MOVEMENT I4I 



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 

 groups, and projecting into the surrounding medium) 

 may be detected in many species by special methods 

 of staining (Fig. 15, B, C, F). 



Movement and Response to Stimuli. — Many kinds 

 of bacteria swim about actively in a liquid medium, 

 in 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, 

 appear to move by wriggling their bodies (rather as 

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

 plasm as a whole. 



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

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

 others (positive and negative chemotaxis). A good 

 example is Bacterium termo, mentioned on p. 113 as 

 specially sensitive to free oxygen and used in Engel- 

 mann's experiment to pick out the particular rays of 

 the spectrum in which the chloroplasts liberate most 

 oxygen. 



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 



