Chap. 5 



ANIMALS AND THEIR ENVIRONMENTS 



77 



Fig. 5.11. Larger free swimmers (nekton) of the open coastal waters. Upper 

 left, dolphins, length up to 12 feet; North Atlantic sea turtle (loggerhead), 100 

 to 200 pounds. Center, swordfish, 250 to 400 pounds. Bottom, blue-fin tuna (or 

 marlin), up to 600 pounds. Not drawn to scale. 



Carbon Cycle. Carbon, a main element in protoplasm and its products, is 

 available only in small amounts. Ordinary air contains about 0.035 per cent 

 of carbon dioxide by volume and only a quarter of this is carbon. From this 

 small amount, plants obtain all they use and in turn become the source of 

 carbon for all organisms. The sources of free carbon dioxide are plant and 

 animal respiration, decay of the bodies of plants and animals, and the release 

 from burning oil and coal. From all these sources it is automatically returned 

 to the atmosphere. The only way that it gets back to protoplasm is by green 

 plants. 



Plants take carbon dioxide {CO 2) from the air and with the help of energy 

 from the sun during photosynthesis, produce the valuable food, carbohydrate. 

 When a carbohydrate unites with oxygen, the energy of action and heat and 

 carbon dioxide are set free, the latter in part a waste product respired into the 

 air. One branch of the cycle is thus complete. In another branch of the circuit, 

 carbon is built into the protoplasm. It is locked within the cells until they die, 

 decompose, and free it into the air to unite with oxygen as carbon dioxide 

 (Fig. 5.15). 



Oxygen Cycle. Plants and animals take oxygen (Oo) from air or water in 



