1. Arachno-: combining forms meaning 'Arachnoid membrane' or 'spider': Arachnoidal, Arachnoidism.

2. Arachnoid cysts are cerebrospinal fluid covered by Arachnoidal cells. They develop at the base of the skull, the surface of the brain, or on the Arachnoid membrane

3. Arachnoid cysts are cerebrospinal fluid covered by Arachnoidal cells and collagen that may develop between the surface of the brain and the cranial base or on the Arachnoid membrane, one of the three meningeal layers that cover the brain and the spinal cord