Found in some jellyfish, polychaetes, spiders and many mollusks. The parts of the vertebrate eye are structurally and functionally diverse. The vertebrate eye consists of a tough outer layer of connective tissue, the sclera , and a thin inner pigmented layer, the choroid . The anterior choroid forms the iris , which regulates the amount of light entering the pupil . The ciliary body produces aqueous humor that fills the cavity between the lens and cornea.
Vitreous humor fills the cavity behind the lens and comprises most of the eye's volume.
Both aqueous humor and vitreous humor help to focus light onto the retina. Controlled by the ciliary muscle. The photoreceptors of the eye are rod cells and cone cells. Their relative numbers in the retina are partly correlated with whether an animal is diurnal or nocturnal. The light absorbing molecule is retinal , synthesized from vitamin A, which is bonded to a characteristic kind of protein, opsin .