PART III 

 THE PATHOGENIC PROTOZOA 



CHAPTER XXVI 



PHYLUM IV. PROTOZOA 



This division includes the most primitive organisms belonging to the 

 animal kingdom. While some can be detected b.y sharp eyes as tiny 

 swinmiing specks, most all are so small that they can only be seen with 

 the aid of the microscope. The individual animal is constituted by a 

 single cell, which, with a difference in development, characteristicall}^ 

 distinguishes the Protozoa from other animal groups. In most cases 

 they live independently of each other, but not rarely a number are 

 associated in colonies. Each individual in such a colony is, as a rule, 

 physiologically complete, that is, performing within itself all of the 

 functions necessary to its life and reproduction. The colonization, 

 however, tends to a degree of differentiation and interdependence, and 

 in certain cases there are morphological and physiological differences 

 among the individuals so grouped, these usually being related primarily 

 to the functions of nutrition and reproduction. 



The protozoan colony may be said to differ from the metazoan in that 

 each cell of the colony represents an animal which may live unassociated 

 with other cells, while in the metazoan the individual is comprised l\v 

 the aggregation of cells among which there is a morphological differentia- 

 tion corresponding to special functions which are distributed among 

 adaptively specialized cell-groups; the bod\'-cells are not capable of free 

 existence and they can only live as integral parts of the metazoan. 

 The Protozoa being single-celled animals, there is a further fundamental 

 difference in their development, since it essentially follows that there is 

 no formation of germ layers as occurs in all Metazoa. The division or 

 budding of the protozoan cell results directlj' in a new generation and 

 not in the development of germinal tissue layers, though the new cells 

 may remain aggregated to form a colony. 



While the Protozoa are referred to as the most simple rejiresentatives 

 of the animal kingdom, they present, nevertheless, considerable differ- 

 ences in form and modification of the cytoplasm, the functions of mo- 

 tion, alimentation, excretion, and reproduction being performed by a 



