01 n. Any species or marine hydroids, of the genus Hydractinia and allied genera. These hydroids form, by their rootstalks, a firm, chitinous coa…
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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.(Zool.) See: Athecata