system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do shells form? Why are there so many types? Why do all the curly shells curl in the same direction?
gpt
Mollusks like snails and clams have very delicate bodies, so they need protection from the elements and predators. As mollusks develop in the sea, their mantle tissue absorbs salt and chemicals. They secrete calcium carbonate, which hardens on the outside of their bodies, creating a hard shell. The shell stays attached to the mollusk but it is not actually part of its living body because it is made of minerals, not mollusk cells (unlike most animal structures). The mollusk continues to take in salt and chemicals from the sea and secrete calcium carbonate, which makes its shell grow even bigger. When a mollusk dies it discards its shell, which eventually washes up on the shore. This is how seashells end up on the beach.