<P> The vitreous is the transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye . It is produced by certain retinal cells . It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the hyaluronic acid), no blood vessels, and 98--99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibres with the mucopolysaccharide hyaluronic acid, and also a wide array of proteins in micro amounts . Amazingly, with so little solid matter, it tautly holds the eye . </P> <P> Photoreception is phylogenetically very old, with various theories of phylogenesis . The common origin (monophyly) of all animal eyes is now widely accepted as fact . This is based upon the shared genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto - eye believed to have evolved some 540 million years ago, and the PAX6 gene is considered a key factor in this . The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race" among all species that did not flee the photopic environment . Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce . Hence multiple eye types and subtypes developed in parallel (except those of groups, such as the vertebrates, that were only forced into the photopic environment at a late stage). </P> <P> Eyes in various animals show adaptation to their requirements . For example, the eye of a bird of prey has much greater visual acuity than a human eye, and in some cases can detect ultraviolet radiation . The different forms of eye in, for example, vertebrates and molluscs are examples of parallel evolution, despite their distant common ancestry . Phenotypic convergence of the geometry of cephalopod and most vertebrate eyes creates the impression that the vertebrate eye evolved from an imaging cephalopod eye, but this is not the case, as the reversed roles of their respective ciliary and rhabdomeric opsin classes and different lens crystallins show . </P> <P> The very earliest "eyes", called eyespots, were simple patches of photoreceptor protein in unicellular animals . In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell . These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the light source . </P>

What structure changes its shape to focus light on the back of the eye