NERVOUS SYSTEM OF AURELIA AURITA. 
565 
might be regarded as belonging to such a sheath. The fact of the distinction between 
fibre and sheath not being visible in the unstained preparation would be accounted 
for, if we suppose them to possess much the same index of refraction. 
Course and distribution of the nerve-fibres . — The nerve-fibres are not uniformly 
distributed over the whole subumbrella. They are met with in least abundance near 
the margin, but are not wholly absent even here, and I have sometimes thought 
I could trace a delicate fibre into a tentacle. But I have not been able to detect 
anything of the nature of a marginal ring of nerve-fibres like that described by 
Haeckel in the craspedote forms. 
At certain parts the fibres come together to form wonderfully intricate interlace- 
ments (fig. 7), whilst at other places only a few fibres may be seen crossing the field 
of the microscope (fig. 3). Neither individually nor collectively do the fibres appear 
to have any special relation to the nutritive tubes. Nor are they especially numerous 
in the neighbourhood of the lithocysts, although a certain number of nerve-fibres may 
generally be traced converging towards each of these structures (fig. 4). I have never 
been able to follow them directly, either into the marginal bodies or into the nerve- 
epithelium near; at the same time it is probable that some of these converging nerves, 
if they do not enter or issue from the lithocyst itself, at least come into relation with 
the thick nerve- epithelium which is found near the base of that organ. The diffi- 
culties of observation are here very considerable, owing partly to the fineness of the 
converging fibres, and partly to the fact that the tissues near the margin, and especially 
near the lithocysts, are apt, more than elsewhere, to become uniformly darkly stained 
by the reagent employed. 
At the central part of the umbrella some of the fibres may be observed to sweep 
round the margins of the four genital sacs, and to pass between these towards the 
polypite, but it has not been possible to trace them actually into the latter. 
The general direction of the fibres over the whole subumbrella is radial, but this 
direction is greatly obscured by bendings and intercrossings. Moreover, a few nerves 
pass in the direction of the muscular fibres, and therefore parallel with the circum- 
ference of the swimming-bell. 
If we trace out the course of the individual nerve-fibres more closely (as has been 
done with the fibre marked xx. in figs. 11-16), we are struck with certain remark- 
able facts. In the first place, each fibre is entirely distinct from, and nowhere 
structurally continuous with, any other fibre. Secondly, each fibre is provided with a 
bipolar nerve-cell (fig. 13), which is interpolated in or near the centre of the fibre, 
each end of the fibre representing the prolongation of one of the poles of the nerve- 
cell. Thirdly, each nerve-fibre is of limited length (seldom exceeding 4 millims. from 
end to end), and in most cases tapers at either extremity to a gradual termination. 
Lastly, it may be mentioned that the fibres are rarely branched ; and where they are 
so (as in fig. 12) the branches do not join with other nerve-fibres, but after a longer 
or shorter course end in a tapering extremity like the unbranched fibres. 
