In Biology operculum (Latin for "little lid") has been used to describe several completely separate features.

Gastropods

The operculum (plural : opercula or operculums) of gastropods is a corneous plate at the opening of the shell, attached dorsally to the foot. This fingernail-like structure seals the aperture, serving as a cover against predators when the snail body is retracted. It also enables the snail to survive periods of drought.

The operculum has a concentric structure and a nucleus near the parietal margin (close to the umbilicus).

There are two types of operculum :

• The first type consists of a thin to rather thick corneous material. This matter is supple, single layered, and circular to subcircular in shape.
• The second type has a multi-layered structure, with a corneous base and a calcareous overlay which is sometimes carved with spiral structures and grooves.

The operculum sculpture helps us to tell apart and sort related groups (genera) of land operculates.

Pulmonates (lunged snails) lack an operculum, but have instead an epiphragm (a membranaceous or calcareous septum).

Gastropod: external link

• Pictures of diverse gastropod opercula

This site also provides fairly extensive material on the growth, development, and physiology of the gastropod operculum as well as hundreds of pictures of both the operculum and it's host shell.

Fishes

The operculum in fishes is the hard bony flap covering the gills.

Botany

• a flap of the spore-bearing sporangium of a moss, covering the peristome (appendages surrounding the mouth of a moss capsule)
• the cap of the ascus in certain ascomycetous fungi
• the covering of a pyxidium (capsule whose upper part falls off when the seeds are released) of a plant, such as the plantain.

Brain

The operculum is part of the inferior frontal gyrus of the frontal lobe in the brain. One famous part of the operculum is Broca's area which plays an important role in speech production.