WHAT ARE BACTERIA ? 39 



in which motion is very slight, if not entirely absent, the 

 flagella serve to set up currents round the organism by which 

 food is brought to, and excretions removed from, the bacillus. 

 These flagella may be met with as a single pair, or we may 

 have three or four pairs attached to the same organism. They 

 stain like the membranes, and appear to develop only in those 



Photo-micrograph of Spirillum Undala, with pair of well-marked flagella at each 

 end, x looo. 



organisms that have special affinity for oxygen, for as soon 

 as the ciliated forms reach the surface of a fluid, they lose 

 their cilia or they become much less active. This latter, 

 however, seems to be the more probable explanation, for 

 Loffler has shown that many organisms are provided with 

 cilia, which at one time were supposed to possess nothing of 



