Arthropod is an invertebrate animal having an exoskeleton (externalskeleton), a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda (from Greek ἄρθρον árthron, “joint“, andποδός podós “leg“, which together mean “jointed leg”), and include the insects,arachnids, and crustaceans. Arthropods are characterized by their jointed limbs and cuticles, which are mainly made of α-chitin; the cuticles of crustaceans are also biomineralized with calcium carbonate. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. The arthropodbody plan consists of repeated segments, each with a pair of appendages. They are so versatile that they have been compared to Swiss Army knives, and it has enabled them to become the most species-rich members of allecological guilds in most environments. They have over a million described species, making up more than 80% of all described living animal species, and are one of only two animal groups that are very successful in dry environments – the other being the amniotes. They range in size from microscopic plankton up to forms a few meters long.