400 MICROORGANISMS 



different forms of bacteria (Fig. 271). Sometimes you will 

 have all three forms on your slide at once. The three forms 

 are: first, A, spheres, called cocci (singular, coccus) ; second, 

 B, rods called BACILLI (singular, bacillus}', third, C, spirals 

 called SPIRILLA (singular, spirillum). Some of each of these 

 forms possess locomotor, or swimming, organs in the form of 

 FLAGELLA (B, Fig. 271) and are therefore able to swim through 

 the water like an animal. Others lack these organs and are 

 therefore non-motile. The relative abundance of the three 

 different forms is indicated by the work of Migula, a German 

 bacteriologist, who, in the year 1900 enumerated and described 

 833 bacilli, 343 cocci, and 96 spirilla. 



* ' C 



FIG. 271. Cocci, spirilla, and bacilli. 



Since the structure of bacteria is very simple and since all 

 the different kinds fall under one of these three different forms, 

 it is obvious that the different kinds of bacteria can not be 

 distinguished merely by microscopic examination. Bacteri- 

 ologists have devised many ways of distinguishing them, but 

 these methods are too difficult and too intricate for us to 

 enter into. It is enough to say that bacteria differ from each 

 other physiologically rather than structurally. Two bacteria 

 may look for all the world alike under the microscope, and yet 

 their waste products and secretions may differ very widely. 

 These products of the one may be a violent poison in our bodies, 

 while those of the other may be perfectly harmless to us. 



344. Where Bacteria may be Found. It would be easier 

 to say where bacteria are not found, for they are found 

 almost everywhere. They are doubtless absent in the midst 

 of deserts, in the depths of the sea, in the frigid regions 

 near the poles, and in the tissues of healthy animals and 

 plants, but in almost any other place on or near the surface 



