The previously unknown life cycle of the parasitic hydroid Hydrichthys mirus is described. The adult medusa has 5-6 tentacles and could be referred to Leuckartiara. Another species of Hydrichthys has previously been shown to have a Stomotoca-like medusa, characterized by the possession of two tentacles. It is proposed that Hydrichthys originated from the Leuckartiara lineage and that, through pedomorphosis in at least one species, the medusa retained the two-tentacle state of the newly-released medusa of all pandeids, thus becoming referable to Stomotoca. It is suggested that Stomotoca has hitherto constituted apolyphyletic taxon, embracing parallel lineages of pandeids, each retaining juvenile features. An additional genus LARSONIA, is introduced to accommodate species with Stomotoca-like medusae and Hydrichthys-like hydroids. The Larsonia type species is L. pterophylla. Descriptors: STOMOTOCA; LEUCKARTIARA; LARSONIA PTEROPHYLLA new genus figs. 1-3.