1.  arachnoid(n = noun.body) arachnoid membrane - the middle of the 3 meninges;
is a kind of meninges, meninx
is a part of leptomeninges

2.  arachnoid(n = noun.animal) arachnid - air-breathing arthropods characterized by simple eyes and four pairs of legs;
is a kind of arthropod
is a member of arachnida, class arachnida
has particulars: daddy longlegs, harvestman, phalangium opilio, scorpion, false scorpion, pseudoscorpion, whip-scorpion, whip scorpion, spider, acarine
Derived form adjective arachnoid₁

   arachnoid(a = adj.pert) arachnidian, spiderlike, spiderly, spidery - relating to or resembling a member of the class Arachnida;
Derived form noun arachnoid₂

1.  Resembling a spider's web; cobweblike. [1913 Webster]

2.  Pertaining to a thin membrane of the brain and spinal cord, between the dura mater and pia mater. [1913 Webster]

3.  Covered with, or composed of, soft, loose hairs or fibers, so as to resemble a cobweb; cobwebby. [1913 Webster]

1.  The arachnoid membrane. [1913 Webster]

2.  One of the Arachnoidea. [1913 Webster]

arachnoid, n. & adj.
--n. Anat. (in full arachnoid membrane) one of the three membranes (see MENINX) that surround the brain and spinal cord of vertebrates.
--adj. Bot. covered with long cobweb-like hairs.

Etymology:
mod.L arachnoides f. Gk arakhnoeides like a cobweb f. arakhne: see ARACHNID

arachnoid membrane