Protonephridia and Metanephridia - their relation within the Bilateria

Two different kinds of nephridia occur within the Bilateria, protonephridia closed up by a terminal 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 features. Therefore, it is hypothesized that protonephridia are homologous organs throughout the Bilateria. They must have evolved once as one pair of monociliated organs orinatinng from the ectoderm and consistin of one terminal, one duct and one nephropore cell In the ground pattern of the Bilateria the cilium of the terminal cell has only one rootlet and is surrounded by resumably eight strengthened and elongated microvilli. Cilium and microvilli extend into the hollow cyinder 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 metanehridia 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 horonids the rotonephrdial duct of the larva is retained in the postlarva and acquires a coelothelially derived funnel, whereas in annelids the metanephridia are uniform organs orihating from a solid anlage, which is a repetition of the protonehridial anlage of the larva. The differences contradict a homology of the metanephridia in Annegda and Phoronida. We therefore have to conclude that metanephridia must have evolved indeendently, at least two times. The comparative analysis of nephridia in the Bilateria allows the following hyothesis: Pro tonephridia were evolved in a monohasic acoelomate organism in the stem fineage 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 modified into meta nephridia. Accordin to the differences between the metanehridia of phoronids and annelids, we emphasize that. tiere is no possibility to trace back all bilaterian taxa with a coelom to a common stem species.