Operculum (animal)

An operculum is an anatomical feature, a stiff structure resembling a lid or a small door that opens and closes, and thus controls contact between the outside world and an internal part of an animal.

Examples include:

A live individual of the sea snail Rapana venosa retracted into the shell, with the operculum closing the aperture

Firing sequence of the cnida in a hydra's nematocyst ^{[

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Operculum (lid)
"Finger" that turns inside out
/ / / Barbs
Venom
Victim's skin
Victim's tissues