Merriam-Webster Collegiate Dictionary - arachnoid

 I. adjective  Date: 1789  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. covered with or composed of soft loose hairs or fibers  II. noun  Etymology: New Latin ~es, from Greek arachnoeides, like a cobweb, from arachne spiderweb  Date: circa 1847 an ~ membrane  III. adjective  Etymology: New Latin Arachnida + English -oid  Date: 1852 resembling or related to the arachnids