Medusae with gonads either on manubrium, on manubrial wall with perradial continuation along radial canals, or on radial canals only. Marginal tentacles peripheral, hollow. Marginal sense organs, when present, ecto-endodermal closed statocysts. Hydroids solitary or colonial, small, sessile, with or without tentacles, without perisarcal thecae.
Two families, the Olindiidae (with gonads on radial canals only, except the freshwater genus Limnocnida; with internal statocysts; without ocelli), and the Proboscidactylidae (with gonads on the manubrium extending over the proximal part of radial canals; 4-6 or more radial canals, usually branched; without statocyst and without ocelli).