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Concept
Exoskeleton
Summary
An exoskeleton (from Greek έξω éxō "outer" and σκελετός skeletós "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. Some large exoskeletons are known as "shells". Examples of exoskeletons within animals include the arthropod exoskeleton shared by chelicerates, myriapods, crustaceans, and insects, as well as the shell of certain sponges and the mollusc shell shared by snails, clams, tusk shells, chitons, and nautilus. Some animals, such as the turtle, have both an endoskeleton and an exoskeleton.
Exoskeletons contain rigid and resistant components that fulfill a set of functional roles in many animals including protection, excretion, sensing, support, feeding, and acting as a barrier against desiccation in terrestrial organisms. Exoskeletons have roles in defense from pests and predator and in providing an attachment framework for musculature.
Arthropod exoskeletons contain chitin; the addition of calcium carbonate makes them harder and stronger, at the price of increased weight. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles. These structures are composed of chitin and are approximately six times stronger and twice the stiffness of vertebrate tendons. Similar to tendons, apodemes can stretch to store elastic energy for jumping, notably in locusts. Calcium carbonates constitute the shells of molluscs, brachiopods, and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria. One mollusk species, the scaly-foot gastropod, even uses the iron sulfides greigite and pyrite.
Some organisms, such as some foraminifera, agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to a common misconception, echinoderms do not possess an exoskeleton and their test is always contained within a layer of living tissue.
Exoskeletons have evolved independently many times; 18 lineages evolved calcified exoskeletons alone.
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