A unicellular organism, also known as a single-celled organism is an organism that consists of only one cell, in contrast to a multicellular organism that consists of multiple cells. Historically simple single celled organisms have sometimes been referred to as monads 
Prokaryotes, most protists, and some kinds of fungi are unicellular. Although some of these organisms live in colonies, they are still unicellular. These organisms live together, and each cell in the colony is the same. However, each cell must carry out all life processes in order for that cell to survive. In contrast, even the simplest multicellular organisms have specialized cells that depend on each other in order to survive.
Most unicellular organisms are of microscopic size and are thus classified as microorganisms. However, some unicellular protists and bacteria are macroscopic and visible to the naked eye. Examples include:
- Valonia ventricosa, with a diameter of 1 to 4 centimeters it is one of the largest single-celled organisms.
- Syringammina fragilissima
- Thiomargarita namibiensis
- ^ Monad, Biology online, http://www.biology-online.org/dictionary/Monad, retrieved 30-06-2011
- ^ Max Planck Society Research News Release Accessed 21 May 2009
- ^ Researchers Identify Mysterious Life Forms in the Extreme Deep Sea. Accessed 2011-10-24.
- ^ Bauer, Becky (October 2008). "Gazing Balls in the Sea". All at Sea. http://www.allatsea.net/article/October_2008/Gazing_Balls_in_the_Sea. Retrieved 27 August 2010.
- ^ John Wesley Tunnell, Ernesto A. Chávez, Kim Withers (2007). Coral reefs of the southern Gulf of Mexico. Texas A&M University Press. p. 91. ISBN 1585446173. http://books.google.com/books?id=tu0sqBp8eAAC&pg=PA91.
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