It is obvious that bacteria which can move about in a liquid have an advantage in obtaining food, since they do not need to wait for it to be brought to them. This advantage is probably slight. [Illustration: FIG. 21.--A bacterium showing a single flagellum at the end--monotrichic.] [Illustration: FIG. 22.--A bacterium showing a bundle of four flagella at the end--lophotrichic.] An organism may have only one flagellum at the end. It is then said to be monotrichic (Fig. 21) (μόνος = alone, single; τριχος = hair). This is most commonly at the front end, so that the bacterium is drawn through the liquid by its motion. Rarely it is at the rear end. Other bacteria may possess a bundle of flagella at one end and are called _lophotrichic_ (Fig. 22) (λοφος = tuft). Sometimes at approaching division the flagella may be at both ends and are then _amphitrichic_ (Fig. 23) (αμφι = both). It is probable that this condition does not persist long, but represents the development of flagella at one end of each of a pair resulting from division of an organism which has flagella at one end only. In many bacteria the flagella arise from all parts of the surface of the cell. Such bacteria are _peritrichic_ (Fig. 24) (περι = around). The position and even the number of the flagella are very constant for each kind and are of decided value in identification. [Illustration: FIG. 23.--A bacterium showing flagella at each end--amphitrichic.] [Illustration: FIG. 24.--A bacterium showing flagella all around--peritrichic.] Flagella are too fine and delicate to be seen on the living organism, or even on bacteria which have been colored by the ordinary stains. They are rendered visible only by certain methods which cause a precipitate on both bacteria and flagella which are thereby made thick enough to be seen (Chapter XIX, p. 210). The movement of liquid around a bacterium caused by vibrations of flagella can sometimes be observed with large forms and the use of “dark-field” illumination. Flagella are very delicate and easily broken off from the cell body. Slight changes in the density or reaction of the medium frequently cause this breaking off, so that preparations made from actively motile bacteria frequently show no flagella. For this reason and also on account of their fineness the demonstration of flagella is not easy, and it is not safe to say that a non-motile bacterium has no flagella except after very careful study.