The Polychaeta have traditionally been divided into free-living and sedentary forms, the Errantia and Sedentaria respectively. While this is the most widely used system of ordinal classification, it is generally agreed that it is an unnatural one. However, no other proposed system has been widely accepted. Polychaete systematics is still very poorly understood.
Polychaetes are usually considered to be the most "primitive" of living annelids, that is, they are believed to most closely resemble the common ancestor of all annelids, and perhaps of all segmented animals.