THE CRUSTACEA 



145 



simply membraneous. The last somite or joint, the true 

 " tail," is, however, calcined. 



The two last pairs of " walking " legs are aborted in size 

 and form, and are turned up, folded on themselves, over the 

 back. The large claw at the " wrist " bends, hingelike, in 



Fig. 58. The Common Hermit Crab. Pagurm Beruhardus. | Natural 

 size. Right - hand one ensconced in whelk shell ; left - hand cne 

 extracted. 



a lateral direction only ; other crustaceans move their claws 

 from this joint in an upward and inward direction. The 

 abdomen is curled towards the right, so as to fit into the 

 whorl of the shell which the crab makes its home, and the 

 side appendages of the tail or " tail fins," are developed 

 on one side into a hooklike arrangement, so as to hold on 

 to the central part (the columnella) of the shell. The 



