82 GENERAL BIOLOGY 



round, berry-like heap. These three genera include all our 

 common allies of Vorticella of colonial habit. 



It is not to be overlooked, while studying protozoans, 

 that even in these forms, there is a foreshadowing of the 

 principal organs of the higher animals. The long esophagus 

 of Epistylis is prototype of the alimentary canal ; the con- 

 tractile vacuole, forerunner of a sort of rudimentary circula- 

 tory apparatus; and the myonemes constitute a sort of 

 elemental muscular system. 



THE LIFE PROCESS IN PLANT AND ANIMAL CELL. 



We have seen that in many algae and in most protozoans 

 the cell is an independent organism: all functions of plant 

 or animal are performed by it. Even when such cells 

 are grouped together to form a larger organism, their union 

 is for the most part a loose one, and their physiological 

 independence is little impaired. To the cell, then, we must 

 go to learn what are plant and animal functions, and how 

 they are performed. 



How does the cell live and grow? This is a hard question, 

 answered as yet only in part. The answer so far as avail- 

 able is best stated in terms of matter and energy. 



Matter. — The bodies of living beings are composed of a 

 few chemical elements, such as are common in soil and 

 water ever3rwhere. This is readily determinable by chemi- 

 cal analysis. In all living substance there are nine chemical 

 elements constantly occurring, three others (the three last 

 named below) that are nearly always present, and a 

 number of others occur here and there. The twelve are: 



Carbon C Sulphur . . . . S Magnesium . . Mg 



Hydrogen . . . H Phosphorus . P Sodium .... Na 



Oxygen O Potassium. . . K Chlorine .... CI 



Nitrogen . . . . N Iron Fe Calcium. .... Ca 



