THE EARTHWORM AND ITS ALLIES 135 



very great. Thus they may be kept for months in a 

 moist vessel without food, or with only filter paper, with- 

 out starving. On the other hand, they die in a dry atmos- 

 phere in a few hours, whereas they may be submerged in 

 water for several days without injury. Very remarkable is 

 their power of healing after injury. If an earthworm be 

 cut in two near the middle, and the halves be kept under 

 favorable conditions, each half may develop its missing 

 organs so that two complete worms will result. The 

 anterior half of one worm may be attached to the hinder 

 end of a second worm by the cut edges, owing to the 

 fact that the cut edges grow together. This operation 

 is called grafting. 



Economics. Earthworms are, to a certain extent, in- 

 jurious to vegetation, since they eat tender seedlings and 

 roots, but, on the other hand, they are almost indispensable 

 to agriculture. Their burrows permit rain to percolate 

 deep into the ground, instead of running off on the sur- 

 face. They keep the soil loose, facilitating the penetration 

 of the roots of plants. The earth that passes through 

 their bodies is ejected on the surface of the ground near 

 the openings of their burrows, and is called a " casting." 

 By means of castings the deeper-lying earth is brought to 

 the surface, and the surface layer of rich earth, called 

 " vegetable mould," is in this way increased in thickness 

 by additions to its upper surface. The thickness of the 

 layer of mould which the castings of one year, if uniformly 

 spread out, would make has been estimated by Darwin to 

 be in England about two-tenths of an inch. Most of these 

 castings are merely taken from the deeper-lying mould, 

 but they are enriched by the intestinal secretions in pass- 

 ing through the body of the worm. These intestinal secre- 



