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Thenephridium (pl.:nephridia) is aninvertebrate organ, found in pairs and performing a function similar to thevertebrate kidneys (which originated from the chordate nephridia). Nephridia removemetabolic wastes from an animal's body. Nephridia come in two basic categories:metanephridia andprotonephridia. All nephridia- and kidney- having animals belong to thecladeNephrozoa.
Ametanephridium (meta = "after") is a type ofexcretorygland found in many types ofinvertebrates such asannelids,arthropods andmollusca. (In mollusca, it is known as theBojanus organ.)
A metanephridium typically consists of the nephrostome (a ciliated funnel) opening into thebody cavity, connected to aduct which may be variously glandularized, folded or expanded (vesiculate) and which typically opens to theorganism's exterior. These ciliated tubules pump water carrying surplusions,metabolic waste,toxins fromfood, and uselesshormones out of the organism by directing them down funnel-shaped bodies callednephrostomes. This waste is passed out of the organism at thenephridiopore. The primaryurine produced by filtration of blood (or a similarly functioning fluid) is modified into secondary urine throughselective reabsorption by thecells lining the metanephridium.
Thesaccate metanephridia are excretory glands which function similarly to the metanephridia. They are found in thearthropods:coxal glands of arachnids, antennal (or green) glands and maxillary glands of crustaceans, etc.
The saccate metanephridia filter the fluid of thehemocoel, as opposed to the metanephridia which filtercoelomic fluid. In a saccate metanephridium, there is a ciliated funnel covered with a membrane that helps to filter the hemocoel ofheavy particles (such asproteins andcarbohydrates) before the fluid even enters the funnel. Inside the funnel, the fluid is further processed through selective reabsorption, and eventuallyexcreted from thenephridiopore.
InCrustacea, the saccate metanephridia are associated with the antennae and form theantennal gland. In freshwatercrustacea, 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 arehypertonic to the surrounding water.
Aprotonephridium (proto = "first") is found in the phylaPlatyhelminthes,Nemertea,Rotifera andChordata (lancelets). They have the same anatomy as themetanephridia but with the internal ciliated funnel blocked by terminal cells: either aflame cell (if ciliated) or asolenocyte (if flagellated). Thus theirtubules lack internal openings, while retaining their opening to the organism's exterior. They function inosmoregulation (ionoregulation).
Each terminal cell has one or morecilia and their beating inside the protonephridial tube creates an outward going current and hence a partialpressurization in the blind of the tube. Because of this, pressurization driveswaste fluids 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 smallmolecules to pass, but largerproteins 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 thenephridiopore.
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 asflatworms. 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 phylaPlatyhelminthes and are also called blind tubules. These tubules bear a tuft ofcilia orflagellum. 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.
Cephalopod anatomy | ||||||||||||||
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| Shell |
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| Mantle & funnel |
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| Head & limbs |
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| General | ||||||||||||||
Developmental stages:Spawn →Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult •Egg fossils •Protoconch (embryonic shell) | ||||||||||||||