Cartoon depiction of the structural and physiological components in the mammalian nephron (top), and the planarian protonephridia (bottom). Both the nephron and protonephridia are made up of several different subregions. Water and small molecules are filtered out of the bloodstream by the glomerulus in mammals and flame cells in planarians (blue). Useful substances are then reabsorbed into the bloodstream as they travel along the tubule (pink, red, gray and green; each color represents an area where different molecules are taken back up into the body). Finally, water and metabolic wastes pass into the collection duct (purple) and are excreted. Vu et al. suggest that these similarities make the planarian protonephridia a good model of the mammalian nephron.