Spirorbis tridentatus

Levinsen, 1883

Description:
Body twisted to the left, with 3 thoracic chaetigers.
Talon conical in side-view (narrow in young specimens) and with indistinct lateral protuberances in 'face'-view. 4-5 pairs of radioli, with long filamentous tips.
Collar slightly asymmetrical.
Collar chaetae on convex side with a toothed fin separated by a gap from the finely toothed (non-cross-striated) blade, those in the concave fascicle smaller with a reduced gap or none at all as in simple bordered chaetae. 2nd and 3rd segments with notopodial capillary chaetae, some winged; 3rd 3rd segment also with Apomatus-chaetae. Two pairs of thoracic neuropodial tori, with uncini; 3 rows of teeth on the uncini. Abdominal neuropodial chaetae geniculate with strong teeth along the distal part. Abdominal tori long and bear up to 30 uncini, similar to those on Spirorbis spirorbis (S. tridentatus-detail).

Size:
Up to 5.5 mm for 29 chaetigers.

Tube:
Shiny, usually with rounded longitudinal ridges, coil diameter 3 mm, the last whorl widening abruptly towards mouth.

Colour:
Transparent.

Habitat:
Under stones and in rock-crevices; lower shore and (more sparsely) to a depth of 30 m.

Distribution:
North Atlantic, English Channel, North Sea, Skagerrak, Kattegat, northern Ă–resund.

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