404 



Comparative Animal Physiology 



for use in air and one for water. The curvatures of the respective lens and 

 corneal surfaces are such that proper focusing occurs. 



Spectacles. Whenever an eye is free to rotate under a fixed transparent 

 covering through which the light passes, the covering is referred to as a spec- 

 tacle or goggle. A spectacle may be simply the corneal surface which does not 

 rotate with the eve (lamprey, lungfishes, eels, etc.), or it may be a window 

 in a movable lower lid (some turtles and lizards), or a window in a struc- 

 ture formed by the permanent fusion of both lids (snakes, certain hsh, and 

 many lizards)' The latter type of spectacle forms a permanent secondary 

 protective covering which is separated from the cornea by a liquid. This 

 type is one commonly seen in snakes and spectacled lizards. In these forms 

 the outer surface is periodically replaced when the skin is shed. 



[Higher P l ocentals| 



COLO/? VISION: 



PresenI H 



Probably present — ^ 

 Possibly present — [ I 



— D 



Absent - 



Hogfishes| 



Fig. 114. The probable phylogenetic tree of vertebrate color vision. From Walls."" 



ADAPTATIONS TO PHOTIC QUALITY. For civilizcd man color vision is largely of 

 aesthetic value. However, in lower animals it serves a very distinct purpose, 

 in that it increases the visibility of objects and therebv enhances the ability 

 of the animal to obtain food or to escape enemies. The probable phylogenetic 

 distribution of color vision according to Walls is shown in Figure 114. This 

 chart is based largely on well controlled behavior experiments by numerous 

 authors prior to 1942. Since that time it has been demonstrated by Granit'"'' 

 with electrical methods (e.g.. Fig. 142), that the neurones of the optic nerve 



