Any one of numerous species of tubicolous annelids of the genus Serpula and allied genera of the family Serpulidæ. 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.  [1913 Webster]