Most snails, we are told, have an operculum. A flattened piece of hard material that acts like a door to the opening of the snail shell. While probably an effective barrier to predation, many land snails have abandoned the operculum. Instead, they seal their aperture with a mucous-based material called an epiphragm. In my opinion, the casting away of the operculum is probably the single biggest step towards being successful land invertebrates snails have taken since the evolution of the Pulmonata.
That's all I have to say about that right now - but an out loud thought I have regards extra space in the body whorl of operculates versus non. I suspect non-operculate snails have more extra space in their body whorl, but I don't have the data to back up that hypothesis. Anyone want to write a paper?