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 are 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 are 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- 
sirens are 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 are furnished with extensions 
which have no blood vessgls. 
H 
