nephridiostome

anatomy

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July 1, 2026

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The term "nephridiostome" refers to the internal opening of a nephridium, which is a unit of the excretory system in many invertebrates. This structure allows fluid from the body cavity to enter the nephridial tubule for waste filtration and expulsion from the body.123

Nephridia are crucial for eliminating metabolic wastes, preventing the need for all cells to be in direct contact with the external environment for diffusion. There are two main types: protonephridia, found in simpler invertebrates like flatworms, and metanephridia, present in more complex, segmented animals such as earthworms. In earthworms, the nephridiostome is the entry point for body fluid into the tubule, where useful substances are reabsorbed, and waste is processed into a dilute urine before excretion.13

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excretion
The nephridia of annelids, nemertines, flatworms, and rotifers
…an internal opening called the nephridiostome. As the fluid passes along the tubule, probably driven by cilia, its composition is modified. In the two lower regions of the tubule the fluid becomes progressively more dilute, presumably as a result of the reabsorption of salts. Finally, a very dilute urine passes…
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…an internal opening called the nephridiostome. As the fluid passes along the tubule, probably driven by cilia, its composition is modified. In the two lower regions of the tubule the fluid becomes progressively more dilute, presumably as a result of the reabsorption of salts. Finally, a very dilute urine passes…
Read More