36 INTRODUCTION TO PROTOZOA 



resembling the nematocysts of jellyfishes. Some of the latter 

 have recently been elevated to the rank of a separate class, 

 Cnidosporidia, by the German parasitologist, Braun. Many 

 other difficulties in connection with the classification of the 

 Protozoa as outlined above could be cited, but since no more 

 acceptable classification has yet been proposed this classification 

 is followed here. 



The class Sarcodina consists in the main of free-living forms 

 occurring in the ocean, fresh water and soil. Many of the marine 

 forms are furnished with calcareous shells which are largely in- 

 strumental in building up chalk deposits. The majority of the 

 parasitic species belong to the genus Endamoeba. 



The class Flagellata contains some of the most primitive as 

 well as some very highly specialized kinds of animals. Many of 

 the free-living forms possess chlorophyll and are included by 

 botanists in the Plant Kingdom. There could be little question 

 about their vegetable nature were it not for the fact that there is 

 every gradation between those which are t\'pical plants in form 

 and function and those which are equally' typical animals in 

 every respect. The parasitic species are all of distinctly animal 

 nature, some ingesting and devouring solid food, others absorb- 

 ing food by osmosis. With the flagellates were once included, 

 also, the spirochsetes on account of a supposed relationship with 

 the trypanosomes, but this theory has long since been exploded, 

 and the spirochsetes are now usually looked upon as only dis- 

 tantly related to the flagellates. 



The class Ciliata is least important of the four classes of Pro- 

 tozoa from the parasitologist's point of view. There is only 

 one species of ciliate, Balantidium coli, which is common and 

 widespread enough and pathogenic enough in its effects to deserve 

 serious consideration as a human parasite. A few other intestinal 

 ciliates have been discovered in man but they are of little im- 

 portance. 



The class Sporozoa contains parasitic forms exclusively, but 

 fortunatel}^ man is peculiarly exempt from the attacks of all but 

 a few species. Among the few, however, are included the ma- 

 larial parasites, which rank among the first of pathogenic organ- 

 isms as regards significance to the human race as a whole. It is 

 possible that the undiscovered parasites of such diseases as 

 Rocky Mountain spotted fever, yellow fever and dengue, and 



