Webster's English Dictionary

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arachnoid

1. arach.noid \*-'rak-.no.id\ n [NL arachnoides, fr. Gk arachnoeide-s, like 
   a cobweb, fr. ara]chne- spider, spider's web : a thin membrane of the brain 
   and spinal cord that lies between the dura mater and the pia mater
2. arachnoid aj 1: of or relating to the arachnoid membrane  2: covered 
   with or composed of soft loose hairs or fibers 
3. arachnoid aj [deriv. of Gk arachne-] : resembling or related to the 
   arachnids