Patterns in the sand

I think I've mentioned before that my knowledge of all things marine is minuscule. So when I do a beach walk it's like a mystery tour. I've got no idea what any of the small creatures on the sand or in the rock pools are. (I think that sentence is grammatically incorrect but it can stay - it's the way I talk, and a diary should be idiosyncratic.)

So, what could have made these tracks in the sand I wonder? Perhaps a crab?

And this one? We dug up the little bump at the end of the trail and found the shell. And buried it again.