294 THE MECHANICAL FUNCTIONS. 



increase of one-fifth of its former dimensions. 

 When the animal has attained its full size, no 

 operation of this kind is required, and the same 

 shell is permanently retained. 



A provision appears to be made, in the inte- 

 rior of the animal, for the supply of the large 

 quantity of calcareous matter required for the 

 construction of the shell at the proper time. A 

 magazine of carbonate of lime is collected, pre- 

 vious to each change of shell, in the form of two 

 rounded masses, one on each side of the stomach. 

 In the crab these balls have received the absurd 

 name of crab's eyes; and during the formation 

 of the shell they disappear. 



It is well known that when an animal of this 

 class has been deprived of one of the claws, that 

 part is in a short time replaced by a new claw, 

 which grows from the stump of the one which 

 had been lost. It appears from the investigations 

 of Reaumur, that this new growth takes place 

 more readily at particular parts of the 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 sponta- 

 neous 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 



