All parts of a unicellular organism are in close contact with the external environment. This is not the case with most of the cells in a multicellular animal. These cells are bathed in tissue fluids which provide their immediate environment. Known as the internal environment, tissue fluids play an important role in the functioning of the cells of multicellular animals. The conditions in the internal environment, such ad temperature, pH and concentration of solutes, must he maintained at levels that are optimal for the cells. Poisons and unwanted substances can pollute the internal environment and harm the cells, and hence, the animal as a whole.
To get rid of unwanted materials from the internal environment, multicellular animals have developed excretory systems and mechanisms.
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