The 3-eyed, intertidal and almost-terrestrial snail Cerithidea scalariformis has been the subject of several posts (for example, this one and this one).
The picture below shows one snail photographed from below while it was crawling on a glass plate. Note the snout all the way in the front and the foot behind it, both in contact with glass.
Also note how short the foot is relative to the shell. One consequence of the discordant sizes of the foot and the shell is that the former cannot lift the latter above the substrate. The snail drags its shell behind it. Look at the picture below.
To pull its shell through wet sand and mud, its usual habitat, Cerithidea scalariformis must spend quite a bit of energy. There must, therefore, be a counterbalancing advantage to having a shell much longer than the foot that is supposed to pull it. Otherwise, evolution would have long ago done something about it.
One possible advantage of having a foot much shorter than one's shell is that when the snail is withdrawn into its shell, the foot, the last part of a snail to enter its shell, can withdraw far behind the aperture. And that is good when the aperture is under attack by a predator like a crab that can break open the aperture.
I don't actually know how far into its shell Cerithidea scalariformis can withdraw. I intend to find out in the near future.
Here is the follow-up post.