Hi there I've had this happen to me before with my gals. It's the baby snails which would of killed your adult. It is always a good idea to keep babies seperate from adults. The babies start of I think just looking for somewhere safe to hide which happened to be inside the adults shell. But whilst in there they would of got hungry and instead of coming out they would of eaten A little bit of the older guy. Contrary to popular belief snails can eat meat. They eat meat for the protein which helps with growth which babies especially need. You could try fish flales which contain protein and hopefully satisfy their meat tendscies. Babies are cute but I oringally kept mine with 6 adults and they systematcily killed each snail in turn by just hiding in their shells. I learnt the hard way never keep them together till the babies are no longer able to fit inside the shell of the adult. Hope this helps.
This is a thread from 2008, so it's unlikely the original poster will see your response. However I've raised hundreds of babies of many different species in tanks with adult snails, and have never seen a single one climb inside an adult's shell, so I'm extremely surprised to hear that apparently 6 snails of yours were killed in this way (and that you didn't separate them after it happened the first or second time!). Did you get any pictures of the babies inside the adult shells? Would be interesting to see something so strange!