8 
BACTERIOLOGY. 
Mor¬ 
phology 
The morphological characters of bacteria—that 
is, their size and shape—vary greatly, and upon this 
basis it is convenient to subdivide them into three 
types:— 
A. Coccus; pleural form, Cocci. 
B. Bacillus; plural form, Bacilli. 
C. Spirillum; plural form, Spirilla. 
The cocci are shaped like berries, that is, about 
spherical. They may be flattened on one side or 
concave, or split like a coffee-bean. They may be 
arranged in pairs called diplococci; in fours, tetra- 
cocci; or in cubes, sarcinse. They are commonly ar¬ 
ranged in long strings or chains termed streptococci, 
or in masses often likened to bunches of grapes, 
staphylococci. The bacilli are rod-shaped, sometimes 
slightly curved, and vary greatly in length, from 
Viooo to V25000 °f an inch. They occasionally form 
in chains or rows. The spirilla are spiral or cork- 
screw-shaped, as the name implies. They vary both 
in length and in the number of spirals. Of these 
three types the bacilli are by far the most numerous 
and the spirilla the least numerous. The types are 
not interchangeable; so it is not possible for a coccus 
to become a bacillus or a bacillus a spirillum. 
In order to see them it is necessary to use a micro¬ 
scope of high magnifying power; indeed, it is highly 
probable that some forms of bacteria are so small that 
they cannot be seen with any of the microscopes that 
we have. 
