438 ADAPTATIONS TO MEDIA AND SUBSTRATES 



touch and smell. It is perhaps because of this handicap in sensitivity that 

 the turtles have developed the most completely transparent corneae, 

 humors, and lenses of any vertebrates. 



Amphibious Squamates — Among the lizards there is at least one 

 conspicuously amphibious form, the marine iguana of the Galapagos 

 Islands, Amblyrhynchus cristatus. These great lizards feed mostly upon 

 bottom sea-weeds at some distance from shore; but it was Charles Dar- 

 win who first demonstrated that their every instinct is to cling to the 

 land or make for shore when they are attacked or frightened. It is be- 

 lieved that they feed in the sea out of dire necessity rather than choice. 

 Their eyes have never been studied, but it is unlikely that they are any- 

 thing but aerial in their adaptations. It will be recalled that the very 

 thoroughly aquatic sirenians, with similar feeding habits, get along with 

 eyes which can only be very poor-sighted under water. 



There are many amphibious snakes. The most completely aquatic of 

 them, the marine cobras (Hydrophiinae) and the fluviatile Homa- 

 lopsinas, are practically unknown, ophthalmologically. The river snakes 

 —Acrochordus javanicus for example — have the eyes toward the top of 

 the head, but this does not necessarily mean that they are ever used out 

 of water any more than does the same situation in the angler-fishes and 

 star-gazers. 



Years ago, Beer studied Natrix tessellatus, a European relative of our 

 common water snakes, and found it to differ in two respects from ter- 

 restrial colubrids. The lens was not completely firm, and when removed 

 from the eye it took on the shape it has in accommodation, just as does 

 a human lens. This unusual softness permits the tessellatus lens (and 

 those of our Natrix species, rainbow snakes, etc.?) to be squeezed by the 

 pupillary sphincter as in the turtles, thus greatly extending the range of 

 accommodation. In other snakes it is only those circular muscle fibers 

 massed toward the root of the iris which are much concerned with accom- 

 modation, and the process (see p. 282) changes only the position of the 

 lens and not its form. The sea-snakes contract their pupils to stenopaic 

 pinholes when out of water, thus solving their problem somewhat as the 

 seals (v. i.) have done. 



Amphibious Birds — The birds had no sooner come into existence as a 

 group than some of them, like the extinct Hesperornis, promptly took to 

 the water. Many groups, and many scattered species, have become more 

 or less aquatic since. Some are very decidedly so, and can fly as well (or 



