The metanephridia are a type of excretory gland found exclusively in invertebrates belonging to the group annelid worms or some molluscs and arthropods.
Metanephridiums have the function of eliminating waste resulting from the metabolic process and are responsible for maintaining the concentration of all kinds of substances within the worm's body regulated.
Within the animal kingdom, both vertebrate and invertebrate animals can be found. This second group is characterized because the animals that compose it do not have a vertebral column or an articulated internal skeleton. It includes worms or annelids, mollusks and arthropods.
Most invertebrates have a waste excretion system composed of nephridiums, which can be protonephridium (flame cells) or metanephridium.
These systems are glands that perform a similar function to that of the kidneys in other animals. Unlike the protonephridia, the tubules of the metanephridia lack flaming cells and open directly into the body cavity known as the coelom.
The cilia that line the interior of each tubule create an empty space through which fluids can be led to the outside.
During this process, the cells that line the walls of the metanephridiums are responsible for reabsorbing important nutrients still contained in the fluids as they pass through the tubule.
Structure of metanephridiums
Metanephridiums are glands that perform the function of the kidneys in worms or annelids. Its structure is composed of a group of tubules, usually a pair of them for each coelom (a cavity dedicated to facilitating the transport of substances into the body of the annelid). These tubules are open at both ends.
The first end of the tubules is located inside the cavity of the coelom, forming a structure similar to that of a funnel.
At this end it is known as a nephrostoma and it has several cilia that surround it, directing the flow of substances into the coelom. The other end is called the nefidiopore and is located outside the body.
The nephrostoma is a cilia-filled duct that opens inside the coelom. On the other hand, the nephriodopore has numerous glands, in this way its size can increase or decrease thanks to the action of vesicles that help it eliminate everything that is inside.
The tubules of metanephridiums have the ability to transport liquids through a pumping system and the action of the cilia found in them.
By being able to transport water, they allow excess ions, toxins, wastes and hormones to be eliminated through the nephriodopore.
The urine produced by the annelid blood filtration process is transformed into secondary urine with the help of the cells that cover the metanephridium.
In this way, the chemical composition inside the body of the annelids is regulated, extracting only those products that do not contribute anything and whose concentration is high.
Function
Metanephridia serve the same function as flamboyant cells in other invertebrates. These are responsible for removing waste from the body of annelids, some mollusks and arthropods.
They are more complex excretory structures than protonephridiums, since they are open at both ends, giving way more quickly and easily to the fluids found in them. On the other hand, they are highly vascularized, which is why they can contribute to the urine manufacturing process.
By having a double opening that allows them to be connected to the exterior and to the coelom, the metanephridiums are able to receive the waste materials collected in the coelom, transporting it, filtering it a second time and finally sending it abroad for disposal. In other words, the metanephridiums drain the liquid present into the coelom.
When the waste fluids or urine pass from the coelom to the metanephridiums, their concentration is isotonic, however, when passing through the tubules of the metanephridiums, all the salts are removed, leaving the urine as a more dilute substance.
In this way, the function of metanephridiums can be understood as if they were kidneys, since their objective is to filter and reabsorb the substances contained in the urine, forming an initial solution through a filtration process that will later be modified by a process of reabsorption as it passes through the tubules.
Appearance
The appearance, shape and size of both the coelom and the metanephridiums can vary depending on the species of invertebrate that has them.
Annelids or worms
In annelids the coelom is elongated, therefore, their body has different sets of metanephridiums, usually one pair for each ring of the body.
Mollusks
In the case of mollusks, the coelom appears as a cavity that includes both the pericardium and the kidneys, therefore, the metanephridiums look as if they were a kidney in the body of the mollusks.
Arthropods
Few arthropods use the coelom and metanephridium system to carry out the process of excretion of waste substances.
However, those that do so have small nephridium tubes connected to the coelom, which is a small-sized, thin-walled sac connected to the inner terminal of the excretory or metanephridium tubules.
Regardless of the appearance or size of the coelom and metanephridia, the functions carried out by this system are always the same within the body of any invertebrate.
This is how the metanephridiums are responsible for evacuating the solutions contained inside the coelom, moving them slowly to the outside, while filtering the nutritive substances still present.
In this way, metanephridiums will always be linked to the process of urine formation, its filtration, reabsorption and subsequent evacuation out of the body.
References
- Britannica, TE (2017). Encyclopædia Britannica. Obtained from Nephridium: britannica.com
- Spain, G. d. (sf). Biosphere Project. Obtained from EXCRETOR SYSTEMS IN INVERTEBRATES: resources.cnice.mec.es
- Fanjul, ML, & Hiriart, M. (1998). Nephridios. In ML Fanjul, & M. Hiriart, Functional Biology of Animals (pp. 507-509). Siglo Vientiuno Editores.
- Hine, R. (2015). Metanephridium. In R. Hine, A Dictionary of Biology (p. 368). Oxford: Oxford University Press.
- Recio, CG (November 26, 2015). Paradais-Sphynx. Obtained from Excretory apparatus of animals. System types and examples.:parais-sphynx.com
- Schmidt-Nielsen, K. (2002). Animal Physiology: Adaptation and Environment. New York: Cambridge University Press.