Class of annelid worms which possess distinct metameric segmentation. All have bristly parapodia (movable, paired appendages) on each body segment. Eyes may be present. Males and females occur. Most are marine, although some occur in fresh water and on land. The group first appeared in the Cambrian, and impressions of polychaete worms are among the fauna of the Burgess Shale. However, polychaetes are represented in the fossil record largely by burrows (e.g. Skolithos), tubes, and scolecodont assemblages.