CHAPTER XLIV 



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 



