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.