Norman the Slug with the Silly Shell is an easy book to link this to. Use Pritt sticks so the they can be taken home immediately without worrying too much about the thing disintegrating withint five minutes. Could also link it to What the Ladybird Heard and turn the snail into a ladybird.
Hermit crabs, like this one collected in Moorea, usually protect their soft, vulnerable abdomens from predators by reusing empty snail shells. They are picky home owners and they will trade shells with other crabs to get a better fit or a less damaged shell. This specimen shows the crab without its customary borrowed shelter.
Pale yellow sea slug with bright orange and blue ruffle down its back and bright orange spots on the end of its antennae. Sea slug looks like a snail without its shell and is only a couple inches long.