xii PHYLUM CHORDATA 453 



they simply bury in the earth, leaving them to be hatched 

 by the heat of the sun. Some, however, are viviparous ; in 

 all cases the young are left to shift for themselves as soon as 

 they are born. 



Snakes are also usually extremely active and alert in their 

 movements; and most are very intolerant of cold, under- 

 going a hibernation of greater or less duration during the 

 winter season. Many live habitually on the surface of the 

 ground — some kinds by preference in sandy places or 

 among rocks, others among long herbage. Some (tree- 

 snakes) live habitually among the branches of trees. 

 Others (fresh-water snakes) inhabit fresh water; others 

 (sea-snakes) live in the sea. The mode of locomotion of 

 snakes on the ground is extremely characteristic, the 

 reptile moving along by a series of horizontal undulations 

 brought about by contractions of the muscles inserted into 

 the ribs, any inequalities on the surface of the ground 

 serving as fulcra against which the free posterior edges 

 of the ventral shields (which are firmly connected with the 

 ends of the ribs) are enabled to act. The burrowing blind- 

 snakes and other families of small snakes feed on insects 

 and worms. All the rest prey on vertebrates of various 

 kinds — fishes, frogs, lizards, snakes, birds and their eggs, 

 and mammals. The pythons and boas kill their prey by 

 constriction, winding their body closely round it and draw- 

 ing the coils tight till the victim is crushed or asphyxiated. 

 Some other non-venomous snakes kill with bites of their 

 numerous sharp teeth. The venomous snakes sometimes, 

 when the prey is a small and weak animal such as a frog, 

 swallow it alive : usually they first kill it with the venom of 

 their poison-fangs. 



When a venomous snake strikes, the poison is pressed out 

 from the poison-gland by the contraction of the masseter 



