Body is short, thick, maggot-like. Segmentation is rather indistinct.
Prostomium is roundly conical, fused to peristomium, with a single nuchal organ. Eyes absent. Peristomium without chaetae, about as long as the first chaetiger; with a strong pharynx.
Parapodia rudimentary, biramous, with 1 to 5 chaetae per bundle. Parapodial lappets and cirri absent.
Chaetae weakly bent, with 4 to 7 big teeth (C. serratus-detail).
Pygidium rounded, without cirri.
Up to 2.8 mm for 15 chaetigers.
Whole body with greenish, brown, blue or black glands.
Eulittoral to upper sublittoral. Between Lithothamnion and sponges. Also in coarse sand and gravel mixed with mud.
Pacific, Atlantic Ocean to Mediterranean, Black Sea, English Channel and Helgoland.