How does a snail get its shell?
And the answer: It makes its own shell.
As a snail grows, its shell grows too. Snails secrete calcium carbonate through their glands, which slowly adds layers to their shells. Like a tree, you can roughly determine the age of a snail by the number of layers or rings of its shell.
Though snails do not hatch from their eggs fully shelled, these mollusks enter the world with a protoconch, the first component of a functioning shell. Survival instincts push the young snail to then turn and eat the calcium-rich egg from which they just hatched, and thus the snail sets out on the lifelong process of ingesting foods with enough calcium to keep their shell on the upward spiral (pun intended).
If you were to slice open a snail shell, you'd notice several main layers in the cross-section. At the surface, there's the periostracum, a thin outer layer usually made with organic material. Below that, you've got a layer of hard calcium sitting atop a blanket of nacre, a resilient composite material better known as "mother of pearl." One aspect that sets snails apart from other shelled creatures is the lack of blood vessels or nerves in the shell. Unlike turtles that fix cracks through cellular repair, calcium and protein secretions from snails' mantles can be used to help strengthen the damaged area.
Snails are among some of the most highly varied species. Check out this video of the deadly Cone Snail below for a taste of these mollusks' variability and evolved specialization.