What is "arachnida"

Arachnida \A*rach"ni*da\, n. pl. [NL., fr. Gr. ? spider.] (Zo["o]l.) One of the classes of Arthropoda. See Illustration in Appendix.

Note: They have four pairs of legs, no antenn[ae] nor wings, a pair of mandibles, and one pair of maxill[ae] or palpi. The head is usually consolidated with the thorax. The respiration is either by tranche[ae] 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.