Order Catenulida Meixner, 1924
Delicate worms with pharynx simplex, simple gut and poorly differentiated parenchyma. Vegetative reproduction via paratomy common. Usually lacking statocyst. Unpaired protonephridial system arising caudally and running dorsally forward and then ventrally backwards to discharge posteriorly. Eggs entolecithal, testes dorsal with penis antero-dorsal, male organs often lacking. Free-living, freshwater (marine groups have been described).
Catenulidae Graff, 1905
Chordariidae Marcus, 1945
Stenostomidae Vejdovsky, 1880
Retronectidae Sterrer and Rieger, 1974
Tyrrheniellidae Riedl, 1959
According to Ax (1984) these are perhaps the simplest of the flatworms.