The Collaborative International Dictionary of English v.0.48:

Medusoid \Me*du"soid\, a. [Medusa + -oid.] (Zool.)
   Like a medusa; having the fundamental structure of a medusa,
   but without a locomotive disk; -- said of the sessile
   gonophores of hydroids. -- n. A sessile gonophore. See
   Illust. under Gonosome.
   [1913 Webster]

WordNet (r) 3.0 (2006):

medusoid
    adj 1: relating to or resembling a medusa
    n 1: one of two forms that coelenterates take: it is the free-
         swimming sexual phase in the life cycle of a coelenterate;
         in this phase it has a gelatinous umbrella-shaped body and
         tentacles [syn: medusa, medusoid, medusan]