No. 19.] 



ECHINODERMS OF CONNECTICUT. 



103 



symmetrical, with both mouth and anus, and is known as a pluteus 

 (Fig. 16). It bears absolutely no resemblance to the adult 

 urchin. The free-swimming pluteus lives and feeds at the sur- 

 face for a period of several weeks, the time depending some- 

 what on the temperature and other conditions, and varying in 

 different species. In this time it may have been carried by cur- 

 rents and tides to a considerable distance from its place of origin. 



Fig. 17. Arbacia punctulata. Side 

 view of young urchin immediately after 

 the metamorphosis. The spines re- 

 maining from the pluteus are in process 

 of absorption. (After Brooks.) 



When ready to assume the adult form, the pluteus settles to 

 the bottom, absorbs its larval organs, and by a complicated meta- 

 morphosis assumes the condition of a young urchin (Figs. 17 

 and 18). The young urchin differs from the adult in having 

 the whole upper surface covered by the plates which will event- 

 ually form only the periproct of the adult, and by the relatively 

 enormous tube-feet. The animal is now radially symmetrical, and 

 creeps about on the sea bottom after the manner of the adult 

 urchin. It feeds upon diatoms and other minute organisms until, 



