The water expulsion vesicle (also known as a contractile vesicle) is an important osmoregulatory organelle in freshwater protists, which are hyperosmotic to surrounding water. Water constantly diffuses into cell and could cause the cell to burst if there wasn't some way to get it out. That is what water expulsion vesicle does. A network of collecting tubules extends throughout cells, and rich supply of mitochondria on the surface of tubules provides energy required to pump water from cytoplasm into the tubules. Tubules dump the water they've collected into water expulsion vesicle, which slowly fills. Once full, vesicle's membrane fuses with the outer plasma membrane creating a pore out of which excess water flows.