In this article we will discuss about the definition and classification of protozoa.
Meaning of Protozoa:
The protozoa are the simplest animals and are usually microscopic in size. A protozoan body is regarded as a single cell but functionally it is an entire individual, performing all the vital activities which characterise a living animal. It is for this reason that they are regarded as acellular animals rather than as unicellular ones.
Moreover, there may be more than one nucleus in the protozoan body, but none of them is in sole charge of a particular part of the animal. In the cellular animals (metazoa), each nucleus is in absolute charge of a particular part of the body, known as the cell. In the acellular protozoa, the body is not subdivided into cells, and the portions of protoplasm set apart for carrying out special functions are regarded as organelles.
At present about 30,000 species of protozoa are known to us. They occur throughout the earth, in fresh and salt water, in damp places, and in decaying organic matter. Many are free-living but a few occur in colonies. Some protozoa live as unwanted guests or parasites within the body of other animals; they often cause serious diseases, such as malaria, kalaazar, sleeping sickness, dysentery, etc., in their host.
Classification of Protozoa:
The phylum protozoa has been divided into four classes. The classification is based principally on their mode of locomotion.
Class I:
Rhizopoda (Rhiza= root; podus = foot). Protozoa having peculiar temporary organelles for locomotion, called pseudopodia or false feet. Examples—Amoeba proteus; Entamoeba histolytica (causing dysentery).
Class II:
Mastigophora (Mastix =whip; phoros=bearer) or Flagellata (flagellum = whip). Protozoa that move by the lashing of whip-like organelles called flagella. Example—Euglena viridis; Trypanosoma gambiensi (causing African sleeping-sickness); Leishmania donovani (causing kala-azar).
Class III:
Sporozoa (Spora —seed; animal). Parasitic protozoa possessing no locomotor organelles and reproducing by means of spores. Example—Monocystis gregarina; Plasmodium vivax (causing malaria).
Class IV:
Ciliophora (Cilium— eyelash). Protozoa that move by hair-like cilia. Example—Paramoecium caudatum (the slipper- animalcule); Vorticella campanula (the bell-animalcule).