An exoskeleton is an external skeleton that supports the body of an organism. Many insects, crustaceans, and mollusks have exoskeletons. The insect in this photo has an exoskeleton. Exoskeletons contain chitin and calcium carbonate which make them rigid. Since exoskeletons are rigid, this prevents growth. Therefore, organisms with closed exoskeletons must shed them and then grow a new soft exoskeleton. This allows them to grow until the new exoskeleton has hardened. However, organisms with open exoskeletons can add new material to their exoskeleton while they grow.