FUNCTIONS OF ANIMAL ORGANISMS 461 



at the end of it. In the sponges, too, digestion is intracellular. In 

 coelenterates and ctenophores digestion is begun in the gastrovascular 

 cavity and is, therefore, partly extracellular, but it is completed intracel- 

 lularly; the same thing is true of flatworms, except in the case of cestodes, 

 where a digestive system is entirely lacking. From the roundworms 

 onward digestion takes place in the alimentary canal, is extracellular, 

 and is caused by a variety of enzymes. 



A number of the most important digestive enzymes have already 

 been mentioned in connection with the different phyla. In the proto- 

 zoans proteins form almost the exclusive food, though under certain 

 conditions these organisms can utilize fats and starches. ' In passing 

 gradually to the higher phyla the number of enzymes increases and the 

 variety of foods which can be digested and absorbed also becomes 

 greatly increased. 



In mammals the stomach is largely mechanical in its function, 

 though it also carries on digestion. It serves in many cases to hold a 

 considerable supply of food, which is gradually reduced to liquid form, 

 being changed to the consistency of thick pea soup, in which state it is 

 known as chyme. The hydrochloric acid of the gastric juice serves to 

 convert the pepsin-containing secretions of the gland cells to active 

 pepsin, to create proper conditions for the activity of these secretions, 

 and also to control the action of the sphincter muscle which guards the 

 pylorus, the opening into the intestine, and which is normally in a state 

 of tonic, or continuous, contraction. The acid inhibits the sphincter 

 muscle, which dilates and permits a portion of the chyme to pass, after 

 which the sphincter again closes and remains closed until the chyme in 

 the intestine has become alkaline and that of the stomach has reached 

 the necessary degree of acidity. Then the pylorus is again opened and 

 another portion of the chyme is passed on. 



In the intestine the chyme is slowly passed along by peristalsis. 

 During the time that the chyme is in this part of the alimentary canal 

 the amino acids and sugars are absorbed into the blood; the fatty acids 

 and glycerin, into the lymphatics; and water, into both. The fatty 

 acids and glycerin are recombined in the process of absorption, appearing 

 in the lymphatics in the form of fats, changing the lymph to what is 

 known as chyle. 



It may be added here that the amino acids are taken up by the various 

 cells of the body and are either used in growth or are immediately oxi- 

 dized; the fats are also taken up by the cells and oxidized, releasing energy 

 in the form of heat, or are stored; while the sugars are stored in the 

 liver, to be passed into the blood and used, especially by the muscles, as 

 needed. 



509. Circulation. — Circulation takes place in protozoans only by 

 currents within the cytoplasm. In sponges it is effected through the 



