INTRODUCTION 



25 



narily studied under the head of geology, and the geologist uses it 

 in determining the relative ages of the rock strata which compose 

 the crust of the earth. 



Classification of the Animal Kingdom 



Very few people realize how many different kinds of animals 

 there are and how greatly they vary in size, structure, ajid habits 

 of life. The estimated number of kinds is all the way from 1,000,000 

 to 10,000,000. To date, approximately 840,000 species have been 



Cbordata 

 (Frog) 



Annelida 

 {Lumbricus) 



Bryo^oa 

 (Bugulaj 



Rotifera 



Platyhelminthes 

 (Tapeworm) 



/^rthropoda 

 (Crayfbh) 



MoUusca 

 (Snail) 



Echinodermata 

 {Starfish) 



NemathMnthes 

 {Ascahs) 



Fori f era 

 (5cypha) 



Coelenterafca 

 (Hydra) 



Ctenophcra 



(Be roe) 

 sea walnut 



Protozoa 

 (Paramecium) 



ANIMAL KINGDOM 



Fig. 2. — Phylum relations in the animal kingdom. 



named and described. In order that different forms of animals may 

 be known and definitely recognized, a scheme of grouping related 

 kinds has been devised. 



The entire kingdom is divided into two subkingdoms: Protozoa, 

 or all single-celled animals, and Metazoa, the many-celled animals. 

 The secondary groups are phyla, and they in turn are divided into 

 classes. The principal groups subordinate to the class are order, 

 family, genus, and species. The principal phyla are listed and de- 

 scribed briefly: 



Phylum Protozoa. — Individuals consist either of a single cell or of 

 aggregates of cells, by each of which are performed all the essential 



