I love shell doors. No idea why, but somewhere in a box there’s a jam jar of shell doors that I collected in Eritrea. These, however, are not Eritrean. Being here at home they could be either from the Canary Islands, Greece, Sardinia or Cuba. I’m not really sure.
A shell door is actually called an operculum – if you google “shell door” you get all sorts of engineering stuff that has nothing to do with these natural beauties. Operculum however means “lid” and wikipedia has a lovely page about them.
In brief – some molluscs have little trapdoors made of calcium (like their shell) stuck to the bottom of their foot that yes, might give them extra security when they need to retreat from a hungry predator, but especially keep them from drying out when they find themselves out of the water (low tide, summer etc).
They can be used as decorative elements (inserted in rings, earrings, necklaces, masks etc), as paperweights (the larger ones) and when they are ground to powder they can be burnt as incense, described as having a strong musky smell of burnt hair. I guess I won’t be doing that anytime soon.