106 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



called ''cellulase" or "cytase." It is interesting and signifi- 

 cant in this connection that those animals (the herbivora) 

 in which cellulose forms the chief article of diet do not pos- 

 sess any cytase-producing glands, but depend upon certain 

 bacteria present in their intestines to supply the enzyme for 

 cellulose digestion. 



Lipase is an enzyme which has the ability to split fat 

 into glycerin and fatty acid. It is produced in the stomach 

 and pancreas of the higher animals and is present in some 

 plants. A few bacteria are known that have the ability 

 to split fat into the same components. 



Pepsin and trypsin (proteases) are proteolytic enzymes 

 produced in the stomach and intestines of the higher ani- 

 mals. These enzymes have been little studied in the micro- 

 organisms. Marshall states, however, that "there is so 

 far as can be determined, httle appreciable difference be- 

 tween the proteolytic enzymes obtained from different 

 organisms, whether low or high in the plant or animal 

 world, consequently many experiences with animal pep- 

 sin and trypsin can be applied to microbial enzymes." 



The blood-clotting enzyme, thrombase, does not occur 

 in microorganisms, according to Marshall. 



Rennet is a substance present in the stomach of calves. 

 Rennet is found in many species of microorganisms. Ren- 

 net-forming bacteria are found in milk and dairy products, 

 in soil and other habitats. 



Pigment formation is an activity that is widely distrib- 

 uted in plants and animals. Many bacteria are known 

 in which pigments are produced. 



A few strains of bacteria are known that have the prop- 

 erty of decomposing hemoglobin. One of these forms is 

 Streptococcus hemolyticus. Certain functions in the mam- 

 mahan body are of a similar nature. It is beheved that 

 the spleen has the property of destroying red blood cor- 

 puscles when their hemoglobin content falls to a certain 



