272 Adjustment to Conditions of Aquatic Life 



II 



The mammals that live in the water are two small 

 orders of whales, Cetacea and Sirenia, and a few 

 scattering representatives of half a dozen other orders. 

 Tho few in number they represent almost the entire 

 range of mammalian structure. They vary in their 

 degree of fitness for water life from the shore-haunting 

 water-vole, that has not even webbing between its toes, 

 to the ocean going whales, of distinctly fish-like form, 

 that are entirely seaworthy. It is a fine series of 

 adaptations they present. 



For all land-animals, returned to the water to live, 

 there are two principal problems, (i) the problem of 

 getting air and (2) the problem of locomotion in the 

 denser medium. Warm-blooded animals have also 

 the problem of maintaining the heat of the body in 

 contact with the water. To begin with the point last 

 named, aquatic mammals have solved the problem of 

 heat insulation by developing a copious layer of fat and 

 oils underneath the skin. This development culminates 

 in the extraordinary accumulation of blubber in arctic 

 whales. 



No aquatic mammals have developed gills. They all 

 breathe by means of lungs as did their terrestrial ances- 

 tors. All must come to the surface for air. Their 

 respiratory adaptations are slight, consisting in the 

 shifting of the nostrils to a more dorsal position and 

 providing them with closable flaps or valves, to prevent 

 ingress of the water during submergence. 



It is with reference to aquatic locomotion that 

 mammals show the most striking adaptations. About 

 in proportion to their fitness for life in the water they 

 approximate to the fish-like contour of body that we 

 have already discussed (page 249) as stream-like form. 

 Solidity and compactness of the anterior portion of the 



