Arachnoid
ARACH'NOID, adjective [Gr. a spider, and from; Heb. to weave, that is to stretch, to draw out; Eng. reach.]
In anatomy, the arachnoid tunic, or arachnoid is a semitransparent thin membrane which is spread over the brain and piamater, and for the most part closely connected with the latter. The term has also been applied to that capsule of the crystalline lens, which is a continuation of the hyaloid membrane.
ARACH'NOID, noun A species of madrepore found fossil.