6 PROTOZOOLOGY 



adopted, since it includes undoubted animals and plants, thus creat- 

 ing an equal amount of confusion between it and the animal or the 

 plant. Calkins (1933) excluded chromatophore-bearing Mastigoph- 

 ora from his treatment of Protozoa, thus placing organisms similar 

 in every way, except the presence or absence of chromatophores, in 

 two different (animal and plant) groups. This intermingling of char- 

 acteristics 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 several forms in which the 

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

 called colonial Protozoa (p. 173), are well represented by the mem- 

 bers 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 a few 

 forms there may be a differentiation of the individuals into repro- 

 ductive and vegetative cells. Unlike the cells in a metazoan which 

 form tissues, these vegetative cells of colonial Protozoa are not so 

 dependent upon other cells as are the cells in Metazoa; therefore, 

 they do not form any true 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 metazoan, but never reaching the gastrula stage. Thus, colonial 

 Protozoa are only cell-aggregates without histological differentiation 

 and may thus be distinguished from the Metazoa. 



An enormous number of species of Protozoa are known to man. 

 From comparatively simple forms such as Amoeba, up to highly 

 complicated organisms as represented by numerous ciliates, the 

 Protozoa vary exceedingly in their body organization, morphological 

 characteristics, behavior, habitat, etc., which necessitates a tax- 

 onomic arrangement for proper consideration as set forth in detail 

 in Chapters 8 to 44. 



Relationship of protozoology to other fields of 

 biological science 



A brief consideration of the relationship of Protozoology to 

 other fields of biology and its possible applications may not be 

 out of place here. Since the Protozoa are single-celled animals 

 manifesting the characteristics common to all living things, they 

 have been studied by numerous investigators with a view to dis- 

 covering the nature and mechanism of various phenomena, the 



