under--

~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, sarcinae; 6, staphylococci.]

_Sarcinae_, 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,
spirochaetae.]

_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
leptothriciae (_vide infra_).

~3. Spirilla~ (Fig. 84, 4 to 6).--Curved and twisted filaments.
Classified, according to shape, into--

Spirillum.
Vibrio (comma).
Spirochaeta.

Many Spirochaetes 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 sulph