a. [ Hydra + -oid. ] (Zool.) Related to, or resembling, the hydra; of or pertaining to the Hydroidea. -- n. One of the Hydroideas. [ 1913 Webster ]

‖ 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 ]