BACTERIA. 31

these*conditions the bacteria appear as solidly-colored
spheres, rods, or spirals, as the case may be.

The cell-walls of some of the bacteria seem at times to
undergo a peculiar gelatinous change or to allow the ex-
^.udation of gelatinous material from the protoplasm, so
that the individuals appear surrounded by a distinct halo
or capsule. This is not only a peculiarity of certain indi-
viduals, but one which only takes place when they develop
under certain conditions; thus, Friedlander points out
that the capsule of his pneumonia bacillus, when it was
found in the lung or in the " prune-juice" sputum, was
very distinct, while it could not be demonstrated at all
when the organisms grew in gelatin.

From the cell-walls of many bacteria numerous deli-
cate straight or wavy filaments project. These are called
cilia or fiagella, and seem to be organs of locomotion.
Sometimes they are only observed projecting from the
ends or from one end; sometimes they are so numerous
and so regular in their distribution as to give the organ-
isms a woolly appearance.

Many of the bacteria which are thus supplied with
flagella are actively motile and swim about like mi-
croscopic serpents. In all probability the locomotory
powers of the bacteria are not entirely dependent upon
the presence of the flagella, but may sometimes be due
to contractility of the protoplasm within an elastic cell-
wall. The micro-organisms most plentifully supplied
with them are. those of the rod and spiral shape. Only
one of the spherical forms, Micrococcus agilis of Ali-
Cohen, has been shown to have flagella. This and one
other species are probably the only motile'cocci. Ob-
serving that the organisms known to be most active are
those best supplied with flagella, it is reasonable to con-
clude that the motility is dependent upon the flagella.

The presence of flagella, however, does not necessarily
imply motility, for some of the bacilli amply provided
with these appendages are not motile. The flagella may
not only serve as organs of locomotion, and be of use to