Bob's not dead!!!
I am such a dumb ass! He had just moulted and had gone way inside his shell, so far in that I couldn't see him at all.
I had put what I thought was his empty shell on the kitchen counter, and when I saw it on the floor earlier thought that one of the kids had knocked it down...until I picked it up and saw Bob's pale pink legs sticking out!
Yay! Bob's alive!