"Paramecium" and "Amoeba" possess large contractile vacuoles (average diameter of 13 and 45 µm, respectively), which are relatively comfortable to isolate, manipulate and assay. The smallest known contractile vacuoles belong to "Chlamydomonas", with a diameter of 1.5 µm. In "Paramecium", which has one of the most complex contractile vacuoles, the vacuole is surrounded by several canals, which absorb water by osmosis from the cytoplasm. After the canals fill with water, the water is pumped into the vacuole. When the vacuole is full, it expels the water through a in the cytoplasm which can be opened and closed. Other protists, such as "Amoeba", have CVs that move to the surface of the cell when full and undergo exocytosis. In "Amoeba" contractile vacuoles collect excretory waste, such as ammonia, from the intracellular fluid by both diffusion and active transport.