178 GENERAL BIOLOGY OF MICRO-ORGANISMS 



around the inside of the larger portion of the tube after the 

 manner of an Esmarch roll-tube. The bacteria which are filtered 

 out by the sugar will develop as so many colonies upon the 

 solidified medium. 



Many important micro-organisms, and certainly some germs 

 of disease, are borne through the air by the winged insects, and 

 to a less extent by birds. The microbes are found not only on 

 the feet and outer body surfaces of these carriers, but they also 

 occur on and in the mouth parts, in the alimentary canal and 

 sometimes in the interior of the animal's body tissues. Certain 

 pathogenic micro-organisms (Plasmodium, Trypanosoma) are 

 known to be transmitted from one person to another almost 

 exclusively by biting insects, and the importance of these carriers 

 in air-borne disease of both animals and plants, is being recognized 

 more and more. 



Micro-organisms of Water and Ice. The water of rivers, 

 lakes and the ocean always contains bacteria. The number 

 of organisms varies greatly in different places and under different 

 conditions. The number of different species found in water 

 is also very large. Some of these, the natural water bacteria, 

 including many bacilli which produce pigment and some cocci 

 and spirilla, seem to live in surface water as their natural habitat. 

 With the addition of putrescible material these forms are in- 

 creased in number and certain of them (Proteus group, fluorescing 

 bacteria) become numerous. Soil bacteria are numerous in 

 waters during floods and after rain, and they may persist for 

 some time. Intestinal bacteria occur in waters which receive 

 sewage or are otherwise contaminated with excreta. They 

 persist only a relatively short time. Certain intestinal protozoa, 

 Entamaba, Aalantidtum, seem also to occur in waters at times. 

 Ground-water 1 contains few or no bacteria under normal condi- 

 tions, and is therefore suitable for a source of water-supply, 

 when a sufficient amount is available. The possibility of contami- 



1 Ground-water is the water which originally derived from rain or snow sinks 

 through superficial porous strata, like gravel, and collects on some underlying, 

 mpervious bed of clay or rock. 



