CHAPTER 10 



CLASSIFICATION AND GENERAL ORGANIZATION OF 



ANIMALS 



The principles of classification have been illustrated in particular 

 groups of animals in the accounts of Vertebrata and other Chor- 

 data, and in the Protozoa. The distinction between Protozoa and 

 Metazoa has also been explained. The classification of IMetazoa is 

 indicated in this chapter in order that the student may understand 

 something of the Animal Kingdom as a whole before undertaking 

 the study of representative animals that is outhned in the chapters 

 immediately following. 



Classification 



The Principal Types of Animals. — It is necessary to classif}- 

 animals, if only for the purpose of Usting the various types, and 

 hence there have been many classifications in the history of 

 Zoolog3\ Gradually, however, these have been refined to a 

 measure of agreement, until at present there is no wide diversity 

 of opinion regarding the nature of the major subdivisions, or 

 phyla. The Animal Kingdom may be divided, according to most 

 zoologists, into a relatively small number of these larger groups, 

 the phyla, which are in turn variously subdivided. As Usted in 

 most of the current texts, these phyla are as follows : 



Phylum Protozoa, the single-celled animals. 



Phylum Mesozoa, a small group of very simple organization. 



Phylum Pari f era, the sponges. 



Phylum Coelenterata, the polyps, jellyfish, corals, etc.^ 



Phylum Platyhelminthes, the flat worms, and nemerteans.^ 



Phylum Nemathelminthes, the round wonns. 



^ The Ctenophora, or sea-walnuts, which would here be listed as a subdivision 

 of the Coelenterata, are classified by some zoologists as a separate phylum. 

 * The nemerteans may be placed in a Phylum, Nemertinea. 



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