Any one of numerous species of tubicolous annelids of the
   genus Serpula and allied genera of the family Serpulidae. They secrete
   a calcareous tube, which is usually irregularly contorted, but is
   sometimes spirally coiled. The worm has a wreath of plumelike and often
   bright-colored gills around its head, and usually an operculum to close
   the aperture of its tube when it retracts.