An organ capable of detecting light. The simplest eyes simply detect whether the surroundings are light or dark. Compound eyes are found among the arthropods (insects and kin), and are composed of many simple facets which give a pixelated image (not multiple images as is often believed). In most vertebrates and some mollusks the eye works by projecting images onto a light-sensitive retina, where the light is detected and trasmitted to the brain via an optic nerve. The eye is typically roughly spherical, filled with a gel-like substance, with a focusing lens and often a muscle called the iris that controls how much light enters.