570 
PROFESSOR E. A. SCHAFER ON THE 
distance into contact with the corresponding stratum just described as underlying the 
entodermic cells of the lithocystic canal (fig. 18). Moreover, indications can some- 
times be obtained of fibres bridging across the mesoderm which separates the ectoderm 
of the fovea nervosa superior from that covering the base of the lithocyst, so that all 
the tracts of modified epithelium with fibrous substratum, whether ectodermic or 
entodermic in their origin, are probably to be regarded as forming collectively a 
rudimentary nervous centre. 
The ectodermic covering of the lithocyst . — The cells which compose the ectodermic 
covering of the lithocyst form a single layer, which is continuous with the general 
ectodermic covering of the under surface of the umbrella. But the layer varies very 
greatly in thickness, in conformity with the varying length of the epithelium cells 
and with the greater or less development of the granular (fibrous) stratum which lies 
immediately beneath them. This is by far most strongly marked over the basal and 
intermediate portions of the lithocyst, gradually disappearing in the terminal portion, 
where the ectoderm is represented by a layer of small flattened cells covering the 
clump of entodermal otolithic cells, and separated from them by a very thin 
mesoclermic stratum. Some of these cells contain the brown pigment before mentioned, 
and near the distal of the two furrows which encircle the lithocyst there are transitional 
forms' between them and the greatly elongated pigmented cells of the intermediate 
portion immediately to be described. 
The epithelium of the basal portion cpiite resembles that already described as lining 
the fovese. The fixed branched extremities of the cells pass as before through the 
granular-looking sub- epithelial stratum, and having reached its deeper portion turn 
sharply round to course either peripherally or centrally, and some, it may be, also 
laterally, within the layer in question. How these and the other fibres of the similar 
layers before described terminate must for the present remain undecided. They 
seem gradually to shade off and disappear beneath the neighbouring epithelium 
cells. They are very much less defined than the fibres of the general nervous plexus 
of the subumbrella ; moreover, they unite as before said into an irregular network, 
and altogether have far less decidedly the character of nerve fibres. The stratum 
which they form resembles in its half-granular half-fibrous appearance the neuroglia 
of the grey matter of the Vertebrate brain ; and the tracts in question with their 
elongated ciliated cells and the granular substratum bring forcibly to mind the 
appearance presented by the central nervous system of the Vertebrate embryo at the 
time when differentiation into cells and fibres is just beginning. Altogether there can 
be very little doubt that we here meet with the first beginnings (in a phylogenetic 
sense) of a central nervous apparatus. It is interesting to remark that although 
characteristic nerve-cells are met with on the nerves of the subumbrella, yet in these 
specialized parts, which may be regarded as the representatives not only of a central 
nervous system but also of sense-organs, definite nerve-cells are altogether wanting. 
The ectodermic cells of the intermediate portion of the lithocyst resemble, on the 
