PHYLUM ECHINODERMATA 213 



one species with the spermatozoa of another species, and the 

 fertilization of enucleated fragments of sea-urchins' eggs with 

 spermatozoa of another species. 



9. THE POSITION OF ECHINODERMS IN THE ANIMAL KINGDOM 



Echinoderms and ccelenterates, because of their radial sym- 

 metry, were at one time placed together in a group called RADIATA. 

 The anatomy of the adult and the structure of the larvae, how- 

 ever, show that these phyla really occupy widely separated posi- 

 tions in the animal kingdom. The adult echinoderms cannot 



Blastoidea 

 Echinoidea Holothurioidea Cystoidea Crinoidea 



Ophiuroidea Protechinoidea Carpoidea 



\ X X 



Asteroidea Protopelmatozoa (Thecoidea ?) 



First Fixed Ancestor 



t 



Protoccelomata 



FIG. 152. Diagram showing the probable relations of the classes of 

 Echinoderms. (After MacBride.) 



be compared with any other group of animals, and we must look 

 to the larvae for signs of relationship. The bilateral larva is 

 either a modification for a free-swimming life or an indication 

 of the condition of its ancestors. The latter view is accepted 

 by most zoologists. The ancestors of echinoderms were doubt- 

 less bilateral, worm-like animals which became fixed and were 

 then modified into radially symmetrical adults. The probable 

 relations of the classes of echinoderms are shown in Figure 152 

 (MacBride). 



