570 William Patten 



HiCKsoN (15 and 16) and Carrière (19) have only been able to 

 add to the excellent work of Hensen a few corrections of minor im- 

 portance, concerning the strncture of the lens, cornea etc. 



Sharp (18), withoiit making any contribntion to oiir knowledge on 

 the subject, has come to the conclusion that the external pigmented 

 ring of the eye is really the sensitive part, while the eye itself, he 

 considers to have another function thau that of vision. 



It is interesting to follow the fluctuations in Zoological opinion, 

 caused by the accession of new knowledge concerning these organs. 

 At first, the superficial resemblance between these structures and the 

 human eye was so great that an equivalent fimctional power was 

 likewise tacitly ascribed to them. With increased knowledge, the 

 structural similarity was seen to disappear, and less importance 

 was then attached to their fimctional powers, until, finally, the 

 careful researches of Hensen showed a remarkable discrepancy be- 

 tween their highly complicated structnre, and their apparent lack of 

 visual power. This difference is heightened by the great number of 

 eyes present, which appear to far exceed the simple requirements of 

 such an animai.; 



Thus many were led to believe that the organs in question were 

 not eyes at all, but luminous organs, or they were placed in the 

 category of structures, the functions of which were unknown. 



General structure. 



After comparing the complex organs just described with the sim- 

 pler eyes of Arca^ and the infinitely less complicated ones of Avicula, 

 we are led to expect a corresponding difference in their functional 

 powers. How far this expectation is realized will be seen from what 

 follo WS. 



If we study the structure of the eyes of Pecte7i, we shall find that 

 the parts really have the function that their names and composition 

 indicate. 



We see a Constant purpose in view ; the concentration of the rays 

 of light, and formation ofinverted Images of external objects upon a 

 sensitive nervous layer, the arrangement of whose Clements shows a 

 definite relation to the direction of the rays of light. 



First, let US consider the cornea and the colorless opening, or pu- 

 pi 1, surrounded by the pigmented iris. The curvature of the former 



