fami.
arts), the foxes (C. vulpes, etc.), the Cape hunting dog
(Zycaon), the bush-dog (Le¢icyon) of Guiana and Brazil, and
the primitive Otocyon megalotzs from S. Africa. In the dog
the dental formula is Pe ; the upper carnassial or fourth pre-
molar has a stout bilobed blade, the lower carnassial or first
molar has a compressed bilobed blade. The skull is more
elongated than in the cats; the orbits are very widely open
posteriorly; the clavicles are very small; the limbs are
digitigrade ; there are five toes on the fore-feet, but the short
thumb does not reach the ground; there are only four toes
on the hind-feet, but in domestic dogs the rudiment of the
hallux is sometimes enlarged as the ‘‘dew-claw” ; the claws
are non-retractile and blunt.
778 MAMMALIA,
(3) ARCTOIDEA—Bear-like Carnivores
The tympanic bulla shows no trace of an internal septum; the
paroccipital process of the exoccipital is quite apart from the
bulla, and widely separated from the mastoid process of
the periotic. The limbs are plantigrade or sub-plantigrade,
and always bear five toes. There is no caecum.
Family Ursida—Bears. The molars have broad tuberculated crowns
used for grinding. The three anterior premolars are usually
rudimentary. The auditory bulla is depressed. In relation to
the character of the teeth, it should be noted that the diet is at
least in part vegetarian ; even the polar bear eats herbs in the
summer. Ursus, aM absent from Ethiopian and Australian
regions, represented in the Neotropical region by only one
species, elsewhere widespread.
Family Procyonide—The Himalayan Panda (.4/urus fulgens), the
American raccoon (Procyon).
Family Mustelide—The otter (Zzéra), the sea-otter (Latax /utris),
the skunk (Mefhztz's), the badger (AZe/es), the ratel (Mel/zvora),
the marten, sable, polecat, stoat, weasel (A/ustela).
CREODONTA (extinct)
In Eocene and early Miocene strata, in Europe and America, there
are remains of what seem to be generalised Carnivora, ancestral to the
modern types, and apparently related to Insectivora as well. Those
included in the sub-order Creodonta have strong canines but no single
carnassials, while the molars are often like those of Marsupials. The
brain seems to have been small.
Examples.—Ayenodon, Proviverra, Arctocyon.
Order PINNIPEDIA. Seals, Eared Seals, and Walruses
Marine Carnivores, unable to move readily on land, but coming
ashore for breeding purposes. They feed for the most part on fish,
molluscs, and crustaceans. Absent from the tropics, they are repre-
sented on most of the coasts in temperate and Arctic zones. Many are
inarkedly gregarious.
The upper parts of the limbs are included within the skin and generaP
contour of the body. There are five well-developed digits connected
by a web of skin. In the hind-foot the first and fifth toes are generally
stouter and longer than the rest. There are no clavicles. The tail is
very short.
The small milk-teeth are absorbed before or immediately after
birth, The incisors are always fewer than 3 ; there are no carnassials ;
the back teeth have pointed cusps, often sloping slightly backwards.
The cranial cavity is rounded ; there is a characteristic interorbital
constriction.
The brain is large and well convoluted. The eyes are large and
prominent, with a flat cornea. The external ear is small or absent.
INSECTIVORA. 779
The cecum is very short. The kidneys are divided into lobules.
The mamme are two or four in number, and lie on the abdomen.
The young are ‘‘ precocious.”
Family Otariide.—Eared or fur-seals, connecting the Pinnipeds with
the Fissipeds. The hind-feet can be turned forward and used on
land in the normal fashion. The palms and soles are naked.
There is a small external ear. The testes lie in an external
scrotum,
2u 47%, Pacific and S, Temperate seas.
Family Trichechidae—Walruses, intermediate between the Otariidae
and the seals. The hind-feet can be turned forwards and used
on land. The upper canines form large tusks; the other teeth:
are small, single rooted, and apt to fall out ; those generally in
use are a but the dentition of the foetus is ee.
The jaw seems relatively short, an adaptation perhaps to mussel-
crushing instead of fish-catching.
There are no external ears.
The walrus or morse, Zrichechus (Arctic).
Family Phocidee—Seals, the most specialised Pinnipeds, The hind-
limbs are stretched out behind, and the strange jumping move-
ments on land are effected by the trunk, sometimes helped by-
the fore-limbs. The palms and soles are hairy. There are well-
developed canines; the upper incisors have pointed crowns ;.
there are 2 back teeth. There is no external ear, The testes.
are abdominal.
The sea-lion O¢aria,
The common seal (Phoca), ae the grey seal (Halicharus),.
the monk seal (Afonachus), the large elephant seal (Macrorhinus
leoninus).
Order INSECTIVORA
This order includes hedgehog, mole, shrews, and related:
mammals usually of small size. There is much diversity:
of type, so that a statement of general characters is very
difficult.
Most Insectivores run about on the earth; the mole
(Zalpa), and others like it, are burrowers; Potamogale,
Myogale, and others are aquatic; Zupaia and its relatives.
live like squirrels among the branches,
Most feed on insects; some arboreal forms eat leaves.
as well; the moles eat worms; Fotamogale is said to feed.
on fish.
The body is usually covered with soft fur, but the hedge-
hog (Z7inaceus) is spiny, and so to a less extent is Centetes,
the ground-hog of Madagascar. The digits, usually five in
780 MAMMALIA.
number, are clawed, and the animals walk in plantigrade
or semi-plantigrade fashion. In most, the mamme are
thoracic or abdominal.
The cranial cavity is small; the skull is never high; the
facial region is long; the zygomatic arch is slender or
incomplete. Except in Potamogale, there are clavicles.
There are never fewer than two pairs of lower incisors.
The enamelled molars have tuberculated crowns and well-
developed roots. In many cases it is not easy to distinguish
the usual division of the teeth into incisors, canines, pre-
molars, and Bee but in many the dentition is typical—
4,154 3=
In the hedgehog, according to Leche, i. 3, pm. 2, m. 1-3,
of the upper jaw, and i. 3, c., pm. 3, m. 1-3, of the lower
jaw, are persistent milk-teeth, but, according to others, the
milk-teeth are represented by mere rudiments (“ prelacteal
germs”), and the functional teeth correspond to the perma-
nent set of other mammals.
The cerebral hemispheres are smooth, and leave the
cerebellum (and sometimes the corpora quadrigemina)
uncovered; the olfactory lobes are large; the corpus
callosum is short and thin. Thus, as regards the brain, |
the Insectivora represent a low grade of organisation.
The stomach is a simple sac; the intestine is long and
simple, but the vegetarian forms have a cecum. In most
there are odoriferous glands, axillary in shrews, but usually
near the anus.
The testes are inguinal or in the groin, or near the
kidneys, not in a scrotum. The penis may be pendent
from the wall of the abdomen, but is usually retractile.
There is a bicornuate uterus. Several and usually many
offspring are born at once.
The allantoic placenta is discoidal and deciduate. There
is a provisional yolk-sac placenta.
Insectivora are represented in the temperate and tropical
zones of both hemispheres, but not in S. America (except
in the Northern Andes) nor Australia. In the former
continent their place is taken by the insectivorous opossums.
Examples.—The hedgehogs (Zr¢maceus), throughout Europe, Africa,
and most of Asia, dentition ree the shrews (Sorex), in
CHIROPTERA. 783
Europe, Asia, and N. America, dentition ae ; the moles ( Za/pa),
throughout the Palzarctic region; the tailless tenrec (Cemdetes)
of Madagascar; the §. African golden moles (Chrysochloris),
probably the most primitive of all Eutheria ; the African jumping
shrew (Macroscelides) ; the Oriental tree-shrews ( 72paza).
Order GALEOPITHECIDA&
It seems justifiable to recognise a separate order for the very
divergent Galeofithecus, from the Malay Archipelago and the
Philippines. They are arboreal vegetarian animals. The fore- and
hind- limbs are connected by a parachute, and the animals can glide
from tree to tree, ‘sometimes traversing a space of seventy yards with:
a descent of only about one in five.” The structure of the incisors
is unique among Mammals. They are expanded laterally, compressed
from before backwards, and furnished with many cusps. The lower
are pectinated, the flattened crowns being penetrated by numerous
vertical slits, and the outer of the two upper pairs have double roots.
The dentition is an ‘The molars are multicuspidate. The orbit
has an almost complete bony ring. There is a tympanic bulla. The
cerebral hemispheres have a few furrows. There is a simple stomach
and a large sacculated cecum. The testes are scrotal, the penis
pendulous. There are two pairs of pectoral mamme, and one young
one at a birth.
Order CHIROPTERA. Bats
Bats are specialised Mammals related to Insectivores.
They have the power of flight, the fore-limbs being modified
as wings. The wing is formed by a fold of skin which
usually begins from the shoulder, extends along the upper
margin of the arm to the base of the thumb, thence between.
the long fingers, and along the sides of the body to the hind-
legs or even to the tail. Contrasted with the wing of a
bird, that of a bat has a rudimentary ulna beside a long
curved radius, a wrist with six bones, five free digits, four
of which have very long metacarpals, while the thumb is
short. The phalanges are usually reduced to two. The
pectoral girdle is strong; there is a long curved clavicle, a
large triangular scapula, a long coracoid process ; the pre-
sternum bears a slight keel on which are inserted some of
the muscles used in flight. The thumb is always clawed ;
the other digits are unclawed, except in most frugivorous.
bats, where the second digit bears a claw.
The hind-limb is relatively short and weak, the pelvic
girdle is also weak, and in most cases the pubic symphysis
782 MAMMALIA.
is loose in the males, unformed in the females. The knee is
turned backwards like the elbow; the ankle has a cartil-
aginous prolongation or calcar, which supports the fold of
skin between limb and tail; the five toes are clawed.
The vertebral column is short; there is little mobility
between the vertebrz ; neural spines are absent behind the
third cervical, except in Pteropide ; the caudal vertebrze are
very simple. The ribs are usually flat. The maximum
dentition is 353 ; the milk-teeth are very different from the
permanent set. All the bones are slender, and the long
‘bones have relatively large medullary canals.
The cerebral hemispheres are smooth, or with few con-
-volutions, and leave the cerebellum uncovered. The spinal
‘cord is at first very broad, but narrows rapidly behind the
neck. The sense of touch is remarkably developed in the
hot skin of the wing, the large mobile external ears, the
whisker hairs of the snout, and in the strange plaited “ nose
leaves” around the nostrils. Even when deprived of sight,
‘hearing, and smell, bats will fly about in a room without
‘striking numerous wires stretched across it. The stomach
is usually simple, but there is a long pyloric diverticulum,
filled with coagulated blood, in the blood-sucking Desmodus.
The whole gut is very short in insectivorous forms. There
iis never more than a very short czecum.
The temperature of the body is high. The testes are
abdominal or inguinal; the penis is pendent. The uterus
is simple, bicornuate, or duplex. There is usually but one
offspring at a time, and there are never more than two.
The mamme are two in number, thoracic, generally post-
axillary in position. As in Insectivora and Rodentia, the
yolk-sac forms a provisional placenta, and the allantoic
placenta is discoidal and deciduate. “ What looks like
menstrual flux has been noticed in some bats. In most
European bats sexual union occurs in autumn, but the
sperms are simply stored in the uterus, for ovulation and
fertilisation do not take place till spring—after the winter
sleep. In exceptional cases, especially in young forms
which were not mature in autumn, pairing occurs in spring.
Fossil Chiroptera occur in Upper Eocene strata, but are
quite like the modern forms.
PROSIMIZA,
783
SuB-ORDER MEGACHIROPTERA.
SUB-ORDER MICROCHIROPTERA.
| Frugivorous bats, usually large.
The molars have smooth crowns,
with a longitudinal groove,
The thumb is clawed, and generally
also the second digit.
| The tail, if present, is below, not
) bound up with the interfemoral
membrane.
The pyloric part of the stomach is
in most cases much elongated.
Found in warm and tropical parts
of the eastern hemisphere.
Examples. —
The ‘‘ flying-foxes’’ or fox-bats
(Pteropus), large, tailless bats,
distributed from Madagascar
to India, Ceylon, Malaya,
S. Japan, Australia, Poly-
nesia, The largest species
(P. edulis) measures 5 ft.
across its spread wings. Den-
2132
2133
tition, —=.
isverycommon. Xantharpyia
egyptiaca inhabits the Pyra-
mids.
In India, Cynopterus marginatus
Usually insectivorous bats, small
in size.
The molars have cusped crowns,
with transverse grooves.
In the hand the thumb only is
clawed,
The tail, if present, is bound up
with the interfemoral membrane,
or lies along its upper surface.
Except in one family, the stomach
is simple.
Found in the tropical and temper-
ate regions of both hemispheres.
Examples,—
The horseshoe-bats (Aznolo-
phus); the common pipistrelle
(Vesperugo pipistrellus); the
genus Vespertilio, with four
British species; Vampyrus
spectrum, a large Brazilian
form, which seems to have been
erroneously credited with
blood-sucking habits; the
common vampire (Desmodus
rufus), an American bat—a
formidable blood-sucker.
Order Prosim1i& (Syz. LEMUROIDEA, Lemurs)
These monkey-like animals are sometimes ranked with
monkeys as a sub-order of Primates ; but there seems more
warrant for placing them in a separate order. They agree
with monkeys in many respects, ¢.g. in having pollex and
hallux opposable, flattened digits, pectoral mammee (except
in Chiromys), and a ‘Simian fissure” in the brain. They
differ from monkeys (Anthropoidea) in the following
points: The cranial cavity is usually elongated, and the
face more fox-like than monkey-like ; the orbit opens freely
into the temporal fossa (except in Zarsius); the lachrymal
foramen lies in front of the orbit; the first pair of upper
incisors is separated in the middle line (except in Zarszus) ;
the second digit of the foot always bears a pointed claw,
but the others usually have flat nails; the cerebral hemi-
784 MAMMALIA,
spheres are but slightly convoluted, and do not completely
overlap the cerebellum (except in Indrisinz); the middle
or transverse portion of the colon is almost always folded
or convoluted on itself; there may be abdominal and
inguinal as well as pectoral mammz ; the uterus is bicor-
nuate; the urethra perforates the clitoris (except in
Chiromys); the placenta is diffuse and non-deciduate except
in Zuarsius, where it is metadiscoidal and deciduate.
Among other features we may note that the Lemurs are
plantigrade and usually pentadactyl; the tail (sometimes
reduced) is never prehensile; the mandibles are often
unfused at the junction; in the Madagascar forms the
tympanic remains a half-ring within the bulla which is
due to the periotic; the carpus has a centrale usually
free ; there is a large cecum without a vermiform appendix ;
there are often retia mirabilia on some of the arteries and
veins.
The lemurs are small, furry quadrupeds, with fox-like
faces but the general appearance of monkeys. Most are
nocturnal, all arboreal. They feed on fruits and leaves,
on eggs and small animals. Most are loud-voiced. They
are usually uniparous.
A. Madagascar Lemurs, with the tympanic annulus free in the bulla.
Family Lemurinz, with long faces. Some have interesting tufts of
vibrissze on the forearm, and a strange forearm gland, with spines
in the male. Family Indrisine, with short faces, cerebrum
covering cerebellum. Family Chiromyine, with one type
Chiromys, the Aye-Aye, highly specialised, e.g. with very long
slender third finger, with a flat nail on the thumb only, with
1113
om)
rodent-like permanent incisors (
1003
, with inguinal mamme.
B. Ethiopian and Oriental Lemurs, with the tympanic sharing in
making the bulla,
Family Galagine, with one type Gadago, with elongated calcaneum
and navicular. It occurs right across Africa. Family Lorisine.
Asiatic and African.
C. The aberrant Indo-Malayan Zars¢us, with many peculiarities, e.g.
the orbit communicates with the temporal fossa only by a fissure,
the upper incisors are close together, the calcaneum and navicular
are greatly elongated like the calcaneum and astragalus in the
frog, the placenta is metadiscoidal and deciduate as in monkeys,
The lemurs are interesting, both because they link the Anthropoidea
to lower Mammals, and because of their distribution. In Eocene.
ANTHROPOIDEA. 735
times or even earlier they appeared in Europe and N. America, and
were then of more generalised type. In the latter continent they
became extinct ; ‘but in the Old World they appear to have migrated
southwards at an early period into Ethiopian and Oriental regions,
They reached Madagascar at a time when that island was connected to:
the continent, and before the advent of the larger carnivores. There
they have been isolated and have developed in a fashion comparable
to that which has occurred in the case of the Australian Marsupials.
Of fifty living species:thirty-six are confined to Madagascar, and these
are very abundantly represented. Outside of Madagascar lemurs
maintain a precarious footing in forests or islands, and are usually
few in number. They are handicapped by the absence of defensive
weapons, the frequent slowness of movement, and the feeble intelli-
gence ; they-are saved by their arboreal and usually nocturnal habits,
by their quiet movements, and by their shyness.
Order ANTHROPOIDEA (= PRIMATES or SIMIz)
This order includes five families.
Family 5. Hominide. Man.
» 4 Anthropomorphidee or Simi-
ide. Anthropoid Apes. | Old World
» 3» Cercopithecide. Baboons, { Catarrhina.
etc.
» 2. Cebide. American Seige New World
» 1. Hapalide. Marmosets. Platyrrhina.
The following characteristics are generally true :—
The body is hairy, least so in man; the incisors do not
exceed =; the molars are 2, except in the marmosets,
where they are = ; the back teeth are bunodont, the premolars
with two cusps, the molars usually with four; the cranial
cavity is relatively large; the axis of the orbit is directed
forward, and the orbit is closed off from the temporal fossa
by ingrowths of frontal and jugal meeting the alisphenoid ;
the lachrymal foramen is infra-orbital; the clavicles are well
developed ; the radius and ulna move freely on one another
in pronation and supination ; the scapnoid, the lunar, and
usually the os centrale are distinct; there are usually five
fingers and toes, but the thumb may be absent or rudi-
mentary; the thumb (or pollex) if present is opposable
except in marmosets; the big toe (or hallux) is opposable
50
786 MAMMALIA.
except in man; the nails are almost invariably flat, except
in marmosets; the cerebral hemispheres have in most cases
numerous convolutions, and usually cover the cerebellum ;
the stomach is simple except in Semnopithecus and its
relatives, in which it is sacculated ; there is a cecum which is
often large; there are two mamme on the breast; the
uterus is simple; the testes lie in a scrotum; the penis is
pendent; the placenta is metadiscoidal, being developed by
the concentration of the villi from a diffuse area into a well-
defined disc. Most Anthropoidea are arboreal, gregarious,
uniparous, and tropical or sub-tropical.
CONTRAST BETWEEN PLATYRRHINA AND CATARRHINA
The New World Platyrrhina are in many ways so different from the
‘Old World Catarrhina that a twofold (diphyletic) origin of the monkey
order is not improbable. There are no transitional forms, and the
distribution of the extinct representatives corresponds with that of the
living forms.
PLATYRRHINA
Broad cartilaginous _internarial
septum.
Nostril directed outwards.
Tympanic bone not more than a
ring. No bony external audi-
tory meatus.
‘Tympanic bulla.
Alisphenoid usually meets the
parietal on the side of the
skull, and the orbital plate of
the jugal meets the parietal.
A large orbito-temporal fora-
men.
‘Three premolars.
Tail often prehensile, with
never fewer than 14 verte-
bree,
No cheek-pouches.
No ischial callosities.
CATARRHINA
Narrow.
Downwards.
Forms a bony external auditory
meatus.
None.
Frontal usually meets the squa-
mosal, and the jugal does not
meet the parietal, being
hindered by the frontal and
alisphenoid.
Small.
Two premolars.
Tail not prehensile, sometimes
practically absent.
Usually present, except in Apes.
Present, except in Gorilla, Orang,
and Chimpanzee.
HAPALIDA7—CEBID. 787
PLATYRRHINA. CATARRHINA,
No sigmoid flexure in the colon A sigmoid flexure.
descendens.
Never more than a_ slight The cecum is conical; with a
narrowing at the end of the vermiform appendix in Apes.
cecum, which is usually bent
like a hook.
No hints of a ‘secondary dis- A “‘ secondary discoidal placenta”
coidal placenta.” (only hinted at in Anthropoid
Apes).
Family 1. Hapatip# (= Arctopithecini). Marmosets
The marmosets are the smallest monkeys, not much
larger than squirrels. They live in companies in the
Neotropical forests, especially in Brazil, and feed on insects
and fruit.
In addition to the general Platyrrhine characters, the
following are noteworthy.
Their dentition, 2737, is distinctive, for other Anthropoidea
2132
have 4 molars. The molars have three main tubercles
instead of the usual four. The pinna of the ear is very
hairy. The tail is long, bushy, and non-prehensile. The
pollex is long, but not opposable; all the digits have a
pointed claw except the short opposable hallux. The
cerebral hemispheres have few convolutions. The marmo-
sets often bear three young ones at a birth, whereas the
other monkeys usually bear but one. There are two
genera, Hapale and Midas.
Family 2. Cesip#. American Monkeys
The American monkeys occur throughout tropical
America, but are most at home in Brazil. In addition to
the general Platyrrhine characters, the following are note-
worthy. The tail is long except in Brachyurus, and is
often prehensile. The digits have nails, not claws; the
thumb if present is opposable. The pinne are more or
less naked. The dentition is characteristic, for there are
six back teeth ; the formula being 2133. All are uniparous.
788 MAMMALIA.
Examples.—The howling monkeys (AZycetes or Alouata), with
diverticula from the larynx and enormously dilated hyoid,
protected by the expanded mandibles; the sakis (P2thecia),
with very long non-prehensile tail ; the spider-monkey (A7ze/es),
with exceedingly prehensile tail and a thumbless hand; the
capuchins (Cedzs), often imported into Europe.
Family 3. CERCOPITHECIDE (=Cynomorph Catarrhina).
Old World Monkeys
The Old World monkeys are plantigrade quadrupeds,
and the snout or muzzle often justifies the term Cynomorph ,
or dog-like. Besides the general Catarrhine characters,
the following are noteworthy: The sternum is long and
narrow; there are 19-20 dorso-lumbar vertebre; the
foramen magnum is directed backwards; the arms are
shorter than the legs; the hairs of the arm are all directed
towards the hand ; the skin forms callosities, often brightly
coloured over the ischia; there are usually cheek-pouches ;
the caecum is conical and without a vermiform appendix.
In the sub-family Cercopithecinee there are cheek-pouches, the
stomach is simple, and the fore- and hind- limbs are almost equal.
Examples.—The African baboons (Cynocephalus or Papio), e.g. the
mandrill (C. mazmon), notable for the bright colours of the
face and hips in the adult males ; the macaques (JZacacus), all
1. Asiatic except the tailless Barbary ape (JZ. zuus) of N. Africa
and Gibraltar; the African Cercopithecus.
In the sub-family Semnopithecinze there are no cheek-pouches, the
stomach is sacculated in a complex fashion, and the hind-limbs are
longer than the fore-limbs.
Examples.—The sacred Indian apes (Semmnopithecus), the African
Colobus, and the proboscis monkey (Wasalzs) of Borneo.
Family 4, ANTHROPOMORPHIDZ or SIMUD#& (= Anthropo-
morph Catarrhina). Anthropoid Apes
This family includes the Gibbons (Ay/odates), the Orang
(Simia), the Chimpanzees (Anthropopithecus), and the Gorilla
(Gorilla). As they are most like man, they are called
Anthropoid.
Along with the general Catarrhine characters the following
are noteworthy: The sternum is short and broad; there
are 16-18 dorso-lumbar vertebre ; the arms are longer than
the legs; the hairs of the upper arm are directed down-
ANTHROPOMORPHIDA. 789
wards, those of the forearm upwards ; except the plantigrade
gibbons, the apes tend to walk on the edges of their feet ;
there are no cheek-pouches ; there are no ischial callosities
except in gibbons ; the ceecum has a vermiform appendix.
The Gibbons (/Zy/odaées) live in §.-E. Asia, especially in the Malayan
region. The largest attains a height of 3 ft. They walk erect with
Fic. 418.—Skull of Orang-Utan.
p., Parietal; /, frontal ; sg., squamosal ; 7., jugal ; 7z., maxilla.
the hands reaching the ground. The skull is not prolonged into a
vertical crest. There is an os centrale in the carpus. The hallux is
well developed. The only flat nails are those of pollex and hallux.
There are 13 ribs and 18 dorso-lumbar vertebre. ‘There are small
ischial callosities,—the only instance in Anthropoids. They are mainly
arboreal in their habits. They feed on fruits, leaves, shoots, eggs,
790 MAMMALIA.
young birds, spiders, and insects. Their voice is powerful, and one
species (the Siamang) has a laryngeal sac. As regards teeth, the
gibbons are most like man. Some authorities rank the gibbons in a
separate family apart from the three other Anthropoids.
The Orangs (.S¢za) live in swampy forests in Sumatra and Borneo.
The males measure over 4 ft. They walk on their knuckles and
on the outer edges of the feet. The skull is prolonged into a vertical
crest. There are but slight supra-orbital ridges. The canines are very
large. There are twelve ribs as in man, and sixteen dorso-lumbar
Fic. 419.—Skull of gorilla.
vertebree. The larynx is connected with two large sacs which unite
ventrally. They are arboreal in their habits, and make nests in the
branches. They are exclusively vegetarian. As regards the structure
of the brain, the Orangs are most like man.
The Gorillas (Gorz//a) live in Western Equatorial Africa. They are
larger than all other apes, and larger than man, though not over 54 ft.
in height. The arms reach to the middle of the lower leg, and the
animals walk with the backs of their closed hands and the flat soles ot
their feet on the ground. There are prominent supra-orbital ridges.
The canines of the males are very large. The cervical vertebra bear
very high neural spines, on which are inserted the muscles which
support the heavy skull. There are thirteen ribs, and seventeen
HOMINID.
dorso-lumbar vertebre. There is
no os centrale inthe carpus. They
live in families in the forest, and
feed on fruits. As regards size,
the gorillas are most like man.
The males are much larger than
the females.
The Chimpanzees (Anthropo-
pithecus) live in Western and
Central Equatorial Africa. They
do not exceed a height of 5 ft.
The arms reach a little below the
knee: They walk on the backs
of their closed hands and on their
soles or closed toes. The skull
has no high crests. The supra-
orbital ridges are distinct. The
canines are smaller than in gorilla
or orang. There is no centrale
in the carpus. There are vocal
sacs,
families in the forest, and are
chiefly arboreal, making nests in
trees. They seem to feed on
fruits. In the sigmoid curvature
of the vertebral column the chim-
panzees are most like man.
In connection with the an-
thropoid apes may be noticed
Lithecanthropos erectus, a new
genus erected by Dubois from the
top of a skull, some teeth, and a
femur found by him (fossil) in Java,
and believed to represent a fornt
intermediate between man and the -
Anthropoid apes.
Family 5. Hominip&
Genus Homo
The distinctiveness of man
from his nearest allies de-
pends on his power of build-
ing up ideas and of guiding
his conduct by ideals. But
there are some structural
peculiarities of interest.
The chimpanzees live in’
79%
Fic. 420.—Skeleton of male gorilla,
el., Clavicle; sc., tip of scapula; S.,
presternum; 4., humerus; ~, radius}
w., ulna; £2, ilium; C., coccyx; P.,
pubis; /s., ischium; #., femur; 4,
tibia; 7, fibula.
792 MAMMALIA,
Man alone, after his infancy is past, walks thoroughly
erect. Though his head is weighted by a heavy brain, it
does not droop forwards. With his upright attitude the
increased command of vocal mechanism is perhaps in part
connected. Man plants the soles of his feet flat on the
‘ground; the great toes are often longer, never shorter than
the others, and lie in a line with them; he has a better
theel than monkeys have. The arms are shorter than the
legs. ‘[here is no os centrale. There are 12 ribs and 17
-dorso-lumbar vertebree.
Compared with the anthropoid apes, man has a bigger
forehead, a less protrusive face, smaller cheek-bones and
supra-orbital ridges, no sagittal or occipital crests, pro-
jecting nasals, an early disappearance of the suture
between premaxilla and maxilla, a true chin (hinted at in
the Gibbon), more uniform teeth forming an uninterrupted
horseshoe-shaped series without conspicuous canines. The
‘body is very naked; the legs are relatively longer; the
thallux is practically non-opposable; there are no vocal
sacs ; there is at most a vestige of an os penis.
More important, however, is the fact that the weight of
‘the gorilla’s brain bears to that of the smallest brain of
an adult man the ratio of 2: 3, and to the largest human
brain the ratio of 1 : 3; in other words, a man may have
a brain three times as heavy as that of a gorilla. The brain
-of a healthy human adult never weighs less than 31 or 32
-oz.; the average human brain weighs 48 or 49 02z.;3:
the heaviest gorilla brain does not exceed 20 oz. “The
‘cranial capacity is never less than 55 cubic in. in any
normal human subject, while in the orang and chimpanzee
it is but 26 and 274 cubic in. respectively.”
But, as Owen allowed long since, there is an “all-pervad-
ing similitude of structure” between man and the anthro-
poid apes. As far as structure is concerned, there is much
less difference between man and the gorilla than there is
between the gorilla and the marmoset.
As regards the much-discussed question of a tail in man, it may be
noted that if we define a tail as ¢hat part of the body which contains
postsacral vertebra and sundry other parts of primitive caudal segments,
and which ts, moreover, completely surrounded by integument, then such
‘tails occur always in early embryos of man, and as abnormalities after
HOMINID. 793
birth. The abnormalities may be either altogether soft or they may
contain bone, but in no case adequately known is there any increase in
the number of vertebrae which normally fuse to form the terminal
portion of the human vertebral column, known as the coccyx.
The arguments by which Darwin: and others have sought
to show that man arose from an ancestral type common to
him and to the higher apes, are the same as those used to
substantiate the general doctrine of descent. The Descent
of Man is the expansion of a chapter in the Origin
of Species. The arguments may be briefly summarised :—
(1) Physiological. The bodily life of man is like that of
monkeys; men and monkeys are subject to similar
diseases ; various human traits of gesture, expression, etc.,
are paralleled among the “brutes”; ‘‘theromorphic”
monsters corroborate the alliance.
(2) Morphological. The structure of man is like that of
the anthropoid apes; none of his anatomical distinctions,
except that of a heavy brain, are momentous; there are
about eighty vestigial structures in his musculay, skeletal,
and other systems.
(3) Historical. Certainties in regard to remains of
primitive man are few, but his individual development reads
like a recapitulation of ancestral history.
To many, man seems too marvellous to have been natur-
ally evolved ; to others the evidence seems insufficient ; but
if the doctrine of descent is true for other organisms, it is
likely to be true for man also. ;
As to the antiquity of the human race, it is certain that
men lived in Europe in the later stages of the Ice Age, and
there are indications of human life in Pliocene times: No
fossil remains are known till the Pleistocene. But, as it is
certain that man could not have arisen from any of the
known anthropoid apes, and as it is likely that he arose
from an ancestral stock common to them and to him, it
seems justifiable to date the antiquity of the human race
not later than the time when the anthropoid apes are known
to have been established as a distinct. family. This takes
us back to Miocene ages.
If man was naturally evolved, the factors in the process
require elucidation, but in regard to these we can only
speculate. From what we know of men and monkeys, it
794 MAMMALIA.
seems likely that, in the struggles of primitive man, wits
were of more use than strength. When the habits of
walking erect, of using sticks and stones, of building
shelters, of living in families began—and they have begun
among monkeys—it is likely that wits would grow rapidly.
The prolonged infancy, characteristic of human offspring,
would help to evolve gentleness. But even more important
is the fact that among monkeys there are distinct societies.
Families combine for protection; the combination favours
the development of emotional and intellectual strength.
“Man did not make society ; society made man.”
Finally, it is plain that all repugnance to the doctrine of
descent as applied to man should disappear when we
clearly realise the great truism of evolution, that “there
is nothing in the end which was not also in the beginning.”
CHAPTER XXVII
GEOGRAPHICAL DISTRIBUTION OF ANIMALS.
‘
As similar animals tend to occur where the conditions of
life are similar, we are warranted in speaking of a pelagic
fauna, an adyssa/ fauna, a “ttoral fauna, and so on. Let us.
briefly consider this grouping of animals according to their
haunts.
Pelagic.—The pelagic fauna includes all the animals of
the open sea, both drifters (Plankton) and swimmers.
(Wekion). The physical conditions in’which they live are-
very favourable,—there is room for all, sunshine without
risk of drought, and an evener life throughout the day and!
throughout the year than is to be found elsewhere except in.
the abysses of the deep sea. Moreover, the minute pelagic
Algze afford an inexhaustible food-supply to the animals. It
is not surprising, therefore, to find that the open sea has.
been peopled from the earliest times of which the rocks give
us any life record.
The fauna is representative, exhibiting great variety of
types, from the minute Voctiluca which sets the waves
aflame in the short summer darkness, to the giants of
modern times—the whales. It includes a few genera of
Foraminifera, rich in species, most Radiolarians, Dino-
flagellata, many Infusorians, Meduse and Medusoids,
Siphonophora and Ctenophora, many “‘ worms,” a few Holo-
thurians, a legion of Crustaceans, a few Insects (Halo-
batide), such Molluscs as Pteropods, Heteropods, and
many of the Cephalopods, such Tunicates as Sa/pa and
Pyrosoma, many fishes, a few turtles and snakes, besides
some well-known birds and mammals. There are also
hosts of /avva/ forms which are pelagic for a time.
The fauna of the open sea is representative, but there are
796 GEOGRAPHICAL DISTRIBUTION OF ANIMALS.
few of the types which we can suppose to have lived there
always. It may be that forms like the minute water-fleas
have been there almost from the first, but most bear the
impress of lessons which the open sea could never have
taught them.
Pelagic animals tend to be delicate and translucent ;
many are phosphorescent. The number of species, differing
from one another within a relatively narrow range, is often
enormous; thus about 5000 species of Radiolarians are
known. The huge number of individuals, which fréquently
occur in great swarms, is equally characteristic. Perhaps
both facts indicate that the conditions of life are relatively
easy, as is also implied in the limitless food-supply afforded
by the unicellular Alge. The pelagic fauna is richest in
the colder seas.
Abyssal.—Through the researches of the Challenger and
similar expeditions, we know that there is practically no
depth-limit to the distribution of animal life, though the
population is denser at moderate depths than in the deepest
abysses, and though there is probably a thinly-peopled
zone between the light-limit and the greatest depths.
We know, too, that there are abyssal representatives of
most types from Protozoa to Fishes, and that the distribution
tends to be cosmopolitan, in correspondence with the
uniformity of the physical conditions.
The abyssal fauna includes some Foraminifera and
Radiolarians, many flinty sponges, some corals, sea-
anemones, and Alcyonarians, a few medusz, annelids and
other “worms” on the so-called red clay, representatives
of the five extant orders of Echinoderms, abundant Crusta-
ceans, representatives of most of the Mollusc types, and
peculiarly modified Fishes, some with small eyes, others
with large eyes, which probably catch the fitful gleams of
phosphorescence.
As to the physical conditions, the deep-sea world is in
darkness, for a photographic plate is not influenced below
250-500 fathoms; it is extremely cold, about the freezing-
point of fresh water, for the sun’s heat is virtually lost at
about 150 fathoms; the pressure is enormous—thus at
2500 fathoms it is about 24 tons per square inch; the cold
water in sinking from the polar regions brings down much
LITTORAL. 797
oxygen ; it is quite calm, for even the greatest storms are
relatively shallow in their influence; there are no plants
(except perhaps the resting phases of some Algz), for
typical vegetable life depends upon light, and not even
bacteria, otherwise almost omnipresent, are known to
flourish in the great depths. A strange, silent, cold, dark,
-plantless world! The animals feed upon one another and
upon the débris which sinks from above.
We do not clearly know when the colonising of the depths began,
but there is much to be said for the view that an abyssal fauna was, at
most, scanty before Cretaceous ages. But whensoever the peopling of
the abysses occurred, it must have been gradual. It is likely that most
of the pioneers migrated outwards and downwards from the shore
region (in a wide sense), following the drift of food ; it is possible that
others, e.g. some Crustaceans, sank from the surface of the open sea.
The boreal character of many deep-sea animals has been often remarked,
and it is plausible to suppose that there was a particularly abundant
colonisation in the Polar regions, and a gradual’ spreading towards the
Equator as the Poles became colder. - Perhaps the richness of the fauna
at the Equator may be thought of as in part due to the meeting of two.
great waves of life from the Poles.
The abyssal conditions of life tend to uniformity over vast
areas, just as in the open sea. But, on the whole, life must
always have been harder in the depths that on the surface.
The absence -of plants, for instance, involves a keener
struggle for existence among animals. Thus, although
many abyssal forms, ¢.g. sea-anemones, live a passive séden-
tary life, waiting for food to drop into their mouths, the:
majority are less easy-going. The deep sea has been a
sterner school of life than the surface.
Littoral.—At a very early date the shores were peopled,
and the fauna is very rich and representative. From the
strictly Littoral zone, exposed at low tide, with its acorn-
shells and periwinkles, limpets and cockles, to the Lam-
inarian zone (to 15 fathoms), with its sea-slugs and oysters,
where the great seaweeds wave listlessly amid an extremely
keen battle, to the Coralline zone (15-40 fathoms), with its.
carnivorous buckies, what variety and abundance, what
crowding and struggle ! ;
There are Infusorians and Foraminifera, horny, flinty, and’
calcareous Sponges, zoophytes and sea-anemones, many
“worms,” star-fishes and sea-urchins, crabs and shrimps,
798 GEOGRAPHICAL DISTRIBUTION OF ANIMALS.
acorn-shells on the rocks and sandhoppers among the jetsam,
a few insects about high-tide mark, sea-spiders clambering
on the seaweeds, abundant bivalves and gasteropods, sea-
squirts in their degeneracy, besides fishes, a few reptiles,
numerous shore birds, and an occasional mammal. The
shore fauna is thus very representative, rivalling in its range
that of the open sea, far exceeding that of the abysses.
The conditions of life on the shore are in some ways the
most stimulating in the world. It is the meeting-place of
air, water, and land. Vicissitudes are not exceptional, but
normal. Ebb and flow of tides, fresh-water floods and
desiccation under a hot sun, the alternation of day and night,
felt much more markedly than on the open sea, the endless
variations between gently lapping waves and _ blasting
breakers, the slow changes of subsidence or elevation,—
these are some of the vicissitudes to which shore animals
are exposed. The shore is rich in illustrations of keen
‘struggle for existence and of life-saving shifts or adaptations,
such as masking, protective coloration, surrender of parts,
and “death feigning.” We may think of it as a great school
where many of the primary lessons of life, such as moving
head foremost, were learnt.
Fresh water.—Perhaps the most striking fact in regard
to the animals which live in fresh water is their uniformity.
The number of individuals in a lake is often immense, but
the number of species is relatively small, the number of
types still smaller. In widely separated basins and in
different countries the same forms occur.
We may distinguish a littoral, a surface, and a deep-
water lacustrine fauna. The deep-water forms are chiefly
Rhizopods, Turbellarians, Nematodes, Leeches, Chzetopods,
Amphipods, Isopods, Entomostraca, a few Arachnids, some
insect larvee, and molluscs, and the general opinion is that
these are derivable from the shore fauna of the lake, which
includes similar forms, along with a few others, such as the
fresh-water sponge and Hydra. On the other hand, the
‘surface lacustrine fauna, consisting of water-fleas, Rotifers,
Infusorians, etc., widely and uniformly distributed, is said
not to be derivable from the shore fauna. In transparency,
in gregariousness, in nocturnal habit, and in other ways,
they present a marked analogy with the marine Plankton.
MINOR FAUNAS. 799
How are we to account for their origin and wide distri-
bution ?
1. To explain the uniformity, Darwin referred to the birds which
carry organisms from watershed to watershed, to the carrying power of
‘the wind, and to changes of land level which bring different river-beds
into communication. But this is not enough.
2. It seems very likely that some of the fresh-water forms have
migrated from the sea and seashore through brackish water to rivers
and lakes. As the possibility of making the transition depends on the
constitution of the animal, it is intelligible that similar forms should
succeed in different areas.
3. There seems much force in what Credner and Sollas emphasise,
that many lakes are dwindling relict-seas of ancient origin. Granted a
fairly uniform Pelagic fauna, ¢.g. before Cretaceous times, we can
understand that the conversion of land-locked seas into lakes would
imply a decimating elimination, and, as the conditions of elimination
would be much the same everywhere, the result would be uniformity
in the survivors.
Minor faunas.—(a) Of brackish water.—We are warranted in
‘speaking of a brackish-water fauna, because of its uniformity in widely-
‘separated regions. It does not seem to be a mere physiological
assemblage, varying in each locality, but rather a transition fauna of
ancient date, a relic of a littoral fauna once more uniform. The fact
is that the power to live in brackish water is not very common; it
runs in families.
(4) Cave fauna.—In America, thanks very largely to the labours of
Packard, about 100 cave animals are known; in Europe the number
is about 300, the increase being largely due to the occurrence of about
100 species of two genera of beetles in European caves. In the famous
Mammoth Cave of Kentucky, which has over 100 miles of passages,
with streams, pools, and dry ground, there are over 40 different species
of animals. The temperature is very equable, varying little more than
a degree throughout the year; it is, of course, dark ; and there are no
plants other than a few Fungi. Thus the conditions present some
analogy with those of the deep sea. The fauna is of much interest to
evolutionists, for we wonder how far the peculiarities of the cave-
animals, ¢.g. absence of coloration and frequent blindness, are due to
the cumulative effect of the environment and of disuse, or how far they
represent the survival of germinal variations, and the result of the
cessation of natural selection along certain lines, Have the seeing
animals found their way out, leaving only the blind sports, which crop
up even in daylight? or is the loss of eyes the result of disuse and
absence of stimulus? Or again, if it be granted that pigment is an
-organic constitutional necessity, ¢.g. a waste product, while coloration
is explicable as an adaptation wrought out in the course of natural
elimination, then the question arises, whether the cessation of natural
selection—a condition awkwardly called ‘‘panmixia”—which might
account for the disappearance of the coloration when there is no
premium set upon it, can also account for the loss of pzgmzent—that is, of
‘a character which was not acquired in the course of natural selection ?
800 GEOGRAPHICAL DISTRIBUTION OF ANIMALS.
(see Beddard’s Azdmal Coloration). Our only answer at present is
that there is need for experiment.
(c) Parasitic fauna.—It seems legitimate to rank together those
animals whose habitat is in or on other organisms, from which they
derive subsistence, without in most cases killing them quickly, if at all,
or, on the other hand, rendering them any service. Among ectopara-
sites there are such forms as fish-lice and many other Crustaceans,
numerous insects such as lice and fleas, and Arachnids such as mites.
Among endoparasites there are Sporozoa, some Mesozoa, many
Nematodes, most Trematodes, all the Cestodes, many Crustaceans,
insect larvee, and Arachnids,
The parasitic habit implies degeneration (varying according to the
degree of dependence), great nutritive security, prolific reproduction,
and enormous hazards in the fulfilment of the life history. :
Parasitic animals must be distinguished—(a) from epiphytic or epizoic
animals which live attached to plants or animals, but.are in no way
dependent upon them, e.g. acorn-shells on Norway lobster ; (4) from
commensals (p. 178), who live in some degree of partnership, but without
in any way preying upon one another, ¢.g. crab and sea-anemone ; and
(c) from symbions, who live in close partnership, or symbiosis (p. 119),
e.g. Radiolarians and Algee. But between these habits there are many
gradations, and from close association there is always an easy transition
to parasitism.
Terrestrial The colonising of dry land has doubtless
been a gradual process, as different types wandered inland
from the shore, or became able to survive the drying up
of fresh-water basins. The fauna includes some Pro-
tozoa, eg. Ameba terricola, which lives in moist earth,
some of the Planarians, Nematodes, Leeches, Cheetopods,
and other “ worms,” a few Crustaceans like the wood-lice
(Onzscus), many insects and Arachnids, a legion of slugs
and snails, most adult Amphibians, most Reptiles, many
Birds, and most Mammals. Among Vertebrates certain
fishes are of interest in having learned to gulp mouthfuls
of air at the surface of the water, to clamber on the roots
of the mangrove trees, or to lie dormant through seasons of
drought. But among Vertebrates, Amphibians were the
first successfully to make the transition from water to dry
land.
It is important to bear in mind that many a stock may, in the course
of its evolution, have passed through + variety of environments. Thus
the thoroughly aquatic Celaceans were probably derived from a land
stock common to them and to the Ungulates, and may have passed
through a fresh-water stage. Without going farther back, we have
here an illustration of the zigzag course of evolution.
AERIAL—EVOLUTION OF FAUNAS. 801
We cannot believe in any abrupt transition from the shore to derra
forma. It has been a slow ascent, slow as the origin of dry land
itself. Thus mud-inhabiting worms, dwellers in damp humus, bank-
frequenting animals, those which find a safe retreat in rottenness or
inside bolder forms, dot the path from the shore inland. Many have
lingered by the way, many have diverged into cul-de-sacs, many have
been content to keep within hearing of the sea’s lullaby, which soothed
them in their cradles.
Simroth, in his work on the origin of land animals, seeks to show
that hard skins, cross-striped. muscle, brains worthy of the name, red
blood, and so on, were acquired as the transition to terrestrial life
was effected. Let us take the last point by way of illustration. Iron
in some form seems essential to the making of hemoglobin, but iron
compounds are relatively scarce and not readily available in the sea ;
they are more abundant in fresh water, and yet more so as the land is
yeached. Therefore it is suggested that it was as littoral animals
forsook the shore for the land, va fresh-water paths, that iron, in some
form, entered into their composition, became part and parcel of them,
helped to form heemoglobin or some analogous pigment, and thus opened
the way to a higher and more vigorous life.
Aerial,.—The last region to be conquered was the air.
Insects were the first to possess it, but it was long before
they were followed. The flying-fishes vibrated their fins
above the foam as they leapt; the web-footed tree-frogs,
Draco volans with its skin spread out on elongated ribs, and
various lizards, began to swoop from branch to branch ;
some of the ancient Saurians flopped their leathery skin-
wings ; a few arboreal mammals essayed what the bats
perfected ; and the feverish birds flew aloft gladly.
Perhaps a keen struggle among insects, or such events as floods,
storms, and lava-flows would prompt to flight; perhaps it was the
eager males who led the way; perhaps the’ additional respiratory
efficiency, produced by the outgrowth of wings, gave these a new use.
Perhaps the high temperature of birds—an index to the intensity of
their metabolism—may have had to do with the development of those
most elaborate epidermic growths which we call feathers. But we must
still be resigned to a more or less ingenious ‘‘ perhaps.”
Evolution of faunas.—The problem of the evolution of
faunas is still beyond solution, but various possibilities may
be stated.
(a) According to Moseley, ‘“‘the fauna of the coast has not only
given origin to the terrestrial and fresh-water faunas, it has throughout
all time, since life originated, given additions to the Pelagic fauna in
return for having received from it its starting-point. It has also received
some of these Pelagic forms back again to assume a fresh littoral
51
802 GEOGRAPHICAL DISTRIBUTION OF ANIMALS.
existence. The terrestrial fauna has returned some forms to the shores,
such as certain shore birds, seals, and the polar bear; and some of
these, such as the whales and a small oceanic insect, Halobates, have
returned thence to Pelagic life.”
“The deep sea has probably been formed almost entirely from the
littoral, not in the most remote antiquity, but only after food, derived
from the débris of the littoral and terrestrial faunas and floras, became
abundant in deep water.”
‘*Tt was in the littoral region that all the primary branches of the
zoological family tree were formed ; all terrestrial and deep-sea forms
have passed through a littoral phase, and amongst the representatives
of the littoral fauna the recapitulative history, in the form of series of
larval conditions, is most completely retained.”
(6) According to Agassiz, Simroth, and others, if one may venture to
compress their views into a sentence, a littoral fauna was the original
one, whence have been derived, on the one hand, the Pelagic and
abyssal faunas; on the other hand, the~fresh-water and terrestrial
faunas.
(¢) According to Brooks, a Pelagic fauna was primitive, whence
have been derived the tenants of the shore and the inhabitants of the
deep sea. To the latter, however, a possibility of ascending again is
not denied.
(d) Sir John Murray has emphasised the importance of ‘‘ the mud-
line”—the lower boundary of the littoral area—as an important head-
quarters of animal life, and as the area from which the abysses were
peopled. The possibilities may be expressed in a diagram.
Fresh Water
Shore ay
Open Sea
OUTSTANDING FACTS. 803
MORE DETAILED PROBLEMS OF GEOGRAPHICAL
DisTRIBUTION
Leaving the general, and at present very obscure, problem
of the evolution of faunas, let us briefly notice some of the
more detailed questions of distribution. We shall content
ourselves with stating (1) a few of the outstanding facts ;
(2) the factors determining why some animals are here and
others there ; and (3) the usually recognised zoo-geographical
regions.
Outstanding facts.—(a) Widely separated countries may
' have an essentially similar fauna. Thus, there is much in
common between Britain and Northern Japan, and there
is so much agreement between the North European (Pale-
arctic) and the North American (Nearctic) fauna, that.many
unite the two regions in one (Holarctic).
_ (8) Closely adjacent countries may have quite different
faunas. Thus the Bahamas and Florida, Australia and
New Zealand, are peopled by very different animals. Two
little islands, Bali and Lombok, in the Malay Archipelago,
which are separated by ‘‘Wallace’s Line,” a strait only
fifteen miles wide at its narrowest part, differ from each
other in their birds and quadrupeds more widely than
Britain and Japan.
(c) Regions with very different faunas are in many cases
connected by transition areas. Thus a journey from the
North of. Canada to Brazil would show a fairly gradual
transition from an Arctic to a tropical fauna.
(d) At the same time there are regions whose fauna is
exceedingly distinctive and sharply defined. Thus the
Mammalian fauna of Australia is distinctively Marsupial,
and nowadays the American opossums and Canolestes are
the only Marsupials beyond the Australasian limits.
(e) Another striking fact is the “discontinuous distri-
bution ” of certain types, by which we mean that examples
of a type may occur in widely separated regions without
there being any representatives in the intermediate area.
The general explanation is that the type in question once
enjoyed a wide distribution, as the rock record shows, and
that the conditions favourable to survival have been found
in widely separated places. Thus of the genus Tapir
804. GEOGRAPHICAL DISTRIBUTION OF ANIMALS.
there are some four species in South and Central America,
while the only other species occurs in Malacca and Borneo.
Similarly the Camelide are represented by one genus in
the Old World and another in South America, and the
insectivorous Centetidz are represented by five genera in
Madagascar, and one in Cuba and Hayti.
The factors determining distribution.—There are six factors
which combine to determine the particular distribution of an animal.
‘These may be conveniently considered in pairs.
(a) Distribution is in part determined by the constitution of the
animal and by the physical conditions of the region, Thus snakes
diminish rapidly in numbers towards the poles, their constitution being
in most cases ill-adapted to withstand cold; thus crayfishes are absent
from districts where the fresh water does not contain sufficient lime salts
for their needs.
(4) Distribution is in part determined by the position of the animal’s
original home (which is often an unknown fact), and by the available
means of dispersal. Thus, so far as we know, the Old World has been
the exclusive home of the anthropoid apes, and there they have
remained ; thus bats, being able to fly, have a more cosmopolitan
‘distribution than most other mammals; thus amphibians, being unable
to withstand salt water, are absent from almost all oceanic islands.
(c) Distribution is in part determined by the actual changes (geo-
logical, climatic, etc.) which have affected different regions, and by
,‘ bionomic ” factors, z.¢. the relations between the animal in question
and other organisms, whether animals, plants, or man. Thus it is
plain that we cannot understand the fauna of Australia without knowing
the geological fact that part of this island was once connected with
the Oriental continent by a bridge of land across the Java Sea. The
AustraJasian mammalian fauna consists of survivals and descendants
of Mesozoic Marsupials which have been exterminated everywhere else,
except the American opossums and Cazolestes. The original Australian
mammals were saved, not by any virtue of their own, but by the earth-
change which insulated them. Similarly, it is the geologist who helps
us to understand the faunal diversity on the two sides of ‘‘ Wallace’s
line,” or the absence of amphibians, reptiles, and mammals from the
Canaries. That much will also depend on the animal’s power of
surviving the struggle for existence in different regions is too obvious
to require exposition. We need only think of the way in which man
has in a few years altered the distribution of many birds and mammals,
Bemis lms indeed reducing it to #z/, or increasing it with disastrous
results.
To sum up: the chief factors determining geographical
distribution are—(1) the constitution of the animal, (2) the
physical conditions of the region, (3) the position of the
original home, (4) the means of dispersal, (5) the historical
Z00-GEOGRAPHICAL REGIONS. 805
changes of the earth and its climate, and (6) the bionomic
relations.
Zoo-geographical regions.—I shall simply quote a para-
graph from Professor Heilprin’s work, Zhe Geographical
and Geological Distribution of Animals (Internat. Sci.
Series. London, 1887), a very valuable book for the
student, especially as it considers distribution in space
and time together.
“By most naturalists (Wallace, Sclater, and others) the
terrestrial pottion of the earth’s surface is recognised as
consisting of six primary zoological regions, which corre-
spond in considerable part with the continental masses of
2 lig ag These six regions are—
. The Palearctic, which comprises Europe, temperate
Rais: (with Japan), and Africa north of the Atlas Moun-
tains; also Iceland, and the numerous oceanic islands of
the North Atlantic ;
“2, The £thiopian, embracing all of Africa south of the
Atlas Mountains, the southern portion of the Arabian Pen-
insula, Madagascar, and the Mascarene Islands, and which,
consequently, nearly coincides with the Africa of geo-
graphers ;
“3. The Oriental or Indian, which embraces India south
of the Himalayas, Farther India, Southern China, Sumatra,
Java, Bali, Borneo, and the Philippines ;
“4. The Austrazan, comprising the continent of Aus-
tralia, with Papua or New Guinea, Celebes, Lombok, and
the numerous islands of the Pacific ;
“eo. The Vearetic, which embraces Greenland, and the
greater portion of the continent of North America (excluding
Mexico) ;
“6, The Meotropical, corresponding to the continent of
South America, with Central America, the West Indies, and
the greater portion of Mexico.”
Professor Heilprin makes several modifications on this
scheme of distribution: (@) uniting Palearctic and Nearctic
in one Holarctic realm; (4) establishing a special Poly-
nesian realm for the scattered island groups of the Pacific;
and (¢) defining three transition regions—(1) around the
Mediterranean, intermediate between Palearctic, Ethiopian,
and Oriental, (2) Lower California between Western Hol-
806 GEOGRAPHICAL DISTRIBUTION OF ANIMALS.
arctic and Neotropical, and (3) the Austro-Malaysian Islands
lying to the east of Bali and Borneo, inclusive of the
Solomon Islands, a region intermediate between Oriental,
Australian, and Polynesian. It seems also convenient to
recognise two Polar regions,—Arctic and Antarctic.
It may be useful to map out the divisions as follows :—
ARCTIC
NEARCTIC. PALAARCTIC.
aes wt
| Holarctic. |
Transition Transition
to: to—ORIENTAL.
Transition to Polynesian
and to
Polynesian—NEOTROPICAL. ETHIOPIAN. AUSTRALIAN.
ANTARCTIC
Many authorities use the following arrangement :—
NotToG#a OR SOUTHERN WORLD
1. Australian Region, including three sub-regions,—New Zealand,
Australian, and Papuasian or Austro-Malayan.
u1. Neotropical Region, including two sub-regions, —South American
and Antillean or West Indian.
ARCTOGA OR NORTHERN WORLD
1. Periarctic or Holarctic Region, including two sub-regions,—
Palearctic (Eurasian and Mediterranean) and Nearctic
(Canadian and Sonoran).
. Paleotropical Region, including two sub-regions,— African
(Ethiopian and Malagasy) and Oriental (Indian and
Malayan).
1
oI
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CHAPTER 2A VII
THEORY OF EVOLUTION
In Chapter VI. we indicated the nature of the evidence
which has led naturalists to accept the doctrine of descent
as a modal interpretation of organic nature. The data of
physiology and morphology, combined with what is known
of the history of the race and the development of the
individual, have led us to believe that the forms of life now
around us are descended from simpler ancestors (except in
cases of degeneration), and these from still simpler, and so
on, back to the mist of life’s beginnings. In other words,
we believe that the present is the child of the past and the
parent of the future. This is the general idea of evolution.
But while this general idea, which is a very grand one, is
usually recognised as the simplest interpretation of the
facts, we remain in doubt as to the factors of the process by
which the world of life has come to be what it is. This
uncertainty is in part due to the complexity of the problem,
in part to the relative novelty of the inquiry—for precise
etiology is not yet fifty years old—in part also to the fact
that, while there has been much theorising, there has been
comparatively little experimenting or connected observation
as to the modes and causes of evolution.
With the exception of Dr. Alfred Russel Wallace and a
few others, who believe that it is necessary to postulate
spiritual influxes to account for certain obscure beginnings,
e.g. of the higher human qualities, evolutionists are agreed
in seeking to explain the evolution of plants and animals
as a continuous “natural” ‘process, the end of which was
implicit in the beginning. In so doing, they follow the
method of analysis, endeavouring to explain the facts in
their lowest terms. But as the biologist’s lowest term is
808 THEORY OF EVOLUTION.
living matter, and as one aspect of this is, in favourable
conditions, known as thought, there is no reason to call
the evolutionist’s analysis ‘ materialistic” —if anything
opprobrious be meant by that adjective. The common
denominator of the biologist is as inexpressibly marvellous
as the philosopher’s greatest common measure—if, indeed,
the two are not practically the same.
Two great problems.—Our uncertainty in regard to the
factors of evolution is so great that I cannot venture here
to do more than indicate (2) what the great problems are,
and (4) the general drift of the most important suggestions
which have been made towards their solution.
The two great problems before the evolutionist are :—
(1) What is the nature and origin of variations, ze. of
those organic changes which make an organism
appreciably different from its parents or its
species ?
(2) What are the directive factors which may operate
upon given variations, determining their elimina:
tion or their persistence, and helping towards the
familiar but puzzling result—the existence of
distinct and relatively well-adapted species?
Secure answers to these two questions must be found in
reference to the present; as our data accumulate, it will
be more possible to argue back to the past.
It may be convenient to speak of the factors which cause
variation as przmary or originative, and of the factors which
operate upon or direct the course of variation as secondary
or directive. As far as practical results are concerned, the
two sets of factors are of equal importance.
Nature of variations.—We mean by variations those
changes in organisms which make them appreciably
different from their parents or from their species.
The term of course includes not only material differ-
ences, but also those whose only demonstrable expression
is psychical. Thus an increase in maternal affection is as
important and real a variation as the sharpening of a
canine tooth.
It may also be useful to distinguish variations in size,
symmetry, number of appendages, and so on, from more
qualitative variations in chemical composition, such as the
CHANGES DUE TO ENVIRONMENT. 809
appearance of a new pigment; but this distinction is only
a matter of convenience, as it is only a matter of degree.
Again, variations occur which may be called continuous,
being merely minute increments or diminutions of certain
parental or specific characters. These are related to one
another much in the same way as are the successive stages
in the continuous growth of an individual.
But other variations occur which deserve. to be called
discontinuous. For, without the appearance of transitional
stages, marked variations crop up, reaching with apparent
suddenness to what must be called mew, and. may withal
exhibit a measure of perfectness.
That both kinds of variations occur is a fact of life; .
the possibility of both is probably a primary quality of
organisms ; but we are only beginning to know the relative
frequency of the two kinds and their respective limits (see
Bateson’s Materials for the Study of Variation, 1894, and
De Vries’s Species and Varieties, 1905).
Primary or originative factors,—What causes variation ?
This is the fundamental question, but it is the least
answerable.
It is, indeed, an axiom or a truism that changes in any
animate system are evoked by changes in the larger system
of which the organism forms a part. In other words, the
stimulus to organic change must always be ultimately
traceable to the environment; but this is implied in our
conception of living matter, and does not help us to under-
stand the immediate conditions which lead to the change.
In the absence of sufficiently precise data, we can do
little more than point out various possibilities :—
(2) Changes due to Environment ( = Environmental
Modifications)
There is abundant proof that changes in surrounding
pressure, in the chemical composition of the medium, in
food-supply, in heat, light, etc., may be followed by changes
in the organism upon which these influences play. Changes
in the body of the organism follow changes in the environ-
ment. But (1) it is difficult to discriminate between
changes which may be spoken of as the direct results of
environmental influence, and those to which the organism
810 THEORY OF EVOLUTION.
was already definitely predisposed, and to which the
environmental change supplied only the stimulus. (2) We
have not at present sufficient data to enable us to state that
changes arising in or acquired by the Jody of an individual
organism as the result of surrounding change do as such
in any degree specifically affect the reproductive cells. In
other words, we cannot at present say that “ environmental
modifications” are transmissible. And if they are not,
their importance in evolution is only indirect.
(4) Changes due to Function (= Functional Modifications)
It is an undoubted fact that the bodily structure of an
animal may be changed by the increased use of certain
parts, or the disuse of others,—in short, by some change of
function, which may be directly prompted by some change
in the external conditions of life. But important as these
functional changes and their results are to the ¢vdividual,
we are uncertain as to their importance for the race, for
we do not know to what extent (if any) the results are
transmissible.
(c) Variations due to Changes in the Germ Cells
In many cases of variation, particularly those which
appear in early life, it is not possible to suggest any
environmental or functional condition which may be
regarded as the stimulus or the cause. We are led in
such cases to believe that the variation in bodily structure
or habit is the expression of some novelty in the proto-
plasmic constitution of the germ cells. Then, hiding our
ignorance, we say that the variation is germinal, con-
stitutional, congenital, or blastogenic. It seems to lead
to clearness if we call these germinal changes and their
results variations, keeping the term modifications for those
changes [(2) and (4)] wrought upon the body as the result
of environmental or functional influences.
But why should there be changes in the germ cells?
Perhaps because living matter is very complex and un-
stable, and because it is of its very nature to differentiate
and integrate and grow; perhaps because the immediate
environment of the germ cells (blood, body cavity fluid,
sea-water, etc.) is complex and variable. Moreover, every
SECONDARY OR DIRECTIVE FACTORS. 8rr
multicellular organism, reproduced in the usual way, arises
from an egg-cell fertilised by a spermatozoon, and the
changes involved in and preparatory to this fertilisation,
or “amphimixis,” may make new permutations and com-
binations of living substances or vital qualities not only
possible but necessary.
Secondary or directive factors.—1. Matural Selection.—
The distinctive contribution which Charles Darwin and
Alfred Russel Wallace made to etiology was their theory
of Natural Selection.
By natural selection is meant that process whereby, in
the ordinary course of nature, certain organisms, e.g. certain
members of the same species, are more or less rapidly
and discriminately eliminated, while others are allowed to
survive.
That some forms, ¢g. in one family, should succeed
less well than others, depends obviously on the fact that
all are not born alike,—depends, in other words, on the
fact of variation.
That there should be elimination is necessary—(a) because
a pair of animals usually produce many more than a pair,
and the population tends to outrun the means of subsist-
ence; and (4) because organisms are at the best only
relatively well adapted to their conditions of life, which are
variable. These two primary facts and their subsequent
consequences, ¢g. that some animals feed upon others,
that there may be more males than females, etc., render
some struggle for existence necessary, though this phrase
must be used, as Darwin said, “in a wide and metaphorical
sense,” including all endeavours for the well-being, not
only of the individual, but of its offspring.
The facts then are—that variations constantly occur, that
some members of a species or family are necessarily less
fitly adapted than others, and that the course of nature is
such that these .relatively less fit forms will tend to be
eliminated, while the relatively more fit will tend to survive,
As many variations reappear generation after generation,
and may become gradually increased in amount, the con-
tinuance of the selective or eliminating process will work
towards the origin of new adaptations and new species.
The importance of natural selection as a secondary
812 THEORY OF EVOLUTION.
factor in evolution will vary according to stringency of the
eliminating process, and it must be noted that the ‘struggle
for existence” varies in intensity within wide limits, that
it requires to be investigated for each case, and cannot be
postulated as a force of nature.
The importance of the factor will also depend on the
number, nature, and limits of the variations which occur.
Thus a new species might arise, either by the occurrence of
a discontinuous variation of considerable magnitude, or by
the eliminating process acting for many generations on a
‘series of minute continuous variations.
Darwin also believed in the importance of sexual selection,
in which the females choose the more attractive males, which,
‘succeeding in reproduction better than their neighbours,
tend to transmit their qualities to their numerous male
heirs. But this and other forms of reproductive selection
es be regarded as special cases of natural selection.
2. “ Zsolation.” —Under this title, Romanes, Gulick, and
others include the various ways in which free intercrossing
is prevented between members of a species, eg. by
geographical separation, or by a reproductive variation
‘causing mutual sterility between two sections of a species
living on a common area. Without some “isolation”
tending to limit the range of miutual fertility within a
species, or bringing similar variations to breed together, a
new variation is liable, they say, to be “swamped” by
intercrossing. But definite facts as to this “swamping,”
and in many cases as to the alleged “isolation,” are hard to
find, nor can we say that a strong variation will not persist
unless it be ‘‘isolated.” In fact, much evidence has been
gathered in recent years which shows that certain kinds of
variations are very strongly heritable and do anything. but
“blend.” Romanes’ view, however, was that “without
isolation, or the prevention of free intercrossing, organic
evolution is in no case possible. Isolation has been the
universal condition of modification. Heredity and varia-
bility being given, the whole theory of organic evolution
becomes a theory of the causes and conditions which lead
to isolation.” It must be admitted that some forms of
isolation lead to inbreeding, and this to “ prepotency,”
which often implies the persistence of individual variations.
Primary (Originative) Factors.
SUMMARY OF EVOLUTION THEORIES.
SUMMARY OF EVOLUTION THEORIES
(Axiom or Truism.)
Changes are all ultimately due to the External Influences
and the Nature of the Organism, z.e. of Protoplasm.
813
( Environment.)
Changes in the
environment are
followed by changes
in the organism,
either — | or (4) in
(2) in its | its germ
body, cells,
or (c) in (4) through
(2) (?).
(Result of (a) * En-
- wtronmental Modi-
Jications.”*)
Degree of trans-
missibility unknown.
Such environ-
mental modifications,
IF transmissible, and
if the originating con-
ditions persist for
some time, might
perhaps give rise to
new species, especi-
ally if favoured by
natural selection and
isolation, In the
individual lifetime
Secondary (Directive) Factors.
they may serve to
shield the incipient
stages of vardations
in a similar direction.)
(Organism.)
Germinal varia-
tions arising from
the nature of pro-
toplasm, or from
changes in the
nutritive environ-
ment of the germ
cells, or from the
changes necessarily
associated with fer-
tilisation, may. be
continuous or dis-
continuous, quanti-
tative or qualitat-
ive, etc.
( Variations.)
Such variations
probably supply the
usual material for
the origin of new
species, for the
establishment of
which, more or less
natural selection
(elimination) and
isolation must be
necessary, according
to the nature of the
variation.
(Function. )
Use and disuse of
parts, or change of
‘function (due to
change of environ-
ment or to ger-
minal change), are
followed by changes
in—(q@) the body of
the organism, or (4)
in the gerin cells,
either directly or (?)
through (a).
(Result of (a) ‘“Func-
tional Modifica-
tions.’’)
sear eeneeneeee See eeeneeeeee
Degree Of transmis-
sibility unknown.
eee eee errr ere tee
Such functional
modifications, IF
transmissible, and
if the originating
conditions persist
for some time, might
perhaps give rise to
new species, espe-
cially if favoured
by natural selection
and isolation. In
the individual life-
time they may serve
to shield the in-
cipient stages of
variations in a
similar direction.
‘saBueyD Jo USC
“sotoads jo UISIIQ
APPENDIX
SOME ZOOLOGICAL BOOKS
INTRODUCTORY :—
F. Jeffrey Bell, ‘‘ Comparative Anatomy and Physiology.”
C. Lloyd Morgan, ** Animal Biology.”
T. Jeffery Parker, ‘‘ Elementary Biology.”
T. J. Parker and W. A. Haswell, ‘‘ A Manual of Zoology.”
J. Arthur Thomson, ‘‘ The Study of Animal Life.”
J. G. Needham, ‘‘ General Biology.”
O. Latter, ‘‘Natural History of Common Animals.”
R. Lulham, ‘‘ Introduction to Zoology.”
M. I. Newbigin, ‘‘ Life by the Seashore.”
F. W. Gamble, ‘‘ Animal Life.”
J. Graham Kerr, fe Zoology.”
Huxley’s ‘‘ Crayfish.”
Milne Marshall’s ‘‘ Frog.”
TextT-Books OF ZOOLOGY :—
T.. HH. Huxley, ‘‘ Anatomy of Invertebrates” (1877), and
‘¢ Anatomy of Vertebrated Animals ” (1871).
T. J. Parker and W. A. Haswell, ‘‘ Text-Book of Zoology”
(2 vols., Lond. 1898).
Hatchett Jackson’ s edition of Rolleston’s ‘Forms of Animal Life”
(Oxford, 1888).
A. Sedgwick, ‘‘ Student’s Text-Book of Zoology,” Part I. (Lond.
1898), Part II. (1905), Part III. (1909).
Shipley and MacBride, ‘‘ Zoology, an Elementary Text-Book.”
G. C. Bourne, ‘An Introduction to the Study of the Compara-
tive Anatomy of Animals.”
J. Ritzema Bos, “‘ Agricultural Zoology ” (translated by Davis,
Lond. 1894).
Borradaile, ‘‘Text-Book of Zoology.”
Books as GUIDES TO PRACTICAL WoRK :—
C. Vogt and E. Yung, ‘‘Traité d’Anatomie Comparée pratique”
(Paris, 1885-95) ; also in German.
T. J. Parker, ‘*Zootomy ” (Lond. 1884).
T. J. and W. N. Parker, ‘‘ Practical Zoology.”
816
« APPENDIX.
Huxley and Martin, ‘‘ Practical Biology.”
A. Milnes Marshall and C. H. Hurst, ‘‘ Practical Zoology.”
W. K. Brooks, ‘‘ Handbook of Invertebrate Zoology for
Laboratories and Seaside Work ” (Boston, 1882).
Monographs on Sea-urchins, Lob-worm, Limpet, Ascidian, etc.,
published by Liverpool Biological Committee (Williams
& Norgate, London).
A. Bolles Lee, “‘ Microtomist’s Vade-Mecum ” (4th ed., 1896).
M. F. Guyer, ‘‘ Animal Micrology” (Chicago, 1909).
G. B. Howes, ‘‘ Atlas of Practical Elementary Biology” (Lond.
1885).
GENERAL MORPHOLOGY :—
Ernst Haeckel, ‘‘ Generelle Morphologie ” (Berlin, 1866), re-
printed in condensed form, 1906.
Herbert Spencer, ‘‘ Principles of Biology” (London, 1864-66).
.W. His, ‘‘ Unsere K6rperform ” (1875).
G. Jaeger, ‘‘ Allgemeine Zoologie ” (1878).
P. Geddes, article ‘‘ Morphology ” (‘* Encycl. Brit.”),
CLASSIFICATION, see :—
E. Ray Lankester, article ‘‘ Zoology ” (‘‘Encycl. Brit.’).
W. A. Herdman, ‘‘ Phylogenetic Classification of Animals.”
H. Gadow, ‘‘ Classification of Vertebrata ” (1898).
Articles in last edition of ‘‘ Encyclopzedia Britannica.”
Works ON COMPARATIVE ANATOMY :—
Richard Owen, ‘‘ Comparative Anatomy of Vertebrate Animals *
(4th ed., 1871).
. H. Huxley, of. ct.
. Gegenbaur, ‘‘ Elements of Comparative Anatomy” (trans. by
F, Jeffrey Bell, Lond. 1878. New edition in German, 1898).
Lang, ‘‘ Text-Book of Comparative Anatomy” (trans. by
H. M. and M. Bernard). New German edition, 1900 e¢ seg.
R. Wiedersheim, ‘‘ Comparative Anatomy of Vertebrata” (trans.
by W. N. Parker, Lond. 1886 ; new ed., 1907).
Oppel, ‘‘ Vergleichende mikroskopische Anatomie” (1896-1905).
Reynolds, ‘‘ The Vertebrate Skeleton ” (1897).
Schimkewitsch, ‘‘ Vergleichende Anatomie der Wirbeltiere ”
(1909).
Biitschli, ‘* Vorlesungen itber vergleichende Anatomie ” (3 vols.).
> On
Works ON COMPARATIVE PHYSIOLOGY :—
Claude Bernard, ‘‘ Phénoménes de la Vie Commune aux
Animaux et aux Végétaux ” (1878).
Paul Bert, ‘‘ Lécons sur la Physiologie comparée de la Respira-
tion ” (1870),
SOME ZOOLOGICAL BOOKS. 817
C. F. W. Krukenberg, ‘“ Vergleichend-Physiologische Studien ”
and ‘‘ Vortrige ” (1881-89).
F. Je Bell, ‘‘ Comparative Anatomy and Physiology” (Lond,
1887). .
A. B. Griffiths, ‘‘ Comparative Physiology ” (1891).
Halliburton, ‘‘ Physiological Chemistry ’’ (1891).
Bunge, “‘ Physiological and Pathological Chemistry ” (translated
1890).
M. I. Newbigin, ‘‘ Colour in Nature ” (Lond. 1898).
O. von Fiirth, ‘‘ Vergleichende chemische Physiologie der
niederen Tiere ” (1903).
C. S. Sherrington, ‘‘ Integrative Action of the Nervous System ”
(1906).
M. Verworn, ‘‘ General Physiology.”
Geddes and Thomson, ‘‘ Evolution of Sex.”
M. Hartog, ‘‘ Problems of Life and Reproduction.”
EMBRYOLOGY :—
F. Mz a ‘* Comparative Embryology ” (2 vols., Lond.
1880-81).
M. Foster aN F. M. Balfour, revised by A. Sedgwick and W.
Heape, ‘‘ Elements of Embryology ” (Lond. 1883).
A. C. Haddon, ‘Introduction to the Study of Embryology”
(Lond. 1887).
O. Hertwig, ‘‘Lebrbuch der Entwicklungsgeschichte des
Menschen und der Wirbelthiere” (translated by E. L. Mark,
3rd ed., 1893.)
E. Korschelt and K. Heider, ‘‘ Lehrbuch der Vergleichenden
Entwicklungsgeschichte der Wirbellosen Thiere ” (Jena,
1890-93 ; translated).
Roule, ‘‘Embryologie Générale” (Paris, 1892), and ‘‘ Embryo-
logie Comparée” (Paris, 1894).
. Milnes Marshall, ‘‘ Vertebrate Embryology ” (1893).
. S. Minot, ‘‘ Human Embryology ” (1892).
. B. Wilson, ‘‘The Cell in Development and in Inheritance ”
1900).
R Lilley, ‘«Embryology of the Chick.”
. Hertwig, “ Allgemeine Biologie ” (1906).
Jenkinson’s ‘‘ Experimental Embryology.”
Om Hop
PAL/ONTOLOGY :—
H. A. Nicholson and R. Lydekker, ‘‘ Manual of Palzontology
(2 vols., Lond. and Edin. 1889).
K. A. von Zittel, ‘‘ Handbuch der Palzontologie” (completed
3893). Translated. miaie ;
A. Smith Woodward, ‘‘ Vertebrate Palzeontology ” (1898).
M. Neumayr, ‘‘ Die Stamme des Thierreichs” (vol. i., Wien und
Prag, 1889). . pat
Gaudry, ‘‘ Les Enchainements du Monde Animal” (4889-90).
52
818 APPENDIX.
Carus Sterne (Ernst Krause), ‘‘ Werden und Vergehen” (4th ed.,
Berlin, 1905).
C. Depéret, ‘‘ Les Transformations du Monde animale” (1907 ;
translated).
GEOGRAPHICAL DISTRIBUTION :—
A. R. Wallace, ‘‘ Geographical Distribution” (2 vols., Lond.
1876).
A. Heilprin, ‘‘The Geographical and Geological Distribution of
Animals ” (Lond. 1887).
R. Lydekker, ‘‘ Geographical Distribution of Mammals” (Cam-
bridge).
F. E. Beddard, ‘‘ Geographical Distribution ” (Oxford).
Trouessart, ‘‘ La Geographie Zoologique ” (Paris, 1890).
W. Marshall, in Berghaus’ ‘‘ Physikal Atlas” (Leipzig, 1887).
Books OF NATURALIST TRAVELLERS, ¢.g. :—
Charles Darwin, ‘‘ Voyage of the Beagle” (London, 1844; new
ed., 1890).
H. 5 a **Naturalist on the Amazons” (new ed., Lond.
1892).
T. Belt, ‘‘ Naturalist in Nicaragua” (2nd ed., 1888).
A. R. Wallace, ‘‘ Malay Archipelago” (1869), ‘‘ Tropical Nature ”
(1878), ‘Island Life” (1880).
Wyville Thomson, ‘The Depths of the Sea” (1873), ‘‘ Voyage
of the Challenger” (1885).
H. nN Moseley, ‘‘ Naturalist on the Challenger” (1879, new ed.,
1892).
W. H. Hudson, ‘ Naturalist in La Plata.”
A. E. Brehm, “ From North Pole to Equator” (translation,
edited by J. Arthur Thomson, with bibliography, 1895).
S. J. Hickson, ‘‘ Naturalist in the Celebes.”
A. Alcock, ‘* Naturalist in the Indian Ocean.”
Sir John Murray and Dr. J. Hjort, ‘‘The Deep Sea.”
COMPARATIVE PsyCHOLOGY :—
G. J. Romanes, ‘‘ Animal Intelligence” and ‘‘ Mental Evolution
of Animals.”
C. Lloyd Morgan, ‘‘ Animal Behaviour”; ‘‘ Introduction to
Comparative Psychology” (London, 1894); ‘‘ Habit and
Instinct” (Lond. 1897).
Jennings, ‘‘ Behaviour of the Lower Organisms.”
GENERAL NATURAL History :—
Brehm’s ‘‘ Thierleben”’ (10 vols., Leipzig und Wien).
Cassell’s ‘‘ Natural History ” (edited by P. Martin Duncan, 6
vols,, 1882),
SOME ZOOLOGICAL BOOKS. 819
“‘Standard or Riverside Natural History” (edited by J. S.
Kingsley, 6 vols., 1888).
“*Royal Natural History ” (edited by R. Lydekker, 6 vols.).
Hesse and Doflein, ‘‘ Tierbau und Tierleben ”' (2 vols.).
Hilzheimer, ‘‘ Handbuch der Biologie der Wirbeltiere.”
Nusbaum, Karsten and Weber, ‘‘ Lehrbuch der Biologie,”
W. % Pycraft and others, ‘‘ History of Birds, Reptiles, Fishes ”
2 vols.).
Books on EvoLuTION :—
Charles Darwin, ‘‘ Origin of Species” (1859, etc.).
Alfred Russel Wallace, ‘‘ Darwinism ” (1889).
Herbert Spencer, ‘‘ Principles of Biology” (1866; new ed.,
vol, i., 1898).
Ernst Haeckel, ‘‘ Generelle Morphologie ” (1866).
For more recent books, see J. Arthur Thomson, ‘‘ Study of Animal
Life.” To the list there given must be added Weismann’s ‘‘Germ-
Plasm ” (1893), Bateson’s ‘‘ Materials for the Study of Variation”
(1894), Delage’s ‘‘ L’Hérédité” (1895), Weismann’s ‘‘ Evolution
Theory ” (2 vols., Lond. 1905), De Vries’s ‘‘Species and Varieties”
(1905), T. H. Morgan’s ‘‘ Experimental Zoology” (1906), Thotnson’s
‘© Heredity (1908), Bateson’s ‘‘ Mendel’s Principles of Heredity ”
(1909), Thomson’s ‘‘ Darwinism and Human Life” (1910), Bateson’s
“‘ Problems of Genetics,” Thomson and Geddes’s ‘‘ Evolution,”
Dendy’s ‘‘ Outlines of Evolutionary Biology.”
GENERAL WorRKS OF REFERENCE :—
‘Treatise on Zoology,” by E, Ray Lankester and others (several
volumes published).
W. Hatchett Jackson’s edition of Rolleston’s ‘‘ Forms of Animal
Life” (Oxford, 1888). A very valuable work, with special
bibliographies.
Leunis, ‘‘ Synopsis des Thierreichs” (re-edited by Ludwig,
Hanover, 1886).
Bronn, ‘‘ Klassen und Ordnungen des Thierreichs.”
E. Ray Lankester and others, ‘‘ Zoological Articles reprinted
from ‘Encycl. Brit.’ ” (Lond. 1891).
Yves Delage and others, ‘‘Traité de Zoologie Concréte ” (Paris,
many vols.). *
Shipley and Harmer, ‘‘Cambridge Natural History” (10 vols.),
MONOGRAPHIC SERIES, ¢@.g. :—
“Reports of the Voyage of H.M.S. Challenger.”
“ Fauna und Flora des Golfes von Neapel.”
‘Catalogues of the British Museum—Natural History.”
** Ray Society Publications.”
820 APPENDIX.
RECORDS OF RESEARCH, ¢.2.—
Journal of Royal Microscopical Society (6 parts in the year).
Zoologisches Jahresbericht (Naples ; yearly).
Anatomische Ergebnisse (Merkel & Bonnet ; yearly).
Zoologisches Zentralblatt (monthly).
HisTorRY OF ZOOLOGY :—
W. Whewell, ‘‘ History of Inductive Sciences” (1840).
J. V. Carus, ‘‘ Geschichte der Zoologie ” (1872).
E. Perrier, ‘‘ La Philosophie Zoologique avant Darwin” (1884).
E. Haeckel, ‘‘ Natural History of Creation ” (trans., Lond. 1870).
E. Ray Lankester, article ‘‘ Zoology ” (‘‘ Encycl. Brit.”).
H. A. Nicholson, ‘‘ Natural History: Its Rise and Progress in
Britain ” (1888).
E. Krause, ‘‘ Die Allgemeine Weltanschauungen ” (1889).
H. F. Osborn, ‘‘ From the Greeks to Darwin ” (1894).
J. Arthur Thomson, ‘‘ The Science of Life” (1899).
J. Arthur Thomson, ‘‘ The Progress of Science” (1902).
Locy, ‘* Biology and its Makers.”
INDEX
AARD-VARK. . .
Aard-wolf . 3 3
Abdominal pores .
a8 ribs «
Abducens nerve
Abomasum . ‘
Absorption . .
Abyssal fauna.
Acanthias
A annnthaLaalt.
DALALLIUVUTL « . .
Acarina . ‘ 3
Acephala .
Acetabulum .
Achromatin .
Acineta
Acinetaria
Acipenser
Accela . q
Acontia ; :
Acorn-shell . P :
Acquired characters.
Acrania= i aaa
Acraspeda
Acrodont teeth
Acromion .
Actinia. o
Actiniaria.
Actinomma .
ee ew ee we
Actinophrys.
Actinosphzerium
Actinotrocha (larva).
Actinozoa. . ‘
Adambulacrals. ,
504,
490,
73)
—+>—
PAGE
751 | Adamsia . . .
707 | Adder . f .
543 | Adhesive cells .
620 | Adrenal body of rabbit .
540 | Adrenalin -
758 | Aginopsis 3 .
28 | Ailuroidea . . ‘
796 | Aflurus : 5
567 | A°pyornis i é
244 | Aérial fauna. 3 é
200 | Aithalium ‘i z
568 | Agamidze a s
372 | Aglossa ‘ é
425 | Agouti. 7
706 | Air-bladder of Fishes ‘
46 | Air-sacs of Birds .
Ill 3» 99 Of Lizards
111 | Albumen gland of snail.
570 $5 of bird’s egg .
182 3 of frog’s egg .
162 | Alces . F : ‘
305 | Alcyonacea .
809 | Alcyonaria < F
459 5 and Zoantharia
173 | Alcyonidium. .
625 | Alcyonium . ‘
jos | Alecithal .
162 | Alimentary system ec
174 59 Amphioxus .
III 3 Anodonta .
go 53 Arenicola .
92 3 Ascidian
248 a5 Aurelia
174 33 Balanoglossus
255 99 bee ,
670,
824
Alimentary system of—
Birds .
cockroach .
crayfish :
Crinoidea .
Crustacea
Distomum .
earthworm .
frog . F
haddock.
Helix . .
Hirudo .
Holothurian
Insects
Lizards ‘
Myxine
Nematoda .
Nemerteans
Peripatus
Petromyzon.
pigeon
rabbit .
Rotifera .
scorpion
sea-urchin .
Sepia . é
skate . ‘
spider . .
starfish
Vertebrates .
Alisphenoid canal. 3
Allantois
Alligators
38, 610, 642, 685;
Alloioccela .
Allotheria
Alpaca.
Alternation of generations
”
”
”
”
”
a 2
Altrices
in Aurelia .
in Ceelentera
in Distomum
in Nematodes
in Spongilla
in Tunicates
Ambiens muscle
Amblypoda .
Amblyrhynchus
Ambulacral areas .
Amblystoma. ; ;
ossicles
INDEX
PAGE PAGE
Ametabolic Insects F © 352
651 | Amia . ‘ « » 4571
330 | Amitosis - e » 48
288 | Ammoccetes . F “ - 526
274 | Ammonites . ‘ a - 432
312 | Ammothea . 7 - 379
184 | Amnion . 610, 642, 685, 729
215 | Amniota : : . 610
590 | Amceba 3 ‘ é . 89
555 » functionsof . . 12
386 physiology of . - 22
239 Amphibia i - 578
269 “ classification of 605
344. 33 Fishes compared with 579
625 5 history of ,
519 - life of . js 3 ies
203 Mammals and » 609
199 Amphiblastula ; . . 131
320 | Amphiccelous vertebra . « 1583
524 | Amphidiscs . és ‘ . 130
665 | Amphilina . % ‘ . 196
710 | Amphimixis . ‘ : 811
246 | Amphioxus . "i - 459
365 | Amphipoda . ‘i ; . 308
264 | Amphiporus . ‘ 197
407 Amphisbeenidee ‘ : . 629
542 | Amphistylic . : ‘ . 584
369 | Amphiuma . : » 606
257 | Ampullee of starfish » 258
498 | Anableps, ‘‘placenta” of . 643
755 | Anabolism . : : 33,55
731 | Anaconda . . 634
641 | Anal cerci of cockroach. . 329
182 | Analogous organs. & s 37
743 | Anamnia . ‘: . 610
750 | Anapophyses < : . 700
57 | Anaspides . ‘ e . 308
155 | Anchinia . ‘ 457
141 | Anchitherium 3 . . 762
186 | Ancylus ; é a‘ . 421
205 | Anemonia . é . » 174
130 | Angiostomum . . . 205
453 | Anguis. ‘ ‘ . . 629
679 | Angular bone : 554
659 | Animal Kingdom — General
765 survey of . ‘ 5 . I-19
629 Animalculists ‘ 63
606 | Animals and Plants contrasted 22-24
264 | Annelids . + 209
255 a3 development of . 228
INDEX 825
PAGE PAGE
Annelids and Uenelees + 476 | Appendages of spider . . 367
Anodonta . : » 392 35 of Trilobita . 373
Anolis . : . r - 629 | Appendicular skeleton of
Anomodontia . ‘ 609, 738 Vertebrates . ‘ . 482
Anopheles . : F + 359 | Appendicularia . : » 457
Anoplodium . ‘ » 182 | Appendix vermiformis . . 71
Ant-eaters Fi ‘ + 751 | Apterygota . : . 361
Antedon a - 272 | Apteryx . . ‘ . 687
Antenne of cockroach F » 327 | Aptornis . . F . 689
x Of crayfish - 284 | Apus . é ‘ 299
»» of Myriopods . + 325 | Aquatic Mammals 7 » 735
= of Peripatus . » 319 | Aqueduct of Sylvius » 487.
Ss of Trilobites . » 378 | Aqueductus Vestibuli . » 475
Antennules of crayfish . . 284 | Aqueous humour ofeye. .. 497
Anthomeduse . . » 171 | Arachnactis . . . . 163
Anthozoa . + 170,174 | Arachnoid fluid . . + 470
Anthropoid Apes . ‘ ‘ . 788 55 membrane . - 488
Anthropoidea - 785 | Arachnoidea. ' ‘ - 363
Anthropomorph monkeys » 788 | Araneide . , 2 » 367
Anthropopithecus . + 791 | Arca . ‘ . 3 » 426
Antilocapra . * » 759 | Arcella 4 » 108, 116
Antilope. : ° + 759 | Archeeoceti . F . - 739
Antipatharia . ‘ ‘ « 166 | Archeeopteryx . : 686
Antiquity of man . é » 793 | Archzornithes ‘ . 686
Antlers . ‘ + 758 | Archenteron. + 67, 469
Ants : 5 . » 360 | Archi-Annelida . : + 234
Anura . i - 605 | Archicoele . 7 . « 198
Anus of Vertebrates + 502 | Archigetes . 190, 195
Apes . . : . - 788 | Archipterygium of Fishes - 564
Aphides : : » 360 | Archoplasm , . - 46
Aphrodite . “i ‘ » 233 | Aycifera ‘ ‘i ‘ - 584
Apis . ; : . + 332 | Arctocyon . . . - 778
Aplysia % : » 421 | Arctoidea . . - 778
Apneustic Insects . P - 346 | Arctomys . : ‘5 = 972
Apoda (Echinoderma) . - 272 | Arctopithecini . : » 787
Apoda=Gymnophiona . - 606 | Area apaca . . » 642, 684
Apodemata (crayfish) . - 286 | Area pellucida . - 642, 684
Appendages of Arachnoidea . 363 | Arenicola . ‘ . . 222
we of Arenicola . 224 | Argonauta .. < : » 432
e of cockroach . 327 | Argulus : eg 303
5 of crayfish . . 284 | Argyroneta . ‘ 372
a of Crustacea . 281 | Arion . : . 421
35 of Eurypterina . 377 | Aristotle’s lantern . . 265
33 of haddock » 555 | Arius . F f : 562
- of Insects . - 339 | Armadillos . . : + 75%
es of Limulus - 376 | Arrow-worms . . » 244
a3 of Myriopods . 325 | Artemia 299
«3 ofPeripatus . 319 | Arterial arches of Vertebrates 506
5 of Polycheta . 230 55 system. See Vascular
of scorpion + 365 system.
INDEX
826
PAGE
Arterial system of embryo of
higher Vertebrates 507
Arthrobranchs of crayfish 291
Arthropoda, classes of . 9
, general characters of 280
Articular : : - 554
Artiodactyla . : 754
33 and Perissodac-
tyla contrasted 754
Ascaridze . . . 208
Ascaris . . . . » 202
Ascetta : j ‘4 + 125
Ascidia ‘ : . 444
Ascidiacea 457
Ascon type (of sponge) . 127
Asexual reproduction . + 54
Aspidocotylea . : 190
Aspredo : ‘a ‘ 562
Astacus . : 281
Asteracanthion . : + 255
Asterias fi . : + 255
Asteroidea F + 254
Astronyx . 2 z . 262
Astropecten . . . 260
Astrophyton . . » 262
Astrorhizidee ; + IQ
Atavisms or reversions . . 86
Atax * - 373
Ateles . a ‘ . 788
Athecze . . . . 617
Atlanta ‘ . a + 422i
Atlantosaurus - . 645
Atlas vertebra a : - 700
Atrial cavity F 446, 465
Atriopore of Amphioxus . 460
Attidee . « 372
Auchenia - 756
Auditory capsules of skull » 478
Auditory nerve. « 490
53 organs of Insects + 344
Aulostoma » 244
Aurelia 7 . - 152
s life history of . » 155
+, relatives of - 157
Auricularia (of Holothuria) . 278
Australian region . . 805
Autogamy . ‘ e - 56
Autophage . . . . 679
Autostylic . ‘ . - 584
Autozooid é . . 168
Aves (see Birds) .
Axial skeleton ad Backbone) 478
Axifera
Axis vertebra ‘ 7
Axolotl : . - 606,
Aye-Aye : .
Azygobranchs ,. .
BABIRUSA . . . ‘
Baboons Z . ‘ é
Backbone is
Bactrites .
Baculites ‘ ‘
Badger : ‘
Baleena “ ‘
Baleenoidea .
Balzenoptera .
Balanoglossus ‘i . “
59 Vertebrate char-
acters 0:
Balanus , ;
Baleen . ‘ *
Bandicoot . r .
Barbary ape . :
Barbule E ‘
Barnacle ze c f
Basitemporal . :
Batoidei c .
Bats. ‘ .
Bdellostoma . : ‘ :
Bears . é * «
Beaver ‘ . %
Bee . , ‘
Beetles ‘ 2 5
Belemnites . A “ 2
Belinurus . , . ,
Belodon c .
Beroe . ‘ é .
Bilateral symmetry . :
Bile P 3
Bilharzia. .
Bionomics. See Cicology.
3 Crustacea . F
a Fishes c es
se Insects . z
39 Mollusca . 423,
PA Protozoa .
33 Sponges . ‘
Bipalium * . *
Bipinnaria . .
169
700
607
784
420
755
788
481
432
INDEX
PAGE
Bird lice, . . - 361
Birds ; ‘ ‘ . 647
»» and Reptiles . . 612
», Classification of . - 686
»» courtship of . . . 677
» diet of . ‘ . « 679
» eggs of . A . 678
»» feathers of : - 655
s» flight of - 673
»» general characters ‘of » 648
35 Migration of. . 680
»» moulting of . Py » OF
» nests of ; . 677
>» pedigree of . - 690
song of es 3 - 677
Birgus : . . + 309
Bivalves . + 425
Bivium of starfish . ‘: - 258
Bladder of frog - » 591
»» Of lizard ‘ - 628
» Of Mammals . - 717
+> worms » 190, 195
Blastocyst . ; - 728
Blastoderm . a * s 67
Blastoidea - ‘i a 275
Blastomere . ‘ : +» 471
Blastopore 7 3 - 67
Blastosphere . F . - 67
Blastula 5 - 67
Blatta . . - 326
Blind spot of eye. : : - 497
Blood . . 28, 42
» of frog 591
», of Mammals 696, 712
Boa. a 634
Body cavity . 347
Weis eos of Amphioxus 465
» 9» Of Arenicola 226
» 9 Of Balanoglossus 436
» 99 Of crayfish . + 290
» 9 Ofearthworm . 213
» 9 OfInsects . » 347
» 9 Of Teleostean . 504
x» 9 Of Vertebrates . 503
Bone . ‘ 2 » 42
Bonellia . 5 + 233
Bony pike . : + 571
Book scorpions . - 366
Bos « “ " « 759
Bothriocephalus . fi » 196
Botryllus, .
Bougainvillea .
Bovidee . ,
Brachiodont .
Brachiolaria . 5
Brachionus . 2
Brachiopoda .
Brachyura
Brackish water fauna
Bradypodidze
Bradypus
Brain of pigeon
x» of skate
», Of Vertebrates
»» summary of parts of
Branchellion .
Branchiz = Gills
Branchial arches, .
55 sense organs .
Branchiobdella
Branchiopoda
Branchiostegal rays
Branchipus .
Branchiura
Breast bone .
Brisinga
Brittle-stars .
Bryozoa
Buccinum
Buckie = Buccinum
Budding
Bugs ‘ é
Bulbus arteriosus .
Bulla .
Bunodont .
Bursa Fabricii .
Butterflies a
Byssus . . .
CapniIs flies . .
Ceca . i
Cecilia. .
Ceecum of rabbit .
Ca’ing whale é
Calamoichthys .
Calcaneum .
Calcarea ° :
Callorhynchus ,
Calymene .
Calyptoblastic ,
328
Camelide .
Camelus
Campanularia
Campodea
Campodeiform larva
Cancer .
Canide , ‘
Canis
-Cannon-bone
Capitellide .
Capitulum of rib :
Capra .
Caprella
Capreolus
‘Capybara
‘Carapace of Chelonia
Carcharias
Carchesium ,
Carcinus
Cardium
‘Cardo . é
Caretta
‘Carina of Cirri pedia
», or keel of Birds .
Carinaria
-Carinatee
$5 and Ratitee
‘Carinella
Carinoma
Carmarina
‘Carnassial teeth
Carnivora
Carpo- metacarpus .
Carpus .
Cartilage
-bones
Caryophyllzeus
Cassiopeia
Cassowary
Castor .
Casuarius
Cat.
Catarrhini
Caterpillar
Caudata
Cave fauna
Cavia .
Cebidee
Cebus .
trasted
eT
INDEX
PAGE
190,
755
750
171
361
354
310
777
777
757
231
7OL
Cell cycle among Protozoa
», division . ‘
o3 5 rationale of
», Hucleusof .
», substance of .
», theory :
wall of . J e
Gass p 7 °
», forms of
», structure of . 3
Cellulose in Tunicates .
Cenoplacentalia
Centetes
Centipedes . .
3 and millipedes
Central corpuscles .
Centrolecithal -
Centrosomes . .
Cephalaspis . .
Cephalochorda .
Cephalodiscus
Cephalopoda .
Cephalothorax of crayfish
Cephalothrix
Ceratites
Ceratodus
Ceratosa
Cercarize
Cerci of cockroach
Cercopithecidze
Cercopithecus é .
Cere of pigeon. .
Cerebellum... .
Cerebral hemispheres .
Cerebratulus . :
Cerianthus . .
Cervidee F : ;
Cervus. 5 : ‘:
Cestoda ‘ ‘
Cestracion . ‘ ‘
Cestum Veneris
Cetacea f
Cetochilus a 7
Cheetoderma . a 7
Cheetognatha .
Cheetopoda .
Cheetopterus .
Chalaza % :
Chalicotherium . .
Chalina x ‘ 7
164,
Chameleo . : . F
Charybdea . P .
Chelicerz of Limulus . j
ss scorpion .
spider
uF
Chelifer ‘ : ‘
Chelone 3 a
Chelonia . . ‘
Chelonide . c
Chemotaxis . é
Chernes 7 i
Chevron bones
Chevrotain . :
Chiasma of optic nerves
Chilognatha . a
Chilopoda :
Chimera . . ‘
Chimpanzee . 3 .
Chiromys
Chiroptera
Chiton . :
Chlamydomyxa
Chlamydosaurus
Chlorophyll .
Choanocytes .
Choeropus
Choloepus F
Chondracanthus .
Chondrocranium . .
Chordze tendineze . .
Chordata and WNon- chor-
data . , :
Chordotonal organs ‘ :
Chorion .
Choroid of eye. ‘
»» Plexus . P ‘
Chromatin
Chromosmes. ‘ F
Chrysalis
Chrysaora . f
Chrysochloris ‘
Chyle . :
Chyme .
Cicada .
Cidaris .
Cilia
Ciliary nerve
») process
Ciliata . .
Ciona . é .
INDEX - 829
PAGE PAGE
627 | Circulatory system. See Vas-
174 cular system.
376 | Cirri of Cheetopods ‘ 230
365 », of Crinoids . ‘ 273
367 | Cirripedia . . E + 303
366 | Cirrussac . 3 . . 185
618 | Cladocera ‘ 3 301
613 | Claspers of skate . » 532
618 | Classification, grades of. . <I¢
113 53 of Animals 16, 17
366 3 of organs . - 4
636 59 the basisof . 14
756 | Clavelina . c . « 452
498 | Clavicle ‘ ‘ ‘ 482
325 | Clepsine . . 244
325 | Clio. j 5 5 421
563 | Cliona . é 134
yor | Clitellum of earthworm 5 212
784 | Clitoris ‘ 699:
781 | Cloaca of Vertebrates 503
418 | Clypeaster . 3 3 267
108 | Cneme. ‘ s . . 165
629 | Cnidoblasts . ‘ 142, 146
25 | Cnidocil . . . 142
125 | Cobra . ‘ ‘ 635
747 | Coccidiidia . : ‘ III
750 | Coccidium . 5 » 103
303 | Coccus Insects . f . 360
478 | Cochlea . . 493
712 | Cockle. ‘ . é 427
Cockroach . . - + 326
8 | Cocoon . ‘ . 354
344 | Cod. x 550
731 | Codosiga . ‘ 3 110
497 | Coelentera . i ‘ 137
485 | Ceeliac ganglia . 712
46 | Coelom (see Body cavity) « 347
46 | Coelom pouches . » 69
354 | Ccelomata and Coelentera 12, 138
158 | Coenolestes . : - 747
781 | Coenurus f ‘ fs » 195
28 | Cold-blooded . . . 696
27 | Coleoptera . : . . 360
352 | Collembola . . 361
267 | Collozoum 110
112 | Colobus 5 . 788
490 | Colon , . . 711
497 | Columba é ‘i 654
Ill 5 livia . - 86, 654
457 | Columela of Vertebrate ear . 495
INDEX
330
PAGE
Columella or epi-pterygoid 637
f in corals 165
PF in snail 384
Comatula 272
Commensalism among "Crus-
tacea 316
55 », Fishes 563
of sea-anemone
and hermit- crab 177
Commissures of Mammalian
brain 708
Complemental males among
Cirripedia . 303
Myzostomata 235
Conchifera (see Bivalves) 425
Conchin = Conchiolin 384
Condylarthra - 766
Condyles, occipital 694, 702
Conjugation, dimorphic . 56
o multiple 55
ay of Paramcecium 95
a of Protozoa 55, 117
oe of Vorticella 98
‘Conjunctiva . Fi + 497
Connective tissue . . » 41
Contractile vacuoles. + FIZ
Contraction of muscle . + 42
Conus arteriosus + 505
Convergence. z + 37, 747
Convoluta . 5 . . 182
‘Copepoda . . + 303
‘Coracoid » 482, 705
‘Coral, making of . ‘ » 164
Corallium . 168
Corals . . ‘ 164
‘Cord ylophora s . IZI
‘Corium ; ‘ . + 477
‘Cornea. . a . » 497
Coronary . . : - 604
Coronella. ‘ 2 « 634
Corpora quadrigemina . + 695
‘Corpus callosum . + 695
» geniculatum . » 709
>> luteum A » 725
», striatum » 471, 664
Correlation of organs. - 36
‘Corrodentia . A - 3061
Corticata : 7 + 106
Cotylophora . , - 757
Couguar . + 754
PAGE
Courtship of Birds 3 677
Si of Spiders. 371
Cowper’s glands . 720
Coxa . » 327
Coxal aaa ‘of scorpion + 365
Crab 296
Crangon 309
Crania . 251
Cranial nerves . 489
Craniata 16, 475
Craspedon of swimming- “bell. 1 50
Craspedota . 159
and Acraspeda 159
Crayfish, the fresh water 281
5 the sea . ‘ 281
Cremaster 745
Creodonta 778
Cribriform plate 703
Cricket . 344
Crinoidea 272
Cristatella . ‘ - 250
Crocodiles, Alligators, and
Gavials A . 641
Crocodilia . ; . 635
Crop of earthworm + 205
x, leech A : + 239
» Pigeon. : - 666
Crossopterygii . 569
Crotalus 635
Crumb of bread sponge . 133
Crura cerebri . P 487
Crural glands 321
Crusta petrosa 722
Crustacea 3 - . 281
x general charactersof 281
5 systematic survey of 298
Cryptobranchus . 606
Cryptoniscidee . é + 291
Crystalline style 397
Ctenidia ‘ ‘ 2 382
Cteniza. . . 2 372
Ctenoid scale 565
Ctenophora . 2 174
Cubomedusze 5 174
Cucullanus . 205
Cucumaria . - r 272
Cuma . 3 3 . 308
Cuscus . : : . 748
Cuticle . . ‘ . . 282
Cutis . ‘ - . + 477
Cuttlebone .
Cuttlefish
Cuvierian organs .
‘Cyanea
Cyclas .
Cyclodus, ‘ placenta” of
Cycloid scale
Cyclops 1
Cyclostomata
‘Cydippe .
Cymbulia
‘Cynocephalus
Cynoidea.
Cynomorph Monkeys
Cypridina . .
Cypris .
Cysticercus .
Cystidea
Cysts of Protozoa
Cytoplasm .
DAPHNIA .
Dart sac of snail
Dasypeltis
Dasypodide .
Dasyprocta .
Dasypus .
Dasyuride .
Dasyurus
Dead-Man °s-fingers
Decapoda (Cephalopods)
af (Crustacea) .
Decidua
Decidua reflexa
Deciduate
Deep sea
Deer
Degeneration
Delamination
Delphinoidea
Delphinus
Demodex
Demospongize
Dendroccelum
Dentalium
Dentary .
Dentine
Dentition of Mammals
Dermal denticles
Dermaptera .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
2
.
.
.
.
.
.
.
INDEX
PAGE
- 433 | Dermis of Vertebrates . :
. 402 | Descent, doctrine of . 84,
. 269 | Desmodus - : ;
158, 173 | Deutoplasm . , . .
» 427 | Development of—
- 628 » Amphioxus . 7
« 565 3 Annelids . .
- 303 st Anodonta .
- 516 35 Ascidia .
» 175 si Balanoglossus ‘
+ 420 3 Cheetognatha fi
. 788 3 chick . F -
- 777 9 crayfish % .
. 788 a Crustacea . i
» 301 73 earthworm A
+ 301 xy Echinoderma “
. 195 93 eye. . .
. 275 5% feather . :
. TIS ‘es frog. ‘ ‘
» 44 ug haddock ‘ i
55 hair . ,
+ 301 58 Hydra . - A
+. 392 Ar Insects . r
» 632 aig Mammalian teeth .
» 751 sf Mammals . é
» F773 9 Nemerteans . .
+» 751 5% Peripatus . .
- 746 - Petromyzon . ‘
- 746 or placenta 5 .
» 164 o Polycheta . ‘
+ 432 s'5 Reptilia . .
- 308 o scorpion fe :
« 733 8 skate . . a
+ 732 sii skull . s .
- 734 55 Sponges 4 .
- 796 9 Teeth . ‘ -
- 758 iss Tunicata . is
- 39 - Vertebrata . :
69, 163 | Diapedesis - -
. 770 | Diaphragm . . - 694,
- 770 | Diastema . ‘ . .
+ 373 | Dibranchiata . . .
- 133 | Dichogamy . . - 56,
- 182 | Dichogamy . a fi
» 424 3 in Tunicates .
- 554 | Dicotyles F ri
- 722 | Dicyemide . . .
+ 722 | Dicynodon . . 4
- 532 | Didelphyidz . .
+ 361 | Didelphys 8
832
Differentiation .
Digestion . .
Digitigrade . ‘ ‘
Dimorphism of sexes.
Dinoflagellata
Dinophilus .
Dinornis
Dinosauria
Dinotherium
Diphycercal .
Diphyes :
Diphyodont dentition
Diploblastic . Z
Diplopoda = Millipedes .
Diplotrophoblast .
Diplozoon . : ‘
Dipnoi . . .
Diporpo . .
Diprotodon .
Diprotodontia -
Diptera i . .
Dipterus .
Direct division .
Discoidal segmentation .
Discomedusze ‘
Discophora . ‘
Discus proligerus . .
Distomum
Distribution, geographical
as geological .
Division of tabour
Dochmius . 5
Doctrine of descent
Dodo
Dog .
Dogfish
Dogwhelk
Doliolum is
Dolphin . :
Donkey F
Dorcatherium
Doris . . .
Draco . .
Dracunculus .
Dragon-flies .
Dreissensia .
Drepanophorus
Dromeognathous .
Dromeus .
Dromia . °
. ‘
ee eee
205,
84,
INDEX
Duckmole . . ¥
Ductus botalii
‘3 endolymphaticus
Dugong .
Duodenum
Duplicidentata
Dura mater . ; ‘
Ear of Arenicola .
» of crayfish
», Of Myxine . Z
» of pe aa ‘
Earthworm . ‘
Earwigs
Ecaudata
Ecdysis or moulting 0 of Ar-
thropods .
Echidna ¥
Echinococcus 3
Echinoderma
a contrast between
the classes of. .
Echinoidea .
Echinorhynchus
Echinus
Echiuridz
Ectoderm
Ectoplasm
Ectoprocta .
Edentata.
Edible bird’s nest .
Edrioasteroidea . :
Egg-case of skate .
>» cell ; ‘
Eggs of Birds .
Elasipoda_.
Elasmobranchii
Electric organs
Eledone i ‘
Elephant’s tooth shells .
Elephas ‘
Elginia . .
Elimination . a
Elpidia 2 . .
Embolé 5 ‘ ‘
Embryology .
Emu .
Enaliornis i
Enamel F % ‘
Encystation . . 2
Endoderm ,. .
Endolymph . .
Endoplasm .
Endopodite (cray fish)
Endosternite :
Endostyle of Tunicates .
Enoplidz .
Enteroctele . .
Enteron=gut : .
Enteropneusta .
~Entomostraca . ‘
Entoprocta . 7
Eolis . .
Epanorthide. .
Epeira . . .
Ephemeride . é .
Ephyra. o . .
Epiblast Fs :
Epibolé a . .
Epicoracoid .
Epidermis of Vertebrates
Epididymis of rabbit .
>» * Ofskate .
Epiglottis .
Epimeron (crayfish) :
Epiphragm of snail .
Epiphyses . .
Epiphysis = Pineal "body
Epipodite of crayfish
Epipterygoid
Epipubic bones.
Episternum .
Epistoma (crayfish)
Epithelial tissue .
Equidee
Equus . ‘
Erinaceus.
Eruciform .
Erythrocytes.
Ethiopian region
Eucyrtidium .
Euglena
Bulemeliiasneii.,
Eupagurus .
Euphausia .
Euplectella .
Eurypterina .
Euspongia .
Eustachian tube
Eustrongylus
53
INDEX 833
PAGE
Eutheria . F . + 741
Euthyneura . é + 420
Evolution, evidences of. . 85
5 factorsin . 807
on of sex . ‘ 54
- summary of theories
of . ‘ . 813
Excretion in Animals , » 30
Excretory system of—
we Amphioxus . 458
- Anodonta . + 399
i Arenicola . . 227
<4 Balanoglossus 437
- cockroach , 331
a8 crayfish . - 291
- Crustacea. 4 313
. earthworm . x ay
i frog. 5 596
a haddock ‘ - 558
9 Helix . Re » 388
3 Hirudo. . + 241
% Insects . 4 + 347
ne Myxine. 520
5 Peripatus . + 320
55 Petromyzon . © 525
9 pigeon . 7 671
ay rabbit ‘ « wag
ay scorpion , + 365
33 Sepia . 5 » 409
i skate ‘ + 546
a5 Tunicates . + 449
Vertebrates . + 509
Exopodite (crayfish) - 283
Exoskeleton of Vertebrates . 477
Extinct Reptiles 644
Extinction of Animals . - 80
Eyes of Insects. . + 343
5, Ofpigeon . . - 665
» of Vertebrata A + 495.
FABELLE , . . « YO?
Facial nerve . . . - 490
Feeces . ‘ fs F 28
Fallopian tube, . + 719
Fangs of eee . . + 632
Fat ‘ . we 29
Fatigue 7 . - 26
Feather-stars _ ‘ - 272
Feathers, development of . 656
35 of pigeon + 654
834 IND EX
PAGE PAGE
Feathers, parts of . A . 657 | Foraminifera ‘ » 108
Felide . . . . - 776 | Fore-brain= Prosencephalon » 484
Felis. « : ‘ - 777 | Fore-gut=Stomodeum. . 498
Femur . . ; - 482 | Fornix. : ; 5 » 695
Ferments . ‘ . . 27 | Fossils. ‘ : : 78, 79
Fertilisation . F ‘i . 65 | Fox . * 777
Fibula . : . ‘ . 482 | Fresh-water fauna. 3 . 798
Fierasfer é i . 178 53 mussel ‘ 302
Filaria . . . : . 206 | Fritillaria . . . - 457
Filibranchia . . - 426 | Frog. : . : » 580
Filoplume Z . - 654 | Fuligo . 4 . + 107
Fins. ‘ : . . 564 | Functions, change ‘of 38
Firmisternia . 3 ’ - 586 o of Animals . » 24
Fishes, abyssal . 7 » 563 ae secondary, of organs 38-
>» and ea com-
pared » 579 | GapDUS. 5 . + 550
», colour of . - 560 | Galago. . : : - 784
»,» commensalismin . 563 | Galathea . 4 . + 309
» fins of : $ - 564 | Galea . ri i i + 327
», foodof . 561 | Galeodes . : S - 367
» general characters of . 530 | Galeopithecide . . - 781
yy hermaphrodite . + 561 | Galeopithecus ‘ F - 781
», orders of . 2 566 | Gall-bladder. : : 2 91
+) parental care among . 564 | Galton’s law. . é > 74
»» reproduction of . » 561 | Gamasus . ‘ j + 372
»» sensesof . . + 561 | Gammarus . ‘ ‘ + 308°
», Viviparous. . +» 562 | Ganglion . e ‘ »- 4B
Fission . P é - 54 | Ganoid scales ‘ ‘ « 565
Flagella * . . + 112 | Ganoids F 5 é + 571
Flagellata. . . + 110 | Gapes . Fi 2 : » 208
Flagellum of snail. + 390 | Garden spider . F + 372
Flame cells . . . - 180 | Gasteropoda . + 417
Fleas . : . . + 360 ay life history ‘of + 422
Flies. . . » 360 parasitic . + 422
Flight of Birds : ‘ . 673 Gastraca theory . . . 71
Flocculus - 664, 708 | Gastric filaments . , 154
Floridine in Sponges > » 130 3» juice. : - 27
Floscularia . . 246 >» mill of crayfish 289
Flustra . A . + 250 | Gastroliths of crayfish . » 289
Flying Mammals . . - 735 | Gastrula . : . - 67
Foetal membranes of Birds . 685 | Gavials . ; 7 - 641
5 a ofMammals 730 | Gecarcinus . Fi 310
‘5 9 of Reptiles 643 | Geckonide . , . 628
Follicle cells. . é + 725 | Gemmation= Budding . » 54
Food vacuoles. . + 90 | Genealogical tree . ‘ 15
Foot of Molluscs . . - 388 | Genetta H FI + 977
Foramen magnum . « 702 | Geodesmus . - 182
>, of Monro . » 709 | Geographical distribution » 803
x, Panizze. . - 639 | Geological record . A ree 4
»» triosseum . - 657 | Geophilus . _ . » 325
INDEX
PAGE
Gephyrea . : ‘ » 247
Germ cells . a) 53
» plasm. : . » 73
Germinal vesicle . : » 58
Gestation . . » 737
Gharials = Gavials . . 641
Gibbon : . . . 789
Gill-clefts . . 474
Gills. See Respiratory system.
Giraffe . . 758
Gizzard of cockroach F + 334
x» of crayfish a » 289
x,» Ofcrocodile . » 639
»» Ofearthworm . » 215
», of pigeon. 659, 666
Glass-rope sponge + 133
Glenoid cavity . - 695, 705
Glenoid fossa ‘ «703, 775
Globe fishes . . - 560
Globigerina . . + 109
Glochidium of Anodonta . 400
Glossopharyngeal nerve + 490
Glycogen. . , - 28
Glyptodontide , . + 751
Gnathobdellide . . - 244
Gnathostomata . . » 516
Gnats . i . . » 359
Goldfish 572
Gonads = Reproductive organs 40
Gonapophyses of cockroach . 327
Gonophores . ‘ . » 152
Gordiide . : zi + 208
Gorgonia - = + 169
Gorgonocephalus . . + 262
Gonilla. * x + 790
Graafian follicle 7 . - 719
Graafilla . . . - 182
Grampus . . . + 770
Grantia . . . + 133
Graptolites . . . » 173.
Grass snake . . . + 634
Green gland of crayfish + 291
Gregarina . . . + 102
Grey matter of brain . + 709
Gromia . . - Iog
Gubernaculum . . - 717
Guinea-pig . . . = 973
x» worm . . « 206
Gunda. é . . + 182
Gymnoblastic . . + I7I
Gymnomyxa. .
Gymnophiona
Gymnosomata ,
Gyrodactylus ‘
Happock , . .
Hemadipsa . . y
Hemoceele , q A
Heemocyanin ‘ F
Hemoglobin of blood .
Hemopis . . .
Hag . ff :
Hair ‘ ‘ 699,
Halichoerus . . : '
Halichondria F 5 ‘
Halicore . . .
Haliotis - .
Halisarca ‘ F
Halitherium . "i ‘i
Halobates . - . ‘
Hamadryad . ri . ‘
Hapalide . . , ‘i
Harderian gland . é ‘
Hare . ‘ ‘ A ,
Harelip ai s : F
Harvest men c ‘ F
» Mites , E .
Hastigerina . " . F
Hatteria . + é
Heart of Vertebrates "
Hectocotylus of cuttlefish
Hedgehog, dentition of i
#5 development of .
is placenta of .
Heliopora . .
Heliozoa ss, 3
Helix . ’ o
3, Shell of ‘ ‘
Heloderma . ri 2 ‘
Hemichorda. r " :
I{emimetabolic Insects ; :
Hemiptera . . .
Hepatopancreas . .
Heredity . .
Hermaphrodite duct of snail .
Hermaphroditism . . .
yy of Cestodes .
3 of Cirripedia .
- of earthworm,
3 of leech. .
836
Hermaphroditism of Myxine.
a of Trematoda.
of Tunicata
Herpestes . . .
Flesperornis . . .
Heterocercal
Heteroccela . x
Heteroccelous vertebrae *
Heterodera .
Heterodont dentition
Heteropods . x °
Heteronereis a
Hexacanth . ‘
Hexacoralla . < i
Hexactinellida
Hind-gut = Proctodzeum
Hippocampus of brain .
Hippopotamus
Hippotherium i
Hirndinea :
Hirudo : ‘
Histology 3
Histriodrilus. .
Hoatzin= Opisthocomus
Holoblastic segmentation
Holocephali . . .
Holophytic . . .
Holopneustic F
Holothuria . 7 .
Holothuroidea c
Homarus. ‘
Homo. 5 «
Homocercal . ‘
Homoceela , F
Homodont dentition
Homology of organs
Homoplastic .
Honeycomb bag .
Horns . ‘ . .
Horse .
Huanaco
Humerus
Hyena
Hyzenodon
Hyalea
Hyalonema .
Hybrids ‘
Hydatina
Hydra . : .
», budding off, 5
- ee we
oe | aoe we %
INDEX
PAGE PAGE
520 | Hydra, development of « 147
184 »> minute structure of 145
» 449 », Physiology of ~. 22!
2 997 »» Yeproduction of . 53> 147
. 687 | Hydractinia . ¢ ‘ . 140
- 564 | Hydra-tuba of Aurelia . 156
+ 133 | Hydrochcerus ‘ » 973
. 648 | Hydrocorallinz os Aga
» 208 | Hydroid colonies . % 191
- 723 | Hydromeduse . r s Fo
- 420 | Hydrophis : + 635
» 231 | Hydrophora . - . » 71
- 192 | Hydropotes . . » 758
- 164 | Hydrozoa . ‘ ‘ . 170
- 133 | Hylobates . ; 5 - 789
498 | Hymenoptera 6 . 360
709 | Hyoid arch of Vertebrates 480
« 755 | Hyo-mandibular . F 480
. 762 | Hyostylic . . 5 584
+ 235 | Hypopophysis . 700
. 236 | Hypoblast 40, 70
» 41 | Hypodermis= Epidermis 328
- 234 | Hypophysis . ‘ e » 485
+ 690 | Hypostomata : : 528
- 65 | Hypostome . : . + 144
. 568 | Hypsodont . a é a “72
- 113 | Hypural bone . : + 552
- 346 | Hyracoidea . ‘ 763
. 272 | Hyracotherium . 4 » 762
. 268 | Hyrax . - > ; - 763
. 281 | Hystricomorpha . a « 973
+ 791 | Hystrix ‘ : 773
» 564
- 133 | IANTHINA . . » 420
« 723 | Ichneumon . A oe Fae
14, 37 | Ichthyodorulites . » 568
+ 37 | Ichthyomyzon . ‘ 527
» 758 | Ichthyophis . : 3 607
+ 721 | Ichthyopsida ‘ : » 475
- 761 $3 Sauropsida, and
- 756 Mammalia contrasted » 61f
- 482 | Ichthyopterygium of Fishes . 564
- 777 | Ichthyornis . 4 ‘ - 689
+ 778 | Ichthyosauria FB - » 644
+ 421 | Idotea. . . P - 308
+ 133 | Iguanide . . . + 629
+ 75 | Iguanodon , . . + 645
. 246 | Ilium . . F ‘ . 482
42, 143 | Incus of ear . ‘ + 695, 705
+ 144 | Indirect development . + 253
Indirect division . ‘
Indrisine . 4 :
Infundibulum ‘ é
‘Infusoria .
5 ciliary movement in 112
Ink-bag of Cephalopods 408
Innominate bone . . 706
Insecta ‘ 5 7 325
»» _ Classification of. 360
Insectivora . d . 779
Insects and disease . 358
», and flowers . 357
»» injurious . . 358
»» pedigree of : + 359
Integration of individual 36
Interclavicle . 625
Intervertebral disc 700
Intracellular digestion . 27
Invagination . 67
Invertebrata, classes of . . 8
55 and Vertebrata . 8
Iris. : F : » 407
Ischium . A 482
Isolation . : . 812
Isopleura. H F 417
Tsopoda . . 308
Iter, js ‘ ‘i 487
Ixodes . . 2 5 373
JACKAL f . e 777
Jacobson’s organ . P 710
Jellyfish B . - 152
Jerboa . a ‘ F 972
Jugal . é F ‘ 480
Julus . , ‘ . - 325
KANGAROO . . i 749
Karyokinesis i: . - 48
Katabolism . . 32, 55
Keber’s organ 398
Keratin : . 548
Kidney, functions of. 30
»» Of Vertebrates. 509
King-crab_. 375
Kiwi . . : : 687
Kolga . . . 272
Kowalevskia 457
LaBIAL cartilages of skull 535
», palpofcockroach . 327
INDEX
PAGE
- 48 | Labial palp of Insects
784 | Labium of Insects
485 | Labrum of Insects
Labyrinth of ear .
Labyrinthodontia .
Labyrinthulidea .
Lacerta 5
Lacertilia. .
Lachrymal gland
Lacinia e
Lacteals .
Lactic acid .
Lagena=Cochlea.
Lagomys . ‘
Lambdotherium
Lamellibranchiata
Lamprey .
Lamp,shells .
Lancelet ‘i
Lanice . .
Larva of Amphibians
>, of Anodonta
», of Antedon.
of Ascidian .
- of Aurelia .
», of Chzetopods
», of Crustaceans
» of Holothuria
», of Insects .
», Of lamprey .
- 5, Of Molluscs
», of Nemerteans
3» of Ophiuroids
»» of Polygordius
», of sea-urchin
», Ofstarfish .
Larvacea , ;
Larynx .
Lateral line system
Laurer-Stieda canal
Layers of Coelomata and Hy-
dra contrasted
», the germinal
Leech . ‘ :
Lemming
Lemur. ;
Lemuroidea .
Lens of eye . .
Leopard. .
Lepas . . .
838
PAGE
Lepidoptera. . . » 360
Lepidosiren . . . » 577
Lepidosteus . : : » 571
Lepisma 361
Leptocardii = Cephatochorda 459
Leptodera . » 205
Leptodora . . . + 301
Leptoplana . 7 . 182
Leptostraca . . fs + 306
Lepus . « O73
Leucocytes of blood + 712
Leucon type of sponge . » 127
Leucosolenia 4 ‘ + 133
Lice’. « + 360
Life history of Ameeba . + 90
55 of Anodonta 400
3 of Aurelia . 155
59 of Cestodes 195
39 Coccidium . « 104
0 of Distomum 186
He of Fishes - 562
ee of Frog . 601
5 of Gregarina . 103
+ of Insects . «3352
- of Monocystis . 100
a of Nematodes . 205
a of Paramoecium . 96
of Polystomella 92
35 of Spongilla 130
45 of Tzenia IQI
0 of Trichina 207
35 of Tunicates 450
5 of Vorticella
eee we ew we ew ew we
Ke)
ive)
‘5 of Volvox . 99
Ligula . . 196
Lima . . 5 . 427
Limax. . 421
Limbs and girdles ‘of—
5 a Chelonia . 615
“ <5 Crocodilia. 638
‘3 » frog . 584
a3 ss pedicel 555
9 ‘3 lizard « 625
‘5 5 pigeon . 662
55 5 rabbit » 705
skate « 537
Limbs, Theories as to origin
of . . ‘8 . 564
Limneus. 3 . 186, 4Il
Limnocodium : + 149, 177
INDEX
Limnocnida . - ‘ ‘
Limpet
Limulus
Lineus.
Linguatulida
Lingula
Lion . ‘i
Lipocephala . . .
Lithobius. ‘
Littoral life . ‘
Littorina
Liver fluke .
sy functions of a Fi
»» of Vertebrates . ‘
Lizards .
Llama . ‘ F ‘ .
Lobosa . i ‘ ;
Lobster ‘ ;
Lobworm
Locust .
Loligo . ‘ : .
Lophiodontide . : .
Lophodont . 7 .
Lophophore . 3 ‘
Lophopus
Loxosoma . :
Lucernaria . . .
Luidia . y
Lumbricus . ‘ »
Lung-books of scorpion .
Lungs . F :
a function of . .
Lutra . % f ‘i
Lycaon 7 ‘ . .
Lycosa : -
Lymph ;
‘,, hearts es *
Lymphatic system of frog :
re » Ofrabbit .
ee » of Verte-
brates
Macacus .
Macherodus a
Macrauchenia é
Macrobdella ® . ‘
Macromere . : r ‘.
Macronucleus . ri
Macropodidze 7 . .
Macropus . . . .
Macrorhinus . . F
Macroscelides ‘ .
Macrura 3 ' *
Madreporaria ‘ 7
Madreporic plate of brittle-star
3 >) Of sea-urchin
+> », Ofstar-fish .
Malacobdella : , 5
Malacostraca a
Malapterurus, electric organ of
Malar or jugal. : ‘
Malaria parasites .
Malleus of ear.
Malpighian body of kidney
tubule 5
2 tubules of cock-
roach
Mammalia . 5 j c
33 development of
x general characters
of :
ee 55 classifica-
tion of
life of
” 3
- ar survey of
35 history of
Mammary glands .
Mammillary body :
Mammoth . F
Manatee (Manatus) F
Mandible of crayfish.
Mandibular arch of Verte-
brates . .
Mandril , F : F
Manis . . - -
Mantle of Molluses A .
Manubrium of sternum . .
5 of swimming-bell
Many-plies . . : .
Marmosets . ‘ r
Marmot :
Marsipobranchii (see Cyclo:
stomata) i ‘
Marsupial bones . 7 F
Marsupials . . .
ya families of . .
Marsupium . . .
Marten 7 . . F
Mastodon . 5 ; .
Mastoid process . . .
INDEX
PAGE
779
781
309
164
261
264
255
201
298
32
ai
121
605
511
Maturation . .
Maxillee of crayfish -
», of Insects
>» of Myriopods .
>», Of Vertebrates .
Maxillipedes of anal
May-flies
Meckel’s cartilage.
Medulla . A
Medullary canal
7 groove.
Medusze :
Medusoids
Megachiroptera
Megalopa .
Megalosaurus
Megascolides . .
Megatherium , .
Meibomian gland .
Meiotic division
oe ee we
Meles .
Mellivora , ‘
Membrane bones .
Membranipora .
Mendel’s Law 3
Menognatha. .
Menorhyncha . :
Mento-meckelian . .
Mentum of cockroach .
», _ of Insects :
Mephitis ‘
Merlia . ‘i * s
Mermaid’s purse . .
Mermis ‘ .
on
Meroblastic segmentati
Merozoite . é :
Mesencephalon . :
Mesenchyme cells F
Mesenteric filaments
Mesenteries of sea-anemone .
Mesenteron . .
Mesentery
Mesoderm (or mesoblast)
8 segments
Mesogloea . .
es of Coelentera
a3 of Sponges
Mesonephros .
Mesoplacentalia .
Mesopterygium .
ee ay fe is
iS)
80
BS
840
Mesosma (of scorpion)
Mesosternum
Mesozoa .
Metabola . .
Metabolism . .
Metacarpals . .
Metacoracoid
Metacromion -
Metagenesis, or alteration of
generations
Metagnatha .
Metakinesis . ‘ .
Metamorphosis of —
*3 Anodonta
si Crustacea
93 Echinoderma
35 frog .
43 Insects * *
‘a lamprey .
Tunicata .
Metanephros . .
Metapleural fold . .
Metapterygium . F
Metasoma (scorpion) .
Metatarsals .' . ‘
Metatheria .
Metazoa . a
»» and Protozoa .
Metencephalon .
Microchiroptera
“Microhydra .
-Microlestes .
“Micromere
Micronucleus
Micropyle
Microstoma .
‘Microzooids . a
Midas . ‘
Midge . : .
Mid-gut = Mesenteron 7
Migration of Birds .
Milk . ‘i .
Millepora .
Millipedes .
Milt .
Miracidium , ‘.
Mites . ‘
Mitosis ,
Moa .
Modes of inheritance ‘
INDEX
PAGE
Modification 4 » 74, 810
Moina . . ‘i 5 + 301
Molars. : F ‘ « 923
Mole . i é ‘i . 781
Mollusca, ‘i - 380
55 classification of . 416
iis general characters of 380
‘9 shell of . c + 415
Molluscoidea 7 . » 248
Moloch i . . . 629
Monachus ‘ F 779
Monaxonida . 133
Monera ‘ . 114
Monitor . . ri 629
Monkeys . ‘ : 787
Monocystis . . . 100
Monodon % 770
Monomeniscous eyes 344
Monostomum ‘ 190
Monotremata F 740
Moose . 7 ‘ ‘ - 758
Morphology . i F - 34
Morse . F S . » 97g
Morula ‘ : ‘ . 67
Moschus. F : - 758
Mosquito . 7 359
Mother of pearl . . 415
Moths . . . 360
Moulting of birds . . 679
ay of cuticle of cray-
fish +. 282
Mouth origin of Vertebrate - 498
Mucous canals of skate. 532
x» glands of Myxine 517
3 » of snail. 301
Mud fishes . : 573
» line. : j 802
Miillerian duct. : 51
Multiple conjugation 117
Multituberculata . 738
Murex . , ‘ , 423
Mus . ‘ > 972
Muscle, contraction of . » 42
» fibres 26, 42
»» smooth 26, 42
», striped . ‘ 26, 42
Muscular activity in Animals 26
Muscular system of—
4 Amphioxus . 464
5 Anodonta . 395
Muscular system of—
53 Arenicola .
5 Ascidian
we Aurelia #
8 Balanoglossus
5 Birds . "I
Y cockroach .
4s crayfish ‘
53 earthworm
rog .
$5 haddock ‘
5 Helix . .
ag Hirudo <
” Insects ‘i
ia Myxine Fs
% Peripatus
aa Petromyzon .
59 pigeon . .
a rabbit .
se Sepia . e
aS skate ;
starfish e
Muscular tissue .
Musk deer 7
», glands .
Mussel ‘ ‘
»» fresh-water
Mustela
Mustelus
Mya .
Mycetes
Mycetozoa .
Myelencephalon
Mygale .
Myodes é ‘
MM araeres or Myotomes
Myomorpha .
Myotomes
Myriopoda -.
Myrmecobius
Myrmecophagide .
Myrmecophagus .
Mysis . : :
Mystacoceti . F
Mytilus .
Myxine ‘ .
3 and Petromyzon
trasted . .
Myxospongida . .
Myzostomata . .
con-
INDEX
PAGE
Nais . . . ‘
Naja . , .
Narwhal ¢ a
Nasal capsule : .
Nasilis . : 7
Natural selection . :
Nauplius. .
Nautilus . .
Navel . . ‘ #
Nearctic region . ‘
Nebelia ‘ . .
Nectonema . 7 .
Necturus i 7 Fi
Nekton > . .
Nematocysts. . .
Nematoda . é i
Nematohelminthes
Nemertea or Nemertines
Neomenia
Neomylodon.
Neornithes .
Neotropical region
Nephelis . .
Nephridia of crayfish
<5 of earthworm .
<5 of leech .
a of mussel -
se of Peripatus .
of Vertebrates
Nephridioblast
Nephrops .
Nephrostoma of kidney
Nephthys :
Nereis . . .
Nerve cells . 4
Nerve fatigue .
s» fibres. a
» tissue.
Nerves, cranial
Nervous activities in Animals
Nervous system of—
oe Amphioxus .
5 Anodonta
a Arenicola
Pe Ascidian
a5 Aurelia
x Balanoglossus
‘9 bee.
93 Cephalopoda
i Cheetognatha
842 INDEX
PAGE
Nervous system of— Nucleus pulposus . . .
nf cockroach . + 329 | Nudibranchs. 5 5 .
i crayfish : .- 286 | Nummulites. . - F
55 Crinoid . » 274 | Nymph . S
3 Distomum . - 184 | Nymphon $ Fi
35 earthworm . « 24
35 frog. . » 588 | OBELIA a . . .
55 haddock ‘ + 555 | Obstetric toad ‘ s
55 Helix . . + 385 | Obturator foramen ‘ F
a Hirudo. + 237 | Ocellate : ‘
43 Holothurian . - 269 | Ocelli . . ; ‘
15 Hydra. » 146 | Octanemus . . .
3 Insects . + 343 | Octocoralla . . .
5 Lizards . . 625 | Octopoda . A
i Myxine F » 518 | Octopus .
5 Nematoda . - 203 | Oculomotor nerve. Fi
si Nemerteans . - 197 | Odonata e ‘ ;
ss Peripatus + 320 | Odontoceti . -
- Petromyzon . - 524 | Odontoid process .
5 pigeon . 7 « 664 | Odontolcz , ‘ é
a rabbit . 7 « 707 | Odontophora (see Glosso-
a scorpion : « 365 phora) . .
a8 sea-urchin . - 266 | Odontophore ‘ ‘
si Sepia . : « 405 | CEcology—
a skate . , - 537 os Birds . F .
as spider . A « 368 a Coelentera F
$5 starfish. : » 256 | Cisophagus of Vertebrates .
53 Vertebrates . « 482 | Okapi. 3 ‘
Nervures H ; + 341 | Oikapleura . . ‘ .
Nests of Birds . 677 | Olfactory lobes. 7 .
Neurapophysis = Neural spine 630 >» Nerves. . .
Neurilemma . . 43 | Oligocheta . . .
Neuroblast . . » 222 | Omentum :
Neuro-muscular cells. - 43 | Ommastrephes . .
Neurones . - 42 | Omphaloidean trophoblast .
Neuropodium . - 230 | Omphalopleura 2
Neuroptera . . . » 361 | Oniscus . . . .
Newts . 7 . 606 | Ontogeny’ . a . .
Nictitating membrane of eye 479, | Oocyst ¥ Fi 3 5
665, 698 | Ooze, Atlantic . s 7
Noctiluca . . ° 11r | Opalina s
Nomarthra . . . + 750 | Operculum of Gasteropods 5
Nose, the : é + 492 5 of Limulus . .
ys of Myxine . ‘ » 519 57 of scorpion . .
Nothosaurus. ; » 644 59 of Teleostei. .
Notochord of Balanoglossus « 436 | Ophidia ‘ is :
Notopodium. ‘ a - 230 | Ophiocoma . ‘ ‘ .
Notoryctes . : . + 746 | Ophiopholis . 3 ‘ ,
Nucleus ‘ : + 45 | Ophiothrix . . 5 j
» division of. ‘ - 48 | Ophiuroidea. . . .
Opisthobranchs
Opisthoccelous
Opisthocomus
Oppossums .
Optic chiasma
foramen
lobes .
nerves. .
thalami .
Orca. ‘
Orchestia .
Organs. ‘ .
origin of .
>» rudimentary
Oriental region
Ornithodelphia
Omithorhynchus .
Orthoptera
Orycteropus .
Oscarella
Osculum
2?
Osphradium of Molluscs
Ossicles of ear
Osteoblasts . 3
Ostracoda ‘
Ostracodermi ‘
Ostrea . é
Ostriches . ‘
Otaria . a :
Otocyon =.
Otocysts = Otoliths
Otoliths of Vertebrates .
Otter
Ova of—
», Anodonta .
»» Birds
»» cockroach .
»» crayfish 3
>», earthworm .
», Echinoderms.
», Fishes .
»» fluke.
» frog. .
», Hydra. .
», Monotremes .
oy Myxine.
:, Peripatus
Reptilia ,
Placental Mamma!
.
.
*
.
.
°
.
°
s
686,
695,
INDEX
PAGE
420
571
690
746
709
723
487
-Pallium ‘
Ova of—Vertebrates .
Oviducal gland of skate
Oviduct of Vertebrates .
Oviparous Vertebrates .
Ovis .
Ovo- testis of. snail .
Ovo-viviparous Vertebrates
Ovulation . ‘ i
Ovum, the . 5
maturation of the
membranes
the Vertebrates
» theory ‘ ‘
Oxyuris : . .
Oyster . ‘i 3 .
oP
”
PACHYMATISMA . .
Pacinian corpuscles.
Pagurus . , .
Palearctic region . F
Paleemon :
Paleontological series .
Paleontology ‘ .
Palzeospondylus . .
Paleeostraca . . .
Palzeotherium .
Palato-pterygo quadrate ci
tilage .
Palinurus
Palisade worm
Palmipes .
Palp .
Paludicella .
Paludina ‘
Pancreatic juice .
Panda . ‘ %
Pandalus.
Pangolin .
Panmixia .
Panniculus adiposus
i carnosus
Panorpata . :
Pantopoda .
Papillary muscles .
Parachordals of skull
Paraglosse .
Paramcecium
Parapodia . .
ed
around
‘ar
844
Parasitic fauna.
Parasitism of—
INDEX
PAGE
. 800
ya Acarina . e372
3 Cestoda . "Ig!
5 Cirripedia . + 306
Pr Copepoda . + 303
i Crustacea - 316
+3 Gasteropods . . 422
55 Insects. + 357
‘3 Nematoda 204
4a Pentastomum » 374
Trematoda . . 188
Parasphenoid ¥ 553 38 3
Parasuchus . . . 641
Parazoa . . . » 134
Pareiosaurus . ‘ - 644
Parental care in—
a3 Amphibians. . 608
55 Asteroids . 260
35 Birds F . 679
v9 crocodiles . 640
‘5 Fishes . . 562
Mammals 737
Paroccipital process 702
Parthenogenesis . » 56
9 in Apus. + 300
3 in Artemia . 299
- in Insects . 349
Patella . a ‘ 420
Pathetic nerve 490
Paunch a 758
Pauropoda . 7 : 325
Pauropus 325
Peccaries 755
Pearl . ‘ 4 196
Pecora. + 757
Pecten. 427
>, of eye of Birds . . 665
Pectines of scorpion. - 365
Pectoral girdle . . 482
Pedal ganglion 385, 396, 405
Pedalion . ‘ + 246
Pedicellariz . 264
Pedicellina . s + 250
Pedipalpi 7 . - 366
Pedipalps of scorpion 365
i of spider i 368
Pelagia . . - 141
Pelagic life . ! 795
Pelagonemertes 198, 201
PAGE
Pelagothuria . . 268
Pelecypoda (see vous + 425
Pelias . . ‘ » 634
Pelomyxa . . - 108
Pelvic girdle . i . 482
Penzeus : : + 315
Penella - 287, 303
Penis of Mammals . » 719
Pennatula 5 » 164, 169
Pentacrinus . ; . . 272
Pentastomum . . 374
Pepsin . . s 27
Peragale A . ‘ + 747
Perameles : . 747
Peramelide . : . 747
Perch . . 573
Perching of Birds . . 657
Peribranchial cavity . 446
Pericalpa 7 174
Pericardium . A 504
Perilymph 494
Perineal glands 699
Peripatus F a. 33 I oe 3
55 and.Annelids . 323
Peripheral segmentation 67
Periplaneta . . 326
Periptychus . . . 766
Perisarc ‘ , “ 140
Perissodactyla : 754
Peristaltic action . ‘i » 28
Peritoneum . F TIL
Perivisceral fluids. . 784
Periwinkle . ‘ + 420
Perla ‘ 361
Petaurus 748
Petromyzon . : é ar § 21
on and Myxine con-
trasted . 526
Petrous portion of eres 705
Phacochcerus ‘ + 755
Phacops é 4 377
Phagocytosis . . 292
Phalanger E ‘3 748
Phalangeridze : . 747
Phalanges . . 482
Phalangide . 7 . 307
Phalangium . : 3 «367
Phallusia. ‘ ‘ 457
Pharyngobranchii 5 + 400
Pharynx a 499
Phaseogale , .
Phascolarctos
Phascolomyidz
Phascolomys .
Phascolosoma
Phenacodus . ‘
Phoca . 7
Phocena . .
Phocidze
Pholas . .
Phormosoma ‘s
Phoronidea . 7
Phoronis . .
Phrynosoma. .
Phrynus .
Phyllopoda .
Phyllosoma . .
Phylloxera . :
Phylogeny 7
Physalia .
Physeter : ‘
Physiology . .
Physoclistous ‘
Physostomous .
Phytoptids F
Phytozoa .
Pia mater
Pica é as
Pigeon. E .
Pigeon’s milk
Pigs :
Pilidium larva
Pineal body .
rf in Sphenodon
55 in Iguana .
Pinnipedia .
Pinnotheres . .
Pipa. : ij .
Pipe-fishes . > ‘
Pipistrelle .
Piroplasma . .
Pisces (see Fishes)
Pisiform
Pi-hecanthropus
Pituitary body
Placenta, Hints of a,
Mammalia
of Mammals .
”
INDEX
PAGE
3,
Placentation, classification of
Placoid scales
746
748
747
747
247
694
779
770
Plagraulax . .
Planaria .
Plankton. f
Planorbis
Plantigrade .
Plants and Animals
Planula larva
Plasmodium .
Plastron 4 .
Platanista . .
Platydactylus
Platyhelminthes
Platypus.
Platyrrhina .
Plesiosauria .
Pleura of crayfish .
Pleuracanthus
Pleural membrane
Pleural sacs . .
Pleurobranchs of crayfish
Pleurodont teeth .
Pleuronectidze
Pleuro-peritoneal cavity
Pliosaurus
Ploughshare bone
Plumularia .
Pluteus larva
Pluvianus .
Pneumatic bones .
Pneumoderma
Pneumogastric nerve
Podical plate of
roach
Podobranchs of crayfish
Podura
Poikilothermal
Poison gland of snakes . .
Polar globules
oes EPs
cock--
in earthworm .
» in Vertebrates
Polecat
Polian vesicle (Holothuria)
Polychezeta
| Polycladida .
“Polyclinum .
Polygordius .
Polymeniscous eyes
Polymorphism
Polyodon . .
Polyphemus . .
228,
327
291
339
220
513
7
270
846
Polyplacophora
Polypodium .
Polyprotodontia
Polypterus .
Polyspermy .
Polystomella
Polystomum
Polyzoa
Pond snail . . :
Pons Varolii. zi :
Pontobdella . .
Porcellenaster
Porcellio
Porcupine.
Porifera
Porocytes .
Porpita .
Porpoise.
Portal vein . :
Portuguese man- -of-war .
Portunus. . .
Post-anal gut
»» caval vein
Potamogale .
Powder-down
Praecoces
Preesternum .
Prairie-dog .
Prawn .
Precaval veins F
Preen gland of Pigeon
Prepuce
Priapulide .
Priapulus. ‘
Primary vesicles of brain
Primates. .
Primitive groove
>, streak
Pristis . .
Pro-amnion .
Proboscidea .
Procavia
Proceelous .
Proctodzeum
Procyon ;
Proechidna .
Proneomenia
Pronephros .
Prongbuck .
Propterygium
ae
642,
642,
INDEX
PAGE PAGE
418 | Proscolex . . . + 190
167 | Prosencephalon » 484
746 | Prosimiz + 983
570 | Prostate glands 719
63 | Protandrous. 520
92 | Protective colouring of Insects 357
188 | Proteles é P ‘ e ORF
249 | Proteomyxa . . - 107
421 | Proterosauria ‘i F . 620
709 | Proterospongia . 3 . 134
244 | Proteus . ‘ . 606
260 » animalcule (see
308 Ameeba) . » 89
773 | Protobranchia ‘ a + 426
124 | Protocercal . = ‘i - 564
126 | Protodrilus . ‘ i + 234
173 | Protogynous , ‘ + 453
770 | Protohippus . . . » 762
715 | Protohydra . . © - 149
173 | Protomyxa . . . 107
310 | Protoplasm . 7 + 31, 45, 51
502 | Protopterus . . 575
643 | Protospongia . : 133
779 | Prototheria . 7 . 740
655 | Prototracheata . 318
679 | Protovertebre = Mesoblastic
695 segments . . 601
772 | Protozoa 2 : . 88
309 », and Disease . » 120
627 55 and Metazoa . . 22
657 33 classification of . 106
719 93 functions in the » 412
247 3 general interest of ,. 122
247 93 history of the . . II9
487 1.8 immortality of . 122
785 3 reproduction in » 116
684 es structure of . II4
684 survey of the . + 107
567 Proventriculus ‘i : - 666
643 | Provortex 5 - 182
764 | Psalterium . a js » 758
763 | Pseudaxonia. : ‘ . 169
582 | Pseud-heemal system . » 259
498 | Pseudobranch . + « 556
778 | Pseudogastrula 5 + 432
740 | Pseudolamellibranchia . » 426
424 | Pseudonavicelle , ‘ « IO
509 | Pseudopodia ’ . . 89
759 | Pseudopus . . . - 629
537 | Pseudoscorpionidze . » 366
INDEX
PAGE
Psolus . . . . . 272 | Recapitulation of ancestral
Pteranodon . ‘ ‘ » 645 history . _ A
Pteraster . , . » 260 | Rectal gland . . .
Pterichthys . 5 . » §28 | Redcoral . A . .
Pterobranchia ‘ A . 440 | Redia . . 5 . .
Pterodactyl . . , . 645 | Reducing division . .
Pteropods . ‘ . 421 | Reed . ‘ . . :
Pteropus. 5 : . 783 | Reindeer . . ‘.
Pterosauria . . . . 645 | Relative antiquity .
Pterotrachea . . + 420 Animals . A :
Pterygota. . s . 360 | Renal-portal system . 595,
Pterylee . . - 653 | Reproduction . .
Ptyalin . ‘: . » 27 a9 of Amphibia ‘
Ptychodera . . z » 435 ms of Crustacea.
Puff-adder . 635 a of Fishes . ‘
Pulmonary sacs of Arachnids 363 ty of Insects . .
as », of scorpion . 366 + of Mammals.
a5 a of spider . 369 modes of . F
Pulmonata . 7 + 421 Reproductive system fee
Pupa . . . . 354 io Amphioxus . -
Pupil of eye. . . + 497 a Anodonta . A
Purpura . < 3 + 422 6 Arenicola . .
Pycnogonidze , : - 378 a Ascidian. F
Pycnogonum . . » 379 a Aurelia ‘
Pygostyle . . - . 660 i Balanoglossus F
Pyloric czeca of Insects . + 345 os bee. . ‘
Pyrosoma . .. ¥ . 457 vs cockroach . F
Python . . - » 634 5 crayfish $ 2
' Pythonomorpha . : » 645 53 Crocodilia .
“ iy Distomum . .
QUADRATE . : 5 , a ms ee » .
uagga F . . : 2 i rog. . :
Quege e WA haddock s :
RABBIT : . 5 . 697 5 Helix . - .
Raccoon : . . 778 Pr Hirudo é .
Radial symmetry . ‘ » 35 ‘6 Holothurian , .
Radials ofa fin . 7 » 564 a5 Insects’. : Fi
Radiolaria . s ; » 109 sh Lizards " ,
Radius. i « 482 si Myxine ‘ a
Radula of cuttlefish ‘ - 407 5 Nematoda . A
», of gasteropods . - 4II > Nemerteans . S
»» of snail . + 386 95 Peripatus . .
Raja. See Skate. we Petromyzon “ ‘
Rana. See Frog. aa pigeon. - ‘
Rangifer . . - 758 a rabbit. . F
Rat . ‘ ‘ ‘ «© 735 sa sea-urchin ¥ a
Ratitee . ‘ ci ‘ - 686 38 Sepia . . .
Rattlesnake . ‘ : . 635 i skate . - :
Ray e , : . 507 is spider . 7 a
Razor- shell 3 i ‘i . 427 rr starfish e ‘
PAGE
848
Reproductive system of—
a Vertebrates .
Reptilia
>, and Birds .
a development of.
»» extinct
», Yelationships of
Respiration in Animals
Respiratory system of—
5 Acarina
+3 Amphioxus .
a5 Anodonta.
ey Arenicola
a Ascidian .
- Balanoglossus
5 bee i
‘6 cockroach
55 crayfish :
#4 Crocodilia
5 Crustacea
a? frog i 7
$ haddock ‘
$9 Helix . ,
a5 Insects .
<5 Limulus F
‘5 Lizards ‘
‘5 Myxine
+ Peripatus.
a5 Petromyzon .
ay pigeon . :
‘is rabbit . .
ay scorpion ‘
~~ sea-urchin
55 Sepia . :
i” skate i.
ee spider .
19 starfish . .
ee Tracheata *
33 Vertebrates .
Retia mirabilia of Cetacea
es of Sirenia
Reticulum _ . ‘
Retina . : : -
Retinula zi . .
Reversion . . .
Rhabdites 7 .
Rhabdoceelida . i
Rhabdom . . é
Rhabdonema a .
Rhabdopleura. .
INDEX
PAGE
se Demphorhynalus . :
513 | Rhea . é . .
610 | Rhinoceros . . F .
612 | Rhinoderma. . : .
642 | Rhinolophus f E :
644 | Rhizocrinus . q . .
646 | Rhizopoda F * .
29 | Rhomboidal sinus. : .
Rhopalura_. 5 A
372 Rhynchobdellidee . :
463 | Rhynchocephalia . .
-398 | Rhynchota . :
227 | Rhytina .
446 Ribs of Vertebrates e
437 | Rock-dove . .
337 | Rodentia . . : .
330 | Roe-deer ‘
291 | Rorqual .
640 | Rostrum of crayfish
313 | Rotatoria=Rotifers . .
596 | Rudimentary organs . 309,
556 | Rumen i . .
388 | Ruminants . 2
345 | Rumination , F .
376 :
627 | SABELLARIA & . Fé
519 | Saccocirrus . : . é
320 | Sacculina : . .
525 | Sacculusofear . ‘ ‘
671 3 rotundus. a #
715 | Sacrum . .
366 | Sagitta . , :
267 | Salamander . . . .
408 | Salinella ‘ . . .
543 | Saliva . .
369 | Salivary glands of ant- eaters .
259 7 3, of cockroach,
318 55 », Of Collocalia .
508 . » Of Felix -
768 a8 » of Hirudo .
752 33 », Of Insects ,
758 re xy of rabbit 0
497 | Salmon . i i .
288 | Salpa . . .
86 | Sarcolemma . :
180 | Sarcoptes .
182 | Sauropsida, Ichthyopsida and
288 Mammalia :
205 | Saurure 5 3
442 | Saw-flies
Scales of Birds,
»» Of Fishes ,
»» Of Mammals
» of en
Scallop é
Scalpellum .
Scaphirhynchus
Scaphopoda .
Scapula ‘i
Schizocardium
Schizoceele .
Schizont
Schizopoda . : 5
Sciuromorpha x x
Sciurus : .
Sclerotic . : r
3) ossicles ,
Scolex . ‘
Scolopendra . . .
Scolopendrella
Scorpion 3 ‘
Scrotum “ " i
Scuta . . . .
Seutés, i .
Scyllium s i
Scyphistoma ‘ F
Scyphomedusze
Sea-anemone
», butterflies
53 COWS
+) cucumbers
», horse
», lion ‘
>> mouse
>», Otter °
” pen
3, snakes
y> Squirts (vide T unicata)
urchin :
Seals z
Sebaceous glands ,
Sectorial tooth
Sedentaria
Segmental duct of Vertebrates
Segmentation of Ovum .
ee ewe
.
>, in Amphioxus .
.» », Anodonta .
>> 9 -Ascidian
+> +, Balanoglossus
+> 33 Cheetognatha
INDEX
PAGE
655 | Segmentation of Ovum—
565 »» in Crustacea
720 +9 5) earthworm .
612 sy 9, Echinoderms
426 x 9», fowl . A
305 1» 9 frog. F ,
570 2 9) Gasteropoda
424 »» 93 haddock .
482 9 Hydra
435 »> 93 Insects 5
199 s> 2, Lamprey
105 » »» Monotremata
308 +> 9) Placental Mam-
772 mals . .
772 »> 3) Reptilia
497 +» 99 Scorpion
665 >» 9 Skate .
190 + 9) LTeleosteans .
325 s> 3, Wertebrata .
325 | Seison .
364 | Selachii= Elasmobranchs
717 | Selection . :
303 | Selenodont .
477 | Self-fertilisation in Hydra
567 si 5) Serranus .
157 ats », tapeworms
173 5, Trematoda
159 | Sella turcica.
421 |-Semicircular canals of ea ear
752 | Seminal vesicle
268 | Semnopithecus
562 | Sense organs of—
779 $5 Amphioxus
233 oh Ascidian
778 3 Aurelia
164 a crayfish
635 3 Crocodilia .
443 5a Crustacea .
262 3 frog.
779 ag haddock
721 a hag .
775 ‘5 Helix.
231 es Hirudo .
511 “ Holothurian
65 a Insects
468 5 Lampiey
400 i pigeon.
450 33 rabbit
438 3 Sepia .
245 er skate . ‘
849
PAGE
313
220
275
683
599
423
558
148
246
567
811
724
147
561
191
186 °
237
269
343
524
665
710
406
542
850 INDEX
PAGE
Sense organs of—
5 spider . + 368
starfish ‘ + 256
”
a Vertebrates + 492
Sepia . . : . + 402
Sepiostaire . : : + 405
Serpents. . : - 628
Serpula - j 2 # 233
Serranus. : é - 561
Sertularians . ‘i . . I71
Sesamoid bones . ‘ + 700
Sete . : + 213, 283
Sex, evolution of . . 2 54
Sexual selection . 812
$5 » among birds | 677
33 >». amongspiders 371
»» Yeproduction . 52
‘5 45 divergent
modes of . , é . 56
Sharks. . a - - 567
Shell of—
59 Anodonta . + 393
#5 Argonauta . » 429
3 Cephalopoda - 428
rs Chiton. i » 417
a5 Helix . j - 384
sc Nautilus ‘ « 433
53 Planorbis_ . + 431
‘3 Scaphopoda . + 424
Spirula . » 429
Shell gland or sac . + 301
Shore-crab . ‘ . + 310
Shrews ‘ ‘ : . 780
Shrimp . 3 é + 309
Sida. . é + 301
Silicispongize ‘ A + 133
Simia . ‘; : ‘ . 788
Simiidze : 788
Simplicidentata . ‘ » 773
Sinews . . .
Sinus venosus.. + 505
Siphon of Cephalopods. + 428
», of Gasteropods . + 422
», Of Lamellibranchs . 426
Siphonaptera ‘ + 360
Siphonoglyphes ‘3 ‘
Siphonophore . - 7
Siphonops . . - - 606
Siphonozooid . ‘
Siphuncle of Nautilus ,
Sipunculidz .
Sipunculus .
Siren . E : :
Sirenia
Skate . :
55 development of .
Skeleton of—
3 Amphioxus .
ay Balanoglossus
>» bat .
35 Birds . :
a0 Chelonia .
i crayfish
i Crinoids
a6 Crocodilia .
9x frog.
we haddock
1 Hatteria .
% Lizards
es Mammals ,
49 Myxine
e Petromyzon .
45 pigeon . .
ss rabbit . .
5 sea-urchin .
53 Sepia é
- skate . "
<3 Snakes. <
- starfish . .
33 Vertebrates .
Skin of—
Amphioxus .
ye Anodonta
4 Balanoglossus
a Birds
58 cockroach .
sik crayfish ¥
sti Crocodilia .
a earthworm .
- Fishes .
5 frog. ‘
pe haddock,
x8 Helix . .
ae Insects. ‘
a5 Leech . .
- Lizards ‘
7 Mammals ,
we Myxine 'e
” Peripatus .
os Petromyzon ,
Sleeping-sickness paras:
Sloth animalcules
Skin of—
3 rabbit . -
Sepia . p
sis skate i
st Tunicate .
“3 Vertebrates .
Skull . é
Skull of—
vs Crocodilia .
vi frog. ‘
PP haddock ‘
- lizard . .
ve Mammals .
“ pigeon . ri
55 rabbit . .
i skate . .
33 snakes. .
i
Sloths . A
Slough of snake
Slow-worm .
Slug.
Snail (see Helix)
Snakes.
Solaster
Solen . ‘
Solenia
Solenomya .
Solpuga
Solpugidee or + Solifugse
Somatic cells
Somatopleure
Somites = Segments
Song of birds
Sorex . .
Spadella .
Spatangus .
Spatularia .
Species
Spermathecze
Spermatic cord
Spermatophores
Spermatozoa
Spheeridia
Sphzrozoum
Spheerularia .
Sphenethmoid
Sphenodon *.
Sphenotic
.
.
.
.
.
.
te
INDEX
PAGE
Spicules (of sponge)
- Spider ia s
Spiders
Spinal cord . .
3 ganglia
yy) nerves
Spinning glands of Insects
»» of Spiders
Spiracles of skate .
Spiracular cartilage
Spiral valve .
Spirochete . .
Spirula .
Splanchnopleure .
Spleen. qi
» of frog i
»» of pigeon .
xy of rabbit
», of skate
“Splenial
Sponge, development of a
Sponges ‘ ‘
Spongilla
Sporocyst
Sporozoa
Sporozoite 7
Spring-tails . .
Squamosal . .
Squilla . :
Squirrels
Stagonolepis .
Stapes of ear .
Starfish ‘
Stegocephali .
Stelechotokea
Steller’s sea-cow .
Stenothermal
Sterno-tracheal muscles.
Sternum of Crustacean
ment
of pigeon
Stickleback .
Stigmata. .
Sting of scorpion .
Stinging animals .
» cells %
Stipes . 7 7
Stoat . . .
Stolonifera .
seg
Stomatodeeum = Stomodzeum.
852
Stomato-gastric nerves . ‘
Stomodzum . 5
Stone canal of Starfish a
Strepsiptera . . ‘ :
Streptoneura ‘ . .
Stringops . . :
Strobila . . :
Strongylocentrotus . 7
Strongylus . 7
Struggle for existence . .
Struthio . ‘ . .
Sturgeon. F - ‘
Stylaster . 7
Stylifer 5 :
Suberites . Pe
Submentum . :
Sub-neural ee of As-
cidian .
Substitution of organs ‘
Subzonal membrane 643; 685;
Sudorific glands .
Suide . ‘ ‘ r
Sulcus . 5 ‘ ‘
Sun animalcules . ‘i ‘
Suprarenal bodies ‘ 3
53 body of pigeon.
si x» Ofrabbit .
Surangular . : . .
Surinam toad . :
Sus. , ‘“
Suspensorium . . :
Swim-bladder . . .
Sycon type (of sponge» .
Syllids . . :
Symbiosis. :
Symmetry of Animals . <
Sympathetic nervous system .
Symphyla . ‘ é z
Symplectic .
Synangium . 7 5 .
Synapta . .
Syngamus . : 7
Syn-sacrum . . .
Syrinx . 5 ‘
Systemodon .
TADPOLE of frog
Tenia . :
Teeniole .
Tail of Fishes
INDEX
PAGE
287
498°
258
360
420
674
190
267
208
735
686
570
171
Talpa . .
Tamandua
Tape-worms.
Tapir . .
Tarantula ,
Tardigrada .
Tarsipes
Tarsius , a
Tarso-metatarsus .
Tarsus . ‘
Tasmanian wolf
Tatusia F .
Tealia . P 4
Tectibranchia ‘
Teeth of Crocodilia
», of extinct birds
> Oflizards .
3, of Mammals
3» of Ornithorhynchus
>», Ofskate .
of snakes
Tegenaria .
Teleostei ,
Telolecithal .
Telson . .
Temnocephaloidea
.
Temperature of Birds .
of Mammals
Temporal. See Squamosal
Tenrec.
Tentaculocysts
Tentorium
Teredo, s
Tergum .
Termites.
Terrestrial fauna
Testudo
Tetrabranchiata
Tetranychus.
Tetrapneumones .
Tetrarhynchus .
Tetrodon . .
Tetronerythrin
Thalamencephalon
Thalassicola . :
Thaliacea .
Thecophora . .
Thelyphonus
Theromorpha
Thoracic duct
Thread cells =
Thread-worms .
Thylacinus . ‘
Thymus , és
» Of frog. .
$i of rabbit . 3
8 of skate . x
Thyroid . ‘
» Offrog . ‘
ay of rabbit .
a of skate . *
Thysanoptera F
Thysanura . : .
Tibia . ‘i
Tibio-tarsus . .
Ticks .
Tiedemann’s ‘bodies
Tiger . >. : ‘
Tillotherium a .
Tinamou
Tissues
Titanotherium
Toads . 3 .
Tornaria . :
Torpedo : .
3 electric organ of
Tortoises
Toxodon fi
Trabeculee of skull
-Tracheze 7 “
o «=O. Arachnoidea
8 of Mites . :
» Of Peripatus
5, Of spider .
Tracheal gills
Tracheata .
Trachomedusee .
Trachydosaurus
Trachymedusze
Tragulina .
Trematoda .
Trematoda, classification of .
Treptoplax .
Trichechus .
Trichina
Trichocephalus .
Trichocysts . :
Trichoplax . .
Trichoptera . :
Tricladida . .
Stinging cells’,
INDEX '
PAGE
142 | Triconodon , a
202 | Trigeminal . .
746 | Trilobites . . i
500 | Triploblastic. . ,
596 | Tristomum . ;
712 | Triton . , . <
546 | Tritubercular : 7
499 | Trivium 2 5 5
596 | Trochanter .
717 | Trochlear nerve
546 | Trochosphere
361 | Trombidium. . ‘
361 | Trophoblast . : A
482 | Trophospongia . .
663 | Trophozoite. . “
373 | Tropidonotus
258 | Truncus arteriosus
777 | Trypanosoma
766 | Trypsin .
689 | Tse-tse fly .
40-44 | Tube- feet of brittle-star.
763 a9 of sea-urchin .
605 55 of starfish
439 | Tubercle of rib
567 | Tubifex
532 | Tubipora .
613 | Tubularia .
765 |. Tunicata f .
478 | as classification of
345 | Tunicin 7 : .
363 | Tupaia. -
372 | Turbellaria . . ‘
320 >» Classification
346 | Turtles. A g
370 | Tylenchus . ‘ .
318 | Tylopoda
173 | Tympanic bulla
628 | Tympanum . 3 i.
171 | Typhlopide . F .
756 | Typhlosole . . .
183 | Tyroglyphus. .
188
136 | UINTATHERIUM . .
779 | Ulna . . A
207 | Umbilical cord . 3
206 55 vesicle . F
96 | Umbilicus of embryo .
136 53 of feather .
360 | Umbo. -
182 | Uncinate processes ,
228,
120, 359
ee eee
ies
te} iS)
Oo
+ 393
636, 660
854 INDEX
PAGE PAGE
Ungulata. . . . ‘ 53 | Vascular system of—
Unio . ‘ ‘ » 393 i sea-urchin . - 267
Ureter . . 717, 718 o Sepia . + 408
Urethra ‘ 719 5 skate . A » 544
Urochorda , 443 is spider . : » 369
Urodela 606 is starfish . + 259
Urostyle ‘ + 582 si Vertebrates . 504
Urside . : - 778 | Velarium . ‘ F - 152,
Ursus . . 778 | Velella. : . $ « 193
Uterus . » 719 | Veliger. 416
49 masculinus 719 | Velum. » 150
Utriculus of ear 493 | Venous system. See Vascular
system.
VACUOLES . , j - 44 | Ventricles of brain 487
a contractile . » 90 rr of heart 505
x food . a » 90 | Venus . x r P - 426
Vagina. Z Z ; » 719 >, flower-basket . é- 133
Vagus nerve . P ‘ « 490 » girdle 175
Vampire bat. . . - 783 | Vermiform appendix . » 711
Vampyrus . F . 783 | Vertebra, parts ofa 482, 630, 700
Varanide . . 5 + 629 | Vertebral column. . 481
Variation 54, 151, 808 3 3 of birds 648
Vas deferens. : F - 513 3 45 of crocodile 636
Vascular system of— 53 » Offrog . 582
Amphioxus . - 467 ee ‘9 ofhaddock 552
3 Anodonta . » 398 i “5 of lizard 623
Fe Arenicola 227 35 is of pigeon . 659
Ae Ascidian . + 449. - x6 of rabbit 700
53 Ennai + 437 55 “9 of skate . 533
a bee. + 337 i 9 ofsnakes . 630
i cockroach . » 330 ; Vertebrata P a - 473
4 crayfish a . 290 5 affinities of Annelids
5 Crinoid ‘ . 274 with . 476
a Crocodilia . . 639 55 ancestry of . 475
» ‘Dipnoi. - 575 Ja development of 514
‘3 earthworm . » 216 ss general characters
em frog. ‘ + 591 : of + 473
+5 haddock F - 557 9 ae classifica-
3 Helix . - - 387 tion of. 475
55 Hirudo. 2 - 240 a3 gill-clefts of . + 474
23 Insects. ' + 347 <3 heart of ‘ + 505
i Lizards 3 625 a and Invertebrata . 7
55 Mammalia . +» 695 - nervous system of. 482
sy Myxine ‘ + 520 " notochord of. . 481
33 Nemerteans . + 200 a segmental symme-
$5 Peripatus + 320 “ try of , » 474
55 Petromyzon , + §25 | Vesicule seminales. See Re-
a pigeon . . - 667 productive system.
5 rabbit . F » 712 | Vesiculate . é - « E51
7 scorpion, - 365 | Vespertilio . a « 2a 783
Vesperugo . .
Vestibule. .
Vestigial structures
ss eee
"ee ee
Vibrissze
Vicugna . .
Villi . ‘ . F
Visceral arches . F
clefts j
”
>» nerves
Vitelline membrane of ovum
Vitreous humour . P
Viverra i é ‘
Viviparous Fishes . . .
*8 Insects 7
35 Lizards ‘ :
at Verrtebrates .
Vole : ; 7
Volvox. ; 3 e .
Vorticella . ‘ é ‘
Vulva . ‘ ‘ .
WALKING-STICK insect
i leaf
Walrus
Warm- blooded 4
Wasps . ;! 4 . .
Water bears .
Water scorpion, vascular sys-
tem of—
"5 oe Crinoid
- WP Holothurian
55 As Ophiuroid .
sy 43 sea-urchin .
35 55 starfish
Weasel. . . . .
Web of Spiders 5
Whales é
Wheel animalcule. a :
Zygosphene . . .
855
PAGE
Whelk . é 5 ; 420
Whip scorpions » 366
White matter of brain . 489, 709
» Ofspinalcord . 489
Wing of bat . . i 781
» of Birds S . 663
» Ofinsect . ‘ + 341
», of pterodactyl . 645
Wolf... 777
Wolffian duct é SII
| Wombat . . . 747
Worms 7 7 10, II, 179
XANTHARPYIA . 783
Xenarthra . 750
Xiphisternum . . 695
Xiphosura . : ; 375
Yapock c - 746
Yellow cells. » 113
Yolk 5 - 59
>> sac : 683, 685, 728
1» 9 Placenta. . 643, 733
ZEBRA. ‘ 762
Zeuglodonts. 771
Zozea * 315
Zoantharia 164
Zona radiata of ovum 513
Zoochlorellze . 113
Zoo-geographical regions 805
Zoonerythrin. . : » 130
Zoophytes . qi ‘ a 137
Zygeena ‘ . - 567
Zygantrum . - 630
Zygapophyses ‘ - - 700
Zygomantic arch . + 702, 775
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