Holochroal eyes are compound eyes with many tiny lenses (sometimes more than 15,000, each 30-100μm, rarely larger). They are the oldest and most common type of

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

eye, and found in all orders of

from the Cambrian to the Permian periods. Lenses (composed of

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

) covered a curved, kidney-shaped visual surface in a hexagonal close packing system, with a single corneal membrane covering all lenses. Unlike in schizochroal eyes, adjacent lenses were in direct contact with one another. Lens shape generally depended on cuticle thickness. The lenses of trilobites with thin cuticles were thin and biconvex, whereas those with thick cuticles had thick lenses, which in extreme cases, could be thick columns with the outer surface flattened and the inner surface hemispherical. Regardless of lens thickness, however, the point at which light was focused was roughly the same distance below the lens.

References

Eye Animal anatomy {{trilobite-stub