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
Eodiscina surface. The interlensar sclera is not deeper than the lenses. This type of eye is called abatochroal. A second type of eye, called schizochroal, is unique to the Phacopina suborder, and has up to several hundreds of relatively large lenses (0.05-0.5 mm), that do not touch either, but the corneal membrane extends into the sclera outside the lens, and the sclera is deeper than the lens. The most usual type of trilobite eye, called holochroal, has usually thousands of small hexagonal lenses without an interlensar sclera, and a common cornea. Many later Eodiscina however, lost their visual organs altogether, and