hydractinian

Hy`drac*tin"i*an, n. Etym: [See Hydra, and Actinia.] (Zoöl.) Defn: Any species or marine hydroids, of the genus Hydractinia and allied genera. These hydroids form, by their rootstalks, a firm, chitinous coating on shells and stones, and esp. on spiral shells occupied by hermit crabs. See Illust. of Athecata.

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