Most amphibians, such as frogs and salamanders, have color vision, even though they may be restricted to narrower bands of the visible color spectrum than are humans. There are at least four types of photoreceptors or optical sensors with different chemical composition that allow seeing different amounts of color by amphibians. Since amphibians have been around a long time and have adapted to a variety of environments, their eye configurations vary slightly for different environments. Amphibian eyes contain photoreceptors like those of some fish. Some fish are vulnerable to the changes of light intensity as it varies with water depth. There are some amphibians with nearly-complete 360-degree vision. They have a neural mechanism to contract their pupils, like some species of fish. The following are a few examples of amphibian eyes.
Amphibian eyes focus the same way as a camera: the lens moves along the optic axis of the eyeball toward or from the retina. They have upper and lower eyelids, and nictitating membrane or the third eyelid, which protects the surface of the eyes from drying. Some amphibians, mainly terrestrial ones, have also lachrymal glands, which continuously produce tears to prevent drying out. And frogs, for example, perceive nonmoving objects as background.
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