Animal shell

An animal shell is a hard outer covering produced by an animal for protection or support. Shells include mollusk shells and the bony carapace of turtles.

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Calcium carbonate: A chemical compound found in minerals such as calcite and aragonite, and in many animal shells. Mollusks build much of their shell from this mineral.
Biomineralization: The process by which organisms produce minerals within or around their tissues. It explains how animals form hard mineral-rich shells.
Mollusk shell: A hard covering secreted by the mantle of a mollusk, commonly made largely of calcium carbonate. This is the familiar shell form, distinct from a turtle’s internalized bony armor.
Mollusc: An invertebrate animal group that includes snails, clams, octopuses, and their relatives. Many familiar animal shells are produced by mollusks.
Chitin: A structural polysaccharide found in arthropod exoskeletons and fungal cell walls. Many arthropod coverings combine chitin with proteins and minerals.
Nacre: A layered composite material secreted on the inner surfaces of some mollusk shells. Its microscopic layers help mollusk shells resist fracture.
Arthropod exoskeleton: An external cuticle of arthropods, often containing chitin and periodically shed during growth. It serves protective and supportive roles but is renewed by molting.
Shellfish: A culinary term for aquatic invertebrates, especially mollusks and crustaceans. Harvested shellfish connect shell-bearing animals with food and fisheries.
Collagen: A family of structural proteins that form fibers in connective tissues and bone. Collagenous layers support the mineralized shell of turtles.
Shell growth: The addition of new shell material as an animal develops. Mollusks usually enlarge shells by adding material at the opening.