4 THE PROTOZOA 



Although, as Virchow pointed out, each cell of a tissue is a complete 

 organism performing all of the functions of living matter, some 

 one of these functions predominates over the others and gives to the 

 cell and to the tissues of which it forms a part its special character- 

 istics. In these specialized cells the secondary functions, i.e. those 

 acting only for the good of the cell itself, fall into the background 

 and are not readily investigated. In the Protozoa, on the other hand, 

 no one function predominates, and despite their primitive nature, the 

 protoplasm of which they are composed appears quite similar to that 

 of the most highly specialized tissue-cell. In it, however, lies the 

 secret of digestion and assimilation, of the kidney's secretion, and of 

 muscular contraction. 



The Protozoa invite attention from still another point of view. As 

 the lowest animals they show the beginnings of sex differentiation, of 

 maturation, or the changes which the germ-cells undergo before fer- 

 tilization, and of fertilization, while their union into cell-aggregates 

 or colonies with incipient division of labor among the constituent cells, 

 points the way toward the Metazoa, and makes them significant in 

 the light of evolution. 



As complete primitive organisms, therefore, the Protozoa are impor- 

 tant from many points of view : structurally, they contain in simple 

 form the elements which in higher tissue-cells are moulded into more 

 complicated organs of the cell ; functionally, they epitomize the life 

 activities of even the highest many-celled animals, but their vital pro- 

 cesses are more easily observed and correlated ; theoretically, they 

 occupy a prominent place in questions of phylogeny, of sex, and of 

 reproduction, and finally, placed as they are at the lowest limit of 

 animal life, they must ever be closely connected with problems con- 

 cerning its origin. 



With this conception of the Protozoa in mind the present volume 

 has been written. The work makes no pretence of a comprehensive 

 description of the Protozoa or of any one group, but aims rather to 

 give an intelligible idea of the main types, to point out the problems 

 of biology with which the Protozoa are most closely connected, and, 

 so far as possible in a limited space, to survey the work already 

 accomplished. 



In the present introductory chapter there is a brief historical review 

 of the stages by which the Protozoa have come to be regarded as single 

 cells, and at the same time as complete animal organisms. Here, too, 

 is a short account of the interesting position which the Protozoa have 

 held in the time-honored dispute over the limitations of the animal 

 and plant kingdoms, and in theories of spontaneous generation. The 

 second chapter deals with the general structures and functions of the 

 Protozoa as a group, and introduces the four following chapters, which 



