Source : Webster's Revised Unabridged Dictionary (1913)

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

   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}.