The system naturally groups itself under three heads, the nerve ring, the sensory clubs, and the motor plexus of fibres and ganglia that underlies the epithelium of the subumbrella. The general relations of the nerve ring and of the sensory clubs have been given before in the description of Charybdea Xaymacana, so that we may pass at once to the consideration of the finer details of the nervous tissues.
In the structure of the nerve ring I have found myself unable to come to the same results as those given by Claus, who so far as I know is the only one that has studied the nerve with special reference to its histology. Our difference amounts to this, that he finds two distinct types of cells in the epithelium of the nerve, sensory and supporting, which would make it a receiving as well as transmitting organ, while I have not been able to demonstrate satisfactorily the sensory cells, and, therefore, so far as my own observation is concerned, I am disposed to attribute to the nerve simply the function of conducting impulses. I do not know just how much weight to assign to my inability to find evidence in my sections of the sensory type of cells. Eimer (mentioned by Hesse, ’95, p. 420), the Hertwigs (’78) and Claus (’78) have independently discovered the two types in one medusa or another, and the Hertwigs, at least, have demonstrated them by macerated preparations. So far as Charybdea is concerned, however, Claus had only preserved material and had to rely upon sections, as have I, since the material which I had preserved with especial reference to maceration did not turn out well. The results that we get from sections vary enough for me to believe that Claus interpreted his sections very much by analogy with other forms—as indeed, is suggested by his own words (’78, p. 22): “Da es mir nicht geglückt ist die durch die längere Conservirung in Weingeist fest vereinigten Elemente zu isoliren, habe ich das muthmassliche Verhältniss beider Elemente nach Analogie der mir für die Acalephen bekannt gewordenen Verhältnisse, welche O. und R. Hertwig so schön auch am Nervenring der Carmarina zur Darstellung gebracht haben, zu ergänzen versucht.” There can be no doubt of our having the same structures to deal with, for C. Xaymacana is so much like C. marsupialis as to be perhaps more worthy of being called a variety of the latter than a distinct species.
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.
A comparison, now, with Claus’s figures (’78, Taf. II, Figs. 19-21) will show that, except for the rather unimportant matter of the mucous cells, which he finds regularly and thickly disposed on each side of the nerve (’78, Fig. 21), our only essential difference lies in the matter of sensory cells in the epithelium. His figures show a multitude of spindle-shaped sensory cells whose central ends are continued in processes that bend around into the mass of fibres of the nerve. In his Fig. 20 a relatively small number of nuclei, just one-third as many, are seen attached nearer to the surface, which represent the supporting cells. The plan of structure (as shown in his Fig. 20) is an alternation of (1) supporting cells offering a broad peripheral end to the surface and having the central end continued as a supporting fibre to the gelatinous lamella, and (2) spindle-shaped sensory cells with nuclei at a lower level, which send their peripheral process up between the supporting cells to the surface, while the central process becomes continuous with the nerve fibres, often branching into two processes. In my sections I have not been able to see either a regular alternation of nuclei at different levels, or central processes which unmistakably bend round into the nerve fibres. In every case in which I could trace the central process of a cell clearly it ran to the supporting lamella, and this whether the nucleus of the cell lay near the surface of the nerve or deeper down, as in the somewhat spindle-shaped cell seen on the left of the centre of the nerve in [Fig. 47]. Of course in many cases the central process could not be traced in a section, and this leaves room for the supposition that such were always the sensory cells. From my inability to demonstrate sensory cells in the nerves of Charybdea, I by no means wish to deny their existence; for that remains to be proved, or disproved, by macerations. At any rate, they cannot be so numerous as has been supposed. The position of the nuclei shows that.
The epithelium of the nerve is said by Claus to be ciliated. It has been suggested by Schewiakoff that probably in such cases the sensory cells bear one long cilium, while the supporting cells have many smaller cilia. Unfortunately, I made no observations upon the ciliation of the nervous structures of the living animal, and the traces of cilia that are shown in preparations of preserved material are a poor basis to speculate much on. Claus considers the sensory cells of the epithelium of the nerve a special seat of tactile sensation.
The way in which the nerve reaches the sensory clubs is interesting. Under the topic of the vascular lamellæ it was explained that the sensory clubs and the bottom of the sensory niche from which they spring are parts of the subumbrella. [Fig. 37] reminds at a glance better than any other one drawing how the bottom or inner wall of the niche is completely cut off from the exumbrella by vascular lamellæ above and below the stalk of the club. From this figure, now, it will readily be understood that the nerve in order to pass to the base of the stalk has simply to traverse the gelatine of the subumbrella. This fact, which seems surprising enough at first sight in view of the position of the clubs on the external surface of the umbrella, was correctly pointed out and explained by Claus, but one or two figures will serve perhaps to give a clearer idea of it.
[Fig. 49] is a diagram of the nervous structures in the region of the sensory niche, as they would be seen on the surface of the subumbrella turned toward the bell cavity. The outline of the sensory niche as it is seen through the tissue of the animal is represented by the line osn. The sensory club (scl), and its stalk with a conical basal portion are given by the lightly dotted outline and are also imagined as seen through the animal. The nerve (n), being on the surface of the subumbrella, is shown as a heavy line describing an arch over the outline of the niche. In the middle point of the arch is a slight thickening of the nervous tissue (rg) which shows in section a large increase in the number of ganglion cells, and is the radial ganglion of Claus. The same is seen, exaggerated in size, in [Fig. 12]. From it there extends upward a slender strand of nervous tissue (rn), the radial nerve of Claus. In Charybdea this can be traced but a very short distance. In Tripedalia it is much more distinct and traceable for a longer distance, and I might say in passing that this and the sensory organs in the proboscis are the only differences I have noted between the nervous systems of Tripedalia and Charybdea.
Nerve ring, radial ganglion and radial nerve all lie on the bell cavity surface of the subumbrella. The way, now, in which the nerve ring reaches the base of the stalk is simply by sending two roots through the gelatine of the subumbrella to the conical base of the stalk. These roots are seen in the diagram at rns. After passing through the gelatine the roots come together on the inner side of the base—that is, the side turned toward the bell cavity—and then pass downwards (nst) on the inner side of the stalk of the club to the mass of nervous tissue at its end.
This passage of nervous tissue through the gelatine in order to reach the sensory club is a little hard to grasp at the first, and I have tried to render it more intelligible by a couple of drawings of sections. [Fig. 50] is a transverse section through the upper part of the region of the sensory niche, not quite horizontal (i. e. parallel with the bell margin), but slanting so as to lie on the plane of the reference arrow x-y in [Fig. 49]. The plane passes just through the top of the niche, and in two areas has cut through the roof with its epithelium of ectoderm (ece, ecs) so that the space of the sensory niche (sn) appears. The vascular lamella of the sensory niche (vls) is shown, as in Figs. [13] and [14], running on each side from the endoderm that lines the canal of the sensory club (enc) to the endoderm of the adjacent stomach pocket (enp). By it the gelatine of the exumbrella is separated from that of the subumbrella, and one sees that it is only through the latter that the nerve has to pass in order to reach the base of the sensory club. It is also seen that one part of the roof of the niche which is cut through lies outside of the ring of lamella and is therefore lined with ectoderm of the exumbrella (ece) while the other lies within the ring and is lined with ectoderm of the subumbrella (ecs). Owing to the slanting direction of the cut only the root on one side is cut through. The other is indicated, however, on the right side of the drawing. In this method of passage of nerve fibres, together with the accompanying ganglion cells, directly through the gelatine to the stalk of the sensory club my work is only confirmation and explanation of Claus.