. Elementary biology; an introduction to the science of life . EYES AND LIGHT To us the eye is a seeing organ — that is, a means of distinguishing objects, forms, colors, shades, and Hghts at a distance. It is therefore hard for us to realize, first, how animals can get about with- out such useful or- gans, and, second, how it is possible to be sensitive to light and shade without eyes. Yet many ani- mals are very sensi- tive to light without having any eyes, and many animals get along very well with- out distinguishing between light and darkness. We have already learned that plants are sensit
. Elementary biology; an introduction to the science of life . EYES AND LIGHT To us the eye is a seeing organ — that is, a means of distinguishing objects, forms, colors, shades, and Hghts at a distance. It is therefore hard for us to realize, first, how animals can get about with- out such useful or- gans, and, second, how it is possible to be sensitive to light and shade without eyes. Yet many ani- mals are very sensi- tive to light without having any eyes, and many animals get along very well with- out distinguishing between light and darkness. We have already learned that plants are sensitive to light (p. 38), and that the ameba will respond to sudden changes of illumination (p. 24). From these facts we may infer that protoplasm itself is more or less sensitive to light — that light is a kind of energy that may change the processes that go on in protoplasm. 268. Primitive light perception. In the ameba every part of the body is equally sensitive to light. This is true of the protozoa generally, and also of the simplest plants. There are some one-celled plants, however, in which there is a special region that is particularly sensitive to light. One of the most common of these is the Euglena (see Fig. 93). 229 Fig. 93. Euglena This one-celled organism is capable of moving about by means of the swimming lash, like many animals; it has chlorophyl, like many plants. Near the base of the lash is a reddish speck which is sensitive to light. Although it is often called an eyespot, it is no more like an eye than a grain of powder is like a cannon
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