Mostly small slender bodied mobile animals. The prostomium has 3 antennae and a single pair of simple palps which are fused together in some species. There are usually 1-2 pairs of tentacular cirri. The eversible pharynx may be armed with a single tooth or a ring of smaller teeth. Immediately behind the pharynx is a strongly muscular proventriculus. Parapodia are uniramous and have simple setae as well as compound falcigers and/or spinigers.