zoa" consists in their
simpler structure and mode of growth. They are essentially filaments
which continually multiply by fission--a process often carried so far
that the little organisms present themselves as short rods, or as
curved (comma-shaped), or even spherical particles (micrococci)--and
only in favourable conditions arrest their self-division so as to grow
for a time into the thread-like or filament shape. Often these
filaments are not straight, but spirally twisted, and are called
"spirilla." Some of them are blood parasites, but the larger number
attack the tissues, and others occur in the digestive canal.

The parasitic disease-producing protozoa, on the other hand, are of
softer substance, often have the habit of twisting themselves in a
corkscrew-like manner, and usually are provided with an undulating
membrane or frill, as well as with one or with two whip-like swimming
processes (the latter are present also and are often numerous in the
actively swimming phases of bacteria), and have a more complicated
life-history. They divide, as a rule, longitudinally and not
transversely, and pass from one "host" to a second, where they assume
distinct forms--males and females, which conjugate and break up (each
conjugated or fused pair) into a mass of very numerous, excessively
minute, young. The disease-producing protozoa of this kind are
frequently parasitic in the blood of man and animals, and were only
recently recognised, after the disease-producing bacteria of many
kinds had been thoroughly studied. These animal microbes are often
spoken of as "blood-flagellates" or haemo-flagellata, and the larger
kinds are called "Trypanosomes," or "screw-form parasites," or whilst
a series of more minute ones are called "Piroplasma," or "pear-shaped
parasites." Many, but not all, are found during a certain period of
their life, actually inside the corpuscles of the blood. The fact that
many of these blood-flagellates (if not all) have, besides their life
in the blood of one species of animal, a second period of existence in
the juices or the gut of another animal, has made it very difficult to
trace their migrations, since in the second phase of their history
their appearance differs considerably from that which they presented
in the first. And often they exist in one kind of animal without doing
any harm, and are only poisonous when introduced by insects into the
blood of other kinds of animals!

There is, further, anoth