PROTOZOA - GENERAL CHARACTERS AND CLASSIFICATION:
The name protozoa -- (protos means first and zoan means animal) coined by Goldfuss (1818).It was Von Siebold (1845) who, recognized the unicellular nature of protozoa.Anton von Leuwenhoek first observed protozoan under microscope.
First protozoan observed was Vorticella convellaria.
Protozoa are generally microscopic -- they are found in fresh water,saltwater,dampsoil, while some are parasitic,symbiotic,commensals.
They are called acellualr or non cellular because their bodies are not differentiated into cells,or unicellular and one cell performs all functions.
Protozoa forms a heterogeneous group,the members display an extreme diversity of structure,different types of symmetryand adaptations to various environments.
Many protozoa are complicated in structure. Some protozoa form colonies.Protozoa might be defined as an organism which is made up of mass of protoplasm not divided into cells and which carries on all vital activities of life such as locomotion,feeding,digestion,respiration­,excretion, reproduction etc.
Although it is not divided into cells,it has specialization and division of labour with in its cytoplasmic mass.It is erroneous to think of protozoans as simple animals,for many have complicated structures and are physiologically complex.
Body is naked or covered by a pellicle or a shell made of silica or calciumcarbonate.Giarelia shows bilateral symmetry. Helizoans and radiolarians showspherical symmetry. Sessile forms like choanoflagellates are radially symmetrical. The vital activities of the body are performed by different organelles of thecell,hence division of labour is at sub-cellular level.
Special structures like pseudopodia or whip like flagella or hair like cilia performlocomotion.In some,myonemes are useful in locomotion.
Nutrition is holozoic or holophytic or osmotrophic (saprobic or parasitic).Digestion is intracellular. In some like Euglena,nutrition is miaotrophic,perform more than one type of nutrition.

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