558 COLLEGE ZOOLOGY 



(3) the urinary bladder is absent; and (4) the brain case is 

 closed anteriorly. 



Snakes are covered with scales; those on the head are so 

 regular as to be of importance in classification. On the ventral 

 surface in front of the anus is a single row of broad scales, called 

 abdominal scutes, to which the ends of the ribs are attached. 

 The outer, horny layer of the skin is shed a number of times 

 during the year. Appendages are entirely absent except 

 in a few species, like the python, which possess a pair of 

 short spur-like projections one on either side of the anus, — 

 vestiges of the hind limbs. The eyelids are fused over the 

 eyes, but there is a transparent portion which allows the 

 animal to see. When the skin is being shed, the snake is 

 partially blind. 



There is no tympanic membrane, and the sense of hearing 

 is very slightly developed. The tongue is a slender, deeply 

 notched protrusible structure that can be thrust out even when 

 the mouth is closed, because of the presence of grooves in the 

 jaws. It is very sensitive to vibrations and probably serves as an 

 organ of hearing. The prevalent idea that the tongue can inflict 

 an injury is erroneous. The teeth are sharp and recurved. 

 They are adapted for forcing the food into the throat. In the 

 venomous snakes certain teeth are grooved or tubular, and serve 

 to conduct poison into any object bitten. 



The bones of the skull are so arranged that the jaws are ex- 

 tremely mobile. The snake is on this account able to swallow 

 objects four or five times the diameter of its neck. When 

 swallowing, the glottis is pulled forward, thus preventing the 

 snake from choking. The vertebrae are very numerous — 

 there may be over four hundred — and a large number of ribs 

 are also present. 



Movement on land is accompanied by lateral undulations of 

 the body. The body is drawn forward by pressing the rough 

 posterior edges of the abdominal scutes against the substratum. 

 Snakes cannot move forward on a smooth surface. Most 



