EAKTH-WOKMS. 



227 



loose tesserae ; worms have penetrated the old 

 walls of the ruins, and were found in them, 

 with traces of mold ; and the pavement had 

 sunk considerably in nearly all the rooms. 



The chief share of the work of covering the 

 buildings is attributed to worms. 



Worms also contribute to the disintegration 

 of the rocks and the denudation of the land, 



West. 



FIG. 5. SECTION THROUGH THE FOUNDATIONS or A BURIED ROMAN VILLA AT ABINGEB. A A, vegetable mold ; 

 B, dark earth full of stones, thirteen inches in thickness ; C, hlack mold ; D, broken mortar ; E, black mold; 

 F F, undisturbed sub-soil ; G, tesserae; H, concrete ; I, nature unknown ; W, buried wall. 



by generating humus acids which act on the 

 carbonates, by grinding up in their crops the 

 stones they swallow, and by bringing earth to 

 the surface in their castings, to be blown away 

 by the winds and washed away by the rains 

 into the valleys. They are extraordinarily nu- 

 merous. Hensen says there are 53,767 of them 



South. 



in an acre of garden-soil, and Mr. Darwin is 

 willing to allow half that number, or 26,886, 

 to the acre in corn-fields and pasture-lands ; 

 and as in many parts of England a weight of 

 more than ten tons of dry earth annually passes 

 through their bodies and is brought to the sur- 

 face on each acre, the whole superficial bed of 



North. 



FIG. 6. A NORTH AND SOUTH SECTION THROUGH THE SUBSIDED FLOOR OF A CORRIDOR PAVED WITH TESSERA 

 (Silchester). Outside the broken-down bounding walls, the excavated ground on each side is shown for a 

 short space. Mature of the ground beneath the tesserae unknown. Scale, .fe. 



vegetable mold must pass through them every 

 few years. By triturating this earth, by sub- 

 jecting its minerals to the action of the humus 

 acids, and by periodically exposing the mold to 

 the air, they prepare the ground in an excellent 

 manner for the growth of fibrous-rooted plants 

 and for seedlings. 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 accumulated castings of 

 worms, and are thus brought in a more or less 

 decayed state within reach of the roots of 

 plants. Leaves are digested by them and con- 

 verted into humus. Their burrows, penetrat- 



