ABSTRACT
As illustrated in Figure 19.1, the earliest form of light-sensitive organ was an unstructured678 spot of photoreceptor protein, called eyespot or pigment spot ocellus, able to sense ambient light levels but not to image the environment, as still found in some flatworms, jellyfishes and sea stars. Throughout evolution, some of these organs depressed into a cavity to form a pit eye or stemma, allowing rough directional sensitivity, as in the case of the infrared-sensing pit organs of pit vipers, while others evolved further into the biological equivalent of a pinhole camera, such as that of the nautilus, potentially followed by the development of one or more648 focusing lenses, as in vertebrates.
