770 Affinities of Annelids to Vertebrates. [August, 
The genetic relationship of the two groups he conceives to 
have been that of diverging branches from a common ancestor. 
This hypothetical ancestral form was a segmented annelid-like 
creature with a chorda, gills on all the segments, supported by a 
cartilaginous rod on the side of each segment, with a neural 
mouth like that of an annelid and with a pair of segmental 
organs in each segment opening to the exterior and also inter- 
nally into the digestive tract. From this ancestor the vertebrates 
arise by an interesting series of modifications in structure due to 
“change of function” of the primitive structures; some of the 
segmental organs gave rise to the mouth and branchial clefts, 
while others formed the genital and urinary ducts and even the 
anus; the branchiz were variously modified to form the branchie 
of the vertebrate, its fins and limbs and perhaps the external gen- 
ital organs ; the cartilaginous supporting rods gave rise to the 
ribs, skeleton of the limbs and part of the skull. 
This hypothetical ancestor was more highly specialized than 
the present annelids, which have since degenerated, perhaps (he 
thinks), owing to the sharp competition between the more annelid- 
like and the more vertebrate-like forms of the common ancestral 
group. The whole tenor of Dohrn’s work inclines to the conclu- 
sion that annelids, like ascidians, etc., are degenerate vertebrates. 
However satisfactory in explaining an origin of the vertebrates 
from an hypothetical ancestor this hypothesis may or may not 
be, it apparently is unsatisfactory in so far as it concerns the 
annelids. 
Much of the evidence in support of Hatschek’s views is derived 
_ from a consideration of the development of annelids, both of 
. 
Polychztz and of Oligochetze. 
His comparison of the two groups, annelids and vertebrates, is 
essentially that of Dohrn’s (Figs. 3, 4) except that he does not 
place the point where the primitive cesophagus pierced the ner- 
vous system at the fossa rhomboidalis but at the hypophysis cerebri, 
a position more comparable to the present position of the mouth 
among the annelids, as it is farther forward, in the unsegmented - 
Portion of the head. He reduces the excretory organs in both 
groups to a common type represented by a transient stage in the 
2 larva of certain annelids when the branched excretory tube in the 
head ofa Trochophora larva sends back a branch into the trunk, 
hos apnd tube sie rise to = segmental tube in each 
