Snails usually begin to grow their protective shells before they hatch.  The common garden snail hatches from a tiny round egg.  Hidden under the young snail’s shell is a flap of skin called the mantle.  The mantle oozes a limy liquid that quickly hardens into a shell.

As the snail grows in size, it adds coil after coil to its shell, building from the open end (for mouth) as fast as it needs more from.  The food that is eaten by the snail provides the materials that form the shell.  Without its shell, a snail would have little protection against drying up.–Dick Rogers