CHAPTER III 
THE PROTOZOA 
12. 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. 
13. 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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