CRUSTACEA. 271 



limb, and especially at the joints, and the animal 

 seems to be aware of the greater facility with which 

 a renewal of the claw can be effected at these parts ; 

 for if it chance to receive an injury at the extremity 

 of the limb, it often, by a spontaneous effort, breaks 

 off the whole limb at its junction with the trunk, 

 which is the point where the growth more speedily 

 commences. The wound soon becomes covered 

 with a delicate white membrane, which presents 

 at first a convex surface : this gradually rises to a 

 point, and is found on examination to conceal the 

 rudiment of a new claw. At first this new claw 

 enlarges but slowly, as if collecting strength for the 

 more vigorous effort of expansion which afterwards 

 takes place. As it grows, the membrane is pushed 

 forwards, becoming thinner in proportion as it is 

 stretched, till at length it gives way, and the soft 

 claw is exposed to view. The claw now enlarges 

 rapidly, and in a few days more acquires a shell 

 as hard as that which had preceded it. Usually, 

 however, it does not attain the same size, a circum- 

 stance which accounts for our frequently meeting 

 with Lobsters and Crabs which have one claw much 

 smaller than the other. In the course of the sub- 

 sequent castings, this disparity gradually disappears. 

 The same power of restoration is found to reside 

 in the legs, the antennae, and the jaws."* 



The Class Crustacea is divided into two great 

 sections, or Sub-classes, distinguished chiefly by the 

 texture of their covering. 



* Roget, Bridgewater Treatise, vol. i. p. 294. 



