iv] MODES OF NUTRITION 115 



evolution of such a digestive fluid in the internal 

 cavities evolved for the purpose of increasing the 

 absorptive surface of animals. Small particles taken 

 into them would be ingested by the cells lining their 

 walls just as small particles are ingested by the 

 amoeboid cells of the alimentary canals of many 

 worms, and just as an amoeba ingests its prey. But 

 if the food particles were too big to be ingested in 

 this way? Even then we need not postulate the 

 existence of a digestive fluid, for autolysis of the 

 food organism would occur when it died. Every 

 cell of the body of an animal contains proteolytic, 

 lipolytic, and amylolytic enzymes, and when a tissue 

 dies self-digestion, or autolysis, begins. It is by 

 autolysis that the exudation producing consolidation 

 of the lung disappears in cases of recovery from lobar 

 pneumonia. Or bacterial decomposition of the body 

 of the entrapped food organism would occur. The 

 same result would be attained either by autolysis, 

 bacterial decomposition, or by digestion that is the 

 resolution of the proteids to the stage of amino-acids, 

 that of the fats to fatty acids, and that of the carbo- 

 hydrates to soluble sugars. All these substances 

 would then be absorbed by the walls of the internal 

 cavity in which they lay. 



We may however reasonably suppose that a cell 

 in contact with an eatable organism will be stimulated 



82 



