Light micrograph of Paramecium protozoans around an air bubble (left). Most are paired & undergoing conjugation, a type of sexual reproduction. The ciliates possess two different types of nucleus; a small diploid micronucleus & a larger polyploid macronucleus. During conjugation the cytoplasm of the two conjugants fuses, the macronuclei disintegrate and the micronuclei undergo meiotic division. The haploid products of this division are exchanged & a zygote nucleus is formed in both conjugants. New macronuclei are then formed by mitotic division of the genetic material of the new micronuclei. Magnification: x100 at 35mm size.