operculum
noun
oper·cu·lum ō-ˈpər-kyə-ləm 
plural opercula ō-ˈpər-kyə-lə also operculums
also operculums 1
: a body process or part that suggests a lid: such as
a
: a horny or shelly plate on the posterior dorsal surface of the foot in many gastropod mollusks that closes the shell when the animal is retracted
b
: the covering of the gills of a fish see fish illustration
2
: a lid or covering flap (as of a moss capsule)
Example Sentences
Recent Examples on the Web Scallops, for example, showcase a complex system of camouflage, with a colored shell on top and a white shell on the bottom; most gastropods are born with trapdoor-like devices called an operculum, which protect them from predators. 
Susan Glaser, cleveland.com, 10 Jan. 2018
Susan Glaser, cleveland.com, 10 Jan. 2018 Word History
Etymology
borrowed from New Latin, going back to Latin, "cover, lid," from operīre "to shut, close, cover" (from op-, variant of ob- ob- + pre-Latin *-wer-je- "open or close," as specified by a prefix, going back to Indo-European *Hu̯er-) + -culum, instrumental suffix (going back to Indo-European *-tlom) — more at aperient
First Known Use
1681, in the meaning defined at sense 1
Medical Definition
operculum
noun
oper·cu·lum ō-ˈpər-kyə-ləm
plural opercula -lə also operculums
also operculums : any of several parts of the cerebrum bordering the sylvian fissure and concealing the insula