52 THE BIOLOGY OF AN ANIMAL. 



making a cross-section. It is richly supplied with Hood-vessels that pass 

 down into its cavity from the dorsal vessel (Fig. 39), and its main func- 

 tion is probably to increase the surface for the absorption of food (cf. the 

 " spiral valve " in the intestine of sharks.) 



The outer surface of the stomach-intestine is covered with pigmented, 

 yellowish-brown "chloragogue-cells." These were formerly supposed to be 

 concerned with the secretion of the digestive fluid, and hence are often 

 called " hepatic cells." This, however, is probably an erroneous interpreta- 

 tion, and they are now believed to be concerned with the process of excre- 

 tion (p. 61). 



Digestion. Digestion begins even before the food is taken 

 into the alimentary canal ; before being swallowed, the leaves, 

 etc., are moistened by digestive fluid poured out from the 

 mouths of the worms. The main action, however, doubtless goes 

 on in the anterior part of the stomach-intestine and diminishes 

 as the food passes backward. It has been proved by experiment 

 that the digestive fluid acts on at least two of the three principal 

 varieties of organic food-stuffs, viz. , on proteids and on starch 

 (carbohydrate), and in so far resembles the pancreatic fluid of 

 higher animals, which it further resembles in having an alkaline 

 reaction. Analogy leads us to believe that the digestive fluid 

 has some action also on fats ; but this has not been proved. 



Krukenberg and Fredericq have shown that the digestive fluid of the 

 earthworm contains at least three ferments ; and according to the former 

 author these occur only in the stomach-intestine. They are as follows : 



1. Peptic ferment, which has the property in an acid medium of con- 

 verting proteids into soluble and diffusible peptones; this is therefore 

 analogous to the pepsin of the gastric juice in higher forms. 



2. Tryptic ferment, having a similar action on proteids, but only in an 

 alkaline medium hence analogous to the trypsin of pancreatic juice. 



3. Diastatic ferment, which converts starch into glucose (grape-sugar) 

 in an alkaline medium hence analogous to the ptyalin of saliva and the 

 amylolytic ferment of pancreatic juice. 



Absorption. The ferments of the digestive fluid convert the 

 solid proteids into soluble and diffusible peptones, the starchy 

 matters into sugar (glucose). These products dissolve in the 

 liquids present and are then gradually absorbed by the walls of 

 the intestine as the food passes along the alimentary canal. The 

 precise mechanism of absorption is not yet thoroughly understood, 

 but it is probable that much of the nutriment passes by diffusion 

 (osmosis) into the walls of the stomach-intestine and thence into 



