The Infusoria of many writers comprise many of the lowest forms of plants - such as the Diatoms - together with the Rotifera, a class of minute animals now known to belong to the Annulosa. By modern writers, however, the term Infusoria is used strictly to designate those Protozoa which possess a mouth and rudimentary digestive cavity. They are, for this reason, often called collectively the "stomatode" Protozoa, in contradistinction to the remaining members of the sub-kingdom, which are all "astomatous." The so-called "suctorial" Infusoria (Acinetae), however, appear to have no definite oral aperture; and the same is the case with the parasitic Opalina, though there is great doubt as to the propriety of placing this in the Infusoria at all. The name Infusoria itself is derived from the fact that the members of the class are often developed in organic infusions.

The Infusoria or Stomatode Protozoa may be defined as Protozoa which are mostly provided with a mouth and rudimentary digestive cavity, which do not possess the power of emitting pseudo-podia, but which are furnished with vibratile cilia, or with contractile filaments. They are mostly microscopic in size, the sarcode is differentiated into an ectosarc and an endosarc, and a nucleus and contractile vesicle are present.

Most modern writers regard the Infusoria as strictly speaking "unicellular" animals, each of the simple individuals corresponding morphologically to a single cell. Upon this view which is by no means free from difficulties - the "nucleus" of the Infusorian animalcules really corresponds with the structure known by the same name in an ordinary animal or vegetable cell.

The Infusoria may be divided into three orders - viz., Sudoria, Ciliata, and Flagellata - of which the second comprises the majority of the members of the class, and alone requires much consideration.