The term 'skeletal opening' refers to an exoskeleton, which is a hard outer shell that covers and protects the body of certain animals. Exoskeletons are made up of a series of plates or segments that are connected together to form a protective covering. This type of structure is found in many invertebrates, such as crabs, lobsters, and insects. The skeletal openings allow for movement and flexibility while still providing protection from predators and other environmental factors.