Excretory systems remove excretions from the body and serve as organs of osmoregulation. Active transport and ultrafiltration are the two basic principles of excretion. The structure of filtration organs corresponds with the type of body cavity being present. Both types of organs — protonephridia and metanephridial systems — filter body fluids, either into a canal system (protonephridia) or into the coelom (metanephridial systems), and remove the excretes in a canal system, where modification takes place. Protonephridia and metanephridial systems differ in the location and structure of these two components. There are different models of excretory organ evolution, but the model considered most likely in this chapter is one which describes unique evolution and subsequent differentiation of protonephridia, and multiple evolution of metanephridial systems.
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