CLYPEASTROIDEA 



543 



an iuterradius and the axis of symmetry passes through tlie 

 centre of one radius. We can thus distinguish an anterior group 

 of three radii, or " trivium," from a posterior pair or " bivium." 

 The madreporite lies in the left anterior interradius. The five 

 genitals and five oculars surround a dorso-central plate, which 

 covers the spot which in Endocyclica is occupied by the periproct. 



mouth 



amb. 



mmmim^'^' 



anus. 



Fig. 241. — Oral view of "Sand-dollar" {Eehinarachnms parma), with .s]iiiic 

 amb, Aniljulacral furrow. x 1. 



The whole test is covered with extremely short delicate spines, 

 which form a veh'ety felt-work, and are all of approximately the 

 same length ; they are of a brownish purple colour. The spines 

 on the dorsal surface are all ciliated, and these cilia cause a 

 current of fresh sea-water to How continually over the modified 

 tube-feet. Pedicellariae are scattered amongst the bases of the 



