228 SULPHUR BACTERIA 



In Thiobacillns Bovistus a further step is marked by a slightly 

 greater interdependence of the enclosed cells. In Thiohacillus 

 thiogenus the advance has taken the form of a more complex 

 covering of slime, an inner and an outer layer being distin- 

 guishable, the outer with an obviously protective function. 

 An interesting extension of this line of development is that 

 shown in Thioploca, for here the movements of the filaments 

 inside the slimy colony are not unrelated, showing that 

 the units have already lost their independent character. 

 This is the last stage in the evolutionary series along this 

 line. 



B. Enlargement of the Coccus. — Large cocci are occasionally 

 formed as pleomorphic phases of Lankesteron roseo-persicina, 

 and it is possible that the evolution of such organisms as 

 Chromatium, Achromatium, and Thioporphyra may have re- 

 sulted from the stabilization of similar large forms. Under 

 unfavourable conditions both Chromatiinn and Thioporphyra 

 tend to revert to smaller cocci. 



C. Shortening of the Filament. — The greater freedom of 

 movement possible to short rods in which cilia have developed 

 indicates another line of development. The filaments of such 

 primitive forms as Lankesteron roseo-persicina readily break up 

 into shorter lengths ; these are probably the precursors of 

 such bacteria as Thiobacillus and Thiopseudomonas. Thio- 

 hacillus Bovistus appears to be an intermediate species in this 

 line of development, for in it there is considerable slime for- 

 mation, whilst the cells on the other hand have attained a 

 certain freedom of movement. 



D. Colonial Habit. — In Thiopedia the cocci, although 

 somewhat loosely arranged, develop symmetrically. In 

 Rhodolhiosarcina a further advance is noted, as the cocci are 

 closely associated in regular formation, and whilst in Thio- 

 pedia slime formation is regular, in the other it has become 

 entirely suppressed. The ease with which the disruption of 

 the units of a Sarcina can be brought about so that they exist 

 as uni- and diplo-cocci shows that the grouping of the cells to 

 form colonies is not a racial habit of long duration as time is 

 reckoned in evolution. 



