nictitating membrane vs third eyelid

nictitating membrane and third eyelid both are nouns.

Word	Noun	Adjective	Verb	Adverb
nictitating membrane	Yes	No	No	No
third eyelid	Yes	No	No	No

As nouns, nictitating membrane and third eyelid are synonyms defined as:

nictitating membrane and third eyelid: a protective fold of skin in the eyes of reptiles and birds and some mammals

nictitating membrane (noun)	third eyelid (noun)
a protective fold of skin in the eyes of reptiles and birds and some mammals	a protective fold of skin in the eyes of reptiles and birds and some mammals