A scale-worm with a body of uniform width. Dorsal and ventral surfaces smooth.
Prostomium bilobed, with a peak on each lobe, a papillated median antenna, a pair of lateral antennae, ventrally inserted, and a pair of palps. Two pairs of eyes, the anterior pair on the line of greatest width, the posterior pair near the rear margin.
First segment bearing chaetae and a pair of dorsal and ventral tentacular cirri. Scales overlap, covering the body. The first pair of scales are round, the rest oval to kidney-shaped, a quarter to a half of the margins fringed with small papillae, the surfaces covered with small tubercles and with a row of flattened discs extending over the posterior border. Notopodia are small mounds with a ventral acicular ligule and abundant chaetae. Notopodial chaetae spinose with blunt tips.
Neuropodia well developed with an anterior ligule and chaetae. Neuropodial chaetae of 2 kinds: 1) spinose along most of the length with unidentate tips; 2) spinose on the terminal quarter and with bidentate tips (H. fragilis-detail).
Up to 26 mm for 41 chaetigers.
Dorsal surface brown with two transverse white lines per segment.
Sublittoral to 2900 m, on fine sand.
Arctic, North Pacific, North Atlantic up to English Channel, Skagerrak, Kattegat, Öresund.