An arthropod is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages. Arthropods form the phylum Arthropoda, and include the insects, arachnids, myriapods, and crustaceans. Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. 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, some of which, unlike most animals, are very successful in dry environments. They range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab. Arthropods' primary internal cavity is a hemocoel, which accommodates their internal organs, and through which their haemolymph - analogue of blood - circulates; they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.
†Trilobitomorpha - Includes classes Agnostida, Asaphida, Corynexochida, Harpetida, Nectaspida, Redlichiida, Lichida, Odontopleurida, Phacopida, Proetida and Ptychopariida.
Chelicerata - Includes classes Arachnida, Pycnogonida and Merostomata.
Myriapoda - Includes classes Chilopoda, Diplopoda, Pauropoda and Symphyla.
Crustacea - Includes classes Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Ostracoda and Malacostraca.
Hexapoda - Includes classes Insecta and Entognatha.