Eye

An eye is an organ that is capable of spatial vision, meaning it is able to detect an image. The quality of the image range greatly in quality across the animal kingdom. The pit eye of the planarian flatworm, for instance, has some ability to detect differences in intensity, resulting in a extremely blurry monochromatic image. The eye of an eagle, by contrast, can detect differences in wavelength (color) as well as intensity at incredible resolution (detail). While the eye may seem common, it is by no means a universal organ in the animal kingdom. Of the approximately 30 animal phyla, only about a third have eyes. Another third have no way of detecting light at all, and the last third have organs which can detect the presence of light. These are not considered real eyes because they do not produce an image. An earthworm, for instance, has light sensitive spots that allow it to detect whether it is above or below ground. While flies have true eyes, fly larvae do not; instead, they have a cluster of photoreceptors backed by a pigment molecule which prevents the light from coming in from the back. The larvae wave their heads around, and if they detect a higher intensity light than in the direction they are moving, they will align their bodies away from the brighter light. This tendency is called negative phototaxis.