320 FORMATION OF THE LAYERS. 



covered with cilia, now assumes more or less the form of a cone, 

 at the apex of which is the anus, while the base forms the 

 rudiment of a large prae-oral lobe. The alimentary sack grows 

 forwards and then bends upon itself nearly at right angles, and 

 meets a stomodaeal invagination from the ventral side some way 

 from the front end of the body. 



The alimentary canal soon differentiates itself into three 

 regions (i) oesophagus, (2) stomach, and (3) intestine. With 



FlG. 139. TWO STAGES IN THE DEVELOPMENT OF SfiRPULA. (After StOSsich.) 



m. mouth ; an. anus ; al. archenteron. 



these changes the larva, which in the meantime becomes hatched, 

 assumes the characters of a typical Annelid larva (fig. 139 B). 

 In front is a large prae-oral lobe, at the sides of which the eye- 

 spots soon appear. The primitive segmentation cavity remains 

 as a wide space between the curved alimentary tract and the 

 body walls, and becomes traversed by muscular fibres passing 

 between the two. The original chorion appears to serve as 

 cuticle, and is perforated by the cilia. 



The further changes in this larval form do not present features of 

 general importance. A peculiar vesicle, which in anomalous cases is 

 double, is formed near the anus. If it were shewn to occur widely 

 amongst Chaetopoda, it might be perhaps regarded as homologous with 

 the anal vesicles of the Gephyrea. 



Serpula is one of the few Chaetopoda at present known in 



