78 THE FOOD OF ANIMALS 



for life in fresh water or even in the sea. The prey in most cases 

 consists of living animals, often of relatively large size, and it is 

 clear that a comparatively narrow animal, devoid of any chewing 

 arrangement, must be modified in structure so as to enable it to 

 deal with such prey, which after capture has to be swallowed 

 whole. The mouth, of course, must be capable of great dilata- 

 tion, and this is made possible by the presence of a double hinge- 

 joint where the lower jaw is attached, and by the fact that the 

 two halves of the lower jaw are connected in front by an elastic band 

 instead of being firmly united together as is the rule in backboned 

 animals. Escape of the prey when this is swallowed alive, as is 

 usually the case, is prevented by the sharp, backwardly - curved 

 teeth. There must also be some way of preventing the snake 

 from choking during the slow process of swallowing, and this is 

 found in the peculiar nature of the top of the windpipe, which 

 is drawn out into a cone that protrudes from the corner of the 

 mouth while the prey is passing slowly down the gullet. Not 

 only is the mouth capable of great expansion, but also the gullet 

 and stomach, as well as the part of the body in which they are 

 contained, an important point to notice here being the complete 

 absence of a breast-bone and limb-girdles, which, if present, would 

 prove a serious hindrance to the excessive dilatation which is 

 absolutely necessary. Although an average snake affects meals 

 of a bulky sort, which are digested with comparative ease, long 

 intervals elapse between them. A constant supply of water for 

 drinking is, however, indispensable. 



Snakes have various ways of dealing with their prey, and it 

 will be convenient to take harmless forms first, afterwards con- 

 sidering the nature and mode of action of the poison-apparatus in 

 the venomous species. A convenient point of departure is afforded 

 by the common Grass Snake (Tropidonotus natrix) (see vol. i, 

 p. 233) of Britain, which feeds on frogs, small mammals, birds, 

 and even fishes, for it is an expert swimmer. These are secured 

 by the numerous pointed teeth, and swallowed alive. The unfor- 

 tunate frog, which has so many enemies, from man downwards, 

 is the favourite prey of this snake, which, as so often happens with 

 these reptiles, seems to exert a kind of hypnotic influence upon it, 

 for, instead of trying to escape, it stands still, uttering cries of fear. 

 Next moment it is seized and gradually swallowed, protesting 

 meanwhile in a way which is far from pleasant to witness. 



