The Collaborative International Dictionary of English v.0.48:

Hydromedusa \Hy`dro*me*du"sa\, n.; pl. Hydromedus[ae]. [NL.
   See Hydra, and Medusa.] (Zool.)
   Any medusa or jellyfish which is produced by budding from a
   hydroid. They are called also Craspedota, and naked-eyed
   medus[ae].
   [1913 Webster]

   Note: Such medus[ae] are the reproductive zooids or
         gonophores, either male or female, of the hydroid from
         which they arise, whether they become free or remain
         attached to the hydroid colony. They in turn produce
         the eggs from which the hydroids are developed. The
         name is also applied to other similar medus[ae] which
         are not known to bud from a hydroid colony, and even to
         some which are known to develop directly from the eggs,
         but which in structure agree essentially with those
         produced from hydroids. See Hydroidea, and
         Gymnoblastea.
         [1913 Webster]