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Excretion is the process of removing wastes such as ammonia, urea, and uric acid from the body. Different organisms have evolved different organs and mechanisms to eliminate waste. Vertebrates have kidneys whereas invertebrates and microorganisms have simple mechanisms to excrete metabolic waste products.

The two excretory systems other than the kidneys discussed here are as follows:

1. Flame cells: They are specialized cells that play a role in the excretion of wastes and osmoregulation in Platyhelminthes such as flatworms. They are found throughout the body and line the tubules. Flame cells collect nitrogenous wastes from the surroundings. The cluster of cilia filters the fluids and pushes the waste into the tubules. The waste from the tubules is passed on to the excretory duct and expelled from the excretory pore.
2. Malpighian tubules: These are excretory organs found in arthropods. They are finger-like structures located between the midgut and hindgut. They collect waste such as uric acid, ions, and water from the hemolymph. As the waste passes through the tubules water and important ions get reabsorbed. The pH becomes acidic and the uric acid starts to precipitate. The solid uric acid waste is passed on to the hindgut and eliminated from the body.