hydractinian

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology A marine hydroid of the genus Hydractinia or a closely-related genus