Initially they were considered spirotrichs, then treated as a separate category, before receiving their modern placement.

They are usually bell or disc shaped, with a prominent paroral membrane arising from the oral cavity and circling counter-clockwise around the anterior of the cell, accompanied by a smaller series of membranelles.

The rest of the body is unciliated, except for a telotroch band circling the posterior in mobile species and stages.

These are common in both freshwater and marine environments, and many live attached to aquatic plants and animals.

Most live on the integument or gills of freshwater and marine invertebrates, but other hosts occur, including fish and even other ciliates, and other locations as well.