_Motility._ When a drop of water containing bacteria is placed upon a slide, a clean cover glass superimposed, and the specimen examined under an oil immersion lens, various rapid movements will generally be observed. These are of four kinds: (1) A dancing stationary motion known as _Brownian movement_. This is molecular, and depends in some degree upon heat and the medium of the moving particles. It is non-progressive, and is well known in gamboge particles. (2) An _undulatory_ serpentine movement, with apparently little advance being made. (3) A _rotatory_ movement, which in some water bacilli is very marked, and consists of spinning round, with sometimes considerable velocity, and maintained for some seconds or even minutes. (4) A _progressive_ darting movement, by which the bacillus passes over some considerable distance.
The conditions affecting the motion of bacteria are but partly understood. Heating the slide or medium accelerates all movement. A fresh supply of oxygen, or indeed the addition of some nutrient substance, like broth, will have the same effect. There are also the somewhat mysterious powers by which cells possess inherent attraction or repulsion for other cells, known as _positive_ and _negative chemiotaxis_. These powers have been observed in bacteria by Pfeiffer and Ali-Cohen.
[Illustration: BACILLI, SHOWING FLAGELLA]
The essential condition in the motile bacilli is the presence of _flagella_.[6] These cilia, or hairy processes, project from the sides or from the ends of the rod, and are freely motile and elastic. Sometimes only one or two terminal flagella are present; in other cases, like the bacillus of typhoid fever, five to twenty may occur all round the body of the bacillus, varying in length and size, sometimes being of greater length even than the bacillus itself. It is not yet established as to whether these vibratile cilia are prolongations of capsule only, or whether they contain something of the body protoplasm. Migula holds the former view, and states that the position of flagella is constant enough for diagnostic purposes. They are but rarely recognisable except by means of special staining methods. _Micrococcus agilis_ (Ali-Cohen) is the only coccus which has flagella and active motion.