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Definition of arachnoid. 1  1 : of or relating to a thin membrane of the brain and spinal cord that lies between the dura mater and the pia mater. 2  2 : covered with or composed of soft loose hairs or fibers.
  The arachnids, Arachnida, are a class of invertebrate animals in the subphylum Chelicerata. Arachnids are named for the mythological figure Arachne.They are chiefly te â¦ rrestrial arthropods, some 65,000 to 73,000 species including the spiders, scorpions, harvestmen, ticks, and mites. The arachnids, Arachnida, are a class of invertebrate animals in the subphylum Chelicerata. Arachnids are named for the mythological figure Arachne.They are chiefly te â¦ rrestrial arthropods, some 65,000 to 73,000 species including the spiders, scorpions, harvestmen, ticks, and mites.