CHAPTER XXI. 



PROTOZOA, THEIR CLASSIFICATION AND SPECIES PATHO- 

 GENIC FOR ANIMALS. 



Historical sketch. Our knowledge of protozoa has de- 

 veloped simultaneously with that of bacteria. Nothing was 

 known of either until Van Leeuwenhoek made his observation 

 on the existence of microscopic organisms. They were called 

 Animalculae for many years. With the development of better 

 microscopes and technique for the study of microorganisms, 

 the Schizomycetes, unicellular plants or bacteria, and the 

 Protozoa (first animals), or unicellular animals, were recog- 

 nized and differentiated. The bacteria seem to have been 

 more extensively studied although the protozoa have been clas- 

 sified and many families, genera and species described. 



The protozoa are quite widely distributed in external 

 nature. They are found in largest numbers where the climate 

 is warm and moist. Their functions have not been defined 

 but it is known that some of them live on and in the body tis- 

 sues and that among these there are certain species that pro- 

 duce morbid changes or disease in the host. Here as with 

 bacteria, the disease produced in the host is secondary to the 

 development of the parasite. In an elementary text of this 

 kind it is neither possible nor would it be profitable to take up 

 a, detailed discussion of the classification and life history of 

 protozoa. The fact should be brought to the attention, how- 

 ever, that there is in the external world, in soil and water 

 and on the mucosae of animals, a vast multitude of micro- 

 scopic forms belonging to the animal kingdom and that certain 

 of these are parasitic, and a few pathogenic, for domesticated 

 animals and for man. For a more extended study of Protozoa 

 the student is referred to the general and more specific 



