<P> Exoskeletons contain rigid and resistant components that fulfill a set of functional roles including protection, excretion, sensing, support, feeding and acting as a barrier against desiccation in terrestrial organisms . Exoskeletons have a role in defense from pests and predators, support, and in providing an attachment framework for musculature . </P> <P> Exoskeletons contain chitin; the addition of calcium carbonate makes them harder and stronger . 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 as strong and twice as stiff as vertebrate tendons . Similar to tendons, apodemes can stretch to store elastic energy for jumping, notably in locusts . </P> <P> Many different species produce exoskeletons, which are composed of a range of materials . Bone, cartilage, or dentine is used in the Ostracoderm fish and turtles . Chitin forms the exoskeleton in arthropods including insects, arachnids such as spiders, crustaceans such as crabs and lobsters, and in some fungi and bacteria . 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 species of mollusc, the scaly - foot gastropod, even makes use of the iron sulfides greigite and pyrite . </P> <P> 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, as their test is always contained within a layer of living tissue . </P>

What must a crab do to its skeleton when it needs to grow