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 conditions for existence.

The food of protozoa consists chiefly of other organisms, particularly bacteria, and they are classed with the animals.

The protozoa are the most widely distributed and the most universal of the parasites.