THE SLIT PUPIL 



219 



even then it has considerable length, for it cannot eliminate itself entirely. 

 The arrangement of the iris muscles around a slit pupil, however, is such 

 as to make it easy for the slit to be closed without any impossible degrees 

 of muscle contraction — closed entirely in some instances, or in any case 

 to so small an area that the pupil is far better able to keep pace with 

 intensity-changes than it is in the frog or even in ourselves. (Fig. 85a, b) . 

 The slit pupil is hence in a sense paradoxical, for though it is an adap- 

 tation to nocturnaUty it has nothing whatever to do with seeing in dim 

 light. Hosts of nocturnal species do not have such a pupil, and are well 

 able to see under scotopic conditions. They get along with a circular 

 pupil because they are content to stay out of bright light. Any strongly 

 nocturnal, rod-rich animal which cares or dares to venture out in the sun, 

 — whether a cat stalking the barnyard sparrow, a gecko seeking flies, a 



Fig. 86 — Pupil shades in mammals. After Lindsay Johnson. 



a, 'umbraculum' (operculum) of hyrax, Procavia (an analogous structure occurs in many 

 whales), b, corpora nigra of Gazella dorcas. c, corpora nigra of camel. 



snake seeking warmth, or a shark basking at the surface — needs a slit 

 pupil and will be found to have one. 



Even some diurnal and arhythmic animals have devices for shielding 

 the pupil from intense glare coming directly downward or reflected up- 

 ward from the ground. Among such devices are the pigmentation of the 

 upper cornea in surface-loving needle-fishes and in Torpedo, the expan- 

 sible pupillary opercula of some fishes and whales (Fig. 65, p. 158), the 

 (voluntarily?) expansible 'umbraculum' above the pupil of the hyrax, 

 and 'corpora nigra' along the pupil margins of ungulates (Figs. 85c, 86). 



Distribution and Meanings of Pupil Shapes — Phylogenetically, the 

 slit pupil is first met with in the elasmobranchs (Table VI, next page), 

 the only group of fishes whose pupils have much contractile excursion. 

 Most sharks have practically circular pupils, and a slit is characteristic 



