366 



THE EYE IN EVOLUTION 



Chameleon 



Skink 



shallow temporal one may also be present containing both single and 

 double cones (Underwood, 1951) ; this is the only known occurrence of a 

 bifoveate retina apart from Birds. It is to be noted that with their 

 lateral eyes and small binocular field (about 20°, Kahmann, 1932) 

 binocular fixation with the central foveae of lizards is out of the question; 

 each is used monocularly and independently except, perhaps, for the 

 chameleon with its quite extraordinary ocular movements.^ The 

 shallow temporal fovea in Anolis can, however, be used for binocular 

 vision to assist in its agile arboreal activities. 



The 0^)110 nerve does not have a well-defined and orderly fascicular 

 system and throughout it the oligodendroglial cells are somewhat 

 irregularly scattered (Prince, 1955). 



THE OCULAR ADNEXA. Most lizards j30ssess two eyelids outlining 

 a horizontal palpebral aperture (Fig. 419), and with the exception of an 

 iguanid, Anolis alligator, a species of American " chameleon " in which 

 the two lids move equally, the upper lid is more or less stationary, the 

 lower mobile as is usual in the lower Vertebrates ; the latter is often 

 supported by a tarsal plate of fibrous tissue and moved by a retractor 

 muscle attached to its lower border and arising from the depths of the 

 orbit (Cords, 1922 ; Anelli, 1936). ^ In some forms (Chamceleon) in 

 which the globe is very large, the palpebral aperture is constricted to 

 the size of the pupil and the lids move with the eyeball (Figs. 420 and 

 845). In this lizard the lids are exceedingly soft and thin and rarely 

 close ; when they do they form a horizontal slit at the same time 

 pushing the eye backwards into the orbit. 



In a number of lizards belonging to the families Lacertidse (as Eremias, 

 Cahrita and Ophiops), Tejidte and Scincida?, and in some species as Cordylosaurus, 

 Lanthanotus and soine West Indian members of the iguanid genus, Anolis, there 

 is a transparent window in the lower lid where the scales are reduced or absent 

 throvigh which vision is possible when the lid is drawn upwards ; alternatively, 

 as in the Iguanids, two or three black-bordered scales are semi-transparent, 

 forming, as it were, a window with panes of glass through which some vision is 

 possible (Figs. 435-441). The area involved is small and when the eye is opened 

 the window is concealed in a fold in the lower lid. Most of these lizards live in 

 deserts or a rocky habitat and it is probable that such a window may serve as a 

 protective measure against abrasion by sand or grit (Walls, 1934). In other cases 

 (as the West Indian Anolina?) the animals inhabit dark caves and frequently come 

 out to the sun ; it may be that the black-bordered scales act as dark glasses as a 

 protection against the sun in an animal with a relatively immobile pupil (Plate, 

 1924; Mertens, 1954 ; Williams and Hecht, 1955). In others again, particularly 

 bin-rowing lizards, the skink, Ablepharus, and those which like the geckos crawl 

 in gravel and stubble, as a protective measure the transparent lower lid is fused 

 with the upper to constitute a " secondary spectacle " ^ fitting over the globe 



1 p. 694. 



" Only in some Mammals (the leopard, bat and hedgehog) is cartilage found in 

 tlv- ' :rsal plate, 

 p. 266. 



