CRUSTACEA. 215 



members are in some cases prehensile, in others ambulatory, 

 in others natatory, in others partially branchiferous, and 

 so on. The abdominal sometimes serve the purpose of swim- 

 ming, at others of bearing and protecting the eggs, at others 

 they are partially converted into branchiae. Besides the 

 modifications, some or other of them are, in many forms, 

 either wholly wanting or rudimentary/^ 



Exuviation. 



It is a peculiarity in this class of animals, that after they 

 have, like insects, passed through various metamorphoses, 

 or changes and conditions, — after they have arrived at their 

 adult and ultimate state, shape, and functions, — thei/ con- 

 tinue to increase in size; yet the hard, stony, shelly, or 

 horny covering with which they are invested does not in- 

 crease in size. There is no provision for its enlargement 

 by marginal additions to the plates of which it is formed, 

 such as obtain in the shells of molluscous animals. We 

 have heard of the beautiful lap-dog sold to a lady, which 

 was soon after found to be ill, and on the doctor being sent 

 for was relieved by ripping up ^ false shin in which he had 

 been invested. The poor wretch had, like the Crabs, groivn, 

 while his outer coat had oiot, and the doctor^s scissors were 



