capsule, which cause acute pneumonia; (_d_) gonococci, with the
opposed surfaces flattened, which cause gonorrhoea. _e_, _f_, _g_,
_h_, Rod-shaped bacteria or bacilli: (_e_) diphtheria bacilli; (_f_)
tubercle bacilli; (_g_) anthrax bacilli; (_h_) the same bacilli in
cultures and producing spores; a small group of spores is shown. (_i_)
Cholera spirillæ. (_j_) Typhoid bacilli. (_k_) Tetanus bacillus;
_i_, _j_, _k_ are actively motile, motion being effected by the small
attached threads. (_l_) The screw-shaped spirochite which is the cause
of syphilis.]

The bacteria (Fig. 17) are unicellular organisms and vary greatly in
size, shape and capacity of growth. The smallest of the pathogenic or
disease-producing bacteria is the influenza bacillus, 1/51000 of an
inch in length and 1/102000 of an inch in thickness; and among the
largest is a bacillus causing an animal disease which is 1/2000 of an
inch in length and 1/25000 of an inch in diameter. Among the
free-living non-pathogenic forms much larger examples are found. In
shape bacteria are round, or rod-shaped, or spiral; the round forms
are called micrococci, the rod-shaped bacilli and the spiral forms are
called spirilli. A clearer idea of the size is possibly given by the
calculation that a drop of water would contain one billion micrococci
of the usual size. Their structure in a general way conforms with that
of other cells. On the outside is a cell membrane which encloses
cytoplasm and nucleus; the latter, however, is not in a single mass,
but the nuclear material is distributed through the cell. Many of the
bacteria have the power of motion, this being effected by small
hair-like appendages or flagellæ which may be numerous, projecting
from all parts of the organisms or from one or both ends, the movement
being produced by rapid lashing of these hairs. A bacterium grows
until it attains the size of the species, when it divides by simple