How to mask where I don't want to see the layer and its animation

Hi,

I've been getting some really great help but I want to only see the 'little lines' layers as they pass over the object of which they are in front.  There is a background layer but I've turned if off while I try to find the solution.  I've grouped the layers as you will see - but how do I mask/hide them unless in front of the shape.  I think it may be about black over the whole layer and then rub back to white to show the parts that are to be seen.  Hope this makes sense.  All help is appreciated.