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The close association between two or more organisms of different species, often but not necessarily benefiting each member. The association of algae and fungi in lichens and of bacteria living in the intestines or on the skin of animals are forms of symbiosis. Some scientists believe that many multicellular organisms evolved from symbiotic relationships between unicellular ones and that the DNA-containing organelles within certain eukaryotic cells (such as mitochondria and chloroplasts) are the product of symbiotic relationships in which the participants became interdependent. There are four forms of symbiosis: amensalism , commensalism , mutualism , and parasitism .