As a fish swims over the ocean floor, it’s being watched by hundreds of rocks. The rocks are actually the eyes of a chiton, an armoured relative of snails and other molluscs. Perhaps uniquely among living animals, it sees the world through lenses of limestone, and its eyes literally erode as it gets older.

Chitons are protected by a shell consisting of eight plates. The plates are dotted with hundreds of small eyes called ocelli. Each one contains a layer of pigment, a retina and a lens. People have known about the ocelli for years, but no one knew what they were made from or how much the chitons could actually see with them.

The vast majority of animal lenses are made