304 Descriptive Zoology. 



cell are at first essentially alike in structure and function. 

 To fit themselves for the different kinds of work that they 

 have to do, they grow different. This growing different 

 is called differentiation. Each becomes fitted for one 

 special work ; this is specialization. 



Tissues. In the higher metazoans there are many 

 cells devoted to each of the different kinds of work to be 

 done. This we should naturally expect, for in such a 

 body there are myriads of cells, but only comparatively 

 few kinds of work. As has already been made clear, an 

 amoeba can do nearly all the kinds of work that any 

 animal can do, though in a much simpler way. The func- 

 tions of animals are summed up in saying they move, feel, 

 eat, grow, breathe, protect themselves, and reproduce their 

 kind. Stated more formally, the functions of animals are 

 included in the processes of motion, sensation, support, pro- 

 tection, reproduction, and nuttition (nutrition including 

 digestion, absorption, circulation, respiration, and excretion). 



For instance, muscle cells are cells devoted to the work 

 of motion ; they have largely given up the other functions 

 that they originally possessed. The nerves have lost the 

 ability to change their form, in devoting themselves to 

 the special work of sensation. So, too, with the cells 

 of the supporting and protective tissues. 



A tissue is a group of cells having the same structure 

 and function. 



An amoeba consists of a single cell, but it can do a 

 number of kinds of work. A tissue consists of many cells, 

 but they all do the same kind of work. In other words, 

 an amoeba is simple in structure, but complex in function ; 

 a tissue is complex in structure, but simple in function. 



Organs. In addition to the differentiation of cells into 

 tissues, there is a still further division of labor by having 



