442 ADAPTATIONS TO MEDIA AND SUBSTRATES 



The kingfishers — if our interpretation of their eyes is correct — are 

 quite alone among birds in having clear vision through both air and 

 water without need of accommodating at all as they pass from one 

 medium into the other. In the European Alcedo a. at this, at least, 

 Kolmer found the following situation : 



The axis of the eye is somewhat shorter than its equatorial diameter. 

 Near the ora terminalis, temporally, there is a second fovea which is as 

 well-developed as the central one, and which sits in an outpocketing of 

 the eyeball wall. The body of the lens is nearly symmetrical, but the lens 

 as a whole, due to variation in the thickness of the annular pad, is 

 strongly egg-shaped with its narrow end aimed toward the temporal 

 fovea. The long axis of the lens is thus parallel to the palpebral fissure. 

 The ciliary body is also strongly asymmetrical, narrowed nasally, and 

 has particularly powerful processes around the narrow end of the oval 

 lens. There is a well-developed sphincter iridis, but otherwise the iris and 

 ciliary body are actually poor in muscle elements; and the chorioidal 

 muscle-cells, said to be common in the neighborhood of the fovea cen- 

 tralis in other birds (see p. 281), are lacking. Kolmer 's interpretation fol- 

 lows — how right it is, we cannot be sure at present; perhaps the investi- 

 gator has been more ingenious than the bird itself : 



As the kingfisher descends through the air, the prospective prey is 

 kept in sharp focus on the central fovea of one eye, and the other eye 

 progressively converges in sympathy. The lens is moved nasally as well 

 as squeezed, due to the asymmetry of the ciliary body. This, together 

 with the evaginated location of the temporal fovea, puts the latter so far 

 from the optical center that its line of sight — not in use in air — is ex- 

 tremely myopic. As the bird enters the water, the cornea 'disappears' 

 and at once the prey is sharply registered on the temporal foveae of both 

 eyes and thus is seen binocularly. At the same instant, the line of sight 

 of each central fovea becomes hopelessly hypermetropic and remains so 

 until the bird emerges into the air once more. The kingfisher seems to 

 have improved on Anableps, in devising a lazy way of accomplishing an 

 end which costs the turtle or the cormorant many a calorie. 



Amphibious Mammals — The statement is often made that there is 

 no mammal which cannot swim if tossed into water. There are a great 

 many species which do not have to be thrown in. Brazier Howell, in his 

 work on aquatic mammals, mentions about three score which he terms 

 definitely (though not obliged to be) aquatic in their predilections. 



