THE REPTILES 197 



remain functional throughout life. Corresponding to the 

 shape of the body, these are usually much elongated and 

 ordinarily paired (Fig. 118, l.lng.). The snakes are peculiar 

 in having the left lung rudimentary or even lacking com- 

 pletely, while the right one becomes greatly elongated and 

 extends far back into the body. In nearly all the reptiles 

 the amount of oxygen brought into the lungs is relatively 

 large and the activity of the animal is proportionately 

 great. The circulation of reptiles shows an advance be- 

 yond that of the Amphibia. As in the latter, there are 

 two distinct auricles ; but the chief difference arises from 

 the fact that the ventricle is more or less divided by a par- 

 tition which to a considerable degree prevents the blood 

 returning from the lungs from mixing with the impure 

 blood as it returns from its journey over the body. In the 

 crocodiles and alligators the partition is complete, and the 

 circulation thus approaches close to that of the higher 

 animals. 



186. Hibernation. Attention has already been called to 

 the fact that reptiles are very susceptible to cold, rapidly 

 growing less active as the temperature lowers. When win- 

 ter comes on they seek protected spots, and either alone 

 or grouped together hibernate. The various activities of 

 the body during this period are at very low ebb. The blood 

 barely circulates, breathing is imperceptible, and stiff 

 and insensible to the world about them they remain until 

 the warmth again stirs them to their former activity. 

 Some of our common turtles must also pass a somewhat 

 similar sleep while embedded far down in the mud during 

 the disappearance of the ponds in summer. At such times 

 no food is taken, but owing to their loss in weight it is 

 probable that a slow consumption of the body supplies the 

 small amount of necessary energy. 



187. Nervous system and sense-organs. At first sight one 

 is struck with the small size of the brain of fishes, Am- 

 phibia, and reptiles. Their intelligence likewise is at low 



