What is "trachymedusae"

Trachymedusae \Tra`chy*me*du"s[ae]\, n. pl. [NL., fr.Gr. ? rough + medusa.] (Zo["o]l.) A division of acalephs in which the development is direct from the eggs, without a hydroid stage. Some of the species are parasitic on other medus[ae].

Trachymedusae belong to the phylum Cnidaria and the class Hydrozoa, among the 30 genera are 5 families containing around 50 species in all, the family Rhopalonematidae has the greatest diversity.