In the case of spiral forms it is obvious that there is only one possible grouping, that is, in chains of two or more individuals adherent end to end. A chain of two spirilla might be called a _diplospirillum_ (διπλός = double); of three or more, a _streptospirillum_ (στρεπτός = necklace, chain) (Fig. 41). These terms are rarely used, since spirilla do not ordinarily remain attached. Likewise the bacillus can grow only in chains of two or more, and the terms _diplobacillus_ (Fig. 42), bacilli in groups of two, and _streptobacillus_ (Fig. 43), bacilli in chains are frequently used. Still the terms _thread_, _filament_, or _chain_ are more common for _streptobacillus_. [Illustration: FIG. 43.--Streptobacillus grouping.] [Illustration: FIG. 44.--Typical diplococcus grouping. Note that the individual cocci are flattened on the apposing sides.] [Illustration: FIG. 45.--Long streptococcus grouping.] [Illustration: FIG. 46.--Short streptococcus grouping.] Since the coccus is spherical, _transverse_ division may occur in any direction, though in three planes only at right angles to each other. Division might occur in _one plane only_ as in spirilla and bacilli, or in _two planes only_ or in _all three planes_. As a matter of fact these three methods of division are found among the cocci, but only one method for each particular kind of coccus. As a result there may be a variety of cell groupings among the cocci. When division occurs in one plane only, the possible groupings are the same as among the spirilla or bacilli. The cocci may occur in groups of two--_diplococcus_ grouping (Fig. 44), or in chains--_streptococcus_ grouping (Figs. 45 and 46). When the grouping is in _diplococci_, the individual cocci most commonly appear as hemispheres with the plane surfaces apposed (Fig. 44). Sometimes they appear as spheres and occasionally are even somewhat elongated. The individuals in a streptococcus grouping are most commonly elongated, either in the same direction as the length of the chain, or at right angles to it. The latter appearance is probably due to failure to enlarge completely after division. Streptococci frequently appear as chains of diplococci, that is, the pair resulting from the division of a single coccus remain a little closer to each other than to neighboring cells, as a close inspection of Fig. 45 will show.