Many free-living flatworms, such as Planaria, live in fresh water. These animals excrete water through an elaborate network of tubules running throughout their bodies. The tubules end in flame cells, so called because each cell has a tuft of cilia projecting into the tubule (Figure 51.4). The beating of the cilia gives the appearance of a flickering flame. A flame cell and a tubule together form a protonephridium (plural protonephridia; Greek proto, “before,” + nephros, “kidney”).

Question

For the freshwater planarian, the excretory product would be hypotonic to the interstitial fluid because the animal has to be excreting excess water that is entering its body by osmosis.

Extracellular fluid enters the tubules by filtration. The beating of the cilia causes a slight negative pressure in the tubule, and movements of the animal create positive pressure in the extracellular fluid. This pressure difference causes extracellular fluid to be filtered through tiny spaces between tubule cells. The filtrate flows toward the animal’s excretory pore, and along the way the cells of the tubules modify the composition of the fluid by reabsorption and secretion of specific ions and molecules. Because more ions are reabsorbed than are secreted, the urine that leaves the flatworm’s body is less concentrated than the extracellular fluid. Thus the protonephridium conserves ions and excretes water and wastes.