Body cilindrical, up to 150 mm in length and 35 mm thick; coloured brownish, often with irregular darker spots.
There are 10 small tentacles.
The tube-feet are retractile and are arranged in distinct double series in all five ambulacra. In small specimens, the tube-feet may be arranged in zigzag rows (O. planci tube feet).
The skin is thick, smooth and leathery, with numerous calcareous deposits consisting of (1) plates with more than 4 holes and which are covered with nodules, and (2) small irregular cups in the outer layer of the skin (O. planci spicules).
This epifaunal species is found on sheltered sites, on shells, tube-worms and other sessile invertebrates; from depths of 15-175 m.
In the North Sea this species is known from ....... Elsewhere it is known from the coast of Portugal, off Senegal and the Mediterranean.
This species very much resembles Ocnus lacteus, but can be identified by the calcareous deposits which are larger and have more 4 holes.