The unicellular organisms at the lowest position in the animal kingdom are by no means so well known as the corresponding vegetable organisms. Of late years much attention has been paid to them, and there are indications that there may be in their case a wide field of pathogenesis still to be discovered. The presence of protozoa in the blood in malaria suggests that in some of the specific fevers agents of this class may exist.

In this early stage of the inquiry modes of isolation and of cultivation are still to a large extent to be discovered. It is scarcely to be expected that the same media will be suitable for animal organisms as for vegetable. For a similar reason the morphology and classification of the group is also in an initial stage.