1146 The American Naturalist. [December, 
At the period when the mesoderm-bands are dividing into 
segments the mesenchym elements have so far increased through- 
out the entire trunk as to fill out completely the space between 
ectoderm and entoderm, not occupied by those bands. At the 
same time a portion (subsequently the septal and mesenterial 
muscles) penetrates between the mesodermal segments, thus 
leading to the division of the mesoderm-bands. This fact is also 
to be observed, that the external follows the internal segmenta- 
tion of the body. At this stage in development, the similarity 
between the mesodermal structures of the young Annelid and 
those of an adult Turbellarian cannot be doubted. In the latter, 
as in the former, we find between the skin and the intestine cell 
aggregates, either solid or becoming hollowed out; in the latter the 
sexual organs, in the former the mesoderm somites (while in both 
the head region remains free from such structures)! The struc- 
tures in Annelids and Turbellarians here compared are, in my 
opinion, really genetically connected. All the mesenchym struc- 
tures in the Annelid, both in larval and adult stages, may be 
compared with quite similar structures in the parenchyma of the 
 Turbellarian; the paired, metameric, peritoneal sacs, arising from 
the mesoderm somites, enclosing the body-cavity and producing 
the sexual products in definite areas, may be regarded as sexual 
follicles, with much enlarged cavities and manifoldly differentiated 
walls.? 
In place of a more detailed demonstration, I will here give a 
preliminary sketch of my views on the phylogenetic development 
of the Annelid, such as I gave at the last (eighth) Congress of 
Naturalists, at St. Petersburg, January, 1890. 
*In the Annelid the cephalic lobe d t ofits own, . 
but, as I have repeatedly convinced myself, WOpel ves its peritoneal lining from the growth 
of the walls of the first post-oral pair of somites,—that is, the first pair of the trunk. In 
this process the primitive head-cavity is completely obliterated. 
5After Hatschek had expressed the idea that “the secondary body-cavity is com- 
Nemerteans, but soon completely renounced this “ working hypothesis" in favor of 
Kleinenberg’s recent interpretation of the mesoderm. I had conceived, in the main, the 
views here presented before the publication of this opinion of Berg's, and have as yet 
learned no valid reason for changing my min 
