PHYLUM ECHINODERMA 175 



oral shield and two smaller adoral shields. One of the oral shields 

 becomes modified to form the madreporite. 



Development involves a bilaterally symmetrical larva (Fig. 

 85) known as the pluteus or, better, the ophiopluteus. 



Ophiura, Ophiopholus, and Amphioplus are typical genera of 

 brittle stars. Astrophyton is the basket star with its finely 

 divided arms. 



Class Echinoidea 



The sea urchins and sand dollars are usually globular, hemi- 

 spherical, or disc-shaped. The shape, which is unalterable in any 

 given species, is determined by the arrangement of the skeletal 

 plates. These are immovably united to form a firm shell or test. 



Fig. 85. — Ventral view of young ophiopluteus of Ophiothrix fragilis. {Redrawn 

 from MacBride, courtesy of MacmiUan Co.). 



Spines usually cover most of the test except at the oral and aboral 

 poles. Surrounding the mouth, there is a circular opening where 

 the plates are replaced by a membrane termed the peristome. 

 Normally, the anus occurs at the pole opposite the mouth in a 

 region called the periproct, while in some instances it occurs on 

 the margin of the disc. The skeletal plates are arranged in 

 meridional bands part of which bear openings through which the 

 ambulacral feet protrude and are therefore termed ambulacral 

 areas. The non-perforated plates between two adjacent ambu- 

 lacral areas are designated as an interambulacral area. 



Each ambulacral area terminates at the periproct in a single 

 ocular plate homologous to the ocular plate at the end of each 

 arm in the Asteroidea. A series of genital plates alternate with 

 the ocular plates around the periproct, and each marks the 

 termination of an interambulacral area. One of the genital 

 plates is modified to serve as a madreporite. 



In its fundamental arrangement, the water-vascular system is 

 essentially like that described for the asteroids. From the 

 madreporite, the stone canal leads into the circumesophageal 



