Hillhousia mirabilis, a Giant Sulphur Bacterium. 399 



the microscope, but a pale yellow or yellow-green colour is quite obvious on 

 careful inspection by transmitted daylight. 



Owing to the large quantity of sulphur they contain, the cells are very 

 heavy, sinking in the water almost like sand-grains. 



The cells are motile, exhibiting slow oscillatory and rolling movements. 

 The organism is a peritrichous bacterium with several hundred short cilia 

 disposed all over the exterior of the cell-wall. The cilia can be seen imme- 

 diately on fixation either with a 5-per-cent. carbolic acid solution or with 

 a 40-per-cent. formalin solution. The action of these reagents results 

 in a cessation of the movements of the cilia in from 10 to 20 seconds, during 

 which period many of them are thrown off and become disintegrated. Some 

 of these cilia exhibit a contractile or wriggling movement after having been 

 thrown off, indicating that the contractility of the cilium is not dependent 

 upon attachment at the base. The cilia are plainly observed by the use of 

 the above reagents, but only within a few seconds of the moment of fixation 

 (figs. 15 and 16). 



The cilia can also be clearly seen in active movement by examining the 

 living organism by means of the Zeiss or Leitz dark-ground illuminator. It 

 is likewise possible to demonstrate the presence of cilia by immersing the 

 living organisms in a drop of indian ink. The cilia appear as a halo around 

 the cell, and the minute particles of ink can be seen in rapid motion on the 

 edge of the halo. This movement is of a different character from the 

 Brownian movement of the minute particles in the water. The tiny particles 

 of ink can be seen to be lashed about and driven into small eddies. 



Cell-division is relatively slow, and in no observed instance was a single 

 division completed in less than 24 hours, the time occupied in many cases 

 being as much as 48 hours. 



The Sulphur Globules. — The globules of sulphur are enormously larger and 

 much more densely packed than in any of the other known sulphur bacteria. 

 They are dissolved by glacial acetic acid,* by boiling in a solution of 

 magnesium sulphate ; and also by prolonged boiling in potassium chlorate. 

 They turn almost black on boiling in ferrous sulphate ; and a brownish-black 

 precipitate is found in the cells after boiling in lead acetate. A 2-|-per-cent. 

 solution of commercial formalin entirely removes the globules. Carbon 

 bisulphide penetrates the cell-wall only with difficulty, but after penetration 

 quickly dissolves the sulphur. 



When the living organisms in a pure condition are placed in distilled water 



* This was pointed out by Corsini in the case of Beggiatoa ; cf. A. Corsini, " TJeber die 

 sogenannter Schwefelkornchen die man bei des Familie der ' Beggiatoacse ' antrifft " 

 ' Centralbl. fur Bakteriol.,' II, 1905. 



