6 PROTOZOOLOGY 



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. 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 sug- 

 gests 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. 145), 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 de- 

 pendent 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 metazoan, but never reach- 

 ing the gastrula stage. Thus colonial Protozoa are only cell-aggre- 

 gates without histological differentiation and may thus be distin- 

 guished 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 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- 



