Animals with exoskeletons include all arthropods such as insects, crustaceans and spiders as well as many other invertebrate animals such as shelled mollusks. Shelled mollusks include snails, tusk shells, clams, nautilus and chitons. An exoskeleton is essentially an outer skeleton that protects and supports an animal's body.
In common usage, some larger animals with exoskeletons, such as lobsters, are said to have "shells." Humans and other mammals have internal skeletons called "endoskeletons." Some animals, such as the tortoise, have both an exoskeleton and an endoskeleton. Exoskeletons are composed of a range of materials such as bone, cartilage, dentine, chitin, calcium carbonates, silica, greigite and pyrite.
Not all tough outer layers in animals are exoskeletons. The armadillo has a tough outer layer that is constructed of bone, and the pangolin has one made of hair. Exoskeletons are rigid and can present growth limitations. Therefore, a true exoskeleton must be shed in order for an animal to grow. A new exoskeleton then forms beneath the old one, which is initially soft and pliable. The animal pumps itself up while it is soft to expand it to maximum size and then lets it harden. If the animal does not shed its exoskeleton once it is outgrown, it can suffocate within its own shell and fail to reach maturity.