A group of 34 families of Polychaeta in which the anterior, or cephalic, region is more or less fully exposed and the body is often long, linear to short, and depressed. The segments of these worms are similar or change gradually. The pharynx is often heavily muscularized and eversible, and its inner walls are fortified with calcified or chitinized plates or jaws. Some families are benthic throughout their lives, others are entirely pelagic, and some have pelagic larval and reproductive stages and benthic trophic development. Errantia occur in all seas, at all depths, and in inland seas or lakes. They range in length from a few millimeters to 6 ft (2 m).