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 in front of the rear margin.
First segment bearing chaetae and a pair of dorsal and ventral tentacular cirri. Scales overlap, but leave some posterior segments exposed. The first pair of scales are round, the rest vary from oval to kidney-shaped, the margins with very short papillae on less than a quarter of the circumference, 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 numerous chaetae. Notopodial chaetae spinose with blunt tips.
Neuropodia well developed with an anterior acicular ligule and numerous chaetae. Neuropodial chaetae of 2 kinds: 1) spinose with bidentate tips; 2) a few upper and lower chaetae spinose with unidentate tips (H. extenuata-detail).
Up to 70 mm for 47 chaetigers.
Variable, from grey to red, with plain or patterned scales.
Littoral on rocky shores, to 1250 m on varied bottoms. Sometimes found in the tubes of other polychaetes.
Arctic, North Pacific, North Atlantic up to the Mediterranean, Adriatic, Black Sea, English Channel, probably whole North Sea, Skagerrak, Kattegat, Öresund. South Africa.