WoRMS note details

Hydroid mostly stolonal, occasionally branched, one oral whorl of four to five capitate tentacles, with or without filiform tentacles, medusa buds borne singly on hydranth body immediately above filiform tentacles or in similar position when these are absent.
Medusae adapted for swimming and adhering to surfaces, with bell-shaped umbrella; manubrium cylindrical, in adults with pouches, gonads without perradial separations, mouth short, unbranched lips bearing four to six spherical nematocyst clusters; without apical chamber above manubrium; radial canals bifurcated or simple, final number of canals entering circular canal usually of same number as marginal tentacles; marginal tentacles branching, each with one to six proximal branches ending in an adhesive organ and one to ten more distal branches with nematocyst clusters; abaxial ocelli with lens.
Schuchert, P. (2006). The European athecate hydroids and their medusae (Hydrozoa, Cnidaria): Capitata Part 1. Revue suisse de Zoologie. 113: 325-410. available online at http://www.biodiversitylibrary.org/item/128494#page/101/mode/1up