Column elongated, up to 150 mm long, usually broadest toward the aboral end which may be rounded or bluntly pointed and bears a terminal pore.
Tentacles: Marginal tentacles arranged in four pseudocycles, up to about 70; in full extension they are very long, about 4-5 x disc diameter, and taper evenly to sharp points. Labial tentacles short, generally obscuring the mouth and most of the deeply concave disc.
Colouration: Column brown, buff, or flesh-coloured, usually becoming darker above. Marginal tentacles various shades of translucent brown or buff, usually banded with darker brown. Labial tentacles brown, buff or pinkish, never banded. Other colours may occasionally be present on the disc and tentacles, particularly white or vivid pale green; completely white specimens are also known.
Larval form - Synarachnactis bournei: Column short and squat, sub-spherical, up to 2.5 mm in length. Up to 11 short marginal, and 8 tiny labial tentacles are present, depending on age. Generally colourless and translucent, older larvae becoming brownish or yellowish. In mature larvae, about to settle to the bottom and begin a sessile, benthic existence, the column becomes elongated, up to about 5 mm long.
Adults live buried in mud, sand, or gravel, from about LWST to at least 100 m depth. The tube is long, up to 400 mm or more, and frequently winds around stones if these are present in the substrate. Larvae are frequent in the plankton from January to August, having a planktonic life of about three months.
Adults are locally abundant in many localities on all coasts of the British Isles and in some areas are common on the shore. This species occurs on all western coasts of Europe from Greenland and Spitzbergen south to Biscay. Larvae, but not adults, have been recorded from the Mediterranean.
A related larval form, Synarachnactis membranacea (Haime, 1854) has an elongated column up to 6 mm long, with up to nine short marginal tentacles and apparently lacking labials. The adult of this larva, Cerianthus membranaceus, has been recorded at Roscoff, Brittany, and therefore it may occur in the plankton off South-west England, although it has not yet been reported.