alternative forms: Hydroida.
this order includes the hydras and the free-swimming hydromedusae, 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.