A scale-worm with a body of uniform width. Dorsal and ventral surfaces smooth.
Prostomium bilobed, with a peak on each lobe, a 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, at least half of the margins fringed with short papillae, the surfaces with small tubercles and a row of larger tubercles on the outer border. Notopodia are small mounds with a ventral acicular ligule and many chaetae. Notopodial chaetae spinose with blunt tips.
Neuropodia well developed with an anterior ligule and chaetae. Neuropodial chaetae spinose with bidentate tips, a few in the lower position with unidentate tips (H. impar-detail).
Up to 25 mm for 40 chaetigers.
Scales brownish, sometimes with one or more dark patches. Surface tubercles in these patches often with a halo of pale colour.
Littoral on rocky shores, sublittoral to 655 m on a variety of bottoms.
Arctic, North Atlantic up to the Mediterranean, English Channel, whole North Sea, Skagerrak, Kattegat, Öresund, Belts to Bay of Kiel.