nephridium protonephridium The ends are called flame cells (if cilliated) or solenocytes (if flagellated); they function in osmoregulation and ionoregulation. The terminal cells are located at the blind end of the protonephridium. Each cell has one or more cilia and their beating inside the protonephridial tube creates an outward going current and hence a partial pressurization in the blind of the tube. Because of this pressurization drives waste fluids from the inside of the animal, they are pulled through small perforations in the terminal cells and into the protonephridium. The perforations in the terminal cell are large enough for small molecules to pass, but larger proteins are retained within the animal. From the bottom of the protonephridium the solutes are led through the tube, formed by the canal cells, and exits the animal from a small opening formed by the nephridiopore. Selective reabsorption of useful molecules by the canal cells occurs as the solutes pass down the tubule. Protonephridia are generally found in basal organisms such as flatworms. Protonephridia likely first arose as a way to cope with a hypotonic environment by removing excess water from the organism (osmoregulation). Their use as excretory/ionoregulatory structures likely arose secondarily UBERON:0006973 BILA:0000126 Wikipedia:Nephridium#Metanephridia A network of dead-end tubules lacking internal openings found in the Phylums Platyhelminthes, kinorhyncha and Rotifera.