Studies on the ultrastructure and function of a primitive excretory organ, the protonephridium of the rotifer Asplanchna priodonta

The fresh water animal Asplanchna priodonta has a pair of protonephridia which have been considered excretory organs by many investigators. We have studied the ultrastructure and function of the protonephridium of this animal by using electron microscopy, micropuncture, and ultramicroanalysis of minute samples of body fluid and urine. It was shown that this primitive excretory organ is a syncytium with complex organization. It has a part where filtration of body fluid takes place (terminal organs) and a part where reabsorption of water, sodium and other solutes of the filtered fluid occurs (protonephridial tubule). The body fluid of the animal is hypertonic to the surrounding medium (osmolarity of 81.0 vs. 17.7 mOsm/l). This is maintained by the excretion of urine hypotonic to and with less sodium than the body fluid. When the animal is placed in distilled water a mild dilution of the body fluid occurs initially. The protonephridium reacts by increasing the excretion of water and decreasing the excretion of sodium and other solutes in the urine. Thus, the homeostasis of the body fluid is maintained. This study shows that the primitive protonephridium functions on the same filtration-reabsorption of body fluid (or plasma) basis as the more developed nephridia of higher invertebrates and the nephrons of the vertebrate kidneys. In Asplanchna priodonta, the course of fluid during urine formation is from the body cavity-terminal organ-tubule-exterior. This is identical to the course of fluid from blood-coelomic cavity (or Bowman's capsule)-tubule-exterior found in the excretory organs of the higher invertebrates and vertebrates. It appears that such a course of fluid is basic for the function of excretory organs.