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However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways; Arthropods also have two body elements that are not part of this serially repeated pattern of segments, an acron at the front, ahead of the mouth, and a telson at the rear, behind the anus. Originally it seems that each appendage bearing segment had two separate pairs of appendages; and upper and a lower pair.These would later fuse into a single pair of biramous appendages, with the upper branch acting as a gill while the lower branch was used for locomotion.The number of species remains difficult to determine.
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.
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 cuticle of many crustaceans, beetle mites, and millipedes (except for bristly millipedes) is also biomineralized with calcium carbonate.
Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80% of all known living animal species.Aquatic species use either internal or external fertilization.Almost all arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother.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.Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles.