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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
See more Encyclopedia articles on: Zoology: Invertebrates
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