14 



NATURAL HISTORY 



Animals definitely respond by movement to the stimulus of light, 

 but unlike green plants, some respond positively and others nega- 

 tively. The unicellular Ameba is killed by too much light. Earth- 

 worms and some other animals are definitely repelled by light. The 

 moth, on the other hand, is attracted to light. Although of great 

 importance, light may be injurious to some forms, for bacteria and 

 some animals are killed by long exposure to it. The dangers from 

 certain wave lengths of light are seen in a bad case of sunburn. 



Light influences animals in other ways. Light stimulus coming 

 through the eyes of flounder is said to give rise to changes in the pig- 

 ment of the skin. Thus the surface of the skin takes on the general 

 color and markings of its background. Some animals in caves lack 

 pigment, and there seems to be a general relationship between light 

 and pigment in the skin. There is a day and night rhythm in the lives 

 of many animals. Land snails feed at night, while activities of most 

 birds are confined to the daytime. Bees go to flowers during day- 

 light. Migrations of plankton are influenced by light, many crusta- 

 ceans coming to the surface only at night and going deep down into 

 the water during the daytime. 



A dry alkali lake. 



Life is practically absent in such areas. 

 why this is so.'> 



II riiilil I'll rcc 



Can you explain 



