The endoskeleton is a type of support system found in some animals. It is composed of rigid, hard bones or cartilage that provide a strong, supportive structure that protects internal organs and allows the animal to move. The endoskeleton also helps the animal to stay upright and to provide a point of attachment for muscles. Many animals have an exoskeleton, which is an external covering of hard plates, but the endoskeleton is found only in animals that have an internal skeleton. Examples of animals with an endoskeleton include humans, birds, and reptiles.