Then we have _spiro-bacteria,_ the spirilla and the spirochetae; the
former having short open spirals, the latter long and closely wound
spirals. The _spirillum, volutans_ is often found in drinking water,
and in common with some other specimens of this class is provided with
flagellae, sometimes at both extremities, which furnish the means of
rapid locomotion. The spiro-bacteria multiply by spores, although
little is at present known of their life history. They frequently are
attached together at their extremities, forming zigzag chains.
We have seen that bacteria differ greatly in appearance from the
elongated dot of the bacterium proper, to the elongated rod or
cylinder of the bacillus, and the long spirals of spiro-bacteria. It
is unfortunate that they are not sufficiently constant in habit to
always attach themselves to one or the other of these genera. The
micrococcus has a habit of elongating at times until it is impossible
to recognize him except as a bacterium; while bacilli, again, break up
until their particles exactly resemble micrococci.
Bacteria cannot exist without water; certain forms require oxygen,
while others thrive equally well without it; some thrive in solution
of simple salts, while others require albuminoid material.
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