Protonephridia and Metanephridia

Nephridia take place in two major forms - the protonephridium and metanephridium. Protonephridia are found in flat worms. The protonephridial canals end blindly in structures called flame cells or solenocytes internally. Flame cells comprise central cavities that are continuous with the cavities of tubules and consist of a bunch of cilia - the flame. Actually, the flame cell interdigitates along with the first cell of the tubule, by means of finger shaped processes. In solenocytes, the cell lumen is prolonged into a delicate tube and the flame is reduced to a single flagellum. Flat worms usually have a pair of protonephridia. The protonephridial canals are much branched bearing flame ceIls at the end of branches. The flame cells are found scattered during the parenchyma. The flame cells do the filtration; the filtered fluid is propelled by flagella. The nephridiaI epithelium performs the functions of reabsorption and secretion. These physiological mechanisms are comparable with those performed by a vertebrate kidney.