What is the Difference Between Protonephridia and Metanephridia

The main difference between protonephridia and metanephridia is that protonephridia are a network of dead-end tubules without internal openings, whereas metanephridia are a type of excretory glands with a ciliated funnel opening into the body cavity. Furthermore, protonephridia occur in the phyla Platyhelminthes, Nemertea, Rotifera, and Chordata (Lancelets), while metanephridia occur in the phyla Annelida, Arthropoda, and Mollusca.   

Protonephridia and metanephridia are two types of nephridia that mainly occur in invertebrates. Generally, they are functionally analogous to the vertebrate kidney and are responsible for the removal of metabolic waste from the body. 

Key Areas Covered 

1. What are Protonephridia
     – Definition, Structure, Importance
2. What are Metanephridia
     – Definition, Structure, Importance
3. What are the Similarities Between Protonephridia and Metanephridia
     – Outline of Common Features
4. What is the Difference Between Protonephridia and Metanephridia
     – Comparison of Key Differences

Key Terms 

Acoelomates, Coelomates, Metanephridia, Nephridia, Protonephridia

What are Protonephridia 

Protonephridia are a type of nephridia that occur in invertebrates, including the phyla Platyhelminthes, Nemertea, Rotifera, and Chordata (lancelets). However, protonephridia are homologous organs among bilaterians. The main characteristic feature of protonephridia is that they are closed by a flame cell. Therefore, they do not open to the body cavity. Basically, they might appear as a pair of monociliated organs originated from the ectoderm. Moreover, each protonephridium contains a single terminal, duct, as well as a nephron cell. 

Figure 1: Protonephidium

Furthermore, the terminal cell of the protonephridia is the flame cell. Although the flame cell has only one rootlet, it is surrounded by eight long and strong microvilli. On the contrary, the flame cell connects to an adjacent duct cell by desmosomes.  

What are Metanephridia 

Metanephridia are the other type of nephridia which occur in invertebrates, including the phyla Annelida, Arthropoda, and Mollusca.  Generally, metanephridia are a ciliated funnel, which opens into the body cavity.  This opening connects to a duct, which is folded or expanded. After that, the duct opens into the exterior. Moreover, the main function of these ciliated tubules is to direct excess ions, toxins, useless hormones, and metabolic wastes towards the funnel. 

Figure 2: Metanephridium

Moreover, the saccate nephridia are a type of metanephridia which occur in arthropods: coxal glands of arachnids, antennal (or green) glands and maxillary glands of crustaceans, etc. The main function of this type of metanephridia is to filter the fluid of the hemocoel. 

Similarities Between Protonephridia and Metanephridia 

• Protonephridia and metanephridia are two types of nephridia, which are excretory glands in invertebrates. 
• Both occur in bilaterians. 
• Moreover, they occur in pairs. 
• They are morphologically analogous to kidneys of vertebrates. 
• The main function of nephridia is to remove metabolic waste. 
• Furthermore, they initially filter and subsequently modify body fluids.  

Difference Between Protonephridia and Metanephridia 

Definition 

Protonephridia refer to the tubular, excretory structures in certain invertebrates, usually ending internally in flame cells and having an external pore, while metanephridia refer to primitive excretory organs present in many invertebrates, originating in a ciliated coelomic funnel. Thus, this is the main difference between protonephridia and metanephridia.

Structure 

While protonephridia are a network of dead-end tubules without internal openings, metanephridia are a type of excretory glands with a ciliated funnel opening into the body cavity.  

Flame Cell 

Also, another important difference between protonephridia and metanephridia is that protonephridia are closed by a flame cell, while metanephridia have an opening to the body cavity and are not closed by flame cells. 

Occurrence 

Furthermore, protonephridia occur in the phyla Platyhelminthes, Nemertea, Rotifera, and Chordata (lancelets) while metanephridia occur in the phyla Annelida, Arthropoda, and Mollusca.   

Coelomates and Acoelomates 

Moreover, protonephridia occur in both coelomates and acoelomates, while metanephridia occur in coelomates. 

Conclusion 

Protonephridia is a type of primitive excretory organ which occurs in Platyhelminthes, nemerteans, rotifers, and lancelets. Moreover, it is a network of dead-end tubules. Also, it does not contain an internal opening. On the other hand, metanephridia is another type of primitive excretory organs which occur in annelids, arthropods, and mollusks. However, it is a gland that contains a ciliated funnel opening into the body cavity. Therefore, the main difference between protonephridia and metanephridia is the structure of nephridia. 

References:

1. Bartolomaeus, Th., and P. Ax. “Protonephridia and Metanephridia – Their Relation within the Bilateria.” Journal of Zoological Systematics and Evolutionary Research, vol. 30, no. 1, 2009, pp. 21–45., doi:10.1111/j.1439-0469.1992.tb00388.x.

Image Courtesy:

1. “Flamecell” By Anilocra at en.wikipedia – Transfered from en.wikipedia by User:Vojtech.dostal. (Public Domain) via Commons Wikimedia   
2. “Metanephridium” By KDS4444 – Own work (CC BY-SA 4.0) via Commons Wikimedia