17. Exoskeleton

An exoskeleton is the external skeleton that forms a rigid outer covering for the bodies of arthropods and some other invertebrates. Exoskeletons are composed of chitin, a complex polysaccharide, and calcium carbonate. They contain flexible joints and an inner layer of muscles that allow for mobility of the animal.  The image above shows the molted exoskeleton of a cicada. Exoskeletons do not grow with the animal, therefore they must be shed, allowing the animal to grow before its new exoskeleton hardens.