From Big Medical Encyclopedia

ARACHNOID (Arachnidea) — a class of backboneless animals like arthropod (Arthropoda), a subtype of helitserovy (Chelicerata).

The class P. is divided into several groups: scorpions (Scorpiones), zhgutonogy (Pedipalpi), lozhnoskor-peonies (Pseudoscorpiones), solpu-g, or phalanxes (Solifugae), senokosets (Opiliones), spiders (Aranei), ri-tsinule (Ricinulei), mites (Acarina); as a part of zhgutonogy and mites are distinguished on some independent groups. Items are distributed on all continents. It is known apprx. 50 thousand types. The sizes P. are very various: from 0,1 mm to 17 cm. The body is divided into a cephalothorax and an abdomen, conjoint is more rare. The cephalothorax is formed by 7 segments, bears 6 pairs of extremities. Two front couples are transformed to oral appendages — helitser (upper jaws) and pedi-palpa (mandibles), other four — ambulatory legs. Antennas are absent, their role is often carried out by front legs. The abdomen is segmented, the maximum number of segments reaches 12 (at scorpions); their P.'s most have less. At spiders and mites segments of an abdomen are usually merged. P. with the help of either lungs, or tracheas (two-pulmonary spiders have both that and others) breathe, is more rare the surface of skin. The circulatory system which is not closed is presented by the pulsing back vessel which is reported with a perigastrium. P.'s liver represents set of ferruterous blind outgrowths of intestines; it is the body cosecreting digestive juice. Eliminative organs — koksalny glands and malpigiyeva vessels. The nervous system is presented by nadglotochny nodes and the subpharyngeal nervous weight formed by merge of nodes of a belly nervous chain. Sense bodys are well developed, presented by eyes (usually several couples on a cephalothorax) and various cover sensillas — tactile, seismo - gigro-, thermo - hemoretseptorny, etc. Many P. have poisonous glands (scorpions, spiders, etc.). Item of a razdelnopola. The sexual opening is located in the basis of an abdomen; insemination or without copulation (spermatoforny), or by means of copulative bodies of various structure. P.'s most lay eggs, there are viviparous forms (scorpions, nek-ry mites). The item — land inhabitants; only the few types passed to a water way of life (a water spider, water mites). In the bulk P. — predators, eat arthropods, worms, mollusks. Abenteric digestion by means of the digestive juices brought in the victim at a sting with the subsequent absorption is characteristic. The types feeding on the decaying substances, microflora, mushrooms, seeds (granary mites) exhausting juice of plants (web, galloobrazuyushchy mites) and also parasites of animals and the person are known (see. Akariformny mites , Argasovy mites , Gamazoidny mites , Ixodic mites ). Eating blood, parasitizing on skin or in skin, they exhaust an organism of the owner, often are carriers of causative agents of many diseases of the person and animals (see. Transmissible diseases ). Among P. types are known (e.g., the spider a kara-court), a sting to-rykh not only is painful, but also is deadly (see. are MORE POISONOUS animals Are more poisonous ). Poison of nek-ry spiders possesses hemolitic action (see. Hemotoxins ).

Bibliography: Dogel V. A. Zoology of invertebrates, page 271, M., 1975; Life of animals, under the editorship of JI. A. Zenkevich, t. 3, page 56, M., 1969; Ivanov A. V. Spiders, their structure, a way of life and value for the person, JI., 1965.

A. B. Lange.