[Illustration: FIG. 25.--_Sarcina ventriculi._ One surface only is generally seen. Those cells which are drawn with double contour are seen with the correct focus, and more distinctly than those cells lying deeper drawn with single contour.] [Illustration: FIG. 26.--_Sarcina minuta_: _a-d_ successive stages of one individual (from 4–10 p.m.); _f_ an individual of 32 cells.] The condition termed “Zooglœa,” which reminds us of _Nostoc_, is produced by the cells becoming strongly mucilaginous. A number of individuals in active division are found embedded in a mass of mucilage, which either contains only one, or sometimes more, of the above-named forms. The individuals may eventually swarm out and continue their development in an isolated condition. Such mucilaginous masses occur especially upon moist vegetables (potatoes, etc.), on the surface of fluids with decaying raw or cooked materials, etc. The mucilaginous envelope is thrown into folds when the Bacteria, with their mucilaginous cell-walls, multiply so rapidly that there is no more room on the surface of the fluid. The cells of the Bacteria are constructed like other plant-cells in so far as their diminutive size has allowed us to observe them. The cell-wall only exceptionally shows the reactions of cellulose (in _Sarcina_, _Leuconostoc_; also in a Vinegar-bacterium, _Bacterium xylinum_); a mucilaginous external layer is always present. The body of the cell mostly appears to be an uniform or finely granulated protoplasm. Very few species (_e.g. Bacillus virens_) contain chlorophyll; others are coloured red (purple sulphur Bacteria); the majority are colourless. _Bacillus amylobacter_ shows a reaction of a starch-like material when treated with iodine before the spore-formation. Some Bacteria contain sulphur (see p. 37). The body, which has been described as a _cell-nucleus_, is still of a doubtful nature. Artificial colourings with aniline dyes (especially methyl-violet, gentian-violet, methylene-blue, fuchsin, Bismarck-brown and Vesuvin) play an important part in the investigations of Bacteria. MOVEMENT. Many Bacteria are self-motile; the long filaments of _Beggiatoa_ exhibit movements resembling those of _Oscillaria_. In many motile forms the presence of cilia or flagella has been proved by the use of stains; many forms have one, others several cilia attached at one or both ends (Fig. 23) or distributed irregularly over the whole body; the cilia are apparently elongations of the mucilaginous covering and not, as in the other Algæ of the protoplasm. In _Spirochæte_ the movement is produced by the flexibility of the cell itself. Generally speaking, the motion resembles that of swarm-cells (_i.e._ rotation round the long axis and movement in irregular paths); but either end has an equal power of proceeding forwards. The swarming motion must not be confounded with the hopping motion of the very minute particles under the microscope (Brownian movement).