THE EYE OF PECTEN. 



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 (PL I, fig. 1 c) as it passes round 

 the sides of the eyes, but immediately in front of the 

 eye (PI. I, 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 become 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 epi- 

 thelium ; and a shallow trough runs round the line of its 

 juncture with it (PI. I, fig. 3). The convexity of the 

 cornea is not great, and the dome of it frequently only just 



