Prostomium of this scale-worm bilobed with a small, rounded peak on each lobe, median and lateral antennae and a pair of long palps. Two pairs of eyes, the anterior pair anterior to the line of greatest width, the posterior pair in front of the rear margin. First segment bearing two to four chaetae and a pair of dorsal and ventral tentacular cirri.
14 pairs of scales. Scales are oval to round, the outer lateral and posterior margins fringed with papillae, the surfaces slick, shiny, dotted with cylindrical papillae; without microtubercles. Notopodia are small mounds with a ventral acicular ligule and numerous chaetae. Notopodial chaetae with numerous spinose rows, some short, slightly curved, with short bare tips and some longer, straight. Neuropodia are well developed with slender, finger-like anterodorsal ligule and chaetae in rows. Neuropodial chaetae numerous, mostly very long, filamentous with a long spinose part and capillary tips. Also beneath the acicula a few stouter chaetae present and at the bottom of the bundle a few much shorter, distally capillary chaetae with short spinose part (B. sarsi-detail).
Up to 68 mm for 38 chaetigers.
Dorsum with grey to brown or blackish pigmentation present as one longitudinal and transverse bands per segment. Ventrally grey, iridescent. Scales with irregular brown pigmentation.
On mud or sand; eulittoral to 2324 m. Also in Arenicola burrows.
Arctic, North Pacific, North Atlantic, Baltic to Bay of Kiel, Netherlands (Goeree-Overflakkee).
A variety with 15 pairs of scales, bearing microtubercles, was found near Gotland, Sweden (Pettibone, 1993b).