The excretory ducts of Diphyllobothrium dendriticum (Nitzsch 1824) plerocercoids: ultrastructure and marker distribution.

An electron dense marker, lanthanum nitrate, was injected into the excretory ducts of living plerocercoids of Diphyllobothrium dendriticum and the observations made by electron microscopy. The contractions of the plerocercoid spread the marker into the excretory system, and the distribution was found to be irregular both within the ducts and from one duct to another. Communication between the smallest ducts and the rest of the excretory system was noted. The marker was also introduced from the surrounding medium, in this case the fixative. It was found to flush into the excretory ducts through the pore in the tail and to be distributed into the ducts in the same way as the injected marker. No other pores were observed through the tegument other than the excretory pore in the tail by either method. In ultrastructure the duct wall is similar to that of other cestodes. The distributive role of the excretory system is discussed, including a comparison between the tegument and the duct wall.