A small- to medium-sized species, up to 50-60 mm in length. Trunk is short and tapering. Introvert is long and slender; two to four times the length of trunk when fully everted. Oral disk carries fairly numerous tentacles surrounding mouth and arranged in radial/longitudinal rows; large specimens have 60-70 tentacles. There are no hooks on introvert. Skin of introvert and trunk is finely wrinkled and has minute papillae; characteristically, the skin of the trunk is corrugated showing an irregular pattern of longitudinal zigzag folds. Nephridiopores are ventrolateral and anterior of anus on anterior trunk. Longitudinal muscle layer of body wall is continuous, not collected into bands.
Internally, two retractor muscles are present (T. procera-internal). These are fused for most of their length and are inserted in posterior third of trunk. Intestine is tightly coiled in a double spiral supported by a spindle muscle attached anteriorly near anus, but not posteriorly. Rectal caecum present. One gut-fixing muscle fastens anterior coils of gut to dorsal body wall. Contractile vessel is conspicuous in that its distal half carries numerous branching villi. Two nephridia present hanging freely in coelom.
Spawns in May and June. In the Kattegat population, individuals appear to act as temporary parasites of the polychaete Aphrodita aculeata .
Inhabits muddy sand, in depths of 2-200 m.
In the north-east Atlantic a limited distribution extending from the Skagerrak and northern North Sea to the west coasts of Scotland and Ireland, including the Irish and Celtic Seas. It appears to be absent from the southern North Sea and the English Channel. Elsewhere the species is recorded from off the east coast of North America between 34 and 40°N and off California. The distribution of the species outside of the northeast Atlantic region is uncertain.