Body small and cylindrical, gratually tapering towards the posterior end. Up to 90 chaetigers, seven of them thoracic.
Radioli arise from the paired lobes and number about seven (5-9) on the left side and eight (7-10) on the right side. They are arranged in two semicircles and are not connected by a basal membrane. The two rows of pinnules become larger towards the tips of the radioli, which are free of pinnules to greater of lesser extent.
Operculum without a distal rim but with dark chitinous inwardly curved spines; peduncle arising near dorsal mid-line, D-shaped in cross-section with a curved surface facing mouth.
The collar is high, not lobed and has an entire edge. It is continuous with the thoracic membranes, which are united ventrally on the anterior abdominal segments.
Peristomium without eyes.
Collar chaetae slightly geniculate with large teeth at the knee and smaller, but distinct, ones along the distal part.
The thoracic uncinicerous tori are arranged in two nearly parallel rows, with up to 90 uncini per torus. Uncini with a single row of 6-7 curved teeth, the most anterior is gouged, apparently bifurgated.
Usually about 60 abdominal segments, the anterior two apparently without chaetae. The abdominal uncini in the anterior segments have a single row of curved teeth (6-7), including the anterior gouged one. The posterior uncini are smaller, with two rows of small teeth, 10-12 teeth in each row. The abdominal bundles of chaetae consist of 2-5 geniculate ones (F. enigmaticus-chaetae).
Up to 27 mm long for 40-120 segments.
Tube up to 10 cm long and about 1.4 mm width. Semicircular to circular in cross-section, with fine transverse growth rings and occasional shelf-like rings distally.
Greenish to brownish, female often reddish, crown with green or brown cross-bands, operculum brown with whitish or yellowish ring.
On hard substrata in the warmer and sometimes brackish water of estuaries, docks, lagoons and power station outfalls. Sublittoral.
World-wide in brackish waters of subtropical regions, transported to some harbours along the North Sea coasts. In Vlissingen since 1967. Also Noordzeekanaal, Copenhagen, Marstal, Emden and in France.