CLASSIFICATION. 13 
those species which only form rod-shaped cells, and filaments com- 
posed of rod-like segments ; or straight filaments not distinctly seg- 
mented, which may be rigid or flexible. 
The SPIRILLA are also included in a single genus, embracing all 
of those species in which the filaments are spiral in form and the 
segments more or less spiral or ‘‘ comma-shaped ’’—filaments either 
rigid or flexible. 
This simple morphological classification of the monomorphous 
group of Baumgarten corresponds with the nomenclature now gene- 
rally in use among bacteriologists. We speak of the spherical bac- 
teria as cocc2 or as micrococci, of the rod-shaped bacteria as bacilli, 
and of the spiral bacteria as spir7lla. 
It is true, however, that we are sometimes embarrassed to decide 
whether a particular microérganism belongs to one or the other of 
these morphological groups or so-called genera. Among the bacilli, 
for example, we may have, in the same pure culture, rods of very 
different lengths, some being so short that if alone they might be 
taken for cocci, while others may have grown out into long fila- 
ments. Butif we are assured that the culture is pure the presence 
of rod forms establishes the diagnosis, and usually the cocci-like 
elements, when carefully observed, will be seen to be somewhat 
longer in one diameter than in the other. The German bacterio- 
logists generally insist upon placing among the bacilli all straight bac- 
teria in which, as a rule, one diameter is perceptibly greater than 
that transverse to it; and several species of well-known bacteria 
which were formerly classed as micrococci are now called bacilli— 
e.g., Friedlander’s bacillus (‘‘ pneumococcus”’), Bacillus prodigitosus. 
The distinction made by Cohn and others between the genus 
Bacterium (Duj.) and the genus Bacillus (Cohn) cannot be main- 
tained, inasmuch as we may have short rods and quite long fila- 
ments in the same pure culture of a single species ; and, again, the 
character upon which the genus Vibrio (Ehr.) was established— 
viz., the fact that the filaments are flexible and the movements 
sinuous—is not a sufficient generic or even specific character, for in 
a pure culture there may be short rods which are rigid, and long 
filaments which are flexible and have a sinuous movement. We 
therefore to-day speak of all the elongated forms as bacilli, unless 
they are spiral and have a corkscrew-like motion, in which case they 
are known as spirilla. 
The bacteria are also classified according to their biological char- 
acters, and it will be necessary to consider the various modes of 
grouping them from different points of view other than that which 
relates to their form. This is the more important inasmuch as we 
are not able to differentiate species by morphological characters 
