DIGESTION AND ABSORPTION. 101 



animal may under certain circumstances absorb kinetic energy in 

 the form of heat, but this is available only as a condition, not as 

 a cause of protoplasmic action. In this inability to use kinetic 

 energy the earthworm is typical of animals as a whole. 



Of the organic portion of the food proteids are a sine qua 

 non, and in this respect again the worm is a type of animal life 

 in general. Either the fats or the carbohydrates may be omitted 

 (though the annual probably thrives best upon a mixed diet in 

 which both are present), but without proteids no animal, as far 

 as is known, can long exist. 



General History of the Food. Digestion and Absorption. 

 Lumbricus takes daily into its alimentary canal a certain amount 

 of necessary food-stuffs, but these are not really inside the body 

 so long as they remain in the alimentary canal ; for this is shown 

 by its development to be only a part of the outer surface folded 

 in to afford a safe receptacle within which the food may be 

 worked over. Before the food can be actually taken into the 

 body, or absorbed, it must undergo certain chemical changes col- 

 lectively called digestion (cf. p. 49). A very important part 

 of this process consists in rendering non-diffusible substances dif- 

 fusible, in order that they may pass through the walls of the 

 alimentary canal into the blood. Proteids, for example, have 

 been shown to be non-diffusible (Chap. III). In digestion they 

 are changed by the fluids of the alimentary canal into peptones 

 substances much like proteids, but readily diffusible. In 

 like manner the non-diffusible starch is changed into diffusible 

 sugar and becomes capable of absorption. It is highly probable 

 that all carbohydrates are thus turned into sugar. The fats are 

 probably converted in part into soluble and diffusible soaps which 

 are readily absorbed, but are mainly emulsified and directly passed 

 into the cells of the alimentary tract in a finely divided state. 

 Nothing, however, is known of this save by analogy with higher 

 animals. In all cases digestion takes place outside the body, and 

 is only preliminary to the real entrance of food into the physio- 

 logical, or true, interior. 



Metabolism. After .absorption into the body proper the 

 incoming matters are distributed by the circulation to the ulti- 

 mate living units or cells, and are finally taken up by them and 

 built into their substance. There is reason to believe that each 



