412 ADAPTATIONS TO MEDIA AND SUBSTRATES 



holds the records for bodi depth and duration of submergence, for it has 

 been known to go down, at a speed of eight knots, for more than a mile, 

 and to stay there for 105 minutes. This performance is related to the 

 character of the prey, for the cachalot prefers to feed upon the deep- 

 water giant squid, Architeuthis princeps. 



The several species of beaked whales (Ziphiidae) bridge the gap from 

 Physeter and its pygmy relative Kogia (a latinization of 'codger') to 

 the great family Delphinidae, incorporating the fifty-odd species of dol- 

 phins and porpoises. These forms are relatively small, ranging from little 

 four-foot river dolphins to animals fourteen or more feet long, the very 

 distinct narwhal and beluga being still larger. The delphinids are pelagic, 

 and many are notoriously playful. Porpoises are fond of racing against 

 steamers, and are perfectly capable of keeping up with the swiftest of 

 the fishes, the 50-knot marlins. Their rolling and frequent breaching is 

 mere exuberance, and the eye is probably as completely useless in air as 

 is that of a mysticete. A small, separate family of freshwater dolphins 

 includes the susa, Pldtanista, already described as having eyes which are 

 pecuUarly degenerate (p. 210). 



Optically, the whale eye has reverted to the fish type — perfectly so, in 

 the Odontoceti. Otherwise, like that of the Sirenia, it has greeted the 

 water not as an ancient friend, but as a new enemy. Unable to shake off 

 all of its previous terrestrial modifications, it has superimposed upon 

 them still other changes, to make of itself a terrestrial eye secondarily 

 adapted to water by being shielded from water : 



The same glandular pattern that we noted in the sea-cows has also 

 been independently developed by the whales. Harder's gland is conspic- 

 uous, and if a lacrimal gland is present it also secretes oil. The sebaceous 

 Meibomian glands have disappeared along with the tarsal plates in 

 which they are embedded in other mammals, but the palpebral conjunc- 

 tiva is paved with small oil-glands. This abundant provision for making 

 the exposed part of the eyeball salt- and waterproof, and immune to 

 friction, is aided by the cornification of the corneal epithelium, some- 

 thing which is seen elsewhere only in the seals, and in one or two ant- 

 eating mammals — with a different, but obvious, meaning there. The tear- 

 draining mechanism, sac and duct, has of course vanished. The whales 

 are the only aquatic mammals which are quite devoid of eyelashes, and 

 there is no nictitating membrane, no retractor bulbi. 



No whale is known to be able to rotate the eyeball, though the extra- 

 ocular muscles are present and are often quite massive. Their reten- 



