nictitating membrane(n = noun.body) third eyelid - a protective fold of skin in the eyes of reptiles and birds and some mammals;
is a kind of protective fold
is a part of eye, oculus, optic
has particulars: haw

arachnoid membrane, basilar membrane, cell membrane, chorioallantoic membrane, embryonic membrane, fertilization membrane, fetal membrane, hyaline membrane disease, hyaloid membrane, membrane, membrane bone, mucous membrane, plasma membrane, reissner's membrane, semipermeable membrane, serous membrane, synovial membrane, tympanic membrane, virginal membrane