=Capsule.=--The _capsule_ is a second covering outside the cell wall and probably developed from it (Fig. 19). It is usually gelatinous, so that bacteria which form capsules frequently stick together when growing in a fluid, so that the whole mass has a jelly-like consistency. The term _zoöglœa_ was formerly applied to such masses, but it is a poor term and misleading (zoön = an animal) and should be dropped. The masses of jelly-like material frequently found on decaying wood, especially in rainy weather, are in some cases masses of capsule-forming bacteria, though a part of the jelly is a product of bacterial activity, a gum-like substance which lies among the capsulated organisms. When these masses dry out, they become tough and leathery, but it is not to be presumed that capsules are of this consistency. On the contrary, they are soft and delicate, though they certainly serve as an additional protection to the organism, doubtless more by selective absorption than mechanically. Certain bacteria which cause disease form capsules in the blood of those animals which they kill and not in the blood of those in which they have no effect (_Bacterium anthracis_ in guinea pig’s blood and in rat’s blood). The presence of capsules around an organism can be proved only by staining the capsule. Many bacteria when stained in albuminous fluids show a clear space around them which appears like a capsule. It is due to the contraction of the fluid away from the organism during drying. =Metachromatic Granules.=--The term “_metachromatic_” is applied to granules which in stained preparations take a color different from the protoplasm as a whole (Fig. 20). They vary widely in chemical composition. Some of them are glycogen, some fat droplets. Others are so-called “granulose” closely related to starch but probably not true starch. Others are probably nuclein. Of many the chemical composition is unknown. They are called “Babes-Ernst corpuscles” in certain bacteria (typhoid bacillus). Since they frequently occur in the ends of cells the term “polar granules” is also applied. Their presence is of value in the recognition of but few bacteria (“Neisser granules” in diphtheria). =Flagellum.=--A _flagellum_ is a very minute thread-like process growing out from the cell wall, probably filled with a strand of protoplasm. The vibrations of the flagella move the organism through the liquid medium. Bacteria which are thus capable of independent movement are spoken of as “motile bacteria.” The actual rate of movement is very slight, though in proportion to the size of the organism it may be considered rapid. Thus Alfred Fischer determined that some organisms have a speed for short periods of about 40 cm. per hour. This is equivalent to a man moving more than 200 miles in the same time.