The worm lives inside a smooth tube of fine mud or sand particles held together with mucus.
The tube stands upright with the lower end attached to stones and the upper end protruding from the sea bed. When covered by water, the worm extends its tentacles out of the tube to feed by filtering out small food particles.
At low tide, it withdraws back into the tube which often appears grey and like a small plastic tube.
As a child I used to think the tube was a bit of gray electric cable lying in the mud.