THE THIONIC ACID BACTERIA 225 



gether and covered with a very thin layer of sHme. Pure 

 cultures were not obtained. 



Bacterium retiformans is 2 — 4'5/x long and 0-5 — i-o^ broad. 

 It develops as zoogloea colonies adhering to the walls of the 

 culture flask. It is doubtful whether these two organisms 

 belong to the genus Bacillus (or Bacterium). 



Pseudomoiias bipunctatiis (Gicklhorn) is provisionally 

 placed in this group, but it is doubtful whether this organism 

 should be classed in the group of Bacteria. Its characteristics 

 are rather those of the simpler Flagellates. Furthermore, its 

 physiology differs somewhat from that of the thionic acid 

 bacteria. It is composed of a single colourless transparent 

 cell, which becomes ovoid during division. Its length is 

 8 — I2/X and breadth 3 — 5jLt. It possesses a single, delicate 

 polar cilium, about I2ju. long, which propels the organism from 

 behind at the rate of 600/L1 per minute. Each cell contains 

 two, sometimes three, highly refractive drops. These resemble 

 the large drops found in Microspira vacillans. Their com- 

 position has not yet been ascertained. They are not composed 

 of sulphur, and the sole justification for the inclusion of the 

 organism in the sulphur bacteria rests on the fact that the 

 organisms show a preference for sulphuretted hydrogen. 



After division, many of the daughter cells are temporarily 

 pear shaped, and sometimes even show pointed ends (Fig. 23c). 

 Pure cultures have not been obtained. 



Habitat. — Found in foul mud from the Botanic Gardens 

 of Graz. 



Pseudomonas hyalina (Gicklhorn) resembles the preceding 

 in general form and ciliation, but is smaller, measuring 4 — 6jain 

 length and 2 — 2-5^ in breadth. The reason for its separation 

 from the preceding species lay in the absence of intermediate 

 sizes. As both were found in the same medium, and at the 

 same time, the probabilities are in favour of the two organisms 

 being the same species. 



Habitat. — As the preceding species. 



15 



