THE EYE OF PECTEN. 
451 
figs, 8 and 9. In figs. 8 and 9, the first section is cut through 
the optic nerve, and shows the manner in which the retinal 
branch runs up the side of the eye-cavity ; the second section 
shows the manner in which the branch bends over on to the 
retina and spreads out. The distribution of the comple- 
mentary branch is diagrammaticaily represented in fig. 1 n ; 
it seems to divide into a number of branches which enve- 
lope the eye-cup, and probably send filaments to the cornea, 
lens, tapetum, and epithelium. 
Comparison of the eyes of the three species, P. maximus, 
P. jacohceus, and P. opercularis. — The eye of P. maximus 
is undoubtedly the most highly developed, the eye of P. 
opercularis is the simplest, whilst P. jacohceuSy although 
more like P. opercularis than P. maximus^ shows many 
points in which it is intermediate between the two. 
The lens in P. opercularis is separated from the retina by 
a considerable space (PL XXXV, fig. 10), and consequently 
the chamber containing the humour is relatively large. In 
P.jacohceus the lens is larger than in P. opercularis ^ and 
the chamber consequently smaller ; and in P. maximus the 
lens is very large, and nearly touches the retina, the cham- 
ber of the eye being sometimes very small. A gradation is 
thus observed in the character of this part of the eye in the 
three species. In P. maximus but a small space is filled 
with humour, in P. jacohceus a much larger space is filled 
with it, and in P. opercularis there is a larger space still. 
Again, when the retinas of the three species are compared, 
a similar gradation is found. The retina of P. opercularis is 
comparatively thin, and the concavity and convexities of its 
anterior surface slight. In P. jacohceus the retina is de- 
cidedly thicker, and the anterior surface is more convex at 
its sides than in P .opercularis ; moreover, it may be noticed 
that the delicate membrane which separates the anterior 
from the posterior limbs of the rods has become bent up in 
the regions corresponding with the anterior convexities of 
the retina. In P. maximus all these variations become 
much exaggerated. The retina is much thicker than in 
either of the other species ; and the side convexities of its 
anterior surface are much bolder (PI. XXXV, fig. II, a, 5, c). 
The anterior concavity does not undergo much variation. 
The shape of the membrane separating the anterior and 
posterior limbs of the rods is greatly altered. InP. opercularis 
this membrane is observed, in section, to stretch from side 
to side without any well-marked curves ; in P. jacohceus two 
'well-marked curves, corresponding with the anterior con- 
