The Collaborative International Dictionary of English v.0.48:

Hydroidea \Hy*droi"de*a\, n. pl. [NL. See Hydra, and -oid.]
   (Zool.)
   An extensive order of Hydrozoa or Acaleph[ae]. [Written also
   Hydroida.]
   [1913 Webster]

   Note: This order includes the hydras and the free-swimming
         hydromedus[ae], together with a great variety of marine
         attached hydroids, many of which grow up into large,
         elegantly branched forms, consisting of a vast number
         of zooids (hydranths, gonophores, etc.), united by
         hollow stems. All the zooids of a colony are produced
         from one primary zooid, by successive buddings. The
         Siphonophora have also been included in this order by
         some writers. See Gymnoblastea, Hydromedusa,
         Gonosome, Gonotheca.
         [1913 Webster]