Herbs aquatic. Floating leaves triangular-rounded to cordate, submersed ones lanceolate. Flowers solitary, axillary, mostly submersed and cleistogamous. Calyx teeth 5; tube adnate to ovary. Corolla funnelform, bilabiate; lowest lobe longest, upper 2 forming inconspicuous lip. Stamens 2, included, basally adnate to corolla tube. Ovary inferior, 2-locular, 1 locule reduced, another with 2 ovules. Stigma 2-lobed. Fruiting pedicel nodding. Fruit indehiscent, narrow and long, with 5 hooklike or spiny appendages below calyx lobes, sometimes with 3-5 longitudinal wings. Seeds 1.

One or two species: China, Japan, Korea, Russia (Far East); one species in China.

The genus is sometimes placed in its own family (Trapellaceae, especially if Martyniaceae is separated from the Pedaliaceae).