Difference Between Protonephridia And Metanephridia With Examples

Nephridium is unit of the excretory system in many primitive invertebrates and also in the amphioxus; it expels wastes from the body cavity to the (usually aquatic) exterior. The evolution of nephridia encouraged tissue specialization by eliminating the need for all cells of an organism to be in contact with seawater for diffusion of metabolic wastes.

Nephridia occur in two forms:

• The simpler, more primitive protonephridia, found in flatworms, ribbon worms, and rotifers, are usually scattered among the other body cells.
• The more advanced metanephridia, found in earthworms, it usually arranged in pairs.

What Is Protonephridia?

A protonephridium is a network of dead-end tubules lacking internal openings, found in the phyla Platyhelminthes, Nemertea, Rotifera and Chordata (lancets). The ends are referred to as flame cells (if ciliated) or Solenocytes (if flagellated); they function in osmoregulation.

The terminal cells are located at the blind end of the protonephridium. Each 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 waste 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 small molecules to pass, but larger proteins are retained within the animal.

From the bottom of the protonephridium the solute 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 flatworms. 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.

What You Need To Know About Protonephridia

• Protenephridia refers to the tubular excretory structures in certain invertebrates, usually ending internally in flame cells and having an external pore.
• Protonephridia are a network of dead-end tubules without internal openings.
• Protonephridia occurs in phyla Platyhelminthes, Nemertea, Rotifera and chordata.
• Protonephridia occur in both coelomates and acoelomates.
• Internal opening is absent in protonephridia.
• Selective re-absorption does not take place in protonephridia.
• Flame cells are present in the protonephridia.
• Glandularized structures are not observed in protonephridia.
• Perforations are present protonephridia.

What Is Metanephridia?

Metanephridium is a type of excretory gland found in invertebrates such as annelids, arthropod and mullusca.  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 (vesiculated) 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.

What You Need To Know About Metanephridia

• Metanephridia refers to the primitive excretory organs present in many invertebrates, originating in a ciliated coelomic funnel.
• Metanephridia is composed of ciliated funnel-like opening structures.
•  Metanephridia occur in the phyla Annelida, Arthropoda and Mollusca.
• Metanephridia occur in coelomates. 
• Internal opening is present in metanephridia.
• Selective re-absorption takes place in metanephridia.
• Nephrostomes are present in metanephridia.
• Glandularized structures are observed in metanephridia.
• Perforations are absent in metanephridia.

Also Read: Difference Between Deuterostomes And Protostomes

Difference Between Protonephridia And Metanephridia In Tabular Form