THE ANIMAL CLIMAX-THE CHORDATES 



317 



+«ttiS. 



va&a af Ferenfid . 

 adrenol glond _ 



ranol vein 



Kidney 



urooanitol 



Fig. 13-38. Male frog urinogenital system, showing the kidney and testis enlarged and in cross-section. 



The air passes into the mouth cavity 

 through the nostrils where it is actually 

 swallowed into the trachea and lungs 

 through the glottis, a slit-like opening in 

 the rigid circular larynx (Fig. 13-34). The 

 trachea, into which the glottis opens, is 

 very short and immediately branches into 

 the two thin-walled, sac-like lungs. These 

 are very inefficient organs of respiration 

 when compared to those found in mam- 

 mals. In fact, they are so inefficient that the 

 animal must rely to some extent upon the 

 skin to supplement the lungs in obtaining 

 sufficient oxygen. 



The process of breathing in the frog 

 differs considerably from that in man be- 

 cause of the lack of both ribs and a dia- 



phragm. The air is brought into the mouth 

 through the nostrils by the sudden lowering 

 of the floor of the mouth. The valves in the 

 nostrils are then closed and the floor of the 

 mouth raised which causes the air to be 

 swallowed into the lungs. However, much 

 of the respiration takes place in the mouth 

 alone, for only now and then is the air 

 taken into tlie lungs. Apparently, respira- 

 tion can take place through the lining of 

 the mouth, as well as the lungs and skin. 

 When the animal is quiet and the water is 

 cold it can remain submerged for long pe- 

 riods of time, as through the winter months, 

 receiving all of its oxygen and giving off 

 all of its carbon dioxide through the skin. 

 Exact measurements show that actually 



