Paramecium typically lives in a hypotonic environment which means that
water diffuses from the outside to the inside of the animal by osmosis.
Paramecium removes water by using active transport and has a pair of organelles
just for this purpose. These organelles are called contractile vacuoles.
When Paramecia are examined using the light microscope the contractile
vacuoles(CV) can be seen as a pair of spherical
structures surrounded by a series of radiating ducts. These ducts collect
water and pump it into the central area by means of sets of contractile
fibers. In turn, the central sphere contracts when it is full, forcing
water to the outside of the protist.
Image copyright Ralf Schmode. Used with permission 1998
In addition,
Paramecium and other Protista have a second technique for regulating osmotic
balance. If Paramecium is examined under polarized light, bright "inclusions"
are typically visible as in the picture show here. These are crystals of
salts. If the cytoplasm becomes excessively dilute, then the organism can
release salts from these crystals to make the cytoplasm more hypertonic.
Conversely if the cytoplasm becomes excessively hypertonic, salts can be
taken out of solution by the organism.
Image copyright Ralf Schmode. Used with permission 1998