Species in the other major clade of annelids, Sedentaria (from the Latin sedere, sit), tend to be less mobile than those in Errantia. Some species burrow slowly through marine sediments or soil, while others live within tubes that protect and support their soft bodies. Tube-dwelling sedentarians often have elaborate gills or tentacles used for filter feeding. Although the Christmas tree worm shown in the photo above once was classified as a “polychaete,” current evidence indicates it is a sedentarian. The clade Sedentaria also contains former “oligochaetes.”
Urry, Lisa A.. Campbell Biology (p. 701). Pearson Education. Kindle Edition.