fuel filler problems

Could someone please explain how the tank in a '65 Barracuda vents to the atmosphere? The original tank was missing when I bought my car, but I do have a new replacement. There is only one vent line, and it comes out of the fill tube near the tank, comes up and vents into the fill tube near the top about 1.5" below the fuel cap. the fuel cap is not vented, and I don't see how/where air would enter the tank as the fuel is used up. The service manual mentions to be sure the vent line on the tank does not get clogged with undercoating or the tank will collapse. I could be missing some parts, but have looked at the parts book and I seem to have everything except the rubber seal between the fill tube and tank. The "vent line" can't vent to the atmosphere because the cap seals off the whole tube assembly including the vent line. What am I missing here?