You have to be lucky to actually catch a snail in motion. And when you do, they are typically s-l-o-w. In fact, you'd almost think they were asleep.
Which is what they do for most of the day- sleep. Because snails as members of the mollusk family, need moisture to keep their bodies going. The heat of the sun is very drying, so they usually don't come out until nighttime.
When they do, they extend their "heads", which generally have a pair of tentacles on them, and go looking for vegetation to chew. The eyes of many species are perched at the end of the tentacles, which are hollow, allowing the snail to draw their eyes into their bodies when threatened.
The snail is self-sufficient in several ways. For one, it is a hermaphrodite, so there is no need to seek a mate to fertilize the 30 or so eggs they will lay under a stone. It also has a ready-made retreat, for when the weather turns cold. Snails burrow into ground cover, and draw their body into the shell, sealing the entrance with a layer of slime that hardens, leaving a small space for air.
This same behavior is followed by species that live in desert areas when there is a shortage of water. They will withdraw into their shells, and hibernate or sleep, for as much as 2-3 years, until conditions improve.