In traditional classifications a class of annelid worms in which each body segment has a pair of flattened fleshy lobes (parapodia) bearing numerous bristles (chaetae). All polychaetes are aquatic and most of them are marine. They include the fanworms (Sabella), which construct tubes of sand, etc., in which they live; and the lugworms (Arenicola) and ragworms (Nereis), which burrow in sand or mud. Molecular studies have thrown the evolutionary relationships of annelids into question and shown the polychaetes to be a paraphyletic group. See Annelida.