140 MARINE ANIMALS OF MASSACHUSETTS BAY. 



division of identical parts radiating from a common centre. A 

 comparison of the embryonic and adult forms in these orders, 

 confirms entirely this classification based upon structural features. 

 The Star-fishes, in their earlier stages, resemble the mature Ophi- 

 urans, while the Crinoids, the lowest group of all, retain through- 

 out their whole existence many features characteristic of the 

 embryonic conditions of the higher Echinoderms. In this prin- 

 ciple of classification, already so fertile in results, we may hope to 

 find, in some instances, the solution of many perplexing points 

 respecting the structural rank of animals, the confirmation of 

 classifications already established ; in others, an insight into the 

 true relations of groups which have hitherto been divided upon 

 purely arbitrary grounds. 



