Metamorphosis of the excretory system of Paragonimus ohirai (Trematoda), with special reference to its functional significance.

The fine structure of the excretory system was studied in metacercariae and juveniles of Paragonimus ohirai. The former were in vitro excysted, and the latter were collected from the abdominal cavity of a rat 24 hours postinfection. The terminal organs of the excretory system were composed of a flame cell and the first cell of a tubule. In the excysted metacercaria, there was no space between these cells to allow the passage of fluid. This suggests that the terminal organs of P. ohirai may be inactive in this stage. The excretory bladder was formed of epithelial cells which contained numerous lipid droplets and a large amount of glycogen in the cytoplasm. The bladder of the metacercaria seems to function as a storage area for nutrients and other materials. These characteristics of the metacercaria are considered to be related to the enclosed conditions created by encystment. Excretory organs had undergone cytomorphosis in the juveniles and appear to possess active excretory function. The periflagellar space in the terminal organs was formed, and lipids and concretions were excreted from a thin layer of the excretory epithelia into the lumen.