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.

 

© Webster 1913.

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