Sight, Organs of

(eyes), organs in man and animals that perceive light stimuli. They are found in representatives of all classes of vertebrates and most invertebrates (except sponges).

In multicellular animals the principal element of the organs of sight is the initiallysensing visual cell—the photoreceptor. It perceives light by means of its peripheral end (external segment), which in vertebrates has the shape of a rod or a cone. In the majority of animals the organs of sight are located on the head and are connected to the brain by the optic nerves. Organs of sight are classified as converted or inverted, according to the location of the visual cells in relation to the source of light; in converted organs of sight the receptive end of the visual cell is turned toward the light, and in inverted ones it is turned away from the light.

The simplest organs of sight consist of isolated visual cells located among the epithelial cells on the body surface. Such organs of sight, capable only of distinguishing light from darkness, are known, for example, in earthworms. The increasing complexity of organs of sight in the process of animal evolution occurred by means of the concentration of disconnected visual cells into clusters, their submersion under external covering, and the creation of pigment screens and also of light-refractive, accommodation, oculomotor, and protective devices. In leeches, along with dispersed visual cells, one also finds clusters of them, lined with pigment cells, which isolate the photosensitive cells from lateral light rays. The organs of sight of certain coelenterates and lower worms consist of so-called eyespots that lie in the ectoderm and consist of visual cells and pigment cells lining them. In some cases the pigment may accumulate in the visual cells themselves. In becoming more complex, the organs of sight took on a bubble-like or cup shaped form, for example, in some coelenterates and mollusks; sometimes the cavity of the bubble or the cup is filled with a transparent, gelatinous, light-refractive mass called the vitreous body. More complex organs of sight, equipped with a dioptric, or light-refracting, apparatus, are found in some mollusks, annelid worms, and arthropods. Their visual cells lie under the epithelium and, together with the pigment cells, form the retina. In many arthropods (crustaceans and insects) the organs of sight consist of faceted eyes, which are composed of numerous separate eyelets called ommatidia. Faceted eyes make it possible to perceive the shape of objects; they are adapted to vision at close range and have no accommodation devices.

The most perfected organs of sight belong to man, all the vertebrates (especially birds), and some invertebrate animals (in particular, cephalopods), in which they are in the form of so-called chambered eyes.