The crustacean class of animals, of which the lobster, crab, and
shrimp are familiar examples, have this peculiarity of structure--that
their soft bodies are enclosed within a coat-of-mail formed of
carbonate and phosphate of lime. In fact, they carry their skeleton
outside their bodies, both for defence of the vital parts within, and
for the attachment of the muscles which move their limbs, and every
part of their frame. No warrior of old was ever more completely
enveloped in his hard coat-of-mail, with its jointed greaves and
overlapping scales, than is the lobster in its crustaceous covering;
with this exception, that the warrior could at pleasure unbuckle
himself from his armour, whereas the body and limbs of the crustacea
are completely incased in hollow cylinders, firmly and accurately
jointed, from which there is no such ready release. Now, as this
shelly integument envelops them from their earliest youth, and as it
does not expand and grow, the natural growth of the soft body beneath
would be entirely prevented did not nature supply a remedy of a very
curious kind--the exuviation, or periodical throwing off of the
external crust, and the formation of a larger shell-covering fitted
for the increasing growth of the animal. This is a circumstance which
has long been familiar to naturalists, and indeed the most ordinary
observer must have often remarked in the crabs and lobsters brought to
table, appearances indicative of their change of external coverings.
In the back of the edible crab, may often be noticed a red membrane
lining the inner side of the shell, but so loose as to be readily
detached. Along the greater part of its course this membrane has
already assumed a half-crustaceous consistence, and is just the
preparatory process to the old shell being thrown off by the animal.
There is another curious circumstance which has also been long
known--that crabs and lobsters can renew lost limbs. Some
misconception, however, had existed regarding the manner in which this
was effected, until the observations of the late Sir John Dalyell have
thrown more accurate light on the subject.
This most amiable and eminent zoologist, who was lost to science last
year, afforded a pleasing illustration of the solace and delight which
the pursuit of the study of nature yields to the diligent inquirer
into her mysteries. With a feeble constitution and frame of body,
which precluded his mingling in the more active pursuits of everyday
life, this sedentary philosopher collected around him examples of
minute and curious being from the depths of the ocean, from lake and
river, and for many long years found the delight of his leisure hours
in watching the habits of the animals, and in discovering and
describing many singular circumstances in the constitution of their
bodies, and the peculiar adaptations of their structure and instincts
to their modes of existence. One of his last communications to the
public, imparted with all the modesty and simplicity of true genius,
at the last meeting of the British Association in Edinburgh, was on
this subject of the exuviation of the crustacea.[2]
It appears from Sir John's observations that crustaceans begin to
throw off their shells at a very early period of their life, even in
that embryo state in which they first appear after having left the
egg, and before they have yet assumed the real form of their mature
state. During every successive exuviation in this embryo state, they
assume more and more of their perfect and established form. While the
crab is young and rapidly growing, frequent exuviations take place at
short intervals, from three to five times in the course of one year.
Previous to the change, the animal almost ceases to feed, and becomes
rather inactive; the proper time having at length arrived, exuviation
is effected in the course of a few hours, body and limbs being alike
relieved from their hard covering. Until the new shell acquires
firmness and strength, the creature is very shy, and in the state of
nature, retires into cavities below rocks or heaps of protecting
sea-weed. Sir John had kept for some time one of our smaller species
of shore-crabs (_Carcinus monas_), of medium size, of a brown colour,
with one white limb. One summer evening it was put outside the window
in a capacious glass-vessel of sea-water. In the morning a form
exactly resembling its own, only somewhat larger, lay in the vessel.
This was the same animal, which had performed exuviation, and
extricated itself from the old shell during the night. The resemblance
between both forms was complete--everything was the same, even the
white limb was seen in both. Another specimen kept was of smaller
size, the opposite extremities of the limbs being only thirteen lines
asunder; its colour was green, with three white patches on the back.
In the course of little more than a year five exuviations took place
at irregular intervals, the new shell and animal becoming larger each
time. The third shell came on uniformly green, the white spots being
entirely obliterated. On the fourth exuviation, the limbs expanded two
inches and a half. From the long slender form of the limbs of
crustacea, they are very liable to mutilation. Crabs are also a very
pugnacious family, and in their battles limbs are often snapped off.
These mutilations, however, are readily repaired; although, contrary
to what was the common belief, the restoration takes place only at the
next regular period of exuviation.
The full-grown common crab (_Cancer pagurus_) is of a reddish-brown
colour, the claws tipped black; but some of the young are naturally of
the purest white, which remains long unsullied. This does not arise
from confinement, which, according to Sir John, has no influence on
colour. 'A young white specimen of the common crab was subjected to
observation on 29th September. The body might have been circumscribed
in a circle three-quarters of an inch in diameter, and the extended
limbs by one-and-a-half inch in diameter. Its first exuviation ensued
on 8th November, the second on the 30th of April following, and the
shell then produced subsisted till 12th September, when another
exuviation took place, introducing a new shell of such transparent
white that the interior almost shone through it. All the shells were
white, and increased somewhat in size successively. This last shell of
12th September subsisted until 29th March, being 197 days, when it was
thrown off during another exuviation.'
But what was remarkable, the animal now had only the two large claws,
the other eight limbs were deficient. 'Resting on its breast as it
was, I did not at first discover the fact, that the creature presented
a strange and very uncouth aspect. However, it fed readily, and proved
very tame, though helpless; often falling on its back, and not being
able to recover itself from the deficiency of its limbs. I preserved
this mutilated object with uncommon care, watching it almost
incessantly day and night: expecting another exuviation which might be
attended with interesting consequences, I felt much anxiety for its
survivance. My solicitude was not vain. After the defective shell had
subsisted eighty-six days, its tenant meantime feeding readily, the
desired event took place in a new exuviation on 23d June. On this
occasion a new animal came forth, and in the highest perfection, quite
entire and symmetrical, with all the ten limbs peculiar to its race,
and of the purest and most beautiful white. I could not contemplate
such a specimen of nature's energies restoring perfection, and through
a process so extraordinary, without admiration. Something yet remained
to be established: was this perfection permanent, or was it only
temporary? Like its precursor, this specimen was quite tame, healthy,
and vigorous. In 102 days it underwent exuviation, when it appeared
again, perfect as before, with a shell of snowy white, and a little
red speckling on the limbs. Finally, its shell having subsisted 189
days, was succeeded by another of equal beauty and perfection, the
speckling on the legs somewhat increased. As all the shells had
gradually augmented, so was this larger than the others. The extended
limbs would have occupied a circle of four inches diameter. About a
month after this exuviation the animal perished accidentally, having
been two years and eight months under examination. It was an
interesting specimen, extremely tame and tranquil, always coming to
the side of the vessel as I approached, and holding up its little
claws as if supplicating food.'
The shrimp when in confinement becomes very tame, and readily
exuviates. The process is frequent, the integument separates entire,
and is almost colourless. In female crustaceans the roe is placed
outside the shell to which it adheres. During the period of such
adherence, the female crab, so far as observation goes, does not
change its shell--a marked provision of nature to preserve the spawn.
We may remark that other classes of animals exuviate in a similar
manner to the crustaceans. Thus serpents throw off in entire masses
their scaly coverings, even a slough from the eyes; and various
insects in their larva state are continually throwing off and renewing
their skins.