id, and a
disagreeable odor will be given off. Microscopic examination shows the
first to be free from germs of any kind, while the second is swarming
with various forms of bacteria.

[Illustration: FIG. 8.--Bacteria.]

These little organisms have of late years attracted the attention of
very many scientists, from the fact that to them is due many, if not
all, contagious diseases. The germs of many such diseases have been
isolated, and experiments prove beyond doubt that these are alone the
causes of the diseases in question.

If a drop of water containing bacteria is examined, we find them to
be excessively small, many of them barely visible with the strongest
lenses. The larger ones (Fig. 8) recall quite strongly the smaller
species of oscillaria, and exhibit similar movements. Others are so
small as to appear as mere lines and dots, even with the strongest
lenses. Among the common forms are small, nearly globular cells;
oblong, rod-shaped or thread-shaped filaments, either straight or
curved, or even spirally twisted. Frequently they show a quick
movement which is probably in all cases due to cilia, which are,
however, too small to be seen in most cases.

[Illustration: FIG. 9.--_Euglena_. _A_, individual in the active
condition. _E_, the red "eye-spot." _c_, flagellum. _n_, nucleus. _B_,
resting stage. _C_, individual dividing, x 300.]

Reproduction is for the most part by simple transverse division, as in
oscillaria; but occasionally spores are produced also.

CLASS III.--GREEN MONADS (_Volvocineae_).

This group of the protophytes is unquestionably closely related to
certain low animals (_Monads_ or _Flagellata_), with which they are
sometimes united. They are characterized by being actively motile, and
are either strictly unicellular, or the cells are united by a
gelatinous envelope into a colony of definite form.

Of the first group, _Euglena_ (Fig. 9), may be selected as a type.

This organism is found frequently among other algae, and occasionally
forms a green film on stagnant water. It is sometimes regarded as a
plant, sometimes as an animal, and is an elongated, somewhat
worm-like cell without a definite cell wall, so that it can change
its form to some extent. The protoplasm contains oval masses, which
are bright green in color; but the forward pointed end of the cell
is colorless, and has a little depression. At this end there is a
long vibratile protoplasm