how do in-tank vapour separators work?

For many decades, cars have a vapour separator connected to the charcoal cannister. Some have external devices, and some are inside the fuel tank. I got a tank from a scrap yard and removed the sender and filler elbow to look inside. There are 2 white cylinders on the top of the tank. A pipe from each runs along the top of the tank to a connection on the outside for the breather hose to the charcoal cannister. Now when the tank is full to the brim those cylinders would be in liquid. Beside, they will be sloshed when car is moving. So how do they block the liquid fuel? Is it made of some permeable material that only passes fumes?