THE PHYSIOLOGY OF THE CRAYFISH. THE MECHANISM BY WHICH THE PARTS OF
THE LIVING ENGINE ARE SUPPLIED WITH THE MATERIALS NECESSARY FOR THEIR
MAINTENANCE AND GROWTH.
An analysis of such a sketch of the “Natural History of the Crayfish”
as is given in the preceding chapter, shows that it provides brief
and general answers to three questions. First, what is the form and
structure of the animal, not only when adult, but at different stages
of its growth? Secondly, what are the various actions of which it is
capable? Thirdly, where is it found? If we carry our investigations
further, in such a manner as to give the fullest attainable answers
to these questions, the knowledge thus acquired, in the case of the
first question, is termed the _Morphology_ of the crayfish; in the
case of the second question, it constitutes the _Physiology_ of the
animal; while the answer to the third question would represent what
we know of its _Distribution_ or _Chorology_. There remains a fourth
problem, which can hardly be regarded as seriously under discussion,
so long as knowledge has advanced no further than the Natural History
stage; the question, namely, {47} how all these facts comprised
under Morphology, Physiology, and Chorology have come to be what they
are; and the attempt to solve this problem leads us to the crown of
Biological effort, _Ætiology_. When it supplies answers to all the
questions which fall under these four heads, the Zoology of Crayfish
will have said its last word.
* * * * *
As it matters little in what order we take the first three questions,
in expanding Natural History into Zoology, we may as well follow
that which accords with the history of science. After men acquired a
rough and general knowledge of the animals about them, the next thing
which engaged their interest was the discovery in these animals of
arrangements by which results, of a kind similar to those which their
own ingenuity effects through mechanical contrivances, are brought
about. They observed that animals perform various actions; and, when
they looked into the disposition and the powers of the parts by which
these actions are performed, they found that these parts presented
the characters of an apparatus, or piece of mechanism, the action of
which could be deduced from the properties and connections of its
constituents, just as the striking of a clock can be deduced from the
properties and connections of its weights and wheels.
Under one aspect, the result of the search after the _rationale_
of animal structure thus set afoot is _Teleology_; or the doctrine
of adaptation to purpose. Under another {48} aspect, it is
_Physiology_; so far as Physiology consists in the elucidation of
complex vital phenomena by deduction from the established truths of
Physics and Chemistry, or from the elementary properties of living
matter.
* * * * *
We have seen that the crayfish is a voracious and indiscriminate
feeder; and we shall be safe in assuming that, if duly supplied with
nourishment, a full-grown crayfish will consume several times its own
weight of food in the course of the year. Nevertheless, the increase
of the animal’s weight at the end of that time is, at most, a small
fraction of its total weight; whence it is quite clear, that a very
large proportion of the food taken into the body must, in some shape
or other, leave it again. In the course of the same period, the
crayfish absorbs a very considerable quantity of oxygen, supplied by
the atmosphere to the water which it inhabits; while it gives out,
into that water, a large amount of carbonic acid, and a larger or
smaller quantity of nitrogenous and other excrementitious matters.
From this point of view, the crayfish may be regarded as a kind of
chemical manufactory, supplied with certain alimentary raw materials,
which it works up, transforms, and gives out in other shapes. And the
first physiological problem which offers itself to us is the mode of
operation of the apparatus contained in this factory, and the extent
to which the products of its activity are to be accounted for by
reasoning from known physical and chemical principles. {49}
We have learned that the food of the crayfish is made up of very
diverse substances, both animal and vegetable; but, so far as they
are competent to nourish the animal permanently, these matters all
agree in containing a peculiar nitrogenous body, termed _protein_,
under one of its many forms, such as albumen, fibrin, and the like.
With this may be associated fatty matters, starchy and saccharine
bodies, and various earthy salts. And these, which are the essential
constituents of the food, may be, and usually are, largely mixed up
with other substances, such as wood, in the case of vegetable food,
or skeletal and fibrous parts, in the case of animal prey, which are
of little or no utility to the crayfish.
The first step in the process of feeding, therefore, is to reduce the
food to such a state, that the separation of its nutritive parts,
or those which can be turned to account, from its innutritious, or
useless, constituents, may be facilitated. And this preliminary
operation is the subdivision of the food into morsels of a convenient
size for introduction into that part of the machinery in which the
extraction of the useful products is performed.
The food may be seized by the pincers, or by the anterior chelate
ambulatory limbs; and, in the former case, it is usually, if not
always, transferred to the first, or second, or both of the anterior
pairs of ambulatory limbs. These grasp the food, and, tearing it into
pieces of the proper dimensions, thrust them between the external
maxillipedes, which are, at the same time, {50} worked rapidly to
and fro sideways, so as to bring their toothed edges to bear upon
the morsel. The other five pairs of jaws are no less active, and
they thus crush and divide the food brought to them, as it is passed
between their toothed edges to the opening of the mouth.
As the alimentary canal stretches from the mouth, at one end, to the
vent at the other, and, at each of these limits, is continuous with
the wall of the body, we may conceive the whole crayfish to be a
hollow cylinder, the cavity of which is everywhere closed, though it
is traversed by a tube, open at each end (fig. 6). The shut cavity
between the tube and the walls of the cylinder may be termed the
_perivisceral cavity_; and it is so much filled up by the various
organs, which are interposed between the alimentary canal and the
body wall, that all that is left of it is represented by a system
of irregular channels, which are filled with blood, and are termed
_blood sinuses_. The wall of the cylinder is the outer wall of the
body itself, to which the general name of _integument_ may be given;
and the outermost layer of this, again, is the _cuticle_, which
gives rise to the whole of the exoskeleton. This cuticle, as we have
seen, is extensively impregnated with lime salts; and, moreover, in
consequence of its containing _chitin_, it is often spoken of as the
_chitinous cuticula_.
Having arrived at this general conception of the disposition of the
parts of the factory, we may next proceed to consider the machinery
of alimentation which is {51} contained within it, and which is
represented by the various divisions of the alimentary canal, with
its appendages; by the apparatus for the distribution of nutriment;
and by two apparatuses for getting rid of those products which are
the ultimate result of the working of the whole organism.
And here we must trench somewhat upon the province of _Morphology_,
as some of these pieces of apparatus are complicated; and their
action cannot be comprehended without a certain knowledge of their
anatomy.
The mouth of the crayfish is a longitudinally elongated,
parallel-sided opening, in the integument of the ventral or sternal
aspect of the head. Just outside its lateral boundaries, the
strong mandibles project, one on each side (fig 3, B; _4_); their
broad crushing surfaces, which are turned towards one another, are
therefore completely external to the oral cavity. In front, the
mouth is overlapped by a wide shield-shaped plate termed the upper
lip, or _labrum_ (figs. 3 and 6, _lb_); while, immediately behind
the mandibles, there is, on each side, an elongated fleshy lobe,
joined with its fellow by the posterior boundary of the mouth.
These together constitute the _metastoma_ (fig. 3, B; _mt_), which
is sometimes called the lower lip. A short wide gullet, termed the
œsophagus (fig. 6, _oe_), leads directly upwards into a spacious bag,
the _stomach_, which occupies almost the whole cavity of the head. It
is divided by a constriction into a large anterior chamber (_cs_),
into the under face of which the {52} gullet opens, and a small
posterior chamber (_ps_), from which the intestine (_hg_) proceeds.
In a man’s stomach, the opening by which the gullet communicates
with the stomach is called the _cardia_, while that which places the
stomach in communication with the intestine is named the _pylorus_;
and these terms having been transferred from human anatomy to that
of the lower animals, the larger moiety of the crayfish’s stomach
is called the _cardiac division_, while the smaller is termed the
_pyloric division_ of the organ. It must be recollected, however,
that, in the crayfish, the so-called cardiac division is that which
is actually furthest from the heart, not that which is nearest to it,
as in man.
The gullet is lined by a firm coat which resembles thin parchment.
At the margins of the mouth, this strong lining is easily seen to
be continuous with the cuticular exoskeleton; while, at the cardiac
orifice, it spreads out and forms the inner or cuticular wall of the
whole gastric cavity, as far as the pylorus, where it ends in certain
valvular projections. The chitinous cuticle which forms the outermost
layer of the integument is thus, as it were, turned in, to constitute
the innermost layer of the walls of the stomach; and it confers
upon them so great an amount of stiffness that they do not collapse
when the organ is removed from the body. Furthermore, just as the
cuticle of the integument is calcified to form the hard parts of the
exoskeleton, so is the cuticle of the stomach calcified, or otherwise
hardened, to give rise, in the first {53} place, to the very
remarkable and complicated apparatus which has already been spoken
of, as a sort of _gastric mill_ or _food-crusher_; and, secondly, to
a _filter_ or _strainer_, whereby the nutritive juices are separated
from the innutritious hard parts of the food and passed on into the
intestine.
[Illustration: FIG. 9.—_Astacus fluviatilis._—A, the stomach with
its outer coat removed, seen from the left side; B, the same viewed
from the front, after removal of the anterior wall; C, the ossicles
of the gastric mill separated from one another; D, the prepyloric
ossicle and median tooth, seen from the right side; E, transverse
section of the pyloric region along the line _xy_ in A (all × 2).
_c_, cardiac ossicle; _cpv_, cardio-pyloric valve; _lp_, lateral
pouch; _lt_, lateral tooth, seen through the wall of the stomach
in A; _mg_, mid-gut; _mt_, median tooth, seen through the wall of
the stomach in A; _œs_, œsophagus; _p_, pyloric ossicle; _pc_,
pterocardiac ossicle; _pp_, prepyloric ossicle; _uc_, uro-cardiac
process; _t_, convexities on the free surface of its hinder end;
_v_^1, median pyloric valve; _zc_, zygocardiac ossicle.]
{54}
The gastric mill begins in the hinder half of the cardiac division.
Here, on the upper wall of the stomach, we see a broad transverse
calcified bar (figs. 9–11, _c_) from the middle of the hinder part of
which another bar (_uc_), united to the first by a flexible portion,
is continued backwards in the middle line. The whole has, therefore,
somewhat the shape of a cross-bow. Behind the first-mentioned piece,
the dorsal wall of the stomach is folded in, in such a manner as
to give rise to a kind of pouch; and the second piece, or what we
may call the handle of the crossbow, lies in the front wall of this
pouch. The end of this piece is dense and hard, and its free surface,
which looks into the top of the cardiac chamber, is raised into two
oval, flattened convex surfaces (_t_). Connected by a transverse
joint with the end of the handle of the crossbow, there is another
solid bar, which ascends obliquely forwards in the back wall of the
pouch (_pp_). The end which is articulated with the handle of the
crossbow is produced into a strong reddish conical tooth (_mt_),
curved forwards and bifurcated at the summit; consequently, when the
cavity of the stomach is inspected from the fore part of the cardiac
pouch (fig. 9, B), the two-pointed curved tooth (_mt_) is seen
projecting behind the convex surfaces (_t_), in the middle line, into
the interior of that cavity. The joint which connects the handle of
the crossbow with the hinder middle piece is elastic; hence, if the
two are straightened out, they return to their bent disposition as
soon as they are released. The upper end of {55} the hinder middle
piece (_pp_) is connected with a second flat transverse plate which
lies in the dorsal wall of the pyloric chamber (_p_). The whole
arrangement, thus far, may be therefore compared to a large cross-bow
and a small one, with the ends of their handles fastened together by
a spring joint, in such a manner that the handle of the one makes an
acute angle with the handle of the other; while the middle of each
bow is united with the middle of the other by the bent arm formed
by the two handles. But, in addition to this, the outer ends of the
two bows are also connected together. A small, curved, calcified bar
(_pc_) passes from the outer end of the front crosspiece downwards
and outwards in the wall of the stomach, and its hinder and lower
extremity is articulated with another larger bar (_zc_) which runs
upwards and backwards to the hinder or pyloric crosspiece, with
which it articulates. Internally, this piece projects into the
cardiac cavity of the stomach as a stout elongated reddish elevation
(_lt_), the surface of which is produced into a row of strong sharp,
transverse ridges, which diminish in size from before backwards, and
constitute a crushing surface almost like that of the grinder of an
elephant. Thus, when the front part of the cardiac cavity is cut
away, not only are the median teeth already mentioned seen, but, on
each side of them, there is one of these long lateral teeth.
[Illustration: FIG. 10.—_Astacus fluviatilis._—Longitudinal section
of the stomach (× 4), _c_, cardiac ossicle; _cæ_, cæcum; _c.p.v_,
cardio-pyloric valve; _cs_, cushion-shaped surface; _hg_, hind-gut;
_hp_, aperture of right bile duct; _lp_, lateral pouch; _lt_,
lateral teeth; _mg_, mid-gut; _mt_, median tooth; _œs_, œsophagus;
_p_, pyloric ossicle; _pc_, pterocardiac ossicles; _pp_, prepyloric
ossicle; _uc_, urocardiac process; _v_^1, median pyloric valve;
_v_^2, lateral pyloric valve; _x_, position of gastrolith; _zc_,
zygocardiac ossicle.]
There are two small pointed teeth, one under each of the lateral
teeth, and each of these is supported by {56} a broad plate, hairy
on its inner surface, which enters into the lateral wall of the
cardiac chamber. There are various other smaller skeletal parts, but
the most important are those which have been described; and these,
from what has been said, will be seen to form a sort of hexagonal
frame, with more or less flexible joints at the angles, and having
the anterior and the posterior sides {57} connected by a bent
jointed middle bar. As all these parts are merely modifications of
the hard skeleton, the apparatus is devoid of any power of moving
itself. It is set in motion, however, by the same substance as that
which gives rise to all the other bodily movements of the crayfish,
namely, _muscle_. The chief muscles which move it are four very
strong bundles of fibres. Two of these are attached to the front
crosspiece, and proceed thence, upwards and forwards, to be fixed to
the inner face of the carapace in the front part of the head (figs.
5, 6, and 12, _ag_). The two others, which are fixed into the hinder
crosspiece and hinder lateral pieces, pass upwards and backwards,
to be attached to the inner face of the carapace in the back part
of the head (_pg_). When these muscles shorten, or contract, they
pull the front and back crosspieces further away from one another;
consequently, the angle between the handles becomes more open and the
tooth which is borne on their ends travels downwards and forwards.
But, at the same time, the angle between the side bars becomes
more open and the lateral tooth of each side moves inwards till it
crosses in front of the middle tooth, and strikes against this and
the opposite lateral tooth, which has undergone a corresponding
change of place. The muscles being now relaxed, the elasticity of
the joints suffices to bring the whole apparatus back to its first
position, when a new contraction brings about a new clashing of the
teeth. Thus, by the alternate contraction and relaxation of these
two pair of muscles, the {58} three teeth are made to stir up and
crush whatever is contained in the cardiac chamber. When the stomach
is removed and the front part of the cardiac chamber is cut away,
the front cross-piece may be seized with one pair of forceps and the
hind cross-piece with another. On slightly pulling the two, so as to
imitate the action of the muscles, the three teeth will be found to
come together sharply, exactly in the manner described.
Works on mechanics are full of contrivances for the conversion of
motion; but it would, perhaps, be difficult to discover among these
a prettier solution of the problem; given a straight pull, how to
convert it into three simultaneous convergent movements of as many
points.
What I have called the _filter_ is constructed mainly out of
the chitinous lining of the pyloric chamber. The aperture of
communication between this and the cardiac chamber, already narrow,
on account of the constriction of the walls of the stomach at this
point, is bounded at the sides by two folds; while, from below, a
conical tongue-shaped process (figs. 6, 10, and 11, _cpv_), the
surface of which is covered with hairs, further obstructs the
opening. In the posterior half of the pyloric chamber, its side
walls are, as it were, pushed in; and, above, they so nearly meet in
the middle line, that a mere vertical chink is left between them;
while even this is crossed by hairs set upon the two surfaces. In
its lower half, however, each side wall curves outwards, and forms
a cushion-shaped surface (fig. 10, _cs_) which looks downwards and
inwards. If the {59} floor of the pyloric chamber were flat, a
wide triangular passage would thus be left open in its lower half.
But, in fact, the floor rises into a ridge in the middle, while, at
the sides, it adapts itself to the shape of the two cushion-shaped
surfaces; the result of which is that the whole cavity of the
posterior part of the pyloric division of the stomach is reduced to a
narrow three-rayed fissure. In transverse section, the vertical ray
of this fissure is straight, while the two lateral ones are concave
upwards (fig. 9, _E_). The cushions of the side walls are covered
with short close-set hairs. The corresponding surfaces of the floor
are raised into longitudinal parallel ridges, the edge of each of
which is fringed with very fine hairs. As everything which passes
from the cardiac sac to the intestine must traverse this singular
apparatus, only the most finely divided solid matters can escape
stoppage, so long as its walls are kept together.
Finally, at the opening of the pyloric sac into the intestine, the
chitinous investment terminates in five symmetrically arranged
processes, the disposition of which is such that they must play the
part of valves in preventing any sudden return of the contents of the
intestine to the stomach, while they readily allow of a passage the
other way. One of these valvular processes is placed in the middle
line above (figs. 10 and 11, _v_^1). It is longer than the others and
concave below. The lateral processes (_v_^2,) of which there are two
on each side, are triangular and flat. {60}
[Illustration: FIG. 11.—_Astacus fluviatilis._—View of the roof of
the stomach, the ventral wall of which, and of the mid-gut, is laid
open by a longitudinal incision (× 4). On the right side (the left in
the figure), the lateral tooth is cut away, as well as the floor of
the lateral pouch. The letters have the same signification as in fig.
10.]
The cuticular lining which gives rise to all the complicated
apparatus which has just been described, must not be confounded with
the proper wall of the stomach, which invests it, and to which it
owes it origin, just as the cuticle of the integument is produced
by the soft {61} true skin which lies beneath it. The wall of
the stomach is a soft pale membrane containing variously disposed
muscular fibres; and, beyond the pylorus, it is continued into the
wall of the intestine.
It has already been mentioned that the intestine is a slender
and thin-walled tube, which passes straight through the body
almost without change, except that it becomes a little wider and
thicker-walled near the vent. Immediately behind the pyloric valves,
its surface is quite smooth and soft (figs. 9, 10, and 12, _mg_),
and its floor presents a relatively large aperture, the termination
of the bile duct (fig. 12, _bd_, fig. 10, _hp_), on each side.
The roof is, as it were, pushed out into a short median pouch or
_cæcum_ (_cæ_). Behind this, its character suddenly changes, and six
squarish elevations, covered with a chitinous cuticle, encircle the
cavity of the intestine (_r_). From each of these, a longitudinal
ridge, corresponding with a fold of the wall of the intestine, takes
its rise, and passes, with a slight spiral twist, to its extremity
(_hg_). Each of these ridges is beset with small papillæ, and the
chitinous lining is continued over the whole to the vent, where it
passes into the general cuticle of the integument, just as the lining
of the stomach is continuous with the cuticle of the integument at
the mouth. The alimentary canal may, therefore, be distinguished
into a _fore_ and a _hind-gut_ (_hg_), which have a thick internal
lining of cuticular membrane; and a very short _mid-gut_ (_mg_),
which has no thick cuticular layer. It will be of {63} importance
to recollect this distinction by-and-by, when the development of the
alimentary canal is considered.
[Illustration: FIG. 12.—_Astacus fluviatilis._—A dissection of a
male specimen from the right side (nat. size). _a_, anus; _aa_,
antennary artery, cut short; _ag_, anterior gastric muscles, the
right cut away to its insertion; _bd_, aperture of right bile duct;
_cm_, constrictor muscles of stomach; _cæ_, cæcum; _cpm_, right
cardio-pyloric muscle; _cs_, cardiac portion of stomach; _cm_,
extensor muscles of abdomen; _fm_, flexor muscles of abdomen;
_ga_, gastric artery; _gn._ 1, supraœsophageal ganglion; _gn._
2, sub-œsophageal ganglion; _gn._ 13, last abdominal ganglion;
_h_, heart; _ha_, hepatic artery; _hg_, hind-gut; _iaa_, inferior
abdominal artery; _la_, right lateral aperture of heart; _lr_, left
liver; _mg_, mid-gut; _oa_, ophthalmic artery; _œ_, œsophagus; _pg_,
posterior gastric muscles, the right cut away to its insertion;
_ps_, pyloric portion of stomach; _sa_, sternal artery; _saa_,
superior abdominal artery; _t_ (to the left), telson; _t_ (near the
heart), testis; _vd_, left vas deferens; _vd′_, aperture of left
vas deferens; _2_, right antennule; _4_, left mandible; _9_, left
external maxillipede; _10_, left forceps; _15_, first, _16_, second,
and _20_, sixth abdominal appendages of the left side.]
If the treatment to which the food is subjected in the alimentary
apparatus were of a purely mechanical nature, there would be nothing
more to describe in this part of the crayfish’s mechanism. But,
in order that the nutritive matters may be turned to account, and
undergo the chemical metamorphoses, which eventually change them
into substances of a totally different character, they must pass out
of the alimentary canal into the blood. And they can do this only
by making their way through the walls of the alimentary canal; to
which end they must either be in a state of extremely fine division,
or they must be reduced to the fluid condition. In the case of the
fatty matters, minute subdivision may suffice; but the amylaceous
substances and the insoluble protein compounds, such as the fibrin
of flesh, must be brought into a state of solution. Therefore some
substances must be poured into the alimentary canal, which, when
mixed with the crushed food, will play the part of a chemical agent,
dissolving out the insoluble proteids, changing the amyloids into
soluble sugar, and converting all the proteids into those diffusible
forms of protein matter, which are known as _peptones_.
The details of the processes here indicated, which may be included
under the general name of _digestion_, have only quite recently been
carefully investigated in the crayfish; and we have probably still
much to learn about {64} them; but what has been made out is very
interesting, and proves that considerable differences exist between
crayfishes and the higher animals in this respect.
The physiologist calls those organs, the function of which is to
prepare and discharge substances of a special character, _glands_;
and the matter which they elaborate is termed their _secretion_.
On the one side, glands are in relation with the blood, whence
they derive the materials which they convert into the substances
characteristic of their secretion; on the other side, they have
access, directly or indirectly, to a free surface, on to which they
pour their secretion as it is formed.
Of such glands, the alimentary canal of the crayfish is provided
with a pair, which are not only of very large size, but are further
extremely conspicuous, on account of their yellow or brown colour.
These two glands (figs. 12 and 13, _lr_) are situated beneath, and
on each side of, the stomach and the anterior part of the intestine,
and answer in position to the glands termed liver and pancreas in
the higher animals, inasmuch as they pour their secretion into the
mid-gut. These glands have hitherto always been regarded as the
_liver_, and the name may be retained, though their secretion appears
rather to correspond with the pancreatic fluid than with the bile of
the higher animals.
[Illustration: FIG. 13.—_Astacus fluviatilis._—The alimentary canal
and livers seen from above (nat. size). _bd_, bile-duct; _cæ_,
cæcum; _cs_, cardiac portion of stomach, the line pointing to the
cardiac ossicle; _hg_, hind-gut; _mg_, mid-gut; _pc_, pterocardiac
ossicle; _ps_, pyloric portion of stomach, the line pointing to the
pyloric ossicle; _r_, ridge separating mid-gut from hind-gut; _zc_,
zygocardiac ossicle.]
Each liver consists of an immense number of short tubes, or _cæca_,
which are closed at one end, but open at the other into a general
conduit, which is termed their _duct_. The mass of the liver is
roughly divided into {66} three lobes, one anterior, one lateral,
and one posterior; and each lobe has its main duct, into which all
the tubes composing it open. The three ducts unite together into
a wide common duct (_bd_), which opens, just behind the pyloric
valves, into the floor of the mid-gut. Hence the apertures of the
two _hepatic ducts_ are seen, one on each side, in this part of the
alimentary canal when it is laid open from above. Every cæcum of the
liver has a thin outer wall, lined internally by a layer of cells,
constituting what is termed an _epithelium_; and, at the openings of
the hepatic ducts, this epithelium passes into a layer of somewhat
similar structure, which lines the mid-gut, and is continued through
the rest of the alimentary canal, beneath the cuticula. Hence the
liver may be regarded as a much divided side pouch of the mid-gut.
The epithelium is made up of _nucleated cells_, which are particles
of simple living matter, or _protoplasm_, in the midst of each of
which is a rounded body, which is termed the _nucleus_. It is these
cells which are the seat of the manufacturing process which results
in the formation of the secretion; it is, as it were, their special
business to form that secretion. To this end they are constantly
being newly formed at the summits of the cæca. As they grow, they
pass down towards the duct and, at the same time, separate into their
interior certain special products, among which globules of yellow
fatty matter are very conspicuous. When these products are fully
formed, what remains of the substance of the cells dissolves away,
and {67} the yellow fluid accumulating in the ducts passes into the
mid-gut. The yellow colour is due to the globules of fat. In the
young cells, at the summit of the cæca, these are either absent, or
very small, whence the part appears colourless. But, lower down,
small yellow granules appear in the cells, and these become bigger
and more numerous in the middle and lower parts. In fact, few glands
are better fitted for the study of the manner in which secretion is
effected than the crayfish’s liver.
* * * * *
We may now consider the alimentary machinery, the general structure
of which has been explained, in action.
The food, already torn and crushed by the jaws, is passed through
the gullet into the cardiac sac, and there reduced to a still more
pulpy state by the gastric mill. By degrees, such parts as are
sufficiently fluid are drained off into the intestine, through the
pyloric strainer, while the coarser parts of the useless matters
are probably rejected by the mouth, as a hawk or an owl rejects
his casts. There is reason to believe, though it is not certainly
known, that fluids from the intestine mix with the food while it is
undergoing trituration, and effect the transformation of the starchy
and the insoluble protein compounds into a soluble state. At any
rate, as soon as the strained-off fluid passes into the mid-gut it
must be mixed with the secretion of the liver, the action of which is
probably {68} similar to that of the pancreatic juice of the higher
animals.
[Illustration: FIG. 14.—_Astacus fluviatilis._—The corpuscles of
the blood (highly magnified). _1–8_ show the changes undergone by
a single corpuscle during a quarter of an hour; _9_ and _10_ are
corpuscles killed by magenta, and having the nucleus deeply stained
by the colouring matter. _n_, nucleus.]
The mixture thus produced, which answers to the chyle of the higher
animals, passes along the intestine, and the greater part of it,
transuding through the walls of the alimentary canal, enters the
blood, while the rest accumulates as dark coloured fæces in the hind
gut, and is eventually passed out of the body by the vent. The fæcal
matters are small in amount, and the strainer is so efficient that
they rarely contain solid particles of sensible size. Sometimes,
however, there are a good many minute fragments of vegetable tissue.
The blood of which the nutritive elements of the food {69} have thus
become integral parts, is a clear fluid, either colourless, or of a
pale neutral tint or reddish hue, which, to the naked eye, appears
like so much water. But if subjected to microscopic examination,
it is found to contain innumerable pale, solid particles, or
_corpuscles_, which, when examined fresh, undergo constant changes
of form (fig. 14). In fact, they correspond very closely with the
colourless corpuscles which exist in our own blood; and, in its
general characters, the crayfish’s blood is such as ours would be if
it were somewhat diluted and deprived of its red corpuscles. In other
words, it resembles our lymph more than it does our blood. Left to
itself it soon coagulates, giving rise to a pretty firm clot.
The sinuses, or cavities in which the greater part of the blood is
contained, are disposed very irregularly in the intervals between the
internal organs. But there is one of especially large size on the
ventral or sternal side of the thorax (fig. 15, _sc_), into which
all the blood in the body sooner or later makes its way. From this
_sternal sinus_ passages (_av_) lead to the gills, and from these
again six canals (_bcv_), pass up on the inner side of the inner wall
of each branchial chamber to a cavity situated in the dorsal region
of the thorax, termed the _pericardium_ (_p_), into which they open.
[Illustration: FIG. 15.—_Astacus fluviatilis._—A diagrammatic
transverse section of the thorax through the twelfth somite,
showing the course of the circulation of the blood (× 3). _arb.
12_, the anterior or lower, and _arb′. 12_, the posterior or upper
arthrobranchia of the twelfth somite; _av_, afferent branchial
vessel; _bcv_, branchio-cardiac vein; _bg_, branchiostegite; _em_,
extensor muscles of abdomen; _ep_, epimeral wall of thoracic cavity;
_ev_, efferent branchial vessel; _fm_, flexor muscles of abdomen;
_fp_, floor of pericardium; _gn._ 6, fifth thoracic ganglion; _h_,
heart; _hg_, hind-gut; _iaa_, inferior abdominal artery, in cross
section; _la_, lateral valvular apertures of heart; _lr_, liver;
_mp_, indicates the position of the mesophragm by which the sternal
canal is bounded laterally; _p_, pericardial sinus; _pdb. 12_,
podobranchia, and _plb. 12_, pleurobranchia of the twelfth somite;
_sa_, sternal artery; _saa_, superior abdominal artery; _sc_, sternal
canal; _t_, testis; XII., sternum of twelfth somite. The arrows
indicate the direction of the blood flow.]
The blood of the crayfish is kept in a state of constant circulating
motion by a pumping and distributing machinery, composed of the
_heart_ and of the _arteries_, with {71} their larger and smaller
branches, which proceed from it and ramify through the body, to
terminate eventually in the blood sinuses, which represent the veins
of the higher animals.
When the carapace is removed from the middle of the region which lies
behind the cervical groove, that is, when the dorsal or _tergal_
wall of the thorax is taken away, a spacious chamber is laid open
which is full of blood. This is the cavity already mentioned as the
_pericardium_ (fig. 15, _p_), though, as it differs in some respects
from that which is so named in the higher animals, it will be better
to term it the _pericardial sinus_.
The heart (fig. 15, _h_), lies in the midst of this sinus. It is a
thick muscular body (fig. 16), with an irregularly hexagonal contour
when viewed from above, one angle of the hexagon being anterior and
another posterior. The lateral angles of the hexagon are connected
by bands of fibrous tissue (_ac_) with the walls of the pericardial
sinus. Otherwise, the heart is free, except in so far as it is kept
in place by the arteries which leave it and traverse the walls of
the pericardium. One of these arteries (figs. 5, 12, and 16, _saa_),
starting from the hinder part of the heart, of which it is a sort of
continuation, runs along the middle line of the abdomen above the
intestine, to which it gives off many branches. A second large artery
starts from a dilatation, which is common to it with the foregoing,
but passing directly downwards (figs. 12 and 15, _sa_, and fig. 16,
_st.a_), either on the right or on the left side of the intestine,
{72} traverses the nervous cord (figs. 12 and 15), and divides into
an anterior (fig. 12, _sa_) and a posterior (_iaa_) branch, both of
which run beneath and parallel with that cord. A third artery runs,
from the front part of the heart, forwards in the middle line, over
the stomach, to the eyes and fore part of the head (figs. 5, 12, and
16, _oa_); and two others diverge one on each side of this, and sweep
{73} round the stomach to the antennæ (_aa_). Behind these, yet two
other arteries are given off from the under side of the heart, and
supply the liver (_ha_). All these arteries branch out and eventually
terminate in fine, so-called _capillary_, ramifications.
[Illustration: FIG. 16.—_Astacus fluviatilis._—The heart (× 4). A,
from above; B, from below; C, from the left side. _aa_, antennary
artery; _ac_, alæ cordis, or fibrous bands connecting the heart with
the walls of the pericardial sinus; _b_, bulbous dilatation at the
origin of the sternal artery; _ha_, hepatic artery; _la_, lateral
valvular apertures; _oa_, ophthalmic artery; _s.a_, superior valvular
apertures; _s.a.a_, superior abdominal artery; _st.a_, sternal
artery, in B cut off close to its origin.]
In the dorsal wall of the heart two small oval apertures are visible,
provided with valvular lips (fig. 16, _sa_), which open inwards, or
towards the internal cavity of the heart. There is a similar aperture
in each of the lateral faces of the heart (_la_), and two others in
its inferior face (_ia_), making six in all. These apertures readily
admit fluid into the heart, but oppose its exit. On the other hand,
at the origins of the arteries, there are small valvular folds,
directed in such a manner as to permit the exit of fluid from the
heart, while they prevent its entrance.
The walls of the heart are muscular, and, during life, they contract
at intervals with a regular rhythm, in such a manner as to diminish
the capacity of the internal cavity of the organ. The result is,
that the blood which it contains is driven into the arteries, and
necessarily forces into their smaller ramifications an equivalent
amount of the blood which they already contained; whence, in the long
run, the same amount of blood passes out of the ultimate capillaries
into the blood sinuses. From the disposition of the blood sinuses,
the impulse thus given to the blood which they contain is finally
conveyed to the blood in the branchiæ, and a proportional quantity
of that {74} blood leaves the branchiæ and passes into the sinuses
which connect them with the pericardial sinus (fig. 15, _bcv_), and
thence into that cavity. At the end of the contraction, or _systole_,
of the heart, its volume is of course diminished by the volume of the
blood forced out, and the space between the walls of the heart and
those of the pericardial sinus is increased to the same extent. This
space, however, is at once occupied by the blood from the branchiæ,
and perhaps by some blood which has not passed through the branchiæ,
though this is doubtful. When the systole is over, the _diastole_
follows; that is to say, the elasticity of the walls of the heart
and that of the various parts which connect it with the walls of the
pericardium, bring it back to its former size, and the blood in the
pericardial sinus flows into its cavity by the six apertures. With
a new systole the same process is repeated, and thus the blood is
driven in a circular course through all parts of the body.
* * * * *
It will be observed that the branchiæ are placed in the course of the
current of blood which is returning to the heart; which is the exact
contrary of what happens in fishes, in which the blood is sent from
the heart to the branchiæ, on its way to the body. It follows, from
this arrangement, that the blood which goes to the branchiæ is blood
in which the quantity of oxygen has undergone a diminution, and that
of carbonic acid an increase, as compared with the blood in the heart
itself. For the {75} activity of all the organs, and especially of
the muscles, is inseparably connected with the absorption of oxygen
and the evolution of carbonic acid; and the only source from which
the one can be derived, and the only receptacle into which the other
can be poured, is the blood which bathes and permeates the whole
fabric to which it is distributed by the arteries.
The blood, therefore, which reaches the branchiæ has lost oxygen and
gained carbonic acid; and these organs constitute the apparatus for
the elimination of the injurious gas from the economy on the one
hand, and, on the other, for the taking in of a new supply of the
needful “vital air,” as the old chemists called it. It is thus that
the branchiæ subserve the respiratory function.
The crayfish has eighteen perfect and two rudimentary branchiæ in
each branchial chamber, the boundaries of which have been already
described.
Of the eighteen perfect branchiæ, six (_podobranchiæ_) are attached
to the basal joints of the thoracic limbs, from the last but one to
the second (second maxillipede) inclusively (fig. 4, p. 26, _pdb_,
and fig. 17, A, B); and eleven (_arthrobranchiæ_) are fixed to the
flexible interarticular membranes, which connect these basal joints
with the parts of the thorax to which they are articulated (fig.
4, _arb_, _arb′_, fig. 17, C). Of these eleven branchiæ, two are
attached to the interarticular membranes of all the ambulatory legs
but the last, (=6) and to those of the pincers and of the external
maxillipedes, (=4) and one to that of the {77} second maxillipede.
The first maxillipede and the last ambulatory limb have none.
Moreover, where there are two arthrobranchiæ, one is more or less in
front of and external to the other.
[Illustration: FIG. 17.—_Astacus fluviatilis._—A, one of the
podobranchiæ from the outer side; B, the same from the inner side;
C, one of the arthrobranchiæ; D, a part of one of the coxopoditic
setæ; E, extremity of the same seta; F, extremity of a seta from the
base of the podobranchia; G, hooked seta of the lamina; (A–C, × 3;
D–G, highly magnified). _b_, base of podobranchia; _cs_, coxopoditic
setæ; _cxp_, coxopodite; _l_, lamina, _pl_, plume, and _st_, stem of
podobranchia; _t_, tubercle on the coxopodite, to which the setæ are
attached.]
These eleven arthrobranchiæ are all very similar in structure (fig.
17, C). Each consists of a stem which contains two canals, one
external and one internal, separated by a longitudinal partition. The
stem is beset with a great number of delicate _branchial filaments_,
so that it looks like a plume tapering from its base to its summit.
Each filament is traversed by large vascular channels, which break
up into a net-work immediately beneath the surface. The blood driven
into the external canals of the stem (fig. 15, _av_) is eventually
poured into the inner canal (_ev_), which again communicates with
the channels (_bcv_) which lead to the pericardial sinus (_p_).
In its course, the blood traverses the branchial filaments, the
outer investment of each of which is an excessively thin chitinous
membrane, so that the blood contained in them is practically
separated by a mere film from the aërated water in which the gills
float. Hence, an exchange of gaseous constituents readily takes
place, and as much oxygen is taken in as carbonic acid is given out.
The six podobranchiæ, or gills which are attached to the basal
joints of the legs, play the same part, but differ a good deal in
the details of their structure from those which are fixed to the
interarticular membranes. Each consists of a broad _base_ (fig. 17, A
and B; _b_) beset with many {78} fine straight hairs, or _setæ_ (F),
whence a narrow _stem_ (_st_) proceeds. At its upper end this stem
divides into two parts, that in front, the _plume_ (_pl_), resembling
the free end of one of the gills just described, while that behind,
the _lamina_ (_l_), is a broad thin plate, bent upon itself
longitudinally in such a manner that its folded edge lies forwards,
and covered with minute hooked setæ (G). The gill which follows is
received into the space included between the two lobes or halves
of the folded lamina (fig. 4, p. 26). Each lobe is longitudinally
plaited into about a dozen folds. The whole front and outer face of
the stem is beset with branchial filaments; hence, we may compare one
of these branchiæ to one of the preceding kind, in which the stem
has become modified and has given off a large folded lamina from its
inner and posterior face.
The branchiæ now described are arranged in sets of three for each
of the thoracic limbs, from the third maxillipede to the last but
one ambulatory limb, and two for the second maxillipede, thus making
seventeen in all (3 × 5 + 2 = 17); and, between every two there is
found a bundle of long twisted hairs (fig. 17, A, _cx.s_; D and E),
which are attached to a small elevation (_t_) on the basal joint of
each limb. These _coxopoditic setæ_, no doubt, serve to prevent the
intrusion of parasites and other foreign matters into the branchial
chamber. From the mode of attachment of the six branchiæ it is
obvious that they must share in the movements of the basal joints of
the {79} legs; and that, when the crayfish walks, they must be more
or less agitated in the branchial chamber.
The eighteenth branchia resembles one of the eleven arthrobranchiæ in
structure; but it is larger, and it is attached neither to the basal
joint of the hindermost ambulatory limb, nor to its interarticular
membrane, but to the sides of the thorax, above the joint. From
this mode of attachment it is distinguished from the others as
_pleurobranchia_ (fig. 4, _plb. 14_).
Finally, in front of this, fixed also to the walls of the thorax,
above each of the two preceding pairs of ambulatory limbs, there is a
delicate filament, about a sixteenth of an inch long, which has the
structure of a branchial filament, and is, in fact, a rudimentary
pleurobranchia (fig. 4, _plb. 12_, _plb. 13_).
The quantity of water which occupies the space left in the branchial
chamber by the gills is but small, and as the respiratory surface
offered by the gills is relatively very large, the air contained
in this water must be rapidly exhausted, even when the crayfish
is quiescent; while, when any muscular exertion takes place, the
quantity of carbonic acid formed, and the demand for fresh oxygen,
is at once greatly increased. For the efficient performance of the
function of respiration, therefore, the water in the branchial
chamber must be rapidly renewed, and there must be some arrangement
by which the supply of fresh water may be proportioned to the demand.
In many animals, the respiratory surface is {80} covered with
rapidly vibrating filaments, or _cilia_, by means of which a current
of water is kept continually flowing over the gills, but there are
none of these in the crayfish. The same object is attained, however,
in another way. The anterior boundary of the branchial chamber
corresponds with the cervical groove, which, as has been seen, curves
downwards and then forwards, until it terminates at the sides of the
space occupied by the jaws. If the branchiostegite is cut away along
the groove, it will be found that it is attached to the sides of the
head, which project a little beyond the anterior part of the thorax,
so that there is a depression behind the sides of the head—just as
there is a depression, behind a man’s jaw, at the sides of the neck.
Between this depression in front, the walls of the thorax internally,
the branchiostegite externally, and the bases of the forceps and
external foot-jaws below, a curved canal is included, by which the
branchial cavity opens forwards as by a funnel. Attached to the base
of the second maxilla there is a wide curved plate (fig, 4, _6_)
which fits against the projection of the head, as a shirt collar
might do, to carry out our previous comparison; and this scoop-shaped
plate (termed the _scaphognathite_), which is concave forwards and
convex backwards, can be readily moved backwards and forwards.
If a living crayfish is taken out of the water, it will be found
that, as the water drains away from the branchial cavity, bubbles
of air are forced out of its anterior opening. {81} Again, if,
when a crayfish is resting quietly in the water, a little coloured
fluid is allowed to run down towards the posterior opening of the
branchial chamber, it will very soon be driven out from the anterior
aperture, with considerable force, in a long stream. In fact, as
the scaphognathite vibrates not less than three or four times in
a second, the water in the funnel-shaped front passage of the
branchial cavity is incessantly baled out; and, as fresh water flows
in from behind to make up the loss, a current is kept constantly
flowing over the gills. The rapidity of this current naturally
depends on the more or less quick repetition of the strokes of the
scaphognathite; and hence, the activity of the respiratory function
can be accurately adjusted to the wants of the economy. Slow working
of the scaphognathite answers to ordinary breathing in ourselves,
quick working to panting.
A farther self-adjustment of the respiratory apparatus is gained by
the attachment of the six gills to the basal joints of the legs.
For, when the animal exerts its muscles in walking, these gills are
agitated, and thus not only bring their own surfaces more largely in
contact with the water, but produce the same effect upon the other
gills.
* * * * *
The constant oxidation which goes on in all parts of the body not
only gives rise to carbonic acid; but, so far as it affects the
proteinaceous constituents, it produces {82} compounds which contain
nitrogen, and these, like other waste products, must be eliminated.
In the higher animals, such waste products take the form of urea,
uric acid, hippuric acid, and the like; and are got rid of by the
kidneys. We may, therefore, expect to find some organ which plays the
part of a kidney in the crayfish; but the position of the structure,
which there is much reason for regarding as the representative of the
kidney, is so singular that very different interpretations have been
put upon it.
On the basal joint of each antenna it is easy to see a small conical
eminence with an opening on the inner side of its summit (fig. 18).
The aperture (_x_) leads by a short canal into a spacious sac, with
extremely delicate walls (_s_), which is lodged in the front part of
the head, in front of and below the cardiac division of the stomach
(_cs_). Beneath this, in a sort of recess, which corresponds with
the epistoma, and with the base of the antenna, there is a discoidal
body of a dull green colour, in shape somewhat like one of the fruits
of the mallow, which is known as the _green gland_ (_gg_) The sac
narrows below like a wide funnel, and the edges of its small end
are continuous with the walls of the green gland; they surround an
aperture which leads into the interior of the latter structure,
and conveys its products into the sac, whence they are excreted by
the opening in the antennary papilla. The green gland is said to
contain a substance termed _guanin_ (so named because it is found in
the _guano_ which is the accumulated {83} excrement of birds), a
nitrogenous body analogous in some respects to uric acid, but less
highly oxidated; and if this be the case, there can be little doubt
that the green gland represents the kidney, and its secretion {84}
the urinary fluid, while the sac is a sort of urinary bladder.
[Illustration: FIG. 18.—_Astacus fluviatilis._—A, the anterior part
of the body, with the dorsal portion of the carapace removed to show
the position of the green glands; B, the same, with the left side
of the carapace removed; C, the green gland removed from the body
(all × 2). _ag_, left anterior gastric muscle; _c_, circumœsophageal
commissures; _cs_, cardiac portion of stomach; _gg_, green gland,
exposed in A on the left side by the removal of its sac; _ima_,
intermaxillary or cephalic apodeme; _œs_, œsophagus seen in
transverse section in A, the stomach being removed; _s_, sac of green
gland; _x_, bristle passed from the aperture in the basal joint of
the antenna into the sac.]
* * * * *
Restricting our attention to the phenomena which have now been
described, and to a short period in the life of the crayfish, the
body of the animal may be regarded as a factory, provided with
various pieces of machinery, by means of which certain nitrogenous
and other matters are extracted from the animal and vegetable
substances which serve for food, are oxidated, and are then delivered
out of the factory in the shape of carbonic acid gas, guanin,
and probably some other products, with which we are at present
unacquainted. And there is no doubt, that if the total amount of
products given out could be accurately weighed against the total
amount of materials taken in, the weight of the two would be found
to be identical. To put the matter in its most general shape, the
body of the crayfish is a sort of focus to which certain material
particles converge, in which they move for a time, and from which
they are afterwards expelled in new combinations. The parallel
between a whirlpool in a stream and a living being, which has often
been drawn, is as just as it is striking. The whirlpool is permanent,
but the particles of water which constitute it are incessantly
changing. Those which enter it, on the one side, are whirled around
and temporarily constitute a part of its individuality; and as they
leave it on the other side, their places are made good by new comers.
{85}
Those who have seen the wonderful whirlpool, three miles below the
Falls of Niagara, will not have forgotten the heaped-up wave which
tumbles and tosses, a very embodiment of restless energy, where the
swift stream hurrying from the Falls is compelled to make a sudden
turn towards Lake Ontario. However changeful in the contour of its
crest, this wave has been visible, approximately in the same place,
and with the same general form, for centuries past. Seen from a mile
off, it would appear to be a stationary hillock of water. Viewed
closely, it is a typical expression of the conflicting impulses
generated by a swift rush of material particles.
Now, with all our appliances, we cannot get within a good many miles,
so to speak, of the crayfish. If we could, we should see that it
was nothing but the constant form of a similar turmoil of material
molecules which are constantly flowing into the animal on the one
side, and streaming out on the other.
The chemical changes which take place in the body of the crayfish,
are doubtless, like other chemical changes, accompanied by the
evolution of heat. But the amount of heat thus generated is so small
and, in consequence of the conditions under which the crayfish lives,
it is so easily carried away, that it is practically insensible. The
crayfish has approximately the temperature of the surrounding medium,
and it is, therefore, reckoned among the cold-blooded animals.
If our investigation of the results of the process of {86}
alimentation in a well-fed Crayfish were extended over a longer
time, say a year or two, we should find that the products given out
were no longer equal to the materials taken in, and the balance would
be found in the increase of the animal’s weight. If we inquired how
the balance was distributed, we should find it partly in store,
chiefly in the shape of fat; while, in part, it had been spent in
increasing the plant and in enlarging the factory. That is to say, it
would have supplied the material for the animal’s growth. And this
is one of the most remarkable respects in which the living factory
differs from those which we construct. It not only enlarges itself,
but, as we have seen, it is capable of executing its own repairs to a
very considerable extent.
{87}