Arthropods are the largest phylum of animals and include the insects, arachnids, crustaceans, and others. More than 80% of described living animal species are arthropods, with over a million modern species described and a fossil record reaching back to the early Cambrian.

 

Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, including various symbiotic and parasitic forms. They range in size from microscopic plankton up to forms several meters long.

 

Arthropods are characterized by the possession of a segmented body. They have a dorsal heart and a central nervous system. All arthropods are covered by a hard exoskeleton made of chitin, which provides physical protection and resistance to drying. Periodically arthropods shed this covering when it moults.