MICROORGANISMS AND HUMAN WELFARE 



11 



colorless organisms ; these are the bacteria. A careful study 

 of many kinds of bacteria shows that they have several char- 

 acteristic shapes (see Fig. 7), by means of which they may be 

 roughly classified. Some are rod-shaped (like a firecracker), 

 some are spheri- 

 cal, or egg-shaped, 

 and still others are 

 spiral-shaped. 

 Each bacterium is 

 a tiny bit of trans- 

 lucent protoplasm, 

 inclosed in a cell 

 wall of cellulose. 

 Thus far no nu- 

 cleus has been dis- 

 covered in any 

 kind of bacteria. 

 Because of their 

 cellulose walls, 

 and because of 

 their likeness to ~ 



certain low forms n (1 H 



of green plants, bi- H rl H H 



U UUU 



bacteria bacteria with 



reproducing spores 



FIG. 7. Various forms of bacteria. 



spherical bacteria 

 (cocoi) 



rod-shaped 

 bacteria 

 (bacilli) 



spiral bacteria (spirilla) 



ologists now regard 

 these organisms 

 as plants rather 

 than animals. ' 



Some kinds of bacteria have one or more long, hairlike 

 projections from the ends, called cil'i-a, which give the germs 

 still further resemblance to firecrackers. These cilia lash 

 about rapidly, and thus drive the cell through the water. 

 The spiral bacteria roll over and over, and advance in a spiral 

 path like a corkscrew. 



