CRUSTACEA. 



325 



within the recesses of their selected abode, obtain a secure retreat, 

 which they drag after them wherever they go, until, by growing 

 larger, they are compelled to leave it in search of a more capacious 

 lodging. The wonderful adaptation of all the limbs to a residence 

 in such a dwelling cannot fail to strike the most incurious ob- 

 server. The chela, or large claws, differ remarkably in size ; 

 so that, when the animal retires into its concealment, the smaller 

 one may be entirely withdrawn, while the larger closes and guards 

 the orifice. The two succeeding pairs of legs, unlike those of 

 the lobster, are of great size and strength ; and, instead of being 

 terminated by pincers, end in strong pointed levers, whereby 

 the animal can not only crawl, but drag after it its heavy habit- 

 ation. Behind these locomotive legs are two feeble pairs, barely 

 strong enough to enable the soldier-crab to shift his position in the 

 shell he has chosen ; and the false feet attached to the abdomen 

 are even still more rudimentary in their deyelopement. But the 

 most singularly altered portion of the skeleton is the fin of the 

 tail, which here becomes transformed into a kind of holding ap- 

 paratus, by which the creature retains a firm grasp upon the bottom 

 of his residence. Fig. 154. 



In the Brachyura,) or Crabs, we have at once, in the concentra- 

 tion observable in all parts of the skeleton, an indication of its 



