THE PHYSIOLOGY OF THE CRAYFISH—THE MECHANISM BY WHICH THE LIVING
ORGANISM ADJUSTS ITSELF TO SURROUNDING CONDITIONS AND REPRODUCES
ITSELF.
If the hand is brought near a vigorous crayfish, free to move in a
large vessel of water, it will generally give a vigorous flap with
its tail, and dart backwards out of reach; but if a piece of meat is
gently lowered into the vessel, the crayfish will sooner or later
approach and devour it.
If we ask why the crayfish behaves in this fashion, every one has an
answer ready. In the first case, it is said that the animal is aware
of danger, and therefore hastens away; in the second, that it knows
that meat is good to eat, and therefore walks towards it and makes a
meal. And nothing can seem to be simpler or more satisfactory than
these replies, until we attempt to conceive clearly what they mean;
and, then, the explanation, however simple it may be admitted to be,
hardly retains its satisfactory character.
For example, when we say that the crayfish is “aware of danger,” or
“knows that meat is good to eat,” what {88} do we mean by “being
aware” and “knowing”? Certainly it cannot be meant that the crayfish
says to himself, as we do, “This is dangerous,” “That is nice;” for
the crayfish, being devoid of language, has nothing to say either to
himself or any one else. And if the crayfish has not language enough
to construct a proposition, it is obviously out of the question that
his actions should be guided by a logical reasoning process, such
as that by which a man would justify similar actions. The crayfish
assuredly does not first frame the syllogism, “Dangerous things
are to be avoided; that hand is dangerous; therefore it is to be
avoided;” and then act upon the conclusion thus logically drawn.
But it may be said that children, before they acquire the use of
language, and we ourselves, long after we are familiar with conscious
reasoning, perform a great variety of perfectly rational acts
unconsciously. A child grasps at a sweetmeat, or cowers before a
threatening gesture, before it can speak; and any one of us would
start back from a chasm opening at our feet, or stoop to pick up a
jewel from the ground, “without thinking about it.” And, no doubt, if
the crayfish has any mind at all, his mental operations must more or
less resemble those which the human mind performs without giving them
a spoken or unspoken verbal embodiment.
If we analyse these, we shall find that, in many cases, distinctly
felt sensations are followed by a distinct desire to perform some
act, which act is accordingly performed; {89} while, in other cases,
the act follows the sensation without one being aware of any other
mental process; and, in yet others, there is no consciousness even of
the sensation. As I wrote these last words, for example, I had not
the slightest consciousness of any sensation of holding or guiding
the pen, although my fingers were causing that instrument to perform
exceedingly complicated movements. Moreover, experiments upon animals
have proved that consciousness is wholly unnecessary to the carrying
out of many of those combined movements by which the body is adjusted
to varying external conditions.
Under these circumstances, it is really quite an open question
whether a crayfish has a mind or not; moreover, the problem is
an absolutely insoluble one, inasmuch as nothing short of being
a crayfish would give us positive assurance that such an animal
possesses consciousness; and, finally, supposing the crayfish has a
mind, that fact does not explain its acts, but only shows that, in
the course of their accomplishment, they are accompanied by phenomena
similar to those of which we are aware in ourselves, under like
circumstances.
So we may as well leave this question of the crayfish’s mind on one
side for the present, and turn to a more profitable investigation,
namely, that of the order and connexion of the physical phenomena
which intervene between something which happens in the neighbourhood
of the animal and that other something which responds to it, as an
act of the crayfish. {90}
Whatever else it may be, this animal, so far as it is acted upon
by bodies around it and reacts on them, is a piece of mechanism,
the internal works of which give rise to certain movements when it
is affected by particular external conditions; and they do this in
virtue of their physical properties and connexions.
Every movement of the body, or of any organ of the body, is an effect
of one and the same cause, namely, muscular contraction. Whether the
crayfish swims or walks, or moves its antennæ, or seizes its prey,
the immediate cause of the movements of the parts which bring about,
or constitute, these bodily motions is to be sought in a change which
takes place in the flesh, or muscle, which is attached to them.
The change of place which constitutes any movement is an effect of
a previous change in the disposition of the molecules of one or
more muscles; while the direction of that movement depends on the
connexions of the parts of the skeleton with one another, and of the
muscles with them.
The muscle of the crayfish is a dense, white substance; and if a
small portion of it is subjected to examination it will be found to
be very easily broken up into more or less parallel bundles of fine
fibres. Each of these fibres is generally found to be ensheathed in a
fine transparent membrane, which is called the _sarcolemma_, within
which is contained the proper substance of the muscle. When quite
fresh and living, this substance is soft and {91} semi-fluid, but it
hardens and becomes solid immediately after death.
[Illustration: FIG. 19.—_Astacus fluviatilis._—A, a single muscular
fibre; transverse diameter 1‐110th of an inch; B, a portion of
the same more highly magnified; C, a smaller portion still more
highly magnified; D and E, the splitting up of a part of fibre into
fibrillæ; F, the connexion of a nervous with a muscular fibre which
has been treated with acetic acid. _a_, darker, and _b_, clearer
portions of the fibrillæ; _n_, nucleus of sarcolemma; _nv_, nerve
fibre; _s_, sarcolemma; _t_, tendon; 1–5, successive dark bands
answering to the darker portions, _a_, of each fibrilla.]
Examined, with high magnifying powers, in this {92} condition,
the muscle-substance appears marked by very regular transverse
bands, which are alternately opaque and transparent; and it is
characteristic of the group of animals to which the crayfish belongs
that their muscle-substance has this striped character in all parts
of the body.
A greater or less number of these fibres, united into one or more
bundles, constitutes a muscle; and, except when these muscles
surround a cavity, they are fixed at each end to the hard parts
of the skeleton. The attachment is frequently effected by the
intermediation of a dense, fibrous, often chitinous, substance, which
constitutes the _tendon_ (fig. 19, A; _t_) of the muscle.
The property of the living muscle, which enables it to be the cause
of motion, is this: Every muscular fibre is capable of suddenly
changing its dimensions, in such a manner that it shortens and
becomes proportionately thicker. Hence the absolute bulk of the fibre
remains practically unchanged. From this circumstance, muscular
_contraction_, as the change of form of a muscle is called, is
radically different from the process which commonly goes by the same
name in other things, and which involves a diminution of bulk.
The contraction of muscle takes place with great force, and, of
course, if the parts to which its ends are fixed are both free to
move, they are brought nearer at the moment of contraction: if one
only is free to move that is approximated to the fixed part; and if
the muscular {93} fibre surrounds a cavity, the cavity is lessened
when the muscle contracts. This is the whole source of motor power in
the crayfish machine. The results produced by the exertion of that
power depend upon the manner in which the parts to which the muscles
are attached are connected with one another.
[Illustration: FIG. 20.—_Astacus fluviatilis._—The chela of the
forceps, with one side cut away to show, in A, the muscles, in
B, the tendons (× 2). _cp_, carpopodite; _prp_, propodite; _dp_,
dactylopodite; _m_, adductor muscle; _m′_, abductor muscle; _t_,
tendon of adductor muscle; _t′_, tendon of abductor muscle; _x_,
hinge.]
One example of this has already been given in the curious mechanism
of the gastric mill. Another may be found in the chela which
terminates the forceps. If the {94} articulation of the last joint
(fig. 20, _dp_) with the one which precedes it (_prp_) is examined,
it will be found that the base of the terminal segment (_dp_) turns
on two hinges (_x_), formed by the hard exoskeleton and situated
at opposite points of the diameter of the base, on the penultimate
segment; and these hinges are so disposed that the last joint can
be moved only in one plane, to or from the produced angle of the
penultimate segment (_prp_), which forms the fixed claw of the chela.
Between the hinges, on both the inner and the outer sides of the
articulation, the exoskeleton is soft and flexible, and allows the
terminal segment to play easily through a certain arc. It is by this
arrangement that the direction and the extent of the motion of the
free claw of the chela are determined. The source of the motion lies
in the muscles which occupy the interior of the enlarged penultimate
segment of the limb. Two muscles, one of very great size (_m_), the
other smaller (_m′_), are fastened by one end to the exoskeleton of
this segment. The fibres of the larger muscle converge to be fixed
into the two sides of a long flat process of the chitinous cuticula,
on the inner side of the base of the terminal segment, which serves
as a tendon (_t_); while those of the smaller muscle are similarly
attached to a like process which proceeds from the outer side of the
base of the terminal segment (_t′_). It is obvious that, when the
latter muscle shortens it must move the apex of the terminal segment
(_dp_) away from the end of the fixed claw; while, {95} when the
former contracts, the end of the terminal segment is brought towards
that of the fixed claw.
A living crayfish is able to perform very varied movements with its
pincers. When it swims backwards, these limbs are stretched straight
out, parallel with one another, in front of the head; when it walks,
they are usually carried like arms bent at the elbow, the “forearm”
partly resting on the ground; on being irritated, the crayfish sweeps
the pincers round in any direction to grasp the offending body; when
prey is seized, it is at once conveyed, with a circular motion,
towards the region of the mouth. Nevertheless, these very varied
actions are all brought about by a combination of simple flexions and
extensions, each of which is effected in the exact order, and to the
exact extent, needful to bring the chela into the position required.
The skeleton of the stem of the limb which bears the chela is, in
fact, divided into four moveable segments; and each of these is
articulated with the segments on each side of it by a hinge of just
the same character as that which connects the moveable claw of the
chela with the penultimate segment, while the basal segment is
similarly articulated with the thorax.
If the axes of all these articulations[7] were parallel, it is
obvious that, though the limb might be moved as a whole through a
considerable arc, and might be bent in various {96} degrees, yet all
its movements would be limited to one plane. But, in fact, the axes
of the successive articulations are nearly at right angles to one
another; so that, if the segments are successively either extended
or flexed, the chela describes a very complicated curve; and by
varying the extent of flexion or extension of each segment, this
curve is susceptible of endless variation. It would probably puzzle
a good mathematician to say exactly what position should be given to
each segment, in order to bring the chela from any given position
into any other; but if a lively crayfish is incautiously seized,
the experimenter will find, to his cost, that the animal solves the
problem both rapidly and accurately.
[7] By axis of the articulation is meant a line drawn through
the pair of hinges which constitute it.
The mechanism by which the retrograde swimming of the crayfish is
effected, is no less easily analysed. The apparatus of motion is, as
we have seen, the abdomen, with its terminal five-pointed flapper.
The rings of the abdomen are articulated together by joints (fig. 21,
×) situated a little below the middle of the height of the rings, at
opposite ends of transverse lines, at right angles to the long axis
of the abdomen.
Each ring consists of a dorsal, arched portion, called the _tergum_
(fig. 21; fig. 36, p. 142, _t. XIX_), and a nearly flat ventral
portion, which is the _sternum_ (fig. 36, _st. XIX_). Where these
two join, a broad plate is sent down on each side, which overlaps
the bases of the abdominal appendages, and is known as the _pleuron_
(fig. 36, _pl. XIX_). {97} The sterna are all very narrow, and are
connected together by wide spaces of flexible exoskeleton.
[Illustration: FIG. 21.—_Astacus fluviatilis._—Two of the abdominal
somites, in vertical section, seen from the inner side, to show ×, ×,
the hinges by which they are articulated with one another (× 3). The
anterior of the two somites is that to the right of the figure.]
When the abdomen is made straight, it will be found that these
_intersternal_ membranes are stretched as far as they will yield. On
the other hand, when the abdomen is bent up as far as it will go, the
sterna come close together, and the intersternal membranes are folded.
The terga are very broad; so broad, in fact, that each overlaps its
successor, when the abdomen is straightened or extended, for nearly
half its length in the middle line; and the overlapped surface is
smooth, convex, and {98} marked off by a transverse groove from the
rest of the tergum as an _articular facet_. The front edge of the
articular facet is continued into a sheet of flexible cuticula, which
turns back, and passes as a loose fold to the hinder edge of the
overlapping tergum (fig. 21). This tergal _interarticular membrane_
allows the terga to move as far as they can go in flexion; whilst,
in extreme extension, they are but slightly stretched. But, even
if the intersternal membranes presented no obstacle to excessive
extension of the abdomen, the posterior free edge of each tergum fits
into the groove behind the facet in the next in such a manner, that
the abdomen cannot be made more than very slightly concave upwards
without breaking.
Thus the limits of motion of the abdomen, in the vertical direction,
are from the position in which it is straight, or has even a very
slight upward concavity, to that in which it is completely bent
upon itself, the telson being brought under the bases of the
hinder thoracic limbs. No lateral movement between the somites of
the abdomen is possible in any of its positions. For, when it is
straight, lateral movement is hindered not only by the extensive
overlapping of the terga, but also by the manner in which the hinder
edges of the pleura of each of the four middle somites overlap the
front edges of their successors. The pleura of the second somite are
much larger than any of the others, and their front edges overlap the
small pleura of the first abdominal somite; and when the abdomen is
much flexed, these pleura even {99} ride over the posterior edges of
the branchiostegites. In the position of extension, the overlap of
the terga is great, while that of the pleura of the middle somites is
small. As the abdomen passes from extension to flexion, the overlap
of the terga of course diminishes; but any decrease of resistance
to lateral strains which may thus arise, is compensated by the
increasing overlap of the pleura, which reaches its maximum when the
abdomen is completely flexed.
It is obvious that longitudinal muscular fibres fixed into the
exoskeleton, above the axes of the joints, must bring the centres
of the terga of the somites closer together, when they contract;
while muscular fibres attached below the axes of the joints must
approximate the sterna. Hence, the former will give rise to
extension, and the latter to flexion, of the abdomen as a whole.
Now there are two pairs of very considerable muscles disposed in this
manner. The dorsal pair, or the _extensors_ of the abdomen (fig. 22,
_e.m_), are attached in front to the side walls of the thorax, thence
pass backwards into the abdomen, and divide into bundles, which are
fixed to the inner surfaces of the terga of all the somites. The
other pair, or the _flexors_ of the abdomen (_f.m_) constitute a
very much larger mass of muscle, the fibres of which are curiously
twisted, like the strands of a rope. The front end of this double
rope is fixed to a series of processes of the exoskeleton of the
thorax, called _apodemata_, some of which roof over the sternal
blood-sinuses {100} and the thoracic part of the nervous system;
while, in the abdomen, its strands are attached to the sternal
exoskeleton of all the somites and extend, on each side of the
rectum, to the telson.
[Illustration: FIG. 22.—_Astacus fluviatilis._—A longitudinal section
of the body to show the principal muscles and their relations to
the exoskeleton (nat. size). _a_, the vent; _add.m_, adductor
muscle of mandible; _e.m_, extensor, and, _f.m_, flexor muscle of
abdomen; _œs_, œsophagus; _pcp_, procephalic process; _t,t′_, the two
segments of the telson; _XV–XX_, the abdominal somites; _1–20_, the
appendages; ×, ×, hinges between the successive abdominal somites.]
When the exoskeleton is cleaned by maceration, the abdomen has a
slight curve, dependent upon the form and the degree of elasticity
possessed by its different parts; and, in a living crayfish at rest,
it will be observed that the curvature of the abdomen is still more
marked. Hence it is ready either for extension or for flexion.
A sudden contraction of the flexor muscles instantly increases the
ventral curvature of the abdomen, and {101} throws the tail fin, the
two side lobes of which are spread out, forwards; while the body is
propelled backwards by the reaction of the water against the stroke.
Then the flexor muscles being relaxed, the extensor muscles come into
play; the abdomen is straightened, but less violently and with a far
weaker stroke on the water, in consequence of the less strength of
the extensors and of the folding up of the lateral plates of the fin,
until it comes into the position requisite to give full force to a
new downward and forward stroke. The tendency of the extension of
the abdomen is to drive the body forward; but from the comparative
weakness and the obliquity of its stroke, its practical effect is
little more than to check the backward motion conferred upon the body
by the flexion of the abdomen.
* * * * *
Thus, every action of the crayfish, which involves motion, means the
contraction of one or more muscles. But what sets muscle contracting?
A muscle freshly removed from the body may be made to contract in
various ways, as by mechanical or chemical irritation, or by an
electrical shock; but, under natural conditions, there is only one
cause of muscular contraction, and that is the activity of a nerve.
Every muscle is supplied with one or more nerves. These are delicate
threads which, on microscopic examination, prove to be bundles of
fine tubular filaments, filled with an apparently structureless
substance of gelatinous consistency, the _nerve fibres_ {102} (fig.
23). The nerve bundle which passes to a muscle breaks up into smaller
bundles and, finally, into separate fibres, each of which ultimately
terminates by becoming continuous with the substance of a muscular
fibre (fig. 19, F). Now the peculiarity of a muscle nerve, or _motor_
nerve, as it is called, is that irritation of the nerve fibre at any
part of its length, however distant from the muscle, brings about
muscular contraction, just as if the muscle itself were irritated.
A change is produced in the molecular condition of the nerve at the
point of irritation; and this change is propagated along the nerve,
until it reaches the muscle, in which it gives rise to that change in
the arrangement of its molecules, the most obvious effect of which is
the sudden alteration of form which we call muscular contraction.
[Illustration: FIG. 23.—_Astacus fluviatilis._—Three nerve fibres,
with the connective tissue in which they are imbedded. (Magnified
about 250 diameters.) _n_, nuclei.]
[Illustration: FIG. 24.—_Astacus fluviatilis._—A, one of the (double)
abdominal ganglia, with the nerves connected with it (× 25); B,
a nerve cell or ganglionic corpuscle (× 250). _a_, sheath of the
nerves; _c_, sheath of the ganglion; _co, co′_, commissural cords
connecting the ganglia with those in front, and those behind them.
_gl.c._ points to the ganglionic corpuscles of the ganglia; _n_,
nerve fibres.]
If we follow the course of the motor nerves in a {103} direction
away from the muscles to which they are distributed, they will
be found, sooner or later, to terminate in _ganglia_ (fig. 24 A,
_gl.c_; fig. 25, _gn. 1–13_). A ganglion is a body which is in great
measure composed of nerve fibres; but, interspersed among these,
or disposed around them, there are peculiar structures, which are
termed _ganglionic corpuscles_, or _nerve cells_ (fig. 24, B).
These are nucleated cells, not unlike the epithelial cells which
have been already mentioned, but which are larger {105} and often
give off one or more processes. These processes, under favourable
circumstances, can be traced into continuity with nerve fibres.
[Illustration: FIG. 25.—_Astacus fluviatilis._—The central nervous
system seen from above (nat. size). _a_, vent; _an_, antennary nerve;
_a′n_, antennulary nerve; _c_, circumœsophageal commissures; _gn._
1, supraœsophageal ganglion; _gn._ 2, infraœsophageal ganglion;
_gn._ 6, fifth thoracic ganglion; _gn._ 7, last thoracic ganglion;
_gn._ 13, last abdominal ganglion; _œs_, œsophagus in cross section;
_on_, optic nerve; _sa_, sternal artery in cross section; _sgn_,
stomatogastric nerve.]
The chief ganglia of the crayfish are disposed in a longitudinal
series in the middle line of the ventral aspect of the body close to
the integument (fig. 25). In the abdomen, for example, six ganglionic
masses are readily observed, one lying over the sternum of each
somite, connected by longitudinal bands of nerve fibres, and giving
off branches to the muscles. On careful examination, the longitudinal
connecting bands, or _commissures_ (fig. 24, _co_), are seen to be
double, and each mass appears slightly bilobed. In the thorax, there
are six, larger, double ganglionic masses, likewise connected by
double commissures; and the most anterior of these, which is the
largest (fig. 25, _gn._ 2), is marked at the sides by notches, as
if it were made up of several pairs of ganglia, run together into
one continuous whole. In front of this, two commissures (_c_) pass
forwards, separating widely, to give room for the gullet (_œs_),
which passes between them; while in front of the gullet, just behind
the eyes, they unite with a transversely elongated mass of ganglionic
substance (_gn._ 1), termed the _brain_, or _cerebral ganglion_.
All the motor nerves, as has been said, are traceable, directly or
indirectly, to one or other of these thirteen sets of ganglia; but
other nerves are given off from the ganglia, which cannot be followed
into any muscle. In {106} fact, these nerves go either to the
integument or to the organs of sense, and they are termed _sensory
nerves_.
When a muscle is connected by its motor nerve with a ganglion,
irritation of that ganglion will bring about the contraction of the
muscle, as well as if the motor nerve itself were irritated. Not
only so; but if a sensory nerve, which is in connexion with the
ganglion, is irritated, the same effect is produced; moreover, the
sensory nerve itself need not be excited, but the same result will
take place, if the organ to which it is distributed is stimulated.
Thus the nervous system is fundamentally an apparatus by which two
separate, and it may be distant, parts of the body, are brought into
relation with one another; and this relation is of such a nature,
that a change of state arising in the one part is followed by the
propagation of changes along the sensory nerve to the ganglion, and
from the ganglion to the other part; where, if that part happens
to be muscle, it produces contraction. If one end of a rod of
wood, twenty feet long, is applied to a sounding-board, the sound
of a tuning-fork held against the opposite extremity will be very
plainly heard. Nothing can be seen to happen in the wood, and yet
its molecules are certainly set vibrating, at the same rate as the
tuning-fork vibrates; and when, after travelling rapidly along the
wood, these vibrations affect the sounding-board, they give rise
to vibrations of the molecules of the air, which reaching the ear,
are converted into an audible note. So in the nerve tract: {107}
no apparent change is effected in it by the irritation at one end;
but the rate at which the molecular change produced travels can
be measured; and, when it reaches the muscle, its effect becomes
visible in the change of form of the muscle. The molecular change
would take place just as much if there were no muscle connected with
the nerve, but it would be no more apparent to ordinary observation
than the sound of the tuning-fork is audible in the absence of the
sounding-board.
If the nervous system were a mere bundle of nerve fibres extending
between sensory organs and muscles, every muscular contraction would
require the stimulation of that special point of the surface on which
the appropriate sensory nerve ended. The contraction of several
muscles at the same time, that is, the combination of movements
towards one end, would be possible only if the appropriate nerves
were severally stimulated in the proper order, and every movement
would be the direct result of external changes. The organism would
be like a piano, which may be made to give out the most complicated
harmonies, but is dependent for their production on the depression
of a separate key for every note that is sounded. But it is obvious
that the crayfish needs no such separate impulses for the performance
of highly complicated actions. The simple impression made on the
organs of sensation in the two examples with which we started, gives
rise to a train of complicated and accurately co-ordinated muscular
contractions. To carry the analogy {108} of the musical instrument
further, striking a single key gives rise, not to a single note,
but to a more or less elaborate tune; as if the hammer struck not a
single string, but pressed down the stop of a musical box.
It is in the ganglia that we must look for the analogue of the
musical box. A single impulse conveyed by a sensory nerve to a
ganglion, may give rise to a single muscular contraction, but more
commonly it originates a series of such, combined to a definite end.
The effect which results from the propagation of an impulse along
a nerve fibre to a ganglionic centre, whence it is, as it were,
reflected along another nerve fibre to a muscle, is what is termed a
_reflex action_. As it is by no means necessary that sensation should
be a concomitant of the first impulse, it is better to term the nerve
fibre which carries it _afferent_ rather than sensory; and, as other
phenomena besides those of molar motion may be the ultimate result
of the reflex action, it is better to term the nerve fibre which
transmits the reflected impulse _efferent_ rather than motor.
If the nervous commissures between the last thoracic and the first
abdominal ganglia are cut, or if the thoracic ganglia are destroyed,
the crayfish is no longer able to control the movements of the
abdomen. If the forepart of the body is irritated, for example, the
animal makes no effort to escape by swimming backwards. Nevertheless,
the abdomen is not paralysed, for, if it be irritated, it will flap
vigorously. This is a case of pure {109} reflex action. The stimulus
is conveyed to the abdominal ganglia through afferent nerves, and is
reflected from them, by efferent nerves, to the abdominal muscles.
But this is not all. Under these circumstances it will be seen that
the abdominal limbs all swing backwards and forwards, simultaneously,
with an even stroke; while the vent opens and shuts with a regular
rhythm. Of course, these movements imply correspondingly regular
alternate contractions and relaxations of certain sets of muscles;
and these, again, imply regularly recurring efferent impulses from
the abdominal ganglia. The fact that these impulses proceed from the
abdominal ganglia, may be shown in two ways: first, by destroying
these ganglia in one somite after another, when the movements in
each somite at once permanently cease; and, secondly, by irritating
the surface of the abdomen, when the movements are temporarily
inhibited by the stimulation of the afferent nerves. Whether these
movements are properly reflex, that is, arise from incessant new
afferent impulses of unknown origin, or whether they depend on the
periodical accumulation and discharge of nervous energy in the
ganglia themselves, or upon periodical exhaustion and restoration of
the irritability of the muscles, is unknown. It is sufficient for
the present purpose to use the facts as evidence of the peculiar
co-ordinative function of ganglia.
The crayfish, as we have seen, avoids light; and the slightest
touch of one of its antennæ gives rise to active motions of the
whole body. In fact, the animal’s {110} position and movements are
largely determined by the influences received through the feelers
and the eyes. These receive their nerves from the cerebral ganglia;
and, as might be expected, when these ganglia are extirpated, the
crayfish exhibits no tendency to get away from the light, and the
feelers may not only be touched, but sharply pinched, without effect.
Clearly, therefore, the cerebral ganglia serve as a ganglionic
centre, by which the afferent impulses derived from the feelers and
the eyes are transmuted into efferent impulses. Another very curious
result follows upon the extirpation of the cerebral ganglia. If an
uninjured crayfish is placed upon its back, it makes unceasing and
well-directed efforts to turn over; and if everything else fails,
it will give a powerful flap with the abdomen, and trust to the
chapter of accidents to turn over as it darts back. But the brainless
crayfish behaves in a very different way. Its limbs are in incessant
motion, but they are “all abroad;” and if it turns over on one side,
it does not seem able to steady itself, but rolls on to its back
again.
If anything is put between the chelæ of an uninjured crayfish, while
on its back, it either rejects the object at once, or tries to make
use of it for leverage to turn over. In the brainless crayfish a
similar operation gives rise to a very curious spectacle.[8] If the
object, whatever it {111} be—a bit of metal, or wood, or paper, or
one of the animal’s own antennæ—is placed between the chelæ of the
forceps, it is at once seized by them, and carried backwards; the
chelate ambulatory limbs are at the same time advanced, the object
seized is transferred to them, and they at once tuck it between the
external maxillipedes, which, with the other jaws, begin vigorously
to masticate it. Sometimes the morsel is swallowed; sometimes
it passes out between the anterior jaws, as if deglutition were
difficult. It is very singular to observe that, if the morsel which
is being conveyed to the mouth by one of the forceps is pulled back,
the forceps and the chelate ambulatory limbs of the other side are at
once brought forward to secure it. The movements of the limbs are, in
short, adjusted to meet the increased resistance.
[8] My attention was first drawn to these phenomena by my friend
Dr. M. Foster, F.R.S., to whom I had suggested the desirableness
of an experimental study of the nerve physiology of the crayfish.
All these phenomena cease at once, if the thoracic ganglia are
destroyed. It is in these, therefore, that the simple stimulus set
up by the contact of a body with, for example, one of the forceps,
is translated into all the surprisingly complex and accurately
co-ordinated movements, which have been described. Thus the nervous
system of the crayfish may be regarded as a system of co-ordinating
mechanisms, each of which produces a certain action, or set of
actions, on the receipt of an appropriate stimulus.
When the crayfish comes into the world, it possesses in its
neuro-muscular apparatus certain innate {112} potentialities of
action, and will exhibit the corresponding acts, under the influence
of the appropriate stimuli. A large proportion of these stimuli come
from without through the organs of the senses. The greater or less
readiness of each sense organ to receive impulses, of the nerves to
transmit them, and of the ganglia to give rise to combined impulses,
is dependent at any moment upon the physical condition of these
parts; and this, again, is largely modified by the amount and the
condition of the blood supplied. On the other hand, a certain number
of these stimuli are doubtless originated by changes within the
various organs which compose the body, including the nerve centres
themselves.
When an action arises from conditions developed in the interior of
an animal’s body, inasmuch as we cannot perceive the antecedent
phenomena, we call such an action “spontaneous;” or, when in
ourselves we are aware that it is accompanied by the idea of the
action, and the desire to perform it, we term the act “voluntary.”
But, by the use of this language, no rational person intends to
express the belief that such acts are uncaused or cause themselves.
“Self-causation” is a contradiction in terms; and the notion that
any phenomenon comes into existence without a cause, is equivalent
to a belief in chance, which one may hope is, by this time, finally
exploded.
In the crayfish, at any rate, there is not the slightest reason to
doubt that every action has its definite physical {113} cause, and
that what it does at any moment would be as clearly intelligible, if
we only knew all the internal and external conditions of the case, as
the striking of a clock is to any one who understands clockwork.
* * * * *
The adjustment of the body to varying external conditions, which is
one of the chief results of the working of the nervous mechanism,
would be far less important from a physiological point of view than
it is, if only those external bodies which come into direct contact
with the organism[9] could affect it; though very delicate influences
of this kind take effect on the nervous apparatus through the
integument.
[9] It may be said that, strictly speaking, only those external
bodies which are in direct contact with the organism do affect
it—as the vibrating ether, in the case of luminous bodies; the
vibrating air or water, in the case of sonorous bodies; odorous
particles, in the case of odorous bodies: but I have preferred
the ordinary phraseology to a pedantically accurate periphrasis.
It is probable that the _setæ_, or hairs, which are so generally
scattered over the body and the appendages, are delicate tactile
organs. They are hollow processes of the chitinous cuticle, and their
cavities are continuous with narrow canals, which traverse the whole
thickness of the cuticle, and are filled by a prolongation of the
subjacent proper integument. As this is supplied with nerves, it is
likely that fine nerve fibres reach the bases of the hairs, and are
affected by anything which stirs these delicately poised levers.
{114}
[Illustration: FIG. 26.—_Astacus fluviatilis._—A, the right antennule
seen from the inner side (× 5); B, a portion of the exopodite
enlarged; C, olfactory appendage of the exopodite; _a_, front view;
_b_, side view (× 300); _a_, olfactory appendages; _au_, auditory
sac, supposed to be seen through the wall of the basal joint of the
antennule; _b_, setæ; _en_, endopodite; _ex_, exopodite; _sp_, spine
of the basal joint.]
There is much reason to believe that odorous bodies affect crayfish;
but it is very difficult to obtain experimental evidence of the
fact. However, there is a good deal of analogical ground for the
supposition that some peculiar structures, which are evidently of
a sensory {115} nature, developed on the under side of the outer
branch of the antennule, play the part of an olfactory apparatus.
Both the outer (fig. 26 A. _ex_) and the inner (_en_) branches of the
antennule are made up of a number of delicate ring-like segments,
which bear fine setæ (_b_) of the ordinary character.
The inner branch, which is the shorter of the two, possesses only
these setæ; but the under surface of each of the joints of the outer
branch, from about the seventh or eighth to the last but one, is
provided with two bundles of very curious appendages (fig. 27, A,
B, C, _a_), one in front and one behind. These are rather more than
1‐200th of an inch long, very delicate, and shaped like a spatula,
with a rounded handle and a flattened somewhat curved blade, the
end of which is sometimes truncated, sometimes has the form of a
prominent papilla. There is a sort of joint between the handle and
the blade, such as is found between the basal and the terminal parts
of the ordinary setæ, with which, in fact, these processes entirely
correspond in their essential structure. A soft granular tissue fills
the interior of each of these problematical structures, to which
Leydig, their discoverer, ascribes an olfactory function.
It is probable that the crayfish possesses something analogous to
taste, and a very likely seat for the organ of this function is in
the upper lip and the metastoma; but if the organ exists it possesses
no structural peculiarities by which it can be identified. {116}
There is no doubt, however, as to the special recipients of sonorous
and luminous vibrations; and these are of particular importance,
as they enable the nervous machinery to be affected by bodies
indefinitely remote from it, and to change the place of the organism
in relation to such bodies.
* * * * *
Sonorous vibrations are enabled to act as the stimulants of a special
nerve (fig. 25, _a′n_) connected with the brain, by means of the very
curious _auditory sacs_ (fig. 26, A, _au_) which are lodged in the
basal joints of the antennules.
Each of these joints is trihedral, the outer face being convex;
the inner, applied to its fellow, flat; and the upper, on which
the eyestalk rests, concave. On this upper face there is a narrow
elongated oval aperture, the outer lip of which is beset with a
flat brush of long close-set setæ, which lie horizontally over the
aperture, and effectually close it. The aperture leads into a small
sac (_au_) with delicate walls formed by a chitinous continuation of
the general cuticula. The inferior and posterior wall of the sac is
raised up along a curved line into a ridge which projects into its
interior (fig. 27, A, _r_). Each side of this ridge is beset with a
series of delicate setæ (_as_), the longest of which measures about
1‐50th of an inch; they thus form a longitudinal band bent upon
itself. These _auditory setæ_ project into the fluid contents of the
sac, and their apices are for the most part imbedded in a gelatinous
mass, which contains irregular particles of sand {117} and sometimes
of other foreign matter. A nerve (_n n′_,) is distributed to the sac,
and its fibres enter the bases of the hairs, and may be traced to
their apices, where they end in peculiar elongated rod-like bodies
(fig. 27, C). Here is an auditory organ of the simplest description.
It retains, in fact, throughout life, the condition of a simple sac
or involution of the integument, such as is that of the vertebrate
ear in its earliest stage.
[Illustration: FIG. 27.—_Astacus fluviatilis._ A, the auditory sac
detached and seen from the outside (× 15); B, auditory hair (× 100);
C, the distal extremity of the same more highly magnified. _a_,
aperture of sac; _as_, auditory setæ; _b_, its inner or posterior
extremity; _n n′_, nerves; _r_, ridge.]
{118}
The sonorous vibrations transmitted through the water in which the
crayfish lives to the fluid and solid contents of the auditory sac
are taken up by the delicate hairs of the ridge, and give rise to
molecular changes which traverse the auditory nerves and reach the
cerebral ganglia.
* * * * *
The vibrations of the luminiferous ether are brought to bear upon
the free ends of two large bundles of nerve fibres, termed the optic
nerves (fig. 25, _on_), which proceed directly from the brain, by
means of a highly complex _eye_. This is an apparatus, which, in
part, sorts out the rays of light into as many very small pencils as
there are separate endings of the fibres of the optic nerve, and,
in part, serves as the medium by which the luminous vibrations are
converted into molecular nerve changes.
The free extremity of the eyestalk presents a convex, soft, and
transparent surface, limited by an oval contour. The cuticle in
this region, which is termed the _cornea_, (fig. 28, _a_), is, in
fact, somewhat thinner and less distinctly laminated than in the
rest of the eyestalk, and it contains no calcareous matter. But
it is directly continuous with the rest of the exoskeleton of the
eyestalk, to which it stands in somewhat the same relation as the
soft integument of an articulation does to the adjacent hard parts.
[Illustration: FIG. 28.—_Astacus fluviatilis._—A, a vertical section
of the eye-stalk (× 6); B, a small portion of the same, showing the
visual apparatus more highly magnified; _a_, cornea; _b_, outer dark
zone; _c_, outer white zone; _d_, middle dark zone; _e_, inner white
zone; _f_, inner dark zone; _cr_, crystalline cones; _g_, optic
ganglion; _op_, optic nerve; _sp_, striated spindles.]
The _cornea_ is divided into a great number of minute, usually square
facets, by faint lines, which cross it from side {119} to side
nearly at right angles with one another. A longitudinal section shows
that both the horizontal and the vertical contours of the cornea
are very nearly semicircular, and that the lines which mark off the
facets merely arise from a slight modification of its substance
between the facets. The outer contour of each facet forms part of
the general curvature of the outer face of the cornea; the inner
contour sometimes exhibits a slight deviation {120} from the general
curvature of the inner face, but usually nearly coincides with it.
When a longitudinal or a transverse section is taken through the
whole eyestalk, the optic nerve (fig. 28, A, _op_) is seen to
traverse its centre. At first narrow and cylindrical, it expands
towards its extremity into a sort of bulb (B, _g_), the outer surface
of which is curved in correspondence with the inner surface of the
cornea. The terminal half of the bulb contains a great quantity of
dark colouring matter or pigment, and, in section, appears as what
may be termed the _inner dark zone_ (_f_). Outside this, and in
connection with it, follows a white line, the _inner white zone_
(_e_), then comes a _middle dark zone_ (_d_); outside this an outer
pale band, which may be called the _outer white zone_ (_c_), and
between this and the cornea (_a_) is another broad band of dark
pigment, the _outer dark zone_ (_b_).
When viewed under a low power, by reflected light, this outer dark
zone is seen to be traversed by nearly parallel straight lines,
each of which starts from the boundary between two facets, and can
be followed inwards through the outer white zone to the middle dark
zone. Thus the whole substance of the eye between the outer surface
of the bulb of the optic nerve and the inner surface of the cornea is
marked out into as many segments as the cornea has facets; and each
segment has the form of a wedge or slender pyramid, the base of which
is four-sided, and is applied against the inner surface of {121} one
of the facets of the cornea, while its summit lies in the middle dark
zone. Each of these _visual pyramids_ consists of an axial structure,
the _visual rod_, invested by a sheath. The latter extends inwards
from the margin of each facet of the cornea, and contains pigment
in two regions of its length, the intermediate space being devoid
of pigment. As the position of the pigmented regions in relation to
the length of the pyramid is always the same, the pigmented regions
necessarily take the form of two consecutive zones when the pyramids
are in their natural position.
The visual rod consists of two parts, an external _crystalline cone_
(fig. 28, B, _cr_), and an internal _striated spindle_ (_sp_).
The _crystalline cone_ consists of a transparent glassy-looking
substance, which may be made to split up longitudinally into four
segments. Its inner end narrows into a filament which traverses
the outer white zone, and, in the middle dark zone, thickens
into a four-sided spindle-shaped transparent body, which appears
transversely striated. The inner end of this _striated spindle_
narrows again, and becomes continuous with nerve fibres which proceed
from the surface of the optic bulb.
The exact mode of connection of the nerve-fibres with the visual rods
is not certainly made out, but it is probable that there is direct
continuity of substance, and that each rod is really the termination
of a nerve fibre.
Eyes having essentially the same structure as that of {122} the
crayfish are very widely met with among _Crustacea_ and _Insecta_,
and are commonly known as _compound eyes_. In many of these animals,
in fact, when the cornea is removed, each facet is found to act as
a separate lens; and when proper arrangements are made, as many
distinct pictures of external objects are found behind it as there
are facets. Hence the notion suggested itself that each visual
pyramid is a separate eye, similar in principle of construction to
the human eye, and forming a picture of so much of the external world
as comes within the range of its lens, upon a retina supposed to
be spread out on the surface of the crystalline cone, as the human
retina is spread over the surface of the vitreous humour.
But, in the first place, there is no evidence, nor any probability,
that there is anything corresponding to a retina on the outer face of
the crystalline cone; and secondly, if there were, it is incredible
that, with such an arrangement of the refractive media as exists in
the cornea and crystalline cones, rays proceeding from points in
the external world should be brought to a focus in correspondingly
related points of the surface of the supposed retina. But without
this no picture could be formed, and no distinct vision could take
place. It is very probable, therefore, that the visual pyramids do
not play the part of the simple eyes of the _Vertebrata_, and the
only alternative appears to be the adoption of a modification of the
theory of _mosaic vision_, propounded many years by Johannes Müller.
{123}
Each visual pyramid, isolated from its fellows by its coat of
pigment, may be supposed, in fact, to play the part of a very narrow
straight tube, with blackened walls, one end of which is turned
towards the external world, while the other incloses the extremity of
one of the nerve fibres. The only light which can reach the latter,
under these circumstances, is such as proceeds from points which lie
in the direction of a straight line represented by the produced axis
of the tubes.
[Illustration: FIG. 29.—Diagram showing the course of rays of light
from three points _x_, _y_, _z_, through the nine visual rods
(supposed to be empty tubes) A–I of a compound eye; _a–i_, the nerve
fibres connected with the visual rods.]
Suppose A–I to be nine such tubes, _a–i_ the corresponding nerve
fibres, and _x_ _y_ _z_ three points from which light proceeds. Then
it will be obvious that the only light {124} from _x_ which will
excite sensation, will be the ray which traverses B and reaches the
nerve-fibre _b_, while that from _y_ will affect only _e_, and that
from _x_ only _h_. The result, translated into sensation, will be
three points of light on a dark ground, each of which answers to one
of the luminous points, and indicates its direction in reference to
the eye and its angular distance from the other two.[10]
The only modification needed in the original form of the theory of
mosaic vision, is the supposition that part, or the whole, of the
visual rod, is not merely a passive transmitter of light to the
nerve-fibre, but is, itself, in someway concerned in transmuting
the mode of motion, light, into that other mode of motion which we
call nervous energy. The visual rod is, in fact, to be regarded as
the physiological end of the nerve, and the instrument by which the
conversion of the one form of motion into the other takes place; just
as the auditory hairs are instruments by which the sonorous waves are
converted into molecular movements of the substance of the auditory
nerves.
[10] Since the visual rods are strongly refracting solids, and
not empty tubes, the diagram given in fig. 29 does not represent
the true course of the rays, indicated by dotted lines, which
fall obliquely on any cornea of a crayfish’s eye. Such rays
will be more or less bent towards the axis of the visual rod
of that cornea; but whether they reach its apex and so affect
the nerve or not will depend on the curvature of the cornea;
its refractive index and that of the crystalline cone; and the
relation between the length and the thickness of the latter.
It is wonderfully interesting to observe that, when the so-called
compound eye is interpreted in this manner, {125} the apparent
wide difference between it and the vertebrate eye gives place to a
fundamental resemblance. The rods and cones of the retina of the
vertebrate eye are extraordinarily similar in their form and their
relations to the fibres of the optic nerve, to the visual rods of
the arthropod eye. And the morphological discrepancy, which is at
first so striking, and which arises from the fact that the free ends
of the visual rods are turned towards the light, while those of the
rods and cones of the vertebrate eye are turned from it, becomes a
confirmation of the parallel between the two when the development of
the vertebrate eye is taken into account. For it is demonstrable that
the deep surface of the retina in which the rods and cones lie, is
really a part of the outer surface of the body turned inwards, in the
course of the singular developmental changes which give rise to the
brain and the eye of vertebrate animals.
* * * * *
Thus the crayfish has, at any rate, two of the higher sense organs,
the ear and the eye, which we possess ourselves; and it may seem a
superfluous, not to say a frivolous, question, if any one should ask
whether it can hear and see.
But, in truth, the inquiry, if properly limited, is a very pertinent
one. That the crayfish is led by the use of its eyes and ears to
approach some objects and avoid others, is beyond all doubt; and, in
this sense, most indubitably it can both hear and see. But if the
question {126} means, do luminous vibrations give it the sensations
of light and darkness, of colour and form and distance, which they
give to us? and do sonorous vibrations produce the feelings of noise
and tone, of melody and of harmony, as in us?—it is by no means to
be answered hastily, perhaps cannot be answered at all, except in a
tentative, probable way.
The phenomena to which we give the names of sound and colour are
not physical things, but are states of consciousness, dependent,
there is every reason to believe, on the functional activity of
certain parts of our brains. Melody and harmony are names for states
of consciousness which arise when at least two sensations of sound
have been produced. All these are manufactured articles, products
of the human brain; and it would be exceedingly hazardous to affirm
that organs capable of giving rise to the same products exist in the
vastly simpler nervous system of the crustacean. It would be the
height of absurdity to expect from a meat-jack the sort of work which
is performed by a Jacquard loom; and it appears to me to be little
less preposterous to look for the production of anything analogous to
the more subtle phenomena of the human mind in something so minute
and rude in comparison to the human brain, as the insignificant
cerebral ganglia of the crayfish.
At the most, one may be justified in supposing the existence of
something approaching dull feeling in ourselves; and, to return to
the problem stated in the {127} beginning of this chapter, so far
as such obscure consciousness accompanies the molecular changes of
its nervous substance, it will be right to speak of the mind of a
crayfish. But it will be obvious that it is merely putting the cart
before the horse, to speak of such a mind as a factor in the work
done by the organism, when it is merely a dim symbol of a part of
such work in the doing.
Whether the crayfish possesses consciousness or not, however, does
not affect the question of its being an engine, the actions of which
at any moment depend, on the one hand, upon the series of molecular
changes excited, either by internal or by external causes, in its
neuro-muscular machinery; and, on the other, upon the disposition
and the properties of the parts of that machinery. And such a
self-adjusting machine, containing the immediate conditions of its
action within itself, is what is properly understood by an automaton.
* * * * *
Crayfishes, as we have seen, may attain a considerable age; and there
is no means of knowing how long they might live, if protected from
the innumerable destructive influences to which they are at all ages
liable.
It is a widely received notion that the energies of living matter
have a natural tendency to decline, and finally disappear; and that
the death of the body, as a whole, is the necessary correlate of
its life. That all living things sooner or later perish needs no
demonstration, but it would be difficult to find satisfactory grounds
{128} for the belief that they must needs do so. The analogy of a
machine that, sooner or later, must be brought to a standstill by the
wear and tear of its parts, does not hold, inasmuch as the animal
mechanism is continually renewed and repaired; and, though it is
true that individual components of the body are constantly dying,
yet their places are taken by vigorous successors. A city remains,
notwithstanding the constant death-rate of its inhabitants; and such
an organism as a crayfish is only a corporate unity, made up of
innumerable partially independent individualities.
Whatever might be the longevity of crayfishes under imaginable
perfect conditions, the fact that, notwithstanding the great number
of eggs they produce, their number remains pretty much the same in a
given district, if we take the average of a period of years, shows
that about as many die as are born; and that, without the process of
reproduction, the species would soon come to an end.
There are many examples among members of the group of _Crustacea_
to which the crayfish belongs, of animals which produce young from
internally developed germs, as some plants throw off bulbs which
are capable of reproducing the parent stock; such is the case, for
example, with the common water flea (_Daphnia_). But nothing of this
kind has been observed in the crayfish; in which, as in the higher
animals, the reproduction of the species is dependent upon the
combination of two kinds of living {129} matter, which are developed
in different individuals, termed _males_ and _females_.
These two kinds of living matter are _ova_ and _spermatozoa_, and
they are developed in special organs, the _ovary_ and the _testis_.
The ovary is lodged in the female; the testis, in the male.
The _ovary_ (fig. 30, _ov_) is a body of a trefoil form, which is
situated immediately beneath, or in front of, the heart, between the
floor of the pericardial sinus and the alimentary canal. From the
ventral face of this organ two short and wide canals, the _oviducts_
(_od_), lead down to the bases of the second pair of walking limbs,
and terminate in the apertures (_od′_) already noticed there.
[Illustration: FIG. 30.—_Astacus fluviatilis._—The female
reproductive organs (× 2); _ov_, ovary; _od_, oviduct; _od′_,
aperture of oviduct.]
[Illustration: FIG. 31.—_Astacus fluviatilis._—The male reproductive
organs (× 2); _t_, testis; _vd_, vas deferens; _vd′_, aperture of vas
deferens.]
The _testis_ (fig. 31, _t_) is somewhat similar in form to the ovary,
but, the three divisions are much narrower {130} and more elongated:
the hinder median division lies under the heart; the anterior
divisions are situated between the heart behind, and the stomach and
the liver in front (figs. 5 and 12, _t_). From the point at which
the three divisions join, proceed two ducts, which are termed the
_vasa deferentia_ (fig. 31, _vd_). These are very narrow, long, and
make many coils before they reach the apertures upon the bases of the
hindermost pair of walking limbs, by which they open externally (fig.
31, _vd′_, and fig. 35, _vd_). Both the ovary and the testis are very
much larger {131} during the breeding season than at other times;
the large brownish-yellow eggs become conspicuous in the ovary, and
the testis assumes a milk-white colour, at this period.
[Illustration: FIG. 32.—_Astacus fluviatilis._—A, a two-thirds grown
egg contained in its ovisac (× 50); B, an egg removed from the ovisac
(× 10); C, a portion of the wall of an ovisac with the adjacent
portion of the contained egg, highly magnified; _ep_, epithelium of
ovisac; _gs_, germinal spots; _gv_, germinal vesicle; _m_, membrana
propria; _v_, vitellus; _vm_, vitelline membrane; _w_, stalk of
ovisac.]
[Illustration: FIG. 33.—_Astacus fluviatilis._—A, a lobule of
the testis, showing _a_, acini, springing from _b_, the ultimate
termination of a duct (× 50). B, spermatic cells; _a_, with an
ordinary globular nucleus _n_; _b_, with a spindle-shaped nucleus;
_c_, with two similar nuclei; and _d_, with a nucleus undergoing
division (× 600).]
The walls of the ovary are lined internally by a layer of {132}
nucleated cells, separated from the cavity of the organ by a delicate
structureless membrane. The growth of these cells gives rise to
papillary elevations which project into the cavity of the ovary,
and eventually become globular bodies attached by short stalks, and
invested by the structureless membrane as a _membrana propria_ (fig.
32, _m_). These are the _ovisacs_. In the mass of cells which becomes
the ovisac, one rapidly increases in size and occupies the centre
of the ovisac, while the others {133} surround it as a peripheral
coat (_ep._). This central cell is the _ovum_. Its nucleus enlarges,
and becomes what is called the _germinal vesicle_ (_g.v._). At the
same time numerous small corpuscles, flattened externally and convex
internally, appear in it and are the _germinal spots_ (_g.s._). The
protoplasm of the cell, as it enlarges, becomes granular and opaque,
assumes a deep brownish-yellow colour, and is thus converted into
the _yelk_ or _vitellus_ (_v._). As the egg grows, a structureless
_vitelline membrane_ is formed between the vitellus and the cells
which line the ovisac, and incloses the egg, as in a bag. Finally,
the ovisac bursts, and the egg, falling into the cavity of the ovary,
makes its way down the oviduct, and sooner or later passes out by
its aperture. When they leave the oviduct, the ova are invested by a
viscous, transparent substance, which attaches them to the swimmerets
of the female, and then sets; thus each egg, inclosed in a tough
case, is firmly suspended by a stalk, which, on the one side, is
continued into the substance of the case, while, on the other, it is
fixed to the swimmeret. The swimmerets are kept constantly in motion,
so that the eggs are well supplied with aërated water. {134}
[Illustration: FIG. 34.—_Astacus fluviatilis._—A–D, different stages
in the development of a spermatozoon from a seminal cell; E, a mature
spermatozoon seen from the side; F, the same viewed _on face_ (all
× 850); G, a diagrammatic vertical section of the same.]
The testis consists of an immense number of minute spheroidal
vesicles (fig. 33, A, _a_), attached like grapes to the ends of
short stalks (_b_), formed by the ultimate ramifications of the vasa
deferentia. The vesicles may, in fact, be regarded as dilatations of
the ends and sides {135} of the finest branches of the ducts of the
testis. The cavity of each vesicle is filled by the large nucleated
cells which line its walls (fig. 33, B), and, as the breeding season
approaches, these cells multiply by division. Finally, they undergo
some very singular changes of form and internal structure (fig. 34,
A–D), each becoming converted into a flattened spheroidal body, about
1‐1700th of an inch diameter, provided with a number of slender
curved rays, which stand out from its sides (fig. 34, E–G). These are
the _spermatozoa_.
The spermatozoa accumulate in the testicular vesicles, and give rise
to a milky-looking substance, which traverses the smaller ducts,
and eventually fills the vasa deferentia. This substance, however,
consists, in addition to the spermatozoa, of a viscid material,
secreted by the walls of the vasa deferentia, which envelopes the
spermatozoa, and gives the secretion of the testis the form and the
consistency of threads of vermicelli.
The ripening and detachment of both the ova and the spermatozoa
take place immediately after the completion of ecdysis in the
early autumn; and at this time, which is the breeding season, the
males seek the females with great avidity, in order to deposit the
fertilizing matter contained in the vasa deferentia on the sterna
of their hinder thoracic and anterior abdominal somites. There it
adheres as a whitish, chalky-looking mass; but the manner in which
the contained spermatozoa reach and enter the ova is unknown. The
analogy {136} of what occurs in other animals, however, leaves
no doubt that an actual mixture of the male and female elements
takes place and constitutes the essential part of the process of
impregnation.
Ova to which spermatozoa have had no access, give rise to no progeny;
but, in the impregnated ovum, the young crayfish takes its origin in
a manner to be described below, when the question of development is
dealt with.
[Illustration: FIG. 35.—_Astacus fluviatilis._—The last thoracic
sternum, seen from behind, with the proximal ends of the appendages,
A, in the male, B, in the female, (× 3). _am_, articular membrane;
_cxp_, coxopodite; _st XIV_, last thoracic sternum; _vd_, aperture of
vas deferens.]
{137}