Asexual Reproduction of Paramecium

Paramecium, like other ciliates, usually reproduces by splitting in two, forming two virtually identical individuals from one organism. Most ciliates divide transversely, splitting in two by pinching off in the middle of the long axis of the organism, as can be seen in the photo. The macronucleus does not divide by mitosis, rather it elongates and then splits in two, roughly one half going to the each of the new daughter cells. This type of reproduction involves no exchange of genetic material, and is a type of asexual reproduction. This form of asexual reproduction is often called 'fission'.

Sexual Reproduction of Paramecium

Under certain conditions, such as overcrowding or environmental stress, Paramecium  turns from strictly asexual reproduction to sexual reproduction. Sexual reproduction involves the exchange of genetic material between two individuals of different 'mating strains'. Through a process called conjugation, two paramecia line up side by side and then fuse together. All but one of the cell's micronuclei disintegrate. This micronucleus then divides* into four – one of which will be exchanged during conjugation. 

(* This process involves meiosis, where the diploid micronucleus divides twice to yield four haploid micronuclei. Three of these disintegrate, while one divides again to produce two swapping haploid micronuclei.)

Each mate then swaps one haploid micronucleus with its partner. The new micronucleus fuses with the old to make a diploid micronucleus. It represents a combination of genetic material derived from two genetically different individuals. This chromosome mixing is the basic principle of sexual reproduction. The two mates then separate and go on their way in their watery environment. They begin again to produce multiple copies of themselves through asexual fission.