Any of the free-living, single-celled protozoan s that make up the genus Paramecium .
Most are about the size of the period at the end of this sentence. They vary in shape and are surrounded by a rigid protein layer (pellicle) covered with hundreds of cilia that beat rhythmically to propel them and to direct bacteria and other food particles into their oral groove. Food particles are collected into food vacuole s, where digestion takes place. Two (occasionally three) contractile vacuoles close to the surface near the ends of the cell expand and contract as they discharge metabolic wastes and excess fluid. Paramecia have two kinds of nuclei: a large macronucleus (the centre of all metabolic activities) and at least one small micronucleus (which stores the genetic material necessary for sexual reproduction).