86 



PRIMITIVE ANIMALS 



Fig. 19. Diagrams representing the derivation of the Vertebrate (C) 

 from the Annelid (A) through the intermediate stage (B). In A 

 there is a prostomium pr. containing a dorsally situated brain b., 

 connected by circum-oesophageal commissures passing round the 

 mouth m. with the ventral chain of ganglia v.0. 1 , v.g.' 2 , etc. 

 The mesodermal somites s. 1 are not concentrated in head region. 



