Structure, function and development of segmental organs in Annelida

In Annelida, nearly each segment contains a pair of ducts that either are protonephridia or metanephridia. These segmental organs function as excretory organs and, after having been modified, they may also act as gonoducts during maturity. In certain polychaetous annelids and especially in clitellates this function has been adopted by additional gonoducts which generally are formed at the begining of maturity. At the end of the last century the gonocoel theory tried to explain the relation between gonads, coelomic cavities and nephridia. Using the gonocoel theory axiomatically, Goodrich (1945) assumed that in annelids a pair of protonephridia and a pair of gonoducts represent the primary condition. Evolution of metanephridia on the one hand and the fusion of gonoducts and nephridia on the other hand occurred within the Annelida. Based on recent ultrastructural investigations into the development of different segmental organs, this paper re-evaluates Goodrich's hypothesis. According to these data the segmental organs differentiate from a single anlage. Each consists of three or four cells which line a small lumen filled with microvilli. The duct becomes ciliated and the most proximal cells are separated when the coelom extends by fluid accumulation between the lining cells. During enlargement of the coelomic cavity the proximal part of the anlage is passively opened, so that the cilia face the coelom, to form the funnel. If separation of the proximal duct cells is suppressed, the anlage differentiates into a protonephridium, which secondarily may acquire a funnel during maturity by proliferation of proximal duct cells. Thus, different pathways in nephridial development lead to completely different segmental organs in the fertile adult. Additional gonoducts evolve in different lineages within the annelids.