A DEUTEROSTOME-LIKE NEPHRIDIUM IN THE MITRARIA LARVA OF<i>OWENIA FUSIFORMIS</i>(POLYCHAETA, ANNELIDA)

Abstract

The mitraria larva of O. fusiformis possesses a pair of nephridia which are not typical polychaete larval protonephridia, rather they resemble the pore canal-hydropore complex of deuterostome larvae. An ultrastructural analysis of the nephridium reveals a small body cavity, lined by monociliated podocytes, which opens to the exterior by a monociliated nephridioduct. Extracellular matrix between podocyte pedicels forms the filtration surface and podocyte and nephridioduct cilia produce the pressure gradient driving filtration. Therefore, each nephridium is considered a giant protonephridium. Although this organization is unique to Owenia among the annelids, it is identical ultrastructurally to the hydropore, pore canal, and adjoining coelom of deuterostome larvae, which is also a nephridium. It is concluded that this cytological similarity between the nephridia of the mitraria and deuterostome larvae reflects a cellular homology which has a wide distribution and is of limited usefulness in the recognition of relationships. The anatomical differences between them suggest they are, at best, serially homologous organs. Mitraria nephridia compare more favorably to the typical protonephridia (head kidneys) of polychaete trochophores and larval protonephridia of phoronids. Data support the view that Owenia is primitive among the polychaetes and suggest that the mitraria nephridium represents the plesiomorphic design of protonephridium within the Polychaeta.