It is very important that you repair this as soon as possible. This is causing an open crankcase letting in all sorts of dirt and contaminants into the crankcase. Additionally it is probably allowing unfiltered air in the intake of the engine.
These are both very detrimental to your engines health. See to this problem very, very soon.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in