1. Globular forms, cocci (Figs. [27], [30] c): spherical or ellipsoidal, single cells, which, however, are usually loosely massed together and generally termed “Micrococci.”
2. Rod-like forms: more or less elongated bodies; the shorter forms have been styled “Bacterium” (in the narrower sense of the word), and the term “Bacillus” has been applied to longer forms which are straight and cylindrical (Figs. [28], [29], [30] E).
Fig. 23.—Spirillum sanguineum. Four specimens. One has two cilia at the same end, the sulphur grains are seen internally.
3. Thread-like forms: unbranched, long, round filaments, resembling those of Oscillaria, are possessed by Leptothrix (very thin, non-granular filaments; Fig. [30] A, the small filaments) and Beggiatoa (thicker filaments, with strong, refractile grains or drops of sulphur (Fig. [31]); often self-motile). Branched filaments, with false branching like many Scytonemaceæ, are found in Cladothrix (Fig. [30] B, G).
4. Spiral forms: Rod-like or filamentous bodies, which more or less strongly resemble a corkscrew with a spiral rising to the left. In general these are termed Spirilla (Fig. [23]); very attenuated spirals, Vibriones (standing next to Fig. [30] M); if the filaments are slender and flexible with a closely wound spiral, Spirochætæ (Fig. [24]).
5. The Merismopedium-form, consisting of rounded cells arranged in one plane, generally in groups of four, and produced by divisions perpendicular to each other.
6. The Sarcina-form, consisting of roundish cells which are produced by cellular division in all the three directions of space, united into globular or ovoid masses (“parcels”) e.g. Sarcina ventriculi (Figs. [25], [26]).
Fig. 24.—Spirochæte obermeieri, in active motion (b) and shortly before the termination of the fever (c); a blood corpuscles.