CRUSTACEA. \\ 



in fact, appears to be the centre around which the 

 cephalic organs are concentrated. We can readily 



understand that the hunt for food might tend, in 

 the course of many generations, to bring into activity 

 more and more of the appendages in the vicinity of 

 the mouth, and that this might result, not only in a 

 forward concentration of these appendages, hut also 

 in such a modification of their structure as would lit 

 them to he more useful servants of the mouth. Yet 

 this effort alone would not he sufficient to account for 

 that exceeding concentration of structure observable m 

 the highest Crustacea. Other effects must he taken 

 into consideration, namely, those which would arise from 

 efforts to walk. It is evident this desire could not be 

 gratified without throwing more work on those append- 

 ages which were best suited by their position and struc- 

 ture to bear the weight of the body. The enlarge- 

 ment and increased strength which would surely follow 

 would bring the forward part of the body into greater 

 use, and would render the swimmerets and the abdo- 

 men, which are so powerful and necessarily large in a 

 swimming animal, less and less functionally useful, and 

 cause them to decrease in size in proportion as the 

 animal became more and more a walking type. In 

 order that a swimming Crustacean should change into 

 a walking form, the two ends of the body must be 

 shortened, or, in other words, there must be a con- 

 centration forwards of the posterior part, and a con- 

 centration backwards of the anterior portion, so that 

 the centre of gravity may be brought into proper rela- 

 tion to the bases of support. 



This is admirably illustrated by the crabs, as will be 



