infusoria

Collective term for certain members of the protozoan Phylum Ciliophora, of which it is considered a subclass in certain classification systems. Members naturally inhabit infusions of decomposing organic material and some are cultured by aquarists in order to feed very small fish fry. The name was formerly applied to a much larger group of aquatic microorganisms which also included some other protist groups and unicellular algae.