Historically, sea urchins were known as sea hedgehogs due to their superficial resemblance to the spiny mammalian species. “Urchin” is, in fact, a Middle English term for hedgehog. Sand dollars (like those pictured in this set of illustrations) are flattened, burrowing sea urchins. Dead, sunlight-bleached specimens resemble silver coins, hence their common name. These figures portray a sand dollar’s lifecycle from larva to adult. The center specimen, at two-months old, is only a millimeter long in life, and yet Haeckel managed to portray it in extreme detail. These details were made possible thanks to 19th century advances in the microscope, particularly the invention of the apochromatic lens system, which allows two wavelengths to be brought to the same focus.