BACTERIA 103 



occur have been given special names as follows: A coccus is spherical 

 in form, a bacterium or bacillus is short rod shaped, a bacillus slightly 

 bent is a vibrio, while one more strongly curved is a spirillum; straight 

 thread-like forms are called leptothrix and corkscrew forms are spirochaete. 



FIG. 58. Bacilli of various forms. (From Williams.) 



228. After division the cells may adhere in chains or become free. When 

 the cell walls become gelatinous the cells adhere in a large mass which is 

 known as a zooglea. Spores are formed by the contraction of a part of 

 the protoplasm into a dense mass which then surrounds itself with a cell 



* ->' 



>V^y 



VVT/VV~ 



FIG. 59. Spirilla of various forms. (From Williams.) 



membrane. Because of their mode of formation these are called endo- 

 spores. They are highly resistant. 



229. The physiological differences between bacteria are very great. 

 This is evident in the substances which they excrete and the effect pro- 

 duced by these excretions on the surrounding medium. A number of 



* 



FIG. 60. Staphylococci. Streptococci. Diplococci. Tetrads. Sarcinae. 

 (From Williams.) 



examples will be mentioned. Closteridium butyricum and many other 

 bacteria thrive where there is no free oxygen. This is possible because 

 they have the power of decomposing substances containing oxygen. On 

 the other hand, Bacillus aceti, and others, cause the combination of alcohol 

 and oxygen to form acetic acid (vinegar) . The nitrite bacteria in the soil 

 oxidize ammonia into nitrites and the nitrate bacteria continue the oxi- 



