Eye in Invertebrates

The light receptors of many invertebrates do not form definite images; the simply register light or dark or the direction of a source of light. The simplest such animal eyes are the light sensitive patches found on the flagella, or limblike projections, of the protozoan Euglena and the eyes pots of certain flatworms called planaria. Some organisms that have evolved true eyes have also retained simple photoreceptors of this type. Examples are the so called ocelli found in the lobsters and in the brain area under the skull; these organism can perceive light even when their true eyes have been removed.

Detection of Light
Despite the variety of types of eyes, the chemical process that transforms light into nerve impulses in the eyes is basically similar in all land vertebrates and marine fish, and in some insects. In 1967 George Wald of Harvard shared a Nobel Prize for physiology or medicine for discovering the details of the first step, which occurs in the retina or ommatidium.

The substances in the retina that detect light are called photosensitive, or visual, pigments. The major pigment in the eye is rhodopsin, or visual purple, which is composed of two distinct parts; a protein molecule called opsin, and a molecule made from vitamin A called retinene. When light strikes rhodopsin, the opsin portion, this leads, through a cascade of chemical reactions, to nerve impulse that relay visual information to the animal’s brain.

In the dark, and with the aid of chemical energy obtained from metabolism, retinene and opsin are recombined and rhodopsin is reconstituted. In very intense light, visual purple may be split faster than it can be reconstituted. Vision may then become impaired, for example, as in so called snow blindness. Vision many be similarly impaired if vitamin A is deficient I supply, and a shortage of retinene results in so-called night blindness.