Contractile vacuole - Protozoans
The contractile vacuoles may differ in complexity in various groups of protozoans. In amoebae the vacuoles are carried around in the cytoplasm. 'Small vesicles join them emptying their contents into the vacuoles till the vacuole joins the membrane emptying its content to the outside.
In ciliates (e.g. Paramecium) the contractile vacuoles have a more complex structure. The contractile vacuole is located in a fixed position with an excretory pore leading to the outside surrounded by ampullae of feeder canals. The feeder canals are surrounded by a network of fine canals (20nm) in diameter, which are also connected to the tubular system of endoplasmic reticulum. The ampullae are surrounded by a bundle of fibrils that may have a role in contraction of the ampullae. The ampullae contract thereby, filling the vacuole and when the contents of the vacuole are discharged to the exterior the ampulae get disconnected so that back flow is prevented.
Figure: The contractile vacuole of Amoeba proteus is surrounded by tiny vesicles that fill with fluid, which is then, emptied into the vacuole. Note the numerous mitochondria that are believed to provide energy needed to adjust' the salt - content of the tiny vesicles.