Body thin and cylindrical, with up to 126 segments.
Prostomium is conical, small, without eyes. First segment without chaetae, with two reddish eyes dorsally.
Thorax with 10 chaetigers.
First 4 segments with long, winged capillary chaetae only, segment 5 and following segments with hooks only; thoracic hooks with three rows of small teeth over the large terminal tooth, hooded.
Abdominal segments are longer than thoracic, but become shorter towards the posterior end.
Abdominal noto- and neuropodia with tori, which bear hooks. Abdominal hooks are much shorter than the thoracic hooks. Abdominal tori are set on the posterior 1/3 of the segment; thoracic parapodia in the middle of the segment (M. fragilis-detail).
Genital hooks and gills absent.
Pygidium with 1 ventral anal cirrus .
Up to 50-60 mm for more than 100 segments.
Lives in vertical burrows; various bottoms, mainly in sandy mud. 0-40 m. Produces wormcasts , which resemble those of Heteromastus and Arenicola.
Mediterranean, North Sea and Kattegat.