XIII PHYLUM CHORDATA 367 



already stated, are viviparous ; in all cases the young are left to 

 shift for themselves as soon as they are born. 



Most of the Snakes also are extremely active and alert in their 

 movements; and most are very intolerant of cold, undergoing 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 sur- 

 face 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. 1'he 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 Snake 

 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. 1009, Mc), 



ii ^m 



Fig. 1009.— Poison apparatus of Rattlesnake. A, eye; Gc, poison-duet entering the poison- 

 fang at t ; Km, muscles of mastication partly cut through at * ; Mc. constrictor (masseter) 

 muscle ; Mc'. continuation of the constrictor muscle to the lower jaw ; N. nasal opening ; S, 

 fibrous poison sac ; z, tongue ; za, opening of the poison-duct ; zf, pouch of mucous membrane 

 enclosing the poison-fangs. (From Wiedersheim's Comparative Anatomy.) 



one of the muscles which raise the lower jaw; it is thus forced 

 along the duct (Gc) to the aperture (za), and injected into the 

 wound made by the fang. The effect is to produce acute pain 



