vex lens is no doubt sufficient to bring the light to a focus on 

 the retina, although the retina is in contact with the surface of 

 the lens. Judging from section it seems likely that the refrac- 

 tive indices of the cornea, lens, and retina are practically the 

 same. If this is the case, the only refraction that takes place 

 is when the light enters the cornea. In this case the relative 

 convexity of the outer surface of the cornea determines the 

 focal distance, and the shape of the inner portion of the eye is 

 immaterial as long as the lens and retina are kept in contact. 

 With such an arrangement the more convex the cornea the 

 shorter the lens must be in order to place the retina at the focal 

 distance, and vice versa. 



The nerve that enters the eye on the side next to the shell, 

 just in front of the surface of the retina that is applied to the 

 lens, supplies the retina. The layer that resembles rods is placed 

 on the side turned away from the lens, and it is to these that 

 the nerve fibers are apparently distributed. A pigment layer, 

 often of considerable thickness, lies next to the rods. Another 

 nerve, a branch of the one already described, reaches the eye 

 near its optical axis, and spreads out beneath this pigment layer. 

 I have not traced its distribution. 



Looking directly into it, the eye universally appears blue. 

 The color is probably due to the breaking up of the light 

 reflected from it by the small elements of which it is composed 

 as there seems to be no blue pigment. 



The development of the eye has not been carefully followed. 

 It is noticeable thai the small, presumably young eyes, have 

 proportionately much thicker corneas than the large eyes have. 



It is a difficult matter to determine by experiment how well 

 a scallop sees. If an animal is placed in a position that is illu- 

 minated from one side, and allowed to remain undisturbed for 

 some time, and then a sudden shadow is made to fall over it, 

 it is almost sure to suddenly close its shell. If this be tried 

 several times at short intervals the animal usually soon fails 

 to respond. It is also to be noticed that Pecten irradians when 

 approached in shallow water will either start to swim or close 

 its valves. It is, of course, not at all certain that the stimula- 

 tion that leads to this action is received through the eyes. The 

 response is much more noticeable than with most other shallow 



SO 



