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Nephridia are invertebrate organs which function similarly to kidneys. They remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia.
Metanephridia
A metanephridium (pl. metanephridia) is a type of excretory gland or nephridium found in many types of invertebrates such as annelids, arthropods and molluscs. It 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 through openings known as nephrostomes. This waste is passed out of the body at the nephridiopore. The primary urine produced by filtration of blood (or a similar functioning component) are modified into secondary urine through selective reabsorption by the cells lining the metanephridium.
In many earthworms the nephridial ducts open into the digestive tract instead, a condition known as enteronephry.
Protonephridia
A protonephridium is a network of dead-end tubules lacking internal openings. The ends are called flame cells (if cilliated) or solenocytes (if flagellated); they function in osmoregulation. The terminal cells are located in the bottom of the protonephridium. Each cell has one or several cilia and when they beat inside the protonephridial tube they create an outward going current and hence a partial vacuum in the bottom of the tube. Because of this partial vacuum waste fluids from the inside of the animal are pulled through small perforations in the terminal cells and into the protonephridium. The perforations in the terminal cell are large enough for small molecules to pass, but larger proteins are retained within the animal. From the bottom of the protonephridium the waste fluid is led through the tube, formed by the canal cells, and exits the animal from a small opening formed by the nephridiopore. Protonephridia are found in simpler organisms such as flatworms.
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