This species is highly variable in size, shape and morphology. Littoral specimens are small, not exceeding 20 mm in length whilst those from the sublittoral may attain lengths of 100-300 mm. The trunk is cylindrical, tapering posteriorly and sometimes markedly so forming a long, appendix-like "tail". The tentacular crown becomes more complex with increasing size. In small specimens, a single circle of tentacles surrounds the mouth as in the littoral forms with typically 8 to 16 tentacles but not more than 20. In larger sublittoral forms, particularly from more northerly latitudes, the number of tentacles is considerably greater with additional tentacles developing in radial/longitudinal rows around the margin of the oral disk; in the largest specimens over 100 tentacles may be present. There are no hooks on the introvert. The skin of the introvert and trunk is finely wrinkled, with minute papillae. In large specimens the skin often has a lustrous, "mother-of-pearl" appearance. Large, presumably old, specimens show the greatest morphological variation, particularly in the structure and appearance of the skin. The nephridiopores are ventrolateral, just anterior of the anus on the anterior trunk. The longitudinal muscle layer of the body wall is continuous, not collected into bands.
Internally, four retractor muscles are present, the ventral pair inserted in the middle third of the trunk, the dorsal pair at about, or posterior to, the level of the anus (G. margaritacea-internal). The intestine is tightly coiled in a double spiral supported by a spindle muscle attached anteriorly near the anus, but not posteriorly. The rectal caecum is usually well developed. Gut-fixing muscles fasten the oesophagus and anterior coils of the gutspiral to the body wall in the region between the roots of the dorsal and ventral retractors. The contractile vessel is simple, not readily seen in small specimens. Two nephridia; these hang freely in coelom.
Around southwest England, this species matures at a small size (<10 mm in length) and probably spawns in October or November.
Inhabits mud and sand/gravel from the lower shore (crevices) to about 4600 m
This species is uncommon. Records are confined to the littoral zones of southwest England and western Scotland, plus a few scattered localities in the central and northern North Sea. It has a worldwide distribution, mainly in temperate and polar regions.