1. PROTOZOA

2. They include over 65,000 species and their size, in majority of cases, range between 5 and 250 micrometre in diameter. They represent a diverse group of eukaryotic protists. Mostly they are unicellular, but a few form colonies in which individual cells are joined by cytoplasmic threads or embedded in a common matrix

3. They are distributed in diverse moist habitats, commonly occur in sea, in fresh water and in soil.
Free living protozoa are found in the polar region and at very high altitudes.
Parasitic protozoa may be found in association with most animal groups.
Many protozoa survive dry conditions by the formation of a resistant cyst or dormant stage. Parasitic protozoa can modify their morphology and physiology to cope with a change in host. For example, malarial parasite produces male gametes in response to a drop in temperature on transfer from a warm-blooded mammalian host to a mosquito.

7. Morphology
The protozoan cell is devoid of cell wall. The outermost boundary is made up of a cell unit membrane called plasmalemma. This not only protects the cell from external factors and controls exchange of substances, but it also acts as the site of perception of mechanical or chemical stimuli as well as establishment of contact with other cells.
Sometimes the protozoan cell is surrounded by a non rigid cuticle composed of chitin or chitin like substances.
Some protozoa produce an exoskeleton in the form of shells which are made up of siliceous, calcareous or proteinaceous material.

9. A characteristic type of covering called cystare found in some protozoa. This cyst protects the cell from environmental factors like high light intensity, high temperature, desiccation, anaerobiosis, etc.

11. Like other eukaryotic cells, the cytoplasm of the protozoan cell is more or less homogeneous and the various structures endoplasmic reticulum, golgi bodies, mitochondria, ribosomes, food vacuoles, contractile vacuoles, nucleus, etc are embedded within it.
Nucleus is eukaryotic and may be either one in number (in case of most protozoa) Or more than one (only in the case of some of them) in each cell. Ciliates (Paramecium is a typical example of this class) possesses two kind of nuclei in each cell.
A small ‘micronucleus’which functions principally in sexual processes and a large ‘macronucleus’ that controls metabolism, development and other cellular processes.

14. Reproduction
Protozoa reproduce by both asexual and sexual means, asexual process is more common but the sexual method is common only in some protozoa.

15. Asexually they reproduce by binary fission, multiple fission and budding. The most common method of asexual reproduction is binary fission where the body divides producing two equal or sometimes unequal daughter bodies.

17. Some protozoa reproduce asexually by multiple fission or schizogamy
Some protozoa reproduce asexually by multiple fission or schizogamy. In this case, the nucleus divides mitotically to produce a large number of nuclei before the body divides. Each nucleus with the surrounding cytoplasm forms a daughter protozoan and then they separate.

18. Many protozoa reproduce asexually by budding
Many protozoa reproduce asexually by budding. In this process daughter nuclei formed as a result of mitotic division migrate into a cytoplasmic protrusion (bud) , which is ultimately separated from the parent protozoan by fission

19. Only some protozoans carry out sexual reproduction
Only some protozoans carry out sexual reproduction. The most common method is conjugation in which there is an exchange of gametes between paired protozoa of complementary mating types(conjugants). Conjugation is mostly prevalent among ciliates like Paramecium.

22. Useful protozoa
1.Protozoa play an important role in the economy of nature.
Protozoans serve as food for many small aquatic organisms. Zooplankton are tiny protozoans which live in the sea. They form the principal diet of blue whales, who gulp them in with sea water. Zooplankton that feed on photosynthetic phytoplankton are eaten by insect larvae, small crustaceans, worms, etc which in turn become food for large marine organisms.

23. Useful protozoa
2.Numerous holozoic protozoa feed on putrefying bacteria in various water bodies and thus help indirectly in the purification of water.
3.Biological sewage treatment involves both anaerobic digestion as well as aeration process. In the different stages of sewage treatment processes, both anaerobic and aerobic protozoa contribute. They are the ultimate decomposers in nature, as they feed on bacteria and fungi, which decompose dead organic matter. They are, thus, useful in the treatment of sewage.

24. Useful protozoa
4. Some protozoans live in the body of other organisms and help them. Termites, for example, have protozoans living in their body. The protozoans digest the cellulose in the wood eaten by termites and convert it into carbohydrates that the termites can use.
5.Many protozoa are used as research organisms for biochemical and molecular biological studies because many of the biochemical pathways used by protozoa occur in all eukaryotic cells.

25. Harmful protozoa
some of the human diseases caused by protozoa are very important. The protozoan Endamoeba histolytica causes amoebiasis (amoebic dysentery) in human beings, Giardia causes giardiasis (dysentery), while Plasmodium causes malaria. trypanosome, a parasitic protozoan which lives in the bloodstream of human beings, cattle and other animals, causes a dangerous disease called sleeping sickness.