Protozoa:

Protozoa are tiny microscopic animals, in fact the simplest and most primitive of all animals. Their bodies are not made up of cells, hence they are said to be acellular. Protozoans produce protective and resistant cysts to tide over unfavourable conditions. The commonest method of reproduction is asexual, by fission. Most of the members of this phylum live in water and damp soil, where they help to decompose organic matter by feeding on bits of the remains of dead plants and animals. Digestion is intra-cellular. In nature's complex food-web, they serve as food for small aquatic animals. Some protozoans are parasitic causing diseases in man and in live stock and other animals in which man has an interest. Amoebae and paramecium are examples of free living protozoan. Plasmodium (causing malaria), Entombed (causing amoebic dysentery) and Trypanosoma (causing sleeping sickness) are examples of parasitic protozoans.