RETINAL STRUCTURE AND ANIMAL HABITS 61 



lizards, round-pupilled snakes, and possibly some turtles. 

 The geckos, however, which belong to the lizard family, are 

 primarily nocturnal in their habits and possess a pure rod 

 retina. Examples of the essentially cone-bearing retina of 

 the turtle (Chrysemys picta), and of the pure rod retina of 

 the gecko, are shown respectively in Figures 40 and 41. 



With respect to the turtles, it has been generally held that 

 they possess cones only. Walls (1934a) says that in the 

 photophobic forms such as the limicolous Chelydra, many 

 rods are present, but that very few rods are found in the sun- 

 loving genera such as Chrysemys. According to him (19426) 

 rods are more numerous in the actively nocturnal Pseudemys 

 than in Chelydra. It is of interest in this connection that 

 Crozier, et al. (1938-1939), who studied the flicker response 

 function for Pseudemys, report this 

 retina as possessing cones only. 



In my earlier work on the turtle ret- 

 ina (Detwiler, 1916) I described the 

 presence of single and double cones 

 and the absence of rods in Chrysemys 

 picta. A critical re-examination of the 

 sections reveals the presence of an 

 occasional element which lacks an oil 

 drop, but which possesses a paraboloid 

 and a broader outer segment than the 

 typical cone. This element may well be 

 a rod, but one must not confuse it with 

 the 'accessory' member of a double 

 cone which also lacks an oil globule, 

 but which possesses a paraboloid and 

 an external segment somewhat larger than that of the princi- 

 pal cone (Figure 42, A). This very sparse rod-like element 

 is shown in Figure 42, B. 



Crocodiles and alligators possess a double retina which is 

 very rich in rods. Their eyes possess also a light reflecting 

 tapetum (Laurens and Detwiler, 1921) and are particularly 

 well adapted for nocturnal vision. 



A B 



Fig. 42. A, double 

 cone from the retina of 

 the diurnal turtle (Chry- 

 semys picta); B, rod from 

 same retina. X 480. 



