BREATHING AND HISSING OF SNAKES. 145 



From the elongated form of the puhnonary bag, and 

 the large volume of air which it contains, we can under- 

 stand not only how a temporary suspension of respiration 

 can be supported, but we comprehend how it is that these 

 reptiles can remain under water for long periods, as they 

 often do, — not because they breathe in the water, but 

 because they can for a while do without breathing. 



Snakes have been seen to remain perfectly quiet at the 

 bottom of a clear stream for half an hour or more. Some- 

 times in this totally quiescent state one has been supposed 

 to be dead, until, on a stone being thrown, it has darted 

 away like a fish. None of the aquatic birds or the cetaceous 

 mammalia can remain so long under water without coming 

 to the surface to breathe as serpents can. 



At the Zoological Gardens they remain for hours at a 

 time in their tanks. Often you will see a head peeping 

 out — which, unfortunately, is all we can see — while the bath 

 is being enjoyed, but as often the head is also immersed, 

 though, of course, for a shorter interval, the snake lifting it 

 to breathe occasionally. 



We can imagine also the great assistance in swimming 

 which this long air-receptacle must be, these reptiles deriv- 

 ing from it the same advantage, says Professor Owen, ' as 

 an eel from its swim-bladder.' In chap. xii. is described 

 the almost swimming motion of the more active snakes 

 when gliding through long grass, or effecting progress over 

 a very smooth surface. In the water the action is similar 

 — that is, the progression is by lateral undulations, the tail 

 being the chief propelling power. Whether through the 

 resisting medium of water, or beating the air, so to speak. 



K 



