Hydroid small, simple, mostly solitary, some forming colonies; sessile; with or without tentacles; without theca but having a mucoprotein periderm, cysts and stolons can be covered by perisarc. Medusa mostly with 4 complete radial canals, 6 also possible, often also with incomplete centripetal canals not reaching manubrium; with or without marginal nematocyst ring; gonads along radial canals or exceptionally on manubrium (Armorhydra
); marginal tentacles peripheral, hollow, without true basal bulb, marginal sense organs as internal enclosed statocysts of endo-ectodermal origin, embedded in the mesogloea near ring canal or in the velum; no ocelli; exceptionally reduced medusoids (Monobrachium)
. Some species also in freshwater.