44 ELEMENTARY ZOOLOGY 



the body of Amoeba, and the elliptical disk-like red blood- 

 cells. The cells composing the muscles are, moreover, 

 like neither kind of blood-cells, and the cells of which the 

 liver is composed are not like the cells of the muscles. 

 That is, there are many different kinds of cells in the body 

 of a many-celled animal. While the single cell which 

 composes the whole body of the Amoeba is able to do all 

 the things necessary to maintain life, the various cells in the 

 body of a complex animal are differentiated or specialized, 

 certain cells devoting themselves to a certain function or 

 special work, and others to other special functions. For 

 example, the cells which compose the organs of the 

 nervous system, the brain, ganglia, and nerves, devote 

 themselves almost exclusively to the function of sen- 

 sation, and they are especially modified for this purpose. 

 The highly specialized nerve-cells resemble very little the 

 primitive generalized body-cell of Amoeba. The muscle- 

 cells of the complex animal body have developed to a 

 high degree that power of contraction which is possessed, 

 though in but slight degree, by Amoeba. These muscle- 

 cells have for their special function this one of contraction, 

 and massed together in great numbers they form the 

 strongly contractile muscular tissue and muscles of the 

 body on which the animal's power of motion depends. 

 The cells which line certain parts of the alimentary canal 

 are the ones on which the function of digestion chiefly 

 rests. And so we might continue our survey of the 

 whole complex body. The point of it all is that the 

 thousands of cells which compose the many-celled animal 

 body are differentiated and specialized; that is, have 

 become changed or modified from the generalized primitive 

 amoeboid condition, so that each kind of cell is devoted 

 to some special work or function and has a special struc- 

 tural character fitting it for its special function. In the 

 Protozoan body the single cell can perform and does per- 



