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The nephridium (: nephridia) is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove Metabolism wastes from an animal's body. Nephridia come in two basic categories: metanephridia and protonephridia. All nephridia- and kidney- having animals belong to the clade Nephrozoa.
A metanephridium typically consists of the nephrostome (a ciliated funnel) opening into the 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 , metabolic waste, from food, and useless out of the organism by directing them down funnel-shaped bodies called . This waste is passed out of the organism at the nephridiopore. The primary urine produced by filtration of blood (or a similarly functioning fluid) is modified into secondary urine through selective reabsorption by the cells lining the metanephridium.
The saccate metanephridia filter the fluid of the hemocoel, as opposed to the metanephridia which filter coelomic fluid. In a saccate metanephridium, there is a ciliated funnel covered with a membrane that helps to filter the hemocoel of molecular weight particles (such as and ) before the fluid even enters the funnel. Inside the funnel, the fluid is further processed through selective reabsorption, and eventually excretion from the nephridiopore.
In Crustacea, the saccate metanephridia are associated with the antennae and form the antennal gland. In freshwater crustacea, the saccate metanephridia are especially large due to their role in osmoregulation; crustacea must remove large amounts of water from the tissues, as the cells are hypertonic to the surrounding water.
Each terminal cell has one or more cilia and their beating inside the protonephridial tube creates an outward going current and hence a partial pressurization in the blind of the tube. Because of this, pressurization drives Metabolic waste from the inside of the animal, and they are pulled through small perforations in the terminal cells and into the protonephridium. The perforations in the terminal cell are large enough for small to pass, but larger are retained within the animal. From the bottom of the protonephridium the solutes are led through the tube, formed by the canal cells, and exits the animal from a small opening formed by the nephridiopore.
Selective reabsorption of useful molecules by the canal cells occurs as the solutes pass down the tubule. Protonephridia are generally found in basal organisms such as . Protonephridia likely first arose as a way to cope with a hypotonic environment by removing excess water from the organism (osmoregulation). Their use as excretory and ionoregulatory structures likely arose secondarily.
These are excretory systems in phyla Platyhelminthes and are also called blind tubules. These tubules bear a tuft of cilia or flagellum. An organ of excretion in flatworms: a hollow cup-shaped cell containing a bunch of cilia or flagellum, whose movement draws in waste products and wafts them to the outside through a connecting tubule.
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