Coeloms and nephridia in annelids and arthropods

The Ecdysozoa hypothesis caused a still ongoing debate on the homology of segmentation in Annelida and Panarthropoda. In this study we reconsider the homology of segmental organs and of the mesoderm based on ultrastructural studies of the anatomy of coeloms and nephridia in the course of their development. Our results challenge main traditional views on coelomogenesis in arthropods, including the previous concept of "coelomic sacs" as mesoblastic organs. Transient occurrence of segmental coeloms is refuted for crustaceans but confirmed for chelicerates and insects. When present, the coeloms proper seem to be largely decoupled from organogenesis. Primarily they only give rise to nephridial anlagen and may also act as pathfinding organs for mesodermal blast cells. The main source for pioneer blast cells is consistently apolar segmental mesoderm in arthropods. Its putative origin from coelomic walls is not corroborated by our results, neither is the assumption that the nephridial organs of arthropods are derived from metanephridial systems. Our data on coelomogenesis in yolk-rich embryos of onychophorans cause further ambiguity on ancestral states of mesoderm development in Panarthropoda. Current insights reveal that the body of evidence supporting a homology of coeloms and nephridia in annelids and panarthropods is remarkably poor. Because the coeloms in arthropods show no obvious functional significance and thus are liable to entire suppression, we nevertheless consider historical reasons (rather than convergence) as the more likely explanation for segmental coeloms and nephridia in annelids and pan arthropods, despite marked differences in almost all details of coelomogenesis.