arachnoid vs arachnidian

arachnoid is a noun but arachnidian is not a noun.

Word	Noun	Adjective	Verb	Adverb
arachnoid	Yes	Yes	No	No
arachnidian	No	Yes	No	No

As adjectives, arachnoid and arachnidian are synonyms defined as:

arachnoid and arachnidian: relating to or resembling a member of the class Arachnida

Other synonyms of arachnoid include spiderlike, spiderly, spidery.

arachnoid (noun)	arachnidian (noun)
air-breathing arthropods characterized by simple eyes and four pairs of legs
the middle of the 3 meninges

arachnoid (adjective)	arachnidian (adjective)
relating to or resembling a member of the class Arachnida	relating to or resembling a member of the class Arachnida