Page:A Text-book of Animal Physiology.djvu/48

The 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 1/30000 to 1/10000 of an inch in diameter.

They grow mostly in the longitudinal direction, and reproduce 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 (zoöglœa form) they are surrounded by a gelatinous matter, probably secreted by themselves.

Fig. 29.—Micrococcus, very like a spore, but usually much smaller.
Fig. 30.—Bacterium.
Fig. 31.—Bacillus. The central filament presented this segmented appearance as the result of a process 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.
Fig. 33.—A drop of the surface scum, showing a spirillum aggregate in the resting state.