The skeletal system is the framework of hard, articulated structures that provide physical support, attachment for muscles, and protection for the bodies of animals. The skeletal system of invertebrate animals, particularly those in the phylum Arthropoda, is on the outside of the body, and is referred to as an exoskeleton. That of vertebrate animals in the phylum Vertebrata is on the inside of the body, and is referred to as an endoskeleton.

The external skeleton of such arthropods as crustaceans, insects, and spiders consists of a large number of rigid plates and jointed appendages made of chitin (this is a complex polysaccharide carbohydrate, consisting of numerous linked monosaccharide units). The exoskeleton provides protection, and rigid structures to which muscles can attach, allowing the body parts to be moved in a directed fashion. The exoskeleton of arthropods is divided up into functional regions, such as the abdomen...