Where do seashells come from? [VID: Where do seashells come from? The shells you find on the beach were once home to small, soft-bodied creatures called mollusks. Clams, scallops, mussels and oysters are examples of mollusks. Mollusks live by absorbing salts and chemicals from the seawater around them. As they process these materials, they secrete calcium carbonate. This chemical solidifies outside of their body and gradually forms a hard outer shell. The shell is attached to the mollusk but is