436 
DR. A. A. W. HUBRECHT. 
nerve and stretch vertically downwards towards the pro- 
boscidian sheath. I believe I am justified in regarding these 
processes as the equivalents of the smallest terminal twigs 
of the nerve-branches iu the Hoplonemertini and other 
animals with a dichotomically divided peripheral nervous 
system. At the same time I must regard the nervous tunic 
just sketched as representing a more primitive type of peru 
pheral nerve-system, in which it has not yet come to a locali- 
sation into transverse branches, metamerically placed, hut in 
which one of the layers of the hody-wall is yet in function as 
the recipient and conductor of nervous stimuli. 
The facts observed and above described for the numerous 
species and genera of Schizonemertini are so palpable and 
unmistakable, and at the same time so constant,^ that the 
question immediately presents itself ; — How do the less differ- 
entiated and older types of Nemerteans, the Pal^onemer- 
TiNi, and more especially the genus Carinella, behave in this 
respect ? 
PI. XXXII, fig. 4 and 5, may serve to give the answer to this 
question. We find the whole of the central nervous system 
directly under the epiderm, exteriorly to the muscles of the 
body-wall, as was noted above. A thick basement membrane 
separates the cellular parts of the skin from the subjacent 
muscular tissue, and in this basal membrane we find in 
favorable preparations (those in which this basal membrane 
is somevvhat distended, being preferable) that the sheath of 
nerve-cells {nc.) belonging to the longitudinal trunk is directly 
continuous with a layer {nl.), which here again is provided 
with nuclei of exactly the same character as those of the 
nerve-cells and ensheathes the body exteriorly to the muscles. 
The histological character of this layer corresponds to what 
we have found in the nervous tunic of Schizonemerteans. 
Here, again, a fibrillar structure and the scanty absorption of 
staining reagents would suggest the idea of nervous-tissue, 
even were the direct passage into the nerve-cells of the 
lateral trunks less perfectly demonstrable. In front of 
the brain this layer is continued in the heail, being more 
conspicuous ventrally and laterally than dorsally. Here, 
moreover, the fibrillar structure is far more apparent, and 
the enclosed nerve-cells are less numerous. 
In Polia and Valencinia the same layer is present, only 
’ Tlie opinion on the nervous nature of this layer, first emitted in my 
above-cited paper, ‘Zur Anatomie und Physiologic des Nervensystems der 
Ncmertincn,’ p. 47, has only lately been confirmed for another genus of 
Schizotiemcrteans by Herr Hewoletzky of Yienna (‘ Zoologischer Anzei- 
gcr,’ No. 02, p. 399). 
