292 THE MECHANICAL FUNCTIONS. 



of the moUusca, has been denominated a crvst. 

 A soHd structure of this kind, as we have already 

 seen, does not admit of increase by the exten- 

 sion of its own parts: so that in order to allow 

 of the growth of the parts which it encloses, it is 

 necessary that it be cast off, and exchanged 

 for a new shell of larger dimensions. 



The process by which this periodical casting 

 and renewal of the shell are eifected, has been 

 very satisfactorily investigated by Reaumur. 

 The tendency in the body and in the limbs to 

 expand during gro\A^h is restrained by the 

 limited dimensions of the shell, which resists 

 the efforts to enlarge its diameter. But this 

 force of expansion goes on increasing, till at 

 length it is productive of much uneasiness to the 

 animal, which is, in consequence, prompted to 

 make a violent effort to relieve itself; by this 

 means it generally succeeds in bursting the shell ; 

 and then, by dint of repeated struggles, extricates 

 its body and its limbs. The lobster first with- 

 draws its claws, and then its feet, as if it were 

 pulling them out of a pair of boots : the head 

 next throws off its case, together with its anten- 

 nae ; and the two eyes are disengaged from their 

 horny pedicles. In this operation, not only the 

 complex apparatus of the jaws, but even the 

 horny cuticle and teeth of the stomach, are all 

 cast off along with the shell : and, last of all, the 

 tail is extricated. But the whole process is not 



