how do i remove anti-siphon valve

My fuel line is clamped to a 90 deg. fitting at the tank. I assume this is the anti-siphon valve. There is a large nut that looks like it holds something inside the tank. If i remove this large nut will i lose a part in the tank? How do i inspect the anti-siphon valve? Do i need to pull a long part out of the tank? I only have a few inches between the top of the tank and the boat hull.   Any information would be helpful. thanks

Ron,
Nothing will fall into the tank. Yes, that fitting on the top is the anti-siphon. There's a ball check inside that you want to take a look at.