Umbrella 2.5-6 mm wide, flat, mesoglea very thin; without gastric peduncle; mouth with 4 simple lips; 4 radial canals; gonads spherical to ovoid, sac-like, hanging from middle to end of radial canals. Various nominal species of Obelia hydroids are common throughout the world, but there are still no characters to distinguish their medusa stages with certainty.
According to Zamponi and Genzano (1990), the medusae of Obelia dichotoma Hincks, 1868 and Obelia longissima (Pallas, 1766), whose hydroids are both present in the South Atlantic, can be distinguished by their cnidome, composed of atrichous isorhizas, atrichous anisorhizas and basitrichous isorhizas in Obelia dichotoma, and by microbasic mastigophores and macrobasic mastigophores in Obelia longissima. However, macrobasic mastigophores seem improbable in the genus Obelia, as well as in any other Leptomedusae (contamination?).