Order Macrostomida Karling, 1940
With pharynx simplex and simple gut. Brain not encapsulated, without statocyst. Paired protonephridia. Eggs entolecithal, female gonopore anterior to male (separate or into a common atrium); testes compact, male canal passes directly posterior to gonopore, usually with a hard stylet. Free-living, marine and freshwater.
Dolichomacrostomidae Rieger, 1971
Macrostomidae Beneden, 1870
Microstomidae Luther, 1907
There has been no comprehensive account of all the groups of these simple forms, though accounts of the various families are available.