Hydractinian /(?)/

Hy·drac·tin·i·an

Hydractinian

n.
  1. 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. (Zool.)