What are those little fitting on fuel lines called where the line goes into the tank?

Take a weed eater for example. Hose comes off the primer bulb, to a little male fitting, then there's a small square piece, then another make fitting. The square piece is what makes a half decent seal where the fuel line is going into the gas tank..

What the heck are those things called and where can I get a few dozen in a bulk buy?