(n.) 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.
HYDRACTINIAN
Meaning of HYDRACTINIAN in English
Webster's English dictionary. Английский словарь Webster. 2012