THE VERTEBRATE ANIMAIS 



295 



<^ Snakes. — Probably the most disliked and feared of all animals 

 are the snakes. This feeling, however, is rarely deserved, for, on 

 the whole, our common snakes are beneficial to man. The black 

 snake and the milk snake feed largely on injurious rodents (rats, 

 mice, etc.), the pretty green snake eats injurious insects, and the 

 little DeKays snake feeds partially on slugs. If it were not that 

 the rattlesnake and the copperhead are venomous, they also could 

 be said to be useful, for they live on English sparrows, rats, 

 mice, moles, and rabbits. 



Snakes are almost the only legless vertebrates. Although the Umbs 

 are absent, stiU the pelvic and pectoral girdles are developed. The very 

 long backbone is made up of 

 a large number of ^ertebria?, 

 as many as four hundred 

 being found in the boa con- 

 strictor. Ribs are attached 

 to all vertebrse in the region 

 of the body cavity. 



Locomotion. — Locomotion 

 is performed by puUing and 

 pushing the body along the 

 ground, a leverage being ob- 

 tained by means of the broad, 

 flat scales, or scutes, with 

 which the ventral side of the 

 body is covered. Snakes 

 may move without twisting 

 the body. This is accomplished by a regular drawing forward of the 

 scutes and then pushing them backward rather violently. 



Feeding Habits. — The bones of the jaw are \'ery loosely joined to- 

 gether. Thus the mouth of the snake is capable of wide distention. It 

 holds its prey by means of incurved teeth, two of which (in the poisonous 

 snakes) are hoUow or grooved, and serve as a duet for the passage of 

 poison. The poison glands are at the base of the curved fangs in the 

 upper jaw. The tongue is very long and cleft at the end. It is an organ 

 of touch and taste, and is not, as many people believe, used as a sting. 

 The food is swaUowed whole, and pushed down by ryhthmic contractions 

 of the muscles surrounding the gullet. They usually refuse other than 

 living prey. 



Adaptations. — Snakes are usually protectively colored. They are 

 not extremely prolific animals, but hold their own with other forms of 

 life, because of their numerous adaptations for protection, their noiseless 

 movement, protective color, and, in some cases, by their odor and poison. 



Skull of boa constrictor, two thirds natural size. 

 Note the inpointing teeth. Photograph by 

 Davison. 



