ECHINODERMATA — SEA URCHINS 



165 



but slightly inclined to each other, the plates from the opposite 

 sides alternating, in the very large marginal plates and in 

 having two, instead of four, rows of tube-feet. 



1. Sketch specimen. 



2. Is it a dorsal or ventral view ? 



3. How does it diilfer from Asterias? 



^0m 



CLASS E, OPHIUROIDEA (BRITTLE STARS) 



Ambulacral and digestive systems confined to the central 

 disk; otherwise in external appearance much like the Aster- 

 oidea. Movement by means of 

 the slender, rounded arms which 

 are quite sharply marked off 

 from the central disk. No anal 

 opening present. Madreporite 

 ventral. 



The range of the class is from 

 the Ordovician to the present. 

 Living forms are more com- 

 monly inhabitants of deep than 

 of shallow waters. Ophiopholis 

 aculeata (Fig. 65), the ''brittle 

 starfish," is abundant at a 

 depth of one to one hundred 

 feet in rocky crevices from the coast of New Jersey to the Arctic 

 Ocean and is likewise common on the Pacific coast and in north- 

 ern Europe. 



CLASS F, ECHINOIDEA (SEA URCHINS) 



Free-mo\ing animals, usually with a globular, or disk-shaped, 

 body. Skeleton {test) rigid, formed of firmly united calcareous 

 plates, or more or less flexible with imbricating plates, covered 

 with movable spines. Mouth ventral and central, or anteriorly 

 eccentric, with (except in the adult spatangoids) a compHcated 



Fig. 65. — Ophiopholis aculeata Gray, 

 from the coast of New England. The 

 brittle starfish. Dorsal view (xi). 

 (From Verrill and Smith.) 



