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ADVANCED PHYSIOLOGY 



independent life. Although our own bodies are composed of 

 many millions of these cells, there are some organisms made 

 up of one cell only. These are usually microscopic and are 

 called unicellular animals and plants; although very tiny, 

 each lives an independent life. Some of these animals are 

 shown in Figure 15. They vary in shape and differ in struc- 

 ture. Some of them have "mouths"; others simply take 

 their food in at any part of the body by allowing their pro- 

 toplasm to flow around it. Some of them have organs for 

 locomotion, others do not. Some have shells, while others 

 have no covering at all. But each is a single cell, and each 

 carries out its own life processes, such as respiration, secretion 



FIG. 16. SHOWING THE METHOD OP CELL DIVISION 



and multiplication. The cell cannot be subdivided into 

 smaller units which would be able to sustain independent 

 life. 



Since such cells are the simplest parts into which living 

 matter can be divided, we may call them the units of life 

 and may regard our bodies as a combination of a large num- 

 ber of such units, considering the life of the whole body 

 as the sum of the lives of its different celle. We should 



