22 
rorCLAR SCIENCE REVIEW. 
the eyeball, but at the bottom of the eye it curves iu (like the 
cup-shaped inversion of the bottom of a wine-bottle) and sepa- 
rates the eye from the general cavity of the head. In a physio- 
logical point of view the existence of a sclerotic coat or capsule 
is unimportant ; but anatomically the determination of its true 
homology possesses great interest, as it helps to prove the iden- 
tity of constructive plan which has been so much lost sight of in 
comparing the vertebrate and invertebrate eyes. 
Choroid and Iris , — In our figure a line of darker shading 
sweeping round the inner surface of the cornea and continued 
on the inside of the sclerotic coat and in front of the optic gang- 
lion represents a choroid coat. This is deeply pigmented, just 
as we see it in the vertebrate eye ; and where it lines the cornea 
a stroma of pigmented cells in irregular layers is very conspicu- 
ous after due preparation under the microscope. The same 
characteristic stroma is even more marked where the choroid 
lying on the periphery of the optic ganglion receives additions 
of pigment which line the nerve sheaths of the bacillar stratum. 
Thus the optic ganglion lies really outside the cavity of the eye- 
ball. In the eyes of higher mollusca (cephalopod and pulmo- 
gasteropod) this optic ganglion is seen much more distinctly se- 
parated from the eyeball proper, although covered by a reflection 
of the sclerotic capsule. In the vertebrate eye the separation of 
the optic nerve trunk into separate bundles of fibres occurs just 
as it passes through the choroid coat : and in the invertebrate 
eye the separation of isolated fibres from the ganglion mass 
occurs just in the same place and in a similar manner. But in 
the invertebrate eye the choroid pigment, besides lining the sides 
of the eye, is massed in quantity in the interior, both at the 
bottom of the eye and in bands which run through the optic 
ganglion and also invest the separate nerves. And the dense 
pigment layer covering the outer surface of the optic ganglion 
for a long time misled observers into the belief that rays of light 
could not reach the percipient elements of the retina ; whereas, 
as we now know, these percipient elements are situate in front of 
the optic ganglion, and in fact extend as far as the posterior sur- 
face of the cornea, their outer ends being in direct contact with 
the plane of images formed by the corneal lenses. On referring 
to fig. 5, which is that of a section across the bacilli (percipient 
elements), we see a number of clear circles, the sheaths of these 
bacilli, surrounded by dark lines representing the pigment on 
tlieir outside. Keferring again to fig. (), w^e see how the outer 
and inner ends of these sheaths are imbedded in the thick 
layers of pigment accumulated behind the cornea and in front of 
the optic ganglion. The portion of pigment in front wdiich sub- 
serves the office of iris covers the bulb-like or pear-shaped end 
of the bacillurn except at its point of contact with the cornea; so 
