f the spore.]

While many forms are fixed to the substratum, others are free, being in
this condition either motile or immotile. The chief of these forms are
described below.

[Illustration: FIG. 5.--Characteristic groups of _Micrococci_. (After
Cohn.) A. _Micrococcus prodigiosus._ B. _M. vaccinae._ C. Zoogloea stage of
a _Micrococcus_, forming a close membrane on infusion--Pasteur's
_Mycoderma_. (Very highly magnified.)]

_Cocci_: spherical or spheroidal cells, which, according to their
relative (not very well defined) sizes are spoken of as _Micrococci_,
_Macrococci_, and perhaps _Monas_ forms.

_Rods_ or _rodlets_: slightly or more considerably elongated cells
which are cylindrical, biscuit-shaped or somewhat fusiform. The
cylindrical forms are short, _i.e._ only three or four times as long as
broad (_Bacterium_), or longer (_Bacillus_); the biscuit-shaped ones
are _Bacteria_ in the early stages of division. _Clostridia_, &c., are
spindle-shaped.

_Filaments_ really consist of elongated cylindrical cells which remain
united end to end after division, and they may break up later into
elements such as those described above. Such filaments are not always
of the same diameter throughout, and their segmentation varies
considerably. They may be free or attached at one (the "basal") end. A
distinction is made between _simple_ filaments (_e.g._ _Leptothrix_)
and such as exhibit a false branching (_e.g._ _Cladothrix_).

_Curved_ and _spiral_ forms. Any of the elongated forms described above
may be curved or sinuous or twisted into a corkscrew-like spiral
instead of straight. If the sinuosity is slight we have the _Vibrio_
form; if pronounced, and the spiral winding well marked, the forms are
known as _Spirillum_, _Spirochaete_, &c. These and similar terms have
been applied partly to individual cells, but more often to filaments
consisting of several cells; and much confusion has arisen from the
difficulty of defining the terms themselves.

In addition to the above, however, certain Schizomycetes present
aggregates in the form of plates, or solid or hollow and irregular
branched colonies. This may be due to the successive divisions
occurring in two or three planes instead of only across the long axis
(_Sarcina_), or to displacements of the cells after division.

[Sidenote: Reproduction.]

_Growth and Di