Body long and relatively broad.
Prostomium triangular with four eyes and two frontal horns. Caruncle reaching the posterior margin of chaetiger 2.
Notopodial postchaetal lobes in anterior segments completely fused with the gills; posteriorly only basally fused.
Anterior parapodia with winged capillaries only. From neuropodium 30-57 onwards also 4-10 bidentate hooks and 1-2 spines (M. fuliginosus-detail).
Gills start at segment 1. Anterior gills are small, posteriorly gills become longer and cirriform.
Pygidium with 6-10 cirri.
Up to 114 mm for 230 segments.
Living animal salmon-pink to dark purple.
Eulittoral to bathyal, in muddy sand and under stones.
Whole North Sea, Skagerrak, Kattegat, North Atlantic to the Mediterranean, Red sea, Black sea and North Pacific.