MOVEMENT 14! 



the case on the surface of a protoplasmic unit). This 

 is not composed of cellulose, and is not to be compared 

 with the cell wall of an ordinary plant cell. In some 

 kinds of bacteria the colloid membrane swells very 

 greatly in the liquid medium inhabited by the organism, 

 and thus the individual cell, or group or colony of 

 cells, may become surrounded by a thick mucilaginous 

 investment, making the whole colony quite visible to 

 the naked eye and slimy to the touch. A colony of 

 this kind is called a zooglcea. 



Flagella or cilia (extremely delicate threads of proto- 

 plasm arising from the surface of the cell, singly or in 

 *oups, and projecting into the surrounding medium) 

 lay be detected in many species by special methods 

 >f staining (Fig. 15, B, C, F). 



Movement and Response to Stimuli. Many kinds 

 )f bacteria swim about actively in a liquid medium, 

 most cases no doubt by the beating of their flagella, 

 for these have never been detected in the non-motile 

 forms. Some bacteria, however, especially the spirilla, 

 ippear to move by wriggling their bodies (rather as 

 in eel moves), i.e. by the contractility of the cell proto- 

 )lasm as a whole. 



Motile bacteria move, as a rule, in response to chemical 

 timuli, i.e. towards some substances and away from 

 thers (positive and negative chemotaxis). A good 

 example is Bacterium termo, mentioned on p. 113 as 

 specially sensitive to free oxygen and used in Engel- 

 m's experiment to pick out the particular rays of 

 spectrum in which the chloroplasts liberate most 

 >xygen. 



Nutrition. The foods of bacteria are very various 

 both in kind and in chemical nature, and their habitats 

 naturally correspond with these different foods. Some 



