xii PHYLUM CHORDATA 451 



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 drawing 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 (Fig. 256 Mc\ one of the muscles which raise 

 the lower jaw : it is thus forced along the duct (Gc) to the 



G G 2 



