b SYDNEY J. HICKSON. 



ment, such as 1 have represented diagrammatically in 

 fig. ly, 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. maxiinus than it is in either 

 P.jacoh(Eus or P. opercularis, and in P .Jacobceus 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. 3°. Spindle- 

 shaped nucleated rods. 4°. Molecular and nuclear layer. 

 6°. 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. II, fig. la, 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; 

 this condition occurs especially in the rods at the side con- 



