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(invertebrate zoology)
The name used in some classification schemes to describe a class of primitive arthropods.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(also Arachnida), a class of invertebrates of the phylum Arthropoda. The class embraces approximately 35,000 species and includes the orders Scorpionida, Palpigradi, Pseudoscorpionoidea, Solifugae (Phalangida), Opiliones, Araneida, and Ricinulei; also assigned to the Arachnoidea are the groups Pedipalpi and Acarina.

The arachnids are from 0.1 mm to 17 cm long. The body is usually divided into a nonsegmented cephalothorax and an abdomen, which is only rarely fused. The cephalothorax bears six pairs of appendages: one pair of chelicerae (usually chelate), one pair of pedipalps, and four pairs of walking legs. Many arachnids (Scorpionida, Solifugae, Pseudoscorpionoidea, some Araneida, and some Pedipalpi) have a segmented abdomen. Most Araneida and many Acarina have an unsegmented abdomen, which is sometimes equipped with special appendages—modified abdominal legs (spinnerets in Araneida and pectines in Scorpionida).

Respiration is tracheal or pulmonary; only Dipneumonas and Araneomorphae have both lungs and tracheae. Some arachnids breathe through the surface of their body. Excretion is accomplished by the coxal glands and the Malpighian tubules. The nervous system in most arachnids consists of the supra-esophageal ganglia and the subesophageal nerve complex formed by the fusion of parts of the abdominal ganglia and by its unfused ganglia. In Araneida all abdominal ganglia fuse to form a common mass; in Acarina the abdominal ganglia fuse with the supra-esophageal ganglia. The sense organs consist of eyes and sensory hairs; some arachnids, such as Araneida, react to sounds and odors. Many arachnids, including Scorpionida, Araneida, and some Pseudoscorpionoidea, have poison glands. The sexes are separate. Most species deposit eggs, while some are viviparous.

Arachnids are distributed on all continents. They usually inhabit dry land; only a few species have adopted aquatic life (water spider, water mites). Most arachnids are predators, feeding on other arthropods, worms, mollusks, and small reptiles. The invertebrates are ancient terrestrial arthropods. They are related to Eurypterida; they too trace their ancestry from trilo-bites. The oldest arachnids are known from the Upper Silurian.


Osnovy paleontologii: Chlenistonogie trakheinye i khelitserovye. Moscow, 1962.
Ivanov, A. V. Pauki, ikh stroenie, obraz zhizni i znachenie dlia cheloveka. Leningrad, 1965.
Zhizn’ zhivotnykh, vol. 3. Moscow, 1969.
Dogel’, V. A. Zoologiia bespozvonochnykh, 6th ed. Moscow, 1974.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Image 2 shows a stepped dissection of the cranial fascial layers, including the visible layers of the arachnoid mater just superficial to the brain; the dura mater; the bony cranium; pericranium; galea aponeurotica (with the muscle fibers of frontalis and occipitalis visible anteriorly and posteriorly); and the superficial fascia of the scalp presented continuous with the skin.
Hypertrophic meningitis is a rare disorder characterized by diffuse or focal, linear or nodular thickening of the dura, underlying pia and arachnoid mater and the tentorium.1,2 Numerous causes leading to these changes have been identified (Table).
It has been postulated that following subdural catheter placement, subsequent entry of the catheter into the subarachnoid space occurs due to tearing of the arachnoid mater on administration of local anaesthetic (10).
The pattern of block progression may have represented an initial subdural block followed by penetration of the arachnoid mater by the epidural catheter.
One such plane is the subdural space, which lies between the dura and arachnoid mater. Unintentional injection of local anaesthetic into the subdural space has been seen to result in both a wide dermatomal spread (2-7) as well as an inadequate block (8,9).