The system that structures the body, carries the weight of the body and preserves the body is called the skeleton. There are two types of skeletons.
(i) Exoskeleton: The skeleton that forms the outer structure of the body is called exoskeleton. Exoskeleton of vertebrates like hooves, horns, claws, scales, feathers, beaks etc. The exoskeleton is the chitinous plate of arthropods and the calcareous shell of molluscs.
(ii) Endoskeleton: The skeleton which forms the internal structure of the body is called endoskeleton. The bones and cartilage of vertebrates are the endoskeleton. Invertebrates have endoskeleton. Spicules of Parifera, calcareous plate of Echydermata etc. are endoskeleton. Molluscs do not have skeletons. Earthworms, worms, cockroaches, butterflies etc. do not have skeletons.