There is typically a ventral groove containing the mouth and distinct oral cilia, separate from those of the body.

Members are widely distributed, and include many free-living (typically fresh-water, but many marine) and symbiotic forms.

Most are microphagous, grazing on smaller organisms swept into the mouth by the cilia, but various other feeding habits occur.

The Oligohymenophorea were first proposed in 1974 as one of three classes of ciliates, together with the Polyhymenophorea or spirotrichs and the now abandoned Kinetofragmophora.

Since then the apostomes have been added, but otherwise its composition has remained relatively constant, with the main variations being the positions of the peniculids and plagiopylids.