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.
Fig. 21.—A bacterium showing a single flagellum at the end—monotrichic.
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.
Fig. 23.—A bacterium showing flagella at each end—amphitrichic.
Fig. 24.—A bacterium showing flagella all around—peritrichic.