394 



METAZOAN PHYLA 



two sides of the mouth alternately, the animal gradually forces the 

 object down into its esophagus, through which it is passed by peristaltic 

 movements to the stomach. Since during the process of swallowing food 

 the passage of air through the mouth would be interfered with, the glottis 

 is carried far forward in the floor of the mouth, opening just behind the 

 lower teeth. The cartilages of the trachea prevent it from being closed, 

 and breathing is in this way permitted while the food is being swallowed. 

 Over the sides and back of the body of a snake the scales overlap 

 like shingles on a roof. On the ventral surface, however, there is a series 

 of broad scales known as scutes, each one of which runs from one side of 

 the body to the other and which overlap like the weatherboards on a 



Fig. 283. — Eggs of the bull snake, Pituophis sayi (Schlegel), which are buried in the 

 ground and incubated and hatched by the heat from the sun. {From J. E. Guthrie, The 

 Snakes of Iowa, Bulletin #239, Iowa State College of Agriculture, 1926.) 



house. The posterior margin of a scute can be projected, and as its 

 rough edge is pressed against the surface on which the snake is, it serves 

 as an organ of prehension. As waves of contraction pass from the head 

 backward, while the scutes are used in clinging, a slow forward gliding 

 results, which is the normal mode of locomotion. This is usually accom- 

 panied by lateral convolutions. The latter mode of locomotion is also 

 used on a smooth surface where the scutes do not gain a hold. When the 

 snake is alarmed, it attempts to make more rapid headway by alternately 

 throwing the body into coils and then straightening it again. Swimming 

 is also accomplished by lateral convolutions of the body. 



The whole epidermal covering is shed at one time and several times 

 during a year. After a new horny covering is formed under the old one, 

 the latter is freed along the margins of the jaws and the animal literally 



