No. I.] EYES OF MOLLUSCS AND ARTHROPODS. 75 



cornea and of the outer surface of the lens can be modified by- 

 radiating and circular contractile fibres. The size of the pupil 

 may be modified by increasing the curvature of the cornea. 

 The lens may be raised bodily, or lowered, by the combined 

 action of what I have called the ciliary muscle, and of an 

 elastic cushion, the septal membrane. That the body in ques- 

 tion is a true lens, and that its change of shape and of position 

 is to modify the position of an image, is shown by placing the 

 eye in such a position that one may observe the inverted image 

 of any neighboring object formed by the lens upon the retina. 



On focusing between the argentea and the place where the 

 image formed by the lens is seen with the greatest distinctness, 

 one sees a double image, less distinctly toward the argentea, 

 but increasing in sharpness toward the focal point of the 

 eye, where the two images coincide. The only explanation 

 I have to offer for the origin of the second image is that it is a 

 reflected one of the first, formed by the curved surface of the 

 argentea. 



The eyes oi Pectcn, like the faceted ones oi Area, are disposed 

 more or less distinctly in pairs, and show several peculiarities 

 in arrangement and coloration. 



The eyes of the flat, left valve are larger and more numerous 

 than those of the curved, right one. In Area there is a 

 difference between the eyes of the right and left sides, but none 

 in the shape of the valves. In most Pectens the maximum dif- 

 ference in the shape of the valves is accompanied by a maxi- 

 mum difference in the size and number of the eyes on both 

 sides. One occasionally finds an eye, in those species in which 

 eyes are especially numerous, — Pecten varius and P. oper- 

 cularis, — the pupil of which is entirely covered with pigment. I 

 have taken especial pains to examine these organs, which could 

 no longer function as eyes, and have found that the retina with 

 its rods and nerve-fibres is perfectly developed. The corneal 

 cells are provided with median transverse teeth, and with longi- 

 tudinal folds at their inner ends. The teeth and folds fit into 

 corresponding indentations of neighboring cells, giving firmness 

 and flexibility to the cornea. The longitudinal folds are con- 

 tinuous with fine fibres which cross the pseudo-cornea and unite 

 with the outer surface of the lens. 



Beneath the iris there is a nucleated layer of fibres, some of 



