How do seashells form?
To understand most seashells, you need to understand creatures called mollusks. Their bodies are mostly soft and they have no bones. So they need a hard covering for protection. To make this shell, they use glands that give off liquids that turn hard. But what happens as the mollusk gets bigger? The shell's shape can't be changed. So the mollusk simply makes it bigger by adding more shell to the edge. Some amazing shapes can result. Imagine a shell is like a tiny pocket. Add to the edges and you get a bigger, wider pocket. Shells with two sides, such as clams and scallops, grow like that. Now imagine instead that the pocket turns as it grows. If it turns enough, you get a kind of spiral that gets bigger and bigger. Snail and conch shells are made that way. Most seashells are mostly calcium carbonate, a chemical also common in coral.