vm PHYLUM MOLLUSCOIDA 367 



the presence in both of larval forms which may very well be looked 

 upon as modified trochophores. 



The setae of Brachiopods, sunk in muscular sacs, are marks of 

 annulate affinities, since such organs are found elsewhere only 

 among Chaetopoda and Gephyrea (Sect. X.). The form of the 

 larva tells in the same direction, the eye-bearing head region 

 or prostomium and the provisional seise being very striking charac- 

 ters. But the segmentation of the Brachiopod is quite different 

 from that of the annulate larva, in which new segments are always 

 added behind those previously formed, and in which metamerism 

 always affects the rnesoderm. 



