Seashells are formed out of the calcium-rich outer mantle tissue of mollusks and certain other marine animals. The shells are made almost entirely from calcium carbonate excreted over a protein network generated by the top layer of the animal's body.
Creatures that create seashells are not the only animals that protect themselves with hard exterior body parts. For example, turtles and tortoises also make shells composed mostly of calcium. However, their structure is dramatically different.
Seashells are exoskeletal structures devoid of living cells. Turtle shells are more intimately connected to the turtle and contain blood vessels and other living material, including bones. The shells also contain nerves, while seashells do not.
Mollusks and other shell-bearing marine animals develop shells soon after birth. As they grow, they enlarge their shells by secreting additional protein around the sides of the shell and covering that network with calcium carbonate. This is a gradual and continuous process. Turtle shells also grow gradually, but without chemical secretions. These shells are part of the turtle's skeleton and grow proportionally with its other bones.
When mollusks die, their soft bodies decompose. Their shells, however, remain intact for a long time. These empty shells are often appropriated by hermit crabs, which use them as a temporary home. Once a hermit crab grows too large for a shell, it discards it and searches for a new one.