A shell is a hard, rigid outer layer developed by a wide variety of different animal species, including mollusks, crustaceans, turtles and tortoises, armadillos, and microscopic organisms. Shells are used for protection, locomotion, defence, structure or in ways that relate to feeding. Scientific names for shell types include exoskeleton, armour, test, carapace, cuticle and peltidium.
Seashells are usually primarily composed of calcium carbonate, which can take different crystalline forms, one being nacre otherwise known as mother of pearl.Mollusks can thrive in the sea, in fresh water, or on land.
The majority of shell-forming marine mollusks belong to two classes: Gastropoda (univalves, or snails) and Bivalvia (bivalves, including clams, oysters, and scallops).
A mollusk shell is formed, repaired and maintained by a part of the anatomy called the mantle. Any injuries to or abnormal conditions of the mantle are usually reflected in the shape and form and even color of the shell.