What is the Difference between protonephridia and metanephridia?

Two different kinds of nephridia occur within the Bilateria, protonephridia closed up by a termi- nal cell and metanephridia opening into the coelomic cavity. Both initially filter and subsequently modify intercellular fluids. Whereas metanephridia are strictly correlated to a coelom, proto- nephria occur in acoelomate as well as in coelomate organisms. Protonephridia of different bilaterian taxa correspond to each other in several structural fea- tures. Therefore, it is hypothesized that protonephridia are homologous organs throughout the Bilateria. They must have evolved once as one pair of monociliated organs originating from the ectoderm and consisting of one terminal, one duct and one nephropore cel!. In the ground pattern of the Bilateria the cilium of the terminal cell has only one rootlet and is surrounded by presumably eight strengthened and elongated microvilli. Cilium and microvilli extend into the hollow cylinder of the terminal cell, which is oriented distally and is attached to the adjacent duct cell by desmosomes. This cylinder is perforated by clefts and represents the supporting structure of the filtration barrier consisting of extracellular matrix. In the Annelida and Phoronida, the metanephridia at the postlarval stages are ontogenetically preceded by protonephridia in the larva, but far reaching structural and developmental differ- ences exist between the metanephridia of both. In phoronids the/rotonephridial duct of the larva is retained in the postlarva and acquires a coelothelially derive funnel, whereas in annelids the metanephridia are uniform organs originating from a solid anlage, which is a repetition of the protonephridial anlage of the larva. The differences contradict a homology of the metanephridia In Annelida and Phoronida. We therefore have to conclude that metanephridia must have evolved independently, at least two times. The comparative analysis of nephridia in the Bilateria allows the following hypothesis: Pro- tonephridia were evolved in a monophasic acoelomate organism in the stem lineage of the Bilateria. During the evolution of biphasic life cycles consisting of an acoelomate larva and a coelomate adult, the information about the differentiation of protonephridia has been preserved in the early acoelomate developmental (larval) stages. During postlarval development and the formation of a coelom the protonephridia have either been retained or moditied into meta- nephridia. According to the differences between the metanephridia of phoronids and annelids, we emphasize that there is no possibility to trace back all bilaterian taxa with a coelom to a common stem species.

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sania zulfiqar kaimkhani