In this group, the cells range in different species from .5 //, to 2 M in diameter, but most measure about I /*. Before division they may increase in size in all directions. The species are usually classified according to the method of division. If the cells divide only in one axis, and through the consistency of their envelopes remain attached, then a chain of cocci will be formed. A species in which this occurs is known as a streptococcus. If division takes place irregularly the resultant mass may be compared to a bunch of grapes, and the species is often called a staphylococcus. Division may take place in two axes at right angles to one another, in which case cocci adherent to each other in packets of four (called tetrads) or sixteen may be found, the former number being the more frequent. To all these forms the word micrococcus is often generally applied. The individuals in a growth of micrococci often show a tendency to remain united in twos. These are spoken of as diplococci, but this is not a distinctive character, since every coccus as a result of division becomes a diplococcus, though in some species the tendency to remain in pairs is well marked. The adhesion of cocci to one another depends on the character of the capsule. Often this has a well-marked outer limit (micrococcus tetragenus), sometimes it is of great extent, its diameter being many times that of the coccus (streptococcus mesenterioides). It is especially among the streptococci and staphylococci that the phenomenon of the formation of arthrospores is said to occur. In none of the cocci have endogenous spores been certainly observed. The number of species of the streptococci and staphylococci probably exceeds 150. Usually included "in this group are coccus-like organisms which divide in three axes at right angles to one another. These are usually referred to as sarcince. If the cells are lying single they are round, but usually they are seen in cubes of eight, with the sides which are in contact slightly flattened. Large numbers of such cubes may be lying together. The sarcinae are, as a rule, rather larger than the other members of the group. Most of the cocci are non-motile, but a few motile species possessing flagella have been described.
