ONE-CELLED AND MANY-CELLED ANIMALS 41 



are the so-called white blood corpuscles which have an appear- 

 ance much like that of the Amoebae and have a great deal of 

 freedom or independence in their life. 



The rest of the hundreds or thousands or millions of cells that 

 go to make up a many-celled animal's body differ greatly 

 in appearance and behavior from Amoebae, and differ also 

 greatly among themselves. Besides the amoeboid white cells 

 in the blood there are, in red-blooded animals, many elliptical, 

 disk-like reddish cells and they have an entirely different func- 

 tion from that of the white cells. The cells composing the 

 muscles are, moreover, not like either of the kinds of blood 

 cells; the cells of which the liver is composed are not like the 

 cells of the muscles; and the cells which compose the organs of 

 the nervous system, brain, ganglia and nerves, differ markedly 

 from those of the blood, muscles and liver, and differ also very 

 much among themselves. 



Each of these kinds of cells, and each of the many other kinds 

 that exist in the body of one of the higher animals, has become 

 specialized in order to devote itself to a certain particular func- 

 tion or special work. For example, the cells of the nervous 

 system devote themselves to the function of receiving and 

 transmitting sensation. The muscle cells have developed to 

 a high degree the power of contractility, and they have for their 

 special function this one of contraction. Massed together in 

 great numbers, they form the strongly contractile 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 largely rests. And 

 so we might continue our survey of the whole complex animal 

 body. The point of it all is, however, that the thousands of 

 cells which compose many-celled animal bodies are differenti- 

 ated and specialized. That is, have become changed or modi- 

 fied from the generalized primitive amoeboid cell condition so 

 that each kind of cell is devoted to some special work or func- 

 tion, and has a special structural character fitting it for its 

 special function. 



Organs and Functions. The specialized cells are grouped 

 into tissues and organs. These organs are known to us 



