Body is long, thin, cylindrical; earthworm-like.
Prostomium small, conical, without appendages; can be retracted in peristomium. Two nuchal organs. Young specimen with small eyes.
Thorax with 11 chaetigers. Peristomium biannular, without chaetae. The following 4-5 segments with short, stout capillary chaetae only; the posterior six thoracic segments with tori bearing hooded hooked chaetae dorsally and ventrally, on both sides.
Abdominal segments at first long, cylindrical and biannulate, then shorter and rounded, finally bell-shaped, with swollen parapodial ridges at posterior end. Abdominal hooks are shorter, stouter, more strongly toothed than thoracic. (H. filiformis-detail).
Gills are simple, from about the 80th segment.
Four pairs of genital pores on chaetigers 9-12; genital hooks absent.
Pygidium with one anal cirrus.
Up to 180 mm long and maximal 1 mm wide, for more than 150 segments.
Anterior body red; posterior body yellowish or reddish-green.
Lives in branching burrows, supported by mucus. Produces wormcasts on the surface of the bottom. From upper eulittoral down to lower abyssal (4680 m). Favours muddy sand and tolerates pollution and changing salinity grades.
Arctic, North Pacific, North Atlantic up to Mediterranean and Black Sea, English Channel up to Straight of Dover, whole North Sea, Skagerrak and Kattegat. Persian Gulf, South Africa, Australia, New Zealand.