Who's been living in my shell?

I was walking on the beach yesterday and found the two shells in the picture but guess what? they were empty. That got me thinking about who might have lived in them and how did they end up on the beach? I looked around and saw some rock pools - do you think that might have been where they came from? So I am hoping that you can help me with these questions.

Maybe over the weekend you could go to the beach and rock pools to see what lives there. Can you also look for shells like these and see if you can find out who they belong to? If you post your pictures we can investigate your findings together. You could also draw a picture of what you find if you'd prefer.