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:

Araneina \A*ra`ne*i"na\ ([.a]*r[=a]`n[-e]*[imac]"n[.a]), n. pl.
   [NL., fr. L. aranea spider.] (Zool.)
   The order of Arachnida that includes the spiders.
   [1913 Webster]

   Note: They have mandibles, modified as poison fangs, leglike
         palpi, simple eyes, abdomen without segments, and
         spinnerets for spinning a web. They breathe by
         pulmonary sacs and trache[ae] in the abdomen. See
         Illustration in Appendix.
         [1913 Webster]