Snail shells are formed when a snail continually feeds on calcium. A snail is born with a soft version of its shell that needs calcium to grow into the hardened form.
Snails are very vulnerable when they are first born. They are born underneath soil to protect them from the world until they are more developed and fit for surviving. A snail constantly searches for sources of calcium to strengthen its shell. Snails are one of the few animal species in the world that are able to adapt and evolve with their surrounding conditions over time. Their shell helps snails to do just this.