Estimates of the total species range from 2-10 million, and arthropods live in all major habitats. Appendages are extensions of the main body and are used for locomotion, feeding, reproduction, defense, and sensing the environment. The structure of appendages is often used to classify arthropods. Bodies of arthropods are segmented, although some segments may fuse during development. All organ systems are highly developed.
Arthropods are coelomate and their circulatory system is open.
The external skeleton, of arthropods is made of chitin, which is a long chain, of nitrogen containing sugar molecules. These fibrous molecules are strong and resist chemicals. Chitin may be as soft as the body of a butterfly or, if impregnated with calcium carbonate, as hard as the shell of a lobster. This exoskeleton provides protection, a moisture barrier, and a place for muscle attachment. Although this tough covering limits growth, arthropods periodically shed their exoskeleton and quickly enlarge before the new exoskeleton hardens.