6 PRACTICAL ZOOLOGY 



either because of their use in industries or of their less direct 

 beneficial or injurious qualities. Furthermore, by directing our 

 attention to certain aspects of the life histories and activities 

 of these animals we shall obtain a general knowledge of the laws 

 and principles involved in the study of animals, the study known 

 as Zoology. Throughout our study, however, we should not 

 lose sight of the fact that we ourselves are animals, and that the 

 needs of the living creatures which we dominate are similar to 

 our own needs although the methods of satisfying them may be 

 very different. An animal must be adapted to the conditions 

 within its habitat or it cannot maintain itself. In our studies, 

 therefore, we must learn how the animal is adapted to its particu- 

 lar set of conditions before we can solve the problems involved. 



Finally, there is one phase of animal study that has only re- 

 cently been emphasized, and that is the relation of animals to 

 the community, state, and nation. This subject, which is rather 

 fully treated in this book, we may call " Civic Zoology." 



It will be necessary in the following chapters to speak of 

 certain of the large assemblages of animals, and for this 

 reason a simplified classification is here appended. It is 

 not intended that the student should learn the following 

 classification, but he should use it as a convenient and 

 simple reference. It is often advisable to separate the 

 entire animal kingdom into two subkingdoms, the Inverte- 

 brates and the Vertebrates, because of the relative importance 

 of the latter. The Vertebrates possess a backbone; the Inver- 

 tebrates do not. These subkingdoms may then be divided into 

 eight groups called phyla. The phyla are arranged, from the 

 simplest to the more complex. The numbers refer to the num- 

 ber of known kinds of animals in each phylum. 



SUBKINGDOM I. INVERTEBRATA. -- Animals without backbones. 



518,400. 



Phylum i. Protozoa. Minute, single-celled, or colonial 

 animals. 8500. 



