CLASSIFICATION 35 



evident: one of the difficulties connected with drug treatment of 

 some protozoan diseases is the power of the protozoans to be- 

 come immune to the drug when given in doses which are not 

 destructive to the host; if such immunity is lost during trans- 

 mission by an intermediate host there is no danger of an immune 

 race of the parasite becoming permanently established. 



Classification. It is little wonder that such a varied assem- 

 blage of single-celled animals as constitutes the group Protozoa 

 should be difficult to classify. It is obvious that these simple 

 animals may be profoundly modified by their environment and 

 such modifications can actually be seen in the course of the life 

 history of many. The changes in form undergone by a trypano- 

 some, for instance, under different environmental conditions and 

 at different periods in the life history are represented in Fig. 18. 



It has been the custom among zoologists to divide the Protozoa 

 into four classes, based principally upon the nature of the organs 

 of locomotion. These classes in brief are as follows: Sarcodina, 

 including forms with pseudopodia; Flagellata, including forms 

 with flagella; Ciliata, including forms with cilia; and Sporozoa, 

 a heterogeneous assemblage of parasitic protozoans which as 

 adults have no organs of locomotion, and which reproduce by 

 breaking up into spores. Though other classifications have 

 been attempted, the above system is the one generally used. 

 It is probable that it is not in all respects a natural classifica- 

 tion, and that changes in it will be made with increasing 

 knowledge of Protozoa. A few examples of the difficulties con- 

 nected with this classification may be pointed out. There are 

 protozoans, as Craigia, which are typical Sarcodina during 

 part of their life cycle and typical flagellates during another 

 part, and some, such as certain soil amebae, which readily 

 change from one phase to another under the influence of varying 

 environmental conditions; there are others, as Mastigamceba, 

 which exhibit at once typical pseudopodia and a whip-like organ 

 which can only be regarded as a flagellum; there are species 

 having organs in every way intermediate between flagella and 

 cilia; the Sporozoa contain some species, such as the malaria 

 parasites, Plasmodium, which during a part of their life have 

 typical pseudopodia and suggest relationship with the Sarcodina, 

 others which show striking affinities to the Flagellata, and still 

 others which possess coiled projectile threads in polar capsules, 



