A scallop carries several dozen eyes, each about a millimetre across, arrayed along the edges of both mantle lobes. Each eye contains a layer of guanine crystals at its rear, tiled into a concave mirror. Light passes through the eye, strikes the mirror and is reflected back onto a retina positioned in front of it.
High-resolution imaging published in 2017 showed that the guanine crystals are square-ish tiles arranged in a mosaic whose local orientation varies across the mirror, correcting for aberration and producing a usable image. Each eye also has two retinal layers, which appear to receive light focused from different distances or angles.
What the scallop does with this remains open. The animal has no brain in the vertebrate sense, and behavioural evidence indicates it detects movement and changes in light rather than resolving detailed scenes. It is safer to say the optics are capable of forming an image than to say the scallop perceives one. The eyes are also lost and regrown, and their number rises as the animal grows.