Nephridium

The nephridium (plural nephridia) is an invertebrate organ which occurs in pairs and performs a function similar to the vertebrate kidney. Nephridia remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia, but there are other types.

A metanephridium (meta = "after") is a type of excretory gland found in many types of invertebrates such as annelids, arthropods and mollusca. (In mollusca, it is known as the Bojanus organ.)

A metanephridium typically consists of a ciliated funnel opening into the body cavity or coelom connected to a duct which may be variously glandularized, folded or expanded (vesiculate) and which typically opens to the organism's exterior. These ciliated tubules pump water carrying surplus ions, metabolic waste, toxins from food, and useless hormones out of the organism by directing them down funnel-shaped bodies called nephrostomes. This waste is passed out of the organism at the nephridiopore. The primary urine produced by filtration of blood (or a similar functioning fluid) is modified into secondary urine through selective reabsorption by the cells lining the metanephridium.

The saccate metanephridia are excretory glands which function similarly to the metanephridia. They are found in the arthropods: coxal glands of arachnids, antennal (or green) glands and maxillary glands of crustaceans, etc.

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