arachnid(n = noun.animal) arachnoid - 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

   An arachnidan. Huxley. [1913 Webster]

arachnid, n. any arthropod of the class Arachnida, having four pairs of walking legs and characterized by simple eyes, e.g. scorpions, spiders, mites, and ticks.

Derivative:
arachnidan adj. & n.

Etymology:
F arachnide or mod.L arachnida f. Gk arakhne spider