Definition and Introduction
All unicellular ( or acellular ) eukaryotic animals.
Most primitive (Gr. Protos = first=zoon= animals ) organisms considered animals because of heterotrophic nutrition and motility .Although the whole body is a single minute cell . it performs all vital activities that are essential for an activities that are essential for an independent existence as a complete organism. About 50,000 species (30,000 present and 20,000 extinct ) are so far know, but the total number of individual protozoans in nature must be much more than the of the membrane of any other phylum.
Protozoans were first studied by Leeuwenhoek 1677. The name Protozoa was coined by Goldfuss 1871. The branch of their study is called protozoology .
1. Usually minute (0.001-3.00mm) microscopic, free-living in water , damp soil, or decaying organic matter, Many parasitic in other animals and plants .Some symbiotic or commensal. Solitary or colonial. Free moving or sessile.
2. Body symmetrical or asymmetrical. Shape variable , oval, elongated , spherical, or otherwise , but fixed in most species unstable and ever-changing in some.
3. Body naked with only the plasma membrane , or covered by a thin pellicle . Some with a hard , horny, limy or siliceous covering called shell, test or lorica.
4. Division of labour exhibited by specialized cell structures or organelles formed of macromolecules. Hence , only molecular or protoplasmic level of body organization ,
5. One to several nuclei, mostly similar when more than one, but of two types ( unclear dimorphism ) in some .
6. Locomotory organelles pseudopodia, flagella , or cilia,
7. Nutrition principally heterotrophic ( animals like) holozoic , saprozoic saprophytic (absorbing liquid substances of animal or plant origin from the sur-roundings or parasitic. Some are autotrophic or holophytic ( plant like) but usually , these can also carry some kind of heterotrophic mutrition (mixotrophic) ,Digestion in holozoic protozoans occurs in food vacuoles with the help of lysosomal enzymes.
8. If necessary the quantity of water is regulated ( osmo regulation ) by one or more contractile vacuoles.
9. Gaseous exchange and excretion by diffusion through body surface . Ammonia is principal nitrogenous waste (ammonotelic).
10. Reproduction asexual or sexual. Asexual by fission or budding , sexual by gametes or conjugation.
11. Encystment common for reproduction , dispersal or survival during adverse environmental conditions.