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Chelicerata[kulis´´urAt´u] Pronunciation Key, phylum of the living horseshoe crabs (class Merostomata), the arachnids (class Arachnida), and the sea spiders (class Pycnogonida). 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 merostomates are extinct, the only living representative being Limulus, the horseshoe crab (subclass Xiphosura) and its relatives, which inhabits the soft bottom mud of shallow, coastal seas. Merostomates have five or six gills, which have been modified from body appendages, and a long tail (telson) lacking appendages. The extinct giant water scorpions (subclass Eurypterida) belong to this class. 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 orders Aranaea (spiders), Acarina (ticks and mites), Opiliones (daddy longlegs, or harvestmen), and Scorpionida (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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