ADDITIONAL ANIMAL STUDIES 187 



the snakes which have no appendages. Unlike the turtles the jaws of 

 all other reptiles contain sharp teeth, used in holding their prey, and 

 in the rattlesnake and copperhead some of these teeth are provided 

 with poison glands. None of the other reptiles in the northern part 

 of the United States are in any way dangerous to man. Indeed, 

 many snakes destroy large numbers of rats and mice, while lizards 

 catch large numbers of insects. The hide of the alligator is of 

 considerable value for leather. All reptiles breathe throughout 

 their life by lungs, and most of them reproduce by eggs,, which 

 are hatched by the warmth of the sun. 



F. Mammals 



143. Characteristics of mammals. In this class of vertebrates 

 are included domesticated animals such as the cow, sheep, horse, 

 camel, dog, and cat. Let us consider the structure of some of these 



FIG. 136. The sperm whale. 



annuals to see why they should be grouped together. We are 

 familiar enough with the animals named above to know that they all 

 have a head, neck, trunk, and tail and that these regions are covered 

 with hair. A few mammals, e.g. the baboons, have no tail, and a 

 few are nearly destitute of hair, like the whales (Fig. 136) ; but all of 

 them nourish their young on milk produced. in certain organs 

 known as mammary glands; hence these animals are called 

 mammals. 



The organs of the head, namely the ears, eyes with eyelids, and the 

 nostrils, are prominent in all common mammals, but vary in size and 

 shape. The jaws have teeth set in sockets, but the number and 

 kinds of teeth vary greatly. Rats, rabbits, and squirrels, for 



