The Collaborative International Dictionary of English v.0.48:

Trachymedusae \Tra`chy*me*du"sae\, n. pl. [NL., fr.Gr. ? rough +
   medusa.] (Zool.)
   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 medusae.
   [1913 Webster]