||Hy*droi"de*a, n. pl. [NL. See Hydra, and -oid.] (Zoöl.) An extensive order of Hydrozoa or Acalephæ. [Written also Hydroida.]

&fist; This order includes the hydras and the free-swimming hydromedusæ, 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.