PUPIL SHAPES AND THEIR MEANING 225 



Raja (Fig. 65a, b; p. 158), and by a number of other animals (v. i.). 

 Among the snakes, the vertical pupil is seen in all nocturnal forms 

 excepting very secretive burrowers (e. g., coral snakes) and the cobras, 

 whose nocturnality is far from perfect. All boas and pythons, all pit- 

 vipers, and all vipers except such primitive and crepuscular forms as 

 Causus and Atractaspis, have the slit. So also with a few elapids (e. g., 



Fig. 90 — Effect of variations in the pupil. Modified after Franz. 



a, standard of comparison, b, the pupil has been doubled in width, hence quadrupled in 

 area; but the visual field is thereby only slightly enlarged, c, the multiple pupil, whose 

 effect is to reduce the brightness of the image and improve its resolution, without sacrifice of 

 field width, d, narrowing of field owing to separation of pupil and lens, e, narrowing 

 of field owing to thickening of pupil margin. 



Acanthopbis, Bungarus) and a considerable number of colubrids, par- 

 ticularly back-fanged species. In short, all strongly nocturnal snakes 

 which ever voluntarily hunt or bask in bright light have vertical slit 

 pupils. In some secretive and crepuscular forms, shapes intermediate 

 between the round and the slit forms occur. Thus in the rainbow snakes 



"^ o ^ ^ ^ -^ ^ 



Fig. 91 — Eye of a shark, Scylliorhinus canicula; external view, and stages in the contraction 

 of its pupil. Redrawn from Franz, n- nasal side; /- temporal side. 



the fully contracted pupil is a broad vertical ellipse, and in Arizona it 

 contracts to the shape of an egg with the narrow end pointing downward. 

 Diurnal snakes nearly all have round pupils, though three or four tree- 

 snakes (e. g., Ahcetulla picta) have horizontally oval ones; and in three 

 colubrid genera there is a horizontal keyhole of special significance which 

 may divide into two ovals when it closes (see pp. 185-6 and Fig. 79). 



