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Gunnerus, 1768

Body short, cylindrical, gradually tapering to the posterior end. Usually more than 100 chaetigers, seven of them are thoracic.
12-19 pairs of radioli with thin tips; radioli arranged in two semicircles. Without basal membrane. Terminal filament of radiole is of very variable length, 1-2 times as long as pinnules. Eyes and stylodes absent on radioli.
Dorsal lip of mouth with triangular protrusion laterally; ventral lip with tiny palps.
Operculum with a proximal serrated cup and a distal crown; the crown without a central projection; spines fused to each other for less than one-third of their length.
Peduncle is smooth, circular in cross section, inserted just below first and second normal radiole. Pseudoperculum present.
Collar is low with entire edge and mid-ventral depression; continuous with thoracic membranes, which form an apron across anterior abdominal chaetigers.
The first chaetiger bears finely denticulate capillaries and some characteristic chaetae, which are slightly geniculate, mostly with two (1-3) large teeth at the knee and denticulate distal part.
The rest of the thoracic chaetae are capillaries and chaetae with broad, denticulate wings. Apomatus-chaetae absent.
In abdomen, neuropodial chaetae are flat trumpet chaetae. On the most posterior segments some very long capillary chaetae present.
Thorax with six uncinigerous chaetigers; uncini are triangular with 6-7 teeth in a longitudinal row. Abdominal uncini with five teeth in a row.
Pygidium is bilobed.

Up to 35 mm long for more than 100 segments. Width of thorax up to 1.8 mm.

White and calcareous, about 5 cm long, 1.9 (up to 2.8) mm wide (H. norvegica). Thin-walled, but thicker than in Hydroides elegans , with many sinuous growth lines on the sides. Longitudinal keels absent. Round or subtrapezoidal in cross-section. Surface smooth or with distinct growth rings. Irregularly meandering over substrate, sometimes tightly spiralling around e.g. hydroid stalk.

Abdomen red, crown with red-white cross-bands. Operculum red or with 2 red rings.

On hard substrata; usually sublittoral.

Northeast Atlantic up to Mediterranean, Adriatic and Black Sea. English Channel, North Sea, Skagerrak, Kattegat, the Big Belt and Öresund.

Hydroides norvegica