CRUSTACEANS. 



615 



jsents the smallest changes of form. The Amphipods, on the 

 ler hand, come from the egg in a form similar to that of the 

 rfect animal. Many Isopods, also, undergo only slight changes of 

 form ; many acquire another pair of feet in addition to those which 

 they had on leaving the egg. The young Limuli, according to 

 MILNE EDWARDS, leave the egg without that ensiform appendage 

 )r pointed tail which so strikingly distinguishes the full-grown 

 ill. 



The crustaceans cast their shell several times. In younger indi- 

 Luals these moultings succeed each other at shorter periods, but 

 in full-grown animals, at least in the decapods, the hard calcareous 

 shell is cast off only once a year. In the River-cray the moulting 

 occurs towards the end of summer. The hard shell begins to 

 loosen itself from the body, which in the meantime is replaced 

 by a new covering situated beneath the former. The animal makes 

 many movements and contortions, until at last a fissure occurs on 

 the back between the abdomen and the large cephalothorax, from 

 which the animal with the fore part of its body and its feet makes 

 its appearance. At last the hinder part of the body divests itself 

 of its old covering. For these observations we are indebted to 

 REAUMUR. In the short-tailed crabs the shell splits on each side 

 along the epimera. The cast-off shell presents perfectly the form of 

 the living animal 1 . 



As long as the new shell is still thin and flexible, the crays and 

 crabs are very sensitive. They then conceal themselves in holes, 

 until the new shell has attained sufficient hardness, for which a few 

 days only are requisite. 



With the shell the inner coat or the epithelium of the stomach 

 is renewed in the crays and crabs. When a new internal tunic of 

 the stomach has been formed the old one is cast off and dissolved in 

 the cavity of the stomach. It has been supposed that the two 

 round calcareous plates which are situated on each side of the 



contested the observations of THOMPSON, but afterwards admitted, with that upright- 

 ness which always belongs to the inquirer after truth alone, his mistake respecting this 

 important discovery, and that he had done the English naturalist a wrong ; Op. cit. 

 s. 46. See also the observations of Du CANE on the metamorphoses of Cancer mcenas, 

 Annals of nat. Hist. in. pp. 438 440, PL XI. 



1 See EEADMUR Mem. de VAcad. des Sc. 1718, p. 263 and foil., COLLINSON Phil. 

 Trans. 1746 and 1751, MILNE EDWAKDS Hist. nat. des Crust, i. pp. 53 57; comp. 

 also ETMEB JONES Animal Kingdom, sd edit. 1855, pp. 434 436. 



