94 



COMPARATIVE ZOOLOGY. 



stood. It saponifies part of the fats, so that they are dis- 

 solved, and prevents the food from decomposing during 

 the process of digestion and absorption. The chyle is 

 slowly driven through the small intestine by the creep- 

 ing, peristaltic motion of its walls, 49 the nutritious portion 

 being taken up by the absorbents, as described in the next 

 chapter, while the undigested part remaining is discharged 

 from the large intestine. 50 



CHAPTER XL 



THE ABSORBENT SYSTEM. 



The nutritive matter (chyle), prepared by the digestive 

 process, is still outside of the organism. How shall it 

 enter the living tissue ? 



In animals, like the Infusoria and Polyps, whose digest- 

 ive department is not separated from the body-cavity, the 

 food, as soon as dissolved, mingles freely with the tissues 

 and organs it has to nourish. In the higher Invertebrates 

 having an alimentary canal, the chyle passes, by simple 

 transudation, through the walls of the canal directly into 

 the soft tissues, as in Insects, or is absorbed from the canal 

 by veins in contact with it, as in Sea-urchins, Mollusks, 

 Worms, and Crustaceans, and then distributed through 

 the body. 



In Vertebrates only do we find a special absorbent sys- 

 tem. Three sets of vessels are concerned in the general 

 process by which fresh material is taken up and added to 

 the blood s Capillaries, Lacteals, and Lymphatics. 

 Only the two former draw material from the alimentary 

 canal. 



It is a general law that the food is absorbed as fast as 



