An arthropod is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda (from Greek ἄρθρον árthron, "joint", and ποδός 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 arthropod body plan consists of repeated segments, each with a pair of appendages. Their versatility has enabled them to become the most species-rich members of all ecological 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.