14 VERTEBRATE PHOTORECEPTORS 



less and less. In mammals and primates, with some excep- 

 tions, the anterior and posterior surfaces are even flatter. 



Despite the general tendency for the lens to become less 

 spherical as one ascends the vertebrate scale, there are many 

 exceptions. The retention of a spherical lens by aquatic 

 forms has already been pointed out. The lenses of nocturnal 



forms, regardless of class, also tend 

 towards spheroidal forms. One of 

 the classic examples is the gecko 

 (Figure 13) in which the lens is seen 

 to be almost a perfect sphere. It is 

 to be noted from the figure that the 

 lens is enormous in proportion to 

 other eye structures. Its volume is 

 equal to that of the vitreous. 

 Fig. 13 Diagrammatic whereas in the human eye, the lens 



section of the eye of the gecko *^ ' 



(Gecko swinhonis Giienther) is about one-ninth the size of the 

 showing large, almost spheri- yitreous. It is noteworthy in this 

 cal lens. X 11. 



connection that in the nocturnal 

 lemurs the lens is not only much more biconvex than in pri- 

 mates, but it is relatively larger (Figures 49, 52 and Table 2). 

 One exception to this is Nyctipithecus (owl monkey) which is 

 nocturnal in its habits. Here too the lens is much more bicon- 

 vex than in the diurnal monkeys, and resembles in shape and 

 relative size that of the nocturnal lemuroids. We see, there- 

 fore, that nocturnal forms also possess greater refracting 

 power than do diurnal forms. This of course is an advantage 

 in helping to concentrate the small amount of light available. 

 Another interesting feature of the nocturnal eye, in addi- 

 tion to the large highly curved cornea and large spheroid 

 lens, is the shape of the pupil. Whereas most animals have 

 circular pupils, many nocturnal forms possess oval-shaped, 

 frequently vertical (e.g. geckos, alligators, cats) ; sometimes 

 horizontal (e.g. Tarsius) pupils. Hartridge (1919) in dis- 

 cussing the function of a vertical slit pupil such as in the 

 cat, points out that an oval pupil in which the long axis is 

 vertical will cause the lens system to form images in which 



