An operculum, also called a cats eye shell, is the covering of a gastropods shell aperture (opening), kind of like their “trap door”. This helps keep the animal protected from predators and gives it a defense. Being able to see this live snakeskin turban shell with its vivid green “trap door” closed in contrast with its stunning red snakeskin pattern was a highlight of this particular dive in Fiji! We don’t have snakeskin turbans here in Hawaii, so it was a real treat for me to not only see this

Operculum (fish) - Alchetron, The Free Social Encyclopedia

Operculum in biology

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