268 



HOW DO WE CLASSIFY ANIMALS? 



(6) ; (7) bones ; (8) on the legs (which show 



their (9) to the reptiles) ; (10) but no true 



(11) ; (12) temperature ; a (13). They lay 



(14) covered with a (15) (16). The 



(17) in leg and bill show clearly what kind of (18) 



the bird leads and the (19) of (20) it uses. 



PROBLEM IX. 



WHAT ARE THE CHARACTERISTICS OF 

 MAMMALS? 



Practical Exercise 20. Make brief descriptions in your workbook of two 

 mammals such as a cat and a horse. How do they differ? How are they 

 similar ? 



Mammals. Mammals are characterized by being warm blooded, 

 by having a four-chambered heart, a diaphragm, and well-developed 

 lungs. The most characteristic features, however, are that they 



have a hairy cover- 

 ing at some period 

 of their existence 

 and bring forth their 

 young alive. The 

 young are nourished 

 on milk secreted by 

 glands known as the 

 mammary glands ; 

 hence the term 

 "mammals." Mam- 

 mals are considered 

 the highest of verte- 

 brate animals, not 

 only because of their complicated structure, but because of 

 their mental development. 



Individual project. Visit a museum and study the skeletons and 

 mounted bodies of a seal and of a whale. Why do we consider these 

 animals mammals rather than fishes or amphibians? 



Carnivores. Carnivorous mammals are to a large extent flesh 

 eaters. In a wild state they hunt their prey, which is caught and 

 torn with the aid of well-developed claws and long, sharp canine 

 teeth. 



U. S. Bureau of Biological Survey 

 What kind of food does the black bear eat ? 



