568 
PROFESSOR E, A. SCHAFER ON THE 
layer of thickened ectoderm, the ciliated columnar cells of which (fig. 10, A, B) are 
very narrow and elongated, each having a clear oval nucleus, which causes the portion 
of the cell where it is situated to he bulged out. For convenience of adaptation, there- 
fore, the nuclei of adjoining cells are on different levels, and this gives an appearance 
as if there were several layers of nuclei (fig. 18). The upper or free end of the cell is 
peculiar in presenting a distinct, highly refractive thickened border (fig. 10, A, &.), 
convex outwardly, and from the top of the convexity a single long cilium springs 
(%• 10, A, c.). The attached ends of the cells rest upon a fibrous stratum, often 
granular looking in section, which in fact seems to be formed by the interlacement and 
union of fine fibres prolonged from the branching fixed ends of the columnar cells 
(fig. 10, A,/). These fibres have a certain resemblance to delicate nerve-fibres ; they 
pass from the cells, at first perpendicularly through the stratum in question (fig. 18), 
but they then turn abruptly down towards the bottom of the fovea, passing beneath 
and between the branching processes of the other cells, under which they aid in 
forming in like manner a fibrous stratum. Towards the upper limit of the fovea the 
fibrgus stratum becomes thinner and at last altogether ceases, and the elongated cells 
pass by a gradual transition into the small epithelium cells which cover the upper 
surface of the umbrella. 
Immediately beneath and on the oral side of the attached end of the lithocyst is 
another deep depression of the surface. This, which may be termed fovea nervosa 
inferior (fig. 17, fn.i.), is applied for the greater part of its extent to the process of 
the nutritive canal which passes to the lithocyst. It is lined by a thick epithelium 
with a nervous substratum, quite similar in appearance to that found in the superior 
fovea ; towards the lithocyst this epithelium passes into the ectodermal covering of 
that structure. It is possible that the subumbrellar nerve-fibres, which were before 
described as converging towards the attachment of the lithocyst, end in the fibrous 
substratum of this inferior fovea. 
The lithocyst itself consists of three parts, which may be distinguished as the basal 
(fig. 17, b.), the intermediate (fig. 17, if and the terminal portions (fig. 17, t.), the 
last-named projecting freely under the cushion before mentioned. They are marked 
off from one another by two shallow grooves. The terminal part contains the clump 
of calcareous crystals which give the name of lithocyst to the whole organ. The 
intermediate part has a deep brown colour on the rip-turned surface, especially in 
and near the groove which marks it off from the terminal portion, the colour being 
situated in the ectodermic cells.* 
The interior of the lithocyst is occupied throughout its whole extent by a radial 
prolongation from the marginal nutritive canal (fig. 17, n.c.), which is, of course, lined 
* The presence of calcareous crystals (otoliths ?) in the marginal bodies of the Medusae has led to their 
being commonly regarded as auditory organs, and the presence of pigmented spots (ocelli P) is supposed 
to indicate the existence of a dim visual perception, but there has been hitherto no experimental proof 
of the actual possession of these functions in the case of Aurelia. 
