FUNGI 181 
Bacteria are found almost everywhere—in the air, in 
the water, in the soil, in most foods, and in the bodies of 
plants and animals, as regular inhabitants. Many of them 
are entirely harmless, some are useful, and others are very 
dangerous. A laboratory near Paris, arranged for studying 
bacteria in the air, has found that the average number of 
bacteria in every quart of air in that locality is eighty. 
The highest numbers were found during the autumn, and 
the lowest during the winter; while a wind from the city 
increased the numbers very much. The “pure” water of 
springs and wells contains abundant bacteria, while in 
stagnant water and sewer water they swarm in immense 
numbers. The slimy deposits usually observed about 
“iron” and “sulphur” springs, or in the pipes leading from 
them, are due to the presence of the peculiar bacteria liv- 
ing in such waters. The presence of dangerous bacteria 
in drinking water is probably the most common cause of 
epidemics of infectious diseases, and warnings as to the 
dangerous condition of a city water-supply should always 
be heeded. It is very evident that no sewage should find 
its way into such water-supply. 
It is important to know something about the structure 
and the habits of bacteria, before considering some of their 
important relations toman. They are one-celled and occur 
in three general forms: (1) spherical cells, usually grouped 
in various ways, and including the minutest forms 
(Fig. 121, B); (2) rod-shaped cells, that is, longer than 
broad, the cells remaining separate or attached end to end 
and forming filaments (Fig. 121, F and G@); (3) elongated 
cells, more or less curved, from short curved forms resem- 
bling a comma to long spirals (Fig. 121, J-M@). Many 
bacteria swim more or less actively by means of cilia; and 
this fact first gave the impression that they are minute 
animals—an impression that is still prevalent outside of 
laboratories (Fig. 121). 
