How does a clam make its shell?

Shell of a Clam

If you have ever examined a clam shell, you may have wondered how the shell got bigger as the clam grew.  A clam is born with a shell just the right size for its body.  Inside the protecting shell of the living animal is a fleshy layer of tissue called the “mantle.”  

The mantle oozes a limy shell liquid which quickly hardens and becomes part of the shell.  As long as a clam grows, its shell also grows.  The food that a clam eats provides the minerals that form the shell.  The hard shell serves as a clam’s skeleton, and the soft animal inside can never leave it.–Dick Rogers