The structure of the nerve as I conceive it is given in Figs. 47 and 48. The former represents a cross-section, and shows, as others have pointed out, that the layer of circular muscle fibres (_cm_) is interrupted by the nerve. It is evident that the tissues which elsewhere on the subumbrella were differentiated into muscle epithelium and muscle fibre have here become nerve epithelium and nerve fibre, a point that has not been remarked upon before, so far as I remember, and that may be of interest in connection with the neuro-muscular theory. The epithelium of the nerve (_scn_) is seen to be made up of cells whose inner ends narrow down into a kind of stalk or process that runs to the gelatine of the supporting lamella (_gs_) and there joins a little cone of the gelatine that juts out to meet it. The cells are smaller in general than those that overlie the muscle layer, especially on the two lateral margins of the nerve, where they are more crowded together and overarch the nerve-fibres. The fibres are seen in cross-section between the processes of the cells. They apparently must lie imbedded in some clear, watery fluid that does not show in the preserved material. The processes of the epithelial cells give the fibres the appearance of lying in alveoli, or being divided into strands, and one of these strands (_ax_) is always discernible among the others by reason of its more numerous or finer or more compactly massed fibres. This is the “axis” of Claus. Here and there in its course appear ganglion cells having their long axis in the longitudinal direction of the nerve. Elsewhere, in the nerve as well, and usually nearer to the surface, are found other ganglion cells, mostly bipolar, some multipolar, which are readily distinguishable from those of the axis by the fact that their long axis lies across the nerve. One of these cells is shown in the figure (_gc_). Here and there in the epithelium alongside the nerve are found mucous cells (_mc_), distinguished by their clear contents and by the small exhausted-appearing nucleus at the base with a few threads of protoplasm.
In Fig. 48 I have tried to represent the structure of the nerve by means of a series of five different views such as would be given by focusing at five successive levels. In the first (1) we have the epithelium of the nerve (_scn_ in Fig. 47) in surface view, the cells appearing polygonal in outline, with here and there a mucous cell. In (2) we find a very slight layer of ganglion cells and fibres having a transverse direction (_gc_ and _fp_ in Fig. 47). These are continuous with the plexus of fibres and ganglion cells which lie above the muscle layer all over the subumbrella, and which represent the motor part of the nervous system. This connection with the nerve shows how co-ordination is effected. At the same level are found fibres of the axis also having a longitudinal direction. In (3) is seen the main body of fibres, divided in the osmic preparation from which the drawing was made into irregular wavy strands which are in all probability largely the result of preservation, but are in part also due to the separation by processes of the epithelial cells, as was seen in Fig. 47. The axis is seen with one of its longitudinally directed bipolar ganglion cells; and at the sides the fibres of the circular muscle of the subumbrella. These show a slanting direction to the nerve, due to the fact that the nerve, as mentioned before, has a sinuous course from the margin in interradius to the level of sensory club in perradius. At the next focus (4) we come to the gelatine of the subumbrella (_gs_ in Fig. 47), and below this (5) to the larger polygonal outlines of the endodermal cells of the stomach pocket (_enp_, Fig. 47), which like the ectoderm show mucous cells at irregular intervals.