272 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



the trichobranchia to the phyllohranchia will he very 

 easily effected. 



The shrimp (Ciangon) also possesses ph yllobra nchiae, 

 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 rlways 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 si xth som ite has very 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 lengthof thej^e phalothor ax 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 moyabl.e_exopodite. More- 

 over, the eye-stalks are notjuclosed in a cavity or orbit, 

 and the eyes themselves appear above and in front of 

 the antennules. The external in.a xillip .edes are narrow, 

 and their endopodi tes are more or less leg-like. 



None of these statements apply to the crabs. In these 



