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. ... When a mollusk dies it discards its shell, which eventually washes up on the shore. This is how seashells end up on the beach.

How do seashells get their shape?

Mantle tissue that is located under and in contact with the shell secretes proteins and mineral extracellularly to form the shell. Think of laying down steel (protein) and pouring concrete (mineral) over it. Thus, seashells grow from the bottom up, or by adding material at the margins.

Where do shells come from?

Most shells come from soft-bodied mollusks. Snails, clams, oysters, and others need the hard protection of their shells. This tough outer covering protects the tasty body hiding inside. Other animals, such as crabs and lobsters, also make a tough outer covering, but here we focus on mollusk shells.