BACKBONED ANIMALS WHICH BREATHE IN AIR 



425 



and which is separated by a fold from the mouth-cavity. The 

 mouth can therefore be kept open under water without fear of 



Fig. 549. Head of Crocodile to show Breathing Arrangements, hinder part in section 



BR., Brain; LA., larynx; L.J., lower jaw; N., external nostril; N'., internal nostril; SK., skull. Course of 

 air entering lungs shown by arrows. 



suffocation by entry of water into the lungs. The external 

 nostrils are valvular, and close when their owner sinks below 

 the surface. o T 



Curious modifications of the breathing-organs 

 are found in Snakes. As in the snake-like Amphi- 

 bians (Caecilians), the left lung is reduced to a 

 mere vestige, the right lung being correspondingly 

 enlarged. The reason for this arrangement would 

 appear to be the same in both cases, z>. adapta- 

 tion to the shape of the long narrow body. An 

 ordinary Snake feeds on living animals of rela- 

 tively large size, and, to prevent choking while 

 the tedious process of swallowing is going on, the 

 end of the windpipe is drawn out into a project- 

 ing tube which protrudes from one side of the 

 mouth. 



Snake-like Lizards (Amphisb&no), like true 

 Snakes, possess but one fully-developed lung, that 

 of the right side. Fi s- sso.-Lungs of 



. a Chameleon, showing 



The lungs of Chameleons (fig. 550) suggest outgrowths. TR .,wind- 



S ^. , V & . ' , pipe (trachea). 



the arrangements that Birds possess in a much 



more elaborate condition, for the hinder portions of them grow 



