Most molluscs (slugs, snails and many shellfish) use a radula to scrape algae and other food from the surface of rocks and shells. It’s a large, complex structure which resembles a giant alien tongue with many rows of teeth, and can be more than half the length of the whole animal! But chitons have an even more amazing radula, whose teeth are capped with magnetic iron compound magnetite. This is visible here in black. They are one of the only animals able to produce the magnetite themselves, and it has been suggested they can sense magnetic field directions (though not using the radula).
Image courtesy of Lyle Gordon and the Nikon Small World competition.