Marine protozoans and protozoans that are internal parasites lack contractile vacuoles because the water concentration in their cytoplasm is the same as that in their environment. In these cases, there is no net movement of water into the cell. Many ciliates, protozoans with cilia as a means of movement, are very large cells and may have more than one nucleus. A micronucleus is also present. The micronucleus controls sexual reproduction.
Protozoans have an impact on people in several ways.
One of the primary ways is that several protozoans are human parasites. Parasites live in or on other organisms and, generally, cause harm to the host organism. A sarcodine that causes human disease is Entamoeba which causes amebic dysentary. Plasmodium , a sporozoan, causes malaria. One protozoan is also responsible for destruction of houses.