272 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



the tricliobranchia to tlie pliyllobrancliia will be very 

 easily effected. 



The shrimp {Crangon) also possesses phjdlobranchise, 

 and differs from the prawn chiefly in the character of its 

 locomotive and prehensile thoracic limbs. 



There are yet other very well-known marine animals, 

 which, in common appreciation, are always associated with 

 the lobsters and crayfishes, although the difference of 

 general appearance is vastly greater than in any of the 

 cases which have yet been considered. These are the 

 Crabs. 



In all the forms we have hitherto been considering, 

 the abdomen is as long as, or longer than, the cephalo- 

 thorax, while its width is the same, or but little less. 

 The sixth somite has yery large appendages, which, 

 together with the telson, make up a powerful tail-fin ; 

 and the large abdomen is thus fitted for playing an 

 important part in locomotion. 



Again, the length of the cephalothorax is much greater 

 than its width, and it is produced in front into a long 

 rostrum. The bases of the antennae are freely movable, 

 and they are provided with a movable exopodite. More- 

 over, the eye-stalks are not inclosed in a cavity or orbit, 

 and the eyes themselves appear above and in front of 

 the antennules. The external maxillipedes are narrow, 

 and their endopodites are more or less leg-like. 



None of these statements apply to the crabs. In these 



