WEBSedentaria and Errantia are the two biggest clades of polychaetes, and together they compose clade Pleistoannelida. Sedentaria's most basal clade is Orbiniida . [2] Other groups that are nested within Sedentaria are: Clitellata , the Sabellida / Spionida clade, Opheliida , Echiura , Cirratuliformia , Terebelliformia , Maldanomorpha and the families …