Human eye of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock. The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of the anterior segment and the posterior segment. The anterior segment is made up of the cornea, iris and lens. The cornea is transparent and more curved, and is linked to the larger posterior segment, composed of the vitreous, retina, choroid and the outer white shell called the sclera. The cornea is typically about 11.5 mm (0.3 in) in diameter, and 1/2 mm (500
Arthropod eye more complex schizochroal eye was found only in one sub-order of trilobite, the Phacopina (Ordovician-Silurian). It has no modern counterpart. The eye has up to 700 larger lenses with individual sclera separating each lens. Each lens has a cornea. Schizochroal eyes developed from holochroal ones and were more powerful with overlapping visual fields. They were particularly useful for nocturnal vision and possibly for colour and depth analysis. The lenses of the eye were constructed from single calcite crystals. Early schizochroal eye designs were rather haphazard and irregular, though constrained by the geometrical requirements of packing identical sized lenses on a