The Simplest Sight

The most elementary sight organs are not eyes at all, but light-sensitive areas called eyespots which can only detect differences between light and dark. Many simple forms of marine life, like the one-celled Euglena, have eyespots to orient them toward the light, which they use to manufacture energy in much the same way plants do. The eyespots, which contains one or more specks of pigment, also is useful in directing the organism away from the surface during periods of strong sunlight, when the sun's rays could injure it.

During millions of years of evolution, some marine creatures have developed highly specialized and often complex eyespots. The scallop has from 50 to 200 such spots, each of which functions like a simple but unfocused eye.

Unique among microscopic marine life is the pinhead-sized Copilia, which can actually form images by means of its crude visual system. Some scientists think the Copilia may be an evolutionary link in the progression of the organs of vision from eyespots to more complex eyes.


Life Science Library - Light and Vision