282 PART IV. CLASSIFICATION. 



the most part, merely phases in the life-history of the more com- 

 plex forms. 



The Schizomycetes multiply mainly by cell-division (whence 

 their name), and they do so with great rapidity under favourable 

 conditions. In many forms reproduction is also effected by means 

 of spores (e.g. Bacillus subtilis, Bacillus Anthracis, Clostridium 

 butyricum). Each spore is formed from a cell, the protoplasmic 

 contents contracting from the cell-wall and surrounding them- 

 selves with a thick proper wall ; the spore is set free by the decay 

 of the old cell-wall. Spore-formation generally takes place in 

 the zooglo3a-stage, and is promoted by conditions which are un- 

 favourable to growth and multiplication by division. The vitality 

 of the spores is remarkable, being retained under conditions, such 

 as extremes of temperature, desiccation, etc., which would prove 

 fatal to the organisms themselves. 



A comparatively simple life-history is that of Bacillus subtilis, which 

 makes its appearance in infusions of hay when allowed to stand. The in- 

 fusion gradually becomes turbid, owing to the rapid multiplication of the 

 Bacillus. At first the organisms move actively by means of cilia (Fig. 

 163, B), the motile form: after this the cells cease to be motile, and 

 remain connected into filaments, the bacillus-form: then a membranous 

 scum forms on the surface of the liquid, the zoogloea-form : at this stage 

 spore-formation begins (Fig. 163, C) and the life-cycle is at an end. The 

 spores give rise, on germination, to the motile form, and so the cycle is 

 repeated. 



The most conspicuous feature in the physiolog}- of the Schizomy- 

 cetes is their capacity for decomposing organic compounds, indu- 

 cing various fermentative processes, such as the lactic and the 

 butyric fermentation of various kinds of sugars, etc. (but never 

 the alcoholic fermentation), and the putrefactive fermentation of 

 complex nitrogenous organic substances, such as proteids, etc. 

 Some are parasitic in the bodies of animals, such as Sarcina Ven- 

 triculi, Leptothrix buccalis which causes decay of the teeth, and 

 the various forms of Bacteria which cause Phthisis, Cholera, An- 

 thrax, and other diseases. 



The particular form presented, and the degree of the physiolo- 

 gical activity manifested, at any given time, is determined by the 

 external conditions, such as the nature of the obtainable food, the 

 temperature, the presence or absence of oxygen, etc. ; important 

 variations in any of these conditions may induce change from one 



