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From 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]