PROTOZOA. 145 



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 reproduce 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, 

 excepting 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, 

 and when growth reaches a certain limit the animal (cell) 

 divides, and we have now two individuals in the place of the 

 original one. 



In other Protozoa different regions in the cell may be 

 specialized in different directions and give rise to what 

 may be called cell-organs. A single example must suffice. 

 In the form figured (fig. 2) we have but a single cell, 

 but it is a" cell of definite shape. Externally the body is 

 covered with a denser layer, comparable in position and 

 use to a skin. A little deeper are developed longitudinal 

 lines of contractile material which act in the same way as 

 the muscles of the Metazoa, moving one part on another. 

 Over the outer surface are minute hair-like organs (cilia) 



