ß56 William Patten 



cula remains as a thiu. structureless membrane covering the outer 

 ends of the rods (PI. 32, fig. 132). These organs constitute the simplest 

 invaginate eyes, such as are foimd in Area, Patella etc. An advance 

 in the structure of such an eye is made by an increase in the depth of 

 the optic cup and in the size of the retinidia, accompanied by a reduction 

 in the size of the opening, or pupil of the eye. What seems to have 

 been an incidental result of the deepening of the optic cup was an in- 

 crease in the thickness of the cor ne al cuticula, which may be followed 

 in its growth in various genera of Gasteropods until, finally, it complete- 

 ly fills the optic cup. A discussion of the effect which a Constant increase 

 in the thickness of this refractive substance until it entirely fills the 

 cavity of the eye would have upon the rays of light entering the pupil, 

 would lead us too far into physiological grounds, for this paper. The 

 general result would be, first, to increase the number of direct and re- 

 flected rays of light falling upon the floor of the optic cup : second, to 

 exclude the outer walls more and more from the light, until finally only 

 direct rays fall upon the inner wall of the cup. Either a central, or 

 external part of the enlarged cornea! cuticula. or vitreous body, be- 

 comes hardened into a rounded, or lenticular body to form the lens. 



According as the inner wall of the optic cup becomes more ex- 

 posed to rays of light whose direction is more and more fixed and Con- 

 stant, so a corresponding change in the structure of the rods will appear, 

 in that they become more sharply and regularly defined. the fibrillae 

 of their contained retinidia showing a more perfect cross arrangement, 

 and consequently being more perfectly at right angles to the rays of 

 light. The more complete the arrangement of the retinidial fibrillae. 

 just so much greater will be the effect of this uniformity in direction 

 upon the cuticular substance of the rods supporting the fibres. until . 

 finally, the so-called lamination of the rods, due to the 

 perfectly radiai arrangement of the retinidial fibrillae, 

 is produced. My own observations only extend over those Gastero- 

 pod Mollusca [Haliotis and Patella) in which the eye is very imper- 

 fectly developed. In the higher forms, the observations as yet made 

 are too incomplete to allow a definite conclusion concerningthe structure 

 of the rods and their origin. According to Hilger, they cousist of a 

 sheath and core, formed respectively by the pigmeuted and colorless 

 cells. A rod of that composition has no resemblance to any similar 

 structure of which we kuow, and cannot be compared in any way 

 with the rods of other Mollusca. I believe that Hilger has formed 

 a false conception of the rods in Gasteropods. It seems much more 



