gill arch vs body structure

gill arch and body structure both are nouns.

Word	Noun	Adjective	Verb	Adverb
gill arch	Yes	No	No	No
body structure	Yes	No	No	No

As nouns, body structure is a hypernym of gill arch; that is, body structure 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
body structure: a particular complex anatomical part of a living thing and its construction and arrangement

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

gill arch (noun)	body structure (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