164 



PHYSIOLOGY 



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through the entire length of the 

 animal (Fig. 94). If it is cut 

 open it will be found full of 

 earth. The worm burrows, and 

 as it has no appendage which it 

 can use for this purpose it must 

 eat its way through the ground. 

 The earth passes into the canal 

 at the anterior end and out at 

 the posterior end. During its 

 passage the worm gets from the 

 water and organic substances 

 which the soil contains the food 

 necessary for its life. The con- 

 stant burrowing of worms 

 loosens the soil ; further, in pass- 

 ing through the intestine the soil 

 becomes mixed with digestive 

 juices and is ground into fine 

 particles. In this way the 

 earth-worm increases the fertil- 

 ity of the soil and is of great 

 benefit to farmers. 



The skin is a moist mem- 

 brane. Through it oxygen 

 passes into the blood and carbon 

 dioxide out of it. 



Increase of Digestive Sur- 

 face. — If the inside wall of the 



Fig. 93.— Earthworm, ventral surface, an, anus ; c, girdle ; m, mouth ; o.d., ex- 

 ternal openings of the oviducts ; s, setae ; s.r., openings of the seminal recep- 

 tacles ; s.d., external openings of the sperm ducts. (From Sedgwick and 

 Wilson. 



