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(M. Sars, 1851)

The body is short and oval in shape (S. arcticus-ventral view), with 12-24 segments. Ventral surface smooth, without papillae.
Prostomium small, partially emerging dorsally as a distinct papilla, with one pear-shaped antenna and 4 eyes.
Peristomium limited to lips.
First segment completely fused in front of prostomium, without tentacular cirri.
Parapodia biramous. Notopodia forming long transverse ridges, supported by spines. Neuropodial short and truncate, with multiple aciculae and compound chaetae. Dorsal and ventral cirri absent (S. arcticus-detail).
Pygidium with two anal cirri.
Pharynx eversible, without jaws.

Up to 9 mm for 24 segments.

Depending on host: yellowish, red, brown or violet.

Ectoparasites on sponges or hydrozoans.

Arctic, North Pacific, North Atlantic and Mediterranean. Northern North Sea including Skagerrak and Kattegat.

Spinther arcticus