The Collaborative International Dictionary of English v.0.48:

Gymnochroa \Gym*noch"ro*a\ (j[i^]m*n[o^]k"r[-o]*[.a]), n. pl.
   [NL., fr. Gr. gymno`s naked + chro`a skin, body.] (Zool.)
   A division of Hydroidea including the hydra. See Hydra.
   [1913 Webster]