4 HANDBOOK OF PROTOZOOLOGY 



tozoa which nourish themselves by holophytic and saprozoic 

 methods. Thus no absolute and clean-cut separation is possible 

 between them. Haeckel, therefore, proposed the term Pro- 

 tistan kingdom 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 plant kingdom. 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 the 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 known as colonial Protozoa, are well represented by the 

 members of Volvocidae, in which the individuals are either 

 joined by cytoplasmic threads or embedded in a common 

 matrix. The majority of these cells are alike both in structure 

 and in function, although in several genera there may be differ- 

 entiation of the individuals into reproductive and \egetative 

 cells. Unlike the cells in a metazoan which form tissues, these 

 vegetative cells of colonial Protozoa, however, are not de- 

 pendent upon other cells; hence they do not form any tissue. 

 The reproductive cells produce zygotes through fusion which 

 subsequently undergo repeated division processes and may pro- 

 duce a stage comparable to 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. 



With regard to their habitat, the Protozoa may be divided 

 into free-living forms and those living on or in other organisrns. 

 The vegetative, or trophic, stages of free-living Protozoa have 

 been found in every type of fresh and salt water, in soil and in 

 decaying vegetable matter. While the freshwater inhabiting 

 forms are ordinarily unable to live in salt water, and vice versa, 

 seemingly one and the same species has, in a number of cases, 

 been found in both fresh and salt waters. 



The factors which influence their presence in a given body 

 of water are temperature, chemical composition, kind and 

 amount of food, etc. Excessive cold seems to be less detrimental 



