202 Descriptive Zoology. 



row, and continues back, as a transparent air sac, through 

 most of the length of the body cavity. The left lung is 

 rudimentary, sometimes being so small it is difficult to see. 

 The windpipe is long, beginning very close to the front of 

 the floor of the mouth. This is regarded as an adaptation 

 to the mode of eating, so that the snake may not be suffo- 

 cated during the long and tedious process of swallowing. 



Circulatory System. The heart beat is slow and the 

 circulation not very active. The heart continues to beat 

 long after the head is severed. The temperature of the 

 blood varies with that of the surroundings. 



Excretory Organs. In the posterior part of the body 

 cavity are the two long, slender kidneys, whose ducts open 

 into the cloaca. 



The Eggs and the Young. Eggs are produced in two 

 long, narrow ovaries; the oviducts also open into the 

 cloaca. Some snakes deposit their eggs in the earth, 

 though probably a majority bring forth the young alive. 



Senses of Snakes. Sight and touch are fairly well de- 

 veloped, though a study of snakes does not reveal a keen 

 sense of sight. Some snakes are affected by music, show- 

 ing a sense of hearing. Of their senses of smell and taste 

 we know but little. 



Adaptations of Internal Organs to External Form. We 

 have seen how the external form is adapted to the mode of 

 life. Let us now see how the internal organs are fitted to 

 the necessarily long, narrow space allotted to them. In 

 the first place the body cavity is so long that a moderately 

 long digestive tube is accommodated without the necessity of 

 coiling it, as in many animals. But one lung is developed, 

 and that one is long and narrow, whereas our bodies admit 

 of two relatively wide lungs placed side by side. The liver 



