374 



THE EYE IN EVOLUTION 



Terrapin 



and the absence of an oil-droplet are also present ; these anomalous 

 cells occur particularly in those species which habitually avoid the 

 light (the snapping turtle, Chelydra) or are frankly nocturnal (the 

 terrapin, Pseudemys) (Detwiler, 1916-43 ; Walls, 1934-42). The 

 cones retract slightly on exposure to light (Detwiler, 1916) and, as in 

 lizards, the migration of the retinal pigment is restricted (3-6/x in the 

 tortoise, Detwiler, 1916). 



An area centralis on the visual axis is present in the retina of 



most species where the cones are smaller 

 and more densely packed than else- 

 where and the increased number of 

 nuclei determine a thickening of the 

 nuclear layers ^ ; a fovea, however, is 

 absent except as a rarity when a shallow 

 depression is found. ^ In the central 

 area the ratio of receptor cells to 

 ganglion cells is 1 : 1 , while in the peri- 

 phery it is 3 : 1.^ 



THE OCULAR ADNEXA. Of the twO 



lids the lower is the larger and more 

 mobile and the palpebral aperture, 

 horizontal in the lacertilian eye, is 

 canted so that it runs from the dorso- 

 temporal to the ventro -nasal quadrants 

 of the eye, as if to make it parallel with 

 the surface of the water in aquatic types 

 when swimming with the head raised 

 above the surface. Only rarely is there 

 a transparent window in the centre of 

 the mobile lower lid (the Murray turtle, Chelodina ; the turtle, 

 Emyda). The movements of the lower lid and the semi-opaque 

 nictitating membrane are controlled by two long tendons which arise 

 from a fan-shaped pyeamidalis muscle fixed to the posterior aspect 

 of the globe (Fig. 453) ; the retractor bulbi muscle is powerful and 

 when it contracts the globe is drawn inwards and twisted far round, 

 the lower lid and nictitating membrane covering the eye at the same 

 time. So forceful may this movement be in some turtles that when the 

 lower lid closes against the upper the action is continued so that the 

 latter is pushed back into the orbit. The ocular movements, however, 

 are relatively sluggish, the eyes moving independently of each other. 



Fig. 4.52. — The Visual Cells of 

 THE Snapping Turtle, Chelydra. 



A single cone, a double cone and 

 a rod ( X 1,000) (Gordon Walls). 



' The painted turtle, Chrysemi/s, Detwiler (1943), etc. 



2 The soft-shelled turtle, Emyda, Gillett (1923). 



■ The common European fresh-water turtle, Emys obicularis, Vilter (1949). 



