The third cell form is the _spiral_, typified by a section of a cork-screw and named _spirillum_, plural _spirilla_ (Fig. 37). A very short spiral consisting of only a portion of a turn is sometimes called _vibrio_ (Fig. 38). Vibrios when seen under the microscope look like short curved rods. The distinction between the two can be made only by examining the organism alive and moving in a liquid. The vibrio shows a characteristic spiral twisting motion. Very long, flexible spirals are usually named _spirochetes_ (Fig. 39). The spirochetes are motile but flagella have not been shown to be present. [Illustration: FIG. 33.--Cocci.] [Illustration: FIG. 34.--Bacilli.] [Illustration: FIG. 35.--Short bacilli.] [Illustration: FIG. 36.--Curved bacilli. Only the one in the center of the field is in focus. The others curve out of focus.] Besides the three typical cell forms bacteria frequently show very great irregularities in shape. They may be pointed, bulged, club-shaped or even slightly branched. These peculiar and bizarre forms practically always occur when some of the necessary conditions for normal growth, discussed in Chapters VI and VII, are not fulfilled. They are best regarded as _involution_ or _degeneration_ forms for this reason (Fig. 40). In a very few cases it is not possible to obtain the organism without these forms (the diphtheria group). It is probable that these cell forms are normal in such cases, or else conditions suitable for the normal growth have not been obtained. [Illustration: FIG. 37.--Spirilla.] [Illustration: FIG. 38.--Vibrio forms of spirilla. Compare with Fig. 36.] [Illustration: FIG. 39.--Spirochetes.] [Illustration: FIG. 40.--Involution forms. The organisms are tapering and branched at one end.] CHAPTER IV. CELL GROUPINGS. It has been stated that bacteria reproduce by transverse division, that is, division across the long axis. Following repeated divisions the new cells may or may not remain attached. In the latter case the bacteria occur as separate isolated individuals. In the former, arrangements characteristic of the particular organism almost invariably result. These arrangements are best described as _cell groupings_ or _growth forms_. [Illustration: FIG. 41.--Streptospirillum grouping.] [Illustration: FIG. 42.--Diplobacillus grouping.]