HOW THE FROG BREATHES 



The frog swallows air into the lungs. Lowering the floor of the mouth enlarges the 

 mouth cavity, and air comes into it through the nostrils. The nostrils are closed, and 

 the floor of the mouth is raised. The air is thus forced into the pipe leading to the 

 lungs. If the frog were forced to keep his mouth open, he would suffocate 



Reptiles and all the higher vertebrates breathe entirely by means of lungs. 

 Reptiles swallow air, as do the amphibians. Birds rely solely on rib move- 

 ments, as they have no diaphragm. All mammals breathe like man. Water- 

 snakes and snapping turtles spend most of their time in water, but come to 

 the surface from time to time to breathe. Alligators and crocodiles have 

 raised nostrils, which protrude above the water when the rest of the animal 

 is submerged. Whales, like other mammals, breathe air in lungs. 



In Brief 



Living cells always exchange gases with the liquid which immediately 

 surrounds them. 



In many-celled organisms, cells remote from the surface get their oxygen 

 supply indirectly. 



Roots get oxygen that is dissolved in the soil water which immediately 

 surrounds them. Air diffuses into the leaves and bark of plants through 

 special openings. 



Plants growing in swampy areas have shallow root-systems; roots suffo- 

 cate if submerged too long or too deeply. 



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