116 



ECHINODERMA. ECHINOIDEA 



a line drawn from the summit to the centre of the base. 

 In the regular echinoids both anus and mouth are central 

 — being placed at opposite poles of the test ; in the 

 irregular echinoids the anus is always, and the mouth 

 often, excentric. In the test we may distinguish three 

 parts : a small patch of plates placed at the summit, known 

 as the apical disc or apical system ; a series of plates 

 surrounding the mouth ; and the remainder of the test 

 termed the corona. 



o— 



,-m 



--o 



g- 



Fig. 46. 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, peri- 

 proctal membrane with small plates ; g, genital plates, each with a 

 pore ; m, madreporic plate ; o, ocular plates, x 1-^. 



In a typical echinoid of the regular group (e.g. Echinus) 

 the anus is placed within the apical disc (fig. 46 B), which 

 then consists of the following parts. Near the centre is 

 the anus (a), which is surrounded by a membrane covered 

 with scaly plates and known as the periproct (p). The 



