The Serpulidae are a family of sessile, tube-building annelid worms in the class Polychaeta. The members of this family differ from the sabellid tube worms in that they have a specialized operculum that blocks the entrance of their tubes when they withdraw into the tubes. In addition, serpulids secrete tubes of calcium carbonate. Serpulids are the most important biomineralizers among annelids. About 300 species in the Serpulidae family are known, all but one of which live in saline waters. The earliest serpulids are known from the Permian (Wordian to late Permian).