CH. VI THE CRAYFISH 347 



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 cefhalothora.x and a posterior abdomen 

 (Fig. 86, cth, ab). 



The cephalothorax is unjointed, and is covered by a 

 cuirassdike 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 cara- 

 pace is developed from the dor,sal 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 87, 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. 320), 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. 83, T), a narrow ventral region or stermnn (S), 

 and downwardly directed lateral processes, \k\% pleura (PL). 

 The seventh division of the abdomen is the telson (Fig. 86, t) : 

 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 Astacus is a con- 

 tinuous cuticular structure, discontinuously calcified so as 



