CHAPTER III 



THE SIMPLEST ANIMALS OB PROTOZOA 



21. Single-celled and many-celled animals. In almost 

 every portion of the globe there are multitudes of animals 

 whose body consists of but a single cell ; while those forms 

 more familiar to us, and usually of comparatively large 

 size and higher development, such as sponges, insects, 

 fishes, birds, and man himself, are composed of a multitude 

 of cells. For this reason the animal kingdom has been 

 divided into two great subdivisions, the Protozoa including 

 all unicellular forms and the Metazoa embracing those of 

 many cells. 



22. Single-celled animals. The division of the Protozoa 

 comprises a host of animals, usually of microscopic size, 

 inhabiting fresh or salt water or damp localities on land in 

 nearly every portion of the globe. The greater number 

 wage their little, though fierce, wars on one another with- 

 out attracting much attention ; others, in the sharp struggle, 

 have been compelled to live upon or within the bodies of 

 other animals, and many have become notorious because of 

 the diseases they produce under such circumstances. A 

 few are in large measure responsible for the phosphores- 

 cence of the sea ; and still others have long been favorite 

 objects of study because of their marvelous beauty. Adapted 

 for living under diverse conditions, the bodily form differs 

 greatly, and yet all conform to three or four principal types, 

 of which we may gain a good idea from the study of a few 

 representative forms. 



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