NORMAL FORMS 23 
phological characteristic of certain members of this group. Organ- 
isms exhibiting this paired arrangement are referred to as diplococci. 
The flattening of the proximated surfaces may be associated with an 
elongation of the axes of the organisms parallel to the plane of apposi- 
tion, which leads to "coffee bean" shaped diplococci, exemplified in 
the meningococcus and gonococcus, or to an elongation of the axes 
perpendicular to the plane of apposition, in which event the organisms 
are "lance-shaped" diplococci, as for example the pneumococcus. 
Bacilli.— Bacilli are rod-shaped, cylindrical organisms in which a 
longer and a shorter dimension may be recognized. They are typi- 
cally circular in cross-section. When division is taking place the 
shorter bacilli may be temporarily oval or nearly circular in outline. 
The dimensions of bacilli vary considerably: some are habitually long, 
some are short, some are thick, some are thin. The ends may be 
convex, less commonly flat or even concave. A few bacilli are not 
typically isodiametric, but appear in outline as club-shaped, spindle- 
shaped, or even more or less conical (cuneate) rods. Less commonly, 
slightly curved rods are met wath; the curvature takes place along 
the longer dimension. 
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Fig. 1. — The normal types of bacteria. 1-6, cocci; 7-13, bacilli; 14-16, spirilla; 1, 
micrococcus; 2 and 3, diplococci; 4, tetracoccus; 5, sarcina; 6, streptococcus (the lower 
chain includes an arthrospore) ; 7 and 8, bacilli; 9, 10, 12 and 13, bacilli with various 
granules; 11, streptobacillus; 14, vibrio; 15, spirillum; 16, Spirocliista, Treponema. 
Spirilla. — Spiral bacteria, like the bacilli, exhibit a longer and a 
shorter dimension; unlike the bacilli, the longer axis is curved in three 
planes of space. The curvature may be slight, less than a complete 
turn, in which event the organism is "comma-shaped" when viewed 
under the microscope; it may be a series of open curves, giving the 
organism a sinuous outline; or it may be very much curved, so that the 
organism resembles a somewhat closely coiled spring in general appear- 
ance. As a rule, the curvature is symmetrical and quite uniform in 
each type. 
The cocci, through almost imperceptible morphological gradations, 
merge into the bacilli, and the bacilli, through the slightly curved 
forms, merge into the spirilla. Even in the spirilla slight differences 
in curvature are frequently discernible. Thus, a culture of the cholera 
vibrio may contain occasional straight, uncurved organisms in addition 
to the slightly curved rods which are the characteristic morphological 
