Body short, cylindrical, tapering anteriorly and posteriorly. Eight chaetigers in thorax, three in abdomen.
Two pairs of radioli with a few (often just one) side-branches, ending at the same height as the radioli themselves. Also two tentacular filaments.
Collar dorsally low, medially incised, ventrally high, semicircular. Peristomium with two eyes.
The first chaetiger bears long, winged capillaries, other thoracic segments with 3-5 long, winged and 2-3 short, spatulate chaetae.
Thoracic neuropodia with 3-4 hooks.
Abdominal neuropodia with 3 long capillary chaetae. Thoracic notopodia bear 8-12 uncini with relatively long shafts and five rows of five teeth of same size.
(M. aestuarina-chaetae )
Pygidium conical, without eyes.
Up to 6 mm for 11 segments.
10 mm long, combined with a burrow, protruding from the sediment for about 2 mm.
Body transparent, crown greenish.
In mud and mixed bottoms. 0-20 m. Brackish waters.
North Sea to Baltic (Helsinki).