272 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
the trichobranchia to the phyllobranchia will be very 
easily effected. 
The shrimp (Crangon) also possesses phyllobranchiz, 
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 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 length of the cephalothorax is much greater 
than its width, and it is produced in front into a long 
rostrum. The bases of the antenne 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 
