452 
SYDNEY J. HICKSON. 
vexities of the retina, are observed ; but in P. maximus these 
curves are converted into two distinct folds, which run up 
into the substance of the retina. The membrane between 
the folds does not sink again as low as it is at the com- 
mencement of the folds, and consequently the central 
part of the retina is raised in the form of a table above 
the level of its sides. This elevation of the central part 
of the retina may be also seen in P. jacobaeus, though it 
is not nearly so well marked. The folds which occur in 
P. maximus cause th; rods to appear to be given off in a 
pinniform manner at the sides of the retina, and before 
I found the intermediate condition in P. jacohmis I had 
some difficulty in determining the true relationship between 
the retinas of P. maximus and P. opercularis. (Compare 
a, h, c, fig. 11). 
In addition to those just mentioned there are other minor 
points in which the eyes of these species differ from one 
another, such as in the shape of the cells composing the 
lens and in the distribution of the retinal nerve, &c., but 
they are comparatively slight. 
General considerations . — Plaving thus described, in some 
detail the anatomy of the various parts which compose 
the eyes of Pecten, I shall, before leaving the subject, point 
out some of their interesting morphological peculiarities. 
It is, in itself, a remarkable thing to find a large and 
variable number of eyes situated oii an area at some con- 
siderable distance from any central nerve-ganglion ; and, 
when it is remembered that the class and even family (with 
one other exception, e. g. Spondylus) to which the genus 
belongs, possess no organs of vision at all in the adult con- 
dition, it is altogether surprising that they should be of such 
extraordinary complexity as they have proved to be. The 
high structural development that this eye h.is attained is, 
however, not so remarkable as the fact that in many ways 
it differs from the ordinary Invertebrate eye, and resembles 
that of the Yertebrata. 
In the first place, the lens is built up of a large number of 
distinct nucleated cells, which undergo a flat'ening at its 
circumference very similar to that found in the eye of the 
Yertebrata. Whether the lens is developed from the cells 
of the epiblast, as in the Yertebrata, or from the mesoblast, 
must at present be left unsettled, but it will probably be 
found, when the development of the eye is studied, that in 
this respect also it resembles the eyes of the Yertebrata. 
The tapetum, a structure which is of considerable im- 
