Small floating ferns. Rhizome slender, creeping, short, branching into protostelic stems, bearing short, chlorophyllous and deciduous roots. Leaves all alike, 2 at each node, tiny, bilobed, imbricate, upper lobe papillate and inhabited by colonies of the blue-green algae Anabaena, lower lobe usually a-chlorophyllous. Sori (sporocarps) are formed in 2 or 4 on the lower lobes of the first leaf of each stem. The microspores are joined together into several masses (massulae) in each sporangium by the hardened frothy mucilage. Each massula is covred with barbed hairs (glochidia) which enable the floating massulae to anchor to a floating male prothallus.