4 I'ROTOZOOLOGY 



zoic and of Protophyta, holophytic; but there are large numbers 

 of Protozoa which nourish themselves by holo})hytic method. 

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

 unicellular organisms is not possible. Haeckel coined the name 

 Protista to include these organisms in a single group, but this is 

 not generally adopted, since it includes undoubted animals and 

 plants, thus creating an equal amount of confusion between it and 

 the animal or the plant. Recently Calkins (1933) excluded 

 chromatophore-bearing Mastigophora from his treatment of Pro- 

 tozoa, thus placing organisms similar in every way, except the 

 presence or absence of chromatophores, in two different groups. 

 This intermingling of characteristics between the two groups of 

 microorganisms shows clearly their close interrelationship and 

 suggests strongly their common ancestry. 



Although the majority of Protozoa are solitary and the body 

 is composed of a single cell, there are a few forms in which the 

 body is made up of more than one cell. These forms, which are 

 called colonial Protozoa, are well represented by the members of 

 Phytomastigina, in which the individuals are either joined by 

 cytoplasmic threads or embedded in a common matrix. These 

 cells are alike both in structure and in function, although in 

 several genera there may be differentiation of the individuals into 

 reproductive and vegetative cells. Unlike the cells in a metazoan 

 which form tissues, these vegetative cells of colonial Protozoa 

 are not dependent upon other cells; therefore, they do not form 

 any tissue. The reproductive cells produce zygotes through sexual 

 fusion, which subsequently undergo repeated division and may 

 produce a stage comparable with the blastula stage of a meta- 

 zoan, but never reaching the gastrula stage. Thus colonial Pro- 

 tozoa are'only cell-aggregates without histological differentiation 

 and may thus be distinguished from the Metazoa. 



Between 15,000 and 20,000 species of Protozoa are known to 

 man. From comparatively simple forms such as Amoeba, up to 

 highly complicated organisms as represented by numerous cili- 

 ates, the Protozoa vary exceedingly in their body organization, 

 morphological characteristics, behavior, habitat, etc., which ne- 

 cessitates a taxonomic arrangement for proper consideration as 

 set forth in detail in chapters 7 to 43. 



