Thank you, Wendall.
You won't often see a hermit crab outside of a shell like this. They normally occupy the shells of dead mollusks, like the Hairy Triton Snail shell shown. All you see is their legs, claws and tentacles if they are moving around. As they grow, though, they have to leave their shell and find a larger one which is what I am presuming this one was doing. As kids, we would see much smaller hermit crabs in the tidal pools in coastal California.