STRUCTURE OF THE MEDUSA. 17 
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 Geryonide this swelling is 
still further strengthened by a peculiar modification 
of the other tissues in the neighbourhood, which had 
been previously described by Professor Haeckel. 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 bi-polar, more seldom multi- 
polar. 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 longi- 
tudinally 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. 
