PRACTICAL ZOOLOGY

By F. W. GAMBLE, D.Sc., F.R.S.

UNIVERSITY OF BlRMIHGHAM.

ANIMAL LIFE.

A work written in the first instance for
those who wish to learn or teach such a
survey of the animal pageant as can ally
itself with observation and experiment ; and
in the second place for those who wish to
organise their knowledge of animal life.

Illustrated.

LONDON: JOHN MURRAY.

A JUNIOR COURSE OF

PRACTICAL ZOOLOGY

BY THE LATE

A. MILNES MARSHALL, M.D.,D.Sc.,M.A.,F.R.S.

PROFESSOR IN THE VICTORIA UNIVERSITY ; BEYER PROFESSOR OF ZOOLOGY IN OWENS
COLLEGE, MANCHESTER J FORMERLY FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE

AND THE LATE

C. HERBERT HURST, Ph.D.

LECTURER IN THE VICTORIA UNIVERSITY J DEMONSTRATOR AND ASSISTANT
LECTURER IN ZOOLOGY IN OWENS COLLEGE, MANCHESTER

NINTH EDITION
REVISED BY F. W. GAMBLE, D.Sc, F.RS.

MASON PROFESSOR OF ZOOLOGY AND COMPARATIVE ANATOMY IN THE
UNIVERSITY OF BIRMINGHAM

LONDON

JOHN MURRAY, ALBEMARLE STREET, W,

1920

>/,

First Edition (Smith, Elder & Co.) January 1887

Second Edition .... May 1888

Thhid Edition .... March 1892

Fourth Edition .... May 1895

Fifth Edition .... January 1899

Sixth Edition .... September 1905

Seventh Edition .... January 1912

Reprinted . . . . April 1916

Eighth Edition (John Murray) . May 1918

Ninth Edition .... February 1920

[All rights reserved]

STUDENTS OF OWENS COLLEGE

THIS BOOK IS DEDICATED
BY THEIR SINCERE WELL WISHERS

THE AUTHORS

PREFACE TO THE NINTH EDITION.

A few alterations have been made in this revised edition. The
account of the Tapeworm and the figures illustrating it are
new and are based upon the commonest form occurring in the
Dog. The nomenclature of the Vertebrate Kidney, always
a very difficult subject, has been revised, though I have not
used the term ' opisthonephros,' suggested by Professor
Graham Kerr for the kidney of fishes, as the use of that term
implies a theoretical assumption that should be avoided when
one is dealing with students.

I wish to acknowledge the kind assistance of Mr. F. J.
Meggitt, M.Sc, and of Mr. Harold Jackson, M.Sc.

F. W. Gamble.
February, 1920.

PREFACE TO THE FIRST EDITION

This book has been written in the hope that it may meet the
wants of those who desire to obtain a practical acquaintance
with the elements of animal morphology, and who find the
existing manuals insufficient for their purpose.

The animals selected for description are those which are
generally accepted as suitable types for a junior laboratory
course. They are of convenient size, and can all be obtained
readily and at small cost ; they include characteristic repre-
sentatives of the more important of the great groups of
animals ; and they give opportunity for very varied methods
of examination. A student who works conscientiously
through the book will thus acquire a good insight into the
leading facts of animal structure and a technical knowledge
of the principal methods of research.

In the mode of treatment adopted the practical character
of the book has been made paramount. Directions for dissec-
tion have throughout been printed in italics, and a system
of indentation has been adopted to render the subdivisions,
more distinct. In almost all cases the descriptions are so
arranged that the whole dissection can be performed on a
single specimen : if more than one can be obtained, the order
in which the several sections are taken may be varied ; but
in every case each of the main systems — vascular, nervous,
&c. — should be dissected in its entirety, for in this way alone
can a proper idea of its relations be obtained.

No attempt has been made to write exhaustive descrip-
tions of the several animals ; neither has strict uniformity
of treatment been specially aimed at. Portions of the subject
which experience has shown to present special difficulties
are treated at what may appear undue length ; while, on the

X PREFACE TO THE FIRST EDITION.

other hand, the limits of time ordinarily available for labora-
tory work have led to the almost entire omission of systems,
such as the muscular, which are of subordinate educational
value.

Although this is essentially and professedly a laboratory
text-book, yet morphological explanations have been freely
admitted ; and this from a conviction that a student best
grasps the meaning of anatomical facts if the explanation is
given him while the facts are actually before his eyes.

Illustrations have been introduced somewhat sparingly,
for it is of the utmost importance that they should not be
allowed to replace the drawings which a student must make
from his own dissections. The majority of the figures here
given are new, and have been drawn expressly for the book
by Mr. Hurst or myself. The drawings on the wood were
made by Mr. P. Hundley, and the blocks engraved by Mr. G.
Pearson : both these gentlemen have taken great pains to
render the figures at once faithful and artistic.

A. M. M.
Owens College,
December, 1886.

CONTENTS.

INTRODUCTION.

PAGE

Observation of animals during life — Drawing — Methods of killing
animals — Dissection — Injection — Microscopical examination
— Drawing with the camera lucida — Measuring of micro-
scopical objects — Use of hardening and staining reagents —
Section-cutting — Mounting microscopical objects — Prepara-
tion of skeletons ........ xix

CHAPTER I.
Peotozoa.

Amoeba — Paramecium — Opalina — Vorticella — Monocystis — Try-
panosoma 1

CHAPTER H.
Hydea, Obelia.

Examination of a living specimen — Examination of prepared

specimens 18

CHAPTER TIL

The Livee-fltjke op the Sheep. Fasciola (Disfomum)
Jiepatica.

The mature liver-fluke — Life-history of the liver-fluke . . 34

XI 1 CONTENTS.

CHAPTER IV.

PAOB

The Tapeworm . . . . . . .45

CHAPTER V.

The Leech. Hirudo medicinalia.

External characters — Dissection — Examination of transverse

sections 49

CHAPTER VI.

The Earthworm. Lumbricus lerrestris.

External characters — Dissection of the earthworm — Examination

of transverse sections ....... 66

CHAPTER VII.

The Freshwater Mussel. Anodonta cygnea.

The shell — Dissection of the mussel — Examination of transverse

sections 89

CHAPTER VIII.

The Edible Snail. Helix pomatia.

External characters — Dissection of the snail . . . .117

CHAPTER IX.

The Crayfish. Astacus pallipes.

External characters — Dissection — The arteries as seen in an in-
jected specimen ....... 140

CHAPTER X.

The Cockroach. Periplaneta americana.
Habits — External characters — Dissection 169

CONTENTS. xiii

CHAPTER XI.
The Lancelet. Amphioxus lanceolatus.

PAGE

External characters — Anatomy — Dissection — Examination of

transverse sections ' . .187

CHAPTER XII.

The Dog-fish. Scyllium canicula.

External characters — The skeleton — The abdominal viscera — Dis-
section of the digestive system — Dissection of the respiratory
system — Dissection of the circulatory system — Dissection of
the renal and reproductive systems — Dissection of the nervous
system — Dissection of the sense organs . . . .218

CHAPTER XIII.

The Skeleton of the Rabbit. The Skull of the Dog.

The axial skeleton — The appendicular skeleton . . . .291

CHAPTER XIV.
Dissection of the Rabbit. Lepue cuniculus.

External characters — The abdominal viscera — Dissection of the
digestive system — The thoracic viscera — Dissection of the cir-
culatory system — Dissection of the renal and reproductive
systems — Dissection of the neck — Dissection of the sympa-
thetic nervous system — Dissection of the head — Dissection of
the brain ......... 335

CHAPTER XV

The Skeleton of the Fowl. Oallus domesticus.

The axial skeleton — The appendicular skeleton .... 401

xiv CONTENTS.

CHAPTER XVI.
Dissection of the Pigeon. Columba livia.

PAGE

External characters — Dissection of the pectoral muscles — Dissec-
tion of the air-sacs — Dissection of the digestive system — Dis-
section of the circulatory system — Dissection of the respiratory
system — Dissection of the renal and reproductive systems —
Examination of the brain — The sense organs . . .429

CHAPTER XVII.
The Development of the Chick

Practical instructions — General description — Segmentation of the
egg — Formation of the germinal layers — The primitive streak
— The embryo from the first day to the time of hatching —
Observations on the living egg 464

APPENDIX.

Reagents for killing, hardening, and preserving — Reagents used in
staining — Clearing reagents — Mounting media — Microtomist's
media — Cement for fixing down sections cut in paraffin —
Injection fluids 489

INDEX . 499

LIST OF ILLUSTRATIONS.

FIGUBE PAGE

1-8. Amoeba 2

9. Paramecium aurelia ....... 4

10. VorticeUce. . 9

11. Monocystis ......... 14

12. Hydra : Diagrammatic longitudinal section ... 19

13. „ Section of the body-wall, highly magnified . . 21

14. Obelia . 31

15. Fasciola hepatica : Diagrammatic figure of the digestive, ex-

cretory, and nervous systems .... 36

16. „ The reproductive system . . . . .38
17-21. „ A series of stages in the life-history ... 42

Fig. 17. The free-swimming larva
„ 18. A sporocyst, containing redise
„ 19. A young redia
„ 20. An adult redia
„ 21. A free cercaria
21a. Tapeworm 47

22. Hirudo medicinalis : The ventral surface .... 53

23. „ Diagrammatic figure of the vascular, renal, re-

productive, and nervous systems . . 53

24. „ Diagrammatic transverse section through the

middle of the body 64

25. Lumbricus terrcstris : Longitudinal vertical section through

the anterior part of the animal ... 71

26. „ Plan of the reproductive organs ... 78

27. „ Diagrammatio transverse section through the

middle of the body ..... 85

28. Anodonta cygnea : Dissection from the right side, to show the

♦ heart and gills 98

29. „ Dissection from the right side, to show the

digestive and renal systems . . . 104

30. „ Diagrammatio transverse section through the

middle of the ventricle . . . .114

31. Helix pomatiai) Dissection from the right side, to show the

mantle-cavity ...... 124

32. „ Dissection from the right side, to show the

digestive and reproductive systems . . 128

33. „ Section along the axis of the shell . . 138

34. Astacus pallipes : Transverse section through the abdomen . 142

35. „ The oral appendages 148

36. „ Diagrammatic transverse section through the

thorax 152

xvi LIST OF ILLUSTRATIONS.

FIGURE PAGE

37. Astacus pallipes : Dissection of a male from the right side . 156

38. Periplaneta americana : The head, front view . . .171

39. „ The head, from behind 171

40. „ Dissection of a male from the right side . 176

41. „ Dissection of the hinder part of the abdomen

of a female ...... 185

42. Amphioxus lanceolalus : A young specimen, seen from the

right side ...... 188

43. „ Transverse section through the anterior part of

the pharynx 209

44. , Transverse section through the hinder part of

the pharynx ...... 213

45. „ Transverse section passing through the atrial

pore ....... 215

46. „ Transverse section passing through the anus . 216

47. Scyllium canicula : The skull and visceral skeleton from the

right side 226

48. „ The skull and visceral skeleton from the ventral

surface .232

49. „ The pelvic girdle and finsfrom the ventral surface 237

50. „ The pectoral girdle and fins from the ventral

surface ....... 239

51. Longitudinal vertical section through the head

and anterior part of the body . . . 246

52. „ The venous system ..... 248

53. Scyllium catulus : The arterial system .... 254

54. Scyllium canicula : Transverse section passing through the

posterior dorsal fin . . . . . 259
55-56. „ Dissections of the renal and reproductive

systems of the female .... 265
57-58. „ Dissection of the rena and reproductive

systems of the male ..... 268

59. Scyllium catulus : Dissection of the brain and cranial nerves . 272

60. Lepu8 cuniculus : Selected vertebrae ..... 295

61. „ The skull from the right side . . .303
61a. Canis familiaris : Skull in longitudinal section . . 305
61b. ,, Ventral surface of the skull . . . 308

62. Lepus cuniculus : Skeleton of the left hand. . . . 328

63. „ Skeleton of the left foot . . . .328

64. „ Dissection of a male from the left side . . 340

65. „ Dissection of the neck ..... 374

66. „ Diagrammatic transverse section through the

head, showing the tympanio cavity and

auditory ossicles . . ... . 382

67. „ Dissection of the brain from the right side . 386

68. „ Ventral surface of the brain .... 389

69. „ Dissection of the brain from above . . 393

70. „ Transverse section of the brain, passing through

the third ventricle ..... 396

71. „ Median section of the brain .... 398

72. „ Transverse section of the brain, passing through

the cerebellum ...... 400

LIST OF ILLUSTRATIONS.

xvil

FIGURE

73. Gallus domesticus : The left half of the skeleton,

right side ....

74. The skull from the right side

75. Columba livia : Dissection from the right side

76. Development of the chick : The egg

77-78.

79-81.

82.

83.

84.

85.

86-87.

90.
91.
92.

PAGE

from the

. 402

. 410

. 440

. 467

. 469
469-71

94.

Segmentation of germinal disc

Sections of segmenting blastoderm

Blastoderm at 20th hour . 472

Section of blastoderm at 20th hour . . 472

Section of one-day chick embryo . . . 473

Blastoderm at 24th hour .... 475

Surface view and section of chick embryo of

36 hours 476

Section of embryo (43 hours) . . . 477

Section of embryo (24 hours) . . . 478

Section of embryo (48 hours) . . . 479

Section across head of two-day chick . . 483

Section across head of 60 hours . . . 484

Blastoderm at 36 hours .... 486

Three-day chick . . . . 487

INTRODUCTION

The following pages contain a brief summary of the methods
ordinarily employed in the dissection and microscopical
examination of animals. They are not intended to form an
exhaustive account of anatomical technology ; and in many
cases they are repetitions of the practical directions given in
other parts of the book.

I. ON THE OBSERVATION OF ANIMALS DURING LIFE.

All animals should be carefully observed alive before they
are dissected, for by this means alone is it possible to obtain
a clear idea of the uses and mode of action of many organs,
such as the tentacles of Hydra, the odontophore of a snail,
the scaphognathite of a crayfish, &c.

II. ON DRAWING.

Careful drawings must always be made, both of the living
animal and of all dissections and preparations. These draw-
ings should be made to scale, in a book kept for the purpose,
and on one side of the page only.

Correctness of outline is of more importance than minute
detail ; and the usefulness of the drawings is greatly increased
by the systematic use of diagrammatic colouring. A separate
colour should be employed for each system of organs, the
several parts of which may be indicated by gradations of tint.

As a rule, the more bulky organs, as the liver, should be
coloured with dull tints, and the brighter colours reserved
for the smaller and less conspicuous parts. Arteries are
usually coloured red and veins blue.^

XX INTRODUCTION

III. ON THE METHODS OF KILLING ANIMALS.

The method of killing is of considerable importance, and
careful attention should be paid to the directions given at the
commencement of each chapter.

Two methods of killing animals are in general use, and
the one to be chosen depends upon whether the specimen is
to be (1) dissected and examined temporarily, or (2) con-
verted into permanent microscopical preparations.

For the first purpose chloroform is useful. Three or four
minutes' immersion is sufficient for small animals, after
which the animal should be washed in water. For frogs and
rabbits at least fifteen minutes are needed. This answers well
for the leech and earthworm. For killing crayfish boiling
water may be employed, and snails die well expanded if
drowned in warm water.

For the second purpose a reagent is employed which not
only kills the tissues, but penetrates and ' fixes ' them, so
that they are but slightly affected by subsequent treatment.
There are three chief fixatives : osmic acid, corrosive subli-
mate, and Miiller's fluid, the uses of which are given in the
Appendix. Strong alcohol is the simplest and most gene-
rally applicable, but does not give the best results for histo-
logical work. For Protozoa, osmic acid or the vapour of
glacial acetic acid is useful ; for Hydra, Perenyi's fluid. For
larger animals a mixture of 90 parts concentrated corrosive
sublimate solution and 10 parts glacial acetic acid is a good
reagent. The specimen must be subsequently washed in
alcohol.

IV. ON DISSECTION.

The object of dissection is to separate the several organs
from each other, so far as is necessary to define their boun-
daries and display their relations to each other. It consists
mainly in the removal of the connective tissue which binds
the parts together and obscures their outlines.
The necessary instruments are the following : —
1. Two or three scalpels, of different sizes.

ON DISSECTION. xxi

2. Two pairs of forceps — one large and one small. Both
pairs should be straight, and should have roughened tips to
secure a firmer hold.

3. Two pairs of scissors — one pair large and strong for
general dissection, the other pair small for finer work. The
latter pair should have the blades either bent at an angle —
elbow scissors — or else curved. In selecting scissors, care
should be taken to see that they cut quite to the points of
the blades.

4. A pair of bone-forceps or very stout scissors, for
cutting bone and other hard substances.

5. A pair of stout needles, firmly mounted in handles.

6. A pair of the finest sewing-needles, mounted in wooden
handles. Only about a quarter of an inch of the needle
should project. They are used for teasing histological pre-
parations.

7. A seeker, i.e. a blunt needle mounted in a handle, and
bent at an obtuse angle half an inch from the end.

8. A pocket-lens, containing two or three lenses mounted
in a handle, and giving when combined a magnifying power
of at least six diameters.

9. A razor, and some means for keeping it sharp.

10. A blowpipe of metal or glass, with the end bent at an
angle.

11. A glass tube drawn out to a point at one end, and
fitted with an india-rubber cap at the other : for use in wash-
ing dissections, and in injection.

For the dissection of the larger animals, as the dog-fish
or rabbit, a deal board, about two feet long by a foot and a
half wide, may be used ; to this the animal should be fastened
by pins, or by steel awls with wooden handles.

Smaller animals, and special parts of larger ones, should
be dissected under water, which supports the parts and
greatly facilitates the dissection. For this purpose an ordinary
white pie-dish with sloping sides is well adapted, the bottom
being fitted with a soft deal board weighted with lead, or a
sheet of cork cemented in with marine glue. A similar but
smaller dish may be used for dissecting under spirit.

xxii INTRODUCTION.

Animals, such as the cockroach, which are difficult to fix
with pins, may be cemented down with melted wax, or they
may be half imbedded in a plate of paraffin, and the plate
then fixed down with pins in the dissecting-dish.

For fine work a dissecting microscope affords great
assistance. The pocket-lens may be turned into one by
fitting one end of a wine-cork into the handle or case of the
lens, and passing a stout pin transversely through the other
end. The pin should be stuck upright into the dissecting-
board, with the lens over the part to be dissected : focussing
is effected by sliding the cork up and down the pin. A lens-
holder is, however, more convenient.

The following rules for dissection should be carefully
observed : —

1. Before commencing a dissection, fix the animal down
firmly to the dissecting-board or dish.

2. In fixing an animal with pins, stick them in obliquely,
so that their heads do not get in the way or obscure the
dissection.

3. Dissect under water, unless the animal is too large.
Change the water as soon as it gets dirty. A gentle stream
of water allowed to play upon the dissection is often a valu-
able aid.

4. Never cut away anything until you are quite certain
what it is you are removing.

5. Put the part you are dissecting slightly on the stretch.
This applies more particularly to blood-vessels, nerves, ducts,
and muscles.

6. In cleaning blood-vessels, nerves, &c, dissect along
them, and not across them ; and avoid laying hold of them
with the forceps.

7 The dissection is in many cases greatly facilitated by
placing the specimen in spirit for a day or so before dis-
secting it. In some cases the dissection may with advantage
be performed under spirit.

8. Always keep your instruments clean and sharp. Be
careful not to blunt your fine scissors or scalpels by using
them for cutting hard parts.

ON INJECTION. xxii:

9. If you get in a muddle, stop, and wash the dissection
thoroughly Under the tap before proceeding further.

Successive slices cut from an animal, or part of an animal,
with a razor are often exceedingly instructive. This is especially
the case with the mussel and snail, and with the brains of
the rabbit and pigeon. The specimens must be previously
hardened with spirit or other reagent, and the slices should
be examined in water or spirit.

V. ON INJECTION.

The injection of coloured fluids into the blood-vessels or
ducts of an animal renders them much easier to see, and to
follow to their distribution.

The colouring matter used must not be soluble in, or
affected by, any of the fluids in which the specimen is after-
wards to be dissected, hardened, or preserved. The most
convenient are French blue, Prussian blue, chrome yellow,
vermilion, and carmine.

In the case of the larger animals, starch or freshly prepared
plaster of Paris forms, if coloured, a convenient substance for
injection : it solidifies in the vessels, and so does not escape
if a vessel is cut during the dissection.

For smaller animals, thick gum-water or white of egg
may be injected cold, or a jelly made of gelatin and water
injected warm : the animal should afterwards be put into
alcohol to harden the injection. If the animal is not to be
dissected after injection, water coloured with any of the above
pigments may be used with advantage, and this is particu-
larly useful for the alimentary and excretory systems of the
liver-fluke.

For injecting small animals, a suitable syringe consists of
a glass tube with an india-rubber cap fitted on one end, the
other end being drawn out to a point sufficiently fine to
enter the vessel to be injected. After the tube is drawn out
in the flame and cut off, its sharp edges must be slightly
rounded off by holding for a moment in the flame. Several
such cannulse of various sizes should be kept ready.

xxiv INTRODUCTION.

For injecting larger animals, such as a rabbit, with, plaster
of Paris, the blood must first be washed out of the vessels
by injecting warm salt-solution. The following apparatus is
necessary, and must all be placed on the table ready for use
before the animal is killed ; for the blood coagulates after
death so rapidly as to allow no time to look for instru-
ments.

1. Cannulse : glass ones are the most convenient ; they
should be of various sizes, to fit the several vessels which it is
proposed to inject, and each should have a slight constriction
near the tip, so that it may be firmly secured in the vessel by
a ligature. The largest cannula that will enter the vessel
should be used.

2. A glass syringe, capable of holding about an ounce of
fluid. A larger syringe is liable to get blocked up.

3. Several pieces of strong india-rubber tubing, about
an inch long, to connect the nozzle of the syringe with the
cannula, both of which they must fit rather tightly.

4. Two or three glass plugs to fit the india-rubber tubing.

5. A dish of water or salt-solution, in which all the
cannula and india-rubber connections are laid, to displace
the air from them.

6. Fine plaster of Paris.

7. A mortar and pestle.

8. Colouring matter, such as vermilion previously rubbed
up with water : it must be well shaken before use.

9. A piece of muslin to strain the injection through. It
must be well wetted before use.

10. A jar into which to strain the injection.

11. A jar containing about half a pint of salt-solution
(75 per cent.), at a temperature of 38° to 40° C.

12. Dissecting-board, pins, scalpels, scissors, two pairs of
forceps, seeker, thin string or thread, and two or three pairs
of ■ bull-dogs.' These last are very short spring-forceps, the
spring being so arranged as to close the forceps. They are
convenient to stop the escape of blood or injection from any
vessels that may have been cut.

The injection should be performed close to a large sink,

I

ON MICROSCOPICAL EXAMINATION. XXV

over which is a water-tap with a foot or two of wide india-
rubber tubing attached.

When everything is ready, kill the animal, and as soon as
it is dead, lay it open, cutting as few blood-vessels as possible.
Expose the root of the aorta or other vessel from which it is
intended to inject the animal ; choose a cannula of the right
size ; fit it with an india-rubber connection ; fill it with salt-
solution, and stop the end of the connection with a glass plug.
Pass a ligature round the vessel ; make a longitudinal slit in
the vessel ; insert the cannula ; tighten the ligature upon it,
and tie it with a bow. If the ligature be too tight it will
cut the vessel. Fill the syringe with the warm salt-solution ;
remove the plug from the cannula ; press the body of the
animal slightly, to remove some of the blood from its vessels,
and to get rid of any clot that may have formed close to the
cannula. Inject the salt-solution, to force the remaining
blood from the vessels before it can coagulate ; and wash or
sponge the blood away. If the arteries are being injected,
the vena cava and portal veins should be cut open to allow
free escape of the blood, and vice versa.

Mix the plaster of Paris in the mortar, stirring in the
colour, and making the plaster thin. Strain it rapidly through
the muslin, and inject immediately with the syringe. When
the vessels appear to be well injected, remove the syringe
and insert the glass plug, and wash the animal to get rid of
blood and any injection that may have escaped.

Allow the animal to remain two or three hours in cold
water before dissecting it or putting it into spirit.

VI. ON MICROSCOPICAL EXAMINATION.

The microscope affords the means of investigating the
structure of minute animals, and the finer details of those
of larger size. The microscopical examination of the special
organs of the larger animals is of great importance, and must
on no account be neglected.

The microscope consists of a body and a stand. The body

xxvi INTRODUCTION.

is a tube of metal carrying the lenses, which are the essential
part of the instrument.

The stand supports the body in such a way that the latter
may be moved up and down without any lateral movement.
It also supports a stage, which ought to be horizontal, to
hold the object to be examined. The stage has a round
hole in its centre, through which light may be transmitted
through the object by a mirror fixed below the stage. This
aperture is usually much larger than is necessary ; but the
excess of light, and especially the light far from the optical axis
of the lenses, is cut off either by means of diaphragms, of which
there is a series, of various sizes, fixed in such a way as to
allow of their being easily changed, or by an ' iris-diaphragm.'

The combination of lenses at the upper end of the body
is the eye-piece ; the combination at the lower end is the
objective.

One or two eye-pieces, and two objectives of different
magnifying powers, \ inch or low power, and £ or -| inch or
high power, are required.

An object can be seen clearly through a microscope only
when it is at a certain definite distance from the objective ;
and this distance varies with different objectives and eye-
pieces, and to a slight extent with different observers. An
objective of high power requires to be nearer to the object
than does one of lower power. The regulation of this distance
is called focussing, and is effected in two ways.

The coarse adjustment of focus is made by simply sliding
the body up or down, with a slight twisting movement,
through the tube of the stand in which it is supported, or
else by a rack and pinion worked by a milled head.

The jine adjustment is effected by means of a screw, the
position of which differs in different microscopes, but which
should be so placed that it can readily be worked with either
hand. By turning the head of the screw from left to right, in
the direction of the hands of a watch, the body of the micro-
scope is lowered, and the objective brought nearer to the
object ; by turning in the reverse direction, the objective is
raised.

ON MICROSCOPICAL EXAMINATION. xxvii

In using the microscope the following rules are to be
carefully observed : —

1. Examine every object with the low power first. Having
adjusted the eye-piece and the objective, turn the mirror so
as to reflect the light up the body of the microscope ; place
the object on the stage under the objective, and carefully
lower the body with a screwing motion till the objective is
about a quarter of an inch from the cover-glass ; then look
through the eye-piece, and gradually raise the body till the
object becomes distinctly visible. Focus accurately with the
fine adjustment screw. With the high power, begin with the
objective close to the cover-glass, and then proceed as before.

2. Never examine an object with the high power unless
it is protected by a cover-glass. Take extreme care never to
let the objective touch the cover-glass, and never to touch
the face of the objective or allow any dirt to get on it. The
face of an objective cannot be cleaned without doing harm
to it.

3. Keep both eyes open when looking through the micro-
scope. Also get into the habit of using either eye.

4. When examining an object, keep one hand on the fine
adjustment, and keep screwing it up and down slightly. In
this way parts of the object at different depths are brought
into focus, and a much clearer idea of the object is obtained.

5. With a high power, use a small diaphragm : the amount
of light will be somewhat diminished, but the clearness and
definition much increased.

6. See that the body of the microscope slides smoothly in
its tube. If it does not, remove it, and clean it by rubbing
with a few drops of olive oil ; wipe off the oil before replacing
in the tube. Clean the inside of the tube in the same way.

7. The object may appear indistinct from dirt in any of
the following places — i.e. on the eye-piece, the objective, or
the cover-glass. If it be on the cover-glass, the dimness
varies when the slide is moved ; if on the eye-piece, it varies
when this is rotated ; if not on either of these, it must be on
or in the objective.

The eye-piece and the lower surface of the objective may

xxvm INTRODUCTION.

be cleaned with chamois-leather or silk. If the objective is
smeared with glycerin, wash it carefully, then dry with a
soft handkerchief. Canada balsam, which sometimes gets on
the objective, may be removed with a drop of benzol on a
handkerchief. It is, however, safer to leave this to an
optician or to the demonstrator, as a very small quantity of
benzol getting inside the rim which carries the lens may lead
to the separation of the two lower lenses from each other,
rendering the objective useless.

VII. ON DRAWING WITH THE CAMERA LUCID A.

In making a drawing of an object seen with a microscope,
it is often found difficult to draw all parts of it on the same
scale of magnification ; and as a result, such drawings are
liable to give a false idea of the form of an object, and of the
relative sizes of its parts.

This source of error may be obviated by the use of a
camera lucida, by means of which a sheet of paper laid on
the table at the side of the microscope is visible at the same
time as the object under the microscope. The magnified
image appears to lie upon the paper, and by tracing out its
lines with a pencil an accurate outline is easily obtained.
As, however, the use of the apparatus involves the loss of a
large proportion of the light from the object, only outlines
should be so drawn, and the details should be filled in after
removal of the camera from the microscope.

VIII. ON MEASURING MICROSCOPIC OBJECTS.

The eye-piece micrometer is a plate of glass, the upper
surface of which is either engraved with a scale, or divided
into a number of equal squares by cross-lines ruled upon it.
This is placed between the lenses of the eye-piece of the
microscope in such a position that the scale can be seen
distinctly on looking through the microscope.

The stage micrometer is a slide on which a scale is en-
graved, divided to, say, hundredths of a millimetre.

To measure a minute object, focus it under the micro-

ON THE USE OF REAGENTS. xxix

scope with, the eye-piece micrometer in position ; read off
the apparent size on the scale of the micrometer ; replace
the slide by the stage micrometer and read off upon that the
actual value of the noted number of divisions of the eye-
piece micrometer.

The value of each division on the eye-piece micrometer
will obviously vary when the objective is changed or the
length of the microscope tube is altered.

By means of a camera lucida a scale may be drawn upon
a sheet of paper laid at the side of the microscope, coincid-
ing with the apparent image of the stage micrometer seen
through the microscope, and this scale may then be used to
measure directly the size of an object seen at the same time
through the microscope. One such scale for each power of
the microscope may conveniently be drawn in the notebook
which is to receive the drawings, and the student will then
be able to determine the real size of any object drawn in the
book with the aid of the camera.

Eye-pieces and stages are also constructed in which the
measurement is accomplished by means of a fine screw with
a divided head. To use the micrometer stage, the slide
bearing the object is laid upon it, one edge of the image is
made to coincide with a mark upon the eye-piece micrometer
or a slip of glass in the same position. The screw of the
stage is then turned till the opposite edge of the image
coincides with the same mark, the distance through which
the slide has been moved is then read off on the divided head
of the screw, and this distance is, of course, the diameter of
the object.

IX. ON THE USE OF DISSOCIATING, HARDENING,
£ND STAINING REAGENTS.

Reagents are used for fixing, dissociating, hardening, and
staining specimens. Those required for general use are few
in number, and directions for their preparation will be found
in the Appendix.

1. Fixing has already been explained. To obtain good
results it is necessary to employ either small animals or

xxx INTRODUCTION.

small portions of large ones. Use a relatively large bulk of
the fixing fluid, and wash for several hours according to the
fixative employed.

2. Dissociating or macerating consists in dissolving the
intercellular substance that binds the tissue together. By
the aid of this method, followed by teasing, the separate cells
of a tissue may be isolated. Any fixative used in a diluted
form (about one-tenth) acts as a dissociator. Weak alcohol
(30 per cent.), osmic and acetic acids, and Muller's fluid
are the most useful. For Hydra, osmic-acetic acid is specially
useful.

3. Hardening is necessary in the case of soft animals or
of tissues to be cut into slices. Grades of alcohol 50 per
cent., 70 per cent., and 90 per cent, are successively em-
ployed after fixation and washing. In other cases (e.g. the
fresh-water mussel) a weak solution of chromic acid (J per
cent.) in water serves well. For the brain of the rabbit and
pigeon ordinary methylated spirit is convenient.

4. Staining not only makes the tissues more distinct, but
also affords insight into their minute structure.

Most stains will not penetrate a living organism or tissue.
Methylene blue, however, is an exception, and therefore of
great value. It is employed in very dilute solutions (see
Appendix) for the study of small transparent forms (e.g.
Paramecium, Hydra) and for the tracing out of nerves and
nerve-cells in higher animals. For rapidly staining dead
but fresh tissues and such as have been fixed and washed in
water, safranin and dahlia are useful, especially for Pro-
tozoa, developing spermatozoa of the earthworm, leech, and
crayfish. The stain must only be allowed to act for two or
three minutes, and then the surplus carefully removed by
distilled water.

Carmine is a generally useful stain, especially Grenacher's
borax-carmine. As this is an alcoholic stain, specimens
should be ^placed in it from 50 per cent, alcohol. After the
staining is completed, the object should be transferred to
acid-alcohol for a few minutes or hours according to its size.

Picrocarmine is an aqueous solution and directly applicable

ON CUTTING SECTIONS. xxxi

to objects washed after fixing. Treat subsequently with
50 per cent., 70 per cent., and acid-alcohol.

Hematoxylin (especially Meyer's acid-hsem-alum, see
Appendix) gives excellent results if used with due care, but
a trace of acid or turpentine will completely -destroy the
colour.

X. ON CUTTING SECTIONS.

Many points in anatomy and histology may be best made
out by examination of microscopical sections — that is, slices
cut so thin as to allow of their being examined under the
microscope by transmitted light.

The hardened object, supported, if necessary, between
two pieces of carrot, may be held between the fingers and
thumb, and sections sliced off it freehand with a sharp
razor.

The microtome or section-cutting machine, however, gives
better results. It consists essentially of three parts : a clamp
or other contrivance for holding the object to be cut, means
of guiding the razor, and a ' feeding ' arrangement. These
parts are so arranged that the razor can be drawn smoothly
across the object to be cut, taking off a section at each stroke,
the ' feeding ' arrangement bringing the object into the path
of the razor, and the amount of ' feed ' determining the thick-
ness of the section.

In the simplest form of microtome the razor is drawn by
hand over a smooth plate, the object to be cut being pushed
up through a hole in the centre of the plate by means of a
screw or other ' feeding ' arrangement placed below.

More complicated microtomes have been devised to obviate
the wearing of the edge of the razor by friction upon the
plate ; to render it possible to cut the object in any desired
direction ; to cut more evenly, more easily, and more rapidly ;
and to yield sections in a string or ribbon, so as to simplify
and expedite the mounting of large series of sections.

The preparation of an object for cutting in a microtome
depends to some extent upon the nature of the object and

xxxii INTRODUCTION

the number of sections required, as well as the purpose for
which they are intended. The olyect has in any case to be
supported by impregnating it with some homogeneous sub-
stance which is firm enough without being brittle or too
hard. The freezing method is the simplest of all, but it is
not easily applicable when a large number of sections are
required mounted in consecutive series, nor when very thin
sections are required. The tissue to be cut is first soaked in
gum-water, then placed on the freezing-plate of the micro-
tome and cooled by an ether spray or a freezing mixture, and
cut when just thawing.

The simplest and quickest imbedding method consists in
soaking fresh tissue in acetone, whence it may be transferred
direct to paraffin. More usually the specimen, which may
be stained previously, is dehydrated by soaking in absolute
alcohol ; the alcohol is then removed by soaking in turpentine,
xylol, or, better, oil of cedar wood, and the object is then
transferred to paraffin kept just melted in a water-oven, the
temperature not being allowed to rise above 60° C. Paraffin
melting at 55° or 56° C. is the best, though a softer paraffin
may be used in a cold room, if it be found that the hard
paraffin crumbles before the razor. When the object is
thoroughly permeated, it is transferred to a small trough of
paper with just enough paraffin to form on cooling a block
completely enclosing it. It must be cooled as quickly as
possible by placing the trough in a dish containing cold
water, as the paraffin is liable to crystallise if allowed to cool
slowly. A couple of L -shaped blocks of metal, resting on a
plate of glass or metal, so as to form a box of adjustable size,
may be used in the place of the paper trough. The block
containing the object is fixed in the microtome ; the most
efficient of all contrivances for this purpose is, perhaps, a
grooved metal plate which is simply warmed slightly and
pressed against the end of the block, and then cooled quickly
as soon as the block has begun to melt. The further pro-
cedure depends entirely upon the particular microtome used.
The sections are to be mounted as described in Section XI.

ON MOUNTING MICROSCOPICAL OBJECTS. xxxiii

XI. ON MOUNTING MICROSCOPICAL OBJECTS.

The most convenient slide for microscopical objects is a
slip of glass three inches long and one inch wide, or for large
series of sections the slide may be 1^ inch wide. The glass
should be free from air-spaces and other flaws, and the edges
should be ground.

Cover-glasses must be very thin (No. 1) and free from flaws.
For single objects square covers (£ to ■§ inch) are best, and for
series of sections oblong covers, which should not come within
-jJg- inch of the edge of the slide. Space should also be left at one
end for a label.

' Cells ' are required to protect thick objects from the pres-
sure of the cover-glass. Thin flat rings of tin are good, and
should be cemented to the slide with gold size or ' brown
cement ' ; or a ring of the cement painted on with a brush and
allowed to dry may be used as a cell for thinner objects. No
cell is required for sections. Copper, brass, and vulcanite cells
are to be avoided.

An object may be mounted dry by fixing it to the slide in
the centre of a cell with a very small quantity of collodion or
gum-water, or of solution of shellac in creosote or alcohol.
When the cement is dry, a cover of the same diameter as the
cell is to be laid over it, and secured by varnishing at the edge
with any of the cements sold for the purpose.

More usually it is advisable to mount an object in a liquid
or solid medium. Such are glycerin, which is liquid ; Farrant's
medium, and Canada balsam, which are liquids becoming solid ;
and glycerin-jelly, which is solid when cold.

An object, stained or otherwise, to be mounted in glycerin or
Farrant's medium may be transferred to it direct from water,
great care being taken to avoid introducing air. The drop
of glycerin, &c, used should be just large enough to spread out
to the edge of the cover. When glycerin is used, the cover must
be fixed by means of gold size or other cement applied round
the edge with a small paint-brush.

To mount in glycerin-jelly the object is first soaked in
glycerin and then in melted glycerin-jelly {see Appendix) ; it

C

xxxiv INTRODUCTION.

is then transferred to a drop of the melted glycerin- jelly upon
the slide, and the warm cover, previously wetted with the
melted jelly to aid in excluding air, is immediately placed
upon it and held in position till the jelly solidifies. Avoid
heating more than is necessary. The edges of the cover may
be cemented as above, but this is not absolutely necessary.

To mount an object in Canada balsam, it must, after
staining, be dehydrated by soaking in absolute alcohol ; then
soaked in oil of cedar- wood or xylol, till the alcohol is removed ;
and, lastly, placed in the drop of balsam upon the slide and
covered.

Mounting sections cut in paraffin. Take a clean, dry slide
and spread evenly over it with the finger a very thin layer of
glycerin and albumen (see Appendix), so that the slide appears
to be only slightly greasy. An excess of glycerin and albumen
must be avoided, otherwise the sections will be cloudy. Place
the sections perfectly flat upon the slide, and press them down
gently with the finger. Warm gently in an oven or over a
flame till the paraffin is just melted, and immerse the slide in
xylol for two or three minutes to dissolve out the paraffin from
the sections. Lift the slide out of the xylol, drain and remove
the excess of xylol with a duster, place a drop of balsam over
the sections while still moist with xylol, and cover. In the
case of sections stained with hematoxylin it is advisable,
after removing the excess of xylol, as described above, to run
oil of cedar-wood over the sections in order to completely
remove the xylol, which tends to oxidise and decolorise
hsematoxylin, and thus destroys the stain.

In some cases it is necessary to stain sections on the slide.
The following is a convenient method : Fix the section with
glycerin and albumen, and remove the paraffin by warming
the slide and placing it in xylol as described above. When
the paraffin is all dissolved out, place the slide in absolute
alcohol for a few minutes to remove the xylol. Then pass the
slide gradually through 90 per cent., 70 per cent., 50 per cent,
alcohol. If an alcoholic stain (e.g. borax carmine, alcoholic
safranin, &c.) is to be used, the slide is now placed in stain.
If an aqueous stain (e.g. heemalum, &c.) is to be used, the

ON PREPARING SKELETONS. xxxv

slide must be placed in 30 per cent, alcohol, then in water, and
then into stain. After being in stain the requisite amount
of time, the excess of stain is removed by washing in some
suitable reagent, and the slide is then passed gradually through
50 per cent., 70 per cent., 90 per cent, to absolute alcohol,
where it is allowed to remain a few minutes in order to
thoroughly dehydrate. After dehydration oil of cedar- wood
is poured over the sections till the alcohol is all removed and
the oil flows freely over the slide. The excess of oil is then
removed, balsam placed on the sections, and they are covered
in the usual way.

Curled or crumpled sections should be straightened before
mounting, so that all the parts of the object He in one plane.
This can be done as follows : Smear the slide with glycerin
and albumen, and place a few drops of clean (or distilled)
water on the slide. Place the crumpled sections on the water
and very gradually warm the slide. As the water becomes
warm the creases will disappear, and the sections will lie
perfectly flat on the water. Great care must be taken not to
melt the paraffin, as this causes great contraction and spoils
the sections completely. When the sections are quite flat,
the water may be drained off, or removed by the application
of blotting-paper. The slide is then placed for a few hours in
an oven or incubator whose temperature is lower than the
melting-point of the paraffin used. When quite dry the slide
is warmed to melt the paraffin, placed in xylol, and treated in
the usual way.

XII. ON PREPARING SKELETONS.

Skeletons of rabbits, fowls, &c, may be prepared by mace-
ration or by boiling. Before macerating the bones should
have most of the flesh cut off ; they are then to be put into
cold water and left for several weeks for the remaining flesh
to rot away. They will afterwards require copious washing,
and may then be bleached by exposure to direct sunlight for
two or three weeks, during which time they should be re-
peatedly dipped into water and then left to dry. This same

xxxvi INTRODUCTION.

process also removes much of the offensive smell of freshly
macerated bones. Both these effects may, however, be more
quickly produced by steeping the bones in a clear solution of
bleaching powder, and afterwards washing them in running
water.

Boiling the bones and then picking off the flesh yields
skeletons serving all the purposes of the student, though not
so white as those obtained by maceration.

The preparation of cartilaginous skeletons is described at
p. 223.

XIII. ON OBTAINING AMCEBM FROM SOIL.

Ten grammes of chopped hay are put into a large flask
or beaker with one litre of distilled water and steamed for
three quarters of an hour. Then filter, make the filtered
liquid just alkaline to litmus by adding a few drops of sodium
hydrate and sterilise the alkaline liquid in an autoclave.
When cold, pour a small quantity into three or four Petri
dishes and inoculate each dish with a trace (not more than
half a gramme) of soil, almost any kind of garden or field soil
will answer. Put the dishes into a cool incubator or on the
laboratory table for a day or two, and then examine with a
microscope for amoebae. The amcebse vary in size from
very small forms to others measuring *03-*04: millimetres
in length when extended. They may be found in the en-
cysted condition and in all stages of activity. Subcultures
with fresh hay-infusion may also be made.

For detailed examination, clean cover-glasses may be
dropped on to the surface of the culture medium and then
removed to slides, or a platinum loopful of the surface layer
may be put on to a slide and covered.

PRACTICAL ZOOLOGY

Chapter I.

THE PROTOZOA.

peotozoon is an animal which consists of a single nucleated
)rotoplasmic mass which may be compared to a single cell
>f the tissues of a higher animal.

Protozoa are always of small size, and are usually micro-
;opic.

I. AMCEBA PROTEUS.
This protozoon is found in rivers and in freshwater pools,
imong algae known as ' diatoms ' and ' desmids,' and in damp
situations. Other amoebae may be obtained from soil or by
laking a hay-infusion in pond water and skimming off the
iridescent pellicle that accumulates after four weeks' culture,
[ts indefinite and ever-changing shape is characteristic, and
las gained for it the name * proteus animalcule.' There are
several forms, the largest of which are just visible to the naked
sye, while others are exceedingly minute.

Place a drop of water containing Amcebcs on a slide ;
:over ; and search for the animals with a low power of the
licroscope. When found, examine them with a high power.

l. General Appearance.

The animal is an irregular mass of protoplasm, running
>ut into blunt processes or pseudopodia, the protrusion and
fraction of which cause great changes in the form of the
ill. The pseudopodia may branch slightly.

Structure.

The whole body is a naked mass of protoplasm, of which
two portions are distinguishable.
1. The ectosarc is the clearer outer portion of the body.

THE PROTOZOA.

2. The endosarc is the granular central portion. Its
flowing movements, which accompany the pro-
trusion and retraction of the pseudopodia, are
rendered obvious by the granules contained in it.

Figs, ljto 8.— Amoeba : x 400. (c. h. h.)

Pigs. 2 and 3 were drawn from the same specimen, which was moving
in the direction shown by the arrow . Pig. 4 is a form with branch-
ing pseudopodia. Pigs. 5 to 8 were drawn from one specimen at
intervals of about twenty seconds.

N, nucleus, p, pseudopodium. pv, pulsating vacuole.

Within the endosarc make out the following structures.

a. Vacuoles are spaces filled with fluid, which is less

dense than the surrounding protoplasm. They
contain in many cases algae taken in as food.

b. The pulsating vacuole is a space filled with a watery

fluid. It is fairly constant in position, and
pulsates rhythmically. More than one may be
present. Time its pulsations.

c. The nucleus is a spherical granular body, denser

AMCEBA. 3

or more highly refractive than the rest of the
endosarc. During life it is easily seen in speci-
mens free from food. To stain it, place some
Amoebae on a cover-slip, and invert this over a
bottle of glacial acetic acid. Dry slightly over
a small flame, and stain with hematoxylin.
More than one nucleus may be present, but they
are all equivalent.

Movements.

The change of form already referred to is known as
amoeboid movement,' a form of that elongation and con-
traction seen best in muscle. Pseudopodia are protruded
com any point of the surface, and can be completely with-
drawn, and a slow crawling movement of the whole animal
can be effected by their means. Pseudopodia may also
surround algae and then be withdrawn, bringing the food
into the body.

Draw a specimen of Amoeba several times at intervals of
half a minute, indicating the direction of flow of the particles
)y means of at rows. Compare the drawings, and note the
differences of form even in sluggish specimens in which move-
ment was not obvious.

Feed with indigo or carmine, or other finely divided pig-
ment, and note that food is taken in at almost all parts of the
surface.

D. Reproduction.

Amoeba multiplies by fission ; that is, the whole mass
divides into two, each containing a portion of the nucleus, of
the endosarc, and of the ectosarc of the original cell.

E. Encystation.

Amceba periodically and also under adverse conditions
assumes a spherical shape and secretes a cyst round itself.
These cysts may be either resting-cysts or reproductive cysts
From the former the Amoeba emerges unchanged. From
the latter it issues (in one marine species at least) as a cloud
of excessively minute flagellated spores which conjugate in

THE PROTOZOA.

pairs and form amoebulse.
imperfectly known.

The life-history is however still

II. PARAMECIUM AURELIA.

This is a free-swimming freshwater protozoon found in
abundance among decaying vegetable matter. It differs

n e:p av

Fig. 9. — Paramecium aurelia, seen from above and slightly from the
right side : x 300. ■ (c. h. h.)

AV, anterior pulsating vacuole. EC, ectosarc : the lines running across
it indicate the trichocysts. EN, endosarc. EP, micro-nucleus. FV, food-
vacuole. M, mouth. MY, 'myophan' striation. N, mega-nucleus. OG,oral
groove. PV, posterior pulsating vacuole, in systole. TR, discharged tricho-
cyst threads. X, cilia.

from Amoeba in its more definite shape ; in the more marked
differentiation of ectosarc and endosarc ; in the presence of
cilia and the absence of pseudopodia ; in its active locomotion
by means of cilia ; and in the possession of a definite mouth,
and of definite dorsal and ventral surfaces, and anterior and
posterior ends.

Put a drop of water containing Paramecia on a slide in
weak melted gelatin to limit the movements of the animals ;
cover, and examine with low and high powers.

A. General Appearance.

The animal is an elongated, somewhat flattened body
about too inch in length, rounded at its narrow anterior end,
and bluntly pointed at the broader posterior end.

It swims actively by means of cilia distributed over its

PARAMECIUM.

entire surface. Near the middle of the ventral or oral surface
is an oblique groove, the Vestibule, leading to the mouth.

B. Structure.

The two layers, ectosarc and endosarc, are much more
sharply denned than in Amoeba.
1. The ectosarc is the comparatively firm outer layer, the
elasticity of which preserves the general form of the
body. Its deeper part is marked by longitudinal
or oblique ' myophan ' striations or myonemes, which
vary in distinctness with the movements of the
animal, and form a contractile system. They are
due to differentiation of fibres of the ectosarc.

a. The pellicle is the delicate superficial and stiffer

layer, serving as a protective covering for the
underlying protoplasm, of which it is the dif-
ferentiated external layer.

b. The cilia are very numerous delicate vibratile fila-

ments arising from the layer of ectosarc imme-
diately beneath the cuticle, through which latter
they project. An undulating membrane com-
posed of a partially coalesced tine of cilia lies
in the vestibule and mouth. This serves for
the ingestion of food.

c. The trichocysts are minute oval sacs arranged side

by side in the deeper part of the ectosarc, per-
pendicular to the surface, and in such numbers
as to form an obvious layer. When the animal
is irritated a stiffish thread can be shot out from
each of these trichocysts, and project on the sur-
face beyond the cilia. They are protective and
offensive weapons.

d. Two pulsating vacuoles are situated in the substance
of the ectosarc of the dorsal or aboral region,
one at about a third of the animal's length
from each end. In diastole they are nearly
spherical, but at the moment of systole, or con-
traction, they become stellate, and canals can

6 THE PROTOZOA.

then be seen radiating from them. They also
open to the exterior at the same moment.

e. The oral groove runs obliquely backwards along the

ventral surface from near the anterior end to the
mouth, which is placed a little behind the middle
of the length of the animal. Its cilia are fused
to form an undulating membrane, and drive water
and food particles into the mouth.

f. The mouth is an oval aperture in the ectosarc at

the hinder end of the oral groove, through which
the food passes into the endosarc.

g. The anus is an aperture between the mouth and

the hinder end of the body. It is visible only at
the moment of extrusion of fsecal matter.

2. The endosarc is the more fluid granular protoplasm
forming the central portion of the cell-body.

a. The food-vacuoles are spherical spaces in the

endosarc filled with water containing food-
particles.

b. The circulation of the endosarc is rendered obvious

by the food-vacuoles and the granules, which are
carried round in a definite direction.

c. The mega-nucleus is an elongated ovoid body near

the centre of the cell-body.

d. The micro-nucleus is a much smaller body applied

to one side of the mega-nucleus, and of an entirely
different character. In some species there are
two micro-nuclei.

C. Reproduction.

Fission is the only known method, and is effected in
the following manner. A transverse constriction
appears on the surface of the animal, and deepens
till the body is divided into two, each having a
portion of the original mega-nucleus and micro
nucleus, and each becoming^, perfect Paramecium.

OPALINA. 7

Conjugation is not a mode of reproduction, and is
closely followed by it. Two Paramecia become
united by their ventral surfaces, and, after swim-
ming about together for some time, separate and
become independent again. During and after
this union certain changes occur in the mega-nuclei
and micro-nuclei of both individuals, resulting in
the so-called ' rejuvenescence.'
Kill and stain the Paramecia by running a drop of methyl-
green in acetic acid under the cover-glass ; or fix with acetic
acid vapour and then stain with hcematoxylin.

The nuclei and the discharged trichocysts are well seen
in specimens so treated.

D. Encystation.

Paramecium under adverse conditions secretes a kidney-
shaped cyst around itself. This is however a resting cyst only.

III. OPALINA.

Opalina is cne of many diverse kinds of protozoa usually
found living in large numbers in the large intestine of
the frog. From its large size and the ease and certainty
with which it can be obtained, it is a very convenient form
for examination.

Slit up the large intestine of a freshly pithed frog ; scrape
away the contents together with the epithelial lining of the
intestine, and dilute with salt-solution ('75 per cent.). Examine
a drop of the mixture microscopically.

A. General Appearance.

Opalina is a flattened, asymmetrically oval body, about
31- inch long, covered with cilia, by means of which it swims
actively. It has neither mouth nor anus.

B Structure.

1. The ectosarc is thinner than in Paramecium. There
are no pulsating vacuoles, and no trichocysts.
a. The pellicle, if present at all, is exceedingly thin.

3 THE PROTOZOA.

b. The cilia are of equal size over the whole surface.

c. ' Myophan ' striations are obvious : they are mainly

longitudinal.
2. The endosarc contains no food-vacuoles, owing to the
fluid nature of the food ; this also accounts for the
absence of a mouth, the nutriment being absorbed
by the whole surface of the cell-body.
a. The nuclei are numerous.

C. Reproduction.

This is effected during spring and summer by oblique
or transverse fission.

Kill the Opalince by running a drop of acetic acid (1 per
cent.) under the cover-glass ; and then stain with safranin or
carmine.

D. Conjugation.

In late summer very small opalinaG have been observed
to become encysted, and in this state they leave the frog and
are subsequently swallowed by tadpoles. Within the cyst the
Opalina divides into gametes. Two of these fuse, and the
zygote so produced becomes encysted and subsequently
develops into a multinucleate form.

IV. VORTICELLA.

Vorticella, which is just recognisable with the naked eye,
differs from the two preceding infusorians in being attached
by a contractile stalk to plants or other objects, often to other
aquatic animals ; it may, however, under certain conditions,
become detached from its stalk and swim freely.

Both freshwater and marine forms of Vorticella are known.
The following description applies more particularly to some
of the larger freshwater species, but most of the marine forms
closely resemble these.

Vorticella may be distinguished from allied genera which
are often found with it by its unbranched stalk, and by the
spiral form which this assumes on contraction.

Mount a specimen in a drop of water together with a bit

VORTICELLA.

B XI

Fig. 10. — A group of Vorticellse : x 200. (c. h. h.)

Pigs. I, II, and III show the animal in various positions. IV is a much smaller
specimen drawn to the same scale. V shows a specimen made to contract by
action of alcohol. VI is detached from its stalk, and swimming away freely disc
forward. VII, VIII, IX show three stages of fission. X, XI, and XII show the
separated individual swimming by means of the aboral circle of cilia : XI is slightly
contracted, XII strongly contracted after the cover-glass has been tapped.

A, Food-vacuole discharging contents at anus. C, cilia of the disc. CF, con-
tractile fibre of stalk, d, disc. D', disc retracted. FV, food-vacuole. PH>
pharynx, pv, pulsating vacuole. P, peristome. M, myophan striation. N>
nucleus. V, vestibule.

10 THE PROTOZOA.

of the weed or other substance to which it is attached. Examine

it microscopically.

A. General Appearance.

Vorticella consists of a body, somewhat conical in shape in
the expanded condition, and with its apex or proximal end pro-
longed into the slender stalk by which the animal is attached.

The distal end of the body, or disc, forming the base of
the cone, is bordered by a prominent rim, the peristome.

The various species of Vorticella differ somewhat in form
and proportions from one another.

Vorticella is highly irritable, and when disturbed, as by
lightly touching the cover-glass with a needle, it instantly
contracts. The peristome is suddenly turned in, concealing the
disc and cilia, and at the same moment the stalk is suddenly
coiled up into a corkscrew spiral. If the irritation be discon-
tinued, the stalk gradually straightens out, the disc and peri-
stome become everted, and the ciliary movement is resumed.
B. Structure.

Vorticella is a protozoon, and the whole of the animal,
stalk and all, is one single cell, comparable to a single Amoeba
or Paramecium. The entire animal consists of protoplasm,
in which, as in Paramecium, a distinction may be made be-
tween the outer and firmer layer, or ectosarc, and the central
and more fluid mass, or endosarc. The stalk is a prolongation
of the ectosarc only.
1. The body.

a. The pellicle, or outermost layer of the ectosarc, forms

a protective investment to the whole body.

b. The disc is the distal or free end of the animal : it

is slightly convex.

c. The peristome is the projecting lip or rim surround-

ing the disc, and separated from it by a circular
groove.

d. The vestibule is a conical pit-like depression be-

tween the disc and the peristome : it marks the
1 ventral ' surface of the animal.

e. The pharynx is a narrow continuation of the inner

VORTICELLA. 11

end of the vestibule, extending deep into the
endosarc.

i. The anus is a small opening into the vestibule,
visible only at the moment of extrusion of fsecal
matter.

g. The cilia are arranged in an incomplete ring round
the margin of the disc, in the groove between the
disc and peristome : they also line the vestibule
and pharynx. They produce a current which
runs round the disc in the direction opposite to
that of the hands of a watch, then down the
vestibule near its ventral surface, and, after a
turn in the upper part of the pharynx, up along
the dorsal wall of the vestibule to the exterior-
There is a long undulating membrane projecting
through the external opening. The cilia serve
to sweep food particles into the interior of the
animal. A circular line near the attached end
of the body marks the zone from which the aboral
cilia will develop during fission.
* h. Food-vacuoles, enclosing food particles, may often
be seen circulating in the endosarc.

i. The pulsating vacuole is a spherical cavity, situated
between the disc and the vestibule, and opening
into the latter. It pulsates rhythmically.

k. The mega-nucleus is an elongated curved rod lying in

the endosarc, about the middle of the body.
1. The micro-nucleus is a minute corpuscle applied to

one side of the mega-nucleus. It can only be

demonstrated on well-stained preparations.
m. ' Myonemes ' are developed in the deeper layer of

the ectosarc, especially at the proximal end of

the body, close to the stalk.
2. The stalk.

a. The pellicle forms a thick covering to the stalk :

to its elasticity the straightening of the stalk

after contraction is probably due.

12 THE EROTOZOA.

b. The contractile band running down inside the
cuticular sheath of the stalk is continuous above
with the deeper, ' myoneme,' layer of the ectosarc
of the body. The spiral shortening of the stalk is
due to its contraction. In a specimen in which this
band had been broken, it was seen to be drawn up
suddenly towards the body at each inversion of
the disc, the stalk, however, remaining straight.

C. Reproduction.

1. Fission. The cell-body becomes broader from side to side,

and a constriction appears across its distal surface.
This constriction extends downwards till it reaches
the base of the cone close to the stalk, dividing the
cell-body into halves. Of these, one is like the
parent, and remains attached to the stalk. The
other, while still attached to the base of the first,
develops an aboral circle of cilia near its base,
becomes detached, and swims away by means of the
aboral cilia. After a time it attaches itself by its
base to some submerged object, and forming a stalk
by modification of the aboral cilia and elongation
of the ectosarc it becomes a Vorticella similar to the
parent. The process of fission may be completed
in from one to two hours.

2. Conjugation occurs sometimes, but it is unlike the con-

jugation of Paramecium in two important points.
First, the conjugation is between two dissimilar
forms : an ordinary large-stalked form, and a much
smaller free-swimming form which has originated
by repeated division of a large form. Secondly,
the union of the two is a complete and permanent
fusion, the smaller being absorbed into the larger.

This permanen b fusion of a small active cell with
a relatively large fixed cell, followed by a division of
the fused mass, presents a striking analogy to the
process of sexual reproduction occurring in higher
animals.

MONOCYSTIS. 13

. Encystation may occur after conjugation, and apparently
as the result of it. The disc becomes retracted ;
the cell-body usually separates from its stalk, be-
comes spherical, and secretes a cyst around itself.

D. Action of Reagents.

Kill some Vorticellce with methyl-green containing a trace
of acetic acid ; wash after three minutes with water,
and mount in glycerin or balsam.

f The macro-nucleus will be stained and its shape
and relations can be well studied.

V. MONOCYSTIS. T

Monocystis belongs to a special class of Protozoa, the
Sporozoa or Gregarines. The distinctive characters of this
class are their exclusively parasitic nature, absence of a
digestive tract, and the formation of resistent spores provided
with a thick coat. Monocystis is a common parasite of the
earthworm. It occurs in the vesiculse seminales and also in
the hinder region of the ccelom, where its spores form masses
visible to the naked eye. The attacks of such large numbers
of parasites weaken the reproductive system of the worm,
and in consequence reduce both the size and fertility of
the seminal vesicles. At least three species of Monocystis
are known.

Make a cover-glass preparation of the contents of a vesicula
seminalis taken from an infected worm, according to the
method described on p. 82. Search the preparation for the
free and encysted Monocystis.

The stage in the life-history of Monocystis before spore-
formation is the nutritive phase, and hence the term ' tro-
phozoite ' may then be applied to the animal. To the later
stage the term ' gametocyte ' is applicable.

A. Structure.

Each trophozoite is a single cell with a single nucleus.

14

THE PROTOZOA.

It is an elongate, fusiform organism, varying much
in length according to age. At the earliest stage

Fig. 11. — Monocystis.

A, a free" specimen of Monocystis. N, nucleus. B, two specimens in a
common cyst, c, breaking up into gametes. D, after conjugation : formation
of spores. (A, after Bourne. B & D, after Minchin.) X 35.

it inhabits the central protoplasmic mass or
blastophore about which the developing spermocytes

MONOCYSTIS.

15

of the worm are aggregated, and in consequence
the parasite may often be found covered with hair-
like structures, which in some species are the
adhering spermatids, but are hair-like outgrowths
of the pellicle in others. The trophozoite consists
of granular protoplasm enclosed in a pellicle, and is
capable of slight amoeboid movement. A mouth
is absent, nutrition being effected by osmosis of
the fluid contained in the vesicula seminalis through
any part of the cuticle.

. Reproduction.

When the trophozoite is full grown it becomes a gameto-
cyte and passes through a series of changes that
convert its protoplasm into spores. The changes
may be defined as (1) enclosure of two gametocytes
within a cyst ; (2) formation and fusion of gametes
or reproductive cells produced by the division of
each gametocyte ; (3) the formation of spore-coats
round the fused gametes.

Two gametocytes come together and form a
common enclosing membrane. The nucleus of
each cell divides and subdivides into a great number
of small nuclei, which then travel to the surface of
each gametocyte, where they become surrounded
by a portion of the originally continuous proto-
plasm and form the gametes or germ-cells. So far
the events in each gametocyte have gone on inde-
pendently of those in the other.

Now, however, conjugation of the gametes of
one gametocyte with those of the other takes place.
To allow of this, the partition between the two
gametocytes breaks down and allows free passage
of gametes in each direction, though how the move-
ment of the gametes is performed we do not know.
The nuclei and protoplasm of gametes having fused
in pairs, usually of different origin and of slightly
different character, a coat is formed about the fused

16 THE PROTOZOA.

mass or zygote, and within this spore-coat the zygote
divides four times, producing eight sporozoites. As
all these events take place in the cyst formed about
the two gametocytes, a very large number of spores
are enclosed in such a sphere. Hence the term
' Monocystis.'
Examine prepared sections of a vesicula containing many
Monocystis.

The' fate of these spores, each containing eight
sporozoites, is uncertain. They are probably eaten
together with the worm, by thrushes and other birds,
scattered over the ground with the droppings and
washed into the soil, whence they are again taken
up by earthworms. In the worm the spore-coat is
probably digested, releasing the sporozoites, which
then migrate into the vesicula? and become tro-
phozoites.

VI. TRYPANOSOMA.

Trypanosomes (' Boring-bodies ') are Protozoa living in
the blood or other tissues of animals and of man. The animal
so infected is said to be the ■ host ' and the Trypanosome the
' parasite.' Disease of the host does not necessarily accom-
pany the presence of Trypanosomes, since many affected
animals have in course of time acquired immunity.

There are many kinds of Trypanosomes, but one of the
largest is found in the blood of the dogfish, and another
closely similar form occurs in the skate. The following
description applies to either.

Clean a centrifuge-tube and place in it a few cubic centi-
metres of the blood of a freshly killed dogfish or skate. Insert
the tube into the centrifuge-machine and rotate it for a few
minutes, taking precautions to ensure the safe working of the
apparatus. When the motion has stopped remove the tube. All
the blood corpuscles will now be at the bottom.

With a fine clean pipette suck up a little of the sediment and
place it in a drop of the plasma on a slide. Cover and examine
with low and high power of the microscope. Place another drop on

:

TRYPANOSOMA. 17

a clean cover-glass and invert this over a bottle of glacial acetic
acid. The acetic vapour will kill and fix the cells in a few
minutes. Dry gently over a flame, cover the film so produced
with weak hematoxylin or methyl-blue-eosin. Allow the stain to
act for five minutes. Wash with water and mount in glycerine or
dehydrate with successively strong grades of alcohol, clear with
cedar wood oil, and mount in Canada balsam.

A. General Appearance and Structure.

Trypanosoma consists of an elongated cell (90 /jl in length)
produced at one end into a whip-like ' tail '
(flagellum) and lies coiled up in the blood plasma, but
is continually agitated by a wave-like movement of the
' undulating membrane ' that projects from one side
of its body. No mouth or contractile vacuole is present.
Nutrition is carried on by osmosis through the delicate
pellicle. The cell consists of ectoplasm, endoplasm,
and nuclear bodies, v,

Two nuclei are present. The ■ tropho-nucleus ' consists
of a lens-shaped mass placed near the centre of the
body. The much smaller ' kineto-nucleus ' lies in
front of ohis, and from it is given off the * flagellum'
Which runs down the side of the body, following a
tortuous course, and attached by the undulating mem-
brane for the greater part of its length, but finally
emerging freely.

B. Reproduction.

Fission is the only means of reproduction that is at present
known to occur

C. Life-history.

Trypanosomes are usually carried from one host to another
by an invertebrate that sucks the blood of the infected
host and then, after cultivating the parasite for a time
in its own body, infects the new host by its bite or in
other ways. In the case of fish-Trypanosomes the
carrier is a leech, but the life-history of the dogfish
Trypanosome is not yet worked out.

18

Chapter II

HYDKA.

Hydra is a small freshwater animal, abundant in ponds and
ditches and in slowly moving streams.

It consists of a tubular body, about a quarter of an inch
in length, one end of which is closed, while the opposite end
is open and forms the mouth, a short distance below which is
a circle of tentacles, usually six to eight in number.

Both the tentacles and the body are extremely contrac-
tile. The former, when the animal is disturbed, can be
almost completely withdrawn, and the latter may shrink up
so as to become a mere knob.

Hydra is usually attached by its closed basal end or foot
to water- weeds or other bodies. By contractions of the foot
it can crawl along slowly, and it can also progress more
rapidly by fixing itself alternately by the mouth and foot,
arching the body with a looping movement like a caterpillar.
It is carnivorous, and by means of the nematocysts with
which its tentacles are studded can paralyse and kill active
animals nearly as big as itself and of active habits.

Hydra receives its name from its remarkable power of
recovery from injury. A specimen may be cut into two or
more pieces, either transversely or longitudinally, and each
fragment will not only survive, but within a short time will
become a complete Hydra. The entire animal may even be
regenerated from a single tentacle.

At least three species of Hydra are described as occurring
commonly in this country. Of these H. viridis is distin-
guished by its green colour and its smaller size, H. fusca is
brown in colour, and H. vulgaris almost colourless.

HYDRA.

19

I

It is uncertain, however, how far these species are really-
distinct from one another, and the following description will
apply to any of them.

Fia. 12. — Hydra. A diagrammatic longitudinal section of a specimen
with ripe reproductive organs, and with a fully formed bud : x 12.
(a. m. m.)

A. mouth. B, hypostome. C, enteron or digestive cavity, d, ectoderm.
E, mesogloea. Fi endoderm. G, cavity of one of the tentacles. H, testis.
I, ripe ovum in the ovary. K, fully formed bud, with mouth and tentacles.
L, foot, by which the Hydra is attached to a piece of weed.

I. EXAMINATION OF A LIVING SPECIMEN.

Cut off a small piece of weed with a Hydra attached to it ;
place it on a slide in a drop of water, and put a cover-glass

20 HYDRA.

over it. If the Hydra is detached or fixed to the side of the
tank, take it up with a dipping tube, and place it on a slide
in a drop of water, with a small piece of weed for the animal
to attach itself to, and to protect it from pressure. Cover as
before.

A. General Appearance.

Leave the slide until the Hydra has fully expanded, and
then examine it with a low power.

1 . The body is tubular : its cavity, the coelenteron, opens

to the exterior at the mouth, and serves as the diges-
tive cavity of the animal.

The shape of the body varies very greatly
according to the degree of elongation or contrac-
tion. It is usually more slender in its proximal or
basal half than in the distal portion, but when fully
extended is of nearly uniform diameter through-
out. It is often distended locally by food.

a. The foot is the closed proximal end of the body

which forms a kind of sucking disc for attach-
ment to water- weeds, etc.

b. The mouth is a small aperture at the free or distal

end of the body. It can be greatly dilated, so
as to allow the entrance of food masses of con-
siderable size.

c. The hypostome is the conical part of the body

above the tentacles, with the mouth at its
summit.

2. The tentacles are hollow processes of the body-wall,

usually six to eight in number, and arranged in a
single whorl round the base of the hypostome, which
is the widest part of the body. Their cavities open
into the general digestive cavity of the animal.

The tentacles, which have a warty appearance,
are extremely contractile. When fully extended
they are two or three times the length of the body ;
when fully contracted they are scarcely visible.

STRUCTURE OF BODY-WALL.

21

They are capable of free and active movement, and
are used for capturing and paralysing the prey, which
they then, by their contraction, transfer to the
mouth.

Fig. 13. — Hydra viridis. A small portion of a longitudinal section
through the body-wall : x 800. (a. m. m.)

A, one of the large ectoderm cells. Bi nucleus of a large ectoderm cell.
C, muscular tail of a large ectoderm cell. D, an undischarged nematocyst
lying within its parent cell, or cnidoblast. E, cnidocil. F, a nematocyst with
discharged thread. G, interstitial cell. H , supporting lamella or mesoglcea. I,
endoderm cell. K, vacuole of endoderm cell, l, nucleus of endoderm cell.
M, green-cells. N, flagellum of endoderm cell.

B. Structure.
1. Structure of the body-wall.

Examine the body-wall more closely with the low power,
and note its division into layers.

a. The ectoderm is the outermost layer of the body.

It is colourless, and forms about a third of the
entire thickness of the wall.

b. The endoderm is the inner layer, lining the diges-

tive cavity, and forming about two-thirds of the

22 HYDRA.

thickness of the body-wall. It is coloured green
or brown according to the species of Hydra
examined.

c. The supporting lamella, or mesogloea, is a very
thin gelatinous layer between the ectoderm
and the endoderm, hardly visible with the low
power.

2. Structure of the tentacles.

Examine one of the tentacles with a high power, and note
its division into layers corresponding to those of the body-wall.
Examine (1) the middle of the breadth of the tentacle,, altering
the focus so as to bring the several layers into view in turn ;
and (2) the edge of the tentacle, where by proper adjustment of
the focus a profile view of the cells can be obtained.

a. The ectoderm is a cellular layer made up as follows :

i. The large ectoderm cells form a single layer,
covering the whole surface of both the tentacles
and the body. They are somewhat conical in
shape. Their pouter ends are closely fitted
together, forming a mosaic pattern very clearly
visible in surface views of the tentacle. Their
inner ends, which rest directly on the support-
ing lamella, are narrower, and separated from
one another by intervals of varying extent :
these inner ends are produced into slender
muscular processes or ' tails,' which lie parallel
to the long axis of the tentacle, and closely
applied to, or imbedded in, the mesogloea.

The shape of these large ectoderm cells
varies very greatly with elongation and con-
traction of the tentacle. When contracted they
are widened transversely, and their outlines are
very distinct ; when elongated their length
considerably exceeds their width.

ii . The interstitial cells are small rounded cells placed

STRUCTURE OF TENTACLES.

23

in little clusters between the inner or basal ends
of the large ectoderm cells.
The nematocysts or * thread cells ' are sharply-
defined oval capsules, filled with fluid and en-
closing a long hollow spirally wound filament,
formed by doubling in of the wall of the capsule
at one pole. They develop within certain of
the interstitial cells called cnidoblasts, and lie,
when mature, in and between the large ectoderm
cells, causing marked projections on the surface.

The cnidoblast, or cell within which the
nematocyst is developed, persists as an envelop-
ing capsule, and is produced at its outer or Iree
surface into a small process, the cnidocil.

The nematocysts can be discharged or ex-
ploded, the contained hollow filament being
shot out with great force and velocity. In this
process of discharge, which is brought about by
contraction of the cell in which the nematocyst
lies, the basal part of the thread is discharged
first, and, being provided with barbs, of which
there are three principal and several smaller
ones, fixes itself at once in the body against
which the thread is shot, and into which the
remainder of the filament is then propelled.

The discharge of these filaments appears to
be under the control of the Hydra : they exert
a marked numbing or paralysing influence on
animals into which they are shot, and are the
means by which the Hydra captures its prey.
A nematocyst once discharged cannot be used
again, and is cast off.

There are three kinds of nematocysts : the
large barbed kind ; a smaller kind without bards,
especially abundant on the tentacles and used
for entangling the prey ; and a third kind, used
for temporary attachment.

Nematocysts of both the larger and the

24 HYDRA.

smaller kinds occur in the body-wall, though
less abundantly than in the tentacles.

iv. The * nerve-cells.' Certain small stellate cells
found in the ectoderm are described as nerve-
cells. A connection between them and the cells
enclosing the nematocysts has been traced, and
it is possible that they are concerned in the dis-
charge of the latter.

b. The endoderm consists of a singleT layer of large

granular and vacuolated cells lining the cavity
of the tentacle. Many of these bear flagella.
• Focus the middle of the thickness of the tentacle ; note the

flagella of the endoderm cells, and the stream of nutrient particles

up and down the cavity of the tentacle.

c. The supporting lamella is the thin gelatinous layer

between the ectoderm and the endoderm.
Focus the side of the tentacle at the middle of its thickness,
and the mesogloea will be seen as a thin transparent layer between
the ectoderm and the endoderm.

Sharply tap the microscope so as to cause the Hydra to
retract its tentacles.

In the contracted condition of the tentacle
the ectoderm and endoderm are both strongly
wrinkled transversely, but the mesogloea is not.
This shows that contractility is specially asso-
ciated with the supporting lamella ; the real con-
tracting elements are the muscular ' tails ' of the
ectoderm cells applied to or imbedded in its
substance.

d. Discharge of the nematocysts.

Focus one of the tentacles with the high power ; then place
a drop of acetic acid on the slide, and allow it to run under the
cover -glass.

As the reagent reaches the tentacles these
will be retracted, and their nematocysts dis-
charged in consequence of the irritation. Some

REPRODUCTION. 25

of the nematocysts will remain in the ectoderm
with their threads projecting freely ; others will
separate completely.
Examine, and draw the nematocysts, showing their shape,
the threads with their enlarged basal portions, and the barbs.

Search for specimens with partially discharged threads ;
and note that the threads are turned inside out in the process
of discharge.

0. Reproduction.

Hydra reproduces asexually and sexually.

1. Asexual reproduction.

a. Gemmation or budding consists in the formation
of a hollow outgrowth from the side of the body,
which acquires a mouth and tentacles at its
distal end, and ultimately constricts at its base,
separates from the parent, and becomes an in-
dependent animal.

The rate of budding depends largely on the
supply of food and on temperature. If a rapidly
budding Hydra be transferred to water in which
there is little or no food to be obtained, the
formation of the buds will be stopped, and buds
already formed may even be absorbed.

A single Hydra may give rise to more than
one bud at once, and these may develop second-
ary buds before separating from the parent
animal. In this way temporary colonies may
be formed, which, however, sooner or later
break down into their component units.

b. Fission. A Hydra may be cut in two, and each part
will live and become a perfect animal. The
process of fission is, however, very rare.

2. Sexual reproduction.
Hydra may be of separate sex but is often

hermaphrodite, the same animal having both testes
and ovaries. Contrary to the rule among herma-

26 HYDRA.

phrodite animals, the reproductive organs are
extremely simple in structure, and the animals
are capable of fertilising their own ova.

a. The testes are conical or spherical swellings of the
body-wall, varying in number from one or two
to twenty. They are usually situated near the
oral or distal end of the body, not far below the
tentacles ; but when numerous they may extend
along the greater part of the length of the body.
Each testis is formed by local proliferation of
the interstitial cells of the ectoderm, giving rise
to small conical heaps, which project externally
and are covered on their outer surfaces by
capsules formed from the large ectoderm cells.

The interstitial cells, after repeated divisions,
become converted into spermatozoa by modifi-
cation of their nuclei to form the heads, and
elongation of their protoplasm to form the long
swimming tails. When fully formed they are
discharged by rupture of the capsule at its most
prominent point.
Select a Hydra in which the testes are large ; put it into a
drop of water on a slide ; cover it ; and, by gentle pressure
on the cover-glass, rupture one of the testes, and so discharge the
spermatozoa. Examine with a high power.

The spermatozoa are filiform bodies about
3~i^- inch in length. Each consists of a small
ovoid highly refractive head, containing the
nucleus, and a long active thread-like tail.

b. The ovaries usually develop a little later than the
testes. In Hydra viridis there is seldom more
than a single ovary present at a time, but in the
brown species there may be as many as eight.
They form, when ripe, spherical projections of
the body-wall considerably larger than the testes,
and situated as a rule nearer the basal end of
the animal. In each ovary only a single ovum

PREPARED SPECIMENS. 27

is produced. The ovary is formed, like the testis,
by multiplication of the interstitial cells to form
a small swelling ; one of these cells, near the
centre of the heap, soon becomes larger than the
rest and becomes the ovum. This grows rapidly
at the expense of the interstitial cells of the
ovary, which form a capsule around it and supply
it with nutriment. The ovum is at first amoeboid,
so as to come into contact with a large number
of the nutrient cells. When it has reached its
full size it becomes spherical or ovoid ; the
capsule then thins away and ruptures at its
summit, and, shrinking back, leaves the outer
half of the ovum exposed to the water, and
ready to receive the spermatozoa.

After fertilisation the ovum segments, forms
a hard capsule around itself, becomes detached
from the parent, and, falling to the bottom of
the tank or pond in which the animal is living,
develops after a time into a young Hydra.

II. EXAMINATION OF PREPARED SPECIMENS.

A. Disintegrated Specimens.

Kill a Hydra with 1 per cent, acetic acid and a trace of
osmic acid. Leave it for five minutes, and then examine in
water.

The cells are loosened from one another, and may now be
examined separately.

1. The ectoderm cells.

a. The large ectoderm cells will be found isolated in
various parts of the preparation. They are some-
what conical in shape, their outer ends being
broad and containing the large nuclei. Their
inner ends are much narrower, and are produced
into slender muscular processes or tails. These
tails He on the outer surface of the supporting

28 HYDRA.

lamella, and partially imbedded in its substance.
Their direction is mainly longitudinal, i.e.
parallel to the long axis of the animal, and it is
probably on them that the contractility of the
animal (p. 18) depends.

b. The interstitial cells are much smaller, and usually

remain in small clusters. Within some of them
the early stages of development of the nemato-
cysts may be seen.

c. The nematocysts. In specimens killed with acetic

acid, nematocysts are frequently met with in
which the thread is only partially protruded,
and in which the relations of the capsule to the
thread can be easily determined.

d. The nerve cells. The small stellate cells supposed

to be nerve cells may sometimes be seen. They
are better seen in specimens treated with methy-
lene blue.

2. The supporting lamella may be seen in teased prepara-

tions in the form of transparent shreds or sheets of
greater or less extent, on the outer surface of which
a distinct longitudinal striation is present, caused by
the muscular tails of the large ectoderm cells.

On the inner surface of this lamella trans-
verse fibres occur, which appear to be connected
with the endoderm cells, and are probably muscular
in function.

3. The endoderm cells are very variable in shape, and

during life some of them are amoeboid. Each
commonly contains one or more non-contractile
vacuoles, which may be so large as to reduce the pro-
toplasm of the cell to a very thin peripheral lamella.
The outer end of each cell contains in Hydra viridis
a number of small cells coated with chlorophyll, to
which the green colour of the animal is due. In
the brown species of Hydra similar bodies are
present, but devoid of chlorophyll.

TRANSVERSE SECTIONS. 29

The endoderm cells may also contain, in addition
to particles of food, little clusters of brown or black
granules, probably formed by breaking down of the
chlorophyll grains. The flagella are seldom pre-
served in teased specimens.

B. Transverse Sections.

Examine a series of transverse sections through the body
of Hydra. To prepare these, kill the animal in the expanded
condition by Perenyi's fluid ; transfer first to 70 per cent, and
then to 90 per cent, alcohol; stain with hcematoxylin, and
then imbed in paraffin and cut it into thin sections with a
microtome.

1. The ectoderm.

a. The large ' ectoderm cells are well seen in such

sections ; their muscular tails, which are mainly
longitudinal in direction, are cut transversely,
and appear as a row of highly refractive dots on
the outer surface of the mesoglcea.

At the basal end or foot of the Hydra the
ectoderm cells are more columnar in shape,
contain longitudinal rows of granules, and are
probably glandular.

On the outer surface of the ectoderm cells
there is a thin cuticular layer, more obvious in
the brown than in the green Hydra.

b. The interstitial cells are well seen in sections through

the body or tentacles, but are absent in the foot.

c. The nematocysts are very abundant in the tentacles,

less numerous in the body, and absent in the
foot. In specimens killed with osmic acid, few
if any of them will be discharged.

d. The testis or ovary may be cut across, and should

be looked for.

E

30 HYDRA.

2. The supporting lamella is seen in transverse sections as a

very thin transparent layer between the ectoderm and
the endoderm.

3. The endoderm cells differ greatly in shape in different

parts of the body and at different times. During
life some are amoeboid, and their processes may
extend so far as to almost obliterate the cavity of the
animal. Others contain glandular secretions which
are used for the digestion of food in the ccelenteron.
The nucleus is usually nearer the outer or basal end,
and is flattened in shape. The flagella can very
seldom be seen in sections.

In the foot the endoderm cells are much smaller
than in the body, and form a layer of short columnar
or almost cubical cells containing granular concre-
tions.

OBELIA.

Obelia is a delicate white organism consisting of a branch-
ing stem rising an inch or so from a root-like stolon. The
stem is bent in a zigzag fashion, each angle supporting a
polyp or hydranth comparable to a Hydra. If the buds of
Hydra remained in contact we should have an organism
not unlike Obelia.

Obelia is, however, marine. It lives fixed to the fronds
of seaweed, the shells of lobsters, and the surface of wood-
work immersed in the sea. Its food consists of the minute
animals and plants that accumulate as microscopic debris
along the coast.

A. Structure.

The structure of an Obelia-hydroid is somewhat
similar to that of Hydra. Each polyp consists of a
digestive cavity and a capacious mouth surrounded
by a circle of tentacles that rise at the base of the
lips. Adjacent polyps are united by the central
stem. The outer ectoderm of the mouth, digestive
cavity, and tentacles is lineal by an inner endoderm

OBELIA.

31

layer, but the latter forms a solid strand of cells
down each tentacle. The nematocysts are extremely

ME

EN

Fig. 14. — Obelia.

A, a small portion of the hydroid colony. B, a free medusa in vertical section.
BL, blastostyle. C, calyx. EC, ectoderm. EL, endoderm-lamella.

EN, endoderm. GO, gonads. ME, medusa. M, mouth of manubrium.
NE, nematocysta on the tentacles. OT, otocyst. p, perisarc. ST, stomach.
T, tentacles. V, velum. (Partly after Shipley and McBride.) A., X 50.

B, x 12.

32 HYDRA.

small, and not associated in distinct batteries, and
are all of one kind. Moreover, the cuticle, so
extremely delicate in Hydra, is thickened in Obelia
to form a protective coat, the perisarc, which ex-
pands as a chalice about each polyp.

Obelia is easily found on the fronds of seaweed, and can
be kept alive in sea-water without any special precautions
other than frequent aeration and excluding dust. To fix
colonies use corrosive sublimate and acetic acid ; stain with
nigrosin and mount in balsam.

In the axil or angle between the polyp and the
stem, shoots of a kind differing in many respects
from the ordinary polyps will be found. These are
the blastostyles, or genital sacs, and differ from
the hydroid polyps in having neither mouth nor
tentacles, and in giving rise to buds that acquire
mouth, tentacles, swimming-bell, and reproductive
organs. These buds are jelly-fish or medussB.

B. Structure of a Medusa.

An Obelia-medusa consists of a very shallow
bell, with the clapper hanging down from its centre.
The clapper is a greatly elongated throat or manu-
brium, opening at its lower end by the mouth, and
leading at its upper end into four radial canals that
run to the margin of the umbrella, and are there
connected by a circular canal.

The bell consists of a gelatinous disc provided
with marginal circular muscles by which the pul-
sating movements of the medusa are performed.
Along the edge of the bell, tentacles are distributed
and eight otocysts are found distributed, two in
each quadrant at equal distances from one another.
These organs of balance probably subserve the
functions of rendering Obelia sensitive to water
movement.

OBELIA. 33

Along the radial canals four patches of germ-
cells, either male or female, are found.

Obelia-medusce can be chained by keeping ripe Obelia-
hydroids in a dim light for a few days. Frequently also they
are taken by using a fine muslin net with a bottle tied to re-
place the ordinary bottom of the net, and towed behind a boat.

They may be fixed with weak (2 per cent.) formalin, or
with Flemming's mixture. In hardening, transfer very slowly
from one grade of alcohol to another.

C. Development.

The fertilised egg of Obelia gives rise to an egg-
shaped ciliated body, or ' planula,' composed of an
outer and an inner layer of cells. This settles down,
and by budding establishes the hydroid. Some of
the buds feed ; others have no mouth, and become
medusae. The entire product of the development
of the egg comprises the hydroid colony and its
medusae, and to this the term ■ individual ' is con-
veniently given. The individuals so produced from
the eggs of medusae of a colony form a new genera-
tion. There is, therefore, no ' alternation ' of
hydroid and medusoid ' generations ' as is often
maintained. The medusa is a specialised bud or
organ of reproduction, and may be compared with
the flower of a plant if we could imagine such a
structure to detach itself and to disseminate its
seeds by active or passive movements.

34

Chapter III.

THE LIVER-FLUKE OF THE SHEEP.

Fasciola (Distomum) hepatica.

The adult liver-fluke is a flat unsegmented worm, about an
inch and a half long, living in the bile-ducts of certain
domestic animals, and notably in those of the sheep, in which
it gives rise to the destructive disease known as liver-rot. It
very rarely occurs in man.

The animal is hermaphrodite, and its eggs, which have
thick chitinous shells, are deposited in enormous numbers
in the bile-ducts of the sheep or other host, from which
they pass into the alimentary canal, ultimately escaping with
the faeces. From these fertilised eggs, if deposited in damp
places or in water, larvae are produced which lead a free exist-
ence for a short time, but very soon become parasitic within
the body of Limncea truncatula, an amphibious snail. Two
or more generations produced by development of unfertilised
ova now succeed : and the last of these encyst on grass. These
encysted forms are swallowed with the grass by sheep, and
passing into their bile-ducts become the adult sexually mature
flukes.

This alternation of generations is entirely different from
that of Obelia.

I. THE MATURE LIVER-FLUKE.

Slit open the bile-ducts in the liver of an infected sheep,
and transfer the living flukes to a dish of warm salt-solution
(•75 per cent.) to clean them.

1. Inject the excretory system of one fluke, and the alimentary

EXTERNAL CHARACTERS. 35

canal of another, with a thin injection, such as freshly preci-
pitated Prussian blue, suspended in water. Inject the coloured
fluid, by means of a very fine glass cannula provided with an
indiarubber cap, into the hinder end of the animal; in the
middle line for the excretory system, and about 1 mm. from it for
the alimentary canal. Place the animals between glass slips,
and tie the slips together with cotton. Put the slips with the
animals between them into alcohol (90 per cent.) for twelve hours
or more, when the specimens may be dehydrated, cleared, and
mounted in balsam.

2. Squeeze specimen somewhat more lightly between two
glass slides, and leave it in corrosive acetic mixture (see Appendix)
for five or six hours. Wash well with 50 per cent., 70 per cent.,
80 per cent, alcohol for about two days. Pass gradually down
to water, and stain with weak hcemalumfor twelve hours. Wash
for the same length of time in 1 per cent, alum solution. Then
pass gradually through water, 50 per cent., 70 per cent., 90 per
cent., to absolute alcohol (two hours). Clear in cedar-wood
oil, and mount in balsam.

A. External Characters.

1. In form the animal is flat and oval with a blunt tri-

angular projection forming its anterior end. Its
length is about an inch and a half, its breadth about
half an inch.

2. The mouth is an oval aperture at the anterior end, in

the middle of the cup-shaped anterior sucker.

3. The ventral sucker is a muscular cup in the mid- ventral

line, near the junction of the triangular anterior
portion with the broader part of the body.

4. The genital aperture, through which the penis may be

protruded, is on the ventral surface between the two
suckers, and slightly nearer the posterior one.

5. The cuticle is a thin layer covering the whole animal.

Its surface is covered with minute backwardly
directed, pointed scales, which are best seen in a
specimen kept in spirit and dried at the moment of
examination.

36

THE LIVER-FLUKE.
0

Fia. 15. — Fasciola hepatica from the ventral surface : x 4.
(After Sommer.)

The alimentary and nervous systems only are shown on the left side
of the figure, the excretory system alone on the right side of the figure.
A, right main division of the alimentary canal. C, one of its diverticula.
CS. cirrus-sac. q, left anterior dorsal trunk of excretory system. G, lateral
ganglion. M, main trunk of excretory system. N, lateral nerve. O, mouth.
P, pharynx, s, ventral sucker. Vt left anterior ventral trunk of excretory
system. X, excretory pore.

REPRODUCTIVE SYSTEM. 37

B. The Alimentary Canal.

1. The pharynx is an ovoid muscular mass immediately

behind the mouth.

2. The oesophagus is a very short straight thin- walled tube

behind the pharynx.

3. The intestine, into which the oesophagus opens just; in

front of the genital aperture, divides immediately
into a right and a left limb, each of which runs to
the hinder end of the body, giving off numerous
small caeca on its inner side, and many large branched
ones on its outer side ; the whole intestine forming
a series of csecal tubes ramifying all over the body
without anastomoses.

C. The Excretory System.

This is difficult to see except in injected specimens. It
consists of a network of minute and much-branched ducts,
which commence with slightly dilated ends into which project
long flame-shaped cilia. The small ducts freely anastomose
with one another, and open into larger transverse ducts,
which in turn open into the main duct.

The main duct is a medium tube of considerable size,
formed by the union of four anterior ducts, a dorsal and a
ventral on each side, at about a quarter of the animal's length
from the anterior end : from this point the main duct runs
directly backwards to open to the exterior by a terminal
median pore.

D. The Reproductive System.

This is complicated, as in most hermaphrodite animals.

1. The male organs.

a. The testes are much-branched tubes, lying in the
middle part of the body, and extending over
about half its length and half its width. The two
testes are about equal in extent, one lying behind
the other.

38

THE LIVER-FLUKE.

The vasa deferentia are two in number, one arising
near the middle of each testis. They run forwards
as far as the ventral sucker, where they open into
the vesicula seminalis.

Fm. 16.-

-Fasciola hepatica. Reproductive organs, from the ventral
surface : x 6. (After Sommer.)

CS, cirrus-sac. D, ductus ejaculatorius. F, aperture of the oviduct.
O, ovary. OD, ovarian duct. P, penis. S, shell-gland. T, anterior testis.
U, uterus. VA, vas deferens from anterior testis. VP, vas deferens from
posterior testis. VS, vesicula seminalis. Y, yolk-gland. YD, duct of yolk-
gland.

THE FEMALE ORGANS. 39

c. The vesicula seminalis is an elongated sac lying

above, and somewhat in front of, the ventral
sucker.

d. The ductus ejaculatorius is a fine tube running

from the vesicula seminalis to the end of the
penis. When the penis is withdrawn this duct
is thrown into convolutions.

e. The penis or ' cirrus ' is a large muscular organ

which lies, when withdrawn, within the cirrus-
sac, a space in front of the ventral sucker. It
can be protruded by evagination, and the ejacu-
latory duct then lies within it.

f. The cirrus-sac is a cavity between the ventral sucker

and the genital aperture. The penis lies in it
when withdrawn, as do also the vesicula seminalis
and a small accessory gland which surrounds the
ejaculatory duct.

2. The female organs.

a. The ovary is branched and tubular : it lies on the

right, or rarely on the left side in front of the
anterior testis, and its branches unite to form
the narrow ovarian duct.

b. The yolk-glands are very numerous small rounded

masses, scattered along two areas extending
along the sides of the body from end to end,
and each nearly one quarter of the whole width
of the animal.

c. The vitellarian ducts, or ducts of the yolk-glands,

unite to form an anterior and a posterior duct
on each side. These unite about the junction
of the anterior and middle thirds of the animal,
forming a transverse duct, which runs inwards
to open into the median yolk-reservoir : from
this a single median vitellarian duct runs a short
distance forwards and unites with the ovarian
duct to form the oviduct.

40 THE LIVER-FLUKE.

Close to the junction is the opening of a short
duct, the vagina or 'canal of Laurer,' which
opens externally in the middle line on the dorsal
surface.

d. The shell-gland is a median glandular mass, really

an aggregate of unicellular glands, surrounding
the junction of the ovarian and median vitellarian
ducts.

e. The oviduct, or ' uterus,' is a wide, much-convoluted

tube, commencing at the point of union of the
ovarian and vitellarian ducts in the midst of the
shell-gland, and lying between the shell-gland
and the genital aperture. Its convolutions extend
halfway across the animal, and numerous eggs
can be seen within it.

f. The aperture of the oviduct is at the base of the

penis, and on its left side. When the penis is
fully withdrawn a slight cavity, the 'genital
sinus,' is formed, into the left side of which the
oviduct then opens.

II. LIFE-HISTORY OF THE LIVER-FLUKE.

The free- swimming larvae, may be obtained by removing
e99s from the bile-ducts of an infected sheep and hatching
them in a shallow vessel of water in a warm room. Sporocysts
and redioe may be obtained from specimens of Limn sea trun-
catula, kept in the same vessel.

The main features in the life-history are as follows.

A. The First Generation is produced sexually by the develop-
ment of fertilised ova.

1. The eggs, laid in large numbers in the bile-ducts of the
sheep or other infected mammal, pass with the bile
into the intestine, and so escape from the body.
They are ovoid bodies "005 inch long and '003
inch broad, enclosed in smooth brownish chitinous

LIFE-HISTORY. 41

shells. Within each shell are a single ovum or germ-
cell and a large number of yolk-cells. After the escape
of the egg from the body of the host, segmentation
occurs, an embryo develops, and a circular operculum
at one end of the shell opens to allow it to escape.

2. The free larva is conical, with a short papilla at its

broad anterior end ; the whole surface is covered
with long cilia, enabling the larva to swim rapidly.
Two eye-spots are present, and two 'flame-cells,'
or excretory organs. The ectoderm is a single layer
of flattened cells, usually arranged in five transverse
bands : within is a mass of granular cells. When
this larva meets with Limncea truncatula, a small
amphibious snail, the head-papilla becomes elon-
gated, and by means of it the larva bores its way
into the snail. Unless the larva happens to come
across a Limncea within about eight hours after its
escape from the egg, it dies.

3. Development of the sporocyst. Within the snail, usually

in its pulmonary chamber, the ectoderm cells of the
larva swell, lose their cilia, degenerate, and are
thrown off ; the remainder grows rapidly, and in two
or three weeks becomes an elongated sac, '024 inch
long. This sac, the sporocyst, has an outer struc-
tureless cuticle, a thin muscular layer, and an
epithelial layer lining the cavity.

The eye-spots, though losing their form, persist ;

and ciliated excretory funnels are present. Such

sporocysts sometimes, though rarely, multiply by

transverse fission in the early stages of their

development.

B. The Second Generation consists of Rediae, which develop

within the sporocyst, from cells that may be looked

upon as unfertilised ova, and are themselves parthe-

nogenetic.

1. From the epithelium of the sporocyst, cells are budded off

which segment to form solid masses or morulas lying

42

THE LIVER-FLUKE

LTFE-HISTORY. 43

in the cavity of the sporocyst. Each of these morulae
becomes flattened on one side, and then acquires
a rudimentary gut, and body-cavity. The larvae
increase in size, elongate, and develop into rediae.
The rediae so formed force their way out of the
sporocyst and become free : the wound in the sporo-
cyst heals, and other rediae are formed in the same
way within it.
The free rediee wander about in the snail, increasing in
size and being especially abundant in the liver. The
adult redia (fig. 20) is a cylindrical body about -06
inch long, with a collar -like ridge running round it
near the anterior end, and with a pair of blunt pro-
cesses projecting from the hinder part of the ventral
surface, which aid in locomotion. The body-wall
resembles that of the sporocyst in structure, but is
more muscular, and has definite excretory canals
which commence as funnels with ' flame-shaped '
bunches of cilia.

The alimentary tract is a comparatively short-
blind sac, with walls one cell thick : the mouth is
at the anterior end, and behind it the wall of the sac
is thickened to form a strong muscular pharynx.

Figs. 17 to 20. — Fasciola hepatica. Five stages in the life-history.
(After Thomas.)

Fig. 17. — The free-swimming larva or ' miracidium.'

Fig. 18. — A sporocyst, containing developing rediae.

Fig. 1.9. — A young redia. The shaded area represents the digestive sac.

Fig. 20. — An adult redia, containing one daughter-redia, two cercariae
approaching maturity, and germs in various stages. The shaded
area represents the digestive sac.

Fig. 21. — A free cercaria.

The following letters have the same signification in the five figures: — C»
nearly ripe cercarise. CC, cystogenous cells of cercaria. DR, daughter-redia.
DT, limbs of the digestive tract. F, head-papilla. H, eye-spots. H', the
same degenerating. K', germinal cell. L, epaulet-like cells of first row of
ectoderm cells. M, embryo in optical section; gastrula stage. N, pharynx
of redia. O, digestive sac. OE, oesophagus of cercaria. p, lips of redia.
Q, collar of redia. R, processes of redia serving as feet. S, embryos between
the morula and stages. T, trabeculae crossing body-cavity of redia. u, cells
in redia supposed to be glandular. V, birth-opening, by which cercarisB and
daughter-redise escape from the redia. W, morula, still retained in the body-wall.
W', larger morula. Y , oral sucker of cercaria. Y', ventral sucker. Z, pharynx
of cercaria.

44 THE LIVER-FLUKE.

From the inner surface of the body-wall of the
redia, cells arise which develop into gastrulse as
in the sporocyst : these may become redise like the
parent, or may develop into cercarise. Both rediae
and cercarise may be formed in the same redia.

C. The Third Form is a Cercaria (fig. 21). These are not
necessarily the third generation, for several genera-
tions of rediee may intervene between them and
the sporocyst.

1. Within the parent redia the embryo develops a long tail

near its hinder end, an anterior sucker round the
mouth, and a posterior sucker on the ventral surface.
Its alimentary tract, which is at first solid, becomes
bifurcated to form the two limbs of the intestine,
the portion in front of the bifurcation forming a
pharynx and a short oesophagus. The rudiments
of the genital organs can be recognised. A single
redia may contain about twenty such cercarise at
one time.

2. The ripe cercarise, which measure nearly 1 mm. in

length, including the tail, escape from the redia by
an aperture just behind the collar. At first they
are very active, and work their way out of the snail.
As this snail, Limncea truncatula, is amphibious, they
may be set free either in water or on damp grass.
In either case they shortly lose their tails, and
encyst on grass or some other plant.

3. In the encysted condition the cercarise are swallowed

by sheep with the grass. They then again become
active, escaping from their cysts, and working their
way along the bile-ducts, where they grow rapidly,
and develop in about six weeks into sexually mature
flukes.

i-

45

Chapter IV.

THE TAPEWORM.

The tapeworm is a mouthless organism living when adult in
the alimentary canal of vertebrates. It consists of a chain of
segments. One end of the chain adheres to the lining of the
intestine by means of suckers and hooks, whilst the other end
lies freely in the cavity of the gut. The attached end is called
the ' scolex,' and this scolex buds off the segments, which
increase in size and complexity as they are pushed further and
further away from their point of origin by the interpolation
of new segments. Finally, the larger segments are detached
singly and are discharged with the faeces of the host.

Two common species inhabit the intestine of the dog.
The discharged segments of the one species, Dipylidium
caninum, adhere to the hairs around the dog's anus. A dog-
flea larva feeding on one of these segments introduces into itself
one or more eggs which develop into tailed larvae, or cysti-
cercoids. Should a dog when snapping at the fleas swallow
one containing a cysticercoid, the latter attaches itself to the
dog's intestine, becomes the scolex, and proceeds to bud off
segments, so producing the mature tapeworm or ' Strobila '
(pine-cone). The eggs from the mature segments of the
second species, Taenia serrata, are discharged on to the grass,
and if eaten by a rabbit develop into ' bladder-worms ' (see
p. 345) attached to the mesenteries or inhabiting the liver.
On being eaten by a dog, each bladder- worm excysts, attaches
itself to the dog's intestine and buds off segments as in the
first species. Thus the life-history of a tapeworm consists
of an egg, a larva, a cystic stage, and a segmented adult stage,
passed under different conditions and in different hosts.

F

46 THE TAPEWORM.

Nearly all dogs contain tapeworms, and isolated segments
are easily obtained alive after defalcation. They should be
examined after clearing in glycerine and also fixed with corrosive-
acetic mixture, washed in 70 per cent, spirit, and either stained,
cleared, and mounted or cut into transverse sections. To
obtain a complete chain of segments, and especially the scolex,
a post-mortem must be made of a freshly killed dog or cat.

1. General Structure of the Commonest Dog-tapeworm.

The * scolex ' or attached end is a mobile, muscular, conical
structure supporting four suckers and five circlets of
minute hooks, but with no mouth or sense-organs.
It is followed by the ' neck,' the region of greatest
growth continually producing ' segments ' or ' pro-
glottides.' There is no trace of an alimentary canal.

Each segment contains two sets of hermaphroditic reproduc-
tive organs, but as the male organs mature before the fe-
male ones, the appearance of the segments undergoes a
gradual change as they are pushed further and further
away from their origin in the neck-region.

At first each segment develops the male organs, at a later
stage of growth its female organs appear, still later the
female organs ripen, and lastly the organs consist of
fertilised eggs, the male and accessory female organs
having degenerated.

2. Reproductive System.

Examine segments which have been cleared or stained and
mounted.

The testes consist of minute follicles scattered over one
surface (so-called dorsal). They are united by exces-
sively minute canals into two coiled ducts — the vasa
deferentia. which open on the margin of the opposite
side of the segment, each by an eversible penis.

The female glands on each side consist of a deeply lobed
ovary lying internal to the penis and a yolk-gland
behind it.

BUTTERWORTHsa.

Fig. 21a.— Dipylidium caninum- The commonest dog -tapeworm.

1. — A tapeworm ( X 4 J).

2.— Scolexof same(x 78). (f) rostellum.

3.— Hook from rostellum(X 170).

4. — A mature proglottid (x 31). («) cirrus-sac
(n) lateral nerve ; (pv) ovary ; (<) testes ; (v)
yolk-gland.

5. — The shell-gland and surrounding ducts, (o) ovidact
gland ; («) uterus ; (v) vagina ; (yd) yolk duct.

6, — A detached ripe proglottid ( x 17). (tc) egg capsule?

7. — Egg capsule containing eggs (x 300).

8.— Egg(x 400).

(ex c) lateral excretory oanal ;
(vd) vas deferens; Q/'j)

(ov) ovary; (shg) shell-

48 THE TAPEWORM.

From the ovary arises the oviduct, connected with the
exterior by the vagina, a duct opening behind the
penis. The oviduct is joined by a duct from the yolk-
gland, the junction being marked by the presence of
numerous gland- cells, the shell-gland. From this
junction arises the uterus, a reticulated tube which
the eggs enter after being fertilised by spermatozoa
introduced by the vagina. In larger segments the
uterus extends greatly, but finally breaks down into
numerous capsules each containing a bundle of eggs.
The egg consists of two shells surrounding a small
embryo furnished with three pairs of hooks.

3. Excretory System.

This consists of the same type of organ as that found in
the Liver-fluke, namely ' flame-cells,' canaliculi, and
longitudinal canals. The canals run continuously
throughout the whole chain of segments.

4. Muscular System.

The longitudinal muscles run throughout the length of the
entire organism and break when one or more segments
are discharged. Circular and oblique fibres are also
present and can be easily seen in transverse sections.

5. Nervous System.

This consists of ganglia innervating the muscular scolex,
and of longitudinal lateral nerves that supply the
muscles of the segments. It is best seen in sections.

I

49

Chapter V.
THE LEECH. Hirudo medicinalis.

The leech is an elongated flattened worm, from three to five
inches in length, and provided with a muscular sucker at
each end. The body is marked externally by a series of
transverse constrictions dividing it into rings, or annuli, and
is capable of considerable elongation and contraction.

Leeches occur in freshwater pools and marshes in this
country, but far more abundantly on the Continent. They
either swim freely by vertical undulations of the body, or pro-
gress in a looping manner, attaching themselves alternately
by the anterior and posterior suckers. The leech lives on the
blood of higher animals : it lays its eggs in a cocoon, which
it buries in a hole in the bank of the pond it inhabits.

Leeches are ' segmented animals,' i.e. several of the organs
are repeated, usually in pairs, at regular intervals along part
or the whole of the length of the body. The segments or
somites, as indicated by the internal organs, are much less
numerous than the annuli, five of these latter corresponding
to each somite, except near the ends of the body.

This segmental arrangement affects markedly the nervous,
excretory, and reproductive systems, and to a less degree the
digestive organs.

Leeches intended for dissection should be hilled with chloro-
form.

I. EXTERNAL CHARACTERS.

1. The shape varies greatly with the degree of elongation
or contraction. The body is broadest a little way

50 THE LEECH.

behind the middle of its length, and is oval in trans-
verse section, the dorsal surface being more convex
than the ventral.

2. The annuli, or rings, into which the body is divided by a

series of transverse grooves, are about ninety-five in
number, and occur throughout the whole length of
the body except its extreme ends. They bear small
papillae, which are more obvious when the body is in
the contracted condition.

3. The colour differs greatly in different individuals. The

dorsal surface is darker than the ventral, and is
usually marked by three pale longitudinal stripes on
each side, the middle and outer of which are inter-
rupted by dark dots. These dots are markedly
larger, especially in the middle stripe, at every fifth
annulus, the annulus so marked being the hindmost
of a somite. The most anterior of the five annuli
making up a somite bears a transverse ring of small
white dots. Towards the two ends of the body the
somites are shorter and contain fewer annuli.

4. The suckers.

a. The anterior sucker is oval, with the longer axis

longitudinal. It is placed on the ventral surface
of the anterior end, and is cupped in the centre
to form the buccal cavity. It may be regarded
as formed by the fusion of the prostomium, a part
in front of the mouth, with the first two annuli of
the first somite.

The prostomial portion is very commonly
folded down over the mouth as a kind of lip.

b. The posterior sucker is circular, and larger than

the anterior one. It is composed of six segments
fused together, and is imperforate.

5. External openings.

a. The mouth is a funnel-like depression in the anterior
sucker, at the bottom of which are the three jaws.

EXTERNAL CHARACTERS. 51

b. The anus is a very small aperture on the dorsal
surface in, or just in front of, the constriction
separating the posterior sucker from the body.

c. The genital apertures. The leech is hermaphrodite :
the male and female apertures are separate, and
are both on the ventral surface in the median line.

i. The male aperture is a small hole with tumid
lips at the hinder border of the twenty-fourth
annulus, i.e. in the second annulus of the sixth
somite. From it the muscular penis is often
seen protruding.

ii. The female aperture is less conspicuous than
the male, and is placed one somite further
back, i.e. at the hinder border of the twenty-
ninth annulus, the second annulus of the
seventh somite.

d. The apertures of the nephridia, or excretory organs,

are minute paired openings on the ventral surface
of the body in the hindmost annulus of each
somite from the second to the eighteenth in-
clusive.

Hold the leech in your hand with the ventral surface up-
ward. Dry it with a cloth, and then gently squeeze it : minute
drops of fluid will exude from the nephridial apertures, which
are thus rendered clearly visible.

II. DISSECTION OF THE LEECH.

Stretch the leech slightly with the fingers and fix it under
water with the dorsal surface upward, putting the pins through
the sides of the suckers so as to avoid injury to the nerve-ganglia.

Make a longitudinal incision through the skin, a little to
one side of the middle line, taking care not to damage the ali-
mentary canal, which lies close beneath the skin. Carefully
dissect off the integument from the alimentary canal along the

52 THE LEECH.

whole length of the animal, and pin out the flaps right and
left. Wash thoroughly under the tap. Note at once the dorsal
blood-sinus, which runs along the dorsal wall of the alimentary
canal.

A. The Digestive System.

The alimentary canal of the leech runs straight from
mouth to anus. Along the greater part of its length it is a
wide tube, whose capacity is much increased by paired lateral
diverticula, and is further capable of great distension.

A leech can draw as much as three times its own weight
of blood, a great part of which is often spontaneously dis-
charged. The digestion of a full meal may take as long as
nine months in an adult animal.

Clean the dorsal wall of the alimentary * canal along its
whole length, talcing care not to damage the supra-oesophageal
ganglia. If the canal is empty, its walls are white and readily
distinguished ; if full, it appears red from the contained blood,
and is less easy to dissect. Wash frequently under the tap
during the dissection.

1. The mouth is a conical depression in the anterior sucker,
at the bottom of which are the jaws. These are three
laterally compressed muscular cushions arranged
radially, one being dorsal and anterior, and the other

Fig. 22. — Hirudo medicinalis, seen from the ventral surface. (After
A. G. Bourne.)

The numbers 1 to 23 indicate the somites ; the numbers I to V the five
annuli of the twelfth somite.

A, anterior sucker, with the mouth. B, posterior sucker. G, male aperture.
H, female aperture. N, nephridial apertures.

Fig. 23. — Hirudo medicinalis. A diagrammatic figure of the renal, ner-
vous, and reproductive systems. The animal has been opened along
the mid-dorsal line, pinned out, and the alimentary canal removed.
(After A. G. Bourne.)

The somites are numbered 1 to 23, and their boundaries are indicated by
the dotted lines.

CG, supra-cesophageal ganglion. EP, left epididymis. G 1, the first of
the twenty-three post-oral ganglia. GL, glandular enlargement of the oviduct,
L, lateral vessel. LA, latero-ventral branch of lateral vessel. |_D, latero-
dorsal branch. |_L, latero-lateral branch. N 1, the first of the seventeen
nephridia of the left side, o, nerve-collar. OV, ovisac containing the left
ovary. PE, penis. T 4, the third testis of the left side. VD, vas deferens.

DIGESTIVE SYSTEM.

53

Fig. 22

54 THE LEECH.

two ventrolateral : each cushion is covered by a thin
chitinous cuticle, which is thickened along the free
edge of the jaw, and notched into the sharp teeth
by which the bite of the leech is produced. The
mouth leads, by a very small aperture, into the
pharynx.

Snip away the margin of the anterior sucker so as to expose
the jaws. Examine them from the ventral surface with a pocket
lens : remove one of them and examine it with a low power of
the microscope to see the teeth.

2. The pharynx is an oval sac with very muscular walls.

It is connected with the body- wall by strong radial
muscles, which give it a villous appearance, and by
their contraction dilate its cavity and produce a
sucking action.

3. The salivary glands are very large granular pyriform

cells surrounding the pharynx. Each cell is a

gland in itself, and is produced into a long stalk or

ductile opening on one of the jaws. The secretion

has the power of preventing coagulation of the

blood, and so very greatly facilitates the act of

sucking.

Remove part of the wall of the pharynx, and tease it on a

slide in salt-solution. The large pyriform gland-cells, with

their long ductules, will easily be seen with a low power.

4. The oesophagus is a short narrow tube leading from the

pharynx to the crop.

5. The crop consists of eleven chambers, separated by

partitions, each perforated by a circular or trans-
versely oval opening, surrounded by a sphincter
muscle. The partitions may be demonstrated by
cutting transversely across the dorsal face of one
of the larger posterior chambers, and pulling the
lips of the cut away, so that the partitions lie
nearly horizontal, and their perforations are clearly
seen. From each chamber a pair of lateral sacs,

THE (XELOM. 55

the caeca, are given ofi. They slope more and
more backwards as we pass down, and the two
last are parallel and contiguous to each other,
and to the intestine which lies in a dorsal groove
above them.

Slit up the crop along the mid-dorsal line, wash out its
contents thoroughly, and pass a seeker into the several diver-
ticula of one side, slitting them open along their whole length.

6. The stomach is a small spherical slightly bilobed

dilatation immediately behind the crop, lying be-
tween the basal portions of the backwardly directed
last pair of diverticula, and opening behind into the
intestine.

7. The intestine is a narrow straight tube running from

the stomach to the anus. Its inner wall projects
as a spiral fold into the cavity.

Slit open the intestine with scissors along the mid-dorsal
Une ; wash it thoroughly, and note the spiral folding of its
wall.

B. The Ccelom.

In the leech there is no obvious body-cavity, the space
between the integument and the alimentary canal being filled
up by muscle, connective tissue, and other structures.

A complicated system of tubular channels runs through
all parts of the body, and is filled with a red fluid, the blood,
which contains numerous colourless corpuscles. The prin-
cipal channels are of two kinds, some having muscular,,
others non-muscular walls. Both sets of tubes are parts of
the original body-cavity, or ccelom, which is in some kinds of
leeches more extensive, but is in Hirudo reduced to a system
of canals the two lateral ones of which have acquired mus-
cular walls. There are no true blood-vessels.

The arrangement and relation of these sinuses are best
determined by the microscopical examination of sections,
and will be described later on ; the broad features, most of

56 THE LEECH.

which can be made out without difficulty by dissection, are
as follows :

1. The Sinuses, or non-contractile parts of the ccelom.

a. The dorsal sinus is a longitudinal tube running

along the dorsal surface of the whole length of
the alimentary canal. It communicates pos-
teriorly with the ventral sinus by fine vessels
which pass between the intestine and the terminal
diverticula of the crop. It has already been seen
(p. 52).

Dissect away the alimentary tract completely, from the
oesophagus backwards, taking great care not to damage the
underlying organs. The further dissection is greatly facili-
tated by putting the leech into strong spirit for ten minutes
or so.

b. The ventral sinus is a median tube running along

the body beneath the alimentary canal. Within
it the ventral nerve-chain lies. It communicates
with a series of peri-nephrostomial sinuses,
lying on the dorsal surfaces of the testes ; and
also with the capillary system on the walls of
the crop.

2. The contractile parts of the ccelom.

a. The lateral * vessels ' are a pair of longitudinal

tubes running in a sinuous course along the
sides of the body. They are connected together
by transverse vessels which pass across beneath
the ventral sinus. They send branches to the
intestine, the nephridia, and the reproductive
organs.

b. The cutaneous plexuses are very abundant, and

connect the sinuses and lateral tubes in various
places, putting them in free communication with
one another. The microscopical characters will
be described later on.

EXCRETORY SYSTEM. 57

C. The Excretory System.

The excretory organs of the leech consist of a series of
seventeen pairs of nephridia or segmental organs, lying in
the somites from the second to the eighteenth inclusive. They
he at the sides of and below the alimentary canal, and open
to the exterior by paired apertures on the ventral surface.

1. Structure of a nephridium (figs. 23 and 24).

Each nephridium is a cellular rod, thicker in the
middle than at the ends, and bent on itself so as
to form a loop, the two limbs of which are anterior
and posterior respectively. The looped portion lies
at the side of the alimentary canal, extending up
into the dorsal part of the body ; the ends he in the
ventral region, that of the anterior limb being in
most cases prolonged inwards almost to the median
plane, and that of the posterior limb being rolled up
on itself. From the anterior limb a duct runs back
to a vesicle with muscular walls, which opens to the
exterior on the ventral surface of the body.

The cellular rod is traversed by a complicated
system of ducts, which perforate the individual cells,
and receive minute ductules ramifying in the sub-
stance of the cells.

The whole nephridium is invested by a pig-
mented connective-tissue capsule, and receives a
very abundant supply of blood from branches of the
lateral tubes, the efferent stream passing inwards
through the peri-nephrostomial sinus to the ventral
sinus.
Dissect carefully one of the nephridia about the middle of

the body, noting its several parts, and their relations to one

another and to other organs.

Remove another nephridium from the body, and dissect it

on a slide in a drop of water ; examine it with low and high

powers of the microscope.

Snip off with fine scissors the dorsal half of one of the

testes, with the testis-lobe of the nephridium ; mount it on a

58 THE LEECH.

slide in a drop of salt solution, and examine with low and high
powers.

The entire nephridium consists of the following
parts :

a. The testis-lobe forms the ventral half of the anterior

limb of the loop. It is a cylindrical rod, the
inner end of which lies' in the peri-nephrostomial
sinus on the dorsal surface of the testis. This
end is slightly enlarged, and has a cauliflower-
like appearance, the cells covering it being spongy
and ciliated, and often depressed at their ends.

This inner end of the testis-lobe probably
corresponds to the funnel-like opening of the
nephridium into the body-cavity in the earth-
worm (p. 27).

b. The main lobe is the thickest part of the rod : it is

continuous with the outer end of the testis-lobe,
and forms the anterior limb, the bend, and half
of the posterior limb of the loop.

c. The apical lobe is separated by a slight constriction

from the posterior limb of the main lobe. It runs
downwards and inwards, diminishing slightly in
thickness, and 'ends in a coiled-up extremity, the
apex of which is closely attached to the junction
of the testis-lobe and the main lobe.

d. The vesicle-duct is a short narrow tube arising

from the anterior limb of the main lobe at its
junction with the testis-lobe, and running back-
wards beneath the apical lobe to the vesicle.

e. The vesicle is a somewhat flattened sac with

muscular walls : it lies behind the rest of the
nephridium, and opens to the exterior by a short
duct, the opening being on the ventral surface of
the body and in the hindmost annulus of the
somite in which the nephridium lies.

D. The Reproductive System.

Leeches are hermaphrodite, but do not impregnate their
own ova.

REPRODUCTIVE SYSTEM; 59

1. The male organs.

a. The testes are nine pairs of spherical sacs lying
in the somites from the eighth to the sixteenth
inclusive. They are situated in the ventral region
of the body at the sides of the ventral sinus.
Each pair lies mainly in the second, but partly
in the first annulus of the somite in which it

occurs.

The segmental repetition of the testes is note-
worthy. Sometimes a tenth and even an eleventh
pair of testes may be present, and occasionally
the numbers are unequal on the two sides of the
body.
Remove- one of the testes, and tease it on a slide in salt

solution. Cover, and examine with low and high powers.

Stained preparations can be made by following the directions

given on page 82.

Spermatozoa in various stages of develop-
ment will be seen.

b. The vasa efferentia are a series of short sinuous

ducts which run outwards from the testes, and
open into the vasa deferentia.

c. The vasa deferentia are a pair of longitudinal tubes

which run forwards in a sinuous course along
the ventral body-wall, about midway between the
testes and the vesicles of the nephridia.

In front of the first pair of testes they continue
their course through the seventh somite, and in
the sixth enter the epididymes.

d. The epididymes are a pair of convoluted tubular

bodies with thick white walls lying in the sixth
somite. They secrete a substance by which the
spermatozoa are aggregated into spermaphores.
From the inner side of each a short duct leads
to the base of the penis, the duct of the right side
passing under the nerve-chain.

e. The penis is a pyriform body lying in the sixth

60 THE LEECH.

somite. It is wide at its base, narrow and tubular
in its distal part, and can be protruded some dis-
tance through, the male genital aperture in the
second annulus of this somite.

The female organs lie entirely in the seventh somite.

a. The ovaries are a pair of minute filamentous bodies,

enclosed in small spherical sacs, lying close to
the mid- ventral line.

b. The oviducts are a pair of slender tubes leading

from the sacs of the ovaries, which unite to form
a much-convoluted duct surrounded by uni-
cellular-gland cells, and opening into the vagina.
The right oviduct passes under the nerve-chain.

c. The vagina is a thick-walled tube, opening to the

exterior at the female genital pore in the second
annulus of the seventh somite.

E. The Nervous System.

The nervous system consists of a pair of supra- oesophageal
ganglia, a nerve-collar, and a double gangliated cord ex-
tending the whole length of the ventral body- wall. From the
ganglia nerves run outwards to the various parts of the body.

1. The supra-cesophageal ganglia are a pair of closely

apposed bodies in the dorsal wall of the anterior
end of the pharynx, immediately behind the jaws.
They supply the jaws, the eyes, and other parts of
the anterior end of the body.

2. The nerve-collar is very small, and closely surrounds

the anterior end of the pharynx.

3. The ventral nerve-chain lies in the ventral- sinus, which

must be slit open to expose it. It consists of two
halves very closely apposed in the median plane.
There are twenty-three pairs of ganglia, lying in
the first annuli of their respective somites. The
successive pairs of ganglia are much closer together
at the two ends of the body than they are along the
middle portion of its length.

TRANSVERSE SECTIONS. 61

The first, or sub-cesophageal ganglia, are really
multiple, and give off five pairs of nerves.

The last, or twenty-third, are also multiple, and
give several branches to the posterior sucker.

From each of the other pairs of ganglia two pairs
of nerves are given off, supplying the corresponding
somite.
F. Sense-organs.

1. Eyes. The leech has ten eyes, seen as minute black dots,
arranged round the dorsal edge of the anterior
sucker. Each consists of a pigmented cup, filled
with rod-like bodies, and receiving a nerve at its
base. The eyes differ very little from smaller cup-
like segmental sense-organs found on the first
annulus of each segment throughout the body.
Their structure can only be made out by examination
of microscopical sections.

III. EXAMINATION OF TRANSVERSE SECTIONS.

Many points in the anatomy of the leech can only be
determined by examination of microscopical preparations,
and for this purpose transverse sections of the body are the
most instructive. Of these half a dozen or more should be
examined, as the appearances will necessarily differ according
to the particular organs and parts through which the section
happens to pass.

The following description is a general one, and will apply
to any section through the middle region of the body.

A. Shape.

A transverse section of the body of a leech is oval in out-
fine, the transverse diameter being the greater, and the ventral
surface being flatter than the dorsal.

B. The Integument.

1. The cuticle. The most superficial layer is a thin struc-
tureless elastic cuticle, which is secreted by the
underlying^epidermis, and is shed and renewed at
frequent intervals during fife.

G

62 THE LEECH.

2. The epidermis consists of a single layer of columnar

nucleated cells, wider at their outer than at their
inner ends.
a. The epidermal glands. Certain of the epidermal
cells are modified to form unicellular glands.
These may remain in the epidermal layer as
mucous glands, or they may sink down into the
muscular layers, as in the case of the salivary
glands, and of the clitellar glands of the fifth
to the seventh somites, which latter secrete the
cocoon in which the eggs are laid.

In all cases each cell is a complete gland in
itself, and preserves its communication with the
surface by means of a long slender ductule.

3. The dermis is a layer of some thickness lying between

the epidermis and the muscular coats. It consists of
a jelly-like matrix containing numerous branched
corpuscles. It is traversed by pigmented fibres,
which branch and anastomose very freely, and pene-
trate between the epidermal cells : to these the colour
of the skin is due. There are also in the dermis
irregularly arranged muscle-fibres, chiefly transverse
in direction, and a very abundant capillary plexus,
the branches of which penetrate between the inner
ends of the epidermal cells. It is by means of this
cutaneous capillary system that respiration is
effected.

C. The Muscles.

The muscles of the leech consist of long fusiform cells
arranged either singly or in bundles. Each muscle-cell con-
sists of an outer cortical layer which is striated longitud'nally,
and a central medullary portion composed of granular pro-
toplasm and containing the nucleus. The cortical layer is
sharply marked off from the medullary portion, so that in a
transverse section of a muscle-cell it appears as a ring, radially
striated, and surrounding a central finely granular mass.
1. The outer or circular layer of muscles consists of fibres,

THE ALIMENTARY CANAL. 63

which encircle the body transversely. The layer
itself is not much thicker than the epidermis, but
the transverse muscles of the dermis should, per-
haps, be grouped with it.

2. The middle or oblique layer of muscles is -more deeply

placed, and is separated from the circular layer by
a stratum of connective tissue. The fibres run
obliquely round the body.

3. The inner or longitudinal layer of muscles lies im-

mediately within the oblique layer, and is by far the
thickest of the three layers.

4. The dorso-ventral muscles are bundles of muscle-fibres

crossing the body more or less obliquely from the
dorsal to the ventral surface. They pass between
the bundles of longitudinal muscles and through the
oblique and circular layers, their ends spreading out
just beneath the epidermis.

D. The Alimentary Canal.

A section across the middle portion of the body will cut the
crop, of which the median portion and the lateral diverticula
will in most sections appear separate from each other.

The walls consist of an outer connective-tissue layer with
a few muscular fibres, and an inner lining formed of a single
layer of short columnar epithelial cells. The folds into which
the inner surface is thrown involve the connective-tissue layer
as well as the epithelium.

E. The Lateral Tubes and Sinuses.

The blood-spaces and lateral tubes correspond to the
body-cavity of higher animals. They are readily recog-
nised in sections, owing to the deeply staining clotted blood
with which they are filled.

1. The dorsal and ventral sinuses he in the median plane,
above and below the crop respectively. They have
thin non-muscular walls, and the ventral one encloses
the nerve-chain.

64

THE LEECH

REPRODUCTIVE ORGANS. 65

2. The lateral tubes are large and have distinct muscular

walls.

3. The capillaries are very thin-walled tubes, the distribu-

tion of which can easily be followed owing to the
contained blood. The intercellular plexuses of the
epidermis and of the nephridia should be specially
noticed.

4. The botryoidal tissue consists of a network of vessels,

, the channels of which are irregular in width and
the walls formed of large granular pigmented cells
which project as irregular rounded swellings.

These curiously swollen vessels occur in great
abundance around the crop, immediately within the
longitudinal muscle-layer. They are in free com-
munication with the ordinary blood-capillaries.

. The Reproductive Organs.

1. The testes are seen, if the section happens to pass

through them, as spherical sacs lying at the sides
of the ventral sinus, and containing groups of
spermatozoa in various stages of development.

2. The vasa deferentia lie to the outer sides of the testes :

each is a thick-walled tube, with a small central
lumen.

Fig. 24. — Hirudo medicinalis. A diagrammatic transverse section
through the middle of the body. On the left side of the figure the
whole of a nephridium is drawn in order to show its relations : the
anterior surface of the section is towards the spectator, and the
two limbs of the main lobe of the nephridium are slightly displaced,
being represented as inner and outer, instead of anterior and pos-
terior respectively, (a. m. m.)

A, crop. AL, apical lobe of nephridium. AM, anterior limb of main lobe
of nephridium. B, right diverticulum of crop. C» left diverticulum of crop.
CU, cuticle. D, epidermis. DS, dorsal sinus. E, layer of circular and oblique
muscles. F, longitudinal muscle-layer. G, cauliflower head of nephridium.
I, vesicle of nephridium. K, external aperture of nephridium. |_V, lateral
tube. N, nerve cord, o, testis with developing spermatozoa. P, vas deferens.
PM, posterior limb of main lobe of nephridium. PS, peri-nephrostomial sinus.
R, section of nephridium. S, intracellular ductules of nephridium. TL, testis-
lobe of nephridium. VS, ventral sinus. X, botryoidal tissue. Y, dorco-
ventral muscles.

m

Chapter VI.

THE EARTHWOEM. Lumbricus terrestris.

The earthworm lives in burrows in moist earth, through
which it makes its way by alternate elongation and contrac-
tion of its body, aided by small stiff setae projecting from its
surface. That part of the body most exposed to light is
dark ; the rest is almost colourless.

It is a pinkish worm about six inches long, cylindrical in
front, flattened posteriorly. Its food consists of organic
matter derived chiefly from the soil, which it swallows in
enormous quantities. The worm is hermaphrodite, and lays
its eggs in a chitinous cocoon, which also contains packets of
spermatozoa and an albuminous fluid.

The body is marked by transverse grooves dividing it into
annuli, of which there are about one hundred and fifty. This
external annulation corresponds with an internal segmenta-
tion, the body-cavity being divided into chambers by trans-
verse septa, which are placed opposite the external grooves.
Nearly all the organs exhibit a corresponding segmental
arrangement, there being one pair of nephridia and one
nerve-ganglion in each segment or somite throughout the
greater part of the animal's length, the blood-vessels, muscles
and setae being also disposed segmentally.

Two closely allied genera of large earthworms, Lumbricus
and Allolobophora, occur commonly in this country, each
represented by several species.

The several species differ from each other in the position
and arrangement of certain of the reproductive organs, and
in other minor points. The following description, taken from

EXTERNAL CHARACTERS. 67

Lumbricus terrestris, the largest of the commoner species,
will, however, apply with but slight alterations to any of the
others.

The worm is most conveniently killed by chloroform, or
by warm corrosive sublimate, and kept in 2% Formalin.

I. EXTERNAL CHARACTERS.

1. Form. The body is cylindrical along the greater part of

its length, flattened in its hinder part. It is pointed
in front, blunt behind, and is thickest about one-
third of its length from the anterior end.

2. Colour. The earthworm is of a pale pinkish or yellowish

colour, replaced in front by a dark purplish brown,
which extends a considerable distance backwards as
a mid-dorsal stripe. The hinder end is also tinged
with brown. The dorsal blood-vessel can be seen
through the integument, appearing as a dark-
coloured wavy line along the dorsal surface.

3. The annuli, into which the body is divided externally

by transverse grooves, are larger and more obvious
in front than behind. Each annulus corresponds
to a segment, and is subdivided by smaller grooves,
the number of which varies in different regions of the
body, and also on the dorsal and ventral surfaces.

4. The prostomium, or cephalic lobe, is the rounded lobe

forming the extreme anterior end of the worm. In
Lumbricus it extends backwards on the dorsal sur-
face so as completely to intersect the first annulus
and meet the second. In Allolobophora it extends
about halfway through the first annulus.

5. The clitellum, or cingulum, is the thickened integument

of the dorsal and lateral surfaces of the thirty-second
and five following segments, but varies slightly both
in position and extent in different specimens and at
different times of the year. In other species of earth-
worm it occupies other segments.

68 THE EARTHWORM.

6. The cuticle is a delicate chitinous membrane investing

the whole body of the worm. It may usually be
detected by its iridescence, and can be readily
stripped off a worm which has been killed by
chloroform, or one which has been macerated a few
hours in water.

Examined microscopically it shows a series of
oblique lines, intersecting each other at right angles,
and minute highly refractive thickenings at many of
the points where the lines intersect.

7. The setae are short chitinous spines implanted in the

body-wall, and arranged in two double rows along
each side of the body, each segment, except the first
and last, bearing four pairs. They may be felt on
drawing the worm through the fingers from head to
tail. The setae are to be regarded as special localised
thickenings of the cuticle.

The setce are easily isolated by boiling a bit of a worm in
a solution of caustic potash ; they should then be washed and
mounted in glycerM, or dried and mounted in balsam.

Each seta is sigmoidally curved : it is thickest at
its middle, and tapers towards each end.

8. External apertures.

a. The mouth is a small crescentic aperture on the

ventral surface of the first segment. It is over-
hung by the prostomium.

b. The anus is an oval aperture at the hinder end of

the body.

c. The genital apertures are arranged in four pairs.

i. The apertures of the vasa deferentia are situated
on the ventral surface of the fifteenth segment,
[ one on each side, slightly to the outer side of
the inner double row of setae. They are trans-
verse slits, with tumid lips.

EXTERNAL CHARACTERS. 69

ii. The apertures of the oviducts are similarly
situated in the fourteenth segment. They are
small pores not seen so easily as the male
apertures, their lips not being tumid.

iii. The apertures of the spermothecse or receptacula
seminis, of which there are two pairs, are
situated in the grooves between the segments
and in the line of the outer row of setae, the
first pair between segments ix. and x., the
posterior pair between segments x. and XI.

d. The excretory apertures are best seen in a specimen
which has been hardened in spirit, and dried with
a cloth immediately before examination. Each
segment, except the first three and the last, has
one on each side, either on the ventral surface a
little in front of the outer seta of the inner double
row, or, in some segments, high up on the side
of the animal.

To see these apertures, strip off the cuticle from the anterior
part of the worm, and examine the worm with a pocket lens,
squeezing it slightly so as to cause fluid to exude from the

e. The dorsal pores, though small, are often visible to
the naked eye. They open mid-dorsally in the
grooves between the segments, and place the
ccelom in communication with the exterior.

The first one is between segments x. and xi.
and opens into segment xr. Behind this point
they occur regularly in every intersegmental
groove. »

Make a drawing to show the external characters. This is
best done by dividing two facing pages of a large note-book
transversely into three spaces by two lines half the width of a
page apart. Continue the lines right across from one page to
the other, thus giving the centre lines for the drawings. Now
rule about 35 lines, a quarter of an inch apart, across the centre

70 THE EARTHWORM.

lines to represent the septa ; and draw the anterior part of the
worm from below about one line, leaving the sketch of the dissection
to be drawn about the other.

II. DISSECTION OF THE EARTHWORM.

Extend the animal under water with the dorsal surface
upwards, fixing it down by two pins through the sides of the
fourth segment and two near the hinder end. Cut through
the body-wall from end to end, close to the mid-dorsal line.
Gently raise the flaps with the forceps, and note the trans-
verse septa dividing the body-cavity into segments. Carefully
pin out the flaps right and left with fine pins, stretching them
well as you do so.

It will save much time in determining the positions of
some of the important organs if the pins are placed in definite
segments — say the tenth, fifteenth, and twentieth.

A. The Ccelom.

The ccelom, or body-cavity, is a large space, within which
the alimentary canal and other viscera He. It is divided into
segments by transverse septa, which connect the alimentary
canal with the body- wall. The septa correspond to the ex-
ternal annuli throughout the whole length of the body, except
in two regions : (1) in front of the fourth segment, where they
are absent ; (2) in the region of the gizzard, where they are
slightly irregular in arrangement. The ccelom contains a
fluid with numerous amoeboid corpuscles.

B. The Digestive System.

The alimentary canal is a straight tube running from the
mouth to the anus, and is divisible into the following regions :

1. The buccal cavity is small, and is situated in the first
three segments. The opening of the mouth in the
ventral part of the first segment has soft lips and
no teeth.

DISSECTION.

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72 THE EARTHWORM.

2. The pharynx, which follows the buccal cavity, has thick

muscular walls connected with the body-wall by
strands of muscle which run obliquely backwards
perforating the anterior septa. The pharynx
extends backwards about as far as the sixth or
seventh annulus when the animal is in an average
condition of extension ; but it lies entirely in front
of the septum dividing the fifth segment from the
sixth (fig. 25),

3. The oesophagus is a concertina-like tube extending from

the pharynx to the thirteenth or fourteenth segment.

a. The oesophageal pouches are a pair of short lateral

diverticula of the oesophagus lying in segment x.,
and opening into the oesophagus at the anterior
end of segment xi. At certain seasons they con-
tain concretions of more or less crystalline cal-
careous matter, chiefly calcium carbonate.

To see these oesophageal pouches, press aside the large
white vesiculce seminales which hide them.

b. The oesophageal glands are two pairs of lateral

protuberances on the sides of the oesophagus in
segments xi. and xn. They are hollow, and their
cavities, which are subdivided by a large number
of horizontal lamellae, contain a milky calca-
reous fluid. They are local thickenings of the
glandular walls of the oesophagus, and their
numerous cavities open into the oesophageal
pouches.

C. Both the glands and the pouches are highly vascular
and play an important part in relation to the
lime of the soil and possibly also in regard to
respiration.

4. The crop is a large dilated portion of the alimentary

canal in segments xm. to xvi., but when filled it
encroaches upon the segments in front, pushing the
septa before it (fig. 25).

CIRCULATORY SYSTEM. 73

5. The gizzard extends from the sixteenth segment back-
wards to about the twentieth. Its walls are very
thick and muscular, and it has a thick chitinous
lining.

6 The intestine is a thin-walled sacculated tube running
straight backwards from the gizzard to the anus.

a. The * yellow ' cells are a layer of peritoneal cells
surrounding the intestine and in close relation
with the blood-vessels.

Cut the intestine open along one side, and wash out its
mtents.

b. The typhlosole is a prominent median longitudinal

ridge projecting into the cavity of the intestine
from its dorsal wall, of which it is a fold. Its
surface is marked by transverse folds.

C. The Circulatory System.

. The blood is a fluid coloured bright red by haemoglobin
and containing colourless corpuscles. Several of the larger
vessels contract rhythmically, especially the five pairs of
' commissural ' vessels. The circulation is seen best in a worm
soon after hatching out.

For the dissection of the circulatory system a fresh worm
should be taken and killed with chloroform. Owing to the
1 yellow cells ' overlying the alimentary canal, the dorsal vessel
is not easily traced.

1. The dorsal vessel is a large median vessel running along
the dorsal surface of the alimentary canal through-
out its whole length, and dividing in front into
branches which ramify over the pharynx. It can be
seen through the skin in the living animal, and the
flow of blood in it is from behind forwards.

A large lateral oesophageal vessel runs along
each side of the oesophagus, and communicates with
the dorsal vessel in the tenth segment. Branches of
it ramify over the pharynx and over the oesophageal
glands and pouches.

74 THE EARTHWORM.

From the alimentary canal the blood is returned
to the dorsal vessel by intestinal vessels, of which
there are in the region of the intestine two pairs to
each segment, and which are surrounded by the
1 hepatic cells.'

2. The ventral vessel is a median vessel running back-

wards below the alimentary canal the whole length
of the animal : in it the blood flows backwards.

3. The contractile * commissural ' are five pairs lying in

segments vn. to xi., and connect the dorsal with the
ventral vessel. During life they are rhythmically
contractile, the contractions passing along them from
above downwards, commencing with the hindmost
pair, and driving the blood from the dorsal to the
ventral vessel. In the fresh state each has fre-
quently a moniliform appearance.

4. Neural vessels. These are smaller longitudinal vessels

in close relation with the nerve-cord which runs
along the floor of the body-cavity in the mid- ventral
line (see p. 84).

a. The subneural vessel is median, and lies immedi-

ately beneath the nerve-cord along its whole
length.

b. The lateral neural vessels are a pair of smaller

longitudinal vessels lying along the sides of the
nerve-cord.

5. The nephridial vessels supply the nephridia or excretory

organs (see p. 76).

a. The afferent nephridial vessels arise from the

ventral vessel, one pair in each segment, and run
. outwards to the nephridia, supplying all three
loops.

b. The efferent nephridial vessels return the blood

from the nephridia to the parietal vessels.

EXCRETORY SYSTEM. 75

6. The parietal vessels. In each segment the subneural
and dorsal vessels are connected by a pair of parietal
vessels, which run on the inner surface of the body-
wall, and receive smaller vessels from it.

D. The Excretory System.

This consists of a series of tubular nephridia or ' segmental
organs,' arranged in pairs corresponding to the segments
along the whole length of the animal, excepting the first
three and the last segments, from which they are absent.
They are largest in the region'of the oesophagus.

Each nephridium is a long tube thrown into loops, which
are closely applied together and suspended in the body-cavity
close to its wall. The tube commences with a minute ciliated
mouth — the nephrostome — opening into the body-cavity ; then
follows a long and very slender portion, to which succeeds a
shorter and wider part ; the terminal portion is still wider
and has muscular walls, and opens distally to the exterior at
the aperture already seen on the surface.

1. Examination of a nephridium in situ (fig. 27).

The worm should be opened immediately after being killed
with chloroform and dissected in salt solution. Stretch the
segments thoroughly and examine with a hand-lens.

The nephridia are seen as semi-transparent
loops lying along the inner surface of the body-wall,
and extending from near the mid-ventral fine almost
to the mid-dorsal fine. Each is in relation with
two segments, the funnel and a very short length of
the tube lying in the anterior of the two segments,
and the rest of the tube with the external opening in
the posterior segment.

Examine one of the nephridia more closely, noting its parts.

a. The nephrostome is a minute funnel lying in the
body-cavity close to the mid-ventral line, and

76 THE EARTHWORM.

connected by a short stalk — the first part of the
nephridial tube — with the septum behind it.

b. The looped portion of the tube lies in the segment
behind that containing the funnel. Three loops
can easily be made out, of which the middle
one is the longest and reaches almost to the
mid-dorsal line ; while the outer one is the
widest and leads to the external aperture.

The three loops may be conveniently distin-
guished (see fig. 27) as the long loop, the short
loop, and the terminal loop respectively.

The terminal portion always enters the body-
wall at about the level of the outer seta of the
inner double row. The external pore is con-
nected with it, in those segments in which it
opens dorsally, by a channel in the body-wall.

2. Microscopical structure of a nephridium.

Carefully remove by fine-pointed forceps an entire nephri-
dium with a very small part of the septum which it traverses.
If the worm has been killed with chloroform, the ciliated portions
of the nephridium may be still working. Examine in salt
solution.

Examine with low and high powers,

a. The funnel is ciliated, its border consisting of a row

of large cells with very distinct nuclei. The
nephrostome or open mouth of the funnel can
be easily seen : it is often partially blocked up
by a small mass of ccelomic corpuscles.

b. The first part of the tube, or « narrow tube,' is very

slender, and its relations are difficult to deter-
mine. Starting from the funnel it runs straight
back through the septum into the posterior of the
two segments with which the nephridium is con-
nected. In this segment it runs in a somewhat
irregular course along both limbs of the short
loop, and along the proximal limb of the long

EXCRETORY SYSTEM. 77

loop ; it then bends back sharply on itself, and
returns, following the same path, to the base of
the short loop, where it passes into the second
part of the tube. The duct of the * narrow tube '
is an intracellular one, traversing the individual
cells, which are placed end to end like drain-pipes.
The * narrow tube ' is ciliated along part, but not
the whole of its length.

c. The second part of the tube, or * middle tube,'

forms the proximal limb of the long loop. The
duct is here also an intracellular one : it is much
wider than that of the ' narrow tube,' and is
ciliated along its whole length.

d. The third part of the tube, or 'wide tube,' com-

mences with a dilated part of the ampulla, at
the apex or bend of the long loop : beyond this
it forms the distal limb of the long loop, both
limbs of the short loop, and a portion of the
proximal limb of the terminal loop. The duct
in this portion of the tube is wide, intracellular,
and non-ciliated ; and its walls consist of very
granular protoplasm.

e. The fourth part of the tube, or 'bladder,' forms

part of the proximal, and the whole of the distal
limb of the terminal loop. It is much wider than
the rest of the tube ; it is lined by epithelium ;
and its walls, which are non-glandular, contain
interlacing muscular fibres. The ' bladder ' opens
to the exterior by the aperture already seen.
Its cavity contains parasitic round-worms, not
to be mistaken for cilia.

E. The Reproductive System.

The earthworm is hermaphrodite. The reproductive
organs are lodged in segments ix. to xv., i.e. in the region of
the oesophagus.

H

78

THE EARTHWORM.

The female organs consist of a pair of ovaries ; a pair of
oviducts, with a receptaculum ovorum opening into
each ; and two pairs of spermothecse.

a. The ovaries, which lie in segment xnr., are a pair
of small white pyriform bodies attached to the

Fia. 26. — Lumbricus herculeus. Plan of the reproductive organs as
seen from above after removal of the alimentary canal. The body-
wall is pinned down flat. (c. H. h.)

A, anterior lateral vesicula seminalis. B, first median vesicula seminalis.
O, posterior lateral vesicula seminalis. N , nerve-cord, o, ovary. OD, oviduct.
R, receptaculum ovorum. S, spermotheca. SF, seminal funnel lying in the
median vesicula seminalis. T, testis. VD, vas deferens : the reference line
indicates the point where the posterior vas efferens opens into it. 9 to 1 5, ninth
to fifteenth segment.

REPRODUCTIVE SYSTEM. 79

septum forming the anterior wall of the segment,
and hanging freely from it into the ccelom below
the oesophagus.
To see the ovaries and oviducts stretch the body-wall trans-
versely and longitudinally, press the oesophagus to one side,
and look for them with a hand lens.

b. The oviducts are a pair of short tubes, the wide

mouths of which are placed in the hinder part of
segment xiii., one opposite each ovary.

From the points where they perforate the
septum, the oviducts run outwards and slightly
backwards to their external openings on the
ventral surface of segment xiv.

c. The receptacula ovorum are a pair of reniform sacs,

attached to the anterior wall of segment xiv. just
above the oviducts, into the upper part of the
funnel-like mouths of which they open.

d. The spermothecae or receptacula seminis are two

pairs of globular sacs lying in the hinder parts

of segments ix. and x. respectively. Their ducts,

which are short, run backwards and open to the

exterior in the grooves between the ninth and

tenth, and the tenth and eleventh segments,

respectively, just within the line of the outer

double row of setae.

1 In these the seminal fluid received from

another worm is stored. The spermatozoa are

[ aggregated into ■ spermatophores ' by the glan-

rdular apertures of the spermothecae and in this

[state are fixed to the body of another worm.

In Allolobophora, and exceptionally in Lum-
bricus, the spermothecse are situated in segments
x. and xi., their ducts passing forwards to open
in the same position as above.

2. The male organs consist of two pairs of testes ; two
pairs of seminal funnels, leading into a pair of vasa

80 THE EARTHWORM.

deferentia ; and median and lateral vesiculae semi-
nales.

a. The vesiculae seminales vary greatly in form and

size with the season of the year. When fully
developed they form conspicuous milk-white or
pinkish bodies at the sides of the oesophagus, and
overlapping this.

i. A pair of anterior lateral vesiculae seminales lie

in segment ix. They are opaque white bodies
attached to the posterior wall of the segment,
one at each side of the oesophagus.

Some earthworms have a similar pair in
segment x., but these are not developed in
Lumbricus.

ii. Two pairs of posterior lateral vesiculae seminales

lie in segments xi. and xn. respectively. They
are very much larger than the anterior vesiculae,
especially at certain seasons, and are attached
to the anterior septa of their respective segments
somewhat nearer the median plane than the
anterior vesiculae.

iii. A thin-roofed median vesicula seminalis, of
considerable size, is situated in each of the
segments x. and xi. below the oesophagus,
which must be removed to expose them.
Within each are a pair of testes and a pair of
seminal funnels. With the anterior of these
median vesiculae the anterior lateral vesiculae
and the first pair of posterior lateral vesiculae
communicate. The second pair of posterior
lateral vesiculae open in similar fashion into
the second median vesicula.

The median vesiculae are absent in Allo-
lobophora.

b. The testes are two pairs of small flattened digitate

bodies attached to the septa in front of segments

REPRODUCTIVE SYSTEM. 81

x. and xi. respectively, and projecting freely into
the cavities of the median vesiculse seminales,
not far from the nerve-cord.

To see the testes, open the two median vesiculce seminales
without, injuring the structures within them. Wash carefully.

c. The seminal funnels are large opaque white much-

folded membranous funnels, of which one pair is
situated in each median vesicula seminalis, the
open mouth being turned towards the testes,
and almost enclosing them. Their narrow ends
perforate the hinder septa of their respective
segments (x. and xi.), and are continuous with
the vasa efferentia.

d. The vasa deferentia. From the posterior end of

each seminal funnel a fine convoluted tube, the
vas efferens, runs outwards and backwards on
the ventral body- wall. The two vasa efferentia
of each side unite in segment xn., and the vasa
deferentia so formed run back to open to the
exterior by the apertures already seen on the
ventral surface of segment xv. The vasa de-
ferentia are slightly imbedded in the body- wall,
and their exposure requires care.

3. Examination of the egg-capsule.

Capsules may be obtained under stones and in earth or
dung-heaps tenanted by Lumbricus. They are oval brown
bodies, about the size of a wheat grain. Draw them, noticing the
presence of quite well-developed young in advanced capsules.
Examine their contents.

4. Microscopical examination of the reproductive organs

and of their contents.

a. The ovaries. Remove an ovary entire ; examine in
salt-solution ; then fix, stain with carmine or
hematoxylin, dehydrate, clear, and mount in
balsam.

82 THE EARTHWORM

The proximal and thicker end of the ovary
is a solid mass of cells from which the ova
are formed : the distal portion contains ripe
ova. Each ovum is a large cell with very dis-
tinct nucleus and nucleolus. The nucleolus is
stained by carmine more deeply than the rest of
the cell.

b. The receptacula ovorum. Remove one and examine

it as above.

Ova may be seen in it at certain seasons.

c. The contents of the spermothecse. Remove a spermo-

theca and transfer it to a slide. Tease it in salt-
solution and examine with low and high powers.

The seminal fluid in the spermothecse con-
tains innumerable mature spermatozoa, each of
which is an exceedingly fine filament with a
slightly thickened rod-like head at one end.

d. The testes. Remove a testis ; stain, dehydrate and

clear it, and tease it in balsam : mount another
in salt-solution. Examine with a high power.

The spermogonia are cells, each of which
by division gives rise to spermocytes. Only
the earliest stages of the division occur in the
testes, masses of spermocytes leaving them with
numerous nuclei and tuber culated surfaces.

e. The contents of the vesiculae seminales. Cover-glass

method.

1. Remove a portion of one of the vesiculae seminales

to a slide, tease with needles and dilute to
three or four times its bulk with normal salt-
solution.

2. By means of a pipette spread a small drop of this

liquid evenly over the surface of a clean cover-
glass. Invert this and hold it over the mouth
of a bottle containing glacial acetic acid for
several seconds in order to kill and ' fix ' the

THE NERVOUS SYSTEM. 83

cells (i.e. to coagulate the protoplasm of the
cells).

3. Warm very gently over a flame until the prepara-

tion begins to dry. Then cover with a drop
of dahlia solution (-25 per cent.) and allow to
stain for three minutes.

4. By means of a pipette wash with water and mount

in a small drop of glycerin on a slide.

5. Examine under the microscope and draw the

several stages shown.

The vesiculse contain spermatozoa in all
stages of development : — (1) the massed sper-
mocytes as they leave the testis ; (2) a stage in
which the tubercles, or spermatids, are more
distinct and are produced outwards into fila-
ments ; (3) later stages in which the filaments
are elongated to form the tails of the sperma-
tozoa, while the bodies of the spermatids form
their rod-like heads.

Stages in the life-history of the Protozoon
parasite, Monocystis (p. 15), usually occur in the
vesiculae.

F. The Nervous System.

This consists of a ventral cord, swollen slightly in each
segment ; and a pair of dorsal ganglia in front, connected with
the ventral cord by a pair of connectives running round the
sides of the alimentary canal.

1. The nerve-collar is a small ring of nervous substance
surrounding the buccal sac just in front of the
pharynx, in the third segment. It consists of a pair
of pyriform supra-cesophageal ganglia, united by
their broader ends in the median plane, and sending
large nerves to the prostomium ; a pair of lateral
connectives running round the sides of the buccal
sac ; and a pair of closely apposed ventral ganglia,
the first pair of the ventral series.

84 THE EARTHWORM.

2. The ventral nerve-cord.

Remove the alimentary canal, from the pharynx back-
wards, without injuring the nerve-collar and ventral cord.

The cord runs along the ventral body-wall in
the middle line, giving off three pairs of nerves in
each segment along nearly the whole length of the
body. The cord is slightly swollen opposite the
middle of the segments, the swellings or ganglia
being more conspicuous in the hinder part of the
body.

III. EXAMINATION OF TRANSVERSE SECTIONS.

The structure of the body-wall and alimentary canal, and
many other points in the anatomy of the earthworm, are best
understood from a microscopical examination of transverse
sections of the whole animal.

To prepare these sections, take a worm which has thoroughly
evacuated its food contents and kill with the corrosive-acetic
mixture ; cut out about half an inch from the (esophageal region
and also from the middle, and leave these in 70 per cent, spirit
with trace of iodine added. Stain with hcemalum ; imbed ;
and cut into transverse sections with a microtome. Mount the
sections in order.

Intestinal Region.

A. Shape.

The section is nearly circular in outline. The dorsal
surface is indicated by the typhlosole, the ventral by the
nerve-cord.

B. The Integument.

1. The cuticle is a thin structureless superficial layer, often
separated by the action of the reagents from the
underlying epidermis.

TRANSVERSE SECTIONS.

85

2. The epidermis lies immediately below the cuticle. It
consists of a single layer of columnar cells, set side
by side at right angles to the surface, with nuclei
near their bases. Many of these cells have the
character of ' goblet cells,' being distended by a
granular secretion formed within them ; other cells
lying between these are extremely slender. In the

Fig. 27. — Lumbrieus. A diagrammatic transverse section through the
middle of the body. On the left side of the figure a nephridium is
shown, and on the right side the setse. The septa are not shown,
and parts only of the circulatory system are represented, (a. m. m.)

A, cuticle. B, epidermis, c, coelom. CM. layer of circular muscles.
D, seta. DV, dorsal vessel. E, cavity of intestine. F, typhlosole, filled
with ' hepatic ' cells. G , epithelium of intestine. H , ' hepatic ' cells. | , ventral
nerve-cord. J, nerve. K, ' giant fibres.' |_, nephrostome. |_M, layer of longi-
tudinal muscles. M, the point where the nephridium perforates the septum
between two segments : between the two cut ends the convoluted ' narrow
tube ' is intercalated, which is not shown in the figure. N, the first or long loop
of the nephridium. o, the second or short loop of the nephridium. P, the
third or terminal loop of the nephridium. R , external aperture of the nephridium .
SV, subneural vessel. V, intestinal vessel. VV, ventral vessel.

86 THE EARTHWORM.

clitellum the epidermis is greatly thickened. Blood-
vessels penetrate between the epidermal cells.

3. The dermis is an exceedingly thin layer immediately

beneath the epidermis.

4. The setae are lodged in sacs, which are invaginations of

the skin extending deep into the body, through both
the muscular layers. They are cuticular structures,
and are moved by special muscles. Setae in various
stages of development may be seen within the sacs.

C. The Muscular Layers of the Body-wall.

1. The external layer, which is rather thicker than the

epidermis, consists of transverse fibres running
round the body : it is completely interrupted at the
grooves between the segments. The connective
tissue between the muscle fibres is pigmented
slightly ; in the dorsal wall of the oesophageal
region this pigmentation is very marked.

2. The internal layer is very much thicker than the ex-

ternal. It consists of longitudinal fibres, arranged
on the sides of a series of radial longitudinal sheets,
and having a feather-like appearance in transverse
section.

D. The Coelom, or Body-cavity.

This is the space around the intestine, between it and the
body-wall. On its outer surface it is lined by a thin peri-
toneal epithelium, and contains a colourless fluid, with numer-
ous amoeboid corpuscles.

1. The peritoneal 'yellow' cells He outside the intestine
and surround the vessels which return the blood from
the intestine to the dorsal vessel. They are elon-
gated granular cells of large size, and generally
arranged radially to the surface of the intestine.
They are very abundant in the cavity of the typhlo-
sole, which they almost completely fill.

TRANSVERSE SECTIONS. . 87

E. Intestine.

This occupies the middle of the section. Its diameter
is about half that of the body of the worm.

1. The typhlosole is a fold of the dorsal wall extending

into the intestine from above, narrowing its cavity,
and making it crescentic in transverse section.

2. The epithelium lining the intestine consists of a single

layer of columnar ciliated cells.

3. The muscular layer of the intestinal wall is thin. The

fibres, which are nearly all transverse in direction,
are imbedded in a connective-tissue layer.

F. The Nephridia.

These lie at the sides of the alimentary canal. Their
appearance in transverse sections varies greatly according
to the part of the nephridium through which the section
happens to pass. By reference to the description of a nephri-
dium, on page 76, it will be fairly easy to identify the parts
present in any particular section that may be examined.

G. The Septa.

Owing to their obliquity, parts of the septa are present
in nearly all transverse sections, and lie in the body-cavity
between the body- wall and the alimentary canal. Each
septum is a thin sheet of connective tissue, in which are
circular and radial muscles. The ventral edge of the septum
is not attached to the body-wall near the middle line, a gap
being left around the nerve-cord through which the ccelomic
fluid can pass from one segment to another.

H. The Nerve-cord.

This lies near the ventral surface, just within the longi-
tudinal muscle-layer, and is enclosed in a double connective-
tissue sheath, between the two layers of which is a stratum
of longitudinal muscle-fibres.

88 THE EARTHWORM.

In the sheath lie three longitudinal blood-vessels, a sub-
neural below the cord, and a pair of lateral neural vessels at
the sides. Along the dorsal surface of the cord are three
1 giant-fibres ' of nervous nature. The fibrous connective
tissue of the sheath extends into the cord, dividing its fibres
into two main bundles, and subdividing each of these into
smaller ones. Nerve-cells of large size, each with distinct
nucleus and nucleolus, form a layer along the whole length
of the ventral and lateral surfaces of the cord : they are more
abundant in the ganglionic enlargements.

I. The Blood-vessels.

These are readily recognised by the deeply stained blood
clots with which they are filled. The larger vessels have
muscular walls. Besides the neural vessels just mentioned
the following may be recognised : —

1. The dorsal vessel lies in the body-cavity above the

intestine. It is of large size, and has masses of
1 hepatic ' cells at its sides.

2. The intestinal vessels lie in the walls of the intestine :

they are surrounded by ' hepatic ' cells, and may in
some sections be seen to open into the dorsal vessel.

3. The ventral vessel lies between the nerve-cord and the

intestine, and is connected with the latter by a
median sheet of vascular connective tissue.

4. The nephridial vessels.

5. The capillaries of the skin.

(Esophageal Kegion.

There is no typhlosole. Sections of the reproductive
organs appear and block up the body-cavity. The vesiculse
and seminal funnels are especially obvious.

Chapter VII.

THE FRESHWATER MUSSEL. Anodonta cygnea.

The body of the mussel is enclosed in a shell, the two halves
or valves of which cover the right and left sides of the body
respectively. These valves, which are hinged together along
the median dorsal line, can be opened to a slight extent by
means of an elastic ligament along the hinge-line, and closed
by powerful adductor muscles. When the valves are closed
they completely conceal all parts of the body.

Mussels vary a good deal in size according to their age :
a fair-sized specimen is about four and a half inches long, one
and a half thick, and two and a half from the dorsal to the
ventral edge.

The mussel lives in freshwater ponds and streams, with its
anterior end buried somewhat obliquely in the mud of the
bottom, and the posterior end projecting up into the water.
By means of a muscular foot it can plough its way slowly
along the bottom of the pond. Its food consists of minute
organisms, both animal and vegetable, which are swept in at
the hinder end of the shell by ciliary currents and carried
forwards to the mouth, the stream of water so kept up serving
for respiration as well as nutrition.

The sexes are distinct. The young embryos are retained
within the gills of the mother during the early stages of their
development ; they then hatch and attach themselves to fish.
While attached they increase considerably in size and undergo
a distinct metamorphosis. Finally they detach themselves,
and adopt the mode of life of the adult.

90 ■ THE FRESHWATER MUSSEL.

I. THE SHELL

The shell is a cuticular product formed by the underlying
epidermis. It is, therefore, entirely outside the body, and
incapable of growth except by addition of new rings or new
layers to the part already formed. It consists of an organic
basis impregnated with calcareous salts.

A. The Outer Surface of the Shell.

1. Shape. The anterior end of the shell is rounded, the

posterior end more pointed, and the dorsal hinge-
line straight.

2. The umbo is the oldest part of each valve. It is close

to the hinge-line, and about a quarter of the length
of the shell from its anterior end.

3. The lines of growth are the concentric curves on the

surface of the shell, marking successive stages in
its growth. Each of these lines was at one time the
margin of the shell.

The shell does not grow uniformly in all direc-
tions. Growth is much more rapid towards the
ventral than towards the dorsal surface, and more
rapid posteriorly than anteriorly ; hence the umbo
or oldest part is close to the dorsal edge, and nearer
the anterior than the posterior end.

4. The ligament connects the two valves together along

the mid-dorsal line : it is a median, imperfectly
calcined, and elastic part of the shell itself. It is
very narrow at the anterior part of the hinge-line,
and much broader behind. As the valves are only
kept closed by contraction of the adductor muscles,
they will open as soon as these muscles relax.
Hence in dead specimens the valves always gape
owing to the unopposed action of the elastic liga-
ment.

B. The Inner Surface of the Shell.

Wedge open the valves of the shell with the handle of a

THE SHELL. 91

scalpel or other instrument, so that the ventral edges of the
valves are about half an inch apart, and the adductor muscles
can be clearly seen. Note the mantle-lobes or flaps lining the
shell-valves, and with a scalpel separate the right mantle-lobe
from the shell.

Identify the positions of the adductor muscles by com-
parison with fig. 28, and detach with a scalpel first the anterior
then the posterior adductor muscle from the right valve, keeping
the edge of the knife close to the shell. As soon as this is done
the valves will gape open about half or three-quarters of an
inch, owing to the action of the ligament. Bend back the right
valve of the shell, and remove it completely, cutting through the
ligament along the hinge-line. Clean the detached valve, and
examine it.

1. The inner surface lias a white pearly appearance, except

round the edge. The lines of growth are visible,
but not nearly so plainly as on the outside.

2. The hinge is a smooth longitudinal ridge extending

backwards from the umbo along the dorsal edge
of the shell. When the valves are closed, the
two hinges are in contact opposite the umbo, but
gradually diverge from each other behind it.

3. The ligament, which is very strong and elastic, com-

mences in front of the umbo, and extends back
along the straight dorsal edge of the shell, being
very narrow in front, but wider behind.

4. The muscular impressions are slight depressions in the

surface of the shell at the places of attachment of
the adductor and other muscles.

a. The anterior adductor impression is a large oval

slightly depressed area, with its long axis vertical,
close to the anterior end of the shell, and nearer
the dorsal than the ventral edge.

b. The anterior retractor impression is small, and

continuous with the posterior border of the
anterior adductor impression.

92 THE FRESHWATER MUSSEL.

c. The protractor impression is a separate small

impression, about J inch behind the lower part
of the anterior adductor impression.

d. The posterior adductor impression is near the

dorsal edge and posterior end of the shell, below
and behind the hinder end of the ligament. It
is rather larger than the anterior adductor im-
pression, and is oval in shape, with the longer
axis horizontal.

e. The posterior retractor impression is small, and

continuous with the upper and anterior edge of
the posterior adductor impression.

f. The pallial line, or line of attachment of the mantle

to the shell, runs nearly parallel to the ventral
edge of the shell, and about half an inch from it.
It connects the hinder border of the posterior
adductor impression with the ventral edge of
that of the anterior adductor.

5. The paths of shifting of the muscles are slight triangular
depressions, converging from the margins of the
several muscular .impressions towards the umbo.
They mark the boundaries of the paths along which
the muscles have shifted during the growth of the
shell. In the young state, when the umbo and the
part immediately around it form the whole valve,
adductor and other muscles are present in the same
-relative positions as in the adult : during the growth
of the shell by addition of new matter round its
margin the muscles gradually shift outwards, in-
creasing in size as they do so.

C. The Structure of the Shell.

If the shell be broken across, and the edge examined with
a pocket lens, it is seen to consist of three layers :

1. The periostracum or outer layer is thin, horny, and
uncalcified. To it the brownish-green colour of

THE SHELL. 93

the shell is due. It is commonly, but erroneously,
spoken of as the ' epidermis ' of the shell.

In the growth of the shell this layer is the first to
be formed, and the free edge of the shell, especially
round the ventral border and the posterior end, con-
sists of it alone. It is formed by the thickened edge
of the mantle-fold.

The periostracum is reflected over the edge of
the shell, and lines a narrow strip of its inner sur-
face round the ventral border and at the two ends.

2. The middle or prismatic layer forms about half the

thickness of the shell : it is very densely calcified,
and consists of numerous polygonal prisms placed
side by side, and obliquely to the surface of the
shell.

Like the periostracum, it is formed by the edge
of the mantle. It is visible on the outer surface of
the shell at places where, as at the umbo, the peri-
ostracum has been rubbed off. It can also be seen
round the margin of the shell on its inner surface.

3. The inner, nacreous or pearly layer lines the whole of

the inner surface of the shell except a narrow
marginal strip. Its thickness differs in different
. parts and increases with the age of the mussel. It
consists of a number of superposed laminae laden
with calcareous particles. It is formed by the sur-
face of the body and mantle in contact with the
shell.

Pearls are formed in animals closely allied to
the mu sel by concentric deposits of this nacreous
layer around parasites.

II. DISSECTION OF THE MUSSEL.

In dissecting the mussel, a small brush may with great
advantage be used to remove the mucus and to clean the several
parts.

I

94 THE FRESHWATER MUSSEL.

Leave the animal in the left valve of the shell, and fix it
down firmly to the dissecting -board under water. Pins can
be passed through the ligament.

A. External Characters.

1. The muscles. The anterior and posterior adductors are

easily recognised, and form useful landmarks. Their
fibres run transversely from one valve to the other,
and by their contraction close the shell.

Close to the adductors are the smaller protractor
and retractor muscles of the foot, the origins of
which have already been seen on the inner surface
of the shell.

The protractor is behind the lower border of the
anterior adductor, and the anterior retractor is
behind the upper part of the same muscle. The
posterior retractor is continuous with the upper and
anterior border of the posterior adductor.

2. The right mantle-lobe is the flap of integument which

covers the side of the body : it is in contact during
life with the inner surface of the shell, to which it
is attached along the pallial line. Its inner surface
is ciliated.

Dorsally it is continuous with the body-wall, of
which it is an outgrowth. Its ventral border is free
and grooved, this free border secreting the outer and
middle layers of the shell.

The mantle-lobe begins in front above the middle
of the anterior adductor muscle, and runs round its
anterior border as a narrow fringe. Opposite the
part of the body between the two adductor muscles
the mantle-lobe is very wide, extending from near
the dorsal border to the ventral edge of the shell.

At the hinder end of the animal, below the
posterior adductor, the margin of the mantle which
here bounds the inhalant aperture is very greatly

EXTERNAL CHARACTERS. 95

thickened and pigmented, and bears a fringe of
tentacular processes.

Immediately behind the posterior adductor the
mantle-lobe fuses with its fellow of the opposite side,
and the fused borders run forwards along' the dorsal
surface of the posterior adductor for about half an
inch. They then separate and continue forwards, as
a pair of narrow fringes bounding a shallow groove,
for about half an inch further, where they cease.

3. The left mantle-lobe, which can as yet be only imper-
fectly seen, agrees exactly with the right, the animal
being bilaterally symmetrical.

i. The pallial or mantle cavity is the space between the
right and left mantle-lobes. In it lie the foot and
the greater part of the visceral mass, the gills, and
other organs.

It is divided by a horizontal partition, formed by
the bases of the gills, into two chambers of very
unequal size : — (1) the large ventral branchial
chamber ; (2) the smaller dorsal supra-branchial
passages, which unite behind to form the cloacal
chamber.

5. The pallial openings. If a mussel be placed in a vessel
of water with a layer of mud or sand at the bottom
and left to itself, it will assume, the position described
above, the anterior end being buried somewhat
obliquely in the mud, and the posterior end project-
ing into the water. The valves will be slightly
opened, and streams of water, which can be rendered
clearly visible by adding a little colouring matter,
will pass in and out of the posterior end, the in-
going or inhaled stream being ventral, the exhaled
dorsal.

This current is maintained by the cilia of the
mantle, gills, and palps : it serves to bring water
for nutrition and respiration, and to carry away the

96 THE FRESHWATER MUSSEL.

fsecal matter and excretory and reproductive pro-
ducts.

a. The inhalant aperture, through which the inhaled

stream enters the branchial chamber, is a vertical
slit at the hinder end of the body : it is bordered
laterally by the thickened posterior edges of the
mantle-lobes, which bear the tentacular fringes.
Below, it is incompletely closed by the approxi-
mation of the edges of the mantle-folds ; and
above it is separated from the exhalant aperture
by a horizontal partition formed by the fusion of
the gills of the two sides with each other.

b. The exhalant or cloacal aperture is much smaller

than the inhalant opening, and lies immediately
dorsal to this, and behind the posterior adductor
muscle. It is bounded laterally by the thickened
mantle borders, which have here no tentacles.

Pass a seeker into the cloacal opening, and forwards along
the supra-branchial cavity above the gills. Note the partition
between the exhalant and inhalant apertures.

6. Organs lying in the branchial chamber.

Turn bach the right mantle-lobe as fully as possible.

a. The foot and visceral mass form a large laterally
compressed oblong mass, about half the length
of the shell, and lying between the two adduc-
tors. The upper two-thirds, which are paler in
colour, form the visceral mass : the lower or
ventral third forms a powerful muscular foot,
orange-red in colour, which can be protruded
between the valves in front, and is used by the
animal for working its way along the bed of the
pond in which it lives.

The gills are two pairs of large lamellar organs
lying at the sides of the visceral mass, between
it and the mantle, and extending back beneath

EXTERNAL APERTURES. 97

the posterior adductor to the hinder end of the
mantle-cavity. The outer gill is, in the female,
often greatly distended with embryos.
c. The labial palps are two pairs of triangular folds
lying at the sides of the anterior part of the vis-
ceral mass, between the anterior adductor and the
gills. Each pair consists of a couple of lamellae,
of about equal size, attached along their dorsal
borders to the body-wall, and marked on their
apposed surfaces by close-set transverse ridges.

7. The external apertures.

a. The mouth is in front, between the anterior adductor

and the anterior border of the foot.

Pass a seeker into the mouth, and note that it passes verti-
cally upwards behind the anterior adductor into the stomach.

The mouth is bordered by anterior and pos-
terior lips, formed by the labial palps. The outer
palp is continuous with its fellow of the opposite
side, forming a low ridge-like anterior lip, imme-
diately behind the anterior adductor. In a
similar manner the two inner palps are continuous
with each other, and form a posterior lip in front
of the foot.

The deep groove between the outer and inner
palps of each side thus leads directly into the
mouth. The palps are richly ciliated, so that
any food-particles that come within the range of
their action are swept into the groove and carried
forwards to the mouth.

b. The anus opens into the cloaca! cavity, close to its

external opening.

Insert a seeker into the groove between the margins of the
mantle-lobes above the posterior adductor, and pass it back
over the adductor into the cloacal cavity. Slit up this canal
so as to expose thoroughly the hinder part of the rectum, and
the anus.

98

THE FRESHWATER MUSSEL.

Insert a seeker into the anus and pass it upwards and
forwards along the rectum as it lies on the dorsal surface of
the posterior adductor.

B. The Gills.

Cut away the right mantle-lobe completely with scissors,
cutting along the base of the palps, round the anterior end of
the gills, and back along the attached base of the outer gill. Take
care not to damage the parts to which the mantle is attached.

Fig. 28. — Anodonta cygnea. Diagrammatic view from the right side.
The right valve of the shell and the right mantle-lobe have been
removed, and the pericardial cavity opened from the right side.
The line of attachment of the right mantle-lobe is indicated by the
strong dotted line. (a. m. m.)

A, margin of the left valve of the shell in which the animal is lying. AA,
anterior adductor muscle. AN, anus. AR, right anterior retractor muscle.
AU, right auricle. B, ligament of the shell. C, cloacal cavity. CG, right
cerebral ganglion. D, partition between the inhalant and exhalant apertures.
E, tentacles bordering inhalant aperture. F, foot. H, anterior aorta. IG,
inner gill of right side. IP, inner labial palp of right side. K, kidney. LM,
left mantle-lobe. M, mouth. N, posterior aorta. OG, outer gill of right side.
OP, outer labial palp of right side, p, right protractor muscle. PA, posterior
adductor muscle. PC, pericardial cavity. PG, right pedal ganglion. PR,
right posterior retractor muscle. R, rectum. RM, line of attachment of
right lobe of mantle. RN, space between right and left lobes of mantle above
the posterior adductor muscle. V, ventricle. VM, visceral mass. W, edge of
left mantle-lobe.

THE GILLS. 99

Each gill consists of two lamellae, continuous along their
ventral borders. Each lamella is formed of very numerous
vertical gill-filaments and horizontal bars, united to form a
kind of trellis-work with very small meshes, the greater
number of which are blocked up by fusion of the deeper sur-
faces of the filaments with each other.

The two lamellae of each gill are further connected at inter-
vals by thick vertical ridges, parallel to the vertical filaments.
In the case of the inner gill these ridges are at intervals of
about twenty filaments ; in the outer gill they are more
numerous, occurring at about every seventh filament.

1 . The outer gill.

a. The outer lamella of the outer gill is attached along

the whole length of its dorsal border to the inner
surface of the mantle, close to its fine of origin
from the body- wall. This fine of attachment of
the gill runs obliquely across the body, its anterior
end being near the dorsal surface a little below
and behind the umbo, while its posterior end is
below and behind the posterior adductor.

b. The inner lamella of the outer gill is attached

along its whole length to the outer lamella of
the inner gill, and in front to the body- wall.

Turn the outer gill back, so as to see its attachment to the
inner gill.

2. The inner gill.

a. The outer lamella of the inner gill is attached along

its whole length, as just noticed, to the inner
lamella of the outer gill.

b. The inner lamella of the inner gill differs in its

attachment at different parts of its length.

Turn the inner gill back, so as to expose the dorsal border
of its inner lamella.

100 THE FRESHWATER MUSSEL.

yc

i. The anterior part of the dorsal border, about a
quarter of its total length, is attached to the
side of the visceral mass.
ii. The middle portion has a free edge, and extends
back as far as the hinder border of the foot.

iii. The posterior part, behind the foot, is fused with
the corresponding lamella of the gill of the
opposite side, forming the partition between
the inhalant and exhalant chambers of the
mantle-cavity.

3. The supra-branchial or exhalant passages are wide
longitudinal channels running just above the bases
of the gills, and continuous with the interlamellar
spaces of the gills.

Alongside the foot there are two such passages
on each side, one above the outer gill, and one above
the inner gill. The latter communicates with the
large branchial chamber through the longitudinal
slit between the inner lamella of the gill and the
hinder part of the visceral mass.

Behind the foot there are only three passages,

the two inner ones becoming continuous with each

other ; and beneath the posterior adductor the three

unite to form a wide shallow passage, opening behind

into the cloacal chamber.

Pass a seeker into the cloacal cavity, and then forwards

along the supra-branchial passages above the bases of the

gills. Lay open first the outer passage of the right side, and

then the inner one ; taking care not to damage the kidney, which

lies immediately above them. Note that the inner passages

of the two sides are continuous with each other behind the foot,

and lliat they communicate with the branchial cavity through

the slits between the inner gills and the body.

The respiratory stream of water, kept up by the
cilia on the mantle and gills, enters at the inhalant
aperture, passes forwards along the branchial cavity,

THE CIRCULATORY SYSTEM. 101

and then through the meshes of the gills into the
supra-branchial passages. It then passes backwards
along the supra-branchial passages to the cloacal
cavity, and so out.

Respiration is probably effected mainly by the
inner surfaces of the mantle-lobes, and not by the
gills.

4. The structure of the gills.

Cut the gills across, and examine the cut edges with a pocket
lens. Separate the two lamelloe from each other ; and mount
small pieces of them and examine with the microscope.

For a more minute study of the structure of the gills micro-
scopical sections should be made through various parts.

The individual filaments are slender vertical
laterally compressed rods, the inner edges of which,
i.e. those turned towards the space between the two
lamellae of the gill, are in most cases expanded
laterally, and fused with their neighbours.

The filaments are clothed with a single layer of
ciliated epithelial cells, which are cubical over the
greater part of the surface, but columnar along the
outer edge, a row of cells along each side of this
outer edge having peculiarly long cilia. Each fila-
ment is strengthened by a double chitinous rod,
which lies close to its outer border.

The inner or deeper parts of the bars consist
chiefly of lacunar tissue, i.e. a loose network of
branched cells, the meshes of which contain blood.
Distinct blood-vessels are comparatively few in
number ; they are large, and he principally in the
inter-lamellar rods which bolt together the two
lamellae of each gill.

C. The Circulatory System.
1. The pericardial cavity is an elongated space of con-
siderable size lying along the dorsal surface of the
animal, ventral to the ligament, and above the bases

102 THE FRESHWATER MUSSEL.

of the gills. It contains the heart and is traversed
by the rectum. Its walls are thin and semitrans-
parent behind; thicker and spongy in front.
Through them the pulsations of the heart can be
seen.
Open the pericardial cavity by a longitudinal incision

along the right side, and cut away as much of its walls as is

necessary to expose the heart fully.

2. The heart consists of three chambers, a median ventricle,
and right and left auricles ; the latter returning to
the ventricle the blood from the mantle-lobes and
gills of the right and left sides respectively.

a. The right auricle is a triangular sac with very thin

transparent walls. The base of the triangle lies
along the dorsal border of the gills of the right
side, from which it receives blood ; the apex is
attached to the side of the ventricle about the
middle of its length, and opens into it by a
valved aperture.

To see the auricle, press the gills down gently.

b. The left auricle is similar to the right, and returns

the blood from the left gills and mantle-lobe.

c. The ventricle is an elongated thick-walled muscular

sac of a reddish colour, lying in the middle of
the pericardial cavity, and receiving the auricles
laterally. Its posterior end is enlarged and
bilobed.

The ventricle surrounds the rectum, which
runs straight through the pericardial cavity from
end to end, apparently traversing the ventricle,
though not doing so in reality. In the early
stages the ventricle lies completely below the
rectum, but during development its anterior end
extends up at each side, and the two processes
meet each other and unite above the rectum,
completely surrounding it.

THE CIRCULATORY SYSTEM. 103

3. The arteries.

There are two main^arteries, one arising from
each end of the ventricle, which convey the aerated
blood from the heart to all parts of the body. Their
main branches may be followed by injecting them
from the ventricle.

a. The anterior aorta is a continuation of the anterior

end of the ventricle : it runs forwards along the
dorsal surface of the rectum, enters the visceral
mass, and divides into visceral and pedal
arteries, which supply the anterior two-thirds
of the animal.

b. The posterior aorta arises from the posterior end

of the ventricle, and runs backwards beneath the
rectum: it divides into right and left branches,
which supply the hinder part of the body.

4. The veins, except the largest ones, cannot be dissected

without injecting them, and even then they are
difficult to follow. They may, however, be identified
in sections of hardened specimens (p. 112).

a. The vena cava is a median longitudinal vessel lying

beneath the floor of the pericardium : it receives,
in front, large veins returning blood from the
foot and visceral mass ; and behind, much
smaller vessels from the posterior part of the
body.

b. The afferent branchial veins are a pair of large

vessels, running along the lines of attachment of
the outer and inner gills to each other. Vessels
arise from them which carry blood to the gills.

c. The efferent branchial veins run along the bases

of the outer lamellae of the outer gills : they
return to the auricles the blood from the gills,
and also receive at their anterior and posterior
ends large vessels returning aerated blood from
the mantle-lobes.

104

THE FRESHWATER MUSSEL.

5 The course of the circulation.

The ventricle receives arterial blood from the
auricles, and drives it through the arteries all over
the body. The blood from the foot and viscera is

K R U GO U0

PR \ \ i r P \

SC LM i

Fio. 29.— Anodonta cygnea. Diagrammatic view from the right side.
The right valve of the shell, the right mantle-lobe, the right gills,
and the right auricle have been removed. The alimentary canal
has been laid open from the right side as far as the point where it
enters tho pericardial cavity. The pericardial cavity, the ventricle,
and the right kidney and ureter have been laid open from the right
side. The longer arrow passes from the pericardial cavity through
(he right reno-pericardial aperture into the right kidney. The
shorter arrow passes through the external aperture of the right
ureter, (a. m. m.)

A, margin of the loft valve of the shell in which the animal is lying. AA,
anterior adductor muscle. AN, anus. B, ligament of the shell. BD, bile-duct.
BD', openings of bile-duct into stomach, c, cloacal cavity. D, septum
between tho cloacal and branchial cavities. E, tentacles bordering inhalant
aperture, f, foot. G, line of fusion of inner lamellae of right and left inner
gill*. GO, genital aperture. H, anterior aorta. I, intestine. K, kidney.
L, liver. LG, inner gill of left side. LM, left mantle-lobe. M, mouth. N,
posterior aorta, o, oesophagus, pa, posterior adductor muscle. PC, peri-
cardial cavity. PR, right posterior retractor muscle. R, rectum. RG, genital
gland— ovary or testis, rm, line of attachment of right lobe of mantle. RP,
right rono-poricardial aperture. s, stomach. SC, median supra-branchial
canal, lying above the inner gills. T, typhlosole. U , right ureter. UO, external
opening of right ureter, v', cavity of ventricle.

THE KIDNEYS. 105

returned to the vena cava, from which it passes
through the kidneys to the gills. From the gills it
is returned by the efferent branchial veins to the
auricles. The blood from the mantle-lobes is
returned direct to the auricles, and does not pass
through either the kidneys or the gills.

The circulation is in great part carried on
through irregular channels or lacunae.

D. The Kidneys.

The kidneys, or ' organs of Bojanus,' are paired, and lie
side by side just beneath the pericardium.

Each kidney is a wide thin-walled tube, doubled on itself
so that its two ends are closely approximated. These ends
are anterior, and placed opposite the anterior end of the
pericardium, while the loop is posterior and lies against the
posterior adductor muscle.

The two limbs of the loop are very different : the ventral
limb, or kidney proper, has spongy walls, and is lined through-
out by a darkly pigmented glandular epithelium, so that the
whole limb appears black. The dorsal limb, or ureter, is a
wide thin-walled tube which conveys the excretory products
forwards to the external opening.

Dissect the right kidney as described below, examining
first the ureter, and then the glandular portion.

1. The external opening of the ureter is on the side of the
body, between the two lamellae of the inner gill, and
a short distance in front of the point at which its
inner lamella becomes free from the body.

Turn back the gills of the right side, and pass one blade
of a fine pair of scissors between the lamellce of the inner
gill, through the anterior end of the slit between the inner lamella
and the body. Cut forwards horizontally through the junction
of inner and outer gills parallel to their base, and about an eighth
of an inch above this. Separate the cut edges, and gently brush
the parts clean.

106 THE FRESHWATER MUSSEL.

The opening of the ureter is a small hole, with
rather prominent white lips. Almost immediately
below it is another and less conspicuous opening,
the aperture of the genital duct, which should be
recognised at once.

2. The ureter.

Remove the gills of the right side completely, cutting along
their lines of attachment to the body and mantle. Inflate the
ureter through its external opening, and slit it up along its
whole length.

The ureter is a wide thin-walled passage lying
above and to the outer side of the kidney. Its roof
is continuous with, and inseparable from, the floor
of the pericardium ; and its floor is continuous with
the roof of the kidney. On its inner side it is
separated from the ureter of the other side by the
vena cava. The two ureters communicate with
each other through a large slit-like opening near
their anterior ends.

3. The kidney appears as an elongated black body lying

beneath the ureter. Posteriorly it enlarges con-
siderably, and is in close contact with the posterior
adductor muscle, which it partially embraces.
Lay open the kidney longitudinally with scissors.

Its walls are thrown into thick spongy folds,
clothed with a black glandular epithelium. The
enlarged posterior end opens into the hinder end of
the ureter.

4. The reno-pericardial openings are a pair of crescentic

slits at the anterior end of the pericardial cavity, in
the angle between the rectum and the floor of the
pericardium : they lead from the pericardial cavity
into the anterior ends of the right and left kidneys
respectively.

Find the reno-pericardial opening on the right side, and
pass a seeker through it, and backwards along the kidney. Slit
open the passage, using the seeker as a guide.

NERVOUS SYSTEM. 107

The pericardial cavity is developmentally a part
of the body-cavity or coelom. Hence in the mussel,
as in the earthworm and in many other animals,
the kidneys are tubular organs with glandular walls,
leading from the body-cavity to the exterior.

From the external apertures the excretory pro-
ducts are carried back by the outward respiratory
stream along the supra-branchial passages to the
cloacal cavity, and so out.

E. The Nervous System.

The nervous system of the mussel consists of three main
pairs of ganglia widely distant from each other, but united
by nerve-connectives, and giving off nerves which supply the
various organs.

The ganglia are about the size of large pin-heads, and
are of a bright-orange colour, which renders them easy to see.

1. The cerebral ganglia are placed one on each side of

the hinder border of the mouth, just above the line
of attachment of the mantle-lobe, and below and in
front of the protractor muscle.

The ganglion lies very close to the surface, just beneath the
skin, and is readily exposed if its exact position be determined
first.

The two cerebral ganglia are connected together
by a commissure which runs round the front of the
mouth. Each ganglion also gives nerves to the palps,
the anterior adductor muscle, and other adjacent
parts, and is joined to the other ganglia of its side
by nerve-connectives.

The cerebral ganglia of the mussel probably
correspond to the cerebral (supra-cesophageal) and
pleural ganglia of other molluscs, such as the snail.

2. The pedal ganglia are a pair of closely apposed

ganglia about as large as the cerebral. They are
situated in the foot, just above the junction of the

108 THE FRESHWATER MUSSEL.

visceral portion with the foot proper, and about a
third of the length of the foot from its anterior end.
To find the pedal ganglia, split the anterior part of the
foot with a scalpel in the median plane, and dissect the two
halves apart until the ganglia are met with. The junction
of the muscular and visceral portions is very clearly defined,
and the ganglia lie close to it, in the visceral portion.

Each pedal ganglion gives nerves to the foot.
The ' auditory ' organ or otocyst, which lies a little
behind and below the ganglion, is innervated from
the cerebral ganglion.

3. The cerebro-pedal connectives are a pair of nerve-cords

connecting the cerebral ganglia with the pedal
ganglia. They run in an almost straight course
between the two ganglia, and are easily exposed.

4. The visceral ganglia are a pair of closely apposed

ganglia, on the ventral surface of the posterior
adductor muscle about the middle of its length.

Pass a seeker into the cloacal cavity, and along the median
supra-branchial passage, and lay open this latter, if it has
not already been done. The visceral ganglia are then readily
found in the position noted above.

From the ganglia nerves are distributed to the
posterior adductor, the mantle, and other parts.

5. The cerebro- visceral connectives are a pair of long

nerve-cords which connect the cerebral with the
visceral ganglia.

From each cerebral ganglion the connective
curves upwards and backwards through the visceral
mass, lying close to the surface. It then runs
backwards along the inner surface of the kidney
to the visceral ganglion. The part in relation with
the kidney is very easy to dissect : in front of the
kidney the dissection is more troublesome, but
presents no serious difficulty.

DIGESTIVE SYSTEM. 109

F. The Digestive System.

1. The alimentary canal is a convoluted tube the greater
part of which lies in the visceral mass, imbedded in
the liver and generative organ, while the terminal
portion runs back through the pericardial cavity to
the anus.
Insert a seeker into the mouth, and, using this as a guide,
slit up the oesophagus and stomach with scissors. Slit open
in a similar fashion the whole length of the intestine, begin-
ning at the rectum and working backwards to the stomach
(cffig. 29, p. 104). A vertical longitudinal section of the whole
visceral mass is a useful guide to this dissection.

a. The mouth lies immediately behind the anterior

adductor, and between the lips formed by the
palps : there are no jaws. The food, which con-
sists of minute organisms of various kinds, both
animal and vegetable, is swept in at the hinder
end of the shell by the inhaled stream of water,
and carried forwards to the mouth by the action
of the cilia covering the palps.

b. The oesophagus is a short straight tube passing

almost vertically upwards behind the anterior
adductor.

c. The stomach is a dilated chamber partially sub-

divided by folding of its walls. A crystalline
style is found within it at certain seasons.

d. The intestine arises by a small orifice from the

ventral surface of the stomach, and runs back
wards and downwards into the visceral mass,
where it is closely surrounded by the generative
gland. It follows the course shown in fig. 29,
and finally passes into the rectum.

e. The rectum, which is rather wider than the intes-

tine, runs upwards through the visceral mass to
the anterior end of the pericardial cavity, through
which it runs horizontally backwards, sur-
rounded by the ventricle. Behind the pericardial

K

HO THE FRESHWATER MUSSEL.

cavity it turns slightly downwards, runs over
the posterior adductor, and opens into the cloacal
cavity by a slit-like anus.

The ventral wall of the rectum is folded so
as to form a prominent longitudinal ridge, the
typhlosole, projecting into its cavity. This
typhlosole is largest at the junction of the
rectum with the intestine.

2. The liver is a large many-lobed digestive gland sur-
rounding the stomach and adjacent parts of the
oesophagus and intestine, and opening into the
stomach by several bile-ducts.

G. The Reproductive System.

The sexes are separate. The reproductive organs are
simple, and very similar in the two sexes.

The ovary, or testis, is very large, and, when the sexual
products are ripe, forms the greater part of the visceral
mass between the foot and the kidneys. The ducts of each
side converge to the genital aperture which lies on the side
of the body immediately below the opening of the ureter.

Tease a small portion of the ovary, or testis, in salt-solution
or glycerin ; cover, and examine it with low and high powers.

In the female the eggs, after they escape from the genital
aperture, pass in large numbers into the space between
the^wo lamellae of the outer gill, which they distend very
greatly. Here they develop into embryos known as glochidia,
which are in many respects very unlike the parent. They
have bivalved shells, each valve of which is triangular and
incurved at its apex so as to form a sharp tooth, the teeth of
the two valves constituting an efficient pair of pincers. There
is only a single adductor muscle : the gills are absent or
rudimentary, as also is the foot. A long coiled filament
serves to anchor the glochidium to the gill of the parent.

Take a female in which the outer gills are distended with

TRANSVERSE SECTIONS. Ill

embryos ; cut the gill across ; remove some of the embryos,
mount them on a slide in water, and examine them with a
microscope.

III. EXAMINATION OF TRANSVERSE SECTIONS.

Several points in the anatomy of the mussel, and notably
the relations of the gills, kidneys, and heart, are best studied
by means of a series of transverse sections.

For this purpose take a good-sized specimen, and put it
into \ per cent, chromic acid, wedging the valves slightly open
so that the acid may have free access to the branchial cavity.
After a couple of days transfer it to spirit, and keep it till wanted.

To make the sections, remove both valves of the shell care-
fully ; place the animal on a board, and cut it transversely
with a razor into a series of slices about a quarter of an inch
thick ; arrange the sections in order in a dissecting dish under
water ; examine and draw them.

The most instructive sections are those passing (1)
through the renal and reproductive apertures ; (2) through
the middle of the ventricle*; (3) through the posterior adductor.
The chief features of these sections are as follows :

A. Transverse Section through the Renal and Reproductive
Apertures.

The section passes through the widest part of the body, a
little way behind the umbo.

1. The mantle-folds arise very close to the dorsal surface,

one on each side of the pericardial cavity. The
folded margin which secretes the two outer layers
of the shell should be noticed.

2. The visceral mass is large and laterally compressed,

and lies in the middle of the section between the two
mantle-lobes. The chief part of its substance is
made up of the lobes of the ovary, or testis. The

J12 THE FRESHWATER MUSSEL.

ventral edge of the visceral mass is produced into
the wedge-shaped muscular foot.

3. The alimentary canal. The coils of the intestine are

seen in the visceral mass, imbedded in the genera-
tive gland, and cut across about a half a dozen
times. Lying in the pericardial cavity in the median
line immediately beneath the dorsal surface is the
rectum, in which the typhlosole or fold of its ventral
surface is well seen.

4. The gills lie in the dorsal half of the mantle-cavity,

between the visceral mass and the mantle. The
attachments of their lamellae to the body-wall are
well seen. The inner lamella of each inner gill is
attached to the base of the visceral mass.

At the base of each gill is the supra-branchial
passage, along which the respiratory stream of
water flows backwards to the cloacal cavity.

5. The circulatory system.

a. The pericardial cavity is nearly circular in outline ;

its roof and sides are thick and spongy, forming
what is spoken of as Keber's organ.

b. The anterior aorta lies on the dorsal surface of

the rectum, and its cavity is slit-like in section.

c. The vena cava lies in the median plane between

the excretory organs and the visceral mass.

d. The afferent branchial vessels lie one on each side,

just above the attachment of the outer lamella
of the inner gill and inner lamella of the outer
gill.

6. The excretory organs are paired, and he between the
pericardium and the visceral mass.

a. The glandular portions, or kidneys proper, are cut
across close to their anterior ends. They are
seen as a pair of small tubes, with glandular
longitudinally folded walls, lying side by side just

TRANSVERSE SECTIONS. 113

above the visceral mass and opposite the bases
of the inner gills.

If the section has passed a little in front of
the excretory apertures, the reno-pericardial
passages from the pericardial cavity to the
anterior ends of the kidneys are well seen.

b. The ureters are cut through at the part where they
communicate with each other across the median
plane : they appear as a wide cavity immediately
below the pericardium and above the kidneys.
The external openings are to the outer sides of
the kidneys, between the two lamellae of each
inner gill.

7. The reproductive apertures are immediately ventral to
the excretory apertures ; they may be followed with
a seeker into ducts leading from the genital gland.

B. Transverse Section through the Middle of the Ventricle.

1. The mantle-lobes have the same appearance as in the

previous section.

2. The visceral mass is cut across close to its posterior

end, and hence is much smaller than in the previous
section : it is occupied almost entirely by the ovary,
or testis.

3. The alimentary canal. The intestine is cut across twice

in the visceral mass ; and the rectum with the
typhlosole is cut across as it lies in the pericardial
cavity.

4. The gills. The attachments of the gills are the same

as in the previous section, except that the inner
lamella of the inner gill ends in a free edge above.

At the base of each gill, between its lamellae, is a
wide supra-branchial passage. The inner canal of
each side communicates with the branchial cavity
through the slit between the unattached inner
lamella of the gill and the base of the visceral mass.

114

THE FRESHWATER MUSSEL.

5. The circulatory system.

a. The pericardial cavity is very much larger and has
thinner walls than in the previous section : it is
triangular in shape.

PlO. 30. — Anodonta cygnea. Transverse section of the animal passing
through the middle of the length of the ventricle. The cavity of
the heart is dotted, (a. m. m.)

A, shell. AU, left auricle. B, ligament. BC, branchial cavity. F, foot.
I, intestine. |G, left inner gill. «, left kidney. KO, Keber's organ of the
right side. LM, left mantle-lobe. OG, left outer gill. PC, pericardial cavity.
R, cavity of rectum. RG, genital gland, ovary, or testis. RM, edge of right
mantle-lobe. SC, supra-branchial canal of right outer gill. T, typhlosole. U,
left ureter, y, wall of ventricle. V', cavity of ventricle. VC, vena cava. W,
supra-branchial canal of left inner gill, x, free upper edge of the inner lamella
of the right inner gill ; between this edge and the body is the slit through which
the branchial cavity communicates with the supra-branchial canal of the inner
gill. Y, the right oerebro-visoeral connective.

TRANSVERSE SECTIONS. 115

b. The ventricle surrounds the rectum : its outer wall

is thick and muscular, its inner wall thin.

c. The auricles are a pair of very thin-walled sacs lying

at the sides of the pericardial cavity. Each is
attached along its outer side to the wall of the
cavity, and by its inner border to the ventricle,
into which it opens by a slit-like valved aperture.

d. The vena cava is a median thin-walled tube lying

in the floor of the pericardium.

e. The afferent branchial vessels, right and left, lie

just above the attachment of the outer lamella of
the inner gill and inner lamella of the outer gill.

6. The excretory organs.

a. The kidneys are two wide tubes with thick spongy

walls, whose epithelium is glandular, pigmeDted,
and folded. They lie below the pericardium,
above the visceral mass, and opposite the bases
of the inner gills. Their inner walls are in con-
tact with each other below ; and are separated
dorsally by the vena cava, from which large
sinuses enter them.

b. The ureters are a pair of thin-walled tubes lying

along the dorsal surface and outer sides of the
kidneys, immediately below the pericardium.
They are separated from each other in the median
plane by the vena cava.

7. The nervous system.

The connectives between the cerebral and
visceral ganglia lie side by side, between the two
kidneys and immediately below the vena cava.

C. Transverse Section through the Middle of the Posterior
Adductor Muscle.
1. The posterior adductor is a large mass of transverse
muscular bands running across the dorsal part of
the section.

116 . THE FRESHWATER MUSSEL.

2. The gills are cut across close to their hinder end, and

are small. The inner lamellae of the two inner gills
are united with each other in the median plane,
forming a horizontal septum which separates the
large branchial chamber below from the wide but
shallow supra-branchial or exhalant chamber.

3. The rectum lies immediately dorsal to the adductor

muscle. The typhlosole is still present.

4 The visceral ganglia lie side by side on the ventral
surface of the adductor.

m

Chapter VIII

THE EDIBLE SNAIL. Helix pomaiia.

The snail is a terrestrial animal, feeding on leaves and other
vegetable matter, which it rasps off with its toothed tongue,
or odontophore. The dorsal surface of the body carries a
spirally coiled univalve shell, which lodges most of the viscera,
and within which the entire animal can be withdrawn for
protection. It breathes air by means of a pulmonary chamber,
formed by a fold of skin — the mantle — on the dorsal surface.
It is hermaphrodite, but incapable of self -fertilisation.

The edible snail occurs in many parts of England, and
can be readily obtained from dealers. On account of its
large size it is preferable to the commoner English species,
H. aspersa, to which, however, the following description will
apply with but slight modification.

Snails are best killed by putting them into cold water and
then running a layer of very hot water on to the surface,
when they die in an expanded condition in about two hours.
In winter the mouth of the shell is closed by a plate of hardened
mucus, the epiphragm, which must be removed before placing
the snail in water. When dead the snail should be put into
spirit for a short time before it is dissected, in order to facilitate
the removal of the mucus.

I. EXTERNAL CHARACTERS.

A. Before Removal of the Shell.

1. The shell is a right-handed conical spiral, each turn of
which overlaps and conceals the greater part of the

118 THE SNAIL.

preceding turn. In the fully-formed shell there are
about four and a half turns in the spiral.

The shell is placed obliquely on the back of the
animal with its apex to the right : its mouth is in
front, and faces downwards and to the right. Like
the shell of the mussel, it is a cuticular structure,
i.e. it is outside the epidermis and is non-cellular.
It has therefore no power of interstitial growth, but
can only increase in size by the addition of new
shell-matter round its free edge or mouth.

The shell will be more fully described after its
removal from the body.

2. The part of the animal outside the shell. This consists
chiefly of the head and the foot, and will vary
greatly according to the degree of protrusion of the
animal from the shell. It has a mammillated
surface, capable of holding in the wrinkles a con-
siderable amount of water. By increasing or
diminishing this amount the activity of the snail is
heightened or diminished.

a. The head is the rounded anterior extremity of the
animal. It bears the two pairs of tentacles and
the mouth.

i. The posterior or dorsal tentacles are a pair of
long fleshy cylindrical processes of the dorsal
surface of the head. They are very freely
movable and can be completely withdrawn by
invagination when the animal is disturbed.
Each bears an eye at its extremity, which
is only visible when the tentacle is fully
protruded.

ii. The anterior or ventral tentacles are a pair of
much smaller processes arising from the sides
of the head, below and in front of the larger
tentacles. Like these latter, they can be
completely withdrawn by invagination.

EXTERNAL CHARACTERS. 119

b. The foot is a large flattened muscular expansion of

the ventral wall of the body. It is of an elon-
gated oval shape, commencing in front with a
rounded anterior border immediately below and
behind the head, and narrowing behind to a long
pointed extremity. A median longitudinal groove
runs along its ventral surface.

Watch the under surface of the foot as a snail creeps along
a sheet of glass, and note the waves that pass from behind for-
wards along the muscles.

c. The collar is a thickened fleshy rim surrounding

the mouth of the shell, and separated by a deep
groove from the body which it encircles. It is
thicker at the sides than in front or behind, and
is especially thick on the right side in the angle
of the mouth of the shell.

d. The external apertures.

i. The mouth is a small aperture on the ventral
surface of the anterior end of the head. It
is bordered at the sides by two prominent
lateral lips, and is separated from the foot by
a less conspicuous inferior lip.

Pass a seeker into the mouth, and then upwards and back-
wards into the cavity of the buccal mass.

ii. The genital aperture is on the right side of the
head, behind and below the larger tentacle,
and immediately behind the' posterior edge of
the right Up. From it a groove runs back-
wards along the dorsal surface of the body :
a similar groove runs along the left side.

Pass a seeker into the genital aperture, and backwards
along the genital passage.

iii. The respiratory or pulmonary aperture. On

the right side of the Collar at its widest part
is a deep notch, at the dorsal end of which

120 THE SNAIL.

is a large round hole, the respiratory aper-
ture, leading into the pulmonary chamber.

Wash away the mucus from the collar on the right side ;
clean the notch in the collar, and pass a seeker through the
respiratory aperture into the pulmonary chamber.

iv. The anus is a smaller slit-like hole, immediately
below and to the right of the respiratory
aperture. From it a spout-like groove runs
forwards to the edge of the collar.

Clean the anal opening and groove, and pass a seeker into
the anus and back along the rectum.

v. The aperture of the pedal mucous gland is

immediately below the inferior lip, between
the head and the foot.

Pass a seeker into the aperture, and backwards along the
gland, and lay the gland open along its whole length.

B. After Removal of the Shell.

Twist the animal out of the shell, taking care not to damage
the upper part of the body in so doing. The shell comes off readily
if the snail has been put into spirit for a short time after drowning.

1. The Shell.

a. The colour differs a good deal in different speci-

mens. The ground colour is a yellowish brown,
marked by darker longitudinal bands running
round the spiral, and by obliquely transverse
lines corresponding to the lines of growth.

b. The nucleus is the oldest part of the shell, at the

top of the spiral, corresponding to the umbo of
the mussel-shell.

c. The mouth is oval, and notched posteriorly by the

penultimate turn of the shell. Its lip is smooth
and slightly everted.

THE SHELL. 121

d. The lines of growth are transverse ridges parallel

to the mouth, and indicating successive positions
of the mouth during the growth of the shell.

e. The umbilicus is an opening on the under surface

of the shell leading into the hollow axis or
columella. It is sometimes closed.

Glip away one half of the shell with bone forceps, and
grind the cut edges flat on a file or stone.

f. The columella is the axial portion of the shell

round which it is coiled : it is formed by the inner
wall of the several turns, each of which overlaps
rather more than half of the preceding one.

g. The structure of the shell. The shell, like that of

the mussel, consists of three layers, of which the
outer and middle ones are formed by the collar,
and consequently can only increase by additions
round the mouth of the shell. The inner layer
is formed by the integument covering the vis-
ceral mass,
i. The periostracum or outer layer is horny and
uncalcified. To it the colour of the shell is due.
It is usually rubbed off the uppermost turns
of the shell, leaving the middle layer exposed.

ii. The middle layer is much the thickest of the
three. It is densely calcified, and has an
opaque porcellanous appearance.

iii. The inner or nacreous layer has a glistening,
pearly appearance, and a very smooth surface.
It is formed by the superposition of a number
of delicate densely calcified laminae.

2. The visceral hump is the part of the snail which is per-
manently lodged within the shell. It is the elon-
gated and spirally coiled dorsal part of the animal,
and contains the greater part of the viscera. It is
covered by a very thin layer of integument which
secretes the nacreous layer of the shell, and which

122 THE SNAIL.

is sufficiently transparent to allow the several organs
to be seen through it.

For convenience of description the basal or
largest turn of the spiral, commencing with the
collar, will be spoken of as the first turn, the next
as the second, and so on.

Place the snail in a dissecting dish under water, and
determine the positions of the organs mentioned below. Note
that the outer side of the spiral corresponds to the animal's left
side, and the inner side to the animal's right.

a. The kidney is a large, somewhat triangular body,

of a yellowish- white colour and granular appear-
ance, lying about halfway round the first turn of
the spiral, opposite to the collar.

b. The pericardium, enclosing the heart, is an oval

sac, lying immediately to the left of the anterior
half of the kidney, which is notched to receive it.

c. The mantle, forming the thin roof of the respiratory

cavity, extends forwards along the dorsal surface
and left side of the body, from the pericardium
and kidney to the collar.

d. The liver is a dark reddish-brown mass, which

begins immediately beyond the pericardium and
kidney, and extends to the top of the spiral.

e. The intestine is a thin-walled tube, much paler in

colour than the liver, and appearing on the
surface at one or two places in the first and
second turns of the spiral.

f. The rectum runs along the right border of the first

half-turn of the spiral, just below the ridge-like
thickening which borders the right or inner side
of the spiral.

g. The albumen gland is a large white or yellowish

mass at the junction of the first and second
turns of the spiral. It separates the right and
left lobes of the liver from each other.

MANTLE-CAVITY. 123

h. The hermaphrodite gland is a small yellowish body-
on the inner side of the upper half on the second
turn of the spiral.

i. The columellar muscle, by which the animal can be
retracted within the shell, runs along the right
or inner side of the spiral, and is attached to the
columella in the upper half of the first turn.

II. DISSECTION OF THE MANTLE-CAVITY AND ORGANS
IN RELATION WITH IT.

The respiratory or mantle-cavity of the snail lies along
the dorsal surface, commencing at the collar, and extending
backwards rather more than three-fourths of the way round
the first turn of the spiral. The mantle is a transverse fold
of skin which, arising from the dorsal surface of the visceral
mass, grows forwards along the back, ending in front in a
thickened rim, the collar. Between the mantle-fold and the
back of the animal is the large mantle-cavity, which at first
opens anteriorly by a wide orifice. Fusion of the collar with
the body-wall reduces this orifice to a small round aperture
on the right side, the respiratory or pulmonary aperture,
through which air alternately enters and leaves the cavity
during respiration.

In close relation with the mantle-cavity are the rectum,
the heart, and the kidney and ureter.

Place the snail in a dissecting dish under water, and pin
it firmly down through the foot, with the dorsal surface upward.
Open the mantle-cavity by a transverse incision through its
thin roof, immediately behind the collar. Cut backwards from
each end of the incision along the sides of the cavity to its posterior
end, cutting along the left side close to the junction of the mantle
and body-wall, and along the right side about a quarter of an
inch to the left of the prominent ridge that borders this side,
midway between this and the right border of the kidney.

Turn back the roof of the mantle-cavity, and pin it down
so as fully to expose the pericardium and kidney.

124

THE SNAIL.

A. The Walls of the Mantle-cavity.
1. The roof and sides of the cavity are formed by the
mantle itself, a thin fold of skin which bears on its
under surface a very rich network of large blood-
vessels, especially abundant in front and along the
right side.

r^* These vessels, in which the blood is aerated, unite
to form a large trunk, the pulmonary vein, which

Fio. 31. — Helix pomatia. Dissection from the right side to show the
mantle-cavity and organs in relation with it. The arrow passes
through the pulmonary aperture into the mantle-cavity. The shell
and the upper part of the spiral visceral mass have been removed.
(a. m. m.)

A, auricle. B, anus, c, collar. D, dorsal body-wall forming the floor of
the mantle-cavity, e, eye. F, foot. G, genital aperture. H, groove leading
to genital aperture. K, kidney. |_, lip. P, pulmonary vein. PA, pulmonary
aperture. PC, pulmonary or mantle-cavity, pv, pulmonary vessels in root
of mantle-cavity, pv', cut ends of pulmonary vessels. R, rectum. S, anterior
tentacle. T, posterior tentacle, u, ureter. V, ventricle.

takes back the oxygenated blood to the heart, run-
ning along the left side of the kidney (see p. 139).

The mantle is greatly thickened in front to form
the collar : posteriorly it is thickened by the kidney
and pericardium, both of which he in its substance.

MANTLE-CAVITY. 125

2. The floor of the cavity, which is formed by the dorsal
wall of the body, is muscular, and strongly arched
from side to side. Contraction of its muscles causes
flattening of the floor, and protrusion of the body
further out of the shell, and thus enlarges the cavity,
causing an inrush of air through the pulmonary
aperture. On retraction of the body, the floor resumes
its previous convex shape, diminishing the size of
the cavity and effecting expiration.
k By alternate protrusion and retraction of the
body the air in the mantle-cavity is continually
renewed, and the cavity thus becomes a lung.

B. The Organs in relation with the Mantle-cavity.

1. The pericardial cavity and heart.

a. The pericardial cavity, which has already been seen

from the exterior, is oval, thin-walled, and half
an inch or more in length. It lies in the roof of
the mantle-cavity at its posterior end and left side,
and is in close contact with the left side of the
kidney about the middle of its length.

Open the pericardial cavity to expose the heart.

b. The heart has two chambers, of about equal size.

i. The auricle is proximal, i.e. nearer the collar,
and has thin walls. It receives in front the
pulmonary vein, which returns to it oxygenated
blood from the lung.

ii. The ventricle is the pear-shaped distal division
of the heart, with thick muscular walls. It
receives oxygenated blood from the auricle,
and drives it through the aorta to all parts of
the body.

The blood-vessels will be described later on.

2. The kidney and ureter.

a. The kidney is a large pale yellow triangular body
in the posterior part of the roof of the mantle-

L

126 THE SNAIL.

cavity. Its left side is notched to receive the
pericardium, which lies close against it.

i. The reno-pericardial canal is a narrow ciliated
passage leading from the pericardial cavity to
the kidney. Its pericardial opening is about
the middle of the length of the cavity, oppo-
site the ventricle, and close to the inner or
ventral border of the septum between the
pericardial cavity and the kidney.

The reno-pericardial passage is difficult to make out. To
see it, take a fresh snail, make a small hole into the pericardial
cavity, and inject into it carmine or other colouring matter sus-
pended in water. This will pass into the kidney along the
passage, which can then be followed.

ii. The structure of the kidney. The kidney is a
sac lined by a glandular epithelium, and with
its walls thrown into a large number of
lamellar folds projecting into its cavity.

Slit open the kidney longitudinally; wash it out with a
gentle stream of water, and note its structure.

b. The ureter is a thin-walled duct which commences
at the proximal end of the kidney and runs back
along its right side to the distal end. It then
turns sharply forwards, and runs along the right
side of the mantle-cavity to its anterior end,
lying dorsal to the rectum, and just within the
prominent iidge-like fold that marks the junction
of the roof and right side of the mantle-cavity.

In front it opens into the mantle-cavity imme-
diately above and to the right of the pulmonary
aperture. The opening is slit-like, and is con-
tinued as two divergent grooves with prominent
lips, which run down the right and left sides of
the pulmonary aperture, the right groove being
continuous below with the spout-like aperture of
the rectum.

DIGESTIVE SYSTEM. 127

Insert one blade of a pair of scissors into the pulmonary
aperture from the outside, and cut through the collar, keeping
close to the left side of the aperture. Turn the right lip of
the aperture over to the right side. Pass a seeker into the rectum
from the anus so as to identify it. Note the ureter running
along the right side of the rectum, but dorsal to it in the natural
position of the parts. Trace the ureter forwards to its opening,
and inflate it with a blowpipe. Insert a seeker into the opening,
and, using this as a guide, slit up the ureter along its whole
length, following it back to the hinder end of the mantle-cavity,
and then forwards close alongside the kidney to its anterior
end, into which it opens.

3. The rectum is a wide tube running along the right side
of the mantle-cavity, ventral to the ureter. It opens
in front at the anus, which has already been seen.

Insert a bristle or seeker into the rectum from the anus,
and note carefully the relations of the anus and the respira-
tory and renal apertures.

III. DISSECTION OF THE DIGESTIVE SYSTEM.

Cut away the part of the collar to the left of the pulmo-
nary aperture. Detach the right side of the collar, with the anal
aperture, from the body-wall. Separate the rectum from the
body by cutting along the right side of the mantle, just below
the rectum, as far back as the hinder end of the mantle-cavity.
Turn the rectum back, and pin it down.

Make a median longitudinal incision through the floor of
the mantle-cavity, and continue it forwards along the head to
its anterior end. Dissect the flaps away from the underlying
parts, and pin them out right and left.

Carefully remove the thin skin covering the spiral visceral
mass, and unravel the reproductive organs, which are white
in colour and very bulky, and the alimentary canal, the coils
of which are very intimately connected with the liver.

Arrange the reproductive organs on the right, the digestive
organs on the left side of the dissection.

128

THE SNAIL.

jt« ^ \„

DIGESTIVE SYSTEM. 129

The removal of the integument from the visceral mass requires
care, hut presents no great difficulty. Special care is needed
at the posterior end of the mantle-cavity, where the parts are
rather firmly bound together.

The dissection is greatly facilitated by frequent washing
under the tap.

A. The Alimentary Canal.

The alimentary canal is a convoluted tube, of nearly
uniform size along the greater part of its length, but pre-
senting special features in its anterior portion.

1. The buccal mass or pharynx is the enlarged anterior end

of the canal into which the mouth opens. It forms
a prominent rounded mass in the head, with stout
muscular walls, and will be more fully examined
at a later stage of the dissection (p. 134). It con-
tains the odontophore.

Insert a seeker into the mouth, and note that it passes up-
wards into the buccal mass.

2. The oesophagus is a narrow thin-walled tube, which

arises from the dorsal surface of the buccal mass
and runs backwards, passing into the crop.

3. The crop is a large fusiform thin-walled dilatation of

the alimentary canal, lying partly in front of the
visceral hump, partly in the first turn of the spire.
It is widest about the middle of its length, and
gradually narrows behind.

Fig. 32. — Helix pomatia. Dissection from the right side to show the

digestive and reproductive systems, (a. m. m.)

A, anus. AG, albumen gland. B, buccal mass, c, sac of radula. Cd
cerebral ganglion. D, jaw. E, mouth. F, flagellum. G, genital aperture.
H, crop. HD, hermaphrodite duct. HG, hermaphrodite gland. I, intes-
tine. K, dart sac. L, mucous glands. LD, left bile-duct. |_L, left lobes of
liver. M, female portion of the common duct. N, male portion of the common
duct. O, receptaculum seminus. o', dilated end of receptaculum seminus. P,
penis, retracted. PG, pedal ganglion. R, rectum. RD, right bile-duct.
RL, right lobe of liver, s, ' stomach.' SD, salivary duct. SG, salivary gland.
T, tentacle. VD, vas deferens. NG, viscero-pleural ganglion. Y, eye. 2,
posterior end of foot.

130 THE SNAIL.

4. The ■ stomach ' is a slightly dilated loop of the alimentary

canal, between the crop and the intestine. It lies
close to the surface of the visceral hump, near the
commencement of the second turn of the spire,
and between the right and left lobes of the liver.
Its mucous membrane is thrown into longitudinal
folds.

5. The intestine describes a somewhat S-shaped course,

and is imbedded in the liver, from which it is not
very easy to separate it. From the stomach it runs
back round the first turn of the spiral, lying in a
groove on the outer side of the left lobe of the
liver, to the hinder end of the mantle-cavity, where
it passes into the rectum. It is superficial along a
great part of its course.

6. The rectum has already been traced along the right side

of the mantle-cavity to the anus.

B. The Digestive Glands.

1. The salivary glands are a pair of large white lobulated

glands lying on the upper surface and the sides of
the crop, to the walls of which they are attached
by numerous strands of connective tissue. Their
ducts, which are long and slender, run forwards
and open into the buccal mass, one on each side
of the oesophagus.

2. The liver is a very large bilobed gland, of a reddish-

brown colour, which forms a large part of the
visceral hump.

a. The left lobe of the liver is the larger of the two,
and is partially subdivided into three lobes.
It lies in the upper half of the first turn of the
spire, immediately behind the mantle-cavity,
and is grooved on its outer surface by the
intestine. It has three main ducts, which unite
to open by a single large aperture into the left
side of the stomach.

REPRODUCTIVE SYSTEM. 131

b. The right lobe of the liver occupies the greater
part of the second and upper turns of the spire.
It is separated from the left lobe by the posterior
end of the crop, the stomach and the commence- ,
ment of the intestine, and the albumen gland.
It has one large duct, which opens into the
right side of the stomach, almost opposite to
the left duct, but slightly beyond it.

Slit up the stomach, and wash out its contents. Note the
openings of the bile-ducts, and follow these with a seeker into
the lobes of the liver.

IV. DISSECTION OE THE REPRODUCTIVE SYSTEM.

The snail is hermaphrodite, but does not fertilise its own
ova. As is usual in such cases, the reproductive organs are
extremely complex.

1. The hermaphrodite gland is a small yellowish body

lying on the inner side of the second turn of the
spire, and closely imbedded in the right lobe of
the liver. Within it both ova and spermatozoa are
produced.

Place a small piece of the hermaphrodite gland on a slide
in a drop of water : tease it slightly : cover : and examine
with low and high powers.

The gland consists of a number of finger-like
follicles, in each of which ova are developed in the
outer wall, and spermatozoa in the more central
part. The ova are large round granular cells, with
very large reticulate nuclei : the spermatozoa, which
are generally aggregated in wisp-like bundles, have
small rod-like heads and very long tails.

2. The hermaphrodite duct is a very sinuous duct, of a

whitish colour, lying along the inner side of the
spire close to the columella, and opening into the
inner side of the albumen gland.

132 THESNAIL.

3. The albumen gland is an elongated compact yellowish-

wliite body, varying greatly in size at different
times. It lies in the upper half of the first turn of
the spire along its inner side, between the stomach
and the left lobe of the liver.

4. The common duct arises from the end of the albumen

gland, close to the point of entrance of the herm-
aphrodite duct, and runs forwards through the body
almost to the head, lying along the right side of the
crop and oesophagus, and slightly ventral to these.

It is very wide, and along the greater part of
its length is incompletely divided by a longitudinal
septum into two parallel ducts. Of these one, the
walls of which are thrown into very numerous and
prominent transverse folds, serves for the passage of
the ova ; while the other, which transmits the sper-
matozoa, has a comparatively smooth surface, but
has its walls thickened by a whitish glandular pro-
static mass. At their anterior ends the two ducts
separate completely from each other as oviduct
and vas deferens respectively.

Slit open the common duct longitudinally; and note the
septum, which partially divides it into male and female ducts.

5. The male duct and its accessory organs.

a. The vas deferens is a slender, slightly convoluted

tube of uniform diameter, which runs forwards
along the inner side of the female organs, between
these and the buccal mass, passes under the
retractor muscle of the right upper tentacle, and
then runs back to open into the base of the penis.

b. The flagellum is a long tubular diverticulum, which

arises from the vas deferens just before it reaches
the penis. It lies along the dorsal surface of the
crop and other viscera. In it the spermatozoa
are agglutinated together to form a long rod-
like body, the spermatophore.

REPRODUCTIVE SYSTEM. 133

c. The penis is a muscular tube continuous with the
vas deferens, and lying somewhat obliquely across
the oesophagus just behind the buccal mass : it
opens to the exterior at the genital pore, through
which it can be protruded.

To the penis is attached the retractor penis
muscle, a narrow muscular band, which arises
from the floor of the mantle-cavity, and runs
downwards and forwards to its insertion.

6. The female duct and its accessory organs.

a. The oviduct is a short thick-walled tube, running

forwards from the point at which it leaves the
vas deferens.

b. The vagina is a thick-walled tube into which the

oviduct opens in front, and which runs forwards
to open with the penis at the common genital
pore.

c. The mucous glands are two tufts of tubular glands,

opening by a pair of apertures into the vagina ;
each tuft consists of thirty to forty finger-like
processes.

d. The dart sac is a large pyriform sac opening into

the vagina just beyond the mucous glands. It
has very thick walls, and contains a quad-
rangular calcareous spicule.

e. The spermotheca, or receptaculum seminis, is a long

caecal diverticulum from the hinder end of the
vagina. It lies alongside the common duct, and
its hinder end is expanded into a globular dilata-
tion just behind the pericardium and kidney.
It is of about the same length as the flagellum,
and serves to lodge the spermatophore received
from another snail.

A small diverticulum is very commonly given
off about a third of its length from the vagina.

134 THE SNAIL.

In H. aspersa this is largely developed and forms
the longer portion of the spermotheca.

Care must be taken not to mistake the spermo-
theca for the vas deferens, nor to cut away its
globular dilatation during the dissection of other
parts.

V. DISSECTION OF THE NERVOUS SYSTEM.

The nervous system consists of paired ganglia, of commis-
sures and connectives uniting them, and of nerves running
from these central organs to all parts of the body. The
chief ganglia with their commissures and connectives form
a nerve-collar encircling the anterior end of the alimentary
canal. When the head is protruded this collar surrounds
the oesophagus immediately behind the buccal mass ; but
when the head is retracted the buccal mass is pulled back
through the collar, which then lies in front of it.

The ganglia are enveloped in a rather dense layer of
connective tissue, which must be removed before their shape
can be determined.

1. The supra-oesophageal or cerebral ganglia are a pair of
closely apposed pyriform masses, forming a broad
transverse band across the dorsal surface of the ali-
mentary canal.

From the outer ends of the ganglia large nerves
run to both pairs of tentacles and to the lips.

From their under or ventral surface a pair of buc-
cal nerves run forwards alongside the oesophagus,
and end in a pair of small buccal ganglia, which lie
at the outer sides of the salivary ducts close to their
openings into the buccal mass. The buccal ganglia
are connected by a transverse commissure, and from
them nerves arise supplying the buccal mass.

From the posterior and outer border of each
supra-cesophageal ganglion two stout connectives
pass downwards and slightly backwards to the sub-

SENSE ORGANS. 135

oesophageal ganglia. Between these two on each
side is the slender auditory nerve.

2. The sub-oesophageal ganglia are a number of closely

approximated ganglionic masses, the boundaries of
which are difficult to determine. The anterior aorta
runs through the centre of the mass, marking the
division between the antero-inferior portion, or pedal
ganglia, and the postero -superior portion, or viscero-
pleural ganglia.

a. The pedal ganglia are a pair of closely apposed

masses, from which a number of large nerves
pass to the foot.

b. The viscero-pleural ganglia consist of at least two

pairs of ganglionic masses, closely apposed to each
other and to the pedal ganglia. From them large
nerves pass outwards to the viscera and to the
body-wall.

3. The structure of the ganglia.

Remove one of the ganglia ; place it^ in 1 per cent, acetic
acid for a short time to soften the connective tissue, and then
tease it on a slide in glycerin. Cover; and examine it with
low and high powers.

The nerve-cells are large granular cells with very
large and distinctly reticulate nuclei, and with long
branching processes by which they are connected with
each other and with the nerve-fibres.

VI. THE SENSE ORGANS.

1. The eyes are placed one at the end of each of the larger
tentacles, and are only exposed when these are fully
protruded. Each receives a branch from the large
nerve supplying the tentacle.

Each eye is a small spherical vesicle, the wall of
which is strongly pigmented in its posterior portion,
and lined by a layer of retinal cells. The cavity of

136 THE SNAIL.

the vesicle is partially filled by a large cuticular lens,
which projects into it from its anterior wall.

2. The * auditory ' organs are a pair of small spherical sacs
imbedded into the substance of the pedal ganglia,
and containing numerous minute calcareous otoliths.
They are plabed at the ends of the auditory nerves,
which connect them with the supra-cesophageal
ganglia.

VII. DISSECTION OF THE BUCCAL MASS.

Remove the nerve-collar. Cut across the oesophagus and
salivary glands about a quarter of an inch behind the buccal
mass, and fix down the buccal mass firmly by pins through its
retractor muscles. Note the horny jaw, seen as a dark trans-
verse hoop across the front of the buccal mass : and the sac
of the radula, which forms a rounded projection from its hinder
end.

Slit up the oesophagus with scissors along the mid-dorsal
line, and continue the cut forwards along the buccal mass as
far as the jaw. Separate the two sides so as to expose the cavity
of the buccal mass.

1. The radula is a brownish-yellow chitinous ribbon lying
on the floor of the buccal cavity, and having its free
surface produced into an immense number of minute
backwardly directed teeth, arranged in very regular
transverse rows.

The hinder end of the radula is folded longitudi-
nally on itself, and lodged in a tubular sac, which
projects freely behind the buccal mass, and within
which the radula is formed as a cuticular secretion of
its epithelial lining. The anterior end is opened out
and rests on a cartilaginous cushion. By means of
protractor and retractor muscles the organ can be
protruded from the mouth, and worked backwards
and forwards with a licking or rasping action. As
it wears away in front, it is replaced by growth
forwards from its hinder end.

CIRCULATORY SYSTEM. 137

The radula with the cushions and the muscles
together make up the odontophore.

Dissect out the radula ; boil in 5 per cent, caustic potash ;
and mount it on the slide in a drop of water or glycerin, with
its toothed surface upward. Examine it with low and high
powers.

2. The jaw is a semicircular chitinous bar, strongly ridged
on its posterior surface, against which the odonto-
phore works.

Dissect out the jaw and examine it with a lens or the micro-
scope.

VIII. THE CIRCULATORY SYSTEM.

The dissection of the circulatory system must be performed
on a second snail.

It is greatly facilitated by injecting the vessels with
a coloured fluid. The arteries should be injected from the
ventricle, and the pulmonary vein and its branches from the
auricle. The venous system must be injected from several
places ; the right pulmonary sinus is easily injected both
forwards and backwards, and the pedal sinuses may also be

1. The arterial system.

From the ventricle arises a single large vessel,
the aorta. This gives off almost at once a large
visceral artery, which runs along the upper surface
of the liver to the top of the spiral, supplying both
lobes of the liver, the intestine, and the reproductive
organs.

The aorta itself runs forwards along the right
side of the crop, between it and the common duct,
giving off large branches to the salivary glands and
body-wall. It then continues its course forwards
along the under surface of the oesophagus ; pierces
the sub-oesophageal ganglionic mass, passing between
the pedal and viscero-pleural ganglia ; gives off a

138

THE SNAIL.

large pedal artery which runs back along the foot ;
and finally divides into large branches supplying the
tentacles and buccal mass.

The venous system.

The venous system consists partly of tubular
vessels with definite walls, and partly of irregular
lacunar spaces, the exact relations of which to the
arteries and to the body-cavity are not determined.

Fiu. 33.— Helix pomatia. Section taken along the axis of the shell, and
transversely to the foot. The shell is represented by the thick
black line. (a. m. m.)

A, umbilicus. AG, albumen gland. B, nucleus of shell. C, crop. D,
common duct. E, flagellum. F, foot. G, pulmonary sinus. HG, hermaphro-
dite gland. |, intestine. K, kidney. LL, left lobe of liver. MC, mantle-
cavity or lung. MG, mucous glands. P, pedal gland. PC, pericardial cavity.
PS, pedal sinu3. pv, pulmonary vessels. R, rectum. R|_, right lobe of
liver, rm, retractor muscles, s, spermotheca. SG, salivary gland. U,
ureter, v, ventricle.

CIRCULATORY SYSTEM. 139

Two large irregular pedal sinuses lie in the foot,
one on each side of the pedal gland.

A visceral sinus commences at the top of the
spiral and runs down it, receiving branches from
the various organs, to the posterior end of the
mantle-cavity, where it opens into the pulmonary
sinus, a large vein running all round the mantle-
cavity at the junction of its sides and floor. Along
the right side the pulmonary sinus lies immediately
ventral to the rectum, to which it is closely attached,
and from which it receives numerous small veins.
From the pulmonary sinus the blood is carried to
the roof of the mantle-cavity by the afferent pul-
monary vessels : these run parallel to each other
in the roof of the mantle-cavity, interdigitating with
the efferent vessels, with which- they are connected
by a network of thin-walled vessels in which the
blood is aerated. The efferent vessels unite to form
the pulmonary vein, a large trunk running straight
back in the roof of the mantle-cavity to the'heart.
Just before reaching the auricle, the pulmonary vein
receives a large renal vein from the kidney.

140

Chapter IX.

THE CRAYFISH. Astacus pallipes.

The freshwater crayfish is fairly common in many of the
streams of England and Ireland, hiding in holes under the
banks. It is of a greenish grey tint and is capable of some
degree of colour-change. In form as well as in internal
structure it closely resembles the lobster, to which it is nearly
related. It moves, about slowly, half walking on its long
thoracic legs, half-swimming, but when alarmed darts suddenly
backwards by violent flexion of its tail or abdomen.

The hard external cuticle of the crayfish is transversely
jointed, to allow movements of the body and limbs ; and this
segmentation affects also those internal organs which are
directly concerned with locomotion, i.e. the muscles, and the
nerves and blood-vessels supplying them. No segmentation,
however, is to be seen in the alimentary canal or its glands,
and none in the excretory and reproductive systems : in this
respect the crayfish differs widely from such animals as the
leech, in which these systems are arranged segmentary.

Each segment of the crayfish, except the last, bears a pair
of jointed appendages. These differ greatly in the various
regions of the body, but all are modifications of one common
type, and serially homologous with each other. The anterior
ones form tactile organs ; those in the region of the mouth act
as jaws, and the remainder as organs of locomotion. The
foremost pair of these latter are of great size, and end in
powerful pincers, used for prehension of food and for offen-
sive and defensive purposes.

The crayfish is carnivorous, seizing its food, and tearing it
to pieces by means of the large pincers, aided by the smaller

EXTERNAL CHARACTERS. 141

pincers of the next two pairs of legs, and then cutting it up
into little bits with the jaws, which are placed at the sides
of the mouth. The real chewing is effected afterwards in
the stomach itself.

The sexes are distinct. The male and female are very
similar, but the abdomen is broader in the female.

The eggs are comparatively large, and are carried about by
the female, attached to the abdominal appendages, until the
young are hatched. The newly hatched crayfish differs con-
siderably in form from the adult, but has the full number of
appendages already present. It remains for a time attached
by its pincers to the appendages of the mother, but soon takes
to independent life. To allow for the growth of the animal,
the cuticle, which is incapable of interstitial growth, is cast
off periodically, and a new one secreted. This ecdysis, or
shedding of the cuticle, occurs three or more times during
the first year of the animal's life, and afterwards about once
a year, or less often. It involves not merely the external
investment of the body and limbs, but the covering of the
gills and sense organs, and to a certain extent the lining of
the alimentary canal.

Crayfish may be killed almost instantaneously by drop-
ping them into boiling water.

I. EXTERNAL CHARACTERS.

The body of the crayfish is very obviously divisible into
an anterior un jointed portion, the cephalothorax, and a
jointed and flexible hinder portion, the abdomen.

A. Examination of a Typical Segment of the Body.

Examine in detail the third or fourth segment of the abdomen,
noticing the following 'points :

1. The cuticular investment or exoskeleton of the segment
is hardened by calcification, except the portions at
the joints, which remain soft to allow of movement.
Each hardened portion is called a sclerite.

M

142 THE CRAYFISH.

a. The tergum is the convex dorsal plate of the exo-

skeleton.

b. The sternum is the ventral transverse bar between

the bases of the limbs.

c. The pleura are a pair of plates projecting down at

the sides.

d. The epimera are the portions of hardened cuticle

between the pleura and the bases of the limbs.
They are very small in this segment.

Fia. 34. — Astacus pallipes, var. nobilis. Transverse section through
the abdomen, (c. H. H.)
DA, dorsal abdominal artery. EM, extensor muscles of the abdomen.
EP, epimeron. FM, flexor muscles of the abdomen. M, muscles of the
appendage. N, endopodite. NG, nerve-ganglion. P, protopodite : the
proximal segment, which is very short, has been almost entirely cut away : its
length is indicated by the notch on the outer side of the protopodite. PL,
pleuron. PR, protodceum. s, sternum. T, tergum. V, ventral abdominal
artery. X, exopodite.

e. Soft uncalcified portions connect the segment with

those in front of and behind it, and allow move-
ment of one segment upon another.

f. The joints between the successive segments are

provided with a rounded peg or pivot at each side

EXTERNAL CHARACTERS. 143

on the anterior margin of the tergum, near its
junction with the pleuron. This peg fits into a
corresponding depression in the hinder margin
of the segment next in front, and serves as a
fulcrum in the flexion and extension of the
abdomen ; it also prevents movement of the
abdomen from side to side.

2. The appendages are attached to the sides of the ventral
surface of the segment, between the epimera and
the ends of the sternum, by soft patches of cuticle,
the arthrodial membranes. Each appendage con-
sists of a two-jointed basal portion, the protopodite,
and of two many-jointed filaments borne on its
distal end. These are the endopodite and the
exopodite, the former of which is nearer the median
• plane.

B. The Body.

This is obviously composed of two parts, the cephalothorax
and the abdomen.

1. The cephalothorax is composed of thirteen segments
fused together, the fusion being so complete that
the segmentation is not recognisable on the dorsal
and lateral surfaces, though it can still be seen
below.

The cephalothorax is encased laterally and
dorsally by a large shield, the carapace. This is
divided into cephalic and thoracic portions, covering
respectively the head and the thorax of the animal,
by the shallow cervical groove, which runs trans-
versely across its dorsal surface, and obliquely for-
wards at the sides. The thoracic portion of the
carapace arises as an outgrowth from the hinder
border of the head, which grows back over the
thoracic segments, and is continuous with them
dorsally.

a. The head, or part in front of the cervical groove,

144 THE CRAYFISH.

is composed of five segments. In it are to be
noticed the following structures :

i. The rostrum is a median pointed prolongation
of the carapace, extending forwards from its
anterior end.

ii. The eyes are paired and mounted on movable
stalks, one at each side of the rostrum.

iii. The mouth is far back on the ventral surface of
the head, and almost hidden by the jaws. It
is bounded in front by a fleshy protuberance,
the labrum.

b. The thorax, or part behind the cervical groove, is
composed of eight segments. The following
points are to be noticed in it :

i. The thoracic portion of the carapace is divided
by two longitudinal grooves about half an
inch apart ; the branchio-cardiac grooves,
into a median dorsal portion covering the
heart, and two large plates, the branchio-
stegites, which extend down at the sides of
the thorax and cover the gills.

ii. The sterna of the thoracic segments are all fused
together, except the hindmost, which is mov-
able.

2. The abdomen is composed of seven movable segments,
united by peg-and-socket joints, which allow of
movements in a vertical plane only, the limit of
extension possible being nearly a straight line.

a. The first abdominal segment is smaller than the

succeeding ones, and is devoid of pleura -

b. The segments from the second to the sixth are

essentially like the typical segment already
examined.

c. The last abdominal region, or telson, consists of

four pieces •. — (I) the segment proper traversed

ABDOMINAL APPENDAGES. 145

by the end of the rectum, (2) the post-anal plate,
(3 and 4) the two furcal styles which project
backwards at the junction of (1) and (2).

C. The Appendages.

These are all reducible to a common type composed of the
three parts, protopodite, exopodite, and endopodite, already
seen in the appendage of the typical segment, together with
an epipodite, which may be a simple plate or may bear gill
plumes. From such a typical form all the appendages of
the crayfish can be derived by variations in form and relative
size of the various parts, or by total suppression of one or
more parts.

In the following description the appendages are numbered
from before backwards, and are distinguished by the large
Eoman numerals I. to XIX.

Remove the appendages of the right side one by one, beginning
with the hindmost, and taking special care to remove each appen-
dage entire by cutting round the inner side of its attachment
to the body. The characters of the smaller ones can be best made
out by examination in a drop of water, between two slips
of glass. Place them in corresponding positions and draw
them.

1. The abdominal appendages, excepting the hindmost,
are of small size, and serve in the female to carry
the eggs. The last or twentieth segment has no
appendages.

XIX. The sixth abdominal appendages are broad and
lamellar, and form with the telson the powerful
tail-fin.

i. The protopodite is short, broad and undivided.

ii. The endopodite is a flat oval plate, fringed at its
free edge with setae.

146 THE CRAYFISH.

iii. The exopodite is similar, but larger, and divided
by a transverse binge into two parts.

XVIII. The fifth abdominal appendage is a small swim-
meret.

i. The protopodite is two-jointed, the small proxi-
mal segment being named coxopodite, the
longer distal segment the basipodite.

ii. The endopodite is a many- jointed filament
covered with setae. Its first segment is the
largest.

iii. The exopodite is similar, but smaller.

XVII. and XVI. These two pairs of appendages closely
resemble XVIII.

XV. In the female this resembles XVIII., but is liable
to reduction, and may be absent.

In the male it is normally turned forwards
and is specially modified for transferring the
spermatozoa to the female. The protopodite
and exopodite resemble those of XVIII., but the
large proximal segment of the endopodite is pro-
duced distally on its inner side into a short
plate, rolled longitudinally upon itself to form an
imperfect tube : the remainder of the endopodite
is a small-jointed rod as in XVIII.

XIV. In the female this is reduced or absent.

In the male it is modified for the same purpose
as XV. It is an unjointed rod, the distal portion
of which is flattened and rolled upon itself to
form an imperfect tube. It is probably formed of
the protopodite and endopodite fused together.

2. The thoracic appendages, of which there are eight pairs,
fall readily into two groups : a posterior group of
five pairs of large appendages, XIII. to IX., used for
walking and prehension, and devoid of exopodites;

THORACIC APPENDAGES. 147

and an anterior group of three pairs of maxillipedes
or foot-jaws, VIII. to VI., which possess both exo-
podites and endopodites, and aid in passing food to
the mouth.

The thoracic appendages can be more easily removed, and
their relations determined, if the branchiostegite be first cut
away.

XII. The last but one of the thoracic legs may be taken
as a type of the series.

i. The protopodite consists of two segments, of
which the first, or coxopodite, is short and
broad, and bears a large gill and a tuft of long
fine hairs, the ' coxopoditic setae.' The second
segment, or basipodite, is much smaller, and
bears the endopodite.

ii. The endopodite consists of five segments. Of
these the last is claw-like.

XIII. This, the last thoracic leg, differs from XII. in
having no gill. In the male the coxopodite is per-
forated at its inner angle by the genital pore.

XI. This resembles XII., but the penultimate seg-
ment is prolonged distally so as to form with the
last segment a chele or pair of pincers. The inner
angle of the coxopodite is perforated by the genital
pore in the female.

X. This is like XL, but has no genital pore.

IX. This is similar to X., but much larger and stouter,
the terminal chele being especially large and
powerful.

VIII. The third maxillipede is directed forwards under
the five appendages in front of it.

i. The protopodite consists of coxopodite, bearing

148

THE CRAYFISH.

THORACIC AND HEAD APPENDAGES. 149

a gill and coxopoditic setae; and basipodite,
bearing endopodite and exopodite.

ii. The endopodite consists of five segments. The
first segment is the largest and is fused with
the basipodite.

iii. The exopodite is slender, and consists of a long
proximal segment and a distal many -jointed
filament.

VII. The second maxillipede is smaller and less firmly
calcified than VIII., which it resembles in form ;
but the exopodite is larger than in VIII., and
the endopodite smaller, and all its joints are
movable.

VI. The first maxillipede is smaller and softer than
VII.

i. The two segments of the protopodite are ex-
panded on the inner side, forming a large two-
lobed lamella. A broad plate, the epipodite,
takes the place of a gill.

ii. The endopodite is very small and two-jointed.

iii. The exopodite resembles that of VII.

In the Continental variety (Astacus fluvi-
atilis) a small gill is present attached to this
limb.

3. The head appendages consist of three pairs of oral
appendages or jaws, V. to III., placed at the sides of
the mouth, and two pairs of pre-oral appendages,
II. and I., at the front. of the head.

Fig. 35. — Astacus pallipes, var. nobilis. The oral appendages of the

right side seen from below : x 2£. (c. h. h.)
HI, the mandible. IV and V, the first and second maxillae. VI, VH,

and Vni, the three maxillipedes.

A, region of attachment to body-wall. AB and AD, points of insertion
of abductor and adductor muscles of mandible. B, basipodite. C, D, I, M, P,
joints of endopodite. CX, coxopodite. E, epipodite. F, pivots of mandible.
G, gill. H, coxopoditic setee. N, endopodite. S, scaphognathite. X, first
segment of exopodite.

150 THE CRAYFISH.

V. The second maxilla consists of a protopodite, an
endopodite, and on the outer side an exopodite
which is subservient to respiration, the scapho-
gnathite.

i. The protopodite consists of two segments, coxo-
podite and basipodite, each of which is ex-
panded on its inner side into a bilobed lamella.

ii. The endopodite is small, pointed, and unjointed.

iii. The scaphognathite or exopodite is an elongated
plate attached by the middle of its inner edge
to the outer side of the basipodite. It serves
to keep a current of water flowing through
the gill-chamber (p. 155).

IV. The first maxilla is small. The two segments of
the protopodite extend inwards as thin curved
lamellae, and the endopodite is a small plate to
their outer side. There is no exopodite.

III. The mandible consists of a very stout undivided
protopodite, forming the powerful jaw at the side
of the mouth ; and an endopodite, which forms a
small three-jointed palp, lying in a groove on the
front of the protopodite.

Before removing the mandible, note the position of the two
pivots about which it turns, and the points of insertion of the
tendons of its abductor and adductor.

Immediately in front of the first maxilla and be-
hind the mandible is a small unjointed plate, leaf-
like in form. It is not regarded as a true appendage,
but as an outgrowth of the metastoma, which forms
the posterior and lateral 'borders of the mouth.

II. The antenna consists of protopodite, exopodite,
and endopodite, the last of which forms the long
' feeler.'

i. The protopodite consists of two segments ; the first,

THE GILLS. 151

or coxopodite, being very broad, and projecting
only very slightly from the surface of the head :
on its ventral surface is a tubercle perforated
posteriorly by the renal aperture. The second
segment, or basipodite, is also short and stout,
and bears the endopodite and exopodite.

ii. The endopodite has three stout basal segments and
a long slender many-jointed feeler. The first
segment is divided longitudinally into an inner
and an outer part, movable on each other, and
thus allowing rotation of the next segment.

iii. The exopodite is a stout sharp-pointed blade,
the scaphocerite.

I. The antennule is smaller than the antenna.

i. The protopodite is three-jointed, the basal segment
being much the largest, and containing the
* auditory ' organ, which opens on its dorsal
surface. The aperture is three-cornered and
is guarded by a series of plumose setae, which
extend across it from its outer lip.

ii. The endopodite and exopodite are two many-
jointed filaments, the exopodite being slightly
the larger.

D. The Gills and Gill-chambers.

The gills are highly vascular external outgrowths from
the sides of the thorax and from its appendages. Primi-
tively in them respiration occurs, this process consisting in
an interchange, by diffusion through the thin cuticle, of the
gases dissolved in the water and in the blood. The gills
are enclosed and protected by downward growths of the
carapace at the sides of the thorax, the branchiostegites.

1. The branchial chambers are situated one at each side of
the thorax, between the body and the branchiostegite.

Expose the left branchial chamber by cutting away the

152

THE CRAYFISH.

branchiostegite of that side; and fix the animal on its right

side under water.

The chamber is bounded on its inner side by
the thoracic wall, and on its outer side by the
branchiostegite. It is open below and behind, and
the former opening is partially stopped by the basal
segments of the legs. The greater portion of the

Fig. 3G. — Astacus pallipes. Diagrammatic transverse section through
the thorax. The dotted spaces contain arterial blood. The spaces
shaded with horizontal lines contain venous blood. The arrows
indicate the direction of the blood circulation, (c. H. h.)

AB, arthrobranchia : only one is figured on each side. AF, afferent vessel
of gill. AM, arthrodial membrane. B, branchiostegite. BC, branchio-cardiac
canal. EF, efferent vessel of gill. EM, extensor muscle of the abdomen. ESt
endophragmal skeleton. FM, flexor muscle of the abdomen. H» heart, showing
the dorsal ostia, with their valves. L, liver. N , nerve-connective. P, procto-
deum. PB, podobranchia. PL, pleuro-branchia. PS, pericardial sinus. T,
testis, with roots of vasa deferentia. v, ventral thoracic artery. VA, artery
supplying leg. VS, ventral blood-sinus : it contains numerous large muscles
moving the limb3, which are not represented in the figure. W, one of the
legs, oat off short.

THE GILLS. 153

chamber is occupied by the gills ; but in front of
them, and just behind the line of the cervical groove,
is a channel, the cervical canal. In this canal the
scaphognathite works to and fro with a sculling
movement, driving the water forwards out of the
gill-chamber, and discharging it in front just below
the renal aperture. (See Section E, p. 155.)

2. The gills may be described in three groups according
to their position of attachment.

a. Podobranchiae are gills attached to the coxopodites

of appendages VII. to XII. The coxopodite of
VI. bears a large epipodite in place of a gill.

b. Arthrobranchiae are gills arising from the arthrodial

membranes at the bases of the thoracic appen-
dages. At the base of each of the limbs VIII. to
XII. are two arthrobranchiae, an anterior and a
posterior one ; and attached to the arthrodial
membrane of VII. is a single one.

c. Pleurobranchiae are gills arising from the side-wall

of the thorax, above the ridges which form the
dorsal boundaries of the arthrodial membranes.
The crayfish has only one well-developed pleuro-
branchia on each side, on the thirteenth segment,
but abortive ones are found on the next two or
three segments in front.

Primitively all the gills arose as outgrowths
of the basal joint of the appendages, three to
each. Movements of these appendages induced
shaking of the gills. With the increased size
and strength of the animal, improved respi-
ratory mechanism arose. The branchiostegites
covered over the gills, and the latter shifted
their points of attachment proximally, so that
now only the podobranchiae retain their original
position

154

THE CRAYFISH.

The following table shows the number and
arrangement of the gills in the crayfish :

Rud.=Abortive rudiment. Ep.= Epipodite.

Segment

VI.

VII.

VIII.

IX.

X.

1

1
1

Rud.

XL

1

1

1

Rud.

XII.

XIII.

Totals

Podobranchiae x .
Arthro. anterior
Arthro. posterior
Pleurobranchise .

Ep.
0
0
0

1
1
0
0

1

1
1

°

1
1

0

1

1

1

Rud.

0
0
0

1

6-r*Ep.

5f-H
1 + 3 Rud.

1 See p. 149.

Verify the above table by carefully counting the gills, and
turning them down as they are checked off in the table.

3. The structure of the gills is different in the different
groups.

Remove and examine in turn a pleurobranchia, an arthro-
branchia, and a podobranchia.

a. The pleurobranchise and arthrobranchise resemble

bottle-brushes, each consisting of a series of
delicate branchial filaments arranged upon a
central stem, which is traversed by afferent and
efferent blood-vessels. It is in the branchial
filaments that respiration is effected, diffusion
taking place through the thin cuticle covering
them.

b. A podobranchia is more complicated, and consists of

(i.) a basal plate arising from the outer surface
of the coxopodite, and covered with finely
plumose setae ; (ii.) a stem arising from the
dorsal border of the basal plate close to its
apex ; (iii.) a lamina, which is a corrugated
plate borne on the distal end of the stem,
doubled longitudinally upon itself, and beset
with small hooked setse ; and (iv.) a plume,
which arises from the apex of the stem and
resembles an arthrobranchia. The plume and
the outer face of the stem are covered with
branchial filaments.

CIRCULATORY SYSTEM. 155

E. Demonstration of the Respiratory Current of Water
through the Gill-chamber.

Place a living crayfish in a shallow dish of water ; and,
when the animal is at rest, run into the dish close to the bases
of the hinder legs, by means of a pipette, a few drops of water
coloured with suspended carmine or other pigment. Watch the
currents entering under the edges of the branchiostegites behind,
and issuing in front from the mouths of the cervical canals.

II. DISSECTION OF THE CRAYFISH.

The several systems are described below in the order in
which it will be most convenient to dissect them : this order
may be varied if found desirable.

A. The Circulatory System.

The blood of the crayfish, which is slightly bluish, is
carried from the heart to the various parts of the body by
arteries which open into large lacunar spaces forming the
body-cavity or haemoccel. From this it passes to the gills, in
which it is aerated, and from which it is conveyed through
the branchiocardiac canals to the pericardial sinus. From
the sinus it enters the heart through the apertures in its
walls.
1. The pericardial sinus and heart.
Insert one blade of a stout pair of scissors into the slit
behind the carapace. Cut forwards, holding the blade horizon-
tally, right up to the rostrum, and then across and downwards
to the outer side of the eyes. Remove the carapace completely.
Next cut backwards along the outer edge of the abdominal terga
on each side, avoiding injury to the muscles, and carefully
remove the tergal sclerites. Clean away the pigmented epidermis
with forceps.

a. The pericardial sinus is a cavity of considerable
size lying in the dorsal portion of the thorax
and receiving the arterial blood from the gills.

156

THE CRAYFISH.

CIRCULATORY SYSTEM. 157

b. The heart is a polygonal thick-walled muscular

sac lying in the pericardial sinus. If living it will
be seen to contract rhythmically. It receives
blood from the pericardial sinus by three pairs of
valvular apertures, the ostia, of which the dorsal
pair will be easily seen. The lateral and ventral
pairs will be better seen at a later stage of the
dissection.

c. The roots of the arteries are rather small and trans-

parent, and hence difficult to see. They arise

from the two ends of the heart, and should be

noticed at the present stage of the dissection,

though the arteries themselves are best seen in

an injected specimen, and will be described later

(p. 167).

i. From the anterior end of the heart arise a median

ophthalmic artery and a pair of antennary

arteries ; and behind and below these a pair of

hepatic arteries.

Fig. 37. — Astacus pallipes, var. nobilis. Dissection of a male from
the right side. The right sictaof the body-wall with its appendages
and the right side of the oesophagus, stomach, and mesenteron
have been removed. The thick black line represents the cut edge
of the cuticle, (c. H. h.)

A, endophragmal skeleton. AA, antennary artery. AU. 'auditory' sac.
B, basipodite of the tenth appendage. BD, aperture of left bile-duct into the
mesenteron C, cardiac chamber of the stomach. CM, caecum. CO, cardiac
ossicle. CP, middle joint of tenth appendage. CP9, middle joint of ninth
appendage. CX, coxopodite of tenth appendage. D, terminal joint of
same. DA, dorsal abdominal artery. E, eye. G 1, supra-oesophageal gan-
glion. G 2, sub-oasophageal ganglion. G 3, second post-cesophageal ganglion.
G 8, last thoracic ganglion. G 14, last abdominal ganglion giving off posterior
visceral and other nerves. H, right lateral ostium of the heart. H A, hepatic
artery. |, proctodceum. IS, first joint of tenth appendage. K, kidney.
L, fiver. LT, lateral tooth. M, second joint of tenth appendage. MS,
metastoma. MT, median tooth, m 8, second joint of eighth appendage.
Nl, N 3, etc., endopodites of first, third, etc., appendages. OA, ophthalmic
artery. OC, left para-cesophageal connective : the greater part of the right
one has been removed. OE, oesophagus. PL 16, pleuron of the sixteenth
segment. PO, pyloric ossicle. PPO, prepyloric ossicle. PR, propodite of
the tenth appendage. PTO, pterocardiac ossicle. PY, pyloric chamber of
the stomach, p 3, p 4, etc., protopodites of the third, fourth, etc., appendages.
R, rostrum. SA, sternal artery, s 16, sternum of the sixteenth segment.
T, testis : the right anterior lobe has been removed. T 16, tergum of the
sixteenth segment. U, ureter. UA, aperture of the ureter. UCO, urocardiac
ossicle. V, ventral thoracic artery. VA, ventral abdominal artery. VD, left
vas deferens : the right one has been cut off short. VDA, aperture of the left
vas deferens. X 1, X 2, etc., exopodites of first, second, etc., appendages.
ZO, zygocardiac ossicle. 1 4, the fourteenth appendage.

158 THE CRAYFISH.

ii. From the hinder end of the heart arises the
large median dorsal abdominal artery, and

immediately below this the large median
sternal artery, which latter can be better
seen at a later stage.

d. The alae cordis are three pairs of fibrous bands
which hold the heart in position by connecting it
with the walls of the pericardial sinus.

Remove the branchiostegite and turn down the gills of the
left side, unless this has already been done.

2. The branchiocardiac canals are a series of channels in
the body-wall conveying blood from the gills to the
pericardial sinus. They can be seen through the
transparent body-wall running from the bases of
the gills, and converging to open into the sides of
the pericardial sinus.

Cut a gill off close to its base, and blow into the cut end
of the efferent blood-vessel with a blowpipe, or inject some
coloured fluid into it, and observe the air or coloured fluid
entering the pericardial sinus.

B. The Reproductive System.

Carefully remove the side- wall of the thorax, and expose the
reproductive organs.

a. In the male.

i. The testis is a soft white body lying immediately
below the pericardial sinus, and extending
backwards a little behind it. It consists
of a pair of anterior lobes and a median
posterior lobe.

ii. The vas deferens of each side is a long con-
voluted tube, running from the junction of
the anterior and posterior lobes of the testis
to the external aperture on the coxopodite of
the last thoracic leg. Before the breeding

DIGESTIVE SYSTEM. 159

season it will be found filled with an opaque
white seminal mass.

Treat a small piece according to directions on p. 82 ; cover,
and examine it with low and high powers.

iii. The spermatozoa are flattened spheroidal cells
from which project tangentially a number of
stiff immobile curved processes.

b. In the female.

i. The ovary is a saccular organ situated below and
at the sides of the pericardial sinus, and
extending some distance behind it. Like the
testis, it consists of two anterior lobes and a
posterior one.

ii. The oviduct of each side is a short wide almost
straight tube, running from about the middle
of the length of the ovary, with which it is
continuous, to the external opening on the
coxopodite of appendage XL, the last thoracic
leg but two.

C. The Digestive System.

The alimentary canal of the crayfish is a tube running in a
nearly straight line from the mouth to the anus. Of this tube
the middle portion or mesenteron, which is very short, is alone
formed from the primitive alimentary tract of the embryo,
and the ' liver ' is an outgrowth of it. The stomodceum, or
anterior portion of the canal, and the protodoeum, or hind
portion, which together form almost the whole length of the
canal, are both formed by invagination of the external surface
of the body ; and both have a chitinous fining which is con-
tinuous, at the mouth and anus respectively, with the chitinous
external covering of the body.

1. The digestive glands.

a. The * liver ' is a large bilobed body, of a brown or
greenish colour, lying in the thorax at the sides

160 THE CRAYFISH.

of the alimentary canal. Each lobe is much
branched and hollow, and its secretion enters the
mesenteron by a wide bile-duct, which will be
seen later on in the dissection.

2. The alimentary canal.
Remove the abdominal terga, the genital duct, and the
greater part of the liver of the left side, and as much of the muscles
and cephalic carapace as is necessary to expose the alimentary
canal.

a. The stomodoeum, or first portion of the alimentary

canal, has a chitinous lining.

i. The mouth is an oval aperture on the ventral
surface between the jaws.

ii. The oesophagus is a short wide tube running
vertically upwards from the mouth.

iii. The * stomach,' or rather gizzard, is a spacious
dilatation occupying the greater part of
the head, and extending a short distance into
the thorax. It consists of a larger cardiac
chamber in front and a smaller pyloric cham-
ber behind, separated from each other by a
constriction. Its chitinous lining is thick,
and some parts of it are especially hardened,
forming the gastric mill, a complex mecha-
nism of * ossicles ' supporting three large and
strongly calcified teeth. These structures will
be seen at a later stage of the dissection.

b. The mesenteron or mid-gut, which follows the

stomach, is very short and devoid of chitinous
fining. On each side it receives a wide bile-duct
from the liver, and its dorsal wall is produced
into a small caecum.

c. The protodceum or intestine is a long narrow

straight tube running from the hinder end of
the mesenteron to the anus. It has a chitinous

DIGESTIVE SYSTEM. 161

lining throughout, and its inner surface presents
longitudinal and slightly spiral ridges beset
with small papillae.

The anus is a longitudinal slit on the ventral
surface of the telson : it can be closed by ap-
proximation of its sides.

Remove a piece of the intestine, and examine its inner
surface under water with a low power of the microscope, to
see the ridges and papillce.

d. The gastric mill is a complex structure made up of numerous

sclerites, or hardened portions of the cuticle lining the

stomach, by means of which the chewing of the food is

effected : the principal sclerites will alone be described here

viz. five median dorsal pieces and two pairs of lateral pieces,

the others being small and of less importance.

Remove the entire stomach from the body, avoiding injury to the

structures around the oesophagus ; place it in a dish under water, and

carefully clean away the muscles.

i. The cardiac ossicle is a broad antero-trans verse sclerite in the

roof of the cardiac chamber of the stomach,
ii. The urocardiac ossicle is a median dorsal sclerite in the
hinder wall of the cardiac chamber. Its upper end is
broad, and hinged to the hinder edge of the cardiac
ossicle ; its lower end is narrow, and directed downwards
and backwards,
iii. The pyloric ossicle is a narrow postero -transverse sclerite

in the dorsal wall of the pyloric chamber.
iv. The prepyloric ossicle lies in the anterior wall of the pyloric
chamber, and is hinged above to the anterior border of
the pyloric ossicle,
v. The median teeth and processes are strong calcified sclerites,
at the junction of the cardiac and pylorio chambers.
These are fused with the urocardiac ossicle in front and
the prepyloric behind,
vi. The pterocardiac ossicles are attached one to each end of
the cardiac ossicle by oblique hinges. Their lower ends
are curved backwards,
vii. The zygocardiac ossicles are a pair of stout sclerites, run-
ning from the outer ends of the pyloric ossicle forwards,
downwards, and outwards, to the ventral extremities of
the pterocardiacs. Their ventral margins are curved
inwards to form a pair of stout plates, calcified at their

162 THE CRAYFISH.

inner borders to form the lateral teeth. The inner edge
of each lateral tooth bears a row of denticles, which
decrease in size from before backwards.

viii. The filter. The aperture between the cardiac and pyloric
chambers is much narrowed by lateral folds ; and the
cavity of the pyloric chamber itself is reduced by two
lateral and a median ventral fold to a narrow three-rayed
fissure.

All these folds bear short close-set hair-like setae, which
cross each other from opposite sides so as to form a very
efficient filter or strainer, permitting the passage only of
fluid or of very finely divided solid particles.

The gastric mill is put into action by muscles arising from the
carapace and attached to the cardiac and pyloric ossicles,
which, by their contractions, they pull away from each other.
The cardiac ossicle moving forwards pulls the urocardiac and
the median tooth with it, and the prepyloric ossicle rotates
forwards with them, giving a downward movement to the
hinder part of the tooth, and causing its apex to rotate
forwards. The same movement of the cardiac ossicle
forwards, and of the pyloric backwards, causes the zygo-
cardiacs to pull back the pterocardiacs, which rotate about
the oblique hinges, pressing the zygocardiacs inwards, and
bringing the lateral teeth together in the median plane.

Pull the cardiac and pyloric ossicles forwards and backwards respectively
with forceps, when the three teeth will be seen to come together.

D. The Excretory System.

1. The kidneys, or ' green glands,' are a pair of rounded

and somewhat flattened bodies, of a light-green
colour, lying in the ventral part of the head, in
front of the mouth.

2. The ureters are a' pair of wide thin-walled sacs, lying

one on the dorsal surface of each kidney. Each
ureter communicates directly with the cavity of its
kidney, and opens to the exterior -by an aperture
already seen on the hinder surface of a tubercle on
the basal segment of the antenna.

Inflate the ureter by blowing air in at its external aper-
ture.

I

NERVOUS SYSTEM. 163

E. The Endophragmal Skeleton (fig. 37, p. 156).

In the thorax a series of calcified plates project inwards
from the cuticle of the ventral surface, forming an internal
skeleton^ which gives origin to the muscles of the legs and to
the powerful flexor muscles of the abdomen.

F. The Nervous System.

1. The central nervous system consists of a series of
ganglia arranged in pairs, and united by paired
bundles of nerve-fibres, or connectives, to form a
chain, which runs along the whole length of the
body close to the mid- ventral line. The two ganglia
of each pair are usually fused indistinguishably.

a. The pre-oesophageal ganglionic mass, or ' brain,' is

a white mass of considerable size, situated just
behind and slightly above the bases of the anten-
nules. It gives off nerves supplying the eyes,
the antennules, and the antenna).

b. The para-cesophageal connectives are a pair of long

nerve-cords, on each side of the oesophagus,
connecting the ' brain ' with the hinder part of
the nervous system. They are connected with
each other by a transverse commissure, imme-
diately behind the oesophagus.

c. The post-cesophageal ganglionic mass is an ovoid

mass, lying a short way behind the mouth.
From it arise the nerves supplying the mand-
ibles, both pairs of maxillae, and the first and
second maxillipedes.

d. The thoracic nerve-chain consists of six ganglionic

masses, united by paired connectives, and
supplying the appendages from the third
maxillipedes to the last thoracic legs. The first
one is situated immediately behind the post-
cesophageal ganglionic mass, with which it is
closely connected. The connectives between

164 THE CRAYFISH.

the fourth and fifth ganglionic masses diverge
slightly, and the sternal artery (p. 167) passes
between them.
The thoracic nerve-chain lies beneath the endqphragmal
skeleton, which must be removed to expose it.

e. The abdominal nerve-chain consists of six ganglionic
masses, joined by connectives, and supplying
the abdominal segments and their appendages.
The hindmost ganglionic mass is larger than
the others, and supplies both the nineteenth
segment and the telson.

2. The visceral nervous system.
This can only be dissected in a specimen in which the ali-
mentary canal has been left undisturbed.

a. The anterior visceral nerve is a median nerve

formed, just in front of the oesophagus, by the
union of a median nerve from the ' brain ' with
two pairs of nerves arising from ganglionic
swellings on the para-cesophageal connectives.
From the union of these, the nerve runs up in
front of the stomach to branch out on its dorsal
wall.

b. The posterior visceral nerve arises from the hind-

most ganglionic mass of the abdominal nerve-
chain, and runs forwards along the ventral
surface of ihe intestine.

G. The Organs of Special Sense.

Besides tactile organs, i.e. the antennules, antenna?, and
the palps of the oral appendages, the crayfish has eyes, and
1 auditory ' and olfactory organs.

1. The eyes, on movable stalks, have already been seen at
the sides of the rostrum.

a. External characters of the eye.

Remove one of the eye-stalks, and examine it carefully
with a lens.

THE EYE. 165

The * cornea ■ is tlie transparent, apparently
black, cap of uncalcified cuticle covering the end
of the eye-stalk, and bounded by an oval outline.
Its surface is divided into quadrangular facets by
lines crossing each other nearly at right angles,
each facet corresponding to one of the elements
of which the compound eye consists.

b. Structure of the eye as seen in a longitudinal section.

Carefully divide the eye-stalk and eye longitudinally into
two halves. The calcified portion of the cuticle is best cut with
scissors, the remainder with a sharp knife. Examine one of
the cut surfaces with a strong lens. Better sections are obtained
by fixing the eye of a crayfish just after changing its skin :
imbedding in paraffin or celloidin, cutting and staining. An
allied crustacean, Squilla, is still more suitable.

The eye consists of a number of conical bodies
arranged in a radiate fashion, their bases being
turned outwards and formed by the corneal
facets, and their apices being directed inwards
and connected with the optic ganglion. Each
of these component elements is isolated optically
from its neighbours by pigment.

i. The cornea is thin, and the strips of cuticle
separating its facets scarcely differ from the
facets themselves.

ii. The crystalline pyramids are transparent conical
bodies arranged radially and abutting upon
the cornea, each pyramid corresponding to one
corneal facet. They are separated from each
other by movable layers of ' iris ' pigment.

iii. The inner ends of the pyramids are imbedded in
a movable layer of retinal pigment, in which
lies a series of radially arranged retinulae,
each of the latter being surrounded by a

166 THE CRAYFISH.

layer of brilliant tapetal pigment, either white
or pink,
iv. The optic ganglion (really ganglia) occupies the
eye-stalk. Its surface is deeply pigmented.

v. The optic nerve lies in the axis of the eye-stalk,
and connects the optic ganglion with the
1 brain.'

2. The ' auditory > organs or * statocysts ' are lodged in the
basal segments of the antennules, and open to the
exterior on their dorsal surfaces by three-cornered
apertures, guarded by fringes of plumose setae.

Remove an antennule entire. Holding it firmly between
the fingers, carefully remove with scissors the inner and outer
walls of the basal segment, taking care not to injure the ' auditory '
sac itself Clear away the muscles surrounding the sac, and
the setos from the aperture.

With a fine-pointed syringe wash out the contents of the
' auditory ' sac — a collection of sand and other foreign matter
from the water in which the animal lives, to which the name
1 otolith ' has been given.

Examine the sac in water or glycerin on a slide under a
low power of the microscope.

a. The * auditory * sac is a transparent ovoid chitinous

sac, continuous at its opening with the cuticle of
the antennule.

b. The sensory setae are nearly straight simple-pointed

setae varying greatly in size, the outer ones being
the largest. The larger ones are arranged in
parallel curved lines along the floor and inner
wall of the sac ; of these there are two rows
behind, the number increasing to five or more
in front. Parallel to these runs a row of very
minute setae on the inner side.

c. The sensory nerve supplying these rows of setae

may have been removed with the muscles, but
should be looked for.

THE ARTERIES. 167

3. Olfactory setae. On the ventral surface of each joint of
the exopodite of the antennule are two tufts of fine
sensory setae. Under a low power of the microscope
each is seen to consist of two segments, of which
the distal one is much the longer, and is flattened.
They are probably olfactory in function.

III. THE ARTERIES AS SEEN IN AN INJECTED SPECIMEN.

Bore a small hole in the carapace immediately above the
heart : inject with Prussian blue, using a syringe just large
enough to fill the hole. Dissect from the left side, removing
the left half or more of the carapace, and such parts of the
liver, genital gland and duct, and of the side-wall and muscles
of the abdomen as may be necessary to expose the following
arteries :

1. The ophthalmic artery is a median artery arising from

the anterior end of the heart and running forwards
over the stomach, then curving downwards to supply
the front of the stomach and the oesophagus.

2. The antennary arteries arise from the anterior end of

the heart, one on each side of the ophthalmic artery.
Each runs forwards, outwards, and downwards over
the side of the stomach, to which it sends a gastric
artery : and then forwards to the anterior end of
the head, supplying the kidney, antenna, antennule,
and eye, and sending a branch to the rostrum. On
its way it gives off numerous small arteries to the
muscles of the regions which it traverses.

3. The hepatic arteries arise from the ventral surface of

the heart near its anterior end, and supply the liver,
in which they are buried.

4. The sternal artery is single, and arises from the hinder

end of the heart, whence it runs directly downwards,
passing to the right or left of the intestine, and
between the connectives uniting the fourth and fifth
ganglion-pairs of the thoracic nerve-chain (p. 163).

168 THE CRAYFISH.

It then divides into two arteries, running forwards
and backwards respectively.

a. The ventral thoracic artery runs forwards beneath

the nerve -chain, supplying the sternal region of
the thorax, and the appendages from the hind-
most ambulatory legs to the mandibles.

b. The ventral abdominal artery is a large median

vessel given off by the sternal artery on reaching
the ventral region of the body. It runs back-
wards beneath the nerve-chain, and supplies the
ventral region of the abdomen and the abdominal

The dorsal abdominal artery is a large median artery
arising from the ventral part of the hinder end of
the heart, and running backwards just above the
intestine to the hinder end of the body, supplying
the intestine, muscles, and other parts of the dorsal
region of the abdomen. It communicates at its
hinder end with the ventral abdominal artery.

169

Chapter X.

THE COCKROACH. Periplaneta americana.

The common cockroach or ' black beetle ' of our kitchens and
bakehouses — Periplaneta (Blatta) orientalis — is not a true
beetle, but an insect belonging to the order Orthoptera, of
which grasshoppers, locusts, and crickets are also members.
This species differs only slightly from P. americana, which
has been chosen for description because it is larger and more
easily dissected than the common form. P. americana may
be found in large numbers on board ships and at most docks.

Cockroaches are active insects, of nocturnal habits, hiding
away in the daytime and coming out to feed at night. They
will devour almost any animal or vegetable substance.

The whole animal is covered by a chitinous cuticle which
is thick and hard, and of a dark-brown colour, except at the
joints, where it is soft and paler. By these joints the body is
divided transversely into segments, which are movable on
each other except in the head. As in the crayfish, the seg-
mentation affects certain of the internal organs, as the
muscular and nervous systems, but is not shown by the
digestive, excretory, or reproductive organs.

The female lays its eggs in a hard oblong capsule, in which
they are placed side by side. The young develop without
metamorphosis, i.e. when newly hatched they resemble the
adult in form and habits, though they are devoid of wings.
During growth the cuticle is shed several times. The wings
appear at the last ecdysis, when the animal attains sexual
maturity : the female of P. orientalis is, however, wingless. (

170 THE COCKROACH.

I. HABITS.

Watch the living animal in a glass vessel, noting especially
the positions of its legs and head ; also the mode of using the
long antenna?,, and the maxillary and labial palps.

Watch the respiratory movements of the abdomen. Feed
with sopped bread, and watch the lateral movement of the
jaws.

II. EXTERNAL CHARACTERS.

Kill the animal by pouring two or three drops of chloro-
form upon it.

A. The Body.

The body is divided into head, thorax, and abdomen, each
of which is composed of several segments. A small neck
connects the head with the thorax.

1. The head is broad from side to side, and short from
before backwards, and is produced downwards into
a broad process bearing the jaws. Owing to its
shortness and vertical elongation, it has a deceptive
appearance of being bent downwards and back-
wards beneath the thorax.

a. The epicranium covers the dorsal and posterior

surfaces of the head. It is divided into right
and left halves by a median suture which divides
in front into two, running obliquely outwards to
the fenestra (p. 171).

b. The clypeus is a broad plate covering the front of

the head, below the epicranium.

c. The labrum is a smaller plate hinged to the lower

edge of the clypeus, and forming the anterior or
1 upper ' Up.

d. The gense, or ' cheeks,' are a pair of vertical

plates covering the sides of the head, behind
and below the eyes.

EXTERNAL CHARACTERS.

171

e. The eyes are large black reniform elevations, one

on each side of the head. They are compound,
each having very numerous hexagonal facets.

f. The fenestra are a pair of small white oval patches

to the inner sides of the bases of the antennae.

2. The neck is short and slender : its cuticular covering
is soft and white, with the exception of seven
thickened patches or sclerites, which are hard and
brown. Of these sclerites two are slender trans-

Eig. 38. — Periplaneta americana. The head, front view. (c. h. h.)
A, autenna. CL, clypeus. E, eye. EP, epicranium. F, fenestra. G,
galea. GE,gena. |_, lacinia. LP, labial palp. LP., labrum. MN, mandible
(the left mandible has been removed). MP, maxillary palp. M 2, second
maxilla.

Fig. 39. — Periplaneta americana. The head from behind, (c. h. h.)
E, EP, G, L, LP, MN, and MP, as in the preceding figure.
C, car do. LG,ligula. M, mentum. N, nerve-connective. O, oesophagus.

PG, paraglossa. S, stipes. SD, salivary ducts. SM, submentum. TR,

cervical tracheae.

verse bars on the ventral surface ; a third is median
and dorsal, and almost divided into two longitudi-
nally ; the remaining four are much larger, and
form two pairs at the sides of the neck.

3. The thorax consists of three segments.

a. The prothorax is the first and longest segment of

172 THE COCKROACH.

the thorax. Its tergum, the pronotum, is pro-
longed forwards over the neck.

b. The mesothorax is smaller, and its tergum bears at

its anterior corners a pair of elytra, or wing-
covers. These are folds of skin and cuticle which
extend backwards beyond the hinder end of the
abdomen, and are movably articulated to the
mesothoracic tergum or mesonotum.

[The elytra are abortive in the female of P.
orientalist]

c. The metathorax, or hindmost segment of the thorax,

bears a pair of wings, which are membranous
outgrowths of the body-wall, like the elytra, but
broader, thinner, and more movable. By means
of these the animal is able to fly. When not in
use they are folded longitudinally like a fan
and covered by the elytra.

[The female of P. orientalis has no wings.]

4. The abdomen, which forms rather more than half the
length of the body, is broad from side to side,
especially in the female, and is flattened dorso-
ventrally. Its cuticular investment is softer than
that of the head and thorax. It is composed of
ten distinct segments, but the hinder ones are tele-
scoped within each other, so that the full number
are not seen at once. The podical plates may
represent the tergum of an eleventh segment.
a. The terga are cuticular plates covering the dorsal
surface of the abdomen and overlapping each
other from before backwards. They are broad
from side to side, but short from before backwards.
Of the ten only eight are obvious, the eighth
and ninth being hidden by the seventh through
the telescoping of this part of the body. The
tenth is prolonged backwards into a thin horizontal
plate, deeply notched in its posterior border.

. [In P. orientalis the eighth and ninth terga
are hidden by the seventh in the female only.]

EXTERNAL CHARACTERS. 173

b. The podical plates are a pair of small cuticular

plates at the sides of the anus (fig. 40, p. 176).

To see the podical plates, lift up the tenth tergum with the
forceps.

c. The cerci are a pair of many- jointed styles, attached

under the lateral edges of the tenth tergum and
projecting backwards.

d. The sterna on the ventral surface resemble the terga

on the dorsal surface. The first is rudimentary.

In the male the ninth sternum bears a pair
of styles.

In the female the seventh sternum is pro-
duced backwards into a large boat-shaped pro-
cess, which forms the lower wall of the large
genital pouch. The hinder sterna are modified
in relation with the sexual apparatus, and are
completely hidden by the seventh.

B. The External Apertures.

1. The mouth opens on the ventral surface of the head,

behind the labrum and between the jaws.

2. The anus is at the hinder end of the body, beneath the

tenth abdominal tergum and between the inner
margins of the podical plates.

3. The genital aperture is single, and is placed at the

hinder end of the body, below the anus. It is
larger in the female than in the male, and will be
better seen at a later stage of the dissection.

4. The stigmata, or spiracles, are the respiratory apertures,

twenty in number. There are two large thoracic
ones on each side, one between the prothorax
and the mesothorax, and the other between the
mesothorax and the metathorax. A pair of
abdominal stigmata are situated between the
lateral margins of the tergum, and the sternum
at the anterior end of each of the first eight
abdominal segments.

0

174 THE COCKROACH.

To see the stigmata, examine from the ventral surfaces,
stretching the abdomen slightly. The stigmata are at the apices,
of small backwardly projecting spout-like processes, which lie
just beneath the anterior angles of the several terga.

C. The Appendages.
1. The head appendages are four pairs.

a. The antennae are very long, slender, many-jointed,

and freely movable filaments, inserted into a
pair of oval arthrodial membranes just in front
of and below the eyes.

To see the next three pairs of appendages, fix the animal
down on its back by means of a pair of small pins passed through
the sides of its prothorax. Bend the ventral part of the head
forwards, and fix it in that position by means of crossed pins.

Examine the appendages in situ, noting their positions.

Remove the whole of the labium (sm to pg, fig. 39) without
injury to the structures beneath it. Mount it in glycerin.

Similarly remove and mount the right first maxilla and
the right mandible.

b. The mandibles are a pair of stout jaws, strongly

toothed on their inner margins, and working
sideways. They lie immediately below the
gense, and articulate with the epicranium and
with the clypeus.

c. The first maxillae are situated behind the mandibles.

Each consists of the following parts :

i. The protopodite consists of two segments, the
cardo or proximal piece, and the stipes or
distal piece.

ii. The endopodite has two divisions, an inner and
an outer. The inner division, or lacinia, is a
hard blade pointed at the end, and provided
with stiff setae along its inner border. The
outer division, or galea, is softer and is elon-

APPENDAGES. 175

gated : on the inner surface of its distal end
is an oval cushion covered with short curved
stiff setae, and notched to receive the point of
the lacinia.

iii. The exopodite or maxillary palp is a five- jointed
palp, borne on the outer side of the distal end
of the stipes. Its first two segments are short.

d. The second maxillae are similar in shape to the
first maxillae, but smaller. They are fused
together in the median plane by their prote-
in podites, and together constitute the labium or
posterior lip.

i. The two protopodites are fused in the median
plane to form a two -jointed plate. The sub-
mentum or proximal segment is the larger of
the two, and is very broad from side to side.
The distal segment, or mentum, is smaller,
and bears the endopodites and exopodites.
ii. The endopodites are together sometimes called
the ligula. Each is divided into an outer
division, the paraglossa, corresponding to the
galea of the first maxilla ; and an inner divi-
sion, corresponding to the lacinia.
iii. The exopodites are three-jointed palps corre-
sponding in form and position to those of the
first maxillae. The stump supporting each is
called the palpiger.
The thoracic appendages are three pairs of legs, borne
by the sterna of the thoracic segments. They are very
similar to each other, but the hindmost are rather
the largest. Each leg consists of the following parts :

a. The coxa is the stout flat proximal piece of each leg.

b. The trochanter is a very small piece attached to

the distal end of the coxa.

c. The femur resembles the coxa in form, but is

more slender.

176

THE COCKROACH.

DISSECTION. 177

d. The tibia is a slender straight segment armed with
strong spines.

e. The tarsus, or foot, is six-jointed, each segment
having a soft white setose patch under its distal
end, giving the animal a foothold in climbing

steep or slippery surfaces. The terminal piece
or pulvillus is small and bears a pair of claws.
3 Abdominal appendages are very rare amongst adult in-
sects. The anal cerci already described, and possibly
also the styles on the ninth sternum of the male, may
be appendages, as also may the gonapophyses ; but
it is doubtful whether these really belong to the
same category as the appendages of the head and
thorax.

The gonapophyses belong to the genital appa-
ratus, and will be described later.

III. DISSECTION OF THE COCKROACH.

Fix the animal down with the dorsal surface upwards.
This may be done by drying it with blotting paper, and im-
mersing its under surface in melted paraffin or wax, and holding
it in this position till the wax is cool : or the animal may be
fixed to a board by means of small pins passed through the lateral
regions of the mesothorax and of the abdomen.

Fig. 40. — Periplaneta americana. The male dissected from the right
side. The testes and vasa deferentia are not figured, (c. h. h.)

A, anus. AN, antennary lobe of supra-oesophageal ganglion, with anten-
nary nerve cut short. C, crop. CG, supra-cesophageal ganglion. CL, colon.
CR, anal cercus. CX, coxa. E, elytron. F, femur. FG, frontal ganglion.
FN, visceral nerve. G, gizzard. GP, one of the gonapophyses. G 5, fifth
abdominal ganglion. H, hepatic caecum. HT, heart. I, ileum. L, lingua.
LN, nerve to labrum. LP, labial palp. M, mushroom-shaped gland. MC,
Malpighian caeca opening into junction of mesenteron and ileum : they are much
longer and more numerous than indicated. MD, ejaculatory duct. ME,
mesenteron. M N , maxillary nerve. MP, maxillary palp. PN, labial nerve.
PP, podical plate, diagrammatically drawn. R, rectum. S, sub-cesophageal
ganglion. SD, salivary duct just above its opening into the mouth. SG,
salivary glands. SL, style borne by ninth sternum. SR, salivary receptacle.
ST, one of the abdominal stigmata. T, proximal joint of the tarsus. TB,
tibia. TR, trochanter. T 1, first thoracic tergum, or pronotum. T 3, third
thoracic tergum, or metanotum. VG, visceral ganglion. W, wing. 2, tergum
of second abdominal segment. 8, tergum of eighth abdominal segment. 10,
tergum of tenth abdominal segment.

178 THE COCKROACH.

Cut off the elytra and the wings. Carefully cut through
the terga of the abdomen and thorax close to each side, and
remove the terga, avoiding injury to the heart, which lies just
beneath.

A. The Circulatory System.

1. The heart is a straight chambered tube running along
the mid-dorsal line of the abdomen and thorax. It
receives blood through paired lateral openings or
ostia, arranged segmentally, and drives it forwards
by rhythmical contractions.

B. The Digestive and Excretory Systems.

The alimentary canal, with its glandular appendages, is
situated almost entirely in the abdomen, and is enveloped in
an opaque white dendritic mass, the fat-body.

Expose and unravel the alimentary canal and its appen-
dages. In dissecting the salivary ducts great care must be
taken not to injure the nervous system in the head.

1. The alimentary canal is short, and slightly convoluted
in its hinder part. It may be divided according to
its mode of development into three regions — viz.
the stomodceum and the proctodeum, which are
invaginations of the anterior and posterior ends of
the embryo respectively ; and the mesenteron, or
middle portion, which alone is formed from the
primitive alimentary tract of the embryo.

a. The stomodceum has a chitinous cuticular lining,
continuous with the cuticular investment of the
head ; and is divided into the following regions :

i. The buccal cavity is lodged in the head, and its
posterior wall is raised up to form an elon-
gated fleshy tongue or lingua.

The salivary duct opens by a median
aperture in the angle between the lingua
and the labium (fig. 40).

DIGESTIVE SYSTEM. 179

ii. The oesophagus is a narrow tube running from
the mouth through the nerve-collar and along
the neck. It is laterally compressed in the
cervical region.

iii. The crop is a large thin-walled pyriform sac,
extending through the thorax and far into the
abdomen. It is not separated by any distinct
line of demarcation from the oesophagus.

iv. The gizzard, or proventriculus, is a pyriform sac,
with thick muscular walls. Within it is a
series of six large cuticular teeth, and behind
these six small elevations covered with setae,
which serve as a strainer.

Lay the gizzard open to expose the teeth.

b. The mesenteron is a short narrow tube extending

backwards from the hinder end of the gizzard.

Lay the mesenteron open, and note the funnel-like prolon-
gation of the gizzard extending into it.

c. The proctodceum is the convoluted hinder portion

of the alimentary canal, and is lined by a thin
chitinous cuticle. It is divided into the following
regions :
i. The ileum is a short narrow tube, the commence-
ment of which is marked by the attachment
of the numerous fine thread-like Malpighian
tubules.

ii. The colon is much wider and longer than the
ileum, and is constricted at its hinder end.

iii. The rectum is the short dilated terminal portion
of the alimentary canal. Its wall has six longi-
tudinal ridges, projecting internally. It ends
at the anus, between the podical plates.

2. The digestive glands.

a. The salivary glands are two pairs of diffuse white
glands on the sides of the anterior part of the crop.

180 THE COCKROACH.

The salivary receptacles are a pair of thin-
walled sacs, one on each side of the crop, between
the two glands of its side.

The ducts of the receptacles run forwards
and unite beneath the nerve-cord at the hinder
end of the neck to form a median duct, which
runs forwards to open into the mouth, behind
the base of the lingua.

The ducts of the salivary glands unite to form

one duct on each side : the two ducts thus formed

run forwards and unite beneath the nerve-cord

in the hinder part of the neck to form a median

duct : this runs forwards between the nerve-cord

and the median duct of the receptacles, and opens

into the latter at the anterior end of the neck.

Having made out the anatomical relations of the salivary

glands, receptacles, and ducts, remove the whole apparatus.

Stain it with carmine; dehydrate, mount in balsam, and

examine with the microscope.

Nuclei are visible in the walls of the ducts,
and in the secretory cells of the glands.

The chitinous lining of the ducts shows a
spiral thickening.
b. The * hepatic ' cseca are eight or seven ceecal diver-
ticula of the anterior end of the mesenteron.
They are convoluted and club-shaped.
Note the openings of the hepatic cceca into the mesenteron.
3. The excretory system consists of sixty or more
Malpighian tubules, which are fine filamentous
diverticula of the anterior end of the ileum,
arranged in six bundles. They are very long
and convoluted, and are interlaced among the
tracheae and the filaments of the fat-body all
through the abdomen.

C. The Respiratory System.

The respiratory system consists of a series of tracheal tubes
containing air. These commence at the respiratory apertures,

RESPIRATORY AND NERVOUS SYSTEMS. 181

or stigmata on the sides of the thorax and abdomen, and
ramify through all parts of the body. Expiration is effected
by contraction of the muscles of the body, which compress
the tracheae ; ■ inspiration, by dilatation of the tracheae through
the elasticity of their cuticular lining.

The readiness with which air is carried to all parts of the
body, through the great perfection of the respiratory system,
accounts for the extreme imperfection of the circulatory
system in an animal which is otherwise very highly organised.

1. The tracheal tubes when full of air are silvery white.
Remove a large trachea, and examine it microscopically in

a. The cuticular lining has a very strong spiral

thickening, which gives great elasticity to the
tube.

b. The cells which form the organised wall of the tube,

and secrete the chitinous lining, are thin and not
easily seen till stained.

Stain with carmine, and note the nuclei of the cells.

Remove small portions of muscle from the leg, of the fat-
body, of the salivary gland, etc., and note and examine the fine
tracheal tubes ramifying through them. Stain as above.

D. The Nervous System.

The nervous system consists of supra-oesophageal and
sub-oesophageal ganglia, and para-cesophageal connectives in
the head ; a double ventral chain in the thorax and abdomen ;
and a visceral nervous system in relation with the alimentary
canal (fig. 40, p. 176).

1. The cephalic nervous system.

If the buccal cavity has already been exposed, a second
specimen will probably be necessary for the dissection of the
nervous system of the head.

Fix the head, with the anterior surface upwards, by means
of a fine pin through the upper part of the epicranium and
another between the mandibles.

182 THE COCKROACH.

Remove the clypeus and the anterior portion of the epi-
cranium with scissors or the point of a scalpel, taking care
not to injure the ganglia, which lie close beneath the clypeus.
Carefully expose and clean the following structures, avoiding
injury to the cephalic portion of the visceral nervous system :

a. The supra-oesophageal, or ' cerebral,' ganglia are

a pair of large closely apposed ganglia close to
the anterior surface of the head. They are sup-
ported by an internal chitinous framework, the
tentorium.

i. The * hemispheres ' are the large rounded upper
parts of the ganglia. From them arise the
large optic nerves.

ii. The antennary lobes are the smaller and lower
portions of the ganglia. From them the an-
tennary nerves arise.

Turn the head and fix it with the right side uppermost.
Remove the gena and mandible, and as much as is necessary
of the first maxilla, submentum, and mentum, to expose the
following structures on the right side :

b. The para-cesophageal connectives are a pair of short

thick bands of nervous tissue, running round the
sides of the oesophagus, from the lower part of
the supra-cesophageal ganglia to the sub-cesopha-
geal ganglia. From each a nerve arises supplying
the labrum, as well as a nerve belonging to the
visceral system.

c. The sub-cesophageal ganglia are a pair of large

ganglia lying between the submentum and the
oesophagus. Below they give off several nerves,
of which the two largest on each side supply the
maxillary and labial palps. Above, they give off
connectives, which run back through the neck
and form the anterior part of the ventral chain.

SENSE ORGANS. 183

2. The ventral chain.

Fix the cockroach with the dorsal surface uppermost ;
remove the gizzard, mesenteron, and proctodeum ; and turn
the crop aside.

The ventral chain consists of a double series of
ganglia, with double connectives, running along the
ventral wall of the body.

A pair of closely apposed ganglia he in each of
the three thoracic segments, and a pair of smaller
ganglia in each of the first six abdominal segments.
The last pair are the largest of the abdominal ganglia,
and give off nerves to the various parts of the sixth
and succeeding segments.

3. The visceral nervous system arises as a pair of nerves,

one from the anterior part of each para-cesophageal
connective. The two nerves, after giving off a pair
of the nerves to the labrum, unite in the frontal
ganglion, a small median ganglion on the anterior
wall of the oesophagus, just below the antennary
lobes. From this ganglion a median recurrent nerve
runs backwards on the oesophagus, beneath the
supra-cesophageal ganglia, from which it receives a
pair of ganglionated nerve-trunks ; it then continues
its course backwards, as a median nerve, along the
oesophagus and crop to a small triangular ganglion
on the dorsal surface of the crop, about the middle
of its length. From this ganglion two branches run
obliquely backwards over the surface of the crop.

E. Sense Organs.

1. The tactile organs have already been examined. They

are the antennae, the maxillary palps, the labial
palps, and the anal cerci.

2. The eyes are very large compound organs which agree

in essential characters with those of the crayfish.
The corneal facets are hexagonal.

3. The fenestra are perhaps sense organs, but their func-

tion is unknown.

184 THE COCKROACH.

F. The Reproductive System.

1. In the male.

a. The testes are a pair of organs lying immediately

below the fourth and fifth terga of the abdomen.
They are imbedded in the fat-body, and are
only functional in young males.

b. The vasa deferentia are a pair of exceedingly

narrow tubes leading from the testes to the
vesiculae seminales.

c. The vesiculae seminales, together known as the

* mushroom-shaped gland, ' are two tufts of white
glandular caeca, opening into the slightly dilated
anterior end of the ejaculatory duct.

d. The ejaculatory duct is a short muscular tube

opening behind to the exterior, just below the
anus.

e. The gonapophyses are a series of strong chitinous

hooks and plates, forming a complex copulatory
organ. They are asymmetrical, and surround
the genital aperture.

2. In the female.

a. The ovaries are two sets of elongated tubular organs
in the hinder part of the abdomen. Each set
consists of eight tubes, united in front and
attached by a ligament to the dorsal wall of the
thorax. Behind and below they unite to form
the oviducts. Each tube has a moniliform
appearance, from the bulgings caused by the
contained ova : in the hinder and lower part of
the tube the ova are large and nearly mature ;
those in the upper part of the tube are in various
earlier stages of development.

At the time of laying, one ovum escapes from\
each ovarian tube, and the sixteen are laid side
by side in a capsule.

REPRODUCTIVE SYSTEM.

185

b. The oviducts, formed as above, are a pair of short

wide tubes, which unite, and open to the exterior
by a median vertical slit in the eighth sternum.

c. The spermotheca consists of two unlike and unequal

cseca of small size, opening by a median aperture
on the ninth sternum. It lies close behind the
last nerve-ganglia of the ventral chain.

Fig. 41. — Periplaneta americana. The hinder part of the abdomen of
the female dissected from the right side to show the reproductive
apparatus. The right ovary with its duct has been removed, and
only a small portion of the colleterial glands is shown. The thick
black lines represent the cut edges of the terga, sterna, etc.,
x 8. (c. H. H.)

A, anus, c, anal cercus. CG, colleterial glands. CO, aperture of ducts
of colleterial glands. G, gonapophyses. G 5 and g 6, the last two abdominal
nerve-ganglia. O, left ovary. OD, left oviduct. OD'j median oviduct.
P, podical plate. R, rectum. S> the two divisions of the spermotheca. S 4,
S 5, etc., fourth and following sterna. T 4, T 5, etc., fourth and following terga.

d. The colleterial glands are a pair of much-branched
tubular glands, whose ducts open separately,

186 THE COCKROACH.

but close together, behind the spermotheca, and
between the inner gonapophyses.

e. The gonapophyses are six symmetrically arranged
processes, strongly chitinised on their outer
surfaces. They are outgrowths from the sternal
surface, between the vulva and the anus ; and
aid in depositing the eggs.

187

Chapter XI.

THE LANCELET. Amphioxus hnceolatus.

Amphioxus is a small semi-transparent fish-like animal, about
a couple of inches in length, found in the English Channel,
the Mediterranean and other seas. It is by day of sluggish
habits, and usually remains buried in the sand, either com-
pletely or with the anterior end alone protruding ; but at
night, or if disturbed, it swims actively by rapid lateral
movements of the body.

Though distinctly belonging to the great group of verte-
brate animals, Amphioxus is in several important respects
simpler than any of the more familiar members of the group.
Indeed, in spite of its fish-like appearance and habits, the
differences between Amphioxus and an ordinary fish, such as
a dog-fish or whiting, are of greater morphological importance
than those separating fish from the most highly organised
vertebrates, such as birds or mammals. It is therefore
necessary to divide vertebrates into two main divisions, the
first of which, Acrania, includes Amphioxus and a probably
degenerate group, the Ascidians ; while the second division,
Craniota, includes all remaining vertebrates, from fish to
mammals.

Owing to the small size of Amphioxus, it is not practicable
to determine all the points in its anatomy by dissection. The
method of procedure adopted in the case of the other animals
dealt with in this book will in consequence be departed from
to some extent ; and the present chapter will be arranged
under three heads.

I. A description of the anatomy of Amphioxus.

188

AMPHIOXUS

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EXTERNAL CHARACTERS. 189

II. Directions for the dissection of Amphioxus ; by which
the more salient points in its anatomy may be
determined.

m. Descriptions of transverse sections taken through
different regions, and examined as microscopical
objects.

I. THE ANATOMY OF AMPHIOXUS.

A. External Characters.

1. The shape. Amphioxus is elongated, pointed at both

ends, and compressed from side to side, especially
towards the posterior end. There is no obvious
head, and no trace of limbs.

2. The fins. A low dorsal fin runs along the mid-dorsal

line from end to end of the animal, becoming more
prominent at the hinder end as the upper lobe of
the caudal fin.

The ventral surface bears a median fin in its
posterior third, but in front of this is flattened, so.
that the body is triangular in transverse section.
The sides of this flattened surface are bordered by
the lateral fins, or metapleural folds.

3. The myotomes. The sides of the body are marked by

a series of > shaped lines, formed by septa of
connective tissue which divide the great lateral
muscles of the body into segments or myotomes.

4. The * mouth ' or opening of the oral vestibule is a large

oval aperture on the ventral surface of the anterior
end of the body, and is fringed on each side by a
series of ciliated tentacles or cirri.

5. The anus is on the ventral surface, not far from the

hinder end of the body. It is placed a little to the
left of the median plane, at the base of the caudal
fin.

6. The atrial pore is a median aperture on the ventral

surface, bordered by prominent lips. It is in front

190 AMPHIOXUS.

of the anus, at the junction of the ventral and
lateral fins, and about one-third the length of the
animal from its hinder end. Through it the water
escapes which has been taken in at the mouth with
the food, and which serves for respiration.

B. The Skeletal System.

The skeleton of Amphioxus is extremely simple, and
contains neither bone nor cartilage.

1. The notochord is a median elastic rod which traverses

the body from end to end, lying rather nearer the
dorsal than the ventral surface, and between the
central nervous system and the alimentary canal. It
is in contact with the integument at the two ends of
the body ; and it serves slightly to stiffen the animal.

It consists of cells in which, at an early stage,
large vacuoles appear : these vacuoles increase in
size and ultimately almost completely fill the cells,
rendering their outlines difficult to trace. On the
dorsal and ventral surfaces of the notochord, the
cells are comparatively unaltered.

In higher vertebrates also a notochord is formed
in early stages of development, but it is afterwards
replaced more or less completely by the vertebral
column.

2. The buccal skeleton. At each side of the mouth is a

curved bar, resembling the notochord in structure.
Each bar is divided transversely into a number of
short segments, and from these filaments arise which
support the buccal cirri, which vary from twenty to
forty in number, becoming more numerous as age
advances.

3 The skeleton of the median fins. Along the whole of
the dorsal fin, except at its extreme anterior and
posterior ends, is a longitudinal series of connective-
tissue compartments, filled with a gelatinous sub-
stance. The compartments are more than twice as

SKELETAL AND MUSCULAR SYSTEMS. 191

numerous as the segments of the body. A similar,
but double, structure extends along the ventral fin,
between the anus and the atrial pore.

4. The branchial skeleton. A series of elastic rods are

developed in the walls of the pharynx to support the
gill-arches. Their arrangement will be described
with the alimentary canal.

5. The connective-tissue skeleton. The connective-tissue

of Amphioxus is very dense, and forms a kind of
skeleton, of which the following are the chief parts :

a. The sheath of the notochord is a thick sheath

closely surrounding the notochord.

b. The neural sheath surrounds the central nervous

system, and is continuous below with the noto-
chordal sheath.

c. The septa are sheets of connective tissue which

run outwards from the notochordal and neural
sheaths to the integument. They separate the
successive muscle-segments, or myotomes, from
each other ; and their lines of insertion into the
integument form the > shaped markings along
the sides of the animal. The septa on the two
sides of the body are not paired, but alternate
with each other.

C. The Muscular System.

1. The lateral muscles are by far the largest part of the
muscular system. They form the side- walls of the
body along its whole length, and are divided, as
noticed above, into muscle-segments or myotomes
by the connective-tissue septa. The muscle-fibres
run longitudinally, i.e. parallel to the axis of the
body ; and it is by alternate contractions of these
muscles on opposite sides of the body, aided by
the elasticity of the notochord, that the lateral
undulatory swimming movements of the animal
are produced.

192 AMPHIOXUS.

There have been found to be 61 myotomes on
each side of the body in a considerable number
of specimens.

2. The ventral muscles form a sheet covering the ventral

surface of the body from the mouth to the atrial
pore. The muscle-fibres run transversely from
side to side, and by their contraction expel the
water from the atrial cavity.

3. Smaller muscle-bundles are found in relation with the

mouth and its cirri, with the gill-apparatus, and
with the anus and atrial pore.

Nearly all the muscles are striated.

D. The Digestive and Respiratory Systems.

The alimentary canal is a nearly straight tube, the
anterior part of which is so modified as to allow of the
escape of water taken in at the mouth with the food, and to
utilise this water for respiration, as is the case in aquatic
vertebrates generally.

1. The buccal cavity or oral vestibule is bounded laterally

by folds, within which and near their free margins
are the curved bars which support the cirri. There
are no jaws.

The anterior part of the buccal cavity is lined by
a single layer of short columnar epithelial cells,
some of which bear short flagella. In the hinder
part of the cavity the epithelium is altogether
different, the cells being very long and slender and
provided with long flagella. This epithelium forms
the * wheel-organ,' by which water and floating
food-material are wafted into the pharynx.

2. The velum is a muscular diaphragm between the buccal

cavity and the pharynx, opposite the anterior angle
of the seventh myotome. It is perforated below its
middle by an aperture which leads upwards and
backwards into the pharynx, and the hinder border
of which is fringed with a circle of twelve backwardly
projecting tentacles.

DIGESTIVE SYSTEM. 193

3. The pharynx is a wide sac, forming about half the length
of the alimentary canal. It is attached along its
mid-dorsal line to the under surface of the sheath of
the notochord, and is almost surrounded by a space
— the atrial cavity — which receives the water that
passes out of the pharynx. Its sides are perforated by
a large number of slit-like apertures — the gill-slits —
which run obliquely downwards and backwards. The
parts of the pharyngeal wall between the successive
slits are narrow bars — the gill-arches — each of which
is strengthened by an axial rod of a chitinous sub-
stance. These arches are of two kinds, arranged
alternately : the axial rods of the third, fifth, etc.,
arches being forked at their ventral ends, while the
alternate rods are unsplit. Each double gill-slit is
originally a single one, but becomes divided in the
course of development by the downgrowth of the
unsplit bar from its dorsal end. The split bars may
hence be called primary rods, and the unsplit ones
secondary rods. Like the myotomes, the gill -arches
are not in pairs, but alternate on the two sides
of the body.

The successive gill-arches are connected by
horizontal bars, of which there are usually three or
more crossing each slit, so that the pharynx has the
character of an open meshwork. The inner surface
of each gill-slit is lined by columnar flagellate
cells, which maintain a continual stream of water
through the gill-slits from the pharynx to the atrial
cavity, the stream serving to aerate the blood in
the vessels of the arches as it passes over them.
The inner edge of each bar also bears flagellate
cells separated from those lining the gill-slits by
two groups of cells bearing very short cilia.

Along the mid-dorsal line of the pharynx is the
deep epibranchial groove, lined by a single layer
of long columnar ciliated cells. A band of similar
cells runs along the endostyle or mid-ventral wall

194 AMPHIOXUS.

of the pharynx, which is folded longitudinally in
its hinder part to form a groove. The endostyle
also contains mucous glands.

4. The intestine commences at the hinder end of the
pharynx, close to the dorsal surface, and runs
straight to the anus. It is a direct continuation of
the epibranchial groove, and at its commencement
is very narrow ; further back it dilates consider-
ably ; and this expanded part, which is sometimes
called 'stomach,' is produced into a large pouch-
like sac — the liver — which runs forwards some
distance along the right side of the pharynx, and
ends blindly in front.

E. The Atrial Cavity.

The boundaries and relations of this cavity can best be
determined by the examination of a series of transverse
sections.

The atrial or epipleural cavity is the large place which
surrounds the pharynx at the sides and below : into it the
stream of water passes from the pharynx through the gill-
slits, to escape finally from the atrial pore.

The atrial cavity develops in the embryo as a groove
along the under surface of the body ; by fusion of its lips
the groove becomes a tube, which ends blindly in front, but
retains its opening behind at the atrial pore. The cavity
extends laterally so as to surround the pharynx both below
and at the sides, lying between the pharynx and the muscular
body-wall.

The epithelium lining the atrial cavity is supported by a
layer of connective tissue, and usually contains a good deal
of brown pigment, which renders it easy to trace. It is repre-
sented by the thick black line in figs. 43-46, which show its
distribution in different regions of the body. It lines the
inner surface of the atrial folds, covers the outer surface of
each of the gill-arches, and invests the liver and the anterior
portion of the intestine.

ATRIAL CAVITY AND CCELOM. 195

On the primary gill-arches this layer of atrial epithelium
is wider than on the secondary arches, and towards the dorsal
surface of the pharynx it rises up to form the suspensory folds
of the pharynx, which run across from the dorsal part of the
sides of the pharynx to the inner surface of the atrial folds
and separate the atrial cavity below from the dorsal ccelomic
canals above.

The atrial cavity commences opposite the anterior end of
the pharynx, and extends back almost as far as the anus ;
behind the atrial pore it is confined to the right side of the
body.

F. The Coelom or Body-cavity.

This can only be traced by means of sections.

The ccelom, or body-cavity, is perfectly distinct from the
atrial cavity, though its boundaries are not easy to follow,
and its relations differ considerably in different regions of the
body. It contains during life a coagulable fluid.

1. Behind the atrial pore the ccelom is a cavity of some

width surrounding the intestine and separating it
from the body-wall.

2. In front of the atrial pore it becomes greatly reduced

owing to the increased size of the atrial cavity : it is,
however, easily recognisable as a narrow space im-
mediately surrounding the intestine and the liver.

3 In the region of the pharynx the coelom is much sub-
divided and more difficult to trace. Its chief divi-
sions are as follows : —

a. The dorsal ccelomic canals are a pair of wide

spaces lying at the sides of the dorsal part of
the pharynx, between the atrial folds and the
suspensory folds of the pharynx.

b. The branchial ccelomic canals are a series of tubular

cavities in the primary gill arches, lying within

196 AMPHIOXUS.

the folds into which the atrial epithelium is
thrown along the outer sides of these arches.
These cavities are very small in the ventral
portions of the arches, but widen out towards
their dorsal ends, and open, as shown in figs. 43
and 44, into the large dorsal ccelomic canals
above the suspensory folds. Branchial ccelomic
canals also traverse the skeletal rods of the
secondary gill-arches.

c. The endostylar ccelomic canal is a space, in which

the cardiac aorta runs, below the endostyle.
The branchial ccelomic canals of both primary
and secondary gill-arches communicate with it.

d. A series of perigonadial ccelomic spaces surround

the reproductive organs.

G. The Circulatory System.

The main features of the circulatory system can he made
out by examination of a series of transverse sections.

There is no heart, but the general course of the circulation
is the same as in other gill-breathing vertebrates. The blood
is colourless and contains but few corpuscles.

The principal vessels are as follows :

1. The cardiac aorta is a median longitudinal vessel run-

ning forwards along the ventral wall of the pharynx
in the endostylar ccelomic canal.

2. The aortic arches are a series of vessels running up the

pharyngeal bars or gill-arches. There appear to be
three in each bar, the ccelomic, the somatic, and the
visceral. The ccelomic aortic arch is the outermost
of the three, and in the primary bars lies close to
the atrial epithelium in the wall of the branchial
ccelomic canal, while in the secondary bars it ap-
pears to He within the skeletal rod. The somatic
aortic arch runs along the inner edge of the skeletal
rod. The visceral aortic arch runs near the inner

CIRCULATORY SYSTEM. 197

edge of the bar. The aortic arches open above into
the dorsal aortae. Those of the primary pharyngeal
bars alone arise directly from the cardiac aorta.
The ccelomic aortic arches of adjacent bars com-
municate with each other by vessels in the horizontal
bars of the pharynx.

3. The dorsal aortse are a pair of longitudinal vessels into

which the aortic arches of both primary and second-
ary gill-arches open at their dorsal ends. They lie
just beneath the notochord, one on each side of the
epibranchial groove, and project into the dorsal
ccelomic canals.

The dorsal aortse carry the aerated blood back-
wards : they unite at the hinder end of the pharynx
to form a single vessel, which runs back along the
dorsal surface of the intestine.

4. The portal veins are vessels running in the ccelom along

the ventral surface of the intestine : they collect
the blood from the intestine and carry it forwards
to the liver, where they break up into capillaries.

5. The hepatic veins are vessels which collect the blood

from the liver and carry it backwards along the
dorsal surface of the liver. On reaching the junc-
tion of the liver and intestine they unite to form a
single vessel, which runs forwards along the ventral
surface of the pharynx and becomes the cardiac
aorta.

6. A longitudinal vessel runs along the inner side of each

atrial fold, and supplies the myotomes and repro-
ductive organs.

H. The Lymphatic System.

Distinct from the ccelomic spaces is a system of lymphatic
spaces associated with the myotomes, nerve-roots, and other
parts : of these the most conspicuous are a pair of large
metapleural canals running longitudinally within the meta-
pleural folds.

198 AMPHIOXUS. .

I The Excretory System.

Three kinds of excretory organs have been described in
Amphioxus.

1. The pigmented canals discovered by Lankester are

possibly excretory. These are a pair of short wide
funnels, with deeply pigmented walls, placed in the
twenty-seventh segment of the body, opposite the
hinder end of the pharynx. They lie, one at each
side of the body, in the dorsal ccelomic canals above
the suspensory folds of the pharynx. Each tube is
attached along its outer side to the body-wall, and
opens by its wider end into the atrial cavity ; in
front it is considerably contracted, but appears to
open into the ccelomic canal.

2. Muller's renal papillae are numerous pigmented papillae

on the floor of the atrial cavity.

3. Weiss and Boveri have discovered a series of curved

excretory tubules lying in the dorsal ccelomic
canal along each side of the pharynx. Each tubule
communicates with the atrial cavity by a single
opening on the outer side of a secondary pharyngeal
bar, close to its dorsal end, and contains remarkable
hollow cells within each of which is a nagellum.

K. The Nervous System.

The nervous system of Amphioxus consists of : (1) the
central portion, which, as in other vertebrates, is a tube of
nervous matter lying in a sheath of connective tissue imme-
diately above the notochord, and extending almost the entire
length of the body ; (2) the peripheral portion, which consists
of the nerves connecting the central portion with the various
parts of the body.

The nervous system can be well seen in transverse sections
of adults, or by examination of young specimens mounted
whole. If fresh specimens can be obtained, the entire nervous
system can be isolated by placing them in 20 per cent, nitric

NERVOUS SYSTEM. 199

acid for three days, then washing thoroughly and leaving in
water for a day. By this treatment the connective tissue is
so softened and loosened that on shaking the bottle the nervous
system is readily and completely separated from all the other
parts. It may then be transferred to alcohol, stained, and
mounted in the usual manner.

1. The central nervous system or spinal cord is of nearly

uniform diameter along the greater part of its
length : it diminishes slightly in size towards the
anterior end, and much more markedly near the
posterior end, where it is very slender.

The central canal lies much nearer the ventral
than the dorsal surface : it is very small except at
the extreme front end, where it expands to form the
ventricle. From the anterior end of this a small
diverticulum arises which in early life communicates
by a pore with a ciliated pit on the dorsal surface
and left side of the anterior end of the body.
Through this pit the central canal of the nervous
system at first opens directly to the exterior, but the
pore closes during development.

At intervals along the spinal cord are spots of
black pigment, contained in cells in the floor of the
central canal.

2. The peripheral nervous system consists of nerves of two

kinds : (1) nerves which arise by single roots, and,
except in the case of the first pair, from the dorsal
surface of the spinal cord ; (2) nerves arising by
multiple roots, and from the ventral surface of the
cord.

a. Nerves arising by single roots. These apparently
correspond to the dorsal or sensory roots of the
spinal nerves of other vertebrates. Only the
first two pairs have ganglionic enlargements.
The first and second are purely sensory nerves,
and the remainder are chiefly sensory.

200 AMPHIOXUS.

i. The first pair of nerves arise from the ventral
surface of the anterior end of the central
nervous system, and supply the anterior fin.
Their branches have ganglionic swellings near
their distal ends.

ii. The second pair of nerves are much larger.
They arise from the dorsal, surface of the
spinal cord behind the first pair. Each
divides into three main branches, supplying
the anterior fin. The finer branches have
ganglionic enlargements near their distal
ends.

iii. The third pair of nerves are much smaller
than the second, and arise immediately be-
hind them. They supply the parts about the
mouth.

iv. The fourth pair of nerves arise from the dorsal
surface of the spinal cord, a short distance
behind the third pair ; the nerve of the right
side being a short distance behind that of the
left side.

v. The fifth pair of nerves are still more oblique,
the right nerve arising some distance behind
the left.

The succeeding nerves, along the whole
length of the cord, arise alternately on the
two sides. These nerves run outwards along
the connective-tissue septa separating the
myotomes, and divide into branches sup-
plying the skin, and, to a limited extent,
muscles.

b. Nerves arising by multiple roots. These apparently
correspond to the ventral or motor roots of the
spinal nerves of other vertebrates.

They arise from the ventral surface of the
spinal cord along nearly its whole length, alter-

SENSORY AND REPRODUCTIVE ORGANS. 201

nating with the dorsal nerves ; a ventral nerve
on one side being opposite a dorsal nerve on the
other. Each arises by a large number, thirty or
more, of very slender rootlets, placed one in front
of another, the entire breadth of a ventral root
being nearly equal to the interval between two
successive roots. The several rootlets do not
unite, but pass separately through the connec-
tive tissue sheath of the cord, and end in the
myotomes.
L. The Sense Organs.

The sense organs of Amphioxus are extremely simple in
structure, and differ markedly from those of the higher verte-
brates in being unpaired.

1. The pit-like depression of the skin at the front end

which in the young places the central canal of the
nervous system in communication with the exterior
has been described as an olfactory organ. In the
adult it is connected by a solid stalk with the roof
of the ventricle.

2. Perception of light is probably enhanced by the black

pigment cells scattered along the spinal cord, and
by an especially large pigment-spot in the anterior
wall of the ventricle.
If'. The Reproductive Organs.

The sexes are distinct, but the males and females are
alike except as regards the microscopic structure of their
reproductive organs. There are no genital ducts.

1. The female.

The ovaries are a series of saccular organs of a
horseshoe shape arranged in a row along the inner
surface of the atrial fold on each side of the pharynx,
and extending back as far as the atrial pore. They
He in cavities, which are extensions of the coelom
into the atrial folds : these cavities are easily recog-
nisable while the ovaries are young, but become
almost obliterated when the ova ripen.

202 AMPHIOXUS.

The ova, when mature, are discharged into the
atrial cavity by dehiscence of the proper wall of
the ovary and of the atrial membrane. The dis-
charged ova, together with the ovaries, form a
bulky mass, which causes great distension of the
atrial cavity and distortion of the pharynx and other
organs.

The ova, which measure j^ inch in diameter,
sometimes pass through the gill-slits into the
pharynx, and escape through the mouth ; in other
cases they are discharged through the atrial pore.

2. The male.

The testes are similar in form and position to the
ovaries ; and the spermatozoa, when ripe, are dis-
charged, like the ova, into the atrial cavity, from
which they escape by the atrial pore.

II. DISSECTION OF AMPHIOXUS.

Take an adult specimen which has been macerated for
about a day and a half or longer in 20 per cent, nitric acid,
and dissect it as described below.

Specimens which have been preserved in alcohol serve
perfectly well for this purpose. The effect of the nitric acid is
to soften and loosen the connective tissue, and so render the
dissection much easier,

1. External Characters.

Place the animal under water in a small dissecting dish.
Determine all the external characters, as described on p. 189.

Lay the animal on its left side, and fix it down by two
small pins passed through the body slightly nearer the ventral
than the dorsal border, one pin passing through the buccal
cavity about one-sixteenth of an inch from the anterior end ;
the other through the tail, behind' the anus, and about one-
eighth of an inch from the hinder end of the body.

2. The skin.

With needles or fine forceps strip off tJie skin from the right

DISSECTION. 203

side of the body ; stain and mount a piece, and examine it
under the microscope.

The skin consists of two layers : —

a. The epidermis is a single layer of short columnar

and somewhat dumb-bell-shaped cells, with a
thick cuticle on their outer ends.

b. The dermis is a very thin layer of connective tissue

underlying and .supporting the epidermis. It is
so thin that it is often difficult to recognise it
in specimens treated in this way ; but it will be
easily seen in transverse sections of the animal.

Strip off the myotomes of the right side one by one with
needles or fine forceps ; commencing just behind the anus and
working forwards until nearly the whole of the myotomes of
the right side have been removed, a few only being left at the
anterior and posterior ends of the animal.

In removing a myotome, take hold of its anterior end or
angle, and very carefully draw it off backwards, taking care
not to pull away the reproductive or other underlying organs.

3. The notochord is a conspicuous yellow rod traversing
the animal from end to end : lying rather nearer the
dorsal than the ventral surface, and at the level of
the angles of the myotomes.

L The spinal cord is a white band lying immediately dorsal
to the notochord.

5. The skeleton of the dorsal fin extends along the whole

length of the dorsal surface except the extreme
anterior and posterior ends : it is a firm gelatinous
rod, divided transversely into segments which are
much more numerous than the myotomes.

6. The skeleton of the ventral fin is a similar but double

structure on the ventral surface extending from the
atrial pore to the anus.

7. The reproductive organs are a series of squarish masses

204 AMPHIOXUS.

lying along the ventral edge of the anterior three-
fourths of the length of the animal.

Remove two or three of the reproductive organs ; put them
on a slide in a drop of glycerin : tease them, and examine with
a microscope.

The ovary of the female consists of large rounded
and polyhedral ova, with large nuclei : cf. fig. 44.

The testis of the male has a finely granular ap-
pearance, the spermatozoa, when ripe, consisting of
minute heads with long thread-like tails.

8. The transverse muscles of the ventral body-wall in the

pharyngeal region are well seen, immediately ventral
to the reproductive organs.

Remove the reproductive organs of the right side completely,
so as to expose the alimentary canal along its whole length.

9. The atrial cavity is the space surrounding the walls of

the pharynx at the sides and below. It opens to the
exterior by the atrial pore.

10. The alimentary canal runs nearly straight from mouth

to anus : it consists of the following parts :

a. . The buccal cavity is the most anterior portion : the
mouth, opening into it below, is bordered by
tentacles, which increase in length from before
backwards.

b. The pharynx, which is in part concealed by the liver,
forms about half the length of the alimentary
canal : it is crossed by close-set parallel lines,
oblique to the axis of the body, and marking
the positions of the gill-clefts : these do not
quite reach either the mid-dorsal or mid -ventral
lines, where the wall of the pharynx is formed
by the epibranchial band and the endostyle
respectively.

C, The (Esophagus is a short narrow tube leading from

ALIMENTARY CANAL. 205

the dorsal surface of the hinder end of the
pharynx to the intestine.

d. The intestine is wider in front, and narrows behind,

running back to the anus.

e. The liver is a large saccular diverticulum from the

front end of the intestine, which extends forwards
along the right side of the hinder two-thirds or
more of the pharynx.

To make out the relations of the pharynx, oesophagus, and
intestine to each other, carefully remove the liver.

11. The structure of the alimentary canal.

a. The buccal tentacles.

Remove the entire buccal skeleton, with the tentacles in
both sides, carefully dissecting it with needles from its attach-
ments. Stain it with pier ocar mine ; mount on a slide in
glycerin, and examine with the microscope.

A jointed skeletal bar borders the buccal
aperture on each side, and supports the buccal
tentacles.

b. The pharynx.

Remove a portion of the pharynx carefully : divide it along
the mid-dorsal line, and spread it out flat on a slide ; stain it ;
mount in glycerin or balsam, and examine with the microscope.

The arrangement of the gill-arches and gill-
clefts, and their relation to the endostyle, can be
well made out.

Remove a second length of the pharynx, and with a fine
brush pencil off the epithelium from the gill-arches. Treat a
third portion with a solution of caustic potash. Mount in
glycerin, and examine with the microscope.

The primary or split rods, and the secondary
or unsplit rods (compare p. 193), can be readily
made out ; also the horizontal bars connecting
the successive pairs of primary rods together, and
the relations of the rods to the endostyle.

Q

206 AMPHIOXUS.

12. The nervous system.

Remove the nerve-cord along its entire length : stain it ;
mount on a slide in glycerin, and examine with the micro-

The dorsal roots of the nerves and their
alternate arrangement are well seen (compare
pp. 200, 201).

13. The structure of the notochord.

Remove a piece of the notochord : stain it ; mount in
glycerin, and examine with the microscope.

The notochord breaks up very readily into
transverse discs.

III. EXAMINATION OF TRANSVERSE SECTIONS.

Many points in the anatomy of Amphioxus are determined
far more easily by an examination of transverse sections of
the animal than by dissection. For this purpose a specimen
should be cut by means of a microtome, and the sections mounted
on slides in their proper sequence. It is only necessary to
examine in detail a few of the more typical ones.

A. Transverse Section passing through the Buccal Cavity.

1. General characters. The dorsal surface is indicated by

the laterally compressed dorsal fin : the ventral
surface by the mouth-opening, which is bordered at
the sides by the thickened lips ; within the buccal
cavity are usually seen sections of the tentacles.
About the middle of the section is the notochord,
and above this is the spinal cord. At the sides are
the myotomes, separated from each other by con-
nective-tissue septa.

2. The skin consists of two layers.

a. The epidermis is a single layer of short columnar

cells, with a thick cuticle on their outer ends.

b. The dermis is a very thin layer of connective tissue

underlying and supporting the epidermis.

SECTION THROUGH BUCCAL CAVITY. 207

3. The skeletal system.

a. The notochord is oval in section, the longer axis

being vertical. It is crossed by numerous wavy
transverse bands, the spaces between which are
occupied by a homogeneous substance : this
latter is the coagulated semifluid matter occupy-
ing the vacuoles of the notochordal cells, the
outlines of which are indicated by the wavy
bands.

Near the surface of the notochord, and
especially in the dorsal region, cells may be
seen which have undergone comparatively little
modification.

b. The connective tissue forms a thick deeply staining

investment to the notochord, which is prolonged
upwards to enclose the spinal cord, and out-
wards to form the septa : these latter separate
the myotomes from each other, and are con-
tinuous at the outer borders with the dermis.

c. The buccal skeleton. Sections of the segmented

rods which strengthen the lips are seen bordering
the entrance to the buccal cavity : a smaller rod
occupies the axis of each tentacle. In structure
these rods resemble the notochord.

d. The skeleton of the dorsal fin. The dorsal fin is

filled with a homogeneous gelatinous substance,
which is stained slightly by re-agents.

4. The muscles.

a. The myotomes are seen in section as squarish
masses of muscle lying along the sides of the
body in the compartments formed by the septa.
Owing to the obliquity of the myotomes, three
or more are cut in each transverse section, and
owing to their > shape each myotome is cut
twice. The myotomes of the two sides are not
opposite, but alternate with each other.

208 AMPHIOXUS.

b. Other much smaller muscles are seen in connection
with the buccal skeleton and tentacles.

5. The buccal cavity.

The epithelium lining the buccal cavity con-
sists of a single layer of cells, which in the anterior
and ventral part are short and columnar, but in the
posterior and dorsal region are much elongated,
slender and flagellate.

6. The nervous system.

a. The spinal cord lies in a connective-tissue sheath

immediately above the notochord. In transverse
section it is somewhat triangular, with rounded
angles, and is rather wider from side to side
than it is dorso-ventrally.

The central canal lies nearer the ventral than
the dorsal surface : it is lined by short columnar
ciliated cells, and immediately below it are the
large deeply pigmented cells already mentioned
(p. 199). From the central canal a narrow
vertical slit extends upwards to the dorsal surface
of the cord, both the canal and the slit being
bordered by large nerve-cells. In young speci-
mens the slit is open above, but in adults it is
closed by connective tissue, and by processes of
the nerve-cells growing across it from side to
side. The greater part of the cord has a finely
punctate appearance, caused by the cut ends of
the nerve-fibres.

b. The nerves can only be recognised in sections

which happen to pass through their roots.
i. The dorsal nerves arise by large single roots from

the sides of the cord, about midway between

the dorsal and the ventro-lateral angles,
ii. The ventral nerves arise by very minute and

multiple rootlets from the ventro-lateral angles

of the cord.

SECTION THROUGH PHARYNX.

209

. Transverse Section through the Anterior Part of the
Pharynx.

1. General characters. The section is triangular in shape,
the apex being formed by the dorsal fin, the base by
the ventral surface, and the basal angles by the
metapleural folds. The pharynx, perforated by the
gill-slits, and surrounded by the atrial cavity, lies
below the notochord.

Pig. 43. — Amphioxus lanceolatus. Transverse section through the
anterior part of the pharynx of an adult specimen. The boundary
of the atrial cavity is indicated by a thick black line. (a. m. m.)

A, skeleton of dorsal fin. B, spinal cord. C, notochord. D, connective-
tissue sheath surrounding notochord. E, cavity of pharynx. F, epibranchial
groove of pharynx. G, endostyle, which in this anterior part is flattened out or
even convex. H, atrial cavity. J, transverse muscles in floor of atrial cavity.
M, dorsal coelomic canal. P, metapleural canal. R, left dorsal aorta. S,
endostylar coelomic canal in which the cardiac aorta lies. X, myotome.
Y, suspensory fold of pharynx, separating the dorsal coelomic canal from the
atrial cavity. Z, gill-arch or branchial bar : the triangular white spot repre-
sents the cut surface of the skeletal rod of the arch.

210 AMPHIOXUS.

2. The skin of the ventral surface is thrown into deep

folds to allow for the distension of the atrial cavity
when the reproductive organs attain their full size.
Its general characters are the same as in A,

3. The skeletal system is the same as in A, except that the

branchial skeleton is present, and that there is no
buccal skeleton.

4. The muscular system.

a. The myotomes are similar to those of section A,

but considerably larger.

b. The ventral muscles form a thin sheet in the atrial

fold of each side, extending from the lowest
myotome to the mid-ventral line, where the
muscles of the two sides meet in a median raphe.

c. Small muscular bands are present in the suspen-

sory folds of the pharynx and in the gill-arches.

5. The pharynx is of an inverted heart shape in transverse

section.

a. The branchial bars, or gill-arches. Owing to their
obliquity and extreme closeness to each other,
twenty or more bars may be cut on each side in
a single section. Each bar is clothed on its
outer surface by the atrial epithelium, which is
a single layer of columnar cells. Within this is
the skeletal rod, deeply grooved along its inner
edge. From the rod a thin plate of connective
tissue extends inwards, clothed on each side by
the pharyngeal epithelium, a single layer of
very long columnar flagellate cells.

The branchial bars are alternately large and
small, the difference being especially marked at
their outer edges. The large ones are the
primary, and the small ones the secondary bars.

In the large primary bars a small space is
visible between the atrial epithelium and the
skeletal rod. This space, the branchial ccelomic

SECTION THROUGH PHARYNX. 211

canal, which is part of the coelom, is much
larger in the dorsal than in the ventral bars of
the section, and in the most dorsal ones it is a
space of considerable size, the atrial epithelium
being produced outwards for some distance. The
uppermost two or three of these outward pro-
longations of the atrial epithelium unite together
to form the suspensory folds of the pharynx,
which sling it to the inner surface of the atrial
folds, the branchial ccelomic canals opening here
into the dorsal ccelomic canals (fig. 44).

b. The epibranchial groove is a deep median groove

in the dorsal wall of the pharynx immediately
beneath the notochord. It is lined by columnar
epithelium.

c. The ventral wall of the pharynx is formed by a flat

or slightly convex plate, the endostyle, covered
by a layer of columnar epithelial cells.

6. The atrial cavity surrounds the sides and ventral surface

of the pharynx, lying between it and the atrial folds,
and communicating with the cavity of the pharynx
through the gill-slits. The atrial epithelium consists
of a single layer of pigmented columnar cells, and is
often much folded on the ventral wall.

7. The coelom is much subdivided in this region : its

principal divisions are as follows :

a. The dorsal ccelomic canals are a pair of large

irregular spaces at the sides of the epibranchial
groove, above the suspensory folds of the pha-
rynx, and between these and the atrial folds.

b. The branchial ccelomic canals are the cavities in

the primary branchial bars, between the atrial
epithelium and the skeletal rods. They all open
at the dorsal ends into the dorsal ccelomic
canals.

c. The endostylar ccelomic canal is a median space in

212 AMPHIOXUS.

the ventral region of the endostyle. It encloses
the cardiac aorta.

8. The metapleural canals are a pair of large spaces, trian-

gular in section, lying in the metapleural folds.

9. The circulatory system. The larger vessels can be re-

cognised in sections by the coagulated blood which
they contain.

a. The cardiac aorta is a median vessel lying in the

endostylar ccelomic canal.

b. The dorsal aortse are a pair of vessels lying one on

each side of the epibranchial groove.

c. The aortic arches he in the branchial bars, both

primary and secondary (see pp. 196, 197).

10. The nervous system is the same as in section A.

C. Transverse Section passing through the Hinder Part of
the Pharynx.

1. General characters. The section is larger than B, pass-

ing through the thickest part of the body. It is
triangular in shape, the angles being formed by
the dorsal and lateral fins respectively. The section
passes through the laterally compressed pharynx, on
the right side of which is the liver. The reproduc-
tive organs project into the atrial cavity from the
inner sides of the atrial folds.

The cutaneous, skeletal, muscular, and nervous
systems are practically the same as in section B.

2. The digestive system.

a. The pharynx has the same structure as in section B,

but differs markedly in shape, being greatly com-
pressed laterally, so that its two sides are almost
in contact with each other ; both endostyle and
epibranchial groove are well marked.

b. The liver lies at the right side of the pharynx, and

is oval in section. Its walls are thick, and consist

SECTION THROUGH PHARYNX.

213

of a single layer of very long and slender flagellate
cells, resting on a thin outer wall of connective
tissue.

44. — Aiuphioxus lanceolatus. Transverse section through the
hinder part of the pharynx of an adult female, passing through the
liver and ovaries. The boundary of the atrial cavity is indicated
by a thick black line. (a. m. m.)

A, skeleton of dorsal fin. B, spinal cord. C, notochord. D, connective-
tissue sheath of notochord. E, cavity of pharynx. F, epibranchial groove.
G, endostyle. H, atrial cavity. L, liver. M, dorsal ccelomic canal. N,
branchial ccelomic canal. O, coelomic space surrounding liver. OV, ovary.
P, metapleural canal. R, left dorsal aorta. S, endostylar ccelomic canal. T,
hepatic veins.

3. The atrial cavity has the same general relations as in
section B. The atrial epithelium covers the outer
surfaces of the branchial bars, and the suspensory

214 AMPHIOXUS

folds of the pharynx ; and is continued downwards
over the inner surfaces of the atrial folds, covering
the reproductive organs, and being thrown into folds
along the ventral wall of the atrial cavity. It also
forms an investment to the liver.

4. The coelom is still more subdivided than in section B ;

its main divisions are as follows :

a. The dorsal coelomic canals are a pair of large

spaces above the suspensory folds of the pharynx,
as in section B.

b. The branchial coelomic canals are a series of cavi-

ties in the primary branchial bars, as in section B.

c. The endostylar coelomic canal is a median space in

the endostyle.

d. A space surrounding the liver, between its proper

wall and the atrial epithelium, is part of the
ccelom.

e. Paired spaces immediately surrounding the repro-

ductive organs are also extensions of the coelom.

5. A pair of large metapleural canals lie in the lateral fins.

6. The circulatory system.

a. The cardiac aorta, the dorsal aortse, and the aortic

arches are as in section B.

b. The portal veins he on the liver, in the coelomic

space surrounding it. They are often difficult to
identify in transverse sections.

c. The hepatic veins are three or four vessels lying

side by side on the dorsal surface of the liver,
in the coelomic space between the liver and the
atrial membrane investing it.

7. The reproductive organs occupy similar positions in the

two sexes. They are masses, varying in size accord-
ing to the degree of ripeness of the ova or sperma-
tozoa, lying on the inner sides of the atrial folds,
and projecting into the atrial cavity.

SECTION THROUGH ATRIAL PORE.

215

D. Transverse Section passing through the Atrial Pore.

1. General characters. The section is smaller than the
preceding one, and the myotomes reach much
further down the sides. The intestine is cut across,

Fig. 45. — Amphioxus lanceolatus. Transverse section of an adult
specimen, passing through the atrial pore. The boundary of the
atrial cavity is indicated by a thick black line. (a. m. m.)

A, skeleton of dorsal fin. B, spinal cord, c, notochord. D, connective-
tissue sheath of notochord. H , atrial cavity. K, intestine. O, ccelomic space
surrounding intestine. P, metapleural canal. Q, dorsal aorta. U, portal
veins. V, atrial pore, represented diagrammatically. W, metapleural folds.

but the section lies entirely behind the reproductive
organs. The metapleural folds are present, but are
much smaller than in sections B and C, and between
them in the median plane is the atrial pore.

The skin, and the skeletal, muscular, and nervous

216 AMPHIOXUS.

systems are practically as in section B, except that
the ventral muscles are absent.

2. The intestine is circular in section. Its walls are thick,

and, like those of the liver, consist of a single layer
of very long and slender epithelial cells, resting on
a thin connective-tissue basement membrane.

3. The atrial cavity surrounds the sides and the ventral

surface of the intestine, and opens to the exterior

Fig. 46. — Amphioxus lanceolatus. Transverse section of an adult
specimen, passing through the anus. (a. m. m.)

A, dorsal fin. B, spinal cord. C, notochord. D, connective-tissue sheath
v of notochord. |, ventral fin. K, anus. K', intestine, o, coeloni. Q, dorsal

aorta.

at the atrial pore in the mid-ventral line between a
pair of thickened and prominent lips, which are
prolonged backwards like a spout.

4. The coelom is reduced to a narrow space surrounding

the intestine, between its proper wall and the atrial
membrane.

5. The metapleural canals are very greatly reduced in size.

SECTION THROUGH ANUS. 217

6. The circulatory system.

a. The dorsal aorta is a median vessel between the

notochord and the intestine.

b. The portal or sub-intestinal veins are three or four

vessels lying side by side on the ventral surface
of the intestine, in the ccelomic space surround-
ing it.

E. Transverse Section passing through the Anus.

1. General characters. The section is much smaller than

the preceding ones. It is oval, with median dorsal
and ventral fins, and the notochord occupies its
centre.

The section passes behind the atrial cavity
which is therefore entirely absent in it.

The skin and the skeletal, muscular, and nervous
systems are practically as in section B, except that
the ventral muscles are absent.

2. The intestine lies below the notochord, and the anus is

on the left side of the ventral fin.

3. The coelom is a space surrounding the dorsal and lateral

surfaces of the intestine.

4. The dorsal aorta is a median vessel in the connective

tissue below the notochord.

218

Chapter XII

THE DOG-FISH. Scyllium canicula.

The dog-fish, together with the sharks and skates, belongs
to the group of fish known as Elasmobranchs, the great
majority of which are powerful swimmers, of carnivorous
habits, and exceedingly voracious. One of the chief characters
of the group is that almost the whole of the internal skeleton
remains cartilaginous throughout life : hence Elasmobranchs
are frequently spoken of as cartilaginous fish.

There are several kinds of dog-fish found off our own
coasts. The following description is taken from one of the
most abundant species, but will apply with but slight changes
to any of the common forms. Skates differ markedly from
dog-fish in external appearance, owing to the enormous size
of their pectoral fins, which form the great lateral wing-like
expansions of the body ; as regards internal structure, how-
ever, the differences are comparatively unimportant, and a
skate might easily be dissected by following the directions
given in this chapter. All the measurements here given are
taken from a dog-fish of about two feet in length, which will
be found the most convenient size for dissection.

Scyllium canicula, like most other species of dog-fish, lays
eggs, which are of large size, and enclosed in horny capsules
produced at their angles into long spirally twisted strings
for attachment. In many cases the eggs are retained within
the oviduct during part or the whole of their development ;
and in some few species a still closer relation is established
between the mother and the embryo, similar in many respects
to the placenta of mammals, whereby the embryo obtains

EXTERNAL CHARACTERS. 219

during its development a supply of nutriment direct from
the mother.

I. EXTERNAL CHARACTERS.
Lay the dog-fish on a board, and determine the following

A. General Appearance.

The body is greatly elongated : it is broadest about the
level of the pectoral fins, in front of which it is flattened
dorso-ventrally, ending in the blunt rounded snout. The
hinder part of the body is compressed laterally, and tapers
gradually to the end of the long slender tail, the hindmost
three or four inches of which are bent up at an angle.

The general colour is grey ; paler on the ventral surface,
and marked on the dorsal and lateral surfaces with dark
brown spots which are generally larger and fewer in the female
than in the male. The animal is capable of slightly altering the
intensity of these spots. The entire surface is covered with
small scales, and along each side of the head and body there
runs a slight longitudinal mark, the lateral line.

B. The Fins.

The fins are flattened outgrowths supported by an internal
cartilaginous skeleton, and usually triangular in shape, the
base or attached border being very commonly free at its
posterior end.

1. Median Fins.

a. The dorsal fins are two in number, the anterior

and larger one being placed about the middle of
the length of the body, and the posterior one
a short distance further back.

b. The caudal fin forms a vertical fringe round the

posterior four or five inches of the tail. Its dorsal
portion is narrow, its hinder end truncated, and
its ventral portion partially subdivided into a
small posterior and a large anterior lobe.

220 THE DOG-FISH.

c. The anal fin is on the ventral surface opposite the
interval between the two dorsal fins.

2. Paired fins, representing the limbs of higher verte-
brates.

a. The pectoral fins are the largest of all the fins, and

project horizontally outwards from the sides of
the ventral surface of the body, just behind the
head.

b. The pelvic fins are smaller than the pectoral, and

are placed on the ventral surface of the body, a
little way in front of the middle of its length.
Their inner borders touch each other, and in the
male are fused together behind.

In the male a part of each pelvic fin is
separated from the rest of the fin, and specially
modified to form a copulating organ or * clasper ' :
this is a stout backwardly directed rod, deeply
grooved along its inner and dorsal surface. The
groove leads forwards into a sac which lies just
beneath the skin of the ventral surface of the
body, close to the median line, and ends blindly
about an inch in front of the pelvic girdle.

C. External Apertures.

1. Median apertures.

a. The mouth is a large transverse crescentic opening

on the under surface of the head, a short dis-
tance from its anterior end.

b. The cloacal aperture is on the ventral surface of

the body between the pelvic fins, i.e. in front of
the middle of the length of the body. It gives
exit to fsecal matter from the intestine, and to
the renal and genital products.

2. Paired apertures.

a. The nostrils are a pair of large circular apertures
on the under surface of the head in front of the

EXTERNAL CHARACTERS. 221

mouth, with which they are connected by wide
grooves covered by flaps of skin.

b. The spiracles are a pair of small apertures on the

sides of the head, just behind the openings of the
eyes. They are really a pair of modified gill-
clefts. Through the spiracles sea-water passes
into the pharynx.

Pass a seeker down one of the spiracles into the mouth.

c. The gill-clefts are a series of five vertical slits on

each side of the neck in front of and slightly
above the pectoral fins. They open obliquely
backwards, and during life give exit to the
water passed from the mouth and spiracles over
the gills for the purpose of respiration.

Pass a seeker through the gill-clefts into the mouth.

d. The cloacal pouches are a pair of pocket-like de-

pressions at the sides of the cloacal aperture
and between the pelvic fins.

e. The cloacal papillae are attached in front of the

cloacal pouch, with their apices directed back-
wards. Each is traversed by a canal (peritoneal
canal) which opens anteriorly into the body-
cavity and posteriorly by the abdominal pores
into the cloacal pouches. The pores are minute,
and are only open in mature dog-fish.

3. Scattered apertures.

The apertures of the sensory and ampullary canals
are rows of minute openings arranged symmetri-
cally on the surface of the head. The openings
of the ampullary system are especially abundant
on the snout. They lead into tubes of con-
siderable length, which lie beneath the skin and
are filled with a transparent gelatinous substance.
Squeeze the head so as to press out the gelatinous sub-
stance, and so render the openings of the tubes more obvious.

222 THE DOG-FISH.

D. The Scales and Teeth.

1. The scales. The whole surface of the body is covered

with small placoid scales, which are special de-
velopments of the skin.

a. General arrangement. The scales are larger on the

dorsal than on the ventral surface, and form a
powerful caudal ' rasp ' on the sides of the tail.
Their projecting points are directed backwards,
so that the finger can be readily passed along the
- body from the head to the tail, but meets with
considerable resistance in the reverse direction.

b. Structure of the scales.

Cut out a small piece of skin, and boil it for a few minutes
in a solution of caustic potash to isolate the scales. Mount
them in glycerin, and examine with the microscope.

Each scale consists of a calcified base, usually
four-lobed, from which a pointed spine of den-
tine, capped with * enamel,' projects backwards.

2. The teeth are really the enlarged scales of the skin

covering the jaws. They are arranged in several
rows, and have their points directed backwards.

II. ■THE SKELETON.

The skeleton of the dog-fish forms an important link
between the exceedingly simple condition seen in Amphi-
oxus and the complicated bony framework of the higher
vertebrates.

In the embryo a rod-like note-chord extends down the
back, immediately beneath the central nervous system and
dorsal to the alimentary canal, similar to that of Amphioxus,
except that it does not quite reach to the anterior end of the
body. Around this notochord a sheath of cartilage is formed,
which in the, head gives rise to the base of the skull, and in

VERTEBRAL COLUMN. 223

the body becomes divided transversely into segments, the
centra or bodies of the vertebrae, the flexibility of this region
being thereby increased.

Above the centra neural arches are formed, which sur-
round and protect the brain and spinal cord ; and below them
haemal arches, which tend to encircle the body-cavity.

Besides this axial skeleton a visceral skeleton is developed
around the anterior part of the alimentary canal, and skeletal
elements are also formed in relation with the fins.

The greater part of the skeleton consists throughout life
of cartilage. It is hardened in many places by deposition of
calcareous salts.

The skeleton of the dog-fish is best prepared by taking
a specimen which has been some days in spirit, dipping it in
hot water a few degrees below boiling-point for a few minutes,
in order to soften the connective tissue, and then scraping or
brushing away the soft parts until the skeleton is clean. Speci-
mens so prepared must not be allowed to dry, but should be
kept in weak spirit.

A. The Vertebral Column.

A trunk vertebra consists of a centrum, traversed by the
notochord ; a dorsal or neural arch, surrounding the spinal
cord ; and transverse or haemal processes. In the caudal or
tail vertebrae of the dog-fish the haemal processes unite
ventrally, forming a haemal canal which encloses the caudal
artery and vein.

The structure of the vertebral column is best made out by
cutting sagittal, transverse, and horizontal sections through
various parts of its length with a strong knife, and comparing
these with each other.

1. The centra are short cylinders of cartilage, about as long
as they are wide, connected together by interver-
tebral ligaments of strong connective tissue, which
allow slight movement between the successive
vertebrae and free flexion of the column as a whole.

224 THE DOG-FISH.

Each centrum is a ring, thin at its ends, but
thickened on its inner side in the middle of its
length, so that the cavity is hour-glass-shaped
(fig. 51, p. 247). The inner surface of the ring is
lined by a layer of calcified connective tissue.

The notochord persists throughout the whole
length of the column : it is greatly constricted
opposite the middle of each vertebra, but thickened
almost to the full width of the column in the inter-
vertebral spaces.

2. The haemal arches.

In the anterior part of the vertebral column the
centra are flattened ventrally, and produced laterally
into blunt horizontal transverse processes which
bear at their ends movably articulated cartilaginous
ribs about half an inch in length.

Further back the transverse processes become
more prominent and project ventrally as haemal
processes, causing the under surface of the column
to appear grooved.

In the tail the ribs are absent, and the haemal
processes of each vertebra meet and unite mid-
ventrally, thereby completing the haemal arch, which
encloses the caudal artery and vein.

At the hinder end of the tail, opposite the caudal
fins, the haemal arches are produced downwards into
median haemal spines.

The haemal arches develop independently of the
vertebral centra, but become connected with them
at a very early date.

3. The neural arches are made up of elements of three

kinds : neural processes, neural plates, and neural
spines.

a. The neural processes, forming the bases of the
neural arches, are blunt processes projecting

THE SKUI^L. 225

upwards from the sides of each centrum opposite
its middle.

b. The neural plates are a series of thin hexagonal

plates of cartilage which form the sides of the
neural canal (figs. 48 and 51). On each side
there are twice as many neural plates as there
are vertebrae, the neural plates being placed
alternately opposite the middles of the vertebrae,
and opposite the intervertebral intervals. The
former or vertebral neural plates are fused in
the adult with the tops of the neural processes,
and have their posterior borders notched for the
passage of the ventral roots of the spinal nerves.
The dorsal roots of the nerves pass out through
notches in the posterior borders of the inter-
vertebral neural plates.

c. The neural spines are a series of median nodules

of cartilage wedged in between the dorsal ends of
the neural plates so as to complete the neural
arches above.

Along the greater part of the length of the
column there are two neural spines to each
vertebra, but in places the arrangement is less
regular.

B. The Skull.

The skull of the dog-fish remains cartilaginous through-
out life, thereby escaping the complications that result from
the development of bone : in it, also, the several factors of
which the skull is made up are more easily determined than
in the higher forms. For these reasons it is peculiarly
instructive, affording an important clue to the complex con-
dition met with in higher vertebrates.

The skull consists of (1) an axial tube, the cranium
proper, open at both ends, which lodges the brain, and is to
be regarded as an anterior unsegrhented -continuation of the
vertebral column ; and (2) the olfactory and auditory sense-

226

THE DOG-FISH.

THE SKULL. 227

capsules, which are cartilaginous capsules investing the nose
and the ear respectively. These latter are at first independent
of the cranium, but in the adult fuse with it forming lateral
expansions of its anterior and posterior ends. This fusion
of the sense-capsules and cranium is so complete that they
will be described together.

1. General form of the skull.

The skull is a somewhat oblong box of cartilage,
deeply hollowed at the sides to form the orbits,
which lodge the eyes, and ending in front in a
short-pointed rostrum formed by three converging
rods.

The olfactory capsules are thin-walled lateral
expansions of the anterior end of the skull, in front
of the orbits ; and the auditory capsules are more
massive projections behind the orbits.

Examine the several surfaces of the skull in succession,
identifying the parts described below.

2. The dorsal surface of the skull.

a. The olfactory capsules are a pair of large oval
cartilaginous cups at the anterior end of the
skull : they have very thin walls and are easily
torn. Their long axes are placed somewhat

Fig. 47. — Scyllium canicula. The skull and visceral skeleton with the
anterior part of the vertebral column seen from the right side.
The labial cartilages and gill-rays are omitted, (a. m. m.)

A, auditory capsule. B, post-orbital groove. C, inter-orbital canal. E»
upper jaw. F, lower jaw. G, liyo-mandibular cartilage. H, cerato-hyal. |,
pharyngo-branchial. K, epi-branciiial. |_, cent to- branchial. M, extra-bran-
chial. N, vertebral neural plate. N.C, olfactory capsule. O. centrum of ver-
tebra, p, intervertebral mniral plate. R, neural spine. S» foramen jor the
ventral root of a spinal nerve. T, foramen for the dorsal root of the same
nerve. U» orbital grooves, lodging the ophthalmic branches of the fifth and
seventh nerves. W, aperture at end of orbital groove through which the
ophthalmic branches of the fifth and seventh nerves leave the orbit. Y, foramen
for hyoidean artery, z, ethmo-palatine ligament.

II, optic foramen. Ill, foramen for third nerve. IV, foramen for fourth
nerve. V, foramen for the main branches of the fifth and seventh nerves, and
for the sixth nerve. Va, foramen for the ophthalmic branch of the fifth nerve.
Vila, foramen for the ophthalmic branch of the seventh nerve. IX, foramen
for the ninth or glossopharyngeal nerve.

228 THE DOG-FISH,

obliquely, and they are separated from each
other by a thin median internasal septum.

b. The rostrum is formed by three cartilaginous bars,

of which two run forwards from the upper
surface of the olfactory capsules, converging in
front ; while the third is an anterior prolonga-
tion of the floor of the cranium, or brain-case.

c. The anterior fontanelle is a large, almost circular

hole in the roof of the skull, between and behind
the olfactory capsules. During life it is closed
by a connective-tissue membrane, in connection
with the inner surface of which is the paraphysis.

d. The supra-orbital crests are prominent curved ridges

of cartilage running along the sides of the skull
from the olfactory to the auditory capsules, and
forming the dorsal boundaries of the orbits.
Along the inner side of each crest, between it
and the cranium, is a shallow longitudinal groove
in which lies a bundle of sensory canals.

e. The foramina for the ophthalmic nerves are a pair

of small apertures behind and to the outer sides
of the anterior fontanelle, and to the inner sides
of the supra-orbital crests. Through them the
ophthalmic branches of the fifth and seventh
nerves pass from the orbit to the dorsal surface
of the olfactory capsules.

f. The auditory capsules are large lateral projections

at the hinder end of the skull. They lodge the
auditory organs, certain parts of which can be
recognised without dissection.

i. Between the two auditory capsules there is a
median depression, at the bottom of which
are a pair of holes. Each hole leads down-
wards and outwards into a canal, the aquae-
ductus vestibuli, which opens below into the
internal ear.

THE SKULL. 229

Pass a seeker down the aquceductus vestibuli on one side
into the ear.

ii. From the outer side of the aperture of the aquge-
ductus vestibuli on each side a ridge runs for-
wards and outwards across the skull-roof and
towards the orbit. In this lies the anterior
vertical semicircular canal of the ear, which
can usually be seen through the cartilage,
iii. A similar ridge, running backwards and out-
wards from the same point, lodges the posterior
vertical semicircular canal.

iv. The external or horizontal semicircular canal

can usually be seen through the cartilage,
lying to the outer side of the other two canals,
and with its convexity directed outwards.

3. The side of the skull.

a. The rostrum and olfactory capsule have already

been seen.

b. The orbit is the deep oval concavity at the side of

the skull, extending from the olfactory to the
auditory capsule, and bounded above and below
by the prominent supra-orbital and sub-orbital
ridges. It lodges the eyeball and its muscles,
and is traversed by numerous nerves, the rest
of the space being occupied by the large orbital
blood-sinus.
Identify the following structures within the orbit (see
Jig. 47). '

i. The orbito-nasal foramen is a small round hole
at the anterior inferior angle of the orbit,
through which the veins of the anterior part of
the head communicate with the orbital sinus,
ii. The optic foramen is a large hole about the
middle of the length of the orbit and near its
ventral border. Through it the optic nerve
enters the orbit.

230 THE DOG-FISH.

iii. The foramen for the third nerve is a small hole
in the inner wall of the orbit, about a quarter
of an inch behind the optic foramen and a
little above it.

iv. The foramen for the fourth nerve is a very
small hole near the dorsal border of the inner
wall of the orbit, vertically above the optic
foramen, or very slightly behind this.

v. The foramen for the fifth and seventh nerves is

a large hole at the posterior and ventral angle
of the orbit, immediately in front of the audi-
tory capsule. Through it the main branches
of the fifth and seventh nerves, and also the
sixth nerve, enter the orbit.

vi. The aperture of the inter-orbital canal is a
small round hole in front of and a little
below the foramen for the fifth and seventh
nerves, and below and behind the foramen
for the third nerve. The canal traverses the
base of the skull, and places the orbital blood
sinuses of the two sides in communication
with each other.

vii. The foramen for the hyoidean artery lies a
little in front of, and slightly below, the
opening of the inter-orbital canal. Through
it the hyoidean artery enters the skull.

viii. The foramina for the ophthalmic branches of the
fifth and seventh nerves. These nerves enter
the orbit through two holes near its hinder
end, just in front of the auditory capsule, and
above the foramen for the main branches of
the fifth and seventh nerves : the aperture for
the ophthalmic branch of the seventh nerve
is the larger of the two, and is placed slightly
above and behind that for the fifth. From
these apertures shallow grooves run forwards

THE SKULL. 231

along the inner wall of the orbit, close to its
dorsal border, to the foramen by which the
two nerves leave the orbit : this is situated
at the antero -superior angle of the orbit, and
has already been seen on the dorsal surface
of the skull.

c. The articular surface for the hyo-mandibular car-

tilage is a concave depression on the side of
the hinder end of the skull, below the auditory
capsule.

d. The post-orbital groove is a deep horizontal groove

immediately above the articular surface for the
hyo-mandibular cartilage, and overhung by the
projecting ledge of the auditory capsule which
lodges the horizontal semicircular canal : it
lodges during life the venous canal through
which the orbital sinus communicates with the
anterior cardinal sinus.

The anterior end of the groove is bridged
over by stout ligamentous tissue.

e. The foramen for the glosso-pharyngeal nerve is at

the hinder end of the floor of the post-orbital
groove, behind and below the horizontal semi-
circular canal of the ear.

4. The ventral surface of the skull.

a. The base of the skull is a broad flat plate of

cartilage, crossed almost transversely a little
behind its middle by a pair of grooves in which
the carotid arteries he. At the point of meeting
of the grooves in the middle line is an aperture
through which the internal carotid artery enters
the skull.

b. The apertures of the olfactory capsules are large,

but. are considerably reduced by the nasal car-
tilages, which are liable to be torn in cleaning

THE DOG-FISH.

Fig. 48. — Scyllium canicula. The skull and visceral skeleton seen from
the ventral surface. The labial cartilages, extra-branchials, and
gill-rays of the left side have been removed, (a. m. m.)

A, base of skull. BB, basi-brancliial. BC 1 , BC 5, first and fifth cerato-
branchials. BE 1 , BE 5, first and fifth cpi-branehials. BH 1 , first hypo-branchial -
represented as though seen through the cerato-hyal. BH 2, second hypo-
branchial. BP 1 , BP 5, first and fused fourth and fifth pharyngo-branchials.
E, upper jaw. EB 1, EB 2, EB 3, first, second, and third extra-branchials.
EL, ligament connecting the two halves of the upper jaw. F, lower jaw. FL,
ligament connecting the two halves of the lower jaw. G, gill-ray. HB, basi-
byal. HC, cerato-hyal. MM, hyo-mandibular cartilage. L 1, L 2, labial
cartilages. N, nasal cartilage. NC. olfactory capsule. R, median bar of
rostrum. R|_, lateral bar of rostrum. S, notch for Cuvierian sinus. T,
foramen for neck of Thyroid gland.

VISCERAL SKELETON. 233

the skull. Within the capsules are seen the
large foramina, closed in the natural condition
by fenestrated membranes, through which the
olfactory nerves enter.

5. The posterior end of the skull.

To see this properly the skull must be separated from the
vertebral column.

a. The foramen magnum is the large median hole

through which the spinal cord enters the skull
before expanding into the brain.

b. The notochord enters the base of the skull imme-

diately below the foramen magnum.

c. The condyles are a pair of round prominences

at the sides of the notochord, and below the
foramen magnum : they articulate with the first
vertebra.

d. The foramen for the pneumogastric nerve of each

side is at the bottom of a pit-like depression at
the side of the foramen magnum, below and to
the inner side of the auditory capsule.

C. The Visceral or Pharyngeal Skeleton.

This consists of a series of cartilaginous hoops or arches
placed one behind another, and encircling the anterior part
of the alimentary canal. In conformity with the shape of
the mouth and pharynx, which they surround, the hoops are
wide from side to side, and compressed dorso-ventrally.

The first arch gives rise to both upper and lower jaws.

The second or hyoidean hoop is large, and plays an
important part in slinging the jaws to the skull.

The third and succeeding hoops are more slender : they
surround the pharynx and support the gills.

To see the visceral skeleton properly, a roll of paper should
be passed in at the mouth-opening, and backwards through the
several hoops.

234 THE DOG-FISH.

1. The first or mandibular arch is very greatly modified;

its ventral part is large and stout and forms the
lower jaw. The upper jaw is formed as an out-
growth from its anterior edge.

a. The upper jaw consists of a pair of stout carti-

laginous rods, wide behind and narrowing
towards their anterior ends, where they are
united across the middle line by ligament. They
extend forwards as far as the openings of the
olfactory capsules, and are connected by stout
ethmo-palatine ligaments with the base of the
skull, at the anteroinferior angles of the orbits.
They bear teeth along the ventral surface of the
anterior two-thirds of their length, but these are
lost in skulls prepared as described.

b. The lower jaw consists of a pair of wide flattened

bars of cartilage, which are movably hinged with
the upper jaw behind, and bound together by
a median ligament in front. It lies, when the
mouth is shut, parallel to and immediately
behind the upper jaw. It bears teeth along the
dorsal surface of the anterior two-thirds of its
length on each side, but these are lost in skulls
prepared as described.

2. The second or hyoidean arch is incomplete above : it

consists of a pair of rods the dorsal ends of which
articulate with the sides of the skull, while the
ventral ends are connected together by a median
plate of cartilage. Each rod is further divided into
an upper and a lower segment.

a. The hyo-mandibular cartilage, or upper segment, is
a short stout bar of cartilage, projecting almost
horizontally outwards from the skull, but slightly
downwards and backwards. Its inner and upper
end articulates with a concave surface on the side
of the skull, near its hinder end, and immediately

VISCERAL SKELETON. 235

«

below the post-orbital groove : its outer end
articulates with the lower segment of the arch
opposite the angle of the mouth. To its hinder
border are attached a series of rods of cartilage —
gill-rays — which support the gill-folds.

The hyo -mandibular cartilage in the dog-fish
is spoken of as the suspensorium, because it
forms the skeletal link between the jaws and
the skull.

Skulls, like that of the dog-fish, in which the
hyoidean arch forms the suspensorium of the
lower jaw, are spoken of as hyostylic.

b. The cerato-hyal, or lower segment of the hyoidean

arch, is a longer and more slender bar which runs
forwards and inwards in the floor of the mouth,
just behind the lower jaw, and partly overlapped
by it. It bears gill-ravs along its posterior border.

c. The basi-hyal is a broad median plate of cartilage

lying in the floor of the mouth behind the lower
jaw. It is produced behind into two horns which
are attached by ligaments to the dorsal surfaces
of the ceratohyals. It is perforated by the neck
of the * thyroid gland.'

3. The branchial arches are the remaining five visceral
arches : they diminish in size from before backwards,
and each is divided into four segments on each side.

a. The pharyngo-branchials, the most dorsal elements,

are flattened rods running forwards and outwards
in the roof of the pharynx. The anterior ones
are connected with their fellows of the opposite
side by ligament ; and the last two of each side
are much smaller than the others, and fused
together.

b. The epi-branchials are short broad plates attached

to the outer ends of the pharyngo-branchials.

236 THE DOG-FISH.

•

With the exception of the hindmost, they bear
gill-rays along their posterior borders.

c. The cerato-branchials are stout rods running forwards

and inwards in the ventral wall of the pharynx,
and bearing gill-rays along their posterior borders.
The fifth or hindmost cerato-branchial is shorter,
but much broader than the others : it has no
gill-rays, and its posterior border is deeply notched
to lodge the Cuvierian sinus (p. 250).

d. The hypo-branchials are small rods in connection

with the inner ends of the first, second, third
and fourth cerato-branchials. The first hypo-
branchial, which is very small, is directed for-
wards, and connects the first cerato-branchial
with the basi-hyal. The three hinder hypo-
branchials are larger, and are directed backwards.

e. The basi-branchial is an elongated flattened median

plate, pointed behind. It is connected with the
two hinder pairs of hypo-branchials in front,
and at the sides with the fifth cerato-branchials.
With these latter it strengthens and supports
the dorsal wall of the pericardial cavity (fig. 48,
p. 232).

f. The extra-branchials are three pairs of elongated

curved rods lying along the outer sides of and
beneath the second, third, and fourth branchial
arches.

4. The labial cartilages are two pairs of slender cartila-
ginous rods in the folds of skin at the sides of the
mouth.

D. The Skeleton of the Fins.
1. The median fins.

The skeleton of the dorsal fins consists of a series
of parallel rods of cartilage, the basal cartilages,
sloping upwards and backwards, and calcified along
their axes. The lower or basal ends of the rods

APPENDICULAR SKELETON.

237

often unite together, and the dorsal ends support
one or more rows of closely fitting polygonal plates.
On each side of this median cartilaginous skeleton
is a series of slender elongated horny fibres derived
from the skin.

The other median fins are similar to the dorsal
ones, except that the cartilaginous elements may be
reduced to a longitudinal basal rod, or may even be
completely absent.

Fig. 49. — Scyllium canicula. The pelvic bar and fins of a male seen
from below. In the right fin the cartilaginous skeleton is shown,
hi the left the muscles and the horny dermal fibres, (a. m. m.)

D, basi-pterygiuin. DC) posterior end of the basi-pterygium, forming the
skeleton of the clasper. DF, radial cartilages. ' F, anterior radial, articulating
directly with the pelvic girdle. H, horny fibres. M, muscles. PG, pelvic
girdle.

2. The paired fins are essentially similar to the median
fins, and are possibly to be regarded as local enlarge-
ments of lateral fins, which originally extended along
the sides of the body.

It is convenient to take the posterior or pelvic

fins before the pectoral, as their structure is simpler.

a. The pelvic bar is a nearly straight rod of cartilage,

placed transversely across the ventral region of

the body, about half an inch in front of the

238 THE DOG-FISH.

cloaca. The ends of the bar are produced for-
wards into blunt processes, and bear on their
posterior surfaces the articular facets for the
pelvic fins.

b. The pelvic fins. The cartilaginous skeleton of each

fin consists of a slightly curved basal rod, the
basi-pterygium, which lies along the inner side
of the fin, and articulates in front with the
pelvic girdle. From the outer side of the basi-
pterygium a series of parallel cartilaginous
radial cartilages, about fourteen in number, run
outwards and backwards, supporting at their
ends smaller polygonal plates. The anterior
one or two of the radials may articulate inde-
pendently with the pelvic girdle.

In the adult male the posterior end of the
basi-pterygium is produced backwards into the
skeleton of the clasper.

Besides the cartilaginous skeleton, horny
fibres like those of the median fins are present
in great numbers on both surfaces of the fin.

c. The pectoral girdle, which lies immediately behind

the last branchial arch, is considerably larger
than the pelvic, and consists of a hoop of carti-
lage incomplete above.

The ventral portion of the hoop is produced
forwards into a flattened process, which forms
part of the floor of the pericardial cavity, and is
hollowed dorsally to receive the ventricle of the
heart.

At each side the arch is thickened, and bears
on its outer border a triple articular facet for the
basal cartilages of the fin.

d. The pectoral fin has three basal cartilages, the pro-

pterygium, meso-pterygium, and meta-ptery-
gium, articulating with the three facets on the
pectoral girdle. Of these the pro -pterygium is
the smallest, and the meta-pterygium, which

APPENDICULAR SKELETON. 239

extends along the inner or attached border of the
fin, and corresponds to the basi-pterygium of the
pelvic fin, is much the largest.

From the outer borders of the basal cartilages
a series of close-set radial cartilages diverge : of
these the pro -pterygium supports a single large
one ; the meso-pterygium also supports one,

Fig. 50. — Scyllium canicula. The pectoral girdle and fins of a male
seen from the ventral surface. In the right fin the cartilaginous
skeleton is shown, in the left the muscles and horny dermal fibres.
(a. m. m.) ^

A, pro-pterygium. AF, pro-pterygial radial. B, meso-pterygium. BF,
meso-pterygial radial, c, meta-pterygium. CF, meta-pterygial radial. E,
polygonal cartilaginous plates. H, horny fibres. M, muscles of fin. FG, ven-
tral portion of pectoral girdle. PG', dorsal portion of pectoral girdle.

which is, however, split distally into two or
three ; while the meta-pterygium supports twelve
or more much narrower radials.

To the outer ends of the radials two or more
rows of polygonal plates of cartilage are attached,
forming the peripheral portion of the skeleton.

Horny fibres are present in large numbers,
as in the pelvic fins.

240 THE DOG-FISH.

III. THE ABDOMINAL CAVITY AND VISCERA.

Lay the dog-fish on its back on the dissecting -board, and
pin it down through the fins. Determine the positions of the
pectoral and pelvic girdles by feeling them through the skin,
the pectoral girdle being a little in front of the pectoral fins,
and the pelvic girdle a short distance in front of the cloacal
aperture.

Open the abdominal cavity by a median ventral incision,
extending from the pelvic girdle behind to the pectoral girdle
in front. Cut through the body-wall transversely, just behind
the pectoral girdle, and pin the two flaps down to the dissect-
ing-board. Cut through the pelvic girdle in the median plane,
and prolong the cut backwards to the level of the cloacal aperture,
keeping a little to its right side. Wash or sponge out the abdo-
minal cavity.

The ccelom or body-cavity extends forwards into the head
in the embryo, but in the adult is reduced to the pericardial
cavity which lodges the heart, and the abdominal cavity in
which he the principal viscera. The abdominal cavity is a
wide space extending from the level of the pectoral girdle
backwards as far as, or slightly beyond, the cloacal aperture,
where it communicates with the exterior through the abdo-
minal pores (p. 221).

Pass a seeker from the abdominal cavity through one of
the abdominal pores to the exterior.

Identify the positions and relations of the following organs
before proceeding further with the dissection.

1 . The liver is a very large solid organ of a brownish yellow

colour, attached by a median suspensory ligament to
the anterior wall of the abdominal cavity. It consists
of two lobes of about equal size, which are united
in front, and extend back along the sides of the ab-
dominal cavity for the greater part of its length.

2. The stomach is a wide U shaped tube lying between the

ABDOMINAL VISCERA. 241

lobes of the liver, and extending about two-thirds
the length of the abdominal cavity.

3. The spleen is a large brownish-red body attached to the

loop of the stomach, and sending a long narrow lobe
along its distal limb.

4. The pancreas is a whitish, laterally compressed body,

about an inch and a half long, lying in the angle
between the distal limb of the stomach and the in-
testine. Its anterior end is expanded and gives off
a small ventral lobe, which is closely applied to the
intestine.

5. The intestine is a wide and nearly straight tube run-

ning along the right side of the abdominal cavity
from the stomach to the cloaca.

6. In the female the ovary is single, and attached by a

delicate median fold of peritoneum to the wall of
the abdominal cavity. The contained ova gives its
surface a tuberculated appearance.

In the male the testes are a pair of elongated
solid bodies, of a whitish colour, lying along the
dorsal region of the anterior two-thirds of the ab-
dominal cavity. Their hinder portions fuse together.

7. The peritoneum is a smooth membrane lining the

abdominal cavity, and reflected downwards from the
mid-dorsal line as a double sheet, the mesentery,
which invests the alimentary canal and other
viscera, and attaches them to the body- wall.

The mesentery is well developed at the hinder
end of the abdominal cavity, but is interrupted by
one or more large fenestrse opposite the greater part
of the length of the intestine.

8. The kidneys are covered by the peritoneum, but can be

recognised as a pair of slightly elevated longitudinal
ridges, one on each side of the mid-dorsal line of
the abdominal cavity, and extending nearly its whole
length.

242 THE DOG-FISH.

IV. DISSECTION OF THE DIGESTIVE SYSTEM.

In dissecting the alimentary canal be careful not to injure
the blood-vessels in relation with it : these lie for the most part
in the mesentery.

A. The Alimentary Canal.

1. The mouth and pharynx. The detailed examination of

these parts is best deferred until the circulatory
system is dissected.

2. The oesophagus is a short wide tube, which enters the

abdominal cavity at its anterior end and expands
almost at once into the stomach.

Pass the handle of a scalpel or seeker through the mouth
and down the oesophagus into the stomach.

3. The stomach is U-shaped. Its proximal limb, which is

very wide and directly continuous with the oeso-
phagus, extends back nearly to the hinder end of the
abdominal cavity ; it then turns sharply forwards
to form the distal limb, which is shorter and much
narrower.

4. The intestine is separated from the distal limb of the

stomach by a slight pyloric thickening. Its proximal
part is about equal in width to the stomach, alongside
which it lies ; but it soon dilates to form the wide
ileum, the surface of which is marked spirally by
blood-vessels, corresponding to the line of attach-
ment of the spiral valve. At its hinder end the
intestine narrows to form the rectum, which, after
a course of about an inch and a half, opens to the
exterior at the cloacal aperture.
The spiral valve.
Open the intestine along its whole length by a longitudinal
incision along its right side. Wash out its contents. Carefully
cat away with scissors the ventral wall from the several turns

DIGESTIVE SYSTEM. 243

of the spiral valve, and turn it over to the left side, so as to expose
fully the cavity of the intestine. Wash freely.

The spiral valve is a membranous fold,
attached along one edge to the inner surface of
the intestine, round which it runs spirally. The
fold is about an inch and a half wide at its
anterior end, but gradually diminishes in width
towards the posterior end, and disappears at the
commencement of the rectum. The first turn
of the spiral is a long one, but the succeeding
ones, of which there are usually seven or eight,
are only about a quarter of an inch apart. The
general appearance of the valve is that of a
series of cones one within another, the apices of
the cones being usually directed forwards, but
sometimes at the hinder end backwards.

The valve serves to retard the passage of
food down the intestine, and to increase the
extent of its absorptive surface.

B. The Glands.

1. The liver has been described above.

a. The gall-bladder is a large irregular sac imbedded

in the anterior part of the left lobe of the liver,
close to the median plane.

b. The bile-duct leaves the gall-bladder between the

two lobes of the liver, and almost in the median
plane. It receives ducts from the lobes of the
liver, and runs back along the ventral margin
of the mesentery to the intestine, along which
it runs for a short distance to open in bo the
commencement of the ileum. The bile-duct has
a total length of about three inches.

Find the duct along the edge of the fold of mesentery
between the liver and the intestine : make an incision in its
walls, and inject it with a coloured fluid such as Prussian

244 THE DOG-FISH.

blue injection : follow it forwards to the liver and gall-bladder,
and backwards to its opening into the intestine.

2. The pancreas has been described above (p. 241).

The pancreatic duct runs forwards through the
pancreas close to its ventral border. It leaves
Vthe gland at the posterior angle of its ventral
lobe, and at once enters the ventral wall of the
intestine about an inch beyond the pylorus : it
runs in the wall of the intestine for about half
an inch, and opens just to the outer side of the
line of attachment of the spiral valve.-

Find the duct along the ventral border of the pancreas :
trace it to the intestine : slit open the intestine, and trace the
duct backwards in its wall as a white ridge ending in a very
small papilla. It is easier to follow after injection with a
coloured fluid.

3. The rectal gland is a thick- walled tube, about three-

quarters of an inch long, lying in the abdominal
cavity dorsal to the rectum. From its hinder end a
duct runs forwards along the edge of the mesentery
to open into the dorsal wall of the rectum.

Cut the gland across about the middle of its length. Note
the thickness of its walls and the small size of its cavity. Pass
a bristle down the gland and duct into the rectum ; and slit open
the rectum to see the opening of the duct.

V. DISSECTION OF THE RESPIRATORY SYSTEM.

The dog-fish breathes by gills, which are vascular folds
of the mucous membrane of the pharynx, supported by the
branchial arches. Respiration is effected by rhythmical
compression and expansion of the muscular pharynx causing
water to enter in at the mouth and spiracle, to pass between the
branchial arches, over the gills, and out through the gill-clefts.

1. The gills.

Expose thoroughly the gills on the left side.

CIRCULATORY SYSTEM. 245

The gills are series of closely set, soft, and highly
vascular folds of mucous membrane radiating out-
wards from the cartilaginous branchial arches and
supported by the gill-rays.

Grills are borne on the posterior surface of the
hyoid arch, and on both the anterior and posterior
surfaces of each of the four following arches. The
inner borders of the branchial arches have carti-
laginous processes, which hinder the passage of
food to the gill-clefts. The last branchial arch
bears no gill.

2. The pseudobranchs.

Slit up the spiracle of the left side in the same way as the
gill-clefts.

The pseudobranch is a rudimentary gill, con-
sisting of about half a dozen small parallel folds,
on the anterior wall of the spiracle. Its presence
shows that the spiracle is really a gill-cleft.

VI. DISSECTION OF THE CIRCULATORY SYSTEM.

A. The Pericardial Cavity and Heart.

1. The pericardial cavity.

Lay the dog-fish on its bach on the dissecting-board, and
pin it down through the fins. Open the pericardial cavity by
removing the middle portion of the pectoral girdle, taking care
not to injure the heart, which lies in close contact with it. Cut
away the ventral wall of the cavity so as to expose it fully.

The pericardial cavity is median, and lies be-
tween the gills : it is triangular in shape, with the
apex directed forwards, and is almost completely
filled by the heart, of which the globular ventricle
is the most prominent portion. Its ventral wall is
stiffened by the anterior prolongation of the pectoral

246

THE DOG-FISH.

CIRCULATORY SYSTEM. 247

girdle, and its dorsal wall by the wide basi-bran-
chial plate and the fifth cerato-branchials.

2. The heart is a single tube bent on itself in an S -shape,
and dilated to form a series of chambers separated
by transverse constrictions.

Determine the position and general characters of the
divisions of the heart in order from behind forwards, pressing
them aside when necessary, but making no further dissection.

a. The sinus venosus is a thin-walled transversely

placed tube attached along its whole length to
the posterior and dorsal wall of the pericardial
cavity. It receives laterally the veins returning
blood from the body, and opens in front into the
auricle by a median aperture.

To expose the sinus venosus, press the ventricle slightly
forwards.

b. The auricle is a large thin-walled sac occupying

the dorsal half of the pericardial cavity. It is
triangular in shape, with its posterior , angles
produced into processes which, in the natural
position of the parts, project laterally at the
sides of the ventricle.

To expose the auricle, push the globular ventricle to one
side.

c. The ventricle is a thick-walled globular sac lying

Fig. 51. — Scyllium canicula. A longitudinal and vertical section
through the head and anterior part of the body. The section of the
brain is taken a little to one side of the median plane, and the spinal
cord has been cut across a short way behind the brain. The cut
surfaces of the cartilaginous skeleton are dotted, (a. m. m.)

A, roof of the cranium. B,internasal septum, c, amp ullary canals. D, fold
of the wall of the hemisphere in which lies the choroid plexus. E, aperture
leading from the cerebral ventricle to the olfactory ventricle. F, third ventricle.
G, stalk of the paraphysis. H, optic lobe. I, optic chiasma. IP, inter-
vertebral neural plate. K, pituitary body. |_, cerebellum. M, restiform body.
MN, lower jaw. MX, upper jaw. N, interorbital sinus. NP, vertebral neural
plate. NS, neural spine, o, basi-hyal, supporting the tongue. P, pericardial
cavity. PC, pericardio-peritoneal canal. Q, sinus venosus. R, auricle. S,
ventricle. T, conus arteriosus. TE, teeth, u, cardiac aorta. V, aperture of
afferent branchial vessel. VC, vertebral centrum. W, first branchial cleft.
X, spiracle or hyo-mandibular cleft. Y, notochord. Z, spinal cord.

248

THE DOG-FISH.

VENOUS SYSTEM. 249

ventral to the auricle, and forming the most

conspicuous part of the heart.
d. The conus arteriosus is a cylindrical tube, running

forwards from the ventricle to the anterior end

of the pericardial cavity.
3. The pericardio-peritoneal canal is a median passage,
about an inch long, opening in front into the hinder
end of the pericardial cavity dorsal to the sinus
venosus, and running backwards along the ventral
surface of the oesophagus, to open behind into the
abdominal cavity. Its anterior opening is large and
conspicuous ; the opening into the abdominal cavity,
which may be double, is smaller and slit-like.

Find the pericardial opening in the middle line, imme-
diately dorsal to the sinus venosus. Insert a seeker, and pass
it davn the canal into the abdominal cavity.

B. The Venous System.

The venous system of the dog-fish consists of three great
divisions : (1) the veins opening into the sinus venosus, which
return blood from all parts of the body to the heart ; (2) the
hepatic portal system, which conveys to the liver the blood
from the alimentary canal, pancreas, and spleen ; (3) the renal
portal system, which conveys to the kidneys the blood from
the part of the body behind the cloaca.

1. The veins opening into the sinus venosus.

These veins are most readily traced by following

Fig. 52. — Scyllium canicula. Dissection of the venous system from the
ventral surface. The right half of the lower jaw, the right lobe of
the liver, and the alimentary canal from the oesophagus backwards
have been removed, (a. m. m.)

A, nostril. AS, anterior cardinal sinus. B, right olfactory organ. BV,
subclavian vein. C, left half of lower jaw. D, fifth nerve and buccal branch
of seventh nerve crossing the orbit. DC, Cuvierian sinus. E, right eye. f, cut
edge of floor of mouth. G , first branchial clef t. H, ventricle. HS, left hepatic
sinus. HV, left hepatic vein. |, auricle. JV, left inferior jugular sinus.
JV', opening of right inferior jugular sinus into Cuvierian sinus. K, pericardial
cavity. L, left lobe of liver. |_V, right lateral vein, m, oesophagus, cut across.
MV, hyoidean sinus. N, kidney or metanephros. o, communication between
the right and left posterior cardinal sinuses. OS, orbital sinus. P, post-orbital
sinus. PS, posterior cardinal sinus. PV, posterior cardinal- vein. RS, aper-
tures of renal veins into posterior cardinal veins. SA, aperture from sinus
venosus to auricle. SV, sinus venosus.

250 THE DOG-FISH.

them back from the sinus venosus, and they will be
described in this way, though it must be remembered
that the now of blood in them during life is towards
the sinus venosus. They are chiefly remarkable for
their usually enormous size, the dilated portions
being spoken of as sinuses.

To dissect the veins, slit open the ventral wall of the sinus
venosus transversely, and wash out the contained blood; then
follow the veins with a seeker, slitting them open along their
ventral walls, and washing out the blood. If preferred, they
may be injected with plaster of Paris, or they may be inflated
with air.

a. The hepatic sinuses open into the sinus venosus by

a pair of apertures in its posterior wall close to
the median plane and near the dorsal surface.
The apertures lead into a pair of wide sinuses,
separated from each other by an imperfect
septum, and lying immediately behind the peri-
cardium, between it and the liver, and ventral to
the oesophagus.

Slit open one of the hepatic sinuses, and follow it back
into the corresponding lobe of the liver, noting its large size
and the numerous veins opening into it.

b. The Cuvierian sinuses are a pair of rather narrow

tubes, forming lateral continuations of the sinus
venosus, the wall of the pericardium marking
the boundary between the two. Each is about a
quarter of an inch long, and curves upwards from
the sinus venosus round the side of the oesophagus,
lying in a notch in the posterior border of the
fifth cerato -branchial cartilage. Each Cuvierian
sinus receives about the middle of its course the
inferior jugular sinus, and at its dorsal end the
great anterior and posterior cardinal sinuses.

i. The inferior jugular sinus, which is of small
size, can be traced forwards and inwards

VENOUS SYSTEM. 251

along the outer wall of the pericardial cavity ;
and then forwards, in the floor of the mouth.

Talce especial care not to injure the branchial arteries (p. 255).

ii. The anterior cardinal sinus opens into the
Cuvierian sinus by a small aperture placed
immediately behind the fifth cerato-bran-
chial cartilage, and guarded by a very per-
fect valve of which the ventral flap is much
the larger.

From this opening the sinus extends for-
wards as a wide irregular space, immediately
above the gill-arches. In front of the first
gill this sinus communicates with the orbit
by the narrow post-orbital sinus. This passes
above the spiracle and below the ear, and
opens in front into the large orbital sinus
which surrounds the eyeball and its muscles,
and is usually filled with blood-clots. The
orbital sinuses of the two sides communicate
with each other by the inter-orbital sinus,
which runs across in the floor of the skull,
between the hinder parts of the orbits.

Just in front of the first gill the anterior
cardinal sinus receives the large hyoidean
sinus, which lies in a groove on the outer
surface of the hyo-mandibular cartilage, and
may be followed downwards and inwards to
the floor of the mouth, where it communicates
with the inferior jugular sinus.

Find the opening of the left anterior cardinal sinus into the
Cuvierian sinus ; insert a seeker into it, turn the fish over, and
lay open the cardinal sinus along its whole length by slitting
through its dorsal wall ; wash out the blood-clots, and follow
it forwards to the orbit. Follow the hyoidean sinus, from the
anterior end of the cardinal sinus, downwards along the hyo-
mandibular cartilage to the floor of the mouth.

252 THE DOG-FISH.

iii. The posterior cardinal sinus extends back along
the dorsal surface of the abdominal cavity
close to the median plane, its dorsal wall being
firmly united to the body-wall, and its ventral
surface covered by the peritoneum. Its
anterior end forms a very large thin-walled
sac, three-quarters of an inch or more in
width. In front ib lies at the side of the
oesophagus, and close to the hepatic sinus ;
further back it lies dorsal to the oesophagus,
and alongside its fellow of the other side, with
which it communicates freely. Behind this
point, i.e. about two inches behind the peri-
cardium, the sinuses of the two sides narrow
considerably, and run back side by side to
the hinder end of the abdominal cavity, lying
between the kidneys, from which they draw
blood by numerous renal veins. The right
sinus extends to the hinder end of the kidneys,
but the left usually commences further for-
wards.

Slit open the ventral walls of both sinuses at their anterior
ends, and, wash out the blood; note the enormous size of the
sinuses and their free communication with each other, dorsal
to the oesophagus. Pass a seeker backwards along the right sinus
to its commencement between the posterior ends of the kidneys
{fig. 52) ; but do not lay it open in this hinder part, lest the
reproductive organs and the cloaca should be injured.

At its anterior end, immediately to the
outer side of its opening into the Cuvierian
sinus, the posterior cardinal sinus receives
the subclavian vein, which returns blood
from the pectoral fin and adjacent parts of
the body-wall, and runs in a groove along the
inner surface of the pectoral girdle. There are
two pairs of lateral veins. One pair arises
from the pelvic region and runs forward to

BLOOD-VESSELS. 253

join the subclavian. This, the lateral abdominal
vein, corresponds to the anterior abdominal
vein of the frog. The other paired vein is
the lateral cutaneous vein, which runs more
superficially from the tail right . up to the
Cuvierian sinus.

2. The renal portal system.

The blood from the muscles of the tail is returned
by the caudal vein, which lies below the caudal artery
in the haemal arches of the vertebrae. Opposite the
hinder ends of the kidneys the caudal vein divides into
the right and left renal portal veins, which run forwards
along the dorsal edges of the right and left kidneys
respectively, receiving branches from the body-wall,
and entering the kidneys along their whole length.

Cut across the tail about two inches behind the cloaca.
Find the caudal vein lying in the hcemal arches of the vertebras,
ventral to the caudal artery ; trace the vein forwards with a
seeker, or inject it, and then dissect it out.

3. The hepatic portal system.

The hepatic portal vein is formed by the union

of two veins, one from the main lobe of the spleen

and the other from the intestine. It runs forwards

in the mesentery along the dorsal border of the

pancreas, and, receiving veins from the pancreas

and from the stomach, enters the liver a little to

the right of the median plane.

Find the hepatic portal vein at the point where it enters

the liver, a little to the right of the junction of the two lobes,

and follow it backwards, turning aside the viscera as may be

necessary, but not cutting anything.

C. The Arterial System.

The arterial system may conveniently be described under
three heads : (1) the afferent branchial vessels, which carry
the venous blood from the heart to the gills to be aerated ;

T

254

THE DOGFISH.

A& AH AE

AFFERENT BRANCHIAL VESSELS. 255

(2) the efferent branchial vessels, which collect the aerated
blood from the gills ; and (3) the dorsal aorta, which is formed
by the union of the efferent branchial vessels, and conveys
arterial blood to the body generally.

1. The afferent branchial vessels.

The afferent branchial vessels are most easily found by
making a median incision through the muscles in front of the
pericardium, and parting the muscles gently till the thyroid body
— a red oval body — is found : this overlies the anterior end of
the cardiac aorta, which may then be dissected backwards with
care, so as to avoid injuring the afferent vessels, which it gives
off, and which must now be followed up to the gills.

a. The cardiac aorta is a direct continuation forwards

of the conus arteriosus. It is little more than an
inch in length, and gives off laterally the five
pairs of branchial arteries.

Note the thyroid gland lying just over the anterior end of the
cardiac aorta.

b. The branchial arteries arise in pairs from the

cardiac aorta, and carry the blood from it to the
gills. The two hindmost pairs arise close together

Fig. 53. — Scyllium catulus. Dissection from below to show the arterial
system. The floor of the mouth has been cut through a little to
the left of the median plane, and the left half turned outwards.
The lower jaw, the lower portion of the hyoid arch, and the whole
of the branchial skeleton have been removed on the left side. The
floor of the brain-case and that of the left orbit have been removed.
On the right side the skin and muscles and portions of some of the
gills have been removed. The thyroid gland is not shown, (c. h. h.)

AA, pericardial cavity. AE, sinus venosus. AF, auricle. AH, ventricle.
AK, conus arteriosus. AL, AM, and AN, first, second, and third branchial
arteries, in hyoid and first and second branchial arches respectively. AO, car-
diac aorta. CA, hyoidean artery. CE, external carotid artery. CP, internal
or posterior carotid artery. CV, carotid artery. EA, post-spiracuiar artery.
EB, anterior limb of first efferent branchial loop. EC, communicating vessel
between second and third efferent branchial loops. EE, fourth epibranchial
artery, bringing blood from gills of fourth and fifth clefts. HA, dorsal aorta.
HE, subclavian artery. HF, cceliac artery. NA, nostril. NE, nasal flap:
in this species the two flaps are not united as in S. canicula. o, eyeball. TA,
hyo-mandibular cartilage cut across. VA, spiracle with pseudo-branch. VE
and VF, first and fifth branchial clefts. W, pectoral girdle cut across, with a
portion of the muscles of the pectoral fin.

256 THE DOG-FISH.

or sometimes by a common trunk, immediately
in front of the pericardial cavity. The third pair
arise about a quarter of an inch further forwards.
At its anterior end, about midway between the
pericardial cavity and the mouth, the cardiac
aorta divides into right and left branches, each of
which again divides into the two anterior bran-
chial arteries of its side.

The first or anterior branchial artery runs
along the outer border of the hyoid arch, extend-
ing nearly to its dorsal end, and giving off
branches to the gill borne by the arch.

The four hinder branchial arteries follow
similar courses along the outer borders of the
first four branchial arches, extending almost to
their dorsal ends, and giving off numerous an-
terior and posterior branches to the gills borne
by these arches.

2. The efferent branchial vessels.

The efferent branchial vessels can easily be injected from
the caudal artery in the following manner. Cut across the tail
about a couple of inches behind the cloaca. Find the caudal
artery in the hcemal arches of the vertebral column, lying im-
mediately below the vertebral centra, and dorsal to the caudal
vein. Inject the caudal artery with plaster of Paris, coloured
with carmine, and rapidly strained to remove any particles
large enough to plug the vessels. Lay the animal in cold water
for an hour.

Insert one of the blades of a strong pair of scissors into the
mouth, and cut bach horizontally along the left side through
the gill-arches, as far back as the last gill-arch ; from the hinder
end of this incision cut transversely across the floor of the
pharynx, behind the heart, to the corresponding point on the
opposite side. Turn the floor of the mouth, with the heart, over
to the right side.

Wash the mouth and pharynx thoroughly. Dissect off the
mucous membrane from the roof of the mouth ; find the efferent

EFFERENT BRANCHIAL VESSELS. 257

branchial vessels running inwards and backwards from the
gill-slits, and follow them, on the right side, outwards to the
gills, and inwards to the dorsal aorta in the middle line.

The efferent branchial vessels form loops, one of
which runs round the margin of each of the first
four branchial clefts, and receives the arterial blood
from the gills of that cleft. A single vessel, i.e. a
half-loop, runs along the anterior border of the
fifth branchial cleft, and opens into the loop of
the fourth cleft. Each loop communicates with the
next one about the middle of its length by a short
horizontal vessel.

- From the ventral ends of the loops small arteries
arise, which supply the floor of the mouth and
adjacent parts.

From the dorsal ends of the loops, four main
efferent trunks, or epibranchial arteries, arise on
each side : these run backwards and inwards in the
roof of the mouth to the middle fine, where they
unite in pairs to form the median dorsal aorta.

a. The carotid artery is a small vessel which arises on
each side from the dorsal end of the efferent
vessel of the hyoidean gill, just in front of the
origin of the first efferent trunk. It runs for-
wards and inwards across the ventral surface
of the skull, opposite the hinder border of the
orbit, and divides, about one-third of an inch
from the middle line, into external and internal
carotid arteries. Of these, the former runs for-
wards and outwards across the floor of the orbit,
and supplies the upper jaw and snout ; while
the latter, continuing its course in a groove in
the ventral surface of the skull, passes through
a median foramen into the cranial cavity. In
the foramen the two internal carotids cross each
other, each uniting within the cranial cavity
with the hyoidean artery of the opposite side.

258 THE DOG-FISH.

b. The hyoidean artery arises from the anterior limb
of the first loop about the middle of its length,
in line with the connecting vessels between the
successive branchial loops, and runs forwards to
the anterior wall of the spiracle, where it gives
branches to the pseudobranch. It then con-
tinues its course forwards and inwards across
the floor of the orbit, crossing ventral to the
external carotid artery, and entering the skull
by a small hole, | inch in front of the inter-
orbital foramen, joins with the internal carotid
artery of the opposite side, and divides into
branches supplying the brain.

3. The dorsal aorta and its branches.

The anterior pair of efferent branchial vessels
unite about the level of the inner openings of the
fourth gill-clefts, forming the dorsal aorta, into
which the hinder vessels open in pairs.

In front of the point of union of the first efferent
branchial vessels the aorta is continued forwards
beneath the skull as a slender vessel : after a short
course this divides into right and left branches
which, running outwards, join the carotid arteries
just before their division into external and internal
carotids.

The dorsal aorta runs backwards along the whole
length of the body, lying in the trunk just below the
vertebral column, and in the tail, where it becomes
the caudal artery, in the hsemal arches of the ver-
tebrae. It gives off branches supplying the whole of
the body behind the head.

The anterior part of the aorta should be dissected between
the kidneys ; the posterior part, which is deeply placed between
and behind the kidneys, is best seen by dissecting one of the
kidneys from the body-wall and turning it inwards. The
posterior part is, however, best left until the excretory system
is dissected.

ARTERIAL SYSTEM.

259

The principal branches of the dorsal aorta are
the following :

i. The subclavian arteries are a pair of vessels,
which arise from the dorsal surface of the
aorta between the third and fourth pairs of
efferent branchial vessels, and run outwards
and backwards in the body-wall and along

— • H

Fid. 54. — Scyllium canicula. Transverse section passing through the
posterior dorsal fin. (a. m. m.)

A, horny fibre. B, basal cartilage of the dorsal fin. Ci myotomes. D,
neural spine. E, base of neural arch. F, notochord. g, calcified portion of
centrum. H, haemal arch. I, spinal cord. K, lateral vein. |_, caudal artery. *
M, caudal vein.

the hinder border of the pectoral girdle to
the pectoral fins, which they supply.

ii. The cceliac artery is a large median vessel
arising from the ventral surface of the aorta,
about half an inch behind the openings of
the last pair of efferent branchial vessels. It
runs backwards in the mesentery close to the
dorsal surface of the stomach, and divides

260 THE DOG-FISH.

into two main branches, one of which
supplies the proximal end of the stomach
and the liver, and the other the first part of
the intestine and the pancreas.

iii. The anterior mesenteric artery is a median
vessel arising from the aorta about an inch
and a half behind the coeliac artery. It
supplies the intestine and rectum, and gives
branches to the genital organs.

iv. The lieno-gastric artery is a median vessel
arising from the aorta immediately behind
the mesenteric artery. It supplies the loop
of the stomach between its proximal and
distal limbs, and also the spleen and part of
the pancreas.

v. The posterior mesenteric artery is a small median
vessel arising from the aorta about an inch
and a half behind the lieno-gastric, and run-
ning backwards in the mesentery to the
rectal gland, which it supplies.

vi. The parietal arteries are small paired vessels
arising at intervals along the whole length
of the aorta, and supplying the body-walls.

vii. The renal arteries are small paired vessels
arising from the parietal arteries, and sup-
plying the kidneys, which they enter from
the dorsal surface.

The dissection of the renal arteries should be postponed
until after the dissection of the reproductive organs and cloaca.

To see the arteries, dissect one kidney away from the body-
wall and turn it inwards, so as to expose its dorsal surface.

viii. The iliac arteries are a pair of small vessels
leaving the aorta near the hinder end of the
body-cavity, and running outwards in the
body-walls to the pelvic fins, which they
supply.

THE HEART. 261

D. Dissection of the Heart.

Remove the floor of the mouth, with the heart and peri-
cardium, and pin it down in a dish under water, with the
ventral surface upwards. Open and examine the several
cavities of the heart in the order given below, washing out the
contained blood by a stream of water.

Open the ventricle by a horizontal incision, and continue
the cut forwards along the right side of the conus arteriosus
to its anterior end. Turn the ventral portions of both ventricle
and conus over to the left side.

1. The conus arteriosus contains two sets of valves, each

set consisting of three valves.

The posterior set is just in front of the ventricle,
and the anterior and larger set about the middle of
the length of the conus. Each valve is thick in its
central part, thin and membranous at the edges.

2. The ventricle has very thick muscular and spongy walls,

and a small central cavity. The opening from the
auricle is a transverse slit guarded by a two-lipped
valve, and is placed in the dorsal wall of the
ventricle near its hinder end, and slightly to the
left side.

3. The auricle.

Cut across the conus at its anterior end ; open the auricle by
horizontal incisions along its lateral and- anterior borders, turn
the ventral wall bach, and wash out the blood from the cavity.

The walls of the auricle are thin, with radiating
muscular bands ; the cavity is very large and tri-
angular in shape. The opening leading from the
sinus venosus is a vertical slit in the posterior wall
of the auricle ; and the transverse aperture leading
to the ventricle is in the ventral wall, and slightly
to the left of the median plane.

4. The sinus venosus has already been laid open. Its walls

are thin, with a very feebly developed muscular net-
work. The sinu-auricular aperture is a median

262 THE DOG-FISH.

vertical slit in the anterior wall, to the left of the
ventral end of which is a small aperture from the
coronary sinus, which returns to the sinus venosus
the blood from the walls of the heart.

VII. DISSECTION OF THE RENAL AND REPRODUCTIVE
SYSTEMS.

These two systems have already been seen in part, but
may now be examined more fully. They are originally in-
dependent of each other, but in the course of development
become associated, owing to the genital ducts being formed
from parts of the kidney-ducts.

The kidneys are paired organs, lying side by side along
the whole length of the dorsal wall of the abdominal cavity.
Each consists at an early age of its development of a
number of convoluted tubules, placed one behind another,
and opening at one end into the abdominal cavity : at their
other ends they open into a longitudinal duct, the segmental
duct, which runs the whole length of the kidney, and opens
in front into the abdominal cavity and behind into the
cloaca.

During development, in many vertebrates, each kidney
appears in two portions : an anterior part or pronephros,
which is small and abortive in the dog-fish and other Elas-
mobranchs ; and a posterior portion, the mesonephros or
Wolffian body, including the greater part of the length of the
primary kidney (largely developed in most ordinary fish).

The ducts also undergo division. The segmental duct
becomes split longitudinally into two separate ducts : the
Mullerian duct, which loses its connection with the kidney ;
and the Wolffian duct, into which the kidney tubules open.
At a later stage several of the hinder kidney tubules unite to
form one or more independent ducts, or ureters.

In the female the right and left Mullerian ducts unite
in front, so as to open into the abdominal cavity by a single
median aperture, and become converted into the oviducts ;

EENAL AND"REPRODUCTIVE SYSTEMS.

"263

while the mesonephros and its ducts form the functional
kidney and ureters of the adult.

In the male the Miillerian ducts are abortive and func-
tionless ; the anterior mesonephric tubules become connected
with the vasa efierentia of the testis, conveying the sperma-
tozoa to the mesonephric or Wolffian duct, which acts as vas
deferens. The posterior part of the kidney and its ducts, as
in the female, become the functional kidney and ureters. The
tubules of the kidney have persistent peritoneal funnels
opening into the body-cavity.

TABLE SHOWING THE EMBRYONIC AND ADULT CONDITIONS OF THE
KIDNEYS AND THEIR DUCTS.

Embryo

Female

Male

Pronephros

Wolffian body, 1 divided
or Mesonephros j into
Miillerian duct.
Wolffian duct .

Posterior ducts

Absent.
/Abortive portion.
) Kidney.

Oviduct.

Small in front,

dilating behind to

form urinary sinus.

Ureters.

Absent.

Epididymis.

Kidney.

Abortive.

Vas deferens,

urino-genital sinus,

and sperm-sac.

Ureters.

A. The Female.

1. The ovary, which is single, is an elongated soft lobu-
lated mass, varying a good deal in size, but usually
about two or three inches long in specimens of the
size described. It lies in the abdominal cavity a
little to the right of the median plane, and is
attached to the dorsal wall by a narrow fold of
peritoneum, which is very easily torn. The ova
form conspicuous rounded projections on the sur-
face of the ovary, and when fully formed are half
an inch or more in diameter.

2. The oviducts, which are formed from the Miillerian
• ducts of the embryo, are a pair of tubes running

264 THE DOG-FISH.

along the dorsal wall of the abdominal cavity
ventral to the kidneys, and nob far from the median
plane. Their anterior ends curve inwards round the
sides of the oesophagus, and unite immediately in
front of the liver, to open into the abdominal cavity
by a median ventral slit, through which the ova
enter the oviducts from the abdominal cavity.

About a third of its length from the anterior
end, each oviduct expands to form the thick-walled
oviducal gland, which secretes the horny capsules
of the eggs. Behind the glands the oviducts are
large and dilatable, with longitudinally folded walls :
their posterior ends unite, and open by a large
median aperture into the dorsal wall of the shallow
cloaca, immediately behind the rectum. This
aperture is closed in the young female by a thin
membrane, the hymen.

Find the anterior opening of the oviducts in the suspensory
ligament of the liver, immediately behind the pericardial wall :
insert a seeker through it into the oviducts, and follow one of
these back to the cloaca, slitting through its ventral wall.

3. The kidneys, or mesonephra, extend nearly the whole
length of the abdominal cavity, lying dorsal to the
peritoneum, and one at each side of the vertebral
column. The anterior half or more of each kidney

Fig. 55. — Scyllium canicula. Dissection of a female from the ventral
surface, to show th8 renal and reproductive systems. The right
lobe of the liver and the alimentary canal from the oesophagus to
the rectum have been removed. An arrow is passed up the left
oviduct, which is cut open to show its point, (a. m. m.)

Fig. 56. — Further dissection of the renal and reproductive systems.
The left oviduct is removed ; and the right oviduct and the
urinary sinuses are laid open. (a. m. m.)

A, auricle. AP, abdominal pore. B. pericardial cavity, c, cloaca. E.
oesophagus, f, pelvic fin. G, gill-slit. |_, liver, o, ovary. OD, oviduct.
OD', oviduct laid open. OG, oviducal gland. OV, egg in the oviduct, p,
abdominal opening of oviduct. PG, pelvic girdle. R, rectum. RO, opening
of reatum into cloaca. S, anterior part of kidney. SD, Wolffian duct.
T, hinder part of kidney. TD, posterior ducts or ureters. TO, openings
of ureters into urinary sinus. UP, urinary papilla. US, urinary sinus. V,
ventricle.

RENAL AND REPRODUCTIVE SYSTEMS.

265

266 THE DOG-FISH.

is abortive, and appears as an ill-defined tract of soft
yellowish-brown degenerate gland-tissue, arranged
in patches corresponding to the vertebrae.

The posterior third or so of the kidney is a
well-developed, compact, laterally compressed, and
lobulated mass of a brownish colour, extending
back as far as the level of the cloaca.

Dissect off the 'peritoneum from the ventral surface of one
of the kidneys, so as to expose it along its whole length. At
the hinder end dissect along its outer side from the body-
wall, so as to see its full size.

4. The Wolffian ducts are a pair of straight narrow tubes

lying on the ventral surface of the kidneys along
their whole length. Their posterior ends are en-
larged to form a pair of tubular sacs, the urinary
sinuses, which run back side by side, unite together
posteriorly, and open into the cloaca at the apex of
the small urinary papilla in the posterior margin of
the opening of the oviducts.

Find the opening at the apex of the urinary papilla : pass
a seeker through the opening and forwards along the urinary
sinus of one side : slit up the ventral wall of the sinus along
its whole length.

5. The ureters are slender tubes, usually four to six in

number on each side, which run from the meta-
nephros to open into the dorsal surface of the
urinary sinus by four to six slit-like apertures lying
along the septum between the two sinuses. The
anterior ureter is much longer than the others.

Find the ureters, lying on the ventral surface of the meta-
nephros, and trace them back to the urinary sinus. Slit up
the sinus, unless this has already been done, and examine its
dorsal wall to see the openings of the ureters.

6. The cloaca is a shallow depression between the pelvic

fins. In the female there are three openings into

RENAL AND REPRODUCTIVE SYSTEMS. 267

it : the rectum in front, the genital aperture in the
middle, and the urinary sinus behind.

B. The Male.

1. The testes are a pair of soft elongated bodies lying along

the dorsal wall of the abdominal cavity, and in the
adult united with each other at their hinder ends.
Like the ovary, they are attached to the body-wall
by delicate folds of peritoneum which are very
easily torn, so that in many specimens the testes
appear to lie free in the abdominal cavity.

2. The vasa efferentia are a number of very slender ducts

which arise on each side from the dorsal surface of
the testis, and run forwards in the fold of peritoneum
to the Wolffian body, which they enter at its anterior
end.

3. The kidneys. Each kidney is, as in the female, divided

into an anterior part and a posterior excretory
part. The anterior half differs from that of the
female in being secondarily connected with the
reproductive system forming the epididymis or
genital portion. The hinder part, as in the female,
is a laterally compressed mass of considerable size,
especially thick at its hinder end. This part is the
functional kidney.

Dissect off the 'peritoneum from the ventral surface of one
of the kidneys, so as to expose it along its whole length. At
the hinder end dissect the kidney from the body-wall along its
outer sidet so as to see its full size.

4. The Wolffian ducts are a pair of thick-walled tubes

running in a very sinuous course along the ventral
surface of the genital portion oi the kidney, from
which the ducts receive numerous segmentally
arranged tubules. Opposite the junction of the two
portions of the kidney they dilate rather suddenly
into two wide thin-walled tubes, the vesiculae

268

THE DOGFISH.

RENAL AND REPRODUCTIVE SYSTEMS. 269

seminales, which run straight back along the ventral
surface of the metanephros to open into the urino-
genital sinus, just in front of the cloaca. The walls of
the vesicula seminalis are marked by close-set trans-
verse lines, corresponding to internal foldings of its
lining membrane.

The Wolffian duct acts as vas deferens, conveying

the spermatic fluid from the testis to the exterior.

Dissect the Wolffian duct of one side away from the ventral

surface of the mesonephros, noting the tubules it receives from

this ; and follow it back to the cloaca.

5. The sperm-sacs are a pair of thin-walled tubes lying
along the ventral surfaces and the outer sides of
the vesiculse seminales, to which they are closely
attached, and which they about equal in length.
Each sperm-sac ends blindly in front, and dilates
behind to form the urino-genital sinus.

Dissect away the sperm-sac from the vesicula seminalis
awl follow it back to the urino-genital sinus.

Fio. 57. — Scy Ilium canicula. Dissection of a male from the ventral
surface, to show the renal and reproductive systems. The left
lobe of the liver and the alimentary canal from the oesophagus to
the rectum have been removed. The excretory organs of the left
side only are shown. An arrow passes through the left abdominal
pore into the abdominal cavity, (a. m. m.)

Fio. 58. — FurtLer dissection of the renal and reproductive organs.
The liver and the greater part of the testis have been removed.
The right metanephric duct is shown in its natural position ; the
left duct has been displaced outwards. The urino-genital sinuses
of both sides have been laid open. (a. m. m.)

AP, abdominal pore. CP, clasper. CR, groove in clasper. CS, sac con-
tinuous with groove in clasper. e, oesophagus. F, pelvic fin. G, gill-slit.
H, testis. L, liver. P, abdom nal opening of Miillerian ducts. PD, abortive
Miillerian duct. PG , pelvic girdle. R, rectum, rg, rectal gland.- RO, open-
ing of rectum into cloaca. S, Wolffian body or mesonephros. SD, Wolffian
duct or vas deferens. SO. opening of Wolffian duct into urino-genital sinus.
SS, sperm-sac. T, metanephros. TD, ureter. TD,, ducts from hinder part
of kidney, joining the ureter as it reaches the urino-genital sinus. TO,
opening of ureter into urino-genital sinus. UP, urino-genital papilla. US,
urino-genital sinus. VE, vasa efferentia. VS, vesicula seminalis, the dilated
hinder end of the Wolffian duct.

u

270 THE DOG-FISH.

6. The urino-genital sinus of each side is the posterior
end of the sperm-sac : the vesicula seminalis and
the ureter open into it. About a quarter of an
inch behind these openings the sinuses of the two
sides unite, and then open into the cloaca by a small
aperture at the apex of the urino-genital papilla,
which is situated immediately behind the opening of
the rectum.

Find the opening at the apex of the urino-genital papilla :
pass a seeker through the opening, and forwards along the
urino-genital sinus and sperm-sac of one side : slit up the
ventral wall of the sinus and sac along their whole length :
examine, and pass a seeker into the orifices of the vesicula
seminalis and ureter.

7 The ureters are usually five in number on each side ; they
arise from the outer border of the ventral surface
of the secreting portion of the kidney at intervals
along its length. The four anterior ducts, after
a short course, unite to form a wide thin-walled
tube which runs backwards along the inner side
and dorsal surface of the vesicula seminalis to open
into the urino-genital sinus, immediately to the
inner side of the opening of the Wolffian duct ; just
before reaching the sinus this tube is joined by the
fifth or hindmost duct, from the posterior portion of
the kidney. From three to five smaller ducts open
separately into the urino-genital sinus a little further
back.

The ureters are more easily followed after injecting
them with a coloured fluid from the main ureter, which is
a wide tube, lying along the inner side of the vesicula seminalis.

8. The cloaca is the shallow depression between the pelvic

fins into which, in the male, the rectum opens in
front and the urino-genital sinus behind.

9. The Miillerian ducts are abortive in the male, their

THE NERVOUS" SYSTEM. 271

anterior ends alone being present. A small pocket-
like sac in the suspensory ligament of the liver,
between it and the pericardium, represents the ab-
dominal opening of the oviducts of the female.
From this sac the ducts can be traced a short distance
round the sides of the oesophagus.

10. The claspers (see p. 220).

11. The spermatozoa are delicate thread-like structures,

each with a thickened head round which a spiral
membrane runs.

VIII. DISSECTION OF THE NERVOUS SYSTEM.

For this purpose a dog-fish should be taken which has been
hardened in spirit, the roof of the skull having been previously
removed so as to allow the spirit free access to the brain. The
specimen must be moistened from time to time to prevent the
nerves drying up, or else should be dissected in water or weak
spirit.

The nervous system consists of (1) a central portion, the
brain and spinal cord, lodged within the skull and vertebral
column respectively ; and (2) a peripheral portion, the nerves
themselves, which connect the brain and spinal cord with
all parts of the body.

From the mode of its development the central nervous
system is tubular, and it retains this character throughout
life. The central canal is very small in the spinal cord, but
widens out considerably in the brain, its local dilatations
being spoken of as the ventricles of the brain.

A. The Dorsal Surface of the Brain.

Remove the skin from the dorsal surface of the head. Dissect
away the eyelids on the right side so as to open up the orbit.
Slice away the roof of the cranium so as to expose the brain fully,
slicing through the auditory capsule on the right side, but leaving
it uninjured on the left.

272

THE DOG-FISH.

v.vh.a

THE BRAIN. 273

The brain nearly fills the cranial cavity : it is narrow
between the orbits, but expands considerably in front of
them. As seen from above, the following parts are visible
in order from before backwards.

1. The prosencephalon is the most anterior and' the largest

division of the brain. It is a smooth somewhat
globular mass, lying in the expanded anterior part
of the cranial cavity, between the olfactory capsules
and the orbits. It is marked in front by a shallow
median groove, and is sharply separated behind from
the succeeding part of the brain. It corresponds to
the cerebral hemispheres of higher vertebrates.

2. The olfactory lobes are a pair of large oval masses,

arising by stout stalks from the sides of the
prosencephalon, their distal surfaces being closely
applied to the olfactory capsules.

3. The thalamencephalon is the narrow portion of the brain

immediately behind the prosencephalon ; its roof is
very thin, so that the cavity, or third ventricle,
can be seen through it. From the hinder part of the
roof arises the pineal body, a slender tubular process
which extends forwards over the prosencephalon,

Fig. 59. — Scyllium catulus. Dissection of the brain and cranial nerves
from the dorsal surface. The right eye has been removed. The
cut surfaces of the cartilaginous skull and vertebral column are
dotted. About two-thirds the natural size. (a. m. m.)

B, prosencephalon. BB, pineal body. BC, cerebellum. BO, olfactory
lobe. BS, spinal cord. BT, thalamencephalon. CA, auditory capsule. CO,
olfactory capsule. E, eye. F.1-F.5, first to fifth branchial clefts. H.1-H-4,
first to fourth branchial arches. M H , external or horizontal semi-circular canal .
NC, anterior cardial sinus. N H , opening of hyoidean sinus into anterior cardinal
sinus. N.L, nerve ofthe lateral line. N P, post-orbital sinus. O, olfactory organ.
O I, obliquus inferior. OS, obliquus superior. R|, rectus internus. SD, dorsal
root of spinal nerve. SP, hyomandibular cleft or spiracle. SV, ventral root of
spinal nerve.

II, optic nerve. Ill, third nerve, iv, fourth nerve. V, VII. A, ophthalmic
branches of fifth and seventh nerves, v. B, maxillary branch of fifth nerve.
V. 0, mandibular branch of fifth nerve. VI I . B, palatine branch of seventh nerve.
VII. B1, buccal branch of seventh nerve. VII. 0, hyoidean branch of seventh
nerve. VI I. D, ' pre-spiracular ' branch of seventh nerve. VIII, auditory nerve.
IX, ninth or glossopharyngeal nerve. X, tenth or penumogastric nerve. X. A,
branch of pneumogastric nerve to second branchial cleft. X. B, branch of
pneumogastric nerve to third branchial cleft. X. D, branch of pneumogastrio
nerve to fifth branchial cleft. X. F, visceral branch of pneumogastric nerve.

274 THE DOG-FISH.

and ends in a slightly dilated knob attached
to the membranous roof of the skull some little
distance in front of the brain.

4. The optic lobes are a pair of closely apposed ovoid

protuberances lying in the narrowest part of the
cranial cavity opposite the middle of the orbits.
Their posterior halves are overlapped and concealed
by the cerebellum.

5. The cerebellum is a long flattened oval body arising from

the dorsal surface of the brain immediately behind
the optic lobes, and extending forwards over them,
and backwards for an equal distance over the
medulla oblongata. Its surface is slightly grooved.

6. The medulla oblongata is the part of the brain behind

the cerebellum. Its roof is very thin, and liable to
be torn away so as to expose its cavity, the fourth
ventricle. Its sides are thickened, especially in
front, where they give origin to the roots of the fifth,
seventh, and eighth cerebral nerves. Immediately
above these roots, and below the posterior projec-
tion of the cerebellum, the medulla is produced into
a pair of large lateral wing-like processes, the resti-
form bodies, which are united with each other beneath
the overhanging cerebellum.

B. The Orbit.

This should be examined before the dissection of the nerves
is proceeded with. If the eyelids have been removed as
directed above, no further dissection is necessary. Wash out
the blood-clots from the orbital sinus with a stream of water,
avoiding injury to the nerves.

1. The eyeball is almost hemispherical in shape, its outer

or corneal surface being very flat.

2. The eye-muscles, by which the movements of the eye

are effected, are six narrow muscular bands which
arise from the skull-wall and are inserted into the
eyeball : they are arranged in two groups.

ORBIT AND CRANIAL NERVES. 275

a. The recti muscles are four in number. They arise

close together, near the hinder end of the orbit,
and diverge outwards to be inserted into the
eyeball.
i. The rectus superior runs outwards and for-
wards, to be inserted into the dorsal surface
of the eyeball.
ii. The rectus internus, or anterior, runs forwards
between the skull-wall and the eyeball, and is
inserted into the anterior surface of the latter.

iii. The rectus externus, or posterior, is the hind-
most of the eye-muscles, and runs directly
outwards behind the eyeball, to be inserted
into its posterior surface,
iv. The rectus inferior runs outwards and forwards
beneath the eyeball, and is inserted into its
ventral surface.
The origin of the rectus inferior can he seen from above,
between the other recti muscles ; to see its insertion, turn the
eyeball up.

b. The obliqui muscles are two in number. They

arise close together at the anterior end of the
orbit and run backwards and outwards to the
eyeball,
i. The obliquus superior is inserted into the dorsal
surface of the eyeball, just in front of the
insertion of the rectus superior,
ii. The obliquus inferior is inserted into the ventral
surface of the eyeball, just in front of the
insertion of the rectus inferior.
To see the obliquus inferior, turn the eyeball up.

C. The Cranial Nerves (fig. 59).

These should be dissected before the brain is removed, in
order that their entire course may be seen.

Dissect the nerves on the right side, leaving the left side
for the dissection of the sense-organs.

276 THE DOG-FISH.

The ophthalmic branches of the fifth and seventh nerves,
which run through the orbit close to the skull-wall, and dorsal to
all the eye-muscles, should be identified at once ; and care must
be taken not to injure them (see pp. 278, 279, and fig. 59, p. 272).

A paired pre-olfactory nerve arises at the extreme front

end of the brain, and, resting upon the olfactory lobe,

passes to the nasal septum. The olfactory * nerves '

arise on each side in two large bundles from the

anterior surface of the olfactory lobe, and pass at

once through the sieve-like membrane separating

the cranial cavity from the nose. They end in the

folds of the large olfactory organ.

Dissect the olfactory nerves from the dorsal surface, taking

care to preserve the ophthalmic branches of the fifth and seventh

nerves, which run forwards across the olfactory lobe, but separated

from this by the cartilaginous cranium.

2. The optic " nerve " enters the orbit through a foramen in

the side wall of the skull, near its ventral surface,
and about midway between the origins of the recti
and of the obliqui muscles. In the orbit it is en-
closed in a very tough connective-tissue sheath.
It is not a nerve but a portion of the brain.
Press the eye-muscles aside to see the optic nerve between

the recti and obliqui. The course of the nerve within the skull

will be seen when the brain is removed.

3. The third nerve, or motor oculi, is a rather slender

nerve which, arising from the ventral surface of the
brain below the optic lobes, runs outwards and
slightly backwards to the skull-wall, which it
perforates about a quarter of an inch behind the
optic nerve, and immediately in front of the origins
of the recti muscles.

In the orbit it divides at once into three branches,
of which the first supplies the rectus internus, and
the middle one the rectus superior. The hindmost
branch passes downwards between the rectus superior
and rectus externus close to their origins, and then

CRANIAL NERVES. 277

forwards, ventral to the rectus inferior, which it
supplies, to the obliquus inferior, in which it ends.
As it crosses the rectus inferior it is joined by a
branch from the fifth nerve.

Gently press away the optic lobs, from the skull-wall, so as
to see the third nerve within the skull, and also the point at
which it enters the orbit. In the orbit the branches to the rectus
internus and rectus superior are easily dissected from above.
To trace the branch to the obliquus inferior place the fish on its
side, and turn the eyeball upwards so as to expose the nerve from
below.

4. The fourth or pathetic nerve is a very slender nerve

which arises from the dorsal surface of the brain, in
the angle between the optic lobe and the cerebellum,
and almost in the median plane. From the brain
it runs forwards and outwards to the skull-wall,
which it perforates a little in front of the middle of
the orbit, dorsal to the optic nerve, and immediately
ventral to the ophthalmic branches of the fifth and
seventh nerves. In the orbit it runs outwards and
forwards to the posterior border of the obliquus
superior, in which it ends.

Though very slender, the fourth nerve is easy to trace. Lift
up the front end of the cerebellum to see its origin between the
cerebellum and the optic lobes ; trace it forwards within the
skull, and in the orbit outwards to the obliquus superior.

5. The fifth, seventh, and eighth nerves arise very close

together from the side of the medulla at its widest
part, opposite the posterior part of the cerebellum.
The eighth or auditory nerve, which is the hindmost
of the three, enters the auditory capsule at once ;
while the fifth and seventh nerves pass outwards
through the skull-wall, by a foramen at the posterior
and inner angle of the orbit, immediately behind the
origins of the recti muscles.

278 THE DOG-FISH.

Gently press away the medulla from the skull-wall until
the nerve-roots come into view, and then carefully cut away
the skull-wall and the auditory capsule until the roots are fully

a. The fifth or trigeminal nerve has three main
branches.
i. The ophthalmic branch arises from the anterior
border of the root of the nerve, close to the
brain. It runs forwards and upwards for
about a quarter of an inch within the skull,
and then perforates the skull-wall to enter
the orbit, about and a little in front of the
origins of the recti muscles.

In the orbit it runs forwards close to the
skull-wall, alongside of and immediately below
the ophthalmic branch of the seventh nerve,
and dorsal to all the eye-muscles. At the
anterior and inner angle of the orbit it passes
through a canal between the olfactory capsule
and the cranium, crosses the olfactory lobe
and continuing its course forwards divides into
branches, which supply the skin of the dorsal
surface of the snout.

Gently press away the brain from the skull-wall to see the
root and the part of the nerve within the cranium, taking care
not to confound it with the ophthalmic branch of the seventh
nerve, which lies immediately dorsal to it. In the orbit dissect
from the side, and in front of the orbit from above.

The main stem of the fifth nerve on entering
the orbit (accompanied by the buccal branch of
the seventh) runs forwards and outwards across
its floor as a broad ribbon-like band, easily
mistaken in spirit specimens for a muscle. Near
the outer margin of the orbit this separates
into the maxillary and mandibular branches.
The buccal branch of the seventh nerve lies
between and dorsal to these.

CRANIAL NERVES. 279

Turn the eye up ; trace the nerve across the floor of the
orbit, and follow its branches to their distribution.

ii. The maxillary branch, which is the anterior of
the three, turns over the upper jaw at the
anterior border of the orbit, and divides into
branches which run forwards to supply the
region of the upper jaw.

iii. The mandibular branch, the posterior of the
three, crosses the upper jaw, and then curving
round the angle of the mouth, where it lies
very close to the surface, runs forwards along
the lower jaw. It supplies the muscles moving
the lower jaw.

6. The sixth nerve is very slender, and arises from the

ventral surface of the medulla near the median
plane, and a short way behind the roots of the fifth
and seventh nerves ; it passes out through the same
foramen with these nerves, lying below them and
separated from them by dense connective tissue.
It supplies the rectus externus.

To see the part of the sixth nerve within the skull, cut through
the roots of the fifth and seventh nerves, and press the brain
aside. In the orbit the nerve can be seen along the inferior border
of the rectus externus.

7. The seventh or facial nerve has four main branches.

i. The ophthalmic branch arises from the dorsal
edge of the medulla, immediately behind the
cerebellum. It runs forwards for about a
quarter of an inch within the skull, and then
enters the orbit through a foramen above and
a little behind the origins of the recti muscles.
In the orbit it runs forwards close to the
skull-wall, alongside of and dorsal to the
ophthalmic branch of the fifth nerve. It
supplies the sensory ampullae and sensory
canal of the supra-orbital region.

280 THE DOG-FISH.

To see the root, which is large, 'press the brain away from
the skull. The dissection is the same as for the corresponding
branch of the fifth nerve.

The main stem of the seventh nerve enters
the orbit immediately behind the fifth, and
divides almost at once into three main branches.
Turn the eye up and dissect the nerve from the side, fol-
lowing the branches to their distribution.

ii. The buccal branch arises from the brain along
with the ophthalmic. It accompanies the
maxillary branch of the fifth nerve, lying be-
tween this and the mandibular branch on the
floor of the orbit. It supplies the sensory am-
pullae and the sensory canal of the infra-orbital
region. This nerve is better seen in the skate.

iii. The palatine branch is a slender nerve which
runs forwards and outwards across the floor
of the orbit, immediately behind the main
stem of the fifth nerve : it then turns for-
wards beneath this nerve and runs along the
upper jaw, sending branches to the roof of
the mouth.

iv. The hyoidean or post-spiracular branch is a
much stouter nerve, which runs straight
outwards along the posterior wall of the orbit
towards the spiracle. Continuing its course
outwards and backwards, in close contact
with the anterior wall of the auditory capsule,
it passes along the posterior border of the
spiracle, where it lies just beneath the skin,
and then runs downwards and backwards
along the anterior border of the hyoid arch,
giving off an external mandibular branch to
the ampullae and sensory canals of the hyo-
mandibular region.

Close to its origin it gives off a number of
small * pre-spiracular ' branches, one or more

CRANIAL NERVES. 281

of which, pass to the anterior wall of the
spiracle and to the pseudobranch.

8. The eighth or auditory nerve, immediately after its

origin, enters the auditory capsule through a hole
in the anterior end of its inner wall,, and divides
into branches supplying the several parts of the
internal ear.
Slice away the auditory capsule down to the level of the
root of the auditory nerve, and trace the nerve into it.

9. The ninth or glosso-pharyngeal nerve arises from the side

of the medulla, about an eighth of an inch behind the
auditory nerve and nearer the ventral surface. It
at once enters a foramen on the inner side of the
auditory capsule and about the middle of its length,
and, passing horizontally outwards and backwards
along a canal in the floor of the capsule, emerges
at its posterior and outer angle, opposite the upper
end of the first branchial cleft, where it divides
into two branches,
i. The anterior or pre-branchial branch is the
smaller of the two, and runs down the pos-
terior border of the hyoid arch,
ii. The posterior or post-branchial branch is larger,
and runs down the anterior border of the first
branchial arch.
Press away the medulla from the skull-wall to see the root
of the nerve, about an eighth of an inch behind the auditory
nerve : slice away the auditory capsule horizontally to expose
the nerve in the canal, and then follow the branches along the
branchial arches.

10. The pneumogastric nerve or vagus is a large nerve,

which arises by a number of roots from the side of
the hinder part of the medulla. The root of the
lateral-line nerve is dorsal to that of the glosso-
pharyngeal.

The vagus, accompanied for some distance by
the lateral-fine nerve, runs outwards and backwards

282 THE DOG-FISH.

through the skull-wall, emerging at its hinder end
between the cranium and the auditory capsule. It
then runs horizontally backwards, parallel to the
vertebral column, and a short distance to its outer
side, lying along the inner wall of the anterior cardinal
sinus, and giving off numerous branches.

Press away the medulla from the skull-wall to see the
roots of origin of the vagus. Slice away horizontally the hinder
end of the skull to expose the nerve in this part of its course. Lay
open the anterior cardinal sinus by slitting up its dorsal wall,
wash out the contained blood, and identify the branches of the
vagus running along its inner wall and across its floor. Dissect
these branches out, and follow them to their distribution.

i. The branchial nerves, which are four in number,
run backwards and outwards in the floor of
the anterior cardinal sinus to the upper ends
of the four hinder, i.e. second to fifth, bran-
chial clefts. Each then divides into a smaller
anterior and a larger posterior branch, which
supply the two borders of the corresponding
cleft, just as the glosso-pharyngeal supplies
the two borders of the first branchial cleft.

ii. The visceral or intestinal nerves are continua-
tions backwards of the trunk from which the
branchial nerves arise : they give off several
branches, of which the most important are
the cardiac nerves to the heart, and the
gastric nerves to the stomach.

The nerve of the lateral-line arises dorsal to
the root of the ninth nerve. It runs backwards
above and to the inner side of the branchial
division of the vagus, continuing its course to
the hinder end of the body and parallel to the
lateral fine, the sense-organs of which it supplies.
It is probably to be associated with the seventh
nerve.

DISSECTION OF THE BRAIN. 283

D. Dissection of the Brain.

1. Removal of the brain.

Gut across the medulla a short way behind the roots of
the vagus. Cut through the olfactory nerves so as to separate
the olfactory lobes from the nasal capsules ; and cut through the
several cranial nerves one by one, about midway between the
brain and the skull-wall. Turn out the brain from the cranium,
taking care not to damage the pituitary body : this latter lies
on the under surface of the brain, opposite to the optic lobes,
and in a hollow of the skull-floor, to which it is firmly attached,
and from which it must be dissected away.

Examine the skull after removal of the brain, and identify
the several nerve-stumps and their foramina of exit.

2. The ventral surface of the brain.

Place the brain in spirit, with the ventral surface upwards,
and identify the following structures :

a. The prosencephalon is marked by a shallow median

groove ; posteriorly it is directly continuous
with the thalamencephalon.

b. The olfactory lobes have much the same appearance

as from the dorsal surface.

c. The optic chiasma is formed by the decussation of

the optic nerves on the under surface of the
thalamencephalon. Beyond the chiasma the
nerves diverge outwards towards the foramina
in the skull- wall which lead to the orbits.

d. The infundibulum consists of a pair of prominent
oval swellings, sometimes called lobi inferiores,
on the under surface of the thalamencephalon,
immediately behind the optic chiasma. It is
produced posteriorly into a large thin-walled
pouch, the saccus vasculosus.

e. The pituitary body is a thick-walled median tubular
body lying immediately beneath and attached to
the infundibulum, the posterior end being, in the

284 THE DOG-FISH.

natural condition of the parts, attached to the floor
of the skull.

It is important to remember that the pituitary
body, as here defined, is not developed from the
brain.

f. The roots of the third nerves arise from the ventral

surface of the brain close to the middle line, and
opposite the junction of the infundibulum with
the pituitary body.

Lift up the pituitary body and infundibulum so as to
expose the third nerves fully.

g. The medulla oblongata is narrow in front : it

widens 'considerably opposite the roots of the
fifth and seventh nerves ; and then narrows
again as it passes back into the spinal cord.

h. The roots of the sixth nerves arise from the ventral
surface of the medulla, near the middle line, and
a short distance behind the level of the roots of
the fifth and seventh nerves.

3. The cavities of the brain.

Bisect the brain by a median vertical incision along its
whole length. Slice away the inner surface of one half of the
prosencephalon until the cavity within it is fully exposed.

The brain, as already noticed, is tubular, and its
cavity is continued into all the principal lobes or
processes, which may be regarded as formed by fold-
ings of its wall, or rather by unequal growth at
various parts. The roof of the brain varies greatly
in thickness at different parts, but the floor is nearly
uniform throughout (cf. fig. 51, p. 247).

a. The lateral ventricles are a pair of large laterally
compressed cavities in the prosencephalon, sepa-
rated from each other by a median vertical
septum. Posteriorly they open into the cavity
of the thalamencephalon, and on their outer
sides they are prolonged into the olfactory lobes.

SPINAL CORD AND NERVES. 285

The posterior wall of the prosencephalon, just
in front of and above the thalamencephalon,
is produced on each side into a very thin-walled
fold, which projects forwards into the lateral
ventricle nearly to its anterior end. Between
the two layers of the fold are numerous blood-
vessels forming the choroid plexus.

b. The third ventricle is a cavity within the thalam-

encephalon : it communicates in front with the
lateral ventricles, above with the very narrow
cavity of the stalk of the pineal body, behind
with the cavity beneath the optic lobes, and
below with the infundibulum.

c. The saccus vasculosis is a sac with very thin walls,

standing in this respect in marked contrast to
the infundibulum, which has thick walls. Its
cavity does not communicate with the pituitary
body which lies on its ventral surface. The
pituitary body has only a secondary connection
with the brain.

d. The optic lobes, cerebellum, and restiform bodies

are all hollow outgrowths from the dorsal surface
of the brain. Their walls are of moderate thick-
ness, excepting the dorsal ends of the restiform
bodies, which are very thin.

e. The fourth ventricle is the cavity in the medulla,

beneath and behind the cerebellum : its sides
■ and floor are thick, its roof very thin.

E. The Spinal Cord and Nerves.

Slice away the roof of the neural canal horizontally, so as
to expose the dorsal surface of the spinal cord.

Gently press the cord away from the side of the canal, to
expose the roots of the spinal nerves.

1. The spinal cord lies in the neural canal of the vertebral
column. It is flattened dorso-ventrally ; marked by

X

286 THE DOG-FISH

median longitudinal grooves or fissures along its
dorsal and ventral surfaces ; and traversed through-
out its whole length by a narrow central canal,
continuous in front with the fourth ventricle of the
brain.

2. The spinal nerves arise in pairs from the sides of the
spinal cord. Each nerve arises by two roots, a
dorsal or ganglionated sensory root, and a ventral or
motor one : of these, the dorsal root is some distance
in front of the corresponding ventral one, the succes-
sive dorsal and ventral roots alternating at about
equal intervals along the sides of the spinal cord.

Each dorsal root passes out from the neural canal
through a notch in the hinder margin of the interverte-
bral neural plate. The corresponding ventral root
arises by three or more rootlets, and passes out from
the neural canal through a smaller notch in the
hinder margin of the vertebral neural plate. Out-
side the vertebral column the two roots unite to
form the spinal nerve.

Make out the pectoral plexus arising from the first thirteen
spinal nerves ; and the pelvic plexus arising from nerves 25-35.

IX. DISSECTION OF THE SENSE \ ORGANS.
A. The Nose.

Dissect out one of the olfactory organs from its capsule ;
cut it across transversely about the middle ; wash it, and examine
its structure.

The olfactory organ is a sac, the walls of which are thrown
into a large number of transverse folds attached in pairs to
a median septum. Each fold is covered by a soft mucous
membrane, the surface of which is increased by secondary
foldings. The olfactory nerves enter the organ from the dorsal
surface, and spread out in the folds.

The great size of the olfactory organ, and of the nerves it
receives, is very characteristic of Elasmobranch fishes.

SENSE ORGANS. 287

B. The Eye.

Remove one of the eyes by cutting across the eye muscles
and the optic nerve. Snip off the remains of the muscles, and
divide the eye into an inner and an outer half by cutting all
round it equatorially. Place the two halves under water in a
dissecting dish.

The walls of the eye consist of three concentric layers,
and within its cavity, near the outer side, is the spherical
lens about the size of a pea.

1. The sclerotic, or outermost coat of the eye, is thin and

cartilaginous. It invests the whole of the eye
except the part covered by the cornea.

2. The cornea is an oval patch on the outer surface of the

eye, through which the light enters. It is about
half an inch long and a quarter of an inch broad,
and is almost flat. It consists of transparent con-
nective tissue, and is continuous at its edges with
the sclerotic.

3. The choroid is a black opaque membrane, lying within

the sclerotic. It adheres closely to the sclerotic,
and is firmly attached to the line of union of
sclerotic and cornea. Beyond this line it is con-
tinued into the iris, which lies free behind the
cornea, and is perforated in its centre by an elon-
gated oval slit, the pupil, through which light passes
to the interior of the eye.

The inner surface of the choroid, especially in
the inner half of the eye, is covered by a glistening
silvery membrane, the tapetum.

4. The retina is a delicate membrane lying on the inner

surface of the choroid, and readily separating from
it except at the point of entrance of the optic nerve,
where it adheres firmly, and is continuous with the
nerve. It is the only part of the eye sensitive to
light.

288 THE DOG-FISH.

5. The lens is a solid spherical body, suspended in the eye

immediately behind the pupil.

The spherical shape of the lens, together with
the flatness of the cornea, is very characteristic of
aquatic animals.

6. The falciform process is an ingrowth of the choroid

projecting into the vitreous humor from the blind
spot.

C. The Ear.

The ear consists of a membranous sac, or vestibule, filled
with fluid, and produced into three curved tubes, the semi-
circular canals, which he in planes of which each is at right
angles to the other two. Each canal opens at both its ends
into the vestibule.

The whole sac is lodged within the cartilaginous auditory
capsule, which is firmly united with the side of the hinder
end of the skull.

Scrape clean the hinder end of the skull, so as to expose the
left auditory capsule fully. Note, on the dorsal surface, the
prominent rides lodging the anterior and posterior vertical
semicircular canals, the former running backwards and in-
wards from the hinder margin of the orbit, and the latter back-
wards and outwards to the posterior end of the skull. Note
also, at the inner side of the point of meeting of the two canals,
a hole leading downwards and outwards into a canal, the aquae-
ductus vestibuli.

Hold the skull in your left hand and carefully pare away
the cartilage with a sharp scalpel until the several parts of
the auditory organ are fully exposed. Owing to the trans-
parency of the cartilage, the parts can be seen before they are
met with, and the dissection, though requiring care, is not difficult.

1. The vestibule is a laterally compressed sac lying in the
lower and inner part of the auditory capsule. It is
partially divided into two ; contains milky calcareous
concretions ; and communicates with the dorsal
surface of the head by the aquaeductus vestibuli,

SENSE ORGANS. 289

which is a persistent relic of the involution of the
dorsal integument from which the ear is developed.

Pass a seeker down the aquceductus vestibuli into the
vestibule.

2. The semicircular canals are three in number. They

open at both ends into the vestibule, and each is
dilated at one end into an oval ampulla, which
receives a branch of the auditory nerve.

i. The anterior vertical canal has its ampulla at
the anterior end close to the orbit.

ii. The posterior vertical canal is the longest of
the three, and forms an almost complete circle.
Its ampulla is at its posterior and inferior end,
on the ventral wall of the skull.

iii. The horizontal canal lies in the wing-like lateral
process of the auditory capsule behind the
orbit : its ampulla is at its anterior end, close
to that of the anterior vertical canal.

3. The auditory nerve enters the auditory capsule on its

inner side, and at once divides into branches, which
can be traced to the vestibule and to the ampullae
. of the semicircular canals.

D. The Sensory and Ampullary Canals.

The dog-fish possesses at least two kinds of tegumentary
sensory organs. (1) The sensory canals, a continuous system,
developed along certain lines on the head, the jaws, and
down the sides of the body forming ' the organs of the lateral
line.' These structures are found in almost all fishes. (2)
The blind and more or less short ampullary canals not found
outside the group of Elasmobranchs, and distinguished from
the sensory canals by the swellings or ampullae at their blind
ends.

These ampullae are collected principally into a large mass
at the anterior end of the snout, in front of the cranial cavity ;

290 THE DOG-FISH.

and two smaller masses at the sides of the head, between the
nose and the orbit. From these masses the tubes diverge
to their external openings on the dorsal and ventral surfaces
of the head. The tubes are filled with a transparent gelatinous
matter, which can be squeezed out from their openings.

The function of the two canal systems is not known with
certainty, but they are generally believed to be sense organs
of some kind. The nerves supplying them in the head are
the ophthalmic, buccal, and external mandibular branches of
the seventh nerve.

Squeeze the head of a fresh specimen, so as to make the
gelatinous matter exude, and so render the openings visible.

Remove the shin from the dorsal and ventral surfaces of
the head, an operation much facilitated by dipping the head
into hot water for a few moments ; identify the masses of
ampullce and the nerves in connection with them, and trace
the tubes outwards and backwards to their openings. Some of
the tubes which run along the dorsal surface of the skull, between
the cranium and the orbit, are some inches in length.

Cut out a small piece of the mass of ampullce : put it on a
slide in a drop of water : separate the parts with needles : cover,
and examine under the microscope. Note the ampullce, their
internal divisions, the nerves supplying them, and the tubes.

291

Chapter XIII.

THE SKELETON OF THE RABBIT. Lepus cuniculus.

In the early stages of development the skeleton of the rabbit
consists entirely of cartilage. This primary cartilaginous
skeleton agrees in general plan with the skeleton of the dog-
fish.

The skeleton of the adult rabbit is made up of bone, together
with a small amount of cartilage. The bones are of two chief
kinds :

(1) Cartilage-bones are so called from their arising in
direct relation with the pre-existing cartilage. They make up
the greater part of the adult skeleton.

(2) Membrane-bones are developed independently of the
primary cartilaginous skeleton, and are almost confined to
the head. They are to be regarded as dermal bones which
have lost their connection with the skin, and become in
many cases firmly united with the underlying cartilage
and cartilage-bones : they are morphologically equivalent
to the bony plates forming the bases of the scales in the
dog-fish.

The sesamoid bones form a third class. These are small
nodules of bones developed within the tendons, of certain
of the limb-muscles close to the joints : they modify the
action of these muscles by altering the direction in which
they pull.

The teeth of the rabbit are cutaneous structures, com-
parable to the scales or teeth of the dog-fish ; they are,

292 THE SKELETON OF THE RABBIT.

however, in the adult so intimately associated with the bones
of the jaws that they may conveniently be described with the
skeleton. The hairs and claws, which are special develop-
ments of the epidermis, or surface-layer of the skin, will be
described in the next chapter.

The rabbit's skeleton may be divided, like that of the
dog-fish, into axial and appendicular portions, which will be
dealt with successively.

The following points in the rabbit's skeleton should be
noticed as characteristic of the great majority of mammals,
and separating them from lower vertebrates :

Epiphyses are present at the ends of the vertebral centra,
as well as at the ends of the long bones of the limbs ; and
the successive vertebrae are connected by intervertebral liga-
ments : the cervical vertebrae are seven in number. The
sternum is transversely jointed. In the skull there is a
completely ossified occipital ring, with two condyles : the
cranial and olfactory cavities are separated by a bony cribri-
form plate : there is not a distinct para-sphenoid on the under
surface of the skull : the lower jaw articulates directly with
the squamosal : there are three or four small but distinct
auditory ossicles. The teeth form a single but discontinuous
row round the margin of each jaw. In the shoulder girdle
the coracoid is reduced to a small process of the scapula, and
there is no interclavicle. The ankle-joint is situated between
the tibia and fibula above, and the proximal tarsal bones
below : the fourth and fifth carpalia, and the corresponding
tarsalia, fuse to form a single bone in each limb : the preaxial
digit, corresponding to the thumb, has two phalanges, while
the others have each three. In the rabbit the first digit of
the hind limb, corresponding to the great toe in man, is
abortive : in other mammals it has two phalanges like the
thumb.

Examine the prepared skeletons and make drawings to scale
of the several parts.

VERTEBRAL COLUMN. 293

I. THE AXIAL SKELETON.

A. The Vertebral Column, or * Backbone.'

This consists of a series of bony rings or vertebrae, placed
end to end so as to form a tube, which stiffens the back, and
lodges and protects the spinal cord. The successive vertebrae
are movably articulated with each other, and their centra
are separated by intervertebral discs of fibro-cartilage, except
in the sacrum, where a certain number are fused together to
support the pelvis.

The vertebrae fall readily into five groups, named as fol-
lows from before backwards :

(1) The cervical or neck vertebrae.

(2) The thoracic or chest vertebrae, which bear movably
articulated ribs.

(3) The lumbar vertebrae, which are the largest of the
series, and bear no ribs.

(4) The sacral vertebrae, which are fused together and
support the pelvis.

(5) The caudal or tail vertebrae.

1. General characters of a vertebra.
Examine, as a type, the second lumbar vertebra.

a. The vertebra is tubular, the. cavity or neural canal

being traversed during life by the spinal cord.

b. The centrum or body is the thickened ventral wall

of the tube, forming the floor of the neural canal.
It corresponds in position to the notochord of
the embryo, which it partially replaces.

c. The epiphyses are two thin plates of bone applied

to the ends of the centrum, with which they fuse
after the animal is full grown.
. The neural arch forms the sides and roof of the
neural canal, and presents at each end, close to
the centrum, a pair of intervertebral notches for

294 THE SKELETON OF THE RABBIT.

the passage of the spinal nerves, the posterior
notches being much the deeper.

e. The neural spine or spinous process is a median

dorsal crest, projecting upwards and forwards
from the top of the neural arch.

f. The hypapophysis is a median ventral process from

the under surface of the centrum, projecting
downwards, forwards, and slightly to the right.

g. The transverse processes are a pair of large flattened

processes with expanded ends, projecting for-
wards, downwards, and outwards from the sides
of the centrum.

h. The zygapophyses or articular processes are borne
on the two ends of the neural arch : they articu-
late with those of the adjacent vertebrae, so as to
limit torsion of the vertebral column, and lateral
displacement of the contiguous vertebrae.

i. The pre-zygapophyses, or anterior articular pro-
cesses, project upwards from the anterior end
of the neural arch. The articular surfaces
are concave, and turned inwards towards each
other : above them the processes extend up-
wards in this vertebra to form the stout
metapophyses.

Fio. 60. — Lepus cuniculus. Selected vertebrae, (a. m. m.)
I. First cervical vertebra, or atlas, from the dorsal surface. H.
Second oervical vertebra, or axis, from the right side. IH. Fifth
cervical vertebra ; anterior surface. IV. Fourth thoracic vertebra,
from the right side. V. Fourth thoracic vertebra, and fourth pair of
ribs ; anterior surface. VI. Second lumbar vertebra, from the right
side. VH. Second lumbar vertebra ; anterior surface.

A, articular surface for atlas. AP, anapophysis. AZ, anterior or pre-
zygapophysis. c, centrum. CE, epiphysis of centrum. CR, cervical rib.
F H , facet for capitulum or head of the fourth rib. F H ', facet for capitulum of the
fifth rib. FO, facet for odontoid process. FT, facet for tubercle of fourth
rib. HP, hypapophysis. MP, metapophysis. NS, neural spine, or spinous

process. OP, odontoid process, pz, post-zygapophysi3. RC, capitulum or
head of rib. RP, process of rib for attachment of ligaments. RS, sternal
portion of rib. RT, tubercle of rib. RV, vertebral portion of rib. S, sternum.
TP, transverse process. VA, vertebrarterial canal.

VERTEBRAL COLUMN.

295

296 THE SKELETON OF THE RABBIT.

ii. The post-zygapophyses, or posterior articular
processes, project backwards from the top of
the hinder end of the neural arch. The articular
surfaces are slightly convex, and face outwards
and downwards.

iii. The anapophyses are a pair of small backwardly
directed processes from the hinder end of the
neural arch, below the post-zygapophyses and
above the intervertebral notches.

2 The cervical vertebrae.

These are seven in number in the rabbit, as in
almost all other mammals. As a group they are
characterised by the shortness of their centra ; the
small size of their spinous processes ; and the fusion
of the ribs with the vertebrae to form the perforated
transverse processes, through which the vertebral
arteries run.

a. The first, or atlas vertebra, differs very markedly
from all the others : its more important charac-
ters are as follows :

i. The neural canal is very large, and is divided into
two parts by a transverse ligament, which is
often preserved in the dried skeleton : of
these, the upper and larger is the true neural
canal, giving passage to the spinal cord;
while the lower and smaller one lodges the
odontoid process of the second vertebra.

ii. The centrum is probably represented by the
odontoid process, which has become detached
from the atlas, and fused with the centrum
of the second vertebra. The mid-ventral
portion of the atlas is very narrow, and is
produced backwards into a short process.

iii. The neural spine is an inconspicuous ridge, most
prominent in front.

iv. The transverse processes are broad horizontal

VERTEBRAL COLUMN. 297

lamellae, consisting of the V-shaped cervical
ribs, the space between the articulation of
the rib forming the longitudinal vertebrar-
terial canals. In front of each transverse
process the canal is continued .upwards as a
groove to a foramen in the neural arch, through
which the vertebral artery enters the neural
canal and the first spinal nerve leaves it.

Along the inner and hinder border of each
transverse process is a deep groove, corre-
sponding to an intervertebral notch, for the
passage of the second spinal nerve.

v. The articular surfaces. The anterior end of the
atlas has two very large deeply concave sur-
faces, for articulation with the condyles of
the skull : the posterior end has two much
smaller facets for the second vertebra.

b. The second or axis vertebra.

i. The centrum is broad and flat, and is produced
in front into the conical odontoid process,
which is probably the centrum of the atlas
vertebra. The suture between the odontoid
process and the body of the axis is easily
seen in young rabbits.

ii. The neural spine forms a large vertical crest,
very prominent in front, and bifid behind.

iii. The transverse processes are small backwardly
directed spines, perforated at their bases by
the vertebrarterial canals or rib spaces.

iv. The anterior articular surfaces are large and
convex, and situated at the sides of the ante-
rior end of the centrum.

v. The post-zygapophyses project backwards from
the hinder edge of the neural arch, their
articular surfaces facing downwards and out-
wards.

298 THE SKELETON OF THE RABBIT.

c. The remaining cervical vertebrae : third to seventh,
i. The centra are short, broad from side to side,
thin from above downwards, and obliquely
truncated at the ends. The centrum of the
seventh vertebra bears at its posterior end a
pair of facets, which form parts of the arti-
cular surfaces for the heads of the first pair
of thoracic ribs,
ii. The neural spines are small, excepting that of

the seventh vertebra,
iii. The transverse processes are composed of ribs
fused with the vertebra. They are perforated
at their bases by the vertebrarterial canals :
beyond these they divide into dorsal and
ventral portions, of which the latter, the
inferior lamellae, form prominent bony plates,
except in the seventh vertebra, from which
they are absent,
iv. The zygapophyses are borne on the sides of the
neural arches ; the anterior facing upwards
and inwards, the posterior downwards and
slightly outwards.
3. The thoracic vertebrae.

. These are usually twelve, sometimes thirteen, in
number, and are characterised by bearing movably
articulated ribs. The neural spines are long,
especially in front : that of the tenth or anticlinal
vertebra is nearly vertical, those in front of it
sloping backwards, and those behind forwards.
The posterior two or three thoracic vertebrae are
larger than the others, and gradually approach the
lumbar vertebrae in size and shape.

a. A typical thoracic vertebra : any one of the first

six.

i. The centrum is short, and thick from above

downwards. On each side in front is a small

facet, which, with a similar one on the hinder

VERTEBRAL COLUMN. 299

edge of the vertebra next in front, forms the
articular surface for the head of the rib. At
the posterior end are a similar pair of facets
for the ribs of the succeeding vertebra.

ii. The neural arch is notched, both, in front and
behind, for the passage of the spinal nerves.

iii. The transverse processes are short, stout, and
horizontal, and bear on their under surfaces
the articular facets for the tubercles of the
ribs.

iv. The neural spine is long and slender, and slopes
obliquely backwards.

v. The zygapophyses. The pre-zygapophyses face
upwards and outwards, the post-zygapo-
physes downwards and inwards.

b. The posterior three or four thoracic vertebrae.
i. The centra are larger than in the anterior
thoracic vertebrae : the facet for the head of
each rib is confined to the anterior end of
the centrum, the posterior end having no
articular surface.

ii. The neural spines are lower than in the anterior
vertebrae, but longer from before backwards.

iii. The transverse processes are placed further
back, and have no articular facets for the
ribs.

iv. Metapophyses appear in the ninth thoracic
vertebra, and increase considerably in size in
the succeeding ones. They are usually con-
fluent with the pre-zygapophyses in all except
the ninth thoracic vertebrae.

The lumbar vertebrae.

These are usually seven in number : the most
anterior one may, however, bear ribs, in which case
it is reckoned as a thirteenth thoracic vertebra, and
the lumbar vertebrae are then reduced to six.

300 THE SKELETON OF THE RABBIT.

The characters of a typical lumbar vertebra have
already been described : as a group they are charac-
terised by their large size, and the great development
of their processes, especially the transverse pro-
cesses. They increase in size from before back-
wards, the last but one being the largest. The first
two have hypapophyses : the last one has a short
stout centrum, and a stout crest-like neural spine. '

5. The sacral vertebrae.

Strictly speaking, the sacral vertebrae are those
of which the expanded ribs directly support the
pelvis : and of these the rabbit has only one, or at
most two. A certain number of the succeeding ver-
tebrae, which are fused with each other and with the
last true sacral vertebra, are, however, commonly
included under the same name, the whole fused mass
being spoken of as the sacrum. Beckoned in this
way, the rabbit has three or more sacral vertebra?,
the lines of boundary between which can easily be
made out in spite of their fusion.

a. The first sacral vertebra is much the largest, and is

produced laterally into a pair of large wing-like
expansions which support the pelvis, and are
probably modified ribs fused with the vertebra.
The neural spine is large and nearly vertical.

b. The hinder sacral vertebrae are much smaller, and

decrease in size from before backwards. Their
number is variable, increasing with age through
fusion with the anterior caudal vertebrae.

The anterior outer border of the second sacral
vertebra just touches the pelvis, but it is doubtful
whether any true costal element is present in it.

6. The caudal vertebrae.

These are about sixteen in number : they decrease
in size from before backwards, gradually losing all
their processes, and ultimately the neural arches as

VERTEBRAL COLUMN. 301

well, the hindmost vertebrae being reduced to mere
rod-like centra.

B. The Ribs and Sternum.

These form, with the thoracic vertebrae, the skeletal frame-
work of the wall of the thorax, which plays an essential
part in the mechanism of respiration, besides protecting the
thoracic viscera, and giving origin to some of the extrinsic
muscles of the fore-limbs.

1. The ribs are curved bony rods, movably articulated with
the vertebrae above, and connected at their lower
ends with the sternum. There are in the rabbit
twelve, or sometimes thirteen, pairs.

a. A typical rib, such as the fifth, consists of two

portions of very unequal size, joined together
almost at right angles.

The vertebral portion, which is the larger part
of the rib, is a flattened rod of bone, strongly
curved above, nearly straight below. The
dorsal end or capitulum articulates with a
concave surface, furnished partly by the
centrum of the corresponding vertebra, and
partly by that of the vertebra next in front.
A short way beyond the capitulum, and on
the dorsal surface of the rib, is the tubercle
or surface for articulation with the trans-
verse process of the corresponding vertebra ;
and immediately beyond the tubercle is a
short vertical projection for the attachment
of ligaments,
ii. The sternal portion is a short bar of calcified
or imperfectly ossified cartilage, connecting
the lower end of the vertebral portion with
the sternum.

b. The ribs in general. The ribs increase in length

from the first, which is very short, to the sixth,
behind which they diminish. The first nine pairs

302 THE SKELETON OF THE RABBIT.

of ribs have both capitular and tubercular articu-
lations with the vertebrae : in the remaining three
(or four) pairs, which are more slender than the
anterior ones, the tubercles are absent, and the
capitular articulations, which alone are present,
are confined to the corresponding vertebrae.

The sternal portions of the first seven pairs
articulate directly with the sternum : the remain-
ing ones do not reach the sternum, though those
of the eighth and ninth pairs are closely con-
nected with the ribs in front of them.

2. The sternum is an elongated laterally compressed
median rod, situated in the ventral wall of the
thorax, and divided transversely into 'segments or
sternebrae. It is developed in connection with the
ventral ends of the ribs, and consists at first of right
and left halves.

The anterior segment or manubrium is much the
largest, and is produced ventral! y and in front into
a prominent keel.

Behind the manubrium come four segments of
about equal length, then a very short segment, and
finally the xiphisternum, a long slender bony rod
ending behind in a horizontal plate of cartilage.

The first pair of ribs articulate with the sides of
the manubrium, and the succeeding six pairs be-
tween the several sternebrse.

C. The Skull.

This is made up of cranium proper, olfactory and auditory
capsules, and visceral skeleton. The following description
applies not only to the skull of the rabbit but (with certain
modifications) to that of the dog, which is the better of the
two from its larger size. The differences may be understood
by reference to the figures and descriptions.

In the adult rabbit, the original cartilaginous skull is
almost completely replaced by bones. In addition to these

STERNUM AND SKULL.

303

cartilage-bones, membrane-bones are largely developed ; and
the two groups of bones, though primitively distinct, become
intimately and immovably united to form a complex structure,
in which the several morphological components, and even
the boundaries of individual bones, are difficult to determine,
and of which the essential composition can only be ascer-
tained by a study of the embryonic stages.

In the following description the membrane-bones are
described in connection with the cartilage-bones with which
they are most closely associated, and their names are printed
in italics. Though it is convenient to deal with the membrane-

EM. B.0. B.S. PT.A. RL.

M.N.

Fig. 61. — Lepus cuniculus. The skull from the right side. The middle
portion of the zygomatic arch and the right half of the mandible
have been removed, (a. m. m.)

A, external pterygoid process of ali-sphenoid. AS, ali-sphenoid. B, internal
orbital foramen. BO, basi-occipital. BS, basi-sphenoid. c, occipital con-
dyle. D, mandibular symphysis. EO, ex-occipital. F, frontal. FA, foramen
lacerum anterius. FM, foramen ovale, confluent with the foramen lacerum
medium. G, orbital groove, for ophthalmic division of trigeminal nerve. I,
anterior upper incisor. IF, infra-orbital foramen. IP, interparietal. J, lower
incisor. i_,lacrymalbone. LF, lacrymal foramen. M, maxilla. MN, mandible.
N, nasal bone. OF, optic foramen. OS, orbito-sphenoid. p, parietal. PE, par-
occipital process of ex-occiptal. PL, palatine bone. PM, pre-maxilla. PO,
periotic. PT, pterygoid, s, squamosal. SF, stylo-mastoid foramen. SO,
supra-occipital. T, tympanic bones. ZM, zygomatic process of maxilla, cut
short. ZS, zygomatic process of squamosal, cut short.

304 THE SKELETON OF THE RABBIT.

bones in this way, it must be borne in mind that, inasmuch as
they are primitively independent of the cartilaginous skeleton,
any attempt to group them according to the divisions of this
primary skeleton must be more or less arbitrary.

1. The cranium proper, or brain-case, forms the posterior
part of the skull, and is situated almost completely
behind the orbits, or sockets for the eyeballs. The
anterior or facial part of the skull is not in a line
with the base of the cranium, but is deflected down-
wards through angle of about 60°.

The bones of the cranium proper are arranged
in a series of segments or rings, one in front of the
other : of these, the hindmost or occipital segment
is the most obvious, and is very easily detached from
the rest of the skull as a bony ring.

Care must be taken not to confuse these segments
with vertebrae, with which they do not correspond.

a. The occipital segment consists entirely of cartilage-
bones, which may fuse together inseparably. It
articulates with the first vertebra by means of the
occipital condyles.

i. The basi-occipital is a flat median bone, forming
the floor of the hinder part of the cranial
cavity. It forms the ventral boundary of
the foramen magnum, and a small part of
each occipital condyle.

ii. The ex-occipitals bound the foramen magnum
laterally, and form the greater part of the
condyles. Each is produced at its outer side
into a long downwardly directed par-occipital
process, which is closely applied to the pos-
terior surface of the tympanic bulla.

iii. The supra-occipital is a large median bone,
which completes the occipital segment above.
The whole bone is markedly cancellous, and
its outer surface is raised into a prominent

THE SKULL.

305

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306 THE SKELETON OF THE RABBIT.

shield-shaped elevation, giving attachment to
the muscles raising the head.

b. The parietal segment consists of both cartilage-
bones and membrane-bones. It is situated in
front of the occipital segment, with which it is in
contact above and below ; while at' the sides the
two segments are separated by the interposition
of the auditory capsules and the squamosal
bones.

i. The basi-sphenoid is a median bone placed, directly
in front of the basi-occipital, with which it is
connected by a thin plate of cartilage. Seen
from below it is triangular, with the apex
directed forwards. On its upper or dorsal
surface is a depression, the- sella turcica :
this lodges the pituitary body, and is bounded
behind by the large overhanging posterior
clinoid process ; while its floor remains un-
ossified as the pituitary foramen.

ii. The ali-sphenoids are a pair of wing-like bones
ankylosed with the sides of the basi-sphenoid,
and forming parts of the floor of the cranial
cavity. From the ventral surface of each
ali-sphenoid there projects downwards a ver-
tical plate of bone, the external pterygoid
process, which is placed nearly transversely
to the longitudinal axis of the skull, and is
connected along its inner edge with the pala-
tine bone : it is perforated at its base by three
foramina, of which the innermost and largest
transmits the internal maxillary artery and
vein, and the two outer ones transmit branches
of the mandibular division of the fifth nerve.

Between the ali-sphenoid and the anterior
end of the basi-sphenoid on each side is a large
slit-like notch, the foramen lacerum anterius

THE SKULL. 307

or sphenoidal fissure, through which the third,
fourth, and sixth nerves, and the first and
second divisions of the fifth nerve pass out
from the cranial cavity into the orbit.

At the posterior border of each ali-sphenoid
is a deep notch, the foramen ovale, for the
passage of the third division of the fifth nerve.

iii. The parietals are a pair of flattened mem-
brane-bones, meeting each other in the mid-
dorsal fine at the sagittal suture, and forming
a great part of the roof of the cranial cavity.

From the posterior and outer angle of each
parietal a thin plate runs down on the inner
surface of the squamosal, which latter bone
separates the parietal from the ali-sphenoid.

iv. The interparietal is a small median bone in the
roof of the skull, between the parietals and
the supra-occipital.

The frontal segment surrounds the anterior end of
the brain : it is placed in front of the parietal
segment, with which it articulates closely.

i. The pre-sphenoid is a median laterally compressed
bone in front of the basi-sphenoid, and con-
nected with it by cartilage. It forms the
lower margin of the optic foramen, a large
aperture through which the optic nerves enter
the orbits from the cranial cavity, and the two
orbits communicate in the dried skull with
each other. The upper and posterior border
of the pre-sphenoid is produced backwards
into the small anterior clinoid processes,
which bound the sella turcica in front.

ii. The orbito-sphenoids are a pair of lamellar bones,
which are fused with the pre-sphenoid, the
boundary between the two being indistinguish-
able. They surround the optic foramen, be-

308

THE SKELETON OF THE RABBIT.

Fig. 61b. — Ganie familiaris. Ventral Surface of the Skull.

(After Collinge.)

APF, anterior palatine foramen. AS, ali-sphenoid. ASC, ali-sphenoid
canal, posterior opening. BO, basi-occipital. BS, basi-sphenoid. CF, con
dylar foramen. EAM, external auditory meatus. EO, ex-occipital. F,
frontal. FLA, foramen lacerum anterius. FLM, foramen laceruni
medium. FLP, foramen lncerum posterius. FM, toramen magnum.
FO, foramen ovale. FR, foramen rotundum. JU, jugal. MAI, meatus
auditorius internus MX, maxilla. OC, occipital condyle. OP, optic
foramen. ORF, orbital fossa. p, palatine. PAP, par-occipital process.
PGF, post-glenoid foramen. PS. presphenoid. PT, pterygoid. PMX,
pre-maxilla. SMF, stylomastoid foramen. SO, Bupra-occipital. SQ,
squamosal. TF, temporal fossa.. TY, tympanic. VO, vomer.

THE SKULL. 309

hind which they form part of the wall of the
cranium and of the orbits, uniting suturally
with the ali-sphenoids and with the frontals
and squamosals. In front of the optic
foramen the orbito-sphenoids are continued
forwards as a pair of vertical laminae, diverg-
ing from each other at an acute angle, and
forming parts of the inner walls of the orbits,
iii. The frontals are a pair of large membrane-bones
completing the frontal segment. They unite
suturally with each other in the mid-dorsal
line, with the parietals behind and with the
orbito-sphenoids below.

The frontals form the roof and sides of the
anterior part of the cranial cavity, and meet
each other on its floor in front : they extend
forwards into the region of the nose. The
outer surface of each frontal forms the upper
part of the wall of the orbit, and bears a
prominent crescentic supra-orbital process, in
front of which it sends forwards a slender
process between the maxilla and premaxilla.

d. The ethmoidal region lies entirely in front of the
cranial cavity, and is in close relation with the
olfactory organs,
i. The mes-ethmoid is a median vertical plate of
cartilage, which extends forwards and down-
wards in front of the pre-sphenoid as the
septum narium separating the right and left
olfactory cavities from each other.

Its upper and posterior border is ossified
and expanded laterally into the cribriform
plate, which is placed obliquely across the
anterior end of the cranial cavity, in the plane
of the base of the skull, and is perforated by
a number of holes through which the olfactory
nerves pass from the brain to the nose.

310 THE SKELETON OF THE RABBIT.

2. The sense-capsules. Each of the organs of special sense
has in the embryo rabbit, as in the dog-fish, a
protecting investment of cartilage or strong con-
nective tissue. In the case of the olfactory and
auditory organs these capsules are closely connected
with the cranium, and in the adult are more or less
extensively ossified. Membrane-bones are also de-
veloped in immediate relation with the sense-capsules,
and may conveniently be considered with them.

a. Bones in relation with the olfactory capsules. The

olfactory capsules are very large in the rabbit,
and occupy a great part of the facial portion
of the skull. They are so closely connected
with the ethmoidal region and with the upper
jaw that certain of the bones may be referred,
with equal propriety, to either of these divisions,
i. The nasals are a pair of elongated membrane-
bones, forming the roof of the nasal cavities.
They unite suturally with the frontals behind,
and with each other in the median plane.

ii. The ethmo-turbinals are intricately folded laminaB
of bone, fused to the lower surface of the cri-
briform plate of the ethmoid, and occupying
the upper and posterior part of each nasal
cavity.

iii. The maxillo-turbinals are a pair of somewhat
similar, but more delicate and more complex
bones, lying in the interior portion of the
nasal cavities, in front of the ethmo-turbinals.

iv. The naso-turbinals are a pair of thin laminaB of
bone, folded to form pouches, and fused with
the under surfaces of the nasal bones,
v. The vomers are a pair of slender blade -like bones,
fused with each other along their ventral
margins, and embracing the lower edge of
the mes-ethmoid cartilage. Behind they are

THE SKULL. 311

expanded laterally, and connected with the
ethmo-turbinals.

b. Bones in relation with the optic capsules.

The lacrimals are a pair of small lamellar bones
forming part of the anterior walls of the orbits,
and loosely wedged in between the frontals
and the maxillae. Each is notched at its outer
border for the passage of the lacrymal duct :
this is supported by a process of the bone
which extends forwards into the nose.

The lacrymals correspond to the most
anterior of a series of small bones bordering
the orbits in bony fishes.

c. Bones in relation with the auditory capsules (cf.

fig. 66, p. 382).

i. The periotics are the bones which replace the
cartilaginous capsules enclosing the organs of
hearing in the embryo. Each ossifies from
three centres : but the several bones — pro-otic,
epiotic, opisthotic — early unite to form an
irregular mass, which fits loosely into the
side-wall of the cranium, between the occi-
pital segment and the squamosal and which
readily falls out from the dried skull.

In each periotic there may be distinguished
a dense petrous portion, enclosing the essential
part of the organ of hearing ; and a porous
mastoid portion, which forms its outer and
posterior part, and is produced downwards
into the mastoid process, lying immediately
in front of the par-occipital process of the
ex-occipital bone.

On the inner surface of each periotic, as
seen in a bisected skull, there are two depres-
sions. Of these the upper and larger one,
the floccular fossa, lodges the floccular lobe of

312 THE SKELETON OF THE RABBIT.

the cerebellum. The lower one is divided by
a ridge of bone into two, of which the anterior
one is the opening of the aquseductus Fallopii,
transmitting the facial nerve ; and the posterior
one is that of the meatus auditorius internus,
through which the auditory nerve enters the
internal ear.

The anterior border of the periotic is pro-
duced inwards into a prominent crest of bone
which projects into the cranial cavity, and,
together with a similar but smaller ridge on
the supra-occipital, separates the cerebral and
cerebellar fossae from each other.

On the outer surface of the petrous portion
of the periotic, and seen only after removal of
the tympanic bone, are two small apertures.
Of these the anterior and more conspicuous
one is the fenestra ovalis, in which the stapes
often remains wedged ; while the posterior
and more irregularly shaped one is the fenestra
rotunda, which is close to the posterior edge
of the bone, and overhung by the mastoid
process. Immediately below the fenestra
ovalis is a rounded swelling, the promontory,
within which the cochlea lies,
ii. The tympanies are a pair of somewhat flask-
shaped bones, imperfect on their inner sides,
and closely applied to the outer surfaces of
the periotics. The expanded body of the
flask, or bulla, encloses the tympanic cavity ;
and the upwardly directed neck, or meatus
auditorius externus, supports the cartilage
of the large external ear. Within the bulla,
at the base of the meatus, is an incomplete
bony ring, across which the tympanic mem-
brane is stretched during life.

The Eustachian tube leaves the tympanic
cavity by an aperture at the anteroinferior

THE SKULL. 313

angle of the tympanic bone, between it and
the periotic, and passes forwards to the pos-
terior nasal chamber (p. 381).

The bony canal by which the facial nerve
leaves the skull runs along the posterior
border of the tympanic, between it and the
mastoid process of the periotic, and opens
below at the stylomastoid foramen.

On the ventral surface of the tympanic
bone, near its inner border, is a conspicuous
foramen, through which the internal carotid
artery enters the skull.

iii. The auditory ossicles (see fig. 66) are most
conveniently examined with the bones of the
auditory capsules, on account of their close
connection with the organs of hearing ;
though, morphologically, they belong to the
mandibular and hyoidean arches.

They form on each side a chain of small
bones stretching across the tympanic cavity
from the tympanic membrane to the fenestra
ovalis.

a. The malleus consists of a small body and two
processes. The larger process, or manu-
brium, is blade-like, and is attached along
the greater part of its length to the tym-
panic membrane, the tip being near the
centre of the membrane. The shorter pro-
cess, which is anterior, articulates with a
small fossa on the inner surface of the
tympanic bone.

/?. The incus is a minute somewhat pyriform
bone : its head bears in front a saddle-
shaped articular surface for the body of
the malleus, and is produced behind into
a short process, which articulates with a

314 THE SKELETON OF THE RABBIT.

fossa on the outer surface of the periotic
bone, just above the fenestra ovalis.

The narrow end or stalk of the incus
projects downwards : its tip is bent in-
wards, and has attached to it a very small
bony disc, the os orbiculare.
y. The stapes is a very small stirrup-shaped
bone, articulating by its arch with the os
orbiculare, and attached by its basal plate
to the membrane closing the fenestra ovalis.

3. The jaws. The cartilaginous maxillary and mandibular
arches of the embryo, corresponding to the upper
and lower jaws of the dog-fish, disappear almost
completely during development ; the jaws of the
adult rabbit consisting of membrane-bones deve-
loped around and in front of these arches.
a. Bones in relation with the maxillary arch. These
are very closely connected with the cranium and
with the olfactory capsules,
i. The pterygoids are a pair of narrow vertical
plates of bone, attached to the base of the
skull at the junction of the basi-sphenoid with
the ali -sphenoids. The posterior border of
each pterygoid is free, and ends below in the
curved hamular process, while the anterior
border unites with the palatine and ali-
sphenoid. On the outer side of the pterygoid,
between it and the external pterygoid process
of the ali-sphenoid, is the triangular ptery-
goid fossa.
i. The 'palatines are a pair of nearly vertical plates
of bone, attached above to the ventral border
of the pre-sphenoid, and behind to the ptery-
goids and ali-sphenoids, thus completing the
anterior boundaries of the pterygoid fossae.
They form the side-walls of the posterior
narial passage, which they separate from the

THE SKULL. 315

orbits. In front they are produced inwards
into the horizontal palatal processes, which
meet each other in the median plane in the
roof of the mouth, opposite the grinding teeth,
forming the posterior third of the bridge of
bone supporting the hard palate.

iii. The maxillce are a pair of large irregular bones
forming a large part of the skeleton of the
upper jaw and of the face.

The body, or alveolar portion, of each
maxilla projects into the front part of the
orbit, and is excavated by the alveoli or
sockets for the grinding teeth. Its inner
surface is covered by the palatine behind ;
and in front is produced inwards into the
horizontal palatal process, which meets its
fellow in the median plane, and forms the
anterior part of the bony bridge supporting
the hard palate.

From the outer side of the body arises the
stout zygomatic process, which is prolonged
upwards into the orbital process, forming the
anterior boundary of the orbit. In front,
the maxilla is prolonged into the large fene-
strated facial plate, which forms the lateral
boundary of the olfactory chamber, and is
perforated just above the first grinding tooth
by the infra-orbital foramen, for the passage
of the maxillary division of the trigeminal
nerve.

iv. The premaxillce are a pair of bones forming the
anterior part of the upper jaw, and lodging
the incisor teeth : they articulate with the
maxillae behind, and with each other in the
median plane in front.

The upper and posterior angle of each
premaxilla is produced backwards into a

316 THE SKELETON OF THE RABBIT.

long nasal process, which Jies along the
outer border of the nasal bone, and above
the maxilla and the anterior process of the
frontal.

On the under surface of the skull bounded
laterally by the premaxillae and maxillae,
and behind by the hard palate, is a large
triangular aperture, divided longitudinally
into the slit-like anterior palatine foramina
by the palatal processes of the premaxillse,
a pair of thin longitudinally rolled laminae
attached in front to the premaxillse, and
enclosing the organs of unknown function
known as Jacobson's organs.

v. The jugals or malars are a pair of laterally com-
pressed bars forming the greater part of the
length of the zygomatic arches, which bound
the orbits below. Their anterior ends are
fused with the zygomatic processes of the
maxillse, and posteriorly they articulate along
their dorsal edges with the squamosals.

b. Bones in relation with the mandibular arch.

The uppermost part of the mandibular arch

is said to give rise to the incus and malleus,

which have already been described in connection

with the auditory capsule (p. 313).

i. The squamosals are a pair of slightly curved

bony plates, which complete the side-walls

of the cranial cavity, lying in front of the

periotics, and articulating with the parietals,

frontals, orbito-sphenoids, and ali-sphenoids.

From the hinder border of each squamosal
a slender post-tympanic process arches back-
wards over the external auditory meatus,
lying along the outer surface of the periotic.

From the outer surface of the squamosal
arises a stout zygomatic process, which bears

THE SKULL. 317

on its under surface the articular facet for
the mandible : beyond the facet it bends
downwards, to articulate by an expanded
end with the jugal bone, and so complete the
zygomatic arch.

ii. The mandible, or lower jaw, consists of a pair
of somewhat triangular plates of bone, firmly
united in front, and articulating by their
posterior and upper angles with the zygomatic
processes of the squamosals.

The anterior portion of each half or ramus
is stout : it lodges an incisor tooth in front,
and further back, on its upper margin, the
grinding teeth. The hinder part of the ramus
is much thinner : its postero-inferior border,
or angle, is rounded, and inflected along its
margin ; its posterior border, above the angle,
is deeply notched ; its upper border is
thickened to form the longitudinally elongated
condyle, which articulates with the squamosal
in such a way as to allow free movement of
the jaw backwards and forwards, but very
restricted movement from side to side : its
anterior border is deeply grooved, the outer
lip or coronoid process being inflected.

On the inner surface of each ramus, just
behind the grinding teeth, is the inferior
dental foramen, through which the mandi-
bular nerve enters to supply the teeth.

4. The hyoid bone lies in the floor of the hinder part of
the mouth, between the rami of the mandible.

It consists of a median body, and two pairs of
backwardly projecting cornua, of which the posterior
are the larger.

The anterior cornua are the ventral ends of the
hyoidean arches, the dorsal end of each of which has
aborted. The middle piece has separated off as a

318 THE SKELETON OF THE RABBIT.

small bone (stylo-hyal) attached to the par-occipital
process. The posterior cornua are the lower ends,
and only persistent parts, of the first pair of bran-
chial arches.

5. Apertures on the surface of the skull.

After the several bones have been examined, the
positions of the more important foramina should be
determined, most of which have already been seen.
Except when otherwise stated, the several apertures
are paired.

a. Foramina for the passage of cranial nerves.

i. The optic foramen is a large round median hole
in the orbito-sphenoids : it transmits the optic
nerves, and in the dry skull puts the two
orbits in communication with the cranial
cavity and with each other,
ii. The foramen lacerum anterius, or sphenoidal
fissure, is a vertical slit below and behind the
optic foramen, and between the basi-sphenoid
and ali-sphenoid. Through it the third, fourth,
and sixth nerves, and the ophthalmic and
maxillary divisions of the fifth, enter the
orbit from the cranial cavity. In most mam-
mals the maxillary division of the fifth nerve
leaves the skull by a separate aperture, the
foramen rotundum, which in the rabbit is
confluent with the sphenoidal fissure.
iii. The internal orbital foramen perforates the
inner wall of the orbit, about a quarter of an
inch in front of the optic foramen, and at the
anterior end of a groove which curves back
over the optic foramen towards the sphenoidal
fissure. Through it the nasal branch of the
ophthalmic division of the fifth nerve passes
from the orbit into the cranial cavity, which
it enters just behind the lower edge of the
cribriform plate.

FORAMINA OF THE SKULL. 319

iv. The infra-orbital foramen is on the side of the
face, in front of the zygomatic process of the
maxilla : it transmits the infra-orbital branch
of the maxillary division of the fifth nerve
from the orbit to the face.

v. The anterior palatine foramen or foramen in-
cisivum is a large slit-like aperture, extending
from the bony bridge of the palate almost to
the incisor teeth, and imperfectly separated
from its fellow by the palatal processes of
the premaxillse. Through it pass the naso-
palatine branch of the maxillary division of
the fifth nerve and the naso -palatine canal.

vi. The posterior palatine foramen is a small
hole in the bony palate, at the junction of its
maxillary and palatine portions : it transmits
the anterior palatine branch of the maxillary
division of the fifth nerve, and the blood-
vessels of the palate.

vii. The foramen lacerum medium is an irregular
aperture on the under surface of the skull,
between the ali-sphenoid and the periotic,
just in front of the tympanic bulla. Its an-
terior part, which represents the separate
foramen ovale of other mammals, transmits
the mandibular division of the fifth nerve.

mi. The stylomastoid foramen is a small aperture
about the middle of the posterior border of
the tympanic bone, between it and the mastoid
process of the periotic. Through it the main
trunk of the seventh, or facial, nerve leaves
the skull.

ix. The foramen lacerum posterius is an irregular
fissure on the under surface of the hinder end
of the skull, between the occipital condyle
and the tympanic bulla. It transmits the

320 THE SKELETON OF THE RABBIT

glossopharyngeal, pneumogastric, and spinal
accessory nerves, and the internal jugular
vein,
x. The condylar foramina are a • couple of holes
running obliquely through the ex-occipital,
just in front of the condyle. They transmit
the two divisions of the hypoglossal nerve, and
are best seen through the foramen magnum.

b. Other important apertures in the skull.

i. The anterior narial opening is at the anterior
end of the skull, and is bounded by the nasals
and the premaxillse. It is single in the dried
skull, but divided in the natural condition by
the vertical cartilaginous septum narium.

ii. The lacrymal foramen is formed by a deep
notch in the outer border of the lacrymal bone,
completed in front by the orbital process of
the maxilla. It leads into the canal along
which the lacrymal duct runs into the nose.

iii. The pituitary foramen is a median hole, per-
forating the floor of the pituitary fossa in the
centre of the basi-sphenoid : in the natural
condition it is closed by membrane.

iv. The carotid foramen perforates the ventral sur-
face of the tympanic bone near its inner
border, close to the occipital condyle : through
it the internal carotid artery enters the skull.

v. The opening of the Eustachian canal is on the
under surface of the skull, at the anterior and
inner angle of the tympanic bone, imme-
diately behind and to the inner side of the
foramen lacerum medium : through it the
Eustachian tube passes from the tympanic
cavity to the posterior narial chamber.

vi. The external auditory aperture is the large
upwardly directed opening of the neck of the

THE TEETH. 321

tympanic bone, round the edge of which the
cartilage of the external ear is attached.

vii. The foramen magnum is the large median
hole at the hinder end of the skull, surrounded
by the bones of the occipital segment. It is
traversed by the central nervous system, and
marks the boundary between the brain and
the spinal cord.

The teeth.

The teeth are cutaneous structures developed
from the mucous membrane of the mouth. They
consist of an extremely hard outer layer of enamel,
which is epidermal in origin ; and a central less hard
portion of dentine, which is developed from the
dermis. The dentine is hollow, its cavity containing
the pulp of the tooth, which receives vessels and
nerves through the large aperture at its base. Sur-
rounding the lower part of the tooth, and filling up
the folds of its surface, is a thin layer of bone, the
crusta petrosa or cement.

It is characteristic of the teeth of the rabbit,
which is herbivorous, that the aperture at the base of
each does not become constricted, but remains wide
open, the teeth continuing to grow throughout life :
whereas in most mammals the base of the tooth
sooner or later becomes narrowed to form a root or
fang, the aperture becoming almost obliterated, and
• the growth of the tooth ceasing entirely.

The teeth of the rabbit do not form a continuous
series, as in man, but the front teeth or incisors are
separated by a great gap or diastema from the group
of grinding teeth in the hinder part of the mouth.
The rabbit has no teeth corresponding to the canine
teeth of other mammals.

a. The teeth of the upper jaw.

i. The incisors are two pairs of teeth at the front

322 THE SKELETON OF THE RABBIT.

of the upper jaw, lodged in alveoli in the pre-
maxillse.

a. The anterior incisors are long, and curved
almost into semicircles, the greater part of
each being lodged in an alveolus which
extends almost to the hinder end of the
bone. Along the anterior surface the
enamel is thick, and folded so as to form
a longitudinal groove. Behind and at the
side it is much thinner, and hence wears
away sooner, rendering the cutting edge
chisel-shaped.

y8. The posterior incisors, which are lodged in
alveoli immediately behind the anterior
ones, are much smaller and slightly curved.

ii. The grinding teeth form a set of six teeth on
each side, borne by the maxilla, and fall into
two groups.

a. The premolars are the three anterior grinders
of each side, and are distinguished from
the posterior ones by the fact that, like the
incisors, they are preceded in early life by
' milk ' or deciduous teeth, which in the
rabbit are shed shortly after birth.

Each premolar is flattened from before
backwards, and each of the two hinder
ones has a deep groove along its outer
surface which carries the enamel almost
across the tooth, giving rise to a promi-
nent ridge on the grinding surface. b

(3. The molars are the three posterior grinders on
each side : they differ from the premolars,
in having no deciduous predecessors. The
first two are similar to the premolars ; the
last is small and not grooved.

THE TEETH. 323

b. The teeth of the lower jaw.

i. The incisors are two in number, and project for-
wards from the anterior end of the mandible.
They are similar to the large upper incisors,
but are less strongly curved and are not
grooved : the alveoli in which they are lodged
extend back in the jaw as far as the anterior
grinders.

ii. The grinding teeth form a set of five on each
side, placed opposite those of the upper jaw,
and like these are divisible into two groups.
They differ from the upper grinders in being
curved so as to be convex instead of concave
outwards.

a. The premolars are the two anterior grinders
on each side. They are somewhat similar
to the upper ones, but narrower from side
to side. The outer surface of the first,
which is the larger of the two, is marked
by two deep longitudinal grooves, and that
of the second by a single groove. The
inner surface of each is less deeply grooved
than the outer.

/3. The molars are the three hinder grinders on
each side : they are similar to the second
premolars, but have no deciduous prede-
cessors : the hindmost one is smaller and
simpler than the other two.

II. THE APPENDICULAR SKELETON.

The appendicular skeleton comprises the bones of the two
pairs of limbs, and the girdles which support them. The
bones of the limbs are mostly elongated, and provided with
epiphyses at their ends.

A. The Pectoral Girdle.

This lies outside the ribs, and is attached to the axial

324 THE SKELETON OF THE RABBIT.

skeleton by muscles and ligaments only. In the rabbit, as
in nearly all other mammals, it is very incomplete : the
dorsal or scapular portion on each side is well developed,
but the ventral or coracoid portion is almost entirely absent,
being represented merely by a small knob of bone fused with
the scapula : the clavicles also are imperfect.

i. The scapula is a triangular plate of bone, of
which the apex is directed downwards and for-
wards, and is expanded and hollowed on its
ventral surface to form the shallow glenoid
cavity, with which the head of the humerus
articulates.

Of the three borders, the anterior or
coracoid border is nearly straight ; the pos-
terior or glenoid is slightly concave ; and the
dorsal or suprascapular, which is the shortest
of the three, is convex and supports the car-
tilaginous suprascapular.

The spine is a prominent ridge running
along the outer surface of the scapula, nearly
parallel to and a short distance behind the
coracoid border. It is continued ventrally
into a free downwardly directed process, the
acromion, from the posterior border of which
the long metacromion projects backwards.

The coracoid border is continuous below
with the coracoid process, which overhangs
the glenoid cavity, and is produced inwards
into a blunt hook. This is a distinct bone in
the young rabbit, and is the sole represen-
tative of the coracoid portion of the pectoral
girdle.

ii. The clavicle is a slender'curved cartilage-bone,
rather less than an inch in length, and is
attached to a ligament running from the
scapula to the sternum.

THE FORE-LIMB. 325

Small cartilaginous nodules occur, one at
the outer and two at the inner end of the
clavicle.
B. The Fore-limb.

The fore-limb of the rabbit corresponds to the upper limb
of man, and may be divided, like this, into arm, forearm,
wrist, and hand. The relative positions of these several
divisions are considerably altered from the primitive condi-
tion, and are best understood by comparing them with the
human arm in the following manner :

Extend your right arm horizontally outwards from the
shoulder, with the palm of the hand vertical and the thumb
upwards : there may now be distinguished a ventral or flexor
surface ; a dorsal or extensor surface ; a preaxial border,
along which are the thumb and the radius ; and a postaxial
border, along which are the little finger and the ulna.

Next bend the elbow at right angles, so that the fingers
point forwards, and bring the whole arm down to the side,
rotating it through 90°, so that the preaxial border becomes
the outer and the postaxial border the inner one. Now
pronate the hand, turning it through 180°, so that the thumb
is brought to the inner side, and the palm directed down-
wards. Place the rabbit's skeleton in front of you, with the
vertebral column vertical, the head directed upwards, and
the limbs pointing away from you, and the right fore-limb
will correspond in position to your own right arm. Note
that the preaxial border is on the outer side in the arm, and
on the inner side in the hand, while in the forearm the two
bones cross each other obliquely. The hand in the rabbit is
fixed permanently in the prone position — that is, with the
palm turned downwards.

1. The arm contains only a single bone.

i. The humerus is an elongated bone, articulating
by its enlarged proximal head with the glenoid
cavity of the scapula, and at its distal end, by
the trochlea, with the bones of the forearm.

At the proximal end of the bone, in front
of the head, is the longitudinal bicipital

326 , THE SKELETON OF THE 'RABBIT.

groove, for the tendon of the biceps muscle :
this is bounded on its inner side by the lesser
tuberosity ; and on its outer side by the greater
tuberosity, a large roughened projection con-
tinued as a ridge down the anterior surface
of the bone as far as its middle.

The trochlea at the distal end of the bone
is an oblique pulley-like surface, divided in
front by a ridge, and articulating with the
radius and ulna to form the elbow-joint.
Immediately above the trochlea, in front and
behind, are the supra-trochlear fossae, which
communicate with each other through the
supra-trochlear foramen : the posterior or
olecranon fossa is much the larger of the two,
and lodges the olecranon process of the ulna
when the arm is extended.

2. The forearm contains two bones, of about equal size,
immovably articulated, though not ankylosed, with
each other.

i. The radius, or preaxial bone, lies in front of
the ulna at the proximal end of the forearm,
but in the distal half lies along its inner or
preaxial side.

It is an elongated bone, slightly convex
forwards. Its proximal end or head articu-
lates with both grooves of the trochlea of the
humerus, and also with the anterior surface
of the ulna ; its distal and larger end articu-
lates with the bones of the wrist.

ii. The ulna, or postaxial bone of the forearm, has
a greatly enlarged proximal end, on the an-
terior surface of which are the sigmoid notch
for articulation with the trochlea, and also
two small articular surfaces for the radius.
Behind the elbow -joint, the bone forms the
large laterally compressed olecranon process.

THE FORE-LIMB. 327

The distal end of the ulna is small, and
articulates with the carpus.

3. The wrist. In the rabbit the wrist or carpus consists
of nine small bones, of which one, the pisiform, is
commonly regarded as a sesamoid bone or ossifica-
tion in the tendon of a muscle, but by some ana-
tomists is considered to be a vestige of a sixth digit.
The arrangement of the remaining eight can best
be understood by comparison with more primitive
types.

The typical carpus consists of nine bones,
arranged in a proximal row of three, articulating
with the radius and ulna ; a distal row of five, each
of which supports one of the digits ; and a central
bone wedged in between the proximal and distal
. rows. In the rabbit the arrangement is as follows :

a. The proximal row of carpals consists of the three

typical bones.

i. The radiale, or scaphoid, is on the inner or pre-
axial side, and articulates with the radius.

ii. The intermedium, lunar, or semilunar, is the
middle bone of the three, and articulates
with the radius.

iii. The ulnare, or cuneiform, is the outer or post-
axial bone of the proximal row, and articu-
lates with the ulna.

b. The central bone.

i. The centrale is a small bone, articulating with
the distal surfaces of the intermedium and
radiale, and appearing to belong to the distal
row of carpals.

c. The distal row of carpals consists of four bones,

the two outermost of the typical carpus having
fused to form a single bone,
i. The first carpale, or trapezium, is on the inner or

328

THE SKELETON OF THE RABBIT.

THE FORE-LIMB. 329

preaxial side of the wrist : it articulates with
the radiale, and supports the first metacarpal.

ii. The second carpale, or trapezoid, articulates
with the radiale and centrale, and supports
the second metacarpal.

iii. The third carpale, or os magnum, is very small :
it articulates with the intermedium, the cen-
trale, and the third metacarpal.

iv. The fourth and fifth carpalia are fused to form
the unciform bone, which is much larger :
it articulates with the ulnare, and supports
the fourth and fifth metacarpals.

d. The pisiform is a small bone on the posterior or
flexor surface of the carpus, and articulates with
the distal end of the ulna and with the ulnare.

The hand in the rabbit has five digits, as in man :
the bones of each consist of a proximal metacarpal,
followed by the phalanges, the last of which, the
ungual phalanx, is conical and slightly curved, and
supports the claw.

The pollex, or preaxial digit, corresponding
to the thumb in man, is much shorter than the
others, and has only two phalanges, while each of
the other digits has three.

Sesamoid bones are developed in the tendons

Fig. 62. — Lepus cuniculus. Skeleton of the left hand, extensor sur-
face. The pisiform bone is not seen in this view. Natural size,
(c. H. H.)

A, radius. B, epiphysis of radius. C, ulna. D, epiphysis of ulna. E,
radiale. F, intermedium, g, ulnare. H, centrale. K. 1, K. 2, K. 3, first
three carpalia. K. 4, unciform bone. M.1,M.2, M. 5, first, second, and fifth
metacarpals. PA, proximal phalanges of first and fifth digits. PB, PC, middle
and ungual phalanges of fifth digit. T, claw.

Fig. C3. — Lepus cuniculus. Skeleton of the left foot, extensor surface.
Natural size. (c. h. h.)

E, astragalus. G, calcaneum. H, naviculare. K, process representing
first tarsale. K. 2, K- 3, second and tnird tarsalia. K- 4, cuboid. M. 1 , M- 2,
first (rudimentary) and second metatarsals fused together. M. 5, fifth meta-
tarsal. PA, PC. proximal and ungual phalanges of second toe. T, claw
enclosing ungual phalanx of fifth toe.

330 THE SKELETON OF THE RABBIT.

of the flexor muscles, opposite the metacarpo-
phalangeal and certain other joints.

C. The Pelvic Girdle.

The pelvic girdle, which supports the hind-limb, consists
of two halves, which, though primitively nearly at right
angles to the vertebral column, are in the adult rabbit almost
parallel to it. They are firmly attached to the sacrum in
front, and to each other in the ventral symphysis behind.

Each half, or os innominatum, consists of three bones —
ilium, ischium, and pubes — and presents about the middle of
its outer surface a deep cup-shaped acetabular cavity for the
femur, in the formation of which all the three bones take
part.

i. The ilium is the anterior and dorsal member
of the girdle, corresponding to the scapula
in the pectoral girdle. It is expanded and
blade-like in front, and narrowed behind.
On its inner or sacral surface, a little way
in front of the acetabulum, is a roughened
crescentic surface for articulation with the
sacrum ; while its outer surface is divided by
a longitudinal ridge into ventral or iliac, and
dorsal or gluteal fossae. The ilium forms
about half of the acetabulum, the lines of
boundary between it and the ischium and
pubes being easily seen in the young skeleton.

ii. The ischium forms the posterior and dorsal
portion of the os innominatum, and is
separated from the pubes by the large oval
obturator foramen. It forms about a third
of the acetabulum, behind which it bears
on its inner surface the sharp, backwardly
directed ischial spine. Its posterior part is
flattened, the margin being thickened to
form the ischial tuberosity, which is raised
on its outer surface into a prominent hooked

PELVIC GIRDLE AND HIND-LIMB. 331

process. The ischium corresponds to the
coracoid in the shoulder-girdle.

iii. The pubes is the smallest bone of the three.
Its acetabular portion ossifies separately in
the rabbit, as a small cotyloid bone, com-
pleting the margin of the acetabulum below.
The pubes unites along its ventral border
with its fellow of the opposite side to form
the anterior part of the symphysis, the
hinder portion of which is formed by the
ischium ; the line of union between these
two bones is visible only in the skeletons of
very young rabbits.

D. The Hind-limb.

The hind-limb is much larger than the fore-limb, and,
like this, is divisible into four main portions.

In the hind-limb the departure from the primitive condi-
tion is much less marked than in the fore-limb. The whole
limb has rotated forwards through about a right angle, so that
the preaxial border is directed inwards, and the postaxial
outwards ; the original dorsal surface being turned forwards,
and the ventral surface backwards.

1. The thigh, like the arm, contains a single bone, the
femur. Certain sesamoid bones in connection with
the knee-joint may be considered with it.

i. The femur is an elongated bone, consisting of
a cylindrical shaft and two enlarged ex-
tremities.

The proximal end bears on its inner or
preaxial side the prominent rounded head,
which articulates with the acetabulum to
form the hip-joint. To the outer side of the
head, and forming the end of the bone, is a
large rough process, the great trochanter :
between this and the head, on the posterior

332 THE SKELETON OF THE RABBIT.

surface of the bone, is a deep pit, the trochan-
teric or digital fossa.

On the inner or preaxial side of the bone,
immediately below the head, is a rough ridge,
the lesser trochanter ; opposite to which on
the outer or postaxial side is the prominent
third trochanter.

The lower or distal end of the bone 'bears
two large condyles which articulate with the
tibia. These are separated from each other
by the intercondylar notch, which is con-
tinuous with a deep groove on the front of
the bone, along which the patella slides.

ii. The patella, or knee-pan, is a large sesamoid
bone in the tendon of the extensor muscles of
the leg. It is connected by ligaments with
the tibia, and slides in the groove on the front
of the lower end of the femur.

At the back of the knee-joint are smaller
sesamoids, the fabellae.

2. The leg contains two bones, which are very unequal in
size, and in the adult are fused together distally.

i. The tibia, or preaxial bone of the leg, is a stout
and straight bone, the longest in the body.
Its enlarged proximal end, which is triangular
in section, bears on its upper end two oval
surfaces for articulation with the condyles of
the femur. Below the head is the prominent
cnemial crest, extending some distance down
the anterior surface of the bone.

The distal end of the tibia, which is
indistinguishably fused with the lower end
of the fibula, bears an articular surface for
the astragalus.

ii. The fibula, or postaxial bone, is very slender :
its proximal end is distinct, and articulates

THE HIND-LIMB. 333

with a facet on the outer surface of the head
of the tibia. ,

Its distal portion is completely fused with
the tibia, and bears at its end an articular
surface for the calcaneum.

The tarsus (fig. 63) consists, in the rabbit, of six bones,
arranged, like the bones of the wrist, in a proximal
and a distal row, with a central bone between them.
The typical tarsus exactly corresponds to the typical
carpus in number and arrangement of bones.

a. The proximal row of tarsals consists of two bones,
i. The astragalus, which probably corresponds to

the tibiale and intermedium of the typical
tarsus fused together, lies on the inner or
preaxial side of the tarsus : it articulates by a
prominent pulley-like surface with the distal
end of the tibia.

ii. The calcaneum, or fibulare, is the largest bone
of the tarsus : it lies along the outer side of
the astragalus, articulating with the fibula,
and is produced backwards into the large
calcaneal process or heel.

b. The central bone.

The naviculare, or centrale, is placed immedi-
ately in front of the astragalus. It sends
forwards a large process along the ventral
surface of the foot.

c. The distal row of tarsals consists of three bones, the

innermost or preaxial bone of the typical tarsus
being absent, together with the digit which it
supports, though probably represented by the
process at the proximal end of the second meta-
tarsal ; while the two outer tarsalia, like the
corresponding carpalia, fuse together.

i. The second tarsale, or meso-cuneiform, is very

AA*

334 THE SKELETON OF THE RABBIT.

small : it articulates with the navicular, and
supports the second metatarsal.

ii. The third tarsale, or ecto-cuneiform, lies to the
outer side of the meso-cuneiform. It arti-
culates with the navicular, and supports the
third metatarsal.

iii. The cuboid, formed by the fusion of the fourth
and fifth tarsalia, is a much larger bone, on
the outer side of the foot : it articulates with
the calcaneum and navicular, and supports
the fourth and fifth metatarsals.

4. The foot of the rabbit has only four toes, the preaxial
digit of the typical foot, corresponding to the great
toe in man, having almost completely disappeared.

Each of the four digits consists of a long meta-
tarsal bone, followed by three phalanges, the last of
which bears a claw.

The second (apparent first) metatarsal has at its
proximal end a backwardly directed process, which
articulates with the navicular : this is a distinct bone
in the young rabbit, and probably represents the
ecto-cuneiform and the metatarsal of the lost first
toe or hallux (see fig. 63, p. 328).

335

Chapter XIV.
DISSECTION OF THE BABBIT.

The rabbit, rat or cavy may be taken as a type of the Mam-
malia Bodentia ; and the following points should be noted as
characteristic of the great majority of the Mammalia :

The distinguishing features of the skeleton have been
noted in the preceding chapter.

Externally the body is covered with hair : the urino-
genital and anal apertures are distinct from each other : and
there are large external ears.

As regards the viscera : Mammary glands are present,
opening to the exterior on teats placed on the ventral surface
of the body. The heart has four chambers : there is a single
aortic arch in the adult, formed from the fourth left arch of
the embryo : the blood is hot, and the red corpuscles non-
nucleated. There is a complete diaphragm ; and the lungs,
which are not attached to the ribs, are completely surrounded
by pleurae. The ureters open directly into the bladder. In
the brain there is a corpus callosum, connecting the hemi-
spheres ; and corpora quadrigemina in place of corpora
bigemina, or four optic lobes in place of two. The cochlea
is coiled spirally.

The ova are extremely small, and are retained within the
uterus during almost the whole period of their development.
The embryo receives a constant supply of nutriment direct
from the mother by means of the placenta, a structure in
which the blood-vessels of the embryo and of the mother are
brought into very close relation, so that diffusion can readily
take place from one to the other.

In the following account the several systems are described
one by one, in order that each may be studied in its entirety

336 DISSECTION OF THE RABBIT.

as far as practicable, and the mutual relations of its several
parts demonstrated. This method is the best, and should be
followed whenever possible. If, however, it is intended to
perform the whole dissection on a single rabbit, it will be
found advisable somewhat to modify the order of proceeding.

For most purposes, young immature rabbits are best for
dissection, but for the examination of the reproductive organs,
it is necessary to have specimens six months old. They may be
hilled by pouring a few drops of solution of cyanide of potas-
sium into the mouth, or else by chloroform, or by coal gas in a
lethal chamber. If cyanide is used the mouth must be well
washed after death.

The brain should be removed at once and put into either
strong spirit, if required soon, or still better, into a mixture of
equal parts of one per cent, potass, bichromate and eight per
cent, formalin. This hardening fluid should be renewed at
least once, and allowed to act for three weeks. Then wash the
brain in running water for a day, and keep it in five per cent,
formalin.

I. EXTERNA^ CHARACTERS.

The whole body is covered with hairs variously coloured ;
and is obviously divided into head, neck, trunk, a short tail,
and two pairs of limbs.

A. The Head.

Note the general form of the head. Determine the position
of the mandible, the zygomatic arches, the supra-orbital pro-
cesses of the frontals, and other parts of the skull by feeling
through the skin.

1. The mouth is a small transverse aperture on the under

surface of the front of the head, bounded by the
soft hairy lips. The upper lip is divided by a
median cleft, which connects the mouth with the
nostrils, and leaves the upper incisors exposed.

2. The nostrils are a pair of oblique slits at the end of the

hairy snout.

EXTERNAL CHARACTERS. 337

3. The eyes are large, and placed at the sides of the head.

Each eye is protected by an upper and a lower eye-
lid, fringed with scanty eyelashes ; and by a third
eyelid, or nictitating membrane, a hairless fold of
opaque white skin, which lies within the other eye-
lids, and can be pulled across the eyeball from the
anterior angle or canthus.

4. The vibrissas, or whiskers, are long stiff tactile hairs, at

the side of the snout and above and below the eyes.

5. The external ears, or pinnae, are very large and freely

movable. Each is folded longitudinally, and is sup-
ported by cartilage at its base, where it surrounds
the external auditory aperture.

B. The Trunk.

This is divided into an anterior portion or thorax, stiffened
laterally by the ribs, and a posterior part or abdomen.

Feel through the skin, and determine the positions of the
vertebral column, the ribs, and the sternum.

1. The anus is a small median aperture beneath the base of

the tail.

2. The perinseal pouches are a pair of hairless depressions

of the skin, in front of and at the sides of the anus :
into them open the ducts of the perinseal glands, the
secretion of which has the strong smell characteristic
of the rabbit. These glands should be removed at
once if the rat is under dissection.

3. The urino-genital aperture is situated in both sexes in

front of the anus.

a. In the male the aperture is placed at the end of the

long cylindrical penis, which can be retracted
within a loose sheath of skin, the prepuce.

b. In the female the opening, or vulva, is slit-like^ and

larger than in the male ; and its anterior margin
is produced into a smajl fleshy process, the
clitoris, which corresponds to the penis of the
male.

338 DISSECTION OF THE RABBIT.

4. The scrotal sacs, found in the male only, are a pair of

prominent oval pouches of skin, at the sides of the
penis, into which the testes descend.

5. The teats, or mammae, of the female are four or five pairs

of small papillse, perforated by the openings of the
ducts of the mammary or milk glands. They are
arranged at intervals along the ventral surface of
the thorax and abdomen : the most anterior pair
being about two and a half inches apart and an inch
behind the elbow, and the hindmost pair opposite
the thighs and rather more than an inch apart.

C. The Limbs.

1. The fore-limbs, as far as the elbows, are closely applied

to the sides of the thorax and hidden by the skin of
the trunk. The several bones of the shoulder-girdle
and limb, which have been already studied in the
skeleton, can easily be identified by feeling them
through the skin. The palmar surface of the paw
. is hairy ; the innermost digit or pollex is shorter
than the others, not reaching the ground. All the
digits bear claws.

2. The hind-limbs are much longer than the fore-limbs.

The pelvic girdle and the several bones of the limb
can be felt through the skin. In the foot the inner-
most digit or hallux is absent, and the remaining
four are long and clawed. The plantar surface or
sole of the foot is hairy.

II. THE ABDOMINAL VISCERA.

Remove the shin from the trunk and limbs. Lay the rabbit
on its bach, and fasten it down to the dissecting -board by fins
through the limbs.

A. The mammary glands in the female he immediately
beneath the skin. They are yellowish, fatty-looking,

ABDOMINAL VISCERA. 339

dendriform masses, covering the ventral surface of the
whole length of the thorax and abdomen ; their ducts
may easily be traced, converging to the teats.

B. The Abdominal Viscera in situ.

Open the abdominal cavity by a median incision through
its ventral wall, from the hinder end of the sternum to the pubic
symphysis, taking care not to injure the viscera. From the
anterior end of the incision cut outwards for about two
inches on each side, just behind the ribs, and pin out the flaps
right and left. Note the positions of the following viscera,
without disturbing them :

1. The liver is a large dark-red body at the anterior end of

the abdominal cavity, extending further back on the
left side than on the right.

2. The stomach lies immediately behind the liver and is

partially covered by it : its surface is smooth and of
a bluish white colour.

3. The duodenum is a narrow pinkish tube, a small part of

which is seen to the right of the stomach.

4. The small intestine is a long narrow much-convoluted

tube, part of which is seen at the left side of the
abdominal cavity, behind the liver.

5. The caecum is a very wide tube, of a dark colour, coiled

on itself and marked by a spiral constriction. In the
natural position of the parts it covers almost all the
other viscera behind the stomach. On the right
side the caecum ends in the pale fleshy vermiform
appendix.

6. The colon is a wide tube, with markedly sacculated walls,

running obliquely across the abdomen between the

folds of the caecum.

"%

7. The rectum is a white tube, dilated at intervals by the

faecal pellets : small parts of it are seen to the
right of the stomach, and at the hinder end of the
abdomen. - -

340

DISSECTION OF THE RABBIT.

u ■» I

ABDOMINAL VISCERA. 341

8. The bladder is a thin-walled sac at the posterior end of

the abdomen, just in front of the pubic symphysis.

Turn the alimentary canal over to the right side, -without
cutting or tearing anything, to expose the following parts :

9. The spleen is an elongated dark-red body lying behind

the stomach, and attached to its left or cardiac end.

10. The kidneys are a pair of compact ovoid bodies attached

to the dorsal wall of the abdomen ; the right one
being immediately behind the liver, and the left one
about an inch and a half further back. In front of
each kidney is a small round yellow adrenal body.

11. The diaphragm is a muscular partition forming the

anterior wall of the abdomen, and separating it from
the thorax. To see it the liver should be pressed
back.

12. The peritoneum is a glistening membrane which lines

the abdominal cavity. The mesentery is a double
layer of peritoneum, reflected from the dorsal wall
of" the abdomen, which at its margin supports and
encloses the alimentary canal : between its two layers
blood vessels, lymphatics, and nerves pass to and
from the various organs. The omentum is a special

Fig. 64. — Lepus cuniculus. Diagrammatic view from the left side.
The head and vertebral column are represented in longitudinal
section ; the lungs, the left half of the liver, the stomach, the greater
part of the intestine, and the brain and spinal cord have been
removed, and the loop of the duodenum spread out. (a. m. m.)

A, tongue. AN, anus. B, bladder. BO, basi-occipital. BS, basi-sphenoid.
C, liver. CA, carotid artery. D, diaphragm. DA, aorta. E, duodenum.
ET, ethmo-turbinal. EU, aperture of Eustachian tube. F, pancreas. FF,
floccular fossa. G, gall-bladder. H, bile-duct. HV, opening of hej>atic vein
into posterior vena cava. IV, posterior vena cava. J, organ of Jacobson.
JV, left external jugular vein. K, kidney turned up. L, larynx bisected.
LA, left auricle. |_G, sublingual gland. |_V, left ventricle. |_7, seventh
lumbar vertebra. M, manubrium of the sternum, with the sternal portion of
the first rib. MA, anterior mesenteric artery. MG, submaxillary gland. MT,
maxillo-turbinal. N, posterior nasal chamber, o, optic foramen. P, hard
palate. PA, spermatic artery. PD, pancreatic duct. PG, perinaeal gland.
PN, penis. PS, pre-sphenoid. PV, portal vein. Q, oesophagus. R, hinder
part of the rectum. RG, rectal gland, s, uterus masculinus. SA, left sub-
clavian artery. SR, suprarenal body, displaced. SV, left anterior vena cava.
TN, right tonsil. TR, trachea. T 1 , first thoracic vertebra. T 1 2, twelfth
thoracic vertebra. U, ureter. V, testis, w, epididymis. X, vas deferens.
Y, prostate. Z, pelvic symphysis.

342 DISSECTION OF THE RABBIT.

fold of peritoneum extending backwards from me
stomach and loaded with fat.

13. The coeliacyand mesenteric ganglia and the nerves

connecting them should now be recognised. They
he in the mesentery near the vertebral column
(see p. 347).

14. The portal vein and its branches should be identified

(p. 355) and drawn.

III. THE DIGESTIVE SYSTEM.

In this section the abdominal portion of the digestive
system will alone be considered ; the buccal cavity, pharynx,
salivary glands, and oesophagus will be described later.

External Characters of the Digestive System.

1. The stomach.

Expose the stomach fully, by turning the liver forwards,
and the intestine over to the animaVs left.

The stomach is much dilated at its left or cardiac
end ; narrower towards the right or pyloric end ;
and markedly curved, the concavity being directed
forwards. Near the left end of its anterior border is
the cardia, or opening of the oesophagus ; and from
the thick-walled pyloric end a small aperture, the
pylorus, leads to the duodenum.

2. The duodenum, or first portion of the intestine, runs

from the pylorus along the right side of the ab-
dominal cavity almost to its hinder end : it then
turns back on itself, forming a U-shaped loop. In
• the mesentery connecting the two limbs of the loop
are the large duodenal vessels.

3. The pancreas.

Turn the duodenal loop over to the left side without injuring
the mesentery, and spread it out over the other viscera.

DIGESTIVE SYSTEM. 343

The pancreas is a diffuse fat-like gland, of a
pinkish colour, scattered in the mesentery of the
duodenal loop. It lies mainly around the large
vessels of the loop, and is more compact at its
anterior end.

The pancreatic duct commences in front, and
runs backwards through the gland, receiving smaller
ducts from its several lobes : it opens into the inner
side of the distal limb of the duodenal loop, about
three inches beyond the bend.
Open the duodenum, opposite to the entrance of the pan-
creatic duct ; wash out the contents ; note the aperture of the
duct, and insert a bristle into it.

4. The liver is a large solid organ, thick in the middle
and thinner towards the edges. Its anterior sur-
face is convex and fits against the diaphragm, to
which it is attached by a median vertical fold of
peritoneum, the suspensory ligament : its posterior
surface is concave, and lies against the stomach.
The large size of the liver is due to the need for
extensive stores of glycogen and to a massive
mechanism for regulating the quality of the blood
passed along the portal vein.

a. The lobes of the liver.

The liver is divided into five distinct lobes,
of which one is grooved for the gall-bladder.

b. The gall-bladder is an elongated thin-walled sac,

rather less than an inch in length, of a dark
green or brown colour, and lying in the groove
in the posterior surface of the right central lobe
of the liver.

c. The bile-duct is a narrow tube, about two inches

long, running backwards from the smaller and
dorsal end of the gall-bladder to open into the
dorsal surface of the duodenum, about a third
of an inch beyond the pylorus. It receives

344 DISSECTION OF THE RABBIT.

ducts from the several lobes of the liver, and
lies in the mesentery immediately to the right
of the large portal vein.

Open the duodenum opposite the opening of the bile-duct ;
wash out its contents, and find the aperture of the duct on a
small papilla. The bile-duct is most easily traced from the
duodenum forwards, and may with advantage be first injected
from a point about half an inch from the duodenum.

Unravel the coils of the intestine, freeing them from each
other by cutting through the mesentery along its line of attach-
ment to the intestine. Leave the duodenal loop and the rectum,
with their mesenteries, untouched. Carefully avoid injuring or
removing the cceliac and mesenteric ganglia and the associated
nerves (see p. 347).

Lay out the intestine on the dissecting -board, so as to show
the relations and proportions of its several parts. Avoid all
unnecessary injury to the blood-vessels, and ligature any that
bleed.

5. The small intestine, which is directly continuous with

the duodenum, is about seven or eight feet in
length, and of uniform diameter throughout.

a. Peyer's patches are slightly thickened oval spots,

granular in appearance, and about a third of an
inch in diameter, which occur at intervals along
the whole length of the small intestine, on the
side opposite to the attachment of the mesentery.

b. The sacculus rotundus is the dilated distal end of

the' small intestine, opening into the side of the
caecum about an inch from its proximal ends.
Its walls have the structure of Peyer's patches.

6. The caecum and vermiform appendix.

The caecum is a large thin-walled diverticulum
of the alimentary canal, at the junction of the small
intestine and colon. It is about twenty inches long
and an inch or more in diameter, and is marked
externally by a spiral constriction which runs

DIGESTIVE SYSTEM. 345

twenty to thirty times round it. The small intestine
opens into the side of the caecum about an inch
from its proximal end, a large Peyer's patch in the
wall of the caecum being continuous with the sacculus
rotundus.

Proximally, the caecum passes directly into the
colon, while distally it ends blindly in the thick-
walled finger-like vermiform appendix. This latter
is about four inches long, and its walls resemble
Peyer's patches in appearance and structure.

In the rabbit the caecum is of enormous size, as
in most herbivorous mammals with simple stomachs.

7. The colon is about a foot and a half in length. The

first part of it is very markedly sacculated, the
sacculations lying in three longitudinal rows,
separated by smooth areas of unequal width.
Towards the rectum the walls become smooth.

8. The rectum or terminal portion of the intestine is a

narrow tube, about two and a half feet in length,
pale in colour, and presenting a moniliform appear-
ance, owing to the contained faecal pellets. At its
hinder end it passes through the pelvic cavity to
the anus.

Note. — Numerous semi-transparent sacs usually occur,
attached to the peritoneum in the pelvic and gastric regions.
These are the cysts of a Tapeworm, and they develop to
maturity in a dog.

Internal Structure of the Alimentary Canal.

1. The stomach.

Slit up the stomach along its posterior surface : wash out
its contents, and examine it under water.

The wall of the stomach consists of an outer
peritoneal investment ; a middle muscular layer,
thickened at the pyloric end ; and an inner layer of

346 DISSECTION OF THE RABBIT.

• mucous membrane, which is raised into irregular
longitudinal folds. The pyloric orifice is narrowed
by a prominent muscular rim.

2. The small intestine.

Cut out a small piece from the wall, and examine its inner
surface under water.

The villi are minute tag-like processes of the
mucous membrane, covering its surface, and giving
it a velvety appearance.

Peyer's patches present a honeycombed appear-
ance, the depressions being occupied by smooth
convex lymph-follicles, and bordered by villi.

3. The caecum.

Remove the ccecum from the body in a sink, cutting across
the small intestine and the colon about an inch from it. Slit
it open longitudinally : wash out its contents thoroughly, and
examine under water.

The inner surface is papillose, and raised along
the line of the external constriction into a prominent
spiral fold a quarter of an inch or more in width.
The opening from the sacculus rotundus is small and
circular ; that into the colon is much larger.

Cut the vermiform appendix across to see the thickness
of its wall ; and slit it open to examine its inner surface, which
is similar to that of Peyer's patches, but with smaller follicles.

4. The colon.

Slit open part of the colon ; wash ; and examine under
water, noting the papillose surface and the sacculations of the
wall.

5. The rectum.

Examine a part of the rectum in a similar manner, and
note its smooth mucous membrane.

THORACIC VISCERA. 347

IV. THE ABDOMINAL SYMPATHETIC GANGLIA.

These are well seen in a fresh rabbit, especially in lean
specimens. There are two main ganglia, one, the c celiac,
in front of, and one, the anterior mesenteric, behind the
origin of the anterior mesenteric artery, at the level of the
left adrenal body. To see them, spread the viscera over to
the left side. A small additional ganglion occurs in relation
to the posterior mesenteric artery.

V. THE THORACIC VISCERA.

Open the thorax by a transverse incision just in front of
the diaphragm ; cut through all the ribs except the first along
each side, and remove the ventral wall of the thorax as a tri-
angular piece, taking great care not to injure the vessels and
other structures beneath it.

Identify and examine the following parts without any
further dissection :

1. The thymus is a pale soft glandular-looking body in the

anterior part of the thorax. It is very variable in
size, being larger in the young animal, and some-
times extending back so as to overlap the heart.

2. The heart, enclosed in the thin transparent pericardium,

lies in the middle of the thorax : it is dark in colour
and conical in form ; the apex being directed back-
wards and slightly to the left ; and the base, with
the roots of the great vessels, forwards.

3. The lungs are a pair of pink spongy bodies at the sides

of the thorax, and filling the greater part of its
cavity. Each lung collapses as soon as the side of
the thorax on which it lies is opened.

The lungs he quite free in the thoracic cavity,
except at their roots, where the blood-vessels and
bronchi enter them. The left lung is divided into
two lobes ; and the right into four, of which the
smallest and hindmost He across the median plane
behind the heart, and closely applied to the oesophagus.

348 DISSECTION OF THE RABBIT.

4. The diaphragm is a thin partition, separating the

thoracic cavity from the much larger abdominal
cavity. Its marginal portion is muscular, the fibres
arising from the inner surface of the hinder ribs and
from the vertebral column, and converging to be
inserted into the thinner tendinous central portion.
The muscular margin is specially developed in the
dorsal region, forming two stout pillars arising from
v the lumbar vertebrae.

The diaphragm is arched so as to present a very
convex surface towards the thorax : by contraction
of its muscular portion, in the act of inspiration, this
convexity is diminished and the thoracic cavity
thereby enlarged. The small size of the thoracic
relatively to the abdominal cavity should be noticed.

5. The phrenic nerves are a pair of slender white cords

lying between the heart and the lungs, and easily
seen on pressing these apart. Posteriorly they
divide into branches supplying the muscular portion
of the diaphragm (see fig. 65, p. 374).

Their origin from the fourth cervical nerves will
be seen in the dissection of the neck (p. 377).

6. The pleurse. Each side of the thorax is lined by a

glistening membrane, the pleura, which is reflected
over the lung at its root in the same way as the
peritoneum is reflected over the abdominal viscera.
The right and left pleural sacs are separated from
each dther by the mediastinal cavity, the greater
part of which is occupied by the heart and peri-
cardium, the posterior part remaining vacant.

During life the lungs are distended so as to
almost fill the thoracic cavity, the visceral layer of
the pleurae, covering the lungs, being closely applied
to the parietal layer lining the thorax, so that the
pleural cavities are practically obliterated.

Each pleura being an air-tight sac, the lungs
follow the movements of the ribs and diaphragm ;

HEART AND GREAT VESSELS. 349

so that enlargement of the thoracic cavity causes
expansion of the lungs, and hence inspiration.

7. The oesophagus is a narrow muscular tube, running

through the thorax immediately ventral to the ver-
tebral column, and readily seen on raising the left
lung.

8. The pneumogastric nerves. At the anterior end of the

thorax the pneumogastric nerves He parallel to the
phrenic nerves, and a little nearer the median plane.
Further back the pneumogastrics lie dorsal to the
heart and to the roots of the lungs, and close to the
vertebral column ; the left nerve lying on the oeso-
phagus, the right nerve a little to its right side.
The nerves are readily seen on turning aside the
lungs and heart.

9. The sympathetic ganglia he on each side of the aorta,

and are easily seen in a fresh rabbit as slight thick-
enings of a white thread that runs over the ribs.

VI. DISSECTION OF THE CIRCULATORY SYSTEM.
A. The Heart and the Roots of the Great Vessels.

Remove the thymus and the parietal layer of the pericardium,
and carefully clean the base of the heart and the roots of the great
vessels, avoiding injury to the nerves.

1. The heart. The ventricular portion of the heart, which
forms almost the whole of the exposed ventral sur-
face, is marked by a groove which runs from the
base obliquely backwards and to the right, indicat-
ing the division into right and left ventricles.

The auricles lie at the base of the heart, on its
dorsal aspect, the auricular appendices being the
only parts of them visible on the ventral surface.

a. The right ventricle forms the right side of the
ventricular portion, but does not reach to the
apex. It is soft to the touch.

B B

350 DISSECTION OF THE RABBIT.

b. The left ventricle forms the left side and apex of

the heart ; and is firmer than the right ventricle,
owing to the greater thickness of its walls.

c. The left auricle is anterior to the left ventricle, and

is largely concealed by the great vessels. It is
produced ventrally into the auricular appendix.

d. The right auricle is dorsal to the base of the right

ventricle, and is produced in front into the
auricular appendix.

2. The roots of the great vessels.

a. Opening into the right auricle.

i. The right anterior vena cava returns blood from
the right side of the head, neck, and thorax,
and from the right fore-limb. It enters the
thorax at its anterior end, and runs back
along the inner side of the right lung, to
open into the right auricle by an aperture
in its dorsal wall.

ii. The left anterior vena cava returns blood from
the left side of the head, neck, and thorax,
and from the left fore-limb. It runs back
along the inner side of the left lung, and
crosses between the heart and the root of the
left lung, to open into the left side of the
right auricle.

iii. The posterior vena cava returns blood from all
parts of the body behind the diaphragm. It
enters the thorax through an aperture in the
diaphragm and runs forwards, slightly to the
right of the median plane, to open into the
posterior end of the right auricle.

b. Arising from the right ventricle.

The pulmonary artery conveys the venous blood
from the heart to the lungs. It arises from
the anterior border of the right ventricle in

THE VEINS. 351

the mid-ventral line, and arches over to the
dorsal surface of the left auricle, where it
divides into the right and left branches, sup-
plying the two lungs.

c. Opening into the left auricle.

The pulmonary veins return the blood from the
lungs to the heart. There are two main
veins on each side, which converge to open
into the dorsal surface of the left auricle.

d. Arising from the left ventricle.

The aorta conveys arterial blood from the heart
to all parts of the body. It arises from the
middle of the base of the heart, dorsal to
the root of the pulmonary artery, runs for-
wards about half an inch, and then arches
over to the left side, and runs back along the
left side of the vertebral column. Its thoracic
portion is readily seen on turning aside the
left lung.

The ductus arteriosus is a ligamentous
band, connecting the aorta with the pulmonary
artery, just before the division of the latter
into right and left branches. During em-
bryonic life it forms a tubular connection
between the two vessels.

In cleaning the ductus arteriosus, take care not to injure
the left recurrent laryngeal nerve which loops round it (see
fig. 65, p. 374).

B. The Veins.

The veins form three distinct systems, communicating
with each other only through capillaries, and containing
different kinds of blood.

(1) The first of these systems consists of the three venae
cavse, with the veins opening into them : these bring to the
right auricle venous blood from the body generally, and from
the liver.

352 DISSECTION OF THE RABBIT.

(2) The second or portal system collects the blood from
the walls of the alimentary canal, and from- the pancreas and
spleen, and conveys it to the liver.

(3) The third or pulmonary system conveys arterial blood
from the lungs to the left auricle.

In dissecting the veins it is convenient to follow them from
the larger to the smaller trunks, and they will be described in
this order ; it must be remembered, however, that the flow of
blood is in the reverse direction. Care must be taken not to
cut the veins, especially at the anterior end of the thorax, as
the blood obscures the dissection, and the vessels themselves,
when empty, are difficult to follow, owing to the thinness of
their walls. If by chance a large vein is cut, it must be liga-
tured immediately, to stop the bleeding.

During the dissection of the veins take great care not to
damage or remove the arteries or nerves lying near them.

1. The system of the venae cavse.

a. The right anterior vena cava has already been

seen. It is formed by the union of the jugular

and subclavial veins, immediately in front of

the first rib.

' i. The azygos cardinal vein is a median vein which

runs forwards in the mid-dorsal line of the

thorax, lying close to the vertebral column

and along the right side of the aorta. It

receives branches from the posterior seven

or eight intercostal spaces of both sides of

the body. Opposite the level of the auricles

it passes round the right side of the oesophagus

and trachea, to open into the vena cava close

to the auricle.

ii. The right anterior intercostal vein is a small
vein returning blood from the anterior four
or five intercostal spaces of the right side,
and opening into the vena cava just in front
of the azygos vein.

THE VEINS. 353

iii. The right internal mammary vein runs for
wards on the inner surface of the ventral
thoracic wall, near the middle line, and opens
into the vena cava opposite the first rib.

iv. The right subclavian vein returns blood from the
fore-limb and shoulder.
In dissecting the next two veins, take great care to avoid
injury to the muscles, vessels, and nerves of the neck.

v. The right external jugular vein is a large vein
running along the side of the neck. It is
formed in front by the union, just behind the
angle of the jaw, of the anterior and posterior
facial veins, returning blood from the face
and ear.

vi. The right internal jugular vein is a small vein
returning blood from the brain, and running
down the neck alongside the trachea, to open
into the right external jugular vein close to
its union with the subclavian.

b. The left anterior vena cava resembles the right,

except that it receives no azygos vein, and that
its course in the thorax is somewhat different, as
already described (p. 350).

c. The posterior vena cava is a large median vein,

commencing at the hinder end of the abdomen,
and running forwards close to the vertebral
column and to the right (or left) of the aorta.
In front it leaves the abdominal wall and be-
comes imbedded in the dorsal surface of the
liver, beyond which it enters the thorax through
an aperture in the central tendon of the dia-
phragm, and runs forwards, as already described
(p. 350), to open into the right auricle.

The following are the principal veins opening
into the posterior vena cava :
In dissecting these veins, take care not to damage the
arteries, which accompany and roughly correspond to them,

354 DISSECTION OF THE RABBIT.

or the renal and reproductive organs or the nerves in the
mesentery.

i. The phrenic veins are small veins in the sub-
stance of the diaphragm, opening into the
vena cava as it passes through this.

ii. The hepatic veins are large vessels returning
blood from the several lobes of the liver, and
opening into the portion of the vena cava im-
bedded in the liver. There are four chief ones.

iii. The renal veins are a pair of large veins running
inwards from the kidneys to the vena cava,
the right and shorter one being about three-
quarters of an inch in front of the left.

Each renal vein receives a small vein from
the corresponding adrenal body, and a much
larger one from the dorsal wall of the abdomen.

iv. The spermatic veins in ths male [or ovarian
veins in the female] are a pair of small veins
returning blood from the testes [or ovaries],
and opening into the vena cava near the
hinder end of the abdomen. The left one
frequently opens into the renal vein, or the
ilio-lumbar vein.

v. The ilio-lumbar veins are paired, and return
blood from the hinder part of the abdominal
walls, opening into the vena cava about the
level of the anterior border of the thigh. The
left one often receives the spermatic [or ova-
rian] vein, and opens into the renal vein.

vi. The external iliac veins are a pair of large
veins returning blood from the hind-limbs,
and opening into the vena cava at the hinder
end of the abdomen, about an inch in front
of the pubic symphysis. They receive small
veins from the bladder [and in the female
from the uterus].

THE VELSS. 355

They are direct continuations of the
femoral veins, which lie along the inner or
preaxial borders of the thighs.

vii. The internal iliac veins return blood from the
backs of the thighs, and, running forwards
through the pelvic cavity, unite about half
an inch behind the external iliacs to form the
commencement of the posterior vena cava.

2. The portal system consists of the veins conveying blood
from the whole length of the alimentary canal
from the stomach backwards, and also from the
pancreas and spleen. These veins lie in the mesen-
tery, and converge to form the main trunk, or
portal vein : this lies close alongside the posterior
vena cava and, entering the liver, divides into
branches supplying its several lobes.
The portal system is best seen in a freshly hilled animal
on opening the abdomen and turning the viscera aside. The
veins are large and usually gorged with blood, and care must
be taken not to injure them, as they bleed very freely.

The following are the main trunks which unite
to form the portal vein.

i. The lieno-gastric vein returns blood from the
walls of the stomach and from the spleen.

ii. The duodenal vein returns blood from the duo-
denum and pancreas.

iii. The anterior mesenteric vein is formed by the
union of veins from the whole length of the
small intestine, and from the caecum, the colon,
and the greater part of the length of the
rectum.

iv. The posterior mesenteric vein lies in the meso-
rectum or hindmost part of the mesentery,
and returns blood from the terminal portion
of the rectum.

356 DISSECTION OF THE RABBIT.

3. The pulmonary system consists of the pulmonary veins,
which have already been described (p. 351).

In drawing the veins make a large sketch of the head, neck,
ribs, and abdomen. Then draw the veins in relation to them
and the chief organs. The portal vein had better be drawn
separately.

C. The Arteries.

The aTteries form two distinct systems, of which one, the
aortic system, conveys arterial blood from the left ventricle
to all parts of the body ; while the other, or pulmonary
system, carries venous blood from the right ventricle to the
lungs. The arteries are thicker-walled, and hence paler in
colour, than the veins.

The aortic system should be dissected first. Clean the
aorta along its whole length, and follow the arteries arising
from it to their distribution.

During the dissection of the arteries, take great care not to
damage or remove the nerves lying near them.

1. The aorta arises from the base of the left ventricle, and,

turning towards the left side to form the arch of
the aorta, runs backwards through the thorax and
abdomen. It lies on the ventral surface of the
vertebral column, and divides about an inch in
front of the level of the pubic symphysis into the
two" common iliac arteries.

2. Arteries arising from the aorta in the thorax.

a. The innominate artery arises from the commence-
ment of the arch of the aorta : it gives off, close
to its origin, the left carotid artery, and then
runs forwards a short distance and divides into
the right subclavian and right carotid arteries.
i. The right subclavian artery runs outwards just
in front of the first rib : it gives branches to
the shoulder, and runs on as the brachial artery
into the right fore-limb. Its principal branches
are the following :

THE ARTERIES. 357

a. The vertebral artery arises from the sub-
clavian just beyond its origin, and passing
dorsalwards enters the vertebrarterial canal
of the cervical vertebrae, along which it
passes, supplying the spinal .cord and the
brain.

/?. The internal mammary artery arises from the
subclavian just before it leaves the thorax,
and runs backwards along the inner surface
of the ventral wall of the thorax.

The next two arteries may be better seen during the dis-
section of the neck. If dissected now, the greatest care must
be taken not to injure the nerves (see fig. 65, p. 374).

ii. The right carotid artery runs forwards along-
side the trachea, supplying the neck. The
ramus descendens of the hypoglossal nerve
crosses it. Opposite the angle of the jaw, it
divides into (1) the internal carotid artery,
which enters the skull through the carotid
foramen to supply the brain ; and (2) the
external carotid artery, which supplies the
right side of the head and face.

iii. The left carotid artery corresponds to the right
carotid in its course and distribution.

b. The left subclavian artery arises from the left side

of the arch of the aorta : its course and branches
correspond to those of the right subclavian.

c. The intercostal arteries are a series of small paired

arteries, arising from the dorsal surface of the
aorta, and running outwards on the inner sur-
face of the thoracic walls, one behind each rib.

3. Arteries arising from the aorta in the abdomen.

In examining these arteries the mesentery must not be cut.
a. The cceliac artery is a large median vessel, arising
from the ventral surface of the aorta, about half

358 DISSECTION OF THE RABBIT.

an inch behind the diaphragm. It runs in the
mesentery, and divides into the hepatic artery
supplying the liver, and the lieno-gastric artery
supplying the stomach and the spleen.

b. The anterior mesenteric artery is a large median

vessel, arising from the aorta about three-
quarters of an inch behind the coeliac artery.
It divides into numerous branches, which run
in the mesentery to supply the duodenum,
pancreas, small intestine, caecum, and colon.

c. The renal arteries are paired, the right one arising

just behind the anterior mesenteric artery, and
the left a little further back. Each gives a
small branch to the muscles of the back, and
then runs outwards to the kidney.

d. The spermatic arteries, in the male, are a pair of

very small arteries, which arise about a couple
of inches behind the left renal artery, and run
backwards and outwards along the dorsal sur-
face of the abdomen to enter the epididymes at
their anterior ends.

The ovarian arteries, in the female, arise in
the same position as the spermatic, but run
outwards and slightly forwards to the ovaries.

e. The posterior mesenteric artery is a median vessel,

arising from the aorta about an inch in front of
its division into the common iliacs. It supplies
the hinder part of the rectum.

I. The lumbar arteries are small median arteries,
arising from the dorsal surface of the aorta, and
dividing into right and left branches supplying
the body-walls.

g. The median sacral artery is a small vessel arising
from the dorsal surface of the aorta, shortly
before its division, and running along the ventral
surface of the sacrum to the tail. It is to be re-
garded as the posterior continuation of the aorta.

THORACIC DUCT AND HEART. 359

h. The common iliac arteries, formed by the apparent
bifurcation of the aorta, run backwards and out-
wards to the hind-limbs, along which they are
continued as the femoral arteries. In the abdo-
men each gives off the following branches :

i. The ilio-lumbar artery runs outwards along
the posterior part of the dorsal wall of the
abdomen.

ii. The internal iliac artery runs backwards along
the dorsal wall of the pelvic cavity.

iii. The vesical artery is a small branch which runs
backwards to the bladder. In the female it
also supplies the uterus.

4. The pulmonary artery has already been described
(p. 350).
Follow the branches of the pulmonary artery into the lungs.
Draw the arteries, first sketching the outline and chief viscera.

D. The Thoracic Ducts. *

The thoracic ducts are a pair of thin-walled tubes running
forwards through the thorax, and lying in the fat to the
side of and dorsal to the aorta. Between the third arid
fourth ribs they expand somewhat, and then turning inwards
traverse the thymus and open into the anterior vena cava
close to the subclavian vein.

The thoracic ducts receive lymphatic vessels from almost
all parts of the body, and place these in communication
with the venous system.

E. Dissection of the Heart.

Do not remove the heart until after the dissection of the nerves
of the neck is completed (see pp. 373 and 375). Cut across the
great vessels, about half an inch from the heart. Remove the
heart, and pin it down under water with the dorsal surface
upwards, passing the pins through the ventricles and the arch
of the aorta. Clean the roots of the several vessels.

360 DISSECTION OF THE RABBIT.

] . The auricles.

Cut away the outer walls of both auricles so as to expose
the cavities. Wash out the contained blood.

a. The right auricle. The wall of the auricle is thin ;
that of the auricular appendix is thicker, and
is marked internally by muscular ridges.

i. The orifices of the venae cavse.

The right anterior vena cava opens into
the anterior part of the auricle.

The left anterior vena cava opens into the
left side of the posterior end of the auricle :
just before its opening it receives the
coronary vein, returning blood from the sub-
stance of the heart.

The posterior vena cava opens into the
dorsal surface of the auricle, in front of the
left anterior vena cava.

ii. The Eustachian valve is a membranous fold,
between the orifices of the left anterior vena
cava and of the posterior vena cava. During
foetal life it directs the blood from the pos-
terior vena cava through the foramen ovale
into the left auricle.

iii. The septum auricularum is a thin partition
between the right and left auricles. In it is
an oval patch, the fossa ovalis, close to the
opening of the posterior vena cava. This is
thinner than the rest of the septum, and
is perforated in the embryo by the foramen
ovale, which sometimes persists in the adult,
and is guarded by a membranous valve with
chordae tendineee. Even when the foramen is
closed, the notched edges of the valve, and
remnants of the chordse tendinese are often
recognisable.

iv. The right auriculo-ventricular aperture is a

THE HEART. 361

wide crescentic opening leading into the right
ventricle.

b. The left auricle. The wall of the auricle is thin :
that of the auricular appendix is thicker, and is
marked by a network of muscular ridges.

i. The apertures of the pulmonary veins. The

veins of the two sides unite as they enter the
dorsal surface of the auricle.

ii. The left auriculo-ventricular aperture is a large
circular funnel-like opening leading into the left
ventricle.

2. The ventricles.
Cut across the ventricles about one-third of an inch from
the apex ; note the shapes of the cavities.

The cavity of the right ventricle is crescentic in
section : its wall is comparatively thin, embracing
and half surrounding the left ventricle.

The left ventricle has very thick walls and a
small irregular cavity.

a. The right ventricle.

Remove the auricles, and cut short the aorta and pulmonary
artery. Remove the outer wall of the right ventricle, by longi-
tudinal incisions through its dorsal and ventral walls, and a
transverse incision across its base a short distance behind the
anterior border. Avoid injury to the valve.

i. The tricuspid valve guards the right auriculo-
ventricular aperture, and is formed of three
membranous flaps attached round its margin.
The borders of the flaps project into the
ventricle, and are connected by tendinous
strings, the chordae tendineae, to^muscular
processes of the wall of the ventricle, the
larger of which are known as musculi papil-
lares.

362 DISSECTION OF THE rabbit.

ii. The orifice of the pulmonary artery is at the
left anterior angle of the ventricle, and is
guarded by three pocket-like semilunar valves,
the free edges of which are directed away
from the ventricle.

Note the position of the valves from above ; slit up the
pulmonary artery between two of them, and examine them.

b. The left ventricle.

Remove the outer wall in the same manner as that of the
right ventricle.

i. The mitral valve, guarding the left auriculo-
ventricular aperture, resembles the tricuspid
valve, except that there are only two flaps,
and the musculi papillares are much larger.

ii. The orifice of the aorta is at the base of the
ventricle, immediately to the right of the
mitral valve. It is guarded by three semi-
lunar valves, one of which is dorsal, and the
other two ventro-lateral in position. Imme-
diately beyond the two latter valves the
coronary arteries, which supply the heart,
arise from the aorta.

The heart of a sheep may be dissected with advantage,
and it differs so little from that of the rabbit, that the fore-
going description applies, with very little modification, to it.
The chief differences are the union of the two anterior venae
cavae before entering the auricle, and the presence of a muscu-
lar ' moderator ' band crossing the cavity of the right ventricle
obliquely.

VII. DISSECTION OF THE RENAL AND REPRODUCTIVE

SYSTEMS.

In the rabbit, as in the dog-fish, certain parts of the primi-
tive kidneys and their ducts lose their excretory function, and

RENAL AND REPRODUCTIVE SYSTEMS. 363

become converted into accessory genital organs and genital
ducts. Owing to the close connection of the two systems,
they, may, in the adult, be conveniently considered together,
though it must be remembered that the connection is merely
a secondary one (c/. pp. 262-263).

In the male rabbit the pronephros is absent or abortive ;
the mesonephros, or Wolffian body, acquires an intimate
connection with the testis, and becomes converted into the
caput epididymis, the proximal part of the "Wolffian duct
becoming converted into the cauda epididymis, while its
distal portion becomes the vas deferens.

In the female, the pronephros is absent ; the Mullerian
duct becomes the oviduct ; the mesonephros or Wolffian
body and its duct are absent, or abortive.

In both sexes alike, the metanephros and its duct become
the kidney and ureter respectively.

The bladder of the rabbit arises as a ventral diverticulum
of the rectum, and is at first entirely independent of the
kidney ducts, which open into the dorsal wall of the rectum,
a condition retained throughout life in the frog. During
development the ducts shift round so as to open into the
bladder, which then separates from the rectum and acquires
a separate opening to the exterior, through which the urinary
and genital products leave the body.

It is convenient to describe the two sexes separately, as
they differ very greatly ; and to include in the account cer-
tain glands in the neighbourhood of the rectum.

A. The Male Rabbit.
1. The renal system.

a. The kidneys are a pair of dark-red bodies about an
inch and a quarter long, imbedded in fat in the
dorsal wall of the abdomen, outside the peri-
toneum. Each is of a flattened ovoid shape,
with a notch, the hilus, on its inner side where
the ureter and blood-vessels enter and leave it.

Bisect one of the kidneys, longitudinally, from its outer

364 DISSECTION OF THE RABBIT.

edge ; and turn the central half inwards, so as to expose the
cut surfaces.

i. The cortical substance forms the superficial
layer of the kidney ; the Malpighian bodies,
which are confined to it, give it a dotted
appearance.

ii. The medullary substance, forming the deeper
part of the kidney, is radially striated, and
consists mainly of the non-glandular parts of
the tubules. The inner surface of the medul-
lary substance projects as a conical process,
the pyramid, on which the tubules open.

lii. The pelvis is the expanded anterior end of the
ureter, which lies in the cavity of the kidney,
and receives the urine from the apertures on
the pyramid.

b. The ureters are a pair of slender tubes, running

back from the kidneys along the dorsal wall of
the abdomen, a short distance from the middle
line. At the hinder end of the abdomen they
turn inwards and open into the bladder, opposite
the anterior border of the pubic symphysis.

c. The bladder is a thin-walled muscular sac, of

which the anterior end, in front of the pubic
symphysis, can alone be seen at this stage of
the dissection.

2. The reproductive system.

The testes, in early life, he against the dorsal
wall of the abdomen, close to the kidneys. Before
maturity, however, they separate from the dorsal
wall, and pass through the inguinal canals into the
scrotal sacs

a. The scrotal sacs are a pair of muscular pouch-like
diverticula of the abdominal wall, lying ventral
to the pelvis and close to the median plane. Each

MALE REPRODUCTIVE SYSTEM. 365

scrotal sac communicates with the abdominal
cavity through the narrow inguinal canal.
Slit up one of the scrotal sacs along its ventral surface to
expose the testis lying in it. Lay open the inguinal canal.

b. The testes.

The testes are a pair of elongated ovoid
bodies, of a pink colour, varying greatly in size
at different times, and about an inch and a half
in length when fully developed. They are attached
to the hinder ends of the scrotal sacs, but can
easily be pulled back into the abdominal cavity

c. The epididymes are a pair of irregular masses of

convoluted tubes, lying along the inner edges
of the testes. Each epididymis consists of (1) a
soft pinkish body, the caput epididymis, in close
contact with the anterior end of the testis ; (2) a
narrow band, extending backwards from the caput
along the testis ; and (3) a larger, more conical,
and much-convoluted body, the cauda epididymis,
lying at the posterior end of the testis, and con-
nected by a short cord, the gubernaculum, with
the bottom of the scrotal sac.

The epididymes are the modified Wolffian
bodies of the embryo, the tubules of which grow
into the testes, and so form the vasa efferentia,
through which the spermatozoa escape from the
testes.

The spermatic arteries and veins enter and
leave the epididymes at their anterior ends.

d. The vasa deferentia are a pair of whitish tubes one

of which arises from each cauda epididymis and
runs forwards, alongside the testis, to enter the
abdominal cavity through the inguinal canal.
Within the abdomen it loops round the ureter
from the outer to the inner side, and runs back
to the neck of the bladder. The first part of

C 0

366 DISSECTION OF THE RABBIT.

its course is sinuous, but further forwards it
becomes straighter.

The vasa deferentia are the modified Wolffian
ducts of the embryo.

Dissect both scrotal sacs from the abdominal wall, and
turn them forwards with the testes and vasa deferentia. Deter-
mine the line of the pelvic symphysis and cut it through with
a strong knife placed exactly along the median line. Separate
the two halves. Note the corpora cavernosa, a pair of hard
ligamentous bodies running backwards from the hinder border
of the ischia to the penis ; and cut them across close to the ischia.

Inflate the bladder and urino-genital canal by means of
a blowpipe inserted into the terminal aperture of the penis,
and tie a ligature round the penis half an inch from its end,
to prevent the air escaping.

Clean the various structures in the order given below.

e. The urethra, or urino-genital canal, is the continua-

tion backwards of the neck of the bladder through
the pelvic cavity. It is a straight tube, with thin
and very vascular walls, and lies immediately
dorsal to the pelvic symphysis, and ventral to the
rectum. Behind the symphysis it is continued
into the penis.

f. The penis is an elongated body, projecting from the

ventral surface of the animal, immediately behind
the pelvic symphysis and in front of the anus.
It is traversed by the urethra, through which
the urinary and genital products are discharged.
The posterior wall of the penis is very vascular,
and forms the corpus spongiosum ; while its
anterior surface is stiffened by the corpora
cavernosa. The free end of the penis is soft and
obliquely truncated ; and is invested by a loose
sheath of skin, the prepuce. The external aper-
ture of the urethra is a slit-like opening on its
posterior surface.

MALE REPRODUCTIVE SYSTEM. 367

g. The uterus masculinus is a large median sac,
slightly bilobed in front, attached to the dorsal
surface of the urino-genital canal, into which it
opens, and extending forwards along the dorsal
surface of the bladder to about the middle of its
length.

Note. — The mature rat and cavy have large seminal vesicles
which are not present as such in the rabbit.

h. The posterior ends of the vasa deferentia are

slightly dilated, and run back, side by side, along
the dorsal surface of the neck of the bladder,
between it and the uterus masculinus. They
open separately into the ventral wall of the uterus
masculinus, just in front of its communication
with the urino-genital canal, and opposite the
hinder end of the obturator foramen.

Slit up the uterus masculinus along one side with scissors.
Note the large aperture leading into the urino-genital canal,
and, just in front of this, the small paired openings of the vasa
deferentia.

i. The prostate is a series of glandular masses, grouped
around the dorsal surface and sides of the uterus
masculinus and vasa deferentia, and opening by
short ducts into the urino-genital canal. There
are four or five lobes, the largest of which is an
anterior lobe, closely applied to the dorsal wall
of the uterus masculinus, and bilobed in front :
behind this is a smaller posterior lobe, sometimes
divided into two : a pair of still smaller lateral
lobes lie in the angle between the vasa deferentia
and the uterus masculinus.

k. Cowper's glands are a pair of glandular bodies,
varying greatly in size, and placed on the dor-
sal wall of the urino-genital canal, behind the
prostate.

368 DISSECTION OF THE RABBIT.

1. The perineal glands are a pair of elongated glan-
dular bodies, dark in colour, and about three-
quarters of an inch long. They He at the sides
of the penis, and in the anterior walls of the
perineal sacs, into which their ducts open.

m. The rectal glands are a pair of pale yellowish
bodies, much larger than the perinseal glands,
and lying along the sides and posterior surface
of the last two inches of the rectum.

Slit up with scissors the urino-genital canal along its whole
length, in the mid-ventral line.

n. The aperture of the uterus masculinus is a large
crescentic opening in the dorsal wall of the
urino-genital canal.

o. The verumontanum is a small vascular papilla,
immediately behind the aperture of the uterus
masculinus.

p. The apertures of the ducts of the prostatic glands

are small slit-like openings at the sides of the
verumontanum.

B. The Female Rabbit.

1. The urinary system is the same as in the male, except

that the ureters open into the bladder much further
forwards, close to its anterior end.

2. The reproductive system.

Divide the pelvic symphysis as in the male (p. 366) ; and
inflate the vagina and bladder with a blowpipe, inserted into
the vulva. Ligature the vestibule about an inch in front of
the vulva to prevent the air escaping.

a. The ovaries are a pair of oval bodies, of a pale
yellowish colour, and about three-quarters of an
inch long, attached to the dorsal wall of the
abdomen, behind the kidneys and about an inch
in front of the crests of the ilia.

FEMALE REPRODUCTIVE SYSTEM. 369

On their surfaces the Graafian follicles, each
of which contains an ovum, are visible as small
rounded semi-transparent projections.

b. The oviducts are formed from the Miillerian ducts

of the embryo. Their anterior ends are nar-
row, and form the Fallopian tubes : the middle
portions are wider, and become the uteri, within
which the young are developed ; and the posterior
ends unite to form a median tube, the vagina.

i. The Fallopian tubes are a pair of narrow slightly
convoluted tubes, about the size of ureters.
They he along the free edges of the broad
ligaments, a pair of folds of peritoneum sus-
pending them from the dorsal wall of the
abdomen.

The anterior ends of the Fallopian tubes
form wide membranous funnel-shaped mouths
lying along the outer sides of the ovaries,
and attached to their anterior ends.

ii. The uteri, which are continuous with the pos-
terior ends of the Fallopian tubes, are a pair
of thick-walled tubes which vary enormously
in size according to the presence or absence
of embryos, and the stage of their develop-
ment. The two uteri open by separate aper-
tures into the anterior end of the vagina.

iii. The vagina is a very wide median tube, com-
mencing a little in front of the crests of the
ilia, and extending straight back to unite,
within the pelvic cavity, with the neck of
the bladder, at the commencement of the
vestibule.

c. The vestibule or urino-genital canal is a wide median

tube, commencing opposite the hinder end of
the obturator foramen, and running back ventral
to the rectum through the pelvic cavity, to the

370 DISSECTION OF THE RABBIT.

vulva. Its walls are very vascular, and corre-
spond to the corpus spongiosum in the male.

d. The clitoris is a small rod-like body, corresponding

to the penis of the male, and lying in the anterior
or ventral wall of the vestibule, just within its
external opening. In the rat it is traversed
by the urethra. It ends in a soft conical body,
the glans clitoridis, and is connected with the
ischia by two corpora cavernosa similar to those
of the male, but smaller.

e. Cowper's glands are reduced or absent.

f. The perinseal and rectal glands are similar to those

of the male (p. 368).

Slit up with scissors the vestibule and the hinder part of
the vagina along one side. Note the great vascularity of the
walls, the position and relations of the clitoris, and the opening
of the bladder into the vestibule.

Slit up the anterior part of the vagina along the mid-ventral
line, to expose the openings of the uteri into the vagina. Note
the very prominent lips of these openings.

VIII. DISSECTION OF THE NECK.

A special dissection of the neck is desirable, on account of
the great importance of the structures contained in it, more
particularly the nerves (see fig. 65, p. 374).

Lay the rabbit on its back on a dissecting -board ; and pin
back the head so as to extend the neck fully. Make a median
ventral incision through the skin of the neck, and dissect the
skin away from the underlying parts, taking care not to
damage the muscles. Keep the dissection moist.

A. The Hyoid Bone. . .

The body of the hyoid is a transverse bar of bone, deeply
placed between the rami of the mandible, and about midway
between the hinder end of the mandibular symphysis and
the angles of the jaw.

ORGANS OF THE NECK. 371

The posterior cornua of the hyoid are a pair of slender
rods of bone, about half an inch long, running backwards
and outwards from the outer angles of the body of the hyoid,
parallel to the angles of the jaw.

Cut up in the middle line between the halves of the mandible.
Notice the paired (submaxillary) glands and press them slightly
outwards. Between them is the triangular mass of muscle in
which the hyoid lies imbedded, its posterior cornua conspicuous
by its iridescence. Just to the outer side of these lies the hypoglossal

B. The Blood-vessels of the Neck.

The two most important, on each side, are the following :

1. The external jugular vein has already been seen (p. 353).

2. The carotid artery runs forward along the outer border

of the trachea (see p. 357).

Clean the carotid artery carefully, taking great care to
avoid injuring the nerves which lie close alongside it, and a
nerve, the ramus descendens of the hypoglossal, which crosses
its ventral surface about the level of the thyroid cartilage.

C. The Trachea.

The trachea, or windpipe, is a straight tube running
down the neck almost in the middle line. It is only partially
exposed at present, but will be seen better in the course of
the dissection. Its anterior end, the larynx, which lies
almost immediately behind the hyoid, is dilated, and sur-
rounded on its ventral and lateral surfaces by the wide
thyroid cartilage, which forms a prominent median swelling
in the throat, between the rami of the mandible. Behind
this is the annular cricoid cartilage, which is wide dorsally
and narrow ventrally. Behind the cricoid the trachea narrows
slightly, and is strengthened by a series of cartilaginous rings,
incomplete along the mid-dorsal line. The trachea runs
backwards along the neck, and, entering the thorax, divides

372 DISSECTION OF THE RABBIT.

about the level of the middle of the heart into the right and
left bronchi, which enter the right and left lungs respectively.

D. The Thyroid Body.

The thyroid is a soft vascular body, consisting of a pair
of lateral lobes at the sides of the anterior part of the trachea,
connected by a narrow median lobe, which runs across the
ventral surface of the trachea a short way behind the thyroid
cartilage. The size of the thyroid varies, increasing with the
age and maturity of the rabbit.

E. The Nerves of the Neck.

Stretch the carotid artery on one side, and spread out the
surrounding connective tissue. Note the large nerve running
on its outer side — the vagus or pneumogastric nerve. In fol-
lowing the nerves for the first time, this nerve, the hypoglossal,
and sympathetic are alone of importance. A lymphatic gland
and duct lie close to the origin of the nerves of the neck, and
should not be mistaken for a nerve and its ganglion.

1. The spinal accessory nerve, the eleventh cranial nerve, leaves the
skull by the foramen lacerum posterius, between the auditory
capsule and the ex-occipital bone, in company with the
pneumogastric and glosso-pharyngeal nerves. Outside the
skull it runs almost vertically downwards for about half an
inch, and then divides into branches distributed to the
sterno-mastoid and other muscles.

The sterno-mastoid muscle runs from the front end of the
sternum to the mastoid process of the skull behind the ears.
Turn it back and follow the branches of the nerve to the base
of the skull.

2. The hypoglossal nerve, the twelfth cranial, and the
motor nerve of the tongue, is a stout nerve which
leaves the skull by the condylar foramina, im-
mediately behind the foramen lacerum posterius.
Outside the skull it runs downwards and inwards
for about half an inch, and then turns forwards,
crossing the internal carotid artery near the point
of division of the carotid into internal and external

NERVES OF THE NECK. 373

carotids. It then runs forwards and inwards,
parallel to the posterior cornu of the hyoid, to the
base of the tongue, in the muscles of which it ends.

The hypoglossal nerve is readily found just outside the
posterior cornu of the hyoid ; it should be followed from this
point backwards to the skull, and forwards to the tongue.

The ramus descendens is a branch of the hypoglossal, which
arises from the main nerve just before it reaches the
internal carotid artery. It runs backwards along the
outer side of the carotid for about half an inch. It then
crosses the carotid, about the level of the hinder border
of the thyroid cartilage, and runs backwards, ending in
muscles of the hyoid and thyroid regions. It receives
fibres from the first and second spinal nerves.

The ramus descendens is easily found crossing the carotid
about the point named above, and should be traced backwards
along the sterno-thyroid muscle, and forwards to the point at
which it leaves the main stem of the hypoglossal.

3. The pneumogastric or vagus nerve, the tenth cranial*
is a stout nerve which leaves the skull by the fora-
men lacerum posterius, presenting a very distinct
ganglionic swelling near its origin. It runs down-
wards and backwards until it reaches the carotid
artery, and then straight backwards along the neck,
lying immediately to the outer side of, and slightly
dorsal to, the carotid artery. Entering the thorax,
it runs alongside the oesophagus to the stomach,
where it ends.

Its principal branches are the following :

Find the pneumogastric nerve in the neck, along the outer
■ side of the carotid artery. Follow it backwards along the neck
to the thorax, and forwards to the skull.

a. The anterior laryngeal nerve is a small nerve, arising from
the pneumogastric opposite the upper border of the
thyroid cartilage. It rims almost directly inwards, pass-
ing dorsal to the carotid artery, and ends in branches
distributed to the mucous membrane of the larynx, and
to the crico-thyroid muscle.

374

DISSECTION OF THE RABBIT.

NERVES OF THE NECK. * 375

The sole difficulty in dissecting the anterior laryngeal nerve
lies in the possibility of confusing it with the ramus descendens
of the hypoglossal. This is avoided if it be remembered that,
while both nerves cross the carotid artery, the ramus descendens
lies ventral to the artery, and the anterior laryngeal, dorsal
to it.

The depressor nerve is a very slender branch of
the anterior laryngeal nerve, which runs
backwards along the neck : it lies dorsal to
the carotid artery, and along the inner side
of the main stem of the pneumogastric nerve,
very close to the sympathetic nerve-trunk.
It ends in the heart.

Find the depressor nerve where it leaves the anterior laryngeal
nerve, about the level of the posterior end of the thyroid cartilage,
and follow it backwards along the neck. Its dissection in the
thorax is difficult.

b. The right posterior or recurrent laryngeal nerve
arises from the pneumogastric nerve at the
posterior end of the neck. It loops round the
subclavian artery, and then runs forwards along

Fig. 65. — Lepus cuniculus. Dissection to show the nerves of the
neck. The hinder part of the trachea is pushed slightly to the right
side, and the thyroid body is removed. Two-thirds natural size,
(c. H. H.)

AC> right carotid artery. AD, ductus arteriosus, with loop of left recur -
reut laryngeal nerve passing behind it. AO, arch of aorta. AP, root of pul-
monary artery. AC, right subclavian artery, with loop of right recurrent
laryugeal nerve passing behind it.

MA, mandibular muscle. MD, stylo-hyoid muscle. MH, sterno-hyoid
muscle ; the sterno-thyroid is seen at the hinder end, where both muscles are
cut acro3s. MM, sterno-mastoid muscle. MV, posterior cornu of hyoid. MW,
clavicle.

NG, glosso-pharyngeal nerve, p, anterior and posterior portions of right
pneumogastric nerve. The middle portion has been removed. PA, left pneu-
mogastric nerve. PB, ganglion of the pneumogastric nerve. PG, anterior
laryngeal nerve. PD, depressor nerve. PR, PS, right and left recurrent
laryngeal nerves. Q, spinal accessory nerve. R, hypoglossal nerve. RD,
descending ramus of the hypoglossal : its connections with the first and second
spinal nerves are not shown. SB, great auricular nerve arising from the third
cervical spinal nerve. SC, SD, SE, fourth, fifth, and eighth cervical spinal
nerves. SH, SK, right and left phrenic nerves : the anterior portion of the left
one has been removed. SL, cervical sympathetic nerve. SM, anterior cervical
ganglion. SN, middle cervical ganglion.

TM, submaxillary gland. TN, Wharton's duct. TP, oesophagus. TR,
trachea. TV, lung. TW, diaphragm. TZ, ear, cut off short. VL, VR, left
and right anterior venae cavse.

376 DISSECTION OF THE RABBIT.

the neck, lying alongside the trachea, and
dorsal to the carotid artery, or slightly to its
inner side. It supplies almost all the muscles
of the larynx.

On the left side, the nerve is not given off
until after the pneumogastric has entered the
thorax. It loops round the ductus arteriosus,
and then runs forwards along the neck to the
larynx (see fig. 65).

Find the nerve, as it loops round the subclavian artery, or
on the left side the ductus arteriosus and arch of the aorta ;
and follow it forwards along the neck to the larynx.

The curious course of the recurrent laryngeal
nerves is explained by the shifting backwards
of the heart which occurs during development.
As this shifting involves the roots of the great
vessels, these nerves, which are originally straight,
become pulled out into loops.

c. Branches of the pneumogastric to the heart, lungs,
and oesophagus. These are of small size, and
are given off in the thorax : the cardiac branches
end in a plexus, lying between the roots of the
aorta and pulmonary artery.

Turn the lungs and heart over to one side : follow the
pneumogastric backwards through the thorax, alongside the
cesophagus, and trace out its branches.

4. The cervical sympathetic nerve lies alongside the
trachea, to the inner side of the carotid artery,
and close to the pneumogastric and depressor
nerves.

About the level of the angle of the jaw it
has a small oval swelling, the anterior cervical
ganglion, and at the posterior end of the neck,
a short distance in front of the subclavian
artery, there is a similar swelling, the middle
cervical ganglion. In the thorax this sympa-

SPINAL NERVES. 377

thetic nerve is connected With a chain of ganglia
and connectives which cross the heads of the
ribs.

5. The phrenic nerve is a branch of the fourth cervical
nerve, which arises just beyond the point of emer-
gence of this nerve from the neural canal, and runs
back alongside the vertebral column. Entering the
thorax it continues its course backwards, lying
between the heart and the lungs, and ends in the
diaphragm, which it supplies with motor fibres.

In the neck it crosses the roots of the hinder
cervical nerves almost at right angles. It receives
a small branch from the fifth cervical nerve, and
sometimes one from the sixth, as it passes them.

To expose the phrenic nerve gently press the heart and
lung apart. Follow the nerve back to the diaphragm, and for-
wards along the neck to its origin.

F. The Spinal Nerves.

The spinal nerves arise in pairs from the sides of the
spinal cord. There are eight cervical, twelve thoracic, seven
lumbar, four sacral, and six coccygeal. Each spinal nerve
arises from two roots, and divides into three branches. The
two roots are (a) a dorsal or sensory and ganglionated one,
and (Jb) a ventral or motor. The nerve so formed passes out
of the spinal canal by the intervertebral foramen. Its three
branches are : —

a. The sympathetic nerve, which, together with its

fellows, forms a longitudinal ganglionated trunk
lying under the vertebral column and supplying
the vessels and viscera.

b. The dorsal branch, which supplies the muscles and

skin of the mid-dorsal region.

c. The ventral branch, which supplies the lateral and

ventral muscles and most of the skin.
The chief portions of the sympathetic nervous system have

378 DISSECTION OF THE RABBIT.

already been seen (pp. 347, 376). The dorsal branches of the
spinal nerves are easily found, especially in a fresh rabbit
The ventral branches are seen traversing the intercostal muscles,
and forming the brachial and the pelvic plexus, which can be
dissected both down the limb and back to their origin.

G. The Larynx.

The larynx, or organ of voice, is the anterior part of the
trachea. The thyroid and cricoid cartilages form its outer
wall,, and it is protected in front by the epiglottis.

Make a medium longitudinal section through the anterior
part of the trachea.

1. The epiglottis is a curved plate of cartilage covered by
mucous membrane attached to the anterior and
ventral border of the larynx. When food is being
swallowed it closes over the glottis like a lid, pre-
venting the passage of the food into the larynx.

. 2. The thyroid cartilage forms the most anterior ring of
the trachea : it is about half an inch in length along
the mid-ventral line, but is incomplete dorsally.

3. The cricoid cartilage, or second ring of the trachea, is a

complete ring, lying immediately behind the thyroid.
It is narrow ventrally, but wide dorsally ; and
articulates with the postero -dorsal angles of the
thyroid cartilage.

4. The arytenoid cartilages are a pair of cartilaginous

nodules, articulated to the anterior edge of the
dorsal surface of the cricoid cartilage.

5. The vocal cords are a pair of folds of mucous mem-

brane, strengthened by elastic ligaments. They are
attached dorsally to the arytenoids, and at their
ventral ends to the thyroid cartilage. It is by
their vibration that the ' voice ' is produced.

6. The glottis is the median slit between the vocal cords,

through which the air passes to and from the lungs.

THE HEAD. 379

IX. DISSECTION OF THE HEAD.

Skin the head. Open the jaws, examine the mouth (pp. 379-
380), and with bone forceps inserted between the lower incisors
separate the halves of the lower jaw. With strong scissors
cut close to the left mandibular bone, follow its' course, and
remove it. Insert the bone forceps between the two nasal
bones and remove the left one ; then between the front incisors,
so as to divide the halves of the upper jaw. With strong scissors
cut through the soft palate and nasal cavity a trifle to the left
of the middle line. Cut through the hard palate, ethmoid, &c,
with bone forceps. In this way the left half of the skull may
be removed.

Notice the cartilaginous mesethmoid plate separating the
right from the left nasal cavity. Carefully remove it from
before backwards, noticing and retaining the vascular organ of
Jacobson attached to its ventral surface in the first part of its
course.

Sponge the turbinal bones that now lie exposed. With
scissors carefully expose the nasal passage and follow it back-
wards to the pharynx. Scrape the tongue clean, lay open the
pharynx, the first part of the trachea, and the oesophagus.
Make a drawing of this lateral dissection at least twice the
natural size.

1. The roof of the mouth.

a. The palate, or median part of the roof, is long and

narrow, and separates the buccal from the nasal
cavity. Its anterior half, the hard palate, is
raised into hard transverse ridges, against which
the tongue works ; the posterior half, or the soft
palate, is smooth and soft, and ends behind in a
free notched border, nearly oppo iie the angle of
the jaw.

b. The upper teeth, comprising the incisors at the

front of the palate, and the grinders at the sides
opposite the junction of hard and soft palates,
have already been described (p. 322).

380 DISSECTION OF THE RABBIT.

c. The naso-palatine canals, which connect the nasal

and buccal cavities, open in a pair of grooves
running backwards from the small posterior in-
cisors, and about an eighth of an inch behind
these.

d. The tonsils are a pair of small pits at the sides of

the posterior part of the soft palate, near its
hinder border. The outer wall of each is much
thickened, forming a papilla which nearly closes
the opening.

2. The floor of the mouth.

a. The tongue is an elongated muscular mass, attached

along the greater part of its length to the floor of
the mouth, and produced in front into a free
flattened tip.

The surface of its hinder part is hard and
slightly elevated ; that of the anterior part is
softer and dotted over with small white taste
papillae, which are especially numerous at and
under the tip.

At the sides of the tongue, opposite the last
molar teeth, are a pair of oval patches, the
papillae foliatse, the surfaces of which are crossed
obliquely by close-set parallel ridges, in which
are imbedded numerous taste-bulbs.

About a quarter of an inch above and behind
the papillae foliatse are a pair of circumvallate
papillae, which are small and white, and sur-
rounded by circular grooves.

b. The lower teeth, already described with the skeleton

(p. 323), are placed opposite the corresponding
teeth of the upper jaw, and bite against them.

3. The sides of the mouth are smooth and soft, with the

exception of a hairy patch on each side, between the
incisor and molar teeth.

THE HEAD. 381

4. The pharynx is a continuation backwards of the buccal

cavity, beyond the margin of the soft palate. The
mouth and the posterior narial chamber open
into it in front ; behind it is continued into the
oesophagus ; and ventrally it communicates through
the glottis and trachea with the lungs.

a. The glottis is a large opening in the floor of the

pharynx, a short way behind the tongue, leading
into the trachea or windpipe.

b. The epiglottis is a thin bilobed cartilaginous flap,

forming the anterior boundary of the glottis,
and projecting into the pharynx behind the
tongue, opposite the free edge of the soft palate.
During the act of swallowing it is folded back
over the glottis, so as to close its entrance.

5. The posterior nasal chamber lies above the soft palate,

which separates it from the buccal cavity.

a. The orifices of the Eustachian tubes are a pair of

oval apertures at the sides of the roof of the
chamber, about the middle of its length.

The Eustachian tubes connect the tympanic
cavities with the posterior nasal chamber. The
tympano-Eustachian passage is the modified
hyomandibular pouch of the embryo, corre-
sponding to the spiracle of the dog-fish. Its
connection with the ear is purely secondary, the
passage serving to keep the pressures of air on
the two sides of the tympanic membrane equal.

Pass a seeker backwards and outwards, along one of the
Eustachian tubes, into the tympanic cavity (cf. fig. 66).

b. The organs of Jacobson are a pair of small tubular

bodies enclosed in the palatal processes of the
premaxillse, and lying in the floor of the anterior
part of the nasal cavities. Each communicates

DD

382^

DISSECTION OF THE RABBIT.

Fig. 66. — Lepus cuniculus. Diagrammatic section across the head to
show the relations of the internal ear, tympanic cavity and mem-
brane, and the auditory ossicles. The section is drawn as seen from
the front, and is taken along a line joining the reference letters
SO and MN in fig. 61, p. 303. The external ears are cut short,
and the floccular lobes of the cerebellum, which lie between the
three semicircular canals on each side, are omitted entirely, (a. m. m.)

B, buccal cavity. BO, basi-occipital. c, cochlea. CA, right external carotid
artery. CE, cerebellum. E, external ear or pinna. EM, external auditory meatus.
ET, Eustachian tube. H, body of the hyoid. LA, right lingual artery. M,
malleus : to its inner side are seen the incus and stapes. MN, mandible. MO,
medulla oblongata. N, posterior nasal chamber. P, soft palate. PO, periotic.
S, post-tympanic process of squamosal. SO, anterior vertical semicircular canal.
SO. supra-occipital. T, tympanic bone. TC, tympanic cavity. TM. tympanic
membrane.

THE HEAD. 383

in front with the nostril, and through the naso-
palatine canal with the mouth.

Make a drawing of the dissection to show the farts exposed
in this vertical section of the head.

6. The salivary glands are large and paired.
Dissect from the right side.

a. The parotid gland, the largest of the salivary glands, is a soft

pinkish mass lying in front and below the external audi-
tory meatus, between this and the hinder edge of the
mandible. It is very variable in size, and may extend
beyond the angle of the jaw and some distance forwards
along its inner surface.

The duct of the parotid, or Stenonion duct, leaves the
gland at its anterior border, and runs forwards just
beneath the skin, parallel to and about a quarter of an
inch below the zygomatic arch. It opens into the mouth
by a small aperture on the inner side of the cheek, opposite
the second upper premolar tooth.

b. The infra-orbital gland is an irregular lobulated mass, about

three-quarters of an inch long, lying below and in front
of the eye, partly above the zygomatic arch and partly
hidden by it.

The duct runs doAvnwards to open into the *nouth,
close to the Stenonian duct.

c. The sub-maxillary gland is a compact reddish ovoidal mass,

lying close to its fellow between the angles of the man-
dible, and in front of the larynx (fig. 65, TM).

The duct, Wharton's duct, leaves the outer side of the
gland near its hinder end, and runs forwards along the
inner side of the jaw to open into the floor of the mouth
midway between the lower incisors and the root of the
tongue, the openings of the ducts of the two sides being
about an eighth of an inch apart.

Dissect from the ventral surface. To follow the duct into
the mouth, turn the halves of the lower jaw aside.

d. The sublingual gland is an elongated flattened reddish body,
about three-quarters of an inch long, lying along the
inner side of the ramus of the mandible, between this and
Wharton's duct.

The ducts open separately on the floor of the mouth.

384 DISSECTION OF THE RABBIT.

X. DISSECTION OF THE BRAIN.

The brain should be removed directly after the rabbit is
killed,' and placed at once in strong spirit or the mixture given
below, as it decomposes rapidly. It is well to put a loose pad
of cotton wool in the bottom of the bottle, and place the brain
upon it, in order to prevent flattening from contact with the
glass, and to ensure free access of the spirit to all parts of the
surface ; the brain should be turned over after it has been a
few hours in spirit.

To remove the brain, first shin the head ; then expose the
brain by snipping away, with the bone forceps, the roof and
sides of the skull, beginning at the foramen magnum, and
working forwards. Take special care in the region of the ears
to avoid injury to the fioccular lobes of the cerebellum, which
are lodged in the fioccular fossae of the periotic bones (p. 311).

Having completely removed the roof and sides of the cranium
and cut through the dura mater, a tough fibrous membrane
which lines the cranial cavity, cut across the spinal cord about
half an inch behind the foramen magnum, and turn out the
brain from behind forwards, lifting it up gently with the handle
of a scqlpel.

Note and identify the several nerve-roots as you do so, and
cut through them in order, close to the skull-wall, so as to leave
as much of the roots as possible attached to the brain.

Place the brain in spirit or a mixture of equal parts of one
per cent, potass, bichromate and eight per cent, formalin at
once, handling it with great care, as it is very soft and easily
damaged. Leave it in spirit for three or four days, or in the
mixture three weeks ; after which wash in water for a day, and
then dissect it as described below.

The several divisions of the rabbit's brain are the same
as those of the brain of the dog-fish, the most important
differences between the two being (1) the great size of the
cerebral hemispheres, which overlap and conceal the dorsal
and lateral surfaces of the thalamencephalon and optic lobes ;
(2) the greater size and more complicated structure of the
cerebellum ; (3) the much greater development of the trans-

THE BRAIN. 385

verse commissures connecting the two halves of the brain
with each other.

Remove the hardened brain from spirit and examine it in
a small dish of water or weak spirit.

A. The Brain-membranes.

1. The dura mater is the tough connective-tissue membrane

which lines the cranial cavity. The greater part of
this membrane is left in the skull on removal of the
brain. It projects into the cranial cavity as a
median vertical fold, the falx cerebri, which lies
between the hemispheres ; and a transverse fold,
the tentorium, which separates the hemispheres*
from the cerebellum.

2. The pia mater is a much thinner and very vascular

layer of connective tissue, which closely invests the
brain. The blood-vessels running to and from the
brain he in it.

B. External Characters of the Brain.

Strip off the pia mater from the brain with fine forceps as
completely as possible, taking great care on the base of the brain
not to drag away the nerve-roots.

1. The dorsal surface of the brain.

a. The cerebral hemispheres, which form the anterior
two-thirds of the brain, are a pair of broad
triangular bodies, pointed in front and closely
apposed to each other along the median plane.

Their surfaces are smooth and convex : they
are marked by a few shallow grooves, or sulci ;
and are divided by rather more conspicuous
grooves at their outer edges into anterior or
frontal, and posterior or parietal lobes.

The two hemispheres are connected with
each other by a large transverse commissure,
the corpus callosum, which is asily seen on

386

DISSECTION OF THE RABBIT.

gently separating the hemispheres with the
handle of a scalpel.

b. The olfactory lobes are a pair of large club-shaped

bodies, projecting forwards from beneath the
anterior ends of the hemispheres.

c. The thalamencephalon is covered by the hemi-

spheres, and cannot be seen from the dorsal
surface.

d. The corpora quadrigemina, or optic lobes, are two

pairs of rounded elevations on the dorsal surface
P op

Fig. 67. — Lepus cuniculus. The brain dissected from the right side,
x 2. (c. h. H.)

C, cerebellum. CC, corpus callosum. CP, choroid plexus. CS, corpus
striatum. H, hippocampus major. HA, frontal lobe of right hemisphere.
HL, left hemisphere. M, medulla oblongata. O, olfactory lobe. OP, anterior
optic lobe, p, pineal body. PV, pons Varolii.

II, root of optic nerve. V, root of trigeminal nerve. VI, root of abducent
nerve. VIM, root of auditory nerve ; immediately in front of it is seen the
root of the facial nerve. XI, spinal accessory nerve.

of the brain, and almost completely covered by
the hemispheres.

e. The cerebellum is a large transversely elongated
mass, lying behind the hemispheres. It is
divided into a large median lobe, the vermis,
and two lateral lobes, to the outer sides of
which are the small rounded floccular lobes.

THE BRAINi 387

The whole surface of the cerebellum is
marked by close-set folds, mainly transverse in
direction.

f. The medulla oblongata is the part of the brain

beneath and behind the cerebellum^ It is some-
what flattened dorso-ventrally, is widest in front,
and gradually narrows behind, passing into the
spinal cord.

i. The velum medullse anterius, or valve of
Vieussens, is a thin transparent membrane
connecting the hinder border of the optic
lobes with the cerebellum, and roofing over
the anterior part of the fourth ventricle. It
lies beneath the anterior part of the cere-
bellum, which must be gently pressed back-
wards to expose it.

ii. The velum medullse posterius is a thin trans-
parent membrane forming the roof of the
posterior part of the fourth ventricle : it is
covered in front by the cerebellum.

g. The pineal body is a small rounded median body

lying on the optic lobes, and connected by a
stalk with the roof of the thalamencephalon.

It is visible on the dorsal surface, in the
angle between the hinder ends of the cerebral
hemispheres and the cerebellum, but is very
liable to be torn away with the dura mater. It
has been shown to be a degenerate eye-like organ.

The ventral surface of the brain.

a. The cerebral hemispheres, as seen from below, are
closely apposed in front, but diverge posteriorly.
About the middle of its length the inner
border of each hemisphere presents a notch,
which is continued outwards across the surface
of the hemisphere as a shallow groove, the

388 DISSECTION OF THE RABBIT.

Sylvian fissure, separating the frontal lobe from
the temporal.

b. The olfactory lobes lie, in their hinder portions,

along the under surface of the frontal lobes of
the hemispheres, extending back as far as the
Sylvian fissures. In front, they project some
distance beyond the hemispheres.

c. The infundibulum is a median rounded elevation

lying between the temporal lobes, about the
middle of the length of the brain : at its apex
is the rounded vascular pituitary body. This
latter- is usually left behind in the pituitary fossa
in the base of the skull when the brain is re-
moved, and a slit-like aperture is then left at
the apex of the infundibulum, leading into the
third ventricle.

d. The optic chiasma is a transverse band of nerve-

fibres lying immediately in front of the infundi-
bulum. From its anterior border the optic
nerves arise, and from its outer ends the optic
tracts may be followed backwards towards the
optic lobes by gently pressing the temporal lobes
aside.

e. The corpus mammillare, or corpus albicans, is a

small rounded median elevation immediately
behind the infundibulum.

f. The pons Varolii is a stout band of transverse

nerve-fibres which crosses the ventral surface
of the medulla opposite the posterior ends of the
hemispheres. It connects the two sides of the
cerebellum with each other.

g. The crura cerebri are two large bands of nerve-

fibres, immediately in front of the pons Varolii,
which run forwards, diverging from each other,
and connect the medulla oblongata with the
hemispheres.

THE BRAIN.

389

Fig. 68. — Lepus cuniculus. The ventral surface of the brain. The
greater part of the left temporal lobe has been sliced off horizontally.
The positions of the three semicircular canals of the left side are
indicated by the thick lines surrounding the floccular lobe of the
cerebellum, x 2. (a. m. m.)

CC, cms cerebri. CG, corpus geniculatum. D, descending cornu of left
lateral ventricle, h, hippocampus major. LF, floccular lobe of cerebellum.
LL, lateral lobe of cerebellum. OC, optic chiasma. OT, optic tract, p,
pituitary body. PV, pons Varolii. SA, anterior vertical semicircular canal,
t SH, horizontal semicircular canal. SP, posterior vertical semicircular canal.
T, temporal lobe of cerebral hemisphere.

I, olfactory lobe, with roots of olfactory nerves. II, optic nerve. Ill, third
nerve, or motor oculi. IV, pathetic nerve. V, trigeminal nerve. VI, sixth
or abducent nerve. VII, facial nerve. VIII, auditory nerve. IX, glosso-
pharyngeal nerve, x, pneumogastric nerve. XI, spinal accessory nerve. XII,
hypoglossal nerve.

390 DISSECTION OF THE RABBIT.

h. The medulla oblongata, as seen from the ventral
surface, is broad in front and gradually narrows
behind. It is marked by a median ventral fissure,
bordered by two narrow bands, the ventral
pyramids. At the outer sides of the pyramids,
immediately behind the pons Varolii, are a pair
of oblong patches of transverse nerve-fibres, the
corpora trapezoidea.

C. The Roots of the Cerebral Nerves.

There are twelve pairs of cerebral or cranial nerves in the
rabbit, all of which, with the exception of the fourth pair,
arise from the ventral surface or from the sides of the brain.

Identify, and clean the nerve roots on one side of the brain,
in the order given below, taking great care not to drag them away
from the brain.

1. The olfactory nerve arises, by many roots, from the

ventral surface of the anterior end of the olfactory
lobe. These roots are almost necessarily left be-
hind in taking out the brain.

2. The optic nerve is a stout nerve, running forwards from

the anterior border f'of the optic chiasma, and then
curving outwards.
3 The oculo-motor nerve is small, and arises from the inner
border of the crus cerebri, close to the middle line,
and immediately behind the corpus mammillare.

4, The pathetic nerve is very small. It arises from the

dorsal surface of the valve of Vieussens, close to
the middle line, and just behind the optic lobes.
It then runs round the side of the brain to the
ventral surface, lying between the cerebellum and
the optic lobe.
Find the nerve en the ventral surface, and follow it round
the side of the brain to its origin.

5. The trigeminal nerve is very large, and arises by two

closely apposed roots from the side of the hinder
border of the pons Varolii. Of the two roots the

ROOTS OF THE CEREBRAL NERVES. 391

inner and smaller one is motor, the larger outer one
sensory in function.

6. The abducent nerve is very slender, and arises from the

ventral surface of the medulla, close to the middle
line, and immediately behind the pons Varolii.

7. The facial nerve arises from the outer side of the ante-

rior end of the corpus trapezoideum, immediately
behind the trigeminal nerve.

8. The auditory nerve is large, and arises from the side

of the medulla, immediately behind and to the outer
side of the facial nerve.

9 and 10. The glossopharyngeal and pneumogastric nerves

arise close together, by many slender rootlets, from
the side of the medulla, close behind, and a little
above the root of the auditory nerve.

11. The spinal accessory nerve arises from the side of the

medulla and spinal cord by about ten rootlets, the
most anterior of which is just behind the pneumo-
gastric, and the hindmost as far back as the fifth
spinal nerve.

12. The hypoglossal nerve arises by a number of rootlets

from the ventral surface of the medulla, close to the
middle line, and to the outer side of the ventral
pyramid.

D. The Structure of the Brain.

The brain is, from the mode of its formation, tubular, its
cavity being directly continuous with the central canal of the
spinal cord. From a very early stage it presents a series of
dilatations or vesicles, partially separated from each other
by constrictions. Of these vesicles, the first or most anterior
one is the fore-brain, from which the cerebral hemispheres
and thalamencephalon arise as hollow outgrowths ; the second
vesicle is the- mid-brain or mesencephalon ; and the succeeding
ones, of which there are at least four or five, are spoken of

392 DISSECTION OF THE RABBIT.

collectively as the hind-brain. The cavities of the vesicles
remain in communication with each other throughout life,
and are spoken of in the adult as the ventricles of the brain.

1. Dissection of the Brain.

a. The cerebral hemispheres.

Lay the brain with its dorsal surface upward, and gently
press the hemispheres apart : note the position of the corpus
callosum.

Then, with the back of a knife peel off the portion of the
right hemisphere lying above the corpus callosum. Keep the
brain wet with spirit.

i. The corpus callosum is a broad horizontal band
of fibres connecting the two hemispheres.

ii. The structure of the hemisphere.

a. The outer layer or cortex is a superficial invest-
ment of grey matter, about an eighth of
an inch thick.

/3. The central or medullary portion consists of
white matter, i.e. of medullated nerve-fibres.
The fibres of the corpus callosum spread out
into and are continuous with it.

iii: The lateral ventricle, or cavity of the hemisphere.

Cut through the corpus callosum on the right side, so as
to open the lateral ventricle beneath it. Lift up the corpus
callosum, and carefully cut it away, so as to expose the ven-
tricle fully, leaving a narrow strip along its inner edge.

The lateral ventricle, opposite the middle
of .the hemisphere, is wide from side to side,
but shallow dorso-ventrally. Anteriorly, it
extends forwards as a narrow vertical slit,
the anterior cornu, which lies nearer the inner
than the outer surface of the hemisphere.

Posteriorly, the lateral ventricle curves

THE BRAIN.

393

backwards and inwards towards the hinder
end of the hemisphere as the posterior cornu ;
and from its outer side, a little behind the
middle of the hemisphere, a descending cornu
runs outwards and downwards, and then curves
round in the temporal lobe to the ventral

Fig. 69. — Lepus cuniculus. The brain dissected from above. Enlarged,
(c. H. H.)

C, lateral lobe of cerebellum. CA, pillars of cerebellum. CB, cut edge of
velum medullas posterius. CC, corpus callosum : the right half is removed.
C D, cut edge of corpus callosum. C F, floccular lobe of cerebellum. CS, corpus
striatum. F, anterior limit of body of fornix. H, hippocampus major.
M, medulla oblongata. O, olfactory lobe. OP, anterior optic lobe. P, pineal
body. V, fourth ventricle.

region, running forwards and inwards almost
to the anterior end of the lobe, and ending
opposite the infundibulum.

394 DISSECTION OF THE RABBIT.

Open up the descending cornu along its whole length, by
carefully cutting away the outer and ventral walls of the tem-
poral lobe.

iv. The hippocampus major is a prominent convex
ridge, projecting into the floor and inner side
of the descending cornu, along its, whole
length (cf. fig. 67, p. 386).

v. The corpus striatum is a thickened part of the
wall of the hemisphere, forming the outer side
and floor of the anterior part of the lateral
ventricle. It lies in front of the hippocampus
major, from which it is separated by a deep
groove, running obliquely backwards and out-
wards across the floor of the ventricle.

Cut away carefully the outer wall of the anterior cornu of
the lateral ventricle, including the corpus striatum.

vi. The septum lucidum is the thin vertical inner wall of
the hemisphere. It is placed opposite the corpus
striatum, and ventral to the corpus callosum.

vii. The foramen of Monro is a small hole immediately beneath
the hinder border of the septum lucidum, through
which the lateral ventricle communicates with the
third ventricle, or cavity of the thalamencephalon.

viii. The posterior pillar of the fornix is a narrow band lying
along the anterior edge of the hippocampus major.
Posteriorly it follows the hippocampus into the
descending cornu ; while anteriorly it becomes con •
tinuous with its fellow of the opposite side in the
median plane, forming the body of the fornix, imme-
diately above the foramen of Monro.

ix. The choroid plexus is a thin vascular fold of
the inner wall of the hemisphere project-
ing into the descending cornu in front of
the hippocampus major, and closely applied
to the anterior surface of the latter along its
whole length.

x. There is a median vertical cleft between the right and
left hemispheres, the * fifth ventricle.' It lies below

THE BRAIN. 395

the corpus callosum and above the fornix, and is
bounded laterally by the inner walls of the two
hemispheres (septum lucidum).

It is merely a part of the space between the
two hemispheres, partially enclosed by the corpus
callosum and the fornix ; and is not, properly speak-
ing, a ' ventricle ' at all.

To expose the fifth ventricle from above, carefully remove
the narrow strip of the corpus callosum that has been left along
the inner side of the hemisphere, and the fifth ventricle will be
seen as a narrow cleft between the two hemispheres.

b. The fore-brain, or thalamencephalon.

i. The optic thalami are a pair of large lateral
thickenings of the sides of the fore-brain,
immediately in front of the optic lobes,
and overlapped by the temporal lobes of the
hemispheres. They are separated by grooves
from the corpora striata in front and the optic
lobes behind.

ii. The optic tracts are a pair of narrow bands,
which can be traced backwards and upwards
from the outer ends of the optic chiasma,
over the surface of the optic thalami, to the
anterior borders of the optic lobes.

Take hold of the anterior end of the right optic tract with
the forceps, and strip it off the optic thalamus, so as to see its
origin from the optic lobe.

iii. The pineal body is connected by a tubular stalk
with the hinder end of the thin roof.

Remove the roof of the thalamencephalon so as to open the
third ventricle.

iv. The third ventricle, or cavity of the fore-brain,
is a median vertical cleft between the optic
thalami. It is very narrow from side to side,
and communicates through the foramina of
Monro with the lateral ventricles.

396

DISSECTION OF THE RABBIT.

c. The mid-brain, or mesencephalon.

i. The corpora guadrigemina, or optic lobes, form
the dorsal portion of the mid-brain, and are
now well seen.

ii. The crura cerebri form the base of the mid-
brain : after removal of the optic tracts they
can be traced forwards and outwards from
the anterior border of the pons Varolii to the
optic thalami.

Fig. 70. — Lepus cuniculus. Transverse section of the brain, passing
through the middle commissure of the third ventricle. The shading
indicates diagrammatically the distribution of white and grey
matter. The section is taken somewhat obliquely, so as to show
different parts on the two sides, (a. m. m.)

A, optic thalamus, forming the side-wall of the third ventricle. B, part ol
the third ventricle above the middle commissure. C, middle commissure. D,
infundibulum. E, corpus callosum. F, temporal lobe of the left cerebral hemi-
sphere. G, descending cornu of the right lateral ventricle. H, hippocampus
major.

2. Longitudinal section of the brain.

Divide the brain into right and left halves by a median
longitudinal section. Place the left half in a small dish of
spirit, and examine its inner surface.

If the dissection has been properly performed, the left half

THE BRAIN. 397

of the brain will be uninjured. If it has been damaged, a
second brain should be taken, and bisected as directed above.

a. The cerebral hemisphere.

i. The corpus callosum is seen cut across, and its
boundaries can easily be determined.

ii. The body of the fornix is a thin median band
lying below the corpus callosum. It is con-
tinuous with this latter behind, but in front
diverges from it considerably, the space be-
tween the two being the fifth ventricle.

b. The fore-brain, or thalamencephalon.

i. The third ventricle, though very narrow from
side to side, is of considerable depth from
the dorsal to the ventral surface. Owing to
its narrowness its boundaries are difficult
to determine.

The outline of the third ventricle can be rendered more
obvious by adding a little ink or other colouring matter to the
spirit in which the brain is examined.

ii. The commissures of the third ventricle are

three bands of transverse fibres connecting
the right and left halves of the thalamen-
cephalon with each other. Of the three the
middle one alone really crosses the cavity of
the ventricle.

a. The anterior commissure is a small band of
transverse nerve-fibres in the substance of
the anterior wall of the ventricle, and
immediately in front of the fornix.

/?. The middle commissure is a very large band
of fibres connecting the two optic thalami
across the ventricle. It is nearly circular
in outline, and crosses the ventricle about
its middle, filling up the greater part of it.

E E

398

DISSECTION OF THE RABBIT.

y. The posterior commissure is a small band of
transverse fibres in the roof of the hinder
end of the third ventricle, just in front of
the optic lobes.

iii. The roof of the third ventricle has already been
seen and removed.

Fiq. 71. — Lepus cuniculus. A longitudinal and vertical section of the
brain, taken in the median plane, (a. m. m.)

A, pituitary body. AC, anterior commissure. AF, anterior pillar of the
fornix, seen in the wall of the third ventricle. C, cerebellum. CA, corpus
albicans. CC, corpus callosum. CH, inner surface of left cerebral hemisphere.
F, body of the fornix. FM, foramen of Monro. G, roof of third ventricle.
I, infundibulum. MC, middle commissure. N, anterior optic lobe or ' nates.'
O, olfactory lobe. OC, optic chiasma. ON, left optic nerve. P, pineal body.
PC, posterior commissure. PV, pons Varolii. T, posterior optic lobe, or
' testis.' VA, velum medullae anterius, or valve of Vieussens. VP, velum
medullae posterius.

Ill, third ventricle. IV, fourth ventricle. V, so-called fifth ventricle.

iv. The stalk of the pineal body is a tubular pro-
longation, upwards and backwards, of the
roof of the ventricle, just in front of the
posterior commissure.

v. The optic chiasma appears in this section as a
thickening in the floor of the anterior end
of the ventricle.

THE BRAIN. 399

vi. The infundibulum is a funnel-like depression of
the floor of the ventricle, about the middle
of its length ; to its apex the pituitary body
is attached.

vii. The corpus albicans is a rounded -swelling on
the ventral surface of the floor of the ven-
tricle, in the hinder wall of the infundi-
bulum.
viii. The foramen of Monro is a vertical slit-like
opening at the anterior end of the ventricle,
between and slightly above the anterior and
middle commissures. It leads from the third
to the lateral ventricle.

ix. The anterior pillar of the fornix is a slender
band of nerve-fibres in the side-wall of the
ventricle, running backwards and downwards
from the body of the fornix, beneath the
middle commissure, to the corpus albicans,
in which it ends.

c. The mid-brain, or mesencephalon.

i. The cavity or ventricle of the mid-brain is very

small,
ii. The optic lobes, or corpora Quadrigemina, have

moderately thick walls, and form the roof of

the mid-brain, which is prolonged a short

distance into them.
iii. The crura cerebri, which form the floor of the

mid-brain, are very thick.

d. The hind-brain.

i. The fourth ventricle, or cavity of the hind-
brain, is moderately wide from side to side,
but shallow from above downwards. It has
a very thin roof, but thick floor and sides.

ii. The velum medullae anterius, or valve ol Vieussens,
is a thin membrane extending from the optio lobes
to the cerebellum, and forming the roof of the anterior
half of the fourth ventricle.

400 DISSECTION OF THE RABBIT.

iii. The velum medullse postering is a thin membrane
forming the roof of the posterior half of the
fourth ventricle.

iv. The cerebellum is attached to the dorsal surface
at the junction of the vela medullse anterius
and posterius. It consists of an outer layer
of grey matter, which is very deeply folded,
and a central much-branched mass of white
matter, the section having a tree-like appear-
ance.

Fig. 72. — Lepus cuniculus. Transverse section of the brain, passing
through the widest part of the cerebellum. The shading indicates
diagrammatically the distribution of white and grey matter.
(a. m. m.)

A, vermis, or middle lobe of the cerebellum. B, lateral lobe of the cere-
bellum, c, floccular lobe of the cerebellum. D, transverse fibres of the pons
Varolii, iv, fourth ventricle.

v. The pons Varolii is seen, in transverse section,
crossing the ventral surface of the anterior
part of the hind-brain, opposite the velum
medullse anterius.

3. Transverse sections of the brain.

2/ another brain can be obtained, it should be cut with a
razor into a series of transverse sections, about an eighth of an
inch thick, and the sections arranged in order in a dish of spirit.

Such a series of sections affords very instructive
views of the mutual relations of the several parts of
the brain.

401

Chapter XV.

THE SKELETON OF THE FOWL. Gallus Domesticus.

The fowl's skeleton is, on account of its larger size, more
convenient for study than that of the pigeon. The differences
between the two are of but slight importance, and the fol-
lowing description will practically apply to either.

The bird's skeleton consists essentially of the same parts
as the rabbit's, but presents certain special characteristics, of
which the more important are : (1) The great lightness of
the bones, many of which contain large air cavities ; (2) the
extensive fusion of primitively distinct bones with each
other during development ; (3) the modifications in the
limb-girdles and limbs, fitting birds for bipedal locomotion
and for flight.

As in the case of the rabbit's skeleton, the names of the
membrane-bones are printed in italics, those of the cartilage-
bones in thick type.

I. THE AXIAL SKELETON.

The axial skeleton, as in other vertebrates, is divisible
into skull and vertebral column.

A. The Vertebral Column.

As compared with the rabbit's, the vertebral column of
the bird is distinguished by (1) the great length and flexibility
of the neck, the cervical portion being nearly equal in length
to the whole of the rest of the column ; (2) the rigidity of the
thoracic and abdominal portions of the column, owing to the
fusion of their component vertebrae ; (3) the great length of

402 SKELETON OF THE FOWL.

°P I V N

VERTEBRAL COLUMN. 403

that portion of the vertebral column with which the pelvis is
connected ; (4) the small number of the caudal vertebrae, and
the fusion of the posterior ones to form the pygostyle ; (5) the
absence of epiphyses.

The vertebral column may be divided into cervical,
thoracic, sacral, and caudal regions, the boundaries between
these being ill-defined, and the sacral portion being further
divisible into three sub-sections.

1. The cervical vertebrae. The boundary between cervical
and thoracic vertebras is an arbitrary one, of no
morphological value, and is not easy to determine in
the fowl. It is usual to count as the first thoracic
vertebra the most anterior one which bears a rib
articulating with the sternum, and to name all the
vertebrae in front of this one cervical. Beckoned in
this way there are sixteen cervical vertebrae, the two
hindmost of which bear well- developed and movably
articulated ribs, which, however, do not reach the
sternum.

In the following description the vertebral column
is supposed to be placed horizontally.

a. A typical cervical vertebra : any one from the sixth
to the tenth.

The parts of the vertebra are the same as in
the rabbit, except that there are no terminal
epiphyses. The chief peculiarities are the
following :

Fig. 73. — Gallus domesticus. The left half of the skeleton. The skull,
vertebral column, and sternum are bisected in the median plane,
(c. H. H.)

A, acetabulum. B, cerebral fossa. CB, cerebellar fossa. CL, clavicle.
CO, coracoid. CR, cervical rib. C1 , first cervical vertebra. DE, DM, DO,
preaxial, middle, and postaxial digits of the wing. FE, femur. HC, hypo-
cleidium. HU, humerus. HY, hyoid. IF, ilio-sciatic foramen. IL, ilium.
IS, ischium, l, lacrymal bone. MCu, metacarpal bone of ulnar digit. MN,
mandible. MS, metosteon. MT, tarso-metatarsus. MT 1, metatarsal bone,
of first digit. N, nasal bone. OP, optic foramen, p, premaxilla. PB, pubes.
PL, palatine bone. PY, pygostyle. R, radius. RC, radial carpal bone. S,
keel of the sternum. SC, scapula. T, tibio-tarsus. TH 4, fourth thoracic
vertebra, y, ulna. UC, ulnar carpal bone. UP, uncinate process of rib.
Z, infra-orbital bar. . x. . .

1 , 2, 3, 4, the first, second, third, and fourth digits of the foot.]

404 SKELETON OF THE FOWL.

i. The vertebra is very long and light, as compared
with the cervical vertebra of a rabbit.

ii. The centrum is slender. The terminal arti-
cular surfaces are saddle-shaped, the anterior
one being convex from above downwards,
and concave from side to side ; while the
posterior surface, which is larger and more
prominent, is concave from above downwards,
and convex from side to side. These saddle-
shaped articular surfaces are very, character-
istic of the cervical vertebrae of birds.

iii. The neural arch is shorter than the centrum,
and is deeply notched in the middle line,
both in front and behind. The neural arch
is also notched behind, on both sides, for the
passage of the spinal nerves.

iv. The neural spine is rudimentary in the middle
cervical vertebrae.

v. The transverse processes and cervical ribs. At
the anterior end of the vertebra an irregular
process projects outwards at each side, which
is perforated at its base by a large foramen,
and is produced at its hinder border into a
sharp backwardly directed process.

This process is really a rib, which, as in
the cervical vertebrae of the rabbit, is fused
with the vertebra ; the foramen at its base
transmits the vertebral artery and corre-
sponds to the notch between the capitulum
and tuberculum of the rib.

vi. The zygapophyses.

The pre-zygapophyses are borne on the
upper surfaces of the transverse processes :
their articular surfaces are flat and face up'
wards and inwards.

The post-zygapophyses project backwards
from the posterior border of the neural arch,

VERTEBRAL COLUMN. 405

their articular surfaces facing downwards and
outwards.

The atlas, or first vertebra, is very short and ring-
like. Its ventral portion is thickened, resembling
a centrum, and is notched above to receive the
odontoid process of the axis : its anterior surface
is deeply cupped to articulate with the occipital
condyle of the skull. The posterior border of
the neural arch is produced backwards into zyg-
apophyses, articulating with the second vertebra.

The axis or second vertebra is, with the exception
of the atlas, the smallest of the cervical series :
it has a blunt neural spine, but no transverse
processes, and no ribs. The centrum is produced
in front into the slender odontoid process.

The remaining cervical vertebrse. The third and
fourth cervical vertebrse are shorter than the
succeeding ones, and have short thick ribs, and
lateral wing-like ridges placed obliquely along
their sides. The middle cervicals are the longest
of the series, and the hinder ones are shorter
and more massive. The last cervical vertebra is
fused with the first thoracic.

The neural spines are present, though small,
in the anterior cervical vertebrse : they are well
developed in the hinder ones, but are almost
absent in the middle ones of the region.

The hypapophyses are well developed in the
hindmost half-dozen cervical vertebrse.

The ribs are more than half the length of
the vertebrse in the middle of the neck, but are
blunt and inconspicuous in the twelfth to the
fourteenth vertebrse. In the fifteenth the ribs are
well developed, and movably articulated with
the vertebra ; and in the sixteenth they are still
larger, and resemble the thoracic ribs, but do not
meet the sternum.

406 SKELETON OF THE FOWL.

2. The thoracic vertebrae are five in number. The three

anterior ones are fused together and with the last
cervical vertebra, their centra, arches, and various
processes being confluent, and the neural spines
and hypapophyses forming continuous dorsal and
ventral ridges.

The fourth thoracic vertebra is free ; and the
fifth is fused with the first sacral vertebra, its
transverse processes abutting against and fusing
with the ilia.

3. The sacrum consists of fourteen or fifteen vertebrae,

fused together and supporting the ilia. It may be
divided into the following regions :

a. The three anterior sacral vertebrae are firmly fused

together, and with the last thoracic vertebra.
They have stout transverse processes, which
abut against the ilia, and which in the first one
of two are single, but in the third are divided
into dorsal and ventral portions, the latter being
very stout and projecting directly outwards from
the broad centrum. The neural spines of these
vertebrae are fused to form a vertical crest of
bone, which is continuous in front with that of
the last thoracic vertebra, and fused along its
edge with the dorsal edges of the ilia.

b. Behind these come four or five shorter vertebrae,

with broad centra fused together. Their trans-
verse processes, which have no ventral elements,
are united to form bony plates, which support
the ilia along their outer margins.

Between the bodies of these vertebrae and
the ilia are the deep hollows in which lie the
middle lobes of the kidneys.

c. Next come seven vertebrae, the centra of which are

compressed laterally in the anterior ones, and
dorso-ventrally in the hindmost two or three.
The transverse processes, except in the last, are

RIBS AND STERNUM. 407

double, the dorsal elements fusing to form bony
plates, continuous with those of the vertebrae in
front of them. The ventral elements of the
transverse processes are rod-like in the case of
the first four or five vertebrae, and are regarded
as ribs, because they ossify independently of
the centra : in the hinder vertebrae, they are
smaller. The first pair of these ribs are much
stouter than the rest : they are placed in the
adult opposite the acetabula, with the walls of
which they are connected by bony ridges run-
ning across the inner surfaces of the ilia, and
separating the fossae for the middle and posterior
lobes of the kidneys fiom each other. In the
embryo, and in the young chick, these ribs are
some distance behind the acetabula, and are no
larger than those of the succeeding vertebrae.
From the special relation of the bird's sacrum
to bipedal locomotion, it exhibits a necessary diver-
gence from the sacral vertebrae of other vertebrates,
usually two in number. Huxley considered that
group (b) are the true sacral vertebrae, on the ground
that the spinal nerves associated with them unite
to form the sacral plexus. Gegenbaur and others
hold that two or more of the vertebrae in group
(c) are the true sacral ones, on account of their
possessing separately ossified ribs.
4 The caudal vertebrae consist of four or five free ones, and
a terminal pygostyle. This latter, which supports
the rectrices, or large tail feathers, is much larger
than the others, and triangular in shape. It is
formed by the fusion of five or six vertebrae, which
are distinct in the embryo.

B. The Ribs and Sternum.
1 The Ribs.

a. The cervical ribs. As already noticed, the last two
cervical ribs of each side are alone movably

408 SKELETON OF THE FOWL.

articulated, the remainder being fused with
the transverse processes of the corresponding
vertebrae.

Of these two, the first is small, but has dis-
tinct capitular and tubercular articulations ; the
second is larger and bears on its posterior border
a backwardly projecting uncinate process, which
overlaps the next rib.
b. The thoracic ribs are five on each side, and slightly
increase in size from before backwards : each
consists of vertebral and sternal portions, meet-
ing at an angle,
i. The vertebral portions are flattened and slightly
curved bars of bone, which articulate by
capitular and tubercular surfaces with the
centra and transverse processes of their re-
spective vertebrae. Uncinate processes pro-
ject backwards from the posterior borders of
all, except the last, or sometimes the last two.
ii. The sternal portions are slender, nearly straight
rods of bone, which run forwards and down-
wards, from their junctions with the vertebral
portions, to the side of the anterior part of
the sternum, with which they articulate.
2. The sternum, or breastbone, is very large, and projects
back far beyond the ribs, and over the greater part
of the wall of the abdomen. Its ventral surface is
raised into a prominent vertical crest or keel, which
gives origin to the muscles moving the wings.
i. The body of the sternum is a somewhat boat-
shaped plate of bone : it is wide and stout
in front, and perforated on its concave dorsal
surface by apertures, through which the air-
sacs communicate with the cavity of the
bone : it is narrow in the middle portion of
its length, but widens somewhat behind,
ii. The manubrium is a small vertical plate of

THE SKULL. 409

bone, projecting from the anterior end and
ventral surface of the body.

iii. The carina, or keel, is a very prominent trian-
gular crest projecting downwards from the
ventral surface of the sternum, and slightly
thickened along its anterior border, which is
concave.

iv. The metostea are a pair of large processes,
projecting backwards from the sides of the
anterior end of the body of the sternum.
Each divides, close to its origin, into two
large flattened processes, the external and
middle xiphoid processes, which are ex-
panded at their hinder ends.

v. The costal surfaces are the lateral borders of
the body, in front of the metostea : they are
notched for the sternal ends of the ribs.

vi. The costal processes project forwards and out-
wards, in front of the costal surfaces.

vii. The coracoid grooves are a pair of deep trans-
verse grooves on the anterior end of the body
of the sternum, continuous with each other,
across the median plane, through a hole in
the base of the manubrium. With them the
ventral ends of the coracoid bones articulate.

C. The Skull.

The chief characteristics of the bird's skull are : (1) Its
great lightness ; (2) the very extensive fusion of the bones of
the cranium with each other, most of the cranial sutures
being closed, and the outlines of the bones obliterated, by
the end of the first year ; (3) the large size of the orbits,
which are separated from each other by a thin vertical in-
ter-orbital septum ; (4) the prolongation of the face forwards
into a long conical toothless beak.

The skull consists of the same essential parts as that of
the rabbit, from which, however, it differs in the following

410

SKELETON OF THE FOWL.

important points : (1) There is only a single occipital condyle,
for articulation with the atlas vertebra ; (2) the base of the
skull is strengthened by a large membrane-bone, the basi-
temporal ; (3) the orbits are placed almost entirely in front
of the cranium, instead of below it, and are separated from
each other only by the thin inter-orbital septum ; (4) the
lower jaw is connected with the squamosal by a movably
articulated bone, the quadrate ; (5) there is only a single
auditory ossicle, the columella ; (6) there are no teeth.

Fig. 74.

0 A QJ B Q J D t

-Gallus domesticus. The skull from the right side. (a. m. m.)

A, articular surface of the mandible. AT, anterior tympanic recess, lead-
ing to the Eustachian tube. B, pterygoid. C, occipital condyle. D, palatine.
E, vomer. F, mandibular foramen. FO, fenestra ovalis. FR, fenestra
rotunda. FZ, zygomatic process of frontal bone. G, supra-angular. H,
dentary. is, inter-orbital septum. J, jugal. L, lacrymal. M, maxilla.
MP, maxillo-palatine process of maxilla. N, nasal. OF, optic foramen.
PM, premaxilla. pt, posterior tympanic recess. Q, quadrate. QJ, quad-
rato-jugal. SF, olfactory foramen. SZ, zygomatic process of squamosal.
TF, foramen for fifth nerve.

The fowl's skull consists of cranial and facial portions
which are loosely connected together, and capable of slight
movement upon each other. The cranial portion consists of
the cranium proper, or brain-case, and the auditory capsules :
the facial part is formed by the jaws and the olfactory capsules.

1. The cranial portion of the skull is a somewhat conical
box of bone, the base of the cone forming the hinder

THE SKULL. 411

end of the skull, and the apex being directed for-
wards. In the young skull the outlines of the com-
ponent bones can easily be followed, but in the
adult they are indistinguishable. The following are
the main points to be noticed :

a. The hinder end of the cranium.

i. The foramen magnum is a large nearly circular
aperture, in the middle of the posterior end of
the skull, through which the spinal cord enters
the cranial cavity.

ii. The occipital condyle is a rounded knob, im-
mediately below the foramen magnum, into
which it projects slightly. It is notched in
the mid-dorsal line, and has below it a deep
pit-like depression.

iii. The foramen of exit for the hypoglossal nerve,

on each side, is a small aperture, immediately
to the outer side of the occipital condyle, and
slightly ventral to it

iv. The foramen of exit for the pneumogastric and
glossopharyngeal nerves, on each side, is a
larger hole about an eighth of an inch to the
outer side of that for the hypoglossal nerve,
and midway between the condyle and the rim
of the tympanic cavity.

v. To the outer side of the above foramen, and
between it and the tympanic rim, is a group
of three or four larger holes : some of these
lead into the tympanic cavity ; while the most
ventrally situated one is the entrance to the
carotid canal, which traverses the base of the
skull, and lodges the carotid artery.

vi. The lambdoidal crest is a prominent ridge of
bone surrounding the dorsal half of the hinder
end of the skull, and separating the occipital
"from the parietal region.

412 SKELETON OF THE FOWL.

b. The ventral surface of the cranium.

i. The base of the skull is wide behind. The basi-
sphenoid is covered by the broad transverse
basi-temporal, a membrane bone which forms
part of the boundaries of the tympanic cavities,
and probably corresponds to the posterior part
of the para-sphenoid of the frog.

In front of the basi-temporal the base of
the skull is narrow, and formed by the ros-
trum, or thickened ventral border of the inter-
orbital septum, which probably represents the
anterior part of the para-sphenoid.

ii. The anterior openings of the Eustachian canals

are a pair of small holes, close to the median
plane, and opening into a deep notch between
the anterior end of the basi-temporal bone and
the rostrum.

iii. The foramina of exit for the carotid canals

are a pair of small holes, near the middle line,
and immediately to the outer sides of the
Eustachian apertures. They are connected
with each other by a canal, traversing the
base of the skull.

Pass a bristle along the carotid canal of one side, and through
the transverse canal from side to side.

c. The side of the cranium.

i. The tympanic cavity is a hemispherical depres-
sion, at the side of the hinder end of the skull,
bounded above by the squamosal and below by
the basi-temporal. The tympanic membrane is
attached just within its prominent lip.

Within the tympanic cavity can be seen
the following :

a. The fenestral recess is an oval depression in
the middle of the cup, divided by a parti-
tion into an upper hole, the fenestra ovalis,

THE SKULL. 413

and a lower, the fenestra rotunda. The

former of these lodges during life the inner
end of the auditory ossicle or columella, a
slender bar of bone and cartilage, the outer
end of which is attached to the tympanic
membrane.

6. The posterior opening of the Eustachian canal

is a funnel-like depression at the ventral
and anterior corner of the tympanic cavity.
The tube into which it leads passes forwards
and inwards, between the basi-temporal and
the basi-sphenoid, to open, as already seen,
on the base of the skull, close to the middle
line.

Pass a bristle along the Eustachian canal of one side,
from behind forwards.

y. The anterior tympanic recess is a slit-like
depression, immediately above the posterior
opening of the Eustachian canal, with which
it is often confluent.

S. The posterior tympanic recess is a large
depression close to the dorsal border of the
tympanic cavity, and just above the fene-
stral recess.

ii. The zygomatic process of the squamosal is a
thin bar of bone which projects forwards
almost horizontally from the anterior and
upper edge of the tympanic cavity. Its distal
end often meets, and fuses with, the post-orbital
process of the frontal bone.

iii. The articular surface for the quadrate is a de-
pression on the side of the skull, immediately
in front of the tympanic cavity, between it
and the zygomatic process of the squamosal.

iv. The foramen of exit for the trigeminal nerve
is a round hole in the side of the skull, below

414 SKELETON OF THE FOWL.

and in front of the articular surface for the
quadrate.

v. The orbit is a very large depression in the side
of the skull, which lodges the eyeball : it is
bounded above by the frontal bone ; in front,
by the frontal and lacrymal bones, the latter
of which is produced downwards at its anterior
end into a slender curved process. Behind,
the orbit is bounded by the frontal with its
post-orbital process, and by the ali-sphenoid ;
and on the inner side it is separated from the
orbit of the other side by the inter-orbital
septum, a thin vertical plate of bone, formed
by the coalescence of the pre-sphenoid and
orbito-sphenoid behind, and the mesethmoid
in front and above. The lower edge of the
septum is thickened and covered by a mem-
brane-bone, the rostrum, which forms the
anterior part of the para-sphenoid.

Below, the orbit is very incomplete : its
outer margin is formed by the slender zygo-
matic arch, and its floor, nearer the middle
line, is formed in part by the palatine and
pterygoid bones.

vi. The foramen of exit for the olfactory nerves is

a median opening at the anterior end of the
cranium, and is continued forwards as a
horizontal slit, between the inter-orbital sep-
tum and the frontal bones.

vii. The optic foramen is a large median hole in
the orbito-sphenoid bones, opening into the
posterior part of the orbits, and continued
forwards as a semicircular notch in the inter-
orbital septum.

viii. The foramina of exit for the third and fourth
nerves are small holes, immediately to the

THE SKULL 415

outer side of the optic foramen, and sometimes
confluent with it.

d. The dorsal surface of the cranium.

The dorsal surface of the cranium is rounded,
and formed almost entirely by the parietal and
frontal bones. It is broad behind, narrower in
front, and separated by a sharp, almost trans-
verse line from the facial part of the skull.
The anterior ends of the frontal bones diverge
slightly from each other, and allow the meseth-
moid to come to the surface between them.

2. The facial portion of the skull lies below and in front of
the cranial portion, with which it is only loosely
connected. It consists of (1) the olfactory capsules,
and the bones associated with them ; (2) the skeletal
elements of the jaws, and the hyoid apparatus.

a. The olfactory capsules and associated bones. The

olfactory capsules themselves remain in a car-
tilaginous condition throughout life, and are
therefore absent in macerated skulls. They are
separated only by a thin vertical septum, and
almost completely fill the conical space which,
in the dried skull, lies between the anterior end
of the orbit and the tip of the beak. Each
olfactory capsule has its cavity complicated by
a spirally rolled lamina of cartilage, which pro-
jects inwards from its outer wall.

In relation with the olfactory capsules are the
following bones :
i. The nasals are a pair of thin lamellar bones
which cover the sides of the hinder ends of
the olfactory capsules. Behind, they are fused
with the anterior ends of the frontals : while
their anterior borders are divided by deep
notches into superior and inferior processes
which embrace the external narial openings

416 SKELETON OF THE FOWL.

and are closely connected with processes of
the premaxillce.
ii. The vomer is a slender median bony rod which
lies below the hinder part of the olfactory
capsules, and immediately in front of the
rostrum. It is paired in some birds.

b. The upper jaw. The bones of the upper jaw, as
seen from the ventral surface, form on each side
two bony arcades, which are fused together in
front to form the beak, and are closely approxi-
mated behind, but widely separated from each
other along the greater part of their course.
The inner arcade is formed by the pterygoid
and palatine bones, which articulate with the
rostrum ; the outer arcade consists of the slender
sub-orbital bar and the premaxilla.

The upper jaw is connected with the anterior
end of the frontals, and with the mesethmoid.
Posteriorly, it is slung up to the side of the skull,"
just in front of the tympanic cavity, by the quad-
rate bones,
i. The pterygoid is a short stout bone, forming the
hinder part of the inner arcade. It articu-
lates at its outer and posterior end with the
inner surface of the quadrate, and at its
inner and anterior end with the rostrum,
along which it can slide,
ii. The palatine is a slender horizontal bar of bone
lying in front of the pterygoid, and forming the
greater part of the inner arcade. Behind, it
articulates with the pterygoid, and is expanded
into a broad lamella, which articulates with
the rostrum along its inner edge. In front,
the palatine is wedged in between the maxilla
and the premaxilla.
iii. The premaxilla is a triradiate bone, the anterior
part of which is ankylosed with its fellow in

THE SKULL. 417

the median plane to form the tip of the beak,
while the three processes diverge backwards.

The ascending or nasal process, which is
much the longest of the three, is a long
slender bar of bone, which runs backwards
and upwards, closely applied to its fellow of
the opposite side, and along the inner side
of the nasal bone, to the mesethmoid, with
which it is connected. The bone is very thin
and elastic at its hinder end, so as to allow of
a certain amount of movement of the facial
on the cranial portion of the skull.

The outer or maxillary process runs back-
wards and slightly outwards, forming part of
the margin of the upper jaw. The nasal and
maxillary processes of the premaxilla, together
with the superior and inferior processes of the
nasal bone, surround the large aperture at the
side of the beak in which the external narial
opening lies.

The inner or palatine process, the smallest
of the three, is a narrow plate of bone which
extends horizontally backwards, a short dis-
tance from the median plane. It forms part
of the roof of the mouth, and is connected
behind with the anterior end of the palatine
bone.

iv. The maxilla is a slender rod of bone, lying
behind and to the inner side of the maxillary
process of the premaxilla, and forming part
of the sub-orbital bar. Its anterior end is
expanded into a thin horizontal lamina of
bone, the maxillo-palatine process, which pro-
jects inwards the median plane, lying dorsal
to the palatine bone.

v. The jugal is a very slender rod, which forms the
middle portion of the sub-orbital bar. It lies

418 SKELETON OF THE FOWL.

dorsal to the other two components of the
bar, and its exact limits are not recognisable
in the adult.

vi. The quadrato-jugal is the hindmost of the
three components of the sub-orbital bar. Its
posterior end is thickened, and articulates
with the outer side of the distal end of the
quadrate. In front it extends forwards as far
as the maxilla, being overlapped for a con-
siderable part of its length by the jugal.

c. The mandibular arch is divided into two portions,
of very unequal length, placed almost at right
angles to each other. The proximal part is the
quadrate bone, which connects both the upper
and lower jaws with the cranium ; the distal por-
tion, forming the lower jaw, is of much greater
length, is fused with its fellow in front, and is
composed of several bones firmly united together.

i. The quadrate, which forms the suspensorium, is
a stout triradiate bone. Its proximal or dorsal
arm articulates with a cup-like depression ir*
the squamosal, at the anterior and upper
border of the tympanic cavity, between this
and the base of the zygomatic process.

The distal or ventral arm, which is the
stoutest of the three, is expanded transversely
at its lower end, and bears the articular
surface for the lower jaw ; it also articulates
at its outer edge with the quadrato-jugal, and
along its inner margin with the pterygoid.

The third or anterior arm of the quadrate
is a more slender process, which projects for-
wards and inwards into the hinder part of the
orbit, lying parallel to and above the pterygoid.

ii. The mandible, or lower jaw, consists of a pair
of long laterally compressed rami, thickened
and expanded at their hinder ends, where

THE SKULL. 419

they articulate with the quadrates, and firmly
unite together in the median plane in front.
Like the upper jaw it bears no teeth, but is
covered by a horny epidermal sheath.

Each ramus of the mandible . really con-
sists of five bones, one of which is developed
in the primary cartilaginous arch, while the
others are formed around it.

a. The articular forms the expanded posterior
end of the ramus. On its dorsal surface
is the elongated and slightly concave arti-
cular surface for the quadrate : behind this
the bone is produced backwards and up-
wards into the curved posterior articular
process ; and on the inner side into a similar
but stouter internal articular process.

The articular is continuous in front
with Meckel's cartilage, the cartilaginous
bar of the mandibular arch, which persists,
even in the adult, along the greater part of
the length of the mandible, ensheathed in
membrane-bones.

/J. The angular is a splint-like bone which
underlies the articular, and extends for-
wards along the ventral edge and inner
surface of the proximal half of the lower
jaw.

7. The supra-angular forms the dorsal part of

the posterior third of the mandible, along
both outer and inner surfaces. Its dorsal
border is raised into a small coronoid pro-
cess, about the middle of its length.

8. The dentary is the largest of the bones of

the jaw, and forms the distal half of each
ramus : it is fused with its fellow in front
at the symphysis : and posteriorly it extends

420 SKELETON OF THE FOWL.

back along the ventral border of the supra-
angular, between this and the angular, as
far as the articular.

e. The splenid is a thin lamina of bone, applied
to the inner surface of each ramus of the
jaw, along the middle of its length.

d. The columella and the hyoid apparatus. These
may conveniently be considered together, as the
columella is, according to most authorities, the
uppermost part of the hyoid arch, just as the
quadrate is of the mandibular.

i. The columella is a minute and slender rod of
bone, the proximal end of which is expanded
into an oval plate, fitted into the fenestra
ovalis of the tympanic cavity ; while the
outer or distal end, which is cartilaginous and
divided into three diverging arms, is attached
to the tympanic membrane. It serves to con-
vey the sound-vibrations from the tympanic
membrane to the membrane closing the
fenestra ovalis, and so to the internal ear.

Owing to its small size and slight attach-
ment, the columella is often lost in macerated
skulls : it may, however, sometimes be found
projecting from the fenestra ovalis.

ii. The hyoid apparatus is a bony framework lying
in the floor of the mouth, and supporting the
tongue. It consists of a median jointed rod
of bone and cartilage formed of glosso-hyal,
basi-hyal, and basi-branchial ; to the sides of
which are attached in front the short cerato-
hyals, and further back the elongated and
jointed thyro-hyals, the ventral portions of
the first branchial arches.

PECTORAL GIRDLE. 421

II. THE APPENDICULAR SKELETON.

A. The Pectoral Girdle.

The pectoral girdle is stout, and is firmly connected with
the sternum to give support to the wings. It differs markedly
from that of the rabbit in the large size of the coracoid,
which is almost perpendicular to the scapula ; and in the
completeness of the clavicle.

i. The scapula is a long flattened blade-like bone
which in the natural condition of the parts
lies above the ribs, a short distance from and
almost parallel to the vertebral column. It
is connected by muscles with both the ribs
and the vertebrae. Its anterior end, which
is expanded and firmly connected with the
coracoid, bears on its outer surface a shallow
depression forming part of the glenoid cavity ;
and is produced forwards, at its inner border,
into a process which helps to form the canal
for the tendon of the second pectoral muscle,
ii. The coracoid is a stout straight bone running
from the shoulder to the anterior end of the
sternum. Its upper end is connected behind
and at its inner side with the scapula, and
bears on its outer surface a cup-shaped de-
pression, which forms the greater part of the
glenoid cavity.

Above the glenoid cavity, the coracoid is
produced upwards and inwards into a strong
process, which, with the corresponding pro-
cess of the scapula, completes the foramen
triosseum, through which the tendon of the
second pectoral muscle passes to its inser-
tion into the back of the humerus. This
process also articulates with the clavicle.

The lower end of the coracoid is broad and
flattened, and bears a transversely elongated

422 SKELETON OF THE FOWL.

surface, articulating with the anterior end of
the sternum,
iii. The clavicles are a pair of slender curved bones,
whose upper ends are slightly expanded, and
connected, as described above, with the cora-
coids ; while their lower ends are fused together
to form a median laterally compressed plate.

The two clavicles together form the fur-
cula or ' merrythought.'

B. The Fore-limb or Wing.

The wing of the bird corresponds to the fore-limb of the
rabbit, but differs from it in several respects, the more
important of which are : (1) the presence of only three digits ;
and (2) the fusion of the distal row of carpal bones with
the three persistent metacarpals to form a compound bone,
the carpo-metacarpus.

When the wing is extended for flight, its surfaces and
borders correspond to those of the primitive vertebrate limb,
the preaxial border being directed forwards, the postaxial
backwards, and the dorsal and ventral surfaces upwards and
downwards respectively. When the wing is folded, in the
position of rest, the three main divisions — arm, forearm, and
hand — are bent on each other like the letter Z, the elbow
pointing backwards and the wrist forwards. Owing to a
slight rotation at the shoulder- joint, and a slight obliquity of
the elbow- joint, the dorsal surface of the arm now faces
inwards and upwards ; that of the forearm outwards and
slightly upwards ; while in the hand, which is bent on the
forearm in a position of extreme abduction, the dorsal sur-
face faces almost directly outwards.

1. The arm.

i. The humerus is an elongated slightly curved
bone expanded at both ends.

The proximal end, or head, presents a
large convex vertically elongated surface for
articulation with the glenoid cavity, bordered

THE FORE-LIMB. 423

by two tuberosities, which are preaxial and
postaxial respectively.

The preaxial tuberosity is small, but is
continued along the bone as the prominent
forwardly directed deltoid ridge : . this gives
insertion to the great pectoral and deltoid
muscles ; while a small tubercle at its proxi-
mal end, and partly on the extensor surface,
marks the insertion of the tendon of the second
pectoral muscle, or elevator of the wing.

The postaxial or greater tuberosity is much
larger, and has on its extensor surface a deep
pit, the pneumatic foramen, leading to an
air-cavity in the shaft of the bone.

The distal end of the humerus bears a
trochlear articular surface for the radius and
ulna, the radial surface being preaxial, and
placed obliquely to the long axis of the bone ;
the ulnar surface being postaxial, and trans-
verse to the axis.

2. The forearm has two bones, which are separate from
each other along their whole length,
i. The radius is slender and nearly straight. Its
proximal end, or head, presents a terminal
cup-shaped articular surface for the preaxial
condyle of the humerus ; and its distal end
articulates with the carpus.

ii. The ulna is rather longer and much stouter
than the radius, and is slightly curved. Its
proximal and larger end has a large articular
surface for the postaxial tubercle of the
humerus, beyond which it projects as the
blunt olecranon process. Its distal end articu-
lates with the carpus and with the radius.

3. The wrist consists, in the embryo, of two rows of carpals,
a proximal row of two and a distal row of three. Of

424 SKELETON OF THE FOWL.

these the two proximal carpals persist in the adult,
while those of the distal row fuse with the meta-
carpals. The wrist- joint thus lies in the carpus, and
not between the forearm and carpus, as in the
rabbit.
The proximal row of carpals consists of two small
irregular bones :
i. The radiale lies between the end of the radius

and the manus.
ii. The ulnar e is a larger, more irregular bone,
which articulates proximally with the ulna,
and is notched distally to receive the carpo-
metacarpus. It is very freely movable.

4. The manus. In the embryo there are three separate meta-
carpals. It is not certain whether they correspond
to those of the three outer, middle, or inner digits.
In the adult they fuse together, and with the distal
row of carpals, to form the carpo-metacarpus. A
small nodule of cartilage to the outer side of these
disappears completely, or, according to some autho-
rities, fuses with the postaxial metacarpal.

a. The carpo-metacarpus is an elongated bone, in which

the three component metacarpals can easily be
distinguished,
i. The preaxial metacarpal is very short and
stumpy, forming a small projection on the pre-
axial side of the proximal end of the bone,
ii. The middle-metacarpal is a stout straight bone,
enlarged at its ends, and giving off near its
proximal end a process from its ulnar side,
iii. The postaxial metacarpal is slightly curved,
'and is more slender than the middle one, with
which it is fused at both ends.

b. The phalanges.

i. The preaxial digit has two phalanges, the ter-
minal one being small.

THE PELVIC GIRDLE. 425

ii. The middle digit has three phalanges, the proxi-
mal one being larg3 and lamellar, and the
distal one very small.

iii. The postaxial digit has only a single phalanx.

C. The Pelvic Girdle.

The pelvic girdle consists of the same three elements on
each side — ilium, ischium, and pubes — as the mammalian
pelvis, from which, however, it differs widely in form. The
three component bones are firmly united to form the os
innominatum ; and at the junction of the three is the aceta-
bulum, or socket for the head of the femur, which is partially
membranous at its bottom.

i. The ilium is an elongated lamellar bone, which
extends a considerable distance both in front
of and behind the acetabulum. Its inner
border is connected along nearly its whole
length with the sacral vertebrae and their
processes. The outer surface of the anterior
part is concave, that of the posterior part
convex : the inner surface of the bone forms,
with the adjacent vertebrae, two deeply concave
depressions in which the kidneys he. On the
outer surface, immediately behind and above
the acetabulum, is a projection, the anti-
trochanter, which bears on its outer surface
an articular facet for the great trochanter of
the femur,
ii. The ischium is a flattened lamellar bone pro-
jecting almost horizontally backwards from
the acetabulum, of which it forms about a
third. It is separated from the ilium, just
behind the acetabulum, by the oval ilio-sciatic
foramen, behind which the two bones are fused
together,
iii. The pubes is a long slender bar of bone which
projects downwards and backwards, parallel to

426 SKELETON OF THE FOWL.

the lower border of the ischium. It forms a
very small part of the ventral border of the
acetabulum, in front of which it extends for-
wards as a blunt pre-pubic process.

Behind the acetabulum, the pubes and
ischium are separated by the slit-like obturator
foramen, behind which the two bones are
fused for a short distance. The hinder end of
the pubes is thickened, and extends some dis-
tance beyond the ischium.

D. The Hind-limb.

In the bird's leg there is the same extensive fusion of
the bones which is characteristic of other parts of the skeleton.
There are no free tarsals, the proximal row having fused with
the tibia, and the distal row with the metatarsals.

The position of the limb is the same as in the rabbit ; the
knee pointing forwards, and the preaxial border being internal
along the whole length of the limb.

1. The thigh has only a single bone.

i. The femur has a cylindrical slightly curved
shaft, and enlarged ends. The proximal end
is produced on its inner side into a rounded
head, articulating with the acetabulum. On
the outer or postaxial side of the head is an
irregular process, the great trochanter. Be-
tween this and the head, at the proximal
end of the bone, is the articular surface for
the anti-trochanter of the ilium.

The distal end of the femur presents in
front a deeply grooved pulley-like surface for
the patella, a sesamoid bone in the tendon of
the extensor muscle of the leg : this surface is
bounded laterally by two prominent condylar
ridges, which articulate with the tibia ; the
outer or postaxial condyle having on its pos-
terior surface a deeply grooved facet for the
upper end of the fibula.

THE HIND-LIMB. 427

2. The leg.

i. The tibio-tarsus is a large stout nearly straight
bone, considerably longer than the femur : it
is formed by fusion of the tibia with the
proximal row of tarsals. The proximal ex-
tremity of the bone is much enlarged, and
bears at its end two surfaces which articulate
with the condyles of the femur ; on its ex-
tensor surface is the prominent cnemial crest,
into which the tendon of the extensor muscle
• of the thigh is inserted, beyond the patella.
The distal end of the bone has a prominent
pulley-like articular surface for the tarso-
metatarsus. The distal portion, corresponding
to the tarsal bones, is easily detached in the
young fowl.

ii. The fibula is a slender bone, closely applied to
the outer or postaxial surface of the tibio-
tarsus, with which it is often fused. Its proxi-
mal end is enlarged, and articulates with the
femur ; the rest of the bone is very slender,
and usually does not reach to the ankle.

3. The ankle-joint is between the proximal and distal rows

of tarsals : these at a very early age fuse with the
tibia and metatarsus respectively, so that in the
adult there are no free tarsal bones.

4. The pes. The first four digits are present ; the meta-

tarsal of the first is small and imperfect : those of

the next three are large, and are fused with each

other and with the distal tarsal row. The fifth or

postaxial digit, corresponding to the little toe in man,

is represented by a rudimentary metatarsal.

a. The tarso-metatarsus is a stout straight bone, about

as long as the femur, and formed by the fusion

of the distal tarsal row with the second, third,

fourth, and fifth metatarsals. Its proximal end

is enlarged, and bears terminally two cup-shaped

428 SKELETON OF THE FOWL.

facets for the tibio -tarsus ; on its posterior surface
is a prominent ridge deeply grooved behind, and
traversed at its base by a longitudinal canal, for
the passage of the tendons of the flexor muscles of
the toes. Behind the ridge, two sesamoid bones
are usually present.

In the shaft of the bone, three metatarsals
can easily be distinguished. They do not he
quite parallel to each other, the middle one
being slightly posterior to the other two at its
proximal end, and anterior distally.

At the distal end of the bone, the three meta-
tarsals separate from each other, and each ends
in a pulley -like surface for the corresponding digit.

b. The first metatarsal is incomplete, its distal end

alone being ossified. It is a small irregular
nodule of bone, attached by ligament to the inner
and posterior surface of the tarso-metatarsus, a
short distance above its distal end.

c. The digits. The four preaxial digits are present.

In many domestic fowls, notably Dorkings, an
extra or supernumerary toe is present on the
inner side of the foot. The true fifth, or postaxial
digit, is always absent,
i. The hallux, or great toe, is connected with the
first metatarsal, and directed backwards. It
consists of two phalanges, the last one bearing
a claw,
ii. The second toe is directed forwards and slightly
inwards. It has three phalanges, the last one
clawed,
hi. The third toe is the largest, and is directed for-
wards. It has four phalanges, the terminal
one clawed,
iv. The fourth toe is directed forwards and out-
wards. It has five phalanges, and the distal
one is clawed.

429

Chapter XVI.

DISSECTION OF THE PIGEON. Columba livia.

Birds are in many respects the most highly differentiated
vertebrates. While agreeing in their essential morphological
characters with the other air-breathing vertebrates, and more
especially with reptiles, they present a number of very special
characters, chiefly connected with the power of flight.

The modifications in the skeleton have been noticed in the
preceding chapter ; in the other systems the most noteworthy
features are : (1) The development of feathers, which are very
highly specialised products of the epidermis ; (2) the great size
of the pectoral muscles, by which flight is effected ; (3) the pro*
longation of the bronchi through the lungs into spacious thin-
walled air-sacs, lying between the viscera, and prolonged into
several of the bones ; (4) the persistence of the fourth right
aortic arch, instead A, as in mammals, the fourth left, to form
the arch of the aorta ; (5) the lateral, instead of dorsal, posi-
tion of the optic lobes of the brain ; and (6) in the female, the
disappearance, during development, of the ovary and oviduct
of the right side.

Physiologically, the most interesting points are : the high
temperature of the blood, which exceeds that of mammals,
varying between 100° and 110° F. in different birds ; and the
modification in the mechanism of respiration, expiration
being the active movement, and inspiration almost purely
passive.

Birds are oviparous, and the eggs, which are of large size,
are protected by calcareous shells. The eggs are fertilised in
the upper part of the oviduct before the shell is formed, and
hence commence to develop before they are laid. The egg

GG

430 DISSECTION OF THE PIGEON

contains within itself sufficient nutriment for the develop-
ment of the chick up to the time of hatching : and all that
is necessary to ensure development is that the egg, after it is
laid, should be kept at about the temperature of the parent's
body. This is in most birds effected by incubation, a task
usually fulfilled by the hen bird, but sometimes wholly or
in part by the cock.

The food of the pigeon consists largely of hard seeds, and
small stones are swallowed with it.

Pigeons should be hilled with chloroform, otherwise the
lice with which they are invariably infested may prove a great
ny,isance. Before the dissection is commenced, the bird should
be thoroughly plucked, with the exception of one of the wings,
on which the feathers may be left till later.

I. EXTERNAL CHARACTERS.

Excepting the lower joints of the legs and the toes, nearly
the whole surface of the body is covered with feathers.

A. The Main Divisions of the Body.

1. The head is elongated antero -posteriorly, and produced

in front into a pointed horny beak.

At the base of the beak is a naked swollen patch
of skin, the cere.

At the sides of the head are the large eyes, each
provided with upper and lower lids, and with a
well-developed third eyelid, or nictitating mem-
brane, a semi-transparent fold of skin which can be
drawn across the eye from its anterior angle with
great rapidity.

2. The neck is very long and flexible.

3. The trunk is deep dorso-ventrally, and somewhat com-

pressed from side to side. Along the mid-ventral
line is a prominent ridge, formed by the ventral
border of the carina, or keel of the sternum.

Posteriorly the trunk ends in a short blunt tail.

THE FEATHERS. 431

B. The Limbs.

1. The wing.

The surface of the wing is increased by the two
alar membranes at the armpit and shoulder. Its
inferior concavity grips the air, and the chief work
in flight is done by the front edge of the wing.
Hence the specially long and strong flight-feathers.

Identify the several bones of the wing by feeling them through
the skin. Note also the varying positions of the surfaces of
the several divisions of the wing, when it is extended and folded
respectively (p. 422).

The ala spuria, or bastard wing, is a small tuft
of feathers attached to the phalanx of the preaxial
digit.

2. The leg.

The toes and the tarso-metatarsal region are,
except in some domesticated breeds, devoid of
feathers and covered with large horny epidermal
scales. The ends of the toes are provided with
horny claws.

C. The Feathers.

The possession of feathers, which are specially modified
epidermal structures, is one of the most characteristic fea-
tures of the bird.
1. Structure of a feather.
Pull out one of the large feathers from the wing or tail ;
and note in it the following points :

a. The scapus, or stem, is a stiff axial rod, running
the whole length of the feather. It consists of
two dissimilar portions :
i. The calamus, or quill, is the tubular semi-
transparent proximal portion, the base of
which is inserted into the skin. At the
proximal end of the quill there is a small
aperture, the inferior umbilicus ; and at its
distal end between the quill and the vane,

432 DISSECTION OF THE PIGEON.

there is a second aperture, the superior um-
bilicus.

ii. The rachis, or shaft, is the distal solid portion

of the stem. It is a tapering, flexible, elastic

rod, square in transverse section, and grooved

along its ventral surface.

b. The vexillum, or vane, is the flattened portion of

the feather, attached along the sides of the

rachis. It is made up of barbs and barbules.

i. The barbs are a series of narrow elastic laminae,
attached by their bases along the two sides
of the rachis, perpendicular to the plane of
the vane.

ii. The barbules are much smaller processes, which
form fringes along the sides of the barbs :
they bear hooklets, which hold together the
adjacent barbs, and so give the feather that
resistance to the air on which the bird's power
of flight depends.

2. Varieties of feathers.

a. The quill feathers are the large feathers of the
wings and tail : they are of two kinds,
i. The remiges, or wing-quills, have the inner or
posterior half of the vane much broader than
the outer or anterior half. They are divided
into groups in accordance with the bones to
which they are attached (fig. 75, p. 440).

The primary quills are those attached to
the bones of the manus. They are eleven in
number, of which six are attached to the
middle metacarpal, and are hence called meta-
carpal quills : one, ad-digital, is attached
to the phalanx of the postaxial digit ; two,
mid-digital, to the proximal phalanx of the
middle digit ; and two, pre-digital, one of
which is small, are attached to the distal
phalanx of the middle digit.

EXTERNAL APERTURES. 433

The secondary quills are those attached
to the ulna,
ii. The rectrices, or tail- quills, have the two halves
of the vane about equal in size. They are
used by the bird for steering during flight.

There are twelve in the wild rock-pigeon,
but may be more in domestic breeds.

b. The coverts are the smaller feathers which cover

the bases of the quills of the wings and tail.

c. The contour feathers are the short soft feathers

covering the body generally. The barbules are
less perfectly developed than in the quill-
feathers, so that the barbs easily separate from
each other, especially at the base of the vane.

d. The filoplumes are the minute rudimentary feathers

left in the skin after the bird has been plucked.
Each consists of a hair-like stem, with a very
rudimentary vane at its apex.

3. Arrangement of the feathers.

The feathers are attached to certain definite areas
of the skin, known as pterylse, the intervening tracts
or apteria being devoid of feathers. This arrange-
ment is not very obvious in the pigeon, in which
the feathers are more uniformly distributed than in
many birds. Apteria may, however, be seen in the
mid-ventral and mid-dorsal regions of the body, and
along the outer surfaces of the proximal segments of
the wings.

D. External Apertures.

a. Median apertures.

i. The mouth is a wide slit-like opening in the long
pointed beak.

ii. The cloacal aperture is a transverse slit, with
prominent lips, on the ventral surface of the
body, below the base of the tail.

434 DISSECTION OF THE PIGEON.

b. Paired apertures.

i. The nostrils are a pair of oblique slit-like aper-
tures, between the beak and the cere,
ii. The external auditory apertures are a pair of
circular openings below and slightly behind
the posterior or outer angles of the eyes.
Each is surrounded by a circlet of small back-
wardly directed feathers, the auriculars.

Each leads inwards, and slightly backwards
and downwards, to the tympanic cavity.

II. DISSECTION OF THE PECTORAL MUSCLES.

Remove the shin from the ventral surface of the thorax
and from one shoulder, so as fully to expose the first pectoral
muscle. Clean this muscle along its whole length, defining its
boundaries carefully. Ligature the large blood-vessels.

1. The first or great pectoral muscle is large and trian-

gular, and forms the greater part of the side of the
breast. It arises from the whole length of the ven-
tral half of the keel of the sternum, from the
whole length of the clavicle, and from the lateral
part of the body of the sternum. Its fibres run for-
wards and outwards, converging to a broad flat
tendon, which is inserted into the deltoid ridge of
the humerus. From the anterior border of the
muscle, near the shoulder, a slip is sent off to the
skin.

The great pectoral is the main depressor of the

wing, and the most important of the muscles of

flight.

Cut through the great pectoral muscle, transversely to its

fibres, and about the middle of its length. Turn the two halves

of the muscle aside, so as clearly to define its origin and insertion ;

then cut away the muscle entirely.

2. The second pectoral muscle is a smaller and more

deeply placed muscle, similar in shape to the first
pectoral, and completely covered by it.

THE AIR-SACS. 435

It arises from the dorsal part of the side of the
keel of the sternum, and from the inner part of the
ventral surface of its body. Its fibres run forwards
and outwards, converging to be inserted into a
long tendon, which passes through -the foramen
triosseum, formed by the scapula, coracoid, and
clavicle, and reaches the back of the shoulder- joint,
where it is inserted into the dorsal surface of the
humerus, near the deltoid ridge.

Owing to the course taken by its tendon, the
muscle elevates the wing, and so opposes the great
pectoral muscle.

To see the insertion of the tendon of the second 'pectoral
muscle, remove or push aside the deltoid muscle, which covers
the bach of the shoulder-joint. Test the action of the muscle
by pulling the tendon.

3. The third pectoral muscle is a small triangular muscle,
arising from the outer border of the ventral two-
thirds of the coracoid, and from the costal process
of the sternum. Its fibres run forwards and out-
wards, to be inserted into the postaxial border of
the head of the humerus. It aids the great pectoral
muscle in depressing the wing.

III. DISSECTION OF THE AIR -SACS

The air-sacs are large, but very thin-walled sacs lying
mainly along the sides and dorsal surface of the body-cavity.
They communicate, as already noticed, with the bronchi,
and they play a very important part in the mechanism of
respiration of the bird.

Expiration is effected by the thoracic and abdominal
muscles, which compress the thorax and abdomen, driving
the air from the air-sacs, through the lungs and trachea.

Inspiration is effected by the elastic expansion of the
thorax and abdomen on relaxation of the muscles : this ex-
pansion causes an inrush of air along the trachea, through

436 DISSECTION OF THE PIGEON.

the lungs and into the air-sacs, the lungs being thus filled
with fresh air.

A complete dissection of the air-sacs is not easy to per-
form, and involves a good deal of damage to other structures.
The main features in their distribution and relations can,
however, be easily determined, as described below. The
dissection of the air-sacs must precede that of the other
systems, as if they are once cut into, the walls of the sacs
collapse, and they can no longer be inflated.

The air-sacs are nine in number : a median interclavicu-
lar, a pair of cervical or pre-bronchial, two pairs of thoracic
or intermediate, and a pair of abdominal or posterior.

Make a median ventral incision through the shin of the
neck, and pin out the flaps. Note the cesophagus, a wide
fleshy median tube; and the trachea, which lies to the left of
the oesophagus, and is easily recognised by the close-set rings
by which it is surrounded.

Separate the trachea from the surrounding tissues in the
hinder part of the neck, and pass a ligature round it. Slit
open the trachea about the middle of its length ; pass a blow-
pipe down the trachea through the slit, and tie it firmly in.
Inflate the lungs and air-sacs through the blowpipe and tighten
the ligature round the trachea to prevent the air escaping.

Make a mid-ventral incision through the skin from the
posterior edge of the sternum to the cloaca, and a transverse
incision along the posterior border of the sternum. Carry the
incisions through the muscles of the abdominal walls, taking
care not to injure the viscera beneath, and reflect the flaps
outwards.

1. The posterior or abdominal air-sacs are a pair of large
sacs, with very thin transparent membranous walls,
which he along the dorsal wall of the pelvic
cavity, ventral to the kidneys but dorsal to the
intestine. At the hinder end of the abdomen they
come close up to the ventral surface when inflated.
Each communicates with the posterior border
of the corresponding lung, near its outer angle.

THE AIR-SACS. 437

2. The posterior thoracic air-sacs are a pair of rather

smaller sacs lying along the outer sides of the
anterior ends of the abdominal sacs. They lie
mainly posterior to the lungs, but overlap the
ventral surfaces of their hinder ends. -

Each communicates with the outer and posterior
angle of the corresponding lung.

3. The anterior thoracic air-sacs cover the ventral surfaces

of the anterior two-thirds of the lungs, and extend
back so as to overlap the posterior thoiacic sacs.
They he at the sides of the thorax, and are in exten-
sive contact with the ribs and with the pericardium.
Each communicates with the ventral surface of
the lung, rather in front of the middle of its length,
and not far from the median plane of the body.

4. The interclavicular air-sac is a median sac formed by

the fusion of two originally distinct ones. It con-
sists of (1) a median portion, which surrounds the
hinder end of the trachea, and can easily be seen
in the angle between the two clavicles ; and (2)
lateral diverticula, one of which runs forwards
along each side of the trachea, while others pass
outwards, beneath the coracoid, and open into
large axillary air-sacs in the neighbourhood of
the shoulder- joints, which communicate with the
cavities of the humeri.

The interclavicular air-sac opens into the ventral
surface of each lung, immediately in front of the
point of entrance of the main bronchus.

5. The cervical air-sacs are much smaller, and he in front

of the lungs at the base of the neck, alongside the
vertebral column.

They communicate with the anterior angles or
apices of the lungs.

The position, extent, and relations of the air-sacs can be
best made out in a bird in which they have been distended

438 DISSECTION OF THE PIGEON.

with a solution of chromic acid (2 per cent.), and the whole
bird then thoroughly hardened by immersion in the same
solution for about a week. The specimen is then to be pre-
served in spirit.

V. DISSECTION OF THE DIGESTIVE SYSTEM.

A. The Buccal Cavity.

Open the mouth slightly; cut through the cheek, parallel
to the jaws, for about half an inch on each side ; and turn down
the floor of the mouth so as to expose the cavity fully. Note
the following structures :

1. The posterior narial apertures are a pair of long narrow

apertures lying side by side in the roof of the mouth.
The anterior two-thirds of their length are hidden
by a pair of prominent folds of the mucous membrane
of the palate.

2. The aperture of the Eustachian tubes is a median open-

ing, behind the posterior narial apertures.
Pass a bristle through the aperture, and along the right
and left Eustachian tubes to the tympanic cavities.

3. The tongue is triangular, ending in front in a sharp

forwardly directed point ; and produced behind into
a pair of backwardly directed lobes, fringed with fine
horny processes.

4. The glottis is an oval aperture, with tumid lips, in the

floor of the mouth, just behind the tongue.

5. The entrance to the oesophagus is large and wide, and

situated behind the glottis.

B. The Abdominal Viscera in situ.

Lift up the hinder end of the sternum, and cut through
its sides about midway between the dorsal and ventral surfaces.
Cut through the coracoids and clavicles about the middle of
their length. Detach the sternum with a scalpel from the
underlying parts, and remove it entirely.

DIGESTIVE SYSTEM. 439

1. The falciform ligament is a median vertical sheet of

peritoneum connecting the dorsal surface of the
sternum with the viscera': it is continuous in
front with the pericardium, and behind with the
omentum.

2. The oblique septum, a fibrous membrane attached to

the pericardium, and extending backwards, separat-
ing the air sacs from the general body-cavity.

3. The heart, which is still enclosed in the pericardium, is

large and conical, with the apex directed backwards.
It lies in the anterior part of the thorax.

4. The lungs are spongy bodies lying at the sides of, and

dorsal to, the heart, and very largely concealed by it.

5. The liver is a solid lobed organ, of a dark-red colour,

lying behind and at the sides of the heart.

6. The great omentum is a fold of mesentery loaded with

fat covering the viscera behind the liver. It is con-
tinuous in front with the falciform ligament.

Lift up the great omentum to expose the organs covered by it.

7. The gizzard is a large firm body lying immediately

behind the left lobe of the liver.

8. The duodenum is a loop of intestine lying along the

right side of the abdominal cavity, and embracing
the pancreas.

9. The intestine forms a convoluted mass in the hinder

part of the abdomen.

C. The Alimentary Canal.

Unravel the intestine by cutting through the mesentery
close to it, but taking care not to touch the duodenal loop.

Note the veins in the mesentery, which collect the blood
from the alimentary canal, and converge to form the portal
vein, which enters the liver in the notch between its right
and left lobes.

440

DISSECTION OF THE PIGEON

DIGESTIVE SYSTEM. 441

Note also, and preserve the posterior mesenteric vein,
which runs in the mesenterial fold connecting the rectum
with the dorsal wall of the abdomen.

1. The oesophagus commences at the hinder part of the

buccal cavity, and runs back through the neck and
thorax to the stomach. In the neck it is thin-
walled, and dilates behind to form a large bilobed
sac, the crop. In the thorax it has thick walls.

2. The thyroid gland is a paired structure at the base of

the neck.

3. The stomach, or proventriculus, lies dorsal to the left

lobe of the liver : it is rather wider than the oeso-
phagus, and has thick glandular walls, in which
the gastric juice is secreted. Attached to the right
side of the proventriculus is a small red body, the
spleen.

4. The gizzard is a large hard body, somewhat flattened

laterally, and with very thick muscular walls, the
muscular fibres radiating outwards from a bluish
tendon in the centre of each lateral surface. It
forms a powerful mill for grinding the food.

The proventriculus opens into the gizzard at its

Fig. 75. — Columba livia. Dissection of the male from the right side.
Half the liver and the greater part of the intestine have been
removed. In the right wing the bones and the arrangement of the
primary and secondary feathers are shown. The reproductive
organs are omitted, (a. m. m.)

A, nostril. AD, ad-.digital primary feather. B, external auditory meatus.
BW, bastard wing. C, oesophagus. CA, right carotid artery, d, crop. DA,
aorta. E, keel of the sternum. F, right auricle. G, right ventricle. HV,
hepatic vein. H 1, left bile-duct. H 2, right bile-duct. |, distal end of
proventriculus or stomach. |A, right innominate artery. |V, posterior vena
cava. JA, left innominate artery. Jv, right jugular vein. K, gizzard. |_,
liver. M, proximal limb of duodenum. MD the two mid-digital primary
feathers. MP, the six metacarpal primary feathers. M 1 , preaxial metacarpal.
M 2, middle metacarpal. M 3, postaxial metacarpal. N, cloacal aperture.
N 1 , preaxial digit of manus. O, bursa Fabricii. O 1 , proximal phalanx
of middle digit of manus. O 2, distal phalanx of middle digit of manus. P.
pancreas. PA, right pectoral artery. PD, pre-digital primary feather. PV
portal vein. P 1, first pancreatic duct, p 2, second pancreatic duct. P 3.
third pancreatic duct. Q, pygostyle. R, rectum. RC, radial carpal bone .
RX, rectrices or tail-feathers. R 1, ulnar digit of manus. S, ureter. SA,
right subclavian artery. SV, right anterior vena cava. T, rectal diverticulum.
U, kidney. UC, ulnar carpal bone. V, pelvis. W, lung. X, humerus. Y radius.
Z, ulna.

442 DISSECTION OF THE PIGEON

dorsal border, and rather to the left side ; while the
duodenum leaves it a little to the right side of the
same spot.

Split open the gizzard along its ventral edge, and wash out
the contents. Note the following features :

a. The great thickness of the muscular walls.

b. The thick cuticular lining to the cavity of the

gizzard, in which small stones are usually found
imbedded.

c. The openings of the proVentriculus and duodenum,

placed side by side at the dorsal edge.

5. The duodenum is a U-shaped loop formed by the first

part of the intestine. The pancreas lies between the
two limbs of the loop, the further dissection of which
will be postponed until the liver and pancreas are
examined.

6. The small intestine is a much-convoluted tube, of nearly

uniform diameter, and about two feet and a half in
length.

7. The rectum, or terminal portion of the intestine, is about

an inch and a half long. Its commencement is
marked by a pair of small csecal processes, the rectal
diverticula. Posteriorly, the rectum opens into the
cloaca, the examination of which should be deferred
until the kidneys have been dissected.

D. The Liver.

The liver is divided into a larger right, and a smaller left
lobe, the surfaces of which are hollowed and grooved to
receive the heart, duodenum, and gizzard.

The bile-ducts are two in number, and open into the
duodenum.

1. The left bile-duct is a wide tube, about an inch long,
which, emerging from the notch between the right
and left lobes of the liver, runs backwards to open

CIRCULATORY SYSTEM. 443

into the proximal limb of the duodenum, about half
an inch beyond the gizzard.

2. The right bile-duct is longer and narrower. It leaves
the liver near the dorsal margin of the right lobe,
and runs back to open into the distal limb of the
duodenum, about the junction of its middle and
distal thirds.

E. The Pancreas.

The pancreas is a compact pinkish gland lying in the loop
of the duodenum, along nearly the whole of its length, and
grooved by it.

The pancreatic ducts are three in number, and all open
into the distal limb of the duodenum.

The first two ducts leave the right border of the gland
about the middle of its length, and run forwards, parallel to
each other, to open close together into the distal limb of the
duodenum, about the middle of its length, and close to the
right bile-duct.

The third pancreatic duct is longer than the others. It
leaves the gland near its anterior end, and runs forwards to
open into the distal limb of the duodenum, close to its junc-
tion with the small intestine.

Pin out the duodenum on a dissecting board, and expose
the bile-ducts and 'pancreatic ducts. The dissection is best
performed under water, and from the dorsal surface.

V. DISSECTION OF THE CIRCULATORY SYSTEM.

A. The Heart.

The heart of the bird is large, and is divided into four
chambers, those of the right side being completely separated
from those of the left. The right side of the heart receives
the blood which is brought back by the veins from all parts
of the body, and drives it to the lungs to be aerated : from
the lungs it is returned to the left side of the heart, and
hence driven all over the body through the arteries. The

444 DISSECTION OF THE PIGEON.

right side of the heart therefore contains venous blood, and
the left side arterial blood.

Open the pericardial cavity so as to expose the heart fully ;
and dissect away the connective tissue and fat from the vessels
at the base of the heart.

The heart is conical, with the apex directed backwards
and slightly to the left side. A line of fat, which encircles
it, marks externally the division into auricular and ven-
tricular portions.

1. The auricular portion is anterior in position, and forms

the base of the cone It is thin- walled, dark in
colour, and divided into right and left auricles, the
division being indicated externally by a line of fat.

2. The ventricular portion is posterior, and forms the apex

of the cone. It is much thicker- walled than the
auricular portion, and paler in colour. The division
between right and left ventricles is not obvious
externally.

B. The Veins.

The veins should be dissected before the arteries, as they
lie for the most part nearer the surface. Great care must be
taken not to cut or tear them, as bleeding occurs freely, and is
apt to be very troublesome. If a complete dissection is re-
quired, the venous system should be injected from either the
femoral or the jugular vein.

1. Veins opening into the right auricle. The right auricle
receives the venous blood from the whole of the
body by three large veins, the vense cavae.

a. The right anterior vena cava returns blood from
the right side of the head and neck, the right
wing, and the right side of the breast.

It is a short wide vein, about a quarter of an
inch in length, opening into the right anterior
angle of the right auricle : it is formed by the
union of three principal veins.

THE VEINS. 445

i. The right jugular vein runs along the right side
of the neck, close to the vertebral column, and
dorsal to the oesophagus.

Anteriorly it unites -with the left jugular
vein, on the under surface of the -base of the
skull. In its course down the neck it receives
veins from the side of the neck and from the
crop ; and at the posterior end of the neck it
is joined by the vertebral vein.

ii. The right brachial vein returns blood from the
wing.

iii. The right pectoral vein is the largest of the
three. It returns blood from the pectoral
muscles, and receives also the internal mam-
mary vein, which runs along the inner surface
of the thoracic wall.

b. The left anterior vena cava returns blood from the

left side by veins corresponding to those opening
into the right vena cava. Ab the base of the
heart it runs transversely across the dorsal sur-
face of the left auricle, to open into the left
border of the right auricle.

To see the opening of the left vena cava into the auricle,
lift up the apex of the heart, and turn it forwards.

c. The posterior vena cava returns blood from the

posterior part of the body, including the liver.

It is formed by the union of the two iliac
veins a short distance behind the liver. The
vein, so formed, runs forwards through the sub-
stance of the right lobe of the liver, close to its
dorsal surface : it receives the hepatic veins as
it leaves the liver, and then runs forwards as a
short wide vessel to open into the posterior sur-
face of the right auricle.

Turn the heart forwards, to expose the posterior vena cava
as it enters the right auricle. Follow it bach through the

H H

446 DISSECTION OF THE PIGEON.

liver, slitting it up and washing out the contained blood. Note
the openings of the hepatic veins, and follow them with a seeker
into the liver. Follow the vena cava bach, behind the liver,
to the point where it is formed by the union of the right and
left iliac veins.

2. The veins of the kidneys.

The veins in connection with the kidneys are
best dissected in specimens injected from the
femoral vein. Their relations may, however, be
determined in an uninjected specimen by slitting
them open and following them with a seeker.

The kidneys are a pair of elongated three-lobed
bodies, lying against the ventral surface of the ilia,
at the sides of the sacrum.

The general arrangement of the veins is as follows :

The large veins enter each kidney, the femoral
and the hypogastric, returning blood from the leg
and the intestine respectively. They traverse the
kidney and unite within it. The common trunk
formed by their union is joined by the renal vein,
returning blood from the substance of the kidney
itself, and emerges from the inner side of the kidney
as the iliac vein, which unites with its fellow of the
opposite side to form the posterior vena cava.

a. The posterior mesenteric vein is a median vein

running backwards in the mesentery supporting
the rectum : it collects the blood from the hinder
part of the small intestine, and from the rectum
and cloaca. Opposite the posterior end of the
kidneys it receives a small median caudal vein
from the tail, and at once divides into the right
and left hypogastric veins.

b. The hypogastric vein of each side is joined by the

internal iliac vein from the inner surface of the
pelvic cavity, and then enters the kidney at its
posterior end : it runs forwards through the

THE VEINS. 447

posterior and middle lobes of the kidney, and
joins the femoral vein, in the groove between
the middle and anterior lobes. It receives blood
from the dorsal and outer portions of these two
lobes.

c. The femoral vein returns the blood from the leg.

Entering the kidney at its outer side, between
the middle and anterior lobes, it is joined, as
just noticed, by the hypogastric vein : a little
further on it receives the renal vein, returning
blood from the inner and ventral portions of the
two hinder lobes of the kidney, and smaller
veins from the anterior lobe : and it finally
emerges from the kidney as the iliac vein.

d. The iliac vein, formed in this way by the junction

of the femoral, hypogastric, and renal veins,
leaves the kidney at its inner border, between its
anterior and middle lobes ; and then runs for-
wards, uniting opposite the anterior end of the
kidney with the iliac vein of the other side, to
form the posterior vena cava.

3. The hepatic portal system.

The portal vein lies in the omentum, between
the two bile-ducts, and on reaching the liver divides
into two branches, one supplying each lobe. It
conveys to the liver the blood from the greater part
of the length of the alimentary canal, and is formed
by the union of three chief veins."

a. The gastro-duodenal vein returns the blood from

the right side of the gizzard, from the duodenum,
and from the last loop of the small intestine.

b. The anterior mesenteric vein returns blood from

the greater part of the length of the small intestine

c. The posterior mesenteric vein returns blood from

the hinder part of the small intestine and from
the rectum. The blood in the anterior part of

448 DISSECTION OF THE PIGEON.

this vein flows forwards to the portal vein, that

in the hinder part backwards to the hypogastric

veins.
4. Veins opening into the left auricle.

The pulmonary veins, which are extremely
short, open into the posterior surface of the left
auricle.

Turn the heart forwards : open the left auricle : wash out
the contents ; and pass a seeker along the pulmonary veins
into the lungs.

C. The Arteries.

1. The pulmonary arch is a single trunk, which arises

from the anterior end of the right ventricle, close to
the ventral surface and a little to the left of the
median plane. It divides soon after its origin into
right and left pulmonary branches.

a. The left pulmonary artery is short, and passes

direct to the left lung, which it enters in front
of the pulmonary vein, and ventral to the left
bronchus.

b. The right pulmonary artery passes dorsal to the

other arterial trunks, to enter the right lung in
front of the bronchus.

2. The aorta arises from the base of the left ventricle,

lying between the two auricles, and to the right

of, and slightly dorsal to, the pulmonary artery. It

gives off almost immediately the very large right

and left innominate arteries, and then continues its

course as the dorsal aorta.

a. The left innominate artery is a short wide artery,

which divides, after a course of about a quarter

of an inch, into two vessels of very unequal

size.

i. The left carotid artery, which is much the

smaller of these two vessels, runs forwards

along the neck, lying close to its fellow of

THE ARTERIES. . 449

the opposite side, in a groove along the ventral
surface of the vertebral column.

About half an inch from its origin it gives
off the vertebral artery, which runs forwards
along the side of the neck, in the vertebrar-
terial canal of the cervical vertebrae.

Opposite the angle of the jaw, the carotid
artery divides into external and internal
carotid arteries.

a. The external carotid artery supplies the tongue,
the muscles of the jaws, and other parts
of the head.

ft. The internal carotid artery enters the skull
by a foramen in its base, and supplies the
brain.

ii. The subclavian artery, which is much the larger
of the two branches of the innominate, runs
outwards, and divides, after a course of not
more than an eighth of an inch, into the
brachial and pectoral arteries.

a. The brachial artery runs straight outwards to
the wing, giving a branch to the shoulder-
joint.

/?. The pectoral artery is a large vessel, very-
little smaller than the innominate itself:
it runs outwards and backwards, looping
round the outer side of the sternal end of
the coraco'd, to enter the deeper surface
of the great pectoral muscle, in which it
divides into numerous branches.

b. The right innominate artery has branches exactly

corresponding to those of the left innominate.

c. The dorsal aorta, which is rather smaller than

either of the innominate arteries, arches over to
the right side, crosses the right pulmonary
artery, and then runs backwards along the

450 DISSECTION OF THE PIGEON.

mid -dorsal line of the thorax and abdomen. Its
principal branches are as follows :

i. The coeliac artery is a median vessel which arises
about the level of the auriculo-ventricular
septum of the heart It runs back in the
mesentery, and divides into branches which
supply the stomach, gizzard, and part of the
intestine.

ii. The anterior mesenteric artery is a median
vessel, which arises about a quarter of an
inch behind the cceliac artery, and runs back-
wards in the mesentery, dividing into branches
which supply the greater part of the length
of the intestine.

iii. The anterior renal arteries are a pair of small
vessels, which arise from the aorta opposite
the anterior ends of the kidneys, and supply
their anterior lobes. From the artery of the
left side a branch is given off to the ovary.

iv. The femoral arteries are paired, and arise oppo-
site the anterior lobes of the kidneys. They
pass outwards, dorsal to the kidneys, to supply
the extensor muscles of the thigh. Each gives
off a pubic artery, which runs backwards
along the ventral border of the pubes.

v. The sciatic arteries are a pair of larger vessels
which arise opposite the middle lobes of the
kidneys : they pass outwards and backwards,
between the middle and posterior lobes of
the kidneys, and through the ilio -sciatic
foramina, to supply the flexor muscles of the
thighs and the muscles of the legs.

Each sciatic artery gives off a middle
renal artery to the middle lobe of the kidney,
and a posterior renal artery to the posterior
lobe.

THE HEART. 451

#vi. The posterior mesenteric artery is a median
vessel, which arises opposite the hinder ends
of the kidneys, and supplies the rectum and
cloaca.

vii. The internal iliac arteries are a pair of vessels
arising about the same level as the posterior
mesenteric artery, and running outwards and
backwards along the hinder part of the pelvis.
. viii. The caudal artery is the terminal portion of
the dorsal aorta : it runs along the ventral
surface of the caudal vertebras.

D. Dissection of the Heart.

The heart may be dissected in situ, or the vessels may be
cut across about a quarter of an inch from it, and the heart
removed and placed under water in a dissecting dish.

1. The auricles.

Slit up the outer wall of the two auricles with scissors, and
wash out the contained blood.

a. The right auricle.

i. The openings of the venae cavae.

Pass a seeker into these openings, and note their positions
relatively to the cavities of the heart.

ii. The Eustachian valve is a muscular fold, pro-
jecting into the cavity of the auricle, on the
right side of the opening of the posterior
vena cava.

hi. The septum auricularum is a thin muscular par-
tition dividing the right and left auricles from
each other. Near its centre is a thin oval
patch, the fossa ovalis, marking the position
of the foramen ovale of the embryo.

iv. The right auriculo-ventricular aperture is a large
crescentic opening in the posterior wall of the
auricle.

452 DISSECTION OF THE PIGEON.

b. The left auricle.

i. The openings of the pulmonary veins are in a

small recess of the dorsal wall of the auricle.

ii. The left auriculo-ventricular aperture is cir-
cular.

2. The ventricles.
Cut across the ventricles transversely, about the middle of
their length. Note the shapes of the cavities, and the thick-
ness of their walls.

a. The right ventricle wraps round the right side of

the left ventricle. Its outer wall is compara-
tively thin, and its cavity is crescentic in trans-
verse section.

i. The right auriculo-ventricular valve is formed
by two muscular flaps : one long and external,
the other short, meeting the first at a slight
angle and connected to the ventricular wall
at the base of the conus of origin of the
pulmonary artery.

ii. The aperture of the pulmonary artery is at

the extreme anterior end and left side of the
ventricle.

Pass a seeker from the ventricle along the pulmonary artery,
and lay this open with scissors.

The semilunar valves are three crescentic
pocket-like flaps guarding the entrance to
the artery.

b. The left ventricle has very thick spongy walls,

except at the apex, where it is thin. Its cavity
is somewhat circular in transverse section.

i. The left auriculo-ventricular or mitral valve
is formed by three membranous flaps, which
project into the ventricle. Of these the two
outer form the antero- and postero-lateral

RESPIRATORY SYSTEM. 453

divisions respectively. Their hinder borders
are attached by three sets of chordae tendinese
to muscular processes of the ventricular wall,
the musculi papillares.

ii. The aperture of the aorta is at the anterior end
of the ventricle, and at its right side. In
the natural position it lies dorsal to the aper-
ture of the pulmonary artery.

Pass a seeker from the ventricle along the aorta, and lay
it open with scissors along its right side.

The semilunar valves are three pocket-like
flaps, similar to those of the pulmonary
artery.

VI. DISSECTION OF THE RESPIRATORY SYSTEM.

Remove the heart, if this has not already been done, and
also the alimentary canal and liver. Clean the trachea and
the ventral surface of the lungs.

1. The trachea, or windpipe, is a tube which commences

in front at the glottis, runs back along the neck,
and divides in the thorax into two bronchi, entering
the right and left lungs respectively.

It is surrounded by a series of closely set
rings, which are ossified ventrally, and cartilaginous
dorsally.

In the anterior part of the neck the trachea is
ventral to the oesophagus ; further back it lies along
its left side. In the thorax, where it again lies
ventral to the oesophagus, it divides into the two
bronchi, which run outwards and backwards to
enter the lungs on the ventral surface and near
their anterior ends.

2. The lungs are a pair of spongy bodies, attached to the

dorsal and dorso-lateral walls of the anterior part
of the thorax. They are covered ventrally by the

454 DISSECTION OF THE PIGEON.

pleura, an anterior continuation of the peritoneum.
Their inner edges he close together, separated from
each other in the median plane by the projecting
centra of the thoracic vertebrae, and by the dorsal
aorta.

Remove the pleura from the ventral surface of one lung ;
and separate the lung from the thoracic wall, noting its close
attachment to this wall, and the grooves in its surface which
lodge the ribs.

3. The syrinx, or ' lower larynx,' is the organ of voice of

the , pigeon, and is formed by the dilated hinder
end of the trachea, and the commencement of the
bronchi.

Cut across the trachea about an inch in front of its bifur-
cation. Make two lateral incisions, with scissors, along the
sides of the trachea and bronchi, and turn down the ventral
wall.

i. The tympanum is the cavity of the lower end
of the trachea and commencement of the
bronchi.

ii. The membrana tympaniformis interna is the
mucous membrane lining the inner wall of
the commencement of each bronchus.

iii. The membrana semilunaris is a delicate vertical
fold of mucous membrane extending for-
wards into the tympanum from the angle of
bifurcation of the trachea : by its vibration
the voice is produced.

iv. The pessulus is a slender bar of cartilage run-
ning across the tympanum, in the substance
of the membrana semilunaris, from the dorsal
to the ventral surface.

4. The structure of the lungs.

Pass a seeker along one of the bronchi, and slit it open
with scissors. Follow the branches of the bronchus through

RENAL AND REPRODUCTIVE SYSTEMS. 455

the lungs, and note the openings of the various air-sacs described
above (pp. 435-438).

VII. DISSECTION OF THE RENAL AND REPRODUCTIVE
SYSTEMS.

These may conveniently be considered together, because,
as in the rabbit, the genital ducts are in both sexes formed
from what were originally parts of the excretory system.

In the male the vas deferens is formed from the meso-
nephric or Wolffian duct ; in the female the oviduct is formed
from the Mullerian duct. In both sexes the kidneys and
ureters of the adult are formed on each side from the hind-
most division of the kidney of the embryo, i.e. the meta-
nephros. and the metanephric duct.

A. The Male Pigeon.
1. The renal system.

a. The kidneys are a pair of solid three-lobed bodies,

lodged in cavities bounded by the ilia and
sacrum, immediately behind the lungs. Their
ventral surfaces are covered by peritoneum.
There is no distinction between cortical and
medullary portions, and no ' pelvis.'

b. The ureters are a pair of straight narrow tubes.

Each arises in the substance of the anterior lobe
of its kidney, emerges between the anterior and
middle lobes, and runs backwards along the inner
side and ventral surface of the middle and
posterior lobes, and then straight back to the
dorsal wall of the cloaca.

Slit up one of the ureters, and follow it into the anterior
lobe of the kidney, noting the openings of smaller ducts into it
at intervals along its length.

c. The adrenals are a pair of small elongated yellowish

bodies, attached to the ventral surface of the

456 DISSECTION OF THE PIGEON.

anterior ends of the kidneys, alongside the iliac
veins. They have no physiological connection
with the kidneys.

2. The reproductive system.

a. The testes are a pair of oval bodies attached by a

fold of peritoneum to the inner borders of the
anterior lobes of the kidneys, and lying in con-
tact with their ventral surfaces.

b. The vasa deferentia emerge from the inner sides

of the posterior ends of the testes, and run back
along the outer borders of the ureters as a pair
of sinuous tubes with opaque walls.

Their hinder ends are dilated into small vesi-
culae seminales, which open into the cloaca.

3. The cloaca is a shallow cup-like depression into which

the rectum, ureters, and genital ducts open.

a. The rectum opens in the ventral wall of the cloaca.

Slit up the rectum along om. side, and note its opening into
the cloaca.

b. The urino-genital pouch is a saccular diverticulum

of the dorsal wall of the cloaca, separated from
the rectum by a transverse fold. The ureters
open into it dorsally and close to the middle
line ; and to their outer sides are the openings
of the vasa deferentia, on the apices of a pair of
small papillae.

c. The posterior chamber of the cloaca is a thin-walled

dilatation of the dorsal wall of the cloaca, behind
the urino-genital pouch, and close to the external
cloacal opening : into its dorsal surface a glan-
dular diverticulum, the bursa Fabricii, opens in
young birds, but is usually absent in the adult.

REPRODUCTIVE SYSTEM. 457

B. The Female Pigeon.

1. The renal system is the same as in tht male.

2. The reproductive system.

a. The ovary. In the embryo a pair of ovaries are

present, but of these the right one disappears
during development, and the left alone persists.
This is a large irregular-shaped body, suspended
in a fold of peritoneum opposite, and partly in
front of, the anterior lobe of the left kidney.
Numerous ova in various stages of development
project from its surface.

b. The oviducts. The left oviduct is a wide convoluted

tube, thin-walled in front, thick behind and
lying along the left side of the pelvic cavity. In
front it opens into the body-cavity by a long
oblique slit-like mouth, which is in close contact
with the ovary. Posteriorly it opens into the
cloaca.

The right oviduct is rudimentary : it has the
form of a slender tube, opening behind into the
cloaca, and usually not more than half an inch
in length.

The egg, when discharged from the ovary,
consists of the * yolk ' only : the ' white ' is an
albuminous substance formed round the yolk by
the walls of the middle portion of the oviduct ;
and the shell is secreted by the hindmost part of
the oviduct shortly before the egg is laid.

3 The cloaca is similar to that of the male, but larger.

a. The rectum has the same relations as in the male.

b. The urino-genital pouch. The openings of the

ureters are in the anterior wall of the pouch,
about an eighth of an inch apart. The openings
of the oviducts are to the outer sides of those of
the ureters, the opening of the left oviduct being

458 DISSECTION OF THE PIGEON.

much larger than that of the rudimentary right
one.

c. The posterior chamber is as in the male.

VIII. EXAMINATION OF THE BRAIN.

A. Removal oi the Brain.

Cut through the skin of the top of the head along the middle
line, and turn the flaps aside. Expose and scrape clean the
bones of the roof of the skull. Slice off with a scalpel the skull-
roof, taking care not to injure the brain, which lies very close to
the bone.

Cut away, bit by bit, the roof and sides of the skull, with
the scalpel and stout scissors, so as to expose the brain thoroughly,
leaving one ear uninjured. Remove the neural arches of the
first two vertebrce, and divide the spinal cord transversely. Turn
out the brain carefully, cutting across the several nerve-roots
one by one. Place the brain in a bottle of strong spirit, with a
pad of loose cotton-wool at the bottom, and leave it for two or
three days until it is thoroughly hardened. Examine it in
weak spirit.

B. External Characters of the Brain.
1. The dorsal surface.

a. The cerebral hemispheres are a pair of large pyri-

form bodies closely applied to each other in the
median plane. Their surfaces are nearly smooth,
and their anterior ends bluntly pointed.

b. The olfactory lobes are a pair of small conical bodies

projecting forwards from the anterior ends of the
hemispheres.

c. The pineal body is a small oval body immediately

behind the hemispheres, and in the angle between
them.

d. The optic lobes are a pair of smooth ovoid bodies

at the sides of the brain, behind and rather below
the hemispheres.

THE BRAIN. 459

e. The cerebellum is a median elongated oval body

marked by a number of transverse fissures. In
front it is in contact with the hemispheres ;
behind, it overlaps the medulla ; and laterally
it lies above the optic lobes.

f. The medulla oblongata is thick and wide : its an-

terior end is covered by the cerebellum, and pos-
teriorly it is continuous with the spinal cord.
„ <*

2. The ventral surface of the brain.

a. The optic chiasma, formed by the crossing of the

optic nerve, lies between and slightly in front
of the optic lobes.

b. The infundibulum is a small median process, imme-

diately behind the optic chiasma. It is connected
with the pituitary body, which is usually left in
the skull when the brain is removed.

C. Bisection of the Brain.

Divide the brain into right and left halves by a median
longitudinal section, so as to expose its cavities.

a. The third ventricle, or cavity of the fore-brain, is

of considerable extent from before backwards,
and dorso-ventrally ; but is very narrow from
side to side. It opens in front by lateral aper-
tures, the foramina of Monro, into the lateral
ventricles of the hemispheres. Its roof is pro-
duced upwards into the stalk of the pineal
body, and its floor is depressed to form the
infundibulum.

b. The Sylvian aqueduct, or cavity of the mid-brain,

is small, but communicates laterally with large
cavities in the optic lobes.

c. The fourth ventricle, or cavity of the hind-brain,

is wide from side to side, but shallow dorso-
ventrally.

460 DISSECTION OF THE PIGEON.

IX. THE SENSE ORGANS

A. The, Eye.

1. The eyelids.

The upper eyelid is only slightly movable : the
lower one very freely so.

The nictitating membrane, or third eyelid, is a
fold at the anterior angle of the eye, lying within
the other eyelids, between them and the eyeball. It
can be pulled obliquely downwards and backwards
over the front of the eyeball with great rapidity.

2. The muscles of the eyeball.

Open the orbit by removing its dorsal wall with strong
scissors, taking care not to damage the nictitating membrane.

a. The muscles moving the eyeball are essentially the

same as those of the dog-fish or rabbit.

Remove the eyeball completely, cutting through the muscles
and the optic nerve as far as possible from the eyeball.

b. The muscles of the nictitating membrane are two

in number, and he close to the inner or orbital
surface of the eyeball.
Snip away with scissors the recti and obliqui muscles.

i. The quadratus, or bursalis muscle, is an oblong
sheet of muscle, lying in close contact with the
inner surface of the eyeball. Its fibres arise
along the dorsal edge of the eyeball and run
downwards, ending just above the optic nerve
in a free border, which forms a tubular ten-
dinous sheath for the tendon of the pyramidalis
muscle.

ii. The pyramidalis is a small muscle somewhat
triangular in shape. It arises from the inner
surface of the ventral part of the eyeball,
below, and a little anterior to, the entrance of
the optic nerve. From this origin its fibres

THE EYE. 461

run upwards and backwards, converging to
form a long thread-like tendon, which, passing
through the tubular sheath of the quadratus,
runs down on the inner surface of the eye-
ball, and then under it, to be inserted into
the ventral border of the nictitating mem-
brane.

By the combined action of these two
muscles the rapid movement of the nictitating
membrane is effected.

3. The structure of the eye.

Place the eye under water, and divide it by an equatorial
incision into two halves.

The general structure of the eye is the same as
that of other vertebrates ; but the following points
are of special interest.

a. The pecten is a vascular pigmented fold, which pro-

jects into the cavity of the eye from its orbital
surface, ventral to the entrance of the optic nerve.
It is folded longitudinally like a fan ; and should
be compared with the falciform process of the
dog-fish (p. 288).

b. The lens is, as in terrestrial animals generally,

much flatter than that of the dog-fish.

c. The sclerotic plates are a ring of small flat bones

overlapping each other, and surrounding the
eye, just behind the junction of the cornea and
the sclerotic.

B. The Ear.
1. The tympano-Eustachian passage.

This is the modified hyo-mandibu'ar cleft of the
embryo, corresponding to the spiracle of the dog-fish.
Its connection with the ear is of a purely secondary
character. It serves to keep the air in the tympanic
cavity at the same pressure as outside.

II

462 DISSECTION OF THE PIGEON.

a. The external auditory meatus is the outer part of

this passage, from the exterior to the tympanic
membrane. The external aperture is a circular
hole on the side of the head, behind and below
the posterior border of the orbit.

Cut away the lower jaw. Cut away the external meatus
carefully with scissors, so as to expose the tympanic mem-
brane.

b. The tympanic membrane is a thin transparent

septum separating the external meatus from the
tympanic cavity.

c. The columella is a small rod of bone and carti-

lage lying on the inner side of the tympanic
membrane, but easily seen through it. It lies
almost horizontal, its free anterior end forming
a slight projection in the centre of the membrane.
The columella is formed from the uppermost
part of the cartilage of the hyoidean arch„ and
probably corresponds to part, or the whole, of the
hyo-mandibular cartilage of the dog-fish.

Cut across the tympanic membrane in front of the colu-
mella, to open the tympanic cavity.

d. The tympanic cavity is the middle dilated portion

of the tympano-Eustachian passage.^ On its
inner wall is the fenestra ovalis, into which the
posterior end of the columella is inserted.

e. The Eustachian tube is the innermost division of

the tympano-Eustachian passage. It is a narrow
tube, leading from the anterior and lower angle of
the tympanic cavity, and running forwards and
inwards between the proper base of the skull and
the basi-temporal bone. The Eustachian tubes
of the two sides unite, and open into the roof of
the mouth by a median aperture, just behind the
posterior border of the narial opening, and in

THE EAR. 463

front of the transverse ridge forming the hinder
border of the palate.

2. The auditory organ.

The essential organ of hearing is imbedded in the
side-wall of the skull. It is, however, easy to dissect,
as the layer of bone immediately investing it is very
firm and compact, and so preserves the shape of the
organ, while the rest of the bone is soft and spongy,
and easily removed.

Remove the hinder part of the side of the skull, containing
the auditory organ ; and carefully pick away with scalpel and
forceps the outer spongy layer of bone, until the shape of the
organ can be clearly seen.

a. The anterior vertical semicircular canal lies in the

natural position of the parts, close alongside the
posterior border of the optic lobe and the side of
the cerebellum. #

b. The posterior vertical semicircular canal lies almost

immediately below the anterior vertical canal.

c. The horizontal semicircular canal lies opposite the

lower border of the optic lobe and the hinder end
of the cerebellum.

The three semicircular canals, as in other
vertebrates, are in planes, each at right angles to
the other two.

464

Chapter XVII.
THE DEVELOPMENT OF THE CHICK.

I. PRACTICAL INSTRUCTIONS.

The following apparatus is requisite and should be adjusted
before the eggs are inserted.

An incubator {those made by Hear son are the best), the
temperature of which must be maintained constant at 41 "5° C.
(or 103-105° F,) by a reliable thermostat. Each egg should have
the date of insertion written in pencil. A number of metal
stands with small gas jet under each and connection for gas
tubing, though not essential, are desirable to carry the dishes
of warm salt solution, in which the eggs are to be placed during
examination. A convenient size is 8| inches by 6 inches, with
a flange of 1 inch width to support it, and covered on three sides
in order to screen the flame below from draughts.

A corresponding number of enamelled dishes, each with a
plaster of Paris mould to carry the egg. These moulds are
easily made by pouring the plaster into a small dish and depress-
ing the surface with a pot egg. A thermometer may be placed
in each dish, and warm 75 per cent, salt-solution added. The
dish is placed on the warm stage, the gas jet lit and turned down,
and the dish allowed to rise to the desired temperature. When
this is reached the gas must be reduced to a very minute flame.

Instruments, watch-glasses, and slides are also required,
together with some gummed paper. The most useful fixative
is perhaps picro-sulphuric (100 vols, of concentrated picric
acid and 2 vols, of strong sulphuric acid ; the precipitate is
filtered off and the filtrate diluted with thrice its volume of

PRACTICAL INSTRUCTIONS. 465

water). The excess of this fixative must be removed by repeated
changes of 50 per cent, and then 70 per cent, alcohol.

For general purposes when histological details are not
required, Perenyi's fluid is useful. It fixes rapidly, and the
specimen can be transferred direct to 70 per cent, alcohol. Be*
fore staining in bulk, remove all traces of acid by thoroughly
washing in alcohol or water.

For making sections of the earlier stages {segmentation
and up to twelve hours' incubation) an osmic acid mixture, such
as Flemming's strong mixture (see Appendix), is advisable.
Fixation will be complete in five or ten minutes. The tissues
should then be well washed in water and the sections prepared
as soon as possible.

Another excellent fixative is corrosive-acetic mixture (v.
Appendix). The sections should be treated with a weak solution
of tincture of iodine before staining.

Preparation of Preserved Embryos.

In order to preserve the embryo, the blastoderm must be
removed from the yolk. This operation requires care, and
two dangers have to be particularly avoided : — (1) the crump-
ling of the blastoderm ; (2) the rupture of the yolk. In order
to proceed successfully some operators take the whole yolk out
of the egg and out of the solution, and then place a ring of
gummed paper round the blastoderm. When this has adhered
a circular cut is made along the outer edge of the paper, the
egg is returned to the salt solution, and the blastoderm floated
off into a watch-glass or slide. It is better, however, to proceed
as follows : — Cut round the outer margin of the blastoderm,
after making a window in the upper region of the egg. Lift
the blastoderm and some salt solution carefully on to a watch-
glass, remove any adherent yolk, and straighten out any folds
by sucking with a pipette. The vitelline membrane should be
removed from the blastoderm before the fixing fluid is added.
If the above instructions are carefully performed, the blasto-
derm separated from the yolk will be found adhering to the
vitelline membrane. In order to separate them, hold the vitelline
membrane by forceps with its blastoderm surface downwards

466 DEVELOPMENT OF THE CHICK.

under the salt solution, and with the other hand work a needle
very cautiously along the edge of the blastoderm, between it
and the vitelline membrane, until the blastoderm is quite detached.
The adhesion only occurs near the 'periphery. This operation
may be done in a watch-glass, but if the specimen is to be mounted
whole, it should now be floated on to a slide before fixation, in
order to become well .flattened. The fixative should be added
from a pipette until the whole is well covered.

The length of time of fixation varies with the reagent em-
ployed. Osmic mixtures act very quickly ; sublimate in
half an hour, Perenyi's fluid in an hour, picro-sulphuric in
six hours. Wash well after fixing and transfer to increasing
strengths of alcohol. Keep in 75 per cent.

For staining, weak borax-carmine is most generally useful,
and the embryos can be left in it over night. Grenacher's
Hcematoxylin serves equally well. Differentiate with acid
alcohol, dehydrate, clear and mount in balsam.

To prepare Sections : —

Fix, harden, stain, and dehydrate as before. ' Cut away the
blastoderm from around the embryo. Place in pure cedar-
wood oil for an hour, then transfer to a mixture of cedar-oil
and soft paraffin at the temperature of the bath for half an hour,
and again to pure paraffin for one to four hours, according
to size. The paraffin should have the lowest melting-point
consistent with setting firmly at the temperature of the room.
A paraffin melting at 54° C. will be found generally useful.

Prepare a small paper box by folding up the ends of greased
paper, or use a couple of L-shaped pieces of metal placed together,
so as to form a shallow trough. Warm a needle and pair of
forceps. Pour the paraffin into the box or trough, insert the
embryo, orientate quickly with needle and forceps, and lower the
trough to half its depth into a dish of cold water. Allow to set
thoroughly. Remove the block so formed. Trim it with a
sharp knife so as nearly to expose the embryo, and proceed to fix
it on the carrier of the microtome and cut into sections. Trans-
verse sections are the best to begin with.

PRACTICAL INSTRUCTIONS.

467

Paraffin sections ready cub and stained may easily be
mounted by the following procedure : — Place a clean slide on
a white sheet of paper. Draw a pencil outline and then a
line | inch from one end across the outline to allow for the
label. Make a mixture of distilled water and Mayer's albumen
(v. Appendix) in the proportion of 10 c.c. water to 1 drop of
albumen, and cover the part of the slide on which you wish to mount

Fig. 76; — The Hen's Egg at the time of laying x

BA, blastoderm. SH,
white or albumen. WC>
•vitelline membrane.

egg shell. SM, shell membrane. SV, air chamber. WA.
chalaza, or twisted cord of denser albumen. Y, yolk. Z,

the sections with some of the mixture. Upon this float the sections
in the desired order, and warm over a flame or on the paraffin
oven until the sections are flattened out. Care must be taken not
to melt the paraffin. Then run off the water. Arrange the
sections neatly in order, and leave them till perfectly dry. When
dry, the paraffin can be dissolved out with xylol, and the sections
stained and mounted.

Wash in absolute alcohol and mount in balsam.

468 DEVELOPMENT OF THE CHICK.

II. GENERAL DESCRIPTION.

The hen's egg is formed by the union of the ' mature '
egg (that is, the egg-cell after attaining its full size and form-
ing its polar cells by unequal division), and the ' mature '
spermatozoon — that is, one of the sperm-cells formed by the
final equal division and differentiation of a sperm mother-
cell. These two cells when ready to unite are termed

1 gametes,' and when united they form a * zygcte.' Hence,
strictly speaking, the hen's egg or zygote is a double structure
composed of both male and female nucleus and protoplasm.

All the ova of a hen are produced from her left ovary.
The right ovary and oviduct disappear or shrivel early in life.
Good ' layers ' produce 100 eggs or more in a season, together
with many more immature ova which are absorbed towards
the end of the egg-laying period. The ovary then diminishes
greatly in extent, and the oviduct shrinks from being a tube

2 feet long and \ inch wide, to one 6 inches long and y1^ inch
thick. In order to see the mature ovary, a hen or pigeon
should be examined as on p. 457. The ripe ova are then
seen to be enclosed in vascular follicles, and the whole organ is
in a state of intense anabolic activity.

1. Segmentation (cleavage) of the Egg.

The egg of a bird consists of cytoplasm distended very
greatly by concentric layers of white and yellow yolk, except
for a lens-shaped area in which the nucleus lies. This region
appears as a white disc when viewed from above, and is
known as the ' germinal disc,' as it gives rise to the greater
part of the bird. The early segmentation is confined to this
area. It begins during the passage of the egg down the
oviduct, and is completed by the time the egg is laid. The
first step in the process is the appearance of two vertical,
shallow furrows, at right angles to one another, which penetrate
the surface of the disc, but do not extend to the bottom of the
cytoplasmic disc. Four radial fissures then appear about
midway between the former furrows. These are again

SEGMENTATION (CLEAVAGE) OF THE EGG.

469

followed by others, some of which are again radial in arrange-
ment, whilst others join up two or more radial furrows. In
this way the surface of the disc is, as it were, converted into

Fig. 77. — An early stage in the segmentation of the germinal disc of
the Hen's Egg. (After Coste and Duval.) x 10.

Fig. 78. — A later stage, in which the germinal disc has increased in
size and the segments have, by further division, become smaller
and more numerous. (After Coste and Duval.) x 10.

Both these figures are from eggs taken from the lower part of
the oviduct of the hen.

a mosaic of small polygonal areas, bounded peripherally by
larger irregular segments which form the ■ germ- wall.'

Fig. 79. — Section through the germinal disc and adjacent parts of the
yolk of a Hen's Egg about the middle of its stay in uterus. The
plane of section corresponds to a vertical line drawn through the
centre of Fig. 78. (After Duval.) x 25.

N, nucleus of completed segment. N', nucleus of segment not yet completely
separated from the yolk. VL, vacuole. Y, yolk. ZA, completed blastomere.
ZB, incompletely separated blastomere.

This areolation is an expression of cell-division. The
zygote-nucleus divides before the cleavage of cytoplasm takes

470

DEVELOPMENT OF THE CHICK.

place, and the cleavage of the egg or zygote is a division into
cells. Unlike the cleavage of most animal eggs, that of the
hen is partial or ' meroblastic ' owing to the great mass of
egg-yolk, which appears to prevent the formation of cells
except over the germinal disc. This layer of cells into
which the surface of the disc is transformed is at first a single
one, but by horizontal divisions soon becomes converted into
a double one. The disc is now spoken of as the * blastoderm.'
At or about this stage the egg is laid (figs. 76 and 80).

2. The First Day : Formation of the Germinal Layers.

The blastoderm at the time of laying is a circular patch,
about 3' 5 mm. in diameter, on the surface of the yolk.
Owing to its lower specific gravity the blastoderm is always
uppermost, however the egg be rolled over. It consists of a
marginal white rim, the area opaca, and a central circular,
more translucent portion, the area pellucida.

E B ZL

Fig. 80. — Vertical section of the blastoderm and adjacent part of the
yolk of a Hen's Egg towards the close of segmentation. The
anterior edge ia to the right, the posterior edge to the left hand.
(After Duval.) x 25.

A, blastocoel or segmentation cavity. E, epiblast. N', nucleus of blasto-
mere, which as yet is only incompletely separated from the yolk. VL, vacuole.
Y, yolk. ZL, one of the lower-layer cells or blastomeres.

Sections of the blastoderm show that the disc of cells
is not uniform in structure. It consists of several layers.
Among the lowermost of these a cavity is formed by the
accumulation of fluid. As this expands, the cells above it
are thinned out to form an outer germinal layer or epiblast,
and a lower germinal layer or hypoblast (fig. 81). The cavity

PRIMITIVE STREAK: ORIGIN OF THE MESOBLAST. 471

below the hypoblast is called the subgerminal cavity, and
corresponds to the archenteron of the frog.

After incubation has begun, the blastoderm spreads
rapidly whilst retaining its circular shape. By the end of the

Fig. 81. — Vertical section of the blastoderm and adjacent parts of the
yolk of a Hen's Egg at the time of laying, but before the commence-
ment of incubation. The anterior edge of the blastoderm is to
the right, the posterior edge to the left side of the figure. (After
Duval.) x 25.

BV, subgerminal cavity. E, epiblast. H, hypoblast. N', nucleus in yolk
round which a cell will be formed later. Y, yolk. ZF, formative cell. ZL,
lower-layer cells.

first day it is 20 mm. in diameter ; by the end of the second
it has spread about half-way round the egg. Complete
enclosure of the yolk is not effected until the seventeenth
day.

The Primitive Streak : Origin of the Mesoblast.

Shortly after the beginning of incubation an opaque band
becomes visible in the posterior part of the pellucid area,
which soon acquires an elongated appearance and is known
as the primitive streak. It grows rapidly backwards, and
the pellucid area elongates simultaneously, so that the
primitive streak always lies entirely in that area, which
thus assumes an oval and then a pear-shaped outline.
The appearance of the blastoderm at the twentieth hour is
seen in fig. 82.

As the primitive streak elongates a fine transparent line
may be seen running down its centre. This appearance is
caused by a groove, the primitive groove, which traverses the

472

DEVELOPMENT OF THE CHICK.

length of the primitive streak and terminates in front in
a small pit.

Fig. 82. — A diagrammatic figure of the blastoderm of a Hen"s Egg
about the twentieth hour of incubation. (In part after Duval.)
x 8.

AD, area pellucida : the part left white consists of epiblast and hypoblast
alone : in the hinder part of the pyriform area, covered by the light shading,
mesoblast is present as well. AK, area opaca. M, dotted line indicating the
boundary of the mesoblast. NR, neural plate, the first commencement of the
central nervous system. PS, primitive streak.

Transverse sections through the primitive streak show
that it is a keel-like band of proliferating epiblast (fig. 83),
and that the lower-layer or hypoblast cells have little or no

Fig. 83. — Transverse section across the blastoderm of a Hen's Egg about
the twentieth hour of incubation, the section passing through the
primitive streak about the middle of its length, x 200.

E, epiblast. H, hypoblast. M, mesoblast. PG, primitive groove. PS,
primitive streak.

share in its formation. The cells that it buds off migrate
into the spaces between the epiblast and hypoblast as a
paired sheet of loose tissue. This tissue is the mesoblast,

FATE OF THE GERMINAL LAYERS.

473

and the whole of the middle germinal layer or mesoblast is
derived from the primitive streak by proliferation of the
epiblast in this manner.

Fate of the Germinal Layers.

The epiblast or covering layer of the blastoderm gives
rise to the following organs and tissues of the chick : — The
epidermis and its appendages (scales, feathers, &c.) ; the
nervous system ; the sensory epithelium of the sense-organs ;
the lining of the mouth and of the cloaca.

The hypoblast gives rise to the epithelium of the ali-
mentary canal and of the glands that spring from it.

The mesoblast gives rise to all the connective tissue,
vascular, muscular and skeletal structures, as well as to the
urinary and reproductive organs.

The Notochord and Medullary Folds.

Immediately in front of the primitive streak a less
clearly denned opaque area appears towards the end of the
first day, which is shown by sections to be a forward con-
tinuation of the primitive streak, inseparably fused, however,

Fia. 84. — Transverse section across the body of a Chick Embryo at the
twenty-fourth hour of incubation, x 200.

CH, notochord. E, epiblast. H, hypoblast. M, mesoblast. MA. com-
mencing neural ridge. NG, neural grove. NP> neural plate.

with the hypoblast. This structure, which is called the
* head-process,' is destined J to form the notochord or fore-
runner of the vertebral column.

474 DEVELOPMENT OF THE CHICK.

Examination of the blastoderm in front of the primitive
streak at about the sixteenth hour of incubation shows that the
epiblast is several cells thick (the medullary plate) and is
folded in the middle line to form the first trace of the nervous
system — the * neural ' or ' medullary ' folds— and that the
centre or axis of the head-process possesses for the hinder
part of its length a rod of cells— the ' notochord ' already re-
ferred to. The mesoblast at present spreads out like a pair
of wings. The first definite organs to form are the nervous
system and the notochord. At the end of the first day,
therefore, the embryo is almost all head.

3. The Second Day.

On the first day a semicircular fold, or intucking of the
blastoderm (the ' head-fold ') had appeared in front of the
medullary plate, which makes great progress on this day,
and has the effect of folding off the anterior region of the
embryo from the underlying yolk. The formation of blood-
vessels on the surface of the yolk takes place at the same
time, and by the forty-eighth hour a circulation of blood
within and without the embryo is established.

The chief changes in the embryo are increase in its length
and differentiation of the constituent layers. The develop-
ment of the brain is hastened on. The neural folds become
higher and meet in the mid-dorsal line, forming the neural
crest and tube. The anterior half of this tube expands to
form a succession of swellings — the segments of the brain —
whilst the spinal cord remains slender, and minute, closely seb
thickenings of the neural crest foreshadow the future ganglia.
Two of the brain swellings are especially noticeable. These
are optic vesicles and form the first trace of the eyes. Mean-
while the ears are indicated by a pair of depressions of the
epiblast in the region of the future hind-brain (fig. 86, EI).

The most striking change made in the embryo during the
second day is the multiplication of the six or seven paired
blocks of tissue on either side of the nervous system and
notochord which had appeared on the first day. These
blocks or 'somites' are portions of the mesoblastic sheets

THE EMBRYO ON THE SECOND DAY.

475

cut off by longitudinal and transverse fissures. They develop
in order from before backwards, beginning at the level of
the ear. They give rise to all voluntary muscles, except those
of the head, the axial skeleton of the body, and the dermis.

Fig. 85. — A diagrammatic figure of the blastoderm of a Hen's Egg about
the twenty -fourth hour of incubation. (In part after Duval.) x 8.

AD, area pellucida : the part left white is the proamnion, and consists of
epiblast and hypoblast alone : in the hinder part of the pyriform area pellucida,
covered by the light shading, mesoblast is present as well. AK, area opaca.
BF, commencing fore-brain. M, dotted line indicating the limit to which the
mesoblast has spread. MS, mesoblastic somite or protovertebra. NG, neural
groove. PS, primitive streak.

Transverse sections of a two-day chick show (fig. 90, MP)
that these somites are at first hollow structures. Very
shortly afterwards, by a horizontal slit in the lateral wings
of the primitive streak, the mesoblast from which they are
cut off becomes hollow too. This latter cavity forms the
* ccelom ' or body-cavity of the future chick. The outer wall
of the cavity forms the body wall of the chick, and is there-
fore called the ' somatopleure,' whilst the inner wall, at pre-
sent stretched over the yolk, will form the wall of the gut or
1 splanchnopleure.' The hypoblast, meanwhile, has become a

476

DEVELOPMENT OF THE CHICK.

definite layer and is raised up from the yolk over that portion
of the head which is folded off from the egg ; while elsewhere

Fia. 86. — A Chick Embryo at the thirty-sixth hour of incubation ; seen

from the dorsal surface, x 20.
Flo. 87. — A median longitudinal, or sagittal, section of a Chick Embryo

at the thirty-sixth hour of incubation, x 20.

AN, head fold of the amnion. BF, fore-brain. BH, hind-brain. BM, mid-
brain. BO, optic vesicle. CH, notochord. CP, pericardial cavity. El, audi-
tory pit. GF, fore-gut, or anterior portion of the mesenteron. H, hypoblast.
MS, mesoblastic somite or protovertebra. NS, spinal cord. NT, neurenteric
canal. PS, primitive streak. RV, ventricular portion of tl e heart. SO, so-
matopleure. SP, splanchnopleure. TA, allantois. VV, vitelline veins.

THE BLOOD VESSELS.

477

it is fairly closely adherent to the yolk. This uplifted portion
of the gut is the ' fore-gut.'

Examine a series of sections, or selected sections, of a two-
day-chiclc. Draw and name the parts.

Fig. 88.— Transverse section across the head of a Chick Embryo at the
forty-third hour of incubation. The section is taken immediately
behind the auditory pits and the heart, and passes through the
rudiments of the glosso-pharyngeal nerves, x 100.

A, aorta. BH, cavity of hind-brain. CH, notochord. E, epiblast. h,
hypoblast. ME, somatopleuric layer of mesoblast. MH, splanchnopleuric
layer of mesoblast. n B, neural crest : the part shown in the figure gives rise
later on to the ganglia of the glosso-pharyngeal nerves. VV, vitelline vein.

In the region of the somites the undivided mesoblast is
during the second day differentiated to form the embryonic
kidney or mesonephros (fig. 90, KC). This arises from the
intermediate cell-mass as described on p. 482.

The Blood-vessels.

The vascular system on the second day consists of (1)
paired vitelline veins bringing nutritious blood from the yolk
to the heart ; (2) tubular heart ; (3) paired aorta dividing
into : (4) vitelline arteries. In addition there is a well-
developed circulation in the capillaries of the yolk-sac. The
veins of the embryo have scarcely begun to develop.

KK

478 DEVELOPMENT OF THE CHICK.

4. The Third Day. The Amnion.

As the embryo increases rapidly in weight, especially at
its head-end, it tends to sink down into the soft yolk. This
tendency is probably the reason why certain folds of the
somatopleure (head-fold, lateral folds, tail-fold) gradually
enclose and surround the body. These folds constitute the
' amnion ' or caul (figs. 86, 90, AN), which looks like a sort of
veil. The space between the two layers of the amnion is part
of the ccelom.

Fig. 89. — Transverse section across the head of a Chick Embryo at the
twent\ -fourth hour of incubation, passing through the region of the
mid-brain, x 100.

B, cavity of the mid-brain. CH, notochord, not yet separated from the
hypoblasts wall of the pharynx. E, epiblast. H, hypoblast. NA, neural
ridge. RT, commencing heart. TP, pharynx.

A. External Features.

The great size of the head is a marked feature of this and
succeeding stages, and the flexure both of the fore-brain on
the hind-brain and of the head upon the trunk are equally
noticeable. The head is also twisted over to the left side so
that the body is no longer symmetrical. The trunk has
elongated and the number of body-sigments (as indicated by
the somites) has imreased. There is still no trace of wing
or leg.' The hyoidean and first three branchial clefts become

ANATOMY.

479

distinct. The mouth is formed by perforation of the oral
membrane, a thin partition between the pharynx and a
depression from the surface known as the stomodseum.

B. Anatomy.

The most important feature is the commencing develop-
ment of all the internal organs. The third day is the most
eventful date of the whole incubation period.

aa. Alimentary Canal.

The pharynx undergoes a transformation ; four pairs of
pouches grow outwards towards the surface of the head.

Fig. 90. — A transverse section across the body of a Chick Embryo at
the forty-eighth hour of incubation, x 150.

A, aorta. AN, amnion, c, body cavity or CKlom. CH, notochord. CM,
myocoel, or cavity of mesoblastic somite. H, hypoblast forming roof of mid-
gut. KC, Wolffian duct. KS, nephrostome. ME, somatopleuric layer of
mesoblast. MH, splanchnopleuric layer of mesoblast. MP, muscle plate.
NE, rudiment of spinal ganglion. NS, spinal cord. OE, genital epithelium.
VC, posterior cardinal vein. VV, vitelline vein.

These outgrowths meet with corresponding ingrowths from
the surface. The first pouch is the hyomandibular, and gives
rise to part of the Eustachian tube and tympanic cavity.

480 DEVELOPMENT OF THE CHICK.

The second and third pouches (first and second branchial)
open to the exterior for a few hours, but subsequently close up
and disappear. The fourth pouch (third branchial) does not
open to the surface of the neck and disappears.

These pouches correspond to the spiracle and first three
gill-pouches of the dog-fish, but are at no time respiratory
organs.

The oesophagus, trachea, and lungs begin their develop-
ment at this period ; but the alimentary tract is still a straight
tube, the middle portion of which is open to the yolk-sac.

The formation of the tail-fold causes the terminal portion
of the gut to become tubular in contrast to the open mid-gut.
From this hind-gut there develops on the third day a pouch-
like outgrowth — the * allantois,' which ultimately expands
to form a stalked bladder-like sac, which extends through
the ccelom between the amniotic folds over the surface of the
embryo and yolk-sac, and comes into close contact with
the shell. The allantois is comparable with the bladder of
the frog, but is cast off at the time of hatching. It is very
highly vascular, and possesses the property of absorbing
oxygen from the air-chamber inside the shell and from the
atmosphere that penetrates through the pores of the shell.
The allantois therefore is the respiratory organ of the embryo,
just as the yolk-sac is the alimentary organ until the time of
hatching.

bb. Vascular System.

The heart on the third day is bent into a S -shaped loop
when seen from the left side, with the head pointing to the
right. The lower curve of the above S represents the auricle,
the upper one the ventricle leading to the truncus. From the
truncus six pairs of aortic arches arise and run in the sub-
stance of the visceral arches (mandibular, hyoidean, first,
second, third and fourth branchial), but the third branchial
arch is vestigial. Only three pairs are in existence at a given
time.

These join together on each side of the pharynx, and after
supplying the head form the paired dorsal aorta ; and this

ANATOMY. 481

coursing backwards gives off branches to the trunk and
then divides into arteries for the yolk-sac and allantois
respectively. The veins meantime have developed. These
embryonic veins are very different from • those of the later
bird. They consist of a pair of anterior and a pair of posterior
cardinal veins like those of a dog-fish, and again, like the fish,
these four veins open into a paired ' ductus Cuvieri ' before
reaching the sinus vencsus. There is also a median vein
which receives blcod from the yolk-sac and allantois, and forms
the ' ductus venosus,' which thus returns blood, aerated and
laden with yolk, direct to the heart.

cc. The Muscles, Skeleton and Connective Tissues.

These arise mainly from the ' protovertebrse ' which
increase in number from before backwards. About thirty-
six are present at the end of the third day. The full number
is attained on the sixth day. Their relations are as follows : —

1-4. Occipital.

5-16. Cervical.
17-19. Brachial.
20-25. Abdominal.
26-32. Pelvic.
33-35. Cloacal.
36-52. Caudal.

dd. The Renal Organs.

The kidney of vertebrates shows a division into three
sets of structures (see p. 262), called respectively pronephros,
mesonephros, and metanephros.

The pronephros is the larval kidney, and as few vertebrates
pass through a larval stage in their life-history, this structure
is very imperfect.

The mesonephros is the functional kidney of the adult
fish and amphibian and the embryonic functional kidney of
higher forms. Accordingly it is always well developed, but
for a varying time.

The metanephros is the functional kidney of adult birds,

482 DEVELOPMENT OF THE CHICK.

reptiles and mammals. It does not begin to function fully
until birth.

In the chick the pronephros is, as would be expected,
reduced to a mere vestige. Its tubules occur in segments
5-15 (as reckoned by the somites), but are very imperfect.
They arise at the end of the second day and disappear on
the fourth day. The pronephric duct into which they open
arises partly by their union and partly by an outgrowth
from the anterior tubules which grows backwards until it
acquires an opening into the cloaca. This duct is the Wolffian
duct, and corresponds with the mesonephric duct of the
dog-fish.

The mesonephros arises as a series of tubules in somites
16-32 (fig. 90, KC). Each tubule is a modified part of the
intermediate cell-mass, and opens into the Wolffian duct.
Each has, moreover, a Malpighian body at its commence-
ment. The whole organ receives its blood in part from the
dorsal aortal and in part from the posterior cardinal veins,
which thus form a renal portal system. Degeneration of the
mesonephros begins about the tenth day, and ultimately the
organ loses its excretory function and becomes converted
into the epididymis of the cock and into a functionless vestige
in the hen.

The metanephros begins its development upon the fourth
day, and extends from somites 25-33.

ee. The Organs of Sense.

The Eye. — As in vertebrates generally, the retina or
essential part of the eye is derived from the brain, whilst
the lens is an ingrowth of the surface epiblast. The retina
develops from the optic vesicle (figs. 91, 92). On the second
day these vesicles grow rapidly, and by the end of that day
are constricted at their base and pitted inwards on their outer
surface. The lens arises as a hollow pitting-in of the surface
epiblast, as in fig. 91, OL.

On the third day the optic vesicle has doubled upon
itself to form the retina on the inner side and the pigment
layer on the outer. The stalk by which it is connected with

ANATOMY.

483

the brain becomes the basis of the optic ' nerve,' and along
its tract nerve-fibres grow out from the retinal ganglion-cells
to the brain.

The optic * nerve ' is therefore not a nerve but a tract
leading from the peripheral brain ganglion-cells of the retina
to the visual centres.

Fig. 91. — Transverse section across the head of a Chick Embryo at the
forty-eighth hour of incubation. The section is taken along a line
corresponding to one joining the relerencc letters El and OL. in the
three -day embryo shown in fig. 94. Owing to the cranial flexure,
both fore- brain and hind- brain are cut by the section. The right side
of the section is slightly anterior in position to the left side. x 60.

A, aorta. AC, carotid artery. BF, cavity of fore-brain. BH, cavity of
hind-brain. CH, notochord. El, auditory pit. HM, nyo-mandil>ular cleft.
MN, mandibular arch. OC, cavity of optic cup. OL, inva^inat on of epibla*t
to form the lens. OS, optic stalk. PT, pituitary body. TP, pharynx. VII,
facial nerve.

The further development of the eye takes place gradually.
The rods and cones are the last parts to be formed.

484

DEVELOPMENT OF THE CHICK.

The Ear. — The ears arise during the second day as a
pair of epiblastic depressions at the sides of the hind-brain
(fig. 91, EI). These pits sink inwards and become converted
into the two auditory vesicles, the mouths of which close by
the end of the third day.

These vesicles give rise to the membranous labyrinth.
By foldings of their walls the utriculus becomes constricted

BS

Fig. 92. — Transverse section across the fore-brain and eye of a Chick
Embryo at the sixtieth hour of incubation. On the right side the
section passes through the optic stalk ; on the left side it passes
just behind the stalk, x 45.

BF, cavity of fore-brain. BS, cavity of commencing cerebral hemisphere.
MX, maxillary arch. OC, inner wall of optic cup. OD, outer wall of optic
cup. c L, lens. OS, optic stalk.

off from the sacculus. From the utriculus the semicircular
canals are formed ; from the sacculus the cochlea arises.
These changes take place between the fifth and eleventh
days. The entire epithelial lining of the labyrinth is thus
derived from the epiblast.

The accessory structures of the ear are of very diverse
nature, and are too complex for description in this placa.

OBSERVATIONS ON THE LIVING EGG. 485

The cavity of the middle ear and Eustachian tube develops
in part from the first or hyomandibular visceral cleft.

The succeeding changes in the egg are as follows : —

Fourth day. — The egg albumen becomes restricted to a
small area, which is gradually enclosed by the allantois.

The wings and legs make their appearance as flattened
conical buds. The internal changes described during the
third day are extended. The permanent kidney and its duct,
the ureter, are developed.

Fifth day. — The embryo is so strongly flexed that the
head and tail are almost in contact. The cartilaginous
elements of the skeleton have appeared.

Sixth day. — The characters of a bird are developed in
the wings and feet, skull, and alimentary canal.

Seventh day. — The amnion and allantois commence
rhythmical contractile movements by which the embryo is
rocked to and fro. The head now ceases to grow more rapidly
than the body.

Ninth day. — Feathers begin to protrude through the skin,
though still enclosed within their sheaths.

Twentieth day. — The beak perforates the inner shell-
membrane and the chick breathes air. The allantois circula-
tion ceases and that of the lungs begins.

Twenty-first day. — The chick is hatched.

III. OBSERVATIONS ON THE LIVING EGG
Place an egg of the desired age in a dish of warm salt-solu-
tion, allowing it to rest on the plaster mould and just covering
with fluid. Carefully remove the shell over the uppermost area
so as to make a window an inch or an inch and a half in diameter.
Allow some of the egg-white to escape into the dish, but take great
care to avoid puncturing the yolk. The blastoderm will be found
lying just under the aperture, and if the egg has developed nor-
mally the following structures will be visible and should be
drawn, using a hand-lens when necessary (figs. 85-94). The
large end of the egg should be drawn pointing to the left of the
observer.

End of the first day (fig. 85). — The neural folds- are well

486 DEVELOPMENT OF THE CHICK.

developed and about to meet in the region of mid- and
hind-brains. Four to six pairs of somites have appeared.
The primitive streak is still clearly marked, and the vitelline
veins are established.

At the thirty-sixth hour (figs. 86, 93). — The vascular area
(AV fig. 93) has been organised and the heart and paired aorta
have developed in the embryo. The neural folds have met
along the mid-dorsal fine, except at the hinder end of the
body, and the brain occupies about half the total length of
the tubular nervous system. The first indications of the eye

Fig. 93. — The yolk of a Hen's Egg at the thirty-sixth hour from the
commencement of incubation. The structure of the embryo at this
stage is shown on a larger scale in figs. 86 and 87. xf.

AD, area pellucida of the blastoderm. AK, area opaca. AV, area vasculosa.
EM, embryo. SM, vitelline membrane^ Y, Yolk-sac.

and ear respectively are seen in the optic vesicles and auditory
vesicles (BO and EI fig. 86). The head-fold of the amnion
is rising up. From twelve to fourteen pairs of somites are
present.

At the forty-eighth hour. —The * cranial flexure ' and
twisting of the head over to the left side now render the
embryo asymmetrical. The vascular area has increased,
and the vitelline arteries are distinct and he immediately
behind the vitelline veins. The amniotic folds have increased
in extent and the number of somites is now about 27-28.

OBSERVATIONS ON THE LIVING EGG. 487

Fig. 94. — A Chick Embryo at the end of the third day of incubation.
Owing to the twisting of the fore part of the embryo, the head and
neck are seen from the right side, and the hinder part of the body
from the dorsal surface. The amnion has been removed, x 20.

A, dorsal aorta. A 1 , first or mandibular aortic arch. A 3, third aortic arch,
in the first branchial arch. AC, carotid artery. AV, vitelline artery. BF,
thalamencephalon or fore-brain. BH, medulla oblongata. BL, cerebellum.
BM, mid-brain. BS, cerebral hemisphere. El, auditory vesicle. HC 1, first
branchial cleft. HC 2, second branchial cleft. HM, hyo-mandibular cleft.
MS, mesoblastic somite or protovertebra. NS, spinal cord. OC, optic cup.
OF, olfactory pit. OL, lens. PN, pineal body. RA, auricle of heart. RT,
truncus arteriosus. RV, ventricle of heart. VV, vitelline veins.

488 DEVELOPMENT OF THE CHICK.

At the end of the third day (fig. 94). — The pharynx and
the four pharyngeal pouches (hyomandibular slit, first,
second and third branchial clefts) have appeared, and the
first three aortic arches are established. The regions of the
brain are demarcated ; the eye and ear respectively are now
clearly indicated. The number of somites has greatly
increased. The chief embryonic venous spaces (cardinal
veins) can be easily seen and traced to their opening into
the sinus venosus. The circulation of the blood should
be carefully followed and drawn.

At the end of the fourth or fifth day. — The allantois
grows out of the hind-gut. The wing and leg bud out from
the sides and become indistinctly segmented by joints. No
traces of digits are as yet present. The head is now relatively
enormous, and the cranial flexure, or bending of the fore-
brain upon the hind-brain, passes through an angle of at
least 120°. The amnion is now fully developed and encloses
the entire embryo in a sort of water-bath.

The rapid development of the chick may be best ascertained
by sketching an embryo of, say, about 36 hours in the morning
and making another sketch in the late afternoon. If kept in
warm salt-solution during this interval, the embryo will be found
to have developed considerably. It must, of course, remain
attached to the yolk.

489

APPENDIX

LIST OF REAGENTS.

I. REAGENTS FOR KILLING, HARDENING, PRE-
SERVING, AND DISSOCIATING.

ABSOLUTE ALCOHOL.

Use. — Hardening reagent, but most used for dehydrating speci-
mens before clearing them in oil of cloves or xylol.

ALCOHOL (Spirit).

It is used of the following strengths : 90 per cent., 70 per cent.,
50 per cent., 30 per cent.

Preparation. — 90 per cent, alcohol is the ordinary ' methy-
lated spirit.' The weaker solutions are obtained by
diluting it with water.

Use. — (1) For preserving specimens ; at least 70 per cent.

(2) Hardening reagent ; 90 per cent.

(3) For dissolving the intercellular substance, so that
the cells may be isolated by teasing ; 30 per cent.

CHROMIC ACID, 10 per cent, solution.

Preparation. — Dissolve 10 grms. chromic acid crystals in
100 c.c. water.

490 APPENDIX.

Use. — As a fixing and hardening reagent it must be reduced to
a 1 per cent, or *5 per cent, solution by addition of water
before use ; and osmic acid may be added to it.

CORROSIVE SUBLIMATE, solution.

Preparation. — Saturate water with mercuric chloride : 1 litre
dissolves about 70 grms.

Use. — (1) As a hardening reagent it is used cold.

(2) For killing small animals it may be used warm or
cold.

After use it should be washed out in alcohol, to which a
little tincture of iodine is added.

CORROSIVE- ACETIC, mixture.

Preparation. — Take 90 vols, of concentrated sublimate solution
and add 10 vols, of glacial acetic acid.

Use. — An excellent fixative for general work. Wash out in
successive strengths of 50. 70. and 80 per cent, alcohol.

FLEMMING'S SOLUTION.

Preparation. — Fifteen parts chromic acid (1 per cent), 4 parts

osmic acid (2 per cent.), with or without 1 part glacial acetic

acid.
Use. — Good fixative and hardening reagent. Wash well in

running water for 24 hours. Then place in alcohol,

beginning with 30 per cent.

MULLER'S FLUID.

Preparation. — Dissolve 25 grms. potassium bichromate and
10 grms. sodium sulphate in 1 litre water.

Use. — Hardening reagent. It also dissolves intercellular sub-
stance, so that the cells may be separated by teasing.

OSMIC ACID (Os04), 2 per cent, solution.

Preparation. — Dissolve 1 grm. ' osmic acid ' crystals in 50 c.c.
distilled water. Keep in the dark.

Use. — For killing microscopic animals the vapour is a good
fixative. Before using the solution, it is to be diluted with
1-3 times its bulk of water. Avoid inhaling the vapour.

LIST OF REAGENTS. 491

PERENYI'S FLUID.

Preparation. — Dissolve 1*5 grm. chromic acid crystals in about
half a litre of water. Add 40 c.c. strong nitric acid. When
cold add 300 c.c. 90 per cent, alcohol (methylated spirit),
and make up to 1 litre with water. The solution becomes
violet -coloured.

Use. — Hardening reagent, especially useful for embryos.
Tissues after remaining in the fluid 2-4 hours should be
washed in 70 per cent, alcohol.

PICRIC ACID, saturated solution.

Preparation. — Saturate water with picric acid by shaking it
up with the crystals. Allow to stand : decant before use.

Use.— Hardening reagent. Wash out with alcohol.

PICRIC ACID, Kleinenberg's solution.

Preparation. — To 900 c.c. distilled water add 18 c.c. strong
sulphuric acid, and add picric acid crystals to concentration.

Use. — Hardening reagent. Wash out in alcohol

ACETIC ACID, glacial or 1 per cent, solution.

Preparation. — Dissolve 10 c.c. glacial acetic acid in water :
make up to 1 litre with water.

Use. — (1) For clearing tissues, and especially for rendering
nuclei more distinct.

(2) For killing small animals the vapour of glacial acid
is effective,

FORMALDEHYDE (Formol, formalin).

Preparation. — Dilute the 40 per cent, solution to 4 per cent.

Use. — For preserving animals, especially for hydroids, frog's
spawn, and gelatinous tissues such as snails. A mixture
of equal parts of 1 per cent, potassium bichromate and
8 per cent, formalin is a good hardening reagent for brain's.
Leave them in the mixture 2-3 weeks, then wash for
a day in running water, and keep in 5 per cent, formalin
till required.

492 APPENDIX.

II. REAGENTS USED IN STAINING.

ALUM CARMINE.

Preparation. — Dissolve 200 grms. ammonia-alum in water : boil
10-20 minutes with excess of carmine : filter : make up
to 1 litre with water : add a few drops of carbolic acid to
preserve from mould.

Use. — Stain for hardened tissues. It may be diluted with
4 times its bulk of water or less.

BORAX CARMINE (Grenadier's).

Preparation. — Dissolve 125 grms. carmine and 20 grms. borax
in 500 c.c. water : add 600 c.c. 70 per cent, alcohol : allow
to stand two or three days : filter.

Use. — Stain for tissues which have been hardened. May be
used warm (50° C.) or cold. The tissues should afterwards
be treated with acid alcohol. For staining objects which
are to be mounted whole, such as chick embryos, it should
be diluted with several times its bulk of 40 per cent,
alcohol, and the treatment with acid alcohol should be
prolonged.

ACID ALCOHOL.

Preparation. — To 100 c.c. of 70 per cent, alcohol add 25 c.c.
hydrochloric or nitric acid.

Use. — For differentiating the parts of stained specimens, and
for removal of surplus stain.

PICRO CARMINE.

Preparation. — Dissolve 10 grms. carmine in 40 c.c. strong
ammonia and 2 litres water. Add 50 grms. picric acid.
Shake well a few minutes : allow to stand : and decant.
Allow to stand a few days, stirring occasionally. Evapo-
rate to dryness over a water-bath. To every 2 grms. of dry
residue add 100 c.c. water.

Use. — Stain. It may be used with advantage for specimens
hardened with osmic acid or otherwise. Subsequent

LIST OF REAGENTS. 493

immersion for a short time in acid alcohol improves the
effect in the case of some tissues. If the object be dehy-
drated rapidly, or by means of alcohol containing picric
acid, a double-stained effect may be obtained, some tissues
being stained yellow and others red.

JBRAZILIN.

Preparation. — Prepare the two following solutions A and B.
A: 1 grm. iron-alum dissolved in 100 c.c. 70 per cent,
alcohol. B : 5 grm. brazilin in 100 c.c. 70 per cent, alcohol.

Use. — For staining sections on the slide or minute organisms
(Protozoa, &c). The slides are placed in fluid A for 1-3
hours. Then after slight washing in 70 per cent, alcohol
in solution B for about 18 hours. They are then dehy-
drated and mounted.

GRENA CHER'S HEMATOXYLIN.

Preparation. — 24 c.c. saturated solution of Hematoxylin in
absolute alcohol, 900 c.c. saturated solution of ammonia-
alum in water. Add these together and allow to stand for
a week and filter. Then add 150 c.c. pure glycerin and
300 c.c. 90 per cent, alcohol. It improves with keeping.
For whole mounts dilute with 30 per cent, spirit. Surplus
stain may be carefully removed by acid alcohol of half the
usual strength. When the required depth of stain is
obtained, transfer the object to 70 per cent, alcohol which
contains a trace of ammonia, to neutralise the acid.

MEYER'S ACID H^EMALUM.

Preparation. — Dissolve 1 grm. hsematein in 50 c.c. 90 per cent,
alcohol. Dissolve 50 grms. alum in 1 litre distilled water.
Mix and allow to settle. Add 20 c.c. glacial acetic acid
and a crystal of carbolic acid or thymol.

Use. — For staining tissues in bulk or sections on the slide. In
either case the object must be brought down to water, and
after staining washed in 1 per cent, alum-solution.

It is preferable to Kleinenberg's solution in being (1)
of definite composition, (2) less affected by light, and (3)
keeps better.

LL

494 APPENDIX.

METHYLENE BLUE.

Preparation.— Dissolve TV grm. in 100 c.c. salt-solution.

Use. — For staining small animals in life. It brings out the
gland-cells, muscle-cells, and nerve elements. A small
quantity should be diluted with twenty times its bulk of
distilled water.

METHYLENE BLUE AND EOSIN.

Preparation. — Take saturated solutions (A) of eosin and (B) of
methylene blue in 75 per cent, alcohol.

Use. — For staining blood-films. Take two clean cover-glasses,
and place a small drop in the centre of one of these.
Cover this with the other and after 2 seconds draw apart
laterally. Wave the slips in the air to dry. Dip each
cover-slip in (A) for 1 minute, drain rapidly, dip into
75 per cent, alcohol for 5 seconds and again into fresh
75 per cent, alcohol for 5 seconds. Then for 20 seconds
into solution (B). Dip momentarily into 75 per cent,
alcohol. Press slightly on a pad of filter paper. Wave
till dry. Then mount in a drop of Canada balsam placed
on a clean slide.

METHYL-GREEN.

Preparation. — A saturated water-solution (8 grms. in 100 c.c.)
of methyl-green with a trace of 1 per cent, acetic acid.

Use. — To fix and stain the nuclei of Protozoa.

DAHLIA, *25 per cent, solution.

Preparation. — Dissolve £ grm. in 100 c.c. distilled water.
Use. — For staining Protozoa and cover-glass preparations.

SAFBANIN.

Preparation. — Dissolve |-1 grm. in 100 c.c. water or 50 per
cent, alcohol.

Use. — A nuclear stain for the same objects as Dahlia.

LIST OF REAGENTS. 495

III. CLEARING REAGENTS

(i.e. REAGENTS FOR REMOVING ALCOHOL OR PARAFFIN, AND
RENDERTNO THE OBJECT PERMEABLE BY BALSAM).

OIL OP CLOVES.

Use. — To remove alcohol from an object before mounting it in
balsam. The object must be left in the oil till it becomes
transparent : if left too long the object may become
brittle. It is liable to destroy the colour of objects stained
with haematoxylin.

OIL OF CEDAR-WOOD.

Use. — Same as oil of cloves. It is less liable to injure speci-
mens stained with haematoxylin, and is also useful for
clearing before imbedding.

XYLOL.

Use. — To remove paraffin from sections (see Introduction,
p. xxxiv), which should only remain in it long enough for
the paraffin to be dissolved.

IV. MOUNTING MEDIA.
SALT-SOLUTION, '75 per cent.

Preparation. — Dissolve 7*5 grms. common salt in 1 litre water.
Use. — Living or fresh tissues are usually examined in it.

GLYCERIN. — Used either pure, or diluted with an equal bulk of
water.

Use. — For mounting microscopical objects.

GLYCERIN JELLY.

Preparation. — Soak gelatin in water till soft ; pour off excess
of water ; melt, and, while still fluid but cool, add one-
eighth of its bulk of white of egg : mix thoroughly : boil :
strain through flannel : to strained fluid, while still hot, add

496 APPENDIX.

half its volume of a mixture of equal parts of glycerin
and camphor water.

Use. — For mounting microscopical objects which are too
transparent when mounted in balsam (see Introduction,
p. xxxiii). The edges of the cover-glass require cement.

FARRANT'S MEDIUM.

Preparation. — Dissolve 400 grms. white gum arabic in 400 c.c.
water without heat, stirring occasionally : add 200 c.c.
glycerin : strain through clean flannel if necessary. Keep
in a stoppered bottle with a piece of camphor.

Use. — Same as glycerin-jelly, but it is used cold, and objects
may be transferred to it direct from water. It soon
hardens at the edges : ringing with varnish is therefore
unnecessary.

CANADA BALSAM.

Preparation. — May be used as bought from the chemist, or
diluted to a suitable consistency with benzol. Still better,
evaporate to dryness : powder and dissolve in chloroform,
benzol, or xylol : benzol is best.

Use. — The most useful of all media for mounting stained
objects. (For method of using, see Introduction, p. xxxiv.)

V. MICROTOMIST'S MEDIA.
GUM-WATER.

Preparation. — Dissolve gum arabic in 3 times its weight of
water without heat : allow to stand : strain if necessary.

Use. — For impregnating objects to be cut by the freezing
microtome.

ACETONE.

jj8e, — For impregnating fresh tissues before imbedding in
paraffin.

PARAFFIN.

Melting-point from" 50°tC.

XJse. — For imbedding objects to be cut into sections.

LIST OF REAGENTS. 497

SOFT PARAFFIN.

Melting-point about 40° C.

Use. — To smear the sides of the block that is being cut, so as
to ensure the sections cohering by their edges into a con-
tinuous ribbon.

VI. CEMENT FOR FIXING DOWN SECTIONS
CUT IN PARAFFIN.

GLYCERIN AND ALBUMEN (Mayer's Albumen).

Preparation. — Mix equal volumes of fresh white of egg and
glycerin. Keep a piece of camphor in the bottle.

Use. — For fixing sections in order upon the slide. For full
directions see Introduction, p. xxxiii, and p. 465, where
directions for staining sections on the slide are also given.

VII. INJECTION FLUIDS.

CARMINE INJECTION FLUID.

Preparation. — Rub up carmine in water : add ammonia drop
by drop till the carmine is dissolved : filter : evaporate at
a gentle heat, stirring constantly to get rid of the ammonia.
Rub up the residue with camphor water.

Use. — For injecting small animals and parts of animals. It
must be well shaken before use.

PRUSSIAN BLUE INJECTION FLUID.

Preparation. — To a weak solution of ferric chloride add a few
drops of hydrochloric acid, and then pour in enough of a
stronger solution of potassium ferrocyanide to cause com-
plete precipitation. The fluid should be deep blue, but
transparent, and the precipitate too fine to be seen with
the naked eye.

498 APPENDIX.

PLASTER-OP-PARIS INJECTION.

Preparation. — Rub up fine plaster of Paris with water, in a
mortar, to the consistency of thin cream. Colour to the
required tint with carmine injection fluid, good vermilion
or with French blue. Strain through fine muslin. Starch
may be used in place of the plaster.

Use. — For injecting blood-vessels, &c., of rabbits and other
large animals. It must be used immediately after pre-
paration. (See Introduction, p. xxiii.)

GELATIN INJECTION.

Preparation. — Soak gelatin in warm water till thoroughly soft.
Pour off the superfluous water : melt, and stir into it
carmine injection fluid, or other insoluble colouring
substance, and a trace of carbolic acid ; or if preferred
the gelatin may be soaked in camphor water, and carbolic
acid is then unnecessary. Stir till almost cold. The in-
jection is solid when cold.

Use. — For injection of specimens of which it is proposed to
make sections. It is to be melted, and kept well stirred
during injection. If the specimen is large or difficult to
inject, the whole operation must be performed in a vessel
of water as hot as the hand can bear it.

WH3TE-0F-EGG INJECTION.

Preparation. — Rub up fresh white of egg with carmine injec-
tion, or other colouring fluid which will not cause coagu-
lation of the albumen.

Use. — For injection specimens of which sections are to be cut.
It is used fresh, and the specimen is then placed in alcohol,
or in hot water, to coagulate the albumen.

GUM INJECTION.

Preparation. — Make a thick mucilage of gum arabic in cold
water.

Use. — Inject cold, previously colouring as desired. Plunge
the specimen into alcohol, and the injection will solidify.

INDEX

Abdomen : Crayfish, 144 Cockroach, 172
Abdominal appendage : Crayfish, 145, 146 ; Cockroach, 176
Abdominal pore: Dog-fish, 221, 240

Abdominal viscera: Dog-fish, 240; Rabbit, 338 ; Pigeon, 438
Acetabulum : Rabbit, 330 ; Fowl, 425
Acromion : Rabbit, 324
Adrenal body : Rabbit, 341 ; Pigeon, 455
Air sac : Pigeon, 435
Ala cordis : Crayfish, 158
Ala spuria : Pigeon, 431
Albumen gland : Snail, 122, 132
Alimentary canal, see Digestive System
Ali-sphenoid : Rabbit, 306 ; Fowl, 414
AUantois : Chick, 480
Alternation of generations; Liver-fluke, 34
Alveolus: Rabbit, 315
Amnion : Chick, 478
AMCEBA, 1-3

AMPHIOXUS, 187-217 ; external characters, 1 89 ; skeletal system, 190 ; muscular system,
191 ; digestiveand respiratory systems, 193 ; atrial cavity, 195 ; ccelom,
196 ; circulatory system, 197 ; excretory system. 198 ; nervous system,
198; sense organs, 201 ; reproductive system, 201 ; dissection, 202 ;
transverse sections, 206
Anapophysis, set Vertebra
Angular bone : Fowl, 419
Ankle, see Tarsus
Ankle-joint : Fowl, 427
Annulus : Leech, 47 ; Earthworm, 66
Anodonta, see Mussel
Antenna: Crayfish, 150 ; Cockroach, 174
Antennule : Crayfish, 151
Anti-trochanter : Fowl, 425

Anus: Paramecium, 6; Vorticella, 11; Leech, 48 ; Earthworm, 68 ; Mussel, 97, 110 j
Snail,119 ; Crayfish, 160 ; Cockroach, 173, 179 ; Amphioxus, 189, 217 ; Rabbit, 337
Aorta: Mussel, 103 ; Snail, 137 ; i2a&6&, 351, 356; Pigeon, 449-451
cardiac: Amphioxus, 197, 212; Dog-fish, 255

dorsal: Amphioxus, 196, 211, 216; .Dogr /?«/*, 256, 257; Pigeon, 449
Aortic arch: Amphioxus, 195; Rabbit, 356; Pigeon, 429 ; C7*ic£, 480
Aperture, se« o&o Foramen

auditory: Crayfish, 181; iZo&&&, 320, 337; Pigeon, 434

cloacal: Mussel, 96 ; Dog-fish, 220 ; Pigeon, 433

excretory: Liver-fluke, 37; ZeecA, 48; Earthworm, 69; Mussel, 108, 113;

£ncw7, 126 ; Crayfish, 151
exhalant : Mussel, 97
external : ieecA, 47 ; Earthworm, 68 ; Mussel, 96 ; <Snaz7, 119 ; Cockroach, 173 ;

Dog-fish, 220 ; Pigeon, 433
gonital : Liver-fluke, 35, 40 ; £eec/», 48 ; Earthworm, 68 ; .3/tm<?Z,106, 113 ; -Srai/,
119; Crayfish, 146; Cockroach, 173

500

INDEX.

Aperture, inhalant : Mussel, 96

of Eustachian canal : Rabbit, 320 ; Fowl, 412, 413
of Eustachian tube : Rabbit, 385 ; Pigeon, 438
on surface of skull, see Foramen
posterior narial : Pigeon. 438
reno-pericardial : Mussel, 10fi ; Snail, 125
respiratory Snail, 119, 124; Cockroach, 173
urino-genital : Rabbit, 337 ; Dog-fish, 270
Appendage: Crayfish, 140, 143. 145, 151; Cockroach, 174-177
abdominal : Crayfish, 145 ; Cockroach, 177
head : Crayfish, 149 ; Cockroach, 174
thoracic : Crayfish, 146 ; Cockroach, 175
Apteria: Pigpon. 432
Aquaeductus Pallopii : Rabbit, 312

vetibuli: Dog-fish, 228, 288
Arbor vitao : Rabbit, 405
Area opaca: Cfoc«, 470
Area pellucida : CWrjfc, 470
Arm, see Fore-limb
Arteries : Mussel, 103 ; -SnaiZ, 137 ; Crayfish, 157-167 ; Dog-fish, 253 ; Rabbit, 356 ; Pigeon,

449 ; see aZso Aorta
Artery, brachial: Rabbit, 358 ; Pigeon, 449
branchial : Dog-fish, 255

carotid: Dog-fish, 257 ; Rabbit, 357, 372 ; Pigeon, 448, 449
caudal : Dog-fish. 258 ; Pigeon, 451
coeliac: Dog-fish, 259 ; /Ja&6ft, 357 ; Pigeon, 449
coronary : Rabbit, 362
epihranohial : Dng-flsh, 257
femoral : Pigpon, 450
hepatic: KaV>i<, 358
hyoidean: D»g-ftsh, 258

iliac : Dog-fish, 260 ; Rabbit, 359 ; Pigeon, 450
ilio lumbar : Rabbit, 359
innominate : Rabbit, 350 ; Pigeon, 448, 449
intercostal: Rabbit. 35.
lieno-?astric : Doq-fish, 260 ; Rabbit, 358
lumbar: Rabbit, 358
mammary : Rabbit, 357

mesenteric : Dog-fish, 260 ; Rabbit, 357 ; Pigeon, 450
ovarian : Rabbit, 357
parietal : Dog-fish, 260
pectoral : Pigeon, 449
pnbic: Pigeon, 450
pulmonary: Rabbit, 350; Pigeon, 448
renal: Dog-fish, 260 ; i2a&&ft, 35 7; Pigeon, 450
sacral: Rabbit, 358
sciatic: Pig on, 450
spermatic: Rabbit. 355

subclavian: Dnq-fish, 259 ; Rabbit, 356, 357 ; Pigeon, 449
vertebral : Rabbit, 357 ; Pigeon, 449
vesical : Rabbit. 359
vitelline: CfoVfr, 477
Arthrobranchia : Crayfish. 153, 154
Arthrodial membrane: Crayfish, 143
Articular bone : .fV>«tf, 41 9
Articular process, see Vertebra
Arvtenoid cartilase : Rabbit. 379
Astacus. see Crayfish
Atlas vertebra : Rabbit, 297 ; .FowZ, 405
Atrial cavity : Amphioxus, 194, 204, 211, 213, 216
Atrial epithelium : Amphioxus, 210
Atrial pore: Amphioxus, 189
Anditorv aperture, see Aperture
Auditory capsule: Doa-Hsh. 228; Rabbit, 311

Auditorv orsan : Mussel, 108 ; 3wm7, 135 ; Crayfish, 164 ; Dog-fish, 287 ; Pijeon, 462
Au-h'tory ossicle: /toW«, 313; *W, 413, 420; Pigeon, 462
Auricle: VwsseZ, 102, 115 ; Snai'J, 125 ; Dog-fish, 247, 261 ; iZad&if, 350, 360, £61 ;

Piqpon, 451
Auricular feathers: Pigeon, 434
Axis vertebra : Rabbit, 296 ; iWZ, 405

INDEX. 501

Backbone, see Vertebral column

Barb : Pigeon, 432

Barbule: Pigeon, 432

Basal cartilages : Dog-fish, 2Z0, 2Z8, 239

Basi-brancbial : Dog-fish, 230

Basi-hyal : Dog-fish, 235

Basi-occipital : Rabbit, 304

Basipodite, see Appendage : Crayfish

Basi-pterygium : Dog-fish, 238

Basi-sphenoid : Rabbit, 306 ; Fowl, 412

Basi-temporal : Fowl, 412

Beak: Pigeon, 430

Bile-duct : Mussel, 110 ; Snail, 130, 131 ; Crayfish, 159, 160 ; Dog-fish, 24 \ ; Rabbit, 344 ;

Pigeon, 442
Birds, characters of, 401, 429
Bladder: Earthworm, 77 ; Rabbit, 341, 363, 364 ; see also Sinus, urinary and urino-genital ;

also Nephridium
Blastoderm : Chick, 470
Blastophore : Earthworm, 83
Blastostyle: Obelia, 32
Blatta, see Cockroach

Blood : Leech, 52 ; Earthworm, 73 ; Mammals, 335 • Pigeons, 429
Blood-sinus: L^fcA, 53, 63; Snail, 138. 139 ; Crayfish, 155 ; sec a&o Sinus, Lacuna, Vein
Blood-vessel: Earthworm, 73, 87; Mussel, 103 ; Snai'Z, 137 ; see also Circulatory system,

Artery, Vein
Body-cavity, see Ccelom
Bojanus, organ of: Mussel, 105
Bone, cartilage : Rabbit, 291, 302

membrane : Rahhit, 291, 303

sesamoid : Rabbit, 291, 320; see also Patella
Botryoidal tissue: Leech, 64

Brain : Dog-fish, 271, 283 ; Rabbit, 388 ; Pigeon, 457
Brain, cavity of, see Ventricle
Brain-case, see Cranium

Brain-membrane: Rabbit, 385 ,- ,

Branchia, see Gill

Branchial arch : Dog-fish, 235 ; Chick, 480
Branchial bar: Amphioxus, 193, 210
Branchial chamber : Mussel, 98, 116 ; Crayfish, 151
Branchial vessels : Dogfish, 255, 257
Branchio-cardiac canal : Crayfish, 158
Branchio-cardiac groove : Crayfish, 144
Branchiostegite : Crayfish, 144, 151
Branchu? : Rabbit, 373
Buccal cavity : Earthworm, 70 ; Cockroach, 178 ; Amphioxus, 192, 204, 208 ; Rabbil,ZS2 ;

Pigeon, 438
Buccal mass: AtaO, 129, 130
Buddine : Hydra, 25
Bulla, Tympanic: Rabbit, 312
Bursa Fabricii : Pigeon, 456

Csecum : fla&5#, 339, 345, 346

Calamus: Pigeon, 431

Canal, ampuliarv : Dog-fish, 221, 289

branchio-cardial : Crayfish, 158

carotid : Fowl, 411, 412

central: Amphioxus, 199, 208

Eustachian: Rabbit, 320; .FW, 412, 413

interorbital : Dog-fish, 230

naso-palatine : Rabbit, 383

neural, sec Vertebra

of Laurer : Liver-fluke, 40

pericardio-peritoneal : Dog-fish, 249

reno-pericardial : Mussel, 105 ; SnatJ, 125

semicircular: Doa-flsh, 228, 229, 287, 288 ; Pigeon, 463

sensory Dog-fish, 221, 279, 280, 289

urino-erenitai : Rabbit, 366, 369

vertibrarterial, see Vertebra
Capitulum : Rabbit, 301 ; Pigeon, 408

502 INDEX.

Carapace : Cray-fish, 143

Oardia : Rabbit, 342

Oardo : Cockroach, 174

Carino : Fowl, 409

Oarpo-metacarpus : Fowl, 423, 424

Carpus : typical, 327 ; Rabbit, 327 ; Fowl, 424 ; see also Carpo-metacarpus

Cartilage-bone : Rabbit, 291, 302

Centrum : Bog-fish, 223 ; see also Vertebra, typical

Cephalo-thorax : Crayfish, 143

Cerato-branchial : Bog- fish, 236

Cerato-hyal : Bog-fish, 235

Cercaria: Liver-fluke, 44

Oerci: Cockroach, 173, 177

Cere: Pigeon, 435

Cerebellum : Bog-fish, 273, 285 ; Rabbit, 390, 404 ; Pigeon, 458

Cerebral hemispheres : Bog-fish, 273 ; Rabbit, 389, 391, 395 ; Pigeon, 458

Cervical canal : Crayfish, 152

Cervical groove : Crayfish, 143

Cervical ribs : Pigeon, 404

Chele : Crayfish, 147

Ohiasma, optic: Bog-fish, 283; Rabbit, 392, 403 ; Pigeon, 458

CHICK, 464-488

Chlorophyll: Hydra, 28

Chordae tendineae: Rabbit, 361 ; Pigeon, 452

Choroid : Bog-fish, 287

Choroid plexus : Bog-fish, 284 ; jRa*^, 399

Cilia : Paramecium, 5, 6 ; Opalina, 7 ; Vorticella, 11

Oingulum, see Clitellum

Circulation, course of : Mussel, 104 ; Crayfish, 155

Circulatory system : Earthworm, 73-87 ; Mussel, 101, 112, 113, 115 ; Srurifr, 137 ; Crayfish,
155, 167 ; Cockroach, 178 ; Amphioxus, 197, 212, 217 ; Bog-fish,
245 ; iZo66«, 349 ; Pigeon, 443 ; CMcAr, 480-93

Cirrus : Liver-fluke,- 39 ; Amphioxus, 189

Cirrus-sac : Liver-fluke, 39

Clasper : Bog-fish, 220, 238

Clavicle: -Ra5&#, 324 ; -FotoJ, 422

Clinoid process: Rabbit, 306, 307

Clitellum: Earthworm, 67, 74

Clitoris: -Ra&&«, 337, 370

Cloaca: Bog-fish, 266, 270; Pigeon, 456, 457

Oloacal aperture : Mussel, 96 ; Bog fish, 220 ; Pigeon, 43 3

Cloacal chamber : Mussel, 95, 100

Cloacal papillae : Bog- fish, 221

Oloacal pouch : Bog-fish, 221

Clypeus : Cockroach, 170

Cnemial crest: 7Jo6&t7, 332; .FW, 427

Cnidoblast: Hydra, 23

Cnidocil : Hydra, 23

Cochlea: .Ra&&t<, 312, 335

COCKROACH, 169-186 ; external characters, 170 ; appendages, 174 ; circulatory system,
178 ; digestive system, 178 ; excretory system, 180 ; respiratory system,
180 ; nervous system, 181 ; sense organs, 183 ; reproductive system,
184

Cocoon : Leech, 46 ; Earthworm, 66, 79

Ooelenteron : Hydra, 20

Ccelom : Leech, 52, 63 ; Earthworm,! 0, 85 ; Mussel, 98 ; Amphioxus, 196, 210, 214,216 ,217

Coelomic canal : Amphioxus, 196, 197, 212, 214

Collar: Snail, 119

Colon : Cockroach, 179 ; Rabbit, 339, 345, 347

Columba, see Pigeon

Columella : Snail-shell, 121 ; Fowl, 413, 420 ; Pigeon, 462

Commissure, of brain : Rabbit, 404 ; see also Corpus callosum, Pons Varolii

Condyle, of skull: Bog-fish, 233; Rabbit, 304; Fowl, 411

Conjugation : Paramecium, 6, 7 ; Vorticella, 12 ; Monocystis, 15

Conus arteriosus : Bog-fish, 249, 261

Coracoid : Fowl, 421 ; Rabbit, 292

Coracoid process : Rabbit, 324

Cornea: Crayfish, 166; Cockroach, 183; Bog-fish, 287

Coronoid process: Rabbit, 317; Fowl, 424

Corpus albicans : Rabbit, 388, 399.

INDEX. 503

Corpus callosum : Rabbit, 385, 392, 397

cavernosum : Rabbit, 362, 363, 366

mammillare, see Corpus albicans
Corpus or Corpora quadrigemina : Rabbit, 335, 386 396, 400

spongiosum : Rabbit, 366

striatum : Rabbit, 393

trapezoidea : Rabbit, 390
Cotyloid bone : Rabbit, 331
Covert feathers : Pigeon, 433
Coxa : Cockroach, 174
Coxopodite : 147

Cranium : Dog-fish, 225 ; Rabbit, 304 ; Fowl, 410

CRAYFISH, 140, 168 ; external characters, 141 ; appendages, 145 ; gills, 151 ; circulatory
system, 145 ; reproductive system, 158 ; digestive system, 159 ; excretory
system, 163 ; nervous system, 163 ; sense organs, 165 ; injected specimen,

Cribriform plate : Rabbit, 309
Cricoid cartilage : Rabbit, 373, 379

Crop : Leech, 51 ; Earthworm, 72 ; Snail, 129 ; Cockroach, 179 ; Pigeon, 440
Crura cerebri : Rabbit, 388, 397, 400
Cutaneous blood-vessels : Leech, 51

Cuticle : Liver-fluke, 35, 43 ; Leech, 60 : Earthworm, 68, 84 ; Mussel, see Shell ; Snail,
see Shell ; Crayfish, 140, 141-143 ; Cockroach, 169 ; Pigeon, see Gizzard

Dart-sac : Snail, 133
Dentary bone : Fowl, 419

Dermis : Leech, 70 ; Earthworm, 86 ; Amphioxus, 203, 207

Development: Liver-fluke, 40, 45 ; Mussel, 89-110 ; Crayfish,\i\; Cockroach, 169 ; Dog-
fish, 218, 262, 263 ; Rabbit, 335 ; Bird, 429
Diaphragm : Rabbit, 340, 348

Digestive system : Liver-fluke, 37 ; Redia, 44 ; Cercaria, 44 ; Leech, 49, 63 ; Earthworm,
70, 86 ; Mussel, 109, 112, 113, 116 ; Snail, 127 ; Crayfish, 159 ; Cockroach,
178 ; Amphioxus, 192, 205, 210, 212, 216, 217 ; Dog-fish, 242 ; Rabbit,
342 : Pigeon, 438
Digit : Rabbit, 334 ; Fowl, 424, 428
Disc : Vorticella, 10
Distomum, see Liver-fluke

DOG-FISH, 218, 290 ; external characters, 219 ; skeleton, 222 ; abdomina cavity and
viscera, 240 ; digestive system, 242 ; respiratory system, 244 ; circulatory
system, 245 ; renal and reproductive systems, 262 ; nervous system, 271 ;
sense organs, 286
Duct, bile : Mussel, 100 ; Snail, 130, 131 ; Crayfish, 159, 160 ; Dog-fish, 243 ; Rabbit, 344 ;
Pigeon, 442
common genital : Snail, 132
hermaphrodite : Snail, 131
lacrymal : Rabbit, 31 1
mesonephric, see Wolffian
metanephric : Rabbit, 363 ; Pigeon, 455

Mullerian : Dog-fish, 262, 263, 270 ; Rabbit, 363 ; Pigeon, 455
of salivary receptacles : Cockroach, 180
ovarian : Liver-fluke, 39

pancreatic : Dog-fish, 346 ; Rabbit, 343 ; Pigeon, 443
pronephric, see Mullerian

salivary : Snail, 130 ; Cockroach, 178, 180 ; Rabbit, 387
segmental : Dog-fish, 262
Stenonian : Rabbit, 383
vitellarian : Liver-fluke, 39
Wharton's : Rabbit, 383

Wolffian : Dog-fish, 262-264, 265 ; Rabbit, 363, 365, 366 ; Pigeon, 454, 455 ; Chick, 482
Ductus arteriosus : Rabbit, 351

Ductus ejaculatorius : Liver-fluke, 36 ; Cockroach, 184
Duodenum : Rabbit, 339, 342 ; Pigeon, 439, 440
Dura mater : Rabbit, 384

Ear, see Auditory Organ

EARTHWORM, 66-87 ; external characters, 67 ; ccelom, 70 : digestive system, 70 ; circula-
tory system, 73 ; excretory system, 74 : reproductive system, 77 ;
nervous system, 82 ; transverse sections, 83

504 INDEX.

Ecdysis : Crayfish, 141 ; Cockroach, 169

Ectoderm : Hydra, 21, 22, 24, 27-29

Ectosaro : Amoeba, 2 ; Paramecium, 5 ; Opalina, 7 ; Vorticella, 10

Egg : see Ovum

Elytron : Cockroach, 172

Embryo : Liver-fluke, 42 ; J/uaseZ, 89, 110 ; Crayfish, 141 ; i?a&Z>ft, 335 ; Chick, 473-488

Encystment : Opalina, 8 ; Vorticella, 12

Endoderm : SycZro, 21, 24, 28, 34 ; Obelia, 30

Endophragmal skeleton : Crayfish, 163

Endopodite : Crayfish, 145 ; see also Appendage

Endosarc : Amoeba, 2, 3 ; Paramecium, 6 ; Opalina, 8 ; Vorticella, 10

Endostyle : Amphioxus, 194, 204, 211, 212

Epiblast : Chick, 469, 473

Epibranchial band or groove : Amphioxus, 193, 204, 211, 212

Epibranchial cartilage : Bog- fish, 235

Epicranium : Cockroach, 170

Epidermis : Leech, 60 ; Earthworm, 85 ; Amphioxus, 203, 206 ; Rabbit, 292

Epididymis : Leech, 58 ; Rabbit, 365

Epiglottis : i?a&6ft, 378, 381

Epimeron : Crayfish, 142

Epiotic : Rabbit, 311

Epiphragm : Snail, 117

Epiphysis, of vertebra : Rabbit, 293

Epipleural cavity : Amphioxus, see Atrial cavity

Epipodite : Crayfish, 145, 149

Ethmoidal region : Rabbit, 309

Ethmo-turbinal : Rabbit, 310

Eustachian canal : Rabbit, 320, 381 ; Fowl, 412, 413

Eustachian tube : Rabbit, 381 ; Pigeon, 462 ; Chick, 479, 485

Eustachian valve : Rabbit, 360 ; Pigeon, 451

Excretory system : Liver-fluke, 37, 44 ; Leech, 54 ; Earthworm, 74 ; Mussel, 104 ; SnaiZ,
125 ; Crayfish, 163 ; Cockroach, 180 ; Amphioxus, 198 ; Dog-fish, 262,
271 ; JJa&&a, 362, 368 ; Pi^eore, 454 ; CAfcJSr, 481-482 ; see also
Vacuole, pulsating

Ex-occipial : Rabbit, 304

Exopodite : Crayfish, 143 ; see also Appendage

Exoskeleton : Crayfish, 141-143

Extra-branchial : Crayfish, 141 ; Bog-fish, 236

Eye : Leech, 59 ; Snail, 135 ; Crayfish,163 ; Cockroach,171, 183 ; Amphioxus, 201 : Z>off-
fish, 274, 286 ; flafc&ft, 337 ; Pigeon, 459

Eye : development of, Chick, 482-3

Eyelid : Pigeon, 459

Eye-muscle, see Muscle

Fabella : Rabbit, 332

Fallopian tube : Rabbit, 369

Palx cerebri : Rabbit, 385

Pasciola, see Liver-fluke

Pat-body : Cockroach, 178

Peather : Pigeon, 431

Pemur : Cockroach, 175 ; Rabbit, 331 ; Fowl, 426

Fenestra : Cockroach, 171, 183

Fenestra ovalis : Rabbit, 312 ; -Fowtf, 417, 425

Fenestra rotunda : Rabbit, 312 ; Fowl, 412

Fenestral recess : Fowl, 412

Fibula : Rabbit, 332 ; .Fourf, 427

Filoplume : Pigeon, 433

Pin : Amphioxus, 189, 190, 207 ; Bog-flsh, 219, 236

Fission : Amoeba, 3 ; Paramecium, 6 ; Opalina, 8 ; Vorticella, 12 ; Hydra, 28

Flagellum : Hydro, 27 ; Snat7, 132

Ploccular fossa : Rabbit, 311

Floccular lobe : Rabbit, 386

Pontanelle : Bog-flsh, 228

Food : Amoeba. 3 ; Paramecium, 6 ; Opalina, 8 ; Vorticella, 11 ; Hydra, 21 ; 06eZm, 30 ;

icccA, 46 ; Earthworm, 66 ; Mussel, 89, 108 ; <Sfaat7, 117 ; Crayfish, 140 ; CocA-

roacfc, 169 ; Bog-fish, 218
Foot : Hydra, 20, 30 ; Mussel, 95 ; Srcai7, 118 ; Rabbit, 334 ; fowZ, 427 ; Pigeon, 431
Foramen, anterior palatine : Rabbit, 316, 319
condylar : Rabbit, 319
for internal carotid artery : Bog-fish, 230; Rabbit, 313, 321

INDEX. 505

Foramen, ilio-sciatic : Fowl, 425

incisivum : Rabbit, 319

inferior dental : Rabbit, 317

infra-orbital : Rabbit, 315, 318

internal orbital : Rabbit, 318

laoerum anterius: Rabbit, 306, 318

lacerum medium : Rabbit, 319

lacerum posterius : Rabbit, 319

lacrymal : Rabbit, 320

magnum : Dog-fish, 233 ; Rabbit, 320 ; Fowl, 411 *

obturator : Rabbit, 330 ; Fowl, 425

of Monro : Rabbit, 398, 403 ; Pigeon, 459

optic : Dog-fish, 229 ; Rabbit, 308, 318

orbito-nasal : Dog-fish, 229

ovale : 117 ; Rabbit, 360 ; Pigeon, 451

pituitary : Rabbit, 320

posterior palatine : Rabbit, 319

rotundum : Rabbit, 318

stylomastoid : Rabbit, 313, 319

triosseum: Fowl, 421 ; Pigeon, 435
Foramina of exit for cranial nerves

for 1st cranial nerve (olfactory) : Dog-fish, 231 ; Rabbit, 309 ; Fowl, 414

for 2nd cranial nerve (optic) : Dog-fish, 229 ; Rabbit, 307, 318 ; Fowl, 414

for 3rd cranial nerve (oculo-motor) : Dog-fish, 229 ; Rabbit, 306, 307, 313 ;
Fowl, 414

for 4th cranial nerve (pathetic) : Dog-fish, 229 ; Rabbit, 306, 307, 313 ; Fowl, 414

for 5th cranial nerve (trigeminal) : Dog-fish, 230 ; Fowl, 413

for ophthalmic branch of 5th: Dog-fish, 228, 230 ; Rabbit, 306, 307, 313

for maxillary branch of 5th : Rabbit, 306, 315, 318

for mandibular branch of 5th : Rabbit, 306, 319

for 6th cranial nerve (abducent) : Dog-fish, 230 ; Rabbit, 306, 307, 318

for 7th cranial nerve (facial) : Dog-fish, 230 ; Rabbit, 313, 319

for ophthalmio branch of 7th : Dog-fish, 228, 230

for 8th cranial nerve (auditory) : Rabbit, 311, 312

for 9th cranial nerve (glosso-pharyngeal) : Dog-fish, 231 ; Rabbit, 319 ; Fowl,
411

for 10th cranial nerve (pneumogastric) : Dog-fish, 233 ; Rabbit, 319; Fowl, 'ill

for 11th cranial nerve (spinal accessory) : Rabbit, 319

for 12th cranial nerve (hypoglossal) : Rabbit, 319, 320 ; Fowl, 411
Fore-brain : Rabbit, 391, 395, 397, 399 ; Chick, 475-6, see also Thalamencephalon
Fore-limb : Rabbit, 325, 338 ; Fowl, 422
Fornix : Rabbit, 394, 397, 400
Fossa ovalis : Rabbit, 360 ; Pigeon, 451
FOWL : skeleton of, 341-458 ; vertebral column, 401 ; ribs and sternum, 407 ; skull, 409 ;

appendicular skeleton, 421
Frontal bone : Rabbit, 309 ; Fowl, 414, 415
Frontal segment : Rabbit, 306
Funnel, nephridial : Leech, 55 ; Earthworm, 74, 75

seminal : Earthworm, 80
Furcula : Fowl, 422

Galea : Cockroach, 174

Gall-bladder: Dog-fish, 243; Rabbit, 344

Gallus, see Fowl

Gamete: Monocystis, 15-16; Coccidium, 17 ; Chick, 468

Ganglion, see Nerve-ganglion

Gastric mill : Crayfish, 161

Gastrula : Liver-fluke, 43

Gemmation : Hydra, 25

Gena : Cockroach, 170

Generations, alteration of : Obeila, 33 ; Liver-fluke, 34

Genital sinus : Liver-fluke, 40 ; Dog-fish, 253

Germ-cell : Monocystis, 15 ; Liverfluke, 42

Germinal layers : Chick, 470

Giant-fibres : Earthworm, 88

Gill : Mussel, 96, 98, 112, 116 ; Crayfish, 153 ; Dog-fish, 244

Gill-arch : Amphioxus, 193, 205, 210 ; see also Branchial arch

Gill-ray : Dog-fish, 235, 236

Gill-slit: Amphioxus, 193, 204; Dog-fish, 221

506 INDEX.

Gizzard: Earthworm, 72; Cockroach, 179 ; Pigeon, 439, 440
Gland, albumen : Snail, 122, 131 ; Pigeon, see Oviduct

clitellar : Leech, 60

colleterial : Cockroach, 185

Oowper's : Rabbit, 367, 370

digestive : Crayfish, 159 ; Cockroach, 179, 180 ; Dog-fish, 241, 243, 244

epidermal : Leech, 60

green : Crayfish, 163

hermaphrodite : Snail, 122, 131

infra-orbital : Rabbit, 383

mammary : Rabbit, 328

mucous : Leech, 60 ; Snail, 133

mushroom-shaped : Cockroach, 184

oesophageal : Earthworm, 72

oviducal : Dog-fish, 264

parotid : Rabbit, 383

pedal : Snail, 120

perinaeal: Rabbit, 327, 368, 370

prostatic : Rabbit, 367

rectal: Dog-fish, 244; Rabbit, 368, 370

salivary : Leech, 51, 59 ; Snail, 129 ; Cockroach, 179 2Jo5&tV, 385, 387

shell : Liver-fluke, 40

sublingual: Rabbit, 383

submaxillary : Rabbit, 383

thyroid : see Thyroid body

yolk : Liver-fluke, 39
Glochidium : Mussel, 110
Glottis: .Ra&&&, 380, 384 ; Pigeon, 443
Gonapophysis : Cockroach, 177, 184, 186
Graafian follicle : Pa&&#, 369
Green gland : Crayfish, 163
Growth : Mussel-shell, 89, 93 ; Snail-shell, 120, 121 ; Crayfish, 141

centres of: CM*, 471, 473
Gubernaculum : Rabbit, 365

Haemal arch : Dog-fish, 223, 224
Haemal process : Dog-fish, 224
Haemal spine : Dog-fish, 224

Haemoglobin : Earthworm, 73 — _ ,

Hallux : Rabbit, 331 ; Fowl, 430
Hamular process : Rabbit, 314
Hand : Rabbit, 329

Head : Snail, 118 ; Crayfish, 143 ; Cockroach, 170 ; Pa&&#, 336, 382 ; Pigeon, 430
Head-process: Chick, 473

Heart : Earthworm, 73 ; Mussel, 102, 115 ; SnatZ, 125 ; Crayfish, 155 ; Cockroach, 178 ; Pop-
/isft, 245-249, 261 ; .Ra&oft, 347, 349, 359 ; Sheep, 362 ; Pt^eorc, 439, 443, 496
Helix, see Snail

Hepatic caecum : Cockroach, 180
Hepatic cell : Earthworm, 73, 86
Hermaphrodite duct : Snail, 131
Hermaphrodite gland : Snail, 122, 131
Hind-brain : Rabbit, 387, 388, 397 ; Chick, 476
Hind-limb : Rabbit, 331, 332 ; Fowl, 426 ; Pigeon, 431
Hinge of mussel-shell, 91
Hippocampus major : Rabbit, 397
Hirudo, see Leech
Humerus : Rabbit, 325 ; Fowl, 422

HYDRA, 18-30 ; general appearance, 20 ; structure, 21-25 ; reproduction, 25-27 ; dis-
integrated specimens, 27-29 ; transverse sections, 29, 30
Hydranth, 30
Hymen : Dog-fish, 264
Hyoid bone : Rabbit, 317, 371 ; Fowl, 420

Hyoidean arch : Dog-fish, 234 ; Rabbit, 317 ; Fowl, 420 ; Pigeon, 462
Hyomandibular cartilage : Dog-fish, 234
Hyostylic skull : Dog-fish, 235
Hypapophysis, see Vertebra
Hypoblast : Chick, 470, 473
. Hypobranchial : Dog-fish, 236
Hypobranchial groove : Amphioxus, see EndostyJe
Hypostome : Hydra, 20

INDEX. 507

Ileum : Cockroach, 179 ; Dog-fish, 242

Ilium : Rabbit, 330 ; Fowl, 425

Incisor : Rabbit, 321, 322

Incus: Rabbit, 313

Infundibulum : Dog-fish, 283 ; Rabbit, 392, 403 ; Pigeon, 459

Integument : Leech, 60 ; Earthworm, 85 ; Snail, 121 ; Amphioxus, 202, 203, 206

Internasal septum : Dog-fish, 227

Interorbital canal : Dog-fish, 230

Interorbital septum : Fowl, 409, 414

Interparietal : Rabbit, 307

Interstitial cell : Hydra, 22, 23, 26, 27, 29

Intestines: Liver-fluke, 37, 41; Leech, 51; Earthworm, 72, 86; Mussel, 109, 112;
Sncn'Z, 122, 130 ; Crayfish, 160 ; Cockroach, 179 ; Amphioxus, 195, 205, 216,
217 ; Dog-fish, 241, 242 ; fla&&&, 339, 344, 346 ; Pigeon, 439, 440, 452

Ischium : Rabbit, 330 ; Fowl, 425

Jacobson's organ : Rabbit, 385

Jaw: Leec/j, 49; Snail, 136; Crayfish, 149, 150 ; Dog-fish, 234 ; tfa&&&, 314, 315 ; JW,

421, 425
Jugal : #a&6#, 316 ; Fowl, 417

Kidney : Mussel, 105, 106, 112, 115 ; Snail, 122, 125 ; Crayfish, 163 ; Amphioxus, 198 ;

Dog-fish, 241, 262, 264, 277 ; i2a&&&, 341, 363 ; Pigeon, 455 ; CM*, 480
Killing, mode of : Amoeba, 3 ; Paramecium, 7 ; Opalina, 8 ; Vorticella, 13 ; Hydra, 29 ;
Liver-fluke, 34, 35 ; Z«ecft, 46 ; Earthworm, 67 ; <Snai7, 117 ;
Crayfish, 141 ; Cockroach, 170 ; flaWt*, 336 ; Pigeon, 430

Labial cartilage : Dog-fish, 236

Labial palp : Mussel, 97 ; Cockroach, 17 5

Labium : Cockroach, 175

Labrum : Crayfish, 142 ; Cockroach, 170

Lacinia : Cockroach, 173

Lacrymal bone : Rabbit, 311 ; .FWJ, 415

Lacuna : Mussel, 105

Lacunar tissue : Mussel, 101

Lambdoidal crest, .FowJ, 411

Lamella, inferior : Rabbit, 298

supporting : Hydra, 22
Lamina terminalis : Rabbit, 403
Lancelet, *ee Amphioxus
Larynx : iJo&fcft, 378 ; Pigeon, 454
Lateral line : Dog-fish, 219, 289
LEECH, 46-65 ; external characters, 46 ; digestive system, 49 ; coelom and blood-vessels,

52 ; excretory system, 54 ; reproductive system, 57 ; nervous system, 58 ;

sense organs, 59 ; transverse sections, 59
Leg : Crayfish, 146, 147 ; Cockroach, 175 ; see also Hind-limb
Lens : Dog-fish, 287 ; Pigeon, 461 ; Chick, 482-3
Lepus, see Rabbit

Life-history : Monocystis, 15-16 ; Coccidium, 17 ; Obelia, 33 ; Liver-fluke, 40 ; Tape-
worm, 45 ; Crayfish, 141 ; Cockroach, 169
Ligament, broad : Rabbit, 369

ethmo-palative : Dog-fish, 234

falciform : Pigeon, 438

of jaws : Dog-fish, 233, 234

of mussel-shell, 89, 90

suspensory, of liver : Dog-fish, 240 ; Rabbit, 343
Ligula : Cockroach, 175

Limb, see Fore-limb, Hind-limb, Appendage, Leg
Limnaea truncatula, 42-44
Lingua : Cockroach, 178
Lip : Snail, 119 ; Rabbit, 336
Liver : Mussel, 110 ; Snail, 122, 130 ; Crayfish, 159 ; Amphioxus, 194, 205, 212 ; Dog-fish

240; Rabbit, 339, 343 ; Pigeon, 439, 442
Liver-fluke, 34-45 ; external characters, 35 ; alimentary canal, 37 ; excretory system,
37, 44 ; reproductive system, 37 ; nervous system, 40 ; life-history, 40
Lobi inferiores : Dog-fish, 283

Locomotion : Amoeba, 3 ; Paramecium, 4 ; Vorticella, 12 ; Hydra, 18 ; Leech, 46 ; Earth-
worm, 65 ; Mussel, 89, 96 ; Crayfish, 140 : Amphioxus, 187, 191

508 INDEX.

Lumbricus, see Earthworm

Lung : Snail, 123-125 ; Rabbit, 347 ; Pigeon, 439, 453, 454

Lymphatics: Rabbit, 359

Malleus : Rabbit, 315

Malpighian body : Rabbit, 364

Malpighian tubule : Cockroach, 180

Mammae : Rabbit, 335

Mammalian skeleton : characters of, 292

Mammals : characters of, 335

Mandible: Cray fish, 150 ; Cockroach, 174 ; Rabbit, 317 ; Fowl, 423

Mandibular arch : Dog-fish, 233 ; Rabbit, 316 ; Fowl, 419

Mantle : Mussel, 94, 111 ; Snail, 122

Mantle-cavity : Mussel, 95 ; Snail, 123-127

Manubrium: Obelia, 32

Manubrium mallei : Rabbit, 313

Manubrium sterni : Rabbit, 302 ; Fowl, 408

Manus : Rabbit, 329 ; fowtf, 426

Mastoid portoin ; Rabbit, 311

Mastoid process : Rabbit, 311

Maxilla: Crayfish, 149, 150; Cockroach, 174, 175 ; 2Ja6&t7, 315 ; .FW, 417

Maxillary arch : Rabbit, 314

Maxillipede : Crayfish, 147, 149

Maxillo-palatine process : .Fowtf, 417

Maxillo-turbinal : Rabbit, 310

Meatus auditorius externus : Rabbit, 312 ; Pigeon, 461

Meatus auditorius internus : Rabbit, 312

Meckel's cartilage : Fowl, 419

Mediastinum : Rabbit, 348

Medusa : Obelia, 32

Meganucleus : Paramecium, 6 ; Vorticella, 11

Membrane-bone: Rabbit, 291, 303

Mentum : Cockroach. 175

Mesencephalon : fla&&i«, 391, 396, 399

Mesenteron : Crayfish, 159, 160 ; Cockroach, 178, 179

Mesentery : Dog-fish, 241 ; iJa&Sft, 341

Mesethmoid : Rabbit, 307 ; Fowl, 414

Mesoblast : CMcA;, 471

Mesoblasticsometes: Chick, 475

Mesoglcea, see Supporting lamella

Mesonephros, see Wolffian body

Mesonotum: Cockroach, 172

Meso-pterygium : Dog-fish, 238

Mesothorax : Cockroach, 172

Metacarpus : Rabbit, 329 ; f owJ, 424

Metacromion : Rabbit, 324

Metanephros : Rabbit, 363 ; Pigeon, 455 ; C^icA, 481

Metapleural canal : Amphioxus, 197, 212, 214, 216

Metapleural fold : Amphioxus, 189

Metapophysis, Rabbit, 294 ; see also Vertebra

Meta-pterygium : Dog-fish, 238

Metastoma : Crayfish, 150

Metatarsus : Rabhit, 334 ; Fowl, 427, 428

Metathorax : Cockroach, 172

Metosteon : Fowl, 409

Micro-nucleus : Paramecium, 6 ; Vorticella, 11

Mid-brain, see Mesencephalon

Mid-gut, see Mesenteron

Molar teeth : Rabbit, 322, 323

Monocystis, 13-16

Morula : Liver-fluke, 43

Mouth : Paramecium, 6 ; Hydra, 20 ; Liver-fluke, 35 ; Zeecft, 47, 49 ; Earthworm, 68, 70 ;

Mussel, 96, 107 *, 5?iat7, 119 ; Crayfish, 144, 160 ; Cockroach, 173 ; Anvhioxus, 189,

204. 206 ; Dog-fish, 220 ; JJa&&#, 336 ; Pigeon, 433 ; see ata> Buccal cavity
Muscle, bursalis : Pigeon, 460

columellar : Snail, 122

mandibular: Rabbit, 372

of eyeball : Dog-fish, 274 ; Pigeon, 460

of nictitating membrane: Pigeon, 460

pectoral : Pigeon, 434

INDEX. 509

Muscle, pyramidalis : Pigeon, 460

quadratus: Pigeon, 460

retractor penis : Snail, 133

sterno-mastoid : Rabbit, 372
Muscle-process : Hydra, 28, 29
Muscular impression : Mussel-shell, 91
Muscular system : Leech, 62 ; Earthworm, 86 ; Muscel, 94 ; Amphioxus, 191, £0/ • se*

a/ o Muscle
Musculi papillares : Rabbit, 359 ; Pigeon, 452
Mushroom-shaped gland : Cockroach, 184

MUSSEL, 89-116 ; shell, 90 ; external characters, 94 ; gills, 98 ; circulatory system, 101 ;
excretory system, 105 ; nervous system, 107 ; digestive system, 109; repro-
ductive system, 110 ; transverse sections, 111
Myophan striation : Paramecium, 5 ; Opalina, 7 ; Vorticella, 11
Myotome: Amphyozus, 189, 191, 207

Nacreous layer : Mussel-shell, 93 ; Snail-shell, 121

Nasal bone : Rabbit, 310 ; Fowl, 415

Nasal cartilage ■ Dog-fish, 231

Nasal chamber : Rabbit, 381

Naso-turbinal : Rabbit, 310

Neck : Cockroach, 171 ; Rabbit, 370-378 ; Pigeon, 430

Nematocyst : Hydra, 23, 24, 28, 30 ; discharge of, 24, 25 ; Obelia, 31

Nephridium : Leech, 54, 57, 74 ; Earthworm, 75, 78

Nephrostome : Ear hworm, 75, 77

Nerve, antennary : Cockroach, 182

auditory : Snail, 134 ; Crayfish, 167 ; see also Cranial nerve

branchial : Bog- fish, 282

buccal : Snail, 134 ; Dog-fish, 280

cardiac : Dog-fish, 282 ; Rabbit, 376

cranial : Dog-fish, 275-282 ; see also Nerve roots

1st cranial (olfactory) : Dog-fish, 275

2nd cranial (optic) : Dog-fish, 276

3rd cranial (oculo-motor) : Dog-fish, 276

4th cranial (pathetic) : Dog-fish, 277

5th cranial (trigeminal): Dog-fish, 277, 278

6th cranial (abducent) : Dog-fish, 279

7th cranial (facial) : Dog-fish, 277, 279

8th cranial (auditory) : Dog-fish, 277, 281, 289

9th cranial (glossopharyngeal) : Dog-fish, 281

10th cranial (pneumogastric or vagus) : Dog-fish, 281 ; Rabbit 373-375

11th cranial (spinal accessory) : Rabbit, 372

12th cranial (hypoglossal) : Rabbit, 372

depressor: Rabbit, 375

descendens noni, see Ramus descendens

gastric: Dog-fish, 282

intermediary : Leech, 61

laryngeal, anterior : Rabbit, 376

laryngeal, posterior : Rabbit, 377

mandibular: Dog-fish, 278

maxillary : Dog-fish, 279

of lateral line: Dog-fish, 281, 282

of neck : Rabbit, 271-277

ophtha mic : Dog-fish, 278, 279, 290

optic : Crayfish, 166 ; Cockroach, 182 ; see also 2nd cranial nerve

palatine : Dog-fish, 280

phrenic: Rabbit, 377

post-spiracular ; Dog-fish, 280

pre-spiracular : Dog-fish, 281

pulmonary : Rabbit, 376

recurrent laryngeal : Rabbit, 377

spinal : Dog-fish, 286 ; Rabbit, 377

sympathetic: Rabbit, 376, 377

visceral : Crayfish, 164, 165 ; Cockroach, 183 ; Dog-fish, 282
Nerve-cell : Hydra, 27, 31 ; Leech, 88 ; Earthworm, 135 ; Snail, 135
Nerve-chain, see Nervous system

Nerve-collar : Liver-fluke, 40 ; Leech, 59 ; Earthworm, 86 ; Snail, 134
Nerve-commissure : Mussel, 127
Nerve-connective: Mussel, 108 ; Crayfish, 163, 164 ; Cockroach, 182

M M

510 INDEX.

Nerve-cord, see Nervous system
Nerve-ganglion buccal : Snail, 134

cerebral: Mussel, 107 ; Snail, 134; Cockroach, 182

cervical : Rabbit, 376

coeliac : Babbit, 345

frontal : Cockroach, 183

mesenteric : Rabbit, 345

optic : Crayfish, 164

parieto-splanchnic, see Visceral

pedal : Mussel, 107 ; Snail, 135

post-cesophageal : Crayfish, 162 ; Cockroach, 182

pre-cesophageal : Crayfish, 161 ; Cockroach, 182

sub-cesopbageal : Leech, 59 ; Snail, 134 ; Cockroach, 182

supra-cesophageal : Leech, 59 ; Earthworm, 83 ; Snai7, 134 ; Cockroach, 182

sympathetic : #a&&«, 345, 349, 376

visceral : Mussel, 108, 116

viscero-pleural : Snail, 135
Nerve-roots, cranial : Dog-fish, 274-282 ; Rabbit, 390, 391

spinal : Amphioxus, 199, 200, 206, 208 ; Dog-fish, 286 ; Rabbit, 377
Nervous system : Hydra, 24, 28 ; Liver-fluke, 40 ; Leecft, 58, 65 ; Earthworm, 83, 88 ;
Mussel, 107 ; Snat7, 134 ; Crayfish, 163 ; Cockroach, 181-183 ; Amphi-
oxus, 198, 206, 208 ; Dog-fish, 271 ; Rabbit, 373, 380, 388 ; Pigeon, 457
Neural arch : Dog-fish, 224 ; see also Vertebra
Neural canal : see Vertebra
Neural plate : Dog-fish, 224
Neural process : Dog-fish, 224
Neural spine : Dog-fish, 225 ; see also Vertebra
Nictitating membrane : Rabbit, 337 ; Pigeon, 430, 464, 465
Nose, see Olfactory organ

Nostril : Dog-fish, 220 ; Rabbit, 336 ; Pigeon, 433

Notochord : Amphioxus, 190, 203, 206, 207 ; Dog-fish, 222, 223, 233 ; Chick, 474
Nucleus : Amoeba, 3 ; Paramecium, 6 ; Opalina, 8 ; Vorticella, 11 ; Monocystis, 15 ;
Trypanosoma, 17 ; Snail-shell, 120

OBELIA, 30-33

Oblique septum : Pigeon, 439

Occipital condyle : Dog-fish, 233 ; Rabbit, 306 ; -FVwf, 411
Occipital segment : Rabbit, 304
Odontoid process : Rabbit, 297 ; Fowl, 405
Odontophore : Snail, 129, 136
Oesophageal pouches : Earthworm, 72

CEsophagus : Liver-fluke, 37 ; ieec/i, 51 ; Earthworm, 70 ; Mussel, 109 ; Sram, 129 ; Cray-
fish, 160 ; Cockroach, 179 ; Dog-fish, 242 ; iJa&&&, 349 ; Pigeon, 440
Olecranon process : Rabbit, 326 ; -Fowtf, 423
Olfactory capsule : Dog-fish, 227 ; Rabbit, 310 •, .FcwZ, 415
Olfactory lobe: Dog-fish, 273 ; iia&&&, 386, 388 ; Pigeon, 458
Olfactory organ : Crayfish, 167 ; Amphioxus, 201 ; Dog-fish, 286
Omentum : Rabbit, 342' ; Pigeon, 439
Opalina, 7, 8
Opisthotic : tfafc&ft, 311

Optic chiasma : Z)o^ /Zsft, 283 ; Rabbit, 392, 393 ; Pigeon, 458
Optic foramen : Dog-fish, 229 ; jKo&fttY, 318 ; Fowl, 414
Optic lobe : Dog-fish, 274, 285 ; Rabbit, 390, 404 ; Pigeon, 458
Optic thalamus: Rabbit, 395
Optic tract : i?a&&tf, 388, 395
Oral groove : Paramecium, 6
Orbit : Dog-fish, 229-231, 274 ; Fowl, 414
Orbito-sphenoid : Rabbit, 307 ; Fowl, 414
Os innominatum : Rabbit, 330 ; Fowl, 425
Os orbiculare : Rabbit, 314

Ossicle, auditory : Rabbit, 313 Fowl, 413, 425 ; Pigeon, 462
Ossicles of stomach ; Crayfish, 161
Otocyst : Mussel, 109 ; -Sraat'Z, 135
Ovarian duct : Liver-fluke, 39
Ovary : Hydra, 26, 27 ; Liver-fluke, 39 ; Z«?cA, 58 ; Earthworm, 78, 82 ;" Mussel, 110;

Crayfish, 159; Cockroach, 184; Amphioxus, 201, 203, 204, 214 ;j Dog-fish,

241, 263; iJo6&ft,.369 ; Pigeon, 456
Oviduct: Liver-fluke, 40 ; Zeecft, 58 ; Earthworm, 77 ; Snatf, 133 ; Crayfish, 159 ; CocZ;-
roacft, 185 ; Dog-fish, 263 ; iZa&fcft, 369 ; Pigeon, 456

INDEX. 511

Ovum : Hydra, 27 ; Liver-fluke, 42 ; Earthworm, 81 ; Mussel, 110 ; Snail, 131 ; CocA;-
roach, 184; Amphioxus, 202 ; Dog-fish, 218, 263 ; Rabbit, 335, 369 ; Wd 429 •
Pi>»», 457 ; tfen, 467-8

Palatal process : Rabbit, 315

Palate: Rabbit, 383

Palatine bone: Rabbit, 314 ; ifywZ, 416

Palial cavity, see Mantle-cavity

Pallial line : Mussel-shell, 92

Palp, labial: Mussel, 97; Cockroach, 175; mandibular: 'Crayfish, 150

Paip, maxillary : Cockroach, 175

Palpiger : Cockroach, 175

Pancreas : Dog-fish, 240 ; Rabbit, 342 ; Pigeon, 443

Papilla, circumvallate : Rabbit, 380

foliata : -Raft&ft, 380

urino-genital : Dog-fish, 270
Paraglossa : Cockroach, 175
PARAMECIUM, 4-7
Para-sphenoid : .Few/, 414
Parietal bone : Rabbit, 307 ; Fowl, 415
Parietal segment : Rabbit, 306, 307
Par-occipital process : Rabbit, 304
Patella : Rabbit, 332 ; Fowl, 426
Pearl, 93

Pecten : Pigeon, 466
Pectoral tin : Dog-fish, 220, 238

Pectoral girdle : Dog-fish, 238 ; Rabbit, 323 ; -Fowtf, 421
Pelvic fin : Dog-fish, 238

Pelvic girdle : Dog-fish, 237 Rabbit, 330 ; JW, 425
Pelvis of kidney : Rabbit, 364

Penis: Liver-fluke, 39; Leecft, 58; Snail, 132; Rabbit, 337, 366
Pericardial cavity : Mussel, 101, 112, 114 ; Snat7, 125 ; Dog-fish, 245
Pericardial sinus : Crayfish, 155
Pericardium : Snail, 112 ; Rabbit, 347
Perineal-pouch : Rabbi, 337
Periostracum : Mussel-shell, 92 ; Snail-shell, 121
Periotic : iZo66tY, 311
Periplaneta, see Cockroach
Perisarc : Obelia, 32
Peristome: Vorticella, 10

Peritoneum : Earthworm, 86 ; Dog-fish, 241 ; Rabbit, 341
Pes : i2a&&#, 334 ; .FWJ, 427
Pessulus : Pigeon, 454
Petrous portion: Rabbit, 311
Peyer's patch : iZa&fcft, 344, 346
Phalanges : Rabbit, 329, 334 ; Fowl, 424, 427
Pharyngo-branchial : Dog-fish, 235
Pharynx: Vorticella, 11; Liver-fluke, 37 ; Zeecft, 52; Earthworm, 70; £nai7, 129;

Amphioxus, 193, 205, 210, 212; iZoJ&ft, 384; Cfa'cfc, 479
Pia mater : Rabbit, 389

PIGEON, 429-463 ; external characters, 430 ; pectoral muscles, 434 ; air-sacs, 435 ;
digestive system, 438 ; circulatory system, 443 ; respiratory system, 1^3;
renal and reproductive systems, 454 ; brain, 457 ; sense organs, 459
Pigmented canals : Amphioxus, 198

Pineal body: Dog-fish, 273; Rabbit, 391, 400, 403; Pigeon, 458, 459
Pinna, Rabbit, 337

Pituitary body : Dog-fish, 283, 285 ; Rabbit, 388 " 398 ; Pigeon, 459
Pituitary foramen: Rabbit, 306, 320
Pituitary fossa, see Sella turcica
Placenta : Rabbit, 335
Pleura : Rabbit, 345 ; Pigeon, 453
Pleurobranchia : Crayfish, 153
Pleuron : Crayfish, 142
Podical plate : Cockroach, 172, 173
Podobranchia : Crayfish, 153, 154
Pollex : Rabbit, 329
Pons Varolii: Rabbit, 388, 403
Pore, abdominal: Dog-fish, 22 210
Pore, dorsal : Earthworm, 69

512 INDEX.

Post-axial border of limb : Rabbit, 325, 331 ; Fowl, 422

Post-orbital groove : Dog-fish, 231

Post-zygapophysis, see Vertebra

Pre-axial border of limb : Rabbit, 325, 331 ; Fowl, 422, 427

Premaxilla : Rabbit, 310 ; Fowl, 416

Premolar : Rabbit, 322, 323

Prepuce: Rabbit, 337, 367

Pre-sphenoid : Rabbit, 307 ; Fowl, 414

Pre-zygapophysis, see Vertebra

Primary quill : Pigeon, 432

Prismatic layer : Mussel-shell, 93

Proctodceum: Crayfish, 159, 160: Cockroach, 178, 179

Pronephros: Bog-fish, 262; Rabbit, 363 ; Chick, 482

Pronotum : Cockroach, Y12,

Pro-otic: Rabbit, 311

Pro-pterygium : Dog-fish, 238

Prosencephalon : Dog-fish, 273, 283

Prostate : Rabbit, 367

Prostomium : Leech, 47 ; Earthworm, 68

Prothorax : Cockroach, 171 —

Protopodite : Crayfish, 147 ; see also Appendage

Protovertebra, see Mesoblastic somite

PROTOZOA, 1-17

Proventriculus : Cockroach, 178 ; Pigeon, 440

Pseudobranch : Dog-fish, 245

Pseudopodia, 1-3

Pterygoid bone : Rabbit, 314 ; Fowl, 416

Pterygoid fossa : Rabbit, 314

Pterygoid process : Rabbit, 306

Pteryla : Pigeon, 433 ->,

Pubes : Rabbit, 330 ; Fowl, 425

Pulvillus : Cockroach, 177

Pupil : Dog-fish, 287

Pygostyle : Fowl, 407

Pylorus : Rabbit, 342

Pyramid of kidney : Rabbit, 364

Quadrate: Fowl, 418
Quadrato-jugal : Fowl, 418
Quill. Pigeon, 431
Quill-feather : Pigeon, 432

BABBIT, 291-400 ; vertebral column, 293 ; ribs and sternum, 301 ; skull, 302 ; teeth, 121;
appendicular skeleton, 323 ; external characters, 336 ; abdominal viscera,
338 ; digestive system, 342 ; thoracic viscera, 347 , circulatory system, 349;
renal and reproductive systems, 362 ; dissection of neck, 370 ; dissection of
head, 379 ; dissection of brain, 384
Rachis: Pigeon, 432
Kadial cartilage : Dog-fish, 238, 239
Radius : Rabbit, 326 ; Fowl, 423
Radula : Snail, 136

Ramus descendens of 12th cranial nerve : Rabbit, 371
Receptaculum ovorum : Earthworm, 78, 81
Receptaculum seminis, see Spermotheca
Rectal diverticula : Pigeon, 442
Rectrix : Fowl, 399 ; Pigeon, 343
Rectum : Mussel, 102, 109 ; Snail, 121, 127; Cockroach, 179 ; Dog-fish, 242 ; Rabbit, 339,

345, 347, 368 ; Pigeon, 442, 456, 457
Redia,43,44

Rejuvenescence: Paramecium, 7
Remex : Pigeon, 432
Reno-pericardial canal : Snail, 125
Reproduction : Amoeba, 3 ; Paramecium, 6 ; Opalina, 8 ; Vorticella, 12 ; Hydra .25, 28 ;

Liver-fluke, 40, 44 ; Snail, 131
Reproductive system : Liver-fluke, 37 ; Tapeworm, 46 ; Leech, 57, 64 ; Earthworm, 78 ;
Mussel, 110 ; Snail, 131 ; Crayfish, 158 ; Cockroach, 184 ;
Amphioxus, 201, 203, 214; Dog-fish, 262; Rabbit, 362, 370;
Pigeon, 454-457
Respiration : Leech, 62 ; Mussel, 100 ; Snail, 124, 125 , Crayfish, 151, 154 ; Cockroach,
180; Amphioxus, 193: Dog-fish, 244; Pigeon, 435

INDEX. 513

Bespiratory system : Snail, 123 ; Cockroach, 173, 180 ; Amphioxus, 192, 204, 205 : Dog-fish,

244 ; Pigeon, 435, 453-455 ; see also Gill, Lung
Restiform body : Bog-fish, 274, 285
Eetina : Dog-fish, 287 ; Chick, 482-3
Rib : Dog-fish, 224 ; Rabbit, 301 ; Fowl, 407
Rostrum : Crayfish, 144 ; Dog-fish, 227 ; Fowl, 412, 414

Sacculus rotundus : Rabbit, 345

Saccus vasculosus : Dog-fish, 283

Sacrum : Rabbit, 300 ; Fowl, 406

Salivary receptacle : Cockroach, 180

Scale : Dog-fish, 221

Scaphocerite : Crayfish, 151

Scaphognathite : Crayfish, 150, 154

Scapula : i?a6&t'«, 324 ; Fowl, 421

Scapus : Pigeon, 431

Sclerite : Crayfish, 141, 151 Cockroach, 171

Sclerotic : Dog-fish, 286J

Sclerotic plate Pigeon, 461

Scrotal sac: Rabbit, 338, 364

Scyllium, sec Dog-fish

Secondary quill : Pigeon, 432

Segmental organ, sec Nephridium

Segmentation: Leech, 46; Earthworm, 66; Crayfish, 140; Cockroach, 169 ;~ Chick, 481

Segmentation of skull : Rabbit, 304

Sella turcica : Rabbit, 306

Semicircular canal : Dog-fish, 228, 229, 288, 289 ; Pigeon, 463

Seminal funnel : Earthworm, 80

Sense-capsules ; Dog-fish, 225 ; Rabbit, 310

Sense-organs : ZeecA, 59 ; Mussel, 108 ; <Snot7, 135 ; Crayfish, 165 ; Cockroach, 183 ;

Amphioxus, 201 ; Dog-fish, 286 ; Pigeon, 459 ; CAic*, 482
Septum : Earthworm, 70, 96 ; Amphioxus, 191, 207
Septum auricularum : Rabbit, 360; Pigeon, 451
Septum interorbital : -FW, 409, 414
Septum lucidum : Rabbit, 394
Septum narium : Dog-fish, 227 ; Rabbit, 320
Septum obliquum : Pigeon, 439

Sesamoid bone : -Rotot*, 291, 329 ; see oZso Fabella, Patella
Seta : Earthworm, 68, 85
Shell : JfusseZ, 90 ; Snai7, 117, 120
Shell-gland : Liver-fluke, 40 ; Tapeworm, 46
Sinus, blood: Zeec/j, 52, 54, 63; tfnaiJ, 138; Crayfish, 155

anterior cardinal : Dog-fish, 250, 251

coronary : Dog-fish, 262

Ouvierian : Dog-fish, 250

genital : Dog-fish, 253

hepatic : Dog-fish, 250

hyoidean : Dog-fish, 251

inferior jugular : Dog-fish, 250

inter-orbital : Dog-fish, 230, 251

orbital: Dog-fish, 229, 251

posterior cardinal : Dog-fish, 250, 251

post-orbital : Dog-fish, 251

venosus : Dog-flsh, 247, 249, 261
Sinus, genital : Liver-fluke, 40 ; Dog-fish, 253
urinary : Dog-flsh, 266
urino-genital : Dog-fish, 270
Skate, 218

Skeleton : Amphioxus, 190, 203, 205, 207 ; Dog-fish, 222 ; Rabbit, 291 ; .FotcJ, 311
Skeleton, appendicular : Rabbit, 323 ; .Foutf, 421
axial : Rabbit, 293 ; -FozcZ, 401
branchial : Amphioxus, 193, 205
buccal : Amphioxus, 190, 205, 207
endophragmal : Crayfish, 163
of fins : Amphioxus, 190, 203, 207 ; Dog-fish, 230
mammalian, 292
visceral : Dog-fish, 233
Skin, sec Integument
Skull: Dog-fish, 225 ; Rabbit, 302; 2)o^, 304, 307; Fowl, 409

514 INDEX.

Snail, 117-139 ; external character?, 117 ; mantle -cavity, 123 ; digestive system, 127 ;

reproductive system, 131 ; nervous system, 134 ; sense-organs, 1 35 ; buccal

mass, 136 ; circulatory system, 137
Somatopleur : Chick, 475
Somite : Leech, 46 ; Chick, 475
Spermatid : Earthworm, 83

Spermatophore : Leech, 59 ; Earthworm, 79 ; Snail, 132
Spermatozoa: Hydra, 26 ; Leech, 57 ; Earthworm, 82, 83 ; Snail 131 ; Crayfish, 158 ;

Amphioxus, 202 ; Dog-fish, 271
Spermocyte : Earthworm, 82
Spermogonia : Earthworm, 82

Spermotheca: Earthworm, 79, 93; Snail, 133 ; Cockroach, 185
Sperm-sac : Dog-fish, 269
Sphenoidal fissure : 2?a&&ft, 306, 307
Spinal cord : Amphioxus, 199, 206, 208 ; Dog-fish, 285
Spinous process, see Vertebra
Spiracle : Cockroach, 173, 180 ; Dog-fish, 221
Spiral valve : Dog-fish, 242
Splanchnopleure : Chick, 475
Spleen : Dog-fish, 241 ; Rabbit, 341 ; Pigeon, 440
Splenial : f o«tf, 420
Sporocyst, 43

Sporozoite : Monocystis, 14 ; Coccidium, 17
Squamosal : Rabbit, 316 ; jfoutf, 412, 413
Stapes : Rabbit, 314
Sternebra: Rabbit, 302

Sternum : Crayfish, 142, 144 ; Cockroach, 173 ; #a&&#, 302 ; Fowl, 408
Stigma : Cockroach, 173, 180
Stipes: Cockroach, 174
Stomach : Zeecft, 52 ; Mussel, 109 ; 5nai7, 129 ; Crayfish, 160, 161 ; Amphioxus, 194 ; £o?-

fish, 240; 2ta&&&, 339, 341, 346; Pigeon, 445
Stomodoeum : Crayfish, 159, 160 ; Cockroach, 178 ; Ctacfc, 479
Stylo-hyal : Rabbit, 317
Submentum : Cockroach, 175
Suborbital ridge : Dog-fish, 229
Sucker : Liver-fluke, 35, 44 ; Z^cTj, 47
Sulcus : Rabbit, 389, 399
Supporting lamella : Hydra, 22, 24, 28, 30
Supra-angular: Fowl, 419

Supra-branchial passage or chamber : Mussel, 95, 100, 112, 113, 116
Supra-occipital : Rabbit, 304
Supra-orbital crest : Dog-fish, 228, 229
Supra-orbital process : Rabbit, 309
Suprascapula : Rabbit, 324

Suspensory fold of pharynx : Amphioxus, 195, 211, 213
Swimmeret, see Abdominal appendage
Sylvian aqueduct : Rabbit, 399 ; Pigeon, 459
Sylvian fissure : Rabbit, 387
Symphysis pubis : Rabbit, 331
Syrinx : Pigeon, 454

Tapetum : Dog-fish, 287

Tapeworm, 45-48

Tarso-metatarsus : Fowl, 427

Tarsus : Cockroach, 177 ; Rabbit, 328, 333

Teat: Rabbit, 338

Teeth: Crayfish, 161, 162; Cockroach, 179; Dog-fish, 222, 234; Rabbit, 291, 321

Telson: Crayfish, 144

Tentacle : Hydra, 20, 22 ; Mussel, 97 ; Snail, 118 ; Amphioxus, 189, 204, 205

Tentorium : Cockroach, 182 ; Rabbit, 389

Tergum : Crayfish, 142 ; Cockroach, 172

Testis : .Hydra, 26 ; Liver-fluke, 37 ; icccA, 57, 64 Earthworm, 81, 82 ; Mussel, 110 ;

Crayfish, 158 ; Cockroach, 184 ; Amphioxus, 202, 204, 214 ; Dog-fish, 241,

267 ; iZoW>ft, 364, 365 ; Pigeon, 454
Thalamencephalon : Dog-fish, 273, 285 ; Rabbit, 390, 395, 399 ; Pigeon, 459
Thoracic appendage : Crayfish, 142 ; Cockroach, 175
Thoracic duct : Rabbit, 359

Thorax: Crayfish, 144; Cockroach, 171; iZo6&t<, 337, 347
Thread-cc.ll: Hydra, 23, 24, 28, 30; «ec ateo Trichocyst

1UJDEX. 515

Thymus : Rabbit, 347

Thyroid body : Dog-fish, 255 ; Rabbit, 373 ; Pigeon, 441

Thyroid cartilage : Rabbit, 373, 378

Tibia : Cockroach, 176 ; Rabbit, 332

Tibio-tarsus : Fowl, 432

Tongue : Cockroach, 178 ; Rabbit, 380 ; Pigeon, 438

Tonsil : Rabbit, 380

Trachea : Rabbit, 372 ; Pit/eora, 453

Tracheal tube : Cockroach, 180, 181

Trichocyst : Paramecium, 5, 7

Trochauter : Cockroach, 175 ; i2a&&#, 331 ; .FoW, 426

Trophozoite : 14

Trypanosoma: 16-17

Tubercle: Rabbit, 301

Tympanic bone : Rabbit, 312

bulla : Rabbit, 304, 312

cavity : Rabbit, 312 ; Fowl, 412 ; Pigeon, 462

membrane : Rabbit, 312 ; ifyMtf, 412 ; Pigeon, 461

Tympano-Eustachian passage : Pigeon, 461 ; Chick, 479

Typhlosole: Earthworm, 72, 86; Mussel, 110, 112

Typical segment : Crayfish, 141

Ulna : iZotaft, 326 ; JW, 423
Umbilicus of feather : Pigeon, 431

of snail-shell, 120
Umbo : Mussel-shell, 90
Uncinate process : Fowl, 408
Ureter : Mussel, 105, 106, 113, 115 ; Snail, 126 ; Crayfish 163 ; Dog-fish, 262, 264, 270 ;

Rabbit, 359 ; Pigeon, 455
Urethra: iZoft&ft, 366
Urino-genital canal, see Urethra, Vestibule

pouch : Pigeon, 456, 458 »

Uterus : Liver-fluke, 40 ; Rabbit, 369
Uterus masculinus : Rabbit, 367, 368

Vacuole: Hydra, 21, 30

Vacuole, food : Amoeba, 3 ; Paramecium, 6 ; Vorticella, 11

pulsating : Amoeba, 3 ; Paramecium, 5 ; Vorticella, 11
Vagina : Liver-fluke, 40 ; icecA, 58 ; -Sfaatf, 133 ; .Ratal'*, 369
Vagus, see Nerve, 10th cranial
Valve, Eustachian : Rabbit, 360 ; Pigeon, 451
mitral: Rabbit, 362 ; Pigeon, 457-
of Vieussens : ^afc^, 387, 400

semilunar: -Ro&&i«, 362; Pigeon, 457, 458

spiral : Dog-fish, 242
tricuspid : Rabbit, 361
Vane : Pigeon, 437
Vas deferens : Liver-fluke, 38 ; ieecA, 57, 65 ; Earthworm, 81 ; Snatf, 132 ; Crayfish, 158 ;

Cockroach, 184 ; Dog-fish, 269 ; .Ra&&i<, 365 ; Pigeon, 455
Vas efferens : Zeec/», 57 ; Earthworm, 80 ; Dog-fish, 267 ; i?a&6i'«, 365
Vein, azygos cardinal : Rabbit, 352

brachial: Pigeon, 445

caudal : Dog-fish, 253 ; Pigeon, 451

coronary : Rabbit, 360

duodenal: Rabbit, 355

facial : iJa&&#, 353

femoral : Rabbit, 355 ; Pigeon, 447

gastro-duodenal : Pigeon, 452

hepatic : Amphioxus, 197, 214 ; #a&&ft, 354 ; Pigeon, 445

hypogastric : Pigeon, 451

iliac : Ro&&«, 354 ; Pt'^oii, 451, 452

ilio-lumbar : Rabbit, 354

intercostal: Rabbit, 352

internal iliac : Pigeon, 451

internal mammary : Rabbit, 353 ; Pigeon, 445

jugular : JRa&&«, 353 ; Pigeon, 444

516

INDEX.

Vein, lateral: Dog-fish, 252

lieno-gastric : Rabbit, 355

mesenteric : Rabbit, 355 ; Pigeon, 439, 446, 447
ovarian: Rabbit, 354
pectoral : Pigeon, 445
phrenic : Rabbit, 354

portal: Amphioxus, 197, 214; Dog-fish, 253; Rabbit, 355; Pigeon, 439, 447
pulmonary : 5nai7, 124 ; Rabbit, 351 ; Pigeon, 447
renal: Dog-fish, 252; iia&&&, 354; Pigeon, 446, 447
renal portal : Dog-fish, 253
spermatic: Rabbit, 354
subclavian : Dog-fish, 258 ; tfafc&ft, 353
vertebral: Pigeon, 437
vitelline : C/a'cfc, 477
Veins: J/wsseZ, 103; -Snatf, 138; Dog-fish, 249; 22o&&&, 351; Pigeon, 444; see

also Sinus
Veins of kidney: Pigeon, 446
Volum: Amphioxus, 192
Velum interpositum : Rabbit, 395, 399
Velum medullae : Rabbit, 386, 400
Vena cava : Mussel, 103, 112, 115

anterior : Rabbit, 350, 352, 353 ; Pigeon, 444, 445 ; Chick, 481
posterior: Rabbit, 350, 353; P^ecm, 445 ; Chick, 481
Ventricle of brain : Dog-fish, 248-285 ; Rabbit, 395

lateral : Dog-fish, 294 ; Rabbit, 396 : Pigeon, 459
third : Dog-fish, 273, 284 ; Pa&&#, 400 ; Pigeon, 459
fourth : Dog-fish, 274, 275 ; Pa&&#, 404 ; Pigeon, 459
fifth: flaa&tt, 398
Ventricle of heart : Mussel, 102, 115 ; Snail, 125 ; Dog-fish, 247, 261 ; Rabbit, 349, 350,

361, 362; Pigeon, 452
Ventricle of larynx : Rabbit, 379
Ventricle of nervous system : Amphioxus, 199
Vermiform appendix : Rabbit, 339, 345, 346
Vermis cerebelli : Rabbit, 392
Vertebra anticlinal : Rabbit, 298

caudal : Rabbit, 300 ; Fowl, 407
cervical : Rabbit, 290 ; Fowl, 402
lumbar : i?o66«, 293, 299
sacral : Rabbit, 300 ; Fowl, 406
thoracic : Rabbit, 298 ; Jew*, 406
typical : Rabbit, 293
Vertebral column : Dog-fish, 223 ; Rabbit, 293 ; Powf, 401
Vertebral column, development of : Dog-fish, 222
Vertebrarterial canal, see Vertebra, cervical
Verumontanum : Rabbit, 366
Vesicula seminalis : Liver-fluke, 39 ; Earthworm, 80, 83 ; Cockroach, 184 ; Dog-fish, 258 ;

Pigeon, 456
Vessel, see Blood-vessel
Vestibule : Vorticella, 10 ; Rabbit, 369
Vestibule of ear : Dog-fish, 288
Vexillum : Pigeon, 432
Vibrissa : Rabbit, 337
Villi : Rabbit, 346
Visceral hump : Snail, 121
Visceral mass : Mussel, 97, 101

Visceral nervous system : Crayfish, 154 ; Cockroach, 183
Visceral skeleton : Dog-fish, 233
Vitellarium, see Yolk-gland
Vocal cord : Rabbit, 378
Vocal organ, see Larynx, Syrinx
Vomer: Rabbit, 310; Fowl, 416
VORTICELLA: 8-13
Vulva: Rabbit, 337

Wing : Cockroach, 172 ; Fowl, 422 ; Pigeon, 431 ; CAic*, 485
Wolffian body : Dog-fish, 262, 264, 267 ; Rabbit, 363 ; Chick, 482
Wrist, see Carpus

INDEX. 517

Xiphisternum: Rabbit, 302
Xiphoid process : Fowl, 409

Yolk-cell : Liver-fluke, 42

Yolk-gland : Liver-fluke, 39 ; Tapeworm, 48

Yolk-reservoir : Liver-fluke, 39

Yolk-sac : Chick, 477, 480

Yolk : Hen's egg, 468

Zygapophysis, see Vertebra

Zygomatic arch : Rabbit, 316

Zygomatic process : Rabbit, 315, 316 ; Fowl, 403

Zygote : Coccidium, 17

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