388 Animal Biology 



which the worm crawls. These plastered tunnels prevent their collapse 

 and allow moisture and air to penetrate the soil. In this way the earth- 

 worms cultivate the soil. 



Enzymes of the digestive juices act on foods in a manner similar to 

 that of higher animals, as shown by the following table : 



ENZYME 



FOOD ACTED ON 



PRODUCTS FORMED 



Trypsin 

 Diastase 

 Steapsin 



Proteins 



Carbohydrates 



Fats 



Peptones 



Sugars 



Fatty acids and glycerin 



Absorption in the earthworm occurs through the walls of the 

 intestine, being assisted by the amoeboid action of some of the lining 

 epithelial cells. Some absorbed foods are placed in the coelomic 

 cavity where they are circulated by the coelomic fluid. Other ab- 

 sorbed foods are placed in the circulatory system to be taken to various 

 parts of the body. 



Prostominm 



Buccal cavity 

 \ 



Circumpharyngeal 

 Cerebral gransrlion connective 



In- 



in 

 I. 



V 



Seermental nerve 



Month 



Subpharynfireal ^nslion 



Septal nerve 



Fig. 188. — Earthworm nervous system. Side view of anterior end with the 

 cerebral gangHon and larger nerves. (From Hegner: College Zoology. By per- 

 mission of The Macmillan Company, publishers.) (After Hess.) 



Circulation. — A complex system of blood vessels forms a so-called 

 ''closed type'' of circulatory system. A closed system is one in which 

 the blood flows, more or less continuously, in the vessels, with only a 

 limited amount of its constituents passing in and out through their walls. 

 The more important parts of the system are: (1) a dorsal blood vessel 

 dorsal to the digestive tract, (2) a ventral vessel ventral to the digestive 

 tract, (3) five pairs of pulsating, looplike heart arches in segments VII 



