CLASS OF CBUSTACEA. 445 



into a mass covered by a large carapace or case (Fig. 431) ; 

 this dorsal buckler advances in general more or less in front, 

 descends on each side to the base of the limbs, and backwards 

 as far as the origin of the abdomen (Figs. 418, 421). It results 

 from this arrangement that we can no longer recognise 

 throughout all this part of the body any trace of an annular 

 division ; but beneath, most of the rings, although united 

 together, are still recognisable, and leave at their points of 

 junction the lines of suture, more or less distinct. The eyes 

 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 (Fig. 424), and in general they may be folded or withdrawn 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 



Fig. 431. Crabe Tourteau (C. Pagurus). 



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 pre- 

 hension (Fig. 431). In the decapoda, the best adapted for 

 swimming (such as the craw-fish, the lobster, the langouste, 

 and the palemons), the body is elongated, and the abdomen 

 terminated by a large transverse fin (Fig. 418) ; whilst in 



