The generaZanclea, HalocoryneandZanclellaare surveyed, with description of all known species, including eight new ones ofZanclea(Z.bomala, Z, divergens, Z. fanella, Z. giancarloi, Z. medusopolypata, Z. retraduis, Z. gilii, Z. hirohitoi), two new species ofHalocoryne(H. frasca, H. pirainoid), and two new ones ofZanclella(Z.diabolica, Z. glomboides). For most species, the description covers the whole life cycle. The hydroids of this group can be symbiotic with either bryozoans, bivalves, or corals and only few species are not substrate‐specialist. Symbiotic relationships led to polymorphism and colony integration with the hosts. The morphologies of hydroid and medusa stages are often not consistent, so that species with very derived hydroids have non‐derived medusae (e.g.,Halocoryne pirainoid), or species with very derived medusae have non‐derived hydroids (e.g.,Zanclella diabolica).The architecture of the newly released medusae ofZanclellais exceptional due to a sharp compression of the exumbrella and of the presence of just two radial canals. A phylogenetic scenario is suggested, in spite of the inconsistencies in the rates of change in the two main stages of the cycle.