The _Crustaceans_ are mostly gill-breathers, though some of the aquatic species have no special organs for respiration, but obtain the oxygen necessary for respiration by absorption through their thin, soft skin, while the terrestrial species breathe by means of tracheæ, as we have just observed. 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.