LARVAL FORMS. 315 



than to the Echinoderm larva. Mitraria amongst the Chsetopods 



jtrT) 



Jir.6 -'9. ^ 



Fig. 233. Two stages in the development of Mitraria. (After Metschnikoff.) 

 ?«. mouth; an. anus; sg. supra-oesophageal ganglion; hr. and h. provisional 

 bristles; 'pr.h. prasoral ciliated band. 



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

 differs from a Pilidium 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 circnmoral ciliated 

 ring of the Pilidium ; 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 ^^^: 234. Cyphonautes (larva of Membra- 



/i . ^,1 . nipora). (After Hatscnek.) 



agreement between them IS, \x. < f f^^+ «i„r,/i. ^ „r,^ 



,° , . .' m. mouth; a. anus; /.//. foot gland; x. pro- 



however, the possession m blematical body (probably a bud), 

 common of provisional setae. The aboral apex is turned downwards. 



