CHAPTER III 



ORGANISMS OF ONE CELL. Continued 

 THE UNICELLULAR ANIMALS 



IN cells of tissues in higher animals some one vital function 

 predominates over all others and gives to the cell its particular 

 character. With muscle cells, the function of contractility or 

 movement overshadows all others; with nerve cells, irritability 

 or nervous response to external stimuli is predominant; with 

 epithelial cells, the function of secretion predominates, and so 

 on, each type of cell performing its own work but all working 

 for the good of the organism as a whole. Some of the vital 

 functions indeed are lost with such differentiation ; the function 

 of digestion, for example, is confined to cells of the alimentary 

 tract while the latter have lost all power of independent move- 

 ment. Such cells in which one function overrides all others may 

 be spoken of as physiologically unbalanced cells, while other cells 

 in which the functions or vital activities are equally developed 

 may be spoken of as physiologically balanced. Such balanced 

 cells are illustrated by the entire group of protozoa or animals 

 consisting of one cell only, a group comprising some of the ni' st 

 perfect of single cells and at the same time the simplest in 

 structure of all animals. 



A. AMOEBA PROTEUS 



Few organisms have been studied more frequently than this 

 minute protozoon, and nothing gives a better idea of living 

 matter than this fascinating bit of protoplasm with its enig- 

 matical movement and its vital activities. It is found in the 

 greatest variety of places but is not as plentiful as many text 

 books would lead one to suppose. It may be found, however, 

 among the superficial dead leaves and slime in many ponds or 

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