CHAPTER XXXV 

 ANIMALS. VERTEBRATES 



The study of animals is called zoology. We are not go- 

 ing to make a study of zoology now. That will come 

 later. But, in order to fill out your picture of nature, you 

 need a definite idea of the animal kingdom. 



Just about half a million different kinds of animals have 

 been described by zoologists, and about seven-tenths of these 

 are insects. This has been called the Age of Man and In- 

 sects because in this age these two kinds of animals have 

 been by far the most successful. 



Branches of the Animal Kingdom. The entire ani- 

 mal kingdom is divided into twelve branches. Of these there 

 are two that you should understand. The other ten may 

 wait until you study zoology. These two branches are the 

 jointed-legged animals, and the vertebrates, or animals with 

 skeletons. Of the former there are several classes, three of 

 which you should know. A crayfish or a lobster is an ex- 

 ample of one of these classes, the class called Crustacea; to 

 another spiders belong; while a third is made up of that 

 greatest of all classes, the Insecta. We shall study them in 

 a separate chapter. 



The Vertebrates. Of the animals with skeletons (Ver- 

 tebrates), there are five classes you should remember: 

 fishes, amphibians, reptiles, birds, and mammals. These 



