Hy`drac*tin"i*an (?), n. [See Hydra, and Actinia.] (Zoöl.) 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.