Submerged aquatic herbs without roots. Stems branched. Leaves whorled, divided into linear segments. Flowers minute, solitary, axillary, sessile, unisexual, surrounded by greenish, perianth-like, 8–15-lobed involucre which is fused at base; male and female flowers on separate nodes of same plant; male flowers with 8–24 stamens, anthers almost sessile, 2-locular, extrorse, with 2 short, sharp, prongs on anther tips; female flowers with single superior ovary (within involucre), ovary sessile, 1-locular, ovules solitary, pendulous. Fruit an achene, crowned by persistent style.
1 genus with 6 species world-wide.