It is hypothesized that bivalve-inhabiting hydroids originated from colonial, free-living eirenid hydrozoans, initially appearing as an Eutima species with solitary hydroids producing immature medusae with tentacles and manubrium, and also with derived characteristics of the marginal warts of the mature medusae as the remnants of the tentacular bulbs of the ancestral eirenid, and decreased number of statocysts. The derivate eumedusoid-producing Eugymnanthea evolved then. Deduced from the morphology, life cycles, and geographical distributions of this group and of all the known extant Eutima species, it is proposed that parallel, paedomorphic evolution took place in the marginal regions of the area of distribution of the ancestral-like Eutima at least in the northern hemisphere of both the Pacific and the Atlantic Oceans. Eugymnanthea appeared as a polyphyletic taxon as a result of the parallel evolution of different species of Eutima. It is pointed out that Eugymnanthea and Eutima are to be merged into a single genus in the future.