Prostomium bilobed, with two small eyes. Five antennae, short, smooth or indistinctly annulated. Peristomium two to three times longer than the following segments. No cirri on apodous segment.
Gills start on chaetigers 10-40 as a single filament; with 2-8 filaments where best developed, continuing nearly to the end of the body. Dorsal cirri very short. Chaetae of four kinds: 1) capillary chaetae; 2) comb-chaetae; 3) compound chaetae with elongated tapering blades; 4) stout bidentate hooded hooks which begin on about chaetiger 45. Hooks amber coloured; aciculae black (M. sanguinea-detail). Two long and two short pygidial cirri.
Mandibles slender, anteriorly calcified. Maxillae I: hooked; maxillae II: 3-6 teeth each; left maxilla III: 5-8 teeth; maxillae IV: 3-6 and 6-10 teeth (M. sanguinea-jaw plates).
Up to 600 mm for 500 segments.
Yellowish orange, brownish red, or pinkish grey, strongly iridescent; gills bright red.
In fine sand, mud and mixtures of both, clay, under stones, in crevices of rocks, on wooden pilings, in oyster beds, in sponges, in empty worm tubes, in roots of sea grass (Zostera ), from the intertidal zone to a depth of about 200 m. It lives in well defined burrows which are lined with mucus, mud and sand. It may also form long galleries in fissures of rocks.