Many 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. The beating of the cilia give the appearance of a flickering flame (Figure 51.4). A flame cell and a tubule together form a protonephridium (plural, protonephridia; from the Greek proto, "before," and nephros, "kidney").
Tissue fluid enters the tubules (how it does so is not entirely clear), and the beating of the cilia causes this fluid to flow through the tubules toward the animal's excretory pore. As it flows, the cells of the tubules modify the composition of the fluid. As the modified tubule fluid (urine) leaves the flatworm's body, it is less concentrated than the animal's tissue fluid, so ions are conserved and water is excreted by the protonephridium.
51.4 Protonephridia in Flatworms The protonephridia of the flat-worm Planaria consist of tubules ending in flame cells.The tubule cells modify the composition of the fluid passing through them.
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