Webster's English Dictionary

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arachnid

arach.nid \*-'rak-n*d\ \-n*d-*n\ n [deriv. of Gk arachne- spider] : any of 
   a class (Arachnida) of arthropods comprising mostly air-breathing  
   invertebrates, including the spiders and scorpions, mites, and ticks, and 
   having a segmented body divided into two regions of which the anterior 
   bears four pairs of legs but no antennae - arachnid aj