BACTERIA. 



55 



is not understood. Many of the trick obaderia are surrounded by a mem- 

 brane homologous to the capsule in the lower bacteria and known as a 

 sheath. 



The best-known member of this group is the water-pest bacterium 

 (Crenothrix polyspora), (Fig. 38) an iron bacterium, which has the power 

 of oxidizing certain forms of iron, causing a deposit to accumulate in the 

 water pipes of cities where it may cause considerable trouble. It is prob- 



FlG. 38. Crenothrix polyspora Cohn, Brunnenfaden. 



and Weiss.) 



(After Migula from Schmidt 



able also that this bacterium has had a very important part in the depo- 

 sition of our iron ores, such as those found on the Mesaba range. Another 

 member is the Actinomyces boms (Fig. 98) which is the cause of the 

 common disease in cattle known as lumpy jaw. This bacterium may 

 also infect man. Many other forms of trichobacteria are found in 

 nature and probably play important parts in the chemical transformation 

 of matter 



