200 
MR, G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
sisting of two parts, a central and a peripheral. The central part is localised in the 
margin of the swimming-bell, and there forms a “ nerve-ring ” which is divided by the 
insertion of the veil into an upper and a lower nerve-ring. In many species the upper 
nerve-ring is spread out in the form of a flatfish layer, which is somewhat thickened 
where it is in contact with the veil. In these species the nerve-ring is only indistinctly 
marked off from the surrounding tissues. But in other species the crowding together 
of the nerve-fibres at the insertion of the veil gives rise to a considerable concentration 
of nervous structures; while in others, again, this concentration proceeds to the extent 
of causing a well-defined swelling of nervous tissue against the epithelium of the veil 
and umbrella. In the Geryonidae this swelling is still further strengthened by a 
peculiar modification of the other tissues in the neighbourhood, which has been 
previously described by Professor Hjeckel. In all species the upper nerve-ring lies 
entirely in the ectoderm. Its principal mass is composed of nerve-fibres of wonderful 
tenuity, among which are to be found sparsely scattered ganglion cells. The latter are 
for the most part bipolar, more seldom multipolar. The fibres which emanate from 
them are very delicate, and, becoming mixed with others, do not admit of being further 
traced. Where the nervous tissue meets the enveloping epithelium it is connected 
with the latter from within, but differs widely from it; for the nerve-cells contain a 
longitudinally striated cylindrical or thread-like nucleus which carries on its peripheral 
end a delicate hair, while its central end is prolonged into a fine nerve-fibre. There are, 
besides these, two other kinds of cells which form a transition between the ganglion and 
the epithelium cells. The first kind are of a long and cylindrical form, the free ends of 
which reach as far as the upper surface of the epithelium. The second kind lie for the 
most part deep 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 
muscle tissue 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 uerve-ring, viz. : of nerve-fibres and ganglion cells. Yet there is so dis¬ 
tinct 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 nerve-ring there are numerous nerve-fibres of con- 
