CRUSTACEA. 291 



facility the crab finds in walking side-ways, 

 which it can do with great quickness when 

 urged by a sense of danger. 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, Avhich are attached to the 

 under side of the body, behind the true feet, and 

 which terminate in a tin-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 terminated by a num- 

 ber of flat 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 com- 

 position. They contain, however, some phos- 

 phate of lime, in addition to the carbonate. The 

 calcareous particles are deposited on a membrane 

 of considerable firmness ; and they together com- 

 pose a dense, but thin and fragile structure, 

 which, in order to distinguish it from the sliells 



