Chapter 1 

 Introduction 



PROTOZOA are unicellular animals. The body of a protozoan 

 is morphologically a single cell and manifests all characteristics 

 common to the living thing. The various activities which make up 

 the phenomena of life are carried on by parts within the body or cell. 

 These parts are comparable with the organs of a metazoan which are 

 composed of a large number of cells grouped into tissues and are 

 called organellae or cell-organs. Thus the one-celled protozoan is a 

 complete organism somewhat unlike the cell of a metazoan, each of 

 which is dependent upon other cells and cannot live independently. 

 From this viewpoint, certain students of protozoology maintain 

 that the Protozoa are non-cellular, and not unicellular, organisms. 

 Dobell (1911), for example, pointed out that the term "cell" is 

 employed to designate (1) the whole protozoan body, (2) a part of 

 a metazoan organism, and (3) a potential whole organism (a fertilized 

 egg) which consequently resulted in a confused state of knowledge 

 regarding living things, and, therefore, proposed to define a cell as 

 a mass of protoplasm composing part of an organism, and further 

 considered that the protozoan is a non-cellular but complete organ- 

 ism, differently organized as compared with cellular organisms, the 

 Metazoa and Metaphyta. Although some writers (Hyman, 1940; 

 Lwoff, 1951) follow this view, the great majority of protozoologists 

 continue to consider the Protozoa as unicellular animals. Through 

 the processes of organic evolution, they have undergone cytological 

 differentiation and the Metazoa histological differentiation. 



In being unicellular, the Protozoa and the Protophyta are alike. 

 The majority of Protozoa may be distinguished from the majority of 

 Protophyta on the basis of dimensions, methods of nutrition, direc- 

 tion of division-plane, etc. While many Protophyta possess nuclear 

 material, it is not easy to detect it in many forms; on the other hand, 

 all Protozoa contain at least one easily observable nucleus. The 

 binary fission of Protozoa and Protophyta is longitudinal and trans- 

 verse respectively. Most of Ciliata, however, multiply by transverse 

 division. In general the nutrition of Protozoa is holozoic and of 

 Protophyta, holophytic or saprophytic; but there are large numbers 

 of Protozoa which nourish themselves by the latter methods. Thus 

 an absolute and clean-cut separation of the two groups of unicellular 

 organisms is not possible. Haeckel (1866) coined the name Protista 

 to include these organisms in a single group, but this is not generally 



