488 ZOOLOGY. 



are always carried on the extremities of a pair of moveable 

 appendages which spring from the first segment of the head ; 

 sometimes the length of their peduncle is very considerable 

 (Fig. 510), and in general they may be folded or withdrawn 

 into the cavities performing the office of orbits, and which are 

 formed by the anterior margin of the carapace, shell, or case. 

 The organs of locomotion are also very well developed in these 

 Crustacea: several can run with extreme rapidity, others 

 swim still more swiftly. Their limbs, as we have already 

 said, are five pairs in number, fixed to the five last rings of 

 the thorax ; but in general those of the four last pairs alone 

 serve for locomotion, and those of the first pair terminating 

 in a forceps more or less perfect, become instruments of 

 prehension (Fig, 517). In the decapoda the best adapted 

 for swimming (such as the cray-fish, the lobster, and the 

 pabemons), the body is elongated, and the abdomen ter- 

 minated by a large transverse fin (Fig. 504) ; whilst in 

 those which are formed for running, the crabs, for example, 

 the abdomen is very short, has no terminal fin, and is curved 

 under the thorax. 



Fig. 517. Common edible Crab ; C. Pagurus. 



574. The stomapoda have also the eyes carried on move- 

 able peduncles, the thorax covered entirely or partly by a 

 carapace, and the limbs cylindrical ; but their branchiae are 

 not enclosed in the cavities of the thorax, but float nnder the 

 abdomen, or are altogether wanting. The squilla (Fig. 515), 

 of which we have already spoken, belongs to this order. 



575. In the division edriophthalmia, the head is distinct 



