Hydractinian

Hy`drac`tin´i`an


n.1.(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's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.