• WordNet 3.6
    • n Arachnida a large class of arthropods including spiders and ticks and scorpions and daddy longlegs; have four pairs of walking legs and no wings
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Webster's Revised Unabridged Dictionary
    • n. pl Arachnida (Zoöl) One of the classes of Arthropoda. See Illustration in Appendix.☞ They have four pairs of legs, no antennæ nor wings, a pair of mandibles, and one pair of maxillæ or palpi. The head is usually consolidated with the thorax. The respiration is either by trancheæ or by pulmonary sacs, or by both. The class includes three principal orders: Araneina , or spiders; Arthrogastra , including scorpions, etc.; and Acarina , or mites and ticks.
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Century Dictionary and Cyclopedia
    • arachnida In zoology, a class name used with varying signification. In Lamarck's and Latreille's systems of classification, one of three classes into which the Linnean Insecta were divided, the other two being Crustacea and Insecta; primarily divided into Pulmonaria and Trachearia.
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Chambers's Twentieth Century Dictionary
    • Arachnida a-rak′ni-da a sub-class of Tracheate Arthropoda, embracing spiders, scorpions, mites, &c., first separated by Lamarck from the Insecta of Linnæus
    • n Arachnida and adj. Arach′noid, like a cobweb
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Webster's Revised Unabridged Dictionary
NL., fr. Gr. spider


In literature:

Her prey was some hideous Scorpion, that first-born of the Arachnida.
"Bramble-bees and Others" by J. Henri Fabre
"Sketches of the Natural History of Ceylon" by J. Emerson Tennent
Even towards the nearer provinces of the Myriopoda and Arachnida I can find no bridge.
"Our Common Insects" by Alpheus Spring Packard
As you might gather, I don't care for brothers Arachnidae.
"Tabby" by Winston Marks
Lamarck included the Thysanura and the Myriapoda in his class Arachnida.
"Encyclopaedia Britannica, 11th Edition, Volume 2, Slice 3" by Various
ACAR'IDA, a division of the Arachnida, including the mites, ticks, and water-mites.
"The New Gresham Encyclopedia. Vol. 1 Part 1" by Various
The Arachnida are therefore tritognathous.
"Encyclopaedia Britannica, 11th Edition, Volume 2, Slice 6" by Various

In news:

Scorpions are members of the class Arachnida and are closely related to spiders, mites, and ticks.