Body arched upwards at both ends into a U-shape, the posterior end is produced into a long, conical, tail-like prolongation; up to 200 mm in length; coloured yelowish-brown, larger specimens are often nearly black. At the base of the body, there is a rectangular ventral sole, which serves as a powerfull sucking disk (P. phantapus ventral sole).
This species has a crown of 10 large bushy tentacles, which are coloured white and orange.
The tube-feet are mostly confined to the ventral sole, where they are arranged in distinct rows. Apart from the ventral sole, tube-feet are found only on the soft-skinned introvert.
Outside the ventral sole, the skin is covered with thick, mostly imbricating scales, often with small, rounded grains on their surface. In the ventral sole, the skin contains only very few calcareous deposits which consist of either (1) small cups, or (2) larger spherical/oval bodies of complicated structure (P. phantapus spicules).
Young specimens live attached, by their ventral sole, to stones, shells and other hard substrates. Larger specimens are found living free in muddy or sandy bottoms, with only the anterior and posteror ends protruding above the ground and the ventral sole facing downwards.
In the North Sea this species is found on the British east coast as far south as Yorkshire. Elsewhere it is distributed on the north west coast of Britain and Ireland and northwards to the Arctic. Also known from the American east coast.