74 ECHINODERMATA. ECHINOIDEA. 



dorsal surface of the starfish, and the corona the ambula- 

 cral surface. 



b 



FIG. 19. A. Diagram of the upper surface of a regular echinoid, with 

 the tubercles and spines omitted, a, ambulacral areas; b, inter- 

 ambulacral areas; p, pores in the ambulacral plates. B. Apical 

 disc of Echinus esculentus, Recent, a, anus ; p, periproct; g, genital 

 plates, each with a pore; m, madreporic plate; o, ocular plates, 

 each with a pore. 



In a typical regular echinoid (e.g. Echinus) the anus is 

 placed within the apical disc (fig. 19), which then consists 

 of the following parts. At or near the centre is the 

 anus (a), this is surrounded by a membrane covered with 

 scaly plates and known as the periproct {p}. The peri- 

 proct is encircled by a ring formed of ten plates, five 

 called genital (g} and five ocular (o). The genital plates 

 form the inner part of the ring, they are often more or 

 less hexagonal in outline, and are provided with a pore 

 which serves as the opening for the genital ducts. In 

 some cases one or more of the plates is imperforate, and 



