Freshwater and terrestrial invertebrates have a wide variety of adaptations for maintaining salt and water balance and excreting nitrogen. Here we explore three examples of invertebrate excretory systems: protonephridia, metanephridia, and Malpighian tubules. Each of these systems produces an extract of interstitial fluid lacking large molecules. They then change the solute composition (ions and small molecules) of that fluid to form an excretory product.
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Vertebrates have evolved to occupy fresh and salt water and dry terrestrial habitats.
Metanephridia of annelids filter blood from their closed circulatory systems and process the resulting filtrate by secretion and reabsorption in a system of tubules.
Malpighian tubules use active transport of solutes, including uric acid, to pull extracellular fluid into the tubules osmotically; precipitation of the uric acid enables reabsorption of water.