THE EYE or PECTEN, 
445 
eyes is covered with an epithelium, consisting of columnar, 
non-ciliated, and slightly granular cells bearing nuclei, 
situated near the base of the cells. As this epithelium 
passes over the eye-bulbs, it undergoes two interesting modi- 
fications. It becomes considerably thicker and filled with a 
dark brown pigment (PI. XXXIV, fig. 1 c) as it passes round 
the sides of the eyes, but immediately in front of the 
eye (PL XXXIV, fig. 1 a), it again diminishes in thickness, 
and becomes perfectly transparent. By thus surrounding 
the eye on all sides with a dark-coloured pigment, leaving 
only a round spot in front, clear and transparent, the epi- 
thelium, by limiting the entrance of the light to a small 
diaphragm in front, here performs the function of an iris. 
The epithelium which runs over this transparent part, and 
which forms the epithelial layer of the cornea, differs from 
the ordinary epithelium covering the rest of the mantle in 
that their cells are rather larger, are perfectly transparent in 
the living condition, and the nuclei are large and spherical, 
and situated in the centre of the cells. 
The eye consists of the following parts, which I shall now 
describe in order. The cornea, covered externally by its 
transparent epithelium, protects a large elliptical lens. 
Close up to the lens is the retina, but separated from it by 
the optic nerve, which spreads out over the anterior surface 
of the retina. The retina rests upon a tapetum, and behind 
this, occupying all the posterior concavity of the eye-cup, 
there is a red pigment. 
The cornea — consists of two parts, the outer epithelium, 
which has already been described, and a basement mem- 
brane, consisting of a thin layer of connective tissue. As 
before stated, this epithelium is merely a modification of the 
general epithelium of this part of the mantle; and the pig- 
mented epithelium surrounding the eye-bulbs (in like manner, 
a modification of the same tissue) is continuous with it 
all round its edge. The passage of the cells of the pig- 
mented epithelium into those of the corneal epithelium is 
signalised by two important changes in the characters of the 
cells. In the first place the pigment entirely disappears, 
and the nuclei, which in the former case were obscured by 
the pigment, now becomes apparent, and in the second place 
the cells are considerably diminished in their longitudinal 
axis. The diminution in size of the cells causes the edge of 
the cornea to be sunk below the level of the pigmented e])i- 
thelium ; and a shallow trough runs round the line of its 
juncture with it (Bl. XXXIV, lig. 3). The convexity of the 
cornea is not great, and the dome of it frequently only just 
VOL. XIX. NEW SEK. G G 
