Protozoa:
The term Protozoa comprises all single-celled animals. The single cell performs all the vital functions of the body. As the single cell performs all the vital functions of the body, the protozoa are known as single-celled animals or acellular or non-cellular organisms.
The single-celled animals, mono, di, or multinucleate that perform all the vital activities of the body and the cells may sometimes form colonies but maintain the individual entity are called protozoa.
Characteristics of Protozoa:
1. Generally microscopic and single-celled animals and the single cell performs all the vital activities of the body and so they are also called acellular.
2. The body may be naked or covered by a pellicle.
3. The body may also be enclosed in a shell.
4. Body shape variable, it may be oval, elongated, spherical, or disc-like.
5. Nucleus sac like oval or biconvex and contains a thick endsome.
6. Main locomotory organs are pseudopodia, flagella, or cilia in Sporozoa well defined locomotory organs are absent.
7. Excretion takes place throughout the body surface or through cytopyge or contractile vacuoles.
8. Nutrition holozoic, saprozoic, holophytic, or parasitic.
9. Respiration takes place through the body surface by diffusion.
10. Reproduction is asexual and sexual. Sexual reproduction by alteration of generations.
11. They live in freshwater, in seawater, in the soil as free-living animals and some live as ecto and endoparasites.
12. In higher forms conjugation and syngamy are seen.