gill slit vs complex body part

gill slit and complex body part both are nouns.

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

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

gill slit: one of a series of slit openings in the pharynxes of fishes and aquatic amphibians through which water passes
complex body part: a particular complex anatomical part of a living thing and its construction and arrangement

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

gill slit (noun)	complex body part (noun)
one of a series of slit openings in the pharynxes of fishes and aquatic amphibians through which water passes	a particular complex anatomical part of a living thing and its construction and arrangement