Key Difference – Protonephridia vs Metanephridia
The key difference between Protonephridia and Metanephridia is the type of cells used in excretion. Protonephridia are composed of many flame cells that are ciliated cells used in excretion. Metanephridia are composed of cells that have an internal opening known as nephrostome and an external opening known as nephridiopore.
Nephridia are important structures in excretion. It is important to eliminate the toxic nitrogenous waste from the system. Thus, upon evolution, many types of cells have been studied to have functioned as excretory cells.
CONTENTS
1. Overview and Key Difference
2. What is Protonephridia
3. What is Metanephridia
4. Similarities Between Protonephridia and Metanephridia
5. Side by Side Comparison – Protonephridia vs Metanephridia in Tabular Form
6. Summary
What are Protonephridia?
Protonephridia are cup-shaped structures found in organisms belonging to the phyla Platyhelminthes, Nemertea, Rotifera, and some Chordates such as lancelets. Protonephridia are one of the most primitive types of cells that evolved to function as excretory cells. Protonephridia form a network of tubules that only has an external opening and is devoid of an internal opening. These ends of the protonephridia are referred to as flame cells. These flame cells can either be flagellated or ciliated. Flagellated flame cells known as Solenocytes are mainly involved in ionoregulation. Ciliated flame cells are involved in osmoregulation.
Figure 01: Protonephridia
The beating of the cilia in the flame cells creates a current that travels outward. Due to this, a pressure is built within the blind end of the tube. Due to this pressure, waste fluid is pulled through the perforations into the protonephridium. Then the waste fluid will be passed to the exterior, via the tube through the nephridiopore. Protonephridia also are involved in the removal of excess water from the body when placed in a hypotonic environment.
What is Metanephridia?
Metanephridia is found among many invertebrates such as Annelids, Arthropods and Molluscs. They are more accurately referred to as a type of an excretory gland. Metanephridia is composed of ciliated funnel-like opening structures. These funnel-like structures are called as nephrostome. They open to the coelom of the organism via a duct. This duct is heavily granularized and folded. The duct has an opening to the organism’s exterior.
Figure 02: Metanephridia
The ciliated tubules are involved in pumping toxic waste, water, intracellular protein waste out of the body via the nephrostomes. The waste that passes through the nephrostome is released to the exterior via the nephridiopore. Selective reabsorption takes place where the primary urine is modified to secondary urine by filtration.
What are the Similarities Between Protonephridia and Metanephridia?
- Both Protonephridia and Metanephridia are involved in the elimination of toxic waste and excess water in the body.
- Both Protonephridia and Metanephridia can be ciliated.
- Both structures contain a network of tubules.
- Both structures have an opening to the exterior known as the nephridiopore which the waste fluid is released.
What is the Difference Between Protonephridia and Metanephridia?
Protonephridia vs Metanephridia |
|
| Protonephridia is composed of many flame cells that are ciliated or flagellated cells used in excretion. | Metanephridia are composed of cells that have an internal opening known as nephrostome and an external opening known as nephridiopore. |
| Type of Cells Involved | |
| Flame cells are present in the protonephridia. | Nephrostomes are present in metanephridia. |
| Presence of Flagellated Cells | |
| Present in solenocytes | Absent |
| Structure | |
| Protonephridia are cup-shaped cells. | Metanephridia are funnel-like structures. |
| Selective Re absorption | |
| Selective reabsorption does not take place in protonephridia. | Selective reabsorption takes place in metanephridia. |
| Internal Opening | |
| Internal opening is absent in protonephridia. | Internal opening is present in metanephridia. |
| Glandularized Structures | |
| Glandularized structures are not observed in protonephridia. | Glandularized structures are observed in metanephridia. |
| Presence of Perforations | |
| Perforations are present in protonephridia. | Perforations are absent in metanephridia. |
| Examples | |
| Organisms of the phyla Platyhelminthes, Nemertea, Rotifera and some Chordates such as lancelets have protonephridia. | Organisms belonging to the phyla Annelida, Arthropoda and Mollusca, have metanephridia. |
Summary – Protonephridia vs Metanephridia
Protonephridial and the Metanephridial systems are two primitive systems used by organisms for the excretion process. Protonephridia are composed of ciliated or flagellated flame cells that aid in the release of waste fluid via the nephridiopore. Metanephridia are composed of funnel-like structures known as nephrostome with an interior opening that collects the waste fluid from the body cavity. The waste fluid then passes through the tubules and are released via the nephrostome. This is the difference between protonephridia and metanephridia.
Reference:
1.Baeumler, Natalie, et al. “Development of the Excretory System in a PolyplacophoranMollusc: Stages in Metanephridial System Development.” Frontiers in Zoology, BioMed Central, 2012. Available here
2.Ax, P. “Protonephridia and Metanephridia ‐ Their Relation within the Bilateria.” Journal of Zoological Systematics and Evolutionary Research, Wiley/Blackwell (10.1111), 27 Apr. 2009. Available here
Image Courtesy:
1.’Flamecell’By Anilocra (Public Domain) via Commons Wikimedia
2.’Figure 41 02 02’By CNX OpenStax , (CC BY 4.0) via Commons Wikimedia