Return to Table of Contents

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.

[2] Report of British Association, 1851. Pp. 120-122.