336 ELEMENTS OF COMPARATIVE ZOOLOGY. 



last reaches the adult condition in which it resembles closely 

 the parent. These metamorphoses at times give us clues 

 as to the past history of the group. Thus the larvas of 

 Echinoderms (p. 291) and the tadpoles of the Anura (p. 

 50) point to the fact that the first group has descended from 

 markedly bilateral ancestors, and that the radiate condition 

 of the adult has been secondarily acquired; while the his- 

 tory of the frog is evidence that these batrachians have 

 sprung from tailed water-breathing ancestors. In the In- 

 sects, on the other hand, the larval and pupal stages have 

 far less significance, but apparently have been introduced 

 into the history the better to adapt these forms to the 

 various conditions of their existence. 



