416 



COLLEGE ZOOLOGY 



nary bladder is absent; and (4) the brain 

 case is closed anteriorly. 



Snakes are covered with scales; those on 

 the head are usually so regular as to be of 

 importance in classification (Fig. 287B,C). 

 On the ventral surface in front of the cloacal 

 opening 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 possesses a pair of short spurlike pro- 

 jections, one on each side of the cloacal 

 opening— vestiges of the hindlimbs (Fig. 

 287A). 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 



Ilium 



Femur 



Figure 287. A, vestigial hindlimb and girdle bones of the python. The skeletons of nearly all 

 snakes are without limbs but the pythons are among the exceptions. These remnants of hind- 

 limbs suggest that the ancestors of snakes traveled on legs. B, scales on anterior end of the 

 hognose snake or puff adder. C, scales on anterior end of black snake or blue racer. (A after 

 photo, courtesy of Chicago Natural History Museum; B, C, after photo, courtesy of General 

 Biological Supply House.) 



there is much doubt regarding the existence 

 of a sense of hearing. The tongue is a slen- 

 der, 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 serves as an auxiliary olfac- 

 tory organ, carrying odorous particles to the 

 paired organs of Jacobson in the roof of the 

 mouth. The prevalent idea that the tongue 

 can inflict an injury is erroneous. Further- 

 more, there appears to be no good evidence 

 that the tongue is sensitive to vibrations. 

 The teeth are sharp and curve inward (Fig. 

 288). They are adapted to prevent food 

 from slipping forward, once swallowing has 

 commenced. In the venomous snakes certain 



teeth are grooved or tubular and serve to 

 conduct venom into any animal bitten. 



Snakes do not chew their food but swal- 

 low it whole. They can eat animals much 

 larger than their own bodies (Fig. 290). 

 Some of the structural adaptations making 

 this possible are: (1) the lower jaw joins 

 with the skull very loosely, by means of two 

 slender bones (quadrates); (2) furthermore, 

 the lower jaw can spread at the anterior 

 midpoint, allowing for lateral expansion; 

 and (3) the bones of the palate are movable. 



Movement on land is accompanied by 

 several types of motion, but the two prin- 

 cipal ones are: lateral undulations of the 

 body and the shifting of the abdominal 



