18 ANIMAL PHYSIOLOGY. 
Tar BACTERIA. 
The bacteria include numberless varieties of organisms of 
extreme minuteness, many of them visible only by the help of 
the most powerful lenses. Their size has been estimated at 
from gotry to re$sa Of an inch in diameter. 
They grow mostly in the longitudinal direction, and repro- 
duce by transverse division, forming spores from which new 
generations arise. : , 
Some of them have vibratile cilia, while the cause of the 
movements of others is quite unknown. 
As in many other lowly forms of life, there is a quiescent 
as well as an active stage. In this stage (zodglea form) they 
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Fia. 29.—Micrococcus, very like a spore, but usually much smaller. 
Fic. 30.—Bacterium. 
Fie. 31.—Bacillus. The central filament presented this segmented appearance as the result of 
a a ere of transverse division occurring during ten minutes’ observation. 
Fig. 32.—Spirillum ; various forms. The first two represent vibrio, which is possibly only a 
stage of spirillum. 
Fic. 33.—A drop of the surface scum, showing a spirillum aggregate in the resting state. 
are surrounded by a gelatinous matter, probably secreted by 
themselves. 
