and the animal. In the former class belong the bacteria, and in the
latter the protozoa. The two classes are not sharply differentiated, but
in general the vegetable parasites are less highly organized than the
animal.

BACTERIA

SHAPE.--Bacteria are composed of single cells and are consequently
called unicellular organisms. Under the microscope individual cells are
seen to differ in size, shape, and structure. In shape bacteria show
three different types; the rod-shaped (bacillus), the spherical
(coccus), and the spiral (spirillum). The organisms causing typhoid
fever for example are a variety of bacilli, those causing pneumonia are
cocci, while those causing Asiatic cholera are spirilla.

[Illustration: FIG. 1.--BACILLI OF VARIOUS FORMS. (_Williams._)]

SIZE.--Bacteria vary greatly in size. Average rod-shaped bacteria are
about 1/25000 of an inch long, but there are undoubtedly organisms so
small that they cannot be seen, even by means of the strongest
microscopes we now possess.

[Illustration: STAPHYLOCOCCI. STREPTOCOCCI. DIPLOCOCCI. TETRADS.
SARCINAE. FIG. 2.--(_Williams._)]

MOTION.--The power of motion in certain species of bacteria is due to
hair-like appendages called flagella. These flagella by a lashing
movement somewhat resembling the action of oars enable the organisms to
move through fluids.

MULTIPLICATION.--After bacteria have fully developed, each cell divides
into two equal parts; the process of division is called fission. Each
of these two parts rapidly grows into a full-sized organism. Then
fission again takes place, so that four bacteria replace the original
one. In each of the four, fission occurs again, and so the process of
multiplication continues. As bacteria develop they group themselves in
characteristic ways. Some, like the streptococci, arrange themselves in
chains; the diplococci, in pairs; the tetrads, in groups of four; others
in packets called sarcinae, and still others, the staphylococci, form
masses supposed to resemble bunches of grapes.

[Illustration: FIG. 3.--SPIRILLA OF VARIOUS FORMS. (_Williams._)]

[Illustration: FIG. 4.--BACTERIA SHOWING FLAGELLA. (_Williams._)]

Under favorable conditions fission occurs rapidly; in some types a new
generation may appear as often as every 15 minutes. Enormous
multiplication would result if nothing occurred to check the process.
But in nature such increase never continues unhindered, and bacteria,
acting upon their food substances