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The phylum Arthropoda is typically subdivided into four subphyla, of which one is extinct: [129] Artiopods are an extinct group of formerly numerous marine animals that disappeared in the Permian–Triassic extinction event, though they were in decline prior to this killing blow, having been reduced to one order in the Late Devonian extinction.
Phylum: Arthropoda: Clade: Arachnomorpha: Subphylum: ... (living) arthropods are a monophyletic group and are divided into three ... The diagram below summarizes ...
Arthropods form the phylum Arthropoda. They are distinguished 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. Arthropods are bilaterally symmetrical and their body possesses an external skeleton.
Arthropods (/ ˈ ɑːr θ r ə p ɒ d / ARTH-rə-pod) are invertebrates in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin , often mineralised with calcium carbonate , a body with differentiated ( metameric ) segments , and paired jointed appendages .
As in all arthropods, the chelicerate body has a very small coelom restricted to small areas round the reproductive and excretory systems. The main body cavity is a hemocoel that runs most of the length of the body and through which blood flows, driven by a tubular heart that collects blood from the rear and pumps it forward.
Diagram of a spider leg and pedipalp – the pedipalp has one fewer segment. Arachnid legs differ from those of insects by the addition of two segments on either side of the tibia, the patella between the femur and the tibia, and the metatarsus (sometimes called basitarsus) between the tibia and the tarsus (sometimes called telotarsus), making a total of seven segments.
There are seven main taxonomic ranks: kingdom, phylum or division, class, order, family, genus, and species. In addition, domain (proposed by Carl Woese ) is now widely used as a fundamental rank, although it is not mentioned in any of the nomenclature codes, and is a synonym for dominion ( Latin : dominium ), introduced by Moore in 1974.
Other members of the class have an entirely different mode of life. Barnacles of the superorder Rhizocephala, including the genus Sacculina, are parasitic castrators of other arthropods, including crabs. The anatomy of these parasitic barnacles is greatly reduced compared to their free-living relatives.