A large species, up to about 350 mm in length, with a cylindrical trunk. The oral disk carries a large tentacular lobe, which is highly folded, its margin sinuous or scalloped but not markedly incised to form tentacles. The introvert is without hooks but is covered with large flattened papillae, which are triangular in shape and backwardly pointing. The surface of the trunk has a characteristic pattern of rectangles defined by grooves formed by the underlying circular and longitudinal muscle bands but this pattern may not be discernible in small specimens (<35 mm long). The nephridiopores are ventrolateral and anterior to the anus on the anterior trunk. The body wall is thick with circular, diagonal and longitudinal muscle layers collected into bands and containing longitudinal canals, which communicate with the coelom at intervals. The longitudinal muscle bands number 28-34 in the anterior trunk region, fewer in the posterior trunk (S. nudus-detail).
Internally, four retractor muscles are present, all inserted in the anterior trunk at about level of the anus, their bases each extending across 4-8 muscle bands (S. nudus-internal). The intestine is loosely coiled in a double spiral, fastened to the body wall by numerous fixing muscles and supported by a spindle muscle attached anteriorly near the anus, but not posteriorly; the intestine also has a characteristic minor coil (the so-called "post-oesophageal" or "Sipunculus-loop" between the oesophagus and the main spiral. The rectal caecum is variable in size but usually small. There are two simple contractile vessels on the oesophagus, one dorsal and one ventral. The two nephridia are attached for about one quarter of their length. A pair of "racemose" glands is held in thin mesenteries, one on either side of the rectum.
A closely related species, Sipunculus norvegicus , occurs in the deeper waters of the northeastern Atlantic. It is chiefly distinguished from S. nudus in having fewer longitudinal muscle bands (22-24).
Inhabits sand, from the lower shore to about 700 m depth.
In European waters S. nudus is found from the west coast of Ireland and southern North Sea to the eastern Mediterranean. It has a world-wide distribution, mainly in temperate and tropical seas.