Body broad anteriorly, narrowing posteriorly.
Prostomium anteriorly incised, without antenna, posteriorly with a caruncle reaching chaetiger 2-5. Two long palps present. 0-4 eyes.
First notopodium with chaetae.
Notopodial postchaetal lobes in anterior segments triangular, in posterior segments short, cylindrical. Neuropodial postchaetal lobes high but short.
Fifth chaetiger with a few lanceolate chaetae anteriorly, 6-16 modified chaetae in a row and capillary chaetae ventrally. Modified chaetae thick and blunt with bent tips and subdistal cavities with a membrane. 7th and following neuropodia with 3-7 strongly bent hooded hooks. Neuropodial capillaries also present but disappearing towards the body end. 45th and following notopodia with capillaries and a few needle-like chaetae, that are about as thick, but shorter than the capillaries (P. caeca-chaetae).
Gills start at segment 8 or 9, disappearing in mid-body or posterior segments.
Pygidium with a dorsally incised funnel. Ventral and lateral incisions often also present.
Up to 60 mm for 130 segments.
In lithothamnia and kelp holdfasts.
North Sea, Skagerrak, Kattegat, Öresund, Arctic, North Pacific, Atlantic to South Africa, Mediterranean, Red Sea, Indian Ocean.