WHERE ANIMALS LIVE 3 



If animals were entirely restricted to the land for their homes, 

 only a comparatively small part of the world would be inhabited, 

 since about three fourths of our globe is covered with water. 

 Most of this is sea water, which contains about three and one 

 half per cent of salt, whereas a comparatively small area is cov- 

 ered by the fresh water in lakes, ponds, and streams. A study of 

 the animals that live in sea water and in fresh water soon reveals 

 the fact that, with few exceptions, those accustomed to salt 

 water perish almost at once if transferred to fresh water, and 

 vice versa, those living normally in fresh water die very quickly 

 if placed in the sea. Thus are the habitats of animals strictly 

 limited. Some of the common animals that live in the sea are 

 whales, sea turtles, many fish, a host of crabs, lobsters, and 

 similar forms, cuttlefishes, oysters, many clams and snails, 

 some worms, the starfish and its near relatives, the jellyfishes, 

 corals, sponges, and thousands of different kinds of minute 

 animals that can be seen clearly only with the aid of the micro- 

 scope. 



Since most of us do not live on the seacoast, we are naturally 

 more familiar with the animals that live in fresh water. Fish, 

 crayfish, aquatic insects, the young of the mosquito, and the tad- 

 poles of frogs and toads are abundant fresh-water forms. They 

 do not, however, occur everywhere, but each kind of animal is 

 restricted to a rather definite habitat. For example, some fish 

 live in only the deepest parts of lakes, others prefer slow-flowing 

 streams, and many select the rapid waters of rivers and brooks. 

 Similarly, with other fresh-water inhabitants, each has its own 

 sort of habitat from which it very seldom strays. 



This might seem to complete the list of available habitats for 

 animals, but there is one mode of existence that from the stand- 

 point of human welfare is probably more important for us to 

 know about than any other. This is the parasitic existence led 

 by thousands of forms, like the tapeworm, liver fluke, hookworm, 

 and trichina, and that vast army of invisible foes called germs 

 which are responsible for such diseases as malaria and yellow 



