132 QUARTERLY CHRONICLE. 
poda will be found all that is known concerning it in the 
higher mollusca. ‘he remarkable circumstance that the 
bacillar layer forms the innermost stratum of the retina in- 
stead of the outermost, as in the Vertebrata, seems to be as 
clearly established in the Gasteropoda as in the Cephalopoda. 
In the retina of Pteroceras four layers may be dis- 
tinguished—the most external 0:005 mm. thick. The 
external, which may be termed a “ basal membrane,” 
is a homogeneous membrane 0:005 mm. thick. To this 
succeeds a layer of fine granular fibrillar nerve-substance, 
which is thickest at the bottom of the eye; from this: 
layer proceed in a radial direction elongated nucleated 
cells containing pigment towards their apices; this layer may. 
be termed the “layer of retinal cells.” Internally to this 
succeeds a layer of clear substance (somewhat granular in the 
prepared eyes), which exhibits no trace of cells or nuclei, but 
consists of closely packed cylinders, each of which at its inner 
end supports a sort of cap of more strongly refractive material ; 
this layer corresponds in all respects with the bacillar layer 
of the Cephalopoda. ‘The entire retina is surrounded by an 
outer coat. 
The nerve-layer consists of delicate fibres glued together 
by a somewhat granular interstitial substance, the fibrils 
being collected into minute indistinctly separate bundles, 
which cross each other frequently, but all run parallel with 
the surface of the basal membrane. 
The cells composing the cellular layer are of several 
forms:—1. Acuminated cells, which become gradually 
attenuated as they approach the rods. 2. A second sort are 
distinguished as the “‘ broad-ended cells,” which differ from 
the former in the circumstance that they are always broadly 
truncated at the end directed towards the “rods.” 3. The 
third kind of cells is very peculiarly formed; they are long 
delicate filaments, which exhibit at one point a fusiform en- 
largement, and close to their junction with the rods again 
widen, at the same time that they become occupied with pig- 
ment. Closer inspection shows that the fusiform;enlargement 
is a nucleus having strong power of imbibing colouring matter, 
and which is surrounded by an enlargement of the fibre 
itself. Besides these three forms are found, but much more 
sparingly, very slender cells with a minute nucleus. The 
bacillar layer consists of cylindrical thick-walled tubes, which 
at the bottom of the eye are 0-097 mm. long and 0:001 mm. 
thick, and at the periphery 0°054 mm. and 0°010 mm. They 
are constituted of a gelatinous substance, which becomes 
granular in spirit. 
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