384 METAZOAN PHYLA 



the ribs extended beyond the sides of the body and covered by a thin 

 membranous fold of the skin, forming a sort of ghding plane. In tropical 

 America are large lizards known as iguanas which are a favorite article 

 in the native diet. The horned toads of the West are lizards. The only 

 poisonous lizard known is the Gila monster found from southwestern 

 United States to Central America. The largest lizards in the world are 

 the monitors of the Lesser Sunda Islands in the Malay Archipelago which 

 reach a length of 15 feet. 



416. Snakes. — Snakes differ from lizards and chameleons in the 

 structure of the lower jaws; in the absence of both free hmbs and girdles, 

 though in a few forms a trace of the pelvic girdle is present; and in the 

 absence of a urinary bladder. 



Owing to the fact that the lower jaws on the two sides are but very 

 loosely connected in the median line and that they are also loosely 

 attached to the quadrate bone, which is in turn loosely attached to the 

 skull, the mouth may be greatly expanded. Since there is no sternum 

 and the ribs are not attached ventrally, the throat and body are capable 

 of great distension. Thus snakes, which always swallow their food 

 whole, are able to ingest objects actually much greater in diameter than 

 the head or body of the snake itself. During the process of swallowing, 

 the teeth, which point backward, are used to hold the prey and those 

 of the two sides are brought into use alternately. While the teeth of the 

 jaws on one side are holding the prey, those of the jaws on the other side 

 are loosened and that side of the mouth is carried forward over the 

 object being swallowed, after which those teeth are again set in. Then the 

 teeth on the first side are loosened and that side of the mouth is carried 

 still farther ahead and those teeth in turn set in. Thus by working the 

 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 

 Uke 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 

 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 ghding 

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

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



