WATER TURNED TO "BLOOD." 155
A microscopical examination of the red water showed that
the colour was due to the presence in countless myriads of a
minute organism, Chromatium Okenii, one of the sulphur bac-
teria. This organism only measures from 1/2000 to 1/1500 of
an inch in length, so that it will pass quite freely through a silk
gauze net of the finest mesh, and consequently will not usually
be noticed by the "pond-life" collector, even when present
in a pond in fair numbers. On the other hand, considered as a
bacterium, it is somewhat of a giant. It consists of a sausage-
shaped body, usually about twice as long as broad, with a tendency
to be slightly curved. It is provided with a single flagellum,
but this, unlike what occurs in the majority of flagellated forms.
Fig. 1. Chromatium Okenii.
is not situated at the anterior, but at the posterior end, so that
it propels the organism forward from behind. With dark-ground
illumination the "light space," or area covered by the flagellum
when in motion, can be seen to be of an elongated hour-glass
shape, but with the distal end somewhat open. The position of
the vibrating flagellum as a whole, as shown by the movements
of its light-space, can be varied with regard to the axis of the
body and it thus serves as propeller and rudder in one. The
body of C. Okenii is uniformly coloured a reddish purple and
it usually contains a number of spherical highly refracting gran-
ules of variable size, which are said to be droplets of liquid
sulphur.