Order 7. =Sirosiphoniaceæ.= The individuals are formed of multicellular threads with longitudinal divisions; true branching and heterocysts, and often distinct differentiation into apex and base. Reproduction by synakinetes, rarely by resting akinetes and ordinary reproductive akinetes. _Hapalosiphon_, _Stigonema_, _Capsosira_, _Nostocopsis_, _Mastigocoleus_. Family 2. =Bacteria.=[6] The Bacteria (also known as Schizomycetes, and Fission-Fungi) are the smallest known organisms, and form a parallel group to the Blue-green Algæ, but separated from these Algæ by the absence of their colouring material; chlorophyll is only found in a few Bacteria. The various forms under which the vegetative condition of the Bacteria appear, are termed as follows: 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_). [Illustration: 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). [Illustration: FIG. 24.--_Spirochæte obermeieri_, in active motion (_b_) and shortly before the termination of the fever (_c_); a blood corpuscles.] All Bacteria are unicellular. In the case of the micrococci this is self-evident, but in the “rod,” “thread,” and “spiral” Bacteria, very often numerous cells remain united together and their individual elements can only be recognised by the use of special reagents.