~1. Cocci.~ (Fig. 83).--Rounded or oval cells, subdivided according to the arrangement of the individuals after fission, into-- _Diplococci_ and _Streptococci_, where division takes place in one plane only, and the individuals remain attached (a) in pairs or (b) in chains. _Tetrads_, _Merismopedia_, or _Pediococci_, where division takes place alternately in two planes at right angles to each other, and the individuals remain attached in flat tablets of four, or its multiples. [Illustration: FIG. 83.--Types of bacteria--cocci: 1, Diagram of sphere indicating planes of fission; 2, diplococci; 3, streptococci; 4, tetrads; 5, sarcinæ; 6, staphylococci.] _Sarcinæ_, where division takes place in three planes successively, and the individuals remain attached in cubical packets of eight and its multiples. [Illustration: FIG. 84.--Types of bacteria--bacilli, etc.: 1, Bacilli; 2, diplobacilli; 3 streptobacilli; 4, spirilla; 5, vibrios; 6, spirochætæ.] _Micrococci_ or _Staphylococci_, where division takes place in three planes, but with no definite sequence; consequently the individuals remain attached in pairs, short chains, plates of four, cubical packets of eight, and irregular masses containing numerous cocci. ~2. Bacilli~ (Fig. 84, 1 to 3).--Rod-shaped cells. A bacillus, however short, can usually be distinguished from a coccus in that two sides are parallel. Some bacilli after fission retain a characteristic arrangement and may be spoken of as _Diplobacilli_ or _Streptobacilli_. Leptothrix is a term that in the past has been loosely used to signify a long thread, but is now restricted to such forms as belong to the leptothriciæ (_vide infra_). ~3. Spirilla~ (Fig. 84, 4 to 6).--Curved and twisted filaments. Classified, according to shape, into-- Spirillum. Vibrio (comma). Spirochæta. Many Spirochætes appear to belong to the animal kingdom and are grouped under protozoa; other organisms to which this name has been given are undoubtedly bacteria. Higher forms of bacteria are also met with, which possess the following characteristics: They are attached, unbranched, filamentous forms, showing-- (a) Differentiation between base and apex; (b) Growth apparently apical; (c) Exaggerated pleomorphism; (d) "Pseudo-branching" from apposition of cells; and are classified into-- 1. Beggiotoa. } Free swimming forms, which 2. Thiothrix. } contain sulphur granules. 3. Crenothrix. } 4. Cladothrix. } These forms do not contain 5. Leptothrix. } sulphur granules. 6. Streptothrix. A group which exhibits true but not dichotomous branching, and contains some pathogenic species. The morphology of the same bacterium may vary greatly under different conditions.