vi FORM AND FUNCTION 97 



Lungs of Lower Vertebrate Animals. 



In comparing reptiles and birds in a previous chapter, 

 I said nothing about the lungs, because I thought it 

 would be more intelligible after some account of the 

 machinery of breathing and its working had been 

 given. 



If birds really had reptilian ancestors, it would be 

 very odd if existing reptiles had no trace of any 

 development similar to the air-sacks that in birds are 

 so characteristic a feature. There is one reptile that 

 has unmistakable air-sacks — the Chameleon. They 

 are small, it is true, but in their nature the same as 

 the bird's. The snake's one fully developed lung (the 

 other has shrunk to insignificance) is suggestive of 

 a bird's ; it is a bag the walls of the front part of 

 which arc full of blood vessels. The hinder part is 

 simply a reservoir of air. The same is the case with 

 the lizard's lungs. In crocodiles, they arc more com- 

 plicated, not at all like mere bags as they are in 

 snakes and lizards. In this point, too, crocodiles come 

 nearer to birds than other reptiles. It is curious that 

 the swim-bladder of fishes, like lungs, an outgrowth 

 from the alimentary canal, but, unlike lungs, an out- 

 growth from its dorsal (or hinder) wall, often has its 

 anterior half covered with blood vessels, while the 

 hinder part is simply a membranous bag. The lepido- 

 sirensare fish, which, if left in the mud when their river 

 dries up, become air-breathers ; they have true lungs, 

 pouches opening from the ventral (or front) wall of 

 the gullet, and these arc furnished with extensions 

 which have no blood vessels. 



H 



