THE EARTHWORM AND OTHER SEGMENTED WORMS 171 



ice when used in climbing up in the burrows, since the body of 

 the animal fits its burrow so snugly. 



DIGGING THE BURROW. When digging in soft earth, the 

 worm simply forces its way through by alternate extensions 

 and contractions of the body, but in harder soil it must eat its 

 way through. The body of the worm is like a double tube 

 (Fig. 97), a small one represented by the straight alimentary 

 canal within the larger one, the body wall. The earth that is 

 eaten in digging passes into and directly through the alimentary 

 canal, reaching the surface in the form of castings. The cast- 

 ings of earthworms are the dark heaps of earth so often to be 

 seen on the ground after damp weather. 



Food. Digging in this way not only results in a burrow for 

 the worm, but provides food for it as well, since the soil con- 

 tains the decaying vegetable matter or humus upon which it 

 feeds, and the animals are careful to make their burrows in soil 

 containing a good supply of humus. Besides this, other food 

 is gathered usually at night when the worms are active. The 

 animals crawl out on the surface, and holding fast to the top 

 of their burrows with their tails, explore the neighborhood for 

 pieces of leaves which they drag into their holes. These leaves 

 when decomposed serve as food for the worms. 



Digestion. The alimentary canal, as usual in higher animals, 

 may be separated into distinct parts, and is accompanied by 

 glands which secrete juices that are discharged into it. The 

 food, after being sucked in by the muscular pharynx, passes 

 into the oesophagus, where it is mixed with a secretion from the 

 calciferous glands; this secretion neutralizes the acids in the 

 food. It then enters the crop, a thin-walled storage place. 

 From the crop it passes into the muscular gizzard, where it is 

 ground up, a process often aided by minute solid particles, like 

 grains of sand, that are swallowed with the food. Next it enters 

 the intestine, where the digestion and absorption chiefly take 

 place (Fig. 96, i). 



Circulation and Excretion. A complicated system of blood 



