Body U-shaped, dorsally arched, ventrally flat.
Sixteen thoracic chaetigerous segments.
Epidermis densely papillated with filamentous papillae, encrusted with sand.
Prostomium and peristomium without eyes, with two grooved palps and 30-100 short filamentous gills in two groups, retractile.
Notopodial chaetae long capillaries, 2-11 per segment.
Neuropodial chaetae of the first chaetiger capillary, the others 3-4 very brittle, distally bent spines (B. villosa-detail).
Up to 40 mm for 35 segments.
Living animal greenish. In alcohol grey or brownish.
Sublittoral to 2000 m.
Antarctic and Subantarctic, Arctic, North Pacific, North Atlantic to Mediterranean, North Sea to Öresund.