4 SYDNEY J. HICKSON. 



reaches the level of a line drawn from the highest points of 

 the pigmented epithelium on either side of it. This appear- 

 ance is not often seen in sections, as the pigmented epi- 

 thelium rapidly shrinks, when the tissue dies, and under 

 most reagents ; but I am fully persuaded of the accuracy of 

 this statement from an examination of the eyes of living 

 specimens of Pecten maximus and sections of Pecten oper- 

 cular is. 



The delicate epithelial cells of the cornea, in consequence 

 of being entirely unprotected by any membrane similar to 

 the conjunctiva of the higher animals, are quite naked, and 

 very liable to injury from the rough edges of the tentacles 

 vs^hich surround them. The arrangement just described, 

 however, probably prevents the tentacles from coming into 

 immediate contact with them. The little trough which runs 

 round the margin of the cornea always contains a little 

 liquid, even when the eye itself is removed from the water ; 

 and the pressure of the tentacles when folding over the eye 

 causes it to spread out as a thin layer over the cornea, and 

 thus the cells are prevented from coming into immediate 

 contact with the tentacle. 



Thus, the two remarkable modifications, namely, the pre- 

 sence of a large quantity of pigment, and a greater longi- 

 tudinal axis of the cells which the pigmented epithelium 

 exhibits, are of considerable value to the eye, firstly, to pre- 

 vent very divergent rays from entering, and secondly, to pre- 

 vent any damage to the cornea caused by the rubbing of the 

 adjacent tentacles over the sides of the eye. 



The second layer of the cornea is about half as thick as 

 the epithelial layer, and, like it, is perfectly colourless and 

 transparent. It consists merely of a thin continuation of 

 the connective tissue of the stalk. It may be called the 

 basement membrane of the corneal epithelium, as from the 

 absence of any definite cellular elements its only function 

 probably is to support these cells. 



Beyond the cornea this membrane becomes much thicker, 

 and supports the pigmented epithelium, and at the same 

 time structural elements make their appearance in it. From 

 thence it passes into ihe connective tissue of the eye-stalk 

 without further modifications. 



The lens — is one of the most interesting parts of the eye. 

 It is comparatively large, and is composed of a number of 

 nucleated cells. In the fact that the lens is formed by 

 more than one cell the eye of Pecten bears an interesting 

 resemblance to that of the Vertebrata. The shape of the lens 

 has been a subject of much dispute amongst the authors 



