The Collaborative International Dictionary of English v.0.48:

Hydrosome \Hy"dro*some\, Hydrosoma \Hy`dro*so"ma\, n. [NL.
   hydrosoma. See Hydra, and -some body.] (Zool.)
   All the zooids of a hydroid colony collectively, including
   the nutritive and reproductive zooids, and often other kinds.
   [1913 Webster]