NERVOUS SYSTEM OF AURELIA AURITA. 
567 
end ; it passes at either pole sometimes abruptly, but more often gradually into the 
nerve-fibre. 
Cells are frequently met with, especially in young specimens, which closely resemble 
the nerve-cells just described, with the exception that the nerve-fibre processes are of 
much more limited length, or even, in some cases, altogether absent. They are, no 
doubt, ectoderm cells, in process of development into nerve-cells and nerve-fibres : 
they are usually more abundant in the neighbourhood of the polypite than elsewhere 
(fig. 3). The arms of the polypite itself generally show a number of cells of this 
description, but they are entirely destitute of nerve-fibre prolongations. 
Function of the subumbrellar nerves. — Each nerve-fibre may be regarded as serving 
to connect a part of the muscular sheet nearer the circumference with other parts 
nearer the centre of the swimming-bell, and thus to bring the contraction of different 
zones of muscle more nearly in correspondence. Further, as a result of the interlace- 
ments which occur, and the closely parallel course which the fibres take in them, it is 
reasonable to conclude that nervous impulses are transmitted by some means or 
another from fibre to fibre. If so, the result would be the same as if an actual 
net-work of nerves existed, viz., the production of a general co-ordination in the 
contractions — not absolute, it is true, but often nearly so. Lastly, since the nerve- 
cells are brought into close relation with the exterior of the body through the 
medium of the ectodermic epithelial cells, amongst which they he, any stimulus 
affecting these is communicated by means of the nerve-fibre processes to the parts 
of the muscular sheet to which these fibres themselves are distributed, and, if there 
is any means of transmission from fibre to fibre, it will rapidly spread over the general 
subumbrellar plexus to the whole of the contractile tissue. 
The Lithocysts ancl Tracts of Nerve- epithelium. 
The lithocysts of Aurelia are eight thumb-shaped projections (fig. 17) at the 
circumference of the umbrella, each being situate in one of the marginal bodies and 
having a horizontal direction with a slight upward curvature. They are covered 
above by a cushion-like prominence (fig. 17, c.) of the upper edge of the umbrella, 
and are further hidden by a tongue-shaped fold on either side, into which a coecal 
protrusion of the marginal canal extends.* 
At the base of the cushion above referred to, there is seen a deep pit which extends 
down towards the attached end of the hthocyst. This pit it will be convenient to 
distinguish by the name of fovea nervosa superior (figs. 1 7, IS, f.n.s.). It is lined by a 
# These diverticula of the nutritive canal, after passing a certain distance laterally into the substance 
of the fold, contract abruptly and the narrowed portion then passes a variable distance towards the end 
of the fold. The larger portions of the diverticula often present, on one or both sides, a solid cellular axis, 
projecting into the lumen of the canal, and formed apparently by an infolding of the entodermal wall, but 
its purpose and the precise mode of its formation I have been unable to elucidate. 
MDCCCLXXVIIJ. 4 D 
