Body fusiform, up to 120 mm in length; coloured reddish.
There are 20 tentacles arranged in 2 circles, the outer one bearing 5 pairs of larger tentacles and the inner ring bearing 5 pairs of smaller tentacles. The tentacles of each pair in the inner ring are placed close together, so that they appear to be united at the base.¨
Large, relatively few tube-feet are scattered over the body, except in young specimens where the tube-feet are confined to the ambulacra.
Body wall is thin and rather transparent. Calcareous deposits are common in the skin of young specimens, but in older animals these deposits are found only in the skin of the introvert and posterior part of the body. The calcareous deposits consist of (1) tables with a low spire and a rather large disc with serrate edge. Deposits in the tentacles consist of (2) larger, fenestrated plates (T. pellucidum spicules).
This species is found on sandy or muddy bottoms, where it covers itself with shell fragments; at depths of 10-380 m.
In the North Sea this species has been recorded from the Shetland Islands down to Northumberland on the British east coast. Elsewhere it is distributed from the Arctic down to the Channel, and from Greenland to Florida.
Thyonidium drummondii is similar, but larger with a darker skin and spicules with a longer spire.