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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.
Arachnids (Arachnida) are a group of arthropods that include spiders, ticks, mites, scorpions and harvestmen. Scientists estimate that there are more than 100,000 species of arachnids alive today. Arachnids have two main body segments (the cephalotorax and the abdomen) and four pairs of jointed legs.