548 



INTERRELATIONSHIPS OF ORGANISMS 



very small changes in temperature, hardly more than 5°C. at most. In 

 most parts of the United States the annual temperature range in lakes 

 and deep ponds is only 15 to 30°C. 



The medium and respiration. All organisms, irrespective of the 

 medium in which they live, must be able to secure oxygen and excrete 

 carbon dioxide. Except for a few protozoa, a few algae, and a number 

 of bacteria, which are able to liberate combined oxygen from various 

 of its compounds, all organisms depend upon a gaseous interchange with 

 the medium that surrounds them to meet this need. 



Fig. 32.4. One of the largest food strainers. The right whale (Balaena bisayensis) has 

 "whalebone" (baleen) strainers hanging from the upper jaw. A mouthful of seawater is 

 strained out through these, leaving the food organisms inside the mouth. The chief food 

 of the whalebone whales is small shrimplike crustaceans which are often enormously 

 abundant in ocean plankton. {Courtesy American Museum of Natural History.) 



For terrestrial animals living in an atmosphere that is one-fifth oxygen 

 and much less than 1 per cent carbon dioxide, the chief problems of 

 respiration are to provide a sufficiently extensive respiratory surface for 

 gaseous interchange and to protect such a surface from too great a water 

 loss. (All membranes permeable to oxygen and carbon dioxide are also 

 freely permeable to water vapor.) Many small terrestrial animals solve 

 this problem by utilizing the body surface for respiration and avoid 

 desiccation by remaining within permanently damp situations. A much 

 more successful and less restricted terrestrial existence has been achieved 

 in other animals by the development of special, internal respiratory 

 surfaces that are protected from evaporation and must be aerated by 

 special breathing movements. Such structures are the lungs of the verte- 

 brates, the tracheae of insects, and the modified internal gills of arachnids 

 ("book lungs") and of terrestrial mollusks. 



