The element stored in the shells and skeletons of living organisms is calcium.

Here's why:

* Calcium carbonate (CaCO3) is the primary component of shells in many marine organisms like mollusks, crustaceans, and corals.

* Calcium phosphate (Ca3(PO4)2) is the main component of bones and teeth in vertebrates.

Calcium plays a crucial role in providing structural support and rigidity to these structures.