k 296 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



EXTERNAL CHARACTERS 



(PL I, figs. 1, 2, 3). 



The whole of the exterior of the animal is covered 

 b}^ a thick continuous chitinous exoskeleton or shell, 

 which is highly calcified except between the movable 

 somites of the abdomen and between the movable podo- 

 meres in the appendages. 



The body may be conveniently divided into an 

 anterior region — the Cephalon, a middle region — the 

 Thorax, and a posterior region — the Abdomen. As in 

 all the Decapoda the Cephalon and Thorax are fused to 

 form the Cephalothorax. 



The Cephalothorax is by far the largest portion of 

 the body, and is the only part seen from the dorsal 

 surface. The Abdomen is much reduced and is a flap- 

 like structure closely applied to the ventral region of 

 the cephalothorax between the bases of the walking legs. 



There is every reason to believe that the crabs and 

 their relatives have arisen from primitive Crustaceans 

 having a body divided up into a' number of movable 

 segments or somites. Extreme specialisation has taken 

 place, especially in the cephalothoracic region, and it is 

 in the Abdomen that one sees the nearest approach to 

 this primitive external segmentation. A careful exami- 

 nation, however, reveals the fact that there are five 

 somites in the cephalic region, eight in the thorax and 

 six in the abdomen — nineteen in all. 



Before entering on a description of the more complex 

 parts it will be useful to examine the structure of a 

 typical abdominal somite. 



The third abdominal somite of the female may be 

 taken as a type (see text fig. 1). 



This somite is flattened dorso-ventrally. On the 



