NERVOUS SYSTEM OF AURELIA AURITA. 
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by entoderm cells. This prolongation remains hollow in the basal and in the inter- 
mediate portions, but into the terminal part it is continued as a solid projection, which 
expands into the clump of otolithic cells. These, therefore, are continuous with, and 
are doubtless derived from the entoderm, for they have no connexion whatever with 
the ectoderm, being separated from it by a thin layer of the jelly-like tissue (meso- 
derm ?). This is a noteworthy fact, for in nearly all animals in which an auditory 
organ containing otoliths is developed, these particles are, so far as is at present 
known, formed in connexion with cells derived solely from the ectoderm. The 
lithocyst consists then (1) of a central part or core derived from the entoderm, and 
(2) of a covering formed of ectodermic cells, the two being separated from one another 
by a thin layer of the jelly-like mesoderm.* 
Entodermic part or core of the litliocyst . — As already mentioned, the central 
canal of the lithocyst is prolonged directly from the circular nutritive canal of the 
margin, and it extends as far as the junction of the intermediate with the terminal 
portion. It is lined by a columnar ciliated epithelium, the cells of which are much 
longer than those found in the ordinary nutritive canals. Its lumen is partially filled 
with rounded cells, which towards its extremity entirely occlude it, so that the 
canal is prolonged into the terminal portion of the lithocyst by a somewhat narrowed, 
solid stalk, which widens out as the otolothic clump. Of the cells which form the 
stalk the external are much elongated, and are disposed in a radiating manner, whilst 
those in the centre are rounded or polygonal, and the more distal already have small 
crystals developed in their interior. The peripheral otolithic cells of the clump itself 
have also a radiate arrangement and an oblong appearance in longitudinal section 
(fig. 17), whilst the internal ones are more irregularly packed, and many of them 
almost regularly dodecahedral in shape. In teased (osmic) preparations their flattened 
sides sometimes are seen to have the bases of columnar cells applied to them, the ends 
of the latter tapering to fine branching processes (fig. 1 0, E). These (nerve fibre ?) 
processes seem to become lost in a network having a granular appearance in section, 
underlying the tapering ends of the columnar entodermal cells, like the stratum already 
described as existing under the ectodermal cells of the fovea; nervosa; and that imme- 
diately to be described in connexion with the ectodermic part of the lithocyst. 
Opposite the fovese the fibrous stratum under the ectodermic cells comes for a certain 
* In one specimen of Aurelia , sections of which were prepared, there was observed a small sup- 
plementary organ (fig. 20, V) a short distance from one of the ordinary lithocysts, remarkable for several 
reasons. In the first place, it projected from the upper edge of the umbrella-margin, and in place of 
being covered with a cushion it was free superiorly, resting below on the prolonged lower edge of the 
umbrella. In the second place it was of simple structure, consisting merely of a bell-shaped ectodermal 
prominence, enclosing a diverticulum of the marginal canal ; both its ectodermal and entodermal layers 
(which were separated by a thin layer of jelly) being distinctly thickened. But the most interesting 
fact was that the entoderm at the apex of the bell was prolonged through the ectodermic covering, in the 
form of a short stalk, bearing a small bunch of otolithic cells, which could thus project, naked and 
without an ectodermic covering, into the surrounding medium. 
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