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AN ELEMENTARY COURSE

OF

PRACTICAL ZOOLOGY

AN ELEMENTARY COURSE

OF

PRACTICAL ZOOLOGY

° cs BY THE LATE

T. JEFFERY PARKER, D5Sc., F.R.S.

_—
PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO
DUNEDIN, NEW ZEALAND

> AND

W. N. PARKER, Px.D.

PROFESSOR OF ZOOLOGY AT THE UNIVERSITY COLLEGE OF SOUTH WALES
AND MONMOUTHSHIRE, IN THE UNIVERSITY OF WALES

With One Hundred and Fifty-six MMlustrations

London
MACMILLAN AND CO., Limirep
NEW YORK: THE MACMILLAN COMPANY

1900

Ricuarp Ciay anp Sons, Limitep,
LONDON AND BUNGAY.

PREFACE

In the early part of 1897, my brother and I had arranged
to collaborate in writing a practical text-book of Elementary
Zoology, adapted more particularly to the requirements of
Students pursuing courses in the subject as laid down by
various examining bodies. We had, however, only reached
the stage of deciding on a general plan at the time of my
brother’s death in November of the same year.

The following are the chief points on which we had
agreed :—

1. To adopt the method pursued in Huxley and Martin’s
Elementary Biology of giving a connected account of each
example. 2. To give brief practical directions which should
serve mainly as a guide, the student being able to refer, in
case of difficulty, to the descriptive accounts preceding
them. 3. In the larger animals, to arrange for as much work
as possible to be done on one specimen: there is much to
be said in favour of this plan apart from the fact that the
average student cannot give sufficient time to the subject
to dissect a fresh specimen for each system of organs.
4. To begin the course of instruction by an introductory
study of one of the higher animals ; to include in this in-
troduction the elements of Histology and Physiology ; and

vi PREFACE

to select the Frog for the purpose: after trying various
methods, I have found this plan to be the most satisfactory
in practice. 5. To give drawings and diagrams of difficult
dissections, and of details which the beginner cannot as a
rule make out satisfactorily for himself; but otherwise to
limit the number of illustrations so as not to tempt the
student to neglect observing the things themselves. 6. To
include a short account of methods and technique, limited
to the barest essential outlines, sufficient for a student
working by himself to make out the things described, but
not going into such details as would naturally be learnt in a
properly organised laboratory.

In the meantime, my brother had in preparation a Biology
for Beginners, in which he intended to carry out the
plan, suggested in the preface to his Zlementary Biology,
of giving a simple account, with practical directions, of one
of the higher animals and one of the higher plants, as
an introduction to the study of Biology. The animal he
selected was the Frog, and the manuscript of the greater
part of this section of the book was already finished and
the rest in rough draft. He had previously suggested that
some of this work might be utilised for our proposed Prac-
tical Zoology ; and I found that, with certain additions and
with modifications in the arrangement, the whole of it was
exactly the kind of introduction I had in view for the first
part of our book. Some of the illustrations that my brother
had intended to insert in the Biology for Beginners I have
found it advisable to omit, and even now the figures in the
introductory part are purposely nearly as numerous as those
in the rest of the book. But apart from these various minor

PREFACE vil

modifications, Chapters I—-XII in Part I are almost en-
tirely taken from my brother’s manuscript. _ :

We felt that there could be no object in entirely re-
writing the descriptions of several familiar animals already
given in my brother’s published works; and in Part II. I
have intentionally made use of these descriptions, borrowing
very freely from the Elementary Biology, as well as (with
Professor Haswell’s permission) from the TZextbook of
Zoology ; and, to a less extent, from the Zootfomy.

The practical directions are mainly based on a series of
Laboratory-instructions I drew up some years ago for the
use of my junior classes, which consist principally of
students preparing for the Intermediate Science examin-
ation of the University of Wales, the Preliminary Scientific
examination of the London University, and the first ex-
amination of the Conjoint Board of the Royal Colleges
of Surgeons and Physicians. The time such students can
devote to an elementary course in the subject is limited ;
and throughout the book I have borne in mind that the
main object of teaching Zoology “as a part of a liberal
education is to familiarise the student not so much with the
facts as with the ideas of the science,” but at the same
time that he should be provided with a sound basis of facts
so arranged, selected, and compared as tocarry out this
principle.

Our original intention was to include one or more examples
of each of the larger phyla, and also to add a practical
exercise after each type, giving general directions for the
examination of an allied form for comparison. But I
found that this would be impossible within the space of a

viii PREFACE

single volume, and it was therefore necessary to limit the
descriptions mainly to those animals to which the students
for whom the book is chiefly intended have to give special
attention. This has resulted in rather a heavy balance on
the side of Vertebrates; but on the whole, I think that if
sufficient work is done on the lower animals to illustrate
certain main facts and generalisations, a comparative study
of several Vertebrates forms as good a training as any for
beginners—more especially in the case of medical students.

I am indebted to Mr. H. Spencer Harrison, B.Sc.,
Demonstrator of Biology in this College, for much assistance
in testing and improving the practical instructions, as well
as for various suggestions and for help while the work was
passing through the press. The new figures were re-drawn
from the originals by Mr. M. P. Parker.

W. N. PARKER.

UNIVERSITY COLLEGE, CARDIFF,
Novenber, 1899.

. THE FROG (continued): THE JOINTS AND MUSCLES

CONTENTS

PREFACE .... . .

Part I

CHAPTER I

SCOPE OF THE SCIENCE OF BIOLOGY—THE FROG : PRELIMINARY
SKETCH OF ITS STRUCTURE, LIFE-HISTORY, AND VITAL
FUNCTIONS. . 2... s. eee tae eS aN ph a tak Sate o.

HINTS ON DISSECTION . SH eS o

CHAPTER II

THE FROG (continued): GENERAL INTERNAL STRUCTURE
PRACTICAL DIRECTIONS .. . Say raat’ Sei

CHAPTER III

THE FROG (continued): THE SKELETON .
PRACTICAL DIRECTIONS .. ...

CHAPTER IV

PRACTICAL DIRECTIONS ... 2... ae

CHAPTER V

THE FROG (continued): WASTE AND REPAIR OF SUBSTANCE
—THE DIGESTIVE ORGANS—NUTRITION. 2... 2 ee ee
PRACTICAL DIRECTIONS 2. + se ee eee eee

PAGE

16-
31

66.
76

x CONTENTS

CHAPTER VI

THE FROG (continued): THE VASCULAR SYSTEM-—THE CIRCU-
LATION OF THE BLOOD -.... «eee
PRACTICAL DIRECTIONS 3 ; a ar

CHAPTER VII

THE FROG (continued): ‘THE MICROSCOPICAL EXAMINATION
OF THE SIMPLE TISSUES .
PRACTICAL DIRECTIONS .. ts

CHAPTER VIII

‘YHE FROG (continued) : THE MICROSCOPICAL EXAMINATION OF
THE COMPOUND TISSUES—GLANDS—SECRETION AND AB-
SORPTION . . oan . soe

PRACTICAL DIRECTIONS

CHAPTER IX

THE FROG (continued): RESPIRATION AND EXCRETION... .
PRACTICAL. DIRECTIONS: 405-20 Gh ee a ewe ee

CHAPTER X

THE FROG (contizucd): THE NERVOUS SYSTEM . 2 3
PRACTICAL DIRECTIONS See dhas ‘ i

CHAPTER XI

THE FROG (continued): THE ORGANS OF SPECIAL SENSE
PRACTICAL DIRECTIONS .. . eae a

CHAPTER XII

THE FROG (continued): REPRODUCTION AND DEVELOPMENT.
PRACTICAL DIRECTIONS ......

CHAPTER XIII

THE FROG (continued): MEANING OF THE TERM SPECIES—THE
PRINCIPLES OF CLASSIFICATION—EVOLUTION-—ONTOGENY
AND PHYLOGENY—HEREDITY AND VARIATION—STRUGGLE
FOR EXISTENCE—SELECTION—ORIGIN OF SPECIES

PAGE

78
98

104
119

126
135

141
152

154
175

179
191

193
210

215

CONTENTS xi

Part II

CHAPTER I

AMCEBA—UNICELLULAR AND MULTICELLULAR ANIMALS . . 229
PRACTICAL DIRECTIONS i ae . * 238

CHAPTER II
HAMATOCOCCUS AND EUGLENA—MONADS AND BACTERIA—DIF-
FERENCES BETWEEN ANIMALS AND PLANIS—SAPROPHYTES 240
PRACTICAL DIRECTIONS... : . e259

CHAPTER III

PARAMCECIUM, OPALINA, VORTICELLA AND ITS ALLIES—PARA-
SITIC AND COLONIAL ORGANISMS—BIOGENESIS AND ABIO-
GENESIS—CLASSIFICATION OF THE UNICELLULAR ORGAN-

ISMS EXAMINED : ; : bi &e chm oor
PRACTICAL DIRECTIONS ...... j . . 286
CHAPTER IV,

HYDRA: BOUGAINVILLEA—ALTERNATION OF GENERATIONS—

CHARACTERS OF THE PHYLUM C@LENTERATA. ... 289

PRACTICAL DIRECTIONS Byres ; . 314,
CHAPTER V

THE EARTHWORM—CHARACTERS OF THE PHYLUM ANNULATA 318

PRACTICAL DIRECTIONS me os «gta 2 ». Sat
CHAPTER VI

THE CRAYFISH—CHARACTERS OF THE PHYLUM ARTHROPODA. 346

PRACTICAL DIRECTIONS... ... .... sey B72

CHAPTER VII
THE FRESH-WATER MUSSEL—CHARACTERS OF THE PHYLUM
MOLLUSCA—ENUMERATION OF THE CHIEF PHYLA OF THE
ANIMAL KINGDOM .. ~~... 0 6 ee ee ee ee e381
PRACTICAL DIRECTIONS .. . he ois Spe ae Annes 397

xii CONTENTS

CHAPTER VIII

PAGE
CHARACTERS OF THE PHYLUM VERTEBRATA—AMPHIOXUS 403
PRACTICAL DIRECTIONS , . Se a . . 410
CHAPTER IX
CHARACTERS OF THE CLASS PISCES—THE DOGFISH Pa 414
PRACTICAL DIRECTIONS . .. . oa . er 456
CHAPTER X
CHARACTERS OF THE CLASS MAMMALIA—THE RARBIT... . 466
PRACTICAL DIRECTIONS . ae a die a 525
CHAPTER XI
THE MINUTE STRUCTURE OF CELLS—CELL DIVISION—OOGENE-
SIS AND SPERMATOGENESIS—MATURATION AND FERTILI-
ZATION OF THE OVUM—EFFECT OF FOOD-YOLK ON DE-
VELOPMENT—FORMATION OF THE CHIEF ORGANS OF THE
ADULT IN VERTEBRATES, AND OF THE AMNION, ALLANTOIS
AND PLACENTA... 2. 2 + © oe . 541
PRACTICAL DIRECTIONS . a . 585

INDEX. . oat ag Ee Boe Ee ee RSS

AN ELEMENTARY COURSE OF
PRACTICAL ZOOLOGY

PART I

CHAPTER I

SCOPE OF THE SCIENCE OF BIOLOGY—THE FROG: PRELIMIN-
ARY SKETCH OF ITS STRUCTURE, LIFE-HISTORY, AND
VITAL FUNCTIONS—HINTS ON DISSECTION

Biology, Zoology and Botany.—There is a good deal of
misconception as to the scope of the science of Biology.
One often meets with students who think that while the
study of animals as a whole is Zoology and the study of
plants as a whole Botany, Biology is the study of a
limited number of animals and plants, treated as if they
had no connection with anything else,—even with one
another.

This is quite wrong. Biology is the master-science which
deals with all living things, whether animals or plants, under
whatever aspect they may be studied. Physiology, treated
for practical purposes as a separate subject, is a branch of

biology ; so is anatomy, to which the medical student
Pract. Zoo. B

2 SCOPE OF THE SCIENCE OF BIOLOGY CHAP.

devotes so much time; so are botany and zoology, in the
ordinary sense of the words, z.e. the study of the structure,
the mutual relations, and the arrangement or classification
of plants and animals. But biology may also be pursued,
and very profitably pursued too, quite independently of
teachers, class-rooms, and examinations. The country boy
who knows the song of every bird, its nesting place, the
number of its eggs, the nature of its food, the lurking place
of the trout in the stream or the frogs in the marsh ; who
has watched the ants with their burden of grain, or the bees
with their loads of honey or pollen; has begun the study of
biology in one of its most important branches. The in-
telligent gardener who observes the habits of plants, their
individual tastes as to soil, moisture, sunshine and the like,
is also something of a biologist without knowing it. So
also is the collector of eggs, shells, or insects, provided he
honestly tries to learn all he can about the things he collects,
and does not consider them merely as a hoard or as objects
for barter. Indeed, all that is often spoken of as natural
history, so far as it deals with living things—plants and animals
—and not with lifeless natural objects, such as rocks and
minerals, is included under the head of biology.

What then is the connection between biology in this wide
sense and the kind of thing you are expected to learn in a
limited number of lessons ? Simply this :—In the class-room
nature cannot be studied under her broader aspects : indeed,
much out-door natural history cannot be ¢augft at all, but
must be picked up by those who have a love of the subject,
a keen eye, and patience. But there is one thing we can
do within the narrow limits of the class-room : we can con-
fine ourselves to some department of biology small enough
to be manageable: we can take, for instancé, one or more
familiar animals and plants, and, by studying them in some

I THE STUDY OF ZOOLOGY 3

detail, get some kind of conception of animals and plants
as a whole. This book deals with the zoological side of
biology only ; and what we have now to do is, in fact, what
you have often done in the study of English: you take a
single verse of a poem at a time, analyse ‘it, parse it, criticise
its construction, try to get atits exact meaning. If you have
any real love of literature this detailed study of the part will
not blind you to the beauty of the whole. And so if you
have any real love of nature, the somewhat dry and detailed
study we have now to enter upon should serve to awaken
your interests in the broader aspects of biology by showing
you, in a few instances, what wonderful and complex things
animals are.

One word of warning before we begin work. You must
at the outset disabuse your mind of the fatal error that
zoology or any other branch of natural science can be learnt
from books alone. In the study of languages the subject
matter is furnished by the words, phrases, and sentences of
the language; in mathematics, by the figures or other symbols.
All these are found in books, and, as languages and mathe-
matics are commonly the chief subjects studied at school,
they tend to produce the habit of looking upon books as
authorities to which a final appeal may be made in disputed
questions. But in natural science the subject-matter is
furnished by the facts and phenomena of nature ; and the
chief educational benefit of the study of science is that it
sends the student direct to nature, and teaches him that a
statement is to be tested, not by an appeal to the authority
of a teacher or of a book, but by careful and repeated
observation and experiment.

The object of this book, therefore, is not only to give
you some idea of what animals are, but also to induce
you to verify the statements contained in it for your-

Bs2

4 THE FROG CILAP.

self. The description of each animal you should follow
with the animal before you; and if you find the account in
the book does not agree with what you see, you must
conclude, not that there is something wrong with your
subject, but either that the description is imperfect or
erroneous, or that your observation is at fault and that
the matter must be looked into again. In a word, zoology
must be learnt by the personal examination of animals: a
text-book is merely a guide-post, and al] doubtful points
must be decided by an appeal to the facts of nature.

It matters very little what animal we choose as a starting-
point—-a rabbit, a sparrow, or an earthworm—one will serve
almost as well as another to bring out the essential nature
of an animal, how it grows, how it is nourished, how it
multiplies. On the whole, one of the best subjects to begin
with is a frog: partly because it is easily obtained, partly
because its examination presents no difficulties which an
intelligent student may not be expected to surmount by
due exercise of patience.

Let us therefore begin our studies by catching a frog and
placing it in a convenient position for examination, as, for
instance, under an inverted glass bell-jar or even a large
tumbler.

External Characters.—Notice, first of all, the short,
broad ¢vunk, passing inserisibly in front into the flattened
Aead—there being no trace of a neck—and ending behind
without the least vestige of a tail: these constitute the axza/
parts of the animal. In the ordinary squatting position the
back has a bend near the middle, producing a peculiar
humped appearance. The head ends in front in a nearly
semicircular szout, round the whole edge of which extends
the huge slit-like mouth. On the top of the fore-end of the

I EXTERNAL CHARACTERS 5

snout are the two small os¢ri/s, one on each side of the
middle line; and, some distance behind them, the large,
bright, prominent eyes, in which we can distinguish, as in
our own eyes, a coloured ring or 77s, surrounding a
roundish black space or pupil. The eyelids, however, are
rather different from our own: the upper is fairly well
developed, but the lower is a mere fold of skin, incapable
itself of being folded over the eye, but produced into a thin
transparent skin, the mécttating membrane, which can be
drawn upwards over the eye. The entire absence of eye-
brows and eyelashes is a point worthy of notice.

Extending backwards from the eye is a large black patch,
in the middle part of which is a circular area of tightly
stretched skin, reminding one of the parchment of a
tambourine: this is the drum-membrane, or tympanic
membrane, a part of the ear. Here again we see a striking
difference from our own organs: in ourselves the drum-
membrane, instead of being flush with the surface of the
head, is placed at the inner end of a deep passage or tunnel,
the entrance to which is guarded by the large external ear.
Of the latter there is no trace in the frog.

Attached to the trunk are two pairs of offshoots or append-
ages, the arms and legs, or fore- and hind-limbs, in which
the resemblance to our own limbs will be at once obvious.
The arms are very short: each consists of an upper arm, a
fore-arm, and a hand, the latter provided with four fgers,
which are slender and tapering and have no nails. The
legs, on the other hand, are very long: each consists of a
stout ¢high, a long shank, with a well-marked “calf,” and
a very curious foot. The ank/e-region is long—almost like
a second shank—and has no heel: it is followed by five
toes, the first or innermost short, the second of moderate
length, the third longer, the fourth longer still, and the fifth

6 THE FROG CHAP.

of about the same length as the third. All the toes are
joined together by thin transparent weds, and, like the
fingers, have no nails. The name digit is conveniently
applied both to fingers and toes. Between the bases of
the thighs, at the hinder end of the trunk, is a small
aperture, the vert or anus.

In the squatting posture the body is raised upon the
arms, which are kept slightly bent at the elbows, with the
fingers spread out and directed forwards. In this position
the innermost of the four fingers corresponds with our own
index finger, the frog having no thumb. The hind-limb,
under similar circumstances, is bent into a sort of Z, the
knee being directed forwards and the ankle-joint backwards.
The toes are turned forwards, and the inner one, which is
the smallest of all, corresponds with our own great toe.

Owing to the bent position of the limbs, we cannot very
well, as in our own arms and legs, speak of their upper and
lower ends. It is therefore customary to call the end of a
limb, or of any division of a limb, which is nearest to the
trunk, the groxdmal end, that which is furthest away the
distal end. Thus the proximal end of the fore-arm is the
elbow region, the distal end of a digit is its tip.

The whole body, including head, trunk, and limbs, is
covered with a soft, slimy skin, of a brown colour, irre-
gularly spotted with brown or black on the upper or dorsal
surface, and whitish on the under or ventral surface. The
colouring is, however, not constant; in a frog kept in the
dark the black spots increase to such an extent that the
whole animal becomes almost black, while if kept in full
daylight a corresponding brightening of the tints takes
place. Moreover, the spots and patches of brighter colour
are very variable: if you examine a dozen specimens you
will see at once that no two are alike in this respect The

I MOVEMENTS AND GROWTH 7

large black patch situated behind the eye and containing the
tympanic membrane, is, however, always present, and is one
of the chief distinguishing marks of the common British frog
as compared with other kinds, such as the “ edible frog” of
the Continent.

Sexual Characters.—As in so many of the more familiar
animals there are two sexes of frogs, easily distinguished
from one another. If you examine several of them you will
find that a certain number have on the palm of the hand,
towards the inner side, a large swelling, rather like the
ball of our own thumb, but much more prominent.and of
a black colour. Frogs having this structure are males ;
there is no trace of it in the females.

Actions performed by the Living Frog.—Kept under
suitable conditions a frog very soon shows evidences of life.
If touched or otherwise alarmed it attempts to escape by
making a series of vigorous leaps—suddenly extending the
hind-legs and jumping to a considerable height. Thrown
into water it swims by powerful strokes of the hind-limbs.
It has thus, like so many living things with which we are
familiar, the power of voluntary movement.

If kept under observation for a sufficient time—weeks or
months—it will be found that frogs gvow until they reach a
certain limit of size. Growth, in the case of the frog, is an
increase in size and weight affecting all parts of the body, so
that the proportions remain practically unaltered, and no
new parts are added.

Careful observation shows that the throat is constantly
rising and falling, and the nostrils opening and shutting.
These movements, like the expansion and contraction of
the human chest, are respiratory or breathing movements,
and serve to pump air into and out of the lungs.

It requires frequent watching and sharp observation to see

8 THE FROG CHAP.

a frog feed. It lives upon insects, worms, slugs, and the
like. Opening its mouth it suddenly darts out a tolerably
long, nearly colourless, and very sticky fongue ; if the prey
is a small insect, such as a fly, it adheres to the end, and
the tongue is quickly drawn back into the mouth, the
whole operation being performed with almost inconceivable
rapidity.

Like other animals the frog discharges waste matters
from its body. Its droppings or feces, discharged from the
vent, are black and semi-solid. From the same aperture, it
expels periodically a quantity of clear fluid, the «zie, which
is perfectly clear and colourless, and contains little beyond
water.

Sometimes a frog will escape from confinement, leaving
its damp box or vivarium for the warm, dry atmosphere of
an ordinary room. When this happens the animal is
usually found next morning dead and shrunken, and with
its naturally moist skin dry and hard. From this it may
be inferred that there is a constant evaporation of water
from the skin, which, under ordinary circumstances, is
checked by a damp atmosphere or by occasional immersion
in water.

Hibernation.—In winter frogs bury themselves in damp
places and become sluggish, all manifestations of life becom-
ing hardly apparent until the following spring, when they
emerge from their holes. In this way they escape the
dangers of frost which would otherwise be fatal to them.
This suspension of activity during winter is known as
hibernation, or the winter sleep.

Reproduction and Development.—If you examine a
number of frogs towards the end of winter—about February
in England—you will find that the full-grown females are
distinguished from the males, not only by the absence of

I EGGS AND TADPOLES 9

the pad on the hand, but by the swollen condition of the
trunk, due to the interior being distended with eggs. After
a time the eggs are laid, being passed out of the vent by
hundreds ; each is a little globular body about #gth inch in
diameter, half black and half white, and surrounded by a
sphere of clear jelly, by means of which the eggs adhere
together in large irregular masses, the well-known “ frog-
spawn.” As the eggs are laid the male passes out of his
body, also by the vent, a milky fluid, the m/f or spermatic
uid, which gets access to the eggs and zmpregnates or
Jertilises them. Without impregnation they are incapable
of developing.

Neither male nor female takes the slightest care of the
eggs when once they are deposited and fertilised. They
are simply left in the water unprotected in any way ; and,
naturally enough, the mortality among them during the
course of development is very great, the majority being
eaten or otherwise destroyed, and only a very small per-
centage coming to maturity.

The first noticeable change in the spawn is that the
sphere of jelly surrounding each egg swells up so as to
acquire several times the diameter of the enclosed egg.
The egg itself, or embryo, as it must now be called,
gradually becomes entirely black, then elongates, and takes
on the form of a little creature (Fig. 1, 1), with a large
head, a short tail, and no limbs; which, after wriggling
about for a time, escapes from the jelly and fixes itself, by
means of a sucker on the under side of its head, to a
water-weed. Great numbers of these ¢adpo/es, as the free-
living immature young or /arve of the frog are called, may
be seen attached in this way. At first they are sluggish and
do not feed, but, before long, they begin to swim actively
by lashing movements of their tails, and to browse on the

10 THE FROG CHAD.

weeds. They are thus in the main vegetable-feeders, not

carnivorous, like the adult frog. On each side of the head

appear three little branched tufts or g7é/s, which serve as

Fic. 1.—Stages in the life-history of the Common Frog, from the newly-hatched
tadpoles (7) to the young frog (8); 2a is a magnified view of 2. (After Mivart).

respiratory organs (2, 2“), the tadpole, like a fish, breath-
ing air which is dissolved in the water. After a while the
gills shrivel up and the tadpole then comes periodically to
the surface to breathe, lungs having in the meantime made

I SUMMARY OF CHAPTER 1

their appearance. A little pair of hind-limbs appears at the
root of the tail, and a pair of fore-limbs behind the head
(5, 6). As these increase in size the tail slowly dwindles,
the head and trunk assume the characteristic frog-form, and
the little animal now comes on land and hops about as a
small, tailed frog (7). As growth goes on the tail further
diminishes and finally disappears altogether, the transforma-
tion or metamorphosis being thus completed (8).

Death and Decomposition.—Frogs may live for many
years, but, sooner or later, either in the ordinary course of
nature or by accident, they die. The heart stops beating,
the flesh undergoes what is called “ death-stiffening,”
becoming hard and rigid, and all vital manifestations cease.
Before long the process of decomposition ensues, the flesh,
viscera, etc., soften and emit a bad smell, and in course of
time rot away completely, leaving only the bones.

Summary of Chapter.—The very brief and cursory study
we have made so far shows us (1) that a frog has certain
definite parts arranged in a particular way; (2) that it
performs characteristic movements, some of them, such as
leaping and swimming, voluntary ; others, such as breathing,
involuntary ; (3) that it takes in sod@ food, consisting mainly
of vegetable matter in the tadpole, of living animals in the
adult; (4) that it gives off waste matters; (5) that it
reproduces its kind by laying eggs, which develop only if
impregnated ; (6) that it undergoes a transformation or
metamorphosis, the egg giving rise toa /arva, the tadpole,
which, after living for a time the life of a fish, gradually
changes into a frog.

12 PRACTICAL WORK CHAP.

HINTS ON DISSECTION

Instruments and other Requisites for Dissection.’—In
order to carry out the dissection of the frog and other animals success-
fully it is necessary to be provided with proper tools. The most
important are—

1. Three or four sharp dissecting knives, or scalpels, of different sizes,

2. A large and a small pair of straight dissecting forceps; the small
pair should have a peg on one leg fitting into a hole on the other, to
prevent the points crossing ; the points should be roughened.

3. A large and a small, fine-pointed pair of dissecting scéssors; the
small pair for the more delicate work, and the large pair for coarser
work and for cutting through bones. For the latter purpose a pair of
bone-forceps is useful, but is not necessary in the case of such a small
animal as the frog.

4. A seeker, z.e., a blunt needle mounted in a handle.

5. Three or four frodes: a seeker or knitting needle, or a thin slip
of whalebone will answer for some purposes, but the most generally
useful form of probe is made by sticking the end of a hog’s bristle into
melted sealing-wax, and immediately withdrawing it so as to affix a
little knob or guard.

6. An anatomical blowpipe, or, failing this, a piece of glass-tubing,
6 or 8 inches long, with one end drawn out in the flame until it is not
more than sth to s'5th of an inch in diameter.

7. An ordinary ‘‘ mediczéne-dropper,” or ‘ feeder” of a self-feeding pen
(see Fig. 25), made of a piece of glass-tubing about 3 inches long,
drawn out in the flame at one end, and thickened at the other, so as to
form a collar, over which an india-rubber cap—an ordinary non-
perforated teat—is fixed. This is useful for washing fine dissections, as
well as for injecting.

8. A dissecting dish, Get a common white pie-dish, about 6 or 8
inches long, with rather low sides. Cut out a piece of self-coloured
(brown) cork-carpet or thick linoleum the size of the bottom of the dish,
and a piece of sheet lead of the same size, and fasten the two together
by three or four ties of copper wire or strong thread. Place this in the

1 A suitable box of dissecting instruments can be bought from most
scientific instrument makers for about £1. (For further apparatus re-
quired in connection with injection and microscopical work, see pp. 99,
119, and 135.)

I HINTS ON DISSECTION 13

dish with the lead (which is simply to keep the cork-board from float-
ing in water) downwards. Or, place a few strips of sheet lead in the
bottom of the dish, and then pom in some melted paraffin-wax into
which a little lamp-black has been stirred, so as to make a layer half an
inch or more in thickness.

For larger animals than the frog, in addition to a larger dish, a
dissecting board will also be required. Get a piece of soft deal or pine
about 18 inches x 11 inches and @ inch thick, and nail round its edge
a strip of wood about # inch x 4 inch, so as to form a projecting rim.

9. A magnifying glass. Any good pocket lens or a common watch-
maker’s glass will answer the purpose. As it is often desirable to
have both hands free while using the lens, a stand of some kind is
useful. One of the simplest is made by fixing a piece of thick flexible
wire, 6 or 8 inches long, into a heavy block of wood, 3 or 4 inches in
diameter ; the free end of the wire is bent into a loop to carry the lens,
which can thus, by bending the wire, be raised or lowered as required.
Or, get a piece of narrow clock-spring, about 13 inches long, and rivet
one end of it to the outside of the rim ofa watchmaker’s glass, and
the other to a small piece of zinc or brass; on passing the spring round
the head, the lens is kept in place at the eye without exertion.

10. Medium and small-sized pzzs. Large blanket pins are useful for
fixing down larger animals.

11. A small sponge and a duster.

12. One or more wide-mouthed bottles or jars, containing a preserva-
tive in which to place your subjects after each day’s work. The most
convenient preservative for the purpose in most cases is the fluid sold as
Jormaline,! which can be diluted with water as it is wanted. For pre-
serving your dissections from day to day, a 1 per cent. solution of
formaline is strong enough in many cases—z.e., I cubic centimetre of
formaline to 99 c.c. of water, or three-quarters of a dram of formaline to
half a pint of water. For permanent preservation, a stronger solution
—2 to 4 per cent., according to circumstances—should be used, or
methylated spirit. If formaline is not available, use strong methylated
spirit (¢.e., about 90 per cent.) diluted with one-third of its bulk of
water.

13. A plentiful supply of clean water.

14. An ounce or two of chloroform.

1 A 40 per cent. solution of the gas formic aldehyde.

14 PRACTICAL WORK CHAR.

Rules to be observed in Dissection.—Many of the parts and
organs of animals are bound together by means of a substance known as
“« connective-tissue,” and the main object of dissection is to tear away
and remove this substance so as to separate the parts from one
another. :

The subject should be firmly fixed down in the dissecting-dish or on
the dissecting-board by means of pins, inserted obliquely, so that they
do not interfere with the dissection. The dissecting-dish must always
be used for finer dissections, which should be done under water; only
just enough water being put into the dish to cover the dissection, which
should be washed under the tap from time to time.

When dissecting a part keep it on the stretch, and avoid fingering it or
damaging it with the forceps. :

Never remove anything until you know what you are removing.
Dissect along, and not across, such structures as blood-vessels and nerves.

See that your instruments are kept clean and sharp, and never use the
smaller scissors and scalpels for coarse work.

Drawing.—You should make a point of drawing as many of your
preparations, as well as of the living animals, as possible: an accurate
sketch, taken from Nature, no matter how rough, is of more value in
teaching observation and in impressing the facts on your memory than
the examination and copying of more perfect drawings made by others.
Any one can soon learn to make sketches of this kind, even without
having any previous knowledge of drawing.

Each sketch should be made to scale, and small objects should be en-
larged several times ; it is much easier to insert details ina large drawing
than ina small one. Mark the scale against each drawing—e.g., x 2,
x $.

Using a rule and compasses, first sketch in an outline of the principal
parts with a Zard pencil ; if your object is bilaterally symmetrical, draw
a faint line down the middle of the paper, and then sketch in one side
first. When you have sketched in all the outlines correctly, go over
them again with a softer pencil, so as to make them clear and distinct.
Do not attempt any shading unless you have some knowledge of
drawing.

Then tint the various parts in different colours, using very Light tints
except for such structures as vessels and nerves. It is as well to keep
to the same colours for the corresponding organs or tissues in all the
animals you examine : thus you might in all cases colour the alimentary

I HINTS ON DRAWING 15

canal yellow, the arteries red, the veins blue, glands brown, cartilage
green, and so on.

Make your drawings on one side of the page only; the opposite side
can then be used for explanations of the figures.

Never insert on your original sketches anything you have not actually
seen ; you can copy as many other figures as you like from various

sources, but these should be kept apart from your own original
drawings.

Directions for the examination of the external characters of the adult
frog, as described in this chapter, will be given at the end of Chapter
II. ; and of the eggs and tadpoles in Chapter XII.

CHAPTER II
THE FROG (continued): GENERAL INTERNAL STRUCTURE

You have now seen that a frog can perform a number of
very complicated actions; and, if you have any curiosity
in these matters, you will probably want to know some-
thing of the mechanism by which these actions are
brought about. Now, the best way to understand the
construction of a machine, such -as a clock or a steam-
engine, is to begin by taking it to pieces; and, in the
same way, you can find out the parts of which the
living machine we call a frog is made, and the way they
are related to one another, only by taking it to pieces, or
dissecting it.

First notice, in addition to the external characters described
in the last chapter, that the various parts of the body
are strengthened or stiffened, as in ourselves, by a number of
bones, which together form the greater part of the skeleton.
It is quite easy to ascertain by feeling that the head con-
tains a hard sku//; the lower jaw, a lower-jaw-bone or
mandible; that running through the back is a jointed
vertebral column, or back-bone; that the region of the
chest is protected by a sternum, or breast-bone; and that
each division of the limbs has its own bone or bones.

The Mouth-Cavity.—There are also several points to

CHAP, II THE FROG: MOUTH AND PHARYNX 17

be observed in the interior of the mouth. All round the
edge of the upper jaw is a row of small conical eeth (Fig.
7). There are no teeth in the lower jaw; but on the
roof of the mouth, a short distance behind the snout, are
two little patches of teeth, called the vomerine teeth (vo. 7).
Just behind these are two apertures, called the zferna/
nostrils (p. na): a guarded bristle passed into one of the
external nostrils and pushed gently backwards and down-
wards, will be found to enter the mouth by the correspond-
ing internal nostril.

Behind the internal nostrils are two large hemispherical
projections, due to the roof of the mouth being bulged out
by the huge eyes, as can be readily made out ve pushing
the eyes from outside.

On the floor of the mouth is the large, flat songue
(¢ng), remarkable for the fact that it is attached at its front
end, its hinder end being free and double-pointed. When
the frog uses it to catch insects it is suddenly thrown for-
wards, almost like a released spring. Just behind the back-
wardly-turned tip of the tongue is an oval elevation,
having on its surface a longitudinal slit, called the glottis
(gz), which leads, as we shall see afterwards, into the lungs.

The back of the mouth narrows considerably, and the
soft skin or mucous membrane lining it is here thrown into
folds. A probe gently pushed backwards passes, as we
shall see, into the stomach. The narrowed region of the
mouth is the throat, or sZarynx. On its upper wall, near
the angles of the mouth, are two pits: a guarded bristle
passed into one of these will be found to come into
contact with the corresponding tympanic membrane,
which will be pierced if sufficient force is used. The
pits are known as the Eustachian recesses or tubes (eus. t).

Dissection of the Frog: Skin and Muscles.—If a slit is
Pract. ZOOL, C

18 THE FROG CHAP.

made in the skin of the belly, and a probe pushed in under it,
it will be seen that the skin, instead of being firmly attached
to the underlying flesh, as in a rabbit or a sheep, is for the
most part quite loose, a spacious cavity lying between it
and the flesh. Not, however, a single continuous cavity
for the whole body: the probe, gently pushed in various
directions, is stopped, in front, at about the level of the
arms ; behind, at the junction of the thighs with the trunk ;
and at each side, along an oblique line joining the
armpit with the thigh. Moreover, by opening the
skin of the back, throat, and limbs, and inserting the
probe as before, similar cavities will be found in these
regions, all separated from one another by partitions,
along which the skin is firmly united to the underlying
flesh. It will be noticed also that the probe, when with-
drawn from any of these cavities, is wet. The cavities
contain a watery fluid, called Zymph, and are hence known
as subcutaneous lymph sinuses (Fig. 7, d. ly. s, v. ly. 5).
When the skin is removed it will be seen that under
the skin and separated from it by the lymph-sinuses is a
nearly colourless, semi-transparent, fibrous substance, the
flesh. At first this appears to be continuous over the whole
body, but, by careful dissection with a sharp scalpel, a
very delicate, transparent skin, called the fascia, can be
separated from the flesh, which is then seen to consist of
anumber of separate bands (Fig. 2, pct, my. hy, etc. ; see
also Fig. 16), covered as aforesaid by the fascia, and
separated from one another by a kind of packing substance,
also very delicate and transparent, and known as
connective-tissue. These bands or sheets are the muscles,
and the whole of the flesh is made up of distinct muscles,
readily separated from one another when once the requisite
anatomical skill is attained. Here and there—for instance

IT BODY-WALL 19

on the top of the head and the front of the shanks—there
are no muscles, and the bones are covered only by skin and
connective tissue.

Passing along the middle line of the belly is a dark
longitudinal streak (Fig. 2, add. v) : this is a dlood-vesse/, the

Fic. 2.—A frog with the skin (sk) of the ventral surface cut through and turned
back right and left, soas to expose the muscles. Of these the mylohyoid (my. /y),
pectoralis (fcz), external oblique (ext. 062), and rectus abdominis (7ct. aba)
are lettered. On the right side (left in the figure), the posterior portion of the
pectoral muscle is cut away, its two ends (fct’ Act”) only being left. The
cartilaginous extremity of the breast-bone (xiphisternum, x. s¢) is shown, as
well as the abdominal (aéd. v), musculo-cutaneous (#.c¢.v) and subclavian
(scé. v) veins, and the cutaneous artery (c. @).

abdominal vein. On each side of the body another vein (v7.

c. v) is seen forming a loop, one limb of which is on the

turned-back flap of skin, while the other passes between

the muscles not far from the armpit: this vessel is the

musculo-cutaneous vein, Both these veins, and many others
C2

20 THE FROG CHAP;

which will be seen in the course of the dissection, are thin-
walled tubes full of d/o0d, as will be proved if you should
happen to cut one of them, when the blood will escape in
considerable quantity.

Between the right and the left fore-limbs the ventral
region of the trunk is protected by certain bones which
form part of the shoulder-girdle: projecting backwards from
this in the middle line is a flat, heart-shaped plate of a
softer, gristle-like substance, known as cartilage (compare
Fig. 12). Immediately between the thighs a cartilage called
the pubis, part of the Ap-girdle (Fig. 14), can be felt.
Between the shoulder and hip-girdles the ventral body-wall
is soft, being formed only of muscle and connective tissue.

The Abdomen and its Contents.—By cutting through
the muscles of the belly or abdomen, a large cavity, the dody-
cavity or celome, is exposed, in which .are contained
numerous structures presently to be described. In order,
however, to open the whole of the cavity the ventral
part of the shoulder-girdle must be removed.

In the middle line, between the fore-limbs, and there-
fore covered in the entire animal by the shoulder-girdle, is
a pink conical body (Figs. 3 and 4, v) connected in
front with a thin-walled bag, (7. az, 72 av) of a purplish
colour. The whole thing is the feart: the pink
posterior portion is called the ventricle; the purple
anterior part consists of two chambers, the auricles.
The heart is enclosed in a transparent, membranous bag,
the pericardium (ped).

Just behind or posterior to the heart are two
large masses (/7), usually of a dark reddish-brown
colour; these are the right and left lobes of the Zver.
They extend forwards, one on each side of the heart:
between them is a globular bag of a greenish colour (Fig. 3,

II DISSECTION OF A MALE FROG 21

‘pu

Fic. 3.—Dissection of a Male Frog. The enteric canal and liver are displaced to
the animal’s left, and part of the liver is cut away. Some of the muscles are
cut away and certain nerves and blood-vessels traced into ‘the head and
limbs. : . : 5 p .

abd. v. abdominal vein; 47. brachial artery, vein, and nerve 5 c@/. mes. splanchnic
or cceliaco- mesenteric artery; cf. ad. right fat-body; dz. duodenum;

. ext. ju. external jugular vein; _/#. femoral vein; g7. d¢. gall-bladder; 4. pf.

. hepatic portal vein; 44. v. hepatic vein; 2~. liver; 1. c. v. musculo-cutaneous
. vein; my. Ay. mylohyoid muscle; gcd. pericardium ; Z.c. hy. anterior cornu of
hyoid; f¢.cv. postcaval vein; fx/.a. pulmonary artery; Az. v. pulmonary
vein ; fv. pelvic vein; r.aw. right auricle; vct. rectum; 7. Ad. right kidney;
r. ing. right lung ; ~. pr. cv. right precaval vein; +. p¢. right renal portal
vein; ~. sfy. tight spermary ; s. zt. ileum; sfZ. spleen; s¢. stomach; s. v.
sinus venosus; ¢y. @. conus arteriosus; z#. 6/. urinary bladder; w7. ureter;
v. ventricle ; vs. sz. seminal vesicle. :

22 THE FROG CHAP.

gl. bl), the gall-bladder. In front of the liver and left
and right of the heart are two thin-walled, transparent
sacs (r. lng, /. Ing) with a honeycombed surface, the /ungs.
Their appearance varies very much according to their state
of distension. When full of air they are an inch or more in
length in a full-sized frog, and protrude freely as soon as
the abdomen is opened: when empty they hardly show
unless the liver is turned aside.

Emerging from beneath the left lobe of the liver (beneath
in the present position of the animal, actually above) is a
wide, whitish tube (Fig. 3, s¢) which almost immediately
turns to the right (the frog’s right, not yours), so as to
form a U-shaped bend (s¢,dm). This is the stomach,
which is connected with the pharynx by a short tube
called the gullet or esophagus (compare Fig. 7, gui, st),
and which varies considerably in size according to
whether it is empty or distended with food. The stomach
becomes continuous with a narrower tube, the first part of
which (dm) passes forwards parallel with the stomach,
thus forming the narrow limb of the U, while the rest of
it (s.2a/) is thrown into a rather complex coil. This
tube is the small intestine; the part in immediate con-
nection with the stomach (dm) is distinguished as the
duodenum and the coiled part as the z/eum. Between
the stomach and duodenum, in the bend of the U, is a
small yellowish-white body of irregular form, the pancreas
(Figs. 7, 62, and 18, P).

The stomach and intestine are kept in place and
suspended to the dorsal wall of the body-cavity by a delicate
membrane, the mesentery (Fig. 5, mes), which is folded in
correspondence with the various coils. As we shall see, the
mesentery is really a portion of a thin, moist membrane,
the peritoneum, with which the body-cavity is lined.

II ABDOMINAL VISCERA 23

The small intestine becomes continuous, posteriorly, with
a much wider tube (Figs. 3 and 7, zc¢), lying against the
dorsal wall of the abdomen, and called the /arge intestine or
rectum. It is continued into a short tube, the cloaca (c/),
which passes backwards, between the backbone above and
the pubis below, to open externally by the vent. Thus the
mouth-cavity, pharynx, gullet, stomach, small intestine, rec-
tum, and cloaca form a continuous tube, opening externally
at each end, by mouth and anus respectively, and, for the
greater part of its extent, contained within the body-cavity.
The whole tube is known as the exteric or alimentary canal.

Attached to the mesentery, close to the anterior end of the
rectum, is a rounded body of a deep-red colour, the spleen
(Figs. 3 and 7, sf/). Quite at the posterior end of the
abdominal cavity a very thin-walled and very transparent
sac (#.62) will be seen, connected with the ventral surface of
the cloaca, and varying very much in size according to its
state of distension. This is the urinary bladder, which
communicates by an aperture (Fig. 7, 42’) with the cloaca,
and when distended will be seen to bea bilobed sac of
considerable size.

If your specimen should be an adult female, and the
time of year approaching the breeding season, you will
already have observed, as the most prominent organs in the
body, two large, lobed structures of a dark colour, protrud-
ing one on each side, and partly obscuring the view of the
other organs. Each (Fig. 4, 2 ovy) contains an immense
number of small globular bodies, half black and half white,
and is suspended to the roof of the body-cavity by a sheet
of peritoneum. ‘These bodies are the ovaries, or organs for
the manufacture of the eggs ; the rounded bodies of which
they are largely composed are the eggs themselves. To
each ovary is attached a curious structure (cq. ad) of a

syst. &r

|
coed. mes
|
\ A\—
plev vy i

nha. ;

Fic. 4.—Dissection of a y gz
have been cut through, and tk
The liver is removed, with the
postcaval vein (f¢.cz). The v

ption of a small portion (ir) S
cle of the heart (v) is turned

CHAP. II ABDOMINAL VISCERA 25

the abdominal vein (aéd. v) is severed and turned backwards. The right ovary
and fat-body are removed, and the right oviduct (7. ovd) is slightly displaced
outwards.

abd. vy. abdominal vein; c@/. mes. splanchnic or cceliaco- mesenteric artery ;
cp. ad. corpus adiposum, or fat-body; d. ao. dorsal aorta; gz/. gullet;
Au. cut end of humerus or upper-arm bone ; 7. az, left auricle ; 2. dug. left lung ;
2, oud. \eft oviduct ; 2. ovd’. its opening into the body cavity ; 2. ovd”. its pos-
terior dilatation ; 2 ovy. left ovary ; Zr. portion of liver ; Az. cv. postcaval vein ;
pt. cv’. its anterior portion passing between the liver and the heart ; ». az. right
auricle; ref. rectum; ~. Ad. right kidney; 7. dwg. right lung; 7. gt. renal
portal vein; .ovd. right oviduct ; ~ ozd’. its opening into the body cavity ;
vr. ova, its posterior dilatation ; syst. ¢~. systemic trunks at their point of union ;
wu. 6l. urinary bladder ; “7. ureter ; v. ventricle.

bright yellow colour, and produced into a number of
streamer-like lobes ; this is the fatdody.

By lifting up either of the ovaries there is seen beneath it
—in the natural position of the parts above or dorsal to it—
a greatly convoluted colourless tube (4 ovd, r. ovd) of about
the same diameter as the intestine. This is the ovtduct,
through which the eggs pass from the ovary to the cloaca.
If the specimen is allowed to remain long in water the
oviducts will be found to swell and finally to become dis-
integrated ; this is due to the fact that in them is formed the
jelly in which the laid eggs are enclosed, and which, as we
have seen, swells in water.

In the male there is seen, on turning the intestines aside,
a pair of yellow ovoidal bodies (Fig. 3, 7. sfy) about half an
inch long, attached by peritoneum to the dorsal wall of the
body-cavity. These are the sfermaries or testes ; they manu-
facture the spermatic fluid or milt by which the eggs are
impregnated. To the anterior end of each is attached a
fat-body (cp. ad), like that of the female. In young speci-
miens of both sexes the reproductive organs—spermaries,
ovaries, and oviducts—are very small.

When the intestine is turned aside there will also be seen,
in both sexes, a pair of flattened, irregularly-oval bodies (Figs.
3 and 4, ~. &d) lying in the posterior part of the abdominal
cavity just above or dorsal to the ovaries or spermaries.

26 THE FROG CHAP.

These are the Adveys. With the outer edge of each is
connected a tube, the wzefer (wr), by which the urine,
formed in the kidneys, is carried to the cloaca (Fig. 7).

It has been pointed out that the abdomen is lined
by peritoneum, and that the various organs are suspended
by folds of the same membrane, called, in the case of the
enteric canal, the mesentery. The relations of this mem-
brane are best seen in a diagrammatic transverse section of -

Wee abd.v

Fic. 5.—Diagrammatic transverse section through the trunk of a frog, to show the
relations of the peritoneum.
abd.wv. abdominal vein; d. ao. dorsal aorta; 7/. ilium; z##. intestine; kd.
kidney ; 7. muscles of back ; wz’. muscles of abdomen ; tes. mesentery ; 2. fer.
parietal layer of peritoneum ; p- per’. the same, turning down to cover the
kidney; ft. cv. postcaval vein; sk. skin; s.cuz. ly.s. sub-cutaneous lymph-
sinuses; spy. spermary ; s.v. dy. s. sub-vertebral lymph smus; w. st. urostyle
(part of the iertabral column); wv. fer. visceral layer of peritoneum, investing in-
testine ; 7. fer’. the same, investing spermary.

the body (Fig. 5), though many points can be perfectly
well made out from the actual specimen. The body-cavity
is lined by what is called the fariefal /aver of the
peritoneum (f. fer), which adheres closely except in the
middle dorsal region, where it leaves the body-wall and
becomes closely applied to the ventral surface of the

II PERITONEUM—NEURAL CAVITY 27

kidneys and reproductive organs. Leaving these, the
peritoneum of the right approaches that of the left side, and
the two, coming into contact, form a double vertical sheet, the
mesentery (mes), which extends ventrally towards the enteric
canal. On reaching the latter, the two layers diverge again
and surround the canal, forming the visceral layer of perito-
neum (zv. ger). The liver, oviducts, etc., are suspended and
covered in the same way. Thus the lining of the body-
cavity, the investment of the various organs contained in it,
and the folds by which they are suspended, are all parts of
one continuous membrane. The space left between the two
diverging layers of peritoneum, in the mid-dorsal region,
contains lymph, and is known as the sud-vertebral lymph
sinus (s.v. ly. 5).

We have already noticed the abdominal .and musculo-
cutaneous veins. Other veins of greater or less size will be
seen everywhere, passing, for instance, to the head and
limbs (Fig. 3), and in the mesentery. Running parallel
with many of the veins are smaller vessels, many of which
have pigment in their walls, and which are of distinctly
stouter texture. These are the av/erzes. They contain little
blood in the dead animal, and, owing to the stoutness
and elasticity of their walls,do not collapse when empty.
Hence they are quite easy to see in a frog from which all
the blood has been drained, while the thin-walled veins are
almost invisible under like circumstances. Finally, there
will be seen in many parts of the body, often lying parallel
to an artery and a vein, white cords, the erves.

The Neural Cavity and its Contents.—By turning the
frog with its back upwards and cutting through the muscles
of the back and the arches of the vertebre (see Fig. 6), as
well as, in front, the roof of the skull, you will see that the
backbone contains a distinct cavity, the neural canal, in

28 THE FROG CHAP.

which lies a white rod, made of the same soft, pulpy sub-
stance as the nerves, and called the spinal cord (Fig. 6,
sp. cd), which ends behind in a thread-like prolongation
(f 4, some distance in front of the thighs. It will also be
found that the neural canal is continued, with a. slightly

Fic. 6.—Dissection of a Frog in which the entire neural canal has been opened from
above, and the brain (47) and spinal cord (sf. cd) laid bare. The brain consists
of olfactory lobes (o/f 2), cerebral hemispheres (c7d. 2), diencephalon (die),
optic lobes (oft. 2), cerebellum (ci), and medulla oblongata (sed. 061), which
will be referred to in Chapter X. The spinal cord ends in a delicate prolonga-
tion, the filum terminale (4 4. The nasal bones (za), eyes (e), auditory
region of the skull (az), transverse processes of the nine vertebrz (7. s—v. 9),
urostyle (w. st) and ilia (72) are indicated in outline, and serve as landmarks.
(After Howes, slightly altered.)

increased diameter, into the skull, and that the spinal cord

becomes continuous with the drain (ér), a complex organ

formed of several parts, which will be referred to hereafter.
General Structure of the Limbs.—A transverse section

II SUMMARY OF CHAPTER 29

cut across one of the legs, at about the middle of the thigh,
will show in the middle of the cut surface the thigh-bone,
around it the flesh or muscle, and around this again the
skin. Similar cuts through various parts of both fore- and
hind-limbs show that these appendages of the body are solid,

ret ts.
sp.ca ling ning kd,

PMX _ fr =. a
MAG —— as

a
eusl bhy|
vlys

vhys

Fic. 7.—Dissection of a Male Frog from the left side. The left fore- and hind-limbs
and the left sides of the head and trunk have been cut away, the enteric canal
and liver are displaced downwards, and the mouth, pharynx, and cloaca laid
open.

an. anus; 8. d. bile-duct ; 4. hy. body of hyoid ; 42. urinary bladder ; 2’. its opening
into the cloaca; c. avt. conus arteriosus ; cédiz. cerebellum ; c/. cloaca; cx. 3,
centrum of third vertebra ; cf. aa’. corpus adiposum ; c7. /. cerebral hemisphere ;
d. ly. s. dorsal lymph sinus; dv. duodenum; ¢f/. cov. epi-coracoid; ems. ¢.
Eustachian recess ; #R. PA. fronto-parietal ; ¢7. glottis; gwd. gullet ; IL. ilium ;
IS. ischium ; Ad. kidney ; 2. az. left auricle ; 7. dug. left lung ; 7. liver ; M.MCK.
mento-meckelian bone; x#.a@.7, arch of first vertebra; off 2. olfactory lobe ;
oft. 2. optic lobe ; O. ST. epi- and omo-sternum ; Acad. pericardium ; PAZX. pre-
maxilla; fx. pancreas; /. za. internal nostril; Az. pubis; vcf. rectum; 7. dug.
right lung; s. z7#¢. ileum; sf. cd. spinal cord ; SPH.ETH. sphenethmoid ;
spl. spleen; s¢. stomach; s. v. sinus venosus; ¢zg. tongue; Zs. spermary 3 77.
ureter ; 77”. its aperture into the cloaca; UST. urostyle: v. ventricle ; v. Zy.s.
ventral lymph sinus; vo. z. vomerine teeth; vs. sem. seminal vesicle. (From
Parker and Haswell’s Zoology.)

containing no cavities, except the sub-cutaneous lymph-
sinuses previously observed.

Summary.—We thus get a notion of the general plan of
construction of a frog as follows. It consists of a central or

30 THE FROG CHAP.

axtal portion, the head and trunk, and of two pairs of lateral
offshoots or appendages, the fore- and hind-limbs. The trunk
is hollowed out into two cavities: the abdominal or body-
cavity (celome) below, and the xeural canal above ; of these
the neural cavity alone is continued into the head. The
abdominal cavity contains the greater part of the enteric canal,
the liver, gall-bladder, pancreas, spleen, lungs, heart, kidneys,
urinary bladder, and reproductive organs. The neural
canal contains the brain and spinal cord. The anterior
end of the enteric canal is continued forwards into the head,
forming the mouth-cavity, and opens externally by the
mouth aperture; its posterior end opens externally by the
anus. The enteric canal passes through the containing
body-cavity, having no communication with it. The lungs
open into the pharynx, and thus communicate with the ex-
terior not only by the mouth but also by the nostrils. The
kidneys, bladder, and oviducts communicate with the
cloaca, and thus with the exterior through the anus. Neither
the neural nor the abdominal cavity has any communi-
cation with the exterior. The walls of the head and trunk
consist largely of muscles and bones covered with skin.
The limbs are solid outgrowths of the trunk, formed also
mainly of muscle, with bony supports and a covering of
skin.

Organs.—Notice that the body consists of various definite
structures, or organs as they are technically termed, which
have various purposes or functions to perform. The enteric
canal, together with the liver and pancreas, are organs of
digestion ; the lungs and skin, organs of respiration or breath-
ing ; the heart and blood-vessels organs of circulation, serving
as they do to propel and conduct the blood through the body ;
the kidneys, aided by the skin, organs of excretion, for getting
rid of waste matters ; the ovaries and spermaries, organs of

Ir PRACTICAL DIRECTIONS 31

reproduction ; the muscles, organs of movement; the brain
and spinal cord, together with the nerves, organs of control,
serving to direct or control the actions of the body; the
skin, nose, eye, and ear, sensory organs, by which communi-
cations are kept up with the external world.

Tissues.—Notice also that the various parts of the body
are built up of different materials, or //sswes as they are
called. We have already distinguished muscle, bone,
cartilage, connective-tissue and nervous ‘tissue. Other
tissues we shall meet with in the course of a more careful
examination.

PRACTICAL DIRECTIONS.

To kill a Frog for Dissection.—Place a frog ona plate, and cover
it with a tumbler, or put it into a stoppered bottle. Soak a little bit of

cotton-wool or sponge in chloroform, and push it under the edge of the
tumbler, or drop it into the bottle. Ina few minutes the vapour will
make the animal quite insensible, and a somewhat longer exposure will
kill it painlessly.

External Characters.—Observe the voluntary and the involuntary
respiratory movements of the living animal, and compare with a dead
frog when making out the external characters (pp. 4-8) and the position
of the various parts of the skeleton (p. 16).

Sketch the entire animal from the side or from above.

The Cavity of the Mouth.—Gently open the mouth of 2 dead
frog as wide as possible, and make out the points described on pp. 16
and 17. Sketch.

The Body-wall,—Lay the frog on its back in the dissecting-dish, and
fix it firmly by sticking pins through the skin of the arms and legs.
With the forceps, held in the left hand, pinch up the skin of the
abdomen near the middle line between the thighs, and make a nick init
with the points of the scissors. Then, holding the edge of the hole thus
made with the forceps, pass in a probe and push it forwards as far as it
will go without opposition. Note :—

The sub-cutaneous lymph-sinuses, and the underlying muscle.

With the scissors extend the incision made in the skin of the belly
forwards, in a straight line, to the chin, Holding up the edge of the

32 THE FROG CHAP,

skin with a forceps, cut through, with a scalpel, the partitions between
adjacent lymph-sinuses, so as to separate the whole of the skin of the
ventral surface from the muscle, and, having done so, pin back the flaps,
right and left (see Fig. 2). Similar cuts should be made in the skin of
the limbs and back. Observe—

The fascia, the muscles of the body-wall, the abdominal and
musculo-cutaneous veins, and the shoulder-girdle and pubic region of the
hip-gtrdle.

The Abdomen and its Contents.—Pinch up the muscles on one
side of the abdominal vein with the forceps, and make an incision in
them by a single snip of the scissors. Then, holding the edge of the
wound with the forceps, extend the cut forwards to the shoulder-girdle
and backwards to the pubis. Keep the cut parallel to the abdominal vein,
and be careful not to wound the latter. You will find that the incision
thus made opens a large dody-cavzty or celome, in which a number of
structures, the abdominal viscera, are contained. Note that the body-
wall consists of three layers : (1) skz, (2) muscles, with their fascéa, and
(3) peritoneum.

So far, however, the cavity is not thoroughly opened. Lift up the side
of the abdominal wall to which the abdominal vein is attached, and
very carefully separate the vein by tearing through, with a needle or
the point of a scalpel, the connective tissue by which it is attached to
the inner face of the muscles: ov, in order to prevent the possibility of
injuring the vein, cut through the muscles of the body-wall longitu-
dinally on the other side of the abdominal vein, so as to leave a narrow
strip of muscle attached to it. Then make two cross-cuts, starting from
the anterior end of the longitudinal incision, and extending outwards
towards the fore-limbs: take care not to injure the musculo-cutaneous
veins, and pin back the two flaps into which the soft abdominal wall is
now divided (Figs. 3 and 4). Next dissect away the muscles covering
the shoulder-girdle, so as to expose the bones: identify the bones
called coracotd and clavicle (compare a skeleton and Fig. 12). With
the strong scissors cut through both these bones as near as possible to the
shoulder-joint: then lift up the middle portion of the shoulder-girdle
thus separated, and carefully dissect it away from the underlying parts.

Having thus exposed the whole of the abdominal cavity, pour just
enough water into the dissecting-dish to cover the animal, first washing
away any blood which may have escaped from cut vessels. If your
specimen is a female, dissolve a little common salt in the proportion of

Ir PRACTICAL DIRECTIONS 33

I per cent. in the water, or mix it with about one-third of its bulk of
methylated spirit, in order to prevent the excessive swelling of the
oviducts. If, however, you wish to make out the blood-vessels in this
specimen without injecting them, it is as well to defer putting water
into the dish until a later stage of the dissection.

Note—

1. The peritoneum-partetal and vesceral layers.

2. The fericardium, containing the heart. If not already opened,
the pericardium should be slit through, so that the aurdc/es and ventricles
can be plainly seen. If the frog has been killed quite recently, you will
be able to observe the pzz/satdon of the heart.

3. The right and left lobes of the Zver, and the gal/-bladder.

4. The two /ungs: if contracted, inflate with a blowpipe through the
glottis.

5. The enteric or alimentary canal, consisting of gullet or esophagus,
stomach, small intestine (duodenum and tleum), and large intestine or
rectum communicating with the cloaca, which will be seen at a later
stage, and which opens to the exterior by the vent.

6. The mesentery.

7. The pancreas,

8. The spleen.

9. The urinary bladder, If collapsed, insert a blowpipé into the
vent and inflate. (You will very likely find some small parasitic flat-
worms, called Polystomum integerrinum, in the bladder ; each worm
has a ring of suckers round the hinder end.)

10. In the male the sfermartes and fat-bodées, and in the female the
ovaries, fat-bodies, and oviducts,

11. The Acdneys and wreters,

12. The mode of suspension of all these organs (p. 26), and the
position of the sub-vertebrallymph-sinus. In order to clearly understand
the relations of these parts, a thick transverse section should be made
through another frog in the region of the kidneys and examined under
water (compare Fig. 5).

Sketch the contents of the abdomen 77 sztz.

The Neural Cavity and its Contents.—Now turn the frog with
its back upwards, and pin it firmly to the bottom of the dissecting-dish
or to the dissecting-board as before. Pinch up the skin, make a
longitudinal cut through it from the snout to within a short distance of
the vent, and turn the flaps right and left. The muscles of the back

Pract. Zoot, D

34 THE FROG CH. 11

will be exposed, and, in front, the roof of the skull, which lies just
beneath the skin. Carefully dissect away the muscles along the middle
line of the back until the vertebral column is seen. Comparea prepared
skeleton and Fig 8,and make out the arches of the vertebre. Between
the first of these and the back of the skull is a slight space: insert one
blade of the strong Scissors into this, directing the points backwards,
and cut through the arch of the first vertebra, first on one, then on
the other side, and finally detach and remove the little piece of bone.
The xeural canal will then be exposed, in which lies the sfzal cord
(compare Fig. 6).

Work backwards, cutting away the arches of the remaining vertebra,
and you will find that the spinal cord ends behind in a thread-like pro-
longation.

Next, using the scissors in the same manner, cut away, bit by bit, the
roof of the skull: two large bones—the /ronto-pardetals (see Figs. 8
and 9), forming a considerable part of the roof, can be more easily re-
moved by raising them up with the edge of a scalpel. Note the cavity
of the skudd and its contained drain.

General Structure of the Limbs.—With a strong knife, cut
across one of the legs at about the middle of the thigh. Notice the
thigh-bone, muscles, and skin. Sketch.

Now preserve your specimen in formaline (1 or 2 per cent.), or spirit
(70 per cent. ).

CHAPTER III
THE FROG (continued): THE SKELETON

Ir you have followed the description given in the preced-
ing chapter with a frog before you, testing every statement
as you proceeded by reference to the specimen, you will
now have a very fair notion of the general build of the
animal. The next thing to do is to study its various parts
in somewhat greater detail.

As the bones and cartilages form the framework on which
all the other parts are supported, it is convenient to begin
with them. You may study them on a prepared skeleton, but
.a far better plan is to make a skeleton for yourself as
directed on p. 53.

Parts of the Skeleton.—The skeleton consists of the
following regions :-—

1. The skull (Figs. 8and 9): a complex mass of mingled
bone and cartilage, enclosing the brain and the organs of
smell and hearing, and supporting the upper jaw. Connected
loosely with the skull, but really forming part of it, are the
lower jaw and the tongue cartilage.

2. The vertebral column or backbone, consisting of nine
movably united pieces, the vertebra (Fig. 8 v.1—v.g), and
of a long bony rod, the urosty/e (usT).

D2

36 THE FROG CH. IIT

3. The shoulder-girdle or pectoral arch, an inverted arch
of bone and cartilage nearly encircling the anterior part of
the trunk and giving attachment to—

4. The bones of the fore-limbs.

5. The fip-girdle or pelvic arch, an apparatus shaped
somewhat like a bird’s “merrythought”: it is attached in
front to the ninth vertebra and behind gives attachment to—

6. The bones of the Aznd-limbs.

The Vertebral Column.—The essential structure of a
vertebra may be best studied by examining any of the nine
from the second to the seventh : the first, eighth, and ninth
present certain peculiarities, and are best left till last.

The whole vertebra (Fig. 8, B) has something the form of
a signet-ring with its sides produced into two outstanding
projections. The part corresponding with the stone of the
ring is ventral in position, and is called the dody or centrum
(cn), the form of which is procelous, 7.e., its anterior face is
concave, its posterior face convex, and both faces are
covered with a thin layer of cartilage. The part corre-
sponding with the circle of the ring is the meural arch
(gd, 7m): it arches over the spinal cord and is produced
in the middle line above into a blunt projection, the meurat
Spine (nm. sp). From the arch is given off, on each side, the
large outstanding projection already referred to, the
transverse process (tr. pr), which is tipped with cartilage in
the second, third, and fourth vertebre.

The neural arch gives off from its anterior face, just above
the origin of the transverse processes, a pair of small shelf-
like projections, the articular processes or sygapophyses (a. 2yg).
Each has its upper surface flat and smooth, and covered with
a thin layer of cartilage. A similar pair of processes spring
from the posterior face of the arch, but have the smooth,
cartilage-covered surface looking downwards.

sus Ps

Ky

Fic. 8.—A, skeleton of Frog from the dorsal aspect; B, anterior face of the fourth

ag

vertebra. In A the left half of the shoulder-girdle and the left fore- and hind-
limbs are removed, as also are the membrane-bones of the left side of the skull.
Cartilaginous parts are distinguished by dotting. The names of cartilage bones
are in thick capital letters, those of membrane bones in italic capitals, other
references in small italics.

. ¢. Ay. anterior horn of hyoid ; ac¢d. acetabulum; AST. astragalus; a.zyg. anterior

articular processes, or zygapoph sis; 4.y. basi-hyal; ©. calcar; CAL. cal-
caneum; cz. centrum; E. 06. exoccipital; FE. femur; fon, fox.’ fontan-
elles; #R. PA. fronto-parietal; HU. humerus; IL. ilium; 4. lamina of
neural arch; JZX. maxilla; #.sf. neural spine; off cf. olfactory capsule;
ot. pr. otic process; f.c. hy. posterior horn of hyoid; Ad. pedicle of neural
arch ; PALX. premaxilla: PR. OT. pro-otic; QU.JU, quadrato-jugal ; RA. UL.
radio-ulna; SP. ETH. sphenethmoid ; SQ. squamosal; S. SCP. supra-scapula ;
sus. suspensorium ; TI. PI. tibio-fibula; 47.47. transverse process ; UST. uro-
style; V. 1, cervical vertebra-; V. 9, sacral vertebra; VO. vomer; /—V digits.
(From Parker and Haswell's Zoology, after Howes, slightly altered.)

38 THE FROG CHAP,

When two vertebra are placed in position, the convex
posterior face of the foremost centrum fits into the concave
anterior face of its successor, like a cup and ball, and at
the same time the posterior articular processes of the first
fit over the anterior articular processes of the second. All
the touching surface is, as we have seen, capped with
cartilage, and the vertebrze can be moved upon one another,
either up and down or from side to side.

The centra and articular processes are the only parts of
the vertebree which are in contact when the bones are in their
natural positions. Large gaps or notches are left between
the dorsal portions of the arches (see Fig. 8, A) to allow of
the requisite amount of up and down movement, and there
are similar gaps between the sides of the arches, bounded by
the articular processes above and the centra below. These
are called the zztervertebral foramina: through them the
nerves pass from the spinal cord.

The only differences of importance between the vertebree now under
consideration is in the form of their transverse processes, which are
specially large in the third, short and devoid of cartilaginous tips in the
fifth, sixth, and seventh.

The first vertebra (v. 1) has no transverse processes, and
its anterior face bears, instead of the ordinary articular pro-
cesses, a pair of obliquely placed, oval, slightly concave sur-
faces or facets, covered with cartilage, and serving for the
articulation of the condyles of the skull, presently to be
described. The transverse processes of the ninth vertebra
are very long and strong, directed backwards, and tipped
with cartilage: to them the arms of the pelvic-girdle are
articulated.

The eighth vertebra differs from its predecessors in having its body
concave behind as well as in front. Corresponding with this, the ninth

i AXIAL SKELETON 39

(v.9.) has its centrum convex in front, while behind it presents two
little rounded elevations placed side by side.

It will be seen that the vertebrz are all corresponding
structures, following one another in a regular series from
before backwards. A correspondence of this kind, in
which there is a repetition of similar parts along the body, is
termed a serial homology, and thus not only the vertebree as
a whole, but also their various parts are serially homologous,
each to each, the correspondence being disturbed only by the
first vertebra, in which the transverse processes are absent
and the anterior face modified for articulation with the skull.

The urostyle (usT) is a long bone, the anterior part of
which has somewhat the appearance of a small vertebra with
no transverse processes, and having the posterior face of its
centrum produced backwards into a long projection. On its
front face it has a double concavity for articulation with the
double convexity on the ninth vertebra. Near its anterior
end there is on each side a small aperture for the last spinal
nerve.

The skull is a very complex structure, consisting partly
of bone, partly of cartilage. .It is divided into the following
regions :—

1. The drain-case or cranium, a sort of oblong box con-
taining the brain (Figs. 8 and 9): it forms the middle portion
of the skull and is a direct forward continuation of the
vertebral column.

2. The auditory capsules, a pair of outstanding masses
arising, right and left, from the posterior end of the brain-
case. They lodge the organs of hearing.

3. The olfactory capsules (olf. cp), smaller masses pro-
ceeding from the anterior part of the brain-case and united
with one another in the middle line. They lodge the organs
of smell.

40 THE FROG CHAP.

4. The suspensoria (sus), a pair of outstanding rods,
springing from the outer and upper portions of the auditory
capsules, and directed downwards, outwards and backwards.
To them the ends of the lower jaw are attached.

5. The upper jaw, a half-circle of bone and cartilage,
united in front to the olfactory capsules and behind to the
auditory capsules and suspensoria. On each side of the
skull, between the cranium and upper jaw, is a large space,
the orbit, in which the eye is contained.

6. The dower jaw, a roughly semicircular bar of bone and
cartilage, articulated at each end with the corresponding
suspensorium.

7. The tongue-cartilage or Ayord (b. hy), a shield-shaped
cartilage connected by delicate curved rods (a. ¢. Ay) with
the auditory capsules.

On the posterior surface of the brain-case is a large hole,
the foramen magnum (fig. 9, Cc, for. mag), on each side of
the lower edge of which is an oval elevation covered with
cartilage, the occipital condyle (oc. cn). The foramen magnum
leads into the cavity in which the brain is contained. If the
first vertebra is placed in its natural position with regard to the
skull it will be seen that the foramen magnum corresponds
with the neural canal of the vertebra, and that the condyles
fit into its articular surfaces. Thus the skull readily moves
up and down upon the vertebra, the condyles acting as
rockers ; a space between the neural arch and the dorsal
edge of the foramen magnum covered by membrane in
the fresh state, allows of the requisite amount of play.

The discrimination of the separate bones of the skull is rather
difficult, and may very well be omitted by the beginner at the present
stage.

The occipital condyles are borne on a pair of irregular bones
(EX.OC) which bound the sides of the foramen magnum, nearly uniting

41

SKULL

II

above and below it, and extending over a considerable portion of the

posterior surface of the auditory capsule.

pitals,

These bones are the exocc7-

In front of each exoccipital is another irregular bone (PR.OT)

forming the front part of the auditory capsule, and called the pro-ofie.

Cpasaae Apysys ‘samozy Jaye ‘[[aasepy pue JayIeg wo17)
[vsowenbs “Os ¢ plowqieueyds *Aag ‘gg
LO*d * eylixewasd yyy : arpad ‘pag
“Kd ‘averipenb-ojyjed -2d zug Sauteed “ZR Gg { ssao0id
°u2 ‘90 {SaAJaU YWadas pue yYY
¢ UBI[axoVU-oJUSUT
Syeidioc0x9 ‘99 “xg
UIOD IOLIAIUe “hy +2 -w

*JIWOA "G4 { whtiosusdsns
:yesnf-oyerpenb -7/°76 { pos
£ plody jo nuiod so11a3sod

sus Ssadeys ‘gps §
-Siaid “9 Zq !9n0-01d
! plousydsered -“os
sarnsdeo Arojoejjo “g2 “yo ‘ ajApuos eyidi990
Suauresoy odo ‘2 *aar
Teratred-ojuoy “Pg "yy Sumuseur usuresoy «Sy of
SB[PUINOD “TQO ‘-prody jo Apoq “dy +g §

S[eseu pay ¢ Byixew “yyy

‘(2MBy 30 1J2]) Opis 1YSt1 ay3 UO Paaoural are sau sueIqMaUT ayy
V “Borg ay} jo [INAS—"6 ‘org

apis Yea] oy woy ‘g :

Each pro-otic is separated from the corresponding exoccipital, in young

frogs, by a band of cartilage, but in old specimens the two bones are

more or less completely united.

In the disarticulated skull it can be made out that the exoccipital and

42 THE FROG CHAP,

pro-otic of each side enclose a cavity; in this the organ of hearing is
contained. The exoccipital is perforated, just in front of the condyle,
by a large aperture through which two nerves, the glossopharyngeal and
the vagus, pass on their way from the brain. The pro-otic is similarly
perforated or notched for the trigeminal and facial nerves (Vz. 5, 7).

The dorsal surface of the brain-case is covered by two longish, flat
bones (FR. PA). In the young condition each of these consists of two
distinct bones, the front one the /ronta/, the hinder the fariefal, As
the young frog grows the frontal and parietal of each side becomes com-
pletely fused, forming a single frozto-pardetal. On the upper surface of
each olfactory capsule is a roughly triangular bone, the nasal ia ), in
front of which is the corresponding nostril.

The ventral surface of the brain-case is covered by a dint bone
(PA.SPH) having the shape of a T. The stem extends forwards in
the middle line as far as the olfactory capsules, while the ‘arms stretch
outwards beneath the auditory capsules. This very characteristic bone
is the parasphenoid. On the under surface of the olfactory capsules,
corresponding tc the nasals above, are a pair of irregular bones, the
vomers (VO). Their outer edges are notched and help to bound the
internal nostrils; their posterior ends bear the vomerine teeth.

The anterior end of the brain-case is surrounded by a bone
(SP.ETH) which extends forwards into the region of the olfactory
capsules, and is partly covered by the fronto-parietals above and by the
parasphenoid below. This is the givdle-bone or sphenethmoid. In the
disarticulated skull it is seen to, have a very peculiar shape. Its
posterior half encloses a single cavity in which the fore-end of the brain
(Fig. 6, off. 2) is lodged. Its anterior half encloses two cavities, right
and left, separated from one another by a vertical partition, and serving
to lodge the posterior ends of the olfactory sacs or organs of smell.
Each of these cavities communicates with the single posterior cavity by
a small hole through which the nerve of smell passes.

Between the girdle-bone in front and the pro-otic behind, the side-
walls of the skull are formed of cartilage perforated by a rounded
aperture, the optic foramen (Nv. 2) for the nerve of sight.

Forming the outer part of the suspensorium is a hammer-shaped bone,
the sguamosal (SQ); its head is applied to the auditory capsule and
projects forwards into the orbit.

The upper jaw is formed of three bones on each side. In front is the
premaxilla (PALX), « short bone, sending off an upward process

Il SKULL 43

towards the nostril. Next follows the maxz//a (7X), a long, curved
bone, forming the greater part of the upper jaw, and joined at its
posterior end to a small, slender bone, the guadrato-jugal (QU JU),
which is firmly connected with the lower end of the suspensorium.
Both premaxilla and maxilla are produced below into a prominent edge
from which spring a number of small conical teeth, arranged in a
single row.

Besides these three bones there are two others which seem, as it were,
to brace the upper jaw to the brain-case and suspensorium. The
palatine (PAL) is a' narrow, rod-like bone, placed transversely behind
the olfactory capsule. The pterygoid (PTG) is « large, three-rayed
bone ; one ray is directed forward and connected with the outer end of
the palatine and with the inner face of the maxilla; another passes
backwards and inwards and is connected with the auditory capsule; the
third extends backwards and outwards and forms the inner and ventral
portion of the suspensorium. The main mass or core of the suspen-
sorium, between the squamosal outside and the pterygoid within, is a
rod of cartilage (ss), which is continued forwards by a bar (fad. gz)
supporting the pterygoid and palatine.

There is an important distinction to be drawn between the bones of
the skull which can be made out only by the exercise of a good deal of
care and patience. By softening the connective-tissue which binds the
bones together it is possible to remove the majority of them without
injuring the underlying cartilage (compare the right and left sides of the
skull in Figs. 8 andg, A and C), provided, of course, that the opera-
tion is skilfully performed : these bones are the nasals, vomers, fronto-
parietals, parasphenoid, premaxille, maxille, quadrato-jugals, palatines,
pterygoids, and squamosals. A sort of foundation or groundwork (left
side of figure in Figs. 8 and 9, A; right side in Fig. 9, C) is then left
behind, consisting mainly of cartilage, but containing the exoccipitals,
pro-otics, and girdle-bone. These five bones cannot be removed
without pulling the cartilaginous’ groundwork or chondrocranium to
pieces. We thus get a distinction between cartz/age-bones which are
actually continuous with the cartilage and form part of the chondro-
cranium, and memdbrane-bones which lie outside the chondrocranium,
united to it only by connective tissue.

The chondrocranium has a. cartilaginous roof, underlying the fronto-
parietals ; it is pierced by one large (Fig. 8, fo) and two small ( fo’)
spaces, called fontanelles, covered by membrane. It has also a cartila-

44 THE FROG CHAP,

ginous floor (Fig. 9, A) underlaid by the parasphenoid. The olfactory
capsules (o/f. cp) also have a cartilaginous roof and floor of irregular form,
with the posterior end of which is united the cartilaginous pa/ato-guadrate
bar (fad. gz), with which the palatine and pterygoid bones are connected.
Posteriorly this bar is joined to the cartilaginous groundwork or core
of the suspensorium (szs), which unites above with the auditory capsule
by two processes (Fig. 9, C, of. pv, ped) and below furnishes an articular
surface for the lower jaw.

Notice that in describing the vertebral column no distinction was
drawn between cartilage-and membrane-bones. As a matter of fact the
vertebree and the urostyle are all cartilage-bones ; each consists, in the
tadpole, of cartilage which subsequently undergoes ossdficatéon, 7.¢.,
is replaced by bone in which a deposition of lime salts takes place.

The lower jaw (Fig. 9, B) consists of two halves, or
rami, united with one another in front by ligament. At
its posterior end each half bears on its upper surface a
shallow pit, by which it articulates with the suspensorium,
and a little in advance of this pit is an elevation of the
dorsal edge of the jaw, called the coronary process.

Each half of the lower jaw consists ofa cartilaginous core called Afeckel’s
cartilage, which furnishes the articular surface referred to, and in front is
ossified as a small cartilage-bone, the mezto-meckelian (M.MCK). Out-
side the cartilage are two membrane-bones. One, the angzlo-splenial,
extends along the inner surface and lower edge of the jaw and forms the

coronary process, while the dentary (DN7) forms the outer surface of
the anterior half of the jaw.

The hyoid is a thin, shield-shaped plate of cartilage
(Figs. 8 and 9, 4. 2y) produced, both in front and behind,
into a pair of processes or horns, as well as into less impor-
tant offshoots. The azterior horns (Fig. 9, a.c.hy) are long,
delicate, cartilaginous rods which curve backwards and then
upwards, finally uniting with the auditory capsules. The
posterior horns (p.c.hy) are short bony rods which pass
backwards, diverging as they go, one on each side of the
glottis.

lI TYMPANIC CAVITY 45

Two apparently insignificant structures connected with
the skull must be described because of their connection with
the organ of hearing. Behind the suspensorium is a recess,
roofed over by the squamosal, and, in the entire frog,
converted by muscle and other tissues into a chamber, the
tympanic cavity (Fig. 10, tymp. cav), bounded externally by
the tympanic membrane, and communicating with the mouth

memblabh

Zymp. aL
AY pew

ost SCIOV

Fic. 10.—Transverse section (diagrammatic) through the head of a Frog at the
level of the tympanic cavity. The various parts of the skull shown in section
are black, the muscle, &c., grey, and the skin and mucous membrane white.

an. tymp. tympanic ring; 4. ky. body of hyoid; dxc. cav. buccal cavity ;
ch. plx. choroid plexus ; col. columella ; exs. ¢. Eustachian tube ; fen. ov. fenestra
ovalis ; sed. 0b7, medulla oblongata ; sem. lab, membranous labyrinth ; nd.
mandible; Mv. V///. auditory nerve; 9. st. omosternum; zg. pterygoid ;
gu. ju. quadrato-jugal ; s#f. stapes ; tywp.cav. tympanic cavity ; typ. mem.
tympanic membrane. (From Parker and Haswell’s Zoology.)

by the Eustachian tube. Supporting the tympanic mem-
brane, as the frame of a tambourine supports the parch-
ment, is a cartilaginous ring, the tympanic ring (shown in
section in Fig. 10, az. fymp). Stretching across the tym-
panic cavity from the outer wall of the auditory capsule to
the tympanic membrane is a small, hammer-shaped rod, the
columella (co/), having a bony handle and a cartilaginous

46 THE FROG CHAP.

head, the latter firmly fixed to the inner face of the tym-
panic membrane. The inner end of the handle is tipped
with cartilage, and is attached to a small cartilaginous nodule,
the stapes (st), which is inserted into an aperture in the
auditory capsule known as the fenestra ovalis (fen. ov). With
care the columella can easily be removed with small forceps,
in a wet skull, and examined under a magnifying glass.
The shoulder-girdle has the form of an inverted arch
encircling the anterior region of the trunk, and having its
dorsal ends turned inwards so as partly to cover the second
to the fourth vertebree (Figs. 8 and 11). The dorsal region,

HOW $.5¢p

U3
SCD

J gt

Az cor
€f2.COor

Fic. 11.—Diagrammatic transverse section through the shoulder-girdle.
cor. coracoid ; ef. cor. epicoracoid ; g/. glenoid cavity ; 4. humerus ; s¢f. scapula ;
s. scp. supra-scapula ; v. 7, third vertebra.
on each side, is formed by a broad plate, the supra-scapula
(s. sc), or upper blade-bone. It is mostly formed of bone,
but its free edge consists of cartilage which, when dried, is
seen to be impregnated with a granular deposit of lime-salts.
This rough, brittle tissue is called cakified cartilage, and is
distinguishable from true bone, which has usually a smooth,
enamelled surface. ; :
Connected with the ventral end of the supra-scapula and
passing vertically downwards is a flat bone, broadened at
each end, the scapuda or blade-bone (Fig. 11, sep: Fig. 12, 5S).

111 SHOULDER-GIRDLE 47

From its rower end two bones (Fig. 12, C4, Co; Fig. 11,
cor) pass directly inwards, parallel with one another, to end
in a plate of cartilage (Co!), which meets with its fellow
of the opposite side in the middle line of the chest (w).
The more anterior of these (C7) is a narrow bone and
is called the clavicle or collar bone, the posterior one is

Fic. 12.—The shoulder-girdle of the Frog from the ventral aspect.

Co. coracoid ; Co’. epicoracoid ; CZ. clavicle ; ZZ. epi- and omo-sternum ; G. glenoid
cavity ; /e. fenestra between procoracoid and coracoid ; KC. cartilage separating
scapula and clavicle; A. xiphisternum; 7. junction of epicoracoids; S,
scapula; S?. sternum. (From Wiedersheim’s Comparative Anatomy.)

broader and is known as the coracoid (Co). Between the
scapula on the one hand and the clavicle and coracoid on
the other, there is a cartilaginous interval (AC), the posterior
edge of which is scooped out into a depression, the glenoid
cavity (Fig. 12, G; Fig. 11, g/), for the articulation of the
upper-arm bone. ,

48 THE FROG CHAP,

Connected with the median ventral portion of the shoulder-
girdle is the s¢erzum, or breast-bone, which consists of two
separate parts, one extending forwards, the other backwards,
in the middle line, and each formed of a flattened bony rod
(Zp, S?#), tipped with a flat plate of cartilage.

The anterior bony rod is called the efzsternum (Hp), its terminal
cartilage the omosternum ; the posterior bony rod is the sterzmz (St.), the
bilobed cartilage at its end the xiphisternum (Ku.), The cartilages
uniting the inner or ventral ends of the clavicles and coracoids are dis-
tinguished as the epdzcoracozds (Co).

All the bones of the shoulder-girdle and sternum are cartilage-bones
except the clavicle. This can be removed, and is seen partly to surround
a bar of cartilage, the procoracofd, which stretches between the scapula
and the epicoracoid and is .ordinarily completely concealed by the
clavicle.

The Fore-limb.—The upper arm is supported by a single
bone, the Aumerus (Fig. 8, HU), the first example we have
had of what is conveniently called a Jong bone. It consists
of a roughly cylindrical shaft, formed of dense bone, and of
two ex¢remities—the proximal of partially calcified cartilage,
the distal of spongy or cancellated bone. The proximal
extremity or Aead is convex, and fits into the glenoid cavity
of the shoulder-girdle (Fig. 11); the distal extremity. or
condyle is almost globular, and is articulated with the bone
of the fore-arm.

In alongitudinal section of a humerus which has not been
allowed to dry you will see that the shaft (Fig. 13, sh) is not a
solid rod, but a tube, containing a cavity, the marrow-cavity.
In this way the weight of the bone is diminished without its
strength being impaired. The marrow-cavity contains a
substance called bone-marrow, composed chiefly of connective
tissue and fat, with blood-vessels. The proximal end of the

hollow shaft is, as it were, plugged by the cartilaginous
extremity.

MT ‘FORE-ARM 49

The fore-arm is also supported by a single bone, the vadio-
ulna (Fig. 8, RA. UL). Its proximal end is concave and
articulates with the almost globular condyle of the humerus:
the outer or posterior edge of the concavity is produced into
a short process, the olecranon or elbow. The distal end is

Fic. 13.—Longitudinal sections of the principal long bones of the Frog.
A, humerus; B, radio-ulna; C, femur; D, tibio-fibula. cx. condyle; 4 foramen
for artery ;_/2. fibula; Aa. head ; 7. marrow ; o/. olecranon process ; #. bony par-
tition ; va. radius; sh. shaft; 72. tibia; #2. ulna.

incompletely divided into two articular surfaces, and between
these is a groove passing for some distance towards the
proximal end of the bone. ‘A section shows that at this end
there are two distinct marrow-cavities, indicating that the bone
is really double.. That this is the case is proved by the

Pract. Zoot, E

50 THE FROG CHAP.

examination of a very young frog, in which the single fore-
arm bone is represented by two distinct cartilages, the radius
on the inner or thumb-side, and the w/za on the outer or
little-finger side. The olecranon is a process of the ulna.

The skeleton of the hand is divisible into three regions :
the carpus or wrist, the me¢acarpus or mid-hand, and the
phalanges or finger-bones. The carpus consists of six
small irregular bones, arranged in two rows (Fig. 8). The
proximal row articulates with the radio-ulna, while to the
distal row are attached the metacarga/s, which together
constitute the metacarpus. Four of these are long rod-like
bones and support the bases of the four fingers or digits :
to them are attached the phalanges, of which the first or
innermost digit (II), has two, the next two,-and the remain-
ing two digits three apiece. A very small metacarpal, with
a single phalanx (I), occurs on the radial side and is con-
cealed by the skin in the entire frog: it corresponds with
our own thumb, so that the apparent first digit of the frog
is really the second or index-finger.

The Hip-girdle—This, as we have seen, has somewhat
the form of a bird’s merrythought. It consists of two long
arms (Fig. 8, IL: Fig. 14, Z/), which are articulated with the
transverse processes of the ninth vertebra, and sweeping
backwards, unite in a disc-shaped mass, having on each side
of it a deep, hemispherical cavity, the acetabulum (Fig. 8,
acth ; Fig. 14, G), for the articulation of the thigh bone.

Two sutures, or lines of separation, nearly at right angles
to one another, divide the disc-shaped portion into three
parts. One of these, dorsal and .anterior in position, is
continued into one of the arms of the hip-girdle and forms
half of the acetabulum ; this is the zwm (Fig. 14, ZZ, P).
The second, posterior in position, is the zschium (Is) ; like
the ilium it is made of true bone. The third, or pudis (Kn),

It HIND-LIMB 51

is ventral, and is formed of calcified cartilage. Originally each
of these elements is paired, z.¢., there is an ilium, an ischium,
and a pubis on each side, the three together forming the
innominate, but in the adult the right and left ischia and
pubes become united in the median
plane, the ilia only remaining free.
The Hind-limb.—The thigh, like
the upper arm, is supported by a
single long bone, the femur (Fig. 8,
FE), having a gently curved shaft
and extremities of calcified cartilage.
Its rounded proximal extremity, or
head, fits into the acetabulum : its
distal end articulates with the single
bone of the shank, the “d70-ftbula
(TI. FI). This, the longest bone
in the body, also has a shaft and — Fi. 14.—The pelvic girdle of

a ) Peay the Frog seen from the
extremities, and is further distin- right side.

: é G. acetabulum ; A. pubis ;
guished by grooves running from Gee vee
each end towards the middle of the
shaft. Sections show that the grooved portions of the bone
contain a double marrow-cavity (Fig.13, D), and in the young
animal there are found two shank-bones which afterwards
unite, the /#4za on the inner side, the #4z/a on the outer side.

The foot, like the hand, is divisible into three regions :
the ¢arsus or ankle, the me/atarsus or mid-foot, and the
phalanges or toe-bones. The tarsus, like the carpus,
consists of two rows, but with only two bones in each.
Those of the proximal row (as¢vagalus and calcaneum) are
greatly elongated (AST., CAL.), and furnish an additional
segment to the limb, thus increasing the frog’s leaping
powers : those of the distal row are very small.

The metatarsals are five in number: those of the first

E 2

52 THE FROG. CHAP.

and second digits (I, IT) bear-two’ phalanges each, those
of the third and fifth, three each, and that of the
fourth, four. Attached to the inner side of the tarsus
is a little claw-like structure (C) composed of two or three
bones and called the cadar or spur.

Notice the striking correspondence in __ structure
between the fore- and hind-limbs,.a correspondence which
extends also, though less obviously, to the. limb-girdles.
The humerus corresponds or is serially homologous (p. 39)
with the femur, the radius with the tibia, the ulna with the
fibula, the carpals with the tarsals, the metacarpals with
the metatarsals, and the phalanges of the fingers with
those of the toes. Then in the limb-girdles the’ glenoid
cavity corresponds with the acetabulum, the scapula and
supra-scapula (being above the articular cavity) with the
ilium, the procoracoid and clavicle (being ventral. and
anterior in position) with the pubis, and the coracoid with
the ischium. Thus not only are the limbs: and limb-
girdles serially homologous structures, but their several parts
are also serially homologous, each to each.

Nature of Bone.—It is a mistake to suppose that
bones are made exclusively of hard mineral mattér, like
‘rocks or stones. If one of the long bones, for example,
.is put into weak acid, bubbles of gas will rise from the
‘bone, showing that the phosphate and carbonate of lime,
of which, it is partly composed, is being decomposed with
.the liberation of carbonic acid. When the liberation of
.bubbles is over, the bone will be found to be unaltered
-in form, but to be-quite flexible instead of haid ‘and rigid.
‘It'can’ be bent in any direction, and a ‘bone of sufficient
‘length, such as a sheep’s rib, can be tied in a knot. This
shows that the bone contains’ a large amount’ of organic
-or animal matter. On the other hand, a bone. may be

It PRACTICAL DIRECTIONS 53

completely cadined or heated to redness in a closed vessel,
when its animal matter is completely consumed and its
mineral matter left. Under these circumstances it becomes
very brittle, falling to pieces at a touch, and its appearance
is far more altered than by the removal of the mineral
matter.

PRACTICAL DIRECTIONS,

Preparation of the Skeleton.—Kill a frog with chloroform (p. 31),
open the abdomen according to the directions on p. 32, but without
cutting the shoulder-girdle, and remove the contained organs. Then,
the frog being firmly pinned down, remove the skin and gradually
cut away the flesh from the bones with scalpel and scissors. In the
case of the long bones of the limbs, it is best to cut through the muscles
near one end of the bone and then gradually to strip them back towards
the other end until the bone is exposed. The process is facilitated -by
dipping the frog occasionally into boiling water : this softens the connec-
tive tissue- by which the bones and muscles are bound together, and
thus allows them to be more readily separated. While at work keep
Fig. 8 before you, and be particularly careful not to injure those parts
of the skeleton which are made of carté/age (dotted in the figure),
and are therefore easily cut. The most important of these cartilaginous
parts are the Ayozd or tongue-cartilage (4.4y), lying in the floor of the
mouth, -the omosternum (Fig. 12, in front of Zp), which projects in
the middle line in front of the shoulder-girdle, and the x7phisternum
(An), which extends backwards from the. same region. Great care
will also be required in cleaning the bones of the hands and
feet, since the fine cords or ¢esdors which pass to them from the
muscles’are very: strong, and if pulled upon with much force are sure to
bring away the small toe-bones with them: they should be separated as
far as possible and then cut off, close to the bones, with scissors.

Keep all the parts of the skeleton together, avoiding separation of the
various bones until the general characteristics of the entire- skeleton
have been made out: the only part which cannot be kept in connection
with the rest is the shoulder-girdle, which will come apart, together with
the fore-limbs.

Examination of the Skeleton.—You should have two skeletons
to examine—one dried, after it has been thoroughly cleaned, and one

54 THE FROG CHAP. III

which has been kept from the first in spirit or formaline: the latter is
the more instructive. An additional skull should be carefully cleaned,
and then boiled until the numerous bones become separated from one
another or d¢sarticulated.

After observing the form and relations of different parts of the
skeleton as described on pp 35 and 36 (Fig. 8, A), they may be sepa-
rated from one another for more detailed examination. The individual
vertebree should be strung on a piece of wire or string so as to prevent
their being lost or misplaced.

With the specimen before you, work through the characters of the
axial skeleton (pp. 36—46): if you omit the details given in small type
(pp. 40—44) at the present stage, do not forget to examine them
subsequently. Make sketches of—a. Any one of the vertebre from
the first to the seventh, from the side and from the front and back3
6. the first vertebra; «. the urostyle; @. the skull from above and
from below ; and e. the hyoid.

It requires considerable skill to make a satisfactory preparation of the
chondrocranium, but if you wish to attempt it, procure a large skull
which has not been dried, and boil it in water. Carefully separate, by
means of a scalpel, most of the membrane-bones (p. 43); the palatines,
pterygoids, and quadratojugals, and the dentaries and angulosplenials
cannot well be disarticulated without destroying the soft cartilaginous
parts beneath them.

Make out—1. The dvaiz-case and its fontanelles and nerve apertures.
2. The olfactory capsules. 3. The audztory capsules. 4. The palato-
quadrate bar (to which the palatine, pterygoid, and quadratojugal bones
have been left attached). 5. The mandibular or Mecke?’s cartilage
(to which the angulosplenial and dentary have been left attached).
6. The cartilage-bones (exoccipitals, pro-otics, sphenethmotd, and mento-
meckelians). 7. The columella, stapes, and fenestra ovalis. Sketch
from above and from below.

Now proceed to examine the appendicular skeleton (pp. 46—52),
and sketch the shoulder-girdle and fore-limb, and the hip-girdle and
hind-limb.

Split one of the longer limb-bones (e.g., humerus or femur)
longitudinally with a knife, and note the sarrow-cavity in the shaft
(Fig. 13). Place another of the long bones in 10 per cent. hydro-
chloric acid for an hour or two; wash thoroughly in water and
examine.

CHAPTER IV
THE FROG (continued) : THE JOINTS AND MUSCLES

In the previous chapter the bones—more than 150 in
number—which together constitute the greater part of the
skeleton of the frog, have been considered as so many
separate parts, fitting into or against one another in certain
ways. We must now see how they are joined together in
the entire animal so as to afford the requisite support, and,
at the same time, to allow of free movement.

The Hip-joint—Let us begin by a study of the hip-joint
(Fig. 15).

The acetabulum (acéé), as you have already seen (p. 50),
isa hemispherical depression on the outer surface of the
hip-girdle. It is formed of cartilage, continued into a pro-
jecting rim round the edge of the cavity. The head of the
femur (Zd@) is also formed of cartilage, and fits accurately
but rather loosely in the acetabulum.

The acetabulum is lined, and the head of the femur is
covered, by a thin skin of connective tissue, the pevichon-
drium (p. cha), which, in both cases, is continued on to the
adjacent bone, where it receives the name of fertosteum
(2. ost).

Attached all round the rim of the acetabulum is a strong
sheet of connective tissue called the capsular hgament

56 THE FROG CHAP.

(cps. lg), forming a short, fibrous tube. The other end of
this tube is fixed to the femur, just below the head, the
ligament being continuous, in each case, with the perichon-
drium. There is thus a space between the head of the
thigh bone and the acetabulum, closed all round by the
capsular ligament. This space is filled with a delicate, fibrous,
closed bag, the synovial capsule (sy. cps), one side of which

Pp.cst eps.lg
ha

Fic. 15.—Horizontal section of the Frog’s hip-joint.
actb. acetabulum ; cfs. Ze. capsular ligament ; 4d. head of femur ; 77. ilium ; med.
marrow ; #.chd. perichondrium; J. os¢, periosteum; fw. pubis; sh. shaft of
femur ; sy. cfs. synovial capsule.

chd actb

fits closely into the acetabulum, while the other as closely
invests the head of the femur. The capsule is filled with a
watery fluid, the syzov/a, and thus forms a buffer or water-
cushion between the adjacent bones. Thus the synovial
capsule keeps the two bones slightly apart and prevents
friction, while the capsular ligament keeps them together
and prevents dislocation.

It is obvious that, in such a joint as this, movement is
possible in all directions. The femur can be inclined either
upwards, downwards, or sideways, and is capable of a certain
amount of rotation. The joint is, in fact, a cup-and-ball

Iv : JOINTS 57

Joint, and is capable of movement in any plane. A similar
but less perfect cup-and-ball joint is that of the shoulder,
in which the cup is furnished by the glenoid cavity, the ball
by the head of the humerus.

Other Joints.—The elbow- and knee-joints are constructed
on the same general plan, but, owing partly to the form of
the adjacent surfaces, partly to the mode of attach-
ment of the ligaments, they are capable of movement
in one plane only, z.e., upand down, but not from side to
side. They are therefore distinguished as hinge-joints.

The vertebre are connected with one another in a similar
way. Between the convex hinder face of one centrum and
the concave front face of its successor is a synovial capsule,
and the two centra are bound together by ligament, a
shallow cup-and-ball joint, with a very limited range of
movement, being produced. There are also synovial
capsules between the articular processes, which, being in
contact with one another by flat surfaces and working mainly
from side to side, form glading-joints. Thereare also strong
ligaments connecting the neural arches with one another and
joining the first vertebra to the skull.

In all cases where free movement is necessary the
joints are formed in the same way; the bones are bound
together by ligaments, and a synovial capsule is interposed
between their adjacent surfaces. When little or no move-
ment is required, as between the bones of the shoulder- and
hip-girdles, the union is effected by cartilage or ligament
only, and there is no synovial capsule. Such joints are
therefore distinguished as zamoveable or imperfect joints.

The Muscles.—We see then that the bones of the skeleton
are attached to or articulated with one another by means
of ligaments, so arranged, in most cases, as to allow of more
or less frée movement between the bones. We must now

Fic. 16.—The muscles of the Frog from the ventral aspect. On the left side (right
of figure) many of the superficial muscles have been cut and reflected to show
the deep layer.

add. brev. adductor brevis ; add. long. adductor longus ; add. mag. adductor mag-
nus ; ded. deltoid ; ext. cv. extensor cruris ; evz. ¢vs. extensor tarsi; FE. femur ;

CHAP, IV MUSCLES 59

gn. hy. genio-hyoid ; gstv. gastrocnemius ; Ay. ¢7. hyoglossus ; zzs. en. tendinous
Inscription ; 2. a/b, linea alba; my. hy. mylo-hyoid ; ob¢. int. obliquus internus ;
o6/. ext. obliquus externus; o0.s¢. omosternum ; J. c. Ay. posterior cornu of
hyoid ; Act. pectoralis; Actw. pectineus ; ger. peroneus; rct. add. rectus ab-
dominis ; rect. int. may. rectus internus major; sar. sartorius; sd. 22. sub-
mentalis ; sev. te. semi-tendinosus ; #7. anz. tibialis anticus ; 216, post. tibialis
posticus; TI. FI. tibio-fibula; vas¢. zat. vastus internus; +. s¢. xiphisternum.
(From Parker and Haswell’s Zoology.)

try to find out how the movements are effected in the
living frog.

It was pointed out in the second chapter that the
flesh is made up of distinct bands or sheets, the muscles,
some of which came under your notice in your first dissec-
tion. It is quite easy to convince yourself that the whole of
the flesh has this character by skinning a frog and carefully
removing the fascia (p. 18) which covers the muscles and
the more delicate web of connective tissue which forms a sort
of packing substance between them. After noticing some
of the muscles shown in Fig. 16, especially those of the leg,
concentrate your attention on the muscle marked gs¢7, a
prominent spindle-shaped mass of flesh forming the calf of
the leg, and known as the gastrocnemius (gstr).

The spindle shaped, fleshy mass or ée//y of this muscle is
continued at either end into a band of strong, tough con-
nective tissue, the ¢exdon (Fig. 17). The tendon at the
proximal end is flat and is attached to the distal end of
the femur and to the proximal end of the tibio-fibula, in
each case becoming continuous with the periosteum of the
bone. The tendon at the distal end has the form of a stout
cord and is distinguished as the /endo Achillis ; it corresponds
with the strong tendon just above the heel of the human
foot. At its distal end it is continued into a broad sheet of
connective tissue, the A/antar fascia, which spreads over the
whole sole or plantar surface of the foot.

If the foot is bent upon the shank as in the ordinary
sitting position of the frog, and the gastrocnemius pulled

60 THE FROG CHAP, .

upwards or towards the thigh, the foot will instantly be
bent backwards, so as to come into a straight line with
the shank, the action being one of those performed by the
living frog when leaping. It will be seen that the proximal
tendon is attached to a relatively fixed point: it is
distinguished as the tendon of origin, or the muscle is.
said to arise from the femur and tibio-fibula. The distal.
tendon is attached to a relatively moveable part, the foot,
and is called the tendon of zsertion, the muscle being said
to be zzserted into the plantar fascia.

Muscular Contraction._-Obviously, however, there is:
nothing to pull upon the muscle from outside in the living.
frog: We must, therefore, try to form some idea as to-
how the action of bending the foot, roughly imitated in the-
dead subject, is performed during life. If the -gastro-
cnemius be exposed in a recently killed frog, the foot bent
up as before, and a smart pinch be given to the belly of
the gastrocnemius, the foot will be bent back, although
no pull has been exerted on the muscle. The same thing
will happen if you drop on the gastrocnemius a single drop
of weak acid or of a strong solution of common salt, or if
you touch it with a hot wire, or if you apply to it the
electrodes from an induction coil so as to pass an electric
current through it.

Careful observation shows that what happens under either
of these circumstances is, that the belly of the muscle
decreases in length and at the same time increases in
breadth, so as to become shorter and thicker (Fig. 17).
The result of this must necessarily be to cause its two ends to
approach one another. As the tendon of origin is attached
to the femur, which we suppose to be fixed, it is unable to-
move, and the insertion is therefore drawn upwards, bringing
with it the moveably articulated foot. In fact exactly the same

Vv MUSCULAR CONTRACTION 61

thing takes place as when we raise our own forearm, This
action is performed by means of the biceps muscle which
arises from the scapula and is inserted into the fore-arm.
When the latter is raised we feel a lump rise on the front of
the upper arm due to the thickening of the biceps.

Fic. 17-—Diagram of apparatus for demonstrating the contraction of the gastroc-
nemius muscle. ,

A, upright bearing two adjustable horizontal arms. To the upper of these.(B) is fixed
by a clamp, the femur (/@), having the gastrocnemius (gs¢7) in connection with
it. To the lower arm (C) is fixed a light lever (L) moveable in a vertical plane,
and having the tendon of insertion of the muscle attached to it by a thread.
The dotted lines show the form of the gastrocnemius and the position of the lever
during contraction of the muscle. sc. zv. the sciatic nerve.

This shortening and thickening of* the miuscle is termed
a contraction. Do not fail to notice that this word is used
in a special sense. When we say that a red-hot bar of iron

‘contracts on cooling we mean that it becomes smaller in all

dimensions—undergoes dan actual decrease in bulk. But in

“muscular contraction there is no alteration in bulk: the

62 THE FROG CHAP.

decrease in length is balanced by an increase in thickness,
as when a stretched piece of india-rubber is relaxed.

The external influence by which a contraction is induced
is called a stimulus. As we have seen, a stimulus may be
produced by actual contact of some external object
(mechanical stimulus), or by chemical action (chemical
stimulus), or by heat (thermal stimulus), or by an electrical
current (electrical stimulus).

Relation of Muscle and Nerve.—Evidently, however, we
have by no means got to the bottom of the matter. In the
living frog movements are always going on, and are all due
to the contraction of muscles, and yet no stimuli of the
kind enumerated are applied to any of them. As the
muscles retain the power of contraction for some little time
after the death of the animal, it is easy to make such experi-
ments as that described in the next paragraph.

Running longitudinally between the muscles on the dorsal
side of the thigh is a shining white cord, the sctatic nerve
(Fig. 17, sev), accompanied by a vein: it gives off
branches to the muscles and skin, and, amongst others, one
to the gastrocnemius. If this nerve be carefully separated
as it traverses the thigh and pinched with the forceps, the
gastrocnemius will contract just asif the stimulus had been
applied to it directly, and the same will happen if a chemical,
thermal, or electrical stimulus be applied.

Thus a stimulus applied to the nerve of a muscle has the
same effect as if applied to the muscle directly : it gives rise
to a nervous impulse, which, travelling along the nerve,
induces contraction of the muscle.

Once more, however, external stimuli are not applied to
the frog’s nerves during life, and it is obvious that we must
carry our inquiry a little further. The sciatic nerve if traced
upwards will be found to pass into the trunk (Fig. 51, S¢),

Iv MUSCULAR SYSTEM 63

and finally to join the spinal cord, which, as we have seen,
is in connection with the brain. In the living frog nervous
impulses originate in one or other of these organs, without
the direct intervention of an external stimulus, and are con-
ducted along the nerves to the muscles. But further con-
sideration of this subject must be deferred until we have
made a special study of the nervous system.

The Muscular System in General.—All over the body
the muscles, though varying greatly in form—some being
elongated and band-like (Fig. 16, sav), others spindle-shaped
(gastr), others in the form of broad flat sheets (wy. hy, ob/.
ext)—have the same general relation to the skeleton as in
the case of the gastrocnemius. Each muscle arises or has
its origin in a relatively fixed part, and is inserted into a
relatively moveable part. As each muscle contracts in one
direction only, it follows that the more complex the move-
ments any part is capable of performing, the more numerous
must be its muscles. For instance, the femur, which, as we
have seen, is capable of universal movement, has no fewer
than nine muscles, arising from various parts of the hip-girdle,
inserted into it. Even the minute phalanges of the fingers
and toes all have their little slips of muscle by which the
various movements of grasping and relaxing, approximating
and separating the digits are effected.

There are certain terms applied to muscles which it is useful to know.
A muscle which raises a part, ¢.g., the lower jaw, is called a evator,
one which lowers a part a depressor. A muscle which serves to straighten
one part upon another, ¢.g., to bring the shank into line with the thigh,
is an extensor, one which bends one part on another is a flexor, A
muscle which draws a limb towards the trunk is an adductor, one which
draws it away an adductor, one which rotates one part upon another, e.g.,
the femur on the pelvis, a rotator.

Thus all the complex and accurately adjusted movements
of the frog are performed by the contraction of its numerous

64 THE FROG CHAP,

muscles, acting either singly or in concert. The contractions
of these muscles are brought about by nervous impulses sent
from the brain or spinal cord along nerves which branch
out and are distributed to the muscles, thus bringing the
whole of the complex machinery which affects the movements
of the animal under the direct control of its will.

PRACTICAL DIRECTIONS.

The Joints and Ligaments.

1. The Azp-jocnt. Strip off most of the muscles from the thigh and
the adjacent parts of the pelvis; decalcify in weak acid and then wash
thoroughly. Cut the femur through lengthwise and continue the section
through the pelvic girdle (Fig. 15).

Note: a. The cartdlage of the acetabulum and head of the femur;
the perichondrium and periosteum. (6) The capsular “igament and its

‘relations to the syoudal capsude. Observe that the hip-joint is a cap
and ball joint. Sketch.

In a prepared wet skeleton—

2. Examine and compare a hzuge-joznt (e.g., elbow or knee).

3. Examine the cartilaginous union between the bones of the shoulder-
or hip-girdle (csmoveable or imperfect joints).

4. Examine the joints and ligaments of the vertebral column.

The Muscles.

1. Remove the skin from part of the body and legs of a preserved
frog (the one you have already dissected will do). Then clear away the
fascia here and there and’ separate some of the muscles by dissecting
away the connective tissue which binds them together. Notice the
different forms of the muscles in different parts. (Compare Fig. 16
and p. 63). ;

2. In the hind-leg carefully dissect away the connective tissue
investing .the gas¢rocnemzus muscle (Figs. 16 and 17), and trace it
upwards’ towards the thigh, and downwards towards the foot, cutting

‘away any of the other muscles which obscure the view. Notice the
belly, and the tendons of origin and insertion (p. 60). Sketch.

Bend the foot upon the shank as in the ordinary sitting position of the
frog. Hold the thigh firmly with one hand, and with the other take hold

Iv PRACTICAL DIRECTIONS 65

of the gastrocnemius and pull it upwards or towards the thigh. Note
the result.

3. Ina recently killed frog (see p. 103) expose the gastrocnemius as
directed above, and with the small forceps give a sharp pinch to the belly
of the muscle. Note the coztraction following the s¢¢mulus.

Then remove the skin on the dorsal side of the thigh, and separate
the muscles in this region so as to expose the sczatdc nerve (Figs. 17 and
51). Trace this towards the shank and notice its branch going to the
gastrocnemius. Carefully separate the nerve as it traverses the thigh
and pinch it with the forceps, noting again the contraction following
the stimulus,

Pract Zoow B

CHAPTER V.

THE FROG (continued): WASTE AND REPAIR OF SUBSTANCE —
THE DIGESTIVE ORGANS—NUTRITION,

Waste and Repair.—The effects of prolonged muscular
exertion are familiar to every one. Sooner or later sensa-
tions of fatigue, hunger, and thirst are produced, accom-
panied bya loss of weight. Indeed, however little exertion
we make and however often we feed, our weight always goes
down between meals and rises again when we take food.
The loss of substance, of which the diminution in weight
is the index, takes place largely in the form of perspiration,
a fluid consisting of water, with certain organic and inorganic
matters in solution. A further loss is due to the air
breathed out from the lungs; this is always moist, z.e.,
contains a good deal of water, and is further distinguished
by containing a considerable volume of the gas carbon
dioxide, or carbonic acid (CO,). Besides these two constant
sources of loss, there is an intermittent loss in the urine,
which consists of water, containing certain matters in
solution, the most characteristic of which are two complex
substances called urea (CON,H,) and uric acid (C;H,N,O,).
Both of these, as well as carbon dioxide, act as poisons if

CHAP. V WASTE AND REPAIR 67

allowed to remain in the system. Lastly there is an inter-
mittent source of loss in the waste matters or feeces which
are passed out from the intestine.

These losses are made good in two ways. Firstly, by
breathing, in which process we constantly inhale pure air
and replace the poisonous carbonic acid by oxygen. Secondly
by eating and drinking, by which, at intervals, we make good
the loss of solids and liquids. Just as a clock is constantly
running down and has to be wound up in order to keep
it going, so our bodies run down by loss of substance
between meals, and require to be wound up by the repair of
substance which results from food and drink.

The same thing is true of the frog. Every one of its
numerous and often vigorous movements is done at the
expense of a certain waste of substance. The various
tissues of the body are constantly undergoing a process of
wear and tear, expressed, not as in machines of human
construction, by a wearing away of surfaces and a loosening
of bolts and screws, but bya slow and almost imperceptible
dwindling, the lost material being carried off principally in
the form of water, carbon dioxide, and urea or some allied
compound containing nitrogen.

Food of the Frog: general characteristics of the Diges-
tive Process.—As we have seen, the food of the frog
consists of worms, slugs, insects, and the like. These it
catches and swallows whole, the stomach often becoming
immensely distended with numbers of captured animals.
After remaining for some time in the stomach the carcases
are found to have undergone a marked change. Their soft
parts become softer and slimy and finally semi-fluid, and in
this way the food undergoes gradual disintegration. The
quantity of food in the stomach decreases, some of it is’
passed into the intestine, which it traverses from duodenum

F 2

68 THE FROG CHAP,

to rectum, and certain portions of it are finally ejected from
the vent in the form of feces.

It is not difficult to assure one’s self that the weight of the
feeces passed during a certain time is very much less than
that of the food swallowed during the same time. Obviously
some constituents of the food have disappeared during its
progress through the enteric canal. The character of the
feecal matter, moreover, is very different from that of the food;
the only portions of the swallowed animals discoverable
in the rectum are bits of their hard parts; for the rest, the
feeces form a pulpy, black mass. That this change is due
to certain definite chemical processes taking place in the
enteric canal may be inferred from the fact that the contents
of the stomach, as well as the walls of that organ, have
an acid reaction, and turn blue litmus paper red. On
the other hand the contents of the small intestine are,
to a greater or less extent, alkaline, restoring reddened
litmus paper to its original blue colour.

It is also obvious that there must be some definite
mechanism for propelling the food from one end of the
enteric canal to the other; its passage through so long,
narrow, and coiled a tube can certainly not be accounted
for by supposing it to be merely pushed onwards as fresh
food is swallowed.

In order to understand the various processes connected
with digestion we must make a renewed and more careful
examination of the organs concerned, after removing them
from the body.

The Digestive Organs.—Arising from the gall-bladder
and passing backwards to the duodenum is an extremely
delicate tube (Fig. 18, Dc, De), the common bile-duct, which
opens into the duodenum. By gently squeezing the gall-
bladder a drop of greenish fluid may be made to ooze out

Vv DIGESTIVE ORGANS 69

of the end of the duct (D2?) into the intestine ; this fluid is
the dive.

Very careful dissection shows that the common bile-duct

Fic. 18.—Stomach and duodenum of Frog with liver and pancreas.

De, De.1 common bile duct; Dc.? its opening into the duodenum ; D. cy. cystic
ducts ; DA, Dh.) hepatic ducts ; Dw. duodenum ; G. gall-bladder; L, L1, L%, L3,
lobes of liver, turned forwards; ZA%. duodeno-hepatic omentum, a sheet of
peritoneum connecting the liver with the duodenum ; JZ. stomach ; P. pancreas ;
1, pancreatic duct; Py. pylorus. (From Wiedersheim’s Comparative
Anatomy.)

is joined by several other tubes which are traceable into the
liver and, are called hepatic ducts (Dh, Dh'). The tubes
going to the gall-bladder are called cystic ducts (D.cy) ; by

70 THE FROG CHAP,

their union with the hepatic ducts the common bile duct is
formed.

The bile is manufactured in the liver, and the gall-bladder
is merely a reservoir in which it is stored up, to be
discharged into the intestine when required for digestion.

It requires still more accurate observation to show that
the pancreas also discharges a fluid, the pancreatic juice, into
the intestine. A very delicate branching tube, the pax-
creatic duct (P'), joins the bile-duct, into which it discharges
the pancreatic juice, the two fluids entering the intestine
together. Both fluids are digestive juices, i.e., liquids which,
as we shall see, act ina particular way upon the food.

By cutting open the enteric canal and ‘examining its inner
surface under water with a magnifying glass it is seen that the
wall of the canal consists of two layers, easily separable
from one another. The outer or muscular layer (Fig. 19 A,
musc), is tough and strong, the inner layer or mucous
membrane (m.m), is soft and slimy. Between the two is
very loose connective tissue, the swdmucosa-—really a part of
the mucous membrane—which, being easily torn, allows of
the ready separation of the muscular and mucous layers.

In the stomach the mucous membrane is raised into
longitudinal folds (7), in order to allow of distension ; in an
empty stomach they are well marked, and give the cavity a
star-like cross-section (Fig. 40) ; in one full of food they are
entirely obliterated and the walls of the organ so stretched
as to be almost transparent. Anteriorly the ridges thin out
and disappear at the cardia or junction between the gullet
and stomach ; posteriorly they converge, as the stomach nar-
rows, towards the pylorus (Fig.19 A, Ay), or junction with the
duodenum, Here the muscular coat is greatly thickened in
a ring-like form, forming the pyloric valve ( py.v), by which
the aperture of communication between the stomach and

v DIGESTIVE ORGANS 71

intestine is greatly narrowed and only small particles can
‘pass through: In the duodenum (dz) the mucous membrane
is raised into little tuft-like elevations (7”) ; in the ileum the
ridges (B, 7”) become longitudinal again ; in the rectum (7c)
they are absent.

When food is taken into the stomach, a fluid, the gastric
Juice, oozes from the mucous membrane. It is this fluid

i
PY?
Fic. 19.—Portions of the enteric canal of the Frog in longitudinal section.
A, stomach and duodenum ; B, part of ileum and rectum. dz. duodenum ; 7Z. ileum ;
m. 7. mucous membrane ; #zsc. muscular layer ; py. pylorus; py. v- pyloric

valve ; ~. longitudinal ridges (rugz:) of stomach ; 7. transverse ridges of duo-
denum; 7. longitudinal ridges of ileum; cz. rectum ; s¢. stomach.

which reduces the slugs, insects, etc., to the pulpy condition
referred to above: it is, like the bile and pancreatic fluid,
a digestive juice.

General Properties of Food—We must now devote a
little attention to the characters of the food itself and to the
precise nature of the changes brought about by the digestive
process.

72 THE FROG CHAP.

As we have seen, the frog is a carnivorous animal. Now
the digestible part of the substance of animals consists
mainly of two classes of chemical compounds, called protedds
and fats. The most familiar example of a proteid is white
of egg : other proteids, of varying composition, are found in
muscle, in blood, and in other animal tissues. All are
composed of the five chemical elements—carbon, oxygen,
hydrogen, nitrogen, and sulphur—-the five elements being
combined in the following proportions :—

Carbon from 51°5 to 54°5 per cent
Hydrogen ap SOS 350 FBS G3
Oxygen 9» 20°95, 23°75 os
Nitrogen ERR TPO a,
Sulphur iy OTP yy BIO. yy

Fats differ from proteids in containing no nitrogen or
sulphur : they are formed of carbon, oxygen, and hydrogen,
the number of atoms of hydrogen being always more than
twice as great as the number of atoms of oxygen.

It will be noticed that two important articles of diet are
absent from the above list, namely sugar and starch—the
latter the largest constituent of flour, oatmeal, rice, &c.
The vegetable substances used as food by animals, such as
corn and grass, contain these bodies in varying proportions
in addition to vegetable proteids, and there is no doubt
that the frog must eat a small quantity of such vegetable
food, if only in the stomachs of the herbivorous animals upon
which it preys. Now, starch and sugar belong to a group of
compounds called carbohydrates, composed of carbon,
oxygen, and hydrogen, but differing from fats in that the
number of atoms of hydrogen is always exactly double that
of the atoms of oxygen, as in water. Lastly, the food always
contains a certain quantity of saline or mineral matters, as
well as water.

v FOODS 73

Diffusible and Non-diffusible Foods.—These four classes
of food materials—proteids, fats, carbohydrates, and
minerals—may be arranged in two groups according to a
certain physical peculiarity. If a solution of common salt
is placed in a vessel with a bottom made of bladder, called a
dialyser, which is floated in a larger vessel of pure water, it
is found that, after a certain lapse of time, the water in the
outer vessel has become salt. The sodium chloride has, in
fact, passed by diffusion through the bladder. The same
thing will happen if a solution of sugar is placed in the inner
vessel : salt and sugar are both diffusible substances, capable
of passing through an animal membrane.

On the other hand, if the inner vessel contains white of
egg, or oil, or starch well boiled in water, no diffusion takes
place. Hence proteids, fats, and starch are non-diffusible
foods, and are thus sharply distinguished from salt and
sugar, which are diffusible.

The mucous membrane of the stomach and intestine are
animal membranes having the same physical properties as
bladder. We may consequently infer that any salt or sugar
contained in the enteric canal will diffuse through the mucous
membrane and make its way, as we shall see more particularly
hereafter, into the blood, thus serving to nourish the whole
body. Proteids, fats, and starch, on the other hand, will
be incapable of diffusing, and will, therefore, unless
some change happens to them, be absolutely useless
as nutriment. For, since the enteric canal communi-
cates with the quter world at both ends, the food, para-
doxical as it may sound, is practically outside the body
as long as it remains in the canal: it is only when
it is absorbed into the blood or lymph that it is ac-
tually, in the strict sense, taken into the body. Thus,
if proteids, fats, and starch are to be of any use to the

74 THE FROG CHAP,

* frog, they must, in some way, be rendered capable of
being absorbed.

Action of Digestive Juices.—This is exactly what is done
by the digestive juices. If white of egg or any other proteid
is mixed with gastric juice and kept at a suitable tem-
perature, it is converted into a form of proteid called
peptone, which is capable of diffusing through an animal
membrane. The change is effected by means of a substance
called Zepsin, contained in the gastric juice, in which there
is also a certain proportion of hydrochloric acid. To. this
the acid reaction of the gastric juice already alluded to (p.
68) is due.

By means of the gastric juice the bodies of the animals
swallowed by the frog have their proteids largely converted
into peptones, which, being diffusible, pass through the
mucous membrane as readily as sugar or salt. Hence
the great diminution in the bulk of the food during its
sojourn in the stomach: a large proportion of it is absorbed
there and then, and only a comparatively small quantity
is passed through the pyloric valve into the intestine, where
it becomes alkaline, owing to the action of the fluid which
enters the duodenum through the bile duct, and which,
as we have seen (p. 70), consists of bile and pancreatic
juice.

Pancreatic juice has a similar effect on proteids, the
change being effected by a substance called ¢rypsin, which,
however, acts in a alkaline solution. It also has the
property of converting starch into sugar, and of splitting
up fats into fatty acids and glycerine, both of which are
diffusible. The substances by which these changes are
effected are called by the general name of ferments:
pepsin and trypsin are proteolytic or proteid-converting
ferments, and the pancreatic juice also contains an amylolytic

v DIGESTION 75

ferment, which converts starch into sugar, and a fat-
decomposing ferment.
The exact mode of absorption of the fats is not thoroughly
understood. It is usually supposed that only a small pro-
portion of them are decomposed into fatty acids and gly-
cerine, and that the greater part is merely broken up into
particles so small that they can be taken up by the epi-
thelial cells of the intestine. This emulsification of fat is
effected by the combined action of the pancreatic juice and
bile, the fats being reduced to the condition in which they
exist in milk and in the emulsions of cod-liver oil so much
used in place of the natural form of that medicine.
Thus during the passage of the food through the intestine
the remainder of the proteids, the whole of the fats, and any
‘starch which may be present, are rendered capable of
being absorbed: they pass through the mucous membrane
into the blood, and by the time the rectum is reached all
the nutriment is extracted from the food, and there re-
mains only a small quantity of indigestible matter, which is
passed out in the form of feces. ;

Peristaltic Movements.—The passage of the food
through the enteric canal is effected by the contraction of its
muscular layer, which is really double, and which is composed
of muscular fibres (see Chapter vii., Figs. 39 and 40). Inthe
inner layer these fibres have a transverse direction, encircling

_the tube, and by their contraction narrowing it ; the outer
layer.consists of longitudinal fibres, which by their contraction

-shorten.it. . By the alternate contraction and relaxation of

‘the two layers are produced a series of fertstaltic movements,
not unlike those by which an earthworm makes its way over
the ground : they can be seen in a freshly-killed frog, and
still better in a rabbit or rat.

Summary of Chapter.—The body is constantly under-

76 THE FROG CHAP.

going waste, and in consequence needs continual repair.
The waste-matters chiefly take the form of carbon dioxide,
water, and urea. Repair is effected partly by breathing (see
Chapter ix), partly by feeding. Food, consisting of proteids,
fats, carbohydrates, inorganic substances, as well as water, is
taken into the enteric canal, where, by the action of the three
digestive juices—gastric juice, pancreatic juice, and bile—it
is converted partly into a solution (peptones, sugar, fatty acids,
glycerine), partly into an emulsion (fats). As it is driven
along the canal, from stomach to rectum, by the action of
the muscular coat, the dissolved or emulsified substances
gradually disappear from the canal, and are absorbed into
the system. Finally, the indigestible constituents are ex-
pelled as feces.

Our next task must be to learn something of the process
of absorption, and of the means by which the digested food
is conveyed to the various parts of the body, so as to supply
them with the means of repairing the waste they are
constantly undergoing. For this purpose we must now
study what is called the vascular system, t.e. the heart, the
blood-vessels, and the various cavities containing lymph.

PRACTICAL DIRECTIONS.

The Digestive Organs.—Pin down under water, with the ventral
side uppermost, as before, the specimen already dissected, or another in
which the body-cavity has been opened in the same way. Note the
positions of the Aostcaval vein (Figs. 3 and 4, Zt. cv), the hepatic portal
vein (Fig. 3, Ap. pt), the aorta (Fig. 4, d. ao), and the splanchnic or
caliaco-mesenteric artery (Figs. 3 and 4, ca. mes). Then, taking care
not to injure the aorta, remove the greater part of the digestive organs,
including the liver, by cutting through the gullet and rectum and severing
the mesentery, cutting through the postcaval also where it enters the liver.
(The cloaca will be examined at a later stage). Pin the organs in the dis-
secting dish as nearly as possible in their natural position. Turn the

v PRACTICAL DIRECTIONS 77

lobes of the liver forwards (z.e., towards the head) and after making out
the relations of the parts already examined 27 sz¢z« (pp. 20—23), note—

The common bile-duct, formed by the union of the Aefatic and cystic
ducts, and the point at which it enters the duodenum (Fig. 18). Make
a small slit in the duodenum just opposite its entrance, and gently squeeze
the gall-bladder between your finger and thumb, so as to force a drop of
bile into the intestine. (The pascreatzc duct and its communication with
the bile-duct cannot easily be made out by dissection). Sketch the whole
dissection.

Now remove the liver, unravel the intestine by tearing through the
mesentery, and lay open the enteric canal by inserting one blade
of the scissors into the gullet, and cutting through the whole tube in a
longitudinal direction. Test the contents of the stomach and intestine
with litmus paper—(for this experiment a freshly killed frog is of course
necessary)—and then pin out your dissection with the inner surface
upwards, wash it under the tap, and examine under water with a
magnifying glass) Make out—

1. The cardia, pylorus, and pyloric valve.

2. The mucous membrane, and its different appearances in the stomach,
small intestine, and large intestine.

3. The muscular layer, covered externally by the peritoneum.

Make a simple déalyser (p. 73) by tying a piece of wet bladder firmly
over one end of a wide glass tube about six inches long. Into this put a
solution of sugar or salt, and immerse the tube up to the level of the
solution in a rather larger vessel of distilled water, and leave it for a
short time: taste the water in the outer vessel. Then place some white
of egg in the dialyser, and test for albumen by heating some of the water
in the outer vessel over a flame: if albumen is present, it will become
coagulated and form a cloud in the water.

CHAPTER VI.

THE FROG (continued): THE VASCULAR SYSTEM—THE CIR-
CULATION OF THE BLOOD.

In our preliminary examination.of the frog (Chap. II) we
learned one or two facts about the vascular system. We
found that there is a heart within a pericardium, two sets of
vessels, arteries and veins, containing red blood, and a set
of irregular cavities or sinuses, containing lymph. We must
now try to get some more accurate and detailed information
on these matters.

General Characteristics of Blood and Lymph.—It will
be convenient to begin by studying certain easily verified
characteristics of the blood. Frog’s blood may be used, or
as it is as well to have a considerable quantity, that of some '
larger, freshly-killed, red-blooded animal, such as a rat or
rabbit.

When first drawn from the heart or vessels the blood will
be seen to be a fluid, nearly as mobile as water or milk;
it “ finds its level,” like any other liquid, and can be readily
poured from one vessel to another. In a few minutes,
however, it undergoes a change; it ceases to be fluid, and
coagulates, or “sets” into a jelly, which if turned out of the
vessel, retains the shape of the latter. Before long a further
change takes place; the jelly begins to shrink, drops of

CHAP. VI HEART 79

yellowish fluid appear on its surface and gradually run to-
gether into larger and larger drops. The jelly contracts still
further, and finally draws itself away from the walls of the
vessel and floats in the accumulated fluid, still retaining the
form of the vessel, but being greatly reduced in size. The
process of coagulation of the blood is now complete ; the
red, jelly-like substance is called the c/o, the yellowish fluid
the serum.

When first drawn from a vein the blood is deep purple in
colour, and the clot retains for a time the same hue. But
before long, all parts of it which are fully exposed to the air
take on a bright scarlet colour. We may therefore distinguish
between red, or aérated, and purple, or on-aérated blood.

Lymph also coagulates on standing, producing a colourless
clot. It is practically blood mus its peculiar red colouring
matter, the properties of which, as well as the real nature of
coagulation, will be discussed in the next chapter.

The Heart: external characters.—Some of the divisions
of the heart have already been noticed (p. 20). The ventricle
(Figs. 3, 4, 7, 20, 21 and 22, v), is a conical body of a pinkish
colour, having its bluntly-pointed apex directed backwards.
To its broad base is attached the dark-coloured, thin-walled
auricular division, actually consisting of two chambers, the
right and left auricles (r. au, 7. au), but appearing single in
the entire heart. Passing obliquely across the auricles is a
cylindrical structure, the conus arteriosus (c. art); it starts
from the right side of the base of the ventricle, and passes
forwards and to the left, finally dividing near the anterior
boundary of the auricles into two branches, which extend
respectively right and left.

By lifting up the ventricle, or turning it to one’side (Figs.
3 and 4), there is seen in the dorsal part of the pericardial
cavity a thin-walled chamber (s. v) of a dark colour, con-

80 THE FROG CHAP. VI

nected with the right side of the auricular division. This is
the sinus venosus.

The Arteries.—The two branches of the conus arteriosus
just referred to soon branch again. Each divides into three
vessels, often spoken of as artertal arches, called respectively
the carotid trunk (Fig. 20, car. tr.) the systemic trunk (syst. tr)
and the pulmo-cutaneous trunk (pul. cu. tr). All these con-
form to the definition of an artery given on p. 27, ie,
they are stout, elastic vessels, containing little blood after
death, and not collapsing when empty.

The carotid trunk divides immediately into two, a Angual
artery (2g), which can be traced to the tongue, and a
carotid artery (car), which branches repeatedly, its ultimate
ramifications going to various parts of the head. At the
origin of the carotid is a little rounded mass with a sponge-
like interior, the carotid plecus (car. gl).

The systemic or aortic trunk extends outwards, in contact
with the gullet, then sweeps upwards, backwards, and inwards
—i.e., towards the middle line—and finally joins with its
fellow of the opposite side to form a single median vessel,
the dorsal aorta (Figs. 4, 5, and 20, @. ao), which passes
backwards, just beneath the vertebra] column and between
the kidneys.

As it sweeps round the gullet, the systemic trunk gives off
a vertebral artery (Fig. 20, vert) to the vertebral column and
part of the head, a subclavian artery to the fore-limb, and
an esophageal artery to the gullet.

From the point of union of the two aortic trunks springs
a single splanchnic or celiaco-mesenteric artery (cel. mes); it
divides into several branches, which are traceable to the
liver (4p) stomach (gs), duodenum (dw), spleen (sp/) and
ileum (772).

The dorsal aorta gives off on each side four vena/ arteries

|
tg |

car |

‘eartr

Fic. 20.—The arterial system of the frog, with the heart, lungs, kidneys and left
testis, supposed to be removed from the body and viewed from the ventral
aspect.

car carotid artery; car.g/. carotid plexus; c.a7v¢. conus arteriosus; car. tr.
carotid trunk ; ceé. mes. splanchnic or coeliaco mesenteric artery ; cz. cutaneous
artery ; @. ao, dorsal aorta ; dz. duodenal artery ; gs. gastric artery ; 4. hepatic
artery ; 72. iliac artery; zw. intestinal arteries; 4d. kidney; 2. az. left auricle ;
dg. lingual artery ; Zug. lung ; pz/. pulmonary artery ; Az. cz. tv. pulmo-cutaneous
trunk ; #7. az. right auricle ; 77. renal arteries; sf2. splenic artery; syst. tr.
systemic trunk ; sfvz. spermatic artery ; ¢s. testis; v. ventricle ; vevt. vertebral
artery, just behind which are seen the subclavian and cesophageal arteries. (From
Parker and Haswell’s Zoology.) :

Pract. Zoou. G

82 THE FROG CHAP. VI

(vn) to the kidneys, and spermatic arteries (spm) in the male, or
ovarian in the female, to the reproductive organs, and at the
posterior end of the abdominal cavity divides into the
right and left z/ac arteries (2) which go to the hind limbs,

The pulmo-cutaneous trunk divides into two main
branches, the pulmonary artery (pul) which goes to the
lung, and the cutaneous artery (cu) which forms an extensive
system of branches over the skin.

With proper care all these arteries can be traced into
the various organs to which they are distributed, when they
will be found to branch repeatedly, sending ramifications to
all parts. The iliac artery, for instance, may be followed
along the whole length of the leg, giving off branches to
all the muscles, to the skin, and to the digits, with their
intervening web.

The Veins.—Since every part of the body has its vein
as well as its artery, there is a rough correspondence
between the two kinds of blood-vessel. The arrange-
ment of the principal trunks is, however, very different in
the two cases.

On each side of the base of the heart is a large vein
called the precaval or vena cava anterior (Fig. 21, pr. cv):
by turning the ventricle aside, the two precaval veins can
be seen to join the anterior end of the sinus venosus (s.v).
Each precaval is formed by the confluence of several
veins, of which the most important are the external jugu-
Jar (ext, ju) from the lower jaw and tongue, the znternal jugu-
lar (int. ju) from the brain, eye, etc., the subclavian (br),
from the fore-limb, and the mzsculo-cutaneous (mu. cit), already
noticed, from the superficial parts of the head and part of
the skin and muscles of the abdomen.

With the posterior end of the sinus venosus is connected
a single large vein, the fostcaval or vena cava posterior

Fic. 21.—The venous system of the frog, with the heart, lungs, liver, kidneys and
right testis, supposed to be removed from the body and viewed from the dorsal
aspect.

abd, abdominal vein ; 57. subclavian vein ; cd. cardiac vein ; ds. 226. dorso lumbar
vein ; dz. duodenal vein; ext. yu. external jugular vein ; i. femoral vein; gs.
gastric vein ; 4A. hepatic vein ; 4. t. hepatic portal vein ; zzz. intestinal veins ;
int. ju. internal jugular veirf; 4d. kidney ; 72. aw. left auricle; Zag. lung ; lw.
liver ; 7s. cz. musculo-cutaneous vein ; 47. cv. precaval vein; Az. cv. postcaval
vein; fu. pulmonary vein; fv. pelvic vein; 7. az. right auricle; 7. renal
veins; vz. #7. renal portal vein; sc. sciatic vein; sfé. splenic vein; spzz.
spermatic vein; s.v. sinus venosus ; ¢s, testis ; ves, vesical veins (from bladder).
(From Parker and Haswell’s Zoology.)

G2

84 THE FROG CHAP.

(pz. cv), a wide vessel lying between the kidneys and
extending forwards to the liver (Fig. 4). It runs parallel with
and beneath, 7.e., ventral to, the dorsal aorta (Fig. 5), from
which it is at once distinguished by its greater diameter.
Posteriorily it is formed by the confluence of four renal
veins (Fig. 21, xz) from each kidney, and it also receives
in the male, spermatic veins (spm) from the spermaries, and
in the female, ovarian veins from the ovaries. Anteriorly
it perforates the liver (/vr), receiving two hepatic veins (hp)
from that organ, and finally enters the sinus venosus.

We have now to consider a striking want of correspondence
between the arterial and venous systems of the frog. As
you will remember, the dorsal aorta, after giving off the
renal arteries, passes backwards and divides into the two
iliac arteries for the legs. You might naturally expect a
somewhat similar arrangement with the veins, especially if
you have studied human physiology and learnt how the
posterior (or inferior) vena cava of man is formed by the
confluence of the veiris from the legs, and receives higher
up those from the kidneys. In the frog, as we have just
seen, the postcaval does not reach to the hinder boundary
of the kidneys, and the renal veins are the only vessels
entering its posterior end.

In the frog,as a matter of fact, the connections of the
veins of the legs are very peculiar. You remember the
abdominal vein seen in our preliminary dissection (Figs. 2,
3 and 4, abd.v). This vessel, if traced backwards, is found
to fork at the posterior end of the abdomen—or more accu-
rately, the single abdominal vein is found to be formed by
the confluence of two fe/vic veins (Fig. 21, £v), which can be
traced along the base of the leg (compare Fig. 3). Towards the
front of the thigh is the principal vein of the leg, the femoral
vein (Fig. 21, fm), which on emerging from the leg, divides

vI VEINS 85

into two branches. One of these is the pelvic vein already
seen ; it unites with the corresponding vessel of the opposite
side to form the abdominal vein (add). The other branch of
the femoral is called the renal portal vein (rn.pt) ; it passes
directly forwards, receiving the sctatic vein (sc) from the back
of the leg, and then goes along the outer border of the
kidney, finally branching out in that organ. It also receives
a large vein (ds. 7mé) from the muscles of the back.

The abdominal vein receives, near its posterior end,
small veins (ves) from the urinary bladder. It passes
forwards, as already seen, receiving veins from the abdo-
minal muscles, and, quite at its anterior end, a small vein
(cd) from the heart. It then divides into two branches
which enter the liver and branch out in that gland.

The veins from the stomach (gs), the intestine (du, in?),
spleen (sf/), and pancreas run in the mesentery alongside
the corresponding arteries. Near thé liver they all unite to
form a large vessel, the hepatic portal vein (hp. pt), which
enters and branches out into the ‘liver, first sending off a
branch which joins the abdominal.

Thus the veins from ‘all parts of the body, except the
lungs, ultimately discharge into the sinus venosus. The
veins from the lungs are quite singular in their course ; they
unite, in each lung, to form a single pulmonary vein ( put),
which passes behind, or dorsal to, the sinus venosus, and
discharges, with its fellow of the opposite side, into the left
auricle.

Character of the Blood in the Arteries and Veins.—There
are certain differences between the arteries and veins in
respect of the blood they contain. As a rule, the veins
contain purple or non-aérated blood, the arteries more or
less thoroughly aérated or scarlet blood. But there are
certain exceptions. As we shall see in a later chapter, the

86 THE FROG CHAP.

blood is aérated in the lungs and skin; hence the blood
returned from those organs by the pulmonary and musculo-
cutaneous veins is aérated. On the other hand, the blood
in the pulmo-cutaneous artery is non-aérated.

Flow of Blood.—We must now try to understand the
function of this complicated blood-system and the reason
why every part of the body has two vessels, an artery and a
vein. That there is some kind of movement of the blood
has been hinted in the foregoing description, in which
arteries have been described as branching out to various
parts, veins as formed by the confluence of smaller veins
from various"parts.

Were an artery to be cut in a living frog, the blood would

be found to flow out in a series of jerks corresponding with .
the beats of the heart. Moreover the blood would flow

from the side of the cut nearest to the heart, and the flow

might be stopped by tying or compressing the artery on that

side, z.e., between the heart and the cut. Evidently, then,

the blood in the living animal flows from the heart along

the arteries to the various parts of the body, and is propelled

by the pulsation of the heart.

If a vein were cut the result would be very different.
The blood would flow in a comparatively slow stream and
without jerks ; it would flow, moreover, from the side of the
cut furthest from the heart, so that, in order to stop the
bleeding, the vein must be.tied or compressed on the far
side of the cut. The blood in the veins flows, therefore,
towards the heart in an even stream, unaffected by the
heart’s pulsations.

Thus the blood is driven by the heart to the various
parts of the body through the arteries, and is returned from
the various parts of the body to the heart by the veins. Two
questions thus naturally arise: how is it that the blood

vI STRUCTURE OF HEART 87

takes this direction and not the other, and how does it make

its way from the artery of a given organ into the vein?
Internal Structure of the Heart.—To answer the first

question—why the blood leaves the heart by the arteries

Fic. 22. —The heart of the frog from the ventral aspect, with the cavities laid open.
a, a,’ bristle passed into left carotid trunk ; au, v. v, auriculo-ventricular valves ;
4. 6." bristle in left systemic trunk; ¢, ¢, “bristle in left pulmo-cutaneous trunk ;
car, a, carotid artery; car. gi. carotid plexus ; c. avt, conus arteriosus ; cav. tr.
carotid trunk ; Zaz. left auricle; Zg. a. lingual artery ; 2.u. longitudinal valve ;
pul. cu. tr, pulmo-cttaneous trunk ; pul. v. aperture of pulmonary veins ; 7. az.
right auricle ; s. az. ap. sinu- -auricular aperture ; spf. aur. septum auricularum

W, Ve ‘valves 3) zt, ventricle, (From Parker and Haswell’s Zoology.)

and returns to it by the veins, and not vce versé—we must
examine the heart itself in some detail.

The ventricle is a hollow structure with thick spongy walls
and a small cavity (Fig. 22, v¢), and there are two perfectly

88 THE FROG CHAP,

distinct auricles, the right (7. az), considerably larger than the
left (2 az), separated from one another by a vertical partition
(spt. aur).

You have already seen that the conus arises from the
right side—the frog’s right, not yours—of the base of the
ventricle. A little to the left-of this point there is an
aperture through which a bristle can be passed from the
ventricle into either of the auricles. Both auricles, then,
communicate with the ventricle by a single auriculo-
ventricular aperture. This is guarded by two little mem-
branous flaps (az. v. v), which spring, one from the dorsal,
one from the ventral edge of the aperture, and hang down
into the ventricle, to the walls of which they are attached
by little tendinous cords, represented in the figure by white
streaks. Thus the flaps have the character of folding doors
or valves opening only one way ; they readily flap backwards,
Z.¢., into the ventricle, but are prevented from flapping
forwards or into the auricles by the tendinous. cords
attached to their backs. The two flaps are the auriculo-
ventricular valves. Their mode of action is easily understood.
If the auricles, being full of blood, contract and squeeze
themselves together, the pressure will force aside the valves
and allow the blood a free passage into the ventricle. On
the other hand, if the ventricle contracts, the blood, getting
behind the valves, will force them together and close the
aperture, the tendinous cords preventing their being driven
into the auricles by undue pressure.

In the interior of the conus is a longitudinal fold or valve
(2.v), which traverses it obliquely, attached to its dorsal wall
and free ventrally ; and there are also three little semilunar
or watch-pocket shaped valves (v) guarding the aperture
between the ventricle and the conus : they are arranged with
their edges turned forwards or towards the cavity of the

VI CIRCULATION 89

conus, so that any pressure of fluid from the side of the
ventricle must force them back and allow ready ingress into
the conus, while pressure in the opposite direction must fill
them, bringing their edges together, and so barring the
passage.

Anteriorly the longitudinal valve projects in the form of
a free flap, and at the same level is a semilunar valve (2):
the two together separate the conus proper from a small
chamber, the d/bus aorte, from which the right and left
carotid (a, a’) and systemic (4, 2’) trunks arise. The pulmo-
cutaneous trunks (c) spring from the conus by an aperture
(c') situated just posteriorly to the valve o', and_ itself
guarded by a small valve.

In the dorsal wall of the right auricle is a large transverse
aperture (s,a@u.ap). This leads into the sinus venosus : it is
therefore called the sizu-auricular aperture; its two edges
are produced into flaps, the sézx-auricular valves, which
allow free passage from the sinus to the right auricle, but
prevent any flow in the opposite direction.

Valves of the Veins.—In addition to the valves of the
heart, many of the veins contain small watch-pocket valves,
all arranged with their concavities directed towards the
heart, so as to allow of a free passage in that direction. Any
attempt to flow in the opposite direction, z.e., from the larger
to the smaller veins, will result in filling the valves, bringing
their edges into contact with the opposite wall of the vein,
and thus effectually blocking the passage.

Circulation of the Blood.—We see then that an inves-
tigation of the structure of the heart shows that fluid can
traverse it in one direction only, viz., from the sinus to the
right auricles, from the auricle to the ventricle, from the
ventricle to the conus, and from the conus to the bulbus
aorte, and so to the arteries. The valves in the veins are so

(eye) THE FROG CHAP. VI

arranged as to allow the blood in these vessels to flow only
towards the heart. The experiment of cutting the vessels
shows that the blood in the arteries does actually flow from
the heart, that in the veins towards the heart. We thus
demonstrate that there is not merely a movement but a true
circulation of the blood, the current starting from the heart,
passing by the arteries to all parts of the body, and being
returned to the heart by the veins.

Action of the Heart.—The circulation of the blood is
effected by the pulsation of the heart. This organ is
made of muscle ; each of its cavities is to be considered
as a bag, the walls of which are formed of muscular fibres
crossing one another in various directions and encircling
the cavity. We have seen that when an ordinary spindle-
shaped muscle contracts, its two ends are brought nearer
together. When a hollow muscular bag contracts the effect
will be to squeeze the walls together and so diminish the
cavity. Hence when any chamber of the heart contracts
it must expel a part or the whole of its contained blood.
The contraction of the chambers of the heart takes place in
regular order: first the sinus, then the two auricles together,
then the ventricle, and lastly the conus. The contraction
in each case is visible as a sort of throb and is followed
by a period of rest, during which the chamber regains its
former dimensions.

The course of the blood through the heart will now be
clear. When the sinus (Fig. 23, s.v) contracts, the con-
tained blood, which, coming by the precavals and postcaval,
is non-aérated, is acted upon in all directions and might
therefore be forced either into the three great veins ( f7.cv.0,
ptcv.v) or into the right auricle (7.aw). But the veins are
full of blood steadily flowing towards the heart, and any
regurgitation is further prevented by their valves: the right

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92 THE FROG CHAP.

or testis; CZ. Hd. of head; Cp. H. L. of hind-limb; Cp. Ka. of kidney;
Cp. Lng. of lung; Cp. Lor. of liver ; Cf. Px. of pancreas; Cp. Sk. of skin;
Cp. Spl. of spleen; cz. a. cutaneous artery; cz. v. cutaneous vein; Cx. Gl.
cutaneous gland; d. ao, dorsal aorta; Ext. C. enteric canal; £f, Ent.
epithelium of enteric canal; Ep, Lng. of lung; Zf. Sé. of skin ; "ED. Ur. T.
of urinary tubule; g/m. glomerulus ; tl.a. iliac artery; ind. a. artery to
stomach and intestine; z#¢.v. vein from stomach and _ intestine ; Jt V.
jugular vein ; 4A. a. hepatic artery (to liver) ; 44, pt. v. hepatic portal vein ; Ag. v.
hepatic vein; 2, au, left auricle; Lg. lung; Lur.C. liver cells; ly. cp. “lymph
capillaries ; dy. v. lymphatic vessels ; Mlp. Cp. Malpighian capsule ; #s¢, nephros-
tome; 7. ty. At, posterior lymph- -heart 5 3 Px.C. cells of pancreas; Pa. D. pan-
creatic duct; #7. cv.v. precaval vein; Af.cv.v. post caval vein; ful. a. pul-
monary artery ; pul. cu. tr. pulmo-cutaneous trunk; pzd.v. pulmonary vein;
yan, right auricle; 7#.f¢.v. renal portal vein; sc/.a, subclavian artery ;
s.¢. ly. $. sub-cutaneous lymph-sinus ; scd.v, subclavian vein ; sf. @. splanchnic
artery; sé. v. splenic artery ; s/. v. splenic vein; s.v. sinus venosus ; syst. tr.
systemic trunk; U. Bé. urinary bladder; U7. ureter ; v. valve in vein; vs. a.
vesical artery (to bladder); vs. v. vesical vein; v7. ventricle,

auricle, on the other hand, has finished its contraction and is
now relaxing ; it is therefore empty. Thus, on the principle
of least resistance, the contraction of the sinus fills the right
auricle with blood from the great veins, and the sinus itself
is refilled from the same source as soon as it begins to relax.

Immediately after the sinus has ceased to contract the
two auricles contract together: the right, as we have seen,
has just been filled from the sinus, the left (2 az) is full of
aérated blood brought to it by the pulmonary vein ( pul. v).
The presence of the sinu-auricular valves prevents the blood
in the right auricle from being forced back into the sinus :
that in the left auricle is prevented from being forced back
into the pulmonary veins by the steady onward flow in the
latter. On the other hand the ventricle is beginning to
relax and is empty. Consequently the auriculo-ventricular
valves are forced back into the ventricle (v/) and the blood
from both auricles flows into and fills that chamber, the
right half of which becomes filled with non-aérated, the left
with aérated blood, the two taking an appreciable time to
mingle.

The instant it is thus filled the contraction of the ventricle
‘begins. As it does so the blood, getting behind the auriculo-

vI CIRCULATION 93

ventricular valves, forces them together, and thus prevents any
backward flow into the auricle. At the same time the
semilunar valves at the entrance of the conus (¢. art) are
pushed aside and the blood flows into that chamber. Since
the conus opens from the right side of the ventricle, the blood
first entering it will be non-aérated ; there will then follow a
certain amount of mixed blood ; and finally, as the ventricle
reaches the limit of its contraction, the aérated blood
from its left side will be forced into the conus. (Compare
Fig. 22.)

Last of all the chambers: of the heart, the conus begins
its contraction. The semilunar valves are immediately
filled with blood, and, closing together, stop all backward
flow into the ventricle. Two alternative courses are now
open to the blood: it can pass either directly from the
conus into the pulmo-cutaneous trunk ( pw/. cw. tr), or make
its way into the bulbus aortz (4. av). As a matter of fact it
takes the former course, owing to the circumstance that
there is little resistance in the limited blood-system of the
lungs, while that in the systemic and carotid trunks is very
great. Hence the blood just received into the conus from
the ventricle, which, as we have seen, is non-aérated, goes
immediately to the lungs and skin to be aérated.

Before long—in a fraction of a second—the flow of
blood into them increases the pressure in the pulmonary
vessels, and at the same time the blood is continually flow-
ing onwards—z.e., away from the heart—in the systemic and
carotid trunks. Consequently the pressure in these vessels
rapidly diminishes, and the blood soon forces aside the
valves between the conus and the bulbus and fills the latter.
Here again the question of pressure comes in. It is easier
for the blood to make its way into the wide systemic trunks
(syst. tr) uniting immediately into the long dorsal aorta

94 THE FROG CHAP,

(d.ao) than into the comparatively narrow carotid trunks
(car. tr), obstructed by the carotid plexuses. Hence, the
non-aérated blood having been mostly driven into the
pulmo-cutaneous trunk, the mixed blood, from the middle
part of the ventricle, goes into the systemic trunk, and
thence to the various arteries supplying the limbs (scd. a, 72. a)
and the viscera (sf. a, etc.). Finally, when the pressure is
sufficiently raised in the systemic trunks the remaining
blood, which, coming from the left side of the heart, is
aérated, is pumped into the carotid trunks (car. ¢r) and
thence to the head.

Thus, owing to the arrangement of the valves, and to the
varying pressures in different parts of the vascular system,
the non-aérated blood returned from the various parts of
the body to the heart is mostly sent to the lungs and skin to
be aérated. Mixed blood is sent to the trunk, limbs, and
viscera, while for the head with its contained brain—the
directing and controlling organ of the whole animal—a
special supply of pure, aérated blood is reserved.

We see then that the course of the circulation may be
proved, as a simple matter of induction, from the structure
of the heart and its valves, the direct observation of its
beat, and the manner in which the flow from cut vessels
takes place. It was by observation and experiments of this
kind that thec irculation of the blood in the higher animals
was demonstrated by William Harvey in the seventeenth
century. But the final and most conclusive proof of the
circulation—from directly observing the flow—became
possible only after the invention of the microscope. This
instrument, by furnishing a sufficiently high magnifying
power, allows us to see for ourselvest he actual movement
of the blood in an animal or organ of sufficient transparency ;
and, at the same time, clears up the question, previously

VI CAPILLARIES 95

insoluble, of how the blood, having reached a given part or
organ by the arteries, finds its way into the veins to begin
its return journey.

The Circulation in the Frog’s Web.—There are three
parts in the frog transparent enough to allow of the blood-
flow being seen in them—the web of the foot, the tongue,
and the mesentery. Of these the web is the most con-
venient, and can be examined under the microscope without
any injury to the animal.

The Capillaries.—If you have the makings of a naturalist,
you will acknowledge the sight to be one of the most won-
derful you ever:saw. In the thickness of the web is an
irregular network of minute blood-vessels, called capillaries
(Fig. 24), and through them the blood is seen to flow with
great rapidity, its course being made especially evident by
the minute particles or corpuscles it contains, the structure
of which we shall study later on. You will also notice much
larger vessels, the smallest arteries and veins. The arteries
(a2) are distinguished by the fact that the blood in them
flows in the direction from the leg towards the margin of the
web, while in the veins (v) it takes the opposite direction.
You must remember, however, that under the microscope
everything is reversed ; right appears left and left right, and
a current actually flowing towards the observer appears to
go in the opposite direction.

By careful examination you will see that both arteries and
veins aré in connection, by minute branches, with the
capillary network, and will be able to trace the blood from
an artery, through the capillaries, into a vein.

The same thing can be seen in other transparent organs ;
and by injecting the vascular system with a fluid injection-
mass, such as gelatine suitably coloured, it can be proved
that all parts of the body are permeated with a capillary

96 THE FROG CHAP.

network into which the blood is passed by the arteries, and
from which it is received into the veins.

Fic. 24.—Blood-vessels of the web of a frog’s foot seen under a low magnifying power.

a. small arteries ; v. small veins. The minute tubes joining the arteries to the

veins are the capillaries. The arrows show the direction of the circulation.

In the small portion marked off, the pigment cells, which occur throughout
the web, are also represented. (From Huxley's Physiology.)

Thus by means of the microscope we are able to take the
final step in demonstrating the circulation. The fact that

VI LYMPHATIC SYSTEM 97

the blood can flow in one direction only is proved by the
disposition of the valves of the heart and of the veins, but
the passage of the blood from the smallest arteries to the
smallest vein by a connecting system of minute tubes or
capillaries can be proved only by the employment of con-
siderable magnifying powers. We see that the vascular system
of the frog is a closed system of vessels : the blood is every-
where confined within definite tubes through which it flows
in a definite direction, never escaping, as in some of the
lower animals, into large irregular spaces among the tissues.

The Lymphatic System. — Included in the vascular-
system are certain cavities and vessels containing lymph,
and together constituting the Lymphatic system, We have
already noticed the subcutaneous lymph sinuses (p. 18, Fig.
23, s.c. dy. s) and the sub-vertebral lymph sinus (p. 27, Fig. 5,
s.v. ly. s). There are also found in nearly all parts of the body,
delicate, thin-walled, branching tubes, the Zymphatic vessels
(Fig. 23, 4y. v). Unlike the blood-vessels, the lymphatics
are all of one kind, there being no distinction into anything
of the nature of arteries and veins. They arise in Zymph-
capillaries (ly. cp), which are, as it were, interwoven with the
blood-capillaries, but have no connection with them. By
the lymph-capillaries the fluid which has exuded from the
blood: in its passage through the tissues is taken up and
passed into the lymphatic vessels or sinuses, and these in their
turn finally communicate with certain transparent muscular
organs called fymph-hearts. Of these there are two pairs. The
anterior lymph-hearts (a. Zy. A) lie, one on either side,
beneath the scapula and just behind the transverse process of
the third vertebra: the posterior pair (f. Zy. Af) are situated
one on each sidé of the posterior end of the urostyle.
These organs pulsate regularly, like miniature hearts, and
pump the lymph into the veins, the anterior pair communi-

PRACT. ZOOL, H

98 THE FROG CHAP,

cating with the subclavian, the posterior with-the renal portal
vein,

The lymphatics of the enteric canal have an important function to
perform in that they absorb the fatty portions of the food (p. 75). The
fluid they contain has a milky appearance, owing to the presence of
minute suspended fat-globules, and for this reason they receive the name
of lactéeals.

The celome (Fig. 23, ce?) is really a great lymph-sinus.
It communicates with the veins of the kidneys through
certain microscopic apertures called xephrostomes (ust).

The spleen (p. 23, and Fig. 3, 567) has important relations
with the blood- and lymph-vessels, and probably acts as a
blood-filter, removing particles in the blood which are no
longer wanted.

PRACTICAL DIRECTIONS.

The Vascular System.

a, Let some blood from a frog—or better from the veins of some larger,
freshly-killed, warm-blooded animal, such as a rat or a rabbit—flow
directly into a white cup or porcelain capsule. Note that it soon
coagulates, and soon afterwards separates into c/ot and serum. Notice
also the difference in colour between the blood freshly drawn from a
vein, and that soon assumed by exposed portions of the clot.

4. Pin a freshly-killed frog to the dissecting board, dorsal side
upwards, and cut through the skin of the back along the middle line.
The posterior lymph-hearts (p. 97, Fig. 23) will then be seen. To make
out the azterdor lymph-hearts, carefully separate the supra-scapule from
the vertebral column. Some of the chief Zymph-sdnuses have already
been seen ; special methods are required to trace the lymph-vessels.

c. Now turn the frog the other way upwards, pin it down in the dissect-
ing dish, and open-the body-cavity as before (p. 32), taking great care
not to cut the abdominal, musculo-cutaneous, and other veins, Slit open
the pericardium and remove as much of it as possible, so as to
expose the entire heart. The structure of the heart and the course of
many of the blood-vessels can also be made out in the specimen from

VIO. INJECTION OF BLOOD-VESSELS ‘99

which you have already removed the alimentary canal. In the following
dissections, use a dissecting lens whenever necessary.

I. In the heart (Figs. 3, 4, 7, 20 and 21), notice again the ventricle,
and the vzghé and deft auricles (appearing single in the entire heart), and
make out also the conus arteriosus, dividing into two distally, and the
sinus venosus (dorsal).

If the heart is still beating, notice the order of contraction of its
different divisions (p. 90).

Injection of the Arteries.—The tracing of the arteries is greatly
facilitated by filling them with some coloured substance. The operation
requires, therefore, a coloured fluid or dz/ection-mass capable ot
traversing the arteries, and some contrivance by which it can be injected
into them.

The most convenient injection-mass is made as follows :—

1. Grind up in a mortar 4 grammes of ‘‘ French blue” (to be had at
the oilman’s), with 4 cubic centimetres of glycerine and the same
quantity of methylated spirit.

2. Grind up 50 grammes of common laundry starch, with 50 cubic
centimetres of water and 25 of methylated spirit, and add to the
mixture the colour as prepared in (1), Mix thoroughly and strain
through muslin.

This injection-mass will keep for an indefinite period in a stoppered
bottle, requiring only to be stirred up when used. If it is considered
too troublesome to make, a simpler but less satisfactory mass may be
made by simply stirring up some French blue in water in the proportion
of a teaspoonful to a tumbler.

For injecting the mass into the blood-vessels, the most satisfactory
instrument is a brass injecting syringe, holding about one ounce,
provided with nozzles of various sizes. This is, however, expensive,
and an ordinary glass syringe, to be had of any druggist, will answer the
purpose very fairly if provided with a proper nozzle or cannula. This
latter is made by drawing out one end of a piece of glass tubing about
two inches long until it is fine enough to pass into the conus arteriosus :
a piece of india-rubber tubing is then used to connect it with the fixed
nozzle of the syringe. A still simpler injecting apparatus is furnished
by a common ‘‘ medicine dropper” (see p. 12, and Fig. 25). By alterna-
tive squeezing and releasing the cap, fluid is drawn into or expelled
from the tube.

Having provided these requisites proceed as follows. Open the

H 2

100 THE FROG CHAP,

abdomen of a freshly-killed frog in the usual way, taking great care not
to injure the blood-vessels.. Remove the middle portion of the
shoulder-girdle so as to expose the heart,
lay open the pericardium, and with a
single snip of the scissors cut off the pos-
terior half of the ventricle, allowing the
blood to escape freely. Pass a piece of
thread (not cotton), about six inches long,
round the heart, at about the junction of
the auricles and ventricle, and give it a
single loose tie, as shown in Fig. 25,
When the bleeding has ceased, fill the
medicine dropper, or syringe. with injec-
a tion-mass and pass the narrow end of the
F former, or the nozzle of the latter, through
' the cut end of the ventricle into the conus
—take care not to push it into one of the
auricles instead—and tighten the thread so
as to keep it in place. Then squeeze the
cap of the medicine dropper, or push in the
piston of the syringe, and if the operation is
successful, you will see the blue injection
pass from the conus into the arterial trunks,
and thence into the various arteries of the
body. The contrast between the arteries,
filled with the blue mass, and the veins,
BG et ne ie filled with blood, is then very striking,
: foes po ee we particularly in the mesentery. When the
eo ‘ndiaceabher aay arteries are well-filled, withdraw the nozzle
its pointed end (dotted) is from the heart and instantly draw the thread
passed through the cutend ;, A
of the ventricle (v) into tight and knot it so as to prevent escape of
ee . ine ne the injection. Then place the whole frog in
thread. spirit (methylated spirit 3 parts, water I
part), for a few hours, after which time the
injection mass will be found to have set hard enough to allow of the
arteries being conveniently traced.
Injection of the Veins.—The veins are much more difficult to
inject than the arteries, but if you wish to make a double injection on
the same specimen, colour the injection-mass with vermilion or carmine

vi PRACTICAL DIRECTIONS IOI

in the case of the arteries instead of with French blue, using the latter
for the veins. The operation is best performed by inserting the nozzle
mto an incision in the abdominal vein: by directing the nozzle for-
wards, the portal vein will be injected: by directing it backwards
the pelvic and renal portal veins. But for a really satisfactory prepara-
tion, it is best to inject from the heart through the conus, as directed
above, with a weak, warm solution of gelatine (in the proportion
of one part of gelatine to two parts of water), coloured with pre-
cipitaled carmine. In this case the injection mass, containing only
microscopic particles, passes from the arteries through the capillaries
into the veins, keeping throughout to the course taken by the blood
during life, and therefore unimpeded by the valves of the veins. A
syringe must be used, since the medicine-dropper will not give sufficient
pressure, and the animal should be placed in warm water during the
process.

If. Now make out the course of the chief vecns (p. 82, Fig. 21): (if
not injected, put no water into the dissecting-dish at present) :—

1. The two frecavals, and the external jugular, internal jugular,
sub-clavian, and musculo-cutaneous.

2. The Jostcaval, to see which turn the viscera on one side (Figs. 3
and 4). Note the rexa/, spermatic or ovarian, and hepatic veins.

3. The hepatic portal vein and its factors.

4. The abdominal vein and pelvic veins.

5. The veins from the hind legs can be more easily seen at a later
stage, after the alimentary canal is removed, and so their examination is
best left until certain of the arteries have been traced (or use the
specimen you have dissected previously for this purpose). Remove the
skin from the thigh, place the frog on its side, and make out the
JSemoral, pelvic (already seen), renal portal, and sciatic vedn, as well as
a large vein from the muscles of the back.

6. The two pulmonary veins.

Make a sketch of the heart and as many of the veins as you have
followed out up to this point, inSerting the others after removing the
alimentary canal (see p. 102).

III. The chief arteries may now be followed out (p. 80, Fig. 20) :—

Note the carotid, the systemic, and the pulmo-cutaneous trunk, arising
from the conus arteriosus, and then trace each of these out as follows :—
_ 1. The carotid trunk gives off a lingual artery, and is continued into
the head as the carotid artery, having at its origin the carotid plexus.

102 THE FROG CHAP,

2. The systemze trunk unites with its fellow to form the dorsa: aorta,
first giving off vertebral, subclavian, and esophageal artertes. From the
point of union of the two systemic trunks arises the splanchnic or
caliaco-mesentertc artery. After following this out Lo its distribution,
remove the alimentary canal as directed on p. 76, when the following
branches of the dorsal aorta will be more plainly seen:—the renal,
Spermaitc or ovaitan, and dzac artertes.

3. The peelmo-cutancous trunk divides into a pulmonary artery, passing
along the outer side of the corresponding lung, and a ceédéaneous artery.

Sketch the heart and chief arteries, and then make out and sketch
the renal portal system (p. 101), if you have not already done so,

IV.1 Cut out the heart of a frog preserved in formaline, taking great
care not to injure it. Fasten it out in a dissecting-dish with the ventral
surface upwards, by sticking very small pins through the arteries and
veins—not through the heart itself. Pinch up the ventricle with fine
forceps, and with small scissors gradually snip away its ventral wall,
noting that it is a hollow structure with thick, spongy walls and a small
cavity, which will probably be full of clotted blood. Wash this out,
and then procced to open the auricles in a similar way, and to wash out
the blood they contain. Observe the right and /eft auricles, separated
by a partition, Slit open the conus arteriosus, and continue the cut
forwards Lo the origin of the main arteries. Examine with a lens and
make out (p. 87, Fig. 22) :—

1. The aurzculo-ventricular aperture and its valves.

2. The longitudinal valve and the three small semz/enar valves in the
conus arteriosus.

3. The orégins of the carotid and systemic trunks from the bulbus
aorte, and the small aperture leading into the pulno-cutancous trunks.

4. The séxe-auricular aperture and its valves.

5. The aperture in the left auricle leading into the pulmonary veins.

Sketch.

Turn over the heart, so that its dorsal surface is upwards, and cut away
enough of the dorsal wall of the sinus venosus to show the sinu-
auricular aperture from the other side.

1 On account of its small size, the examination of the structure of the
frog’s heart is somewhat difficult, and the student is advised to dissect

first the heart of a larger animal, such asa dogfish or a rabbit. (See
Part II.)

vI PRACTICAL DIRECTIONS 103

V. Get a piece of thin board—e.g., the side of a cigar box—about six
inches long by three wide. At the middle of one of the narrow sides
make a round hole about half an inch in diameter, and about half an
inch from each end of the same side make a notch or rather slit, with a
penknife. This is called a ‘‘ frog-board.”

Next get as light coloured a frog as possible. Chloroform it as directed
on p. 31, but remove it from the influence of the anzesthetic as soon as
it is insensible, when the brain may be destroyed by the operation of
pithing, so that there may be no suspicion of the frog feeling any
inconvenience from this harmless experiment. To pith the frog, feel
with the finger the joint between the skull and first vertebra on the
dorsal side, and with a sharp scalpel make a small cut through the
skin and underlying tissue, so as to expose the spinal cord in this
region; then rapidly insert a blunt instrument, such as a seeker
or a small piece of wood, into the cranial cavity, and move it
about until the brain is completely destroyed. Lay the frog on the
frog-board with a piece of wet rag wrapped loosely round the body, and
take one or two turns around both frog and board with a piece of tape—
you must avoid tying it tightly or the circulation will be impeded.
Stretch out one leg, and selecting the most transparent web, tie a piece
of thick soft silk round each of the two toes by which it is bounded.
Adjust the leg so that the web comes just over the hole in the frog-
board, and bring the two pieces of silk through the slits, regulating them
until the web is evenly stretched out over the hole. Lastly, place the
frog-board on the stage of the microscope,! with the hole over the
aperture in the stage, and either fix it with the clips or rest the opposite
end on some support: adjust the mirror so as to illuminate the web from
beneath, and examine it with the low power. Note the network of
capillaries and the cérculation of the blood through the arteries, capil-
laries and veins (Fig. 24).

1 A brief description of the compound microscope will be given at
the end of the next chapter.

CHAPTER VII

THE FROG (continued): THE MICROSCOPICAL EXAMINATION
OF THE SIMPLE TISSUES.

BEFORE carrying our enquiries any further into the
anatomy and physiology of the frog it will be necessary to
devote some consideration to its microscopic structure or
histology, since there are many matters in connection with
the various organs which can be further elucidated only by
the examination of the minute structure of the organs as
revealed by the microscope (see p. 119).

Let us, first of all, examine a drop of the frog’s blood
under the low power of the microscope. It will at once be
seen that the blood is not a simple homogeneous fluid, but
that it contains a large number of minute solid bodies
floating in it. These are called by the general name of
blood-corpuscles: the fluid part of the blood in which they
float is called the A/asma. At first, owing to currents in the
fluid, the corpuscles will be found to move to and fro, but
after a time they come to rest. Under the high power you
will notice that the corpuscles are of two kinds. The greater
number of them are regularly oval in form (Fig. 26, C), and
of a yellow colour. If the drop of blood is thick enough
in one part for the corpuscles to lie over one another, so

CHAP. VII BLOOD CORPUSCLES 105

that the light passes through two or three layers of them to
reach the eye, they will appear red : they are. hence called red
corpuscles, Frequently they are seen turned on edge (D),
and their appearance in this position shows them to be flat
oval discs with a swelling in the centre. They are about
oth of a millimetre (about ;,';5th inch) in long diameter.

Among the red corpuscles are found, in much smaller
numbers, bodies (Fig. 26, A) not more than half the long dia-
meter of the red corpuscles in size, quite colourless, distinctly
granular—so as to have the appearance of ground glass—

aA nu

D

Fic. 26.—Blood corpuscles of the frog, highly magnified.
A, colourless corpuscle; B, the same in process of division; C, red corpuscle,
surface view ; D, the same, edge view. xz. nucleus. (From Parker’s Biology.

and with a slightly irregular outline. These are the colour-
less corpuscles or leucocytes. They are not flat, like the red
corpuscles, but have the form of irregular lumps.

The plasma, like the leucocytes, is quite colourless, so that
the colour of the blood is seen to be due entirely to the
large number of red corpuscles it contains.

If the drop of blood has been prepared and examined
under the high power with sufficient rapidity, a remarkable
phenomenon can be made out with regard to the colourless
corpuscles. This can be most easily demonstrated by making
a series of outline sketches of the same leucocyte at intervals
of a minute or two. You will then notice that the sketches
all differ from one another: in one there will perhaps be a
little projection going off to the right; in the next this will
have disappeared and a similar projection will have appeared

106 THE FROG CHAP.

on the left, and so on. Asa matter of fact, as long as the
blood is quite fresh, the leucocytes are in constant movement,
sending out and withdrawing little processes of their substance
called pseudopods or “false feet,” by means of which they
can crawl slowly along like independent living things.
These very peculiar and characteristic movements are called
amaboid movements. Occasionally a leucocyte may be seen
to elongate itself and divide into two (Fig. 26, B): this is a
case of what is called scmple fission. ‘The red corpuscles
neither move nor divide.

If a drop of some dye or staining fluid be run in under
the cover-glass, the corpuscles will be seen to become rather
faint in outline, very transparent, and lightly tinted ; but the
most obvious effect is that in the middle of each is seen a
rounded or oval granular body (zz) deeply stained by the
dye, so as to make a very well-defined coloured area in the
interior of the corpuscle. This body is called the nucleus :
it_is present both in the red and the colourless corpuscles.

By adding to a fresh drop of blood, in the same manner,
a drop of weak acetic acid, the nucleus again becomes
distinct, while the body of the corpuscle is rendered very
transparent and almost invisible: indeed it finally disappears
altogether. It is thus proved that the corpuscles, both red
and colourless, consist of two substances, an internal
nucleus, stained by dyes, and unaffected by weak acids ;
and an external substance, called protoplasm, but slightly
affected by dyes, and soluble in weak acids. Both nucleus
and protoplasm consist mainly of proteids (p. 72), together
with water and a small proportion of mineral matters.

When distilled water is added to a drop of blood on the
slide, the corpuscles are seen to swell up and become partly
dissolved, and the red colouring matter of the red corpuscles
is dissolved out, the plasma becoming tinged with yellow.

VII CHARACTERS OF BLOOD 107

Thus the colouring matter is evidently a distinct substance
from the protoplasm, and is called haemoglobin. It is
characterised, among other things, by a strong attraction for
oxygen: in combination with that gas it assumes a bright
scarlet colour: when deprived of oxygen, it becomes
purple. This affinity for oxygen accounts for the change
undergone by the blood when exposed to the air, as
described on p. 79.

Coagulated blood, as seen under the microscope, is
characterised by the plasma being traversed by extremely
delicate threads, forming a sort of network in which the
corpuscles are entangled. These threads are formed of a
substance called /id7iz, which is separated from the plasma
during coagulation, the remaining or fluid portion of the
plasma constituting the serwzm. We may therefore express
the coagulation of the blood in a diagrammatic form as
follows :—

Fresh Blood. Coagulated Blood.
Serum

Plasma. .... .{ Hose a

Corpuscles aa ae os ee

Having observed the microscopic characters of a drop of
blood, let us examine once more the circulation in the web,
this time under the high power (Fig. 27). The red corpuscles
(/) can be seen streaming through the vessels, those in the
capillaries in single file, those in the arteries and veins two
or more abreast: as they pass through narrow capillaries or
round corners, they become bent or squeezed (G, #).
The leucocytes (Z) travel more slowly and often stick to the
sides of the vessels. ;

Columnar Epithelium.—By carefully teasing out a small
piece of the inner surface of the mucous membrane of the

108 THE FROG CHAP,

Fic. 27.—The circulation in the frog’s web, under a high power.

A, wall of capillaries ; 4, tissue of the web'in which the capillaries lie ; C, epiderm
cells; D, their nuclei; /, pigment cells; F, red corpuscles ; G, //, red corpuscles
being syueezed through a narrow capillary; AY, capillary seen through the
epiderm ; /, colourless corpuscles. (From Huxley's Physiology.)

vir EPITHELIUM 109

intestine into the smallest possible particles it will be
found that the process has detached numerous, minute,
conical bodies, about 34 mm. (1, in.) in length, polygonal

in transverse section, and having
one end flat and the other
pointed (Fig. 28). These
bodies are called epithelial cells :
in the natural position they lie
closely applied to one another,
like the blocks of a wood pave-
ment, their flattened ends facing
the cavity of the intestine, while
their pointed ends abut against

Fic, 28.—Columnar epithelial cells
from the frog's intestine.

m. droplet of mucus exuding
from cell ; 227. nucleus.

the submucosa (p. 70). Thus the epithelial cells together
form an epithelium or epithelial layer of the mucous mem-
brane directly bounding the cavity of the enteric canal.
Each cell consists of protoplasm and contains a rounded,
granular nucleus (7), which is made very conspicuous by
staining, and in which are one or more small bodies or xueleoli.

Fic. 29.—Ciliated
epithelial cells
from the mu-
cous membrane
of the frog's
mouth,

(From Parker's
Biology, after
Howes.)

Certain of these cells have a space towards
their free ends containing slime or mucus,
and thus have the form of little cups or gob-
lets: they are known as gob/et-cells (see right
hand cell in Fig. 28).

Ciliated Epithelium.—By the same method
the mucous membrane of the mouth is also
seen to be lined by an epithelium, but the
cells comprising it (Fig. 29) are shorter in
proportion to their length, and each is pro-
duced on its free surface into a number of

delicate, transparent threads of protoplasm called c/a, which,
in the living condition are in constant movement, lashing
backwards and forwards like minute whip-lashes, or, more

110 THE FROG CHAP.

accurately, like the blades of grass in a field when acted
upon by a strong wind. If you happen to get under the
microscope a good-sized bit of mucous membrane with the
cells in position, you will see that the cilia produce a strong
current by which small particles are swept along, while
detached cells swim about, like little independent animals,
by the action of their own cilia. These cé/ated epithelial
cells, like the columnar cells previously described, are made
of protoplasm and each contains a nucleus with one or two
nucleoli clearly brought into view by staining.

The action of the cilia can be demonstrated, on a large
scale, by placing a freshly-killed frog on its back, turning
back or cutting away the lower jaw, and placing a very small
cube of cork on the roof of the mouth near to the projection
due to the eyes. The cork will be slowly swept back
towards the throat.

Squamous or Pavement Epithelium.—By scraping the
outer surface of a piece of skin with a sharp knife, and
examining the scrapings in a
drop of water, after staining
them, the superficial layer of
the skin will be found to be
made up of flattened, roughly
hexagonal plates (Fig. 30 and
Fic. 30.—Squamous epithelial cells Bs: 20 re D) ah closely so

from the frog’s skin. 77. nuclei. — gether, like the tiles of a

mosaic pavement. Each plate
has a_nucleus, and, from its flattened form, is distinguished
as a sguamous or scale-like epithelial cell.

Meaning of the word “ Cell.”—We see thus that the body
of the frog is partly made up of -distinct elements, which,
under a considerable diversity of form, exhibit the same
essential structure, Each consists of a mass of living proto-

VII UNSTRIPED MUSCLE III

plasm, containing in its interior a rounded body, the
nucleus, specially distinguished by the affinity of parts of its
substance for colouring matters. To a body having this
essential structure, whatever its form, the name ce is
applied.

Unstriped Muscle-——Examination of a teased preparation
of the muscular coat of the intestine, stomach, or urinary
bladder will show that it is com-
posed of delicate fibres (Fig. 31)
tapering at both ends, and with
a nucleus in the middle. These
are called smooth or unstriped
muscular fibres: they are ob-
viously cells which have under-
gone a great elongation in length.

During the peristaltic move-
ments of the intestine (p. 75) each
fibre alternately contracts and
relaxes, becoming shorter and
thicker during the former pro-
cess, like the large muscles of
the body (p. 60). The move-
ments, in this case, however, are
not under the control of the will,
and unstriped muscular tissue is

therefore often spoken of as Fic. 31.—Unstriped muscular
fibres from the frog's intestine.

involuntary muscle. To the right are shown fibres from
ga the longitudinal and circular
Contractility of Protoplasm. layers (see Chap. VIII) cross-

‘ ing one another; to the left

—We have now studied three isolated fibres. (After Howes.)

different kinds of movement in

cells :—muscular movement in the unstriped muscle-fibres,
ciliary movement in the ciliated epithelial cells, and ameboid
movement in the colourless blood-corpuscles. Muscular

112 THE FROG CHAP.

movement is due to the fibre undergoing a sudden shorten-
ing in a particular direction and a consequent approxima-
tion of its two ends. Ciliary movement is due to the
alternate bending and straightening of the cilia; and the
bending of a cilium in a particular direction is caused by
the protoplasm of which it is composed shortening or
contracting on the side towards which it bends. Amceboid
movement is the protrusion and withdrawal of irregular
processes of the cell: this results from the protoplasm
undergoing a contraction or squeezing in a given direction,
as a consequence of which one part of its substance is
drawn in and another pushed out. Hence all three kinds
of movement are movements of contraction; and con-
tractility, or the power of contraction, may be considered
as a general property of protoplasm.

Striped Muscle.—If a small piece of any of the body
muscles is carefully teased out with the grain, “e., in the
direction of the length of the fibres, so as to break away the
connective tissue binding them together, the fibres will
readily separate from one another, and they will be seen to
be long and cylindrical. Under the microscope each fibre
shows a delicate transverse striation (Fig. 32), being made
up of alternate bright (4) and dim (d) bands—or more
accurately discs, the fibre being cylindrical—set at right
angles to its length. Hence the ordinary body-muscles or
voluntary muscles are composed of striped muscular fibres
In addition to the transverse striation a fainter longitudinal
striation is more or less distinctly visible.

Each fibre is covered by a delicate membrane (s) called
sarcolemma, beneath which nuclei (#) occur at intervals.

1 The muscles of the heart, although not under the control of the

will, are transversely striated ; but their structure differs from that of
erdinary striped voluntary muscle,

vu STRIPED MUSCLE 113

It will be seen that striped muscle, unlike the tissues
previously considered, does not appear to be composed
of cells, although the occurrence of nuclei seems to hint
at their presence. In the embryo, however, the muscle is
formed of ordinary nucleated cells, which, as growth goes

Fic. 32.—A, part o. a fresh muscular fibre of a frog. B, the same after treatment
with distilled water followed by methyl green.
&. bright bands; @. dim bands ; 2. nuclei ; s, s’. sarcolemma, rendered visible as a
minute blister (s’) by absorption of water and by the rupture of the muscle-
fibre at s> (A, from Huxley’s Physiology.)

on, increase in length while their nuclei multiply by fission,
each enormously elongated cell thus containing a consider-
able number.

Connective Tissue—We will next examine a piece of the
delicate web of connective tissue which binds the muscles
together.

Under the high power, connective tissue is seen to be

PRACT. ZOOL. I

114 THE FROG CHAP.

composed of a sort of irregular network of delicate bundles
of wavy fibres called white connective tissue fibres (Fig. 33, w),
which cross one another in all directions. Amongst them
are found single fibres sweeping across the field in bold

33-—Connective tissue from between muscles of frog's leg.
¢. cells; ¢. noe fibres ; w. white fibres :—all of which are imbedded in a delicate
matrix.

curves and called e/astic fibres (e): it is owing to their
elasticity that the tissue cannot be spread out when wet.
Scattered among the fibres are numérous nucleated cells (¢)
of very varied and irregular form: these are the connective
tissue cells. The fibres, as well as the cells, are imbedded
in a soft, homogeneous ground-substance or matrix.

vil CARTILAGE “its

Thus connective tissue consists partly of cells, but
between these and forming the main substance of the tissue,
isa matrix or ¢xtercelludar substance, enclosing fibres. In
the embryo the tissue consists of closely packed cells, but, as
development proceeds, these separate from one another, and
the ground-substance is formed between them.

Cartilage.—The ordinary clear or hyaline variety of this
tissue-is conveniently studied by examining a piece of the
thin edge of the omo- or xiphi-sternum, or by taking a thin
section with a razor of the head of the humerus or femur.

Cartilage consists of a tough, elastic, transparent,
homogeneous matrix (Fig. 34, m) containing numerous

Fic. 34.—Section of cartilage, from the head of the frog’s femur. |
c. cells; c’, cells undergoing fission ; c. s. empty cell-space ; 7. matrix.

cavities or cel/-spaces (c.s), in each of which is a nucleated
cell (c). The cell-spaces are in many cases arranged in
groups of two or four, sometimes close together, sometimes
with a narrow space of matrix or intercellular substance
between them. This is due to the fact that cartilage grows
by the cells undergoing binary fission, so that two célls are

formed in one cell-space: the two then gradually separate
I 2

116 THE ‘FROG CHAP.

from one another and intercellular-substance is formed
between them. In the embryo, this tissue consists entirely
of closely packed cells which gradually separate and form a
structureless matrix which is firm and elastic, and which in
some parts (pp. 46 and 48) may become calcified.
Bone.—As we have already seen (p. 52) bone is formed
of two constituents, a basis of animal matter in which mineral
matter—calcium phosphate and carbonate—is deposited.
In microscopic eximination we may therefore investigate

Fic. 35.—Lransverse section of dry femur of frog.
c. canaliculi; Zc. lacune ; 27. lamella ; #7. marrow cavity.

either the mineral matter by examining dried bone, or the
animal matter by examining decalcified bone.

A thin section of a dried long bone, such as the femur,
shows that it is formed of very numerous thin layers or
lamella (Fig. 35, 4m), surrounding and concentric with the
marrow cavity. The lamella contain numerous cavities,
the /acune (dc), with delicate, branching tubes, the canalicult

VII BONE 117

(c), radiating from them in all directions, Both lacunz and
canaliculi commonly appear black, owing to their being filled
either with air or with bone-dust produced in grinding the
section.

In a section of decalcified bone (Fig. 36) the marrow is
seen to be surrounded by lamelle of a delicate fibrous
substance, arranged in two layers, an outer (2), having the
periosteum () closely investing it, and an inner (é’), in
contact with the marrow. In the fibrous substance of the
lamellz are cell-spaces, corresponding with the lacunz of the

Fic. 36.—Transverse section of decalcified frog’s femur under a low power, B, portion
of the same under a high power.
, outer, and 4’. inner layer of bone; .¢. bone cells; #. marrow; 0. layer of
osteoblasts in connection with periosteum ; 0’, layer of osteoblasts in connection
with marrow ; Z. periosteum. (After Howes.)

dried bone, and each containing a bone-cell (b,c), which sends
off delicate branched processes of its protoplasm into the
canaliculi. Thus the bone, like connective tissue and
cartilage, consists of cells with an intercellular substance :
the latter is in the form of concentric layers and is impreg-
nated with lime-salts.

The long bones of the frog grow in two directions.
Between the periosteum and the bone is a layer of cells, the

118 THE FROG CHAP,

asteollasts (0), by which new lamellz of bone are formed on
the outside of that already existing: thus the outer layer of
bone (4) grows from within outwards. Between the marrow
and the inner surface of the bone is another layer of osteo-
blasts (0’) which forms new lamella on the inner side of
the existing bone, so that the inner layer (4’) grows from
without inwards.

Summary.—The various simple tissues studied in the
present chapter consist either entirely of cells, or of cells
separated by an intercellular substance. Formed entirely
of cells are the various kinds of epithelium—columnar,
ciliated, and squamous, and unstriped muscle. In striped
muscle the cellshave elongated into fibres and their
nuclei have multiplied. Of tissues consisting of cells with
intercellular substance, connective tissue has the matrix-soft
and homogeneous, with fibres imbedded in it ;-in hyaline
cartilage it is structureless and tough, though elastic; and
in bone laminated and calcified. In the blood, the plasma
may be looked upon as a kind of liquid intercellular
substance.

Cells, wherever they occur, have the same essential struc-
ture, being formed of protoplasm with a nucleus. In
nearly all cases they increase by binary fission, first the
nucleus and then the protoplasm dividing into two.

The distribution of the various tissues throughout the
body is worth noting. Epithelium always bounds a free
surface—covering the outer surface of the body or lining
the inner surface of the enteric canal. Striped muscle forms
the “ flesh,” unstriped muscle the outer layer of the enteric
canal (p. 70). Bone and cartilage form the framework of
the body, while connective tissue is the packing between
the other tissues, . :

VI THE MICROSCOPE 119

N

PRACTICAL DIRECTIONS,

le

Fic. 37.—Diagram of compound
microscope.

a, stand ; 4. pillar; 4’. moveable

portion of pillar, raised and

General Structure of the Compound Microscope.—The com-
pound microscope, with which you must now become acquainted,
consists of a strong stand (Fig. 37, a)
from which rises a vertical J2//ar (6). :
To the latter are attached—a_ horizontal . a
plate or stage (¢), perforated in the cen- i
tre with an aperture (@), the size of
which can be varied by means of a S
diaphragm: an adjustable mérror (e), S
placed below the stage: and a vertical
tube (f) attached above the stage by a
horizontal arm. Two combinations o. I
lenses are used : an odjectzve or object- k
glass (4), consisting of a metal tube with
two or more lenses fixed into it, which J’
screws into the lower end of the tube:
and an ocw/ar or eye-piece (2), consisting Le
of a metal cylinder with a lens at each
end, which slides into the upper end of z a Z
the tube. It is this arrangement of 3
lenses which forms the essential feature
of the compound microscope: the ob-
ject, placed on the stage, is magnified c
by the objective, and the magnified [
image, thrown into the interior of the
tube, is further enlarged by the ocular.
The object is drought cnfo pocus—i.e.,
placed at such a distance from the
objective that a perfectly clear and well-
defined image is obtained—in one ot
two ways. The tube can be raised or
lowered either by sliding it up and
down in an outer tube or collar (g), or,

lowered by fine adjustment ;
¢. stage ; ¢. aperture in stage ;
e. mirror; 4. tube; 7/’. milled
ring for raising and lowering
tube; g. collar; 4. objective ;
7, ocular; & screw of fine
adjustment.

in the more expensive instruments, by a rack and pinion: this move-

ment forms the coarse adjustment.

In addition, all good microscopes

have-a fixe adjustment, usually consisting of a spring concealed in the

120 THE FROG CHAP,

pillar, and acting upon the horizontal arm which carries the tube: it is
worked by a screw (2), and by means of it the tube can be adjusted to
within yg$yath of an inch.

When the object is transparent—as in most cases with which we shall
have to deal—it is placed over the hole in the stage on a glass slide,
and illuminated from below by adjusting the mirror until a beam of light
from the window or lamp is reflected vertically upwards: a small hole
in the diaphragm should be used with the high power. The object is
thus said to be viewed by ¢ransmitted light. In the case of opaque
substances the mirror is not used, and the object is illuminated by the
light falling upon it directly: it is then said to be viewed by reflected
light,

A student’s microscope should have two objectives, one—the /ow
power—magnifying about 80, the other—the Azgh power—about 300 to
400 diameters. One eye-piece is quite sufficient, and a sliding coarse
adjustment is nearly as convenient as a rack and pinion, besides being
cheaper and less likely to get out of order. The mistake often made in
choosing a microscope is to get one of elaborate construction, the money
going largely in brass-work. The proper thing is to get the simplest
form of stand consistent with perfect rigidity, fitted with the best
possible fine adjustment and lenses: to save on either of the latter is
false economy.

Reauisites for Microscopic Work.—In addition to the micro-
scope, the following will be required before starting work :—

1, A few sides or slips of glass, 3 inches long by 1 inch wide, which
can be obtained from an optician. They must be thoroughly cleaned
before being used.

2. A supply (about 4 oz.) of cover-glasses, small pieces vu. very thin
glass, to be had at the optician’s. The most convenient size is 2 inch
square. They are best cleaned by being soaked for a few minutes in
strong nitric acid, and then thoroughly washed under the tap, after
which they should be dried by being placed flat on a clean surface and
rubbed with a handkerchief: if held between the finger and thumb, they
are very liable to be broken.

3. One or two thin g/ass rods, about 6 inches long and 3th inch in
diameter : and one or two azpping-tubes, or pieces of glass tubing about
6 inches long and th inch in diameter. The ends both of rods and
tubes should be rounded off in the flame of a blow-pipe.

4. Half a dozen dissecting needles, made by sticking a fine sewing

vu PRACTICAL DIRECTIONS 121

needle into the end of a wooden penholder, allowing the point to project
about half an inch.

5. A few reagent-bottles for holding the various fluids used for apply-
ing what are called mcro-chemical tests to the tissues. Special bottles
can be bought for the purpose, but sufficiently convenient ones can be
made from ordinary one-ounce phials, fitted with sound corks. Bore a
hole lengthwise through the cork, and insert into the hole a piece of narrow
glass rod pointed at the end just long enough to reach nearly to the bottom
of the bottle when the cork is inserted. This arrangement allows of the
ready application of a single drop of fluid to the object under examination.

6. The following micro-chemical reagents :—

a. Salt solution. Dissolve 0°75 gramme of sodium chloride in 100 c.c.
of distilled water, so as to make a # per cent: solution.

b. Acetze acid, 1 per cent. One c.c. of strong acetic acid to 99 c.c.
of distilled water.

c. Distilled water.

d. Solutions of one or two aniline dyes. For fresh tissues, dissolve
enough methyl green in distilled water to make a deep bluish-green
solution, and add 1 per cent. of strong acetic acid. For preserved
tissues, make a saturated solution of magenta or safranén in strong
alcohol, and dilute with an equal bulk of water.

e. Glycerine, 50 per cent. Equal parts of pure glycerine and distilled
water,

Microscopical examination of the simple tissues.

For the following work, a freshly-killed frog must be used.

(If you wish to measure each kind of tissue element, you must learn to
use a micrometer, which consists of a circular piece of glass, marked at
regular intervals with lines or squares, the distance between which can
be calculated by comparing them with u scale engraved on a slide
known as a stage-micrometer.)

1. The blood,—Have ready a clean dry slideand cover-glass. In a
freshly-killed frog open a vein or make an incision in the heart, and
with a clean glass rod remove a drop of blood to the middle of a slide.
Take hold of the edge of the cover-glass with small forceps, and sup-
porting it with a mounted needle, gently lower it on to the drop of .
blood until the latter is spread out into an even, transparent, yellowish
film. This operation of covering the drop of blood requires a little
practice: if not done quickly, there is danger o. the blood coagu-
lating before it is covered, in which case it will not spread out into a

122 THE FROG CHAP,

transparent layer: i: the cover is lowered too suddenly, bubbles of
air are commonly included.

Examine your preparation, first of all, under the low power, and
learn to recognise the appearance of air-bubbles, Note the colourless
plasma and the numerous minute dlood-cells or corpuscles.

Now replace the low by the high power. Bear in mind that the
higher the power, the shorter the focal distance. With the low power
you will probably find that the object is in focus when about half an
inch from the bottom lens of the objective. The high power, on the
other hand, has to. be brought to within about 34th of an inch of the
cover-glass, which is therefore liable to be broken and the lens to be
injured by careless focussing. The safest plan is to lower the tube,
keeping your eye at the level of the stage, until the objective almost
touches the cover-glass: then, looking through the microscope, very
slowly raise the tube by means of the coarse adjustment, until the object
comes into view. Note (Fig, 26) :—

uw, The numerous flat, oval, ved corpuscles, each with a central
swelling in which the zzclews is contained, Sketch,

6. The colourless corpuscles ox leucocytes, much less numerous, having
a granular appearance, and an irregular or rounded outline.

Focus a colourless corpuscle under the high power, and note its
ameboid movements : sketch its outline rapidly but accurately, and after a
minute or two make another sketch, and then another, until some half-
dozen outline drawings of the corpuscle have been obtained: then
compare your sketches.

Now place on the slide, against one edge of the cover-glass, a drop of
methyl-green, and against the opposite edge a small strip of blotting
paper. Ifthe blood is sufficiently fresh—and if it has coagulated
you must get another drop—the blotting paper will slowly absorb the
blood on one side, and the methyl-green will be drawn in and will
gradually mingle with the blood. When this has taken place, put a
drop of salt solution in the place of the methyl-green, and allow it to
be drawn across so as to remove the superfluous stain: then remove the
blotting paper, and examine the blood once more under the high
power. Notice the szc/ews present in each’kind of corpuscle, and the
surrounding Zrotoplasm. Sketch.

To a fresh drop of blood add in the same manner a drop of 1 per cent.
acetic acid. Note that the body of the corpuscles becomes transparent,
while the nucleus is rendered distinct.

vil PRACTICAL DIRECTIONS 123

To another drop of blood add distilled water. The corpuscles be-
come swollen up and partly dissolved, and the colouring matter
(Aemoglobin) of the red corpuscles is dissolved out into the plasma.

Examine some coagulated blood under the microscope, and note the
threads of 4772 in which the red corpuscles are entangled, and the
Serum,

Examine once more the circulation in the web (p. 103), using the high
power, and follow the course of both red and colourless corpuscles
through the vessels and capillaries. By focussing to the surface of the
web the flattened epithelial cells of the epidermis or outer skin can be
seen, and at a deeper ‘level the black pigment cells (see p. 128, and
compare Figs. 24 and 27).

2. Columnar epithelium.—Take a small piece of frog’s intestine,
and place it for 24 hours in a mixture called Ranvier’s alcohol, consisting
of one part of methylated spirit and two parts of water. With fine
scissors snip off a very small piece—not larger than a pin’s head—from
the inner surface of the mucous membrane, place it on a slide in a drop
of water, and with two dissecting needles tease it out by tearing it into
the smallest possible particles. The operation is best done under a lens.
Then put on a cover-glass, and examine first with the low and then with
the high power. (Remember that a cover-glass must a/ways be used
with the high power.)

Note the minute, more or less conical ced/s of columnar epithelium
(Fig. 28), each containing a xzclews. Observe the god/et-cells amongst
the ordinary columnar cells. Stain with magenta, which is more effective
than methyl-green in specimens previously treated with alcohol, and
wash with water. (Salt-solution need only be employed in the case of
fresh or living tissues.) Sketch.

3. Ciliated epithelium.—Snip off a very small bit of mucous
membrane from the mouth of a recently killed frog, and tease it out in
salt-solution (Fig. 29).

Note the form of the cells and their nuclei: they are relatively
shorter than the columnar cells just examined, and each bears a number
of delicate vibratile ci/éa at its free end. Observe the movements of
the cilia, Sketch.

Treat with methyl-green, magenta, or acetic acid, when the nucleus
will become more apparent.

Perform experiment described on p. 119 to show the action of the cilia
as a whole in the entire animal.

124 THE FROG CHAP.

4- Pavement or squamous epithelium.—Take a bit of frog’s skin
which has been kept for a day or two in Ranvier’s alcohol, scrape the
outer surface with a sharp knife, and examine the scrapings in a drop of
water, afterwards staining with magenta. (Fig. 30.)

Note the flattened cells fitting together like tiles in a pavement, each
one with its nucleus. Sketch.

5. Unstriped Muscle.—Snip off a small piece from an inflated
urinary bladder of a frog which has been preserved in formaline, and
wash with water. Ov, snip off a very small piece—not bigger than a
pin’s head—from the muscular coat of the intestine or stomach, or
from a urinary bladder, which has been in Ranvier’s alcohol for at
least twenty-four hours. Then tease out in a drop of water very
thoroughly. Note the elongated unstriped muscular fibres tapering at
both ends, and each containing a nucleus (Fig. 31). Stain with
magenta. Sketch.

6. Striped Muscle.—Snip off a small piece—about 4th inch long
—from any of the body-muscles of a freshly-killed frog, put it on-a
slide in a drop of salt solution, and tease it out, with the grain, 7.c.,
in the direction of the length of the fibres. The fibres will readily
separate from one another: the teasing process must be stopped as
soon as they are apart, and care must be taken not to tear or crush
the individual fibres, which are large enough to be readily distin-
guishable with a magnifying-glass.

Observe under the low power of the microscope the long cylindrical
fibres (Fig. 32, A), bound together by connective tissue, and showing a
distinct ¢ransverse striation and a less distinct longitudinal striation.
Examine a single fibre under the high power (Fig. 32, B), and make
out the sarcolemma and the numerous zzclez, which will be rendered
more distinct by the addition of methyl-green or acetic acid. Sketch.

7. Connective-tissue.—Carefully separate two of the muscles of
the leg in a fresh frog, and note the delicate web of connective tissue
between them: or, note the fine strands of connective tissue between
the skin and the muscles of the body wall. With fine forceps lift up
a small shred of this, snip it off with scissors, and place it on a dry
slide. Then, with two needles, spread it out into a thin, even layer,
breathing on it occasionally to prevent drying. Lastly, place a drop of
salt solution on a cover-glass, and quickly lower it on the preparation.
The reason for this procedure is that if connective tissue is placed in
fluid, it contracts into a little lump, which is too opaque for examination
and cannot be readily spread out,

VII PRACTICAL, DIRECTION 125,

Examune first with the low and then with the high power, and note
the bundles of white connective-tissue hbres, and the elastic fibres.
(Fig. 33). Sketch.

Add acetic acid: the white fibres will be dissolved, the elastic fibres
more readily distinguished, and the cosnective-tissue cells seen ; the latter
and the delicate ground-substance will be rendered more distinct by
staining with methyl-green. Sketch.

8. Cartilage.—Snip off the thin edge of the omo- or xiphi-sternum
and examine it as before in a drop of salt solution. Ov, cut a thin
section of the head of the humerus or femur with a razor.

Note the transparent, homogeneous ma¢réx, containing numerous ced/-
spaces or lacune, in each of which is a nucleated cell: observe here
and there the groups of cells formed by binary fission (Fig. 34). Stain
as before. Sketch.

9. Bone.—For the examination of dried bone cut a very thin slice of
one of the dried long bones with a fret-saw: fasten it to a slide with
Canada balsam, and, when the balsam has dried quite hard, rub down the
section on a hone until it is thin enough to be quite transparent. O7,
a transverse section of a bone from a larger animal may be prepared in
the same way or bought from a dealer in microscopic objects.

a, Examine first a transverse section of dry frog’s bone (e.g., femur or
humerus), and note the marrow-cavity, the Zamel/e, and the Jacune and
canaliculi ; the two last will probably appear black, owing to their being
filled either with air, or with bone-dust produced in grinding the section
(Fig. 35). A section of human bone, such as is usually supplied ready
prepared, or of the bone of some other larger animal, shows a more
complicated structure: instead of a single system of lamella, the bone
consists of a number of such systems, each surrounding a central canal,
in which blood vessels and nerves run, and which corresponds to the
marrow-cavity in the simpler frog’s bone described above. Sketch.

6. Compare with a section of decalcified frog’s bone,’ and notice—the
fibrous /amel/e arranged in two layers, the outer of which is closely
invested by the periosteum ; the cell-spaces or lacuna, containing bone-
cells; and the outer and inner layers of osteoblasts (Fig. 36). Sketch.

(For the histology of wervoes t¢ssue see Chapter X.)

1 The method of preparing sections of this and other tissues will be
described at the end of the next chapter.

CHAPTER VIIL

THE FROG (continued): THE MICROSCOPIC EXAMINATION
OF THE COMPOUND TISSUES—GLANDS—SECRETION AND
ABSORPTION.

WitH the exception of the tissues of the nervous system,
which will be described later on (Chap. X), we have now
studied the principal simple tissues by the method of dis-
sociation, i.e. by separating their constituent parts. We have
now to consider the way in which these tissues are combined
in the various organs, and for this purpose must adopt some
method of examination by which they are seen in their
natural relations.

The method adopted for this purpose is that of section
cutting. You know how, by cutting sections, in various
directions, of a bit of twig, the arrangement and natural
relations of its various parts—wood, bark, and pith—can be
ascertained. The same thing applies to the organs of the
frog and other animals, but owing to their soft and non-
resistant texture, it is impossible to cut them into sections
thin enough for microscopic examination without special
preparation. The methods employed are by no means easy
for the beginner, especially without verbal instruction and
the resources of a biological laboratory, but in the event of
your wishing to make the preparations described and

CHAP. VIII SKIN 127

figured in this chapter for yourself, the directions given on
pp. 135—139 are simple enough to be carried out with very
limited appliances.

|

Onl

4

{

!

= MY

Fic. 38.—Vertical section of the frog’s skin, highly magnified.

PD. derm, formed of Af 4/7’. Af". horizontal, and v./. vertical fibres of connective
tissue, and containing 4.7. blood-vessels, and Ag. pigment cells. E, epiderm,
consisting of #z. 2. active or Malpighian layer, and 4. Z. horny layer of epithelial
cells; c. gZ. cutaneous gland in section ; c. g/’. in surface view ; @. duct. (After

Howes.)

The Skin.—A vertical section of the skin, z.e. one taken at
right angles to its surface, will be seen to have the following

structure.

128 THE FROG CHAP

The skin is clearly divisible into two layers, an outer,
the epiderm (Fig. 38, E) and an inner, the derm (D). The
epiderm is built up of several layers of epithelial cells.
These differ greatly in form according to their position,
those in the lower or internal layer (#. 7) being columnar,
while those in the upper or external layer (4. 7) are squamous,
and have their protoplasm converted into horny matter so
as to furnish a comparatively hard and insensitive covering
to the body.

The horny layer is cast off periodically in shreds, and to
make up for this, the cells of the inner or deep layer
multiply by binary fission, the increase in their number
necessarily resulting in a pushing upwards of the super-
jacent layers. There is thus a constant travelling of cells
from the inner to the outer surface of the epiderm: as
they pass towards the outer surface they become more and
more flattened, and at last squamous and horny. The
whole process takes place in such a way that the multi-
plication of the columnar cells in the lower layer is just
sufficient to make good the loss of the squamous cells in the
superficial layer.

The derm (D) is formed of connective tissue, the fibres of
which are mostly horizontal (2. f,4./’, 4./”), or parallel to the
surface of the skin, but at intervals are found bands of
vertical fibres (v.f). The derm also differs from the
epiderm in having an abundant blood-supply (4. a), capillaries
ramifying through it in all directions. It also contains
nerves, the ultimate fibres of which have been traced into
the deeper layers of the epiderm. Imbedded in the derm,
especially in its external portion, are irregular cells (fg), the
protoplasm of which contains an intensely black pigment. It
is to these pigment-cells, already referred to on p. 123, that
the black patches in the frog’s skin are due.

VIII CUTANEOUS GLANDS 129

In this, as well as in the other sections described in the
present chapter, the structure of the nuclei of the various
cells can be more easily made out than in the fresh prepara-
tions you have already examined. Each nucleus will be
seen to be enclosed by a definite xuclear membrane, and to
contain in its interior a number of minute bodies, which
‘take up the stain more deeply than the rest of the nucleus.
One or more of these bodies may correspond to the nucleoli
already seen (p. 109), but many of them, are of a different
nature and can often be seen to form a network : the material
of which these are composed is known as chromatin, while
the semi-fluid substance which surrounds them and forms
the ground-work of the nucleus may be distinguished as
the achromatin.

Cutaneous Glands—Secretion.—In the superficial part
of the derm are seen numerous rounded spaces (c. gé, ¢. gl’),
each of which can be proved, by taking sections in various
directions, to be a nearly globular cavity, from which a
narrow canal (d), like the neck of a flask, passes through
the epiderm to open on the external surface. Both the
body and the neck of the flask are lined with epithelium,
the cells lining the body being nearly cubical, those of the
neck squamous.

These structures‘are called cutaneous glands: they per-
form the function of manufacturing the slimy fluid which,
as we have seen, is constantly exuding on the surface of
the skin. The epithelial cells of the gland have the power
of forming minute droplets of the fluid out of the materials
supplied to them by the blood: the droplets escape from
the cells and accumulate in the interior of the gland, whence
the fluid is finally discharged by the duct.

The cells lining the duct are continuous on the one hand
with those of the gland, and on the other with those of the

Pract. Zoot. K

go THE FROG CHAP,

epiderm. The whole gland with its duct is to be looked
upon as a depression of the skin, lined by pushed-in epiderm
cells.

Epithelial cells having the power of manufacturing and
discharging a specific substance are called g/and-cells, and
the process of manufacture is known as secretion. We
have already met with isolated gland-cells in the case of the

Fic. 39.—A, transverse section of frog's intestine; B, small portion of the same
highly magnified.
b.v. blood-vessel ; c. mz. circular layer of muscle; ef. epithelium ; 2. 1. longi-
tudinal layer of muscle-fibres ; Ay. peritoneum ; 7. muscular layer; s. 2. sub-
mucosa. (After Howes.)

goblet-cells of the intestine (p. 109), which secrete mucus ;
but commonly, as in the present instance, gland-cells are
aggregated into a definite organ called a gland.

The Intestine.—A transverse section of the intestine
shows also a very definite and characteristic combination of
simple tissues. The mucous membrane, like the skin, is
composed of two layers, an epithelial layer (Fig. 39, ¢),
corresponding to the epiderm, and a connective tissue layer

VII GASTRIC GLANDS 131

(s.m), corresponding to the derm and called the submucosa.
The epithelium consists of a single layer of cells only (B, ¢),
all columnar, and with their long axes at right angles to the
elevations into which, as we have seen (p. 71), the mucous
membrane is thrown. Amongst the ordinary epithelial cells,
numerous mucus-secreting goblet-cells will be recognised.
The submucosa, like the derm, contains blood-vessels, lym-
phatics, and nerves.

The muscular layer (A, 7) is also divisible into two: an
outer layer of Jongitudinal fibres (B, 4m), running parallel
with the long axis of the tube, and an inner, much thicker
layer of circular fibres (¢.m) which encircle it, and conse-
quently lie at right angles to the longitudinal fibres. Thus
in a transverse section, such as Fig. 39, the fibres of’ the
circular layer are cut longitudinally, those of the longitudinal
layer transversely, while the opposite would be the case in a
longitudinal section.

The peritoneum (gv) which, as we have seen (p. 27)
forms an outer covering to the intestine, is formed of an
inner layer of connective tissue and an outer of squamous
epithelium.

The Stomach.—Transverse sections of the stomach (Fig.
40) show it to differ from the intestine not only in the much
greater thickness of all its layers, but in the fact that the
epithelium, instead of simply forming an even layer over the
ridges of mucous membrane, is sunk into the submucosa in
the form of simple or branched tubes, the gastric glands
(g. g/). These differ from the cutaneous glands in being
not flask-shaped but test-tube shaped, each being a long,
narrow tube, with an extremely small cavity (B and c, ¢).
They are lined by a single layer of gland-cells, and open
by minute apertures (#2) on the surface of the mucous
membrane.

K 2

132 THE FROG CHAP.

The cells of the gastric glands have the power of forming,
out of the materials supplied to them by the blood, the
gastric juice, by which, as we have seen (p. 74), proteids
are digested. Thus, while the raw material supplied to both
cutaneous and gastric glands is the same, the manufactured
article is entirely different in the two cases. Each kind of

Fic. 4o.—A, part of a transverse section of the frog’s stomach ; B, one of the gastric
glands in longitudinal section, highly magnified; C, transverse section of a
gastric gland.

b.v. blood-vessel; c. cavity of gastric gland; c.#. circular muscles; c. 1. 12.
circular layer of muscularis mucosz; ¢f. epithelium; g. g/. gastric glands ;
Z.m. longitudinal muscles ; 2. 7.7, longitudinal layer of muscularis mucose ;
mm. mouth of gastric gland ; #7. nucleus; 4~. peritoneum ; s. 7. submucosa.

gland-cell has the faculty of picking and choosing, the
material supplied being worked up in the one case into the
cutaneous secretion, in the other into gastric juice.

VIII LIVER 133

The submucosa of the stomach is traversed by a narrow
band of unstriped muscle, the muscularis mucose, formed,
like the main muscular layer, of an outer layer of longitudinal
(Z.m.m) and an inner of circular (¢.7.m) fibres.

A muscularis mucosze is also present in the intestine, but as it is very
thin, it may easily be overlooked.

The Liver.—Sections of the liver show it to be made up
of innumerable large, polyhedral cells (Fig. 41, ¢), which are

c

Fic 41.—A, portion ot a section of the frog’s liver; B, smal’ portion of the same,
showing the origin of a bile-duct.
4.c. blood-capillaries, in section; 4.4. bile-passages ; c. liver cells; @, smallest
bile-duct ;-%. nuclei. (After Hoffmann.)

so arranged as to bound extremely fine channels or 47/e-
passages (bp). These are found to open into one another,
and finally to discharge into definite tubes (B, 2), lined with
epithelium. These, in their turn, unite into larger and larger
tubes, which form the hepatic ducts and ultimately open into
the common bile-duct (p. 68).

The liver-cells are glandular and secrete the bile, which,
as it is formed, drips into the bile passages and passes into

134 THE FROG CHAP.

the hepatic ducts, thence making its way either directly into
the intestine or into the gall-bladder (p. 70). The whole
liver, which is the largest gland in the body, is traversed by
a complex network of capillaries (4.c), supplied partly by
the hepatic artery, partly by the portal vein; and, from the
blood thus supplied, the liver-cells obtain the materials
necessary to enable them to discharge their function of
secreting the bile.

The liver-cells have, however, other functions, one of
which is to manufacture, in addition to bile, a substance

Fic. 42.—A, small portion of a section of the frog’s pancreas ; B, diagram showing
the connection between the lobules and ducts.
¢, connective tissue covering of the gland ; @. duct; 2. lobules; #2. nuclei.

called g/ycogen or animal starch. This is stored up in the
cells in the form of minute insoluble granules, which, being
afterwards transformed into soluble sugar, pass into the
blood and so to the tissues.

The Pancreas.—Sections of this gland (Fig. 42) show
it to be made up of microscopic masses or /odules (/), each
of which consists of a cluster of gland-cells enclosing a very
narrow central space. The cavities of adjacent lobules com-

VI NUTRITION 135

municate with one another and open into tubes or ducts (d)
lined with epithelium, which join with one another and
finally discharge into the bile-duct as it traverses the pancreas
(p. 70). The pancreas is distinguished as a racemose or
grape-bunch gland: the duct is comparable to the branched
stalk and the lobules to the grapes.

Connection of the foregoing facts with the physiology
of nutrition.—You will now be able to understand more
clearly the various processes connected with the nutrition
of the frog, hitherto studied without the aid of histology.

When the food enters the enteric canal the various gland-
cells are stimulated into activity, and the gastric juice, bile,
and pancreatic juice are poured out and mingle with the
food, which is digested in the manner already described.
The soluble products of digestion—peptones, sugar, salts,
fatty acids, and glycerine, diffuse through the epithelium of
the enteric canal into the blood-capillaries of the underlying
submucosa, and the blood, now loaded with nutriment, is
carried by the portal vein to the liver and thence by the
hepatic and postcaval veins to the heart (see Fig. 23). At
the same time the fats make their way into the lymph-
capillaries and are finally pumped, by the lymph-hearts,
into the veins. Thus the products of digestion all find
their way ultimately into the blood, and are distributed,

through the circulatory mechanism, to all parts of the
body.

PRACTICAL DIRECTIONS.

Materials required for the Preparation and Sectioning
of Animal Tissues.—In addition to the requisites mentioned in
chapters I and VI, the following will be required :—

a. Corrosive sublimate: a saturated solution in water. Care should
be taken in using this solution, as it is a very virulent poison.

136 THE FROG CHAP.

6. Absolute alcohol,

(. Turpentine.

d. Paraffin: sold in two grades, ‘‘ hard” and ‘‘soft.” It is best to
get an ounce or two of each.

e. A solution of Canada balsam in chloroform or turpentine. This
should be kept in a small glass bottle, with a ground glass cap—not a
stopper or cork.

Jf. Asolution of akoholic borax-carmine. This may be bought ready
mixed, or made as follows :—Grind up in a mortar 2 grammes of car-
mine and 4 grammes of borax, and dissolve in 100 c.c. of distilled
water: to this solution add an equal volume of 70 per cent. alcohol:
allow to stand for a day or two and filter.

& A water-bath,in which melted paraffin may be kept at a constant
temperature. For a make-shift you can use a saucepan with a flat
piece of tin over it; fill the saucepan about half full of water, and
heat it over a spirit lamp or a small oil-lamp or gas-burner, regulating
the distance of the flame so as to keep the temperature of the water at
about 55° C. (131° F.).

h. Two or three watch-g/asses or other small shallow vessels for con-
taining melted paraffin.

2, A sharp, flat-ground razor.

7. A section-lifter, made by beating out flat about half an inch of the
end of a piece of stout copper wire, about 6 in. long, and bending the
flattened portion at an obtuse angle with the rest.

Preparation of Tissues for Section Cutting.

a, Fixing, hardening, and decalcifying.

Sections may be cut from specimens which have been carefully pre-
served in alcohol,—first in 70 per cent., and after a day or two trans-
ferred to 90 per cent. But certain other reagents are more effective in
effecting the fixzng of the tissues—z.e., in quickly killing and coagulating
the protoplasm of the cells with a minimum of shrinkage, and of these
the one most generally useful is a solution of corrosive sublimate (see
above), in which, from a freshly-killed frog, place small pieces of the
various organs and tissues to be examined—e.g., skin, intestine, stomach,
liver, pancreas, kidney, ovary, testis, and spinal cord, as well as
the inner half of the eyeball. The intestine and stomach should be
first washed out in salt solution, and then cut into pieces about % inch
long ; the liver should be {cut into pieces not more than } inch cube.
After about half-an-hour to two hours, according to the size of the

VI SECTION-CUTTING 137

piece, place in water under a tap, and wash thoroughly for a quarter
of an hour or more, until the corrosive sublimate is removed.

After washing, transfer to 50 per cent. alcohol for a few hours,
and then to 70 per cent. for twenty-four hours, after which they may
be stained at once (see below), or transferred to strong methylated
spirit (90-93 per cent.), in which they may be kept until wanted. This
completes the process of hardenzng : it is done gradually, by alcohols of
increasing strength, in order to avoid shrinkage.

In order to decalczfy such tissues as bone, from which the lime-salts
must be extracted before cutting into sections, place a small piece for a
few days in 70 per cent. alcohol, to which 2 per cent. of strong nitric
acid has been added: then wash thoroughly, transfer to alcohol, and
stain.

6. Staining.—Place the organs, cut into convenient sizes for im-
bedding—z.e., not more than 4 inch long, and, in the case of such
organs as the liver, 4 inch in thickness—into borax-carmine for one or
two days, or even more. They will become stained throughout, and
the difficulty of staining the sections after cutting will thus be avoided.
After staining, place them in weak alcohol (50-70 per cent.), slightly
acidulated with hydrochloric acid: if a watch-glass or some such vessel
is used, it is sufficient to dip the end of a glass rod into acid and stir it
round in the alcohol. The effect of the acid is to remove much of the
colour from the protoplasm of the cells, leaving the nuclei brightly
tinted. After half-an-hour or less in the acid‘alcohol, place the tissues
once more in strong methylated spirit.

c. Dehydrating.—Transfer from methylated spirit to absolute alcohol,
which must be kept in a stoppered or tightly corked bottle, as it will
otherwise deteriorate by absorption of water from the air. It has the
effect of withdrawing the last traces of water from the tissues, an abso-
lutely necessary step in order that they may be permeated with paraffin.

a. Imbedding.—Transfer the objects from absolute alcohol to turpen-
tine. This fluid acts as an intermediary between alcohol and paraffin,
being freely miscible with both : it gradually replaces the alcohol in the
tissues, rendering them transparent. If they are not transparent in the
course of an hour or so, the process of dehydration has not been com-
plete, and they must be returned to absolute alcohol. In the mean-
time, melt some paraffin over the water-bath, using various mixtures
of hard and soft according to the season: in a cold room in winter soft
paraffin will be hard enough; in the height of summer hard paraffin

138 THE FROG CHAP.
alone will be suitable. The temperature of the water-bath must never
he allowed to rise more than a degree or two above the melting point
of the paraffin. Transfer the objects from turpentine to melted paraffin
and keep them in it for some hours—the time varies according to the
size of the piece—until they are thoroughly permeated.

If you wish to cut sections by hand, get some ordinary medium-sized
bottle-corks : around each wind a piece of paper, allowing it to project
about $ an inch beyond one end of the cork, and fixing it with a pin,
asin Fig. 43, A. Into the little cylindrical vessel or imbedding box
thus formed pour some melted paraffin, and immediately transfer to it,
by means of a warmed section-lifter or forceps, one of the prepared
pieces, adjusting its position by
means ofa heated needle. When
the paraffin is quite cold remove
the paper, and you will have
fixed to the cork a solid block
of paraffin containing the object
to be cut.

e. Section-cutting.—Pare away
the block of paraffin until the
object comes into view: then
trim the block, as in Fig. 43, B,
until its upper surface, with the
object in the middle, is not more

Fic. 43-—A, imbedding box made by wrap- Hold the

ping paper round a cork; B, cork after
removal of the paper, showing the
paraffin pared down to a convenient
size for sectioning. a, object to be
cut.

than 4 inch square.
cork firmly in the left hand, with
the wrist resting on the table,
and with a razor cut the thin-

nest possible slices of the paraffin
block, including the imbedded object. The razor must be held firmly
grasped at the junction of blade and handle, and kept with the surface
of the blade parallel with that of the block : use almost the whole extent
of the edge for each section. With a little practice you will be able
to cut sections so thin as to be quite transparent under the high power.?

‘1 Jf you are working in a properly furnished laboratory you will
probably learn how to cut sections with a wzcrotome, or section-cutting
machine, which gives much ‘better results and is absolutely necessary
when a complete series of sections of the same object is required.

VIII PRACTICAL DIRECTIONS 139

fi Clearing and Mounting.—Place the section on a slide and warm
it gently on the water-bath until the paraffin melts, and then add a
large drop of turpentine in order to dissolve the paraffin. Then draw
off the turpentine with blotting-paper and replace it by a fresh drop,
repeating the process until all the paraffin is dissolved : put on a cover-
glass and examine.

If you wish to be sure that the parts of your sections are not displaced
in mounting, or to mount several sections on your slide, the latter should
first be smeared over with a very thin layer of a mixture of collodion
and oil of cloves, in equal parts: then place the sections on the slide,
warm, and immerse the whole slide in a small vessel of turpentine,
leaying it until all the paraffin is dissolved.

In order to make a permanent preparation, remove the paraffin
with turpentine, as above, draw off the turpentine, place a drop of
Canada balsam on a cover-glass and very gently lower the cover-glass
on the object, spreading out the balsam in a thin, even layer. Before
long, the balsam will have set quite hard, and the sections may be pre-
served for an indefinite period ; the balsam will set more quickly if you
leave your preparations over the water-bath for a short time.

Remember that object, razor, slide, and cover must be kept free
from water, the presence of which, from the stage of dehydration on-
wards, is fatal to success.

Examination of Compound Tissues.

Examine the following sections, prepared as described above, first
with the low, and then with the high power, noting the parts
enumerated in each case, as well as the structure of the nuclei (zzc/ear
membrane, chromatin, and achromatin.)

1. Vertical section of skin (Fig. 38).

a, Epiderm, stratified, divisible into outer (horny) and inner (JZa/-
pightan) layers.

4, Derm, connective-tissue fibres, blood-vessels and pigment-cells.

c. Cutaneous glands with their ducts. The apertures of the ducts on
the surface you will probably have noticed already in your preparation
of the epithelial cells of the skin (p. 124).

Sketch.

2. Transverse section of intestine (Fig. 39).
a. Mucous membrane : a superficial epithelial layer of columnar cells,

140 THE FROG CHAP, VIII

with goblet cells amongst them; and a deeper connective-tissue laver,
the swbmucosa, enclosing blood-vessels, nerves, and lymphatics.

6. Muscular layer: an external longitudinal, and an internal circular
layer of unstriped muscular fibres.

«. Peritoneal layer. This is very thin, and a careful examination
of good preparations is required in order to make out its structure
(p. 131).

Sketch,

3. Transverse section of stomach (Fig. 40).—After recognising
the layers as above, note :—

a. The gastric glands, and 6. The muscularis mucose.

Sketch.

4. Sections of liver (Fig. 41).

a. Polyhedral gland-cells ; 6. Bile passages and ducts; c. Blood capil-
lardes and vessels,

Sketch.

5- Sections of pancreas (Fig. 42).

a. Lobules, each consisting of gland-cells ; b. Ducts.

Sketch.

CHAPTER IX.
Tut FRoG (continued) : RESPIRATION AND EXCRETION.

In the fifth chapter it was pointed out that a continual
waste of substance goes on in the frog’s body, the lost
material taking the form of three chief waste products or
products of excretion, water, carbon dioxide, and urea. It was
further stated that these substances are got.rid of by means
of the lungs, the kidneys, and the skin.

The Organs of Respiration.—At their anterior ends the
two lungs open into a small, thin-walled chamber (Fig. 44,
2. tr. c), which as it corresponds both with the larynx or
organ of voice, and the trachea or windpipe in ourselves, is
called the Jaryngo-tracheal chamber: it communicates with
the pharynx through the glottis (g/). The walls of the
chamber and the edges of the glottis are supported by
cartilages (a7).

The structure of the lung is best made out by distending
it with air, and then placing it in spirit until thoroughly
hardened: its walls contain so much elastic tissue that if
cut when fresh, it contracts immensely, and its structure is
then difficult to see. The inner surface of the lung is raised
up into a complex network of ridges (A. /mg), which
project into the interior and produce the appearance of an

142 THE FROG CHAP,

irregular honeycomb. All these ridges are abundantly
supplied with blood vessels fed by the pulmonary artery, the
blood being carried away by the pulmonary vein.

The main substance of the lung is made of connective
tissue containing elastic fibres and unstriped muscle, and
traversed by a network of capillaries. Its cavity is lined
by a layer of pavement epithelium, and its outer surface
is covered with peritoneum.

Respiratory Movements.—In breathing, the frog depresses
the floor of the buccal cavity (Fig. 45, 4), and, the mouth

Fic. 44.—The respiratory organs of the frog from the ventral aspect ; B, the laryngo-
tracheal chamber in longitudinal section, with the right lung.
ar, the arytenoid, or principal cartilage of the larynx ; 4. Ay. body of hyoid ; gi.
glottis; Z dg. left lung; 2. ¢7.c. laryngo-tracheal chamber; .c. Av. posterior
horn of hyoid; ~. Zug. right lung, laid open in A to show its internal surface ;
v.cd. vocal cord. (After Howes).

being kept shut, air is drawn in through the nostrils. The
floor of the mouth is then raised (4), by muscles attached
to the hyoid. At the same time the anterior end of the
lower jaw presses upon the moveable premaxillz (fm), the
upward processes of which (p. 42, Figs. 8 and 9, PAZX) act
upon certain cartilages in connection with the external
nostrils in such a way as to produce closure of these
apertures (Fig. 45, B). The gullet (gw/) is so contracted,
except during the act of swallowing, as to be practically

IX RESPIRATORY MOVEMENTS 143

closed. Thus when the floor of the mouth is raised the air
contained in it can escape in one way only, viz., through the
glottis into the lungs.

Thus 7zspiration, or breathing in, is produced by the
buccal cavity acting as a force-pump: the lowering of its
floor draws in air through the nostrils, the raising of its floor
forces the imprisoned air into the lungs. LZxfiration, or

ln.

Ge SF

Fic. 45.—Diagrams illustrating the respiratory movements of the frog. In A the
floor of the mouth is depressed and air is being drawn in through the nostrils ;
in B the floor of the mouth is raised, the nostrils are closed, and air is being forced
into the lungs.

e.n. external nostril; g?. glottis; gwd. gullet; z.. internal nostril ; Zmg. lung ;

off. s. olfactory sac; Amz. premaxilla ; tug. tongue.

breathing-out, is due to a contraction of the elastic lungs,
accompanied by a slight lowering of the tip of the lower
jaw: the latter movement releases the premaxillz and thus
opens the external nostrils.

144 THE FROG CHAP,

Respiration.—By these alternate movements of inspira-
tion and expiration fresh air passes into the lungs at regular
intervals, while part of the air already contained in them is
expelled. Now we saw, when studying the blood (p. 79),
that dark purple blood drawn from a vein becomes bright
scarlet when exposed to air, and we subsequently learnt
(p. 107) that this change is due to the absorption of oxygen
by the red corpuscles.

The blood brought to the lungs by the pulmonary artery
is, as we have seen (p. 94), non-aérated, being the impure
blood returned by the three caval veins to the right auricle.
When this blood is pumped into the capillaries of the lungs
it is separated from the air contained in those organs only
by the extremely thin walls of the capillaries themselves
and the equally delicate pavement epithelium lining the
lungs (Fig. 23, p. 142, Zp. lag, Cp. dmg). Under these
circumstances an interchange of gases takes place between
the air and the blood: the hemoglobin of the red
corpuscles absorbs oxygen, and the carbon dioxide in the
blood, derived from the waste of the tissues, is given off into
the cavities of the lungs. The blood in the pulmonary
capillaries thus become aérated and is returned as red blood
to the left auricle: at the same time it loses carbon dioxide,
together with a certain amount of water, and these waste
substances are expelled from the body with the expired air.

Voice.—It was mentioned above (p. 141) that the glottis
and laryngo-tracheal chamber are supported by cartilages.
The largest of these are a pair of semilunar arytenoid
cartilages (Fig. 44, ar), which bound the glottis to right and
left. The mucous membrane on the inner or adjacent faces
of the arytenoids is raised into a pair—right and left—of
horizontal folds, the vocal cords (v.cd). By means of muscles
these folds can be stretched and relaxed, and can be brought

IX KIDNEYS 145

into either a parallel or a divergent position. When they
are parallel the air, passing to and from the lungs, sets their
edges in vibration and gives rise to the characteristic croak,
the pitch of which can be slightly altered by stretching or
relaxing the cords.

Structure of the Kidneys.—The form and situation of
the kidneys (Figs. 3,4, and 7, 4d) have already been referred
to. Each is a flattened organ of a deep reddish-brown
colour, its inner: edge nearly straight but for one or two
notches, its outer edge curved. Its ventral face is covered

anki *

Fic. 46.—Transverse section of frog's kidney.
gf. glomerulus ; 7. cf. Malpighian capsule; xs¢. nephrostome; Zev. peritoneum
covering ventral face of kidney ; fe7’. fold of peritoneum supporting its outer
border ; 427’. fold supporting its inner border; UR. ureter; ws. ¢v. urinary
tubules, (After Marshall and Bles.)

by peritoneum (Fig. 46, Zev), continued on the one hand
into the parietal layer (Fig. 5, 2. ger), of that membrane, on
the other into the mesentery (mes.) ; its dorsal face is bathed
by the lymph of the subvertebral sinus (sv. Zy.s.) From the
posterior end of its outer edge a delicate tube, the ureter
(Figs. 3, 4, and 7, ur), passes backwards and opens into the
dorsal wall of the cloaca. On its ventral face is a singular
yellowish-white stripe, the adrenal or supra-renal body, an
organ of unknown function (Fig. 7, between the lines from
kd and és).

PRACT. ZOOL. L

146 THE FROG

CHAP.

A thin section shows the whole kidney to be made up of
a tangled mass of microscopic tubes (Fig. 46, 7. 7), so

Fic. 47.—Diagram of a single urinary tubulé
with its blood-vessels, to illustrate the
structure of the frog’s kidney.

af.v. afferent vessel of glomerulus ; ¢. capil-
lary network of kidney; efv. efferent
vessel of glomerulus; g/. glomerulus;
m.cp. Malpighian capsule, showing epi-
thelium ; zs¢. nephrostome showing cilia ;
v.a, renal artery; ~ ft. v. renal portal
vein; 7. v. renal vein; ~~. ureter; 17. fu.,
ur. tu’, ur. te.”, ur. tu.'", different portions
et senate tubule, showing epithelium and
cilia.

twisted together that
any section cuts them
in various planes, some
transversely, when they
appear as circles, others
longitudinally or ob-
liquely. Amongst these
urinary tubules or ne-
phridia, as they are
called, are seen globu-
lar sacs, the AZalpighian
capsules (m. cp), each
having in its interior a
little irregular bunch,
known as the glomerulus
(gé). Very accurate ex-
amination of numerous
sections, as well as of
teased-out specimens,
shows that each Mal-
pighian capsule (Fig. 47,
m. cp), is connected with
aurinary tubule (a7. tz),
to which it forms a
blind, bulb-like end.
The tubule itself winds
through the substance of
the kidney, is joined by

other tubules, and finally discharges into the ureter (wr).
The tubules are lined with somewhat cubical cells of
glandular epithelium, which, in some parts (ur. tu, ur. tu”)

IX EXCRETION 147

are ciliated. The Malpighian capsules are lined with
flattened cells of pavement epithelium.

The arrangement of the bloodvessels is peculiar. Like
other organs, the kidney is permeated by a network of
capillaries (¢f) which form a close mesh between the urinary
tubules, so that the cavity of the tubule is separated from
the blood only by the thickness of the gland-cells and of the
capillary wall. The capillary network is supplied partly by
the renal arteries (7. a), partly by the renal portal vein (7.27. v),
and is drained by the renal veins (7.v). It is in the
behaviour of the renal arteries that the chief peculiarity of
the kidney-circulation lies. On entering the kidneys they
break up into smaller and smaller arteries, but each of the
ultimate branches (af v), instead of discharging into the
general capillary network, passes to a Malpighian capsule, in
the interior of which it breaks up into a little bunch of
coiled capillaries (g/), the glomerulus. From this the blood
is carried off by a minute vessel (ef v) by which it is poured
into the general capillary network and finally discharged
into the renal vein (7. z).

Renal Excretion. While circulating through the glomer-
ulus, water and certain soluble matters are separated from
the blood and make their way into the Malpighian capsule
and thence into the urinary tubule. As the blood circulates
through the general capillary network, the gland-cells of the
tubules excrete, out of the materials brought to them by
the blood, the nitrogenous waste matter zea, in the formation
of which the liver plays an important part ; it is discharged
from the cells into the cavity of the tubule where it is
dissolved in the water separated in the glomerulus. In this
way the wvine is formed. Accumulating in the tubules it
makes its way into the ureter and thence drop by drop into
the cloaca, whence it is either expelled at once, or stored for
a time in the bladder.

L 2

148 THE FROG CHAP.

Note that the formation of urine is a process of secretion
of asimilar nature to the secretion of gastric juice, bile, &c.
The fluid secreted, is, however, of no further use to the
animal, and would, in fact, act asa virulent poison if retained
in the system. It is therefore got rid of as soon as possible.
Secretions of this kind, consisting not of useful but of
harmful or waste matters, are distinguished as excretions.

Bile is also in part an excretion as it contains pigments due to the
disintegration of hemoglobin, and thus by its means the effete colouring
matters of the blood are passed into the intestine and got rid of.

Pulmonary and cutaneous Excretion.—The lungs, be-
sides being organs of respiration, take their share in excretion,
since they get rid of the important waste product, carbon
dioxide, together with a considerable quantity of water.
Similar functions are discharged by the skin, which is also an
organ both of respiration and of excretion. Interchange of
gases take place between the outer air and the blood in the
capillaries of the derm: the carbon dioxide of the non-aérated
blood brought to the skin by the cutaneous artery (p. 93
and Fig. 23) is exchanged for oxygen, and the blood, in the
the aérated condition, is returned by the musculo-cutaneous
vein to the heart. Moreover the cells of the cutaneous
glands separate water and other constituents from the blood,
and the fluid thus formed is poured out on the surface of
the body. Here it serves to keep the skin moist, and is
finally lost, either by evaporation or by mingling with the
water in which the frog is immersed. The cutaneous
secretion has also poisonous properties, and so probably
serves as a defence against some of the animal’s enemies.

Summary of the processes of Nutrition—We are now
in a position to understand the general features of the
whole complicated series of processes which have to do with
the nutrition of the frog, which are collectively spoken of

IX NUTRITION 149

as metabolism. These processes are illustrated in the
diagram (Fig. 23, p. 91), which should be constantly
consulted in connection with the following summary.

All parts of the body are placed in communication with
one another by means of the bloodvessels, through which a
constant stream of blood is flowing in a definite direction.

In all parts waste of substance (destructive metabolism) is
continually going on, and the waste products, water, carbon
dioxide, and some nitrogenous substance which ultimately
takes the form of urea—are passed either directly into the
blood, or first into the lymph and thence into the blood.

At the same time the tissues withdraw nutrient materials
from the blood, whereby the waste of substance is made
good, and the cells and other elements adequately nourished
(constructive metabolism). Oxygen is also withdrawn from
the blood ; like the air supplied to a fire it is essential to the
oxidation or low temperature combustion with which the
waste of the tissues is associated. By the withdrawal of its
oxygen the hemoglobin of the blood alters its colour from
scarlet to purple.

Thus the blood as it passes through the body is constantly
being impoverished by the withdrawal of nutrient matters
and of oxygen and as constantly being fouled by the
discharge into it of waste products. It reaches the capil-
laries of an organ as bright red, aérated blood, and leaves
it as purple, non-aérated blood.

These changes, by which the blood loses nutrient matters
and oxygen, and gains waste products, takes place all over
the body. The converse processes by which nutrient matters
and oxygen are absorbed and waste products got rid of are:
carried on in certain definite portions of the circulatory
system.

In the walls of the enteric canal (Fig. 23, Zz. C), the

150 THE FROG CHAP.

products of digestion pass into the blood (C. £xz¢. C), or in
the case of fats, first into the lacteals and ultimately into the
blood. In this way the due proportion of nitrogenous and
other food materials is kept up.

In passing through the capillaries of the lungs (Cf. Zug)
carbon dioxide is exchanged for oxygen and a certain
amount of water is given off. In the capillaries of the skin
(Cp. Sk), a large quantity of water and smaller proportions
of other waste matters are got rid of. In the kidneys
(Cp. Kd), a considerable quantity of water, together with the
bulk of the urea, are removed from the blood and finally
expelled from the body.

Note that all these changes are produced by the special
activity of particular groups of epithelial cells, which,
however alike they may be in general appearance and
structure, have a marvellous selective faculty peculiar to
themselves. Like all other parts of the body, they are
constantly undergoing the usual processes of waste and
repair, withdrawing nutrient matters and oxygen from the
blood, and passing waste matters into it. But, in addition
to the ordinary processes of nutrition, each particular group
of cellshas the power of withdrawing a specific substance
from the blood or of passing substances into it. Thus the
epithelial cells of thé enteric canal (Zp. yn) pass in
digested food, those of the skin (Z/. S%) and glomeruli
(gim) withdraw water, those of the urinary tubules
(Zp. Ur. T), urea, and so on. Similarly, the various gland-
cells, such as those of the liver (Zur. C), pancreas (Px. C),
gastric, and cutaneous (Cz. G/) glands withdraw specific
substances, or excretions, which are discharged on the free
surface of the epithelium and serve various purposes.

We see that the blood loses—(r), nutrient matters and
oxygen all over the body; (2), water in the skin, lungs, and

IX NUTRITION 151

kidneys ; (3), carbon dioxide in the lungs and skin ; (4)"urea,
principally in the kidneys ; and (5), various substances in
the glands. It gains (1), waste products all over the body ;
(2), nutrient matters in the enteric canal ; (3), liver-sugar in
the liver ; (4), oxygen in the lungs and skin. _ It is therefore
richest in oxygen and poorest in carbon dioxide as it leaves
the lungs and skin, Ze, in the pulmonary aud musculo-
cutaneous veins; richest in nutriment as it leaves the
enteric canal, 7.¢., in the portal vein; poorest in urea as it
leaves the kidneys, z.e., in the renal veins ; poorest in water
as it leaves the skin and kidneys, #.2., in the cutaneous and
renal veins.

In this way a single closed system of pipes not only
supplies all parts of the body with everything necessary for
their sustenance, but serves also as a drainage system to
carry away their various waste products.

Notice that we must distinguish between the nutrition,
respiration, and excretion of the frog as a whole, and of its
various parts. Every one of the thousands of cells, fibres,
&c., in the entire body is nourished, breathes, and excretes,
taking its nourishment and oxygen directly from the blood,
and discharging its waste products into it. What are
called the organs of nutrition and respiration are special
portions of the body set apart for taking in fresh supplies of
food or of oxygen for the organism as a whole, such supplies
being finally distributed by the blood-system. Similarly,
what are called the organs of excretion are special portions
of the body by which the waste products, collected by the
blood from all parts of the organism, are finally discharged.

Evolution of Heat.—The oxidation of the tissues, like
that of coal or wood in a fire, is accompanied by a rise in
temperature. But in the frog, as in other cold-blooded
animals, the evolution of heat is never sufficient to raise the

152 THE FROG CHAP.

temperature of the body more than very slightly above
that of the surrounding medium. In warm-blooded animals,
such as ourselves, the temperature is regulated, according
to the season, by a greater or less evaporation of water
from the surface of the body. In the frog this is not
the case. “The temperature of the animal is always
nearly the same as that of the air or water in which it
lives, and hence the frosts of winter would be fatal to it,
but for the habit of hibernation (p. 8).

Death and Decomposition—The decomposition under-
gone by a dead frog (p. 11), may be looked upon as an ex-
cessive process of waste unaccompanied by repair. Owing
to the action of certain microscopic plants known as
Bacteria, which will be referred to again in Part IL, the
proteids undergo oxidation, amongst the principal products
of which are water, carbon dioxide, ammonia, and certain
gases of evil odour, such as sulphuretted hydrogen and
ammonium sulphide. The gases escape into the air, while
the ammonia is finally converted into nitrous and nitric
acids. These, combining with certain substances in the
soil, give rise to salts called nitrates and nitrites, which
furnish one of the chief sources of the food of plants.

PRACTICAL DIRECTIONS.

The Organs of Respiration and of Voice.—Pin out a frog in
the usual way (pp. 31 and 32), remove the heart, and make out the
precise relations of the lungs, first distending them with air through the
glottis. The specimen already used for the dissection of the vascular
system or alimentary canal will serve the purpose. Harden thoroughly
in spirit and note (Fig. 44) the Zaryzgo-tracheal chamber, which communi-
cates with the pharynx through the glottis on the one hand, and on the
other with both lungs. Observe also the Zostertor horns of the hyoid
which embrace the glottis, and then separate them from the laryngo-

PRACTICAL DIRECTIONS 153

tracheal chamber, so as to remove the latter, together with the lungs,
from the body.

Then dissect off what remains of the mucous membrane of the pharynx
around the glottis, and notice the small /aryngeal muscles in connection
with the laryngo-tracheal chamber: remove these, and pin the
respiratory organs down under water, ventral surface uppermost, by
means of a pin through each lung. Cut away the ventral wall of one
lung, so as to expose the cavity and its connection with the laryngo-
tracheal chamber. (You will very probably find some sarasztes in the
lungs—small worms called Ascarzs nigrovenosa, belonging to the group
of Nematode worms.) Note :—

1. The two arytenoid cartilages, and a ring-shaped cartilage sur-
rounding the base of the lungs.

2. The xetwork of ridges on the inner surface of the lungs. Examine
with a lens. Sketch.

3. The vocal cords. Observe these first in their natural position,
and then with the scissors cut through the laryngo-tracheal chamber
along the line of the glottis so as to divide it into right and left halves
and thus expose the vocal cords from their surface. Sketch.

The Kidneys.—a. Examine these organs ¢# sééz (Figs. 3, 4, and 7)
and note :—

1. Their form and position, and the relations of the peritoneum,
which covers them on the ventral side only. (See Fig. 5).

2. The wreters (their openings into the cloaca may be scen at a later
stage).

3. The yellowish adrenals.

Sketch.

4, Examine under the microscope a transverse section of the kidney,
prepared as directed on p. 136, and make out (Fig. 46) :—

1, The wrénary tubules, cut through in various planes.

2. The Malpighian capsules and their glomerule.

3. Blood-capillaries and vessels.

Sketch a portion under the high power. Compare with a section ot
kidney in which the blood-vessels have been injected with coloured
gelatine (p. 101).

CHAPTER X.
THE FROG (continued): THE NERVOUS SYSTEM,

In a machine of human construction, such as the engine of
a steamer, the proper working of the whole depends, pro-
vided the parts of the machine itself are in good order, upon
two things—the stoking or regulation of the fires, and the
turning of certain cocks and levers by the engineer. In that
very complex machine the frog, we have already studied
what corresponds to stoking, viz., feeding and breathing.
We must now direct our attention to what may be considered
roughly to correspond with the work of the engineer—the
means by which the whole complex machinery is kept under
control, and its various parts made to work together toa
common end.

How does it come about, for instance, that the various
digestive glands begin to secrete actively as soon as food is
taken into the enteric canal? How is it that a touch on any
part of the body, or even the sight of an enemy, is followed
instantaneously by a series of vigorous muscular movements
so ordered as to facilitate escape from the source of danger ?

In the fourth chapter (p. 62) we got so far as to learn
that muscular contractions are induced by xervous impulses
travelling from the brain or spinal cord, along the nerves, to
to the muscles. It may therefore be inferred that the con-

cH. X SPINAL CORD 1se

trolling apparatus, by which the functions of the body are
regulated, is lodged in either the brain or the spinal cord, or
both.

Divisions of the Nervous System.—The nervous system

‘is divisible into (1) the central nervous system, consisting of
the brain and spinal cord (Figs. 6 and 7), and (2) the peri-
pheral nervous system, consisting of the nerves which pass
from the central nervous system to the various parts of the
body. The nerves are divisible into (1) cerebral nerves
(Fig. 53), arising from the brain, and (2) spinal nerves
(Fig. 51) arising from the spinal cord.

The Spinal Cord.—In form the spinal cord (Figs. 6 and
7, sp. ed) is irregularly cylindrical. It is continuous in front
with the brain, and tapers off posteriorily into a fine thread-
like portion, the flum terminale (ft), while opposite the
fore-limbs, and just anteriorly to the filum terminale, it pre-
sents a couple of enlargements, known respectively as the
brachial and sciatic swellings. “Along its dorsal surface runs
a delicate longitudinal line, the dorsal fissure (Fig. 48, af),
and a distinct groove, the ventral fissure (uf), extends along
its lower surface.

The cord is covered with a delicate pigmented mem-
brane known as the fa mater (p.w) and the neural canal in
which it lies is lined with a stout, tough membrane, the dura
mater (Fig. 52, d.m). Between the two is a cavity filled with
a lymphatic arachnoid fluid, which, like the pericardial fluid,
preserves the contained organ from shocks.

Examination of a transverse section of the cord under a
low power will show that the dorsal fissure is an extremely
narrow vertical wall formed by an extension inwards of the
pia mater. The ventral fissure is a distinct cleft. Thus the
two fissures divide the cord into paired half cylinders, right
and left, joined in the middle by a narrow bridge. This

156 THE FROG CHAP,

bridge is traversed from end to end by a very narrow longi-
tudinal canal, the central canal (c.c), so that the cord is not
a solid cylinder, but a tube with an extremely narrow cavity
and excessively thick walls.

The section also shows that the cord is not homogeneous,
but is composed of two different substances. Its outer part
is pure white and shining in the fresh cord, and is hence

Fic. 48.—Transverse section of spinal cord of frog.
c¢.¢. central canal; ¢./ dorsal fissure ; d. 2. dorsal horn of grey matter‘ d. 7. fibres
of dorsal root of spinal nerve ; v. c. nerve cells of dorsal horn; #zv.c’. nerve cells
of ventral horn; 4. w. pia mater; v.f. ventral fissure ; v. 2. ventral horn of grey
re - 7. fibres of ventral root of spinal nerve; 2.7. white matter, (After
owes.

called the white matter (w.m). Its internal substance has
a pinkish colour when fresh, and is called the grey matter
(d.h, v.h). The grey matter has a squarish outline in trans-
verse section. It surrounds the central canal, and is con-
tinued upwards and downwards, forming what are called the
dorsal (a.h) and ventral (v.h) horns of the grey matter.

The Brain.—In front the spinal cord passes insensibly
into the brain (Fig. 49), which is of somewhat greater
diameter than the cord, and is made up of several very dis-

x BRAIN 187

tinct parts or divisions. The hindermost division is called
the Ju/b, or medulla oblongata (Med. ob!) ; this appears to be
simply a widening of the spinal cord (Sf. cd), except that
on its dorsal surface is a triangular body (D, ch. pix”) of a
reddish colour in the fresh condition, and called the Zos-
terior choroid plexus: it is simply a thickening of the pia
mater containing abundant blood-vessels.

The choroid plexus forms a kind of lid to a triangular
cavity (A and D, v*) excavated in the dorsal region of the
medulla oblongata, and called the fourth ventricle. The apex
of the cavity, which is directed backwards, opens into the
central canal of the spinal cord (Fig. 50, v4, ¢c), and the
the fourth ventricle is to be looked upon simply as the
anterior part of the central canal which has become
widened out and is covered only by a thickened portion
of the pia mater.

The fourth ventricle is bounded in front by a narrow
ledge of nervous matter (Fig. 49, C2), which would be hardly
worthy of being considered as a special division of the
brain but for the fact that the corresponding part in many
animals—e.g., dogfish, rabbit, man—is a large and important
structure. It is called the cerebellum.

In front of the cerebellum comes a pair of rounded
elevations, the optic lobes (Figs. 49 and 50, Of¢. 7). Each
contains a cavity, the optic ventricle (Fig. 50, Opt. v),
communicating with a narrow median passage, the fer
(Figs. 49 D and 50, 2), which is continuous behind with the
fourth ventricle. The bulb is continued forwards beneath
the optic lobes as the crura cerebri (Cr. C).

In front of the optic lobes is an unpaired structure, the
diencephalon or ’tween brain (Di). On its upper surface is
a small rounded vascular body, the anterior choroid plexus
(Fig. 49 A, D, ch. pix), formed, like the posterior choroid

opich in

CerH forM Opt CEL ny eh ple
Pm LEM pin Com 7 y

pa | fe

Fic. 49.—Brain of frog. A, from above ; B, from below; C, from the side; D, in
longitudinal vertical section.

Cb, cerebellum ; Cer. H/. cerebral hemispheres ; ch. fZx.1 anterior and ch. pix?

posterior choroid plexus ; com. transverse bands of nerve fibres or commissures

a
a
“

BRAIN 159

connecting the left and right halves of the brain; Cx.C. crura cerebri; D7.
diencephalon ; for. AZ. foramen of Monro ; 7. iter, or aqueduct or Sylvius ; inf.
infundibulum ; AZed. 067. medulla oblongata ; OU 2. olfactory lobe; off. ch.
optic chiasma; Of7."2. optic lobe ; eft.7. optic ventricle ; Ain. stalk of pineal
body , fi#. pituitary body ; Sf. cd. spinal cord ; v7. third ventricle ; 74. fourth

ventricle ; I—X, cerebral nerves; 7. S/. 2S/., first and second spinal nerves ;

(A—C, after Gaupp ; D, from Wiedersheim, after Osborn).

plexus, of a thickening of pia mater, containing numerous
blood-vessels. It helps to roof over a narrow slit-like
cavity, the <¢hird ventricle
(Figs. 49 D and 50, v°), the
sides of which are formed by
thickenings of nervous matter,
the optic thalamt (Dr). On
the ventral surface of the brain Lat
the diencephalon is continued
into a funnel-like extension,
the znfundibulum (Fig. 49, 77/),
to which is attached a rounded
structure, the prtuttary body
(pit). On the dorsal surface,
just behind the choroid plexus,
is the delicate stalk (f7) of
the pineal body—the vestige of

Med. ob!

c.e

a sensory apparatus (Fig. 148 = Fic. s0.—Diagrammatic horizontal

3 é section of frog’s brain.
C), part of which in some .c. central canal ; Cer. /7. cerebral

hemisphere ; Dz. diencephalon ;

s

lizards, for example, has the for. M. foramen of Monro 7. iter}
7 Lat.v. lateral ventricle ; Med.

structure of an eye, and which obi. medulla oblongata; Wr. 7,
: . olfactory nerve ; O/f L. olfactory

was probably functional in the lobe ; Odf v. olfactory ventricle ;
Opt. 2. optic lobe; Of2. v. optic

ancestors of the frog. We ventricle ; SZ. cd. spinal cord ;
: v. 3, third ventricle; 7. 4, fourth

shall meet with other examples ventricle. (After Ecker and Wie-

dersheim.)

of such vestigial organs in the
course of our studies.

In front of the ’tween-brain comes a pair of long, oval
bodies, wider behind and narrower in front. These are the
cerebral hemispheres (Cer. H). Each contains a cavity, the

160 THE FROG CHAP,

lateral ventricle (Fig. 50, Lat. v), which communicates with
the third ventricle by a small aperture, the foramen of Monro
(for. M).

Lastly, each cerebral hemisphere is continued forwards by
a rounded offactory lobe ( Olf. 1), which is fused with its fellow
of the opposite side, the single mass lying in the posterior
compartment of the girdle bone. The lateral ventricles are
continued forwards into the olfactory lobes, forming the
small olfactory ventricles (Fig. 50, O/f. v).

The brain, like the spinal cord, is formed of grey and
white matter, but their relations are different. In the olfac-
tory lobes, cerebral hemispheres, and ’tween-brain the white
matter is internal, and the grey forms a thin outer layer or
cortex. In the optic lobes and medulla the grey matter is
mainly around the ventricles, and the white matter more
external.

Like the spinal cord, the whole brain is covered with pia
mater, densely pigmented in the region of the optic lobes,
and the cranial cavity in which it is contained is lined with
dura mater.

The Spinal Nerves.—The spinal nerves arise symmetri-
cally from the spinal cord on the two sides of the body, and
pass out at the neural canal through the intervertebral
foramina (p. 38).

There are altogether ten pairs of spinal nerves in the adult
frog (Fig. 51, —X’), each of which on leaving the neural
canal divides into a smaller dorsal and a larger ventral
branch (Fig. 53, 1 ss—-5 sf). The first pair leave the cord
through the intervertebral foramina between the first and
second vertebra. They pass at first directly outwards, the
large ventral branch of each, known as the Ayfoglossal,
turning forwards, and going to the muscles of the tongue
(Fig. 51, Z, Fig. 53, 1 Sf).

The second pair (Fig. 51, Z/) are very large ; they emerge

x SPINAL NERVES 161

between the second and
third vertebre, and each
is soon joined by the small
third nerve (777) which
emerges between the third
and fourth vertebree, as well
as by a small branch or two
from the first, thus forming
a simple network or plexus
—the brachial plexus (br.
p4), from which pass off
nerves to the fore-limb,
supplying both skin and
muscles.

The fourth, fifth, and
sixth nerves take a very
similar course. The fourth
(ZV) emerges between the
fourth and fifth vertebra,
the fifth (V) between the
fifth and sixth, and the
sixth ( VZ) between the sixth
and seventh. They all pass
obliquely backwards, and
supply the walls of the
body, being distributed to
both skin and muscles.

The seventh, eighth, and
ninth nerves supply the
muscles and skin of the
hind limbs. The seventh
(VIZ) leaves the neural

canal between the seventh
PRACT. ZOOL.

i | lst
sez

Fic. 51.—The ventral branches of the

spinal nerves and the sympathetic of
the frog, from below: shown on the
right side only.

J—X, spinal nerves ; Ao. systemic arch ;
br. pl. brachial plexus ; C. calcareous
bodies which surround the spinal
ganglia; D. Ao. dorsal aorta; fem.
femoral nerve ;-//. A. iliac artery ; sez.
sciatic nerve; scz. 2. sciatic plexus;
Sk. skull; Sf. A. splanchnic artery ;
Sy. sympathetic cord ; Sy. ¢. commun.-
cating branches between the sympa-
thetic and spinal nerves; Sy. g.
sympathetic ganglia; Us?¢. urostyle ;
V-—V%, centra of vertebrae ; Vg. vagus
nerve, with its ganglion. (After
Gaupp, slightly modified.)

M

162 THE FROG CHAP.

and eighth vertebra, the eighth (VZ/Z) between the eighth
and ninth, and the ninth (ZX) between the ninth vertebra
and the urostyle. They all pass almost directly backwards,
and are united with one another by oblique cross branches
so as to form the setatic plexus (sci. pl), from which are
given off, amongst others, two nerves to the leg, the largest
of which, the seéatic nerve (Sc‘) being that already mentioned
in the chapter on the muscular system.

The tenth (X) is a very small nerve. It emerges through
a small aperture inthe side of the urostyle, and supplies
the cloaca, urinary bladder, and adjacent parts. It is con-
nected by cross branches with the ninth.

It will be noticed that while the large ventral part of the
first spinal nerve—the hypoglossal—supplies muscles only,
and is therefore a motor nerve, all the others go to both
muscles and skin, and are therefore both motor and sensory,
or mixed nerves. They all branch out in a complex manner,
and are traceable to the remotest parts of the body.

The Sympathetic Nerves.—On either side of the dorsal
aorta is a very delicate nerve, having at intervals little swell-
lings called ganglia, each of which is connected with a spinal
nerve by acommunicating branch (Figs. 51 and 53 Sy, Sy.g,
Sy.c). In front of the point where the dorsal aorta (D. Ao)
is formed by the union of the two systemic trunks (4o),
these two sympathetic nerves, as they are called, are con-
tinued forward, one on either side of the vertebral column,
towards the head, when they enter the skull and become
connected with certain of the cerebral nerves.

Each sympathetic nerve has altogether nine or ten ganglia,
each connected with one of the spinal nerves, and from
the ganglia branches are given off which supply the heart
and blood-vessels, the stomach, liver, kidneys, reproductive
organs, rectum, and urinary bladder.

x SPINAL AND CEREBRAL NERVES 163

Origin of the Spinal Nerves.—The mode of origin of the
nerves from the spinal cord is peculiar and characteristic,
Traced towards the cord, each nerve is found, on reaching
the intervertebral foramen from which it emerges, to divide
into two—a dorsal root which springs from the dorsal, and
a ventral root which arises from the ventral region of the
cord (Fig. 52, dx, vr) The dorsal root is distinguished
from the ventral by being dilated into a ganglion (gn). In
Fig. 51 these ganglia lie hidden within certain calcareous
bodies (C) in this region.

Fic. 52.—Transverse section through the vertebral column and spinal cord, to show
the mode of origin of the spinal nerves.

c.c. central canal; c#. centrum; @./. dorsal fissure ; d. 7.!dura mater ; d. 7. dorsal
root ; g. #. grey matter ; g#. ganglion of dorsal root; ~. a. neural arch ; 2. sf.
neural spine ; f. 7. pia mater (the reference line should stop at the margin of
the cord); # nerve trunk; 7». fy. transverse process; v. “ ventral fissure ;
v.r. ventral root ; w.7z. white matter. (After Howes.)

Cerebral Nerves.—There are ten pairs of cerebral nerves
some of which are purely sensory, some purely motor, some
mixed.

The first or olfactory nerves (Fig. 49, /) arise from the
olfactory lobes, and pass through the holes in the trans-
verse partition of the girdle-bone. Each is distributed to
the mucous membrane of the nasal sac or organ of smell
of the same side, and is purely sensory.

The second or optic (Fig. 49 2/7) is a large nerve which
springs from the ventral surface of the ’tween-brain. At

M 2

164. THE FROG CHAP.

their origin the right and left optic nerves have their
fibres intermingled, forming a structure something like a
St. Andrew’s Cross and called the optic chiasma (oft. ch),
the other limbs of the cross passing upwards and _ back-
wards to the optic lobes. The optic nerve makes its
exit from the brain-case through the optic foramen, and is
distributed to the retina, a delicate membrane which
lines the eyeball, and is, as we shall see, the actual organ
of sight. This nerve also is purely sensory.

The third or oculomotor (Fig. 49, £77) is a small nerve
arising from the crura cerebri beneath the optic lobes. It
passes through a small hole in the cartilaginous side of
the skull and supplies four out of the six muscles by
which the eyeball is moved, and is purely motor.

The fourth or pathetic (IV) is a very small nerve leaving
the dorsal surface of the brain between the optic lobes and
the cerebellum, and making its exit from the skull just above
the optic nerve. It is also purely motor, supplying one of
the muscles of the eye—the superior oblique.

The fifth or trigeminal (Figs. 49 and 53, /) is a large nerve
arising from the side of the medulla oblongata. Its root
dilates to form a large ganglion, the Gasserian ganglion,
and leaves the skull by the large aperture noticed in the
pro-otic bone. It owes its name to the fact that it soon
divides into three main branches, one, the ophthalmic (Fig.
53, +), going to the skin of the snout, another, the maxillary
(V*) to the upper lip and lower eyelid, and the third or
mandibular (V*) to the muscles and skin of the lower jaw.
The trigeminal is a mixed nerve.

The sixth or abducent (Fig. 49, V7) is a very small motor
nerve arising from the ventral aspect of the bulb and
supplying one of the muscles of the eyeball called the
posterior rectus.

x CEREBRAL NERVES 165

The seventh or facial nerve (Figs. 49 and 53, VZZ) arises
just behind the fifth and soon joins the Gasserian ganglion.
Both it and the sixth leave the skull by the same aperture

Fic. 53.—Dissection of the head and anterior part of the body of the frog from the
deft side, to show the distribution of the fifth, seventh, ninth and tenth cerebral
nerves, as well as of the hypoglossal and part of the sympathetic.

Ao. systemic arch ; 67.42. brachial plexus; D, Ao. dorsal aorta; dz. duodenum ;
Hi. heart ; Hy. body of hyoid ; A’y1. anterior, and Hy?. posterior horns of hyoid ;
L. lung ; W. olfactory sac ; Ox. orbit ; PZ. pulmonary artery ; Sf. A. splanchnic
artery; St. stomach ; Sy. sympathetic ; //. cut end of optic nerve ; V1. ophthal-
mic, V2, maxillary, and 3. mandibular branch of trigeminal; V//1. palatine,
and V/J2, hyomandibular branch of facial; “XY. glossopharyngeal ; . vagus ;
Xed. cardiac, Xgas. gastric, X/ar. laryngeal, and Xfx/. pulmonary branch of
vagus ; 7 sf. first spinal nerve (hypoglossal); 25f.—5 sf. second to fifth spinal
nerves. (After Howes, slightly modified).

as the fifth. It divides into two branches, one of which,
the palatine (Fig. 53, VZZ') supplies the mucous membrane
of the roof of the mouth, and the other, or Ayomandibular
(Vil) sends a branch to the skin and muscles of the

166 THE FROG CHAP,

lower jaw and to the muscles of the hyoid. It is a mixed
nerve.

The eighth or auditory nerve (Figs. 10 and 49, VZZZ)
arises from the medulla just behind the seventh, passes
through an aperture in the inner wall of the auditory cap-
sule, and is distributed to the auditory organ or membranous
labyrinth (see Chapter XI). It is the nerve of hearing,
and is purely sensory.

The ninth or glossopharyngeal (Figs. 49 and 53, £X ) arises
behind the auditory nerve. It sends a branch to join the
facial and supplies the mucous membrane of the tongue and
pharynx as well as certain small muscles connected with the
hyoid. It is also a mixed nerve.

The zenth or vagus (Figs. 49 and 53, X) is a large nerve
arising in common with the ninth, and dilating, shortly after
leaving the skull, into a vagus ganglion. It supplies the
larynx (X/ar), the heart (Xcd@), the lungs (X gu/), and the
stomach (Xgas), and is therefore often known as the pxeumo-
gastric. It has thus an extraordinarily wide distribution, being
in fact the only cerebral nerve which supplies parts beyond
the head. It is a mixed nerve, and contains many motor
fibres, but its branches—some of which have to do with the
regulation of the heart’s contraction and with respiration—
are better described as efferent and afferent than as motor
and sensory: the meaning of these terms will be explained
later on. The ninth and tenth nerves leave the skull close
together through the aperture noticed in the exoccipital bone.

The sympathetic nerve (Sy) extends forwards from its junction with
the first spinal nerve, joins the vagus, and finally ends anteriorly in the
Gasserian ganglion.

Microscopic Structure of Nervous Tissue.—Examination
of a piece of nerve under the microscope shows it to be

x NERVE-FIBRES AND CELLS 167

composed, like striped muscle, of cylindrical fibres, bound
together by connective tissue. The latter is much more
abundant than in muscle, and in particular forms a thick
sheath round the nerve which must be torn off before the
nervejibres are reached.

Each fibre (Fig. 54, A) is a cylindrical cord in which
three parts can be distinguished. Running along the axis
of the fibre is a delicate protoplasmic strand, the xeuraxds
or axisjibre (mx). Around this is a sheath formed of a
fatty substance and known as the medullary sheath (m.s) ;+
and finally, investing the whole fibre is a delicate, structure-
less membrane, the xeurilemma (ne). At intervals the
medullary sheath is absent, and a xode is produced, where
the fibre consists simply of the neuraxis covered by the neuri-
lemma. Directly beneath the neurilemma nuclei are found
at intervals.

In the ganglia are found, not only nerve-fibres, but
nerve-cells (Fig. 54): these are cells of a relatively large
size, each with a large nucleus and nucleolus. In the
spinal ganglia (B) the cell-body is produced into two pro-
cesses, which may be united at their base. One of these
processes is continuous with the neuraxis of a nerve-fibre ;
the other is also a protoplasmic process which passes into
the spinal cord and sends off branches, each branch finally
ending in a complicated branch-work or arborisation, which
is interlaced, but not actually continuous with, a similar
arborisation arising from a nerve-cell in the spinal cord or
brain (Fig. 55).

The white matter of the brain and spinal cord consists of
nerve-fibres, those in the cord having a longitudinal direction ;
the grey matter contains numerous much-branched (“ multi-

1 The medullary sheath may be absent in certain nerve-fibres (¢.2., in
the sympathetic and olfactory nerves.)

168 THE FROG CHAP.

polar ”) nerve-cells (Figs. 48 and 54, A), each continuous
with a neuraxis and enclosed in a tissue formed partly

Fic. 54.—A, nerve-cell from the grey matter of the spinal cord of a frog, and the
nerve-fibre arising from it; B, cell from the ganglion of a dorsal root.
ne. neurilemma; mz. nucleus; x. neuraxis; 7. s. medullary sheath. (After
Howes.)

of the axis-fibres of nerves which enter the grey from the
white matter, losing their medullary sheath as they do so,

x REFLEX ACTION 169

and partly of a delicate fibro-cellular tissue called xeurogia,
in which the other elements are imbedded.

Functions of the Nervous System: Reflex Action.—In
the fourth chapter you learned that a muscle may be
made to contract by a stimulus applied either to the muscle
itself or to its nerve. You are now in a position to pursue
the subject of the control of various parts of the body by
the nervous system a little further.

A frog is first chloroformed, and then either decapitated
or pithed, z.e., the medulla oblongata is severed and the brain
destroyed (p. 103): there can thus be no question either of
sensation or of voluntary action on the frog’s part. It is
then hung up by a hook or string, so that the legs are
allowed to hang freely. If one of the toes is pinched
with the forceps, the foot will be drawn up as if to avoid
the pinch ; or, if some very weak acid be applied to a toe,
the foot will again be withdrawn, being raised every time
it is touched with the acid with the regularity of a machine.
Again, if acid be applied to various parts of the body, the
foot of the same side will immediately try to rub off the .
irritating substance, or if that foot be held down, the other
will come into play.

Movements of this kind are called reflex actions: the
stimulus applied to the skin is transmitted by sensory nerve-
fibres to the spinal cord, where it is, as it were, reflected
in another form, and passed along motor fibres to one or
more muscles, causing them to contract (p. 60).

As already stated, the spinal nerve-trunks are mixed, 7e.,
contain both sensory and motor fibres. It has been found
by numerous experiments that as the nerve approaches
the spinal cord these two sets of fibres separate from one
another, the sensory passing into the cord by the dorsal
root, the motor by the ventral root. As a consequence of

170 THE FROG CHAP,

this, if the dorsal root be cut and its proximal or central
end—z.e., the end in connection with the cord—stimulated,
muscular contraction will follow just as if the stimulus had
been applied to the skin supplied by the nerve in question.
If the other cut end—the distal or peripheral end—be
stimulated, there is no result. On the other hand, if the
ventral root be cut and its distal end stimulated, the

Fic. 55.—Diagram illustrating the paths taken by the nervous impulses.

¢.c. central canal ; cod. collaterals ; c. cor¢. cell in rind or cortex of the cerebral
hemisphere ; c. g. smaller cerebral cell; d.c. cells in dorsal horn of grey matter ;
d.+. dorsal root ; g. ganglion of dorsal root ; g. ¢. cell in ganglion of dorsal root ;
g.m. grey matter; M. muscle; 7. c. cell in medulla oblongata ; #7. motor
fibre ; S. skin; s.£ sensory fibre ; sf. c. spinal cord ; vc. cells in ventral horn
of grey matter; v.7. ventral root ; w.2. white matter. The arrows indicate
the direction of the impulses.

muscles supplied by it will contract, while stimulation of the
proximal end produces no result.

Very accurate observations have shown that the connec-
tion between the motor and sensory fibres is as follows (Fig.
55). A motor fibre (7) is traceable from the nerve-trunk
through the ventral root (z.7) into the white matter; and
then, its medullary sheath being lost, passes into the ventral
horn of the grey matter, its neuraxis being directly con-

x REFLEX ACTION 171

tinuous with the axis-fibre process of one of the large motor
nerve cells (v.c): the remaining processes of these cells simply
branch out in the neuroglia. The sensory fibres (sf) are
traceable into the dorsal root (¢7); in passing through the
ganglion of the root (g) they are found to be continuous
with its simple (“bipolar”) nerve-cells (g.c), and then pass
into the cord. Instead, however, of entering the grey matter
at once, they pass forwards as well as backwards for some
distance, along the white matter of the cord, giving off
numerous branches, or col/aterals (col), which, losing their
medullary sheaths, enter the dorsal horn of the grey matter
and branch out into a complex series of fine fibres which
interlace with the similar arborisations of the nerve-cells
(p. 167).

The path of a nervous impulse will now be obvious. The
stimulus applied to the skin (Fig. 55, S) is conducted by a
sensory fibre to the nerve-trunk and by the dorsal root to
the spinal cord ; it then passes along the white matter of
the cord, enters the grey matter, and is conducted by the
collaterals to the nerve-cells of the ventral horn, either
directly, or after passing through the cells of the dorsal
horn: from one of the cells of the ventral horn it is con-
ducted by an axis-fibre process continuous with the neur-
axis of a nerve-fibre, which, leaving the cord by a ventral
root, passes along the nerve-trunk and finally goes to a
muscle (M) as a motor fibre.

It will be noticed that a single stimulus applied to the
skin may result in the contraction of numerous muscles—as,
e.g., when the application of a drop of acid to the toe causes
the lifting of the leg, and that the movements are of such a
nature as to withdraw the part stimulated from the irritating
substance. Moreover, as shown by the experiment of apply-
ing acid to various parts of the body, the movements are

172 THE FROG CHAP.

varied according to circumstances ; if one leg is prevented
from rubbing off the irritating substance, the other imme-
diately comes into play. Obviously, then, a simple stimulus
reaching the spinal cord may be transmitted to numerous
motor cells of the ventral horn, and through these to
numerous motor nerves the particular nerve affected differing
according to circumstances (compare Fig. 55). The spinal
cord, therefore, is able, in response to a stimulus reaching it
by a sensory nerve, to originate motor impulses causing
complex muscular movements so adjusted as to serve
definite purposes. Without such external stimulus, how-
ever, the spinal cord of a brainless frog is quite inactive, and
the body of the animal will remain without movement until
it dries up or decomposes.

In the uninjured frog, z.e., the frog with its brain intact, the
case is very different. The animal no longer acts like an
unintelligent machine, each stimulus producing certain in-
evitable movements and no others; but a single stimulus
may produce varied movements, the nature and direction of
which cannot be predicted ; the animal will probably give a
series of leaps, but the number and extent of these varies
according to circumstances.

This is explained by the fact that certain nerve-fibres
of the cord pass forwards to the brain, and that the nerve-
cells in the grey matter of the cord are in communication—
owing to the interlacing of their branching processes with
those of the collaterals—with similar cells in the grey matter
of the brain (Fig. 55, .c, ¢.g, ccort). In certain of these
brain-cells (¢.cor¢), voluntary impulses originate and exercise
a controlling effect upon the cells of the spinal cord, so that
these latter do not constitute, as in the brainless frog, a
machine every movement of which can be accurately pre-
dicted.

x REFLEX AND VOLUNTARY ACTION 173

Moreover, it can be shown by experiment that the process
of originating voluntary impulses is not performed by the
whole brain, but is confined to the cerebral hemispheres.
If the.hemispheres and optic lobes are removed so as to
leave nothing but the bulb and cerebellum, the frog no
longer lies in any position in which it may be placed, ex-
hibiting no movements beyond the beating of the heart, as
is the case when the whole brain is removed. It sits up in
the ordinary attitude, breathes, swallows food placed in the
mouth—while making no attempt to feed itself, turns over
and sits up if placed on its back, and swims if placed in
water. , If left alone, however, it remains in the sitting
posture until it dies. Hence the bulb and cerebellum
are evidently concerned with the co-ordination of muscular
movements, but have no power of originating impulses. If
the optic lobes as well as the medulla oblongata and cere-
bellum are left, the animal is affected by light,.is able to per-
form complex balancing movements, and will even croak
when stroked in a particular way. There is still, however,
no voluntary action ; without the application of stimuli, the
animal sits motionless until it dies.

To sum up in the language of the illustration with which
this chapter was begun, comparing the frog with an engine
of human construction :—the grey matter of the brain may
be compared with the engineer; much of the work of the
engine may go on without him—certain levers, valves, &c.,
acting automatically ; but it is only by his controlling intel-
ligence that the whole mechanism is adapted to the circum-
stances of the moment.

So far, we have considered the nervous system only in its
relations to the skin or general surface of the body and to
the muscles or organs of movement. The other parts of the
body are, however, under nervous control.

174 THE FROG CHAP.

It has been mentioned that the heart continues to beat in
a frog when the brain has been removed: not only so, but
it pulsates with perfect regularity when removed from the
body. This is due to the fact that the muscles of the
heart, like the cilia of ciliated epithelium, have the power
of contracting rhythmically quite independently of the
nervous system, although the heart contains nerve-cells
which were formerly supposed to serve as a_ special
nervous system for this organ, originating all its motor
impulses. It is, however, under the control of the central
nervous system. We have seen that it is supplied by a
branch of the vagus; when this is stimulated, the heart stops
in the dilated state and begin to beat again only after a
certain interval. A feebler stimulus to the vagus will not
actually stop the heart, but will diminish the rate and the
strength of its contractions and consequently the amount of
blood propelled through the body. The vagus is accom-
panied by a branch of the sympathetic which has an exactly
opposite effect ; z.e., stimulation of it accelerates the heart’s
action. In this way, the general blood supply of the body
is regulated by the central nervous system.

The blood supply of the various parts and organs is
regulated by the vaso-motor nerves. These are traceable
through the sympathetic into the spinal cord by the ventral
roots: distally they send branches to the muscular coat of
the arteries. Under ordinary circumstances, a constant
succession of gentle stimuli pass along these from a group of
nerve-cells in the medulla oblongata, and as a result, the
arteries are ordinarily in a state of slight contraction. By
various circumstances these stimuli may be diminished for any
given artery and at the same time stimuli pass down another
kind of vaso-motor fibres: the artery will then dilate and
the blood supply of the organ to which it is distributed will

x AFFERENT AND EFFERENT NERVES 175

be temporarily increased. For instance, the presence of
food in the stomach acts, through the central nervous
system, upon the cceliac branch of the splanchnic artery,
causing a dilatation of all its capillaries and promoting an
increased secretion of gastric juice. The secretion of other
glands is regulated in the same way. In some cases, how-
ever, it has been proved that the nerves of a gland do not
act simply by producing dilatation of the capillaries, but have
a direct effect upon the gland-cells, causing an increased
secretion.

You will thus note that there are nerve-fibres carrying
impulses to the central nervous system which have nothing
to do with sensation, and fibres carrying impulses from the
central nervous system which have nothing to do with
motion, but result in increased secretion or in stoppage of
motion. It is therefore best to use the term afferent (which
includes sensory) for a nerve carrying an impulse to the
brain and spinal cord, and efféren¢ (including motor) for
one carrying an impulse in the other direction,

PRACTICAL DIRECTIONS.

I. The central nervous system (Fig. 6). Lay bare the brain
and spinal cord as directed on p. 33, noting the dura mater and pia
mater: the latter is densely pigmented over parts of the brain. The
specimen in which this operation has already been performed will
do, if the dissection has been done carefully.

Observe the origins of the cerebral and spinal nerves, noting the
long dorsal and ventral roots of the latter (compare Fig. 52) which pass
backwards for some distance before making their exit from the neural
canal; and also the gazgééa on the dorsal roots, lying just outside the
canal and each hidden in a whitish calcareous body in this region
(Fig. 51, C). (The ganglia, however, can be more easily made out ata

176 THE FROG CHAP.

later stage). Then sever the nerves very carefully from the brain and
spinal cord and remove the whole central nervous system from the neural
canal: it is best examined after hardening in formaline or spirit. Lay
it in a dissecting dish, under water, and make out its several parts as
follows :—

a. The spinal cord,

1. Note its cylindrical form, the drachial and scéatic swellings, the
filum terminale, and the dorsal and ventral fissures.

2, Examine a transverse section of the spinal cord, prepared as
described on p. 136, under the low power of the microscope, and make
out the dorsal and ventral fissures, the central canal, and the relations
of the grey and whzte matter (Figs. 48 and 52). Sketch.

b. The brain (Fig. 49).

Beginning from the posterior end of the brain, where it passes into
the spinal cord, make out its several divisions as follows :—

1. The 42/6 or medulla oblongata, with the posterior choroid plexus
on its dorsal side : remove the latter, and notice that it roofs over the
cavity of the fourth ventricle.

2. The small ledge-like cerebellum.

3. The two rounded oftzc dobes, and the crura cerebrz beneath
them. :

4. The diencephalon, formed of a right and left optic thalamus. On
its dorsal side is the anterior choroid plexus, roofing in the third
ventricle; and on its ventral side the zzfundzbulum, to which the
pituitary body is attached ; and, more anteriorly, the optic chiasma.

5. The cerebral hemispheres, continuous in front, with

6. The olfactory lobes, which are fused together in the middle line.

Sketch the whole central nervous system from above, and also the
brain from below and from the side.

With the small scissors or a sharp scalpel, snip off a small piece of
the wall of the hemisphere and optic lobe of one side—say the left, so
as to expose the Jateral ventricle and the optic ventricle (Fig. 50). Then
with a sharp scalpel divide the whole brain into two by a longitudinal
vertical cut very slightly to the left of the middle line, so as to reduce
it toa longitudinal section (Fig. 49, D). Examine the cut surface of
the right side under water, and make out as much as possible of the
relations of the ventricles of the brain :—viz., the fourth ventricle, the
iter and optic ventricle, and the third ventricle, which communicates
with the /ateral ventricle through the foramen of Monro. Sketch.

x PRACTICAL DIRECTIONS 177

Il. The Peripheral Nervous System.

a. The spinal nerves.

Fasten out a frog with the ventral side uppermost, and remove the
heart, enteric canal, reproductive organs, kidneys, and lungs with great
care, leaving behind the systemic trunk and dorsal aorta (Fig. 51). (One
of the specimens already dissected will probably serve the purpose if
the previous directions have been accurately followed.) Note the speza/
nerves passing outwards from the vertebral column on either side, and
the calcareous bodies close to their points of exit, covering up the
ganglia of the dorsal roots (p. 163). If the centra of the vertebre
are removed, the nerve-roots and their origins from the spinal cord
can be made out: the removal of the centra is rendered easier if the
frog is first decalcified by being placed in 10 per cent. nitric acid for
twenty-four hours and then thoroughly washed in running water.

Confine your attention to the large ventral branches of the ten pairs
of spinal nerves, as described on p. 446.

6. The sympathetic nerves (Figs. 51 and 53.)

Examine the systemic trunk and dorsal aorta carefully with a lens.
Closely connected with it will be seen on either side a sympathetéc nerve-
cord, covered by pigmented connective-tissue. Carefully dissect the
cord away from the aorta, and note the gamgléa and the branches
(rami commundcantes) connecting them with the spinal nerves. Sketch
the spinal nerves and sympathetic.

c. The cerebral nerves (Fig. 53.)

The dissection of these in the frog is not an easy task for a beginner,
and directions for their examination in larger animals will be given in
subsequent chapters. The origin of some of them from the brain, and the
apertures through which certain of them pass out from the skull, have
already been seen.

Ill. The Microscopic Structure of Nervous Tissue.

a. Examine your transverse section of the spinal cord (Fig. 48) under
the high power of the microscope, and observe—

1. The verve-cells, present in the grey matter only (compare Fig. 54,
A). Note their branched form and their nuclei ; the larger motor cells
are seen in the ventral horns of the grey matter. Sketch.

2. The xerve-fibres, in both grey and white matter, cut across trans-
versely as well as in other directions, and each showing a deeply-
stained central zeuraxzs. Sketch.

Pract. Zoo. N

178 THE FROG CH. X

4. Tease up a fresh spinal (Fig. 54, B) or sympathetic ganglion in salt-
solution, and stain with methyl-green. Compare the form of the nerve-
cells with those in the spinal cord. Sketch.

c. Cut off a very small piece of any fresh nerve (e.g., sciatic), and
tease it out carefully, 2 a longitudinal direction, in salt solution. Note
that it is made up of cylindrical, unbranched xerve-jibres, bound together
by connective tissue.

Examine a single fibre under the high power (Fig. 54, A), and make
out the seurdlemma, the medullary sheath, and the modes: at the nodes,
the zeuraxzs can also be seen. Sketch.

Tease out another piece of fresh nerve in chloroform, so as to partially
dissolve the medullary sheath, and note the central xeuraxds. Sketch.

Tease out in glycerine a piece of nerve which has been treated with
a I per cent. solution of osmic acid in water for an hour or two and
then well washed in water, The medullary sheath will appear nearly
black, and the neurilemma, with its underlying zzzc/ez, as well as the
nodes, can be plainly seen. Sketch.

Reflex Action. The experiment described on p. 169 should be
seen.

CHAPTER XI
THE FROG (continued); THE ORGANS OF SPECIAL SENSE

In the previous chapter you have learnt how the nervous
system controls the various functions of the body and how
voluntary action is absolutely dependent upon the con-
nection of the brain, through the spinal cord, with the
nerves. Obviously, in order that the power of voluntary
action should be of full use to its possessor, some means of
communication with the external world is not only desirable
but necessary ; the frog, in order to adjust its actions to the
circumstances in which it from time to time finds itself,
must be able to distinguish friends from enemies, suitable
from unsuitable food, darkness from light, heat from cold.

The avenues of communication between the animal and
its surroundings are, as in ourselves, the senses of touch,
taste, smell, sight, and hearing.

The sense of touch, including that of ‘emperature, is
lodged in the whole extent of the skin, which, as you have
already learnt, is abundantly supplied with sensory nerves.
Many of the nerves terminate in connection with what are
known as ¢actile-cells—large flattened cells arranged in groups
just below the epiderm and around which the ultimate fibres
of the sensory nerves are distributed. Stimuli applied to

the skin, either by direct touch or by the heat of the sun,
N 2

180 THE FROG CHAP.

are transmitted to the tactile-cells and thence through the
sensory nerves to the brain. Notice that the stimulus is
transmitted to the nerve-ends through the epithelial cells of
the skin; if the skin be wounded and a stimulus applied
directly to the tactile-cells or the nerves, the sensation is
one, not of touch, but of pain.

The sense of taste is lodged in the mucous membrane
of the mouth, especially in the tongue and in the neighbour-
hood of the vomerine teeth, but extending also as far back
as the gullet. Certain of the epithelial cells have an
elongated form and are arranged in groups known as
taste-buds, to which the fibres of the ninth and palatine
branch of the seventh cerebral nerves, or nerves of taste, are
distributed ; on the tongue, these taste-buds are situated on
papille of the mucous membrane. In this case the stimulus
is supplied, not by direct touch or by alteration of tempera-
ture, but by the contact of sapid or tasty substances. As
before, the stimulus is applied to epithelial cells, and by
them transmitted to the nerves and so to the brain, when
the sensation of taste becomes manifest. Thus, just as
common sensation may be abolished in any part of the
body in three ways—by destruction of the skin, by cutting
the sensory nerve, or by destroying the cerebral hemispheres
—so the sense of taste is lost if either the mucous mem-
brane of the mouth is injured, or if the glossopharyngeal
and palatine nerves are cut, or if the cerebral hemispheres
are destroyed.

The sense of smell is lodged in the zasal or olfactory sacs,
which are enclosed in the olfactory capsules of the skull and
separated from one another by a partition, the zasa/ septum.
Each sac has two apertures, the external nostril, opening
on the surface of the snout, and the zzternal nostril,
opening into the mouth (p. 17). The sacs are lined by a

XI NOSE AND EYE 181

delicate mucous membrane, some of the epithelial cells of
which are of the ordinary columnar type, while others are
extremely slender and produced into delicate processes at
their free ends. With these latter the fibres of the olfactory
nerve are connected, and they are distinguished as o/factory
cells (Fig. 56). As the epithelial cells of the skin are af-
fected by direct contact or by heat, so
the olfactory cells are affected by the
minute particles given off from odorous
bodies: the contact of these particles
acts as a stimulus, which, being trans-
mitted by the olfactory nerve to the brain,
gives rise to the sense of smell. This
sense can be destroyed, as in the case of
feeling and taste—either by destruction of
the olfactory mucous membrane, or by
cutting the olfactory nerves, or by de-
stroying the brain.

The organ of sight or eye of the frog
is so similar in structure to that of man,
that the reader may be referred for details
both of structure and of function to the "p65, Epithelial
text-books of Physiology, and it will only — forty mucous mem-

brane of an Am-

j ‘ 1 j phibian.
be necessary to give a brief outline here. rt ay

Each eye (compare Fig. 57) is a nearly cays Zi ofactory
globular organ, and when removed from on ee Vente:
the orbit and cleaned by dissecting away
the attached muscles, &c., two regions can be distinguished
in it—an opaque portion of a dark bluish colour which forms
some two-thirds of the entire globe and is hidden within the
orbit in the entire animal; and a clear, transparent, some-
what flattened portion which is directed outwards and freely

exposed between the eyelids in the living frog. The outer

182 THE FROG CHAP.

coat of the concealed portion of the eyeball is the sclerotic
(Sc2), and is formed of cartilage in the frog ; its dark colour
is due to the presence of a layer of black pigment which
forms one of the internal coats ; this will be referred to here-
after. Entering the sclerotic on its inner side, z.e., the side
next the braincase, will be seen the cut end of the optic

“Cm

Fic. 57.—Diagrammatic horizontal section of the human eye.

c. cornea ; Ch, choroid (dotted); C,P. radiating folds of the choroid, known as the
ciliary processes ; ¢. c, €. ¢7. conjanctiva; J.iris; Z.lens; 0.4. optic nerve ; os.
ora serrata, a wavy line forming the boundary of the visual portion of the
retina; Z.c. ®. anterior non-visual portion of retina; PZ. pigmented epithe-
lium (black); R. retina; Scd. sclerotic; Sg. suspensory ligament of lens;
V.. H. vitreous chamber. (From Foster and Shore's Physiology.)

nerve (O.4V). The transparent, exposed portion of the
eyeball is the cornea (c), a superficial thin layer of which, or
conjunctiva (e.¢., é. f.), is continuous with the lining of the
eyelids and thus with the skin covering the head ; through
it can be seen the coloured part of the eye, or z7zs (/), with
a black spot—really a hole —in its centre, the pupz/,

XI EYE 183

The interior of the globe (V. #) is filled with a colour-
less, transparent jelly, the wetreous humour, surrounding
which, everywhere but on its external face, is a thin semi-
transparent membrane, reddish when perfectly fresh, but
becoming grey soon after death; this is the retina (R).
Between the retina and the sclerotic is a membrane called
the choroid (Ch), the inner face of which, z.¢., that in contact
with the retina, is coloured black. It is this layer of black
pigment which gives the dark tint to the semi-transparent
sclerotic in the entire eye ; strictly speaking, it is the inner-
most layer of the retina, but actually it adheres to the
choroid and appears like the innermost layer of that coat.
The retina is readily detachable from the choroid, but at
the place where the optic nerve enters (64nd spot) it be-
comes continuous with the fibres of the latter, which pass
through the sclerotic and choroid. The choroid is made
up of connective tissue and contains numerous blood-
vessels as well as pigment-cells.

Lying just internal to the pupil is a nearly globular body,
perfectly transparent when fresh, the cvystalline lens (L) ; it
is kept in place by a delicate membrane, the capsule of the
lens. The iris, which covers the outer face of the lens
except where it is perforated by the pupil, is covered on its
inner surface with black pigment, and is continuous all
round its outer margin with the choroid. Between the iris
and the cornea is a space, the agueous chamber of the eye,
which contains a watery fluid, the agueous humour. The
main cavity of the eyeball, containing the vitreous humour,
is called the witreous chamber (V. #1).

The actual relations of these parts in the entire eye are
best grasped in a vertical section, such as is represented in
Fig. 57. The main part of the eyeball forms a chamber,
enclosed by the sclerotic, darkened internally by the choroid,

184 THE FROG CHAP.

and lined by the retina. Into the outer side of this dark
chamber is let a transparent window, the cornea; behind
which, and separated from it by a space containing the
aqueous humour, is a vertical curtain, the iris, perforated by
an aperture, the pupil. Behind the iris and in close contact
with it is the lens, and filling the whole of the dark chamber
between the lens and iris in front, and the retina behind and
at the sides, is the vitreous humour.

The whole eye thus has the structure of a camera ob-
scura. The cornea, aqueous humour, lens, and vitreous
humour are a series of lenses, so arranged that the rays of
light from an external object are refracted and brought to a
focus on the retina, where they form a greatly diminished
and inverted image of the object. The iris is provided with
muscles, by means of which the pupil can be enlarged or
diminished ; it therefore acts as a diaphragm and regulates
the amount of light entering the eye. Attached to the
capsule of the lens are delicate muscles, by means of which
the lens can be made more or less convex ; in this way the
focus of the entire apparatus can be altered according to
whether the object viewed is nearer or farther from the eye.
This arrangement for accommodation is, however, much
less highly developed in the frog than in man and the
higher animals, in which the relatively smaller lens is
flatter and distinctly biconvex in form (Fig. 57). Thus the
various parts of this complicated organ are so adjusted as
to bring the images of external objects to an accurate focus
on the back part of the interior of the eyeball, z¢., on the
retina.

A vertical section of the retina (Fig. 58) reveals a very
complex structure. On its inner face, z.¢., the surface in
contact with the vitreous humour, is a layer of nerve-fibres
(zf-), formed by the ramifications of the optic nerve, which,

XI RETINA 185

passing through the sclerotic and choroid, perforate the
retina, and spreads out over its inner surface. Next comes
a layer of nerve-cells (g), and then several layers of fibres
and nuclei (gz, zc) ; and finally, forming the outer surface of
the retina proper, is a layer of delicate,
transparent bodies called, from their
form, the vods (rv) and cones (c) ; these
are known from their development to
be modified epithelial cells. The whole
of these structures are supported by
a complex framework of connective
tissue. In close contact with the outer
or free ends of the rods and cones is
a layer of cells the protoplasm of
which is filled with a dense black
pigment. It is this prgmentlayer
(p. ep), which, as we have seen, is
often counted as part of the choroid.|
In spite of its complex structure the
retina is not much more than }th mm.
(qdoth inch) thick, and is perfectly

Fic. 58.—Vertical section

transparent. Hence, when an image ,, Of oes retina.
is formed on it, the rays of light easily eee
penetrate its whole thickness until pet Le ase
they are stopped by the opaque layer ee mee
of pigment. The rays are thus en- epithelium 3: 7., ‘rods:

(After Howes.)
abled to stimulate the rods and cones,

and the stimulus is transmitted through the layers of nuclei
and nerve-cells to the fibres of the optic nerve, along which
it is conveyed to the brain. Thus the actual organ of sight
is not the eye as a whole, but the retina: all the rest is
to be looked upon as an accessory apparatus, for focussing
and for regulating the admission of light,

186 THE FROG CHAP.

As-with the other sense-organs, sight may be destroyed
by injury to the retina or actual organ of sight, by cutting
the optic nerve, or by destroying the brain. But unlike the
other sense-organs already considered, that of sight has a
complex accessory or focussing apparatus in connection
with it, and sight may also be rendered impossible by injury
to the cornea or lens.

It is an obvious advantage to an organ of sight such as
the frog’s that it should be capable of movement in any
direction, so as to allow the light from any object to enter
the pupil. Asa matter of fact, the animal can direct its
gaze through a very wide range by means of eight muscles
attached to the eyeball in the orbit. One of these, the
levator bulbi, raises the whole eye, causing it to project further
on the surface of the head. Another, the retractor bulbt,
withdraws it, causing it to bulge into the mouth. Four
others (compare p. 164 and Fig. 117), the superior, inferior,
anterior and posterior 7vecé/, rotate it respectively upwards,
downwards, forwards, and backwards. And finally, two obligue
muscles, the superior and inferior, produce a rotation along
an axis joining the optic nerve with the middle of the cornea.

The conjunctiva, which covers the outer side of the eye
and lines the eyelids, is kept moist by the secretion of a
lachrymal gland, known as the Harderian gland, situated
between the eyeball and the orbit in the antero-ventral
aspect. The excess of this secretion is carried away into
the olfactory chamber by means of a tube, the zaso-
lachrymal duct.

Each organ of hearing, like that of sight, consists of an
essential portion and an accessory apparatus. The essential
organ of hearing is a structure called the membranous laby-
vinth, contained within the auditory capsule of the skull
(Fig. 10), and consisting of a kind of bag of very peculiar

XI EAR 187

and complicated form (Fig. 59). It is made up, in the
first place, of two somewhat ovoid sacs separated by a
constriction :*the dorsal one is called the uériculus (w), the
ventral the sacculus (s), and from the latter a small process,
the cochlea (2), projects backwards, and a narrow canal, the
endolymphatic duct (d.c), upwards. With the utriculus are

Fic. 59.—External view of left organ of hearing of a Vertebrate (semi-diagrammatic).
aa, ampulla of anterior canal, ae, of external canal, and af. of posterior canal ;
ass. apex of utriculus ; ca. anterior semicircular canal; ce. horizontal canal; cf.
posterior canal ; czs. constriction between sacculus and utriculus ; de, se. endo-
lymphatic duct; 2 cochlea; vec, sf, ss, wu. utriculus; s. sacculus. (From
Wiedersheim’s Vertebrata.)

connected three tubes, called, from their form, the semz-
circular canals, each of which opens into the utriculus at
either end. One of them, the anterior canal (ca), is
directed forwards ; another, the posterior canal (¢4), back-
wards ; both these are vertical in position and are united
to one another at their adjacent ends, The third, the

188 THE FROG CHAP.

external canal (ce), is directed outwards and has a hori-
zontal position. Each canal has one end dilated into
a bulb-like swelling or ampulla (aa, ap, ae); those of
the anterior and external canals are anterior in position,
while that of the posterior canal is posterior.

The whole of this apparatus is filled with a fluid, the
endolymph, in which are contained calcareous particles, the
otoliths or ear-stones. It is made of connective tissue and

Fig. 60. —Toneitainel section through an ampulla.
a. e. auditory epithelium; @.%. auditory hairs ; c. part of semicircular canal; cv.
acoustic spot and ridge; c¢. connective tissue; ¢. epithelium; 7. nerve;
z, junction with utriculus. (From Foster and Shore’s Physiology.)

lined with epithelium, the cells of which are cubical for the
most part; but in certain places the wall is thickened,
forming what are called acoustic spots, of which there is
one to each ampulla, situated on a ridge (Fig. 60), while
others occur in the utriculus and sacculus. On these acoustic
spots the epithelial cells are greatly elongated and produced
at the surface into delicate processes called auditory hairs:
to these cells the fibres of the auditory nerve are distributed.

XI EAR 189

The membranous labyrinth does not fit tightly into the
cavity of the auditory capsule in which it is contained ; but
a space is left between it and the surrounding bone and
cartilage (Fig. 10). This space is filled by a fluid called
perilymph, by which the membranous labyrinth is surrounded
and protected from shocks. As you learnt in studying the
skull, the outer wall of the auditory capsule is perforated by
a small aperture, the fenestra ovalis (Fig. 10, fen. ov), in
which is fixed the stapes (st), a small nodule of cartilage
connected with a bony rod or columella (Figs. 9 and 10,
col), the cartilaginous hammer-shaped outer end of which,
or extra-columella, is fixed to the inner side of the tympanic
membrane (Fig. 9, tymp. memb). The columella lies in the
tympanic cavity (tymp. cav), which is bounded externally
by the tympanic membrane, internally by the auditory
capsule, and at the sides chiefly by muscles and connective
tissue ; while below it communicates with the mouth by
the Zustachian tube (eus. 2).

When sound-waves impinge on the tympanic membrane,
the vibrations to which they give rise are transmitted by
the columella to the stapes, and so to the perilymph.
Thence they are communicated to the endolymph and act
as stimuli to the auditory cells of the acoustic spots, and
the impulses being carried to the brain by the auditory
nerve, give rise to the sensation of sound. Whether or not
all the acoustic spots are truly auditory in function is not
known. It seems certain that the semicircular canals are
really organs for the maintenance of equilibrium.

The sense of sound can be destroyed by injury to the
membranous labyrinth, by cutting the auditory nerve, by
destroying the brain, or—to a great extent at least—by
injury to the tympanic membrane or columella.

Notice that the general plan of all the sensory organs,

190 THE FROG CHAP.

those of the skin, eye, and ear, is the same. They
consist of certain peculiarly modified epithelial cells,
specially sensitive to impulses of particular kinds, and in
communication, by means of an afferent nerve, with nerve-
cells of the brain. The three things—sensory cell, afferent
nerve, and brain—-form a chain, every link of which is
necessary for the performance of the sensory function, so
that the particular sense in question may be destroyed, not
only by destruction of the sense-organs in the strict sense,
but also by section of the afferent nerve or by destruction
of the brain.

General Physiology-Summary.—Before going on to the
next chapter it will be as well to take a final glance at the
physiological processes of the frog as a whole (compare
Fig. 23). The enteric canal is the manufactory in which
the raw material of the food is worked up into a form in
which it can be used by the various parts of the body. The
circulatory organs are the communicating system by which
the prepared food is taken to all parts; and they also form
a drainage system by which waste matters are collected
from all parts and finally ejected by the three main sewers,
the skin, lungs, and kidneys. The skin and lungs, besides
getting rid of waste matters, serve for the supply of oxygen
—a necessary form of gaseous food. The central nervous
system forms a sort of headquarters staff by which the
entire body is controlled, the means of communication
being the nerves, and the muscles the executive by which
the orders from headquarters are executed. And finally
the sense-organs may be looked upon as the various branches
of an intelligence department by which the headquarters
are informed of what is going on outside.

XI PRACTICAL DIRECTIONS . 191

PRACTICAL DIRECTIONS.

The Organs of Special Sense.

I. Olfactory organ. Notice again the external and dnternal nostrils.
Then remove the skin covering the snout, dissect off the nasal bones,
and open up the offactory sacs. Note the pigmented olfactory epithelium
lining these, and make out the offactory nerves and nasal septum.
Sketch.

Il. Eye.

a. Notice again the eyelds, irzs and pupil. Then remove the skin
covering the head so as to expose the nearly globular eyedad/s, lying in
the ordzts.

In the antero-ventral angle of the orbit make out the Harderian

gland, and the eye-muscles passing from the walls of the orbit to
the eye-ball. The four recté and two obl¢gue muscles can be
more easily seen ona larger animal, and directions will be given for
their examination in Part II.; but if you make a dissection of
them in the frog, you should note at the same time the evator and
rectractor bulbz, the latter underlying the eyeball, and the former
situated internally to the recti muscles.
* 6. Remove the eyeball from a freshly-killed specimen, noticing as
you do so the oftze nerve, which is surrounded by the rectiand retractor
bulbi muscles: dissect away these ‘muscles and note the cartilaginous
sclerotic, the cornea, tris, pupil, and the cut end of the optic. nerve.

c. Divide the eyeball into an inner and an outer hemisphere by a rapid
cut with scalpel or scissors taken vertically, midway between the cornea
and the optic nerve, through the wtreous chamber. Place them both in
a watch-glass or small dissecting dish, under water, and examine with a
lens (compare Fig. 57). In the inner hemisphere note the wireous
humour, retina, pigmented choroid, and blind spot or entrance of the
optic nerve ; and in the outer hemisphere, the crystad/ine lens and the
margin of the retina, or ora serrata. Sketch. Remove the lens, and
notice the zvzs, continuous with the choroid, the pzpz/, and the agueous
chamber.

d, Examine sections through the wall of the inner hemisphere of the
eyeball, prepared as directed on p. 136, first under the low, and then
under the high-power of the microscope. Note :—

1. The cartilaginous sclerotec. *

192 THE FROG CHAP. XI

2. The choroid, enclosing pigment-cells and blood-vessels.

3. The reténa (Fig. 58) composed of a number of layers: notice
the pigment epithelium, the rod- and cone-layer, and the various other
layers of the retina, the innermost of which is composed of nerve-fibres
continuous with the optic nerve.

Sketch.

The anatomy of the eye can be more easily made out by dissecting
that of an ox or sheep, which is essentially similar to that of the frog,
and directions for the examination of which will be given in Part II.

Ill. Auditory organ.

Notice again the tympanic membrane and tympanic ring, and then
carefully cut away the former so as to expose the ¢ympanzc cavity.
Observe the Eustachian tube, the fenestra ovals, and the relations of
the stapes, columella, and extra-columella (Fig. 10).

The essentzal part of the auditory organ (membranous labyrinth) is
very small in the frog, and can be more satisfactorily studied in a good-
sized fish (e.g. Dogfish or Cod). Directions for the preparation of the
membranous labyrinth of the Dogfish will be given at a later stage,
but if you have sufficient time and patience to dissect it out in the frog,
proceed as follows :—

Place the head ofa large frog in nitric acid (about ro per cent.) until
the bone is dissolved. Wash well in water so as to remove the acid,
and dissect away the muscle, etc., from the auditory capsule until the
latter is thoroughly exposed. Then with a sharp scalpel slice away the
roof of the capsule until the cavity it contains is exposed. Proceed now
with great caution, removing the cartilage and decalcified bone, bit by
bit, until the cavity is sufficiently enlarged to bring the membranous
labyrinth into view (compare Figs. 10 and 59.) Observe the u¢réculus,
sacculus, otoliths, and the three semzczrcular canals with their ampulle.
Sketch.

CHAPTER XII.
THE FROG (continued): REPRODUCTION AND DEVELOPMENT.

So far we have considered those parts and organs of the
frog which have to do with its welfare as an individual. We
have now to consider the organs which are connected with
the welfare of the frog as a race, that is, with the propagation
of its kind.

The position of the reproductive organs has already been
seen (pp. 23 and 25): they must now be examined in more
detail. The essential part of these organs in each sex is
a pair of bodies known as gonads, called in the male
spermaries ox testes, and in the female ovaries.

Reproductive Organs of the Male.—The spermaries
(Fig. 3, ~. spy, Fig. 5, spy, and Fig. 7, ¢s) are a pair of ovoid
bodies, each attached by a fold of the peritoneum to the cor-
responding kidney, and having connected with it a fat body
(cp. ad). From the inner margin of each spermary spring a
number of delicate tubes, the efferent ducts (Fig. 61, @)
which run in the fold of the peritoneum to the kidney.
Entering this organ near its inner edge, they open into a
longitudinal tube (Z) from which transverse tubes pass hori-
zontally across the kidney to open into the ureter (Ur).
The milt, or spermatic fluid (p. 9) is thus carried off by the
same duct as the urine ; the ureter is therefore often called the

Pract. Zoou. re)

194 THE FROG CHAP.

urinogenital duct. On the outer side of the ureter, and com-
municating with it by numerous short ducts, is a glandular
body, the seminal vesicle (Figs. 3 and 7, vs. sm), which serves
to store up the spermatic fluid.

The spermary itself contains a narrow, irregular, central
cavity, from which the efferent ducts proceed and into
which open a number of short tubes or cvypts (Fig. 62, A),

Fic. 61.—Spermary and kidney of freg showing the relations of the cfferent ducts
(semidiagrammatic). .
C. transverse tubes in kidney; Ho. spermary; Z. longitudinal tube; MV. kidney ;
g. efferent ducts of spetmary; Uy. ureter (urinogenital duct). (From Wieder-
sheim’s Anatomy.)

These are lined with epithelium (¢¢), the cells of which
divide and subdivide, forming groups of smaller cells. Each
of the latter undergoes a remarkable change, becoming con-
verted into a rod-like body, produced into a long thread,
which performs lashing movements, very much like those of
the cilia in ciliated epithelium. These bodies are called
sperms or spermatozoa (Fig. 62, 'A, sf, and B); in spite of

XII REPRODUCTIVE ORGANS 195

their peculiar form, they are cells, the rod-like portion, or
head, being the nucleus, and the delicate vibratile part, or
tail, the protoplasm. In the breeding season the cavities
of the testes are full of sperms floating in a fluid. Thus
the spermatic fluid, like the blood, owes its distinctive
character to the cells floating in it.

Fic. 62.—A, transverse section of a crypt of the spermary. B, stages in the
development of the sperms.
SP. bundles of sperms; 4e. germinal epithelium. (A, after Blomfield; B, after
Howes).

Reproductive Organs of the Female—FEach ovary
(Fig. 4, 2 ovy), is a greatly folded sac with thin walls and
a large cavity, divided up by partitions. It is attached to
the dorsal body-wall by a fold of peritoneum. As we have
seen (p. 23), its surface is studded all over with little
rounded projections, each of which is an ov/sac, and contains
anegg. The egg or ovum (Fig. 63) isa large globular cell with
a clear nucleus (zw) containing numerous nucleoli (zw’),
and having its protoplasm (97) full of yolk-granules— grains
of proteid material which serve as nutriment for the growing

02

1096 THE FROG CHAP.

embryo. It is covered with a delicate membrane, the v7ted-
line membrane. By the time the egg is mature a superficial
deposit of pigment takes place over one hemisphere.

In the young condition all the epithelial cells forming the
walls of the ovary are alike, but as the organ reaches
maturity, certain of them (e) enlarge, and give rise to the

by

Fic. 63.—Transverse section of frog’s ovary.

5. v. blood vessels ; c. ¢. connective tissue ; ef. outer layer, and ef’, inner layer of
epithelium ; ¢f”. outer layer of ovisac, continuous with ef’.; ¢f’. germinal epi-
thelium, derived from ef; ef”. follicular epithelium, derived from ef; x.
nucleus of ovum; zz’. nucleoli; 0. young ovum; /~. protoplasm of ovum
containing yolk-granules. (After Marshall.)

ova, while others form an investment or ‘liicle (ep"”)
for each ovum. :

The oviduct (Fig. 4, 7 ovd), as you have seen, is a
long and greatly convoluted tube lying above or dorsal to
the ovary. Its anterior end narrows considerably, runs
parallel with the gullet, passes to the outside of the root of
the lung, and then opens into the ccelome by a small aper-
ture (x. ovd1). The greater part of the oviduct is about as

wide as the small intestine, and is thick-walled and lined

XII FERTILIZATION 197

with gland-cells, which secrete the jelly (p. 9) surround-
ing the eggs when laid. Posteriorly it suddenly dilates into
a wide, thin-walled chamber (7. ovd") which opens into the
dorsal wall of the cloaca. Notice that there is thus no
connection between the generative organs and the kidneys
in the female, such as occurs in the male, the ureters serving
as renal ducts only.

In the breeding season the ovisacs burst and set free the
eggs into the coelome, whence by Some means or other they
find their way into the small openings of the oviducts, and so
into these tubes, when each becomes surrounded by its
little sphere of jelly secreted by the gland-cells. Passing
down the oviducts the eggs accumulate in the dilated
extremities, which they distend enormously, so that just
before laying, the abdomen of a female frog is nearly filled
with these two great egg-reservoirs ; the ovaries, having lost
so many of their eggs, are correspondingly reduced in size,
and all the other organs are squeezed out of place.

Fertilization—The eggs are now laid, and immediately
the spawn is passed from the oviducts of the female into the
water, the male sheds over them a quantity of spermatic fluid
(p. 9). The sperms, swimming actively through the water,
enter the spheres of jelly and come into contact with
the eggs. A single sperm then penetrates the vitelline
membrane of an ovum, loses its tail, and its head coming
into contact with the nucleus of the egg, fuses or conjugates
with it, so that a single nucleus is formed by the union of
the egg-nucleus with the sperm-nucleus. We may speak
of conjugating cells in general as gametes, the sperm in this
case being the male gamete, and the ovum the female
gamete, the body formed by the fusion of two gametes
being known as a zygote.

This process is known as ‘fertilization or impregnation.

198 THE FROG CHAP. XII

Without it, as we have seen, the egg is incapable of develop-
ment; after it has taken place, the egg-—or more strictly,
the vosperm, since it is now an ovum plus a sperm—is
potentially a young frog, since if left undisturbed in water,
it will in course of time give rise to a tadpole, which in its
turn will change into a frog.

It must be remembered in the first place that the fertilized
egg is a single cell, comparable with a blood corpuscle or
an epithelial cell. It is, however, peculiar in two respects :
first in having its nucleus derived in part from a sperm, so
as to contain matter from both the male and the female
parent; and secondly in having its protoplasm distended
with yolk-granules to such an extent that instead of being a
minute body visible only under the microscope, it is easily
visible by the naked eye. The yolk is not equally distributed :
on one hemisphere it is less abundant than elsewhere,
and it is this more protoplasmic hemisphere which is super-
ficially blackened by a layer of pigment, and which always
floats upwards in the water when the egg is laid.

Segmentation of the Oosperm.—Almost directly after
being laid and fertilized, the egg undergoes a remarkable
change. A furrow appears all around it, as if made with
a blunt instrument, and deepening gradually, at last divides
the oosperm into two hemispheres in contact with one
another by their flat faces (Fig. 64, A). The examination
of sections shows that this process is preceded by the
division of the nucleus into two; its final result is the
separation of the originally one-celled egg into two cells.
Now if you refer to Chapter VIII, you will be reminded
of the fact that the epithelial cells of the skin multiply by a
similar process of s¢mp/e fission, or division into two ; the
nucleus in each case dividing first and afterwards the
protoplasm.

Fic. 64.—Development of the frog.
A—F, segmentation ; G, overgrowth of ectoderm ; H, I, establishment of germinal

layers; J, K, assumption of tadpole-form and establishment of nervous system,
notochord, and enteric canal; L, newly-hatched tadpole.

61. cel. segmentation cavity ; d/A, 4/f’. blastopore ; 674, 47%. gills ; 7. cZ. branchial
arches ; ¢. eye; ect. ectoderm ; evd. endoderm ; ext. enteron ; _f é7. fore-brain ;
h. by. hind-brain ; #.4 . mid-brain ; md. /. medullary fold ; md. gv. medullary
groove; mes. mesoderm; mg. large lower cells; zz. small upper cells;
uch. notochord ; 2. ¢. ¢. neurenteric canal; pcadiz. proctodeum ; pty. invagina-
tion of ectoderm which will form the pituitary body; 7c¢. commencement of
rectum ; s&. sucker; sf. cd. spinal cord; stds. stomodzum ;: Z. tail; yh.
yolk-cells ; y&. AZ. yolk-plug which fills the blastopore. (A—D, F—H, and J
rom Ziegler’s models; E, I, K, and L after Marshall.)

200 THE FROG CHAP.

The furrow which effects this division of the oosperm
passes through both black and white poles, so that each of
the two cells formed is half black and half white. Soon a
second furrow is formed at right angles to the first, being
like it, meridional, z.e., passing through the poles (Fig. 64,
B). It divides what we must now call the ewdryo into four
cells, each half black and half white. A third furrow is
then formed, passing round the equator, but nearer the
black than the white pole (C). It therefore divides the
embryo into eight cells, four upper black and four lower
white, the latter being obviously the larger.

This process of division or segmentation of the oosperm
continues, the black cells dividing more rapidly than the white,
so that before long the embryo consists of a mass of cells, the
polyplast, somewhat resembling a mulberry, and therefore
often called a morula, one hemisphere being composed of
small cells containing much protoplasm and little yolk, and
externally pigmented (Fig. 64, D—F, m7), and the other
of larger cells containing little protoplasm and much yolk,
and not pigmented (mg). As segmentation- goes on, a
cavity appears in the interior of the embryo; it is called
the segmentation cavity (E, b/. cel), and is due to the fact
that the work of segmentation produces a waste of sub-
stance, which there is at present no means of making good,
so that for a time, while the size of the embryo remains the
same, its bulk diminishes, some of its substance being used
up ; in other words, it feeds on itself.

Segmentation now proceeds to such an extent that the
black cells become too small to be seen except with a lens
of tolerably high magnifying power, so that with the amount
of magnification used in Fig. 64, G, the black hemisphere
shows no division into cells. At the same time the black
hemisphete gradually encroaches on the white until only a

XII DEVELOPMENT 201

small circular space—the d/astopore, filled by a mass of yolk-
cells—the yotk-plug, is left uncovered (Fig. 64, G, H, v2. £/).
Development of the Chief Organs.—In the meantime
a second cavity appears in the interior of the embryo, and
as it increases in size, the segmentation cavity undergoes a
proportional diminution and finally disappears. This new

MES

Fic, 65.—Transverse section through a frog-embryo during the formation or the
medullary canal.
c. coelome ; ect. ectoderm ; end. endoderm ; ev¢. enteron 3 wes. mesoderm 3 es! its
outer (parietal) layer ; es. its inner (visceral) layer; md. medullary fold ;
md, gr. medullary groove ; ch, notochord ; y. yolk-cells. (After Marshall.)

cavity, or archenteron (Fig. 64, I and 65, enz), is the rudiment
of the enteric canal: it begins to be formed as a narrow
slit at the edge of the blastopore through which it at
first communicates with the exterior, but it soon becomes
closed: at present there is neither mouth nor anus.

By this time the cells of which the embryo is composed
have assumed different forms and become arranged in a

202 THE FROG CHAP.

very definite manner. First of all there are several layers
of small pigmented cells, derived from the black cells of
earlier stages, which cover the whole embryo with the excep-
tion of the yolk-plug: these constitute the ectoderm (Fig. 64,
G, H, I, ect). Forming the roof of the enteric cavity are
other layers of cells derived from the yolk-cells of earlier
stages and forming the exdoderm (1, end): the floor of the
archenteron is at present formed of unaltered yolk-cells.
Between the ectoderm and the yolk-cells on the ventral
aspect are several layers of small cells which gradually
spread until they form a complete layer between the ecto-
derm and the endoderm: these constitute the mesoderm (I,
mes). The ectoderm, endoderm, and mesoderm are known
as the three embryonic tissues or germinal layers.

While these processes have been proceeding, a longi-
tudinal groove appears on the surface of the embryo : this
is the medullary or neural groove, and is bounded by a
pair of medullary folds (Figs. 64 H and 65, md.gr, maf).
The medullary groove marks the dorsal surface of the
embryo and extends backwards to the blastopore, which
now marks its posterior end. As development goes on, the
folds approach one another and unite, so as to convert the
groove into a canal: the cells lining this canal give rise to
the central nervous system (Fig. 64 K, fdr, m.dr., h.br,
sp.cad), and its cavity becomes the central canal of the spinal
cord and the ventricles of the brain. The anterior end of
the hollow medullary cord thus developed enlarges to form
three vesicles, one behind the other, and known respectively
as the fore-brain, mid-brain, and hind-brain, each containing
a ventricle. The fore-brain eventually gives rise to the
cerebral hemispheres, produced in front into the olfactory
lobes, and to the diencephalon ; the mid-brain to the optic
lobes and crura cerebri; and the hind-brain to the medulla

XII DEVELOPMENT 203

oblongata and cerebellum. It will be seen (Fig. 65) that
the medullary groove is lined by ectoderm and _ that
therefore the whole central nervous system is ectodermic
in origin ; and this is also true of the nerves.

A longitudinal thickening of the endoderm on the dorsal
side of the archenteron becomes constricted off as a solid
tod of cells, lying between the medullary cord and the
archenteron. This is the zo/ochord (Figs. 64 K and 65,
ach) ; it forms the primary axial skeleton around which the
vertebral column is subsequently developed. The meso-
derm is for some time a solid mass occupying all the space
between the ectoderm and endoderm, so that there is no
body-cavity. But at a later stage a cavity appears divid-
ing the mesoderm into two layers, one in contact with the
ectoderm (parietal layer, Fig. 65, mes*), the other with the
endoderm (visceral layer, mes*). ‘The space between them
is the calome (c). The dorsal part of the mesoderm on
either side of the medullary cord and notochord becomes
divided transversely, and gives rise to muscle-segments or
myomeres (Fig. 64, L).

The embryo now begins to elongate in a definite direc-
tion (Fig. 64, J): its dorsal surface, still marked by the
medullary grove, which now soon becomes closed, is slightly
concave, its ventral surface very convex. The blastopore
closes up, the yolk-plug becoming entirely covered by ecto-
derm; but for a short time the medullary canal and
archenteron still communicate by the meurenteric canal
K, ec). This soon disappears, and at the hinder end
of the embryo a little conical outgrowth forms the
rudiment of the tail (4). The opposite end is rounded, and
on its ventral surface appears a little half-moon shaped
groove (which afterwards becomes subdivided into two,
Fig. 66, C), the rudiment of the sucker by which the tad-

204 THE FROG CHAP

pole attaches itself to weeds (J and L, s#). Just above the
sucker a depression—the mouth-pit or stomodeum (L, stdm)
—makes its appearance, and is the first indication of the
mouth. A similar depression below the tail—the proctodeum
(gcdm)—marks the anus: both at first are mere blind pouches
and have no communication with the enteric canal. The
head-region is further marked by two pairs of vertical ridges
separated by depressions: the ridges are the branchial or
gill arches (J, 5r.cl), and the depressions the branchial clefts.

Further elongation takes place (Fig. 64, L), the head
becomes distinctly marked off, the tail extends considerably
beyond the anus, a thickening appears which marks the
position of the eye (e), and a depression that of the ear (just
above 47? in the figure) ; and from each branchial arch arises
a little tuft, the rudiment of one of the external gills (br),
bér*), In this condition the tadpole is hatched: it is still
unable to feed, the stomodzum not yet being in com-
munication with the enteric cavity; it is nourished therefore
entirely by the yolk with which a large portion of the body
is still filled.

Up to the stage shown in Fig. 64, I, the cells of which the
tadpole is composed, although distinguishable into ecto-
derm, endoderm, and mesoderm, are all more or less similar :
there are no muscles, no cartilage, no bone, no connective
tissue. But shortly after the stage referred to, the permanent
tissues begin to be formed: the outer ectoderm cells take
on the form of epidermis, the endoderm cells become the
epithelium of the enteric canal, and from offshoots of the
enteric cavity—all of course lined by endoderm—are formed
the lungs, the liver, the pancreas, and the urinary bladder.

The mesoderm undergoes much more extensive changes,
giving rise to all the connective tissue, cartilage, bone, and
muscle, as well as to other parts—v.e., to by far the greater

XII DEVELOPMENT 205

part of the permanent tissues. In certain places the em-
bryonic mesoderm cells, hitherto in close contact, separate
from one another and become branched, while between them
appear intercellular substance and fibres crossing one
another in various directions: in this way the connective
tissue which form so large a part of the adult body is
produced.

The various parts of the skeleton first arise by the conver-
sion of portions of the mesoderm into cartilage. The cells
retreat from one another and between them a clear sub-
stance, the matrix, makes itsappearance. For a considerable
time almost the whole skeleton consists of cartilage, but
subsequently much of this tissue is replaced by bone
developed from a layer of cells on the inner surface of the
perichondrium. The bone thus formed in connection with
cartilage is cartilage-bone: membrane-bones arise in the
connective tissue outside the cartilage (compare p. 43).

In the place where voluntary muscles are to appear the
mesoderm cells elongate, their nuclei multiply by fission, and
their protoplasm gradually becomes converted into the
striated substance so characteristic of the adult muscular
tissue.

These examples will suffice to illustrate the fact, which
further study would show to be true, that all the permanent
tissues are either—like epithelium—formed entirely of actual
cells, or are—like connective tissue and striped muscle—de-
rived from cells. The entire embryo in its earliest condition—
the oosperm stage—is a single cell,which multiplies repeatedly
by simple fission, forming a group of cells; and these, by
assuming various forms and undergoing various changes,
give rise to all the complex tissues—differing from one
another both structurally and functionally—of the adult
animal, So that every cell, fibre, or what not in the frog is

206 THE FROG CHAP.

a linear descendant, through repeated fission, of the oosperm ;
and the oosperm itself is the product of the fusion of two
cells, one—the ovum—derived from the female, the other
—the sperm—from the male parent. Thus in passing from
the oosperm to the adult animal there is a gradual structural
or morphological differentiation accompanied by a differen-
tiation of function or dtvision of phystological labour.

The expression “division of physiological labour” was
invented by the great French physiologist, Henri Milne-
Edwards, to express the fact that a sort of rough correspon-
dence exists between lowly and highly organised animals
and plants on the one hand, and lowly and highly organised
human societies on the other. In primitive communities
there is little or no division of labour: every man is his own
butcher, baker, soldier, doctor, etc., there is no distinction
between “classes” and ‘t masses,” and each individual is to
a great extent independent of all the rest. Whereas in
complex civilised communities society is differentiated into
politicians, soldiers, professional men, mechanics, labourers,
and so on, each class being to a great extent dependent on
every other.

Structure and Metamorphosis of the Tadpole —Develop-
ment of other Organs —Metamorphosis.—A sketch of the
further development of the tadpole and of its metamorphosis
into the frog has already been given (pp. 9-11, Fig. 1), and
it is now necessary to add a few details.

A third pair of branchial arches appear behind the two
already noticed, and on each a third external gill, The first
two pairs increase greatly in size and all the gills become
branched (Fig. 66, A). At the same time the branchial clefts
come to open into the pharynx, and a current of water
enters the mouth and passes out by the branchial clefts, thus
providing the gills with a constant supply of aerated water for

XII DEVELOPMENT 207

purposes of respiration. A thin median fold of the skin, the
tail fin (Fig. 1), arises all round the tail, both above and
below, and the tail now forms a powerful swimming organ.
The stomodeum (Fig. 66, B—E) and, at a still earlier
period, the proctodzeum (p. 204) open into the enteric cavity,
so that there is now a complete enteric canal: it grows
much faster than the body generally, and becomes coiled like
a watch-spring (D, E). The mouth is very small, and is
bounded by lips beset with little horny projections or papillz
and provided with a pair of horny jaws (C—E) with which
the tadpole now browses upon the water-weeds which form
its staple food. It is on account of this diet that the in-
testine is of such great proportionate length: vegetable diet
contains less nourishment, bulk for bulk, than animal, and
a longer intestine is required to extract all the nutriment
from it.

Soon the external gills show signs of shrivelling, and
on the inner portions of the branchial arches Gnternal gills
(Fig. 66, D+) are developed, like those of a fish, A fold
of skin appears on either side in front of the gills and
gradually grows backwards, covering the branchial clefts and
the external gills, which latter soon disappear entirely:
this is the operculum or gill-cover (B, C); it is quite like
that of an ordinary fish, such as a cod or a perch, except
that it is not supported by bone. The current of water
from the pharynx now, of course, makes its final exit by a
wide cleft between the edge of the operculum and the flank.
The operculum gradually extends, and its free edge unites
with the wall of the body posterior to the gills, so that the
originally large aperture through which the water from the
pharynx passes is much reduced. After some time, a com-
plete union of the operculum with the flank takes place on
the right side, the opercular aperture becoming closed: on the

208 THE FROG CHAP,

left side the spout-like aperture remains open for some time
longer, and the water from both sides passes out through it
(D, E). All this time the tadpole is to all intents and
purposes a fish ; apart from the possession of gills and a

Fic. 66.—Stages in the later development of the tadpole.

A, stage with external gills ; B, stage showing the formation of the operculum ;
C, later stage, in which the external gills are disappearing ; D, stage with in-
ternal gills and budding limbs ; D1, dissection of same stage to show the heart,
gills and lungs; E, later stage, in which the limbs are further differentiated.
(After Howes.)

tail-fin, the structure of the circulatory organs and various
other parts is more like that of a fish than of a frog.

The lungs (D!) now appear, and the tadpole is for a time
truly amphibious, rising periodically to the surface of the
water to breathe air. At a later stage, however, the single

XI DEVELOPMENT 209

opercular aperture on the left side closes, and thenceforth
respiration is purely aerial.

In the meantime the limbs are developed. The hind-
limbs appear as little rounded buds, one on either side
of the root of the tail (D): they gradually elongate and
divide into thigh, shank and foot (E). The fore-limbs
appear beneath the operculum and are therefore hidden
at first (E): at a later stage each divides into upper arm,
fore-arm, and hand, and emerges from its concealment.
As the limbs increase, the tail undergoes a progressive
shrinking ; its tissues become, as it were, digested, and
are carried off by the blood; so that for a time the
metamorphosing tadpole feeds largely on its own tail.

The mouth widens, the horny jaws and papille disappear
and teeth are formed. The suckers vanish, and the intestine
not only begins to grow less rapidly than the body, but even
becomes reduced in length and loses its spiral arrangement,
while vegetable is exchanged for animal diet. The dark
colour of the tadpole gradually gives place to the bright hues
of the frog. The little tailed frog can now leave the. water
and hop about on land: its tail is soon absorbed, and the
metamorphosis is complete (Fig. I, 7, 8).

It will be noticed that in the course of development of
the frog a substitution or replacement of organs occurs.
Thus, for instance, the notochord is replaced by a vertebral
column, the gills by lungs, and the horny jaws by teeth.

Fate of the Germinal Layers.—In concluding this chapter
we may enumerate in rather more detail the various parts
and organs which are derived from each of the three
embryonic tissues respectively. From the ectoderm are
formed—the epidermis ana the cutaneous glands; the
whole of the nervous system, central and peripheral, and the

essential parts of the sensory organs, as well as the crystalline
Pract. Zoou. Pp

210 THE FROG CHAP.

lens of the eye; and also the epithelium lining the mouth
(stomodzeum) and outer part of the cloaca (proctodzeum).
The endoderm gives rise to the epithelium lining the enteric
canal and its various offshoots, including the lungs, urinary
bladder, gastric glands, bile and pancreatic ducts, as well as
the glandular cells of the liver and pancreas; and to the
notochord. From the mesoderm are developed the various
parts which are situated between the ectoderm and endo-
derm with the exception of the notochord, vz., the connec-
tive tissue, cartilage, bone, striped and unstriped muscles,
circulatory organs and peritoneum, as well as the urinary
and reproductive organs and the accessory parts of the
sensory organs.

PRACTICAL DIRECTIONS.

Organs of Reproduction.

Open a frog in the usual way: cut through the gullet, rectum, and
mesentery, and remove all the digestive organs. In the female take
especial care not to injure the roots of the lungs in severing the gullet.
One of the specimens you have already dissected and kept in the pre-
servative fluid should still contain the urinogenital organs intact in the
case of the male: in the female, the ovaries and greater part of the
oviducts will have been removed, but the relations of the two ends of
each oviduct can still be made out.

Examine the reproductive organs, which will now be freely exposed,
under water—or in the case of the female, in 1 per cent. salt solution.

I. Male Organs. (Figs. 3, 5, 7 and 61.)

1. Notice again the spermaries or destes, each supported by a fold
of peritoneum connecting it with the corresponding kidney ; the efferent
ducts ; the branched fat-body ; the ureter (urinogenttal duct); and the
seminal vesicle. Sketch after examining the cloaca (see below).

2. Tease up a bit of the spermary of a recently-killed frog in salt
solution, and notice the form and movements of the sperms (Fig. 62, B).
Sketch. :

3. Examine under the microscope a transverse section of the spermary,
prepared as directed on p. 136, and note (Fig. 62, A) the numerous

XII PRACTICAL DIRECTIONS 211

seminal tubes or crypts which open into it. Under the high power,
observe the epithelial cells (germznal epithelium), lining the tubes and
their subdivision into smaller cells, which eventually give rise to the
sperms, the tails of which project freely into the cavities of the tubes.
Sketch.

II. Female Organs.

1. Notice again (a) the ovardes, varying in size and appearance ac-
cording to the time of year, and each suspended by a fold of peritoneum :
they are studded all over with ovzsacs, each of which contains an egg or
ovum, pigmented when ripe ; and (4) the ovdducts. (Fig. 4.) Trace the
convoluted and glandular middle portion of the oviduct forwards, and
make out the anterior thin-walled portion, running parallel to the gullet
and opening into the ccelome by a small aperture at the base of the
lung ; then trace the middle portion backwards, and notice the thin-
walled, dilated, posterior portion, which opens into the cloaca. Sketch
after examining the cloaca (see below).

If a female frog is examined in the spring, just before the eggs are
laid, the ovaries will be seen to be reduced in size, and the posterior
portions of the oviducts filled with eggs, each surrounded with a gela-
tinous coat secreted by the middle portion of the oviduct.

2. Examine under the microscope a section of the ovary, prepared as
before (p. 136), first with the low, and then with the high power
(Fig. 63). Note the epithelial cells (including the germzna/ epztheliunt)
forming the walls of the ovary, and the ovdsacs in different stages of
development. Each ovisac contains an ovz, surrounded by follicle-
cells. In the ova, note the protoplasm and its contained yolk-granutles,
the vitelline membrane, the nucleus, and the nucleoli. Sketch.

Cloaca.

Carefully cut through the pelvic symphysis in the middle line with a
scalpel and press apart the two innominate bones, so as to expose the
ventral surface of the cloaca. Inflate from the vent, and note that the
large intestine is continuous with the cloaca and that the urinary
bladder opens into it on the ventral side.

The entire urinogenital apparatus, together with the cloaca, may
now be removed from the body, first cutting through the skin round the
vent. Pin down under water, insert the small scissors into the vent,
and slit open the cloaca very slightly to one side of the middle line,

/s0 as not to injure the connection between the bladder and cloaca.

P 2

212 THE FROG CHAP.

Notice the openings into the bladder and large intestine respectively,
and also :—

In the male, the apertures of the ureters (urinogenital ducts), situated
on two small papille, lying close to one another on the dorsal side of
the cloaca: insert a bristle into the ureter. Sketch. eo

In the female, the two small apertures of the ureters, and just in front
of these, the two large apertures of the oviducts, all situated on the
dorsal wall of the cloaca. Insert bristles into the ureter and oviduct.
Sketch.

Impregnation and Development.

In the early spring (end of February or beginning of March), look
about in ponds and ditches for frogs which have taken to the water for
the purpose of laying eggs: This process can be watched more con-
veniently by catching a few frogs, male and female, and putting them
in a large vessel of water or aquarium. If you have been unsuccessful
in procuring frogs or frogs’ spawn, toads will do equally well: their
eggs are laid a few weeks later, and are arranged, each surrounded by
its gelatinous envelope, not in clumps, like those of the frog, but in a
string, like the beads on a necklace.

For purposes of observation, the spawn is best kept in a glass vessel,
together with some water-weeds. Put only a small quantity of spawn
in one vessel; if the water begins to get foul, change it at once.
Examine the eggs every day with a magnifying-glass as development
proceeds, and keep some of the tadpoles alive until metamorphosis
takes place in June.

If you wish to see some of the stages in the development of the frog
during other times of the year, you must obtain some preserved eggs
and embryos, and, if possible, you should also examine the series of wax
models, made on an enlarged scale, which are to be seen in most
zoological museums.

' The following are some of the more important ‘things to be noticed.
(Sketch a series of stages) :—

Before hatching.

I. The unsegmented oosperm, black above, and white below.

II. Early stages in segmentation, during which the individual cells
can be made out by means ofa lens. (Fig. 64 A-F).

III. Later stages in segmentation, during which the individual cells
cannot be distinguished without the aid of a microscope. Note the
gradual enclosure of the white by the black hemisphere, until only a small

XII PRACTICAL DIRECTIONS 213

rounded yolk-glug is left, filling in an aperture—the Jd/astopore, in the
black layer or ectoderm, as it is now called. The yolk-plug is continuous
with a mass of yolk-cells, enclosed by the ectoderm, from which the
endoderm and mesoderm are developed. (Fig. 64, G-I).

IV. The gradual flattening and elongation of the embryo, and the
position of the blastopore at the posterior end of the dorsal side; the
ormation of the medullary folds and groove along the dorsal side, and
the closure of the blastopore. (Fig. 64, H-J).

V. The closure of the medullary groove, the formation of the head
with its sackers and of the faz/ with its ¢az/-fin; as well as the
appearance of the eyes, ears, branchial arches, external gills, and the
involutions for the #ouwth and anus, which, however, do not open until
after the tadpole is hatched. (Fig. 64, K-L).

After hatching.

VI. The appearance of the horny jaws and gafzlle, and the further
development of the head, external gills, and tail. (Fig. 1, 1-2 and
Fig. 66 A-B).

VII. The formation of the oferculum, the closure of the right
opercular aperture, and the disappearance of the external gills. The
bud-like rudiments of the Azzd-dmbs, and their further development.
The fore-limbs remain for a long time hidden beneath the operculum.
(Fig. 1, 3-4 and Fig. 1, 66, C-E).

Metamorphosis.

VIII. The gradual change in the form and colour of the head and
body; the widening of the mouth and loss of the horny jaws and
suckers ; the appearance of the fore-limbs from beneath the operculum,
and the shrinking of the tail (Fig. 1, 5-8).

Dissection of tadpole.

Pin down a full-sized tadpole under water in a small dissecting dish,
with the ventral side uppermost, inserting small pins through the tail
only. Carefully dissect off the ventral body walls, and note the coiled
intestine, the heart, the internal gills, etc. (Fig. 66, D, D*, E).
Sketch.

It is rather a difficult task, and requires much time, to prepare sections
of the early stages of the frog-embryo in order to make out the formation
of the ectoderm, endoderm, and mesoderm, the relations of the seg-
mentation cavity and archenteron, and the development of the central
nervous system and notochord (Figs. 64, I, K, and 65); and directions
for the study of the development of the chief organs of the chick will be

214 THE FROG CHAP. XII

given in a subsequent chapter. But if you wish to make the attempt,
proceed as follows :—

Take out a few eggs every day from the time they are laid until the
tadpoles are hatched : place these on a tile or piece of glass, and with a
pair of needles dissect off the gelatinous covering. Then carefully transfer
the segmenting eggs or embryos into corrosive sublimate for a quarter
to half an hour, and after washing in running water, put them into
weak and then strong alcohol; then stain, imbed, and cut sections as
directed on p. 136.

CHAPTER XIII.

THE FROG (continued): MEANING OF THE TERM SPECIES—
THE PRINCIPLES OF CLASSIFICATION—EVOLUTION—
ONTOGENY AND PHYLOGENY—HEREDITY AND VARIA-
TION—STRUGGLE FOR EXISTENCE—SELECTION— ORIGIN
OF SPECIES.

TuE frog which you have been studying is the only one
commonly found in Great Britain. Another kind, very
similar to it, is, like the common frog, abundant in Ger-
many, and other parts of the European continent, but is rare
in this country, only occurring in parts of the eastern counties ;
and various other frogs, differing in certain minor respects
from these are found in different parts of the world. This
fact is expressed in the language of systematic zoology by
saying that there are various species of frogs, belonging to
the same genus, which are distinguished from one another
by certain definite characteristics as regards form, structure,
and colour.

According to the system of d¢xomial nomenclature intro-
duced by Linnzus, each kind of animal receives two names—
one, the generic name, common to all the species of the
genus ; the other the specific mame, peculiar to the species in
question. Both generic and specific names are Latin in
form, and are commonly Latin or Greek in origin, although

216 THE FROG CHAP.

frequently modern names of persons or places with latinized
terminations are employed. In giving the name of an animal,
the generic name is always placed first, the specific name
following it, and being written as a rule with a small letter.
Thus the common English frog is called Rana temporaria,
and the continental form referred to above, often spoken of
as the edible frog, Rana esculenta.

You will probably have noticed certain differences in
colour and markings in the different individual frogs you
have examined, and it is matter of common observation
that no two individuals of a species are exactly alike. In
the case of human beings and many of the more familiar
animals this is very apparent to every one: in other cases a
more careful examination of the individuals is necessary in
order to tell them apart ; thus, for instance, the individuals in
a flock of sheep appear all alike to the casual observer, but
the shepherd can easily distinguish them from one another.
These differences we designate zxdividual variations, and it
is often difficult to decide whether two kinds of animals
should be considered as distinct species, or as varieties of a
single species, and no universal rule can be given for deter-
mining this point. Among the higher animals, mutual fer-
tility is a fair practical test, the varieties of a species (e.g.,
common pigeon, fowl, dog, horse) usually breeding freely
with one another and producing fertile offspring, while dis-
tinct species usually do not breed together, or else produce
infertile hybrids or mz/es. Compare, for instance, the fer-
tile mongrels produced by the union of the various breeds of
domestic dog with the infertile mule produced by the union
of the horse andass. But this rule is not without exception,
and in the case of wild animals is, more often than not, impos-
sible of application ; failing it, the only criterion of a “ good
species” is usually the presence of constant differences from

XII MORPHOLOGY AND PHYSIOLOGY 217

allied species, whereas if there is a complete series of grada-
tions between two forms, they will be considered to form a
single variable species.

In the previous chapters it will have been evident that an
animal may be studied from two chief points of view, firstly,
from the point of view of its s¢rwcture, and secondly from that
of the functions performed by its various parts and the way
in which these work together for the welfare of the whole.
The branch of Zoology dealing with the former is known as
Morphology, and with the latter Physiology.

It is evident that a knowledge of Morphology is necessary
as a preliminary to the study of Physiology, and also that, as
animals have to be distinguished from one another largely
by structural characters, the foundations of a scientific Zoo-
logy must be laid in Morphology, which, as we have seen,
deals not only with the external characters and the anatomy
and histology of the adult animal, but also with the changes
undergone during the development of the egg into the adult
form, #.¢., with embryology. Given a sound knowledge of
the anatomy, histology, and embryology of animals, their
classification may be attempted : that is, we may proceed to
arrange them in groups and sub-groups, each capable of
accurate definition. In doing so we must be careful to dis-
tinguish between Aomologous parts, or those which correspond
structurally (compare p. 39), and amalogous parts, which
have merely a similar function, and are of no value for
purposes of classification. Thus the fore-limbs of a frog
are homologous with—ve., are formed on a similar plan
to—the wings of a bird, although differing from them in
function ; they are only analogous to those of an insect,
for though the function is similar in both, the structure is
widely different. In the same way the wing of the bird
is only analogous to that of an insect.

218 THE FROG CHAP.

The general method of classification employed by zoolo-
gists may be illustrated by reference to the different kinds
of frogs already referred to in explanation of the terms
genus and species.

The common frog (Rana temporaria) is distinguished
from the “edible frog” (2. esculenta) by its smaller size and
brown colour, by the large black patch in the tympanic
region, and by the rudimentary character of a pair of infla-
table vocal sacs at the sides of the head in the male, which
are very large and highly distensible in #. escudenta. On the
other hand, these two frogs agree with one another and with all
the other species of the genus Raza, in having teeth on the
upper jaw, and in not having the transverse processes of the
sacral vertebra dilated. Comparing all these with our
English toads, of which there are two species, the common
toad and the rarer “ natterjack,” we find that in them the
skin is comparatively dry and covered with glandular warts,
the hind-limbs are proportionally shorter, there are no
teeth, and the transverse processes of the sacral vertebra are
more or less dilated. These differences are so great as not
only to necessitate placing the toads in another genus, the
genus Bufo, but also to relegate the frogs just mentioned
to one family—the Ranida, and the toads to another—the
Bufonide. All frogs and toads, however, agree with one
another in having no tail in the adult, while the trunk
is relatively short and broad, and the hind-limbs are longer
than the fore-limbs. They therefore differ fundamentally
from such animals as the common English newts and the
salamanders, which retain the tail throughout life, and in
which the fore- and hind-limbs are of approximately equal
size. The differences here are obviously far greater than
those between the families mentioned above, and are
emphasised by placing the frogs and toads in the erder

XI CLASSIFICATION 219

Anura, the newts and salamanders in the order
OUrodela.

The UOvodela and Anura, although differing from one
another in many important respects, agree, ¢.g., in possess-
ing gills during part or the whole of their existence, and
in nearly always possessing lungs. They usually pass
through a metamorphosis, the young being hatched in the form
of gilled larvee; their skin is soft and glandular, and the toes
are in nearly all cases without claws. These and numerous
other structural characters separate them from reptiles, in
which gills are never developed, and the young do not pass
through a metamorphosis, while the skin is provided with
scales and the toes have claws. The differences here are
considerably more important than those between the orders
referred to above, and are expressed by placing the latter
in the class Amphibia, while reptiles constitute the class
Reptilia. In the same way the finned, water-breathing
fishes form the class Pisces, the feathered birds the class
Aves, and the hairy animals which suckle their young the
class Mammalia.

Mammals, Birds, Reptiles, Amphibians, and Fishes all
agree with one another in the possession of red blood and
an internal skeleton, an important part of which in the
embryo is the notochord (p. 203), which is nearly always
replaced in the adult by a backbone or vertebral column ;
and in never having more than two pairs of limbs. They
thus differ in some of the most fundamental features of
their organisation from such animals as Crayfishes, Insects,
Scorpions, and Centipedes, which have colourless blood,
a jointed external skeleton, and numerous limbs. These
differences—far greater than those between classes — are
expressed by placing the back-boned animals in the phylum
or sub-kingdom Ver/ebrata, the many-legged armoured forms

220 THE FROG CHAP.

in the phylum Arthropoda. Similarly, soft-bodied animals
with shells, such as mussels and snails, form the phylum
Mollusca ; various worms, such as the earthworm, the phylum
Annulata; polypes and jelly-fishes the phylum Ca/enterata ;
the simplest animals, mostly minute, such as Ameba, the
phylum frotozoa. Finally, the various phyla recognised
by zoologists together constitute the kingdom Azimadia.

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

Thus the zoological position of the common frog is
expressed as follows :—

Kingdom—ANIMALIA.
Phylum—VERTEBRATA.
Class—AMPHIBIA.
Order—ANURA.
Family—-Ranide.
Genus—fana.
Species—temporaria.

Let us now briefly consider some of the reasons which
have led zoologists to adopt the system of classification now
in general use.

“It is obvious that there are various ways in which animals
may be classified, and the question arises—which of these,
if any, is the right one? Is there any standard by which
we can judge of the accuracy of a given classification, or

XHI EVOLUTION 221

does the whole thing depend upon the fancy of the classi-
fiers, like the arrangement of books in a library? In other
words, are all possible classifications of living things more
or less artificial, or is there such a thing as a xatural
classification ?

Suppose we were to try and classify all the members of a
given family—parents and grandparents, uncles and aunts,
cousins, second cousins, and so on. Obviously there are a
hundred ways in which it would be possible to arrange
them—into dark and fair, tall and short, curly-haired and
straight-haired, and so on. But it is equally obvious that
all these methods would be purely artificial, and that the
only natural way, z.e., the only way to show the real con-
nection of the various members of the family with one
another would be to classify them according to blood-
relationship ; in other words, to let our classification take the
form of a genealogical tree.

There are two theories which attempt to account for the
existence of the innumerable species of living things which
inhabit our earth: the theory of special creation and the
theory of evolution.

According to the theory of creation, all the individuals
of every species existing at the present day—the tens of
thousands of dogs, frogs, oak-trees, and what not, are
derived by a natural process of descent from a single indi-
vidual, or from a pair of individuals—in each case precisely
resembling, in all essential respects, their existing descend-
ants—which came into existence by a process outside the
ordinary course of nature and known as creation. On this
hypothesis each species of frog is derived from a common
ancestral pair which came into existence, independently of
the progenitors of all the other species, at some previous
period of the earth’s history.

222 THE FROG CHAP.

Notice that on this theory the various species of frogs are
no more actually ve/afed to one another than is either of
them to a Newt, or for the matter of that, to Homo. The
individuals of any one species are truly related since they
all share a common descent, but there is no more relation-
ship between the individuals of any two independently
created species than between any two independently manu-
factured chairs or tables. The words affinity, relationship,
etc., as applied to different species are, on the theory of
creation, purely metaphorical, and mean nothing more than
that a certain likeness or community of structure exists ;
just as we might say that an easy chair was more nearly
related to a kitchen chair than either of them to a three-
legged stool.

We see, therefore, that on the hypothesis of creation, the
varying degrees of likeness and unlikeness between the
species receive no explanation, and that we get no absolute
criterion of classification: we may arrange our organisms,
as nearly as our knowledge allows, according to their re-
semblances and differences, but the relative importance of
the characters relied on becomes a purely subjective
matter.

According to the rival theory—that of Descent or Organic
Evolution, with which the name of Darwin is inseparably
connected—every species existing at the present day is
derived by a natural process of descent from some other
species which lived at a former period of the world’s history.
If we could trace back from generation to generation the
individuals of any existing species, we should, on this
hypothesis, find their characters gradually change, until
finally a period was reached at which the differences were
so considerable as to necessitate the placing of the ancestral
forms in a different species from their descendants at the

X11 EVOLUTION 223

present day. And in the same way, if we could trace back
the species of any one genus, we should find them gradually
approach one another in structure until they finally con-
verged in a single species, differing from those now existing,
but standing to all in a true parental relation.

It will be seen that, on this hypothesis, the relative like-
ness and unlikeness of the various species of frogs are
explained as the result of their descent with greater or less
modification or divergence of character from the ancestral
form: and that we get an arrangement or classification in
the form of a genealogical tree, which, on this hypothesis,
is a strictly natural one, since it shows accurately the
relationship of the various species to one another and to
the parent stock. So that on the theory of evolution, a
natural classification of any given group of allied organisms
is simply a genealogical tree, or, as it is usually called, a
phylogeny.

Now it is evident that the only way in which we could
be perfectly sure of an absolutely natural classification of
the species of any kind of animal—the frog, for example—
would be by obtaining specimens as far back as the distant
period when the genus first came into existence.

Forming part of the solid crust of the earth are a series of
sedimentary or stratified rocks, and the researches of geolo-
gists have shown that these present a general order of
succession, the lowest, when undisturbed, being in every
case older than the more superficial layers. Imbedded
in these rocks are found the remains of various extinct
animals in the form of what are called /ossi/s ; and it might
perhaps, on first considering the subject, be supposed that,
had a process of evolution taken place, we ought to be
able to find in the rocks belonging to the various geological
ormations a complete series of animal remains, represent-

224 THE: FROG CHAP.

ing all the stages in the evolution of the highest from the
lowest forms. But owing to various causes which we can-
not further consider here, the record of the succession of
life on the globe is very imperfect and incomplete in the
case of any individual species: but the evidence furnished
by Paleontology—as the study of fossils is called—is very
important in supplying proofs that there has been a gradual
evolution of the higher from the lower forms.

Further evidence in the same direction is furnished by
Morphology and Embryology, as will be more apparent to
you when you have studied a number of other kinds of
animals. You have, however, seen that the frog begins
life as a single cell, and that it is possible to trace a series of
modifications which gradually convert the unicellular oosperm
into a tadpole—which is to all intents and purposes a fish,
and that the tadpole subsequently undergoes metamor-
phosis into the more highly organised frog. According to
the ¢heory of recapitulation, these facts indicate that the
frog repeats, during its single life, the series of changes
passed through by its ancestors in the course of ages. In
other words, ontogeny, or the evolution of the individual,
is, in its main features, a recapitulation of phylogeny, or
the evolution of the race. At the same time you must
bear in mind that it is not always an easy matter to deter-
mine which characters are of phylogenetic significance, and
which have been secondarily acquired owing to various
causes. To take an example:—the horny jaws of the
tadpole might be taken to indicate that frogs were de-
scended from ancestors with horny jaws and without teeth,
though in all probability these organs have been secondarily
acquired as adaptations in connection with the habits of the
tadpole, and have therefore, unlike the gills, for instance,
no ancestral significance.

XIII ORIGIN OF SPECIES 225

It is obvious that the evolution of one species from
another presupposes the occurrence of variations in the
ancestral form. As we have seen, such individual variation
is of universal occurrence. This may be expressed by
saying that heredity, according to which the offspring tends
to resemble the parent in essentials, is modified by varia-
bility, according to which the offspring tends to differ from
the parent in detail. If, from any cause, any well-marked
individual variation is perpetuated, there is produced what
is known as a variety of the species, and according to the
theory of the origin of species by evolution, such a variety
may, in course of time, become a new species. Thus a
variety is an incipient species, and a species is a (relatively)
permanent variety.

One other important factor in evolution must be briefly
referred to before concluding this chapter. In order that
every species of animal and plant may flourish, certain
conditions are necessary. Thus the frog requires, for
example, a moist place to live in, and water in which to
lay its eggs. For spots presenting the necessary favourable
conditions, there is constantly going on a competition
between individuals of one species and between the mem-
bers of different species. The nature of this struggle is
well seen when a piece of garden-ground is allowed to
run to waste. Its surface is soon overgrown by weeds of
many kinds, which kill out nearly all the original garden-
plants by depriving them of light and food. By and by the
more hardy weeds exterminate and replace such weaker
forms as may first have obtained a footing, till an entirely
new set of weeds may take the place of those that first
appeared.

A struggle for existence goes on on all sides among

animals. To begin with, before there is any struggle for
Pract, Zoou, Q

226 THE FROG CHAP.

existence in the strict sense, there is—particularly in animals
which, like the frog, produce eggs in great numbers annually
—a very great indiscriminate destruction of ova and young
embryos. Only a few of them reach maturity; a large
proportion are destroyed at one stage or other, some
failing to reach a spot favourable for their further de-
velopment, others becoming the food of other animals.
But such of the young as are less adapted to escape
the various dangers to be encountered, and less fitted to
procure the necessary food, are more likely to be destroyed.
This is one phase—and the most important, perhaps, of all
—of the struggle for existence amongst animals. But there
is also a struggle for existence, not only between individual
animals of the same kind, but between animals of different
kinds. ‘This struggle, in so far as it relates to the competi-
tion for food and shelter, is more severe between nearly
related species ; for in such a case the food and the favour-
able conditions required are the same, or nearly so, in the
two competitors. Again, a struggle for existence of a con-
stant and severe kind also goes on between carnivorous
animals and the animals on which they prey—e.g., between
the frog and the insects and worms on which it feeds, and
between the snake and the frog: a struggle in which the
defensive qualities of the prey—such as swiftness, power of
eluding observation, or of resisting attack—are opposed to
the predatory powers of the attacker.

There can be little doubt that, in the long run, such
individuals will survive as are best fitted to cope with the
conditions to which they are subject. According to
Darwin’s theory of Natural Selection, it is assumed that
such surviving individuals would transmit their special
properties to their progeny, and there would thus be a
gradual approximation towards a perfect adaptation of the

XIII NATURAL SELECTION 227

species to its surrounding conditions by virtue of this
“survival of the fittest.”

Let us suppose the conditions to change. Gradual alter-
ations in climate and other conditions are known to take
place, owing to subsidence or elevation of the land. But
conditions might be changed in many other ways: some
animal or plant previously used as food might become ex-
terminated, or a new enemy might find its way into the
district inhabited by the species. Then such individuals
as presented variations which enabled them better to cope
with the new surroundings, would have the advantage over the
others, and would have a much better chance of surviving
and leaving progeny. The useful variations thus produced
and transmitted to the progeny would tend to increase,
generation after generation, until a form sufficiently distinct
to be regarded as a new species had become developed from
the original one.

That very different varieties of animals and plants can,
and have been produced, in a comparatively short time, by
man selecting those forms which tend to vary in a desired
direction, is a well-known fact. All the breeds and varieties
of our domestic animals have been produced by this process
of artificial selection ; and “if man can by patience select
variations useful to him, why, under changing and com-
plex conditions of life, should not variations useful to
Nature’s living products often arise, and be preserved or
selected ?”

It does not come within the scope of the present work to
discuss either the causes of variability or those which deter-
mine the elevation of a variety to the rank of a species:
both questions are far too complex to be adequately treated
except at considerable length, and anything of the nature
of a brief abstract would only be misleading. As a pre.

228 THE FROG CHAP, XIII

liminary to the study of Darwin’s Origin of Species, the
student is recommended to read Romanes’s Euidences of
Organic Evolution, in which the doctrine of Descent is
expounded as briefly as is consistent with clearness and
accuracy.

PART 11

CHAPTER I
AMCEBA—-UNICELLULAR AND MULTICELLULAR ANIMALS.

From your study of the frog you will have learnt some of
the more important facts with regard to the Morphology and
Physiology of a comparatively highly-organised animal, and
will have overcome a number of preliminary difficulties in
acquiring a knowledge of zoological terminology and
technique. You will now, therefore, be in a better position
to undertake a systematic and comparative examination of
a number of other animals—some much less complicated,
some more complicated, than the frog—working upwards
from the simple to the complex forms and beginning with a
very instructive animalcule belonging to the genus Ameda.

Amcebee are often found in the slime at the bottom of
pools of stagnant water, adhering to weeds and other sub-
merged objects. They are mostly invisible to the naked
eye, rarely exceeding 1th of a millimetre (zjoth inch) in
diameter, so that it is necessary to examine them entirely by
the aid of the microscope. Though they can be seen and
récognised with the low power, the high power is necessary
for the accurate examination of their structure.

Fic. 67.—A, Ameba qguarta, a living specimen, showing granular endoplasm sur-
rounded by clear ectoplasm, and several pseudopods (sd), some formed of
ectoplasm only, others containing a core of endoplasm. ‘The larger Lodies in the
endoplasm are mostly food-particles (300 diameters).

B, the same species, killed and stained with carmine to show the numerous nuclei
(uz) present in this species (compare p. 269) (X 300).

CHAP. I AMCEBA 231

C, Ameba proteus, a living specimen, showing large irregular pseudopods,
nucleus (7), contractile vacuole (c. vac), and two food vacuoles (/ vac), each
containing a small infusor (see p. 261) which has been ingested as food. The
letter @ to the right of the figure indicates the place where two pseudopods have
united to enclose the food vacuole. The contractile vacuole in this figure is
supposed to be seen through a layer of granular protoplasm, whereas in the
succeeding figures (D, E, and G) it is seen in optical section, and therefore
appears clear.

D, an encysted Amceba, showing cell-wall or cyst (cy), nucleus (#2), clear con-
tractile vacuole, and three microscopic plants (diatoms) ingested as food.

E, Ameba proteus, a living specimen, showing several large pseudopods (sa),
single nucleus (#2) and contractile vacuole (c. vac), and numerous food-particles
embedded in the granular endoplasm (x 330).

F, nucleus of the-same after staining, showing a ground substance of achromatin,
containing deeply-stained granules of chromatin, and surrounded by a distinct
membrane (X ro10).

, Ame@ba verrucosa, living specimen, showing wrinkled surface, nucleus (72),
large contractile vacuole (c. vac), and several ingested organisms (x 330).
# Say of the same, stained, showing the chromatin aggregated in the centre
X O10).

I, Ame@ba proteus, in the act of multiplying by binary fission (x 500). (From
Parker's Biology: A, B, E, F, G, and H after Gruber; C and I after Leidy; D
after Howes.) <

Examined under the high power (Fig. 67), the ‘Amceba
appears like a little shapeless blob of jelly, nearly or quite
colourless, and closely resembling a colourless blood cor-
puscle or leucocyte of one of the higher animals (p. 105).
The central part of it, or endopflasm, is granular and semi-
transparent—something like ground-glass—while surround-
ing this inner mass is a border of perfectly transparent and
colourless substance—the ectoplasm.

One very noticeable thing about the Amceba is that, like
the leucocyte, it is never of quite the same form for long
together, owing to the protrusion of pseudopods (psd),
by means of which it creeps along slowly. The occur-
rence of amaboid movements is alone sufficient to show
that it is an organism, or‘ living thing, and no mere mass
of dead matter. Moreover, it consists of protoplasm
and contains a #ucleus (C—H, au), which is rendered more
apparent by staining and consists of chromatin and achro-
matin. The Amceba is thetefore a ce// (compare p. 106).

A very important difference is thus at once seen between
the Amceba and the frog: the Amoeba is unicellular, Ze. it
consists of a single cell, while the frog is, as we have seen

232 AMCEBA CHAP.

a multicellular animal, built up of innumerable cells which are
incapable of an independent existence for any length of time.

Besides the nucleus, there is another structure frequently
visible in the living Amceba and not present in the leuco-
cyte. This is a clear, rounded space in the ectoplasm (« vac),
which periodically disappears with a sudden contraction and
then slowly reappears, its movements reminding one of the
beating of a minute colourless heart. It is called the
contractile vacuole, and consists of a cavity in the ectoplasm
containing a watery fluid.

We must now study the Physiology of our animalcule.
First of all, as we have already seen, it is contractile :
although it has no muscles, it can move about from place to
place. Its movements, like the voluntary movements of the
frog (pp. 7, 173), May occur without the application of any
external stimulus, 7.e., they are spontaneous or automatic; or
they may be induced by external stimuli—by a sudden
shock or by coming in contact with an object suitable for
food. Movements of this latter kind, like those resulting
from the stimulation of the nerves in a brainless frog, are
the result of the z7itabiiity of the protoplasm ; the animal-
cule is therefore both automatic and irritable, although it
possesses neither nerves nor sense-organs.

Under certain circumstances an Amceba temporarily loses
its power of movement, draws in its pseudopods, and
becomes a globular mass around which is formed a thick,
shell-like coat, called the cyst or cell-wall (Fig. 67, D, cy).
This is formed by the protoplasm by a process of secretion
(p. 130): its composition is not known ; it is certainly not
protoplasmic, and very probably consists of some_nitro-
genous substance allied in composition to horn and to the
chitin (see Chapters V & VI) which forms the external shell
of crustaceans, insects, ‘&c.

I NUTRITION 233

The formation of the cyst is probably of great importance
in preserving the animalcule from destruction by drought, so
that should the pool in which it is living dry up, it may still
remain alive, protected by its shell-like case, until the con-
ditions for its active life are once more restored, when it
escapes from this evcysted condition by the rupture of the
cell-wall.

Very often an Amceba in the course of its wanderings
comes in contact with a still smaller organism of some kind
or other. When this happens the Amoeba may be seen to
extend itself round the lesser organism until the latter
becomes sunk in its protoplasm in much the same way as a
marble might be pressed into a lump of clay (Fig. 67, c, a).
The diatom or other organism becomes in this way com-
pletely enclosed in a cavity or food-vacuole (f vac), which
also contains a small quantity of water necessarily included
with the prey. The latter is taken in by the Amceba as
food: so that the Ameeba, like the frog, feeds. It is to
be noted that the reception of food takes place in a
particular way, viz. by dzgestion—i.e. it is enclosed entire
by the organism.

When the prey is thus ingested, its protoplasm becomes
digested, any insoluble portions being passed out or egested,
as feeces (p. 75), from the surface of the Amceba as it creeps
slowly on. Note that all this is done without either ingestive
aperture (mouth), digestive cavity (stomach), or egestive
aperture (anus): the food is simply taken in by the flowing
round it of protoplasm, digested as it lies enclosed in the
protoplasm, and those portions for which it has no further
use are got rid of by the Amceba flowing away from them.

We have seen that the frog possesses certain digestive
glands, the function of which is to secrete digestive fluids
which have an important chemical action on the food

234 AMCEBA CHAP.

swallowed, rendering it soluble and diffusible before it
passes through the epithelial cells of the intestine into the
blood: the gastric juice, for example, has the power of
converting proteids into peptones by means of the ferment
pepsin (p. 74); the digestion here takes place outside the
cells, z.e. is extracellular. There can be little doubt that
the protoplasm of Ameeba is able to render that of its
prey soluble and diffusible by the agency of some sub-
stance analogous to pepsin, and that the dissolved matters
diffuse through the body of the Amceba until the latter is,
as it were, soaked through and through with them. The
process of digestion in Amceba thus takes place within a
single cell, ze. it is zxtracedlular.

It has been proved by experiment that proteids are the
only class of food which Amceba can make use of: it is
unable to digest either starch or fat (p. 72). Mineral matters
must, however, be taken with the food in the form of a
weak watery solution, since the water in which the ani-
malcule lives is never absolutely pure.

The Amceba being thus permeated, as it were, with a
nutrient solution, the elements of the solution, hitherto
arranged in the form of peptones, mineral salts, and water,
become rearranged in such a way as to form new particles
of living protoplasm, which are deposited among the pre-
existing particles. In a word, the food is assimilated, or
converted into the actual living substance of the Ameeba,
which must therefore grow, if nothing happens to counteract
this formation of new protoplasm.

We have seen, however, that work results in a propor-
tional amount of waste (p. 66), and just as in the frog or in
ourselves, every movement of the Amceba, however slight,
is accompanied by a proportional oxidation or low tempera-
ture combustion of the protoplasm, z.e. the constituents of the

I EXCRETION AND RESPIRATION 235

protoplasm combine with oxygen, forming waste or excretory
matters—carbon dioxide, water, and certain nitrogenous
substances of simpler constitution than proteids, such as
urea. These products of excretion, formed in the case
of Amceba without the agency of any special excretory
organs (¢.g. kidneys), are given off partly from its general
surface, but partly, it would seem, by the agency of the
contractile vacuole, by means of which the water taken in
with the food is also got rid of.

With this breaking down of proteids the vital activity of
all organisms are invariably connected. Just as useful
mechanical work may be done by the fall of a weight from a
given height to the level of the ground, so the work done by
the organism is a result of its complex proteids falling, so to
speak, to the level of simpler substances. In both instances
potential energy or energy of position is converted into
kinetic or actual energy.

The statement just made that the protoplasm of Amceba
constantly undergoes oxidation presupposes a constant sup-
ply of oxygen. The water in which the animalcule lives
invariably contains that gas in solution, and diffusion takes
place, oxygen passing into the interior of the Amceba while
carbon dioxide passes out into the water. This is the process
of breathing or respiration (p. 144), and it occurs in Amoeba
without the agency of lungs or other respiratory organs.
Thus the carbon dioxide is got rid of, and at the same time
a supply of oxygen is obtained for further combustion. The
oxidation of the protoplasm of the Amceba is doubtless
accompanied by an evolution of heat, as in higher animals
(p. 1§1), although this has never been proved.

We thus see that a very elaborate series of chemical pro-
cesses is constantly going on in the interior of Ameeba, as in
the frog, the whole series of which is spoken of collectively

236 AMCEBA CHAP.

as metabolism—constructive and destructive (p. 149). Living
protoplasm is thus the most unstable of substances ; it is
never precisely the same thing for two consecutive seconds ;
its existence, like that of a waterfall or a fountain, depends
upon the constant flow of matter into it and away from it.

It follows from what has been said that if the income of
an Ameeba, z.e., the total weight of substances taken in (food
plus oxygen plus water) is greater than its expenditure or
the total weight of substances given out (feeces plus excreta
proper Aus carbon dioxide) the animalcule will grow : if less
it will dwindle away: if the two are equal it will remain of
the same weight or in a state of physiological equilibrium.

It is evident that Amoeba must also be able to perform
the function of reproduction. You have learnt that the cells
of the frog multiply by s¢mpé or binary fission (p. 106): the
nucleus first divides into two, and then the surrounding
protoplasm ; and precisely the same thing occurs in Amceba,
the reproduction of which therefore takes place by the
simplest method known, without any special repro-
ductive organs. The animalcule simply divides into two
Amcebee, each exactly like itself; and in doing so ceases to
exist as a distinct individual. Instead of the successive
production of offspring from an ultimately dying parent, we
have the simultaneous production of offspring by the division
of the parent, which does not die, but becomes simply
merged in its progeny. There can be no better instance of
the fact that reproduction is discontinuous growth.

From this it seems that an Amoeba, unless suffering a
violent death, is practically immortal, since it divides into
two completely organised individuals, each of which begins
life with half of the entire body of its parent, there being
therefore nothing left of the latter to die: it therefore

I MULTIPLICATION 237

appears certain that ‘death has no place as a natural re-
current phenomenon” in that organism.

It is said that occasionally two Amoeba come into contact
and undergo complete fusion, just as the gametes of the frog
(sperm and ovum) unite in the processes of fertilization
(p. 197). This process of conjugation has been more accu-
rately observed in other unicellular organisms (pp. 268 and
278), and it is important to bear in mind that reproduction can
take place in all these quite independently of such a process.

Amceba may also be propagated artificially. If a speci-
men is cut into pieces, each fragment is capable of develop-
ing into a complete animalcule provided it contains a
portion of nuclear matter, but not otherwise. From this it
is obvious that the nucleus exerts an influence of the utmost
importance over the vital processes of the organism.

If an Amceba does happen to be killed and to escape
being eaten it will, like a dead frog, undergo gradual decom-
position, becoming converted into various simple substan-
stances of which carbon dioxide, water and ammonia are the
chief (p. 152).

Death results if the temperature to which an Amceba is
exposed reach about 40°C., and at freezing point its move-
ments cease entirely and it becomes inert.

We thus see that complex organs, composed of various
tissues, each consisting of cells of characteristic form, are
not necessary in order that the vital functions may be
performed : the only essential is nucleated protoplasm. As
we pass from the unicellular Amoeba to the higher multi-
cellular animals we shall find—just as we found in tracing
the development of the frog from the unicellular oosperm
(p. 206),—that a differentation of structure accompanied by a
division of physiological labour becomes more and more

238 AMCEBA CHAP,

marked, some cells giving rise to organs of locomotion,
others to organs of reproduction, and so on. But every
function necessary for the life of an animal—or a plant—is
due in the first instance to protoplasm, and a simple cell,
like the Amceba, can perform them all.

In the next two chapters we shall study certain other
unicellular organisms which show an advance on Amceba in
possessing a certain amount of morphological and physio-
logical differentiation. But the structural differentiations, as
they are merely parts of one cell, cannot be spoken of as
“organs” in the sense in which we have used the word
hitherto, as they are not composed of numerous cells.
They are, however, organs in the physiological sense, as
they perform different functions.

PRACTICAL DIRECTIONS.!

Ameba.

Examine a drop of water containing Ameebze, from the bottom of a
pond, with the low power, first putting on a cover-glass: if the water
does not contain small particles of sand or mud, place a small piece of
paper under the edge of the cover so as to avoid crushing the organisms,
When you have found a specimen, put on the high power, and note—

1. The zrregular and changing form of the animal, the protoplasm
running out into blunt psezdopods. ?

z. The granular character of the protoplasm, the granules usually

1You should, if possible, try and obtain specimens of Amoebze and
the other fresh-water organisms described in this and the two following
chapters for yourself, by collecting stagnant pond-water, together with
a little of the mud at the bottom and some water-weeds, and letting
it stand for a few days ina glass jar or bottle. If you are unable to
find the organisms you require, they, as well as most of -the other
animals described in this book, may be obtained from dealers in
Natural History objects (see ¢.g. the advertisements in Mature).

I PRACTICAL DIRECTIONS 239

not extending to the periphery, so that a clear ectop/asm can be dis-
tinguished from a granular exdoplasm. The granules render the flowing
movements of the protoplasm visible.

3. The food-vacuoles in the protoplasm, containing fluid, and often
also food particles.

4. The contractile vacuole, containing fluid, and its rhythmical con-
tractions. !

5. The protrusion and retraction of the pseudopods. Sketch a
specimen several times at short intervals, noting the direction in which
the granules flow.

6. Look out for specimens undergoing mzzeltiplication by binary
fisston, and also for excysted individuals.

7. Run a little dry carmine or indigo under the cover-glass, and
note that the particles can be taken in at all parts of the surface.

8. Stain with methyl-green (see p. 121), This will kill the animal,
and render the szac/ews distinct.

9. Permanent preparations, showing the nucleus, may be made on
the slide as follows :—

Place a drop of water containing Amoebze on a slide, and soak up
with blotting-paper as much of the water as is possible without carry-
ing the Amcebe along with it. Fix (see p. 136) with a drop of
absolute alcohol, stain (a staining-fluid called picrocarmine is better
than borax-carmine for this purpose), wash carefully with weak and
then with absolute alcohol, and add a drop of turpentine—or better,
oil of cloves. Soak off the excess of oil of cloves with blotting-paper,
and mount in Canada balsam.

CHAPTER II

HAMATOCOCCUS AND EUGLENA—-MONADS AND BACTERIA—
DIFFERENCES BETWEEN ANIMALS AND PLANTS—SAPRO-

PHYTES.,

THE rain-water which collects in puddles, open gutters,
&c., is frequently found to have a green or red colour. The
colour is due to the presence of various organisms—
plants or animals—one of the commonest of which is
Hematococcus (or, as it is sometimes called, Profoccocus or
Spherella) pluvialis. -

Like Amoeba, Hzematococcus is so small as to require
a high power for its examination. Magnified three or four
hundred diameters it has the appearance (Fig. 68, a) of
an ovoidal body, somewhat pointed at one end, and of a
bright green colour, more or less flecked with equally bright
red.

Like Amceba, moreover, it is in constant movement, but
the character of the movement is very different in the two
cases. An active Heematococcus is seen to swim about
the field of the microscope in all directions and with
considerable apparent rapidity. We say apparent rapidity
because the rate of progression is magnified to the same
extent as the organism itself, and what appears a racing
speed under the microscope is actually a very slow crawl

CHAP, II 1leMATOCOCCUS 241

when divided by 300. It has been found that such
organisms as Hzematococcus travel at the rate of one foot

in from a quarter of an hour to an hour ; or, to express

»\ B

‘iG. 68.—A, Mematococcus pluviaiis, motile phase. Living specimen, showing
protoplasm with chromatophores (chy) and pyrenoids (fy7), cell-w ‘all (c. w)
connected to cell-body by protoplasmic filaments, and flagella (/7), The scale
to the left applies to I

B, resting stage of the same, ‘showing nucleus (vz) with nucleolus (’), and thick
cell-wall (c. zw) in contact with the protoplasm.

C, the same, showing division of the cell-body in the resting stage into four
daughter-cells.

D, the same, showing the development of flagella and detached cell-wall by the
daughter cells before their liberation from the enclosing mother-cell-wall.

E, Hematococcus lacustris, showing nucleus (zz), single large pyrenoid (fy7),
‘and contractile vacuole (c. vac).

¥, diagram illustrating the movement of a flagellum ; ad. its base
ent positions assumed by its apex. (From Parker’s Biology :

3 ¢, ¢', ce”. differ-
, after Biitschli.)

the fact in another and fairer way, that they travel a distance

equal to two and a half times their own diameter in one

second. In swimming the pointed end is always directed
Pract. Zoot. R

242 HAMATOCOCCUS cHaP.

forwards, and the forward movement is accompanied by a
rotation of the organism upon its longer axis.

Careful watching shows that the outline of a swimming
Hematococcus does not change, so that there is evidently
no protrusion of pseudopods, and at first the cause of
the movement appears rather mysterious. Sooner or later,
however, the little creature is sure to come to rest, and there
can then be seen projecting from the pointed end two exces-
sively delicate colourless threads (Fig. 68, a, 72), each about
half as long again as the organism itself ; these resemble the
cilia on the epithelial cells lining the frog’s mouth (p. 109),
except that they are few in number, and do not vibrate
rhythmically ; they are therefore usually distinguished as
Jiagella. Ina Hematococcus which has come to rest these
can often be seen gently waving from side to side: when
this slow movement is exchanged for a rapid one the whole
organism is propelled through the water, the flagella acting
like a pair of extremely fine and flexible fins or paddles.
Thus the movement of Hzmatococcus is not ame@botd, i.e.,
produccd by the protrusion and withdrawal of pseudopods,
but is cary, Ze, due to the rapid vibration of cilia or
flagella. ,

By staining and other tests it is shown that Heematococcus,
like Ameba, consists of protoplasm, and that the flagella
are simply filamentous processes of the protoplasm.

The green colour of the body is due to the presence of
a special pigment called chlorophyll, the substance to which
the colour of leaves is due. That this is something quite
distinct from the protoplasm may be seen by treatment
with alcohol, which simply kills and coagulates the proto-
plasm, but completely dissolves out the chlorophyll, pro-
ducing a clear green solution. The solution, although green
by transmitted light, is red under a strong reflected light,

1 CHROMATOPHORES 243

and is hence fluorescent: when examined through the
spectroscope it has the effect of absorbing the whole of the
blue and violet end of the spectrum as well as a part of the
red. The red colour which occurs in so many individuals,
sometimes entirely replacing the green, is due to a colouring
matter closely allied in its properties to chlorophyll, and
called hematochrome.

At first sight the chlorophyll appears to be evenly distri-
buted over the whole body, but accurate examination under
a high power shows it to be lodged in a variable number
of irregular structures called chromatophores (Fig, 68, A, chr),
which together form a layer immediately beneath the sur-
face. Each chromatophore consists of a protoplasmic
substance impregnated with chlorophyll.

After solution of the chlorophyll with alcohol a nucleus
(B, zz) can be made out; like the nucleus of Amceba, it is
rendered more distinct by staining. Other bodies which
might easily be mistaken for nuclei are also visible in the
living organism. These are small ovoidal structures (a, pyr)
with clearly defined outlines occurring in varying numbers
in the chromatophores. When treated with iodine they
assume a deep, apparently black, but really dark blue colour.
The assumption of a blue colour with iodine is the charac-
teristic test of the carbohydrate, starch (p. 72), as can be
seen by letting a few drops of a weak solution of iodine
fall upon some ordinary washing starch. The bodies in
question have been found to consist of a proteid substance
covered with a layer of starch, and are called pyvenoids.

In Hematococcus pluvialis there is no contractile vacuole,
but in another species, 7. /acustris, this structure is present
as a minute space near the anterior or pointed end (Fig. 68,
E, ¢. vac).

There is still another characteristic structure to which no

R 2

244 HAMATOCOCCUS CHAP

reference has yet been made. This appears at the first view
something like a delicate haze round the red or green body,
but by careful focussing is seen to be really an extremely thin
globular shell (a, ¢.ze), composed of some colourless trans-
parent material, and separated, by a space containing water,
from the body, to which it is connected by very delicate
radiating strands of protoplasm. It is perforated by two
extremely minute apertures for the passage of the flagella.
Obviously we may consider this shell as a cyst or ced/-
wadl, differing from that of an encysted Amceba (Fig. 67, D)
in not being in close contact with the protoplasm.

A more important difference, however, lies in its chemical
composition. The cyst or cell-wall of Amceba, as stated in
the preceding chapter (p. 232), is very probably nitrogenous ;
that of Heematococcus, on the other hand, is formed of a
carbohydrate called ceZ/u/vse, allied in composition to starch,
sugar, and gum, and, like starch, having the formula C,H,,0,.
Many vegetable substances, such as cotton, consist of
cellulose, and wood is a modification of the same com-
pound. Cellulose is stained yellow by iodine, but iodine
and sulphuric acid together turn it blue, and a similar
colour is produced by a solution of iodine and potassium
iodide in zinc chloride known as Schulze’s solution. These
tests are quite easily applied to Hzematococcus: the proto-
plasm stains a deep yellowish brown, around which is seen
a sort of. blue cloud, due to the stained and partly-dissolved
cell-wall.

It has been stated that in stagnant water in which it has
been cultivated for a length of time Hzematococcus some-
times assume an amceboid form. In any case, after lead-
ing an active existence for a longer or shorter time, it
comes to rest, loses its flagella, and forms a thick cell-
wall of cellulose (Fig. 68, B), thus becoming encysted. So

a NUTRITION 245

that, as in Ameeba, there is an alternation of an active or
motile with a stationary or resting condition.

In the matter of nutrition, the differences between Hama-
tococcus and Ameeba are very marked, and indeed funda-
mental. As we have seen, Heematococcus has no pseudopods,
and therefore cannot take in solid food after the manner
of Amceba ; moreover, even in its active condition, it is
usually surrounded by a cell-wall, which of course quite
precludes the possibility of ingestion, As a matter of
observation, also, however long it is watched it is never
seen to feed in the ordinary sense of the word. Never-
theless it must take in food in some way or other, or the de-
composition of its protoplasm would soon bring it to an end.

Heematococcus lives in rain-water. This is never pure
water, but always contains certain mineral salts in solution,
especially nitrates, ammonia salts, and often sodium chloride
or common table-salt. These salts can and do diffuse into
the water which is a constituent part of the protoplasm of
the organism, so that we may consider its protoplasm to be
constantly permeated by a very weak saline solution, the
most important elements contained in which are oxygen,
hydrogen, nitrogen, potassium, sodium, calcium, sulphur,
and phosphorus. It must be remarked, however, that the
diffusion of these salts does not take place in the same
uniform manner as it would through parchment or
other dead membrane. The living protoplasm has the
power of determining the extent to which each constituent
of the solution shall be absorbed.

If water containing a large quantity of Hzematococcus
is exposed to sunlight, minute bubbles are found to appear
in it, and these bubbles, if collected and properly tested,
are found to consist largely of oxygen. Accurate chemical

246 H.A.MATOCOCCUS CHAP.

analysis has shown that this oxygen is produced by the
decomposition of the carbon dioxide contained in solution
in rain-water, and indeed in all water exposed to the air ; the
gas, which is always present in small, quantities in the
atmosphere, being very soluble in water.

As the carbon dioxide is decomposed in this way, its
oxygen being given off, it is evident that its carbon must be
retained. As a matter of fact it is retained by the organism,
but not in the form of carbon; in all probability a double
decomposition takes place between the carbon dioxide
absorbed and the water contained in its protoplasm, the
result being the liberation of oxygen in the form of gas and
the simultaneous production of some extremely simple
form of carbohydrate, ze, some compound of carbon,
hydrogen and oxygen with a comparatively small number
of atoms to the molecule.

The next step seems to be that the carbohydrate thus
formed unites with the ammonia salts or the nitrates absorbed
from the surrounding water, the result being the formation
of some comparatively simple nitrogenous compound.
Then further combinations take place, substances of greater
and greater complexity are produced, sulphur from the
absorbed sulphates enters into combination, and proteids
are formed. From these, finally, fresh living protoplasm
arises.

From the foregoing account, which only aims at giving
the very briefest outline of a subject as yet imperfectly
understood, it will be seen that, as in Amceba, the final result
of the nutritive process is the manufacture of protoplasm,
and that this result is attained by the formation of various
substances of increasing complexity. But it must be noted
that the steps in this process of constructive metabolism
are widely different in the two cases. In Amceba we start

1 NUTRITION 247

with living protoplasm—that of the prey—-which is killed
and broken up into diffusible proteids, these being after-
wards re-combined to form new molecules of the living pro-
toplasm of Amceba. So that the food of Amceba is, to
begin with, as complex as itself, and is first broken down by
digestion into simpler compounds, these being afterwards
re-combined into more complex ones. In Haematococcus,
on the other hand, we start with extremely simple
compounds, such as carbon dioxide, water, nitrates, °
sulphates, &c. Nothing which can be properly called
digestion, 7.c.,a breaking up and dissolving of the food, takes
place, but its various constituents are combined into sub-
stances of gradually increasing complexity, protoplasm, as
before, being the final result.

To express the matter in another way: Amceba can only
make protoplasm out of proteids already formed by some
other organism : Heematococcus can form it out of simple
liquid and gaseous inorganic materials.

Speaking generally, it may be said that these two methods
of nutrition are respectively characteristic of the two great
groups of living things. Animals require solid food con-
taining ready-made proteids, and cannot build up their pro-
toplasm out of simpler compounds. Green plants, ze., all
the ordinary trees, shrubs, weeds, &c., take only liquid and
gaseous food, and build up their protoplasm out of carbon
dioxide, water, and mineral salts. The first of these methods
of nutrition is conveniently distinguished as o/ozoic, or
wholly-animal, the second as holophytic, or wholly-vegetal.

It is important to note that only those plants or parts of
plants in which chlorophyll is present are capable of holo-
phytic nutrition. Whatever may be the precise way in which
the process is effected, it is certain that the decomposition
of carbon dioxide which characterises this form of nutrition

248 HAMATOCOCCUS CHAP,

is a function of chlorophyll, or to speak more accurately, of
chromatophores, since there is reason for thinking that it
is the protoplasm of these bodies and not the actual green
pigment which is the active agent in the process.

Moreover, it must not be forgotten that the decomposition
of carbon dioxide is carried on only during daylight, so that
organisms in which holophytic nutrition obtains are depend-
ent upon the sun for their very existence. While Amceba
derives its energy from the breaking down of the proteids
in its food (see p. 235), the food of Hzematococcus is too
simple to serve as a source of energy, and it is only by the
help of sunlight that the work of constructive metabolism
can be carried on. This may be expressed by saying that
Heematococcus, in common with other organisms contain-
ing chlorophyll, is supplied with kinetic energy (in the form
of light or radiant energy) directly by the sun.

As in Ameeba, destructive metabolism is constantly going
on side by side with constructive. The protoplasm becomes
oxidised, water, carbon dioxide, and nitrogenous waste
matters being formed and finally got rid of. Obviously
then, absorption of oxygen must take place, or in other
words, respiration must be one of the functions of the pro-
toplasm of Hzematococcus as of that of Amceba. In many
green, 7.c., chlorophyll-containing, plants, this has been proved
to be the case: respiration, z.¢., the taking in of oxygen and
giving out of carbon dioxide, is constantly going on, but
during daylight is obscured by the converse process—the
taking in of carbon dioxide for nutritive purposes and the
giving out of the oxygen liberated by its decomposition. In
darkness, when this latter process is in abeyance, the occur-
rence of respiration is more readily ascertained.

Owing to the constant decomposition, during sunlight, of
carbon dioxide, a larger volume of oxygen than of carbon

It METABOLISM 249

dioxide is evolved ; and if an analysis were made of all
the ingesta of the organism (carbon dioxide A/vs mineral
salts f/vs respiratory oxygen) they would be found to con-
tain less oxygen than the egesta (oxygen from decomposition
of carbon dioxide //zs water, excreted carbon dioxide, and
nitrogenous waste) ; so that the nutritive process in Hzma-
tococcus is, as a whole, a process of deoxidation. In
Amoeba, on the other hand, the ingesta (food p/us respi-
ratory oxygen) contain more oxygen than the egesta (faeces
plus carbon dioxide, water, and nitrogenous excreta), the
nutritive process being therefore on the whole one of
oxidation. This difference is, speaking broadly, character-
istic of plants and animals generally ; animals, as a rule,
take in more free oxygen than they give out, while green
plants always give out more than they take in.

But destructive metabolism is manifested not only in the
formation of waste products, but in that of substances
simpler than protoplasm which remain an integral part of
the organism, viz., cellulose and starch. The cell-wall is
probably formed by the conversion of a thin superficial
layer of protoplasm into cellulose, the cyst attaining its final
thickness by frequent repetition of the process. The
starch of the pyrenoids is apparently formed by a similar
process of decomposition or destructive metabolism of pro-
toplasm.

We see then that destructive metabolism may result in
the formation of (a) waste products and (b) plastic products,
the former being got rid of as of no further use, while the
latter remain an integral part of the organism.

Let us now turn once more to the movements of Hazemato-
coccus, and consider in some detail the manner of their
performance.

250 HAEMATOCOCCUS CHAP.

Each flagellum (Fig. 68, a, 77) is a thread of protoplasm
of uniform diameter except at its distal or free end, where
it tapers to a point. The lashing movements are brought
about by the flagellum bending successively in different
directions (F). Thus the ciliary movement of Haematococ-
cus, like the amoeboid movement of Amceba, is a phenome-
non of contractility. Imagine an Amceba to draw in all its
pseudopods but two, and to protrude these two until they
became mere threads; imagine further these threads to
contract rapidly and more or less regularly instead of slowly
and irregularly ; the result would be the substitution of
pseudopods by flagella, z.e., of temporary slow-moving pro-
cesses of protoplasm by permanent rapidly-moving ones.

To put the matter in another way: in Ameceba the func-.
tion of contractility is performed by the whole organism ;
in Hzematococcus it is discharged by a small part only, viz.,
the flagella, the rest of the protoplasm being incapable of
movement.

Heematococcus multiplies after becoming quiescent or
in the encysted condition (Fig. 68, c, Dp) ; as in Ameeba its
protoplasm undergoes simple or binary fission, but with the
peculiarity that the process is immediately repeated, so that
four daughter-cells are produced within the single mother-
cell-wall. By the rupture of the latter the daughter-cells
are set free as the ordinary motile form, acquiring their
flagella and -detached cell-wall before making their
escape (D).

Under certain circumstances the resting form divides into
eight or more instead of four daughter-cells, and these when
liberated are found to be smaller than the ordinary motile
form, and to have no cell-wall. Hzematococcus therefore
occurs, in the motile condition, under two distinct forms, ze.,
is dimorphic: the larger or ordinary form with detached

Ir EUGLENA 251

cell-wall is called a megazootd, the smaller form without a
cell-wall a microzooid.

We will now examine another small organism which is
often found in puddles and pools, frequently in such vast
numbers as to give the water a greencolour. This organism
is known as Luglena viridis.

Euglena is also microscopic, its length varying from
zymm.tozmm. The body is spindle-shaped, wide in the
middle and narrow at both ends (Fig. 69, a—E): one
extremity is blunter than the other, and from it proceeds
a single long flagellum (72) by the action of which the
organism swims with great rapidity, the flagellum being,
as in Heematococcus, directed forwards. Besides its rapid
swimming movements, Euglena frequently performs slow
movements of contraction and expansion, something like
those of a short worm, the body becoming broadened out
first at the anterior end, then in the middle, then at the
posterior end, twisting to the right and left, and so on (a—D).
These movements are so characteristic of the genus that the
name euglenoid is applied to them.

The body consists of protoplasm covered with a very
delicate membrane or cuticle which is often finely striated,
and is to be looked upon as a superficial hardening of the
protoplasm. The green colour is due to the presence of
chlorophyll, which tinges all the central part of the body,
the two ends being colourless. It is difficult to make out
whether the chlorophyll is lodged in one chromatophore or
in several.

In Hematococcus we saw that chlorophyll was asso-
ciated with starch (p, 243). In Euglena there are, near the
middle of the body, a number of grains of paramylum
(H, ~), a carbohydrate of the same composition as starch

CHAP.

EUGLENA

252

(CgH,,O0,), but differing from it in remaining uncoloured

by iodine,

5 Csqary eye Wyosing woy ‘Fy fyuay aplaeg reye ‘Q—y 3 KFopo1g ssaxicd

worq) “(¢) serpoq-umypAwered puv ‘(4) moasasar ‘Qv2-2) ajonoea apyouUCD Butmoys ulloy ease ‘ye
*s[[ao-aa7ySnep ayy Jo (2x2 +2)

SHOAIaSaI puv (77) Tau ayz pur “(49) [[ea-[]eo 10 ysXo Surmoys ‘uorssy Areurq Jaye wioy Suysar ‘sy
“G) wnyaSey jo uIS110 pue (sv) jalnd (22)

yimour {(9v2 '9) MNoarasar puv (3¥) 1ods-juswSid Surmoys “| Jo pua Jolie sy1 Jo AMAIA pasrepua ‘7
“MN woy Surisurds (Z/) unpjesey a]Surs v yA yaT[ns puv Yods yuawsid

Sututofpe yA (v2 *9) aponova apovsWNOD ayy Jo wOArasar (zz) sNaponu ayy SUMoys ‘Maia pasrrpua ‘sp
*syUaUIaAOW prloualsne

ansteyuirys ey3 Aq psonpoid wioy jo saSuvyo aqy Surmoys ‘wstueS10 Surat, ayy jo sata Moy ‘Q—y

‘SipLia vuayEn gy —69 “O17

Water containing Euglena gives off bubbles of oxygen in
sunlight : as in Heematococcus the carbon dioxide in solution

in the water is decomposed in the presence of chlorophyll,

iM NUTRITION 283

its oxygen evolved, and its carbon combined with the
elements of water and used in nutrition. For a long time
Euglena was thought to be nourished entirely in this way,
but there is a good deal of reason for thinking that this is
not the case.

When the anterior end of a Euglena is very highly
magnified it is found to have the form shown in Fig. 6g, r.
It is produced into a blunt snout-like extremity, at the base
of which is a conical depression (@. 5) leading into the soft
internal protoplasm :—just the sort of depression one could
make in a clay model of Euglena by thrusting one’s finger
or the end of a pencil into the clay. From the bottom of
this tube the flagellum arises, and by its continual move-
ment gives rise to a sort of whirlpool in the neighbourhood.
By the current thus produced, minute solid food-particles
are swept down the tube and forced into the soft internal
protoplasm, where they doubtless become digested in the
same way as the substances ingested by an Amceba. That
solid particles are so ingested by Euglena has been proved
by diffusing finely powdered carmine in the: water, when
the coloured particles were seen to be swallowed in the way
described.

The depression in question serves therefore as a gu//e¢, and
its external aperture or margin (m7) as a mouth. Euglena,
like Amoeba, takes in solid food, but instead of ingesting
it at almost any”part of the body, it can do so only at one
particular point where there is a special ingestive aperture
or mouth. This is clearly a case of specialisation or differ-
entiation of structure: in virtue of the possession of a mouth
and gullet, Euglena is more highly organised than Amceba.

It thus appears that in Euglena nutrition is both holozoic
and holophytic (p. 247): very probably it is mainly holophytic
during daylight and holozoic in darkness.

254 EUGLENA CHAP.

Near the centre of the body or somewhat towards the
posterior end is a nucleus (£, 2) with a well-marked
nucleolus, and at the anterior end is a clear space (¢. vac),
looking very like a contractile vacuole. It has been shown,
however, that this space is in reality a non-contractile cavity
or reservoir (H, 7) into which the true contractile vacuole
(¢. vac) opens, and which itself discharges into the gullet.

In close relation with the reservoir is found a little bright
red speck (fg), called the pigment spot or stigma. It con-
sists of hematochrome (p. 243), and is curiously like an
eye in appearance, so much so that it is often known as the
eye-spot. Recent experiments seem to show that it is
specially sensitive to light, and is therefore a true eye in the
sense of a light-perceiving organ, although having no actual
visual function.

As in Heematococcus a resting condition alternates with
the motile phase: the organism loses its flagellum and
surrounds itself with a cyst of cellulose (c, cy), from
which, aftér a period of rest, it emerges to resume active
life.

Reproduction takes place by simple binary fission of the
resting form, the plane of division being always longitudinal
(c). Sometimes each product of division or daughter-cell
divides again: finally the two, or four, or sometimes even
eight daughter-cells emerge from the cyst as active Euglene.
A process of multiple fission has also been described,
numerous, simple, minute, active bodies or spores being pro-
duced, which gradually assume the ordinary form and size.

We have seen that typical animal-cells, such as those of
the frog (Part I, Chap. VII) are not provided with a
cellulose cell-wall and do not ‘contain chlorophyll. It is

tr ANIMALS AND PLANTS 255

characteristic, on the other hand, of most plant-cells—which
also consist of nucleated protoplasm—that they are sur-
rounded with a cellulose cell-wall, and that, in the case of
green plants, they contain chlorophyll. Speaking generally,
the nutrition of animals is holozoic, and that of green plants
holophytic ; and in correspondence with this difference in
the character of the food, most animals have an ingestive
aperture or mouth for taking in the solid food, and some
kind of digestive cavity, either permanent (stomach), or
temporary (food-vacuole) ; they also have, as a rule, some
kind of excretory apparatus. Moreover, animals are usually
capable of automatic movement, while in most plants the
organism, as a whole, exhibits no automatism, but only the
slow movements of growth.

' Let us now apply these definitions to the simple forms
described above and see how far they will help us in placing
those organisms in one or other of the two “kingdoms”
(p. 220) into which living things are divided.

Ameeba has a cell-wall, probably nitrogenous, in the rest-
ing condition: it ingests solid proteids, its nutrition being
therefore holozoic: it has a contractile vacuole : and it per-
forms amceboid movements. It may therefore be safely
considered as an animal.

Hzematococcus has a cellulose wall ; it contains chloro-
phyll and its nutrition is purely holophytic: a contractile
vacuole is present in H. lacustris but absent in H. pluvialis :
and its movements are ciliary.

Euglena has a cellulose wall in the encysted state: in
virtue of its chlorophyll it is nourished by the absorption of
carbon dioxide and mineral salts, but it can also ingest solid
food through a special mouth and gullet; it has a contractile
vacuole, and performs both euglenoid and ciliary movements.

256 MONADS CHAP,

In both these organisms we evidently have conflicting
characters: the cellulose wall and holophytic nutrition
would place them both among plants, while from the con-
tractile vacuole and active movements of both genera, and
from the holozoic nutrition of Euglena, we should group
them with animals. That the difficulty is by no means
easily overcome may be seen from the fact that both genera
are claimed at the present day both by zoologists and by
botanists.

Another mode of nutrition occurs in certain organisms
which must now be referred to very briefly.

When animal or vegetable matter is placed in water and
allowed to stand at the ordinary temperature, the well-known
process of decomposition or putrefaction (p. 11 and 152)
sooner or later sets in, the water becoming turbid and
acquiring a bad smell. A drop of it examined under the
microscope is then found to teem with very minute unicellular
organisms, some of which are known as Monads, much smaller
than Euglena. Like Heematococcus, the Monad swims about
by means of two flagella, but it contains no chlorophyll. The
putrefying infusion in which it lives contains proteids in
solution, in part split up by the process of decomposition into
simpler compounds, some of which are diffusible. As the
Monad contains no chlorophyll, its nutrition is evidently not
holophytic, and, apart from the fact that it possesses neither
mouth nor pseudopods, observation seems to show pretty
conclusively that it is not holozoic.

There remains only one way in which nutrition can take
place, namely, by absorption of the proteids and other
nutrient substances in the solution: the Monad may be
said to’live immersed in an immense cauldron of broth which
it is for ever imbibing, not by its mouth, for it has none, but
by the whole surface of its body. This is the saprophytic

II BACTERIA 257

mode of nutrition, and resembles that which occurs in certain
plants which contain no chlorophyll—the fungi. It will be
seen that the main difference between saprophytic and
holozoic nutrition is that in the former digestion, #.¢., the
process of rendering food-stuffs soluble and diffusible, takes
place outside the body, so that constructive metabolism can
begin at once.

In the Monad, the characters are again conflicting: the
probable absence of cellulose, the presence of a contractile
vacuole, and the flagella all have an ‘‘animal” look, but
the mode of nutrition is that of a fungus.

Decomposition, as already stated (p. 152), is due in the first
instance to the action of certain other minute organisms,
known as Bacteria, which appear in the earlier stages of the
putrefaction of an organic infusion. The nutrition of these
organisms, like that of Monads, is usually saprophytic and
the movements are generally ciliary ; but as they have a
definite cell-wall and no contractile vacuole, they are almost
universally classed amongst plants, while Monads are as
constantly included in the animal kingdom.

We see then that while it is quite easy to divide the
higher organisms into the two distinct groups of plants and
animals, any such separation is by no means easy in the
case of the lowest forms of life. It was in recognition of
this fact that Haeckel proposed, many years ago, to institute
a third “ kingdom,” called Pro¢ista, to include all unicellular
organisms. Although open to many objections in practice,
there is a great deal to be said for the proposal. From the
strictly scientific point of view it is quite as justifiable to
make three subdivisions of living things as two: the line
between animals and plants is quite as arbitrary as that
between protists and plants or between protists and animals,

and no more so: the chief objection to the change is that it
Pract. Zoor. Ss

258 ‘ANIMALS AND PLANTS CHAP.

doubles the difficulties by making two artificial boundaries
instead of one.

The important point for the student to recognise is
that these boundaries ave artificial, and that there are no
scientific frontiers in Nature. As in the liquefaction of
gases there is a “critical point” at which the substance
under experiment is neither gaseous nor liquid: as in a
mountainous country it is impossible to say where mountain
ends and valley begins: as in the development of an animal
it is futile to argue about the exact period when, for instance,
the egg becomes a tadpole or the tadpole a frog: so in the
case under discussion. The distinction between the higher
plants and animals is perfectly sharp and obvious, but when
the two groups are traced downwards they are found
gradually to merge, as it were, into an assembtage of organ-
isms which partake of the characters of both kingdoms, and
cannot without a certain violence be either included in or
excluded from either. When any given “ protist” has to
be classified the case must be decided on its individual
merits: the organism must be compared in detail with all
those which resemble it closely in structure, physiology, and
life-history: and then a balance must be struck and the
doubtful form placed in the kingdom with which it has, on
the whole, most points in common.

It will no doubt occur to the reader that, on the theory
of evolution (p. 222), we may account for the fact of the
animal and vegetable kingdoms being related to one another
like two trees united at the root, by the hypothesis that the
earliest organisms were protists, and that from them animals
and plants were evolved along divergent lines of descent.

ll PRACTICAL DIRECTIONS 259

PRACTICAL DIRECTIONS.

Heematococcus. Examine a drop of water containing Hzmato-
coccus, first with the low power, and then, after putting on a cover-
glass, with high power. Note—

1. Their rounded form and green (or red) colour ; the thick ced/ulose
cell-wall; the protoplasm enclosing (a) chromatophores, containing
chlorophyll (the red colour is due to another colouring matter, hemato-
chrome), and (6) a central szzcleus, seen better later; and in the active
forms, the two flagella. Sketch.

2. Dissolve out the chlorophyll by adding alcohol ; the nucleus will
then be visible, and may be made more distinct by staining with
methyl-green, magenta, or iodine. After treatment with iodine, a
bluish colouration will be seen around the small starch-containing

pyrenotas.
3. Treat some specimens either with strong iodine solution and then

with sulphuric acid (75 per cent.), or with Schulze’s solution, which is
prepared as follows :—Make about 25 cc. of a syrupy solution of zinc
chloride, and let it stand for a short time ; then pour off the clear fluid,
saturate it with potassium iodide and add iodine until the solution has
about the colour of sherry. Note the colouration of the cell-wall.

4. In the living specimens note also the mode of division into
‘4 megazooids or into 16 or 32 microzooids, and observe their move-
ments when set free. Sketch.

Euglena. Examine a drop of water containing Euglena with the
low power, then cover, and put on the high power. Note—

1. The spindle-shaped form of the body, and its changes of form in
contraction and expansion.

2. The long flagellum.

3. The superficial cuticle, and the mouth and conical depression
(gullet) at the anterior end.

4. The central part of the body, which contains chlorophyll, except
at the two ends. Near the middle is a zucleus enclosing a nucleolus,
and near the anterior end a non-contractile space into which a contrac-
tile vacuole opens, close to which a red pigment-spot is situated; the
colour of this is due to hematochrome. Grains of Jaramylum may be
recognised near the centre of the. body. Sketch before and after
-Staining as above,

260 PRACTICAL DIRECTIONS CH. II

5. Look for specimens in the resting condition, and observe if any
of them are undergoing division. Sketch.

Pacteria and Monads, Examine a drop of water containing some
putrefying animal or vegetable substance. An infusion of hay is con-
venient for this purpose :—pour boiling water on a handful of hay and
strain the fluid through blotting-paper; shortly afterwards decom-
position will set in.

The first organisms to appear in such a putrefying infusion are Bacteria,
which are so minute as to appear as mere specks under the high power
—some of them being only 34, mm. (zz45y inch) in length, or even
less. Careful examination will show globular, rod-like, and spiral
forms, all capable of movement at one phase of their existence. Monads
will appear somewhat later, and may be recognised by their ovoidal
form and their two flagella, which, however, like those of bacteria, can
only be seen under a very high power.

. CHAPTER III

PARAMECIUM, OPALINA, VORTICELLA AND ITS ALLIES—
PARASITIC AND COLONIAL ORGANISMS—BIOGENESIS AND
ABIOGENESIS—CLASSIFICATION OF THE UNICELLULAR
ORGANISMS EXAMINED.

We have now to consider certain organisms in which
differentiation has gone much further than in the unicellular
forms already considered: which have, in fact, acquired
many of the characteristics of the higher animals and plants
while remaining unicellular (compare p. 238). The study
of several of these more or less highly differentiated though
unicellular forms will occupy the present chapter.

It was mentioned above that, in the earlier stages of the
putrefaction ofan organic infusion, bacteria only were found,
and that later, monads made their appearance (p. 257).
Still later organisms much larger than monads are seen,
generally of an ovoidal form, moving about very quickly,
and seen by the use of a high power to be covered with in-
numerable fine cilia. These are called ciliate Infusoria, in
contradistinction to monads which are often known as
flagellate Infusoria:: many kinds are common in: putrefying
infusions, some occur in the intestines of the higher animals

262 PARAMCECIUM CHAP. III

while others are among the commonest inhabitants of both
fresh and salt water.

A very common ciliate infusor is the beautiful “ slipper
animalcule,” Paramecium, which from its comparatively large
size and from the ease with which all essential points of its
organization can be made out is a very convenient and
interesting object of study.

Compared with the majority of the organisms which have
come under our notice it may fairly be considered as gigantic,
being no less than 3—} mm. (200—26op) in length: in
fact it is just visible to the naked eye as a minute whitish
speck.

Its form (Fig. 70, 4) can be fairly well imitated by making
out of clay or stiff dough an elongated cylinder rounded at
one end and bluntly pointed at the other ; then giving the
broader end a slight twist ; and finally making on the side
rendered somewhat concave by the twist a wide shallow
groove beginning at the broad end and gradually narrowing
to about the middle of the body, where it ends in a tolerably
deep depression.

The groove is called the duccal groove (A & B, buc. gr):
at the narrow end is a small aperture the mouth (m7), which,
like the mouth of Euglena (Fig. 69), leads into the soft
internal protoplasm ofthe body. The surface of the creature
on which the groove is placed is distinguished as the ventral
surface, the opposite surface being upper or dorsal ; the
broad end is anterior, the narrow end posterior, the former
being directed forwards as the animalcule swims. These
descriptive terms being decided upon, it will be seen
that the. buccal groove begins on the left side of the
body, and gradually curves over to the middle of the
ventral surface.

c.vac

1
[70 270971

Fic. 70.—Paramacium caudatum.
A, the living animal from the ventral aspect, showing the covering of cilia, the
buccal groove (to the right) ending posteriorly in the mouth (zh) and gullet
(gul); several food vacuoles (/ vac), and the two contractile vacuoles (c. vac).

264 PARAMCECIUM CHAP.

B, the same in optical sections showing cuticle (cw), cortex (cort), and medulla (med);
buccal groove (6uc. gv), mouth, and gullet (gw); numerous food vacuoles
(/. vac) circulating in the direction indicated by the arrows, and containing
particles of indigo, which are finally ejected at an anal spot ; meganucleus (xz),
micronucleus (fa. x), and trichocysts, some of which (¢7ch) are shown with

_ their threads ejected. The scale to the right of this figure applies to A and B.

C, a specimen killed with osmic acid, showing the ejection of trichocyst-threads,
which project considerably beyond the cilia.

D. diagram of binary fission ; the micronucleus (fa. 2), has already divided, the
meganucleus (zz) is in the act of dividing. (From Parker's Biology: D, after
Lankester.)

As the animal swims its form is seen to be permanent,
exhibiting no contractions of either an amceboid or a
euglenoid nature. It is however distinctly flexible, often
being bent in one or other direction when passing between
obstacles such as entangled masses of weed. This perma-
nence of contour is due to the presence of a tolerably firm
though delicate cuticle (B, cw) which invests the whole surface.

The protoplasm thus enclosed by the cuticle is distinctly
divisible into two portions—an external somewhat dense
layer, the cortical layer or cortex (cort), and an internal more
fluid material, the medullary substance or medulla (med). It
will be remembered that a somewhat similar distinction of
the protoplasm into two layers is exhibited by Amoeba
(p. 231), the ectoplasm being distinguished from the endo-
plasm simply by the absence of granules. In Paramcecium
the distinction is a far more fundamental one: the cortex is
radially striated and is comparatively firm and dense, while
the medulla is granular and semi-fluid, as may be seen from
the fact that food-particles (f vac, see p. 266) move
freely in it, whereas they never pass into the cortex. The
medulla has a reticular structure similar to that of the
protoplasm of the ordinary animal-cell, consisting of a
delicate granular network the meshes of which are filled
with a transparent material. In the cortex the meshes of
the network are closer, and so form a comparatively dense
substance. The cortex also exhibits a superficial oblique
striation, forming what is called the myophan layer.

III STRUCTURE 265

The mouth (mz) leads into a short funnel-like tube, the
gullet (gv/), which is lined by cuticle and passes through the
cortex to end in the soft medulla, thus making a free com-
munication between the latter and the external water.

The cilia with which the body is covered are of approxi-
mately equal size, quite short in relation to the entire animal,
and arranged in longitudinal rows over the whole outer sur-
face. They consist of prolongations of the cortex, and each
passes through a minute perforation in the cuticle. They
are in constant rhythmical movement, like the cilia on the
epithelial cells of the frog’s mouth (p, 109), and are thereby
distinguished from the flagella of Heematococcus, Euglena,
&c., which exhibit more or less intermittent lashing move-
ments.

Near the middle of the body, in the cortex, is a large oval
nucleus (B,.) which is peculiar in taking on a uniform tint
when stained, showing none of the distinction into.chroma-
tin and achromatin which is so marked a feature in many of
the nuclei we have studied. It has alsoa further peculiarity :
against one side of it in P. caudatum is a small oval structure
(pa. nu) which is also deeply stained by, ¢.g., magenta or
carmine. This is the mzcronucleus: it is to be considered
as a second, smaller nucleus, the larger body being dis-
tinguished as the meganucleus. In the closely allied
P. aurelia, there are two micronuclei.

There are two contractile vacuoles (¢. vac), one situated at
about a third of the entire length from the anterior end of
the body, the other at about the same distance from the
posterior end: they occur in the cortex.

The action of the contractile vacuoles is very beautifully
seen in a Paramcecium at rest: it is particularly striking in a
specimen subjected to slight: pressure under a cover glass,
but is perfectly visible in one which has merely temporarily

266 PARAMCECIUM CHAP.

suspended its active, swimming movements. It is then seen
that during the diastole, or phase of expansion of each
vacuole, a number—about six to ten—of delicate, radiating,
spindle-shaped spaces filled with fluid appear round it, like
the rays of a star (upper vacuole in a & B): the vacuole
itself contracts or performs its systo/e, completely disappearing
from view, and immediately afterwards the radiating canals
flow together and refill it, becoming themselves emptied and
therefore invisible for an instant (lower vacuole in a & B)
but rapidly appearing once more. There seems to be no
doubt that the water taken in with the food is collected
into these canals, emptied into the vacuole, and finally dis-
charged into the surrounding medium.

The process of feeding can be very conveniently studied
in Paramcecium by placing in the water some finely-divided
carmine or indigo. When the creature comes into the
neighbourhood of the coloured particles, the latter are swept
about in various directions by the action of the cilia: some
of them however are certain to be swept into the neigh-
bourhood of the buccal groove and gullet, the cilia of
which all work downwards, 7.e., towards the inner end of the
gullet. The grains of carmine are thus carried into the gullet,
where for an instant they lie surrounded by the water of
which it is full: then, instantaneously, probably by the con-
traction of the tube itself, the animalcule performs a sort of
gulp, and the grains with an enveloping globule of water or
Jood-vacuole are forced into the medullary protoplasm.
This process is repeated again and again, so that in any
well-nourished Paramcecium there are to be seen numerous
globular spaces filled with water and containing particles of
food—or in the present instance of carmine or indigo. At
every gulp the newly-formed food-vacuole pushes, as it were,
its predecessor before it : contraction of the medullary pro-

U1 NUTRITION 267

toplasm also takes place in a definite direction, and thus a
circulation of food-vacuoles is produced, as indicated in
Fig. 70 8 by arrows.

After circulating in this way for some time the water of
the food-vacuoles is gradually absorbed, being ultimately
excreted by the contractile vacuoles, so that the contained
particles come to lie in the medulla itself (refer to figure).
The circulation still continues, until finally the particles are
brought to a spot situated about half way between the mouth
and the posterior end of the body: here if carefully watched
they are seen to approach the surface and then to be suddenly
ejected. The spot in question is therefore to be looked upon
as a potential azus, or aperture for the egestion of feeces or
undigested food-matters. It is a potential and not an actual
anus, because it is not a true aperture but only a soft place
in the cortex through which, by the contractions of the
medulla, solid particles are easily forced.

Of course when Paramcecium ingests, as it usually does,
not carmine but minute living organisms, the latter are
digested as they circulate through the medullary protoplasm,
and only the non-nutritious parts cast out at the anal spot.
It has been found by experiment that this infusor can digest
not only proteids but also starch and perhaps fats. The
nutrition of Paramcecium is therefore characteristically
holozoic.

It was mentioned above (p. 264) that the cortex is ra-
dially striated in optical section. Careful examination with
a very high power shows that this appearance is due to the
presence in the cortex of minute spindle-shaped bodies (a
and 8, ¢vch) closely arranged in a single layer and perpen-
dicular to the surface. These are called /richocysts.

When a Paramecium is killed, either by the addition of
some poisonous reagent or by simple pressure of the cover

268 PARAMECIUM CHAP.

glass, it frequently assumes aremarkable appearance. Long
delicate threads suddenly appear, projecting from its surface
in all directions (c) and looking very muchas if the cilia had
suddenly protruded to many times their original length. But
these filaments have really nothing to do with the cilia ; they
are contained under ordinary circumstances in the trichocysts,
probably coiled up; and by the contraction of the cortex
consequent upon any sudden irritation they. are projected i in
the way indicated. In Fig. 70 8, a few trichocysts (¢rch) are
‘shown in the exploded condition, z.e., with the threads pro-
truded. Most likely these bodies are weapons of offence like
the very similar, structures (nematocysts) found in polypes

(see p. 295, Fig. 75).

Parameecium multiplies by simple fission, the division of
the body being always preceded by the elongation and
subsequent division of the mega- and micronucleus
(Fig. 70, D).

Conjugation (p. 237) also occurs, usually after multi-
plication by fission has gone on for some time. Two Para-
meecia come into contact by their ventral faces, and in each
of these conjugating individuals or gametes the meganucleus
and micronucleus undergo a somewhat complicated series of
changes, the essential part of the process being the fusion of
two products of the division of the micronuclei, one from
each gamete, each of which then contains a single nuclear
body, the conjugation-nucleus, formed by the union of nuclear
matter derived from two distinct individuals, and therefore
comparable to the nucleus of the oosperm in the higher
animals (p. 197). In this case, however, the two entire
gametes do not unite into one, but separate after the process
iscompleteand begin once more to lead an independent exist-
ence, when once more ordinary transverse fission takes place.

I OPALINA 269

It will be noticed that, in the present instance, conjugation
is not a process of multiplication : it has been ascertained
that during the time two infusors are conjugating each
might have produced several thousand offspring by con-
tinuing to undergo fission at the usual rate. The importance
of the process lies in the exchange of nuclear material
between the two conjugating individuals: without such
exchange these organisms are said to undergo a gradual
process of senile decay characterized by diminution in size
and degeneration in structure.

Some ciliate Infusoria are parasites. Parasites are organisms
which live in association with other organisms, the ready-
digested food of which they utilize or even nourish them-
selves from the tissues of the forms they infest. It will be
interesting to compare Paramcecium with an Infusor which
lives in the intestine of the common frog, and is known
as Opalina ranarum.

Opalina has a flattened body with an oval outline (Fig. 71,
A, B), and full-sized specimens may be as much as one
millimetre in length. The protoplasm is divided into cortex
and medulla, and is covered with a cuticle; the cilia are
equal-sized and uniformly arranged in longitudinal rows over
the whole surface.

On a first examination no nucleus is apparent, but after
staining, a large number of globular nucleican be seen (x, 7) :
these nuclei multiply within the body of the infusor.

The ‘presence of numerous nuclei in Opalina is a fact
worthy of special notice. The unicellular organisms we have
studied are uninucleate as well as unicellular (compare,
however, Fig. 67, B): the higher animals consist of numerous
cells each with a nucleus: Opalina, on the other hand, is
multinucleate but its protoplasm is undivided, so that it

270 OPALINA CHAP.

presents a condition of things intermediate between the
unicellular and the multicellular types of structure.

Fic. 71.—Ofalina ranarum.
A, living specimen, surface view, showing longitudinal rows of cilia.
B, the same, stained, showing numerous nuclei (#7) in various stages of division.
C, 1—6, stages in the division of a nucleus.
D, longitudinal fission.
E, transverse fission.
F, the same in a specimen reduced in size by repeated division.
G, fal product of successive divisions.
H, encysted form.
I, uninucleate form produced from cyst.
K, the same after multiplication of the nucleus has begun. (From Parker's Biology:
A—C, after Pfitzner ; D—K, from Saville Kent, after Zeller.)

There is no contractile vacuole, and no trace of either
mouth or gullet, so that the ingestion of solid food is impos-
sible. The creature lives, as already stated, in the intestine

Il] NUTRITION AND REPRODUCTION 271

of the frog: it is therefore, like the worms you have probably
noticed in the frog’s urinary bladder and lungs (pp. 33 and
153), an internal parasite, or endoparasite, having the frog
as its Aost. The intestine contains the partially-digested
food of the frog, and it is by the absorption of this that the
Opalina is nourished. Having no mouth, it feeds solely by
imbibition: whether it performs any kind of digestive
process itself is not certainly known, but the analogy of
other mouthless parasites leads us to expect that it simply
absorbs food ready digested by its host, upon which it is
dependent for a constant supply of soluble and diffusible
nutriment.

Thus Opalina, in virtue of its parasitic mode of life, is
saved the performance of certain work—the work of digestion,
that work being done for it by its host. This is the essence
of internal parasitism: an organism exchanges a free life,
burdened with the necessity of finding food for itself, for
existence in the interior of another organism, on which, in
one way or another, it levies blackmail.

Note the close analogy between the nutrition of an internal
parasite like Opalina and the saprophytic nutrition of a
monad (p. 256). In both, the organism absorbs proteids
rendered soluble and diffusible, in the one case by the
digestive juices of the host, in the other by the action of
putrefactive bacteria.

The reproduction of Opalina presents certain points of
interest largely connected with its peculiar mode of life. It
is obvious that if the Opalinee simply went on multiplying,
by fission or otherwise, in the frog’s intestine, the population
would soon outgrow the means of subsistence: moreover.
when the frog died there would be an end of the parasites,
What is wanted in this, as in other internal parasites, is some

272 OPALINA CHAP,

mode of multiplication which shall serve as a means of dis-
persal, or in other words, enable the progeny of the parasite
to find their way into the bodies of other hosts, and so start
new colonies:instead of remaining to impoverish the mother
country.

Opalina multiplies by a somewhat peculiar process of
binary fission : an animalcule divides in an oblique direction
(Fig. 71 p), and then each half, instead of growing to the
size of the parent cell, divides again transversely (E). The
process is repeated again and again (F), the plane of division
being alternately oblique and transverse, until finally small
bodies are produced (c), about j,-3'5 mm. in length, and
containing from two to four nuclei.

If the parent cell had divided simultaneously into a num-
ber of these little bodies the process would have been one of
multiple fission (p. 254): as it is, it forms an interesting link
between simple and multiple fission.

Opalina ranarum multiplies in this way in the spring—z.e.
during the frog’s breeding season. Each of the small pro-
ducts of division (c) becomes encysted (H), and in this
passive condition is passed out with the frog’s excrement,
probably falling on to a water-weed or other aquatic object.
Nothing further takes place unless the cyst is swallowed by
a tadpole, as must frequently happen when these creatures,
produced in immense numbers from the frogs’ eggs, browse
upon the water-weeds which form their chief food.

Taken into the tadpole’s intestine, the cyst is burst or
dissolved, and its contents emerge as a lanceolate mass of
protoplasm (1), containing a single nucleus and covered with
cilia. This, as it absorbs the digested food in the intestine’
of its host, grows, and at the same time its nucleus divides
repeatedly (kK) in the way already described, until by the time
the animalcule has attained the maximum size it has also

II VORTICELLA 273

acquired the large number of nuclei characteristic of the
genus.

Here, then, we have an interesting case of progressive
differentiation or development (p. 9): the organism begins
life as a very small uninucleate mass of protoplasm, and as it
increases in size increases also in complexity by the repeated
binary fission of its nucleus.

The next organism we have to consider is a ciliated infusor
even commoner than those just described. It is hardly
possible to examine the water of a pond with any care with-
out finding in it, sometimes attached to weeds, sometimes
to the legs of water-fleas, sometimes to the sticks and stones
of the bottom, numbers of exquisitely beautiful little creatures,
each like an inverted bell with a very long handle, or a wine-
glass with a very long stem. These are the well-known
“ bell-animalcules ;” the commonest among them belong to
various species of the.genus Vorticella.

The first thing that strikes one about Vorticella (Fig. 72, )
is the fact that it is permanently fixed, like a plant, the
~ proximal or near end of the stalk being always firmly fixed
to some aquatic object, while to the distal or far end the
body proper of the animalcule is attached.

But in spite of its peculiar form it presents certain very
obvious points of resemblance to Paramcecium and Opalina.
The protoplasm is divided into cortex (c, cov?) and medulla
(med), and is invested with a delicate cuticle (cw). There is
a single contractile vacuole (c. vac) the movements of which
are very readily made out owing to the ease with which the
attached organism is kept under observation. There is a
meganucleus (z«) remarkable for its elongated band-like
form, and having in its neighbourhood a small rounded

micronucleus. Cilia are also present, but the way in which
Pract. Zoou. T

e

Fic. 72.—l orticedla.
\, living specimen fully expanded, showing stalk (s¢) with axial fibre (ax. /),
peristome (fer), disc (7), mouth (wth), gullet (gz2/), and contractile vacuole.

13, the same, bent on its stalk and with the disc turned away from the observer.

C, optical section of the same, showing cuticle (cz), cortex ‘(cort), medulla (wed),

. nucleus (vz), gullet (g22), several food-vacuoles, and anus (az), as well as the
structures shown i in A,

DI, a half-retracted and D? a fully-retracted specimen, showing the coiling of the
stalk and overlapping of the disc by the peristome.

E}, commencement of binary fission; E2, completion of the process; E#, the
barrel-shaped product of division swimming freely in the direction indicated
by the arrow.

Fl, a specimen dividing into a megazooid and several microzooids (#7); F?,
Ajvision into one mega- and one Mic rospoid:

G1, G2, two stages in conjugation showing the gradual absorption of the micro-
gamete (mm) into the megagamete.

H!. multiple fission of encysted form, the nucleus dividing into numerous masses ;
H2, spore formed by multiple fission ; H3—H7, development of the spore ; H4 is
undergoing binary fission. (From Parker's Biolog : E—H after Saville Kent.

CHAP. III VORTICELLA 275

they are disposed is very peculiar and characteristic. To
understand it we must study the form of the body a little
more closely.

The conical body is attached by its apex or proximal
end to the stalk: its base or distal end is expanded
so as to form a thickened rim, the per7stome ( per), within
which is a plate-like body elevated on one side, called
the dsc (d), and looking like the partly raised lid of a
chalice. Between the raised side of the disc and the peri-
stome is a depression, the mouth (mh), leading into a
conical gullet (gid).

There is reason for thinking that the whole proximal region
of Vorticella answers to the ventral surface of Paramcecium,
and its distal surface with the peristome and disc to the
dorsal surface of the free-swimming genus: the mouth is to
the left in both. 2:

A single row of cilia is disposed round the inner border
of the peristome and continued on the one hand down the
gullet, and on the other round the elevated portion of the
disc ; the whole row of cilia thus takes a spiral direction
The rest of the body is completely bare of cilia.

The movements of the cilia produce a very curious optical
illusion: as one watches a fully-expanded specimen it is
hardly possible to believe that the peristome and disc are
not actually revolving—a state of things which would imply
that they were discontinuous from the rest of the body. As
a matter of fact the appearance is due to the successive
contraction of all the cilia in the same direction, and is
analogous to that produced by a strong wind on a field of
corn or long grass. The bending down of successive
blades of grass produces a series of waves travelling across
the field in the direction of the wind. If instead of a field
we had a large circle of grass, and if this were acted upon

iP 2

276 VORTICELLA CHAP.

by a cyclone, the wave would travel round the circle, which
would then appear to revolve.

Naturally the movement of the circlet of cilia produces a
small whirlpool in the neighbourhood of the Vorticella, as
can be seen by introducing finely-powdered carmine into the
water. It is through the agency of this whirlpool that food
particles are swept into the mouth, surrounded, as in
Parameecium, by a globule of water: the food-vacuoles
(c) thus constituted circulate in the medullary protoplasm,
and the non-nutritive parts are finally egested at an anal
spot (az) situated near the base of the gullet.

The stalk (a, s¢) consists of a very delicate, transparent,
outer substance, which is continuous with the cuticle of the
body and contains a delicate axial fibre (ax. f) running along
it from end to end in a somewhat spiral direction. This
fibre is a prolongation of the cortex of the body (c): under
a very high power it appears granular or delicately striated,
the striz being continued into the cortex of the proximal
part of the body.

A striking characteristic of Vorticella is its extreme
irritability, z.e., the readiness with which it responds to any
external stimulus. The slightest jar of the microscope, the
contact of some other organism, or even a current of water
produced by some free-swimming form like Paramcecium, is
felt directly by the bell-animalcule, and is followed by an
instantaneous change in the relative position of its parts.
The stalk becomes coiled into a close spiral so as to have
a mere fraction of its original length, and the body from
being bell-shaped becomes globular, the disc being with-
drawn and the peristome closed over it (p!, D?).

The coiling of the stalk leads us to the consideration of
the particular form of contractility called muscular, which is
met with in multicellular animals, e.g. the frog (p. 60). It

rT AXIAL FIBRE 2.7

was mentioned above that while the stalk in its fully
expanded condition is straight, the axial fibre is not straight,
but forms a very open spiral, z.z., it does not lie in the centre
of the stalk, but at any transverse section is nearer the
surface at one spot than elsewhere, and this point as we
ascend the stalk is directed successively to all points of the
compass.

Now suppose that the axial fibre lndergoes a sudden
contraction, that is to say, a decrease in length accompanied
by an increase in diameter, since as we have already
seen there is no decrease in volume in protoplasmic
contraction. There will naturally follow a corresponding
shortening of the elastic cuticular substance which forms the
outer layer of the stalk. If the axial fibre were entirely
towards one side of the stalk, the result of the contraction
would be a flexure of the stalk towards that side, but, as its
direction is spiral, the stalk is bent successively in every
direction, that.is, is thrown into a close, spiral coil.

The axial fibre is therefore a portion of the protoplasm
which possesses the property of contractility in a special
degree; in which moreover contraction takes place in a
definite direction— the direction of the length of the fibre—
so that its inevitable result is to shorten the fibre and con-
sequently to bring its two ends nearer together. This is the
essential characteristic of a muscular contraction, and the
axial fibre in the stalk of Vorticella is therefore to be looked
upon as the first instance of a clearly differentiated muscle
which has come under our notice amongst unicellular
animals.

There are some interesting features in the reproduction of
Vorticella. It multiplies by binary fission, dividing through
the long axis of the body (Fig. 72, Et, £*). Hence it is
generally said that fission is longitudinal, not transverse, as

278 VORTICELLA ‘CHAP

in Paramcecium. But on the theory (p. 275) that the peris
tome and disc are dorsal and the attached end ventral,
fission is really transverse in this case also.

It will be seen from the figures that the process takes place
by a cleft appearing at the distal end (z!) and gradually
deepening until there are produced two complete and full-
sized individuals upon a single stalk (£). This state of
things does not last long : one of the two daughter-cells takes
on a nearly cylindrical form, keeps its disc and peristome
retracted, and acquires a new circlet of cilia near its proximal
end (z?): it then detaches itself from the stalk, which it
leaves in the sole possession of its sister-cell, and swims about
freely for a time in the direction indicated by the arrow.
Sooner or later it settles down, becomes attached by its
proximal end, loses its basal circlet of cilia, and develops a
stalk, which ultimately attains the normal length.

The object of this arrangement is obvious. If, when a
Vorticella divided, the plane of fission extended down the
stalk until two ordinary fixed forms were produced side by
side, the constant repetition of the process would so increase
the numbers of the species in a given spot that the food-
supply would inevitably run short. This is prevented by
one of the two sister-cells produced by fission leading a free
existence long enough to enable it to emigrate and settle in
a new locality, where the competition with its fellows will be
less keen. The production of these free-swimming zooids
is therefore a means of dispersal (p. 272): contrivances
having this object in view are a very general characteristic
of fixed as of parasitic organisms.

Conjugation occasionally takes place, and presents certain
peculiarities. A Vorticella divides either into two unequal
halves (F?) or into two equal halves, one of which divides
again into from two to eight daughter-cells (r!). There are

II CONJUGATION 279

thus produced from one to eight mcrozooids which resemble
the barrel-shaped form (x%) in all but size, and like it become
detached and swim freely by means of a basal circlet of cilia.
After swimming about for a time, one of these microzooids
comes in contact with an ordinary form or megazooid, when
it attaches itself to it near the proximal end (c!), and under-
goes gradual absorption (G?), the mega- and microzooids
becoming completely and permanently fused to form a
zygote (p 197). Asin Paramcecium, conjugation is followed
by increased activity in feeding and dividing.

Notice that in this case the conjugating bodies or gametes
are not of equal size and similar characters ; but one, which
is conveniently distinguished as the mzcrogamete (= micro-
zooid) is relatively small and active, while the other or
megagamete (= megazooid, or ordinary individual) is rela-
tively large and passive. As we have seen in the case of
the frog (pp. 195 and 196), this differentiation of the
gametes is precisely what we get in the higher animals,
and, in fact, in almost all organisms with two sexes: the
microgamete being the male, the megagamete the female
conjugating body (see p. 197).

The result of conjugation is somewhat different in the two
cases already studied : in Paramcecium no zygote is formed,
conjugation being a mere temporary union (p. 268): in
Vorticella the zygote is an actively moving and feeding
body, indistinguishable from an ordinary individual of the
species.

Vorticella sometimes encysts itself (Fig. 72, H'), and the
nucleus of the encysted cell has been observed to break up
into a number of separate masses, each doubtless surrounded
by a layer of protoplasm. After a time the cyst bursts, and
a number of small bodies or spores (a?) emerge from it, each
containing one of the products of division of the nucleus.

280 VORTICELLA CHAP.

These acquire a circlet of cilia (H%), by means of which they
swim freely, and they are sometimes found to multiply by
simple fission (H*). Finally, they settle down (H5) by the
end at which the cilia are situated, the attached end begins
to elongate into a stalk (H°), this increases in length, the
basal circlet of cilia is lost, and a ciliated peristome and
disc are formed at the free end (H’). In this way the
ordinary form is assumed by a process of development
(p. 273), and, moreover, the free-swimming young (H%), to
which the spores formed by division of the encysted proto-
plasm give rise, differ strikingly in form and habits from the
adult. This is expressed by saying that development is in
this case accompanied by a metamorphosis, this word literally
meaning simply a change, being always used in zoology to
express a striking and fundamental difference in form and
habit between the young and the adult; as, for instance,
between the tadpole and the frog (p. 9) or between the
caterpillar and the butterfly. Itis obvious that in the
present instance metamorphosis is another means of ensuring
dispersal.

In Vorticella, as we have seen, fission results not in the
production of equal and similar daughter-cells, but of one
stalked and one free-swimming form. It is however quite
possible to conceive of a Vorticella-like organism in which
the parent cell divides into two equal and similar products,
each retaining its connection with the stalk. If this process
were repeated again and again, and if, further, the plane of
fission were extended downwards so as to include the distal
end of the stalk, the result would be a branched, tree-like
stem with a Vorticella-like body at the end of every
branch.

As a matter of fact, this process takes place not in Vorti-
cella itself, but in some nearly allied infusors, such as

I COLONIES 281

- Carchesium and Epistylis. Each of these forms consists of a
main stem attached by its proximal end and giving off at its
distal end several branches, on each of which numerous
bell-animalcules with short stalks are borne, like foxgloves or
Canterbury-bells on their stem.

We see, then, that Carchesium and Epistylis differ from all
our previous types in being compound organisms. The entire
“tree” is called a colony or stock, and each separate bell-
animalcule borne thereon is an ¢ndividual or x0otd,
morphologically equivalent to a single Vorticella or
Parameecium. The colony is therefore an zndividual of a
higher grade than the zooid, and such a multicellular animal
as a frog is an individual of a higher grade still.

As in Vorticella, the stem of Carchesium consists of a
cuticular sheath with an axial muscle-fibre which, at the distal
end of the main stem, branches like the stem itself, a
prolongation of it being traceable to each zooid; so that
the muscular system is common to the whole colony,
and any shock causes a general contraction of all the
zooids. The stalk of Epistylis, on the other hand, is non-
contractile.

The study of the foregoing living things and especially of
Bacteria (p. 152), the smallest and probably the simplest of all
known organisms, naturally leads us to the consideration
of one of the most important problems of biology—the
problem of the origin of life.

In all the higher organisms we know that each individual
arises in some way or other from a pre-existing individual :
no one doubts that every bird now living arose by a process
of development from an egg formed: in the body of a
parent bird, and that every tree now growing took its origin
either from a seed or from a bud produced bya parent plant.

282 BIOGENESIS CHAP,

But there have always—until comparatively recently, at any
rate—been upholders of the view that the lower forms of life,
bacteria, monads, and the like, may under certain circum-
stances originate independently of pre-existing organisms :
that, for instance, in a flask of some organic infusion
boiled so as to kill any living ‘things present in it, fresh
forms of life may arise de movo—may in fact be created
then and there.

We have therefore two theories of the lower organisms,
the theory of Bvogenesis, according to which each living
thing, however simple, arises by a natural process of bud-
ding, fission, spore-formation, or what not, from a parent
organism: and the theory of Adsogenests, or as it is some-
times called Spontaneous or Eguivocal Generation, accord-
ing to which fully formed living organisms sometimes
arise from not-living matter.

In former times the occurrence of abiogenesis was uni-
versally believed in. The expression that a piece of meat
has “bred maggots” ; the opinion that parasites such as the
gall-insects of plants or the tape-worms in the intestines of
animals originate where they are found ; the belief still held
in some rural districts in the occurrence of showers of frogs,
or in the transformation of horse-hairs kept in water into
eels ; all indicate a survival of this belief.

As accurate enquiries into these matters were made, the
number of cases in which equivocal generation was supposed
to occur was rapidly diminished. It was not surprising
however, considering the rapidity with which Bacteria and
Monads were found to make their appearance in organic
substances and infusions, that many men of science imagined
them to be produced abiogenetically. The rapid multiplica
tion of these forms means, of course, that a certain amount
of fresh living protoplasm has been formed out of the

III SPONTANEOUS GENERATION 283

constituents of the hay-infusion, through the agency in the
first instance of a single living Bacterium. The question
naturally arises, Why may not the formation of protoplasm
take place independently of this insignificant speck of living
matter ?

It must not be thought that this question is in any way
a vain or absurd one. That living protoplasm has at some
period of the world’s history originated from not-living
matter seems a necessary corollary of the doctrine of
evolution, and is obviously the very essence of the doctrine
of special creation (p. 221); and there is no @ priori reason
why it should be impossible to imitate the unknown con-
ditions under which the process took place. But at present
we have absolutely no data towards the solution of this
fundamental problem.

Experiments conducted with proper precautions, however,
all tell the same tale: they prove conclusively that in
putrescible infusions that have been properly steri/ized,—z.e.
thoroughly boiled so as to kill any organisms they may
contain—and adequately protected from the entrance of
atmospheric germs, no micro-organisms ever make their
appearance. So that the last argument for abiogenesis has
been proved to be fallacious, and the doctrine of biogenesis
shown, as conclusively as observation and experiment can
show it, to be of universal application as far as existing
conditions known to us are concerned. It is also
necessary to add that the presence of microbes in consider-
able quantities in our atmosphere has been proved
experimentally.

There is another question intimately connected with that
of Biogenesis, although strictly speaking quite independent
of it. It is a matter of common observation that, in both
animals and plants, like produces like: that a cutting from

284 HOMOGENESIS CHAP.

a willow will never give rise to an oak, nor a snake emerge
from a hen’s egg. In other words, ordinary observation
teaches the general truth of the doctrine of Homogenests.

But there has always been a residuum of belief. in the
opposite doctrine of eferogenesis, according to which the
offspring of a given animal or plant may be something
utterly different from itself, a plant giving rise to an animal
or vice versa, a lowly to a highly organised plant or animal
and so on. Perhaps the most extreme case in which hetero-
genesis was once seriously believed to occur is that of the
“‘barnacle-geese.” Buds of a particular tree growing
near the sea were said to produce barnacles, and these
falling into the water to develop into geese. This sounds
absurd enough, but within the last twenty years two or three
men of science have described, as the result of repeated
observations, the occurrence of quite similar cases among
microscopic organisms. For instance, the blood corpuscles
of the silkworm have been said to give rise to fungi, Euglene
to thread-worms, and so on.

It is proverbially difficult to prove a negative, and it might
not be easy to demonstrate, what all competent naturalists
must be firmly convinced of, that every one of these sub-
posed cases of heterogenesis is founded either upon errors
of observation or upon faulty inductions from correct
observations.

It is obvious that the only way in which a case of hetero-
genesis could be proved would be by actually watching the
transformation, and this no heterogenist has ever done; at
the most, certain supposed intermediate stages between the
extreme forms have been observed—say, between a Euglena
and a thread-worm—and the rest of the process inferred.
On the other hand, innumerable observations have been
made on these and other organisms, the result being that

ur HETEROGENESIS 285

each species investigated has been found to go through a
definite series of changes in the course of its development,
the ultimate result being invariably an organism resembling
in all essential respects that which formed the starting point
of the observations: Euglenz always giving rise to Euglenz
and nothing else, Bacteria to Bacteria and nothing else, and
so on.

There are many cases which imperfect knowledge might
class under heterogenesis, such as the origin of frogs from
tadpoles or of jelly-fishes from polypes (see Chapter IV.),
but in these and many other cases the apparently anomalous
transformations have been found to be part of the normal
and invariable cycle of changes undergone by the organism
in the course of its development: the frog always gives
rise ultimately to a frog, the jelly-fish toa jelly-fish. If
a frog at one time produced a tadpole, at another a trout,
at another a worm: if jelly-fishes gave rise sometimes to
polypes, sometimes to infusoria, sometimes to cuttle-fishes,
and all without any regular sequence—¢hat would be
heterogenesis.

It is perhaps hardly necessary to caution the reader against
the error that there is any connection between the theory of
heterogenesis and that of organic evolution. It might be
said—if, as naturalists tell us, dogs are descended from
wolves and jackals, and birds from reptiles, why should not,
for instance, thread-worms spring from Euglene or Infusoria
from Bacteria? To this it is sufficient to answer that the
evolution of one form from another takes place by a series
of slow, orderly, progressive changes going on through a
long series of generations (p. 222); whereas heterogenesis
presupposes the casual occurrence of sudden transformations
in any direction—/.e., leading to either a less or a more highly
organised form—and in the course of a single generation.

286 PROTOZOA CHAP,

Each of the organisms which we have studied in this
and the two previous chapters consists of a single cell—or
in the case of Carchesium and Epistylis of a colony of cells
to a large extent independent of one another. They are
therefore placed in the lowest primary division of the animal
kingdom—the pbylum Protozoa (p. 220). This phylum is
subdivided into a number of classes, examples of certain of
which we have examined. Those in which, like the Amceba,
the amceboid form is predominant constitute the class
Rhizopoda : those in which, like the Monads and Euglene
(Flagellata), the flagellate form is predominant are often
included with the ciliated forms (Ciliata)—such as Para-
meecium, Opalina and Vorticella—in a single class, the
Lnfusoria.

The animals above the Protozoa are classed, as we have
seen, in a number of different phyla, but as they are all
multicellular they are often spoken of collectively as the
Metazoa, one of the simplest of which we must next
examine.

PRACTICAL DIRECTIONS.

Paramoecium.—Spread a little cotton-wool on a slide over a drop
of water containing Parameecia, in order to entangle them in its meshes,
and put on a cover-glass. Examine first with the low power and then
with the high power. Note—

1. The elongated form of the ‘animal; its anterior (more rounded)
and posterior (more pointed) end; its flattened dorsal and ventral
surfaces ; and its buccal groove and mouth, on the ventral surface.

2. The active movements, due to the cz//a covering the body.

3. The marked distinction between cortex and medulla.

4. The characters of the elastic cortex :—(a) the superficial cutzcle,
and deeper striated layer ; (4) the cé/za arising from the deeper layer,
and projecting through the cuticle ; (c) the ¢rzchocysts—small oval sacs,
imbedded in the deeper layer ; (d) the two spherical contractile vacuoles,
situated in the deeper layer on the dorsal side: note that canals radiate

I PRACTICAL DIRECTIONS 287

from them when they contract; (e) the cilia lining the buccal groove.
(The potential azus behind the mouth can only be seen at the moment
of defeecation.) Compare the mode of feeding with that of Amceba.

5. The characters of the meduila :—(a) The food vacuoles and their
circulation ; (4) the meganzucleus and micronucleus, which can be better
seen when stained. Sketch.

6. Add methyl green. Then note again the structure of cortex and
medulla, as well as—(a) the oval meganucleus, near the middle of the
body ; (4) the wzcronucleus, a smaller body, close to the meganucleus ;
(c) the extruded ¢réchocysts. Sketch.

7. Look out for specimens undergoing transverse /isszon, and also for
others in process of conjugation. Sketch.

Permanent preparations ‘may be made as directed in the case of
Ameeha (p. 239).

Opalina.—Make an incision in the rectum or small intestine
of a freshly-killed frog, and mount a little of its contents in water on
a slide. Note the oval and flattened form of the Opaline and their
uniform covering of cz/a; the cuticle, cortex and medulla; and the
absence of a mouth and contractile vacuole. Stain as before, and
make out the numerous zzc/ez, Look out for the products of division
free and encysted. :

Vorticella. Mount some specimens in a drop of water, and examine
with the low power. The Vorticella will be seen to have the form
of a wine-glass or bell with along stem. The stem frequently contracts
spirally, the edge of the bell being at the same time turned in, so as to
give the animal a rounded form. Put on the high power and note—

1. The form of the bell, its thickened rim or Zerzstome, and the dzsc,
which forms a cover to the bell.

2. The mouth and gullet, opening on one side between the peristome
and disc, which is here slightly raised. (The eas opens:into the oral
depression, but can only be seen at the moment of defzcation.)

3. The séugle row of cétia round the peristome and extending down
the gullet on the one hand, and on to the raised portion of the disc on
the other. Run ina little finely powdered indigo or carmine under the
cover-glass, and note the currents produced by the cilia: the granules
of pigment will be carried down the gullet.

4. The contractions of the bell and stalk.

5. The structure of the cortex, which is similar to that of Para-
meoecium, except that the cilia have a restricted distribution, and that

288 PRACTICAL DIRECTIONS CHAP. III

there are no trichocysts. Moreover, in the stalk (into which the
medulla does not extend) the deeper layer of the cortex gives rise to a
central contractile axzal fibre, by means of which the stalk can contract
spirally.

6. The medulla, which circulates and contains food-vacuoles, as in
Parameecium. The single spherical contractzle vacuole is also situated
in it, as well as an elongated and curved meganucleus and a small
micronucleus (not easy to distinguish). Sketch.

7. Make preparations as directed under Paramcecium. Sketch.

8. Look out for specimens undergoing /isszoz, noting the different
stages, and the second, aboral ring of cilia on ove of the daughter
individuals, which eventually becomes detached. Note these /ree
swimming forms, and also search for conjugating individuals—a small
free Vorticella uniting with a large stationary one. Sketch.

Carchesium or Epistylis,—You will very likely find specimens of
one of these, or of an allied genus, amongst the Vorticelle. Note
that several individual zoozds are borne upon a branched stalk, together
forming a colony.

CHAPTER IV

HYDRA: BOUGAINVILLEA—-ALTERNATION OF GENERATIONS—
CHARACTERS OF THE PHYLUM CQ:LENTERATA.

A CAREFUL search in ponds will often result in the
capture of some small organisms known as “ fresh-water
polypes,” belonging to the genus Aydra.

Although far from uncommon in pond-water, Hydra is not
always easy to find, being rarely abundant and by no means
conspicuous. In looking for it the best plan is to fill either
a clear glass bottle or beaker, or a white saucer, with weeds
and water from a pond and to let it remain undisturbed for a
short time. If the gathering is successful there will be seen
adhering to the sides of the glass, the bottom of the saucer,
or the weeds, little white, tawny, or green bodies, about as
thick as fine sewing cotton, and 2—6 mm. in length. They
adhere pretty firmly by one end, and examination with a
pocket lens shows that from the free extremity a number of
very delicate filaments, barely visible to the naked eye, are
given off.

Under the low power of a compound microscope a Hydra
(Fig. 73) is seen to have a cylindrical oody attached by a
flattened base to a weed or other aquatic object, and bear-
ing at its opposite or distal end a conical structure, the
U

Pract. Zoot,

290 HYDRA CHAP,

. Fic. 73.—Hydra,

c. A, two living specimens of H. viridis attached to a bit of weed. The larger
specimen is fully expanded, and shows the elongated body ending distally in the
hypostome (Ay), surrounded by tentaeles (2), and three buds (4d.1, Ad.?, 6@.3) in
different stages of development ; a small water-flea (a) has been captured by one
tentacle. The smaller specimen (to the right and above) is in a state of complete
retraction, the tentacles (¢) appearing like papilla.

EB, A. fusca, showing the mouth (th) at the end of the hypostome (Ay), the

~ circlet of tentacles (4), two spermaries (sAy), and an ovary (ovy,
C, a Hydra creeping on a flat surface by looping movements.
D, a specimen crawling on its tentacles. (From Parker’s Biology: C and D after
‘W. Marshall.)

hypostome (Ayp), at the apex of which is a circular aperture,
the mouth (th). At the junction of the hypostome with
the body proper are given off from six to eight long delicate

Ivo HYDRA 291

tentacles (¢) arranged in a circlet or whorl. A longitudinal
section shows that the body is hollow, containing a spacious
cavity, the exteron (Fig. 74, a, ent. cav), which communicates
with the surrounding water by the mouth. The tentacles
are also hollow, their cavities communicating with the
enteron.

Thus it will be seen that the Hydra is not d/aterally
symmetrical, like the frog—i.e., equally divisible into two
lateral halves by a median vertical plane passing through
the axis of the body,—but is radially symmetrical, t.e., the
body is divisible into similar parts radiating from a common
central axis.

There are three kinds of Hydra commonly found: one,
#7. vulgaris, is colourless or nearly so ; another, H. fusca, is
of a pinkish-yellow or brown colour ; the third, HZ. viridis, is
bright green. In the two latter it is quite evident, even
under a low power, that the colour is in the inner parts of the
body-wall, the outside of which is formed by a transparent
colourless layer (Fig. 73).

An examination of the living animal shows, in the first
place, that its form is continually changing. At one time (Fig.
73, A, left-hand figure) it extends itself until its length is
fully fifteen times its diameter and the tentacles appear like
long delicate filaments: at another time (right-hand figure)
it contracts itself into an almost globular mass, the tentacles
then appearing like little blunt knobs.

Besides these movements of contraction and expansion,
Hydra is able to move slowly from place to place. This it
usually does after the manner of a looping caterpillar (Fig.
73, C): the body is bent round until the distal end touches
the surface: then the base is detached and moved nearer
the distal end, which is again moved forward, and so on.
It has also been observed to crawl like a cuttle-fish (D)

U 2

292 HYDRA CHAP.

by means of its tentacles, the body being kept nearly
vertical.

It is also possible to watch a Hydra feed. It is a very
voracious creature, and to see it catch and devour its prey is
a curious and interesting sight. In the water in which it
lives are always to be found numbers of “ water-fleas,”
minute animals of about a millimetre or less in length,
belonging to the class Crustacea.

Water-fleas swim very rapidly, and occasionally one may
be seen to come in contact with a Hydra’s tentacle. In-
stantly its hitherto active movements stop dead, and it
remains adhering in an apparently mysterious manner to the
tentacle. If the Hydra is not hungry it usually liberates its
prey after a time, and the water-flea may then be seen to
drop through the water like a stone for a short distance, but
finally to expand its limbs and swim off. If however the
Hydra has not eaten recently it gradually contracts the ten-
tacles until the prey is brought near the mouth, the other
tentacles being also used to aid in the process. The water-
flea is thus forced against the apex of the hypostome, the
mouth expands widely and seizes it, and it is finally passed
down into the digestive cavity. Hydra can often be seen
with their bodies bulged out in one or more places by
recently swallowed water-fleas.

The precise structure of Hydra is best made out by cutting
it into a series of extremely thin sections and examining them
under a high power of the microscope. The appearance
presented by a vertical section through the long axis of the
body is shown in Fig. 74, 4

The whole animal is seen to be built up of cells, each
consisting of protoplasm with a large nucleus (B—p, zz), and
with or without vacuoles. As in the case of most animal cells,
there is no cell-wall,

Iv STRUCTURE 293

The essential feature in the arrangement of the cells is
that they are disposed in two layers round the central diges-
tive cavity or enteron (4, ev. cav) and the cavities of tenta-
cles (ext. cav'). So that the wall of the body is formed
throughout of an outer layer of cells, the ectoderm (ect), and
of an inner layer, the exdoderm (end), which bounds the
enteric cavity (compare p. 202). Between the two layers
is a delicate transparent membrane, the mesoglea, or sup-
porting lamella (msgl). A transverse section (B) shows that
the cells in both layers are arranged radially.

Thus Hydra is a two-layered or diploblastic animal, and
may be compared to a chimney built of two layers of
radially arranged bricks with a space between the layers
filled with mortar or concrete.

Accurate examination of thin sections, and of specimens
teased out or torn into minute fragments with needles,
shows that the structure is really much more complicated
than the foregoing brief descripton would indicate.

The ectoderm cells are of two kinds. The first and most
obvious (8, écf, and c) are large cells of a conical form, the
bases of the cones being external, their apices internal.
Spaces are necessarily left between their inner or narrow
ends, and these are filled up with the second kind of cells
(nz. c), small rounded bodies which lie closely packed be-
tween their larger companions and are distinguished as
interstitial cells. ,

The inner ends of the large ectoderm cells are continued
into narrow, pointed prolongations (c, #. pr), placed at right
angles to the cells themselves and parallel to the long axis of
the body. There is thus a layer of these longitudinally-
arranged muscle-processes lying immediately external to the
mesogloea (B, 7. pr). They appear to possess, like the axial

msyl-

we?

SCALE FoR A
ENS
Too — Wem

Fic. 74.—Hydra.
.\, vertical section of the entire animal, showing the €pody-wall composed of ecto-
derm (ect) and endoderm (ed), enclosing an enteric cavity (ext. cazv), which,
as well as the two layers, is continued (ev. cav’) into the tentacles, and opens

CHAP. IV NEMATOCYSTS 295

externally by the mouth (v¢%) at the apex of the hypostome (Ay). Between the
ectoderm and endoderm is the mesogloea (ssg7), represented by a black line.
In the ectoderm are seen large (zc) and small (z¢c’) nematocysts ; some of the
endoderm cells are putting out pseudopods (sd), others flagella (7). Two
buds (éd.1 4d2.) in different stages of development are shown on the left side, and
on the right a spermary (sfy) and an ovary (ovy) containing a single ovum (oz).

B, portion of a transverse section more highly magnified, showing the large ecto-
derm cells (ec¢) and interstitial cells (¢¢.c); two cnidoblasts (cz) enclosing
nematocysts (fc), and one of them produced into a cnidocil (cc); the layer
of muscle-processes (#2. 7) cut across just external to the mesogloea (msg);
endoderm cells (e#d) with large vacuoles and nuclei (zz), pseudopods (sd)
and flagella (7). The endoderm cell to the right has ingested a diatom (a),
and all enclose minute black granules.

C twoof the large ectoderm cells, showing nucleus (#) and muscle-process (7. $7).

I), an endoderm cell of 4. viridis, showing nucleus (zz), numerous chiomato-
phores (chr), and an ingested nematocyst (7/c).

E, one of the larger nematocysts with extruded thread barbed at the base.

F, one of the smaller nematocysts.

oe noe sperm, (From Parker's Biology: D after Lankester ; F and G after

owes,

fibre of Vorticella (p. 276), a high degree of contractility, the
almost instantaneous shortening of the body being due, in
great measure at least, to their rapid and simultaneous
contraction. It is probably correct to say that, while the
ectoderm cells are both contractile and irritable, a special
degree of contractility is assigned to the muscle-processes,
the cells themselves being eminently irritable, the slightest
stimulus applied to them usually being followed by an
immediate contraction of the whole body.

Imbedded in and between some of the large ectoderm cells
are found clear, oval sacs (##c), with very well-defined walls,
and called ¢hread-cells or nematocysts. Both in the living
specimen and in sections they ordinarily present the ap-
pearance shown in Figs. 74, B, and 75, 4, but are frequently
met with in the condition shown in Figs. 74, E, and 75, B,
that is, with a short conical tube protruding from the mouth
of the sac, armed near its distal end with three recurved
barbs besides several similar processes of smaller size, and
giving rise distally to a long, delicate, flexible filament.

Accurate examination of the nematocysts shows that the
structure of these curious bodies is as follows. Each con-
sists of a tough sac (Fig. 75, a), one end of which is turned

296 HYDRA CHAP

Fic. 75.—Hydra

A, a nematocyst contained in its cnidoblast (czd), showing its coiled filament
and the cnidocil (czc).

B, the same after extrusion of the thread, showing the larger and smaller barbs at
the base of the thread ; 2, the nucleus of the cnidoblast.

C, a cnidoblast, with its contained nematocyst, connected with one of the processes
of.a nerve-cell (zv.¢). (From Parker’s Bzology : after Schneider.)

in as a hollow pouch : the free end of the latter is continued
into a hollow coiled filament, and from its inner surface

IV NEMATOCYSTS 267

project the barbs. The whole space between the wall of
the sac and the contained pouch and thread is tensely filled
with fluid. When pressure is brought to bear on the outside
of the sac the whole apparatus goes off like a harpoon-gun
(B), the compression of the fluid forcing out first the barbed
pouch and then the filament, until finally both are turned
inside out.

It is by means of the nematocysts—the resemblance of
which to the trichocysts of Paramcecium (p. 267) should be
noted—that the Hydra is enabled to paralyse its prey. Prob-
ably some specific poison is formed and ejected into the
wound with the thread : in the larger members of the group
to which Hydra belongs, such as jelly fishes, the nematocysts
produce an effect on the human skin quite like the sting of
a nettle. .

The nematocysts are formed in special interstitial cells
called cnidoblasts (Figs. 74, B, and 75, cxdZ), and are thus
in the first instance at a distance from the surface. But the
cnidoblasts migrate outwards, and so come to lie quite
superficially either in or between the large ectoderm cells.
On its free surface the cnidoblast is produced into a delicate
pointed process, the czédoci? or “ trigger-hair” (cnc). In all
probability the slightest touch of the cnidocil causes con-
traction of the cnidoblast, and the nematocyst, thus com-
pressed, instantly explodes.

Nematocysts are found in the distal part of the body, but
are absent from the foot or proximal end, where also there
are no interstitial cells. They are especially abundant in the
tentacles, on the knob-like elevations of which—due to little
heaps of interstitial cells-—they are found in great numbers.
Amongst these occur small nematocysts with short threads
and devoid of barbs (Fig. 74, a, zéc’ and r).

In connection with the cnidoblasts small irregular cells

298 HYDRA CHAP,

with large nuclei occur (Fig. 75, c, zv.¢); they are supposed
to be xerve-cells, and to constitute a rudimentary nervous
system (compare p. 167).

The ectoderm cells of the foot differ from those of the rest
of the body in being very granular (Fig. 74, 4). The
granules are probably the material of the adhesive secretion
by which the Hydra fixes itself, and these cells are therefore
glandular (p.130)}

The endoderm consists for the most part of large cells
which exceed in size those of the ectoderm, and are re-
markable for containing one or more vacuoles, sometimes
so large as to reduce the protoplasm to a thin superficial
layer containing the nucleus (Fig. 74, a and B, ezd). Then
again, their form is extremely variable, their free or inner
ends undergoing continual changes of form. This can be
easily made out by cutting transverse sections of a living
Hydra, when the endoderm cells are seen to send out long
blunt pseudopods (sd) into the digestive cavity, and now
and then to withdraw the pseudopods and send out from
one to three long, delicate flagella (7). Thus the endoderm
cells of Hydra illustrate in a very instructive manner the
essential similarity of flagella and pseudopods already re-
ferred to (p. 250). In the hypostome the endoderm is thrown
into longitudinal folds, so as to allow of the dilatation of
the mouth in swallowing.

Amongst the ordinary endoderm cells are found long
narrow cells of an extremely granular character. They are
specially abundant in the distal part of the body, beneath
the origins of the tentacles, and in the hypostome, but are
absent in the tentacles and in the foot. There is no doubt
that they are gland-cells, their secretion being a fluid used
to aid in the digestion of the food. a.

Iv SYMBIOSIS 299

In Hydra viridis the endoderm-cells (Fig. 74, p) contain
chromatophores ! (chr) coloured green by chlorophyll, which
performs the same function as in plants (p. 247), so that
in this species holozoic is supplemented by holophytic
nutrition. In H. fusca bodies resembling these chromrato-
phores are present, but are of an orange or brown colour,
and devoid of chlorophyll.

Muscle-processes exist in corinection with the endoderm
cells, and they are said to take a transverse or circular
direction, 7.e, at right angles to the similar processes of
the ectoderm cells.

When a water-flea or other minute organism is swallowed
by a Hydra, it undergoes a gradual process of disintegration.
The process is begun by a solution of the soft parts due to
the action of a digestive fluid secreted by the gland-cells of
the endoderm ; it is apparently completed by the endoderm

1 In the substance of certain animals—e.g., the order Radiolaria of
the class Rhizopoda (p. 286), small Hzematococcus-like cells occur,
which have been proved to be independent organisms, called Zooxan-
thella. Sucha living together of two organisms is known as symdzosis.
It differs essentially from parasitism (see p. 271), in which one organism
preys upon another, the host deriving no benefit but only harm
from the presence of the parasite. In symbiosis, on the contrary, the
two organisms are in a condition of mutually beneficial partnership.
The carbon dioxide and nitrogenous waste given off by the Radiolarian
serve as a constant food-supply to the Zooxanthella: at the same time
the latter by decomposing the carbon dioxide provides the Radiolarian
with a constant supply of oxygen, and also with two important food
stuffs—starch and proteids, which, afler soiution, diffuse from the proto-
plasm of the Zooxanthella into that of the Radiolarian. The Radio-
larian may therefore be said to keep the Zooxanthelle constantly
manured, while the Zooxanthella in return supply the Radiolarian
with abundance of oxygen and of ready-digested food. There is some
reason for believing that the chromatophores of Hydra viridis are also
to be regarded as symbiotic organisms.

300 HYDRA CHAP.

cells seizing minute particles with their pseudopods and
engulfing them quite after the manner of Amcebz. It is
often found that the protrusion of pseudopods during
digestion results in the almost complete obliteration of the
enteric cavity. :

It would seem therefore that in Hydra the process of
digestion or solution of the food is to some extent zxtra-
cellular, i.e., takes place in the interior of the cells them-
selves, as ¢e.g., in Amoeba or Paramcecium: it is however
mainly extra-cellular or enteric, t.e., is performed in a special
digestive cavity lined by cells (p. 131).

The ectoderm cells do not take in food directly, but are
nourished entirely by diffusion from the endoderm. ‘Thus
the two layers have different functions : the ectoderm is pro-
tective and sensory ; it forms the external covering of the
animal, and receives impressions from without ; the endoderm,
removed from direct communication with the outer world,
performs a nutrient function, its cells alone having the power
of digesting food.

The essential difference between digestion and assimilation
is here plainly seen: all the cells of Hydra assimilate, all
are constantly undergoing waste, and all must therefore form
new protoplasm to make good the loss. But it is the endo-
derm cells alone which can make use of raw or undigested
food : the ectoderm has to depend upon various products. of
digestion received by diffusion or osmosis from the endoderm.

It will be evident from the preceding description that
Hydra is comparable to a colony of Amoebz in which par-
ticular functions are made over to particular individuals—
just as in a civilised community the functions of baking and
butchering are assigned to certain members of the com-
munity, and not performed by all. Hydra is therefore an

Iv REPRODUCTION 301

example of izdividuation - morphologically it is equivalent
to an indefinite number of unicellular organisms: but,
these acting in concert, some taking one duty and some
another, form, physiologically speaking, not a colony of
largely independent units (compare p. 281), but a single
multicellular individual.

Hydra has two distinct methods of reproduction, asexual
and sexual.

Asexual multiplication takes place by a process of budding.
A little knob appears on the body (Fig. 73 a, 4d'), and is
found by sections to arise from a group of ectoderm cells ;
soon however it takes on the character of a hollow out-
pushing of the wall containing a prolongation of the enteron,
and made up of ectoderm, mesogloea, and endoderm. (Fig.
74, A, 6d"), In the course of a few hours this prominence
enlarges greatly, and near its distal end six or eight hollow
buds appear arranged in a whorl (Figs. 73, a, and 74, a,
éd*), These enlarge and take on the characters of ten-
tacles, and a mouth is formed at the distal end of the bud,
which thus acquires the character of a small Hydra (Fig. 73,
A, 6d), Finally the bud becomes constricted at its base,
separates from the parent, and begins an independent ex-
istence. Sometimes, however, several buds are produced at
one time, and each of these buds again before becoming
detached : in this way temporary colonies are formed. But
the buds always separate sooner or later, although they
frequently begin to feed while still attached.

It is a curious circumstance that Hydra can also be mul-
tiplied by artificial division: the experiment has been tried
of cutting the living animal into pieces, each of which was
found to grow into a perfect individual

The sexual organs or gonads (p. 193) are of two kinds,

302 HYDRA CHAP.

spermaries and ovaries. Both are found in the same indi-
vidual, Hydra being hermaphrodite or monecious.

The spermaries (Figs. 73, B, and 74, A, spy) are white
conical elevations situated near the distal end of the body :
as a rule not more than one or two are present at the same
time, but there may be as many as twenty. They are per-
fectly colourless, even in the green and brown species, being
obviously formed of ectoderm alone.

In the immature condition the spermary consists of a little
heap of interstitial cells covered by an investment of some-
what flattened cells formed by a modification of the ordinary
large cells of the ectoderm. When mature each of the small
internal cells becomes converted into a sperm (p. 194),
consisting of a small ovoid head formed from the nucleus of
the cell, and of a long vibratile tail formed from its proto-
plasm (Fig. 74, G). By the rupture of the investing cells or
wall of the spermary the sperms are liberated and swim
freely in the water.

The ovaries (Figs. 73, B, and 74, A, ovy) are found
nearer the proximal end of the body, and vary in number
from one to eight. When ripe an ovary is larger than a
spermary, and of a hemispherical form. It begins, like the
spermary, as an aggregation of interstitial cells, so that in
their earlier stages the sex of the gonads is indeterminate.
But while in the spermary each cell is converted into a
sperm, in the ovary one cell (Fig. 74, a, ov), soon begins
to grow faster than the rest, and becomes amceboid in
form, sending out pseudopods amongst its companions
and ingesting the fragments into which they become broken
up, thus continually increasing in size at their expense.
Ultimately the ovary comes to consist of this single amceboid
ovum and of a layer of superficial cells forming a capsule
for it (compare p. 195). As the ovum grows, yolk-granules

Iv DEVELOPMENT 303

are formed in it, and in Hydra viridis it also acquires
green chromatophores.

When the ovary is ripe the ovum draws in its pseudopods
and takes on a spherical form: the investing layer then
bursts so as to lay bare the ovum and allow of the free access
to it of the sperms. One of the latter conjugates with the
ovum producing an vosperm (p. 198) or unicellular embryo.

The oosperm undergoes segmentation, dividing into
a number of cells which constitute a morula or polyplast
(p- 200), the outermost cells of which become changed into
a hard shell or capsule. The embryo, thus protected, falls
to the bottom of the water, and after a period of rest
develops into a Hydra, its cells becoming differentiated
into ectoderm and endoderm, the enteron and mouth
being formed, and the tentacles budding out round the
latter.

It was stated on p. 301 that in a budding Hydra the buds
do not always become detached at once, but may them-
selves bud while still in connection with the parent,
temporary colonies being thus produced.

Suppose the state of things to continue indefinitely : the
result would be a tree-like colony or compound organism
consisting of a stem with numerous branchlets each ending
in a Hydra-like zooid. Such a colony would bear much the
same relation to Hydra as Carchesium or Epistylis bears to
Vorticella (p. 281).

Asa matter of fact this is precisely what happens in a
great number of animals allied to Hydra and known by the
name of Zoophytes or Hydroid polypes.

Every one is familiar with the common Sertularians of the
sea-coast, often mistaken for sea-weeds: they are delicate,
much-branched, semi-transparent structures of a horny

304 BOUGAINVILLEA CHAP. IV

consistency, the branches beset with little cups, from each
of which, during life, a Hydra-like body is protruded.

A very convenient genus for our purpose is Bougainvillea,
a hydroid polyp found in the form of little tufts a few
centimetres long, attached to rocks and other submarine
objects. Fig. 76, a, shows a colony of the natural size, B, a
part of it magnified : it consists of a much-branched stem of
a yellowish colour attached by root-like fibres to the support.
The branches terminate in little Hydra-like bodies called
hydranths (B, hyd), each with a hypostome (Ay) and circlet of
tentacles (¢). Lateral branchlets bear bell-shaped structures
or meduse (med): these will be considered presently.

Sections show that the hydranths have essentially the
structure of a Hydra, consisting of a double layer of cells
—ectoderm and endoderm—separated by a_ supporting
lamella or mesogloea and enclosing a digestive cavity (evt.
cav) which opens externally by a mouth placed at the
summit of the hypostome.

The tentacles, however, differ from those of Hydra in two
important respects. In the first place they are solid: the
endoderm, instead of forming a lining to a prolongation of
the enteron, consists of a single axial row of large cells with
thick cell-walls and vacuolated protoplasm. Then in the
position of the muscle-processes of Hydra there is a layer of
spindle-shaped fibres, many times longer than broad, and pro-
vided each with a nucleus. Such mascle-fibres are obviously
cells greatly extended in length (p. 111), so that the ectoderm
cell of Hydra with its continuous muscle-gvocess is here
represented by an ectoderm cell with an adjacent muscle-
cell. We thus get a partial intermediate layer of cells
between the ectoderm and endoderm, in addition to the
gelatinous mesogloea ; and so, while a hydroid polyp is, like
Hydra, diploblastic (p. 293), it shows a tendency towards the

‘h yl
entcay.

ji hyd :

Fic. 76.—Bougainvillea rantosa.

A, a complete living colony of the natural size, showing the branched stem and
root-like organ of attachment.

B, a portion of the same magnified, showing the branched stem bearing hydranths
(Ayd) and medusz (wed), one of the latter nearly mature, the others im-
mature; each hydranth has a circlet of tentacles (¢) surrounding a hypo-
stome (Ayf), and contains an enteric cavity (e¢. cav) continuous with a narrow
canal (ez. cav’) in the stem. The stem is covered by a cuticle (cz).

C, a medusa after liberation from the colony, showing the bell with tentacles (Z),
velum (v), manubrium (sd), radial (vad.c) and circular (c’7.c) canals, and
eye-spots (oc). (From Parker’s Biology, after Allman.)

x

Pract. Zoou

306 BOUGAINVILLEA CHAP.

assumption of a three-layered or /riploblastic condition
{compare p. 202).

The stem is formed of the same layers and contains a
cavity (ent. cav') continuous with those of the hydranths,
and thus the structure of a hydroid polyp is, so far, simply
that of a Hydra in which the process of budding has
gone on to an indefinite extent and without separation of
the buds.

There is however an additional layer added in the stem
for protective and strengthening purposes. It is evident
that a colony of the size shown in Fig. 76, a, would, if formed
only of soft ectodermal and endodermal cells, be so weak as
to be hardly able to bear its own weight even in water. To
remedy this a layer of transparent, yellowish substance of
horny consistency, called the cuticle, is developed outside
the ectoderm of the stem, extending on to the branches and
only stopping at the bases of the hydranths and meduse.,
It is this layer which, when the organism dies and decays,
is left as a semi-transparent branched structure resembling
the living colony in form except that hydranths and meduse
are-wanting. The cuticle is therefore a supporting organ
or skeleton, not, like our own bones, formed in the interior
of the body (exdoskeleton, p. 16), but like the shell of a
crayfish or lobster, lying altogether outside the soft parts
(exoskeleton).

As to the mode of formation of the cuticle :—we saw that
many organisms, such as Hematococcus and Ameeba, are
able to form a cyst or cell-wall, by secreting or separating
from the surface of the protoplasm a succession of layers
either of cellulose or of a transparent horn-like substance
(pp. 232 and 244). But Amceba and Hematococcus are
unicellular, and are therefore free ‘to form this protective
layer at all parts of their surface. The ectoderm cells of

Iv MEDUSE 307

Bougainvillea, on the other hand, are in close contact with
their neighbours on all sides and with the mesoglcea at their
inner ends, so that it is not surprising to find the secretion
of skeletal substance taking place only at their outer ends.
As the process takes place simultaneously in adjacent cells,
the result is a continuous layer common to the whole
ectoderm instead of a capsule to each individual cell. It is
to an exoskeletal structure formed in this way, ze. by the
secretion of successive layers from the free faces of adjacent
cells, that the name cuticle is in strictness applied in multi-
cellular organisms.

The medusz (xz, wed, and c), mentioned above as occur-
ring on lateral branches of the colony, are found in various
stages of development, the younger ones having a nearly
globular shape, while when fully formed each resembles a
bell attached by its handle to one of the branches of the
colony and having a clapper in its interior. When quite
mature the medusz become detached and swim off as little
jelly-fishes (c).

The structure of a medusa must now be described in
some detail. The bell or wsdredla (c) is formed of a gela-
tinous substance (Fig. 77, b, sg) covered on both its inner
surface or szb-umbrella and on its outer surface or ex-wm-
brella by a thin layer of delicate cells (ect). The clapper-like
organ or manubrium (Fig. 76, c and Fig. 77 D and D’, md)
is formed of two layers of cells, precisely resembling the
ectoderm and endoderm of Hydra, and separated by a thin
mesoglcea ; it is hollow, its cavity (Fig. 77, ez¢. cav) open-
ing below, z.e. at its distal or free end, by a rounded aperture,
the mouth (mth), used by the medusa for the ingestion of
food. At its upper (attached or proximal) end the cavity of
the manubrium is continued into four narrow, radial canals

(Fig, 76, c, vad. c, and Fig. 77, p and p’ rad) which extend
X 2

A

“
OF, /
My

~s
wou
gs
z
oe D

Ny,
M,
"Ny

MEL,

m

“ny
m4

\,
M1

RO

oo
Ss

Fic. 77-—Diagrams illustrating the derivation of the medusa from the hydranth.
In the whole series of figures the ectoderm (ec?) is dotted, the endoderm (end)
striated, and the mesogleea (wsg7) black.

A, longitudinal section of a simple polyp, showing the tubular body with enteric

cavity (e#t. cav), hypostome (vf), mouth (a2h.), and tentacles (2).

A’, transverse section of the same through the plane a4,

B, the tentacular region is extended into a hollow disc.
C, the tentacular region has been further extended and bent into a Lell-like form

CHAP. IV MEDUS4& 309

the enteric cavity being continued into the umbrella (e#¢. cav’); the hypostome
now forms a manubrium (1276).

C’, transverse section of the same through the plane a4, showing the continuous
cavity (ez. cav’) in the umbrella.

D, fully formed medusa; the cavity in the umbrella is reduced to the radial
(vad) and circular (civ. c) canals, the velum (v) is formed, and a double nerve-
ring (zz, zu’) is produced from the ectoderm.

D’, transverse section of the same through the plane a4, showing the four radial
canals (vad) united by the endoderm-lamella (exd. lam), produced by partial
obliteration of the continuous cavity (ext. cav’)in C. (From Parker’s Brology.)

through the gelatinous substance of. the umbrella at equal
distances from one another, like four meridians, and finally
open into a circular canal (cir. c) which runs round the edge
of the umbrella. The whole system of canals is lined by a
layer of cells (Fig. 77, D and v’, evd@) continuous with the
inner layer or endoderm of the manubrium ; and extending
from one canal to another in the gelatinous substance of the
umbrella, is a delicate sheet of cells, the exdoderm-lamella
(D', end. lam).

From the edge of the umbrella four pairs of tentacles
(Figs. 76, c and 77, #) are given off, one pair corresponding
to each radial canal, and close to the base of each tentacle
is a little speck of pigment (Fig. 76, oc), the oce//us or eye-
spot. Lastly, the margin of the umbrella is‘continued inwards
into a narrow circular shelf, the velzm (v).

At first sight there appears to be very little resemblance
between a medusa and a hydranth, but it is really quite
easy to derive the one form from the other.

Suppose a simple polyp or Hydra-like body with four
tentacles (Fig. 77, 4, 4’) to have the region from which the
tentacles spring pulled out so as to form a hollow, trans-
versely extended disc (B). Next, suppose this disc to become
bent into the form of a cup with its concavity towards the
hypostome, and to undergo a great thickening of its meso-
gloea. A form would be produced like c, 7.e. a medusa-like
body with umbrella and manubrium, but with a continuous
cavity (c’, en¢. cav’) in the thickness of the umbrella instead

310 BOUGAINVILLEA CHAP,

of four radial canals. Finally, suppose the inner and outer
walls of this cavity to grow towards one another and meet,
thus obliterating the cavity, except along four narrow radial
areas (D, vad) and a circular area near the edge of the
umbrella (a7. c). This would result in the substitution
for the continuous cavity of four radial canals opening on
the one hand into a circular canal and on the other into
the cavity of the manubrium (ez. cav), and connected with
one another by a membrane—the endoderm-lamella (p' end.
/am)—indicating the former extension of the cavity.

It follows from this that the inner and outer layers of the
manubrium are respectively endoderm and ectoderm: that
the gelatinous tissue of the umbrella is an immensely
thickened mesogloea : that the layer of cells covering both
inner and outer surfaces of the umbrella is ectodermal: and
that the layer of cells lining the system of canals, together
with the endoderm-lamella, is endodermal.

Thus the medusa and the hydranth are similarly con-
structed or Homologous structures (p. 217), and the hydroid
colony is dimorphic (p. 250), bearing zooids of two kinds.
In some allied forms, this individuation may go still further,
the zooids being of very various forms and performing
diverse functions: such a colony is said to be polymorphic.

Sooner or later the medusz separate from the hydroid
colony and begin a free existence. Under these circum-
stances the rhythmical contraction—z.e. contraction taking
place at regular intervals—of the muscles of the umbrella
causes an alternate contraction and expansion of the whole
organ, so that water is alternately pumped out of and drawn
into it. The obvious result of this is that the medusa is
propelled through the water by a series of jerks. The
movement is performed by means of the muscle-processes

IV. MEDUSA 311

and muscle-fibres of the sub-umbrella and velum, some of
which differ from the similar structures in the hydranth in
exhibiting a delicate transverse striation.

There is still another important matter in the structure of
the medusa which has not been referred to. At the junction
of the velum with the edge of the umbrella there lies, imme-
diately beneath the ectoderm, a layer of peculiar, branched
cells, containing large nuclei and produced into long fibre-
like processes. These merve-cel/s (p. 167) are so disposed
as to form a double ring round the margin of the bell, one
ring (Fig. 77, p, zv) being immediately above, the other
(xz") immediately below the insertion of the velum. An
irregular network of similar cells and fibres occurs on the
inner or concave face of the bell, between the ectoderm and
the layer of muscle-fibres. The whole constitutes the
nervous system of the medusa ;.the double nerve-ring is the
central, the network the peripheral nervous system (p. 155).

Some of the processes of the nerve-cells are connected
with ordinary ectoderm-cells, which thus as it were connect
the nervous system with the external world : others, in some
instances at least, are probably directly connected with
muscle-fibres.

We thus see that while the manubrium of a medusa has
the same simple structure as a hydranth, or what comes to
the same thing, as a Hydra, the umbrella has undergone a.
very remarkable differentiation of its tissues. Its ordinary
ectoderm cells, instead of being large and eminently con-
tractile, form little more than a thin cellular skin or ep7th-
elium (p. 109) over the gelatinous mesogloea: they have
largely given up the function of contractility to the muscle-
processes or fibres, and have taken on the functions of a
protective and sensitive layer.

Similarly the function of automatism, possessed by ‘the

312 BOUGAINVILLEA CHAP.

whole body of Hydra, is made over to the group of specially
modified ectodermal cells which constitute the central
nervous system. If a Hydra is cut into any number of
pieces each of them is able to perform the ordinary move-
ments of expansion and contraction, but if the nerve-ring
of a medusa is removed by cutting away the edge of the
umbrella, the rhythmical swimming movements stop dead :
the umbrella is in fact permanently paralysed.

It is not, however, rendered incapable of movement, for
a sharp pinch, ze. an external stimulus, causes a siagle con-
traction, showing that the muscles still retain their irritability.
But no movement takes place without such external stimu-
lus, each stimulus giving rise infallibly to one single con-
traction: the power possessed by the entire animal of
independently originating movement, 7.e. of supplying its
own stimuli, is lost with the central nervous system (compare
p. 172).

Another instance of morphological and_ physiological
differentiation is furnished by the pigment spots or ocelli
(Fig. 76, C, oc) situated at the bases of the tentacles. They
consist of groups of ectoderm cells in which are deposited
granules of deep red pigment, and which serve as organs of
sight.

The two forms of zooid—hydranth and medusa (p. 304)—
are respectively nutritive and reproductive in function, the
reproductive zooids becoming detached and swimming off
to found a new colony elsewhere : the hydranths are purely
nutritive zooids ; the meduse, although capable of feeding,
are specially distinguished as reproductive zooids. The
gonads are found in the walls of the manubrium, between
the ectoderm and endoderm, some meduse producing
ovaries, others spermaries only. Thus while Hydra is mon-
cecious (p. 302), Bougainvillea is, like the frog, dectous,

IV ALTERNATION OF GENERATIONS 313

certain individuals producing only male, others only female
products,

The medusze, when mature, become detached and swim
away from the hydroid colony. The sperms of the males
are shed into the water and carried to the ovaries of the
females, where they fertilize the ova, converting them, as
usual, into oosperms.

The oosperm undergoes segmentation, forming a poly-
plast or morula (p. 200): ectoderm and endoderm become
differentiated, and the ectoderm cells acquire cilia, by means
of which the embryo now swims freely in the water. An
enteron appears in the endoderm, and in this stage
the embryo, which has an elongated form, is known as a
planula., It then loses its cilia and settles down on a rock,
shell, sea-weed, or other submarine object, assuming a
vertical position with its broader end fixed to the support.

The attached or proximal end widens into a disc of
attachment, a dilatation is formed a short distance from the
free or distal end, and a thin cuticle is secreted from the
whole surface of the ectoderm. From the dilated portion
short buds arise in a circle : these are the rudiments of the
tentacles: the narrow portion beyond their origin becomes
the hypostome. Soon the cuticle covering the distal end
is ruptured so as to set free the growing tentacles: an
aperture, the mouth, is formed at the end of the hypostome,
and the young hydroid has very much the appearance of a
Hydra with a broad disc of attachment, and with a cuticle
covering the greater part of the body. Extensive budding
next takes place, the result being the formation of the
ordinary hydroid colony.

Thus from the oosperm or impregnated egg-cell of the
medusa the hydroid colony arises, while the medusa is
produced by budding from the hydroid colony. We have

314 COLLENTERATA CHAP.

what is called an alternation of generations, the asexual genera-
tion or agamobium (hydroid colony) giving rise by budding
to the sexual generation or gamobium (medusa), which in its
turn produces the agamobium by a sexual process, z.e. by
the conjugation of ovum and sperm.

Hydra and Bougainvillea both belong to the simplest
class—the Aydrozoa—of the phylum Coelenterata : this
phylum includes all the polypes or zoophytes, the jelly-fishes,
and the anemones and corals. In all there is an ectoderm
and an endoderm, separated by a mesogloea, which may
consist, as in Hydra, of a structureless membrane containing
no cells, or may be gelatinous as in the medusa, and may
even contain cells, thus assuming more the character of an
intermediate cell-layer or mesoderm. There is no body-
cavity or celome (p. 20) surrounding the digestive cavity
or enteron, and tentacles are present round the mouth.
Organs of offence occur in the form of thread-cells or nema-
tocysts. - 7

In all the higher phyla a definite mesoderm is developed
in the embryo in addition to the ectoderm and endoderm
(triploblastic condition), and in nearly all cases there is a
definite cavity or ccelome present in the mesoderm: hence all
these animals are often included together as the Ceelomata.

PRACTICAL DIRECTIONS.

Hydra.

Examine some living Hydree in a vessel of water, with the naked eye
or with a pocket lens, and note the differences in form according to
the degree of contraction. The animal is usually attached to foreign
bodies (weeds, &c.), at one end, and at the other end a number of tenta-
cles (usually six to eight) are given off. In the expanded state the body and
tentacles are greatly elongated and thread-like, while when contracted
the body is more globular, and the tentacles appear like small knobs,

Iv PRACTICAL DIRECTIONS 315

o
Note the brown colour in H. fusca, and the green colour in i. v7r7idis.
Observe the method of seizing food. Place a specimen on a slide in a
drop of water, together with a small piece of water-weed or paper to
prevent crushing, and then put on a cover-glass. Wait till the animal
is fully expanded, and then examine with the low power. Note :—

1. The dody, enclosing the digestive cavity or enteron, which opens
by the mouth on the free or distal end of the animal, at the summit of a
conical hypostome. At the proximal end is the flattened foot or disc of
attachment.

2. The /entacts, arranged in a single circlet or whore around the
hbase of the hypostome. They are hollow, and their cavities communi-
cate proximally with the general digestive cavity of the body. On their
surface are a number of small knobs.

3. The great contractility of the animal, and especially of the ten-
tacles.

4. The structure of the body-wall, which is made up of (a) an outer
layer of colourless cells (ectoderm); and (6) an inner layer (brown in
HT, fusca and green in H. wiredis) of cells (endoderm) lining the diges-
tive cavity. Between these two layers is 2 thin gelatinous non-cellular
supporting lamella or mesogiva, not easily seen with the low power.
(The tentacles have a similar structure, the details of which cannot be
made out with the low power.) Sketch.

Put on the high power and examine a tentacle, focussing on to the
surface as well as deeper, so as to get an optical section. Note :—

5. The relations of the ectoderm, endoderm, and supporting lamella,
and the nuclei of the ectoderm and endoderm cells.

6. The structure of the ectoderm :—(a) large conical cells, with their
broader ends outwards, arranged in a single row, and differing in form
according to the state of contraction, Thé spaces between the inner
narrower ends of these are filled up with (4) smaller rounded dterstztzal
cells (absent on the foot) ; (c) chread-cells or nematocysts (Fig. 75)—oval
capsules containing a spirally-wound thread, developed within certain
of the interstitial cells called czzdoblasts, and when fully formed, found
imbedded in or between the large ectoderm-cells. They are much more
numerous on the tentacles than on the body, causing the knobs referred
to above. Each cnidoblast gives rise to a small process—the trigger-
hair or cxddocé/, which projects from the surface. Notice the discharged
thread-cells, and observe that each consists of a flask-like base (to which
part of the protoplasm and the nucleus of the burst cnidoblast usually

316 HYDRA CHAP,

remains attached) and a long filament, with three large and several
smaller spines or barbs at its proximal end. (Smaller thread-cells,
with thicker threads and no spines, are also present’; some of these
have long, spirally coiled threads, others shorter, straight threads. These
can be seen better later on.)

"7, The endoderm, consisting of a single layer of large amceboid cells,
which in ZH. vzrzdzs contain ‘green chromatophores. Note the currents
in the tentacles, which are produced by long vibratile flage//a present
on many of the endoderm cells.

8. The thin transparent sapporting lamella. Sketch.

9. Treat a specimen with methyl-green. A slight pressure on the
cover-glass will crush the animal, and render the interstitial cells and
thread-cells especially distinct. Note also other isolated cells of the
ectoderm and endoderm. Sketch.

19. Examine a specimen with buds in different stages ot development,
and note as much as possible of the mode of asexual reproduction by
gemmation. Sketch.

11. If none of your specimens bear sexual organs, try and procure a

mounted preparation which shows them, and examine first with the
low, and then the high power. Note—(a) The spermaries—several
conical swellings, usually not far below the tentacles. They are covered
with large ectoderm cells, and contain numerous interstitial cells, each
of which eventually gives rise to a sferm with a ‘‘ head” and vibratile
“tail.” These are discharged at the apex of the cone, which when
ripe may be ruptured by a slight pressure on the cover-glass. (4) The
ovaries (sometimes only one), generally situated near the proximal end
of the body. They are larger than the spermaries and more spherical,
but at first have a similar structure. When ripe a single ov is found
in each. Sketch.
{© Place some Hydre in a watch-glass with 4 very small amount o
water, and when they have expanded, pour quickly over them a warm
saturated solution of corrosive sublimate in alcohol. Wash several
times with weak alcohol, stain for a few minutes with borax-carmine,
and wash with weak and then with stronger alcohol. Place in‘absolute
alcohol for a few minutes, and afterwards in turpentine or oil of cloves ;
mount in balsam. Work through §§ 5-8 again, noting especially the
characters of the various cells and their nuclei, as well as—

12. The contractile processes coming off from the inner ends of the

iv PRACTICAL DIRECTIONS 317

large ectoderm cells. These extend longitudinally, and lie against the
outer surface of the supporting lamella. Sketch.

Examine transverse sections through the body or tentacles, prepared
as directed on p. 136, after killing and fixing the: specimens as
above. Work through 6—8 again, noting the various cells and their
nuclei, &c. Observe especially—

13. (a) The contractile processes of the ectoderm cells, which will be cut
across transversely, so as to appear as dots just outside the supporting
lamella ; (4) the amebotd and vacuolated character of the endoderm cells.
(Special methods of preparation are necessary in order to show the
flagella.) Sketch.

Bougainvillea.’

If possible, examine first alive, and then kill and stain as directed in
the case of Hydra. Examine under the microscope and note :—-

a. Colonial stage. 1. Thehydranths, attached to acommon branching
stem: compare their structure with that of Hydra. 2. The medusa-
buds. 3. The cuticular exoskeleton supporting the colony.

b. Medusa stage. 1. The umbrella, tentacles, velum, manubrium,
mouth, radial and cércelar canals, ocelli, and gonads.

1 Specimens living or preserved, both of the colonial and medusa
stage of Bougainvillea or some allied form (e.g. Ode//a), can be obtained
from any Marine Biological Laboratory ; or the fresh-water Cordylophora
will answer the purpose as far as the colony is concerned, but it has nu
medusa stage.

CHAPTER V

THE EARTHWORM—CHARACTERS OF THE PHYLUM ANNULATA

THE general form and appearance of an earthworm are
familiar to every one. In this country there are a number
of different species of earthworms belonging to several
genera, the commonest of which are Lumbricus and Alo-
lobophora ; but the differences between these are of minor
importance to the beginner, and any one of the common
forms will serve our purpose,

Earthworms burrow into the soil and lve on decaying
leaves and other organic matter, which they swallow to-
gether with a considerable quantity of earth. This earth,
mingled with the undigested’ portions of the food, is
passed from the body on to thé surface of the ground in the
form of the well-known little heaps or “ castings ” which you
must have noticed in gardens and fields, especially after
rain, when the worms come more frequently to the surface.
In this way, a quantity of finely divided earth, mixed with
the feces of the worms, is constantly being spread
out on the surface of the soil, and Darwin calculated
that on an average a layer of earth about one-
fifth inch in thickness or about ten tons an acre is thus
brought to the surface in the course of a year. Earth-
worms are therefore good friends to the gardener and

CHAP. V THE EARTHWORM 319

agriculturist, as they are continually ploughing and
manuring the soil, and in doing so, they gradually cover up
stones and other objects lying on the surface.

The body of the earthworm is long and narrow, ap-
proximately cylindrical in shape, and bilaterally symmetrical
(p. 291): in the common forms it reaches a length of
about six inches. Anteriorly it is bluntly pointed, while
more posteriorly it is somewhat flattened, its greatest
diameter being reached at about a third of the entire
length from the anterior end. In the ordinary creeping
movements of the animal, which are effected by the
alternate contraction and extension of its body, the an-
terior end is directed forwards. The colour is pinkish in
most species, and is paler on the lower or ventral than
on the upper or dorsal side.

The surface of the body is distinctly marked by trans-
verse annular grooves into Jbody-segments or metameres
(Fig. 79), the number of which is about 150, more or less :
the segments are rather longer towards the anterior end
than they are further back. At the extreme anterior end
is a small finger-shaped ead-lobe or prostomium, which
overhangs the south, situated on the antero-ventral surface
of the next segment, which is therefore called the jez7-
stomium, and is counted as the first metamere. The anus is
a slitlike aperture on the hinder surface of the last or
anal segment. ‘The earthworm is thus a metamerically seg-
mented animal, and the segments are serially homologous
with one another (p. 39).

In adult worms a prominent glandular swelling is
noticeable on the dorsal and lateral surfaces of the body,
extending through about six segments beginning at
about the thirtieth; this is known as the ctellum, and,
as we shall see, is important in the process of im-

326 THE EARTHWORM CHAP.

pregnation and in forming a case or coccoon for the eggs.
On the ventral part of this region are some small
glandular swellings, which are more conspicuous in young
worms before the clitellum is developed.

The whole of the body is invested with a delicate, iri-
descent membrane or cuticle (p. 306) formed as asecretion of
the egzderm or outer epithelial layer of the body (compare p.
128). Every segment, except the first and the last, is provided
with eight small cuticular spines or se¢@ (Fig. 78, se¢)—slightly
curved bodies with tapering ends composed of a horn-like
substance called chztin—each of which is developed in a small
sac formed as an involution of the epiderm, and is provided
with muscles by means of which it can be protruded and re-
tracted. These sete are arranged in couples, forming two
double rows along each latero-ventral region of the body,
and their points can be distinctly felt on drawing the worm
through the fingers : they serve to prevent the animal from
slipping backwards as it moves along on the surface of the
ground or in its burrows.

We have seen that the earthworm takes in its food,
together with quantities of earth, by the mouth, and after
retaining it for a longer or shorter time in the body,
expels it by the anus. It is obvious, therefore, that there
must be some kind of digestive cavity into which the
food passes by the mouth, and from which effete matters
are expelled through the anus. Sections (Figs. 78) show
that this cavity is not a mere space excavated in the interior
of the body, but a definite tube, the exteric or alimentary
canal (p. 23), which passes in a straight line from mouth to
anus, and is separated in its whole extent from the walls of
the body by a wide space, the dody-cavity or celome (cel), as
in the frog (p. 20). So that the general structure of the
earthworm might be imitated by taking a wide tube,

v TRANSVERSE SECTION 321

stopping the ends of it withcorks, boring a hole in each
cork, and then inserting through the holes a narrow tube of
the same length as the wide one. The outer tube would
represent the body-wall, the inner the enteric canal, and the
cylindrical space between the two the ccelome. The inner
tube would communicate with the exterior by each of its
ends, representing respectively mouth and anus; the space
between the two tubes, on the other hand, would have no
communication with the outside. A transverse section of
the body has, therefore, the general character of two con-
centric circles.

It will be remembered that a transverse section of Hydra
has the character of two concentric circles, formed re-
spectively of ectoderm and endoderm (Fig. 74, p. 293), the
two layers being, however, only separated by the thin
mesogleea. At first sight then, it seems as if we might
compare the earthworm to a Hydra in which the ecto-
derm and endoderm, instead of being in contact, .were
separated by a wide interval; we should then compare
the body-wall of the earthworm with the ectoderm of Hydra,
and its enteric canal with the endoderm. But this com-
parison would only express part of the truth.

A thin transverse section (Fig.. 78) shows the body-wall of
the earthworm to consist of four distinct layers. Outside is
a thin transparent cuticle (cz) showing no structure beyond
a series of intersecting oblique lines. Next comes a layer
of epithelium, the efzderm, or deric epithelium (epid). Within
this is a very thin connective tissue layer representing the
derm (p. 128), and a double layer of muscle-fibres by means of
which the movements of the body are produced—an outer,
in which the fibres extend transversely round the body
(circ. mus), and a much thicker inner layer consisting of

longitudinal fibres, in section arranged like the barbs of a
Pract. Zoot. Y

322 THE EARTHWORM CHAP.

feather on a central axis (Jong. mus). Finally, within the
muscular layer and lining the ccelome is a thin perttoneal
membrane (parietal layer, compare p. 26), on the inner

dors.v

SUB ILVOSE
Fic. 78.—Lumbricus, transverse section‘of the middlefregion ofthe body-, -x.«
¢irc. mus. layer of circular muscular fibres!; cad. ccoelome ; cut. cuticle ; dors. v.
dorsal vessel; efid. epiderm ; ext. nefh. nephridropore; hep. layer of yellow
cells; long. mus. longitudinal muscles ; sefh. nephridium ; xephrost. nephros-
tome ; erv.co, nerve cord; set. sete; sub. mn. vess. sub-neural vessel; typh.
typhlosole ; vent. v. ventral vessel. (From Parker and Haswell’s Zoology, after
Marshall and Hurst.)

surface of which is a very thin layer of cells the celomic
epithelium.

A transverse section of the intestine shows an inner
layer of ciliated, columnar enteric epithelium (compare p.
109), a thin middle layer composed of muscle-fibres and

v TRANSVERSE SECTION 323

connective tissue, and an outer layer of large ye//ow cells,
the function of which is not thoroughly understood, and
which correspond to a special development of the ccelomic
epithelium covering the visceral} layer of the peritoneal
membrane which invests the intestine.

We are now in a better position to compare the trans-
verse section of Hydra and of the earthworm. The
epiderm of the earthworm being the outermost cell-layer
is to be compared with the ectoderm of ‘Hydra, and its
cuticle with the layer of the same name which, though
absent in Hydra, is present in the stem of hydroid polypes,
such as Bougainvillea (p. 306). The enteric epithelium
of the earthworm, bounding as it does the digestive
cavity, is clearly comparable with the. endoderm of
Hydra, So that we have the double layer of muscle-
fibres and the two layers of peritoneum not represented
in Hydra, in which their position is occupied merely
by the mesogloea. The muscle-fibres are not of the striped
kind, like those in the corresponding position in the frog
(p. 112).

But it will be remembered that in Medusz there is some-
times found a layer of separate muscle-fibres between the
ectoderm and the mesoglcea,, and it was pointed out
(p. 304) that such fibres represented a rudimentary inter-
mediate cell-layer or mesoderm. We may therefore con-
sider the muscular layer and the peritoneum of the
earthworm as mesoderm, and we may say that in this
animal, as in the frog (p. 203 and Fig. 65), the mesoderm
is divisible into an outer or parietal layer, an inner or
visceral layer.

The parietal layer is in contact with the ectoderm or deric
epithelium, and with it forms the body-wall; the visceral

layer is in contact with the endoderm or enteric epithelium,
¥2

324 THE. EARTHWORM CHAP.

and with it forms the enteric canal. The ccelome separates
the parietal and visceral layers from one another, and is
lined throughout by ccelomic epithelium.

The relation between the diploblastic polype and the
triploblastic worm may therefore be expressed in a tabular
form as follows—

Lydroid Earthworm
Cuticle . . . . . . . Cuticle.
Ectoderm . . . . . . Deric epithelium or epiderm.
Connective tissue and muscle-
Parietal fibres.
layer Peritoneum with its coelomic
Mesoderm. . epithelium (parietal layer).
(rudimentary) Peritoneum with its coelomic
Visceral epithelium (visceral layer).
layer Connective tissue and muscle-
fibres.
Endoderm . . . . . . Enteric epithelium.

Strictly speaking this comparison does not hold good of
the anterior and posterior ends of the worm: at both mouth
and anus the deric passes insensibly into the enteric epithe-
lium, and the study of development shows that the cells
lining both the anterior and posterior ends of the canal are
ectodermal (compare pp. 204 and 207). For this reason the
terms deric and enteric epithelium are not mere synonyms
of ectoderm and endoderm respectively.

It is important that you should, ‘before reading further,
understand clearly the general composition of a tripoblastic

1 It will be seen that the relations of these layers in the earthworm
and frog are similar, except that in the latter the cuticle is wanting
(compare Figs. 5, 38, and 39).

v CCELOME 325

animal as typified by the earthworm, which may be sum-
marised as follows. It consists of two tubes formed of
epithelial cells, one within and parallel to the other, the
two being continuous at either end of the body, where the
inner tube (enteric epithelium) is in free communication
with the exterior ; the outer tube (deric epithelium) is lined
by a layer of connective tissue and muscle-fibres, within
which is a thin peritoneum lined by ccelomic epithelium, the
three together forming the body-wall; the inner tube
(enteric epithelium) is covered externally by a layer of
muscle-fibres and connective tissue and a thin _peri-
toneum covered by ccelomic epithelium, which form with
it the enteric canal ; lastly, the body-wall and enteric canal
are separated by a considerable space, the ccelome.

The enteric canal is not, as might be supposed from the
foregoing description, connected with the body-wall only at
the mouth and anus, but is supported in a peculiar way.
There is no dorsal mesentery as in the frog (p. 27), but a
series of transverse vertical partitions or septa (Fig. 79)
extend right across the body-cavity, each being perforated
by the canal. The septa are regularly arranged and cor-
respond in position with ‘the external grooves by
which the body is divided into metameres. Thus
the transverse or metameric segmentation affects the
coelome as well as the body-wall, the former being divided
up into a series of chambers, which, however, communicate
with one another ventrally, where the septa are incomplete.
Each septum is composed of a sheet of connective tissue
and muscle-fibres, and is covered on both sides by ccelomic
epithelium. The coelome communicates with the exterior by
a series of dorsal pores situated in the grooves between all
the segments except about the first ten.

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DESIAO *¢

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CHAP. V ENTERIC CANAL 427

The digestive canal is not a simple tube of even calibre
throughout, but is divisible into several portions. The
mouth is bounded by a soft lip and leads into a small
buccal cavity, which communicates with a thick-walled
pharynx (Fig. 79, ph), extending through about five seg-
ments and connected with the body-wall by a number of
radially arranged muscle-fibres, the septa being absent in
this region. When the worm feeds, the buccal cavity is
everted, and the muscles serve to draw it and the pharynx
back again, as well as to dilate the pharynx. The latter is
followed by a narrow gullet or wsophagus (oes.) extending
through about eight segments, which is provided towards its
hinder end with three lateral pairs of glandular swellings of
a yellowish colour—the esophageal glands: these contain a
calcareous substance which serves to neutralise the organic
acids present in the food swallowed. The first two pairs
communicate with the third pair, which open into the
cesophagus (oes. g/). The gullet opens into a dilated,
thin-walled receptacle, the crop (cr), and this, again, com-
municates posteriorly with a large gizzard (giz) with thick
and muscular walls, which in about the zoth segment com-
municates with the z#testine (int). The intestine has a
similar character throughout, and extends from the gizzard
to the anus: its dorsal wall is folded inwards so as to pro-
duce a longitudinal ridge or ¢yph/osole (Fig. 78, typh), which
serves to increase the absorptive surface and in the interior
of which the yellow cells are very numerous.

Certain of the cells lining the enteric canal, and especially
those along the typhlosole, are very granular, and like the
endoderm cells of the hypostome of Hydra (p. 298) are
to be considered as unicellular glands. They secrete a
digestive juice which—mixing with the various substances
containing organic acids taken in by the mouth, and

328 THE EARTHWORM CHAP,

neutralised by the calcareous secretion of the cesophageal
glands—dissolves the proteids and other digestible parts so
as to allow of their absorption. It is very probable that
the process is purely extra-cellular or enteric, the food being
dissolved and rendered diffusible entirely in the cavity of the
canal (p. 300). By the movements of the canal—caused
partly by the general movements of the body and partly
by the contraction of the muscles of the canal and septa,
aided by the action of the cilia, the contents are gradually
forced backwards and the earth and other indigestible
matters are expelled at the anus.

The ccelome is filled with a colourless transparent celomic
fiuid in which are suspended amceboid corpuscles or leuco-
cytes like those of the frog’s blood and lymph (p. 105). The
function of this coelomic fluid is probably to distribute the
digested food in the enteric canal to all parts of the
body. In Hydra, where the lining wall of the digestive
cavity is in direct contact with the simple wall of the
body, the products of digestion can pass at once by
diffusion from endoderm to ectoderm; but in the present
case a means of communication is wanted between the
enteric epithelium and the comparatively complex and
distant body-wall. The peptones and other products of
digestion diffuse through the enteric epithelium into the
coelomic fluid, and by the continual movement of the
latter—due to the contractions of the body-wall—are
distributed to all parts. Thus the external epithelium
and the muscles, as well as the nervous system and re-
productive organs not yet described, are wholly de-
pendent upon the enteric epithelium for their supply of
nutriment.

The earthworm, like the frog, possesses a series of b/ood-

v VASCULAR SYSTEM 329

vessels, containing red blood, the whole of which form a
single closed {vascular system, there being no com-
munication between them and any of the other cavities of
the body. The main trunks have a longitudinal direction,
the chief ones being a large dorsal vessel, running along the
dorsal surface of the enteric canal, and a ventral or sub-
intestinal vessel, below the canal (Fig. 78. dors. v, vent. Vv).
In addition to these there are three smaller longitudinal
trunks in relation to the nerve-cord, which, as we shall see,
extends along the ventral side of the ccelome: these are a
median subneural (sub. n. vess) and two lateral neural vessels.
All these longitudinal trunks give off branches to the
various parts of the body, and certain of them are connected
with one another by a pair of lateral commissural vessels:
in the region of the gullet there are about five or six
pairs of large vessels connecting the main dorsal and
ventral trunks ; and the dorsal and subneural trunks are also
connected in each segment all along the body by a pair of
smaller commissural vessels, running in the inner surface of
the body-wall.

Notice that there is here no distinction into arteries and
veins, as in the frog (p. 27), and also that there is no heart.
The vessels gradually divide up into smaller and smaller
branches in the various parts of the body, and then again
unite to form larger and larger vessels which eventually
open into one or other of the main trunks.

The circulation of the blood is effected by the rhythmical,
peristaltic contraction (p. 75) of certain of the larger
vessels: thus the dorsal trunk. contracts from behind
forwards, and the large commissural vessels—often spoken
of as “hearts ”—-which connect it anteriorly with the ventral
trunk, from above downwards, so that the blood passes for-
wards in the dorsal, backwards in the ventral vessel. The

330 THE EARTHWORM CHAP.

blood from the enteric canal is returned by a series of vessels
into the dorsaltrunk. The vessels of the excretory organs, to
be described presently, arise from the ventral trunk, and the
blood from these organs is returned into the commissural
vessels in the body-wall which connect the dorsal and
subneural trunks. By means of branches of these parietal
vessels the body-wall is plentifully supplied with blood.

The red colour of the blood is due to hemoglobin (p.
107), which is not, as in the ‘frog, contained in red blood-
corpuscles, but is dissolved in the plasma, in which, how-
ever, minute colourless corpuscles can be recognised. The
function of hemoglobin in the process of respiration has al-
ready been described (p. 144); but in the earthworm, as in
many other lower animals, there are no specialised res-
piratory organs (lungs or gills), the necessary exchange of
gases being performed by the entire surface of the body,
the minute branches of the blood-vessels in the body-wall
being only separated from the air by the single layer of
epidermic cells, and even penetrating amongst the latter in
the region of the clitellum: this is an exceptional occur-
rence, for as we have seen, capillaries do not, as a general
rule, extend amongst epithelial cells (compare, eg., Figs.

38—40).

In discussing in a previous chapter the differences between
plants and animals, we found (p. 255) that in the unicellular
organisms previously studied, the presence of an excretory
organ in the form ofa contractile vacuole was a characteristic
feature of such undoubted animals as the ciliate Infusoria.
But the reader will have noticed that Hydra and its
allies have no specialised excretory organ, waste pro-
ducts being apparently discharged from any part of the
surface. In the earthworm we meet once more with an
animal in which excretory organs are present, although,

v NEPHRIDIA 331

in correspondence with the complexity of the animal itself,

they are very different from
the simple contractile vacu-
oles of Paramcoecium or
Vorticella, and are more

nearly comparable with
those of the frog (p.
146).

The excretory organs of
the earthworm consist of
little tubes called xephridia,
of which each metamere—-
except the first three and
the last—possesses a pair,
one on either side (Figs. 78,
79, and 80 neph). You
will remember that in the
frog all the nephridia are
connected together to form
a pair of kidneys, each with
a single duct communi-
cating with the cloaca (p.
145). In the earthworm
each nephridium is a long
and extremely delicate tubc ,
arranged in three main
loops (Fig. 80), opening at
one end into the ccelome
by a xephrostome and at
the other communicating
with the exterior directly

(Fig. 78).

Fic. 80.—A nephridium of Lasmbricus,

showing the three main loops into which
the different parts of the tubule are ar-
ranged, as well as the different portions
of the tubule.

a. nephrostome ; 4. 3.4. slender portion

of the tubule into which the nephrostome
opens; c.c. second ciliated portion; d.
glandular portion ; ¢. muscular portion ;
z’. end of ¢ at which the nephridiopore
opens. (From Gegenbaur.)

The tubes are

1 In tne frog the nephrostomes lose their connection with the ne-
phridia, and open in the adult into the renal veins (Fig. 47, p. 146).

332 THE EARTHWORM CHAP.

attached to the posterior faces of the septa. Each
nephrostome (a) is ciliated, and projects through the cor-
responding septum so as to communicate with the segment
of the body-cavity next in front of that in which the main
part of the tubule is situated. The nephrostome opens into
a long and slender transparent part of the tube lined with
ciliated cells in part of its course and extending along the
first and second loops (4); this part is succeeded by a
wider, ciliated portion in the second loop (¢), which com-
municates with a still wider portion (d) lined by granular,
non-ciliated, glandular cells, also lying in the second
loop; the glandular portion opens to a much wider
muscular part of the tube (e) which constitutes the third
loop and opens into the exterior by a small pore—the
nephridiopore—near the outer seta of the inner couple
(Fig. 78).

Thus the nephridia, which are abundantly supplied with
blood-vessels, are lined in part by gland-cells and in part
by cilia which work towards the exterior. Water and
nitrogenous waste from all parts of the body pass by diffu-
sion into the blood and are conveyed to the nephridia,
the gland-cells of which withdraw the waste-products and
pass them into the cavities of the tubes, whence they are
finally discharged from the body. The granular yellow cells
on the walls of the intestine also appear to contain ex-
cretory products, which become set free in the body cavity
and are thence got rid of through the nephridia. It will
be noticed that a certain amount of loss of the ccelomic
fluid must take place through the dorsal p6res as well
as through the nephridia.

In discussing the hydroid polypes we found that one of
the most important points of difference between the loco-
motive medusa and the fixed hydranth was the presence in

Vv NERVOUS SYSTEM 333

the former of a well-developed nervous system (p. 311)
consisting of an arrangement of peculiarly modified cells,
to which automatic action was seen to be due. It is
natural to expect in such an active and_ otherwise
highly-organised animal as the earthworm a nervous sys-
tem of a considerably
higher degree of com-
plexity than that of a
medusa.

The central ner-
vous system consists
of two parts, the drain = sen ganz
and the ventral nerve-
cord. The brain (Fig.
79 or, and Fig. 81,
cer. gang) consists of
a pair of white pear-
shaped swellings or
gangha situated on
the dorsal side of
the buccal sac where

-prost

Ne CO

Fic. 81.—Anterior portion of nervous system of

it 1s continued into Lumbricus.
cer. gang. cerebral ganglia or brain; com. ceso-
the pharynx. The phageal connectives ; ze. co. ventral nerve-cord ;
prost. prostomium. (From Parker and Has-
ventral nerve - cord well’s Zoology, after Leuckart.)

(x. ¢ and ne. co) is a

longitudinal band extending along the whole middle
ventral line of the body, internally to the longitudinal
muscular layer, from the third to the anal segment, and
slightly swollen in each segment. The brain is connected
with the anterior end of the ventral nerve-cord by a pair
ofnervous bands, the esophageal connectives (com) which
pass respectively right and left of the buccal sac, and thus
form a erve-collar,

334 THE EARTHWORM CHAP.

It is to be noted that one division of the central nervous
system—the brain—lies altogether above and in front of the
enteric canal, the other division—the ventral nerve-cord—
altogether beneath it, and that, in virtue of the union of the
two divisions by the cesophageal connectives, the enteric
canal perforates the nervous system. Both brain and cord
are composed of delicate nerve-fibres and‘of nerve-cells, the
latter being situated in the ventral and lateral regions
of the cord along its whole length, so that there is
here hardly any distinction into ganglia and connectives,
although the swellings are often spoken of as ganglia.
Along the dorsal side of the cord are three transparent
tube-like structures, known as giant-fibres, the function of
which is not known (Fig. 78). The whole cord is enclosed
in a sheath consisting of connective tissue and muscular
fibres.

The peripheral nervous system consists of a number of
nerves, both sensory and motor (p. 162), which arise from
the central nervous system and supply the various parts of the
body. From the brain a number of nerves are given off
to the prostomium, and from each ganglionic enlarge-
ment two pairs of nerves can be traced into the body-
wall, while between these enlargements one pair is given
off which supply mainly the septa.

Comparing the nervous system of the earthworm with
that of a medusa it is important to notice the con-
centration of the central nervous system in the higher type,
and the special concentration at the anterior end of the
body to form a brain. When, again, we compare the central
nervous system of the earthworm with that of the frog (pp. 28
and 155) several important points of difference are noticeable.
In the former it lies freely in the ccelome, and with the
exception of the brain, is situated on the ventral side of the

Vv NERVOUS SYSTEM 335

body; while in the frog it is enclosed in a neural canal
and is dorsal in position. The brain of the frog is a
complicated structure, and the whole nervous system is
hollow, there being ventricles in the brain and a central
canal surrounded by nerve-cells in the spinal cord; while
in the earthworm the brain consists merely of a pair of
cerebral ganglia, and it and the ventral cord are solid.

The whole nervous system is capable of originating auto-
matic action. It is a well-known fact that if the body of
an earthworm is cut into several pieces each performs in-
dependent movements ; in other words, the whole body is
not, as in the higher animals, paralysed by removal of the
brain (p. 172). There can, however, be little doubt that
complete co-ordination, #.¢, the regulation of the various
movements to a common end, is lost when the brain is
removed,

The earthworm is devoid of organs of sight or hearing.
It exhibits sensitiveness to bright light, which may be due
to direct action on the central parts of the nervous system.
The sense of hearing appears to be absent; but a faculty
analogous to taste or smell, enabling the animal to dis-
tinguish between different kinds of food, is well developed.
Groups of narrow sensory cells in the epiderm, which are
most abundant on the prostomium and peristomium, have
probably to do with this faculty.

There are two matters of general importance in connection
with the structure of the earthworm to which special at-
tention must be drawn.

Notice in the first place how in this type, far more than
in Hydra, we have, as in the frog, certain definite parts
of the body set apart as organs (p. 30) for the performance
of particular functions: it ,is clear that differentiation of

336 THE EARTHWORM CHAP.

structure and division of physiological labour play a far
more obvious and important part than in any of the
lower organisms described in the four previous chapters.

Notice in the second place the vastly greater complexity
of microscopic structure, the body being divisible into é/ssues
(p. 118) each clearly distinguishable from the rest. We have
epithelial tissue with its cuticle, muscular tissue, and nervous
tissue, as well as blood and ccelomic fluid. One result of
this is, that, toa far greater extent than in Hydra, we can
study the morphology of the earthworm, as we have done
that of the frog, under two distinct heads: anxatomy and

histology (p. 104).

Asexual reproduction does not take place normally in
the earthworm, but it frequently happens by accident that
a worm is cut into two or more parts. When this occurs,
each end is able to reproduce the missing portion: this
process is known as regeneration.

The earthworm, like Hydra, is monoecious or herm-
aphrodite (p. 302), and besides the essential organs of sexual
reproduction—ovaries and spermaries—which are, as in the
frog, developed from certain parts of the ccelomic epi-
thelium, it possesses various accessory organs. The whole
reproductive apparatus is situated in segments 9-15.

The ovaries (Figs. 79 ov, and 82 0) are a pair of minute
bodies about x mm. in length, attached by a short stalk,
one on either side, to the posterior face of the septum
separating segments twelve and thirteen, not far from the
nerve-cord. The proximal end of each ovary, nearest the
stalk, is composed of a mass of undifferentiated cells of
germinal epithelium (compare Figs. 62 and 63): nearer its
middle, certain of these are seen to increase in size so as
to be recognisable as young ova: while the distal end

v REPRODUCTIVE ORGANS

contains the ripe ova, arranged in a single row, each

closed in a vitelline
membrane and con-
taining a large nucleus
and nucleolus and a
number of granules of
food-yolk (p.195). The
eggs are discharged
into the ccelome and
are received into the
female gonoducts or
oviducts (Fig. 79 0.d.,
and Fig. 82) — two
short tubes, each with
a wide ciliated mouth
placed opposite the
corresponding ovary.
The oviduct perforates
the next following sep-
tum (Ze. that between
segments thirteen and
fourteen) to open by
a minute aperture on
the fourteenth seg-
ment, near the inner
double row of sete.
Connected with the
mouth of each oviduct
is a small egg-sac (Fig.
79, 7. 0, Fig. 82, e. 5),
developed as an out-

337

en-

Fic. 82. Diagrammatic longitudinal section o

=

part of a Lumbricus, showing segments
g—15 and the contained generative organs
of one side :X3. In the body-wall the cuticle
is indicated by a clear space, the circular
muscles by irregular dots, the longitudina]
muscles by dotted longitudinal lines, and the
peritoneal membrane by a thin line.

S. egg-sac 5 o.,ovary ; sf. aperture of anterior
spermotheca—both spermothece are indicated
by dotted lines ; sf. s. posterior sperm-sac, the
anterior and middle sacs are not lettered ;
ss. sperm-sac; 7z. anterior spermary—the
posterior is not lettered; 9. aperture of ovi-
duct; 6. aperture of spermiduct. The ovi-
duct, spermiduct, and seminal funnels are
indicated by thick lines. (From the Cambridge
Natural History, after Hesse.)

growth from the same septum and extending back into
the cavity of segment fourteen.

Pract. Zoou.

Z

338 THE EARTHWORM CHAP,

Certain globular sacs called spermothece (Fig. 82) also
belong to the female part of the reproductive apparatus.
Of these there are usually two pairs (sometimes more than
two in Allolobophora) situated in the ninth and tenth
segments, and opening to the exterior between the ninth
and tenth, and tenth and eleventh segments respectively ;
their function will be mentioned presently.

The earthworm possesses two pairs of very minute sperm-
aries attached to the posterior face of the septum between the
ninth and tenth, and tenth and eleventh segments respectively
(Figs. 79 ¢e, and 82, 7). They have a flattened form and
their free or distal ends are produced into finger-shaped
processes. Behind each spermary, and in the same seg-
ment, is a ciliated seminal funnel opening into the ceelome
and produced backwards through the septum next behind
into an efferent duct, the two ducts of either side communi-
cating with a main spermiduct or vas deferens (Figs. 79 and
82). Each of these extends backwards in the ventral body-
wall to open by a tumid lip on the fifteenth segment, near
the inner couple of sete.

The most prominent portions of the reproductive appar-
atus are certain large whitish bodies—the sferm-sacs or
seminal vesicles (Figs. 79, ves. sem, and 82, sp. s) which are very
apparent in the adult worm as soon as the ccelome is cut open
in this region. Of these there are three (Lumbricus) or four
(Allolobophora) pairs, situated in segments g—12. Theyarise
as outgrowths of the septa, and communicate with theccelome ;
but in Lumbricus, the anterior pairand the two posterior pairs
respectively become joined across the middle line so as to
form two median sperm-reservoirs (Figs. 79, med. ves. sem,
and 82, s.s), each of which encloses a coelomic cavity in which
one pair of spermaries and seminal funnels becomes en-
closed. oe

v CROSS-FERTILISATION 339

The cells of which the spermaries are composed do not
develop into sperms in the testes themselves, but pass into
the sperm-sacs, where they undergo division into rounded
masses of cells looking very much like asegmenting oosperm
in the polyplast stage. Each of these products of division of
the testicular cells becomes elongated, and gradually takes
on the form of a sperm with a-rod-like head and a vibratile
tail (compare pp. 194 and 302). When set free, the sperms
pass into the spermiducts through the ciliated funnels.

It is well known that many flowers (the reproductive
organs of higher plants) contain the generative cells of both
sexes, enclosed within the ovules and anthers respectively ;
yet in very many cases self-fertilization does not occur owing
to contrivances of various kinds for its prevention. It has
been proved in numerous instances that cross-fertilization—
7.¢., the impregnation of the ovum in one individual by the
male cell of another—is of great importance in keeping up
the strength and vigour of the plant from generation to genera-
tion. The same is true amongst animals; and though in
some moncecious forms, such as the Hydra, there is no special
arrangement for the prevention of self-impregnation if the
male and female gametes of the same individual ripen at
the same time, in others, such as the earthworm, the ova are
always fertilized by the sperms from another individual.

This is effected in the earthworm in the following way.
Two individuals, their anterior ends pointing in opposite
directions, become applied together by their ventral sur-
faces and attached to one another in this position by a
viscid secretion from the clitellum. The sperms are then
passed from the male apertures of one into the spermothecz of
the other individual, and the two worms afterwards separate.
The clitellum then secretes a tough chitinous tube or cocoon

zZ2

340 THE EARTHWORM CHAP

which forms a broad ring round the body in this region, and
which is gradually slipped forwards. As it passes over the
apertures of the oviducts and spermothecx, ova and sperms
(the latter derived from the other individual) are passed into
it, as well as albumen secreted by certain glands present in
this region. When the worm has entirely withdrawn itself
from the cocoon, the latter closes up at the ends in virtue of
its elasticity, and the eggs, after fertilization, undergo seg-
mentation.

The cells of the polyplast soon become differentiated into
an outer ectoderm and an inner endoderm enclosing the
archenteron, which communicates with the exterior by the
blastopore (compare p. 201). A mesoderm (p. 202) is then
developed, and each layer gradually gives rise to the cor-
responding parts in the adult animal, much as in the frog
(compare p. 209), except that the greater part of each
nephridium is apparently derived from the ectoderm, only
the inner end of the tube, not the whole of it, arising from
the mesoderm: the mesoderm undergoes segmentation,
the ccelome appearing in it as a cavity (p. 203)—or rather
as a series of cavities, one in each segment. The young
worm is then hatched, and it is to be noticed that it passes
through no metamorphosis (p. 11).

In the marine worms belonging tu the same class as the earthworm,
on the other hand, the young is hatched in the form of a larva known

as the ¢rochosphere, which swims by means of cilia arranged in circles

round the body, and gradually undergoes metamorphosis into the adult
form.

There are a number of different kinds of animals
commonly known as “worms,” but many of these (e.g. the
parasitic worms in the lungs and bladder of the frog,
the tapeworms, &c.), are very different from the earth-
worm in structure, and are placed in several different

v CLASSIFICATION 341

phyla. The earthworm is a member of the phylum
Annulata, which also includes a number of other worms living
in the sea and in fresh-water, as well as the leeches, &c. In
all these the body is elongated and divided into metameres;
there is usually an extensive ccelome ; and the nervous
system and nephridia are similar to those of the earthworm.
The class Chetofoda, in which the earthworms and their
fresh-water and marine allies are included, receives its name
from the fact that all its members are provided with cuticular
sete, which in the order to which the marine forms belong
(Polychaeta) are usually long, and of varied forms, and are much
more numerous than in the earthworms and fresh-water
worms, which constitute the order Oligocheta. This order
includes several families, both Lumbricus and Allolobophora
belonging to the family Lumbricde.

PRACTICAL DIRECTIONS.
EARTHWORM.

Select a large earthworm, and after noting its movements and
mode of progression, kill by immersion in spirit for a few minutes and
then place in a dish and let the tap run on it for a short time.

A. External Characters :-—

1. Note:—a. The form and colour of the body and its division into
metameres ; 6. the anterior end terminating in the prostomdum and fol-
lowed by the erdstomium ; ¢. the clétellum; and d. the last or anal
segment.

2. If the worm be drawn through the fingers backwards, the sete
will be felt : examine with a lens and observe their position and the
number in each segment.

3. Make out the following apertures: a. the mouth; 0. the anus ;
c. the dorsal pores (p. 325); @. the two apertures of the spermzducts,
with thickened lips, on the fifteenth segment.

(It requires careful examination to see the other apertures, viz,—
those of the ovdducts, spermothece, and nephridia).

Sketch from below or from the side,

342 THE EARTHWORM CHAP,

8. Dissection :—

I. Take a freshly-killed worm in the left hand, and carefully insert
the point of the fine scissors into the integument about one-third of the
way down the body, close to the middle dorsal line. Place a drop of
the celomic fluid which exudes on a slide, add a drop of salt solution,
and cover. Examine with the low and high powers, and note the
structure and movements of the amabotd corpuscles. Sketch at
intervals.

II. Continue the cut forwards to the prostomium, keeping very slightly
to one side of the median dorsal line, and taking care that the point
of the scissors does not penetrate deeper than the integument : note the
iridescent cuticle. Place the animal in a dish with just enough water
to cover it, and carefully insert a pin between the integument and the
yellow intestine on either side, near the posterior end of the incision,
so as to expose the calome: note the septa connecting the body-wall
with the intestine. Then insert more pins, obliquely, so as to ex-
pose the ccelome and enteric canal up to the anterior end, taking
especial care not to tear the ventral parts of the septa and to stretch
the animal longitudinally as much as possible. Then note :—

1. The sferm-sacs—three or four pairs of large white bodies in
segments IX-XII, and varying greatly in size and form according to
the size of the animal. If your specimen is a Lumbricus, you will
notice that the anterior and the two posterior pairs are respectively
united across the middle line, beneath the enteric canal, to form
the two sperm-reservoirs.

2. The enteric canal and its subdivisions:—a. buccal sac ; 6,
pharynx; c. gullet (largely hidden by the sperm-sacs); d. crop;
gizzard ; and e. zntestine, covered with a layer of yellow cells.

3. The dorsal blood-vessel, containing red blood, and giving off
branches to the enteric canal; the large rhythmically contractile
commissural vessels connecting the dorsal with the ventral vessel: the
latter will be seen later.

4. A pair of small whitish coiled bodies, the zephrzdia, attached to
the posterior face of each septum exposed (except the first three), on
either side of the alimentary canal. Carefully remove one of these in
the region of the intestine—(take hold of the septum with the fine for-
ceps, and cut around the nephridium with the small scissors)—mount in
salt solution or water, and examine first with the low power, and then
with the high power. Note that the nephridium consists of a long

v PRACTICAL DIRECTIONS 343

coiled tube, plentifully supplied with blood-vessels, and that long
yibratile cilia can be seen in partsof it. (For details see § VI).

Adda little methylated spirit to the water in your dissecting dish and
sketch your dissection.

5. The ovardes—Examine segment XIII closely, being very careful
not to injure its contents, and the ovaries may then be seen projecting
backwards into this segment, one on either side, just in front of the
crop. They can easily be recognised by their shape, and by the fact
that they hang freely into the ccelome, as can be seen by touching them
with a seeker. Carefully seize the septum between segments XII and
XIII with the small forceps, and cut around the attachment of an
ovary so as to remove it. Stain with methyl-green and mount in
glycerine (or else fix, stain, and mount in balsam, as directed on
p. 136). Note the mass of undifferentiated cells at the proximal, at-
tached end of the ‘ovary, and the gradual development of the ova
towards the distal, narrower end. Examine an ovum, and observe the
nucleus, nucleolus, and granules of food-yolk. Sketch the ovary.
=:(The ov¢ducts and ovésacs are not easy to make out in dissec.
tions).

6. The globular sfermothece (usually two pairs) in segments IX
and X.

III. Tease out a small portion of a sperm-sac, stain with magenta,
and mount in glycerine. The following stages in the development of
the sperms can then be made out :—a. The sperm-mother-cells (devel-
oped in the spermary) in different stages of «livision: the products of
division, each with a nucleus, become arranged in a single peripheral
row, the central mass of protoplasm remaining undivided. 4. The
gradual elongation of these small cells; and ¢. the conversion of each
into a sgerm, the nucleus forming the rod-like ‘‘head,” and the proto-
plasm giving rise to the delicate ‘‘tail.” @. Free sperms (also to be
found in the spermothecz). Sketch a series of stages.

It is difficult to make out the two pairs of spermardes and the
spermiducts by dissection, and they can be more easily studied by
examining transverse sections, prepared as directed below. (In Lum-
bricus the spermaries and seminal funnels are enclosed within the
median sperm-reservoirs.) The spermiducts are partly embedded in the
body-wall.

IV. Remove the sperm-sacs carefully, and make out further details
as regards the enteric canal (see § II, 2). Note the esophageal glands,

344 THE EARTHWORM CHAP,

Cut open part of the intestine along one side, and observe the thick
dorsal fold or typhlosole projecting into it. Sketch.

V. 1. Note the small cerebral gangléa ox brain on the dorsal side of
the buccal sac, and then cut through the anterior part of the pharynx
just behind the brain. Carefully remove the alimentary canal, noting as
you do so the ventral or sub-intestinal blood-vessel. The nervous-system
will now be exposed. Observe again the paired cerebral ganglia,
from which arise a pair of conmectzves, forming a small xerve-ring or
collar around the buccal sac, and continuous ventrally with the veztral
nerve-cord, consisting of lateral halves fused together and extending along
the whole length of the ventral body-wall, passing through spaces
in the septa, and expanding slightly in each segment, so as to form
ganglionic swellings. Three pairs of nerves are given off in each
segment. Sketch.

2. Remove the nerve-ring and a small portion of the ventral cord,
and examine with the low power. Sketch.

3. A lateral neural vessel can be seen close to the ventral cord on
either side. Remove a portion of the cord, and note the szd-neural
vessel,

VI. Further details as regards the structure of the nephridia are best
made out on a worm which has been preserved in spirit. Very care-
fully remove the enteric canal as directed above, so as not to injure the
septa more than necessary: the nephrostomes can then be seen with a
lens, looking like small whitish dots. Remove an entire nephridium
carefully as before (§ II. 4), stain, and mount in glycerine or balsam. Note
the three loops, and—a. the ciliated nephrostome; 6. the first, slender
part of the tube with its cilia; «. the second wider, ciliated part ;
d, the third still wider, glandular part ; and e. the fourth much wider,
muscular part, which opens on to the exterior by the wephridrofore,
Sketch.

VII. Remove a small piece of the integument containing sete, and
separate the latter out with needles. Mount in water, and examine.
Sketch.

Cc. Transverse Sections.—For the preparation of these, it is best
to keep a worm in coffee-grounds or small pieces of blotting-paper
moistened with water, in order that the gritty contents of its intestine
may be replaced bya soft substance which will not blunt the razor.
Kill the worm, cut a small piece about 3 inch in length from the region
of the intestine,« and fix, stain, and cut into transverse sections as

Vv PRACTICAL DIRECTIONS 345

directed on p, 136. Examine a section with the low power, and
note :—

1. a, The thin czdtzcle ; 6. the epederm, enclosing goblet-cells (unicel-
lylar glands); ¢. the very thin derm; @. the set, with their sacs and
muscles, if your section passes through one or more of them. ,

a. The muscles of the body-wall, a. The external circular layer; and
& the thicker /ongitudinal layer, appearing feather-like in transverse
section, and broken up into bands at the lines of the dorsal pores and
setze. Note that the muscles are unstriped.

3. The calome and perttoneal membrane.

4. The zwzestine, with its thick dorsal typhlosoe. It is lined by a
single layer of columnar cells (exterze eptthelium), outside which is a
thin muscular and connective tissue layer. Externally to this, again,
are the elongated and granular yellow cells, which are especially
abundant in the typhlosole.

5. The dorsal, ventral, and cntestinal blood-vessels.

6. The ventral zerve-cord, just internal to the longitudinal muscular
layer. It is enclosed in a muscular and connective-tissue sheath, im-
bedded in which the swb-neural and lateral neural vessels can be seen.
Along the dorsal side are three clear-looking “‘ g7ant fibres.” Observe
the serve-cel/s along the cord ventrally and laterally, the #e7ves coming
off from the cord, and the symmetrical halves of which the cord is
composed.

7. The nephridia :—these will be seen cut through in various
planes.

The thin seg¢a will be cut through in different directions, and their
relations are therefore not easily seen in sections ; note the circular and
radial muscular fibres in the septa. Sketch the lateral half of your
section, and then put on the high power and work through §§1-7 again.
Sketch as many details as possible.

(If time permits, prepare and examine a series of transverse sections
through the genztal region, and observe any important points not
already made out in your dissection, Note especially the spermarzes
and spermdducts).

CHAPTER VI

THE CRAYFISH.—CHARACTERS OF THE PHYLUM
ARTHROPODA.

WE have now to study an animal formed ona very similar
plan of structure to the Earthworm as regards segmentation
and arrangement of many of the organs, but which reaches
in every respect a far higher grade of organisation.

The common British Fresh-water Crayfish is usually known
as Astacus fluviatilis, and is found in many of the streams and
rivers of England and Ireland, hiding under stones, or in
holes, into which it darts very suddenly on the approach of
danger. Its ordinary creeping movements are slow, and are
effected by means of a number of jointed limbs, for which
reason it is included, together with insects, spiders, scorpions,
&c, in the phylum Arthropoda.

In colour, the Crayfish is greenish grey, and in form it is
very similar to the marine Lobster (Homarus vulgaris), to
which the following description will apply almost equally
well.

In addition to the presence of paired limbs or appendages,
one of the most striking points of difference between an
Earthworm and a Crayfish is the smaller and constant
number of segments or metameres in the latter, as well
as the fact that certain of these are more or less

CH. VI THE CRAYFISH 347

firmly united with one another—/.e., have undergone
concrescence. The result of this fusion of the segments is
that two distinct regions can be distinguished in the
body—an anterior cephalothorax and a posterior abdomen
(Fig. 86, cth, ad).

The cephalothorax is unjointed, and is covered by a
cuirass-like structure, the carapace; and the abdomen is
divided into distinct segments, movable upon one another
in a vertical plane. The cephalothorax is divided into two
regions, an anterior—the ead, and a posterior—the ¢horax,
by a transverse depression, the cervical groove. The cara-
pace is developed from the dorsal and lateral regions of
both head and thorax ; it is free at the sides of the thorax,
where it forms a flap or gi//cover (Fig 87, Ad) on each side,
separated from the actual body-wall by a narrow space in
which the gills are contained.

The limbs spring from the ventral surface. Both trunk
and appendages are covered with a sort of shell, formed of
chitin (p. 320), strongly impregnated with carbonate of lime
in most parts, so as to be hard and but slightly elastic.

The abdomen is made up of seven segments: the first
six of these are to be considered as metameres in the sense
in which the word is used in the case of the earthworm.
Each has a ring-like form, presenting a broad dorsal region or
tergum (Fig. 83, T), a narrow ventral region or sternum (5S),
and downwardly directed lateral processes, the A/eura (PL).
The seventh division of the abdomen is the fe/son (Fig. 86, ¢):
it is flattened horizontally and divided by a transverse groove
into anterior and posterior portions. All seven segments
are calcified, and are united to one another by uncalcified
articular membranes: the first segment is similarly joined
to the thorax. Thus the exoskeleton of Astacus is a con-
tinuous cuticular structure, discontinuously calcified so as

348 THE CRAYFISH CHAP.

to have the character of a hard, jointed armour. Tufts of
minute feather-like cuticular structures, or se/@, are present
on various parts of the exoskeleton both of the body and
appendages.

It has been stated that the abdominal segments are
movable upon one another in a vertical plane: z.e., the whole

Fic. 83.—Transverse section of abdomen of Crayfish.

DA. dorsal abdominal artery ; EM. dorsal muscles of the abdomen; EP. space
between the pleuron and the appendage ; FM. ventral muscles of the abdomen ;
M. muscles of the appendage ; N. endopodite ; NG. nerve-ganglion ; P. protopodite ;
PL. pleuron ; PR. hind-gut ; S. sternum; T. tergum; V. ventral abdominal artery ;
X. exopodite. (From Marshall and Hurst's Zoology).

abdomen can be extended or straightened, and flexed or bent
under the cephalothorax ; the segments are incapable of
movement from side to side. This is due to the fact that,
while adjacent segments are connected dorsally and ven-
trally by flexible articular membranes, they present at each
side a joint, placed at the junction of the tergum and

VI EXOSKELETON 349

pleuron, and formed by a little peg-like process of one seg-
ment fitting into a depression or socket in the other. A
line drawn between the right and left joints constitutes the
axts of articulation, and the only possible movement is in a
plane at right angles to this axis.

Owing to the presence of the carapace the thoracic region
is immovable, and shows no distinction into segments either
on its dorsal (tergal) or lateral (pleural) aspect. But on the
ventral surface the sterna of the thoracic segments are
clearly marked off by transverse grooves, and the hindmost
of them is slightly movable. Altogether eight thoracic
segments can be counted.

The ventral and lateral regions of the thoracic exoskeleton
are produced into the interior of the body in the form of a
segmental series of calcified plates, so arranged as to form
a row of lateral chambers in which the muscles of the limbs
lie and a median tunnel-like passage or sternal canal, con-
taining the thoracic portion of the nervous system (Fig.
87). The entire endophragmal system, as this series of plates
is called, constitutes a kind of internal skeleton.

The ead exhibits no segmentation: its sternal region is
formed largely by a shield-shaped plate, the ef7stoma, nearly
vertical in position. The ventral surface of the head is, in
fact, bent so as to face forwards instead of downwards. The
cephalic region of the carapace is produced in front into a
large median spine, the rostrum: immediately below it is
a plate from which spring two movably articulated
cylindrical bodies, the eye-stacks, bearing the eyes at their
ends.

The appendages have very various forms, and are all, like
the abdomen, jointed or segmented, being divisible into
freely articulated limb-segments or fodome res. You will
at once notice the long feelers attached to the head, the

350 THE CRAYFISH CHAP. VI

five pairs of legs springing from the thorax, and the little
fin-like bodies arising from the sterna of the abdomen. It
will be convenient to begin with the last-named region.

The third, fourth, and fifth segments of the abdomen
bear each a pair of small appendages, the swimming feet or
pleopods (Fig. 83, P, N, X). A pleopod (Fig. 84, 10) consists
of an axis or protopodite having a very short proximal (Fig. 84
pr. 1) and a long distal (fv. 2) podomere, and bearing at its
free end two jointed plates, fringed with sete, the endopodite
(en) and exopodite (ex). These appendages act as fins,
moving backwards and forwards with a regular swing, and
probably aiding in the animal’s forward movements.

In the female a similar appendage is borne on the
second abdominal segment, while that of the first is
more or less vestigial (p. 159). In the male the first
and second pleopods (9) are modified to form incomplete
tubes which serve to transfer the spermatophores (p. 368)
to the body of the female. The sixth pair of pleopods (11)
are alike in the two sexes: they are very large, both
endopodite and exopodite ‘having the form of broad,
flat plates: in the natural position of the parts they lie
one on each side of the telson, forming with it a large
fivelobed tail fin: they are therefore conveniently called
uropods or tail-feet. The telson itself bears no appendages.

The thoracic appendages are very different. The four
posterior segments bear long, slender, jointed /egs (8),
with which the animal walks: in front of these is a pair of
very large legs terminating in huge claws or che/e, and
hence called chelipeds (Fig. 86, 4f 4). The three anterior
thoracic segments bear much smaller appendages, more
or less leg-like in form, but having their bases toothed to

serve as jaws: they are distinguished as maxi/lipeds or foot-
jaws.

J
5. 2" Maxilla 6.1" Maxilliped i
7, 39 Maxilliped

9.Copulatory Organs 10.Swimming Foot

1l.Uropod

Fic. 84.—The principal appendages of the Fresh-water Crayfish, placed in the
same position, with the protopodite (#7) and epipodite (ef) downwards, the
endopodite (ev) to the left, and the exopodite (¢) to the right.

The protopodite is typically’ formed of two podomeres (/”. 1, f. 2), the endopodite
of five (en. 1—en. 5); a gill (¢) may be attached to the epipodite.

The three proximal segments of the antennule are marked 1—3, its flagella 77. t
and_ 7. 2; the distal end of the endopodite of the antenna is a flagellum (7).
(The tufts of threads in 7 and 8 are very long sete which extend between the
gills),- (From Parker and Haswell’s Zoology, after Huxley.)

>. The structure of these appendages is best understood by
a consideration of the ¢hird maxilliped (Fig 84, 7). The

352 THE CRAYFISH CHAP.

main portion of the limb is formed of seven podomeres
arranged in a single series, strongly calcified, and—with the
exception of the second and third, which are fused—move-
ably articulated with one another. The second podomere,
counting from the proximal end, bears a many-jointed
feeler-like organ (ex), and from the first springs a thin, folded
plate (ef) having a plume-like gill (g) attached to it. The
first two segments of the axis form the protopodite (g7. 1, 2),
its remaining five segments the endopodite (em. 1, 5), and
the feeler, which is directed outwards, or away from the
median plane, the exopodite (ex). The folded plate is
called the epipodite: in the natural position of the parts it is
directed upwards, and lies in the gill-cavity between the
proper wall of the thorax and the gill-cover.

The five Zegs (8) differ from the third maxilliped in their
greater size, and in having no exopodite : in the fifth or last
the epipodite also is absent. The first three of them have
undergone a curious modification, by which their ends are
converted into pincers or che/e: the fourth segment of the
endopodite (sixth of the entire limb, en. 4) is produced dis-
tally so as to form a claw-like projection (en. 4’), against
which the terminal segments (ez. 5) bites. The first leg is
much stouter than any of the others, and its chela is of
immense size and forms an important weapon of offence
and defence. The second maxillited resembles the third,
but is considerably smaller : the 7s¢ (6) has its endopodite
greatly reduced, the two segments of its protopodite large
and leaf-like, and no gill is connected with the epipodite.

The head bears a pair of mandibles and two pairs of
maxilie in relation with the mouth, and in front of that
aperture a pair of antennules and.one of antenne. The
hindmost appendage of the head is the second maxilla (5),
a leaf-like jappendage, its protopodite being cut up into

vi APPENDAGES 353

lobes, while the exopodite is modified into a boomerang:
shaped plate, which, as we shall see, is an important
accessory organ of respiration. The first maxilla (4) isa very
small organ, having neither exopodite nor epipodite. The
mandible (3) is a large strongly calcified body, toothed along
its inner edge, and bearing on its anterior border a little
three-jointed, feeler-like body, the Za/p, the two distal seg-
ments of which represent the endopodite, its proximal
segment, together with the mandible proper, the protopodite.

The antenna (2) is of great size, being nearly as long as
the whole body. It consists of an axis of five podomeres,
the fifth or last of which bears a long, flexible, many-jointed
structure, or flagellum (72), while from the second segment
springs a scale-like body or sguame (ex). Itis fairly obvious
that the two proximal segments represent the protopodite,
the remaining three, with the flagellum, the endopodite, and
the squame the exopodite.

The antennule (1) has an axis of three podomeres ending
in two many-jointed flagella (7. 1, 72. 2), which are some-
times considered as corresponding to the endopodite and
exopodite. But in all the other limbs, as we have seen,
the exopodite springs from the second segment of the axis,
and the probabilities are that there is no exact corre-
spondence between the parts of the antennule and those of
the remaining appendages.

The eye-stadks, already noticed, arise just above the an-
tennules, and are formed each of a small proximal and a
large distal segment. They are sometimes counted as
appendages serially homologous with the antennze and legs,
but are more properly to be looked upon as articulated pro-
cesses of the prostomium. It is probable that the anten-
nuies are also -prostomial and not metameric structures :

assuming this to be the case, it will be seen that the
Pract. Zoon. AA

354 THE CRAYFISH CHAP.

body of the crayfish consists of a prostomium, eighteen
metameres, and a telson, which is probably composed
of an anal segment p/ws a post-anal extension. The
prostomium bears eye-stalks and antennules : the first four
metameres are fused with the prostomium to form the head,
and bear the antenne, mandibles, first maxilla, and second
maxille: the next eight metameres (5th—1x2th) consti-
tute the thorax, and bear the three pairs of maxillipeds and
the five pairs of legs: the remaining six metameres (13th—
18th), together with the anal segment, constitute the
abdomen, and bear five pairs of pleopods and one of
uropods.

The articulation of the various podomeres of the append-
ages is on the same plan as that of the abdominal segments
(p. 348). The podomeres are, it must be remembered, rigid
tubes: they are connected with one another by flexible
articular membranes (Fig. 85, a7¢. #), but at two points the
adjacent ends of the tubes come into contact with one
another and are articulated by peg-and-socket joints (%), the
two joints being at opposite ends of a diameter which forms
the axis of articulation. The two podomeres can therefore
be moved upon one another in a plane at right angles to
the axis of articulation and in no other direction, the joints
being pure hinge-joints. As a rule the range of movement
is from the perpendicular to a tolerably extensive flexion on
one side—the articulations are single-jointed, like our own
elbows and knees. The whole limb is, however, capable of
universal movement, owing to the fact that the axes of articu-
lation vary in direction in successive joints: the first
joint of a limb bending, for instance, up and down, the
next obliquely, the next backwards and forwards, and so on.
In some cases—e.g., the pleopods—peg-and-socket joints are
absent, the articulation being formed merely by an annular

vI EXOSKELETON AND MUSCLES 355

articular membrane, movement being therefore possible in
any plane.

Sections show the body-wall to consist of an integument
composed of a layer of deric epithelium (epiderm) secreting
a thick cuticle, and a layer of connective tissue forming
the derm, beneath which is a very thick layer of large and
complicated muscles which fill up a great part of the interior
of the body. Neither on the epiderm nor elsewhere are
there any cilia, the absence of these structures being
generally characteristic of Arthropods.

The cuticle is of great thickness, and except at the joints
between the various segments of the body and limbs, is
impregnated with lime salts so asto forma hard, jointed
armour. It thus constitutes a cuticular exoskeleton, forming
a continuous investment over the whole body but discon-
tinuously calcified. It is shed entire and renewed
periodically—once a year during adult life—the process
being known as ecdysis, growth taking place during the
period between ecdysis and renewal while the animal is
soft.

The muscular system shows a great advance in complexity
over that of the earthworm: and consists entirely of
transversely striated fibres (compare p. 112). In the abdo-
men the muscles are of great size, and are divisible into a
smaller dorsal and a larger ventral set. The dorsal muscles
(Figs. 83 and 86, em) are paired longitudinal bands, divided
into segments called myomeres (p. 203), and inserted by
connective tissue into the anterior border of each segment :
anteriorly they are traceable into the thorax, where they arise
from the side-walls of that region. When these muscles
contract they draw the anterior edge of each tergum under
the posterior edge of its predecessor, and thus extend or
straighten the abdomen,

. AA2

356 THE CRAYFISH ~ 2 CHA.

The ventral muscles (Iigs. 83 and 86, fv) are extraordin-
arily complex and cannot be described in detail here. They
partly aid the dorsal muscles in extending the abdomen, but
are chiefly important in producing an approximation of the
sterna,.and thus in flexing the abdomen. The ventral
muscles are, like the dorsal, traceable into the thorax, where
they arise from the endophragmal system (p. 349). The
flexor muscles are immensely powerful, and produce, when
acting together, a sudden and violent bending of the abdomen
upon the cephalothorax causing the crayfish to dart back-
wards with great rapidity.

It will thus be seen that the body-muscles of the crayfish
cannot be said to form a layer of the body-wall, as in the
earthworm (Fig. 78), but constitute an immense fleshy mass,
filling up the greater part of the body-cavity, and leaving a
very small space around the enteric canal.

In the limbs the essential arrangement of the muscles in
relation with the joints in Arthropods is more easily seen
(Fig. 85): each podomere is acted upon by two muscles
situated in the next proximal podomere. These muscles are
inserted, by chitinous and often calcified tendons, into the
proximal edge of the segment to be moved, the smaller (ex/)
on the extensor, the larger (77) on the flexor side, in each
case half-way between the two hinges, so that a line joining
the two muscular insertions is at right angles to the axis of
articulation.

The digestive organs are constructed on the same general
plan as those of the earthworm, but present many striking
differences. The mouth lies in the middle ventral line of
the head and is bounded in front by a shield-shaped process,
the /abrum, at the sides by the mandibles, and behind by a
pair of delicate lobes, the pavagnatha. It leads by a short
wide gu/let (Fig. 86, @) into a capacious gizzard, usually

Iv ENTERIC

spoken of as the stomach,
which occupies a great part of
the interior of the head, and
is divided into a large ante-
rior division (cs), and a smaller
posterior division (fs): the
latter passes into the ¢ntestine,
which consists of a narrow
and very short md-gut (md)
from which a somewhat wider
hind-gut (hd) extends to the
anus (an), situated on the
ventral surface of the telson.
The outer layer of the en-
teric canal consists of connec-
tive tissue containing striped
muscular fibres: within this is
a single layer of columnar
epithelial cells, none of them
glandular. In the gullet and
gizzard and in the hind-gut
the epithelium secretes a layer
of chitin, which thus consti-
tutes the innermost layer of
those cavities. It is proved
by development that the mid-
gut, which has no chitinous
lining, is the only part of the
enteric canal developed from

Fic. 85.—A leg of the Fresh-water Cray-
fish with part of the exoskeleton
removed to show the muscles.

CANAL 357

en. 2—en. 5, segments of endopodite ; /. hinges; avt.7z. articular membrane pert,
extensor muscle ; 7. flexor muscle. (From Parker and_Haswell’s Zoology.)

A A 2*

358 THE CRAYFISH CHAP.

the enteron of the embryo : the
gullet and gizzard (fore-gut)
arise from the stomodeum,
the hind-gut from the proc-
todeum (p. 204). Thus only
a very small portion of the
enteric epithelium is endo-
dermal.

In the anterior division of
the gizzard the chitinous
lining is thickened and _cal-
cified in certain parts, so as
to form a complex articulated
framework, the “ gastric mill,”
on which are borne a median
and two lateral ¢ee‘, strongly
calcified and projecting into
the cavity of the gizzard.
Two pairs of strong muscles
arise from the carapace, and
are inserted into the gizzard
(Fig. 86): when they contract
Fic. 86.—Dissection of Fresh-water Cray-

fish made by removing the exo-

skeleton with the appendages, and the
muscles, digestive gland and excretory
organ, of the right side.

aa. antennary artery ; a. abdomen ; az.
anus ; 4.d. aperture of right digestive

duct exposed by removal of gland ; 4/.

4, cheliped ; 4. ventral nerve cord.

cs. anterior division of gizzard; cth.

cephalothorax ; ¢7. dorsal muscles ;
fm. ventral muscles; g. brain; /.
heart ; 4d. hind-gut ; @. left digestive
gland ; md. mid-gut; 0. right lateral
ostium of heart ; oa. ophthalmic artery
oaa. dorsal abdominal artery ; @. gullet ; A/. 1—5, pleopods ; AZ. 6, uropod ; As.
posterior division of gizzard ; s.a. sternal artery; ¢. (near heart), spermary ; ¢.

(below anus), telson ; aa. ventral abdominal artery; v. d. spermiduct ; vdo.
male genital aperture. (From Lang, after Huxley.)

VI DIGESTION 359

they move the mill in such a way that the three teeth meet
in the middle line and complete the comminution of the food
begun by the jaws. The separation of the teeth is effected
partly by the elasticity of the mill, partly by delicate muscles
in the walls of the gizzard. The posterior division of the
gizzard forms a strainer: its walls are thickened and pro-
duced into numerous sete, which extend quite across the
narrow lumen and prevent the passage of any but finely
divided particles into the intestine. Thus the gizzard
has no digestive function, but is merely a masticating
and straining apparatus—in fact it is not a stomach at
all in either the embryological or in the physiological sense.
On each side of the anterior division is found, at certain
seasons of the year, a plano-convex mass of calcareous
matter, the gastrolith or ‘crab’s-eye,” which apparently
merely serves to store up reserve calcareous material for
use after the next ecdysis.

The digestion of the food, and to some extent the absorp-
tion of the digested products, are performed by a pair of
large glands (Fig. 86, /r), lying one on each side of the
gizzard and anterior end of the intestine. They are formed
of finger-like sacs or ceca, which discharge into wide ducts
opening into the mid-gut, and are lined with glandular
epithelium derived from the endoderm of the embryo.
The glands are often spoken of as the liver, but as the
yellow fluid they secrete digests proteids as well as fat, the
name hepato-pancreas is often applied to them, or they may
be called simply digestive glands. The crayfish is car-
nivorous, its food consisting largely of decaying animal
matter.

The digestive organs and other viscera are surrounded by
an irregular cavity, which is in free communication with the
blood-vessels and itself contains blood. This cavity is not

360 THE CRAYFISH

CHAP.

lined by epithelium, and is to be looked upon as a large

blood-sinus, and not as a true ccelome.

There are well-developed respiratory organs in the form

Fic. 87.—Transverse section of thorax of Crayfish,
diagrammatic.
abm. ventral abdominal muscles; 2/4 leg; dz.
ventral nerve cord ; d@. intestine ; dé. dorsal
abdominal muscles; ef. wall of thorax; /.
heart; &. gills; Ad. gill-cover; 2 digestive
glands; ov. ovary; fc. pericardial sinus ; sa,
sa. sternal artery; vs. ventral blood-sinus.
The arrow shows the direction of the blood-
current. (From Lang's Comparative Ana-
tony.)

of gills (Figs. 84, g,
and 87, £), contained
in a narrow branchial
chamber, bounded
internally by the
proper wall of the
thorax, externally by
the gillcover or
pleural region of the
carapace. Each gill
consists of a stem
giving off numerous
branchial filaments,
so that the whole
organ is plume-like.
The filaments are
hollow and commu-
nicate with two
parallel canals in the
stem — an external,
the afferent branchial
vet, and an internal,
the efferent branchial
vein (Fig. 89). The
gill isto beconsidered
as an out-pushing of

the body-wall specially modified for respiration (compare p.
204), and it contains the same layers—a thin layer of chitin
externally, then a single layer of epithelial cells, and beneath
this connective tissue, hollowed out for the blood channels.

VI RESPIRATORY AND EXCRETORY ORGANS 361

According to their point of origin the gills (Figs. 84 and 87),
are divisible into three sets—first, podobranchs, or foot-gills
springing from the epipodites of the thoracic appendages,
from which they are only partially separable; secondly,
arthrobranchs, or joint-gills, springing from the articular
membranes connecting the thoracic appendages with the
trunk ; and thirdly, slewrobranchs, or wall-gills, springing from
the lateral walls of the thorax, above the attachment of the
appendages. ‘The total number of gills is eighteen, besides
two filaments representing vestigial (p. 159) or vanishing
gills, which are represented by functional GrEans in some
allied forms.

The water in the branchial chamber is constantly renewed
in the living Crayfish by the action of the plate attached
to the second maxilla (p. 352), the movement to and fro of
which bales out the water in front, and consequently causes
fresh water to flow in behind. Thus a fresh supply of water,
containing air in solution, is continually being passed over
the gills. The fact that the podobranchs are attached to the
bases of the limbs must also result in bringing their surfaces
more easily in contact with the water when the animal uses
its legs in walking.

The excretory organs differ both in position and in form
from those of the earthworm. At the base of each antenna
is an organ of a greenish colour, the amtennary or green
gland (Fig. 88), by which the function of renal excretion
is performed. The gland is cushion-shaped, and contains
canals and irregular spaces lined by glandular epithelium :
it discharges its secretion into a thin-walled sac or urinary
bladder (bl), which opens by a duct on the proximal seg-
ment of the antenna. The green glands are to be looked
upon as corresponding to peculiarly modified nephridia.

The circulatory organs are in a high state of development

362 THE CRAYFISH CHAP,

The heart (Fig. 86, 2) is situated in the dorsal region of
the thorax, and is a roughly polygonal, muscular organ
pierced by three pairs of apertures or ostia (0), guarded
by valves which open inwards. It is enclosed in a spacious
pericardial sinus (Fig. 87, pc) which contains blood. From
the heart~- spring a number of narrow arteries (compare
Pp. 27), which serve to convey the blood to various parts of
the body. At the origin of each artery from the heart are
valves which allow of
the flow of blood in
one direction only,
viz., from the heart
to the artery. From
the anterior end of
the heart arise five
vessels—the median
opthalmic artery (Fig.
86, oa), which passes

Hee Ler aa ae of excretory organ of Crayfish. forwards to the eyes ;
2. bladder ; c.g. outer or cortical green portion ; 7 -
d. duct; 5. yellowish sac-like portion ; 2. p paired antennary ae

white tubular portion. (From Parker and / }
Haswell, after Marchal.) tertes (2a), going to

the antennules, an-
tenne, green glands, &c., and sending off branches to the
gizzard ; and paired hepatic arteries, going to the digestive
glands. The posterior end of the heart gives off two unpaired
arteries practically united at their origin—the dorsal abdominal
artery (0 aa), which passes backwards above the intestine,
sending branches to it and to the dorsal muscles; and the
large sternal artery (Figs. 86 and 87, sa), which extends di-
rectly downwards, indifferently to right or left of the intestine,
passing between the connectives uniting the third and fourth
thoracic ganglia, and then turns forwards and runs in the
sternal canal, immediately beneath the nerve-cord, sending

vI CIRCULATORY ORGANS 363

off branches to the legs, jaws, &c. At the point where the
sternal artery turns forward it gives off the median ventral
abdominal artery (Fig. 86, uaa), which passes backwards
beneath the nerve-cord, and supplies the ventral muscles,
pleopods, &c.

All these arteries branch extensively in the various organs
they supply, becoming divided into smaller and smaller off-
shoots, which finally end in microscopic capillaries (p. 95).
These latter end by open mouths, which communicate with
the lood-sinuses—spacious cavities lying among the muscles
and viscera, and all communicating, sooner or later, with the
sternal sinus, a great median canal running longitudinally
along the thorax and abdomen, and containing the ventral
nerve-cord and the sternal and ventral abdominal arteries.
In the thorax the sternal sinus (Fig. 87, vs, and Fig. 89, sé.s),
sends an offshoot to each gill in the form of a well-defined
vessel, which passes up the outer side of the gill, and is called
the afferent branchial vein (af. br. v). Spaces in the gill-
filaments place the afferent in communication with the
efferent branchial vein (ef. br.v), which occupies the inner
side of the gill-stem. The eighteen efferent branchial veins
open into six branchiocardiac veins (br. c. v) which pass
dorsally in close contact with the lateral wall of the thorax
and open into the pericardial sinus.

The whole of this system of cavities is full of blood, and
the heart is rhythmically contractile. When it contracts the
blood contained in it is prevented from entering the peri-
cardial sinus by the closure of the valves of the ostia, and
therefore takes the only other course open to it, wz., into
the arteries. When the heart relaxes the blood in the
arteries is prevented from regurgitating by the valves at
their origins, and the pressure of blood in the pericardial
sinus forces open the valves of the ostia and so fills the

364 THE CRAYFISH CHAP.

heart. Thus in virtue of the successive contractions of the
heart and of the disposition of the valves, the blood is kept
constantly moving in one direction—from the heart by
the arteries to the various organs of the body, where it
receives carbon dioxide and other waste matters ; thence by
sinuses into the great sternal sinus ; from the sternal sinus
by afferent branchial veins to the gills, where it exchanges

s ‘UsT sts

Fic. 89.—Diagram illustrating the course of the circulation of the blood in the
Crayfish. Heart and arteries red; veins and sinuses containing non-aérated
blood blue ; veins and sinuses containing aérated blood pink. The arrows show
the direction of the flow.

The blood from the pericardial sinus (fcd. s) enters the heart (A¢) by a valvular
aperture (vl.) and is propelled into arteries (a), the orifices of which are guarded
by valves (v?.); the ultimate branches of the arteries discharge the blood into
sinuses (s), and the sinuses in various parts of the body debouch into the sternal
sinus (sé. s); thence the blood is taken by the afferent branchial veins
(af br. v) into the gills, where it is purified and is returned by efferent branchial
veins (ef 7. v) into the branchiocardiac veins (47. c. v) which open into the
pericardial sinus. (From Parker and Haswell’s Zoology.)

carbon dioxide for oxygen ; from the gills by efferent branchial
veins to the branchiocardiac veins, thence into the peri-
cardial sinus, and so to the heart once more.

It will be seen that the circulatory system of the crayfish,
like that of the frog, consists of three sections—(r) the Zeart,
or organ of propulsion ; (2) a system of out-going channels,
the arteries, which carry the blood from the heart to the

vi NERVOUS' SYSTEM 365

body generally ; and (3) a system of returning channels—
some of them, the s/wzses, mere irregular cavities, others,
the zervs, with definite walls—these return it from the
various organs back to the heart. The respiratory organs, it
should be observed, are interposed in the returning current, .
so that blood is taken both to and from the gills by veins.

Comparing the blood-vessels of the Crayfish with those of
the earthworm, it would seem that the ophthalmic artery,
heart, and dorsal abdominal artery together answer to the
dorsal vessel, part of which has become enlarged and muscu-
lar, and discharges the whole function of propelling the
blood. The horizontal portion of the sternal artery, together
with the ventral abdominal, represent the ventral vessel, while:
the vertical portion of the sternal artery is a commissure,
developed sometimes on the right, sometimes on the left side,
its fellow being suppressed.

The blood when first drawn is colourless, but after: ex-
posure to the air takes on a bluish-grey tint. This is owing
to the presence of a colouring matter called hemocyanin,
which becomes blue when combined with oxygen; it is a
respiratory pigment, and serves, like hemoglobin (pp. 107 and
330), as a carrier of oxygen from the external medium to the
tissues. The hemocyanin is contained in the plasma of the
blodd : the corpuscles are all leucocytes (pp. 105 and 328).

The nervous system consists, like that of the earthworm,
of a brain (Fig. 86, g) and a ventral nerve-cord (4), united
by cesophageal connectives. But the ganglia of the ventral
nerve-cord are more distinct, and to them the nerve-cells are
confined, the longitudinal connectives between them consist-
ing of nerve-fibres only. The brain supplies not only the
eyes and antennules, but the antennz as well, and it is found
by development that the two pairs of ganglia belonging to
the antennulary and antennary segments have fused with the

366 THE CRAYFISH CHAP,

brain proper. Hence we have to distinguish between a
primary brain or avchi-cerebrum—the ganglion of the prostom-
jum, and a secondary brain or syz-cerebrum formed by the
union of one or more pairs of ganglia of the ventral cord
with the archi-cerebrum. A further case of concrescence of
ganglia is seen in the ventral nerve-cord, where the ganglia
of the last three cephalic and first three thoracic segments
have united to form a large compound swb-esophageal
ganglion. All the remaining segments have their own
ganglia, with the exception of the telson, which is supplied
from the ganglion of the preceding segment. There is a
visceral system of nerves supplying the gizzard, originating
in part from the brain and in part from the cesophageal
connectives.

The eyes have a very complex structure. The chitinous
cuticle covering the distal end of the eye-stalk is transparent,
is divided by delicate lines into areas or facets which are
mostly square, and constitutes the cormea. Beneath each
facet of the cornea is an apparatus called an ommatideum,
consisting of an outer segment or vitreous body having a
refractive function, and an inner segment or vefinula, en-
closing a striated body, the ~zabdome, and forming the actual
visual portion of the apparatus. The ommatidea are optic-
ally separated from one another by black pigment, so that
each is a distinct organ of sight, with a very limited visual
area, and the entire eye is called a compound eye. The optic
nerve (0f.7) dilates to form an optic ganglion (of. gz) in
close connection with the inner ends of the ommatidea ; the
latter are thus turned towards the light, and are not, like
the rods and cones of the vertebrate eye (p. 185), covered
by a layer of nerve-fibres, &c., through which the light must
first penetrate. ;

Each antennule bears two sensory organs, to which are

VI SENSORY ORGANS 367

usually assigned the functions of smell and hearing respec-
tively. The olfactory organ is constituted by a number of
extremely delicate olfactory sete, borne on the external
flagellum, and supplied by branches of the antennulary
nerve. The auditory organ is a sac formed by invagination
of the dorsal surface of the proximal segment, and is in

Fic. 90.—A, Longitudinal section of eyestalk ; B, a single ommatideum ; a, vitreous
body; 4, retinula; cz, cornea, continuous with cxf, cuticle of eyestalk ;
m, muscles ; 072, ommatidea ; of. g7, optic ganglion; #f., optic nerve. (After
Howes.)
free communication with the surrounding water by a small
aperture. The chitinous lining of the sac is produced into
delicate feathered auditory sete, supplied by branches of
the antennulary nerve, and in the water which fills the sac
are minute sand grains, which take the place of the otoliths
(p. 188) found in most auditory organs, but which, instead
of being formed by the animal itself, are taken in after each
ecdysis when the lining of the sac is shed. Many of the

368 THE CRAYFISH CHAP.

setee on the body generally have a definite nerve-supply,
and are probably facti/e organs.

The crayfish is dicecious (p. 312), and presents a very
obvious sexual dimorphism or structural difference between
male and female, apart from the actual organs of repro-
duction. The abdomen of the female is much broader
than that of the male: the first and second pleopods of
the male are modified into tubular or rather spout-like
organs (p. 350); and the reproductive aperture is situ-
ated in the male on the proximal podomere of the fifth
leg, in the female on that of the third. *°

The spermary (Fig. 86, ¢) lies in the thorax, just beneath
the floor of the pericardial sinus, and consists of paired
anterior lobes and an unpaired posterior lobe. From each
side goes off a convoluted spermiduct or vas deferens (vd),
which opens on the proximal segment of the last leg (vdo).
The sperms are curious, rounded, zon-motile bodies produced
into a number of stiff processes: they are aggregated into
vermicelli-like spermatophores by a secretion of the vas
deferens.

The ovary is also a three-lobed body, and is similarly situ-
ated to the spermary : from each side proceeds a thin-walled
oviduct, which passes’ downwards, without convolutions, to
open on the proximal segment of the third or antepenulti-
mate leg. The eggs are of considerable size and contain a
large amount of yolk.

Both ovary and spermary are hollow organs, discharging
their products internally. Their cavities, lined by germinal
epithelium (p. 336), represent the ccelome, and their ducts
are organs of the same general nature as nephridia (p. 331).
The ova, when laid, are fastened to the sete on the
pleopods of the female by the sticky secretion of glands
occurring both on those appendages and on the segments

VI DEVELOPMENT 360

themselves: they are fertilized immediately after being laid,
the male depositing spermatophores on the ventral surface of
the female’s body just before oviposition.

The process of segmentation of the eosperm presents
certain striking peculiarities. The nucleus divides repeatedly
(lig. 91 A, vv), but no corresponding division of the pro-
toplasm takes place, with the result that the polyplast-stage
(p. 200), instead of being a heap of cells, is simply a multi-
nucleate body (compare p. 269). Soon the nuclei thus
formed retreat from the centre of the embryo, and arrange
themselves in a single layer close to the surface (B): around

Fic. 91.—Two stages in the early development of the Crayfish.

In A the products of division of the nucleus (21) are seen in the centre of the
yolk; in B the nuclei have become arranged in a peripheral layer, each sur-
rounded by protoplasm, so as to form the blastoderm. (From Parker and
Haswell’s. Zoology, after Morin.)

each of these protoplasm accumulates, the central part of
the embryo consisting entirely of yolk-material. We thus
get a superficial segmentation, characterised by a central mass
of yolk and a superficial layer of cells collectively known as
the blastoderm. From this the ectoderm and endoderm are
derived, the latter enclosing a relatively small enteron com-
municating with the exterior through a blastopore (p. 201).

Very soon the embryo becomes triploblastic, or three-
layered, by the budding off of cells from the endoderm
in the neighbourhood of the blastopore: these accumulate
between the ectoderm and endoderm, and constitute the

mesoderm.
Pract. Zoot. BB

370 THE CRAYFISH CHAD.

Before long the blastopore closes, and a stomodzeum and
proctodeum (p. 204) are formed as invaginations of the
‘ectoderm, which eventually communicate with the enteron,
forming a complete enteric canal (p. 357). On each side of
the mouth or aperture of the stomodeal depression (Fig. 92)
three elevations appear, the rudiments of the anten-
nules (a1), antennz (a?), and mandibles (m): in front of
them is another pair of elevations on which the eyes (4)
subsequently appear. An unpaired elevation (7-4) behind
the mouth, having the anus or aperture of the procto-
deeal depression at its summit (4), is the rudiment of
the thorax and abdomen. The embryo is now in the
nauphius stage.

Many allied forms are hatched in the form of a free-swimming larva
(compare pp. 11 and 340), to which the name sazpl7us is applied,
characterised by the presence of three pairs of appendages used for
swimming, and_becoming the antennules, antennz, and mandibles of
the adult. In the crayfish there is no free larva, and the nauplius
stage is passed through before hatching.

The embryo is gradually transformed into the crayfish
by the appearance of fresh appendages, in regular order,
behind the first three ; by the elongation of the rudiment of
thorax and abdomen ; and by the gradual differentiation of
the appendages. When hatched the young animal agrees in
all essential respects with the adult, but its proportions are
very different, the cephalothorax being nearly globular and the
abdomen small. For some time after hatching the young
crayfishes cling in great numbers to the pleopods of the
mother by means of the peculiarly hooked chelz of the first
pair of legs.

All the members of the phylum Arthropoda (p. 346) are
characterised by each typical segment of the body beating a

Vi CLASSIFICATION 371

Pe)

pair of appendages divisible into podomeres ; in addition
to this, there is an almost universal absence of cilia in the
phylum, the sperms are usually non-motile, the muscles are
nearly always of the striped kind, and the body-cavity, which

Fic. 92.—Early embryo of the Crayfish in the nauplius stage.

A in the upper part of the figure is the eye ; 7. the labrum overhanging the mouth,
on each side of which are the rudiments of the antennules (al.), antennz (a2.),
and mandibles (#z.); behind them is the rudiment of the thorax and abdomen
(7A) with the anus (A). The rudiments of the first three pairs of ganglia
(G. ga”, gmt,) are seen through the transparent ectoderm. (From Lang, after
Reichenbach.)

does not represent a true ccelome, is largely represented by
blood-spaces in free communication with the circulatory
system.

The phylum is divided into several classes, which are all
air-breathing except the Crustacea, the class to which the
Crayfish belongs and which also includes the Lobsters,

Crabs, Shrimps, Wood-lice, Barnacles, Water-fleas, &c.
BB 2

372 THE CRAYFISH CHAB,

PRACTICAL DIRECTIONS.
CRAYFISH

In a living specimen note the cephalethorax, abdomen, jointed
appendages, and exoskeleton, as well as the mode of walking and
swimming. Holding the animal out of water between the thumb and
finger, observe that bubbles are continually being formed on either side
of the lower part of the head (respiratory movements, see § B, 1). Kill
with chloroform (p. 31), and at the end of the day’s work, preserve your
dissection in 3 per cent. formaline or 70 per cent. spirit.

A. External Characters.

1. Note again the cephalothorax and abdomen, and that the abdomen
consists of seven movable segments or wzefameres. Examine the ventral
side of the cephalothorax, and note that it also is composed of a number of
segments all fused together, except the ventral part of the last.

2. Examine the third or fourth abdominal segment closely, and note
that it is connected with the segments in front and behind by a sort of
peg- and socket-joint on either side, and that the chitinous exoskeleton
at the joints is soft and pliable, forming an articular membrane, while
elsewhere it is calcified. Distinguish between the dorsal convex
tergum, the ventral sternum, and the pleuron projecting ¢ sownwards on
either side from the tergum. ~* zt

3. Examine the appendages of the same segment : _tHey are attached to
the sterna, near the pleura, by articular membranes. Each consists of a
basal or proximal portion—the protopodite, to which two distal, many«
jointed parts are attached—an inner endopodite, and an outer exopodite.
The cuticle covering the segments of the limb, or podomeres, is more or
less calcified, and the distal segments are covered with feathery se¢e.

4. The second to the sixth abdominal segments are essentially similar
to one another, except as regards the appendages of the second in the
male and of the sixth in both sexes (see p. 350). The first abdominal
segment is smaller than the others, and its pleura are reduced. The
sixth abdominal appendages are very large, and, together with the aza/
segment or telson, form the ¢azl-fir.

5. The cephalothorax is reckoned as consisting of a prostomdum and
12 metameres, which are completely fused together dorsally and laterally,
forming a large calcified shield—the carapace. Thus the entire number
of segments is 20 (prostomium + 18 metameres + telson). The
sternal region of the head is bent upwards.

6. Note :—a, the transverse cervical groove on the dorsal surface of the

VI PRACTICAL DIRECTIONS 373

carapace, extending forwards laterally and forming the boundary between
the head (prostomium + 4 metameres) and the shorax (8 metameres) ;
6, the two longitudinal dranchzocardiac grooves on the tergal region
of the thoracic portion of the carapace, about finch apart: the part of
the exoskeleton between these cavers the heart, and the part below each
groove forms a large plate, the g¢//-cover, at the side of the thorax ;
¢, the rastrum, movable eye-stalks, and epistoma (p. 349).

7. Note the following apertures: Afedian—a, the mouth, on the
ventral surface of the head, between the jaws; 6, the anus, on the
ventral side of the telson: Pacred—c, the auditory aperture, on the
dorsal side of the basal joint of the smaller feeler or antennule (this
will be seen better later on); @, the vesal aperture, on a conical yentral
elevation of the basal joint of the larger feeler (antenna) ; e, the gezztal
aperture, in the male on the basal joint of the last thoracic leg, and in
the female on the last thoracic leg but two.

B. Respiratory Organs.

1. Carefully cut away the left gill-cover with scissors, and fix the
animal under water on its right side, so as to expose the left gdd/-
chamber containing the feathery-looking gz//s. The inner wall of the
chamber is formed by the proper wall of the thorax, and the chamber is
open behind and below. In front of the gills is a groove, in which a
flattened plate (see p. 353) works backwards and forwards during life,
driving the water out in front, and causing the bubbles already noticed,

2. The g#s are 18 in number, and each has the form of a bottle-
brush. The six podobranchs are external to the arthrobranchs and
pleurobranchs (p. 361), and each is attached to a large folded and
corrugated epzpodzte (p. 352). The gills are related to definite metameres,
as will be seen from the following table, in which ef stands for epipodite,
and v for the vestige of a gill. Note that the first pair of thoracic
limbs bears.a simple large epipodite only.

SRORREIS ae | Bly 1 ae | ve | | OWE | ee | Aroma

Podobranchs...| o+ef rep | rtep|t+ep|1+ep} +e 1+4¢p ° 6+7¢ep
Arthrobranchs | o t 2 2 2 2 2 ° It

Pleurobranchs | o ° ° ° ° v v I 1+2u

ToTAL.....| 6p | 26h | 3-+ep|3tep| ates |atotes|atotep) 2 ||s8+20+7ep

374 THE CRAYFISH CHAP.

3. Turn down the podobranchs and make out the ‘relations of the
arthrobranchs from the above table. Then turn these down, or cut
them off, and note the single complete pleurobranch and the twa
vestigial ones. Cut off an arthrobranch and examine its structure, noting
the afferent and efferent blood-vessels in its stem. Sketch.

4. Note the dranchiocardiac veins on the inner side of the thoracic
wall. Blow air or inject French blue into the cut bases of the gills
removed, and note that the branchiocardiac trunks extend upwards to
the pericardial-sinus (see below) from the gills.

C. General Dissection.

Holding the animal in your left hand, insert a scalpel carefully
beneath the hinder edge of the carapace on the dorsal side, so as ta
separate the exoskeleton from the soft integument, and then with the
large scissors cut along the outer side of each branchiocardiac groove,
and remove the median portion of the carapace. Note the pigmented
integument and then remove it, when some of the nearly colourless blood
will ooze out.

I. 1, Examine a drop of blood under the microscope, adding salt
solution. Note the amceboid nucleated corpuscles. Sketch.

2. The gericardial sinus will now be exposed, containing the heart
with three pairs of valvular os¢za (only the dorsal ostia can be seen at
present), through which the blood enters the heart fram the’pericardial
sinus.

Inject some French blue (see p. 99) into the heart through one of
the ostia, so as to fill the arteries (tying is unnecessary). Then remove
the dorsal part of the exoskeleton and integument bit by bit, all along
the thorax and abdomen, as well as the pair of longitudinal extensor
muscles lying just beneath the dorsal integument of the abdomen. Pin
down under water, dorsal surface uppermost, and note :—

3. The absence of a continuous muscular layer in the body-wall and
of a true coelome, and the presence of irregular spaces (dlood s¢nuses)
between the viscera and muscles.

4. The delicate arteries, arising from the anterior and posterior
ends of the heart :—a, the anterior median ophthalmdc artery, running
forwards to the eye-stalks; 4, the paired antennary artery, on either
side of a, and passing forwards and downwards to.supply the gizzard,
renal organ, feelers, &c. ; c, the hepatic artery (also paired), rather
further back and more ventral, extending into and supplying the diges-
tive gland ; @, the median dorsal abdominal artery, arising from the

VI PRACTICAL DIRECTIONS 375

posterior end of the heart, and running along the dorsal side of the
intestine, giving off branches in each metamere ; ¢, the sternal artery,
arising just beneath a and passing directly ventralwards to one side of
the intestine (this will be seen better later on: compare Figs, 86 and
87): it perforates the ventral nerve-chain, supplies the segments and
appendages of the thorax, and gives off a ventral abdominal artery,
supplying the segments and appendages o. the abdomen (this artery can
be seen in injected specimens through the transparent cuticle).

5. Note the position of the following parts before dissecting further :—
a, the gészara,, a large sac in the head, with two pairs of muscles passing
to the integument (now cut through); 4, the adductor muscles of the
mandible, just external to a; ¢, the paired, brownish or greenish dges-
tive gland on either side of, and extending further back than the
gizzard; above it are d@, the gonuds, on either side of and behind the
pericardial sinus. In the male, the ssermary is small and whitish, and
each spermzduct is a coiled, densely white tube ; in the female, the ovary
‘3 a larger, brownish organ, containing prominent ova. (In both sexes,
the paired character of the gonads is partly lost by fusion: a pair of
anterior lohes and a single posterior lobe can be seen in each.) Sketch,

By slightly raising the surrounding parts the gonaducts can be seen to
pass ventralwards to their external apertures (p. 373), the oviducts
being thin-walled and straight, White masses, the sermatophores
(p. 368), will very likely be found stuck on to the sternal region of the
body,

6. Tease up a small portion of the spermary or of a spe:matophore ;
stain, and mount in glycerine, Examine under the microscope and note
the rounded and flattened sperms each with « number of stiff, curved
processes coming off from the periphery. The sperms are 707-motzle.
Sketch,

Remove the heart and reproductive organs carefully, noting the
sternal artery (see above) as you do so, and taking especia] care not to
injure the surrounding parts. Examine the heart under water, and
note the six ostia.

Il. - Zhe enteric canal, Note—t. The oval mouth, bounded by the
Jabrum in front, leading into a short and wide gullet (this will be seen
later on), which dilates to form the large gzzzard (Fig. 86), filling up
a considerable portion of the head and extending into the thorax: a
transverse constriction divides it into an anterior and a ‘posterior
portion 3 both gullet and gizzard are lined by chitin. The chitinous.

376 THE CRAYFISH CHAP.

cuticle of the gizzard ds calcified in places to form the sclerifes or
so-called ‘‘ ossicles” of the gastric mill. Note the two median
sclerites bounding the transverse constriction in front and: behind
respectively. To them the anterior and posterior pairs of muscles
(p. 375, § 5) are attached,

« 2, Following on the gizzard is the short, thin-walled mzd-guz, on the
dorsal side of which is a small c@eum. It has no chitinous lining, and
the large duct of the digestive gland opens into it on either side. The
digestive gland is made up of three main lobes on either side and con-
sists of.a number of small blind tubes.

3. The Aénd-gut, which runs straight to the anus. Its inner surface
is raised into longitudinal ridges which take a slightly spiral course,
and it is lined by a thin chitinous cuticle.

4. Carefully press the gizzard backwards and note a, the drazz, just
behind the bases-of the small feelers ; 4, the gz//et; and c, a pair of
white nerve cords (conmectéves) coming off from the brain and em-
bracing the gullet. Taking care not to injure these parts of the
nervous system, cut through the gullet, just above the connectives, and
then gently remove the whole enterie canal together with the digestive
gland from the body, cutting through the intestine just in front of the
anus. Examine the whole digestive system under water.

5. Note again the mid-gut and the digestive glands and ducts; then
remove the digestive gland of one side, and sketch the enteric canal
from the same side. Slit up the hind-gut so as to see the ridges and
cuticle.

6. Clean the walls of the gizzard and note the other sclerites of the

gastric mill.
- Articulated to each end respectively of the two median sclerites
already referred to, in each lateral wall of the gizzard, is a lateral
sclerite, the two articulating with one another at their other ends, so
that these six sclerites together form a sort of hexagonal frame. Two
other median sclerites, arising from the median ones above-mentioned
respectively, extend downwards into the constriction between the two
portions of the gizzard, and these join below at anangle, where they
bear a median tooth. Each -of the posterior lateral sclerites bears a
lateral tooth.

Cut open the anterior end of the gizzard, and note the strongly calci-
fied, brownish, medan tooth, and the two large Jateral teeth. Seize
hold of the two median assicles with two pairs of forceps, one in each

A PRACTICAL DIRECTIONS 377

hand, and pull gently backwards and forwards (in the direction in
which the muscles pull). It will then be seen that the median and
Jateral teeth come together in the middle line so as to act as a gastric
mill, Note the slit-like lumen of the part of the gizzard behind this
and the arrangement of the seé which act as strainers. Make sketches
as you proceed.

III. 1, The chief muscles of the body are :—a, the paired and seg-
mented dorsal extensor, arising from the side walls of the thorax, and
extending into the abdomen above the intestine, giving off slips to each
segment of the abdomen (this has already been removed); and 4, the
large and complex ventral muscles, the lateral halves of which are not
separate from one another, the fibres being interwoven, somewhat like
those of a rope ; slips are given off tothe abdominal sterna. These act
mainly as a flexor of the abdomen.

2. Muscles pass from the body to the proximal joints of the limb,
and between successive podomeres :—these latter will be examined at
a later stage (§ D).

3. Note again the paired adductor of the mandible (p. 375), and
trace its calcified tendon downwards to its -insertion on to the
mandible.

4. Tease out a small piece of muscle so as to separate its fibres from
one another. Stain, and mount in glycerine. Note the ‘ransverse
striations, sarcolemma, and nuclet (compare Fig. 32). Sketch.

Remove the muscles of the body described above, noting the sternal
artery (p. 375), and taking especial care to leave the abdominal nerve:
cord 27 szte when removing the large ventral muscles. Note that
in the thorax, the nerve-cord passes into a sternal canal, formed by a
series of ingrowths of the exoskeleton—the endophragmal system—from
which the muscles passing to the thoracic limbs arise. Insert the
scissors into the sternal canal, and cut away and remove its roof, bit by
bit. The whole of the central nervous system will then be exposed,

IV. Observe that a more marked distinction into gangla- and
connectives is seen than in the case of the Earthworm, and that the
fusion of the two’ lateral halves of the cord or chain has only affected
the ganglia, the connectives being double all the way along..

1. Note:—a, The drazn, or fised, compound supra-esophageal ganglia;
b, the esophageal connectives, and c, the postoral ventral nerve-cord,
consisting of a large compound szd-asophageal ganglion and of 12
segmental ganglia, united by paired connectives. Beneath the cord, the

378 THE CRAYFISH CHAP.

sternal and ventral abdominal arteries (p. 375) will be seen, the sternal
artery passing between the connectives joining the fifth and sixth
postoral ganglia.

2. The brain gives off nerves to the eyes and the two pairs of
feelers: the subcsophageal ganglion supplies the mandibles and
four following pairs of appendages with their segments. Each of
the other ganglia supplies one segment (with its appendages) only,
except in the case of the last or 6th abdominal ganglion, which
gives off nerves to the telson. (A small avtertor visceral nerve,
arising from the brain and cesophageal connectives and supplying the
fore-gut, will have been removed, and a small posterior visceral
nerve, supplying the hind-gut, arises from the last abdominal
ganglion.) Sketch.

V. The antennary or green-glands are situated just behind the bases
of the large feelers. Blow through the renal aperture of one side (p. 373)
and note the duct and wrzénary bladder on the dorsal side of the gland.
Then cut through the duct, remove the whole gland, and examine under
water. Sketch.

D. The Appendages.—Remove the appendages of one side,
beginning with the last, one by one, cutting through the articular
membrane with a scalpel, and then taking hold of the basal joint with
the forceps and pulling the appendage away. Work through the de-
scription on pp. 350-353 and sketch typical appendages from each region.

Note the delicate Jaragnatha behind the mouth and the /abrum
in front of it (p. 356).

The arrangement of the joints and muscles of the limbs can be well
seen by examining the large first leg or cheliped. Note that each joint
works in a different plane, and then cut away the exoskeleton
from one side of the two or three distal podomeres, so as to
expose the muscles (compare Fig. 85). Then remove these, and note
their chitinous tendons. Observe that the flexor muscle and its tendon,
which moves the pincers, is much-larger than ‘the extensor muscle. -

E. Sensory Organs.

1. Tactile organs. Snip off some setee from the body or appendages.
Examine under the microscope. Sketch.

2. “ Olfactory” organs. Examine the outer flagellum of the
antennule under the low power, and note the tufts of spatula- -like
olfactory sete on the ventral surface. Sketch.

3. Auditory organ. Carefully cut away the convex ventral side of

VI PRACTICAL DIRECTIONS 379

the basal segment of the antennule with scissors, so as to expose the
auditory sac. Cut this out and place it on a slide, carefully removing
the muscles surrounding it, as well as the sete around its aperture.
Note the contained grains of sand (o¢o//ths), and then wash them
away. Stain with magenta and mount in glycerine, flattening the sac
out with a cover-glass. Note that the sac is an involution of the
integument lined by cuticle, and that it contains simple, jointed azdztory
sete of various sizes, arranged in rows, and that branches of the
antennylary nerve run up the stem of each seta. Sketch.

4. The Eyes. Remove one of the eye-stalks, and note the apparently
black, yncalcified, oval portion of the cuticle (cormea) at its distal end.
Strip this off, and nate that it is transparent. Then wash off any
pigment which may have come away with it and mount in water.
Observe the corneal facets. Then cut the eye-stalk into two longitudinal
halves with a knife, and examine with a Jens under water. Note the
optic nerve entering the stalk, and enlarging to form the oftzc ganglion,
from which a number of bodies (ommatzdea) radiate outwards to the
corresponding facets of the cornea. The ommatidea are separated
from one another by pigment. Sketch.

gj. Examine longitudinal sections of the eye-stalks, decalcified
and prepared as directed on p. 136, and note in detail the above
parts. Eacheommatideum lies beneath the corresponding corneal
facet, and is made up of an outer w2treous body or crystalline cone, and
an inner vet¢nzla formed of sensory cells and enclosing a transversely-
striated, spindle-shaped, refractive body or rabdome, and closely con-
nected with the optic ganglion. Note also the pzgment between the
ommatidea. Sketch, “

F. Structure of the Exoskeleton.

In order to follow this out in greater detail, proceed as follows :—

1. Cut through the thorax and abdomen of a crayfish transversely,
and note the relations of the hard and soft parts. (Compare Figs. 83
and 87).

2; Dip a crayfish into hot water, so that the soft parts come away
easily. Open up the cephalothorax from the dorsal side, separate
some of the abdominal segments, and clean thoroughly. Examine
the joints and make out the relations of the exdophragmal system
(p. 377), looking something like a lattice-girder.

3. Examine your preparation of the eye once more under the micro-
scope and notice the part where the section passes through the quter

‘

380 THE CRAYFISH CHAP. VI

wall of the eye-stalk, so as to make out the microscopic structure of the
integument and exoskeleton, which apart from the calcification, is similar
in all parts. Notice the epéderm, and the thick, laminated, chitinous
cuticle, the superficial layer of which.is uncalcified throughout.

G. Dissection from the Side.

Cut through the carapace on the dorsal side as before, but be careful
to keep your cut very slightly to one side—say the left—of the
middle line, and when the pericardium is exposed, inject the arteries
through the heart as before (p. 374). Then remove the gill-cover of the
same side, and examine the gills once more.

Remove and examine again all the appendages of the same side, and
then carefully continue cutting longitudinally through the exoskeleton,
both dorsally and ventrally, close to the middle line, taking care you do
not injure any median organs. Remove the entire exoskeleton of
this side, as well as the dorsal and ventral muscles, cutting through the
latter in the median line very carefully.

After your dissection has been pinned down under water, the green-
gland, digestive gland, and gonad of the same side should also be
removed, and the gizzard cut open. Then tidy up the dissection,
which will now be reduced to a longitudinal section like that represented
in Fig. 86.

Once more carefully follow out the structure and relations of all the
organs exposed, and sketch your dissection,

CHAPTER VII

THE FRESH-WATER MUSSEL.— CHARACTERS OF THE PHYLUM
MOLLUSCA—-ENUMERATION OF THE CHIEF PHYLA OF THE
ANIMAL KINGDOM.

In the mussel we meet with an entirely new type of
structure: the animal, like the worm and crayfish, is bi-
laterally symmetrical, but there is no trace of metameric
segmentation ; the power of locomotion is greatly restricted,
and food is obtained passively by ciliary action, as in Infu-
soria, not by the active movements of definite seizing organs
—tentacles, limbs, or protrusible mouth—as in most of the
Metazoa (p. 286).

Fresh-water mussels are found in rivers and lakes in
most parts of the world. Axodonta cygnea, the swan-mussel,
is the commonest species in England ; but the pearl-mussel,
Unio margaritifera, is found in mountain streams, and other
species of the same genus are universally distriduted.

The mussel is enclosed in a brown shell formed of two
separate halves or valves hinged together along one edge.
It lies on the bottom, partly buried in the mud or sand,
with the valves slightly gaping, and in the narrow cleft thus
formed a delicate, semi-transparent substance is seen, the
edge of the mantle or pallium. The mantle really corisists
of separate halves or odes corresponding with the valves of

382 THE MUSSEL CHAP.

the shell (compare Fig. 95), but in the position of rest the
two lobes are so closely approximated as to appear simply
like a membrane uniting the valves. At one end, however,
the mantle projects between the valves in the form of two
short tubes, one (Fig. 93, ex. sf) smooth-walled, the
other (77. sph) beset with delicate processes or tentacles.
By diffusing particles of carmine or indigo in the water it can
be seen that a current is always passing in at the fringed tube
—hence called the zzhalant siphon, and out at the smooth or
exhalant siphon. Frequently a semi transparent, tongue-like
body (/#) is protruded between the valves at the opposite
side from the hinge and at the end furthest from the siphons:
this is the foot, by means of which the animal is able slowly
to plough its way through the sand or mud. When the
mussel is irritated the foot and siphons are withdrawn
and the valves tightly closed. Ina dead animal, on the
other hand, the shell always gapes, and it can then be seen
that each valve is lined by the corresponding lobe of the
mantle, and that the exhalant siphon is formed by the union
of the lobes above and below it and is thus an actual tube ;
but that the boundary of the inhalant siphon facing the
gape of the shell is simply formed by the approximation
of the mantle-lobes, so that this tube is a temporary one. _

The hinge of the shell is dorsal, the gape ventral, the end
bearing the siphons posterior, the end from which the foot
is protuded anterior: hence the valves and mantle-lobes are
respectively right and left.

In a dead and gaping mussel the general disposition of
the parts of the animal is readily seen. The main part of
the body lies between the dorsal ends of the valves: it is
produced in the middle ventral line into the keel-like foot :
and on each side between the foot and the corresponding
mantle-lobe are two delicate, striated plates, the gid/s or

vi SHELL 383

ctenidia, as they ate often called. Thus the whole animal
has been compared to a book, the back being represented
by the hinge, the covers by the valves, the fly-leaves by the
mantle-lobes, the two first and the two last pages by the
gills, and the remainder of the leaves by the foot. (Fig. 95).

When the body of the mussel 1s removed from the shell
the two valves are seen to be united, along a straight Ainge-
line, by a tough, elastic substance, the Ainge-ligament (Fig.
95, 4g) passing transversely from valve to valve. It is by
the elasticity of this ligament that the shell is opened : it is”
closed, as we shall see, by muscular action: hence the mere
relaxation of the muscles results in opening the shell. In
Anodonta the only junction between the two valves is
afforded by the ligament, but in Unio each is produced into
strong projections and ridges, the Ainge-teeth, separated by
grooves or sockets, and so arranged that the teeth of one
valve fit into the sockets of the other.

The valves are marked externally by a series of concentric
lines parallel with the free edge or gape, and starting from
a swollen knob or elevation, the wmdo, situated towards the
anterior edge of the hinge-line. These lines are Ames of
growth. The shell is thickest at the umbo, which represents
the part first formed, and new layers are deposited under
and concentrically to this original portion as secretions from
the mantle, the shell being, like the armour of the crayfish, a
cuticular exoskeleton. As the animal grows each layer projects
beyond its predecessor, and in this way successive outcrops
are produced, giving rise to the markings in question. In
the region of the umbo the shell is usually more or less
‘eroded by the action of the carbonic acid in the water.

The inner surface of the shell also presents characteristic
markings. Parallel with the gape, and ata short distance
from it, is a delicate streak caused by the insertion into the

384 THE MUSSEL CHAP,

shell of muscular fibres from the edge of the indnitle : the.
streak is hence called the pal/ial line. Beneath the anterior
end of the hinge the pallial line ends in an oval mark, the
anterior adductor impression, into which is inserted one of the
muscles which close the shell. A similar, but larger, posterior
adductor impression lies beneath the posterior end of the
hinge (compare Fig 93, a. ad, ~. ad). Two smaller markings
close behind the anterior adductor impression, dorsal and
ventral respectively, mark the origin of the anterior retractor
and of the protractor muscle of the foot: one just anterior
to the posterior adductor impression, that of the posterior
retractor of the foot. From all these impressions faint con-
verging lines can be traced to the umbo: they mark the
gradual shifting of the muscles during the growth of the
animal.

The shell consists of three layers, the outer layer, as in
the crayftsh, being uncalcified. Outside is a brown horn-
like layer, the fertostracum, composed of conchiolin, a
substance allied in composition to chitin. Beneath this is
a prismatic layer formed of minute prisms of calcium car-
bonate, separated by thin layers of conchiolin ; and, lastly,
forming the internal part of the shell is the macre, or
“mother-of-pearl,” formed of alternate layers of carbonate
of lime and conchiolin arranged parallel to the surface.
The periostracum and the prismatic layer are secreted
from the edge of the mantle only, the pearly layer from the
whole of its outer surface. The hinge-ligament is continu-
ous with the periostracum, and is to be looked upon simply as
a median uncalcified portion of the shell, which is therefore,
in strictness, a single continuous structure.

By the removal of the shell the body of the animal is seen
to be elongated from before backwards, narrow from side to
side, produced on each side into a mantle-lobe, and con-

VII MUSCLES 385

tinued ventrally into a keel-like vésceral mass, which passes
below and in front into the foot (Fig. 93, f#). Thus each
valve of the shellis in contact with the dorso-lateral region
of the body of its own side together with the corresponding
mantle-lobe, and it is from the epithelium covering these
parts that the shell is formed as a cuticular secretion.
The whole space between the two mantle-lobes, contain-
ing the gills, visceral mass, and foot is called the mantle-
cavity.

A single layer of epithelial cells, the deric epithelium or
epiderm, covers the whole external surface—i.e. the body
proper, both surfaces of the mantle, the gills, and foot;
that of the gills and the inner surface of the mantle is cilia-
ted. Beneath the epiderm come connective and muscular
tissue, which occupy nearly the whole of the interior of the
body not taken up by the viscera, the coelome being, as we
shall see, much reduced. The muscles are all unstriped,
and are arranged in distinct bands or sheets, many of them
being very large and conspicuous. The largest are the anterior
and posterior adductors (Figs. 93 and 95, a. ad, p. ad), great
cylindrical muscles which pass transversely across the body
and are inserted at either end into the valves of the shell,
which are approximated by their contraction. Two muscles
of much smaller size pass from the shell to the foot, which
they serve to draw back : they are the axferior and posterior
retractors of the foot. A third foot-muscle arises from the
shell close to the anterior adductor, and has its fibres spread
fan-wise over the visceral mass which it serves to compress,
thus forcing out the foot and acting as a protractor of that
organ. The substance of the foot itself consists of a
complex mass of fibres, the zxtrinsic muscles of the foot,
many of which also act as protractors. Lastly, all along

the border of, the mantle is a row of delicate pallial muscles,
Pract. Zoou. : me

386 THE MUSSEL CHAP.

which, by their insertion into the shell, give rise to the
line already seen.

The ce/ome is reduced toa single ovoidal chamber, the
pericardium (Figs. 93 and 95, fc), lying in the dorsal region
of the body and containing the heart and part of the intes-
tine: it is lined by coelomic epithelium, and does not corre-
spond with the pericardial sinus of the crayfish, which is
a blood-space (p. 362). In the remainder of the body the
space between the epiderm and the viscera is filled by
the muscles and connective tissue.

The mouth (Fig. 93, mth) lies in the middle line just
below the anterior adductor. On each side of it are two
triangular flaps, the internal and external /adral palps ; the
external palps unite with one another in front of the mouth,
forming an upper lip; the internal are similarly united
behind the mouth, forming a lower lip: both are ciliated
externally. The mouth leads by a short gu//et (guZ) into a
large stomach (st), which receives the ducts of a pair of
irregular, dark-brown, digestive glands (d. gl). The intestine
(ént), which is lined by a ciliated epithelium, goes off from
the posterior end of the stomach, descends into the visceral
mass, where it is coiled upon itself, then ascends parallel to
its first portion, turns sharply backwards, and proceeds, as
the rectum (rct), through the pericardium where it traverses
the ventricle of the heart, and above the posterior adductor
finally discharging by the azws (a) into the exhalant siphon,
or cloaca. The wall of the rectum is produced into a
longitudinal ridge, or ¢vph/osole (Figs. 93 and 95, ¢y), and
two similar ridges begin in the stomach and are continued
into the first portion of the intestine. The stomach con-
tains at certain seasons of the year a gelatinous rod, the
crystalline style.

The gills consist, as we have seen, of two plate-like

87

GILLS

VII

side between the visceral mass and the

each

bodies on

mantle: we have thus a 77gf¢t and a “eft outer, and a right

and a left tuner gill (Fig. 98, ext. gl, int. of.)

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the surface (Fig. 93), each gill presents a delicate double

striation, being marked by faint lines running parallel with,

and by more pronounced lines running at right angles to,

cc @

388 THE MUSSEL CHAP.

the long axis of the organ. Moreover, each gill is double,
being formed of two similar plates, the ¢ner and outer
Zamellz, united with one another along the antericr, ventral,
and posterior edges of the gill, but free dorsally. ‘The gill
has thus the form of a long and extremely shallow bag open
above (Figs. 94 and 95) : its cavity is subdivided by vertical
plates of tissue, the ¢ter-lamellar junctions (7. 1. 7), which

ib
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‘bj

Fic. 94.—Diagram of the structure of the gill of Anodonta.

The gill is mad up of V-shaped gill-filaments (/) arranged in longitudinal series
and bound together by horizontal inter-filamentar junctions (7. 4. 7) which cross
them at right angles, forming a kind of basket-work with apertures, the ostia
(os), leading from the outside and opening (os’) into the cavity of the gill. The
latter is divided by vertical partitions, the inter-lamellar junctions (7. 2,7) into
compartments or water-tubes (z.¢) which open also into the supra-branchial
chamber ; 4. v7. blood-vessels. (From Parker and Haswell’s Zoology.)

extend between the two lamellz and divide the intervening
space into distinct compartments or qwater tudes (Figs. 93 and
94, w. ¢), closed ventrally, but freely open along the dorsal
edge of the gill. The vertical striation of the gill is due to
the fact that each lamella is made up of a number of close-set
gilljilaments (Fig. 94, f ): the longitudinal striation to the
circumstance that these filaments are connected by horizontal
bars, the ¢éer-filamentar junctions (1. f.7). At the thin free

VII GILLS 389

or ventral edge of the gill the filaments of the two lamella
are continuous with one another, so that each gill has actually
a single set of V-shaped filaments, the outer limbs of which
go to form the outer lamella, their inner limbs the inner
lamella. Between the filaments, and bounded above and
below by the inter-filamentar junctions are minute apertures
or ostia (0s), which lead from the mantle-cavity through a
more or less irregular series of cavities into the interior: of
the watey-tubes. The filaments themselves are supported
by chitinous rods, and covered with ciliated epithelium, the
large cilia of which produce a current running from the
exterior through the ostia into the water-tubes, and finally
escaping by the wide dorsal apertures of the latter. The
whole organ is traversed by blood-vessels (4. 7).

The mode of attachment of the gills presents certain
features of importance (compare Fig. 95, A, B, C). The
outer lamella of the outer gill is attached along its whole
length to the mantle: the inner lamella of the outer, and
the outer lamella of the inner gill are attached together to
the sides of the visceral mass a little below the origin of the
mantle : the inner lamella of the inner gill is also attached to
the visceral mass in front, but is free further back. The gills
are longer than the visceral mass, and project behind it, below
the posterior adductor (Figs. 93 and 95 C), as far as the pos-
terior edge of the mantle: in this region the inner lamellz of
the inner gills are united with one another, and the dorsal
edges of all four gills constitute a horizontal partitién between
the pallial cavity below and the exhalant chamber or cloaca
above. Owing to this arrangement it will be seen that the
water-tubes all open dorsally into a supra-branchial chamber
(Fig. 95, s. dv. c), continuous posteriorly with the cloaca and
thus opening on the exterior by the exhalant siphon.

The physiological importance of the gills will now be

390 THE MUSSEL CHAP,

obvious. By the action of their cilia a current is produced
which sets in through the inhalant siphon into the pallial
cavity, through the ostia into the water-tubes, thence into the
supra-branchial chamber, and out at the exhalant siphon.
The in-going current carries with it not only oxygen for the
aération of the blood, but also diatoms, infusoria, and other
microscopic organisms, which are swept into the mouth by
the cilia covering the labial palps. The out-going current
carries with it the various products of excretion and the
feeces passed into the cloaca. The action of the gills in
producing the food current is of more importance than their
respiratory function, which they share with the mantle.

The excretory organs are a single pair of curiously-modified
nephridia, situated one on each side of the body just below
the pericardium. Each nephridium consists of two parts, a
brown, spongy, glandular portion or kidney (Figs. 93 and 95,
kd), and a thin-walled, non-glandular part or d/adder (b/).
The two parts lie parallel to one another, the bladder being
placed dorsally and immediately below the floor of the
pericardium : they communicate with one another posteriorly,
while in front the kidney opens into the pericardium (7. Z. a.),
and the bladder on the exterior by a minute aperture (7. a),
situated between the inner gill and the visceral mass. Thus
the whole organ, often called after its discoverer, the organ of
Bojanus, is simply a tube bent upon itself, opening at one
end into the ccelome, and at the other on the external
surface of the body: it has therefore the normal relations of a
nephridium (p. 331). Two bladders communicate anteriorly,
and their epithelium is ciliated, producing an outward current.

It seems probable that an excretory function is also discharged by a
large glandular mass of reddish-brown colour, called the pericardial
gland ork vcber’s organ (Fig. 95, 4.0). It lies in the anterior region of the
body just in front of the pericardium, into which it discharges.

VII TRANSVERSE SECTIONS 391

A through the anterior part of

FIG. 95.—Anodonta. Three transverse sections.
the visceral mass ; B,-through the’posterior part of the visceral mass ; C, through

the posterior adductor muscle.
au, auricle; 42. urinary bladder; ext. g7. external gill; /¢. foot; 7.2.7. inter-
lamellar junction ; w¢. intestine; zz¢. ¢/. internal gill; 4d. kidney; &. 0. peri-
cardial gland ; 2g. ligament ; #. mantle ; #. ad. posterior adductor ; fc. pericar-
dium ; cz. rectum ; s. év. ¢. supra-branchial chamber ; s/. shell; ¢y. typhlosole ;
i vena cava; v.gn. visceral ganglion, v. m2, visceral mass.

v. ventricle; vc.
(From Parker and Haswell’s Zoology, after Howes, slightly altered.)
CG ak

392 THE MUSSEL CHAP.

The circulatory system is well developed. The Hear? lies
in the pericardium, and consists of a single ventricle (Figs.
93 and 95 B, wv) and of right and left auricles (au). The
ventricle is a muscular chamber which has the peculiarity
of surrounding the rectum (7c): the auricles are thin-walled
chambers communicating with the ventricle by valvular
apertures opening towards the latter. From each end of the
ventricle an artery is given off, the anterior aorta (Fig 93,
a. ao) passing above, the posterior aorta (p. ao) below the
rectum. From the aorte the blood passes into arteries (Fig.
96, art.,1 art”) which ramify all over the body, finally forming
an extensive network of vessels, many of which are devoid
of proper walls and have therefore the nature of sinuses.
The returning blood passes into a large longitudinal vein,
the vena cava (Figs. 95 and 96, v. c), placed between the
nephridia, whence it is taken to the kidneys themselves
(Fig. 96, “ph. v), thence by afferent branchial veins (af. br. v)
to the gills, and is finally returned by efferent branchial veins
(ef. dr. v) to the auricles. The mantle has a very extensive
blood supply, and probably acts as the chief respiratory organ
(p. 390) : its blood (a7?) is returned directly to the auricles
without passing through either the kidneys or the gills.
The blood is colourless and contains leucocytes. There is
no communication between the blood-system and the
pericardium.

The nervous system is formed ona type quite different
from anything we have yet met with. On each side of the
gullet is a small cerebro-pleural ganglion (Fig. 93, ¢. pl. gn),
united with its fellow of the opposite side by a nerve-cord,
the cerebral commissure, passing above the gullet. Each
cerebro-pleural ganglion also gives off a cord, the cerebro-
pedal connective, which passes downwards and backwards to
a pedal ganglion (pd. gn) situated at the junction of the

vu NERVOUS SYSTEM 393

visceral mass with the foot: the two pedal ganglia are so
closely united as to forma single bilobed mass. From each
cerebro-pleural ganglion there further proceeds a long cerebro-
visceral connective, which passes directly backwards through
the kidney and ends in a visceral ganglion (Figs. 93 and 95,

Fic. 96.—Diagram of the circulatory system of Anodonta.

The blood received from the auricles (az) is pumped by the ventricle (~) into the
aorta (ao) and thence passes to the mantle (a7¢1.) and to the body generally
(arf2.), The blood which has circulated through the mantle is returned directly
to the auricle : that from the body generally is collected into the vena cava (v. c),
passes by nephridial veins (zfA. v) to the kidneys, thence by afferent branchial
veins (aft br. v) to the gills, and is returned by efferent branchial veins (ef 47. v)
to the auricles ; fc. pericardium. (From Parker and Haswell’s Zoology.)

C, v. gn) placed on the ventral side of the posterior adductor
muscle. The visceral, like the pedal ganglia, are fused
together. The cerebro-pleural ganglia supply the labial
palps and the anterior part of the mantle; the pedal, the
foot and its muscles ; the visceral, the enteric canal, heart,

394 THE MUSSEL CHAP.

gills, and posterior portion of the mantle. The nerve-cells
are confined to the ganglia.

It will be seen that the cerebral commissures and cerebro-
pedal connectives, together with the cerebro-pleural and
pedal ganglia, form a nerve-ring which surrounds the gullet :
the cerebro-pleural ganglia may be looked upon as a supra-
cesophageal nerve mass corresponding with the brain of the
Earthworm and Crayfish, and the pedal ganglia as an infra-
cesophageal mass representing the ventral nerve-cord.

Sensory organs are poorly developed, as might be ex-
pected in an animal of such sedentary habits. In connec-
tion with each visceral ganglion is a patch of sensory
epithelium forming the so-called olfactory organ or, better,
osphradium, the function of which is apparently to test the
purity of the water entering by the respiratory current. Close
to the pedal ganglion a minute “ auditory sac” or ofocyst is
found, the nerve of which is said to spring from the cerebro-
pedal connective, being probably derived from the cerebral
ganglion. Sensory cells, probably tactile, also occur round
the edge of the mantle, and especially on the tentacles of
the inhalant siphon. | ,

The sexes are separate. The gonads (Fig. 93 gon)
are large, paired, racemose (p. 135) bodies, occupying a con-
siderable portion of the visceral mass amongst the coils of
the intestine: the spermary is white, the ovary reddish. The
gonad of each side has a short duct which opens (g.af) on
the surface of the visceral mass, just in front of the renal
aperture.

In the breeding season the eggs, extruded from the genital
aperture, pass into the supra-branchial chamber, and so to
the cloaca. There, in all probability, they are impregnated
by sperms introduced with the respiratory current. The
oosperms are then passed into the cavities of the outer gills,

VII DEVELOPMENT 395

which they distend enormously. Thus the outer gills act as
brood-pouches, and in them the embryo develops into the
peculiar larval form presently to be described.

Asin the frog and earthworm the cells formed by the
segmentation of the oosperm are of two sizes, small cells
composed entirely of protoplasm, and large cells loaded with
yolk-granules. The large become enclosed by the small
cells, but the enteron thus formed is very small and quite
unimportant during early larval life, the young mussels being

‘sm.

ID. --

- Fic. 97.—A, advanced embryo of Anodonta enclosed in the egg-membrane. B, free

larva or glochidium.
f. byssus; g. lateral pits; s. shell; sz. hooks ; s#, adductor muscle ; so. sensory
. hairs ; w. ciliated area. (From Korschelt and Heider. 4
nourished, after the manner of parasites (p. 271), by a
secretion from the gills of the parent.
The dorsal surface of the embryo is soon marked out by

the appearance of a deep depression, the she//-gland, which

. secretes, in the first place, a single median shell. This is,

however, soon replaced by a bivalved /arval shell (Fig. 97,
5) of triangular form, the ventral angles being produced into
hooks (sk). The body at the same time becomes cleft from
below upwards (a), forming the right and left mantle-lobes.
On the ventral surface, between the lobes of the mantle, is

396 THE MUSSEL CHAP.

formed a glandular pouch, which secretes a bunch of silky
threads, the dyssus (f). The larva is now called a glochidium,
the subsequent history of which furnishes another example
of a means of ensuring dispersal in a sedentary animal (com-
pare pp. 272 and 278).

The glochidia, entangled together by means of their
byssal threads, escape from the gills of the parent by the
exhalant siphon, and eventually attach themselves by their
hooked valves to the body of a passing fish, such as a
stickleback. Here they live for a time as external parasites,
gradually undergoing metamorphosis ; and finally drop from
the host and assume the sedentary habits of the adult.

This mode of development is exceptional amongst bivalves, in most
of which (e.g. oyster) the egg gives rise to a larva resembling the
trochosphere of many annelids (p. 340), the prostomial’ region then

growing out into a thickened rim which bears the circlet of cilia in front
of the mouth, the larva at this stage being distinguished as a velzger.

The Mussel belongs to the phylum Mollusca, which
includes, in addition to the bivalved “ shell-fish ”—such as
mussels, oysters, cockles,—the snails, slugs, whelks, peri-
winkles, &c.—most of which possess a univalved shell, as
well as the cuttle-fishes and their allies. These are all
sharply distinguished from the Arthropods by the absence
of segmentation, and by having, as a rule, an exoskeleton in
the form ofa shell. The bivalves are included in the class
Pelecypoda or Lamellibranchiata, the essential structure of
which you will have learnt from your study of the Mussel.

You have now examined examples of the following chief divisions
or phyla of the animal kingdom (compare p. 219):

Protozoa. Annulata. Mollusca.
Coelenterata. Arthropoda. Vertebrata.

In addition Lo these, you will probably have seen, when dissecting the
frog, certain parasites. One of these, Polystomum (p. 33), belongs

vil PRACTICAL DIRECTIONS 397

to a lowly group of worms of a flattened form, constituting the phylum
Platyhelminthes, which includes the parasitic liver-flukes and tape-
worms as well as certain free-living forms; another, Ascards (p. 153),
belongs to the phylum Nemathelminthes in which all the parasitic
thread-worms are placed. Apart from certain other smaller groups,
which include such animals as ‘‘ wheel-animalcules,” ‘‘sea-mats,”
‘**Jamp-shells,” &c., there only remains one other of the larger phyla of
which you will not have studied an example, viz., the phylum
Echinodermata, which is constituted by the star-fishes, sea-urchins,
sea-cucumbers, &c. : these are all inhabitants of the sea, and are peculiar
in exhibiting a more or less pronounced radial arrangement of their
parts, and in possessing a curious calcareous exoskeleton developed
within their integument, consisting of small particles or of definitely-
shaped plates. All the phyla with the exclusion of the Vertebrates are
spoken of collectively as the Zzvertebrata.

PRACTICAL DIRECTIONS.
FRESH-WATER MUSSEL.

Examine a living specimen in a vessel of water with some sand at the
bottom, and note the greenish-brown dzvalve shell, the foot, and the
manner in which the animal buries itself, anterior end downwards,
with the Jalal openings projecting posteriorly. Observe the currents
of water passing in at the fringed zzhalant aperture, and out at the
exhalant aperture: these can be more easily observed if a little finely-
powdered indigo or carmine is placed in the water.

In order to kill the animal in as fully-extended a condition as possible,
place it in a saucepan or beaker of water, and heat over a gas-burner
or spirit-lamp until the water is warmed up to a temperature of about
40° C., when the foot will be protruded. Keep at this temperature
until the mussel is dead. The animal is best preserved from day to day
in 3 per cent. formaline, but 70 per cent. spirit will answer the purpose.

A. External characters.

Compare the shell with the carapace of the crayfish, and observe that
it covers the whole body and is densely calcified except along the dorsal
hinge-line, where it forms an elastic Agament connecting the two lateral
valves of the shell. Note the “es of growth, and the wmbo near the
hinge-line. The anterior end is rounded, the posterior end more
pointed, Jn dead specimens the valves gate, owing to the elasticity of

398 THE MUSSEL CHAP

the ligament and to the relaxation of the adductor muscles (see below),
and they can than be seen to be lined by a fold of the integument, the
mantle or pallium. By wedging the valves open still further, the azterior
and posterior adductor muscles are seen connecting the two valves; also
the foot, visceral mass and gzlls, between the two mantle folds.

B. Dissection.

I. Carefully insert a scalpel between the left valve and mantle-edge,
and separate them all round the margin of the valve. Then cut through
the anterior and posterior adductor muscles close to the shell, and
remove the left valve, cutting through the ligament.

1. Compare the positions of the cut muscles on the body with the
muscular impressions on the shell:—a. the large antertor adductor near
the dorsal and anterior end; and close behind it—d. the protractor and
c. the anterior retractor muscles of the foot, the latter dorsal to the
former ; d. the large posterior adductor near the dorsal and posterior
end, on the upper and anterior margin of which is ¢. the fosterior
retractor. Note -also the thickened ventral edge of the mantle, the
corresponding pad/zal line on the valve, as well as the lines from the
muscular impressions converging towards the umbo, the smooth
longitudinal Ainge (with hznge-teeth in Unio), the Ligament, and the
iridescent inner layer of the shell. Sketch the inner surface of the valve.

2. Break the shell across, and examine the edge with a lens, noting
the three layers of which it is composed :—a. the thin outer uncalcified
pertostracum ; b. the thicker middle présmatic layer; and c¢. the inner
pearly or nacreous layer.

Pin down the animal firmly under water, leaving it in the right valve :
insert the pins through the ligament and muscular part of the foot, and
obliquely against the edges of the shell. Note :—

"3. The Zt mantle-lobe uniting with the right above the middle of the
anterior adductor muscle anteriorly, and behind the posterior adductor
muscle posteriorly. Just behind the posterior adductor muscle the mar-
gins of the mantle are much thickened ; and in life, the two approxim-
ated mantle-edges here separate to form the exhalant and inhalant
apertures, the latter of which is provided with short ¢ez/acles.

4. Turn back the left mantle-lobe and note the mantle-cavity and its
contents :—a. the foot and wdsceral mass; 6. the left pair. of gills
at the sides of the visceral mass; and anteriorly «. the left pair of
small, triangular, /abzal palps. (Note that there is no distinct head).
These parts are situated between the two mantle-folds in the larg

vu PRACTICAL DIRECTIONS 399

ventral mantle-cavity. Note also the position of the pericardium on
the dorsal side of the gills, the perécardial gland, and the left excretory
organ (nephridium), between the pericardium and gills, and extending
back to the posterior adductor muscle.

5. Pass a seeker into the exhalant aperture and note that it enters a
short exhalant siphon or cloaca, separated from the mantle-cavity by
the bases of the gills. Slit this chamber open, and note that it extends
above the posterior adductor muscle, as well as above the gills. The
hinder end of the rec¢ze will he seen just above the posterior adductor
muscle, opening by the avzzs into the cloaca. Insert a seeker into the
mouth, between the anterior adductor muscle and the anterior edge of
the foot.

II. 1. Remove the left mantle-lobe, cutting very carefully along the
bases of the palps and gills. Then slit open the pericardium longi-
tudinally, a short distance dorsally to the gills, and note the rectum,
ventricle, and delicate /eft aurzcle. Cut away sufficient of the wall of
the pericardium to expose these parts, taking care not to injure the
auricle. Note—

2. The rectum, which runs straight through the pericardium, and in
its middle is surrounded by the elongated muscular venéricle, which is
bilobed posteriorly. The thin-walled, triangular Ze/t aurzcle can be seen
opening into the ventricle by a valvular aperture : the base of the auricle
is attached along the dorsal border of the gills. (Note the r¢ght auricle
on the other side). Inject French-blue (p. 99) into the left auriculo-
ventricular aperture, and note «. the axtertor aorta above the rectum,
and 6. the fostertor aorta below the rectum. In the middle line of the
floor of the pericardium the vera cava can be seen.

3. Examine the gz//s. Note the left outer and inner gill, and that in
the female the former is often distended with eggs or darve. Each gill
consists of an ozéer and an zzner lamella, forming a kind of trelliswork,
with small meshes or os¢#a, separated by horizontal and vertical bars,
Cut away a piece of the outer lamella, noting that the two lamell of
each gill are united at intervals by zeterlamellar junctions.

4. Make out the mode of attachment of the gills (see p. 389, and
Fig. 95). Sketch your dissection with the gills in their natural
position.

5. Pass a seeker forwards from the exhalant siphon along the. supra-
branchial passages—two longitudinal canals at the bases of the gills
communicating with the zwterlamedlar spaces or water tubes (the inner

400 THE MUSSEL CHAP,

passage unites with its fellow behind the visceral mass, see Fig. 95).
The inner canal also communicates with the branchial chamber through
the slit between the visceral mass and the middle part of the lamella of
the inner gill. Cut open the canals carefully, and examine their
relations.

6. Turn up the gills, and insert the scissors in the slit just mentioned ;
cut forwards through the inner lamella of the inner gill for a short dis-
tance. The veval aperture or nephridtopore can then be seen opening
into the suprabranchial passage, and just below and in front of it is the
genital aperture. Then note the paired reno-fericardial aperture (ne-
phrostome) in the anterior end of the pericardium, just below the point
at which the rectum enters it: this can be more easily seen if the rectum
is cut through and raised up.

7. Blow through the nephridiopore, and note the thin-walled sac-like
bladder into which it opens: this lies just below the pericardium, and
communicates with its fellow anteriorly. Just beneath and internal
to the bladder is the glandular portion of the kidney, which is dark in
colour and extends further back than the bladder, beyond which it forms
a large mass just in front of the posterior adductor muscle : it communi-
cates with the hinder end of the bladder. Sketch.

III. 1. Remove both gills carefully, cutting along their bases; stain
and mount a small piece of one lamella. Note that the lamella is made
up of close-set vertical bars or gill-filaments, connected by numerous
tranverse inter-filamentar jzerctzors ; the bars are covered with ciliated
epithelial cells, and each is strengthened by pairs of small chitinous
rods. The whole gill is traversed by blood vessels. Sketch.

2. Asmall portion of a gill should be preserved, stained, inbedded,
and -cut into sections (p. 136) passing transversely through the gill-
filaments. Compare with Fig. 94, and sketch.

3. Mount in salt-solution a small piece of fresh gill, and also of the
mantle, and observe the movements of the cilia.

IV. If your specimen is a female, and contains ova or larve in the
outer gill, examine some under the microscope. The ova are provided
with a canal or mzcropyle perforating the vitelline membrane, for the
entrance of the sperms. Note the form of the larve or glochidia
(Fig. 97).

V. The zervous system consist of three pairs of small orange-coloured
ganglia, with connectzves between them.

1. Cut away the left labial palps very carefully and look for the left

VII PRACTICAL DIRECTIONS 401

cerebro-pleural ganglion, situated just below the attachment of the
mantle lobe close to the mouth and directly beneath the epithelium :—
it may already have been exposed by removing the mantle-lobe. The
two cerebro-pleural ganglia are connected by a commissure above the
gullet.

2. On the under surface of the posterior adductor muscle are the two
closely applied vzsceral ganglia, which will be at once seen when the
membranous covering of this muscle is dissected off.

3. The pedal ganglia are more difficult to find, as they are more
deeply situated, at the junction of the foot and visceral mass. Slit up
the foot by a median longitudinal incision, and carefully dissect the
two halves apart until the two closely apposed ganglia are seen; then
remove the left half of the foot.

4. Carefully dissect out the ceredro-fedal connectzve, which runs
straight from the cerebro-pleural to the pedal ganglion. Then follow
out the cerebro-visceral connective from behind forwards :—each extends
forwards from the corresponding visceral ganglion along the inner
surface of the renal organ, and then passes slightly downwards across
the visceral mass to the cerebro-pleural ganglia. Sketch.

VI. The sensory organs are not easy to make out. In order to see
the osphradium and tactéle-cells, it-is necessary to cut microscopic
sections. The ofocyst may be found by examining under the microscope
a portion of the tissue just behind the pedal ganglion ; but is is much
more easily observed in the small fresh-water bivalve Cyc/as, often to
be found in ponds: it is only necessary to examine the entire animal
in water under a low power after removing the shell in order to see
the two globular otocysts, each lined by epithelial cells and containing
an otolith, which is in constant, trembling motion.

VII. The alimentary organs consist of « gullet, stomach, paired
digestive-gland, and coiled zxtestine, imbedded in solid tissue in the
visceral mass, the hinder part of the intestine or rectum alone being
surrounded by a ccelomic cavity (pericardium). The anus has already
been seen.

1. Dissect away the epiderm to the left side of and above the stomach
(if this has not already been done), and note the brown dégestcve gland
in the anterior dorsal part of the visceral mass. Then insert a seeker
into the stomach through the mouth, to act as a guide, and slit up the
short gullet and stomach. Note the ducts of the digestive gland
opening into the stomach.

Pract.ZooL, D-D

403 THE MUSSEL CHAP. Vil

2. In the same way slit up the whole zzéestzne, either from the
stomach backwards or from the rectum forwards, using the seeker as a
guide the whole way, and first examining Fig. 93 to see the direction
which the coils take. Note the ventral ¢yph/osole in the rectum,
beginning at the last coil. Sketch.

VIII. 1. The gonads appear very similar in both sexes, and fill up a
large part of the visceral mass between the coils of the intestine : their
ducts open at the genztal apertures (§ II., 6).

2. Tease up a bit of the ovary or spermary in salt solution, stain with
magenta, and examine.

C. Transverse sections (Fig. 95).

With a sharp scalpel or razor cut a specimen hardened in formaline
or spirit into transverse sections about 4 inch thick, passing obliquely
through—

1. The anterior part of the pericardial cavity, visceral mass, and
foot. Note the relations of the mantle-folds, the gonads, the enteric
canal (including the rectum with its typhlosole), the gills and supra-
branchial canals, the anterior aorta and wena cava, and the kedneys
and bladders.

2. The ventricle, auricles, and hinder part of the visceral mass.
Note the relations of the mavtle-folds, enteric canal, gills, suprabranchial
canals, vena cava, and kidneys and bladders.

3. The posterior adductor muscle, hinder part of gills, and rectum,
Note the relations of the various parts. Sketch the three sections.

CHAPTER VIII

CHARACTERS OF THE PHYLUM VERTEBRATA

AMPHIOXUS.

FRoM your study of the frog, you have learnt something
about a vertebrate animal, and we will now examine
afew more examples of the phylum Vertebrata, which,
as we have seen (p. 219), includes several classes, the
chief of which are—the Pisces, Amphibia, Reptilia, Aves,
and Mammata. Leaving aside the lampreys and _ their
allies, which present certain peculiarities and are therefore
placed in a class by themselves, these all agree with: one
another and resemble the frog in the following essential
characters.

They all possess :—a vertebral column—or at any rate 4
notochord (p. 203), which is nearly always replaced by a
vertebral column in the adult—and a s&u// with upper and
lower jaws ; a hollow, dorsal, nervous system, consisting of
brain and spinal cord; paired olfactory organs, eyes, and
auditory organs, which take on a close connection with the
skull; a pharynx, which, at an early stage at any rate, is
perforated by a small number (never more than seven) of
paired gi//clefts (p. 204); a mouth which is ventral and
anterior, and an avws which is ventral and posterior ; Azdneys
which are composed of numerous urinary tubules or neph-
ridia ; a chambered Zeart and red blood-corpuscles ; a liver,

DD 2

404 THE LANCELET CHAP,

and a hepatic portal system ; usually two pairs of limbs, and

never more than two

3 gon.
3 or. hd. oral hood ; vent. f. ventral
's Zoology, after Kirkaldy.)

notochord

3 cir. cirri; dors. 7. dorsal fin; dors. Av. dorsal fin-rays
(From Parker and Haswell

98.—A mphioxus lanceolatus, side view.

7 Fic.
atrp. atriopore ; cd. /. caudal fin

gonads ; s2¢f2. metap!

leure ; #zyom. myomeres; ch.

ra
>
§ &
a
y
9g
®

fin ; vent. A 7. ventral fin-rays,

an. anus;

it differs from all the
important points.

pairs; and a series of Jody-muscles
which are divided into segments
or myomeres (p. 203), at any rate in
early stages.

Now in various parts of the
world, occurring only rarely off the
English coasts, certain small fish-
like animals called lancelets are
found, the commonest species of
which is known as <Amphioxus
lanceolatus (Fig. 98). This animal,
which possesses a median fin like
that of the tadpole (p. 207), is not
more than a couple of inches in
length, and lives in the sea near the
shore, burrowing in the sand ; it is
especially interesting owing to the
fact that it presents certain charac-
teristics indicating a near relation-
ship to the primitive ancestors of
Vertebrates. It possesses a noto-
chord, a dorsal hollow nervous
system, a pharynx perforated by
gill-slits, a hollow outgrowth of the
intestine representing a simple
liver, with a hepatic portal system,
and a series of nephridia. But
higher Vertebrates in the following

-- The epiderm consists of a single layer of cells. There
is no distinct head and no skull; the persistent noto

405

CHARACTERS

GENERAL

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406 THE LANCELET CHAP,

(Fig. ror), and the colourless blood contains no red
corpuscles. The nephridia remain distinct, not being
united into a single kidney on either side: they are situated
anteriorly, in the neighbourhood of the pharynx (Fig, 99,
nph) ; the gonads (Figs. 98 and 99, gon, and Fig. 100 A, g)
are metamerically arranged, and have no ducts.

In certain of its characters Amphioxus resembles the members of
a group of animals—the 7i»/cata, commonly known as ‘‘ sea-squirts,”
in which the body is enclosed in a “test” or mantle, consisting largely
of cellulose (p. 244). These, like Vertebrates, possess a notochord and a
dorsal, hollow nervous system in young stages, and in the adult retain
numerous pharyngeal gill-slits: they are almost certainly degenerate
descendants of primitive animals from which the Vertebrata also arose.

These numerous and marked differences between the
lancelet and the higher Vertebrates make it necessary to
place Amphioxus in a separate division of the Vertebrata,
called—from one important negative character—the
Acrania, while all the other Vertebrates, which possess
skulls, are included in the division Craniata.

The external appearance of Amphioxus is represented in
Fig. 98. In addition to the points already referred to, it will
be seen that the mouth is surrounded by a fold, the oval hood
(or. hd), from which a number of tentacles or e777 (cir) are
given off ; and that there is a /atera/ or metapleural fold (mtpl)
along either side of the body extending backwards as far as
the atrial pore, in addition to the median fin-fold (dors. f,
ca. f, vent. f) extending round the tail as a caudal fin. In
the young animal the gill slits open directly to the exterior,
but a median canal is subsequently formed along the
ventral side of the body, and as this extends inwards to
form the atrium, it gradually surrounds the pharynx at the
sides, pushing the coelome before it, so that the latter
becomes greatly reduced in this region (Fig. 100 A, co).

The skeleton is very simple ; besides the notochord there

VIII SKELETON AND GILL-BARS 407

are rod-like bars of a substance somewhat resembling
cartilage supporting the oral hood and cirri (Fig. 99, s),
chitinoid rods (47.7) supporting the gill-bars between the

Fic. 100,—Amphioxus lanceolatus., A, transverse section of the pharyngeal region.

a. dorsal aorta; 4. atrium; ¢. notochord; co. coelome; e. endostyle; g. gonad

(ovary) ; #4. branchial septa; 4d. pharynx; J. liver ; #zy. myomere; 2. nephri-
dium ; 7. spinal cord: 57, sz, dorsal and ventral spinal nerves.

B, transverse section of the intestinal region. a¢v. atrium; ca/. ceelome; d. ao.

dorsal aorta ; 272. intestine ; myo. myomere ; zch. notochord ; ze. spinal cord ;

s. int. v. sub-intestinal vein. (From Parker and Haswell’s Zoology: A, from
Hertwig, after Lankester and Boveri; B, partly after Rolph.)

clefts, and short rods of connective-tissue, the /iz-rays,
in the dorsal and ventral fins (Figs. 98 and 99, dors. f7,
vent. f.r).

In the course of development, each primary gill-slit
becomes divided into two by the growth, from above down-
wards, of a tongue-like process, the secondary gill-bar or

septum (Fig. 99, 47. 7. 2), so that in the adult the slits and
D_D 2*

408 THE LANCELET CHAP.

intervening bars are seen to be arranged in couples, the
supporting rods (4r. 7. 7) of the primary bars (dr. sep. Z)
being forked below. A further complication is produced
by the formation of transverse connections, supported by
skeletal rods, between the gill-bars. The gill-slits are more
numerous than the muscle-segments or myomeres (myom),
and owing to their obliquity, a large number of them always
appear in a transverse section (Fig. 100, A). =

On the ventral wall of the pharynx is a longitudinal groove,
the endostyle (Fig. 100 A, ¢), lined by ciliated and glandular
cells; andasomewhat similar groove, the epipharyngeal groove,
extends along the dorsal aspect of the pharynx: these serve
as channels along which the particles of food pass into the
intestine. The minute mouth (Fig. 99, m¢z) leads from the
cavity of the oral hood into the pharynx, and is surrounded
by a membrane, the vedum (v/), produced at its edge into
a number of tentacles (27. 2).

The zntestine is straight (Fig. 99, it) and the Aepatic
cecum, or rudimentary “ver, arises from its anterior end on
the ventral side, and extends forwards to the right of the
pharynx (Fig. 99, and Fig. roo, /).

Although no heart is present, the d/ood-vessels present
some similarities to those of the Craniata. Their arrangement
may be seen from the accompanying diagram (Fig. 101),
which should be compared later on with Fig. 110. The
nephridial tubes (Fig. 99 mph) are situated above the pharynx
on either side and open on the one hand into the reduced
ccelome in this region, and on the other into the atrium
(Fig. 100, 7).

The central canal of the spinal cord widens out in front
(Fig. 99, 47) to form a small cerebral ventricle (en. coe) in
the front wall of which is an unpaired pigment spot,
representing a simple kind of eye (¢. sf), and at its anterior

Vu GONADS 409

end is a ciliated depression usually known as the olfactory
pit (off. ~). There is no auditory organ. The dorsal and
ventral nerves (Fig. 100 A, sz) remain separate, and do
not unite to form a nerve-trunk (compare p. 163).

The sexes are distinct, and the metamerically arranged
gonads (Fig. 98-100, gon. g) are situated in pouches, the
cavities of which represent part of the ccelome and which
project into the atrium on either side of the pharynx. When

as
HJ
hepv sinky
afbra nat epporl.v

fora’ Y20 opbra br

Fic. ror.—Diagram of the vascular system of Amphioxus.
af. br. a. afferent branchial artcries; dr. cé. gill-slits ; cf. interstinal ‘capillaries ;
d. ao. paired dorsal aortze; d. ao’. median dorsal aorta; ef 47.4. ‘efferent
branchial arteries ; ep. fort. v. hepatic portal vein ; hep. v. hepatic vein; zz.
intestine ; éx. liver; Z#. pharynx: s. iv¢.v. sub-intestinal vein; v. ao. ventra
aorta. (From Parker and Haswell’s Zoology.)

ripe, the eggs and sperms are shed into the atrium and find
their way out through the atriopore.

The oosperm undergoes segmentation, and gives rise to a
simple /avva, which gradually develops into the adult form.
The embryology of Amphioxus, which is very instructive,
will be referred to in greater detail in Chapter_XI.

tne

410 THE LANCELET CHAP

PRACTICAL DIRECTIONS.
AMPHIOXUS.

A. External Characters.

Examine an entire specimen and note :—1, the form of the body ;
4, the continuous medzan fins—dorsal, caudal, and ventral; 3, the
paired Jateral fin or metapleure, extending along the body anterior
to the ventral fin; 4, the oraZ hood, anterior and ventral, with its
ctrrz; 5, the anus, a short distance from the posterior end, just on
the left side of the caudal fin; 6, the median and ventral atriopore,
at the junction of the lateral and ventral fins ; 7, the myomeres.

B, Anatomy.

I. Place a young specimen for a short time in absolute alcohol, and
then transfer to oil of cloves. If the specimen is a very small one,
it may with advantage be slightly stained first. Then transfer to a
hollow slide or, if you use an ordinary slide, support the cover-glass
on two small pieces of wood of the thickness of the specimen, and
mount. The preparation may be examined in oil of cloves, but it is
better to make a permanent preparation and replace the cloves by thick
balsam. Examine, and note in addition to the above points :—

1. The notochord, extending from the anterior to the posterior end or
the body, rather nearer the dorsal than the ventral side.

2. The neztral canal enclosing the spinal cord, lying just above the
notochord, and having pigment in its walls.

3. The oral skeleton, consisting of a segmented basal bar along the
sides of the oral hood, with a rod passing down each cirrus.

4. The longitudinal series of connective tissue compartments, which
are filled with gelatinous substance, forming the fiz-rays.

5. The elastic, horny reds supporting the gz//-bars, and the arrange-
ment of the gill-slits in pairs:—the primary gill-bars separating the
successive pairs, the rods supporting them being forked ventrally ; and
the secondary gill-bars between the members of a pair, the rods of these
being unsplit ventrally. Note also the horizontal bars connecting the
primary and secondary gill-bars and making the walls of the pharynx
appear like a meshwork.

6. The cavity of the oral hood, bounded by lateral folds, the
muscular ve/um between it and the pharynx, and the minute mouth.

7. The zzéesténe, running straight from the hinder end of the pharynx

Vu PRACTICAL DIRECTIONS 4ir

to the anus, and giving off near its anterior end the hepatie cecum
or éver, extending forward on the right side of the pharynx.

8. The myomeres and the zntermuscular septa between them, arranged
like a series of V’s, the apices of which point forwards. The muscular
fibres run longitudinally from septum to septum.

9. The gonads (ovardes or spermaries), arranged in a single row on
either side of and rather further back than the pharynx, extending as
far as the atriopore. Note in the female the large ova, which, when
ripe, cause a great distension of the body ; and in the male the minute
sperm-cells, the structure of which cannot he made out in entire
specimens. Sketch.

IT. Cut a specimen into short pieces, about an eighth of an inch in
length, and select portions from—a, between the hinder end of the
atrium and the anus (Fig. 100, B) or through the latter; 4, just in
front of or through the atriopore ; c, through the anterior part of the
pharynx ; and d, through the posterior part of the pharynx (Fig. 100,
A.). Stain and imbed these, and prepare a few sections from each
(p. 136). Examine in the order given above, first with the low, and
then with the high power. Sketch a section from each region.

uw. I. The oval form of the section, the median dorsal and ventral
fins, and the integument (columnar epiderm and thin derm).

2. The myomeres cut across in various planes, and appearing as
squarish masses separated by the septa. The muscle-fibres are mostly
striated.

3. The central sotochord, oval in transverse section, with transverse
wavy lines indicating the boundaries of the vacuolated notochordal
cells. It is surrounded by a connective tissue sheath, continuous with
the connective tissue investment of the neural canal above, and with
the intermuscular septa at the sides: the latter pass into the derm
peripherally.

4. The spinal cord, lying in the neural canal. It is ovoid in trans-
verse section, and has a dorsal fissure (see p. 155) extending downwards
nearly to the cestral canal, which is nearer the ventral than the dorsal
surface. See if your section happens to pass through a dorsal or a
ventral nerve. The dorsal nerves arise by large single roots from
about the middle of the sides of the cord, while the ventral nerves have
numerous fine roots, and arise from the ventro-lateral angles of the
cord. The dorsal and ventral nerves are not in the same transverse

Jane (compare p. 163), but haye an alternating arrangement,

412 THE LANCELET CHAP,

5. The gelatinous connective tissue forming the fiz-rays.

6. The zzzestzne, with its single layer of long epithelial cells. If the
section passes through the anus, the latter will be seen opening on the
left side of the ventral fin.

7. The celome, surrounding the intestine, and also enclosing the
subintestinal or portal veins (Fig. 101).

8. The dorsal aorta, just below the notochord.

d, Go over 2, 3, 4, 5, 6, and 8 again—all much as in a. Note—

1. The dorsal and /ateral fins (metapleural folds), ventral body-wall,
small metapleural lymph-canals, and a¢rzopore, if present in your section.

2. The ce/ome, a narrow space round the intestine, in which three or
four sebintestinal (portal) verns can be seen.

3. The atrdal cavity just outside the coelome, and separated from it
by the atvzal membrane. If the section passes through the atriopore,
it will be seen in the mid-ventral line, putting the atrial cavity in com-
munication with the exterior.

c. Note—1. The triangular form of the body in transverse section ;
the dorsal fin, the ventral surface, with its folded integument and thin
transverse ventral muscles; the metapleural folds and their contained
large canals. Then go over a. 2, 3, 4, and 5 again, which are all much
as in a and 4.

2. The pharynx, lying just below the notochord, and extending nearly
to the ventral body walls. Note—(a) the deep, dorsal epzbranchial
groove ; (b) the ventral evdostyle projecting into the pharyngeal cavity,
and enclosing a coelomic canal; (c) the gz//-bars, a great number of
which will be cut through obliquely, longer Jrzmary alternating with
smaller secondary ones. Note the skeletal rods near their outer surfaces,
and, in the primary bars, the small ca@/omdzc spaces on the outer sides of
the rods; (@) the dorsal suspensory folds of the pharynx, which enclose
the two dorsal calomic canals.

3. The atrial cavity, surrounding the pharynx except at the mid-
dorsal line where the pharynx is attached to the sheath of the noto-
chord, and communicating with the cavity of the pharynx through the
gill-slits. It is enclosed by the two atréal folds, which are united in
the mid-ventral line; the atrial epithelium is often much folded ven-
trally.

4. The single ventral aorta, in the endostylar ccelomic canal, and the
patred dorsal aorte, on either side of the epibranchial groove,

@, Note again- a. 2, 3, 4, 53 ¢ I, 3, 4, and also :—

VIII PRACTICAL DIRECTIONS 413

1. The pharvynx, much as in ¢ 2, but laterally compressed. The ev-
dostyle is now converted into a groove.

2. The hepatec caecum, on the right side of the pharynx, and covered
by the atrial epithelium. Note the calomze space around it, in which
are the fortal veins, and the hepatzc vezns (dorsal side).

3. The gonads (ovaries ox spermartes)—large masses on either side o.
the pharynx, projecting into the atrial cavity and covered by the atrial
folds. The spaces around them represent portions of the ccelome.
Note the small sferm-ced/s in the male, and in the female, the large ova.

(The nephridia can only be made out satisfactorily in good sections of
very well preserved specimens.)

CHAPTER IX
CHARACTERS OF THE CLASS PISCES—THE DOGFISH

THE class Pisces (see p. 403) includes a number of aquatic
Vertebrates which present a considerable amount of differ-
ence in form and structure. Excluding, however, one small
group, the ‘‘ Mud-fishes” or Dzpuoi—which differ so much
from other fishes that they are sometimes placed in a
separate class, intermediate between Fishes and Amphibians
—the Pisces are distinguished from the Amphibia, as a
whole, by certain constant characteristics, of which the
following are the chief.

The organs of respiration and locomotion are adapted for
life in the water. The former consist, as in the tadpole
(p. 207), of a series of vascular processes, the g///s, attached
to the arches separating the gill-clefts and persisting
throughout life; and lungs are never developed. The
pectoral and pelvic limbs have the form of paddle-like fxs,
which, like the median fins (p. 406), are supported by skele-
tal jin-rays; the median fin is usually subdivided into
separate dorsal, ventral, and caudal portions. In addition
to the endoskeleton, there is usually an exoskeleton, de-
veloped in the derm and consisting of sca/es ; and peculiar
integumentary sense-organs, supplied by special nerves not
represented in terrestrial Vertebrates, are always present.

CHAP. XI ELASMOBRANCHS 4is

The cerebellum is relatively large ; internal nostrils and a
tympanic cavity and membrane are not present. There is
only ove auricle in the heart and no post-caval vein. A
urinary bladder’ developed as an outgrowth of the enteric
canal (p. 210), is wanting. There is never such a marked
metamorphosis as in the case of the frog.

The two most important sub-classes of the Pisces are the
LElasmobranchit and the Teleostomi. The Elasmobranchs
are all marine forms, and include the dogfishes, sharks,
rays, and skates: their endoskeleton is composed almost
entirely of cartilage, like that of the tadpole. The Teleo-
stomi, in which the skeleton is mainly or to a large extent
bony, include by far the greater number of fishes—both
marine. and fresh water forms—such as the Salmon, Cod,
Herring, Perch, as well as the Sturgeon and its allies.

‘« The Dogfishes are small Sharks, and include a number
of genera and species. They are all powerful swimmers,
and feed voraciously on other fishes, crustaceans, etc.

The commonest British forms are the Rough Hound
(Scyllium canicula), the Lesser Spotted Dogfish (S. catulus),
the Piked Dogfish (Acanthias vulgaris), and the Smooth
Hound (Mustelus vulgaris). The following description,
though referring mainly to Scy//‘um, will apply, in essential
respects, to any of these.

External Characters and General Structure.-— The
dogfish has a spindle-shaped body, ending in front in a
bluntly-pointed snout and behind tapering off into an up-
turned tail. On the ventral surface of the head is the large,
transversely elongated mouth (Fig. 108), supported by a
pair of. jaws which work in a vertical, and not, like those of
the crayfish, in a transverse plane: they are, in fact, like
those of the frog, portions of the skull, having nothing to

416 THE DOGFISH CHAP.

do with limbs. They are covered with teeth which vary
in form in the different species. In front of the mouth, on
the ventral surface of the snout, are the paired nostrils, each
leading into a cup-like zasal or olfactory sac, and (in
Scyllium) connected with the mouth by a groove. The
eyes are placed one on each side of the head, above the
mouth: they are protected by folds of the skin form-
ing upper and lower eye/éds, the latter of which can be
closed over the eye. Behind the mouth are five pairs of
slit-like apertures arranged in a longitudinal series: these
are the gél/-clefts or external branchial apertures (p. 403).
Just behind each eye is a small aperture, the spzracle:
like the gill-clefts, it communicates with the pharynx, and
it is found by development to be actually the functionless
first gill-cleft.

On the ventral surface of the body, about half-way
between its two ends, is the vent or anus, leading into the
cloaca (p. 23), and on either side of it a small pouch into
which opens a minute hole, the abdominal pore, commu-
nicating with the ccelome, which is therefore not a com-
pletely closed cavity, as in the frog. From the end of the
snout to the last gill-cleft is considered as the head of the
fish ; from the last gill-cleft to the anus as the ¢vunk; and
the rest as the ¢az/.

A number of symmetrically arranged, minute apertures on
the skin of the head, particularly numerous on the snout,
lead into a series of tubes known as sensory canals, which
are situated beneath the skin in this region; and a single
tube, known as the /ateral line canal, the position of which
is indicated by a very faint longitudinal line, extends along
either side of the body and tail. The whole apparatus
constitutes an important, but imperfectly understood, in-

IX FINS AND SKIN 417

tegumentary sense-organ: it is represented in the tadpole,
but disappears at metamorphosis.

Springing from the body are a number of flattened folds,
called the jizs, divisible into median and p.1ired. The
median folds are not continuous, as in the tadpole and
Amphioxus, but are subdivided into several distinct parts,
viz., two dorsal fins along. the middle line of the back,
a caudal fin lying along the ventral edge of the up-
turned tail, and a ventral fin behind the anus. The paired
folds are the pectoral fins, situated one on each side of the
trunk just behind the last gill-cleft, and the pelwic fins,
one on each side of the vent: these correspond to the
pectoral and pelvic limbs of the frog. In the male there
is connected with the inner border of each pelvic fin a
grooved, rod-like structure known as the c/asfer, which
serves as a copulatory organ.

It is very possible that the paired fins, like the median fins, are
specialised portions of a primarily continuous fin-fold—extending along
either side of the body like the lateral or metapleural folds of
Amphioxus.

The fish swims by vigorous strokes of the tail: the
pectoral fins are used chiefly for steering, and the dorsal
and ventral fins serve, like the keel of a boat, to maintain
equilibrium.

The skin or external part of the body-wall consists, as
usual, of two layers, an outer layer of deric epithelium or
epiderm (Fig. 102, Der. Epthm), formed, like that of the
frog, of several layers of cells; and an inner layer of con-
nective tissue, the derm. In the derm are innumerable
close-set, calcified bodies, each consisting of a little irregular
plate of bone produced into a short spine—composed, like
the teeth present in the frog and most Vertebrates, of a calci-
fied tissue harder than bone known as dentine capped with

Pract. ZooL. EE

418 THE DOGFISH CHAP.

a still harder tissue called exame/—which projects through
the epiderm and gives a rough, sandpaper-like character to
the skin. These placoid scales or dermal teeth together con-
stitute the exoskeleton of the dogfish: it is a discontinuous,
mainly dermal, exoskeleton (p. 429), and not a continuous
cuticular one like that of the crayfish.

Beneath the derm is the muscular layer, which, as in
Amphioxus and in the tail of the tadpole, is metamerically
segmented. The muscles are divided into myomeres, follow-
ing one another from before backwards, and having a zigzag
disposition. The fibres composing them are longitudinal,
and are inserted at either end into fibrous partitions or
myocommas which separate the myomeres from one
another.t The muscular layer is of great thickness, espe-
cially its dorsal portion. The fibres of all the body muscles
are, as in the frog and Vertebrates generally, of the striped
kind.

There is a large celome (Figs. 102 and 108), which, as in
other Vertebrates, is confined to the trunk, both head and
tail being, in the adult, accelomate. The cavity is divisible
into two parts: a large abdominal cavity, containing most of
the viscera, and a small anterior and ventral compartment,
the pericardial cavity (Fig. 108, ped. cav), containing the
heart and communicating with the abdominal cavity by a
canal which opens on the ventral surface of the gullet.
Both are lined by ccelomic epithelium (Fig. 102, Cel.
£pthm) underlain by a layer of connective tissue, a strong
lining membrane being thus produced, called, as in the
frog, peritoneum in the abdominal, pericardium in the peri-
cardial cavity.

Another very characteristic Vertebrate feature is that the

1 In the adult frog a segmentation can still be seen in the rectus
muscle of the abdomen (Fig. 16).

Ix ENDOSKELETON 419

dorsal body-wall is tunnelled, from end to end, by a median
longitudinal zeuva/ cavity, in which the central nervous
system is contained. The greater part of the cavity is
narrow and cylindrical, and contains the spinal cord (Figs.

ro2 and 108, sf. cd): its
anterior or cerebral portion
is dilated, and contains the
brain.

Skeleton. —- Imbedded in
the body-wall and extend-
ing into the fins are the
various parts of the emdo-
skeleton. This character-
istic supporting framework is
mainly composed, as in the
tadpole and in embryos of
Vertebrates generally, of
cartilage, which may be more
or less impregnated with
lime salts, so as to have, in
part, the appearance of bone,
but in most parts differing in
structure from true bone and
consisting merely of calcified
cartilage (p. 46).

The entire skeleton con-
sists of separate pieces of
cartilage, calcified or not,
and connected with one

another by ligaments (p. 57) :

Fic. 102. — Diagrammatic transverse
section through the trunk of a
female dogfish. The ectoderm is
dotted, the endoderm radially
striated, the mesoderm evenly
shaded, and the ccelomic epithelium
represented by a beaded line.

Card. V. cardinal vein ; Ceé. ccelome ;
Cal. Epthm. parietal, and Cel,
Epthi'. visceral layer of coelomic
(peritoneal) epithelium; D. Ao.
dorsal aorta; Derm.derm; Deri.
F. R. dermal fin-ray ; D. /, dorsal
fin; /. iat. V. ventral intra- intes-
tinal vein; /#¢, intestine; K. kid-
ney; Zaft. V. lateral vein; M.
myomeres ; 4. A. neural arch; V
c@, central canal of spinal cord ;
Nh. nephridium; Ozvd. oviduct ;
Ovy. ovary; Sp. Ca. spinal cord ;
Ur. ureter; V’. Cent. vertebral cen-
trum. (From Parker's Elementary
Biology.)

as in the frog, it is divisible

into skull, vertebral column, and skeleton of the paired jins,

with their arches or girdles; in connection with the skull

are certain cartilaginous visceral arches, forming the upper
EE2

420 THE DOGFISH CHAP, IX

and lower jaws and supporting the gills; and there are also
skeletal parts in the median fins.

The cranium or brain-case (Fig. 103, Cr) is an irregular,
cartilaginous box containing a spacious cavity for the brain,
very similar to the chondrocranium of the frog (p. 43). It
is produced into two pairs of outstanding projections: a
posterior pair, called the auditory capsules (aud. cp), for the
lodgment of the organs of hearing, ridge-like projections
on which indicate the position of the semicircular canals
(p. 187); and, in front of the brain-cavity, an anterior
pair, the olfactory capsules (olf. cp), for the organs of smell,
open below, and separated from one another by a septum.
Between the olfactory and auditory capsules, on either side,
the cranium is hollowed out into an ovéz¢ (or), bounded by
a supra-orbital and a sub-orbital ridge, for the reception of
the eye. In front the brain-case is produced into three carti-
laginous rods forming the rostrum (rv) and supporting the
snout. On its posterior face is the foramen magnum
(p. 40), on each side of which is an oval condyle for articu-
lation with the first vertebra. On the roof of the skull,
between and behind the olfactory capsules, there is a fon-
taneile (p. 43), closed over by connective tissue only ; and
between the two auditory capsules is a depression into
which open the two endolymphatic ducts of the ears
(p. 187). The nerve apertures will be referred to at a later
stage (Dp. 445).

In the human and other higher vertebrate skulls the
upper jaw, as we have seen to be the case in the frog (Fig.
9), is firmly united to the cranium, and the lower alone is
free. Butin the dogfish both jaws (ms. 7, 7. 7) are connected
with the cranium by ligaments (2g, Ze’) only, and each consists
of strong, paired (right and left) moieties, united with one
another by fibrous tissue. The upper jaw, which corre-

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423 THE DOGFISIi CHAP.

sponds to the palatoguadrate cartilage of the frog (p. 44),
presents at its posterior end a rounded surface against
which fits a corresponding concavity on the lower jaw, so that
a free articulation is produced, the lower jaw, or AMeckel’s
cartilage (p. 44) working up and down in the vertical plane.

The upper and lower jaws of either side correspond to
the first of a series of seven pairs of visceral arches, and are
therefore often spoken of as the mandibular arch. The
remaining six pairs have the form of cartilaginous half-hoops,
lying in the walls of the pharynx, and united with one
another below so as to form a basket-like apparatus support-
ing the gills. The second of these arches is distinguished
as the Ayotd, and is situated immediately behind the jaws.
It consists of two parts: a strong, rod-like hyomandibular
(Fig. 103, Ay. wm), which articulates above with the auditory
capsule and is connected below by fibrous tissue with the
jaws, thus helping to suspend them to the cranium and
serving as a suspensorium (p. 40); and a hyotd cornu or
horn, which curves forwards inside the lower jaw, and is
connected with its fellow of the opposite side by a
median dasi-hyal plate which supports the tongue.

The remaining five arches (47. a. 1—br. a. 5) are called
the dranchial arches. Each is formed of several separate
pieces, united by fibrous tissue so as to render possible the
distension of the throat during swallowing: the fifth is
connected below with its fellow by a large median dasi-
branchial plate, which supports the roof of the pericardial
cavity. Both they and the hyoid give attachment to delicate
cartilaginous dranchial rays (br. 7, br. r', Fig. 109, 7) which
support the gills.

In the frog-tadpole the gills are similarly supported by cartil-
aginous arches, which become greatly reduced and modified at meta-

IX VERTEBRAL COLUMN 423

morphosis : the tongue skeleton of the adult (p. 44) represents the
hyoid and first branchial arch.

In addition to the parts of the skull described above there are certain
small cartilages of minor importance in relation with the nostrils,
spiracle, mouth, and outer sides of the branchial arches (2.g¢., 2 and
ex, br in Fig. 103).

The vertebral column has the general character of a
jointed tube surrounding the spinal portion of the neural

Fic. 104.—Vertebree of Scyllinm canicula.
A and B from the trunk, C and D from the middle of the tail; A and C two
vertebra in longitudinal section ; B and D single vertebre viewed from one end.
4. bony portion of centrum; c. centrum; for. foramen for dorsal, and for". for
ventral root of spinal nerve ; 4.a@. heemal arch; 4. c. heemal canal; 4. sf. hemal
spine ; 7. 2. Z. interneural plate ; 7. a. neural arch ; 2, c. neural canal ; x. Z. neural
plate ; 2. sf. neural spine ; zc. intervertebral substance (remains of notochord)
tr. pr. transverse process ; 7. proximal portion of rib.

canal. Lying beneath this cavity, z.e., between it and the
coelome, isa longitudinal row of biconcave or amphicelous
discs, the vertebral centra (Fig. 104, ¢; Fig. 108, cz): they

are formed of cartilage, but have their anterior and posterior
EE **

424 THE DOGFISH CHAP.

faces converted into bone. The biconcave intervals
between them are filled with a soft ¢ntervertebral substance
(Fig. 104 zc), which is also present between the first
vertebra and the skull and which represents part of the
embryonic notochord (see pp. 203 and 404). The centra
are united by ligament, so that the whole vertebral chain
is very flexible.

In the frog it will be remembered that the completely ossified centra
are procalous, and are articulated with one another (pp. 36 and 57).

Connected with the dorsal aspect of the series of centra
is a cartilaginous tunnel, consisting of the neural arches
(Figs. 104 and 108, 7. a) enclosing the spinal cord: it is
divided into segments, corresponding with, but usually twice
as numerous as the centra owing to the presence of inter-
calary pieces. Arising from each centrum on either side is
a neural process, which in the adult fuses with a neural plate
(Fig. 104, 7.) and is perforated posteriorly for the exit of
the ventral root of a spinal nerve ( for’) ; and fitting between
two consecutive neural plates is an intercalary piece, the
interneural plate (i.n.~), perforated by an aperture for the
dorsal root of a spinal nerve (for). The arch is completed
above by the zexral spines (n. sp), which fit-in between the
neural and interneural plates respectively, and are thus, like
the lateral elements of the arch, twice as numerous as the
centra. The first vertebra has facets for articulation
with the condyles of the skull.

In the anterior part of the vertebral column the centra
give off paired outstanding transverse processes, (Fig. 104
B, tr. pr), to the end of each of which is articulated a
short, cartilaginous rod, the 77 (7). Further back the
transverse processes are directed downwards, instead of
outwards, and in the whole caudal region they unite

IX VERTEBRAL COLUMN 425

below, forming Aemad arches and spines (Fig. 104 D, 4%. a,
A. sp., and Fig. 108, 4. a), which together constitute a kind
of inverted tunnel in which lie the artery and vein of
the tail. In the region of the caudal fin the hzmal spines
are elongated and act as supports for the fin. A centrum,
together with the corresponding neural arch and transverse
processes, or heemal arch, form a vertebra or single segment
of the vertebral column.

In the frog we have seen that there are no independent ribs, and that
the caudal vertebree are represented by a single bone, the urostyle (p. 39).

It should be noticed that in the vertebral column we have
another instance of the metameric segmentation of the
vertebrate body. The vertebrz do not, however, correspond
with the myomeres, but alternate with them. ‘The myo-
commas are attached to the middle of the vertebra, so
that each myomere acts upon two vertebrae and thus
produces the lateral flexion of the body.

In the embryo dogfish, as in the tadpole, before the development of
the vertebral column, an unsegmented, cellular rod with an elastic
sheath, the notochord, resembling that of Amphioxus (p. 404), lies be-
neath the neural cavity in the position occupied in the adult by the line
of centra, by the development of which it is largely replaced. Carti-
lages appear round the notochord, which on the one hand give rise to
the arches, and on the other constrict the notochord at regular inter-
vals, so as to replace it completely in those regions which will form the
middle parts of the vertebral bodies, leaving the notochordal cells in
the biconvex spaces between the centra (Fig. 104, zc). Thus much of
the notochord persists as the soft intervertebral substance.

The skeleton of the median fins consists of a series of
parallel cartilaginous rods, the fix-rays or prerygiophores, the
proximal ends of which are more or less fused together to
form basal cartilages or Jasalia. The free edges of the fins

426 THE DOGFISH CHAP.

are supported by a double series of delicate horn-like fibres,
the dermal fin-rays.

The paired fins are also supported proximally by cartila~
ginous pterygiophores, fused together at their bases to form
basal cartilages which articulate with the corresponding
arch or girdle, and distally by horny, dermal fin-rays. The
pelvic arch (Fig. 105, BP) is a transverse bar of cartilage
situated just in front of the vent, and representing the pubic
and ischiatic portions of the girdle in the frog, an iliac
region, extending dorsally, and coming into connection

Fic. 105.—Diagram of the Elasmobranch pelvic arch (BP, Cep, PP, Sy) and fin.

Bas. basal cartilage; Fo’. nerve foramen; /. iliac process; Pre. anterior ray

articulating directly with the arch; Rad. the remaining radial cartilages. (From
Wiedersheim.)

with the vertebral column, being hardly represented. On
its posterior edge are articular facets for the pelvic fins, _
each of which has a single very large basal cartilage (Bas),
but one or two of the anterior rays (70) may articulate
separately with the arch. In the male, the skeleton of the
clasper (p. 417) is connected with the distal end of the
basal cartilage.

The shoulder-girdle (Fig. 106) is a strong inverted
arch of cartilage situated just behind the last branchial

ix PECTORAL FIN 427

arch. On each side of its outer surface it presents three
articular facets for the pectoral fin; the presence of
these allows of the division of each side of the arch
into a narrow, pointed, dorsal portion corresponding to
the scapular region of the frog (fc¢. g), and a broader

Fic 106.—Ventral view of pectoral arch o. S¢yddium with right pectoral fin.

The pectoral arch is divisible into dorsal or scapular (c¢. g) and ventral or cora-
coid (Zcz.g’) portions separated by the articular facets (a7¢. /) for the fin.
The pectoral fin is formed of three basal cartilages (4s. 1—3) and numerous
radials (vad); its free edge is supported by dermal rays (¢.4 7», (Modified from
Marshall and Hurst.)

ventral portion, answering to the coracoid ( Act. g') united
in the middle line with its fellow of the opposite side: there
is no sternum. The pectoral fin is formed of ptery-
giophores (vad), fused proximally to form basals which are
three in number (ds. 1.—3), the third, like the main
basal of the pelvic fin, being the largest and supporting
the greater number of the cartilaginous rays, which give
rise distally to a series of polygonal plates.

428 THE DOGFISH CHAP.

It will be noticed that while the skeleton of the crayfish is a series of
articulated tubes, with the muscles inside them, that of the dogfish and
frog is a series of articulated rods with the muscles outside. The joints,
formed by two rods applied at their ends and bound together by
ligament, are not confined to movement in one plane, like the hinge-
joints of the crayfish, but are capable of more or less rotatory move-

ment.

Digestive organs.—The mouth, as we have seen, is a
transverse aperture bounded by the upper and lower jaws.

Fic. 107.—Diagram of the development of a tooth.
Bg, Bg. mesoderm; DS. dentine; £M. epithelium of mouth ; AZa. epithelium of
enamel organ ; O. odontoblasts ; SA. dental lamina; 7X. dental papilla. (From
Wiedersheim’s Comparative Anatomy.)

In the mucous membrane covering the jaws are im-
bedded large numbers of Zee¢h—conical, calcified bodies,
with enamelled tips, arranged in transverse rows. They
are to be looked upon as special developments of the

1X DIGESTIVE ORGANS 429

placoid scales or dermal teeth enlarged for the purpose
of seizing prey, and are continually renewed on the inner
sides of the jaws as they are worn away on the outer sides.

The teeth, in Vertebrates generally, are developed in the following
manner. The ectodermal epithelium of the mouth (stomodeum, p. 204)
—or in the case of the dermal teeth of the dogfish that covering the body
generally—grows inwards to form a ridge or dental lamina (Fig. 107,
S) which projects into the underlying mesodermal connective tissue and
becomes enlarged distally to form a bell-shaped examel-organ, into the
base of which a mesodermal dental papilla (ZK) extends : the superficial
part of this papilla forms a layer of cells known as odontoblasts (OQ).
The dentine (DS) is formed, in successive layers, from the odontoblasts,
and gradually accumulates between them and the epithelium lining the
interior of the enamel organ (fa), which gives rise, also in successive
layers, to the enamel, or hardest part of the tooth. Around the base
of the papilla more or less bony matter—the cement—is formed. It
will thus be seen that while the teeth are mainly mesodermal structures,
a part of them—the enamel—is ectodermal in origin.

The mouth leads into an oral cavity, on the floor of
which is a rudimentary éongue (Fig. 108, éng) capable of
very little movement, and which passes insensibly into the
throat or pharynx (ph), distinguished by having its walls
perforated by five pairs of slits, the ¢xternal branchial aper-
tures (i. br. a) as well as by the inner opening of the
spiracle (sf). The pharynx is continued by a short gullet
(guZ) into a capacious, U-shaped stomach, consisting of a
wide cardiac division (cd. s¢) and a narrow pyloric division
(pyl. st). The pyloric division communicates by a narrow
valvular aperture with the zutestine (int), a wide, nearly
straight tube having its lining membrane produced into
a spiral fold, the spiral valve (sp. vl), which practically
converts the intestine into a very long, closely-coiled tube,
and greatly increases the absorbent surface. Finally the
intestine opens into a large chamber, the ¢/oaca (cl), which
communicates with the exterior by the vent.

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CHAP. IX DIGESTIVE ORGANS 431

From the gullet backwards the enteric canal is contained
in the abdominal division of the ccelome, to the dorsal wall
of which it is suspended by a median mesentery (Figs. 102,
and 108, mes). The greater part of the canal is developed
from the enteron of the embryo, and is consequently lined by
endoderm ; the oral cavity is formed from the stomodzum,
and the cloaca from the proctodeum (p. 204). Outside the
enteric epithelium are connective tissue and muscular layers,
the latter formed of unstriped fibres: it is generally charac-
teristic of vertebrates that the voluntary muscles are striped,
the involuntary unstriped (compare pp. 111 and 112).

The immense “ver, divided into two lobes (4/, 7./r), is
situated below the stomach along the whole length of the
abdomen, to the wall of which it is attached by a fold of
peritoneum. It discharges its secretion, the dz/e, into the
anterior end of the intestine by a tube, the dz/e-duct, which
gives off a blind offshoot terminating in a gallbladder
(p. 69).

-The pancreas (pan) lies against the anterior end of the
intestine, with which it communicates independently by the
pancreatic duct. Opening into the hinder part of the in-
testine or rectum, is a small finger-like rectal gland (ret. g/),
the function of which is uncertain. In addition to these
there are, as in all Vertebrates, minute tubular gastric glands
sunk in the mucous membrane of the stomach (p. 131).

The spleen (sf/) is an irregular, dark-red, gland-like body,
of considerable size, attached by peritoneum to the stomach
(compare pp. 23 and 98).

Other so-called ‘‘ductless glands’’—which are also represented in
the frog, and the functions of which are not thoroughly understood—
are the /kyrod in the throat and the ¢hymus in connection with the
dorsal ends of the branchial arches; there are also suprarenal and’
interrenal bodies in relation with the kidneys (compare p. 145).

432 THE DOGFISH CHAP.

Respiratory Organs. The gi//s consist of five pairs of
pouches, ‘each opening by one of the internal branchial
apertures (Fig. 108, 7. dv. a) into the pharynx, and by one
of the external branchial apertures on the exterior. The
walls of the pouches, or zzter-branchial septa, are supported
by the visceral arches and _ bran-
chial rays (Fig. 103, dr. r and
109, 7), and are lined with mucous
membrane raised into horizontal
ridges, the branchial filaments,
which are abundantly supplied with
blood-vessels and are the actual
organs of respiration. As the fish
swims, water enters the mouth and
passes by the internal clefts into the
branchial pouches, and thence out-
wards by the external clefts, aconstant

, supply of oxygen being thus ensured.
Fic. 109.—Transverse section z
through a gill of an Ela- The gill-pouches are developed as

a, afferent branchial artery; offshoots of the pharynx, and

&. branchial arch; 4/1, 4 ; i P
anterior, and 42. pos. the respiratory epithelium is there-

teri hemibranch ;_ 4.
septum ; 7. branchial fore endodermal, not ectodermal,

ray ; v. efferent branchial

astery, (From R. Hert- aS in the crayfish and mussel (com-
rane ORE) pare also pp. 204 and 207).

As already mentioned, the walls of the pharynx are supported by the
cartilaginous visceral arches, which surround it like a series of incom-
plete hoops, each half-arch being embedded in the inner or pharyngeal side
of an interbranchial septum. Thus the visceral arches alternate with the
gill-pouches, each being related to the posterior set of filaments of one
pouch and theanterior set of the next. Anentire gill or holobranch therefore
consists of two half-gills or heszbranchs—the sets ot branchial filaments
belonging to the adjacent sides of two consecutive gill-pouches (Fig. 109).
On the other hand, a gill-pouch is equivalent to the posterior hemibranch
of one gill and the anterior hemibranch of its immediate successor.

1X HEART 433

The first pouch is situated between the hyoid and the first branchial
arch, and the hyoid thus bears a hemibranch only. The first four
branchial arches bear each a holobranch, and the fifth is without gill-
filaments. There is a vestigial hemibranch, or psendobranch, on the
anterior wall of the spiracle.

Now it is known that parts which have become useless tend to dis-
appear more or less completely (¢.g. pineal body of the frog—p. 159,
and certain of the gills in the crayfish—p. 361). In some cases, how-
ever, such vanishing parts take on new relations with other organs and
thus once more become useful in other ways, undergoing a change of
function, Thus in higher Vertebrates the spiracle is utilised in con-
nection with the auditory organ, and instead of disappearing entirely,
as do the other gill-clefts, it Bee rise to the tympanic cavity and
Eustachian tube.

Circulatory organs. The heart is situated in the peri-
cardial cavity or anterior compartment of the coelome, and
is a large muscular organ composed of four chambers.
Posteriorly and dorsally is a small, thin-walled s¢zus venosus
(Fig. 108 and 110, s. v), opening in front into a single,
capacious, thin-walled auricle (au) ; two auricles are present
only in those Vertebrates which possess lungs. The auricle
communicates with a very thick-walled ventricle (v), from
which is given off in front a tubular chamber, also with
thick muscular walls, the conus arteriosus (c. art). There
are valves between the sinus and the auricle, and between
the auricle and ventricle, and the conus contains three
longitudinal rows of valves : all the valves are arranged so as
to allow of free passage of blood from sinus to auricle,
auricle to ventricle, and ventricle to conus, but to prevent
any flow in the opposite direction.

The conus gives off in front a single blood-vessel (v. ao),
having thick elastic walls composed of connective and
elastic tissue and unstriped muscle. This vessel, the ventral
aorta, passes forwards beneath the gills, and gives off on each
side paired lateral branches, the afferent branchial arteries

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CHAP, IX CIRCULATORY ORGANS 435

(Fig. 110,"a. dr.a). Each afferent artery passes to the corre-
sponding gill, and there branches out into smaller and
smaller arteries, which finally open into a network of delicate
capillaries (p. 95), with which the connective tissue of the
branchial filaments is permeated. The blood in these
respiratory capillaries is therefore brought into close relation
with the surrounding water, and as the blood flows through
them it exchanges its carbon dioxide for oxygen, obtained
from the air dissolved in the water.

From the respiratory capillaries the blood is collected
into minute arteries which join into larger and larger
trunks, and finally unite into efferent branchial arteries’ (e.
br. a) by which the purified blood is carried from the
gills. The efferent arteries of the right and left sides unite
in a median longitudinal artery, the dorsal aorta (d. ao),
which passes backwards, immediately beneath the vertebral
column, to the end of the tail.

From the efferent branchial arteries and the dorsal aorta
are given off numerous arteries supplying the whole of the
body with blood. The most important of these are paired
carotid arteries (c. a) to the head, and sudclavians (scl. a) to
the pectoral fins ; unpaired splanchnic arteries (cla, ms.a), to
the enteric canal, liver, pancreas, and spleen; numerous
paired venals (r. a) to the kidneys, spermatic (sf. a) or
ovarian arteries to the gonads, and a pair of iliacs (72. a) to
the pelvic fins. The posterior part of the dorsal aorta,
supplying the tail, is contained in the hzemal canal of the
caudal vertebrze, and is often spoken of as the caudal artery
(ed. a).

The arrangement of the arteries in the tadpole (p. 206) is very similar
to that described above, and the diagram (Fig. 110) would serve almost
equally well for a tadpole as for afish. In the former there are four
pairs of afferent and efferent branchial arteries (corresponding to the

F F*

436 THE DOGFISH CHAP.

second to the fifth in Fig. 110) in relation with the corresponding
branchial arches and their gills. The afferent and efferent vessels at
first communicate with one another through the respiratory capillaries,
but later on cach afferent becomes directly connected with the corre-
sponding efferent artery; and at metamorphosis, when the gills gradually
disappear, all the blood thus passes
directly from the ventral to the dorsal
aorta, through the four arterial arches
(Fig. 111). The first arch (1) gives
rise in the adult frog to the carotid
trunk (ca), and loses its connection with
the second at the dorsal end; the
second (2) forms the systemic trunk ;
the third (3) disappears ; and the fourth
(4), losing its connection with the
dorsal aorta, forms the pulmo-cutaneous
trunk (2).

Having now traced the main course
and arrangement of the chief arteries,
there are a few minor points of detail
to be noticed in the dogfish (Fig. 112).
The five afferent branchial arteries
(af. br 15) of either side do not arise
regularly and symmetrically fron: the
Big ite Diagtain ofthe arterial ventral aorta, as represented in the
arches of an Amphibian. diagrammatic Fig. 110, The anterior
an ang = Nee aru end of the ventral aorta divides into
sponding branchial arches; yight and left branches, each of which

ao, dorsal aorta ; c. a. carotid ; ae
artery; &, embryonicarterial again subdivides to form the first two
oat We byes a afferent branchial arteries, which supply
monary artery; sf.’ ventral respectively the hemibranch on the

aorta. (From Wiedersheim's 7

Anatomy, after Boas.) hyoid arch and the holobranch of the
first branchial arch. The third afferent
branchial artery arises from about the middle of the ventral aorta, and
supplies the holobranch of the second branchial arch ; a short distance
behind it, the fourth and fifth come off close together, and supply the
gills on the third and fourth branchial arches respectively : it will be

remembered that the fifth branchial arch bears no gill-filaments. After

Ix CIRCULATORY ORGANS 437

aeration, the blood from each hemibranch passes into an efferent
branchial artery (ef. br), which, except in the case of the last (ef 47°),
joins with its fellow of the same cleft and thus forms a loop surround-
ing the cleft, the two halves of adjacent loops being connected in the
middle by a commissural vessel. From the upper ends of each of the
four loops arises an eptbranchial artery (ep. br) which is connected with

Fic. 112.—The heart and branchial arteries of Scyddium, from the side,
af, by, \—5, afferent branchial arteries ; az. auricle ; ¢. a. conus arteriosus ; ¢/.1—5,
branchial clefts ; cor. coronary artery ; d. ao. dorsal aorta; d.c. dorsal carotid
artery ; ef br. 19, efferent branchial arteries ; ef. 6r.1—4, , epibranchial arteries ;
nin. mandibular artery : sp. spiracle ; s. c/. subclavian artery ; ; s. v. Sinus venosus ;
v. ventricle ; v. ao. ventral aorta; v. ¢. ventral carotid artery.

the dorsa/ aorta (d. ao), the blood from the last hemibranch passing
into the fourth loop.

From the dorsal end of the first efferent branchial, a dorsal carotid
artery (d. c) is given off: this passes forwards and inwards, gives off a
cranch to the upper jaw and snout, and then runs inwards inagroove on
the skull-floor, which it penetrates in the middle line so as to reach the
tranial cavity. A vessel arises from the middle of the first efferent
branchial, and supplies the pseudobranch, from which the bload is col-
lected by a ventral carotid artery (v.c) which passes through the orbit
into the cranium, giving off branches to the brain and anastomosing
with the dorsal carotid. From the ventral end of ‘the first efferent

branchial a small mandibular artery (mn) passes to the lower jaw. The
FR*t*

438 THE DOGFISH CHAP. 1X

dorsal aorta is continued forwards, anteriorly to the first epibranchial
artery, as a slender vessel which soon bifurcates and anastomoses with
the dorsal carotid. From the ventral ends of the efferent branchial loops
small arteries are given off which supply the lower parts of the head,
the branchial region, and the heart (cor). The subclavian arteries
(s. cZ) arise from the aorta just before it is joined by the last epi-
branchials.

In the short-bodied frog we have seen (p. 80) that there is only a
single splanchnic artery, which soon divides into a cceliac and a mesen-
teric. In the dogfish there are four splanchnic arteries, arising sepa-
rately from the dorsal aorta, viz., a ce/iac, supplying the proximal ends-
of the stomach and intestine, the liver, and pancreas ; an andertor mesen-
teric, arising a short distance further back and supplying the intestine,
&c. ; a Leno-gastric, coming off from the aorta close behind the anterior
mesenteric and going to the spleen and part of the stomach and pan-
creas ; and a small posterzor mesenteric supplying the rectal gland.

As in all Vertebrates, the arteries branch and branch again
in the various parts to which they are distributed, their ulti-
mate ramifications opening into a capillary network with
which all the tissues except the cartilages and epithelia are
permeated. From these systemic capillaries the blood is
collected into larger and larger efferent, thin-walled, trunks or
veins, parts of many of which are greatly dilated to form sinuses.

The blood from the head is brought back by a pair of
jugular veins (Figs. 110, 7. v. and 113 jug. v): each of these
enters a large precaval sinus ( pr. cv. v, dct. c) which passes
vertically downwards and enters the sinus venosus. The blood
from the tail is returned by a caudal vein (cd. v, caud. v)
lying immediately beneath the caudal artery in the hxemal
canal ; this vessel enters the coelome and then divides into
right and left branches, the venal portal veins (r. p. 2,
r. port. v), which pass to the kidneys and join with the
capillaries of these organs, the impure blood brought from
the tail mingling with the pure blood of the renal arteries
(Fig. 110, 7a). From the kidneys the blood is returned

bru

ornv3

brot

‘ bros
infijugv infyug.u

rn port.v

Fic. 113.—Diagram of the chief veins together with the ventral aorta and afferent
branchial arteries of a dogfish (Cheiloscyllium).

ali. enteric canal; 47. v.—ér. v.5 afferent branchial arteries; caud.v. caudal
vein; dct. c. precaval sinus ; £z. heart; 4. dort. v. hepatic portal vein ; hep. s.
hepatic sinus; imf.jug.v. inferior jugular vein or sinus; ug. jugular vein or
sinus ; Zat. v. lateral vein; Ziv. liver; 2. curd. s. left cardinal sinus and vein;

7. port. v. left renal portal vein; efh. kidney; ~. cavd.s. right cardinal sinus
and vein; ~ ort. v. right renal portal vein. (From Parker and Haswell's

Zoology.)
FRF*e**

440 THE DOGFISH CHAP.

into a pair of cardinal veins (crd. v) which pass forwards,
become swollen to form sinuses (Fig. 113, 2. card. s, r. card. s),
receive veins from the reproductive organs, muscles, &c., and
finally enter the precaval sinus.

From the stomach, intestine, spleen, and pancreas the
blood is collected by numerous veins, which all join to form
a large hepatic portal vein ((Fig. 110, #. ~. 0, Fig. 113,
h. port. v). This behaves in the same way as the renal
portal: instead of joining a larger vein on its way to the
heart, it passes to the liver and breaks up to connect with
the capillaries of that organ; its blood, deprived of oxygen
but loaded with nutrient matters from the enteric canal,
mingling with the oxygenated blood brought to the liver by
a branch of the cceliac artery. After circulating through
the capillaries of the liver the blood is taken by a pair of
hepatic sinuses (h. v, hep. s) to the sinus venosus.

The course and arrangement of the veins, like that of the arteries, is
very similar to that existing in the tadpole, in which several important
changes occur at metamorphosis. With the disappearance of the tail
and caudal vein, the renal portal veins receive their blood from the
hind-limbs only. The hinder parts of the two cardinal veins, situated
between the kidneys, fuse into one, and their anterior parts disappear,
a new vessel being developed which conducts the blood from the fused
cardinals to the sinus venosus: the whole of the great vein thus
formed is the fostcaval (p. 82, fig. 21), which is present in all
Vertebrates above the fishes.

The zliac veins of the dogfish (Fig. 110, z/. v) pour the
blood from the pelvic fins into the /ateral veins (Figs. 110
and 113, /a¢. v)—paired trunks running forwards in the side
walls of the body to the sinus venosus, and receiving at
their anterior ends the subclavian veins (sci. v) from the
pectoral fins. In addition to the dorsally situated jugular
veins, there are paired inferior jugulars (Fig. 113, inf. jug. v),

IX CIRCULATORY ORGANS 441

bringing back the blood from the ventral parts of the head,
and each opening into the corresponding precaval.

As we have seen, several of the veins, ¢.g., the precavals,
jugulars, cardinals, and the genital veins, are dilated into
spacious cavities called sinuses (Fig. 113). These are,
however, of a totally different nature from the sinuses of the
crayfish, which are mere spaces among the tissues devoid of
proper walls. In the dogfish, as in the frog and Vertebrates
generally, the blood is confined throughout its course to
definite vessels ; ‘the heart, arteries, capillaries, and veins
invariably forming a closed system of communicating
tubes.

The general course of the circulation will be seen to agree with that
already described in the frog, as well as in the crayfish and mussel, z.¢.,
the blood is driven by the contractions of the heart through the
arteries to the various tissues of the body, whence it is returned tg the
heart by the veins or sinuses (Fig. 114). But whereas in both ‘cray-
fish and mussel the respiratory organs are interposed in the returning
current—both their afferent and efferent vessels being veiris, in the dog-
fish they are interposed in «the outgoing current—their afferent and
efferent vessels being arteries. An artery, it must be remembered, is
a vessel taking blood from the heart to the tissues of the body and
having thick walls; a vein is a thin-walled vessel bringing back the
blood from the tissues to the heart.

Moreover, the circulation in the dogfish is, as in the frog, compli-
cated by the presence of the two portal systems, renal and hepatic. In
both of these we have a vein, renal portal or hepatic portal, which,
instead of joining with larger and larger veins and so returning its
blood directly to the heart, breaks up, after the manner of an artery,
in the kidney or liver, the blood finding its way into the ordinary venous
channels after having traversed the capillaries of the gland in question.

Thus an ordinary artery arises from the heart or from an artery of
higher order and ends in capillaries; an ordinary vein arises from a
capillary network, and ends in a vein of higher order or in the heart.
But the hepatic and renal portal veins end in capillaries after the man-
ner of arteries, and the efferent branchial arteries begin in capillaries
after the manner of veins.

442 THE DOGFISH CHAP.

With regard to the general morphology of the blood-system, the
dorsal aorta with the caudal artery may be considered as a dorsal vessel
(compare Earthworm, p. 329, Crayfish, p. 365, and Amphioxus, Fig.
101); the caudal vein, hepatic portal vein, and ventral aorta as together
representing a ventral vessel; the afferent and efferent branchial
arteries as commissural vessels; and the lateral veins as lateral vessels.
It will be seen that the heart of Vertebrates is a muscular dilatation of
the ventral vessel, as is also shown by a study of its development.

The blood, like that of the frog (pp. 104106), consists
of a colourless plasma containing red corpuscles—the colour

Fic. 114.—Diagram illustrating the course of the circulation in the dogfish. Vessels
containing oxygenated blood red ; non-oxygenated blood blue.

B. capillaries of the body generally ; E. of the enteric canal ; G. of the gills; K. of
the kidneys; L. of the liver; T. of the tail. a. 4x. a, afferent branchial arteries ;
au. auricle ; c. a. conus arteriosus ; d. ao. dorsal aorta ; ¢. bv. a. afferent branchial
arteries ; 4. f.v. hepatic portal vein ; 2.v. hepatic vein ; dc. lacteals ; Zy. lymph-
atics ; fv. cv.v. precaval vein; 7.f.v. renal portal veins; s. v. sinus venosus ;
v. ventricle ; v. ao. ventral aorta. The arrows show the direction of the current.
(From Parker and Haswell’s Zoology.)

of which is due to haamoglobin—and leucocytes. It must be
remembered that the ventral aorta and the afferent branchial
arteries (Fig. 114), like the pulmonary artery of the frog
(p. 144), contain venous blood. As in the frog, there are
in addition to the blood-vessels, a set of /ymphatic vessels.
Nervous System.—The nervous system is constructed on
a similar plan to that of the frog (compare Part I, Chapter X)

IX BRAIN 443

and of Vertebrates in general. The central nervous system
is dorsal in position and consists of a dvaiz contained within
the cranial cavity, and continuous posteriorly with a spinal
cord contained in the neural canal of the vertebral column:
it consists of grey and white matter, and its cavity or
neurocele, lined with epithelium, gives rise to the ventricles
of the brain and to the central canal of the spinal
cord.

In correspondence with the form of the body, the spinal
cord is relatively much longer than in the frog, and it is not
swollen opposite the paired appendages.

In the brain (Fig. 115) the 4/6 or medulla oblongata
(VZ#) broadens out anteriorly to form lateral swellings, and
its contained fourth ventricle (F. rho) is roofed over by the
pia mater. The cerebellum (HF), which is very small in the
frog, is here relatively enormous, and its surface is marked
by slight grooves: it overlaps the bulb behind and the optic
lobes in front, and contains a ventricle communicating
with the fourth ventricle. The oval optic lobes (MH) are
hollow, their cavities communicating with the median
ventricle or z¢er (p. 157) ; and ventrally to them are the crura
cerebri. The diencephalon (ZH) is relatively narrower than
in the frog. From its thin roof, which covers over the
third ventricle, is a delicate tube-like structure (G), which
extends upwards and forwards and ends in a small knob
attached ‘to the roof of the skull: this is the pzmeal body
(p. 159). From the ventral surface of the diencephalon
arises the infundibulum, with an oval swelling on either
side, to which is attached the prtudtary body with a vascular
sac on each side of it and a median tubular body on its
ventral surface. In front of the infundibulum is the optic
chiasma. OT

Apart from the large size of the cerebellum, the most

144 THE DOGFISH

CHAP.

marked difference between the brain of the dogfish and that
of the frog is seen in its anterior portion. In the frog, the

Fic. 115.—Dorsal view of the brain of Seydlium
canicula.

The posterior division of the brain is the medulla
oblongata (Vf), enclosing the fourth ventricle
(F. rho). The large cerebellum (7/7) nearly
covers the optic lobes (7H) The diencephalon
(ZH) shows in the middle the third ventricle,
and the place of attachment of the pineal stalk
(Gp). The prosencephalon (V7) gives off the
olfactory lobes (770. L.ol). The origins of
the following nerves are shown :—optic (/7),
trochlear (//"), trigeminal (VY), facial (VZ/),
auditory (V///), glossopharyngeal (7X), and
vagus (X). (From Wiedersheim’s Anatomy.)

diencephalon is con-
tinuous _ anteriorly
with the paired cere-
bral hemispheres
(159, Fig. 49): in the
dogfish there is in
this region a relatively
smaller, unpaired por-
tion of the brain,
marked in front by
a slight groove, and
known as the Jrosen-
cephalon (VH),which
represents the cere-
bral hemispheres of
the higher Verte-
brates but which does
not become subdi-
vided externally into
paired lobes. Ante-
riorly it gives off,
right and left, a large,
oval olfactory lobe (L.
of) each connected
with the prosen-
cephalon by a short,
stout stalk (Z7o) and
applied distally to
the corresponding ol-
factory capsule. The

prosencephalon contains paired lateral ventricles, which

Ix NERVES 445

communicate posteriorly with the third ventricle and ante-
riorly are continued into the olfactory lobes.

Each spinal nerve arises, as in the frog (p. 163), by two
roots, which, however, are not in the same transverse plane,
the dorsal root being slightly anterior to the corresponding
ventral root, so that successive dorsal and ventral roots of
either side alternate with one another: they also alternate
on the right and left sides. As already mentioned (p. 424),
the two roots of each nerve passes out from the neural
canal independently, uniting on the outside of the
canal to form the spinal nerve. A sympathetic is repre-
sented. Q

The origin and distribution of the cerebral nerves is in
the main similar to that already described in the case of the
frog (p. 163), the chief differences, characteristic respectively
of air-breathing and of branchiate Vertebrates in general,
being ase follows.

In fishes, there are certain nerves, usually considered as
belonging to the facial and vagus, which supply the sensory
canals of the integument (Fig. 116, VZZop, b, e.m, and XZ):
these organs are not present in terrestrial forms, and their
flerves are consequently also wanting. The vagus, more-
over, gives off a series of branchial branches (47.1—-*) to
the gills instead of a pulmonary branch, and the glosso-
pharyngeal (ZX) is also a branchial nerve.

The olfactory nerves (Fig. 116, I) arise from the olfactory lobe o
each side, which is situated in a large aperture in the skull communicat-
ing between the cranial and olfactory cavities. The ofézc nerve (II) is
continued outwards from the optic chiasma, and passes through a
foramen in about the middle of the orbit, towards the ventral side. The
oculomotor (III), arising from the crura cerebri, makes ils exit from the
skull a short distance behind and slightly above the optic nerve. The
tathetic (1V), coming off from the dorsal side of the front end of
the bulb and supplying the superior oblique muscle, pierces the

cHaP,

THE DOGFISH

446

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Ix CEREBRAL NERVES 447

cranial wall almost directly above the optic foramen (compare also Fig.
117). All the other nerves arise from the ventro-lateral regions of the
medulla oblongata, the abducent, supplying the external rectus muscle,
coming offnearer the middle line than, and anterior to, the others. The.
abducent (V1) and the main parts of the ¢rigeménal (V) and factal(VII)
nerves pass out through a single foramen in the skull in the posterior
and ventral part of the orbit, just anterior to the auditory capsule. A
short distance above this foramen are two others, the ventral slightly
anterior to the dorsal: these transmit the ophtha/mdc branches (see
below) of the trigeminal and facial (V of, VII of) respectively, and
from them grooves pass along the dorsal side of the orbit to an aperture
just behind the olfactory capsule, the nerves emerging again on the
dorsal side of the skull. The auditory nerve passes through ~ large
foramen on the inner side of the auditory capsule to supply the mem-
branous labyrinth. The glossopharyngeal (IX) emerges behind the
auditory capsule at the posterior end of a horizontal groove in this
region, and the vagws (X) passes out through a foramen between the
glossopharyngeal and the foramen magnum.

The nerves supplying the integumentary sensory organs are as
follows: (1) The ophthalmic branch of the facial (VII of) runs, as we
have seen, dorsally to the similarly named branch of the trigeminal,
close under the skin, and supplies the sensory tubes and ampullz (see
p- 448) of the upper part of the snout ; those of the lower part of the
snout are innervated by (2) a duccal bvanch (VIL 6), which extends
along the floor of the orbit just above the maxillo-mandibular division
of the trigeminal ; and those in the region of the hyomandibular by a
small (3) external mandibular branch (VII e. m), arising from the large
hyomandibular nerve (see below). The lateral line canal, extending
along the body and tail, is supplied by (4) the dateral branch of the vagus
(X 2), which runs backwards to the inner side of the rest of the nerve
and dorsally to the spinal nerves, along the inner side of the body-wall,
giving off branches which extend outwards between the great lateral
muscles to the lateral canal.

The other branches of the facial are :—a small palatine (VII ), which
extends along the floor of the orbit, just behind the trigeminal, and sup-
plies: the roof of the mouth; andalarge hyomandibular (VII hy) which
passes behind the spiracle—first giving off small presperacular branches
(VII g. s) to its anterior wall, extends along the anterior border of the
auditory capsule and the posterior wall of the orbit, just beneath the

448 THE DOGFISH CHAP.

skin, to the anterior side of the hyoid arch: it thus forks over the
spiracular or mandibulo-hyoid cleft.

The glossopharyngeal (IX) forks above the first gill-cleft, thus giving
rise to two branches, one passing down the posterior side of the hyoid,
and the other down the anterior side of the first branchial arch. The
main part of the vagus extends backwards to the outer side of the
lateral nerve and gives off four branchial nerves (X br. 14) forking
over the second to the fifth gill-clefts respectively, and is then con-
tinued into the wésceral nerves (X v), which supply the stomach and
heart.

Sensory Organs.—The dogfish possesses, as we have seen,
a series of peculiar cutegumentary sense-organs supplied by
the nerves just described, the function of which is not
known with certainty. They are situated within a number
of epithelial canals, developed from the epiderm, the open-
ings of which on the head have already been noticed
(p. 410).

The tubes are of two kinds, known respectively as sensory and am-
bullary canals: the former, which are present in all Vertebrates with
gills (p. 414), are all continuous with one another and are situated along
certain definite regions in the head and jaws, a canal extending
along the body and tail as the Jateral line canal. The ampullary
canals, which are peculiar to Elasmobranch fishes, and which contain
a gelatinous material, are not continuous with one another, but run side
by side, converging to form large masses in the snout and at the sides
of the head; at their blind ends they are swollen to form ampulle, to
which the nerves are distributed. The sensory cells are arranged in
little conical masses in the lining epithelium of the canals or of the
ampulle, a section of one of which nearly resembles that of an ampulla
of a semicircular canal of the ear (Fig. 60).

The olfactory organs are a pair of cup-like sacs in the
snout, enclosed by the olfactory capsules and opening
externally on the ventral side by the xostrils. Notice that
there are no internal nostrils, as in the frog: these are only
present in Vertebrates which possess lungs. The sacs are
lined by the olfactory epithelium, which is supplied by the

IX SENSORY ORGANS 449

olfactory nerves and is raised up into ridges $0 as to increase
the surface.

The structure of the eye, as well as of the accessory
apparatus in connection with it, is in all essential re-
spects the same as in the frog (p. 181), except for the
differences in the eyelids (pp.5 and 410), the absence of
a lachrymal apparatus (p. 186), and for the fact that the
four recti muscles
(Fig. 117) do not en-
sheath the optic nerve,
which emerges into
the orbit a short dis-
tance in front of their
point of origin.

The membranous
labyrinth of the ear
(compare Fig. 509, p.
187) is also very simi-

lar to that of the frog,
Fic. 117.—Semidiagrammatic figure of the eye-
but being larger, and muscles and their nerves of an Klasmobranch.

1 III, oculomotor, JV. pathetic, and V/. abducent

the auditory capsules meee ae posterior rectus muscle ; 2.0. in-

j 7 erior oblique; 7.7. inferior rectus; 7. 7.

being composed en ene rectus : or.wall of orbit i a 0. superior

1 i i oblique ; s. 7. superior rectus. rom Parker
tirely of cartilage, it and Haswell’s Zoology.)

can be dissected out
with comparative ease by slicing away the capsule with
a knife.

A tube given off from the sacculus, called the endolymphatic duct
(Fig. 59), which in the frog communicates with the lymphatic
system, opens to the exterior on the top of the head in the dogfish,
and thus the endolymph isin free communication with the surrounding
sea-water.

As we have seen, the membranous labyrinth is the essesttéal part of
the ear, and it, together with its enclosing capsule, is often spoken

Pract. Zoot. GG

450 THE DOGFISH CHAP.

of as the zz¢ernalear. In the frog there is also an accessory apparatus—
the tympanic cavity and membrane, together with the columella—which
is called the mzddle ear (compare p. 49 and 189).

Urinogenital organs.—In order to understand the mor-
phology of the Addneys, and the close relations existing
in most Vertebrates between them and the generative
organs, it is necessary to know something of the develop-
ment of these parts. In the embryo, the kidneys appear
in the form of separate, segmentally arranged tubes having
the general character of nephridia, opening on the. one
hand by nephrostomes into the ccelome, and on the other
into a longitudinal duct which discharges into the cloaca.
Thus the primitive structure of the vertebrate kidney
furnishes another example of metamerism, which can no
longer be distinctly recognised in the adult kidney. (Figs.
46 and 47.)

At a later stage of development in most vertebrate orders
two longitudinal ducts can be recognised on either side,
which in some cases (e.g. Dogfish) are formed by the
subdivision of the single primary duct. These are known
respectively as the Wolffian and the Millerian ducts: the
former takes on the function of a spermiduct in the male,
although it may (¢.g., in the frog, p. 193) retain also its
function as a ureter; the latter gives rise to the oviduct
in the female.

In the dogfish the kidneys (Fig. 118, ¢f, 2) are long,
narrow, lobulated organs, lying close to the vertebral
column on either side, covered ventrally by the thick
peritoneum, and extending primarily along almost the
whole length of the ccelome. But in the course of de-
velopment, certain important modifications take place in
them and in their ducts (Wolffian ducts). In the
male, about the anterior half of the kidney takes on a

IX URINOGENITAL ORGANS 451

close relation with the generative organs, and gives rise to
a glandular body—the epididymis (A, ep)—with which the
long, convoluted Wolffian duct (sé@), serving exclusively
as a spermiduct, is closely connected ventrally : in the female,
this part of the kidney and its duct becomes vestigial (B,/’).
The hinder half of the embryonic kidney in each sex is
retained in the adult as the renal organ (4), which is some-
what swollen posteriorly. The ureters (#7) are independently
developed tubes, about five in number on each side. In
the female they open separately into the swollen persistent
posterior ends of the Wolffian ducts, which unite together to
form a median urinary sinus (B, u.s), opening by a single
aperture into the cloaca: while in the male (4, wr) most of
them unite to form a wide main ureter before communicat-
ing with a similar median sinus, which, as it receives the
products both of the spermaries and kidneys, is called the
urinogenital sinus (u.g.5).

The sfermaries are a pair of large, elongated, soft
organs united with one another posteriorly, and suspended
to the dorsal body-wall by a fold of peritoneum. From
the anterior end of each (A, 4s) arise delicate efferent ducts
(ef. d), which pass to the epididymis to become connected
with the convoluted spermiduct. The latter dilates pos-
teriorly, where it underlies the functional kidney, forming
an elongated, spindle-shaped seminal vesicle (s.v) which opens
(s.v’) into the base of a thin-walled blind reservoir of about
the same length, the sperm-sac (sf. s); and just to the inner
side of its aperture are the openings of the ureters (w7’). The
sperm-sac is continuous posteriorly with the urinogenital sinus,
the opening of which into the cloaca is situated on a papilla.

The female Scyllium has a single ovary.(B,ov), suspended
by a fold of peritoneum. In the adult it is studded all
ever with rounded ova in different stages of development,

GG2

Fic. 118.—The urinogenital organs of Scyilium canicula from the ventral side.
A, male, and B, female. Only the anterior end of the gonad is represented in each
figure, and except that in B both kidneys are shown, the organs of the right

CHAP, IX DEVELOPMENT 453

size only are drawn. In A the seminal vesicle and sperm-sac are dissected away

from the kidneys and displaced outwards, and the ureters inwards.

a, ~. depression into which the abdominal pore opens ; cé. cloaca; c/s. clasper ;
ef. d. efferent ducts ; ef. epididymis ; %. kidney ; 4’. vestigial anterior portion of
the kidney in the female, represented in the male by the epididymis ; /~. anterior
portion of liver; 2.d. vestigial Miillerian duct in the male; a@s. gullet; ov.
ovary ; ova’, its coelomic aperture ; ovd”. the common aperture of the oviducts
into the cloaca; ~. rectum; sk.g7. shell-gland ; sAd. spermiduct ; sg. 5. sperm-
sac; S.v. seminal vesicle; s.v. its aperture into the urinogenital sinus; ¢s.
spermary 3 7. g.s. urinogenital sinus; #7, ureters; 7’. their apertures into the
urinogenital sinus ; 7, .s. urinary sinus,

varying in diameter from 12-14 mm. downwards: in other
Vertebrates which produce large eggs, a similar reduction of
one ovary may take place (¢.g., Birds). The ovdducts (ovd)
are paired, and extend along the whole length of the dorsal
wall of the coelome, below the kidneys: anteriorly they
unite with one another below the gullet and just in front of
v, . :

the liver, where they communicate with the ccelome by
a common aperture (ovd’) ; posteriorly they open together by
a single aperture (ovd”) into the cloaca, behind the rectum
(r). About the anterior third of each oviduct is narrow and
thin-walled ; the posterior two-thirds is wide and distensible,
and at the junction of the two parts is a yellowish, glandular
mass, the shell-gland (sh. g).

Development.—Impregnation is internal, and is effected
through the agency of the claspers of the male. The eggs,
when ripe, break loose from the surface of the ovary into
the coelome, and thence pass, through the common oviducal
aperture, into one or other of the oviducts, where fertilization
occurs. As it passes into the dilated portion of the oviduct,
the oosperm of Scyllium becomes surrounded first by a
gelatinous substance, and then by a horny egg-shell or
“ Mermaid’s purse ” 1 secreted by the shell-gland, and having
the form of a pillow-case produced at each of its four angles
into a long, tendril-like process. The eggs are laid among
sea-weed, to which they become attached by their tendrils.

1 An egg is contained in the oviduct: figured (Fig, 118 B).

454 THE DOGFISH CHAP,

In Acanthias and Mustelus (p. 415) a mere vestige of the
egg-shell is formed, and the eggs undergo the whole of
their development in the oviducts, the young being even-
tually born alive with the form and proportions of the adult.

The great size of the egg is due to the immense quantity
of yolk it contains: its protoplasm is almost entirely
aggregated at one pole in the form of a small disc. When
segmentation of the oosperm takes place it affects the
protoplasm alone, the inactive yolk taking no part in the
process (compare Crayfish, p. 369). The polyplast stage con-
sequently consists of a little mass of cells, the d/astoderm

Fic. 119.—Section of the upper part of the cosperm of a Dogfish which has undergone
segmentation to form the blastoderm. The blastoderm is formed of asingle layer
of ectoderm cells (white), and of several rows of cells (shaded) which subsequently
give rise to endoderm and mesoderm.

sg. segmentation cavity ; below the blastoderm is the unsegmented yolk containing
scattered nuclei (z). (From Balfour.)

(Fig. 119), at one pole of an undivided sphere of yolk.
The cells of the blastoderm become differentiated into the
three embryonic layers—ectoderm, mesoderm, and endo-
derm. At the same time the blastoderm extends in a
peripheral direction so as to gradually cover the yolk, and
its middle part becomes raised up into a ridge-like thickening,
which is moulded, step by step, into the form of the embryo
fish. The head, trunk, and tail acquire distinctness, and
become more and more completely separated off from the
bulk of the egg, the latter taking the form of a yolk-sac
(Fig. 120, A, y&.s) attached by a narrow stalk to the
ventral surface of the embryo.

IX DEVELOPMENT 455

In this condition the various parts of the adult fish can
be recognised, but the proportions are different and the
head presents several peculiarities. The gill-filaments (67. /’)
are so long as to project through the external branchial
apertures and the spiracle in the form of long threads,
abundantly supplied with blood-vessels, and apparently
serving for the absorption of nutriment—the albumen in the
egg-shell in the case of Scyllium, secretions of the oviduct

Fic. rzo.—A, embryo of ScyMium with yolk-sac’(x 14); B, under-side of head,
enlarged. 4.7 branchial filaments protruding through gill-clefts; 47. 7’.
branchial filaments protruding through spiracle ; cd. caudal fin; d. / dorsal
fins ; ¢. eye ; ex. br. ap. external branchial apertures ; 7z¢#. mouth ; za. nostrils ;
pet. f. pectoral fin; fv. f pelvic fin; s¢. yolk-stalk; vf. ventral fin; yk. s,
yolk-sac. (From Parker’s Biology, after Balfour, slightly altered.)

in the viviparous forms referred to above. Besides this
mode of nutrition, the yolk-sac communicates with the in-
testine by a narrow duct (s¢), through which absorption of
its contents is constantly going on. By the time the young
fish is ready to be hatched or born, the greater part of the
yolk-sac has been drawn into the coelome, a mere remnant
of it still dangling from the ventral surface of the body.

456 THE DOGFISH CHAP.

PRACTICAL DIRECTIONS.

Dogfishes are best preserved in 4 per cent. formaline, which has the
additional merit over spirit of not coagulating the blood, so that the
vessels can be injected in preserved specimens. They can be obtained,
fresh or ready preserved, from any Marine Biological Station.

A. External characters: see pp. 415—418. Sketch from the side.

Isolate some of the dermal teeth by boiling a small piece of skin in
caustic potash, and make out the bony basal plate, and the spine com-
posed of dentine tipped with enamel. Sketch.

B. Skeleton, (If you are working on a fresh fish, and wish to
dissect the soft parts before preservation, the examination of the
skeleton may be postponed until later.)

It is advisable to have one skeleton prepared entire, and one in
which the parts have been disarticulated. Obtain a common butcher’s or
cook’s pointed knife (a strong pocket-knife will do) for cutting through
the rough skin and for the coarser work of preparation. Prepare as
directed on p. 53, and when the greater part of the muscles has been
removed, disarticulate the skull from the vertebral column, leaving the
branchial apparatus attached to it, and also remove the paired fins and
their arches. Disarticulate the hyomandibular cartilage from the
cranium so as to separate the visceral arches, including the jaws
(compare Fig. 103): these should then be thoroughly cleaned without
further immersion in hot water, as the cartilages of which they are com-
posed come apart very easily. The other parts may be dipped into
hot water for a few seconds from time to time, but care should be taken
that the more delicate elements do not thereby become separated. It is
useful to prepare a second cranium as well as a few trunk- and caudal
vertebrze, which should be bisected vertically into right and left halves,
When prepared, the skeleton should be kept in weak spirit or forma-
line, and not allowed to dry, or the cartilages will of course shrink,
unless the following method is resorted to :—

Thoroughly clean a skeleton, or typical parts of it (¢.g. skull, limb-
skeleton, and a few trunk- and caudal vertebree), and then transfer from
weak into strong methylated spirit for a day or so, and afterwards into
absolute alcohol for a few hours. Place in a vessel filled with turpen-
tine for another day, and then transfer into melted paraffin in the water-
bath until the parts are thoroughly permeated, after which they should

Ix PRACTICAL DIRECTIONS 457

be suspended in the water-bath in order to drain off the superfluous
paraffin, and then allowed to cool. Any superfluous paraffin still
remaining may then be removed with a hot wire.

With the specimens before you, work through pp. 419—427, noting
first of all the relations of the parts in the entire skeleton (viz., cranial
and visceral portions of the skull, trunk- and caudal vertebrae, and the
skeleton of the median and paired fins). When examining the skull,
note the nerve foramina (pp. 445—447).

Sketch—(a) the skull (including visceral arches) from the side, and
the cranium in longitudinal section ; (4) trunk- and caudal vertebrz
from the side or in longitudinal section and from the anterior or
posterior face ; (c) the pectoral arch, from the side, with the pectoral
fin attached ; and (d@) the pelvic arch and fin.

C. General dissection: Enteric Canal; &c.

I.—Fix the animal down on the dissecting board with the ventral
surface uppermost by means of strong pins inserted through the paired
fins, and make a median longitudinal cut with a common knife (see
Pp. 456) through the skin and underlying muscular layer which is closely
connected with the skin, from the pectoral to the pelvic arch. At
each end of this incision cut through the body-walls transversely, and
reflect and pin down the two flaps. Cut through the pelvic arch
slightly to one side of the median line, so as not to injure the cloaca.
The abdominal cavity, lined by the serétoneum, will then be exposed.
(In the course of your dissection you will probably find many parasitic
thread-worms belonging to the phylum Memathelmznthes (see p. 397).
Make out :—

1. The ver, with the gal/-bladder: partly embedded in it close to the
junction of its two lobes; the gullet, U-shaped stomach, and the
branches of the vagus nerve on its walls; the wide zztestine, narrow-
ing into a short vectem posteriorly; the cloaca; the pancreas, spleen,
and rectal gland; and the incomplete mesentery. Pass a seeker back-
wards, on one side of the cloaca, through an abdominal pore.

2. In the male, the sfermardes, fused together posteriorly ; and in the
female, the single ovary and the ovzducts and shell-glands. The peri-
toneum covering the Aédneys is so thick that at present they can only be
recognised as slightly convex ridges.

II.—Remove the skin from the dorsal surface of the head between and
slightly in front of and behind the eyes, and then slice away part of the

458 THE DOGFISH CHAP.

roof of the skull with a knife until the dvazx is exposed, being careful not
to injure some nerves which you will see close beneath the skin on
either side of the brain-case. Then cut off the tail transversely, a short
distance behind the pelvic fins, and on the cut surface note—

1. The 2etegement, in which runs the sensory canal of the latera.
line.

2. The centrum and neural and hemal arches of the vertebra, and the
soft zntervertebral substance (remains of the xotochord) ; the spinal cord ;
and the caudal artery and vein.

3. The myomeres and myocommas ; and if your section passes through
a dorsal fin, the cartdlaginous pterygiophores and the horny %n-rays.
Sketch.

III.—The dorsal aorta and its branches may now be injected (see
p- 99) through the cut end of the caudal artery, into which a cannula
should be inserted for some distance (tying is unnecessary). Now return
to the examination of the abdominal viscera, and make out :—

1. The dz/e-duct, opening into the intestine just behind the pylorus.
The pancreatic duct runs in the wall of the intestine, and careful dis-
section is required to make out its course (see § IV, 1).

2. The hepatic portal vein and its factors, entering the liver near the
median plane. If the blood has escaped from it, try to blow it up
with a blowpipe.

3. The position of the dorsal aorta, which “will be seen better at a
later stage, but the chief branches of which should now be traced to
their distribution, as follows: a, the ca/ac artery, extending down-
wards and backwards along the stomach from above the posterior end
of the gullet; 4, the anterior mesenteric artery, arising about 14 inch
behind the cceliac ; ¢, the eno-gastric artery, arising close behind the
anterior mesenteric; and d, the small postertor mesenteric artery, pas-
sing downwards to the rectal gland.

4. The large hepatzc stnus, immediately in front of the liver, below
the gullet : slit it open, and note the veins entering it from the liver.
On either side of the gullet in this region, along the dorsal surface of
the ccelome, a capacious cardinal sinus will be seen: make an aperture
in this, and pass a seeker backwards, noting that the sinus narrows into
the cardinal vein, which passes along the inner side of the correspond-
ing kidney and parallel to the aorta. The gendtal (spermatéc or ovarian)
sinus communicates with the cardinal.

5. The /ateral veins (Figs. 110 and 113), running on either side of

Ix PRACTICAL DIRECTIONS 489

the body, just beneath the peritoneum. Cut through the body-wall on
one side, a short distance behind the pectoral fin; insert a cannula,
directed forwards, into the cut end of the lateral vein, and inject. The
vein will then be seen running forwards as far as the pectoral arch,
when it turns towards the dorsal side.

6. In the female, the united anterior ends of the ovzdzcts and their
ceelomic aperture, ventral to the gullet and just in front of the liver.

IV.—Taking care not to injure the anterior ends of the oviducts and
to leave part of the hepatic sinus zz sz¢z, remove the liver, together
with the stomach and intestine, without injuring the bile-duct, cutting
through the stomach at its junction with the gullet and the intestine
iust in front of the rectal gland. Wash out the portion of the enteric
canal thus removed under the tap, fill it with water, and place the
whole under water in a dissecting-dish. Cut away portions of the wall
of both stomach and intestine, and make out—

1. The course of the dz/e ducts and pancreatic ducts, and their apertures
into the intestine.

2. The pyloric valve, and the spzral valve of the intestine, which
makes about seven or eight close turns, appearing like a series of cones
one within the other.

3. The characters of the mzcous membrane of the stomach and
intestine.

Sketch your dissection.

D. Urinogenital organs.

I.—After noting again the gonads, and in the male the delicate
efferent ducts of the spermaries (Fig. 118, A), remove in the male all but
the anterior ends of the latter, and in the female the entire ovary ; then
carefully dissect away the thick peritoneum covering the kidneys, noting
as you do so the dorsal aorta and its various branches, and once again
the cardinal veins (C, § III, 4), which may be-inflated with air. The
renal portal veins are not easily traced without injection, which may be
done from the cut end of the caudal vein.

Note—

1. The brownish 2dzeys, and in the male their anterior, whitish
sexual part—the epzdidymes.

2. In the male: a, the convoluted sfermdduct, indistinguishable from
the epididymis anteriorly and enlarging posteriorly to form the elongated
seminal vesicle; and 6; the grooved, eversible c/asgers, which are

460 THE DOGFISH CHAP.

supported by cartilages and have each a gland at the base of the
groove.

II.—Cut through the skin round the vent, and dissect the
entire cloaca and the kidneys (together with the epididymes in the
male) away from the body, and pin them down, ventral side upper-
most, under water. Clear away with great care the connective tissue
which binds the ureters and generative ducts to the kidneys posteriorly,
slit open the cloaca, and make out—

1. In the male (Fig. 118, A) the aperture of the rectum and the
urinogenital papilla, Insert the small scissors into the aperture at the
apex of the latter, and slit open the wrznogenttal sinus, continuing the
cut into the two sperm-sacs ; make out the apertures of the semdnal
vesicles and ureters. Pass a seeker or probe into each of these
apertures (the main ureter may be injected), and then dissect out, on
one side—a, the elongated and pointed, thin-walled sperm-sac ; and 4, the
delicate ureters, three or four of which unite to form a widish common
tube, situated towards the inner border of the kidney, before opening
into the urinogenital sinus. Sketch.

2. In the female (Fig. 118, B), the thin-walled, anterior united ends
of the oviducts, their thick-walled posterior portion, the shed/-glands, the
apertures of the rectum and oviducts into the cloaca, and the w7znary
papilla. Insert the point of the scissors into the aperture on the apex
of the latter, and slit open the wrzzary sinus, in which several
openings of the zzeters will be seen on either side. Cut open the
oviducts, and note, if present, the eggs enclosed in horny egg-cases.
Sketch.

E. Circulatory‘ and Respiratory Organs, &c.

I.—After noting the sgzval nerves, exposed by the removal of the
kidneys, the body may be cut through first behind the pectoral arch, and
the posterior portion thrown away. Pin down the head and anterior
portion of the body, ventral side uppermost, make a median longi-
tudinal incision through the skin from the lower jaw to the pectoral arch,
and dissect it away on either side as far as the gill clefts. Then,
without injuring the lateral vein (p. 458), remove the middle portion of
the pectoral arch and expose the perzcardzal cavity and heart. Insert a
seeker, pointing backwards, along the dorsal side of the heart, through

1 See also C. § III.

Ix PRACTICAL DIRECTIONS 461

the canal which communicates between the pericardial and abdominal
cavities ; it opens on the ventral side of the gullet by two apertures.
Make out—

1. The form and relations of the chambers of the heart (szreas venosus,
auricle, ventricle and conus arteriosus).

2. The ventral aorta, to expose which the muscles in front of the
pericardium must be carefully removed ; but before doing so, it is better
to inject the ventral aorta, cutting a small hole in the ventricle, and in-
serting and tying the cannula into the conus arteriosus: use a d/ue
injection if you have already used red for the dorsal aorta.

3. The five afferent branchial arteries (compare Fig. 112): trace
these outwards, and note their distribution. Sketch your dissection.

II.—Cut through the ventral aorta at its junction with the conus
arteriosus and through both ends of the sinus venosus, carefully separ-
ating the latter from the walls of the pericardium and noting the
entrance of the hefatzc séxus (p. 458). Remove the entire heart, pin
it down under water, ventral side uppermost, and cut open the ventricle
and conus arteriosus. Note—

1. Their cavities and walls; the azriculo-ventricular aperture and
valves ; the valves in the conus arterdosus, of which there are two sets,
consisting of three in each set. Sketch.

2. The cavity and walls of the azrzcle and the s¢xu-auricular aper-
ture, which are best made out by turning the heart over, with the dorsal
side uppermost, before cutting open the auricle. Sketch.

3. Insert a seeker into one of the cut distal ends (still left 2 sd¢z) ou
the sinus venosus, and slit it up so as to expose the grecaval sinus of the
same side; by means of a seeker find the apertures into it of the
following veins or sinuses (compare Fig. 113)—a, the jugular ; 6, the
inperior jugular ; ¢, the cardinal; and d, the lateral vein.

III.—Insert the scissors into each external gill-cleft of one side, one
by one, and extend them by cutting dorsally and ventrally, so as to
expose the gi//-pouches, communicating with the pharynx by the zzfernal
gill-clefts. Make out—

1. The branchial filaments, and observe that there are four complete
gills on the first four branchial arches, and a half-gill or hemibranch on
the posterior face of the hyoid arch. Note also the psewdobranch on the
anterior side of the spiracle.

2. The structure of the gills. Remove two entire gills; dissect one,
and cut the other across transversely (compare Fig. 109), noting the

462 THE DOGFISH CHAP.

relations of the septum, cartilaginous branchial arch and rays, branchial
filaments, and single afferent and paired efferent branchial artery.
Sketch.

IV.—Cut through the floor of the pharynx and mouth close to the
middle line, just on one side of the ventral aorta, and extend the cut
through the lower jaw. On one side, turn the floor outwards, and pin
it back in this position, so as to expose the roof of the mouth and
pharynx and the internal gill-clefts ; dissect away the mucous membrane
lining the roof, and trace out on one side (Fig. 112)—

1. The epzbranchial and efferent branchial arteries, and the dorsal
aorta,

2. The carotid and subclavian arteries, &c.

F, Nervous System and Sense-Organs.

I.—Remove with the knife the rest of the skull-roof and a few of the
anterior neural arches, so as to expose the entire brain and the anterior
part of the spinal cord. In doing so, be careful not to injure the con-
tents of the orbit, the nerves referred to on p. 458, or the auditory
capsule of one side. After noting the dura mater and pea mater, make
out—

1. The subdivisions of the drazz (olfactory lobes, prosencephalon,
diencephalon, optic lobes, cerebellum, and medulla oblongata). Sketch.

2: The origins of the cerebral nerves from the brain and the points
at which they penetrate the walls of the skull (pp. 445-447).

3. The spinal cord, and the alternating dorsal and ventral roots of the
spinal nerves. Then cut through the spinal cord just behind the
medulla oblongata, and through the origins of the cerebral nerves.
Remove the brain and place it in formaline or spirit.

II.—Carefully dissect away the skin covering the head and pharyn-
geal region on the undissected side, expose the orbit, and remove
the delicate connective tissue surrounding its contents. Pin down
firmly, and dissect out the following from the side (compare Fig. 116
and pp. 445-449).

1. The ophthalmic division of the facéal nerve, and immediately
below it that of the ¢rigemzna: trace them backwards to their
foramina in the skull-wall and forwards through a canal between the
olfactory capsule and the cranium, to their distribution.

2. The large mass of sensory (ampullary) canals on the dorsal side of
the snout.

1X PRACTICAL DIRECTIONS 463

3. The four vect¢ and the two oddque eye-muscles (Fig. 117), and the
nerves (III, IV, VI) supplying them.

4. The eye, and the oftze nerve—anterior to the recti muscles. The
eye may now be removed by cutting through the muscles and optic
nerve, and dissected as‘directed on p. 191, noting its structure as
before. :

5. The large, flat, maxtllo-mandibular division of the trigeminal,
running forwards and outwards along the floor of the orbit, and there
dividing into maxtlary and mandibular branches.

6. The faccal nerve, entering the orbit close behind the maxillo-mandi-
bular nerve, and giving off: behind the spiracle—a large Ayoman-
dilufar branch, passing along the anterior border of the auditory capsule
and posterior wall of the orbit, and down the anterior side of the hyoid
arch just beneath the skin: and in front of the spiracle—a alatine
and prespiracular \ranches. Of the branches to the sensory
canals, the ophthalmic has already been seen; the dzccal and external
mandibular require very careful dissection in order to make them out
satisfactorily.

7. The glossopharyngealand vagus nerves. To expose these, slice away
sufficient of the auditory capsule (noting as you do so the semzcevcular
canals and the exdolymphatéc duct) to expose the foramina by which they
emerge from the skull, behind the auditory capsule, and separate the
mass of muscles lying alongside the vertebral column from the branchial
apparatus, by dissecting away the connective tissue. Trace the glosso-
pharyngeal to its bifurcation over the first gill-cleft, and in the vagus
follow out—a, the four branchial branches, forking over the remaining
gill-clefts ; 4, the wésceral branch ; and c, the lateral ine branch, above
and to the inner side of the branchial branches,

8. Separate some of the ampullary sensory tubes trom one another,
and note the ampud/e and the nerves supplying them.

g. Carefully slice away the cartilage of the auditory capsule of the
side you have not already dissected so as to expose the membranous
labyrinth. Examine under water, and make out the vestibule with its
contained ofolithic mass, the three semicircular canals with their
ampulla, and the branches of the auddtory nerve (compare Fig. 59).

III. Now examine the preserved brain from above, from below, and
from the side, making out, in addition to the parts already noticed
(§F,L1—

1, The optee chiasma, infundibulum—with an oval swelling and a

464 THE DOGFISH cHapP.

vascular sac on either side, pétudtary body, crura cerebri, and, as far as
possible, the origins of the nerves. Sketch from below.

2. On one side of the brain, cut into the olfactory lobe, prosence-
phalon, optic lobe, and cerebellum from above, so as to expose the o/fac-
tory ventricle, lateral ventricle, optic ventricle, and cerebellar ventricle.
Then bisect the entire brain into right and left halves with a sharp
scalpel, and examine the uninjured half in longitudinal section, noting
in addition to the parts mentioned above, the third ventricle, foramen of
Monro, tter, and fourth ventricle. Sketch.

G. Transverse Sections.—Cut thick transverse sections of an en-
tire dogfish with a knife through—a. the anterior, and 4. the posterior
part of the head (pharyngeal region); +. about the middle of the
body ; and d. the tail. Make out the relations of the various parts and
organs, and sketch the lateral half of each section.

A more satisfactory method than this is to obtain a very young dog-
fish, not more than } inch in diameter, and after cutting it transversely
into pieces about inch in thickness in the regions named above, stain,
imbed, and mount a few sections from each piece (see p. 136). These
can first be examined with a lens or with the low power of the micro-
scope, and then, by putting on the high power, important points in the
histology can be made out. In addition to the minute structure of the
tissues and organs described in Part I. of this book, the structure of the
notochord (p. 425), integumentary sense-organs (pp. 414, 418), dermal
teeth (p. 417), &c., should be studied,

H. Side Dissection.—It is very instructive to supplement and
recapitulate your work on the anatomy of the dogfish by dissecting
another specimen from the side (compare Fig. 108), as in the case of the
crayfish. Open up the abdominal cavity as before, very slightly to the
left side of the middle line. Continue the cut forwards through the
pectoral arch and backwards through the pelvic arch: the arteries may
be injected at this stage. Then dissect off the left half of each arch
with the corresponding fin, and cut away the left body-wall. Cut
through the skin in the mid-dorsal line, and then, working from the
left side, remove the left half of the skull and visceral arches,
as well as the left side of the vertebral column, so as to expose
the brain and spinal cord. Remove the left kidney (and left ovi-
duct if your specimen is a female). Pin down under water, and tidy up
your dissection so as to reduce it to a neat longitudinal section, in

IX PRACTICAL DIRECTIONS 465

which all the unpaired soft organs are left intact. Without tearing
the mesentery, pin out the liver, stomach, and intestine beyond the
ventral limits of the body-wall, so that the other abdominal organs are
not hidden. Follow out the relations of the various organs as before,
and sketch your dissection.

It is important at this stage to refresh your memory of the anatomy
of the Frog, and to compare it with the Dogfish, by making a dis-
section from the side as directed above (compare Fig. 7). Sketch.

Pract. Zoou

CHAPTER X
CHARACTERS OF THE CLASS MAMMALIA—THE RABBIT

BEFORE examining a Rabbit, as an example of the highest
class of Vertebrates—the Mammatia, it will be well to re-
capitulate some of the characters of the frog, the organisa-
tion of which is higher than that of a fish.

The frog, taken as an example of the class Amphibia, differs
from a fish in the following points, amongst others. Its
paired limbs have not the form of paddle-like fins, the fore-
limb consisting of upper arm, fore-arm, wrist and hand, and
the hind-limb of thigh, shank, ankle, and foot, each with
its characteristic skeletal parts; it has no median fin in
the adult, and that of the tadpole is not supported by fin-
rays ; there is no hard, dermal exoskeleton ; respiration in the
adult is pulmonary, and internal nostrils are present ; there
are two auricles in the heart, and the cardinal veins are
replaced by a postcaval; there is a urinary bladder formed
as an outgrowth of the cloaca. Moreover the skeleton,
unlike that of the dogfish, is in the adult composed mainly
of bone.

In all these characters the frog resembles the rabbit. But
the Mammal differs from the Amphibian in many important
respects, some of the chief of which are :—the presence o
an epidermic exoskeleton consisting of airs; the high

CHAP, X MAMMALIA 467

temperature of the blood, which remains almost uniformly
within a few degrees of 100° Fahr., and does not vary to
any appreciable extent with the temperature of the air; the
absence of nuclei in the red corpuscles of the blood; the
presence of mammary glands beneath the skin in the female,
which secrete milk for nourishing the young ; the subdivision
of the body-cavity into two portions—+horax and abdomen—
by a transverse partition, the déaphragm ; the presence of two
ventricles as well as of two auricles in the heart, and of a
single systematic left aortic arch; the higher differentation
of the brain, and also of the skeleton; and the mode of
articulation of the lower jaw. The teeth, again, of the
rabbit, like those of the largé majority of Mammals, are
differentiated into front-teeth for biting or seizing the food
and into cheek-teeth or grinders, and their succession is
limited to two sets; an external ear or inna is present ;
there is no cloaca, the anus and urinogenital apertures
opening separately on the exterior, while the ureters open
directly into the bladder ; the ova are minute, and the young
undergo their early development in the oviduct, where they
are nourished by diffusion from the blood-system of the
parent by means of an organ known as the placenta, and
after birth, they are suckled by the mother.

Bearing in mind these essential characters of the higher
Mammalia as compared with the Vertebrates previously
studied, we can now proceed to examine the structure of the
rabbit in greater detail.

External characters. The Rabbit (Lepus cuniculus) is a
very abundant and widely distributed animal which in the
wild state makes burrows in the earth in which the young
are born; there are a number of varieties, the habits and
general appearance of which have been modified by domesti-
cation (compare p. 227).

HH 2

468 THE RABBIT CHAP.

In addition to head, trunk, and short ¢az/, the rabbit
possesses a distinct eck, and the whole animal, including the
limbs and even the soles of the feet, is covered with a soft
fur consisting of hairs (Fig. 121). In the wild rabbit, the
fur is of a brownish colour, lighter below, and white under
the tail: in the many domesticated varieties the colour is
very varied.

The hairs correspond to modified epidermic cells, which become
converted into a horny material ; they are developed in tube-like involu-
tions of the epiderm called Aatr-sacs, into the swollen base of each of
which a mesodermal hair-gapilla projects, the substance of the hair
being formed from the epidermic cells covering the papilla. Into the
hair-sacs open the ducts of sebaceous glands, the secretion of which
serves to lubricate the hair.

There are five digits in the hand or manus, and four in
the foot or ges, each terminated by a pointed and curved
horny claw, developed, like the hairs, from the epiderm.
Along the ventral surface of the body in the female are four
or five pairs of papille—the eats, on which open the ducts
of the milk-glands, which correspond to modified sebaceous
glands. The various parts of the skeleton (Fig. 121) can
be felt through the skin, and in addition to those already
enumerated in the Vertebrates previously studied, it will be
noticed that the anterior part of the trunk, or ¢horax, is sur-
rounded by 77és, many of which meet below with a breast-bone
or sternum, and which are absent in the posterior part of
the trunk, or abdomen.

Beneath the anterior end of the snout is the transverse
mouth which has a narrow gape and is bounded by upper
and lower lips: the upper lip is divided by a longitudinal
cleft which is continuous with the oblique, slit-like external
nostrils. Just inside the lips are the upper and lower front
teeth orzctsors, which are chisel-shaped, and behind them the

x EXTERNAL CHARACTERS 469

hairy integument is continued on either side into the cavity
of the mouth. The eyes are protected by movable upper and
lower hairy eyelids, as wellas by a hairless third eyelid or
nictitating membrane (compare p. 5), supported by carti-
lage and situated in the anterior corner of the eye, over
which it can be partly drawn: it corresponds to the little
red lump in the inner corner of the human eye. On the
upper lip and above and below the eye are certain very long
and stiff hairs—the “whiskers” or vébrisse, and behind the

Fic. 121.—Rabbit.
Lateral view of skeleton with outline of body. (From Parker and Haswell’s
Zoology.)

eyes are a pair of long and movable external ears or pinnae:
these are supported by cartilage and are somewhat spout-
shaped, leading to the external auditory openings. _

Below the root of the tail is the azws, and in front of
and below this, the uzixogenital aperture, the space between
them being known as the Jevine@um. On either side of these
apertures is a hairless depression of the skin on which open
the ducts of the perineal glands, the secretion of which has
a strong and characteristic odour. In the female the slit-

470 THE RABBIT CHAP. X

like urinogenital aperture is called the /va; in the male
the aperture is smaller and situated on the conical apex of a
cylindrical organ, the fezés, which can be retracted within a
fold of skin, the foreskin or prepuce. On either side of
the penis is an oval pouch of the skin, the scrotal sac, not
very apparent in young animals, in each of which a sfermary
or destis is contained.

Skeleton. The skeleton of the rabbit consists almost
entirely of bone, but it must be remembered that in addi-
tion to certain cartilages described below, all articular
surfaces are covered or lined by a thin layer of cartilage, and
that the various parts of the skeleton are connected together
by ligaments.

In the skull, both cartilage and membrane bones (p. 43)
are much more numerous than in the frog, and the structure
of the entire skull is far more complicated and highly
differentiated. A posterior, relatively large, cranial region, in
the side walls of which auditory capsules are embedded, can
be distinguished from an anterior, somewhat conical, facial
region, constituting the skeleton of the snout (Fig. 122),
Just behind the junction of these two regions on either side
is a large ordit, separated from its fellow bya thin znterorbital
septum, perforated by a foramen for the optic nerve (0/7. fo),
At the sides of the foramen magnum are the two
rounded occipital condyles ; the auditory apertures (aud. me)
are situated at the sides of the posterior part of the cranium,
and the external nostrils open at the anterior end of the
snout. Most of the bones remain more or less distinct
throughout life, and are in contact along lines or szfwres, many
of which are wavy or zig-zagged: others, again, become
completely fused together, in the adult so that their limits
are no longer distinguishable.

Fic. 122.—Skull of Rabbit. A, lateral view with lower jaw ; B, ventral view.
ang. proc. angular process of mandible ; as. alisphenoid (external pterygoid pro-
cess) ; aud. me. external auditory aperture in tympanic bone 3 6.0c. basioccipital ;

472 THE RABBIT CHAP.

4. sph. basisphenoid ; cond. is Ween y tees ee Prone just below the condyle;
Yr. frontal ; int. pa. interparietal ; 7u. jugal ; lachrymal ; max. maxilla; zas.
nasal ; oft. fo. optic foramen ; 0. sph. pie ieth wher pa. parietal ; pal. palatine ;
pal. max. palatine process of maxilla ; 3 par. oc. paroccipital process ; of exoccipital ;
pal, p. max. palatine process of premaxilla ; pmax. premaxilla’; peri. periotic ;
pt. pterygoid ; r2 #.sg. post-tympanic process of sq 3 S. oc. supr

sg. squamosal ; ty.du/. tympanic bulla; vo. vomer ; zyg. max. zygomatic process
of maxilla. (From Parker and Haswell’s Zoology.)

The upper jaw forms part of the facial region, which
encloses the olfactory chambers, and the Jower jaw, consist-
ing of a single bone on either side, articulates directly with
the sides of the cranium without the intervention of a
hyomandibular, as in the dogfish (p. 422) or of a quadrate
cartilage, as in the frog (p. 44). The rest of the visceral
portion of the skull, representing the hyoid and first branchial
arch, forms the so-called Ayo/d bone, which is embedded in
the base of the tongue (Fig. 125, 2y).

The bones! which form the walls of the brain-case are
arranged in three rings or segments, the middle and post-
erior of which are separated by the auditory bones (Figs.
123 and 122, A, fe72).

The posterior, or occipital segment, consists of three bones,
which in the adult become completely united with one another.
The lower of these is the dastoccipital (0. oc), a flattened bone
bounding the foramen magnum below, and forming the
hinder part of the base of the skull and the lower part of
each occipital condyle (oc.c). The two exoccipitals (e. 0c)
bound the foramen magnum at the sides, and form the
upper part of the occipital condyles: each is produced
downwards into a paroccipital process (par. oc) which fits
closely against the posterior surface of a swollen bone (ty. dud)
to be described presently, which is produced into a tube
surrounding the auditory aperture (azd. me). The occipital
segment is completed above by the supraoccipital (s. ac),

1 In the following description, the membrane bones are distinguished
by an asterisk from the cartilage bones.

x SKULL 473

bounding the foramen magnum above ; it has a pitted surface
and is marked externally by a shield-shaped prominence. -4

The middle, or parietal segment, consists of five bones,—a
bastsphenoid ( b.sph) below, an alisphenoid (a.sph.a.s) on either
side, and two parietals * (pa) above. The broad posterior
end of the basisphenoid is connected with the basiocci-

Fic. 123.—Skull of Rabbit in longitudinal vertical section. The cartilaginous nasal
septum is removed,

a. sph, alisphenoid ; ¢. oc. exoccipital ; ¢. #6, ethmo-turbinal ; e¢%. ethmoid ; 77. fossa
for flocculus of brain ; z. incisors ; 7. molars; zx. ¢. maxillary turbinals ; x. 2d.
naso-turbinals ; oc. ¢. occipital condyle ; Aa. palatine ; ee palatine portion of

H

the bony palate; A. 7. premolars ; 4. sf. presphenoid ; I. point at which the
olfactory nerves leave the skull; II. optic foramen; si. sphenoidal fissure
(compare Fig. 122 A, just below of¢. fo.) ; V mx. foramen for mandibular division
of trigeminal ; VII. for facial nerve; VIII. for auditory nerve; IX, X, XI, for
glossopharyngeal, vagus, and spinal accessory; XII. for hypoglossal. Other
letters as in Fig. 122: the unlettered line above 4, sf points to the sella turcica,
or depression in which the pituitary body lies.

pital by a thin plate of cartilage, and tapers in front to a
blunt point: it is perforated at about its middle by an oval
pituitary foramen, and on its upper surface is hollowed out
to form a depression in which the pituitary body lies.
The alisphenoids are wing-like bones, directed upwards
and outwards, and firmly united with the basisphenoid : each
is produced ventrally into a pterygoid process (as), consist-

474 THE RABBIT CHAP

ing of two laminz which converge and unite with one
another anteriorly. The parietals are a pair of thin, slightly
arched bones, forming a considerable part of the roof of the
brain-case, and united with one another by suture along the
middle line; the outer edge of each gives off a thin,
ventral process which is covered by the squamosal (s¢),
a bone which will be referred to presently and which
separates the parietal from the alisphenoid. Interposed
between the parietals and the supraoccipital is a small
median interparietal* (int. pa).

The frontal segment also consists of five bones—a
presphenoid, (p. sph) two orbitosphenoids (0. sph) and two
frontals* (fr). The small presphenoid is laterally com-
pressed and is connected with the basisphenoid by cartilage,
so that in the dry skull there is a considerable interval
between the two bones; it forms the inferior and anterior
boundary of the optic foramen (of¢. fo, ZZ), which puts the
two orbits in communication with one another and both in
communication with the cranial cavity. The orbitosphenoids
are two wing-like laminz directed outwards and slightly
backwards, and completely fused with the presphenoid ;
they surround the rest of the optic foramen. The frontals
form the roof and side-walls of the anterior part of the
brain-case and are united by suture with one another in the
middle line and with the parietals behind; below .they
meet with one another anteriorly on the floor of the brain-
case and unite with the presphenoid by suture ; the outer
part of each forms a prominent crescentic ridge, the
supra-orbital process.

The brain-case is closed in anteriorly by a bone riddled
with numerous small holes for the passage of the olfac-
tory nerves: this is the cribriform plate of the ethmoid
(ech).

x SKULL 475

It will be remembered that in the frog the occipital region is ossified
by exoccipitals only, the parietals and frontals of either side are fused
together, there are no ali- or orbito-sphenoids, the cartilaginous walls of
the anterior part of the cranium are ossified as a sphenethmoid, and
that the floor of the skull is supported by membrane bone, the para-
sphenoid.

The auditory capsules are comparatively loosely wedged
in laterally between the parietal and occipital segments ; in
the embryo each is ossified from three centres, instead of
one (the pro-otic) as in the frog, but these early unite to
form the feriotic bone (peri), as the ossified auditory capsule
is called. The internal or fefrous portion of this bone
(Fig. 123) encloses the membranous labyrinth of the ear
and is very dense and hard ; posteriorly it is produced out-
wards as the porous mastoid portion, which is visible on the
outer side of the skull (Fig. 124 a). Closely applied to the
outer surface of each periotic is a bone called the ¢ympanic,*
consisting of a tubular portion above—the edge of which sur-
rounds the auditory opening (aud. me) to which the cartilage
of the pinna is attached, and of aswollen portion, or ¢ympanic
bulla (ty. bul) below: this encloses the tympanic cavity, and
in it, at the base of the tubular portion, is an incomplete
bony ring to which the tympanic membrane is attached
(Fig. 133). The tympanic is incomplete on its inner side,
where its cavity is closed by the outer wall of the periotic,
and between the two, at the antero-inferior angle of the
former, is the aperture by which the Eustachian tube
leaves the tympanic cavity (compare p. 45). When the
tympanic is removed, two small holes are seen on the outer
wall of the periotic: the anterior of them is the fexestra ovalis
and is plugged by the s¢ages—which together with two small
bones, the malleus and incus (Fig. 133), forms the chain of
auditory ossicles to be described later in connection with

476 THE RABBIT CHAP.

the organ of hearing; the posterior aperture is called the
fenestra rotunda. On the internal or cranial surface of the
periotic is a large depression (/7) which lodges the flocculus
of the cerebellum (Fig. 131).

The olfactory capsules are roofed in by two long and
narrow zasal bones* (nas), which meet together in the
middle line and unite by suture with the frontals posteriorly.
Their side walls are formed by the bones which bear the
teeth of the upper jaw—the premaxille (f. max) and
maxille (max), and in the median line below is a single
long and slender bone, deeply grooved on its upper surface,
and formed by the fusion of the two womers* (vo). The
two nasal chambers are separated from one another in the
middle line by a median vertical plate of cartilage, the zasa/
septum (Fig. 125 2.s) embraced below by the vomer. This
cartilage together with the cribriform plate and a median
vertical plate of bone (e¢#) extending forwards from the
latter into the septum, constitutes the mesethmoid. Within
the nasal chambers certain scroll-like folds of the mucous
membrane (Fig. 125) are present in order to increase
the surface, and in these, cartilages are developed. The
cartilages become ossified, and the resulting ¢urbinal bones
unite with certain of the bones enclosing the olfactory organs,
and are named accordingly. The ethmoid turbinals (Fig.
123, ¢.26) or true olfactory scrolls are two complicated,
folded bones united to the cribriform plate of the ethmoid,
and are covered in the fresh condition by the olfactory
epithelium ; the maxillo-turbinals (mx. tb) are similar but
more complex bones in the antero-ventral part of the nasal
cavities; and the zaso-turbinals (n.¢b) are thin, folded
bones, much less complex, and fused with the inferior
surface of the nasals.

In the front wall of each orbit, fitting comparatively loosely

x SKULL 477

between the frontal and maxilla, is a small bone,the dachrymal,*
(Fig. 122 A, dr) with a notch near its outer border through
which the naso-lachrymal duct passes (compare p. 186.)

As in the frog, the chief bones of the upper jaw on either
side are the premaxilla * (p. max) and the maxilla * (max),
and nearer the middle line are the falatine® (pal)
and pterygoid* (pt): in the embryo the position
of the two last mentioned bones is taken by
cartilage representing the upper jaw of the dogfish (com-
pare Figs. 103 and 9). The premaxille, in which the
sockets for the front or incisor teeth are situated, form the
anterior boundary of the snout, and articulate with one
another in the median line and with the maxilla behind :
each gives off a nasal process passing backwards between
the nasal and maxilla to the frontal, and a palatine process
(gal. p. max) extending backwards along the palate in
contact with its fellow of the opposite side. The maxillz
are large and irregular bones, parts of the sides of which are
fenestrated, and in which the cheek-teeth are situated. From
the inner and inferior edge of each, opposite the first two
cheek-teeth, a horizontal palatine process (pal. max) is
given off, which, articulating with its fellow of the opposite
side, forms the anterior part of the bony support of the
hard palate—this is of much less extent in the rabbit than
in most mammals: from its outer side arises a sygomatic
process (zyg. max) which forms the anterior part of the
strong zygomatic arch extending below and externally to the
orbit.

The palatines are thin, nearly vertical, bony la-
mine, internal to the maxillz, to which they are at-
tached in front, while above they join the presphenoid
and the pterygoid process of the alisphenoid. They
bound the passage of the internal nostrils, and from the

478 THE RABBIT CHAP,

inner and anterior region of each is given off, op-
posite the third cheek-tooth, a horizontal, inwardly
directed process (fa/’), which, articulating in the middle
line with its fellow of the opposite side and in front
with the palatine process of the maxilla, forms the pos-
terior part of the bony support of the hard palate. The
pterygoids are small irregular plates of bones attached
to the posterior edge of the corresponding palatine and the
pterygoid process of the alisphenoid; each ends ventrally
in a backwardly curved process.

The sguamosals* (sg) are a pair of plates which overlap
and complete the side-walls of the brain-case (p. 474) in
front of the periotics: they articulate with the frontals,
parietals, orbitosphenoids, and alisphenoids. From the
outer face of each is given off a strong zygomatic process,
which bears on its under surface the articular facet for
the lower jaw, and further back a slender process
(p.45g:) arises which is applied to the outer surface
of the periotic.

The zygomatic processes of the squamosal and maxilla
respectively are united by a flat bar of bone, the jugal* (ju),
which in the adult is fused with the last-named bone.
All these three bones therefore take part in forming the
zygomatic arch.

Most of the apertures for the transmission of the cerebral nerves have
so far not been mentioned : the branches of the olfactory nerve, as we
have seen, pass out through the numerous apertures in the cribriform
plate (Fig. 123, e¢2), and the optic foramen (II) is situated between the
orbitosphenoid and presphenoid. Behind and below the optic foramen
is a vertical aperture—the sphenozdal fissure (sph. f )—between the basi-
sphenoid and alisphenoid, which transmits the third, fourth, and sixth
nerves, as well as the opththalmic and maxillary divisions of the fifth.
Between the periotic and alisphenoid is a large space (Vz), through
the anterior part of which the mandibular division of the trigeminal

x SKULL 479

leaves the skull.1_ Between the mastoid portion of the periotic and.the
posterior border of the tympanic, at the juncture of the tubular and
bulbous portions of the latter bone, is a small aperture—the stylomastoid
foramen—which transmits the seventh nerve: it and the eighth (VII,
VIII) enter the periotic just below the depression for the flocculus of the
cerebellum (77). A-space (IX, X, XI) between the occipital condyle and
tympanic bulla gives exit to the ninth and tenth, as well as to the
eleventh nerve—which is not represented in the dogfish and frog; and
the hypoglossal, which in mammals is counted as the twelfth cerebral
nerve, passes out through two small apertures (XII) in the exoccipital,
just anterior to the condyle. Various other apertures will be noticed in
the skull and jaws: through some of these branches of certain of the
above-mentioned nerves pass, while others transmit blood-vessels.

The lower jaw or mandible (Fig. 122 A) consists of two
halves or rami, each corresponding essentially to the dentary
of the frog, which unite with one another in front, at the
symphysis, by a rough surface, while behind they diverge
like the limbs of the letter V. Each ramus is a vertical plate
of bone, broad behind and tapering towards the front, where
it bears the incisor teeth: further back, on its upper margin,
are the sockets for the cheek-teeth, and behind them is an
ascending portion which bears the condyle (cor) for articula-
tion with the facet on the squamosal : in front of the condyle
is a curved coronoid process. ‘The postero-inferior border,
which is rounded and inflected, is known as the angular
process (ang. pro).

The Ayotd is a small bone situated at the root of the
tongue, anterior to the larynx (Fig. 125 Ay). It consists of
a stout body or dast-hyal, a pair of small anterior horns,
representing the ventral ends of the hyoid arch of lower
Vertebrates, and a pair of longer, backwardly-projecting pos-

1 In most mammals (e.g. dog, cat), the maxillary division of the
trigeminal passes out through a separate foramen, behind the

sphenoidal fissure ; and the anterior part of the space referred to above
is separated off as is a distinct foramen for the mandibular division,

480 THE RABBIT CHAP.

terior horns or ¢hyvo-hyals, attached to the larynx and repre-
senting the lower ends of the first branchial arch.

The vertebral column includes about forty-five bony
vertebre, each consisting of a centrum, a neural arch, and
various processes (compare pp. 36—38), but becoming
simplified towards the end of the tail. The centra have
flat anterior and posterior surfaces, and are not con-
nected by synovial articulations, as in the frog, but inter-
posed between them are elastic ¢xtervertebral discs of fibro-
cartilage. In addition to the ossification which gives rise to
the main part of the centrum, a separate flat disc of bone
(Fig. 124 ef) is formed on the anterior and posterior
surface of each. These efiphyses are characteristic of the
vertebree of all or nearly all mammals: they unite com-
paratively late with the centrum proper, and so in dis-
articulated skeletons of young animals they often come
away from the main mass of the centrum and remain attached
to the intervertebral discs.

In correspondence with the differentiation of the parts
of the body, the vertebral column is divisible into five
regions (Fig. 121): the cervical in the neck, including seven
vertebre, the first two of which—called respectively the
atlas and axis—are peculiarly modified in order to allow
the skull free movement ; the ¢horacic in the thorax, twelve
or thirteen in number, and bearing vds; six or seven
lumbar in the abdominal region: three or four sacral in
the sacral region: and about fifteen or sixteen cauda/ in the
tail.

Examining one of the anterior thoracic vertebre first (Fig. 124), we
see that the cestrzm (c) is continuous above with the ~eural arch (n. a),
the lower part of which, on either side, presents an anterior and a
posterior notch (z. v. 2), so that when the vertebree are in their

x VERTEBRAL COLUMN 481

natural position, an ¢fervertebral foramen is formed for the passage
of « spinal nerve. The roof of the arch is continued into a long
neural spine (1. sp) projecting upwards and backwards, and just
above the intervertebral notches are a pair of anterior and posterior
articular processes or sygapophyses (pr. 2, pt. 3), which articulate
synovially with the vertebree next in front and behind respectively.
The articular surface of each pre-zygapophysis looks upwards and out-
wards, that of the post-zygapophysis downwards and inwards. Arising
laterally from each side of the
arch is an outstanding ¢razs-
verse process (t. pr), on the
under surface of which is
an articular tubercular facet,
(¢. f) with which the upper
fork of the rib (p. 483) arti-
culates. The lower fork or
head of the rib articulates
with a facet (c.f) formed
partly by-the anterior edge of
the corresponding centrum
just at the base of the neural

arch, and partly by the pos- Fic. 124.—Fifth alae serebia of the
. rabbit, from the left side (x 14).
terior edge of the centrum on centrum; ¢.f”. capitular half-facet for fifth,
next in front, so that each and c.f". for sixth rib; ¢f. epiphysis ;
: Z.v.#,. intervertebral notch; 2. a. neural
centrum bears half a capétz- arch ; 2. sf, neural spine ; Jr. 2. pre-zyga-

eae pophysis ; pt. 2. post-zygapophysis; 7.4
lar facet, as it is called, on tubercular facet for fifth rib; 4. p>, trans-

either side, both anteriorly verse process,

and posteriorly (c. 7’, «. f” ).

There are no free ribs in the vertebra of other regions, in which,
however, they are represented in the embryo, but early fuse with the
corresponding transverse processes.

The first cervécal vertebra, or aé¢/as, is ring-shaped, and its lower
portion is narrow and unlike the other centra. The neural spine is
small, and the transverse processes are broad horizontal plates, each
perforated at its base by a wertebrartertal canal, through which the
vertebral artery runs. On the anterior face of the lateral parts of the
atlas are two concave articular facets for articulation with the occipital
condyles of the skull, and on its posterior face are two smaller acets for
articulation with the second vertebra. The second cervical vertebra, or

Pracr. Zoot. I]

482 THE RABBIT CHAP.

axis, has ils centrum produced anteriorly into a conical odontodd process,
which fits into the lower part of the ring of the atlas and is held in its
place by a ligament extending transversely across the latter: it is ossi-
fied from a distinct centre, and probably represents the true centrum
of the atlas. The neural spine of the axis is elongated and compressed,
and ils transverse processes small and perforated each bya vertebrarterial
canal. Zygapophyses are present only on the posterior face of the
arch. In all the other cervical vertebrae, the transverse processes are
also perforated by the vertebrarterial canal, and except in the seventh
or last, are divided into dorsal and ventral lamelle. The zygapophyses
resemble those of the thoracic vertebra described above. The seventh
cervical vertebra has a longer spine than the others, and bears a pair
of half facets on the posterior surface of its centrum with which the first
pair of ribs in part articulate.

fA typical ¢horacic vertebra has already been described. In the tenth,
the neural spine is vertical, and in the remaining two or three, which are
larger than the others, it slopes forwards. In the posterior three or
four there are no tubercular facets, the ribs in this region not being
forked; and the capitular facets are entire, and are situated on the
corresponding centrum only. Additional processes are present above
the pre-zygapophyses from the ninth thoracic vertebra onwards.

The Zumbar vertebre are relatively large, increasing in size from
before backwards, and their various processes are greatly devel-
oped. The neural spines are directed upwards and forwards, the
transverse processes are large and project outwards, downwards, and for-
wards. As in the posterior thoracic vertebrze, there are stout processes
above the pre-zygapophyses, and there is also a pair of more slender
processes below the post-zygapophyses and a median ventral process
projecting downwards from the centrum in the first two.

Thesacral vertebre are fused together to form the sacrum,which supports
the pelvic arch. The first—and to a less extent the second also—has
large, expanded, transverse processes which articulate with the ilia:
these are the sacral vertebrae proper, the others, which decrease in size
from before backwards, are really the anterior caudal vertebrae which
fuse with the true sacral vertebrze to form a compound sacrum.

The more anterior caudal vertebrz resemble those of the sacral
region, but on passing backwards all the processes are seen to diminish
in size, until nothing but the centra are left at the end of the
tail.

x STERNUM AND PECTORAL ARCH 483

There are twelve or occasionally thirteen pairs of ribs,
which have the form of curved rods, situated in the walls of
the thorax, and articulating with the thoracic vertebre
above and—-in the case of the first seven—-with the breast-
bone or sternum below: the remaining ribs do not reach
the sternum (Fig. 121).

Each rib consists of a bony, dorsal, vertebral portion, and of a ventral,
sternal portion consisting of cartilage which is calcified or only
incompletely ossified. The dorsal end—the head or cagitudum of the
rib—articulates with the capitular facet on the centra, and the first nine
have also a éudercle, ashort distance from the capitulum, which articulates
with the tubercular facet ; just externally to the tubercle is a short,
vertical process. :

The sternum, which is developed in the embryo by the
fusion of the ventral ends of the ribs, consists of six seg-
ments or s¢ernebra, the first of which, or manudrium, is larger
than the rest, and has a ventral keel. With the last is con-
nected a rounded, horizontal, cartilaginous plate, the x7phi-
sternum. The ribs articulate between the successive
sternebrae except in the case of the first pair, the articula-
tions of which are on the manubrium.

The chief bone of the pectoral arch is the flat, triangular
scapula, the coracoid portion (compare p. 47) becoming early
fused with it and forming a small, inwardly curved, coracoid
process, situated anteriorly to the glenoid cavity at the lower
end or apex of the scapula: the apex lies over against the
first rib, and the bone inclines upwards and backwards to
its dorsal base, which in the fresh condition, consists of a strip
of cartilage, the supra-scapula. On its outer surface is a
prominent ridge or spzze, the free ventral edge of which is
called the acromion, from which a process, the metacromion,
projects backwards. The collar-bone or clavicle is never
strongly developed in Mammals in which the fore-limb only

112

484 THE RABBIT CHAP.

moves in one plane—forwards and backwards: in the
rabbit it is a small, curved, rod-like bone, attached by
fibrous tissue at one end to the sternum and at the other
to the coracoid process of the scapula, there being small
cartilages at either end of it.

The relative positions of the bones of the fore-limb are at first sight
somewhat difficult to understand owing to their having become altered
in the course of development. In your own fore-arm the bones can
be rotated on one another, so that the thumb can be made to point
outwards or inwards ; while in the rabbit the first digit has permanently
the same position, pointing inwards. To understand this, extend your
arm outwards with the thumb pointing away from the ground. The
back of the hand and arm, continuous with the dorsal surface of the
body, or back, is its dorsal surface ; the palm of the hand, and the sur-
face of the arm continuous with the: chest, is its vertral surface: the
border of the arm and hand continuous with the thumb is the fre-
axtal border; and that continuous with the little finger the postaxzal
border. This position is called the position of szpznation; if
the fore-arm and hand be now rotated, so that the thumb points
inwards, the position is that of proxatéon. While in this position,
bend the elbow at right angles and bring it inwards close to the body ;
the preaxial border of the hand will now be on the inner side, and
an examination of the bones of the fore-arm shows that they cross
on another. It is in this position that the bones of the rabbit’s fore-
limb are permanently fixed (Fig. 101, and compare Fig. 8).

The proximal extremity of the Awmerus bears a rounded
head for articulation with the glenoid cavity, in front of
which is a groove for the tendon of the biceps muscle
(p. 61); and certain /wderosities for the attachment of mus-
cles will also be observed. Its distal extremity presents a
large, pulley-like surface or ¢voch/ea for the articulation of
the bones of the forearm, and a deep depression or fossa,
perforated bya foramen, on its posterior side, for the recep-
tion of the end of the ulna. The radius is the shorter,
inner (preaxial) bone of the fore-arm, and is slightly curved.

BONES OF THE FORE-LIMB 485

Its head presents a large double surface for articulation with
the trochlea of the humerus, and its distal extremity a pair
of slight concavities for the bones of the carpus: the shaft
is flattened where it abuts against the corresponding flat-
tened surface of the w/va. Near the proximal end of the
last-mentioned bone is a cavity for the articulation of the
humerus, and proximally to this, at the elbow, the ulna is pro-
duced to form a large olecranon process, which is received
into the fossa on the humerus when the limb is extended :
its small distal end articulates with the carpus.

The carpus consists of a proximal and a distal row of
small, nodular bones, which articulate with one another
where they are in contact. The bones of the proximal row,
beginning at the inner (preaxial) side, are the radiale and
intermedium, articulating with the radius, and the w/nare,
articulating with the ulna. In the distal row are five bones,
the middle one of which is distinctly proximal to the other
four, so as really to lie in the middle of the carpus: this is
the centrale, the others constituting a row of distal carpals.
Of these the first three articulate with the corresponding
digits, the fourth, on the outer (postaxial) side, supporting
the fourth and fifth digits, and really consisting of two
carpals fused together. A small bone, the péséform, articu-
lating with the ulna and ulnare on the ventral side, is
usually looked upon as a sesamotd bone, 7.e., an ossification
in the tendon of a muscle ; but it may represent the vestige
of a sixth digit.

The hand or manus consists of five digits, each made up
of a metacarpal and phalanges, articulating with one another.
The innermost (preaxial) digit—the thumb or fo//ex—is the
shortest, and the third the longest: the former has two
phalanges, the others three each, the distal or wagual
phalanx of all the digits having a conical form, its dorsal

486 THE RABBIT CHAP,

surface being grooved for the firmer attachment of the horny
claw. Small sesamoid bones are situated on the under or
palmar side of the joints of the digits.

The ends of the long bones in both limbs are separately ossified as
epiphyses (compare p. 480), which eventually unite with the shaft of the
bone in question.

The pelvic arch consists of two lateral halves or zanominate
ones, the long axis of which is almost parallel with that of
the vertebral column (Tig. 121), and which are firmly united
anteriorly and internally with the transverse processes of the
sacral vertebree by a rough surface, while ventrally they are
connected together by cartilage at the pelvic sympAysis. On
the outer surface of each innominate bone, at about the
middle of its length, is a deeply concave cup, the acetabulum,
for articulation with the head of the femur: in it, in young
rabbits, a triradiate suture can be seen, marking the
boundaries of the three bones of which the innominate is
composed (p. 50). Of these, the antero-dorsal is the 7//um,
which is connected with the sacrum. ‘The postero-ventral
portion of the innominate is perforated by a large aperture—
the obturator foramen, through which a nerve of that name
passes, the bone above and behind it being the ¢schzwa, and
that below it the pds. Behind the obturator foramen the
ischium has a thickened posterior edge or /uderosity, and
then curves round and becomes continuous with the pubis,
both bones taking part in the symphysis.

In young rabbits it will be noticed that the part of the pubis which
enters the acetabulum consists of a’small, distinct epiphysis.

The hind-limb has undergone rotation forwards (Fig. 121), so as to
be brought, like the fore-limb, into a plane parallel with the median
vertical plane of the body; but the rotation being forwards, and the
bones of the shank not being crossed, the preaxial border is internal

in the whole limb, and the original dorsal surface looks, on the whole,
forwards.

x BONES OF THE HIND-LIMB 487

Close to the proximal end of the femur, on its inner
(preaxial) border, is a rounded, projecting Aead for articulation
with the acetabulum : the actual end of the bone is formed
by a strong process, the great trochanter, while just distal to
the head is a /esser trochanter, and opposite this, on the
outer (postaxial) side, a ¢*hird trochanter. ‘The distal end of
the bone bears two large condyles, separated from one
another by a notch, for articulation with the tibia: this
notch is continuous with a groove extending for a short
distance along the anterior (dorsal) surface of the femur in
which a large sesamoid bone, the knee-cap or fate/a, slides :
the patella lies in the tendon of the extensor muscles of the
leg, and is connected by ligament with the tibia. Two other
sesamoid bones, the fase//e, occur on the opposite side of
the knee-joint.

The “dia, or inner (preaxial) bone of the shank, is much
larger than the fdu/a, the distal half of which in the adult
becomes completely fused with it. The proximal end of the
tibia bears two slightly concave articular surfaces for the
condyles of the femur, and distally it articulates with the
tarsus: a prominent ridge—the cnemial crest, extends along
the proximal end of its anterior (dorsal) surface. The
distinct part of the slender fibula is attached proximally to
the tibia.

The /arsus consists of six bones arranged in three rows.
In the proximal row are two tarsals, of which the inner
(preaxial) or astragalus—probably corresponding to two
bones fused together, the ‘déale and intermedium—has
a large pulley-like surface for articulation with the tibia:
while the outer (postaxial) —the fbu/are or calcaneum—artic-
ulates with the fused end of the fibula, and is produced
into a strong heel or cakaneal process. In the middle
row is a single bone, the cevfra/e of the tarsus, and the distal

488 THE RABBIT 2 CHAP.

row is made up of three bones, the true first, together with
the corresponding digit (Aa//ux), being absent as a distinct
bone. The second (apparent first) distal tarsal articulates
proximally with the centrale, and distally with the
innermost (preaxial) metatarsal: the third (apparent
second) with the astragalus and the corresponding meta-
tarsal: the fourth (apparent third), which corresponds to
the two outer (postaxial) tarsals fused together, with the
centrale and calcaneum and the remaining two digits.

The foot or pes consists of four metatarsals with their
phalanges, of which there are three to each digit. The
metatarsal of the hallux, together with the corresponding
distal tarsal, is probably represented by a distinct ossification
which in the adult becomes fused with the second (apparent
first) metatarsal, and forms a process on that bone which
articulates with the centrale. The phalanges are similar to
those of the manus, and sesamotd bones are also present on
the under surface of the foot.

Muscles and Body-wall.—It will be remembered that in
Amphioxus and the dogfish the muscles of the trunk arc
divided up into myomeres (pp. 404 and 418), while in the
adult frog the only indication of such a segmentation of the
muscles is seen in the recti of the abdomen (Fig. 16, 7c. abd).
In the rabbit nearly all trace of a segmentation of the
muscles has also disappeared, and the muscular system,
although similar in its general arrangement to that of
the frog (compare Fig. 16), is more complicated and
highly differentiated. We shall have occasion to notice
certain of the muscles in the course of our examination
of other organs.

Immediately beneath the skin, which consists of epiderm
and derm (p. 128), the whole ventral region of the trunk

x BODY-WALL AND CCELOME 489

and neck is covered by a thin cutaneous muscle, by means of
which the rabbit is able to twitch its skin. Internally to
this muscle in the female are the #ammary glands (p. 467),
which, when secreting, appear as long, whitish, branched
masses, the ducts of which can be traced to the eats, on
the apices of which they open by numerous small apertures.

A whitish band of connective tissue passes along the mid-ventral line
of the abdomen from the xiphisternum to the pubis; this separates two
longitudinal bands of muscle, the rect? abdomznds, from one another ;
and laterally to them, the abdominal wall consists of three thin layers
of muscle with their fibres running in different directions—the externa/
obligue, the tnternal oblique, and the fransversal’s, the latter being
lined on its inner surface by the Zerzéoneum. A fibrous cord, known as
Poupart’s ligament, beneath which the blood-vessels and nerves pass
outwards to the leg, extends upwards and forwards from each pubis
to the anterior part of the corresponding ilium. In the thorax the
muscles of the body-wall are broken: up into separate portions by the
ribs, and thus form a series of .2zdercostal muscles, which are, like the
oblique muscles of the abdomen, arranged in two layers, external and
internal, and are important in respiration.

Extending from the thorax to the fore-limb of either side
are the large pectoral muscles ; and a number of other muscles
can be seen in the neck, in the ventral middle line of which,
covered by the cutaneous muscle, the windpipe or ¢vachea is
visible. The trachea is strengthened by a series of carti-
laginous rings, and ends in front in the /arynx, situated
between the two rami of the mandible; and just in
front of the larynx is the hyoid bone (p. 479), embedded
in a mass of muscle.

The Cclome and its contents.—On cutting open the
body-cavity, it will be seen.to be divided into two main
chambers—the ¢horacic and abdominal cavities—by means of
the diaphragm (Fig. 125,@). The relatively small thorax—
which is lined by a serous membrane corresponding to the

490 THE RABBIT CHAP. X

peritoneum of the abdomen and known as the s/evva—con-
tains the lungs, as well as the heart enclosed in a pericardium,
on the ventral surface of which is an organ known as the
thymus (see p. 431): the gullet and main blood-vessels
also pass through the thorax. The abdomen encloses the
greater part of the enteric canal, together with the liver,
and pancreas, the spleen, and the urinogenital organs.
The diaphragm is convex on its anterior side, towards
the thorax: it consists of a central, thin, semdinous portion
into which radial muscles are inserted. ‘These arise
from the vertebral column and posterior ribs, and are
especially strong on the dorsal side, where they form two
bands known as the fé//ars of the diaphragm. When the
muscles contract, the diaphragm is made flatter, and thus
the thoracic cavity is enlarged.

Digestive organs.—The mouth-cavity (Fig. 125) is large,
and the small gape is bounded by upper and lower lips, be-
hind which are the incisor teeth (7). On either side of the
cavity are the borders of the upper and lower jaws from
which the cheek-teeth project: these are separated from
the incisors by a considerable interval or d@astfema. Close
behind the upper incisors are a pair of small openings
leading into the vaso-palatine canals (n. p. c), which communi-
cate with the nasal cavities but must not be confounded
with the internal nostrils. The roof of the oral cavity is
formed by the fa/ate, the anterior part of which, or hard
palate (fh. p), is transversely ridged and partly supported
by bone (4. 2’, p. 477); while the posterior part, or soft
palate (s. ~) is smooth, its hinder, free edge forming a
pendulous flap, the ve/um palati, on each side of which
is a tonsil consisting of connective and lymphatic tissue and
having the form of a small pit with a broad papilla on its

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3)

492 THE RABBIT CHAP.

of Eustachian tube ;_77¢. Fallopian tube; ¢. 4. gall-bladder ; 4. 7. hard palate ;
A. p’. bones of hard ‘palate ; Ay. hyoid ; 7. incisors ; 7/. ileum; 7.%. passage of
internal nostrils; 7. Jacobson's organ; &. left kidney ; 4. az. left auricle; /zg.
left lung, seen through pleura ; /~. liver; 7.4. fourth lumbar vertebra; ¢. v7,
left ventricle ; 7, #6. maxillary turbinal; 2. 4. ¢. naso-palatine canal; 7. A/.
naso-pharynx ; 2. 2b, naso-turbinal ; 7. s. nasal septum (middle part cut away);
as, gullet; o/. 2. olfactory lobe; ov. left ovary; #. a. pulmonary artery ; Ac.
pericardium; fer. g?. perineal gland; 4%. pharynx; fhr. 2. phrenic nerve
(origin not shown); /. 7. a. posterior mesenteric artery; /. mr. premaxilla;
pu. pancreas ; fu.d. pancreatic duct; fv.c. left precaval; 2. sy. pelvic sym-*%
physis ; A¢.c. postcaval ; py. st. pyloric region of stomach ; ~. ribs; ~c¢. rectum ;
v. gl. rectal gland; ». vv. right ventricle; sk. / floor, and sk. 7. roof of skull ;
sé. gd. sublingual gland ; s. vx. g/. submaxillary gland ; s. A. soft palate, ending
in the’velum palati, on the lower side of which a tonsil is seen ; sf. c. spinal cord ;
sp.. spinal (lumbar) nerves; s. 7. sacculus rotundus of ileum; s. v. first sacral
vertebra ; sy. sympathetic (the anterior end shown on the right side, the rest on
the left); 2. tongue; 747. thyroid ; ¢#. 7.9 ninth thoracic vertebra ; ¢/iy. thymus ;
ty. trachea ; #.d/. urinary bladder ; #7. ureter; w¢. uterus; vag. vagina; vb,
vestibule ; vg. vagus (the anterior end shown on the right side, the rest on the
left) ; v7. vulva.

outer margin. The ¢ozgue (¢) lies on the floor of the mouth

to which it is attached below, its anterior, rounded end

being free: the surface of its posterior part is elevated, and

elsewhere—but more particularly on the tip—its covering

of mucous membrane is produced into minute, finger-

shaped pape, on which some of the microscopic organs -
of taste are situated (compare p. 180). ‘Taste-organs are also

present on a pair of e/rcumvadllate papille on the dorsal side

of the tongue towards its posterior end, and on a pair of
transversely ridged areas—the folate papille, situated

laterally, slightly anterior to the» former. The main

substance of the tongue is composed of muscles, some

extrinsic—z.e. arising from other parts, and others in-

trinsic—/.e. entirely confined to the organ in question.

The ¢eeth (Figs. 122 and 123), as we have seen, are not all
alike, as in the dogfish and frog: there are zwczsors and cheek-
zeeth or grinders, the latter being divisible into two series—
the premolars and the molars. In most Mammals there is
also a pair of canine teeth, situated between the incisors and
premolars, and these are especially long and pointed in such
carnivorous animals as the dog and cat. In the dogfish
and frog, again, the teeth are continually renewed as they

x TEETH 493

become worn out, but in Mammals there are never more
than two functional sets, which are known respectively as
the deciduous or “milk”-teeth, and the successional or
permanent teeth: certain of the former may even be
absorbed before birth, as is the case with the incisors of the
rabbit. The incisors and premolars
(and in Mammals in which they
are present the canines also) have
deciduous predecessors, the molars
developing behind the premolars
and having no predecessors.

All the teeth are embedded in
sockets or alveoli of the jaw-bones,
and each contains a pulp-cavity
(Fig. 126, PA) extending into it
from the base and containing blood-
vessels and nerves. In the case of
the rabbit, the aperture of the pulp-
cavity (PH) remains wide open in
each tooth, and the substance of
the tooth is continually added to 1-226 Longitudinal section

of a mammalian tooth, semi-

5 ; diagrammatic.
at its base as it wears away at PH. pulpreavity PH’. open-
is rae ing of same ; ZB. dentine;
the other end: in many Mammals, 93 a Zs. “Guam:

however (¢.g., dog, cat, man), the oe Wiedersheim’s Ana-
aperture becomes narrowed and

growth ceases after a time, the base of the tooth forming one
or more roots or fangs. The main substance of each tooth
is formed of dentine (ZB), into which the pulp-cavity extends
for a considerable distance and round which the examed
(ZS) forms an external layer, which may become more or less
folded inwards (as in the cheek-teeth and front upper incisors
of the rabbit), the cement (ZC) extending into the folds

(compare p. 429).

494 THE RABBIT CHAP.

The number of the various teeth in the jaws is con-
veniently expressed by a dental formula, in which the kind
of tooth (incisor, canine, premolar, or molar) is indicated
by the initial letter ¢, ¢, pw, or m, and the whole formula
has the arrangement of four vulgar fractions, in each of
which the numerator indicates the number of teeth in the
upper, the denominator that of those in the lower jaw, only
those of one side being indicated, since the teeth of the right
and left sides are always the same. Thus the dental formula
of the rabbit is 72, ¢9, pm3, m} = 28.

The anterior incisors in the upper jaw of the rabbit are long and
greatly curved. They are surrounded by enamel, which is much
thicker on the anterior surface, where it presents a median
groove; the posterior upper incisors are much smaller and are
situated behind the anterior ones. In the lower jaw the single pair of
long and curved incisors have no median groove, and they bite between
the anterior and posterior upper incisors: owing to the thick layer of
enamel anteriorly, they, like the large upper incisors, wear away less
quickly in this region, and thus remain sharp, like a chisel, at their
biting edges. The premolars and molars, on the other hand, are
modified for grinding the food, to do which satisfactorily it is necessary
that they should have broad crowns with a surface which remains uneven.
This is effected in most of the cheek-teeth by the enamel becoming in-
voluted along the outer side in a longitudinal direction, so as to form a
groove extending into the dentine almost to the other side, the groove
becoming filled up with cement. As the enamel is harder than the
dentine and cement, it thus gives rise to ridges as the crown wears.
The first upper premolar and the last upper and lower molars are sim-
pler than the others, and the first lower premolar presents two grooves.

Connected with the mouth-cavity are several pairs of
salivary glands, not present in the other Vertebrates we have
examined, the secretion of which—sa/va—contains a
ferment called ptyadin, which is capable of converting starch
into sugar. The food taken into the mouth is ground up

x ENTERIC CANAL 495

or masticated and mixed with the saliva before passing
down the gullet, and thus digestion begins in the
mouth.

The zxfra-orbital salivary gland is a large, lobulated, pinkish mass
situated in the antero-ventral region of’ the orbit, below and in front of
the eyeball: its duct passes downwards to open into the mouth, nearly
opposite and externally to the second premolar. The parotid gland is
a soft, irregular, flattened organ, lying close beneath the skin, just
below and in front of the base of the external ear; ils duct passes for-
ward and opens close to the duct of the infra-orbital gland. The szd-
maxillary gland (Fig, 125, s.mx. gl), is a reddish, ovoidal, compact
body situated inside the angle of the lower jaw and near the middle
line, somewhat anterior to the larynx: its duct runs forward to open
into the mouth a short distance behind the lower incisors. The sad-
lingual gland (si. gl) is an elongated structure situated on the inner
side of the mandible, and having several ducts opening be ae
into the mouth.

The oral cavity is continued backwards as the pharyns:
(ph): this begins at. the velum palati, above which. it ex-
tends forwards as the xaso-pharynx (n. ph) ; the latter is*con-
tinuous with the passage of the internal nostrils, and ‘into
it open the Eustachian tubes (evs, compare pp. 17 and 45).
On the floor of the pharynx, behind the base of the tongue,
is the g/ottis, which leads into the larynx and is guarded: in
front by an elastic, leaf like, cartilaginous flap, the efig/ottis
(epg): this projects upwards towards the velum palati and
is capable of being pressed backwards over the glottis during
the passage of food from the mouth to the gullet.

The gudle¢ (Figs. 125 and 127, @s) is a narrow but dilatable
tube which passes backwards along the neck and through
the thorax, entering the abdomen through an aperture
in the diaphragm and then opening into the stomach
(py. st, ea. st), a wide, curved sac, elongated transversely
and greatly dilated at the cardiac end, which lies towards the

Fic. 127.—The stomach duodenum, posterior portion of rectum, and_jiver (in outline)
of the Rabbit, with their arteries, veins and ducts. .4, the coeliac artery ot
another specimen (both x 3). The gullet 1s cut through and the stomach some-

x ENTERIC CANAL 497

what displaced backwards to show the ramifications of the coeliac artery (ca. a);
the duodenum is spread out to the right of the subject to show the pancreas ( fm) ;
the branches of the bile-duct (c. 4. @), portal vein (J. v) and hepatic artery (4. a)
are supposed to be traced some distance into the various lobes. of the liver.

a, m. a, anterior mesenteric artery ; caz. caudate lobe of liver with its artery, vein
and bile-duct ; ¢.6.d. common bile-duct ; cd.s¢. cardiac portion of stomach ;
¢. 72,a@. common iliac artery ; ce.a. coeliac artery; cy.a@. cystic artery; cy.d.
cystic duct ; @. ao. dorsal aorta ; dz. proximal, and @z’. distal limbs of duodenum ;
du. a. and dy, a. duodenal eae du. h. a. (in A), duodeno-hepatic artery ; ¢. a.
oer artery and vein ; g. 4. gall bladder ; 2. a. hepatic artery ; 4. d. left hepatic ‘
duct ; Zc. left central lobe of liver, with its artery, vein, and bile-duct ; Ze. 7.
lieno-gastric vein ; 2. 7, left lateral lobe of liver, with its artery, vein, and bile-duct;
ms. branch of mesenteric artery and vein to duodenum ; ms. 7. mesentery of
the rectum ; 7. v. chief mesenteric vein ; as. gullet ; 4. 7. a. posterior mesenteric
artery ; £.2.u, posterior mesenteric vein ; ##. pancreas ; 2, d@. pancreatic duct ;
p.v. portal vein ; Ay. st. pyloric portion of stomach; 7c?. rectum; ~ c. right
central lobe of liver, with artery, vein, and bile-duct ; sfg. Spigelian lobe of
liver with its artery, vein, and bile-duct; sf2. spleen; sf. a. splenic artery,
(From Parker’s Zootomty.)

left side of the body: the pyloric end, from which the duo-
denum arises, towards the animal’s right, is less dilated and
has much thicker muscular walls. The mucous membrane of
the stomach, in which the microscopic gastric glands (p. 131)
are contained, is raised into ridges or ruge, and there
is a circular pyloric valve at the entrance to the intestine.
The duodenum (dw) extends backwards along almost the
whole length of the abdomen and then turns forwards again,
forming a slightly coiled, U-shaped loop, and becoming
continuous with the very long and coiled, second portion of
the small intestine or zlewm (Fig. 125, 22), which finally
dilates to form a rounded sac (s. ~) opening into the proximal
end of the dark-coloured co/on (col) or first portion of the
large intestine. The colon has a much greater diameter
than the small intestine, and presents a series of saccula-
tions arranged in three rows, separated by flat regions
of its wall: it passes insensibly into the second portion of
the large intestine or vectum (rct), which is of about the
same diameter as the small intestine, and is. recognisable by
its rounded swellings containing the pill-like feces: it passes
into the pelvic cavity to open by the anus (a). At the junction

of the ileum and colon is a relatively enormous blind-gut or
Pract. Zoo, KK

498 THE RABBIT CHAP.

cecum (cec)—a structure not met with in either the dogfish
or the frog, and only reaching such a relatively large size
as in the rabbit in certain other herbivorous Mammals in
which the stomach has a simple form: in those which
possess a complicated stomach (v7z., Ruminants) the caecum
is comparatively small. It is continuous with the proxima]
end of the colon, which contains an intra-colic valve and
into which the round sac at the distal end of the ileum
opens by a circular aperture provided with an 7/eo-cole
valve. From this point arises the thin-walled caecum, which
lies coiled on itself amongst the folds of the rest of the
intestine : it is about an inch in diameter, and a spiral con-
striction is seen on the outside marking the attachment, on
the inside, of a sféval valve—like that of the dogfish’s in-
testine but narrower—which makes about twenty-four turns
and ends at the base of a blind, finger-shaped process, the
vermiform appendix, which forms the apex of the caecum.
The whole canal is supported by a mesentery (p. 27) which
has a very complicated arrangement in correspondence with
the numerous folds of the intestine.

It will be noticed that the intestine is much more differentiated as
regards its subdivisions than in the Vertebrates previously examined, and
also that it is relatively much longer, being fifteen or sixteen times as
long as the body.

On cutting open the small intestine, its mucous membrane is seen to
be raised into minute, finger-shaped elevations or vz//, and here and
there certain patches present a honeycombed appearance : these portions
are known as Peyer’s patches, and, like the tonsils, thymus, and spleen,
consist of masses of lymphoid follicles composed of a connective-tissue
framework in which numerous leucocytes are imbedded. Other
so-called ‘‘lymphatic glands” or adeno7ds are present in the mesentery
and elsewhere. Peyer’s patches also occur in the proximal end of the
colon, close to the ileo-colic aperture ; and the round sac with which the
colon communicates, as well as the yermiform appendix, are lined with

x LIVER, PANCREAS, AND SPLEEN 499

similar lymphoid tissue. The mucous membrane of the colon has no
villi, but, like that of the spiral valve, is raised into small papille ;
while that of the rectum is smooth.

‘Phe ver (Figs. 125, /v, and 127) is a large organ, consist-
ing of five lobes. Its anterior surface is convex and is
applied to the diaphragm, its posterior concave surface
fitting against the stomach. A median vertical fold of the
peritoneum attaches it to the diaphragm, and marks the
boundary between the right and /eft central lobes (Fig. 127
7, 1c), Externally to the left central lobe, between it and
the cardiac end of the stomach, is the /ft lateral lobe (7. I),
and externally to the right central lobe the caudate lobe
(caz), applied to the pyloric end of the stomach and hollowed
posteriorly, when it fits over the right kidney: a small
Spigelian lobe (spg) fits closely against the concave anterior
surface of the stomach. The d7/e-duct is made up of several
hepatic ducts (h. d@) from the various lobes of the liver, as
well as of a cystic duct (cy. d) from the pear-shaped gad/-
bladder (g. 6), which is imbedded in the right central lobe
of the liver: the common bile-duct (c.b.d) thus formed opens
into the dorsal side of the duodenum by a prominent aper-
ture a short distance beyond the pylorus.

The pancreas (Figs. 125 and 127 pz) isa diffuse gland,
consisting of a number of small lobules looking not unlike
masses of fat, spread all over the mesentery which connects
the two limbs of the duodenum. The small ducts from the
various lobules run together to form the main pancreatic duct
(gn. @) which opens into the distal limb of the duodenal
loop a couple of inches or so beyond the bend.

The spleen (Fig. 127, sp/) is a long, flat body of a dark
red colour, attached to the cardiac end of the stomach by a
sheet of peritoneum.

KK 2

500 THE RABBIT CHAP.

Organs of respiration and voice.—Owing to the presence
of a neck, the lungs are situated some distance from the
glottis (Fig. 125), and, instead of a short laryngo-tracheal
chamber, as in the frog (p. 141), there is a windpipe or
trachea (tr) extending along the neck (p. 489), just ventrally
to the gullet, its anterior end forming the /avynx or organ of
voice, and communicating with the pharynx through the
glottis. The cartilaginous rings of the trachea are incom-
plete dorsally, and the cartilages of the larynx are more
highly differentiated than in the frog, apart from the presence
of an epiglottis (epg, p. 495). The largest and most anterior
laryngeal cartilage is the ¢#yroid, which, like the epiglottis,
is peculiar to Mammals: it has the form of a broad ring,
incomplete dorsally, and is the part of the larynx which can
be felt externally. The second cartilage is the cricoid,
represented in the frog by a ring-shaped cartilage at the base
of the lungs (p. 153): its form is somewhat like that of a
signet ring, being broad dorsally—where it lies mainly
between the edges of the thyroid—and narrow ventrally.
A pair of arytenoid cartilages are articulated to the dorsal
and inner surface of the cricoid, and each is produced
into a projecting process situated between the two edges of
the thyroid cartilage. The vocal cords (p. 144) are a pair
of elastic folds extending across the cavity of the larynx
from the thyroid below to the arytenoids above, each
bounded in front by a depression.

In the position of rest, the vocal cords lie at an acute angle to one
another, as in the frog; they can be brought into parallelism and
regulated by the action of a number of intrinsic and extrinsic muscles,
and are set in vibration by the respiratory current of air.

After entering the thorax, the trachea divides into two
bronchi (Fig. 125, br), one entering each lung and giving off
branches to its different lobes : the bronchi, like the trachea,

x RESPIRATORY ORGANS 501

are supported by incomplete cartilaginous rings at their
anterior ends, but these gradually disappear after they have
entered the lungs.

The elastic Zengs (Figs. 125 and 128, /mg) are not hollow
sacs, like those of the frog, but are spongy bodies, of a light
pink colour, situated on either side of and above the
heart, and filling the greater part of the thoracic cavity but
collapsing as soon as the wall of the thorax is perforated.
Each is subdivided into two main lobes, and the right lung
has in addition two small accessory lobes, an anterior and a
posterior, the latter lying in the median line, behind the
heart, and being closely applied to the gullet.

Each pulmonary artery (Fig. 125, f.a,) crosses the main bronchus
anteriorly to the point at which it branches into the various lobes,
except in the case of the anterior accessory lobe, the bronchus to which
comes off in front of the artery and may even arise from the trachea
before its bifurcation. Microscopic examination shows that the bronchi
divide and subdivide to form a ramifying system of tubes, each ultimate
branch of which opens into a minute chamber or zzfenddbulum, which
in structure closely resembles a frog’s lung in miniature.

The parietal layer of the pleura (p. 490) lines the cavity of
the thorax, and is reflected over each lung at the entrance of
the bronchus to form the visceral layer: in the median line
it forms a vertical partition, the mediastinum, with which it
is continuous ventrally to the vertebral column above and
beneath the pericardium below (Fig. 128). Thus each lung
(2. Ing, r. ing) has its own separate pleural cavity (2. pl, r. pl),
separated from its fellow by the right and the left medias-
tinum, the space between which is called the mediastinal
space. The anterior and dorsal parts of this space are narrow
and enclose the posterior part of the trachea and the bronchi,
as well as the gullet (ves) and main blood-vessels (aoz‘,
az.v, pt.cav); its middle part is wide, and encloses the heart

502 THE RABBIT CHAP.

(2. vent, r. vent), the mediastinum here fusing with the
visceral layer of the pericardium (v#s. per) and thus ob-
literating the space ; below this it again narrows to form the
ventral mediastinal space (v. med), in which the thymus
(p. 490) is situated.

In the entire animal, the air-tight pleural cavities are

Fic. 128.—Diagrammatic transverse section of the rabbit's thorax in the region of
the ventricles, to show the relations of the pleurze and mediastinum (dotted line),
etc. The lungs are contracted.

aort. dorsal aorta ; az. v. azygos vein ; cez¢. centrum of thoracic vertebra ; 2. dug.
left lung ; 2. Ad. left pleural cavity ; /. vemt. left ventricle ; zy. spinal cord ; oes.
gullet; Jar. fer. parietal layer of pericardium; z. cav. postcaval, close to
its entrance into right auricle; ~ Zug. right lung; 7” f/. right pleural cavity ;
r. vent, right ventricle; s¢. sternum; vis. fer. visceral layer of pericardium ;
v. med. ventral mediastinal space. (From Parker and Haswell’s Zoology.)

completely filled by the lungs, so that the parietal and vis-
ceral layers of the pleurz are practically in contact, there
being only a lubricating serous fluid lymph between them.

The pressure of the air in the bronchial cavities of the
lungs is therefore sufficient to keep them distended ; but as

x HEART 503

soon as the pleural cavities are perforated, the pressure is
equalised, and the elasticity of the lungs comes into play,
causing them to collapse. When the muscles of the dia-
phragm contract (p. 490) air is drawn into the lungs, and
this process is aided by the external intercostal muscles
(p. 489) and, in forced respiration, by other muscles of the
body-wall also. The mechanism of respiration may there-
fore be compared with a suction-pump, while that in the
frog resembles a force-pump (p. 142).

On either side of the larynx isa soft, vascular, gland-like thyrod body,
consisting of two lateral portions connected ventrally by a median
bridge. Its function is not thoroughly understood: morphologically it
represents a gland developed from the pharynx, but it loses its con-
nection with the latter and thus has no duct. The glandular vesicles of
which it is composed give rise to an albuminous substance containing
iodine, which is passed into the blood and lymph; if extirpated in
the living animal, various functional disturbances result. We are also
ignorant of the function of the thymus, which is largest in young
animals, becoming reduced in size in adults.

Organs of circulation.—The Zearé, as in all Vertebrates,
is enclosed in a pericardium consisting of parietal and
visceral layers (Fig. 128), between which is a serous peri-
cardial fluid. There is a complete separation between the
arterial and venous blood in the heart, for in addition to
an auricular septum, as in the frog (p. 88), the ventricular
portion is divided into right and left chambers by a partition
(Fig. 128), the arterial blood from the lungs entering the
left auricle and thence passing into the left ventricle to be
pumped into the aorta, and the venous blood entering the
right auricle and thence into the right ventricle to pass to
the lungs through the pulmonary artery. A distinct conus
arteriosus and sinus venosus (pp. 79 and 433) can no longer
be recognised, the former having become practically absorbed

504 THE RABBIT CHAP.

into the ventricular portion of the heart, and the latter into
the right auricle ;' so that the aorta—together with the caro-
tids—and the pulmonary artery now arise directly from
the left and right ventricles respectively, and the pre-
cavals and postcaval enter the right auricle independently
(Figs. 125 and 130).

The line of separation between the two ventricles can be
seen externally as an oblique depression extending from the
base of the heart backwards and to the right, but not
reaching the apex, which is formed by the left ventricle only.
The small, irregular cavity of the latter is enclosed by very
thick muscular walls, and is partly surrounded by the right
ventricle, the cavity of which is crescentic in transverse
section (Fig. 128), while its walls are much thinner than those
of the left ventricle, as it has only to pump the blood to the
lungs. The auricles have thin walls: each is produced into
a little flap or appendix which envelops the base of the cor-
responding ventricle, and the walls of which are strengthened
by a network of muscular bands.

In the auricular septum is a thin, oval area, the fossa ovals (Fig.
129, f. ov), which in the embryo is perforated, and so allows the blood
from the body to pass directly into the right auricle without going to
the lungs, which are not, of course, functional until the animal is born.

The two auriculo-ventricular apertures are guarded by
valves—that of the left side, or mfral valve, consisting of
two membranous flaps, that of the right, or ¢vicuspid valve
(Fig. 129, ¢7é. v) of three flaps: the valves are attached by
their bases to the margins of the apertures, their apices ex-
tending into the corresponding ventricles. Attached to their
edges are tendinous cords arising from conical elevations
of the ventricular walls known as papillary muscles, which
are much larger in the left ventricle than in the right

x HEART 505

(m. fap): these serve to prevent the valves from being
pushed into the auricles when the ventricles contract.

The right ventricle narrows towards its base, on the
ventral side of the heart, to form a conical prolongation
from which arises the pulmonary artery (Figs. 125 and 130,
p.a), its aperture being guarded by three pocket-like, semi-
lunar valves (Fig. 129, sem. v): the aperture of the aorta from

SECTILYV

Fic. 129.—Heart of the rabbit, seen from the right side, the walls of the right auricle
and right ventricle partly removed so as to expose the cavities.
ao. aorta; f ov. fossa ovalis ; 2. fr. c. opening of left precaval; 1. pap. papillary
muscles; fz¢.c. postcaval; Af.c’. opening of postcaval; ~ fr.c. right pre-
caval; ». gud. right pulmonary artery; sem. v. semilunar valves; ¢7i. v.
tricuspid valve. (From Parker and Haswell’s Zoology.)

the left ventricle is similarly provided with three semilunar
valves. The two precavals (4 fr. ¢, ~ pr. c) and the post-
caval ( f4. c) communicate, as we have seen, directly with
the right auricle, the right precaval opening into it anteriorly,
the left precaval posteriorly, the aperture of the postcaval
being just anterior to that of the left precaval. The
pulmonary veins from each lung unite and open together
into the left auricle.

506 THE RABBIT CHAP.

Membranous folds, the Eustachian and Thebestan valves, extend
into the right auricle between the apertures of the postcaval and left
precaval: in the embryo these help to conduct the blood through the
aperture in the auricular septum (p.504) : they afford another example of
vestigial structures, representing the remains of the sinu-auricular valve.

You will remember that in the frog (p. 80) there are
two systemic trunks, representing the second arterial arch of
the tadpole and fish (p. 435, and Fig. 111), and uniting
above to form the dorsal aorta. In the Mammal, one of
these—the right—disappears in the course of development
and all the blood from the left ventricle passes into the
single deft aortic arch (Figs. 125 and 130) from the base of
which both carotid arteries arise, the aortic arch then
curving over the left bronchus to pass into the dorsal aorta
(d. ao). :

Close to the origin of the aortic arch, just beyond its
semilunar valves, two small coronary arteries are given off to
the walls of the heart ; and more anteriorly, at the curve of
the arch, arise the vessels which supply the head and fore-
limb. There is a certain amount of variation as to the origin
of these, which is asymmetrical, and is usually as follows.
Springing from the arch of the aorta towards the right side
is an innominate artery (Fig. 130, in), which gives off close
to its origin the ft common carotid (/.c.c), and then, passing
forwards, divides into the right common carotid (r.cc) and
the right subclavian (s. cla), the left subclavian (67) taking
its origin independently from the left side of the arch.
Each common carotid passes forwards along the neck, close
to the trachea, and at about the level of the larynx divides
into an izternal carotid (¢. c), which supplies the brain, and
an external carotid (e.c), which goes to the head and face.
Each subclavian forms several branches, the most important
of which are a brachial (br) to the fore-limb, a vertebral

ARTERIES 507

(vr) which passes through the vertebrarterial canal of the
cervical vertebra (p. 481) and supplies the spinal cord and
brain, and an anteriorjepigastric or internal mammary (a. epg)
running along the inner side of the ventral wall of the
thorax. The aorta gives off, in the thorax, a series of small
paired intercostal arteries (t.cs) to the body-walls, and then
passes into the abdomen, between the pillars of the
diaphragm.

A short distance behind the diaphragm the cvfiac artery
(Figs. 127 and 130, cw) arises, and supplies the liver}
stomach, and spleen ; and about half or three quarters of an
inch further back is the anterior mesenteric artery (a.m.a),
the branches of which pass to the small intestine, pancreas,
cecum, and colon. Close behind the anterior mesenteric is
the right—and rather further back the left—vena/ artery (Fig.
130, 7), and still more posteriorly, a posterior mesenteric
(p. m) to the rectum, and a pair of spermatic or ovarian arteries
(spm) to the spermaries or ovaries, as the case may be.
A small caudal artery (m. sc), corresponding to the caudal
continuation of the aorta, arises from the dorsal surface of the
posterior part of the latter just in front of a pair of large
common iliac arteries (¢. tl. a), which appear like a bifurcation
of the aorta. These are continued outwards and backwards
towards the hind-limbs, each giving off an ¢4o-lumbar artery
(.2) to the dorsal body-wall and then dividing into an
internal tliac (t. tl. a) passing along the dorsal side of the
pelvic cavity, and an external iliac (e. tJ. a) which gives off an
artery to the bladder, (s.vs) and in the female one to
the part of the oviduct known as the uterus (#); and
then, passing beneath Poupart’s ligament (p. 489) to the
hind-limb, becomes the femoral artery (fm. a), from the
proximal end of which a posterior epigastric (p. epg)
runs forwards in the ventral abdominal wall. Small Zemdbar

pm

spm spin
mse
i wl
ewly Le |
Pe ees ita ewla
Peps = pepy

fm Jo ' , } . fa

Fic. 130.—The vascular system of the Rabbit from the ventral side. The heart
is somewhat displaced towards the left of the subject ; the arteries of the right
and the veins of the left side are in great measure removed.

CHAP, X VEINS 509

a. epg. anterior epigastric artery ; @/- anterior facial vein; @. 2. anterior mesen-
teric artery; @.f4, anterior phrenic vein; az. 7. azygos vein; 4. right
brachial artery; c.¢/@. common iliac artery; c. 7/.v. hinder end of post-
caval; ca. coeliac artery; @.ao. dorsal aorta; e.c. external carotid artery ;
e.id. a, external iliac artery ; ¢.7/.v. external iliac vein ; e. 7%. external jugular
vein ; fm. a. femoral artery ;_/7. v. femoral vein ; 2. v. hepatic veins ; z.¢. internal
carotid artery ; 7.cs. intercostal vessels; 7.7. internal jugular vein; 7. 2. ilio-
lumbar artery and vein; iz. innominate artery; 7. az. left auricle; ¢.¢.c. left
common carotid artery ; 7. 7. c. left precaval vein; 7. v. left ventricle ; 72. sc.
caudal artery; #.a. pulmonary artery ; 4. efg. posterion epigastric artery and
vein; ~.f posterior facial vein; /. 2. posterior mesenteric artery; /. px.
posterior phrenic veins ; fzc. postcaval vein; #.v. pulmonary vein; 7. renal
artery and vein; ». az. right auricle; 7.c.c. right common carotid artery ;
x, pre. right precaval vein ; 7.7. right ventricle ; sed. a. right subclavian artery ;
sed. 7, right subclavian vein; sf. spermatic artery and vein; s.vs. vesical
artery and vein; #/. uterine artery and vein; 7. vertebral artery. (From
Parker’s Zootomy.)

arteries are also given off from the aorta to the walls of the
abdomen.

The pulmonary artery (p.a) divides soon after its origin
from the right ventricle into two branches, one supply-
ing each lung. Just before its bifurcation it is connected
by a short cord, known as the ductus arteriosus, with the
aorta: this is the solid vestige of the embryonic connection
between the fourth arterial arch and the aorta (compare p.
436, and Fig. 111).

Each precaval (1 pr. or. pr. c) receives—a_ sub-
davian (s. cd. v) from the fore-limb; an external jugular
(e. jw) from the head, running along the neck just
beneath the skin; a small anterior epigastric from the
ventral thoracic wall, as well as small vessels from some
of the anterior intercostal spaces (¢. cs) and the anterior
surface of the diaphragm (a. p2); and a small izterna/l
jugular (i. ju) from the brain, opening into the cor-
responding external jugular nearly opposite the sub-
clavian, An azygos vein (az. v), representing part of the right
cardinal of the embryo (compare p. 440) and receiving blood
from the posterior intercostal spaces, also opens out the
base of the right precaval.

There is no renal portal system, as in the dogfish and frog

510 THE RABBIT CHAP,

(pp. 438 and 85). A pair of zvternal iliac veins (7. 7. v) in the
pelvic cavity unite to join a median vessel (¢ 7/ v), the
hinder end of the postcaval, which receives on either side
an external iliac (e. ii. v), constituted by—a femoral vein
(fm. v) from the hind-limb; a fosterior epigastric ( p. epg),
from the ventral walls of the abdomen, entering the femoral
just external to Poupart’s ligament ; and by small veins from
the bladder as well as from the uterus in the female.
Slightly in front of the external iliacs the postcaval receives
a pair of large ¢o-lumbar veins (¢.2) from the body-walls :
the left iliolumbar sometimes runs forwards to open into the
corresponding renal vein. Rather more anteriorly still are
a pair of spermatic (spm) or ovarian veins, and a large renal
vein (r) enters the postcaval from each kidney. As the
postcaval passes through the dorsal border of the liver, it
receives several large hepatic veins (Figs. 127 and 130, 4. 2’)
from the lobes of that organ. Other small veins from the
body-walls and from the posterior surface of the diaphragm
also open into the postcaval, which then passes through the
central tendon of the diaphragm and runs forward in the
mediastinal space (Fig. 128, Az. cav) to open into the right
auricle.

The hepatic portal vein (Fig. 127, ~. v) is a large vessel
situated in the mesentery, ventral to the postcaval. An-
teriorly it passes into and divides up in the liver, sending a
branch to each lobe : posteriorly it is constituted by a large
anterior mesenteric vein (m. v) returning the blood from the
small intestine, colon, and cecum, and by smaller veins
from the stomach, spleen, and duodenum, as well as by a
posterior mesenteric vein (p. m. v) from the rectum.

The pulmonary veins have already been described (p. 505). In the
freshly-killed animal a number of the delicate, transparent, /ymphatic
vessels (p. 97) can be made out, those from the intestine. (/actea/s)

x BRAIN 511

running in the mesentery. They come into connection with numerous
adenoids (p. 498) in the mesentery and elsewhere, and most
of them communicate with a main trunk—the thoractc duct—which
extends from the abdomen through the thorax on the left and upper
side of the aorta. The thoracic duct also receives the lymphatics from
the right side of the head and neck and the fore-limb, and opens into
the veins at the junction of the left external jugular and subclavian : the
lymphatics of the right side of the head and neck and right fore-limb
communicate with the corresponding veins of the right side.

Nervous System.—The brain (Figs. 125, 131, and 132)
reaches a much higher development than in the other Verte-
brates we have already studied. The prosencephalon is sub-
divided into two cerebral hemispheres (ch), of much larger
relative size than those of the frog (Fig. 49) and forming
about two-thirds of the whole brain. They are closely applied
to one another along their flat internal surfaces, and are roughly
conical in form, narrower in front ( frontal lobes), broadening
out posteriorly (parietal lobes) where they overlap the dien-
cephalon and optic lobes and abut against the cerebellum,
and produced downwards into the prominent ¢emporal lobes
which partly overlap the crura cerebri below. ‘Their ex-
ternal layer or cortex is formed of grey matter, and their
surfaces are smooth, except for the presence of slight fissures
between the lobes: in many Mammals the surface of the
hemispheres is highly convoluted, z.e. raised into numerous
winding elevations or gy7t, separated by narrow grooves or
sulci. A broad transverse band of nerve-fibres forms a com-
missure connecting the two hemispheres known as the
corpus callosum (Fig. 131 cp. ¢, and Fig. 132, ¢. c/): this
structure is confined to the Mammalia, and is even wanting
in certain of the lower members of the class. The o/factory
lobes (olf) are club-shaped, and extend backwards along the
ventral surface of the hemispheres in the form of narrow
bands as far as the temporal lobes,

512 THE RABBIT CHAP.

The diencephalon consists of a right and left optic thalamus
(0. th.) between which is the slit-like ¢hird ventricle (v")

pn.
ond P

Fic. 131. Two dissections of the Rabbit's brain, from above (nat. size), In A, the
left hemisphere is dissected down to the level of the corpus callosum ; on the
right side the lateral ventricle is exposed. In B, the hemispheres are dissected
down to a little below the level of the anterior end of the corpus callosum ; only
the frontal lobe of the left hemisphere is retained, of the right a portion of the
temporal lobe also is left ; the choroid plexus and pineal body are removed, as
well as the greater part of the body of the fornix and the whole of its left posterior
pillar ; the cerebellum is removed with the exception of a part of its right lateral
lobe with the flocculus.

. €0. anterior commissure ; @./o. anterior pillar of fornix 5 a. 2. anterior peduncles
of cerebellum; 4./0. body of fornix ; cl. central lobe of cerebellum ; cé2. its
lateral lobe ; ¢. gz. elevation on the optic thalamus ; c /. cerebral hemisphere ;
ch, pl. part of choroid plexus ; cf. cl. corpus callosum ; cf. s. corpus striatum 3 ¢. 7s.
and d. /. elevations on the bulb ; 77. flocculus ; //. #7. hippocampus ; 2.co. middle
commissure ; 0. Z!. 0.-¢2. optic lobes ; o/f. olfactory lobe ; 0. 74. optic thalamus ; 0.
tr. optic tract (continuation of chiasma); /.co. posterior commissure; 7./0.
posterior pillar of fornix ; 7. pineal body ; Ad. 2. its peduncle ; f. fz. posterior
peduncles of cerebellum ; 4. za. fibres of pons Varolii forming middle peduncles
of cerebellum ; sf, 2. septum lucidum ; s¢. 2, line on corpus callosum ; ¢.'s. band
of white matter lying beneath choroid pleaus; 7. 7. valve of Vieussens; 7°.
third ventricle ; v4. fourth ventricle. (From Parker's Zoofomry.)

=

roofed over by a thin membrane, on the upper surface of which
is a vascular choroid plexus (Fig. 132, v/. ip), and from the

Xx BRAIN 513

hinder part of which arises a stalk bearing at its end a
small, rounded pineal body (pn). The floor of the
diencephalon is produced downwards to form the
infundibulum (inf), to which the pituitary body (pty) is at-
tached. In front of the infundibulum is the optic chiasma
(0. ch) and behind it a small, rounded lobe (¢. ma).

Each otic Jobe is divided into two by a transverse furrow,
so that there are four rounded elevations in this region—an
anterior, larger pair (. 2),and a posterior, smaller pair (a. 2).
Below the optic lobes are the crura cerebri (c. c)—two
strong, diverging bands passing forwards and outwards from
the bulb to the hemispheres.

The dub or medulla oblongata. (m.o) is slightly flattened
dorso-ventrally, and passes behind into the spinal cord, the
dorsal and ventral fissures of which are continued into
it: the fourth ventricle (v*) which it contains is roofed
over by the thin pia mater only (p. 155). Ventrally its
anterior border is marked by a stout band of nerve-fibres
running transversely, and known as the fons Varoli ( p. va).
The cerebellum.is connected with the dorsal surface of
the brain by three pairs of peduncles (Fig. 131, a. px, p. va,
p. pn), and consists of a median central lobe (cb.1) and of
two lateral lobes (cb.2), on the outer side of each of which
isa smaller floccular lobe (fl). The grey matter is super-
ficial, and the surface is marked by numerous folds
which in section present a tree-like pattern (ardor vite),
brought about by the arrangement of the grey and white
matter (Fig. 132).

The fourth ventricle is not prolonged into the cerebellum
to any extent: it is continued forwards as the s#r, from
which no optic ventricles are given off (compare pp. 151 and
443) and which passes into the narrow but deep third
ventricle in front (Fig. 132): this is bounded anteriorly by

Pract. Zoon, LL

514 THE RABBIT CHAP.

a thin wall, the damina terminadis (/. 4), and extends into the
infundibulum below. At its anterior end are the foramina
of Munro (7. m), leading into the middle of the /atera/
ventricles in the hemispheres (Fig. 131). In this region
each lateral ventricle is broad from side to side, but narrow
from above downwards ; it extends forwards into the frontal

Fic. 132.—Rabbit. Longitudinal vertical section of the Rabbit's brain (nat. size),

Letters as in preceding figure ; in addition cd, central lobe of cerebellum, showing
arbor vita; c.c. crus cerebri; ¢. 4.1 parietal, and c. 4.2 temporal lobe ot
cerebral hemisphere ; ¢. 7a. elevation behind the infundibulum ; / 7. foramen
of Monro; 7#/. infundibulum ; @.z. lamina terminalis; Zy. part of hippocampus ;
m.o, medulla oblongata; o.ch. optic chiasma; /y. pituitary body; v7. 7/.
choroid plexus; //. optic nerve. (From Parker's Zootomzy.)

lobe, backwards into the parietal lobe, and downwards into
the temporal lobe. The olfactory lobes are solid.

A prominent, convex ridge of white matter—the Azppocamipus (Fig.
131, 4g. m.) projects into the inner side and floor of each lateral ven-
tricle where it descends into the temporal lobe, and closely applied to it
is a continuation of the choroid plexus (cZ. 7), which passes from the
the roof of the third ventricle into the lateral ventricle through the
foramen of Monro. In front of the hippocampus the outer side and
floor of the anterior part of the lateral ventricle is thickened to form an
eminence of grey matter, the corpus striatum (cp. s) Just beneath the
corpus callosum the internal wall of each lateral ventricle is thin, and
is known as the septem lucidum (sp. Zu); and below it and above the
foramina of Monro is another commissure known as the body of the
fornix (Figs. 131 and 132, 6. Jo) which is continuous on each side with

Xx SPINAL CORD AND NERVES 515

two bands—one (posterior pillar) lying along the anterior edge of the
hippocampus, and the other (anterior pillar) passing backwards in the
side walls of the third ventricle. Connecting the two optic thalami
are three tranverse bands of nerve fibres, known respectively as the
anterior (a. co), neiddle (m. co) and posterior ( p. co) commedssures : the
middle commissure, which is much the largest, is not represented in the
lower Vertebrata.

The spinal cord (Fig. 125, sf. c) is similar in structure
to that already described in other Vertebrates (pp. 155 and
443). It extends through the entire neural canal, ends in a
Jilum terminale, and is swollen opposite the fore- and hind-
limbs, where the nerves arise which form the limb-plexuses
(pp. 161 and 162). -

The dorsal and ventral roots of the spinal nerves lic in the same
transverse plane, as in the frog (p. 163), but are relatively shorter than
in that animal ; and after uniting to form the nerve-trunks, pass directly
outwards through the intervertebral foramina. The dvachzal plexus is
formed from the four posterior cervical and the first thoracic nerve, and
gives off a number of nerves to the shoulder and fore-limb. The sezatzc
or Zumbo-sacral plexus is constituted by the two or three hindermost
lumbar and the first two or three sacral nerves, and gives off branches to
the pelvic region and hind-limb, the chief of which are a femoral and
teroneal going to the dorsal extensor muscles, and a large se¢atéc and an
obturator (which passes through the obturator foramen, p. 486) supplying
the ventral flexor muscles. Arising from the fourth cervical spinal
nerve of either side is a phrenic nerve (Fig. 125, phr. 2), which passes
backwards, between the heart and lungs, to supply the muscles of the
diaphragm ; and a large auricular nerve, arising from the third cervical
nerve, supplies the external ear.

In addition to the ten cerebral nerves enumerated in the
frog (p. 163) and dogfish (p. 445), two others—the spzna/
accessory and the hypoglossal (represented by the first spinal
nerve in the frog, p. 160) emerge from the skull and are
counted respectively as the eleventh and twelfth cerebral
nerves. The former arises from the side of the spinal cord

LL 2

516 THE RABBIT CILAP.

and bulb by numerous fibres, the posterior of which are op-
posite the fifth spinal nerve, from which point it runs forwards
between the dorsal and ventral roots and leaves the skull
together with the glossopharyngeal and vagus (p. 479), sup-
plying certain muscles of the neck and shoulder. The
hypoglossal arises by a number of fibres from the ventral
surface of the bulb, passes out through the condylar foramen
and supplies the muscles of the tongue as well as certain
muscles of the neck.

The origin and distribution of the first ten pairs of cerebral nerves
correspond in their main features with those seen in the frog. The facial
is almost entirely a motor nerve and is chiefly important in supplying
the facial muscles, which are very highly developed in Mammals.

The relations of the sympathetic nerves (Fig. 125, sy) are
also essentially similar to those occurring in the frog (p. 162).
Each passes backwards along the neck close to the vagus
(vg) and alongside the carotid artery, enlarging at each end
of this region to form an anterior and a posterior cervical
ganglion. In the thorax it runs just beneath the heads
of the ribs, having a ganglion in each intercostal space:
it then passes into the abdomen, lying close to the centra
of the vertebre and having ganglia at intervals. From all
the sympathetic ganglia branches are given off connecting
them with the spinal nerves, others going to the blood-
vessels: others again, in the thorax and abdomen, are
connected with plexuses from which nerves pass to the
heart and abdominal viscera. In the abdomen these
plexuses can be seen in the mesentery, a large ce/iac plexus
being present close to the origin of the cceliac and mesen-
teric arteries.

Sensory Organs.—The sense of touch is situated in micro-
scopic ¢acti/e organs in the skin, and groups of cells, called

Xx SENSE-ORGANS 517

taste-buds, are present on the papilla of the tongue (p. 492)
and on the soft palate (compare pp. 179 and 180).

The organs of smell are situated in the olfactory capsules,
the form of which has already been described (p. 476).
They open externally by the external nostrils, and are pro-
duced backwards above the palate into the passage of the
internal nostrils, which communicate with the naso-pharynx
(Fig. 125 ¢. 7, 2. ph, p. 495). The olfactory epithelium,
supplied by the olfactory nerves, is situated on the ethmo-
turbinal (e. #4): the mucous membrane of the maxillo-tur-
binal (7. #%) probably serves merely to warm the inspired air.

On the ventral side of the nasal septum is a pair o1 small, tubular
structures known as Jacodson’s organs (Fig. 125, 7), lined by epithelium
and enclosed in cartilages situated just to the inner side of the palatine
processes of the premaxillee (p. 477). Each of them opens anteriorly
into the corresponding naso-palatine canal (p. 490), and receives a
special branch of the olfactory nerve. The function of these organs is
not understood.

The structure of the eye (Fig. 57) is similar to that already
described in other Vertebrates (pp. 181 and 449), except that
the sclerotic is not cartilaginous, but is composed of dense
fibrous tissue, and the lens is relatively smaller than in the
dogfish and frog and is markedly biconvex in form, the outer
surface being rather flatter than the inner: it is capable of
adjustment by means of the a/ary muscles contained in the
radiating cary processes (C.P) into which the choroid is
thrown just externally to the iris (compare p. 184).

The eyelids have already been described on p. 469. The four rectd
murtscles ensheath the optic nerve, as in the frog (p. 186, compare Fig.
117), but the sugerzor oblique, instead of arising—like the zferdor oblique
~—in the anterior part of the orbit, takes its origin further back, neay the
recti, passes forwards through a fibro-cartilaginous pulley at the anterior
angle of the orbit, and then backwards and outwards to its insertion on
the eyeball.

518 THE RABBIT CHAP.

Between the wall of the orbit and the eyeball are two
glands, the secretion of which, passing through ducts per-
forating the conjunctiva lining the eyelids, serves to keep
the outer surface of the eye moist, and is then conducted
into the nasal chambers by means of the naso-lachrymal
duct (pp. 186 and 477). These two glands correspond to
special differentiations of a primarily continuous structure :
one, the Harderian gland—already met with in the frog—is
situated in the antero-ventral region of the orbit: the other,
or Jlachrymal gland proper, in its postero-dorsal region.
Besides these, a series of small Merbomian glands are present
on the inner side of the edges of the eyelids, and produce
a fatty secretion.

The essential part of the auditory organ consists, as in
other Vertebrates, of the membranous labyrinth with its
three semicircular canals (pp. 186 and 449) enclosed in the
auditory capsule (periotic bone, p- 475), and constituting the
internal ear. he small outgrowth of the sacculus seen in
the dogfish and frog, and known as the cochlea (Fig. 59, 7),
is represented by a relatively larger structure, coiled on itself
ina spiral manner. ‘The part of the periotic bone which
directly surrounds the cavity in which the membranous
labyrinth lies is especially hard, and when the outer portion
of the bone is cut away, is seen to form a sort of cast of the
enclosed organ, the form of which it repeats: this is known
as the dony labyrinth (Fig. 133). Internally it is separated
from the membranous labyrinth bya narrow space all round,
containing the perilymph (p. 189) and only shut off from
the tympanic cavity at the fenestra ovalis and fenestra
rotunda (p. 475) by a membrane which closes them.

The membranous cochlea does not run up the middle of the spiral or
the bony cochlea, but is attached between its outer wall and a spiral
shelf arising from its inner wall. Thus the entire cochlea shows

x EAR 519

three cavities in transverse section—a middle, the membranous cochlea
or scala media, and a scala vestibulé and a scala tympani on either side of
it respectively, which communicate with one another at the apex of the
cochlea and with the perilymphatic cavity surrounding the rest of the
membranous labyrinth at its base, where the scala tympani abuts
against the membrane of the fenestra rotunda, and the scala vestibuli
against that of the fenestra ovalis. On the wall of the scala media which
separates it from the scala tympani is a specially modified series of
auditory cells forming what is known as the organ of Corti, which
receives nerve-fibres from a branch of the auditory nerve extending along
the spiral shelf of the cochlea.

The middle ear (p. 450) is constituted by the tympanic
cavity in the tympanic bulla (p. 475), and communicates
with the pharynx by the Eustachian tube (Fig. 133, E).
The tympanic membrane (M), situated obliquely at the
boundary of the bulbous and the tubular portions of the
tympanic bone, separates the middle ear from the external
ear, consisting of the auditory passage (Ex) and the pinna
(p. 469).

The fenestra ovalis is plugged by a small stirrup-shaped
bone, the s¢apes (Fig. 133, O!), one of the three auditory
ossicles (p. 475) connecting the internal ear with the tym-
panic membrane, and probably corresponding morpho-
logically to the cartilaginous stapes of the frog (Fig. 10,
stp): with it is connected a small staped/us muscle, serving to
keep the membrane of the fenestra ovalis on the stretch.
‘The middle bone of the chain is the zvews (Fig. 133, 07), a
short process of which is articulated to the stapes by the
intermediation of a small bony nodule, while its body
articulates with the outer bone of the series, the mad/eus
(0%). Arising from the body of the malleus is a handle-like
process or manubrium, which is attached to the tympanic
membrane (M): this has the form of the roof of a tent, and
is kept on the stretch by a small muscle, the /exsor tympant,

520 THE RABBIT CHAP.

arising from the wall of the tympanic cavity and inserted on
to the manubrium of the malleus.
The study of development indicates that the malleus corresponds to

the articular part of Meckel’s cartilage of lower Vertebrates, and the
incus to the quadrate (p. 44): the articulation of the bony mandible

lic. 133.-—Diagram of the mammalian bony labyrinth, tympanic cavity, and external
auditory passage.
Cch. bony cochlea; Z, Eustachian tube ; Zx. external auditory passage ; Z. bony
semicircular canals; JZ. tympanic membrane; 4. auditory nerve; Ol, stapes ;
O2, incus; 03, malleus. (After Headley.)

with the squamosal in Mammals has rendered these parts unnecessary
for their original purpose, and they have undergone a change of function,
forming an accessory part of the auditory apparatus.

Urinogenital Organs.—The A7zdveys (Fig. 125, 2) are of a
somewhat compressed, oval shape, with a notch or Az/vs on
the inner side. They are in close contact with the dorsal
wall of the abdominal cavity, the right being somewhat in
advance of the left. Towards the hilus, the tubules of

x URINOGENITAL ORGANS 52!

which the kidney is composed (p. 146) converge to open
into a wide chamber or Ze/v7s, which forms the dilated com-
mencement of the ureter. When the kidney is cut across,
its substance is seen to be divided into a central mass or
medulla and a peripheral portion or cortex. The former
appears radially striated, owing to the tubules in this region
being straight and converging to open on the surface of a
conical process or pyramid, which projects into the pelvis:
the cortex contains the coiled portions of the tubules and
the Malpighian bodies, and thus has a dotted appear-
ance. The ureter (Figs. 125 and 134, wv) runs backwards
along the dorsal wall of the abdomen to open into the
urinary bladder (u. b/, bl), a pyriform sac with elastic walls
which vary in thickness according as the organ is dilated or
contracted: in the male the openings of the ureters are
situated nearer the posterior, narrower end or neck than in
the female. Near the front end of each kidney, towards its
inner side, is a small yellowish adrenal or supra-renal body
(Fig. 125, adr).

In the male Rabbit the spermaries are oval bodies which
in the young animal are situated close to the kidneys, on the
dorsal wall of the abdomen, but which pass backwards and
downwards as the animal approaches maturity until they
come to lie each in a scrotal sac (p. 470), situated at the side
of the urinogenital opening. The cavity of each scrotal sac
is in free communication with the cavity of the-abdomen by
an opening—the inguinal canal. <A convoluted epididymis
(p. 451), closely adherent to the spermary and connected
with the distal end of the scrotal sac, forms the proximal
part of the spermiduct or vas deferens (Fig. 134 A, v. a),
which, together with the blood vessels and nerves of the
spermary, passes through the inguinal canal: it then loops
round the corresponding ureter, and extends back between

522° ‘THE RABBIT CHAP.

the neck of the bladder and a median sac on the dorsal side
of the latter, known as the uéerus masculinus (uw. m). The
neck of the bladder is continued backwards, through the

ur

Fic. 134.—The urinogenital organs of the Rabbit. A, of male; B, of female : from
the left side (half nat. size). The kidneys and proximal ends of the ureters, in A
the spermaries, and in B the ovaries, Fallopian tubes, and uteri are not shown.

an, anus; 6/. urinary bladder ; c. c. corpus cavernosum ; c. s. corpus spongiosum ;
c.gl. Cowper’s gland ; g.c/. glans clitoridis ; ¢.. apex of penis ; A.g7. perineal gland ;
p. gl. aperture of its duct’on the perineal space ; #7. anterior, 4’. posterior, and
pr". lateral lobes of prostate; vcf. rectum; 7. ¢7/. rectal gland; #. g. @. urino-
genital aperture ; 2. 2. uterus masculinus ; 77. ureter 3 7a. vagina 3 7. vestibule ;
z.d. vas deferens. (From Parker's Zeotonzy.)

pelvic cavity, as the wrtnegenital canal or urethra, on the
dorsal side of which, just in front of a rounded elevation, is
an aperture by means of which the uterus masculinus and
vasa deferentia open into it.

x URINOGENITAL ORGANS 523

A prostate gland ( pr), consisting of several lobes, is imbedded in the
walls of the uterus masculinus and opens by small ducts on either side
of the elevation just referred to; and a pair of smaller, ovoid Cower’s
glands communicate with the urinogenital canal further back.

The terminal part of the urethra traverses the copulatory
organ or fens (p. 470), the posterior or dorsal wall of which
is constituted by a soft vascular portion, the corpus spongt-
osum (¢. s), while the opposite surface is strengthened by two
harder bodies, the corpora cavernosa (c. ¢), which are closely
applied together through the greater portion of their length,
but diverge proximally and are attached to the ischia-

In both sexes a pair of ferdneal glands (p. gl) open on the perineal
spaces (p. 469) at the sides of the penis, and two larger rectal glands
(7. gf) lie at the side of the rectum.

In the female the ovaries (Figs. 125 and 135, ov) are small,
ovoid bodies attached by peritoneum to the dorsal wall of
the. abdomen behind the kidneys, the ovarian follicles or
ovisacs (p. 195) forming very small, rounded projections on
their outer surface.

The oviducts, instead of remaining separate along their
whole length, are fused proximally to form a wide, median
portion, the vagina (Figs. 134 B and 135, va), opening
into the urinogenital canal or vestibule (vb), with
which the bladder communicates and which opens ex-
ternally at the vw/va (Fig. 134 B, w. g.@). Into the other or
distal end of the vagina, the paired, thick-walled wer? (Fig.
135, 7. wt, 2. wZ), or middle portions of the oviducts, open by
separate thick-walled apertures. The eggs undergo develop-
ment in the uteri, which vary in size according to whether
or not they contain embryos, and according to the stage of
development of these. Each uterus is continued forwards as
a narrow, slightly coiled tube—the anterior section of the
oviduct, or Fallopian tube (ft. ¢) which communicates with

524 THE RABBIT CHAP.

the ccelome by a small aperture (j#. 7’) surrounded by a
wide, membranous funnel with thin walls and folded margins,
which is applied to the outer surface of the corresponding
ovary. On the ventral or anterior wall of the hinder or
proximal end of the urinogenital canal is a small, hard, rod-
like body, the clitoris (Fig. 125, ¢/), corresponding to the

Lt Lut
# :

/ ;
ti Lt un

Fic. 135.—The anterior end of the vagina, with the right uterus, Fallopian tube
and ovary of the Rabbit (nat. size). Part of the ventral wall of the vagina is
removed, and the proximal end of the left uterus is shown in longitudinal section.

fl. t. Fallopian tube; (77. 2’. its coelomic aperture; 2. 2¢. left uterus 3 2. 2f’. aper-
ture of same (0s uteri) into vagina; ov. right ovary; 7. wt. right uterus; 7”. a2’.
right os uteri; s. vaginal septum ; va. vagina. (From Parker’s Zoozomy.)

penis of the male, and composed of two small corpora
cavernosa (Fig. 134 B, cc) attached at their proximal ends
to the ischia. ‘

The Rabbit is viviparous. ‘The minute ova undergo de-
velopment in the uterus, in which each develops into a
fetus, as the intra-uterine embryo is termed, and is nourished
by means of an organ known as the Adacenta, which will be
described in the next chapter. The young animal escapes

DEVELOPMENT AND CLASSIFICATION 525

from the uterus in a condition in which all the parts have
vecome fully formed except that it is practically hairless ;
the eyelids are at first coherent. As many as eight or ten
young are produced at a birth, and the period of gestation,
ie., the time elapsing between the fertilization of the ovum
and the birth of the young animal is thirty days. Fresh
broods may be born once a month throughout a considerable
part of the year, and, as the young Rabbit may begin breed-
ing at the age of three months, the rate of increase is very
rapid.

The class Mammalia is divided into a number of orders,
that to which the Rabbit belongs being called the Rodentia,
and also including rats and mice, squirrels, beavers, porcu-
pines, and many others. All these are vegetable feeders and
are mostly of small size. They possess no canine teeth,
and their incisors, which are adapted for gnawing, are never
more than two in number in the lower jaw, and in most
of them there are only two in the upper jaw also.

PRACTICAL DIRECTIONS

The specimen used should be over three months old. Place it in a
sufficiently large jar or box with a close-fitting lid together with a piece
of cotton-wool well soaked in chloroform, arid leave it until a short time
after all movements have ceased. A pair of bone-forceps (p. 12) will
be required.

A. External characters (see pp. 467-470).

Microscopic preparations of the skin of a Mammal should be examined,
and the hazrs, and hair-sacs, and sebaceous glands (p. 468) noted. (For
mode of preparation, see p. 136).

B. Skeleton. If more convenient, the detailed examination of the
skeleton may he postponed until after the soft parts have been

dissected.
The skeleton of a young rabbit, about six weeks old, as well as that

526 THE RABBIT CHAP.

of an adult, should be obtained for examination: the former is the more
important of the two for making out the individual bones, which should
all be separated from one another by prolonged maceration in water, or
by boiling for a short time. In the adult skeleton the bones are best
kept in their natural connection. An additional skull should be
prepared, and a longitudinal vertical section made of it with a fine saw ;
but as it is difficult to do this very accurately and perfectly in the
rabbit, owing to the delicacy of some of the bones, the skull of a young
dog or cat may be used for the purpose.

Work through the account of the skeleton given on pp. 470-488, as
well as of the teeth on pp. 492-494, comparing the form, number and
arrangement of the teeth in the herbivorous rabbit and the carnivorous
dog or cat, noting the presence of canznes in the two last mentioned.
Sketch typical parts of the skeleton.

C. Superficial dissection, and injection of the arteries.

Fix the animal on its back on the dissecting-board by inserting large
pins through the limbs. If you wish to inject the arteries, cut through
the skin on the inner side of one of the thighs, reflect it, and with the
seeker expose the femoral artery (p. 507); if your specimen is a small
one, expose one of the carotid arteries (p. 506) instead, by carefully
cutting through the skin along the middle ventral line of the neck.
Pass a piece of thread round the artery thus laid bare and make a small
slit in it with the fine scissors distally to the thread. Insert and tie in
the cannula, and first inject about half a small syringe-full of strong
formaline (1 part formaline and 2 parts water), following this with the
coloured starch injection-mass (p. 99), which will force the formaline
into the capillaries and help to preserve the specimen: this method is
of special advantage for the subsequent examination of the enteric canal,
especially if it is not possible to get to the end of § VI, p. 530, on the
first day; the veins need not be specially injected, as they will be
naturally injected with blood.

Make a median longitudinal incision through the skin in the sternal
region, and continue the cut backwards to the pelvic symphysis and
forwards to the mandibular symphysis; then dissect away the skin
from the underlying muscles over the whole ventral surface, being
careful not to injure any of the larger blood-vessels (e.g., the jugular
veins in the neck). Note :—

1. The thin cutaneous muscle, part of which you have very likely

Xx PRACTICAL DIRECTIONS 527

removed together with the skin ; and in the adult female, the mammary
glands (p. 489): trace the main ducts of some of these to their apertures
on the teats.

2. In the neck :—a, the ¢rachea and larynx (p. 489), to see which
and the following structures clearly the cervical portion of the cutaneous
muscle should be dissected away and the underlying parts carefully
separated with a seeker; 4, the hyozd bone, situated in a mass of muscle
just anterior to the larynx ; ¢, the submaxillary glands (p. 495); and d,
the large external jugular veins (p. 509).

3. In the thorax :—a, the sternum and x7phisternum ; 6, the small
clavicles (p. 483) 3 ¢, the pectoral muscles ; d, the vertebral and sternal
portions of the +zbs and the external intercostal muscles; and e, the

~ blood-vessels and nerves going to the fore-limbs.

4. In the abdomen :—a, the muscles of the abdominal wall, separated
in the middle ventral line by a whitish, fibrous band; 6, Poupart’s
ligament (p. 489); ¢, the blood-vessels and nerves passing beneath
Poupart’s ligament to the hind-limbs.

IT. On the same day on which the animal is killed, remove the skin
from the dorsal surface of the head and anterior part of the neck, and
remove the muscles covering the anterior cervical vertebrae. A
membrane between the skull and first vertebra will then be exposed : cut
through this, and part of the sp¢va/ cord will be seen. With the bone-forceps
cut away the arches of the first three or four vertebrae and the roof of
the skull—taking particular care in the auditory region—so as to expose
the brain: note the olfactory lobes, cerebral hemispheres, cerebellum, and
medulla oblongata, and also the dura mater and pia mater. Then cut
through the spinal cord about a quarter of an inch beyond its junction
with the brain, lever up the latter with the handle of a scalpel, and
carefully cut through all the nerves arising from it and separate it
from the cranial walls, working from behind forwards. Dissect away
the olfactory lobes from their attachments, remove the entire brain and
place it in 3 per cent. formaline.

The animal may be preserved from day to day by wrapping a cloth
round it soaked in 3 per cent. formaline or strong methylated spirit,
and placing it in a closed vessel.

D. Dissection of the abdomen.
I. Once more pin the animal down, ventral side uppermost, and

make an incision along the middle line of the abdominal muscles from

528 THE RABBIT CIIAP,

the xiphisternum to the pelvic symphysis, so as to open up the
abdominal cavity. From the anterior extremity of the incision make
transverse cuts, and turn back the flaps of muscle.! Note :—

1. u, The pertloncum ; b, the diaphragm, with its muscles and
central tendon; c, the pink Zeugs, seen through the latter: make a
small aperture in the diaphragm on one side of the median line and
note the collapse of the corresponding lung.

2. u, The “ver, stomach, small intestine, cecum, colon, and rectum
(pp. 495-499); 4, the zrdvary bladder ; c, the scrotal sacs in the male ;
and—by turning aside the intestines—d, the zzdneys ; and e, the ovaries
and ovzducts (varying much in size according to age) in the female.

3. a, The characters and relations of the lobes of the Liver (compare
Fig. 128), and the folds of peritoneum which attach the liver to the
diaphragm ; 4, the gal/-bladder ; e, the gullet, and its entrance into—e,
the stomach (cardiac and pyloric portions) ; 7, the spleen.

4. a, The parietal and visceral layers of the ferttoneum and the
various subdivisions of the mesentery ; 6, the U-shaped duodenum,
passing into the z/ewm, and the connection of the latter with the
proximal end of—c, the cecum, at the distal end of which, is the
vermiform appendix, while proximally it passes into—-d, the colon,
continuous with—/, the vectzm,which enters the pelvic cavity to open by
the anus.

II. Turn over the stomach to the animal’s right side and make
out—

1. a, The Jostcaval vet, passing from the pelvis forwards, near the
ventral surface of the backbone, through a notch in the liver to the
diaphragm; 4, the dorsal aorta, running partly above, partly alongside, the
postcaval ; ¢, the cw/ac artery, given off from the aorta about an inch
posterior to the diaphragm—trace its main branches; d, the anterior
mesenteric artery, arising about half an inch further back than the
coeliac—trace its main branches.

2. u, The left Azdney, with its artery, vein, and zreter; 6, the
yellowish left adrenal, anterior to the origin of the renal artery and
vein; c, the left ovary, Fallopian tube, and uterus in the female.

1 You will very probably notice a number of transparent, rounded
vesicles, about } inch in diameter, in the abdomen. These are tape-
worms (phylum Platyhelminthes, p. 397) in the encysted stage, which
develop into the adult form when swallowed by a dog.

x PRACTICAL DIRECTIONS 529

3. @, The small posterior mesenteric arlery, \eaving the aorta 3, short
distance posteriorly to the left kidney and supplying the rectum ; 4, the
posterior mesenteric vein, in the mesentery of the rectum.

III. Turn the intestines over to the animal’s left side and spread out
the duodenum, putting its mesentery slightly on the sketch but taking
care not to rupture it, so as to make out—

1, ad, The large avtertor mesenteric vein, into which the posterior
mesenteric vein opens ; 4, the Jaxcreas and its duct, opening into the
distal limb of the duodenal loop an inch or so beyond the bend.

2. a, The right &zdney, partly overlapped by the caudate lobe of the
liver, and its zveter ; b, the right adrenal,

IV. Replace the intestines in their natural position ; cut through the
gullet close to the diaphragm, draw the stomach backwards, turn for-
wards the lobes of the liver, and dissect out the following structures
(Fig. 127) :—

1. The common bcle-duct—made up of cysttc and hepatic ducts, and its
entrance into the duodenum.

2. a, The large hepatic portal veer, running in the mesentery ventrally
lo the postcaval, made up by the union of the mesenteric, gastric,
and splenic veins, and entering the liver, sending a branch to each lobe ;
4, the transparent Lymphatic vessels (lacteals) in the mesentery.

V. Tie two pieces of thread, about half-an-inch apart, round the portal
vein just before it enters the liver (the hepatic artery and bile-duct may
be included in the ligature) ; cut through the rectum just anterior to the
pelvic cavity, and through the portal vein between the ligatures, as well
as through the mesenteric attachments of the stomach and intestine,
and remove them from the body entire. Unravel the intestine by
cutting or tearing the mesentery—except that part of it in which the
pancreas lies—and spread it out on the dissecting-board or in a large
dissecting dish, arranging it so that the various subdivisions may easily
be distinguished from one another. In addition to the relations of
these, note :—

1. The sacculus rotundus (Fig. 125, s. x), and the characters of the
cecum, which proximally passes insensibly into the colon.

2. The length of the intestine as a whole (about fifteen or sixteen
times that of the animal), and the relative lengths of the five divisions.

Sketch the entire canal.

VI. Remove and cut open the stomach and parts of the small intes-
tine, colon, and rectum: wash thoroughly, and examine under water.

Pract. Zoot. MM

530 THE RABBIT CHAP.

Remove the caecum tugether with a small portion of the ileum and

colon, wash it out by directing. a stream of water through it, distend

with water (it is better to harden it first with formaline) and examine in

a dish of water; with the scissors cut away a piece of the wall here and
ere, between the constrictions and in the appendix. Note :—

1. The mucous membrane, muscular coat, and peritoneum.

2. The longitudinal ridges or ruge@ in the stomach, and the pyloric
valve.

3. The aperture of the bile-duct, and the viii and Peyer's patches
(p. 498) in the small intestine.

4. «a, The Peyer’s patches and intra-coléc valve in the proximal part
of the colon ; 4, the thick Zymphord ¢téssue in the walls of the sacculus
rotundus ; ¢, the 2/eo-coltc aperture and valve.

5. The sperval valve of the ceecum, and the dwyphotd tissue of the
vermiform appendix.

6. The characters of the mucous membrane of the caecum and large
intestine.

(Microscopic sections of the small intestine—injected and uninjected,
should also be examined, compared with Fig. 39, and the oJ and
intestinal glands noted.)

VII. Ligature the postcaval at the points where it enters and leaves
tne liver: remove the entire liver, noting as you do so the aorta,
gullet, postcaval, hepatic veins, and phrenic vetns. Sketch the liver from
the posterior surface. Then return to the examination of the abdomen,
and make out (Fig. 130)—

1. The venal and spermatic (or ovarian) artertes and veins.

2. The caudal artery, arising from the dorsal side of the aorta.

3. The common tliac arteries, each of which gives off an z/io-lumbar
branch and divides into an external and an internal zliac, the former be-
coming continuous, beyond Poupart’s ligament, with the femoral. (The
course of these arteries and of the corresponding veins can be more easily
traced after the urinogenital organs have been removed—see § IX.)

4. The zléo-lumbar veins, and the trifurcation of the postcaval into the
external iliacs and a median vein formed by the fusion of the two zzfer-
nal iltacs.

Sketch all these: vessels later, after the removal of the urinogenital
organs,

VIII. Dissect away the peritoneum and fat from the kidneys, ureters,
and genetative organs, and trace each ureter from the Az/us of the

x PRACTICAL DIRECTIONS 531

kidney backwards to the bladder, which, if contracted, may be inflated
from the urinogenital aperture. Then make out—

A. In the male.

1. The fends, with the prepuce, and the aperture of the wénogendtal
canal, which latter is lined by the soft corpus spongiosum, and strength-
ened dorsally by the corpora cavernosa, which diverge proximally and
are attached to the ischia.

2, The scrotal sacs, each communicating with the abdominal cavity by
the zxguznal canal, through which the corresponding sgermzduct and
the spermatic artery and vein pass to the sgermary, to see which and
the efdaidymzs the scrotal sac should be slit open along its ventral wall.

3. The course of the spermiducts.

B. In the female. 2

1. The vzlva; and the clitoris, with its corpora cavernosa, on the
ventral wall of the zwrénogenttal canal.

2. The vagina, utert, and Fallopian tubes with their funnels.

3. The ovarces, studded with ovesacs.

IX. Dissect away the kidneys, ureters—and in the male the scrotal
sacs, in the female the ovaries—from the surrounding parts: cut through
the pelvic symphysis with a knife, and sever the attachment of the
corpora cavernosa to the ischia. Remove the whole of the urino-
genital organs, together with the posterior end of the rectum, from the
body, and after noting § VII, 3 and 4, pin them out in a dissecting-dish,
with the ventral side uppermost, taking care to preserve the natural
relations of the parts. Dissect away all the superfluous connective
tissue and fat, separate the rectum at its cut end from the urinogenital
canal, and turn it on one side, noting the rectal and perineal glands.
Make out, in addition to the parts referred to in § VIII :—

A. In the male. The uterus masculinus and prostate, and the spermz-
ducts passing between the former and the bladder.

Sketch the entire dissection. Then slit open the uterus masculinus so
as to see the openings of the spermiducts, remove the rectum and rectal
glands, and make out the relations of the corpus spongiosum. Make a
median incision along the whole length of the penis, beginning at the
apex and cutting through the fibrous septum betweeen the two corpora
cavernosa: continue the incision forwards, so as to open the bladder
along its ventral wall, and note and indicate on your sketch the opening
of the ureters into the bladder, and the crescentic aperture of the uterus
masculinus into the urinogenital canal, just at the anterior edge of a

cushion-like fold.
M M 2

532 THE RABBIT CHAP.

B. In the female. The wide wrinogenttal cana: with its vascular
walls.

Sketch the entire dissection.

Make a median longitudinal incision through the urinogenital canal,
and continue it forwards until the cavity of the bladder is exposed : slit
open the vagina and one of the uteri and Fallopian tubes in the same
way. Note and indicate on your sketch the openings of the ureters into
the bladder, the connection of the neck of the bladder with the urino-
genital canal, the large aperture in the latter leading into the vagina,
and the thick-lipped aperture (0s zferz), communicating between the
vagina and thick-walled uterus, between which and its fellow is a vesti-
gial septum, indicating the primarily paired character of the vagina.
Insert a seeker into the coelomic aperture of the Fallopian tube. If the
uteri contains embryos they present a series of swellings ; cut each
swelling open from the ventral side, and note the foetus attached toa
discoid placenta (p. 524) ; remove each foetus, together with its placenta,
and preserve it in corrosive sublimate (p. 135). Microscopic sections of
the ovary should be examined, and the hollow ovzsacs noted, each
consisting of follicular cells enclosing a minute ovum.

With a scalpel or razor cut across one of the kidneys through the
hilus, parallel to the dorsal and ventral faces of the organ, and note the
pelvis, and its prolongations—the calzces, the wrdnzary pyramid, and the
cortical and medullary portions. Sketch. Examine also microscopic
preparations of injected and uninjected kidney (compare p. 146).

X. Dissect out the Zmébo-sacral plexus in the pelvic cavity (p. 515), and
then cut through the body just behind the diaphragm ; the abdomen
will be no longer required, but you should retain one of the hind-limbs
if you wish to examine any of its muscles and joints (compare p. 64).

E. Dissection of the thorax and neck.
I. Dissect away the pectoral muscles, cut through all the vertebral

ribs of the left side, except the last five, at about a quarter of an inch
from their junction with the sternal ribs: from the posterior end of the
incision thus made, cut downwards (¢.e., towards the sternum) for about
an inch, and then forwards, through the sternal ribs. Turn forward
the flap thus separated and carefully dissect it away from the underlying
tissues at the anterior end, so as to detach it altogether without injuring
the jugular and brachial veins. Note in the ¢horacie cavity thus laid
open :—
1. The tericardium and heart, and the ¢hymus.

x PRACTICAL DIRECTIONS 533

2. The collapsed Zexzgs: make a small aperture in the trachea and
inflate.

3. The farietal and visceral layers of the pleura and the media-
stinum (Fig. 128) To make out the relations of these and of the
mediastinal space more accurately, cut away a flap from the right wall
of the thorax, as you have already done on the left side, leaving the
sternum intact.

4. The relations of the pericardium, to see which cut through the
posterior end of the sternum: separate the mediastinum from the dorsal
surface of the bone, turn the latter forwards and remove it: then cut
the pericardium open longitudinally, noting the pericardial fluid.

II. Dissect away the pericardium, the thymus, and any fat about the
base of the heart which may obscure the vessels arising from it. Follow
out these vessels to the head by clearing away the connective tissue,
fat, &c., by which they are surrounded, but taking care not to injure
any of the nerves of the neck. Make out :—

1. a, The eft and right ventricles and auricles, and the ramifications
of the coronary artery and vein ; 6, the two bronché.

2. a, The pulmonary artery and its division into left and right trunks ;
6, the pulmonary veins (the course of which will be better seen at a
later stage); c, the two precaval veins, each formed by the union of a
subclavian, an external jugular, and a smaller znternal jugular ; d, the
thoracic portion of the postcaval vein.

3. a, The gullet (cervical and thoracic portions); 64, the phrenic
nerves (p. §1§), and the thoracic portions of the vagz (Fig. 125);
¢, the arch of the aorta, continuous with the dorsal aorta, and the short
ligament (vestige of the foetal dzctus arteriosus) connecting the former
with the pulmonary artery ; @, the thoracic portions of the sympathetic
nerves and their ganglia, lying on the heads of the ribs; ¢, the azygos
vein (p. 509), best seen by turning the heart and lungs over to the left
side of the animal.

4. a, The zznominate artery, giving off the ft and right common
carotids and the right subclavian ; b, the left subclavian artery; c, the
division of each common carotid, at about the level of the anterior end
of the larynx, into an external and an zyfernal carotid.

Sketch the heart and origins of the main vessels.

III. Now dissect out certain of the nerves and other structures in the
neck, as follows :-—

1. The vagus (Fig. 125), running to the outer side of each common

534 THE RABBIT CHAP.

carotid: with the seeker carefully separate it from the carotid, and trace
it forwards to the angle between the head and neck~—at which point it
enlarges to form a ganglion, and backwards to the thorax. Branches are
given off to the larynx (as¢erior and postertor ox recurrent laryngeal
nerves) and to the heart (depressor) nerves, but the dissection of these
may be omitted by the beginner.

2. The sympathetic (Fig. 125), running on the dorsal side of the
carotid artery. It is most easily distinguished by seizing the carotid
with the small forceps just at the junction of the head and neck and
putting it slightly on the stretch: the vagus ganglion will then be seen
on the outer side of the artery, and the more elongated anferdor cervical
ganglion of the sympathetic on its inner side: trace the sympathetic
nerve from this ganglion backwards to the thorax, at the anterior end
of which (close to the subclavian artery) it enlarges to form the Jostertor

cervical ganglion, and then becomes continuous with the thoracic portion --: -

of the cord.

3. The hyfoglossal nerve will be seen at about the level of the vagus
ganglion, passing forwards to the tongue.

4. Note—a, the thyrozd gland ; b, the hyrozd and cricotd cartilages of
the larynx; ¢, the drachzal and vertebral branches of the subclavian
artery; and d, the brachial plexus (p. 515).

IV. Remove the heart and lungs from the body, together with the
posterior end of the trachea and recognisable portions of the aorta,
precaval, and postcaval. Fasten out the organs under water with their
ventral surface uppermost, and after making out the course of the pu/-
monary arteries and veins, cut through them close to the lungs and
separate the latter from the heart. Note :—

1. The two mazi Jobes of each lung and the two accessory Jobes on the
right side.

2. The cartzlages of the trachea and bronchz ; trace the bronchi for a
short distance into the lungs. Sketch.

V. Cut away the outer walls of both auricles so as to expose their
cavities, taking care not to injure the veins which enter them. Wash
out the contained clots of blood, and note—

1. u, The appendzx of each auricle, and the network of muscular
bands in its walls; 4, the septam aurtcularum and fossa ovalis—best
seen from the left side by holding the heart between your eyes and the
light.

2. a, The auriculo-ventricular apertures; b, the apertures of the

x PRACTICAT, DIRECTIONS 535

precaval and postcaval veins ; c, the Eustachian vawe; a, the apertuie
of the coronary vein within the tunnel-like opening of the left precaval ;
é, the apertures of the pulmonary vetns.

Then cut away both auricles so as to expose the bases of the ventricles,
and remove all but about an eighth of an inch of the aorta anc pul-
monary artery; pour water into the ventricles through the auriculo-
ventricular apertures, and squeeze the ventricles, noting—

3. a, The decuspid and tricuspid valves, and the semélunar valves at
the origins of the aorta and pulmonary artery respectively ; 4, the aper-
tures of the coronary arteries just on the distal side of the aortic valves.
Sketch. ;

Now remove the outer walls of both ventricles by making first a
transverse incision along the base of each, and then, from its extrem-
ities, converging incisions nearly to the apex of the heart. Make out :—

4. The relative thickness of the walls of the right and left ventricles,
the form of the septum ventriculorum, the cavities of the two ventricles,
and the muscular ridges in their walls.

5. The flaps of the ¢rzcuspzd and becuspid valves, and their fendznous
cords and papillary muscles,

6. The apertures of the aorta and pulmonary artery into the left and
right ventricle respectively.

Sketch.

F. Dissection of the head.

I. Carefully dissect the skin away from one side of the head; either
side will do, but if you are using the head from which you have already
removed the brain, choose the side on which the auditory region is least
damaged: Notice the Afecbomian glands (p. 518). Cut away, with bone
forceps, the supraorbital process of the frontal, being careful not to injure
any of the contents of the orbit. Note—

1. a, A large mass of muscle (masseter) covering the posterior half
of the mandible, on which branches of the facza/ nerve will be seen ;
4, the thin, irregular parotéd gland, at the base of the pinna, and the
large 7ufraorbital gland, \ying mainly within the orbit below (p. 495).

2. The four vecéz and the two obdzgue muscles of the eye. Note the
course of the superior oblique through its tendinous loop (p. 517).

3. The dachrymal and Harderian glands, situated in the postero-
superior and antero-inferior regions of the orbit respectively.

II. Remove the eyeball by cutting through the muscles and optic nerve;

536 THE RABBIT CHAP.

noting the retractor bulbé muscle around the latter. (For the structure
of the eye, see p. 538.) Cut off the pinna, and clear away the muscles,
&e., covering the tympanic bone: remove the entire tympanic and
periotic bones (p. 475) in one piece, Jay open the external auditory
passage, and very carefully and gradually cut away the outer wall of
both tubular and bulbous portions of the tympanic with the bone-
forceps. Note :—

1. The ¢ympanic membrane and the handle of the mad/eus ; and after
cutting away the former so as not to injure the latter, and removing
rather more of the bulla, make out—

2. The tympanic cavity; the malleus, incus, and stafes ; the lenson
tympani and stapedius muscles; and the aperture of the Zustachian
tube. Remove the auditory ossicles, noting as you do so the fenestra
ovalés and fenestra rotunda, and examine them under the low power
of the microscope: by cutting through the periotic with the bone
forceps, the position of the membranous labyrinth with the cochlea may
be made out.

III. Dissect off the muscles covering the mandible on the side you
are working ; detach its ascending portion from the muscles inserted on
to its inner surface, remove it entire, cutting through the symphysis,
and clear away the underlying muscles. Pass a probe from the cut end
of the gullet forwards into the mouth: lay open the gullet along this,
and pull the tongue downwards, so as to get a view into the interior of
the mouth (Fig. 125). Note :—

1. a, The palate and velum palati ; b, the tongue and its papille
(p. 492); ¢, the pharynx; d, the glottis and efigloltis ; e, the con-
tinuation of the pharynx above the velum palati (#aso-pharynx), with
which the internal nostrils communicate ; f, the positions of the /eeth
(incisors and grinders); g, the apertures of the saso-falatine canals
(p. 490).

Then remove the nasal, premaxilla, and maxilla of the same side,
noting the o/factory nerves and the ¢urb7nals ; and after passing a probe
backwards from the externa! nostril into the nasal chamber as far as
it will go, remove the turbinals on this side, so as to expose the
cartilaginous zasal septum. Also cut away all the remaining bone on
the same side of the middle line, slit up the naso-pharynx, and reduce
your dissection to a neat longitudinal section, noting—

2. The cranial cavity, the roof of which will have been cut away if
the brain has been removed.

Xx PRACTICAL DIRECTIONS 537

3. The aperture of the Eustachian tube: pass a probe from it into
the tympanic cavity.

4. The position of Jacobson’s organ, on the ventral side of the nasal
septum ; and after cutting away the latter, the e¢hmo-turbénals, maxillo-
turbinals, and naso-turbinals of the other side.

Sketch the entire longitudinal section.

IV. Remove the larynx with the anterior part of the trachea. Make
a longitudinal vertical section of it, keep one half entire, and from the
other dissect away the muscles and mucous membrane so as to see the
cartilages clearly. Examine first the cartilages and then the soft parts,
noting the relations of the ¢hyrozd,-crivord, and arytenoid cartilages, the
epiglottis, and the vocal cords. Sketch.

G. The Brain.

I.—External characters. Note :—

1. The medulla oblongata, with its dorsal and ventral fissures and
the pons Varolzd ; the convoluted cerebe//um, consisting of a central lobe
and two /ateral lobes and flocculz.

2. The optzc lobes, seen on slightly separating the hemispheres and
cerebellum ; the crzsra cerebré.

3. The olfactory lobes and cerebral hemispheres ( frontal, partetal, and
temporal lobes)—by gently separating them, the corpus callosum will be
seen—and just behind them is the peneal body; the dzencephalon is
covered above by the hemispheres, below it is visible and is continued
into the z¢nzfundibulum; the pituitary body is nearly always left
behind in the brain-case when the brain is removed, and a small
aperture is seen in the centre of the infundibulum where it was
attached ; the oftée chiasma.

4. The origins of the nerves from the brain; these cannot, of course,
be plainly made out unless the nerves have been carefully cut through
when removing the brain. Compare pp. 163-166, and note in addition
the spinal accessory and hypoglossal.

Sketch the brain from above and from below.

II. Carefully remove successive slices from one of the hemispheres
nearly parallel to its upper surface down to the level of the corpus
callosum, noting the fibres of the latter and the relations of the grey
and white matter in the hemispheres. Cut through the corpus
callosum longitudinally, near the middle line, and remove sufficient
of the hemisphere already dissected to expose the J/ateral ventricle

538 THE RABBIT CHAP,

re

and its extensions into the frontal, parietal, and temporal lobes.
Note :—

1. The heppocampus, the corpus striatum, the chorotd plexus, and
the septum lucidum (p. 514). Then cut away the rest of the same
hemisphere so as to expose—

2. a, The diencephalon and its two lateral halves (optic thalamz)
between which the ¢hzvd ventricle is enclosed ; from its roof the stalk
of the pineal body arises ; 4, the two pairs of optic lobes. Cut away the
lateral half of the cerebellum of the same side, and note—

3. The valve of Vieussens, the fourth ventricle, and the tree-like
appearance (arbor vite) of the cerebellum in section.

III. Now reduce the whole brain to a longitudinal vertical section
(Fig. 132), cutting it through with a sharp knife very slightly to that
side of the median line which you have already dissected, so as to
expose the median ventricles in section. Make out, on the uninjured
half, the relations of the third ventricle, tter, fourth ventricle, foramen
of Monro, infundibulum, optic chiasma, corpus callosum and fornix,
lamina terminalis, antertor, middle, and posterior commissure, crura
cerebri, valve of Vieussens, pons Varolit, &c. Sketch.

IV. A series of thick transverse sections should be made through
another thoroughly hardened brain, and the relations of the parts
already seen by dissection examined.

Hi. The Eye.

On account of its larger size, it is better to substitute the eye of an ox
or sheep—which can be obtained fresh from a butcher—for that of the
rabbit, Two specimens should be obtained, and the fat, muscles,
and any portions of the eyelids remaining attached cleared away.
1, After noting the conjunctiva, cornea, sclerotic, optic nerve, tris, and
pupil, insert the scissors into the margin of the cornea and cut round
and remove it, so as to expose the agueous chamber, noting the agzeous
humour, as well as the ris, les, and pupil (compare Fig. 57).
Sketch.

Insert the handle of a scalpel between the iris and sclerotic and
separate off the latter from the choroid around the outer half of the eye-
ball, first making four radial cuts in the sclerotic, beginning at its
margin, at equal distances from one another. Turn back the four flaps
thus made, and insert pins through them so as to fix [the eye firmly
down under water with the iris uppermost : note the cz/zaryv muscle around

x PRACTICAL DIRECTIONS 530

the ouler margin of the iris, the chorozd, and the ceHary vessels and
nerves. Then make two radial cuts, a short distance from one another,
through the iris, and turn back the portion between the cuts, noting the
ciliary processes. Sketch.

Cut through the choroid and the parts enclosed by it, horizontally,
around the line up to which it has been already exposed, so as to
separate the eye-ball into an inner and outer hemisphere : in doing so,
the gelatinous vztreors humour must be cut with the scissors. Examine
and sketch both sections, noting in the outer hemisphere—the reé7a,
stopping short at the outer margin of the ciliary processes, and the /evs
with its cafsz/e ; and in the inner hemisphere—the reéz7a with its blood-
vessels, and the d/d-spot (or point of entrance of the optic nerve).
Then remove the retina and observe the choroid with its iridescent
tapetum,

2. Cut your second specimen into vertical halves with the scissors,
making the cut pass through the cornea, pupil, and optic nerve. As it
is difficult to cut through the lens without disturbing its relations, it
should be carefully separated from one half, and left entire on the other
half. Examine the relations of the parts once more, compare Fig. 57,
and sketch.

I. Dissection of typical flexor and extensor muscles and
joints of the fore-limb.

1. Expose the dcceps (chief flexor of the fore-arm). Its ovdgzz is
fromthe anterior edge of the glenoid cavity on the pectoral arch: it
arises by a single long ¢ezdon, working in the bicipital groove of the
humerus. It is spindle-shaped, consisting of a single de//y, and is
znserted on to the proximal end of the radius.

2. Expose the érdceps (chief extensor of the fore-arm). It arises
by three main heads from the pectoral arch and humerus, and is inserted
on to the olecranon process of the ulna.

3. Remove the muscles from the shoulder-joint, and note the
capsular ligament (p. 55). Cut through this so as to open the synovza/
capsule, and note the synovial membrane and fluid and the cartilaginous
articular surfaces of the glenoid cavity and head of the humerus
respectively : or, prepare as directed on p. 64 in the case of the hip-
joint of the frog.

4. Asan example of a muscle with a multiple insertion, dissect out
the extensor communis digitorum (dorsal). It arises from the distal

540 THE RABBIT CITAP. X

end of the humerus, and at the distal end of the fore-arm divides into
four tendons, which pass through the asnular ligament to digits 2-5
and are inserted into the middle and distal phalanges. Note the
sesamotd bones on the palmar side of the joints of the digits.

J. Side dissection.

Obtain another rabbit, and make a dissection from the side (compare
Fig. 125) as directed in the case of the dogfish and frog (p. 464).
Make out the structure and relations of all the principal organs once
more, and sketch your dissection,

K. Transverse sections.

The examination of transverse sections is most easily and satisfactorily
done by preparing microscopical sections of a foetus (p. 532) not more
than a couple of inches in length, as directed in the case of the young
dogfish (p. 464) : a foetal rat or mouse will answer the purpose equally
well,

With a sharp knife, cut transverse sections, about {th inch thick, from
the following regions: @, snout ; 4, cranial region ; c, neck ; d, thorax ;
and ¢, abdomen. Stain and imbed, and then cut and mount one or
two sections of each, examining them with a low power and making
sketches.

Then put on the high spower, and make out as much as possible of
the histology of the various organs.

CHAPTER XI

THE MINUTE STRUCTURE OF CELLS: CELL-DIVISION:
STRUCTURE OF THE OVUM: SPERMATOGENESIS: MATU-
RATION AND FERTILIZATION OF THE OVUM: SEGMEN-
TATION OF THE OOSPERM: EFFECT OF FOOD-YOLK ON
DEVELOPMENT : FORMATION OF THE CHIEF ORGANS OF
THE ADULT VERTEBRATE, AND OF THE AMNION, ALLAN-
TOIS, AND PLACENTA.

Structure of the Cell.—We have learnt in previous chap-
ters that all organisms are formed essentially of one or more
cells (compare pp. 110 and 118); that the cell consists of
protoplasm, and contains a nucleus consisting of chromatin
and achromatin (p. 129), in which one or more zueleolt
can usually be distinguished ; and that cells multiply by a
process of dinary fission (p. 236). It will now be necessary
to study the structure of a typical animal cell and of its
mode of division in somewhat greater detail.

There seems to be a good deal of variation in the precise
structure of various animal and plant-cells, but the more
recent researches show that in the cell-body or protoplasm
two constituents may be distinguished, a clear semi-fluid
substance, traversed by a delicate sponge-work. Now
under the microscope the whole cell is not seen at once
but only an optical section of it—that is, all the parts which

542 STRUCTURE OF CELLS CHAP.

are in focus at one time (see e.g. p. 315): by altering the
focus we view the object at successive depths, each view
being practically a slice parallel to the lenses of the
instrument. This being this case, protoplasm presents the
microscopic appearance of a clear or slightly granular
matrix traversed by a delicate network. In the epithelial
cells of animals the protoplasm is bounded externally by a
cell-membrane of extreme tenuity: in amceboid cells the
ectoplasm or transparent, non-granular portion of the cell
consists of clear protoplasm only, the granular endoplasm
alone possessing the sponge-work.

It is quite possible that the reticular character of the cell may be
merely the optical expression of an extensive but minute vacuolation,
or may be due to the presence of innumerable minute granules developed
in the protoplasm as products of metabolism.

The nucleus is spherical or oval in form, and is enclosed
in a delicate nuclear membrane (Fig. 136, A, mu.m).
The achromatin is a homogeneous semi-fluid substance
which forms the groundwork of the nucleus: it resembles
the clear cell-protoplasm in its general characters, amongst
other things in being almost unaffected by dyes. The
chromatin (chr) takes the form of a network or sponge-work
of very variable form, and is distinguished from all other
constituents of the cell by its strong affinity for aniline and
other dyes (compare Part I., Chapters VII and VIII).
Frequently, as we have seen, one or more minute globular
structures, the xucleoli (nu'), occur -in the nucleus either
connected with the network or lying freely in its meshes :
they also have a strong affinity for dyes although they
often differ considerably from the chromatin in their micro-
chemical reactions.

In the body of some cells and possibly of all there is

Xl STRUCTURE OF NUCLEI 543

Fic. 136.—Diagrams illustrating the process of indirect cell-division or mitosis.
A, the resting cell; B,C, D, successive phases in the formation and arrangement
of the chromatin-loops and of the nuclear spindle ; E, F, G, separation of the two
sets of daughter chromosomes and their passage towards the poles of the spindle ;
H, division of the cell-body and formatioa of the-two nuclei; I, division of a
plant-cell, showing the formation of the cell-walk (c.w) by means of a cell-
plate (c. pl)

centrosome; chy. chromatin; sz’. nucleoli; #z.az. nuclear membrane; s.
astrophere; sf. spindle. (From Parker's Siolagy, altered from Flemming,
Rabl, etc.)

>

found a globular body, surrounded by a radiating arrange-
ment of the protoplasm and called the astrosphere (s) : it
lies close to the nucleus, and contains a minute granule

544 CELL-DIVISION CHAP.

known as the central particle or centrosome (c). In many
cases two astrospheres and centrosomes are found in each
cell (B).

Cell-division. The precise changes which take place
during the fission of a cell are, like the structure of the cell
itself, subject to considerable variation. We will consider
what may probably be taken as a typical case (Fig. 136).

First of all, the astrosphere, with its centrosome, divides
(Bg), and the products of its division gradually separate from
one another (c), ultimately passing to opposite poles of the
nucleus (p). At the same time the network of chromatin
divides into a number of separate filaments called chvomo-
somes (B, chr), the number of which appears to be constant
in any given species of animal or plant, although it may
vary in different species from two to twenty-four or more.
Soon after this the nuclear membrane and the nucleoli dis-
appear (B, C), and the nucleus is seen to contain a spindle-
shaped body (sf) formed of excessively delicate fibres which
converge at each pole to the corresponding astrosphere.
The precise origin of this nuclear spindle is uncertain :
it may arise either from the nuclear matrix or, more
probably, from the protoplasm of the cell: it is not affected
by colouring matters.

At the same time each chromosome splits along its whole
length so as to form two parallel rods or loops in close
contact with one another (B): in this way the number of
chromosomes is doubled, each one being now represented
by a pair.

The divided chromosomes now pass to the equator of
the spindle (pb) and assume the form of more or less
V-shaped loops, which arrange themselves in a radiating
manner so as to present a star-like figure when the cell is

XT MITOSIS 545

viewed in the direction of the long axis of the spindle. Every-
thing is now ready for division, to which all the foregoing
processes are preparatory.

The two chromosomes of each pair now gradually pass to
opposite poles of the spindle (rf, Fr), two distinct groups
being thus produced (Gc) and each chromosome of each
group being the twin of one in the other group. Perhaps
the fibres of the spindle are the active agents in this process,
the chromosomes being dragged in opposite directions by
their contraction : on the other hand it is possible that the
movement is due to the contractility of the chromosomes
themselves.
~ After reaching the poles of the spindle the chromosomes
of each group unite with one another to form a network (H)
around which a nuclear membrane finally makes its appear-
ance (1). In this way two nuclei are produced within a
single cell, the chromosomes of the daughter-nuclet, as well
as their attendant centrosomes, being formed by the binary
fission of those of the mother-nucleus.

But ari passu with the process of nuclear division,
fission of the cell-body is also going on. This takes place
by a simple process of constriction (H)—in much the same
way as a lump of clay or dough would divide if a loop of
string were tied round its middle and then tightened.

In comparatively few cases the dividing nucleus instead of
going through the complicated processes just described
divides by simple constriction. We have therefore to dis-
tinguish between direct and ¢ndirect nuclear division. To the
latter very elaborate method the name mz¢osis or haryoktnests
is applied : direct division is then distinguished as amifotic.

In this connection the reader will not fail to note the
extreme complexity of structure revealed in cells and their

nuclei by the highest powers of the microscope. When the
Pract. Zoot. NWN

546 STRUCTURE OF OVUM CHAP.

constituent cells of the higher ‘animals and plants were dis-
covered, during the early years of the present century, by
Schleiden and Schwann, they were looked upon as the
ultima Thule of microscopic analysis. Now the demonstra-
tion of the cells themselves is an easy matter, the problem
is to make out their ultimate constitution. What would be
the result if we could get microscopes as superior to those
of to-day as those of to-day are to the primitive instruments
of eighty or ninety years ago, it is impossible even to con-
jecture. But of one thing we may feel confident—of the
great strides which our knowledge of the constitution
of living things is destined to make during the next half
century,

Structure of the ovum. The striking general resemblance
between the cells of the higher animals and entire unicellular
organisms has been commented on as a very remarkable
fact: there is another equally significant circumstance to
which we must now advert.

All the higher animals begin life as an egg, which is either
passed out of the body of the parent, as such, as in earth-
worms, crayfishes, frogs, birds, &c. (ovzparous forms), or
undergoes development within the body of the parent, as
in some dogfishes (p. 454) and nearly all mammals
(viviparous forms).

The structure of an egg is, in essential respects, the same
in all animals from the highest to the lowest (compare
p. 195’. It consists (Fig. 137) of a more or less globular
mass of protoplasm, in which are deposited particles of a
proteinaceous substance known as yo/k-granules. Within
the protoplasm is a large nucleus containing chromatin
as well as one or more nucleoli—which are often known as
germinal spots, the entire nucleus of the ovum being called

XI MODIFICATION OF OVUM 547

the germinal vesicle. An investing cuticular membrane may
or may not be present. In other words the egg, as we have
already seen, is a cell.

The young or immature ova of all animals present this
structure, but in many cases certain modifications are
undergone before the egg is fully formed. l’or instance, the
protoplasm may throw out pseudopods, the egg becoming
ameeboid (p. 302); or, as
mentioned above and as is
usually the case, the sur-
face of the protoplasm
may secrete a cell-wall,
often of considerable thick-
ness, and known as the
vitelline membrane (p. 196
and Fig. 137), which may
be perforated at one pole by
an aperture, the mcropyle
(p. 400). The most extra-

Fic. 137-—Ovum of a Sea-urchin (7 o0-
Pneustes lividus), showing the radially-

ordinary modification takes
place in some Vertebrata,
such as dogfishes (p. 454)

striated cell-wall (vitelline membrane),
the protoplasm containing yolk granules
(vitellus), the large nucleus (germinal
vesicle) with its network of chromatin,
and a large nucleolus (germinal spot),

(From Balfour, after Hertwig.)

and birds. In a hen’s egg,

for instance (Fig. 138), the yolk-granules increase immensely,
swelling out the microscopic ovum until it becomes what
we know as the “yolk” of the egg: around this layers of
albumen or “white” are deposited by the glands of the
oviduct and finally the shell-membrane and the shell.
Hence we have to distinguish carefully in eggs of this
character between the entire “egg ” in the ordinary accep-
tation of the term, and the ovum or egg-cell. But com-
plexities of this sort do not alter the fundamental fact that
all the higher animals begin life as a single cell, or in other

NN 2

548 SEX-CELLS CHAP,

words, that multicellular animals, however large and complex
they may be in their adult condition, originate as unicellular
bodies of microscopic size; and the same is the case with
plants.

Spermatogenesis and Oogenesis. In the preceding
chapters it has more than once been stated that sperms

Fic. 138.—Semi-diagrammatic view of the egg of the fowl at the beginning cf
incubation.

a. air-space ; a/b. dense layer of albumen; a/s’. more fluid albumen ; 2/. blasto-
derm ; ch. chalaza, a twisted cord of the dense layer of albumen at either end
of the egg, formed as the latter rotates down the oviduct ; s#. shell ; sk. 77. shell-
membrane ; sk.2. 7, sh.nt. 2, its two layers separated to enclose air-cavity. (From
Parker and Haswell’s Zoology, after,Marshall, slightly altered.)

arise from ordinary undifferentiated cells in the spermary,
and that ova are produced by the enlargement of similar
cells in the ovary. Fertilization has also been described
as the conjugation or fusion of ovum and sperm (compare

xr SPERMATOGENESIS 549

p- 197). We have now to consider in greater detail what is
known as to the precise mode of development of sperms
(spermatogenesis) and of ova (oogenesis), as well as the exact
steps of the process by which an oosperm or unicellular
embryo is formed by the union of the two sexual elements.

Both ovary and spermary are at first composed of cells of
the ordinary kind,.the primitive sex-cells ; and it is only by
the further development of these that the sex of the gonad
is determined.

In the spermary the sex-cells (Fig. 139, a) undergo re-
peated fission, forming what are known as the sperm-mother-
cells (B). These have been found in several-instances to be
distinguished by a peculiar condition of the nucleus. We
saw (p. 544) that the number of chromosomes is constant
in any given animal, though varying greatly in different
species. In the formation of the sperm-mother-cells from
the primitive sex-cells the number becomes doubled: in the
case of the mole-cricket, for instance, shown in Fig. 139, the
ordinary cells of the body, including the primitive sex-cells,
contain twelve chromosomes, while the sperm-mother-cells
contain twenty-four.

The sperm-mother-cell now divides (c), but instead of its
chromosomes splitting in the ordinary way (p. 544, Fig 136)
half of their total number—in the present instance twelve—
passes into each daughter cell: thus two cells are pro-
duced having the normal number of chromosomes. The
process of division is immediately repeated in the same pecu-
liar way (D), the result being that each sperm-mother-cell gives
rise to a group of four cells having half the normal number
of chromosomes—in the present instance six. The four cells
thus produced are the immature sperms (): in the majority
of cases the protoplasm of each undergoes a great elonga-
tion, being converted into a long vibratile thread, the ¢a// of

550 SPERMATOGENESIS CHAD.

the sperm (F, G), while the nucleus becomes its more or less

spindle-shaped Aead and the centrosome takes the form of a

small éxtermediate piece at the junction of head and tail.
Thus the sperm or male gamete is a true cell, specially

Fic. 139.—Spermatogenesis in the Mole-Cricket (Gr-yl/otalpa).

A, primitive sex-cell, just preparatory to division, showing twelve chromosomes
(chr); c. the centrosome. B, sperm-mother-cell, formed by the division of A,
and containing twenty-four chromosomes ; the centrosome has divided ‘into two.
C, the sperm-mother-cell has divided into two by a reducing division, each
daughter-cell containing twelve chromosomes. D, each daughter-cell has divided
again in the same manner, a group of four sperm-cells being produced, each
with six chromosomes. E, a single sperm-cell about to elongate to form a sperm.
F, immature sperm ; the six chromosomes are still visible in the head. G, fully
formed sperm. (From Parker's Byology, after vom Rath.)

modified in most cases for active movement. This actively
motile, tailed form is, however, by no means essential: in

XI -OOGENESIS 551

some animals (e.g. Crayfish, p. 368) the sperms are non-
motile.

The peculiar variety of mitosis described above, by
which the number of chromosomes in the sperm-mother-
cells is reduced by one-half, is known as a reducing
division.

As already stated, the ova arise from primitive sex-cells,
precisely resembling those which give rise to sperms. These
divide and give rise to the egg-mother-cells in which, as in
the sperm-mother-cells, the number of chromosomes is even-
tually doubled. The egg-mother-cells do not immediately
undergo divisidn, but remain passive and increase in size
by the absorption of nutriment from surrounding parts: in
this way each egg-mother-cell becomes an ovum. Sometimes
this nutriment is simply taken in by diffusion or osmosis,
in other cases the growing ovum actually ingests neighbour-
ing cells after the manner of an Amceba. Thus in the
developing egg the processes of constructive are vastly in
excess of those of destructive metabolism.

We have seen (p. 249) that the products of destructive
metabolism may take the form either of waste products
which are got rid of, or of plastic products which are stored
up as an integral part of the organism. In the developing
egg, in addition to increase in the bulk of the protoplasm
itself, a formation of plastic products usually goes on to an
immense extent. In plants the stored-up materials may
take the form of starch, of oil, or of proteid substance: in
animals it consists, as mentioned above, of rounded or angular
grains of proteid material, known as yolk-granules. These
being deposited, like plums in a pudding, in the proto-
plasm, have the effect of rendering the fully-formed egg
Opaque, so that its structure can often be made out only
in sections.

552 MATURATION CHAP. XI

Maturation of the ovum.—tThe fully-formed ovum as
described on p. 546, is, however, incapable of being
fertilized or of developing into an embryo: before it is ripe
for conjugation with a sperm or able to undergo the first
stages of segmentation it has to go through a process known
as maturation.

Maturation consists essentially in a twice-repeated process
of cell-division. The nucleus (Fig. 140, A) loses its mem-
brane, travels to the surface of the egg, and takes on the
form of an ordinary nuclear spindle. Next the protoplasm
grows out into a small projection or bud, into which one end
of the spindle projects. Nuclear division then takes place,
one of the daughter nuclei remaining in the bud ( fo/),
the other in the ovum itself. Nuclear division is followed
as usual by division of the protoplasm, and the bud becomes
separated as a small cell distinguished as the first polar
cell.

In some cases development from an unfertilized female gamete takes
place, the process—which is not uncommon among insects (¢g. the
cummon little green plant-louse or /f/7s) and crustaceans (e.g. water-
fleas) —being distinguished as farthenogenests. It has been proved in
many instances and is probably generally true that in such cases the egg
begins to develop after the formation of the first polar cell. Thus in
parthenogenetic ova it appears that maturation is completed by the
separation of a single polar cell, after which the ovum centains the
number of chromosomes normal to the species.

In the majority of cases, development takes place
only after fertilization, and in these maturation is not
complete until.a second polar cell (x, pol) has been formed
in the same manner as the first. The ovum has now lost a
portion of its protoplasm together with three-fourths of its
chromatin, half having passed into the first polar cell and
half of what remained into the second: the remaining one-

*

Fic. 140. The maturation and impregnation of the animal ovum (diagrammatic).

A, the ovum, surrounded by the vitelline membrane (sew), in the act of forming
the first polar cell (fo); Q cenxé. centrosome. B, both polar cells (fod) are
formed, the female pronucleus (Q Avo.) lies near the centre of the ovum, and
one of the several sperms is shown making its way into the ovum at the micropyle
(microp). C, the head of the sperm has become the male pronucleus (¢ Aron),
its intermediate piece the male centrosome (6 cez?); other structures as before.
D, the male and female pronuclei are in the act of conjugation. EK, conjugation
is complete, and the segmentation nucleus (seg. zzc/) formed. (From Parker
and Haswell’s Zoology.)

554 FERTILIZATION CILAP.

fourth of the chromatin becomes enclosed as a nucleus,
which is distinguished as the female pronucleus (B, 2 proit).

The formation of both polar cells takes place by a
reducing division (p. 551) ; so that, while the immature ovum
contains double the number of chromosomes found in the
ordinary cells of the species, the mature ovum, like the
sperm, contains only one-half the normal number.

In some animals the first polar body has been found to divide after
separating from the egg. In such cases the egg-mother-cell or imma-
ture ovum gives rise toa group of four cells—the mature ovum and
three polar-cells, just as the sperm-mother-cell gives rise to a group
of four cells, all of which, however, become sperms (Fig. 139).

Fertilization of the ovum.—Shortly after maturation, the
ovum is fertilized by the conjugation with it of a single
sperm. As we have found repeatedly, sperms are produced
in vastly greater numbers than ova, and it often happens
that a single egg is seen quite surrounded with sperms, all
apparently about to conjugate with it (Fig. 140, B). It has,
however, been found to be a general rule that only one of
these actually conjugates: the others, like the drones in
a hive, perish without fulfilling the one function they are
fitted to perform.

The successful sperm (B) takes up a position at right
angles to the surface of the egg and gradually passes
through the micropyle (micvop) or works its way through
the vitelline membrane until its head lies within the egg-
protoplasm. The tail is then lost, and the head, accom-
panied by the intermediate piece or centrosome, penetrating
deeper into the protoplasm, takes on the form of a rounded
body, the male pronucleus (c, d pron).

The two pronuclei approach one another (p) and finally
unite to form what is called the segmentation-nucleus (E, seg.
uct), the single nucleus of what is not now the ovum but

XI SEGMENTATION OF OVUM 555

the vcosferm—the impregnated egg or unicellular embryo
(compare pp. 197 and 198).

The fertilizing process is thus seen to consist essentially
in the union of two nuclear bodies, one contributed by the
male gamete or sperm, the other by the female gamete or
ovum. It follows from this that the essential nuclear matter
or chromatin of the oosperm—often spoken of as the germ-
plasm—is derived in equal proportions from each of the two
parents. Morcover, as both male and female pronuclei con-
tain only half the number of chromosomes found in the ordi-
nary cells of the species, the union of the pronuclei results
in the restoration of the normal number to the oosperm.

Fertilization being thus effected, the process of segmenta-
tion or division of the oosperm takes place as described in
previous chapters.

Different types of ova and their segmentation.—Before
passing on to consider further details in the process of
development of the oosperm, we must briefly refer to some
differences already noted in the ova of different animals.

We have seen that in all cases the immature egg is a
simple, minute cell, but that owing to the deposition of
yolk-granules in its protoplasm, it may reach a compara-
tively large size (e.g. Crayfish, Dogfish, Bird). The pres-
ence of a greater or less amount of yolk in the ovum
results, as we know, in very considerable differences as
regards its mode of segmentation, as well as in its subsequent
development. The minute eggs of Amphioxus and _ the
Rabbit, for instance, which are each only ,'g mm. (about
giz inch) in diameter, contain so comparatively small an
amount of food-yolk as not to interfere materially with the
process of segmentation: such ova are called aéectthal.
When the quantity of food yolk is relatively greater, it may

556 SEGMENTATION OF OVUM CHAD,

become accumulated towards the centre of the egg, even-
tually leaving a layer of protoplasm comparatively free from
yolk round the periphery (centrolecithal ova, e.g. Crayfish,
Fig. 91); or, as in the case of ‘elolecithal ova (Figs. 64,
11g, and 138), the yolk-granules may become aggregated
more at the lower than at the upper pole (Frog), until
in the most extreme cases there is only a layer of yolkless
protoplasm—the germinal disc (Dogfish, Bird)—lying at the
upper pole of the egg.

As yolk is an inert substance, the more of it an egg
contains the less actively can the latter divide, and the
quantity may be relatively so great in some parts as to pre-
vent segmentation in these parts altogether. We can there-
fore distinguish between Ao/ob/astic oosperms, which undergo
entire segmentation (e.g., Hydra, Earthworm, Mussel, Am-
phioxus, Frog, Rabbit), and meroblastic oosperms, in which
segmentation is limited to that part of the egg in which the
protoplasm is comparatively free from yolk (e.g. Crayfish,
logfish, Bird), this portion, after segmentation, being known
as the d/astoderm. In the centrolecithal ovum it is evident
that the segmentation must be superficial or peripheral
(p. 369), and in the meroblastic telolecitha. ovum discord
or restricted to a small germinal disc at its upper pole
(Figs. 119 and 138). In the case of holoblastic ova the
segmenting cells or d/astomeres may be equal, or nearly equal,
in size (e.g., Amphioxus, Rabbit); or if the yolk is pre-
sent in greater quantities towards the lower pole, unequal,
(e.g. Earthworm, Frog).

The influence of the food-yolk in modifying the carly
processes of development is thus evidently very great, and
in order to understand these processes in their simplest form
it is necessary to select for our study an alecithal holo-
blastic egg, such as that of the lancelet.

XI DEVELOPMENT OF AMPHIOXUS 557

Development of Amphioxus. The oosperm of Am-
phioxus (Fig. 141, A) undergoes binary fission (B), each

B

es
SS
YY

ty

SR
Ss
IS
Se
C\ 2
ats
ee.
LY
So
0

Fic. 141.—Stages in the segmentation of th. oosperm of 4 mphioxus.
D, represents the -four-celled stage (C) from above S}H, vertical section of G; K,
vertical section of the blastula stage (I). (From Korschelt and Heider, after
Hatschek.) _ . :

558 GASTRULA CHAP.

of the two resulting cells dividing again into two (C, D).
This process is continued until a globular mass of cells or
blastomeres is produced by the repeated division of the
one cell which forms the starting point of the series.
Owing to there being rather more yolk at the lower than at
the upper pole, the lower cells are slightly larger than the
upper, so that the segmentation is not quite equal (E—K).
The embryo has now arrived at the polyplast or morula
stage, and sections show that it is hollow, the blastomeres
being arranged in a single layer around a central segmenta
tion-cavity (p. 200): such a hollow polyplast is often known
as a blastula (K). The lower side of the blastula then
becomes tucked in, or 7vvagrnated, the result being that the
single-layered sphere is converted into a double-layered cup
(Fig. 142). This process can be sufficiently well imitated
by pushing in one pole of a hollow india-rubber ball with
the finger. The resulting embryonic stage is known as the
gastrula (Fig. 142, C): its cavity is the primitive enteron
or archenteron (p. 201), and is bounded by the invaginated
cells which now constitute the ezdoderm, the remaining cells
forming the outer wall of the gastrula being the ectoderm
(p. 202). The two layers are continuous at the aperture
of the cup, the gastrula-mouth or blastopore. Between the
ectoderm and endoderm is at first a space, the greatly dimi-
nished segmentation-cavity, which gradually becomes entirely
obliterated, so that the ectoderm and endoderm are in con-
tact (A.B). The general resemblance of the gastrula to a
simplified Hydra,! devoid of tentacles, will at once be ap-
parent, and the stage in the development of the frog’s egg re-

1 It must, however, be remembered (pp. 303 and 313) that the
ectoderm and endoderm of Hydroids are differentiated before the
mouth is formed, so that the mouth does not correspond to the blasto-
pore of the gastrula. :

XI NERVOUS SYSTEM 559

presented in I'ig. 64, F’, though much modified by the quantity
of food-yolk, will be seen to correspond to the gastrula-stage,
As in the frog, the blastopore soon closes, the mouth and
anus being subsequently formed from the stomodeum and
the proctodzeum respectively (p. 204).

The gastrula becomes elongated, flattened on one side,

Stiemee te

Wwe

Hee ORY
>

SGI UICITESSICIC
OS i

Fic. aie stages in the formation of the gastrula of Am /phioxrus. In A,
the nuclei of the endoderm have been omitted. (From Korschelt and Heider,
after Hatschek.)

and convex on the other. The flattened side corresponds

to the dorsal surface of the adult, and the blastopore

now comes to be situated, as in the frog-embryo (Fig. 64

H, K), at the posterior end of the dorsal surface. A

medullary plate and groove (Fig. 143, mp) are then formed,

the central nervous system being developed in a manner
essentially similar to that already described in the
case of the tadpole (p. 202), except that the central
canal of the medullary cord (7) is formed after the

560 NOTOCHORD CHAP.

plate has become separated from the outer ectoderm.
In the mid-dorsal line a thickening of the endoderm
(ch) soon becomes constricted off to form the xotochord

Fic. 143.—Four stages in the lJevelopment of the notochord, nervous system, and
. mesoderm of Amphioxus.

ak, ectoderm ; ch notochord ; dh. cavity of archenteron ; 4d. ridge of ectoderm

growing over medullary plate ; 7&. endoderm ; ¢2/. ccelome ; 2%. ceelomic pouch ;

mk, pariétal layer of mesoderm; 2&2. visceral layer ; 27. medullary plate ; 7.

medullary cord; 2s. protovertebra. (From Korschelt and Heider, after Hatschek.)

(pp. 203, 404 and 425), and on either side of this
a series of hollow endodermic pouches arise, arranged
metamerically (Figs. 143, ma, and 144, us, mk). The.

XI MESODERM AND CCELOME 561

cavities of these, which subsequently give rise to the
ceelome (/#, wsh), are thus at first in free communication with
the archenteron and are known as enferocales 3 from their
walls the sesoderm is derived. Subsequently the communi-
cations between the enteric and enteroccelic cavities become

\H

Fic. 144.—Embryo of Amphioxus. :

A, in vertical section, slightly to the left of the middle line; B, in horizontal
section, a, ectoderm; cz. neurenteric canal; d&, wd. archenteron; é%. endo-
derm ; #z4. mesodermal folds; #, medullary canal; zs. first coelomic pouch ;
ush. coelomic cavity; V. anterior; H. posterior end. (From Korschelt and
Heider, after Hatchek.)

closed, and the paired pouches gradually extend between the
ectoderm and endoderm, both dorsally and ventrally (Fig.
143, C, D), their outer walls (parietal or somatic layer of the
mesoderm, m2!) being in contact with the ectoderm and
forming with it the somatopleure or body-wall, and their
inner walls (visceral or splanchnic layer of the mesoderm,

Pract. Zoor. (ome)

562 SEGMENTATION IN VARIOUS TYPES CHAP.

mk) in contact with the endoderm and with it forming the
splanchnopleure or wall of the enteric tube (compare p. 203).
Thus the body-wall and the enteric canal are separated
by a cavity, the celome (D, Zh), which, much as in the adult
Earthworm, is divided into a series of metamerically ar-
ranged portions: later on, however, the adjacent walls of
these ccelomic sacs disappear, and the ccelome becomes a
continuous cavity.

The embryo Amphioxus is hatched soon after reaching
the gastrula-stage, when it moves about by means of cilia
developed on the ectoderm cells and has to get its own
living, having by this time used up its small reserve of
yolk. It then passes through a complicated series of
larval stages, gradually leading up to the adult form.

Early development of other types.—The presence of a
greater amount of food-material in the egg renders it
possible for the embryo to go on developing further than
the gastrula-stage before being hatched, and as a general
rule, the greater the relative quantity of yolk present in the
ovum of an animal, the less clearly can a gastrula-stage be
recognised.

In the earthworm and mussel the segmentation is entire, but unequal,
and the larger, lower cells become invaginated to form the endoderm
and archenteron while the smaller upper cells give rise to the ectoderm.
In the earthworm the blastopore does not become closed, but gives
rise to the mouth.

In the frog (p. 201) the archenteron arises by « split appearing
amongst the yolk-cells, beginning at the edge of the blastopore
and gradually extending forwards : the process is probably supplemented
by a limited amount of invagination of the ectoderm. Thearchenteron
is at first a very narrow cleft, but soon widens considerably (Fig. 64,
F, ent): for some time it does not actually communicate with the ex-
terior, the blastopore (64) being filled up by a yolk-plug (y4. 9/).
As the archenteron extends forwards, and the relatively small segment}

GASTRULA-STAGE 563

ation-cavity (64 ca) gradually disappears, the edges of the:lower margin
of the blastopore approach one another, and uniting in the median
plane, give rise to a vertical groove, the primitive groove, as it is
called.

In the centrolecithal egg of the crayfish (J'ig. 91) a gastrula-stage is
formed by invagination, but as the centre of the oosperm is filled with
solid yolk in the place of a segmentation-cavity containing fluid, the
invagination only extends a short distance inwards, the archenteron

Fic. 145.—Two stages in the development of the: blastoderm of the chick, at about
the twentieth and twenty-fourth hour of incubation respectively ; diagrammatic.
ar. op. area opaca; ar. pl. area pellucida; Ad. head; med. gr. medullary groove ;
mes. mesoderm, indicated by ,dotted outline and deeper shade; fv. a7. pro-
amnion; gy. st. primitive streak and groove: #7. v. mesodermal segments or
protovertebree, (From Marshall's Ziudbryology, in part after Duval.)

being relatively very small and the ectoderm separated from the
endoderm by the yolk.

The gastrula-stage is much less clearly distinguishable in the
segmenting eggs of the dogfish and bird (pp. 454 and 547), in which
the relatively enormous mass of unsegmented yolk is, as in the crayfish,
sufficient to nourish the embryo until it has’ reached a stage closely
resembling the adult in almost every essential respect except size. A
blastopore can sometimes be recognised in such cases, but in the
embryo of the common fowl it is only represented by a primétive groove
(see above and Fig. 145 gr. st). The blastoderm soon becomes differ-

0 0 2

564 SEGMENTATION IN RABBIT CHAP.

entiated intu an cuter ectoderm and an inner, lower layer of cells
(compare Fig. 119), between which and the yolk the enteric cavity is
formed: a segmentation-cavity is hardly recognisible. As the embryo
develops, it becomes folded off from the yolk, which forms a yo/k-sac on
its ventral side (Figs. 120 and 154).

The minute egg of the rabbit and of most other Mammals, although
alecithal and undergoing a holoblastic segmentation, has presumably

_ Fic. 146.—Oosperm of rabbit 70-90 hours after impregnation.
dv. cavity of blastodermic vesicle (yolk-sac) ; cf. outer layer of cells (trophoblast) ;
Ay. inner mass of cells of the embryonic area ; Zf. albuminous envelope. (From
Balfour, after E. van Beneden.)

been derived from a meroblastic type with abundant yolk like that
of the bird, and some Mammals living in Australia at the present day
still possess eggs of this type. In the higher Mammalia the yolk has
disappeared, as it is no longer needed, the embryo, as we have seen,
being nourished by means of a placenta, which will be described pre-
sently. The early processes of development are therefore somewhat
peculiar, and though the segmentation is holoblastic, the subsequent
development is essentially similar to that of the bird, the embryo
beginning to appear in a mass of cells (Fig 146, Ay’) attached te

XI MESODERM AND CCELOME 565

the upper pole of a large blastoder mie vesicle (bz), representing the yolk-
sac of a bird but containing a fluid instead of yolk’ and being
surrounded by a-layer of cells known as the 4ophob/ast (ep).

In Amphioxus alone amongst the triploblastic animals
described in this book, does the mesoderm arise as a serics
of enteroccelic pouches : it is usually at first solid, and may
be budded off from the endoderm, from the lip of the
blastopore or primitive groove at the junction of the ecto-
derm and endoderm, or both endoderm and mesoderm may
be differentiated at the same time from the lower layer-
cells or yolk-cells (e.g. frog); or, finally, it may arise in
all these ways (e.g. fowl, rabbit). The ccelome is formed
by a split taking place in the mesoderm on either side
(Figs. 65, mes, and Fig. 147, msd,som, spl ), the split gradually
extending with the extension of the mesoderm between
the ectoderm and endoderm. Thus the ccelome is formed,
not as an enteroccele, but as a schizocele.

In Vertebrates each mesoderm-band becomes differ-
entiated into a dorsal portion, the vertebral plate, which
soon loses its coelomic space, and a ventral portion, the
lateral plate, which is divided into parietal and visceral
layers by the coelome (Figs. 143 D and 147). The vertebral
plate undergoes metameric segmentation, becoming divided
into a row of squarish masses, the mesodermal segments or
protovertebre (pr.v), from the dorsal portions of which
the muscular segments or myomeres are formed (p. 203),
and from their ventral portions the vertebral column, the
segmentation of which alternates with that of the myo-
meres.

Development of the chief organs in the Craniata
(compare pp. 201-210).—The nervous system, as well as
the essential parts of the sensory organs are,as we have

566 NERVOUS SYSTEM CH. XI

seen, in all cases formed from the ectoderm (pp. 202, 209,
and Figs. 64, 65, and 143); and in craniate Vertebrates the
anterior end of the hollow medullary tube becomes dilated,
forming three bulb- like? swellings—the fore-brain (Fig.
148, A, f. 4), mid-brain (m. 4), and hind-brain (4. 4). Soon
a hollow outpushing grows forwards from the first vesicle

KS
TILA
af
a

2 <aigeeents

Fic. 147.—Transverse section of embryo of frog.
cal. ccelome ; cad’. prolongation of ccelome into protovertebra (the reference line
should end at the space) ; ez¢. mesenteron (archenteron) ; 7sd@. mesoderm ; 2ch.
notochord ; g7. 7. protovertebra ; ; sg.@. pronephric duct; som. somatic layer
of mesoderm ; sp.c. spinal cord; sfZ. splanchnic layer’ of mesoderm ; yk.
yolk-cells. (From Parker and Haswell’s Zoology, after Marshall.)

(B, prs. en), and the third gives off a similar hollow out-
growth (cb/m) from its dorsal surface. The brain now con-
sists of five divisions: the pvosencephalon (prs. cn) and
diencephalon (dien) derived from the fore-brain, with the
pineal apparatus (pz. 6, pa. e) and the infundibulum
and pituitary body (é#f. pty): the mid-brain or mesen-
cephalon (m. 6) which gives rise to the optic obes and crura
cerebri: and the epencephalon or cerebellum (cbhlm) and

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568 NERVOUS SYSTEM CHAP.

metencephalon or medulla oblongata (med. obf) derived from
the hind-brain. The original cavity of the brain becomes
correspondingly divided into a series of chambers or
ventricles (compare Figs. 148 and 50), all communicating
with one another, and called respectively the fore-ventricle
or prosocele, third ventricle or adiacele, mid-ventricle or
mesoccele (iter and optic ventricles or optocales), cerebellar
ventricle or epicale, and fourth ventricle or me‘acele,

In some fishes (e.g. dogfish, Fig. 115) the brain consists
throughout life of these five divisions, but in most cases
(Figs. 49 and 131), the prosencephalon grows out into
paired lobes, the right and left cerebral hemispheres or
parencephala (Figs. 148, I-L, ¢ 2), each containing a cavity,
the dateral ventricle or paracale (pa. coe) which communi-
cates with the diaccele (a7. cw) by a narrow passage, the
foramen of Monro (fm). From the prosencephalon or
the hemispheres are given off a pair of anterior prolonga-
tions, the olfactory lobes or rhinencephala (olf. 1), each
containing an olfactory ventricle or rhinocele (rh. ce),

In the preceding description the brain has been described as if its
parts were in one horizontal plane; but, asa matter of fact, at a very
early period of development the anterior part becomes bent down over
the end of the notochord, so that the whole organ assumes a retort-
shape, the axis of the fore-brain being strongly inclined to that of the
hind-brain. The bend is known as the cerebral flexure (Fig. 155): it is
really permanent, but as the hemispheres grow forwards parallel to the
hind-brain and the floor of the mid-brain and hind-brain thickens, it
becomes obscure and is not noticeable in the adult.

The ganglia of the dorsal roots of the spinal nerves are developed
from a paired sezral rdage formed close to the junction of the medullary
plate and outer ectoderm, and the dorsal roots themselves appear as
outgrowths from their ganglia (see Fig. 147, above sf. c): the
ventraljoots arise as direct outgrowths from the medullary cord. Certain
of the cérebral nerves are developed in an essentially similar manner to

XI NOSE AND EYE 569

the dorsal roots of the spinal nerves, while others arise as direct ventral
outgrowths from the brain, like the ventral roots,

The offactory organs arise as sac-like invaginations of the
ectoderm, one on cither side of the snout, and become
enclosed by the cartilaginous olfactory capsules, developed,
with the rest of the skeleton, from the mesoderm. The
aperture of invagination gives rise to the external nostril,

opt.c’
ope.c: 7 7

tnvl

opt.st

Fic. 149.—Early (A) and later (B) stages in the development of the eye,
dien. diencephalon ; zzv. 2. invagination of ectoderm to form lens; 2. lens ; oft.c
" outer layer of optic cup; ofz.c’. inner layer; oft. st. optic stalk ; oft. v. optic
vesicle ; #4. pharynx ; Jty, pituitary body. (From Parker and Haswell’s Zoology
altered from Marshall.)

the internal nostrils (in air-breathing forms) being developed
subsequently.

The mode of development of the paired eye of vertebrates
is peculiar and characteristic.

At an early stage of development a hollow outgrowth—the
optic vesicle (Fig. 149, A, oft. v)—is given off from each side
of the fore-brain and extends towards the side of the head,
where it meets with an in-pushing of the ectoderm (¢zz. 7)
which becomes thickened, and finally, separating from the
ectoderm, forms a closed, spherical sac (B, /) with a very

570 EYE CHAP.

small cavity and thick walls (compare Fig. 64, I, ¢). This
body is the rudiment of the lens: as it enlarges it pushes
against the optic vesicle and causes it to become in-
vaginated (8), the single-layered optic vesicle thus be-
comes converted into a two-layered optic cup (oft. ¢, opt. c’),
its cavity, originally continuous with the diaccele, becoming
obliterated. Between the edge of the cup and the lens, on
the ventral side, is a small space which gradually extends
towards the stalk of the cup, and thus gives rise to a slit
. in the wall of the latter: this

a choroid fissure (Fig. 150, aus),
i as it is called, soon becomes
y closed by the union of its

edges. The outer layer of
u the optic cup becomes the

pigment-layer of the retina
(p. 183) : from its inner layer
the rest of that membrane—

aus
Fic.5150.—Plastic representation of the

optic cup and lens. 1 ;
iA ile well OF opude oan aa including the rods and cones
choroid fissure; g?. cavity of optic —is formed. The stalk of the

cup; /. space between the two .
walls,which afterwards disappears ; optic cup occupies, in the

76. inner wall of optic cup ; Z. lens;

Sn. stalk of optic cup (rudiment of embryonic eye, the place of

optic nerve), (After Hertwig.) :

the optic nerve, but thé actual
fibres of the nerve are formed from the nerve-cells of the
retina and grow inwards to the brain.

During the formation of the lens, mesoderm extends in
between the ingrowth from which it arises and the external
ectoderm ; from this the main substance of the cornea and
its inner or posterior epithelium are formed, the adjacent
ectoderm becoming the external epithelium, i.e. that of
the conjunctiva (p. 182). Mesoderm also makes its way
into the optic cup, through the choroid fissure, and gives
rise to the vitreous humour. Lastly, the mesoderm imme-

XI EAR 571

diately surrounding the optic cup is differentiated to form
the choroid, the iris, and the sclerotic.

Thus the eye of Vertebrates has a threefold origin: the sclerotic,
choroid, iris, vitreous humour, and the greater part of the cornea are
mesodermal: the lens and external epithelium of the cornea are
derived from the ectoderm of the head: the retina and optic nerve are
developed from a hollow pouch of the brain, and are therefore in their
ultimate origin ectodermal. The sensory cells of the retina—the rods
and cones—although not directly formed from the external ectoderm, as
in Invertebrates, are ultimately traceable into the superficial layer of
ectoderm since they are developed fromthe inner layer of the optic
vesicle, which is a prolongation of the inner layer of the brain, the latter
being continuous, before the closure of the medullary groove, with the
ectoderm coyering the general surface of the body (compare Fig. 149).

The organ of hearing, like that of smell, arises in the
embryo as a paired invagination of the ectoderm in the
region of the hind-brain, a shallow depression being formed
(Figs. 64 L and 155, av.s) which deepens and becomes
flask-shaped ; and finally, as a rule, loses its connection with
the external ectoderm, becoming a closed sac surrounded by
mesoderm in which the cartilaginous auditory capsule is
subsequently developed. At first simple, it soon becomes
divided by a constriction into dorsal and ventral compart-
ments, from the former of which arise the utriculus and
semicircular canals, and from the latter the sacculus and
cochlea.

The mode of development of the enteric canal has
already been dealt with (pp. 204—210). The first traces
of the 4ver and pancreas are seen as simple offshoots of the
mesenteron (archenteron), which gradually become branched
in a complicated manner, the numerous lobules being more
or less closely connected together by mesoderm. The
gill-pouches arise as paired outgrowths of the endoderm
lining the ;pharynx which come into contact with the

572. RESPIRATORY AND CIRCULATORY ORGANS cHap.

ectoderm, the latter becoming perforated to form the
external branchial apertures. Gill-clefts appear in the cm-
bryo of reptiles, birds, and mammals—animals in which gills
are never developed (Fig. 155); but they early disappear
with the exception of the first cleft, corresponding with the
spiracle of the dogfish, which gives rise in all Vertebrates
above fishes to the tympano-eustachian passage (p. 433):
the branchial skeleton, as we have seen, undergoes a cor-
responding reduction or modification (pp. 422 and 479).
In air-breathing Vertebrates the /umgs arise as a ventral
outgrowth of the pharynx.

The circulatory organs are developed from the meso-
derm, the ear? arising in the visceral layer on the ventral
side of the pharynx. It has at first the form of a straight
tube, which soon becomes twisted into an S-shape and in
which transverse constrictions are formed dividing it into the
different chambers. The auricular and ventricular portions
are each at first single, but from the Amphibia onwards the
former subsequently becomes divided into two by a septum
and the ventricle is similarly subdivided in birds and mam-
mals. The modification of the arterial arches in the ex-
amples studied has already been described (pp. 435 and 506).

In the meroblastic eggs of the dogfish and bird the
dorsal aorta, in addition to its other branches, gives rise to
paired wtediine arteries: these vessels branch up over the extra-
embryonic part of the blastoderm (p. 578), which spreads
over the yolk, and take an important share in the absorp-
tion of the latter bythe embryo. From this avea vasculosa
(Fig. 154), the blood is returned by wéelline veins into
vessels which eventually give rise to the hepatic portal veins.
The other chief veins in all embryonic Craniates are, as in
the dogfish, the jugulars and the cardinals. In all Verte-
brates above the fishes, the cardinals became subsequently

XI URINOGENITAL ORGANS 573

more or less entirely replaced functionally by the develop-
ment of a postcaval (compare p. 440): the anterior part of
one or both cardinals may, however, persist as the azygos
vein or veins (e.g. Rabbit, p. 509).

Urinogenital organs. The excretory organ, speaking of
craniate Vertebrates as a whole, consists of three parts, all
paired and situated along the dorsal wall of the coelome:
the fore-hidney or pronephros (Fig. 151, A, p. nph), the mid
hidney ox mesonephros (ms. nph) and the hind-kidney or meta-
nephros (mt. nph). Each of these is provided with a duct,
the pro- (sg.d), meso- (msn.d), and metanephric (mt. n. d) ducts,
which open into the cloaca. The gonads (gon) lie in the
coelome suspended to its dorsal wall by a fold of peritoneum :
they are developed as ridges covered by coelomic epithelium
(compare pp. 194—196 and 336).

The’ pronephros is nearly always functionless in the adult
and often even in the embryo, and usually disappears alto-
gether: in the young tadpole it acts as the sole excretory
organ for some time. The mesonephros is usually the
functional kidney in the lower Craniata, in which as a rule
no metanephros is developed (see p. 576), and the meso-
nephric duct acts as a ureter, often in addition carrying off
the seminal fluid of the male (¢.g. frog). In the higher
forms the mesonephros is replaced in its excretory function
by the metanephros, the metanephric duct being the ureter
(e.g. Rabbit).

The development of the kidney reveals a resemblance to
the nephridia of worms which would hardly be suspected
from its adult structure. The pronephros (Fig. 151 A, p.
nph) originates as two or threc coiled tubes formed from
mesoderm in the body-wall at the anterior end of the
ceelome ; they are arranged metamerically and each opens

574 URINOGENITAL ORGANS CH. XI

into the ccelome by a ciliated funnel (ast). Obviously
such tubes are nephridia (compare p. 331); their chief
peculiarity is that their outer ends do not open directly on
to the exterior, but into a longitudinal tube, the pronephric
duct (sg.2), which passes backwards and discharges into the
cloaca. It seems probable that this arrangement is to be
explained by supposing that the nephridia originally opened
externally into a longitudinal groove, which, by the apposition
of its edges, was converted into a tube. All the nephridia
of the pronephros open, by their ciliated funnels, into the
narrow anterior end of the ccelome, into which projects a
branch of the aorta ending in a single large glomerulus
(p. 146).

The pronephros soon degenerates, its nephridia losing
their connection with the duct (B), but in the mean-
time fresh nephridia appear in the segments posterior
to the pronephros and together constitute the mesonephros
or Wolffian body (ms. nph), from which the permanent
kidney is formed in most of the lower Craniata (eg.
frog). The mesonephrie nephridia open at one end into
the duct (sg. d), at the other, by ciliated funnels (s¢), into
the ccelome; a short distance from the funnel each gives
off a blind pouch which dilates at the end and forms a
Malpighian capsule (m.c), and a branch from the aorta
entering it gives rise to a glomerulus.

In some forms (eg. Dogfish, p. 450) the pronephric
duct now becomes divided by a longitudinal partition into
two tubes: one retains its connection with the mesonephros
and is now known as the mesonephric or Wolffian duct
(C, ms. 2. d@): the other, or Aillerian duct ( p. n.d), has no
connection with the nephridia, but opens into the ccelome
in the region of the vanishing pronephros, and assumes the
functions of an oviduct in the female. In some Craniata the

( cl

Lat.bl

Fic. 151.—Diagrams illustrating the development o: the urinogenitaljorgans ot
Craniata. vir?

A, ‘pronephros and its duct B, atrophy of pronephros, development of meso-
nephros ; C, differentiation of Wolffian and Miillerian ducts D, development of
metanephros, male type; E, female type.

al. bl. allantoic bladder; az. anus; c?. cloaca; gon. gonad ; ‘i#¢. intestine 5 #.¢
Malpighian capsule; ms. #. d. mesonephric (Wolffian) duct; ys. uph. meso-

576 MUSCLES AND SKELETON CHAP.

nephros ; #¢. 2, d. metanephric duct ; #z¢. #4. metanephros ; zs¢. nephrostomes ;
ow, ovary ; 4.2.d. Miillerian duct 5 p nph. pronephros; sg. d. pronephric duct ;
z. spermary ; v.¢. efferent ducts. F: rom Parker and Haswell’s Zoology).

Miillerian duct appears quite independently of the Wolffian
duct ; the latter is then simply the pronephric duct after
the union with it of the mesonephric tubules.

In the higher Vertebrata (Reptiles, Birds, and Mammals)
a diverticulum (D, E, mz. 2. d@) is given off from the pos-
terior end of the Wolffian duct, which grows forwards and
becomes connected with a series of posterior nephridia. In
this way is formed a metanephros (mt. nph), which gives nse
to the permanent kidney, and a metanephric duct (mé. 2. @)
which becomes the ureter. The Wolffian body ceases to
discharge a renal function, becomes in the female a purely
vestigial organ, and in the male gives rise to the epididymis
(pp. 451 and 521), which receives the efferent ducts from
the spermary and from which the Wolffian duct (spermiduct
or vas deferens) arises.

The homology of the hinder part of the kidney in the dogfish (p. 451)
is differently interpreted by zoologists. It is usually considered as
corresponding to part of the mesonephros, but in its differentiation from
the anterior part of this organ and in the development of special ureters
it resembles the metanephros of higher Vertebrates.

The majority of the muscles are developed, as we have
seen (pp. 203 and 565) from the mesodermal segments
others arising from the parietal and visceral layers of the
mesoderm,

The first part of the endoskeleton to arise is the endo-
dermic notochord (pp. 203 and 560), in the mesoderm
surrounding which cartilage appears and undergoes seg-
mentation, giving rise to the vertebra, the notochord be-
coming constricted by the ingrowing cartilage, and eventually
disappearing more or less completely (compare pp. 425
and 565): it at first extends into the head as far as the

x1 SKULL 377

pituitary body (Fig. 152 C). The cranial cartilage does not
become segmented, but gives rise to a pair of horizontal
bars, the parachordals (PE): these are continued forwards,
diverging around the pituitary body, as the ¢vabecula crantt
(Zr), and thus a support is formed for the developing
brain. The two parachordals and trabecule then unite

Fic. 152.—A and B, two stages in the development of the chondrocranium.

A. eye; AF. antorbital process; B. basal plate, formed from the parachordals ;
C. notochord; Cz. anterior process of trabecula; WV, V&. nose; O. ear; O27.
position of foramina for olfactory nerves ; PZ. parachordal cartilage; P/. post-
orbital process; PR. pituitary space; S. nasal septum; 7+. trabecula. (From
Wiedersheim’s Azatomy.)

respectively with one another, and so form a firm
floor (B) for the future brain-case, which is gradually
developed by the floor growing up on either side and
eventually meeting to a greater or less extent above the
brain : there is never, however, a complete cartilaginous roof
to the cranium, parts of which are only membranous and

form the fontanelles (pp. 43 and 420). In the meantime
Pract. Zoov. PP

578 EMBRYONIC MEMBRANES CHAP

the cartilaginous sense-capsules are developed, the olfactory -
and auditory capsules uniting with the brain-case in front
and behind respectively. ;

The visceral skeleton is formed as a series of cartilaginous
bars within the visceral arches, the first of which forms the
mandibular arch, the second the hyoid, and the others the
branchial arches.

The 4imts appear as small buds (Fig. 155) composed
of ectoderm with a core of mesoderm, and their skeleton
arises by the formation of cartilage at their bases, which
extends inwards to form the arches, and outwards to form
the skeleton of the free portions of the limbs.

As we have seen, the endoskeleton may remain more or
less entirely cartilaginous in the adult (e.g. Dogfish), but in
higher forms extensive processes of ossification set in, certain
bones (cartilage-bones) replacing this cartilage to a greater
or less extent, and others being formed in the surrounding
connective-tissue (compare p. 43)

Development of the Amnion, Allantois and Placenta.—
We must now consider some important and characteristic
structures which are developed in the embryos of the higher
Craniata (Reptiles, Birds, and Mammals), and known as
embryonic membranes. Taking the chick as a convenient
example, these are formed as follows.

The blastoderm gradually extends peripherally so as to
cover the yolk, and thereby becomes divisible into an
embryonic portion, from which the embryo is formed, and
an extra-embryonic portion which invests the yolk-sac and
takes no direct share in the formation of the embryo
(Fig. 154). The extension of the ectoderm and endoderm
takes places regularly and symmetrically, but the meso-
derm, while extending equally in the lateral and posterior

XI AMNION AND ALLANTOIS 579

regions, grows forwards in the form of paired prolongations
which afterwards unite, so that for a time there is an area of
the blastoderm in front of the head of the embryo formed
of ectoderm and endoderm only, and called the pvo-amnion
(Fig. 145 pr. am).

Before the embryo has begun to be folded off from the
yolk the rudiment of one of the two embryonic membranes,
the amnion, has appeared. A crescentic amniotic fold (Fig.
153, A, am. f) arises in front of the head-end of the embryo
from the region of the pro-amnion: it consists at first of
ectoderm only, the mesoderm not having yet spread into the
pro-amnion. The fold is soon continued backwards along
the sides of the body (B) and round the tail (A), but in
these regions (am. /’) it consists from the first of ectoderm
plus the parietal layer of mesoderm, #.¢., it is a fold of what
may be called the embryonic body-wall or somatopleure
(p. 561). -Its cavity is a prolongation of the space between
the parietal and visceral layers of mesoderm, 7.e., is an
extension of the extra-embryonic ccelome.

The entire amniotic fold gradually closes in above (C),
forming a double-layered dome over the embryo. Its inner
layer, formed of ectoderm internally and mesoderm exter-
nally, is the amnion (am), the cavity of which becomes filled
with a watery amniotic fluid, serving as a protective water-
cushion to the enclosed embryo. Its outer layer, formed of
ectoderm externally and mesoderm internally, is the serous
membrane (sr. m): this comes to lie just beneath the vitelline
membrane, with which it subsequently fuses.

The second of the embryonic membranes, the allantois,
is developed as an outpushing of the ventral wall of the
mesenteron (archenteron) at its posterior end (C, a//), and
consists, therefore, of a layer of visceral mesoderm lined
by endoderm. It has at first the form of a small, ovoid sac

PP 2

Fic. 153.—Diagrams illustrating the development of the foetal membranes of a bird.

A, early stage in the formation of the amnion, longitudinal vertical section ; B.

CH. XI ALLANTOIS 581

slightly later stage, transverse section; C, stage with completed amnion and
commencing allantois ; D, stage in which the allantois has begun to envelop the
embryo and yolk-sac. The ectoderm is represented by a blue, the endoderm by
a red line ; the mesoderm is grey. ‘
az. allantois ; ad/’. the same growing round the embryo and yolk-sac ; az. amnion ;
am.f, amniotic fold; az. anus; 4 brain; ca. coelome; c@/’. extra-embryonic
coelome ; At. heart ; ms. ent, mesenteron ; #z¢k, mouth ; zch. notochord ; sp. cd.
spinal cord; sv. 2. serous membrane; uzb.d. umbilical duct ; v#. . vitelline
membrane ; y%. yolk-sac. (Reduced from Parker and Haswell’s Zoology.)

having the precise anatomical relations of the urinary bladder
of the Frog. It increases rapidly in size (Figs. 155 and 154,
all), and makes its way, backwards and to the right, into the
extra-embryonic coelome, between the amnion and the serous

Fic. 154.—Egg of fowl, at the sixth day of incubation, with embryo and feeta
appendages.
a. air-space ; ad, allantois ; az. amnion; av. vasc. area vasculosa ; ed, embryo ;
yk. yolk-sac. (From Parker and Haswell’s Zoology, after Duval.)

membrane (Fig. 153, C, D). Arteries pass to it from the
dorsal aorta, and its veins, joining with those from the yolk-sac,
take the blood through the liver to the heart. Next, the
distal end of the sac spreads itself out and extends all round
the embryo and yolk-sac (D, a//’), fusing, as it does so,
with the serous and vitelline membranes, and so coming to
lie immediately beneath the shell-membrane. It finally en-

closes the whole embryo and yolk-sac, together with the re-

582 ALLANTOIS CHAP.

mains of the albumen, which has, by this time, been largely
absorbed. The allantois serves as the embryonic respiratory
organ, gaseous exchange readily taking place through the

Fic. 155.—Chick at the fifth day of incubation.
all, allantois ; am. cut edge of amnion; az. s. auditory sac ; £ dr. fore-brain ; f /.
fore-limb ; 4. 6. hind-brain ; 4. 2. hind-limb; Az, heart; Zy. hyoid arch ; #7. 67.
mid-brain ; 7”. mandibular arch; ¢. tail. (From Parker and Haswell’s Zoology,
after Duval.)

porous shell; its cavity is an embryonic urinary bladder,
excretory products being discharged into it from the
kidneys.

At the end of incubation the embryo breaks the shell by means of a
little horny elevation or caruncle at the end of the beak. By this time
the remainder of the yolk-sac has been drawn into the ccelome, and the
ventral body-walls have closed round it. On the shell being broken
the allantois gradually shrivels up, respiratory movements begin, the

XI PLACENTA 583

aperture in the shell is enlarged, and the young bird is hatched and
begins a free life. ;

In the higher Mammalia the allantois takes on a further
important function. The mode of development of the
amnion and allantois in the Rabbit is similar to that
described above in the case of the Bird. But the later-
history of the allantois is widely different, owing to the
modifications which it undergoes in order to take part in
the formation of the placenta, the structure by means of
which the foetus receives its nourishment from the walls of
the uterus, with which the blastodermic vesicle (p. 565) ~
becomes adherent. The foetal part of the placenta is formed’
from the outer layer of the amnion (serous membrane,
Fig. 156, sk) in a limited disc-shaped area where the distal .
portion of the allantois -coalesces with it. The membrane: »
thus formed (charion) develops vascular processes—the
chorionic vitli ( pl)—which are received into depressions—
the uterine crypts—in the mucous membrane of the dorsal -
wall of the uterus which constitutes the maternal portion of
the placenta. The completed placenta. with its villi is sup-
plied with blood by the allantoic vessels, and the blood
supply of the uterus is at the same time greatly increased ;
the foetal and maternal capillaries and sinuses are thus
brought into intimate relation with one another in_ the
placenta, and diffusion can take place between them,
- nutrient matter and oxygen diffusing from the blood of
the mother into that of the foetus, while excretory sub |
stances- pass from the blood of the foetus into that of the-
mother.

The discoidal placenta of the Rabbit is of the type termed.
deciduate, the villi of the placenta being so intimately con-
nected with the uterine mucous membrane that a part of the
latter comes away with it at birth in the decidua, or after.

584 * PLACENTA ‘ CHAP.

birth, which is attached to the newly-born young by the
umbilical cord, consisting. of the stalks. of the allantois (a)
and flattened yolk-sac (ed, fd) twisted together. The cord
is gnawed through by the parent-rabbit, the blood-vessels
being compressed in the process; and it soon shrivels up
and comes away at the navel or wmbilicus, which repre-

“pl

Fic. 156.—Diagrammatic longitudinal section of the foetus and embryonic mem-
branes of a rabbit.

«, (on right) amnion ; a. (on left) stalk of allantois; @/. allantois with blood-vessels.; ©
c. embryo ; ds. cavity of the flattened yolk-sac (blastodermic vesicle) ; ed. endo-
dermal layer of yolk-sac; ed’. inner portion, and ed”. outer portion of endoderm~
lining the compressed yeavity of the yolk-sac; /#. vascular layer of yolk-sac; ‘
pl. placental villi; ~. ‘space filled with fluid between the amnion, the allantois,.
and the yolk-sac ; sk. serous membrane ; s¢. margin of vascular area of yolk-sac,
(From Balfour, after Bischoff.) %

’

sents the point of connection between the foetus and the
placenta. The intra-abdominal portion of the allantois is
represented by a cord or ligament, the wachus, which connects
the navel with the apex of the bladder, so that only.a
small portion of ‘the allantoic outgrowth, and not the whole
of it as in the frog, persists in the adult.

XI PRACTICAL DIRECTIONS 585

PRACTICAL DIRECTIONS.

A. Special methods are required to follow out the details of the
structure and division of nuclei, but if you have not the opportunity
of examining preparations illustrating these, a good deal may be made
out as regards the chromatin and the behaviour of the chromosomes
in the process of mitosis by the following simple method.

Obtain a young, gilled-larva or tadpole of the common newt (p. 218),
kill, and place it in corrosive sublimate for about half an hour: wash
thoroughly in water and transfer first into weak, and then into strong
alcohol (p. 137). Stain entire, either with a solution of hematoxylin,
which can be bought ready prepared, or alwm-carmine, which you can
prepare yourself by dissolving 200 grams of ammonia-alum in water,
boiling the solution and adding carmine to excess, filtering, and diluting
with three or four times its bulk of water ; a drop or two of carbolic acid
should be added to the solution.

Strip off small pieces of the skin of the stained preparation, and after
putting them through weak and strong alcohol, transfer to absolute
alcohol, then to turpentine or oil of cloves, and mount in Canada balsam
(p. 139). Examine, comparing Fig. 136 and pp. 541-545, and sketch
as many different stages as possible.

_ B. Observation of the details concerned in the process of maturation
and fertilization of the ovum is too difficult for the beginner, but Zo/ar
cells may easily be seen in the living, freshly-laid eggs of one of the
common pond-snails (e.g. Liémnaeus stagnal’s), in which some of the
stages of segmentation can also be observed. Keep some of these snails
in a glass vessel with water weeds, and notice, that the eggs, when laid,
are enclosed, a number together, in a common gelatinous envelope.
Separate the eggs from one another with needle, and examine in water
under the microscope. Note that the small egg is surrounded by fluid
enclosed in a relatively large egg- case; and observe the minute polar
cells at the periphery ¢ of the ovum.

C. A series of models of the development of Amphtoxus, to be found
in most ‘museums, should be carefully - cuuaiaal (compare Figs 141-
144).

D. In order to follow out the development of the chief organs in a

586 PRACTICAL DIRECTIONS CHAP.

Vertebrate animal, it is necessary to make a number of serial transverse
sections.

. For this purpose chick-embryos (see below) are, on the whole, the
most convenient and satisfactory, but if you also wish to make sections
of embryos of the frog, proceed as directed on p. 214.

Serial sections of embryos in different stages should be mounted on
the same slide, after smearing it with collodion and oil of cloves (p. 139).
It is a matter of some difficulty to make satisfactory sections of the early
stages: the most important stages for the present purpose subsequently
to segmentation are from the time when the embryo begins to become
elongated up to hatching.

A number of fresh, impregnated fowl’s eggs should be obtained and
placed in an incubator at a temperature of from 37 to 40 C. or under a
“*broody ” hen, first marking each with the date. One or two should
be examined each day or oftener for the first four or five days of incuba-
tion. To expose the embryo, place the egg in a dish of warm water
(temperature as above), in order that, after the first day, the beating
of the heart and the circulation of the blood may not be stopped.
With the forceps, tap the surface of the egg lying uppermost so as to
break the shell into small pieces, which can then be removed: cut
away sufficient of the shell-membrane with the scissors to expose the
entire embryo and blastoderm. The early stages are difficult to observe,
and the most important of those referred to below are from the end of
the first to the third day of incubation. As the medullary groove
only closes gradually from before backwards in the body-region, sections
showing different stages in the development of the central nervous system
may be obtained from the same embryo at these stages. Prepare as
above.

1. Examine first an waducubated egg, as directed above, and make out
ts structure (compare Fig. 138). (The blastoderm undergoes seg-
mentation before the egg is laid.) :

2. First day of incubation (18-20 hours). Examine with a lens and
compare Fig 145 A. Then carefully cut round the blastoderm with fine
scissors, float it off in the water, preserve, stain, and mount entire in
Canada balsam. (The removal and preparation of the blastoderm at this
stage is rather difficult, and sections of the next stage will illustrate the
chief points equally well).

XI PRACTICAL DIRECTIONS 587

3. End of first day (about 24 hours). Examine as before, com-
paring Fig. 145 B. Sketch, and then preserve, stain, and eventually
cut sections. Note the ectoderm, endoderm, mesoderm, medullary groove
and its closure to form the medzdlary cord ; the neural ridges (p. 568) ;
the notochord ; the tardetal (somatic) and vésceral (splanchnic) layers
of the mesoderm, and the ca/ome (compare Figs. 64, 65, and 147).
Sketch one or two typical sections.

4. Second day. Examine, prepare, cut sections, and sketch as before,
noting the fore-, mzd-, and hind-brain, the optic vesicles, lens, auditory
pits, heart and vitelline vessels, and the head-fold of the amnion, as well
as the increase in the number of mesodermal segments (compare Figs.
148, 149, and 153).

5. Third day. Examine, prepare, and sketch as before, noting, in
addition to the points already referred to, the further gradual folding
off of the embryo from the yolk (compare p. 454) and extension of the
annon and area vasculosa ; the cerebral flexure; the visceral arches
and clefts and the arterial arches ; the mesonephric duct; and the
further development of the eye and ear (compare Figs. 147, 148, 149,
“150, 153, and 155). i

6. Fourth to sixth days. Observe the further development of the
parts already seen and also the “mé-buds and the allantois (compare
Figs. 153-155). Sketch.

1. Examine and compare a few later stages.

E. Obtain some advanced embryos of the rabbit (p. 542) or rat.
and examine zx sz¢z in the uterus before removing and preserving them.
Note the avzzzon and examine the A/acenta and its connection with the
uterine wall and with the foetus. Sketch.

INDEX

INDEX

(The numbers in italics refer to practical directions.)

A

ies abdominal cavity, 20,
347, 418, 467, 489

Abdominal pore, 416

Abiogenesis, 282

Acanthias, 415, 454

Acetabulum, 50, 486

Achromatin, 129, 231, 541, 542

Acoustic spots, I

Acrania, 406

Acromion, 483

Adaptation, 224

Adenoids, 498

Adrenal bodies, 145, 431, 521

Agamobium, 314

Alimentary canal, see Enteric canal

Allantois, 579, 583, 587

ALLOLOBOPHORA, see Earthworm.

Alternation of generations, 314

Alveoli of jaws, 493

Amnion, 579, 587

AMCBA: occurrence and general
characters, 229; movements,
232; resting condition, 232;
nutrition, 233; growth, 234;
excretion, 235 ; respiration, 235 ;
metabolism, 236; reproduction,
236 ; immortality, 236 ; conjuga-
tion, 237; death, 237 ; animal or

plant ? 255; practical directions,
S

23

Ameeboid movements, 106, 231

Amphibia, 219, 403, 466

Amphiccelous, 423

AMPHIOXUS, see Lancelet

Ampulla, of semicircular canals,
138

Ampulla, of sensory: canals of the
integument, 448

Anal segment, 319

Analogous, analogy, 217

Anatomy, 217

Animals and Plants : comparison of
typical forms, 255; discussion of
doubtful forms, 255, 257 ; bound-
aries artificial, 258

Ankle, see Tarsus

Annulata, 220, 341

ANODONTA, see Mussel

Anura, 219

Antenna, 352

Antennary gland, 361

Antennule, 352

Anus, 6, 267, 276, 319, 357, 386,

" 416, 429, 469, 497

Aorta, aortic arches, see Arteries.

Aperture, or apertures (see also
Foramen, and under Nephridium,
Kidney, Pores, Reproductive
organs, &c.) auditory, Cray/ish,
367 ; Rabbit, 470, 472; cloacal,
see anus 3 exhalant and ‘inhalant,
382

592 INDEX

Aphis, 552

Appendages, lateral, and their skele-
ton: Frog, 5, 28, 36, 48, 51;
Crayfish, 346, 349 3 Dogfish, 417,
426; Rabbit, 484, 486 ; Develop-

ment of in Vertebrates, 578,
587
Aqueous chamber and humour,
183

Arachnoid fluid, 155

Archenteron, 201, 558

Archicerebrum, 366

Area vasculosa, 572, 581, 587

Arm, see Fore-lim

Arterial arches, 80, 436

Arteries: Frog, 27, 80; Crayfish,
362; Mussel, 392 ; aa 433-
438; Rabbit, 503-506; Vertebrate
embryo, 572

Arthobranch, 361

Arthropoda, 220, 346, 370.

Articular membrane, 347, 354

Articular processes, see Zygapo-
physes™

Artificial selection, 227

Arytenoid cartilage, 144, 500

Ascaris, 153

Asexual generation, see Agamo-
bium

Asexual reproduction, see Fission,
Budding, Spore

Assimilation, 149, 234, 300

ASTACUS, see Crayfish

Astragalus, 51, 487

Astrosphere, 543

Atlas vertebra, 481

Atrial pore, 405

Atrium, 404, 406

Atrophy, see Vestige

Auditory capsule, 39, 41, 420, 475

Auditory organ: frog, 45, 186;
Cravfish, 367 ; Mussel, 394; Dog:
Jish, 449; Rabbit, 518; Develop-
ment of, in Vertebrates, 571,
587 ‘

Auditory ossicles, 475, 519

Auricle, see Heart

Automatism, see Movements

Aves, 219, 403

Axial fibre: of Vorticella, 276; of
Carchesium, 281

Axial parts, 4

Axis fibre, see Neuraxis

Axis vertebra, 482

B

Bice 152; structure, 2573
nutrition, 257 ; animals or plants?
257; rapid multiplication, 282 ;
practical directions, 260

Backbone, see Vertebral column

Basal cartilages of fins, or basalia,
425-42

Bell of medusze, see Umbrella

Bilateral symmetry, 291

Bile, 69, 431

Bile-duct, 68, 431, 499

Bile-passages, 133

Binomial nomenclature, 215

Biogenesis, 282

Biology, 1

Bird, development of, 563, 572, 576,
578; 587

Birds, see Aves

Bladder, see Gall-bladder and Uri-
nary bladder

Blastoccele, see Segmentation-cavity

Blastoderm, 369, 454, 563, 586

me embryonic and extra-
oe portions, 572,

57
Blastodermic vesicle, 565, 584
Blastomere, 556
Blastopore, 201, 340, 369, 558, 562
Blastula, 558
Blind spot, 539
Blood: #7og, 20, 78, 85; Earth-
worm, 329, 330; Crayfish, 365;
Mussel, 392; Lancelet, 405 ; Dog-
fish, 442; Mammalia, 467
Blood-corpuscles: colourless, see
Leucocytes ; red, 105, 442
Blood-sinus, see Sinus
Blood-vessels: Frog, 78, 1073
Earthworm, 329; Crayfish, 362 3

INDEX 503

' Mussel, 392; Lancelet, 408;
Dogfish, 433-442 $ Rabbit, 503
Body-cavity, see Ccelome
Body of vertebra, see Vertebra
Body-segments, see Metamere
Bojanus, organ of, 390
Bone : cartilage and membrane, 43,
472; nature of, $23 microscopi-
cal structure of, 116
Bones, see Endoskeleton, Skull,
Vertebra, Ribs, Sternum, and
under individual bones of limbs
Botany, 1
BOUGAINVILLEA :
general characters, 304; micro-
scopic structure, 304; structure
of Medusa, 307 ; nervous system,
311; organs of sight, 308, 312;
reproduction and development,
312; alternation of generations,
3143 practical directions, 777.
Brachial plexus, see Nerve-plexus
Brain : Frog, 28, 156, 202; ELarth-
WOrM, 3333 Crayfish, 365; Mus-
sel, 392; Lancelet, 405, 408;
Dogfish, 443; Rabbit, 501; de-
velopment of, in Vertebrates, 202,
566, 587
Brain-case, see Skull
Branchia, see Gill
Branchial apertures, arches, clefts,
and septa: Zadpole, 204, 206 ;
Lancelet, 405, 407 ; Dogfish, 416,
422, 429, 432; development of,
-571-
Branchial rays, 422
Branchial vessels: Crayjish, 360,
363 3 Mussel, 392 ; Lancelet, 409 ;
Dogfish; 4333 Tadpole, 435
Breast-bone, see Sternum
Bronchus, 500 _
Buccal cavity, 16, 327, 429
Buccal groove, 262
Bud, budding, 301, 303, 313
Bufo, Bufonide, 218
‘Bulb, see Medulla oblongata
Bulbus aorte, 89
Bulla, tympanic, 475
. Byssus, 396

Pract. ZOOL.

occurrence and ,

Cc

Cece 498

Calcaneum, 51, 487
Calcar, 52
Canal: central, of spinal cord, 156,
408, 443; naso-palatine, 4903.
neural, see Vertebral column;
neurenteric, 203; radial and circu-
lar of Medusa, 307 ; semicircular of
ear, see Auditory organ and Mem-
branous labyrinth; sensory, of
Dogfish, 416, 448; sternal, of
Crayfish, 349 ; vertebrarterial, 481
Canaliculi, see Bone
Canine teeth, 492
Capillaries, 95, 363, 435, 438
Capitular facet and capitulum, 481
483
Carapace, 347
Carbohydrates, 72
Carbon dioxide, 66
Carchesium, 281, 288
Cardiac division, see Stomach.

“Carotid arch, see Arteries of Frog,

Dogfish, and Rabbit
Carpus, 50, 485 ;
Cartilage, 20,°35, 115, 419, 470;
calcified, 46, 48, 419

_ Caruncle, 582

Castings.of earthworm, 318

Cell, 110, 231, 540; see under
various types

Cell-colony, see Colony

Cell-differentiation, see Differentia-
tion, and under development of.
various types

Cell-division, 544, 585

Cell-membrane or wall,
542

Cellulose, 244, 254, 405

Cement, 429, 493

Centrosome, 544

Centrum, see Vertebra

Cephalothorax, 347

Cerebellum: Frog, 157; Daath,
443, 513

Cerebral flexure, 568

Cerebral ganglion, see Brain

QQ

232, 244,

594 : : INDEX,

‘Cerebral hemispheres :
Rabbit, 511

Cerebral nerves, see Nerves

Cerebral vesicles, 566

Cerebro-pleural ganglion, 392

Cervical groove, 347

Cheetopoda, 341

Chalaza, 548

Change of function, 433

Chela, 352

Cheliped, 350

Chiasma, optic, 164, 443, 513

Chick, see Bird

Chitin, 320, 347

Chlorophyll, 242, 251

Choroid,: 183, 517

Choroid-fissure, 570

Choroid-plexus, 157, 512

Chromatin, 129, 231, 541, 542

Chromatophores, 243, 299

Chromosomes, 544

Cilia, and ciliary movement, 109,
242, 250, 265, 273, 275

Cilia, absence of in Crayfish, 355

Ciliary folds, muscles, nerves, and
vessels, 184, 517

Ciliata, Ciliate Infusoria, 261, 286

Circulation of blood, 86, 89, 329,

363, 392, 409, 441, 503

Circulatory organs, see Blood-
vessels and Lymphatic system

Cirri, 406

Clasper, 417, 426, 453

‘Class, 219

Classification, 217, 220, 223, 396, 403

Clavicle, 47, 483

Clitellum, 319; 339

Clitoris, 524

Cloaca, 23, 386, 429

Cloacal aperture, see Anus

Cnidoblast, 297

Cnidocil, 297

Coagulation of blood, 78, 107

Cochlea, 187, 518

Cocoon, 339

Ceelenterata, 314

Coelome, 20, 203, 314, 320, 325,
340, 360, 368, 386, 418; Develop:
ment of, 203, 561, 565

Frog, 1593

Coelomata, 314 '

Coelomic epithelium, 322, and see
Epithelium

Coelomic fluid, 328

Collaterals, 171

Colon, 497

Colony, Colonial Lorganiams, 281

Columella, 45, 189

Commissures, see Blood- vessels and
Nervous system; of Brain, 515

Conchiolin,' 384

Concrescence, 347

Condyle, occipital, see Skull

Cones of retina, 185

‘Conjugation, 197, 237, 268, 278

Conjunctiva, 182

Connectives, 333, 365

Connective-tissue, 18, 113

Contractility, nature of, 112

Conus arteriosus, 79, 88, 433

Coracoid, 47, 427

Coracoid process, 483

Cordylophora,. 317

Cornea, 182, 366

Corpus adiposum, see Fat-body ;
callosum, 511; cavernosum and
spongiosum, 523, 524; striatum,
514

Corpuscles, see Blood- -corpuscles
and Leucocytes

Cortex of Brain, 160, 511

Cortical layer, 264, 269, 273 2

Craniata, 406 ‘

Cranium, see Skull _

CRAYFISH : general characters, 346,
372;' limited’ number and con-
crescence of metameres, 346;
appendages, 348, 349, 378 exo-
skeleton, 347, 349, 355, 372,
379 muscular system, 355, 374,
3755377 3 enteric canal, 356, 375 3.
gills, 360, 3773; kidney, 361, 7783
blood-system, 361, 374, 3753
nervous system, 365, 377; sense”
organs, 366, 378; reproductive
organs, 368, 375; development,
369, 563

Creation, 221

Cribriform plate, 474

INDEX : 595.

Cricoid cartilage, 500
Crop, 327
Cross-fertilization, 339
Crura cerebri, 157, 443, 513, 566
Crustacea, 292, 371
Crystalline style, 386
$s lens, see Lens of Eye
Ctenidia, 383
Cutaneous glands:
Kabbit, 468
Cuticle: in unicellular animals, 251,
264, 273; in multicellular animals,
306, 320, 321, 355, 383
Cyst, see Cell-wall and Encystation
syste duct, see Bile-duct

Frog, 1293

D

Dee cells and nuclei, 250,
254, 545
Death, 11, 152
Decalcifying, directions for, 737:
Decidua, 583
Decomposition, 11,, 1513; and see
Putrefaction . ;
Degeneration, 406 ; and see Vestige
Dehydrating, directions for, 7.377
Dental formula, 494
Dental lamina and papilla, 429
Dentine. 417, 429, 493
Deric epithelium, see Epiderm
Derm, 128, 355, 417
Dermal teeth, 418
Descent, doctrine of, see Evolution
Development, meaning of the term,
_9, 273, 280. For development
of the various multicellular types,
see under their names. Practical
work, 272-274, 585
Dialyser, 73, 77
Diaphragm, 467, 489, 490
‘Diastema, 490
Diastole of heart, 90 ; of contractile
vacuole, 266
Diencephalon, 157, 443, 512, 566
‘Differentiation, 204, 206, 237, 261
Diffusion, 73, 551, 583
Digestion, 68; intra- and extra-
cellular, 300, 328

Ch

Digestive glands, 359, 386, and see
Digestion, Enteric canal, Liver,
Glands, Pancreas

Digestive ayssemt, § ace Enteric canal

Digits, 56, 485, 48 ’

Dimorphic, Paes 250,310

Dicecious, 312, 368

Diploblastic, 293, 304, 324

Directions for dissecting, zg; for
drawing, 74; for killing, 77,239, ‘
259, 287, 316, 341, 372. 391;
5253; for preparing skeletons,
53s 379; 456, 5253 for injecting
blood-vessels, 99, 374, 3979:
458, 459s 5265. for microscopic
work, z79 ;/ for, histological and
embryological work, 735, 585

Disc of vorticella, 275 :

Dispersal, 272, 278, 396

Dissecting instruments, &c., 12

Distal, 6

Distribution of food- materials, 148

' Divergence of character, 223

Division of ‘physiological labour,
206, 237

DocFisH : General characters, 415,
456; exoskeleton, 417, 4503
endoskeleton, 419,, 456; enteric
canal, 428, 457, 459; gills, 432,
461; blood-system, 433, 458,
459, ¢613 nervous system, 442,
462; kidneys, 450, 4593. repro-
ductive organs, 451, 459, 460;
sense-organs, 448, 462, 463; de-
velopment, 453, 563, 572

Dorsal,

Drum- membrane, see
membrane

Tympanic

Duct, see under names of individual

duets and glands
Ductus arteriosus, 533
Duodenum, 22, 497
Dura mater, 15 5

E

& ar, see Auditory organ’; in-

ternal, middle, and external, 450,
467, 469, 518, 519

QQ2.,

596

EARTHWORM: general characters,
318, 397; metameric segmenta-
tion, 319; ccelome and enteric
canal, 320, 325, 328, 342, 343,
345 ; cell-layers, 321, 345; blood-
‘system, 328, 342, 344, 345 5 ne-
phridia, 331, 342, 344, 3453
nervous system, 333, 344; differ-
entiation of organs and tissues,
335; reproduction and reproduc-
tive organs, 336, 742, 747; de-
velopment, 340, 562

Ecdysis, 355

Echinodermata, 397

Ectoderm, 202, 209, 293, 304, 307,
340, 369, 454, 558

Ectoplasm, 231

Efferent duct of spermary, 193, 338,
45r

Egestion, 233

Egg of fowl, 547, 555, 556

Egg-cell, see Ovum

Egg-sac, 337

. Elasmobranchii, 415

Embryo, 9, 200; and see under
varlous types

Embryology, 217

Embryonic membranes, 578, gap

Emulsification of fats, 75

Enamel, 418, 429, 493

Enamel-organ, 429

Encystation, 232, 244, 254, 272, 279

Endoderm, 202, 210, 293, 298, 304,
307, 340, 369, 454, 558, 565

Endoderm-lamella, 309, 310

Endolymph, 188

Endolymphatic duct, 187, 420, 449

Endoparasite, see Parasite

Endoplasm, 231 ;

Endophragmal system, 349

Endopodite, 350:

Endoskeleton : Frog, 16, 35, 205;
Lancelet, 406; ‘Dogfish, 419;
Rabbit, 470; development of in
Vertebrates, 576

Endostyle, 408

Energy, conversion of potential into
kinetic, 235 ; source of, in chloro-
phyll-containing organisms, 248

INDEX

Enteric canal: Fvog, 23, 2043
Earthworm, 320, 3273; Crayfish,
356 ; Mussel, 386 ; Lancelet, 405,
408 ; Dogfish, 429; Rabbit, 490-
499; development of in Verte-
brates, 204-210, 571

Enteric epithelium, 322, and see
Epithelium

Enteroccele, 561

Enteron or enteric cavity, 291, 304, °
308, 353

Epencephalon, 566

Epiderm: Frog, 128; Earthworm,
320, 321; Crayfish, 355; Mussel,
385; Lancelet, grr; Dogfish,
417; Rabbit, 468

Epididymis, 451, 521, 576 «

Epiglottis, 495, 500

Epipharyngeal grocve, 408

Epiphysis of Vertebra, 480, 486

- Epipodite, 352
‘Epistoma, 349

Epistylis, 281, 288

Epithelial cells: columnar, 107 ;
ciliated, 109; glandular, 130 e
seg. 3 squamous, 110; stratified,
128

Epithelium, 109, 311 ; coelomic, 322,
and see Peritoneum ; deric—see ~
Epiderm ; enteric, 322, and see
Endoderm.

Equivocal generation, see Abio-
genésis

EUGLENA : occurrence and general
characters, 251 ; movements, 251;
structure, 251; nutrition, 252;
resting stage, 254; reproduction,
254; animal or plant? 255; ;
practical directions, 259

Eustachian tube or recess, 17, 189,
495, 519

Eustachian valve, 506

Evolution: organic, 221, 285; of
animals and plants, 258

Excretion, 148, 150, 235

Excretory organs, see Kidney and
Nephridium, and compare Con-
tractile vacuole

Exopodite, 350

INDEX

Exoskeleton: cuticular, 306, 355,
383; dermal, 418, 429; epider-
mal, 466, 468

Expiration, 143

Extra-cellular digestion, 234

Eye: Frog, 4, 181; Crayfish, 366 ;
Dogfish, 416, 449; Rabbit, 517 ;
development in Vertebrates, 569

Eye: compound, 366

Eyelids, 5, 416, 469

Eye-muscles, see Muscles of eye

Eye-spots or ocelli: Euglena, 254;
Medusa, 309, 312; Lancelet, 408

Eye-stalks, 349, 353

F

Favaue, 487

Feces, 8
Fallopian tube, 523
Family, 218
Fascia, 59 118,
Fat-body, 25
Fats, 72
Femur, 51, 487
Fenestra ovalis, 46, 189, 475, 518
Fenestra rotunda, 476, 518
Ferment, fermentation : amylolytic,
743 peptonizing or proteolytic,
743 putrefactive, 256,257
‘Fertilization, 197, 554, and see also
Conjugation, and under geveIoR-
ment of various types
Fibrin, 107
Fibula, 51, 487
Filum terminale, 155} 515
_ Fingers, see Digits
Fin-rays, 407, 414, 425, 426, 427);
dermal, 426
Fins : Tadpole, 207; Lancelet, 404,
"406 ; Dogyish, 417, 425
? Fishes, see Pisces
Fission, 106, 198, 236, 250, 268,
272, 277; multiple, 254, 272
Fissures of spinal cord, 155
Fixing, directions for, 7,36
Flagellata, Flagellate
261, 286°,
Flagellum, 242, 251, 256, 298; of
antenna and antennule, 353

Infusoria,

597

Flocculus, 476, 513

Foetus, 524

Follicle, ovarian, see Ovisac

Fontanelle, 43, 420

Foods, 67, 72

Foot, of mussel, 382; and see Pes

Foramen : lachrymal, 477 ;° inter-
vertebral, 38, 424, 481 ; magnum,
40, 420, 470; obturator, 486 ; of
Monro, 160, 514

Foramina for cerebral nerves, see
Skull

Fore-brain, 202, 566

Fore-gut, 358 -

Fore-limb or fin, see ‘Kapendates

Fornix, 514 we

Fossa ovalis, 504

Fossils, 223

Froc: Preliminary account, 43
mouth cavity, 16; skin and.
muscles, 17, 77; abdomen and its
contents, 20, 32; neural cavity
and its contents, 27, 737; struc-'
ture of limbs, 28, 7¢; skeleton,
35, 5P3 joints, 55; O43 muscles,
57, 64; enteric canal and diges-
tion, 67, 76; vascular system,
78, 98; circulation of blood, 89,
14033 lymphatic system, 97, 983
simple tissues, 104, 727; com-
pound tissues and glands, 126,
7439; lungs and larynx, 141, 752;
kidneys, 145, 2537; structure and
functions of nervous system, 154,
755 sense-organs, 179, 1973
reproductive organs, 193; 270;
fertilization of eggs, 9, 197; de-
velopment, 9,°198, 272, 274, 562,
586; metamorphosis, 11, 206,
272; Classification, 215; side
dissection, 465; summary of
characters, 466

Function, see Physiology

'G

Cyat-bieaeer, 22, 69, 431, 499
Gallus, see Bird

Gamete, 197, 268

Gamobium, 314

598

Ganglion; 163) 167 ; and see Nerve-
ganglia

Gastric glands, see Glands

Gastric juice, 7X, 74, 132

Gastric mill, 358

Gastrolith, 359

Gastrola, 558 -

-Gastrocnemius, 59

Gemmation, see Budding

Generation, asexual, see Aga-
mobium

Generation, sexual, see Gamobium

alternation of,see Alter-
nation of generations

Generative organs, see Reproductive
organs

Genus, 215, 218

Germinal disc, 556

Germinal epithelium, 194, 196, 336

59 vesicle and spots, 546.

Germplasm, 555

Gestation, 525

Giant-fibres, 334

Gills: Tadpole, 10, 204, 207 ; Cray-
Jish, 360; Mussel, 382, 386;
Dogfish, 432, 455 ;

Gill-arches and clefts, see Branchial
apertures, arches, clefts and septa

Gill-cover, 347, 360

Gill-rays, see Branchial rays.

Gizzard : ae dae 327; Cray-
Ssh, 356, 3

Gland-cells : Hayden, 298, 209;
Earthworm, 327, 339; and see
Glands and Goblet-cells.

Glands: Cowper’s, 523; digestive,
see Enteric canal; gastric, 131,
431, 497 ; green, 361 ; Harderian,
186, 518; lachrymal, 518 ; mam-
mary, 467, 468, 489 ; Meibomian,
518 ; cesophageal, 327 ; perineal,
469, 523; prostate, 523; race-
mose, 135; rectal, 523 ; salivary,
494; and see kidney, liver, pan-
creas, &c. :

Glenoid cavity, 47

Glochidium, 396

Glomerulus, 146, 574

Glottis, 17, 495

Glycogen, 134

a

‘

INDEX

Goblet-cells, 109, 130, 131

Gonad, 193: and see Reproductive
organs

Gonaduct, see Reproductive organs

Grey matter of spinal cord and

brain, 156, 160, 167

Growth, 236

Gryllotalpa, 550

Gullet, 253, 265, wee ; and see
Enteric canal

H

H zemal arch and spine, 425

Heematochrome, 243

Hamarococcus: general eharac-
ters, 240; rate of progression,
240; ciliary movements, 242,
250; colouring matter, 242:
motile and stationary phases,
2443 nutrition, 245; source of
energy, 248; reproduction, 250;

dimorphism, 250; animal or
plant? 255; practical directions,
259

Hemocyanin, 365

Hemoglobin, 107, 330

Hairs, 466, 468

Hallux, 487

Hand, see Manus

Hardening, directions for, 736

Head, 4, 347, 349,416, 468:

Heart, 20, 79, 87, 362, 392, 433,

503 ; development of, 442, 572;
pulsation of, go

Heat, evolution of, 151

Hemibranch, 432°

Hepatic ducts, see Bile duct

Hepatic cecum, 408

Hepatic portal system, see Portal
system

Hepato-pancreas, 359

Heredity, 225

Hermaphrodite, see Moncecious

Heterogenesis, 284

Hibernation, 8

Higher (tripoblastic) animals, uni-
formity in general structure, 324

Hind-brain, 202, 566

INDEX

Hind-limb or fin, see Appendages
Hind-gut, 3
aes of ‘Rcd shell,
393
Hip-girdle, see Pelvic arch
Hippocampus, 514
Histological methods, 120, 1735,
585
Histology, 104
‘Holobranch, 432
‘Holophytic nutrition, 247, 253
‘Holozoic nutrition, 247, 253
Homogenesis, .284
Homology and homologous, 217,
. 310
‘Homology, serial, 39, 52
‘Host, 271
Humerus, 48, 484
Hybrids, 216
HypRA: occurrence and general
characters, 289; species, 291 ;
movements, 291; mode of feed-
ing, 292; microscopic structure,
292; digestion, 299; aséxual,.
artificial, and sexual reproduction,
301; development, 303; pee
cal directions, 774
Hydranth, 304
Hydroid polypes, 303
Hydrozoa, 314
Hyoid, 40, 44, 422, 472, 479
‘Hyomandibular, 422
Hypobranchial groove, see Endo-
style
Hypostome, 289, 304, 308

I i

‘

eae 22, 497

Tlium, 50, 486 ~

Imbedding, directions for, 177
Immortality, 236

Impregnation, see Fertilization |
Incisors, 468, 492, 494

Income and expenditure, 148, 236,

iis AG
Incubation, 58
TIncus, see Auditory ossicles

599"

Individual, 220, 281

Individuation, 281, 301

Infundibulum, of brain, 159, 443;
513, of lung, 501

Infusoria, 261, 286

Ingesta and. -egesta, balance of, 236

Ingestion, 8, 233

Inguinal canal, 521

Injection of blood-vessels, 99, 374,
399; 458, 459, FOI, 526

Innominate bone, 51, 486

Insertion of muscle, 60 -

Inspiration, 143 i

Integument, structure of, 127

Integumentary sense-organs, 414,

44
Intercellular substance, 115,117
Interneural plate, 424
Interstitial cells, 293
Intervertebral discs, 480
Intervertebral foramina, see Fora-
men '
Intervertebral substance, 424
Intracellular digestion, 234
Interrenals, 431
Intestine, 22; see its ‘various sub-
divisions and Enteric canal
Invagination, 558 '
Iris, 5, 182, 571
Irritability, 60, 169, 232, 276° eX
Ischium, 50, 486

J

acobson’s organ, 517
Jaws, 17, 352, 415, 420, 472; 477,

479
Joints, 55, 348, 354

é :
\

: K aryokinesis, see Mitosis

Keber’s organ, see Pericardial gland

Kidney: Frog, 26, 1453; Cray/ish,
361 ; Mussel, 390; Dogfish, 450;
Rabbit, 520; development of, in
Vertebrates, 573

600
L

Lovie palp, see Palp

Labrum, 356

Lachrymal ducts and glands, see
Naso-lachrymal ducts and glands

Lacune, see Bone

Lamelle of gills (Mussel), 388

Lamellibranchiata, 396

Lamina terminalis, 514

LANCELEr: general characters,
404; fins, 4053 skeleton, 405 ;
gill-slits and bars, 407; enteric

canal, 408; blood- vessels, 408 ;.

nephridia, 408 ; nervous system,
408 ; gonads, 409; development,
409, 557; practical directions,
FlO-4lZ

Larva, 9, 349, 379, 395, 396, 409

Laryngo-tracheal chamber, 141

Larynx, 489, 500

Lateral line, 416, 448

Legs, see Appendages

Lens of eye, 183, 517, 569

LeEPuws, see Rabbit :

Leucocyte, 105, 328

Life, origin of, 281; and see Bio-
genesis

Life-history, 8, 206; and see under
various types

Ligaments, 55, 57

Limbs, see Appendages

Lips, 468

Liver, 20, 133, 204, 359, 408, 431,
499 ;

Lobster, 346

Lumbricide, 341

LuMBRICUS, see Earthworm |

Lungs, 10, 22, 141, 204, 208, 501

Lymneus, 585

Lymph, 18

Lymph-hearts, 97, 98

Lymphatic glands, see Adenoids

Lymphatic system, 18, 97, 442

M

M alleus, see Auditory ossicles

Malpighian capsule, 146, 574
Mammalia, 219, 403, 466

*

INDEX '

Mammary glands, see Glands

Mandible, 16, 44, 352, 422,-479

Mantle, 381

Mantle-cavity, 385

Manubrium: of Medusa, 307; of
malleus, 519; of sternum, 483.

Manus, 5, 50, 463, 485

Marrow-cavity, 48

Matrix, see Intercellular substance’

Maturation of ovum, 552

Maxilla: Crayfish, 352

Maxilliped, 350

Meckel’s cartilage, 44, 422

Mediastinum, 501

Medulla oblongata, 157, 443, 513

Medullary: cord, folds, groove, and
plate, 202, 559; sheath, 167;
substance of Infusoria, 264, 269,
273

Medusa, 304, 307-312

Megagamete, 279

Meganucleus, 265, 273

Megazooid, 251, 279,

Membranous labyrinth, 186, 449,
518

Mesencephalon, see Mid-brain

Mesenteron, see Archenteron

Mesentery, 22, 431, 498

Mesoderm, 202, 203, 210, 340, 369,
454, 559, 565; vertebral and

“ lateral plates of, 565

Mesodermal sepments, 565

Mesogloea, 293, 307 =

Mesonephros, 573, 574

Metabolism, 149, 236

Metacarpus, 50, 485

Metamere, Metameric
tion, 319, 346, 354

Metamorphosis, 11, 209, 280

Metanephros, 573, 576

Metapleural fold, 406

Metatarsus, 51, 488

Metazoa, 286

Microgamete, 279

Micrometer, 727

Micromillimetre (u) = zdg¢ of a
millimetre, or ¢g$45 Of an inch

Micronucleus, 265, 275

Micropyle, 400, 547, 554

Microtome, 738

segmenta-

INDEX | 601

Microzooid, 251, 279

Mid-brain, 202

Mid-gut, 357

Milk-glands, see Glands

Milk-teeth, 493

Milt, see Spermatic fluid

ss Mitosis; 545, 585 -

Molars, 492, 494 ;

Mollusca, 220, 396

MONADS: occurrence and general
characters, 256; movements, 256;
nutrition, 256 ; animals or plants?
2573 rapid multiplication, 282 ;
practical directions, 260 -

Moneecious, 302

'**Morphology, 217

Morula, see Polyplast
‘Mother of pearl, see Nacre
seas sections, directions for,

139
Mouth, 4, 16, 253, 265, 275, 290,
am 319, 356, 386, 408, 415,

‘Meath: -cavity, see: Buceal’ cavity

Movement, spontaneous, volun-
tary, and involuntary, 7, 11, 112,
172, 232, 312, 335, 346, 4173

“and see under various types

Mucous membrane, 17

Mule, see Hybrid

Miillerian duct, 450, 574

Multinucleate, 269, 369

Muscle-fibres: striped, 112, 355;
unstriped, ° I1t, 131, 133; and
see under various types

Muscle-processés, 293, 299, 304 -

Muscles: Frog, 18, 57,59, 63, 2053
Crayfish, 321, 355 3 Mussel, 385 ;

Rabbit, 489 5 ‘and’ ‘see Myo-
mere
-- Muscles :

of eye, 186, 449, 5175

of middle ear, 519 «

Muscles, papillary, 504

-- Muscular contraction, 160, 276

Muscular impressions on-shell, 384

Muscular layers of enteric “canal,
579,75 |

‘Muscular system, ‘see ends various
types ; development of, in Verte-
brates, 203, 565, 576 *.

; Nemerheiatares

XN

Muscularis mucosz, 133

MUssEL: general characters, 381,
397 ; mantle, shell, and foot 381,.
397-398 ; food-current, 382, 397 3
gills, 382, 386, 398, 399, 400;
muscles, 385, 798; enteric canal,
386, 399, gor; nephridia, 390,
goo; blood-system, 392, 3799;
nervous system, 392, goo ; sense-
organs, 394. gor; gonads,. 394,
go2; development and meta-
morphosis, 395, goo, 562

Mustelus, 415, 454

“Myocomma, 418

Myomere, 203, 355, 404, a 555
Myophan layer, 264

N

: N acre: nacreous layer, 384 |

Nasal organ, see Olfactory organ
Naso-lachrymal duct, 186, 477, 518
Naso-palatine canals, see Canals
Naso-pharynx, 495

Natural History, 2

Natural selection, 226

Nauplius, 370

Neck, 463

397; 457
Nematocyst, 295

.Nephridiopore, 332

Nephridium, 146, 331, 361, 368,
390, 404, 406, 408, 450, 574

Nephrostome, 145, 146,’ 3315 574

Nerve-cells, 167, 298

Nerve-collar, 333

Nerve- cord: ventral, 333, 365

.Nerve-fibre, 167

Nerve-foramina, 478 ; and see Skull :
and Vertebral column

_Nerve-ganglia, 162, 163, 164, oe

333, 365, 306, 392

Nerve-plexus :. brachial, 161, 515;
coeliac, 516; sciatic or lumbo-
sacral, 162, 515

’ Nerve-roots, 163, 409
Nerves:

afferent and efferent, 166,

1753 cerebral, 163, 445, 515,,
568 ; of lateral line, 447 ; motor -

602 INDEX’

and sensory, -162, 169, - 3343
sciati¢, 62, 162, 5153 -spinal,
160; 409, 445, 515, 5085. sym-

.. pathetic, 162, 445, 516; vaso-
motor, 174; visceral, 366

Nervous impulse, 62, 154

Nervous system: Frog, 155; Hydra,
298; Bougatnoiliea, 311 ; Earth-
worm, 333 3 Crayfish, 365; Mus-
sel, 392; Lancelet, 404, 408;
Dogfish, 4423 Rabbit, 511; de-
velopment of, in Vertebrates, 565,
587

Neural arch, spine, and canal, see
Vertebra

‘Neural plate and process, 424

Neuraxis, 167

Neurenteric canal, see Canal

Neurilemma, 167

Neuroceele, 443; and see Canal,
central of spinal cord, and Ven-
tricles of brain

Neuroglia, 169

Newt, 218

Nictitating membrane, 5, 469

Nodes of nerve-fibre, 167

Nose, see Olfactory organ

Nostrils, 5, 17, 180, 416, 448, 468,

470, 517

Notochord, 203, 404, 425, 560,
576, 587.

Nuclear division, 544,’ 585 5 in-
direct, 545

Nuclear membrane, 542; spindle,

544 oat

Nucleolus, 109 ef seg., 541, 542

Nucleus, 106 -¢ seg., 129, 231, 243,
254, 265, 269, 273

Nucleus, conjugation-, 268; seg-
mentation-, 554

Nutrition, 673 3 and see andet various

types

Ohi, 317

Occipital condyle, see Stull
Ocellus, see Eye-spot-
Oesophagus, see Gullet

oO

Odontoblasts, 429
Odontoid process, 482
Olecranon, 49, 485
Olfactory capsule, 39, 42, 420, 476
” lobe, 160, 444, 511, 514,
568

» organs: Frog, 180; Cray-
Jish, 367; Mussel, 3043 Dogfish,
416, 448; Rabsit, 511, 517; de-
velopment of, in Vertebrates, 569

Olfactory pit, 408

Oligocheta, 341

Ommatideum, 366

Ontogeny, 224.

Oogenesis, 196, 548

Oosperm, 198; and
various types

Oosperm, holoblastic and mero-
blastic, 556

OPALINA: occurrence and general
characters, 269; structure and
division of nuclei, 269; parasitic
Nutrition, 271; reproduction,
271; means of dispersal, 272;
development, 254; practical di-
rections, 287

Optic chiasma, see Chiasma; cup, ~
570 ; lobe, 157, 443, 513, 566;
thalamus, 1593 vesicle, 569

Oral cavity, see Buccal cavity

Oral hood, 406

Orbit, see Skull

Order, 2318

Organ, 30, 151, 238, 335

Organism, 231

Origin of muscles, 60
1 Of species, 225

Osmosis, see Diffusion

see under

-Osphradium, 394

Ossicles : of gizzard (Cray/ish). 358,
_ 376; auditory (Aabbit), see Audi-
~ tory ossicles

Ossification, 44

Ostia: of heart, 362 ;

Otocyst, 394

of gills, 389

_Otoliths, 188

Ovary : Frog, 23, 193, 1953; and see
Reproductive organs

Oviduct, 25, 196, 337, 368, 453,
523, 574

INDEX 603

Ovisac, 195, 523

Ovum, 195, 546, 5553
under various types

Ovum : alecithal, centrolecithal, and
telolecithal, 556

Oxidation of protoplasm, 149, 248 '

and’ see

;

P

Pies, 224

Palate, 490 .

Pallial line, 384

Pallium, see Mantle

Palp: Crayfish, 353 ; Mussel, 386
Pancreas, 22, 70, 134, 204, 431,

499. lt.
Pancreatic juice, 70
Papille of tongue, 180, 492
Parachordal, 577
Paragnatha, 356
Paramylum, 251
PARAMECIUM: structure, 262;
mode of feeding, 266; reproduc-
tion, 268; conjugation, 268 ;
practical directions, 286
Parasite, parasitism, 33, 153, 269,
27
Parietal fer of. peritoneum, 26,
322; see also Mesoderm
Parthenogenesis, 552
_ Patella, 487
* Pectoral arch, 20, 36, 46, 426, 483
»» fin or limb, see Appendages
Pedal ganglion, 392
Pelecypoda, 396 |
Pelvic arch, 20, 36, 50, 426, 486
3, fin or limb, see Appendages
» Pelvis of kidney, 521
Penis, 470, 523
Pepsin, peptone, 74
Pericardial gland, 390
. Pericardial sinus, 362
Pericardium, 20, 386, 418, 490,
503
Perichondrium, 55
Perilymph, 189
Perinzeum, 469

Periosteum, 55

Periostracum, 384

Peristaltic movements, 75

Peristome, 275

Peristomium, 319

Peritoneum, 22, 26, 322, 418, 490

Pes, 5, 51, 468, 488

Peyer’s patches, 498

Phalanges, 50, 51, 485, 488

Pharynx, 17, 327, 405; 408, 429, NS
495 :

Phylogeny, 223, 224

Phylum, 219

Physiology, 1, 217

. Pia mater, 155

Pigment-cells, 128

‘Pigment-layer of retina, 185

Pigment-spot, 254

Pineal body, 159, 443 513, 566 \

Pineal eye, 159, 567

Pinna, 467, 469, 519

Pisces, 219, 403; general caseaciers :
of, 414

Pithing, directions for, 703

Pituitary body, 159, 443s 513, 566 .

‘Placenta, 467, 524, 583

Planula, 313.

Plasma, 104

Plastic products, 249, 551

Platyhelminthes, 397

Pleopod, 350

Pleura, pleural mon EHC 490

Pleurobranch, 361

Pleuron, Crayfish, 347

Podobranch, 361

Podomere, 349

Polar ae 552, 585

Pollex, 48 5,

Polycheta, 341

Polymorphic, polymorphism, 310

Polyplast, 200, 558 ; and see. under i

' various types ;

Polystomum, 33

Pons Varolii, 513

Pores, dorsal, 325

Portal system: sepatic, 85, 440, 5103.
renal,.85, 438

Post-axial and pre-axial borders of
limb, 484, 486

e

6g 8

Poupart’s ligament, 489
Premolars, 492, 494
Prepuce, 470
Preservative fluids, 737
Primitive groove, 563
Prismatic layer, 384
Proamnion, 579
Processes of skull, 472 ef seg.
-Proccelous, 36
Proctodzeum, 204, 358, 431
Pronephric duct, 573
Pronephros, 573
Pronucleus, male and female, 554
Prosencephalon, 444, 566
Prostate, see Gland
Prostomium, 319
Proteids, 72
Protista, 257
_Protococcus, see Hzmatococcus
Protoplasm, 106, 541; and see
Cell
Protopodite, 350.
Protovertebra, 565
Protozoa, 220, 286
Proximal, 6
Pseudobranch, 433
Pseudopod, 106, 231, 250, 298
Beyecrhor 425-427
Pubis, 50, 486
Pulmonary artery and vein; see
_Arteries and Veins
Pupil, 5, 182
Putrefaction, 256, 257, 261, and see
Decomposition
Pyloric division and valve, of
stomach, see Stomach
Pyrenoid, 243

co

R

Risse general characters, 467,
525 3 skeleton, 470, 525 ; muscles
and body wall, "488, 520, 527, 532,
535s 539 3 coelome, 489, 528, 5.32,
digestive organs; 490, 528-530 ;
‘respiratory,and vocal organs, 489,
500, 533 534, 5373 circulatory

INDEX

organs, 503, 527-5353 nervous
system, 511, 527, 532534: 5373
sense - organs, 516, 536-5983 —
urinogenital organs, 520, 528-
532; development, 524, 564, 578,
583, 587

Racemose, 135

Radial canals, see Canals

Radial symmetry, 291 :

Radiolaria, symbiotic relations with
Zooxanthella, 299

Radio-ulna, 49.

Radius, 50, 484

Rana esculenta, 216

RANA TEMPORARIA, see Frog

Ranide, 218 :

Reagents, hardening, preserving, . .
mounting, and staining, 7.5

Recapitulation, theory of, 224

Rectal gland, 431

Rectum, 23, 386, 431, 497

Reducing division, 551, 554

Reflex action, 169, 312

Regeneration, 336

Renal portal system,
system,

Reproduction, 8, and see under.
various types, and also Asexual:
reproduction =

Reproductive organs, 23, 25, 193, |
302, 312, 336, 368, 394, 409, 451,
521, 573

Reptilia, 219, 403 : \

Respiration, 141, 144, 235, 330

Respiratory movements, 7,
593s

Retina, 183. 184, 570

Retinula, 366

Rhabdome, 366

see Portal

142,

-Rhinencephalon, 568

Rhizopoda, 286

Ribs, 424, 468, 483

Rocks, sedimentary and stratified,”
223

Rodentia, 525

Rods and cones, 185

Rostrum, 349, 420

Rudiment, often used for Vestige
(7.2)

INDEX ; 605

Decals, 187 <
acculus rotundus, 497, 498, 529
‘Sacrum, 482
Salamander, 218
Salivary glands, see Glands
Saprophytic nutrition, 256, 257
Sarcofemma, I12
' Scales, 414, "418
Scapula, 46, 427, 483
Schizoccele, 565, 587
Sclerite, 3776 -
Sclerotic, 182, 517, 571
Scrotal sac, 470, 521
_ SCYLLIUM, see Dogfish
me cutting, directions for, 126,
13
Secretion, 130
Segment, see Metamere, Podomere
egmentation-cavity, 200, 558, 562
Segmentation of cosperm, 198, 200,
212, 303, 313, 340, 369, 585"
Segmentation, equal and unequal,
550; discoid, 556
33 superficial, 369, 556
45 metameric, see Me-
tamiere _
-nucleus, 554
Selection, natural and artificial, 226,
227 :
Self-fertilization, 339
Seminal funnel, 338
: 5 vesicle, 194, 338, 451
Sense-organs and cells, 179, 308,
312, 335, 414, 416, 448, 516
Septd, of Earthworm, 325

Septum: lucidum, 514; nasal, 42,
476

Serous membrane of embryo, 579,
583

Sesamoid bones, 485; 487, 488
% Seta, 320, 348, 367
‘Sex-cells, primitive, 549
‘Sexual characters, external, 7, 368,
417, 469
‘ Sexual, generation, see Gamobium
Sexual organs, see Reproductive
organs

Sexual reproduction, see under
various types’

Shank, 5

Shell, 381 ; ; larval, 305,

Skell of egg, 4

Shel]-gland of Mussel-embryo, 3953
of Dogfish, 453

Shoulder-girdle, see Pectoral arch':

Sinus : blood, 360, 362, 364, 438,
4413 lymph, 18, 27; urinary and
urinogenital. 451 ; venosus, 80, 89

Siphon, inhalant and exhalant, 382.

Skeleton, see Endo- and Exo- ©
skeleton

Skin, see Integument

Skull; Frog, 16, 35, 393; Dogyish,
420; Rabbit, 4705 development
of, 576

Smell, organ of, see Olfactory organ

Snout, 4

Somatic layer of mesoderm, 561

Somatopleure, 561

* Spawn, 9

Species, 215 e¢ seg. ; ‘origin of, 222,
225

Sperm, or spermatozooid, 194, 368,
549 ; and see under various types

Spermary, 25, 193, 1943 and see.
Reproductive organs

Spermatogenesis, 194, 548

Spermatophore, 368

Spermotheca, 338

Sperm-reservoir, 338

Sperm-sac, 338, 451

Spinal cord, 28, 155, 443, 515

Spiracle, 416, 433

Splanchnic layer of mesoderm, 561

Splanchnopleure, ‘562 °

Spleen, 23, 98, 431, 499

Spontaneous generation, see Abio-
genesis

Spores, 254, 279

Stalk of Vorticella, 273, 276

Stapes, 46, 189, 473, 519

Starch, 72, 243

Sterilised infusions, 283.

Sternebrze, 483

Sternum, 16, 48, 347, 483

Stimulus, -various kinds of, 62

606

Stock, see Colony

‘Stomach, 22, 70, 357, 386, 429, 495

Stomodzeum, 204, 358, 429, 431

Struggle for existence, 225

Substitution of organs, ‘209

Sucker,, 203

Supporting lamella, see Mesogloea

Suprarenals, see Adrenals

Suspensorum, 40, 422

Sutures, 50, 470

Swimmeret, see Pleopod

Symbiosis, 299

Sympathetic, see Nerves

Symphysis, 486

Syn-cerebrum, 366

Synovial capsule, 56

Systemic arch, ¥0, 436, 506

Systole : of heart, go ; of contractile
vacuole, 266 :

a

cee organs, 179, 368, 394, 516
Tadpole, 9, 203 et seg.

Tail, 9, 209. 406, 416, 468
Tapetem, 539

Tarsus, 51, 487

Taste-organs, 180, 492, 517
Teasing, directions for, 72?

Teats, 468, 489

Teeth, 17, 358, 428, 468, 492
Teleostomi, 415
Telson, 347

Tendon, 59

Tentacles, 290, 291, 304

Tergum, 347
Testis, see Spermary
Thalamencephalon,

- phaton
Thigh, 5

Thoracic duct, 511
Thorax, 347, 467, 489
Thread-cell, see Nematocyst
Thymus, 431, 490, 503
Thyroid, 431, 503

Thyroid ene 500
Tibia, 51, 487

see Dience-

INDEX

Tibio-fibula, 51

Toad, see Bufo

Toes, see Digits

Tissues, enumeration of, 31 ,

Tongue, 8, 17, 429, 492

Tongue-cartilage or bone, see Hyoid

Tonsil, 490

Trachea, 489, 500

Trabeculze cranii, 577

Transverse process, see Vertebra

Trichocyst, 267

Triploblastic 306, 314, 324

Trochanter, 487 -

Trochosphere, 340

Trophoblast, 565

Trunk, 4, 416, 468

Trypsin, 74

Tubercle, and Tubercular facet, 481,
483

Tunicata, 406

Turbinals, 476, 517

Tympanic cavity, membrane, and
ring, 5, 45, 189, 433, 475, 519

Typhlosole, 327, 386

U

| Lie, 50, 485

Umbilical cord, 584
: Umbilicus, 584
Umbo, 383
Umbrella, 307
Unicellular, 231
UNIO, see Mussel
Uraclkius, 584
Urea, 66, 147
Ureter, 26 ; and see under various
types
Urethra, see Urinogenital canal
Uric acid, 66 ~~
Urinary bladder, 23, 361, 390, 521
», tubules, see Nephridium
Urine, 8, 66, 147
Urinogenital aperture, 469

i canal, 522, 523
35 duct, 194
es organs, 193, 450, 520

INDEX

Urinogenital organs, development
of, 573

Urodeles, 219

Uropod, 350

: Urostyle, 35, 39

Uterine crypts, 583

Uterus, 523 -

»» | Masculinus, 522
Utriculus, 187

Vv

V acuole : contractile, 232, 243,
ae 265, 2733 food-, 233; 266,

Tastoas 523
Valve : of Vieussens, 512, 5983 spiral
429, 498 ; ileo-colic, 498 *
Valves: of heart, 88, 433, 504, 506;
of shell, 381 ; ‘of veins, 89
Variability, 225
Variation, individual, 216, 225,
Variety, 225:

Vascular system, see Blood-vessels,
Arteries, Veins.
Vas deferens, see

Wolffian duct
Vasa efferentia, see efferent ducts
Veins : Frog, 19, 82; Crayfish, 363;

Spermiduct,

Mussel, 3923 Dogfish, 438 ;
Rabbit, 504, 509 ; embryo Verte.
brate, 572

Veliger, 396

Velum : of Medusa, 309 ; of Lance-
let, 408

Velum palati, 490

Vena cava, see Veins

Vent, see Anus

Ventral, 6

Ventricle, see Heart

Ventricles of brain, 157, 159, 408,
443, 512+514, 568

Vermiform appendix, 498

Vertebrata, 219 ; general charactets

of, 403
Vertebra and vertebral column, 16,

, 35» 36, 423, 480, 576

607

Vertebrarterial canal, see Canal

Vessels; see Blood-vessels

Vestibule, see Urinogenital canal

Vestige, vestigial, 1 59> 361

Vibrissze, 469

Villi: of intestine, 498 ; of chorion,
583

Viscera, abdominal, 20, 429, 495

Visceral arches and clefts, 419,
422, 571, and see Branchial aper-
tures

Visceral ganglion, 393

Visceral layer of peritoneum, 27,
323

Visceral mass, 385

Vitelline membrane, 196, 547,579

Vitreous body of compound eyes

66

‘|, chamber and humour,’ 183,

Vocal cords, 144, 500
3» sacs, 218

VoRTICELLA: occurrence and
general characters, 273; struc-
ture, 2733 reproduction, 2773
conjugation, 278 ; means of dis-
persal, 278 ; encystation, 2793
spore- formation, 2793 . meta-
morphosis, 230 ; practical direc-
tions, 287

Vulva, 470, 523

Ww

WV aste-products, 8, 66, 249

White matter of brain and spina.

cord, 156, 160
Wolffian body, see Epididymis

an duct, 450, 574
Work and waste, 66, 148,.234
Worms, 340
Wrist, see Carpus
}

X.

‘ N

x iphisternum, 48, 483

608 INDEX

Y ay

¥ attow cells: of Radiolaria, L vsit 281; and see eon
299 ; of Earthworm, 323, 327, 332 and Microzooid ° ee

Yolk, yolk-granules or spheres, 195, Zoology, 1 %
546 ; and see under various Zoophytes, see Hydroid polypes e

types Zooxanthella, 299
_ Yolk-cells, 200, 202 Zygapophysis, 36, 481...
Yolk-plug, 201 Zygoma’: zygomatic arch, 477, 478.
Yolk-sac, 454 : Zygote, 197, 279
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M.A., Christ’s College; Rotifers, &c., Marcus Harroc, M.A., Trinity
College, D.Sc. (Lond.), (Professor of Natural History in the Queen’s College,
Cork); Polyehaet Worms, W. B. Benuam, D.Sc. (Lond.), Hon. M.A.
(Oxon.), Aldrichian Demonstrator of Comparative Anatomy in the University
of Oxford; Earthworms and Leeches, F. E. Benvarv, M.A. (Oxon.),
F.R.S. (Prosector to the Zoological Society); Gephyrea, A. E. Suiptey,
M.A., Christ’s College ; Polyzoa, S. F. Harmer, M.A, King’s College.

(Ready.
VOLUME III.

Molluses, A. H. Cooke, M.A., King’s College; Brachiopods (Recent), A. E.
Surptey, M.A., Christ’s College ; Brachiopods (Fossil), F. R. C. Rrep,
M.A., Trinity College. [Ready.

VOLUME Iv.

Spiders, Mites, &c., C. Warsurton, M.A., Christ’s College (Zoologist to the-
Royal Agricultural Society) ; Scorpions, Trilobites, &c., M. Lauriz, B A.,
King’s College, D.Sc, (Edinb.), (Professor of Zoology in St. Mungo’s College,
Glasgow) ; Pyenogonids, &c., D’Arcy W. Tuompson, C.B., M.A., Trinity
College (Professor of Zoology in University College, Dundee); Crustacea,
W. F. R. Wetvon, M.A., F.R.S., St. John’s College (Jodrell Professor of
Zoology in University College, London).

VOLUME V.

Peripatus, A. Sepcwick, M.A., F.R.S., Trinity College; Centipedes, &c.,
F. G. Sincvatir, M.A., Trinity College; Inseets, Part 1, D. SHarp, M.A.,
F.R.S. (Ready.

VOLUME VI.

Inseets, Part II., Hymenoptera continued (Tubulifera and Aculeata), Coleoptera,
Strepsiptera, Lepidoptera, Diptera, Aphaniptera, Thysanoptera, Hemiptera,
Anoplura. Davip SHarp, M.A.Cantab., M.B.Edin., F.R.S.

Belanoglossus, &0., S. F. Hanuen, Se-D., FR.S.. King’

elanoglossus, &c., S. F. Harmer, Sc.D., F.R.S.. King’s College ; Ascidians
and Amphioxus, W. A. Herpman, D.Sc. (Lond.), ERS. (Professor of
‘Natural History in University College, Liverpool); Fishes, T. W. Brivce,
Sc.D., Trinity College (Professor of Zoology in the Mason University College,

Birmingham).
VOLUME VIII. f
Amphibia and Reptiles, H. Gapow, M.A., F.R.S., King’s College.

VOLUME IX. e
Birds, A. H. Evans, M.A., Clare College. With numerous Illustrations by G. E.
Lopce. [Ready.
VOLUME X

Mammals, F. E. Bepparv, M.A. Oxon., F.RS. (Prosector to the Zoological:
Society).

MACMILLAN AND COG., Lrp., LONDON.

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