1890.] Annelid Descent. 1153 
In the digestive tract it is especially the origin of the pharyn- 
geal apparatus that seems to require explanation ; but even here 
the matter is pretty simple if we regard this as an originally cir- 
cular evaginable part of the foregut epithelium, provided with 
radiating muscle cells and covered by circular and longitudinal 
muscle layers, much as we still find in Annelids. Such a condi- 
tion may without difficulty be derived from the Turbellarian 
pharynx. But the armament of teeth and their retraction into 
special sacs of the pharynx are doubtless acquisitions of a later 
period in the phylogenetic history of our worm. 
But little is to be said concerning the phylogenetic develop- 
ment of the remaining regions of the digestive tract; yet this 
much is probable, that the Turbellarian-like ancestor of the 
Annelids had no such branched intestine as the present Planar- 
ians, which have acquired it along with the flattening and 
broadening of the body, but had a simple intestinal tube, as in 
the Nemerteans, ending posteriorly in an anus. 
As a direct corollary from the history of the Annelid body, 
given here in its general outlines, there results a very definite 
conception of the morphological signification of the mesoderm, 
as I have already stated in my above-mentioned communication. 
Thus, if in Annelids the peritoneal sacs, with all their deriva- 
tives, as well as the segmental cavities in them, are to be derived 
from the sexual glands of their ancestors, then their stages of 
development in ontogeny, the mesoderm somites and mesoderm- 
bands, and finally, to be consistent, also in general the secondary 
or coelomic mesoderm of all Metazoa which have it, must have 
the original signification of a sexual tissue or of gonads.” 
10 One of the best evidences would be furnished by a case in which the secondary 
mesoderm was entirely devoted to the formation of the sexual glands of the adult animal. 
Such a case seems actually presented, according to the account of S. F. Harmer, in the 
chus arise directly from the ectoderm by invagination, would furnish very strong evidence 
on the other side; yet I have convinced myself by my own observations upon the same 
animal that there is an error here, and that the organs mentioned are formed as usual 
from the peritoneum. 
