SECTION IV 

 THE ANIMAL KINGDOM 



CHAPTER 19 



THE ANIMAL KINGDOM 



In an earlier chapter, emphasis was placed on the tremendous 

 variety of animal forms. From the single-celled protozoans to the many- 

 celled vertebrates there is uncounted variation and adaptation of the 

 many animals to the innumerable demands of the varying environments 

 in which they dwell. In spite of this seemingly endless variety, it is 

 possible to recognize related groups of animals. In fact, it is possible 

 to distinguish a relatively small number of phyla. While the animals 

 within each phylum vary considerably, they do have many characters 

 in common. 



The various phyla are separated from each other by several funda- 

 mental characters, the combination of which is unique to each phylum. 

 The most important of these distinguishing characteristics are as fol- 

 lows : 



Cellular Organization. — One major subkingdom of the animal 

 kingdom is composed of animals which consist of but a single cell. 

 These are the protozoans. All the life processes of these forms are 

 carried on within a single cell membrane. While in this subkingdom, 

 cells sometimes mass together as colonies, there is no differentiation 

 among the vegetative cells. 



Contrasting with the protozoans are the metazoans which are 

 composed of many cells showing both histological and physiological dif- 

 ferentiation. Within this latter group, grades of organization can also 

 be recognized. In some there is only differentiation of tissues, while 

 in most, organs and organ systems are found. 



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