class Arachnida vs arachnid

class Arachnida and arachnid both are nouns.

Word	Noun	Adjective	Verb	Adverb
class Arachnida	Yes	No	No	No
arachnid	Yes	No	No	No

class Arachnida (noun)	arachnid (noun)
a large class of arthropods including spiders and ticks and scorpions and daddy longlegs; have four pairs of walking legs and no wings	air-breathing arthropods characterized by simple eyes and four pairs of legs