Body long and narrow with a constriction around the 9th segment.
Prostomium somewhat broader than long, anteriorly bluntly truncated; broadest at the hind margin (M. alleni-head).
Two long, papillose palps.
Thoracic parapodia with slender triangular dorsal postchaetal lamellae, short truncated ventral postchaetal lamellae, tiny papilliform dorsal cirri and finger-like ventral cirri .
Thoracic chaetae all winged capillaries. Abdominal parapodia with low postchaetal lamellae both dorsally and ventrally and very big triangular dorsal and much smaller triangular interramal lamellae.
Abdominal chaetae are tridentate hooded hooks (M. alleni-parapodium); (M. alleni-chaetae).
Up to 40 mm for 70 segments.
Living animal yellowish with a broad dark red band on the posterior thoracic segments.
On sand and mud; upper to middle sublittoral.
Eastern North Atlantic to Mediterranean and Adriatic, English Channel, North Sea, Skagerrak and Kattegat.