Life

Heterotrophic Protista

These Protista are also known as Protozoa, which means “first animals”; in fact, the term does not mean that all protozoa are the ancestors of the animals, but refers to the fact that they ingest food, which is typical of animals.
Some of these Protista are predators and feed on bacteria and other Protista, i.e. they feed on dead organic matter, while others are parasites that exploit the resources of several organisms.
They can be divided into three main groups depending on how they move:
• Amoeboid protozoa: they move with the aid of the pseudopodia, which means “false feet’, i.e. extensions of the cellular cytoplasm that allow the cell to move. The pseudopodia are also used to take in the food particles by phagocytosis (absorption of particles by a cell). This group includes amoebas: they have no external coatings, live in still water and damp soils and some species can be parasitic, such as Entamoeba histolityca, which transmits a disease, amoebic dysentery. On the contrary, Foraminiferida are provided with a calcareous coating with many holes, out of which protrude the pseudopodia, and always live in the sea. Heliozoia and Radiolaria, instead, differ from the others organisms for they have special props in their pseudopodia that keep them fairly stiff.
• Flagellate protozoa: they are equipped with between one and many flagella, through which they move. This group includes organisms that live on their own as well as parasitic ones, some of which transmit intestinal infections, such as the Trichomonas or Tripanosoma; this protozoan lives in Central Africa and moves from the body of the tsetse fly to the body of man or cattle, transmitting the serious sleeping sickness, which causes about 5,000 deaths a year. Other protozoa are symbionts, i.e. they benefit their hosts, such as the Tryconimpha, that lives in the stomach of termites, helping them digest the wood they feed on.
Sporozoa: they have no organs to move. They are all parasites of animals, of other protozoa and man. They can produce spores. This group includes the Plasmodium malariae, which causes the disease transmitted by the female Anopheles, and the Toxosplasma gondii, which transmits toxoplasmosis.
• Ciliates: they are considered the most developed group because of their complexity. They can live either in freshwater or salty water, and move with the aid of the cilia, that in some species cover all the cell, while in others they only cover some areas. In these protozoa, the food is not fed into any point of the cell, but in a specific area, the cytopharynx, near an opening between the cilia, which flutter to move in the food particles. This class includes the paramecium, the vorticella and the Balantidium coli, which causes intestinal lesions in man.