THE LOBSTER AS A TYPE OF CRUSTACEA
The most noticeable feature distinguishing the Lobster[2] (Fig. 1) at
first sight from other familiar animals is the jointed shelly armour
that encases its body and limbs. Over the fore part of the body this
armour is continuous, forming a shield, or _carapace_, which projects in
front, between the eyes, as a toothed beak, or _rostrum_; on the hinder
part--the tail, or _abdomen_--it is divided into six segments, or
_somites_, connected with each other by movable joints. Each of these
somites carries on the under-side a pair of fin-like limbs, or
_swimmerets_, the last pair of which (uropods) are much larger than the
others, and are spread out at the sides of a middle tail-plate, or
_telson_, forming what is known as the _tail-fan_. Since the fore part
of the body also has a series of paired limbs, constructed, as will be
shown later, on the same plan as the swimmerets, it is concluded that
this part also is built up of somites, which have become soldered
together. That this conclusion is correct is shown by comparison with
some of the lower Crustacea in which this part of the body is divided up
into eight separate somites, like those of the abdomen, each carrying,
in place of the swimmerets, a pair of walking legs. In front of these
eight somites, forming what is called the _thorax_, is the head--a part
of the body which is never, in any Crustacean, broken up into distinct
somites, but which, since it carries five pairs of appendages, must
consist of at least five somites. The part of the body covered by the
Lobster's carapace includes both the head and the thorax, and is known,
therefore, as the _cephalothorax_. It is necessary to bear in mind that
the parts of the body to which the names head, thorax, and abdomen, are
applied in Crustacea are by no means exactly equivalent to those which
bear the same names in Insects, for example, and that, beyond a rough
similarity in position, they have no sort of relation to the parts so
named in the body of a vertebrate animal.
[2] The account given here of the structure of the Lobster applies
almost equally well to the River Crayfish or the Norway Lobster. The
student is recommended to follow the description with a specimen of
one of these animals before him.
[Illustration: FIG. 1--THE COMMON LOBSTER (_Homarus gammarus_,) FEMALE,
FROM THE SIDE. (From British Museum Guide.)]
There are altogether twenty pairs of appendages attached to the body of
the Lobster. In front of the head are the stalked _eyes_ (of which the
nature will be discussed later) and two pairs of feelers--the
_antennules_ and _antennae_ (sometimes called the first and second
antennae). Near the mouth on the under-side of the head are three pairs
of jaw-appendages--the strong _mandibles_ and the flattened, leaf-like
_maxillulae_ and _maxillae_. Following these are the appendages of the
thorax, of which the first three are intermediate in form between the
true jaws and the legs, and are therefore termed foot-jaws, or
_maxillipeds_. The remaining five pairs of thoracic limbs are the
_legs_, the first pair forming the large and powerful pincer-claws, or
_chelipeds_, while the others are the walking legs. The six pairs of
swimmerets on the abdomen have already been mentioned.
If one of the somites of the abdomen be separated from the others, it
will be seen (Fig. 2) to consist of a shelly ring, to which the two
swimmerets are attached, wide apart, on the under-side. The arched
upper part of the ring is known as the _tergum_, and the more flattened
under-part as the _sternum_. On each side the tergum overlaps the
sternum, and hangs down as a side-flap, or _pleuron_. On the upper side
of the abdomen the terga of the somites overlap, the front part of each
being pushed under the tergum in front when the abdomen is straightened,
and only exposed to view when the abdomen is bent. Below, the sternum of
each somite is seen to be only a narrow bar, connected with those in
front and behind by soft membrane, and there is no overlapping. At the
sides the somites are connected together by hinge-joints, which allow
them to move only in a vertical plane. Thus the abdomen can be
straightened out or bent downwards and forwards, but cannot be moved
from side to side. In life the Lobster can swim backwards through the
water by vigorously flapping the abdomen.
[Illustration: FIG. 2--ONE OF THE ABDOMINAL SOMITES OF THE LOBSTER, WITH
ITS APPENDAGES, SEPARATED AND VIEWED FROM IN FRONT. (From British Museum
Guide.)]
The carapace which covers the upper side of the head and thorax is not
formed, as might be thought, simply by the terga of the somites becoming
soldered together. This is shown by a comparison with certain
shrimp-like Crustacea (_Mysidacea_) in which the carapace arises, like a
fold of the skin, from the hinder edge of the head, and envelops, like a
loose jacket, the distinctly segmented thorax. In the Lobster this fold
has become adherent to the thoracic somites down the middle of the back,
but at the sides it hangs free, enclosing on each side a cavity within
which lie the gills.
It seems at first sight strange to include in the same category as
"limbs" or "appendages" organs which differ so much in form and function
as do the swimmerets, the walking legs, the jaws, and the antennae.
Nevertheless it can easily be demonstrated that all of them are
constructed on the same general plan, and arise in the embryo from
rudiments which are, for the most part, exactly alike. This is expressed
in technical language by saying that the appendages of the whole series
are _homologous_ with one another. A full discussion of this interesting
fact would require more space than can be devoted to it here, but a few
examples may be given to illustrate what is meant by the "serial
homology" of the appendages in Crustacea.
If one of the swimmerets be detached from the third abdominal somite, it
will be seen (Fig. 2) to consist of a stalk, known as the _protopodite_,
bearing two branches, of which that on the outer side is called the
_exopodite_, and that on the inner side the _endopodite_. The
protopodite consists of two segments, the first very short, and the
second much longer. It can easily be seen that the side-plates of the
tail-fan (the middle plate, as already mentioned, is the telson) are
simply the swimmerets of the sixth abdominal somite. They are much
larger than the other swimmerets, and have the endopodite and exopodite
broadened out into large plates; while the protopodite is very short,
and not divided into segments.
[Illustration: FIG. 3--THIRD MAXILLIPED OF LOBSTER. (From British Museum
Guide.)]
If now the third maxilliped (Fig. 3) be examined, it will be found that,
like the swimmeret, it consists essentially of two branches springing
from a stalk of two segments. The exopodite, however, is much smaller
than the endopodite, and it ends in a flexible lash made up of many
small segments. The endopodite forms the main part of the limb, and has
five segments, so that, with the two segments of the protopodite, there
are _seven_ segments in the main axis of the limb; the second and third
segments are partly soldered together, but the line of union can be
plainly seen. Attached to the outer side of the first segment is a
membranous plate, known as the _epipodite_, on which is inserted, near
its base, a brush-like structure, which is one of the gills. In the
natural position the epipodite and its gill lie in the gill chamber,
hidden from view by the side-flap of the carapace.
[Illustration: FIG. 4--WALKING LEGS OF LOBSTER
A, Of first pair; B, of third pair]
The legs (Fig. 4) can, without difficulty, be seen to consist each of
seven segments like those of the maxillipeds, but there is no exopodite.
In the young Lobster, when just hatched from the egg, however, each of
the legs has a large exopodite like that of the third maxilliped. These
exopodites, which are used in swimming, are afterwards lost as the
animal grows; but their presence in the young is interesting as
confirming the conclusion that the legs, like the maxillipeds, are built
on the same plan as the swimmerets. The large claws, and also the first
and second pairs of walking legs, end in pincers, or _chelae_, the
penultimate segment projecting in a thumb-like process against which the
last segment works. Each leg, except those of the last pair, has on its
first segment an exopodite with a gill like those of the maxilliped.
[Illustration: FIG. 5--APPENDAGES OF LOBSTER IN FRONT OF THIRD
MAXILLIPED
A, Eye-stalk; B, antennule; C, antenna (the flagellum is cut short); D,
mandible; E, maxillula; F, maxilla; G, first maxilliped; H, second
maxilliped. _en_, Endopodite; _ep_, epipodite; _ex_, exopodite; _gn_,
gnathobases, or jaw-plates; _p_, palp of mandible; _sc_, scaphognathite]
Following the series of appendages forwards from the third maxilliped
(Fig. 5), it is easy to trace the gradual reduction of the endopodite
and exopodite; while the two segments of the protopodite become
flattened and broadened inwards to form the jaw-plates. The mandibles
(Fig. 5, D), which are the chief organs of mastication, consist mainly
of the much enlarged basal segment of the protopodite, with a strongly
toothed inner edge, where it works against its fellow of the opposite
side; and the rest of the limb is reduced to a small sensory "palp,"
which represents the second segment of the protopodite and the
endopodite.
The antennae (Fig. 5, C) can be shown, without difficulty, to conform to
the same plan of structure as the other appendages. The two segments of
the protopodite are short, but distinct; the endopodite forms the long
lash, or _flagellum_, composed of very numerous small segments; the
exopodite is reduced to a small movable scale or spine.
The antennules (Fig. 5, B) seem at first sight to present the
two-branched type of structure in its simplest form; but there is
considerable doubt as to whether the two lashes which each bears on a
three-segmented stalk are really equivalent to the endopodite and
exopodite.
The movable stalks which carry the eyes (Fig. 5, A) have been considered
by some to belong to the series of the appendages, and to be, in fact,
modified limbs. If this be the case, we have here the greatest
simplification which the limb undergoes in the Lobster, for each
eye-stalk consists only of two segments: the first small and
incompletely formed, the second in the form of a short cylinder, having
the eye at its end. There are, however, reasons for doubting whether the
eye-stalks are really appendages.
The hard outer covering of the Lobster not only protects and gives
support to the internal organs, but also affords points of attachment
for the muscles by means of which the animal moves. In other words, it
plays the part of a skeleton; but since, unlike the skeleton of
vertebrate animals, it is _outside_ instead of _inside_ the soft parts
of the body, it is known as an _exoskeleton_. Closer examination shows
that this outer covering is really continuous over the whole of the body
and limbs, but is thin and soft at the joints, allowing the parts to
move one upon another. It is composed of a horn-like substance known as
_chitin_, which, except at the joints, is hardened by the deposition in
it of carbonate and other salts of lime.
As this external covering does not increase in size after it has been
formed, and as it cannot stretch to any great extent, the Lobster
requires to cast its shell at intervals as it grows. In this process of
_moulting_ the integument of the back splits between the carapace and
the first abdominal somite. The body and limbs are gradually worked
loose and withdrawn through the opening, leaving the cast shell with all
its appendages almost entire. The new covering, which had been formed
underneath the old before moulting, is at first quite soft, and the
animal rapidly increases in size owing to the absorption of water. The
shell then gradually hardens by the deposition of lime salts.
[Illustration: FIG. 6--DISSECTION OF MALE LOBSTER, FROM THE SIDE. (From
British Museum Guide.)]
The internal anatomy (Fig. 6) presents many points of interest which can
only be briefly touched on here. The food-canal consists of a short
gullet leading into a capacious stomach, from which the straight
intestine runs to the vent on the under-side of the telson. The stomach
has a most remarkable and complicated structure. It consists of two
chambers, a larger in front and a smaller behind, which are lined by a
continuation of the chitinous outside covering of the body. This
chitinous lining is thickened in places to form a system of plates and
levers connected with three strong teeth set in the narrow opening
between the two chambers. By the action of muscles attached to certain
of these plates the teeth work together so as to divide up the food more
finely than had been done by the mandibles and other jaws. The whole
apparatus, in fact, serves as a kind of gizzard, and is known as the
_gastric mill_.
A small part of the intestine at the hinder end is lined, like the
stomach, by a continuation of the chitinous covering, which is turned in
at the vent. This lining and that of the stomach, with the plates and
teeth of the gastric mill, are cast and renewed when the shell is
moulted.
On each side of the food-canal in the thorax lies a large mass of soft
tissue, yellowish-green in colour. This is the _digestive gland_, or
"liver," which secretes the digestive juice, discharging it into the
food-canal by a short duct on each side just behind the stomach.
The _heart_ lies in the middle of the back, just under the hinder part
of the carapace, and gives off, in front and behind, a number of
arteries which carry the blood to the various organs of the body. From
the smaller branches of these arteries the blood passes, not, as in
vertebrate animals, into capillaries, but into the spaces lying between
the organs of the body, and it finds its way back to the heart, not in
definite veins, but by ill-defined venous channels which open into the
pericardium, or space surrounding the heart. From the pericardium the
blood enters the heart by six openings in its walls, each guarded by a
pair of valves which close when the heart contracts, and prevent the
blood from returning to the pericardium.
[Illustration: FIG. 7--GILLS OF THE LOBSTER, EXPOSED BY CUTTING AWAY THE
SIDE-FLAP OF THE CARAPACE (BRANCHIOSTEGITE)]
The venous channels which convey the blood back to the heart are so
arranged that most of the blood passes first through the _gills_, for
the purpose of respiration, before it reaches the heart and is again
distributed through the body. These gills, as already mentioned, lie in
the two branchial chambers under the side-flaps of the carapace (Fig.
7), and are attached, some to the epipodites of the thoracic limbs (as
described above), and some to the soft membrane of the joints between
the limbs and the body; while others are attached to the side-wall of
the thorax itself. Each gill is somewhat like a bottle-brush in shape,
consisting of a central stalk set round with rows of soft hair-like
processes. As the blood streams through the minute channels inside these
filaments, it is separated only by a thin membrane from the surrounding
water, and the absorption of oxygen and discharge of carbon dioxide can
go on easily. For this purpose, however, it is necessary that the water
within the gill chamber should be constantly renewed, and this is
effected in the following way: the front part of the gill chamber forms
a narrow channel running forward under the side-wall of the carapace.
Within this channel lies a large plate known as the _scaphognathite_,
attached to the outer side of the maxilla, which during life is
constantly in movement, causing a current of water to flow forwards
through the channel. The water enters the gill chamber by the narrow
slit-like space between the lower edge of the carapace and the bases of
the legs, and is discharged in front at the sides of the head, where its
movement is helped by the vibrating exopodites of the maxillipeds.
At the sides of the stomach, in the front part of the head, lie a pair
of glands which, from their colour, are known as the _green glands_.
These are the excretory organs, corresponding in function to the kidneys
of the higher animals. Each has connected with it a thin-walled bladder,
which opens to the outside through a small perforation on the
under-side of the first segment of the antenna.
The chief part of the _nervous system_ is the ventral nerve-chain, which
runs along the under-side of the body. This is a long cord having at
intervals a series of knots or swellings, the ganglia or nerve-centres,
from which nerves are given off to the appendages and to the organs of
the body. In the hinder part of the thorax and in the abdomen there is a
ganglion in each somite, but in front these ganglia become crowded
together and coalesced, so that we find only a single large ganglion,
corresponding to the somites from that of the mandibles to that of the
third maxillipeds. Between the ganglia the cord is really double,
although for the greater part of its length the two parts are more or
less completely fused into one. In front of the head and above the
gullet is a ganglion which sends nerves to the eyes, antennules and
antennae, and is known as the _brain_, although it is, perhaps, hardly so
important as that name would suggest. It is connected with the ventral
chain by two cords that pass on either side of the gullet.
The _eyes_, as already mentioned, are set on movable stalks, so that
they can be turned in any direction at the will of the animal, and are
of the type known as "compound eyes." If the convex black area at the
end of the eye-stalk be examined with a strong lens, it will be seen
that the membrane which covers it is divided up into a beautifully
regular series of square facets. This membrane is a thin and transparent
continuation of the chitinous covering of the body, and if it be
stripped off and examined under a microscope, it will be found that each
facet is capable of acting as a lens and forming an image of external
objects. It is not to be supposed, however, that the Lobster sees a
separate image in each of the facets, some thirteen thousand in number,
which go to make up each eye. In the interior of the eye, at some
distance from the surface, are a large number of rod-like bodies,
connected with the fibres of the optic nerve, and believed to be the
actual organs for the perception of light. Each rod corresponds to one
of the facets, and as it lies at the bottom of a long conical tube, of
which the walls are covered with dark pigment, it can only receive light
from a single point in line with the axis of the tube. In this way the
image of any object will be built up, like a mosaic, out of the
impressions of light and darkness received through the separate facets,
and transmitted to the underlying rods. It has been shown in some
Crustacea that, when the animal is in a very dim light, the curtain of
pigment separating the tubes is partially withdrawn, so that the light
from each facet can reach, not one, but several rods. In this way the
images of objects received are much brighter, although they are less
sharply defined.
It might be thought that in animals like the Lobster, enclosed in a
hard shelly covering, the sense of _touch_ must be very dull, if not
altogether absent. This, however, is not the case. What is probably a
very delicate tactile sense is provided for by the numerous hairs which
are found, of many sorts and sizes, all over the body and limbs. Each of
these hairs is really a hollow outgrowth of the chitinous covering,
containing a delicate prolongation of the soft tissues underneath, and
also supplied, in many if not in all cases, with a nerve-fibre, so that
the slightest movement of the hair caused by contact with a solid body
is perceived by the animal. Many of these hairs are themselves beset
with delicate secondary hairs, arranged so that the whole looks like a
feather or like a bottle-brush. These hairs are adapted for detecting
slight movements or vibrations in the surrounding water.
Whether Crustacea living in water can _hear_, in the sense in which the
word is used of animals living in air, is doubtful; but it is certain
that they are extremely sensitive to vibrations only a little coarser,
so to speak, than those we know as sound. The Lobster, and many other
Crustacea, do indeed possess a structure which was long supposed to be
an organ of hearing, and may possibly in part fulfil that function,
although it is now known that that is not its only or even its chief
use. It consists of a small cavity in the basal segment of the stalk of
the antennule, opening to the outside by a narrow slit on the upper
surface of the segment. The cavity is lined by a delicate continuation
of the chitinous covering of the body, and has on its inner surface a
series of feathered hairs of the kind described above, which are richly
supplied with nerve-fibres from a large nerve entering the base of the
antennule. Within the cavity, and for the most part entangled among
these hairs, are a number of grains of sand. When the Lobster moults,
the lining membrane of this cavity is thrown off like the rest of the
exoskeleton, and with it the contained sand-grains. While the shell is
still soft after moulting, and the lips of the slit are not rigid, as
they afterwards become, fresh sand-grains find their way into the cavity
to take the place of those which have been cast off. Perhaps, like some
other Crustacea, the Lobster buries its head in the sand to insure that
some grains may find their way in; for its pincers are too clumsy for it
to pick up sand-grains and to place them in the cavity, as some Prawns
have been seen to do. At all events, if a freshly moulted Prawn be
placed in a vessel of sea-water, and supplied, instead of sand, with
powdered glass or metal filings, particles of glass or metal will after
a short time be found in its antennular cavities. This habit has been
utilized in a very ingenious experiment by which the function of these
organs was demonstrated. A Prawn had been induced in this way to place
particles of iron filings in the cavities, and a strong electro-magnet
was brought near the side of the vessel in which it was kept. It was
observed that the Prawn, which had been swimming in the usual horizontal
position, at once turned the under-side of its body towards the magnet,
and swam about on its side as long as the magnet was in action. When the
current exciting the magnet was cut off, the animal resumed its ordinary
position. This experiment shows that these organs, to which we may now
give their proper name of _statocysts_, are organs for perceiving the
direction of the force of gravity. The magnetic force acted on the
particles of iron in the same way that the force of gravity acts on the
sand-grains in normal conditions, and the Prawn felt the weight of them,
so to speak, pulling towards the side instead of the bottom of the
vessel, and turned its body accordingly, to swim, as it supposed, right
side up. It is now known that those parts of the human ear called the
"semicircular canals" have a somewhat similar function as "organs of
orientation," although to animals walking on the solid ground this
function is not so important as it doubtless is to animals swimming in
water.
The sense of _smell_ is believed to have its seat chiefly in the
antennules. The outer branch of each antennule bears tufts of peculiar
hairs, in which the chitinous covering is extremely delicate, so that
substances dissolved in the water can easily pass through and affect
the nerve-endings within. These hairs are known as "olfactory
filaments."
The sense of _taste_ in aquatic animals is, perhaps, not sharply defined
from that of smell, but it is not very rash to assume that certain hairs
on the mouth parts and on the fleshy upper and lower lips which bound
the opening of the mouth have to do specially with this sense.
The relative importance of the various senses in the Lobster is well
illustrated in the following account of its habits given by Dr. H. C.
Williamson in the Report of the Scottish Fishery Board for 1904. After
noticing that, in daylight at least, the Lobster appears to be
"purblind," only distinguishing light from shadow, Dr. Williamson goes
on: "It tests a shadow with its antennae, or sometimes, when a strong
shadow is thrown on it, it jumps at it with its chelae outstretched and
snapping. It is dependent on its antennae for guiding it in safe places.
It is especially careful in testing any hole before it is satisfied with
it. It discovers the cavity by means of its antenna, which is waved well
out to the side and in front as it walks. It searches the innermost
depths of the hole with the antenna, and then inserts its chela. If the
examination with the chela is also satisfactory, it immediately turns
and backs smartly into the hole. In feeding it is guided to the food by
the antennules. A piece of food which is dropped near a Lobster may fall
quite unnoticed unless it happens to touch the antenna or the [legs].
It is not seen at all. But sooner or later, according as the distance is
short or great, the scent of the food, carried by the currents set up by
the exopodites of the maxillipeds, reaches the Lobster. The Lobster is
immediately excited, although previously it was lying quite inert in its
hole. It whips the water with its antennules in a staccato fashion, and
feels about with the antennae and chelae, at first without leaving its
hole. At once both antennules are seen to be whipping in the direction
in which the food is lying, and an active search is made with the
antennae. If they do not succeed in locating the bait, the Lobster rather
reluctantly leaves its hole, but cautiously, feeling all round about
with its antennae. It goes off straight in the direction in which the
food is lying, and, if it misses it with its antennae and chelae, walks
over it and gets it with its chelate [walking legs]; it usually picks up
its food with the second [walking leg]. Meanwhile the expected feast has
by association stimulated the maxillipeds, which are actively working as
if they were already masticating the food. Once the food is seized it is
conveyed to the maxillipeds, and the Lobster retreats to its hole, there
to enjoy its meal."
Lobsters, like most other Crustacea, are of separate sexes. The females
(see Plate I.) may be distinguished from the males by the fact that the
abdomen is broader and has deeper side-plates, and by differences in
the form of the first two pairs of swimmerets. In the female the first
pair, which have only one branch, are short and slender filaments, while
in the male they are stout and peculiarly twisted rods. The second pair
in the female are similar in form to the succeeding pairs, but in the
male they have an additional lobe on the inner branch. The openings of
the generative organs will be found in the male on the basal segments of
the last pair of legs, while in the female they occupy the same position
on the legs of the last pair but two. The testis of the male lies in the
thorax, just below the heart. The ovary, which has the same position in
the female, is usually much more conspicuous, and from its red colour in
the cooked Lobster it is known as the "coral." On the under-side of the
thorax of the female, between the last two pairs of legs, is a
three-lobed structure enclosing a cavity known as the "sperm-receptacle."
Its function is to receive the fertilizing substance from the male, and
to retain it until the eggs are ready to be deposited.
[Illustration: _PLATE I_
MALE AND FEMALE LOBSTERS, SHOWING THE DIFFERENCE IN THE RELATIVE
BREADTH OF THE ABDOMEN IN THE TWO SEXES. THIS FIGURE ALSO
ILLUSTRATES THE DISSIMILARITY OF THE LARGE CLAWS AND THE FACT THAT
THE "CRUSHING-CLAW" MAY BE ON EITHER THE RIGHT OR LEFT SIDE OF THE
BODY
(_From Brit. Mus. Guide_)]
In the Lobster, as in many other Crustacea, the eggs are carried by the
female until they hatch. After being extruded from the oviducts, they
are attached by a kind of cementing substance to the swimmerets, where
they hang in bunches. The swimmerets are kept constantly moving, so that
the eggs may obtain the oxygen necessary for the developing embryos
within. A female Lobster carrying eggs in this way is said by the
fishermen to be "in berry," and may carry, according to its size, from
about 3,000 to nearly 100,000 eggs. A period of about ten months elapses
between the deposition of the eggs and hatching.
[Illustration: FIG. 8--FIRST LARVAL STAGE OF THE COMMON LOBSTER. x 4.
(After Sars.)]
The young Lobster when first hatched (Fig. 8) differs considerably in
general appearance from the adult animal. The abdominal somites have a
row of spines down the middle of the back, and the telson has a forked
shape. There are no swimmerets, but, as already mentioned, the legs bear
large exopodites, which are used like oars, and by means of these the
larval Lobster swims about at the surface of the sea. The claws or chelae
are at first hardly larger than the other legs, but later they increase
in size, the swimmerets are developed, the exopodites of the legs are
lost, and the young Lobster, sinking to the bottom of the sea, takes on
the creeping habits and gradually assumes the shape of the adult.
In many Crustacea the changes of shape or metamorphoses undergone after
hatching are much greater than in the Lobster. Some of these changes and
their probable significance will be considered at greater length in a
later chapter.
The two large claws of the Lobster (see Plate I.) are not quite alike in
size or in shape. The smaller of the two has the inner edges of the
fingers sharp and set with saw-like teeth; the larger has the fingers
armed with blunt rounded knobs. The larger claw is adapted for crushing
the shells of the animals on which the Lobster feeds, while the smaller
serves for holding and tearing the prey. In the Lobster, as in many of
the higher Crustacea in which this asymmetry occurs, the larger claw may
be indifferently on either side of the body. There are certain cases,
however, among Crabs where the large claw is constantly on the same side
of the body, or, in other words, all the individuals are either
right-handed or, more rarely, left-handed.
If a Lobster be caught by one of its claws or by a leg, it very readily
parts with the limb in its struggles to escape; and if one of the limbs
be crushed or otherwise injured, it is often cast off by the animal. The
separation always takes place at the same point, near the base of the
limb, and is not simply due to the limb breaking at its weakest part. It
is a reflex act, brought about by a spasmodic contraction of some of the
leg muscles. At the place of separation, corresponding to the junction
of the second and third segments of the limb, which, as already
mentioned, are soldered together, the internal cavity is crossed by a
transverse partition, having only a small aperture in the centre through
which the nerves and bloodvessels pass. When the limb is cast off, this
small opening quickly becomes closed by a clot of blood, and further
bleeding is stopped. If, as sometimes happens, a limb which has been
seriously injured is not cast off, the animal not infrequently bleeds to
death. This power of self-mutilation or _autotomy_, as it is called, is
frequently used by Crustacea as a means of escaping from their enemies,
and is closely connected with the power of _regeneration_ of lost
appendages. Beneath the scar which forms on the stump of a separated
limb a sort of bud grows, and gradually assumes the form of the lost
segments. At the next moult this straightens out, and, increasing in
size at succeeding moults, it ultimately provides, in normal cases, a
new member similar in every detail to that which had been lost.
Occasionally it happens, under circumstances not yet altogether
understood, that the process of regeneration may, so to speak, go wrong,
and in this way various malformations and abnormalities result. For
instance, it has been found that, if the larger, crushing claw of a very
young Lobster be removed by operation or by accident, the limb which
grows in its place may assume the form of the smaller, toothed claw.
Further, in some other Crustacea (but not in the Lobster, except in the
very youngest stages), it is found in such cases that, after removal of
the large claw, the claw of the other side assumes at the next moult the
form of a crushing claw, so that there is a "reversal of asymmetry."
A still more remarkable change sometimes occurs when one of the
eye-stalks is injured. If only the tip of the eye-stalk be cut off, so
that the nerve-ganglion which lies in the basal part of the stalk
remains uninjured, it will be found that a normal eye is in course of
time regenerated. If, however, the whole eye-stalk be amputated, and
with it the optic ganglion, there grows in its place, not a new
eye-stalk, but a segmented appendage similar to one of the flagella of
the antennules. This fact is considered by some zoologists to indicate
that the eye-stalks are, like the antennules, true appendages,
homologous with the mouth parts and limbs, but this is a much-disputed
question into which we cannot enter further here.
Lobsters vary a good deal in colour, but as a rule a living Lobster is
of a more or less mottled dark blue, becoming nearly black on the back,
and shaded off into orange yellow or red on the under-side. This
coloration resides in the shell, and does not change much after the
shell has hardened. In this respect the Lobster is unlike many of the
smaller Crustacea which have a thin and more or less transparent
exoskeleton, and in which the colour resides in certain living cells
(chromatophores) of the underlying skin. Many of these Crustacea possess
the power of changing their colours to a remarkable degree, by the
expansion and contraction of the branched chromatophores.
The question which is often asked, "Why does a Lobster turn red when it
is boiled?" is one to which it is not easy to give a simple answer. A
chemical change takes place under the influence of heat in the pigment
of the shell, which changes it from blue to red; how slight the change
is, is perhaps shown by the fact that occasionally living Lobsters are
found of a red colour almost as brilliant as that which is assumed on
boiling.
[Illustration: FIG. 9--SIDE-VIEW OF ROSTRUM OF (A) COMMON LOBSTER
(_Homarus gammarus_) AND (B) AMERICAN LOBSTER (_Homarus americanus_)]
The Common Lobster is found on the coasts of Western Europe, from Norway
to the Mediterranean, living in shallow water, generally a little way
below low-tide mark, wherever a rough, rocky bottom affords suitable
lurking-places. On the Atlantic coast of North America, Lobsters are
also found abundantly in similar situations. These American Lobsters, if
examined carefully, will be found to differ from the European kind in
certain small details of structure, of which the most conspicuous is the
presence, on the under-side of the rostrum, of two spines or teeth. In
the European Lobsters the under-side of the rostrum is smooth (Fig. 9).
In the nomenclature of technical zoology, these two kinds or _species_
of Lobster are said to constitute (along with a third species found at
the Cape of Good Hope) the genus _Homarus_, the European species being
known as _Homarus gammarus_, and the American as _Homarus americanus_.
The so-called "Norway Lobster" or "Dublin Prawn," which differs from the
Common Lobster in having large kidney-shaped eyes and long and slender
claws, and in many other details of structure, is placed in a distinct
genus, and is known as _Nephrops norvegicus_. The genera _Homarus_ and
_Nephrops_, together with some others, constitute the family Homaridae,
which again is grouped with other families in a tribe, Nephropsidea,
forming a part of the order Decapoda. These groups are intended to
express the varying degrees of resemblance and difference in structure
between the species of animals which make up the class Crustacea. Since
we have good grounds for believing that all these species have arisen by
some mode of evolution, this classification also represents the varying
degrees of actual relationship between the different forms, so far as
this relationship can be discovered. In the next chapter a brief sketch
of the chief subdivisions of the Crustacea is given, with such details
as to the characteristics of each as are necessary to render
intelligible the succeeding chapters on their habits and modes of life.