444 ADAPTATIONS TO MEDIA AND SUBSTRATES 



is greatly developed, and further studies may complete a picture of equal 

 visual capacity in the two media, in each of which the beaver certainly 

 performs as though it had excellent vision. 



The otter has repeated the device of the turtles and the cormorants. 

 The ciliary muscle is very well developed, and in addition there is an 

 enormous sphincter in the iris which squeezes the anterior portion of the 

 lens. The range of accommodation is unknown, but in air the eye is 

 emmetropic or slightly hypermetropic, and the otter is known to hunt 

 under water largely by sight despite the small size of the eye. It is entirely 

 likely that the focusing power of the cornea is not at all missed. The 

 adnexa of the otter appear to be quite unmodified. The nasolacrimal 

 duct is nowadays stated to be present, though this was once denied. 



The Pinnipedia, first cousins of the terrestrial carnivores, are more at 

 home in the water than any other mammals except the whales. As with 

 the whales, there are two large divisions of the group which differ some- 

 what in the character of their adaptations. The Phocidse or 'true' seals 

 are extremely clumsy on land owing to the profound modification of 

 their limbs; and they have larger eyes in keeping with their habit of feed- 

 ing upon relatively small prey caught in comparatively deep water. The 

 elephant seal feeds at depths of three hundred to seven hundred feet. 

 The Otariidse (sea-lions or eared seals) are more comfortable on land, 

 being still able to turn their hind feet into something like the standard 

 mammalian walking position. They feed on fairly sizable squids and 

 fishes, and are not believed to swim very deeply. Intermediate between 

 these two families in many structural respects come the moUuscivorous 

 Odobaenidae, the walruses. 



The visual axes of pinnipeds are canted upward to some extent rather 

 than downward as in the strictly water-seeing cetaceans. This is prob- 

 ably related to their vital need of spying out the landing place before 

 crawling out onto it — their terrestrial clumsiness is considerable of a 

 hostage to fortune in the form of the nearest white bear. The eyes aim 

 strongly laterally, the binocular field being about as wide as in the aver- 

 age terrestrial carnivore. In the elephant seal, the young animal has 

 strongly frontal eyes which swing farther laterally during growth — the 

 reverse of the usual ontogenetic change in the attitude of the optic axes. 

 The lid opening is shorter than the diameter of the cornea, which com- 

 pared with that of a fish is relatively small to begin with. But this is no 

 sign of degeneracy — the seals roll and wriggle so much, in their acrobatic 

 swimming, that they would probably be hard to approach unseen even if 



