I took a couple of pictures at the beach recently, of an extremely well-camouflaged invert. Can you see it in this first picture? (Clicking on any of the images will take you to flickr, where you can inspect the full size to your heart's content.)
Not sure what I was looking at yet? Let's zoom in a little.
Still not sure? Here's your final chance. After you look at this one, go back to the start and see if you can find it again. Even when you know it's there, it's not totally easy.
This little guy is (I'm fairly sure) a pallid ghost crab (Ocypode pallidula). It's the smaller of the two species of ghost crab found in the Hawaiian islands. Watching them at the beach can be kind of fun - your eyes tend to catch them when they're moving, but it's easy to lose them when they stop.