Medusae usually flattened, with central lens-shaped mass of mesoglea and much thinner sides. Margin of umbrella divided by peronial grooves so umbrella margin is lobed. Tentacles solid, inserted on exumbrella at some distance from margin, just above peronial grooves; sometimes small secondary tentacles on margin itself. Manubrium very broad and short, with entire circular periphery, or with perradial or interradial peripheral pouches. Generally without radial canals; circular canal absent or looped into marginal flaps to form a "peripheral canal system". Gonads on manubrium walls and/or on manubrial pouches. Marginal sense organs free ecto-endodermal statocysts. With or without otoporpae. Cnidome atrichous isorhizae and apotrichous isorhizae (Cnidocyst types). Reproduction by direct development, by reduced polyps, or by complex parasitic larval development.