environmental conditions than are the vegetative forms, the cysts are responsible for
the transmission of most protozoan infections.
2-22. CLASSIFICATION OF PROTOZOA
The pathogenic protozoa of man can be grouped into four major classes:
a. Class Rhizopoda (ameba). These organisms are characterized by finger-
like protoplasmic processes known as pseudopods (false feet), which serve as organs
of locomotion and for the engulfment of food. They multiply by binary fission during the
motile stage, in which they are known as trophozoites. Many species form cysts, which
consist of several nuclei and a resistant membrane (figure 2-11B). The trophozoites are
very susceptible to injurious agents and are easily destroyed; however, the cysts are
quite resistant. The cysts, therefore, are responsible for transmitting disease. Amebae
of man are found primarily in the digestive tract.
b. Class Ciliata (ciliates). The ciliates are characterized by an oval, uniformly
shaped cell having two nuclei (macronucleus and micronucleus) and numerous short,
hair-like appendages (cilia) for locomotion (figure 2-12 A). Some species multiply by
binary fission and some by conjugation. Cysts are not commonly encountered, but
when found, they are double walled, spherical bodies. The only species pathogenic to
man is Balantidium coli, a natural inhabitant of the large intestine of the domestic hog.
c. Class Mastigophora (flagellates). The flagellates are distinguished by
having, at some time during the life cycle, relatively long, filamentous protoplasmic
processes of locomotion known as flagella (figure 2-13 A). The flagella may be multiple
or single. Multiplication is by binary fission. Some species produce cysts, while others
do not. Flagellates of man are normally found in the intestinal tract, the genital tract,
and the circulatory system.
Figure 2-11. Entamoeba histolytica.