Rag worm/sand worm cast

If you ever wonder what the wading birds like plovers and sanderlings are trying to dig out the sand at low tide, it is the sand worms (among other sand-buried organisms).
Here you don´t see the animal but the sand that the lugworm extrudes from its burrow.
The lugworm lies in this burrow with its head at the base of the head shaft, swallowing sand from time to time. This makes the column of sand drop slightly, so there is a periodic sinking of the sand in the saucer-shaped depression. When it first digs its burrow the lugworm softens the sand in its head shaft by pushing its head up into it with a piston action.