UNICELLULAR FORMS 73 



relationship is called symbiosis. Symbiotic relations are not confined 

 to this one case, however, for the phenomenon is not a rare one 

 among other forms. In Paramoecium bursaria it has been found 

 that when the animals are infected with zoochlorella they may 

 live without an external food supply for over a year. If the zoo- 

 chlorella are destroyed and no external food supply is present, the 

 paramcecia very soon die. The relations of the two organisms are 

 that the paramoecium, in its respiratory metabolism, affords the 

 plant cell a constant supply of carbon dioxide, which, in the pres- 

 ence of sunlight and water, it converts into a carbohydrate. The 

 plant cell thus supplies an internal source of carbohydrate for 

 the energy-requiring processes of the paramoecium, for apparently 

 the plant manufactures enough carbohydrate for both its own and 

 the requirements of the animal cell. Thus, in the absence of bacteria 

 or other outside food, the paramoecium subsists on the food made 

 by the zoochlorella. In turn, the plant cells have at hand a constant 

 supply of carbon dioxide for the photosynthetic processes. 



Symbiotic relationships similar in nature are not unusual; some- 

 times two plants are associated, as in the lichens, sometimes two 

 animals are associated, as in the intestinal Protozoa in certain in- 

 sects; sometimes, the association is that of a plant and an animal, 

 as in this example. A less intim.ate association, which is merely 

 mechanical in nature, is that of commexsalism. In such associa- 

 tions one animal merely serves as the motive power to carry an- 

 other type of animal to its food supply, or one animal follows the 

 other around and thus finds its food. An example of commensalism 

 is the shark sucker, a type of fish that attaches itself to the dogfish 

 shark and is carried about. When the shark makes a kill, the shark 

 sucker is present and thus is assured of food. 



Parasitism. Sometimes an invading animal or plant lives on 

 the substance of, and harms the supporting animal or plant. This 

 is termed parasitism. The invading organism is called a parasite 



