Chelicerata (redirected from Cheliceriform)

Chelicerata (kəlĭs'ərät`ə), 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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Chelicerata [kə‚lis·ə′räd·ə]

(invertebrate zoology)

A subphylum of the phylum Arthropoda; chelicerae are characteristically modified as pincers.

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Chelicerata

A subphylum of the phylum Arthropoda. The Chelicerata can be defined as those arthropods with the anteriormost appendages as a pair of small pincers (chelicerae) followed usually by pedipalps and four pairs of walking legs, and with the body divided into two parts: the prosoma (corresponding approximately to the cephalothorax of many crustaceans) and the opisthosoma (or abdomen). There are never antennae or mandibles (lateral jaws). The Chelicerata comprise three classes: the enormous group Arachnida (spiders, ticks, mites, scorpions, and related forms); the Pycnogonida (sea spiders or nobody-crabs); and the Merostomata (including the Xiphosurida or horseshoe crabs).

Both Merostomata and Pycnogonida are marine, but the enormous numbers and varied forms of the Arachnida are almost entirely terrestrial. The respiratory structures of chelicerates include gills, book-lungs, and tracheae. Sexes are normally separate, with genital openings at the anterior end of the opisthosoma. Some mites and other small chelicerates are omnivorous scavengers, but the majority of species of larger chelicerates are predaceous carnivores at relatively high trophic levels in their particular ecotopes. See Arthropoda