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.