Annual herbs, glabrous, erect or prostrate (not in Victoria) of marshy or aquatic habitats; stems and branches quadrangular. Leaves opposite, usually decussate, sessile; stipules absent. Inflorescence axillary, consisting of cymes (in Victoria), umbels or bracteate heads. Flowers actinomorphic, pedicellate; hypanthium usually campanulate, glabrous, faintly 8-veined; appendages present or absent; sepals 4, small; petals usually 4, small (in Victoria) or absent; stamens 4–8 (4 in Victoria); ovary usually 4-celled. Fruit an irregularly dehiscent hyaline capsule; seeds minute, obovoid.
About 30 species, cosmopolitan; 8 species in Australia.