Paramecium caudatum

Paramecium caudatum are unicellular organisms belonging to the genus of paramecium of the phylum Ciliophora. They are less than 0.25mm in length and are covered with minute hair-like projections called cilia. The cilia are used in locomotion and feeding. They are often called slipper animalcules because of their slipper-like shape.

P. caudatum feed on bacteria by driving them into the gullet with cilia. Their natural habitat is fresh water. They take in water from the hypotonic environment via osmosis and use bladder-like contractile vacuoles to accumulate excess water from radial canals and periodically expel it through the plasma membrane by contractions of the surrounding cytoplasm. The contractile vacuoles also serve to maintain osmotic pressure. When moving through the water, they follow a spiral path while rotating on the long axis. When they encounter an obstacle, they exhibit the so-called "avoidance reaction"; they back away at an angle and start off in a new direction.

Paramecium have 2 nuclei (a large macronucleus and a single compact micronucleus). They cannot survive without the macronucleus and cannot reproduce without the micro-nucleus. Reproduction is either accomplished by binary fission (asexual), conjugation (sexual), or (rarely) by endomixis, a process involving total nuclear reorganization of individual organisms. During binary fission a fully grown organism divides into two daughter cells. Conjugation consists of the temporary union of 2 organisms and the exchange of micro-nuclear elements. Without the rejuvenating effects of conjugation a paramecium ages and dies. Only opposite mating types, or genetically compatible organisms, can unite in conjugation.

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