Some creatures, mammals, for instance, have a skeleton of bones inside their body. But other creatures have that skeleton outside their body. Among such creatures are the mollusks, a group of sea creatures that includes the clam, mussel, and oyster.
The shell of a mollusk is part of the creature, just as our bones are part of us. A mollusk cannot leave its shell and come back again.
The mollusk has glands that take particles of limestone and other substances from the water and deposit them around the soft parts of its body to form a shell.
The shell keeps growing as the mollusk grows inside it. The ridges you find on seashells are really “growth lines,” showing the successive stages of the shell’s growth.