Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

Arachnida
     n : a large class of arthropods including spiders and ticks and
         scorpions and daddy longlegs; have four pairs of walking
         legs and no wings [syn: {class Arachnida}]