THE ANIMAL AND ITS ENVIRONMENT 



91 



about 0.38 micron (violet light) to 0.76 

 micron ( red ) . This extremely narrow range 

 includes the so-called visible spectrum to 

 which our eyes are sensitive. Most animals 

 seem to be sensitive to it also, but some 

 animals are known to respond to wave 

 lengths to which we are insensitive. On the 

 other hand, we respond to wave lengths 

 that some animals are unaware of. Our re- 

 ceptors pick up only about 1/125 of the 

 total range of ethreal vibrations which are 

 constantly being showered on our bodies. 

 Even though visible light is composed of 

 such a small segment of this range, all ani- 

 mals are profoundly affected by it. 



Light has a direct bearing on the orienta- 

 tion of some animals. Moths, for example, 

 fly toward a light, whereas pillbugs avoid 

 bright light. The advantage to the animal 

 in the former case is questionable, but in 

 the latter it has a distinct advantage be- 

 cause the pillbug breathes by means of gills 

 and must seek out damp places. Dark 

 places are more apt to be damp than 

 brightly lighted areas. 



The reproductive cycle of some animals, 

 particularly birds, is definitely influenced 

 by light. If daylight is supplemented by 

 artificial illumination the reproductive or- 

 gans are stimulated to work longer, hence 

 more eggs and young. Farmers have taken 

 advantage of this fact by installing in their 

 chicken houses lights which burn long after 

 the sun goes down. Linked with this is the 

 stimulus that causes migration in at least 

 some birds. Bees are known to determine 

 direction by the angle of the sun (see p. 

 236). 



Some of the lower vertebrates, particu- 

 larly fish and amphibians, have the ability 

 to change color. They usually attempt to 

 match the background upon which they 

 are resting, obtaining the obvious advan- 

 tage of camouflage ( Fig. 5-3 ) . This is done 

 by condensing or spreading out the pig- 

 ments that are confined to special skin cells 

 called chromatophores. Experimentation 

 has shownti that at least one mechanism in- 



». ' iff %. 



Fig. 5-3. A case of concealment by acquiring the color 

 and position of the surrounding environment. Note 

 how the upper part of this swamp eel (Fluta alba) 

 resembles the surrounding eelgrass. 



volves the amount of light that enters the 

 eyes of the animal. This is discussed more 

 fully on p. 430. 



Chemical cycles 



The elements of which all organisms are 

 composed come from the environment and 

 return to the environment upon the death 

 and subsequent decomposition of the or- 

 ganism. There is, then, a constant cycle 

 of the elements. An atom of carbon resid- 

 ing in a protein molecule that goes to make 

 up one of our muscle cells, let us say, may 

 have been incorporated into any carbon- 

 containing molecule of thousands of plants 

 and animals before us, and will become a 

 part of thousands of living things following 

 us. It might be thought of as a kind of 

 "reincarnation," so to speak, but not the va- 

 riety that usually comes to mind when this 

 word is mentioned. All elements found in 

 protoplasm follow specific cycles, two of 



