\Hy`drac*tin"i*an\, n. [See {Hydra}, and
{Actinia}.] (Zo["o]l.)
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}.