CHAP, vn THE CRAYFISH 361 



firmly united with one another i.e., have undergone 

 concrescence. The result of this fusion of the segments is 

 that two distinct regions can be distinguished in the 

 body an anterior cephalothorax and a posterior abdomen 

 (Fig. 93, cth, ab). 



The cephalothorax is unjointed and is covered by a 

 cuirass-like structure, the carapace ; and the abdomen is 

 divided into distinct segments, movable upon one 

 another in a vertical plane. The cephalothorax is 

 divided into two regions, an anterior the head, and a 

 posterior the thorax, by a transverse depression, the 

 cervical groove. The carapace is developed from the 

 dorsal and lateral regions of both head and thorax ; it is 

 free at the sides of the thorax, where it forms a flap or gill- 

 cover (Fig. 94, kd) on each side, separated from the actual 

 body- wall by a narrow space in which the gills are contained. 



The limbs spring from the ventral surface. Both 

 trunk and appendages are covered with a sort of shell 

 formed of chitin (p. 328), strongly impregnated with 

 carbonate of lime in most parts, so as to be hard and 

 but slightly elastic. 



The abdomen is made up of seven segments : the first 

 six of these are to be considered as metameres in the 

 sense in which the word is used in the case of the 

 earthworm. Each has a ring-like form, presenting a 

 broad dorsal region or tergum (Fig. 90, T), a narrow 

 ventral region or sternum (S), and downwardly directed 

 lateral processes, the pleura (PL). The seventh division 

 of the abdomen is the telson (Fig. 93, /) : it is flattened 

 horizontally and divided by a transverse groove into 

 anterior and posterior portions. All seven segments 

 are calcified and are united to one another by uncalcified 

 articular membranes : the first segment is similarly joined 

 to the thorax. Thus the exoskeleton of the crayfish is 

 a continuous cuiiicular structure, discontinuously calci- 



