578 William Patten 



under side of the iris has little, or no pigment, and then the red tape- 

 tum may easily be seen through the transparent epidermis, giving- to the 

 eyes a characteristic red color. In ali cases, the cells on that side of the 

 iris toward the light are darker, and completely fiUed with pigment ; 

 on the opposite side, only the inner ends of the cells are colored. 



In the young eyes of Pecten Jacohaeus and P. opercularis^ the pìg- 

 mented ring, or iris, is broader on the branchiale than on the shell side. 

 In the development, the pigment first appears on the branchial side and 

 grò WS around the eye. But it often happens that the closure of the ring 

 is not completed even in the fully formed eyes. The incomplete closure 

 gives rise to a narrow, colorless fissure, the choroid fissure , appear- 

 ing like a triangulär exteusion of the cornea toward the shell side of 

 the iris. By selecting those eyes with a large choroid fissure, one may 

 look through it into the eye, and study the shape of the lens and the 

 general features of the retina. 



Since the general structure of the eye has long been so well 

 known, it will not be uecessary to give an introductory description, 

 before discussing, in detail, the structure of the various parts. 



The beautiful silvery, emerald, violet and purple reflections, caused 

 by the combined effect of the tapetum, argentea and lens, early excited 

 the admiration of Zoologists, and, among ali the remarkable objects 

 which the prying eyes of the older, or more recent Naturalists bave 

 managed to discover, few are more beautiful, or worthy of admiration 

 than these. When one looks into the pupil, at an angle of about 30^ to 

 the optic axis, the emerald and violet reflections are predominant ; if, on 

 the other band, one looks at an angle of 90", or directly into the eye, the 

 former color disappears, and only the brilliant sparkle of the refractive 

 lens, or the white light of the argentea, is seen. 



The tapetum, as is well known, is somewhat concave, and fur- 

 nishes a perfect mirror for incident rays. But if the microscope, with 

 which one looks directly into the eye, is focused upon the argentea, 

 the red light from the underlying tapetum may be distinctly seen, thus 

 indicating that, although light may enter the eye from below, very little 

 may pass the argentea from above. When the lens is removed, the sil- 

 very glance of the argentea is retained, but the violet and emerald colors 

 bave disappeared. 



The whole external surface of the eye is covered with a continuous 

 layer of columnar epithelium, lo west near the base of the stalk, whence 

 it gradually increases in height as far as^the iris, at whose edge it is 



