538 Our Surroundings 



It is interesting, in considering our balanced world, to note 

 how the various types of living things are fitted to take their part 

 in the carbon-oxygen cycle. Animals with lungs, such as the 

 frog or the dog, breathe in oxygen and exhale carbon dioxide. 

 Insects take in oxygen through tiny holes, or spiracles, in the sides 

 of their bodies, and through them give off carbon dioxide. Fishes 

 and other water animals take oxygen from the water by means 

 of feather-like organs called gills, and through them give out 

 carbon dioxide. Trees take in oxygen through minute pores located 

 in their growing, or cambium, layers, and also excrete carbon 

 dioxide through these pores. In all living things, oxygen is taken 

 in and used in oxidation, and carbon dioxide is given off. 



In the interrelation of plants and animals the dependence of 

 all animals on plant life for food is clearly evident. At first one 

 might think that some animals live almost wholly on animal food 

 and therefore are not dependent on plants, but a little reflection 

 shows that they really are dependent on plants, since the animals 

 on which they feed derive their nourishment from plants. 



Many animals depend on plants not only for food, but for 

 shelter and protection as well. Nearly all wild animals seek the 

 forest or the jungle as a refuge from their enemies, and many 

 make their homes there. Birds build their nests in the branches of 

 trees or, like the woodpecker, in holes in the trees. Bees often 

 make their homes in hollow trees. So do squirrels. Many kinds 

 of insects live on trees, shrubbery and grass. 



Relation of Animals to Man. Most of the animals of the 

 world are regarded either as the friends or as the enemies of man. 

 Many, like the domesticated animals of home and farm, are very 

 useful. Others do harm, causing disease, death or loss of property. 



Domestic Animals. Early in his history man tamed certain 

 animals and made them serve him. They were important factors 

 in his rise to civilization, and they are still very important. With- 

 out horses, cows, sheep, hogs and fowls, farming would be greatly 

 restricted and the food supply of the world would be dangerously 

 reduced. 



Birds. Birds in their relation to man are both beneficial and 

 harmful. They are beneficial when they destroy insects, rats and 



