PROTOZOOLOGY >. ^:,; x 



PART I 



GENERAL DESCRIPTION OF THE PROTOZOA 



ORGANIZATION AND LIFE-HISTORY OF THE PROTOZOA. 



During the latter part of the seventeenth century Antoni van 

 Leeuwenhoek (1632-1723), working with a simple microscope, investigated 

 free-living Protozoa and studied the parasitic forms in the intestine of 

 frogs. He also found that he himself was infected with one of these 

 organisms, which, as Dobell (1920) has pointed out, was probably the 

 well-known Giardia intestinalis. The great Dutch microscopist thus not 

 only discovered free-living Protozoa, but was the first to study parasitic 

 forms, and he can be justly regarded as the father of Protozoology and 

 of its more specialized branch. Medical Protozoology. Since Leeuwen- 

 hoek's day an ever-increasing number of investigators, availing themselves 

 of the experiences of those who had gone before them and of the steady 

 improvement in the microscope, have brought to light an enormous 

 assemblage of minute living creatures, many of which, like the bacteria, 

 were quite beyond the scope of the simple magnifying apparatus used by 

 Leeuwenhoek and other early workers. These minute organisms absorb 

 nourishment and grow, and finally, as in higher animals, reproduce by 

 detaching portions of their bodies to form those of their offspring, while 

 any remaining portion dies. It may be that the entire body of the parent 

 is used up in the production of progeny, or only a small portion of it, as 

 in higher animals, but in either case, extending from parent to offspring, 

 there is a continuity which entitles all living creatures to be regarded as 

 immortal in that a j^ortion at least of the living matter is handed on from 

 one generation to another, unless accidental death prevents reproduction. 

 The fact that all the complex mechanisms of life are concentrated in 

 these minute portions of living matter has led observers to seek in them . 

 an explanation of the phenomena of life in general. A single organism 

 may be kept under observation for the whole of its individual existence, 

 and the visible changes undergone by it during its life, which is terminated 

 by its final production of offspring, may be actually followed under the 

 microscope. It seems evident that beyond the scope of the microscope 

 there exist organisms, or stages of development of visible organisms, 



3 



