e been recorded as attacking man. In some instances the
parasite is always present in the host and some hosts may harbor several
different species of Sporozoa.

Very little work had been done on this group of parasites prior to 1900.
Since that time most of the species that we now know have been
discovered, and within the last few years the life-histories of many of
these have been worked out quite completely. No other group of animals
is being studied more to-day by both the physicians and biologists.

The Sporozoa vary greatly in appearance, organization and life-history.
They are so very plastic that they can adapt themselves readily to their
various hosts, hence we have a great variety of forms. But they all
agree in certain characters; all take their food and oxygen and carry on
excretory processes by osmosis, _i.e._, through the body-wall; all are
capable of some kind of locomotion, some have one or more flagella,
others move by a pseudopod movement. Some are capable of moving from
cell to cell in the body as do the white blood-corpuscles. They all
agree in the production of spores--hence the name.

At certain stages in their development the nucleus within the body of
the organism divides again and again until there are a great many
daughter nuclei, each accompanied by a small mass of protoplasm, often
inclosed in a little sac or cyst of its own. This is the process of
spore-formation and we see that from a single individual we may have by
division, not two animals as in the amoeba, but a score or more of
them. The little cysts or capsules that inclose them enable them to
resist without injury many vicissitudes that would otherwise destroy
them. They may dry up or freeze or lie for a long time in the ground or
water until the time comes when they are introduced into another host.

The class Sporozoa is divided into five small groups or orders. Nearly
all of these contain forms that are of more or less importance, but the
ones that live in the blood-cells (_Haemosporidiida_) are of the most
interest to us because the parasites that cause the malarial fevers and
various other diseases belong here. These are dependent on two hosts for
their existence, the sexual generation usually occuring in an insect or
other invertebrate and the asexual generation in some vertebrate.

CHAPTER III

TICKS AND MITES

The other group or Phylum of animals with which we will be particularly
concerned is known as the Arthr