Medusae flatter than a typical bell, the umbrella usually hemispherical or discoidal. Gonads confined to radial canals, exceptionally extending to proximal part of manubrium. Marginal sense organs, when present, ectodermal velar statocysts, rarely cordyli, occasionally ocelli. Marginal tentacles peripheral and hollow (except in Obelia). Cnidome often microbasic mastigophores. Hydroids thecate, generally protected by rigid perisarc formed by hydrotheca, nematotheca, and gonotheca, rarely with naked hydranths.