62 



VERTEBRATE PHOTORECEPTORS 



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The situation in snakes has been studied by Walls (1932, 

 19426) who examined the eyes of many forms for the presence 

 of visual purple, which is diagnostic for the presence of rods. 

 He found visual purple in most forms with vertical pupils 

 (vertical slit pupils being characteristic of nocturnal forms). 

 In four vertical-pupilled forms he found no visual purple. 

 No visual purple was found in round-pupilled forms, most of 

 which are considered by herpetologists as strictly diurnal. 

 Walls calls attention to the fact that 

 |. ^^ some round-pupilled forms are ap- 



parently nocturnal, but these also gave 

 negative results. He suggested that 

 nocturnal activities in the absence of 

 visual purple may be carried on 

 primarily through olfaction. 



Thus despite the general opinion 

 that snakes are essentially ' cone ' ani- 

 mals, it is seen from Walls' observa- 

 tions that rods occur also among many 

 of these forms. 



Most teleost fishes possess a double 

 retina, as do amphibians, birds, and 

 most mammals. Many fishes have a 

 predominance of rods (particularly the 

 pike and the eel) . Among the elasmo- 

 branchs the presence of a few cones 

 has been described (Greeff, 1899), but in my own preparations 

 I have not been able to identify any. This characteristic rod 

 retina shows also the complete lack of pigment in the epithe- 

 lial pigment layer (Figure 43). 



Among amphibians, the frog has rather small cones and 

 large rods of two types: 1) violet or red rods with long cylin- 

 drical outer segments, and 2) smaller green rods with shorter 

 external segments and long internal segments (Figure 44). 

 Upon the basis of structural differentiation of the retina, 

 birds become divisible into two functional groups, viz., 

 day birds and night birds. It is a well-known fact that ducks 



Fig. 43. Photomicro- 

 graph of the elasmo- 

 branch retina, showing 

 absence of cones and lack 

 of pigment in the epithe- 

 lial pigment layer. X 455. 



