32 PATHOGENIC BACTERIA. the organism by conveying it from an area where the nutrition is less to one where it is greater, but, as Wood- head points out, may, in the non-motile species, serve to stimulate the passage of currents of nutrient material past the organism, so as to increase the food-supply. * The flagellate bacteria have a greater number of repre- sentatives 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. 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. 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.