\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}.