20S THE MECHANICAL FUNCTIONS. 



wards, the lobster can only swim backwards; and it is as- 

 sisted in this action by five pair of false feet, which are at- 

 tached to the under side of the body, behind the true feet, 

 and w^hich terminate in a fin-shaped expansion, giving them 

 the effect of oars. The extremity of the tail is still more 

 expressly formed for giving effect to the stroke, being ter- 

 minated by a number of fiat scales, which, when expanded, 

 present a broad surface to the water. 



The calcareous coverings of these Crustacea are analogous 

 to shell both in structure and composition. They contain, 

 however, some phosphate of lime, in addition to the carbo- 

 nate. The calcareous particles are deposited on a membrane 

 of considerable firmness; and they together compose a dense, 

 but thin and fragile structure, which, in order to distinguish 

 it from the shells of the mollusca, has been denominated a 

 crust. A solid structure of this kind, as we have already 

 seen, does not admit of increase by the extension 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 ofi', and ex- 

 changed for a new shell of larger dimensions. 



The process by w^hich this periodical casting and renewal 

 of the shell are effected, has been very satisfactorily investi- 

 gated by Reaumur. The tendency in the body and in the 

 limbs to expand during growth 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 ani- 

 mal, 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, to- 

 gether with its antennas; 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 



