232 THE PATHOGENIC FLAGELLATES 



disease-causing forms is more characteristic of protozoa than of bac- 

 teria, but the formation of toxins and the installation of immunity 

 give no light on either side. So, too, the passive carriage or active 

 multiplication within the insect host, which Stiles ('06) regarded as a 

 sufficient test of the plant or animal nature of spirochetes, only pushes 

 the problem a step farther back, for some spirochetes, at least, multiply 

 in the insect host and some trypanosomes are apparently carried and 

 transmitted in a passive state. 



On the whole, therefore, while again repeating that the controversy 

 now has only an academic importance, the weight of evidence favors 

 the view that spirochetes as a group are structurally (ectoplasmic) 

 more complex and more plastic and variable in form than bacteria, 

 while functionally they have a more complicated life history. On the 

 other hand, their structures (endoplasmic especially) are much less 

 complex than in protozoa, and their life history, so far as it is known, 

 more simple than that of the known protozoa. Until further obser- 

 vations on the life histories of different species are made we are justi- 

 fied in doing no more than to place the spirochetes as an intermediate 

 group between the bacteria and the protozoa, but leaning more toward 

 the latter, and in this sense they are included under the name spiro- 

 chetida in our classification. 



