The process of molting is a precarious one, not infrequently resulting in death. The crust being too solid to admit of a continuous growth, and increase in size being rapid, frequent shed dings of the skin are necessary. In the lobster, the old skin being detached from the under cellular layer by the secretion of the new cuticle beneath, ruptures between the thorax and abdo men, and the lobster draws itself out of the rent, shedding not only the entire skin and every hair, but also the lining of the mouth, throat and fore stomach, and likewise the out let of the intestine. In about three weeks after the casting of the shell the new one becomes solid and hard. In the crayfish the old skin is loosened and pushed away from the cellular layer beneath by the growth of temporary, short stiff hairs, which disappear after the skin is shed.
The Crustacea are a very ancient type. The earliest remains are found in the Cambrian rocks, but are very scanty compared with the trilobites. They comprise traces of barnacles, Ostracoda or small shelled forms, Phyllocarida, and an obscure form supposed to be allied to the modern freshwater Apus. In the Devonian
Period shelled phyllopods (Estheria) appeared, and in the Carboniferous arose an order (Syn cardia), reptesented by an ancient form (Anas tides) still living in a lake in Tasmania. From this group the existing Schizopoda or opossum shrimps (Mysis), the Squilla, and the ordinary shrimps and crabs, are supposed to have de scended. Isopoda also appeared as early as the Devonian. A shrimp-like Crustacean occurs in the Devonian, and true crabs date from the Jurassic.
The Crustacea are divided into 11 orders, the Branchiopoda, Phyllopoda, Ostracoda, Cope poda, Cirrepedia or barnacles, Arthrostraca, Cumacea, Phyllotarida, Syncarida, Schizopoda, Stomatopoda and Decapoda. More than 5,000 species are known. See BARNACLE; CRAB; Fist' Licx.i HERMIT CRAB; SHRIMP.
Bibliography.—In addition to general text books, consult Huxley, 'The Crayfish) (London 1880) ; Kingsley, 'Crustacea,) in Standard Natural History (Vol. II, Boston 1884) ; Steb bing, 'A History of Crustacea) (London 1893) ; Smith and Weldon, 'Crustacea,) in Cambridge Natural History (Vol. IV, London 1909) ; Cal man, 'Life of Crustacea) (New York 1911).