Dimorphic nucleus means two types of nuclei are present in P. caudatum – large macronucleus and small micronucleus. The macronucleus is roughly kidney-shaped and with inconspicuous nuclear membrane. Macronucleus is the somatic or vegetative nucleus and controls the day-to-day metabolic activities of the cell. The micronucleusis lodged in a depression on the surface of the macronucleus.

It is usually spherical, with a nuclear membrane and with diploid number of chromosomes. It controls the reproductive activities of the organism. Amoeba, Trypanosoma and Plasmodium have only one nucleus.