Noun
One of the classes of Protozoa, including a large number of species, all of minute size.
Source: Webster's dictionaryAquarium use Infusoria are used by owners of aquariums to feed fish fry ; newly hatched fry of many common aquarium species can be successfully raised on this food during early development due to its size and nutritional content. Source: Internet
In modern formal classifications, the term is considered obsolete; the microorganisms previously included in the Infusoria are mostly assigned to the kingdom Protista which itself is a polyphyletic assemblage of groups. Source: Internet