PERIPHERAL NERVOUS SYSTEM IN NEMERTINES. 435 
and then traces of the circumference of a cell enclosing this 
nucleus are visible. No difference can be detected between 
these nuclei and those of the nerve-cells of the brain- and 
marrow- trunks. The bulk of the tissue which remains 
colourless has a finely punctate-fibrillar appearance, resem- 
bling in this respect the fibrillar nerve-substance as we find 
it in the centre of the longitudinal trunks of these worms. 
A characteristic difference is, however, noticed in the fact 
that the separate cells appear to be more numerous in this 
fibrillar plexus than they are in the fibrous centre of the 
longitudinal trunks. 
This curious layer forms a cylindrical sheath throughout 
the whole body between the longitudinal and circular mus- 
cular layers. The median dorsal nerve (which in a former 
paper I proposed to call the nerve for the proboscidian 
sheath) appears as a simple thickening of this layer, and 
laterally the longitudinal marrow-trunks may in the same 
way be looked upon as parallel thickenings of the same 
tissue, as the ganglion-cells in the longitudinal nerve-trunks 
gradually merge, without any considerable difference in 
histological character, into the cells of this layer. In the 
sheath of ganglion-cells belonging to each longitudinal 
trunk the boundaries of the cells are, however, more con- 
spicuous and always visible, and this is not so distinctly the 
case in the circular layers. This cylindrical tunic of distinct 
tissue, which in front is applied to, when not in direct com- 
munication with the tissue of the brain (PI. XXXIII, figs. 9 
and 10), must no doubt be regarded, after what we have 
noticed about its histological character, as a nervous layer. 
Its constituents are nerve-cells as well as fibrillar nerve- 
tissue. 
PI. XXXIII, fig. shows a small part of this nervous 
tunic in a horizontal section through the back of the animal ; 
is the median dorsal nerve ; nl, the fibrillar plexus 
with the separate cells, forming together a layer, differing 
in thickness according to the size of the animal, and always 
only partly visible in a single section. The longitudinal 
{LM.) and circular {TM.) muscles are only sparingly indi- 
cated in the figure, as are also the thin fibres w’hich 
directly traverse the nervous layer (cf. figs. 6 and 7). 
The hypothesis of the nervous nature of this layer is strongly 
confirmed by processes which are sent out by it into the 
adjacent muscular tissue, and which generally carry one (fig. 
8) or more nerve-cells (fig. 11) ; sometimes none. At other 
times these ramifications have a still more marked fibrillar 
appearance ; for example, those wdiich leave the median dorsal 
