32 PATHOGENIC BACTERIA. 



and in fermenting and decaying materials than among 

 those inhabiting the bodies of animals. This is an 

 additional fact in favor of the view that locomotion and 

 flagella are provisions favorable to the maintenance of 

 the species by keeping the individuals supplied with 

 food. 



It may be added that such parasitic disease-producing 

 bacteria as do not habitually gain access to the tissues, 

 but inhabit the intestine, as the bacillus of typhoid fever 

 and the spirillum of cholera, are actively motile, like 

 the saprophytes, while those habitually entering the tis- 

 sues and multiplying there are motionless and without 

 flagella. Of course this example is open to criticism, 

 because the spirillum of relapsing fever, which has never 

 been found elsewhere than in the blood and spleen of 

 affected animals, is actively motile. 



One of the linear organisms, known as the Bacillus 

 megatherium, has a distinct but limited ameboid move- 

 ment. 



The commonly observed dancing movement of the 

 spherical forms seems to be the well-known Brownian 

 movement, which is simply a physical phenomenon. It 

 is sometimes difficult to determine whether an organism 

 is really motile or whether it is only vibrating. In the 

 latter case it does not change its relative position to 

 surrounding objects. 



When speaking of the movement of bacteria, we usually 

 refer to individuals, but in some cases to the colonies, it 

 being a peculiarity of organisms of the proteus group 

 that their colonies upon 5 per cent, gelatin show definite 

 migratory tendencies. The active movement of the bac- 

 teria within the colony causes its shape to change con- 

 stantly, so that it bears a faint resemblance to an ameba, 

 and by the changing shape and active movement of its 

 component individuals succeeds in moving about from 

 place to place upon the surface of the gelatin. 



The bacteria are so minute that a special unit of meas- 

 urement has been adopted by bacteriologists for their 



