‖ n. pl. [ NL. See Hydra, and -oid. ] (Zool.) An extensive order of Hydrozoa or Acalephæ. [ Written also Hydroida. ] [ 1913 Webster ]

☞ 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. [ 1913 Webster ]

n. [ Hydro-, 2 + oxide. ] (Chem.) A hydrate; a substance containing hydrogen and oxygen, made by combining water with an oxide, and yielding water by elimination. The hydroxides are regarded as compounds of hydroxyl, united usually with basic element or radical; as, calcium hydroxide ethyl hydroxide. [ 1913 Webster ]