Thecate hydroids with shallow, saucer- or basin-shaped hydrothecae. Hydrotheca radially symmetrical, usually with diaphragm; margin entire, without operculum. Hydranth large and usually not completely retractable into hydrotheca, with conical hypostome and one circlet of filiform tentacles. Nematophores present or absent. Gonophores in form of fixed sporosacs or rarely, freed as medusae.