Annual or perennial, aquatic or marsh herbs. Stems erect or creeping and rooting at the nodes. Submerged leaves laciniate, pinnatifid; aerial leaves opposite to whorled, entire, laciniate or pinnately-divided. Flowers solitary, axillary or in terminal spikes or racemes. Corolla 5-lobed, 2-lipped. Stamens 4, included, didynamous. Style filiform. Capsule ellipsoid to spherical.
Worldwide: c. 35 species in the Old World tropics and subtropics.