LARVAL FORMS. 



315 



than to the Echinoderm larva. Mitraria amongst the Chsetopods 



jurb 



:B 



(/ 



FIG. 233. Two STAGES IN THE DEVELOPMENT OF MITKAKIA. (After Metschuikoff.) 



HI. mouth; an. anus; sfj. supra-cesophageal ganglion; br. and I. provisional 

 bristles; pr.b. praeoral ciliated band. 



(fig. 233) has, indeed, nearly the form of a Pilidiuni, and mainly 

 differs from a Pilidiuni in the possession of an anus and of provisional 

 bristles; the same may be said of Cyphonautes (fig. 234) amongst 

 the Polyzoa. 



The existence of these two forms appears to shew that the prae- 

 oral ciliated ring of the 

 Trochosphere may very 

 probably be derived directly 

 from the circumoral ciliated 

 ring of the Pilidiuni ; the 

 other ciliated rings or 

 patches of the Trochosphere 

 having a secondary origin. 



The larva of the Brachi- 

 opoda (fig. 220), in spite of 

 its peculiar characters, is, 

 in all probability, more 

 closely related to the Chae- 

 topod Trochosphere than to 



any other larval type. The 

 most conspicuous point of 

 agreement between them is, 



1 - . 



however, the possession in 



common of provisional setae. 



FIG. '234. CYPHONAUTES (LAKVA OF MEMBHA- 

 NIPORA). (After Hatschek.) 



HI., mouth; '. anus; f.f/. foot gland; x. pro- 

 blematical body (probably a bud). 



The aboral apex is turned downwards. 



