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Chelicerata

Chelicerata (kəlĬs´ərät´ə), subphylum of Arthropoda, including the horseshoe crabs (order Xiphosura), the arachnids (class Arachnida), and the sea spiders (class Pycnogonida). The extinct giant water scorpions (order Eurypterida, not true scorpions) also are chelicerates. The chelicerates are characterized by the absence of antennae and jaws and the presence of feeding structures (chelicera), which are modified pincerlike appendages used mainly for grasping and fragmenting food.

Nearly all the xiphosurans are extinct, the only living representative being Limulus, the horseshoe crab and its relatives, which inhabits the soft bottom mud of shallow, coastal seas. Members of class Pycnogonida are commonly known as sea spiders. These exclusively marine carnivores are spiderlike in appearance and range in body length from 4/100 in. (1 mm) to 4 in. (5 cm); the leg spread is sometimes over 2 ft (61 cm). Most sea spiders have four pairs of legs. They feed with a sucking proboscis on algae and other invertebrates and are found in oceans all over the world.

The largest class of chelicerates, class Arachnida, includes the subclass Acari (or Acarina, ticks and mites) and the orders Araneae (spiders), Opiliones (daddy longlegs or harvestmen), and Scorpiones (scorpions), among the most important. Arachnids are predominantly terrestrial, and most are carnivorous, with the digestion of prey starting outside the body. The body is composed of an unsegmented anterior region (prosoma), with a pair of chelicera, a pair of leglike appendages (pedipalps), four pairs of walking legs, and a posterior region (opisthosoma); it is equipped with book lungs or tracheae, for respiration. Arachnids are an ancient group, their fossil records dating back to the Carboniferous period.

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Chelicerata

Chelicerata (phylum Arthropoda) A subphylum of arthropods in which the body comprises a cephalothorax or prosoma and an abdomen or opisthosoma, the first pair of feeding structures are the chelicerae that give the subphylum its name, and there are no antennae. The chelicerates are known as fossils from the early Palaeozoic, and originated as marine organisms, but today there are only five marine species: the horseshoe crabs (Merostomata). The subphylum includes the classes Arachnida, Merostomata, and Pycnogonida.

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Chelicerata

Chelicerata A phylum of invertebrate animals that includes arthropods belonging to the classes Arachnida (spiders and scorpions), Merostomata (horseshoe crabs), and Pycnogonida (sea spiders). The body of these animals is divided into an anterior prosoma and a posterior opisthosoma and bears six pairs of appendages, typically comprising chelicerae, pedipalps, and four pairs of walking legs.

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Chelicerata

Chelicerata (phylum Arthropoda) Subphylum comprising a diverse group of animals which possess an anterior prosoma (fused head and thorax), a posterior opisthosoma (abdomen), and a pair of jointed pincers. The subphylum includes the spiders, mites, and scorpions, as well as the kingcrab (Limulus) and the eurypterids (Palaeozoic water scorpions).

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