392 



THE EYE IN EVOLUTION 



Head of 



Dasypeltis 



Head of the 

 horned viper 



Head of the 

 puff adder 



The retina has the usual vertebrate structure (Figs. 473-4),i but 

 the visual elements show a remarkable variation which has been most 

 thoroughly studied and integrated by Walls (1932-42) (Figs. 475-80). 

 In the primitive Boidse (boas, pythons, etc.) two elements only are 

 present, rhodopsin-bearing rods and single cones without oil-droplets 

 or paraboloids. In most Colubridae, on the other hand, the retina con- 

 tains cones only, three types being present — Type A, a stumpy, fat, 

 single cone ; Type B, a double cone ; and Type C with the structure of 

 the single cones of the boids. In diurnal colubrids and elapids (cobras), 

 the relatively poor C-cone is eliminated ; in nocturnal varieties all 

 three elements become more slender and in some the C-cone contains 

 rhodopsin and becomes a rod {Tarbophis, the egg-eating snake, 

 Dasypeltis, etc.). In the vipers (Viperida^^) the same change has occurred 

 but some C-cones remain, while others appear as rods, four elements 

 thus being present ; while in the Crotahdse (rattle -snakes, moccasins) 

 the rods greatly outnumber the cones. It is interesting that in some 

 forms these four elements are all distinctive (the puff-adder, Bitis 

 arietans) while in others (the common British adder, Vipera berus) the 

 transmutation from the Type C cone to its rod-form is seen in all 

 gradations. 



As we have noted, a temi^oral fovea occurs in certain tree-snakes (Drtjnphis) ^ 

 and in the African bird-snake, Thelotornis kirtlandi (comjiare Fig. 807). 



The optic nerve is primitive in its construction unlike that of all 

 other Reptiles and resembling that of the dipnoan, Neoceratodus,^ the 

 fibres being compactly segregated by septa into fasciculi each with a 

 central ependymal core (Prince, 1955). Afferent fibres are present, and 

 although the majority of fibres cross at the chiasma, some uncrossed 

 fibres are present which terminate in the lateral geniculate nucleus 

 {Natrix {Tropldonotus) natrix, Armstrong, 1951 ; Prince, 1955). 



THE OCULAR ADNEXA. Although snakes are popularly considered 

 Hdless, the eyelids are present but have fused over the eye to form a 

 hard and horny " spectacle " * fitting over the globe like a contact 

 lens and separated from the cornea by a closed conjunctival sac. This 

 structure has excited interest from early times (Blumenbach, 1788 ; 

 Soemmerring, 1818) and has been fully discussed by Schwarz-Karsten 

 (1933) and Walls (1934). The nictitans, at one time assumed to form 

 the spectacle, is absent. Embryologically, as in all Vertebrates, the 

 hds develop as a lid-fold without commissures surrounding the eye, 

 but in snakes this fold gradually grows over the cornea, the palpebral 

 aperture at the same time closing and moving dorsally as it does so ; 

 ihe lower lid thus takes the greatest share in the process. Closure is 



■■ Leydig (1853), Hulke (1864), Schultze (1866-67), Hoffmann (1876), Heinemann 

 (i Franz (1913), Verrier (1933). Kahmann (1933). 



■. 388. 3 p. 314. * p. 266. 



