THE PART PLAYED BY WORMS. 43 



can not work. It is probable that many monoliths and 

 some old walls have fallen down from haying been under- 

 mined by worms. 



THEY PREPARE THE GKOUXD FOR SEED. 



Worms prepare the ground in an excellent 

 manner for the growth of fibrous-rooted plants 

 and for seedlings of all kinds. They periodically expose 

 the mold to the air, and sift it so that no stones larger 

 than the particles which they can swallow are left in it. 

 They mingle the whole intimately together, like a gar- 

 dener who prepares fine soil for his choicest plants. In 

 this state it is well fitted to retain moisture and to absorb 

 all soluble substances, as well as for the process of nitrifi- 

 cation. The bones of dead animals, the harder parts of 

 insects, the shells of land-mollusks, leaves, twigs, etc., 

 are before long all buried beneath the accumulating cast- 

 ings of worms, and are thus brought in a more or less 

 decayed state within reach of the roots of plants. Worms 

 likewise drag an infinite number of dead leaves and other 

 parts of plants into their burrows, partly for the sake of 

 plugging them up and partly as food. 



The leaves which are dragged into the burrows as 

 food, after being torn into the finest shreds, partially di- 

 gested, and saturated with the intestinal and urinary se- 

 cretions, are commingled with much earth. This earth 

 forms the dark-colored, rich humus which almost every- 

 where covers the surface of the land with a fairly well- 

 defined layer or mantle. Von Hensen placed two worms 

 in a vessel eighteen inches in diameter, which was filled 

 with sand, on which fallen leaves were strewed ; and 

 these were soon dragged into their burrows to a depth of 

 three inches . After about six weeks an almost uniform 

 layer of sand, a centimetre ('4 inch) in thickness, was 



