The Collaborative International Dictionary of English v.0.48:

Serpula \Ser"pu*la\, n.; pl. Serpulae, E. Serpulas. [L., a
   little snake. See Serpent.] (Zool.)
   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.
   [1913 Webster] Serpulian