32 PATHOGENIC BACTERIA. 



locomotion, and be of use to the organism by conveying 

 it from an area where the nutrition is less to one where 

 it is greater, but, as Woodhead points out, may, in the 

 non-motile species, serve to stimulate the passage of cur- 

 rents of nutrient material past the organism, so as to in- 

 crease the food-supply. The flagellate bacteria have a 

 greater number of representatives among those whose 

 lives are spent in water 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. 



In carrying the argument a little farther 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. Of 

 course this example is open to criticism, because the spi- 

 rillum of relapsing fever, which has never been found 

 elsewhere than in the blood and spleen of affected ani- 

 mals, is actively motile, while the Bacterium coli com- 

 munis, which is always present in the intestine, is non- 

 motile. 



One of the linear organisms known as the Bacillus meg- 

 atherium has a distinct but limited ameboid movement. 



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. 



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

 urement has been adopted by bacteriologists for their 

 estimation. This is the micro-millimeter (//), or one- 

 thousandth part of a millimeter, and about equivalent 

 to the one-twenty-five-thousandth of an inch. 



