DIGESTION AND ABSORPTION. lOl 



animal may under certain circumstances absorl) kinetic enerirs' 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 wIkjIc. 



Of the organic portion of the food proteids are a »iti£ qmv 

 non^ and in this respect again the worm is a type of aTiimal life 

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

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

 which both are present), but without 2)roteids no animal, as far 

 as is known, can long exist. 



General History of the Food. Digestion and Absorption. 

 LxiDibricus 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 actuallv taken into the 

 body, or absorhed^ 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 exam})le, 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 diff"usible. \\\ 

 like manner the non-diffusible starch is chano^ed into diffusible 

 sugar and becomes capable of absorption. It is higlily j)robable 

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

 probably converted in part into soluble and diff"usi])le soaps whieh 

 are readily absorbed, but are mainly enuilsitied and directly passed 

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

 Nothino^, however, is known of this save by analoi^v with hii^her 

 animals. In all cases digestion takes place outside the ho(fy^ and 

 is only preliminary to the real entrance of food into the physit>- 

 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 



