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 life are much more marked in 
marine than in fresh-water forms. Marine turtles have 
their limbs turned into paddles ; fresh-water Chelonians 
swim by limbs 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 
have 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 like water-shrews, have only slightly 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 life 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 little modified, 
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