e from forms seen with
difficulty under the highest power of the microscope to forms readily
seen with the unaided eye. Their structure in general is more complex
than is the structure of bacteria, and many show extreme
differentiation of parts of the single cells, as a firm exterior
surface or cuticle, an internal skeleton, organs of locomotion, mouth
and digestive organs and organs of excretion. They are more widely
distributed than are the bacteria, and found from pole to pole in all
oceans and in all fresh water. There are many modes of multiplication,
and these are often extremely complicated. The most general mode and
one which is common to all is by simple division; a modification of
this is by budding in which projections or buds form on the body and
after separation become new organisms. In other cases spores form
within the cell which become free and develop further into complete
organisms. These simple modes of multiplication often alternate in the
same organism with sexual differentiation and conjugation. There is
never a permanent sexual differentiation, but the sexual forms develop
from a simple and non-sexual organism. Usually the sexual forms
develop only in a special environment; thus the protozoon which in man
is the cause of malaria, multiplies in the human blood by simple
division, but in the body of the mosquito multiplication by sexual
differentiation takes place. Under no conditions is multiplication so
rapid as with the bacteria, and in general the simpler the form of
organism the more rapid is the multiplication. It is common to all of
the protozoa to develop forms which have great powers of resistance,
this being due in some cases to encystment, in which condition a
resistant membrane is formed on the outside, in others to the
production of spores. A fluid environment is essential to the life of
the protozoa, but the resistant forms can endure long periods of
dryness or other unfavorable environmental conditions. The universal
distribution of the protozoa is due to this; the spores or cysts can
be carried long distances by the wind and develop into active forms
when they reach an environment which is favorable. Their distribution
in water depends upon the amount of organic material this contains. In
pure drinking water there may be very few, but in stagnant water they
are very numerous, living not on the organic material in solution in
this, but on the bacteria which find in such fluid favorable