282 



SYSTEMATIC ZOOLOGY. 



the lower surface; the vent is on the margin of the disc, 

 or near the margin on the lower surface. It is interradial 

 in position. Comparisons with forms like these, or better 



a A a 



FIG. 96. A, oral, and B, aboral surfaces of sand-dollar (Echinarachnius). a, 

 vent; g, genital pores; i, ambulacral areas; m, madreporite; o, mouth. 



with Spatangoids (infra), show why the odd or unpaired 

 ray of a starfish is called the anterior ray (see Laboratory 

 work). In a few of the sand-cakes the margin*of the disc 

 is notched, while in others there may be perforations ex- 

 tending 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. 



CLASS V. HOLOTHURIDEA (SEA-CUCUMBERS). 



The Holothurians are cylindrical Echinoderms, with 

 mouth and vent at the ends of the body, and usually with 

 the ambulacra scattered over the surface in such a way as 

 to make the comparison with a cucumber most apt. Around 



