446 
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 ; hut 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 
which 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 the 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 tire eye of Pecten bears an interesting 
resemblance to thatqf the Vertebrata. The shape of the lens 
has been a subject of much dispute amongst the authors 
