CHAPTER II. 



Origin and structure of the head. 



Brain and cerebral ganglia. — As pointed out above, the 

 great and unsurmountable difficulty connected until now with 

 the Annelidan theory of the origin of Vertebrates is the 

 derivation of the head of the latter from that of Annelids. 

 If we homologize the central nervous system of Vertebrates 

 to that of Annelids and especially the brain of the former 

 to the cerebral ganglia, we find the difficulty before us that 

 we have to assume — as DOHRN did — that originally in the 

 region of the hindbrain or the medulla oblongata the central 

 nervous system must have been pierced by the gut, and 

 that the mouth had originally a dorsal position but after- 

 wards has atrophied and been replaced by a new, ventral, 

 mouth, opposite the old one. A last trace of the former 

 mouth passage of the gut through the nervous system was 

 viewed by DOHRN (1875, p. 3) in the fossa rhomboidea 

 between the crura cerebelli. 



Beard (1888, p. 22) imagined the cerebral ganglion ta 

 have got totally lost, he considered the ventral ganglion 

 chain of Annelids to be the homologue of the whole central 

 nervous system of Vertebrates, and found the rest of 

 the old mouth again in the hypophysis. MlNOT (1897) sug- 

 gested still another way of getting out of the difficulty by 



