Published: May 2005
Did You Know?
In Did You Know? the National Geographic magazine team shares extra information we gathered to expand your knowledge of our featured subjects.

The simplest types of light receptors are known as ocelli—they are so basic that they cannot even be called eyes. Ocelli are light-sensitive regions in single-celled organisms or light-sensitive cells in animals. They usually detect whether the animal's surroundings are light or dark, although some more advanced types can sense the direction of a light source. Corals and sea anemones have simple ocelli—usually of alternating pigment and sensory cells—that are scattered over the animals' entire body surfaces. Some corals and sea anemones cannot perceive color, but others have ocelli containing pigments that specifically absorb certain wavelengths of light. These are usually the blue-green wavelengths that penetrate farthest through water, ensuring that the animals capture the maximum amount of light available.

Some active swimmers, such as jellyfish, have pigmented ocelli. These ocelli contain two different types of pigmented cells: some that shade and some that are light-sensitve. This type of ocellus allows an animal to not only perceive the direction of a light source, but also, because of its multicellular nature, to detect an object moving past it—the shadow is detected by one cell after another as it moves across the ocellus.

—Nora Gallagher