Body slender, thread-like.
Prostomium rounded, slightly broader than long, with four eyes in a trapezoid arrangement. Sometimes with 2-3 extra, small eye-spots on the prostomium. Palps hardly visible dorsally. Median antenna at least 2x as long as lateral ones. Lateral antennae up to 4x as long as prostomium. Nuchal epaulettes short, half-moon shaped.
Dorsal tentacular cirri a little shorter than lateral antennae, ventral much shorter. Dorsal cirri of first segment about as long as median antenna. All other dorsal cirri equal in size and about half the length of segment width. Often the dorsal cirri of the second segment are a bit longer than the next.
Blades of compound chaetae with a larger secondary tooth an a smaller terminal tooth. Posterior parapodia from segment 30 and further with an additional needle shaped chaeta. Bayonet-chaetae from the first segment.
Pharynx S-shaped, with 9 papillae around the opening. Trepan with alternating 9 large and 9 smaller teeth.
Body up to 18 mm long for 78 segments.
Pink or transparent, often with brownish pigmentation or on the bases of the parapodia greenish or grey; upper half of parapodia with a weakly brownish lateral line.
Between algae, on oysterbanks. Vertical distribution from eulittoral down to 150 m.
Arctic, North Pacific, North Atlantic up to English Channel, North Sea and Skagerrak, Kattegat, Öresund.