222 ADAPTATIONS TO NOCTURNAL ACTIVITY 



only of those forms which come frequently to the surface or into shallow 

 water — Scylliorbinus, Lamna, Selache, etc. Most of the elasmobranchs 

 whose eyes aim strongly upward are among the Batoidei — the skate-ray- 

 torpedo group. With the exception of the mantas, the batoids have oper- 

 cula (Fig. 65a, b; p. 158), which slope downward and slightly outward 

 over the pupil, and expand in bright light to block the aperture. The oper- 

 cular margin may be serrated, as in Raja, or smooth as in Dasyatis. It is 

 not unusual for rays to bask at the surface in summer, and they are then 

 exposed to especially strong light, considering the fact that their retinas 

 are pure-rod. The monk-fish (Squatina, a sort of imitation ray which is 

 really a shark) and the electric ray Torpedo have slits — horizontal in the 

 latter as in Selache and Sphyrna, diagonal in Squatina as in Scylliorhinus, 

 Lamna, Orectolobus, Gingylostoma, etc. At least one shark (Prionace 

 glauca) and some rays (e. g., Dcemomanta aljredi, Cephaloptera giorna) 

 have pupils which close to vertical slits. Deep-sea forms, like the less noc- 

 turnal of the littoral and pelagic species, naturally have roundish pupils, 

 which in Etmopterus and the chimaeras are extremely large and are prac- 

 tically immobile. The slit form of pupil is well established in the elasmo- 

 branchs, but in this group its orientation has never settled down to the 

 vertical position which is almost universal in land animals for a reason 

 which will appear later. 



No chondrostean, holostean, or teleostean fishes have slit pupils, 

 though those of Acipenser and Piabuca are broad ellipses, with the long 

 axis vertical and with more or less of a point at each end. In the Amer- 

 ican shovel-nosed sturgeon (Scaphirhynchus platorynchus) the pupil is 

 a canted square with rounded corners. Only a very few teleosts (e. g., 

 Anguilla, Encheliophis) have contractile pupils. The retinae of teleosts, 

 except in deep-sea species of course, are never pure-rod as are those 

 of practically all elasmobranchs; and moreover they have the photo- 

 mechanical changes to rely on. The pupillary opercula present in many 

 flatfishes, and ir others {e. g., Plecostomus) which live on the bottom in 

 shallow water, are in the same category as the umbraculum of the hyrax — 

 these devices are parasols for diurnal eyes which are exposed directly to 

 high intensities. Their mechanism of expansion has yet to be elucidated. 

 In one of the batfishes, Halieutichthyes aculeatus, fixed superior and in- 

 ferior opercula overlap as the pupil closes (taking three seconds or so to 

 do so), and the end result is about as in Scylliorhinus (Fig. 91, p. 225). 



One lungfish, Protopterus, has a most peculiar pupil : the iris is quite 

 devoid of muscle elements, and yet the pupil can become a narrow hori- 



