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The Collaborative International Dictionary of English v.0.48:

Hydractinian \Hy`drac*tin"i*an\, n. [See Hydra, and Actinia.] (Zool.) 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. [1913 Webster]