ARACHNIDA, the name applied to the class of arthropods represented by the scorpion, spider and mite. The body is divided into two regions, the cephalothorax and abdomen, the head having been in embryonic life folded back over and fused with the thorax. There are no antenna, the first pair of appendages resembling mandibles, and called chelicera; the second pair end in a large forceps, or chelm, or in a palpus-shaped appendage called ((pedi palpi." The head appendages are not differ entiated into antenna, mandibles, maxilla and maxillipeds, as in Crustacea. There are four pairs of legs ending in a pair of minute claws. On the abdomen there are no appendages. The respiratory organs are spiracles opening, into tracheas or air-tubes, or pouches containing numerous leaves or their sacs, resembling the leaves of a book, and hence called abook lungs" The eyes are simple, never compound, two being situated in the middle of the head, others on each side of it. There may be as many as five pairs of nephridia. The genital outlet is single instead of being double, as in Limulus. They have a pair of malpighian tubes or urinary vessels, but, like Limulus, possess two large digestive glands, the The young are hatched in the form of the adult, there being no metamorphosis except a slight one in the mites. Their embryos have, on at least six abdominal segments, rudiments of limbs, which indicate their descent from ani mals like Limulus. All of the Arachnida are terrestrial, none of them living in or near fresh water, except a few mites. Their embryology is like that of Limulus, which suggests that the Arachnida have descended from the Merosto mata (q.v.) Whether we take into account the mode of development or the very primitive nature of the appendages, it appears that the Arachnida are much less closely allied to in sects than was formerly supposed. On the other hand, they differ from the merostomes, and especially their living representative, the king crab (q.v.), in having no gills. Their embryology and morphology tend to show that the class has probably descended from limuloid ancestors, of which there are examples in the Silurian rocks, intermediate between limuloid (Xiphosura) and eurypteroid (Eurypterida) forms. The characters in which Arachnida re
semble insects, as respiration by trachea and the presence of urinary tubes (which do not occur in Crustacea or in any other marine or branchiate arthropods), are probably adaptive and were acquired during a change from a marine to a terrestrial life, and not primitive heirlooms. Arachnids also show their later origin than merostomes by the fact that their sexual ducts (oviduct, etc.) are in most cases single, unpaired and in all cases open exter nally by a common single genital aperture in the median line of the body, at the base of the abdomen. In this respect Limulus, with its pair of genital (male and female) openings, situated each at the end of a papilla, which are placed widely apart at the base of the first abdominal legs, is decidedly more archaic. The Arachnida are divided into six groups or orders : (1 ) Scorpionida (scorpions) ; (2) Pseudoscorpaonida (false- or book-scorpions); (3) Pedipalpida (Phrynus or scorpion-spiders) ; (4) Solpugida (galeodes) ; (5) Phalangida (harvest men) ; (6) Araneida (spiders); (7) Acarida (mites and ticks), the latter compris ing the aberrant and degenerate forms, many of them parasitic; while of somewhat doubtful relationship to the Acarina are three aberrant groups: the Linguatulida, Pycnogonida, and Tardigrada. See MITE; SCORPION; SPIDERS.
Several of the Arachnids are of interest in medicine. A few of the scorpions are poison ous, their sting even causing death, although it is quite certain that there are no poisonous spiders north of Mexico. A number of the mites are found in man. One, the Demodes cares, is frequently found in the sebaceous fol licles of the skin, notably in comedos, or black heads. The itch mite (Sarcoptes scabeii) bores little canals under the skin, in which it deposits its eggs. The itch is a common disease of Europe, and is becoming more frequent in this country. Sulphur ointment is one of the best parasiticides for this small nuisance.