364 



THE EYE IN EVOLUTION 



types, showing a variation in configuration from typical cones to rods 

 (Walls, 1934) (Figs. 431-3). In the great majority of lizards of diurnal 

 habit there are typical single and double cones ; the single cones have 

 a yellow oil-droplet ; of the double cones, one element has an oil- 

 droplet and the other a voluminous paraboloid (Krause, 1863). In some 

 geckos. Underwood (1951) described another type of double visual cell 

 wherein each member possessed a paraboloid and an ellipsoid while the 



Figs. 431 to 433. — Visual Cells of Lizards. 



Fig. 431.— The cones of 

 a diurnal lizard, Crota- 

 jjhytus. 



Fig. 432.— The cones of 

 a nocturnal lizard, 

 Xantitsia. 



Fig. 433.— The rods of 

 a gecko, Coleonyx ( X 

 1,000) (Gordon Walls). 



larger member had an oil-droplet.^ In some nocturnal species the drop- 

 lets are discarded (the worm-lizard, Aniella ; the poisonous Gila mon- 

 ster of Mexico and Arizona, Heloderma) or colourless (the night lizard, 

 Xantusia, Heinemann, 1877), but the outer segments of the visual 

 cells, both single and double, are elongated and rod-like although 

 rhodopsin is lacking. In the nocturnal geckos, however, both elements 

 are frankly slim and rod -like and the long outer segments contain an 

 abundance of visual purple ; these should therefore be considered as 

 rods (Detwiler, 1923 ; Walls, 1942). There is little convergence in the 

 retina ; Vilter (1949), indeed, found that the ratio between visual cells 

 and ga- . lion cells was approximately unity. 



Tn Aristelliger Underwood noted occasional trijjle visual cells. 



