372 



THE POPULAR SCIENCE MONTHLY. 



traced to its adult form ; and we have the converse of the case of en- 

 toconcha where the true affinities of a greatly modified adult are shown 

 by its younger stages. 



I give four figures to illustrate one such instance, which is not by 

 any means exceptional or extreme ; and in certain groups of animals, 

 such as the Crustacea, such cases are quite numerous. 



Fig. 5 is a magnified side-view of a crustacean, Sergestes, which 

 is not quite full grown, but still essentially like the adult ; and Fig. 6 



Fig. 



is a similar view of a closely related form, Lucifer, in a similar stage 

 of development. Their close relationship is obvious at a glance, and 

 their resemblances, which are much more conspicuous than their dif- 

 ferences, are rendered more obvious by careful study ; but the case is 



