262 THE MECHANICAL FUNCTIONS. 



satisfactorily investigated by Reaumur. Tlie ten- 

 dency in the body and in the Hmbs to expand 

 during growth is restrained by the limited dimen- 

 sions of the shell, which resists the efforts to enlarge 

 it. But as this force of expansion goes on increas- 

 ing, it is at length productive of much uneasiness 

 to the animal, which is, in consequence, prompted 

 to make a violent eftbrt 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 withdraws 

 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 antennae ; 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 accomplished without long- 

 continued efibrts. Sometimes the legs are lacerated 

 or torn off, in the attempt to withdraw them from 

 the shell ; and in the younger Crustacea the opera- 

 tion is not unfrequently fatal. Even when success- 

 fully accomplished, it leaves the animal in a most 

 languid state : the limbs, being soft and pliant, are 

 scarcely able to drag the body along. They are 

 not, however, left altogether without defence. For 

 some time before the old shell was cast off, pre- 

 parations had been making for forming a new one. 

 The membrane which lined the shell had been 

 acquiring greater density, and had already col- 

 lected a quantity of liquid materials proper for the 

 consolidation of the new shell. These materials 



