2 COLLEGE ZOOLOGY 



the other ten chief divisions are composed entirely of inverte- 

 brates. 



It is therefore of considerable importance at the very beginning 

 to learn something of the characteristics and habitats of the 

 thousands of living creatures that form the basis for the study of 

 zoology. In the following, paragraphs a few facts about each 

 main group are presented in such a way as to give a bird's-eye 

 view of the entire animal kingdom. 



(i) The Vertebrates. — The members of this group possess a 

 bony axis of vertebrae called the vertebral column or backbone 

 (Fig. 418). They are the most highly developed of all animals, 

 and include man. The vertebrates may be subdivided into 

 seven assemblages, each containing numbers of more or less 

 familiar forms. 



At the top of the series are placed the Mammalia (Chap. 

 XXI), usually known as animals or beasts. Among the repre- 

 sentative mammals are man, the apes, monkeys, bats, moles, 

 rats, mice, rabbits, dogs, cats, cows, sheep, horses, whales, sloths, 

 opossums, and the peculiar duckbill (Fig. 513) and spiny ant- 

 eater of Australia. They are vertebrates which possess hair, 

 and, with a few exceptions, nourish their young with milk se- 

 creted by mammary glands. They breathe air by means of 

 lungs, and are said to be warm-blooded, since their body tem- 

 perature is nearly ioo° F., regardless of the temperature of the 

 surrounding medium. 



The members of the group Aves or Birds (Chap. XX) are 

 characterized by the presence of feathers; no other animals 

 possess these structures. Birds are air-breathers and warm- 

 blooded, having a higher body temperature than any other or- 

 ganisms. They are all terrestrial, though many of them are 

 adapted to life on or near the water. The majority of the birds 

 are able to fly long distances, but some of them, like the ostrich 

 and the auk, are flightless. 



Reptiles (Chap. XIX) are remarkably diversified in form, 

 and occupy many kinds of habitats. Most of them live on land, 



