§ IX.] Aquatic Saprophytes. Spirillum. 83 



constant in their forms and distinctions. They are often found 

 by themselves in the waters of bogs ; when they occur in large 

 quantities, and comparatively unmixed with other species, they 

 form dense swarms which, in S. Undula especially, are of a 

 beautiful dark reddish-brown colour. Single live rods are, 

 under the microscope, colourless and homogeneous. When 

 killed and treated with colouring reagents (iodine, anilin-dyes), 

 they exhibit a remarkable separation in the case of both species 

 into short irregular transverse zones of alternately darker and 

 lighter colour — a phenomenon which must not be confounded 

 with the transverse segmentation into distinct cells mentioned 

 above. Finally, both species are distinguished by the extreme 

 vivacity of their movements, and dart like meteors, says Cohn, 

 across the field of vision, the slender Spirillum tenue affording 

 in this way a very elegant display. 



Many other forms of the kinds previously described, and 

 among them of endosporous Bacilli, might be mentioned as 

 living in water. We still desiderate such investigation of these 

 forms as would enable us to give a more exact account of them 

 and of the decompositions which they may effect ; the scattered 

 particulars which are known of them have no interest for us on 

 the present occasion. Of the germs of Bacteria which may be 

 found even in the purest waters when exposed to the air and 

 to dust I have already spoken in the fifth Lecture. 



IX. 



Saprophytes which excite fermentation. Fermentations 

 of urea. Nitrtflcation. Acetous fermentation. Vis- 

 cous fermentations. Formation of lactic acid. Keflr. 

 Bacillus Amylobacter. Decompositions of proteid. 

 Bacterium Termo. 



We have now to consider saprophytic forms which are known 

 to be the causes of distinct processes of decomposition or 

 fermentation, and as examples of more general interest we select 



G 2 



