A scale-worm with an elliptical body. Dorsal surface smooth; ventral surface densely papillate.
Prostomium round, papillate with a long median antenna on a large papillate base, a pair of palps, a pair of large eye stalks each with 2 eyes, and a papillate facial tubercle. Posterior to the median antenna a ridge extends posteriorly and laterally along the posterior margin (H. hystrix-detail).
First segment well developed with a pair of dorsal and ventral tentacular cirri and numerous chaetae of two kinds: 1) fine capillaries; 2) stout capillaries.
Scales overlap, covering the body. They occur on segments 2, 4, 5 and 7 then alternately to the 25th and then on segments 30 and 31. The first pair of scales are round, attached in the middle. the rest oval to rectangular attached by the stalk and also along the dorsal surface of the notopodium. The margins are smooth and the surfaces with small tubercles.
Notopodia with scales have two groups of chaetae, the upper group has chaetae in a semicircle in the indentation of the scale, the lower group has chaetae in a bundle. Notopodia without scales have one mound with chaetae in a bundle. Notopodia chaetae of 3 kinds: 1) fine capillaries; 2) stout spines on segments without scales and on upper mounds of segments with scales; 3) harpoon chaetae on the lower mound of segments with scales.
Neuropodia project beyond notopodia with few chaetae. Neuropodial chaetae of two kinds: 1) stout, curved unidentate with a prominent lower tooth, with or without denticles on the inner curved surface; 2) laterally spinous chaetae on 2nd and 3rd segments, although sometimes absent (H. hystrix-chaetae).
Up to 60 mm, width up to 20 mm for 34 chaetigers.
Body and scales pale.
On gravel or rough ground; sublittoral to 650 m.
Widely distributed in the northern hemisphere, Mediterranean, English Channel, North Sea, Irish Sea and West coast of Scotland. Also reported from Southern Africa, Red Sea and Indian Ocean.