UNICELLULAR FORMS 



8i 



Anaerobes. Now it follows that if an organism is equipped 

 with an enzyme system that will break up compounds which contain 

 potential energy and thus release energy for the organism, that 

 particular organism may be expected to survive and in fact to thrive 

 in the absence of free oxygen, since the union of its food with free 

 oxygen is not necessary for supplying energy. Both plants and ani- 

 mals that flourish in the absence of 



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COCCI 



BACILLI 



air-contammg oxygen are common. 

 These are called anaerobes. Some 

 types are killed by atmospheric 

 oxygen; they are obliged to live 

 shielded from the air. These are the 

 OBLIGATORY ANAEROBES. Othcrs peri- 

 odically live in the absence of oxy- 

 gen; these are facultative ana- 

 erobes. 



Bacteria. We return now to the 

 parasitization of animals by micro- 

 scopic forms. The most important of 

 these are plant parasites which be- 

 long to the Schizophyta, the fission 

 plants. These are the bacteria (p. 77). 

 Not all bacteria are parasitic on 

 Man, nor do all those which do 

 parasitize Man cause disease. Bac- 

 teria are small single-cell plants, so 

 small that none of them can be distinguished by the unaided eye 

 and so small that some of them are beyond the powers of the 

 microscope. A single definite nucleus is not distinguishable; what 

 is taken to be nuclear material in some forms is scattered through- 

 out the cell. They reproduce rapidly under suitable conditions. 

 There are very many varieties known and perhaps many others 

 unknown; they parasitize all sorts of animals and plants, feed on 



SPIRILLI 



Fig. 26. — Various forms of 

 bacteria. (From several au- 

 thors.) 



