176 ^^^ Philippine Journal of Science i9i8 



of protozoa have been made by amateur workers. There is 

 scarcely a person whose interest may not be stirred if given the 

 opportunity to watch the rangings of protozoa through the 

 microscopic jungles. The series of papers of which this forms 

 the first is aimed as much to stimulate the interest of the ama- 

 teur microscopist in a study of our local species of protozoa and 

 a search for new species, as it is designed to be of aid to the 

 general zoologist and the workers in medicine and other allied 

 fields. 



The question "What is a protozoon?" has been answered in 

 a number of ways by different writers. Colloquially they are 

 generally spoken of as "single-celled animals," "the simplest 

 forms of animal life," or "man's ultimate ancestor," definitions 

 that, in the light of our present knowledge of these organisms, 

 are decidedly inadequate. One of the best, if not the best be- 

 cause it is so comprehensive, is that of Calkins,^ which is as 

 follows : 



A protozoon is a primitive animal organism usually consisting of a 

 single cell, whose protoplasm becomes distributed among many free living 

 cells. These reproduce their kind by division, by budding, or by spore 

 formation, the race thus formed passing through different form changes 

 and the protoplasm through various stages of vitality collectively known 

 as the life cycle. 



A brief analysis of this definition should serve to bring out 

 several points, which it is important should be borne in mind by 

 any person who undertakes the study or identification of 

 protozoa. 



A protozoon is a pHmitive animal organism. — The Protozoa 

 are primitive in the sense that their organization as compared 

 with the Metazoa is relatively simple. They have no organic, 

 circulatory, or nervous systems as we understand those things 

 by a study of animals higher in the scale. Their life processes 

 are wholly of a cellular nature, but notwithstanding this they 

 exhibit all the physiological functions manifested by the higher 

 animals. They are considered as animals because, among other 

 things, the animal type of nutrition predominates among them. 

 There are, to be sure, certain species found among the Mycetozoa 

 and the Phytomastigophora that show marked plant character- 

 istics, but which, at the same time, show sufficient animal char- 

 acteristics to justify their classification with the latter group. 



Usually consisting of a single cell. — In a general sense a pro- 

 tozoon taken at random at some stage in its life cycle is a single- 



' Calkins, G. N., Protozoology. Lea & Febiger, Philadelphia (1909), 17. 



