Explain the main features of protozoans and their groups.

(N/A) $\rightarrow$ All protozoans are heterotrophs and live as predators or parasites.
$\rightarrow$ They are believed to be primitive relatives of animals.
$\rightarrow$ They were first studied by Leeuwenhoek.
$\rightarrow$ They can cause several diseases in humans and animals.
$\rightarrow$ General characteristics of protozoans:
$\rightarrow$ They are microscopic,unicellular,and colourless organisms with diverse shapes.
$\rightarrow$ Locomotion occurs with the help of finger-like pseudopodia,flagella,or hair-like cilia.
$\rightarrow$ Respiration occurs through the general body surface.
$\rightarrow$ Reproduction occurs by binary fission,multiple fission,or budding.
$\rightarrow$ Sexual reproduction occurs by syngamy and conjugation.
$\rightarrow$ Amoeboid protozoans: These organisms live in freshwater,seawater,or moist soil. They move and capture prey by extending pseudopodia (false feet),as in Amoeba. Marine forms often have silica shells on their surface. Nutrition is holozoic. Some,such as Entamoeba,are parasites.
$\rightarrow$ Flagellated protozoans: Members are either free-living or parasitic. They possess flagella for locomotion. Nutrition is holozoic,saprobic,or parasitic. Examples include Trypanosoma (sleeping sickness) and Leishmania (kala-azar).
$\rightarrow$ Ciliated protozoans: These are aquatic,actively moving organisms due to the presence of thousands of cilia. They have a cavity (gullet) that opens to the outside of the cell surface. Examples include Paramecium.