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Protonephridia and metanephridia work by creating a filtrate of the body fluids that is modified by the secretion and reabsorption of specific substances before being excreted. Insect Malpighian tubules actively secrete uric acid and other solutes into closed tubules.
learning outcomes
You should be able to:
Describe the structure and function of a protonephridium.
Diagram the process of urine formation by a metanephridium.
Explain how Malpighian tubules enable insects to live in very arid habitats.
How do protonephridia enable flatworms to live in fresh water?
Freshwater flatworms have to deal with a constant influx of water into their extracellular fluids by osmosis. The flame cells make it possible to filter that extracellular fluid into the protonephridia without loss of solutes, and the protonephridia can further process the filtrate by secretion and reabsorption to increase elimination of wastes and conserve solutes.
What are the relationships among blood plasma, coelomic fluid, and urine in an annelid worm?
The blood plasma is filtered in capillaries to produce the coelomic fluid, and the two are similar in composition. The coelomic fluid is processed in the metanephridia through processes of tubular secretion and reabsorption to become the urine that is excreted from the body.
How does the insect excretory system conserve water?
The insect excretory system eliminates nitrogenous waste as uric acid. By actively reabsorbing Na+ and K+ in the hindgut and rectum, the insect concentrates the waste fluid to the point that uric acid precipitates out of solution thus lowering the osmolarity of the contents of the hindgut and rectum and enabling more reabsorption of water due the active reabsorption of Na+ and K+.
Having described how several invertebrate groups handle nitrogen excretion, we will next consider the nephron—