Body long and relatively broad.
Prostomium without eyes, anteriorly straight with frontal horns. Caruncle reaching chaetiger 1.
Notopodial postchaetal lobes in anterior segments fully fused to the gills, posteriorly becoming lanceolate and fused only basally.
Anterior parapodia with capillary chaetae only. From neuropodium 30-57 onwards also 20-25 tridentate hooded hooks (M. vulgaris-detail).
Gills from 1st segment and present on all chaetigers almost to the posterior end of the body. They are cirriform and reach the mid-line of the body.
Pygidium with 15-30 filamentous cirri.
Up to 160 mm for 350 segments.
Pale salmon-pink to yellowish, posteriorly greenish brown, gills bright red.
Eulittoral to 200 m.
North Sea, Skagerrak, Kattegat, Arctic, North Pacific, North Atlantic to Mediterranean, Black Sea, Chile.