1885.] Affinities of Annelids to Vertebrates. 769 
Semper finds an identity in fundamental plan between annelid 
and vertebrate. Though establishing so many homologies be- 
tween the two groups, he does not derive one from the other, but 
inclines to the view that both are descendants of a segmented 
planarian-like ancestral form which on the one hand gave rise to 
the vertebrate group by the enlargement of its anterior ganglion 
to form a brain and by the union of its separate nerve cords along 
the dorsal side to form the spinal cord, while on the other hand 
the annelid was formed by the acquirement of a new mouth 
piercing the nerve cord or passing between the still separate 
nerve cords and by the closure of the primitive mouth. 
Dohrn’s views on the affinities of the annelids to the verte- 
brates are very interesting, if perhaps of less importance as being 
purely hypothetical. 
$ D 
— 
V 
Fic. 3.—Diagram i vertebrate according 2 Dohrn. M, mouth; ‘A, anus; M, 
primitive mouth ; D, dorsal, V, ventral surfac 
M` V 
D A. 
Fic. 4.—Diagram of an annelid according to Dohrn. M, mouth; A, anus; D, 
dorsal, H ventral surface. 
Homologizing the dorsal surface of one group with the ventral 
of the other, he finds the necessity of supposing a new mouth to 
have existed in one group, and unlike Semper regards the verte- 
brate as the form in which this new structure was formed (Figs. 
3, 4). Seeking a trace of the primitive mouth of the vertebrate 
homologous with the mouth of the annelid, which is on the neu- 
ral side, he finally concluded that the fossa rhomboidalis of the 
vertebrate represented the area of the nervous system formerly 
pierced by the cesophagus on its way to a neural mouth, there 
being thus a circum-cesophageal ring formed homologous with 
that of the annelid. The present mouth of the vertebrate was 
thus a new formation having no homologue among the annelids. 
