612 



THE EYE IN EVOLUTION 



Dasyurus 



Hyaena 



Basking shark 



Coney 



snakes ; and in diurnal Sciuridse, particularly the marmot. A practically pure- 

 cone retina is found in Chelonians. 



The characteristics of an arhythinic eye are necessarily a compromise 

 between the first two types, but several expedients are available to 

 protect an eye adapted to scotopic vision from excessive light. A 

 purely diurnal eye (such as that of most Birds or some lizards and 

 snakes with pure-cone retinae) is completely incapacitated in dim 

 illumination, but an eye that is essentially nocturnal can be made into 

 a useful organ in the brightest daylight. These expedients are both 

 optical and retinal in nature. 



{a) A markedly contractile pupil is the simplest and most 

 common optical device. With this simple expedient alone a nocturnal 

 animal may be rendered arhythmic. From the structural point of 

 view, the easiest plan is contraction to a stenopoeic slit as is commonly 

 seen in some selachian Fishes, many Reptiles {Sphenodon, Crocodi- 

 lians, many lizards and nocturnal snakes) in some Marsupials {Dasyurus, 

 Trichosurus), the dormouse, Glis, the small Carnivores (such as the 

 cat), Pinnipedes, the hyaenas, and most Prosimians (Figs. 758-9). Such 

 a slit-pupil, if the aperture is sufficiently narrow, will allow an 

 essentially nocturnal animal to hunt effectively in bright daylight (the 

 nocturnal geckos or the cat) or to bask in comfort in the sun (the croco- 

 dile, a nocturnal snake or a basking shark). Only a few species have a 

 sufficiently powerful sphincter to effect the mechanically more difficult 

 feat of contracting a round pupil to a stenopoeic pin-hole — the teleostean 

 pearl-fish, Encheliophis ; sea-snakes (Hydrophinae) ; the two-toed 

 sloth, Choloepus ; the African jumping hare, Pedetes ; and, above all, 

 the Prosimians, particularly Tarsius, the large round pupil of which 

 contracts to a pin-hole 05 mm. in diameter (Fig. 760). Other similar 

 expedients are less common and include the expansible operculum 

 associated with the pupil of many skates and rays, teleostean flat- 

 fishes and Cetaceans, the umbraculum of the coneys (Hyracoidea), 

 or the parasol provided by the corpora nigra of the Ungulates. ^ 



(6) An occLusiBLE TAPETUM is a second expedient adopted by 

 certain Fishes to achieve some degree of arhythmicity ; in dim light 

 the reflecting surface is exposed in which case the tapetum acts as in 

 a nocturnal eye, but in bright light it is covered with migrating pigment 

 so that in these circumstances the animal is not dazzled. Such a 

 structure is seen in the elaborate choroidal tapetum of Selachians in 

 which the mirror-like guanine plates can be covered when necessary 

 by the migration of the melanin in the choroidal chromatophores (Fig. 

 300),'^ and the retinal tapetum of certain Teleosts of the minnow 

 (Cyprinidae) and perch (Percidae) families-'^ wherein the guanine crystals 

 in the inner halves and processes of the cells of the pigmentary 

 1 p. 649. * p. 283. » p. 305. 



