The eyes of most cephalopods, fish, amphibians and snakes have fixed lens shapes, and focusing is achieved by telescoping the lens in a similar manner to that of a camera. Muscles around the iris change the size of the pupil, regulating the amount of light that enters the eye and reducing aberrations when there is enough light. Such eyes are typically spheroid, filled with the transparent gel-like vitreous humour, possess a focusing lens, and often an iris. The cone cells (for colour) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals which are transmitted to the brain via the optic nerve to produce vision. In most vertebrates and some molluscs, the eye allows light to enter and project onto a light-sensitive layer of cells known as the retina. The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved in 96% of animal species in six of the ~35 main phyla. The first proto-eyes evolved among animals 600В million years ago about the time of the Cambrian explosion. In other organisms, particularly prey animals, eyes are located to maximise the field of view, such as in rabbits and horses, which have monocular vision. The visual fields of many organisms, especially predators, involve large areas of binocular vision for depth perception. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment and to the pretectal area to control the pupillary light reflex.Д¬omplex eyes distinguish shapes and colours. The most simple eyes, pit eyes, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. Image-resolving eyes are present in molluscs, chordates and arthropods.

Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.

In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes detect light and convert it into electro-chemical impulses in neurons. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision.

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