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wards has atrophied and been replaced by a new, ventral, 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 gangfion to - 
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. MINOT (1897) sug- 
gested still another way of getting out of the difficulty by 
