BACTERIA. 31 



these conditions the bacteria appear as solidly-colored 

 spheres, rods, or spirals, as the case may be. 



The cell-walls of some of the bacteria seem at times to 

 undergo a peculiar gelatinous change or to allow the ex- 

 udation of gelatinous material from the protoplasm, so 

 that the individuals appear surrounded by a distinct halo. 

 This is not only a peculiarity of certain individuals, but 

 one which only takes place when they develop under 

 certain conditions; thus, Friedlander points out that the 

 capsule of his pneumonia bacillus, when it was found in 

 the lung or in the "prune-juice" sputum, was very dis- 

 tinct, while it could not be demonstrated at all when the 

 organisms grew in gelatin. 



From the cell-walls of many bacteria numerous deli- 

 cate straight or wavy filaments project. These are called 

 cilia or flagella, and seem to be organs of locomotion. 

 Sometimes they are only observed projecting from the 

 ends or from one end; sometimes they are so numerous 

 and so regular in their distribution as to give the organ- 

 isms a woolly appearance. 



Many of the bacteria which are thus supplied with 

 flagella are actively motile and swim about like mi- 

 croscopic serpents. In all probability the locomotory 

 powers of the bacteria are not entirely dependent upon 

 the presence of the flagella, but may sometimes be due 

 to contractility of the protoplasm within an elastic cell- 

 wall. The micro-organisms most plentifully supplied 

 with them are those of the rod and spiral shape. Only 

 one of the spherical forms, Micrococcus agilis of Ali- 

 Cohen, has been shown to have flagella. This and one 

 other species are probably the only motile cocci. Ob- 

 serving that the organisms known to be most active are 

 those best supplied with flagella, it is reasonable to con- 

 clude that the motility is dependent upon the flagella. 



The presence of flagella, however, does not necessarily 

 imply motility, for some of the bacilli amply provided 

 with these appendages are not motile (bacillus coli com- 

 munis). The flagella may not only serve as organs of 



