240 GENERAL ZOOLOGY 



secretion, and sometimes even hard particles from the 

 calciferous glands, mixes with it. This probably serves to 

 neutralize whatever acid may be present, and helps to 

 maintain the contents of the esophagus in an alkaline con- 

 dition. Thus the digestive fluid of the intestine, which is 

 alkaline in chemical nature, can act without interference. 

 The crop is a temporary reservoir for the food, and the 

 gizzard is the only place in the entire digestive system 

 adapted for dividing large particles of food into minute 

 pieces. Earthworms are known to swallow small, rough 

 pebbles, and even sharp pieces of glass, for the purpose of 

 using them in the gizzard to grind into bits other particles 

 which are swallowed as food. The strong muscles in the 

 wall of the gizzard, by contracting, grind the contents to- 

 gether, and every particle is worn smaller. 



The process of digestion in an earthworm begins when, 

 on seizing food with its lip, the animal pours out a secretion 

 from glands in the pharynx. Digestion continues as the food 

 is being drawn into the pharynx, and while on its way 

 through the esophagus. 



The digestive fluid of the earthworm resembles the diges- 

 tive fluid of the higher animals, in being capable of acting 

 on the three important classes of organic foods. 



Circulation. Whenever an animal body is too large or com- 

 plicated for the food to reach all the cells directly, we find 

 a circulatory system developed. The circulatory system of 

 an earthworm is complete and rather complex. The dorsal 

 blood vessel extends the length of the animal, along the 

 middle, between the body wall and the alimentary canal. 

 The blood flows toward the anterior region in somewhat 

 regular "pulses," carried along by waves of muscular con- 

 traction in the wall of the blood vessel itself. Between the 

 pharynx and the crop are situated five pairs of "hearts." 

 These short tubes receive most of the blood that comes 



