Edited sentence: ) Large colonies of encrusting species often have "chimneys", gaps in the canopy of lophophores, through which they swiftly expel water that has been seived, and thus avoid re-filtering water that is already exhausted.New chimneys appear near the edges of expanding colonies, at points where the speed of the outflow is already high, and do not change position if the water flow changes.They are formed by patches of non-feeling heterozooids