Source : Webster's Revised Unabridged Dictionary (1913)

Hydractinian \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}.