720 EVOLUTION OF ANIMALS Part V 



behind it. Such breaks occur in definite cleavage planes through the middle 

 of a vertebra. The lost part of the tail is replaced on a simpler plan without 

 true vertebrae and often with a different kind of scales. 



Snakes. Snakes travel on the ground. They also climb trees and the "flying 

 snake" of India is a glider. Snakes work their way into crevices and holes 

 made by other animals and the "earth snakes" of southern India are blind 

 burrowers. They swim easily and a few tropical ones spend most of their lives 

 in the water. Nevertheless, the real home of the great majority is the surface 

 of the ground, in touch with earth and plants. There they hunt their prey, 

 waiting for it or silently slithering after it. 



Snakes are superlatively streamlined examples of an efficiency of omission. 



Fig. 35.8. Skeleton of cobra. The skeletons of nearly 

 all snakes are without limbs, limb girdles and sternum. 

 Pythons are among the exceptions. (Courtesy, Rand: 

 The Chordates. Philadelphia, The Blakiston Co., 1950.) 



Their heads are wedge-shaped without extensions of ears or feelers. They are 

 without limbs, limb girdles and sternum, the absence of the latter a convenience 

 since all snakes swallow whole animals, mice, rabbits, or sheep (Fig. 35.9). 

 Only primitive snakes, the pythons and boas, have vestiges of hind legs. 

 The complexity of the middle ear is reduced, without a membranous eardrum 

 and located far back on the head, an advantage since snakes open their mouths 

 back to their internal ears. There are no movable eyehds, but the delicate 

 cornea is protected by a hard transparent cover that is shed when the outer 

 skin is molted. The eye is never left unprotected. There are no vocal cords 

 and no voice. There is no urinary bladder; metabolic waste is semisolid and 

 water is conserved as it is in birds. 



In spite of all these omissions, snakes have all the essential structures of 

 such wide-bodied relatives as the turtles. The pairs of lungs, kidneys, and 



