CKUSTAtKA. 261 



The Lobster, on the contrary, is better formed for 

 swimming than for walking. The hinder part of 

 its body is divided into segments, which play upon 

 each other by a remarkable kind of mechanism, 

 the margins of each portion overlapping the suc- 

 ceeding segment, and partly enclosing it. The 

 tail is the principal agent used in swimming, and 

 the whole force of the muscles is bestowed upon its 

 movements. As it strikes the water from behind 

 forwards, the lobster can only swim backwards ; 

 and it is assisted in this action by five pair of false 

 feet, which are attached to the under side of the 

 body, behind the true feet, and which terminate in 

 a tin-shaped expansion, acting like an oar. The 

 extremity of the tail is still more expressly formed 

 for giving effect to the stroke, being terminated by 

 a number of f^at 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 composi- 

 tion : they contain, however, some phosphate of 

 lime, in addition to the carbonate. The calcareous 

 particles are deposited on a membrane of consider- 

 able tirnniess; 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 off, and 

 exchanged for a new shell of larger dimensions. 



The process by which this periodical casting and 

 renewal of the shell are clfected, has been verv 



