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(kəlĭs'ərät`ə), subphylum of ArthropodaArthropoda
[Gr.,=jointed feet], largest and most diverse animal phylum. The arthropods include crustaceans, insects, centipedes, millipedes, spiders, scorpions, and the extinct trilobites.
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, 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 crabhorseshoe crab,
large, primitive marine arthropod of the family Limulidae, related to the spider and scorpion and sometimes called a king crab (a name also used for the largest of the edible true crabs).
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 and its relatives, which inhabits the soft bottom mud of shallow, coastal seas. Members of class Pycnogonida are commonly known as sea spiderssea spider,
common name for members of the class Pycnogonida, long-legged, rather spiderlike organisms of the subphylum Chelicerata, widely distributed in marine waters. Most are tiny, from 1 to 9 mm (0.04–0.36 in.
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. These exclusively marine invertebrates are spiderlike in appearance, mostly carnivorous, 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 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. Arachnidsarachnid
, mainly terrestrial arthropod of the class Arachnida, including the spider, scorpion, mite, tick, harvestman (daddy longlegs), and a few minor groups. The body is divided into a cephalothorax with six pairs of appendages, and an abdomen.
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 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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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

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a subphylum of invertebrates of the phylum Arthropoda. The body consists of a cephalothorax (prosoma) with six pairs of appendages (chelicerae, pedipalps, and four pairs of legs) and an abdomen (opisthosoma), on which there are appendages only in Xiphosura. Antennae are absent. In many mites and ticks the number of legs is reduced.

Fossil aquatic Chelicerata are known from the Cambrian, and terrestrial species are known from the Devonian. The subphylum includes two classes: Merostomata, which live only in seas, and Arachnoidea, which are mainly terrestrial.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


(invertebrate zoology)
A subphylum of the phylum Arthropoda; chelicerae are characteristically modified as pincers.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.