HOW ANIMALS BREATHE. 



747 



using the skin instead. Even the frogs and toads can long survive 

 the removal of their lungs. Many species of this class aid respiration 

 by swallowing air. Frogs and toads force the air into their lungs by 

 a swallowing action, made necessary by the absence of ribs ; and they 

 can be suffocated, and turtles also, by holding their mouth open. 



In the amphibians, we have a class of animals making use of four 

 distinct means of respiration ; three of them skin, gills, and lungs 

 in about equal degree. It might, therefore, be supposed that their res- 

 piration should be more active and the aeration of their blood more 

 complete than in other vertebrates. Nevertheless, quite the reverse is 

 the truth, for the reason that any function is better performed when 

 localized or " specialized." 



This latter fact is illustrated in the reptiles, which have a circula- 

 tion as incomplete as the frog, with respiration more active, although 



Fig. 7. Longitudinal Section of a Bleak, s, anterior; s', posterior portion of the air-bladder ; 

 <x, oesophagus ; /, air-passage of the air-bladder. (After Semper.) 



possessing only lungs. These are, of course, better developed in the 

 reptile, where they are large sacs, having the interior surface increased 

 by foldings, producing sacculi. Serpents have the left lung undevel- 

 oped, the right one forming a long, cylindrical sac capable of holding 

 a large amount of air. By this means water-snakes are rendered 

 buoyant, and fitted for long submergence. The last fact is also true 

 of turtles. 



Fig. 8 Dipnoi {Lepidosiien anneclens), one of the Mud-Fishes, using the Air-bladder as 



a Lung. 



Lizards and crocodiles have two lungs, usually somewhat divided, 

 and extending through the whole trunk. By their inflation the chame- 

 leon can give itself a plump appearance. 



Reptiles have only a slight motion of the ribs, and are remarkable 

 as a class for feebleness of respiration, considering that their lungs are 



