The mussel's foot is that long, skinny dark red/brown organ coming out of the mussel. The snail is under it, drilling into it to eat it. I like to think the mussel is trying to reach up and move away to escape from the snail, but I'm really not sure why its foot is out like that. Plus, it wouldn't even work since the snail is stuck on it. When I touched the foot, it retracted immediately. It's unusual for the foot to be out all the way like that, so I had to take a photo!
If you are curious, the preliminary results from my experiment are showing no effect of pH treatment on the number or size of mussels drilled. But, it's still going, so who knows what the final results will be!