Submerged, rootless, monoecious freshwater plants. Leaves verticillate, 2-4 times forked, segments linear dentate. Flowers actinomorphic, solitary, axillary, unisexual. Perianth valvate, segments 9-12, persistent, narrow. ♂: stamens 8-24; anthers nearly sessile rather broad, connective pointed, the 2 cells mostly crowned by a minute bristle; ovary rudiment absent. ♀: ovary superior, sessile, 1-celled with 1 ovule; style persistent, subulate, sulcate towards the apex; stamen rudiments absent. Fruit oblong, compressed, warty, not dehiscent, near the base with 2 straight or curved soft spines, or unarmed. Distr. Ca 2 spp., both ubiquitous.