Holochroal eye: found in nearly all Orders few to very many lenses (to >15,000!) lenses typically small, numerous one corneal layer covers all lenses lenses in direct contact with others no sclera between lenses corneal membrane covers surface only. Schizochroal eye: found in some Phacopida only typically fewer lenses (to ca 700) lenses much larger, fewer each lens bears an individual cornea lenses separated from each other sclera between lenses very deep corneal membrane extends into sclera. Abathochroal eye: found in Cambrian Eodiscina only few lenses (to ca 70) lens size small, not numerous each lens bears an individual cornea lenses separated from each other interlensar sclera not deeper than lenses corneal membrane ends at lens margin. All early trilobites (Cambrian), had holochroal eyes and it would seem hard to evolve the distinctive phacopid schizochroal eye from this form. The answer is thought to lie in ontogenetic (developmental) processes on an evolutionary ...