CRUSTACEA. 167 



abdominal nervous chords and the branchiae, and give origin to the 

 muscles of the legs. 



The seven abdominal segments are always united by flexible joints, 

 and have no apodemata. In those Crustacea in which the thorax 

 and its cephalic shield are small, the abdomen is long, and character- 

 izes the great tribe of Podophthalnm called Macroura ; in those in 

 which the thorax and its shield are greatly expanded, the abdomen is 

 very short, as in the crabs, or the tribe hence called Srachi/ura ; this 

 alternating excess and arrest of development of the opposite divisions 

 of the trunk well illustrate the law of organic equivalents. 



The appendages of the abdominal segments are developed, like 

 those of the thoracic ones, from the inferior arcs : they are usually 

 broad, flat, ciliated plates, which may sometimes aid in respiration, 

 are more commonly organs for swimming, form a temporary place 

 of attachment and protection to the ova, and are restricted to this 

 use in the females of the Brachyura, in which the abdomen or 

 tail is concealed by being bent forwards upon the sternum. In the 

 hermit crabs, which present an anomalous softness of their abdominal 

 segments, the last pair of appendages form the claspers by which the 

 parasite holds fast by the columella of the shell it may have selected 

 for its abode. 



The calcified integument of the Crustacea being inelastic, inex- 

 tensile, and not endowed with inherent powers of growth ; being like- 

 wise so disposed in the Podophthalma, as to inclose and protect the 

 greater part of the body by a few large shield-like plates, must needs 

 be thrown off' to allow of the growth of the animal. This moult of 

 the external integument has been observed in a few species of Crus- 

 tacea to take place annually ; and a like ecdysis is probably common to 

 all the class. Reaumur has given the best account of the process 

 in the craw-fish {Astacus Jiuviatilis) . It takes place generally in the 

 month of August. The animal previously retires to some place of 

 concealment, is quiet and fasts for a few days, during which time the 

 old calcified epiderm is loosened from the corium by the formation of a 

 new membranous layer beneath. As this begins to harden, the animal 

 takes measures to rid itself of its old crust ; it rubs its legs against each 

 other, throws itself upon its back, contracts and swells out the body, 

 with alternate violent inflections and intervals of rest. The carapace is 

 thus separated from the abdominal segments, is pushed upwards, and 

 the animal liberates its head with the eyes and antennae, which are 

 provided with new sheaths : then the more diflficult operation of freeing 

 its extremities takes place, the old solid coverings of which split length- 

 wise, and are shaken off. Finally, the crawfish creeps out of the re- 

 mainder of its old shell by withdrawing the abdominal segments ; 



M 4 



