ECHINOIDA. 



289 



ORDER II. CLYPEASTROIDEA (Sand-cakes). 



In the "Sand-cakes" and "Sand-dollars" we have 

 urchins in which the test is disc-shaped and the ambulacra 

 are confined to the upper surface. The mouth is in the 

 centre of the lower surface; the vent is on the margin 



a A 



FIG. 128. .4, oral, and B, aboral surfaces of Sand-dollar (Echinarachnius). 

 a, vent; 0, genital pores; z, ambulacral areas; m, madreporite ; o, 

 mouth. 



of the disc, or near the margin on the lower surface. It is 

 interradial in position. In a few of the sand-cakes the 

 margin of the disc is notched, while in others there may 

 be perforations extending through from upper to lower 

 surface. 



ORDER III. SPATANGOIDS (Heart-urchins). 



In these the body, flat below, arched above, has a heart- 

 shaped outline, and both mouth and vent are eccentric in 

 position upon the lower surface. The ambulacra are all on 

 the upper surface, but the anterior row is lacking. 



