4i8 
SYDNEY J. HICKSON. 
ment, such as 1 have represented diagrammatically in 
fig. 1/, which, I believe, forms a support for the lens. This 
ligament is usually broken by the action of reagents, and 
then hangs down by the side of the cavity, and thus becomes 
difficult to observe ; at the same time the lens sinks down, 
and rests upon the anterior surface of the retina. 
The lens is suspended in the space which corresponds 
with the vitreous humour in the higher animals. This space 
is filled with an aqueous humour in Pecten. The lens is 
larger, and, consequently, the space occupied by aqueous 
humour relatively smaller in P. maximus than it is in either 
P,jacoh( 2 Us or P. opercularis, and in P.jacohceus it is larger 
than in P. opercularis. 
The retina — does not line the concavity of the eye-cup, as 
it does in most well-developed eyes, but is nearly flat, and 
a considerable space is left between it and the floor of the 
cup, which is filled up by the red pigment. In conse- 
quence of this the retina appears in section to be a thick 
band crossing the eye from side to side. Thus, just as the 
lens was remarkable for the way in which it approached the 
retina by hanging back into the cavity, so the retina is re- 
markable for the manner in which it leaves the posterior 
concavity of the eye-cup to approach the centre. The eye 
of Pecten, in fact, presents the interesting peculiarity of the 
approach of the lens and the retina towards the centre, so 
that in P. maximus they almost touch. 
The anterior surface of the retina is convex at the sides 
and concave in the middle, but these convexities and con- 
cavities vary in different species. The different layers of the 
retina will be described from behind forwards, as it will be 
easier to trace the transitions in that way than if described 
from before backwards. They are — 1°. Posterior limbs of 
the rods. 2°. Anterior limbs of the rods. 8°. Spindle- 
shaped nucleated rods. 4°. Molecular and nuclear layer. 
5°. Nerves. 
The posterior limbs of the rods stand upon a membrane, 
which runs along the posterior side of the retina; at their 
anterior ends they pierce a very delicate membrane, and pass 
into the anterior limbs of the rods. The anterior' limbs are 
about twice as long as the posterior limbs, and are usually 
smaller in diameter, and situated farther apart than the pos- 
terior limbs. That they are circular in section may be seen 
from PI. XXXV, fig. 7<z, which is a drawing of a section 
made at right angles to the eye-stalk. The anterior limbs 
of the rods are sometimes swollen so as to appear oval ; 
