THE PROTOZOA 



INTRODUCTION 



" In the clearest waters and in muddy pools, in acid as well as alkaline waters, in brooks, 

 lakes, rivers and seas, often, also, in the interior fluids of living plants and animals, abun- 

 dantly in living men, and periodically borne on the dusts and vapors of our atmosphere, 

 there exists a world unknown to the ordinary senses of man, of minute, peculiar forms of 

 life." — C. G. Ehrenberg, 1838. 



Beyond the ordinary range of unaided vision there exists a world 

 of minute animal organisms, technically known as the Protozoa. 1 

 They abound in the dust of the air, in the sea, in freshet and ditch, 

 in brackish and potable waters — wherever, in short, there is air and 

 moisture, while even air is apparently superfluous for the vast majority 

 of parasitic forms which make their homes in the living bodies of 

 higher plants and animals. Their beauty, their varied modes of life, 

 the suddenness of their appearance and disappearance, the simplicity 

 of their structure, and modes of reproduction, combine to make them, 

 even to the superficial observer, a fascinating group. Apart from 

 their superficial attraction, however, the Protozoa have a deeper sig- 

 nificance to the student of zoology. As the name Protozoa indicates, 

 they are primitive animals, and in the scale of living things they are 

 not far removed from the colorless bacteria on the one hand, and the 

 primitive green plants on the other. Their chief significance, how- 

 ever, and the main feature which distinguishes them from the higher 

 animals or Metazoa, centres in the fact that they consist of but a sin- 

 gle cell within the confines of which are carried on all of the essential 

 vital functions which characterize the highest many-celled animals. 



In their main characteristics these cells do not differ from those 

 which make up the tissues and the body of higher animals. Like a 

 tissue-cell the protozoon consists of protoplasm differentiated into 

 nucleus and cell-body or cytoplasm, both parts being variously modi- 

 fied in the several types (Fig. 1). Unlike tissue-cells, however, the 

 Protozoa are not specialized for the performance of any one function. 

 They invite attention, therefore, from both the morphological or 

 structural and the physiological or functional points of view. Mor- 

 phologically they are equivalent to the isolated epithelial, muscle- or 

 nerve-cell ; physiologically, they are equivalent not merely to the 

 muscle- or nerve-cell, but to the entire group of cells which collec- 



1 The term Protozoa was first used in its modern sense by von Siebold ('45). 

 B I 



