gill arch vs complex body part

gill arch and complex body part both are nouns.

Word	Noun	Adjective	Verb	Adverb
gill arch	Yes	No	No	No
complex body part	Yes	No	No	No

As nouns, complex body part is a hypernym of gill arch; that is, complex body part is a word with a broader meaning than gill arch:

gill arch: one of the bony or cartilaginous arches on each side of the pharynx that support the gills of fishes and aquatic amphibians
complex body part: a particular complex anatomical part of a living thing and its construction and arrangement

Other hypernyms of gill arch include anatomical structure, bodily structure, body structure, structure.

gill arch (noun)	complex body part (noun)
one of the bony or cartilaginous arches on each side of the pharynx that support the gills of fishes and aquatic amphibians	a particular complex anatomical part of a living thing and its construction and arrangement