THE EARTHWORM 



199 



which have the digestive action. These are called ferments, or 

 enzymes. The peculiar quality of an enzyme is to cause a 

 chemical change in another substance without itself losing 

 any of its own properties. Thus, trypsin, the enzyme which 

 acts on proteids, can do so and still remain trypsin. Similarly, 

 the enzyme diastase acts upon starch, and steapsin upon fats. 



Let us consider for a moment why 

 it is necessary for the earthworm, 

 or any animal, to secrete elaborate 

 chemical mixtures like digestive 

 fluids. If we were to examine the 

 alimentary canal of the earthworm, 

 we should find that there are no 

 openings leading from the canal to 

 the body-cavity which might permit 

 food to pass directly to different 

 organs. The wall of the canal is 

 thin, but it is made of cells which 

 are packed closely together. Water 

 will pass through this membrane 

 easily, but certain other substances, 

 although in a liquid state, will not 

 pass through so readily. Those solu- 



f. ,.? j.1 ' u i 



tions which pass through an animal- 

 membrane readily are called crystal- 



7 . , ,. , r> i 



loids; for example, solutions of salt or 

 sugar. Liquids which do not pass readily through an animal- 

 membrane are known as colloids; for example, solutions of 

 meat-juice or starch. The action of the digestive fluid is a 

 double one : it changes the state of the solid food physically, 

 by rendering it liquid ; it changes the state of organic foods 

 '(both solid and liquid) chemically, giving at the same time to 

 each of the altered food-substances a new physical property, 

 namely, that of being able to pass through the intestine-wall. 



i- 101. Experiment show- 

 ing Osmosis 



bottle; 2, water; 3, tube; 



4> piece of sheep , s intest ine 

 containing salt-solution; 



5, level of salt-solution 



