POLYZOA. 



559 



meet in front and behind, but gape ventrally. At the aboral apex 

 there is an opening between the two valves, through which a ciliated 

 disc (ob) projects. 



In the larva of Alcyonidium mytili (Fig. 447) we find all tl>e parts 

 present in Cyphonautes, except the bivalve shell. But it must be 

 noted that the alimentary canal is partly aborted, there being no 

 rectum or anus. 

 Also the aboral 

 structure (r) which 

 corresponds to the 

 ciliated disc is 

 larger, and sur- 

 rounded by a 

 groove called the 

 mantle-groove (^>). 



The larva of 

 Flustrella is inter- 

 mediate between 

 the above forms; 

 it has the shell 

 plates but not the 

 complete alimen- 

 tary canal. 



Finally, in some 

 Cheilostomata the 

 specialisation of 

 the larva is carried 

 still further in the 

 great development 

 of the ciliary circle, 

 and in the absence 

 of an alimentary 

 canal. In these 

 larvae (Fig. 448) 

 there is a pyri- 

 form organ (o) 



FIG. 449. Metamorphosis of the larva of Schizoporella (?) uni- 

 cornis (after Barrois, from Korschelt and Heider). A, first 

 stage of the metamorphosis, showing the evagination of the 

 adhesive organ s. B, next or so-called umbrella stage ; the 

 velum has spread out and become bent towards the sucker, 

 giving the umbrella shape, c cells of velum ; o pyriform organ ; 

 p mantle-groove ; r ciliated disc ; s evaginated sucker ; v vesti- 

 bule or concavity of umbrella the edge of the umbrella 

 eventually applies itself to the sucker, and the inner walls of 

 the umbrella fuse and disappear ; x unknown organ. 



with its tuft of 



cilia; an adhesive organ (s), an aboral, ciliated disc (r) surrounded 

 by a mantle-groove (p\ but there is no trace of alimentary canal, 

 mouth, or anus. 



After a certain duration of free life, the object of which is to 



