Body is very small.Thorax with 8, abdomen with 3 chaetigers.
Three pairs of radioli with side-branches ending at the same height as the radioli themselves. Collar only ventrally developed, building a triangular lip (F. stellaris-anterior end ). Peristomium with one or two pairs of eyes.
Thoracic notopodia with 4-6 long, winged capillary chaetae. The 2nd and 8th chaetiger have additionally 2-3 short capillaries. Segments 3-7 also with two spatulate chaetae on notopodia.
Thoracic neuropodia with 7-9 hooks with small teeth over the large terminal tooth.
Abdominal neuropodia with 2-3 capillary chaetae. Notopodia with 20-25 uncini with 10-11 rows of small teeth, 3-4 teeth in each row (F. stellaris-chaetae).
Pygidium short and rounded, with a pair of eyes.
Up to 4.5 mm.
Cylindrical, encrusted with mud, detritus or sand; often in colonies.
Transparent, yellowish to pink, with brown pigment on the anterior segments.
On sand or mud, usually between algae, ascideans, serpulid tubes or epibiontic. In low water. Tolerates low salinities.
Arctic, North Pacific, North Atlantic to the Mediterranean, English Channel, North Sea to the Baltic, Subantarctic and Antarctic.