unknown vent
 

Hey, new to forums. I got me a 70 cutlass fast back not to long ago similer to my first car. I pulled the interier to find a long oval alumanum tub inseat behind the back seat coming from the floor going 12 inch up. I looked under the car and there are a a set of lines going from the gas tank up the tub, (one on each side of the gas tank) then back out. I've not seen this before, what is the purpose of this?