1907.] NATURAL SCIENCES OF nilLADELPIIIA. 551 



of these platodes corresponds to that of annehds and mollusks. These 

 facts must tend to confirm the view that in their early history these 

 platodes were closely related to the two last mentioned phyla. 



On the other hand, in the development of the entire alimentary 

 canal from a portion of the mesentoblast, 4.d, and in the consequent 

 degeneration of the "macromeres" and of the remaining cells of the 

 fourtii quartet, this polyclad is unique. This peculiar development of 

 the alimentary tract shows that the cleavage of the polyclads, while 

 closely resembling that of other groups, is not a generalized t}'pe. 



