THALLOPHYTA. 623 



conditions remain favourable for further growth. When the substratum is 

 exhausted, &c., spores are formed which can remain for long periods — until, indeed, 

 circumstances are again favourable for renewed development. These spores may- 

 arise either inside the bacterial cells ( = endospores), the protoplasm contracting 

 somewhat and forming a new wall around itself, or the cells may become trans- 

 formed entirely into spores ( = arthospores), the wall becoming specially thickened. 

 In no case is there any sexual process. Whilst many Bacteria are only known 

 under certain forms and are regarded as species of the genera Micrococcus, Bae- 

 krium, Bacillus, Spirillwm, &c., others are known which, in the course of their 

 development, pass through several such forms, and are termed pleomorphic. That 

 all Bacteria are thus pleomorphic seems improbable, though the discovery that 

 pleomorphism existed at one time led to the wildest generalizations. Large 

 numbers of Bacteria display an active movement which, though formerly attri- 

 buted to various contractions of their bodies, are now known to be due to cilia. 

 These cilia may be borne in tufts of 5 or 6 at the two ends of the organism, as in 

 Spirillura Undula, or they may be solitary at one extremity, as in the Cholera 

 Bacterium (Spirochcete cholerce asiaticce), or they may be scattered over the surface 

 of the organism, as in the Hay-bacillus, Bacillus subtilis. It is due to the extreme 

 ■fineness of these cilia that they were not recognized long ago. 



Though the forms under which Bacteria occur are relatively few, their mode of 

 life and special activity is exceedingly varied. The interest attaching to Bacteria 

 rests largely on their eifects on the substratum from which they draw their food. 

 Taking, iirst, the saprophytes. These split up their substratum into simple sub- 

 stances. In some cases there is a complete oxidation, with production of carbon 

 dioxide and water; in others this is only partial, as in some of the cases of 

 fermentation, e.g. when alcohol is oxidized into acetic acid by the activity of the 

 vinegar organisms Bacillus and Micrococcus aceti {cf. figs. 368^ and 368*). Or 

 there may be a decomposition unaccompanied by simple oxidation, as in many 

 cases of fermentation, e.g. as when sugar is split into alcohol and carbon dioxide. 

 Often these operations are accompanied by the development of a foul-smelling gas, 

 when we speak of putrefaction. The number of saprophytic Bacteria which excite 

 ■characteristic splittings in their substrata is considerable. In addition to those 

 •already quoted, we may mention Bacillus Ainfiylohacter, the organism of butyric 

 acid fermentation; Bacillus lacticus, which causes milk to become sour; Leuconostoc 

 mesenterioides, which has the power of converting large quantities of sugar into a 

 gelatinous mass in a very short space of time. Again, in a number of forms the 

 production of a special colouring matter is associated with the activity of the 

 organisms, as is the case with Micrococcus prodigiosus (cf. fig. 368 '), the "blood- 

 portent" which makes its appearance on various starchy food-stufFs, and Beggiatoa 

 "roseo-persicina, found on decaying vegetable matter in water, and known as 

 "peach-mud". Many Bacteria are parasitic in the bodies of animals, and some 

 among them are harmless. This is the case with Sarcina ventriculi (fig. 368^°), 

 known only in the human alimentary canal in the form of packets of cells. Harm- 



