Black-eyed hermit crab, Pagurus armatus.
Not only is the shell she's wearing all broken and jagged at the front (you'd think that would be scratchy!), but it's broken off just above the first turn of the spiral, so she's going around with her tail end exposed to view. I've left her a few similar shells, whole ones, in the next size up for when she decides it's time for a change.
*It's almost impossible to tell a male from a female, they say. Females may be smaller, so I'm going for "she".