CHAPTER VIII 
DISEASES CAUSED BY PROTOZOA 
GENUS SPIROCHAETA 
General consideration of the Spirocheta. The genus Spirochaeta 
is given by Migula as one of the Spirillaceae. The investigations that 
have been made, since this classification was published, on the mode 
of reproduction and other biological phases of these organisms have 
led systematists to look upon the genus as belonging to the protozoa 
rather than to bacteria. The genera Spirochaeta and Spirillum were 
first described by Ehrenberg. The essential point of difference 
between them was the inflexibility of the spirillum and the flexibility 
of the spirocheta. Schaudinn described an undulating membrane 
on the spirocheta. The constancy of this morphological element has 
been questioned by equally eminent authority. 
Calkins has pointed out the relation of spirocheta to bacteria and 
protozoa in the following paragraph: 
“From the foregoing review of the structures and life histories of the spirochetes 
there is little that is definite to determine the natural affinities of these spirilliform 
organisms. The plastic nature of the body and polymorphism are protozoan charac- 
ters. The structure of the so-called flagellum is a point in favor of the bacterial nature, 
but the highly kinetic membrane is an equally strong point in favor of the protozoa. 
The nucleus or its equivalent is more like that of the bacteria than like the morphological 
nucleus of the protozoa; but there are protozoa with distributed nuclei, so that this 
character is not distinctive. The physiological characteristics are quite as typical of 
protozoa as they are of bacteria; division, so often a subject of acrimonious and contra- 
dictory statements, is not decisive, for many protozoa divide transversely (all ciliates 
and Ozyrrhis and Polykrikos among flagellates), while some bacteria are said to divide 
longitudinally. Cultivation on artificial media, thus far unsuccessful with spirochetes, 
is now, thanks to the excellent work of Novy and MacNeal and their followers, no 
longer a distinctive feature, for trypanosomes, like most bacteria, may be so cultivated. 
The results of plasmolysis, urged by Novy and Knapp (’06) as an argument in favor of 
the bacterial nature of spirochetes, have but little value, for the time factor necessary to 
plasmolyse is a purely relative matter dependent upon the nature and resistance of the 
cell membrane. Differences among the bacteria themselves, in this respect, as Prowa- 
zek, Siebert, and many others have pointed out, are quite as marked as the differences 
between undoubted protozoa and spirochetes. The periodicity of symptoms in the 
hosts of disease-causing forms is more characteristic of protozoa than of bacteria, but 
the formation of toxins and the installation of immunity give no light on either side. 
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