Exoskeletons are hard, external structures that support and protect an animal's body. Exoskeletons provide protection from predators but limit the growth of the animal. Many animals enlarge or shed their exoskeletons as they grow.
Insects are an example of animals that have an exoskeleton. They do not have internal bones, and their muscles are attached to the exoskeleton. When a muscle contracts, it pulls on the exoskeleton and causes the insect to move. Most insects lose their exoskeleton in a phase called molting. During this phase, insects are very vulnerable to predators because the new exoskeletons have not hardened enough to protect them.