258 THE SEA SHORE 



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 

 tracheae, 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 antennas, 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 with- 

 out 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 



