Phylum Protozoa:
Charachterisitics, Classificiaon and Examples of Phylum Protozoa;

1. The term protozoa (proto, first + zoa, animal) has traditionally referred to chemoorganotrophic protists. The term “chemoorganotrophic” refers to those organisms that use organic compounds as a source of energy, electrons, and carbon for biosynthesis. The study of all protists, regardless of their metabolic type, is called protistology.
By definition, a protozoan is a complete organism in which all life activities are carried on within a single plasma membrane.
Characteristics of Protozoans:
Protozoans exhibit mainly two forms of life; free-living (aquatic, freshwater, seawater) and parasitic (ectoparasites or endoparasites).
2. The body is unicellular (without tissue and organs).
3. They are small, usually microscopic, not visualize without a micorscope.
4. They are the simplest and primitive of all animals.
5. They have a simple body organization. i.e. with a protoplasmic grade of organization.
6. They have one or more nuclei which are monomorphic or dimorphic.
7. Body naked or bounded by a pellicle, but in some forms may be covered with shells and often provided with an internal skeleton.
8. They are solitary (existing alone/single) or colonial (individuals are alike and independent).
9. Body shape variables may be spherical, oval, elongated or flattened.
10. Body symmetry either none or bilateral or radial or spherical.
11. Body form usually constant, varied in some, while changing with environment or age in many.
12. Body protoplasm is differentiated into an outer ectoplasm and inner endoplasm.
13. The single-cell body performs all the essential and vital activities, which characterize the animal body; hence only subcellular physiological division of labor.
14. Locomotory organs are fingers like pseudopodia, whip-like flagella, hair-like cilia or none.
15. Nutrition may be holozoic (animal-like), holophytic (plant-like), saprozoic or parasitic.
16. Digestion occurs intracellularly which takes place inside the food vacuoles.
17. Respiration occurs by diffusion through the general body surface.
18. Excretion occurs through the general body surface, but in some forms through a temporary opening in the ectoplasm or through a permanent pore called cytopyge.
19. Contractile vacuoles perform osmoregulation in freshwater forms and also help in removing excretory products.
20. Reproduction asexual (binary or multiple fission, budding, sporulation) or sexual (conjugation (hologamy), game formation (syngamy)).
21. The life cycle often complicated with alternation of asexual and sexual phases (alternation of generation).
22. Encystment commonly occurs to resist unfavorable conditions of food, temperature, and moisture, and also helps in dispersal.
23. The single-celled individual not differentiated into somatoplasm and germplasm; therefore, exempt from natural death which is the price paid for the body.
24. Examples: Euglena, Amoeba, Plasmodium, Paramecium, Podophyra, etc.
MAINTAINING OF BODY FUNCTIONS:
1. SUPPORT:
They body of protozoan is supported by pellicle and cytoplasm.
Pellicle:
A regular arrangement of microtubules, called the pellicle. Pellicle underlies the plasma membrane of many protozoa. The pellicle is rigid enough to maintain the shape of the protozoan, but it is also flexible.
Cytoplasm:
The cytoplasm of a protozoan is differentiated into two regions. The portion of the cytoplasm just beneath the pellicle is called ectoplasm (ectos, outside + plasma, to form). It is relatively clear and firm. The inner cytoplasm, called endoplasm (endon, within + plasma, to form), is usually granular and more fluid.
OSMOREGULATION:
The maintenance of water and salt balance in the body is called Osmoregulation. Most marine protozoa have solute concentrations similar to that of their environments. Freshwater protozoa, however, must regulate the water and solute concentrations of their cytoplasm. Water enters freshwater protozoa by osmosis because of higher solute concentrations in the protozoan than in the environment. Contractile vacuoles or water expulsion vacuoles remove this excess water. In some protozoa, contractile vacuoles form by the coalescence of smaller vacuoles. In others, the vacuoles are permanent organelles that collecting tubules radiating into the cytoplasm fill. Contracting microfilaments have been implicated in the emptying of contractile vacuoles.
INGESTION OF FOOD:
Most protozoa absorb dissolved nutrients either by active transport or by ingesting whole or particulate food through endocytosis. Some protozoa ingest food in a specialized region analogous to a mouth, called the cytopharynx.
DIGESTION:
Digestion and transport of food occurs in food vacuoles that form during endocytosis. Enzymes and acidity changes mediate digestion. Food vacuoles fuse with enzyme-containing lysosomes and circulate through the cytoplasm, distributing the products of digestion. After digestion is complete, the vacuoles are called egestion vacuoles. They release their waste contents by exocytosis, sometimes at a specialized region of the plasma membrane or pellicle called the cytopyge.
RESPIRATION & EXCRETION:
Protozoa are in small, they have a large surface area in proportion to their volume. This high surface-area-to-volume ratio facilitates two other maintenance functions: gas exchange and excretion.
Gas exchange involves acquiring oxygen for cellular respiration and eliminating the carbon dioxide produced as a by-product.
Excretion is the elimination of the nitrogenous by-products of protein metabolism. The primary by-product in protozoa is ammonia. Both gas exchange and excretion occur by diffusion across the plasma membrane.
REPRODUCTION:
Both asexual and sexual reproduction occur among the protozoa.
Asexual Reproduction:
Thera are following methods of asexual reproduction in protozoans:
1. Binary Fission:
It is One of the simplest and most common forms of asexual reproduction is binary fission. In binary fission, mitosis produces two nuclei that are distributed into two similar- sized individuals when the cytoplasm divides. During cytokinesis, some organelles duplicate to ensure that each new protozoan has the needed organelles to continue life. Depending on the group of protozoa, cytokinesis may be longitudinal or transverse.
Budding:
During budding, mitosis is followed by the incorporation of one nucleus into a cytoplasmic mass that is much smaller than the parent cell.
Multiple fission:
Multiple fission or schizogony occurs when a large number of daughter cells form from the division of a single protozoan. Schizogony begins with multiple mitotic nuclear divisions in a mature individual. When a certain number of nuclei have been produced, cytoplasmic division results in the separation of each nucleus into a new cell.
Sexual reproduction:
Sexual reproduction requires gamete formation and the subsequent fusion of gametes to form a zygote. In most protozoa, the sexually mature individual is haploid. Gametes are produced by mitosis, and meiosis follows the union of the gametes. Ciliated protozoa are an exception to this pattern. Their sexual reproduction takes place by conjugation.