A medium- to large-sized species, up to 100 mm in length with a stout, tapering trunk. The oral disk has a swollen rim (cephalic collar) and carries a horseshoe-shaped crescent of tentacles situated dorsally to the mouth and enclosing a large nuchal organ. The tentacles number from 12-16 in small specimens, up to 30-60 in medium to large individuals. On the anterior part of the introvert, there are numerous hooks, each with a curved tip and a wide triangular base arranged in rings. The number of rings varies from 10-60 depending on size of specimen and often the posterior rings are incomplete through wear. The whole body surface has dome-shaped papillae of varying size, each capped with a dark ring around a central clear spot; the largest papillae occur over the base of the introvert and over the posterior trunk where they are conspicuous in being darker coloured. The nephridiopores are ventrolateral and just anterior to the anus on the anterior part of the trunk. The longitudinal muscle layer of the body wall is collected into 20-30 bands, joined by numerous anastomoses. In small specimens these bands may not be readily apparent.
Internally, four retractor muscles are present, each inserted by several roots onto the longitudinal muscle bands in the middle third of the trunk (P. granulatum-internal); the ventral retractors are thicker and have wider bases than the dorsal retractors. The intestine is loosely coiled into a double spiral supported by a strong spindle muscle attached anteriorly near the anus and also at the posterior end of the trunk. The rectal caecum is usually small. The contractile vessel is simple. A fixing muscle fastens the oesophagus and the rectum to the body wall between the roots of the dorsal retractors. Two nephridia; these are fastened to the body wall over their anterior halves.
P. granulatum is rather variable in its external appearance and thus has many synonyms. In larger specimens, the papillae are normally darker and more conspicuous than in small specimens; in the latter, the introvert often has dark transverse bands particularly over the dorsal surface.
Spawning appears to occur in August or September.
Inhabits muddy sand/gravel, from the lower shore to 90 m depth. Intertidally it occurs under stones, in crevices and amongst Lithothamnion .
The species ranges from the coastal waters of Norway (one specimen) and France to the Cape Verde Isles and throughout the Mediterranean. It has a restricted distribution in British seas in that it is common along the west coast of Ireland but elsewhere only a few specimens have been recorded, originating from Shetland, Orkney and the Hebrides. It is also reported from scattered localities in the Indo-West Pacific region.