CLYPEASTROIDEA 



543 



an interradius and the axis of symmetry passes through the 

 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 iu 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. 



ifpp^ 



anus. 



Fig. 241. Oral view of "Sand-dollar" (Echinarachnius parma) t with spines. 

 amb, Ambulacra! furrow, x 1. 



The whole test is covered with extremely short delicate spines, 

 which form a velvety 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 



