PREFACE V 



may not lend themselves to the culture method, and then onlv upon 

 the condition of having other living things as food. Many observers 

 have found that intestinal amebae, and others that feed on bacteria, 

 will thrive on solid culture media provided the latter are seeded with 

 bacteria, and this fact is of the greatest importance in obtaining 

 material for study. Other amebae cannot be cultivated in this way, and 

 it is quite probable, as Liihe maintains, that many parasitic protozoa, 

 especially the intracellular parasites, such as the coccidia, will never 

 be successfully cultivated. 



There is need, furthermore, of caution in studying protozoa under 

 such artificial conditions, for they are extremely sensitive to variations 

 and are readily adapted to new conditions. The reactions, both 

 morphological and physiological, of protozoa under such conditions 

 of study require careful control. 



The study of protozoa, therefore, even w^hen it is possible to apply 

 bacteriological methods, is fundamentally different from the study of 

 bacteria as at present carried on. The latter, dependent upon growth 

 conditions, colony formation, reactions to media, etc., are essentially 

 physiological and based upon the functions of the organisms. The 

 study of protozoa, on the other hand, is essentially morphological, or 

 based upon the structures of the protozoan cell, and involves the 

 changes in cell structures which an individual undergoes during various 

 phases of vitality. Hence it becomes necessary, first of all, to know 

 the life history of the protozoon and the fundamental modifications 

 which its protoplasm assumes. Modern protozoology, therefore, has 

 demanded as a basis for genera and species of protozoa a knowledge 

 of the complete life cycle, and as a basis for classification not the struc- 

 tures of the single cells, but the structures which the protoplasm may 

 assume throughout its entire life history from fertilization to death or 

 until the next fertilization. 



The present volume, finally, does not aim at being an exhaustive 

 treatise on the protozoa; it aims, rather, to give an introduction to the 

 study of modern protozoology as seen from the author's point of view; 

 and for numerous omissions, incomplete references, etc., he can only 

 plead the excuse of a large subject crowded into a limited space. 



G. N. C. 



New York, 1909 



