(M. Sars, 1835)
Body long and cylindrical with 32 segments. Body covered with small filiform or conical papillae.
Prostomium as long as broad, in the middle with a short tip. One pair of relatively large eyes and often an additional pair of very small eyes.
Palps club-shaped, not much longer than the prostomium.
4 pairs of club-shaped gills on segments 2-5, clearly longer than the segment width. 3rd segment dorsolaterally with on each side 1 filiform papilla below the gill.
Ventral chaetae start at the 4th segment. Dorsal chaetae normally start at the 5th segment. The first notopodia with a single denticulate capillary chaeta; the following notopodia with 2-7 of such chaetae. Neuropodia bear 1-2 compound chaetae with a short lamella on the concave side of the curved tip (M. clavicornis-chaeta).
Pygidium rounded, with papillae.
Up to 7-10 mm for 45 segments.
Brownish or blackish pigment in between the segments on the anterior body. Phosphorescent.
Low water, between seaweeds.
North Atlantic up to Mediterranean, Adriatic Sea, northern North Sea, Skagerrak and Kattegat.