CHAPTER VI 

 IRRITABILITY 



Protoplasmic Motion. — Living matter is irritable; 

 that is, it has the power to move in response to a stimu- 

 lus. The character of the motion varies. Sometimes 

 the protoplasm circulates within the cell. This flowing 

 movement aids in the distribution of nutritive sub- 

 stances to all parts of the cell and in the concentration 

 of wastes. It may also bring about the movement of the 

 cell from place to place. The ameba and the white blood 

 corpuscles, for example, creep slowly along in the direc- 

 tion in which the protoplasm flows (Figs. 16, 50 B). 



Ciliary Motion. — Sometimes the cells have hair-like 

 projections from their surface which move to and fro. 

 By the concerted movement of these cilia a free cell 

 easily moves from place to place like a boat propelled 

 by oars (Fig. 2). If the ciliated cells form the surface 

 of a membrane like that covering the gills, or lining the 

 oesophagus, of many animals, the moving of the cilia pro- 

 duces a current which, passing over the surface of the 

 membrane, carries along any substances contained in 

 the liquid that bathes it. 



Muscular Contraction. — In many-celled animals some 

 cells have been so differentiated that they are able to 

 change their form by contracting and relaxing. These 



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