CHAPTER 7 

 REPRESENTATIVE SINGLE-CELLED ANIMALS 



The preceding chapters are essentially an extended introduction 

 to the subject of zoology. An attempt has been made to review 

 and expand the student's knowledge of familiar zoological subject 

 matter, namely, the structure and functions of higher animals, 

 with a view to making this the point of departure for a survey of 

 further aspects of zoological science. 



The Protozoa, or unicellular animals, are naturally chosen to serve 

 as an introduction to this more comprehensive study; although 

 their organization and activities are not so simple as might be 

 supposed from superficial examination. The Protozoa are often 

 referred to as " animals reduced to the lowest terms." This is a 

 good characterization, because of their unicellular state; but 

 when we examine the more specialized Protozoa we see that 

 they are more complex than any single cells that occur in many 

 celled animals. The Protozoa are of further interest because in 

 classification (p. 240) they may be placed over against the rest of 

 the animal world. If we proceed to the classification of animals 

 by asking what is the most fundamental difference between the 

 animals of difTerent kinds, the greatest difference, in the opinion 

 of many zoologists, Hes between the unicellular and the multicel- 

 lular state. The Animal Kingdom may be divided into two great 

 groups: (1) the Protozoa, or unicellular animals; and (2) the Meta- 

 zoa, or multicellular animals. While this distinction cannot be 

 sharply drawn, because of the existence of certain intermediate 

 types which are " colonies " of cells, it is good for practical pur- 

 poses; although it Taa,y seem to over-emphasize the Cell Doctrine 

 and to disregard the organization of animals as individuals irre- 

 spective of their cellular state (c/. p. 194). 



Classification of Protozoa 



The Protozoa have been variously classified into groups com- 

 parable with the classes in other phyla of the Animal Kingdom 



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