440 ADAPTATIONS TO MEDIA AND SUBSTRATES 



believed by non-scientific (and some scientific) observers to have their 

 habit of eating pebbles for the purpose of ballasting, as in the case also 

 of the elephant seal. They feed largely upon the same squids and 'krill' 

 as do the baleen whales, but catch them individually and visually, rather 

 than by trawling, for which they have no equipment. 



Next come the cormorants, loons, auks, sea-ducks and diving ducks 

 in general. All of these are able to secure sharply-focused images in both 

 air and water, though not all by the same means. Last come the many 

 species of plungers exemplified by the terns. These probably have no 

 special ability to see in water, and characteristically make only a blind 

 stab for the fish which they have spotted from the air. They often miss, 

 as compared with the birds which beat the fishes at their own game of 

 underwater swimming. 



The ocular devices employed by the birds with truly amphibious vision 

 fall into three categories, exemplified respectively by the cormorants, the 

 diving ducks, and the kingfishers. The simplest of these devices is that 

 of the cormorant, and is developed to about the same degree also in the 

 booby : 



The cormorant compensates for the loss of the cornea in exactly the 

 same way as does the turtle. Its iris is the most muscular one in all the 

 vertebrates, and the deformation of the lens by the powerful sphincter 

 is extreme (Fig. 149). During the process, the pupil at first closes slightly, 

 but then enlarges again as the iris tissue is rolled outward by its pressure 

 against the lens. The lens is as soft as that of a turtle, which probably 

 cannot be said of that of any other bird. In contrast to the few diopters 

 of accommodation of land birds, the cormorant has 40-50 diopters — two 

 or three times the range of the human infant, which is the most accom- 

 modating (?) of mammals, but owes its extensive range to the juvenile 

 pliability of the lens and not to any real need. 



The diving ducks, loons, and auks have much more powerful iridic 

 sphincters than do the non-diving ducks and land birds. They have an 

 action approaching that in the cormorants, whereas in other birds the 

 iris molds the lens only passively as the lens is pressed against it by the 

 ciliary processes. But these birds are as well off under water as the cor- 

 morant, and at less expense of muscular energy. Ischreyt, the leading 

 student of their eyes, found that in all of them the nictitating membrane 

 has a clear, lens-like central window, composed of highly refractive ma- 

 terial which is capable of bending light rays even under water. If we 

 compare the devices of Anableps and the kingfishers (r. i.) with bifocal 



