1152 The American Naturalist. . [December, 
As regards the ectodermal structures, I wish to be as brief as 
possible here, and to postpone their more particular consideration 
to my more detailed publication. 
In my opinion, the permanent nervous system of the Annelids 
is undoubtedly to be derived quite directly from the condition 
found in the Turbellarians. We are to assume here that in the 
parenchymatous ancestor, probably, the fusion of ganglia around 
sense organs to make up the chief centers (as shown in the onto- 
geny of Annelids) had already been completed. I regard the 
entire larval system, including the ring nerve of the ciliated band 
and its ganglia, as a special modification of a still older, originally 
diffuse, subcutaneous nerve-cell plexus. In accordance with this, 
the ciliated band would not at all have the significance which has 
been often ascribed to it, but, like the larval form itself, would 
be only a secondarily acquired peculiarity of an embryo forced 
into a pelagic life. | 
The setz are characteristic of the Annelids; but even in the 
Turbellaria similar, though quite superficial, skin formations 
occur, as for example in the Enantia spinifera described by V. 
Graff. From such dermal armament, at first irregularly distribu- 
ted, may have arisen the true Chetopodia. And here it is to be 
noted that in Enantia the cuticular hooks occur laterally upon 
the entire margin of the body, with the sole exception of the 
head region, just as the Chzetopodia are confined strictly to the 
trunk of the Annelid. 
We may regard the head tentacles and trunk cirri as having 
arisen as evaginations of specially sensitive regions of the integu- 
ment; and since vascular loops were drawn into such hollow 
processes, they became capable of serving at the same time as 
respiratory organs for the body. The fact that in the trunk the 
dorsal cirri, or some of them, became true dorsal branchiz had . 
its origin in that these were least exposed to injury in occasional 
movements of the worm upon solid substances, and thus admitted 
= of a thining of the integument necessary for respiration. The 
ventral processes came much more into contact with the sub- 
| and hence became the bearers of an increased sense of 
