What is the Difference Between Protonephridia and Metanephridia?

Protonephridia and metanephridia are two types of nephridia, which are excretory organs found in invertebrates. They have similar functions to the vertebrate kidney, removing metabolic wastes from the organism's body. The main differences between protonephridia and metanephridia are:

Cell Type: Protonephridia are composed of many flame cells, which are ciliated or flagellated cells used in excretion. Metanephridia, on the other hand, are composed of cells that have an internal opening known as nephrostome and an external opening called the nephridiopore.
Phyla: Protonephridia are found in phyla such as Rotifera, Platyhelminthes, Nemertea, and Chordata (lancets). Metanephridia are found in phyla like Arthropoda, Annelida, and Mollusca.
Structure: Protonephridia have either flame cells or solenocytes and do not have many glandularized structures. Metanephridia have funnel-shaped structures that open into the body cavity or coelom and possess glandularized structures.
Excretion Process: In protonephridia, the activity of cilia or flagella generates an outward current, leading to a partial pressure that propels waste metabolites out of the organism. In metanephridia, the excretory product, derived from blood filtration, is further modified into urine due to selective reabsorption in the cells lining the metanephridium.

In summary, protonephridia and metanephridia are two different types of nephridia that serve as excretory organs in invertebrates. They differ in the types of cells used for excretion, the phyla in which they are found, their structure, and the excretion process.