ANIMALS AND THEIR ENVIRONMENT 285 



The color of flatfishes, certain shrimps, and some other animals 

 changes to correspond to the background on which they rest. When 

 on a dark background, the pigment diffuses so as to fill the cells that 

 contain it and in the aggregate makes the animal dark. When on a 

 light background, the pigment collects into small knots, leaving much 

 of the surface exposed; hence the animal is pale. These changes may be 

 a concealing adaptation helping the animals to escape enemies. 



]Many animals respond to light with changes of behavior, some of 

 which are of ecological significance. Isopods, the "pill bugs" or "sow 

 bugs" that live under boards or stones or in other dark places, are driven 

 into these places by their negative reaction to light. Such situations 

 are generally moist, which is necessary for an animal which, like the pill 

 bugs, respires by means of gills. Most other Crustacea live in water, but 

 some of the pill bugs have taken to land and have done so by utilizing 

 damp places. Their crevices also doubtless give them some useful 

 protection. 



Some animals change their response to light according to certain 

 other conditions. A species of thrips, a minute flower-inhabiting insect, 

 crawls away from the soui'ce of light when it is quiet but is positive to 

 light when mechanically disturl^ed. Under ordinary conditions these 

 reactions drive the insect into the flower (a clover head, for example) 

 ^vhere its food is; but if the flower is vigorously shaken, as by a grazing 

 animal, it crawls out. Probably their lives are often saved by this 

 behavior. 



A more complicated adaptation involving response to light is exhibited 

 by a parasitic copepod (crustacean) named Lernaeopoda. This animal 

 is free-living in its larval stage but must attach itself to the gills or some 

 other part of the brook trout to complete its development. During 

 the day the larval copepod, because it is positive to light, swims near the 

 surface of the water, but at night it sinks to the bottom because it is 

 heavier than water. The brook trout likewise swims near the surface 

 in the daytime, either in response to light or in deliberate search for food 

 organisms which are located there, but at night settles to the bottom 

 because of its high specific gravity. Day and night, therefore, fish and 

 copepod are brought together — an arrangement highly satisfactory for 

 the parasite but not so advantageous for the host. 



Moisture. — All organisms contain in their protoplasm a certain 

 amount of water, usually a very large amount. Without it they are 

 unable to function as living things. Many of them are so constructed 

 as to be unable to maintain this required water without living directly 

 in water. Probably no animal can endure complete desiccation, though 

 there are some that can exist for a long time in situations regarded as dry. 

 Protozoa may secrete a thick wall (cyst) and lie in dry hollows (former 



