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]

The Collaborative International Dictionary of English v.0.48:

Arthrogastra \Ar`thro*gas"tra\, n. pl. [NL., fr. Gr. 'a`rqron
   joint + ? stomach.] (Zool.)
   A division of the Arachnida, having the abdomen annulated,
   including the scorpions, harvestmen, etc.; pedipalpi.
   [1913 Webster]