The Sea Shore

by William S. Furneaux

Most of them are covered with a calcified skin, as in the case of crabs and lobsters; but many are protected with a chitinous or horny covering such as we observe in shrimps and prawns. In either instance the hardened integument constitutes what is known as the exo-skeleton. None of the crustaceans have an internal skeleton of any kind, though some of the inner parts are supported by extensions of the hard skin that penetrate into the body.

It will be readily understood from the nature of the exo-skeleton of the crustacean, and especially of the more or less rigid calcareous covering of the crab and the lobster, that a uniform growth of the body is absolutely impossible, and, in fact, that an increase in size cannot take place without an occasional casting of the hard coat of mail. Hence we find most crustaceans throwing off their coverings at intervals, and growing by fits and starts during the periods between the ‘moultings’ and the hardening of the newly exposed skin.

When a crab or a lobster is about to undergo the process of moulting, it retires to a secluded niche in the rock, where it is not so easily found by its numerous enemies—a necessary precaution, since the creature in its soft or unarmoured condition is eagerly devoured by fishes and other marine animals—and there awaits the first stage of the ordeal. Presently the skin splits; and, after a time, the crustacean succeeds in extricating itself from its shell, which is cast off in a perfect condition, every joint being entire, even to the coverings of the antennæ, the stalked eyes, and other delicate appendages. And not only this, for the portions of the shell that penetrate inward into the body are also discarded, as well as the linings of the stomach and the gills; and these cast-off coats of crabs and lobsters—especially the former—may often be found in the most perfect condition on the sea shore, being washed up without injury on the sandy beach, or found in the very niche in which the creature changed its attire.

If one examines the powerful pincers of a crab or lobster, a thin plate of considerable size will be seen to extend within from the movable ‘jaw’ to give attachment to the muscles by which it is moved, and it seems impossible that this can be removed with the cast skin without considerable injury to the new claw that is already formed, though as yet in a soft condition, within the old and hard one. But it has been observed that this plate actually cuts through the new claw, and that the claw thus divided almost immediately closes up and unites again.

The moulting process being over, the crustacean’s body extends itself within the new, yielding skin; and, the latter becoming gradually hard by the deposition of carbonate of lime, the creature is able, after a period of rest, to roam at large again, without much fear of injury, until the time for the next moulting has arrived.

Those who have made but a slight acquaintance with the common crustaceans of our shores must have noted the frequency with which imperfect specimens occur—specimens with missing appendages, or with a well-formed limb on one side of the body opposed to a puny and almost useless fellow on the opposite side. As to the loss of appendages, this matter will be readily understood by those who have watched crustaceans, and especially crabs and lobsters, in their native element, so often do these pugnacious creatures become engaged in furious broils with their neighbours. And, when we are at work at the collection of various species on the sea shore, how often do we find that a creature escapes from our grip by leaving us in possession of a severed limb, while the owner retreats rapidly among the stones and weeds apparently none the worse for its trifling loss! This is, in fact, a very common method of securing its escape from an enemy; and it appears that many crustaceans have the power of thus rendering a seized limb so brittle that it may be snapped off with the greatest of ease.

We have spoken of the loss thus sustained as a trifling one; and so it is, for crustaceans have the faculty of reproducing lost appendages; and though the loss may be one of considerable inconvenience at first, a new limb eventually appears in the place of each one so willingly discarded.

When such mutilations occur, it will be observed that the severed limb invariably breaks away at the end of the first or basal joint—a point where the bloodvessels are so narrow and contractile that but little loss of blood takes place when the rupture is made—and it has been said that the animal would soon bleed to death if the fracture were to take place at any other point. As it is, the wound soon heals, but no trace of a new limb is to be seen, at least without dissection, until the time of the next moult. The part is developing, however, beneath the cover of the basal joint; and when the moulting period arrives, the new limb, still very small, is exposed to view. It then rapidly enlarges, though not to anything like its proper size, and its surrounding skin becomes hardened by the deposit of the calcareous secretion simultaneously with that of the rest of the body. Further enlargements of the new appendage take place at subsequent moults, with the final result that it is but slightly inferior to its fellow either in size or in power.

The eye of a crustacean is a very complicated structure, commonly described as a compound eye. It consists of a large number of conical, radiating, crystalline rods, collected together into a mass that presents a convex outer surface. This surface is covered with a transparent layer of chitin which naturally presents a more or less distinct netted appearance, the bases of the rods being in contact with its inner surface, and visible through it. Each rod is surrounded by a layer of pigment that prevents light from passing from one to another, and the optic nerve passing into the base of the compound structure sends a sensitive filament into each one.