Eyes in Other Animals
The camera type of eye, which forms excellent images, is found in all vertebrates, in cephalopods (such as the squid and octopus), and in some spiders. In each of those groups the camera type of eye evolved independently. In some species, e.g., kestrels, the eye can perceive ultraviolet light, an aid to tracking prey.
Simple eyes, or ocelli, are found in a great variety of invertebrate animals, including flatworms, annelid worms (such as the earthworm), mollusks, crustaceans, and insects. An ocellus has a layer of photosensitive cells that can set up impulses in nerve fibers; the more advanced types also have a rigid lens for concentrating light on this layer. Simple eyes can perceive light and dark, enabling the animal to perceive the location and movement of objects. They form no image, or a very poor one.
The compound eye is found in a large number of arthropods, including various species of insects, crustaceans, centipedes, and millipedes. A compound eye consists of from 12 to over 1,000 tubular units, called ommatidia, each with a rigid lens and photosensitive cells; each omnatidium is surrounded by pigment cells and receives only the light from its own lens. The lenses fit together on the surface of the eye, forming the large, many-faceted structure that can be seen, for example, in the fly. Each ommatidium supplies a small piece of the image perceived by the animal. The compound eye creates a poor image and cannot perceive small or distant objects; however, it is superior to the camera eye in its ability to discriminate brief flashes of light and movement, and in some insects (e.g., bees) it can detect the polarization of light. Because arthropods are so numerous, the compound eye is the commonest type of animal eye.
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