Body heavy but fragile.
Prostomium with four eyes in a trapezoid arrangement and sometimes with two small ocelli. It is divided into three parts by a Y-shaped groove. Palps shorter than prostomium. Median antenna not much longer than lateral antennae. Lateral antennae a little longer than prostomium.
Tentacular cirri not much longer than antennae. Tentacular segment with an occipital flap, extending dorsally over the prostomium.
Dorsal cirri fusiform, pointed, often weakly annulated, not much longer than segment width. Ventral cirri rounded triangular. One stout acicula. Blades of compound chaetae relatively long, unidentate (O. gibba-detail). In posterior parapodia an additional upper simple chaeta is present.
Pharynx short and narrow, with 6-7 large teeth between two lateral folds.
Body up to 25 mm long for 48 segments.
Whitish to brown with purple spots on segments and gills.
North Atlantic up to Mediterranean, English Channel up to Straight of Dover and northern North Sea.