Body elongate, cilindrical, tapering towards the ends; up to 200 mm in length; coloured whitish.
There are 20 finely branched, red or pink tentacles, arranged in two rings: the outer one consisting of 5 pairs of larger tentacles and the inner ring of 5 pairs of smaller ones. The tentacles of each pair in the inner ring are not placed so close that they appear to be united at the base.
Very numerous and fine tube-feet arranged in more or less distinct series all over the body, excepts for young specimens where the tube-feet are confined to the ambulacra.
The body wall is thick and not transparent. Calcareous deposits are found only in the skin of the introvert. These deposits consist of (1) small, delicate tables with a high, slender spire. This spire may sometimes be reduced, with only the disk being left (2) (T. commune spicules).
This species is found on sandy or muddy bottoms where it usually covers itself with shell fragments; at depths of 10-380 m.
It is known from the Shetland Islands down to Northumberland on the British east coast. Elsewhere distributed from the Arctic down to the Channel, and from Greenland to Florida.
Thyonidium hyalinum is similar, but smaller with a more transparent skin and spicules with a shorter spire.