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An arthropod (/ˈɑːrθrəpɒd/, from Greek ἄρθρον arthron, "joint" and πούς pous, "foot" (gen. ποδός)) is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. 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. Arthopods are bilaterally symmetrical and their body possesses an external skeleton. Some species have wings.

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 percent of all described living animal species, some of which, unlike most other animals, are very successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.

Arthropods' primary internal cavity is a haemocoel, 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. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many bristles known as setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection.

The evolutionary ancestry of arthropods dates back to the Cambrian period. The group is generally regarded as monophyletic, and many analyses support the placement of arthropods with cycloneuralians (or their constituent clades) in a superphylum Ecdysozoa. Overall, however, the basal relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. 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. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions.

Arthropods contribute to the human food supply both directly as food, and more importantly indirectly as pollinators of crops. Some species are known to spread severe disease to humans, livestock, and crops.

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Lateral view of two fairy shrimp. The upper one has a large egg sac rising from the front of its abdomen, while the lower one has enlarged antennae.
Chirocephalus diaphanus is a widely distributed European species of fairy shrimp that lives as far north as Great Britain, where it is the only surviving species of fairy shrimp and is protected under the Wildlife and Countryside Act 1981. It is a translucent animal, about 0.5 in (13 mm) long, with reddened tips to the abdomen and appendages. The body comprises a head, a thorax bearing 11 pairs of appendages, and a seven-segmented abdomen. In males, the antennae are enlarged to form "frontal appendages", while females have an egg pouch at the end of the thorax.

The life cycle of C. diaphanus is extremely fast, and the species can only persist in pools without predators. The eggs tolerate drying out, and hatch when re-immersed in water. C. diaphanus was first reported in the scientific literature in 1704, but was only separated from other species and given its scientific name in 1803; the specific epithet diaphanus refers to the animal's transparency.

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An medium-sized orange-brown butterfly with darker brown wing margins and a few wing spots.

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A mass of pinkish animals. Dark spots are visible on many of their tails.
Cscr-featured.svg Credit: Hans Hillewaert

Squilla mantis is a species of mantis shrimp chiefly found and fished in the Mediterranean Sea. It grows up to 200 millimetres (8 in) long, and is of the spearer type. It is generally dull brown in colouration, but has two brown eye spots, circled in white, at the base of the telson.

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indicates an extinct taxon.

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