PROTOZOA. 



In contrast to the Metazoa, which have just been defined 

 as animals made up of many cells, and these cells arranged 

 in two or three layers and grouped into tissues, comes the 

 group of Protozoa, which may be defined as animals each 

 consisting of a single cell. A little thought will show that 

 this difference is in reality very great. In the metazoan 

 certain groups of cells become adapted (specialized) for the 

 performance of certain work in the body, and the more 

 specialized they become the more restricted are they in 

 their lines of work. Thus in man the cartilage and bone- 

 cells are solely for the support of the body, muscle-cells for 

 the moving of parts or of the body as a whole. When, 

 however, we turn to the Protozoa, composed of but a single 

 cell, we find that this one cell has to do all the work which 

 in the Metazoa is shared by the several groups of cells. It 

 has to feed, to move, to excrete waste matters, and to re- 

 produce its kind. In a word, the cells of the Metazoa are 

 differentiated in various directions; those of the Protozoa 

 are undifferentiated. 



The Protozoa show great variety in shape, appearance, 

 and habits. In some there is no differentiation between the 

 different regions of the cell which composes the body, ex- 

 cepting the fact that a nucleus is usually (if not always) 

 present. Food may be taken in at any point ; any portion 

 may be used for locomotion; and indigestible portions may 

 pass out anywhere on the surface. By feeding they grow, 



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