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The second kind lie for the most part under the upper surface. They are of a large size, and present, coursing towards the upper surface, a long continuation, which at its free extremity supports a hair. In some cases this continuation is smaller, and stops short before reaching the outer surface. Drs. Hertwig observe that in these peculiar cells we have tissue elements which become more and more like the ordinary ganglion-cells of the nerve-ring the more that their long continuation towards the surface epithelium is shortened or lost, and these authors are thus led to conclude that the upper nerve-ring was originally constituted only by such prolongations of the epithelium-cells, and that afterwards these prolongations gradually disappeared, leaving only their remnants to develop into the ordinary ganglion-cells already described.
Beneath the upper nerve-ring lies the lower nerve-ring It is inserted between the muscletissue of the veil and umbrella, in the midst of a broad strand wherein muscle-fibres are entirely absent. It here constitutes a thin though broad layer which, like the upper nerve-ring, belongs to the ectoderm. It also consists of the same elements as the upper nerve-ring, viz. of nerve-fibres and ganglion-cells. Yet there is so distinct a difference of character between the elements composing the two nerve-rings, that even in an isolated portion it is easy to tell from which ring the portion has been taken. That is to say, in the lower nervering there are numerous nerve-fibres of considerable thickness, which contrast in a striking manner with
the almost immeasurably slender fibres of the upper nerve-ring. A second point of difference consists in the surprising wealth of ganglion-cells in the one ring as compared with the other. Thus, on the whole, there is no doubt that the lower nerve-ring presents a higher grade of structure than does the upper, as shown not only by the greater multiplicity of nerve-cells and fibres, but also by the relation in which these elements stand to the epithelium. For in the case of the lower nervering, the presumably primitive connections of the nervous elements with the epithelium is well-nigh dissolved—this nerve-ring having thus separated itself from its parent structure, and formed for itself an independent layer beneath the epithelium. The two nerve-rings are separated from one another by a very thin membrane, which, in some species at all events, is bored through by strands of nervefibres which serve to connect the two nerve-rings with one another.
The peripheral nervous system is also situated in the ectoderm, and springs from the central nervous system, not by any observable nerve-trunks, but directly as a nervous plexus composed both of cells and fibres. Such a nervous plexus admits of being detected in the sub-umbrella of all Medusa, and in some species may be traced also into the tentacles. It invariably lies between the layer of muscle-fibre and that of the epithelium. The processes of neighbouring ganglion cells in the plexus either coalesce or dwindle in their course to small fibres : at the margin of the umbrella these unite themselves with the elements of the nerverings. There are also described several peculiar tissue elements, such as, in the umbrella, nervefibres which probably stand in connection with epithelium-cells; nerve-cells which pass into musclefibres, similar to those which Kleinenberg has called neuro-muscular cells; and, in the tentacles, neuro-muscular cells joined with cells of special sensation (Sinneszellen).
No nervous elements could be detected in the convex surface of the umbrella, and it is doubtful whether they occur in the veil.
In some species the nerve-fibres become aggregated in the region of the generative organs, and in that of the radial canals, thus giving rise in these localities to what may be called nerve-trunks. But in other species no such aggregations are apparent, the nervous plexus spreading out in the form of an even trellis-work.
In the covered-eyed Medusæ the central nervous system consists of a series of separate centres which are not connected by any commissures. These nerve-centres are situated in the margin of the umbrella, and are generally eight in number, more rarely twelve, and in some species sixteen. They are thickenings of the ectoderm, which either enclose the bases of the sense-organs, or only cover the ventral side of the same. Histologically they consist of cells of special sensation, together with a thick layer of slender nerve-fibres. Ganglioncells, however, are absent, so that the nerve-fibres are merely processes of epithelium-cells.
Drs. Hertwig made no observations on the peripheral nervous system of the covered-eyed Medusa; but they do not doubt that such a system would admit of being demonstrated, and in this connection they cite the observations of Claus, who describes numerous ganglion-cells as occurring in the subumbrella of Chrysaora. Here I may appropriately state that before Drs. Hertwig had published their results, Professor Schäfer, F.R.S., conducted in my laboratory a careful research upon the histology of the Medusæ, and succeeded in showing an intricate plexus of cells and fibres overspreading the sub-umbrella tissue of another covered-eyed Medusa (Aurelia aurita).* He also found that the marginal bodies present a peculiar modification of epithelium tissue, which is on its way, so to speak, towards becoming fully differentiated into ganglionic cells.
Lastly, returning to the researches of Drs. Hertwig, these authors compare the nervous system of the naked-eyed with that of the covered-eyed Medusæ, with the view of indicating the points which show the latter to be less developed than the former. These points are, that in the nervecentres of the covered-eyed Medusa there are no true ganglion-cells, or only very few; that the mass of the central nervous system is very small; and that the centralization of the nervous system is less complete in the one group than in the other. In their memoir these authors further supply much interesting information touching the structure of the sense-organs in various species of Medusæ ; but it seems scarcely necessary to extend the present résumé of their work by entering into this division of their subject.
* See “Observations on the Nervous System of Aurelia aurita,”. Phil. Trans., pt. ii., 1878.
In a later publication, entitled “ Der Organismus der Medusen und seine Stellung zur Keimblättertheorie,” Drs. Hertwig treat of sundry features in the morphology of the Medusae which are of great theoretical importance; but here again it would unduly extend the limits of the present treatise if I were to include all the ground which has been so ably cultivated by these industrious workers.
It will presently be seen in how striking a manner all the microscopical observations to which I have now briefly alluded are confirmed by the physiological observations-or, more correctly, I might say that the microscopical observations, in so far as they were concerned with demonstrating the existence of nerve-tissue in the Medusæ, were forestalled by these physiological experiments; for, with the exception of Professor Haeckel's work on Geryonidæ, they were all of later publication. But in matters of scientific inquiry mere priority is not of so much importance as it is too often supposed to be. Thus, in the present instance, no one of the workers was in any way assisted by the publications of another. In each case the work was independent and almost simultaneous.
The remark just made applies also to the only research which still remains to be mentioned. This is the investigation undertaken and published by