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Multicellular organisms are those organisms consisting of more than one cell, and having differentiated cells that achieve specialized functions. Most life that can be seen with the naked eye is multicellular, as are all animals and plants. A group of similarly differentiated cells performing a function in a multicellular organism is known as a tissue. Although some single-celled organisms, such as mycobacteria, have differentiated cells, the differentiation is less dramatic than that usually found in multicellular organisms.

A multicellular organism can work on a number of levels. For instance, a sponge works on a cellular level as the different cells are almost like many separate organisms co-operating jointly for a common goal. This is illustrated by the fact you can put a sponge in a blender and the cells of the sponge will aggregate again to form another sponge. They have no tissues, organs or organ systems which require a higher level of organization. More advanced organisms such as jellyfish, coral and sea anemones work on a tissue level of organization, as their cells can aggregate to form different tissues. For instance, a jellyfish has an outer epidermis and an inner gastrodermis, an example of different tissues to serve a different purpose.