COMPARATIVE SURVEY OF THE TWO METHODS 161 



some deep-sea fishes (Fig. 84c, p. 213), the lens then being huge and 

 filling the anterior chamber. The teleost pupil may close slightly in 

 accommodation, probably passively due to its elasticity, when unblocked 

 by the receding lens; but this is quite meaningless not only because the 

 lens needs no differential stopping (being fixed in shape), but because 

 the active accommodation of teleosts is for distant, not near, objects 

 (see Fig. 98, p. 251). 



The pupils of amphibians have more excursion than those of teleosts, 

 though not as much as they would need if the amphibians did not have 

 rather good photomechanical changes. Thus the frog, despite its poten- 

 tially sensitive duplex retina and its extremely large rods (Fig. 64, 

 p. 148) , has a pupil identical in behavior with that of a pure-cone grass 

 snake devoid of photomechanical changes. If the frog also lacked retinal 

 photomechanical changes, his pupil would have to close farther than it 

 does to permit him to be out where the snake could see him! A few anur- 

 ans have peculiarly shaped pupils. That of Bombina contracts to a play- 

 ing-card 'heart'; and in those whose retinae are probably the most sensi- 

 tive, the spade-foot toads (Scaphiopus spp.), the contracted pupil is a 

 vertical lozenge, the playing-card 'diamond'. The two other suits of the 

 deck are apparently not represented among amphibian pupils, but there 

 are still other weird shapes whose meaning is quite unknown (Fig. 87, 

 p. 223). 



The weak amphibian sphincter pupillae is replaced by a much more 

 powerful one in the reptiles and here, as in birds also, the iris and ciliary 

 muscles are of the striated variety. This change may have been inevitable 

 upon the supervention of a control which is almost completely nervous 

 and sometimes voluntary, though the return to smooth intra-ocular 

 muscles in the mammals argues against this supposition. At any rate, 

 the sauropsidan iris is capable of extremely rapid action, though par- 

 ticular species do not necessarily ever tax this capacity. The turtle pupil, 

 fish-like, is blocked by the lens and does not respond to light at 

 all, contracting only as an accessory to accommodation. Turtles have 

 practically pure-cone retinae with slight, slow retinal migrations or none 

 at all. Their insensitive eyes require neither type of protection from 

 strong light, and in turn limit their possessors to photopic vision and to 

 dependence upon olfaction in dim light or muddy waters. 



In diurnal lizards the iris is but slightly responsive to light, as is true 

 also of diurnal snakes, some of which have quite motionless pupils. 

 Nocturnal lizards are usually pure-rod, and nocturnal snakes are rod- 



