Metanephridia

Metanephridia are different than protonephridia because they obtain fluids directly from the body cavity or coelom. Earthworms have one pair of metanephridia per segment. What makes their system interesting is that the entrance to each pair of metanephridia is located in one segment and the remaining excretory system is located in the following segment.

• What is the advantage of having the entrance to an excretory organ in one segment and the rest of it in another segment, and so ad infinitum?
• Why are earthworms restricted to semiaquatic environments?

Earthworms are similar to advanced animals in that blood is pumped under pressure which causes fluids and small molecules to pass through capillary walls and enter the coelom. The coelomic fluid is then picked up by a ciliated funnel called the nephrostome. From the nephrostome, fluids move through the collecting tubules and bladder before being eliminated. As the coelomic fluid passes through the metanephridium, ions and essential molecules are reabsorbed leaving a hypotonic fluid that leaves the earthworm through the nephridiopore. In addition, select molecules can be secreted into the metanephridia for elimination.

• What role does sodium have in the excretion of an earthworm?
• Do lobsters have metanephridia?
• How do metanephridia differ from true nephridia?

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