Body elongate, cylindrical, tapering anteriorly and posteriorly. Prostomium bluntly conical, with or without eyes on the posterior margin; the species occurring in the North Sea have no antennae or palps. Peristomium of two rings. There is pair of dorsal sensory pits between prostomium and peristomium. No cushionlike upper lip like in family Lumbrineridae. Peristomium is not fused ventrally to the apodous segment, which is without cirri.
Parapodia are small, uniramous or subbiramous, with notopodia smaller than neuropodia. The first segment (following the peristomium) has neither parapodia nor chaetae. Gills absent. Chaetae winged capillaries. Dorsal cirri absent or rudimentary. Ventral cirri absent (Oenonidae).
Pygidium with two or four anal cirri.
Pharynx eversible with a jaw apparatus consisting of a pair of mandibles, one to five pairs of maxillae, maxillary carriers and a flat, ovate plate. Mandibles sometimes absent. Maxillary carriers with an unpaired middle piece, highly sclerotised, slender free rods loosely attached to the posterior end of the first maxillae.
Both free-living and parasitic forms. The free-living forms burrow in sandy and muddy sediments without constructing permanent burrows like the lumbrinerids to which they are superficially similar. Most free-living forms are carnivorous. The parasitic forms are endoparasitic in the body cavity or in the peri-intestinal blood sinus of other polychaetes, sometimes also in echiurans. Some species may be both free-living and parasitic. The parasitic forms are usually of small size and have poorly developed jaws; the chaetae may be reduced.
After: Fauchald and Rouse, 1997, George and Hartmann-Schröder, 1985 and Hayward and Ryland, 1990.
The following taxa of this family occur in the region: