382 PRIMITIVE RADIAL SYMMETRY OF ECHINODERMATA. 



and has not always the form of a complete ring. In Pilidium 

 proper (fig. 232 A) it is a simple ring surrounding the edge of 

 the oral disc. In Muller's larva of Thysanozoon (fig. 222 B) it is 



B 



FlG. 233. TWO STAGES IN THE DEVELOPMENT OF MlTRARIA. (After Metschnikoff.) 



m. mouth; an. anus; sg. supra-cesophageal ganglion; />/-. and b. provisional 

 bristles ; pr.b, proeoral ciliated band. 



inclined at an axis to the oral disc, and might be called praeoral, but 

 such a term cannot be properly used in the absence of an anus. 



FIG. 234. CYPHONAUTES (LARVA OF MEMBRANIPORA). (After Hatschek.) 



m. mouth ; a . anus ; f.g. foot gland ; jr. problematical body (probably a bud). 



The aboral apex is turned downwards. 



