436 ADAPTATIONS TO MEDIA AND SUBSTRATES 



perhaps there also a predisposing factor in the raising of the eyes. On 

 the other hand Anableps, being a top-minnow with a terminal mouth to 

 begin with, has never needed to raise its eyes completely out of water in 

 order to feed from the surface film. Like Anableps, Mugil corsula peri- 

 odically dips its head under water to moisten the cornea. 



Amphibians and Crocodilians — Probably none of the Amphibia or 

 Crocodilini are capable of air-and-water vision. In both groups the eyes 

 are raised in the head so that they, and the nostrils, can be in air while 

 the rest of the body floats awash in concealment from enemies and prey. 

 The implication is that the eyes are adjusted primarily for aerial vision 

 and are of little or no use under water; and what little information we 

 have bears this out. In amphibious and terrestrial amphibians the eye 

 takes on its aerial adjustments during metamorphosis: the lids develop, 

 the primary spectacle becomes a part of the cornea, and the latter be- 

 comes arched, while the lens departs to some extent from the perfectly 

 spherical form which it has in the aquatic tadpole. The refractive index 

 of the lens remains fairly high, however, with a value of 1.44-1.45 in 

 common frogs. These animals are emmetropic in air, but have insufficient 

 accommodation to be anything but strongly hypermetropic in water. 

 Though the ranid frogs are the most amphibious of amphibians, they 

 have less accommodation than the strictly terrestrial bufonid toads, 

 which may have as much as five diopters. Some tree-frogs, just as ter- 

 restrial as the toads, may however have none at all. 



Whether the crocodilians are emmetropic in either air or water is not 

 known, but they have so little accommodation that they could not pos- 

 sibly have clear vision through both media. Their nocturnality and crude 

 central images make this deficiency of no consequence to them. Spend- 

 ing much time basking out of water in dry, sunlit places, the crocodilians 

 have much more perfectly 'terrestrial' adnexa than do the Amphibia. 



Turtles — With the turtles, we come to the first group of amphibious 

 vertebrates in which we can be sure that a perfect focus is attainable 

 whether the head is immersed or above the water surface. They supple- 

 ment the already superb sauropsidan machinery of accommodation (see 

 pp. 269-79) with the powerful sphincter iridis muscle, which squeezes the 

 front of the lens (Fig. 148) into a curvature of very short radius — a regu- 

 lar 'anterior lenticonus'. The range of accommodation is thus very great, 

 easily sufficient to cancel the loss of the corneal surface. The deformation 

 of the lens is facilitated by its extreme softness, which is maximal for all 



