Arachnida

from WordNet (r) 3.0 (2006)

Arachnida
    n 1: a large class of arthropods including spiders and ticks and
         scorpions and daddy longlegs; have four pairs of walking
         legs and no wings [syn: {Arachnida}, {class Arachnida}]

from The Collaborative International Dictionary of English v.0.48

Arachnida \A*rach"ni*da\, n. pl. [NL., fr. Gr. ? spider.]
   (Zool.)
   One of the classes of Arthropoda. See Illustration in
   Appendix.
   [1913 Webster]

   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.
         [1913 Webster]