PART I — PROTOZOA 



CHAPTER ni 

 INTRODUCTION TO PROTOZOA 



Place of Protozoa in the Animal Kingdom. — It is usual for 

 zoologists at the present time to divide the entire Animal King- 

 dom into two great sub-kingdoms, the Protozoa and the Metazoa. 

 These groups are very unequal as regards number of species. 

 The Metazoa include all the animals with which the majority 

 of people are familiar, from the simple sponges and jellyfishes, 

 through the worms, molluscs, and the vast horde of insects and 

 their allies, to the highly organized vertebrate animals, including 

 man himself. The Protozoa, on the other hand, include only 

 microscopic or almost microscopic animals, the very existence of 

 which is absolutely unknown to the average lay person. Al- 

 though some Protozoa are readily visible to the naked eye there 

 are others, such as the yellow fever organism, which are too small 

 to be seen even under the highest power of the microscope. There 

 is no question but that in point of numbers of individuals the 

 Protozoa exceed the other animals, millions to one; a pint jar 

 of stagnant water may contain many billions of these minute 

 animals. About 10,000 species of Protozoa have been described, 

 but it is probable that there are thousands more which are not 

 yet known to science. 



The distinction between the Protozoa and Metazoa is based 

 on a characteristic which is of the most fundamental nature. 

 The Protozoa are animals which perform all the essential func- 

 tions of life within the compass of a single cell. The Metazoa, 

 on the other hand, are many-celled animals, with specialized 

 cells set apart to perform particular functions. A protozoan 

 cell, even though sometimes living in a colony of individuals 

 which are all bound together, can live its life and reproduce its 

 kind quite independently of any other cells, having in itself the 

 powers of digestion, respiration, excretion and secretion, sensi- 



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