The arrangement of the Crustacea in systems of natural history has
undergone numerous changes; they were placed by Linnæus among the
insects:—others considered them to have more analogy to the spiders.
Lamarck was the first who made the Crustacea an independent CLASS
forming two _orders_, which he has named after the arrangement of the
branchiæ, or gills, by which they breathe, namely,—the _Crustacea
homobranchiæ_, the distinguishing marks of which he describes in this
manner: branchiæ hidden under the lateral margins of a kind of cuirass,
covering the body of the animal, with the exception of the tail; the
mandibles always furnished with feelers, the eyes placed on footstalks,
the head not distinct from the trunk, and possessing ten feet to assist
them in their movements. Secondly, the _Crustacea heterobranchiæ_, in
which the branchiæ are external, in various situations, but never under
the lateral margin of a cuirass; they are either under the belly or the
tail, adhering to the feet, or confounded with them: the eyes are in
general fixed, _sedentary, not on footstalks_.
_ORDER CRUSTACEA HOMOBRANCHIÆ_,
(_Shell-Fish with concealed gills._)
The first of these orders, the _homobranchial_ Crustacea, includes most
of the larger kinds of shell-fish, as, for instance, crabs, lobsters,
and cray-fish; shrimps and prawns are also in this division.
Their organization is much more perfect than that of the other order,
and, according to Lamarck, it is among these animals that the last
appearance of the organ of hearing is seen, in tracing the animal
kingdom from the most perfect animals to those whose formation is
apparently less complex.
The body of these creatures appears to be composed of only two
principal parts, the body and the tail; for the head is so intimately
united and confounded with the trunk, as to appear to be merely a
portion of it. The two eyes are fixed at the top of two moveable
supports, and are placed in a hollow prepared for their reception, on
each side of a projecting portion of the shell that covers the head.
The antennæ, which are usually four in number, are placed about this
spot; they are inserted beneath the stems that support the eyes. The
two outermost of these antennæ are generally the longest.
The branchiæ, or organs by which they breathe, assume a form somewhat
pyramidical, arranged like a series of leaves, or the web of a feather;
they are placed in the interior of the shell along each side, and are
so arranged as to adhere to the roots of the feet, so that each of
these feet has a hidden branchia attached to its base.
The mouth is composed of a fleshy lip, projecting between the
mandibles; of two hard triangular mandibles, more or less notched at
their extremity, and each having a kind of feeler inserted on the upper
part; they also possess a little tongue between these mandibles, at
the root of which is the opening to the stomach: they have besides two
pair of jaws, like leaves, the borders of which are fringed, and six
other members, which Lamarck calls _foot-jaws_, from their bearing
some resemblance to legs, or feet. From this it appears that the
parts of the mouth in the Crustacea form a complicated apparatus, and
accordingly we find the whole tribe exceedingly voracious, the Crabs
in particular, feeding upon any animal substance, putrid or not, that
may come within their reach. Some of the species are well known as
articles of food, but they are not equally wholesome at all seasons of
the year.
_THE CRAB TRIBE._
The genera of the Crustacea are so extremely numerous, and the
knowledge we possess of their natural history is so scanty, being
generally confined to their form and colour, that we shall limit
ourselves to a description of some of the best known and most useful
species.
[Illustration: THE ZOEA OF THE COMMON CRAB.]
The Crabs are a very numerous tribe, and contain many singular
species, as far as regards their form, which assumes an endless
variety of curious shapes, sometimes elegantly decorated with the most
brilliant colours; this more particularly applies to those which are
found in the seas of hot climates.
The singular little animal represented in the preceding page, was
placed by naturalists among the _Crustacea_, and considered a perfect
animal of a distinct species; it was named Zoea. Mr. Thomson, the
experienced naturalist we have already noticed, was the first to
discover the real nature of this little creature. We cannot do better
than to introduce the subject by the following observations of the
author of this discovery, showing the reason so little is known
respecting these inhabitants of the deep.
“The sea (which is the habitation of the greater part of the
_Crustacea_) to the casual observer offers nothing but an immense body
of water, here and there presenting a solitary whale, or a vagrant
troop of some of the smaller cetaceous animals; the appearance of a
fish of almost any other kind in the track of a vessel over a vast
expanse of the open ocean, is regarded, even by the mariner, as a
kind of phenomenon, and creates an interest not to be appreciated by
those who have not engaged in distant voyages. The fathomless parts
of the ocean certainly do not offer the same profusion of inhabitants
with the shores of islands and continents, or those parts where the
bottom is within reach of the sounding-line, or where the surface is
interspersed with fields of Sargosa[6]. On due examination, however, we
shall not fail to find it everywhere peopled by a considerable variety
of animals, either of small size, or possessed of such a degree of
translucency as to render them invisible, or scarcely perceptible, even
when on or near to its surface; that it should possess its share of
the organized beings which we see spread over every other part of the
surface of our globe, is a conclusion we might arrive at indirectly,
from the consideration of oceanic fishes and birds being observed in
those parts of the ocean most distant from the land, and the provident
care of the Deity, which we invariably witness throughout the domain of
nature, to furnish food for all, even the meanest of his creatures; the
more minute and invisible inmates of the sea, then, must constitute the
food of oceanic fishes and birds.
“Few of these marine animals, except some of the larger and most
conspicuous, have as yet been observed, so that the investigation of
them holds out a promise of a rich harvest to the naturalist, and
a vast field of exploration, replete with novelty and interest; to
accomplish this, however, he must use the greatest diligence, seizing
every opportunity, when the way of a ship does not exceed three or
four miles per hour, to throw out astern a small towing-net of gauze,
bunting, or other tolerably close material, occasionally drawing it
up, and turning it inside out into a glass vessel of sea-water, to
ascertain what captures have been made. When a ship goes at a greater
rate, and in stormy weather, a net of this kind may be appended to the
spout of one of the _sea-water_ pumps, and examined three or four times
a day, or oftener, according to circumstances.”
Although naturalists were decided in calling the Zoea a crustaceous
animal, they were still far from agreeing as to the place in the
system it ought to occupy, for the different species were so unlike
each other: but it will be no longer a matter of surprise, when it
is known, that this singular creature is not a perfect animal, but
merely the larva, or imperfect state of the Common Crab. This fact is
perfectly new, and interesting in a double point of view, not only
proving their real nature, but also that the Crustacea are not, as
described in most systems, animals undergoing no metamorphosis, and on
that account to be separated from the insects, but that they do undergo
a metamorphosis, and that of a most wonderful nature.
It was in the Spring of 1822 that Mr. Thomson first met with Zoeæ,
in the harbour of Cove, and that in considerable abundance; the year
following, at the same season, one of considerable size occurred;
this was considered a fit object for experiment, and was carefully
supplied with fresh sea-water, from May 14th to June 15th, when it died
in the act of changing its skin. That portion of its new form which
it had been able to disengage was sufficient to show that it bore a
great resemblance to the division of the Crustacea, in which the crabs
and lobsters are placed. “This proof,” says the author, “might be
considered incomplete, if I had not had the good fortune to succeed in
hatching the _ova_ of the Common Crab, during the month of June, which
presented exactly the appearance of the _Zoea taurus_.”
The Common Crab, _Cancer major_, is so well-known, that any description
of its appearance would be useless.
One singular part of the history of these creatures is, the power that
is possessed by them, of changing their shell once in every year; this
power is providentially bestowed upon them, to enable them to increase
in size, a thing that would otherwise be utterly impossible, from the
peculiar nature of the hard coat in which they are enclosed.
The Crab, in order to prepare for the extraordinary change it is about
to undergo, when shifting its shell, chooses a close and well-secured
retreat, in the cavities of rocks, or under great stones, where it
creeps in and remains during the operation. The time of the year
when this occurs is about the beginning of the Summer, at which time
their food is in plenty, and their strength and vigour in the highest
perfection. But soon all their activity ceases; they are seen forsaking
the open parts of the deep, and seeking some retired situation among
the rocks, or some outlet where they may remain in safety from the
attacks of their enemies. For some days before their change, the animal
discontinues its usual voraciousness; it is no longer seen laboriously
harrowing up the sand at the bottom, or fighting with others of its
kind, or hunting its prey; it lies torpid and motionless, as if in
anxious expectation of the approaching change. Just before casting
its shell, it throws itself upon its back, strikes its claws against
each other, and every limb seems to tremble; its feelers are agitated,
and the whole body is in violent motion; it then swells itself in an
unusual manner, and at last the shell is seen beginning to divide at
its junctures, particularly at those of the belly, where it was before
seemingly united. It also seems turned inside out; and its stomach
comes away with its shell. After this, by the same operation, it
disengages itself of its claws, which burst at the joints; the animal,
with a tremulous motion, casting them off, as a man would kick off a
boot that was too big for him.
Thus, in a short time, this wonderful creature finds itself at liberty;
but in so weak and enfeebled a state that it continues for several
hours motionless. Indeed, so violent and painful is the operation,
that many of them die under it; and those which survive, are in such
a weakly condition for some time, that they neither take food, nor
venture from their retreats. Immediately after this change, they have
not only the softness, but the timidity of a worm. Every animal of
the deep is then a powerful enemy, which they can neither escape nor
oppose; and this, in fact, is the time when the dog-fish, the cod,
and the ray, devour them by thousands. But this state of weakness
continues for a very short time; the animal, in less than two days, is
seen to have the skin that covers its body almost as hard as before;
its appetite appears to increase; and, strange to behold! the first
object, it is said, that tempts its gluttony, is its own stomach, which
it was lately disengaged from. This it devours with great eagerness.
In about forty-eight hours, in proportion to the animal’s strength,
the new shell is perfectly formed, and as hard as that which was but
just thrown aside. Previous to the time of moulting or changing their
skin, a flat chalky stone is found on each side of the stomach; this
is believed to form a store of earthy matter for the renewal of the
shell. These stones, popularly known as _crabs’ eyes_, were long
highly esteemed for their medicinal properties, but in reality are not
superior to so much chalk.
When completely equipped in its new dress, the dimensions of the
old shell being compared with those of the new, it will be found
the creature has increased in size nearly one-third, and it appears
wonderful how the old shell could have contained it.
Many of the cold-blooded animals have the power of reproducing a limb,
or a portion of one, if by any accident it has been lost. This faculty
of reproduction is possessed by the Crab in great perfection; but it
has also a surprising power in itself, voluntarily to break off its
own legs and claws. It seems this takes place when any serious injury,
by bruising, has happened to any of its members. After it has received
the hurt it bleeds, and gives signs of pain, by moving the wounded limb
from side to side, but afterwards holds it quite still, in a direct and
natural position, without touching any part of its body or its other
legs with it. Then, on a sudden, with a gentle crack, the wounded part
of the leg drops off at the next joint to the one injured; this appears
to be more easily done with respect to the smaller legs, than in the
case of an injury occurring to those which bear the pincers.
When the leg has dropped off, a mucus, or jelly, is discharged on
the remaining part of the joint next the body, which, as a natural
styptic, instantly stops the bleeding; this gradually hardens and grows
callous, becoming a new leg in miniature, which at every change of the
creature’s shell increases rapidly in size.
Crabs are naturally very quarrelsome, and with their claws fight and
kill each other; and if by chance any of their limbs should be so
bruised, as to have taken away from the creature the power of breaking
off its claws, the protecting jelly is not produced, and the animal
bleeds to death. An experiment was made to give some idea of the
tenacious disposition of this creature, by obliging a Crab, with one
of its great claws, to lay hold of one of its smaller ones; the silly
creature did not distinguish that itself was the aggressor, but exerted
its strength, and soon cracked the shell of its own small leg, which
bled freely; but feeling itself wounded it succeeded in breaking off
its limb in the usual manner,—still, however, holding fast for a
length of time the part of the wounded leg which had come away.
The curious shuffling walk of the Crab is well known, but it does not,
as it is said, walk exactly backwards.