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The below mentioned article provides study notes on Protozoa.
The name Protozoa (Gr., protos = first; zoon = animal) comes from Goldfuss (1818) who, however, used the name simply for the lower groups of zoophyta including protozoans, sponges, coelenterates, rotifers and bryozoans, It was von Siebold (1845) who, recognising the unicellular nature of the Protozoa, first used and defined the name Protozoa in the present sense.
Protozoa are generally microscopic in size, they are found in freshwater, saltwater and damp soil, while some are parasitic, symbiotic and commensals. They are called a cellular or non-cellular because their bodies are not differentiated into cells; parts of their bodies are specialised for various functions, these parts are called organelles in contrast to the organs of Metazoa.
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Thus, Protozoa may be regarded equivalent, not to a metazoan cell, but to an entire metazoan. Protozoa have undergone cytological differentiation, whereas Metazoa have undergone histological differentiation during the course of evolution. Consequently the term unicellular used for Protozoa is unfortunate.
Protozoa forms a heterogeneous group, the members display an extreme diversity of structure, different types of symmetry and adaptations to various environments. Many Protozoa are extremely complicated in structure. Some Protozoa form colonies. A colony has several individuals either attached to each other or enclosed within a gelatinous envelope and joined together by protoplasmic connections.
A colony differs from a metazoan- because its cells are functionally independent of each other. A protozoan, then might be defined as an organism which is made up of a mass of protoplasm not divided into cells and which carries on all the vital activities of life such as locomotion, feeding, digestion, egestion, respiration, excretion and reproduction, etc.
Although it is not divided into cells, it has specialisation and division of labour within its cytoplasmic mass. It is erroneous to think of protozoans as simple animals, for many have complicated structures and are physiologically complex. In as much as Protozoa are not made up of cell, they represent what is called the protoplasmic level of organisation.
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From this standpoint and others, many biologists place them close to the common ancestor of many-celled forms. Some Protozoa are quite close to the plants and may be considered as connecting link between animals and plants.
The number and variety of Protozoa is very great. About 50,000 species have been named although over 20,000 of them are fossil forms. The immense number of Protozoa have traditionally been separated by their means of locomotion into four classes Sarcodina with pseudopodia, Mastigophora with flagella, Ciliata with cilia, and Sporozoa with no organelles for locomotion.
Recently the Committee on Taxonomy and Taxonomic Problems of the Society of Protozoologists published a revised classification of the Protozoa. According to Honigberg (1964) Protozoa have been divided into four subphyla, viz., Sarcomastigophora, Sporozoa, Cnidospora and Ciliophora.
Ten representative types have been described in detail which are studied in various Indian Universities. The types described herein are Euglena, Trypanosome, Amoeba, Entamoeba, Elphidium (= Polystomella), Monocystis, Eimeria, Plasmodium, Paramecium and Vorticella.
