The eyes of trilobites (Trilobita) are compound and made up of radially disposed visual units. Most trilobite eyes are ‘holochroal’ (i.e. many polygonal lenses are in contact with one another and covered by a single cornea). ‘Schizochroal’ eyes occur in the Phacopina (i.e. the lenses are large, separated from one another, each has its own cornea, and each lens is in two parts); it has been shown that this arrangement produces a sharp focus.