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

 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.