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trilobite
A trilobite is a marine arthropod of the Paleozoic with a segmented body divided lengthwise into three lobes, a hard shield over the head, and jointed legs beneath. Around twenty thousand species have been described. Many could roll into a ball like a woodlouse, and their eyes were made of calcite crystals, which means they are the only animals known to have looked at the world through stone.
Etymology
The name is Greek for three-lobed, given for the lengthwise division rather than the more obvious segmentation, which trips people up constantly. The three lobes run head to tail, not front to back: a raised central axis with a flatter lobe on each side of it.
Notes
The calcite lenses are the remarkable part. They are oriented so that the crystal axis points the right way for a clear image, which does not happen by accident, and some species had compound eyes with hundreds of individually corrected lenses of a design not worked out on paper until the seventeenth century. Because the eye is mineral, fossil trilobites preserve their optics intact and it is possible to work out what one could see.