610 



THE EYE IN EVOLUTION 



Myliohatis 



Armadillo 



intermediate types exist ^ ; in general it may be said that in most 

 vertebrate types the retina is duplex, but in nocturnal species the 

 retina becomes rod-rich or pure-rod with a corresponding reduction 

 or elimination of cones. 



A PXJBE-ROD RETINA is Seen only in a few entirely nocturnal animals ^ — the 

 Selachians (except a few species, e.g., the ray, Myliohatis, the dogfishes, Mustelus 

 and Squatina); a few deep-sea Teleosteans ; among the Reptiles, some nocturnal 



Figs. 756 and 757. — Retina of Diurnal and Nocturnal Animals. 



RECEPTORS 



(mostly rods) 

 summoted 

 extensively 



BIPOLAR CELLS 



finally 



sunnmated 



but little 



in: 



GANGLION CELLS* 



BIPOLAR CELLS 



finally 



summated 



extensively 



in: 



GANGLION CELLS 



Fig. 756. 



Fig. 757. 



Cseciliaii 



The diagrams represent two related species, one of which is diurnal and 

 the other nocturnal. The different ratios of visual-cell types call for different 

 relative numbers of the various conductive-cell types, leading to varying 

 degrees of summation in the optic nerve fibres and producing characteristic 

 differences in the i-elative thickness of the retinal layers (Gordon Walls). 



lizards (nocturnal geckos) and snakes (some nocturnal colubrids as Hypsiglena 

 and Phyllorhynchus) ; the Monotreme, echidna ; the armadillo, Dasypus ; and 

 the bats (Chiroptera). It is presumed without clear histological proof to occur 

 in a few other types — in the Chimseras, in Lepidosiren among the lung-fishes, in 

 the Cascilians among Amphibians, and in a few Mammals such as the hedgehog, 

 the shrew, ? in the chinchilla, in the seals (Phocidte), the whales (Cetacea), the 

 nocturnal lemuroids, Tarsius, and the night-monkey, Nyctipithecus. 



A ROD -RICH retina with a few cones occurs in certain Teleosteans such 

 as the burbot. Lota (810,000 rods and 3,400 cones per sq. mm., Wunder, 1925), 

 in the ccelacanth, in Sphenodo7i, and in some nocturnal Rodents {Rattus, Mus, 

 Cavia, etc., in which the projaortion of rods to cones is about 100 : 1). 



251. 



603. 



