THE ANIMALS OF LAKES AND RIVERS 193 



A very large number of birds haunt the vicinity of 

 fresh water, being attracted to it, especially at certain 

 seasons, by the relative abundance of food there. 

 Such forms, however, live on the surface rather than 

 in the water. 



In regard to reptiles it is interesting to note that 

 among the Chelonia, as among mammals, the adapta- 

 tions to the aquatic hfe are much more marked in 

 marine than in fresh-water forms. Marine turtles have 

 their limbs turned into paddles ; fresh-water Chelonians 

 swim by Hmbs which are only slightly modified as com- 

 pared with terrestrial forms. This is quite parallel to 

 the conditions which occur in mammals. Such typically 

 marine mammals as seals. Cetaceans, and Sirenians 

 ha.ve their limbs (when present) turned into paddles. 

 Such fresh-water carnivores as otters, fresh-water 

 rodents like water-voles and beavers, fresh-water in- 

 sectivores hke water-shrews, have only sUghtly modified 

 limbs, retaining many features found in terrestrial 

 types. This is equivalent to saying that for an air- 

 breathing animal the modifications which fit an animal 

 for life in the sea must necessarily be more profound 

 than those which fit it for hfe in fresh water, which 

 usually occurs in relatively small masses, and where 

 the animal inhabitants often retain considerable depen- 

 dence upon the land. 



Among the Chelonia a considerable number spend 

 at least a great part of their time in the water. As 

 two examples we may take the American Snapping 

 Turtle (Chelydra serpentina), a form reaching a con- 

 siderable size, which is found in lakes, ponds, and rivers 

 throughout the greater part of North America. It has 

 strongly webbed feet and is carnivorous. As might be 

 deduced from its limbs, which are but littje modified, 



1404 N 



