Ecologic Study of Living Organisms 635 



flagellated protozoan animals which prepare the wood for absorption 

 by the termites. In turn for their labors, the Protozoa are given pro- 

 tection by the termites. This mutual benefit results in a distribution of 

 both the termites and their Protozoa in such a way that would not be 

 possible if symbiosis did not exist. In a similar manner, certain green 

 algae (plants) live symbiotically in the body of certain species of Hydra 

 (animal). The green algae manufacture food through the process of 

 photosynthesis in addition to giving oxygen to the Hydra. The latter 

 gives protection and carbon dioxide to the algae. This symbiotic rela- 

 tionship between these two species of plant and animal causes a dis- 

 tribution of both of them that would not exist if symbiosis were not 

 practiced. In the construction of the plants, known as lichens, the 

 green, chlorophyll-bearing algae live symbiotically with the colorless 

 fungi. In this case, two different species of plants live together so as to 

 be mutually beneficial. 



Commensalism literally means "eating at a common table," although, 

 in a more general application, it means the association of two species 

 of organisms, in which one species benefits and the other at least is not 

 harmed. The sea anemone (Fig. 95) may attach itself to the shell of a 

 crab, giving some protection to the crab in return for its food which the 

 crab shares with it. The sea anemones are distributed by the crabs as 

 the latter move from place to place. The various interrelationships of 

 various living plants and animals are considered in more detail in other 

 chapters. 



(e) Distribution Affected by Parasitism, Saprophytism, and Preda- 

 ciousness: These types of association of living organisms also influence 

 the ecologic relationships of the organisms in question. Parasitism is 

 the association of two organisms of different species in which the one, 

 known as the parasite, lives at the expense of the other, known as the 

 host. If the parasite lives within the host, it is known as an endopara- 

 site, such as the liver fluke which lives in the body of a snail or sheep 

 or the parasitic tapeworms or roundworms which live within the bodies 

 of other animals. Various species of roundworms may be parasites 

 within the bodies of plants. If the parasite lives externally on its host, 

 it is known as an ectoparasite. Examples of ectoparasites are lice which 

 live externally on the skins of dogs, cats, and men, the biting lice living 

 on the surface of birds, plant lice (aphids) living on the surface of 

 plants, certain fungi (plants) which cause "athlete's foot" living para- 

 sitically on the body of man. Some species of fungi live parasitically 



