i6 



The Liv'mg Things of the Earth unit i 



chimpanzee 



Robin 



Lizard 



Frog 



Codfish 



Fig. 13 Exmnpks of each of the five chief classes of vertebrates. 



The Vertebrates 



CLASS -MAMMALS 



How we can recognize mammals. A4am- 

 mals have a backbone; they are verte- 

 brates. But thev differ from the other 

 vertebrates in that they have hair or fur. 

 Some mammals have very httle hair; 

 there is little hair on an elephant's body 

 and even less on a whale's. But every 

 vertebrate with any hair at all is a mam- 

 mal. The other striking distinguishing 

 characteristic of all mammals is the 

 lummnary or milk glands by which the 

 young are fed. Mammals breathe by 

 means of lungs and they are warm- 

 blooded (that is, their body temperature 

 is fairly constant; it does not change 

 much with changes in the temperature 

 of the surroundings) but these are not 

 characteristics that make them different 

 from all other vertebrates because birds, 

 too, have lungs and are warm-blooded. 

 Mammals also have two pairs of legs 

 but so do all frogs and some reptiles as 

 well. There are about 4000 species of 

 mammals. Because of their complex 

 structure they are spoken of as the 



"highest" animals. This would be a good 

 time for \'ou to begin Exercise i. 



Man and the apes. Mammals are sub- 

 divided into groups (called orders). The 

 group most important to us is the one 

 containing ourselves. All mammals are 

 somewhat like man in structure but the 

 great apes, such as the chimpanzees, the 

 gorillas, and the orangutans, resemble 

 man in structure much more closely 

 than do any other animals. For this 

 reason man and the apes are placed in 

 the same group. The monkeys also belong 

 to this group. 



Mammals with grinding teeth. This 

 is a large group. It really includes sev- 

 eral orders. You probably know giraffes, 

 deer, buffalos, cows, gazelles and goats; 

 horses and zebras; elephants; and rhi- 

 noceroses. Most of these animals have 

 single or double hoofs. The hoof is an 

 enlarged and thickened toenail. How- 

 ever, elephants, rhinoceroses, and some 

 others lack a hoof and hav^e several toes. 



All of them have grindinir teeth used 

 in chewing grass and leaves. Many of 

 them, such as cows, sheep, deer, and 



