Sand masons live in tubes made from sand and shell grit, with a frayed opening. It sticks its head containing long white tentacles out of the tube to catch food particles in the water. Sometimes the tubes are so close together that they form reefs. They catch sand whereby sandy islands develop. These areas on the bottom of the North Sea are very desirable as hideouts for small marine animals. Other scavengers profit from the multitude of food that stays behind between the tubes.