Low-growing herbs. Roots produced at end of leafless branches (rhizophores); rhizomes present in most species. Stems erect, lax or creeping; unbranched or much-branched. Leaves arranged spirally or in four rows; all alike or dimorphic with larger leaves flattened into one plain; leaves very small, bract-like, with minute ligule. Sporangia opening with a slit across top; megasporangia containing 4 spores; microsporangia with numerous tiny spores; all spores tetrahedral to globose.
About 700 species world-wide, mostly in the tropics; 8 native and 1 naturalised species in Australia; 3 in Victoria.