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 



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

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

 bristles ; pr.b. prasoral 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 ; x. problematical body (probably a bud). 



The aboral apex is turned downwards. 



