I'm back in Mexico where I'm getting my sailboat Honu ready for another Sea of Cortez adventure. All I really want to do, though, is collect trap doors. Those might seem kind of bulky on a 37-foot boat, but these aren't your average trap doors. Mine are round, calcium carbonate disks, the largest an inch in diameter.

The trap doors I'm finding are the flat lids some snails have to close the opening of their shells. A common term for them is cat's eyes; the scientific name is opercula (singular, operculum). I call them treasures. To me these shells are as good as gold doubloons.

The opercula I'm finding belong to turban snails, so-called for their spiral shells' wraparound design. I know my cat's eyes come from turban snails because while snorkeling, I'm finding the living snails with their doors shut tight. Many are oddly belly-up, swaying in the water as if dead yet holding their doors closed firmly with their muscular feet.

At first I thought these snails got upside down and hadn't yet righted themselves. But after turning several over and tucking them in rock cracks, I saw there was no end to these turned-up turbans and stopped my rescue efforts. Maybe these are aged snails dying natural deaths. I don't know, but whatever is happening, after they die, they leave lovely legacies.

The outer part of these turbans' shells is rough and covered with algae and other marine growth. The inside is stunning mother-of-pearl. When empty, on sunny days, the rainbow whites of these interiors shine through the water like little spotlights.

Turban snails, and dozens of other snail species, live in this nutrient-rich water by the tons. Really. Living snails, and the shells of dead ones, are everywhere, in some places creating entire beaches.

Besides that being shell-collector heaven, it's also hermit crab heaven. Hermit crab populations are limited by the number of shells the crabs can find as they grow, but there is definitely no housing shortage here. During a low tide last week, hermit crabs wearing a dozen kinds of shells grazed on the rocks by the thousands, some clustered in groups. It seemed odd to see such a variety of species huddled together, but of course that was a facade. The crabs inside these shell homes were the same species.

Talk about live rock. It looked like those boulders were going to get up and walk away.

Turban snails have beautiful round opercula with a slightly raised spiral pattern on the outside and a flat spiral pattern on the inside. The foot of the snail attaches to the inside, and when the animal is threatened it withdraws its body into its shell house and pulls the door shut behind it. In a living snail getting it open takes a tool.

But the days of killing snails for their shells is over. Each species has a job to do on the reef, and to keep our reefs alive and healthy, we must let them do it.

Both hermit crabs and turban snails are shallow-water vacuum cleaners, keeping our shores clean by vacuuming up algae and dead plants and animals. When they die of natural causes, they become gems like no other.

Opercula aren't the treasures Hernandez Cortez was looking for when he explored this area, but they sure are mine. Here I've struck it rich.

Susan Scott can be reached at www.susanscott.net.