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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.
Arachnoids are smaller than most coronae, but have a similar genesis. They features differ from coronae in that they contain a network of both radial and concentric structures that gives the appearance of a spider sitting on interconnected webs of bright lines, hence the name arachnoid, which is Greek for spider.